Steel plate and method for manufacturing the same

Abstract

The present invention provides a steel plate and a manufacturing method thereof. The steel plate has a specified chemical composition, and the steel structure is, by 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, primary martensite: 0 to 10%, the total of pearlite and cementite: 0 to 5%, and the remaining part: bainite. Moreover, when performing EPMA analysis on the surface of the steel plate, the area ratio of the region where the Al S / Si S ratio is 0.2 or less is 50% or less, and the tensile strength of the steel plate is 980 MPa or more.

Description

Technical Field

[0001] The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly to a high-strength steel sheet used as an automotive steel sheet and a method for manufacturing the same. Background Art

[0002] In recent years, from the viewpoint of restricting greenhouse gas emissions associated with global warming countermeasures, there has been a demand to improve the fuel efficiency of automobiles. In order to achieve vehicle body weight reduction and ensure collision safety, the application of high-strength steel sheets has become increasingly widespread. In particular, recently, the demand for ultra-high-strength steel sheets with a tensile strength of 980 MPa or more has been increasing. In addition, for parts of the vehicle body that require rust prevention, high-strength hot-dip galvanized steel sheets with hot-dip galvanization applied on the surface are also required.

[0003] Hot-dip galvanized steel sheets used as automotive parts require not only strength but also various workabilities required for part forming, such as stamping formability and weldability. Specifically, from the viewpoint of stamping formability, the steel sheet is required to have excellent elongation (total elongation in a tensile test: El) and stretch flange formability (hole expansion ratio: λ).

[0004] Generally, as the strength of the steel sheet increases, the stamping formability deteriorates. As a means of achieving both high strength and stamping formability of steel, transformation-induced plasticity (TRIP) steel sheets utilizing transformation-induced plasticity of retained austenite are known.

[0005] In Patent Documents 1 to 3, a high-strength TRIP steel sheet in which the tissue constituent fraction is controlled within a specified range and the elongation and hole expansion ratio are improved is disclosed. In addition, in Patent Document 4, a high-strength steel sheet is described, which has a specified chemical composition, contains ferrite with an average crystal grain size of 2 μm or less at a volume fraction of 15% or less, retained austenite with an average crystal grain size of 2 μm or less at a volume fraction of 2 to 15%, martensite with an average crystal grain size of 3 μm or less at a volume fraction of 10% or less, and the balance being bainite and tempered martensite with an average crystal grain size of 6 μm or less, and on average contains 10 or more cementite particles with a particle size of 0.04 μm or more in the bainite and tempered martensite grains. It is described that the high-strength steel sheet has a tensile strength of 1180 MPa or more, and has high elongation and hole expandability and excellent bending workability as a result.

[0006] In Patent Document 5, a TRIP steel sheet is disclosed, which improves stretch flange formability by restricting the area ratio of massive (low aspect ratio) retained austenite.

[0007] A high-strength TRIP steel sheet is disclosed in Patent Document 6. By controlling the amounts of dissolved Si and dissolved Mn contained in retained austenite to be above specified values, it has a large amount of work hardening in the initial stage of forming and excellent shape freezing property and workability.

[0008] In addition, for steel sheets for automobiles, excellent welding workability is required in addition to stamping formability. In particular, in the welding of hot-dip galvanized steel sheets to each other or the welding of hot-dip galvanized steel sheets to non-coated steel sheets, it is necessary to suppress liquid metal embrittlement (LME) cracking. This phenomenon is cracking that occurs when tensile stress generated by welding acts on a place where zinc liquefied by heat absorption during welding infiltrates along grain boundaries into the interior of the steel sheet and embrittles it.

[0009] It is disclosed in Patent Document 7 that the more Si contained in steel, the more likely the above-mentioned LME cracking occurs. Therefore, a TRIP steel sheet is disclosed in this document, in which Al having the same effect is added to the TRIP steel instead of a part of Si added to obtain retained austenite. In addition, TRIP steel sheets in which a part of Si is replaced by Al are also disclosed in Patent Documents 8 and 9.

[0010] In addition, a method for manufacturing a hot-dip galvanized steel sheet excellent in LME cracking resistance is disclosed in Patent Document 10, which is characterized by controlling the atmosphere during heating and annealing in a hot-dip galvanizing production line.

[0011] Prior art documents

[0012] Patent documents

[0013] Patent Document 1: International Publication No. 2013 / 051238

[0014] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006-104532

[0015] Patent Document 3: Japanese Unexamined Patent Application Publication No. 2011-184757

[0016] Patent Document 4: Japanese International Publication No. 2017 / 179372

[0017] Patent Document 5: International Publication No. 2018 / 190416

[0018] Patent Document 6: International Publication No. 2013 / 018741

[0019] Patent Document 7: International Publication No. 2018 / 202916

[0020] Patent Document 8: Japanese Unexamined Patent Application Publication No. 2011-17046

[0021] Patent Document 9: International Publication No. WO2013 / 144377

[0022] Patent Document 10: International Publication No. WO2018 / 234938 Summary of the Invention

[0023] Problems to be Solved by the Invention

[0024] In the present technical field, there has been a continuous demand for a steel sheet that is excellent in both high strength and stamping formability and also excellent in resistance to LME cracking. Regarding the steel sheets of the prior art, there is still room for improvement from these viewpoints.

[0025] Therefore, an object of the present invention is to provide a steel sheet excellent in stamping formability and resistance to LME cracking at the spot welded portion and a method for manufacturing the same.

[0026] Means for Solving the Problems

[0027] The present inventors repeatedly conducted in-depth studies to achieve the above object, and as a result, obtained the following knowledge.

[0028] Regarding the resistance to LME cracking at the spot welded portion, it was found that the average composition of the steel is of course important, and the chemical composition of the surface layer portion of the steel sheet is also extremely important. Specifically, it was found that when the ratio of the Al concentration (Al S ) to the Si concentration (Si S ) on the steel sheet surface is in a specified range, the resistance to LME cracking is greatly improved. The detailed mechanism thereof is not yet clear, but it is considered that Al on the surface layer of the steel sheet may suppress the intrusion of liquid Zn into the steel sheet. More specifically, when performing EPMA analysis on the steel sheet surface, it was found that when the area ratio of the region where the Al S / Si S ratio is 0.2 or less is 50% or less, an improvement effect appears. In addition, regarding stamping formability, it was found that in particular, reducing the proportion of unrecrystallized ferrite in ferrite is important. Specifically, it was found that by reducing the proportion of unrecrystallized ferrite in ferrite to 50% or less, the steel structure becomes more isotropic, and therefore, in addition to an increase in elongation, the hole expansion property can also be improved. In addition, regarding the improvement of the resistance to LME cracking, in order to increase the Al concentration on the steel sheet surface, a relatively large amount of Al needs to be added, but this will generate fine and relatively large amounts of AlN particles. In order to avoid significantly suppressing the recrystallization of ferrite due to its pinning effect, it is effective to add Ti to fix the dissolved N in the steel as TiN.

[0029] The present invention was achieved based on the above knowledge, and specifically, it is as follows.

