Steel sheet, method for producing same, and component

Abstract

This steel sheet has a specific chemical composition. In a part at the position of 1 / 4 the sheet thickness, the retained austenite is 3-20% and the martensite is 30-75%. The number of retained austenite having an aspect ratio of 3.0 or more in the part at the position of 1 / 4 the sheet thickness is 80% or more of the total number of retained austenite in the part at the position of 1 / 4 the sheet thickness. The average major axis length of retained austenite having an aspect ratio of 3.0 or more in the part at the position of 1 / 4 the sheet thickness is less than 15 µm. The diameters of inscribed circles in all regions consisting of martensite and retained austenite in the part at the position of 1 / 4 the sheet thickness and in a part at the position of 1 / 8 the sheet thickness are less than 2.0 µm and 1.8 µm or less, respectively. The ratio of the average major axis length Ls of retained austenite in the part at the position of 1 / 8 the sheet thickness to the average major axis length Lc of retained austenite in the central part in the sheet thickness direction is 0.93 or less.

Description

Steel plates and methods for manufacturing the same, as well as parts

[0001] This disclosure relates to steel sheets, methods for manufacturing the same, and components including the steel sheets. This application claims priority under Japanese Patent Application No. 2025-006257, filed in Japan on January 16, 2025, which is incorporated herein by reference.

[0002] High-strength steel plates are used as automotive steel sheets to reduce carbon dioxide emissions by making cars lighter and improving fuel efficiency, while also ensuring the safety of passengers.

[0003] In recent years, in order to ensure sufficient corrosion resistance of vehicle bodies and parts, high-strength alloyed hot-dip galvanized steel sheets have been used in addition to high-strength hot-dip galvanized steel sheets (see, for example, Patent Document 1).

[0004] High-strength steel sheets for automobiles require not only strength but also formability necessary for forming parts. Strength and formability are in a trade-off relationship. On the other hand, TRIP (Transformation-Induced Plasticity) steel sheets are known as a means of achieving both of these properties, utilizing the transformation-induced plasticity of retained austenite. TRIP steel sheets are steel sheets that have relatively high C, Si, and Mn content compared to ordinary high-strength steel sheets, and also contain retained austenite, resulting in excellent energy absorption capacity and formability.

[0005] From the perspective of formability, uniform elongation is an important indicator. Increasing the retained austenite fraction in the metal structure of steel sheets contributes to improving uniform elongation. A commonly known technique for increasing the retained austenite fraction is to increase the Si content in the steel.

[0006] Furthermore, in addition to the retained austenite fraction, there are also known techniques to improve the stability of retained austenite and enhance uniform elongation by increasing the aspect ratio of retained austenite to control its morphology so that it becomes so-called acicular austenite (acicular γ) (see, for example, Patent Document 2).

[0007] On the other hand, a high Si content in steel can reduce its liquid metal embrittlement (LME) resistance. To address this issue, a technology is known that enables the creation of TRIP steel sheets with excellent LME resistance while reducing the Si content of the steel (see, for example, Patent Document 3).

[0008] Here, liquid metal embrittlement (LME) refers to the embrittlement phenomenon of the weld that occurs during spot welding. For example, when spot welding galvanized steel sheets (hot-dip galvanized steel sheets, electro-galvanized steel sheets, or alloyed hot-dip galvanized steel sheets) together, cracks called metal embrittlement cracks (LME cracks) may occur in the weld. These LME cracks occur when the heat generated during spot welding melts the zinc in the galvanized layer, the molten zinc penetrates the grain boundaries of the steel sheet structure at the weld, and tensile stress acts on the weld in that state. LME cracks can occur even if one of the steel sheets to be welded is a cold-rolled steel sheet without galvanizing, as long as the other is a galvanized steel sheet. Furthermore, LME cracks are particularly noticeable when spot welding TRIP steel sheets.

[0009] International Publication No. 2018 / 043453, International Publication No. 2022 / 019209, International Publication No. 2022 / 149502

[0010] Automotive steel sheets require not only the strength, formability, and LME resistance properties mentioned above, but also properties that suppress deformation and fracture during collisions (hereinafter also referred to as collision properties).

[0011] After automotive steel sheets are formed into parts, these parts, which are under pre-strain, may undergo bending deformation due to collisions or other events. From the perspective of ensuring passenger safety, it is important to suppress the fracture of parts that have undergone bending deformation.

[0012] In the case of automotive steel sheets, if a coarse, hard phase (martensite) is present in the base material structure, strain concentrates at the interface between this hard phase and soft phases such as ferrite, pearlite, and bainite. Crack initiation is promoted at the strain-concentrated areas, resulting in a lower bending strain at which fracture occurs. In other words, crack initiation is more likely to occur at strain-concentrated areas than at other areas. Therefore, in order to suppress fracture during bending deformation of formed parts, it is desirable to suppress the formation of coarse martensite in the steel sheet material and the formed parts.

[0013] Thus, in order to improve the impact resistance of automotive steel sheets, suppressing coarse martensite during annealing in the steel sheet manufacturing process and during part forming after steel sheet manufacturing is effective. However, in recent years, as mentioned above, there has been a trend to reduce the Si content in steel in order to improve LME resistance. In such low-Si steel, the two-phase region temperature (Ac1 point) becomes higher, and the maximum heating temperature during annealing required to obtain the desired metallic structure also becomes higher. When the maximum heating temperature during annealing is high, coarse retained austenite and coarse martensite tend to form after annealing. In particular, in the case of TRIP steel having acicular γ, acicular γ precipitates while being held in the α / γ two-phase region temperature during the annealing process, but if the maximum heating temperature during annealing is high, austenites connect with each other at the triple point of the large-angle grain boundary, and coarse massive austenite (massive γ) is generated. Such massive γ is prone to transforming into coarse martensite when subjected to strain during forming. Therefore, the proportion of coarse martensite in the metal structure of the part after forming increases. As a result, fracture due to bending deformation during a car collision is more likely to occur at an earlier stage, and good collision resistance cannot be obtained. In other words, to improve the collision resistance of automotive steel sheets, it is necessary to suppress fracture due to bending deformation after pre-straining (i.e., after part forming).

[0014] As described above, conventionally, no steel sheet had been disclosed that possessed high strength while also having excellent formability, LME resistance during spot welding, and impact resistance.

[0015] In view of the above, the present invention aims to provide a steel sheet and a method for manufacturing the same that have high strength and excellent formability (especially uniform elongation), impact resistance, and LME resistance.

[0016] This invention is based on the above findings, and its gist is as follows.

[0017] (1) A steel sheet according to one aspect of the present invention has a chemical composition in mass% of: C: 0.10 to 0.30%, Si: 0.10 to 1.20%, Al: 0.30 to 1.50%, Mn: 1.0 to 4.0%, P: 0.100% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0200% or less, Ni: 0 to 1.00%, Mo: 0 to 0.50%, Cr: 0 to 2.000%, Ti: 0 to 0.100%, B: 0 to 0.0100%, Nb: 0 to 0.10%, V: 0 to 0.50%, Cu: 0 to 1.00%, W: 0 to 0.20%, Ta: 0 to 0.100% It contains Co: 0-3.00%, Mg: 0-0.050%, Ca: 0-0.050%, Y: 0-0.050%, Zr: 0-0.500%, La: 0-0.050%, Ce: 0-0.050%, Sn: 0-0.100%, Sb: 0-0.500%, and As: 0-0.100%, with the remainder being Fe and impurities, and in the metallographic structure at 1 / 4 of the plate thickness, the volume fraction of retained austenite is 3-20%, the volume fraction of martensite is 30-75%, and the remainder is one or more of ferrite, bainite, and pearlite. The number of retained austenite particles with an aspect ratio of 3.0 or higher in the first quarter of the plate thickness is 80% or more of the total number of retained austenite particles in the first quarter of the plate thickness, the average major axis length of retained austenite particles with an aspect ratio of 3.0 or higher in the first quarter of the plate thickness is less than 15 μm, the diameter of the inscribed circle of all regions consisting of martensite and retained austenite in the first quarter of the plate thickness is less than 2.0 μm, the diameter of the inscribed circle of all regions consisting of martensite and retained austenite in the eighth of the plate thickness is 1.8 μm or less, the ratio Ls / Lc of the average major axis length Ls of retained austenite particles with an aspect ratio of 3.0 or higher in the eighth of the plate thickness to the average major axis length Lc of retained austenite particles with an aspect ratio of 3.0 or higher in the center of the plate thickness is 0.93 or less, and the plate thickness is between 1.0 and 2.5 mm.(2) The steel sheet described in (1) above may have the following chemical composition in mass%, Cu: 0 to 0.50%, W: 0 to 0.10%, Co: 0 to 0.50%, Zr: 0 to 0.050%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, and As: 0 to 0.050%. (3) The steel sheet described in (1) or (2) above may have a ratio of 1.1 or more of the number of microstructures in 1 / 8 of the sheet thickness to the number of microstructures in the center of the sheet thickness. (4) The steel sheet described in any of (1) to (3) above may have a low Mn layer with a thickness of 4.0 μm or more from the surface of the steel sheet and a soft layer with a thickness of 10 to 100 μm from the surface of the steel sheet. (5) The steel sheet described in any of (1) to (4) above may have a hot-dip galvanized layer on the surface of the steel sheet. (6) The steel sheet described in any of (1) to (4) above may have an alloyed hot-dip galvanized layer on the surface of the steel sheet. (7) A method for manufacturing a steel sheet according to one aspect of the present invention comprises: a slab heating step of heating a slab having the chemical composition described in (1) above at a temperature range of 1100°C or higher for 1200 seconds or more; a hot rolling step of hot rolling the slab to make a hot-rolled steel sheet; a winding step of cooling the hot-rolled steel sheet to a temperature of 400°C or lower and winding it up; a cold rolling step of cold rolling the hot-rolled steel sheet after the winding step at a reduction ratio of 0 to 30% to make a cold-rolled steel sheet; and an annealing step of annealing the cold-rolled steel sheet, wherein the hot rolling step includes a rolling pass in which at least two rolling passes are performed consecutively in a temperature range of 1010 to 1180°C, with a reduction ratio of 40% or more per pass, and the inter-pass time is 3 to 20 seconds, and the total reduction ratio in the temperature range of over 960°C and below 1010°C is 0% or more and less than 5%; and the annealing step is A method for manufacturing a steel sheet, comprising: heating the cold-rolled steel sheet from Ac1-30°C to a maximum temperature greater than Ac1 and less than or equal to Ac3 at an average heating rate of 2.0°C / s or less, while simultaneously applying a bending and unbending motion to the cold-rolled steel sheet; holding the cold-rolled steel sheet in the temperature range greater than Ac1 and less than or equal to Ac3 for 10 to 400 seconds; cooling the cold-rolled steel sheet after holding in the temperature range from Ac1 to less than Ms at an average cooling rate of 4°C / s or more; and reheating the cooled cold-rolled steel sheet and holding it at 480°C or less for 80 seconds or more.(8) The method for manufacturing steel sheets described in (7) above, in the hot rolling step, the amount of water and water pressure of the cooling water for the rolling rolls in the temperature range of 1010 to 1180°C are set to 8.0 L / min or more and 1.0 × 10, respectively. ６ kgf / m ２ The above may be used. (9) In the method for manufacturing a steel sheet described in (7) or (8) above, the cold-rolled steel sheet may be heated in the annealing step in an atmosphere having a dew point of -20°C to 20°C, containing 0.1 to 30% hydrogen by volume, with the remainder being nitrogen and impurities. (10) The method for manufacturing a steel sheet described in any of (7) to (9) above may include a hot-dip galvanizing step in which the cold-rolled steel sheet after the annealing step is controlled to a temperature range of (galvanizing bath temperature -40)°C to (galvanizing bath temperature +50)°C, and then immersed in a hot-dip galvanizing bath to form a hot-dip galvanized layer on the surface of the cold-rolled steel sheet. (11) The method for manufacturing a steel sheet described in (10) above may include an alloying step in which the hot-dip galvanized layer is alloyed by heating the hot-dip galvanized layer to a temperature range of 300 to 500°C. (12) A part according to one aspect of the present invention includes a steel sheet described in any of (1) to (6) above.

