Steel sheet, component including same, and production method for steel sheet

Abstract

Provided is a steel sheet that is characterized by having a prescribed chemical composition and a metal structure that is, by area%, 40%–80% ferrite, 15%–55% bainite, and 5%–20% martensite, the ferrite including TiC precipitates that have a diameter of 2.0–8.0 nm at a number density of at least 1.0×1016 / cm3, and the standard deviation of the hardness of the ferrite and the bainite being no more than 0.40 GPa. Also provided are a component that includes the steel sheet and a production method for the steel sheet.

Description

Steel plate, part including same, and method of manufacturing steel plate

[0001] The present invention relates to a steel sheet, a part including the same, and a method for manufacturing the steel sheet.

[0002] In recent years, the automotive industry has been seeking to reduce the weight of vehicle bodies in order to improve fuel efficiency. Increasing the strength of the steel sheets used is one effective way to achieve both lightweight vehicle bodies and crashworthiness, and against this background, the development of high-strength steel sheets has been progressing. However, as strength increases, the formability of steel sheets generally decreases. For this reason, in the development of high-strength steel sheets, it is important to increase strength while maintaining a certain level of formability.

[0003] In this regard, for example, Patent Document 1 discloses a steel sheet having a predetermined chemical composition, wherein the microstructure in the range from the surface at a position ⅛ of the sheet thickness in the sheet thickness direction to the position ⅜ of the sheet thickness in the sheet thickness direction from the surface contains, in volume fractions, 10 to 75% ferrite, 20 to 90% martensite, 0 to 5% retained austenite, 0 to 5% total of bainite and bainitic ferrite, and 0 to 5% pearlite, wherein the proportion of unrecrystallized ferrite in the ferrite is 0 to 25%, cementite contained in the martensite satisfies a predetermined formula, and the density of transition carbides contained in the martensite is 1.0×10 13 pieces / m 3 or more, and the density of coarse inclusions with a circle equivalent diameter of 10 μm or more is 0.50 pieces / mm 2 The maximum Vickers hardness Hv is less than or equal to the surface parallel to the surface at a position 1 / 4 of the plate thickness in the plate thickness direction from the surface. max and the minimum value Hv of the Vickers hardness min and when a distribution map of the Vickers hardness is created, the average value of the minimum distance between the peaks of the Vickers hardness is 1.00 mm or less. Furthermore, Patent Document 1 teaches that the above-mentioned configuration can provide a steel sheet excellent in formability, strength, and impact resistance.

[0004] Patent Document 2 describes a dual-phase steel sheet having a predetermined chemical composition, in which the main phase of the microstructure at a position one-quarter of the sheet thickness is polygonal ferrite precipitation-strengthened by Ti carbide, and the second phase is a dual-phase structure consisting of a plurality of dispersed low-temperature transformation products with an area fraction (fsd (%)) of 1 to 10%, the average crystal diameter of the low-temperature transformation products being 3 to 15 μm, and the average nearest neighbor distance between each low-temperature transformation product being 10 to 20 μm. Patent Document 2 also teaches that the above configuration makes it possible to obtain a high-strength dual-phase steel sheet having a tensile strength of 540 MPa or more, excellent uniform elongation, excellent burring workability, and excellent notch fatigue properties, and also excellent surface properties.

[0005] Patent Document 3 describes a steel sheet having a predetermined component composition, in which the Mn segregation degree is 1.5 or less in a region within 100 μm from the surface in the sheet thickness direction, and in which oxide-based inclusions with particle major axes of 5 μm or more are present in a plane parallel to the sheet surface of the steel sheet within 100 μm from the surface in the sheet thickness direction. 2 Patent Document 3 describes a high-strength steel sheet in which the number of inclusions per steel sheet is 1,000 or less, the proportion of oxide-based inclusions having a composition of 50% by mass or more of alumina, 20% by mass or less of silica, and 40% by mass or less of calcia, based on the total number of oxide-based inclusions having a particle major axis of 5 μm or more, and the proportion of oxide-based inclusions having a composition of 50% by mass or more of alumina, 20% by mass or less of silica, and 40% by mass or less of calcia, based on a metallographic structure containing, by volume, 25 to 100% of the total of martensite and bainite phases, less than 75% (including 0%) of ferrite phase, and less than 15% (including 0%) of austenite phase, and the tensile strength is 980 MPa or more. Patent Document 3 also teaches that a high-strength steel sheet with excellent bendability (bending workability) can be obtained by reducing the number of inclusions in the steel sheet surface layer (a region within 100 μm from the steel sheet surface) and controlling the composition of the inclusions within an appropriate range, and by reducing the degree of Mn segregation in the steel sheet surface layer.

[0006] Patent Document 4 describes a high-strength hot-rolled steel sheet characterized by having a microstructure in which the steel sheet has a predetermined component composition, the total volume fraction of the ferrite phase and the bainite phase in the entire steel sheet is 95% or more, the volume fraction of the ferrite phase in the entire steel sheet is 50 to 90%, the ferrite phase contains precipitates of less than 20 nm in size containing 650 to 1100 ppm of Ti, and the bainite phase has a ΔHv (the difference between the maximum and minimum Vickers hardness values ​​of the bainite phase measured at 1 / 4 positions of the sheet thickness in the sheet thickness cross section along the rolling direction) of 150 or less. Furthermore, Patent Document 2 teaches that if a microstructure is formed that is mainly composed of a ferrite phase and a bainite phase, in which precipitates of a size of less than 20 nm containing 650 to 1100 ppm of Ti are precipitated in the ferrite phase, and the ΔHv of the bainite phase is set to 150 or less, a TS of 780 MPa or more can be ensured, and excellent stretch flangeability (hole expandability) and impact resistance can both be achieved.

[0007] Patent Document 5 discloses a steel sheet having a predetermined chemical composition, in a cross section perpendicular to the rolling direction of the steel sheet, where W and t are the width and thickness of the steel sheet, the metallographic structure at a position of ¼W or ¾W from the end face of the steel sheet and at a position of ¼t or ¾t from the surface of the steel sheet is, in area %, ferrite: 5 to 70%, bainite: 30 to 95%, retained austenite: 2% or less, martensite: 2% or less, and pearlite: 1% or less, and the total of ferrite and bainite: 95% or more, the ferrite has precipitates containing Ti within its grains, and the number density of the precipitates containing Ti is 1.0 × 10 16 ~50.0 x 10 16 pieces / cm 3The hot-rolled steel sheet described in Patent Document 5 has a tensile strength (TS) of 780 MPa or more, a uniform elongation (u-EL) of 7000 MPa·% or more, a product of the tensile strength (TS) and the hole expansion ratio (λ) of 50000 MPa·% or more, and a standard deviation of the nano-hardness (TS×λ) of 50000 MPa·% or more.

[0008] Patent Document 6 discloses a steel sheet having a predetermined chemical composition, in a cross section perpendicular to the rolling direction of the steel sheet, where W and t are the width and thickness of the steel sheet, the metallographic structure at a position of ¼W or ¾W from the end face of the steel sheet and at a position of ¼t or ¾t from the surface of the steel sheet is, in area %, ferrite: 5 to 70%, bainite: 30 to 95%, retained austenite: 2% or less, martensite: 2% or less, and pearlite: 1% or less, and the total of ferrite and bainite: 95% or more, the ferrite has precipitates containing Ti within its grains, and the number density of the precipitates containing Ti is 1.0 × 10 16 ~50.0 x 10 16 pieces / cm 3 The hot-rolled steel sheet described in Patent Document 6 has a tensile strength (TS) of 780 MPa or more, a uniform elongation (u-EL) of 7000 MPa·% or more, a product of the tensile strength (TS) and the hole expansion ratio (λ) of 50000 MPa·% or more, and a standard deviation of the nano-hardness (TS×λ) of 50000 MPa·% or more.

[0009] International Publication No. 2020 / 208979 International Publication No. 2014 / 051005 International Publication No. 2017 / 115748 Japanese Patent Application Laid-Open No. 2011-068945 International Publication No. 2018 / 179389 International Publication No. 2018 / 179391

[0010] In relation to the hole expandability and impact resistance properties described in Patent Document 4, for example, if the hole expandability decreases, it may be impossible to form the desired part shape in automobile suspension parts, etc. Furthermore, for parts that require impact resistance, plastic deformation occurs when they receive an impact that exceeds the yield strength (specifically, the yield stress, yield point, or 0.2% proof stress), so from the perspective of ensuring automobile collision safety, it is necessary to improve not only the tensile strength but also the yield strength, and therefore it is necessary to increase the yield ratio, which is the ratio of the yield strength to the tensile strength.

[0011] Therefore, an object of the present invention is to provide a steel plate having high strength and high hole expandability and yield ratio, a part including the same, and a method for manufacturing the steel plate.

[0012] To achieve the above object, the inventors conducted research focusing on the metallographic structure of steel sheets, particularly hot-rolled steel sheets. Specifically, the inventors first discovered that by configuring the metallographic structure of a steel sheet having a predetermined chemical composition to mainly contain ferrite, bainite, and martensite, it is possible to improve the hole expandability and yield ratio while maintaining a relatively high level of strength of the steel sheet. In addition, the inventors discovered that by making TiC precipitates having a moderate diameter present in ferrite at a predetermined number density, the ferrite can be precipitation-strengthened, thereby further increasing the strength of the steel sheet while reducing the difference in hardness between ferrite and bainite, thereby increasing the hole expandability and yield ratio, and further reducing the variation in hardness between ferrite and bainite, thereby more significantly improving the hole expandability, and thus completed the present invention.

