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

Abstract

The present invention provides: a steel sheet that is characterized by having a metal structure which has a specific chemical composition and contains 20-50% by area of ferrite, 40-70% by area of bainite, and 5-20% by area of martensite, wherein the number density of TiC precipitates having a diameter of less than 10.0 nm is 1.0 × 1014 precipitates / cm3 or more, the number density of TiC precipitates having a diameter of not less than 10.0 nm but less than 30.0 nm is 1.0 × 1010 to 1.0 × 1014 precipitates / cm3, and the ratio of the average hardness of bainite and martensite to the average hardness of the ferrite is 0.80 to 1.20; a component including the same; and a method for manufacturing a steel sheet.

Description

[Technical Field]

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

[0002] In recent years, the automotive industry has been seeking to reduce the weight of vehicle bodies in order to improve fuel efficiency. Increasing the strength of the steel sheets used is one effective way to achieve both a lighter vehicle body and collision safety, and against this background, the development of high-strength steel sheets is underway.

[0003] On the other hand, many automotive components are made by press forming, and it is generally known that the formability of steel sheets decreases as the strength increases.

[0004] In this regard, for example, Patent Document 1 describes a high-strength hot-rolled steel sheet characterized by having a microstructure with a predetermined chemical composition, in which the total volume fraction of ferrite and bainite phases in the entire structure is 95% or more, the volume fraction of the ferrite phase in the entire structure is 50 to 90%, precipitates of less than 20 nm in size containing 650 to 1100 ppm of Ti are precipitated in the ferrite phase, and the bainite phase has a ΔHv of 150 or less. Patent Document 1 also teaches that if a microstructure is formed that is mainly composed of ferrite and bainite phases, in which precipitates of less than 20 nm in size containing 650 to 1100 ppm of Ti are precipitated in the ferrite phase, and the bainite phase has a ΔHv of 150 or less, a TS of 780 MPa or more can be ensured, thereby achieving both excellent stretch flangeability and impact resistance.

[0005] Patent Document 2 describes a steel sheet that satisfies, in mass%, C: 0.015 to 0.10%, Si: 2% or less, Mn: 2% or less, and Ti: 0.08 to 0.2%, has a granular bainitic ferrite structure, and contains 5 × 10 precipitates with a circle equivalent diameter of less than 0.03 μm. 6 pieces / mm 2 The number of precipitates with a circle equivalent diameter of 0.03 μm or more is 2 × 10 6 pieces / mm 2The document describes a high-strength hot-rolled steel sheet characterized by the following: Furthermore, Patent Document 2 teaches that the above configuration can improve both stretch flangeability and elongation to obtain a high-strength hot-rolled steel sheet with excellent formability.

[0006] Patent Document 3 describes a hot-rolled steel sheet having a predetermined composition and a multi-phase structure including a tempered bainite phase and a ferrite phase in a volume fraction of 30% or more but less than 45%, and wherein both the ferrite phase and the tempered bainite phase contain MX precipitates (where M represents a metal element and X represents carbon or nitrogen) having an outer diameter of less than 10 nm. Patent Document 3 also teaches that the above configuration makes it possible to obtain a hot-rolled steel sheet with an extremely good balance of strength and ductility, having a tensile strength of 1180 MPa or more and an elongation at break of 15% or more, while suppressing the amount of alloying elements.

[0007] Patent Document 4 describes a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, characterized in that the steel sheet has a predetermined chemical composition, a microstructure in which the combined volume fraction of bainite and bainitic ferrite phases in the entire structure is 50% or more, the combined volume fraction of bainite, bainitic ferrite, and polygonal ferrite phases is 95% or more, and the total Ti content in precipitates with an average diameter of less than 20 nm is 50% or more of the total Ti content in all precipitates in the steel. Patent Document 4 also teaches that by forming a microstructure without using Mo, which is mainly composed of bainite and bainitic ferrite phases and the substantial remainder being polygonal ferrite phase in which Ti-containing precipitates with a size of less than 20 nm are precipitated, a tensile strength of 780 MPa or more can be consistently ensured, and excellent elongation and stretch flangeability can be obtained. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-068945 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-307919 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-138449 [Patent Document 4] Japanese Patent Application Laid-Open No. 2011-122188 Summary of the Invention [Problem to be solved by the invention]

[0009] As described above, it is known that the formability of steel sheets decreases as their strength increases, and properties such as elongation and stretch flangeability (hole expandability) as described in Patent Documents 1 to 4 decrease. For example, if the hole expandability decreases, it may not be possible to process the steel sheets into the desired shape, such as for automobile suspension parts. For this reason, in the development of high-strength steel sheets such as high-strength hot-rolled steel sheets, it is important to increase the strength of the steel sheets while ensuring a certain level of properties appropriate for the application, such as the above-mentioned elongation and hole expandability.

[0010] Therefore, an object of the present invention is to provide a steel sheet having high strength and improved elongation and hole expandability, a part including the same, and a method for manufacturing the steel sheet. [Means for solving the problem]

[0011] In order to achieve the above object, the present inventors have conducted research focusing on the metallographic structure of steel sheets, particularly hot-rolled steel sheets. As a result, the present inventors have found that by configuring the metallographic structure of a hot-rolled steel sheet having a predetermined chemical composition with a structure mainly composed of ferrite and bainite and utilizing precipitation strengthening by adding Ti, it is possible to improve strength and elongation, and also to reduce the difference in hardness between ferrite and the hard phases bainite and martensite, thereby improving hole expandability, and have completed the present invention.

[0012] The present invention, which has achieved the above object, is as follows. (1) In mass%, C: 0.030~0.150%, Si: 0.01 to 1.00%, Mn: 0.50 to 3.00% Ti: 0.05 to 0.20% Al: 0.01 to 0.40%, P: 0.100% or less, S: 0.010% or less, N: 0.0100% or less, O: 0.010% or less, Nb: 0 to 0.15%, V: 0~1.00%, Cr: 0~2.00%, Ni: 0-2.00% Cu: 0-2.00% Mo: 0-1.00%, B: 0~0.0100%, Sn: 0 to 1.00% Sb: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, Bi: 0 to 0.010% REM: 0 to 0.0100%, As: 0~0.010%, Zr: 0 to 0.010% Co: 0-2.00%, Zn: 0 to 0.010%, W: 0 to 1.00%, and The balance has a chemical composition consisting of Fe and impurities, Area % Ferrite: 20-50%, Bainite: 40-70%, and Martensite: Contains 5-20% The number density of TiC precipitates with a diameter of less than 10.0 nm is 1.0 × 10 14 pieces / cm 3 or more, and the number density of TiC precipitates with a diameter of 10.0 to less than 30.0 nm is 1.0 × 10 10 ~1.0×10 14 pieces / cm 3 and A steel plate characterized by having a metal structure in which the ratio of the average hardness of bainite and martensite to the average hardness of ferrite is 0.80 to 1.20. (2) The steel sheet according to (1) above, characterized in that the chemical composition contains, in mass %, Al: 0.20 to 0.40%. (3) The chemical composition is in mass%: Nb: 0.001 to 0.15%, V: 0.001 to 1.00%, Cr: 0.001 to 2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Mo: 0.001 to 1.00%, B: 0.0001~0.0100%, Sn: 0.001 to 1.00%, Sb: 0.001 to 1.00%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, Bi: 0.001 to 0.010%, REM: 0.0001 to 0.0100%, As: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Co: 0.001 to 2.00%, Zn: 0.001 to 0.010%, and W: 0.001 to 1.00% The steel sheet according to (1) or (2) above, characterized in that it contains at least one of the following: (4) The steel sheet according to any one of (1) to (3) above, characterized in that the area ratio of ferrite containing TiC precipitates with diameters of 10.0 to less than 30.0 nm is 10 to less than 50% of the total ferrite. (5) The steel sheet according to any one of (1) to (4) above, characterized in that it has a tensile strength of 780 MPa or more. (6) The steel sheet according to any one of the above (1) to (5), characterized in that it has a thickness of 1.0 to 8.0 mm. (7) A part comprising the steel sheet according to any one of (1) to (6) above. (8) A hot rolling process comprising heating a slab having the chemical composition described in any one of (1) to (3) above and then finish rolling the slab, wherein the hot rolling process satisfies the following conditions (a) to (c): (a) The heating temperature of the slab is 1150 to 1300°C. (b) The holding time in the temperature range of 1150 to 1300°C is 1000 to 4000 seconds, and (c) The finishing temperature of the finish rolling is 850 to 950°C. An intermediate cooling process in which the finish-rolled steel plate is primarily cooled to an intermediate air-cooling temperature of 600 to 750 ° C. at an average cooling rate of 30 to 200 ° C. / second, and then intermediate air-cooled at an average cooling rate of 15 ° C. / second or less for 3 to 15 seconds; The intermediately air-cooled steel sheet is then cooled for a second time at an average cooling rate of 50 to 200°C / sec, and then coiled at a coiling temperature of 20 to 290°C. A method for manufacturing a steel sheet, comprising: [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a steel sheet, particularly a hot-rolled steel sheet, having high strength and improved elongation and hole expandability, a part including the same, and a method for manufacturing the steel sheet. DETAILED DESCRIPTION OF THE INVENTION

[0014] <Steel plate> The steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, has, in mass%, C: 0.030~0.150%, Si: 0.01 to 1.00%, Mn: 0.50 to 3.00% Ti: 0.05 to 0.20% Al: 0.01 to 0.40%, P: 0.100% or less, S: 0.010% or less, N: 0.0100% or less, O: 0.010% or less, Nb: 0 to 0.15%, V: 0~1.00%, Cr: 0~2.00%, Ni: 0-2.00% Cu: 0-2.00% Mo: 0-1.00%, B: 0~0.0100%, Sn: 0 to 1.00% Sb: 0 to 1.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Hf: 0 to 0.0100%, Bi: 0 to 0.010% REM: 0 to 0.0100%, As: 0~0.010%, Zr: 0 to 0.010% Co: 0-2.00%, Zn: 0 to 0.010%, W: 0 to 1.00%, and The balance has a chemical composition consisting of Fe and impurities, Area % Ferrite: 20-50%, Bainite: 40-70%, and Martensite: Contains 5-20% The number density of TiC precipitates with a diameter of less than 10.0 nm is 1.0 × 10 14 pieces / cm 3 or more, and the number density of TiC precipitates with a diameter of 10.0 to less than 30.0 nm is 1.0 × 10 10 ~1.0×10 14 pieces / cm 3 and The alloy is characterized by having a metal structure in which the ratio of the average hardness of bainite and martensite to the average hardness of ferrite is 0.80 to 1.20.

