High-strength steel sheet and method for manufacturing same

Abstract

The purpose of the present invention is to provide: a high-strength steel sheet that has a tensile strength of 980 MPa or greater and that is excellent in ductility, fatigue strength, and blanking properties; and a method for manufacturing the same.　The high-strength steel sheet is characterized by: having a prescribed component composition; the main phase being a bainite phase composed of granular bainite and upper bainite having an area ratio of 85% or greater; the area ratio of a hard second phase that includes fresh martensite and / or a residual austenite phase being less than 15%; the area ratio of the granular bainite to the area ratio of the upper bainite in the bainite phase being 5% to 40%; the difference |Hv1 - Hv2| between the hardness Hv1 at the 1 / 2 position the sheet thickness and the hardness Hv2 at the 1 / 4 position of the sheet thickness being 30% or less in relation to 0.3 TS; and the average crystal grain size of the upper bainite phase being 7 μm or less.

Description

High strength steel plate and method for manufacturing the same

[0001] The present invention relates to a high-strength steel sheet (particularly a hot-rolled steel sheet) suitable for use as an automotive component, which has improved fatigue resistance and punchability in addition to strength and ductility, and a method for producing the same.

[0002] In recent years, from the perspective of global environmental conservation, CO ２ There is a demand for reducing emissions. In particular, there is a strong demand for improved fuel efficiency in automobiles, and efforts are being made to reduce the weight of automobile bodies. Increasing the strength of steel sheets, which are used as raw materials for automobile components, and reducing their thickness is one effective way to reduce the weight of automobile bodies without reducing their strength. In particular, steel sheets with a tensile strength of 980 MPa or more are expected to be a material that can dramatically improve automobile fuel efficiency through thinner thickness.

[0003] On the other hand, increasing the tensile strength of a steel sheet reduces its ductility, thereby worsening its press formability. Automotive parts, particularly suspension parts and other undercarriage parts, must be formed into complex shapes to ensure rigidity, so materials for automotive parts require high press formability, particularly excellent ductility.

[0004] Furthermore, as automotive parts become thinner, their durability decreases. Therefore, if the fatigue strength of the steel plate is low, the durability of the parts may fall short of the design target as the driving distance increases. However, increasing the strength of steel plate generally does not necessarily increase its fatigue strength. Furthermore, increasing the strength of the material increases the likelihood of defects with irregularities on the punched edge. These defects initiate cracks, leading to a deterioration of fatigue resistance.

[0005] Thus, steel sheets having a tensile strength of 980 MPa or more are required to have excellent fatigue strength and punchability in addition to excellent ductility. Various studies have been conducted to date to improve the fatigue strength and punchability while increasing the tensile strength of steel sheets (Patent Documents 1 to 3).

[0006] Patent Document 1 discloses a technology relating to a high-strength hot-rolled steel sheet that is excellent in formability and fatigue resistance by controlling the manufacturing conditions of hot rolling, making the main phase ferrite, and controlling the shape and dispersion form of inclusions.

[0007] Patent Document 2 discloses a technique relating to a high-strength hot-rolled steel sheet that is excellent in punching fatigue resistance and workability by controlling the shape and hardness of martensite in the center of the sheet thickness.

[0008] Patent Document 3 discloses a technology relating to a high-strength hot-rolled steel sheet that is excellent in hole expandability and punchability by controlling the manufacturing conditions of hot rolling, making the main phase bainite, and controlling the shape and dispersion form of the hard second phase as well as the amount of precipitates.

[0009] Japanese Patent Application Laid-Open No. 2014-31560 Japanese Patent Application Laid-Open No. 2015-214718 Republished No. 2017-017933

[0010] However, the conventional techniques described in Patent Documents 1 to 3 have the following problems.

[0011] The techniques described in Patent Documents 1 and 2 do not provide a tensile strength of 980 MPa or more.

[0012] The technology described in Patent Document 3 provides a hot-rolled steel sheet having a tensile strength of 980 MPa or more, excellent fatigue strength, and excellent punchability. The technology described in Patent Document 3 provides a hot-rolled steel sheet having a tensile strength of 980 MPa or more, excellent punchability, and excellent hole expandability. However, the technology does not mention fatigue strength, which is the subject of the present application, and good fatigue strength is not necessarily obtained.

[0013] As described above, in the prior art, there is no established technology for producing a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent ductility, fatigue strength, and punchability.

[0014] Therefore, an object of the present invention is to provide a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent ductility, fatigue strength and punchability, and a method for manufacturing the same.

[0015] In the present invention, excellent ductility means a uniform elongation of 6.0% or more, and excellent fatigue strength means a fatigue strength of 2×10 to tensile strength in a plane bending test at a stress ratio of −1. ６ The ratio of the plane bending fatigue strength to the number of times the material is punched (fatigue limit ratio) is 0.50 or more. Excellent punchability means that when punched using a 10 mmφ punch at three or more clearances, including 10% and 20%, within a clearance range of 10 to 20%, there are no cracks, chips, brittle fractures, or secondary shear surfaces on the end surfaces of the punched holes.