[0018] According to the above-described embodiment of the present invention, it is possible to provide a steel sheet having high strength and excellent formability, impact resistance, and LME resistance, as well as a method for manufacturing the same.

[0019] This diagram illustrates the test method for evaluating LME resistance.

[0020] The following describes a steel sheet, a method for manufacturing the same, and a component including the steel sheet according to this embodiment. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention. Also, the numerical limit ranges described below, separated by "~", include both a lower limit and an upper limit. Numerical values ​​indicated as "less than" or "greater than" do not include the numerical range. In the following description, percentages relating to the chemical composition of the steel sheet are mass percentages unless otherwise specified.

[0021] The steel sheet according to this embodiment has a chemical composition in mass%, of which C: 0.10-0.30%, Si: 0.10-1.20%, Al: 0.30-1.50%, Mn: 1.0-4.0%, P: 0.100% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0200% or less, Ni: 0-1.00%, Mo: 0-0.50%, Cr: 0-2.000%, Ti: 0-0.100%, B: 0-0.0100%, Nb: 0-0.10%, V: 0-0.50%, Cu: 0-1.00%, W: 0-0.20%, Ta: 0-0.100%. It contains Co: 0-3.00%, Mg: 0-0.050%, Ca: 0-0.050%, Y: 0-0.050%, Zr: 0-0.500%, La: 0-0.050%, Ce: 0-0.050%, Sn: 0-0.100%, Sb: 0-0.500%, and As: 0-0.100%, with the remainder being Fe and impurities, and in the metallographic structure at 1 / 4 of the plate thickness, the volume fraction of retained austenite is 3-20%, the volume fraction of martensite is 30-75%, and the remainder is one or more of ferrite, bainite, and pearlite. The number of retained austenite particles with an aspect ratio of 3.0 or higher in the first quarter of the plate thickness is 80% or more of the total number of retained austenite particles in the first quarter of the plate thickness, the average major axis length of retained austenite particles with an aspect ratio of 3.0 or higher in the first quarter of the plate thickness is less than 15 μm, the diameter of the inscribed circle of all regions consisting of martensite and retained austenite in the first quarter of the plate thickness is less than 2.0 μm, the diameter of the inscribed circle of all regions consisting of martensite and retained austenite in the eighth of the plate thickness is 1.8 μm or less, the ratio Ls / Lc of the average major axis length Ls of retained austenite particles with an aspect ratio of 3.0 or higher in the eighth of the plate thickness to the average major axis length Lc of retained austenite particles with an aspect ratio of 3.0 or higher in the center of the plate thickness is 0.93 or less, and the plate thickness is between 1.0 and 2.5 mm.

[0022] A steel plate according to one aspect of the present invention will be described below.

[0023] <Metallic Structure> First, the metallic structure of the steel sheet according to this embodiment will be described. Hereinafter, the microstructure fraction will be expressed as a volume fraction, so the unit "%" of the microstructure fraction means volume percent. When the microstructure fraction is identified by image processing, the area fraction will be considered as the volume fraction. Unless otherwise specified, the metallic structure of the steel sheet according to this embodiment will represent the metallic structure at a position 1 / 4 of the thickness from the surface of the steel sheet (1 / 4 depth position, 1 / 4 of the thickness).

[0024] The reason for defining the metallic structure at the 1 / 4 mark of the sheet thickness as a representative structure of the steel sheet in this embodiment is that the microstructure (components) of the steel sheet changes in the thickness direction because the area near the surface and the area near the center of the sheet thickness are affected by decarburization and central segregation during slab casting, respectively. Therefore, in this embodiment, the structure at the 1 / 4 mark of the sheet thickness is selected as the representative structure of the steel sheet. In the case of plated steel sheets, the outermost layer that serves as the reference in the thickness direction is the surface of the base steel sheet (the interface between the plating layer and the base steel sheet).

[0025] [Retained Austenite: 3-20%] Retained austenite is a microstructure that contributes to improved elongation (especially uniform elongation) due to the TRIP effect. To obtain this effect, the volume fraction of retained austenite should be 3% or more. Preferably, the volume fraction of retained austenite is 5% or more, more preferably 7% or more. On the other hand, if the volume fraction of retained austenite is excessively high, the grain size of retained austenite will increase. Such retained austenite with large grain sizes undergoes a processing-induced transformation into coarse martensite due to the strain applied during part formation. In this case, coarse martensite is prone to crack initiation, and the formability, especially hole-expanding ability, deteriorates, which is undesirable. Furthermore, coarse martensite present in the metal structure of the part becomes a crack initiation point due to bending deformation during impact, hindering impact resistance. For this reason, the volume fraction of retained austenite should be 20% or less. Preferably, the volume fraction of retained austenite is 18% or less, more preferably 16% or less.

[0026] Furthermore, in the steel sheet according to this embodiment, as described later, the stability of retained austenite is enhanced not only by controlling the volume fraction of retained austenite but also by controlling the aspect ratio of the retained austenite. The high stability of the retained austenite suppresses work-induced transformation to martensite, thereby improving formability, particularly uniform elongation. In addition to controlling the aspect ratio of retained austenite, controlling its size prevents the martensite generated by work-induced transformation during part formation from becoming coarse lumps. This improves impact resistance (bending characteristics after pre-straining of the steel sheet).

[0027] [Martensite: 30-75%] Martensite is generally classified into fresh martensite and tempered martensite, but in this embodiment, no distinction is made between them.

[0028] Fresh martensite is a rigid structure with a high dislocation density, and therefore contributes to improved tensile strength.

[0029] Tempered martensite, like fresh martensite, is a structure consisting of lath-like grains and contributes to improved tensile strength. However, unlike fresh martensite, tempered martensite is a hard structure containing fine iron-based carbides internally due to tempering. Tempered martensite can be obtained by tempering fresh martensite, which is formed by cooling after annealing, through heat treatment or other methods.

[0030] To obtain the above-mentioned effects of martensite, the volume fraction of martensite is 30% or more, preferably 40% or more. However, if the volume fraction of martensite is excessively high, the ductility of the steel sheet may deteriorate. Therefore, the volume fraction of martensite is 75% or less.

[0031] [Remainder: Ferrite, Bainite, and Pearlite] The remainder of the material, other than the retained austenite and martensite mentioned above, consists of one or more of the following: ferrite, bainite, and pearlite.

[0032] Ferrite is a soft tissue and is thus easily deformed, contributing to improved elongation. However, in order to obtain the desired high strength, it is preferable to limit the volume fraction of ferrite.

[0033] Bainite is a structure obtained by holding at a temperature range of 350 to 450 °C for a certain period of time after annealing. Bainite is softer than martensite and thus contributes to improved elongation. However, in order to obtain the desired high strength, it is preferable to limit the volume fraction as in the case of ferrite.

[0034] Pearlite contains hard iron carbide and is a structure that serves as a starting point for void generation during hole expansion.

[0035] For the above reasons, in the steel sheet according to the present embodiment, the total volume fraction of ferrite, bainite, and pearlite is preferably 50% or less. In order to increase the strength, the total volume fraction of ferrite, bainite, and pearlite may be 40% or less in total. In order to obtain the effects of the present embodiment, ferrite, bainite, and pearlite are not essential. That is, in the steel sheet according to the present embodiment, the volume fraction of ferrite, bainite, and pearlite may be 0%.

[0036] [Ratio of the number of retained austenite with an aspect ratio of 3.0 or more in the 1 / 4 thickness part of the plate thickness: 80% or more of all retained austenite in the 1 / 4 thickness part of the plate thickness] Retained austenite improves stability when subjected to strain by being needle-shaped. Specifically, retained austenite transforms stepwise into martensite from the grain boundaries, and strain occurs with this transformation. As the transformation progresses, the dislocations generated near the grain boundaries move through the grains to the opposite grain boundaries, and the dislocations accumulate. When the retained austenite is needle-shaped, the distance from the grain boundaries where dislocations occur to the grain boundaries where dislocations accumulate is short. Therefore, a repulsive force occurs between the accumulated dislocations and the newly generated dislocations, and the strain generated by the martensite transformation is not allowed. Since the martensite transformation is inhibited by the above mechanism, the stability of the retained austenite is improved.

[0037] In the steel sheet according to this embodiment, retained austenite is made needle-shaped by the method described later. However, retained austenite that is generated without shape control does not become needle-shaped, and in this case, there is variation in the stability of the retained austenite, resulting in a deterioration of uniform elongation.

[0038] In this embodiment, "retained austenite with an aspect ratio of 3.0 or higher" is defined as "acinetically shaped retained austenite." When retained austenite with an aspect ratio of 3.0 or higher in the first quarter of the plate thickness accounts for 80% or more of the total retained austenite in that first quarter of the plate thickness, the uniform elongation is improved. The percentage of retained austenite with an aspect ratio of 3.0 or higher in the first quarter of the plate thickness is preferably 83% or more of the total retained austenite in that first quarter of the plate thickness, and more preferably 85% or more. There is no particular upper limit to the percentage of retained austenite with an aspect ratio of 3.0 or higher in the first quarter of the plate thickness relative to the total retained austenite in that first quarter of the plate thickness; ideally, it is 100%. The method for determining the percentage of retained austenite with an aspect ratio of 3.0 or higher will be described later.

[0039] [Average major axis length of retained austenite with an aspect ratio of 3.0 or more in the 1 / 4 thickness part of the plate thickness: less than 15 μm] Retained austenite with an aspect ratio of 3.0 or more, that is, acicular retained austenite, has high stability as described above. In the present embodiment, by reducing the average major axis length of this acicular retained austenite in the 1 / 4 thickness part of the plate thickness, the stability of the acicular retained austenite can be further enhanced, and more excellent uniform elongation can be obtained. Furthermore, by reducing the average major axis length of the acicular retained austenite, the refinement of the metal structure of the steel plate can be achieved, so that the collision resistance characteristics can also be improved. When the average major axis length of the acicular retained austenite in the 1 / 4 thickness part of the plate thickness is less than 15 μm, the uniform elongation and the collision resistance characteristics can be further improved. The average major axis length of the acicular retained austenite in the 1 / 4 thickness part of the plate thickness is preferably 13 μm or less, and more preferably 10 μm or less. The lower limit of the average major axis length of the acicular retained austenite in the 1 / 4 thickness part of the plate thickness is not particularly defined, and is, for example, 3 μm or more.

[0040] [Diameter of the inscribed circle of all regions composed of martensite and retained austenite in the 1 / 4 thickness part of the plate thickness: less than 2.0 μm] When coarse martensite or retained austenite exists in the metal structure of the steel plate, strain concentrates at the interface between the hard phase (martensite), including martensite formed by processing-induced transformation accompanying part forming, and the soft phase, and the generation of microcracks at the interface is promoted. Such microcracks significantly reduce the fracture limit. Therefore, in order to improve the collision resistance characteristics, that is, to suppress fracture during bending deformation of the formed part, it is effective to suppress the generation of coarse martensite and coarse retained austenite in the steel plate used as the material.

[0041] If the diameter of the inscribed circle of all regions consisting of martensite and retained austenite in the last quarter of the sheet thickness is less than 2.0 μm, it can be determined that the formation of coarse martensite and coarse retained austenite is suppressed throughout the steel sheet. As a result, strain concentration at the interface between the hard phase and the soft phase can be suppressed during steel sheet processing, and impact resistance can be improved. The diameter of the inscribed circle of all regions consisting of martensite and retained austenite in the last quarter of the sheet thickness is preferably 1.5 μm or less, and more preferably 1.0 μm or less. Since a smaller diameter of the inscribed circle is preferable, no lower limit is particularly set, for example, it can be 0.10 μm or more.

[0042] Furthermore, by further refining the microstructure of the steel sheet surface, impact resistance and LME resistance can be improved. From this viewpoint, it is preferable that the diameter of the inscribed circle of all regions consisting of martensite and retained austenite in the surface layer (i.e., 1 / 8 of the sheet thickness) is 1.8 μm or less. More preferably, the diameter of the inscribed circle in the surface layer is 1.5 μm or less.

[0043] [Ls / Lc: 0.93 or less] When Lc is the average major axis length of retained austenite with an aspect ratio of 3.0 or more at the center of the plate thickness, and Ls is the average major axis length of retained austenite with an aspect ratio of 3.0 or more at 1 / 8 of the plate thickness, the ratio of Ls to Lc, Ls / Lc, is 0.93 or less. In this embodiment, Ls / Lc is an index representing the distribution of acicular retained austenite in the plate thickness direction.

