STEEL PLATE HAVING EXCELLENT TOUGHNESS AND RESISTANCE TO HYDROGEN-INDUCED CRACKING, AND STEEL PIPE FOR LINE PIPE

Abstract

 A steel plate and a steel pipe having excellent hydrogen-induced cracking resistance and toughness are achieved. Further, the steel plate and steel pipe are achieved that can evaluate the hydrogen-induced cracking resistance based on the internal quality of a cast strip without performing a hydrogen-induced cracking test after rolling. The steel plate having the excellent hydrogen-induced cracking resistance and toughness satisfies the specified contents of C, Si, Mn, P, S, Al, Ca, N, and O, and further contains the specified content of one or more elements selected from the group consisting of REM and Zr, with the balance being iron and inevitable impurities. The steel plate is further characterized by that the ratio (Ca/S) of the Ca to the S is 2.0 or more; the Ca, the S, and the O satisfy the relationship of (Ca - 1.25S)/O ≤ 1.80; an Ar gas content in a steel is 0.50 µL/cm³ or less; and a decrease in the amount of Ca obtained by subtracting a Ca concentration in a slab from a Ca concentration in a molten steel in a tundish is a threshold value Ca_dropθ or less, the threshold value Ca_dropθ being a maximum decrease in the amount of Ca that avoids the occurrence of hydrogen-induced cracking in the steel plate obtained by rolling the slab.

Description

Technical Field

[0001] The present invention relates to a steel plate that has excellent hydrogen-induced cracking resistance and toughness, and is suitable for use in line pipes for transportation and tanks for storage of natural gas and crude oil, and to a steel pipe for line pipes with excellent hydrogen-induced cracking resistance and toughness, obtained by using the steel plate.

Background Art

[0002] With the development of degradation resources containing hydrogen sulfide, mainly, line pipes for transportation and tanks for storage of oil, gas, etc., require so-called sour resistance, such as hydrogen-induced cracking resistance or stress-corrosion cracking resistance. Hereinafter, a steel plate that exhibits the adequate sour resistance is sometimes referred to as a "sour-resistant steel plate" in some cases. Hydrogen-induced cracking (hereinafter sometimes referred to as "HIC") is known as a crack caused by the penetration of hydrogen thereinto due to a corrosion reaction with the hydrogen sulfide or the like, and the collection and gasification of the hydrogen at non-metallic inclusions, such as MnS or Nb(C, N).

[0003] HIC is also known to have a tendency to occur in segregation zones, including a center segregation and internal cracks of a cast strip, particularly, at an inclusion, such as MnS, as a starting point. For this reason, some techniques for enhancing HIC resistance have been proposed. For example, Patent Document 1 discloses that a steel material has improved HIC resistance by suppressing segregation degrees of Mn, Nb, and Ti at the center in the thickness direction of a steel plate. Patent Document 2 discloses a method for suppressing HIC that would occur in MnS or a Ca-based acid sulfide as a starting point, by using a parameter formula that includes the contents of Ca, O, and S.

[0004] These methods suppress the occurrence of a large amount of HIC, but in some cases, fine HIC can occur locally at a number of sites.

[0005] Meanwhile, a steel plate is subjected to melting, casting, and hot-rolling, and then it undergoes an HIC test before being dispatched as a product. However, it takes several weeks to obtain the result of the HIC test. Once the HIC occurs during the HIC test, the above-mentioned steel plate cannot be dispatched as a product with excellent hydrogen-induced cracking resistance. Because of this, the steel plate needs to be manufactured again, that is, melted again to produce a product, and then the product needs to undergo the HIC test again. This increases the manufacturing time period and might possibly result in missing the deadline or the like.

[0006] For this reason, it is considered that if the HIC resistance can be evaluated at the stage of a cast strip after the casting without performing the HIC test after hot rolling, the manufacturing time period can be significantly shortened. As mentioned above, HIC occurs at segregation zones (center segregation, internal cracks) or inclusions, such as MnS, as a starting point. Thus, if these can be evaluated at the stage of the cast strip, the evaluation of the HIC resistance is considered to be possible based on the evaluation results.

[0007] For example, in a conventional method that involves performing an HIC test after rolling, a long procedure A-1 from casting to dispatching is carried out in the following way. In contrast, when the HIC resistance can be evaluated at the stage of the cast strip, the steps of "Sample Preparation (for HIC test) → HIC Test" in performing the HIC test can be omitted as illustrated in a procedure B-1, so that products can be dispatched at an early stage.
Procedure A-1: Casting → Rolling → Sample Preparation (for HIC test) → HIC Test → Dispatching
Procedure B-1: Casting → Evaluation of HIC Resistance → Rolling → Dispatching

[0008] If the result of the HIC test is no good (NG), the conventional method would be to perform the following procedure A-2, where it takes a long time to perform steps from the casting to re-melting. In contrast, when the HIC resistance can be evaluated at the stage of the cast strip as illustrated in the following procedure B-2, even if the evaluation result is NG, the steps of "Rolling → Sample Preparation (for the HIC Test) → HIC Test" in the procedure A-2 below can be omitted, which enables a quick start of re-melting.
Procedure A-2: Casting → Rolling → Sample Preparation (for HIC test) → HIC Test → Re-Melting
Procedure B-2: Casting → Evaluation of HIC Resistance → Re-Melting

[0009] As such a method, Patent Document 3 discloses a method in which internal cracks are evaluated at the stage of the cast strip. In this method, the possibility of a hot charge rolling (HCR) operation is determined based on the evaluation result of internal cracks.

[0010] Furthermore, Patent Documents 4 to 8 disclose methods for evaluating the quality of a cast strip before rolling, although they are not intended to evaluate CaO inclusions. For example, techniques mentioned in Patent Documents 4 to 7 evaluate the quality of a cast strip based on the content of inclusions or the content of elements in the cast strip or molten steel in a tundish or the like. In a technique mentioned in Patent Document 8, the quality of the cast strip is evaluated from an analysis result of the molten steel in the tundish (primary determination). If the determination accuracy does not meet the predetermined accuracy, the quality of the cast strip is evaluated from an analysis result of a cast strip sample (secondary determination).

Prior Art Document

Patent Document

[0011] 

Patent Document 1: JP 2010-209461 A

Patent Document 2: JP H6-136440 A

Patent Document 3: JP 2006-198649 A

Patent Document 4: JP S62-277539 A

Patent Document 5: JP 2002-214222 A

Patent Document 6: JP H10-122854 A

Patent Document 7: JP H10-249505 A

Patent Document 8: JP 2000-292418 A

Disclosure of the Invention

Problems to be Solved by the Invention

[0012] Although the techniques mentioned in Patent Documents 3 to 8 are not intended to evaluate CaO inclusions as mentioned above, an evaluation method for CaO inclusions is considered to include evaluation of the content of inclusions or content of elements and the like in the cast strip or molten steel in the tundish, like Patent Documents 3 to 8.

[0013] To evaluate the CaO inclusions at the stage of the cast strip, it is necessary to analyze the CaO content or Ca concentration in a position where a CaO accumulation zone is generated. However, there are variations of the position where the CaO accumulation zone is generated, in the dimensions of a cast strip in the width direction, thickness direction, and casting direction, which makes it difficult to predict the position. Furthermore, even the analysis of a predetermined part of the cast strip does not necessarily lead to the analysis result that shows the CaO content in the CaO accumulation zone. Thus, the analysis result of the cast strip cannot be used to evaluate the CaO inclusions.

[0014] It is also considered that the CaO inclusions are evaluated from the content of inclusions, the content of elements, or the like in the molten steel in the tundish. However, CaO inclusions are aggregated and accumulated after being charged into a mold. Thus, even though no CaO accumulation zone is evaluated to be present based on the CaO content or Ca concentration in the molten steel in the tundish, CaO inclusions can be aggregated thereafter, causing the HIC.

[0015] The present invention has been made in view of the foregoing circumstance, and it is an object of the present invention to achieve a steel plate and a steel pipe that have excellent hydrogen-induced cracking resistance and toughness, and further to achieve a steel plate and a steel pipe that enable the evaluation of the HIC resistance by the internal quality of a cast strip without performing an HIC test.

Means for Solving the Problems

[0016] A steel plate having excellent hydrogen-induced cracking resistance and toughness according to the present invention that can solve the above-mentioned problem includes, in percent by mass:

0.02 to 0.15% of C;

0.02 to 0.50% of Si;

0.6 to 2.0% of Mn;

more than 0% and 0.030% or less of P;

more than 0% and 0.003% or less of S;

0.010 to 0.08% of Al;

0.0003 to 0.0060% of Ca;

0.001 to 0.01% of N;

more than 0% and 0.0045% or less of O; and

one or more elements selected from the group consisting of:

more than 0% and 0.02% or less of REM and more than 0% and 0.010% or less of Zr, with the balance being iron and inevitable impurities, wherein

a ratio (Ca/S) of the Ca to the S is 2.0 or more,

the Ca, the S, and the O satisfy a formula below: (Ca -1.25S)/O ≤ 1.80,

an Ar gas content in a steel is 0.50 µL/cm³ or less, and

a decrease in an amount of Ca obtained by subtracting a Ca concentration in a slab from a Ca concentration in a molten steel in a tundish is a threshold value Ca_dropθ or less, the threshold value Ca_dropθ being a maximum decrease in an amount of Ca that avoids occurrence of hydrogen-induced cracking in the steel plate obtained by rolling the slab.

[0017] The threshold value Ca_dropθ may be a value previously determined by a method including following (i) to (iii):

(i) the Ca concentration in the molten steel in the tundish and the Ca concentration in the slab are measured, and the decrease in the amount of Ca is calculated by subtracting the Ca concentration in the slab from the Ca concentration in the molten steel in the tundish;

(ii) a hydrogen-induced cracking test is performed on a steel plate that is obtained by rolling a slab, which has been cast on the same casting conditions as the above-mentioned slab; and

(iii) the maximum decrease in an amount of Ca that avoids the occurrence of hydrogen-induced cracking is determined from the decrease in an amount of Ca measured by the step (i) and a result of the hydrogen-induced cracking test shown in the step (ii).

[0018] The slab cast on the same casting conditions as the above-mentioned slab may be the slab in which the decrease in the amount of Ca is measured.

[0019] The Ca concentration in the slab may be a minimum Ca concentration of two or more Ca concentrations obtained by examining the Ca concentration at two or more different positions in the thickness direction of the slab.

[0020] The threshold value Ca_dropθ may be 4 ppm (ppm by mass).

[0021] The steel plate may further include one or more of the elements (A) and (B) below as another element:

(A) in percent by mass, one or more elements selected from the group consisting of more than 0% and 0.005% or less of B, more than 0% and 0.1% or less of V, more than 0% and 1.5% or less of Cu, more than 0% and 1.5% or less of Ni, more than 0% and 1.5% or less of Cr, more than 0% and 1.5% or less of Mo, and more than 0% and 0.06% or less of Nb; and

(B) in percent by mass, one or more elements selected from the group consisting of more than 0% and 0. 03% or less of Ti and more than 0% and 0.01% or less of Mg.

[0022] The steel plate is suitable use in line pipes and pressure containers. The invention also includes a steel pipe for a line pipes, formed of the steel plate.

