High-strength steel plate and method for manufacturing the same

Abstract

The present invention provides a high-strength steel sheet having a tensile strength of 1180 MPa or higher, and excellent formability, impact resistance, and axial crushing properties. Therefore, the composition, in mass%, includes C: 0.15-0.30%, Si: 0.10-1.50%, Mn: 1.50-4.00%, P: 0.020% or less, S: 0.0020% or less, sol.Al: 0.100% or less, and N: 0.0150% or less. The total content of ferrite and bainite is 10% or less, tempered martensite is 80% or more, retained austenite is 10.0% or less, and fresh martensite is 10% or less. The C concentration in retained austenite is 0.50% by mass or less, and the Fe carbide content is 0.10-0.30% by mass.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel plate and a method for manufacturing the same. [Background technology]

[0002] Conventionally, efforts have been made to reduce the weight of automobile parts by increasing the strength of the steel sheets used in them, thereby making the parts thinner. For example, Patent Documents 1 and 2 disclose high-strength steel sheets with a tensile strength (TS) of 1180 MPa or higher. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2020 / 158063 [Patent Document 2] International Publication No. 2011 / 093319 [Overview of the project] [Problems that the invention aims to solve]

[0004] Generally, increasing the strength of steel sheets can sometimes reduce their formability. Furthermore, from the perspective of ensuring the safety of occupants during a collision, steel plates used in automotive parts are required to have excellent collision resistance and fracture resistance (i.e., axial crushing resistance) during impact.

[0005] Therefore, the present invention aims to provide a high-strength steel sheet having a tensile strength (TS) of 1180 MPa or more, and excellent formability, impact resistance, and axial crushing properties. [Means for solving the problem]

[0006] As a result of diligent research, the inventors of this invention discovered 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 [4]. [1] A high-strength steel sheet comprising a steel sheet, wherein the composition of the steel sheet is, in mass%, C: 0.15% to 0.30%, Si: 0.10% to 1.50%, Mn: 1.50% to 4.00%, P: 0.020% or less, S: 0.0020% or less, sol.Al: 0.100% or less, and N: 0.0150% or less, with the remainder being Fe and unavoidable impurities, and the microstructure of the steel sheet is, in area percentage, ferrite and bainite totaling 10% or less, tempered martensite 80% or more, retained austenite 10.0% or less, and fresh martensite 10% or less, the C concentration in the retained austenite is 0.50% by mass or less, and the Fe carbide content in the steel sheet is 0.10% by mass or more and 0.30% by mass or less. [2] The above component composition is further defined in mass% as follows: Ti: 0.100% or less, Nb: 0.100% or less, V: 0.100% or less, B: 0.0050% or less, Cu: 1.000% or less, Cr: 1.000% or less, Co: 0.500% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: The high-strength steel sheet described in [1] above, which contains at least one element selected from the group consisting of 0.100% or less, W: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.100% or less, and Bi: 0.200% or less. [3] The high-strength steel sheet according to [1] or [2] above, further comprising a zinc plating layer disposed on the surface of the steel sheet. [4] A method for producing a high-strength steel sheet as described in [1] or [2] above, comprising hot rolling and cold rolling a steel slab having the component composition described in [1] or [2] above to obtain a cold-rolled steel sheet, and heat treatment of the cold-rolled steel sheet, wherein in the heat treatment, the cold-rolled steel sheet is A c3The cold-rolled steel sheet is held at a temperature Ta between 150°C and 900°C, cooled from temperature Ta to a cooling stop temperature T1 between 150°C and Ms point - 70°C at an average cooling rate CR1 of 5°C / s or more, heated from the cooling stop temperature T1 to a temperature T2 greater than the cooling stop temperature T1 but less than or equal to the cooling stop temperature T1 + 70°C, and heated from temperature T2 to a temperature less than or equal to the cooling stop temperature T2. A method for manufacturing a high-strength steel sheet, comprising: cooling the cold-rolled steel sheet at an average cooling rate CR2 of 1°C / s or more from a temperature T1 of 30°C or more to a temperature T3 of 30°C or less than or equal to the above cooling stop temperature T1+30°C; holding the cold-rolled steel sheet at the above temperature T3 for 50 s to 1000 s; and cooling the cold-rolled steel sheet from the above temperature T3 to a temperature of 50°C or less at an average cooling rate CR3 of 1°C / s or more, wherein the sum of the above temperatures T2 and T3 is 350°C or more and 550°C or less. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a high-strength steel sheet having a tensile strength (TS) of 1180 MPa or more, and excellent formability, impact resistance, and axial crushing properties. [Brief explanation of the drawing]

[0008] [Figure 1] This is a chart illustrating an example of heat treatment. [Modes for carrying out the invention]

[0009] [High strength steel plate] The high-strength steel sheet of this embodiment comprises at least a steel sheet, the steel sheet satisfying the component composition and microstructure described later. As a result, the high-strength steel sheet of this embodiment has a tensile strength (TS) of 1180 MPa or more, and is excellent in formability, impact resistance, and axial crushing properties.

[0010] Tensile strength (TS) is measured by a tensile test, which will be described later. A TS of 1180 MPa or higher indicates high strength. Excellent formability means that the total elongation (El) measured by the tensile test described below is 5.0% or more, and the hole expansion rate (λ) measured by the hole expansion test described below is 30% or more. Excellent impact resistance means that the yield stress (YS) measured by the tensile test described below is 800 MPa or more, the work hardening (WH) amount when 1% strain (pre-strain) is generated is 50 MPa or more, the WH amount when 2% strain (pre-strain) is generated is 150 MPa or more, and the bake hardening (BH) amount is 150 MPa or more. Excellent axial crushing characteristics mean that the VDA bending angle α measured by the VDA bending test described below is 60° or more.

