High-strength steel sheet and method for manufacturing same

Abstract

This invention provides a high-strength steel sheet which has tensile strength of 1180 MPa or more and is excellent in formability, collision resistance, and delayed fracture resistance. The component composition thereof contains, in terms of mass%, 0.15%-0.30% of C, 0.10%-1.50% of Si, 1.50%-3.50% of Mn, 0.020% or less of P, 0.0020% or less of S, and 0.100% or less of sol. Al, and 0.0150% or less of N. The total content of ferrite and bainite is 3%-30%, the content of tempered martensite is 60% or more, the content of retained austenite is 1%-10%, and the content of fresh martensite is 15% or less. The C concentration in retained austenite is 0.60% by mass or less. The proportion of ferrite and bainite having an aspect ratio of 2 or less and having a long side length of 10 μm or less is 70% or more. The reduction rate of retained austenite when 2% strain is generated in the steel sheet is 10% or more.

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 Document 1 discloses a high-strength steel sheet with a tensile strength (TS) of 1180 MPa or more.

[0003] International Publication No. 2020 / 045219

[0004] Generally, increasing the strength of steel sheets can reduce their formability. Furthermore, parts obtained by processing steel sheets with a TS of 1180 MPa or higher may experience delayed fracture (a phenomenon where parts suddenly break) due to hydrogen penetration into the steel sheet. Moreover, from the perspective of ensuring occupant safety during collisions, steel sheets used in automotive parts are required to have excellent collision resistance.

[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.

[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 3.50% 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 account for 3% to 30%, tempered martensite for 60% or more, retained austenite for 1% to 10%, and fresh martensite for 15% or less, the carbon concentration in the retained austenite is 0.60% by mass or less, and of the ferrite and bainite, the proportion of those with an aspect ratio of 2 or less and a long side of 10 μm or less is 70% or more, and the reduction rate of retained austenite when a 2% strain is applied to the steel sheet is 10% or more. [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 ｃ３A point or above ｃ３ The cold-rolled steel sheet is held at a temperature T1 of 50°C or less above the Ms point, and then cooled from temperature T1 to a temperature T2 of 400°C to 500°C, 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 then held at temperature T2 for 200 s to 300 s, and then cooled from temperature T2 to a cooling stop temperature T3 of 100°C or less below the Ms point at an average cooling rate CR2 of 5°C / s or more. On the other hand, a method for manufacturing a high-strength steel sheet, wherein 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 s to 1000 s, and 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, the P expressed by the following formula (1) is between 40,000 and 100,000. P = T2 × t2 - T1 × t1 ... (1)

[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 delayed fracture resistance.

[0008] This is a schematic diagram showing the microstructure in an SEM image. This is another schematic diagram showing a different microstructure in an SEM image. This is a chart showing 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 delayed fracture resistance.

[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 hole expansion ratio (λ), measured by a hole expansion test described later, is 20% or higher. Excellent impact resistance means that the yield stress (YS), measured by a 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 a delayed fracture test described later, no fracture occurs even after 72 hours.

[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. If the C content is too low, sufficient strength and amount of tempered martensite cannot be obtained, and TS and YS will decrease. For this reason, the C content is 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, the strength of the tempered martensite will increase excessively, making it easy for λ to decrease, and the retained austenite will become excessively stable, reducing the WH amount. In addition, the Ms point will decrease due to the increase in the C content. In this case, when cooling to the cooling stop temperature T3 described later, the martensitic transformation will not proceed sufficiently, and the untransformed austenite that did not undergo martensitic transformation will undergo martensitic transformation during cooling after holding at temperature T4 described later, and the amount of fresh martensite will increase. For this reason, the C content is 0.30% or less, preferably 0.28% or less, and more preferably 0.25% or less.

[0014] (Si: 0.10% to 1.50%) 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 structure to 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 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 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 3.50% 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, bainite transformation will be suppressed, for example, when holding at temperature T2 as described later. For this reason, the Mn content is 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, 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, 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 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 0.005% 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 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 0.060% or less, and may also be 0.040% or less or 0.020% or less. 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.

[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 0.0045% 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 and facilitate composite structure formation. However, if these elements are present in excessive amounts, the formability of the steel sheet may decrease. For this reason, 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. On the other hand, from the viewpoint of obtaining the effects of adding these elements, the Cu and Cr content is preferably 0.050% or more, more preferably 0.080% or more, even more preferably 0.150% or more, may be 0.300% or more, and may be 0.500% or more.

