Manufacturing method for high-strength steel plates

Abstract

To provide a method for producing a high-strength steel sheet excellent in sheet passing property when cold rolling a hot-rolled steel sheet.SOLUTION: A hot-rolled steel sheet having by mass% C: 0.20-0.50%, Si: 0.5-2.5%, Mn: 2.5-5.0%, P: 0.100% or less, S: 0.0500% or less, Al: 0.01-0.50% and N: 0.010% or less is subjected to a softening treatment (Temperature: Ta-50-Ta+50°C, Duration: ta hour or less) and then cold-rolled to obtain a cold-rolled steel sheet. The cold-rolled steel sheet is heated at a temperature T1 in an austenite single-phase region for 15-1000 seconds, cooled to a temperature T2 of Ms-50°C or more and less than Ms°C, raised to a temperature T3 of 500°C or less and held for 15-1000 seconds, cooled to a temperature T4 of Ms-200°C or more and less than the temperature T2, raised to a temperature T5 of 500°C or less and held for 15-1000 seconds. Ta=600-[Si%]×100 ta=10-[Si%]×4SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing high-strength steel sheets. [Background technology]

[0002] In recent years, improving the fuel efficiency of automobiles has become a crucial issue from the standpoint of protecting the global environment. Therefore, there is a growing movement to reduce the weight of the vehicle itself by increasing the strength of body materials such as steel plates, thereby thinning body components.

[0003] Patent Document 1 discloses a method for manufacturing high-strength steel sheets, which involves "hot-rolling a steel billet having a composition in mass% of C: 0.20% to 0.50%, Si: 0.5% to 2.5%, Mn: 2.5% to 5.0%, P: 0.100% or less, S: 0.0500% or less, Al: 0.01% to 0.50%, and N: 0.010% or less, with the remainder being Fe and unavoidable impurities, and then cold-rolling the resulting "cold-rolled steel sheet" and subjecting it to a specific heat treatment ([Claim 1]). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2021-123801 [Overview of the project] [Problems that the invention aims to solve]

[0005] When hot-rolled steel sheets obtained by hot rolling are cold-rolled, there are cases where the sheet feedability is insufficient (such as when the hot-rolled steel sheet cracks or breaks, or when the hot-rolled steel sheet is too hard to roll). Therefore, the present invention aims to provide a method for manufacturing high-strength steel sheets that exhibits excellent sheet-feedability when hot-rolled steel sheets are cold-rolled. [Means for solving the problem]

[0006] As a result of diligent research by the inventors, we found that the above objective can be achieved by adopting the following configuration, and thus completed the present invention. In other words, the present invention provides the following [1] to [3]. [1] A hot-rolled steel sheet is obtained by hot-rolling a steel billet having a composition in mass% of C: 0.20% to 0.50%, Si: 0.5% to 2.5%, Mn: 2.5% to 5.0%, P: 0.100% or less, S: 0.0500% or less, Al: 0.01% to 0.50%, and N: 0.010% or less, with the remainder being Fe and unavoidable impurities. The hot-rolled steel sheet is then subjected to a softening treatment in which it is heated at a treatment temperature Ta1 of Ta-50°C to Ta+50°C for a treatment time of ta hours or less. A method for manufacturing a high-strength steel sheet, comprising: cold rolling the hot-rolled steel sheet that has undergone the softening treatment described above to obtain a cold-rolled steel sheet; heating the cold-rolled steel sheet at a temperature T1 in the austenite single-phase region for 15 seconds to 1000 seconds; then cooling it to a temperature T2 between Ms-50°C and Ms°C; then raising the temperature to a temperature T3 of 500°C or lower and holding it for 15 seconds to 1000 seconds; then cooling it to a temperature T4 of Ms-200°C or higher and below the temperature T2; and then raising the temperature to a temperature T5 of 500°C or lower and holding it for 15 seconds to 1000 seconds. Here, when the content of component X in the above component composition is given as [X%] in unit mass%, Ta, ta, and Ms are calculated in units of °C, time, and °C, respectively, by the following formula. Ta=600-[Si%]×100 ta=10-[Si%]×4 Ms=550-35×[Mn%]-13×[Si%]-10×[Cr%]-12×[Mo%]-600×{1-exp(-0.96×[C%])} [2] The method for producing a high-strength steel sheet according to [1] above, wherein the above component composition further contains, by mass%, at least one selected from the group consisting of Ti: 0.100% or less, Nb: 0.100% or less, V: 0.100% or less, Ta: 0.100% or less, W: 0.500% or less, Cu: 2.00% or less, Ni: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Cr: 1.000% or less, Mo: 1.000% or less, B: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.100% or less, REM: 0.0050% or less, and Ca: 0.0050% or less. [3] The method for manufacturing a high-strength steel sheet according to [1] or [2] above, wherein the hot-rolled steel sheet subjected to the softening treatment has a carbide diameter of 50 nm or less. [Effects of the Invention]

[0007] According to the present invention, the sheet-feedability of hot-rolled steel sheets during cold rolling is excellent. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic diagram showing the temperature pattern of heat treatment after cold rolling. [Modes for carrying out the invention]

[0009] [Manufacturing method for high-strength steel plates] The manufacturing method for high-strength steel sheets according to this embodiment (hereinafter also referred to as "this manufacturing method") will now be described. In this manufacturing method, generally speaking, a hot-rolled steel sheet is first obtained by hot-rolling a steel billet having a specific composition. Next, the hot-rolled steel sheet is subjected to a softening treatment described later, and then cold-rolled to obtain a cold-rolled steel sheet. After that, the cold-rolled steel sheet is subjected to a specific heat treatment. In this way, a high-strength steel sheet (a heat-treated cold-rolled steel sheet) is obtained. Hereafter, "high-strength steel sheet" or "cold-rolled steel sheet" will also be simply referred to as "steel sheet."

[0010] <Component composition of steel billet> First, let's explain the composition of the steel billet (slab). Unless otherwise specified, "%" in the composition means "% by mass".