[0011] 〈Steel sheet〉 First, the steel sheet included in the high-strength steel sheet of the present embodiment will be described. [[ID=1 + 2]]The thickness of the steel sheet is not particularly limited, and is, for example, 0.3 mm or more and 2.8 mm or less.

[0012] 《Component composition》 First, the component composition of the steel sheet will be described. The unit “%” in the component composition means “mass %” unless otherwise specified.

[0013] (C: 0.15% or more and 0.30% or less) C is an element that contributes to ensuring the amount of tempered martensite and improving the strength. Also, as C increases, the amount of dissolved C in the tempered martensite increases, contributing to an improvement in the BH amount. If the content of C is too low, TS and YS will decrease. Therefore, the content of C is 0.15% or more, preferably 0.17% or more, and more preferably 0.20% or more. On the other hand, if the content of C is too high, Fe carbides will increase. Therefore, the content of C is 0.30% or less, preferably 0.28% or less, and more preferably 0.25% or less.

[0014] (Si: 0.10% or more and 1.50% or less) Si is an element that suppresses the precipitation of Fe carbides. To avoid excessive precipitation of Fe carbides, the Si content should be 0.10% or more, preferably 0.20% or more, and more preferably 0.30% or more. On the other hand, if there is too much Si, the amount of retained austenite and the concentration of C in the retained austenite may increase excessively. For this reason, the Si content should be 1.50% or less, preferably 1.20% or less, and more preferably 1.00% or less.

[0015] (Mn: 1.50% or more and 4.00% or less) Mn is an effective element for improving hardenability, suppressing bainite transformation, and ensuring a certain amount of tempered martensite. For this reason, the Mn content is 1.50% or more, preferably 1.80% or more, and more preferably 2.00% or more. On the other hand, if the Mn content is too high, the distribution of carbon from the surrounding tissue to untransformed austenite is promoted, increasing the amount of retained austenite. For this reason, the Mn content should be 4.00% or less, preferably 3.80% or less, and more preferably 3.50% or less.

[0016] (P:0.020% or less) Although phosphorus (P) is an element that strengthens steel, a high content of P can degrade properties such as TS, λ, and delayed fracture resistance. For this reason, the P content should be 0.020% or less, preferably 0.015% or less, and more preferably 0.010% or less. Although it is not necessary to include phosphorus (P), P is an element that is inevitably included in the manufacturing process, and from the viewpoint of manufacturing costs, its content is preferably 0.001% or more, and more preferably 0.002% or more.

[0017] (S:0.0020% or less) S improves scale delamination during hot rolling and suppresses nitriding during heat treatment, but it may reduce λ and other properties. For this reason, the S content is 0.0020% or less, preferably 0.0015% or less, and more preferably 0.0010% or less. Although sulfur is not required to be included, it is an element that is inevitably included in the manufacturing process, and from the viewpoint of manufacturing costs, its content is preferably 0.0001% or more, and more preferably 0.0005% or more.

[0018] (sol.Al: 0.100% or less) Al is an element that suppresses the formation of carbides and promotes the formation of retained austenite. It is also an element added as a deoxidizing agent in the steelmaking process. However, if there is too much Al, the number of inclusions in the steel sheet will increase, degrading El, and the excessive increase in retained austenite will lead to a decrease in YS. For this reason, the content of sol.Al should be 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less. On the other hand, from the viewpoint of stable deoxidation, the sol.Al content is preferably 0.003% or more, more preferably 0.007% or more, and even more preferably 0.010% or more.

[0019] (N:0.0150% or less) N is an element that forms nitrides such as BN, AlN, and TiN in steel, and it is an element that reduces the hot ductility of steel and lowers the surface quality. Furthermore, steel containing B may lose the effect of B through the formation of BN. For this reason, the N content is 0.0150% or less, preferably 0.0120% or less, and more preferably 0.0100% or less. While it is not necessary to include nitrogen (N), from the standpoint of manufacturing costs, the N content is preferably 0.0001% or more, and more preferably 0.0010% or more.

[0020] (arbitrary element) The composition of the steel sheet may further contain the elements listed below.

[0021] ((Ti: 0.100% or less, Nb: 0.100% or less, and V: 0.100% or less)) Ti, Nb, and V form fine precipitates that increase strength. However, too much of these elements can reduce moldability. Therefore, when adding these elements, the content of Ti, Nb, and V is preferably 0.100% or less, more preferably 0.080% or less, even more preferably 0.050% or less, and may be 0.030% or less, respectively. On the other hand, from the viewpoint of obtaining the effects of adding these elements, the content of Ti, Nb, and V is preferably 0.003% or more, more preferably 0.005% or more, and even more preferably 0.010% or more, respectively.

[0022] ((B:0.0050% or less)) B is an element that improves the hardenability of steel and has the advantage of easily generating tempered martensite. However, if there is too much B, not only will the effect of adding B saturate, but it may also lead to a significant decrease in hot ductility and cause surface defects. For this reason, when adding B, the B content is preferably 0.0050% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. On the other hand, from the viewpoint of obtaining the effect of adding B, the B content is preferably 0.0005% or more, more preferably 0.0008% or more, and even more preferably 0.0010% or more.

[0023] ((Cu: 1.000% or less and Cr: 1.000% or less)) Cu and Cr not only act as solid solution strengthening elements, but also stabilize austenite during cooling in heat treatment, facilitating the formation of composite structures. However, too much of these elements can reduce the formability of the steel sheet. Therefore, when adding these elements, the Cu and Cr content is preferably 1.000% or less, more preferably 0.800% or less, and may also be 0.400% or less, or 0.200% or less, respectively. On the other hand, from the viewpoint of obtaining the effects of adding these elements, the content of Cu and Cr is preferably 0.050% or more, more preferably 0.080% or more, and even more preferably 0.100% or more, respectively.

