Steel sheet, member, and method for producing same

Abstract

Provided are: a steel plate having a tensile strength (TS) of 780 MPa or more, high ductility and stretch flangeability, and excellent axial crushing characteristics; a member; and a method for producing the steel plate and the member. A steel sheet having a component composition in which C, Si, Mn, P, S, sol.Al, and N are contained in specific amounts, the steel sheet has a steel structure containing, in area ratio, 10-60% of a structure comprising at least one of ferrite and bainite ferrite, 20-80% of a structure comprising at least one of tempered martensite and lower bainite, 5% or more of retained austenite, and 10% or less of fresh martensite, and the average C concentration in the retained austenite is 0.60 mass% or more. In a region within 100 [mu] m in the sheet thickness direction from the surface of the steel sheet, the content of Fe element present as carbide in the tempered martensite is 0.20 mass% or less on average.

Description

Technical Field

[0001] This invention relates to high-strength steel sheets and components with excellent formability, suitable for use in various applications such as automobiles and home appliances, and methods for manufacturing the same. Background Technology

[0002] In recent years, due to the increasing demand for lightweight automobile bodies, the application of high-strength steel sheets with a tensile strength (TS) of over 780 MPa in automotive frame components and seat components is being promoted. However, when these high-strength steel sheets are cold-formed, they are prone to crushing due to reduced ductility and elongation flange properties. Furthermore, when necking occurs during stamping, the sheet thickness becomes locally thinner, resulting in product defects similar to cracks. Therefore, to obtain better stamping formability, it is necessary to evaluate the ease of necking. Based on these considerations, the ductility index should be the elongation before necking occurs (uniform elongation: U-El), rather than the elongation at fracture (total elongation: T-El).

[0003] Furthermore, from the perspective of occupant safety, it is necessary to suppress deformation around the cab during a collision. Therefore, energy-absorbing components such as longitudinal beams are required to absorb collision energy through the deformation of each component during a collision. However, high-strength steel plates with tensile strengths of 780 MPa or higher have the following problems: due to the reduction in axial crushing characteristics, the parts subjected to single-processing based on forming become the starting point during a collision, which can easily cause component fracture and prevent them from stably exerting their collision energy absorption capacity.

[0004] As mentioned above, in order to apply high-strength steel sheets with strengths of 780 MPa or higher to energy absorption components in automobiles, in addition to ductility and elongation flange properties, excellent axial crushing characteristics are also required.

[0005] In this context, for example, Patent Document 1 discloses, by mass%, the following components: C: 0.15% or more and 0.30% or less; P: 0.040% or less; S: 0.0100% or less; N: 0.0100% or less; O: 0.0060% or less; one or two of Si and Al: totaling 0.70% or more and 2.50% or less; one or two of Mn and Cr: totaling 1.50% or more and 3.50% or less; Mo: 0% or more and 1.00% or less; Ni: 0% or more and 1.00% or less; Cu: 0% or more and 1.00% or less; Nb: 0% or more and 0.30% or less; Ti: 0% or more and 0.30% or less; V: 0% or more and 0.30% or less; B: 0% or more and 0.0050% or less; Ca: 0% or more. Furthermore, the steel sheet has the following composition: 0.0400% or less, Mg: 0% or more and 0.0400% or less, and REM: 0% or more and 0.0400% or less, with the balance consisting of Fe and impurities. As a percentage of the overall microstructure, it contains one or both of ferrite and granular bainite, totaling 10% or more and 50% or less; one or both of upper bainite and lower bainite, totaling 10% or more and 50% or less; tempered martensite: greater than 0% and 30% or less; retained austenite: 5% or more; and one or more of pearlite, cementite, and martensite, totaling 0% or more and 10% or less. The area percentage of the ferrite is less than 25% of the total area percentage of the ferrite and granular bainite. Therefore, a steel sheet with a tensile strength of 980 MPa or more and excellent elongation and porosity can be obtained.

[0006] Patent document 2 discloses that, as a steel composition, by mass percent, it contains C: 0.07–0.20%, Si: 0.1–2.0%, Mn: 2.0–3.5%, P: 0.05% or less, S: 0.05% or less, Sol.Al: 0.005–0.1%, with the balance consisting of Fe and unavoidable impurities. In the steel microstructure, by area percentage, it contains ferrite: 60% or less, tempered martensite: 40% or more, and fresh martensite: 10% or less. Furthermore, the void number density of the bent portion in the VDA bending test is 1500 voids / mm². 2 Therefore, a high-strength hot-dip galvanized steel sheet with a tensile strength of over 980 MPa and excellent fracture resistance upon impact can be obtained.

[0007] Patent document 3 discloses a steel structure containing, by mass percent, 0.10-0.40% C, 0.5-4.0% Mn, 0.005-2.5% Si, 0.005-2.5% Al, 0-1.0% Cr, with the balance consisting of iron and unavoidable impurities, limited to P: 0.05% or less, S: 0.02% or less, and N: 0.006% or less. The structure also contains 2-30% retained austenite by area ratio, with martensite limited to 20% or less. The average particle size of the cementite is 0.01 μm or more and 1 μm or less, and the cementite contains 30% or more and 100% or less cementite with an aspect ratio of 1 or more and 3 or less. This structure yields a high-strength cold-rolled steel sheet with excellent strength, ductility, and porosity.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent No. 6338038

[0011] Patent Document 2: Japanese Patent No. 6795122

[0012] Patent Document 3: Japanese Patent No. 4903915 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] In Patent Document 1, by effectively utilizing granular bainite, the hardness difference between different phases in the composite structure steel sheet is minimized, thus suppressing the decrease in elongation flange properties associated with the increase of ferrite. This allows for the manufacture of steel sheets with excellent ductility and elongation flange properties, but axial crushing characteristics are not considered.

[0015] In Patent Document 2, steel sheets with excellent axial crushing properties can be manufactured by reducing the porosity. However, since it does not contain a bainite phase with a hardness between ferrite and tempered martensite, the large hardness difference between the different phases and the low elongation flangeability become problems. In Patent Document 3, it is described that steel sheets with excellent ductility and elongation flangeability can be obtained by controlling the shape of cementite, but axial crushing properties are not considered. During bending deformation, cracks are generated starting from cementite, resulting in poor axial crushing properties.

[0016] The present invention was made to solve the above-mentioned problems, and aims to provide steel plates, components, and methods for manufacturing the same, having a tensile strength (TS) of 780 MPa or more, high uniform elongation and elongation flangeability, and excellent axial crushing characteristics.

[0017] It should be noted that, in this invention, tensile strength is determined by tensile testing according to JIS Z 2241 (2011).

[0018] Furthermore, in this invention, high ductility refers to the uniform elongation (U-E1) measured by tensile testing according to JIS Z 2241 (2011).

[0019] (A) When TS is above 780MPa and below 980MPa, U-El ​​is above 16.0%;

[0020] (B) When TS is above 980MPa and below 1180MPa, U-El ​​is above 11.0%;

[0021] (C) Within the range of TS above 1180MPa, U-El ​​is above 8.0%.

[0022] Furthermore, in this invention, high elongation flange performance refers to the following: after punching a 100mm × 100mm square sample using a punching tool with a punch diameter of 10mm and a die diameter of 10.3mm (13% clearance), a conical punch with a 60-degree apex angle is used to enlarge the hole until a crack penetrating the plate thickness is generated, so that the burrs generated during the forming of the punched hole are on the outside. d0 is set as the initial hole diameter (mm), d is set as the hole diameter when the crack is generated (mm), and the hole enlargement rate λ (%) = {(d-d0) / d0} × 100 is 30% or more.

[0023] In addition, in this invention, excellent axial crushing characteristics refer to steel plates with a VDA bending angle α of 70° or more in VDA bending.

[0024] Here, the VDA bending test refers to the bending test (Verband der Automobilindustrie: VDA bending test) according to the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry, which is a three-point plate bending test characterized by very narrowly spaced rollers and a sharp punch. The VDA bending test is conducted as follows: a 60mm×60mm square test piece is used, with the bending ridge direction parallel to the rolling direction, supported by rollers with a roller diameter D of 30mm and a roller spacing L of (plate thickness a0×2)+0.5mm, and pressed from above by a punch with a tip r of 0.4mm at a stroke speed of 20mm / min. The VDA bending angle α refers to the bending angle (° (more specifically, the unit is "° / mm", hereinafter referred to as "°")) calculated using formulas (1) to (5) based on the stroke S (mm) at the maximum load in the above bending test, which can be used as an indicator of axial crush characteristics. It should be noted that the stroke S (mm) at maximum load is the length (mm) the punch moves from the start of the test until the moment the maximum load is achieved.

[0025] c(mm) = D / 2 + r + a0 …(Equation 1)

[0026] p(mm) = D / 2 + L / 2 …(Equation 2)

[0027]

[0028]

[0029]

[0030] Methods for solving problems

[0031] The inventors have conducted in-depth research on steel plates with high strength, excellent ductility and elongation flange properties, and excellent axial crushing characteristics, and have drawn the following conclusions.

[0032] (1) After hot rolling and cold rolling of a steel billet with a specified composition, from the viewpoints of strength, elongation flange and axial crushing characteristics, at temperatures above 750°C and A c3 Annealing is performed at an annealing temperature below a certain temperature (°C), and the annealing temperature is cooled at a specified cooling rate, thereby controlling the total area ratio of ferrite and bainitic ferrite to be more than 10% and less than 60%.

[0033] (2) During the cooling after the above annealing treatment, the cooling stop temperature is set above 200°C and below (martensitic phase transformation start temperature Ms point (°C) - 30°C), thereby partially converting austenite into martensite and / or bainite.

