Steel plate and its manufacturing method

Abstract

The present invention provides a steel sheet which has a predetermined chemical composition, wherein the steel structure thereof contains, in volume%, from 1% to 50% of ferrite with the proportion of non-recrystallized ferrite in the ferrite being from 0% to 50%, 1% or more of tempered martensite, 5% or more of residual austenite, from 0% to 10% of fresh martensite, and a total of from 0% to 5% of pearlite and cementite, with the balance being made up of bainite. If the surface of this steel sheet is subjected to EPMA analysis, the area ratio of the region where the AlS / SiS ratio is 0.2 or less is 50% or less, and the tensile strength is 980 MPa or more. The present invention also provides a method for producing this steel sheet.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a manufacturing method thereof, and more particularly to a high-strength steel sheet used mainly as a steel sheet for automobiles and a manufacturing method thereof. [Background technology]

[0002] In recent years, there has been a demand for improved automobile fuel efficiency in light of greenhouse gas emission regulations as part of measures to combat global warming, and the use of high-strength steel sheets has been expanding to reduce the weight of vehicle bodies and ensure collision safety. In particular, there has been a growing need for high-strength steel sheets with a tensile strength of 980 MPa or more. Furthermore, high-strength hot-dip galvanized steel sheets with a hot-dip galvanized surface are required for areas of the vehicle body that require corrosion resistance.

[0003] Steel sheets used for automotive parts are required to have not only strength but also various workability required for part forming, such as press formability and weldability. Specifically, from the perspective of press formability, steel sheets are required to have excellent elongation (total elongation in tensile tests: El) and stretch flangeability (hole expansion ratio: λ).

[0004] Generally, as the strength of steel increases, its press formability deteriorates. TRIP (Transformation Induced Plasticity) steel, which utilizes the transformation-induced plasticity of retained austenite, is known as a means of achieving both high strength and press formability in steel.

[0005] Patent Documents 1 to 3 disclose high-strength TRIP steel sheets with improved elongation and hole expansion ratio by controlling the structural fraction within a predetermined range. Patent Document 4 also describes a high-strength steel sheet with a predetermined chemical composition, including 15% or less ferrite by volume fraction with an average grain size of 2 μm or less, 2 to 15% retained austenite by volume fraction with an average grain size of 2 μm or less, 10% or less martensite by volume fraction with an average grain size of 3 μm or less, and the remainder being bainite and tempered martensite with an average grain size of 6 μm or less, and containing an average of 10 or more cementite particles with a grain size of 0.04 μm or more within the bainite and tempered martensite grains. It also describes that the high-strength steel sheet has a tensile strength of 1180 MPa or more, high elongation and hole expansion property, and therefore excellent bending workability.

[0006] Patent Document 5 discloses a TRIP steel sheet in which the area ratio of massive (low aspect ratio) retained austenite is limited to improve stretch flangeability.

[0007] Patent Document 6 discloses a high-strength TRIP steel sheet that exhibits a large amount of work hardening in the early stages of forming, excellent shape fixability, and formability, achieved by controlling the amounts of solute Si and solute Mn contained in retained austenite to be equal to or greater than predetermined values.

[0008] In addition to press formability, automotive steel sheets are also required to have excellent weldability. In particular, when welding hot-dip galvanized steel sheets together, or when welding hot-dip galvanized steel sheets to non-galvanized steel sheets, it is necessary to suppress liquid metal embrittlement (LME) cracking. This phenomenon occurs when zinc, which has been converted into a liquid phase by welding heat input, penetrates and embrittles the steel sheet along the grain boundaries, causing cracks when tensile stress generated by welding acts on these embrittled areas.

[0009] Patent Document 7 discloses that LME cracking is more likely to occur in steels with a higher Si content. Patent Document 7 therefore discloses TRIP steel sheets in which Al, which has a similar effect, is added to replace some of the Si added to TRIP steels to obtain retained austenite. Patent Documents 8 and 9 also disclose TRIP steel sheets in which Al is added to replace some of the Si.

[0010] Furthermore, Patent Document 10 discloses a method for producing a hot-dip galvanized steel sheet having excellent resistance to LME cracking, characterized by controlling the atmosphere during heat annealing in a hot-dip galvanizing line. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] International Publication No. 2013 / 051238 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-104532 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-184757 [Patent Document 4] International Publication No. 2017 / 179372 [Patent Document 5] International Publication No. 2018 / 190416 [Patent Document 6] International Publication No. 2013 / 018741 [Patent Document 7] International Publication No. 2018 / 202916 [Patent Document 8] Japanese Patent Application Laid-Open No. 2011-17046 [Patent Document 9] International Publication No. 2013 / 144377 [Patent Document 10] International Publication No. 2018 / 234938 Summary of the Invention [Problem to be solved by the invention]

[0012] In this technical field, there is a continuing need for steel sheets that have both high strength and press formability and are excellent in LME cracking resistance, and there is still room for improvement in these respects even in steel sheets of conventional technologies.

[0013] Therefore, an object of the present invention is to provide a steel sheet having excellent press formability and excellent resistance to LME cracking in spot welds, and a method for manufacturing the same. [Means for solving the problem]

[0014] The present inventors have conducted extensive research to achieve the above object and have come to the following findings.

[0015] It was found that the average composition of the steel is important for the LME crack resistance of spot welds, but the chemical composition of the surface layer of the steel sheet is also extremely important. Specifically, the Al concentration (Al S ) and the Si concentration on the steel sheet surface (Si S It was found that when the ratio of Al to Al is within a certain range, LME cracking resistance is significantly improved. Although the detailed mechanism is not clear, it is thought that Al in the surface layer of the steel sheet may inhibit the penetration of liquid Zn into the steel sheet. More specifically, when the steel sheet surface was analyzed by EPMA, it was found that Al S / Si S We found that an improvement in the area ratio of the region with a ratio of 0.2 or less is achieved when the area ratio is 50% or less. Furthermore, we found that reducing the proportion of unrecrystallized ferrite in the ferrite is particularly important for press formability. Specifically, we found that reducing the proportion of unrecrystallized ferrite in the ferrite to 50% or less makes the steel microstructure more isotropic, thereby improving elongation and hole expandability. Furthermore, while adding a relatively large amount of Al is necessary to increase the Al concentration at the steel surface in order to improve LME cracking resistance, we found that adding Ti to fix the solute N in the steel as TiN is effective in avoiding the formation of fine and relatively large amounts of AlN particles, which have a pinning effect that significantly suppresses ferrite recrystallization.