[0024] ((Co:0.500% or less)) Co is an element that improves hardenability. However, too much Co can increase the amount of coarse precipitates and inclusions. Therefore, when adding Co, the Co content is preferably 0.500% or less, and more preferably 0.300% or less. On the other hand, from the viewpoint of obtaining the effect of adding Co, the Co content is preferably 0.001% or more, and more preferably 0.003% or more.

[0025] ((Ni:1.000% or less)) Ni is an element that improves corrosion resistance. However, if there is too much Ni, scale may form unevenly during heating, which can cause surface defects. For this reason, when adding Ni, the Ni content is preferably 1.000% or less, more preferably 0.500% or less, even more preferably 0.200% or less, and may also be 0.100% or less, 0.050% or less, or 0.010% or less. On the other hand, from the viewpoint of obtaining the effect of adding Ni, the Ni content is preferably 0.001% or more, and more preferably 0.002% or more.

[0026] ((Mo: 1.000% or less)) Mo improves the hardenability of steel. In addition, Mo contributes to improved delayed fracture resistance by precipitating fine carbides containing Mo, which act as hydrogen trapping sites, and by refining the martensite. However, if there is too much Mo, the delayed fracture resistance may deteriorate due to the formation of coarse precipitates. For this reason, when adding Mo, the Mo content is preferably 1.000% or less, more preferably 0.800% or less, and may also be 0.300% or less, 0.100% or less, 0.050% or less, or 0.020% or less. On the other hand, from the viewpoint of obtaining the effect of adding Mo, the Mo content is preferably 0.003% or more, and more preferably 0.005% or more.

[0027] ((Sb: 0.200% or less and Sn: 0.200% or less)) Sb and Sn are effective elements in suppressing decarburization of the steel sheet surface, which occurs due to nitriding and oxidation, up to a thickness of several tens of micrometers. This suppression helps ensure the strength and material stability of the steel sheet. However, too much of these elements can lead to a decrease in toughness. Therefore, when adding these elements, the content of Sb and Sn is preferably 0.200% or less, more preferably 0.100% or less, even more preferably 0.050% or less, and may be 0.035% or less, respectively. On the other hand, from the viewpoint of obtaining the effects of adding these elements, the content of Sb and Sn is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more, respectively.

[0028] ((Ta: 0.100% or less and W: 0.100% or less)) Ta and W contribute to increased strength by forming carbides and carbonitrides. In addition, they partially dissolve in Nb carbides and Nb carbonitrides to form composite precipitates. However, if these elements are present in too large an amount, the additive effect will saturate, and manufacturing costs will also increase. Therefore, when adding these elements, the content of Ta and W is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.050% or less, respectively. On the other hand, from the viewpoint of obtaining the effects of adding these elements, the content of Ta and W is preferably 0.003% or more, more preferably 0.005% or more, and even more preferably 0.010% or more, respectively.

[0029] ((Ca: 0.0050% or less, Mg: 0.0050% or less, and REM: 0.0050% or less)) Ca, Mg, and REM are elements used for deoxidation. However, too much of these elements can cause defects on the surface or inside the steel sheet. Therefore, when adding these elements, the content of Ca, Mg, and REM is preferably 0.0050% or less, more preferably 0.0025% or less, and even more preferably 0.0010% or less, respectively. On the other hand, from the viewpoint of obtaining the effects of adding these elements, the content of Ca, Mg, and REM is preferably 0.0001% or more, and more preferably 0.0003% or more, respectively. REM (rare earth metals) refers to Sc (atomic number 21), Y (atomic number 39), and lanthanides from La (atomic number 57) to Lu (atomic number 71). The REM content is the total content of one or more elements selected from REM. While there are no particular limitations on REM, La and / or Ce are preferred.

[0030] ((Zr: 0.100% or less, Te: 0.100% or less, and Hf: 0.100% or less)) Zr, Te, and Hf spheroidize the shape of nitrides and other materials, improving the ultimate deformability of steel sheets. However, too much of these elements can increase the amount of coarse precipitates. Therefore, when adding these elements, the content of Zr, Te, and Hf is preferably 0.100% or less, more preferably 0.090% or less, and even more preferably 0.080% or less, respectively. On the other hand, from the viewpoint of obtaining the effects of adding these elements, the content of Zr, Te, and Hf is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.020% or more, respectively.

[0031] ((Bi:0.200% or less)) Bi is an element that reduces segregation. However, too much Bi can increase the amount of coarse precipitates. Therefore, when adding Bi, the Bi content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less. On the other hand, from the viewpoint of obtaining the effect of adding Bi, the Bi content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.

[0032] (Remainder: Fe and unavoidable impurities) The steel sheet contains the elements mentioned above in its composition, with the remainder being Fe and unavoidable impurities. Preferably, the steel sheet contains only the elements mentioned above and the remainder being Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. The total content of unavoidable impurities is preferably 0.100% or less.

[0033] Microorganisms I will now explain the microstructure of steel sheets.

[0034] (Ferrite and bainite: less than 10% in total) Ferrite and bainite are softer than tempered martensite. A high proportion of these structures leads to a decrease in TS and YS, and the hardness difference between them and the surrounding hard structures such as martensite causes cracks to propagate from the interface, resulting in a decrease in λ and axial crushing properties. In addition, there is concern that the distribution of carbon from ferrite and bainite to untransformed austenite will increase the carbon concentration in retained austenite. Therefore, the total area ratio of ferrite and bainite is 10% or less, preferably 7% or less, more preferably 5% or less, and may even be 0%.