[0030] (1) A steel plate having a chemical composition containing the following components by mass%:

[0031] C: 0.15 - 0.30%,

[0032] Si: 0.30 - 1.50%,

[0033] Mn: 1.40 - 3.49%,

[0034] P: 0.050% or less,

[0035] S: 0.0100% or less,

[0036] Al: 0.30 - 1.50%,

[0037] Ti: 0.001 - 0.100%,

[0038] N: 0.0100% or less,

[0039] O: 0.0100% or less,

[0040] Cr: 0 - 1.00%,

[0041] Mo: 0 - 1.00%,

[0042] Cu: 0 - 1.00%,

[0043] Ni: 0 - 1.00%,

[0044] Co: 0 - 1.00%,

[0045] W: 0 - 1.00%,

[0046] Sn: 0 - 1.00%,

[0047] Sb: 0 - 0.50%,

[0048] Nb: 0 - 0.200%,

[0049] V: 0 - 1.00%,

[0050] B: 0 - 0.0050%,

[0051] Ca: 0 - 0.0100%,

[0052] Mg: 0 - 0.0100%,

[0053] Ce: 0 - 0.0150%,

[0054] Zr: 0 - 0.0100%,

[0055] La: 0 - 0.0150%,

[0056] Hf: 0 to 0.0100%,

[0057] Bi: 0 to 0.0100%,

[0058] REM other than Ce and La: 0 to 0.0100%, and

[0059] The balance: Fe and impurities,

[0060] In the range of 1 / 8 to 3 / 8 of the thickness centered at a position 1 / 4 of the thickness from the surface of the steel sheet, the volume percentages of the steel structures are as follows:

[0061] Ferrite: 1 to 50%,

[0062] The proportion of unrecrystallized ferrite in ferrite: 0 to 50%,

[0063] Tempered martensite: 1% or more,

[0064] Retained austenite: 5% or more,

[0065] Primary martensite: 0 to 10%,

[0066] The total of pearlite and cementite: 0 to 5%, and

[0067] The balance: bainite,

[0068] And, when performing EPMA analysis on the surface of the steel sheet, the area ratio of the region where the Al S / Si S ratio is 0.2 or less is 50% or less, and the tensile strength of the steel sheet is 980 MPa or more,

[0069] wherein, Al S is the surface Al concentration (mass%), and Si S is the surface Si concentration (mass%).

[0070] (2) The steel sheet according to (1) above, wherein the chemical composition contains one or more selected from the following components by mass%:

[0071] Cr: 0.001 to 1.00%,

[0072] Mo: 0.001 to 1.00%,

[0073] Cu: 0.001 to 1.00%,

[0074] Ni: 0.001 to 1.00%,

[0075] Co: 0.001 to 1.00%,

[0076] W: 0.001 to 1.00%,

[0077] Sn: 0.001 to 1.00%,

[0078] Sb: 0.001 to 0.50%,

[0079] Nb: 0.001 to 0.200%,

[0080] V: 0.001 to 1.00%,

[0081] B: 0.0001 to 0.0050%,

[0082] Ca: 0.0001 to 0.0100%,

[0083] Mg: 0.0001 to 0.0100%,

[0084] Ce: 0.0001 to 0.0100%,

[0085] Zr: 0.0001 to 0.0100%,

[0086] La: 0.0001 to 0.0100%,

[0087] Hf: 0.0001 to 0.0100%,

[0088] Bi: 0.0001 to 0.0100%, and

[0089] REM other than Ce and La: 0.0001 to 0.0100%.

[0090] (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 ferrite is 10% or less,

[0091] [N] - (14.01 / 47.88)·[Ti] ≤ 0 (1)

[0092] wherein, [N] is the N content (mass %), and [Ti] is the Ti content.

[0093] (4) The steel sheet according to any one of (1) to (3) above, which has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on its surface.

[0094] (5) The manufacturing method of the steel sheet according to any one of (1) to (3) above, which includes the following steps:

[0095] (A) A hot rolling process that satisfies the following conditions (A1) to (A4), which includes rough rolling and finish rolling of a slab having the chemical composition described in any one of the above (1) to (3).

[0096] (A1) In rough rolling, at least two passes of rolling are performed where the steel plate temperature is 1050 - 1200 °C and the reduction ratio per pass exceeds 20%.

[0097] (A2) In rough rolling, within 10 seconds after passing through a rolling pass where the steel plate temperature is 1050 - 1200 °C and the reduction ratio exceeds 20%, at least one high - pressure water descaling (also called descaling) is performed. This high - pressure water descaling satisfies a pressure of 10 MPa or more, a distance between the steel plate and the nozzle tip of 500 mm or less, and an angle formed between the direction of the nozzle and the thickness direction of the steel plate of 3 - 15 degrees.

[0098] (A3) In finish rolling, within 3.0 seconds after passing through a rolling pass where the steel plate temperature is 950 - 1100 °C and the reduction ratio is 30% or more, at least one high - pressure water descaling is performed. This high - pressure water descaling satisfies 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 formed between the direction of the nozzle and the thickness direction of the steel plate of 3 - 15 degrees.

[0099] (A4) After the final descaling is completed, the elapsed time (seconds) until the steel plate reaches 700 °C satisfies the following formula (2).

[0100]

[0101] t: Elapsed time (seconds) since the end of the final descaling

[0102] T(t): Steel plate temperature (°C) at elapsed time t

[0103] t f : Elapsed time (seconds) until the steel plate reaches 700 °C after the end of the final descaling

[0104] (B) A pickling process, which includes performing pickling treatment for 30 seconds or more. In this pickling treatment, at least one bending and reverse bending deformation is applied to the obtained hot - rolled steel plate, and then the hot - rolled steel plate is passed through an aqueous solution at a temperature of 70 - 90 °C containing 1.0 - 5.0 mol / L of HCl and less than 3.0 mol / L of Fe at an average speed of 10 m / min or more. 2+ of

[0105] (C) A cold rolling process, in which the hot - rolled steel plate after pickling treatment is cold - rolled with a reduction ratio of 30 - 75%.

[0106] (D) A heat treatment process satisfying the following conditions (D1) to (D5), which includes heat-treating the obtained cold-rolled steel sheet,

[0107] (D1) The average heating rate between 650 and Ac1 °C is 1.0 to 5.0 °C / second,

[0108] (D2) Hold for 1 to 500 seconds (uniform heat treatment) at the maximum heating temperature of Ac1 + 30 to 950 °C,

[0109] (D3) Cool the cold-rolled steel sheet after the uniform heat treatment in such a way that the average cooling rate in the temperature range of 550 to 650 °C is 10 to 100 °C / second (first cooling),

[0110] (D4) Stop cooling between Ms - 150 and Ms °C (second cooling),

[0111] (D5) Heat the cold-rolled steel sheet after the second cooling to the temperature range of 330 to 450 °C, and then hold in the said temperature range for 50 to 1000 seconds (low-temperature holding).

[0112] (6) According to the method for manufacturing a steel sheet described in the above (5), it further includes: performing hot-dip galvanizing or alloyed hot-dip galvanizing on the steel sheet after the first cooling of (D3), the second cooling of (D4), or the low-temperature holding of (D5).

[0113] Advantages of the Invention

[0114] According to the present invention, a steel sheet with excellent stamping formability and resistance to LME cracking at the spot weld portion can be obtained. Detailed Embodiments

[0115] 《Chemical Composition》

[0116] First, the reasons for defining the chemical composition of the steel sheet in the embodiments of the present invention as above are explained. In addition, in this specification, "%" indicating the chemical composition refers to "mass %" unless otherwise specified. Also, in this specification, "~" indicating a numerical range is used to mean including the values described before and after it as the lower limit value and the upper limit value without special explanation.

[0117] [C: 0.15 to 0.30%]

[0118] C (carbon) is an essential element for ensuring the strength of the steel plate. To fully achieve the above effects, the C content is set at 0.15% or more. The C content can also be 0.16% or more, 0.18% or more, or 0.20% or more. On the other hand, if an excessive amount of C is contained, workability such as stamping formability and weldability may deteriorate. Therefore, the C content is set at 0.30% or less. The C content can also be 0.28% or less, 0.27% or less, or 0.25% or less.

