THICK HOT-ROLLED STEEL SHEET HAVING EXCELLENT PROCESSABILITY AND EXCELLENT STRENGTH/TOUGHNESS AFTER THERMAL TREATMENT, AND METHOD FOR PRODUCTION OF THE STEEL SHEET

Abstract

 Disclosed is a thick hot-rolled steel sheet which has a tensile strength of 440 to 640 MPa, preferably 490 to 590 MPa, an elongation of 20% or more, excellent uniformity in the thickness-wise direction, high strength, excellent processability, and excellent strength/toughness after a thermal treatment, and is therefore suitable for use as a structural member for an automobile, a construction machine or the like. Also disclosed is a method for producing the steel sheet. A steel material is provided, which contains 0.10 to 0.20% of C, in which the contents of Si, Mn, Al, P, S and N are adjusted within proper ranges, and which further contains 0.01 to 0.15% of Ti and 0.0010 to 0.0050% of B. The steel material is subj ected to the hot-rolling under the conditions of a rolling finishing temperature for the fmish rolling of 820 to 880°C. The resulting product is cooled at a cooling rate of 15 to 50°C/s and a cooling finishing temperature of 500 to 600°C, and is then wound-up in a coli-like shape. In this manner, the steel sheet can have a structure which is uniform in the thickness-wise direction and is composed of a bainitic ferrite phase. The steel sheet has excellent uniformity in the thickness-wise direction so that the fluctuation in the hardness in the thickness-wise direction falls within the range up to 10% above or below the average value, and has a tensile strength of 440 to 640 MPa and an elongation of 20% or more. The steel sheet may be subjected to a further thermal treatment, thereby producing a highly-strong, highly-tough, thick and large-sized structural member having a tensile strength of 980 MPa or more and a vTrs of -60°C or lower.

Description

Technical Field

[0001] The present invention relates to a hot rolled thick steel sheet and a method for manufacturing the steel sheet. "A hot rolled thick steel sheet" herein is a hot rolled steel sheet having a sheet thickness of 6 mm or more and 12 mm or less, which is a relatively thick hot rolled steel sheet. Such a hot rolled thick steel sheet is suitably used as a material for manufacturing structural components of, for example, automobiles and construction equipment (hereinafter also referred to as construction machines).

Background Art

[0002] In recent years, regulations of emissions limit law for automobiles have been tightened in terms of global environmental protection and weight reduction of a car body has been promoted to improve fuel economy. Automobile components are also not exceptions and weight reduction of automobile components has been strongly demanded. Similarly, weight reduction of structural components of construction machines or the like has been also strongly demanded. This is because large, heavy, and thick-walled materials having a sheet thickness of about 6 mm or more and 12 mm or less and a length of 10 m are often used for the structural components of automobiles, construction machines, and the like. If a highly strengthened steel sheet is used to reduce the weight of components, the formability of a steel sheet such as elongation is decreased, which poses a problem in that the degree of difficulty in processing is considerably increased. In addition, there is a problem in that fatigue strength is not improved at stress concentration zones such as holes opened for weight reduction and weld zones that inevitably exist. Therefore, unlike other small thin-walled parts, large thick-walled parts such as structural components of automobiles, construction machines, and the like had a tensile strength of at most about 540 MPa even after being strengthened.

[0003] In recent years, die quench, in which parts are quenched while being pressed, has been put to practical use as a means for strengthening small thin-walled parts. However, when die quench is applied to large thick-walled parts, there are various problems in that huge equipment needs to be prepared, desired strength cannot be achieved because parts are not quenched to their center due to their thick wall, and the brittle failure unique to thick-walled parts is caused when the parts are as quenched. Thus, die quench is unsuitable for large thick-walled parts.

[0004] However, weight reduction of structural components of automobiles, construction machines, and the like has been strongly demanded and strengthening of components has been desired. Therefore, particularly for components for which high strength is demanded, a material is processed into a shape of components and heat treatment such as quenching and tempering is then performed to achieve high strength and high toughness of components. Thus, in addition to strength and elongation, excellent component strength and toughness achieved by heat treatment performed after a material is processed into a shape of components have been demanded for a hot rolled steel sheet that is a raw material.