[0024] ((Co: 0.500% or less)) Co is an element that improves hardenability. However, if there is too much Co, coarse precipitates and inclusions may increase. Therefore, when adding Co, the content of Co is preferably 0.500% or less, and more preferably 0.300% or less. On the other hand, from the viewpoint of obtaining the addition effect of Co, the content of Co 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 be unevenly generated during heating, which may cause surface defects. Therefore, when adding Ni, the content of Ni is preferably 1.000% or less, more preferably 0.500% or less, still more preferably 0.200% or less, and may even be 0.100% or less, 0.050% or less, or 0.010% or less. On the other hand, from the viewpoint of obtaining the addition effect of Ni, the content of Ni 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. Also, Mo contributes to the improvement of the stress corrosion cracking resistance by precipitating fine carbides containing Mo that become hydrogen trap sites and refining martensite. However, if there is too much Mo, the stress corrosion cracking resistance may deteriorate due to the formation of coarse precipitates. Therefore, when adding Mo, the content of Mo is preferably 1.000% or less, more preferably 0.800% or less, and may even be 0.450% or less, or 0.250% or less. On the other hand, from the viewpoint of obtaining the addition effect of Mo, the content of Mo 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 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 be 0.030% 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 0.090% or less, and even more preferably 0.080% 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 0.010% 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, 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 other materials, improving the ultimate deformability of the steel sheet. However, if these elements are present in too much of each, the amount of coarse precipitates may increase. For this reason, 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 effect 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, if there is too much Bi, the amount of coarse precipitates may increase. For this reason, 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 described above as its component composition, with the remainder being Fe and unavoidable impurities. Preferably, the steel sheet contains only the elements described above and the remainder, with the remainder being Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. Preferably, the total content of unavoidable impurities is 0.100% or less.

[0033] 《Microstructure》 This section explains 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 them. 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 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, as ferrite and bainite increase, when cooling stops, carbon may be distributed from them to the surrounding untransformed austenite, potentially increasing the carbon concentration in the retained austenite. In addition, 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 this reason, the total area ratio 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 desired TS and YS, and from the viewpoint of reducing the hardness difference with the soft structure 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 is, for example, 98% or less, may be 95% or less, or 90% or less.

[0036] (Retained austenite: 1% to 10%) Retained austenite transforms into hard martensite during processing, increasing the hardness difference with the soft structure, which tends to lead to a decrease in YS and λ. For this reason, the area percentage of retained austenite is 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, which tends to decrease the WH content. For this reason, the area percentage 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 the soft tissue leads to a decrease in YS and λ. For this reason, 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] (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: 0.60 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 martensite transformation, and the WH content will not increase. In addition, if the C concentration in retained austenite is too high, processing will 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 C concentration in retained austenite is 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 mass% or more, may be 0.10 mass% or more, may be 0.15 mass% or more, or may be 0.20 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 holding at temperature T1, ferrite and bainite transformations proceed during cooling to temperature T2, and then bainite transformation proceeds during holding at temperature T2. Of these, the bainite generated during holding at temperature T2 is particularly effective in improving delayed fracture resistance. As mentioned above, if there is a large amount of soft microstructure such as ferrite and bainite, YS and λ tend to decrease. If this is elongated in one direction, stress concentration makes it easier for cracks to occur around it, further decreasing the YS and λ. Furthermore, the ferrite and bainite generated during the cooling described above often have a shape that is elongated in one direction (large aspect ratio). In contrast, the bainite generated during holding at 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 generated during holding at temperature T2 described above is the most abundant of 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(described as "」) is 70% or more, preferably 75% or more, and more preferably 80% or more. The upper limit is not particularly limited, and R ＢＦ is, for example, 98% or less, may be 96% or less, or may be 94% or less.