[0016] The present invention has been realized based on the above findings, and is specifically as follows. (1) In mass%, C: 0.15~0.30%, Si: 0.30 to 1.50% Mn: 1.40-3.49%, P: 0.050% or less, S: 0.0100% or less, Al: 0.30~1.50%, Ti: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0~1.00%, Mo: 0-1.00%, Cu: 0-1.00% Ni: 0 to 1.00% Co: 0 to 1.00%, W: 0~1.00%, Sn: 0 to 1.00% Sb: 0 to 0.50% Nb: 0 to 0.200%, V: 0~1.00%, B: 0~0.0050%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Ce: 0 to 0.0150% Zr: 0 to 0.0100%, La: 0 to 0.0150%, Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM other than Ce and La: 0 to 0.0100%, and The balance has a chemical composition consisting of Fe and impurities, The steel structure in the range of 1 / 8 to 3 / 8 thickness, centered on 1 / 4 thickness from the surface, is, in volume %, Ferrite: 1 to 50%, The proportion of unrecrystallized ferrite in ferrite: 0 to 50% Tempered martensite: 1% or more, Retained austenite: 5% or more, Fresh martensite: 0-10% Sum of pearlite and cementite: 0-5%, and Remainder: Bainite When the surface was analyzed by EPMA, Al S / Si S A steel plate in which the area ratio of the region where the ratio is 0.2 or less is 50% or less and the tensile strength is 980 MPa or more. Here, Al S is the surface Al concentration (mass%), and Si S is the surface Si concentration (mass%). (2) The chemical composition is in mass%: Cr: 0.001 to 1.00%, Mo: 0.001 to 1.00%, Cu: 0.001 to 1.00%, Ni: 0.001 to 1.00%, Co: 0.001 to 1.00%, W: 0.001 to 1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.50% Nb: 0.001 to 0.200%, V: 0.001 to 1.00%, B: 0.0001~0.0050%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Ce: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, La: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, Bi: 0.0001 to 0.0100%, and REM other than Ce and La: 0.0001 to 0.0100% The steel sheet according to (1) above, comprising one or more selected from the group consisting of: (3) The steel sheet according to (1) or (2) above, wherein the chemical composition satisfies the relationship of the following formula (1), and the proportion of unrecrystallized ferrite in the ferrite is 10% or less. [N]-(14.01 / 47.88)·[Ti]≦0···(1) Here, [N] is the N content (mass %), and [Ti] is the Ti content. (4) The steel sheet according to any one of the above (1) to (3), which has a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer on its surface. (5) (A) a hot rolling step comprising rough rolling and finish rolling a slab having the chemical composition described in any one of (1) to (3) above, wherein the hot rolling step satisfies the following conditions (A1) to (A4): (A1) In the rough rolling, the steel sheet temperature is 1050 to 1200°C, and rolling is performed at least twice with a reduction rate per pass exceeding 20%; (A2) In rough rolling, high-pressure water descaling is performed at least once within 10 seconds after the steel plate has passed through a rolling pass in which the steel plate temperature is 1050 to 1200°C and the reduction ratio is more than 20%, with the pressure being 10 MPa or more, the distance between the steel plate and the nozzle tip being 500 mm or less, and the angle between the nozzle direction and the thickness direction of the steel plate being 3 to 15 degrees. (A3) In finish rolling, high-pressure water descaling is performed at least once within 3.0 seconds after the steel plate has passed through a rolling pass with a pressure of 2 MPa or more, a distance between the steel plate and the nozzle tip of 400 mm or less, and an angle between the nozzle direction and the thickness direction of the steel plate of 3 to 15 degrees, at a steel plate temperature of 950 to 1100°C, and a reduction ratio of 30% or more. (A4) The time (seconds) elapsed until the steel plate reaches 700°C after the final descaling is completed satisfies the following formula (2):

number

[0017] According to the present invention, a steel sheet having excellent press formability and LME cracking resistance in spot welds can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0018] "Chemical composition" First, the reasons for limiting the chemical composition of the steel sheet according to the embodiment of the present invention as described above will be explained. Note that, in this specification, all "%" used to define the chemical composition is "% by mass" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0019] [C: 0.15~0.30%] Carbon (C) is an essential element for ensuring the strength of steel sheets. To fully obtain this effect, the C content is set to 0.15% or more. The C content may be 0.16% or more, 0.18% or more, or 0.20% or more. On the other hand, excessive C content may deteriorate workability, such as press formability, and weldability. For this reason, the C content is set to 0.30% or less. The C content may also be 0.28% or less, 0.27% or less, or 0.25% or less.

[0020] [Si: 0.30~1.50%] Silicon (Si) is an element that suppresses the formation of iron carbides and contributes to improving strength and formability. To fully obtain these effects, the Si content is set to 0.30% or more. The Si content may be 0.40% or more, 0.50% or more, or 0.70% or more. On the other hand, excessive addition may promote LME cracking during welding. Therefore, the Si content is set to 1.50% or less. The Si content may also be 1.40% or less, 1.20% or less, or 1.00% or less.

[0021] [Mn: 1.40~3.49%] Manganese (Mn) is a powerful austenite-stabilizing element and is effective in increasing the strength of steel sheets. To fully obtain this effect, the Mn content is set to 1.40% or more. The Mn content may be 1.50% or more, 1.70% or more, or 2.00% or more. On the other hand, excessive addition may deteriorate workability such as press formability, weldability, and even low-temperature toughness. Therefore, the Mn content is set to 3.49% or less. The Mn content may also be 3.20% or less, 3.00% or less, or 2.90% or less.

[0022] [P:0.050% or less] P (phosphorus) is a solid-solution strengthening element and is effective in increasing the strength of steel sheets, but excessive addition degrades weldability and toughness. Therefore, the P content is limited to 0.050% or less. The P content is preferably 0.045% or less, 0.035% or less, or 0.020% or less. The P content may be 0%, but excessively reducing the P content increases the cost of dephosphorization, so from an economical standpoint, the lower limit is preferably set to 0.001%.

[0023] [S:0.0100% or less] S (sulfur) is an element contained as an impurity and forms MnS in steel, which deteriorates toughness and hole expandability. Therefore, the S content is limited to 0.0100% or less, which is a range in which deterioration of toughness and hole expandability is not significant. The S content is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less. Although the S content may be 0%, excessively reducing the S content increases the cost of desulfurization, so from an economical standpoint, the lower limit is preferably set to 0.0001%.

[0024] [Al: 0.30-1.50%] In addition to generating retained austenite, Al (aluminum) also contributes to the formation of Al on the steel sheet surface. S / Si S At least 0.30% Al is added to increase the ratio and improve LME cracking resistance. The Al content may be 0.40% or more, 0.50% or more, or 0.60% or more. On the other hand, adding excessive Al not only saturates the effect and leads to unnecessary cost increases, but also raises the transformation temperature of the steel, increasing the load during hot rolling and, as a result, may deteriorate the mechanical properties of the steel sheet. Therefore, the upper limit of the Al content is 1.50%. The Al content may be 1.40% or less, 1.20% or less, or 1.00% or less.