[0044] The surface layer of the steel sheet is the area where processing strain is high during bending deformation. Therefore, by refining the microstructure of the steel sheet surface, particularly the needle-like retained austenite, the occurrence and propagation of microcracks at the interface between the hard and soft phases can be further suppressed, thereby improving impact resistance. Furthermore, as the microstructure is refined, the penetration of liquid metal generated during welding into the grain boundaries of the steel sheet's metal structure is dispersed, thus improving LME resistance. In other words, in this embodiment, by making the size of needle-like retained austenite in the surface layer (1 / 8 of the sheet thickness) smaller than in the center of the sheet thickness (1 / 2 of the sheet thickness), impact resistance and LME resistance can be improved.

[0045] In this embodiment, Ls / Lc is 0.93 or less. Preferably, Ls / Lc is 0.90 or less, and more preferably 0.85 or less. There is no particular lower limit for Ls / Lc, for example, 0.75 or more.

[0046] [Ratio of the total number of microstructures in the 1 / 8 portion of the plate thickness to the total number of microstructures in the center of the plate thickness: 1.1 or more] The number of microstructures correlates with the size of the microstructure; the smaller the microstructure size, the greater the number of microstructures. Here, we will explain the "number of microstructures (microstructure size)" in this embodiment. In the steel plate that is the subject of this embodiment, the fineness of the metallic microstructure is generally expressed by the "grain size". However, the metallic microstructure of the steel plate in this embodiment is composed of multiple phases, and the microstructure is divided by boundaries different from simple (general) crystal grains. Taking this situation into consideration, in this embodiment, the fineness of the metallic microstructure of the steel plate is quantitatively defined by the "number of microstructures (microstructure size)". In other words, the "number of microstructures (microstructure size)" in this embodiment is conceptually the same as the "grain size" and represents the fineness of the metallic microstructure. That is, the fact that the total number of microstructures in the 1 / 8 portion of the plate thickness is 1.1 times or more than the total number of microstructures in the center of the plate thickness indicates that the microstructure in the 1 / 8 portion of the plate thickness is finer than the microstructure in the center of the plate thickness. The closer to the surface in the thickness direction, the higher the strain during bending deformation, making cracks more likely to occur. When the microstructure in the last 1 / 8 of the plate thickness is finer than in the center, the strain can be distributed more uniformly within the surface layer, suppressing localization of strain. As a result, the maximum bending angle increases during bending deformation after tensile pre-strain is applied. Furthermore, the refinement of the microstructure disperses the penetration of liquid metal generated during welding into the grain boundaries of the steel sheet, further improving LME resistance.

[0047] This paper describes the identification of ferrite, bainite, pearlite, retained austenite, and martensite, the calculation of the volume fraction of each tissue, the average major axis length of retained austenite, and the calculation of the inscribed circle of the hard phase.

[0048] The volume fraction of retained austenite can be calculated by measuring the diffraction intensity using X-rays. In X-ray measurements, the surface of a sample cut from a steel plate is removed from the surface to a depth of 1 / 4 of the plate thickness by mechanical and chemical polishing. At the polished surface (1 / 4 depth), Mo-Kα rays are used to determine the integral intensity ratio of a total of five diffraction peaks: (200) and (211) of the bcc phase, and (200), (220), and (311) of the fcc phase. The structural fraction (volume fraction) of retained austenite is then calculated from the obtained integral intensity ratio.

[0049] The volume fractions of other metallic structures are determined by the following procedure. First, the volume fraction of fresh martensite is determined by the following procedure. A sample is taken so that the cross-section of the steel sheet parallel to the rolling direction is the observation surface, and then the observation surface of the sample is etched with a repeller solution. Next, a secondary electron image is taken at 3000x magnification using a field emission scanning electron microscope (FE-SEM) for a region (100 μm × 100 μm area) centered at a depth of 1 / 4 of the sheet thickness from the surface, with a length of 100 μm in the thickness direction and a length of 100 μm perpendicular to the thickness direction. Fresh martensite and retained austenite are not etched by repeller etching. Therefore, in the captured secondary electron image, the area fraction of the region with high brightness and in which the underlying structure is not revealed by etching is considered to be the total area fraction of fresh martensite and retained austenite. The volume fraction of fresh martensite is calculated by subtracting the volume fraction of retained austenite measured by X-ray as described above from this total area fraction.

[0050] The volume fractions of ferrite, pearlite, and tempered martensite can be determined from secondary electron images captured by FE-SEM. The observation surface is a cross-section of the steel sheet parallel to the rolling direction. Polishing and nital etching are performed on the observation surface, and a secondary electron image is captured using FE-SEM at a magnification of 3000x or more in a 100 μm × 100 μm region, which is a region of 100 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction, centered at a depth of 1 / 4 of the thickness from the surface. By leaving multiple indentations around the region observed by the aforementioned repera etching, the same region observed by repera etching can be confirmed. Here, regions that have a substructure within the grains and where carbides precipitate with multiple variants are identified as tempered martensite. Regions where cementite precipitates in a lamellar pattern are identified as pearlite. Regions with low brightness and no substructure are identified as ferrite. The fractions of these tissues are calculated by image analysis using the Trainable Weka Segmentation software in "ImageJ Fiji" according to the above criteria, thereby determining the volume fraction of each tissue. The volume fraction of tempered martensite and the volume fraction of fresh martensite obtained above are then added together to determine the volume fraction of martensite. Furthermore, the volume fraction of bainite is calculated by dividing 100% by the sum of the volume fractions of retained austenite, martensite, ferrite, and pearlite, which are determined using the method described above. If the sum of the volume fractions of retained austenite, martensite, ferrite, and pearlite (VT %) exceeds 100%, the volume fraction of bainite is set to zero, and the volume fraction of each tissue obtained by observation is multiplied by 100 / VT to correct the sum of the volume fractions to 100%.

[0051] The number of retained austenite particles, the number of retained austenite particles with an aspect ratio of 3.0 or greater, and the length of the long axis of retained austenite particles with an aspect ratio of 3.0 or greater are determined by EBSD analysis using FE-SEM. First, the 1 / 4 portion of the plate thickness will be explained. Specifically, a sample is taken with the plate thickness cross section parallel to the rolling direction of the steel plate as the observation surface. After polishing the observation surface of the sample, the strain-influenced layer is removed by electropolishing. An EBSD analysis is then performed on a 100 μm × 100 μm region, which is a region of 100 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction, centered at a depth of 1 / 4 of the plate thickness from the surface, with a measurement step of 0.05 μm. The measurement magnification can be selected from 1000 to 9000 times, for example, 3000 times, the same as the observation of the SEM-backscattered electron image described above. Next, the number of regions where measurement points identified as FCC iron by the EBSD analysis software OIM analysis are continuous in the vertical, horizontal, and diagonal directions of the measurement step is counted as a single independent retained austenite, and the total number of retained austenites in the 1 / 4 portion of the plate thickness is obtained. Furthermore, using the image analysis software "ImageJ Fiji", the shape of each independent retained austenite region is approximated as an ellipse, and the major axis length and minor axis length of each region are determined. The value obtained by dividing the calculated major axis length by the minor axis length is taken as the aspect ratio of each independent retained austenite. Furthermore, retained austenites with an aspect ratio of 3.0 or greater are extracted, and their number is counted to obtain the number of retained austenites with an aspect ratio of 3.0 or greater. In addition, the arithmetic mean of the major axis lengths of each retained austenite with an aspect ratio of 3.0 or greater is taken to obtain the average major axis length of retained austenites with an aspect ratio of 3.0 or greater in the 1 / 4 portion of the plate thickness.

[0052] Measurements similar to those performed on the 1 / 4 portion of the plate thickness were carried out at the center and 1 / 8 portion of the plate thickness to obtain the average major axis length Lc of retained austenite with an aspect ratio of 3.0 or higher at the center of the plate thickness, and the average major axis length Ls of retained austenite with an aspect ratio of 3.0 or higher at 1 / 8 portion of the plate thickness.

[0053] The diameter of the inscribed circle of all regions consisting of martensite and retained austenite in the 1 / 4 thickness portion of the plate is determined by observing the observation surface used to determine the volume fractions of ferrite, pearlite, and tempered martensite, as described above. It is important to note that the "all regions consisting of martensite and retained austenite" defined for determining the diameter of the inscribed circle is a hard structure, and basically consists mainly of the "fresh martensite," "tempered martensite," and "retained austenite" defined for determining the volume fractions, but it is a region that takes into account that it may not be a simple sum of these due to different observation methods. Specifically, a 100 μm × 100 μm region, which is a region of 100 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction, centered at a depth of 1 / 4 thickness from the surface after polishing and nital etching, is observed with an FE-SEM at a magnification of 3000x or more. Because high-carbon structures are difficult to etch, when observed with FE-SEM after Nital etching, the structures that appear to stand out can be identified as fresh martensite, tempered martensite, and retained austenite. In other words, among the areas determined by FE-SEM observation to be structures other than ferrite and pearlite, structures that appear to stand out with relatively less corrosion than the surrounding areas correspond to the "regions consisting of martensite and retained austenite" in this embodiment. These regions may include areas with low brightness in some parts of their interior, but they can generally be observed as regions with relatively high brightness. For each of these individual regions, an inscribed circle is drawn and the diameter of the inscribed circle is determined. Depending on the shape of the structure corresponding to the "regions consisting of martensite and retained austenite," multiple inscribed circles may be drawn, but the inscribed circle is defined as the one that touches the outer circumference of the structure at two or more points and has the largest diameter. In other words, the inscribed circle can also be expressed as the largest inscribed circle that can be drawn in each region containing martensite and / or retained austenite.

[0054] The number of microstructures in the center of the plate thickness and in the 1 / 8 portion of the plate thickness is determined using the same method as when determining the volume fraction of the metallic microstructure in the 1 / 4 portion of the plate thickness. A straight line is drawn on the image of each observation surface, and the number of times it intersects with the microstructure is used to determine the number of microstructures. Specifically, in a 100 μm × 100 μm image that has been polished and etched with Repera solution, "fresh martensite and retained austenite" and "other microstructures" are identified. A straight line with a length of 100 μm is drawn horizontally in the vertical center of the image, and each time the line intersects with a microstructure, it is counted as one intersection. The number of these two microstructures, "fresh martensite and retained austenite" and "other," is then measured. Next, in the same region, identify "ferrite," "pearlite," "tempered martensite," and "other structures" in a 100 μm x 100 μm image obtained by polishing and nital etching. Draw a 100 μm long line horizontally in the vertical center of the image, counting each intersection with a structure as one occurrence, and count the number of each of the four structures: "ferrite," "pearlite," "tempered martensite," and "other." Then, add the "number of two structures" and the "number of four structures" obtained above to determine the number of structures in the observed area. Perform the same procedure for the center of the plate thickness and for the 1 / 8 portion of the plate thickness, and determine the number of structures in each area.

[0055] [Thickness of the low-Mn layer: 4.0 μm or more from the surface] The steel sheet according to this embodiment may have a low-Mn layer with a thickness of 4.0 μm or more from the surface. In other words, the low-Mn layer may be formed to a depth of at least 4.0 μm from the surface of the steel sheet. The low-Mn layer is a so-called internal oxide layer, which is a layer in which at least a part of the grain boundaries is covered with an oxide of an easily oxidizable element such as Si or Mn. By covering the grain boundaries with an oxide, it is possible to suppress the penetration of molten metal into the grain boundaries during welding and to further suppress LME cracking during welding. To obtain this effect to the fullest extent, it is preferable that the thickness of the low-Mn layer be 4.0 μm or more. On the other hand, if the thickness of the low-Mn layer is excessively thick, the elongation may decrease. Therefore, it is preferable that the upper limit of the thickness of the low-Mn layer be 15.0 μm or less. Here, in the case of plated steel sheets, the surface refers to the surface of the base steel sheet (the interface between the plating layer and the base steel sheet).