[0013] The present invention, which has achieved the above object, is as follows. (1) Chemical composition, in mass%, is: C: 0.03 to 0.10%, Si: 0.010 to 0.100%, Mn: 0.50 to 3.00%, Ti: 0.05 to 0.20%, Al: 0.20 to 0.40%, P: 0.100% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0 to 0.050%, V: 0 to 1.000%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.000%, B: 0 to 0.0100%, Sn: 0 to 1.000%, Sb: 0 to 1.000%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, Bi: 0 to 0.010%, REM: 0 to 0.0100%, As: 0 to 0.010%, Zr: 0 to 0.010%, Co: 0 to 2.000%, Zn: 0 to 0.010%, W: 0 to 1.000%, and the balance: Fe and impurities, and the metallographic structure contains, in area %, 40 to 80% ferrite, 15 to 55% bainite, and 5 to 20% martensite, and the ferrite contains 1.0 x 10 TiC precipitates having a diameter of 2.0 to 8.0 nm. 16 pieces / cm 3and the standard deviation of the hardness of ferrite and bainite is 0.40 GPa or less. (2) The chemical composition is, in mass%, Nb: 0.001 to 0.050%, V: 0.001 to 1.000%, Cr: 0.001 to 2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Mo: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Sn: 0.001 to 1.000%, Sb: 0.001 to 1.000%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, Bi: 0.001 to 0.010%, (1) The steel sheet according to (1) above, characterized by containing at least one of REM: 0.0001 to 0.0100%, As: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Co: 0.001 to 2.000%, Zn: 0.001 to 0.010%, and W: 0.001 to 1.000%. (3) The steel sheet according to (1) or (2) above, characterized by an average ferrite grain size of 5.0 μm or less. (4) The steel sheet according to any one of (1) to (3) above, characterized by having a tensile strength of 780 MPa or more. (5) The steel sheet according to any one of (1) to (4) above, characterized by having a sheet thickness of 1.0 to 8.0 mm. (6) A part, characterized by including the steel sheet according to any one of (1) to (5) above.(7) A hot rolling process comprising heating a slab having the chemical composition described in (1) or (2) above and then finish rolling the slab, and satisfying the following conditions (a) to (e): (a) the heating temperature of the slab is 1200 to 1300°C; (b) the holding time in the temperature range of 1200 to 1300°C is 1000 to 4000 seconds; (c) the finish rolling is performed using a tandem rolling mill consisting of five or more rolling stands, and the total reduction in the rolling passes of the front stages other than the last three stages is 60 to 90%; (d) the total reduction in the rolling passes of the last three stages is more than 50%; and (e) the finish rolling temperature is 900 to 1000°C. a primary cooling step of subjecting a finish-rolled steel sheet to an intermediate air-cooling temperature of 705 to 750°C at an average cooling rate of 50 to 200°C / sec, followed by intermediate air-cooling for 3 to 10 seconds; and a secondary cooling step of subjecting the intermediate air-cooled steel sheet to an average cooling rate of 50 to 200°C / sec, followed by coiling at a coiling temperature of 20 to 300°C.

[0014] According to the present invention, it is possible to provide a steel plate having high strength and high hole expandability and yield ratio, a part including the same, and a method for manufacturing the steel plate.

[0015] <Steel Sheet> A steel sheet according to an embodiment of the present invention, particularly a hot-rolled steel sheet, has a chemical composition, in mass %, of C: 0.03 to 0.10%, Si: 0.010 to 0.100%, Mn: 0.50 to 3.00%, Ti: 0.05 to 0.20%, Al: 0.20 to 0.40%, P: 0.100% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0 to 0.050%, V: 0 to 1.000%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.000%, B: 0 to 0.0100%, Sn: 0 to 1.000%, Sb: 0 to 1.000%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, Bi: 0 to 0.010%, REM: 0 to 0.0100%, As: 0 to 0.010%, Zr: 0 to 0.010%, Co: 0 to 2.000%, Zn: 0 to 0.010%, W: 0 to 1.000%, and the balance: Fe and impurities, and the metallographic structure contains, in area %, 40 to 80% ferrite, 15 to 55% bainite, and 5 to 20% martensite, and the ferrite contains 1.0 x 10 TiC precipitates having a diameter of 2.0 to 8.0 nm. 16 pieces / cm 3 The ferrite and bainite are present at a number density of 0.40 GPa or more, and the standard deviation of the hardness of the ferrite and bainite is 0.40 GPa or less.

[0016] As the strength of steel sheets increases, their workability, such as hole expandability, generally decreases. For example, to manufacture automobile suspension components with complex shapes, such as lower arms and trailing arms, steel sheets are required that have high strength, for example, a tensile strength of 780 MPa or more, which enables weight reduction, while also having excellent hole expandability. As mentioned above, in order to ensure automobile collision safety, it is necessary to improve not only tensile strength but also yield strength. Therefore, it is necessary to increase the yield ratio, which is the ratio of yield strength to tensile strength. Therefore, there is a high demand for materials that improve hole expandability while ensuring sufficient strength of the steel sheet and have a high yield ratio from the perspective of automobile collision safety, etc. Therefore, the present inventors conducted research, focusing in particular on the metallographic structure of the steel sheet, in addition to determining the appropriate chemical composition of the steel sheet. To explain in more detail, first, the inventors discovered that by configuring the metal structure of a steel plate having a predetermined chemical composition to contain mainly ferrite, bainite, and martensite, more specifically, by configuring it to contain, in area percentages, ferrite: 40 to 80%, bainite: 15 to 55%, and martensite: 5 to 20%, it is possible to increase the hole expandability and yield ratio while maintaining the strength of the steel plate at a relatively high level.

[0017] On the other hand, in a three-phase structure primarily composed of ferrite, bainite, and martensite, strength is generally ensured by the hard bainite and martensite, while deformation is borne by the soft ferrite. However, if the difference in hardness between the hard phases bainite and martensite and the soft phase ferrite becomes large, this can result in a decrease in hole expandability and yield ratio. In particular, if the difference in hardness becomes too large, deformation concentrates in the soft ferrite, causing early cracking, resulting in a particularly significant decrease in hole expandability. Therefore, the inventors have investigated further improvements in hole expandability and realization of a high yield ratio from the perspective of reducing the difference in hardness between each phase in a metal structure containing such a three-phase structure. As a result, the inventors have discovered a method for precipitation strengthening the softest ferrite in the three-phase structure, more specifically, by forming TiC precipitates with a diameter of 2.0 to 8.0 nm in the ferrite at a concentration of 1.0 × 10 16 pieces / cm 3 It has been found that by precipitating ferrite at a number density of 1.0 × 10 or more, it is possible to not only contribute to improving the strength of the steel sheet as a whole but also to sufficiently reduce the difference in hardness between bainite, which is present in a relatively large amount in the hard structure, and ferrite, which is the softest of the three-phase structure. 16 pieces / cm 3To achieve this number density, the chemical composition of the steel sheet must be appropriate, in addition to the manufacturing method described in detail below. For example, Si and Al contained in steel have the effect of suppressing cementite precipitation. Therefore, by including these elements in the steel in a predetermined amount or more, more specifically, by including Si and Al in an amount of 0.010 mass% or more and 0.20 mass% or more, respectively, it is possible to suppress the consumption of C in the steel for forming cementite, thereby promoting the formation of TiC precipitates during cooling after hot rolling. As a result, the inventors have discovered that, even though a metallographic structure is formed using a three-phase structure containing relatively large amounts of bainite and martensite to achieve high strength and therefore prone to a relatively large difference in hardness, a steel sheet can be obtained with further improved hole expandability and yield ratio by increasing the hardness of ferrite through precipitation strengthening using TiC precipitates of a specific diameter and number density.

[0018] As described above, increasing the hardness of ferrite through precipitation strengthening using TiC precipitates of a specific diameter and number density can reduce the difference in hardness between bainite and ferrite, thereby improving hole expandability and yield ratio. However, even if the hardness difference is sufficiently reduced throughout the metallographic structure, if even a portion of the metallographic structure has a large difference in hardness, such a portion may become the origin of fracture, potentially resulting in reduced hole expandability. Therefore, the present inventors conducted research focusing on the hardness distribution of the metallographic structure in order to further improve the hole expandability of steel sheets. As a result, the present inventors discovered that, in addition to reducing the hardness difference between bainite and ferrite using the TiC precipitates described above, reducing the variation in the hardness of ferrite and bainite within a predetermined range, more specifically, controlling the standard deviation in the hardness of ferrite and bainite to 0.40 GPa or less, can result in a steel sheet with significantly improved hole expandability. By combining the reduction in the hardness difference between bainite and ferrite due to the use of TiC precipitates with the reduction in the hardness variation between ferrite and bainite, not only can the hardness difference be reduced throughout the metal structure, but also the increase in local hardness differences within the metal structure can be reliably reduced, resulting in a more significant improvement in the hole expandability of the steel sheet. Therefore, with the steel sheet according to the embodiment of the present invention, it is possible to achieve particularly excellent hole expandability in addition to a high yield ratio despite its high strength. Therefore, the steel sheet according to the embodiment of the present invention can be effectively used in parts that require both the contradictory properties of high strength and excellent workability, and furthermore, impact resistance, and is therefore particularly useful in the automotive field.

[0019] Hereinafter, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0020] [C: 0.03 to 0.10%] C is an element effective in increasing the strength of steel sheet. Furthermore, C forms carbides and / or carbonitrides with Ti and Nb in steel, contributing to precipitation strengthening based on the formed precipitates and to refinement of the structure due to the pinning effect of the precipitates. To fully obtain these effects, the C content is set to 0.03% or more. The C content may be 0.04% or more, 0.05% or more, or 0.06% or more. On the other hand, excessive C content may result in a decrease in hole expandability and yield ratio due to the formation of cementite. Therefore, the C content is set to 0.10% or less. The C content may be 0.09% or less, 0.08% or less, or 0.07% or less.