[0119] [Si: 0.30 - 1.50%]

[0120] Si (silicon) is an element that inhibits the formation of iron carbide and contributes to the improvement of strength and formability. To fully achieve these effects, the silicon content is set at 0.30% or more. The Si content can also 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 at 1.50% or less. The Si content can also be 1.40% or less, 1.20% or less, or 1.00% or less.

[0121] [Mn: 1.40 - 3.49%]

[0122] Mn (manganese) is a strong austenite stabilizing element and is effective for strengthening the steel plate to high strength. To fully achieve these effects, the Mn content is set at 1.40% or more. The Mn content can also be 1.50% or more, 1.70% or more, or 2.00% or more. On the other hand, excessive addition sometimes deteriorates workability such as stamping formability, weldability, and even low-temperature toughness. Therefore, the Mn content is set at 3.49% or less. The Mn content can also be 3.20% or less, 3.00% or less, or 2.90% or less.

[0123] [P: 0.050% or less]

[0124] P (phosphorus) is a solid-solution strengthening element and is effective for strengthening the steel plate to high strength, but excessive addition deteriorates 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 can also be 0%, but to extremely reduce the P content, the cost of dephosphorization becomes high. Therefore, from an economic point of view, it is preferable to set the lower limit at 0.001%.

[0125] [S: 0.0100% or less]

[0126] S (sulfur) is an element contained as an impurity, and forms MnS in steel, which deteriorates toughness and hole expansion. Therefore, as a range in which the deterioration of toughness and expandability is not significant, the S content is limited to 0.0100% or less. The S content is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less. The S content may be 0%, but if the S content is extremely reduced, the desulfurization cost will become high, so from the perspective of economic efficiency, it is preferred to set the lower limit to 0.0001%.

[0127] [Al: 0.30~1.50%]

[0128] In order to generate retained austenite and to make the Al S / Si S In order to improve the LME cracking resistance by increasing the Al content, at least 0.30% Al (aluminum) is added. The Al content may also be 0.40% or more, 0.50% or more, or 0.60% or more. On the other hand, even if Al is added excessively, the effect will be saturated, which will not only lead to an increase in cost in vain, but also increase the phase transformation temperature of the steel, increase the load during hot rolling, and as a result, may reduce the mechanical properties of the steel sheet. Therefore, the Al content is set at 1.50% as the upper limit. The Al content may also be less than 1.40%, less than 1.20%, or less than 1.00%.

[0129] [Ti: 0.001~0.100%]

[0130] Ti (titanium) is a carbonitride-forming element, and contributes to the high strength of the steel sheet through precipitation strengthening. In an embodiment of the present invention, Ti is added to fix the solid solution N in the steel as TiN. In an embodiment of the present invention, since Al is added in a relatively large content of 0.30% or more, a lot of fine AlN is sometimes generated without adding Ti. In this case, due to the pinning effect generated by the fine AlN particles, the recrystallization of ferrite is significantly suppressed during annealing after cold rolling, and the ductility and hole expansion of the steel sheet are reduced, resulting in a decrease in stamping formability. Therefore, in order to fix the solid solution N in the steel as TiN and suppress the generation of such fine AlN particles, the Ti content is set to 0.001% or more. In an embodiment of the present invention, in order to improve the LME cracking resistance, Al needs to be added, but since the addition of such Al sometimes leads to a decrease in stamping formability, it is important to add Ti from the perspective of taking into account both LME cracking resistance and stamping formability, as described above. The Ti content may also be 0.003% or more, 0.005% or more, 0.010% or more, or 0.015% or more. Furthermore, when Ti is added in a manner satisfying the following formula (1), a greater effect can be obtained, and the proportion of unrecrystallized ferrite in ferrite described in detail later can be reliably reduced, for example, the proportion of unrecrystallized ferrite in ferrite can be reduced to less than 40%, preferably to less than 30% or less than 10%.

[0131] [N]-(14.01 / 47.88)·[Ti]≤0(1)

[0132] Here, [N] is the N content (mass %), and [Ti] is the Ti content. On the other hand, even if Ti is added excessively, the effect will be saturated, which will not only lead to an increase in cost in vain, but also the ductility and hole expansion of the steel sheet may be deteriorated due to the large amount of TiC precipitation. 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.

[0133] [N: 0.0100% or less]

[0134] N (nitrogen) is an element contained as an impurity. When its content is high, coarse nitrides may be formed in the steel, which may deteriorate the bendability and hole expansion. 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 in order to extremely reduce the N content, the cost of removing N will increase, so from the perspective of economic efficiency, it is preferred to set the lower limit to 0.0001%.

[0135] [O: 0.0100% or less]

[0136] Oxygen (O) is an element contained as an impurity. When its content is high, sometimes coarse oxides are formed in the steel, deteriorating bendability and hole expansion properties. 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 can also be 0%, but from the viewpoint of manufacturing cost, it is preferable to set the lower limit at 0.0001%.

[0137] The basic chemical composition of the steel sheet according to the embodiment of the present invention and the slab used in its manufacture is as described above. Furthermore, the steel sheet and the slab may also contain the following optional elements as needed. In addition, the lower limit of the content in the case of not containing the optional element is 0%.

[0138] [Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, and B: 0 to 0.0050%]

[0139] 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 for strengthening the steel sheet. Therefore, one or more of these elements may be added as needed. However, when these elements are added excessively, the effect saturates and the cost increases in vain. Therefore, their contents are set as Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, W: 0 to 1.00%, Sn: 0 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, and B: 0 to 0.0050%. Each element can be 0.001% or more, 0.005% or more, or 0.010% or more. In particular, the B content can also be 0.0001% or more or 0.0002% or more. Similarly, the B content can also be 0.0030% or less, 0.0010% or less, less than 0.0005%, 0.0004% or less, or 0.0003% or less.

[0140] [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%, and REM other than Ce and La: 0 to 0.0100%]

[0141] Ca (Calcium), Mg (Magnesium), Ce (Cerium), Zr (Zirconium), La (Lanthanum), Hf (Hafnium), and REM (rare earth elements) other than Ce and La are elements that contribute to the fine dispersion of inclusions in steel, and Bi (Bismuth) is an element that reduces the microsegregation of substitutional alloy elements such as Mn and Si in steel. Since they respectively contribute to the improvement of the workability of the steel sheet, one or more of these elements can also be added as needed. However, excessive addition will cause deterioration of ductility. Therefore, the upper limit of its content is set to 0.0150% or 0.0100%. In addition, each element can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0142] In the steel sheet of the embodiment of the present invention, the remaining part other than the above elements is composed of Fe and impurities. Impurities refer to components such as those mixed in due to various factors in the manufacturing process, represented by raw materials such as ores and scraps, during the industrial manufacturing of steel sheets.

[0143] 《Steel Structure Inside the Steel Sheet》

[0144] Next, the reasons for limiting the internal structure of the steel sheet of the embodiment of the present invention will be described.

[0145] [Ferrite: 1 - 50%]

[0146] Ferrite is a soft tissue with excellent ductility. In order to improve the elongation of the steel sheet, the ferrite content is set to 1% or more in terms of volume %. The ferrite content can also be 3% or more, 5% or more, or 10% or more in terms of volume %. However, if ferrite is excessively contained, it is difficult to ensure the desired steel sheet strength. Therefore, its content is set to 50% or less in terms of volume %, and can also be 45% or less, 40% or less, or 35% or less.