[0015] As mentioned above, it is known that properties such as elongation and hole expandability decrease as the strength of a steel sheet increases. Therefore, it is generally difficult to improve elongation and hole expandability while maintaining sufficient strength of the steel sheet. Therefore, the inventors conducted research, focusing not only on the chemical composition of the steel sheet but also on the metallographic structure of the steel sheet. More specifically, the inventors first discovered that the strength and elongation of a steel sheet having a predetermined chemical composition can be improved by configuring the metallographic structure of the steel sheet to be mainly composed of ferrite and bainite, more specifically, by configuring the metallographic structure to contain 20 to 50% ferrite and 40 to 70% bainite by area percentage, and utilizing precipitation strengthening by adding Ti.

[0016] Next, the inventors further investigated the effect of TiC precipitates obtained by adding Ti on the properties of steel sheets. As a result, the inventors classified TiC precipitates into TiC precipitates with smaller diameters (i.e., TiC precipitates with a diameter of less than 10.0 nm) and TiC precipitates with larger diameters (i.e., TiC precipitates with a diameter of 10.0 to less than 30.0 nm) and found that by appropriately controlling their number densities, strength and elongation can be further improved, and the difference in hardness between ferrite and the hard phases bainite and martensite can be reduced, thereby significantly improving hole expandability. More specifically, the inventors found that the number density of TiC precipitates with a diameter of less than 10.0 nm can be reduced to 1.0 × 10 14 pieces / cm 3 or more, and the number density of TiC precipitates with a diameter of 10.0 to less than 30.0 nm is controlled to 1.0 × 10 10 ~1.0×10 14 pieces / cm 3 By controlling the ratio of the average hardness of bainite and martensite to the average hardness of ferrite to the desired strength and elongation, it is possible to control the ratio of the average hardness of bainite and martensite to the average hardness of ferrite to the range of 0.80 to 1.20, thereby significantly improving the hole expandability.

[0017] While not intending to be bound by any particular theory, it is believed that while both TiC precipitates with a diameter of less than 10.0 nm and TiC precipitates with a diameter of 10.0 to 30.0 nm can contribute to improving the strength and hardness of the metallographic structure, TiC precipitates with a diameter of less than 10.0 nm are more effective in improving strength and hardness than TiC precipitates with a diameter of 10.0 to 30.0 nm. Therefore, it is believed that by having TiC precipitates of different sizes present in the steel sheet at appropriate number densities, regions with locally different strength and hardness are formed in the steel sheet. In relation to this, it is believed that strength and elongation can be improved by a mechanism similar to that of so-called dual phase (DP) steel, in which a soft phase consisting of ferrite and a hard phase consisting of martensite coexist despite the steel sheet having a structure mainly composed of ferrite and bainite. In addition, in embodiments of the present invention, it is believed that a steel sheet with significantly improved elongation can be obtained by including a relatively large amount of ferrite, i.e., 20 to 50% by area of ​​ferrite.

[0018] As described above, in the embodiment of the present invention, it is important to have both TiC precipitates with diameters of less than 10.0 nm and TiC precipitates with diameters of 10.0 to 30.0 nm present in the steel sheet at appropriate number densities. However, if the number density of TiC precipitates with diameters of less than 10.0 to 30.0 nm becomes too high, much of the Ti and C in the steel sheet will be consumed in the formation of TiC precipitates with diameters of less than 10.0 to 30.0 nm. This naturally reduces the number density of TiC precipitates with diameters of less than 10.0 nm, which have a greater strength-improving effect, and as a result, it is thought that the strength of the steel sheet cannot be sufficiently improved. In addition, if the number density of TiC precipitates with diameters of less than 10.0 nm becomes too high, the proportion of ferrite containing TiC precipitates with diameters of less than 10.0 nm, which have a greater hardness-improving effect, also decreases. In this case, the hardness of the soft ferrite phase cannot be sufficiently improved. For this reason, it is thought that it becomes impossible to control the ratio of the average hardness of bainite and martensite to the average hardness of ferrite within the range of 0.80 to 1.20, and as a result, it becomes impossible to sufficiently improve the hole expandability of the steel sheet. Furthermore, even when the proportion of martensite in the metal structure becomes too high, exceeding 20 area %, the average hardness of bainite and martensite becomes very high, making it difficult to control the ratio of the average hardness of bainite and martensite to the average hardness of ferrite within the range of 0.80 to 1.20.

[0019] Therefore, in an embodiment of the present invention, the metal structure is configured to contain, by area %, ferrite: 20 to 50%, bainite: 40 to 70%, and martensite: 5 to 20%, and the number density of TiC precipitates with a diameter of 10.0 to less than 30.0 nm is set to 1.0 × 10 10 ~1.0×10 14 pieces / cm 3 In this regard, it is considered particularly important to control TiC precipitates with a diameter of less than 10.0 nm, which have a greater effect on improving strength, within the range of 1.0 × 10 14 pieces / cm 3It is believed that the TiC precipitates can be present in the steel sheet at a sufficient number density as described above, and that by combining them with TiC precipitates having a diameter of 10.0 to less than 30.0 nm, it is possible to significantly improve the strength and elongation of the steel sheet by the same mechanism as in DP steel as described above. 14 pieces / cm 3 By limiting the range to 0.80 to 1.20, the proportion of ferrite containing TiC precipitates with a diameter of less than 10.0 nm, which has a greater effect on improving hardness, can be sufficiently increased. Therefore, it is possible to control the ratio of the average hardness of bainite and martensite to the average hardness of ferrite within the range of 0.80 to 1.20, which is believed to significantly improve the hole expandability of the steel sheet. Therefore, according to the steel sheet according to the embodiment of the present invention, it is possible to significantly improve the elongation and hole expandability despite the high strength, for example, tensile strength of 780 MPa or more. Therefore, the steel sheet according to the embodiment of the present invention can reliably achieve the contradictory properties of high strength and excellent formability, and is therefore particularly useful in the automotive field, where both properties are required.

[0020] Hereinafter, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower limit and upper limit, unless otherwise specified.

[0021] [C: 0.030~0.150%] C is an element effective in increasing the strength of steel sheet. Furthermore, C forms carbides and / or carbonitrides with Ti and Nb in steel, contributing to precipitation strengthening based on the precipitates formed. To fully obtain these effects, the C content is set to 0.030% or more. The C content may be 0.040% or more, 0.050% or more, 0.060% or more, or 0.070% or more. On the other hand, excessive C content may result in a decrease in hole expandability due to the formation of cementite. Therefore, the C content is set to 0.150% or less. The C content may be 0.140% or less, 0.120% or less, 0.100% or less, or 0.080% or less.

[0022] [Si: 0.01 to 1.00%] Si is an effective solid-solution strengthening element for increasing strength. Si also inhibits cementite precipitation. Therefore, the inclusion of Si can suppress the consumption of C in steel for the formation of cementite, thereby promoting the formation of TiC precipitates during cooling after hot rolling. To fully achieve these effects, the Si content is set to 0.01% or more. The Si content may be 0.02% or more, 0.03% or more, 0.04% or more, or 0.06% or more. On the other hand, excessive Si content can cause poor surface quality, known as Si scale, or excessive martensite formation. Excessive martensite formation can increase the hardness difference between ferrite and the hard phases bainite and martensite, potentially reducing hole expandability. Therefore, the Si content is set to 1.00% or less. The Si content may be 0.90% or less, 0.80% or less, 0.60% or less, 0.40% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[0023] [Mn: 0.50~3.00%] Mn is an element that is effective in increasing strength as an element for hardenability and solid solution strengthening. To fully obtain these effects, the Mn content is set to 0.50% or more. The Mn content may be 0.70% or more, 1.00% or more, 1.20% or more, or 1.50% or more. On the other hand, excessive Mn content may result in the formation of large amounts of MnS, which may reduce toughness. Therefore, the Mn content is set to 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[0024] [Ti: 0.05~0.20%] Ti is an element that precipitates finely in steel as carbide (TiC), improving the strength of steel through precipitation strengthening and increasing the hardness of ferrite. Ti also fixes C by forming carbides, suppressing the formation of cementite, which is detrimental to hole expandability. Furthermore, Ti also contributes to suppressing recrystallization. To fully achieve these effects, the Ti content is set to 0.05% or more. The Ti content may be 0.08% or more, 0.10% or more, 0.11% or more, 0.12% or more, 0.13% or more, or 0.14% or more. On the other hand, excessive Ti content may cause coarsening of carbides, preventing the desired precipitation strengthening of ferrite. In addition, as TiC precipitates become coarser, the number density of TiC precipitates also decreases, making it impossible to sufficiently increase the hardness of ferrite through precipitation strengthening. Therefore, the Ti content is set to 0.20% or less. The Ti content may be 0.18% or less, 0.17% or less, 0.16% or less, or 0.15% or less.

