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 delayed fracture resistance. Therefore, the composition, in mass%, contains C: 0.15-0.30%, Si: 0.10-1.50%, Mn: 1.50-3.50%, P: 0.020% or less, S: 0.0020% or less, sol.Al: 0.100% or less, and N: 0.0150% or less. The total composition is 3-30% ferrite and bainite, 60% or more tempered martensite, 1-10% retained austenite, and 15% or less fresh martensite. The C concentration in retained austenite is 0.60% by mass or less. Of the ferrite and bainite, 70% or more have an aspect ratio of 2 or less and a long side of 10 μm or less. When a 2% strain is applied to a steel plate, the reduction rate of retained austenite is 10% or more.

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 Document 1 discloses a high-strength steel sheet with a tensile strength (TS) of 1180 MPa or higher. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2020 / 045219 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Generally, increasing the strength of steel sheets can sometimes reduce their formability. Furthermore, for parts obtained by processing steel plates with a pressure of 1180 MPa or higher using a TS (Total System), there is a concern that delayed fracture (a phenomenon in which parts suddenly break down) may occur due to hydrogen penetrating the steel plate. 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 properties.

[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 delayed fracture resistance. [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 steel sheet comprising, the component composition of the steel sheet is, in mass%, C: 0.15% to 0.30%, Si: 0.10% to 1.50%, Mn: 1.50% to 3.50%, 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 3% to 30%, tempered A high-strength steel sheet having 60% or more martensite, 1% to 10% retained austenite, and 15% or less fresh martensite, with a carbon concentration of 0.60 mass% or less in the retained austenite, and with 70% or more of the ferrite and bainite having an aspect ratio of 2 or less and a long side of 10 μm or less, and with a reduction rate of 10% or more of the retained austenite when a 2% strain is applied to the steel sheet. [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 c3 A point or above c3Maintain at a temperature T1 below +50°C, cool the cold-rolled steel sheet from the temperature T1 to a temperature T2 of 400°C or higher and 500°C or lower, provided that the average cooling rate CR1 from the temperature T1 to 500°C is 10°C / s or higher, hold the cold-rolled steel sheet at the temperature T2 for 200 s or longer and 300 s or shorter, cool the cold-rolled steel sheet from the temperature T2 to a cooling stop temperature T3 below Ms point - 100°C at an average cooling rate CR2 of 5°C / s or higher, heat the cold-rolled steel sheet from the cooling stop temperature T3 to a temperature T4 that exceeds the cooling stop temperature T3 and is 150°C or higher and 300°C or lower, hold the cold-rolled steel sheet at the temperature T4 for 10 s or longer and 1000 s or shorter. When the holding time at the temperature T1 is t1 in units of s and the holding time at the temperature T2 is t2 in units of s, P represented by the following formula (1) is 40,000 or higher and 100,000 or lower. A method for manufacturing a high-strength steel sheet. P = T2 × t2 - T1 × t1…(1)

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 higher and excellent formability, collision resistance characteristics, and delayed fracture resistance characteristics.

Brief Description of the Drawings

[0008] [Figure 1A] It is a schematic diagram showing the structure in the SEM image. [Figure 1B] It is a schematic diagram showing another structure in the SEM image. [Figure 2] It is a chart diagram showing an example of heat treatment.

Modes for Carrying Out the Invention

[0009] [High-Strength Steel Sheet] The high-strength steel sheet of the present embodiment includes at least a steel sheet, and the steel sheet satisfies the component composition and microstructure described below. Thereby, the high-strength steel sheet of the present embodiment has a tensile strength (TS) of 1180 MPa or higher and is excellent in formability, collision resistance characteristics, and delayed fracture resistance characteristics.

[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 moldability means that the hole expansion ratio (λ), measured by the hole expansion test described later, is 20% or more. Excellent impact resistance means that the yield stress (YS), measured by the tensile test described later, is 750 MPa or higher, and the work hardening (WH) amount is 100 MPa or higher. Excellent delayed fracture resistance means that, in the delayed fracture test described later, no fracture occurs even after 72 hours.

[0011] <Steel plate> First, let's describe the steel plate that makes up the high-strength steel plate of this embodiment. The thickness of the steel plate is not particularly limited, but is, for example, between 0.3 mm and 2.8 mm.