[0035] (Tempered martensite: 80% or more) To obtain the desired TS and YS, the area ratio of tempered martensite is 80% or more, preferably 85% or more, and more preferably 90% or more. There is no particular upper limit; the area ratio of tempered martensite may be, for example, 99% or less, 98% or less, or 97% or less.

[0036] (Residual austenite: 10.0% or less) Retained austenite can transform into hard martensite during processing, increasing the hardness difference between it and the softer structure, which can lead to a decrease in YS and λ. Therefore, the area percentage of retained austenite is 10.0% or less, preferably 9.0% or less, more preferably 8.0% or less, even more preferably 7.0% or less, particularly preferably 5.0% or less, and may even be 0%.

[0037] (Fresh martensite: 10% or less) Fresh martensite is a hard tissue, and an increase in the hardness difference between it and the soft tissue leads to a decrease in YS, El, and λ. Therefore, the area ratio of fresh martensite is 10% or less, preferably 8% or less, more preferably 5% or less, and may even be 0%.

[0038] The total area percentage of ferrite and bainite, retained austenite, and fresh martensite is preferably 20% or less, but may be 15% or less, or 10% or less.

[0039] (Remnant tissue) The steel sheet may have structures other than the tempered martensite and other structures described above (remaining structures). Examples of remaining structures include pearlite and cementite. The area ratio of the remaining structures is preferably 5% or less.

[0040] (C concentration in retained austenite: 0.50% by mass or less) If the carbon concentration in retained austenite is too high, the retained austenite becomes stable. As a result, the retained austenite does not undergo martensitic transformation during processing, and the amount of WH does not increase. For this reason, the carbon concentration in retained austenite is 0.50% by mass or less, preferably 0.45% by mass or less, and more preferably 0.40% by mass or less. On the other hand, the lower limit is not particularly limited. The C concentration in the retained austenite may be, for example, 0.05% by mass or more, and may also be 0.10% by mass or more.

[0041] (Fe carbide content: 0.10% by mass or more, 0.30% by mass or less) Fe carbides (Fe elements present as carbides) in steel sheets can become crack initiation points during bending, leading to a decrease in axial crushing properties. Furthermore, excessive consumption of carbon due to Fe carbide precipitation reduces the amount of dissolved carbon in tempered martensite, resulting in a decrease in BH content. Therefore, the Fe carbide content is 0.30% by mass or less, preferably 0.27% by mass or less, and more preferably 0.25% by mass or less.

[0042] On the other hand, the precipitation of Fe carbides suppresses the distribution of C from the surrounding structure to the untransformed austenite, thereby preventing an increase in the C concentration in the retained austenite. Therefore, the Fe carbide content is 0.10% by mass or more, preferably 0.12% by mass or more, and more preferably 0.15% by mass or more.

[0043] Fe carbides include, for example, cementite (θ), epsilon (ε), eta (η), and chi (χ) carbides.

[0044] (Measurement method) The area ratios for each organization are calculated as follows:

[0045] ((Area ratio of ferrite, bainite, and tempered martensite)) The steel plate is cut to expose a cross-section perpendicular to the surface of the steel plate and parallel to the rolling direction. The exposed cross-section is mirror-polished and etched with 3 vol% nital to create an observation surface. The observation surface is observed at a position 1 / 4 of the plate thickness using a scanning electron microscope (SEM) at a magnification of 5000x to obtain SEM images of any 10 fields of view. In SEM images, ferrite and bainite (F+B) are the darkest regions and contain almost no carbides internally. Tempered martensite (TM) is also a dark region, but it can be distinguished from ferrite and bainite (F+B) because it has a lath-like substructure and contains carbides internally. Massive structures (MA), consisting of fresh martensite and retained austenite, are white, massive regions and contain no substructure internally. For the 10 SEM images obtained, the area percentage of each tissue (F+B, TM, and MA) is calculated using Adobe Photoshop® (manufactured by Adobe Systems). The average value of the 10 fields of view is adopted as the area percentage.

[0046] ((Area ratio of retained austenite)) The steel plate is ground and polished in the thickness direction so that the measurement surface is located at the 1 / 4 point of its thickness. An X-ray diffraction (XRD) pattern is obtained from the resulting measurement surface using a Mo tube as the X-ray source. More specifically, the integrated reflectance intensities of the (200), (220), and (311) faces of fcc iron (austenite), and the (200), (211), and (220) faces of bcc iron (ferrite) are measured. The intensity ratio of the integrated reflectance intensities of each face of fcc iron to the integrated reflectance intensities of each face of bcc iron is determined. The average of the nine intensity ratios is adopted as the volume fraction of retained austenite. The volume fraction of retained austenite is considered to be the area fraction of retained austenite.

[0047] ((C concentration in retained austenite)) An X-ray diffraction (XRD) pattern is obtained from the measurement surface of the steel plate (at the 1 / 4 thickness position) using a Co tube as the X-ray source. At this time, the lattice constant A of the austenite is determined from the peak angle of the (220) plane of austenite, and the C concentration (unit: mass%) in the retained austenite is calculated based on the following formula. In the following formula, Mn%, Si%, and Al% are the content (unit: mass%) of Mn, Si, and Al in the composition of the steel plate, respectively. C concentration in retained austenite = A - {(0.3572 + 0.0012 × Mn% - 0.00157 × Si% + 0.0056 × Al%)} / 0.033

[0048] ((Area ratio of fresh martensite)) The area ratio of fresh martensite is calculated by subtracting the area ratio of retained austenite obtained by XRD from the area ratio of MA (fresh martensite and retained austenite) obtained by SEM imaging.