[0034] The martensite and bainite generated in this way, during the over-aging process after cooling stops, enrich carbon into the surrounding untransformed austenite, ensuring a specified C concentration in the residual austenite, which helps improve ductility. Furthermore, a dwell time can be performed during cooling, holding the material for 10 seconds to 60 seconds within a temperature range from 500°C to above the martensitic transformation initiation temperature Ms and above 320°C. This ensures more bainite is retained, further improving ductility.

[0035] (3) In the above-mentioned over-aging process after cooling is stopped, the temperature is heated to a temperature of 300°C or higher and 450°C or lower. Then, it is cooled at an average cooling rate of 0.5°C / s or higher to a temperature 30°C or higher and 150°C lower than the above-mentioned heating temperature, and then cooled at an average cooling rate of 0.1°C / s or higher to a temperature of 150°C or higher and 350°C or lower. Then, it is maintained in the temperature range of 150°C or higher and 350°C or lower for 20 seconds or more and 1000 seconds or less. As a result, the precipitation of carbides in the tempered martensite on the surface of the steel plate, which becomes the starting point for crack propagation during VDA bending, can be minimized, and the axial crushing characteristics can be improved. Furthermore, the aforementioned heat treatment process (heating to 300–450°C after cooling is stopped, and then maintaining it at a temperature range of 150°C to 350°C for 20 seconds to 1000 seconds) can promote carbon distribution from the surrounding microstructure to the untransformed austenite region, thereby further increasing the carbon concentration of the retained austenite and improving ductility.

[0036] Thus, especially during the cooling process after annealing and the over-aging process, by appropriately controlling the temperature history, it is possible to simultaneously reduce carbides in tempered martensite and increase the carbon concentration of retained austenite. As a result, it is possible to manufacture steel sheets with a specified tensile strength TS, and excellent ductility, elongation flange, and axial crushing characteristics.

[0037] The present invention is based on the above insights, and specifically, the following solutions are provided.

[0038] [1] A steel plate having a composition comprising, by mass%, C: 0.10% or more and 0.30% or less, Si: 0.5% or more and 2.0% or less, Mn: 1.5% or more and 3.0% or less, P: 0.10% or less, S: 0.020% or less, sol.Al: 1.00% or less, N: 0.015% or less, with the balance being Fe and unavoidable impurities, and having a composition by area of ​​either ferrite or bainitic ferrite. The steel microstructure is characterized by the following composition: 10% to 60% of the upper structure, 20% to 80% of the microstructure consisting of one or more of tempered martensite and lower bainite, 5% or more of retained austenite, and 10% or less of fresh martensite. The average C concentration in the retained austenite is 0.60% by mass or more, and the average Fe content in the tempered martensite in the form of carbides is 0.20% by mass or less in a region within 100 μm of the steel plate surface in the thickness direction.

[0039] [2] The steel plate according to [1] above, wherein the above composition further contains at least one selected from the following by mass%: Ti: less than 0.100%, Nb: less than 0.100%, V: less than 0.100%, B: less than 0.0050%, Cr: less than 1.000%, Cu: less than 1.000%, Sb: less than 0.200%, Sn: less than 0.200%, Ta: less than 0.100%, Ca: less than 0.0050%, Mg: less than 0.0050%, REM: less than 0.0050%.

[0040] [3] The steel plate according to [1] or [2] above, wherein a zinc coating is present on the surface of the steel plate.

[0041] [4] A component made of steel plate as described in any one of [1] to [3] above.

[0042] [5] A method for manufacturing a steel plate, wherein a steel billet having the composition described in [1] or [2] above is hot-rolled and cold-rolled, and then, for the obtained cold-rolled steel plate, it is heated at 750°C or above and A c3After annealing at an annealing temperature below a certain temperature (°C), the material is cooled at an average cooling rate of 3°C / s to 100°C / s within a temperature range from the aforementioned annealing temperature to a temperature T1 above 200°C and below (Ms point (°C) - 30°C). Then, it is heated at an average heating rate of 2°C / s within a temperature range from the aforementioned temperature T1 to a temperature T2 above 300°C and below 450°C. Finally, it is heated from the aforementioned temperature T2 to a temperature range from (T2 - 150°C) to (T2 - 30°C). Cooling is performed at an average cooling rate of CR2 of 0.5°C / s or more within the temperature range up to temperature T3. Cooling is performed at an average cooling rate of CR3 of 0.1°C / s or more within the temperature range from the above temperature T3 to a temperature T4 that is above 150°C and below 350°C and below the above temperature T3. The temperature range is maintained for 20s or more and 1000s or less within the temperature range of 150°C and below 350°C and below the above temperature T4. Cooling is performed at an average cooling rate of CR4 of 1°C / s or more to a temperature of 50°C or below.

[0043] [6] A method for manufacturing a steel plate, wherein a steel billet having the composition described in [1] or [2] above is hot-rolled and cold-rolled, and then, for the obtained cold-rolled steel plate, it is heated at 750°C or above and A c3 After annealing at a temperature below a certain temperature (°C), the annealer is cooled at an average cooling rate of CR5 of 5°C / s or more and 100°C / s or less within a temperature range from the annealing temperature to 500°C. Then, it is cooled for 10 seconds to 60 seconds at an average cooling rate of CR6 of CR6 of CR6 of CR7 ... The temperature is heated at an average heating rate of 2°C / s or more within a temperature range up to temperature T2. It is cooled at an average cooling rate of CR2 at a temperature range from the above temperature T2 to a temperature T3 (T2-150°C or higher and (T2-30°C or lower). It is cooled at an average cooling rate of CR3 at a temperature range from the above temperature T3 to a temperature T4 (150°C or higher and 350°C or lower than T3) at an average cooling rate of CR4. It is held at a temperature range of 150°C or higher and 350°C or lower than T4 for 20 seconds or more and 1000 seconds, and then cooled to a temperature of 50°C or lower at an average cooling rate of CR4 at a temperature range of 1°C / s or higher.

[0044] [7] According to the steel plate manufacturing method described in [5] above, after cooling at an average cooling rate CR1 of 3°C / s or more and 100°C / s or less, the steel plate is subjected to hot-dip galvanizing treatment or alloyed hot-dip galvanizing treatment.

[0045] [8] According to the steel plate manufacturing method described in [6] above, after cooling at an average cooling rate CR7 of 3°C / s or more and 100°C / s or less, the steel plate is subjected to hot-dip galvanizing treatment or alloyed hot-dip galvanizing treatment.

[0046] [9] In the steel plate manufacturing method described in [5] or [6] above, the steel plate is subjected to electro-galvanizing treatment after being cooled at an average cooling rate of CR4 of 1°C or higher.

[0047]

[10] A method for manufacturing a component, comprising a step of forming a component by performing at least one of forming or joining processes on a steel plate as described in any one of [1] to [3].

[0048] Invention Effects

[0049] According to the present invention, a high-strength steel sheet with a tensile strength (TS) of 780 MPa or more and excellent ductility, elongation flange and axial crushing characteristics can be obtained.

[0050] The steel plate of the present invention has excellent axial crushing characteristics, and is therefore suitable for energy absorption components.

[0051] By applying the steel sheet obtained according to the present invention to automotive parts, it is possible to reduce the weight of the vehicle and potentially improve fuel efficiency. Attached Figure Description

[0052] Figure 1 This is a micrograph of an example of the steel plate of the present invention.

[0053] Figure 2 This is a diagram illustrating the method for manufacturing the steel plate of the present invention.

[0054] Figure 3 This is a diagram used to illustrate the calculation method for the VDA bending angle. Detailed Implementation

[0055] The present invention will now be described in detail.

[0056] The steel plate of the present invention comprises, by mass percent, C: 0.10% or more and 0.30% or less, Si: 0.5% or more and 2.0% or less, Mn: 1.5% or more and 3.0% or less, P: 0.10% or less, S: 0.020% or less, sol.Al: 1.00% or less, N: 0.015% or less, with the balance being Fe and unavoidable impurities, and contains, by area percent, one or more of ferrite and bainitic ferrite. The steel microstructure is characterized by: 10% to 60% or more of tempered martensite and lower bainite; 20% to 80% or more of tempered martensite and lower bainite; 5% or more of retained austenite; and 10% or less of fresh martensite. The average C concentration in the retained austenite is 0.60% by mass or more, and the average Fe content in the tempered martensite as carbides is 0.20% by mass or less in a region within 100 μm of the steel plate surface along the thickness direction.

[0057] First, the composition of the steel plate of the present invention will be described. It should be noted that, unless otherwise specified, in the following description, the "%" used as the unit for the content of the constituent elements refers to "mass %".

[0058] [C: Above 0.10% and below 0.30%]

[0059] Carbon (C) is essential for ensuring the amount of tempered martensite and the amount of retained austenite stable at room temperature. It is also the element required to stabilize retained austenite and improve ductility by enriching it within the retained austenite. When the C content is less than 0.10%, these effects cannot be fully realized, making it difficult to ensure the strength and workability of the steel sheet. Therefore, the C content needs to be set at 0.10% or more, preferably 0.12% or more, and more preferably 0.15% or more.

[0060] On the other hand, when the carbon content exceeds 0.30%, the amount of carbides in the tempered martensite increases, and cracks originate from the carbides during bending deformation. Therefore, not only do the axial crushing characteristics deteriorate, but the ductility also decreases due to the excessive increase in strength. Therefore, the carbon content needs to be set to 0.30% or less, preferably 0.28% or less, and more preferably 0.25% or less.