[0011] 《C: 0.20% or more and 0.50% or less》 C is an element essential for ensuring the amount of tempered martensite and martensite in the steel sheet and for strengthening the steel sheet. Also, C is an element essential for ensuring the amount of retained austenite in the steel sheet and for improving the ductility of the steel sheet. From the viewpoint of ensuring the strength and workability of the steel sheet, the amount of C is 0.20% or more, preferably 0.21% or more, and more preferably 0.22% or more. On the other hand, if the amount of C is too large, the steel sheet will embrittle. Therefore, the amount of C is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

[0012] 《Si: 0.5% or more and 2.5% or less》 Si is a useful element that contributes to improving the strength of the steel sheet by solid solution strengthening. Also, Si is a useful element that contributes to ensuring the amount of retained austenite in the steel sheet by suppressing the formation of carbides in the steel sheet. From the viewpoint of obtaining the addition effect of Si, the amount of Si is 0.5% or more, preferably 0.6% or more, and more preferably 0.7% or more. On the other hand, if the amount of Si is too large, the steel sheet will embrittle. Therefore, the amount of Si is 2.5% or less, preferably 2.2% or less, and more preferably 2.0% or less.

[0013] 《Mn: 2.5% or more and 5.0% or less》 Mn is an element that stabilizes retained austenite, is effective for ensuring good ductility, and is also an element that increases the strength of steel by solid solution strengthening. Also, by concentrating Mn in the retained austenite, a large amount of retained austenite can be ensured. From the viewpoint of obtaining such an addition effect of Mn, the amount of Mn is 2.5% or more, preferably 2.6% or more, and more preferably 2.8% or more. On the other hand, if Mn is added excessively, the workability will deteriorate due to the segregation of Mn. Therefore, the amount of Mn is 5.0% or less, preferably 4.8% or less, and more preferably 4.5% or less.

[0014] 《P: Below 0.100%》 P is an element useful for strengthening steel. However, if the amount of P is too large, embrittlement occurs due to grain boundary segregation. Therefore, the amount of P is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less. On the other hand, excessive reduction of the amount of P leads to a significant increase in cost. Therefore, the amount of P is preferably 0.001% or more, and more preferably 0.003% or more.

[0015] 《S: Below 0.0500%》 S becomes inclusions such as MnS and causes cracking when evaluating elongation flange properties. Therefore, it is preferable to reduce the amount of S as much as possible. For this reason, the amount of S is 0.0500% or less, preferably 0.0300% or less, and more preferably 0.0100% or less. On the other hand, excessive reduction of the amount of S leads to a significant increase in cost. Therefore, the amount of S is preferably 0.0003% or more, more preferably 0.0005% or more, and even more preferably 0.0008% or more.

[0016] 《Al: 0.01% or more and 0.50% or less》 Al is a useful element added as a deoxidizer in the steelmaking process. To obtain the addition effect of Al, the amount of Al is 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the amount of Al is too large, the risk of slab cracking during continuous casting increases. Therefore, the amount of Al is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

[0017] 《N: Below 0.010%》 N is an element that most significantly deteriorates the aging resistance of steel, and it is preferable to reduce it as much as possible. For this reason, the amount of N is 0.010% or less, preferably 0.008% or less, and more preferably 0.007% or less. On the other hand, excessively reducing the amount of nitrogen leads to a significant increase in costs. For this reason, the amount of nitrogen is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.

[0018] Other ingredients The component composition may further contain, in mass%, at least one selected from the group consisting of the components listed below.

[0019] (Ti:0.100% or less) Ti is useful for precipitation strengthening of steel. However, if the amount of Ti is too high, the workability and shape retention may be insufficient. Therefore, when including Ti, from the viewpoint of improving workability and other properties, the amount of Ti is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less. On the other hand, when Ti is included, in order to obtain the effect of adding Ti, the amount of Ti is preferably 0.005% or more, more preferably 0.008% or more, and even more preferably 0.010% or more.

[0020] (Nb:0.100% or less) Nb is useful for precipitation strengthening of steel. However, if the amount of Nb is too high, the workability and shape retention may be insufficient. Therefore, when including Nb, from the viewpoint of improving workability and other properties, the amount of Nb is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less. On the other hand, when Nb is included, in order to obtain the effect of Nb addition, the amount of Nb is preferably 0.005% or more, more preferably 0.008% or more, and even more preferably 0.010% or more.

[0021] (V:0.100% or less) V is useful for precipitation strengthening of steel. However, if the amount of V is too high, the workability and shape retention may be insufficient. For this reason, when including V, from the viewpoint of improving workability and other properties, the amount of V is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less. On the other hand, when V is included, in order to obtain the effect of V addition, the amount of V is preferably 0.005% or more, more preferably 0.008% or more, and even more preferably 0.010% or more.

[0022] (Ta:0.100% or less) Like Ti and Nb, Ta contributes to increasing the strength of steel sheets. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides, forming composite precipitates such as (Nb, Ta)(C, N). This is thought to significantly suppress the coarsening of precipitates and stabilize the contribution to strength through precipitation strengthening. For this reason, Ta may be included as needed. However, even if Ta is included in excess, the effect of stabilizing the precipitates will saturate, and the alloy cost will also increase. Therefore, the amount of Ta is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.060% or less. On the other hand, when Ta is included, in order to obtain the effect of adding Ta, the amount of Ta is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.

[0023] (W:0.500% or less) W is effective in strengthening steel by precipitation and may be included as needed. However, if the amount of W is too high, the area ratio of hard martensite becomes excessive, which can lead to an increase in microvoids at the grain boundaries of martensite during hole expansion tests, further promoting crack propagation and reducing tensile flangeability. Therefore, the amount of W is preferably 0.500% or less, more preferably 0.400% or less, and even more preferably 0.300% or less. On the other hand, when W is included, in order to obtain the effect of adding W, the amount of W is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.

[0024] (Cu:2.00% or less) Cu is an element that suppresses the formation of pearlite during cooling after heating at temperature T1, as described later. However, if the amount of Cu is too high, the amount of hard martensite becomes excessive, making it difficult to obtain the necessary workability. For this reason, when including Cu, from the viewpoint of improving workability, the amount of Cu is preferably 2.00% or less, more preferably 1.00% or less, and even more preferably 0.80% or less. On the other hand, when Cu is included, in order to obtain the effect of adding Cu, the amount of Cu is preferably 0.01% or more, more preferably 0.08% or more, and even more preferably 0.10% or more.