[0005] To meet such a demand, for example, Patent Document 1 discloses a method for manufacturing a thin steel sheet including a step of hot-rolling a steel material at a coiling temperature of 720°C or less, the steel material containing C: 0.10 to 0.37% and proper amounts of Si, Mn, P, S, and Al and containing B and N so as to satisfy 14B/10.8N: 0.50 or more, wherein BN that is an intrasteel precipitate has an average grain size of 0.1 µm or more, and prior austenite after quenching has a grain size of 2 to 25 µm. According to the technology described in Patent Document 1, a thin steel sheet having excellent hardenability at a low temperature for a short time after processing, excellent toughness after quenching, and little variation of characteristics according to quenching conditions can be manufactured.

[0006] Patent Document 2 discloses a method for manufacturing a thin steel sheet that is excellent in toughness for impact after quenching, the method including a step of hot-rolling a steel material at a coiling temperature of 720°C or less, the steel material containing C: 0.10 to 0.37% and proper amounts of Si, Mn, P, S, Al, and Ti and containing B and N so as to satisfy effective B amount: 0.0005% or more, wherein TiN that is an intrasteel precipitate has an average grain size of 0.06 to 0.30 µm, and prior austenite after quenching has a grain size of 2 to 25 µm. According to the technology described in Patent Document 2, a thin steel sheet having excellent hardenability at a low temperature for a short time after processing, excellent toughness for impact after quenching, and little variation of characteristics according to quenching conditions can be manufactured.

Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-309344

Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-309345

Disclosure of Invention

[0007] However, the technologies described in Patent Documents 1 and 2 focus on a relatively thin hot rolled steel sheet having a thickness of about 2.4 mm. When the technologies described in Patent Documents 1 and 2 are applied to manufacturing of a relatively thick hot rolled steel sheet used for large thick-walled parts such as structural components of automobiles, construction machines, and the like, the microstructure changes in its sheet thickness direction and the strength and ductility are decreased in the center in the sheet thickness direction. Therefore, a hot rolled steel sheet having a uniform microstructure in the sheet thickness direction and desired strength and ductility cannot be obtained. Furthermore, desired strength and toughness after heat treatment cannot be achieved.

[0008] To obtain a desired uniform microstructure in the center in the sheet thickness direction, a hot rolled thick steel sheet used for large thick-walled parts such as structural components of automobiles, construction machines, and the like needs to be quenched after hot rolling. However, quenching after hot rolling causes the cooling rate at an outer layer of the steel sheet (particularly around the edges in a sheet width direction) to become too high, which causes martensitic transformation. As a result, the outer layer of the steel sheet is hardened, and a hot rolled steel sheet partially having large deviation of hardness along thickness is obtained. When such a hot rolled steel sheet (coil) is cut into raw materials for components, inhomogeneous deformation (called a camber when a hot rolled steel sheet is slit in the width direction) is caused and the dimensional accuracy of the cut materials is decreased. Consequently, the dimensional accuracy of components is decreased.

[0009] An object of the present invention is to provide a high strength hot rolled thick steel sheet that is excellent in strength and toughness after heat treatment; that solves such problems of the related art; that has a tensile strength of 440 to 640 MPa, preferably 490 to 590 MPa, and an elongation of 20% or more (gauge length GL: 50 mm) required for large thick-walled parts; and whose deviation of hardness along thickness is within 10% from the average. Another object of the present invention is to provide a method for manufacturing the hot rolled thick steel sheet. A hot rolled thick steel sheet that is "excellent in strength and toughness after heat treatment" herein is a hot rolled steel sheet having high strength and high ductility, specifically a tensile strength of 980 MPa or more and an elongation of 15% or more (GL: 50 mm) in a typical water quenching and tempering treatment (about 930°C heating water quenching-about 200°C tempering); and having a high toughness, specifically a ductile-brittle fracture transition temperature vTrs of -60°C or less in a Charpy impact test.

[0010] The heat treatment conditions applied to components composed of the steel sheet of the present invention are not limited to the above-described typical water quenching and tempering treatment (about 930°C heating water quenching-about 200°C tempering). For example, desired heat treatment conditions such as about 930°C heating water quenching-about 400°C tempering can be used.