[0147] [Proportion of unrecrystallized ferrite in ferrite: 0 - 50%]

[0148] By increasing the proportion of recrystallized ferrite, that is, reducing the proportion of unrecrystallized ferrite in ferrite, the steel structure can be made more isotropic. Therefore, in addition to the improvement of elongation, the hole expansion property can also be improved. Therefore, in order to obtain excellent elongation and hole expansion property, the proportion of unrecrystallized ferrite in ferrite is limited to 50% or less in terms of volume %. The proportion of unrecrystallized ferrite in ferrite can be 40% or less, 30% or less, or 20% or less in terms of volume %. If this proportion is set to 10% or less in terms of volume %, particularly excellent elongation and hole expansion property can be obtained. The lower limit is not particularly limited and can also be 0%. For example, the proportion of unrecrystallized ferrite in ferrite can be 1% or more, 2% or more, or 3% or more in terms of volume %.

[0149] [Tempered martensite: 1% or more]

[0150] Tempered martensite is a high-strength and tough structure, which is a necessary metal structure in the embodiments of the present invention. In order to balance the strength and elongation at a high level, the content of tempered martensite is set to 1% or more by volume%. The content of tempered martensite is preferably 5% or more, and may also be 10% or more or 20% or more. There is no particular limitation on the upper limit. For example, the content of tempered martensite may also be 90% or less, 80% or less, 70% or less, or 50% or less by volume%.

[0151] [Retained austenite: 5% or more]

[0152] Retained austenite improves the ductility of the steel sheet through the TRIP effect in which it transforms into martensite due to strain-induced phase transformation during the deformation of the steel sheet. Therefore, the content of retained austenite is set to 5% or more by volume%, and may also be 8% or more or 10% or more. Since the more retained austenite, the more the elongation increases, there is no need to specify an upper limit value. However, in order to obtain a large amount of retained austenite, a large amount of alloying elements such as C need to be contained. In the present invention, since an upper limit is set for the C content, it is actually difficult to obtain 30% or more of retained austenite. Therefore, the content of retained austenite may be 30% or less, 25% or less, or 20% or less by volume%.

[0153] [Primary martensite: 0 - 10%]

[0154] In the embodiments of the present invention, primary martensite refers to untempered martensite, that is, martensite without carbide. Since this primary martensite is a brittle structure, it becomes the starting point of fracture during plastic deformation, deteriorating the local ductility of the steel sheet. Therefore, its content is set to 0 - 10% by volume%. The content of primary martensite is preferably 0 - 8% or 0 - 5% by volume%. The content of primary martensite may also be 1% or more or 2% or more by volume%.

[0155] [Total of pearlite and cementite: 0 - 5%]

[0156] Pearlite contains hard and coarse cementite, which becomes the starting point of fracture during plastic deformation, deteriorating the local ductility of the steel sheet. Therefore, its content combined with cementite is set to 0 - 5% by volume%, and may also be 0 - 3% or 0 - 2%.

[0157] [Bainite: the remainder]

[0158] The remaining part of the metal structure of the embodiment of the present invention is composed of bainite. The bainite of the remaining part of the structure may be any one of upper bainite with carbides between laths, lower bainite with carbides within laths, bainite ferrite without carbides, and granular bainite ferrite in which the lath boundaries of bainite are restored and become indistinct, or a mixed structure thereof. The bainite content of the remaining part may also be 0%. For example, the bainite content of the remaining part may be 1% or more, 5% or more, or 10% or more by volume%. The upper limit is not particularly limited. For example, the bainite content of the remaining part may be 70% or less, 60% or less, 50% or less, or 40% or less by volume%.

[0159] The fraction of the steel structure is evaluated by SEM-EBSD method (electron backscatter diffraction method) and SEM secondary electron image observation. First, a sample is collected with the plate thickness cross-section parallel to the rolling direction of the steel plate, that is, the plate thickness cross-section at the central position in the width direction, as the observation surface. After the observation surface is mechanically polished and processed into a mirror surface, electrolytic polishing is carried out. Then, in one or more observation fields in the range of 1 / 8 thickness to 3 / 8 thickness centered at a position 1 / 4 thickness away from the surface of the steel plate on the observation surface, the crystal structure and orientation analysis are carried out on a total area of 2.0×10 -9 m 2 or more. The data obtained by the EBSD method is analyzed using "OIM Analysys 6.0" manufactured by TSL Corporation. In addition, the scoring interval distance (step) is set to 0.03 to 0.20 μm. The region determined to be FCC iron from the observation results is set as retained austenite. Furthermore, the grain boundary map is obtained with boundaries having a crystal orientation difference of 15 degrees or more as grain boundaries.

[0160] Next, the same specimen as the one on which EBSD observation was performed was subjected to nitric acid ethanol etching, and secondary electron image observation was performed on the same field of view as the EBSD observation. In order to observe the same field of view as during EBSD measurement, marks such as Vickers indentations can be made in advance. Based on the obtained secondary electron images, the area fractions of ferrite, retained austenite, bainite, tempered martensite, primary martensite, and pearlite were measured respectively, and these were regarded as volume fractions. A region in the grain with a lower structure and in which cementite precipitates in multiple variants was judged to be tempered martensite. A region in which cementite precipitates in a layered manner was judged to be pearlite (or the sum of pearlite and cementite). A region with low brightness and in which no lower structure was visible was judged to be ferrite. A region with high brightness and in which the lower structure was not revealed by etching was judged to be primary martensite and retained austenite. A region that did not conform to any of the above regions was judged to be bainite. The volume fractions of each were calculated by the point counting method as the volume fractions of each tissue. For the volume fraction of primary martensite, it can be obtained by subtracting the volume fraction of retained austenite determined by X-ray diffraction method.

[0161] In addition, a region in the EBSD in the grain judged to be ferrite with a grain average misorientation (Grain average misorientation; GAM) exceeding 0.6 was judged to be unrecrystallized ferrite, and a crystal with a grain average misorientation of 0.6 or less was judged to be recrystallized ferrite. At this time, the measurement step size was set to 0.10 μm, and the θ step size during the Hough transform of the EBSD pattern was set to 1°.

[0162] The volume fraction of retained austenite was measured using the X-ray diffraction method. In the range of 1 / 8 thickness to 3 / 8 thickness centered at the position 1 / 4 thickness from the surface of the steel plate, the surface parallel to the plate surface was processed into a mirror surface, and the area fraction of FCC iron was measured by the X-ray diffraction method, and this was used as the volume fraction of retained austenite.

[0163] The steel plate according to the embodiment of the present invention may have a zinc-containing coating on at least one surface, preferably on both surfaces. This coating can be a hot-dip galvanized layer or an alloyed hot-dip galvanized layer with any composition known to those skilled in the art, and may contain additive elements such as Al and Mg in addition to containing Zn. In addition, the coating amount is not particularly limited and can be a normal coating amount. The steel plate according to the embodiment of the present invention is of course not limited to the above-mentioned coated steel plate, and also includes a steel plate without coating. This is because, even for a steel plate without coating, for example, when spot welding with a galvanized steel plate, it is possible for the molten zinc on the galvanized steel plate to invade the uncoated steel plate, resulting in LME cracking.

[0164] [Al S / Si SArea ratio of the region where the ratio is 0.2 or less: 50% or less

[0165] In an embodiment of the present invention, in order to improve the LME resistance, when analyzing the steel plate surface with EPMA (Electron Probe Micro Analyzer), it is necessary to use Al S / Si S The area ratio of the region where the ratio is 0.2 or less is limited to 50% or less. Here, Al S is the surface Al concentration (mass%), and Si S is the surface Si concentration (mass%). In the case of subjecting a conventional steel plate to hot-dip galvanizing or alloyed hot-dip galvanizing, etc., sometimes the zinc in the molten coating due to the heat during welding may invade the grain boundaries of the welded part structure, resulting in LME cracking inside the steel plate. However, according to the embodiment of the present invention, by limiting the area ratio of the region where the Al S / Si S ratio is 0.2 or less to 50% or less, it is possible to suppress the invasion of liquid Zn into the steel plate interior by the action of Al concentrated on the steel plate surface layer. As a result, the LME cracking resistance of the steel plate can be significantly improved. The area ratio of the region where the Al S / Si S 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 also be 0%. For example, the area ratio of the region where the Al S / Si S ratio is 0.2 or less may also be 1% or more, 2% or more, or 3% or more.