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

[0013] (C: 0.15% or more and 0.30% or less) Carbon (C) is an element that contributes to ensuring the quantity and improving the strength of tempered martensite. If the C content is too low, sufficient strength and quantity of tempered martensite cannot be obtained, and 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 carbon content is too high, the strength of the tempered martensite increases excessively, which tends to lower λ, and the retained austenite becomes excessively stable, reducing the WH content. In addition, the Ms point decreases with increasing carbon content. In this case, during cooling to the cooling stop temperature T3 described later, the martensitic transformation does not proceed sufficiently, and the untransformed austenite that did not undergo martensitic transformation undergoes martensitic transformation during cooling after holding at temperature T4 described later, increasing the amount of fresh martensite. For this reason, the carbon content 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 suppresses the formation of carbides in martensite and bainite, promoting the formation of retained austenite, and also promotes the distribution of C from the surrounding microstructure to the untransformed austenite, thereby increasing the C concentration in the retained austenite. As a result, when the retained austenite is stabilized, it does not transform into martensite even under high load stress, and the YS (Yield Saturation) increases. For this reason, 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 the Si content is too high, it is likely to lead to a decrease in λ due to excessive strength. Also, the amount of retained austenite and the carbon concentration 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 3.50% 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 bainite transformation will be suppressed, for example, when held at temperature T2 as described later. For this reason, the Mn content should be 3.50% or less, preferably 3.20% or less, and more preferably 3.00% 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 (total speed), λ (laminar), 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 the product does not need to contain phosphorus (P), P is an element that is inevitably included in the manufacturing process. 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, it may lead to a decrease in YS due to an excessive increase in the amount of retained austenite. 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 0.005% or more, more preferably 0.010% or more, and even more preferably 0.018% 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 0.060% or less, and may also be 0.040% or less, or 0.020% 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 0.0045% or less, and even more preferably 0.0040% 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 0.0012% or more, and even more preferably 0.0020% 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, even more preferably 0.700% or less, and may be 0.350% 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, even more preferably 0.150% or more, and may also be 0.300% or more, or 0.500% 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. Furthermore, 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 0.450% or less, or 0.250% or less. On the other hand, from the viewpoint of obtaining the effect of adding Mo, the Mo content is preferably 0.010% or more, preferably 0.050% or more, and more preferably 0.090% or more.

[0027] ((Sb: 0.200% or less and Sn: 0.200% or less)) Sb and Sn are effective elements in suppressing decarburization of steel sheets to a thickness of several tens of micrometers caused by nitriding and oxidation on the surface of the steel sheet. This suppression helps to 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 0.030% 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 0.090% 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 Ta and W is preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.045% 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: 3% to 30% in total) Steel sheets containing ferrite and bainite tend to have superior delayed fracture resistance compared to those without these elements. For this reason, the total area ratio of ferrite and bainite should be 3% or more, preferably 5% or more, and more preferably 10% or more. On the other hand, since ferrite and bainite are soft structures, excessive amounts of them may lead to a decrease in TS and YS, as well as a decrease in λ due to an increased hardness difference with the hard phase. Furthermore, an increase in ferrite and bainite may cause carbon to be distributed from them to the surrounding untransformed austenite when cooling stops, potentially increasing the carbon concentration in the retained austenite. In addition, the stabilized untransformed austenite may undergo martensitic transformation during cooling after holding at temperature T4, as described later, potentially increasing the amount of fresh martensite. For these reasons, the total area percentage of ferrite and bainite should be 30% or less, preferably 25% or less, and more preferably 20% or less.

[0035] (Tempered martensite: 60% or more) From the viewpoint of obtaining the desired TS and YS, and from the viewpoint of reducing the hardness difference with the soft tissue and improving λ, the area ratio of tempered martensite is 60% or more, preferably 65% ​​or more, and more preferably 70% or more. There is no particular upper limit. The area ratio of tempered martensite may be, for example, 98% or less, 95% or less, or 90% or less.

[0036] (Residual austenite: 1% to 10%) Retained austenite transforms into hard martensite during processing, increasing the hardness difference with the softer structure, which tends to lead to a decrease in YS and λ. For this reason, the area percentage of retained austenite should be 10% or less, preferably 7% or less, and more preferably 5% or less. On the other hand, if the steel sheet contains no retained austenite at all, the martensitic transformation of retained austenite does not occur during processing, so the WH content tends to decrease. For this reason, the area ratio of retained austenite is 1% or more, preferably 2% or more, and more preferably 3% or more.