[0011] To achieve the objects described above, the inventors of the present invention have eagerly studied about the factors that affect the strength and formability (ductility) of a relatively thick hot rolled steel sheet having a sheet thickness of 6 mm or more and 12 mm or less and also the factors that affect the strength and toughness after heat treatment. Consequently, they found that, with a composition including proper amounts of Ti and B in a low-carbon steel with C: 0.10 to 0.20% by mass and a low N content of 0.005% by mass and with a bainitic ferrite single phase that is a uniform microstructure across the entire thickness, the deviation of hardness along thickness comes within 10% from the average and the microstructure after heat treatment becomes uniform martensite across the entire thickness while desired high strength and excellent formability are achieved, whereby a hot rolled thick steel sheet that is excellent in strength and toughness after heat treatment can be obtained. They also found that, by adjusting a cooling rate after hot rolling to 15 to 50°C/s on a surface temperature basis, the microstructure can form a bainitic ferrite single phase that is uniform across the entire thickness, whereby the deviation of hardness along thickness comes within 10% from the average.

[0012] The present invention has been completed with an additional investigation on the basis of the above-described findings. The summary of the present invention is as follows.

(1) A high strength hot rolled thick steel sheet with a sheet thickness of 6 mm or more and 12 mm or less that is excellent in strength and toughness after heat treatment includes a composition including C: 0.10 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.10%, N: 0.005% or less, Ti: 0.01 to 0.15%, and B: 0.0010 to 0.0050% by mass with the balance Fe and incidental impurities; and a bainitic ferrite phase having an area ratio of 95% or more, wherein a deviation of hardness along thickness is within 10% from an average; and a tensile strength of 440 to 640 MPa and an elongation of 20% or more (gauge length GL: 50 mm) are satisfied.

(2) A method for manufacturing a high strength hot rolled thick steel sheet that is excellent in strength and toughness after heat treatment includes the steps of hot-rolling a steel material at a finisher delivery temperature of 820 to 880°C in finish rolling to obtain a hot rolled steel sheet having a sheet thickness of 6 mm or more and 12 mm or less, the steel material having a composition including C: 0.10 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.10%, N: 0.005% or less, Ti: 0.01 to 0.15%, and B: 0.0010 to 0.0050% by mass with the balance Fe and incidental impurities; cooling the hot rolled steel sheet at a cooling rate of 15 to 50°C/s on a surface temperature basis until a surface temperature reaches a temperature range of 550 to 650°C; and coiling the hot rolled steel sheet in the temperature range, wherein a deviation of hardness along thickness is within 10% from an average; and a tensile strength of 440 to 640 MPa and an elongation of 20% or more (gauge length GL: 50 mm) are satisfied.

[0013] According to the present invention, a hot rolled thick steel sheet with a sheet thickness of 6 mm or more and 12 mm or less that has desired high strength and excellent formability, specifically a tensile strength of 440 to 640 MPa and an elongation of 20% or more, and that has uniform hardness distribution in a sheet thickness direction, specifically whose deviation of hardness along thickness is within 10% from the average, can be manufactured easily and stably. This produces industrially significant effects. Furthermore, the hot rolled steel sheet of the present invention is excellent in strength and toughness after heat treatment. Therefore, according to the hot rolled steel sheet of the present invention, large thick-walled parts (products) having high strength, high ductility, and high toughness such as structural components of automobiles, construction machines, and the like can be manufactured easily and stably by processing a hot rolled steel sheet into a desired shape and then performing heat treatment.

Best Mode for Carrying Out the Invention

[0014] Because the hot rolled steel sheet of the present invention is mainly used for large structural components of automobiles, construction machines, and the like, the sheet thickness is limited to 6 mm or more and 12 mm or less.

[0015] The reason for limiting the composition of the hot rolled steel sheet of the present invention will be described first. Hereinafter, % by mass is simply expressed as %.