[0025] [Ti: 0.001~0.100%] Titanium (Ti) is a carbonitride-forming element and contributes to increasing the strength of steel sheets through precipitation strengthening. In an embodiment of the present invention, Ti is added to fix solute N in the steel as TiN. In an embodiment of the present invention, Al is added at a relatively high content of 0.30% or more. Therefore, if Ti is not added, a large amount of fine AlN may be formed. In such a case, the pinning effect of the fine AlN particles significantly suppresses ferrite recrystallization during annealing after cold rolling, reducing the ductility and hole expandability of the steel sheet and, as a result, reducing press formability. Therefore, the Ti content is set to 0.001% or more to fix solute N in the steel as TiN and suppress the formation of such fine AlN particles. In an embodiment of the present invention, the addition of Al is necessary to improve LME cracking resistance. However, the addition of Al may also reduce press formability. Therefore, the addition of Ti, as described above, is important from the perspective of achieving both LME cracking resistance and press formability. The Ti content may be 0.003% or more, 0.005% or more, 0.010% or more, or 0.015% or more. Furthermore, when Ti is added so as to satisfy the following formula (1), a greater effect is obtained, and the proportion of unrecrystallized ferrite in ferrite, which will be described in detail later, can be reliably reduced. For example, it is possible to reduce the proportion of unrecrystallized ferrite in ferrite to 40% or less, preferably 30% or less, or 10% or less. [N]-(14.01 / 47.88)·[Ti]≦0···(1) Here, [N] is the N content (mass%), and [Ti] is the Ti content. However, adding too much Ti not only results in a saturated effect and an unnecessary increase in costs, but also leads to the precipitation of large amounts of TiC, which may deteriorate the ductility and hole expandability of the steel sheet. Therefore, the Ti content is set to 0.100% or less. The Ti content may also be 0.090% or less, 0.080% or less, or 0.050% or less.

[0026] [N:0.0100% or less] N (nitrogen) is an element contained as an impurity, and if its content is high, it can form coarse nitrides in the steel, which can deteriorate bendability and hole expandability. Therefore, the N content is limited to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The N content may be 0%, but if the N content is reduced too much, the cost of denitrification will be high, so from an economical standpoint, the lower limit is preferably set to 0.0001%.

[0027] [O:0.0100% or less] O (oxygen) is an element contained as an impurity, and if its content is high, it may form coarse oxides in the steel, which can deteriorate bendability and hole expandability. Therefore, the O content is limited to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The O content may be 0%, but from the viewpoint of manufacturing costs, the lower limit is preferably set to 0.0001%.

[0028] The basic chemical compositions of the steel sheet according to the embodiment of the present invention and the slabs used in the production thereof are as described above. Furthermore, the steel sheet and slab may contain the following optional elements as needed. Note that when the optional elements are not contained, the lower limit of the content is 0%.

[0029] [Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00% and B: 0-0.0050%] Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), Co (cobalt), W (tungsten), Sn (tin), Sb (antimony), Nb (niobium), V (vanadium), and B (boron) are all elements effective in increasing the strength of steel sheets. Therefore, one or more of these elements may be added as needed. However, adding excessive amounts of these elements saturates the effect and unnecessarily increases costs. Therefore, the contents of these elements are limited to Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0-1.00%, Ni: 0-1.00%, Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0-0.50%, Nb: 0-0.200%, V: 0-1.00%, and B: 0-0.0050%. Each element may be 0.001% or more, 0.005% or more, or 0.010% or more. In particular, the B content may be 0.0001% or more or 0.0002% or more. Similarly, the B content may be 0.0030% or less, 0.0010% or less, less than 0.0005%, 0.0004% or less, or 0.0003% or less.

[0030] [Ca: 0-0.0100%, Mg: 0-0.0100%, Ce: 0-0.0150%, Zr: 0-0.0100%, La: 0-0.0150%, Hf: 0-0.0100%, Bi: 0-0.0100% and REM other than Ce and La: 0-0.0100%] Ca (calcium), Mg (magnesium), Ce (cerium), Zr (zirconium), La (lanthanum), Hf (hafnium), and other rare earth elements (REMs) contribute to the fine dispersion of inclusions in steel, while Bi (bismuth) reduces the microsegregation of substitutional alloying elements such as Mn and Si in steel. Since each element contributes to improving the workability of steel sheets, one or more of these elements may be added as needed. However, excessive addition can cause a deterioration in ductility. Therefore, their content should be limited to 0.0150% or 0.0100%. Furthermore, each element may be present in an amount of 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0031] In the steel sheet according to the embodiment of the present invention, the balance other than the above-mentioned elements consists of Fe and impurities, which are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing the steel sheet.

[0032] "Steel structure inside steel plate" Next, the reasons for limiting the internal structure of the steel plate according to the embodiment of the present invention will be described.

[0033] [Ferrite: 1 to 50%] Ferrite has excellent ductility but is a soft structure. To improve the elongation of the steel sheet, the ferrite content is set to 1% or more by volume. The ferrite content may be 3% or more, 5% or more, or 10% or more by volume. However, if the ferrite content is excessive, it becomes difficult to ensure the desired strength of the steel sheet. Therefore, the ferrite content is set to 50% or less by volume, and may be 45% or less, 40% or less, or 35% or less.

[0034] [Ratio of unrecrystallized ferrite to ferrite: 0-50%] Increasing the proportion of recrystallized ferrite, i.e., reducing the proportion of unrecrystallized ferrite in the ferrite, can make the steel structure more isotropic, thereby improving not only elongation but also hole expandability. Therefore, to obtain excellent elongation and hole expandability, the proportion of unrecrystallized ferrite in the ferrite is limited to 50% or less by volume. The proportion of unrecrystallized ferrite in the ferrite may also be 40% or less, 30% or less, or 20% or less by volume. Setting this proportion to 10% or less by volume can obtain particularly excellent elongation and hole expandability. The lower limit is not particularly limited, and may even be 0%. For example, the proportion of unrecrystallized ferrite in the ferrite may be 1% or more, 2% or more, or 3% or more by volume.

[0035] [Tempered martensite: 1% or more] Tempered martensite is a high-strength and tough structure, and is an essential metal structure in embodiments of the present invention. To achieve a high level of balance between strength and elongation, the tempered martensite content is set to 1% or more by volume. The tempered martensite content is preferably 5% or more, and may be 10% or more or 20% or more. There is no particular upper limit, but the tempered martensite content may be, for example, 90% or less, 80% or less, 70% or less, or 50% or less by volume.

[0036] [Residual austenite: 5% or more] Retained austenite improves the ductility of steel sheets through the TRIP effect, which transforms into martensite through stress-induced transformation during deformation of the steel sheet. Therefore, the retained austenite content is set to 5% or more by volume, and may be 8% or more or 10% or more. Since the greater the amount of retained austenite, the greater the elongation, so there is no need to specify an upper limit. However, to obtain a large amount of retained austenite, it becomes necessary to include a large amount of alloying elements such as C. In the present invention, since an upper limit is set on the C content, it is practically difficult to obtain 30% or more of retained austenite. Therefore, the retained austenite content may be 30% or less, 25% or less, or 20% or less by volume.