[0037] (Fresh martensite: 15% or less) Fresh martensite is a hard tissue, and an increase in the hardness difference between it and soft tissue leads to a decrease in YS and λ. Therefore, the area ratio of fresh martensite is 15% or less, preferably 13% or less, more preferably 10% or less, even more preferably 8% or less, particularly 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 40% or less, but may be 36% or less, or 33% 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: 0.60% by mass or less) If the carbon concentration in retained austenite is too high, the retained austenite becomes stable. As a result, even during processing, the retained austenite does not undergo martensite transformation, and the WH content does not increase. In addition, if the carbon concentration in retained austenite is too high, processing can cause the retained austenite to transform into fresh martensite, increasing the strength difference with tempered martensite, which may lead to a decrease in YS and λ. For this reason, the carbon concentration in retained austenite should be 0.60 mass% or less, preferably 0.55 mass% or less, and more preferably 0.50 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% by mass or more, 0.10% by mass or more, 0.15% by mass or more, or 0.20% by mass or more.

[0041] (R BF (70% or more) In the heat treatment described later, cold-rolled steel sheets are heated and held at a temperature T1. After being held at temperature T1, ferrite and bainite transformations occur during cooling to temperature T2, and then bainite transformation continues during holding at temperature T2. Of these, the bainite formed during holding at temperature T2 is particularly effective in improving delayed fracture resistance. Furthermore, as mentioned above, if there is a large amount of soft tissue such as ferrite or bainite, YS and λ tend to decrease. If this tissue is elongated in one direction, stress concentration can easily cause cracks to form around it, further reducing the YS and λ values. Furthermore, the ferrite and bainite formed during the cooling process described above often have a shape that is elongated in one direction (large aspect ratio). In contrast, the bainite formed during the holding process at the temperature T2 described above often has an isotropic shape and a small aspect ratio. From the above, from the viewpoint of obtaining good delayed fracture resistance and further from the viewpoint of obtaining good YS and λ, it is preferable that the bainite produced during holding at the aforementioned temperature T2 is abundant among the ferrite and bainite. Specifically, the proportion of ferrite and bainite with an aspect ratio of 2 or less and a long side of 10 μm or less ("R BF The percentage (referred to as ") is 70% or more, preferably 75% or more, and more preferably 80% or more. There is no particular upper limit, R BF For example, it may be 98% or less, 96% or less, or 94% or less.

[0042] (R γ (10% or more) For example, when a steel sheet is deformed by press forming, its strength increases due to work hardening (WH). Work hardening (WH) of steel sheets occurs not only due to deformation of the steel sheet, but also due to the transformation of retained austenite into hard fresh martensite. In other words, the greater the rate of reduction of retained austenite (the amount of martensitic transformation), the higher the amount of WH. From the viewpoint of impact resistance, a large amount of WH is preferable. Therefore, a 2% strain is induced in the steel sheet by a tensile test, and the reduction rate of retained austenite at that time (denoted as "R γ ") is determined. From the viewpoint of obtaining a good WH amount, R γ is 10% or more, preferably 13% or more, and more preferably 15% or more. The upper limit is not particularly limited, and R γ is, for example, 40% or less, may be 35% or less, or may be 30% or less. Note that the C concentration in retained austenite varies depending on the grains of retained austenite, and the stability of each grain may also vary accordingly. Therefore, the amount of retained austenite that transforms into martensite during processing can be grasped more accurately by R γ .

[0043] (Measurement method) The area ratio of each structure, etc. is determined as follows.

[0044] ((Area ratio of ferrite, bainite and tempered martensite and R BF )) The 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 corroded using 3 vol% nital to obtain an observation surface. The position at 1 / 4 of the plate thickness of the observation surface is observed at a magnification of 5000 times using a scanning electron microscope (SEM) to obtain SEM images of any 10 fields of view. In the SEM image, ferrite and bainite (F + B) are the darkest regions and have almost no carbides inside. Tempered martensite (TM) is a black region, but since it has a lath-like substructure and carbides inside, it can be distinguished from ferrite and bainite (F + B). The massive structure (MA) composed of fresh martensite and retained austenite is a white massive region and has no substructure inside. 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.