[0025] (Ni: 1.00% or less) Ni is an element that stabilizes retained austenite, which is effective in ensuring better ductility. Furthermore, Ni is an element that further increases the strength of steel through solid solution strengthening. For this reason, Ni may be included as needed. However, if the amount of Ni is too high, the area ratio of hard martensite becomes excessive, which can lead to an increase in microvoids at the grain boundaries of the martensite during hole expansion tests, further promoting crack propagation and reducing hole expansion ability. Therefore, the amount of Ni is preferably 1.00% or less, more preferably 0.70% or less, and even more preferably 0.40% or less. On the other hand, when Ni is included, in order to obtain the effect of Ni addition, the amount of Ni is preferably 0.005% or more, more preferably 0.01% or more, and even more preferably 0.10% or more.

[0026] (Sn: 0.200% or less, Sb: 0.200% or less) Sn and Sb may be included as needed to suppress decarburization in the surface layer of the steel sheet, which occurs due to nitriding and oxidation of the steel sheet surface, in an area of ​​several tens of micrometers. This prevents a decrease in the area ratio of martensite on the steel sheet surface, which is effective in ensuring strength and material stability. However, adding excessive amounts of Sn and Sb may lead to a decrease in toughness. Therefore, the amounts of Sn and Sb are preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less, respectively. On the other hand, when Sn and Sb are included, in order to obtain the effects of adding Sn and Sb, the amounts of Sn and Sb are preferably 0.001% or more, and more preferably 0.002% or more, respectively.

[0027] (Cr:1.000% or less) Cr is an element that suppresses the formation of pearlite during cooling after heating at temperature T1, as described later. However, if the amount of Cr is too high, the amount of hard martensite becomes excessive, making it difficult to obtain the necessary workability. For this reason, when including Cr, from the viewpoint of improving workability, the amount of Cr is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less. On the other hand, when Cr is included, in order to obtain the effect of adding Cr, the amount of Cr is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more.

[0028] (Mo: 1.000% or less) Mo is an element that suppresses the formation of pearlite during cooling after heating at temperature T1, as described later. However, if the amount of Mo is too high, the amount of hard martensite becomes excessive, making it difficult to obtain the necessary workability. For this reason, when Mo is included, from the viewpoint of improving workability, the amount of Mo is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less. On the other hand, when Mo is included, in order to obtain the effect of adding Mo, the amount of Mo is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more.

[0029] (B:0.0050% or less) B is a useful element for suppressing the formation and growth of polygonal ferrites from austenite grain boundaries. However, if the amount of B is too high, the processability may be insufficient. Therefore, when B is included, from the viewpoint of improving processability, the amount of B is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less. On the other hand, when B is included, in order to obtain the effect of adding B, the amount of B is preferably 0.0003% or more, more preferably 0.0004% or more, and even more preferably 0.0005% or more.

[0030] (Mg:0.0050% or less) Mg is effective in improving the adverse effect on the pore-expanding properties of sulfides by spheroidizing them. Therefore, Mg may be included as needed. However, if the amount of Mg is too high, inclusions and other defects may increase, potentially causing defects on the surface and inside the steel sheet. Therefore, the amount of Mg is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less. On the other hand, when Mg is included, the amount of Mg is preferably 0.0005% or more, and more preferably 0.0008% or more, in order to obtain the effect of adding Mg.

[0031] (Zr:0.100% or less) Zr is effective in improving the adverse effect on the pore-expanding properties of sulfides by spheroidizing them. Therefore, Zr may be included as needed. However, if the amount of Zr is too high, the number of inclusions and other defects may increase, potentially causing defects on the surface and inside the steel sheet. Therefore, the amount of Zr is preferably 0.100% or less, preferably 0.080% or less, and more preferably 0.060% or less. On the other hand, when Zr is included, in order to obtain the effect of adding Zr, the amount of Zr is preferably 0.0005% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.

[0032] (REM:0.0050% or less) Rare earth metals (REMs) are effective in improving the adverse effects on the pore-expanding properties of sulfides by spheroidizing them. Therefore, REMs may be included as needed. However, if the REM amount is too high, inclusions and other defects may increase, potentially causing defects on the surface and inside the steel sheet. Therefore, the REM amount is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less. On the other hand, when REM is included, the amount of REM is preferably 0.0003% or more, and more preferably 0.0005% or more, in order to obtain the effect of REM addition.

[0033] (Ca:0.0050% or less) Ca is an effective element for improving workability by controlling the morphology of sulfides. However, too much Ca may adversely affect the cleanliness of the steel. Therefore, when Ca is included, the amount of Ca is preferably 0.0050% or less, more preferably 0.0045% or less, and even more preferably 0.0040% or less. On the other hand, when Ca is included, in order to obtain the effect of Ca addition, the amount of Ca is preferably 0.0003% or more, more preferably 0.0005% or more, and even more preferably 0.0008% or more.

[0034] 《Remainder》 The remainder of the component composition consists of Fe and unavoidable impurities.

[0035] <Hot rolling: Manufacturing of hot-rolled steel sheets> Hot-rolled steel sheets are obtained by hot-rolling a steel billet having the above-described composition. The conditions for hot-rolling are not particularly limited and can be those of a conventional method, but preferred conditions are as follows. First, the steel billet is heated to a temperature range of 1100°C to 1300°C, and then the hot rolling is completed in a temperature range of 870°C to 950°C. In this way, a hot-rolled steel sheet is obtained.