[0021] [Si: 0.010 to 0.100%] Si is an effective solid-solution strengthening element for increasing strength. Si also inhibits cementite precipitation. Therefore, the inclusion of Si can inhibit the consumption of C in the steel to form cementite, thereby promoting the formation of TiC precipitates during cooling after hot rolling. To fully achieve these effects, the Si content is set to 0.010% or more. The Si content may be 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, excessive Si content may cause surface quality defects known as Si scale. Therefore, the Si content is set to 0.100% or less. The Si content may be 0.090% or less, 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[0022] [Mn: 0.50 to 3.00%] Mn is an element that is effective in increasing hardenability and strength as a solid solution strengthening element. To fully obtain these effects, the Mn content is set to 0.50% or more. The Mn content may be 0.60% or more, 0.70% or more, 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more. On the other hand, excessive Mn content may result in the formation of large amounts of MnS, which may reduce toughness. Therefore, the Mn content is set to 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[0023] [Ti: 0.05 to 0.20%] Ti is an element that finely precipitates in steel as carbide (TiC), improving the strength of steel through precipitation strengthening and increasing the hardness of ferrite. Ti also forms carbides to fix C and suppress the formation of cementite, which is harmful to hole expandability. To fully obtain these effects, the Ti content is set to 0.05% or more. The Ti content may be 0.08% or more, 0.10% or more, 0.12% or more, or 0.14% or more. On the other hand, excessive Ti content may cause the carbides to become coarse, making it impossible to achieve the desired precipitation strengthening in ferrite. In addition, as the TiC precipitates become coarser, the number density of the TiC precipitates also decreases, making it impossible to sufficiently increase the hardness of ferrite through precipitation strengthening. Therefore, the Ti content is set to 0.20% or less. The Ti content may be 0.18% or less, 0.17% or less, 0.16% or less, or 0.15% or less.

[0024] [Al: 0.20 to 0.40%] Al is an element that acts as a deoxidizer for molten steel. Furthermore, Al also has the effect of suppressing cementite precipitation. Therefore, the inclusion of Al can suppress the consumption of C in the steel to form cementite, thereby promoting the formation of TiC precipitates during cooling after hot rolling. To fully obtain these effects, the Al content is set to 0.20% or more. The Al content may be 0.22% or more, 0.25% or more, or 0.28% or more. On the other hand, excessive Al content may form coarse oxides, resulting in reduced toughness and ductility. Therefore, the Al content is set to 0.40% or less. The Al content may be 0.38% or less, 0.35% or less, or 0.32% or less.

[0025] [P: 0.100% or less] Excessive P content may adversely affect weldability, etc. Therefore, the P content is set to 0.100% or less. The P content may be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0026] [S: 0.0100% or less] Excessive S content may result in the formation of large amounts of MnS, which may reduce toughness. Therefore, the Si content is set to 0.0100% or less. The S content may be 0.0050% or less, 0.0030% or less, or 0.0020% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0027] [N: 0.010% or less] Excessive N content may form coarse nitrides and reduce toughness. Therefore, the N content is set to 0.010% or less. The N content may be 0.008% or less, 0.005% or less, or 0.003% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0028] [O: 0.010% or less] O is an element that is mixed in during the manufacturing process. Excessive O content may form coarse inclusions, reducing the toughness of the steel plate. Therefore, the O content is set to 0.010% or less. The O content may be 0.008% or less, 0.006% or less, or 0.004% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long refining time, resulting in reduced productivity. Therefore, the O content may be 0.0001% or more or 0.0005% or more.

[0029] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.

[0030] [Nb: 0 to 0.050%] Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel, contributing to the refinement of the structure through a pinning effect and thus to the increase in strength of the steel sheet. The Nb content may be 0%, but to achieve this effect, the Nb content is preferably 0.001% or more. The Nb content may be 0.005% or more, 0.010% or more, 0.012% or more, 0.015% or more, or 0.020% or more. On the other hand, excessive Nb content may cause the formation of coarse carbides in the steel, reducing the ductility of the steel sheet. Therefore, the Nb content is preferably 0.050% or less. The Nb content may be 0.040% or less, 0.030% or less, or 0.025% or less.

[0031] [V: 0 to 1.000%] V is an element that contributes to improving strength through precipitation strengthening and the like. The V content may be 0%, but to obtain such an effect, the V content is preferably 0.001% or more. The V content may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, even if V is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the V content is preferably 1.000% or less. The V content may be 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[0032] [Cr: 0 to 2.00%] Cr is an element that improves the hardenability of steel and contributes to improving its strength. The Cr content may be 0%, but to achieve this effect, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive Cr content may saturate the effect and increase manufacturing costs. Therefore, the Cr content is preferably 2.00% or less. The Cr content may be 1.50% or less, 1.00% or less, 0.90% or less, 0.80% or less, 0.70% or less, 0.60% or less, 0.50% or less, 0.40% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0033] [Ni: 0 to 2.00%] [Cu: 0 to 2.00%] Ni and Cu are elements that contribute to improving hardenability or strength by solid solution strengthening. The Ni and Cu contents may be 0%, but to achieve these effects, the contents of these elements are preferably 0.001% or more, and may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive inclusion of these elements may saturate the effects and increase manufacturing costs. Therefore, the Ni and Cu contents are preferably 2.00% or less, and may be 1.50% or less, 1.00% or less, 0.90% or less, 0.80% or less, 0.70% or less, 0.60% or less, 0.50% or less, 0.40% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0034] [Mo: 0 to 1.000%] Mo is an element that improves the hardenability of steel and contributes to improving strength. The Mo content may be 0%, but to achieve this effect, the Mo content is preferably 0.001% or more. The Mo content may be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, excessive Mo content may increase deformation resistance during hot working and increase equipment load. Therefore, the Mo content is preferably 1.000% or less. The Mo content may be 0.900% or less, 0.800% or less, 0.700% or less, 0.600% or less, 0.500% or less, 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[0035] [B: 0 to 0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness. While the B content may be 0%, to achieve this effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, excessive B content may saturate the effect and increase manufacturing costs. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0050% or less, 0.0040% or less, 0.0030% or less, 0.0020% or less, 0.0015% or less, 0.0012% or less, 0.0010% or less, 0.0009% or less, 0.0008% or less, 0.0007% or less, or 0.0006% or less.

[0036] [Sn: 0 to 1.000%] [Sb: 0 to 1.000%] Sn and Sb are elements effective in improving corrosion resistance. The Sn and Sb contents may be 0%, but to obtain such effects, the contents of these elements are preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, excessive inclusion of these elements may result in a decrease in toughness. Therefore, the Sn and Sb contents are preferably 1.000% or less, and may be 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.080% or less.

[0037] [Ca: 0 to 0.0100%] [Mg: 0 to 0.0100%] [Hf: 0 to 0.0100%] Ca, Mg, and Hf are elements that can control the morphology of non-metallic inclusions. The Ca, Mg, and Hf contents may be 0%, but to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, or 0.0010% or more. On the other hand, even if these elements are contained in excess, the effects saturate, and including more than necessary in the steel sheet increases manufacturing costs. Therefore, the Ca, Mg, and Hf contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0038] [Bi: 0 to 0.010%] Bi is an element effective in improving corrosion resistance. The Bi content may be 0%, but to obtain this effect, the Bi content is preferably 0.001% or more. The Bi content may be 0.002% or more or 0.003% or more. On the other hand, even if an excessive amount of Bi is contained, the effect saturates, and containing more Bi than necessary in the steel sheet increases the manufacturing cost. Therefore, the Bi content is preferably 0.010% or less. The Bi content may be 0.005% or less or 0.004% or less.

[0039] [REM: 0 to 0.0100%] REM is an element that can control the morphology of nonmetallic inclusions. While the REM content may be 0%, to achieve this effect, the REM content is preferably 0.0001% or more. The REM content may be 0.0005% or more or 0.0010% or more. On the other hand, if REM is contained in an excessive amount, the effect saturates, and adding more REM than necessary to the steel sheet increases manufacturing costs. Therefore, the REM content is preferably 0.0100% or less. The REM content may be 0.0050% or less, 0.0030% or less, or 0.0020% or less. In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic numbers 57 to lutetium (Lu) with atomic numbers 71. The REM content is the total content of these elements.

[0040] [As: 0 to 0.010%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain this effect, the As content is preferably 0.001% or more. The As content may be 0.002% or more or 0.003% or more. On the other hand, even if an excessive amount of As is contained, the effect saturates, and containing more As than necessary in the steel sheet increases the manufacturing cost. Therefore, the As content is preferably 0.010% or less. The As content may be 0.008% or less or 0.005% or less.

[0041] [Zr: 0 to 0.010%] Zr is an element that can control the morphology of non-metallic inclusions. The Zr content may be 0%, but to obtain this effect, the Zr content is preferably 0.001% or more. The Zr content may be 0.002% or more or 0.003% or more. On the other hand, even if Zr is contained in an excessive amount, the effect saturates, and adding more Zr than necessary to the steel sheet increases the manufacturing cost. Therefore, the Zr content is preferably 0.010% or less. The Zr content may be 0.008% or less or 0.005% or less.

[0042] [Co: 0 to 2.000%] Co is an element that contributes to improving hardenability and / or heat resistance. The Co content may be 0%, but to obtain these effects, the Co content is preferably 0.001% or more. The Co content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Co content may deteriorate hot workability and increase raw material costs. Therefore, the Co content is preferably 2.000% or less. The Co content may be 1.000% or less, 0.500% or less, 0.300% or less, or 0.200% or less.

