STEEL SHEET AND METHOD FOR PRODUCING IT

Abstract

This steel sheet has a predetermined chemical composition and includes, as a metallographic structure, ferrite, bainite, and pearlite in a total volume percentage of 0% or more and 50% or less, residual austenite in a volume percentage of 3% or more and 20% or less, and a remainder of fresh martensite and quenched martensite, wherein the residual austenite with an aspect ratio of 3.0 or more occupies 80% or more of the total residual austenite by area ratio, the steel sheet includes an internal oxide layer with a thickness of 4.0 µm or more from the surface of the steel sheet and a decarburized layer that has a thickness of 10 µm or more and 100 µm or less from the surface of the steel sheet, and an amount of diffusible hydrogen included in the steel sheet is 1.00 ppm or less by mass.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a steel sheet and a manufacturing method thereof. This application claims priority based on Japanese Patent Application No. 2021-001682, filed on January 7, 2021, the contents of which are incorporated herein by reference.

[0002] High-strength steel sheets are used as steel sheets for automobiles in order to reduce the weight of automobiles to improve fuel efficiency, reduce carbon dioxide emissions, and ensure the safety of passengers. In recent years, in order to ensure sufficient corrosion resistance of vehicle bodies and parts, high-strength galvannealed steel sheets have also been used as steel sheets for automobiles, in addition to high-strength hot-dip galvanized steel sheets (see, for example, Patent Document 1).

[0003] Furthermore, high-strength steel sheets used for automotive parts are required to have not only strength but also properties (formability) necessary for part forming, such as uniform elongation. Strength and formability are in a trade-off relationship, but a TRIP (Transformation Induced Plasticity) steel sheet, which is a high-strength steel sheet that utilizes the transformation-induced plasticity of retained austenite, is known as a means for achieving both.

[0004] However, when galvanized steel sheets (hot-dip galvanized steel sheets, electrogalvanized steel sheets, or galvannealed steel sheets) are spot-welded together or when cold-rolled steel sheets are spot-welded to galvanized steel sheets for the assembly of vehicle bodies and / or parts, cracks called liquid metal embrittlement (LME) cracks may occur at the spot welds. LME cracks occur when the heat generated during spot welding melts the zinc in the galvanized layer, causing the molten zinc to penetrate into the grain boundaries of the steel sheet structure at the weld, and tensile stress acts on this state. Even if one of the steel sheets is a cold-rolled steel sheet that has not been galvanized, LME cracks may occur when the molten zinc from the galvanized steel sheet comes into contact with the cold-rolled steel sheet during spot welding if the other is a galvanized steel sheet. Furthermore, LME cracks occur significantly, particularly when high-strength TRIP steel sheets (transformation-induced plasticity steel sheets) are spot-welded. High-strength TRIP steel sheet is a steel sheet that has higher concentrations of C, Si, and Mn than ordinary high-strength steel sheets and contains retained austenite, thereby having excellent energy absorption capacity and press formability.

[0005] Furthermore, for ultra-high-strength steel sheets with tensile strengths exceeding 980 MPa, it is necessary to address not only formability but also the problem of hydrogen embrittlement cracking of steel sheets. Hydrogen embrittlement cracking is a phenomenon in which a steel component subjected to high stress during use suddenly fractures due to hydrogen infiltrating from the environment. This phenomenon is also called delayed fracture due to the fracture mode. It is generally known that hydrogen embrittlement cracking of steel sheets becomes more likely as the tensile strength of the steel sheet increases. This is thought to be because the higher the tensile strength of the steel sheet, the greater the residual stress in the steel sheet after part formation. The susceptibility to this hydrogen embrittlement cracking (delayed fracture) is called hydrogen embrittlement resistance. In the case of automotive steel sheets, hydrogen embrittlement cracking is particularly likely to occur in bent sections where large plastic strain is applied. Therefore, when using high-strength steel sheets for automotive components, not only formability such as elongation, bendability, and hole expandability, but also hydrogen embrittlement resistance of bent sections is required. High-strength steel plates used in car bodies are easily embrittled by hydrogen in the steel, and are prone to cracking and breaking even at low stress when subjected to stress such as bending deformation.

[0006] To address these issues, for example, Patent Document 2 discloses a high-strength steel sheet that has excellent ductility and hole expandability, excellent chemical conversion treatability and plating adhesion, and good fatigue properties and hydrogen embrittlement resistance in bent portions.

[0007] International Publication No. WO 2018 / 043453 International Publication No. WO 2019 / 187060

[0008] However, in the case of automobiles, punching is performed when forming parts. As a result of investigations by the present inventors, it was found that although the high-strength steel plate of Patent Document 2 has excellent hydrogen embrittlement resistance in a bent portion, there is a concern that hydrogen embrittlement may occur at the punched end surface when punching is performed, and that the steel plate may not be able to meet the recent demand for higher crashworthiness.

[0009] As described above, conventionally, no steel sheet has been disclosed that has high strength, as well as excellent formability, crash resistance (particularly crash resistance at punched portions), and LME resistance during spot welding. In view of the above, an object of the present invention is to provide a steel sheet that has high strength, as well as excellent formability (particularly uniform elongation), crash resistance (particularly at punched portions), and LME resistance during spot welding, and a method for manufacturing the same.

[0010] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0011] [1] A steel sheet according to one embodiment of the present invention has a chemical composition, in mass%, of C: 0.10 to 0.40%, Si: 0.10 to 1.20%, Al: 0.30 to 1.50%, Mn: 1.0 to 4.0%, P: 0.0200% 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 2.00%, T i: 0-0.100%, B: 0-0.0100%, Nb: 0-0.10%, V: 0-0.50%, Cu: 0-0.50%, W: 0-0.10%, Ta: 0-0.100%, Co: 0- 0.50%, Mg: 0-0.050%, Ca: 0-0.0500%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.0500%, Ce: 0-0.050%, Sn: [2] The steel sheet according to the above item [1] may have a hot-dip galvanized layer on the surface, and a decarburized layer on the surface, and the amount of diffusible hydrogen contained in the steel sheet is 1.00 ppm or less by mass. [3] The steel sheet according to the above item [1] may have a hot-dip galvanized layer on the surface. [3] In the steel sheet according to the above [1], the surface may have a galvannealed layer.[4] A method for producing a steel sheet according to another aspect of the present invention includes a hot rolling step of hot rolling a slab having the chemical composition described in [1] to obtain a hot rolled steel sheet, a coiling step of cooling the hot rolled steel sheet at a cooling rate of 5°C / s or more and coiling it at 400°C or less, a cold rolling step of pickling the hot rolled steel sheet after the coiling step and then cold rolling it at a reduction of 0.5% to 20.0% to obtain a cold rolled steel sheet, a hydrogen content reduction step of leaving the cold rolled steel sheet in the atmosphere for one hour or more and for t hours or more represented by the following formula (1), and a annealing step of annealing the cold rolled steel sheet after the hydrogen content reduction step. and an annealing process, in which the cold-rolled steel sheet is bent and unbent at 150 to 400°C, heated in an atmosphere having a dew point of -20 to 20°C and containing 0.1 to 30.0 volume % hydrogen, with the remainder being nitrogen and impurities, the cold-rolled steel sheet after heating is held at a holding temperature of Ac1 to Ac3°C for 1 second to 1,000 seconds, the cold-rolled steel sheet after holding is cooled to 100 to 340°C at an average cooling rate of 4°C / s or more, and the cooled cold-rolled steel sheet is reheated and held at 350 to 480°C for 80 seconds or more. t = -2.4 × T + 96 (1) where T is the average air temperature (°C) during storage. [5] The method for producing a steel sheet according to the above [4] may further include a hot-dip galvanizing step of controlling the temperature of the cold-rolled steel sheet after the annealing step to a range of (galvanizing bath temperature - 40) ° C. to (galvanizing bath temperature + 50) ° C., and then immersing the cold-rolled steel sheet in a hot-dip galvanizing bath to form a hot-dip galvanized coating on the surface of the cold-rolled steel sheet. [6] The method for producing a steel sheet according to the above [5] may further include an alloying step of heating the hot-dip galvanized steel sheet to a temperature range of 300 to 500 ° C. to alloy the coating layer.

[0012] According to the above-described aspects of the present invention, it is possible to provide a steel sheet having high strength, as well as excellent formability, crash resistance, and LME resistance during spot welding, and a method for manufacturing the same.

[0013] FIG. 1 is a diagram illustrating a test method for evaluating molten metal embrittlement cracking resistance (LME resistance).

[0014] A steel sheet according to one embodiment of the present invention (steel sheet according to this embodiment) and a method for manufacturing the same will be described below. The steel sheet according to this embodiment has a predetermined chemical composition described below, and in its metallographic structure, the volume fraction of ferrite, bainite, and pearlite is 0% or more and 50% or less in total, the volume fraction of retained austenite is 3% or more and 20% or less, and the balance is one or both of fresh martensite and tempered martensite, the retained austenite having an aspect ratio of 3.0 or more accounts for 80% or more of the total retained austenite in terms of area fraction, the steel sheet has an internal oxidation layer having a thickness of 4.0 μm or more from the surface thereof, and a decarburized layer having a thickness of 10 μm or more and 100 μm or less from the surface thereof, and the steel sheet contains 1.00 ppm or less of diffusible hydrogen by mass.

