High-strength steel sheet and method for producing same

Abstract

Provided is a high-strength steel sheet having a tensile strength of not less than 1180 MPa and exhibiting excellent shapability, impact resistance properties, and axial collapse properties. To this end, the component composition thereof comprises, in mass%, 0.15-0.30% C, 0.10-1.50% Si, 1.50-4.00% Mn, not more than 0.020% P, not more than 0.0020% S, not more than 0.100% sol. Al, and not more than 0.0150% N. The total of ferrite and bainite is not more than 10%, tempered martensite is not less than 80%, residual austenite is not more than 10.0%, and fresh martensite is not more than 10%. The content of an Fe carbide is 0.10-0.30 mass% and the concentration of C is not more than 0.50 mass% in the residual austenite.

Description

High-strength steel plate and method for manufacturing the same

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

[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.

[0003] International Publication No. 2020 / 158063, International Publication No. 2011 / 093319

[0004] Generally, increasing the strength of steel sheets can reduce their formability. Furthermore, from the perspective of ensuring occupant safety during collisions, steel sheets 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.

[0006] The inventors, after diligent study, found that the above objective can be achieved by adopting the following configuration, and thus completed the present invention. That is, the present invention provides the following [1] to [4]. [1] A steel plate comprising the component composition of the steel plate, 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. A high-strength steel sheet containing Al: 0.100% or less and N: 0.0150% or less, with the remainder being Fe and unavoidable impurities, wherein the microstructure of the steel sheet is such that, by area percentage, ferrite and bainite together make up 10% or less, tempered martensite makes up 80% or more, retained austenite makes up 10.0% or less, and fresh martensite makes up 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 as follows 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: [1] The high-strength steel sheet according to [1] above, comprising 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 the high-strength steel sheet according to [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, heat treatment of the cold-rolled steel sheet, and 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 and 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 to a temperature T3 between 30°C and 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.

[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.

[0008] This is a chart illustrating an example of heat treatment.

[0009] [High-Strength Steel Sheet] 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 characteristics.

[0010] Tensile strength (TS) is measured by a tensile test 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 later is 5.0% or higher, and the hole expansion ratio (λ) measured by the hole expansion test described later is 30% or higher. Excellent impact resistance means that the yield stress (YS) measured by the tensile test described later is 800 MPa or higher, the work hardening (WH) amount when a 1% strain (pre-strain) is introduced is 50 MPa or higher, the WH amount when a 2% strain (pre-strain) is introduced is 150 MPa or higher, and the bake hardening (BH) amount is 150 MPa or higher. Excellent axial crushing resistance means that the VDA bending angle α measured by the VDA bending test described later is 60° or higher.

[0011] <Steel Plate> First, the steel plate provided in the high-strength steel plate of this embodiment will be described. The thickness of the steel plate is not particularly limited, and is, for example, 0.3 mm or more and 2.8 mm or less.

[0012] 《Component Composition》 First, let's explain the component composition of steel sheets. Unless otherwise specified, the unit "%" in component composition refers to "mass percent".

[0013] (C: 0.15% or more and 0.30% or less) C is an element that contributes to securing the amount of tempered martensite and improving its strength. In addition, an increase in C increases the amount of solid-solution C in the tempered martensite, thereby contributing to an improvement in the BH content. If the C content is too low, TS and YS will decrease. For this reason, the C content should be 0.15% or more, preferably 0.17% or more, and more preferably 0.20% or more. On the other hand, if the C content is too high, Fe carbides will increase. For this reason, the C content should be 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 is 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 C concentration in the retained austenite may increase excessively. For this reason, the Si content is 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 element that is effective in 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 structure to untransformed austenite is promoted, and the amount of retained austenite increases. For this reason, the Mn content is 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, if its content is high, it may degrade properties such as TS, λ, and delayed fracture resistance. For this reason, the P content is 0.020% or less, preferably 0.015% or less, and more preferably 0.010% or less. Although it is not necessary to include 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, etc. For this reason, the S content is 0.0020% or less, preferably 0.0015% or less, and more preferably 0.0010% or less. Although S is not required to be included, S 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 that is 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 the amount of retained austenite will lead to a decrease in YS. For this reason, the content of sol. Al is 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 content of sol. Al 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 is an element that reduces the hot ductility of steel and lowers the surface quality. In addition, 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. Although it is not necessary to contain N, from the viewpoint of manufacturing cost, the N content is preferably 0.0001% or more, and more preferably 0.0010% or more.