[0056] The thickness of the low-Mn layer is determined by the following method: The concentration distribution of Mn is continuously measured using a high-frequency glow discharge emission spectrometer (GDS) over a distance of 120 μm from the surface of the steel sheet toward the center of the sheet thickness. In the case of plated steel sheets, the concentration distribution of Fe is also measured simultaneously with the measurement of the Mn concentration distribution, and the position where the Fe concentration first exceeds 80% by weight in the Fe concentration distribution is defined as the interface between the plating layer and the base steel sheet (the surface in the plated steel sheet). In the steel sheet according to this embodiment, the Mn concentration increases toward the interior of the sheet thickness and becomes constant from a certain point, so this constant concentration is defined as the representative concentration inside the steel sheet. When the Mn concentration increases toward the interior of the sheet thickness from the surface, the position where the Mn concentration first reaches 90% of the representative concentration inside the steel sheet is defined as X1, and the distance from the surface to X1 is defined as the thickness of the low-Mn layer.

[0057] When analyzing using high-frequency glow discharge analysis (HDS), known HDS analysis methods can be used. Specifically, the surface of the steel plate is placed in an Ar atmosphere, and a voltage is applied to generate a glow plasma. The steel plate surface is then sputtered, and analysis is performed in the thickness direction. The elements contained in the material (steel plate) are identified from the emission spectral wavelengths of the elements emitted when atoms are excited in the glow plasma, and the amount of each element in the material is estimated from the emission intensity of the identified elements using a pre-created calibration curve. Data in the thickness direction can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth using a standard sample beforehand, the sputtering time can be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. Commercially available analytical instruments can be used for HDS analysis.

[0058] [Thickness of the softened layer: 10 to 100 μm from the surface] To improve the bendability after processing, it is effective to soften the surface layer of the steel sheet. The means of softening the surface layer of the steel sheet are not particularly limited, but for example, it may be by providing a decarburized layer on the surface layer of the steel sheet.

[0059] In the steel plate according to this embodiment, in order to obtain the above effect, a soft layer having a thickness of 10 μm or more from the surface of the steel plate may be provided. That is, the soft layer may be formed to a depth of at least 10 μm from the surface of the steel plate. On the other hand, if the thickness of the soft layer exceeds 100 μm, the strength may be insufficient. Therefore, it is preferable that the thickness of the soft layer be 100 μm or less.

[0060] The thickness of the soft layer is determined by the following method. In the steel sheet according to this embodiment, the soft layer is defined as the region on the surface side of the steel sheet (excluding the plating layer) from the position where the hardness is 80% of the average hardness inside the steel sheet. In this embodiment, the average hardness inside the steel sheet and the hardness at each position in the thickness direction of the steel sheet (hardness profile in the thickness direction) are determined as follows.

[0061] A sample is taken from the cross-section of a steel plate, which is used as the observation surface. The observation surface is polished to a mirror finish, and then chemical polishing is performed using colloidal silica to remove the surface processing layer. Using a microhardness tester, a square pyramidal Vickers indenter with a vertex angle of 136° is pressed into the observation surface of the obtained sample at a depth of 5 μm from the surface (in the case of plated steel plates, the interface between the base steel plate and the plating layer) and down to a depth of 1 / 8 of the plate thickness, at a pitch of 10 μm in the thickness direction of the steel plate. At this time, the indentation load is set so that the Vickers indentations do not interfere with each other. For example, the indentation load is 20 gf. After that, the diagonal length of the indentations is measured using an optical microscope or scanning electron microscope, and converted to Vickers hardness (Hv).

[0062] Next, the measurement position is moved 10 μm or more perpendicular to the thickness direction, and the same measurement is performed from a starting point at a depth of 10 μm from the surface down to a depth of 1 / 8 of the thickness. Next, the measurement position is moved another 10 μm or more perpendicular to the thickness direction, and the same measurement is performed from a starting point at a depth of 5 μm from the surface down to a depth of 1 / 8 of the thickness. Next, the measurement position is moved another 10 μm or more perpendicular to the thickness direction, and the same measurement is performed from a starting point at a depth of 10 μm from the outermost layer down to a depth of 1 / 8 of the thickness. By repeating this, the Vickers hardness is measured at five points for each depth position. In this way, hardness measurement data at 5 μm intervals in the thickness direction is obtained. The reason for not simply setting the measurement interval to 5 μm intervals is to avoid interference between indentations. The average value of the five points at the same depth position is taken as the hardness at that thickness position. The hardness profile in the thickness direction is obtained by interpolating between each data point with a straight line.

[0063] Furthermore, the hardness of at least five points in the range of 1 / 8 to 3 / 8 depth of the observation surface is measured using a microhardness measuring device in the same manner as described above, and the value obtained by averaging the values ​​is defined as the average hardness inside the steel plate. In the hardness profile in the thickness direction obtained as described above, the soft layer is defined as the depth from the surface to the point where the hardness first exceeds 80% of the average hardness inside the steel plate.

[0064] <Plate Thickness> The plate thickness of the steel plate according to this embodiment is 1.0 to 2.5 mm. If the plate thickness of the steel plate is less than 1.0 mm, the rolling load may become excessive, making hot rolling difficult. Therefore, the plate thickness of the steel plate according to this embodiment is 1.0 mm or more. Preferably, it is 1.2 mm or more. On the other hand, if the plate thickness exceeds 2.5 mm, it may be difficult to obtain the above-mentioned metal structure. Therefore, the plate thickness is 2.5 mm or less. Preferably, it is 2.2 mm or less.

[0065] Next, the reasons for limiting the chemical composition of the steel sheet according to this embodiment will be explained. Hereinafter, % in relation to the component composition refers to mass %.

[0066] <Chemical Composition> C: 0.10-0.30% C is an element that improves the strength of steel sheets. In addition, C stabilizes austenite, making it an effective element for obtaining retained austenite, which contributes to improved ductility. Therefore, C is effective in achieving both strength and formability. When the C content is 0.10% or more, sufficient retained austenite can be obtained, and the desired tensile strength and ductility can be secured. The C content is preferably 0.12% or more. On the other hand, if the C content exceeds 0.30%, weldability and LME resistance may deteriorate, as well as hole-expanding properties. Therefore, the C content should be 0.30% or less. The C content is preferably 0.28% or less.

[0067] Si: 0.10-1.20% Si inhibits the formation of cementite, thus promoting the enrichment of C in austenite and is an effective element for generating retained austenite after annealing. If the Si content is less than 0.10%, it becomes difficult to obtain the effects of the above action, and sufficient uniform elongation cannot be obtained, which is undesirable. Therefore, the Si content should be 0.10% or more. The Si content is preferably 0.50% or more, more preferably 0.60% or more. On the other hand, if the Si content exceeds 1.20%, LME cracking is more likely to occur during welding, and the chemical conversion treatment properties and plating properties may deteriorate significantly. Therefore, the Si content should be 1.20% or less. The Si content is preferably 1.10% or less, more preferably 1.00% or less.

[0068] Al: 0.30-1.50% Al is an element that has the effect of deoxidizing molten steel. Also, like Si, Al suppresses the formation of cementite, so it is an effective element for promoting the concentration of C in austenite and producing retained austenite after annealing. If the Al content is less than 0.30%, these effects cannot be obtained sufficiently, so the Al content should be 0.30% or more. The Al content is preferably 0.40% or more, more preferably 0.50% or more. On the other hand, if the Al content is too high, coarse Al oxides are formed and the workability of the steel sheet decreases. Also, a high Al content worsens castability. Therefore, the Al content should be 1.50% or less. The Al content is preferably 1.40% or less, more preferably 1.30% or less.

[0069] Mn: 1.0-4.0% Mn has the effect of improving the hardenability of steel and is an effective element for obtaining the metal structure of this embodiment. The metal structure of this embodiment can be obtained by setting the Mn content to 1.0% or more. The Mn content is preferably 1.3% or more. On the other hand, if the Mn content increases excessively, the effect of improving hardenability decreases due to Mn segregation, and the material cost increases. For this reason, the Mn content is set to 4.0% or less. The Mn content is preferably 3.5% or less.

[0070] P: 0.100% or less. P is an element that segregates in the center of the steel plate thickness, reducing toughness and making welds brittle. If the P content exceeds 0.100%, the weld strength and hole-expandability decrease significantly. Therefore, the P content should be 0.100% or less. Preferably, the P content is 0.0500% or less. A lower P content is preferable, but reducing the P content to less than 0.0001% in practical steel plates significantly increases manufacturing costs, making it economically disadvantageous. Therefore, the P content may be 0.0001% or more.

[0071] S: 0.0200% or less. S is an element that reduces weldability and also reduces manufacturability during casting and hot rolling. Furthermore, S forms coarse MnS, which is an element that causes a decrease in hole-expanding properties. When the S content exceeds 0.0200%, the decrease in weldability, manufacturability, and hole-expanding properties becomes significant. Therefore, the S content should be 0.0200% or less. A lower S content is preferable, but reducing the S content to less than 0.0001% in practical steel sheets significantly increases manufacturing costs, making it economically disadvantageous. Therefore, the S content may be 0.0001% or more.

[0072] N: 0.0200% or less. N forms coarse nitrides, reducing bendability and hole-expandability, and is also a cause of blowhole formation during welding. If the N content exceeds 0.0200%, the decrease in hole-expandability and the occurrence of blowholes become significant. Therefore, the N content should be 0.0200% or less. A lower N content is preferable, but reducing the N content to less than 0.0001% in practical steel sheets significantly increases manufacturing costs, making it economically disadvantageous. Therefore, the N content may be 0.0001% or more.

[0073] O: 0.0200% or less. O is an element that forms coarse oxides, reducing bendability and hole-expandability, and is also a cause of blowhole formation during welding. If the O content exceeds 0.0200%, the decrease in hole-expandability and the occurrence of blowholes become significant. Therefore, the O content should be 0.0200% or less. A lower O content is preferable, but reducing the O content to less than 0.0005% in practical steel sheets significantly increases manufacturing costs, making it economically disadvantageous. Therefore, the O content may be 0.0005% or more.

[0074] In the chemical composition of the steel sheet according to this embodiment, the remainder, excluding the above-mentioned elements, consists of Fe and impurities. Impurities are, for example, elements that are introduced from the steel raw materials and / or during the steelmaking process, and their presence is permissible as long as it does not significantly degrade the properties of the steel sheet according to this embodiment.

[0075] The chemical composition of the steel sheet according to this embodiment may, for the purpose of improving various properties, contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ni: 1.00% or less, Mo: 0.50% or less, Cr: 2.00% or less, Ti: 0.100% or less, B: 0.0100% or less, Nb: 0.10% or less, V: 0.50% or less, Cu: 0.50% or less, W: 0.10% or less, Ta: 0.100% or less, Co: 0.50% or less, Mg: 0.050% or less, Ca: 0.050% or less, Y: 0.050% or less, Zr: 0.050% or less, La: 0.050% or less, and Ce: 0.050% or less. Furthermore, the chemical composition of the steel sheet according to this embodiment may contain one or more of Sn, Sb, and As in place of a portion of Fe. When one or more of Sn, Sb, and As are included, it is preferable that the proportions are Sn: 0.050% or less, Sb: 0.050% or less, and As: 0.050% or less.

[0076] Ni: 0-1.00% Ni is an element effective in improving the strength of steel sheets. The Ni content may be 0%, but to obtain the above effect, it is preferable that the Ni content be 0.001% or more. The Ni content is more preferably 0.01% or more. On the other hand, if the Ni content is too high, the elongation of the steel sheet may decrease, and the formability may decrease. For this reason, the Ni content should be 1.00% or less.

[0077] Mo: 0-0.50% Mo, like Cr, is an element that contributes to increasing the strength of steel sheets. This effect can be obtained even in trace amounts. The Mo content may be 0%, but in order to obtain the above effect, the Mo content is preferably 0.01% or more. On the other hand, if the Mo content exceeds 0.50%, coarse Mo carbides may be formed, which may reduce the cold formability of the steel sheet. For this reason, the Mo content should be 0.50% or less.

[0078] Cr: 0-2,000% Cr is an element that improves the hardenability of steel and contributes to increasing its strength, and is an effective element for obtaining the above-mentioned metallic structure. Therefore, Cr may be included. The Cr content may be 0%, but in order to obtain the above-mentioned effects sufficiently, it is preferable to have a Cr content of 0.01% or more. On the other hand, if Cr is included in excess, the effects due to the above action will saturate and it will become uneconomical. Therefore, the Cr content should be 2,000% or less.