[0036] <Softening treatment for hot-rolled steel sheets> As mentioned above, when cold rolling (described later) is performed on hot-rolled steel sheets, there are cases where the sheet's pulsability is insufficient (such as when cracks or fractures occur in the hot-rolled steel sheet, or when the hot-rolled steel sheet is too hard to roll). In hot-rolled steel sheets, carbides such as cementite precipitate at the grain boundaries, and it is presumed that these carbides cause cracking and fracture of the hot-rolled steel sheet during cold rolling. In particular, high-Si steel, which has a high Si content, is prone to embrittlement, making it more likely to have poor treadability. When hot-rolled steel sheets that have cracks or fractures are cold-rolled, the rolls used for cold rolling may also be damaged.

[0037] Therefore, in this manufacturing method, the hot-rolled steel sheet is subjected to a softening treatment, as described later, before cold rolling. Hot-rolled steel sheets that have undergone softening treatment are less prone to cracking or fracturing during cold rolling and exhibit superior treadability. This is presumably because the softening treatment eliminates or reduces the carbides that cause cracking and other problems.

[0038] Softening treatment is a process in which hot-rolled steel sheets are heated at a treatment temperature Ta1 for a treatment time ta1. The processing temperature Ta1 is between Ta-50°C and Ta+50°C. The processing time ta1 is less than or equal to ta time. Ta (unit: °C) and ta (unit: hour) are calculated using the following formulas. Ta=600-[Si%]×100 ta=10-[Si%]×4 In the above formula, [Si%] represents the Si content (unit: mass%) in the component composition described above.

[0039] For the reason that it provides better piping properties, the treatment temperature Ta1 for the softening treatment is preferably Ta-30°C or higher, more preferably Ta-20°C or higher, and even more preferably Ta-10°C or higher. For similar reasons, the processing temperature Ta1 for the softening treatment is preferably Ta+30°C or lower, more preferably Ta+20°C or lower, and even more preferably Ta+10°C or lower.

[0040] For the reason that it provides better treadability, the treatment time ta1 for the softening treatment is preferably ta-0.5 hours or less, more preferably ta-1 hour or less, and even more preferably ta-1.5 hours or less. On the other hand, the processing time ta1 for the softening treatment is preferably 0.5 hours or more, more preferably 1 hour or more, and even more preferably 1.5 hours or more.

[0041] The atmosphere used when performing the softening treatment is, for example, a reducing atmosphere. An example of a reducing atmosphere is an atmosphere consisting of 10 to 30 volume percent hydrogen gas and the remainder nitrogen gas. The hot-rolled steel sheet, which has undergone softening treatment, is then cooled to, for example, room temperature (23±5℃) in order to be subjected to cold rolling. For hot-rolled steel sheets that have undergone softening treatment, pickling may be optionally performed.

[0042] Physical properties of hot-rolled steel sheets that have undergone softening treatment. (Charpy impact value) Charpy impact value (unit: J / cm) 2 ) is the absorbed energy (unit: J) determined by the Charpy impact test, and the cross-sectional area (unit: cm²) of the test specimen. 2 This is the value obtained by dividing by ).

[0043] Hot-rolled steel sheets with a high Charpy impact value are said to absorb impact energy easily and are less prone to fracture (less likely to crack or break during cold rolling). Therefore, the Charpy impact value can be an indicator of whether or not a hot-rolled steel sheet has good pulsability during cold rolling. The Charpy impact value of a hot-rolled steel sheet that has undergone softening treatment is 30 J / cm². 2 The above is preferable, 40 J / cm 2 The above is more preferable: 50 J / cm 2 The above is even more preferable, 60 J / cm 2 The above are particularly preferable.

[0044] The Charpy impact test will be conducted in accordance with JIS Z 2242:2018 "Charpy impact test method for metallic materials" (test temperature: 5°C, notch shape: V-notch, impact blade radius: 2 mm).

[0045] (Diameter of carbide) As mentioned above, in hot-rolled steel sheets that have undergone softening treatment, the carbides shrink. Therefore, the diameter of the carbides can also serve as an indicator of whether or not hot-rolled steel sheets have good pulverability during cold rolling. In a hot-rolled steel sheet that has undergone softening treatment, the diameter of the carbides is preferably 50 nm or less, more preferably 40 nm or less, and even more preferably 30 nm or less.

[0046] The diameter of the carbide is determined as follows: The hot-rolled steel sheet is polished to expose the cross-section at the 1 / 4 thickness point (a position corresponding to 1 / 4 of the sheet thickness in the depth direction from the surface of the hot-rolled steel sheet) as the observation surface. The observation surface is then etched with 3 volume% nital, and then observed at 10,000x magnification using a scanning electron microscope (SEM) to obtain an SEM image. In SEM images, the diameter of carbides (specifically, cementite, an iron carbide) is determined. More specifically, for each carbide, the maximum and minimum lengths are measured, and the sum of these two values ​​divided by 2 ((maximum length + minimum length) / 2) is considered to be the diameter of that carbide. The average of the diameters of any 100 carbides is used as the carbide diameter (in nm) of the hot-rolled steel sheet.

[0047] (Vickers hardness) Hot-rolled steel sheets with a low Vickers hardness are easier to cold-roll and have excellent pulverability. Specifically, the Vickers hardness of the hot-rolled steel sheet that has undergone softening treatment is preferably less than 380 HV, more preferably less than 370 HV, and even more preferably less than 360 HV.

[0048] The Vickers hardness of hot-rolled steel sheets is determined according to JIS Z 2244, with a test force F of 9.8 N.

[0049] <Cold rolling: Manufacturing of cold-rolled steel sheets> Next, the hot-rolled steel sheet that has undergone softening treatment is subjected to cold rolling to obtain a cold-rolled steel sheet. As mentioned above, hot-rolled steel sheets that have undergone softening treatment have excellent ductility.

[0050] A reduction ratio of 30% or more is preferable for cold rolling. This allows for the formation of fine austenite during the heat treatment described later, ultimately resulting in fine retained austenite and martensite. This not only improves the balance between strength and ductility, but also potentially improves elongation flangeability and bendability. On the other hand, while there is no particular upper limit to the reduction ratio in cold rolling, from the viewpoint of the load applied during cold rolling, it is preferable to have a reduction ratio of 85% or less, and more preferably 75% or less. In this manufacturing method, the hot-rolled steel sheet that is cold-rolled is softened, which allows for cold rolling at a high reduction ratio and results in thinner cold-rolled steel sheets.