[0049] ((Fe carbide content)) The Fe carbide content (Fe element present as carbides in the steel sheet) in a steel sheet is determined by electrolysis in an electrolyte solution with the steel sheet as the anode. More specifically, first, a 30mm x 30mm test piece is taken from a steel plate (after removing any zinc plating layer present on its surface). Next, using a 10% AA electrolyte (10% acetylacetone by volume - 1% tetramethylammonium chloride by mass - methanol) as the electrolyte, the test specimen was subjected to a current density of 20 mA / cm². 2 Constant current electrolysis is performed under conditions of an electrolysis time of 30 minutes. In this way, the region including the surface of the steel plate (for example, the region within 100 μm in the thickness direction from the surface of the steel plate) is electrolyzed to expose residues such as Fe carbide. After electrolysis, the test specimen is removed from the electrolyte and immersed in methanol, then subjected to ultrasonic vibration to remove the residue from the surface of the electrolyzed specimen. The residue is then filtered using a 0.2 μm pore size filter to collect it. The amount of Fe (in mass%) in the collected residue is determined by ICP (inductively coupled plasma) emission spectrometry. The determined amount of Fe is considered to be the Fe carbide content in the steel plate.

[0050] <Zinc plating layer> In addition to the steel plate described above, the high-strength steel plate of this embodiment may further include a zinc plating layer disposed on the surface of the steel plate. The zinc plating layer is formed by a zinc plating process described later. The zinc plating layer is not particularly limited and may include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer (alloyed hot-dip galvanized layer), or an electroplated zinc layer. The zinc plating layer may contain elements such as Al and Mg. The composition of the zinc plating layer is not particularly limited, and a general composition can be used. The amount of zinc plating deposited on one side is, for example, 20 g / m². 2 More than 80g / m 2 The following applies:

[0051] [Manufacturing method for high-strength steel plates] Next, a method for manufacturing the high-strength steel plate of this embodiment described above will be explained. In general terms, first, a steel slab having the above-mentioned component composition is subjected to hot rolling and cold rolling to obtain a cold-rolled steel sheet. Next, the cold-rolled steel sheet is subjected to heat treatment.

[0052] <Hot rolling> Hot rolling should be carried out according to standard procedures. For example, first, a steel slab having the above-mentioned component composition is heated and held at a temperature of 1100°C to 1300°C. The holding time is preferably 20 minutes to 300 minutes. Subsequently, the steel slab is hot-rolled at a finish rolling exit temperature, for example, between Ar3 and Ar3 + 200°C, to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet is then wound at a winding temperature, for example, between 400°C and 720°C. From the viewpoint of suppressing variations in sheet thickness and ensuring stable high strength, a winding temperature of 430°C to 530°C is preferable.

[0053] <Cold rolling> Next, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The rolling ratio (cumulative rolling ratio) is, for example, 30% to 85%, and from the viewpoint of stably ensuring high strength and minimizing anisotropy, 35% to 85% is preferable. If the rolling load is high, a softening annealing treatment may be performed in a continuous annealing line (CAL) or box annealing furnace (BAF) at a temperature of 450°C to 730°C.

[0054] <Heat treatment> Next, the cold-rolled steel sheet is subjected to heat treatment under the conditions described below. Figure 1 is a chart illustrating an example of heat treatment. As shown in Figure 1, in general terms, the heat treatment involves holding the cold-rolled steel sheet at a temperature Ta, then cooling it to a cooling stop temperature T1, then heating it to a temperature T2 and cooling it again, holding it at a temperature T3, and then cooling it again. From the viewpoint of ensuring good productivity and desired heating and cooling rates, heat treatment is preferably carried out on a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL).

[0055] 《Maintaining at temperature Ta》 First, the cold-rolled steel sheet is heated to a temperature Ta in the austenite single-phase region and held at that temperature. This ensures that the resulting microstructure contains a small amount of ferrite and bainite while maintaining a certain amount of tempered martensite. Specifically, the temperature Ta is A c3 It is above a certain score, A c3 A temperature of +10°C or higher is preferred. c3 A temperature of +20°C or higher is more preferable. On the other hand, if the temperature Ta is too high, the amount of Fe carbide precipitation may increase, which can reduce the axial crushing characteristics. For this reason, the temperature Ta should be 900°C or lower, preferably 890°C or lower, and more preferably 880°C or lower.

[0056] A c3 The point is determined by measuring the volume change of a cylindrical test specimen (3 mm in diameter x 10 mm in height) when it is heated from room temperature to a predetermined temperature using a Formaster testing machine.

[0057] The holding time at temperature Ta is, for example, 10 seconds or more, preferably 50 seconds or more, and more preferably 100 seconds or more. On the other hand, the holding time at temperature Ta is, for example, 300 s or less, preferably 250 s or less, and more preferably 200 s or less.

[0058] Cooling from temperature Ta to cooling stop temperature T1 Next, the cold-rolled steel sheet, held at temperature Ta, is cooled to the cooling stop temperature T1 at an average cooling rate CR1.

[0059] If the cooling stop temperature T1 is too high, a large amount of lumpy, untransformed austenite remains, resulting in an increase in the amount of fresh martensite and soft bainite in the final microstructure. For this reason, the cooling stop temperature T1 should be below -70°C from the Ms point, preferably below -85°C from the Ms point, and more preferably below -100°C from the Ms point. On the other hand, if the cooling stop temperature T1 is too low, the fresh martensite produced by cooling will not be sufficiently tempered. For this reason, the cooling stop temperature T1 is 150°C or higher, preferably 175°C or higher, and more preferably 200°C or higher.