[0016] To achieve the above object, the inventors conducted extensive research to improve the ductility, fatigue strength, and punchability of hot-rolled steel sheets while maintaining a tensile strength of 980 MPa or more. As a result, the inventors determined that the main phase was bainite, and the area ratios of soft granular bainite and upper bainite formed near the Bs point were controlled within a certain range. Furthermore, the hard second phases, fresh martensite and retained austenite, were controlled to less than 15%. Furthermore, segregation was reduced by setting the difference |Hv1-Hv2| between the hardness Hv1 at the half-thickness position and the hardness Hv2 at the quarter-thickness position at 0.3TS (MPa) to 30% or less. Furthermore, the average grain size of the upper bainite was set to 7 μm or less. It was found that these results enable the production of steel sheets with excellent ductility, fatigue strength, and punchability while maintaining a tensile strength of 980 MPa or more.

[0017] The upper bainite phase herein is an aggregate of lath-shaped bainitic ferrite, specifically a structure having Fe-based carbides and / or retained austenite phase between lath-shaped bainitic ferrite.

[0018] Unlike lamellar (layered) ferrite and polygonal ferrite in pearlite, lath-shaped bainitic ferrite has a lath-like shape and a relatively high dislocation density inside, and therefore the two can be distinguished using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0019] When a residual austenite phase exists between the laths, only the lath-shaped bainitic ferrite portion is regarded as upper bainite and is distinguished from the residual austenite phase. Fresh martensite is martensite that does not contain Fe-based carbides.

[0020] Granular bainite refers to a structure composed of granular bainitic ferrite with Fe-based carbides and / or retained austenite phase between the granular bainitic ferrite. The aspect ratio is 2 or less, and the grains have grain boundaries that do not have the following orientation relationships between the <axial direction> and the <rotation angle>: <15 16 18> - 60°, <6 15 15> - 56°, <6 7 7> - 50°, <1 0 1> - 60°, or <1 2 2> - 17°. The crystal orientation is determined using electron backscatter diffraction (EBSD) patterns. The fresh martensite and / or retained austenite phase has low clarity (IQ value). Prior austenite grains have a large aspect ratio and are large crystal grains, so in this study, crystal grains with an aspect ratio of 2 or more and a size of 10 μm or more were considered to be prior austenite grains. Therefore, upper bainite grains were excluded from crystal grains with a misorientation of 15° or more, and of the remaining crystal grains, crystal grains with a high IQ value and an aspect ratio of 2 or less for 10 μm or less were determined to be granular bainite.

[0021] Furthermore, the fresh martensite and / or retained austenite phase has brighter contrast in an SEM image than the upper bainite phase, the lower bainite and / or tempered martensite phase, and the polygonal ferrite phase, and therefore the fresh martensite phase and / or retained austenite phase can be distinguished from these structures using an SEM.

[0022] The fresh martensite phase and the retained austenite phase have similar contrast in SEM, but can be distinguished from each other using electron beam reflection diffraction.

[0023] The present invention was made based on the above findings and further studies, and is summarized as follows: [1] A steel sheet having a composition containing, by mass%, C: 0.04 to 0.18%, Si: 0.1 to 2.5%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.010 to 1.0%, and N: 0.01% or less, with the balance being Fe and unavoidable impurities, and the microstructure has a bainite phase consisting of upper bainite and granular bainite in an area ratio of 85% or more as the main phase, and fresh martensitic steel sheet. [2] A high-strength steel plate according to [1], wherein the area fraction of a hard second phase containing bainite and / or a retained austenite phase is less than 15%, the area fraction of granular bainite relative to the area fraction of upper bainite in the bainite phase is 5% to 40%, the difference |Hv1 - Hv2| between the hardness Hv1 at the half-thickness position and the hardness Hv2 at the quarter-thickness position is 30% or less for 0.3TS, and the average crystal grain size of the upper bainite phase is 7 μm or less. Group a: one or more selected from V: 0.005 to 0.5%, Ti: 0.005 to 0.2%, and Nb: 0.005 to 0.1%; Group b: one or more selected from Cu: 0.005 to 0.5%, Ni: 0.005 to 0.5%, Cr: 0.005 to 1.0%, and Mo: 0.005 to 0.5%; Group c: B: 0.0002 to 0.005%, Group d: One or two selected from Sb: 0.001 to 0.1%, and Sn: 0.001 to 0.1%, Group e: One or two or more selected from Ca: 0.0001 to 0.005%, Mg: 0.0001 to 0.005%, and REM: 0.0001 to 0.005%.[3] A method for producing a high-strength steel plate according to [1] or [2], wherein a molten steel material having the above-mentioned component composition is cast while being rotated in a horizontal plane relative to a mold by an induction electromagnetic stirrer at a rotation speed of 10 cm / s or more, the obtained steel material is heated to 1150°C or more, the heated steel material is rough-rolled to form a steel plate, the steel plate is finish-rolled under a finish-rolling end temperature condition of (RC-50)°C or more and (RC+100)°C or less, and the steel plate after the finish rolling is cooled after the finish rolling. a time to start of cooling: 2.0 seconds or less, an average cooling rate at the surface to Bs: 30°C / s or more, a residence time at a temperature of (Bs-100)°C or more and Bs°C or less: 3.0 seconds or more and 10.0 seconds or less, a cooling stop temperature: (Bs-250)°C or more and (Bs-100)°C or less, the cooled steel sheet is coiled under a coiling temperature condition: (Bs-250)°C or more and (Bs-100)°C or less, and cooled to (Bs-400)°C or less at an average cooling rate of 1°C / s or less. Note that RC and Bs are defined by the following formulas (1) and (2), respectively. RC (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V (1) Bs (°C) = 830 - 270 × C - 90 × Mn - 70 × Cr - 37 × Ni - 83 × Mo - 20 × Cu (2) Here, each element symbol in the above formulas (1) and (2) represents the content (mass%) of each element, and if no element is contained, it is set to 0.