C: 0.10 to 0.20%

[0016] C is an element that forms a carbide in a steel and effectively contributes to an increase in the strength of a steel sheet. In quenching treatment, C is an element that facilitates martensitic transformation and effectively contributes to strengthening of a microstructure caused by a martensitic phase. In the present invention, a C content of 0.10% or more is necessary. When the C content is less than 0.10%, it is difficult to achieve desired sheet strength (tensile strength: 440 MPa or more) and desired strength after heat treatment (tensile strength: 980 MPa or more). On the other hand, when the C content is more than 0.20%, the sheet strength and the strength after heat treatment become too high, which reduces formability and toughness, thereby decreasing weldability. Thus, the C content is limited to 0.10 to 0.20%.
Si: 0.01 to 1.0%
Si is an element that effectively contributes to an increase in the strength of steel through solution hardening. A Si content of 0.01% or more is necessary to produce such an effect. On the other hand, when the Si content is more than 1.0%, unevenness called a red scale is formed on a surface and surface properties are degraded. This decreases elongation and fatigue strength. Thus, the Si content is limited to 0.01 to 1.0%. Preferably, the Si content is 0.35% or less.
Mn: 0.5 to 2.0%
Mn is an element that effectively contributes to an increase in the strength of steel through solution hardening and an increase in the strength of steel through the improvement in hardenability. A Mn content of 0.5% or more is necessary to produce such an effect. On the other hand, when the Mn content is more than 2.0%, segregation appears markedly and it is difficult to form a bainitic ferrite single phase across the entire thickness. Consequently, the characteristics of a steel sheet and the quality of a material after heat treatment are degraded. Thus, the Mn content is limited to 0.5 to 2.0%. Preferably, the Mn content is 1.0 to 2.0%.

P: 0.03% or less

[0017] P increases the strength of steel through solution hardening, but produces segregation and decreases the uniformity of the quality of a material, thereby significantly decreasing toughness after heat treatment. Therefore, the P content is preferably reduced as much as possible in the present invention, but excess reduction increases material costs. When the P content is more than 0.03%, segregation appears markedly. Thus, the P content is limited to 0.03% or less. Preferably, the P content is 0.02% or less.

S: 0.01% or less

[0018] S is present as a sulfide in steel and decreases ductility, thereby reducing bending workability and the like. Therefore, the S content is preferably reduced as much as possible, but excess reduction increases material costs. When the S content is more than 0.01%, toughness after heat treatment is significantly reduced. Thus, the S content is limited to 0.01% or less in the present invention. Preferably, the S content is 0.005% or less.

Al: 0.01 to 0.10%

[0019] Al is an element that functions as a deoxidizer. Such an effect markedly appears when an Al content is 0.01% or more. However, an Al content of more than 0.1% decreases formability and hardenability. Thus, the Al content is limited to 0.01 to 0.1%. Preferably, the Al content is 0.05% or less.

N: 0.005% or less

[0020] N decreases formability by forming nitrides such as TiN and AlN in steel. N also reduces the amount of B solid solution that is effective for improving hardenability by forming BN during quenching. Such an adverse effect of N is permissible when the N content is 0.005% or less. Thus, the N content is limited to 0.005% or less in the present invention.

Ti: 0.01 to 0.15%

[0021] Ti is an element that effectively contributes to allowing a microstructure after hot rolling to be constituted by bainitic ferrite and that contributes to producing an effect of improving hardenability through a B solid solution because Ti forms a nitride prior to B. Such effects are produced when a Ti content is 0.01% or more. However, a Ti content of more than 0.15% increases deformation resistance during hot rolling and excessively increases rolling load, thereby decreasing toughness after heat treatment. Thus, the Ti content is limited to 0.01 to 0.15%. Preferably, the Ti content is 0.03 to 0.10%.

B: 0.0010 to 0.0050%

[0022] B is an element that suppresses the formation of polygonal ferrite and pearlite during cooling performed after hot rolling and that effectively contributes to improving hardenability and toughness during heat treatment. In the case where a thick steel sheet having a thickness of 6 mm or more is used, such effects markedly appear when a B content is 0.0010% or more. On the other hand, a B content of more than 0.0050% increases deformation resistance during hot rolling and excessively increases rolling load. In addition, such a B content forms bainite and martensite after hot rolling and poses a problem such as sheet cracking. Thus, the B content is limited to 0.0010 to 0.0050%. Preferably, the B content is 0.0015 to 0.0040%.

[0023] The balance other than the components described above is Fe and incidental impurities. For example, Cu: 0.3% or less and Cr: 0.3% or less are permissible as incidental impurities.

[0024] The hot rolled thick steel sheet of the present invention has the above-described composition and a bainitic ferrite single phase across the entire thickness. A single phase herein is constituted by a bainitic ferrite phase having an area ratio of 95% or more. A bainitic ferrite phase includes needle-shaped ferrite and acicular ferrite. Note that 5% or less of a polygonal ferrite phase, a pearlite phase, a cementite phase, a bainite phase, a martensite phase, and the like on an area ratio basis are permissible as a microstructure other than the bainitic ferrite phase.