[0037] [Fresh martensite: 0-10%] In an embodiment of the present invention, fresh martensite refers to martensite that has not been tempered, i.e., martensite that does not contain carbides. Because this fresh martensite has a brittle structure, it becomes the origin of fracture during plastic deformation and deteriorates the local ductility of the steel sheet. Therefore, its content is set to 0 to 10% by volume. The fresh martensite content is preferably 0 to 8% or 0 to 5% by volume. The fresh martensite content may be 1% or more, or 2% or more by volume.

[0038] [Total of pearlite and cementite: 0-5%] Pearlite contains hard and coarse cementite, which acts as a fracture origin during plastic deformation and deteriorates the local ductility of the steel sheet. Therefore, its content, including cementite, is set to 0 to 5% by volume, and may be 0 to 3% or 0 to 2%.

[0039] [Bainite: Remainder] The remainder of the metal structure according to the embodiment of the present invention is composed of bainite. The bainite in the remainder structure may be any of upper bainite having carbides between laths, lower bainite having carbides within the laths, bainitic ferrite having no carbides, and granular bainitic ferrite in which the lath boundaries of bainite are restored and blurred, or a mixture thereof. The bainite content in the remainder may be 0%. For example, the bainite content in the remainder may be 1% or more, 5% or more, or 10% or more by volume. There is no particular upper limit, but the bainite content in the remainder may be, for example, 70% or less, 60% or less, 50% or less, or 40% or less by volume.

[0040] The fraction of the steel structure is evaluated by SEM-EBSD (electron backscatter diffraction) method and SEM secondary electron image observation. First, a sample is taken from a thickness cross section of the steel sheet parallel to the rolling direction and at the center position in the width direction as the observation surface. The observation surface is mechanically polished to a mirror finish, and then electrolytically polished. Next, a total of 2.0 × 10 -9 m 2 The above area is subjected to crystal structure and orientation analysis using the SEM-EBSD method. The data obtained by the EBSD method is analyzed using TSL's "OIM Analysys 6.0." The step distance between grade points is set to 0.03 to 0.20 μm. Regions that are determined to be FCC iron from the observation results are considered to be retained austenite. Furthermore, a grain boundary map is obtained by defining boundaries where the crystal orientation difference is 15 degrees or more as grain boundaries.

[0041] Next, the same sample as used for EBSD observation is subjected to nital etching, and secondary electron images are observed over the same field of view as the EBSD observation. To observe the same field of view as the EBSD measurement, it is a good idea to mark the image with a Vickers indentation or other markings in advance. From the obtained secondary electron image, the area fractions of ferrite, retained austenite, bainite, tempered martensite, fresh martensite, and pearlite are measured and used as volume fractions. Regions with intragranular substructure and cementite precipitated in multiple variants are considered to be tempered martensite. Regions with lamellar cementite precipitated are considered to be pearlite (or the sum of pearlite and cementite). Regions with low brightness and no visible substructure are considered to be ferrite. Regions with high brightness and no visible substructure are considered to be fresh martensite or retained austenite. Regions that do not fall into any of the above categories are considered to be bainite. The volume fractions of each structure are calculated using the point-counting method. The volume fraction of fresh martensite can be determined by subtracting the volume fraction of retained austenite determined by X-ray diffraction.

[0042] Furthermore, among the crystal grains judged to be ferrite, areas where the Grain Average Misorientation (GAM) in EBSD exceeds 0.6 are judged to be unrecrystallized ferrite, and crystals below 0.6 are judged to be recrystallized ferrite. At this time, the measurement step size is 0.10 μm, and the θ step size when performing Hough transform on the EBSD pattern is 1°.

[0043] The volume fraction of retained austenite is measured by X-ray diffraction. In the area of ​​1 / 8 to 3 / 8 thickness from the surface of the steel plate, centered on 1 / 4 thickness, the surface parallel to the plate surface is mirror-finished, and the area fraction of FCC iron is measured by X-ray diffraction, and this is taken as the volume fraction of retained austenite.

[0044] A steel sheet according to an embodiment of the present invention may have a zinc-containing coating layer on at least one surface, preferably both surfaces. The coating layer may be a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer having any composition known to those skilled in the art, and may contain additional elements such as Al and Mg in addition to Zn. The coating weight of the coating layer is not particularly limited and may be a general coating weight. Naturally, the steel sheet according to an embodiment of the present invention is not limited to such coated steel sheets, but also includes uncoated steel sheets. This is because, even with uncoated steel sheets, for example, when spot welding is performed with a galvanized steel sheet, LME cracking may occur due to the penetration of molten zinc from the galvanized steel sheet into the uncoated steel sheet.

[0045] [Al S / Si S The area ratio of the area where the ratio is 0.2 or less: 50% or less In an embodiment of the present invention, in order to improve LME resistance, when the surface of the steel sheet is analyzed by EPMA (electron probe microanalyzer), Al S / Si S The area ratio of the region where the ratio is 0.2 or less must be limited to 50% or less. S is the surface Al concentration (mass%), and Si S is the surface Si concentration (mass%). When conventional steel sheets are subjected to hot-dip galvanizing or galvannealed hot-dip galvanizing, zinc in the coating layer melted by the heat during welding may penetrate into the grain boundaries of the welded structure, causing LME cracking inside the steel sheet. However, according to the embodiment of the present invention, Al S / Si S By limiting the area ratio of the region where the ratio is 0.2 or less to 50% or less, the Al concentrated in the surface layer of the steel sheet can suppress the penetration of liquid Zn into the steel sheet, and as a result, the LME cracking resistance of the steel sheet can be significantly improved. S / Si SThe area ratio of the region where the ratio is 0.2 or less is preferably 30% or less, more preferably 20% or less, and most preferably 10% or less. The lower limit is not particularly limited and may be 0%. For example, Al S / Si S The area ratio of the region where the ratio is 0.2 or less may be 1% or more, 2% or more, or 3% or more.

[0046] Al S and Si S is measured using EPMA as described above. If the target is a plated steel sheet, the surface of the steel sheet is measured after removing the plating with a 5% hydrochloric acid solution containing an inhibitor. If EPMA measurement reveals an average of 0.2 mass% or more of Zn, it is determined that the plating has not been sufficiently removed, and the sample is prepared and measured again with EPMA. The measurement area is 30 mm x 30 mm, and the measurement interval is 30 μm. The mass% of Al and Si is obtained at each measurement point (beam diameter: 30 μm), and the Al content of each measurement point is then calculated. S / Si S Calculate the number of measurement points where the value is 0.2 or less, and divide by the total number of measurement points to obtain Al. S / Si S The area percentage of the region where the ratio is 0.2 or less is determined.