[0079] Ti: 0-0.100% Ti is an element that contributes to increasing the strength of steel sheets through precipitation strengthening, fine grain strengthening by suppressing the growth of ferrite crystal grains, and dislocation strengthening through the suppression of recrystallization. The Ti content may be 0%, but in order to obtain the above effects sufficiently, the Ti content is preferably 0.001% or more. For even higher strength of the steel sheet, the Ti content is more preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.100%, the precipitation of carbonitrides increases and the formability deteriorates. For this reason, the Ti content should be 0.100% or less.

[0080] B: 0-0.0100% B is an element that suppresses the formation of ferrite and pearlite in the metal structure during the cooling process from the austenite temperature range, and promotes the formation of low-temperature transformation structures such as bainite or martensite. B is also an element beneficial for increasing the strength of steel. This effect can be obtained even in trace amounts. The B content may be 0%, but to obtain the above effect, it is preferable to have a B content of 0.0001% or more. On the other hand, if the B content is too high, coarse B oxides are formed, and these B oxides can become the starting point for void formation during press forming, which may reduce the formability of the steel sheet. For this reason, the B content should be 0.0100% or less. Identifying B below 0.0001% requires careful analysis. If the B content is below the detection limit of the analytical instrument, the B content may be considered to be 0%.

[0081] Nb: 0-0.10% Nb is an element that contributes to increasing the strength of steel sheets through precipitation strengthening, fine grain strengthening by suppressing the growth of ferrite crystal grains, and dislocation strengthening through the suppression of recrystallization. The Nb content may be 0%, but to obtain the above effects sufficiently, it is preferable that the Nb content be 0.01% or more. For even higher strength of the steel sheet, it is more preferable that the Nb content be 0.05% or more. On the other hand, if the Nb content exceeds 0.10%, the precipitation of carbonitrides increases and the formability deteriorates. For this reason, the Nb content should be 0.10% or less. From the viewpoint of formability, it is preferable that the Nb content be 0.06% or less.

[0082] V: 0-0.50% V is an element that contributes to increasing the strength of steel sheets through precipitation strengthening, fine grain strengthening by suppressing the growth of ferrite crystal grains, and dislocation strengthening through the suppression of recrystallization. The V content may be 0%, but in order to obtain the above effects sufficiently, the V content is preferably 0.01% or more, and more preferably 0.02% or more. On the other hand, if the V content exceeds 0.50%, carbonitrides precipitate excessively and the formability deteriorates. For this reason, the V content should be 0.50% or less. The V content is preferably 0.40% or less.

[0083] Cu: 0-1.00% Cu is an element that contributes to improving the strength of steel sheets. This effect can be obtained even in trace amounts. The Cu content may be 0%, but in order to obtain the above effect, it is preferable that the Cu content be 0.01% or more. On the other hand, if the Cu content is too high, the productivity of hot rolling may decrease due to red-hot brittleness. The Cu content should be 1.00% or less. The Cu content is preferably 0.50% or less, and may also be 0.05% or less, 0.04% or less, or 0.03% or less.

[0084] W: 0-0.20% W is an element effective in improving the strength of steel sheets. The W content may be 0%, but to obtain the above effect, it is preferable that the W content be 0.01% or more. On the other hand, if the W content is too high, many fine W carbides will precipitate, reducing the elongation due to an excessive increase in the strength of the steel sheet, and potentially reducing the cold workability of the steel sheet. The W content should be 0.20% or less. The W content is preferably 0.10% or less, and may also be 0.05% or less, or 0.03% or less.

[0085] Ta: 0-0.100% Like W, Ta is an element effective in improving the strength of steel sheets. The Ta content may be 0%, but to obtain the above effects, it is preferable that the Ta content be 0.001% or more. On the other hand, if the Ta content is too high, many fine Ta carbides may precipitate, leading to an excessive increase in the strength of the steel sheet, which reduces elongation and may decrease the cold workability of the steel sheet. For this reason, the Ta content should be 0.10% or less. The Ta content is preferably 0.020% or less, and more preferably 0.010% or less.

[0086] Co: 0-3.00% Co is an element effective in improving the strength of steel sheets. The Co content may be 0%, but to obtain the above effect, it is preferable that the Co content be 0.01% or more. On the other hand, if the Co content is too high, the elongation of the steel sheet may decrease, and the formability may decrease. The Co content should be 3.00% or less. The Co content is preferably 2.00% or less, 1.00% or less, or 0.50% or less, and may also be 0.30% or less or 0.15% or less.

[0087] Mg: 0-0.050% Mg is an element that controls the morphology of sulfides and oxides and contributes to improving the bendability of steel sheets. This effect can be obtained even in trace amounts, so the Mg content may be 0%, but in order to obtain the above effect, it is preferable that the Mg content be 0.0001% or more. On the other hand, if the Mg content is too high, the cold formability may decrease due to the formation of coarse inclusions. For this reason, the Mg content should be 0.050% or less. Preferably, the Mg content is 0.040% or less.

[0088] Ca: 0-0.050% Like Mg, Ca is an element that can control the form of sulfides even in trace amounts. The Ca content may be 0%, but to obtain the above effect, it is preferable that the Ca content be 0.0010% or more. On the other hand, if the Ca content is too high, coarse Ca oxides will be generated, and these coarse Ca oxides can become the starting point for crack formation during cold forming. For this reason, the Ca content should be 0.050% or less. The Ca content is preferably 0.040% or less, and more preferably 0.030% or less.

[0089] Y: 0-0.050% Y, like Mg and Ca, is an element that can control the form of sulfides even in trace amounts. The Y content may be 0%, but to obtain the above effects, it is preferable that the Y content be 0.001% or more. On the other hand, if the Y content is too high, coarse Y oxide may be generated, which may reduce cold formability. For this reason, the Y content should be 0.050% or less. Preferably, the Y content is 0.040% or less.

[0090] Zr: 0-0.500% Zr, like Mg, Ca, and Y, is an element that can control the form of sulfides even in trace amounts. The Zr content may be 0%, but to obtain the above effects, it is preferable that the Zr content be 0.001% or more. On the other hand, if the Zr content is too high, coarse Zr oxides may be generated, which may reduce cold formability. The Zr content should be 0.500% or less. Preferably, the Zr content is 0.050% or less, 0.040% or less, and may also be 0.030% or less, 0.020% or less, or 0.015% or less.

[0091] La: 0-0.050% La is an element that is effective in controlling the morphology of sulfides even in trace amounts. The La content may be 0%, but to obtain the above effect, it is preferable that the La content be 0.001% or more. On the other hand, if the La content is too high, La oxide may be generated, which may reduce cold formability. For this reason, the La content should be 0.050% or less. Preferably, the La content is 0.0400% or less.

[0092] Ce: 0-0.050% Ce is an element that can control the form of sulfides even in trace amounts and also contributes to improving LME resistance. To fully obtain the above effects, it is preferable that the Ce content be 0.001% or more. The Ce content may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, if the Ce content is too high, the steel sheet may become brittle and the elongation of the steel sheet may decrease. Therefore, the Ce content should be 0.050% or less. The Ce content may be 0.040% or less, 0.020% or less, or 0.010% or less.

[0093] Sn: 0-0.100% Sn is an element that may be present in steel sheets when scrap is used as a raw material. Excessive Sn content can lead to a decrease in the cold formability of the steel sheet due to ferrite embrittlement. The Sn content should be 0.100% or less. Preferably, the Sn content is 0.050% or less, 0.040% or less, and may be 0%. A lower Sn content is preferable. However, reducing the Sn content to less than 0.001% would lead to an excessive increase in refining costs, so the Sn content may be 0.001% or more.

[0094] Sb: 0-0.500% Sb, like Sn, is an element that can be contained in steel sheets when scrap is used as a raw material for steel sheets. Sb is an element that can strongly segregate at grain boundaries, causing embrittlement of grain boundaries, reduced elongation, and reduced cold formability. The Sb content should be 0.500% or less. Preferably, the Sb content is 0.050% or less, 0.040% or less, and may be 0%. A lower Sb content is preferable. However, reducing the Sb content to less than 0.001% would lead to an excessive increase in refining costs, so the Sb content may be 0.001% or more.

[0095] As: 0-0.100% As, like Sn and Sb, is an element that can be contained in steel sheets when scrap is used as a raw material for steel sheets. As is an element that strongly segregates at grain boundaries and can cause a decrease in cold formability. The As content should be 0.100% or less. Preferably, the As content is 0.050% or less, 0.040% or less, and may be 0.030% or less, 0.020% or less, 0.010% or less, or 0%. A lower As content is preferable. However, reducing the As content to less than 0.001% would lead to an excessive increase in refining costs, so the As content may be 0.001% or more.

[0096] The chemical composition of the steel sheet according to this embodiment can be determined by the following method. The chemical composition of the steel sheet described above can be measured using general chemical composition methods. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-non-dispersive infrared absorption method. If the steel sheet has a plating layer, decarburization layer, oxide layer, etc. on its surface, these surface layers can be removed by mechanical grinding or the like before the chemical composition analysis is performed.

[0097] A zinc plating layer (hot-dip galvanized layer or electro-galvanized layer) may be formed on the surface (both sides or one side) of the steel sheet according to this embodiment. The hot-dip galvanized layer may be an alloyed hot-dip galvanized layer formed by alloying the hot-dip galvanized layer. The chemical composition of the hot-dip galvanized layer of the steel sheet according to this embodiment is not particularly limited and may be a known plating layer. Furthermore, it is not prevented that the steel sheet according to this embodiment has another plating (for example, aluminum plating).

[0098] If the hot-dip galvanized layer is not alloyed, the Fe content in the hot-dip galvanized layer is preferably less than 7.0% by mass. If the hot-dip galvanized layer is an alloyed hot-dip galvanized layer, the Fe content is preferably 6.0% by mass or more. Alloyed hot-dip galvanized steel sheets have better weldability than hot-dip galvanized steel sheets.

[0099] In the steel sheet according to this embodiment, a zinc plating layer is provided, and an additional upper plating layer may be provided on the zinc plating layer for the purpose of improving paintability, weldability, etc. Furthermore, the zinc-plated steel sheet may be subjected to various treatments, such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc.

[0100] <Characteristics> [Tensile Strength] In the steel sheet according to this embodiment, considering its contribution to improving the fuel efficiency of automobiles, the tensile strength (TS) is 980 MPa or higher. There is no particular upper limit to the tensile strength.

[0101] [Formability] In the steel sheet according to this embodiment, the target uniform elongation (%) is 7.5% or more from the viewpoint of formability. There is no particular upper limit to the uniform elongation (%).

[0102] Tensile strength and uniform elongation are measured by taking a JIS No. 5 tensile test specimen from the steel plate as specified in JIS Z 2241:2011 and performing a tensile test in accordance with JIS Z 2241:2011.

[0103] [Collision Resistance Characteristics] The steel sheet according to this embodiment exhibits excellent collision resistance characteristics. The collision resistance characteristics of the steel sheet according to this embodiment are evaluated by taking a VDA bending test specimen from a steel sheet that has been subjected to tensile pre-strain by a tensile test and performing a VDA bending test. The VDA bending test is performed in accordance with the VDA standard (VDA 238-100) specified by the German Association of the Automotive Industry.

[0104] [LME Resistance] The steel plate according to this embodiment has excellent LME resistance. LME resistance is evaluated by the following method. First, two steel plates, for example, one of which is a galvanized steel plate, are welded using a servo motor pressurized single-phase AC spot welding machine (power frequency 50 Hz) at a pressure of 450 kgf (4413 kg・m / s). ２Spot welding is performed while applying pressure. The welding conditions are a current of 6.5 kA, an electrode inclination angle of 3°, no upslope, a current application time of 0.4 seconds, and a holding time after the end of current application of 0.1 seconds. When the plated steel sheet is spot welded in this manner, if no cracks longer than 100 μm occur in the central region of the nugget, it is evaluated as having excellent LME resistance.

[0105] Next, a method for manufacturing steel sheets according to this embodiment will be described.