[0025] By forming a bainitic ferrite single phase across the entire thickness, a hot rolled thick steel sheet can be provided that has desired high strength and high ductility, specifically a tensile strength of 440 MPa or more and 640 MPa or less and an elongation of 20% or more (GL: 50 mm), that is excellent in formability such as a flexural property, and that can be processed into large thick-walled parts such as structural components of automobiles, construction machines, and the like. When the area ratio of the bainitic ferrite phase is less than 95%, both the desired high strength and high ductility cannot be achieved. When the phase fraction of the bainitic ferrite phase is decreased to less than 95%, the uniformity of the microstructure is reduced. As a result, cambering or the like is caused when cutting and the dimensional accuracy is reduced, thereby decreasing formability such as a flexural property. To judge whether a bainitic ferrite single phase is formed across the entire thickness, the area ratios of a bainitic ferrite phase are obtained at a depth of 0.1 mm from the surface, at a position of a quarter the way through the sheet thickness, and at a position of a half the way through the sheet thickness. When the area ratios are 95% or more at all of the three positions, it is judged that a bainitic ferrite single phase is formed across the entire thickness.

[0026] A preferable method for manufacturing a hot rolled thick steel sheet according to the present invention will now be described.

[0027] A molten steel having the above-described composition is preferably smelted by a typical smelting method using a converter, a vacuum melting furnace, or the like to make a steel material such as a slab through a typical casting method such as continuous casting or an ingot making-blooming method. However, a method for making a steel material is not limited to this in the present invention, and any typical method for making a steel material can be suitably applied.

[0028] A steel material having the above-described composition is hot-rolled to obtain a hot rolled thick steel sheet having a sheet thickness of 6 mm or more and 12 mm or less. When the sheet thickness is more than 12 mm, a sufficient reduction ratio is not achieved in hot rolling and the microstructure is coarsened after the hot rolling, which tends to produce martensite during cooling. Thus, the sheet thickness is preferably 12 mm or less. The heating temperature for hot rolling is not particularly limited, and a finisher delivery temperature in hot rolling described below needs only to be ensured. The heating temperature is preferably 1000 to 1300°C, which is a typical heating temperature. When the heating temperature is more than 1300°C, crystal grains are coarsened and hot formability is easily decreased. On the other hand, when the heating temperature is less than 1000°C, deformation resistance is excessively increased and a burden on rolling equipment is increased, which easily poses a problem such as a difficulty in rolling. In addition, when the heating temperature is less than 1000°C, TiC that is present in a steel material is insufficiently melted, which easily causes a difficulty in achieving a desired microstructure and desired strength after hot rolling.

[0029] In the hot rolling, the finisher delivery temperature of finish rolling is 820 to 880°C.

[0030] When the finisher delivery temperature of finish rolling is 820°C or more, ferrite transformation is suppressed in the following cooling step. As a result, a bainitic ferrite phase (bainitic ferrite single phase) having an area ratio of 95% or more can be formed. When the finisher delivery temperature of finish rolling is less than 820°C, ferrite transformation is facilitated in the following cooling step. As a result, a bainitic ferrite single phase is not easily formed. On the other hand, the finisher delivery temperature of finish rolling is more than 880°C, not only ferrite transformation but also bainitic ferrite transformation is suppressed. As a result, a bainitic ferrite single phase is not easily formed and a bainite phase and a martensite phase are easily formed. The formation of a bainite phase and a martensite phase may excessively increase the strength of a steel sheet and cause cracking on a steel sheet in coiling or rewinding of a coil. For this reason, the finisher delivery temperature of finish rolling is limited to 820 to 880°C.

[0031] After the completion of rolling, the hot rolled steel sheet is cooled at a cooling rate of 15 to 50°C/s on a sheet surface temperature basis until a surface temperature reaches a temperature range of 550 to 650°C.

[0032] To form a bainitic ferrite single phase across the entire thickness of a steel sheet, a cooling rate is adjusted so as to be 15°C/s or more on a sheet surface temperature basis in the cooling performed after the completion of rolling. When the cooling rate is less than 15°C/s on a surface temperature basis, a polygonal ferrite phase is easily precipitated, for example, in the center in a sheet thickness direction, which makes it difficult to form a uniform bainitic ferrite single phase in a sheet thickness direction. On the other hand, when the cooling rate is more than 50°C/s on a surface temperature basis, martensite is produced on an outer layer and a uniform bainitic ferrite single phase cannot be formed in a sheet thickness direction. Consequently, the deviation of hardness along thickness becomes significant and it is difficult to adjust the deviation of hardness along thickness to be within 10% from the arithmetic mean hardness (average) in a sheet thickness direction. In the cooling, water cooling is adopted. The cooling rate is preferably adjusted by changing the amount and time of water injection. For this reason, in the cooling performed after the completion of rolling, the cooling rate is adjusted to 15 to 50°C/s on a sheet surface temperature basis. The above-described cooling rate on a surface temperature basis is an average value of actually measured surface temperatures between the finisher delivery temperature of finish rolling and the cooling stop temperature.