Effects of the Invention

[0023] The invention can provide the steel plate and steel pipe that surely have the excellent hydrogen-induced cracking resistance and toughness. Further, the present invention can provide the steel plate and steel pipe in which the HIC resistance can be evaluated by the internal quality of a cast strip without performing the HIC test. These steel plates are suitable for use in line pipes for transportation and pressure containers, such as tanks for storage, of natural gas and crude oil, and the like.

Brief Description of the Drawings

[0024] 

Fig. 1 is a schematic diagram for explaining the flow of CaO inclusions.

Fig. 2 is a diagram showing Ca concentration distribution in various slabs.

Fig. 3 (a) is a cross-sectional view of a slab, and Fig. 3 (b) is a cross-sectional view of a product.

Fig. 4 is a cross-sectional view of the slab.

Fig. 5 is a diagram for explaining an examined surface of the slab.

Fig. 6 is a diagram showing the result of determination of a threshold value in an example according to a first embodiment, specifically, showing the relationship among a Ca concentration Ca_TD1 in a molten steel in a tundish, a Ca concentration Ca_S1 in a slab, and HIC test results thereof.

Fig. 7 is a diagram showing the result of determination of a threshold value in an example according to a second embodiment, specifically, showing the relationship among a Ca concentration Ca_TD1 in a molten steel in a tundish, a minimum value Ca_min1 of the Ca concentration in a slab, and HIC test results thereof.

Mode for Carrying Out the Invention

[0025] The inventors have diligently studied to solve the foregoing problems. First, the inventors have performed a HIC test specified by the National Association of Corrosion and Engineer (NACE) standard TM0284, on various steel plates to evaluate the HIC resistance thereof. The NACE test is a test that involves preparing a solution at pH2.7 that contains 5% sodium chloride (NaCl) solution + 0.5% acetic acid, followed by saturating the solution with hydrogen sulfide gas, then exposing a test specimen to the solution for 96 hours, and finally evaluating the occurrence of HIC in the test specimen.

[0026] Then, the inventors have performed the Charpy impact test according to ASTM A370 for examining the Charpy impact properties after the HIC test, on a surface layer part of a steel plate (for example, see CAMP-ISIJ Vol. 24(2011)-P671), which is known to have its hydrogen concentration particularly increased during the HIC test. The result shows that there are variations in the Charpy impact absorbed energy value.

[0027] The cause for this result, particularly, regarding a specimen having a low Charpy impact absorbed energy value and further variations in the Charpy impact absorbed energy value has been examined. Consequently, it is found that there occurs fine HIC that cannot be observed with a microscope at a magnification of 100X specified by the NACE test (that is, the fine HIC has a size of an observation limit or less). Furthermore, as a result of considering the cause for the occurrence of the fine HIC mentioned above, Ar gas in steel is first found to cause such HIC.

[0028] For this reason, when examining the relationship between the Ar gas content in a steel and variations in the toughness of a surface layer part of a steel plate obtained after the HIC test, it is found that there is an obvious correlation between both these factors. As a result of further examination in detail, when the Ar gas content is high, the following facts are revealed: Ar bubbles remaining in a slab are not completely crimped during hot-rolling and are left as fine defects in the steel plate; and fine HIC occurs at the Ar bubble as a starting point, causing separation in the Charpy impact test, which is cracking in parallel with the surface of the steel plate, thereby resulting in reduced Charpy impact absorbed energy.

[0029] Next, to obtain a high-level stable toughness of the surface layer part of the steel plate after the HIC test, it is considered how much Ar gas content should be restricted in the steel, as evaluated in Examples to be mentioned later. Consequently, it is found that the Ar content in the steel should be set at 0.50 µL (microliters)/cm³ or less. The Ar content is preferably 0. 30 µL/cm³ or less, and more preferably 0.25 µL/cm³ or less. In practice, it is difficult to restrict the Ar content in the steel to zero because Ar needs to be blown into a molten steel during manufacturing steps, including, for example, the prevention of clogging of an injection nozzle, the reflux at a ruhrstahl-heraeus (RH) for degassing, and stirring or the like of steel in a tundish (TD) for flotation separation of inclusions.

[0030]  Note that the above-mentioned "Ar gas content in the steel" is determined by a method mentioned in Examples below.

[0031] Next, the inventors have focused on the tendency for HIC to occur at a MnS inclusion as a starting point. As a result, it is conceived that by causing a steel to contain a rare earth element or Zr, which has a desulfurization effect, the formation of MnS can be suppressed, and the hydrogen-induced cracking resistance can be improved. Further, an appropriate content of such an element is found to efficiently exhibit its desulfurization effect as mentioned later.

[0032] Furthermore, the inventors have focused on the tendency for HIC to occur at a CaO accumulation zone generated as the starting point during producing a cast strip. Consequently, attention is paid to the "decrease in the amount of Ca that is obtained by subtracting the Ca concentration in the slab from the Ca concentration in the molten steel in the tundish", and which can evaluate the presence or absence of the CaO accumulation zones. It is found that if the decrease in the amount of Ca at the stage of the slab is restricted to a predetermined threshold value or less, a steel plate with higher hydrogen-induced cracking resistance can be obtained, so that products can be dispatched at an early stage. This matter will be described in detail below.

[0033]  First, the composition of a steel will be described below. Note that the notation "%" for each component means "% by mass", while the notation "ppm" means "ppm by mass".

[0034] To ensure excellent HIC resistance and toughness, the composition of the steel needs to be controlled. Furthermore, to ensure the high strength, excellent weldability, and the like, which are other properties required as, for example, the steel for line pipes, the composition of the steel plate needs to be as follows. The reasons for specifying the contents of the respective components, including the aforesaid REM and Zr, will be described below.

Composition

[C: 0.02 to 0.15%]

[0035] Carbon (C) is an element essential to ensure the strength of a base metal and a weld bead. Thus, the C content needs to be 0.02% or more. The C content is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, an extremely high C content degrades the heat-affected zone (HAZ) toughness and the weldability of the steel. Any excessive C content is more likely to form NbC or island-shaped martensite, which possibly becomes as the starting point of HIC or a fracture propagation route. Thus, the C content needs to be 0.15% or less. The C content is preferably 0.12% or less, and more preferably 0.10% or less.

[Si: 0.02 to 0.50%]

[0036] Silicon (Si) has a deoxidation function and is effective in improving the strength of a base metal and a weld bead. To exhibit these effects, the Si content is set at 0.02% or more. The Si content is preferably 0.05% or more, and more preferably 0.15% or more. However, an extremely high Si content degrades the weldability and toughness of the steel. Any excessive Si content forms island-shaped martensite to generate and propagate HIC. Accordingly, the Si content needs to be suppressed to 0.50% or less. The Si content is preferably 0.45% or less, and more preferably 0.35% or less.

[Mn: 0.6 to 2.0%]

[0037] Manganese (Mn) is an element that is effective in improving the strength of a base metal and a weld bead. In the present invention, the Mn content is set at 0.6% or more. The Mn content is preferably 0.8% or more, and more preferably 1.0% or more. However, an extremely high Mn content forms MnS, degrading not only the hydrogen-induced cracking resistance, but also the HAZ toughness and weldability. Thus, the upper limit of Mn content is set at 2.0%. The Mn content is preferably 1.8% or less, more preferably 1.5% or less, and still more preferably 1.2% or less.

[P: more than 0% and 0.030% or less]

[0038] Phosphorus (P) is an element inevitably contained in steel. When the P content exceeds 0.030%, the roughness of a base metal and a HAZ are significantly degraded, and the hydrogen-induced cracking resistance of the steel is also degraded. Thus, in the present invention, the P content is restricted to 0.030% or less. The P content is preferably 0.020% or less, and more preferably 0.010% or less.

[S: more than 0% and 0.003% or less]

[0039] Sulfur (S) is an element that forms a large amount of MnS to significantly degrade the hydrogen-induced cracking resistance when contained in a large amount. Thus, in the present invention, the upper limit of S content is 0.003%. The S content is preferably 0.002% or less, more preferably 0.0015% or less, and still more preferably 0.0010% or less. Thus, the S content is desirably low from the viewpoint of improving the hydrogen-induced cracking resistance.

[Al: 0.010 to 0.08%]

[0040] Aluminum (Al) is a strong deoxidizing element. When the Al content is low, the Ca concentration in the oxide tends to increase, that is, the Ca-based inclusions are more likely to be formed at a superficial layer of a steel plate, causing fine HIC. Thus, in the present invention, the Al content needs to be 0.010% or more. The Al content is preferably 0.020% or more, and more preferably 0.030% or more. On the other hand, when the Al content is extremely high, an Al oxide is formed in a cluster shape and becomes a starting point of hydrogen-induced cracking. Thus, the Al content needs to be 0.08% or less. The Al content is preferably 0.06% or less, and more preferably 0.05% or less.

[Ca: 0.0003 to 0.0060%]

[0041] Calcium (Ca) serves to control the form of a sulfide and has an effect of suppressing the formation of MnS by forming CaS. To obtain this effect, the Ca content needs to be 0.0003% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when the Ca content exceeds 0.0060%, HIC occurs at many sites from the Ca-based inclusions as the starting point. Thus, in the present invention, the upper limit of Ca content is set at 0.0060%. The Ca content is preferably 0.0045% or less, more preferably 0.0035% or less, and still more preferably 0.0025% or less.

[N: 0.001 to 0.01%]

[0042] Nitrogen (N) precipitates as TiN in a steel microstructure, preventing austenite grains in a HAZ zone from being coarsened and further promoting ferrite transformation to thereby improve the toughness of the HAZ zone. To obtain these effects, the N content needs to be 0.001% or more. The N content is preferably 0.003% or more, and more preferably 0.0040% or more. An extremely high N content, however, degrades the toughness of the HAZ by the presence of the solid-solute N. The N content needs to be 0.01% or less. The N content is preferably 0.008% or less, and more preferably 0.0060% or less.

[O: more than 0% and 0.0045% or less]

[0043] An oxygen (O) content is desirably low from the viewpoint of improving the cleanliness of a steel. An extremely high O content degrades the toughness of the steel, and additionally causes HIC at an oxide as a starting point, thereby degrading the hydrogen-induced cracking resistance. In this regard, the O content needs to be 0.0045% or less, and is preferably 0.0030% or less, and more preferably 0.0020% or less.

[Ca/S in terms of mass ratio: 2.0 or more]

[0044] As mentioned above, S forms MnS as a sulfide-based inclusion, and HIC might occur at the MnS as a starting point. Thus, the sulfide-based inclusion in the steel has its form controlled as CaS by adding Ca, thereby rendering S harmless for the HIC resistance. To sufficiently exhibit these effects, the Ca/S needs to be set at 2.0 or more. The Ca/S is preferably 2.5 or more, and more preferably 3.0 or more. Note that the upper limit of Ca/S is approximately 17 based on the Ca content and S content specified by the present invention.

(Ca - 1.25S)/O ≤ 1.80

[0045] To avoid the occurrence of HIC due to a Ca-based oxysulfide, it is effective to suppress, especially, CaO that is the most likely to form aggregates among Ca-based inclusions. For this reason, a Ca content (Ca - 1.25S) that is obtained by subtracting a content in Ca present as a sulfide (CaS) in the steel from the total Ca content in the steel must not be excessive relative to the O content. When the Ca content (Ca - 1.25S) is excessive relative to the O content, CaO is more likely to be formed as an oxide-based inclusion, which makes it easier for aggregates of the CaO (coarse Ca-based inclusions) to be formed in a larger amount at a superficial layer of a steel plate. Since these coarse Ca-based inclusions each serve as the starting point of HIC, the (Ca - 1.25S)/O needs to be 1.80 or less in order to obtain the excellent HIC resistance. (Ca - 1.25S)/O is preferably 1.40 or less, more preferably 1.30 or less, still more preferably 1.20 or less, and particularly preferably 1.00 or less. Like CaO, to suppress Al₂O₃ that tends to form aggregates, the lower limit of (Ca - 1.25S)/O is approximately 0.1.