[0051] <Heat treatment for cold-rolled steel sheets> After cold rolling, the cold-rolled steel sheet is subjected to heat treatment. The technical significance of heat treatment in this invention is, in general terms, as follows: Typically, high-carbon and high-manufacturer steels offer high strength, but their high alloying element content results in a very slow bainite transformation rate, and the transformation initiation line during isothermal holding shifts to longer time intervals. Consequently, the bainite transformation is delayed, making it difficult to utilize retained austenite. Therefore, taking advantage of the fact that the rate of bainite transformation is significantly accelerated when martensite and untransformed austenite coexist, heating, cooling, and stopping of cooling are performed in a short time, and then reheating is repeated twice. In other words, martensite nucleation due to cooling promotes bainite transformation during reheating through a swing-back effect, and once the bainite transformation has progressed, martensite nucleation occurs again due to cooling. This allows for sufficient utilization of the bainite transformation in a short time during the second reheating, and by securing retained austenite that contributes to improved ductility, high ductility is achieved. In addition, by preventing the formation of martensite, good elongation flange properties can also be achieved. If reheating is not performed, or if reheating is performed only once, the proportion of martensite with a large aspect ratio increases, which may result in insufficient ductility and elongation flangeability. Therefore, repeating the reheating process twice is important to obtain the desired microstructure and material.

[0052] Figure 1 is a schematic diagram showing the temperature pattern of the heat treatment after cold rolling. The heat treatment follows the temperature pattern (thermal history) shown in Figure 1. A more detailed explanation follows.

[0053] Heating at a temperature T1 in the austenite single-phase region for 15 seconds to 1000 seconds. First, the cold-rolled steel sheet is heated at a temperature T1 in the austenite single-phase region for 15 seconds to 1000 seconds. The high-strength steel sheets obtained by this manufacturing method mainly consist of tempered martensite and bainite obtained by the transformation of austenite. In order to ensure sufficient tempered martensite and bainite content in the steel sheets and to increase their strength, it is preferable to minimize the amount of ferrite. For this reason, heating (annealing) at a temperature T1 in the austenite single-phase region is necessary. The temperature T1 is preferably above Ac3 (austenite transformation point), and more preferably above (Ac3 + 15°C).

[0054] Ac3 (unit: °C) is calculated using the following formula. Ac3=937.2-436.5×[C%]+56×[Si%]-19.7×[Mn%]-4.9×[Cr%]-16.3×[Cu%]+38 .1×[Mo%]+124.8×[V%]+136.3×[Ti%]-19.1×[Nb%]+198.4×[Al%]+3315×[B%] In the above formula, [X%] represents the content (in mass%) of component X in the component composition described above. The [X%] of component X that is not present is considered to be 0.

[0055] The temperature T1 is not particularly limited as long as it is in the austenite single-phase region, but if it is too high, the growth of austenite grains will be significant. This increases the area ratio of tempered martensite, increases the proportion of martensite with a large aspect ratio, decreases the area ratio of retained austenite, and may result in insufficient ductility and elongation flangeability. For this reason, the temperature T1 is preferably 1000°C or lower, and more preferably 950°C or lower.

[0056] If the temperature T1 is below the austenite single-phase region, or if the heating time at temperature T1 is too short, the reverse transformation to austenite may not proceed sufficiently during heating at temperature T1, resulting in a higher ferrite area ratio and making it difficult to ensure sufficient strength. Therefore, the heating time at temperature T1 is 15 seconds or more, preferably 30 seconds or more, and more preferably 60 seconds or more.

[0057] On the other hand, if the heating time at temperature T1 is too long, the austenite will coarse during heating. This increases the area ratio of martensite, increases the proportion of martensite with a large aspect ratio, decreases the area ratio of retained austenite, and may result in insufficient ductility and elongation flangeability. Therefore, the heating time at temperature T1 is 1000 seconds or less, preferably 800 seconds or less, and more preferably 600 seconds or less.

[0058] Cooling to a temperature T2 between Ms-50℃ and Ms℃. Temperature T2 is important for achieving both strength and processability. Cold-rolled steel sheets heated at temperature T1 are cooled to temperature T2, which is the cooling stop temperature between Ms-50°C and Ms°C. Cooling to below Ms°C causes a portion of the austenite to undergo martensite transformation. The martensite formed during cooling to temperature T2 becomes the nucleus for bainite transformation during holding at temperature T3, as described later. This is important for promoting bainite transformation during holding at temperature T3 and for promoting carbon enrichment in the retained austenite.

[0059] If the temperature T2 is below Ms-50°C, the amount of untransformed austenite that becomes martensite at temperature T2 becomes excessive, resulting in a large amount of tempered martensite and a decrease in retained austenite, making it impossible to achieve both excellent strength and workability. For this reason, the temperature T2 should be Ms-50°C or higher. On the other hand, if the temperature T2 exceeds Ms°C, martensite cannot be secured at the time cooling stops. This delays the bainite transformation during holding at temperature T3, ultimately making it difficult to secure the desired amount of retained austenite. For this reason, the temperature T2 is kept below Ms°C.

[0060] Ms (unit: °C) is calculated using the following formula. Ms=550-35×[Mn%]-13×[Si%]-10×[Cr%]-12×[Mo%]-600×{1-exp(-0.96×[C%])} In the above formula, [X%] represents the content (in mass%) of component X in the component composition described above. The [X%] of component X that is not present is considered to be 0.

[0061] 《Increase temperature to T3 below 500℃ and hold for 15 seconds to 1000 seconds》 The cold-rolled steel sheet, cooled to temperature T2, is then heated to a temperature T3 of 500°C or less, and held at temperature T3 for a period of 15 seconds to 1000 seconds. Holding at temperature T3 tempers the martensite generated by cooling from temperature T1 to temperature T2, transforms the untransformed austenite into bainite, and concentrates the solid solution carbon in the austenite. Through these processes, the austenite is stabilized.