[0043] [Zn: 0 to 0.010%] Zn is an element that can be contained in steel sheet when scrap or the like is used as the steel raw material. Therefore, the Zn content is preferably 0.010% or less, and may be 0.008% or less, or 0.005% or less. The Zn content may be 0%, but reducing it to less than 0.001% requires a long refining time, resulting in a decrease in productivity. Therefore, the Zn content may be 0.001% or more, 0.002% or more, or 0.003% or more.

[0044] [W: 0 to 1.000%] W is an element that improves the hardenability of steel and contributes to improving strength. The W content may be 0%, but to obtain this effect, the W content is preferably 0.001% or more. The W content may be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive W content may reduce weldability. Therefore, the W content is preferably 1.000% or less. The W content may be 0.800% or less, 0.500% or less, 0.300% or less, or 0.200% or less.

[0045] In the steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. The impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing the steel sheet.

[0046] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0047] [Metal Structure] The metal structure of the steel sheet according to the present invention includes, by area percentage, 40-80% ferrite, 15-55% bainite, and 5-20% martensite. By configuring the metal structure of the steel sheet to primarily contain these three structures, it is possible to increase the strength of the steel sheet while improving the hole expandability and yield ratio. In addition to containing these three structures at the specific area percentages described above, by utilizing ferrite precipitation strengthening, which will be described in detail later, it is possible to further increase the strength of the steel sheet and appropriately reduce the hardness difference in the metal structure. For example, if the area percentage of ferrite is low, the proportion of hard phases bainite and martensite, particularly the proportion of bainite, will be high. Therefore, even with ferrite precipitation strengthening, the hardness difference in the metal structure, more specifically, the hardness difference between ferrite and bainite, may not be appropriately reduced. In such cases, the desired hole expandability and / or yield ratio may not be achieved. Therefore, the area fraction of ferrite needs to be 40% or more, and may be, for example, 45% or more, 50% or more, or 55% or more. On the other hand, if the area fraction of ferrite is too high, the proportions of the hard phases bainite and martensite will be low, and as a result, it may be impossible to achieve a desired strength, for example, a tensile strength of 780 MPa or more. Therefore, the area fraction of ferrite is set to 80% or less, and may be, for example, 75% or less, 70% or less, 65% or less, or 60% or less.

[0048] From the viewpoint of improving tensile strength, it is preferable that the area fractions of the hard phases, bainite and martensite, are high. From this viewpoint, for example, the area fraction of bainite may be 18% or more, 20% or more, 22% or more, 25% or more, 28% or more, 30% or more, 32% or more, or 35% or more. Similarly, the area fraction of martensite may be 8% or more, 10% or more, or 12% or more. On the other hand, from the viewpoint of reducing the difference in hardness in the metal structure and further improving the hole expandability and yield ratio, it is preferable that the area fractions of bainite and martensite are low. From this viewpoint, for example, the area fraction of bainite may be 52% or less, 50% or less, 48% or less, 45% or less, 42% or less, 40% or less, or 38% or less. Similarly, the area fraction of martensite may be 18% or less, 16% or less, or 14% or less.

[0049] As described above, the metal structure of the steel plate according to the embodiment of the present invention includes ferrite, bainite, and martensite, and may also include other remaining structures. However, the area ratio of the remaining structure is preferably small, and may even be 0%. The area ratio of the remaining structure is not particularly limited, and may be, for example, 0 to 5%, 0 to 4%, or 0 to 3%. In other words, the total area ratio of ferrite, bainite, and martensite may be, for example, 95 to 100%, 96 to 100%, or 97 to 100%. The lower limit of the remaining structure may be 1% or 2%. When a remaining structure is present, the remaining structure may include at least one of pearlite and retained austenite, or may be at least one of them.

[0050] [Identification of Metallic Structure and Calculation of Area Ratio] The structure is observed using a scanning electron microscope. Prior to observation, the sample for structure observation is wet-polished with emery paper and diamond abrasives with an average particle size of 1 μm, and the observation surface is mirror-finished. The structure is then etched with a 3% nitric acid alcohol solution. The observation magnification is 2000x, and 10 random images are taken of a 30 μm x 40 μm field of view at a position 1 / 4 of the plate thickness from the surface. The structure ratio is determined using the point counting method. A total of 225 lattice points, spaced 3 μm vertically and 4 μm horizontally, are defined for the obtained structure image, and the structure present below the lattice points is identified. The structure ratio contained in the steel is calculated from the average value of the 10 images. Ferrite is a massive crystal grain that does not contain iron-based carbides with a major axis of 100 nm or more. Bainite is a collection of lath-shaped crystal grains that either does not contain iron-based carbides with a major axis of 20 nm or more, or contains iron-based carbides with a major axis of 20 nm or more, where the carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction. Here, a group of iron-based carbides elongated in the same direction refers to iron-based carbides whose elongation directions differ by no more than 5°. Bainite grains surrounded by grain boundaries with a misorientation of 15° or more are counted as one bainite grain. Furthermore, martensite, which contains a large amount of solute carbon, is brighter and appears whiter than other structures, making it distinguishable from other structures. When structures other than ferrite, bainite, and martensite are present, the area fraction of the remaining structure is determined by subtracting the total area fraction of ferrite, bainite, and martensite from 100%. Although it is not necessary to specifically identify the remaining structure, when the remaining structure includes pearlite and retained austenite, the pearlite has a unique structure in which cementite precipitates in a lamellar form, and therefore can be identified using a scanning electron microscope. Furthermore, the volume fraction of the retained austenite can be calculated by X-ray diffraction measurement, and since the volume fraction of the retained austenite is equivalent to the area fraction, this can be used as the area fraction of the retained austenite.

[0051] [Number density of TiC precipitates with a diameter of 2.0 to 8.0 nm in ferrite: 1.0 × 10 16 pieces / cm 3In the steel sheet according to the embodiment of the present invention, TiC precipitates having a diameter of 2.0 to 8.0 nm are present in ferrite at a concentration of 1.0 × 10 16 pieces / cm 3 The TiC precipitates are present at a number density of 2.0 to 8.0 nm or more. Here, TiC precipitates include not only TiC but also composite carbides containing Ti and elements other than Ti, such as V and Nb. The method for identifying TiC precipitates with a diameter of 2.0 to 8.0 nm in ferrite and the method for measuring their number density will be described later. By allowing TiC precipitates with a diameter of 2.0 to 8.0 nm to exist in ferrite at such a number density, the hardness of the ferrite can be increased through precipitation strengthening. More specifically, by increasing the hardness of ferrite and reducing the difference in hardness with bainite, a hard phase that is relatively abundant in the metal structure, the difference in hardness in the metal structure primarily composed of ferrite, bainite, and martensite can be reduced. As a result, the hole expandability and yield ratio of the steel sheet can be further improved. Naturally, precipitation strengthening by TiC precipitates also contributes to improving the overall strength of the steel sheet. If the diameter of the TiC precipitates is smaller than 2.0 nm, the TiC precipitates cannot adequately act as obstacles to dislocation motion, and therefore the effect of improving the hardness of ferrite through precipitation strengthening cannot be fully achieved. In addition, the effect of improving the strength of the steel sheet may not be fully achieved. On the other hand, if the diameter of the TiC precipitates is too large, the desired precipitation strengthening in ferrite may not be achieved.

[0052] Without intending to be bound by any particular theory, this is thought to be because the strengthening mechanism changes in relation to dislocation motion as TiC precipitates become coarser. For example, dislocation lines no longer pass across the TiC precipitates, but instead pass through them, leaving loops of dislocation lines around the coarse TiC precipitates, resulting in a smaller amount of precipitation strengthening. In addition, as the TiC precipitates become coarser, the number density of the TiC precipitates also decreases significantly, making it impossible to sufficiently increase the hardness of ferrite through precipitation strengthening. Therefore, in order to effectively increase the hardness of ferrite through precipitation strengthening, it is effective to control the diameter of the TiC precipitates to a range of 2.0 to 8.0 nm. In order to improve the hardness of ferrite to a desired level through precipitation strengthening, it is important to control the number density of TiC precipitates having such a diameter within a predetermined range. From this perspective, TiC precipitates with a diameter of 2.0 to 8.0 nm are incorporated into ferrite at a density of 1.0 × 10 as described above. 16 pieces / cm 3 In order to further enhance the effect of improving the hardness of ferrite, the higher the number density, the more preferable. 16 pieces / cm 3 That's it, 1.5 x 10 16 pieces / cm 3 That's it, 2.0 x 10 16 pieces / cm 3 That's it, 5.0 x 10 16 pieces / cm 3 or more or 10.0 x 10 16 pieces / cm 3 On the other hand, since there is a limit to the contents of C and Ti, which are the supply sources of TiC precipitates, if the number density becomes too high, it may become difficult to control the diameter of the TiC precipitates within a desired range. Therefore, the number density is not particularly limited as long as the diameter is 2.0 to 8.0 nm, but may be, for example, 75.0 × 10 16 pieces / cm 3 Below, 50.0 x 10 16 pieces / cm 3 Below, 30.0 x 10 16 pieces / cm 3 or less or 20.0 x 10 16 pieces / cm3 In the steel sheet according to the embodiment of the present invention, when measured by a three-dimensional atom probe measurement method described in detail later, the number of TiC precipitates having a diameter of 2.0 to 8.0 nm is 1.0 × 10 16 pieces / cm 3 It is sufficient that the particles are present in the ferrite at the above number density, and therefore, as long as the above requirements for diameter and number density are satisfied, for example, coarse TiC precipitates may be present in the ferrite.

