Steel plate

Abstract

A steel sheet according to one aspect of the present invention comprises specified chemical components, in which a metal structure in a part corresponding to a depth of 1 / 4 of the thickness of the steel sheet comprises, in percentage by volume, ferrite in an amount of 10% or more and less than 50%, granular bainite in an amount of 5% or more and less than 40%, martensite in an amount of 30% to 55% inclusive, upper bainite and lower bainite in a total amount of less than 30%, pearlite in an amount of less than 10%, and retained austenite in an amount of less than 5%, the ratio of the number of the martensite parts adjacent to the ferrite to the number of the metal structure parts adjacent to the ferrite is 30% or less in a part corresponding to a depth of 1 / 4 of the thickness of the steel sheet, and the difference between a maximum value and a minimum value of Vickers hardness at a load of 50 gf is 60 HV or less when the Vickers hardness is determined at 30-μm intervals from a origin position corresponding to a depth of 100 μm from the surface of the steel sheet as observed in the thickness direction to a thickness center position.

Description

[Technical field]

[0001] The present invention relates to a steel sheet having excellent formability and a maximum tensile strength of 900 MPa or more, the main uses of which are automobile parts and the like. [Background technology]

[0002] In order to reduce carbon dioxide emissions from automobiles, efforts are being made to reduce the weight of automobile bodies while ensuring safety by using high-strength steel sheets. However, in general, increasing the strength of steel sheets reduces their formability. It is difficult to achieve both strength and formability in high-strength steel sheets. In addition, automobile parts have stress concentration areas such as holes. When steel sheets for automobile parts are press-formed, fractures are likely to occur at the stress concentration areas. For this reason, the industry has been eager to develop steel sheets that can impart high deformability to the stress concentration areas. However, the deformability at the stress concentration areas also tends to decrease as the strength of the steel sheet increases. To solve these problems, several measures have been proposed.

[0003] For example, Patent Document 1 discloses that the metal structure of the steel sheet is a composite structure of ferrite, which is a soft structure, and martensite, which is a hard structure, thereby achieving both strength and elongation. However, since the metal structure of the steel sheet of Patent Document 1 is a combination of a soft structure and a hard structure, the hardness difference between the two structures is large. When the hardness difference between the structures is large, voids are likely to occur at the interface between the structures, and hole expandability may be impaired. Therefore, the steel sheet described in Patent Document 1 is required to have improved hole expandability.

[0004] Patent Document 2 discloses that the metal structure of a steel sheet is a single structure of upper bainite or lower bainite, which has a hardness intermediate between ferrite and martensite, thereby reducing the difference in hardness between the structures and improving strength and hole expandability. However, since upper bainite or lower bainite is a structure composed of bainitic ferrite containing many dislocations and hard cementite, the steel sheet of Patent Document 2 is required to have improved elongation.

[0005] Patent Document 3 discloses that the metal structure of a steel plate is composed of ferrite, upper bainite or lower bainite, and martensite, thereby reducing the difference in hardness between the structures and suppressing deterioration of elongation while ensuring strength and hole expandability. However, since the upper bainite or lower bainite is composed of bainitic ferrite containing many dislocations and hard cementite, the steel plate of Patent Document 3 is required to have improved elongation.

[0006] Patent Document 4 discloses that by forming a metal structure having hard martensite, soft ferrite, and granular bainite having a hardness intermediate between martensite and ferrite, the difference in hardness between the structures is reduced, and high strength, elongation, and hole expandability are obtained. Patent Document 5 discloses a steel sheet having a metal structure having a predetermined chemical composition and represented by area fractions of ferrite: 30% to 50%, granular bainite: 5% to 20%, martensite: 30% to 55%, bainite: less than 35%, and retained austenite and pearlite: 10% or less in total. However, according to the findings of the present inventors, the techniques disclosed in Patent Document 4 and Patent Document 5 do not optimize the cooling rate after hot rolling and the heating rate during annealing. Therefore, the steel sheets disclosed in Patent Document 4 and Patent Document 5 have poor deformability at the stress concentration portion. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 07-011383 [Patent Document 2] Patent No. 2616350 [Patent Document 3] Japanese Patent Application Publication No. 07-207413 [Patent Document 4] International Publication No. 2018 / 051402 [Patent Document 5] International Publication No. 2018 / 138898 Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide a steel plate having high levels of strength, formability, and deformability at stress concentration portions. [Means for solving the problem]

[0009] The gist of the present invention is as follows.