[0106] The steel sheet according to this embodiment can achieve the above-described effects regardless of its manufacturing method, as long as it has the above-described characteristics. However, the following method is preferable because it allows for the stable production of the steel sheet according to this embodiment. Specifically, the preferred method for producing the steel sheet according to this embodiment includes: a slab heating step of heating a slab having the above-described chemical composition at a temperature of 1100°C or higher for 20 minutes or more; a hot rolling step of hot rolling the slab to produce a hot-rolled steel sheet; a winding step of cooling the hot-rolled steel sheet to a temperature of 400°C or lower and winding it; a cold rolling step of cold rolling the hot-rolled steel sheet after the winding step at a reduction ratio of 0 to 30% to produce a cold-rolled steel sheet; and an annealing step of annealing the cold-rolled steel sheet. The hot rolling process includes a rolling pass in which at least two rolling passes are performed consecutively in a temperature range of 1010 to 1180°C, with a reduction ratio of 40% or more per pass, and the inter-pass time is 3 to 20 seconds, and the total reduction ratio in the temperature range of over 960°C and below 1010°C is 0% or more and less than 5%. The annealing process involves heating the cold-rolled steel sheet from Ac1-30°C to a maximum temperature greater than Ac1 and less than or equal to Ac3, at an average heating rate of 2.0°C / s or less, while simultaneously applying a bending and unbending motion to the cold-rolled steel sheet. The cold-rolled steel sheet is then held in the temperature range greater than Ac1 and less than or equal to Ac3 for 10 to 400 seconds. After holding in this temperature range, the cold-rolled steel sheet is cooled from Ac1 to less than the Ms point at an average cooling rate of 4°C / s or more. The cooled cold-rolled steel sheet is then reheated and held at 480°C or less for 80 seconds or more. Preferred conditions for each step are described below.

[0107] [Slab Heating Process] In the slab heating process, the slab having the above-mentioned chemical composition is heated at a temperature of 1100°C or higher for 1200 seconds or more.

[0108] The heating temperature of the slab should be 1100°C or higher to ensure sufficient solid dissolution of coarse inclusions. The heating temperature of the slab may also be 1150°C or higher. There is no specific upper limit for the heating temperature, but it is generally acceptable to keep it below 1300°C.

[0109] Furthermore, the holding time at the above heating temperature should be 1200 seconds or longer in order to bring the slab to the desired temperature in its center and to allow the carbides formed during the casting stage to dissolve completely. There is no upper limit specified for the holding time, but to suppress excessive scale loss, it is preferably 10000 seconds or less, and more preferably 6000 seconds or less.

[0110] Furthermore, while it is preferable to cast the slabs using a continuous casting method from a productivity standpoint, they may also be manufactured by ingot casting or thin slab casting. The slabs are not particularly limited as long as they have the chemical composition described above, and any slab manufactured by a conventional method is acceptable. For example, the slabs may be slabs manufactured by a general method, such as continuous casting slabs.

[0111] [Hot Rolling Process] In the hot rolling process, the heated slab is hot-rolled to produce a hot-rolled steel sheet. Specifically, the hot rolling process can be defined as a high-temperature rolling process in which the heated slab is rolled in the temperature range of 1010 to 1180°C, a medium-temperature rolling process in which the slab after the high-temperature rolling process is rolled in the temperature range of 960 to 1010°C, and a low-temperature rolling process in which the slab is rolled in the temperature range of 960°C or lower. The high-temperature rolling process is a rolling process in which rolling is performed two or more times with a reduction ratio of 40% or more and a time between passes of 3 to 20 seconds. The medium-temperature rolling process is a rolling process in which rolling is performed with a total reduction ratio of 0% or more and less than 5%.

[0112] Note that the reduction ratio in the hot rolling process is the thickness of the sheet before rolling, t ０ The thickness of the sheet after rolling is t １ When this is the case, (1-t １ / t ０ It can be expressed as ) × 100 (%).

[0113] (High-temperature rolling process) The high-temperature rolling process is a process in which the slab is passed through multiple rolling stands in succession at a temperature range of 1010 to 1180°C, and rolled multiple times.

[0114] The temperature range of 1010 to 1180°C is a temperature range in which recrystallization is promoted (also known as the complete recrystallization range). By performing two or more rolling operations consecutively in this temperature range with a relatively high reduction ratio and a predetermined inter-pass time, recrystallization can be promoted, and fine, equiaxed prior austenite grains can be obtained.

[0115] In the high-temperature rolling process, at least two rolling passes are performed consecutively, each with a reduction ratio of 40% or more, with an inter-pass time of 3 to 20 seconds. In other words, the high-temperature rolling process involves performing m rolling passes (m≧2) in the temperature range of 1010 to 1180°C, and includes at least one rolling pattern in which the reduction ratio of the nth pass (n≦m-1) is 40% or more, the reduction ratio of the (n+1)th pass (n+1)th roll is 40% or more, and the inter-pass time between the nth and (n+1)th rolls is 3 to 20 seconds. If the reduction ratio per pass is less than 40%, the refinement of the obtained prior austenite grains may be insufficient, resulting in a coarse microstructure after annealing. The reduction ratio per pass is preferably 45% or more. Also, if the inter-pass time is too long, grain growth of recrystallized grains occurs between passes, leading to coarsening, which may result in a coarse microstructure after annealing. The inter-pass time is preferably 18 seconds or less. Furthermore, if the next rolling is performed with a short inter-pass time, strain accumulates without recrystallization, making it difficult to obtain equiaxed grains in the subsequent low-temperature rolling process. Therefore, the inter-pass time should be 3 seconds or more. Preferably, the inter-pass time is 5 seconds or more.

[0116] In this embodiment, by making the prior austenite grains fine and equiaxed during the high-temperature rolling process and the medium-temperature rolling process, it is possible to suppress the linking of transformed austenite grains to form a lump of austenite during the subsequent annealing process. By suppressing the linking to lump of austenite, the aspect ratio of the retained austenite can be set to 3.0 or higher and the average major axis length to less than 15 μm.

[0117] In the hot rolling process, for the amount of cooling water of the rolling rolls, the total amount and water pressure at the inlet side and the outlet side shall be 8.0 m ３ / min or more and 1.0×10 ６ kgf / m ２ or more, respectively. Thereby, the surface layers of the slab and the hot rolled sheet are cooled more than the inside, the surface layer becomes a finer hot rolled structure, and the surface structure after annealing becomes fine.

[0118] (Intermediate temperature rolling process) The intermediate temperature rolling process is a process of rolling the slab after the hot rolling process in a temperature range exceeding 960°C and less than 1010°C with a total reduction ratio of 0% or more and less than 5%. In the intermediate temperature rolling process, the total reduction ratio shall be 0% or more and less than 5%. The temperature range exceeding 960°C and less than 1010°C is a temperature range in which recrystallization hardly progresses (also referred to as a partial recrystallization range). By reducing the reduction ratio or not performing rolling in such a temperature range, the austenite grains can be made equiaxed, and by combining with the above-mentioned hot rolling process, an equiaxed and fine hot rolled structure can be obtained.

[0119] When the reduction ratio in the partial recrystallization range is 5% or more, the crystal grains become flat, and the structure after annealing may not be equiaxed and fine.

[0120] (Cold rolling process) The cold rolling process is a process of rolling the slab after the hot rolling process or the intermediate temperature rolling process to a desired sheet thickness in a temperature range less than 960°C. In order to obtain the desired sheet thickness, the total reduction ratio may be determined as appropriate. For example, the total reduction ratio in the temperature range less than 960°C may be 20% or more and less than 80%.

[0121] [Winding Process] The winding process involves cooling the hot-rolled steel sheet (hot-rolled steel sheet) to a temperature of 400°C or lower and winding it up. By setting the winding temperature to 400°C or lower, ferrite and pearlite transformations are suppressed in the hot-rolled sheet, resulting in lath-like martensite and bainite (low-temperature transformation structure). Such lath-like low-temperature transformation structure in the hot-rolled sheet, when subjected to the appropriate cold-rolling and annealing processes described below, favorably contributes to the formation of needle-like austenite, which is an effective structure in the steel sheet according to this embodiment. A winding temperature exceeding 400°C is undesirable because it leads to the formation of ferrite and pearlite. Therefore, in this embodiment, the winding temperature is set to 400°C or lower, preferably 350°C or lower.

[0122] [Cold Rolling Process] In the cold rolling process, the hot-rolled steel sheet after the winding process is pickled, and then cold-rolled to a reduction ratio of 0-30% to produce cold-rolled steel sheet. Pickling is a process to remove oxides from the surface of the hot-rolled steel sheet, and the pickling can be done once or multiple times.

[0123] In the cold rolling process, rolling is performed with a reduction ratio of 30% or less, or not at all, allowing the lath-like low-temperature transformation structure obtained in the hot rolling process to be carried over to the subsequent annealing process without alteration. In other words, to increase the ratio of acicular retained austenite to total retained austenite in the final metal structure of the steel sheet, and to obtain retained austenite of a smaller size, it is effective to omit the cold rolling process, or, if performed, to do so with a low reduction ratio.

[0124] If the reduction ratio in the cold rolling process exceeds 30%, the amount of strain introduced increases excessively, and furthermore, the lath-like low-temperature transformation structure obtained in the hot rolling process collapses, making it difficult to obtain the desired structure in the final metal structure after annealing. For example, ferrite interface migration may be promoted during the heating process of the annealing process, making it impossible to obtain acicular austenite. In other words, a smaller reduction ratio in the cold rolling process is preferable. Specifically, the reduction ratio in cold rolling is 30% or less. Preferably it is 28% or less, and more preferably 25% or less. In this embodiment, cold rolling may not be performed. That is, the lower limit of the reduction ratio is 0%.

[0125] Note that the reduction ratio in the cold rolling process is the thickness of the sheet before rolling. ０ The thickness of the sheet after rolling is t １ When this is the case, (1-t １ / t ０ It can be expressed as ) × 100 (%).

[0126] [Annealing Process] The annealing process includes a heating process in which the cold-rolled steel sheet is heated from (Ac1 point - 30)°C to a maximum temperature greater than Ac1 point and less than or equal to Ac3 point; a soaking process in which the cold-rolled steel sheet is kept in the temperature range of greater than Ac1 point to Ac3 point; a cooling process in which the cold-rolled steel sheet after the soaking process is cooled to less than the Ms point in the temperature range of less than 350°C; and a reheating process in which the cooled cold-rolled steel sheet is reheated and kept at 480°C or less.

[0127] In this embodiment, the values ​​of Ac1 point (°C) and Ac3 point (°C) are determined by the following formulas.

[0128] Ac1=723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr+290×As+6.38×W...(1)

[0129] Ac3=910-203×√C+44.7×Si-30×Mn+700×P-20×Cu-15.2×Ni-11×Cr+31.5×Mo+400×Ti+104×V+120×Al...(2)

[0130] Here, C, Si, Mn, P, Cu, Ni, Cr, As, W, Mo, Ti, V, and Al in each formula represent the content [mass%] of each element.

[0131] (Heating Process) In the heating process of the annealing process, the cold-rolled steel sheet is heated at an average heating rate of 2.0°C / s or less in the temperature range from (Ac1 point - 30)°C to the highest temperature which is above Ac1 point and below Ac3 point. Here, the temperature rise continues without stopping from (Ac1 point - 30)°C to the highest temperature, and the average heating rate is calculated for that temperature rise period.

[0132] In this embodiment, the maximum temperature is set above the Ac1 point in order to generate austenite during the heating process. On the other hand, if the maximum temperature is above the Ac3 point, the microstructure becomes a single-phase austenite structure and coarses (becomes agglomerate). Therefore, needle-shaped austenite cannot be obtained. As is also related to the conditions of the soaking process described later, the maximum temperature is preferably (Ac1 + 30)°C or higher, and more preferably (Ac1 + 40)°C or higher, from the viewpoint of ensuring the volume fraction of retained austenite after annealing. Furthermore, from the viewpoint of more stably generating needle-shaped austenite along the lath of the low-temperature transformation structure, the maximum temperature is preferably (Ac3 - 5)°C or lower, and more preferably (Ac3 - 10)°C or lower. Moreover, in the chemical composition of the steel sheet of this embodiment, the Si content is relatively lower than that of conventional TRIP steel from the viewpoint of LME resistance, so the Ac1 point is higher than in the conventional method. If the Ac1 point is high, the annealing temperature required to obtain the desired metal structure (i.e., the maximum temperature) also becomes high, leading to increased coarsening of the austenite and a higher proportion of massive austenite. Furthermore, the Ac1 point is the temperature at which austenite formation begins. Therefore, in this embodiment, by keeping the average heating rate from Ac1-30°C to the maximum temperature relatively low (increasing the residence time in the temperature range from Ac1-30°C to the maximum temperature), we can ensure that austenite formation takes place primarily, thereby promoting the formation of a large number of austenites while suppressing the growth of austenite. As a result, a sufficient amount of fine retained austenite can be secured in the final metal structure.