[0045] In this case, for ferrite and bainite, in addition to the area ratio, the aspect ratio and the length of the longer side are determined. Figures 1A and 1B are schematic diagrams showing tissue (ferrite or bainite) in SEM images. As shown in Figures 1A and 1B, the longest imaginary straight line crossing a single tissue is defined as the long side (a) of the tissue. The longest (but not the shortest) straight line perpendicular to the long side (a) is defined as the short side (b) of the tissue. The ratio of the long side (a) to the short side (b) (a / b) is then determined as the aspect ratio of the tissue. In this way, for all ferrite and bainite in the SEM image, the length of its long side (a) and its aspect ratio (a / b) are determined in the same manner as the area ratio. Furthermore, the area ratio (S) of ferrite and bainite in the SEM image is defined as having an aspect ratio of 2 or less and a long side of 10 μm or less. BF ) proportion (R BF ) is calculated from the following formula. The average value of 10 fields of view is R BF They will be hired as such. R BF =( S BF / S)×100

[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] ((R γ )) A tensile test (see [Examples]) described later is performed to induce a 2% strain (pre-strain) in the steel plate. Then, the area ratio of retained austenite (V2) is determined in the same manner as described above. Using the area ratio of retained austenite (V1) before the tensile test, the reduction rate of retained austenite (R) is calculated based on the following formula. γ ) R γ = 1 - (V2 / V1) × 100

[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 2 is a chart illustrating an example of heat treatment. As shown in Figure 2, in general terms, the heat treatment involves heating the cold-rolled steel sheet, holding it at a temperature T1, then cooling it to a temperature T2, then cooling it to a cooling stop temperature T3, and finally heating it again to a temperature T4. 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 T1》 First, the cold-rolled steel sheet is heated and held at a temperature T1. When the temperature T1 is at a certain level, ferrite and bainite decrease, and tempered martensite increases. Therefore, the temperature T1 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 T1 is too high, the amount of ferrite and bainite decreases too much. Therefore, the temperature T1 should be A c3 The temperature is below +50°C, A c3 A is preferable to be below +40℃. c3 A temperature of +30°C or lower is more preferable.

[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 t1 at temperature T1 is not particularly limited as long as it satisfies equation (1) described later, but for example it can be 5 seconds or more, 10 seconds or more, or 15 seconds or more. On the other hand, the holding time t1 at temperature T1 is, for example, 100 s or less, but may also be 90 s or less, or 80 s or less.

[0058] Cooling from temperature T1 to temperature T2 Next, the cold-rolled steel sheet, which has been held at temperature T1, is cooled to temperature T2, which will be described later. In this case, if the average cooling rate CR1 from temperature T1 to 500°C is too slow, ferrite and bainite transformations may occur during cooling. From the viewpoint of suppressing ferrite and bainite to a certain amount, the average cooling rate CR1 is 10°C / s or more, preferably 12°C / s or more, and more preferably 15°C / s or more. While there is no particular upper limit, from an equipment standpoint, the average cooling rate CR1 is preferably 100°C / s or less, more preferably 80°C / s or less, and even more preferably 60°C / s or less.

[0059] 《Maintaining at temperature T2》 Next, the cold-rolled steel sheet, cooled to temperature T2, is held at temperature T2. This is done to obtain the required bainite. For this reason, temperature T2 is 400°C or higher, preferably 415°C or higher, and more preferably 430°C or higher. The holding time t2 at temperature T2 is 200 s or higher, preferably 215 s or higher, and more preferably 230 s or higher. On the other hand, if the temperature T2 is too high or the holding time t2 is too long, excess bainite will be produced. For this reason, the temperature T2 should be 500°C or less, preferably 485°C or less, and more preferably 470°C or less. The holding time t2 should be 300 s or less, preferably 285 s or less, and more preferably 270 s or less.

[0060] Cooling from temperature T2 to cooling stop temperature T3 Next, the cold-rolled steel sheet held at temperature T2 is cooled to the cooling stop temperature T3. This causes the untransformed austenite to transform into martensitic Therefore, the cooling stop temperature T3 is below Ms point -100°C, preferably below Ms point -125°C, and more preferably below Ms point -150°C. Similarly, the average cooling rate CR2 is 5°C / s or more, preferably 10°C / s or more, and more preferably 50°C / s or more. On the other hand, the cooling stop temperature T3 is not particularly limited in its lower limit; for example, it may be 70°C or higher, 90°C or higher, or 110°C or higher. Also, the average cooling rate CR2 may be 120°C / s or lower, 100°C / s or lower, or 80°C / s or lower.