[0061] [Si: 0.5% to 2.0%]

[0062] Si is an element useful for improving ductility and axial crushing properties by inhibiting the formation of carbides in martensite and bainite, promoting the formation of retained austenite, and improving the stability of retained austenite. The Si content needs to be set at 0.5% or more, preferably 0.6% or more, and more preferably 0.7% or more.

[0063] On the other hand, when the Si content exceeds 2.0%, excessive strength leads to a decrease in ductility and elongation flange properties. Furthermore, there are concerns about increased rolling load during hot rolling, brittle cracking due to liquid metal during welding with galvanized materials, and deterioration of chemical conversion treatment properties. Therefore, the Si content is set to 2.0% or less, preferably 1.5% or less, and more preferably 1.0% or less.

[0064] [Mn: 1.5% or more and 3.0% or less]

[0065] Mn is effective for ensuring strength by maintaining a specified area ratio of tempered martensite and / or bainite. Furthermore, Mn is an important element for stabilizing retained austenite by lowering its Ms point, thereby increasing its area ratio and improving ductility. Therefore, the Mn content needs to be set at 1.5% or more, preferably 1.8% or more, and more preferably 2.0% or more.

[0066] On the other hand, when the Mn content exceeds 3.0%, it leads to excessive strength and significantly delays the bainitic phase transformation, thus reducing ductility. Furthermore, Mn segregation in the thickness direction becomes significant, raising concerns about reduced material stability. Therefore, the Mn content is set to 3.0% or less, preferably 2.8% or less, and more preferably 2.5% or less.

[0067] [P: below 0.10%]

[0068] Phosphorus (P) is an element that strengthens steel, but a high P content deteriorates spot weldability. Therefore, the P content is set to 0.10% or less, preferably 0.02% or less. It should be noted that P can be omitted, but from a manufacturing cost perspective, the P content is preferably 0.001% or more.

[0069] [S: below 0.020%]

[0070] Sulfur (S) is an element that improves the peeling properties of oxide scale during hot rolling and inhibits nitriding during annealing, but it has a significant negative impact on flexibility, porosity, and spot weldability. To reduce these negative effects, the S content is set to at least 0.020% or less. In this invention, from the viewpoint of suppressing the reduction in flexibility and porosity, the S content is preferably set to 0.0020% or less, and more preferably less than 0.0010%. It should be noted that S may be absent, but from the viewpoint of manufacturing cost, the S content is preferably 0.0001% or more. The S content is more preferably 0.0005% or more.

[0071] [sol.Al: 1.00% or less]

[0072] Al is an effective element for inhibiting carbide formation and promoting the formation of retained austenite. Furthermore, Al is added as a deoxidizer in the steelmaking process. Therefore, there is no specific lower limit for the sol.Al content, but for stable deoxidation, it is preferably set to 0.005% or more, and more preferably 0.01% or more.

[0073] On the other hand, when the sol.Al content exceeds 1.00%, the number of inclusions in the steel plate increases, leading to a deterioration in ductility. Therefore, the sol.Al content is set to 1.00% or less, preferably 0.50% or less. More preferably, the sol.Al content is 0.25% or less, and even more preferably 0.10% or less.

[0074] [N: below 0.015%]

[0075] Nitrogen (N) is an element that forms nitrides such as BN, AlN, and TiN in steel, and it reduces the thermal ductility and surface quality of steel. Furthermore, in steel containing boron (B), the effects of boron are negated by the formation of BN. When the N content exceeds 0.015%, the surface quality deteriorates significantly. Therefore, the N content is set to 0.015% or less, preferably 0.010% or less. More preferably, the N content is 0.005% or less, and even more preferably 0.002% or less.

[0076] It should be noted that it may also be free of N, but from the point of view of manufacturing cost, the N content is preferably 0.0001% or more, and more preferably 0.001% or more.

[0077] The balance other than those mentioned above consists of Fe and unavoidable impurities. The steel sheet of the present invention preferably has a composition containing the above-mentioned basic components, with the balance consisting of iron (Fe) and unavoidable impurities.

[0078] The steel plate of the present invention may contain the following optional elements in addition to the above-mentioned components.

[0079] [Ti: below 0.100%, Nb: below 0.100%, V: below 0.100%]

[0080] Ti, Nb, and V form fine precipitates during hot rolling or annealing, thereby increasing strength. To achieve this effect, it is preferable that Ti is 0.010% or more, Nb is 0.020% or more, and V is 0.020% or more.

[0081] On the other hand, when the contents of Ti, Nb, and V exceed 0.100%, the formability decreases. Therefore, when Ti, Nb, and V are added, their contents are preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.050% or less.

[0082] [B: Below 0.0050%]

[0083] Bo (B) is an element that improves the hardenability of steel and has the advantage of easily forming tempered martensite and / or bainite with a specified area ratio. To achieve this effect, Bo is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0084] On the other hand, when the content of B exceeds 0.0050%, not only does its effect become saturated, but it also leads to a significant reduction in thermal ductility and causes surface defects. Therefore, when B is added, the content of B is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0025% or less.

[0085] [Cr: less than 1.000%, Cu: less than 1.000%]

[0086] Cr and Cu not only function as solid solution strengthening elements, but also stabilize austenite and facilitate the formation of composite structures during the cooling process of annealing. Therefore, although it is not necessary to contain Cr and Cu, in order to achieve such effects, the contents of Cr and Cu are preferably 0.005% or more, more preferably 0.008% or more, and even more preferably 0.010% or more, respectively.

[0087] On the other hand, when both Cr and Cu contents exceed 1.000%, the formability of the steel sheet decreases. Therefore, when Cr and Cu are added, their contents are preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.400% or less. The contents of Cr and Cu are preferably 0.200% or less, more preferably 0.100% or less.

[0088] [Sb: below 0.200%, Sn: below 0.200%]

[0089] Sb and Sn are effective elements for suppressing decarburization in a region of approximately tens of μm on the surface of steel plates caused by nitriding and oxidation. Suppressing such nitriding and oxidation prevents a reduction in the amount of martensite formed on the steel plate surface, thereby ensuring the strength and material stability of the steel plate. Therefore, while the addition of Sb and Sn is not always necessary, when added, the content of Sb and Sn is preferably 0.002% or more, more preferably 0.004% or more, and even more preferably 0.006% or more, respectively.

[0090] On the other hand, if any of these elements is present in excess exceeding 0.200%, it will lead to a decrease in toughness. Therefore, when Sb and Sn are added, the contents of Sb and Sn are preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.040% or less, respectively.

[0091] [Ta: Below 0.100%]

[0092] Like Ti and Nb, Ta forms alloy carbides and alloy carbonitrides, contributing to increased strength. Furthermore, it is believed to have the following effect: a portion of it dissolves in Nb carbides and Nb carbonitrides, forming composite precipitates such as (Nb,Ta)(C,N), thereby significantly suppressing precipitate coarsening and stabilizing the contribution rate to the strength increase of the steel sheet due to precipitation strengthening. To achieve this effect, Ta is preferably 0.005% or more.

[0093] On the other hand, adding excessive Ta not only saturates the precipitate stabilization effect but also increases the alloy cost. Therefore, when Ta is added, the Ta content is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.050% or less.

[0094] [Ca: less than 0.0050%, Mg: less than 0.0050%, REM: less than 0.0050%]

[0095] Ca, Mg, and REM are elements used for deoxidation and are effective elements for shaping sulfides into spherical forms and improving the adverse effects of sulfides on local ductility and extended flange properties. To achieve such effects, it is preferable that the Ca content is 0.0001% or more, the Mg content is 0.0001% or more, and the REM content is 0.0001% or more.

[0096] On the other hand, when Ca, Mg, and REM are added in excess exceeding 0.0050%, it causes an increase in inclusions and other contaminants, resulting in surface and internal defects. Therefore, the contents of Ca, Mg, and REM are preferably 0.0050% or less, more preferably 0.0025% or less, and even more preferably 0.0010% or less, respectively.

[0097] The Ca content is more preferably below 0.0008%.

[0098] The Mg content is more preferably below 0.0008%.

[0099] The REM content is more preferably below 0.0008%.

[0100] It should be noted that, in this invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content in this invention refers to the total content of one or more elements selected from the above REMs. There is no particular limitation on REMs, but La and / or Ce are preferred.

[0101] When the optional elements described above are contained in a value below a suitable lower limit, the presence of optional elements in a value below the lower limit does not impair the effects of the present invention. When the optional elements described above are contained in a value below a suitable lower limit, the optional elements are contained as unavoidable impurities.

[0102] Next, the steel microstructure of the present invention will be described.

[0103] [Area fraction of microstructure composed of one or more of ferrite and bainitic ferrite: 10% or more and 60% or less]

[0104] The ferrite and bainitic ferrite formed during annealing and cooling contribute to improved ductility and enrich carbon in the surrounding untransformed austenite, thus aiding in the stabilization of the retained austenite. This effect can be achieved by ensuring that the combined area fraction of ferrite and bainitic ferrite is 10% or more.

[0105] On the other hand, when the combined area fraction of ferrite and bainitic ferrite exceeds 60%, a hardness difference arises between it and the surrounding hard phases such as martensite. During bending processing, cracks propagate from the interface with the hard phases, sometimes leading to a decrease in extension flange properties and axial crushing characteristics. Therefore, the area fraction of the microstructure composed of one or more of ferrite and bainitic ferrite is set to be 10% or more and 60% or less. This area fraction is preferably 15% or more, more preferably 20% or more. Furthermore, this area fraction is 55% or less, more preferably 50% or less.

[0106] Furthermore, although not specifically limited, in ferrite and bainitic ferrite, the ferrite content, based on the area ratio of the entire microstructure, is preferably 15% or more, more preferably greater than 15%, and even more preferably 20% or more. Additionally, in ferrite and bainitic ferrite, the ferrite content, based on the area ratio of the entire microstructure, is preferably 40% or less, more preferably 30% or less.