[0025] [Al: 0.01 to 0.40%] Al is an element that acts as a deoxidizer for molten steel. To fully achieve this effect, the Al content is set to 0.01% or more. The Al content may be 0.05% or more, 0.06% or more, 0.07% or more, 0.08% or more, 0.09% or more, 0.10% or more, or 0.15% or more. Al also has the effect of accelerating ferrite transformation and bainite transformation. To fully achieve this effect, the Al content is preferably set to 0.20% or more. The Al content may be 0.22% or more, 0.25% or more, or 0.28% or more. On the other hand, excessive Al content may form coarse oxides, resulting in reduced toughness and ductility. Therefore, the Al content is set to 0.40% or less. The Al content may be 0.38% or less, 0.35% or less, or 0.32% or less. In an embodiment of the present invention, the ratio of the Ti and Al contents, i.e., the Ti / Al ratio, is not particularly limited, but may be, for example, 0.13 to 18.00. The Ti / Al ratio can be appropriately determined, taking into consideration, among other things, the promotion of ferrite transformation due to the inclusion of Al and the improvement in ferrite hardness due to precipitation strengthening based on TiC precipitates. For example, the Ti / Al ratio may be 0.14 or more, 0.15 or more, 0.16 or more, 0.18 or more, 0.20 or more, 0.25 or more, 0.30 or more, 0.40 or more, or 0.50 or more. Similarly, the Ti / Al ratio may be 16.00 or less, 14.00 or less, 12.00 or less, 10.00 or less, 8.00 or less, 6.00 or less, 5.00 or less, or 3.33 or less.

[0026] [P:0.100% or less] Excessive P content may adversely affect weldability, etc. Therefore, the P content is set to 0.100% or less. The P content may be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0027] [S:0.010% or less] If S is contained in excess, a large amount of MnS may be formed, which may reduce toughness. Therefore, the Si content is set to 0.010% or less. The S content may be 0.005% or less, 0.003% or less, or 0.002% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0028] [N:0.0100% or less] Excessive N content can form coarse nitrides and reduce toughness. Therefore, the N content is set to 0.0100% or less. The N content may be 0.0080% or less, 0.0050% or less, or 0.0030% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0029] [O:0.010% or less] O is an element that is mixed in during the manufacturing process. Excessive O content can cause the formation of coarse inclusions, which can reduce the toughness of the steel plate. Therefore, the O content is set to 0.010% or less. The O content may be 0.008% or less, 0.006% or less, or 0.004% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long refining time, which leads to reduced productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0030] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.

[0031] [Nb: 0-0.15%] Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel, contributing to the refinement of the structure through a pinning effect, thereby increasing the strength of the steel sheet. Nb also contributes to the suppression of recrystallization. The Nb content may be 0%, but to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may be 0.005% or more, 0.01% or more, 0.02% or more, 0.03% or more, or 0.04% or more. On the other hand, excessive Nb content may cause the formation of coarse carbides in the steel, reducing the ductility of the steel sheet. Therefore, the Nb content is preferably 0.15% or less. The Nb content may be 0.12% or less, 0.10% or less, 0.08% or less, or 0.05% or less.

[0032] [V:0~1.00%] V is an element that contributes to improving strength through precipitation strengthening and the like. The V content may be 0%, but to obtain such effects, the V content is preferably 0.001% or more. The V content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if V is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the V content is preferably 1.00% or less. The V content may be 0.50% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[0033] [Cr:0~2.00%] Cr is an element that improves the hardenability of steel and contributes to improving its strength. The Cr content may be 0%, but to achieve this effect, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if Cr is added excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the Cr content is preferably 2.00% or less. The Cr content may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0034] [Ni: 0-2.00%] [Cu: 0-2.00%] Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening. The Ni and Cu contents may be 0%, but to obtain such effects, the contents of these elements are preferably 0.001% or more, and may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if these elements are contained in excess, the effects may saturate and manufacturing costs may increase. Therefore, the Ni and Cu contents are preferably 2.00% or less, and may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0035] [Mo: 0-1.00%] Mo is an element that improves the hardenability of steel and contributes to improving its strength. The Mo content may be 0%, but to achieve this effect, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, excessive Mo content may increase deformation resistance during hot working, resulting in increased equipment load. Therefore, the Mo content is preferably 1.00% or less. The Mo content may be 0.80% or less, 0.50% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[0036] [B: 0~0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness. The B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if B is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.

[0037] [Sn: 0~1.00%] [Sb: 0~1.00%] Sn and Sb are elements effective in improving corrosion resistance. The Sn and Sb contents may be 0%, but to obtain such effects, the contents of these elements are preferably 0.001% or more, and may be 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, excessive inclusion of these elements may result in a decrease in toughness. Therefore, the Sn and Sb contents are preferably 1.00% or less, and may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.

[0038] [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 contents may be 0%, but to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, or 0.0010% or more. On the other hand, even if these elements are contained in excess, the effects saturate, and adding more than necessary to the steel sheet increases the manufacturing cost. Therefore, the Ca, Mg, and Hf contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0039] [Bi: 0~0.010%] Bi is an element effective in improving corrosion resistance. The Bi content may be 0%, but to obtain this effect, the Bi content is preferably 0.001% or more. The Bi content may be 0.002% or more. On the other hand, even if an excessive amount of Bi is contained, the effect saturates, and adding more Bi than necessary to the steel sheet increases the manufacturing cost. Therefore, the Bi content is preferably 0.010% or less. The Bi content may be 0.005% or less or 0.003% or less.

[0040] [REM:0~0.0100%] REM is an element that can control the morphology of nonmetallic inclusions. The REM content may be 0%, but to achieve this effect, the REM content is preferably 0.0001% or more. The REM content may be 0.0005% or more, or 0.0010% or more. On the other hand, if REM is contained in an excessive amount, the effect saturates, and adding more REM than necessary to the steel sheet increases production costs. Therefore, the REM content is preferably 0.0100% or less. The REM content may be 0.0050% or less, 0.0030% or less, or 0.0020% or less. In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0041] [As: 0~0.010%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain this effect, the As content is preferably 0.001% or more. The As content may be 0.002% or more, or 0.003% or more. On the other hand, even if an excessive amount of As is contained, the effect saturates, and adding more As than necessary to the steel sheet increases the manufacturing cost. Therefore, the As content is preferably 0.010% or less. The As content may be 0.008% or less, or 0.005% or less.

[0042] [Zr: 0~0.010%] Zr is an element that can control the morphology of nonmetallic inclusions. The Zr content may be 0%, but to obtain this effect, the Zr content is preferably 0.001% or more. The Zr content may be 0.002% or more, or 0.003% or more. On the other hand, even if an excessive amount of Zr is contained, the effect saturates, and adding more Zr than necessary to the steel sheet increases the manufacturing cost. Therefore, the Zr content is preferably 0.010% or less. The Zr content may be 0.008% or less, or 0.005% or less.

[0043] [Co: 0-2.00%] Co is an element that contributes to improving hardenability and / or heat resistance. The Co content may be 0%, but to obtain these effects, the Co content is preferably 0.001% or more. The Co content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Co content may deteriorate hot workability and increase raw material costs. Therefore, the Co content is preferably 2.00% or less. The Co content may be 1.00% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0044] [Zn: 0-0.010%] Zn is an element that can be contained in steel sheets when scrap or the like is used as a steel raw material. Therefore, the Zn content is preferably 0.010% or less, and may be 0.008% or less, or 0.005% or less. The Zn content may be 0%, but reducing it to less than 0.001% requires a long refining time, resulting in reduced productivity. Therefore, the Zn content may be 0.001% or more, 0.002% or more, or 0.003% or more.

[0045] [W:0~1.00%] W is an element that improves the hardenability of steel and contributes to improving its strength. The W content may be 0%, but to obtain this effect, the W content is preferably 0.001% or more. The W content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive W content may deteriorate weldability. Therefore, the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0046] In the steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. The impurities are components that are mixed in during the industrial production of steel sheet due to various factors in the production process, including raw materials such as ore and scrap.