[0053] [Calculation of Diameter and Number Density of TiC Precipitates] The diameter and number density of TiC precipitates are calculated using a three-dimensional atom probe measurement method as follows. First, a needle-shaped sample is prepared from the sample to be measured by cutting and electropolishing, optionally using a focused ion beam (FIB) processing method in combination with electropolishing, with at least one end of the rod-shaped sample being sharpened to a point. The needle-shaped sample is prepared using the method used to identify ferrite in the metallographic structure identification and area fraction calculation described above, so that ferrite falls within the range of the three-dimensional atom probe measurement. Five or more samples are measured, and the number density of TiC precipitates with a diameter of 2.0 to 8.0 nm is calculated from the average of the measurement results for three samples closest to the average of the five measurement results. In three-dimensional atom probe measurement, the accumulated data can be reconstructed to obtain an actual atomic distribution image in real space. In the case of TiC precipitates with a Na-Cl structure, the unit cell is 4.33 Å, so the interatomic distance between Ti and Ti is 4.33 × √2 = 6.1 Å. Therefore, if multiple Ti atoms exist at approximately the same coordinate position (7 Å or less), these Ti atoms are determined to be in the same precipitate, and the number of Ti atoms determined to be in the same precipitate is counted. If this number is 50 or more, the precipitate is determined to be a TiC precipitate. The diameter of the TiC precipitate is the circle-equivalent diameter calculated from the number of Ti atoms constituting the observed TiC precipitate and the lattice constant of the TiC precipitate, assuming that the TiC precipitate is spherical. The following describes a method for determining the diameter (circle-equivalent diameter) R of the TiC precipitate using the number of Ti atoms in the TiC precipitate obtained by three-dimensional atom probe measurement. Three-dimensional atom probe measurement measures the number N of all atoms in a target sample, but in reality, it is not possible to detect all atoms N in a target sample using three-dimensional atom probe measurement. Since each device has its own specific atomic detection rate α (= number of detected atoms / total number of atoms), the number N of atoms that would have been present can be calculated from the actual measurement value n. In other words, the total number of atoms N = n / α.Next, with respect to the total number of atoms N, it is assumed that the TiC precipitate of the Na—Cl structure has eight Ti atoms in the unit lattice, and the lattice constant a of the Na—Cl structure is 4.33 Å, and the diameter (circle-equivalent diameter) R of the TiC precipitate is calculated using the following formula: Diameter of TiC precipitate R = {(6 / 8) × (1 / π) × N × a}. 3} (1 / 3) Next, for each of the five samples, the number of TiC precipitates with a diameter R in the range of 2.0 to 8.0 nm is measured among the diameters R of the TiC precipitates calculated by the above formula. Then, the number density of TiC precipitates with a diameter of 2.0 to 8.0 nm is calculated for each of the five samples, using the measurement field of view (the range in which atoms are ionized from the sample surface during three-dimensional atom probe measurement) as the denominator and the number of TiC precipitates with a diameter in the range of 2.0 to 8.0 nm as the numerator. Then, as described above, the average value of the measurement results (number density of TiC precipitates with a diameter of 2.0 to 8.0 nm) for the three samples closest to the average value of the measurement results (number density of TiC precipitates with a diameter of 2.0 to 8.0 nm) for the five samples is taken as the number density of TiC precipitates with a diameter of 2.0 to 8.0 nm.

[0054] [Standard Deviation in Hardness of Ferrite and Bainite: 0.40 GPa or Less] In the steel sheet according to the embodiment of the present invention, the standard deviation in hardness of ferrite and bainite is controlled to 0.40 GPa or less. By controlling the standard deviation in hardness of ferrite and bainite within this range, the variation in hardness of ferrite and bainite in the metal structure can be reduced. Therefore, by combining this with the reduction in the hardness difference between bainite and ferrite due to the use of TiC precipitates as described above, not only can the hardness difference be reduced throughout the metal structure, but also the increase in local hardness differences in the metal structure can be reliably reduced, resulting in a more significant improvement in the hole expandability of the steel sheet. From the viewpoint of further improving the hole expandability, the lower the standard deviation in hardness of ferrite and bainite, the more preferable it may be, for example, 0.38 GPa or less, 0.35 GPa or less, 0.32 GPa or less, 0.30 GPa or less, 0.28 GPa or less, or 0.26 GPa or less. The lower limit is not particularly limited, but for example, the standard deviation of the hardness of ferrite and bainite may be 0.05 GPa or more, 0.10 GPa or more, or 0.15 GPa or more.

[0055] [Method for determining the standard deviation in the hardness of ferrite and bainite] The standard deviation in the hardness of ferrite and bainite is determined as follows. First, a sample is cut out from the steel plate so that a cross section of the plate thickness perpendicular to the surface can be observed. The cross section of the sample is wet polished with emery paper and polished with diamond abrasive grains having an average particle size of 1 μm to a mirror finish. The mirror-finished cross section is subjected to a test in accordance with ISO 14577-1:2015 using a microhardness tester in a 30 μm × 30 μm region including ferrite and bainite at a depth of 1 / 4 of the plate thickness from the surface. Specifically, a triangular pyramidal indenter is indented with a load of 1000 μN, and the nanoindentation hardness is measured, obtaining a total of 25 measured values. Next, indentations were made in different regions at a depth of 1 / 4 of the plate thickness from the surface using a microhardness tester with a triangular pyramidal indenter at a load of 1000 μN, and nanoindentation hardness was measured, obtaining a total of 25 measured values. Finally, the standard deviation of the hardness of ferrite and bainite was determined based on the total of 50 nanoindentation hardness measurements obtained. The 30 μm × 30 μm region containing ferrite and bainite can be identified by measuring the same sample in advance using a scanning electron microscope (SEM). Specifically, in SEM observation, martensite containing a large amount of solute carbon appears brighter and whiter than other structures. Therefore, a 30 μm × 30 μm region that does not include such bright, white-appearing regions can be identified in advance using SEM observation, and this region can be measured using the microhardness tester described above.

[0056] [Average grain size of ferrite: 5.0 μm or less] In the steel sheet according to the embodiment of the present invention, the average grain size of ferrite is preferably 5.0 μm or less. By controlling the average grain size of ferrite within such a fine range, it is possible to further improve the strength, hole expandability, and / or yield ratio of the steel sheet. The average grain size of ferrite may be 4.5 μm or less, 4.2 μm or less, 4.0 μm or less, 3.8 μm or less, 3.5 μm or less, 3.2 μm or less, 3.0 μm or less, or 2.8 μm or less. The lower limit is not particularly limited, but for example, the average grain size of ferrite may be 0.5 μm or more, 1.0 μm or more, 1.5 μm or more, or 2.0 μm or more.

[0057] [Measurement of Average Grain Size of Ferrite] The average grain size of ferrite in the steel sheet according to the embodiment of the present invention is determined by analyzing the ferrite separated by the point counting method performed in the identification of the metallographic structure and calculation of the area ratio described above using an SEM-EBSD device (for example, JSM-7001F manufactured by JEOL Ltd.). Specifically, the average grain size of ferrite is determined by the following formula (1) by analyzing the grain size of the BCC phase defined by high-angle grain boundaries with a tilt angle of 15° or more using the SEM-EBSD device for ferrite separated by the point counting method after taking five images in connection with the identification of the metallographic structure and calculation of the area ratio. In the above formula, d represents the average grain size of ferrite, Ai represents the area of ​​the i-th grain (i = 1, 2, ..., N), di represents the circle equivalent diameter of the i-th grain, and N represents the number of grains included in the evaluation area of ​​the average grain size of ferrite.

[0058] [Thickness] The steel sheet according to the embodiment of the present invention is not particularly limited, but generally has a thickness of 1.0 to 8.0 mm. For example, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less. The steel sheet according to the embodiment of the present invention encompasses various steel sheets, and may be, for example, a hot-rolled steel sheet, a cold-rolled steel sheet, or a hot-rolled steel sheet or a cold-rolled steel sheet that has been subjected to a surface treatment such as plating.

[0059] As described above, the steel sheet according to the embodiment of the present invention can achieve a high yield ratio and particularly excellent hole expandability despite its high strength. Therefore, the steel sheet according to the embodiment of the present invention can reliably achieve a high level of the contradictory properties of high strength and excellent workability, while also achieving excellent impact resistance. Therefore, the steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields where these properties are required, and is particularly useful in the automotive field. In a preferred embodiment, an automotive part, particularly an automotive suspension part, is provided that includes the steel sheet according to the embodiment of the present invention. Examples of automotive suspension parts include lower arms and trailing arms. These automotive parts, particularly automotive suspension parts, may include the steel sheet according to the embodiment of the present invention in at least a portion thereof, and therefore, at least a portion of these parts will satisfy the chemical composition and metallographic characteristics described above. In portions of the steel sheet that are not in direct contact with the mold during forming, such as press forming, and that are subjected to a relatively small degree of processing, the characteristics of the metallographic structure do not change significantly before and after forming.

[0060] [Mechanical Properties] [Tensile Strength: TS] Steel sheets having the above-described chemical composition and metallographic structure can achieve high tensile strength, specifically, tensile strength of 780 MPa or more. The tensile strength is preferably 800 MPa or more, 820 MPa or more, or 840 MPa or more. Despite having such extremely high tensile strength, steel sheets according to embodiments of the present invention can achieve improved hole expandability and a high yield ratio due to the specific combination of chemical composition and metallographic structure described above. The upper limit of tensile strength is not particularly limited, but the tensile strength of the steel sheet may be, for example, 1180 MPa or less, 980 MPa or less, 940 MPa or less, 900 MPa or less, or 860 MPa or less. Tensile strength is measured by taking a JIS No. 5 test piece in a direction (C direction) where the longitudinal direction of the test piece is preferably parallel to the rolling direction of the steel sheet, and conducting a tensile test in accordance with JIS Z 2241:2022. When the rolling direction of the steel plate cannot be specified, a JIS No. 5 test piece may be taken from any direction within the surface of the steel plate.