[0042] (R γ : 10% or more) For example, when a steel sheet is deformed by press forming or the like, the strength of the steel sheet increases due to work hardening (WH). The work hardening (WH) of the steel sheet occurs not only due to the deformation of the steel sheet but also due to the transformation from retained austenite to hard fresh martensite. That is, the larger the reduction rate of retained austenite (the amount of martensite transformation), the higher the WH amount. From the perspective of impact resistance characteristics, a large WH amount is preferable. Therefore, a 2% strain is generated in the steel sheet by a tensile test, and the reduction rate of retained austenite at that time (denoted as "R γ ") is obtained. From the perspective 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 thus the stability of each grain may also vary. Therefore, R γ can more accurately grasp the amount of retained austenite that transforms into martensite during processing.

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

[0044] ((The area ratio of ferrite, bainite, and tempered martensite and R ＢＦ)) Cut the steel plate to expose a cross section perpendicular to the surface of the steel plate and parallel to the rolling direction. Polish the exposed cross section to a mirror finish and etch it with 3 volume% nital to create an observation surface. Observe the observation surface at a position 1 / 4 of the plate thickness using a scanning electron microscope (SEM) at 5000x magnification 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.

[0045] At this time, 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 of the imaginary lines crossing a single tissue is taken as the longer side (a) of that tissue. Also, the longest (not the shortest) of the lines perpendicular to the longer side (a) is taken as the shorter side (b) of that tissue. The ratio of the longer side (a) to the shorter side (b) (a / b) is then determined as the aspect ratio of that tissue. In this way, for all ferrite and bainite in the SEM image, the length of the longer side (a) and the aspect ratio (a / b) are determined in the same way as the area ratio. Then, the area ratio (S) of ferrite and bainite with an aspect ratio of 2 or less and a longer side of 10 μm or less is determined relative to the area ratio (S) of all ferrite and bainite in the SEM image. ＢＦ The proportion of (R ＢＦ ) is calculated from the following formula. The average value of 10 fields of view is R ＢＦ To be adopted as R. ＢＦ= (S ＢＦ / S) x 100

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

[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 subjected to heat treatment under the conditions described below. Figure 2 is a chart showing an example of the 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 then heating it again to a temperature T4. From the viewpoint of ensuring good productivity and desired heating and cooling rates, it is preferable to carry out the heat treatment on a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL).

[0055] 《Holding at Temperature T1》 First, the cold-rolled steel sheet is heated and held at temperature T1. By keeping the temperature T1 at a certain level, the amount of ferrite and bainite decreases and the amount of tempered martensite increases. Therefore, 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 will decrease too much. Therefore, the temperature T1 should be A ｃ３ The temperature is below +50°C, A ｃ３ A is preferably below +40°C. 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 is 5 s or more, may be 10 s or more, or may be 15 s or more. On the other hand, the holding time t1 at temperature T1 is for example 100 s or less, may be 90 s or less, or may be 80 s or less.

[0058] Cooling from Temperature T1 to Temperature T2 Next, the cold-rolled steel sheet held at temperature T1 is cooled to temperature T2, which will be described later. At this time, if the average cooling rate CR1 from temperature T1 to 500°C is too slow, ferrite transformation and bainite transformation 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. There is no particular upper limit, but from an equipment viewpoint, 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] <Holding 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, temperature T2 is 500°C or lower, preferably 485°C or lower, and more preferably 470°C or lower. The holding time t2 is 300 s or lower, preferably 285 s or lower, and more preferably 270 s or lower.

[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. At this time, if the cooling stop temperature T3 is too high, or if the average cooling rate CR2 from temperature T2 to the cooling stop temperature T3 is too slow, the martensitic transformation will not proceed sufficiently. The untransformed austenite that did not undergo martensitic transformation will undergo martensitic transformation during the cooling after holding at temperature T4, as described later, and the amount of fresh martensite will increase. For this reason, the cooling stop temperature T3 is Ms point - 100°C or lower, preferably Ms point - 125°C or lower, and more preferably Ms point - 150°C or lower. Similarly, the average cooling rate CR2 is 5°C / s or higher, preferably 10°C / s or higher, and more preferably 50°C / s or higher. On the other hand, the lower limit of the cooling stop temperature T3 is not particularly limited, for example, 70°C or higher, and may be 90°C or higher, or 110°C or higher. Furthermore, the average cooling rate CR2 is, for example, 120°C / s or less, but may also be 100°C / s or less, or 80°C / s or less.

[0061] 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.