[0047] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0048] [Metal structure] The metallographic structure of the steel sheet according to the present invention includes, in area percentages, 20-50% ferrite, 40-70% bainite, and 5-20% martensite. By incorporating a predetermined amount of martensite into the metallographic structure of the steel sheet, while maintaining a structure primarily composed of ferrite and bainite, the strength of the steel sheet can be increased while improving elongation. By incorporating these three structures at the specific area percentages described above and utilizing precipitation strengthening by TiC precipitates, which will be described in detail later, the strength and elongation of the steel sheet can be further increased, and the hardness difference between ferrite and the hard phases bainite and martensite can be reduced, significantly improving hole expandability. For example, a low ferrite area percentage may result in a high proportion of the hard phases bainite and martensite, particularly the proportion of bainite, which may result in reduced elongation. Alternatively, a low ferrite area percentage may not be able to adequately reduce the hardness difference between ferrite and the bainite and martensite even through precipitation strengthening by TiC precipitates. In such cases, the desired hole expandability cannot be achieved. Therefore, the area fraction of ferrite must be 20% or more, and may be, for example, 25% or more, 30% or more, or 35% or more. On the other hand, if the area fraction of ferrite is high, the proportion of hard phases, bainite and martensite, decreases, and as a result, a desired strength, for example, a tensile strength of 780 MPa or more, may not be achieved. Therefore, the area fraction of ferrite is set to 50% or less, and may be, for example, 49% or less, 48% or less, 47% or less, 46% or less, 45% or less, 44% or less, 42% or less, 40% or less, or 38% or less.

[0049] From the viewpoint of improving tensile strength, it is preferable that the area fractions of the hard phases, bainite and martensite, are high. From this viewpoint, for example, the area fraction of bainite may be 42% or more, 45% or more, 48% or more, 50% or more, 55% or more, or 58% or more. Similarly, the area fraction of martensite may be 6% or more, 7% or more, 8% or more, 9% or more, 10% or more, 11% or more, or 12% or more. On the other hand, from the viewpoint of further improving elongation and / or further reducing the hardness difference in the metal structure to further improve hole expandability, it is preferable that the area fractions of bainite and martensite are low. From this viewpoint, for example, the area fraction of bainite may be 68% or less, 65% or less, 62% or less, or 60% or less. Similarly, the area fraction of martensite may be 18% or less, 16% or less, or 14% or less.

[0050] [Remaining tissue] As described above, the metal structure of the steel sheet according to the embodiment of the present invention includes ferrite, bainite, and martensite, and may also include other residual structures. However, the area ratio of the residual structure is preferably small, even 0%. The area ratio of the residual structure is not particularly limited, and 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 structure may be 1% or 2%. When a residual structure is present, the residual structure includes or is at least one of pearlite and retained austenite, preferably pearlite. For example, when the residual structure includes retained austenite, the area ratio of the retained austenite is 2% or less, or 1% or less, preferably 0.5% or less.

[0051] [Identification of metal structure and calculation of area ratio] Structural observations are performed using a scanning electron microscope. Prior to observation, the sample for structural observation is wet-polished with emery paper and diamond abrasives with an average particle size of 1 μm. The observation surface is then polished to a mirror finish, and the structure is then etched with a 3% nitric acid alcohol solution. The magnification for observation is 2000x, and 10 random images are taken of a 30 μm x 40 μm field of view at a position 1 / 4 of the plate thickness from the surface. The structural ratio is determined using the point counting method. A total of 225 grid points, spaced 3 μm vertically and 4 μm horizontally, are defined on the obtained structural image. The structure present below the grid points is identified, and the structural ratio contained in the steel is calculated from the average of the 10 images. Ferrite is a massive crystal grain that does not contain iron-based carbides with a major axis of 100 nm or more. Bainite is a collection of lath-shaped crystal grains that either does not contain iron-based carbides with a major axis of 20 nm or more, or contains iron-based carbides with a major axis of 20 nm or more, but the carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction. Here, a group of iron-based carbides elongated in the same direction refers to iron-based carbides whose elongation directions differ by less than 5°. A single bainite grain is counted as a single bainite grain when surrounded by grain boundaries with a misorientation of 15° or more. Furthermore, martensite, which contains a large amount of solute carbon, appears brighter than other structures and can be distinguished from other structures. When structures other than ferrite, bainite, and martensite are present, the area fraction of the remaining structure is determined by subtracting the total area fraction of ferrite, bainite, and martensite from 100%. Although it is not necessary to specifically identify the remaining structure, when the remaining structure includes pearlite and retained austenite, the pearlite has a unique structure in which cementite precipitates in a lamellar form, and therefore can be identified using a scanning electron microscope. Furthermore, the volume fraction of 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.

[0052] [Number density of TiC precipitates with a diameter of less than 10.0 nm: 1.0 × 10 14 pieces / cm 3 End] [Number density of TiC precipitates with diameters of 10.0 to less than 30.0 nm: 1.0 × 10 10 ~1.0×10 14 pieces / cm 3 ] In the metal structure of the steel sheet according to the embodiment of the present invention, the number density of TiC precipitates having a diameter of less than 10.0 nm is 1.0 × 10 14 pieces / cm 3 or more, and the number density of TiC precipitates with a diameter of 10.0 to less than 30.0 nm is 1.0 × 10 10 ~1.0×10 14 pieces / cm 3 Here, TiC precipitates include not only TiC but also composite carbides containing Ti and elements other than Ti, such as V or Nb, i.e., (Ti,Nb,V)C, and even NbC. When the steel sheet according to the embodiment of the present invention contains Nb within a range of 0.15% or less, NbC may precipitate in addition to (Ti,Nb)C, but the amount of NbC precipitates is extremely small compared to (Ti,Nb)C precipitates. Therefore, even if NbC precipitates are included in the TiC precipitates, the effect of NbC precipitates on the number density of TiC precipitates is substantially negligible. By having TiC precipitates of different sizes present in the steel sheet at the respective number densities described above, it is possible to form regions in the steel sheet that have locally different strengths and hardnesses. In relation to this, despite the structure being primarily composed of ferrite and bainite, it is possible to improve strength and elongation through an action mechanism similar to that of so-called DP steel, in which a soft phase of ferrite and a hard phase of martensite are mixed. Furthermore, by including 20 to 50% by area of ​​ferrite as described above, it is possible to obtain a steel sheet with significantly improved elongation. In other words, unless either the number density of TiC precipitates with a diameter of less than 10.0 nm or the number density of TiC precipitates with a diameter of 10.0 to 30.0 nm is satisfied, sufficient improvement in strength and / or elongation may not be achieved.

[0053] From the viewpoint of further improving the strength, the higher the number density of TiC precipitates with a diameter of less than 10.0 nm, which has a greater effect of improving the strength, the more preferable it is. For example, the number density of TiC precipitates with a diameter of less than 10.0 nm is 2.0 × 10 14 pieces / cm 3 That's it, 5.0 x 10 14 pieces / cm 3 That's it, 10.0 x 10 14 pieces / cm 3 That's it, 20.0 x 10 14 pieces / cm 3 or more, or 30.0 x 10 14 pieces / cm 3 The upper limit is not particularly limited, but for example, the number density of TiC precipitates with a diameter of less than 10.0 nm is 200.0 × 10 14 pieces / cm 3 Below, 180.0 x 10 14 pieces / cm 3 Below, 150.0 x 10 14 pieces / cm 3 or less, or 100.0 x 10 14 pieces / cm 3 In addition, from the viewpoint of improving hole expandability, it is preferable not to generate an extreme difference in hardness in the metal structure, and therefore the higher the number density of TiC precipitates with diameters of 10.0 to less than 30.0 nm, which have a relatively low hardness improvement effect, the more preferable it is. Therefore, for example, the number density of TiC precipitates with diameters of 10.0 to less than 30.0 nm is 5.0 × 10 10 pieces / cm 3 That's it, 1.0 x 10 11 pieces / cm 3 That's it, 5.0 x 10 11 pieces / cm 3 or more, or 1.0 x 10 12 pieces / cm 3 On the other hand, from the viewpoint of increasing the number density of TiC precipitates having a diameter of less than 10.0 nm to further improve strength, it is preferable to reduce the number density of TiC precipitates having a diameter of 10.0 to less than 30.0 nm so that most of the Ti and C in the steel sheet are consumed in the formation of TiC precipitates having a diameter of less than 10.0 nm. Therefore, for example, the number density of TiC precipitates having a diameter of 10.0 to less than 30.0 nm is 8.0 × 10 13pieces / cm 3 Below, 6.0 x 10 13 pieces / cm 3 Below, 5.0 x 10 13 pieces / cm 3 Below, 3.0 x 10 13 pieces / cm 3 or less, or 1.0 x 10 13 pieces / cm 3 It may be the following:

[0054] [Measurement of the number density of TiC precipitates with diameters less than 10.0 nm and 10.0 to 30.0 nm] The number density of TiC precipitates with diameters less than 10.0 nm and between 10.0 and 30.0 nm is determined using the extraction residue method as follows. First, the surface of the test material is mechanically ground to a position one-quarter of the plate thickness. Next, electrolytic extraction is performed in two steps using a 10% AA-based electrolyte (10% acetylacetone (AA) by volume, 1% tetramethylammonium chloride (TMAC) by mass, methanol) containing 0.05% sodium dodecyl sulfate (SDS) as a dispersant. This exposes the TiC precipitates on the sample surface. More specifically, in the first step (pre-electrolysis), electrolysis is performed at 500 mA and 450 C (coulombs) using a 0.05% SDS-10% AA-based electrolyte to remove any dirt or oxide layer from the sample surface. After preliminary electrolysis, the specimen was removed from the electrolyte, its surface washed with methanol, and then immersed in a separately prepared 0.05% SDS-10% AA electrolyte. The second stage of electrolytic extraction (main electrolysis) was performed at 500 mA and 3600 C (coulombs). Approximately 1 g of specimen was electrolyzed. After main electrolysis, the specimen was immersed in a Hansen Solubility Parameter (HSP) dispersion (50% ethylene glycol / 50% dimethyl sulfoxide) and ultrasonicated for 2-3 minutes to disperse and recover the extracted fine TiC precipitates. The size distribution of the fine TiC precipitates recovered in the HSP dispersion was measured using inductively coupled plasma mass spectrometry (ICP-MS) using field-flow fractionation (FFF) (FFF-ICP-MS analysis) in the range of 0-500 nm. Finally, the chromatograms obtained from the FFF-ICP-MS analysis were quantified by size using the flow injection (FI) method, and the number densities of TiC precipitates with spherical equivalent diameters of less than 10.0 nm and 10.0 to 30.0 nm were determined. Since nitrides such as TiN and NbN precipitate preferentially, it is possible to calculate the number density of TiC precipitates by excluding these nitrides from the precipitates based on the N analysis value.