[0024] According to the present invention, it is possible to provide a high-strength steel sheet having a tensile strength of 980 MPa or more and excellent ductility, fatigue strength and punchability, and a method for producing the same.

[0025] When the high-strength steel sheet of the present invention is applied to automobile underbody parts such as suspensions, structural parts, frame parts, and truck frame parts, safety can be ensured and the weight of the automobile body can be reduced, thereby providing significant industrial benefits.

[0026] FIG. 1 is a schematic diagram showing the shape of a test piece for a plane bending fatigue test in the present invention.

[0027] Hereinafter, embodiments of the present invention will be described. Note that the following description shows examples of preferred embodiments of the present invention, and the present invention is not limited to the following embodiments.

[0028] The steel sheet has the following chemical composition: In the following description, "%", which is the unit of content of elements in the chemical composition, means "mass %" unless otherwise specified.

[0029] <C: 0.04 to 0.18%> C is an element effective in improving hardenability, thereby promoting the formation of bainite and improving strength. It also lowers the Bs point and causes bainite to transform at a lower temperature, thereby suppressing the formation of granular bainite. If the C content is less than 0.04%, this effect is not sufficiently achieved, and a tensile strength of 980 MPa or more cannot be obtained. Therefore, the C content is 0.04% or more, preferably 0.05% or more, and more preferably 0.06% or more. On the other hand, if the C content exceeds 0.18%, a hard second phase containing fresh martensite and / or retained austenite is excessively formed, resulting in insufficient fatigue strength. Therefore, the C content is 0.18% or less, preferably 0.17% or less, and more preferably 0.15% or less.

[0030] <Si: 0.1 to 2.5%> Si solid-solution strengthens steel and contributes to improving the strength of steel. Therefore, the Si content is 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more. On the other hand, Si is an element that promotes ferrite formation, and if the Si content exceeds 2.5%, ferrite is formed and fatigue strength decreases. Therefore, the Si content is 2.5% or less, preferably 2.3% or less, and more preferably 2.0% or less.

[0031] <Mn: 1.0 to 3.0%> Mn is an element that stabilizes austenite and is effective in suppressing the formation of ferrite and improving strength. It also lowers the Bs point and causes bainite to transform at a lower temperature, thereby suppressing the formation of granular bainite. If the Mn content is less than 1.0%, this effect is not sufficiently achieved, and ferrite and granular bainite are formed, preventing a tensile strength of 980 MPa or more and excellent fatigue strength from being obtained. Therefore, the Mn content is 1.0% or more, preferably 1.2% or more, and more preferably 1.5% or more. On the other hand, if the Mn content exceeds 3.0%, the hard second phase, including fresh martensite and / or retained austenite, increases, preventing sufficient fatigue strength from being obtained. Therefore, the Mn content is 3.0% or less, preferably 2.8% or less, and more preferably 2.5% or less.

[0032] <P: 0.1% or less> Since P deteriorates weldability, it is desirable to reduce the amount as much as possible. In the present invention, a P content of up to 0.1% is permissible. Therefore, the P content is set to 0.1% or less. Although there is no particular lower limit, a P content of less than 0.003% leads to an increase in refining costs, so the P content is preferably 0.003% or more, and more preferably 0.005% or more.

[0033] <S: 0.01% or less> S deteriorates weldability, so its amount is preferably reduced as much as possible, but in the present invention, an S content of up to 0.01% is acceptable. Therefore, the S content is set to 0.01% or less. Although there is no particular lower limit, an S content of less than 0.0001% leads to a decrease in production efficiency, so the S content is preferably 0.0001% or more, and more preferably 0.0005% or more.

[0034] <Al: 0.010 to 1.0%> Al acts as a deoxidizer and is an element effective in improving the cleanliness of steel. If the Al content is too low, this effect is not necessarily sufficient. Therefore, the Al content is 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, Al is an element that promotes ferrite formation, and if the Al content exceeds 1.0%, ferrite is generated, reducing fatigue strength. Therefore, the Al content is 1.0% or less, preferably 0.8% or less, and more preferably 0.5% or less.

[0035] <N: 0.01% or less> N combines with elements that form nitrides to precipitate as nitrides, contributing to the refinement of crystal grains. However, N tends to combine with Ti at high temperatures to form coarse nitrides, and excessive N content reduces fatigue strength. Therefore, the N content is 0.01% or less, preferably 0.008% or less, and more preferably 0.006% or less. There is no particular lower limit, but an N content of less than 0.001% leads to a decrease in production efficiency, so 0.001% or more is preferred.

[0036] The balance is Fe and unavoidable impurities.

[0037] The above-mentioned components are the basic composition of the high-strength steel sheet of the present invention. If necessary, the following elements may be further contained.