[REM: more than 0% and 0.02% or less]

[0046] A rare earth metal (REM) is an element that is effective in enhancing the hydrogen-induced cracking resistance by suppressing the formation of MnS through the desulfurization effect as mentioned above. To exhibit such effects, the REM content is preferably 0.0002% or more. The REM content is more preferably 0.0005% or more, and still more preferably 0.0010% or more. On the other hand, if REM is contained in a large amount, its effects are saturated. Thus, the upper limit of the REM content needs to be 0.02%. From the viewpoint of preventing the clogging of an immersion nozzle during casting to enhance the productivity, the REM content is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.0050% or less. Note that in the present invention, REM means lanthanoid elements (15 elements from La to Lu), scandium (Sc), and yttrium (Y).

[Zr: more than 0% and 0.010% or less]

[0047] Zirconium (Zr) is an element that serves to form an oxide and disperse it finely in steel, while improving the HIC resistance by the desulfurization effect, thereby contributing to improving the HAZ toughness. To exhibit these effects, the Zr content is preferably set at 0.0003% or more. The Zr content is more preferably 0.0005% or more, still more preferably 0.0010% or more, and yet more preferably 0.0015% or more. On the other hand, the addition of an excessive content of Zr forms coarse inclusions to degrade the hydrogen-induced cracking resistance of a steel plate and the toughness of a base metal. Thus, the Zr content needs to be 0.010% or less. The Zr content is preferably 0.0070% or less, more preferably 0.0050% or less, and still more preferably 0.0030% or less.

[0048] The components of the steel (steel plate, steel pipe) in the present invention have been mentioned above, with the balance being iron and inevitable impurities. In addition to the elements mentioned above, the steel further includes:

(a) one or more elements selected from the group consisting of B, V, Cu, Ni, Cr, Mo, and Nb in the following contents, thereby making it possible to enhance the strength and toughness; and/or

(b) one or more elements selected from the group consisting of Ti and Mg in the following contents, thereby making it possible to improve the HAZ toughness and to promote the desulfurization, thus further improving the HIC resistance. These elements will be described in detail below.

[B: more than 0% and 0.005% or less]

[0049] Boron (B) enhances the hardenability of a steel and the strength of a base metal and a weld bead. Furthermore, B is bonded to N to precipitate BN while the heated HAZ zone is cooled in welding, thus promoting ferrite transformation from the inside of an austenite grain. In this way, B improves the HAZ toughness. To obtain these effects, the B content is preferably 0.0002% or more. The B content is more preferably 0.0005% or more, and still more preferably 0.0010% or more. However, any excessive B content degrades the toughness of a base metal and a HAZ zone, thus leading to degradation in the weldability. Thus, the B content is preferably 0.005% or less. The B content is more preferably 0.004% or less, and still more preferably 0.0030% or less.

[V: more than 0% and 0.1% or less]

[0050] Vanadium (V) is an element effective in improving the strength of steel. To obtain this effect, the V content is preferably 0.003% or more, and more preferably 0.010% or more. On the other hand, when the V content exceeds 0.1%, the weldability and the toughness of a base metal would be degraded. Thus, the V content is preferably 0.1% or less, and more preferably 0.08% or less.

[Cu: more than 0% and 1.5% or less]

[0051] Copper (Cu) is an element effective in improving the hardenability of steel. To obtain this effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when the Cu content exceeds 1.5%, the toughness of steel is degraded. Thus, the Cu content is preferably 1.5% or less. The Cu content is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Ni: more than 0% and 1.5% or less]

[0052] Nickel (Ni) is an element effective in improving the strength and toughness of a base metal and a weld bead. To obtain these effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.05% or more, and still more preferably 0.10% or more. However, an extremely high Ni content leads to an excessively expensive steel for a structure. From the economical aspect, the Ni content is preferably 1.5% or less. The Ni content is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Cr: more than 0% and 1.5% or less]

[0053] Chromium (Cr) is an element effective in improving the strength of steel. To obtain such an effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when the Cr content exceeds 1.5%, the HAZ toughness of the steel is degraded. Thus, the Cr content is preferably 1.5% or less. The Cr content is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Mo: more than 0% and 1.5% or less]

[0054] Molybdenum (Mo) is an element effective in improving the strength and toughness of a base metal. To exhibit this effect, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.05% or more, and still more preferably 0.10% or more. However, when the Mo content exceeds 1.5%, the HAZ toughness and weldability of the steel are degraded. Thus, the Mo content is preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.50% or less.

[Nb: more than 0% and 0.06% or less]

[0055] Niobium (Nb) is an element effective in enhancing the strength of steel and the toughness of a base metal without degrading its weldability. To obtain this effect, the Nb content is preferably 0.002% or more. The Nb content is more preferably 0.010% or more, and still more preferably 0.020% or more. However, when the Nb content exceeds 0.06%, the toughness of the base metal and HAZ is degraded. Thus, in the present invention, the upper limit of Nb content is preferably set at 0.06%. The Nb content is more preferably 0.050% or less, still more preferably 0.040% or less, and yet more preferably 0.030% or less.

[Ti: more than 0% and 0.03% or less]

[0056] Titanium (Ti) precipitates as TiN in steel, thereby preventing austenite grains in a HAZ zone from being coarsened during welding and thereby promoting the ferrite transformation. Thus, Ti is an element that is effective in improving the toughness of the HAZ zone. Furthermore, Ti exhibits the desulfurization effect, and thus is an element that is effective in improving the HIC resistance. To obtain these effects, the Ti content is preferably 0.003% or more. The Ti content is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, any excessive Ti content leads to an increase in the amount of solid-solute Ti and precipitated TiC, thus degrading the toughnesses of a base metal and a HAZ zone. Thus, the Ti content is preferably 0.03% or less, and more preferably 0.02% or less.

[Mg: more than 0% and 0.01% or less]

[0057] Magnesium (Mg) is an element that is effective in improving the toughness of steel through refinement of crystal grains, and also effective in improving the HIC resistance because of its desulfurization effect. To obtain these effects, the Mg content is preferably 0.0003% or more. The Mg content is more preferably 0.001% or more. On the other hand, an excessive Mg content saturates its effect. Thus, the upper limit of the Mg content is preferably 0.01%. The Mg content is more preferably 0.005% or less.

[0058] The steel plate of the present invention is a steel plate having high hydrogen-induced cracking resistance in which a decrease in an amount of Ca obtained by subtracting a Ca concentration in a slab from a Ca concentration in a molten steel in a tundish is a threshold value Ca_dropθ or less. The term "threshold value Ca_dropθ" as used herein means the maximum decrease in the amount of Ca previously determined and which avoids the occurrence of hydrogen-induced cracking in a steel plate obtained by rolling the slab.

[Decrease in Amount of Ca]

[0059] The decrease in the amount of Ca obtained by subtracting the Ca concentration in the slab from the Ca concentration in the molten-steel in the tundish is set to be the predetermined threshold value or less as mentioned above, thereby making it possible to produce the steel plate having the high hydrogen-induced cracking resistance and to dispatch products at an early stage, which will be mentioned later. First, the reason for setting the decrease in the amount of Ca as an evaluation index will be described below.

[0060] The inventors have focused on MnS inclusions and progressed their studies regarding addition of Ca to a molten steel during a secondary refinement to suppress the formation of MnS.

[0061] When an added amount of Ca to the molten steel is appropriate, CaO-Al₂O₃ inclusions are formed in the molten steel. The CaO-Al₂O₃ has good wettability to the molten steel, and thus is not aggregated in the molten steel and remains fine without adversely affecting the HIC resistance.

[0062]  However, when an added amount of Ca to the molten steel is not appropriate, for example, when adding Ca in an excessive amount that exceeds a predetermined amount required to suppress MnS formation and modify Al₂O₃, pure CaO inclusions are also formed in the molten steel, in addition to CaO-Al₂O₃ inclusions. The pure CaO inclusion has inferior wettability to the molten steel and thereby is more likely to be aggregated in the molten steel. The aggregated CaO becomes each coarse inclusion, inducing HIC.

[0063] The coarsened CaO inclusion has a smaller density than the molten steel, and thus most of CaO inclusions are allowed to float and then separated. However, as shown in Fig. 1, parts of CaO inclusions receive a buoyant force while falling down deeply into a cast strip along the flow of the molten steel within a mold, and then trapped in a solidification shell to form a CaO accumulation zone. The CaO accumulation zone acts as the starting point of HIC.

[0064] Thus, if an appropriate added amount of Ca to the molten steel can be determined in advance, the occurrence of HIC due to the CaO inclusions can be suppressed. For this purpose, it is necessary to precisely identify the content of inclusions in the molten steel before addition of Ca, the composition of the inclusions, and the sulfur concentration in the molten steel. However, in practice, it is impossible to identify these factors in advance, and thus the added amount of Ca is set at an amount enough to suppress the formation of MnS. Consequently, the added amount of Ca tends to become excessive, and thereby a CaO accumulation zone is more likely to be formed.

[0065] If the CaO accumulation zones constantly occur in the same position, an accumulation degree of CaO inclusions can be identified by analyzing the Ca concentration in that position. Furthermore, whether the CaO accumulation zone occurs in a cast strip can also be presumed from the accumulation degree of CaO.

[0066] However, as mentioned above, the positions where the CaO accumulation zones occur differ in the thickness direction of a cast strip, depending on casting conditions (casting speed, angle of a discharge port of an immersion nozzle, and the like). For example, as shown in Fig. 2, three slabs (A to C) with different casting conditions (casting speed and angle of the discharge port of the immersion nozzle) differ from one another in the position (e.g., positions a to c) at the high Ca concentration where the accumulation zone occurs. In this way, the positions of the CaO accumulation zones cannot be predicted. Thus, it is difficult to evaluate whether the CaO accumulation zone occurs or not based on the accumulation degree (Ca concentration).

[0067] Then, the inventors have changed their viewpoints on examination positions for the Ca concentration and focused on the position with a low Ca concentration. It is considered that when a CaO accumulation zone occurs, the Ca concentration at the CaO accumulation zone becomes high, while in a position where no CaO accumulation zone occurs, the Ca concentration becomes relatively low. Taking this into account, the inventors have examined the relationship between the "Ca concentration in an arbitrary position in the thickness direction of a slab" and the "Ca concentration in a molten steel in a tundish" when CaO accumulation zones occur. As a result, in the position where no CaO accumulation zone occurs, the "Ca concentration in the slab" is relatively low, and thereby a "value obtained by subtracting the 'Ca concentration in the slab' from the 'Ca concentration in the molten steel in the tundish'", that is, "a decrease in an amount of the Ca concentration from the tundish to the slab" is found to become large.