[0133] To fully obtain the effects described above, the average heating rate is preferably 1.5°C / s or less, more preferably 1.2°C / s or less. The lower limit of the average heating temperature is not particularly limited, but it may be 0.1°C / s or higher, for example, for productivity reasons.

[0134] Furthermore, during the heating process, the cold-rolled steel sheet is subjected to bending and unbending at least for one stage of the process. By repeatedly applying compressive and tensile deformation to the steel sheet through bending and unbending, a significant amount of strain is introduced, particularly to the surface layer of the steel sheet. As a result, the frequency of austenite formation increases, especially in the surface layer of the steel sheet. This makes it possible to reduce the size (especially the average major axis length) of the needle-shaped retained austenite in the surface layer of the annealed steel sheet compared to the center of the sheet thickness, thereby improving impact resistance and LME resistance.

[0135] Bending and unbending should be performed with a roll with a radius of 1500 mm or less. If the roll radius exceeds 1500 mm, it becomes difficult to efficiently introduce dislocations into the steel sheet structure through bending and unbending deformation, so a roll radius of 1500 mm or less is preferable. In addition, the bending and unbending process should be performed at least once.

[0136] Furthermore, during the heating process, it is preferable to heat the cold-rolled steel sheet in an atmosphere with a dew point of -20°C to 20°C, containing 0.1 to 30% hydrogen by volume, with the remainder being nitrogen and impurities. Heating in an atmosphere containing 0.1 to 30% hydrogen, with the remainder being nitrogen and impurities, and with a dew point of -20 to 20°C, prevents the diffusion of easily oxidizable elements to the surface of the steel sheet and promotes the formation of a low-Mn layer on the surface of the steel sheet (promotes internal oxidation).

[0137] If the hydrogen content is less than 0.1%, the oxide film present on the surface of the steel sheet cannot be sufficiently reduced, and an oxide film may form on the steel sheet. In this case, internal oxidation and decarburization reactions may be insufficient. Furthermore, there is a concern that the chemical conversion treatment properties and plating adhesion of the final steel sheet will decrease. Moreover, if the hydrogen content exceeds 30%, safe operation becomes difficult. For this reason, the hydrogen content in the atmosphere (H ２The content is preferably 0.1% to 30% by volume. Furthermore, if the dew point of the atmosphere is below -20°C, external oxidation of Si and Mn occurs on the surface of the steel sheet, and internal oxidation and decarburization reactions may be insufficient. In this case, LME resistance and impact resistance may be slightly reduced. Also, if the dew point is above 20°C, an oxide film is formed on the steel sheet, reducing chemical conversion treatment properties and plating adhesion, and the decarburization reaction proceeds excessively, which may result in insufficient strength of the steel sheet obtained after annealing.

[0138] Generally, annealing furnaces used in the annealing process are broadly classified into three zones: a preheating zone, a heating zone, and a soaking zone. In this embodiment, the atmosphere during the heating process is set to the above conditions, so the control of the atmosphere is mainly carried out in the heating zone of the annealing furnace. However, if the steel sheet temperature is within the heating temperature range in the preheating zone and soaking zone as well, atmosphere control may also be carried out in these annealing furnaces.

[0139] (Soaking Process) In the soaking process, the cold-rolled steel sheet is held for 10 to 400 seconds in a temperature range above Ac1 and below Ac3. By holding the cold-rolled steel sheet under these conditions, needle-shaped austenite is formed along the lath of the low-temperature transformation structure formed up to the winding process. From the viewpoint of ensuring a volume fraction of retained austenite after annealing, the temperature range in which the holding time should be controlled for the soaking process is preferably (Ac1 + 30°C) or higher, and more preferably (Ac1 + 40°C) or higher. Furthermore, from the viewpoint of more stably forming needle-shaped austenite along the lath of the low-temperature transformation structure, the temperature range in which the holding time should be controlled for the soaking process is preferably (Ac3 - 5°C) or lower, and more preferably (Ac3 - 10°C) or lower.

[0140] If the holding time is less than 10 seconds, sufficient austenite will not form during the holding process. As a result, a nearly single-phase ferrite structure will be obtained, and the desired metallic structure cannot be acquired. Furthermore, if the holding time exceeds 400 seconds, the metallic structure will coarseen and productivity will decrease. From the perspective of suppressing the coarsening of ferrite and austenite during the holding process, the holding time may be limited to 300 seconds or less.

[0141] Furthermore, the temperature of the cold-rolled steel sheet during the soaking process does not need to be constant. In other words, the terms "soaking" and "holding" used above do not mean holding at a constant temperature, but rather staying within a specific temperature range for a specific time. To put it another way, as long as the desired microstructure ratio can be obtained, the temperature of the steel sheet during the soaking process may vary within a temperature range above Ac1 and below Ac3.

[0142] (Cooling Process) In the cooling process, the cold-rolled steel sheet after the soaking process is cooled from the Ac1 point to a temperature below the Ms point. At this time, the temperature decrease is allowed to continue without stopping until the temperature reaches any temperature below the Ms point from the Ac1 point, and the average cooling rate is calculated for that temperature decrease period. In this embodiment, the average cooling rate in the cooling process is set to 4°C / s or higher. By cooling at such an average cooling rate, ferrite transformation during cooling can be suppressed, and the desired amount of martensite and retained austenite can be obtained in the microstructure at the time cooling stops. If the average cooling rate is less than 4°C / s, ferrite transformation may not be suppressed. If ferrite transformation is not sufficiently suppressed, the desired amount of martensite and retained austenite cannot be obtained in the final microstructure, and the proportion of remaining microstructures such as pearlite may become excessively large. For these reasons, the average cooling rate in the cooling process is preferably 8°C / s or higher, and more preferably 10°C / s or higher. The upper limit of the average cooling rate in the cooling process is not particularly limited, but it may be 100°C / s or lower. The cooling stop temperature during the cooling process is not particularly limited as long as it is below the Ms point, but it may be cooled to a temperature range of 100 to 340°C. The Ms point is the temperature at which martensitic transformation begins. Therefore, by utilizing the volume expansion that occurs during martensitic transformation, the Ms point can be determined by experimentally simulating the heat pattern from the heating process to the soaking process for each steel type component during the annealing process, and then measuring the thermal expansion coefficient during gas rapid cooling. In this embodiment, the Ms point is the temperature at which a rapid change in thermal expansion begins to occur when cooling from the austenite single-phase region at a cooling rate of 30°C / s or more. In a graph showing the relationship between temperature and thermal expansion coefficient, a tangent line is drawn to the curve of the linear thermal expansion change (representing the thermal expansion change of FCC crystal) when cooling from the austenite single-phase region at a cooling rate of 30°C / s or more, and the point at which the thermal expansion curve begins to deviate from that tangent line is defined as the Ms point.

[0143] (Reheating Process) In the reheating process, in order to increase the stability of the austenite, the cold-rolled steel sheet after the cooling process is reheated in a temperature range of 480°C or lower and held at that temperature range for 80 seconds or more. The reheating temperature range is preferably 340°C or higher, more preferably 350°C or higher, and even more preferably 370°C or higher, or 380°C or higher. During the holding process, carbon (C) is concentrated in the austenite, stabilizing the retained austenite, and hydrogen (H) diffuses and is released outside the steel sheet, improving various properties. If the maximum temperature reached by reheating is less than 350°C, the desired amount of retained austenite may not be obtained, and furthermore, sufficient diffusion of hydrogen may not occur. For this reason, it is preferable that the maximum temperature reached by reheating be 350°C or higher. On the other hand, if the maximum temperature reached by reheating exceeds 480°C, the retained austenite may decompose into ferrite and cementite. For this reason, the maximum temperature reached by reheating should be 480°C or lower. Preferably, it is 450°C or lower. If the holding time in the temperature range below 480°C is less than 80 seconds, carbon may not be sufficiently concentrated in the untransformed austenite, and hydrogen may not be released to the outside of the steel sheet. By making the holding time in the above temperature range 80 seconds or longer, the carbon concentration in the austenite can be increased, and the desired amount of retained austenite can be stably secured after final cooling. To stably obtain the above effect, it is preferable to make the holding time 100 seconds or longer. There is no need to limit the upper limit of the holding time, but since productivity will decrease if the holding time is excessively long, the holding time may be 1000 seconds or less.

[0144] The conditions for cooling the cold-rolled steel sheet to room temperature after the reheating process are not limited, but in order to stably obtain the desired metal structure, the cold-rolled steel sheet after the reheating process may be cooled so that the average cooling rate to the Ms point is 2°C / s or more.

[0145] (Plating Process) The steel sheet manufacturing method according to this embodiment may further include a hot-dip galvanizing process in which a plating is formed on the surface of the cold-rolled steel sheet during the cooling process after annealing, during the reheating process, or after the reheating process. Furthermore, an alloying process may further include an alloying process in which the plating layer is alloyed after the hot-dip galvanizing process. The hot-dip galvanizing method and the alloying method are not particularly limited, and conventional methods can be used. As an example of a hot-dip galvanizing method, one method is to stop cooling in the middle of the cooling process at a temperature range of (galvanizing bath temperature - 40)°C to (galvanizing bath temperature + 50)°C, control the temperature to this range, and immerse the sheet in the hot-dip galvanizing bath to form a hot-dip galvanized layer. As an example of an alloying method, one method is to alloy the hot-dip galvanized layer at a temperature range of 300 to 500°C. When plating is performed during the annealing process, it is conceivable that the thermal history due to the plating process will overlap with the thermal history described in the annealing process, or that the thermal history corresponding to each process will be repeated multiple times. In such cases, it is sufficient to control the process so that the necessary conditions for each process are satisfied during the first period of the period during which the temperature requirements for each process can be calculated.

[0146] Next, a part according to this embodiment will be described. The part according to this embodiment includes the steel sheet according to this embodiment described above. Specifically, a part manufactured using the steel sheet according to this embodiment has the same chemical composition as the steel sheet described above. In addition, the part may contain a mixture of processed and unprocessed parts. The unprocessed parts have the same metal structure as the steel sheet described above. In the processed parts, the metal structure is basically the same as the steel sheet described above, but in areas that have undergone heavy processing or welding, at the edges of the part, or where red rust has occurred, the metal structure described above may not be present, or it may be difficult to determine. Therefore, when measuring the metal structure and mechanical properties of a part, these areas should be avoided, and measurements should be taken only in the unprocessed parts. If there are no unprocessed parts, measurements should be taken only in the parts that have not undergone heavy processing. Unprocessed or unprocessed parts refer to, for example, flat parts of the part, parts where the increase or decrease in plate thickness due to processing is small, and parts that have not undergone punching, hole widening, bending, etc. For example, in the case of the above-mentioned part, the test piece is taken from the flat, largest surface area, near its center of gravity, and then investigated.

[0147] Specifically, the following areas on a part should be avoided for measurement: (i) Welds: within 20 mm of the toe of spot welds, and within 20 mm of the toe of arc / laser welds. (ii) Machined parts: machined parts with a radius of curvature of less than 15 mm, and within 5 mm of the said machined parts. (iii) Ends: ends within 5 mm of the cut end face of the part. (iv) Red rust: within 5 mm of areas where red rust is visible to the naked eye.

[0148] The present invention will be described in more detail with reference to examples.

[0149] Slabs having the chemical composition shown in Table 1 were cast. The cast slabs were hot-rolled to a thickness of 2.8 mm. After hot-rolling, the hot-rolled steel sheets were cooled and coiled under the conditions described in Table 2A, and then cold-rolled under the conditions described in Table 2A to obtain cold-rolled steel sheets with the thicknesses shown in Table 3.