[0107] [Area ratio of microstructure composed of one or more of tempered martensite and lower bainite: 20% or more and 80% or less]

[0108] To achieve the specified strength and elongation flange properties, the total area ratio of tempered martensite and lower bainite is set to 20% or more. This total area ratio is preferably 30% or more, and more preferably 40% or more.

[0109] On the other hand, when the combined area ratio of tempered martensite and lower bainite exceeds 80%, the ductility decreases due to excessive strength enhancement. Therefore, the combined area ratio of tempered martensite and lower bainite is set to 80% or less. This combined area ratio is preferably 70% or less, and more preferably 60% or less.

[0110] [Area ratio of retained austenite: 5% or more]

[0111] To ensure high ductility, the retained austenite content is set to 5% or more in terms of area ratio relative to the overall steel structure. More preferably, it is 7% or more, and even more preferably, it is 10% or more.

[0112] The amount of retained austenite includes retained austenite existing between bainitic ferrite. The area ratio of retained austenite existing between bainitic ferrite can also be determined in the same manner as the area ratio of other retained austenite, according to the determination method described later.

[0113] On the other hand, an excessive increase in the amount of retained austenite can sometimes lead to a decrease in strength, a reduction in the formability of the extended flange, and a deterioration in resistance to delayed fracture. Therefore, although there is no upper limit specified for the area ratio of retained austenite, it is preferably 20% or less, and more preferably 15% or less.

[0114] [Fresh martensite surface area: below 10%]

[0115] Since this can sometimes lead to a decrease in ductility and extended flange properties, the area fraction of fresh martensite is set to 10% or less, preferably 8% or less, and more preferably 5% or less. Alternatively, the fresh martensite can be 0%.

[0116] It should be noted that, in order to ensure the specified strength, ductility, elongation flange formability, and axial crushing characteristics, the proportion of tempered martensite, lower bainite, retained austenite, and fresh martensite in the remaining ferrite and bainitic ferrite structure is preferably set to a total of more than 60%.

[0117] The average carbon concentration in the retained austenite is above 0.60% by mass.

[0118] By increasing the carbon concentration in the retained austenite, the retained austenite is stabilized, thereby improving ductility. Therefore, to obtain sufficient ductility, the average carbon concentration in the retained austenite is set to 0.60% by mass or more, preferably 0.80% by mass or more.

[0119] There is no particular upper limit, but when the average C concentration in the retained austenite is excessively increased, the retained austenite may not undergo phase transformation during deformation, and the effect of improving ductility cannot be fully obtained. Therefore, the average C concentration in the retained austenite is preferably 2.0% by mass or less, and more preferably 1.5% by mass or less.

[0120] [The content of Fe element existing in tempered martensite in the form of carbides in a region within 100 μm of the steel plate surface along the thickness direction is less than 0.20% by mass on average.]

[0121] Carbides precipitated in tempered martensite become crack initiation points during bending deformation, leading to a decrease in axial crushing characteristics. Furthermore, carbide precipitation is a phenomenon that competes with carbon distribution; carbon is consumed in carbide precipitation, potentially hindering carbon enrichment into untransformed austenite. Therefore, the average content of Fe element present in tempered martensite in carbide form is preferably 0.20% by mass or less, more preferably 0.15% by mass or less. It should be noted that while no specific lower limit is specified, in this invention, the aforementioned Fe element content is substantially 0.001% by mass or more.

[0122] As described above, the steel microstructure of the present invention is a steel microstructure containing ferrite, bainitic ferrite, tempered martensite, lower bainite, retained austenite, and fresh martensite (including 0%). The steel microstructure of the present invention can be a steel microstructure composed of ferrite, bainitic ferrite, tempered martensite, lower bainite, retained austenite, and fresh martensite (including 0%).

[0123] In addition, the steel structure of the present invention may contain less than 5% pearlite and cementite as phases other than those mentioned above, which will not impair the effect of the present invention.

[0124] Next, the method for measuring steel microstructure will be explained.

[0125] The area ratio of the microstructure consisting of one or more of ferrite and bainitic ferrite, the microstructure consisting of one or more of tempered martensite and lower bainite, and the blocky microstructure consisting of fresh martensite and retained austenite described later, MA (martensite austenite constituent), was determined by the following method: a section of the plate thickness perpendicular to the surface of the steel plate and parallel to the rolling direction was cut out, mirror-polished, etched with 3% nitric acid ethanol solution, and observed at 1 / 4 of the plate thickness using SEM at 5000x magnification, with the field of view set to 20μm×20μm, for 10 fields of view.

[0126] Figure 1 An example of a SEM image of the steel microstructure of a steel plate is shown. For example... Figure 1 As shown, ferrite and bainitic ferrite ( Figure 1 In the diagram, reference symbols F and BF represent the darkest regions that appear most equiaxed and almost entirely devoid of carbides under SEM. Tempered martensite ( Figure 1 In the middle (referencing the symbol TM), and lower bainite are regions within the SEM containing lath-like substructures and carbide precipitation. MA (fresh martensite and / or retained austenite) Figure 1 In SEM, reference symbols FM and RA refer to whitish, blocky areas that appear to have no internal subtissues.

[0127] The area ratio and average C concentration in the retained austenite were calculated as follows: The steel plate was ground / sharpened along the thickness direction to 1 / 4 of the plate thickness, and the area ratio and average C concentration in the retained austenite were determined by X-ray diffraction intensity measurement. Using a Mo tube spherical X-ray source, the volume ratio of the retained austenite was calculated based on the intensity ratio of the diffraction intensity of the (200), (220), and (311) planes of austenite relative to the (200) and (211) planes of ferrite. The average C concentration in the retained austenite was calculated as follows: Using a Co tube spherical X-ray source, the lattice constant A of austenite was determined from the peak angle of the (220) plane of austenite, and the average C concentration in the retained austenite was calculated by the following formula (6). It should be noted that Mn%, Si%, and Al% in the following description represent the mass content of each element (Mn, Si, Al) in the steel plate.

[0128] Average C concentration (mass%) in retained austenite = A - {(0.3572 + 0.0012 × Mn% - 0.0157 × Si% + 0.0056 × Al%)} / 0.033 … (Equation 6)

[0129] The area ratio of fresh martensite (FM) is obtained by the following equation (7), which is the value obtained by subtracting the area ratio of retained austenite (RA) from the area ratio of MA (fresh martensite and / or retained austenite).

[0130] It should be noted that the "area ratio" of RA (retained austenite) in this invention can be considered to be equal to the "volume ratio" of retained austenite determined by XRD.

[0131] FM(%) = MA(%) - RA(%) …(Equation 7)

[0132] The average content of Fe element in tempered martensite in the form of carbides within a region of 100 μm from the surface of the steel plate along the thickness direction was determined by residue analysis. A 10% by mass AA electrolyte (acetylacetone-1% by mass tetramethylammonium chloride-methanol) was used, with the current density set at 20 mA / cm².2 Electrolysis was performed for 30 minutes, dissolving up to a 100 μm region from the steel plate surface. The sample was then immersed in methanol, and the residue (undissolved precipitates from the electrolysis) stripped by ultrasonic stirring was filtered through a 0.2 μm filter. The amount of Fe in the collected residue was determined by ICP-luminescence analysis and converted to its mass percentage in the steel plate. It should be noted that the collected residue consisted entirely of precipitates present within a 100 μm region from the steel plate surface. In this invention, carbides (bainite, ferrite, etc.) outside the tempered martensite are present in very small amounts compared to those within the tempered martensite. Furthermore, Fe does not exist in the form of iron compounds other than carbonaceous compounds. Therefore, the average content of Fe present as carbides in the tempered martensite within a 100 μm region from the steel plate surface along the plate thickness direction can be obtained through the above determination.

[0133] The steel plate of the present invention described above can also be a steel plate with a zinc coating on its surface (one side or both sides). The coating can be any one of hot-dip galvanizing or electroplating.

[0134] The thickness of the steel plate of the present invention is preferably set to 0.5 mm or more. Alternatively, the thickness is preferably set to 2.0 mm or less.

[0135] Next, the method for manufacturing the steel plate of the present invention will be described.

[0136] Hereinafter, the case in which the process continues after annealing without the specified dwell process will be described as the first embodiment, and the case in which the process continues after annealing with the specified dwell process will be described as the second embodiment.

[0137] It should be noted that, in this invention, the specific temperature in each process refers to the surface temperature of the slab (steel billet) or steel plate.

[0138] in addition, Figure 2 This diagram illustrates the method for manufacturing the steel sheet according to the present invention, specifically showing the time-varying surface temperature of the slab (steel billet) or steel sheet. The following description includes the time-varying temperature to provide a detailed explanation of each step. Figure 2 (a) Shows the time-varying surface temperature of the slab (steel billet) or steel sheet in the steel sheet manufacturing method of the first embodiment (without a dwell process). Additionally, Figure 2 (b) Shows the time change of the surface temperature of the slab (steel billet) or steel plate in the steel plate manufacturing method of the second embodiment (in the case of a dwell process).