[0060] The Ms point (martensitic transformation onset temperature) is determined in a Formaster tester by holding a cylindrical test specimen (3 mm in diameter x 10 mm in height) at a predetermined temperature and then measuring the volume change when it is rapidly cooled using helium gas. The Ms point can change depending on factors such as the heat treatment conditions applied to the steel sheet, even if the composition of the steel sheet is the same.

[0061] If the average cooling rate CR1 is too slow, bainite transformation occurs during cooling, increasing the amount of soft bainite. For this reason, the average cooling rate CR1 should be 5°C / s or higher, preferably 7°C / s or higher, and more preferably 10°C / s or higher.

[0062] On the other hand, the average cooling rate CR1 is not particularly limited to an upper limit, but is preferably 100°C / s or less, more preferably 75°C / s or less, and even more preferably 50°C / s or less.

[0063] Heating from cooling stop temperature T1 to temperature T2 Next, the cold-rolled steel sheet, cooled to the cooling stop temperature T1, is heated to temperature T2 without being held at the cooling stop temperature T1. This promotes the precipitation of Fe carbides. From the viewpoint of controlling the Fe carbide content and the C concentration in the retained austenite to a desired range, temperature T2 is greater than the cooling stop temperature T1°C, preferably T1 + 10°C or higher, and more preferably T1 + 20°C or higher. On the other hand, from the viewpoint of suppressing excessive precipitation of Fe carbides, the temperature T2 is preferably T1 + 70°C or lower, more preferably T1 + 60°C or lower, and more preferably T1 + 50°C or lower.

[0064] Cooling from temperature T2 to temperature T3 Next, the cold-rolled steel sheet, heated to temperature T2, is cooled to a temperature below T2, T3, at an average cooling rate CR2, without being held at temperature T2.

[0065] If the temperature T3 is too low, the fresh martensite will not be sufficiently tempered during holding at temperature T3 (described later). For this reason, temperature T3 is preferably 30°C or higher than the cooling stop temperature T1, preferably 20°C or higher, and more preferably 10°C or higher.

[0066] On the other hand, if the temperature T3 is too high, excess Fe carbide will precipitate. For this reason, the temperature T3 should be less than the temperature T2, preferably 10°C or less below T2, and more preferably 20°C or less below T2. For similar reasons, temperature T3 is less than or equal to the cooling stop temperature T1 + 30°C, preferably less than or equal to the cooling stop temperature T1 + 20°C, and more preferably less than or equal to the cooling stop temperature T1 + 10°C.

[0067] The average cooling rate CR2 is preferably 1°C / s or higher, more preferably 2°C / s or higher, and more preferably 5°C / s or higher, from the viewpoint of suppressing the precipitation of Fe carbides during cooling. While there is no particular upper limit, from the viewpoint of suppressing the cost of cooling using cooling equipment, the average cooling rate CR2 is preferably 20°C / s or less, more preferably 15°C / s or less, and even more preferably 10°C / s or less.

[0068] 《Maintaining at temperature T3》 The cold-rolled steel sheet, cooled to temperature T3, is held at temperature T3. This converts the hard fresh martensite into tempered martensite. If the holding time t3 at temperature T3 is too short, the fresh martensite will not be sufficiently tempered. Therefore, the holding time t3 is 50 seconds or more, preferably 100 seconds or more, and more preferably 200 seconds or more. On the other hand, if the holding time t3 at temperature T3 is too long, excess Fe carbides will be generated in the tempered martensite. For this reason, the holding time t3 should be 1000 s or less, preferably 900 s or less, and more preferably 800 s or less.

[0069] Cooling from temperature T3 to a temperature of 50°C or lower. Next, the cold-rolled steel sheet, held at temperature T3, is cooled to a temperature of 50°C or lower at an average cooling rate CR3. This prevents an increase in Fe carbides due to excessive tempering. For this reason, the average cooling rate CR3 is 1°C / s or higher, preferably 2°C / s or higher, and more preferably 5°C / s or higher. The average cooling rate CR3 is not particularly limited to an upper limit, for example, 80°C / s or less, preferably 70°C / s or less, and more preferably 60°C / s or less.

[0070] Cold-rolled steel sheets cooled to a temperature of 50°C or lower may be subjected to skin pass rolling for purposes such as adjusting surface roughness and flattening. A skin pass elongation rate of 0.05% to 0.50% is preferred. Flattening may be performed using a leveler. Furthermore, after the heat treatment or skin pass rolling described above, a low-temperature heat treatment at 100 to 300°C may be performed for a processing time of 30 seconds to 10 days. This removes hydrogen that has penetrated the steel sheet, reducing the hydrogen content to, for example, less than 0.1 ppm by mass.

[0071] 《Sum of temperature T2 and temperature T3》 By increasing temperatures T2 and T3, the precipitation of Fe carbides is promoted, and the distribution of C from the surrounding microstructure to untransformed austenite is suppressed. For this reason, the sum of temperatures T2 and T3 is 350°C or higher, preferably 375°C or higher, and more preferably 400°C or higher.

[0072] On the other hand, if temperatures T2 and T3 become too high, the Fe carbide content becomes too high. For this reason, the sum of temperatures T2 and T3 should be 550°C or less, preferably 525°C or less, and more preferably 500°C or less.

[0073] <Zinc plating treatment> The cold-rolled steel sheet may be subjected to zinc plating by a conventionally known method during the heat treatment (for example, while being held at a temperature T3).