[0061] 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.

[0062] Heating from cooling stop temperature T3 to temperature T4 Next, the cold-rolled steel sheet, cooled to the cooling stop temperature T3, is heated and held at a temperature T4 above the cooling stop temperature T3, without being held at T3. This tempers the fresh martensite obtained by cooling to the cooling stop temperature T3, obtaining a sufficient amount of tempered martensite. For this reason, the temperature T4 is 150°C or higher, preferably 175°C or higher, and more preferably 200°C or higher. On the other hand, if the temperature T4 is too high, the strength of the tempered martensite may decrease excessively, leading to a decrease in TS. Furthermore, the distribution of carbon to the untransformed austenite is promoted, increasing the carbon concentration in the retained austenite. For this reason, the temperature T4 should be 300°C or lower, preferably 275°C or lower, and more preferably 250°C or lower.

[0063] 《Maintaining at temperature T4》 As described above, holding at temperature T4 tempers the fresh martensite obtained by cooling to the cooling stop temperature T3, thereby obtaining a sufficient amount of tempered martensite. For this reason, the holding time t4 at temperature T4 is 10 s or more, preferably 100 s or more, and more preferably 200 s or more. On the other hand, if the retention time t4 is too long, there is a concern that the retained austenite will decompose into carbides, and the desired amount of retained austenite may not be obtained. For this reason, the retention time t4 is 1000 s or less, preferably 800 s or less, and more preferably 600 s or less.

[0064] Furthermore, the cold-rolled steel sheet held at temperature T4 is then cooled to a temperature of, for example, 50°C or lower. The average cooling rate at this time is, for example, 1°C / s or more, may be 2°C / s or more, or 5°C / s or more. On the other hand, the average cooling rate at this time is, for example, 80°C / s or less, may be 70°C / s or less, or 60°C / s or less.

[0065] 《P》 As the temperature T2 and retention time t2 increase, bainite transformation is promoted. On the other hand, as the temperature T1 and retention time t1 increase, ferrite transformation and bainite transformation are suppressed. Therefore, the value of P, expressed by the following equation (1) (the value obtained by subtracting the product of temperature T1 and holding time t1 from the product of temperature T2 and holding time t2), is adjusted. This controls the amount of ferrite and bainite within the desired range. Specifically, P is 40,000 or more, preferably 50,000 or more, and more preferably 60,000 or more. On the other hand, P is 100,000 or less, preferably 90,000 or less, and more preferably 80,000 or less. P = T² × t² - T⁻¹ × t⁻¹ …(1)

[0066] <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 of T4).

[0067] 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.

[0068] 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.

[0069] 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.

[0070] Now, let's consider, for example, the case where zinc plating is performed while the material is held at a temperature T4. In this case, as long as the heat treatment conditions described above are satisfied, there may be a period of time during the zinc plating process when the temperature T (for example, the temperature of the zinc plating bath) exceeds temperature T4. In other words, if the total holding time t4 within the temperature range T4 is between 10s and 1000s, then zinc plating at a temperature exceeding T4 may be included during the holding period at T4. [Examples]

[0071] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below.

[0072] <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 steel slabs having the component composition shown in Table 1 below.

[0073] <Heat treatment> Next, the cold-rolled steel sheets were subjected to heat treatment under the conditions shown in Table 2 below. Furthermore, the cold-rolled steel sheets that were held at temperature T4 were subsequently cooled to a temperature of 50°C or lower at an average cooling rate of 20°C / s.

[0074] <Zinc plating treatment> For some cold-rolled steel sheets (CR), a hot-dip galvanizing treatment was performed while holding at a temperature T4 to form a hot-dip galvanized layer. In other words, hot-dip galvanized steel sheets (GI) were 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 hot-dip galvanized coating per side is 58 g / m². 2 That was the case.

[0075] For a portion of the cold-rolled steel sheet (CR), an alloying hot-dip galvanizing treatment was applied to form an alloying hot-dip galvanized layer. That is, alloying 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.

[0076] 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.

[0077] 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."