[0015] <Metallic Structure> First, the metallic structure (microstructure) of the steel sheet according to this embodiment will be described. Hereinafter, the structural fraction will be expressed as a volume fraction, and the unit "%" of the structural fraction means volume % unless otherwise specified. In the case where the structural fraction is identified by image processing, the area fraction is regarded as the volume fraction. The metallic structure of the steel sheet according to this embodiment represents the metallic structure at the quarter thickness portion (a depth position of one-quarter of the plate thickness from the surface in the plate thickness direction) unless otherwise specified. The reason for specifying the metallic structure at the quarter thickness portion is that in the plate thickness direction, the microstructure (components) of the steel sheet may be significantly different from other portions near the surface and near the plate thickness center due to decarburization and Mn segregation, respectively, and the metallic structure at the quarter thickness portion is a representative structure of the steel sheet.

[0016] [Ferrite, Bainite, and Pearlite: Total of 0 to 50%] Ferrite is a soft structure that is easily deformed and contributes to improving elongation. However, to obtain the desired high strength, the volume fraction of ferrite must be limited. Bainite is a structure obtained by holding the steel at 350°C or higher and 450°C or lower for a certain period of time after annealing. Bainite is softer than martensite and contributes to improving elongation. However, to obtain the desired high strength, the volume fraction must be limited, as with ferrite. Pearlite contains hard iron carbides and is a structure that serves as the starting point for void generation during hole expansion. For the above reasons, in the steel sheet according to this embodiment, the total volume fraction of ferrite, bainite, and pearlite is set to 50% or less. To increase strength, the total volume fraction of ferrite, bainite, and pearlite may be 40% or less. Since ferrite, bainite, and pearlite are not essential to obtain the effects of this embodiment, the lower limit is 0%.

[0017] [Retained austenite: 3 to 20%] Retained austenite is a structure that contributes to improving elongation (particularly uniform elongation) due to the TRIP effect. To achieve this effect, the volume fraction of retained austenite is set to 3% or more. The volume fraction of retained austenite is preferably 5% or more, more preferably 7% or more. On the other hand, if the volume fraction of retained austenite is excessive, the grain size of the retained austenite becomes large. Such retained austenite with a large grain size becomes coarse and hard martensite after deformation. In this case, it is likely to become a starting point for cracks, and hole expandability deteriorates, which is undesirable. For this reason, the volume fraction of retained austenite is set to 20% or less. The volume fraction of retained austenite is preferably 18% or less, more preferably 16% or less.

[0018] Furthermore, in the steel sheet according to this embodiment, as will be described later, the stability of the retained austenite is improved by controlling not only the volume fraction of the retained austenite but also the aspect ratio of the retained austenite. High stability of the retained austenite can suppress stress-induced transformation to the hard fresh martensite phase, thereby improving uniform elongation.

[0019] [Remainder: Fresh Martensite and / or Tempered Martensite] The remainder other than the above-mentioned ferrite, bainite, pearlite, and retained austenite is one or both of fresh martensite and tempered martensite. Fresh martensite has a high dislocation density and a hard structure, which contributes to improving tensile strength. Tempered martensite, like fresh martensite, is a collection of lath-shaped crystal grains and contributes to improving tensile strength. On the other hand, unlike fresh martensite, tempered martensite is a hard structure containing fine iron-based carbides inside due to tempering. Tempered martensite is obtained by tempering fresh martensite formed by cooling after annealing, etc., through heat treatment, etc. Considering the volume fractions of ferrite, bainite, pearlite, and retained austenite, the total volume fraction of fresh martensite and tempered martensite is 30 to 97%.

[0020] The identification of ferrite, bainite, pearlite, retained austenite, fresh martensite, and tempered martensite in the metal structure and the calculation of their volume fractions will be described.

[0021] The volume fraction of retained austenite can be calculated by measuring diffraction intensity using X-rays. In measurements using X-rays, a sample cut out from a steel plate is mechanically polished and chemically polished to remove the surface from the surface to a depth position of 1 / 4 of the plate thickness, and X-ray diffraction is performed on the polished surface (1 / 4 depth position) using MoKα radiation. The structural fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200), (211) of the bcc phase and (200), (220), and (311) of the fcc phase. A five-peak method is generally used as a calculation method.

[0022] The volume fraction of fresh martensite is determined by the following procedure. A sample is taken so that the observation surface is a cross-section of the steel sheet thickness parallel to the rolling direction. The observation surface of the sample is etched with a RePella solution, and a 100 μm × 100 μm region within 1 / 8 to 3 / 8 of the sheet thickness from the surface, centered at a depth of 1 / 4 of the sheet thickness from the surface, is observed at 3000x magnification using a field emission scanning electron microscope (FE-SEM), and the volume fraction is determined from the resulting secondary electron image. Because fresh martensite and retained austenite are not corroded by RePella corrosion, the area fraction of the uncorroded region is the total area fraction of fresh martensite and retained austenite. The area fraction of this uncorroded region is considered to be the total area fraction of fresh martensite and retained austenite, and the volume fraction of fresh martensite is calculated by subtracting the volume fraction of retained austenite measured by X-rays from this total area fraction.

[0023] The volume fractions of ferrite, bainite, pearlite, and tempered martensite can be determined from secondary electron images obtained by observation using an FE-SEM. The observation surface is a cross-section of the steel sheet parallel to the rolling direction. The observation surface is polished and nital etched, and a 100 μm × 100 μm region within 1 / 8 to 3 / 8 of the sheet thickness from the surface, centered at a depth of 1 / 4 of the sheet thickness from the surface, is observed at 3000x magnification. By leaving multiple indentations around the region observed by the aforementioned Repela corrosion, it is possible to confirm the same region as observed by Repela corrosion. In observation, the inside of the grain boundaries of ferrite are captured with uniform contrast. Bainite is a collection of lath-shaped crystal grains that either do not contain iron-based carbides with a major axis of 20 nm or more within the interior, or contain iron-based carbides with a major axis of 20 nm or more within the interior, with the carbides belonging to a single variant, i.e., a group of iron-based carbides elongated in the same direction. Here, the term "iron-based carbide groups elongated in the same direction" refers to iron-based carbide groups whose elongation directions differ by no more than 5°. Tempered martensite is a collection of lath-shaped crystal grains, and contains iron-based carbides with a major axis of 20 nm or more, but the cementite in the structure has multiple variants. Furthermore, the region where cementite precipitates in a lamellar form is pearlite. Based on these differences, each structure is identified, and the area ratio is calculated by image processing. In this embodiment, the value of the area ratio calculated by image processing, as described above, is considered to be the volume ratio.

[0024] [Proportion of retained austenite with an aspect ratio of 3.0 or more: 80 area % or more of the total retained austenite] By making the retained austenite acicular, the stability of the retained austenite when strained is improved. Specifically, the retained austenite transforms from the grain boundaries to martensite in stages, and strain occurs as a result of this transformation. As the transformation progresses, dislocations generated near the grain boundaries move through the grains to the grain boundaries on the opposite side, resulting in the accumulation of dislocations. When the retained austenite is acicular, the distance from the vicinity of the grain boundaries where dislocations occur to the grain boundaries where dislocations accumulate is short. Therefore, a repulsive force is generated between the accumulated dislocations and newly generated dislocations, and strain caused by martensitic transformation is not tolerated. The martensitic transformation is inhibited by the above mechanism, thereby improving the stability of the retained austenite. In the steel sheet according to this embodiment, the retained austenite is acicular using a method described below. However, retained austenite generated without shape control does not form an acicular structure, and the stability of each retained austenite varies, resulting in poor uniform elongation. Furthermore, hydrogen is likely to remain inside austenite. However, acicular austenite has a larger surface area than blocky austenite, which promotes hydrogen diffusion within the austenite during the holding process described below. This reduces the amount of diffusible hydrogen in the steel sheet. In this embodiment, "retained austenite having an aspect ratio of 3.0 or more" is defined as "acicular retained austenite." When retained austenite having an aspect ratio of 3.0 or more accounts for 80% or more of the total retained austenite, uniform elongation is improved, and hydrogen embrittlement resistance is also improved. The retained austenite having an aspect ratio of 3.0 or more preferably accounts for 83% or more, more preferably 85% or more, of the total retained austenite. There is no particular upper limit to the proportion of retained austenite having an aspect ratio of 3.0 or more in the total retained austenite, and ideally it is 100%. The "proportion" referred to here is the area ratio, as will be described later. There is no upper limit to the aspect ratio of retained austenite that defines the area ratio, but if the aspect ratio is high, it may become the origin of voids when the retained γ transforms, which may reduce the uniform elongation.Therefore, preferably, the proportion of retained austenite with an aspect ratio of 3.0 to 8.0 is 80% or more.