[0047] [Mechanical properties] Steel sheets according to embodiments of the present invention can achieve excellent mechanical properties, such as high strength, specifically a tensile strength (TS) of 980 MPa or more. The tensile strength is preferably 1080 MPa or more, more preferably 1180 MPa or more. The upper limit is not particularly limited, but for example, the tensile strength may be 1500 MPa or less, 1400 MPa or less, 1300 MPa or less, or 1250 MPa or less. Steel sheets according to embodiments of the present invention can also achieve high ductility, more specifically, a total elongation (El) of 10.0% or more, preferably 12.0% or more, more preferably 15.0% or more or 20.0% or more. The upper limit is not particularly limited, but for example, the total elongation may be 40.0% or less or 30.0% or less. Tensile strength and total elongation are measured by taking JIS No. 5 tensile test specimens from the steel sheet in a direction perpendicular to the rolling direction and conducting a tensile test in accordance with JIS Z2241:2011. Furthermore, the steel sheet according to the embodiment of the present invention can achieve high hole expandability, more specifically, a hole expansion ratio (λ) of 20% or more, preferably 25% or more, and more preferably 30% or more. While there is no particular upper limit, the hole expansion ratio may be, for example, 80% or less or 70% or less. The hole expansion ratio is measured by performing the "JFS T 1001 Hole Expansion Test Method" of the Japan Iron and Steel Federation standard. The steel sheet according to the embodiment of the present invention can achieve a high level of balance between tensile strength (TS), total elongation (El), and hole expansion ratio (λ), thereby achieving press formability suitable for use as an automotive component.

[0048] Plate Thickness The steel sheet according to the embodiment of the present invention has a thickness of, for example, 1.0 to 6.0 mm. Although not particularly limited, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more. Similarly, the thickness may be 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less.

[0049] <Manufacturing method> Next, a method for manufacturing a steel sheet 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, but is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0050] (A) Hot rolling process First, in the hot rolling process, a slab having the same chemical composition as that described above for the steel sheet is heated before hot rolling, and then subjected to rough rolling and finish rolling. The heating temperature of the slab is not particularly limited, but is generally preferably 1150°C or higher in order to sufficiently dissolve borides, carbides, etc. Note that the steel slab used is preferably cast by a continuous casting method from the viewpoint of manufacturability, but may also be produced by an ingot casting method or a thin slab casting method.

[0051] [Rough rolling] In this method, a heated slab is subjected to rough rolling at least twice, preferably at least three times, at a steel sheet temperature of 1050 to 1200°C and a rolling reduction per pass exceeding 20%. This promotes strain-induced precipitation of AlN during rough rolling. If the rough rolling conditions do not satisfy the above range, strain-induced precipitation of AlN during rough rolling will be insufficient, and AlN will precipitate during the subsequent finish rolling. The AlN precipitated during finish rolling is finer and has a higher number density than the AlN precipitated during rough rolling. Such AlN suppresses recrystallization during cold rolling annealing due to its pinning effect. As a result, the proportion of unrecrystallized ferrite in the ferrite will increase, and the final steel sheet may not achieve sufficient elongation or hole expandability. Because the temperature in rough rolling is higher than that in finish rolling, applying a relatively high load in rough rolling can promote the precipitation of coarser AlN, which in turn makes it possible to suppress the precipitation of fine AlN with a high number density in the subsequent finish rolling at a lower temperature. Rough rolling can be performed using a tandem system consisting of multiple rolling stands, or a reverse mill system in which a single rolling stand moves back and forth.

[0052] [Finishing rolling] Finish rolling can be performed, for example, using a tandem system consisting of multiple rolling stands. The finish rolling conditions are not particularly limited, but may be performed within the following ranges: a finish rolling entry temperature of 950 to 1100°C, a finish rolling exit temperature of 850 to 1000°C, and a total rolling reduction of 80 to 95%. If the finish rolling entry temperature exceeds 1100°C, if the finish rolling exit temperature exceeds 1000°C, or if the total rolling reduction is less than 80%, the grain size of the hot-rolled steel sheet may become coarse, potentially causing coarsening of the final product sheet structure. On the other hand, if the finish rolling entry temperature falls below 950°C, if the finish rolling exit temperature falls below 850°C, or if the total rolling reduction exceeds 95%, the texture of the hot-rolled steel sheet may develop, potentially resulting in the anisotropy of the final product sheet.

[0053] [Descaling] Descaling is carried out at least once each during rough rolling and finish rolling, more specifically immediately after a specific rolling pass during rough rolling and immediately after a specific rolling pass during finish rolling. In rough rolling, high-pressure water descaling is carried out at least once within 10 seconds after the steel sheet has passed through a rolling pass where the steel sheet temperature is 1050 to 1200°C and the reduction ratio exceeds 20%, with a pressure of 10 MPa or more, a distance between the steel sheet and the nozzle tip of 500 mm or less, and an angle between the nozzle direction and the thickness direction of the steel sheet of 3 to 15 degrees. The upper limit of the descaling pressure in rough rolling is not particularly limited, but may be, for example, 20 MPa or less. Similarly, in finish rolling, high-pressure water descaling is performed at least once within 3.0 seconds after the steel sheet has passed through a rolling pass with a reduction ratio of 30% or more, where the steel sheet temperature is 950 to 1100°C, with a pressure of 2 MPa or more, a distance between the steel sheet and the nozzle tip of 400 mm or less, and an angle of 3 to 15 degrees between the nozzle direction and the thickness direction of the steel sheet. The upper limit of the descaling pressure in finish rolling is not particularly limited, but may be, for example, 20 MPa or less or less than 10 MPa. If descaling that satisfies the above conditions is not performed, Si that cannot dissolve in the scale will be expelled from the scale as the scale grows and will be excessively concentrated in the steel sheet surface. As a result, the surface Si concentration Si S increases, and the surface Al concentration Al S Al, which is the ratio of S / Si S The area with a small ratio increases, and the Al S / Si S The area ratio of the region where the ratio is 0.2 or less falls outside the desired range. In order to reduce or suppress such concentration of Si in the surface layer of the steel sheet, it is particularly important to perform descaling as early as possible after passing through a rolling pass in both rough rolling and finish rolling.

[0054] [Time elapsed after final descaling until the steel plate reaches 700°C] After the final descaling is completed, the steel sheet is cooled to satisfy the following formula (2). If the value of formula (2) exceeds 0.30, the scale will grow again excessively, and the Si expelled from the scale will be concentrated again in the steel sheet surface layer. As a result, SiS increases, and Al S / Si S The area with a small ratio increases, and the Al S / Si S The area ratio of the region where the ratio is 0.2 or less is outside the desired range. On the other hand, if the ratio is below 0.03, the concentration of Al in the surface layer becomes insufficient. S / Si S The area ratio of the region where the ratio is 0.2 or less falls outside the desired range.

number

[0055] [Winding temperature] After finish rolling, the hot-rolled steel sheet is cooled to, for example, 700°C or less and then wound into a coil. The coiling temperature does not need to be particularly limited, but is preferably 450 to 680°C. If the coiling temperature is below 450°C, the strength of the hot-rolled sheet becomes excessive, which may impair cold rolling properties. On the other hand, if the coiling temperature exceeds 680°C, alloy elements such as Mn are concentrated in the cementite, which may delay the dissolution of cementite in the final annealing step and cause a decrease in strength. The lower limit of the coiling temperature may be 500°C. Similarly, the upper limit of the coiling temperature may be 650°C or 600°C.