[0055] [The area ratio of ferrite containing TiC precipitates with diameters of 10.0 to 30.0 nm is 10 to 50% of the total ferrite] In the metal structure of the steel sheet according to the embodiment of the present invention, the area ratio of ferrite in which TiC precipitates with diameters of 10.0 to less than 30.0 nm are present is preferably 10 to less than 50% of the total ferrite. 14 pieces / cm 3 Simply limiting the area ratio to the range below 0.80 to 1.20 results in a sufficiently high proportion of ferrite containing TiC precipitates with diameters of less than 10.0 nm, which have a greater effect on improving hardness. However, by additionally controlling the area ratio of ferrite containing TiC precipitates with diameters of 10.0 to 30.0 nm to less than 10 to 50% of the total ferrite, the proportion of ferrite containing TiC precipitates with diameters of less than 10.0 nm, which have a greater effect on improving hardness, can be directly and reliably increased. Therefore, it becomes possible or easier to more reliably control the ratio of the average hardness of bainite and martensite to the average hardness of ferrite within the range of 0.80 to 1.20, which significantly improves the hole expandability of the steel sheet.

[0056] From the viewpoint of further improving hole expandability, it is preferable to increase the proportion of ferrite containing TiC precipitates with diameters of less than 10.0 nm. In this regard, the lower the area fraction of ferrite containing TiC precipitates with diameters of less than 10.0 to 30.0 nm, the better. Therefore, for example, the area fraction of ferrite containing TiC precipitates with diameters of less than 10.0 to 30.0 nm may be 45% or less, 40% or less, 35% or less, or 30% or less of the total ferrite. On the other hand, if the area fraction of ferrite containing TiC precipitates with diameters of less than 10.0 to 30.0 nm becomes too low, the proportion of bainite and martensite containing TiC precipitates with diameters of less than 10.0 to 30.0 nm may also become too low. In such cases, the proportion of TiC precipitates with diameters of less than 10.0 nm in the entire metallographic structure may increase, resulting in a large hardness difference within the metallographic structure. Therefore, the area ratio of ferrite containing TiC precipitates with diameters of 10.0 to less than 30.0 nm is set to 10% or more of the total ferrite, and may be, for example, 12% or more, 15% or more, 20% or more, or 25% or more.

[0057] [Measurement of ferrite area ratio where TiC precipitates with diameters of 10.0 to less than 30.0 nm exist] The area fraction of ferrite containing TiC precipitates with diameters of 10.0 to less than 30.0 nm is determined using a transmission electron microscope (TEM) as follows. First, a sample taken from a steel sheet is processed into a 0.1 mm-thick thin section including the 1 / 4 position of the sheet thickness, and then the sample is prepared by electropolishing. This sample is then observed using a 200 kV transmission electron microscope, with the electron beam incident direction on the ferrite adjusted to obtain sufficient contrast within a range of 10 degrees from the

[0001] zone axis, to observe the TiC precipitates. Next, to adjust the observation conditions, a bright-field image of the structure is taken at 10,000x magnification, including ferrite and bainite. The brightness of the observed image (output as a bmp image file) is displayed on a gradation scale of 0 to 255, and the imaging conditions are adjusted so that the average brightness is between 100 and 155. Next, ten ferrite microstructure photographs (bright-field images) were taken at a magnification of 100,000 times. The area ratio of ferrite where TiC precipitates with a circle-equivalent diameter of less than 10.0 to 30.0 nm were observed relative to the entire ferrite was determined using the point counting method. In the point counting method, a 100 nm × 100 nm grid was set on the transmission electron microscope photograph. Within the grid, the number of regions where TiC precipitates with a circle-equivalent diameter of less than 10.0 to 30.0 nm were observed within ferrite grains was counted (if even one TiC precipitate with a circle-equivalent diameter of less than 10.0 to 30.0 nm was observed within a ferrite grain, the region was considered to contain the TiC precipitate), and the number of regions where ferrite grains were located was counted. The ratio of these was taken as the area ratio of ferrite where TiC precipitates with a circle-equivalent diameter of less than 10.0 to 30.0 nm were observed. When counting, TiC precipitates located on the lines of the grid frame are not taken into account, and only TiC precipitates located within the grid frame are considered. Finally, the area ratio of ferrite containing TiC precipitates with diameters of 10.0 to 30.0 nm or less is determined by arithmetically averaging the 10 ferrite area ratios obtained. Ferrite is identified in TEM observations using brightness. Specifically, the average brightness of the entire image is calculated, and areas with brightness higher than this average brightness are identified as ferrite. TiC precipitates are also identified by analyzing the crystal structure from the TEM electron diffraction image.Specifically, if the presence of TiC can be confirmed from the electron diffraction image of two or three precipitates present in a ferrite grain, the precipitates observed in the ferrite grain are determined to be TiC precipitates.

[0058] [Ratio of the average hardness of bainite and martensite to the average hardness of ferrite: 0.80 to 1.20] In the metallographic structure of the steel sheet according to the embodiment of the present invention, the ratio of the average hardness of bainite and martensite to the average hardness of ferrite, i.e., (average hardness of bainite and martensite) / (average hardness of ferrite), is 0.80 to 1.20. By controlling the ratio of the average hardness of bainite and martensite to the average hardness of ferrite within this range, the difference in hardness between ferrite and the hard phases bainite and martensite in the metallographic structure can be sufficiently reduced. As a result, the hole expandability of the steel sheet can be significantly improved. From the viewpoint of further increasing the hole expansion ratio, the ratio of the average hardness of bainite and martensite to the average hardness of ferrite is preferably as close to 1.00 as possible. Therefore, the ratio of the average hardness of bainite and martensite to the average hardness of ferrite may be 0.82 or more, 0.85 or more, 0.88 or more, or 0.90 or more, or may be 1.15 or less, 1.10 or less, 1.05 or less, or 1.00 or less.

[0059] [Method for determining the ratio of the average hardness of bainite and martensite to the average hardness of ferrite] The ratio of the average nanohardness of ferrite to that of bainite is determined as follows. First, a sample is cut from a steel plate so that a cross-section perpendicular to the surface can be observed. The cross-section of the sample is wet-polished with emery paper and then polished to a mirror finish with diamond abrasives having an average particle size of 1 μm. The mirror-finished cross-section is then indented at a depth of 1 / 4 of the plate thickness from the surface using a microhardness tester with a test load of 1000 μN at measurement intervals of 5 μm, resulting in a total of 100 measurements of nanohardness. Next, the same sample is measured using a scanning electron microscope. Based on the obtained microstructural analysis results, only measurement points with indentations within the ferrite, bainite, and martensite are extracted. Finally, the arithmetic average of the nano-hardnesses for the ten extracted ferrite samples is taken as the average hardness of the ferrite, and the arithmetic average of the nano-hardnesses for the ten extracted bainite samples and martensite samples is taken as the average hardness of the bainite and martensite. The ratio of these (average hardness of bainite and martensite) / (average hardness of ferrite) is determined as the ratio of the average hardness of bainite and martensite to the average hardness of ferrite.

[0060] Plate Thickness The steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 8.0 mm, although it is not particularly limited thereto. 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, 5.5 mm or less, 5.0 mm or less, 4.4 mm or less, 4.2 mm or less, or 4.0 mm or less.

[0061] As described above, the steel sheet according to the embodiment of the present invention is capable of achieving excellent elongation and hole expandability despite its high strength, and therefore can reliably achieve a high level of compatibility between the contradictory properties of high strength and excellent formability. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful for use in components in technical fields where compatibility between these properties is required. In a preferred embodiment, an automobile part, particularly an automobile suspension part, is provided that includes the steel sheet according to the embodiment of the present invention. Examples of automobile suspension parts include lower arms and trailing arms. These automobile parts, particularly automobile suspension 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 metallographic characteristics described above. In parts of the steel sheet that do not directly contact the mold during forming, such as press forming, and that are relatively lightly processed, the characteristics of the metallographic structure do not change significantly before and after forming.