[0074] Zinc plating is a process such as hot-dip galvanizing. In the hot-dip galvanizing process, it is preferable to immerse the steel sheet in a zinc plating bath and then adjust the amount of zinc plating layer formed by gas wiping or the like. The temperature of the zinc plating bath is, for example, 440°C to 500°C. The amount of Al in the zinc plating bath is, for example, 0.10% by mass to 0.22% by mass.

[0075] The zinc plating process may also be a process in which an alloying treatment is performed after hot-dip galvanizing (alloyed hot-dip galvanizing). The processing temperature for the alloying treatment is, for example, 460°C to 590°C.

[0076] The zinc plating process may also be electro-zinc plating. Examples of electro-zinc plating processes include Zn-Ni electro-alloy plating and pure Zn electro-plating.

[0077] Now, let's consider, for example, the case where zinc plating is performed while the material is held at temperature T3. In this case, as long as the heat treatment conditions described above are satisfied, there may be a period of time when the zinc plating temperature T (for example, the temperature of the zinc plating bath) exceeds temperature T3. That is, if the total holding time t3 within the range of temperature T3 is within the range of 50 s or more and 1000 s or less, zinc plating treatment at a temperature exceeding temperature T3 may be included during the holding at temperature T3.

Examples

[0078] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0079] 〈Manufacture of cold-rolled steel sheet〉 For a steel slab having the component composition shown in Table 1 below, hot rolling (holding temperature: 1250 °C, holding time: 60 min, finish rolling exit side temperature: 1150 °C, coiling temperature: 550 °C) and cold rolling (rolling ratio (cumulative reduction ratio): 50%) were carried out to obtain a cold-rolled steel sheet (sheet thickness: 1.4 mm).

[0080] 〈Heat treatment〉 Next, the cold-rolled steel sheet was heat-treated under the conditions shown in Table 2 below. In addition, the holding time at temperature Ta was set to 100 s in all examples.

[0081] [[ID= twenty-six]]〈Zinc plating treatment〉 For some of the cold-rolled steel sheets (CR), hot-dip zinc plating treatment was carried out during holding at temperature T3 to form a hot-dip zinc plating layer. That is, a hot-dip galvanized steel sheet (GI) was obtained. A zinc plating bath containing 0.20% by mass of Al, with the balance being Zn and inevitable impurities (bath temperature: 470 °C) was used. The adhesion amount per side of the hot-dip zinc plating layer was 58 g / m 2 It was.

[0082] For another part of the cold-rolled steel sheets (CR), alloyed hot-dip zinc plating treatment was carried out to form an alloyed hot-dip zinc plating layer. That is, an alloyed hot-dip galvanized steel sheet (GA) was obtained. A zinc plating bath (bath temperature: 470°C) containing 0.14% by mass of Al, with the remainder being Zn and unavoidable impurities, was used. The alloying treatment was carried out at 550°C. The amount of alloyed hot-dip galvanized layer deposited on one side is 45 g / m². 2 That was the case.

[0083] Furthermore, some of the cold-rolled steel sheets (CR) were subjected to electro-galvanizing (pure Zn electroplating or Zn-Ni electroalloy plating) to form an electro-galvanized layer. In other words, electro-galvanized steel sheets (EG) were obtained. The amount of electroplated zinc coating per side is 50 g / m². 2 That was the case.

[0084] Hereinafter, heat-treated cold-rolled steel sheets (CR), as well as hot-dip galvanized steel sheets (GI), alloyed hot-dip galvanized steel sheets (GA), and electro-galvanized steel sheets (EG) will all be simply referred to as "steel sheets."

[0085] <Observation of Microtissues> For the obtained steel sheets, the area ratios of ferrite and bainite (F+B), tempered martensite (tempered M), retained austenite (retained γ), and fresh martensite (FM) were determined according to the method described above. Furthermore, the C concentration and Fe carbide content in the retained austenite (retained γ) were determined according to the method described above. The results for all of these are shown in Table 3 below.

[0086] <evaluation> The obtained steel plates were subjected to the tests described below to evaluate various properties. The results are shown in Table 3.

[0087] Tensile Test Tensile testing was conducted in accordance with JIS Z 2241:2022. Specifically, from the obtained steel plate, a test specimen (JIS No. 5 test specimen) was taken with the longitudinal direction perpendicular to the rolling direction. Using the taken test specimen, a crosshead speed of 1.67 × 10⁻¹⁰ was applied.-1 Tensile tests were conducted under conditions of mm / s to measure tensile strength (TS) [MPa], yield stress (YS) [MPa], and total elongation (El) [%].

[0088] Furthermore, the WH weight [MPa] was determined by tensile testing. Specifically, a tensile test was conducted to induce a 1% or 2% strain (pre-strain) in the test specimen, and the value obtained by subtracting YS from the final load [MPa] at that time was determined as the WH quantity.

[0089] Furthermore, the BH amount [MPa] was determined by tensile testing. Specifically, first, a tensile test was conducted to induce a 2% strain (pre-strain) in the specimen. Next, after unloading, a heat treatment simulating seizing (holding at 170°C for 20 minutes) was performed. After that, another tensile test was conducted to determine the upper yield stress [MPa]. Then, the BH amount was calculated by subtracting the final load [MPa] used to induce the pre-strain from the determined upper yield stress. If no upper yield point was observed, the YS after the heat treatment was used as the BH amount.

[0090] For the reason of high strength, a TS of 1180 MPa or higher is preferable. For the sake of superior moldability, El is preferably 5.0% or higher. For the reason of excellent impact resistance, YS is preferably 800 MPa or higher, WH amount (pre-strain 1%) is preferably 50 MPa or higher, WH amount (pre-strain 2%) is preferably 150 MPa or higher, and BH amount is preferably 150 MPa or higher.