[0025] The area ratio of retained austenite with an aspect ratio of 3.0 or more to the total retained austenite is determined by EBSD analysis using an FE-SEM. Specifically, a sample is taken with the observation surface at the thickness cross section parallel to the rolling direction of the steel sheet. The observation surface of the sample is polished, and the strain-affected layer is removed by electrolytic polishing. EBSD analysis is then performed on a 100 μm × 100 μm region within 1 / 8 to 3 / 8 of the sheet thickness from the surface, centered at a depth position of 1 / 4 of the sheet thickness from the surface, with measurement steps of 0.05 μm. The measurement magnification is selected from a range of 1000 to 9000 times; for example, 3000 times, the same as the observation of the SEM-backscattered electron image described above, may be used. A retained austenite map is created from the measurement data, and retained austenite with an aspect ratio of 3.0 or more is extracted to determine the area ratio (area of ​​retained austenite with an aspect ratio of 3.0 or more / area of ​​total retained austenite).

[0026] [Internal Oxidation Layer Thickness: 4.0 μm or More from the Surface] The steel sheet according to this embodiment has an internal oxidation layer with a thickness of 4.0 μm or more from the surface (the internal oxidation layer is formed to a depth of at least 4.0 μm from the surface). The internal oxidation layer is a layer in which at least a portion of the grain boundaries is covered with an oxide of an easily oxidizable element such as Si or Mn. Covering the grain boundaries with oxides suppresses the penetration of molten metal into the grain boundaries during welding and makes it possible to suppress LME cracking during welding. If the thickness of the internal oxidation layer is less than 4.0 μm, the above effects cannot be sufficiently obtained. Therefore, the thickness of the internal oxidation layer is set to 4.0 μm or more. On the other hand, if the thickness of the internal oxidation layer is too thick, uniform elongation decreases. Therefore, it is preferable that the upper limit of the internal oxidation layer is set to 15.0 μm or less. However, in the case of a plated steel sheet, the surface refers to the surface of the base steel sheet (the interface between the plated layer and the base steel sheet).

[0027] The thickness of the internal oxide layer is determined by the following method. When the thickness of the steel sheet (or the thickness of the base steel sheet in the case of a plated steel sheet) is t, the position t / 2 from the surface in the thickness direction is defined as the thickness center C. The measurement plane is a cross section of the thickness parallel to the rolling direction of the steel sheet, and the Mn concentration distribution is continuously measured using a high-frequency glow discharge optical emission spectrometer (GDS) over a distance of 120 μm from the surface toward the thickness center C, with the surface of the steel sheet as the origin. The formation of the internal oxide layer depletes dissolved Mn around the oxide, reducing the Mn concentration. Therefore, the Mn concentration is low in the internal oxide layer and increases from the internal oxide layer toward the interior of the sheet thickness, eventually reaching a constant concentration. Therefore, the concentration at this constant point is defined as the representative concentration within the steel sheet. As the Mn concentration increases from the internal oxide layer toward the interior of the sheet thickness, the position X1 is defined as the position where the Mn concentration is 90% of the representative concentration within the steel sheet, and the distance from the surface to X1 is defined as the thickness of the internal oxide layer. When analyzing using high-frequency glow discharge analysis, a known high-frequency GDS analysis method can be used. Specifically, a method is used in which the surface of a steel sheet is placed in an Ar atmosphere, a voltage is applied to generate glow plasma, and the steel sheet surface is sputtered while analysis is performed in the depth direction. The elements contained in the material (steel sheet) are identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the amount of the elements contained in the material is estimated from the emission intensity of the identified elements. Depth data can be estimated from the sputtering time. Specifically, the relationship between sputtering time and sputtering depth can be determined in advance using a standard sample, allowing the sputtering time to 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 equipment can be used for high-frequency GDS analysis.

[0028] [Decarburized Layer Thickness: 10 μm or More and 100 μm or Less from the Surface] To improve bendability after bending, softening the surface layer of a steel sheet is an important requirement. Providing a decarburized layer on the surface of a steel sheet is considered as a means of softening the surface layer of a steel sheet. Furthermore, the presence of a decarburized layer on the surface of a steel sheet improves hydrogen embrittlement resistance after bending. While the detailed mechanism by which the presence of a decarburized layer improves hydrogen embrittlement resistance after bending is unclear, it is thought that decarburization reduces the amount of retained austenite in the surface structure, thereby reducing the amount of fresh martensite generated by strain-induced transformation during bending, thereby improving hydrogen embrittlement resistance. To achieve the above-described effect, the steel sheet according to this embodiment has a decarburized layer that is 10 μm or more thick from the surface of the steel sheet (the decarburized layer is formed to a depth of at least 10 μm from the surface). If the thickness of the decarburized layer is less than 10 μm, the above-described effect cannot be sufficiently achieved. On the other hand, if the thickness of the decarburized layer exceeds 100 μm, the strength is insufficient. Therefore, the thickness of the decarburized layer is set to 100 μm or less.

[0029] The thickness of the decarburized layer is determined as follows. In the steel sheet according to this embodiment, the decarburized layer is defined as the region (excluding the coating layer) on the surface side of the steel sheet relative to the deepest position where the average hardness is 80% or less of the average hardness inside the steel sheet. In this embodiment, the average hardness inside the steel sheet and the average hardness at each position in the thickness direction of the steel sheet are determined as follows. A sample is taken from a cross-section of the steel sheet thickness parallel to the rolling direction, and the observation surface is polished to a mirror finish. Further, chemical polishing is performed using colloidal silica to remove the surface-worked layer. Using a microhardness tester, a square pyramidal Vickers indenter with an apex angle of 136° is pressed into the observation surface of the obtained sample at a 10 μm pitch in the thickness direction of the steel sheet, starting from a depth of 5 μm from the surface (the interface between the base steel sheet and the coating layer in the case of a coated steel sheet) to a position ⅛ of the sheet thickness from the surface. The indentation load is set so that the Vickers indentations do not interfere with each other. For example, the indentation load is 20 gf. The diagonal length of the indentation is then measured using an optical microscope or scanning electron microscope, and converted into Vickers hardness (Hv). Next, the measurement position is moved by 10 μm or more in the rolling direction, and the same measurement is performed from a starting point 10 μm deep from the surface to a position 1 / 8 of the plate thickness. Next, the measurement position is moved by 10 μm or more in the rolling direction, and the same measurement is performed from a starting point 5 μm deep from the surface to a position 1 / 8 of the plate thickness. Next, the measurement position is moved by 10 μm or more in the rolling direction, and the same measurement is performed from a starting point 10 μm deep from the outermost layer to a position 1 / 8 of the plate thickness. By repeating this process, Vickers hardness is measured at five points for each depth position. In this way, hardness measurement data is effectively obtained at a 5 μm pitch in the depth direction. The measurement interval is not simply set to 5 μm pitch to avoid interference between indentations. The average value of five points at the same depth position is taken as the hardness at that thickness position. The hardness profile in the depth direction is obtained by linearly interpolating between the data points. In addition, the hardness is measured at at least five points in the range from 1 / 8 to 3 / 8 thickness, centered at the 1 / 4 thickness position, using a microhardness measuring device in the same manner as above, and the average value obtained is used as the average hardness inside the steel sheet.The region on the surface side of the steel plate from the deepest position where the average hardness is 80% or less of the average hardness inside the steel plate determined as described above is defined as the decarburized layer.

[0030] In the steel sheet according to this embodiment, a decarburized layer defined as above exists in a region having a thickness of 10 to 100 μm from the surface in the sheet thickness direction. In other words, a decarburized layer having a hardness of 80% or less of the average hardness inside the steel sheet exists in the surface layer portion of the steel sheet, and the thickness of the decarburized layer is 10 to 100 μm.

[0031] [Amount of diffusible hydrogen contained in steel sheet: 1.00 ppm or less] The smaller the amount of diffusible hydrogen in the steel sheet, the better the crash resistance properties. In the steel sheet according to this embodiment, the amount of diffusible hydrogen in the steel sheet is set to 1.00 ppm or less by mass so that the steel sheet has excellent crash resistance properties even when it has high strength. If the diffusible hydrogen amount exceeds 1.00 ppm, the crash resistance properties decrease. The diffusible hydrogen amount is preferably 0.80 ppm or less. Hydrogen embrittlement resistance is sometimes evaluated by the critical diffusible hydrogen amount, but in the steel sheet according to this embodiment, the amount of diffusible hydrogen in the steel sheet is controlled from the perspective of reducing the amount of hydrogen during production.

[0032] The amount of diffusible hydrogen in the steel sheet is measured by a temperature-rising hydrogen analysis method using a gas chromatograph (heating rate: 100°C / hour, measurement up to 300°C), and the amount of hydrogen released from the steel material from room temperature to 200°C is defined as the amount of diffusible hydrogen.

[0033] Next, the reasons for limiting the chemical composition of the steel sheet according to this embodiment will be described. Hereinafter, % in the chemical composition means % by mass.