[0038] V, Ti, and Nb are elements that form carbides and are effective in improving strength through precipitation strengthening. Therefore, when V, Ti, and Nb are contained, the respective contents are preferably set to V: 0.005-0.5%, Ti: 0.005-0.2%, and Nb: 0.005-0.1%. If the V, Ti, and Nb contents exceed the respective upper limits, the carbides may become coarse and punchability may deteriorate. Therefore, the V content is preferably set to 0.05% or more, more preferably 0.1% or more, and more preferably 0.3% or less. The Ti content is more preferably set to 0.01% or more, and more preferably 0.1% or less. The Nb content is more preferably set to 0.01% or more, and more preferably 0.08% or less.

[0039] Cr, Ni, Cu, and Mo are elements that stabilize austenite and are effective in suppressing the formation of ferrite and improving strength. Furthermore, by lowering the Bs point and transforming bainite at a lower temperature, the formation of granular bainite is suppressed. Therefore, when Cr, Ni, Cu, and Mo are contained, the respective contents are preferably set to Cr: 0.005 to 1.0%, Ni: 0.005 to 0.5%, Cu: 0.005 to 0.5%, and Mo: 0.005 to 0.5%. If the contents of Cr, Ni, Cu, and Mo exceed the respective upper limits, excessive formation of a hard second phase containing fresh martensite and / or retained austenite may occur, making it impossible to obtain the steel structure of the present invention. The Cr content is preferably 0.01% or more, more preferably 0.3% or more, and more preferably 0.8% or less. The Ni content is preferably 0.01% or more, more preferably 0.05% or more, and more preferably 0.3% or less. The Cu content is preferably 0.01% or more, more preferably 0.05% or more, and more preferably 0.3% or less. The Mo content is preferably 0.01% or more, more preferably 0.05% or more, and more preferably 0.3% or less.

[0040] When B is contained, it is preferably 0.0002 to 0.005%. B is an element that segregates at austenite grain boundaries and suppresses the formation of ferrite, thereby further promoting the formation of upper bainite and granular bainite, and is effective in further improving the strength of the steel sheet. To achieve these effects, the B content is preferably 0.0002% or more. Therefore, the B content is 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0007% or more. On the other hand, if the B content exceeds 0.005%, the above effects saturate. Therefore, the B content is 0.005% or less, preferably 0.004% or less, and more preferably 0.003% or less.

[0041] Sb is an element that suppresses de-elementation from the surface of a steel material when the steel material is heated, and is therefore effective in suppressing a decrease in the strength of the steel. Therefore, when Sb is contained, the content is preferably set to 0.001 to 0.1%. If the Sb content exceeds the above upper limit, it may lead to embrittlement of the steel sheet. The Sb content is more preferably set to 0.005% or more, and more preferably set to 0.05% or less.

[0042] Sn is an element that suppresses the formation of pearlite and is effective in suppressing a decrease in the strength of steel. In order to obtain this effect, when Sn is contained, the content is preferably set to 0.001 to 0.1%. If the Sn content exceeds the above upper limit, it may lead to embrittlement of the steel sheet. The Sn content is more preferably set to 0.005% or more, and more preferably set to 0.05% or less.

[0043] Ca, Mg, and REM are elements effective in improving punchability by controlling the morphology of inclusions. To achieve this effect, when Ca, Mg, and REM are contained, the respective contents are preferably set to Ca: 0.0001-0.005%, Mg: 0.0001-0.005%, and REM: 0.0001-0.005%. On the other hand, if the Ca and REM contents exceed the above upper limits, the amount of inclusions increases, which may result in a deterioration of fatigue strength. The Ca content is more preferably 0.0005% or more and more preferably 0.003% or less. The Mg content is more preferably 0.0005% or more and more preferably 0.003% or less. The REM content is more preferably 0.0005% or more and more preferably 0.003% or less. Note that REM (rare earth elements) is a collective term for Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here refers to the total content of these elements. Note that for each component of groups a to e (V, Ti, Nb, Cr, Ni, Cu, Mo, B, Sb, Sn, Ca, Mg, REM), when the content is less than the above-mentioned lower limit, the component is considered to be contained as an unavoidable impurity.

[0044] Next, the microstructure of the high strength steel sheet of the present invention will be described.

[0045] The high-strength steel sheet of the present invention has the following microstructure: a bainite phase consisting of upper bainite and granular bainite accounts for 85% or more of the area fraction thereof as the main phase; the area fraction of a hard second phase containing fresh martensite and / or a retained austenite phase is less than 15%; the area fraction of granular bainite relative to the area fraction of upper bainite in the bainite phase is 5% or more and 40% or less, and the average grain size of the upper bainite phase is 7 μm or less; and the difference |Hv1−Hv2| between the hardness Hv1 at 1 / 2 the sheet thickness position and the hardness Hv2 at 1 / 4 the sheet thickness position is 30% or less with respect to 0.3TS.

[0046] <Main Phase: 85% or More of Bainite Phase> The microstructure of the high-strength steel plate of the present invention contains a bainite phase consisting of upper bainite and granular bainite as the main phase. If the bainite phase is less than 85% in area fraction, excellent punchability cannot be obtained. Therefore, the bainite phase is set to 85% or more, preferably 90% or more, and more preferably 95% or more.

[0047] <Hard second phase: fresh martensite and / or retained austenite phase less than 15%> If the hard second phase is controlled to be fresh martensite and / or retained austenite phase and the area fraction is less than 15%, no macroscopic stress concentration occurs at the phase interface during fatigue testing, and fatigue cracks do not become initiation points, resulting in excellent fatigue strength. Therefore, the area fraction of the hard second phase including fresh martensite and / or retained austenite phase is set to less than 15%, preferably 10% or less, and more preferably 5% or less.