[0020] (Optional elements) 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 and increase strength. However, if these elements are present in too much, moldability may decrease. For this reason, 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 effect 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, etc. However, if there is too much B, not only will the effect of adding B become saturated, 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 and facilitate composite structure formation. However, if these elements are present in too much, the formability of the steel sheet may decrease. For this reason, when adding these elements, the content of Cu and Cr 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, if there is too much Co, the amount of coarse precipitates and inclusions may increase. For this reason, 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 Ni is added, 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 Ni addition, 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, may be 0.300% or less, may be 0.100% or less, may be 0.050% or less, and may be 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 to a thickness of several tens of micrometers caused by nitriding and oxidation, and this suppression makes it easier to ensure the strength and material stability of the steel sheet. However, if there is too much of these elements, it may lead to a decrease in toughness. For this reason, 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 also be 0.035% or less, respectively. On the other hand, from the viewpoint of obtaining the effect 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 solid-solve in Nb carbides and Nb carbonitrides to form composite precipitates. However, if there is too much of these elements, the additive effect will saturate and the manufacturing cost will increase. For this reason, 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 additive effect of 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, if these elements are present in excessive amounts, defects may occur on the surface or inside the steel sheet. For this reason, 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. Note that 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 not particularly limited, La and / or Ce are preferred as REM.

[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 improve the ultimate deformation ability of the steel sheet. However, if these elements are too much, coarse precipitates may increase. 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 still more preferably 0.080% or less, respectively. On the other hand, from the viewpoint of obtaining the addition effect of these elements, the content of Zr, Te, and Hf is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.020% or more, respectively.

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

[0032] (Balance: Fe and inevitable impurities) The steel sheet contains the above-mentioned elements as its component composition, and the balance consists of Fe and inevitable impurities. The steel sheet preferably contains only the above-mentioned elements and the balance, and the balance is Fe and inevitable impurities. Examples of inevitable impurities include Zn, Pb, As, Ge, Sr, and Cs. The total content of inevitable impurities is preferably 0.100% or less.

[0033] 《Microstructure》 The microstructure of the steel sheet will be described.

[0034] (Ferrite and bainite: 10% or less in total) Ferrite and bainite are softer than tempered martensite. If these microstructures are present in large amounts, it will not only cause a decrease in TS and YS, but also a hardness difference with the surrounding hard microstructures such as martensite, leading to crack propagation from the interface between the two, resulting in a decrease in λ and axial crushing properties. In addition, there is concern that C will be distributed from ferrite and bainite to untransformed austenite, increasing the C concentration in the 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. The upper limit is not particularly limited, and the area ratio of tempered martensite may be, for example, 99% or less, 98% or less, or 97% or less.

[0036] (Retained austenite: 10.0% or less) Retained austenite may transform into hard martensite during processing, increasing the hardness difference with soft microstructures, which may lead to a decrease in YS and λ. Therefore, the area ratio 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 microstructure, and an increase in the hardness difference between it and soft microstructures will lead 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 ratio of ferrite and bainite, retained austenite, and fresh martensite is preferably 20% or less, and may be 15% or less or 10% or less.

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

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

[0041] (Fe carbide content: 0.10% by mass or more and 0.30% by mass or less) Fe carbides in steel sheets (Fe elements present as carbides) can become the starting point for cracks during bending, leading to a decrease in axial crushing properties. Furthermore, if carbon is excessively consumed due to the precipitation of Fe carbides, the amount of solid-solution carbon in the tempered martensite decreases, and the amount of BH decreases. For this reason, 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, thus preventing an increase in the C concentration in the retained austenite. For this reason, 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) carbides, and chi (χ) carbides.