[0033] The above-described cooling stop temperature is in a temperature range in which the surface temperature of a steel sheet is 550 to 650°C. When the cooling stop temperature is less than 550°C on a surface temperature basis, a bainite phase and a martensite phase are produced and a bainitic ferrite single phase cannot be formed. Furthermore, cracking is caused on a hot rolled steel sheet during coiling and the formability of a steel sheet is decreased due to too high strength. On the other hand, when the cooling stop temperature is more than 650°C, a polygonal ferrite phase and a pearlite phase are produced and a bainitic ferrite single phase cannot be formed. In addition, the strength of a steel sheet may fall short of desired strength. Thus, the cooling stop temperature after the completion of rolling is limited to a temperature range of 550 to 650°C.

[0034] After the cooling is stopped, the hot rolled steel sheet is coiled in the temperature range. When the coiling temperature is less than 550°C, a bainite phase and a martensite phase are produced and a bainitic ferrite single phase cannot be formed. On the other hand, when the coiling temperature is more than 650°C, a polygonal ferrite phase and a pearlite phase are produced and a bainitic ferrite single phase cannot be formed. Consequently, the desired strength of a steel sheet cannot be achieved and the uniformity in a sheet thickness direction is decreased. Thus, the coiling temperature is limited to a temperature range of 550 to 650°C on a sheet surface temperature basis.

Example

[0035] After a steel material (steel slab) having a composition shown in Table 1 was heated to heating temperature shown in Table 2, it was hot-rolled under the finish rolling conditions shown in Table 2 to obtain a hot rolled steel sheet having a sheet thickness shown in Table 2. After the completion of finish rolling, the hot rolled steel sheet was cooled under the conditions shown in Table 2 and coiled at a coiling temperature shown in Table 2.

[0036] The obtained hot rolled steel sheet was evaluated for strength, ductility, the uniformity of hardness in a sheet thickness direction, and formability (bending workability) by performing a microstructure observation, a tensile test, a hardness test, and a bending test. Furthermore, after a test panel was prepared from the obtained hot rolled steel sheet and then pickled to remove scales on the steel sheet surface, heat treatment (quenching-tempering treatment) was performed. The test panel was evaluated for strength, ductility, and toughness after heat treatment by performing a microstructure observation, a tensile test, and an impact test. The heat treatment was constituted by quenching and tempering. In the quenching treatment, the test panel was heated to 930°C and held for 10 minutes, and then quenched in water at 20°C. In the tempering treatment, the test panel was heated to 200°C and held for 60 minutes, and then cooled in the air. After the cooling, a test piece was prepared from the test panel to perform the tests. The test methods are as follows.

(1) Microstructure Observation

[0037] After a test piece for microstructure observation was prepared from the obtained hot rolled steel sheet, sheet sections that were parallel to the rolling direction of the test piece were polished and corroded with nital. The metal microstructure was observed (the number of fields of view: 10 spots each) and imaged using a scanning electron microscope (SEM) (magnification: 3000 times) at a depth of 0.1 mm from the surface, at a position of a quarter the way through the sheet thickness, and at a position of a half the way through the sheet thickness. The kinds of phases and the phase fraction (area ratio) of each phase were measured using an image analysis apparatus. The area ratio of a bainitic ferrite phase was calculated by averaging measured values of 10 observed fields. When the area ratios (averages of the measured values in 10 fields) of a bainitic ferrite phase measured at a depth of 0.1 mm from the surface, at a position of a quarter the way through the sheet thickness, and at a position of a half the way through the sheet thickness were all 95% or more, it was judged that a bainitic ferrite phase having an area ratio of 95% or more (bainitic ferrite single phase) was formed across the entire thickness.