[0139] <First Implementation>

[0140] In the manufacturing method of the steel plate of the first embodiment of the present invention, as well as... Figure 2 As shown in (a), a steel billet having the above-mentioned composition is subjected to hot rolling and cold rolling, and then, for the resulting cold-rolled steel sheet, it is heated to above 750°C and A c3 After annealing at a temperature below a certain temperature (°C), the material is cooled at an average cooling rate of 3°C / s to 100°C / s within a temperature range from the annealing temperature to a temperature T1 above 200°C and below (Ms point (°C) - 30°C). Then, it is heated at an average heating rate of 2°C / s within a temperature range from T1 to a temperature T2 above 300°C and below 450°C. Finally, it is heated from temperature T2 to a temperature range above (T2 - 150°C) and below (T2 - 30°C). Cooling is performed at an average cooling rate of CR2 of 0.5℃ / s or more within the temperature range up to temperature T3. Cooling is performed at an average cooling rate of CR3 of 0.1℃ / s or more within the temperature range from temperature T3 to temperature T4, which is between 150℃ and 350℃ and below temperature T3. The temperature range is maintained for 20s or more and 1000s or less within the temperature range between 150℃ and 350℃ and below temperature T4. Cooling is performed at an average cooling rate of CR4 of 1℃ / s or more to a temperature below 50℃.

[0141] (Hot rolling process)

[0142] Hot rolling can be carried out using conventional methods. For example, it can be carried out by heating a steel billet with the above-specified composition to a heating temperature of 1100°C or higher, setting the soaking time to 20 minutes or more, setting the finishing mill exit temperature to above the Ar3 phase transformation point, and setting the coiling temperature to above 400°C.

[0143] The billet heating temperature can be set below 1300℃. The soaking time can be set below 300 minutes. Additionally, the finishing mill exit temperature can be set below the Ar3 phase transformation point + 200℃. Furthermore, the coiling temperature can be set below 720℃.

[0144] The winding temperature is preferably controlled from the viewpoint of suppressing sheet thickness variation and ensuring stable high strength. Specifically, the winding temperature is preferably set to 430°C or higher. Alternatively, the winding temperature is preferably set to 530°C or lower.

[0145] (Cold rolling process)

[0146] In cold rolling, for example, the rolling rate (cumulative rolling rate) can be set to 30% or more. Alternatively, the rolling rate can be set to 85% or less. It is preferable to control the rolling rate from the viewpoint of consistently ensuring high strength and reducing anisotropy. Specifically, the rolling rate is preferably set to 35% or more. It should be noted that when the rolling load is high, softening annealing treatment can be performed at temperatures above 450°C and below 730°C using a CAL (continuous annealing line) or BAF (box annealing furnace).

[0147] (Annealing process)

[0148] The steel billet with a specified composition is hot-rolled and cold-rolled, and then annealed under the following specified conditions. The annealing equipment is not particularly limited, but from the viewpoint of ensuring productivity and the desired heating and cooling rates, it is preferable to use a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL).

[0149] [Annealing process: Annealing temperature is above 750℃ and A] c3 [Temperature (°C) and below]

[0150] From the perspective of improving ductility, in order to ensure the ferrite and bainitic ferrite with a specified area ratio and tempered martensite, the annealing temperature is set to above 750℃ and A c3 Below a certain temperature (°C). To ensure that the combined area fraction of ferrite and bainitic ferrite is 10% or more, the annealing temperature is preferably set to achieve A. c3 The annealing method is adjusted to be used in the austenite + ferrite dual-phase region below a certain temperature. The preferred annealing temperature is (A). c3 Temperature (°C) below -5°C, more preferably (A) c3 Temperature (°C) below -10°C.

[0151] On the other hand, if the annealing temperature is excessively lowered, an excessive amount of ferrite and bainitic ferrite will be generated, and the required tempered martensite will not be obtained, leading to a decrease in strength and elongation flange properties. Furthermore, below 750°C, sufficient recrystallization may not be possible, thus inheriting hot-rolled microstructures such as carbide residues generated during hot rolling. Therefore, the annealing temperature is set to 750°C or higher, and A... c3 Temperature (°C) or below. The annealing temperature is preferably 755°C or higher, more preferably 760°C or higher.

[0152] It should be noted that A c3 Temperature can be determined by measuring the volume change when the temperature is raised from room temperature (25°C) to the austenitic single-phase region using a cylindrical test piece (3 mm in diameter × 10 mm in height) on a Formastor testing machine.

[0153] [First cooling process: Cooling is performed at an average cooling rate of CR1 of 3°C / s or more and 100°C / s or less, within the range from the annealing temperature to a temperature T1 above 200°C and below the martensitic transformation initiation temperature Ms (°C) - 30°C.]

[0154] After the above annealing treatment, rapid cooling is required to obtain the specified area ratio of tempered martensite. When the average cooling rate CR1 of the temperature range from the annealing temperature to a temperature T1 (cooling stop temperature T1) above 200°C and below the martensitic transformation start temperature Ms(°C) - 30°C is less than 3°C / s, excessive ferrite is generated during cooling, thereby reducing strength, elongation flangeability, and axial crushing characteristics. Therefore, from the viewpoint of controlling the amount of ferrite generated, the average cooling rate CR1 of the temperature range from the annealing temperature to the cooling stop temperature T1 (cooling stop temperature T1) above 200°C and below Ms(°C) - 30°C is set to 3°C / s or more. The average cooling rate CR1 is preferably 5°C / s or more, and more preferably 8°C / s or more.

[0155] On the other hand, if the average cooling rate CR1 becomes too high, the plate shape deteriorates. Therefore, the average cooling rate CR1 is set to 100°C / s or less. The average cooling rate CR1 is preferably 50°C / s or less.

[0156] Furthermore, to ensure the specified amount of retained austenite, the temperature T1 (cooling stop temperature T1) is set to 200°C or higher. Preferably, the temperature T1 (cooling stop temperature T1) is 220°C or higher, more preferably 240°C or higher. When the cooling stop temperature T1 exceeds (Ms point (°C) - 30°C), a large amount of blocky, untransformed austenite remains, increasing the amount of fresh martensite during final cooling and reducing the elongation flangeability. Therefore, the cooling stop temperature T1 is set to (Ms point (°C) - 30°C) or lower. Preferably, the cooling stop temperature T1 is (Ms point (°C) - 35°C) or lower.

[0157] It should be noted that the martensitic transformation initiation temperature Ms (°C) can be calculated as follows: using a Formastor testing machine, and measuring A... c3 The same cylindrical test piece was held at the specified annealing temperature, and the volume change was measured when it was quenched with helium gas. The martensitic transformation initiation temperature Ms (°C) was then determined from this.

[0158] Here, the average cooling rate CR1 is "(annealing temperature (cooling start temperature) (°C) - (Ms point (°C) - 30°C) (cooling stop temperature (°C))) / (cooling time (seconds) from annealing temperature to (Ms point (°C) - 30°C))"

[0159] [Heat treatment process: Heating is carried out from temperature T1 to temperature T2, which is above 300°C and below 450°C, at an average heating rate of 2°C / s or more.]

[0160] By heating for a short time within a temperature range from the aforementioned temperature T1 (cooling stop temperature T1: above 200°C and below (martensitic transformation start temperature Ms point (°C) - 30°C)) to a temperature T2 above 300°C and below 450°C, carbide precipitation can be suppressed, while carbon can be enriched from the surrounding microstructure into untransformed austenite, thereby ensuring high ductility. When the temperature T2 is below 300°C, the enrichment of carbon into untransformed austenite is not sufficient, and the tempering of martensite is not sufficient, resulting in excessive strength increase of tempered martensite, which is disadvantageous from the viewpoint of improving ductility and extension flange properties.

[0161] On the other hand, when the heated temperature T2 exceeds 450°C, the untransformed austenite decomposes, and the amount of blocky fresh martensite increases. This reduces ductility and elongation flangeability, and excessive carbide precipitation worsens axial crushing characteristics. Therefore, the temperature T2 is set to 300°C or higher and 450°C or lower. Preferably, the temperature T2 is 330°C or higher, more preferably 350°C or higher. Preferably, the temperature T2 is 420°C or lower, more preferably 400°C or lower.

[0162] Furthermore, when the average heating rate from temperature T1 to temperature T2 (above 300°C and below 450°C) is slow, the axial crushing characteristics may deteriorate not only due to increased carbide precipitation, but also because the enrichment of carbon in the retained austenite decreases with the increase of carbides, leading to a decrease in ductility. Therefore, the average heating rate for the temperature range from temperature T1 (cooling stop temperature T1) to temperature T2 (above 300°C and below 450°C) is set to 2°C / s or more. From the viewpoint of suppressing carbide precipitation, the average heating rate is preferably set to 5°C / s or more, and more preferably 10°C / s or more. The upper limit of the above-mentioned average heating rate is not particularly limited, but it is preferably 50°C / s or less, and more preferably 30°C / s or less.

[0163] Here, the average heating rate is defined as “temperature T2 (heating stop temperature) (°C) - temperature T1 (heating start temperature) (°C)) / (heating time from temperature T1 to temperature T2 (seconds))”.

[0164] [Second cooling process: Cooling is performed at an average cooling rate of CR2 of 0.5℃ / s or higher within the temperature range from T2 to T3 (T2-150℃ or higher and T2-30℃ or lower).]

[0165] [Third cooling process: Cooling is performed at an average cooling rate of CR3 of 0.1℃ / s or higher within a temperature range from T3 to T4, which is above 150℃ and below 350℃.]

[0166] In the over-aging process after cooling stops (from the above-mentioned heating process to the holding process after the third cooling process), the fresh martensite is transformed into martensite by tempering. With this phase transformation, carbides are precipitated and carbon is distributed to the untransformed austenite.

[0167] The precipitation rate of carbides tends to be delayed as the heating temperature decreases after cooling stops. Therefore, by rapidly cooling to a temperature range that can suppress carbide precipitation at a certain cooling rate, the precipitation of carbides can be suppressed and the axial crushing characteristics can be improved.