[0068] Thus, when the above-mentioned "decrease in the amount of the Ca concentration from the tundish to the slab" is large, it can be considered that the accumulation zone does not occur in the position, but a CaO accumulation zone occurs in another position. Therefore, HIC can be evaluated to occur. On the other hand, when the "decrease in the amount of the Ca concentration from the tundish to the slab" is small, it can be inferred that there is almost no difference between the Ca concentration in the tundish and the Ca concentration in the slab, that is, there is no position with the high Ca concentration in the slab. In this case, since any CaO accumulation zone is considered not to occur in the slab, HIC can be evaluated not to occur.

[0069] In the present invention, the HIC resistance is evaluated by using a value obtained by subtracting the "Ca concentration in the slab" from the "Ca concentration in the molten steel in the tundish" (hereinafter referred to as the "decrease in the amount of Ca"), which is associated with the presence or absence of the CaO accumulation zones.

[Determination of Threshold Value of Decrease in Amount of Ca]

[0070] Next, a description will be given on the way to determine a threshold value Ca_dropθ of the decrease in the amount of Ca, i.e., the maximum decrease in the amount of Ca that avoids the occurrence of HIC in a steel plate obtained by rolling a slab in order to determine whether the obtained steel plate has excellent HIC resistance or not.

[0071] The threshold value Ca_dropθ is determined previously, but a method for determination thereof is not particularly limited to the following method. A method for previously determining the threshold value Ca_dropθ will include the following processes (i) to (iii).

(i) The Ca concentration in the molten steel in the tundish and the Ca concentration in the slab are measured. A decrease in an amount of Ca is calculated by subtracting the Ca concentration in the slab from the Ca concentration in the molten steel in the tundish.

(ii) A hydrogen-induced cracking test is performed on a steel plate that is obtained by rolling a slab, which has been cast on the substantially same casting conditions as the slab.

(iii) The maximum decrease in the amount of Ca that avoids the occurrence of hydrogen-induced cracking is determined from the decrease in the amount of Ca measured in the step (i) and a result of the hydrogen-induced cracking test shown in the step (ii).

[0072] Specifically, a method for determining the threshold value Ca_dropθ will be described in detail below by taking first and second embodiments as examples.

[First Embodiment]

(Examination of Ca Concentration in Molten Steel in Tundish)

[0073] Molten steel is taken out of the tundish, and its Ca concentration (Ca_TD1) is analyzed. The molten steel in the tundish is constantly supplied from a ladle, so that the Ca concentration (Ca_TD1) remains constant even after taking out the molten steel.

(Examination of Ca concentration in Slab)

[0074] Next, a Ca concentration (Ca_S1) in a slab is examined. As shown in Fig. 3(a), a sample is taken out of a region R4 (hereinafter referred to as a "reference-side region R4") ranging from the reference-side surface of the slab to D/2 in the thickness direction thereof, and a Ca concentration Ca_S1 in the region R4 is analyzed. The "reference-side region R4", as shown in Fig. 3(a), is in a range from D/2 to D in the thickness direction of the slab oriented from an opposite-reference-side surface thereof.

[0075] As mentioned above, the density of the CaO inclusion is smaller than that of the molten steel, so that the CaO inclusion in the molten steel float while receiving the buoyant force due to a difference in density between the CaO inclusions and molten steel. In a continuous casting machine provided with a curved portion and a horizontal portion, as illustrated in Fig. 1, after CaO inclusions float, they will be trapped in a solidification shell on the opposite-reference-side, whereby a CaO accumulation zone occurs on the opposite-reference-side of the slab, but does not occur on the reference side thereof.

[0076] For this reason, in the present invention, as shown in Fig. 3(a), the Ca concentration Ca_S1 is examined within the "range from the reference-side surface to D/2 in the thickness direction (reference-side region R4)" where no CaO accumulation zone occurs, that is, a range of -0.50D from the center in the slab thickness D toward the reference-side surface in examples to be mentioned later. Based on the Ca concentration Ca_S1 in the reference-side region R4, the "decrease in the amount of Ca" in the position where no CaO accumulation zone occurs can be calculated to precisely evaluate the presence or absence of the CaO accumulation zones.

[0077] Here, a "decrease Ca_drop1 in Ca" is calculated by subtracting the "Ca concentration Ca_S1 in the slab" from the "Ca concentration Ca_TD1 in the tundish".
Ca_drop1 is represented by the following formula.

(Rolling)

[0078] A slab obtained through casting on the same casting conditions as the slab in which the Ca concentration Ca_S1 is measured is hot-rolled to produce a steel plate for measurement of a threshold value. For example, the rolling is performed on the following conditions. Specifically, after heating the slab to a temperature of 1050 to 1250°C, the hot-rolling is performed on the slab through two or more passes. In each pass, a surface temperature of the steel plate becomes 900°C or higher, a cumulative rolling reduction is 40% or more at an average steel plate temperature of 1,000°C or higher, which is determined by calculation to be mentioned below, and a rolling reduction per pass is 10% or more. Subsequently, another hot-rolling is performed such that a cumulative rolling reduction at 700°C or higher and lower than 900°C is 20% or more, and that a rolling-end temperature is 700°C or higher and lower than 900°C. Then, water-cooling on the steel plate is started from a temperature of 650°C or higher and stopped at a temperature of 350 to 600°C. Further, subsequently, the steel plate is air-cooled to the room temperature. The average steel plate temperature is determined in the following way. Specifically, based on data including a rolling pass schedule during rolling and a cooling method (water-cooling or air-cooling) between the passes, the temperature at an arbitrary position of the steel plate in the thickness direction is determined by using an appropriate calculation method, such as a finite difference method. Then, the average steel plate temperature is defined as the average of the determined temperatures of the steel strip in a range from the front to back surface thereof.

(HIC Test)

[0079] An HIC test is performed on the steel plate to check the presence or absence of HIC occurrence. The HIC test is performed by a method specified by the National Association of Corrosion and Engineer (NACE) standard TM0284-2003, as mentioned in examples below.

[0080] As shown in Fig. 3(b), a region to be subjected to the HIC test is a region R41 excluding the vicinity of the center in the thickness direction of the product region R40 and corresponding to the opposite-reference-side region. This is because, as shown in Fig. 1, the coarsened CaO accumulation zones are more likely to be formed on the opposite-reference-side of the slab, and thereby HIC due to the CaO tends to occur in the region corresponding to the vicinity of the opposite-reference-side surface. Note that HIC due to the segregation tends to occur at the center in the thickness direction, so that the HIC due to the CaO cannot be evaluated at the center. For this reason, the occurrence of HIC is examined in the region R41 excluding the vicinity of the center in the thickness direction of the slab.

(Determination of Threshold Value)

[0081] Subsequently, a threshold value Ca_dropθ of the decrease in the amount of Ca that avoids the occurrence of HIC is determined based on the "decrease Ca_drop1 in Ca" and "the result of the HIC test". By comparing a plurality of decreases Ca_drop1 in Ca with the results of the HIC tests, the "threshold value Ca_dropθ" is defined as the maximum decrease in the amount of Ca that never causes the HIC at all. In particular, the measurement results and test results of a plurality of slabs are used to obtain the threshold value with higher accuracy, which can suppress the misjudgment of the presence or absence of HIC occurrence.

[Second Embodiment]

[0082] A second embodiment that differs from the first embodiment in the calculation method for a decrease in an amount of Ca will be described below with reference to Fig. 4. The same components as those in the above-mentioned first embodiment will be briefly described. Also in Fig. 4, the same components as those in the above-mentioned first embodiment are denoted by the same reference characters, and the description thereof will be omitted as appropriate.

(Examination of Ca Concentration in Molten Steel in Tundish)

[0083] A Ca concentration (Ca_TD1) in the molten steel in the tundish is examined.

(Examination of Ca concentration in Slab)

[0084] Next, as shown in Fig. 4, samples are taken out of two or more different examination positions in the thickness direction of each of slabs obtained through casting in the same charge, and a Ca concentration of each sample is analyzed. The minimum Ca concentration (Ca_min1) is selected from the two or more Ca concentrations obtained (Ca_S1, Ca_S2, ...).

[0085] Then, a value obtained by subtracting the "minimum Ca concentration Ca_min1 in the slab" from the "Ca concentration Ca_TD1 in the tundish" is used to calculate the "decrease Ca_drop11 in Ca".
Ca_drop11 is represented by the following formula.

[0086] Suppose that an examination position of the Ca concentration is set at one site within the entire range in the thickness direction of the slab. If the examination position corresponds to an accumulation zone, an extremely high Ca concentration is detected. The decrease in the amount of Ca calculated from such a high Ca concentration is small, which might lead to the determination that no CaO accumulation zone occurs, resulting in the evaluation of no HIC occurring. However, in practice, some accumulation zones are generated, which can also be thought to cause HIC.

[0087] Thus, in this embodiment, the Ca concentration in the slab is examined at different two or more positions of the slab in its thickness direction. The CaO accumulation zone is present in a specific position in the thickness direction that is determined depending on the casting conditions. By changing the examination position in the thickness direction, a position where the CaO accumulation zone does not occur can also be covered by the examination.

[0088] The two or more Ca concentrations (Ca_S1, Ca_S2, ...) include not only the Ca concentration in the accumulation zone, but also the Ca concentration where no accumulation zone occurs. However, the minimum Ca concentration (Ca_min1) is selected from these concentrations, so that the Ca concentration in the position where no accumulation zone occurs can be selected. Based on this concentration, the decrease in the amount of Ca in the position where no CaO accumulation zone occurs can be calculated, thereby making it possible to precisely evaluate the presence or absence of the CaO accumulation zones.

[0089] Here, the formation mechanism of the CaO accumulation zone is the same as that of each of a CaO inclusion and an Al₂O₃ inclusion. The thickness of the accumulation zone of Al₂O₃ inclusions is reported to be 10 mm (see reference: ISIJ International, Vol. 43(2003), No. 10, p.1548-1555). From this report, the thickness of the accumulation zone of the CaO inclusion can also be estimated to be 10 mm. As such, as shown in Fig. 4, when respective examination positions for the Ca concentration are spaced apart from each other by more than 10 mm in the thickness direction, even if one of the examination positions is in the accumulation zone, the other examination positions are located where no accumulation zone occurs. For this reason, two or more examination positions are preferably spaced apart from each other by more than 10 mm in the thickness direction. Note that Fig. 4 shows two examination positions, a distance I between the two examination positions being more than 10 mm in the thickness direction (distance I in the thickness direction between two examination positions > 10 mm).

[0090] As shown in Fig. 1, CaO inclusions are trapped in a wide range located in the vicinity of the curved portion of a casting route. Thus, in regions R1 and R2, each ranging from a corresponding one of both ends in the width direction to D/2 of the slab shown in Fig. 4, the CaO accumulation zone occurs widely in the thickness direction. Thus, within the regions R1 and R2, there is a possibility of failing to examine a position where no accumulation zone occurs even if the examination position for the Ca concentration is changed in the thickness direction. For this reason, the Ca concentration examination position is preferably set at a region R3 with a width W-D that is mainly cooled only from the wide surface side, i.e., that excludes the regions ranging from both ends to D/2 in the width direction.

(Rolling)

[0091] A slab obtained through casting on the same casting conditions as the slab in which the Ca concentration Ca_S1 or the like is measured as mentioned above is hot-rolled to produce a steel plate for measurement of a threshold value.

(HIC Test)

[0092] The HIC test is performed on the steel plate to check the presence or absence of HIC occurrence in the "region R41 corresponding to the vicinity of the opposite-reference-side surface". The HIC test is performed by a method specified by the NACE standard TM0284-2003, as mentioned in examples below.