[0062] If the temperature T3 is too high, the bainite transformation is suppressed, making it difficult to secure the desired amount of retained austenite. For this reason, the temperature T3 should be 500°C or lower, and preferably 420°C or lower. On the other hand, if the temperature T3 is too low, the diffusion rate of solid-solution carbon decreases, reducing the amount of carbon enrichment in the austenite, which can make it difficult to obtain the required carbon concentration in the retained austenite. For this reason, a temperature T3 of 250°C or higher is preferable, 280°C or higher is more preferable, and 300°C or higher is even preferable.

[0063] If the holding time at temperature T3 is too short, the bainite transformation will be insufficient, and the desired microstructure cannot be obtained. As a result, the processability may be insufficient. For this reason, the holding time at temperature T3 should be 15 seconds or more, preferably 50 seconds or more, and more preferably 100 seconds or more.

[0064] A holding time of 1000 seconds at temperature T3 is sufficient, due to the accelerating effect of the bainite transformation by the martensite generated at temperature T2. If the holding time at temperature T3 is too long, a stable retained austenite may not be obtained as the final microstructure, and as a result, the desired ductility may not be achieved. For this reason, the holding time at temperature T3 should be 1000 seconds or less, preferably 700 seconds or less, and more preferably 600 seconds or less.

[0065] Cooling down to temperature T4, where Ms is above -200℃ and temperature T2 is below that temperature. Once the holding period at temperature T3 is complete, martensite, which will serve as the nucleus for bainite transformation, is generated again from the austenite that has not undergone bainite transformation. For this purpose, the cold-rolled steel sheet is cooled to temperature T4, which is the cooling stop temperature below the aforementioned temperature T2. If temperature T4 is higher than temperature T2, the bainite transformation will be insufficient after raising the temperature to T5 (described later) and holding it there, and retained austenite, which contributes to improved ductility, will not be obtained. For this reason, temperature T4 is lower than temperature T2. On the other hand, if the temperature T4 is below Ms-200°C, when cooling stops at temperature T4, the amount of martensite generated becomes excessive, and the amount of tempered martensite after raising the temperature to T5 also becomes excessive, reducing the amount of retained austenite. As a result, it is not possible to achieve both excellent strength and workability. For this reason, the temperature T4 is Ms-200°C or higher.

[0066] 《Increase temperature to T5 below 500℃ and hold for 15 seconds to 1000 seconds》 The cold-rolled steel sheet, cooled to temperature T4, is then heated again to a temperature of T5 (below 500°C) and held at temperature T5 for a period of 15 to 1000 seconds. Holding at temperature T5 tempers the martensite generated by cooling from temperature T3 to T4, and transforms the untransformed austenite into bainite. Thus, at the end of holding at temperature T5, a larger amount of solid solution carbon can be concentrated in the austenite than after holding at temperature T3, leading to improved austenite stabilization and ultimately securing retained austenite that contributes to improved ductility.

[0067] If the temperature T5 is too high, the bainite transformation is suppressed, and the amount of retained austenite decreases. For this reason, the temperature T5 should be 500°C or lower, and preferably 420°C or lower. On the other hand, if the temperature T5 is too low, the diffusion rate of solid-solution carbon decreases, reducing the amount of carbon enrichment in the austenite, which can make it difficult to obtain the required carbon concentration in the retained austenite. For this reason, a temperature T5 of 250°C or higher is preferable, 280°C or higher is more preferable, and 300°C or higher is even preferable.

[0068] If the holding time at temperature T5 is too short, the bainite transformation will be insufficient, and the desired microstructure cannot be obtained. As a result, the processability may be insufficient. For this reason, the holding time at temperature T5 should be 15 seconds or more, preferably 50 seconds or more, and more preferably 100 seconds or more.

[0069] A holding time of 1000 seconds at temperature T5 is sufficient, due to the accelerating effect of the bainite transformation by the martensite generated at temperature T4. If the holding time at temperature T5 is too long, a stable retained austenite may not be obtained as the final microstructure, and as a result, one or both of the desired strength and ductility may not be achieved. For this reason, the holding time at temperature T5 should be 1000 seconds or less, preferably 700 seconds or less, and more preferably 600 seconds or less.

[0070] Other conditions Other conditions include the heating rate and cooling rate, for example, as follows: Cold-rolled steel sheets are heated, for example, from room temperature to temperature T1. The heating rate to temperature T1 is preferably 0.1°C / second or more, more preferably 0.5°C / second or more. On the other hand, it is preferably 40°C / second or less, and more preferably 30°C / second or less.

[0071] The cooling rate from temperature T1 to temperature T2 is preferably 5°C / second or more, and more preferably 10°C / second or more. On the other hand, it is preferably 50°C / second or less, and more preferably 40°C / second or less.

[0072] The heating rate from temperature T2 to temperature T3 is preferably 5°C / second or more, and more preferably 10°C / second or more. On the other hand, it is preferably 100°C / second or less, and more preferably 50°C / second or less.

[0073] The cooling rate from temperature T3 to temperature T4 is preferably 5°C / second or more, and more preferably 10°C / second or more. On the other hand, it is preferably 50°C / second or less, and more preferably 40°C / second or less.

[0074] The heating rate from temperature T4 to temperature T5 is preferably 5°C / second or more, and more preferably 10°C / second or more. On the other hand, it is preferably 100°C / second or less, and more preferably 50°C / second or less.

[0075] After being held at temperature T5, the cold-rolled steel sheet is cooled to, for example, room temperature. Suitable cooling methods include, for example, air cooling, gas cooling, furnace cooling, and water cooling. At this time, the cooling rate from temperature T5 is preferably 5°C / second or more, more preferably 10°C / second or more. On the other hand, it is preferably 1000°C / second or less, and more preferably 100°C / second or less.

[0076] In the heat treatment described above, the holding temperature does not have to be constant as long as it is within the temperature range mentioned above, and may fluctuate within that temperature range. As long as the above-mentioned thermal history is satisfied, the heat treatment can be performed using any equipment. The surface of a heat-treated cold-rolled steel sheet may be subjected to temper rolling for shape correction. A hot-dip galvanized steel sheet may be obtained by plating a heat-treated cold-rolled steel sheet. Furthermore, an alloyed hot-dip galvanized steel sheet may be obtained by alloying it.