[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 to a temperature T4 above the cooling stop temperature T3 and held there, without being held at the cooling stop temperature T3. This tempers the fresh martensite obtained by cooling to the cooling stop temperature T3, and obtains 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, and the TS may decrease. Furthermore, the distribution of C to the untransformed austenite is promoted, and the C concentration in the retained austenite increases. For this reason, the temperature T4 is 300°C or lower, preferably 275°C or lower, and more preferably 250°C or lower.

[0063] 《Holding 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 holding time t4 is too long, there is a concern that the retained austenite will decompose into carbides, and the desired amount of retained austenite will not be obtained. For this reason, the holding 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 values ​​of temperature T2 and holding time t2 increase, bainite transformation is promoted. On the other hand, as the values ​​of temperature T1 and holding time t1 increase, ferrite transformation and bainite transformation are suppressed. Therefore, the value of P (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 to a 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 - T1 × t1 ... (1)

[0066] <Zinc Plating Treatment> During the heat treatment (for example, while holding at temperature T4), the cold-rolled steel sheet may be subjected to zinc plating treatment by a conventionally known method.

[0067] 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.

[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 treatment may also be electro-zinc plating. Examples of electro-zinc plating treatments include Zn-Ni electro-alloy plating and pure Zn electro-plating.

[0070] Now, consider, for example, the case where zinc plating is performed while holding at 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. That is, as long as the total holding time t4 within the range of temperature T4 is between 10 s and 1000 s, zinc plating at a temperature exceeding temperature T4 may be included during the holding at temperature T4.

[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 a steel slab 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. The cold-rolled steel sheets held at temperature T4 were then 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. 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.

[0075] 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.

[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. 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.

[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 each be simply referred to as "steel sheets."

[0078] <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 R in retained austenite (retained γ) were determined according to the method described above. ＢＦ and R γ The following was determined. The results are shown in Table 3 below. Note that if the area fraction of retained austenite was 0%, the C concentration in the retained austenite could not be measured, so "-" is written in the "C concentration in retained γ" column in Table 3 below.

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

[0080] 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 conditions of mm / s to measure tensile strength (TS) [MPa] and yield stress (YS) [MPa].

[0081] Furthermore, the WH amount [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 WH amount was determined by subtracting YS from the final load [MPa] at that time.

[0082] For the reason of high strength, TS is preferably 1180 MPa or higher. For the reason of excellent impact resistance, YS is preferably 750 MPa or higher, and WH is preferably 100 MPa or higher.

[0083] 《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 20% or more.

[0084] 《Delayed Fracture Test》 The delayed fracture resistance was evaluated by conducting a delayed fracture test as follows. 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. 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 fracture occurred 72 hours after the start of immersion, it is "C"; if fracture did not occur 72 hours after the start of immersion but occurred 96 hours after the start of immersion, it is "B"; and if fracture did not occur even after 96 hours after the start of immersion, it is "A". "A" or "B" is preferred, and "A" is more preferred, due to their superior resistance to delayed fracture.

[0085]

[0086]

[0087]

[0088] <Summary of Evaluation Results> As is clear from the results shown in Tables 1 to 3 above, steel sheets No. 1, 5, 9, 15, 18, 22, 25, 27 and 30-35 had a total strength (TS) of 1180 MPa or higher, and were found to have excellent formability, impact resistance, and delayed fracture resistance. In contrast, steel sheets 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 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 3.50% 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 account for 3% to 30%, tempered martensite for 60% or more, retained austenite for 1% to 10%, and fresh martensite for 15% or less, the carbon concentration in the retained austenite is 0.60% by mass or less, the proportion of ferrite and bainite having an aspect ratio of 2 or less and a long side of 10 μm or less is 70% or more, and the reduction rate of retained austenite when a 2% strain is applied to the steel sheet is 10% or more.

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 A point or above ｃ３ The cold-rolled steel sheet is held at a temperature T1 of 50°C or less above the Ms point, cooled from 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 temperature T2 for 200 s or more and 300 s or less, the cold-rolled steel sheet is cooled from temperature T2 to a cooling stop temperature T3 of 100°C or less below the Ms point 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 above the cooling stop temperature T3 and 150°C or more and 300°C or less, the cold-rolled steel sheet is held at temperature T4 for 10 s or more and 1000 s 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, expressed by the following formula (1), is between 40,000 and 100,000. P = T2 × t2 - T1 × t1 ... (1)

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