(Determination of Threshold Value)

[0093] Subsequently, a threshold value Ca_dropθ of the decrease in the amount of Ca that avoids the occurrence of HIC is determined based on the "decrease Ca_drop11 in Ca" and the "result of the HIC test". In this embodiment, the "threshold value Ca_dropθ" is defined as the maximum decrease in the amount of Ca that never causes the HIC at all.

[Measurement of Decrease in Amount of Ca of Charge as Determination Target]

[0094] A Ca concentration Ca_TD11 in a molten steel of the charge as a determination target in the tundish is examined. For example, like the second embodiment, the Ca concentration is examined at different two or more positions in the thickness direction of the slab cast in the same charge. Then, the minimum Ca concentration (Ca_min11) is selected from two or more Ca concentrations (Ca_S11, Ca_S12, ...). The two or more examination positions are preferably spaced apart from each other by more than 10 mm in the thickness direction.

[0095]  Then, the "decrease Ca_drop in Ca" as the determination target is calculated by subtracting the "minimum Ca concentration Ca_min11 in the slab" from the "Ca concentration Ca_TD11 in the tundish". Ca_drop is represented by the following formula.

[Evaluation of Decrease in Amount of Ca of Charge as Determination Target]

[0096] The Ca_drop as the determination target is compared with the threshold value Ca_dropθ. When the Ca_drop is the threshold value Ca_dropθ or less, the obtained steel plate is determined to have excellent HIC resistance. When the Ca_drop exceeds the threshold value Ca_dropθ, the obtained steel plate is determined to be inferior in the HIC resistance.

[0097] The examination position (examined surface) of the slab is preferably a stationary part, but may be a non-stationary part. The term "non-stationary part" as used herein means a part that is cast when the casting condition is varied, for example, a part that is cast at an initial stage of casting, such as when the casting speed increases, or a part that is cast at the end of casting, such as when the casting speed decreases. When intended to examine the non-stationary part, as shown in Fig. 5, a part adjacent to the region subjected to the HIC test is preferably examined. Such a part exhibits substantially the same HIC resistance as that in the HIC test result and can be evaluated more precisely.

[0098] The steel plate in the present invention is a steel plate in which the "decrease Ca_drop in Ca" is calculated by subtracting the Ca concentration in the slab from the Ca concentration in the tundish at a stage of the slab before rolling, and the "decrease Ca_drop in Ca" satisfies the following formula: Ca_drop ≤ threshold value Ca_dropθ. It is considered that the steel plate in the present invention satisfies the relationship of Ca_drop ≤ threshold value Ca_dropθ as mentioned above, and that no CaO accumulation zone is generated in the slab, so that no HIC occurs.

[0099] Accordingly, in this embodiment, the "decrease in the amount of Ca concentration from the tundish to the slab" is used for evaluation of the HIC resistance. This can precisely evaluate the internal quality (accumulation degree of CaO inclusions) of the cast strip. Thus, based on the evaluation result, the HIC resistance can be evaluated at the stage of the cast strip. Consequently, the HIC test that would require several weeks can be omitted, thereby significantly shortening a time period from the manufacture to dispatching.

[0100] A method for manufacturing a steel sheet in the present invention is not particularly limited as long as the method can produce the steel plate in which an Ar gas content in the steel satisfies the above-mentioned specified content. The following method is recommended as a method for easily obtaining the specified steel sheet.

[Manufacturing Method]

[0101] To achieve the above-mentioned Ar content, in a continuous casting step, it is recommended that the number density of inclusions having a major axis of 3 µm or more and a melting point of 1550°C or higher is preferably 3 inclusions/cm² or more in a mold.

[0102] The inclusions with a melting point of 1550°C or higher exist as solids in a mold and thus have bad wettability to the molten steel, whereby the inclusions are aggregated together while trapping therein Ar gas. Eventually, the inclusions have their volumes expanded and thereby can easily float. The relatively coarse inclusions having the major axis of 3 µm or more are in contact with each other in the mold to be more coarsened while trapping therein Ar bubbles, so that the flotation separation of the Ar bubbles in the mold can be promoted. Consequently, the Ar gas content in the steel can be decreased. In particular, when using Ar gas in a step before charging the molten steel into the mold, Ar gas tends to remain in the steel. Thus, the flotation separation of Ar gas with the inclusions is effective.

[0103] Examples of inclusions having the melting point of 1550°C or higher include Al₂O₃, CaO, and a complex inclusion thereof. For a complex inclusion or the like-whose melting point is uncertain, the quantitative analysis of the inclusion is performed by an energy-dispersive X-ray spectroscopy (EDX) or the like. An artificial inclusion that imitates the composition of the analyzed inclusion is made. The temperature at which the artificial inclusion starts to melt is measured with a laser microscopy or the like, whereby a melting point of the artificial inclusion can be recognized. More simply, by using the fact that a liquid inclusion in the mold is observed in a spherical shape after being solidified, an inclusion having an aspect ratio of 1.3 or more may be deemed as an inclusion having the melting point of 1550°C or higher.

[0104] The number density of the inclusions is more preferably 5 inclusions/cm² or more, and still more preferably 10 inclusions/cm² or more. Any excessive number density of the inclusions degrades the toughness of a base metal and a HAZ zone. Thus, the upper limit of the number density of the inclusions is approximately 100 inclusions/cm².

[0105] Specific means for achieving the above-mentioned number density of the inclusions is a method that involves, for example, setting a reflux time in RH at 45 minutes or less in a refining step, adding Ca to the steel in the RH and maintaining the steel until 15 minutes or more has elapsed, and thereafter performing the following step(s):

(a) in a continuous casting machine employing a hot reusing tundish, casting the steel by using the tundish that is left for 30 minutes or more after the end of previous charge casting; and/or

(b) adding, for example, 0.04 kg/ton or more of Al metal to the molten steel in the hot reusing tundish (for example, approximately 0.2 kg/ton of Al can be added, in which the upper limit of the added amount of Al is approximately 0.50 kg/ton or less, and preferably 0.40 kg/ton or less), followed by casting the molten steel.

[0106] Another means for decreasing an Ar gas content in the steel also includes restricting and/or stopping the use of Ar gas in an injection nozzle, the RH, and/or the tundish. However, in order to suppress the clogging of the injection nozzle to prevent the reduction in the yield, it is effective to blow Ar into the injection nozzle from a position located higher by 50 mm or more than an upper part of a discharge port of the injection nozzle. Thus, stopping of the use of Ar in the injection nozzle is not recommended. Note that to achieve the specified Ar gas content, an Ar blowing amount (flow rate) into the injection nozzle is recommended to be preferably 9.0 L(liters)/t(ton) or less (more preferably 6.0 L/t or less). Further, the gas used for blowing into the injection nozzle is also considered to change from Ar gas to nitrogen gas. However, in the use of nitrogen gas, the N content in a steel plate cannot be controlled, easily degrading the toughness of the steel plate, which is not preferable.

[0107] In the present invention, a step to be executed after casting in the way mentioned above is not particularly limited. In the step, hot-rolling can be performed by a normal method, thereby manufacturing a steel plate. Further, the steel plate can be used to manufacture a steel pipe for line pipes by a general method. A steel pipe for line pipes obtained by using the steel plate according to the present invention also has excellent HIC resistance and toughness.

[0108] The present application claims priority to Japanese Patent Application No. 2014-266492 filed on December 26, 2014 and Japanese Patent Application No. 2015-208022 filed on October 22, 2015, the disclosure of both of which is incorporated herein by reference in its entirety.

Examples

[0109] The present invention will be more specifically described below by way of Examples, but is not inherently limited to the following Examples. Various modifications can be obviously made to these Examples as long as they are adaptable to the above-mentioned and below-mentioned concepts as appropriate, and thus they are included within the scope of the present invention.

(1) Casting

[0110] Tables 1-1 to 4 and Figs. 6 and 7 show the experimental conditions and results for determining the threshold value. Slabs, each having a slab thickness D of 280 mm and a slab width W of 2100 mm, were obtained by continuous casting. The casting conditions in the first embodiment are shown in Tables 1-1 and 1-2, and the casting conditions in the second embodiment are shown in Tables 2-1 and 2-2. In Examples, 25 charges for each were manufactured to obtain each of a steel plate of API (The American Petroleum Institute) X65 grade and a steel plate of API X70 grade.

[0111] Here, the conditions shown in Tables 1-1, 1-2, 2-1, and 2-2 will be described.

<Component of Molten Steel in Tundish>

[0112] The concentrations of C, Mn, Nb, P, and Ca were measured by an emission spectroscopy. The S concentration was very low and thus was difficult to measure by the emission spectroscopy. Then, the S concentration was measured by using a combustion-infrared absorption method.

<Casting Conditions>

- Specific Water Content

[0113] 

- Casting Speed

[0114] The casting speed is a drawing speed of the cast strip [m/min.], and was calculated from the diameter (circumferential length) and the rotational speed (the number of revolutions per unit time) of a roll (major roll) in contact with the cast strip.

(2) Examination of Decrease in Amount of Ca

[0115] When the entire length of the slab was 10 m, the molten steel was taken out of the tundish, and the Ca concentration Ca_TD1 in the molten steel in the tundish was examined. After casting, a Ca concentration Ca_S1 in the slab or Ca_min1 was examined. Tables 1-1 and 1-2 show examination positions and the Ca concentrations Ca_S1 when examining the Ca concentrations in the reference-side regions R4 of the slabs. Tables 3-1, 3-2, and 4 show examination positions and the Ca concentrations at the respective sites when examining the Ca concentrations at two to ten different sites (the total number of N shown in Tables 3-1, 3-2, and 4 = any one of 2 to 10) in the thickness direction of the slab. In Tables 3-1, 3-2, and 4, the Ca concentration in each of samples No. 51 to 57 and 69 to 100 was measured at two sites. In each of samples No. 58 to 64, the Ca concentration was checked at any one of three to eight sites. In each of samples No. 65 to 68, the Ca concentration was checked at ten sites. Among the measured plurality of Ca concentrations, the minimum Ca concentration (minimum value Ca) is shown for each sample in these tables. The two to ten sites were positioned spaced apart from one another by more than 10 mm in the thickness direction.