[0166] Al S and Si S are measured using EPMA as described above. In the case of a plated steel plate, the steel plate surface after removing the coating with a 5% hydrochloric acid aqueous solution added with an inhibitor is measured. At this time, when the average value of Zn detected by EPMA measurement is 0.2 mass% or more, it is judged that the coating removal is insufficient, and the sample adjustment and EPMA measurement are performed again. The measurement area is 30 mm × 30 mm, the measurement interval is 30 μm, the mass% of Al and Si at each measurement point (beam diameter: 30 μm) is obtained, and the Al S / Si S at each measurement point is calculated. The number of measurement points with a value of 0.2 or less is calculated and divided by the total number of measurement points, thereby determining the area ratio of the region where the Al S / Si S ratio is 0.2 or less.

[0167] [Mechanical properties]

[0168] The steel sheet according to an embodiment 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. For example, the tensile strength can be 1500 MPa or less, 1400 MPa or less, 1300 MPa or less, or 1250 MPa or less. Similarly, the steel sheet according to an embodiment of the present invention can achieve high ductility. More specifically, it can achieve 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. For example, the total elongation can be 40.0% or less or 30.0% or less. The tensile strength and the total elongation are measured by taking a JIS No. 5 tensile test piece in a direction perpendicular to the rolling direction of the steel sheet and performing a tensile test in accordance with JIS Z2241:2011. In addition, the steel sheet according to an embodiment of the present invention can achieve high hole expansion property. More specifically, it can achieve a hole expansion rate (λ) of 20% or more, preferably 25% or more, more preferably 30% or more. The upper limit is not particularly limited. For example, the hole expansion rate can be 80% or less or 70% or less. The hole expansion rate 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 an embodiment of the present invention can improve the balance of the tensile strength (TS), the total elongation (El), and the hole expansion rate (λ) at a high level, and thus can achieve preferable stamping formability for use as automotive parts.

[0169] [Plate thickness]

[0170] The steel sheet according to an embodiment of the present invention has a plate thickness of, for example, 1.0 to 6.0 mm. There is no particular limitation, and the plate thickness can also be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more. Similarly, the plate thickness can also be 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less.

[0171] [Manufacturing method]

[0172] Next, the manufacturing method of the steel sheet will be described. The following description is intended to illustrate the characteristic method for manufacturing the steel sheet according to an embodiment of the present invention, and is not intended to limit the steel sheet to the steel sheet manufactured by the manufacturing method described below.

[0173] 《(A) Hot rolling process》

[0174] First, in the hot rolling process, for the steel plate, a slab having the same chemical composition as that described above is heated before hot rolling, and then rough rolling and finish rolling are carried out. The heating temperature of the slab is not particularly limited, but in order to fully dissolve borides, carbides, etc., it is generally preferably set to 1150 °C or higher. In addition, from the viewpoint of manufacturability, the used steel billet is preferably cast by the continuous casting method, but it can also be manufactured by the ingot casting method or the thin slab casting method.

[0175] [Rough rolling]

[0176] In this method, the heated slab is rolled at least twice, preferably at least three times, in rough rolling at a steel plate temperature of 1050 - 1200 °C with a reduction ratio per pass exceeding 20%. Thereby, strain-induced precipitation of AlN in rough rolling is promoted. When the rough rolling conditions do not satisfy the above range, the strain-induced precipitation of AlN in rough rolling becomes insufficient, and AlN precipitates in the subsequent finish rolling. The AlN precipitated in finish rolling is finer and has a higher number density than the AlN precipitated in rough rolling. Such AlN suppresses recrystallization during cold rolling annealing through the pinning effect. As a result, the proportion of unrecrystallized ferrite in ferrite becomes high, and it may not be possible to achieve sufficient elongation and hole expansion in the finally obtained steel plate. Since the temperature in rough rolling is higher than that in finish rolling, by performing rolling with a relatively high load in rough rolling, the precipitation of coarser AlN can be promoted, and as a result, the precipitation of fine and high-number-density AlN in the subsequent lower-temperature finish rolling can be suppressed. Rough rolling can be carried out by a tandem method formed by multiple rolling mill stands, or by a reversible rolling mill method in which one rolling mill stand reciprocates.

[0177] [Finish rolling]

[0178] Finish rolling can be carried out, for example, by a tandem method formed by multiple rolling mill stands. The finish rolling conditions are not particularly limited. For example, it can be carried out within the range that satisfies the conditions of an inlet side temperature of 950 - 1100 °C, an outlet side temperature of 850 °C - 1000 °C, and a total reduction ratio of 80 - 95%. When the inlet side temperature of finish rolling is higher than 1100 °C, or the outlet side temperature of finish rolling is higher than 1000 °C, or the total reduction ratio is lower than 80%, sometimes the crystal grain size of the hot rolled steel plate coarsens, and it may cause coarsening of the structure of the final product plate. On the other hand, when the inlet side temperature of finish rolling is lower than 950 °C, or the outlet side temperature of finish rolling is lower than 850 °C, or the total reduction ratio is higher than 95%, since the texture of the hot rolled steel plate is developed, the anisotropy of the final product plate may become obvious.

[0179] [Scale removal]

[0180] In rough rolling and finish rolling, more specifically, immediately after a specific rolling pass in rough rolling and immediately after a specific rolling pass in finish rolling, descaling is carried out at least once each. In rough rolling, within 10 seconds after passing through a rolling pass where the steel plate temperature is 1050 - 1200°C and the reduction ratio exceeds 20%, high-pressure water descaling is carried out at least once. This high-pressure water descaling satisfies the conditions that the pressure is 10 MPa or more, the distance between the steel plate and the nozzle tip is 500 mm or less, and the angle formed by the direction of the nozzle and the plate thickness direction of the steel plate is 3 - 15 degrees. The upper limit of the descaling pressure in rough rolling is not particularly limited and can be, for example, 20 MPa or less. Similarly, in finish rolling, within 3.0 seconds after passing through a rolling pass where the steel plate temperature is 950 - 1100°C and the reduction ratio is 30% or more, high-pressure water descaling is carried out at least once. This high-pressure water descaling satisfies the conditions that the pressure is 2 MPa or more, the distance between the steel plate and the nozzle tip is 400 mm or less, and the angle formed by the direction of the nozzle and the plate thickness direction of the steel plate is 3 - 15 degrees. The upper limit of the descaling pressure in finish rolling is not particularly limited and can be, for example, 20 MPa or less or less than 10 MPa. In the case where descaling that satisfies the above conditions is not carried out, Si that cannot be dissolved in the scale is discharged from the scale as the scale grows and accumulates excessively on the steel plate surface layer. As a result, the surface Si concentration Si S increases, and the ratio with the surface Al concentration Al S , that is, Al S / Si S becomes smaller in more areas, and the area ratio of the region where this Al S / Si S ratio is 0.2 or less goes beyond the desired range. To reduce or suppress this concentration of Si on the steel plate surface layer, it is particularly important to carry out descaling at the earliest stage after passing through the rolling pass in both rough rolling and finish rolling.