[0062] [Mechanical properties] [Tensile strength: TS] Steel sheets having the above-described chemical composition and metallographic structure can achieve high tensile strength, specifically, 780 MPa or higher. The tensile strength is preferably 800 MPa or higher, 820 MPa or higher, or 840 MPa or higher. Despite having such extremely high tensile strength, steel sheets according to embodiments of the present invention can achieve improved elongation and hole expandability due to the specific combination of the chemical composition and metallographic structure described above. The upper limit of the tensile strength is not particularly limited, and the tensile strength of the steel sheet may be, for example, 1180 MPa or lower, 980 MPa or lower, 940 MPa or lower, 900 MPa or lower, or 860 MPa or lower. Tensile strength is measured by taking a JIS No. 5 test piece in a direction in which the longitudinal direction of the test piece is preferably parallel to the rolling direction of the steel sheet (C direction) and conducting a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel sheet cannot be identified, the JIS No. 5 test piece may be taken from any direction within the steel sheet plane.

[0063] [Total elongation: tEL] Steel sheets having the above chemical composition and microstructure can achieve not only high tensile strength but also improved total elongation, more specifically, a total elongation of 14.0% or more. The total elongation is preferably 15.0% or more, more preferably 16.0% or more, and most preferably 18.0% or more. There is no particular upper limit, but the total elongation may be, for example, 30.0% or less or 25.0% or less. The total elongation is measured by taking a JIS No. 5 test piece in a direction in which the longitudinal direction of the test piece is preferably parallel to the rolling direction of the steel sheet (C direction) and conducting a tensile test in accordance with JIS Z 2241:2022. When the rolling direction of the steel sheet cannot be specified, the JIS No. 5 test piece may be taken from any direction within the steel sheet plane.

[0064] [Hole expansion ratio: HER] A steel sheet having the above-described chemical composition and metallographic structure can achieve high hole expandability, specifically, a hole expansion ratio of 100% or more. The hole expansion ratio may be preferably 105% or more, more preferably 110% or more or 115% or more. The upper limit of the hole expansion ratio is not particularly limited, but may be, for example, 150% or less, 140% or less, or 130% or less. The hole expansion ratio is determined as follows. First, a test piece 100 mm wide x 100 mm long is taken from the steel sheet, and a punched hole (initial hole: hole diameter d0 = 10 mm) is made using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%). Next, with the burr facing the die side, the initial hole is expanded using a conical punch with an apex angle of 60° until a crack penetrating the sheet thickness occurs. The hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio HER (%) of each test piece is calculated using the following formula: This hole expansion test is carried out three times, and the average value is determined as the hole expansion ratio HER. HER=100×{(d1-d0) / d0}

[0065] <Steel sheet manufacturing method> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below. More specifically, although the following specifically describes the manufacture of a hot-rolled steel sheet, the steel sheet according to an embodiment of the present invention encompasses any steel sheet having the chemical composition and metallographic structure described above, i.e., not only a hot-rolled steel sheet but also a cold-rolled steel sheet, a plated steel sheet, and the like. Therefore, the following description merely describes a preferred manufacturing method when the steel sheet according to an embodiment of the present invention is a hot-rolled steel sheet.

[0066] A method for manufacturing a steel sheet according to an embodiment of the present invention includes: A hot rolling process comprising heating a slab having the chemical composition described above in relation to the steel plate and then finish rolling the slab, wherein the following conditions (a) to (c) are satisfied: (a) The heating temperature of the slab is 1150 to 1300°C. (b) The holding time in the temperature range of 1150 to 1300°C is 1000 to 4000 seconds, and (c) The finishing temperature of the finish rolling is 850 to 950°C. An intermediate cooling process in which the finish-rolled steel plate is primarily cooled to an intermediate air-cooling temperature of 600 to 750 ° C. at an average cooling rate of 30 to 200 ° C. / second, and then intermediate air-cooled at an average cooling rate of 15 ° C. / second or less for 3 to 15 seconds; The intermediately air-cooled steel sheet is then cooled for a second time at an average cooling rate of 50 to 200°C / sec, and then coiled at a coiling temperature of 20 to 290°C. The present invention is characterized in that it includes:

[0067] [Hot rolling process] [(a) Slab heating temperature: 1150-1300℃] [(b) Holding time in the temperature range of 1150-1300°C: 1000-4000 seconds] First, a slab having the chemical composition described above in relation to the steel sheet is heated. From the viewpoint of productivity, the slab is preferably cast by a continuous casting method, but it may also be produced by an ingot casting method or a thin slab casting method. The slab contains a relatively large amount of alloying elements to obtain a high-strength steel sheet. Therefore, the slab must be heated before hot rolling to dissolve the alloying elements in the slab, and Ti, in particular, must be sufficiently dissolved. If the heating temperature is too low, the alloying elements may not dissolve sufficiently in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. In particular, if the heating temperature is too low, Ti may not dissolve sufficiently, resulting in Ti remaining as coarse carbides (TiC) in the final structure. In this case, even with the intermediate air-cooling process after the hot rolling process, it becomes difficult to finely precipitate TiC precipitates in the steel and improve the strength of the steel sheet through precipitation strengthening. For this reason, the heating temperature is preferably 1150°C or higher. The upper limit of the heating temperature is not particularly limited, but is preferably 1300°C or less from the viewpoint of the capacity of the heating equipment and productivity. Furthermore, by setting the holding time in the temperature range of 1150 to 1300°C to 1000 seconds or more, the alloying elements can be reliably solid-dissolved in the slab, and Ti in particular can be sufficiently solution-dissolved. If the holding time is short, Ti may remain in the final structure as coarse carbide (TiC). The upper limit of the holding time is not particularly limited, but is preferably 4000 seconds or less from the viewpoint of productivity, etc. When rough rolling is performed, holding in the temperature range of 1150 to 1300°C may be performed after rough rolling.

[0068] [Rough rolling] In this method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0069] [(c) Finishing temperature of finishing rolling: 850-950°C] In this manufacturing method, the finish rolling temperature is important for controlling the metallographic structure of the steel sheet. If the finish rolling temperature is low, the degree of austenite processing increases, excessively promoting ferrite nucleation and accelerating ferrite transformation, which can lead to reduced strength. Furthermore, excessive ferrite transformation can increase the hardness difference between ferrite and the hard phases bainite and martensite, resulting in reduced hole expandability. For this reason, the finish rolling temperature is set to 850°C or higher. On the other hand, if the finish rolling temperature is high, recrystallization is promoted, resulting in insufficient ferrite transformation in the subsequent intermediate air-cooling process. This can lead to the failure to achieve the desired microstructure fraction in the final metallographic structure and / or the failure to form TiC precipitates at the desired diameter and number density.

[0070] More specifically, hot rolling crushes the crystals in steel, disrupting the orderly arrangement of Fe atoms within the crystals, resulting in numerous discontinuous structures called deformation bands and the formation of numerous step-like irregularities (ledges) at the grain boundaries. The presence of these deformation bands and ledges allows for the formation of numerous new ferrite crystals from these bands, thereby accelerating the ferrite transformation. However, if the finishing temperature of the finish rolling is higher than 950°C, austenite recrystallizes during hot rolling, and in this case, the only locations where the arrangement of Fe atoms is disrupted are the grain boundaries. As a result, new ferrite crystals can only form at the austenite grain boundaries, preventing sufficient promotion of the ferrite transformation during the subsequent intermediate air-cooling process. Therefore, to promote the ferrite transformation during the intermediate air-cooling process, it is necessary to maintain the austenite grains as unrecrystallized grains during hot rolling. In this regard, because Ti has a recrystallization-suppressing effect, it is important in this manufacturing method to include 0.05% or more Ti while controlling the end temperature of finish rolling to 950°C or less. This combination suppresses recrystallization during hot rolling and sufficiently promotes ferrite transformation in the subsequent intermediate air-cooling process, ensuring that the final metal structure has a ferrite area percentage of 20% or more. Nb, like Ti, also has a recrystallization-suppressing effect, so adding Nb can further promote ferrite transformation. In addition, as mentioned above, Al also has the effect of promoting ferrite transformation and bainite transformation. Therefore, adding Al in an amount of 0.20% or more can further promote ferrite transformation.

[0071] Furthermore, in the subsequent intermediate air-cooling step, as will be explained in detail later, it is necessary to promote ferrite transformation and properly generate TiC precipitates. However, if the end temperature of finish rolling is higher than 950°C, coarse TiC precipitates will precipitate during rolling, making it difficult to generate TiC precipitates with the desired diameter and number density in the subsequent intermediate air-cooling step. Therefore, in the hot rolling step, it is important to set the end temperature of finish rolling to 950°C not only to suppress recrystallization and promote ferrite transformation, but also from the viewpoint of properly generating TiC precipitates. Preferably, the end temperature of finish rolling is 880 to 920°C.