[0056] "(B) Pickling process" The hot-rolled steel sheet obtained in the hot rolling process was treated with 1.0 to 5.0 mol / L of HCl and less than 3.0 mol / L of Fe. 2+ The hot-rolled steel sheet is subjected to pickling treatment at least once before pickling, by passing it through an aqueous solution containing Fe at a temperature of 70 to 90°C at an average speed of 10 m / min or more for 30 seconds or more. In this process, in order to efficiently remove the Si-enriched layer that forms at the interface between the scale and the base steel, the hot-rolled steel sheet is subjected to bending and unbending deformation. When the HCl concentration in the pickling solution is less than 1.0 mol / L or when Fe is present, the hot-rolled steel sheet is subjected to pickling treatment at least once before pickling. 2+If the concentration is 3.0 mol / L or higher, the temperature of the aqueous solution is below 70°C, the average speed of the hot-rolled steel sheet is below 10 m / min, or the pickling time is below 30 seconds, the pickling will not proceed sufficiently, and the Si-enriched layer at the interface between the scale and the base steel will not be removed sufficiently, resulting in the Al content of the steel sheet surface becoming smaller. S / Si S The area with a small ratio increases, and the Al S / Si S The area ratio of the region where the ratio is 0.2 or less exceeds the desired range. 2+ A high concentration is thought to inhibit the chemical reaction between scale and HCl. On the other hand, if the HCl concentration exceeds 5.0 mol / L or the temperature exceeds 90°C, pickling may proceed excessively, resulting in a deterioration in the surface quality of the steel sheet.

[0057] (C) Cold rolling process After pickling, the hot-rolled steel sheet is then cold-rolled. The reduction ratio in cold rolling is 30% or more to promote recrystallization. A reduction ratio of 40% or more is acceptable. However, excessive reduction increases the rolling load and increases the load on the cold rolling mill, so the upper limit is set to 75% or 70%.

[0058] (D) Heat treatment process [Heat treatment: average heating rate between 650 and Ac1°C is 1.0-5.0°C / s] Next, the obtained cold-rolled steel sheet is subjected to a specified heat treatment in the heat treatment process. The average heating rate is set to 5.0°C / sec or less to promote ferrite recrystallization. On the other hand, a rate below 1.0°C / sec hinders productivity. Therefore, the average heating rate between 650 and Ac1 is limited to 1.0 to 5.0°C / sec. Ac1 (°C) is calculated using the following formula. The element symbol in the formula is substituted with the mass% of the element. For elements not contained, 0 mass% is substituted. Ac1(℃)=723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr

[0059] [Soaking treatment: Ac1+30~950℃ maximum heating temperature, held for 1~500 seconds] To ensure sufficient austenitization and obtain the desired structure through subsequent cooling, the steel sheet is heated to at least Ac1 + 30°C and soaked at this temperature (maximum heating temperature). Insufficient austenitization may result in the formation of a large amount of ferrite in the final structure. However, excessively high heating temperatures not only lead to deterioration in toughness due to coarsening of austenite grains but also damage to the annealing equipment. Therefore, the upper limit is set to 950°C, preferably 900°C. A short soaking time does not allow sufficient austenitization, so the soaking time must be at least 1 second. The soaking time is preferably 30 seconds or more or 60 seconds or more. On the other hand, a long soaking time impairs productivity, so the upper limit is set to 500 seconds, preferably 300 seconds. During soaking, the steel sheet does not necessarily need to be maintained at a constant temperature; it may fluctuate within a range that satisfies the above conditions.

[0060] [First cooling: average cooling rate of 10-100°C / sec in the temperature range of 550-650°C] Next, the cold-rolled steel sheet after the soaking treatment is cooled so that the average cooling rate in the temperature range of 550 to 650°C is 10 to 100°C / second (first cooling). If the average cooling rate is less than 10°C / second, the desired ferrite fraction may not be obtained. The average cooling rate may be 15°C / second or more, or 20°C / second or more. The average cooling rate may also be 80°C / second or less, or 60°C / second or less.

[0061] [Second cooling: Cooling stops between Ms-150 and Ms℃] In order to transform a portion of the untransformed austenite into martensite, the steel is cooled to a temperature range of martensite transformation start temperature (Ms) -150 to Ms°C (second cooling). The martensite formed here is tempered by subsequent reheating and holding treatment to become tempered martensite. If the cooling stop temperature exceeds Ms°C, tempered martensite is not formed, and the desired metal structure cannot be obtained. On the other hand, if the cooling stop temperature is below Ms -150°C, the untransformed austenite is excessively reduced, and the desired retained austenite content cannot be obtained. The preferred range of the cooling stop temperature is Ms -120 to Ms -20°C, and more preferably -100°C to Ms -40°C.

[0062] This second cooling may be performed continuously with the first cooling, or may not be continuous with the first cooling. For example, after the first cooling, the cooling may be stopped at a temperature higher than Ms, hot-dip galvanizing may be performed, and then the second cooling may be performed.

[0063] Martensitic transformation occurs after ferrite transformation and / or bainite transformation. Carbon partitions into austenite during this transformation. Therefore, Ms does not match the Ms obtained when the steel is heated to a single austenite phase and then rapidly cooled. In an embodiment of the present invention, Ms is determined by measuring the thermal expansion temperature. For example, Ms can be determined by using a device capable of measuring the amount of thermal expansion during continuous heat treatment, such as a Formastor testing machine, to reproduce the heat cycle from the start of heat treatment (equivalent to room temperature) to cooling to the above-mentioned Ms or below, and measuring the amount of thermal expansion during that period. In a temperature-thermal expansion curve obtained by simulating a heat cycle using a thermal expansion measuring device, the steel sheet thermally shrinks linearly during the second cooling, but deviates from the linear relationship at a certain temperature. The temperature at this point is Ms in an embodiment of the present invention.

[0064] [Low temperature holding: The cold-rolled steel sheet after the second cooling is heated to a temperature range of 330 to 450°C, and then held at that temperature range for 50 to 1000 seconds] After the second cooling, the steel is reheated to a temperature between 330°C and 450°C and held there. In this process, in order to obtain the desired retained austenite content, carbon is concentrated in the austenite, stabilizing the austenite (austempering), while simultaneously tempering the martensite formed during the second cooling. If the holding temperature is less than 330°C or the holding time is less than 50 seconds, carbon is not sufficiently concentrated in the austenite, making it difficult to obtain the desired retained austenite content. On the other hand, if the holding temperature exceeds 450°C or the holding time exceeds 1000 seconds, austenite decomposes into cementite, and the desired retained austenite content cannot be obtained.