[0181] [Elapsed time from the end of the final descaling until the steel plate reaches 700°C]

[0182] After the end of the final descaling, cooling is carried out in a manner that satisfies the following formula (2). When the value of formula (2) exceeds 0.30, the scale regrows excessively, and Si discharged from the scale accumulates again on the steel plate surface layer. As a result, Si S increases, and there are more regions where the Al S / Si S ratio is small, and the area ratio of the region where this Al S / Si S ratio is 0.2 or less goes beyond the desired range. On the other hand, if the Al S / Si S ratio is lower than 0.03, the concentration of Al on the surface layer becomes insufficient. Therefore, it is still AlS / Si S The area ratio of the region with a ratio of 0.2 or less becomes outside the desired range.

[0183]

[0184] t: Elapsed time (seconds) since the end of final descaling

[0185] T(t): Steel plate temperature (°C) at elapsed time t

[0186] t f : Elapsed time (seconds) until the steel plate reaches 700 °C after the end of final descaling

[0187] [Winding temperature]

[0188] The hot-rolled steel plate after finish rolling is wound into a coil after being cooled to 700 °C or lower, for example. The winding temperature does not need to be particularly limited, but is preferably 450 to 680 °C. When the winding temperature is lower than 450 °C, the strength of the hot-rolled plate becomes too high, and sometimes the cold rollability may be impaired. On the other hand, when the winding temperature is higher than 680 °C, due to the concentration of alloy elements such as Mn in cementite, the dissolution of cementite may be delayed in the final annealing process, resulting in a strength decrease. The lower limit of the winding temperature can be 500 °C. Similarly, the upper limit of the winding temperature can be 650 °C or 600 °C.

[0189] <<(B) Pickling process>>

[0190] Perform pickling treatment for 30 seconds or more. In this pickling treatment, the hot-rolled steel plate obtained in the hot-rolling process is passed through an aqueous solution containing 1.0 to 5.0 mol / L of HCl and less than 3.0 mol / L of Fe 2+ at a temperature of 70 to 90 °C at an average speed of 10 m / minute or more. At this time, in order to efficiently remove the Si-concentrated layer formed at the interface between the scale and the steel, at least one bending and reverse bending deformation is imparted to the hot-rolled steel plate before pickling. If the HCl concentration in the pickling solution is lower than 1.0 mol / L, or the Fe 2+ concentration reaches 3.0 mol / L or more, or the temperature of the aqueous solution is lower than 70 °C, or the average speed of the hot-rolled steel plate is lower than 10 m / minute, or the pickling time is less than 30 seconds, pickling cannot be carried out sufficiently, and the Si-concentrated layer at the interface between the scale and the steel cannot be removed sufficiently. Therefore, the area ratio of the region with a small Al S / Si S becomes larger, and the area ratio of the region with an Al S / Si S ratio of 0.2 or less exceeds the desired range. In particular, it is considered that if the Fe in the aqueous solution 2+If the concentration is high, the chemical reaction between the scale and HCl will be hindered. On the other hand, if the HCl concentration exceeds 5.0 mol / L or the temperature exceeds 90 °C, pickling will proceed excessively, and sometimes the surface quality of the steel sheet will deteriorate.

[0191] "(C) Cold Rolling Process"

[0192] The pickled hot-rolled steel sheet is then cold-rolled. In order to promote recrystallization, the cold rolling reduction rate is set to 30% or more. The reduction rate can also be 40% or more. On the other hand, excessive reduction will cause excessive rolling load and increase the load on the cold rolling mill, so its upper limit is set to 75% or 70%.

[0193] "(D) Heat Treatment Process"

[0194] [Heating treatment: The average heating rate between 650 and Ac1 °C is 1.0 - 5.0 °C / second]

[0195] Next, the obtained cold-rolled steel sheet is subjected to a specified heat treatment in the heat treatment process. In order to perform recrystallization of ferrite, the average heating rate is set to 5.0 °C / second or less. On the other hand, if the average heating rate is lower than 1.0 °C / second, productivity will be hindered. Therefore, the average heating rate between 650 and Ac1 is limited to 1.0 - 5.0 °C / second. Ac1 (°C) is calculated by the following formula. Substitute the mass % of the element into the element symbol in the following formula. Substitute 0 mass % for the elements not contained.

[0196] Ac1 (°C) = 723 - 10.7×Mn - 16.9×Ni + 29.1×Si + 16.9×Cr

[0197] [Homogenization heat treatment: Hold for 1 - 500 seconds at the maximum heating temperature of Ac1 + 30 - 950 °C]

[0198] In order to fully perform austenitization to obtain the desired structure in the subsequent cooling treatment, the steel sheet is heated to at least Ac1 + 30 °C or more, and homogenization heat treatment is performed at this temperature (maximum heating temperature). If austenitization is insufficient, a large amount of ferrite may sometimes be generated in the final structure. However, if the heating temperature is excessively increased, not only will the toughness deteriorate due to the coarsening of the austenite grain size, but it will also cause damage to the annealing equipment. Therefore, the upper limit is set to 950 °C, preferably set to 900 °C. If the homogenization time is short, austenitization cannot be fully performed, so it is set to at least 1 second or more. The homogenization time is preferably 30 seconds or more or 60 seconds or more. On the other hand, if the homogenization time is too long, productivity will be hindered, so the upper limit is 500 seconds, preferably 300 seconds. It is not necessary to keep the steel sheet at a constant temperature during homogenization, and it can also vary within the range that satisfies the above conditions.

[0199] [First Cooling: The average cooling rate in the temperature range of 550 to 650 °C is 10 to 100 °C / second]

[0200] Next, the cold-rolled steel sheet after homogenization heat treatment is cooled (first cooling) such that the average cooling rate in the temperature range of 550 to 650 °C is 10 to 100 °C / second. If the average cooling rate is less than 10 °C / second, the desired ferrite fraction may not be obtained. The average cooling rate can be 15 °C / second or more, or 20 °C / second or more. Additionally, the average cooling rate can be 80 °C / second or less, or 60 °C / second or less.

[0201] [Second Cooling: Cooling is stopped between Ms - 150 and Ms °C]

[0202] In order to transform a part of the untransformed austenite into martensite, it is cooled to the range of martensite start temperature (Ms) - 150 to Ms °C (second cooling). The martensite formed here is tempered by subsequent reheating and holding treatments to become tempered martensite. When the cooling stop temperature exceeds Ms °C, tempered martensite is not formed, so the desired metal structure cannot be obtained. On the other hand, when the cooling stop temperature is lower than Ms - 150 °C, the untransformed austenite is excessively reduced, and thus the desired retained austenite content cannot be obtained. The preferred range of the cooling stop temperature is Ms - 120 to Ms - 20 °C, more preferably - 100 °C to Ms - 40 °C.

[0203] This second cooling can be carried out continuously or discontinuously with the first cooling. For example, after the first cooling, cooling can be stopped at a temperature higher than Ms, hot-dip galvanizing treatment can be carried out, and then the second cooling can be carried out.

[0204] In addition, martensite transformation occurs after ferrite transformation and / or bainite transformation. Along with the above transformations, C is partitioned to austenite. Therefore, it is inconsistent with Ms when heated to austenite single phase and quenched rapidly. Ms in the embodiments of the present invention can be determined by measuring the thermal expansion temperature. For example, Ms can be obtained in the following manner: using a device such as a dilatometer that can measure the thermal expansion amount during continuous heat treatment, reproducing the thermal cycle from the start of heat treatment (equivalent to room temperature) to cooling below the above Ms, and measuring the thermal expansion amount during this period. In the temperature - thermal expansion curve when simulating the thermal cycle with a thermal expansion measuring device, the steel sheet linearly thermally contracts during the second cooling, but deviates from the linear relationship at a certain temperature. The temperature at this time is Ms in the embodiments of the present invention.