[0072] [Intermediate air cooling process] In the subsequent intermediate cooling step, the finish-rolled steel sheet is primarily cooled on a run-out table (ROT) at an average cooling rate of 30 to 200°C / s to an intermediate cooling temperature of 600 to 750°C, and then intermediate cooling at an average cooling rate of 15°C / s or less for 3 to 15 seconds. Primary cooling to an intermediate cooling temperature of 600 to 750°C at an average cooling rate of 30 to 200°C / s can promote ferrite transformation. Subsequently, intermediate cooling in this temperature range, preferably in the 650 to 720°C temperature range, at an average cooling rate of 15°C / s or less for 3 to 15 seconds can promote ferrite transformation due to slow cooling, and TiC precipitates with diameters of 10.0 to less than 30.0 nm can be primarily produced at high temperatures above 700°C, and similarly, TiC precipitates with diameters of less than 10.0 nm can be primarily produced at low temperatures below 700°C. In this regard, the area ratio of ferrite containing TiC precipitates with diameters of less than 10.0 to 30.0 nm can also be controlled within a desired range. As a result, strength and other properties are improved by precipitation strengthening based on TiC precipitates, and sufficient precipitation strengthening of ferrite reduces the difference in hardness between ferrite and bainite and martensite, thereby improving hole expandability. More specifically, to appropriately control the formation and grain growth of TiC precipitates during intermediate air cooling, the intermediate air cooling temperature must be set to a relatively high temperature range, i.e., a temperature range of 600 to 750°C. However, in this case, a relatively large amount of ferrite tends to form. Therefore, in this manufacturing method, the average cooling rate during primary cooling from finish rolling to the intermediate air cooling temperature is set to 30°C / s or more, thereby suppressing excessive ferrite formation during primary cooling and promoting ferrite transformation by the subsequent intermediate air cooling at a high temperature, thereby appropriately forming TiC precipitates and growing their grains. If the average cooling rate is less than 30°C / sec, excessive ferrite may be formed or a relatively large number of coarse TiC precipitates may be formed, making it difficult to obtain desired properties such as elongation. On the other hand, if the average cooling rate for the primary cooling exceeds 200°C, the formation of ferrite is excessively suppressed, and the area ratio of ferrite in the final metal structure becomes less than 20%, resulting in a decrease in properties such as elongation.Therefore, the average cooling rate of the primary cooling is set to 200°C / sec or less, and preferably 150°C / sec or less.

[0073] If the intermediate cooling temperature exceeds 750°C or the intermediate cooling time exceeds 15 seconds, excessive ferrite is formed or TiC precipitates become coarse. If excessive ferrite is formed, the desired metal structure containing ferrite, bainite, and martensite in a specific ratio cannot be formed in the final steel sheet. Furthermore, if the TiC precipitates become coarse, the TiC precipitates cannot be formed with the desired diameter and number density. As a result, the appropriate strength-improving and hardness-improving effects of the TiC precipitates cannot be obtained, resulting in reduced strength, elongation, and / or hole expandability. On the other hand, if the intermediate cooling temperature is less than 600°C, ferrite formation is suppressed, resulting in a ferrite area ratio of less than 20% in the final metal structure, and properties such as elongation are reduced. Furthermore, if the average cooling rate during intermediate cooling exceeds 15°C / s or the intermediate cooling time is less than 3 seconds, the formation and grain growth of TiC precipitates are suppressed, making it impossible to form the TiC precipitates with the desired diameter and / or number density. In this case, the appropriate strength-improving and hardness-improving effects of TiC precipitates cannot be obtained, resulting in reduced strength, elongation, and / or hole expandability. In addition, ferrite formation may be excessively suppressed, making it impossible to form the desired metal structure containing ferrite, bainite, and martensite in specific proportions in the final steel sheet.

[0074] In contrast, in the intermediate cooling step, primary cooling is performed to an intermediate cooling temperature of 600 to 750°C, preferably 650 to 720°C, at an average cooling rate of 30 to 200°C / sec, preferably 50 to 150°C / sec, and then intermediate cooling is performed at an average cooling rate of 15°C / sec or less, preferably 10°C / sec or less, for 3 to 15 seconds, preferably 4 to 12 seconds, thereby precipitating ferrite at a desired rate and generating TiC precipitates with an appropriate diameter and number density, and finally forming TiC precipitates with a diameter of less than 10.0 nm at a density of 1.0 × 10 14 pieces / cm3 and TiC precipitates with a diameter of 10.0 to 30.0 nm or less. 10 ~1.0×10 14 pieces / cm 3 and further restricting the area ratio of ferrite where TiC precipitates having a diameter of less than 10.0 to 30.0 nm exist to 10 to less than 50% of the entire ferrite. As a result, the strength, elongation, and hole expandability of the steel sheet can be significantly improved by appropriately utilizing the strength-improving effect and hardness-improving effect of the TiC precipitates.

[0075] [Cooling process] In the subsequent cooling step, the steel sheet after intermediate air cooling is secondarily cooled at an average cooling rate of 50 to 200°C / s and then coiled at a coiling temperature of 20 to 290°C. By subjecting the steel sheet after intermediate air cooling to such a relatively fast average cooling rate, bainite and martensite can be appropriately precipitated, thereby enabling the final steel sheet to have a metallographic structure containing ferrite, bainite, and martensite in specific proportions. In contrast, if the average cooling rate in the secondary cooling is less than 50°C / s, C may be concentrated in the austenite while the steel sheet is cooled to the transformation point, resulting in bainite and / or martensite transformation, making it impossible to obtain the desired metallographic structure in the final steel sheet. In such cases, bainite and / or martensite are formed in relatively large amounts, making it impossible to achieve the desired elongation. Alternatively, the relatively large amount of bainite and / or martensite may increase the ratio of the average hardness of bainite and martensite to the average hardness of ferrite, resulting in reduced hole expandability. On the other hand, if the average cooling rate in the secondary cooling exceeds 200°C, bainite will not be sufficiently formed and / or martensite will be excessively formed, and similarly, the desired metal structure will not be obtained in the final steel sheet. Therefore, the average cooling rate in the secondary cooling is set to 200°C / sec or less, and preferably 180°C / sec or less.

[0076] Furthermore, if the coiling temperature exceeds 290°C, the austenite is cooled to the transformation point while C is concentrated, which may cause bainite and / or martensite transformation, making it impossible to obtain the desired metal structure in the final steel sheet. In such cases, a relatively large amount of bainite and / or martensite is formed, making it impossible to achieve the desired elongation. Alternatively, the relatively large amount of bainite and / or martensite formed may increase the ratio of the average hardness of bainite and martensite to the average hardness of ferrite, resulting in a decrease in hole expandability. Preferably, the coiling temperature is 280°C or lower or 260°C or lower. On the other hand, if the coiling temperature is too low, excessive water cooling or the like may be required, reducing productivity. Furthermore, this may cause the steel sheet to become embrittled. Therefore, the coiling temperature is set to 20°C or higher.

[0077] According to the steel sheet manufactured by the above manufacturing method, the metal structure is configured to contain, by area %, ferrite: 20 to 50%, bainite: 40 to 70%, and martensite: 5 to 20%, and the number density of TiC precipitates with a diameter of 10.0 to 30.0 nm or less is 1.0 × 10 10 ~1.0×10 14 pieces / cm 3 and the area ratio of ferrite in which TiC precipitates with a diameter of less than 10.0 to 30.0 nm are present can be limited to 10 to less than 50% of the entire ferrite. 14 pieces / cm 3The TiC precipitates can be present in the steel sheet at a sufficient number density as described above, and in combination with TiC precipitates having a diameter of 10.0 to 30.0 nm or less, the strength and elongation of the steel sheet can be significantly improved. Furthermore, as described above, by limiting the area fraction of ferrite containing TiC precipitates having a diameter of 10.0 to 30.0 nm to less than 10 to 50% of the total ferrite, the proportion of ferrite containing TiC precipitates having a diameter of less than 10.0 nm, which has a greater effect on improving hardness, can be sufficiently increased. Therefore, the ratio of the average hardness of bainite and martensite to the average hardness of ferrite can be controlled within the range of 0.80 to 1.20, which significantly improves the hole expandability of the steel sheet. Therefore, steel sheets manufactured by the above manufacturing method can reliably achieve the contradictory properties of high strength and excellent formability, and are therefore particularly useful in the automotive field, where both properties are required.

[0078] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way. [Example]

[0079] In the following examples, steel sheets according to embodiments of the present invention, particularly hot-rolled steel sheets, were produced under various conditions, and the tensile strength (TS), total elongation (tEL), and hole expandability (HER) of the obtained steel sheets were investigated.

[0080] First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Table 1. These slabs were heated under the conditions shown in Table 2 and then hot-rolled. Hot rolling was carried out by performing rough rolling and finish rolling, and the end temperature of the finish rolling was as shown in Table 2. Next, the finish-rolled steel sheet was primarily cooled to an intermediate air-cooling temperature under the conditions shown in Table 2, and then intermediate air-cooled. Finally, the intermediate air-cooled steel sheet was secondarily cooled to a coiling temperature under the conditions shown in Table 2, and then coiled at the coiling temperature to obtain a steel sheet having a thickness of 2.5 mm.

[0081] [Table 1]

[0082] [Table 2]

[0083] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0084] [Tensile strength (TS) and total elongation (tEL)] The tensile strength (TS) and total elongation (tEL) were measured by taking a JIS No. 5 test piece with a length of 200 mm and a thickness of 2.5 mm from a direction in which the longitudinal direction of the test piece was parallel to the direction perpendicular to the rolling direction of the steel plate (C direction) and conducting a tensile test in accordance with JIS Z 2241: 2022. More specifically, the test was conducted at room temperature in the range of 10 to 35°C, and a tensile test force was applied to the test piece, and strain was applied until fracture occurred.