[0048] The remaining structure other than upper bainite, granular bainite, fresh martensite and / or retained austenite phase is permissible if it is 3% or less. Examples of the remaining structure include lower bainite, ferrite, tempered martensite, pearlite, etc.

[0049] <Area ratio of granular bainite to the area ratio of upper bainite in the bainite phase is 5% or more and 40% or less> Granular bainite has better ductility than upper bainite. If the area ratio of granular bainite to the area ratio of upper bainite in the bainite phase is less than 5%, excellent ductility cannot be obtained. Therefore, the area ratio of granular bainite to the area ratio of upper bainite in the bainite phase is 5% or more, preferably 8% or more, and more preferably 10% or more. On the other hand, if the area ratio of granular bainite to the area ratio of upper bainite exceeds 40%, significant deterioration in fatigue strength occurs. Therefore, the area ratio of granular bainite to the area ratio of upper bainite is 40% or less, preferably 35% or less, and more preferably 30% or less.

[0050] <Average grain size of upper bainite phase is 7 μm or less> Grain boundaries have the effect of making it difficult for the slip deformation to propagate to adjacent grains, thereby improving fatigue strength. Therefore, the average grain size of the upper bainite phase is 7 μm or less, and preferably 6 μm or less. Although there is no particular lower limit, if the average grain size is too small, it may lead to a decrease in ductility, so the average grain size is preferably 1 μm or more, and more preferably 2 μm or more.

[0051] <The difference |Hv1-Hv2| between the hardness Hv1 at the half-thickness position and the hardness Hv2 at the quarter-thickness position is 30% or less relative to 0.3TS> When the segregation of elements such as Mn is excessive at the half-thickness position, a hard second phase containing fresh martensite and / or retained austenite is locally generated, resulting in a difference in hardness from the matrix structure, making cracks and roughness more likely to occur during punching. If the difference |Hv1-Hv2| between the hardness Hv1 at the half-thickness position and the hardness Hv2 at the quarter-thickness position exceeds 30% relative to 0.3TS, punchability will be significantly degraded. Therefore, the difference |Hv1-Hv2| between the hardness Hv1 at the half-thickness position and the hardness Hv2 at the quarter-thickness position is 30% or less relative to 0.3TS, preferably 25% or less, and more preferably 20% or less. The lower limit does not need to be particularly limited and may be 0%. TS stands for tensile strength. The hardnesses Hv1, Hv2, and TS are measured by the methods shown in the examples.

[0052] The high-strength steel sheet of the present invention has a tensile strength of 980 MPa or more, and also has excellent ductility, excellent fatigue strength, and excellent punchability. Therefore, the high-strength steel sheet of the present invention has high tensile strength, and even when thinned, it ensures safety and can be applied to parts for trucks and passenger cars. In the present invention, the area fractions of each microstructure and the mechanical properties described above are values ​​measured by the methods described in the Examples.

[0053] Next, a method for manufacturing a high-strength steel plate according to an embodiment of the present invention will be described. In the following description, unless otherwise specified, the temperature indicated in "°C" represents the surface temperature of the object (steel material or steel plate).

[0054] The high-strength steel sheet of the present invention can be manufactured by sequentially performing the following processes (1) to (6). Each process is described below. (1) Melting and Casting (2) Heating (3) Hot Rolling (4) Cooling (First Cooling) (5) Coiling (6) Cooling (Second Cooling) (1) Melting and Casting The method for producing the molten steel is not particularly limited. For example, molten steel having the above-described composition can be obtained by melting using a known method such as a converter. Scrap may also be used as the raw material. The resulting molten steel is then subjected to continuous casting to produce a steel material. Induction electromagnetic stirring has the effect of dividing columnar crystals growing from the mold and generating equiaxed crystals at the 1 / 4 to 1 / 2 plate thickness positions, thereby reducing the segregation of elements that occurs during solidification and promoting the formation of granular bainite. If the rotation speed during electromagnetic stirring is less than 10 cm / s, the columnar crystals are not divided sufficiently, and this effect cannot be achieved. Therefore, the turning speed in the horizontal plane is 10 cm / s or more, preferably 15 cm / s or more, and more preferably 20 cm / s or more.

[0055] On the other hand, the upper limit of the rotation speed during electromagnetic stirring is not particularly limited, but from a manufacturing standpoint, it is preferably 50 cm / s or less, more preferably 45 cm / s or less, and even more preferably 40 cm / s or less. After being produced by the continuous casting method, the steel material may be directly subjected to the subsequent heating step, or the steel material may be cooled to become a hot or cold piece and then subjected to the heating step. The chemical composition of the finally obtained high-strength steel plate is the same as the chemical composition of the steel material used.