[0061] [Yield Ratio: YR] Steel sheets having the above chemical composition and metallographic structure can achieve not only high tensile strength but also a high yield ratio; more specifically, a yield ratio of 0.70 or more can be achieved. The yield ratio is preferably 0.75 or more, more preferably 0.80 or more. There is no particular upper limit, but the yield ratio may be 0.90 or less or 0.85 or less, for example. The yield ratio is determined by the following formula based on the tensile strength and 0.2% proof stress measured by taking a JIS No. 5 test piece in a direction (C direction) where the longitudinal direction of the test piece is preferably parallel to the rolling direction perpendicular to the rolling direction of the steel sheet, and conducting a tensile test in accordance with JIS Z 2241:2022. When the rolling direction of the steel sheet cannot be specified, the JIS No. 5 test piece may be taken from any direction within the steel sheet plane. Yield ratio YR = 0.2% proof stress / tensile strength TS

[0062] [Hole expansion ratio: λ] Steel sheets having the above chemical composition and metallographic structure can achieve high hole expandability, specifically, a hole expansion ratio of 80.0% or more. The hole expansion ratio is preferably 85.0% or more, more preferably 90.0% or more. There is no particular upper limit to the hole expansion ratio, but for example, the hole expansion ratio may be 150.0% or less, 130.0% or less, or 120.0% or less. The hole expansion ratio is determined as follows by conducting a hole expansion test in accordance with JIS Z 2256:2020. First, a test piece having a width of 100 mm and a length of 100 mm is taken from the steel sheet, and a punched hole (initial hole: hole diameter D o Next, the initial hole is expanded with a conical punch having an apex angle of 60° until a crack penetrating the plate thickness occurs, with the burr facing the die side. h The hole expansion ratio λ (%) of each test piece is calculated using the following formula: λ = (D mm) = (D h -D o ) / D o ×100

[0063] <Method for Manufacturing Steel Sheet> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below. More specifically, although the following specifically describes the manufacture of a hot-rolled steel sheet, the steel sheet according to an embodiment of the present invention encompasses any steel sheet having the chemical composition and metallographic structure described above, i.e., not only a hot-rolled steel sheet, but also a cold-rolled steel sheet, a plated steel sheet, and the like. Therefore, the following description merely describes one example of a preferred manufacturing method when the steel sheet according to an embodiment of the present invention is a hot-rolled steel sheet.

[0064] A method for producing a steel plate according to an embodiment of the present invention includes a hot rolling process that includes heating a slab having the chemical composition described above in relation to the steel plate and then finish rolling the slab, and satisfies the following conditions (a) to (e): (a) the heating temperature of the slab is 1200 to 1300°C; (b) the holding time in the temperature range of 1200 to 1300°C is 1000 to 4000 seconds; (c) the finish rolling is performed using a tandem rolling mill consisting of five or more rolling stands, and the total reduction in the rolling passes of the front stages other than the last three stages is 60 to 90%; (d) the total reduction in the rolling passes of the last three stages is more than 50%; and (e) the finish rolling temperature is 900 to 1000°C. The method is characterized by including an intermediate air-cooling step in which the finish-rolled steel sheet is primarily cooled to an intermediate air-cooling temperature of 705 to 750°C at an average cooling rate of 50 to 200°C / sec, and then intermediate air-cooled for 3 to 10 seconds, and a cooling step in which the intermediate air-cooled steel sheet is secondarily cooled at an average cooling rate of 50 to 200°C / sec, and then coiled at a coiling temperature of 20 to 300°C. Each step will be described in detail below.

[0065] [Hot Rolling Process] [(a) Slab Heating Temperature: 1200-1300°C] [(b) Holding Time in the 1200-1300°C Temperature Range: 1000-4000 Seconds] First, a slab having the chemical composition described above in relation to the steel sheet is heated. From the viewpoint of productivity, the slab is preferably cast by a continuous casting method, but may also be produced by an ingot casting method or a thin slab casting method. The slab used contains a relatively large amount of alloying elements to obtain a high-strength steel sheet. Therefore, the slab must be heated before being subjected to hot rolling to dissolve the alloying elements in the slab. If the heating temperature is too low, the alloying elements may not dissolve sufficiently in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. Therefore, the heating temperature is preferably 1200°C or higher. The upper limit of the heating temperature is not particularly limited, but is preferably 1300°C or lower from the viewpoints of the capacity of the heating equipment and productivity. Furthermore, by setting the holding time in the temperature range of 1200 to 1300°C to 1000 seconds or more, the alloying elements can be reliably dissolved in the slab. There is no particular upper limit to the holding time, but from the viewpoint of productivity, it is preferably 4000 seconds or less. When rough rolling is performed, holding in the temperature range of 1200 to 1300°C may be performed after rough rolling.

[0066] [Rough rolling] In this method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0067] [(c) Total Reduction in Rolling Passes Prior to Finish Rolling: 60 to 90%] The heated slab, or the slab that has been subjected to rough rolling as needed, is then subjected to finish rolling. In this manufacturing method, finish rolling is performed using a tandem rolling mill consisting of five or more rolling stands, more specifically, 5 to 8 rolling stands. In this manufacturing method, in the finish rolling performed on the heated slab, the total reduction in the rolling passes prior to the last three stands (final three stands) is controlled to 60 to 90%. By performing rolling at such a high reduction in the prior rolling passes, recrystallization can be promoted to refine the metal structure, and in particular, the average grain size of ferrite in the final metal structure can be reduced. Refining the metal structure through such recrystallization is highly advantageous in forming a desired metal structure and improving properties such as hole expandability and yield ratio. If the total reduction ratio in the preceding rolling passes is less than 60%, the desired metal structure containing ferrite, bainite, and martensite in specific proportions cannot be obtained, and properties such as hole expandability and / or yield ratio may be reduced. Therefore, the total reduction ratio in the preceding rolling passes is set to 60% or more, preferably 70% or more. On the other hand, if the total reduction ratio in the preceding rolling passes is too high, the rolling load becomes excessive, increasing the load on the rolling mill. For this reason, the total reduction ratio in the preceding rolling passes is set to 90% or less.

[0068] [(d) Total Reduction in the Last Three Rolling Passes of Finish Rolling: More than 50%] In the finish rolling of this manufacturing method, the total reduction in the last three rolling passes is controlled to more than 50%. By performing rolling under a relatively high pressure of more than 50% even in the last rolling pass of finish rolling, recrystallization can be further promoted and austenite grains can be refined. In this regard, it is possible to reduce the variation in hardness of ferrite and bainite in the final metal structure to within a predetermined range, more specifically, to control the standard deviation in hardness of ferrite and bainite to 0.40 GPa or less. More specifically, ferrite nucleates primarily from austenite grain boundaries. Therefore, by promoting recrystallization and refining austenite grains, the number of austenite grain boundaries can be increased, thereby increasing the number of ferrite nucleation sites. In this regard, it is possible to increase the ferrite growth rate in the subsequent intermediate air-cooling step, allowing ferrite to be generated all at once in a relatively short period of time. Furthermore, in the intermediate cooling step, not only ferrite but also TiC precipitates are formed, as will be described in detail later. While not intending to be bound by any particular theory, it is believed that by rapidly forming ferrite in a short period of time during the intermediate cooling step, it is possible to suppress variations in the diameter and number density of TiC precipitates formed in ferrite during the same intermediate cooling step. As a result, it is believed that it is possible to control the standard deviation in hardness of ferrite and bainite in the finally obtained metal structure to 0.40 GPa or less.

[0069] If the reduction ratio in each of the last three rolling passes is 50% or less, recrystallization cannot be sufficiently promoted, resulting in insufficient refinement of austenite grains. As a result, in the finally obtained metal structure, it becomes impossible to control the standard deviation in the hardness of ferrite and bainite to 0.40 GPa or less. The total reduction ratio in the last three rolling passes of finish rolling is preferably 55% or more or 60% or more. There is no particular upper limit, but the total reduction ratio in the last three rolling passes of finish rolling may be, for example, 90% or less or 85% or less.

[0070] [(e) End Temperature of Finish Rolling: 900 to 1000°C] In this manufacturing method, in addition to controlling the reduction ratios in the front and rear stages of finish rolling, the end temperature of finish rolling is also important for controlling the metallographic structure of the steel sheet. If the end temperature of finish rolling is low, the metallographic structure may become nonuniform, resulting in reduced strength and / or the variation in the hardness of ferrite and bainite may not be reduced to within the desired range, resulting in reduced hole expandability. For this reason, the end temperature of finish rolling is set to 900°C or higher. Preferably, the end temperature of finish rolling is set to 920°C or higher. On the other hand, if the end temperature of finish rolling is too high, the austenite grains after recrystallization become coarse, the number of austenite grain boundaries decreases, and the number of ferrite nucleation sites decreases. As a result, the growth rate of ferrite cannot be increased in the subsequent intermediate air-cooling process, and the standard deviation in the hardness of ferrite and bainite in the final metallographic structure may not be able to be controlled to 0.40 GPa or less. Therefore, the finishing temperature of the finish rolling is set to 1000° C. or less, and preferably, the finishing temperature of the finish rolling is set to 980° C. or less.