[0205] [Low-temperature Holding: The cold-rolled steel sheet after the second cooling is heated to the temperature range of 330 to 450 °C, and then held in the said temperature range for 50 to 1000 seconds]

[0206] After the second cooling, reheating is carried out and maintained in the range of 330°C to 450°C. In this treatment, in order to obtain the desired retained austenite content, while concentrating carbon in austenite and stabilizing austenite (austenite tempering), the martensite formed during the second cooling is tempered. When the holding temperature is lower than 330°C or the holding time is shorter than 50 seconds, the concentration of carbon in austenite becomes insufficient, and it is difficult to obtain the desired retained austenite content. On the other hand, when the holding temperature exceeds 450°C or the holding time exceeds 1000 seconds, decomposition of austenite into cementite occurs, so the desired retained austenite content still cannot be obtained.

[0207] [Hot-dip galvanizing]

[0208] In the case of manufacturing a hot-dip galvanized steel sheet, after the first cooling, the steel sheet is immersed in a hot-dip galvanizing bath. The immersion in the plating bath can be carried out between the first cooling and the second cooling, or between the second cooling and the low-temperature holding, or after the low-temperature holding. Alternatively, after cooling to room temperature and completing the heat treatment process at one time, the plating treatment can be carried out on another production line. No matter which timing among after the first cooling (between the first cooling and the second cooling), after the second cooling (between the second cooling and the low-temperature holding), after the low-temperature holding, and after the heat treatment process is selected for the plating treatment, a steel sheet having the same steel structure as the cold-rolled steel sheet without the plating treatment can be obtained in a manner that does not affect the finally obtained steel structure. Regarding the temperature of the steel sheet when immersed in the hot-dip galvanizing bath, the influence on the steel sheet properties is small, but if the temperature difference between the steel sheet temperature and the plating bath temperature is too large, the plating bath temperature will change, which may affect the operation. Therefore, it is preferable to heat and cool in such a way that the steel sheet temperature reaches the plating bath temperature - 20°C to the plating bath temperature + 20°C. The hot-dip galvanizing can be carried out according to a conventional method. For example, the plating bath temperature is 440 to 470°C, and the immersion time is 5 seconds or less. The plating bath preferably contains 0.08 to 0.2% Al, but in addition, Fe, Si, Mg, Mn, Cr, Ti, and Pb can also be contained as impurities. In addition, it is preferable to control the unit area weight of the coating layer by a known method such as gas wiping. The unit area weight is preferably 25 to 75 g / m per single side 2 。

[0209] [Alloying treatment]

[0210] If necessary, alloying treatment can also be performed on a hot-dip galvanized steel sheet having a hot-dip galvanized layer. In this case, if the alloying treatment temperature is lower than 460°C, the alloying rate becomes slow, which not only impairs productivity but also causes uneven alloying treatment. Therefore, the alloying treatment temperature is preferably 460°C or higher. On the other hand, if the alloying treatment temperature exceeds 600°C, the alloying will proceed excessively, and the adhesion of the coating on the steel sheet may deteriorate. Therefore, the alloying temperature is preferably 600°C or lower.

[0211] Finally, it is cooled to room temperature to produce the final product. For flattening correction and surface roughness adjustment of the steel sheet, temper rolling can also be performed. In this case, in order to avoid deterioration of ductility, the elongation is preferably set to 2% or less.

[0212] Examples

[0213] Hereinafter, examples of the present invention will be described. The conditions in the examples are one example of the conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one example. The present invention can adopt various conditions as long as it does not deviate from the gist of the present invention and can achieve the object of the present invention.

[0214] Steel having the chemical composition shown in Table 1 is cast to produce slabs. The remaining portion other than the components shown in Table 1 is Fe and impurities. These slabs are hot-rolled under the conditions shown in Tables 2 and 4, including rough rolling and finish rolling performed in a tandem manner formed by a plurality of rolling mills, to manufacture hot-rolled steel sheets. Scale removal in rough rolling is performed at least once under the conditions of a pressure of 15 MPa, a distance between the steel sheet and the nozzle tip of 400 mm, and an angle formed by the direction of the nozzle and the plate thickness direction of the steel sheet of 15 degrees. Similarly, scale removal in finish rolling is performed at least once under the conditions of a pressure of 3 MPa, a distance between the steel sheet and the nozzle tip of 300 mm, and an angle formed by the direction of the nozzle and the plate thickness direction of the steel sheet of 10 degrees. Then, cooling and coiling are performed under the conditions shown in Table 2. Next, after applying at least one bending reverse bending deformation using a tension straightening machine, the hot-rolled steel sheet is pickled to remove the scale on the surface. After that, cold rolling is performed. The thickness after cold rolling is set to 1.6 mm in all cases. Furthermore, for the obtained steel sheets, heat treatment is performed on the cold-rolled steel sheets under the conditions shown in Table 2 and on the hot-dip galvanized steel sheets under the conditions shown in Table 4, respectively. Hot-dip galvanizing is performed between the first cooling and the second cooling in the heat treatment process, and alloying treatment is performed as needed. In Tables 2 to 5, CR represents a cold-rolled steel sheet without hot-dip galvanizing, GI represents a steel sheet with hot-dip galvanizing, and GA represents a steel sheet with alloyed hot-dip galvanizing.

[0215] JIS No. 5 tensile test pieces were collected from the steel sheet obtained as described above in a direction perpendicular to the rolling direction, and a tensile test was conducted in accordance with JIS Z2241:2011 to measure the tensile strength (TS) and the total elongation (El). In addition, the "JFS T 1001 Flanging Test Method" of the Japan Iron and Steel Federation standard was carried out to measure the flanging rate (λ). Steel sheets with a TS of 980 MPa or more and TS×El×λ 0.5 / 1000 of 90 or more were judged to have good mechanical properties and excellent stamping formability for use as automotive parts.

[0216] In addition, in order to evaluate the liquid metal embrittlement (LME) cracking resistance of the spot welds, test pieces with a width of 150 mm and a length of 50 mm were collected, and a two-piece group spot welding test was carried out. The plate group was a two-piece group of the steel sheet shown in Tables 3 and 5 and a commercially available 270 MPa grade alloyed hot-dip galvanized steel sheet, and welding was carried out in a state with a 3° chamfer. The testing machine used was a servo motor-driven stationary type spot welding machine. The power supply was set to single-phase alternating current 50 Hz, the pressing force was 400 kgf, the energization time was 20 cycles, and the holding time was 5 cycles. The welding current value was set to the current value when the diameter of the molten weld reached 4 times √t (t: plate thickness / mm). Electrodes made of chromium copper with a tip diameter of φ6 mm and a tip radius of curvature of R40 mm were used. After welding, the cross-section of the weld was observed, and those with cracks of 0.1 mm or more were judged as × (unqualified), and those without were judged as ◎ (qualified). The results are shown in Tables 3 and 5.

[0217]

[0218]

[0219]

[0220]

[0221]

[0222]

[0223]

[0224]

[0225]

[0226]

[0227]

[0228]

[0229]

[0230] In Comparative Examples 2 to 8 and 10 to 16, since the descaling conditions in rough rolling or finish rolling, the cooling conditions after the end of final descaling, or the pickling conditions were not controlled within the specified ranges, the area ratio of the region where the Al S / Si S ratio is 0.2 or less exceeded 50%, and as a result, cracking occurred at the spot weld portion.