[0150] Subsequently, annealing was performed under the conditions described in Table 2B. After the reheating process, the steel sheets were cooled so that the average cooling rate to the Ms point was 2°C / s or higher. Then, in some cases, hot-dip galvanizing or alloyed hot-dip galvanizing was performed under conventional conditions. Steel sheets No. 1 to 39 shown in Table 3 were obtained by the above manufacturing method.

[0151]

[0152]

[0153]

[0154]

[0155] <Measurement of Metallographic Structure> Test specimens for SEM observation were taken from the obtained steel sheets (annealed steel sheets or steel sheets plated after annealing), and after polishing the longitudinal section parallel to the rolling direction, the metallographic structure at 1 / 4 of the sheet thickness was observed in the manner described above, and the volume fraction of each structure was measured by image processing. In addition, the volume fraction of retained austenite was determined by X-ray diffraction in the manner described above. The volume fractions of each structure are shown in Table 3. In Table 3, "B" represents the volume fraction of bainite (%), "α" represents the volume fraction of ferrite (%), "P" represents the volume fraction of pearlite (%), "γ" represents the volume fraction of retained austenite (%), and "M" represents the volume fraction of martensite (%).

[0156] Furthermore, from the obtained steel plates, the proportion of retained austenite with an aspect ratio of 3.0 or higher relative to the total retained austenite was determined by EBSD analysis using FE-SEM in the manner described above. The results are shown in Table 3.

[0157] Furthermore, the diameter of the inscribed circle of the region consisting of martensite and retained austenite, the average major axis length of retained austenite with an aspect ratio of 3.0 or higher, Ls / Lc, the thickness of the soft layer, and the thickness of the low-Mn layer were measured from the obtained steel sheet in the manner described above. The results are shown in Table 3.

[0158] <Characteristics> In addition, the tensile strength (TS) of the obtained steel plate, uniform elongation (%) as an indicator of formability, LME resistance of the spot welded area, and impact resistance assuming a part with pre-strain due to processing were evaluated using the following methods.

[0159] (Tensile Strength) Tensile test specimens of JIS No. 5 were taken from the obtained steel plates perpendicular to the rolling direction, and the tensile strength was measured by performing a tensile test in accordance with JIS Z 2241:2011. A tensile strength of 980 MPa or higher was considered acceptable. The results of the tensile strength measurement are shown in Table 3.

[0160] (Uniform Elongation) A JIS No. 5 tensile test specimen, as specified in JIS Z 2241:2011, was taken from the obtained steel sheet perpendicular to the rolling direction, and uniform elongation was measured by performing a tensile test in accordance with JIS Z 2241:2011. A uniform elongation (%) of 7.5% or higher was considered to indicate excellent formability. The results are shown in Table 3.

[0161] (Collision Resistance) From the obtained steel plate, a JIS No. 5 test piece was taken so that the tensile direction was the rolling direction, and a tensile test was performed on the test piece at a constant strain rate of 0.001 / s. The testing machine was stopped when a tensile pre-strain of 0.5% was applied by stroke control. From this tensile test piece with the applied tensile pre-strain, a VDA bending test piece with the same width but a parallel section length of 60 mm was cut out by machining. Using the obtained VDA bending test piece, a bending test was performed under the following measurement conditions based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry, and the maximum bending angle α was measured. Note that, due to the correspondence when cut from the tensile test piece, the bending deformation was in a direction perpendicular to the rolling direction of the steel plate. In this example, the displacement at the maximum load obtained in the bending test was converted into an angle according to the VDA standard, and the maximum bending angle α (deg) was determined. A specimen was deemed acceptable if its maximum bending angle α (deg) was equal to or greater than α1 (deg) as defined by the following formula. If the tensile strength of the test specimen exceeded 1180 MPa, formula (3) was used for evaluation; if it was 1180 MPa or less, formula (4) was used. Here, in formulas (3) and (4), t is the thickness of the steel plate (mm), and TS is the tensile strength (MPa).

[0162] <Measurement Conditions> Specimen dimensions: 60 mm (rolling direction) x 25 mm (direction perpendicular to rolling) Bending edge: Punched in so that the bending edge is perpendicular to the rolling direction Test method: Roll support, punch pressing Roll diameter: φ30 mm Punch shape: Tip radius = 0.4 mm Distance between rolls: 2.0 x plate thickness t (mm) + 0.5 mm Pressing speed: 20 mm / min Testing machine: SHIMADZU AUTOGRAPH 20kN

[0163]

[0164]

[0165] (LME resistance) A 50 mm x 80 mm test specimen was taken from the obtained steel plate. In addition, a slab having the chemical composition of A in Table 1 was cast, and after applying the manufacturing conditions of Example No. 1, it was immersed in a hot-dip galvanizing bath to produce a hot-dip galvanized steel plate (mating material). A 50 mm x 80 mm test specimen was taken from the manufactured steel plate (mating material).

[0166] Test pieces were taken from each of the steel plates No. 1 to 39, and the mating steel plate was placed on top of them. Two steel plates were then spot-welded together as shown in Figure 1. Specifically, the hot-dip galvanized steel plate used as the mating material was steel plate 1d in Figure 1, and the steel plates under evaluation (No. 1 to 39) were used as steel plate 1e. The two plates were stacked and spot-welded using a pair of electrodes 4a and 4b. The welding conditions were a servo-motor-operated single-phase AC spot welding machine (power frequency 50 Hz) with a pressure of 450 kgf (4413 kg・m / s²). ２ While applying pressure, the current value was set to 6.5 kA, the electrode tilt angle θ to 3°, there was no upslope, the energizing time was 0.4 seconds, and the holding time after the energizing ended was 0.1 seconds.

[0167] After spot welding, the microstructure of the nugget center of the steel plate joint was observed using an optical microscope at magnifications ranging from 200x to 1000x. Based on the observation results, a rating of "A" was given if no cracks occurred, "B" if cracks less than 100 μm in length were observed, and "C" if cracks 100 μm or longer were observed. A rating of A or B indicated excellent LME resistance.

[0168] As shown in Table 3, in the embodiment according to the present invention, the tensile strength was greater than 980 MPa, the uniform elongation (%) was greater than 7.5%, the impact resistance was excellent, and the LME resistance (length of cracks after spot welding) was evaluated as A or B.

[0169] Furthermore, in the case of plated steel sheets that have undergone hot-dip galvanizing or hot-dip galvanizing and alloying treatment with the above-mentioned steel sheets, the tensile strength was greater than 980 MPa, the uniform elongation (%) was greater than 7.5, the impact resistance was excellent, and the crack length after spot welding was evaluated as A or B.

[0170] On the other hand, comparative examples No. 22 to 39 had either a chemical composition or a microstructure that fell outside the scope of the present invention, and were inferior in at least one of the following: tensile strength, uniform elongation, impact resistance, and LME resistance.

[0171] 1d, 1e steel plate 4a, 4b electrode

Claims

1. The chemical composition, in mass%, is as follows: C: 0.10-0.30%, Si: 0.10-1.20%, Al: 0.30-1.50%, Mn: 1.0-4.0%, P: 0.100% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0200% or less, Ni: 0-1.00%, Mo: 0-0.50%, Cr: 0-2.000%, Ti: 0-0.100%, B: 0-0.0100%, Nb: 0-0.10%, V: 0-0.50%, Cu: 0-1.00%, W: 0-0.20%, Ta: 0-0.100%. It contains Co: 0-3.00%, Mg: 0-0.050%, Ca: 0-0.050%, Y: 0-0.050%, Zr: 0-0.500%, La: 0-0.050%, Ce: 0-0.050%, Sn: 0-0.100%, Sb: 0-0.500%, and As: 0-0.100%, with the remainder being Fe and impurities, and in the metallographic structure at 1 / 4 of the plate thickness, the volume fraction of retained austenite is 3-20%, the volume fraction of martensite is 30-75%, and the remainder is one or more of ferrite, bainite, and pearlite. A steel sheet having a thickness of 1.0 to 2.5 mm, wherein the number of retained austenite particles with an aspect ratio of 3.0 or more in the first quarter of the sheet thickness is 80% or more of the total number of retained austenite particles in the first quarter of the sheet thickness, the average major axis length of retained austenite particles with an aspect ratio of 3.0 or more in the first quarter of the sheet thickness is less than 15 μm, the diameter of the inscribed circle of all regions consisting of martensite and retained austenite in the first quarter of the sheet thickness is less than 2.0 μm, the diameter of the inscribed circle of all regions consisting of martensite and retained austenite in the eighth of the sheet thickness is 1.8 μm or less, the ratio Ls / Lc of the average major axis length Ls of retained austenite particles with an aspect ratio of 3.0 or more in the eighth of the sheet thickness to the average major axis length Lc of retained austenite particles with an aspect ratio of 3.0 or more in the center of the sheet thickness is 0.93 or less.

2. The steel sheet according to claim 1, wherein the chemical composition is as follows, in mass%, Cu: 0-0.50%, W: 0-0.10%, Co: 0-0.50%, Zr: 0-0.050%, Sn: 0-0.050%, Sb: 0-0.050%, and As: 0-0.050%.

3. The steel sheet according to claim 1 or 2, wherein the ratio of the number of microstructures in 1 / 8 of the sheet thickness to the number of microstructures in the center of the sheet thickness is 1.1 or more.

4. A steel sheet according to any one of claims 1 to 3, comprising a low-Mn layer having a thickness of 4.0 μm or more from the surface and a soft layer having a thickness of 10 to 100 μm from the surface.

5. A steel sheet according to any one of claims 1 to 4, having a hot-dip galvanized layer on its surface.

6. A steel sheet according to any one of claims 1 to 4, having an alloyed hot-dip galvanized layer on its surface.

7. A slab heating step of heating a slab having the chemical composition described in claim 1 at a temperature of 1100°C or higher for 1200 seconds or more; a hot rolling step of hot rolling the slab to make a hot-rolled steel sheet; a winding step of cooling the hot-rolled steel sheet to a temperature of 400°C or lower and winding it up; a cold rolling step of cold rolling the hot-rolled steel sheet after the winding step at a reduction ratio of 0 to 30% to make a cold-rolled steel sheet; and an annealing step of annealing the cold-rolled steel sheet, wherein the hot rolling step includes a rolling pass in which at least two rolling passes are performed consecutively in a temperature range of 1010 to 1180°C, with a reduction ratio of 40% or more per pass, and the inter-pass time is 3 to 20 seconds, and the total reduction ratio in the temperature range of over 960°C and below 1010°C is 0% or more and less than 5%; and the annealing step is A method for manufacturing a steel sheet, comprising: heating the cold-rolled steel sheet from Ac1-30°C to a maximum temperature greater than Ac1 and less than or equal to Ac3 at an average heating rate of 2.0°C / s or less, while simultaneously applying a bending and unbending motion to the cold-rolled steel sheet; holding the cold-rolled steel sheet in the temperature range greater than Ac1 and less than or equal to Ac3 for 10 to 400 seconds; cooling the cold-rolled steel sheet after holding in the temperature range from Ac1 to less than Ms at an average cooling rate of 4°C / s or more; and reheating the cooled cold-rolled steel sheet and holding it at 480°C or less for 80 seconds or more.

8. In the hot rolling process, the water volume and water pressure of the cooling water for the rolling rolls in the temperature range of 1010 to 1180°C shall be 8.0 L / min or more and 1.0 × 10⁻¹⁰ ６ kgf / m ２ The method for manufacturing a steel plate according to claim 7.

9. The method for manufacturing a steel sheet according to claim 7 or 8, wherein in the annealing step, the cold-rolled steel sheet is heated in an atmosphere having a dew point of -20°C to 20°C, containing 0.1 to 30% hydrogen by volume, with the remainder being nitrogen and impurities.

10. A method for manufacturing a steel sheet according to any one of claims 7 to 9, comprising a hot-dip galvanizing step in which the cold-rolled steel sheet after the annealing step is immersed in a hot-dip galvanizing bath to form a hot-dip galvanized layer on the surface of the cold-rolled steel sheet, after controlling the temperature of the cold-rolled steel sheet after the annealing step to a temperature range of (galvanizing bath temperature - 40)°C to (galvanizing bath temperature + 50)°C.

11. The method for manufacturing a steel sheet according to claim 10, comprising an alloying step of heating the hot-dip galvanized layer to a temperature range of 300 to 500°C to alloy the hot-dip galvanized layer.

12. A component comprising a steel plate according to any one of claims 1 to 6.

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