[0077] [High strength steel plate] Next, we will describe the high-strength steel sheet obtained by the above-described manufacturing method (hereinafter also referred to as "this high-strength steel sheet"). This high-strength steel sheet is a so-called cold-rolled steel sheet, possessing the above-mentioned component composition and the microstructure described later. The sheet thickness is not particularly limited, and is, for example, 5.0 mm or less. High strength means that the tensile strength (TS) is 1300 MPa or higher.

[0078] <Microorganisms> Next, the microstructure (steel sheet structure) of this high-strength steel sheet will be described. Hereafter, the area ratios represent the area ratio relative to the entire microstructure. The area ratio of each structure is determined by the method described in the examples below.

[0079] 《Ferrite area ratio: 0% to 10%》 Ferrite is a soft structure that contributes to ductility. However, too much ferrite makes it difficult to achieve the desired tensile strength. Furthermore, during processing, strain concentrates in the soft ferrite mixed in with the hard structure, easily causing cracks to form, resulting in an inability to achieve the desired workability. If the amount of ferrite is small, a small amount of ferrite will be isolated and dispersed within the hard structure, which can suppress the concentration of strain and avoid deterioration of workability. For this reason, the area ratio of ferrite is 10% or less, preferably 7% or less, and more preferably 5% or less. The area ratio of ferrite may be 0%, but from the viewpoint of ensuring workability, it may be 1% or more, and preferably 2% or more.

[0080] Tempered martensite area ratio: 40% or more and less than 80% Tempered martensite contributes to improved strength. Therefore, the area ratio of tempered martensite is 40% or more, preferably 45% or more, and more preferably 50% or more. On the other hand, if there is a large amount of tempered martensite, as will be discussed later, the desired amount of retained austenite cannot be secured, resulting in insufficient workability such as ductility. For this reason, the area ratio of tempered martensite is less than 80%, preferably less than 75%, and more preferably less than 70%.

[0081] Martensite can be identified by microstructure observation. Untempered martensite (as-quenched martensite) does not contain carbides in its structure. On the other hand, tempered martensite contains carbides with random growth directions in its structure.

[0082] 《Bainite area ratio: 5% to less than 20%》 Sufficient promotion of bainite transformation is necessary to enrich the carbon in the untransformed austenite and obtain retained austenite. Furthermore, in order to obtain the desired tensile strength, the area percentage of bainite should be 5% or more, preferably 6% or more, and more preferably 8% or more. On the other hand, too much bainite leads to a deficiency of retained austenite. For this reason, the area ratio of bainite should be less than 20%, preferably less than 18%, and more preferably less than 15%.

[0083] 《Martensite area ratio: 0% to 10%》 Martensite is a hard structure that increases the strength of steel plates. On the other hand, if there is too much martensite, the amount of bainite decreases, making it impossible to secure a stable amount of retained austenite, and thus reducing ductility. In addition, martensite acts as a starting point for voids, reducing elongation and flangeability. Therefore, the area ratio of martensite is 10% or less, preferably 8% or less, and more preferably 6% or less. The area ratio of martensite may be 0%.

[0084] 《Percentage of retained austenite: 10% to 20%》 During processing, retained austenite undergoes martensite transformation due to the TRIP (Transformation Induced Plasticity) effect, resulting in a hard martensite with a high carbon content, which increases strength while simultaneously improving ductility by enhancing strain dispersion. By combining retained austenite with bainite and martensite in this way, good processability can be obtained even in the high-strength region where the tensile strength (TS) is 1300 MPa or higher. Specifically, the TS × El value can be set to 18000 MPa·% or higher, and the TS × λ value can be set to 40000 MPa·%, resulting in an extremely good balance between strength and processability. If the amount of retained austenite is too low, a sufficient TRIP effect cannot be obtained. For this reason, the area ratio of retained austenite should be 10% or more, preferably 11% or more, and more preferably 12% or more. On the other hand, if there is too much retained austenite, the hard martensite that forms after the TRIP effect occurs becomes excessive, degrading toughness and ductility. For this reason, the area ratio of retained austenite should be 20% or less, preferably 19% or less, and more preferably 18% or less.

[0085] 《Ratio (a / b): 0.5 or less》 If the ratio (a / b) of the area ratio of martensite with an aspect ratio of less than 2.0 (a) to the area ratio of all martensite (b) is too large, sufficient ductility and elongation flange properties may not be obtained. This is because if there is a large amount of extremely hard martensite with low deformability and poor toughness, and an aspect ratio of less than 2.0 (i.e., in a lumpy form), the material may not deform uniformly when strain is applied, and as a result, excellent ductility and elongation flange properties may not be obtained. Therefore, the ratio (a / b) is preferably 0.5 or less, and more preferably 0.4 or less. [Examples]

[0086] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0087] 〈Hot rolling: Production of hot-rolled steel sheet〉 A steel slab having the component composition shown in Table 1 below (the balance consists of Fe and inevitable impurities) was heated to 1250 °C and then finish hot-rolled at 870 °C to obtain a hot-rolled steel sheet.

[0088]

Table 1

[0089] 〈Softening treatment for hot-rolled steel sheet〉 The obtained hot-rolled steel sheet was subjected to a softening treatment under a reducing atmosphere (hydrogen gas: 20 vol%, nitrogen gas: 80 vol%) under the conditions (treatment temperature Ta1 and treatment time ta1) shown in Table 2 below. When the softening treatment was not carried out, “-” was described.

[0090] Thereafter, for the hot-rolled steel sheet, the diameter of the carbide (unit: nm), the Charpy impact value (unit: J / cm 2 ) and the Vickers hardness (unit: HV) were determined by the method described above. The results are shown in Table 2 below (the underline means outside the scope of the present invention or outside the preferred range). From a practical point of view, the Charpy impact value is preferably 30 J / cm 2 or more, and the Vickers hardness is preferably less than 380 HV.