[0078] <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, according to the method described above, the C concentration in retained austenite (retained γ), R BF and R γ They sought it. The results for all of these are shown in Table 3 below. In cases where the area fraction of retained austenite was 0%, the C concentration in the retained austenite could not be measured, so "-" was entered in the "C concentration in retained γ" column in Table 3 below.

[0079] <evaluation> The obtained steel plates were subjected to the tests described below to evaluate various properties. The results are shown in Table 3.

[0080] Tensile Test Tensile testing was performed 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. -1Tensile tests were conducted under conditions of mm / s to measure tensile strength (TS) [MPa] and yield stress (YS) [MPa].

[0081] Furthermore, the WH weight [MPa] was determined by tensile testing. Specifically, a tensile test was conducted to induce a 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.

[0082] For the reason of high strength, a TS of 1180 MPa or higher is preferable. For superior impact resistance, a YS of 750 MPa or higher and a WH of 100 MPa or higher are preferred.

[0083] 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 20% or more is preferable.

[0084] Delayed Destruction Test Delayed fracture testing was conducted as follows to evaluate the delayed fracture resistance characteristics. Specifically, first, the obtained steel plate was sheared to obtain a test specimen (18 mm × 75 mm) with the longitudinal direction perpendicular to the rolling direction. The shearing was performed using a crank press with a rake angle of 0.5° and a clearance of 10%, and the punched end face was ground down to 2 mm to obtain the test specimen. A bending stress of 700 MPa was applied to the center of the obtained test specimen using a four-point bending jig. Next, the test specimen with the applied bending stress was immersed at room temperature (25°C) in a mixture of 0.1 vol% ammonium thiocyanate solution and 50 vol% McIlbain buffer solution (pH: 7). After immersion, the test specimen was observed to check for the occurrence of fracture. Table 3 below indicates the following: if a fracture occurred 72 hours after the start of immersion, it was classified as "C"; if no fracture occurred 72 hours after the start of immersion but a fracture occurred 96 hours after the start of immersion, it was classified as "B"; and if no fracture occurred even after 96 hours after the start of immersion, it was classified as "A". "A" or "B" is preferred, with "A" being more preferred, due to its superior resistance to delayed fracture.

[0085] [Table 1] TIFF0007845595000002.tif225138

[0086] [Table 2]

[0087] [Table 3] TIFF0007845595000005.tif172132

[0088] <Summary of Evaluation Results> As is clear from the results shown in Tables 1 to 3 above, steel plates No. 1, 5, 9, 15, 18, 22, 25, 27 and 30-35 had a TS of 1180 MPa or higher and were found to have excellent formability, impact resistance, and delayed fracture resistance. In contrast, steel plates No. 2-4, 6-8, 10-14, 16-17, 19-21, 23-24, 26, and 28-29 were insufficient in at least one of the following: TS, formability, impact resistance, and delayed fracture resistance.

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 3.50% 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 3% to 30%, tempered martensite accounts for 60% or more, retained austenite accounts for 1% to 10%, and fresh martensite accounts for 15% or less. The C concentration in the retained austenite is 0.60% by mass or less. Of the ferrite and bainite mentioned above, the proportion of those having an aspect ratio of 2 or less and a long side of 10 μm or less is 70% or more. A high-strength steel plate in which the reduction rate of retained austenite when a 2% strain is applied to the steel plate is 10% or more.

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 A point or above ｃ３ Maintain a temperature T1 of +50°C or less. The cold-rolled steel sheet is cooled from the temperature T1 to a temperature T2 of 400°C or more and 500°C or less, provided that the average cooling rate CR1 from temperature T1 to 500°C is 10°C / s or more. The cold-rolled steel sheet is held at the temperature T2 for 200 s to 300 s. The cold-rolled steel sheet is cooled from the temperature T2 to a cooling stop temperature T3 below the Ms point -100°C at an average cooling rate CR2 of 5°C / s or more. The cold-rolled steel sheet is heated from the cooling stop temperature T3 to a temperature T4 that is greater than the cooling stop temperature T3 and between 150°C and 300°C. The cold-rolled steel sheet is held at the temperature T4 for 10 seconds or more and 1000 seconds or less. A method for manufacturing a high-strength steel plate, wherein, when the holding time at temperature T1 is t1 in units of s and the holding time at temperature T2 is t2 in units of s, P, represented by the following formula (1), is 40,000 or more and 100,000 or less. P=T2×t2−T1×t1…(1)

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