[0065] [Hot-dip galvanizing] When manufacturing hot-dip galvanized steel sheets, the steel sheet is immersed in a hot-dip galvanizing bath after the first cooling step. Immersion in the galvanizing bath may be performed between the first cooling step and the second cooling step, between the second cooling step and the low-temperature hold, or after the low-temperature hold. Alternatively, the steel sheet may be cooled to room temperature and subjected to a heat treatment step, followed by galvanization in a separate line. Regardless of whether the galvanization is performed after the first cooling step (between the first cooling step and the second cooling step), after the second cooling step (between the second cooling step and the low-temperature hold), after the low-temperature hold, or after the heat treatment step, the final steel structure is not affected, and a steel sheet having the same steel structure as a cold-rolled steel sheet without galvanization can be obtained. The steel sheet temperature during immersion in the hot-dip galvanizing bath has little effect on the steel sheet performance, but if the difference between the steel sheet temperature and the galvanizing bath temperature is too large, the galvanizing bath temperature may change, causing operational problems. Therefore, it is desirable to heat and cool the steel sheet so that the temperature of the galvanizing bath is between -20°C and +20°C above the galvanizing bath temperature. Hot dip galvanizing may be carried out according to a conventional method. For example, the plating bath temperature may be 440 to 470°C, and the immersion time may be 5 seconds or less. The plating bath preferably contains 0.08 to 0.2% Al, but may also contain other impurities such as Fe, Si, Mg, Mn, Cr, Ti, and Pb. The coating weight of the plating is preferably controlled by a known method such as gas wiping. The coating weight is 25 to 75 g / m per side. 2 is preferred.

[0066] [Alloying treatment] If necessary, the hot-dip galvanized steel sheet having the hot-dip galvanized layer formed thereon may be subjected to an alloying treatment. In this case, if the alloying treatment temperature is less than 460°C, not only will the alloying rate be slow, impairing productivity, but uneven alloying treatment will also occur, so the alloying treatment temperature is preferably 460°C or higher. On the other hand, if the alloying treatment temperature exceeds 600°C, the alloying may proceed excessively, resulting in deterioration of the coating adhesion of the steel sheet. For this reason, the alloying temperature is preferably 600°C or lower.

[0067] Finally, the steel sheet is cooled to room temperature to produce the final product. Temper rolling may be performed to flatten the steel sheet and adjust the surface roughness. In this case, it is preferable to keep the elongation to 2% or less to avoid deterioration of ductility. [Example]

[0068] Next, examples of the present invention will be described. The conditions in the examples are examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples of conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0069] Steel having the chemical composition shown in Table 1 was cast to produce slabs. The balance, other than the components shown in Table 1, consisted of Fe and impurities. These slabs were hot-rolled using a tandem rolling system consisting of multiple rolling stands, including rough rolling and finish rolling, under the conditions shown in Tables 2 and 4, to produce hot-rolled steel sheets. Descaling during rough rolling was performed at least once under the following conditions: pressure: 15 MPa, distance between the steel sheet and the nozzle tip: 400 mm, and angle between the nozzle direction and the thickness direction of the steel sheet: 15°. Similarly, descaling during finish rolling was performed at least once under the following conditions: pressure: 3 MPa, distance between the steel sheet and the nozzle tip: 300 mm, and angle between the nozzle direction and the thickness direction of the steel sheet: 10°. The steel sheets were then cooled and coiled under the conditions shown in Table 2. The hot-rolled steel sheets were then bent and unbent at least once using a tension leveler, after which they were pickled to remove surface scale. They were then cold-rolled. The thickness of all sheets after cold rolling was 1.6 mm. Furthermore, the obtained steel sheets were heat treated under the conditions shown in Table 2 for the cold-rolled steel sheets and under the conditions shown in Table 4 for the hot-dip galvanized steel sheets. Hot-dip galvanizing was performed between the first and second cooling stages in the heat treatment process, and alloying treatment was performed as necessary. In Tables 2 to 5, CR indicates cold-rolled steel sheets that were not hot-dip galvanized, GI indicates steel sheets that were hot-dip galvanized, and GA indicates steel sheets that were hot-dip galvanized.

[0070] JIS No. 5 tensile test pieces were taken from the steel sheets obtained in this way in the direction perpendicular to the rolling direction, and tensile tests were conducted in accordance with JIS Z2241:2011 to measure the tensile strength (TS) and total elongation (El). Furthermore, the Japan Iron and Steel Federation standard "JFS T 1001 Hole Expansion Test Method" was conducted to measure the hole expansion ratio (λ). TS was 980 MPa or more, and TS × El × λ 0.5 A material with a γ / 1000 of 90 or more was judged to have good mechanical properties and press formability suitable for use as an automobile component.

[0071] In addition, to evaluate the resistance to liquid metal embrittlement (LME) cracking of spot welds, 150 mm wide x 50 mm long test pieces were prepared and spot welding tests were conducted on pairs of sheets. The sheets were a pair consisting of the steel sheet shown in Tables 3 and 5 and a commercially available 270 MPa-grade galvannealed steel sheet, and were welded at a 3° impact angle. A servomotor-driven stationary spot welding test machine was used. The power source was single-phase AC 50 Hz, pressure 400 kgf, current application time 20 cycles, and hold time 5 cycles. The welding current was set to a value that resulted in a fused nugget diameter of four times √t (t: sheet thickness / mm). A chromium-copper electrode with a tip diameter of 6 mm and a tip curvature radius of 40 mm was used. After welding, the nugget cross-sections of the samples were observed. Those with cracks of 0.1 mm or greater were rated as × (fail), while others were rated as ⊿ (pass). The results are shown in Tables 3 and 5.

[0072] [Table 1]

[0073] [Table 2-1]

[0074] [Table 2-2]

[0075] Table 2-3

[0076] Table 2-4

[0077] Table 3-1

[0078] Table 3-2

[0079] Table 4-1

[0080] Table 4-2

[0081] Table 4-3

[0082] Table 4-4

[0083] Table 5-1

[0084] Table 5-2

[0085] In Comparative Examples 2 to 8 and 10 to 16, the descaling conditions in rough rolling or finish rolling, the cooling conditions after the completion of final descaling, or the pickling conditions were not controlled within the predetermined ranges, and therefore, Al S / Si S The area ratio of the region where the ratio was 0.2 or less exceeded 50%, resulting in cracks occurring in the spot welds.

[0086] In Comparative Example 9, the number of rolling passes exceeding a rolling reduction of 20% in the rough rolling was small, resulting in an increased unrecrystallized fraction and poor press formability. In Comparative Example 9, strain-induced precipitation of AlN in the rough rolling was insufficient, resulting in the precipitation of fine and relatively large amounts of AlN particles in the subsequent finish rolling. It is believed that the pinning effect of these AlN particles suppressed ferrite recrystallization during heat treatment. In Comparative Example 17, the maximum heating temperature in the heat treatment step was low, resulting in an increased ferrite content and poor press formability. In Comparative Example 18, the average heating rate in the heat treatment step was high, resulting in an increased unrecrystallized fraction and poor press formability. In Comparative Example 19, the cooling stop temperature in the heat treatment step was high, resulting in no tempered martensite being formed and poor press formability. In Comparative Example 20, the cooling stop temperature in the heat treatment step was low, resulting in a reduced retained austenite content and poor press formability. In Comparative Example 21, the low-temperature holding temperature in the heat treatment step was low, so sufficient retained austenite content was not obtained, and press formability was poor. In Comparative Example 22, the low-temperature holding time in the heat treatment step was short, so similarly sufficient retained austenite content was not obtained, and press formability was poor. In Comparative Example 23, the average cooling rate in the temperature range of 550 to 650°C in the heat treatment step was low, so the ferrite content increased, and press formability was poor. In Comparative Example 32, the low-temperature holding temperature in the heat treatment step was high, so sufficient retained austenite content was not obtained, and press formability was poor. In Comparative Examples 40 to 49 and 66 to 75, the chemical compositions were not controlled within the specified range, so press formability or LME cracking resistance of spot welds was poor. In particular, in Comparative Examples 48 and 74, the absence of Ti increased the unrecrystallized rate, and press formability was poor. This is thought to be because, due to the absence of Ti, the solute N in the steel could not be fixed as TiN, and a relatively large number of fine AlN particles were generated, and the pinning effect of these particles suppressed the recrystallization of ferrite during heat treatment.