[Table 1-1]

Sample No.	Component of molten steel in tundish						Casting conditions		Product grade	Examination position of Ca concentration in slab		Ca concentration in slab Cas1 [ppm]
	C (% by mass)	Mn (% by mass)	Nb (% by mass)	S (ppm by mass)	P (ppm by mass)	Ca CATD1 (ppm by mass)	Specific water content [L/kg- steel]	Casting speed Vc [m/min]		Distance from the center in the thickness direction*
										In terms of D (Thickness D)	[mm]
1	0.06	1.32	0.036	6	58	33	0.4	1.0		-0.49D	-69	32
2	0.05	1.28	0.037	6	60	27	0.4	1.1		-0.47D	-66	22
3	0.05	1.30	0.037	6	41	30	0.4	1.3		-0.21D	-29	28
4	0.06	1.27	0.037	5	68	28	0.6	1.0		-0.03D	-4	23
5	0.06	1.32	0.038	4	47	30	0.6	1.1		-0.48D	-67	28
6	0.06	1.33	0.036	5	57	31	0.6	1.2		-0.16D	-22	23
7	0.05	1.31	0.036	4	54	31	0.8	1.0		-0.32D	-45	25
8	0.06	1.33	0.037	7	65	33	0.8	1.1		-0.37D	-52	30
9	0.05	1.28	0.040	3	70	34	0.8	1.3		-0.20D	-28	28
10	0.05	1.33	0.038	4	65	34	1.0	1.0		-0.36D	-50	32
11	0.06	1.31	0.035	3	52	33	1.0	1.1		-0.26D	-36	25
12	0.06	1.31	0.036	7	42	32	1.0	1.3		-0.13D	-18	31
13	0.06	1.29	0.037	6	63	28	1.2	1.2	X70	-0.00D	0	26
14	0.06	1.30	0.039	7	61	30	1.2	1.1		-0.21D	-29	28
15	0.06	1.27	0.037	3	57	30	1.2	1.3		-0.32D	-45	24
16	0.05	1.29	0.036	6	48	31	1.4	1.0		-0.31D	-43	28
17	0.05	1.30	0.038	5	46	26	1.4	1.1		-0.22D	-31	18
18	0.05	1.32	0.037	7	49	26	1.4	1.2		-0.03D	-4	20
19	0.05	1.31	0.037	7	42	34	1.4	1.3		-0.21D	-29	27
20	0.06	1.34	0.038	3	59	28	1.4	1.0		-0.29D	-41	25
21	0.05	1.27	0.036	6	66	32	1.4	1.1		-0.19D	-27	26
22	0.06	1.29	0.036	5	44	34	1.4	1.2		-0.47D	-66	29
23	0.05	1.27	0.035	3	67	31	1.4	1.3		-0.30D	-42	30
24	0.05	1.29	0.037	3	63	31	1.4	1.0		-0.46D	-64	28
25	0.05	1.28	0.035	5	46	29	1.4	1.1		-0.37D	-52	28

*: Positive (+) is defined as the direction from the center in the thickness direction toward an opposite-reference-side surface, while positive (-) is defined as the direction from the center toward a reference-side surface.

[Table 1-2]

Sample No.	Component of molten steel in tundish						Casting conditions		Product grade	Examination position of Ca concentration in slab		Ca concentration in slab Cas1 [ppm]
	C (% by mass)	Mn (% by mass)	Nb (% by mass)	S (ppm by mass)	P (ppm by mass)	Ca CATD1 (ppm by mass)	Specific water content [L/kg- steel]	Casting speed Vc [m/min]		Distance from the center in the thickness direction*
										In terms of D (Thickness D)	[mm]
26	0.06	1.28	0.033	5	52	29	0.4	1.0		-0.37D	52	23
27	0.06	1.25	0.033	4	61	26	0.4	1.1		-0.35D	49	23
28	0.06	1.27	0.034	7	45	31	0.4	1.3		-0.23D	32	27
29	0.06	1.23	0.035	7	51	31	0.6	1.0		-0.01D	1	25
30	0.06	1.27	0.031	7	66	29	0.6	1.1		-0.40D	56	25
31	0.05	1.26	0.031	4	56	27	0.6	1.2		-0.49D	69	24
32	0.05	1.24	0.035	5	43	30	0.8	1.0		-0.17D	24	24
33	0.06	1.25	0.030	4	70	29	0.8	1.1		-0.15D	21	26
34	0.06	1.26	0.034	6	64	33	0.8	1.3		-0.36D	50	31
35	0.05	1.27	0.035	6	48	33	1.0	1.0		-0.37D	52	29
36	0.06	1.20	0.034	5	50	31	1.0	1.1		-0.40D	56	28
37	0.06	1.27	0.032	3	63	32	1.0	1.3		-0.10D	14	28
38	0.05	1.22	0.035	7	44	33	1.2	1.2	X65	-0.24D	34	28
39	0.06	1.25	0.034	3	50	27	1.2	1.1		-0.37D	52	23
40	0.05	1.26	0.031	4	42	25	1.2	1.3		-0.26D	36	22
41	0.05	1.25	0.034	7	50	33	1.4	1.0		-0.39D	55	30
42	0.06	1.23	0.034	4	68	29	1.4	1.1		-0.41D	57	21
43	0.06	1.23	0.033	4	50	33	1.4	1.2		-0.25D	35	29
44	0.05	1.20	0.034	6	58	34	1.4	1.3		-0.25D	35	32
45	0.06	1.23	0.033	5	43	29	1.4	1.0		-0.13D	18	27
46	0.06	1.28	0.034	3	64	33	1.4	1.1		-0.49D	69	26
47	0.05	1.26	0.034	7	46	26	1.4	1.2		-0.49D	69	20
48	0.06	1.28	0.032	5	54	29	1.4	1.3		-0.17D	24	26
49	0.06	1.23	0.034	5	52	30	1.4	1.0		-0.10D	14	28
50	0.05	1.28	0.035	5	44	26	1.4	1.1		-0.27D	38	19

*: Positive (+) is defined as the direction from the center in the thickness direction toward an opposite-reference-side surface, while positive (-) is defined as the direction from the center toward a reference-side surface.

[Table 2-1]

Sample No.	Component of molten steel in tundish						Casting conditions		Product grade
	C (% by mass)	Mn (% by mass)	Nb (% by mass)	S (ppm by mass)	P (ppm by mass)	Ca CaTD1 (ppm by mass)	Specific water content [L/kg- steel]	Casting speed Vc [m/min]
51	0.06	1.32	0.036	6	58	33	0.4	1.0
52	0.05	1.28	0.037	6	60	27	0.4	1.1
53	0.05	1.30	0.037	6	41	30	0.4	1.3
54	0.06	1.27	0.037	5	68	28	0.6	1.0
55	0.06	1.32	0.038	4	47	30	0.6	1.1
56	0.06	1.33	0.036	5	57	31	0.6	1.2
57	0.05	1.31	0.036	4	54	31	0.8	1.0
58	0.06	1.33	0.037	7	65	33	0.8	1.1
59	0.05	1.28	0.040	3	70	34	0.8	1.3
60	0.05	1.33	0.038	4	65	34	1.0	1.0
61	0.06	1.31	0.035	3	52	33	1.0	1.1
62	0.06	1.31	0.036	7	42	32	1.0	1.3
63	0.06	1.29	0.037	6	63	28	1.2	1.2	X70
64	0.06	1.30	0.039	7	61	30	1.2	1.1
65	0.06	1.27	0.037	3	57	30	1.2	1.3
66	0.05	1.29	0.036	6	48	31	1.4	1.0
67	0.05	1.30	0.038	5	46	26	1.4	1.1
68	0.05	1.32	0.037	7	49	26	1.4	1.2
69	0.05	1.31	0.037	7	42	34	1.4	1.3
70	0.06	1.34	0.038	3	59	28	1.4	1.0
71	0.05	1.27	0.036	6	66	32	1.4	1.1
72	0.06	1.29	0.036	5	44	34	1.4	1.2
73	0.05	1.27	0.035	3	67	31	1.4	1.3
74	0.05	129	0.037	3	63	31	1.4	1.0
75	0.05	1.28	0.035	5	46	29	1.4	1.1

[Table 2-2]

Sample No.	Component of molten steel in tundish						Casting conditions		Product grade
	C (% by mass)	Mn (% by mass)	Nb (% by mass)	S (ppm by mass)	P (ppm by mass)	Ca CaTD1 (ppm by mass)	Specific water content [L/kg- steel]	Casting speed Vc [m/min]
76	0.06	1.28	0.033	5	52	29	0.4	1.0
77	0.06	1.25	0.033	4	61	26	0.4	1.1
78	0.06	1.27	0.034	7	45	31	0.4	1.3
79	0.06	1.23	0.035	7	51	31	0.6	1.0
80	0.06	1.27	0.031	7	66	29	0.6	1.1
81	0.05	1.26	0.031	4	56	27	0.6	1.2
82	0.05	1.24	0.035	5	43	30	0.8	1.0
83	0.06	1.25	0.030	4	70	29	0.8	1.1
84	0.06	1.26	0.034	6	64	33	0.8	1.3
85	0.05	1.27	0.035	6	48	33	1.0	1.0
86	0.06	1.20	0.034	5	50	31	1.0	1.1
87	0.06	1.27	0.032	3	63	32	1.0	1.3
88	0.05	1.22	0.035	7	44	33	1.2	1.2	X65
89	0.06	1.25	0.034	3	50	27	1.2	1.1
90	0.05	1.26	0.031	4	42	25	1.2	1.3
91	0.05	1.25	0.034	7	50	33	1.4	1.0
92	0.06	1.23	0.034	4	68	29	1.4	1.1
93	0.06	1.23	0.033	4	50	33	1.4	1.2
94	0.05	1.20	0.034	6	58	34	1.4	1.3
95	0.06	1.23	0.033	5	43	29	1.4	1.0
96	0.06	1.28	0.034	3	64	33	1.4	1.1
97	0.05	1.26	0.034	7	46	26	1.4	1.2
98	0.06	1.28	0.032	5	54	29	1.4	1.3
99	0.06	1.23	0.034	5	52	30	1.4	1.0
100	0.05	1.28	0.035	5	44	26	1.4	1.1

[Table 3-1]

Sample No.	The number N in total	Examined contents of slab
		Minimum Ca concentration Camin1 [ppm]	Examination position of Ca concentration and Ca concentration
			N=1		N=2		N=3		N=4		N=5
			Distance from the center in the thickness direction* [mm]	Ca concentration [ppm]	Distance from the center in the thickness direction* [mm]	Ca concentration [ppm]	Distance from the center in the thickness direction* [mm]	Ca concentration [ppm]	Distance from the center in the thickness direction* [mm]	Ca concentration [ppm]	Distance from the center in the thickness direction* [mm]	Ca concentration [ppm]
51	2	30	10	30	20	37	-	-	-	-	-	-
52	2	25	10	25	40	30	-	-	-	-	-	-
53	2	27	-10	27	60	31	-	-	-	-	-	-
54	2	26	-10	26	80	33	-	-	-	-	-	-
55	2	25	130	25	100	26	-	-	-	-	-	-
56	2	30	-130	30	-120	30	-	-	-	-	-	-
57	2	26	-130	26	-110	27	-	-	-	-	-	-
58	3	31	10	31	30	31	50	30	-	-	-	-
59	4	32	-10	32	-30	31	-50	32	-70	33	-	-
60	5	30	-50	30	-10	30	10	30	50	30	90	31
61	6	30	-100	30	-80	30	-40	31	-20	31	20	29
62	7	28	10	28	30	28	50	28	70	28	90	27
63	7	25	10	25	20	25	30	26	40	25	50	25
64	8	28	10	28	20	28	30	27	40	28	50	33
65	10	29	10	29	20	29	30	29	40	35	50	29
66	10	30	10	30	-130	29	-110	30	-70	30	-50	30
67	10	25	10	25	20	25	30	25	40	24	50	26
68	10	23	10	23	20	23	-130	23	-120	23	-80	23
69	2	31	20	31	50	31	-	-	-	-	-	-
70	2	20	40	20	80	21	-	-	-	-	-	-
71	2	29	60	29	100	29	-	-	-	-	-	-
72	2	32	80	32	120	33	-	-	-	-	-	-
73	2	30	-20	30	-50	30	-	-	-	-	-	-
74	2	27	-40	27	-80	28	-	-	-	-	-	-
75	2	27	-60	27	-130	28	-	-	-	-	-	-

*: Positive (+) is defined as the direction from the center in the thickness direction toward an opposite-reference-side surface, while positive (-) is defined as the direction from the center toward a reference-side surface.

[Table 3-2]

Sample No.	The number N in total	Examined contents of slab
		Minimum Ca concentration Camin1 [ppm]	Examination position of Ca concentration and Ca concentration
			N=6		N=7		N=8		N=9		N=10
			Distance from the center in the thickness direction* [mm]	Ca concentration [ppm]	Distance from the center in the thickness direction* [mm]	Ca concentration [ppm]	Distance from the center in the thickness direction* [mm]	Ca concentration [ppm]	Distance from the center in the thickness direction*	Ca concentration [ppm]	Distance from the center in the thickness direction* [mm]	Ca concentration [ppm]
51	2	30	-	-	-	-	-	-	-	-	-	-
52	2	25	-	-	-	-	-	-	-	-	-	-
53	2	27	-	-	-	-	-	-	-	-	-	-
54	2	26	-	-	-	-	-	-	-	-	-	-
55	2	25	-	-	-	-	-	-	-	-	-	-
56	2	30	-	-	-	-	-	-	-	-	-	-
57	2	26	-	-	-	-	-	-	-	-	-	-
58	3	31	-	-	-	-	-	-	-	-	-	-
59	4	32	-	-	-	-	-	-	-	-	-	-
60	5	30	-	-	-	-	-	-	-	-	-	-
61	6	30	50	30	-	-	-	-	-	-	-	-
62	7	28	110	28	130	28	-	-	-	-	-	-
63	7	25	60	31	70	25	-	-	-	-	-	-
64	8	28	60	28	70	27	80	28	-	-	-	-
65	10	29	60	30	70	30	80	29	90	29	110	28
66	10	30	0	30	40	38	60	31	90	31	110	30
67	10	25	60	25	70	32	80	25	90	26	110	26
68	10	23	-50	22	-20	29	0	24	10	23	30	23
69	2	31	-	-	-	-	-	-	-	-	-	-
70	2	20	-	-	-	-	-	-	-	-	-	-
71	2	29	-	-	-	-	-	-	-	-	-	-
72	2	32	-	-	-	-	-	-	-	-	-	-
73	2	30	-	-	-	-	-	-	-	-	-	-
74	2	27	-	-	-	-	-	-	-	-	-	-
75	2	27	-	-	-	-	-	-	-	-	-	-

*: Positive (+) is defined as the direction from the center in the thickness direction toward an opposite-reference-side surface, while positive (-) is defined as the direction from the center toward a reference-side surface.

[Table 4]

Sample No.	Examined contents of slab
	Minimum Ca concentration Camin1 [ppm]	Examination position of Ca concentration and Ca concentration
		N=1		N=2
		Distance from the center in the thickness direction* [mm]	Ca concentration [ppm]	Distance from the center in the thickness direction* [mm]	Ca concentration [ppm]
76	26	10	26	20	37
77	22	10	22	40	30
78	25	10	25	60	29
79	30	10	30	80	38
80	27	10	27	100	28
81	25	10	25	120	25
82	26	10	26	140	27
83	28	20	28	50	29
84	30	40	38	80	30
85	29	60	34	100	29
86	29	80	29	120	29
87	30	20	30	50	37
88	30	40	30	50	38
89	24	50	24	60	30
90	20	-70	20	-80	20
91	32	-90	32	-100	33
92	27	-110	27	-120	27
93	28	-130	28	-140	28
94	32	-20	32	-30	32
95	26	-10	26	-20	26
96	30	-10	30	-40	31
97	22	-10	22	-60	23
98	25	-10	25	-80	26
99	29	-10	29	-100	29
100	24	-10	24	-120	24

*: Positive (+) is defined as the direction from the center in the thickness direction toward an opposite-reference-side surface, while positive (-) is defined as the direction from the center toward a reference-side surface.

(3) Rolling

[0116] Then, after heating the slab to a temperature of 1050 to 1250°C, the hot-rolling was performed on the slab through two or more passes, in each of which a surface temperature of the steel plate was set at 900°C or higher, a cumulative rolling reduction was 40% or more at an average steel plate temperature of 1,000°C or higher, which was determined by the calculation, and a rolling reduction per pass was 10% or more. Further, another hot-rolling was performed such that the cumulative rolling reduction at a temperature of 700°C or higher and lower than 900°C was 20% or more, and that the surface temperature at the end of the rolling was 850°C. Thereafter, cooling of the rolled steel plate was started at a cooling start surface temperature of 950°C and an average cooling rate of 10°C/s. The cooling was stopped at a temperature of 350 to 600°C. Subsequently, the air-cooling was carried out until the room temperature, thereby eventually producing steel plates with various compositions, each steel plate having a size of 9 to 50 mm in thickness × 2000 to 3500 mm in width × 12000 to 35000 mm in length.

(4) HIC Test

[0117] To determine a threshold value tθ, in Examples, the HIC test was performed after the rolling.

(a) Samples were cut out of the respective steel plates obtained after the rolling, and the HIC test was performed on the samples. The HIC test was performed according to the method specified by the NACE standard TM0284-2003.

(b) After the HIC test, each sample was cut at three sites, and then respective cross sections (three cross sections) were observed with a microscope to confirm the presence or absence of HIC. An observation region was set at the region R41 in the "product region R40 corresponding to the opposite-reference-side region" shown in Fig. 3(b) except for a region ranging from the center in the thickness direction of a product within ±5.3% of the thickness.

(5) Determination of Threshold Value of Decrease in Amount of Ca

[0118] Fig. 6 shows the result of determination of the threshold value in the first embodiment, specifically, showing the relationship among the "Ca concentration Ca_TD1 in the molten steel in the tundish" examined in the process (2), the "Ca concentration Ca_S1 in the slab" shown in each of Tables 1-1 and 1-2, and the HIC test results thereof. Fig. 7 shows the result of determination of the threshold value in the second embodiment, specifically, showing the relationship among the "Ca concentration Ca_TD1 in the molten steel in the tundish" examined in the process (2), the minimum Ca concentration Ca_min1 in the slab shown in each of Tables 3-1, 3-2, and 4, and the HIC test results thereof.

[0119] As can be seen from Fig. 6, in the determination method of the first embodiment, when the decrease in the amount of Ca was 4 ppm or less, no HIC occurred. On the other hand, when the decrease in the amount of Ca exceeded 4 ppm, HIC occurred in some cases, but did not occur in others. This result shows that to surely avoid the occurrence of HIC, the decrease in the amount of Ca satisfies a formula below: decrease in the amount of Ca ≤ 4 ppm. Thus, in Example of the first embodiment, the threshold value of the decrease in the amount of Ca was set at 4 ppm, i.e., Ca_dropθ = 4 ppm.

[0120] Also, as can be seen from Fig. 7, in the determination method of the second embodiment, when the decrease in the amount of Ca was 4 ppm or less, no HIC occurred. On the other hand, when the decrease in the amount of Ca exceeded 4 ppm, HIC occurred in some cases, but did not occur in others. This result shows that to surely avoid the occurrence of HIC, the decrease in the amount of Ca satisfies a formula below: decrease in the amount of Ca ≤ 4 ppm. Thus, also in Example of the second embodiment, the threshold value of the decrease in the amount of Ca was set at 4 ppm, i.e., Ca_dropθ = 4 ppm.

[0121] Note that the "threshold value of the decrease in the amount of Ca" is determined based on all products, regardless of the strength grade thereof. This is because the easiness of occurrence of HIC due to coarse CaO is not related to the strength grade of products.

(6) Evaluation of Slab as Determination Target

[0122] The HIC resistance of each slab as the determination target having the composition shown in Table 5 was evaluated using the threshold value.

[0123] The steel with the composition shown in Table 5 was melted and subjected to continuous casting, thereby producing a slab having the slab thickness D of 280 mm and the slab width W of 2100 mm.

[0124] Then, the Ca concentration Ca_TD11 in the molten steel in the tundish of the charge as the determination target was examined, and the minimum Ca concentration (Ca_min11) in the slab as the determination target was determined, whereby the decrease Ca_drop in Ca of the slab as the determination target was calculated as mentioned above. Then, the threshold value Ca_dropθ = 4 ppm, which was determined in the first and second embodiments as mentioned in the process (5), was used to determine the evaluation of the HIC resistance. Specifically, when the decrease Ca_drop in Ca of the slab as the determination target was 4 ppm or less, the HIC due to CaO did not occur, that is, the HIC resistance was rated as OK. On the other hand, when the decrease Ca_drop in Ca was more than 4 ppm, the HIC due to the CaO occurred, that is, the HIC resistance was rated as NG. These results are shown in Table 6.

[0125] As mentioned above, the steel with the composition shown in Table 5 was melted and subjected to continuous casting, thereby producing a steel strip (slab). Here, the casting was performed such that the number of inclusions having a major axis of 3 µm or more was 3 inclusions/cm² or more in the mold. Specifically, in the control of the number of inclusions in Examples, a reflux time in the RH was set at 5 minutes or more and 45 minutes or less, Ca was added after the RH reflux, and then the steel was maintained for 15 minutes or more and 45 minutes or less. Thereafter, in a continuous casting machine employing a hot reusing tundish, a molten steel was filled in the tundish that was left for 30 minutes or more and 60 minutes or less after the end of casting of the previous charge. Then, 0.04 kg/ton or more (upper limit of approximately 0.50 kg/ton) of Al metal was added to the molten steel in the tundish, which was subsequently cast. Note that the measurement of the number density of inclusions was performed on samples that were taken out of the mold ten minutes after the casting.

[0126] A sample of the molten steel was taken out of the mold, and the number of inclusions of the sample in the mold was observed and counted with a scanning electron microscope (SEM, magnification: 400X, the number of fields of view: 30). Here, an inclusion having a major axis of 3 µm or more, and having an aspect ratio of 1.3 or more which suggests that a melting point thereof is 1550°C or higher, are counted. In this way, the number density of such inclusions was determined. As shown in Table 6, samples having the number density of inclusions of 3 inclusions/cm² or more in the mold determined by the above-mentioned method were rated as "OK", while samples having the number density of inclusions of less than 3 inclusions/cm² were rated as "NG".

[0127] Then, after heating the slab to a temperature of 1050 to 1250°C, each slab was processed by either of two types of hot-rolling and cooling methods, denoted as "TMCP" or "QT" in a "hot-rolling and cooling method" column shown in Table 6. Consequently, steel plates (each having 9 to 90 mm in thickness × 2000 to 3500 mm in width × 12000 to 35000 mm in length) with various compositions were obtained. The above-mentioned "TMCP" was a method that involved: hot-rolling through two or more passes, in each of which a surface temperature of the steel plate was set at 900°C or higher, a cumulative rolling reduction was 40% or more at an average steel plate temperature of 1000°C or higher, determined by the calculation, and a rolling reduction per pass was 10% or more. The TMCP" also involved: another hot-rolling such that a cumulative rolling reduction was 20% or more at a temperature of 700°C or higher and lower than 900°C such that that the surface temperature at the end of the rolling was 850°C. The "TMCP" method further involved: starting to cool the rolled steel plate from a cooling start surface temperature of 950°C at an average cooling rate of 10°C/s and then stopping the cooling at a temperature of 350 to 600°C, followed by air-cooling to the room temperature. The above-mentioned "QT" was a method that involved: hot-rolling such that the surface temperature at the end of the rolling was 850°C or higher, followed by air-cooling to the room temperature; quenching by reheating the rolled steel plate to a temperature of 850°C or higher and 950°C or lower; and tempering the steel plate at 600 to 700°C.

[0128] Then, these respective steel plates were used to measure the Ar gas content in the steel in the following way. The HIC test was performed to evaluate the HIC resistance of the steel plate, and further the Charpy impact test was performed to evaluate the toughness of the steel plate. These results are shown in Table 6.

[Measurement of Ar Gas Content in Steel]

[0129] A test specimen with a size of a product thickness × 15 mm × 15 mm, was cut out of the surface of each steel, and introduced in a vacuum chamber with a degree of vacuum set at 2 × 10^-5 Torr or less. Subsequently, the test specimen was perforated from the surface of the steel plate to the depth of 5 mm under the surface by using the G-straight drill, manufactured by MITSUBISHI MATERIALS Corporation (product ID GSDD3000, diameter D1 : 3.0 mm, groove length L3: 32 mm, entire length: 71 mm, and blade diameter: 3.0 mm), whereby gas components of the steel were extracted. Then, the gas components were subjected to quantitative analysis using a quadrupole mass spectrometer, manufactured by ANELVA Corporation (M-101QA-TDM model) (a measurement range of atomic mass numbers of 1 to 100 amu). Then, the ratio of an Ar content (µL/cm³) to the volume of a part of the steel perforated by the drill mentioned above was determined. This measurement was performed at arbitrary ten positions of the steel plate of each sample, and the "Ar gas content in the steel" was defined as the maximum value among those in the ten positions.

(HIC Test)

[0130] The HIC test was performed using the steel plates. The HIC test was performed according to the method specified by the NACE standard TM0284-2003.

[Charpy Impact Test]

[0131] First, according to ASTM A370, ten Charpy impact test specimens, each having a size of 10 mm in the plate thickness direction × 10 mm in the rolling direction, were taken out of each steel sheet along the direction perpendicular to the rolling direction so that the the center of the test specimen correspond to the position of 6 mm in depth from its surface, and then a notch was formed in each test specimen in the thickness direction of the steel plate. The Charpy impact test was performed on each test specimen according to the ASTM A370 by setting a testing temperature at 20°C, whereby a Charpy absorbed energy and a brittle fracture rate of the test specimen were evaluated. Some of the ten Charpy impact test specimens in total, taken from each steel plate, that had the brittle fracture rate of 5% or less were extracted, and then a standard deviation σ of the Charpy impact absorbed energy values of the extracted test specimens was determined. Steel plates in which their test specimens showed the standard deviation σ of 20 J or less were rated as having excellent toughness, especially as having small variations in the toughness and surely achieving the high toughness.

[Table 6]

Steel type	Hot-rolling and cooling method	Ca concentration CATD11 in molten steel in tundish (% by mass)	Ca concentration Camin11 (% by mass) in slab	Decrease in the amount of Ca (ppm)	Evaluation of HIC resistance of slab	Presence or absence of cracking in HIC resistance test	Number density of inclusions in mold	Ar gas content (µL/cm3)	Standard deviation σ of Charpy absorbed energy (J)
1	TMCP	0.0015	0.0012	3	OK	Absence	OK	0.22	17.2
2	TMCP	0.0022	0.0020	2	OK	Absence	OK	0.41	11.2
3	TMCP	0.0015	0.0012	3	OK	Absence	OK	0.05	8.3
4	TMCP	0.0022	0.0020	2	OK	Absence	OK	0.19	6.1
5	TMCP	0.0017	0.0016	1	OK	Absence	NG	0.61	76.2
6	TMCP	0.0024	0.0022	2	OK	Absence	OK	0.22	16.2
7	TMCP	0.0027	0.0025	2	OK	Absence	OK	0.01	6.5
8	TMCP	0.0017	0.0015	2	OK	Presence	OK	0.08	8.1
9	TMCP	0.0027	0.0022	5	NG	Presence	OK	0.25	24.5
10	QT	0.0014	0.0011	3	OK	Absence	OK	0.13	5.5
11	QT	0.0018	0.0015	3	OK	Absence	OK	0.21	8.8
12	QT	0.0014	0.0011	3	OK	Absence	OK	0.17	7.8
13	QT	0.0015	0.0013	2	OK	Presence	OK	0.24	6.1
14	QT	0.0037	0.0030	7	NG	Presence	OK	0.09	29.3

[0132] Tables 5 and 6 show the following. Steels Nos. 1 to 4, 6, 7, and 10 to 12 satisfied the specified compositions and suppressed the decrease in the amount of Ca of each slab to the threshold value or less, thereby producing the steel plates of the present invention with excellent HIC resistance. Each of these steel plates had the Ar gas content in the steel suppressed within a specified range, and thus stably obtained the excellent toughness while having the excellent HIC resistance.

[0133] In contrast, in steels Nos. 9 and 14, the decrease in the amount of Ca of each slab exceeded the threshold value, so that the evaluation result of the HIC resistance of the slab was rated as NG. In the HIC test after the rolling, some cracks were caused in the steel plates. Thus, these steels Nos. 9 and 14 were confirmed to be inferior in the HIC resistance. Furthermore, the steel type No. 9 was an example in which the chemical composition of the steel plate deviated from the composition range specified by the present invention. That is, in the steel plate made of the steel type No. 9, the contents of REM and Zr were 0%, and the value (Ca - 1.25S) /O deviated from the specified value, resulting in inferior HIC resistance and large variations in the toughness of the steel plate. In the steel type No. 14, the value (Ca - 1.25S) /O deviated from the specified value, resulting in inferior HIC resistance and large variations in the toughness of the steel plate. The steels Nos. 8 and 13 were examples in which the decrease in the amount of Ca of each slab was restricted to be lower than the threshold value, but the chemical composition of the steel plate deviated from the composition range specified by the present invention. That is, in the steel type No. 8, the contents of REM and Zr were 0%, and the value (Ca/S) deviated from the specified value, resulting in inferior HIC resistance. In steel type No. 13, the value (Ca/S) deviated from the specified value, resulting in inferior HIC resistance. In steel type No. 5, the Ar gas content in the steel was excessive, resulting in large variations in the toughness of the steel plate.

[0134] In the examples in which the evaluation of the HIC resistance of the slab was rated as OK, a time period required from starting of casting to completion of a production of the steel plate, that is, a time period until dispatching the steel plate with the sour resistance (casting → rolling → dispatching) was 19 days. In contrast, in cases where the steel plate obtained after the rolling was subjected to the HIC test and then evaluated for the HIC resistance, a time period required from starting of casting to dispatching (casting → rolling → HIC test → dispatching) was 28 days, which was a long duration. In Examples, the HIC test after the rolling was able to be omitted, which could significantly shorten the time period from starting of the casting to dispatching, e.g., from 28 days to 19 days.

[0135] In the examples in which the evaluation of the HIC resistance of the slab was rated as NG, re-melting was started at the stage of the slab. Thus, a time period required from starting of casting to completion of a production of the steel plate, that is, a time period until dispatching the steel plate with the sour resistance (casting → re-melting → rolling → dispatching) was 54 days. In contrast, in cases where the steel plate obtained after the rolling was subjected to the HIC test and then evaluated for the HIC resistance as the product, when the evaluation result was NG, re-melting was started after the HIC test. Eventually, a time period required from starting of casting to dispatching of the steel plate as the product (casting → rolling → HIC test → re-melting → rolling → HIC test → dispatching) was 72 days, which was a longer duration. In Examples, since the HIC test after the rolling was able to be omitted, even though the re-melting was necessary, the time period from starting of the casting to dispatching could be drastically shortened, e.g., from 72 days to 54 days.

[0136] As mentioned above, according to the present invention, the HIC resistance can be evaluated at the stage of the slab as the cast strip without performing the HIC test after the rolling, thereby making it possible to significantly shorten the manufacturing lead time. In Examples, the HIC test is used for both the determination of the threshold value for evaluating the HIC resistance of a slab and the confirmation of HIC. Thus, the determination method of the present invention has high accuracy.

Claims

1. A steel plate having excellent hydrogen-induced cracking resistance and toughness, comprising, in percent by mass:

0.02 to 0.15% of C;

0.02 to 0.50% of Si;

0.6 to 2.0% of Mn;

more than 0% and 0.030% or less of P;

more than 0% and 0.003% or less of S;

0.010 to 0.08% of Al;

0.0003 to 0.0060% of Ca;

0.001 to 0.01% of N;

more than 0% and 0.0045% or less of O; and

one or more elements selected from the group consisting of:

more than 0% and 0.02% or less of REM and more than 0% and 0.010% or less of Zr, with the balance being iron and inevitable impurities, wherein

a ratio (Ca/S) of the Ca to the S is 2.0 or more,

the Ca, the S, and the O satisfy a formula below: (Ca -1.25S)/O ≤ 1.80,

an Ar gas content in a steel is 0.50 µL/cm³ or less, and

a decrease in an amount of Ca obtained by subtracting a Ca concentration in a slab from a Ca concentration in a molten steel in a tundish is a threshold value Ca_dropθ or less, the threshold value Ca_dropθ being a maximum decrease in the amount of Ca that avoids occurrence of hydrogen-induced cracking in the steel plate obtained by rolling the slab.

2. The steel plate according to claim 1, wherein the threshold value Ca_dropθ is a value previously determined by method including following (i) to (iii):

(i) the Ca concentration in the molten steel in the tundish and the Ca concentration in the slab are measured, and the decrease in the amount of Ca is calculated by subtracting the Ca concentration in the slab from the Ca concentration in the molten steel in the tundish;

(ii) a hydrogen-induced cracking test is performed on a steel plate that is obtained by rolling a slab, which has been cast on the same casting conditions as the above-mentioned slab; and

(iii) the maximum decrease in the amount of Ca that avoids the occurrence of hydrogen-induced cracking is determined from the decrease in the amount of Ca measured by the step (i) and a result of the hydrogen-induced cracking test shown in the step (ii).

3. The steel plate according to claim 2, wherein the slab cast on the same casting conditions as the above-mentioned slab is the slab in which the decrease in the amount of Ca is measured.

4. The steel plate according to any one of claims 1 to 3, wherein the Ca concentration in the slab is a minimum Ca concentration of two or more Ca concentrations obtained by examining the Ca concentration at two or more different positions in the thickness direction of the slab.

5. The steel plate according to any one of claims 1 to 3, wherein the threshold value Ca_dropθ is 4 ppm.

6. The steel plate according to any one of claims 1 to 3, further comprising, as another element, in percent by mass, one or more elements selected from the group consisting of:

more than 0% and 0.005% or less of B,

more than 0% and 0.1% or less of V,

more than 0% and 1.5% or less of Cu,

more than 0% and 1.5% or less of Ni,

more than 0% and 1.5% or less of Cr,

more than 0% and 1.5% or less of Mo, and

more than 0% and 0.06% or less of Nb.

7. The steel plate according to any one of claims 1 to 3, further comprising, as another element, in percent by mass, one or more elements selected from the group consisting of:

more than 0% and 0.03% or less of Ti, and

more than 0% and 0.01% or less of Mg.

8. The steel plate according to any one of claims 1 to 3 for use in line pipe.

9. The steel plate according to any one of claims 1 to 3 for use in pressure container.

10. A steel pipe for line pipe, formed of the steel plate according to any one of claims 1 to 3.

Drawing