[0071] [Intermediate Air-Cooling Process] In the next intermediate air-cooling process, the finish-rolled steel sheet is primarily cooled on a run-out table (ROT) at an average cooling rate of 50 to 200°C / s to an intermediate air-cooling temperature of 705 to 750°C, and then intermediate air-cooled for 3 to 10 seconds. Primary cooling to an intermediate air-cooling temperature of 705 to 750°C at an average cooling rate of 50 to 200°C / s suppresses excessive ferrite formation and / or ferrite coarsening, while promoting the precipitation of TiC precipitates by the subsequent intermediate air-cooling at high temperatures. As a result, the ferrite is sufficiently precipitation-strengthened, thereby reducing the difference in hardness between ferrite and bainite and improving the hole expandability and yield ratio. More specifically, in order to promote the formation and grain growth of TiC precipitates during intermediate air-cooling and sufficiently precipitation-strengthen the ferrite, the intermediate air-cooling temperature must be set to a relatively high temperature range, i.e., 705 to 750°C. However, in this case, excessive ferrite is generated, resulting in a ferrite area ratio exceeding 80% in the final metal structure and / or coarsening of the ferrite, making it impossible to obtain the desired properties. Therefore, in the present manufacturing method, the average cooling rate in the primary cooling from finish rolling to the intermediate air-cooling temperature is set to 50°C / s or more to suppress excessive ferrite generation and ensure sufficient precipitation of TiC precipitates by the subsequent intermediate air-cooling at high temperatures. If the average cooling rate in the primary cooling is less than 50°C, excessive ferrite may be generated and / or coarsening of the ferrite may occur. Alternatively, TiC precipitates may coarsen during the subsequent intermediate air-cooling at high temperatures. On the other hand, if the average cooling rate in the primary cooling exceeds 200°C, ferrite generation is excessively suppressed, resulting in a ferrite area ratio of less than 40% in the final metal structure, resulting in a deterioration in properties such as hole expandability. Therefore, the average cooling rate in the primary cooling is set to 200°C / s or less, preferably 160°C / s or less. Although this step is called the intermediate cooling step for convenience, it is a step including primary cooling and intermediate cooling as described above.

[0072] If the intermediate cooling temperature exceeds 750°C or the intermediate cooling time exceeds 10 seconds, excessive ferrite may be formed or TiC precipitates may become coarse. Excessive ferrite formation makes it impossible to form the desired metal structure containing ferrite, bainite, and martensite in a specific ratio in the final steel sheet. Furthermore, coarsening of TiC precipitates may significantly reduce the number density of the TiC precipitates. In such cases, the effect of improving the hardness of ferrite through precipitation strengthening cannot be fully achieved. If the intermediate cooling temperature exceeds 800°C, the intermediate cooling temperature may be higher than the ferrite transformation point, making it difficult for ferrite to form. In this case, excessive bainite is formed, and precipitation strengthening of ferrite may not be able to adequately reduce the hardness difference in the metal structure, more specifically, the hardness difference between ferrite and bainite. As a result, the desired hole expandability and / or yield ratio cannot be achieved. On the other hand, if the intermediate cooling temperature is less than 705°C or the intermediate cooling time is less than 3 seconds, the formation and grain growth of TiC precipitates are suppressed, and the desired diameter and / or number density cannot be obtained. Similarly, in this case, the effect of improving the hardness of ferrite through precipitation strengthening cannot be fully achieved. In addition, ferrite formation may be excessively suppressed, which may result in the failure to form the desired metal structure containing ferrite, bainite, and martensite in specific proportions in the final steel sheet. Furthermore, as described above, the suppression of the formation and grain growth of TiC precipitates and the excessive suppression of ferrite formation may prevent the desired amount of ferrite from being properly precipitation strengthened. In such cases, it may be impossible to reduce the variation in hardness of ferrite and bainite to within the desired range.

[0073] On the other hand, in the intermediate cooling step, primary cooling is performed to an intermediate cooling temperature of 705 to 750°C at an average cooling rate of 50 to 200°C / sec, preferably 50 to 160°C / sec, and then intermediate cooling is performed for 3 to 10 seconds, preferably 4 to 9 seconds, thereby precipitating ferrite at a desired rate and generating TiC precipitates in the ferrite, which then undergo appropriate grain growth to finally produce 1.0 x 10 TiC precipitates having a diameter of 2.0 to 8.0 nm. 16 pieces / cm 3 As a result, the hardness improvement effect of ferrite due to precipitation strengthening is fully exerted, and the difference in hardness between ferrite and bainite in the metal structure is reduced, making it possible to further increase the hole expandability and yield ratio.

[0074] [Cooling Process] In the next cooling process, the steel sheet after intermediate air cooling is secondarily cooled at an average cooling rate of 50 to 200°C / s, and then coiled at a coiling temperature (secondary cooling stop temperature) of 20 to 300°C. Coiling is performed immediately after the secondary cooling. By subjecting the steel sheet after intermediate air cooling to such a relatively fast average cooling rate, bainite and martensite can be appropriately precipitated, making it possible to form a metallographic structure containing ferrite, bainite, and martensite in specific proportions in the final steel sheet. In contrast, if the average cooling rate in the secondary cooling is less than 50°C / s, bainite and / or martensite cannot be appropriately precipitated, and therefore the desired metallographic structure cannot be obtained in the final steel sheet. In such cases, a tensile strength of 780 MPa or more cannot be achieved. Therefore, the average cooling rate in the secondary cooling is set to 50°C / s or more, preferably 70°C / s or more. On the other hand, if the average cooling rate in the secondary cooling exceeds 200°C, bainite will not be sufficiently formed and / or martensite will be excessively formed, and similarly, the desired metal structure will not be obtained in the finally obtained steel sheet. Therefore, the average cooling rate in the secondary cooling is set to 200°C / sec or less, and preferably 180°C / sec or less or 150°C / sec or less.

[0075] On the other hand, if the coiling temperature exceeds 300°C, martensite is not sufficiently generated, and therefore sufficient strength cannot be obtained. On the other hand, if the coiling temperature is too low, excessive water cooling or the like is required, which reduces productivity. It may also cause embrittlement of the steel sheet. Therefore, the coiling temperature is set to 20°C or higher. Note that, although this process is called the cooling process for convenience, it is a process that includes secondary cooling and coiling (excluding primary cooling) as described above.

[0076] According to the steel sheet manufactured by the above manufacturing method, by forming a metal structure containing ferrite, bainite, and martensite in specific proportions, it is possible to improve the hole expandability and yield ratio while maintaining the strength of the steel sheet at a relatively high level. 16 pieces / cm 3 Because TiC precipitates are present at a number density of 0.40 GPa or less, precipitation strengthening not only contributes to improving the overall strength of the steel sheet, but also sufficiently reduces the difference in hardness between the relatively abundant bainite phase in the hard structure and the softest ferrite phase in the three-phase structure, thereby further improving the hole expandability and yield ratio of the steel sheet. In addition, because the standard deviation in the hardness of ferrite and bainite is controlled to 0.40 GPa or less, this, combined with the reduction in the hardness difference between bainite and ferrite due to the use of TiC precipitates, not only reduces the hardness difference throughout the metal structure, but also reliably reduces the increase in local hardness differences within the metal structure, resulting in a more significant improvement in the hole expandability of the steel sheet. Therefore, steel sheets manufactured by the above-described manufacturing method are particularly useful in the automotive field, as they can be effectively used in components that require both high strength and excellent workability, and also in components that require impact resistance.

[0077] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0078] In the following examples, steel sheets according to the embodiments of the present invention were produced under various conditions, and the tensile strength (TS), hole expansion ratio (λ), and yield ratio (YR) of the obtained steel sheets were examined.

[0079] First, slabs having various chemical compositions shown in Table 1 were produced at a steelmaking plant. Then, after reheating to the heating temperatures shown in Table 2, rough rolling was performed to produce rough bars having a thickness of 30 mm. The rough bars were heated to 1250°C at an average heating rate of 20°C / s and held at that temperature for 3600 seconds. Then, using a rolling mill consisting of multiple rolling stands, finish rolling was performed with two or more rolling passes in the front stage and three rolling passes in the rear stage under the conditions shown in Table 2. The end temperatures of the finish rolling were as shown in Table 2. Next, the finish-rolled steel sheets were primarily cooled to an intermediate air-cooling temperature under the conditions shown in Table 2, and then intermediate air-cooled. Finally, the intermediate air-cooled steel sheets were secondarily cooled to a coiling temperature under the conditions shown in Table 2, and then coiled at the coiling temperature to obtain steel sheets having a thickness of 2.5 mm.

[0080]

[0081]

[0082] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0083] [Calculation of Diameter and Number Density of TiC Precipitates] The diameter and number density of TiC precipitates were calculated using the three-dimensional atom probe measurement method described in detail herein, with the device-specific atomic detection rate α set to 0.35. The detection rate α is calculated by dividing the number of detected atoms by the total number of original atoms, where the "total number of original atoms" corresponds to the number of atoms in a standard sample (the number of atoms of which is known in advance). In other words, the detection rate α of the device can be calculated by dividing the number of atoms detected from a standard sample using an actual device by the total number of original atoms in the standard sample.