[0010] (1) A steel sheet according to one embodiment of the present invention has a chemical composition, in mass%, of C: 0.07% or more and 0.15% or less, Si+Al: 0.20% or more and 2.50% or less, Mn+Cr: 1.20% or more and 4.00% or less, P: 0% or more and 0.040% or less, S: 0% or more and 0.010% or less, N: 0% or more and 0.010% or less, O: 0% or more and 0.006% or less, Mo: 0% or more and 0.50% or less, Ti: 0% or more and 0.20% or less, Nb: 0% or more and 0.02% or less, and Nb: 0% or more and 0.030% or less. .20% or less, B: 0% or more, 0.010% or less, V: 0% or more, 0.50% or less, Cu: 0% or more, 1.00% or less, W: 0% or more, 0.10% or less, Ta: 0% or more, 0.10% or less, Ni: 0% or more, 1.00% Below, Sn: 0% or more, 0.050% or less, Co: 0% or more, 0.50% or less, Sb: 0% or more, 0.050% or less, As: 0% or more, 0.050% or less, Mg: 0% or more, 0.050% or less, Ca: 0% or more, 0.0 The metal structure at 1 / 4 of the plate thickness is, in volume fractions, ferrite: 10% or more, less than 50%, granular bainite: 5% or more, less than 40%, martensite: 30% or more, less than 55%, upper bainite and lower bainite: total and the ratio of the number of martensite particles adjacent to the ferrite to the number of metal structures adjacent to the ferrite at 1 / 4 of the plate thickness is 30% or less, and the difference between the maximum and minimum Vickers hardness values ​​under a load of 50 gf measured at 30 μm intervals from a position 100 μm deep from the surface of the steel plate in the plate thickness direction to the center position of the plate thickness is 60 HV or less. (2) In the steel sheet described in (1) above, the chemical composition is, in mass%, Mo: 0.01% or more, 0.50% or less, Ti: 0.001% or more, 0.20% or less, Nb: 0.0001% or more, 0.20% or less, B: 0.0001% or more, 0.010% or less, V: 0.001% or more, 0.50% or less, Cu: 0.001% or more, 1.00% or less, W: 0.001% or more, 0.10% or less, Ta: 0.001% or more, 0.10% or less, Ni: 0.001% or more, 1.00% or less, Sn: 0.001% or more, 0.0 It may contain one or more selected from the group consisting of 50% or less, Co: 0.001% or more, 0.50% or less, Sb: 0.001% or more, 0.050% or less, As: 0.001% or more, 0.050% or less, Mg: 0.0001% or more, 0.050% or less, Ca: 0.001% or more, 0.040% or less, Y: 0.001% or more, 0.050% or less, Zr: 0.001% or more, 0.050% or less, La: 0.001% or more, 0.050% or less, and Ce: 0.001% or more, 0.050% or less. (3) In the steel plate described in (1) or (2) above, a ratio Hv30 / Hvi of a Vickers hardness Hv30 under a load of 0.29 N at a position 30 μm deep from the surface of the steel plate in the plate thickness direction to a Vickers hardness Hvi under a load of 0.29 N at 1 / 4 of the plate thickness may be 0.8 or less, and a tensile strength may be 900 MPa or more. (4) The steel sheet according to any one of the above (1) to (3) may have a hot-dip galvanized layer or a hot-dip zinc alloy plated layer on the surface. (5) The steel sheet according to any one of the above (1) to (3) may have a galvannealed layer on the surface. Effect of the Invention

[0011] According to the present invention, it is possible to provide a steel sheet having formability and deformability at stress concentration portions suitable for use as structural members of automobiles and the like, and having a high tensile strength of 900 MPa or more. [Brief description of the drawings]

[0012] [Figure 1]FIG. 2 is an explanatory diagram showing how granular bainite is generated. [Figure 2-1] 1 is a temperature-time graph showing a heating condition before annealing of the steel sheet according to the present embodiment. [Figure 2-2] 1 is a temperature-time graph that illustrates the cooling conditions after annealing of the steel sheet according to the present embodiment. [Diagram 3] FIG. 1 is a schematic diagram showing the shape of a test piece for evaluating processed portion elongation El2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present inventors have found that it is preferable to make the steel sheet have a metal structure having martensite, ferrite, and granular bainite, and further to control the arrangement of these metal structures. Specifically, the present inventors have found that by controlling the metal structure of the steel sheet so that granular bainite is arranged between ferrite and martensite, it is possible to reduce the number of locations with large hardness differences. This can further improve the hole expandability of the steel sheet. The present inventors have also found that by applying a metal structure with a small hardness difference in the thickness direction to the steel sheet, it is possible to further increase the deformability at the stress concentration portion.