[0044] (Measurement method) The area ratio of each tissue is determined as follows:

[0045] ((Area ratio of ferrite, bainite, and tempered martensite)) A steel sheet is cut to expose a cross section perpendicular to the surface of the steel sheet and parallel to the rolling direction. The exposed cross section is mirror-polished and etched with 3 volume% nital to create an observation surface. The observation surface is observed at a position 1 / 4 of the sheet thickness using a scanning electron microscope (SEM) at a magnification of 5000x to obtain SEM images of any 10 fields. In the SEM images, ferrite and bainite (F+B) are the darkest regions and contain almost no carbides internally. Tempered martensite (TM) is a black region, but it has a lath-like substructure internally and contains carbides, so it can be distinguished from ferrite and bainite (F+B). The massive structure (MA) consisting of fresh martensite and retained austenite is a white massive region and contains 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 fraction of retained austenite)) A steel plate is ground and polished in the thickness direction so that the measurement surface is at the 1 / 4 position 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 intensity of the (200), (220), and (311) planes of fcc iron (austenite), and the (200), (211), and (220) planes of bcc iron (ferrite) are measured. The intensity ratio of the integrated reflectance intensity of each plane of fcc iron to the integrated reflectance intensity of each plane 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 as 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 the austenite, and the C concentration in the retained austenite (unit: mass%) is calculated based on the following formula. In the following formula, Mn%, Si%, and Al% are the content of Mn, Si, and Al in the composition of the steel plate, respectively (unit: mass%). 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) is determined by electrolysis in an electrolyte with the steel sheet as the anode. More specifically, first, a 30 mm x 30 mm test piece is taken from the steel sheet (if a zinc plating layer is present on its surface, this is removed). Next, a 10% AA electrolyte (10 vol% acetylacetone - 1 mass% tetramethylammonium chloride - methanol) is used as the electrolyte, and the test piece is electrolyzed at a current density of 20 mA / cm². ２ Constant current electrolysis is performed under the condition of an electrolysis time of 30 min. 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 carbides. After electrolysis, the test piece is removed from the electrolyte and immersed in methanol and subjected to ultrasonic vibration to peel off the residue from the surface of the electrolyzed test piece. The residue is collected by filtration using a filter with a pore size of 0.2 μm. The amount of Fe (unit: 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 sheet described above, the high-strength steel sheet of this embodiment may further include a zinc plating layer disposed on the surface of the steel sheet. The zinc plating layer is formed by a zinc plating treatment 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 electro-galvanized 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 layer deposited on one side is, for example, 20 g / m². ２ 80g / m or more ２ The following applies:

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

[0052] <Hot Rolling> Hot rolling should be carried out according to the conventional method. 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 min to 300 min. After that, for example, Ar ３ Point or more Ar ３ A steel slab is hot-rolled at a finish rolling exit temperature of 200°C or less to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet is wound at a winding temperature of, for example, 400°C to 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 reducing 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 a box annealing furnace (BAF) at a temperature of 450°C to 730°C.

[0054] <Heat treatment> Next, the cold-rolled steel sheet is heat-treated under the conditions described below. Fig. 1 is a chart showing an example of the heat treatment. As shown in Fig. 1, schematically, in the heat treatment, the cold-rolled steel sheet is held at temperature Ta, then cooled to the cooling stop temperature T1, then heated to temperature T2 and cooled again, held at temperature T3, and then cooled. From the viewpoints of ensuring good productivity and desired heating and cooling rates, the heat treatment is preferably carried out on a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL).

[0055] <Holding at temperature Ta> First, the cold-rolled steel sheet is heated to the temperature Ta in the austenite single-phase region and held at temperature Ta. Thereby, in the finally obtained microstructure, a small amount of ferrite and bainite is made, while ensuring a certain amount of tempered martensite. Specifically, the temperature Ta is at or above the A ｃ３ point, preferably at or above the A ｃ３ point + 10°C, and more preferably at or above the A ｃ３ point + 20°C. On the other hand, if the temperature Ta is too high, the precipitation amount of Fe carbide may increase and the axial crushing property may deteriorate. Therefore, the temperature Ta is 900°C or lower, preferably 890°C or lower, and more preferably 880°C or lower.

[0056] The A ｃ３ point is determined by measuring the volume change of a cylindrical test piece (diameter 3 mm × height 10 mm) from room temperature to a predetermined temperature using a Formaster tester.