[0168] On the other hand, as the heating temperature decreases after cooling stops, the enrichment of carbon into untransformed austenite and the tempering of fresh martensite are hindered, which may lead to excessive strength increase, ductility and deterioration of extended flange properties.

[0169] Based on the above, in order to suppress the precipitation of carbides by cooling to a specified temperature range and promote the enrichment of carbon into untransformed austenite and the tempering of fresh martensite, thereby simultaneously obtaining excellent axial crushing characteristics, ductility and extended flange properties, a second cooling process under specific conditions is performed between the heat treatment process and the third cooling process.

[0170] Specifically, to achieve the above-mentioned effect, the temperature T3 (cooling stop temperature T3) in the second cooling process is set to be above (T2-150°C) and below (T2-30°C). Preferably, temperature T3 is above (T2-120°C), more preferably above (T2-100°C). Preferably, temperature T3 is below (T2-50°C), more preferably below (T2-70°C).

[0171] Furthermore, the average cooling rate CR2 is set to 0.5°C / s or higher, preferably 0.6°C / s or higher, and more preferably 0.7°C / s. No specific upper limit is specified, but as the average cooling rate increases, the carbon distribution to the retained austenite and the tempering of the fresh martensite are delayed, tending to reduce ductility and extension flange properties. Therefore, the average cooling rate CR2 is preferably 5°C / s or lower.

[0172] Here, the average cooling rate CR2 is defined as "(temperature T2 (°C) (cooling start temperature) - temperature T3 (°C) (cooling stop temperature)) / (cooling time from temperature T2 to temperature T3 (seconds))".

[0173] As a second cooling treatment, after cooling to temperature T3, a third cooling treatment is performed to further cool while suppressing carbide precipitation, promoting carbon distribution, and increasing the carbon concentration and amount of retained austenite. At this point, as the cooling temperature decreases, the carbon distribution rate decreases, and it is possible that carbon may not be sufficiently transferred into the untransformed austenite.

[0174] On the other hand, as the cooling temperature increases and the average cooling rate decreases, carbides precipitate during cooling, leading to a deterioration in axial crushing characteristics. Therefore, temperature T4 is set to be 150°C or higher and 350°C or lower. Temperature T4 is preferably 180°C or higher, more preferably 200°C or higher. Furthermore, temperature T4 is preferably 300°C or lower, more preferably 250°C or lower.

[0175] Furthermore, the average cooling rate CR3 is set to 0.1℃ / s or more, preferably 0.2℃ / s or more, and more preferably 0.3℃ / s or more. No specific upper limit is specified, but from the viewpoint of sufficiently promoting carbon distribution to untransformed austenite, the average cooling rate CR3 is preferably CR2 or less and 1℃ / s or less.

[0176] Here, the average cooling rate CR3 is defined as "(temperature T3 (°C) (cooling start temperature) - temperature T4 (°C) (cooling stop temperature)) / (cooling time from temperature T3 to temperature T4 (seconds))".

[0177] [Holding process: Hold at a temperature range of 150°C to 350°C and below temperature T4 for 20 seconds to 1000 seconds]

[0178] Keep the temperature set below the above temperature T4.

[0179] When the temperature is kept below 150°C, the tempering of fresh martensite is not fully carried out, resulting in excessive strength increase and reduced enrichment of carbon into untransformed austenite. Therefore, it is disadvantageous from the point of view of improving ductility and extended flange properties.

[0180] On the other hand, when the temperature is kept above 350°C, the amount of carbide precipitation increases, the axial crushing characteristics deteriorate, and the precipitation of carbides hinders the enrichment of carbon into untransformed austenite, resulting in a reduction of residual austenite and a decrease in ductility.

[0181] Based on the above, the holding temperature is set to 150°C or higher and 350°C or lower, and is set to below temperature T4. The holding temperature is preferably 180°C or higher, more preferably 200°C or higher. The holding temperature is preferably 300°C or lower, more preferably 250°C or lower.

[0182] Furthermore, extending the holding time increases the enrichment of carbon into untransformed austenite, stabilizes the retained austenite, and improves ductility. On the other hand, shortening the holding time tends to reduce the amount of carbide precipitation, thus improving axial crushing characteristics. Based on the above, the holding time is set to be 20 s or more and 1000 s or less. The holding time is preferably 100 s or more, more preferably 200 s or more. The holding time is preferably 700 s or less, more preferably 500 s or less.

[0183] Furthermore, after the first cooling treatment step (after cooling at an average cooling rate CR1 of 3°C / s or higher and 100°C / s or lower), that is, before or after any step from the heating treatment step from the aforementioned temperature T1 to temperature T2 to the holding treatment step of maintaining a temperature of 150°C or higher and 350°C or lower, or during any step, hot-dip galvanizing or further alloying treatment can be performed to form a zinc coating on the steel plate surface. In this case, it is preferable to immerse the steel plate in a galvanizing bath at a temperature of 440°C or higher and 500°C or lower for hot-dip galvanizing, and then adjust the coating adhesion by means of gas wiping or the like. Hot-dip galvanizing preferably uses a galvanizing bath with an Al content of 0.10% by mass or higher and 0.22% by mass or lower. In addition, after the hot-dip galvanizing treatment, an alloying treatment of the zinc coating can be performed. When performing the alloying treatment of the zinc coating, it is preferable to perform it in a temperature range of 470°C or higher and 590°C or lower, and heating to this temperature range will not impair the effects of the present invention, so it can be performed.

[0184] [Fourth cooling process: Cool CR4 to below 50°C at an average cooling rate of 1°C / s or higher]

[0185] Then, from the viewpoint of preventing softening caused by excessive tempering and reduced ductility caused by carbide precipitation, the steel sheet is cooled from the holding end temperature (150°C or higher and 350°C or lower, and temperature T4 or lower) in the aforementioned holding process to a temperature of 50°C or lower at an average cooling rate of 1°C / s or higher. From the viewpoint of stabilizing stamping formability by adjusting surface roughness and flattening the sheet shape, and from the viewpoint of improving yield strength, the steel sheet can be surface-rolled. The surface-rolling elongation is preferably set to 0.1% or higher and 0.5% or lower. In addition, the sheet shape can also be flattened using a leveling machine.

[0186] The average cooling rate CR4 from the aforementioned end temperature to a temperature below 50°C is 1°C / s or more, preferably 5°C / s or more. In this invention, the upper limit of the average cooling rate CR4 is not specified, but from a manufacturing point of view and from the point of view of suppressing the phase transformation from retained austenite to fresh martensite, it is preferably 10°C / s or less.

[0187] Here, the average cooling rate CR4 is defined as "(temperature below 150°C and 350°C and below T4 (°C) (holding end temperature (cooling start temperature)) - 50°C (cooling stop temperature)) / (cooling time (seconds) from the holding end temperature to the cooling stop temperature))".

[0188] From the viewpoint of improving the formability of the extended flange, a low-temperature heat treatment can also be performed at 100–300°C for 30 seconds to 10 days after the above-mentioned heat treatment or after surface rolling. This treatment allows hydrogen that has penetrated the steel sheet during tempering or annealing of the martensite generated during final cooling or surface rolling to escape from the steel sheet. Through low-temperature heat treatment, hydrogen can be reduced to less than 0.1 ppm by mass.

[0189] Alternatively, electroplating can be performed. That is, after the fourth cooling process (cooling at an average cooling rate of CR4 of 1°C or higher), the steel sheet can be electroplated with zinc. After electroplating, from the viewpoint of reducing hydrogen in the steel, the aforementioned low-temperature heat treatment is preferred.

[0190] <Second Implementation>

[0191] In the manufacturing method of the steel plate of the second embodiment of the present invention, as well as... Figure 2 As shown in (b), a steel billet having the above-mentioned composition is subjected to hot rolling and cold rolling, and then, for the resulting cold-rolled steel sheet, it is heated to above 750°C and A c3After annealing at a temperature below a certain temperature (°C), cooling is performed at an average cooling rate of CR5 (between 5°C and 100°C / s) within a temperature range from the annealing temperature to 500°C (first cooling treatment before dwell treatment). Then, cooling is performed at an average cooling rate of CR6 (between 500°C and Ms (°C)) for 10 to 60 seconds (dwell treatment) within a temperature range from 500°C to the dwell stopping temperature T5 (between Ms (°C)). Finally, cooling is performed at an average cooling rate of CR7 (between 3°C and 100°C / s) within a temperature range from the dwell stopping temperature T5 to a temperature above 200°C and below (Ms point (°C) - 30°C) (first cooling treatment after dwell treatment). Finally, cooling is performed at an average cooling rate of CR7 (between temperature T5 and Ms (°C) - 30°C) within a temperature range from T1 to... Heating is performed at an average heating rate of 2°C / s or more within a temperature range of 300°C to 450°C up to T2. Cooling is performed at an average cooling rate of 0.5°C / s or more within a temperature range of 0.5°C / s or more within a temperature range of 0.5°C / s or more within a temperature range of 0.1°C / s or more within a temperature range of 0.1°C / s or more within a temperature range of 150°C to 350°C up to T4 down to a temperature range of 1°C to 350°C up to T4 down to a temperature range of 1°C to 350°C up to T4 down to a temperature range of 1°C to 350°C up to T4 down to a temperature range of 1°C to 350°C up to T2 down to a temperature range of 1°C / s or more within a temperature range of 10°C to 350°C down to T4 down to a temperature range of 10°C to 350°C up to T2 down to a temperature range of 10°C to T4 ...

[0192] In the second embodiment, hot rolling, cold rolling, and annealing can be performed under the same conditions as in the first embodiment.

[0193] In addition, in the second embodiment, the processing in the first cooling process of the first embodiment is set as a first cooling process before the dwell process, a dwell process, and a first cooling process after the dwell process.

[0194] In addition, in the second embodiment, the heating treatment, second cooling treatment, third cooling treatment, holding treatment, and fourth cooling treatment after the first cooling treatment following the dwell treatment can be performed sequentially under the same conditions as the heating treatment, second cooling treatment, third cooling treatment, holding treatment, and fourth cooling treatment in the first embodiment.

[0195] Furthermore, regarding the hot-dip galvanizing process in the second embodiment, it is set to be performed after the first cooling process following the dwell process (after cooling at an average cooling rate of CR7 of 3°C / s or more and 100°C / s or less), instead of after the first cooling process following the first cooling process in the first embodiment (after cooling at an average cooling rate of CR1 of 3°C / s or more and 100°C / s or less). Apart from this, it can be performed under the same conditions as the first embodiment.

[0196] Furthermore, in the second embodiment, other conditions, including the low-temperature heat treatment after all heat treatments and the electroplating treatment, can also be performed under the same conditions as in the first embodiment.

[0197] In this embodiment, the first cooling process before the dwell process, the dwell process, and the first cooling process after the dwell process will be described in detail below.

[0198] [First cooling process before dwell treatment: Cooling is performed at an average cooling rate of CR5 of 5°C / s or higher and 100°C / s or lower within a temperature range from annealing temperature to 500°C.]

[0199] [Dwelling process: Within the temperature range from 500°C to the dwell stopping temperature T5 above Ms (°C) and above 320°C, the dwell time is CR6 for 10 seconds to 60 seconds at an average cooling rate of less than 10°C / s]

[0200] By performing a dwell treatment during cooling, more bainitic ferrite can be obtained by area ratio, carbon is enriched in untransformed austenite, and the residual austenite is stabilized, resulting in steel sheets with better ductility. Therefore, from the viewpoint of maximizing ductility, it is preferable to perform a dwell treatment during cooling.

[0201] When a cooling process is performed with a dwell time, the material is first cooled at an average cooling rate CR5 of 5°C / s or more but less than 100°C / s within a temperature range from the annealing temperature to 500°C. When the average cooling rate CR5 is less than 5°C / s, a large amount of ferrite is formed, leading to a decrease in strength and a decrease in λ. The average cooling rate CR5 is preferably 8°C / s or more.

[0202] On the other hand, when the average cooling rate CR5 is too high, the plate shape deteriorates. Therefore, the average cooling rate CR5 is 100°C / s or less, preferably 50°C / s or less, and more preferably less than 30°C / s.

[0203] Here, the average cooling rate CR5 is defined as "(annealing temperature (cooling start temperature) (°C) - 500°C (cooling stop temperature)) / cooling time (seconds) from annealing temperature to 500°C".

[0204] Next, by cooling at an average rate of CR6 of 10°C or less for 10 to 60 seconds within a temperature range from 500°C to a holding temperature T5 above the martensitic transformation initiation temperature Ms (°C) and above 320°C, the bainitic transformation can be promoted, resulting in the formation of high-C-concentration retained austenite adjacent to the bainitic ferrite. Below the Ms (°C), martensite forms first, and the bainitic transformation is excessive due to the swing-back phenomenon, leading to a decrease in strength and elongation flanger properties.

[0205] On the other hand, when the temperature range exceeds 500°C, the driving force of bainitic phase transformation decreases and the amount of bainitic phase transformation decreases.

[0206] Based on the above, the temperature range is set to be above Ms (°C) and above 320°C and below 500°C. Preferably, the temperature range is above Ms (°C) + 15°C, more preferably above Ms (°C) + 30°C. Preferably, the temperature range is above 350°C, more preferably above 380°C. Furthermore, the temperature range is preferably below 480°C, more preferably below 460°C.

[0207] When the average cooling rate CR6 exceeds 10°C / s, the amount of bainite phase change decreases. Therefore, the average cooling rate CR6 is set to 10°C / s or less. Furthermore, when the residence time is less than 10s, the desired amount of bainitic ferrite is not obtained; when it exceeds 60s, carbon enriches from bainitic ferrite to bulky, untransformed austenite, leading to an increase in the amount of residual bulky structure. Therefore, the residence time is set to 10s or more and 60s or less. From the viewpoint of ensuring both bainitic ferrite and retained austenite, a residence time of 20s or more is preferred. Additionally, from the viewpoint of improving the elongation flangeability by reducing bulky structure, a residence time of 50s or less is preferred.

[0208] Here, the average cooling rate CR6 is "(500℃ (dwell start temperature) (℃) - (dwell stop temperature (℃) above Ms point (℃) and above 320℃))) / (dwell time (seconds) from dwell start temperature to dwell stop temperature))".

[0209] [First cooling process after dwell time: Cooling is carried out at an average cooling rate of CR7 of 3°C / s or more and 100°C / s or less, within the range from the dwell temperature T5 to a temperature T1 above 200°C and below the martensitic transformation start temperature Ms (°C) - 30°C.]

[0210] After the aforementioned dwell treatment, rapid cooling is required to obtain the specified area ratio of tempered martensite and / or bainite. When the average cooling rate CR7 is less than 3°C / s in the temperature range from the dwell stop temperature T5 to a temperature T1 (cooling stop temperature T1) above 200°C but below the martensitic transformation initiation temperature Ms (°C) - 30°C, ferrite forms during cooling, thereby reducing strength, elongation flangeability, and axial crushing characteristics. Therefore, from the viewpoint of suppressing ferrite formation, the average cooling rate CR7 in the temperature range from the dwell stop temperature T5 to the cooling stop temperature T1 (cooling stop temperature T1) above 200°C but below Ms (°C) - 30°C is set to 3°C / s or more. The average cooling rate CR7 is preferably 5°C / s or more, and more preferably 8°C / s or more.

[0211] On the other hand, if the average cooling rate CR7 is too high, the plate shape deteriorates. Therefore, the average cooling rate CR7 is set to 100°C / s or less. The average cooling rate CR7 is preferably 50°C / s or less.

[0212] Furthermore, to ensure the specified amount of retained austenite, the temperature T1 (cooling stop temperature T1) is set to 200°C or higher. Preferably, the temperature T1 (cooling stop temperature T1) is 220°C or higher, more preferably 240°C or higher. When the cooling stop temperature T1 exceeds (Ms point (°C) - 30°C), a large amount of blocky, untransformed austenite remains, increasing the amount of fresh martensite during final cooling and reducing the elongation flangeability. Therefore, the cooling stop temperature T1 is set to (Ms point (°C) - 30°C) or lower. Preferably, the cooling stop temperature T1 is (Ms point (°C) - 35°C) or lower.

[0213] Here, the average cooling rate CR7 is "(stopping temperature T5 (cooling start temperature) (°C) - (Ms point (°C) - 30°C (cooling stop temperature (°C))) / (cooling time from thestopping temperature T5 to the cooling stop temperature (seconds))".

[0214] Next, the components of the present invention and their manufacturing method will be described.

[0215] The component of the present invention is formed by performing at least one of forming and joining processes on the steel plate of the present invention. Furthermore, the manufacturing method of the component of the present invention includes a step of forming and joining processes on the steel plate of the present invention to form the component.

[0216] The steel sheet of the present invention has a tensile strength of 780 MPa or higher and exhibits high ductility, excellent extension flange formability, and excellent axial crushing characteristics. Therefore, components obtained using the steel sheet of the present invention also possess high strength and, compared to conventional high-strength components, exhibit high ductility, excellent extension flange formability, and excellent axial crushing characteristics. Furthermore, lightweighting can be achieved by using components of the present invention. Therefore, components of the present invention can be suitable for applications such as vehicle body frame components. The components of the present invention also include welded joints.

[0217] Forming processes can utilize conventional processing methods such as stamping without limitation. Furthermore, joining processes can utilize conventional welding methods such as spot welding, arc welding, riveting, and rivet joining without limitation.

[0218] Example

[0219] Cold-rolled steel sheets with a thickness of 1.4 mm having the composition shown in Table 1 were processed under the annealing conditions shown in Tables 2 and 3 to manufacture the steel sheets of the present invention and the comparative examples.

[0220] It should be noted that cold-rolled steel sheets are obtained by hot rolling (heating temperature: 1250℃, soaking time: 60 minutes, finishing mill exit temperature: 1150℃, coiling temperature: 550℃) and cold rolling (rolling rate (cumulative reduction rate): 50%) of steel billets with the composition shown in Table 1.

[0221] Table 2 shows the conditions under which no dwell treatment is performed, and Table 3 shows the conditions under which dwell treatment is performed. It should be noted that some steel sheets (cold-rolled steel sheets: CR) undergo hot-dip galvanizing treatment in any process from temperature T1 to holding at a temperature below 150°C and below 350°C after heating, to produce hot-dip galvanized steel sheets (GI).

[0222] Here, steel sheets are hot-dip galvanized by immersing them in a galvanizing bath at a temperature of 440°C to 500°C, and then the coating adhesion is adjusted by methods such as gas wiping. The galvanizing bath uses an Al content of 0.10% to 0.22%. Furthermore, a portion of the hot-dip galvanized steel sheets are alloyed after the above hot-dip galvanizing treatment to produce alloyed hot-dip galvanized steel sheets (GA). Here, alloying is performed at a temperature range of 460°C to 590°C. Additionally, a portion of the steel sheets (cold-rolled steel sheets: CR) are electroplated to produce electro-galvanized steel sheets (EG).

[0223]

[0224]

[0225]

[0226] The steel microstructure was determined using the method described above. The results are shown in Tables 4 and 5. Table 4 shows the conditions under which no dwell treatment was performed, and Table 5 shows the conditions under which dwell treatment was performed.

[0227] (Tensile strength, ductility)

[0228] JIS 5 tensile test pieces and expanded hole test pieces were cut from the obtained steel plate and tensile tests were performed (according to JIS Z2241 (2011)). The tensile strength TS and uniform elongation U-El ​​are shown in Tables 4 and 5.

[0229] A tensile strength of 780 MPa or higher is considered to be of excellent strength.

[0230] In addition, the following conditions are considered to have excellent ductility: the uniform elongation U-El ​​is 16.0% or more when TS is less than 980 MPa, 11.0% or more when TS is 980 MPa or more and less than 1180 MPa, and 8.0% or more when TS is 1180 MPa or more.

[0231] (Extended flange)

[0232] The formability of the extended flange was evaluated using a hole-expanding test according to the Japanese Iron and Steel Federation standard JFST1001. Specifically, a 100mm x 100mm square sample was punched using a punching tool with a 10mm punch diameter and a 10.3mm die diameter (13% clearance). Then, a conical punch with a 60-degree apex angle was used to expand the hole until a crack penetrating the plate thickness was formed, with the burrs generated during hole formation on the outer side. Let d0 be the initial hole diameter (mm) and d be the hole diameter at crack initiation (mm). The expansion rate λ (%) was calculated as {(d-d0) / d0} × 100. The expansion rate λ is shown in Tables 4 and 5.

[0233] Steel plates with a λ of 30% or more are considered to have excellent elongation flange properties.

[0234] (Axial crushing characteristics)

[0235] Figure 3 This is a diagram used to illustrate the calculation method for the VDA bending angle.

[0236] like Figure 3 As shown, the VDA bending angle α, as an evaluation of axial crushing characteristics, is evaluated through a bending test (Verband der Automobilindustrie: VDA bending test) according to the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry. The VDA bending test, defined by the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry, is a three-point plate bending test characterized by very narrowly spaced rollers 10 and sharp punches 11. The VDA bending test is conducted as follows: A 60mm×60mm square test piece is used, with the bending ridge direction parallel to the rolling direction. It is supported by a roll with a roll diameter D of 30mm and a roll distance L (not shown) of (plate thickness a0×2)+0.5mm. A punch with a top radius of curvature r of 0.4mm is pressed in from above at a stroke speed of 20mm / min to conduct the VDA bending test (in the figure, symbol 11A represents the punch before pressing in, and symbol 11B represents the punch after pressing in).

[0237] It should be noted that, Figure 3 For ease of explanation, only the test piece (steel plate X) that can be symmetrically plotted with the pressing axis as the center line is shown in the illustration. A X B ), one side of roller 10.

[0238] VDA bending angle α refers to the bending angle (°) calculated using formulas (1) to (5) based on the stroke S (mm) under the maximum load in the bending test mentioned above. It can be used as an indicator of axial crushing characteristics.

[0239] Steel plates with an α value of 70° or higher are considered to have excellent axial crushing characteristics.

[0240] c(mm) = D / 2 + r + a0 …(Equation 1)

[0241] p(mm) = D / 2 + L / 2 …(Equation 2)

[0242]

[0243]

[0244]

[0245]

[0246]

[0247] The examples of the present invention shown in Tables 4 and 5 satisfy the following conditions: U-El ​​is 16.0% or more when TS is 780 MPa or more and less than 980 MPa; U-El ​​is 11.0% or more when TS is 980 MPa or more and less than 1180 MPa; U-El ​​is 8.0% or more when TS is 1180 MPa or more; and λ is 30% or more, or even α is 70° or more. They have excellent strength, ductility, elongation flange and axial crushing characteristics. In contrast, the comparative examples are worse in one of these aspects.

[0248] Furthermore, it is understood that since the steel plate of the present invention has high strength, high ductility, excellent extension flange forming ability and excellent axial crushing characteristics, the components obtained by forming the steel plate of the present invention, the components obtained by joining the steel plate, and the components obtained by forming and joining the steel plate are also high in strength, high in ductility, excellent extension flange forming ability and excellent axial crushing characteristics, just like the steel plate of the present invention.

[0249] Industrial availability

[0250] The present invention can manufacture steel sheets with excellent ductility, elongation flange properties and axial crushing characteristics for use in automobiles and home appliances, especially suitable for energy absorption components in automobiles, and can be preferably applied to stamping applications of these components.

[0251] Symbol Explanation

[0252] F Ferrite

[0253] BF bainitic ferrite

[0254] TM tempered martensite

[0255] FM Fresh Martensite

[0256] RA Residual Austenite

[0257] α VDA bending angle

[0258] 10 rollers

[0259] D roller diameter

[0260] 11 Punch

[0261] 11A Punch before pressing

[0262] 11B Punch after pressing

[0263] S stroke

[0264] The radius of curvature at the tip of r

[0265] X A steel plate before pressing

[0266] X B Pressed steel plate

[0267] a0 plate thickness

Claims

1. A steel plate comprising, by mass%, C: 0.10% or more and 0.30% or less, Si: 0.5% or more and 2.0% or less, Mn: 1.5% or more and 3.0% or less, P: 0.10% or less, S: 0.020% or less, sol.Al: 1.00% or less, N: 0.015% or less, with the balance being Fe and unavoidable impurities. And it has a steel structure containing, by area ratio, 10% to 60% of ferrite and bainitic ferrite, 20% to 80% of tempered martensite and lower bainite, 5% or more of retained austenite, and 10% or less of fresh martensite. The average C concentration in the retained austenite is above 0.60% by mass. In the region within 100 μm of the steel plate surface along the thickness direction, the average content of Fe element existing in tempered martensite in the form of carbides is less than 0.20% by mass.

2. The steel plate according to claim 1, wherein, The composition further contains at least one element selected from the following (by mass%): Ti: 0.100% or less, Nb: 0.100% or less, V: 0.100% or less, B: 0.0050% or less, Cr: 1.000% or less, Cu: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less.

3. The steel plate according to claim 1 or 2, wherein, The steel plate has a zinc coating.

4. A component made of steel plate according to any one of claims 1 to 3.

5. A method for manufacturing a steel plate, wherein, A steel billet having the composition described in claim 1 or 2 is subjected to hot rolling and cold rolling, and then, for the resulting cold-rolled steel sheet, Above 750℃ and A c3 After annealing at an annealing temperature below a certain temperature (°C), Cooling is performed at an average cooling rate CR1 of 3°C / s or more and 100°C / s or less, from the annealing temperature up to a temperature T1 above 200°C and below (Ms point (°C) - 30°C). Heating is performed at an average heating rate of 2°C / s or more within a temperature range from the stated temperature T1 to a temperature T2 that is above 300°C and below 450°C. Cooling is performed at an average cooling rate of CR2 of 0.5°C / s or higher within a temperature range from the stated temperature T2 to a temperature T3 that is above (T2-150°C) and below (T2-30°C). Cooling is performed at an average cooling rate of CR3 or higher within a temperature range from the temperature T3 to a temperature T4 that is above 150°C and below 350°C and below the temperature T3. Maintain for more than 20 seconds and less than 1000 seconds within a temperature range of 150°C to 350°C and below the temperature T4. CR4 is cooled to a temperature below 50°C at an average cooling rate of 1°C / s or higher.

6. A method for manufacturing a steel plate, wherein, A steel billet having the composition described in claim 1 or 2 is subjected to hot rolling and cold rolling, and then, for the resulting cold-rolled steel sheet, Above 750℃ and A c3 After annealing at an annealing temperature below a certain temperature (°C), Cooling is performed at an average cooling rate CR5 of 5°C / s or higher and 100°C / s or lower within a temperature range from the annealing temperature to 500°C. Within a temperature range from 500℃ to the dwell temperature T5 (above Ms point (℃) and above 320℃), the dwell time is 10s to 60s with an average cooling rate of less than 10℃ / s. Cooling is performed at an average cooling rate of CR7 of 3°C / s or more and 100°C / s or less within a temperature range from the stopping temperature T5 to a temperature T1 that is above 200°C and below (Ms point (°C) - 30°C). Heating is performed at an average heating rate of 2°C / s or more within a temperature range from the stated temperature T1 to a temperature T2 that is above 300°C and below 450°C. Cooling is performed at an average cooling rate of CR2 of 0.5°C / s or higher within a temperature range from the stated temperature T2 to a temperature T3 that is above (T2-150°C) and below (T2-30°C). Cooling is performed at an average cooling rate of CR3 or higher within a temperature range from the temperature T3 to a temperature T4 that is above 150°C and below 350°C and below the temperature T3. Maintain for more than 20 seconds and less than 1000 seconds within a temperature range of 150°C to 350°C and below the temperature T4. CR4 is cooled to a temperature below 50°C at an average cooling rate of 1°C / s or higher.

7. The method for manufacturing a steel plate according to claim 5, wherein, After cooling at an average cooling rate CR1 of 3°C / s or higher and 100°C / s or lower, the steel plate is subjected to hot-dip galvanizing or alloyed hot-dip galvanizing.

8. The method for manufacturing a steel plate according to claim 6, wherein, After cooling at an average cooling rate CR7 of 3°C / s or higher and 100°C / s or lower, the steel plate is subjected to hot-dip galvanizing or alloyed hot-dip galvanizing.

9. The method for manufacturing a steel plate according to claim 5 or 6, wherein, After cooling at an average cooling rate of CR4 of 1°C or higher, the steel sheet is electro-galvanized.

10. A method for manufacturing a component, comprising a step of forming a component by performing at least one of forming or joining processes on a steel plate as described in any one of claims 1 to 3.

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