[0034] <Chemical Composition> C: 0.10 to 0.40% C is an element that ensures a predetermined amount of martensite (fresh martensite and tempered martensite) and improves the strength of the steel sheet. When the C content is 0.10% or more, a predetermined amount of martensite can be obtained and the desired tensile strength can be ensured. The C content is preferably 0.12% or more. On the other hand, when the C content exceeds 0.40%, weldability and LME resistance deteriorate, as well as hole expandability. Hydrogen embrittlement resistance also deteriorates. Therefore, the C content is set to 0.40% or less. The C content is preferably 0.35% or less.

[0035] Si: 0.10% to 1.20% Si is a useful element for improving the strength of steel sheets through solid solution strengthening. Furthermore, Si inhibits the formation of cementite, promoting the concentration of C in austenite and effectively generating retained austenite after annealing. Si also has the effect of segregating carbon (C) at γ grain boundaries during the annealing process described below. A Si content of less than 0.10% is undesirable because it is difficult to obtain the above-mentioned effects, sufficient uniform elongation cannot be obtained, and hydrogen embrittlement resistance deteriorates. Therefore, the Si content is set to 0.10% or more. The Si content is preferably 0.50% or more, more preferably 0.60% or more. On the other hand, a Si content exceeding 1.20% increases the likelihood of LME cracking during welding and significantly deteriorates chemical conversion treatability and platability. Therefore, the Si content is set to 1.20% or less. The Si content is preferably 1.10% or less, more preferably 1.00% or less.

[0036] Al: 0.30% or more, 1.50% or less Al is an element that has the effect of deoxidizing molten steel. Furthermore, like Si, Al suppresses the formation of cementite, and is therefore an effective element for promoting the concentration of C in austenite and generating retained austenite after annealing. In the steel sheet according to this embodiment, the Si content is set within the above-mentioned range to improve LME resistance, and the Al content is set to a relatively high range to increase the volume fraction of retained γ. Specifically, since these effects cannot be sufficiently obtained when the Al content is less than 0.30%, the Al content is set to 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 generated, reducing the workability of the steel sheet. Furthermore, a high Al content deteriorates castability. Therefore, the Al content is set to 1.50% or less. The Al content is preferably 1.40% or less, more preferably 1.30% or less.

[0037] Mn: 1.0 to 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. By setting the Mn content to 1.0% or more, a desired metal structure can be obtained. The Mn content is preferably 1.3% or more. On the other hand, if the Mn content is excessive, the effect of improving hardenability decreases due to Mn segregation, and the material cost increases. Therefore, the Mn content is set to 4.0% or less. The Mn content is preferably 3.5% or less.

[0038] P: 0.0200% or less P is an impurity element that segregates in the center of the steel plate thickness, reducing toughness and embrittling the weld. If the P content exceeds 0.0200%, the weld strength and hole expandability will be significantly reduced. Therefore, the P content is set to 0.0200% or less. The P content is preferably 0.0100% or less. The lower the P content, the better, and 0% is acceptable. However, reducing the P content to less than 0.0001% in practical steel plates will significantly increase manufacturing costs and be economically disadvantageous. Therefore, the P content may be set to 0.0001% or more.

[0039] S: 0.0200% or less S is an impurity element that reduces weldability and manufacturability during casting and hot rolling. S also forms coarse MnS, causing a decrease in hole expandability. If the S content exceeds 0.0200%, the deterioration of weldability, manufacturability, and hole expandability becomes significant. Therefore, the S content is set to 0.0200% or less. The lower the S content, the better, and 0% is acceptable. However, reducing the S content to less than 0.0001% in practical steel sheets significantly increases manufacturing costs and is economically disadvantageous. Therefore, the S content may be set to 0.0001% or more.

[0040] N: 0.0200% or less N is an element that forms coarse nitrides, reduces bendability and hole expandability, and causes blowholes during welding. If the N content exceeds 0.0200%, the hole expandability decreases and blowholes occur significantly. Therefore, the N content is set to 0.0200% or less. The lower the N content, the better, and 0% is acceptable. However, reducing the N content to less than 0.0001% in practical steel sheets significantly increases manufacturing costs and is economically disadvantageous. Therefore, the N content may be set to 0.0001% or more.

[0041] O: 0.0200% or less O is an element that forms coarse oxides, reduces bendability and hole expandability, and causes blowholes during welding. If the O content exceeds 0.0200%, the hole expandability decreases and blowholes occur significantly. Therefore, the O content is set to 0.0200% or less. The lower the O content, the better, and 0% is acceptable. However, 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 set to 0.0005% or more.

[0042] In the chemical composition of the steel sheet according to this embodiment, the balance excluding the above elements is basically Fe and impurities. Impurities are elements that are mixed in from the steel raw materials and / or during the steelmaking process, and whose presence is permitted to the extent that they do not clearly deteriorate the properties of the steel sheet according to this embodiment. On the other hand, the chemical composition of the steel sheet according to this embodiment is such that, in order to improve various properties, part of Fe is replaced with 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, and the like. %, Co: 0.50% or less, Mg: 0.050% or less, Ca: 0.0500% or less, Y: 0.050% or less, Zr: 0.050% or less, La: 0.0500% or less, Ce: 0.050% or less, Sn: 0.05% or less, Sb: 0.050% or less, and As: 0.050% or less. These elements do not necessarily need to be contained, so the lower limit is 0%. Furthermore, as long as they are within the above ranges, the effects of the steel sheet according to this embodiment are not impaired even if these elements are contained as impurities.

[0043] Ni: 0 to 1.00% Ni is an element effective in improving the strength of steel sheet. The Ni content may be 0%, but to obtain the above effect, the Ni content is preferably 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 deteriorate. For this reason, the Ni content is set to 1.00% or less.

[0044] Mo: 0 to 0.50% Mo, like Cr, is an element that contributes to increasing the strength of steel sheets. This effect can be achieved even with a small amount. The Mo content may be 0%, but to achieve 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 are formed, which may reduce the cold formability of the steel sheet. For this reason, the Mo content is set to 0.50% or less.

[0045] Cr: 0 to 2.00% Cr is an element that improves the hardenability of steel and contributes to high strength, and is an effective element for obtaining the above-mentioned metal structure. Therefore, Cr may be contained. The Cr content may be 0%, but in order to fully obtain the above-mentioned effects, it is preferable that the Cr content be 0.01% or more. On the other hand, if Cr is contained in excess, the effects of the above actions will saturate and it will become uneconomical. Therefore, the Cr content is set to 2.00% or less.

[0046] Ti: 0 to 0.100% Ti is an element that contributes to increasing the strength of steel sheet through precipitation strengthening, grain refinement due to suppression of ferrite grain growth, and / or dislocation strengthening through suppression of recrystallization. The Ti content may be 0%, but to fully obtain the above effects, the Ti content is preferably 0.001% or more. To further increase the 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, deteriorating formability. For this reason, the Ti content is set to 0.100% or less.

[0047] B: 0 to 0.0100% B is an element that suppresses the formation of ferrite and pearlite in the metal structure during cooling from the austenite temperature range and promotes the formation of low-temperature transformation structures such as bainite or martensite. Furthermore, B is a beneficial element for increasing the strength of steel. This effect can be achieved even with trace amounts. Although the B content may be 0%, in order to achieve the above effect, it is preferable that the B content be 0.0001% or more. On the other hand, if the B content is too high, coarse B oxides are formed, which may become the origin of voids during press forming, potentially reducing the formability of the steel sheet. Therefore, the B content is set to 0.0100% or less.

[0048] Nb: 0 to 0.10% Nb is an element that contributes to increasing the strength of steel sheets through precipitation strengthening, grain refinement due to suppression of ferrite grain growth, and / or dislocation strengthening through suppression of recrystallization. The Nb content may be 0%, but to fully obtain the above effects, the Nb content is preferably 0.01% or more. To further increase the strength of steel sheets, the Nb content is more preferably 0.05% or more. On the other hand, if the Nb content exceeds 0.10%, the precipitation of carbonitrides increases, deteriorating formability. Therefore, the Nb content is set to 0.10% or less. From the viewpoint of formability, the Nb content is preferably 0.06% or less.

[0049] V: 0 to 0.50% V is an element that contributes to increasing the strength of steel sheets through precipitation strengthening, grain refinement due to suppression of ferrite grain growth, and / or dislocation strengthening through suppression of recrystallization. The V content may be 0%, but to fully obtain the above effects, 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%, excessive precipitation of carbonitrides occurs, deteriorating formability. For this reason, the V content is set to 0.50% or less. The V content is preferably 0.40% or less.

[0050] Cu: 0 to 0.50% Cu is an element that contributes to improving the strength of steel sheet. This effect can be achieved even with a small amount. The Cu content may be 0%, but in order to achieve the above effect, the Cu content is preferably 0.01% or more. On the other hand, if the Cu content is too high, there is a risk of reduced productivity in hot rolling due to red shortness. For this reason, the Cu content is set to 0.50% or less.