[0057] The holding time at temperature Ta is, for example, 10 s or more, preferably 50 s or more, and more preferably 100 s 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 the 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 bulky, 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 is below Ms point - 70°C, preferably below Ms point - 85°C, and more preferably below Ms point - 100°C. On the other hand, if the cooling stop temperature T1 is too low, the fresh martensite produced by cooling is not 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 by measuring the volume change when a cylindrical test specimen (3 mm in diameter x 10 mm in height) is held at a predetermined temperature in a Formaster testing machine and then rapidly cooled using helium gas. The Ms point may vary depending on 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, which has been 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, temperature T2 is less than or equal to the cooling stop temperature T1 + 70°C, 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, which has been 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, more preferably 20°C or higher, and more preferably 10°C or higher.

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

[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. There is no particular upper limit, but from the viewpoint of suppressing the cost of cooling using cooling equipment, the average cooling rate CR2 is preferably 20°C / s or lower, more preferably 15°C / s or lower, and even more preferably 10°C / s or lower.

[0068] 《Holding 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. For this reason, the holding time t3 is 50 s or more, preferably 100 s or more, and more preferably 200 s or more. On the other hand, if the holding time t3 at temperature T3 is too long, an excess of Fe carbides will be generated in the tempered martensite. For this reason, the holding time t3 is 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 more, preferably 2°C / s or more, and more preferably 5°C / s or more. The upper limit of the average cooling rate CR3 is not particularly limited, 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. The skin pass elongation rate is preferably 0.05% to 0.50%. Flattening may be performed using a leveler. Alternatively, after the above-mentioned heat treatment or skin pass rolling, 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] <Total temperature T2 and T3> By increasing temperatures T2 and T3, the precipitation of Fe carbides is promoted, and the distribution of C from the surrounding structure to untransformed austenite is suppressed. For this reason, the total temperature T2 + 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> During the heat treatment (for example, while holding at temperature T3), the cold-rolled steel sheet may be subjected to zinc plating treatment by a conventionally known method.

[0074] The zinc plating process is, for example, hot-dip galvanizing. In hot-dip galvanizing, it is preferable to immerse the steel plate 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 treatment may also be electro-zinc plating. Examples of electro-zinc plating treatments include Zn-Ni electro-alloy plating and pure Zn electro-plating.

[0077] Now, consider, for example, the case where zinc plating is performed while holding 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, as long as the total holding time t3 within the range of temperature T3 is between 50 s and 1000 s, zinc plating at a temperature exceeding temperature T3 may be included during the holding at temperature T3.

[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] <Manufacturing of Cold-Rolled Steel Sheets> Cold-rolled steel sheets (thickness: 1.4 mm) were obtained by hot rolling (holding temperature: 1250°C, holding time: 60 min, finish rolling exit temperature: 1150°C, coiling temperature: 550°C) and cold rolling (rolling rate (cumulative reduction rate): 50%) on a steel slab having the component composition shown in Table 1 below.

[0080] <Heat Treatment> Next, the cold-rolled steel sheets were subjected to heat treatment under the conditions shown in Table 2 below. In all cases, the holding time at temperature Ta was 100 s.

[0081] <Zinc Plating Treatment> For some cold-rolled steel sheets (CR), a hot-dip galvanizing treatment was performed while holding at a temperature T3 to form a hot-dip galvanized layer. That is, a hot-dip galvanized steel sheet (GI) was obtained. A zinc plating bath (bath temperature: 470°C) containing 0.20% by mass of Al, with the remainder being Zn and unavoidable impurities, was used. The amount of galvanized coating on one side of the hot-dip galvanized layer was 58 g / m². ２ That was the case.

[0082] For another portion of cold-rolled steel sheets (CR), an alloying hot-dip galvanizing treatment was performed to form an alloying hot-dip galvanized layer. That is, alloying hot-dip galvanized steel sheets (GA) were 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 alloying hot-dip galvanized layer deposited per side was 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. That is, electro-galvanized steel sheets (EG) were obtained. The amount of electro-galvanized layer deposited on one side was 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 each be simply referred to as "steel sheets."

[0085] <Observation of Microstructure> For the obtained steel sheets, the area percentages 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 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 following tests to evaluate various properties. The results are shown in Table 3 below.