(2) Tensile Test

[0038] A JIS No. 5 test piece (GL: 50 mm) was prepared from the obtained hot rolled steel sheet (or the test panel) such that the pulling direction was perpendicular to the rolling direction. A tensile test was performed in conformity to JIS Z 2241. Tensile characteristics (yield strength YS, tensile strength TS, and elongation El) were obtained to evaluate strength and ductility.

(3) Hardness Test

[0039] A test piece for hardness measurement was prepared from the obtained hot rolled steel sheet, and sheet sections that were parallel to the rolling direction of the test piece were then polished. Vickers hardness HV (load: 9.8 N = 1 kgf) was measured with a 0.2 mm pitch. The hardness measurement was started at a position of 0.2 mm from a surface. When a point to be measured next reached a position within 0.2 mm from another surface, the point was not measured and the hardness measurement was finished. The average hardness (average value) HV_mean of the hot rolled steel sheet was calculated by averaging the obtained hardness values in the sheet thickness direction using an arithmetic mean. In addition, the difference ΔHV between the maximum hardness and the minimum hardness was calculated to obtain [ΔHV/HV_mean] × 100 (%). Thus, the uniformity in the sheet thickness direction was evaluated.

(4) Bending Test

[0040] A test piece for a bending test (size: sheet thickness t x 100 x 200 mm) was prepared from the obtained hot rolled steel sheet such that a direction perpendicular to the rolling direction was a longitudinal direction of the test piece. To measure the minimum bend radius (mm) that does not cause cracking on the outer side of the bent portion, 180 degree bending was performed at various bend radii such as bend radii of 0.5 times, 1.0 time, 1.5 times, and 2.0 times the sheet thickness such that the longitudinal direction of the test piece was a circumferential direction. The minimum bend radius was expressed as a ratio to the sheet thickness of the test piece.

(5) Impact Test

[0041] A V-notch test piece was prepared from the obtained test panel in conformity to JIS Z 2242 such that the longitudinal direction of the test piece was perpendicular to the rolling direction. A Charpy impact test was performed to obtain a ductile-brittle fracture transition temperature vTrs (°C), which is a temperature at which percent ductile fracture is 50%. Thus, the toughness after heat treatment was evaluated.

[0042] Table 3 shows the obtained results.

Table 1

Steel Nos.	Chemical composition (% by mass)
	C	Si	Mn	P	S	Al	N	Ti	B
A	0.10	0.03	1.35	0.015	0.004	0.038	0.0035	0.042	0.0018
B	0.12	0.15	0.83	0.013	0.003	0.042	0.0036	0.035	0.0022
C	0.15	0.03	1.24	0.010	0.003	0.047	0.0042	0.038	0.0016
D	0.16	0.05	1.11	0.013	0.003	0.042	0.0040	0.033	0.0031
E	0.15	1.20	0.71	0.011	0.003	0.033	0.0043	0.045	0.0014
F	0.15	0.03	0.25	0.024	0.004	0.044	0.0047	0.041	0.0013
G	0.15	0.03	2.34	0.013	0.005	0.046	0.0038	0.039	0.0016
H	0.14	0.03	0.84	0.045	0.003	0.039	0.0032	0.037	0.0015
I	0.15	0.05	0.83	0.015	0.012	0.041	0.0041	0.048	0.0019
J	0.16	0.03	0.81	0.012	0.003	0.043	0.0039	0.004	0.0021
K	0.15	0.04	0.89	0.013	0.003	0.046	0.0042	0.16	0.0014
L	0.16	0.03	0.76	0.012	0.004	0.039	0.0044	0.038	0.0003
M	0.15	0.03	0.82	0.011	0.002	0.044	0.0042	0.042	0.0075
N	0.16	0.70	1.24	0.015	0.003	0.047	0.0046	0.052	0.0018
O	0.18	0.03	0.75	0.016	0.002	0.038	0.0042	0.043	0.0016
P	0.20	0.01	0.88	0.018	0.004	0.045	0.0038	0.044	0.0018
Q	0.23	0.02	0.95	0.012	0.003	0.044	0.0036	0.041	0.0019
R	0.08	0.03	0.77	0.011	0.004	0.043	0.0042	0.042	0.0023