[0051] W: 0 to 0.10% W is an element effective in improving the strength of steel sheet. The W content may be 0%, but to obtain the above effect, the W content is preferably 0.01% or more. On the other hand, if the W content is too high, a large number of fine W carbides will precipitate, which may reduce the elongation due to an excessive increase in the strength of the steel sheet and may reduce the cold workability of the steel sheet. For this reason, the W content is set to 0.10% or less.

[0052] Ta: 0 to 0.100% Like W, Ta is an element effective in improving the strength of steel sheet. The Ta content may be 0%, but in order to obtain the above effect, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content is too high, a large number of fine Ta carbides will precipitate, which will excessively increase the strength of the steel sheet, reducing elongation and potentially reducing the cold workability of the steel sheet. For this reason, the Ta content is set to 0.100% or less. The Ta content is preferably 0.020% or less, and more preferably 0.010% or less.

[0053] Co: 0 to 0.50% Co is an element effective in improving the strength of steel sheet. The Co content may be 0%, but to obtain the above effect, the Co content is preferably 0.01% or more. On the other hand, if the Co content is too high, the elongation of the steel sheet may decrease, and formability may be reduced. For this reason, the Co content is set to 0.50% or less.

[0054] Mg: 0 to 0.050% Mg is an element that controls the morphology of sulfides and oxides and contributes to improving the bending formability of steel sheets. This effect can be obtained even with a small amount, so the Mg content may be 0%, but to obtain the above effect, the Mg content is preferably 0.0001% or more. On the other hand, if the Mg content is too high, there is a risk that the cold formability will deteriorate due to the formation of coarse inclusions. For this reason, the Mg content is set to 0.050% or less. The Mg content is preferably 0.040% or less.

[0055] Ca: 0 to 0.0500% Like Mg, Ca is an element that can control the morphology of sulfides with a small amount. The Ca content may be 0%, but in order to obtain the above effect, the Ca content is preferably 0.0010% or more. On the other hand, if the Ca content is too high, coarse Ca oxides are generated, and these coarse Ca oxides can become the starting point for crack generation during cold forming. For this reason, the Ca content is set to 0.0500% or less. The Ca content is preferably 0.0400% or less, and more preferably 0.0300% or less.

[0056] Y: 0 to 0.050% Like Mg and Ca, Y is an element that can control the morphology of sulfides with a small amount. The Y content may be 0%, but in order to obtain the above effects, the Y content is preferably 0.001% or more. On the other hand, if the Y content is too high, coarse Y oxides may be generated, which may reduce cold formability. For this reason, the Y content is set to 0.050% or less. The Y content is preferably 0.040% or less.

[0057] Zr: 0 to 0.050% Like Mg, Ca, and Y, Zr is an element that can control the morphology of sulfides with a small amount. The Zr content may be 0%, but in order to obtain the above effects, the Zr content is preferably 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. For this reason, the Zr content is set to 0.050% or less. The Zr content is preferably 0.040% or less.

[0058] La: 0 to 0.0500% La is an element that is effective in controlling the morphology of sulfides even in small amounts. The La content may be 0%, but in order to obtain the above effect, the La content is preferably 0.0010% or more. On the other hand, if the La content is too high, La oxides may be generated, which may reduce cold formability. For this reason, the La content is set to 0.0500% or less. The La content is preferably 0.0400% or less.

[0059] Ce: 0 to 0.050% Ce is an element that can control the morphology of sulfides even in small amounts and also contributes to improving LME resistance. To fully obtain the above effects, the Ce content is preferably 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 excessive, the steel sheet may become embrittled and the elongation of the steel sheet may decrease. Therefore, the Ce content is set to 0.050% or less. The Ce content may be 0.040% or less, 0.020% or less, or 0.010% or less.

[0060] Sn: 0 to 0.05% Sn is an element that can be contained in steel sheets when scrap is used as a raw material for the steel sheets. Sn has the effect of improving corrosion resistance and may be contained, but it is an element that may cause a decrease in the cold formability of the steel sheets due to the embrittlement of ferrite. Since an Sn content of more than 0.05% causes significant adverse effects, the Sn content is set to 0.05% or less. The Sn content is preferably 0.04% or less, and may be 0%. However, since reducing the Sn content to less than 0.001% results in an excessive increase in refining costs, the Sn content may be set to 0.001% or more.

[0061] Sb: 0 to 0.050% Like Sn, Sb is an element that can be contained in steel sheets when scrap is used as a raw material for the steel sheets. Sb has the effect of improving corrosion resistance and may be contained; however, it is an element that strongly segregates at grain boundaries, which may cause embrittlement of the grain boundaries, reduced elongation, and reduced cold formability. Since an Sb content exceeding 0.050% causes significant adverse effects, the Sb content is set to 0.050% or less. The Sb content is preferably 0.040% or less, and may be 0%. However, since reducing the Sb content to less than 0.001% results in an excessive increase in refining costs, the Sb content may be set to 0.001% or more.

[0062] As: 0 to 0.050% Like Sn and Sb, As is an element that can be contained in steel sheets when scrap is used as a raw material for the steel sheets. As is an element that improves the hardenability of steel and may be contained, but it is an element that strongly segregates at grain boundaries and may cause a decrease in cold formability. If the As content exceeds 0.050%, the adverse effects become significant, so the As content is set to 0.050% or less. The As content is preferably 0.040% or less, and may be 0%. However, since reducing the As content to less than 0.001% leads to an excessive increase in refining costs, the As content may be set to 0.001% or more.

[0063] The chemical composition of the steel sheet according to this embodiment can be determined by the following method. The chemical composition of the above-described steel sheet may be measured by a general chemical composition method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. If the steel sheet has a plating layer on its surface, the plating layer may be removed by mechanical grinding before analyzing the chemical composition.

[0064] A zinc plating layer (hot-dip galvanized layer or electrogalvanized layer) may be formed on the surface (both or one side) of the steel sheet according to this embodiment. The hot-dip galvanized layer may be an alloyed 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, the steel sheet according to this embodiment may also have another plating (for example, aluminum plating, etc.).

[0065] When the hot-dip galvanized layer is not alloyed, the Fe content in the hot-dip galvanized layer is preferably less than 7.0 mass%. When the hot-dip galvanized layer is an alloyed hot-dip galvanized layer, the Fe content is preferably 6.0 mass% or more, more preferably 7.0 mass% or more. A galvannealed steel sheet has better weldability than a hot-dip galvanized steel sheet.

[0066] The steel sheet according to this embodiment may have a zinc-plated layer and an upper plating layer provided on the zinc-plated layer for the purpose of improving paintability, weldability, etc. The zinc-plated steel sheet may also be subjected to various treatments, such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc.

[0067] <Characteristics> [Tensile strength] In the steel sheet according to this embodiment, the tensile strength (TS) is targeted to be 980 MPa or more, taking into consideration the contribution to improving the fuel efficiency of automobiles. The upper limit of the tensile strength is not particularly limited, but may be 1310 MPa or less from the viewpoint of formability.

[0068] [Uniform elongation] In the steel sheet according to this embodiment, the uniform elongation (u-El) is targeted to be 7.0% or more from the viewpoint of formability. There is no particular upper limit for the uniform elongation.

[0069] The tensile strength and uniform elongation are measured by taking a JIS No. 5 tensile test piece described in JIS Z 2241:2011 from the steel sheet in a direction perpendicular to the rolling direction and conducting a tensile test in accordance with JIS Z 2241:2011.

[0070] [Impact Resistance Properties] The steel plate according to the present embodiment has excellent hydrogen embrittlement resistance at the punched end faces, and therefore has excellent impact resistance properties. For example, when the tensile strength when a JIS No. 5 tensile test piece is prepared by forming semicircular punched holes with a diameter of 10 mm at the center of each end thereof and pulling the test piece along JIS Z 2241:2011 is defined as TS1, and the tensile strength when a JIS No. 5 tensile test piece is prepared by forming semicircular reamed holes with a diameter of 10 mm at the center of each end thereof by machining and pulling the test piece along JIS Z 2241:2011 is defined as TS2, and when R = TS1 / TS2, the value of R is preferably 0.93 or more.

[0071] [LME Resistance] The steel sheet according to this embodiment is, for example, a steel sheet obtained by welding two steel sheets, at least one of which is a zinc-plated steel sheet, at a pressure of 450 kgf (4413 kg m / s) using a servo motor pressure type single-phase AC spot welder (power frequency 50 Hz). ２ When plated steel sheets are spot-welded while applying pressure at a welding point (100 kJ / 1000 psi) with a current value 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, it is preferable that no cracks of 100 μm or more in length occur in the central region of the nugget.

[0072] Next, a method for manufacturing a steel sheet according to this embodiment will be described.