[0087] In contrast to this, the steel sheets of the examples have a TS of 980 MPa or more and a TS × El × λ 0.5The test results for LME cracking resistance at spot welds were also good, demonstrating that the steel has excellent press formability and LME cracking resistance at spot welds.

Claims

1. In mass%, C: 0.15-0.30%, Si: 0.30-1.50%, Mn: 1.40-3.49%, P: 0.050% or less, S: 0.0100% or less, Al: 0.30-1.50%, Ti: 0.001 to 0.100%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0-1.00%, W: 0-1.00%, Sn: 0-1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0-1.00%, B: 0 to 0.0050%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Ce: 0 to 0.0150%, Zr: 0 to 0.0100%, La: 0 to 0.0150%, Hf: 0-0.0100%, Bi: 0 to 0.0100%, REM other than Ce and La: 0 to 0.0100%, and The balance has a chemical composition consisting of Fe and impurities, The steel structure in the range of 1 / 8 thickness to 3 / 8 thickness centered on 1 / 4 thickness from the surface is, in volume %, Ferrite: 1 to 50%, Proportion of unrecrystallized ferrite in ferrite: 0 to 50% Tempered martensite: 1% or more, Retained austenite: 5% or more, Fresh martensite: 0 to 10% Sum of pearlite and cementite: 0 to 5%, and Remainder: Bainite When the surface was analyzed by EPMA, it was found that Al S / Si S A steel plate having an area ratio of 50% or less where the ratio is 0.2 or less and a tensile strength of 980 MPa or more. Here, Al S is the surface Al concentration (mass%), and Si S is the surface Si concentration (mass %).

2. The chemical composition is, in mass %, Cr: 0.001-1.00%, Mo: 0.001-1.00%, Cu: 0.001 to 1.00%, Ni: 0.001 to 1.00%, Co: 0.001 to 1.00%, W: 0.001-1.00%, Sn: 0.001 to 1.00%, Sb: 0.001 to 0.50%, Nb: 0.001-0.200%, V: 0.001-1.00%, B: 0.0001 to 0.0050%, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, Ce: 0.0001 to 0.0100%, Zr: 0.0001 to 0.0100%, La: 0.0001 to 0.0100%, Hf: 0.0001-0.0100%, Bi: 0.0001 to 0.0100%, and REM other than Ce and La: 0.0001 to 0.0100% The steel sheet according to claim 1, comprising one or more selected from the group consisting of:

3. 3. The steel sheet according to claim 1, wherein the chemical composition satisfies the relationship of the following formula (1), and the proportion of unrecrystallized ferrite in ferrite is 10% or less. [N]-(14.01 / 47.88)・[Ti]≦0...(1) Here, [N] is the N content (mass %), and [Ti] is the Ti content.

4. The steel sheet according to any one of claims 1 to 3, having a hot-dip galvanized layer or a galvannealed layer on a surface thereof.

5. (A) A hot rolling process comprising rough rolling and finish rolling a slab having the chemical composition according to any one of claims 1 to 3, wherein the hot rolling process satisfies the following conditions (A1) to (A4): (A1) In the rough rolling, the steel sheet temperature is 1050 to 1200 ° C., and rolling is performed at least twice with a rolling reduction rate per pass exceeding 20%; (A2) In rough rolling, high-pressure water descaling satisfying the following conditions: pressure: 10 MPa or more, distance between the steel sheet and the nozzle tip: 500 mm or less, and angle between the nozzle direction and the thickness direction of the steel sheet: 3 to 15 degrees is performed at least once within 10 seconds after the steel sheet has passed through a rolling pass in which the steel sheet temperature is 1050 to 1200°C and the reduction ratio is more than 20%; (A3) In finish rolling, high-pressure water descaling is performed at least once within 3.0 seconds after the steel plate has passed through a rolling pass in which the steel plate temperature is 950 to 1100°C and the reduction ratio is 30% or more, the pressure being 2 MPa or more, the distance between the steel plate and the nozzle tip being 400 mm or less, and the angle between the nozzle direction and the thickness direction of the steel plate being 3 to 15 degrees. (A4) The time (seconds) elapsed until the steel sheet reaches 700°C after the completion of final descaling satisfies the following formula (2): [Equation 1] t: Time elapsed since the end of the final descaling (seconds) T(t): Steel plate temperature at elapsed time t (°C) t f : Time (seconds) elapsed after final descaling until the steel sheet reaches 700°C (B) The obtained hot-rolled steel sheet is subjected to at least one bending and unbending deformation, and then the hot-rolled steel sheet is subjected to a treatment with 1.0 to 5.0 mol / L of HCl and less than 3.0 mol / L of Fe. 2+ a pickling step comprising: passing the steel sheet through an aqueous solution containing the compound at a temperature of 70 to 90°C at an average speed of 10 m / min or more for 30 seconds or more; (C) A cold rolling process in which the hot-rolled steel sheet after pickling treatment is cold-rolled at a reduction ratio of 30 to 75%; (D) A heat treatment step including heat treating the obtained cold-rolled steel sheet, which satisfies the following conditions (D1) to (D5): (D1) The average heating rate between 650 and Ac1°C is 1.0 to 5.0°C / sec; (D2) Ac1+30 to 950 ° C. at a maximum heating temperature for 1 to 500 seconds (soaking treatment); (D3) Cooling the cold-rolled steel sheet after the soaking treatment so that the average cooling rate in the temperature range of 550 to 650 ° C. is 10 to 100 ° C. / sec (first cooling); (D4) Stopping cooling between Ms-150 and Ms ° C. (second cooling); (D5) The cold-rolled steel sheet after the second cooling is heated to a temperature range of 330 to 450 ° C. and then held at that temperature range for 50 to 1000 seconds (low temperature holding). The method for producing a steel sheet according to any one of claims 1 to 3, comprising:

6. The method for producing a steel sheet according to claim 5, further comprising: subjecting the steel sheet after the first cooling in (D3), after the second cooling in (D4), or after the low-temperature holding in (D5) to hot-dip galvanizing or hot-dip galvannealing.

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