[0056] (2) Heating First, the steel material is heated to a heating temperature of 1150°C or higher. After cooling to a low temperature, most of the precipitate-forming elements are present in the steel material as coarse precipitates in a non-uniform manner. If the additive elements are present as coarse and non-uniform precipitates, the additive elements will not be fully effective, and the desired steel structure will not be obtained. Therefore, it is necessary to heat the steel material prior to hot rolling to dissolve the coarse precipitates. For this reason, the heating temperature of the steel material is 1150°C or higher, preferably 1180°C or higher, and more preferably 1200°C or higher. On the other hand, if the heating temperature of the steel material is too high, slab defects will occur and yield will decrease due to scale-off. For this reason, the heating temperature of the steel material is preferably 1350°C or lower, more preferably 1300°C or lower, and even more preferably 1280°C or lower. In terms of uniformizing the temperature of the steel material during heating, it is preferable to heat the steel material to the heating temperature and then hold it at the heating temperature. The time for holding at the heating temperature (holding time) is not particularly limited, but from the viewpoint of improving the temperature uniformity of the steel material, it is preferably 1,800 seconds or more. On the other hand, if the holding time exceeds 10,000 seconds, the amount of scale generation increases. As a result, scale bite and the like are likely to occur in the subsequent hot rolling, leading to a decrease in yield due to poor surface defects. Therefore, the holding time is preferably 10,000 seconds or less, and more preferably 8,000 seconds or less. Note that this heating step also includes a case where the steel material before hot rolling is directly subjected to hot rolling (direct rolling) while still at a high temperature (i.e., while maintaining a temperature within the above heating temperature range) after casting.

[0057] (3) Hot Rolling Next, the heated steel material (or the steel material still at a high temperature after casting) is subjected to hot rolling, which includes rough rolling and finish rolling. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0058] First, a steel material is rough-rolled to obtain a rough-rolled sheet. Before being subjected to finish rolling, the obtained rough-rolled sheet may be subjected to descaling (high-pressure water descaling) in which high-pressure water is sprayed on the inlet side of a finish rolling mill.

[0059] Next, in the present invention, when the temperature RC is defined by the following formula (1), the finish rolling end temperature is set to be (RC-50)°C or higher and (RC+100)°C or lower. RC is the lower limit temperature of austenite recrystallization estimated from the chemical composition. If the finish rolling end temperature is lower than (RC-50)°C, strain relaxation by recrystallization is difficult to occur, and the austenite continues to accumulate strain during cooling, excessively promoting the formation of granular bainite. For this reason, the finish rolling end temperature is set to be (RC-50)°C or higher, preferably (RC-30)°C or higher, and more preferably RC°C or higher.

[0060] On the other hand, if the finish rolling end temperature is higher than (RC + 100)°C, the austenite grains will coarsen and the average grain size of the upper bainite will increase, making it impossible to obtain sufficient fatigue strength. Therefore, the finish rolling end temperature is (RC + 100)°C or lower, preferably (RC + 80)°C or lower, and more preferably (RC + 50)°C or lower. RC is defined by the following formula (1): RC (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V (1) Here, each element symbol in the formula (1) represents the content (mass%) of each element, and if the element is not contained, it is set to 0.

[0061] (4) Cooling (First Cooling) Next, the obtained hot-rolled steel sheet is cooled (first cooling). In this case, the time from the end of hot rolling to the start of cooling (cooling start time) is set to within 2.0 seconds after the end of finish rolling. If the cooling start time exceeds 2.0 seconds, the austenite grains become coarse and the average grain size of upper bainite becomes large, making it impossible to obtain sufficient tensile strength and fatigue strength. Therefore, the cooling start time is set to within 2.0 seconds, preferably within 1.5 seconds, and more preferably within 1.0 seconds.

[0062] In addition, the average cooling rate at the surface from the finish rolling end temperature to Bs is set to 30°C / s or more. If the average cooling rate from the finish rolling end temperature to Bs is too slow, excessive ferrite is generated, and sufficient tensile strength and fatigue strength cannot be obtained. Therefore, the average cooling rate is 30°C / s or more, preferably 40°C / s or more, and more preferably 50°C / s or more. On the other hand, although there is no particular upper limit, if it is too fast, it becomes difficult to control the cooling stop temperature. Therefore, the average cooling rate is preferably 500°C / s or less, more preferably 300°C / s or less, and even more preferably 150°C / s or less.

[0063] Furthermore, during cooling, the residence time at a temperature of (Bs-100)°C or higher and Bs°C or lower is set to 3.0 seconds or higher and 10.0 seconds or lower. If the residence time at a temperature of (Bs-100)°C or higher and Bs°C or lower is less than 3.0 seconds, sufficient granular bainite cannot be obtained. Therefore, the residence time at a temperature of (Bs-100)°C or higher and Bs°C or lower is 3.0 seconds or higher, preferably 4.0 seconds or higher, and more preferably 5.0 seconds or higher. If the residence time at a temperature of (Bs-100)°C or higher and Bs°C or lower exceeds 10.0 seconds, excessive granular bainite is generated, and sufficient fatigue strength cannot be obtained. Therefore, the residence time at a temperature of (Bs-100)°C or higher and Bs°C or lower is set to 10.0 seconds or lower, preferably 9.0 seconds or lower, and more preferably 8.0 seconds or lower.