[0014] In one embodiment of the present invention obtained based on the above findings, in a high-strength steel plate having a tensile strength of 900 MPa or more, both strength and formability are achieved by controlling the volume fraction of granular bainite and the arrangement of ferrite, martensite, and granular bainite.

[0015] Hereinafter, a steel sheet according to one embodiment of the present invention will be described.

[0016] First, the metal structure of the steel sheet according to this embodiment will be described. Hereinafter, the structure fraction is expressed as a volume fraction, so the unit "%" of the structure fraction means volume %. As will be described later, the metal structure is controlled at the 1 / 4 plate thickness portion. All of the structure fractions described below refer to values ​​at the 1 / 4 plate thickness portion.

[0017] Metal structure at 1 / 4 of plate thickness Ferrite: 10% or more, less than 50% Ferrite is a soft structure that is easily deformed, contributes to improving elongation, and promotes the formation of granular bainite. When ferrite is 10% or more, the transformation from austenite to granular bainite is easily promoted. The ferrite content is preferably 12% or more, 15% or more, or 20% or more.

[0018] To ensure tensile strength, the ferrite content is less than 50%, and preferably is 40% or less, 35% or less, or 30% or less.

[0019] Granular bainite: 5% or more, less than 40% Granular bainite is an aggregate of multiple lath-shaped bainitic ferrite. Granular bainite is characterized by its low dislocation density. For example, the dislocation density of granular bainite is 10 13 m / m 3 The difference in the crystal orientation angle of bainitic ferrite at the grain boundaries between these bainitic ferrites (crystal orientation difference) is small, at 5° or less, so granular bainite is a structure that appears as a single mass with a crystal grain size of about 5 to 20 μm. This is because the recovery progresses due to heat treatment, and the crystal orientation difference at the interface between bainitic ferrites becomes about 5° or less, and the interface appears to have disappeared. The inside of granular bainite may contain retained austenite and martensite. However, unlike upper bainite and lower bainite, granular bainite does not contain carbides inside. Therefore, unlike general bainite, granular bainite is softer than general bainite and martensite. An example of a method for identifying granular bainite is described in Kitajima et al., "Distinguishing Ferrite and Granular Bainite Using Electron Channeling Contrast Images" (CAMP-ISIJ, Vol. 26 (2013) 896).

[0020] Upper bainite is an aggregate of multiple lath-shaped bainitic ferrite, and contains carbides at the interfaces of the bainitic ferrite, but does not contain carbides inside each bainitic ferrite.On the other hand, lower bainite is an aggregate of multiple lath-shaped bainitic ferrite like upper bainite, and in addition to containing carbides at the interfaces of the bainitic ferrite, carbides aligned in a specific direction exist inside each bainitic ferrite.

[0021] In addition, since granular bainite is formed in a high temperature region, recovery progresses inside it. Therefore, although granular bainite has a dislocation substructure, it has a lower dislocation density than upper bainite and lower bainite. Therefore, granular bainite is harder than ferrite, which does not contain a dislocation substructure and has a low dislocation density, and is softer than upper bainite and lower bainite. Therefore, granular bainite has better elongation than general bainite.

[0022] In addition, granular bainite reduces the difference in hardness between ferrite and martensite, and therefore suppresses the generation of voids from the interface between ferrite and martensite during hole expansion.

[0023] Granular bainite contains subgrains (areas surrounded by grain boundaries with a crystal orientation difference of approximately 5° or less) and is therefore distinguished from polygonal ferrite, which is a structure that does not contain subgrains.

[0024] Figure 1 shows a schematic diagram of how bainitic ferrite generated from prior austenite grain boundaries changes into granular bainite, upper bainite, and lower bainite during the cooling process, depending on the difference in the generation temperature (°C). As shown in Figure 1(a), in either case, bainitic ferrite 2 is first generated from the prior austenite grain boundary 1 toward the inside of the prior austenite grain. Furthermore, as shown in Figure 1(b), lath-shaped bainitic ferrite 2 is generated adjacent to the previously generated bainitic ferrite 2.

