STEEL SHEET AND METHOD FOR PRODUCING SAME

Abstract

 Provided is a steel plate having high strength of TS ≥ 690 MPa, excellent low-temperature toughness, low yield ratio, and low acoustic anisotropy. A steel plate comprises a chemical composition containing certain elements in certain amounts with a balance consisting of Fe and inevitable impurities. In a microstructure at a depth position of 1/4 of a plate thickness from a plate surface in a plate thickness direction, a ferrite fraction is 5 % to 95 % and a martensite austenite constituent fraction is 1 % to 30 % with residual microstructure consisting of tempered martensite and/or bainite, an average aspect ratio of crystal grains that are regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is less than 2.0, and a number density of crystal grains having an equivalent circular diameter of more than 30 µm among the crystal grains is 250/mm² or less.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a steel plate, particularly a steel plate that can stably ensure excellent strength, low-temperature toughness, and low yield ratio and is suitable for use at low temperatures, and a method of producing the same. The steel plate according to the present disclosure can be suitably used in large low-temperature liquefied gas storage tanks because, for example for liquefied gas storage tanks for ships, stress relief can be performed mechanically instead of by post weld heat treatment.

BACKGROUND

[0002] When carbon steel and/or carbon manganese steel is used for tanks that are classified as independent Type-C tanks and have a design temperature below -10 °C among liquefied gas storage tanks for ships, stress relief after welding is required. Stress relief is usually performed by post weld heat treatment (PWHT), but mechanical stress relief is also possible if the yield ratio of the steel material is 0.8 or less. For large tanks, PWHT is difficult, and accordingly a low yield ratio material that can be subjected to mechanical stress relief is desired. For example, in the case where carbon steel is used for large storage tanks for liquefied CO₂, high-strength steel that ensures excellent toughness at a low temperature of -50 °C and has a tensile strength (hereafter also referred to as TS) of 690 MPa or more is needed. Thus, a steel material having low yield ratio, high strength, and excellent low-temperature toughness is required. Since weld defects are checked by ultrasonic testing during welding process in some cases, the material may also be required to have low acoustic anisotropy.

[0003] For steel plates with low-temperature toughness and low yield ratio, for example, JP 2016-507649 A (PTL 1) discloses a steel plate having an impact toughness of 150 J or more at -75 °C, a yield ratio of 0.8 or less, and a tensile strength of 530 MPa or more.

CITATION LIST

Patent Literature

[0004] PTL 1: JP 2016-507649 A

SUMMARY

(Technical Problem)

[0005] However, the TS of the steel plate described in PTL 1 is 620 MPa at a maximum, and a steel plate with a TS of 690 MPa or more is not yet provided. Conventionally, since carbon steel with low yield ratio, excellent low-temperature toughness, and high strength of 690 MPa or more is unavailable, expensive nickel steel such as 9 % Ni steel needs to be used, causing an increase in material costs.

[0006] It could therefore be helpful to provide a steel plate having high strength of TS ≥ 690 MPa, excellent low-temperature toughness, low yield ratio, and low acoustic anisotropy, and a method of producing the same.

(Solution to Problem)

[0007] Upon careful examination on the chemical composition and microstructure of high-strength steel to achieve low-temperature toughness and low yield ratio, we discovered the following, especially regarding the microstructure: An effective way of achieving desired property improvement is that, in the microstructure at a depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction, the ferrite fraction is 5 % to 95 % and the martensite austenite constituent fraction is 1 % to 30 % with the residual microstructure consisting of tempered martensite and/or bainite, the average aspect ratio of crystal grains that are regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is less than 2.0, and the number density of crystal grains having an equivalent circular diameter of more than 30 µm among the crystal grains is 250/mm² or less.

[0008] Herein, the term "ferrite" refers to BCC phase that remains without reverse transformation even when martensite and bainite are heat-treated to a temperature of Ac₁ point or more and inherits the original lath-like microstructure. By setting this relatively soft ferrite phase to 5 % to 95 % and finely distributing martensite austenite constituent, low yield ratio can be achieved.

[0009] Moreover, when regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more are defined as crystal grains, by suppressing the formation of coarse grains so that the number density of crystal grains having an equivalent circular diameter of more than 30 µm will be 250/mm² or less and limiting the residual microstructure other than ferrite and martensite austenite constituent to be tempered martensite and/or bainite phase, low-temperature toughness can be achieved.

[0010] Furthermore, by setting the average aspect ratio of crystal grains to less than 2.0, reduction in acoustic anisotropy can also be achieved.

[0011] The present disclosure is based on these discoveries. We thus provide the following.

1. A steel plate comprising a chemical composition containing (consisting of), in mass%, C: 0.02 % or more and 0.15 % or less, Si: 0.01 % or more and 0.50 % or less, Mn: 0.05 % or more and 2.50 % or less, Ni: 0.5 % or more and less than 5.0 %, P: 0.03 % or less, S: 0.005 % or less, and N: 0.0010 % or more and 0.0080 % or less, with a balance consisting of Fe and inevitable impurities, wherein in a microstructure at a depth position of 1/4 of a plate thickness from a surface of the steel plate in a plate thickness direction, a ferrite fraction is 5 % to 95 % and a martensite austenite constituent fraction is 1 % to 30 % with a residual microstructure consisting of tempered martensite and/or bainite, an average aspect ratio of crystal grains that are regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is less than 2.0, and a number density of crystal grains having an equivalent circular diameter of more than 30 µm among the crystal grains is 250/mm² or less.