Table 2

Steel panel Nos.	Steel Nos.	Hot rolling conditions					Sheet thickness (mm)	Remarks
		Heating temperature (°C)	Finisher delivery temperature of finish rolling * (°C)	Cooling rate * (°C/s)	Cooling stop temperature * (°C)	Coiling temperature * (°C)
1	A	1200	860	40	640	610	6.0	IE
2	B	1200	855	50	620	590	7.0	IE
3	C	1250	860	30	620	600	8.0	IE
4	C	1250	800	40	630	600	8.0	CE
5	C	1250	920	40	610	580	8.0	CE
6	C	1250	860	5	630	620	8.0	CE
7	C	1250	850	100	600	570	8.0	CE
8	C	1250	855	40	550	500	8.0	CE
9	C	1250	860	45	650	680	8.0	CE
10	C	1250	870	30	690	640	8.0	CE
11	C	1250	870	30	530	560	8.0	CE
12	D	1250	860	15	600	570	7.0	IE
13	E	1250	860	40	630	600	8.0	CE
14	F	1250	860	40	620	590	8.0	CE
15	G	1250	865	40	600	580	8.0	CE
16	H	1250	845	45	630	600	8.0	CE
17	I	1250	850	40	630	610	8.0	CE
18	J	1250	860	40	640	610	8.0	CE
19	K	1250	850	40	620	600	8.0	CE
20	L	1250	855	35	630	600	8.0	CE
21	M	1250	840	40	620	600	8.0	CE
22	N	1250	860	40	550	550	12.0	IE
23	O	1250	855	20	650	650	10.0	IE
24	P	1250	830	40	640	620	8.0	IE
25	Q	1250	860	40	640	620	8.0	CE
26	R	1200	850	45	620	600	8.0	CE

*) on a surface temperature basis IE: Invention Example CE: Comparative Example

[0043] In all Invention Examples, a bainitic ferrite phase having an area ratio of 95% or more (bainitic ferrite single phase) is uniformly formed in a sheet thickness direction, whereby there is provided a high strength hot rolled thick steel sheet with excellent formability that has a tensile strength of 440 MPa or more and an elongation of 20% or more; that is excellent in uniformity because the deviation of hardness ΔHV along thickness is within 10% from the average hardness value (average) HV_mean; and that is excellent in bending workability with a minimum bend radius of 0.5t or less. Furthermore, high strength with a tensile strength of 980 MPa or more, high ductility with an elongation of 15% or more, and high toughness with a vTrs of -60°C or less can be achieved by performing quenching and tempering treatment. In contrast, in Comparative Examples that depart from the scope of the present invention, a uniform bainitic ferrite phase is not formed and "strength or ductility" or "strength and ductility" do not reach the above-described desired values. Furthermore, the deviation of hardness ΔHV along thickness becomes large and the uniformity in the sheet thickness direction is decreased. In addition, one or more of strength, ductility, and toughness after quenching and tempering treatment do not reach the above-described desired values, which provides a hot rolled steel sheet that lacks any of strength, ductility, and toughness after quenching and tempering treatment.

Claims

1. A high strength hot rolled thick steel sheet with a sheet thickness of 6 mm or more and 12 mm or less that is excellent in strength and toughness after heat treatment, the hot rolled thick steel sheet comprising a composition including C: 0.10 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.10%, N: 0.005% or less, Ti: 0.01 to 0.15%, and B: 0.0010 to 0.0050% by mass with the balance Fe and incidental impurities; and a bainitic ferrite phase having an area ratio of 95% or more, wherein a deviation of hardness along thickness is within 10% from an average; and a tensile strength of 440 to 640 MPa and an elongation of 20% or more (gauge length GL: 50 mm) are satisfied.

2. A method for manufacturing a high strength hot rolled thick steel sheet that is excellent in strength and toughness after heat treatment, the method comprising the steps of hot-rolling a steel material at a finisher delivery temperature of 820 to 880°C in finish rolling to obtain a hot rolled steel sheet having a sheet thickness of 6 mm or more and 12 mm or less, the steel material having a composition including C: 0.10 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.10%, N: 0.005% or less, Ti: 0.01 to 0.15%, and B: 0.0010 to 0.0050% by mass with the balance Fe and incidental impurities; cooling the hot rolled steel sheet at a cooling rate of 15 to 50°C/s on a surface temperature basis until a surface temperature reaches a temperature range of 550 to 650°C; and coiling the hot rolled steel sheet in the temperature range, wherein a deviation of hardness along thickness is within 10% from an average; and a tensile strength of 440 to 640 MPa and an elongation of 20% or more (gauge length GL: 50 mm) are satisfied.