[0064] Furthermore, the cooling method is not particularly limited as long as forced cooling is performed to achieve the above average cooling rate. The cooling stop temperature is (Bs-250)°C or higher and (Bs-100)°C or lower. If the cooling stop temperature is less than (Bs-250)°C, the microstructure will become lower bainite. Both lower bainite structures have high strength but lack excellent ductility. For this reason, the cooling stop temperature is (Bs-250)°C or higher, preferably (Bs-220)°C or higher, and more preferably (Bs-200)°C or higher. On the other hand, if the cooling stop temperature is higher than (Bs-100)°C, granular bainite will be excessively formed, making it difficult to obtain excellent fatigue strength. For this reason, the cooling stop temperature is (Bs-100)°C or lower, preferably (Bs-120)°C or lower, and more preferably (Bs-150)°C or lower. Bs is defined by the following formula (2): Bs (°C) = 830 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo - 20 x Cu (2) Here, each element symbol in the above formula (2) represents the content (mass%) of each element, and if an element is not contained, it is set to 0.

[0065] (5) Coiling Next, the cooled hot-rolled steel sheet is coiled under the condition of a coiling temperature of (Bs-250)°C or higher and (Bs-100)°C or lower. If the coiling temperature is lower than (Bs-250)°C, lower bainite and / or tempered martensite will be formed, resulting in insufficient ductility. For this reason, the coiling temperature is (Bs-250)°C or higher, preferably (Bs-230)°C or higher, and more preferably (Bs-200)°C or higher. On the other hand, if the coiling temperature is higher than (Bs-100)°C, granular bainite will be formed in excess, making it impossible to obtain excellent fatigue strength. For this reason, the coiling temperature is (Bs-100)°C or lower, preferably (Bs-120)°C or lower, and more preferably (Bs-150)°C or lower.

[0066] (6) Cooling (Second Cooling) Next, the steel sheet is cooled to (Bs-400)°C or lower at an average cooling rate of 1°C / s or less (second cooling). If the average cooling rate from the coiling temperature to (Bs-400)°C or lower exceeds 1°C / s, lower bainite and / or tempered martensite will form, resulting in insufficient ductility. For this reason, the average cooling rate from the coiling temperature to (Bs-400)°C or lower is 1°C / s or less, preferably 0.8°C / s or less, and more preferably 0.5°C / s or less. There is no particular reason to limit the lower limit of the average cooling rate, but from the viewpoints of manufacturing efficiency and coarsening of carbides, a rate of 0.001°C / s or more is preferable. Cooling can be performed to any temperature below (Bs-400)°C, but cooling to 50°C is preferable. Cooling can be performed in any manner, for example, it may be performed in the state of a wound coil.

[0067] The high-strength steel sheet of the present invention can be manufactured by the above-described procedure. After coiling and subsequent cooling, the steel sheet may be subjected to conventional methods. For example, temper rolling may be performed, or pickling may be performed to remove scale formed on the surface.

[0068] Molten steel having the composition shown in Table 1 was melted in a converter and cast by a continuous casting method at the rotation speed shown in Table 2 to produce a steel slab as a steel material. The obtained steel material was heated to the heating temperature shown in Table 2, and then the heated steel material was subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot-rolled steel sheet. The finish rolling finish temperature in the hot rolling was as shown in Table 2. Next, the obtained hot-rolled steel sheet was cooled under the conditions of the average cooling rate and cooling stop temperature shown in Table 2 (first cooling). The cooled hot-rolled steel sheet was coiled at the coiling temperature shown in Table 2, and the coiled steel sheet was cooled at the average cooling rate shown in Table 2 (second cooling) to obtain a high-strength steel sheet. After cooling, temper rolling was performed and pickling was carried out. Pickling was carried out using a hydrochloric acid aqueous solution with a concentration of 10 mass % at a temperature of 85°C.

[0069]

[0070]

[0071] Test specimens were taken from the obtained high-strength steel plates, and the microstructure and mechanical properties were evaluated according to the procedures described below.

[0072] <Microstructure> Test specimens for microstructure observation were taken from the obtained high-strength steel sheets so that the observation surface was the plate thickness cross section parallel to the rolling direction. The surfaces of the obtained test specimens were polished and further corroded using an etchant (3% nital solution) to reveal the microstructure. Next, 10 fields of view at 1 / 4 the plate thickness were photographed using a scanning electron microscope (SEM) at 5000x magnification to obtain SEM images of the microstructure. The obtained SEM images were analyzed using image processing to quantify the area fractions of upper bainite (UB), polygonal ferrite (F), lower bainite and / or tempered martensite (LB + TM). Furthermore, because it is difficult to distinguish between upper bainite (UB), granular bainite (GB), fresh martensite (FM), and retained austenite phase (γ), they were identified using electron beam reflection diffraction, and their respective area fractions and average grain sizes were determined. The area fractions of each measured microstructure and the average grain size of upper bainite are shown in Table 3. Table 3 also lists the total area ratio (B) of upper bainite and granular bainite and the total area ratio (FM+γ) of fresh martensite and retained austenite phase.

[0073]

[0074] <Tensile test> A JIS No. 5 tensile test piece (JIS Z 2201) was taken from the obtained hot-rolled steel sheet so that the tensile direction was perpendicular to the rolling direction. －３ Tensile tests were conducted in accordance with the JIS Z 2241 standard, where tensile strength and uniform elongation were determined. In the present invention, a tensile strength of 980 MPa or more was considered acceptable. A uniform elongation of 6.0% or more was considered to have excellent ductility.