[0073] The steel sheet according to this embodiment can be manufactured by a manufacturing method including the following steps: (I) a hot rolling step in which a slab having the above-described chemical composition is hot-rolled to obtain a hot-rolled steel sheet; (II) a coiling step in which the hot-rolled steel sheet is cooled at a cooling rate of 5°C / s or more and coiled at 400°C or less; (III) a cold rolling step in which the hot-rolled steel sheet after the coiling step is pickled and then cold-rolled at a reduction of 0.5% to 20.0% to obtain a cold-rolled steel sheet; (IV) a hydrogen content reduction step in which the cold-rolled steel sheet is left in the atmosphere for one hour or more and for t hours or more represented by the following formula (1); and (V) an annealing step in which the cold-rolled steel sheet after the hydrogen content reduction step is annealed: t = -2.4 × T + 96 (1) where T is the average air temperature (°C) during the exposure. Preferred conditions for each step will be described below. For conditions not described, known conditions can be applied.

[0074] [Hot Rolling Process] In the hot rolling process, a slab having the above-described chemical composition (similar to the chemical composition of the steel sheet according to the present embodiment) is hot-rolled to produce a hot-rolled steel sheet. The slab used for hot rolling is not particularly limited as long as it has the above-described chemical composition, and may be produced by a conventional method. The slab may be produced by a common method, such as a continuous casting slab or a thin slab caster. In the hot rolling, rough rolling and finish rolling are performed. In the finish rolling, the slab after rough rolling is rolled using multiple finish rolling mills. The heating temperature and holding time of the slab before hot rolling are not particularly limited. The thickness of the hot-rolled steel sheet obtained by hot rolling is not particularly specified, but if the thickness is less than 1.0 mm, the sheet may break during passing in the annealing process. If the thickness is greater than 6.0 mm, the steel sheet is heavy and may not be taut even when tension is applied during passing, resulting in meandering. Therefore, it is preferable that the thickness is 1.0 to 6.0 mm.

[0075] [Coiling process] The hot-rolled steel sheet (hot-rolled steel sheet) as described above is cooled to a temperature of 400°C or less (coiling temperature) so that the cooling rate from the hot-rolling process end temperature to the coiling temperature is always 5°C / s or more, and the steel sheet is coiled at that temperature. By setting the cooling rate (minimum cooling rate) to 5°C / s or more and the coiling temperature to 400°C or less, ferrite transformation and pearlite transformation are suppressed, and a hard structure (low-temperature transformation structure) that is the source of the acicular structure is obtained. The cooling rate is preferably 10°C / s or more, more preferably 20°C / s or more. The upper limit of the cooling rate is not particularly limited, but may be 100°C / s or less from the viewpoint of manufacturability. At temperatures lower than 400°C, the cooling rate is not limited.

[0076] [Cold Rolling Process] In the cold rolling process, the hot-rolled steel sheet after the coiling process is pickled and then cold-rolled at a reduction of 0.5 to 20.0% to obtain a cold-rolled steel sheet. Pickling is a process for removing oxides from the surface of the hot-rolled steel sheet and may be performed under known conditions. The number of pickling processes may be one or more. Cold rolling imparts strain and increases the number of carbide precipitation sites, thereby promoting the precipitation of iron-based carbides during the heating process in the annealing process described below. These iron-based carbides suppress ferrite interface migration during the heating process, thereby allowing acicular austenite to be obtained during the soaking process. To achieve this effect, the cold rolling reduction is set to 0.5% or more. The reduction is preferably 5.0% or more. On the other hand, if the cold rolling reduction exceeds 20.0%, ferrite interface migration is promoted during the heating process in the annealing process, making it impossible to obtain acicular austenite. For this reason, the reduction rate of cold rolling is set to 20.0% or less, and preferably 18.0% or less.

[0077] [Hydrogen Content Reduction Process] In the hydrogen content reduction process, after the cold rolling process, the steel sheet is left in the air for a period of time t (unit: time) = [-2.4 x T + 96] or more (T is the average air temperature (°C) during leaving) until the annealing process described below. This process makes it possible to reduce the amount of hydrogen that has penetrated into the steel sheet during the heating and pickling processes before the hot rolling. If t (leaving time) is less than -2.4 x T + 96 (hours), the amount of hydrogen cannot be sufficiently reduced. However, if T is 40°C or higher, the leaving time is set to 1 hour or more. In other words, the leaving time is set to 1 hour or more and t hours or more.

[0078] [Annealing step] In the annealing step, the cold-rolled steel sheet after the hydrogen content reduction step is bent and unbent at 150 to 400°C, and then annealed to reduce the amount of hydrogen and H ２ The steel sheet is heated in an atmosphere containing O with the balance being nitrogen and impurities and having a dew point of -20 to 20°C (heating process), then held at an annealing holding temperature T°C of Ac1 to Ac3°C for 1 second or more and 1000 seconds or less (soaking process), cooled at an average cooling rate of 4°C / s or more to a temperature range of 350 to 480°C (cooling process), and held at that temperature range (350 to 480°C) for 80 seconds or more (holding process).

[0079] (Heating Process) During the heating process of the annealing process, the steel sheet is bent and unbent with rolls having a radius of 1500 mm or less while the steel sheet is kept at a temperature of 150 to 400°C. The steel sheet is then heated in an atmosphere with a dew point of -20 to 20°C, containing 0.1 to 30.0 volume percent hydrogen, with the remainder being nitrogen and impurities. Bending and unbending the steel sheet at 150 to 400°C has two effects. First, it allows a sufficient amount of iron-based carbides to precipitate. In this case, the shape of the austenite becomes needle-like during the soaking process described below. Second, by repeatedly subjecting the steel sheet to compressive and tensile deformation, the lattice spacing within the steel sheet can be repeatedly changed, allowing hydrogen in the surface layer to be released outside the steel sheet. Furthermore, hydrogen present within the steel sheet is also diffused to the surface layer. If the bending and unbending temperature is less than 150°C, hydrogen diffusion does not occur sufficiently, resulting in an excessive concentration of diffusible hydrogen in the final steel sheet. Furthermore, if the temperature exceeds 400°C, the dislocations imparted by bending and unbending recover too quickly, failing to obtain a sufficient amount of iron-based carbides and resulting in insufficient acicular austenite. If the roll radius exceeds 1500 mm, it becomes difficult to efficiently introduce dislocations into the steel sheet structure by bending and unbending deformation, so the roll radius is set to 1500 mm or less. Furthermore, by heating in an atmosphere containing 0.1 to 30.0 volume % hydrogen, the balance being nitrogen and impurities, and having a dew point of −20 to 20°C, it is possible to prevent the diffusion of easily oxidizable elements to the steel sheet surface and promote internal oxidation. If the hydrogen content is less than 0.1 volume %, the oxide film present on the steel sheet surface cannot be sufficiently reduced, and an oxide film is formed on the steel sheet. As a result, the chemical conversion treatability and coating adhesion of the steel sheet obtained after heat treatment are reduced. Furthermore, if the hydrogen content exceeds 30.0 volume %, the risk of hydrogen explosion increases during operation. For this reason, the hydrogen content (H ２The Si content is set to 0.1% or more and 30.0% or less by volume. Furthermore, if the dew point of the atmosphere is lower than -20°C, external oxidation of Si and Mn occurs in the surface layer of the steel sheet, resulting in insufficient internal oxidation and decarburization. In this case, LME resistance and crash resistance are reduced. Furthermore, if the dew point is higher than 20°C, an oxide film is formed on the steel sheet, reducing chemical conversion treatability and coating adhesion, and the decarburization reaction proceeds excessively, resulting in insufficient strength of the steel sheet obtained after annealing. Annealing furnaces are broadly divided into three zones: a preheating zone, a heating zone, and a soaking zone. In this embodiment, the atmosphere in the heating zone is set to the above-mentioned conditions. Atmosphere control is also possible in the preheating zone and the soaking zone.

[0080] (Soaking Process) In the soaking process, the cold-rolled steel sheet after the heating process is soaked in a temperature range of Ac1 point to Ac3 point for 1 second to 1000 seconds. Soaking under these conditions generates acicular austenite along the laths of tempered martensite. The specific soaking temperature can be adjusted appropriately based on the Ac1 point (°C) and Ac3 point (°C) expressed by the following formula, taking into account the desired proportion of the metal structure. Ac1 = 723 - 10.7 x Mn - 16.9 x Ni + 29.1 x Si + 16.9 x Cr + 290 x As + 6.38 x W (2) Ac3 = 910 - 203√C + 44.7 x Si - 30 x Mn + 700 x P - 20 x Cu - 15.2 x Ni - 11 x Cr + 31.5 x Mo + 400 x Ti + 104 x V + 120 x Al (3) where C, Si, Mn, P, Cu, Ni, Cr, Mo, Ti, V, and Al are the contents [mass%] of each element. If the soaking temperature is below the Ac1 point or the soaking time is less than 1 second, austenite is not generated during soaking. Therefore, a single-phase ferrite structure is formed, and the desired metal structure cannot be obtained. Furthermore, if the soaking temperature exceeds the Ac3 point, the structure during soaking becomes a single-phase austenite structure, and the hard structure (low-temperature transformation structure) that is the source of the acicular structure is lost. Therefore, acicular austenite cannot be obtained. Furthermore, if the soaking time exceeds 1,000 seconds, productivity decreases. The soaking time in the soaking process may be set to 300 seconds or less in order to suppress coarsening of ferrite and austenite during soaking. The temperature of the steel sheet during the soaking process does not need to be constant. As long as the desired structure ratio can be obtained, the temperature of the steel sheet during the soaking process may vary within the temperature range from the Ac1 point to the Ac3 point.