[0091]

Table 2

[0092] As shown in Table 2 above, the hot-rolled steel sheets of Nos. 1 to 4, 9 to 10, and 13 to 14, in which the treatment temperature Ta1 of the softening treatment is Ta - 50 °C or higher and Ta + 50 °C or lower, and the treatment time ta1 of the softening treatment is ta hours or less, have a carbide diameter of 50 nm or less and a Charpy impact value of 30 J / cm 2 or more.

[0093] In contrast, hot-rolled steel sheets No. 5-7, 11, and 15, where the softening treatment temperature Ta1 is outside the range of Ta-50°C to Ta+50°C, and / or the softening treatment time ta1 exceeds ta hours, have carbide diameters greater than 50 nm and a Charpy impact value of 30 J / cm². 2 It was less than [amount missing]. Furthermore, hot-rolled steel sheets No. 8, 12, and 16, which were not subjected to softening treatment, had a Vickers hardness of 380 HV or higher.

[0094] <Cold rolling: Manufacturing of cold-rolled steel sheets> Next, the hot-rolled steel sheets (excluding hot-rolled steel sheets No. 8, 12, and 16, which were not subjected to softening treatment) were cold-rolled at a reduction ratio of 75% to obtain cold-rolled steel sheets with a thickness of 0.6 mm. At this time, the Charpy impact value was 30 J / cm². 2 For cold-rolled steel sheets (Nos. 5-7, 11, and 15) obtained by cold-rolling hot-rolled steel sheets of less than 50

[0095] Hot-rolled steel sheets No. 8, 12, and 16, which were not subjected to softening treatment, could not be cold-rolled to a reduction ratio of 75% under the same conditions as the other hot-rolled steel sheets that had undergone softening treatment. Therefore, cold rolling was performed with a reduction ratio of 50% to obtain a cold-rolled steel sheet with a thickness of 1.2 mm.

[0096] <Heat treatment for cold-rolled steel sheets> Next, the obtained cold-rolled steel sheet was subjected to heat treatment according to the temperature pattern shown in Figure 1 under the conditions shown in Table 3 below, and then temper-rolled with a reduction ratio of 0.1% to obtain a high-strength steel sheet. Other conditions for the heat treatment of the temperature pattern shown in Figure 1 were as follows: The heating rate from room temperature to temperature T1 was 2°C / second. The cooling rate from temperature T1 to temperature T2 was 20°C / second. The heating rate from temperature T2 to temperature T3 was 30°C / second. The cooling rate from temperature T3 to temperature T4 was 15°C / second. The heating rate from temperature T4 to temperature T5 was 20°C / second. The cooling rate from temperature T5 back to room temperature was 15°C / second.

[0097] [Table 3]

[0098] <High strength steel plate> The area percentages of each microstructure in the obtained high-strength steel sheet, and the ratio (a / b) of the area percentage of martensite with an aspect ratio of less than 2.0 (a) to the area percentage of total martensite (b), were determined by the method described in paragraphs

[0082] to

[0087] of Patent Document 1. The results are shown in Table 4 below.

[0099] Furthermore, the tensile strength (TS, unit: MPa), total elongation (El, unit: %), and hole expansion ratio (λ, unit: %) of the obtained high-strength steel plate were determined by the method described in paragraph

[0088] of Patent Document 1. Furthermore, the product of tensile strength (TS) and total elongation (El) (TS × El), and the product of tensile strength (TS) and hole expansion ratio (λ) (TS × λ) were calculated. The results for all of these are shown in Table 4 below. When TS × El ≥ 18000 [MPa·%], the material was evaluated as having excellent machinability. When TS × λ ≥ 40000 [MPa·%], the processability was evaluated as excellent.

[0100] [Table 4]

[0101] As shown in Table 4 above, all of the high-strength steel plates exhibited good workability.

Claims

1. In mass %, C: 0.20% or more and 0.50% or less, Si: 0.5% or more and 2.5% or less, Mn: 2.5% or more and 5.0% or less, P: 0.100% or less, S: 0.0500% or less, Al: 0.01% or more and 0.50% or less, and N: 0.010% or less, and a steel sheet having a component composition consisting of the balance of Fe and unavoidable impurities is hot-rolled to obtain a hot-rolled steel sheet, the hot-rolled steel sheet is subjected to a softening treatment of heating at a treatment temperature Ta1 of Ta - 50°C or more and Ta + 50°C or less for a treatment time ta1 of ta hours or less in a reducing atmosphere, the hot-rolled steel sheet subjected to the softening treatment is cold-rolled to obtain a cold-rolled steel sheet, the cold-rolled steel sheet is heated at a temperature T1 in the austenite single-phase region for 15 seconds or more and 1000 seconds or less, then cooled to a temperature T2 of Ms - 50°C or more and less than Ms°C, then heated to a temperature T3 of 500°C or less and held for 15 seconds or more and 1000 seconds or less, then cooled to a temperature T4 that is Ms - 200°C or more and less than the temperature T2, and then heated to a temperature T5 of 500°C or less and held for 15 seconds or more and 1000 seconds or less, a method for manufacturing a high-strength steel sheet. Here, when the content of component X in the component composition is [X%] in mass %, Ta, ta, and Ms are each in units of °C, hours, and °C, and are determined by the following formulas. Ta = 600 - [Si%] × 100 ta = 10 - [Si%] × 4 Ms = 550 - 35 × [Mn%] - 13 × [Si%] - 10 × [Cr%] - 12 × [Mo%] - 600 × {1 - exp(-0.96 × [C%])}

2. The component composition further comprises, in mass %, Ti: 0.100% or less, Nb: 0.100% or less, V: 0.100% or less, Ta: 0.100% or less, W: 0.500% or less, Cu: 2.00% or less, Ni: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Cr: 1.000% or less, Mo: 1.000% or less, B: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.100% or less, REM: 0.0050% or less, and Ca: 0.0050% or less, and contains at least one selected from the group consisting of, the method for manufacturing a high-strength steel sheet according to Claim 1.

3. The hot-rolled steel sheet subjected to the softening treatment has carbide diameters of 50 nm or less, the method for manufacturing a high-strength steel sheet according to Claim 1 or 2.

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