[0075] <Plane bending fatigue test> Test specimens having the dimensions shown in FIG. 1 were taken from the obtained hot-rolled steel sheets so that the longitudinal direction of the test specimen was perpendicular to the rolling direction, and a plane bending fatigue test was carried out in accordance with the provisions of JIS Z 2275. The stress loading mode was a stress ratio R = -1 and a frequency f = 25 Hz. The load stress amplitude was changed in six stages, and the stress cycle up to fracture was measured, and an S-N curve was obtained. ６In the present invention, when the value obtained by dividing the fatigue limit by the tensile strength obtained in the tensile test was 0.50 or more, the specimen was evaluated as having excellent fatigue properties.

[0076] <Evaluation of Punchability> Test specimens (size: t (plate thickness) × 30 mm (width) × 30 mm (length)) were taken from the obtained hot-rolled steel sheets. A punched hole was formed in the center of the taken test specimen by punching using a 10 mmφ cylindrical punch at three or more clearances including 10% and 20% within a clearance range of 10 to 20%. The clearance is a percentage [%] of the plate thickness of the test specimen. When cracks, chips, brittle fracture surfaces, and secondary shear surfaces were not visually observed on the end surface of the punched hole, the specimen was evaluated as having excellent punchability, with a rating of ○; when these were observed, the specimen was evaluated as ×.

[0077] <Hardness Measurement> Hardness measurement samples were taken from the obtained high-strength steel sheets so that the thickness cross section parallel to the rolling direction was the hardness measurement cross section, and the hardness (Hv1) at the 1 / 2 thickness position and the hardness (Hv2) at the 1 / 4 thickness position were measured. The Vickers hardness measurement conditions were a load of 100 g and a holding time of 10 s. Measurements were taken at five points in the ±1 / 20 thickness region at measurement intervals of 250 μm or more, and the average was calculated.

[0078] Next, the difference between the obtained Hv1 and Hv2 (|Hv1-Hv2|) was calculated as a percentage relative to 0.3 times the tensile strength obtained in the tensile test, and evaluated.

[0079] The inventive examples are all high-strength steel sheets having a tensile strength of 980 MPa or more and excellent ductility, fatigue strength, and punchability, while the comparative examples outside the scope of the present invention either do not achieve a tensile strength of 980 MPa or more, or do not achieve excellent ductility, fatigue strength, or punchability.

Claims

1. A steel sheet having a composition containing, by mass%, C: 0.04-0.18%, Si: 0.1-2.5%, Mn: 1.0-3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.010-1.0%, and N: 0.01% or less, with the balance being Fe and unavoidable impurities; its microstructure has a bainite phase consisting of upper bainite and granular bainite at an area ratio of 85% or more as the main phase, the area ratio of a hard second phase containing fresh martensite and / or retained austenite phase being less than 15%, and within the bainite phase, the area ratio of granular bainite relative to the area ratio of upper bainite is 5% or more and 40% or less, A high-strength steel plate in which the difference |Hv1-Hv2| between the hardness Hv1 at 1 / 2 the plate thickness position and the hardness Hv2 at 1 / 4 the plate thickness position is 30% or less for 0.3TS, and the average crystal grain size of the upper bainite phase is 7 μm or less.

2. The high-strength steel plate according to claim 1, further comprising, in addition to the above-mentioned chemical composition, one or more of the groups a to e in mass %. Group a: one or more selected from V: 0.005-0.5%, Ti: 0.005-0.2%, and Nb: 0.005-0.1%; Group b: one or more selected from Cu: 0.005-0.5%, Ni: 0.005-0.5%, Cr: 0.005-1.0%, and Mo: 0.005-0.5%; Group c: one or more selected from B: 0.0002-0.005%, Group d: one or two selected from Sb: 0.001-0.1%, and Sn: 0.001-0.1%; Group e: Ca: 0.0001-0.005%, Mg: 0.0001-0.005%, and REM: 0.0001 to 0.005%, one or more selected from the following:

3. A method for producing a high strength steel plate according to claim 1 or 2, comprising the steps of: casting a molten steel material having the above-mentioned chemical composition while rotating it in a horizontal plane relative to a mold at a rotation speed of 10 cm / s or more using an induction electromagnetic stirrer; heating the obtained steel material to 1150°C or more; rough rolling the heated steel material to obtain a steel plate; finish rolling the steel plate under the conditions of a finish rolling end temperature of (RC-50)°C or more and (RC+100)°C or less; and cooling the steel plate after the finish rolling under the conditions of a time from the end of finish rolling to the start of cooling of 2.0 s or less, an average cooling rate on the surface up to Bs of 30°C / s or more, a residence time at a temperature of (Bs-100)°C or more and Bs°C or less of 3.0 s or more and 10.0 s or less, and a cooling end temperature of (Bs-250)°C or more and (Bs-100)°C or less; A method for producing a high strength steel sheet, in which the cooled steel sheet is coiled under the condition of a coiling temperature of (Bs-250)°C or more and (Bs-100)°C or less, and cooled to (Bs-400)°C or less at an average cooling rate of 1°C / s or less. Note that RC and Bs are defined by the following formulas (1) and (2), respectively. RC (°C) = 750 + 120 x C + 100 x N + 10 x Mn + 250 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 750 x Nb + 150 x V ... (1) Bs (°C) = 830 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo - 20 x Cu ... (2) Here, each element symbol in the above formulas (1) and (2) represents the content (mass%) of each element, and 0 is used when the element is not contained.

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