[0085] Hole Expansion Ratio (HER) The hole expansion ratio was determined as follows. 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 10.25 to 11.5 mm (clearance 12.5%) was used to create a punched hole (initial hole: hole diameter d0 = 10 mm). Next, with the burr facing the die, the initial hole was expanded using a conical punch with an apex angle of 60° until a crack penetrating the plate thickness occurred. The hole diameter d1 mm at the time of crack occurrence was measured, and the hole expansion ratio HER (%) of each test piece was calculated using the following formula. This hole expansion test was performed three times, and the average value was determined as the hole expansion ratio λ. λ=100×{(d1-d0) / d0}

[0086] Steel sheets with a tensile strength (TS) of 780 MPa or more, a total elongation (tEL) of 14.0% or more, and a hole expansion ratio (λ) of 100% or more were evaluated as having high strength and improved elongation and hole expansion properties. The results are shown in Table 3.

[0087] [Table 3]

[0088] Referring to Tables 1 to 3, in Comparative Example 15, the heating temperature in the hot rolling process was low, so Ti was not sufficiently dissolved and remained in the final structure as coarse carbides (TiC). As a result, even with the intermediate air-cooling process, TiC precipitates with a diameter of less than 10.0 nm, which have a greater strength-improving effect, could not be formed at the desired number density, resulting in a decrease in TS. In addition, because TiC precipitates with a diameter of less than 10.0 nm, which have a greater strength-improving effect, could not be formed at the desired number density, the hardness difference in the metal structure increased, the ratio of the average hardness of bainite and martensite to the average hardness of ferrite decreased, and the HER decreased. In Comparative Example 16, the intermediate air-cooling temperature was high, so the TiC precipitates coarsened, resulting in a high number density of TiC precipitates with a diameter of 10.0 to 30.0 nm. This is thought to be related to the fact that the proportion of ferrite containing TiC precipitates with a diameter of less than 10.0 nm, which have a high hardness-improving effect, was reduced, and the hardness of the soft ferrite phase could not be sufficiently improved. As a result, the ratio of the average hardness of bainite and martensite to that of ferrite was reduced, and the HER was reduced.

[0089] In Comparative Example 17, the high end temperature of the finish rolling promoted recrystallization, resulting in insufficient ferrite transformation in the subsequent intermediate air-cooling process, and as a result, tEL decreased. In Comparative Example 18, the low intermediate air-cooling temperature suppressed ferrite formation, resulting in a ferrite area ratio of less than 20% in the final metal structure, and tEL decreased. In Comparative Example 19, the long intermediate air-cooling time resulted in excessive ferrite formation, which further coarsened TiC precipitates, and as a result, the number density of TiC precipitates with diameters less than 10.0 nm also decreased. As a result, TS and HER decreased. In Comparative Example 20, the slow average cooling rate in the primary cooling down to the intermediate air-cooling temperature resulted in the formation of a relatively large number of TiC precipitates with diameters of 10.0 to 30.0 nm, and as a result, tEL decreased. In Comparative Example 21, the end temperature of finish rolling was low, which is thought to have increased the degree of austenite reduction, excessively promoting ferrite nucleation and accelerating ferrite transformation. As a result, TS decreased and the difference in hardness between ferrite and the hard phases, bainite and martensite, increased. In other words, the ratio of the average hardness of bainite and martensite to the average hardness of ferrite could not be controlled within the desired range, resulting in a decrease in HER. In Comparative Example 22, the average cooling rate in the secondary cooling after intermediate air cooling was slow, which is thought to have resulted in the formation of relatively large amounts of bainite and martensite due to the concentration of C in austenite. As a result, tEL decreased. In Comparative Example 23, the coiling temperature was high, which is thought to have similarly resulted in the formation of relatively large amounts of bainite and martensite due to the concentration of C in austenite. As a result, the ratio of the average hardness of bainite and martensite to the average hardness of ferrite increased, resulting in a decrease in HER.

[0090] In Comparative Example 24, the average cooling rate during intermediate air-cooling was too fast, which inhibited the formation and grain growth of TiC precipitates. Therefore, TiC precipitates with diameters of less than 10.0 nm could not be formed at the desired number density, and ferrite formation was also inhibited. As a result, it is believed that the TiC precipitates did not provide an adequate hardness improvement effect, and the ratio of the average hardness of bainite and martensite to the average hardness of ferrite increased, resulting in a decrease in HER. In Comparative Example 25, the heating holding time during the hot rolling process was too short, which is believed to be why Ti was not sufficiently dissolved and remained in the final structure as coarse carbides (TiC). As a result, even with the intermediate air-cooling process, TiC precipitates with diameters of less than 10.0 nm, which have a greater strength-improving effect, could not be formed at the desired number density. Meanwhile, the number density of TiC precipitates with diameters of 10.0 to 30.0 nm was somewhat increased. This resulted in a decrease in tEL and HER. In Comparative Example 26, the Si content was high, and therefore martensite was excessively formed, increasing the difference in hardness between ferrite and the hard phases of bainite and martensite. In other words, the ratio of the average hardness of bainite and martensite to the average hardness of ferrite increased, and the HER decreased.

[0091] In contrast, all of the steel sheets according to the examples of the present invention have a predetermined chemical composition, and by appropriately controlling the conditions in the manufacturing method, the steel sheets contain, in area ratios, 20 to 50% ferrite, 40 to 70% bainite, and 5 to 20% martensite, and the number density of TiC precipitates with a diameter of less than 10.0 nm is 1.0 × 10 14 pieces / cm 3 or more, and the number density of TiC precipitates with a diameter of 10.0 to less than 30.0 nm is 1.0 × 10 10 ~1.0×10 14 pieces / cm 3It was possible to obtain steel sheets having a metallographic structure in which the ratio of the average hardness of bainite and martensite to the average hardness of ferrite was 0.80 to 1.20. As a result, despite the high tensile strength of 780 MPa or more, it was possible to significantly improve elongation and hole expandability. Furthermore, the Ti / Al ratios of Steels A to J in Table 1 were 0.16 to 3.33. Furthermore, when a residual structure was present in the invention examples, the residual structure was at least one of pearlite and retained austenite.

Claims

1. In mass%, C: 0.030-0.150%, Si: 0.01-1.00%, Mn: 0.50-3.00%, Ti: 0.05-0.20%, Al: 0.01-0.40%, P: 0.100% or less, S: 0.010% or less, N: 0.0100% or less, O: 0.010% or less, Nb: 0 to 0.15%, V: 0-1.00%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.00%, B: 0 to 0.0100%, Sn: 0-1.00%, Sb: 0 to 1.00%, 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: 0-2.00%, Zn: 0 to 0.010%, W: 0 to 1.00%, and The balance has a chemical composition consisting of Fe and impurities, In area %, Ferrite: 20-50%, Bainite: 40-70%; and Martensite: 5 to 20% The number density of TiC precipitates with a diameter of less than 10.0 nm is 1.0 × 10 14 pieces / cm 3 or more, and the number density of TiC precipitates having a diameter of 10.0 to less than 30.0 nm is 1.0 × 10 10 ~1.0 x 10 14 pieces / cm 3 and A steel plate characterized by having a metal structure in which the ratio of the average hardness of bainite and martensite to the average hardness of ferrite is 0.80 to 1.

20.

2. The steel plate according to claim 1, characterized in that the chemical composition contains, in mass%, Al: 0.20 to 0.40%.

3. The chemical composition is, in mass %, Nb: 0.001 to 0.15%, V: 0.001-1.00%, Cr: 0.001-2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Mo: 0.001-1.00%, B: 0.0001 to 0.0100%, Sn: 0.001 to 1.00%, Sb: 0.001 to 1.00%, 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 2.00%, Zn: 0.001 to 0.010%, and W: 0.001-1.00% The steel sheet according to claim 1, characterized in that it contains at least one of the following:

4. The steel sheet according to any one of claims 1 to 3, characterized in that the area ratio of ferrite in which TiC precipitates having a diameter of 10.0 to less than 30.0 nm are present is 10 to less than 50% of the total area of ​​ferrite.

5. The steel plate according to any one of claims 1 to 3, characterized in that it has a tensile strength of 780 MPa or more.

6. The steel sheet according to any one of claims 1 to 3, having a sheet thickness of 1.0 to 8.0 mm.

7. A part comprising the steel sheet according to any one of claims 1 to 3.

8. A hot rolling process comprising heating a slab having the chemical composition according to any one of claims 1 to 3 and then finish rolling the slab, wherein the following conditions (a) to (c) are satisfied: (a) The heating temperature of the slab is 1150 to 1300°C; (b) the holding time in the temperature range of 1150 to 1300°C is 1000 to 4000 seconds; and (c) The finishing temperature of the finish rolling is 850 to 950°C. An intermediate air-cooling step in which the finish-rolled steel plate is primarily cooled to an intermediate air-cooling temperature of 600 to 750 ° C. at an average cooling rate of 30 to 200 ° C. / second, and then intermediate air-cooled at an average cooling rate of 15 ° C. / second or less for 3 to 15 seconds; A cooling process in which the intermediately air-cooled steel sheet is secondarily cooled at an average cooling rate of 50 to 200°C / sec, and then coiled at a coiling temperature of 20 to 290°C. The method for producing a steel sheet according to any one of claims 1 to 3, comprising:

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