[0081] (Cooling Process) In the cooling process after the soaking process, the cold-rolled steel sheet after the soaking process is cooled to a temperature range of 100 to 340°C so that the average cooling rate is 4°C / s or more for the subsequent holding process. By cooling under these conditions, ferrite transformation during cooling can be suppressed, and the desired amounts of martensite and retained austenite can be obtained in the final structure. If the average cooling rate is less than 4°C / s, ferrite transformation cannot be suppressed. If the cooling stop temperature is less than 100°C, the martensite fraction becomes high. On the other hand, if the cooling stop temperature is more than 340°C, the ferrite, bainite, and pearlite fractions become high, making it difficult to obtain the desired structure.

[0082] (Holding Process) In the holding process, in order to reduce the amount of hydrogen in the steel sheet while increasing the stability of austenite, the cold-rolled steel sheet after the cooling process is reheated to a temperature range of 350 to 480°C and held at that temperature range for 80 seconds or more. If the holding time is shorter than 80 seconds, carbon is not sufficiently concentrated in the untransformed austenite, and hydrogen cannot be released to the outside of the steel sheet. By holding the steel sheet in the above temperature range for 80 seconds or more, the carbon concentration in the austenite can be increased and the desired amount of retained austenite can be secured after final cooling. To stably achieve the above effect, the holding time is preferably 100 seconds or more. There is no need to limit the upper limit of the holding time, but an excessively long holding time reduces productivity, so the holding time may be 1000 seconds or less. If the holding temperature is less than 350°C, the desired amount of retained austenite cannot be obtained, and furthermore, sufficient diffusion of hydrogen does not occur. Therefore, the holding temperature is set to 350°C or higher, preferably 380°C or higher. On the other hand, if the holding temperature exceeds 480°C, the retained austenite decomposes into ferrite and cementite, which is not preferable. Therefore, the holding temperature is set to 480°C or lower, preferably 450°C or lower.

[0083] The conditions for cooling the cold-rolled steel sheet after the holding process to room temperature are not limited, but in order to stably obtain a desired metal structure, the cold-rolled steel sheet after the holding process may be cooled to the Ms point or lower at an average cooling rate of 2°C / s or more.

[0084] When reducing the amount of hydrogen in a steel sheet, as described above, it is important to control the amount of hydrogen at each stage of the hydrogen amount reduction step, bending-unbending in the annealing step, and holding step, and a sufficient effect cannot be obtained by controlling only any one of the stages.

[0085] (Plating Step) The steel sheet manufacturing method according to this embodiment may further include a hot-dip galvanizing step in which a coating is formed on the surface of the cold-rolled steel sheet during the cooling process after annealing, during the holding process, or after the holding process. Furthermore, an alloying step in which the coating layer is alloyed may be further included after the hot-dip galvanizing step. The hot-dip galvanizing method and the alloying method are not particularly limited, and conventional methods can be used. Examples of hot-dip galvanizing methods include a method in which cooling is stopped in the temperature range of (galvanizing bath temperature -40) °C to (galvanizing bath temperature +50) °C during the cooling process, and the steel sheet is immersed in a hot-dip galvanizing bath while maintaining the temperature within this range to form a hot-dip galvanized coating. Examples of alloying methods include a method in which hot-dip galvanizing is performed in a temperature range of 300 to 500 °C.

[0086] The present invention will now be described more specifically with reference to examples.

[0087] Slabs having the chemical compositions shown in Table 1 were cast. The cast slabs were heated to the temperatures shown in Table 2 and then hot-rolled to thicknesses of 1.0 to 6.0 mm. After hot rolling, the hot-rolled steel sheets were cooled and coiled under the conditions shown in Table 2, and then cold-rolled under the conditions shown in Table 2 to obtain cold-rolled steel sheets. These cold-rolled steel sheets were left in the air under the conditions shown in Table 3 to reduce the hydrogen content. Annealing was then performed under the conditions shown in Tables 3 and 4. Examples in which bending and unbending were performed were performed using rolls with a roll radius of 1100 mm in a temperature range of 150 to 400°C. After the holding process, the samples were cooled to the Ms point or below at an average cooling rate of 2°C / s or more. Some samples were then immersed in a hot-dip galvanizing bath after controlling the temperature to a range of (galvanizing bath temperature -40)°C to (galvanizing bath temperature +50)°C. Furthermore, in some of the examples where plating was performed, the steel sheets were heated to a temperature range of 300 to 500°C to alloy the plating layer. In the table, GI indicates examples where hot-dip galvanization was performed, and GA indicates examples where alloyed hot-dip galvanization was performed. Steel sheets with example numbers 1 to 37 were obtained in this manner.

[0088]

[0089]

[0090]

[0091]

[0092] <Metal Structure Measurement> A test piece for SEM observation was taken from the obtained steel sheet (annealed steel sheet or annealed and plated steel sheet), and the longitudinal cross section parallel to the rolling direction was polished. The metal structure was then observed at a position 1 / 4 of the sheet thickness as described above. Image processing was used to measure the area fraction of each structure (ferrite, bainite, pearlite, and the remainder (fresh martensite and / or tempered martensite)), which was used as the volume fraction. X-ray diffraction was also performed as described above to determine the volume fraction of retained austenite. The volume fractions of each structure are shown in Table 5.

[0093] Furthermore, the area ratio of retained austenite having an aspect ratio of 3.0 or more to the total retained austenite was determined for the obtained steel sheets by EBSD analysis using an FE-SEM in the same manner as described above. The results are shown in Table 5.

[0094] The thickness of the decarburized layer and the thickness of the internal oxide layer were measured for the obtained steel sheets in the same manner as described above. The amount of diffusible hydrogen contained in the steel was also measured in the same manner as described above. The results are shown in Table 5.

[0095]

[0096] <Measurement of Properties> The tensile strength (TS), uniform elongation (u-El) as an index of formability, impact resistance assuming punching, and LME resistance of spot welds of the obtained steel sheets were evaluated by the following methods.

[0097] (Tensile strength) (Uniform elongation) JIS No. 5 tensile test pieces described in JIS Z 2241:2011 were taken from the obtained steel sheets in a direction perpendicular to the rolling direction, and tensile tests were performed in accordance with JIS Z 2241:2011 to measure tensile strength and uniform elongation. A tensile strength of 980 MPa or more was considered to be acceptable. Furthermore, a uniform elongation (%) of 7.0% or more was considered to have excellent formability. The tensile strength measurement results are shown in Table 6.

[0098] (Collision Resistance) The collision resistance was evaluated based on the range of the R value shown in the following formula. A 10 mm diameter semicircular punched hole was created at the center of each end of a JIS No. 5 tensile test piece using a punch diameter of 10 mm and a punching clearance of 12±2%, and the test piece was pulled along JIS Z 2241:2011. The tensile strength was defined as TS1. A 10 mm diameter semicircular reamed hole was machined at the center of each end of the JIS No. 5 tensile test piece, and the test piece was pulled along JIS Z 2241:2011. The tensile strength was defined as TS2. R (= TS1 / TS2) was evaluated as follows, and a rating of A or B was considered to have excellent collision resistance. A: R = 0.96 to 1.00 B: R = 0.93 to less than 0.96 C: R = less than 0.93

[0099] (LME Resistance) Test pieces measuring 50 mm x 80 mm were taken from the obtained steel sheets. Furthermore, a slab having the chemical composition A in Table 1 was cast, and after applying the manufacturing conditions of Example No. 1, the slab was immersed in a hot-dip galvanized bath to produce hot-dip galvanized steel sheets (mating members). Test pieces measuring 50 mm x 80 mm were taken from the produced steel sheets (mating members). A test piece taken from each of the steel sheets of Example Nos. 1 to 37 was overlapped with a steel sheet serving as the mating member, and the two steel sheets were spot-welded as shown in FIG. 1 . Specifically, the hot-dip galvanized steel sheet serving as the mating member was used as steel sheet 1d in FIG. 1 , and the steel sheet to be evaluated (Example Nos. 1 to 37) was used as steel sheet 1e. The two sheets were then spot-welded with a pair of electrodes 4a and 4b. The welding conditions were a pressure of 450 kgf (4413 kg m / s ) using a servo motor pressure-type single-phase AC spot welder (power frequency 50 Hz). ２ ), while applying pressure, the current value was 6.5 kA, the electrode inclination angle θ was 3°, there was no upslope, the current flow time was 0.4 seconds, and the holding time after the end of current flow was 0.1 seconds. After spot welding, the structure of the center of the nugget at the joint of the steel sheets was observed using an optical microscope at magnifications of 200 to 1000 times. As a result of the observation, if no cracks occurred, it was evaluated as "A", if cracks less than 100 μm in length were observed, it was evaluated as "B", and if cracks 100 μm or more in length were observed, it was evaluated as "C". A rating of A or B was determined to be excellent in LME resistance.