[0087] Tensile Test The tensile test was conducted in accordance with JIS Z 2241:2022. Specifically, a test specimen (JIS No. 5 test specimen) was taken from the obtained steel plate with the longitudinal direction perpendicular to the rolling direction. Using the taken test specimen, a crosshead speed of 1.67 × 10⁻⁶ was used. －１ Tensile tests were conducted under mm / s conditions to measure tensile strength (TS) [MPa], yield stress (YS) [MPa], and total elongation (El) [%].

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

[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] at the time of pre-strain from the determined upper yield stress. If an upper yield point was not observed, the YS after the heat treatment was used as the BH amount.

[0090] For the reason of high strength, TS is preferably 1180 MPa or higher. For the reason of excellent formability, 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 Expansion Test》 The hole expansion test was conducted in accordance with JIS Z 2256:2010. Specifically, the obtained steel plate was sheared to take a test piece measuring 100 mm x 100 mm. A hole (punched hole) was formed in the taken test piece 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 expanded so that the burrs generated when the punched hole was formed were on the outside, until a crack penetrating in the thickness direction of the test piece occurred. After that, the initial hole diameter was reduced to d ０ Given the hole diameter at the time of cracking as [mm] and d [mm], the hole expansion ratio (λ) [%] was calculated using the following formula: λ = {(d - d ０ ) / d ０} × 100 For the reason of excellent moldability, λ is preferably 30% or more.

[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 60 mm x 60 mm test piece was taken from the obtained steel sheet, and the rolls (roll diameter D: 30 mm, distance between rolls L: thickness of the test piece a) were used in a direction where the rolling direction of the test piece and the axial direction of the rolls were perpendicular. ０ The specimen was placed on a surface of [mm] × 2 + 0.5 mm). Next, the specimen, supported by the roll, was pressed down from above using a punch (punch tip r: 0.4 mm) to determine the stroke S [mm] at maximum load (stroke speed: 20 mm / min). Note that the stroke S at maximum load is the distance the punch moved 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 also determined. Note that the unit of the VDA bending angle α is "° / mm", but for convenience it is written as "°". c = D / 2 + r + a ０ p=D / 2+L / 2 W=√(p ２ + (S - c) ２ -c ２) sin(αc / 2)={p×c+W×(S-c)} / {p ２ + (S - c) ２} cos(αc / 2)={W×p−c×(S−c)} / {p ２ + (S - c) ２ For the reason that it exhibits excellent axial crushing characteristics, a VDA bending angle α of 60° or more is preferable.

[0093]

[0094]

[0095]

[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 total strength (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 properties.

Claims

1. Equipped with a steel plate, 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. A high-strength steel sheet containing Al: 0.100% or less and N: 0.0150% or less, with the remainder being Fe and unavoidable impurities, wherein the microstructure of the steel sheet is such that, by area percentage, ferrite and bainite together make up 10% or less, tempered martensite makes up 80% or more, retained austenite makes up 10.0% or less, and fresh martensite makes up 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, 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, and 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. The high-strength steel sheet according to claim 1 or 2, further comprising a zinc plating layer disposed on the surface of the steel sheet.

4. A method for producing a high-strength steel sheet according to claim 1 or 2, comprising: hot rolling and cold rolling a steel slab having the component composition according to claim 1 or 2 to obtain a cold-rolled steel sheet; heat treatment of the cold-rolled steel sheet; and in the heat treatment, the cold-rolled steel sheet is A ｃ３ The cold-rolled steel sheet is held at a temperature Ta of 150°C or higher and 900°C or lower, cooled from temperature Ta to a cooling stop temperature T1 of 150°C or higher and 70°C or lower at an average cooling rate CR1 of 5°C / s or higher, heated from the cooling stop temperature T1 to a temperature T2 greater than the cooling stop temperature T1 and 70°C or lower, cooled from temperature T2 to a temperature T3 less than T2 and between 30°C or higher and 30°C or lower at an average cooling rate CR2 of 1°C / s or higher, held at temperature T3 for 50 s or more and 1000 s or less, cooled from temperature T3 to a temperature of 50°C or lower at an average cooling rate CR3 of 1°C / s or higher. 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.

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