[0084] [Tensile Strength (TS) and Yield Ratio (YR)] Tensile strength (TS) was measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the steel plate (C direction) and conducting a tensile test in accordance with JIS Z 2241:2022. The gauge length was 50 mm. More specifically, the test was conducted at room temperature in the range of 10 to 35°C, and a tensile test force was applied to the test piece, and strain was applied until fracture. The yield ratio (YR) was determined by the following formula based on the tensile strength (TS) and 0.2% proof stress measured by a tensile test in accordance with JIS Z 2241:2022 using a JIS No. 5 test piece. Yield ratio YR = 0.2% proof stress / tensile strength TS

[0085] [Hole Expanding Ratio: λ] The hole expanding ratio was determined by conducting a hole expanding test in accordance with JIS Z 2256:2020 as follows. First, a test piece having a thickness of 2.5 mm, a width of 100 mm, and a length of 100 mm was taken from a steel plate, and a punching tool having a punch diameter of 10 mm and a die diameter of 10.6 mm (clearance 12.5%) was used to punch a hole (initial hole: hole diameter D o Next, the initial hole was expanded with a conical punch having an apex angle of 60° until a crack penetrating the plate thickness occurred, with the burr facing the die side. h The hole expansion ratio λ (%) of each test piece was calculated using the following formula. This hole expansion test was performed three times, and the average value was determined as the hole expansion ratio λ. λ = (D h -D o ) / D o ×100

[0086] Steel sheets having a tensile strength (TS) of 780 MPa or more, a hole expansion ratio (λ) of 80.0% or more, and a yield ratio (YR) of 0.70 or more were evaluated as having high strength, high hole expandability, and high yield ratio. The results are shown in Table 3.

[0087]

[0088] With reference to Tables 1 to 3, in Comparative Example 11, the total reduction rate in the last three rolling passes of the finish rolling was low, which presumably prevented sufficient promotion of recrystallization and insufficient refinement of austenite grains. As a result, the standard deviation of the hardness of ferrite and bainite in the final metallographic structure exceeded 0.40 GPa, resulting in a decrease in λ. In Comparative Example 12, the total reduction rate in the first rolling passes other than the last three rolling passes of the finish rolling was low, which prevented the desired ferrite area ratio from being obtained. As a result, the difference in hardness between ferrite and bainite could not be appropriately reduced, resulting in a decrease in λ. In Comparative Example 13, the intermediate air-cooling temperature was low, which suppressed the formation and grain growth of TiC precipitates, preventing the desired number density of the TiC precipitates from being obtained. In addition, the formation of ferrite was excessively suppressed, preventing the desired amount of ferrite from being appropriately precipitation-strengthened, and preventing the variation in hardness of ferrite and bainite from being reduced to within the desired range. As a result, the effect of improving the hardness of ferrite by precipitation strengthening could not be fully achieved, and λ decreased. In Comparative Example 14, the end temperature of finish rolling was low, resulting in a non-uniform metal structure, and as a result, the standard deviation in the hardness of ferrite and bainite exceeded 0.40 GPa, and λ decreased. In Comparative Example 15, it is believed that the average cooling rate in the primary cooling to the intermediate air-cooling temperature was slow, causing the ferrite to coarsen. As a result, the standard deviation in the hardness of ferrite and bainite exceeded 0.40 GPa, and λ decreased. In Comparative Example 16, the intermediate air-cooling time was short, which suppressed the formation and grain growth of TiC precipitates, making it impossible to obtain the desired number density. As a result, the effect of improving the hardness of ferrite and the strength of the steel sheet by precipitation strengthening could not be fully achieved, and TS, λ, and YR decreased. In Comparative Example 17, the intermediate cooling temperature was too high, exceeding 800°C, which is thought to have made it difficult for ferrite to form, resulting in excessive bainite formation. As a result, the difference in hardness between ferrite and bainite could not be appropriately reduced, resulting in reduced λ and YR. In Comparative Example 18, the intermediate cooling time was too long, resulting in excessive ferrite formation and reduced TS.Furthermore, the λ and YR also decreased because the desired metal structure containing ferrite, bainite, and martensite in specific proportions was not obtained.

[0089] In Comparative Example 19, the high end temperature of the finish rolling caused the austenite grains after recrystallization to coarsen, reducing the number of austenite grain boundaries and the number of ferrite nucleation sites. As a result, the desired ferrite area ratio could not be achieved even by the subsequent intermediate air-cooling step, and the standard deviation of the hardness of ferrite and bainite in the finally obtained metal structure could not be controlled to 0.40 GPa or less, resulting in reduced λ and YR. In Comparative Example 20, the average cooling rate in the secondary cooling after the intermediate air-cooling was slow, preventing sufficient precipitation of bainite and martensite, resulting in reduced TS. In Comparative Example 21, the high coiling temperature resulted in the area ratio of martensite being less than 5%, resulting in reduced TS. In Comparative Example 22, the low C content prevented TiC precipitates from precipitating at a sufficient number density. As a result, TS, λ, and YR were reduced. In Comparative Example 23, the low Ti content similarly prevented TiC precipitates from precipitating at a sufficient number density. As a result, TS, λ, and YR decreased. In Comparative Example 24, the Si content was low, so the precipitation of cementite could not be sufficiently suppressed, and it is thought that C in the steel was consumed in the formation of cementite. As a result, the formation of TiC precipitates was suppressed, and the number density of the TiC precipitates was 1.0 × 10 16 pieces / cm 3 The results were less than 1.0 × 10 precipitation strengthening could not be fully achieved, and the effect of improving the hardness of ferrite and the effect of improving the strength of the steel sheet were not fully achieved, resulting in a decrease in TS and λ. In Comparative Example 25, the Al content was low, so the precipitation of cementite could not be fully suppressed, and it is believed that the C in the steel was consumed in the formation of cementite. As a result, the formation of TiC precipitates was suppressed, and the number density of the TiC precipitates was 1.0 × 10 precipitation strengthening. 16 pieces / cm 3The hardness of ferrite due to precipitation strengthening and the strength of the steel sheet could not be sufficiently improved, resulting in a decrease in TS, λ, and YR.

[0090] In contrast, all of the steel sheets according to the invention examples have a predetermined chemical composition, and by appropriately controlling the conditions in the manufacturing method, the steel sheets contain, in area ratios, 40 to 80% ferrite, 15 to 55% bainite, and 5 to 20% martensite, and TiC precipitates with a diameter of 2.0 to 8.0 nm are present in the ferrite at an area ratio of 1.0 × 10 16 pieces / cm 3 It was possible to obtain a steel sheet having a metallographic structure in which ferrite, bainite, and martensite exist at a number density of 0.40 GPa or more and the standard deviation in hardness of ferrite, bainite, and martensite is 0.40 GPa or less. As a result, the strength of the steel sheet was improved due to precipitation strengthening of ferrite by TiC precipitates, the difference in hardness between ferrite and bainite was reduced due to this precipitation strengthening, and further, the variation in hardness of ferrite and bainite was reduced, resulting in a high tensile strength of 780 MPa or more, while significantly improving the hole expandability and yield ratio. Furthermore, when a residual structure was present in the inventive examples, the residual structure was at least one of pearlite and retained austenite.

Claims

1. The chemical composition is expressed in mass percent. C: 0.03 to 0.10%, Si: 0.010-0.100%, Mn: 0.50-3.00%, Ti: 0.05-0.20%, Al: 0.20-0.40%, P: 0.100% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0 to 0.050%, V: 0-1.000%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.000%, B: 0 to 0.0100%, Sn: 0-1.000%, Sb: 0 to 1.000%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Hf: 0-0.0100%, Bi: 0 to 0.010%, REM: 0-0.0100%, As: 0 to 0.010%, Zr: 0 to 0.010%, Co: 0-2.000%, Zn: 0 to 0.010%, W: 0-1,000%, and The remainder consists of Fe and impurities. The metallic structure, in area percentage, Ferrite: 40-80%, Bainite: 15-55%, and Martensite: Contains 5-20%, TiC precipitates with a diameter of 2.0–8.0 nm are present in the ferrite, with a total area of ​​1.0 × 10⁻⁶. 16 pieces / cm 3 They exist at the above number density, A steel plate characterized in that the standard deviation of the hardness of ferrite and bainite is 0.40 GPa or less.

2. The aforementioned chemical composition, in mass%, Nb: 0.001 to 0.050%, V: 0.001-1.000%, Cr: 0.001-2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Mo: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Sn: 0.001 to 1.000%, Sb: 0.001 to 1.000%, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001-0.0100%, Bi: 0.001-0.010%, REM: 0.0001-0.0100%, As: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Co: 0.001 to 2.000%, Zn: 0.001 to 0.010%, and W: 0.001-1.000% The steel plate according to claim 1, characterized in that it includes at least one of the following.

3. The steel sheet according to claim 1 or 2, characterized in that the average particle size of the ferrite is 5.0 μm or less.

4. The steel plate according to claim 1 or 2, characterized by having a tensile strength of 780 MPa or more.

5. The steel plate according to claim 1 or 2, characterized by having a plate thickness of 1.0 to 8.0 mm.

6. A component characterized by comprising the steel plate described in claim 1 or 2.

7. A hot rolling process comprising heating a slab having the chemical composition described in claim 1 or 2, and then finishing rolling it, wherein the following conditions (a) to (e) are satisfied: (a) The heating temperature of the slab is 1200 to 1300°C. (b) The holding time in the temperature range of 1200 to 1300°C is 1000 to 4000 seconds. (c) The finish rolling is performed using a tandem rolling mill consisting of five or more rolling stands, and the total reduction ratio in the preceding rolling passes, excluding the last three stages, is 60-90%. (d) The total reduction ratio in the subsequent three rolling passes is greater than 50%, and (e) The finishing rolling temperature is between 900 and 1000°C. An intermediate air cooling process is performed in which the finish-rolled steel sheet is first cooled to an intermediate air cooling temperature of 705 to 750°C at an average cooling rate of 50 to 200°C / second, and then intermediate air-cooled for 3 to 10 seconds, and The intermediate-cooled steel sheet is secondarily cooled at an average cooling rate of 50 to 200°C / second, followed by a cooling process where it is wound at a winding temperature of 20 to 300°C. A method for manufacturing steel plates, characterized by including the following: