Steel plate, parts containing the same, and method for manufacturing steel plate

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

[Technical Field]

[0001] This invention relates to steel plates, parts containing the same, and methods for manufacturing steel plates. [Background technology]

[0002] In recent years, the automotive industry has been demanding lighter vehicle bodies from the perspective of improving fuel efficiency. To achieve both vehicle weight reduction and collision safety, increasing the strength of the steel sheets used is one effective method, and for this reason, the development of high-strength steel sheets is progressing. On the other hand, increasing the strength of steel sheets generally reduces their workability. Therefore, in the development of high-strength steel sheets, it is important to achieve high strength while ensuring a certain level of workability.

[0003] In this regard, for example, Patent Document 1 describes a material having a predetermined chemical composition, in which the microstructure in the range from 1 / 8 of the plate thickness from the surface in the thickness direction to 3 / 8 of the plate thickness from the surface in the thickness direction contains, by volume fraction, ferrite: 10-75%, martensite: 20-90%, retained austenite: 0-5%, bainite and bainitic ferrite total: 0-50%, and pearlite: 0-5%, the proportion of unrecrystallized ferrite in the ferrite is 0-25%, the cementite contained in the martensite satisfies a predetermined formula, and the density of the transition carbide contained in the martensite is 1.0 × 10⁻⁶. 13 pieces / m 3 The above results indicate that the density of coarse inclusions with an equivalent circular diameter of 10 μm or more is 0.50 particles / mm³. 2 The following applies: the maximum Vickers hardness Hv is measured on a plane parallel to the surface at a position 1 / 4 of the plate thickness in the thickness direction from the surface. max and the minimum value of Vickers hardness Hv min A steel sheet is described characterized in that the ratio to is 1.40 or less, and when a Vickers hardness distribution map is created, the average of the minimum distances between Vickers hardness peaks is 1.00 mm or less. Furthermore, Patent Document 1 teaches that the above configuration can provide a steel sheet with excellent formability, strength, and impact resistance.

[0004] Patent Document 2 describes a composite steel sheet having a predetermined chemical composition, wherein at a thickness of 1 / 4 of the sheet thickness, the main phase of the microstructure consists of polygonal ferrite precipitated and strengthened by Ti carbide, and the second phase consists of a composite structure comprising multiple dispersed low-temperature transformation products with an area fraction (fsd(%)) of 1 to 10%, the average crystal diameter of the low-temperature transformation products is 3 to 15 μm, and the average nearest neighbor distance between each low-temperature transformation product is 10 to 20 μm. Furthermore, Patent Document 2 teaches that with the above configuration, a high-strength composite steel sheet can be obtained that has a tensile strength of 540 MPa or more, and at the same time has excellent uniform elongation, burring workability and notch fatigue characteristics, as well as excellent surface properties.

[0005] Patent Document 3 describes a material having a predetermined component composition, with a Mn segregation degree of 1.5 or less in the region within 100 μm from the surface in the thickness direction, and with oxide-based inclusions with a particle length of 5 μm or more on a plane parallel to the surface of the steel plate within 100 μm from the surface in the thickness direction, up to 100 mm 2 The invention describes a high-strength steel sheet having 1000 or fewer inclusions per sheet, with a total number of oxide-based inclusions with a particle length of 5 μm or more, where the alumina content is 50% by mass or more, the silica content is 20% by mass or less, and the calcia content is 40% by mass or less, with a number ratio of 80% or more, and the metallic structure consists of a total of 25-100% martensite and bainite phases by volume, less than 75% (including 0%) ferrite phase, less than 15% (including 0%) austenite phase, and a tensile strength of 980 MPa or more. Furthermore, Patent Document 3 teaches that a high-strength steel sheet with excellent bendability (bendability) can be obtained by reducing the number of inclusions in the surface layer of the steel sheet (the region within 100 μm from the surface of the steel sheet), controlling the composition of those inclusions within an appropriate range, and reducing the degree of Mn segregation in the surface layer of the steel sheet.

[0006] In Patent Document 4, there is described a high-strength hot-rolled steel sheet having a microstructure characterized by having a predetermined component composition, with the total volume ratio of the ferrite phase and the bainite phase in the whole structure being 95% or more, the volume ratio of the ferrite phase in the whole structure being 50 to 90%, precipitates with a size of less than 20 nm containing 650 to 1100 ppm of Ti being precipitated in the ferrite phase, and ΔHv (the difference between the maximum value and the minimum value of the Vickers hardness of 30 bainite phases measured at the position of 1 / 4 of the plate thickness in the plate thickness cross-section along the rolling direction) of the bainite phase being 150 or less. Further, in Patent Document 2, it is taught that if a microstructure is formed mainly of a ferrite phase and a bainite phase, with precipitates of less than 20 nm containing 650 to 1100 ppm of Ti being precipitated in the ferrite phase and ΔHv of the bainite phase being 150 or less, a TS of not less than 780 MPa can be ensured, and excellent elongation flangeability (hole expansion property) and impact resistance characteristics can be achieved simultaneously.

[0007] In Patent Document 5, having a predetermined chemical composition, in a cross-section perpendicular to the rolling direction of the steel sheet, when the width and thickness of the steel sheet are W and t, respectively, at 1 / 4W or 3 / 4W from the end face of the steel sheet and at the position of 1 / 4t or 3 / 4t from the surface of the steel sheet, the metallographic structure 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 in the grains, and the number density of the precipitates containing Ti is 1.0×10 16 ~50.0×10 16 pieces / cm 3and the hot-rolled steel sheet contains TiN precipitates in the steel sheet, the average equivalent circle diameter of the TiN precipitates is 1.0 to 10.0 μm, the average value of the shortest distance between adjacent TiN precipitates is 10.0 μm or more, and the standard deviation of the nano-hardness is 1.00 GPa or less is described. Further, in Patent Document 5, according to the above configuration, the tensile strength (TS) is 780 MPa or more, the product of the uniform elongation u-EL and the tensile strength TS (TS×u-EL) is 7000 MPa·% or more, and the product of the hole expansion rate λ and the tensile strength TS (TS×λ) is 50000 MPa·% or more, and it can be taught that a hot-rolled steel sheet having balanced properties can be obtained.

[0008] In Patent Document 6, when having a predetermined chemical composition and in a cross section perpendicular to the rolling direction of the steel sheet, when the width and thickness of the steel sheet are W and t, respectively, at 1 / 4W or 3 / 4W from the end face of the steel sheet, and at a position of 1 / 4t or 3 / 4t from the surface of the steel sheet, the metallographic structure 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 in the grains, and the number density of the precipitates containing Ti is 1.0×10 16 ~50.0×10 16 pieces / cm 3 and the hot-rolled steel sheet contains TiN precipitates in the steel sheet, the average equivalent circle diameter of the TiN precipitates is 1.0 to 10.0 μm, the average value of the shortest distance between adjacent TiN precipitates is 10.0 μm or more, and the standard deviation of the nano-hardness is 1.00 GPa or less is described. Further, in Patent Document 6, according to the above configuration, the tensile strength (TS) is 780 MPa or more, the product of the uniform elongation u-EL and the tensile strength TS (TS×u-EL) is 7000 MPa·% or more, and the product of the hole expansion rate λ and the tensile strength TS (TS×λ) is 50000 MPa·% or more, and it can be taught that a hot-rolled steel sheet having balanced properties can be obtained.

Prior Art Documents

Patent Documents

[0009] [Patent Document 1] International Publication No. 2020 / 208979 [Patent Document 2] International Publication No. 2014 / 051005 [Patent Document 3] International Publication No. 2017 / 115748 [Patent Document 4] Japanese Patent Publication No. 2011-068945 [Patent Document 5] International Publication No. 2018 / 179389 [Patent Document 6] International Publication No. 2018 / 179391 [Overview of the project] [Problems that the invention aims to solve]

[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 not be possible to form the desired part shape in automobile suspension parts, etc. Furthermore, for parts that require impact resistance, plastic deformation occurs when subjected to an impact exceeding the yield strength (specifically, the yield stress, yield point, or 0.2% proof stress, etc.). Therefore, from the viewpoint of ensuring the collision safety of automobiles, it is necessary to improve not only the tensile strength but also the yield strength, and thus it is necessary to increase the yield ratio, which is the ratio of yield strength to tensile strength.

[0011] Therefore, the present invention aims to provide a steel sheet having high strength, high hole-expandability and yield ratio, a component containing the same, and a method for manufacturing the steel sheet. [Means for solving the problem]

[0012] To achieve the above objective, the inventors focused on the microstructure of steel sheets, particularly hot-rolled steel sheets, and conducted research. Specifically, the inventors first found that by configuring the microstructure of a steel sheet having a predetermined chemical composition to mainly consist of ferrite, bainite, and martensite, it is possible to improve hole-expandability and yield ratio while maintaining a reasonably high level of strength in the steel sheet. In addition, the inventors found that by introducing TiC precipitates with an appropriate diameter at a predetermined number density within the ferrite, the ferrite can be precipitated and strengthened, thereby further increasing the strength of the steel sheet, reducing the difference in hardness between ferrite and bainite to improve hole-expandability and yield ratio, and further reducing the variation in hardness between ferrite and bainite to more significantly improve hole-expandability, thus completing the present invention.

[0013] The present invention, which has achieved the above objectives, is as follows. (1) The chemical composition is expressed in mass%, C: 0.03~0.10%, 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: 0~0.050%, V: 0~1.000%, Cr: 0~2.00%, Ni: 0~2.00%, Cu: 0~2.00%, Mo: 0~1.000%, B: 0~0.0100%, Sn: 0~1.000%, Sb: 0~1.000%, Ca: 0~0.0100%, Mg: 0~0.0100%, Hf: 0~0.0100%, Bi: 0~0.010%, REM: 0~0.0100%, As: 0~0.010%, Zr: 0~0.010%, Co: 0~2.000%, Zn: 0~0.010%, 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 to 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 ferrite and bainite hardness is 0.40 GPa or less. (2) The chemical composition is, in mass%, Nb: 0.001~0.050%, V: 0.001~1.000%, Cr: 0.001~2.00%, Ni: 0.001~2.00%, Cu: 0.001~2.00%, Mo: 0.001~1.000%, B: 0.0001~0.0100%, Sn: 0.001~1.000%, Sb: 0.001~1.000%, Ca: 0.0001~0.0100%, Mg: 0.0001~0.0100%, Hf: 0.0001~0.0100%, Bi: 0.001~0.010%, REM: 0.0001~0.0100%, As: 0.001~0.010%, Zr: 0.001~0.010%, Co: 0.001~2.000%, Zn: 0.001~0.010%, and W: 0.001~1.000% The steel plate according to (1) above, characterized in that it includes at least one of the following. (3) The steel sheet according to (1) or (2) above, characterized in that the average particle size of the ferrite is 5.0 μm or less. (4) A steel sheet as described in any one of the above items (1) to (3), characterized by having a tensile strength of 780 MPa or more. (5) A steel plate according to any one of the above (1) to (4), characterized by having a plate thickness of 1.0 to 8.0 mm. (6) A component characterized by including a steel plate as described in any one of the above items (1) to (5). (7) A hot rolling process that includes heating a slab having the chemical composition described in (1) or (2) above, and then finishing rolling it, and satisfying the following conditions (a) to (e): (a) The heating temperature of the slab is 1200-1300°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 900 to 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 cooling is performed for 3 to 10 seconds, and The intermediate-cooled steel sheet is secondarily cooled at an average cooling rate of 50-200°C / second, followed by a cooling process where it is wound at a winding temperature of 20-300°C. A method for manufacturing steel plates, characterized by including the following: [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a steel sheet having high strength, high hole-expandability and yield ratio, a part containing the same, and a method for manufacturing the steel sheet. [Modes for carrying out the invention]

[0015] <Steel plate> The steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, has a chemical composition in mass%, C: 0.03~0.10%, 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: 0~0.050%, V: 0~1.000%, Cr: 0~2.00%, Ni: 0~2.00%, Cu: 0~2.00%, Mo: 0~1.000%, B: 0~0.0100%, Sn: 0~1.000%, Sb: 0~1.000%, Ca: 0~0.0100%, Mg: 0~0.0100%, Hf: 0~0.0100%, Bi: 0~0.010%, REM: 0~0.0100%, As: 0~0.010%, Zr: 0~0.010%, Co: 0~2.000%, Zn: 0~0.010%, 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 to 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, It is characterized by a standard deviation of 0.40 GPa or less in the hardness of ferrite and bainite.

[0016] As steel sheets become stronger, their workability, such as their ability to expand holes, generally decreases. For example, in order to manufacture parts with complex shapes, such as lower arms and trailing arms in the undercarriage of automobiles, a steel sheet is required that has high strength, such as a tensile strength of 780 MPa or more that enables weight reduction, while also having excellent hole-expandability. Furthermore, as mentioned earlier, from the perspective of ensuring the crash safety of automobiles, 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 yield strength to tensile strength. Thus, there is a high demand for a material that has sufficient strength in steel sheets, improves hole-expandability, and has a high yield ratio from the perspective of automobile crash safety, etc. Therefore, the inventors of the present invention investigated the metallographic structure of the steel sheet in addition to making the chemical composition of the steel sheet appropriate. To explain in more detail, the inventors first found that by configuring the metal structure of a steel sheet having a predetermined chemical composition to mainly consist of ferrite, bainite, and martensite, and more specifically, by configuring it to contain ferrite: 40-80%, bainite: 15-55%, and martensite: 5-20% by area percentage, it is possible to increase the hole-expandability and yield ratio while maintaining the strength of the steel sheet at a reasonably high level.

[0017] On the other hand, in a three-phase structure mainly composed of ferrite, bainite, and martensite, the hard bainite and martensite generally provide strength, while the soft ferrite is responsible for deformation. However, if the difference in hardness between the hard phases of bainite and martensite and the soft phase of ferrite becomes large, this can lead to a decrease in hole-expanding properties and yield ratio. In particular, if the difference in hardness becomes too large, deformation concentrates in the soft ferrite, causing premature cracking, and resulting in a particularly significant decrease in hole-expanding properties. Therefore, the inventors investigated further improvement of hole-expanding properties and the realization of a high yield ratio from the viewpoint of reducing the difference in hardness between each phase in metal structures including such three-phase structures. As a result, the inventors found that precipitation strengthening of the softest ferrite in the three-phase structure, more specifically, precipitation of 1.0 × 10⁻⁶ TiC precipitates with a diameter of 2.0 to 8.0 nm into the ferrite 16 pieces / cm 3 By precipitation-strengthening ferrite at the above number density, it is naturally possible to improve the overall strength of the steel sheet, and it has been found that the difference in hardness between bainite, which is relatively abundant in the hard structure, and ferrite, which is the softest in the three-phase structure, can be sufficiently reduced. In the steel sheet according to the embodiment of the present invention, 1.0 × 10 TiC precipitates with a diameter of 2.0 to 8.0 nm are present in the ferrite. 16 pieces / cm 3In order to achieve the above number density, in addition to the manufacturing method which will be explained in detail later, it is necessary to make the chemical composition of the steel sheet appropriate. For example, Si and Al contained in the steel have the effect of suppressing the precipitation of cementite. Therefore, by including these elements in the steel in amounts above a certain level, more specifically, including Si and Al at a level 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 to form cementite, thereby promoting the formation of TiC precipitates during cooling after hot rolling. As a result, the inventors have found that even though the metal structure is composed of a three-phase structure containing relatively large amounts of bainite and martensite in order to achieve high strength, and therefore the difference in hardness tends to be relatively large, it is possible to obtain a steel sheet 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, by increasing the hardness of ferrite through precipitation strengthening using TiC precipitates of a specific diameter and number density, the difference in hardness between bainite and ferrite can be reduced, thereby improving hole-expanding properties and yield ratio. However, even if the difference in hardness is sufficiently reduced in the overall metal structure, if there are parts in the metal structure with a large difference in hardness, these parts may become the starting point for fracture, and in such cases, hole-expanding properties may decrease. Therefore, the inventors focused on the hardness distribution of the metal structure and conducted an investigation to further improve the hole-expanding properties of steel sheets. As a result, the inventors found that, in addition to reducing the difference in hardness between bainite and ferrite using the TiC precipitates described above, reducing the variation in the hardness of ferrite and bainite to a predetermined range, or more specifically, controlling the standard deviation of the hardness of ferrite and bainite to 0.40 GPa or less, can result in a steel sheet with significantly improved hole-expanding properties. By combining the reduction of the hardness difference between bainite and ferrite due to the use of TiC precipitates with the reduction of variations in the hardness of ferrite and bainite, it is possible not only to reduce the hardness difference of the entire metal structure, but also to reliably reduce the increase in local hardness differences within the metal structure, and as a result, the hole-expanding properties of the steel sheet can be significantly improved. Therefore, according to the embodiment of the present invention, despite being high strength, it is possible to achieve a high yield ratio and particularly excellent hole-expanding properties. For this reason, the steel sheet according to the embodiment of the present invention can be effectively used in parts where both high strength and excellent workability, which are conflicting properties, are required, and furthermore, impact resistance is required, making it particularly useful in the automotive field.

[0019] The steel sheets according to embodiments of the present invention will be described in more detail below. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. In this specification, "~", which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower and upper limits, respectively, unless otherwise specified.

[0020] [C:0.03~0.10%] Carbon (C) is an effective element for increasing the strength of steel plates. Furthermore, C forms carbides and / or carbonitrides with Ti and Nb in the steel, contributing to precipitation strengthening based on these precipitates and to microstructural refinement due to the pinning effect of these precipitates. To fully obtain these effects, the C content should be 0.03% or higher. The C content may also be 0.04% or higher, 0.05% or higher, or 0.06% or higher. On the other hand, excessive C content may reduce hole expansion properties and yield ratio due to cementite formation. Therefore, the C content should be 0.10% or lower. The C content may also be 0.09% or lower, 0.08% or lower, or 0.07% or lower.

[0021] [Si: 0.010~0.100%] Si is an effective element for increasing strength as a solid solution strengthening element. Si also has the effect of suppressing cementite precipitation. Therefore, including Si can suppress the consumption of carbon (C) in the steel for cementite formation, thereby promoting the formation of TiC precipitates during cooling after hot rolling. To fully obtain these effects, the Si content should be 0.010% or higher. The Si content may also be 0.020% or higher, 0.030% or higher, or 0.040% or higher. On the other hand, excessive Si content can cause surface quality defects called Si scale. Therefore, the Si content should be 0.100% or lower. The Si content may also be 0.090% or lower, 0.080% or lower, 0.070% or lower, 0.060% or lower, or 0.050% or lower.

[0022] [Mn: 0.50~3.00%] Mn is an effective element for increasing strength as a hardenability and solid solution strengthening element. To fully obtain these effects, the Mn content should be 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, if the Mn content is excessive, a large amount of MnS may be generated, which may reduce toughness. Therefore, the Mn content should be 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: 0.05~0.20%] Ti (Ti) precipitates finely in steel as carbides (TiC), improving the strength of the steel through precipitation strengthening and increasing the hardness of ferrite. Furthermore, Ti fixes carbon (C) by forming carbides, suppressing the formation of cementite, which is detrimental to hole expansion. To fully obtain these effects, the Ti content should be 0.05% or higher. The Ti content may also be 0.08% or higher, 0.10% or higher, 0.12% or higher, or 0.14% or higher. On the other hand, excessive Ti content can lead to coarser carbides, making it impossible to obtain the desired precipitation strengthening in ferrite. Additionally, the coarser TiC precipitates reduce the number density of TiC precipitates, preventing sufficient hardness increase of ferrite through precipitation strengthening. Therefore, the Ti content should be 0.20% or lower. The Ti content may also be 0.18% or lower, 0.17% or lower, 0.16% or lower, or 0.15% or lower.

[0024] [Al: 0.20~0.40%] Al is an element that acts as a deoxidizing agent for molten steel. Al also has the effect of suppressing cementite precipitation. Therefore, including Al can suppress the consumption of carbon (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 should be 0.20% or higher. The Al content may also be 0.22% or higher, 0.25% or higher, or 0.28% or higher. On the other hand, excessive Al content can lead to the formation of coarse oxides, which can reduce toughness and ductility. Therefore, the Al content should be 0.40% or lower. The Al content may also be 0.38% or lower, 0.35% or lower, or 0.32% or lower.

[0025] [P:0.100% or less] Excessive phosphorus (P) content can negatively affect weldability and other properties. Therefore, the P content should be 0.100% or less. The P content may also 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 lead to increased 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 can lead to the formation of large amounts of MnS, which can reduce toughness. Therefore, the Si content should be 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 lead to increased 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 nitrogen (N) content can form coarse nitrides, reducing toughness. Therefore, the N content should be 0.010% or less. The N content may also 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 lead to increased 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 introduced during the manufacturing process. Excessive O content can lead to the formation of coarse inclusions, which can reduce the toughness of the steel sheet. Therefore, the O content should be 0.010% or less. The O content may also 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 it to less than 0.0001% requires more time for refining, leading to a decrease in 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, if necessary, contain at least one of the following optional elements in place of a portion of the remaining Fe.

[0030] [Nb:0~0.050%] Nb is an element that contributes to the refinement of the microstructure and, consequently, the increased strength of steel sheets by forming carbides, nitrides, and / or carbonitrides in steel through a pinning effect. The Nb content may be 0%, but to obtain such an effect, it is preferable that the Nb content be 0.001% or more. The Nb content may also 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, if the Nb content is excessive, coarse carbides and the like may be formed in the steel, which may reduce the ductility of the steel sheet. Therefore, it is preferable that the Nb content be 0.050% or less. The Nb content may also be 0.040% or less, 0.030% or less, or 0.025% or less.

[0031] [V: 0~1.000%] V is an element that contributes to improving strength through precipitation strengthening, etc. The V content may be 0%, but to obtain such an effect, it is preferable that the V content be 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, if the V content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the V content be 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: 0~2.00%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength. While the Cr content may be 0%, it is preferable that the Cr content be 0.001% or more to obtain such effects. 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 lead to saturation of the effect and an increase in manufacturing costs. Therefore, it is preferable that the Cr content be 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: 0~2.00%] [Cu: 0~2.00%] Ni and Cu are elements that contribute to improved strength through improved hardenability or solid solution strengthening. The Ni and Cu content may be 0%, but to obtain such effects, it is preferable that the content of each element be 0.001% or more, and may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if these elements are included in excess, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the Ni and Cu content be 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: 0~1.000%] Mo is an element that enhances the hardenability of steel and contributes to improving its strength. While the Mo content may be 0%, it is preferable that the Mo content be 0.001% or more to obtain such effects. The Mo content may also be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, if the Mo content is excessive, the deformation resistance during hot working may increase, and the equipment load may become larger. Therefore, it is preferable that the Mo content be 1.000% or less. The Mo content may also 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: 0~0.0100%] B improves low-temperature toughness by segregating at grain boundaries and increasing grain boundary strength. The B content may be 0%, but to obtain this effect, it is preferable that the B content be 0.0001% or more. The B content may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if the B content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the B content be 0.0100% or less. The B content may also 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: 0~1.000%] [Sb: 0~1.000%] Sn and Sb are elements that are effective in improving corrosion resistance. The Sn and Sb content may be 0%, but to obtain such an effect, it is preferable that the content of each element be 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 content of these elements may lead to a decrease in toughness. Therefore, it is preferable that the Sn and Sb content be 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: 0~0.0100%] [Mg: 0~0.0100%] [Hf: 0~0.0100%] Ca, Mg, and Hf are elements that can control the morphology of nonmetallic inclusions. The Ca, Mg, and Hf content may be 0%, but to obtain such an effect, it is preferable that the content of each element be 0.0001% or more, and may be 0.0005% or more, or 0.0010% or more. On the other hand, if these elements are included in excess, the effect will saturate, and including them in the steel sheet more than necessary will lead to an increase in manufacturing costs. Therefore, it is preferable that the Ca, Mg, and Hf content be 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less, each.

[0038] [Bi: 0~0.010%] Bi is an effective element for improving corrosion resistance. While the Bi content may be 0%, it is preferable that the Bi content be 0.001% or higher to obtain this effect. The Bi content may also be 0.002% or higher, or 0.003% or higher. On the other hand, if the Bi content is excessive, the effect will saturate, and including more Bi than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, it is preferable that the Bi content be 0.010% or lower. The Bi content may also be 0.005% or lower, or 0.004% or lower.

[0039] [REM:0~0.0100%] REM is an element that can control the morphology of nonmetallic inclusions. The REM content may be 0%, but to obtain such an effect, it is preferable that the REM content be 0.0001% or more. The REM content may be 0.0005% or more, or 0.0010% or more. On the other hand, if the REM content is excessive, the effect will saturate, and including more REM in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, it is preferable that the REM content be 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 including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[0040] [As: 0~0.010%] As is an effective element for improving corrosion resistance. While the As content may be 0%, it is preferable that the As content be 0.001% or more to obtain this effect. The As content may also be 0.002% or more, or 0.003% or more. On the other hand, if the As content is excessive, the effect will saturate, and including more As than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, it is preferable that the As content be 0.010% or less. The As content may also be 0.008% or less, or 0.005% or less.

[0041] [Zr:0~0.010%] Zr is an element that can control the morphology of nonmetallic inclusions. While the Zr content may be 0%, it is preferable that the Zr content be 0.001% or higher to obtain such an effect. The Zr content may also be 0.002% or higher, or 0.003% or higher. On the other hand, if the Zr content is excessive, the effect will saturate, and including more Zr than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, it is preferable that the Zr content be 0.010% or lower. The Zr content may also be 0.008% or lower, or 0.005% or lower.

[0042] [Co: 0~2.000%] Co is an element that contributes to improving hardenability and / or heat resistance. The Co content may be 0%, but to obtain these effects, it is preferable that the Co content be 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, if the Co content is excessive, the hot workability may decrease, and this can lead to an increase in raw material costs. Therefore, it is preferable that the Co content be 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: 0~0.010%] Zn is an element that can be contained in steel sheets when scrap or similar materials are used as steel raw materials. 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 more time for refining, leading to 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: 0~1.000%] W is an element that enhances the hardenability of steel and contributes to improving its strength. While the W content may be 0%, it is preferable that the W content be 0.001% or more to obtain such effects. The W content may also 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, it is preferable that the W content be 1.000% or less. The W content may also 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 remainder of the elements other than those mentioned above consists of Fe and impurities. Impurities are components that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap.

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

[0047] [Metal structure] The microstructure of the steel sheet according to the embodiment of the present invention includes, by area percent, ferrite: 40-80%, bainite: 15-55%, and martensite: 5-20%. By configuring the microstructure of the steel sheet to mainly consist of these three structures, it is possible to increase the strength of the steel sheet while improving its hole-expanding properties and yield ratio. In addition to including these three structures in the specific area percentages described above, by utilizing ferrite precipitation strengthening, which will be explained in detail later, it is possible to further increase the strength of the steel sheet and appropriately reduce the difference in hardness within the microstructure. For example, if the area percentage of ferrite is small, the proportion of bainite and martensite, which are hard phases, particularly the proportion of bainite, becomes high, and even with ferrite precipitation strengthening, it may not be possible to appropriately reduce the difference in hardness within the microstructure, more specifically, the difference in hardness between ferrite and bainite. In such cases, it becomes impossible to achieve the desired hole-expanding properties and / or yield ratio. Therefore, the area percentage 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 ratio of ferrite becomes too high, the proportion of the hard phases, bainite and martensite, decreases, and as a result, it may not be possible to achieve the desired strength, for example, a tensile strength of 780 MPa or more. Therefore, the area ratio of ferrite should be 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, a higher area ratio of the hard phases, bainite and martensite, is preferable. From this viewpoint, for example, the area ratio 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 ratio 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 hole-expanding properties and yield ratio, a lower area ratio of bainite and martensite is preferable. From this viewpoint, for example, the area ratio 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 ratio of martensite may be 18% or less, 16% or less, or 14% or less.

[0049] The metallic structure of the steel sheet according to the embodiment of the present invention includes ferrite, bainite, and martensite as described above, and may also include other residual structures, but the area ratio of the residual structures is preferably small, and may be 0%. The area ratio of the residual structures is not particularly limited, but may be, for example, 0-5%, 0-4%, or 0-3%. In other words, the total area ratio of ferrite, bainite, and martensite may be, for example, 95-100%, 96-100%, or 97-100%. The lower limit of the residual structures may be 1% or 2%. If residual structures are present, they may include at least one of pearlite and retained austenite, or at least one of them.

[0050] [Identification of metallographic structure and calculation of area ratio] Microstructure observation is performed using a scanning electron microscope. Prior to observation, the sample for microstructure observation is polished using wet polishing with emery paper and diamond abrasive grains with an average particle size of 1 μm to a mirror finish on the observation surface, and then the microstructure is etched with a 3% nitric acid alcohol solution. The observation magnification is set to 2000x, and 10 random images of a 30 μm × 40 μm field of view at a position 1 / 4 of the plate thickness from the surface are taken. The proportion of the microstructure is determined by the point counting method. A total of 225 grid points are set on the obtained microstructure images, spaced 3 μm vertically and 4 μm horizontally, and the microstructure present beneath the grid points is identified. The proportion of the microstructure contained in the steel is determined from the average value of the 10 images. Ferrite is defined as a massive crystalline grain that does not contain iron-based carbides with a major axis of 100 nm or more. Bainite is an aggregate of lath-like crystal grains that 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, and these 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 means that the difference in the elongation direction of the iron-based carbide group is within 5°. Bainite is counted as one bainite grain when surrounded by grain boundaries with an orientation difference of 15° or more. In addition, martensite, which contains a large amount of solid-solution carbon, has a higher brightness and appears whiter than other structures, so it can be distinguished from other structures. If structures other than ferrite, bainite, and martensite are present, the area ratio of the remaining structure is determined by subtracting the total area ratio of ferrite, bainite, and martensite from 100%. While it is not necessary to specifically identify the remaining tissue, if the remaining tissue contains pearlite and retained austenite, pearlite can be identified by scanning electron microscopy because it has a distinctive structure in which cementite precipitates in a lamellar pattern. Furthermore, the volume fraction of retained austenite can be calculated by X-ray diffraction measurement, and since the volume fraction of retained austenite is equivalent to the area fraction, this can be used as the area fraction of retained austenite.

[0051] [Number density of TiC precipitates with a diameter of 2.0-8.0 nm in ferrite: 1.0 × 10 16 pieces / cm 3 [End] In the steel sheet according to the embodiment of the present invention, the ferrite contains 1.0 × 10 TiC precipitates with a diameter of 2.0 to 8.0 nm. 16 pieces / cm 3 These precipitates exist at the number density described above. Here, TiC precipitates include not only TiC but also composite carbides containing Ti and other elements, 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 introducing TiC precipitates with a diameter of 2.0 to 8.0 nm into ferrite at this number density, the hardness of the ferrite can be increased through precipitation strengthening. More specifically, by increasing the hardness of the ferrite and reducing the hardness difference with bainite, a relatively abundant hard phase in the metal structure, the hardness difference in the metal structure, which is mainly composed of ferrite, bainite, and martensite, can be reduced. As a result, it becomes possible to further improve the hole-expanding properties and yield ratio of the steel sheet. 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 movement, and therefore the hardness improvement effect of ferrite due to precipitation strengthening cannot be fully obtained. In addition, the strength improvement effect of the steel sheet may not be fully realized. On the other hand, if the diameter of the TiC precipitates is too large, the desired precipitation strengthening in the ferrite may not be obtained.

[0052] While not intended to be bound by any particular theory, this is thought to be because the coarser size of the TiC precipitates alters the strengthening mechanism in relation to dislocation motion. For example, dislocation lines no longer pass across the TiC precipitates but instead pass around the coarser TiC precipitates, leaving loops of dislocation lines, thus reducing the amount of precipitation strengthening. In addition, as the TiC precipitates coarseen, their number density also decreases significantly, making it impossible to sufficiently increase the hardness of the ferrite through precipitation strengthening. Therefore, to effectively increase the hardness of the ferrite through precipitation strengthening, it is effective to control the diameter of the TiC precipitates within the range of 2.0 to 8.0 nm, and to improve the hardness of the ferrite to the desired level through precipitation strengthening, it is important to control the number density of TiC precipitates with such diameters within a predetermined range. From this perspective, TiC precipitates with a diameter of 2.0 to 8.0 nm are placed in the ferrite as described above, at a density of 1.0 × 10⁻⁶ 16 pieces / cm 3 It is necessary to have them present at the above number density. To further enhance the hardness-improving effect of ferrite, a higher number density is preferable, for example, 1.2 × 10⁻⁶. 16 pieces / cm 3 The above is 1.5 × 10 16 pieces / cm 3 The above is 2.0 × 10 16 pieces / cm 3 The above is 5.0 x 10 16 pieces / cm 3 or more or 10.0 × 10 16 pieces / cm 3 The above is also acceptable. On the other hand, because there are limitations on the content of C and Ti, which are the sources of TiC precipitates, if the number density becomes too high, it may become difficult to control the diameter of the TiC precipitates within the desired range. Therefore, the number density is not particularly limited as long as it satisfies a diameter of 2.0 to 8.0 nm, but for example, 75.0 × 10 16 pieces / cm 3 Below, 50.0 × 10 16 pieces / cm 3 Below, 30.0 × 10 16 pieces / cm 3 The following or 20.0 × 10 16 pieces / cm3 The following may also apply. In the steel sheet according to the embodiment of the present invention, when measured by the three-dimensional atom probe measurement method which will be described in detail later, the number of TiC precipitates with a diameter of 2.0 to 8.0 nm is 1.0 × 10⁻⁶. 16 pieces / cm 3 It is sufficient that the above number densities are present in the ferrite, 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 the 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, and if necessary, by using focused ion beam (FIB) processing in conjunction with electropolishing, to create a rod-shaped sample of 100 nm or more in length and a cross-section of 40 nm × 40 nm or more, with at least one end sharpened. At this time, the needle-shaped sample is prepared so that ferrite falls within the range of three-dimensional atom probe measurement, using the method used to identify ferrite in the metal structure identification and area ratio calculation described above. At least five 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 of three samples that are close to the average of the five measurement results. In three-dimensional atom probe measurement, the accumulated data can be reconstructed to obtain an actual distribution image of atoms in real space. In the case of a TiC precipitate with a Na-Cl structure, the unit cell is 4.33 Å, so the interatomic distance between Ti atoms is 4.33 × √2 = 6.1 Å. Therefore, if multiple Ti atoms are present at approximately the same coordinate position (less than 7 Å), these Ti atoms are judged to be in the same precipitate. The number of Ti atoms judged to be in the same precipitate is counted, and if this number is 50 or more, the precipitate is determined to be a TiC precipitate. The diameter of the TiC precipitate is the equivalent diameter of a circle calculated by assuming the TiC precipitate is spherical, based on the number of Ti atoms constituting the observed TiC precipitate and the lattice constant of the TiC precipitate. The method for determining the diameter (equivalent diameter of a circle) R of the TiC precipitate using the number of Ti atoms of the TiC precipitate obtained by the three-dimensional atom probe measurement method is shown below. The three-dimensional atom probe measurement method measures the total number of atoms N in the target sample, but in reality, the three-dimensional atom probe measurement method cannot detect the total number of atoms N in the target sample. Each instrument has its own unique atomic detection rate α (= number of atoms detected / total number of atoms), so the number of atoms N that would have been present is calculated from the actual measured value n. That is, the total number of atoms N = n / α. Next, assuming that the Na-Cl structure of the TiC precipitate has 8 Ti atoms in its unit cell, and that the lattice constant a of the Na-Cl structure is 4.33 Å, the diameter (circular diameter) R of the TiC precipitate is calculated using the following formula. TiC precipitate diameter R = {(6 / 8)·(1 / π)·N·a} 3} (1 / 3) Next, for each of the five samples, the number of TiC precipitates whose diameter R is within the range of 2.0 to 8.0 nm is measured within the diameter R 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, with the measurement field (the range from the sample surface to which atoms are ionized during three-dimensional atom probe measurement) as the denominator and the number of TiC precipitates with a diameter of 2.0 to 8.0 nm as the numerator. Then, as described above, the average of the measurement results (number density of TiC precipitates with a diameter of 2.0 to 8.0 nm) from three samples that are close to the average of the measurement results (number density of TiC precipitates with a diameter of 2.0 to 8.0 nm) from 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 of the hardness of ferrite and bainite is controlled to 0.40 GPa or less. By controlling the standard deviation of the hardness of ferrite and bainite within this range, the variation in the hardness of ferrite and bainite in the metal structure can be reduced. Therefore, in combination with the reduction of the hardness difference between bainite and ferrite caused by the use of TiC precipitates as described above, it is possible not only to reduce the hardness difference of the metal structure as a whole, but also to reliably reduce the increase in local hardness differences in the metal structure, and as a result, it is possible to more significantly improve the hole-expanding properties of the steel sheet. From the viewpoint of further improving hole-expanding properties, a lower standard deviation of the hardness of ferrite and bainite is preferable, and 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 higher, 0.10 GPa or higher, or 0.15 GPa or higher.

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

[0056] [Average particle size of ferrite: 5.0 μm or less] In the steel sheet according to the embodiment of the present invention, the average particle size of ferrite is preferably 5.0 μm or less. By controlling the average particle size of ferrite within such a fine range, it is possible to further improve the strength, porosity, and / or yield ratio of the steel sheet. The average particle 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 particle 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 particle size of ferrite] The average grain size of the 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 used in the identification of the metal structure and calculation of the area ratio described above, using a SEM-EBSD device (for example, JEOL Ltd., JSM-7001F). Specifically, the average grain size of the ferrite is determined by analyzing the grain size of the BCC phase defined by large-angle grain boundaries with an inclination angle of 15° or more using a SEM-EBSD device for the ferrite separated by the point counting method after taking five images related to the identification of the metal structure and calculation of the area ratio, and is obtained by the following formula (1).

number

[0058] [plate 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 types of steel sheets, and may be, for example, a hot-rolled steel sheet, a cold-rolled steel sheet, or a hot-rolled or cold-rolled steel sheet that has undergone surface treatment such as plating.

[0059] As described above, the steel sheet according to the embodiment of the present invention achieves high strength, a high yield ratio, and particularly excellent hole-expanding properties. Therefore, the steel sheet according to the embodiment of the present invention can reliably achieve a high level of balance between the conflicting properties of high strength and excellent workability, while also achieving excellent impact resistance. For this reason, 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 undercarriage part, is provided that includes the steel sheet according to the embodiment of the present invention. Examples of automotive undercarriage parts include lower arms and trailing arms. These automotive parts, particularly automotive undercarriage parts, only need to include the steel sheet according to the embodiment of the present invention in at least a portion of these parts, and therefore at least a portion of these parts will satisfy the chemical composition and metallic structure characteristics described above. In parts of the steel sheet that do not come into direct contact with the mold during forming such as press forming, and where the degree of processing is relatively low, the metallic structure characteristics do not change particularly before and after forming.

[0060] [Mechanical properties] [Tensile strength: TS] According to the steel sheet having the above chemical composition and metal structure, a high tensile strength, specifically a tensile strength of 780 MPa or higher, can be achieved. The tensile strength is preferably 800 MPa or higher, 820 MPa or higher, or 840 MPa or higher. According to the steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, improved hole-expandability and a high yield ratio can be achieved by specific combinations of the chemical composition and metal structure described above. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the steel sheet may be 1180 MPa or lower, 980 MPa or lower, 940 MPa or lower, 900 MPa or lower, or 860 MPa or lower. The tensile strength is measured by taking a JIS No. 5 test specimen from a direction (C direction) where the longitudinal direction of the test specimen is preferably parallel to the direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel sheet cannot be determined, a JIS No. 5 test specimen may be taken from any direction within the surface of the steel sheet.

[0061] [Yield ratio: YR] According to the steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, the yield ratio can also be increased, more specifically, a yield ratio of 0.70 or higher can be achieved. The yield ratio is preferably 0.75 or higher, more preferably 0.80 or higher. There is no particular upper limit, but for example, the yield ratio may be 0.90 or lower or 0.85 or lower. 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 specimen from the direction (C direction) where the longitudinal direction of the test specimen is preferably parallel to the direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel sheet cannot be determined, a JIS No. 5 test specimen may be taken from any direction within the surface of the steel sheet. Yield ratio YR = 0.2% proof stress / Tensile strength TS

[0062] [Hole expansion ratio: λ] According to the steel sheet having the above chemical composition and metal structure, high hole expansion properties, specifically a hole expansion ratio of 80.0% or more, can be achieved. 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 performing a hole expansion test in accordance with JIS Z 2256:2020. First, a test piece with a width of 100 mm x length of 100 mm is taken from the steel sheet, and a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%) is used to punch a hole (initial hole: hole diameter D o Create a hole (10mm). Next, with the burr facing the die side, use a conical punch with a 60° apex angle to widen the initial hole until a crack occurs that penetrates the thickness of the plate, and then widen the hole diameter D when the crack occurs. h Measure the hole expansion ratio in mm and calculate the hole expansion ratio λ (%) for each test specimen using the following formula. Perform this hole expansion test three times and determine the average value as the hole expansion ratio λ. λ=(D h -D o ) / D o ×100

[0063] <Method of manufacturing steel plates> 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 illustrate characteristic methods for manufacturing a steel sheet according to an embodiment of the present invention, and is not intended to limit the steel sheet to those manufactured by the manufacturing method described below. More specifically, the following describes the manufacturing of a hot-rolled steel sheet, but the steel sheet according to an embodiment of the present invention includes not only any steel sheet having the chemical composition and metal structure described above, i.e., hot-rolled steel sheet, but also cold-rolled steel sheet, plated steel sheet, etc. Therefore, the following description merely describes an 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 manufacturing a steel sheet according to an embodiment of the present invention includes a hot rolling step that satisfies the following conditions (a) to (e): heating a slab having the chemical composition described above in relation to a steel sheet, and then finish rolling it. (a) The heating temperature of the slab is 1200-1300°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 900 to 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 cooling is performed for 3 to 10 seconds, and The intermediate-cooled steel sheet is secondarily cooled at an average cooling rate of 50-200°C / second, followed by a cooling process where it is wound at a winding temperature of 20-300°C. It is characterized by including [specific features]. The following describes each process in detail.

[0065] [Hot rolling process] [(a) Slab heating temperature: 1200~1300℃] [(b) Holding time in the temperature range of 1200~1300℃: 1000~4000 seconds] First, a slab having the chemical composition described above in relation to the steel plate is heated. From the viewpoint of productivity, the slab used is preferably cast by the continuous casting method, but it may also be manufactured by the ingot casting method or the thin slab casting method. The slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to solid dissolve the alloying elements in the slab. If the heating temperature is low, the alloying elements will not sufficiently solid dissolve in the slab, leaving coarse alloy carbides, which may cause brittle cracking during hot rolling. For this reason, the heating temperature is preferably 1200°C or higher. The upper limit of the heating temperature is not particularly limited, but from the viewpoint of the capacity of the heating equipment and productivity, it is preferably 1300°C or lower. In addition, by holding the temperature in the 1200-1300°C range for 1000 seconds or more, the alloying elements can be reliably solid dissolved in the slab. The upper limit of the holding time is not particularly limited, but from the viewpoint of productivity, it is preferably 4000 seconds or lower. When rough rolling is performed, holding at a temperature range of 1200-1300°C may be done after rough rolling.

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

[0067] [(c) Total reduction ratio in the rolling pass preceding the finish rolling: 60-90%] The heated slab, or a slab that has been roughly rolled 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 five to eight rolling stands. In this manufacturing method, in the finish rolling performed on the heated slab, the total reduction ratio in the preceding rolling passes, excluding the last three passes, is controlled to 60-90%. By performing rolling at such a high reduction ratio in the preceding rolling passes, recrystallization can be promoted to refine the microstructure, and in particular, the average grain size of ferrite in the final microstructure can be reduced. Such refinement of the microstructure by recrystallization is very advantageous in forming the desired microstructure and improving properties such as hole-expanding ability and yield ratio. If the total reduction ratio in the preceding rolling passes is less than 60%, the desired microstructure containing ferrite, bainite, and martensite in specific proportions may not be obtained, and properties such as hole-expanding ability and / or yield ratio may decrease. Therefore, the total reduction ratio in the preceding rolling pass should be 60% or more, preferably 70% or more. On the other hand, if the total reduction ratio in the preceding rolling pass is too high, the rolling load will be excessive, and the load on the rolling mill will increase. For this reason, the total reduction ratio in the preceding rolling pass should be 90% or less.

[0068] [(d) Total reduction ratio in the three rolling passes after the finish rolling stage: more than 50%] In the finish rolling of this manufacturing method, the total reduction ratio in the subsequent three rolling passes is controlled to more than 50%. By applying rolling under relatively high pressure of more than 50% even in the rolling passes after the finish rolling, recrystallization can be further promoted and the austenite grains can be refined. In connection with this, it is possible to reduce the variation in the hardness of ferrite and bainite in the final metal structure to within a predetermined range, or more specifically, to control the standard deviation of the hardness of ferrite and bainite to 0.40 GPa or less. To explain in more detail, ferrite is mainly nucleated from austenite grain boundaries. Therefore, by promoting recrystallization and refining the austenite grains, the number of austenite grain boundaries can be increased, thereby increasing the number of ferrite nucleation sites. In connection with this, the growth rate of ferrite can be increased in the subsequent intermediate air cooling process, making it possible to generate ferrite rapidly in a relatively short time. In addition, in the intermediate air cooling process, not only ferrite but also TiC precipitates are generated, as will be explained in more detail later. While not intended to be bound by any particular theory, it is believed that rapidly generating ferrite in a short time during the intermediate air cooling process can suppress variations in the diameter and number density of TiC precipitates generated in the ferrite during the same intermediate air cooling process. As a result, it is believed that the standard deviation of the hardness of ferrite and bainite in the final metal structure can be controlled to 0.40 GPa or less.

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

[0070] [(e) End temperature for finish rolling: 900~1000℃] In this manufacturing method, in addition to controlling the reduction ratio in the pre- and post-finish rolling stages, the finishing temperature of the finishing rolling is also important for controlling the microstructure of the steel sheet. If the finishing temperature of the finishing rolling is too low, the microstructure may become non-uniform, potentially reducing strength, and / or the variation in the hardness of ferrite and bainite may not be reduced to the desired range, potentially reducing hole-expanding properties. For this reason, the finishing temperature of the finishing rolling should be 900°C or higher. Preferably, the finishing temperature of the finishing rolling should be 920°C or higher. On the other hand, if the finishing temperature of the finishing rolling is too high, the austenite grains after recrystallization become coarser, reducing the number of austenite grain boundaries and decreasing the ferrite nucleation sites. As a result, the ferrite growth rate cannot be increased in the subsequent intermediate air-cooling process, and it may not be possible to control the standard deviation of the hardness of ferrite and bainite to 0.40 GPa or less in the final microstructure. Therefore, the finishing temperature of the finishing rolling should be 1000°C or lower. Preferably, the finishing temperature of the finishing rolling should be 980°C or lower.

[0071] [Intermediate air cooling process] The finish-rolled steel sheet is first cooled on a runout table (ROT) at an average cooling rate of 50-200°C / second to an intermediate air-cooling temperature of 705-750°C during the subsequent intermediate air-cooling process, followed by intermediate air-cooling for 3-10 seconds. By first cooling to an intermediate air-cooling temperature of 705-750°C at an average cooling rate of 50-200°C / second, excessive ferrite formation and / or ferrite coarsening can be suppressed, while the subsequent intermediate air-cooling at a high temperature can promote the precipitation of TiC precipitates. As a result, the ferrite is sufficiently strengthened by precipitation, thereby reducing the hardness difference between ferrite and bainite and improving hole-expanding properties and yield ratio. More specifically, in order to promote the formation of TiC precipitates and grain growth during intermediate air-cooling and sufficiently strengthen the ferrite by precipitation, the intermediate air-cooling temperature needs to be set in a relatively high temperature range, namely 705-750°C. However, in this case, excessive ferrite is formed, and the ferrite area ratio in the final metal structure exceeds 80% and / or the ferrite becomes coarse, making it impossible to obtain the desired properties. Therefore, in this manufacturing method, the average cooling rate during primary cooling from finish rolling to the intermediate air cooling temperature is set to 50°C / second or higher to suppress excessive ferrite formation and to ensure that TiC precipitates are sufficiently precipitated by the subsequent intermediate air cooling at a high temperature. If the average cooling rate of primary cooling is less than 50°C, excessive ferrite may be formed and / or the ferrite may become coarse. Alternatively, the TiC precipitates may become coarse during the subsequent intermediate air cooling at a high temperature. On the other hand, if the average cooling rate of primary cooling exceeds 200°C, ferrite formation is excessively suppressed, and the ferrite area ratio in the final metal structure becomes less than 40%, resulting in a decrease in properties such as hole expansion. Therefore, the average cooling rate of primary cooling is set to 200°C / second or less, preferably 160°C / second or less. Although this process is referred to as the intermediate air cooling process for convenience, as mentioned above, it includes both primary cooling and intermediate air cooling.

[0072] If the intermediate air cooling temperature exceeds 750°C or the intermediate air cooling time exceeds 10 seconds, excessive ferrite formation may occur, or TiC precipitates may become coarse. If excessive ferrite formation occurs, it becomes impossible to form the desired microstructure containing ferrite, bainite, and martensite in specific proportions in the final steel sheet. Furthermore, if the TiC precipitates become coarse, the number density of these TiC precipitates may decrease significantly. In such cases, the hardness-enhancing effect of ferrite due to precipitation strengthening cannot be fully obtained. If the intermediate air cooling temperature becomes too high, exceeding 800°C, the intermediate air cooling temperature may exceed the ferrite transformation point, making ferrite formation difficult. In this case, excessive bainite formation occurs, and even with ferrite precipitation strengthening, it becomes impossible to adequately reduce the hardness difference in the microstructure, more specifically, the hardness difference between ferrite and bainite. As a result, the desired hole-expanding properties and / or yield ratio cannot be achieved. On the other hand, if the intermediate air cooling temperature is less than 705°C or the intermediate air cooling time is less than 3 seconds, the formation of TiC precipitates and grain growth are suppressed, and the desired diameter and / or number density cannot be obtained. Similarly, in this case, the hardness-improving effect of ferrite due to precipitation strengthening cannot be fully obtained. In addition, ferrite formation may be excessively suppressed, in which case it becomes impossible to form the desired metallic structure containing ferrite, bainite, and martensite in specific proportions in the final steel sheet. Furthermore, as described above, the suppression of TiC precipitate formation and grain growth, along with excessive suppression of ferrite formation, may make it impossible to properly precipitate and strengthen the desired amount of ferrite. In such cases, it becomes impossible to reduce the variation in hardness of ferrite and bainite to the desired range.

[0073] In contrast, in the intermediate air cooling step, primary cooling is performed to an intermediate air cooling temperature of 705-750°C at an average cooling rate of 50-200°C / second, preferably 50-160°C / second, and then intermediate air cooling is performed for 3-10 seconds, preferably 4-9 seconds, thereby precipitating ferrite in the desired proportion and generating TiC precipitates within the ferrite, which are then appropriately grown to produce TiC precipitates with a diameter of 2.0-8.0 nm, resulting in 1.0 × 10⁻¹⁶ TiC precipitates. 16 pieces / cm 3 This makes it possible to create ferrite at the above number density. As a result, by fully utilizing the hardness-enhancing effect of precipitation strengthening, it becomes possible to reduce the difference in hardness between ferrite and bainite in the metal structure, thereby further improving hole-expanding properties and yield ratio.

[0074] [Cooling process] After intermediate air cooling, the steel sheet is secondarily cooled at an average cooling rate of 50-200°C / second in the next cooling step, and then wound up at a winding temperature (secondary cooling stop temperature) of 20-300°C. Winding is performed immediately after secondary cooling. By secondarily cooling the steel sheet after intermediate air cooling at such a relatively fast average cooling rate, bainite and martensite can be appropriately precipitated, making it possible to form a metal structure in the final steel sheet that contains ferrite, bainite, and martensite in specific proportions. In contrast, if the average cooling rate of secondary cooling is less than 50°C / second, bainite and / or martensite cannot be appropriately precipitated, and therefore the desired metal structure cannot be obtained in the final steel sheet. In such cases, it becomes impossible to achieve a tensile strength of 780 MPa or more. Therefore, the average cooling rate of secondary cooling should be 50°C / second or higher, preferably 70°C / second or higher. On the other hand, if the average cooling rate of the secondary cooling exceeds 200°C, bainite will not form sufficiently and / or martensite will form excessively, making it impossible to obtain the desired metal structure in the final steel sheet. Therefore, the average cooling rate of the secondary cooling should be 200°C / second or less, preferably 180°C / second or less, or 150°C / second or less.

[0075] On the one hand, if the winding temperature exceeds 300°C, sufficient martensite will not be formed, and therefore sufficient strength cannot be obtained. On the other hand, if the winding temperature is too low, excessive water cooling will be required, reducing productivity. It may also cause embrittlement of the steel sheet. Therefore, the winding temperature should be 20°C or higher. Although this process is conveniently called the cooling process, as mentioned above, it is a process that includes secondary cooling and winding (excluding primary cooling).

[0076] According to the steel sheet manufactured by the above manufacturing method, by forming a metallic structure containing ferrite, bainite, and martensite in specific proportions, it is possible to increase the hole-expandability and yield ratio while maintaining a reasonably high level of strength in the steel sheet. Furthermore, 1.0 × 10 TiC precipitates with a diameter of 2.0 to 8.0 nm are present in the ferrite. 16 pieces / cm 3 Because they exist at the above number density, precipitation strengthening naturally contributes to improving the overall strength of the steel sheet. Furthermore, it is possible to sufficiently reduce the hardness difference between bainite, a relatively abundant hard phase in the hard structure, and ferrite, the softest of the three phases. As a result, it is possible to further improve the hole-expandability and yield ratio of the steel sheet. In addition, since the standard deviation in the hardness of ferrite and bainite is controlled to 0.40 GPa or less, in combination with the reduction in the hardness difference between bainite and ferrite caused by the use of TiC precipitates, it is possible not only to reduce the hardness difference in the entire metal structure, but also to reliably reduce the increase in local hardness differences within the metal structure. As a result, it is possible to more significantly improve the hole-expandability of the steel sheet. Therefore, steel sheets manufactured by the above manufacturing method can be effectively used in components where a balance between the conflicting properties of high strength and excellent workability is required, and furthermore, impact resistance is required, making them particularly useful in the automotive sector.

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

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

[0079] First, slabs with various chemical compositions shown in Table 1 were produced in a steelmaking plant. Then, after reheating to the heating temperatures shown in Table 2, rough bars with a thickness of 30 mm were produced by rough rolling. These rough bars were heated to 1250°C at an average heating rate of 20°C / second and held for 3600 seconds. Then, using a rolling mill consisting of multiple rolling stands, finish rolling was performed under the conditions shown in Table 2, with at least two rolling passes in the first stage and three rolling passes in the second stage. The final temperatures of the finish rolling were as shown in Table 2. Next, the finish-rolled steel sheets were primary-cooled to the intermediate air-cooling temperature under the conditions shown in Table 2, followed by intermediate air cooling. Finally, the intermediate-cooled steel sheets were secondary-cooled to the winding temperature under the conditions shown in Table 2, and then wound at that winding temperature to obtain steel sheets with a thickness of 2.5 mm.

[0080] [Table 1]

[0081] [Table 2]

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

[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 detailed herein, with an instrument-specific atomic detection rate α of 0.35. The detection rate α is calculated as [number of detected atoms] / [total number of original atoms], where "total number of original atoms" refers to the number of atoms in a standard sample (a sample with a known number of atoms). In other words, the detection rate α of the instrument can be determined by dividing the number of atoms detected from the standard sample using the actual instrument by the total number of original atoms in that standard sample.

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

[0085] [Hole expansion ratio: λ] The hole expansion ratio was determined as follows by conducting a hole expansion test in accordance with JIS Z 2256:2020. First, a test piece measuring 2.5 mm thick x 100 mm wide x 100 mm long was taken from the steel plate, and a punching tool with a punch diameter of 10 mm and a die diameter of 10.6 mm (clearance 12.5%) was used to punch out a hole (initial hole: hole diameter D o A hole diameter of 10mm was created. Next, with the burr facing the die side, the initial hole was widened using a conical punch with a 60° apex angle until a crack that penetrated the plate thickness occurred, and the hole diameter D at the time of crack occurrence was then made. h The hole expansion ratio λ (%) for each test specimen was calculated by measuring the diameter in mm and 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 with a tensile strength (TS) of 780 MPa or higher, a hole expansion ratio (λ) of 80.0% or higher, and a yield ratio (YR) of 0.70 or higher were evaluated as having high strength, high hole expansion properties, and a high yield ratio. The results are shown in Table 3.

[0087] Table 3

[0088] Referring to Tables 1-3, in Comparative Example 11, the total reduction ratio in the three final rolling passes of the finish rolling process was low, which prevented sufficient promotion of recrystallization and resulted in insufficient refinement of austenite grains. As a result, the standard deviation in the hardness of ferrite and bainite in the final resulting microstructure exceeded 0.40 GPa, and λ decreased. In Comparative Example 12, the total reduction ratio in the rolling passes prior to the three final rolling passes of the finish rolling process was low, preventing the acquisition of the desired ferrite area ratio. As a result, the difference in hardness between ferrite and bainite could not be adequately reduced, and λ decreased. In Comparative Example 13, the low intermediate air cooling temperature suppressed the formation and grain growth of TiC precipitates, preventing the acquisition of the desired number density of TiC precipitates. In addition, ferrite formation was excessively suppressed, preventing adequate precipitation strengthening of the desired amount of ferrite, and thus the variation in the hardness of ferrite and bainite could not be reduced to the desired range. As a result, the hardness improvement effect of ferrite due to precipitation strengthening could not be fully obtained, and λ decreased. In Comparative Example 14, the final finishing temperature was low, resulting in a non-uniform metal structure. Related to this, the standard deviation of the hardness of ferrite and bainite exceeded 0.40 GPa, and λ decreased. In Comparative Example 15, the average cooling rate during primary cooling up to the intermediate air cooling temperature was slow, which is thought to have caused the ferrite to coarseen. As a result, the standard deviation of 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 of TiC precipitates and grain growth, and the desired number density could not be obtained. As a result, the hardness improvement effect of ferrite and the strength improvement effect of the steel sheet due to precipitation strengthening could not be fully obtained, and TS, λ, and YR decreased. In Comparative Example 17, the intermediate air cooling temperature exceeded 800°C, which is thought to have made ferrite formation difficult, and consequently, excessive bainite formation occurred. As a result, the difference in hardness between ferrite and bainite could not be adequately reduced, and λ and YR decreased. In Comparative Example 18, the intermediate air cooling time was too long, resulting in excessive ferrite formation and a decrease in TS.Furthermore, λ and YR decreased due to the inability to obtain the desired metallic structure containing ferrite, bainite, and martensite in specific proportions.

[0089] In Comparative Example 19, the high finishing temperature of the end rolling process likely caused the austenite grains to coarseen after recrystallization, reducing the number of austenite grain boundaries and thus the ferrite nucleation sites. As a result, the desired ferrite area ratio could not be achieved even with the subsequent intermediate air cooling process, and the standard deviation of the hardness of ferrite and bainite in the final resulting microstructure could not be controlled to 0.40 GPa or less, resulting in a decrease in λ and YR. In Comparative Example 20, the slow average cooling rate during the secondary cooling after intermediate air cooling prevented sufficient precipitation of bainite and martensite, resulting in a decrease in TS. In Comparative Example 21, the high coiling temperature resulted in a martensite area ratio of less than 5%, leading to a decrease in TS. In Comparative Example 22, the low carbon content prevented the precipitation of TiC precipitates at a sufficient number density. As a result, TS, λ, and YR decreased. Similarly, in Comparative Example 23, the low titanium content prevented the precipitation of TiC precipitates at a sufficient number density. As a result, TS, λ, and YR decreased. In Comparative Example 24, the low Si content prevented sufficient suppression of cementite precipitation, and it is thought that the carbon in the steel was consumed in cementite formation. As a result, the formation of TiC precipitates was suppressed, and the number density of said TiC precipitates was 1.0 × 10⁻⁶. 16 pieces / cm 3 As a result, the hardness-enhancing effect of ferrite due to precipitation strengthening, and furthermore, the strength-enhancing effect of the steel sheet, could not be sufficiently obtained, and TS and λ decreased. In Comparative Example 25, because the Al content was low, the precipitation of cementite could not be sufficiently suppressed, and it is thought 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 said TiC precipitates was 1.0 × 10⁻⁶. 16 pieces / cm 3As a result, the hardness-enhancing effect of ferrite due to precipitation strengthening, and furthermore, the strength-enhancing effect of the steel plate, could not be fully obtained, and TS, λ, and YR decreased.

[0090] In contrast, the steel sheets in all the examples of the invention have a predetermined chemical composition, and by appropriately controlling each condition in the manufacturing method, they contain, by area ratio, ferrite: 40-80%, bainite: 15-55%, and martensite: 5-20%, with 1.0 × 10 TiC precipitates with a diameter of 2.0-8.0 nm in the ferrite. 16 pieces / cm 3 We were able to obtain a steel sheet having a metallic structure in which the above number densities exist and the standard deviation of the hardness of ferrite, bainite, and martensite is 0.40 GPa or less. As a result, we were able to achieve high strength of 780 MPa or more while significantly improving hole-expanding properties and yield ratio, due to the improvement in the strength of the steel sheet caused by ferrite precipitation strengthening by TiC precipitates, the reduction in the difference in hardness between ferrite and bainite due to said precipitation strengthening, and the reduction in the variation in the hardness of ferrite and bainite. In addition, when residual structures were present in the example of the invention, these residual structures were 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, The standard deviation of hardness for ferrite and bainite is 0.40 GPa or less. A steel plate characterized by having a tensile strength of 780 MPa or more.

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 to 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 plate thickness of 1.0 to 8.0 mm.

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

6. 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:

Images