2. The steel plate according to 1., wherein the chemical composition further contains, in mass%, one or more selected from Al: 0.100 % or less, Nb: 0.1 % or less, Cr: 2.00 % or less, Mo: 1.0 % or less, Cu: 2.0 % or less, V: 0.05 % or less, Ti: 0.03 % or less, B: 0.0030 % or less, Ca: 0.007 % or less, REM: 0.010 % or less, and Mg: 0.007 % or less.

3. A method of producing a steel plate, the method comprising subjecting a steel material having the chemical composition according to claim 1 or 2 to hot rolling, thereafter to first heating retention, thereafter to quenching, thereafter to second heating retention, and thereafter to cooling treatment, wherein in the hot rolling, a finish temperature is 900 °C or more, in the first heating retention, a heating temperature is in a temperature range of Ac₃ point or more and 1000 °C or less, in the quenching, an average cooling rate from 600 °C to 300 °C at a depth position of 1/4 of a plate thickness from a surface of the steel plate in a plate thickness direction is 3 °C/s or more, and a cooling end temperature is 300 °C or less, in the second heating retention, a heating temperature is in a temperature range of Ac₁ point or more and less than the Ac₃ point, and in the cooling treatment, an average cooling rate from 700 °C to 500 °C at the depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction is 3 °C/s or more, and a cooling end temperature is 500 °C or less and 200 °C or more.

(Advantageous Effect)

[0012] The steel plate according to the present disclosure, despite its material being carbon steel or carbon manganese steel, can be used for steel structures that are used in low-temperature environments, for example, large low-temperature storage tanks for ships such as liquefied CO₂ tanks and LPG tanks. The steel plate according to the present disclosure also contributes to lower construction costs than nickel steel. This yields industrially great advantageous effects.

DETAILED DESCRIPTION

[0013] Embodiments of the present disclosure will be described in detail below. The following description shows preferred embodiments of the present disclosure, and the present disclosure is not limited to such.

[Chemical composition]

[0014] A steel plate according to the present disclosure has a certain chemical composition. Preferably, the steel material used for producing the steel plate according to the present disclosure also has the certain chemical composition. Each element contained in the chemical composition will be described below. In this specification, "%" as a unit of content of each element denotes "mass%" unless otherwise specified.

C: 0.02 % or more and 0.15 % or less

[0015] C is an element that has the effect of improving the strength of the steel plate. In order to achieve this effect, the C content is 0.02 % or more. The C content is preferably 0.03 % or more. If the C content is more than 0.15 %, the amount of martensite austenite constituent in the steel plate increases and low-temperature toughness decreases. The C content is therefore 0.15 % or less. The C content is preferably 0.12 % or less.

Si: 0.01 % or more and 0.50 % or less

[0016] Si is an element that acts as a deoxidizer. In order to achieve this effect, the Si content is 0.01 % or more. The Si content is preferably 0.03 % or more. If the Si content is excessively high, toughness decreases. The Si content is therefore 0.50 % or less. The Si content is preferably 0.30 % or less.

Mn: 0.05 % or more and 2.50 % or less

[0017] Mn is an element that enhances the hardenability of the steel and is effective in increasing the strength of the steel plate. In order to achieve this effect, the Mn content is 0.05 % or more. The Mn content is preferably 0.10 % or more. If the Mn content is more than 2.50 %, toughness degrades. The Mn content is therefore 2.50 % or less. The Mn content is preferably 2.00 % or less.

Ni: 0.5 % or more and less than 5.0 %

[0018] Ni is an element effective in improving the low-temperature toughness of the steel plate. In order to achieve this effect, the Ni content is 0.5 % or more. Since Ni is an expensive element, the steel plate costs increase as the Ni content increases. Accordingly, in the present disclosure, the Ni content is less than 5.0 %. The Ni content is preferably 0.8 % or more. The Ni content is preferably 3.5 % or less.

P: 0.03 % or less

[0019] P is an inevitable impurity, and is a harmful element that adversely affects the low-temperature toughness of the steel plate. For example, in order to obtain a sound base metal and weld joint when welding the steel plate to produce a welded structure, it is preferable to reduce the P content as much as possible. The P content is therefore 0.03 % or less. Since lower P content is better from the viewpoint of low-temperature toughness, the lower limit of the P content is not set and may be 0 %. Excessively reducing the P content, however, causes an increase in cost. Accordingly, the lower limit of the P content is preferably 0.001 % from the viewpoint of cost.

S: 0.005 % or less

[0020] S forms MnS in the steel and significantly degrades low-temperature toughness, and accordingly it is desirable to reduce the S content as much as possible with the upper limit being 0.005 %. The S content is preferably 0.002 % or less. Since lower S content is better, the lower limit of the S content is not set and may be 0 %.

N: 0.0010 % or more and 0.0080 % or less

[0021] N forms precipitates in the steel. If the N content is more than 0.0080 %, the toughness of the base metal decreases. N is also an element that forms AlN and thus contributes to grain refinement of the base metal. This effect is achieved when the N content is 0.0010 % or more. The N content is therefore 0.0010 % or more and 0.0080 % or less. The N content is preferably 0.0020 % or more. The N content is preferably 0.0060 % or less.

[0022] In one embodiment of the present disclosure, the chemical composition contains the certain amounts of the elements described above with the balance consisting of Fe and inevitable impurities.

[0023] In another embodiment of the present disclosure, the chemical composition may optionally further contain one or more selected from Cr, Mo, Al, Cu, Nb, V, Ti, B, Ca, REM, and Mg preferably in the following amounts.

Cr: 2.00 % or less

[0024] Cr is an element that can improve the strength of the steel plate without significantly impairing low-temperature toughness. In order to achieve this effect, the Cr content is preferably 0.01 % or more. The Cr content is more preferably 0.30 % or more. If the Cr content is more than 2.00 %, the low-temperature toughness of the steel plate may decrease. The Cr content is therefore preferably 2.00 % or less. The Cr content is more preferably 0.80 % or less.

Mo: 1.0 % or less

[0025] Mo is an element that contributes to improve strength of the steel, and may be optionally added depending on the desired strength. If the Mo content is more than 1.0 %, toughness may degrade. Accordingly, in the case where Mo is added, the Mo content is preferably 1.0 % or less. From the viewpoint of achieving the strength improving effect by Mo, the Mo content is preferably 0.01 % or more.

Al: 0.100 % or less

[0026] Al is an element that acts as a deoxidizer, and is most widely used in the molten steel deoxidation process for high tensile strength steel. In order to achieve this effect, the Al content is preferably 0.001 % or more. The Al content is more preferably 0.010 % or more. If the Al content is more than 0.100 %, the toughness of the base metal may decrease. The Al content is therefore preferably 0.100 % or less. The Al content is more preferably 0.07 % or less.

Cu: 2.0 % or less

[0027] Cu is an element that can increase strength while maintaining high toughness, and may be optionally added depending on the desired strength. If the Cu content is more than 2.0 %, hot brittleness may occur and degrade the surface characteristics of the steel plate. Accordingly, in the case where Cu is added, the Cu content is preferably 2.0 % or less. The Cu content is more preferably 1.0 % or less. In order to achieve the foregoing effect, the Cu content is preferably 0.01 % or more. The Cu content is more preferably 0.10 % or more, and further preferably 0.20 % or more.

Nb: 0.1 % or less

[0028] Nb is an element that contributes to improve strength of the steel, and may be optionally added depending on the desired strength. If the Nb content is more than 0.1 %, the toughness of the base metal may degrade. Accordingly, in the case where Nb is added, the Nb content is preferably 0.1 % or less. From the viewpoint of achieving the strength improving effect by Nb, the Nb content is preferably 0.005 % or more.

V: 0.05 % or less

[0029] V is an effective element that enhances the strength of the steel plate through strengthening by precipitation. If the V content is excessively high, the low-temperature toughness of the steel plate may decrease. Accordingly, in the case where V is added, the V content is preferably 0.05 % or less. The V content is more preferably 0.04 % or less. Although no lower limit is placed on the V content, the V content is preferably 0.010 % or more in order to achieve the foregoing effect.

Ti: 0.03 % or less

[0030] Ti is an element that has the effect of enhancing the toughness of the weld without degrading the mechanical properties of the base metal when welding the steel plate to produce a welded structure. In order to achieve this effect, the Ti content is preferably 0.003 % or more. A Ti content of more than 0.03 %, however, may cause a decrease in toughness. The Ti content is therefore preferably 0.03 % or less.

B: 0.0030 % or less

[0031] B is an element that enhances hardenability when added in a small amount. In order to sufficiently achieve this effect, the B content is preferably 0.0003 % or more. If the B content is more than 0.0030 %, toughness may degrade. Accordingly, in the case where B is added, the B content is preferably 0.0030 % or less.

Ca: 0.007 % or less

[0032] Ca is an element that has the effect of improving the low-temperature toughness of the steel plate by controlling the form of inclusions in the steel. If the Ca content is excessively high, the cleanliness of the steel may be impaired and the Charpy absorbed energy at low temperatures (hereafter also referred to as Charpy toughness) may decrease. Accordingly, in the case where Ca is added, the Ca content is preferably 0.007 % or less. The Ca content is more preferably 0.004 % or less. Although no lower limit is placed on the Ca content, the Ca content is preferably 0.001 % or more in order to achieve the foregoing effect.

REM: 0.010 % or less

[0033] REM (rare earth metal) is an element that has the effect of improving the low-temperature toughness of the steel plate by controlling the form of inclusions in the steel, as with Ca. If the REM content is excessively high, the cleanliness of the steel may be impaired and Charpy toughness may decrease. Accordingly, in the case where REM is added, the REM content is preferably 0.010 % or less. The REM content is more preferably 0.008 % or less. Although no lower limit is placed on the REM content, the REM content is preferably 0.001 % or more in order to achieve the foregoing effect.

[0034] Herein, REM is a generic term for 17 elements including 15 lanthanoid elements and Y and Sc, and these elements may be contained singly or in combination. The REM content is the total content of these elements.

Mg: 0.007 % or less

[0035] Mg is an element that has the effect of improving the low-temperature toughness of the steel plate by controlling the form of inclusions in the steel, as with Ca and REM. If the Mg content is excessively high, the cleanliness of the steel may be impaired and Charpy toughness may decrease. Accordingly, in the case where Mg is added, the Mg content is preferably 0.007 % or less. The Mg content is more preferably 0.004 % or less. Although no lower limit is placed on the Mg content, the Mg content is preferably 0.001 % or more in order to achieve the foregoing effect.

[Microstructure]

(Ferrite fraction: 5 % to 95 %, martensite austenite constituent fraction: 1 % to 30 %, residual microstructure: tempered martensite and/or bainite)

[0036] In the microstructure at a depth position of 1/4 of the plate thickness from the surface of the steel plate according to the present disclosure in the plate thickness direction, the ferrite fraction is 5 % to 95 % and the martensite austenite constituent fraction is 1 % to 30 % with the residual microstructure consisting of tempered martensite and/or bainite. If the ferrite fraction in the microstructure is less than 5 %, a yield ratio of 0.80 or less cannot be achieved. If the ferrite fraction is more than 95 %, the fraction of hard phase such as martensite austenite constituent is low and a yield ratio of 0.80 or less cannot be achieved. If the martensite austenite constituent fraction is less than 1 %, a yield ratio of 0.80 or less cannot be achieved. If the martensite austenite constituent fraction is more than 30 %, toughness degrades. If the residual microstructure is not tempered martensite and/or bainite, for example, if the residual microstructure is as-quenched martensite or bainite, the desired toughness cannot be achieved.

(Average aspect ratio of crystal grains: less than 2.0, number density of crystal grains having equivalent circular diameter of more than 30 µm: 250/mm² or less)

[0037] In the microstructure at a depth position of 1/4 of the plate thickness from the surface of the steel plate according to the present disclosure in the plate thickness direction, when regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more are defined as crystal grains, the average aspect ratio of the crystal grains is less than 2.0, and the number density of crystal grains having an equivalent circular diameter of more than 30 µm among the crystal grains is 250/mm² or less. If the average aspect ratio of the crystal grains is 2.0 or more, the orientation of the microstructure tends to be high, causing an increase in acoustic anisotropy. If the number density of crystal grains having an equivalent circular diameter of more than 30 µm among the crystal grains is more than 250/mm², toughness decreases. The number density of crystal grains having an equivalent circular diameter of more than 30 µm is preferably 150/mm² or less.

[0038] The plate thickness of the steel plate is not limited and may be any thickness. For example, the plate thickness of the steel plate is preferably 6 mm or more and 50 mm or less.

[Mechanical properties]

(Tensile strength)

[0039] The tensile strength of the steel plate is not limited, but is preferably 690 MPa or more. With a tensile strength of 690 MPa or more, the plate thickness can be reduced when the steel plate is used for tanks. The tensile strength of the steel plate is more preferably 720 MPa or more. Although no upper limit is placed on the tensile strength, the tensile strength is preferably 1000 MPa or less.

[0040] The tensile strength can be measured by the method described in the EXAMPLES section below.

(Yield ratio)

[0041] The yield ratio of the steel plate is not limited, but is preferably 0.80 or less. With a yield ratio of 0.80 or less, mechanical stress relief can be performed instead of post weld heat treatment.

(Low-temperature toughness)

[0042] The toughness value of the steel plate is not limited, but the Charpy absorbed energy at -50 °C (vE_-50°C) is preferably 100 J or more in a full-size Charpy impact test. vE_-50°C of the steel plate is more preferably 150 J or more.

[Production method]

[0043] A method of producing the steel plate according to the present disclosure (production method) will be described below. In the following description, the term "temperature" refers to the temperature at the center of the plate thickness unless otherwise specified. The temperature at the center of the plate thickness can be obtained, for example, by heat transfer calculation from the surface temperature of the steel plate measured with a radiation thermometer.

[0044] The production method is a steel plate production method comprising subjecting a steel material having the above-described chemical composition to hot rolling, thereafter to first heating retention, thereafter to quenching, thereafter to second heating retention, and thereafter to cooling treatment. In the hot rolling, the finish temperature is 900 °C or more. In the first heating retention, the heating temperature is in the temperature range of Ac₃ point or more and 1000 °C or less. In the quenching, the average cooling rate from 600 °C to 300 °C at a depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction is 3 °C/s or more, and the cooling end temperature is 300 °C or less. In the second heating retention, the heating temperature is in the temperature range of Ac₁ point or more and less than Ac₃ point. In the cooling treatment, the average cooling rate from 700 °C to 500 °C at a depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction is 3 °C/s or more, and the cooling end temperature is 500 °C or less and 200 °C or more. The steel plate according to the present disclosure can be suitably produced under these conditions.

[0045] Each step will be described in detail below.

[0046] First, the steel material having the above-described chemical composition may be heated before hot rolling. In this case, the heating temperature of the steel material is preferably 900 °C or more and 1250 °C or less. The method of producing the steel material is not limited. For example, molten steel having the above-described chemical composition may be prepared by steelmaking using a conventional method and cast to produce the steel material. The steelmaking may be performed using any method such as a converter, an electric furnace, or an induction furnace. The casting is preferably performed by continuous casting from the viewpoint of productivity, but may be performed by ingot casting and blooming. An example of the steel material is a steel slab.

[0047] The steel material obtained as a result of casting and the like may be heated after cooling, or directly heated without cooling.

[0048] If the heating temperature of the steel material is less than 900 °C, due to high deformation resistance of the steel material, the load on the mill in the subsequent hot rolling may increase and hinder the hot rolling. Therefore, the heating temperature of the steel material is preferably 900 °C or more. If the heating temperature of the steel material is more than 1250 °C, due to noticeable oxidation of the steel, the loss incurred by the removal of the oxide film caused by the oxidation may increase, resulting in a decrease in yield rate. Therefore, the heating temperature of the steel material is preferably 1250 °C or less.

(Hot rolling)

[0049] The steel material having the above-described chemical composition is subjected to hot rolling to obtain a hot-rolled steel plate having a final plate thickness. The finish temperature in the hot rolling is 900 °C or more. If the finish temperature is less than 900 °C, acoustic anisotropy may degrade. The final plate thickness of the hot-rolled steel plate is not limited, but is preferably 6 mm or more and 50 mm or less as mentioned above. After the hot rolling, the hot-rolled steel plate may be optionally cooled. The cooling method is not limited, and may be, for example, air cooling or water cooling.

(First heating retention)

[0050] After the hot rolling, the hot-rolled steel plate is subjected to heating retention in an austenite range (first heating retention). Thus, the heating temperature in the first heating retention is Ac₃ point or more. If the heating temperature of the hot-rolled steel plate is more than 1000 °C, crystal grains coarsen and toughness decreases. Accordingly, the heating temperature in the first heating retention is 1000 °C or less.

(Quenching)

[0051] The hot-rolled steel plate after the first heating retention is subjected to quenching (i.e. accelerated cooling). It is important that, in the quenching, the average cooling rate from 600 °C to 300 °C at a depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction (hereafter such a depth is also denoted as 1/4t) is 3 °C/s or more.

[0052] If the average cooling rate in the quenching is less than 3 °C/s, it is difficult to obtain the desired transformed microstructure, so that sufficient strength cannot be achieved. Although no upper limit is placed on the average cooling rate, if the average cooling rate is more than 200 °C/s, it is difficult to control the temperature at each position in the steel plate, so that the material quality tends to vary in the plate transverse direction and the rolling direction. This is likely to cause variation in material properties such as tensile property and toughness. Therefore, the average cooling rate is preferably 200 °C/s or less.

[0053] If the cooling stop temperature (i.e. cooling end temperature) at a position of 1/4t in the quenching is more than 300 °C, the desired transformed microstructure cannot be obtained. The cooling stop temperature at a position of 1/4t in the quenching is therefore 300 °C or less. By accelerated cooling under such conditions, the hot-rolled steel plate is quenched well.

[0054] The quenching (accelerated cooling) treatment may be performed by any method without limitation. For example, one or both of air cooling and water cooling may be used. For water cooling, any cooling method (for example, spray cooling, mist cooling, laminar cooling, etc.) using water is available.

(Second heating retention)

[0055] The hot-rolled steel plate after the quenching is then subjected to heating retention in a dual-phase temperature range (second heating retention). The heating temperature in the second heating retention is Ac₁ point or more and less than Ac₃ point. If the heating temperature is less than Ac₁ point, martensite austenite constituent cannot be obtained and low yield ratio cannot be achieved. If the heating temperature is Ac₃ point or more, ferrite phase will be less than 5 % and tempered martensite phase will be more than 90 %, and low yield ratio cannot be achieved.

[0056] For heating retention in the dual-phase temperature range, any heating method may be used as long as the heating temperature can be controlled in the foregoing manner. An example of the heating method is furnace heating. The furnace heating is not limited, and a typical heat treatment furnace may be used.

[0057] After the heating temperature is reached, the hot-rolled steel plate may be held in the dual-phase temperature range for any period of time before the below-described cooling treatment is started. The holding time is not limited, but is preferably 5 minutes or more.

[0058] Ac₁ point can be calculated according to the following formula (1):

Ac₁ point (°C) = 750.8 - 26.6 × C + 17.6 × Si - 11.6 × Mn - 22.9 × Cu - 23 × Ni + 24.1 × Cr + 22.5 × Mo - 39.7 × V - 5.7 × Ti + 232.4 × Nb - 169.4 × Al

[0059] Ac₃ point can be calculated according to the following formula (2):

Ac₃ point (°C) = 937.2 - 436.5 × C + 56 × Si - 19.7 × Mn - 16.3 × Cu - 26.6 × Ni - 4.9 × Cr + 38.1 × Mo + 124.8 × V + 136.3 × Ti - 19.1 × Nb + 198.4 × Al + 3315 × B

(Cooling treatment)

[0060] Cooling treatment is then performed. In the cooling treatment, the average cooling rate from 700 °C to 500 °C at a position of 1/4t is 3 °C/s or more, and the cooling end temperature is 500 °C or less and 200 °C or more. If the average cooling rate is less than 3 °C/s, there is a possibility that the desired transformed microstructure cannot be obtained and strength and toughness decrease. If the cooling stop temperature is more than 500 °C, there is a possibility that bainite formed after the cooling stop becomes a main component and low-temperature toughness is not satisfied. If the cooling stop temperature is less than 200 °C, the desired tempering effect cannot be achieved in the end and toughness degrades.

[0061] After the cooling stop (after the end of the cooling treatment), air cooling may be performed in order to improve toughness by self-tempering. The cooling rate in the air cooling is not limited. For example, the cooling rate is typically 1 °C/s or less when the plate thickness is 6 mm to 50 mm.

EXAMPLES

[0062] Steel plates were each produced according to the following procedure, and their properties were evaluated.

[0063] First, molten steel having the chemical composition shown in Table 1 was prepared by steelmaking using a converter, and subjected to continuous casting to obtain a steel slab (thickness: 200 mm) as a steel material. Ac₁ point (°C) calculated according to the foregoing formula (1) and Ac₃ point (°C) calculated according to the foregoing formula (2) are shown in Table 1 (and Table 2).

[0064] Subsequently, the obtained steel slab was heated and then hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel plate having the corresponding plate thickness (final plate thickness). After this, each treatment was performed under the conditions shown in Table 2 to obtain a steel plate.

[Table 2]

[0065] 

Table 2

No.	Steel material			Production conditions									Remarks
	Steel sample ID	Ac1 point (°C)	Ac3 point (°C)	Heating of steel material	Hot	rolling	First heating retention	Quenching		Second heating retention	Cooling treatment
				Heating temperature (°C)	Plate thickness (mm)	Finish temperature (°C)	Heating temperature (°C)	Average cooling rate (*1)(*2) (°C/s)	Cooling end temperature (*1) (°C)	Heating temperature (°C)	Average cooling rate (*1)(*3) (°C/s)	Cooling end temperature (*1) (°C)
1	A	689	830	1200	40	950	900	15	200	780	15	250	Example
2	A	689	830	1100	40	900	850	15	250	760	15	350	Example
3	A	689	830	1000	25	900	950	30	150	790	15	350	Example
4	A	689	830	1200	30	1000	900	25	100	710	25	300	Example
5	A	689	830	1100	40	950	900	15	200	810	25	300	Example
6	A	689	830	1150	40	900	900	15	100	780	15	450	Example
7	A	689	830	1200	40	750	900	15	200	780	15	300	Comparative Example
8	A	689	830	1200	40	1000	1050	15	200	760	15	350	Comparative Example
9	A	689	830	1150	40	950	850	15	100	680	15	300	Comparative Example
10	A	689	830	1100	40	900	900	15	200	860	15	350	Comparative Example
11	A	689	830	1200	40	950	900	0.5 (Air cooling)	200	790	15	300	Comparative Example
12	A	689	830	1150	40	950	900	15	250	770	0.5 (Air cooling		Comparative Example
13	A	689	830	1090	40	900	900	15	200	760	15	100	Comparative Example
14	A	689	830	1120	40	950	980	15	200	820	15	550	Comparative Example
15	A	689	830	1200	40	950	900	15	450	780	15	250	Comparative Example
16	B	664	826	1120	12	950	900	50	250	750	50	450	Example
17	C	700	822	1200	40	950	870	15	100	780	15	300	Example
18	D	697	831	1070	40	900	900	15	200	770	15	350	Example
19	E	710	876	1120	30	900	880	25	200	780	25	400	Example
20	F	671	809	1170	20	950	900	35	200	740	35	360	Example
21	G	673	824	1100	40	900	910	15	250	760	15	300	Example
22	H	689	829	1200	40	950	870	15	100	780	15	320	Example
23	I	693	819	1150	40	950	880	15	100	760	15	350	Example
24	J	720	863	1150	40	950	900	15	200	810	15	350	Example
25	K	624	757	1150	40	950	890	15	200	700	15	230	Example
26	L	716	862	1200	40	950	880	15	250	820	15	320	Example
27	M	719	847	1150	40	900	900	15	200	800	15	300	Example
28	N	715	828	1100	40	950	900	15	250	780	15	300	Comparative Example
29	O	696	867	1200	40	950	900	15	100	800	15	350	Comparative Example
30	P	711	856	1120	40	900	910	15	250	810	15	350	Comparative Example
31	Q	721	843	1070	40	900	890	15	200	770	15	300	Comparative Example
32	R	720	862	1150	40	950	880	15	250	790	15	400	Comparative Example
33	S	690	824	1100	40	900	900	15	100	790	15	320	Comparative Example
34	I	714	843	1160	40	950	890	15	200	780	15	350	Comparative Example
35	U	709	833	1200	40	950	880	15	200	760	15	350	Comparative Example
36	V	703	850	1200	40	980	900	15	200	800	15	300	Comparative Example
37	W	697	842	1200	40	980	920	15	200	800	15	300	Comparative Example
Underlines indicate outside the scope of the present disclosure.
(*1) Depth position of 1/4 of plate thickness
(*2) Temperature range from 600 °C to 300 °C
(*3) Temperature range from 700 °C to 500 °C

[0066] For each of the obtained steel plates, the microstructure, tensile strength (TS), yield ratio (YR), low-temperature toughness (vE_-50°C), and acoustic anisotropy (sound velocity ratio) were evaluated in the following manner. The evaluation results are shown in Table 3.

[Microstructure]

[0067] A test piece for microstructure observation was collected from the steel plate so that a position of 1/4t would be the observation position. The test piece was embedded in resin so that a cross section perpendicular to the rolling direction would be the observation plane, and mirror-polished. After this, nital etching was performed, and then observation was made using a scanning electron microscope with 5000 magnification and an image of microstructure was taken. The obtained image was analyzed to identify microstructure fractions. Microstructures identified are as follows:

• Tempered martensite and/or bainite: hard phase containing cementite
• Martensite austenite constituent: hard phase not containing cementite and having an equivalent circular diameter of 1 µm or less
• As-quenched martensite: hard phase not containing cementite and having an equivalent circular diameter of more than 1 µm
• Ferrite: matrix phase other than the above.

[0068] Further, microstructure analysis was conducted by EBSD. Defining regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more as crystal grains, for a total area of 1 mm × 1 mm, the crystal grain size distribution was calculated and the number density of crystal grains having an equivalent circular diameter of more than 30 µm was measured. In addition, the average value (average aspect ratio) of the aspect ratios (= the length in the rolling direction/the length in the plate thickness direction) of crystal grains obtained in an area of 1 mm × 1 mm was calculated.

[Tensile strength (TS) and yield ratio (YR)]

[0069] A JIS No. 4 tensile test piece perpendicular to the rolling direction was collected from a position of 1/4t of the steel plate. Using the tensile test piece, a tensile test was conducted in accordance with JIS Z 2241 to evaluate the tensile strength (TS) of the steel plate. If the tensile strength was 690 MPa or more, the steel plate was evaluated as having high strength and rated as "pass". The yield ratio (YR) was also evaluated based on the tensile test result. If the yield ratio was 0.80 or less, the steel plate was rated as "pass".

[Low-temperature toughness (vE_-50°C)]

[0070] V-notched test pieces parallel to the rolling direction were collected from a position of 1/4t of the steel plate in accordance with JIS Z 2202. Using the V-notched test pieces, Charpy impact test was conducted in accordance with JIS Z 2242 to determine the Charpy absorbed energy at -50 °C (vE_-50°C). The Charpy absorbed energy can be regarded as an index of the low-temperature toughness of the steel plate. In the Charpy impact test, three test pieces (A, B, and C) were collected per steel plate and measured. The measurement results are shown in Table 3. In the full-size Charpy impact test, if vE_-50°C of each test piece was 100 J or more, the steel plate was evaluated as having excellent Charpy toughness and rated as "pass".

[Acoustic anisotropy (sound velocity ratio)]

[0071] In order to evaluate the acoustic anisotropy of the steel plate, the transverse sound velocity ratio specified in JIS Z 3060 was evaluated. The transverse sound velocity ratio herein is a value defined as CSL/CSC, that is, the ratio of the sound velocity CSL (m/sec) when the vibration direction of the transverse wave is the rolling direction (L direction) to the sound velocity CSC (m/sec) when the vibration direction of the transverse wave is a direction (C direction) orthogonal to the rolling direction. The measurement results are shown in Table 3. If CSL/CSC was 1.02 or less, the steel plate was evaluated as having low acoustic anisotropy and rated as "pass".

[Table 3]

[0072] 

Table 3

No.	Evaluation results											Remarks
	Microstructure					TS (MPa)	YR	vE-50°C (J) (A)	vE-50°C (J) (B)	vE-50°C (J) (C)	Sound velocity ratio
	Ferrite fraction (%)	Martensite austenite constituent fraction (%)	Residual microstructure*	Crystal grains greater than 30 µm (number/mm2)	Average aspect ratio
1	40	13	Tempered martensite/bainite	210	1.5	720	0.69	230	240	235	1.00	Example
2	50	12	Tempered martensite/bainite	170	1.4	710	0.69	230	240	220	1.00	Example
3	30	12	Tempered martensite/bainite	240	1.5	730	0.70	200	210	220	1.00	Example
4	80	10	Tempered martensite/bainite	210	1.4	700	0.71	230	230	220	1.00	Example
5	10	10	Tempered martensite/bainite	210	1.5	740	074	230	220	230	1.00	Example
6	40	5	Tempered martensite/bainite	210	1.4	720	0.77	200	210	210	1.00	Example
7	40	14	Tempered martensite/bainite	130	2.5	760	0.71	250	240	250	1.03	Comparative Example
8	50	12	Tempered martensite/bainite	280	1.5	680	0.71	90	80	70	1.00	Comparative Example
9	100	0	-	170	1.4	700	0.95	200	210	200	1.00	Comparative Example
10	0	5	Tempered martensite/bainite	180	1.5	750	0.85	220	200	230	1.00	Comparative Example
11	30	8	Tempered martensite/bainite	260	1.4	650	0.72	90	100	90	1.00	Comparative Example
12	45	15	Bainite	260	1.5	650	0.71	90	110	80	1.00	Comparative Example
13	50	15	Martensite/bainite	220	1.5	740	0.68	80	90	90	1.00	Comparative Example
14	40	10	Bainite	260	1.4	750	0.73	70	80	80	1.00	Comparative Example
15	40	13	Tempered martensite/bainite	260	1.5	670	0.69	120	80	90	1.00	Comparative Example
16	40	8	Tempered martensite/bainite	220	1.5	780	0.71	210	210	200	1.00	Example
17	30	25	Tempered martensite/bainite	170	1.4	770	0.68	210	200	220	1.00	Example
18	40	10	Tempered martensite/bainite	210	1.4	710	0.71	220	230	240	1.00	Example
19	35	15	Tempered martensite/bainite	180	1.5	720	0.72	180	170	190	1.00	Example
20	40	10	Tempered martensite/bainite	220	1.4	730	0.71	210	210	220	1.00	Example
21	45	15	Tempered martensite/bainite	230	1.5	720	0.72	210	200	215	1.00	Example
22	25	12	Tempered martensite/bainite	170	1.4	720	0.69	150	160	160	1.00	Example
23	35	20	Tempered martensite/bainite	180	1.5	720	0.68	140	150	140	1.00	Example
24	40	10	Tempered martensite/bainite	220	1.4	710	0.72	160	180	200	1.00	Example
25	30	15	Tempered martensite/bainite	190	1.4	720	0.71	255	260	256	1.00	Example
26	20	15	Tempered martensite/bainite	180	1.5	810	0.71	200	220	210	1.00	Example
27	30	13	Tempered martensite/bainite	220	1.4	820	0.69	235	240	250	1.00	Example
28	45	35	Tempered martensite/bainite	210	1.4	760	0.68	60	70	80	1.00	Comparative Example
29	30	1	Tempered martensite/bainite	210	1.5	630	0.78	230	240	220	1.00	Comparative Example
30	20	12	Tempered martensite/bainite	220	1.4	720	0.71	80	90	90	1.00	Comparative Example
31	50	10	Tempered martensite/bainite	190	1.4	710	0.71	80	90	80	1.00	Comparative Example
32	40	10	Tempered martensite/bainite	180	1.5	630	0.71	200	200	210	1.00	Comparative Example
33	40	15	Tempered martensite/bainite	210	1.5	760	0.72	50	60	50	1.00	Comparative Example
34	35	10	Tempered martensite/bainite	190	1.4	730	0.72	60	50	60	1.00	Comparative Example
35	50	15	Tempered martensite/bainite	170	1.5	750	0.71	70	40	60	1.00	Comparative Example
36	25	12	Tempered martensite/bainite	210	1.4	730	0.70	80	90	90	1.00	Comparative Example
37	25	12	Tempered martensite/bainite	260	1.4	730	0.70	80	150	120	1.00	Comparative Example
			*Tempered martensite/bainite: state in which carbides are formed in microstructure
			Martensite/bainite: state in which carbides are not precipitated

[0073] As can be seen from these tables, in each Example, a steel plate having a certain chemical composition and microstructure and having high strength, high low-temperature toughness, low yield ratio, and low acoustic anisotropy was obtained.

Claims

1. A steel plate comprising a chemical composition containing, in mass%,

C: 0.02 % or more and 0.15 % or less,

Si: 0.01 % or more and 0.50 % or less,

Mn: 0.05 % or more and 2.50 % or less,

Ni: 0.5 % or more and less than 5.0 %,

P: 0.03 % or less,

S: 0.005 % or less, and

N: 0.0010 % or more and 0.0080 % or less,

with a balance consisting of Fe and inevitable impurities,

wherein in a microstructure at a depth position of 1/4 of a plate thickness from a surface of the steel plate in a plate thickness direction, a ferrite fraction is 5 % to 95 % and a martensite austenite constituent fraction is 1 % to 30 % with a residual microstructure consisting of tempered martensite and/or bainite, an average aspect ratio of crystal grains that are regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is less than 2.0, and a number density of crystal grains having an equivalent circular diameter of more than 30 µm among the crystal grains is 250/mm² or less.

2. The steel plate according to claim 1, wherein the chemical composition further contains, in mass%, one or more selected from

Nb: 0.05 % or less,

Al: 0.100 % or less,

Cr: 2.00 % or less,

Mo: 1.0 % or less,

Cu: 2.0 % or less,

V: 0.05 % or less,

Ti: 0.03 % or less,

B: 0.0030 % or less,

Ca: 0.007 % or less,

REM: 0.010 % or less, and

Mg: 0.007 % or less.

3. A method of producing a steel plate, the method comprising subjecting a steel material having the chemical composition according to claim 1 or 2 to hot rolling, thereafter to first heating retention, thereafter to quenching, thereafter to second heating retention, and thereafter to cooling treatment,

wherein in the hot rolling, a finish temperature is 900 °C or more,

in the first heating retention, a heating temperature is in a temperature range of Ac₃ point or more and 1000 °C or less,

in the quenching, an average cooling rate from 600 °C to 300 °C at a depth position of 1/4 of a plate thickness from a surface of the steel plate in a plate thickness direction is 3 °C/s or more, and a cooling end temperature is 300 °C or less,

in the second heating retention, a heating temperature is in a temperature range of Ac₁ point or more and less than the Ac₃ point, and

in the cooling treatment, an average cooling rate from 700 °C to 500 °C at the depth position of 1/4 of the plate thickness from the surface of the steel plate in the plate thickness direction is 3 °C/s or more, and a cooling end temperature is 500 °C or less and 200 °C or more.