[0091] Hole widening test The hole expansion test was conducted in accordance with JIS Z 2256:2010. Specifically, the obtained steel plate was sheared to obtain a test specimen measuring 100 mm x 100 mm. A hole (punched hole) was formed in the obtained test specimen using a punching tool (punch diameter: 10 mm, die diameter: 10.3 mm, clearance: 13%). Then, using a conical punch with a 60-degree apex angle, the hole was widened until a crack penetrating the thickness direction of the test specimen occurred, with the burrs generated when the punched hole was formed facing outwards. After that, the hole widening ratio (λ) [%] was calculated from the following formula, with the initial hole diameter being d0 [mm] and the hole diameter at the time of crack occurrence being d [mm]. λ = {(d - d0) / d0} × 100 For the sake of excellent moldability, a λ of 30% or more is preferable.

[0092] VDA bending test The VDA (Verband der Automobilindustrie) bending test is a three-point bending test using rolls spaced very close together and a sharp punch, and was conducted in accordance with the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry. Specifically, first, a 60mm x 60mm test specimen was taken from the obtained steel plate and placed on a roll (roll diameter D: 30mm, distance L between rolls: thickness of the test specimen a0 [mm] x 2 + 0.5mm) with the rolling direction of the test specimen perpendicular to the axial direction of the roll. Next, the test specimen, supported by the roll, was pressed down from above using a punch (punch tip r: 0.4 mm), and the stroke S [mm] at maximum load was determined (stroke speed: 20 mm / min). The stroke S at maximum load is the distance the punch traveled from the start of the test until the maximum load was obtained. Subsequently, the values ​​of c[mm], p[mm], and W[mm] were determined based on the following formula, and the VDA bending angle α was further calculated. Note that the unit of the VDA bending angle α is "° / mm", but for convenience, it is written as "°". c = D / 2 + r + a0 p = D / 2 + L / 2 W=√(p 2 +(Sc) 2 -c 2 ) sin(αc / 2) = {p × c + W × (Sc)} / {p 2 +(Sc) 2} cos(αc / 2) = {W × pc × (Sc)} / {p 2 +(Sc) 2} For the reason that it exhibits excellent axial crushing characteristics, a VDA bending angle α of 60° or more is preferable.

[0093] [Table 1] TIFF0007845596000002.tif225137

[0094] [Table 2]

[0095] [Table 3] TIFF0007845596000005.tif208128

[0096] <Summary of Evaluation Results> As is clear from the results shown in Tables 1 to 3 above, steel plates No. 1-2, 5, 9, 14, 18, 21, 27 and 30-35 had a TS of 1180 MPa or higher, and were found to have excellent formability, impact resistance, and axial crushing properties. In contrast, steel plates No. 3-4, 6-8, 10-13, 15-17, 19-20, 22-26, and 28-29 were insufficient in at least one of the following: TS, formability, impact resistance, and axial crushing characteristics.

Claims

1. Equipped with steel plates, The component composition of the steel plate is, in mass%, C: 0.15% or more and 0.30% or less, Si: 0.10% or more and 1.50% or less, Mn: 1.50% or more and 4.00% or less, P: 0.020% or less, S: 0.0020% or less, Sol. Al: 0.100% or less, N: Contains 0.0150% or less, with the remainder consisting of Fe and unavoidable impurities. The microstructure of the aforementioned steel sheet is such that, by area percentage, ferrite and bainite together account for 10% or less, tempered martensite accounts for 80% or more, retained austenite accounts for 10.0% or less, and fresh martensite accounts for 10% or less. The C concentration in the retained austenite is 0.50% by mass or less. A high-strength steel sheet having an Fe carbide content of 0.10% by mass or more and 0.30% by mass or less.

2. The aforementioned component composition is further expressed in mass%, Ti: 0.100% or less, Nb: 0.100% or less, V: 0.100% or less, B: 0.0050% or less, Cu: 1.000% or less, Cr: 1.000% or less, Co: 0.500% or less, Ni: 1.000% or less, Mo: 1.000% or less Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.100% or less, The high-strength steel sheet according to claim 1, comprising at least one element selected from the group consisting of Bi: 0.200% or less.

3. Furthermore, the high-strength steel sheet according to claim 1 or 2, comprising a zinc plating layer disposed on the surface of the steel sheet.

4. A method for manufacturing a high-strength steel sheet according to claim 1 or 2, A steel slab having the component composition described in claim 1 or 2 is subjected to hot rolling and cold rolling to obtain a cold-rolled steel sheet, and the cold-rolled steel sheet is subjected to heat treatment. In the aforementioned heat treatment, The cold-rolled steel sheet is A ｃ３ Maintain a temperature Ta between 1.5°C and 900°C. The cold-rolled steel sheet is cooled from the temperature Ta to a cooling stop temperature T1 of 150°C or higher and Ms point - 70°C or lower, at an average cooling rate CR1 of 5°C / s or higher. The cold-rolled steel sheet is heated from the cooling stop temperature T1 to a temperature T2 greater than the cooling stop temperature T1 but less than or equal to the cooling stop temperature T1 + 70°C. The cold-rolled steel sheet is cooled from the temperature T2 to a temperature T3 that is less than the temperature T2 and between the cooling stop temperature T1 - 30°C and the cooling stop temperature T1 + 30°C, at an average cooling rate CR2 of 1°C / s or more. The cold-rolled steel sheet is held at the temperature T3 for 50 s to 1000 s. The cold-rolled steel sheet is cooled from the temperature T3 to a temperature of 50°C or lower at an average cooling rate CR3 of 1°C / s or more. A method for manufacturing a high-strength steel plate, wherein the sum of the temperatures T2 and T3 is 350°C or more and 550°C or less.

Images