[0100]

[0101] As shown in Tables 1 to 6, in the examples (Example Nos. 1 to 16) according to the present invention, the tensile strength was greater than 980 MPa, the uniform elongation was greater than 7.0%, the crash resistance index R was rated A or B, and the LME resistance (crack length after spot welding) was rated A or B.

[0102] Furthermore, in the case of plated steel sheets that were subjected to hot-dip galvanizing treatment or hot-dip galvanizing treatment and alloying treatment, the tensile strength was greater than 980 MPa, the uniform elongation was greater than 7.0%, the crash resistance index R was rated A or B, and the crack length after spot welding was rated A or B.

[0103] On the other hand, the comparative examples, Example Nos. 17 to 37, were outside the range of the present invention in terms of either chemical composition or structure, and were inferior in either tensile strength, uniform elongation, impact resistance, or LME resistance.

[0104] In Example No. 17, the minimum cooling rate from the hot rolling end temperature to the coiling temperature was less than 5°C / s. Therefore, the proportion of retained austenite with an aspect ratio of 3.0 or more in the structure after annealing was small, and the amount of diffusible hydrogen contained in the steel was large. As a result, the uniform elongation and crash resistance properties were low. In Example No. 18, the coiling temperature was higher than 400°C. Therefore, the proportion of retained austenite with an aspect ratio of 3.0 or more was small, and the amount of diffusible hydrogen contained in the steel was large. As a result, the uniform elongation and crash resistance properties were low. In Example No. 19, the cold rolling reduction was less than 0.5% in the cold rolling process, and the proportion of retained austenite with an aspect ratio of 3.0 or more in the structure after annealing was small, and the amount of diffusible hydrogen contained in the steel was large. As a result, the uniform elongation and crash resistance properties were low. In Example No. 20, the cold rolling reduction was greater than 20.0% in the cold rolling process, resulting in a small proportion of retained austenite with an aspect ratio of 3.0 or more in the annealed structure, and a large amount of diffusible hydrogen contained in the steel. As a result, the uniform elongation and crash resistance properties were poor. In Example No. 21, the time left in the atmosphere in the hydrogen content reduction process was less than −2.4 × T + 96 (hours), so the amount of diffusible hydrogen could not be sufficiently reduced. As a result, the crash resistance properties were poor. In Example No. 22, bending and unbending were not performed during the heating process in the annealing process, resulting in a small proportion of retained austenite with an aspect ratio of 3.0 or more in the annealed structure, and a large amount of diffusible hydrogen contained in the steel. As a result, the uniform elongation and crash resistance properties were poor. In Example No. 23, the dew point during the heating process in the annealing process was less than −20°C, so the internal oxide layer and decarburized layer were not sufficiently thick. As a result, the LME resistance was poor. In Example No. 24, the dew point exceeded 20°C during the heating process in the annealing step, resulting in an excessively thick decarburized layer. As a result, the tensile strength was low. In Example No. 25, the holding temperature during the soaking process in the annealing step was below the Ac1 point, resulting in the total area fraction of ferrite, bainite, and pearlite exceeding 50% and the volume fraction of retained austenite being 0%. As a result, the tensile strength was low.In Example 26, the holding temperature during the soaking step of the annealing process exceeded the Ac3 point, resulting in a small volume fraction of retained austenite and a small proportion of retained austenite with an aspect ratio of 3.0 or greater. As a result, the crashworthiness and uniform elongation were poor. In Example 27, the average cooling rate during the cooling step of the annealing process was less than 4°C / s, resulting in a total area fraction of ferrite, bainite, and pearlite exceeding 50%. As a result, the tensile strength was poor. In Example 28, the holding temperature during the holding step of the annealing process was less than 350°C, resulting in the retained austenite not stabilizing and a small volume fraction of retained austenite. As a result, the uniform elongation was poor. In Example 29, the holding temperature during the holding step of the annealing process exceeded 480°C, resulting in a total area fraction of ferrite, bainite, and pearlite exceeding 50%. As a result, the tensile strength was poor. In Sample No. 30, the holding time in the holding step of the annealing process was less than 80 seconds, so the retained austenite was not stabilized and the volume fraction of the retained austenite was small, resulting in low uniform elongation.

[0105] In Example No. 31, the C content was less than 0.10%, resulting in low tensile strength. Furthermore, the volume fraction of retained austenite was insufficient. As a result, the uniform elongation was low. In Example No. 32, the C content exceeded 0.40%, resulting in reduced LME resistance. In Example No. 33, the Si content was less than 0.10%, resulting in reduced volume fraction of retained austenite. As a result, the uniform elongation was low. In Example No. 34, the Si content exceeded 1.20%, resulting in reduced LME resistance. In Example No. 35, the Al content was less than 0.30%, resulting in reduced volume fraction of retained austenite. As a result, the uniform elongation was low. In Example No. 36, the Mn content was less than 1.0%, resulting in the total area fraction of ferrite, bainite, and pearlite exceeding 50%. As a result, the tensile strength was low. In sample No. 37, the cold rolling reduction was less than 0.5% in the cold rolling step, and the hydrogen content reduction step was not performed, so the proportion of retained austenite with an aspect ratio of 3.0 or more in the structure after annealing was small, and the amount of diffusible hydrogen contained in the steel was large. As a result, the uniform elongation and crash resistance properties were low.

[0106] 1d, 1e Steel plates 4a, 4b Electrodes

Claims

1. A steel sheet, having a chemical composition by mass%: C: 0.10 to 0.40%, Si: 0.10 to 1.20%, Al: 0.30 to 1.50%, Mn: 1.0 to 4.0%, P: 0.0200% 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 2.00%, Ti: 0 to 0.100%, B: 0 to 0.0100%, Nb: 0 to 0.10%, V: 0 to 0.50%, Cu: 0 to 0.50%, W: 0 to 0.10%, Ta: 0 to 0.100%, Co: 0 to 0.50%, Mg: 0 to 0.050%, Ca: 0 to 0.0500%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.0500%, Ce: 0 to 0.050%, Sn: 0 to 0.05%, Sb: 0 to 0.050%, As: 0 to 0.050%, the balance being Fe and impurities, and in the metallographic structure, the total volume ratio of ferrite, bainite, and pearlite is 0% or more and 50% or less, the volume ratio of retained austenite is 3% or more and 20% or less, the balance being one or two of fresh martensite and tempered martensite, and retained austenite having an aspect ratio of 3.0 or more occupies 80% or more of the total retained austenite by area ratio, having an internal oxide layer with a thickness of 4.0 μm or more from the surface of the steel sheet and a decarburized layer with a thickness of 10 μm or more and 100 μm or less from the surface of the steel sheet, and the amount of diffusible hydrogen contained in the steel sheet is 1.00 ppm or less on a mass basis, a steel sheet.

2. The steel sheet according to claim 1, having a hot-dip galvanized layer on the surface.

3. The steel sheet according to claim 1, having an alloyed hot-dip galvanized layer on the surface.

4. A hot rolling step of hot rolling a slab having the chemical composition according to claim 1 to obtain a hot-rolled steel sheet; a coiling step of cooling the hot-rolled steel sheet at a cooling rate of 5 °C / s or more and coiling it at 400 °C or less; a cold rolling step of cold rolling the hot-rolled steel sheet after the coiling step with a reduction rate of 0.5% or more and 20.0% or less after pickling to obtain a cold-rolled steel sheet; a hydrogen amount reduction step of leaving the cold-rolled steel sheet in the atmosphere for 1 hour or more and for t hours or more represented by the following formula (1); and an annealing step of annealing the cold-rolled steel sheet after the hydrogen amount reduction step. The annealing step includes: applying bending and unbending to the cold-rolled steel sheet at 150 to 400 °C; heating the cold-rolled steel sheet in an atmosphere containing 0.1 to 30.0% by volume of hydrogen with a dew point of -20 °C to 20 °C, the balance being nitrogen and impurities; holding the heated cold-rolled steel sheet at a holding temperature of Ac1 to Ac3 °C for 1 second or more and 1000 seconds or less; cooling the held cold-rolled steel sheet to 100 to 340 °C at an average cooling rate of 4 °C / s or more; reheating the cooled cold-rolled steel sheet and holding it at 350 °C or more and 480 °C or less for 80 seconds or more. A method for manufacturing a steel sheet. t = -2.4×T + 96 (1) Here, T is the average air temperature (°C) during standing.

5. Further, after controlling the cold-rolled steel sheet after the annealing step to a temperature range of (zinc plating bath temperature - 40) °C to (zinc plating bath temperature + 50) °C, dipping it in a molten zinc plating bath to form a molten zinc plating on the surface of the cold-rolled steel sheet, the method for manufacturing a steel sheet according to claim 4 having a molten zinc plating step.

6. Further, having an alloying step of heating the molten zinc-plated steel sheet in a temperature range of 300 to 500 °C to alloy the plating layer, the method for manufacturing a steel sheet according to claim 5.