[0231] In Comparative Example 9, since the number of rolling passes with a reduction ratio exceeding 20% in rough rolling was small, the non-recrystallization rate increased and the stamping formability was poor. Regarding Comparative Example 9, it is considered that the strain-induced precipitation of AlN in rough rolling became insufficient, and fine and relatively large amounts of AlN particles precipitated in subsequent finish rolling. By the pinning effect generated by such AlN particles, the recrystallization of ferrite during heat treatment was suppressed. In Comparative Example 17, since the maximum heating temperature in the heat treatment process was low, the ferrite content increased and the stamping formability was poor. In Comparative Example 18, since the average heating rate in the heat treatment process was high, the non-recrystallization rate increased and the stamping formability was poor. In Comparative Example 19, since the cooling stop temperature in the heat treatment process was high, tempered martensite was not formed and the stamping formability was poor. In Comparative Example 20, since the cooling stop temperature in the heat treatment process was low, the retained austenite content decreased and the stamping formability was poor. In Comparative Example 21, since the low-temperature holding temperature in the heat treatment process was low, a sufficient retained austenite content could not be obtained and the stamping formability was poor. In Comparative Example 22, since the low-temperature holding time in the heat treatment process was short, a sufficient retained austenite content could not be obtained either and the stamping formability was poor. In Comparative Example 23, since the average cooling rate in the temperature range of 550 to 650°C in the heat treatment process was low, the ferrite content increased and the stamping formability was poor. In Comparative Example 32, since the low-temperature holding temperature in the heat treatment process was high, a sufficient retained austenite content could not be obtained and the stamping formability was poor. In Comparative Examples 40 to 49 and 66 to 75, since the chemical composition was not controlled within the specified range, the stamping formability or the LME cracking resistance of the spot weld portion was poor. In particular, in Comparative Examples 48 and 74, since Ti was not contained, the non-recrystallization rate increased and the stamping formability was poor. It is considered that this is because Ti was not added, so the dissolved N in the steel could not be fixed as TiN, fine and relatively large amounts of AlN particles were generated, and by its pinning effect, the recrystallization of ferrite during heat treatment was suppressed.

[0232] In contrast, for the steel sheet of the Example, since TS is 980 MPa or more, and TS×El×λ 0.5Since / 1000 is 90 or more and the test results of the resistance to LME cracking of the spot welded part are good, the stamping formability and the resistance to LME cracking of the spot welded part are excellent.

Claims

1. A steel plate having a chemical composition containing the following components by mass%: C: 0.15 to 0.30%, Si: 0.30 to 1.50%, Mn: 1.40 to 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 to 1.00%, Cu: 0 to 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：0～0.0100％、 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: Fe and impurities, In the range of 1 / 8 thickness to 3 / 8 thickness centered at the position 1 / 4 thickness from the surface of the steel plate, the steel structure is by 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, Primary martensite: 0 to 10%, The total of pearlite and cementite: 0 to 5%, and The balance: Bainite, Further, when performing EPMA analysis on the surface of the steel sheet, the area ratio of the region where the Al S / Si S ratio is 0.2 or less is 50% or less, and the tensile strength of the steel sheet is 980 MPa or more. Among them, Al S is the surface Al concentration in mass%, and Si S is the surface Si concentration in mass%.

2. The steel plate according to claim 1, wherein The Ce content is 0 to 0.0100 mass%, and the La content is 0 to 0.0100 mass%.

3. The steel plate according to claim 1, wherein, The chemical composition contains by mass% one or more selected from the following components: Cr：0.001～1.00％、 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：0.001～1.00％、 Sn: 0.001 to 1.00%, Sb: 0.001 to 0.50%, Nb: 0.001 to 0.200%, V：0.001～1.00％、 B：0.0001～0.0050％、 Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Ce: 0.0001 to 0.0100%, Zr：0.0001～0.0100％、 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%.

4. The steel plate 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) Wherein, [N] is the N content in mass%, and [Ti] is the Ti content in mass%.

5. The steel plate according to claim 2, 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) Wherein, [N] is the N content in mass%, and [Ti] is the Ti content in mass%.

6. The steel plate according to claim 3, 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 in mass %, and [Ti] is the Ti content in mass %.

7. The steel sheet according to any one of claims 1 to 6, which has a hot-dip galvanized layer on its surface.

8. The steel sheet according to any one of claims 1 to 6, which has an alloyed hot-dip galvanized layer on its surface.

9. A method for manufacturing the steel sheet according to any one of claims 1 to 6, which includes the following steps: (A) A hot rolling step that satisfies the following conditions (A1) to (A4), which includes rough rolling and finish rolling of a slab having the chemical composition according to any one of claims 1 to 6. (A1) In rough rolling, at least two rollings are performed at a steel sheet temperature of 1050 to 1200 °C and a reduction ratio per pass exceeding 20%. (A2) In rough rolling, within 10 seconds after passing through a rolling pass at a steel sheet temperature of 1050 to 1200 °C and a reduction ratio exceeding 20%, at least one high-pressure water descaling is performed. This high-pressure water descaling satisfies 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 formed by the direction of the nozzle and the sheet thickness direction of the steel sheet of 3 to 15 degrees. (A3) In finish rolling, within 3.0 seconds after passing through a rolling pass at a steel sheet temperature of 950 to 1100 °C and a reduction ratio of 30% or more, at least one high-pressure water descaling is performed. This high-pressure water descaling satisfies 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 formed by the direction of the nozzle and the sheet thickness direction of the steel sheet of 3 to 15 degrees. (A4) After the final descaling is completed, the elapsed time in seconds until the steel sheet reaches 700 °C satisfies the following formula (2). t: Elapsed time in seconds since the end of the final descaling T(t): Steel sheet temperature in °C at elapsed time t t f : Elapsed time in seconds until the steel plate reaches 700 °C after the final descaling is completed (B) Pickling process, which includes performing pickling treatment for 30 seconds or more. In this pickling treatment, at least one bending and reverse bending deformation is applied to the obtained hot-rolled steel sheet. Then, the hot-rolled steel sheet is passed through an aqueous solution containing 1.0 to 5.0 mol / L of HCl and less than 3.0 mol / L of Fe 2+ at an average speed of 10 m / minute or more, and the temperature of the aqueous solution is 70 to 90 °C. (C) A cold rolling step, in which the pickled hot-rolled steel sheet is cold-rolled at a reduction ratio of 30 to 75%. (D) A heat treatment step that satisfies the following conditions (D1) to (D5), which includes heat-treating the obtained cold-rolled steel sheet. (D1) The average heating rate between 650 and Ac1 °C is 1.0 to 5.0 °C / second. (D2) It is held at the maximum heating temperature of Ac1 + 30 to 950 °C for 1 to 500 seconds, which is called homogenization heat treatment. (D3) The cold-rolled steel sheet after homogenization heat treatment is cooled in such a way that the average cooling rate in the temperature range of 550 to 650 °C is 10 to 100 °C / second, which is called the first cooling. (D4) Cooling is stopped between Ms - 150 and Ms °C, which is called the second cooling. (D5) The cold-rolled steel sheet after the second cooling is heated to the temperature range of 330 to 450 °C, and then held in the said temperature range for 50 to 1000 seconds, which is called low-temperature holding.

10. The manufacturing method of the steel sheet according to claim 9, further comprising: subjecting the steel sheet after the first cooling in (D3), the second cooling in (D4), or the low-temperature holding in (D5) to hot-dip galvanizing.

11. The manufacturing method of the steel sheet according to claim 9, further comprising: subjecting the steel sheet after the first cooling in (D3), the second cooling in (D4), or the low-temperature holding in (D5) to alloyed hot-dip galvanizing.

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