[0025] Here, when the formation temperature (°C) is relatively high (denoted as high temperature in FIG. 1), the interface between the bainitic ferrite 2 formed in a lath shape does not contain carbides such as cementite. Then, by holding at a relatively high temperature, recovery proceeds. As a result, the crystal orientation difference between the interface between the bainitic ferrite 2 formed in a plurality of lath shapes becomes about 5° or less, and the interface appears to have disappeared. Granular bainite can be formed even if the cooling rate is lowered, but when granular bainite is formed by isothermal holding, the progress of recovery is further promoted and the disappearance of the interface becomes noticeable. In this way, as shown in FIG. 1(c), granular bainite 3, which is a structure that appears as a single mass with a crystal grain size of about 5 to 20 μm, is formed as an aggregate of multiple lath-shaped bainitic ferrite 2. Note that the inside of the granular bainite 3 may contain retained austenite or martensite hard structure 4, but unlike upper bainite and lower bainite, carbides are not contained inside.

[0026] On the other hand, when the formation temperature (°C) is lower than when granular bainite 3 is formed, upper bainite or lower bainite is formed. That is, when the formation temperature (°C) is slightly lower than when granular bainite 3 is formed (indicated as "medium temperature" in FIG. 1), carbides 5 such as cementite are contained at the interface between the bainitic ferrite 2 formed in a lath shape. In this case, the interior of the bainitic ferrite 2 does not contain carbides, and upper bainite 6 is formed. When the formation temperature (°C) is even lower (indicated as "low temperature" in FIG. 1), in addition to the carbides 5 such as cementite being contained at the interface between the bainitic ferrite 2 formed in a lath shape, the interior of the bainitic ferrite 2 also contains carbides, and lower bainite 7 is formed.

[0027] When the granular bainite content is 5% or more, the effect of improving elongation and the effect of suppressing voids can be obtained. The granular bainite content is preferably 8% or more, 10% or more, or 15% or more.

[0028] Furthermore, by making the granular bainite content less than 40%, strength can be ensured. The granular bainite content is preferably 35% or less, 32% or less, or 30% or less.

[0029] Martensite: 30% or more, 55% or less Martensite is a structure with high dislocation density and hardness, which contributes to improving tensile strength. By making the martensite content 30% or more, a tensile strength of 900 MPa or more is ensured. The martensite content is preferably 32% or more, 35% or more, or 40% or more.

[0030] Furthermore, when the martensite content is 55% or less, the elongation and hole expandability can be ensured. The martensite content is preferably 50% or less, 45% or less, or 42% or less.

[0031] Upper bainite and lower bainite: less than 30% in total Upper bainite and lower bainite (hereinafter, the term "bainite" may be used collectively to refer to both) have a high dislocation density, which deteriorates elongation. Specifically, bainite has a dislocation density of 1.0×10 14 m / m 3 The upper bainite is composed of bainitic ferrite and cementite, which are about 0.01 to 0.01%. Bainite has a large difference in hardness from ferrite. Therefore, the interface between ferrite and bainite is likely to become the starting point of voids, which deteriorates the hole expandability. By making the upper bainite and the lower bainite less than 30% in total, elongation and hole expandability are ensured. Preferably, the upper bainite and the lower bainite are 25% or less, 20% or less, or 10% or less in total. There is no need to specify the lower limit of the total amount of upper bainite and lower bainite, and it may be specified as 0% or more, 0.2% or more, 0.5% or more, or 1.0% or more, for example.

[0032] Perlite: Less than 10% Pearlite is a structure containing hard cementite, which becomes the origin of voids during hole expansion and deteriorates hole expandability. Therefore, pearlite is less than 10%. Pearlite is preferably 8% or less, 6% or less, or 5% or less. There is no need to specify a lower limit for pearlite, and for example, pearlite may be specified as 0% or more, 0.2% or more, 0.5% or more, or 1.0% or more.

[0033] Retained austenite: Less than 5% The retained austenite is a structure that contributes to improving elongation through transformation induced plasticity (TRIP). However, martensite generated by the transformation induced plasticity of the retained austenite is very hard and becomes the starting point for the generation of voids, which deteriorates the hole expandability. Therefore, the retained austenite is set to less than 5%. The retained austenite is preferably 4% or less, 3% or less, or 2% or less. There is no need to specify the lower limit of the retained austenite, and the retained austenite may be specified to be, for example, 0% or more, 0.2% or more, 0.5% or more, or 1.0% or more.

[0034] In the 1 / 4 region of the plate thickness, the ratio of the number of martensite adjacent to ferrite to the number of metal structures adjacent to ferrite is 30% or less. In the hole expansion test, the area damaged by punching is further deformed, which causes voids to form and connect, leading to fracture. In other words, in order to improve the hole expansion property, it is necessary to suppress the formation and connection of voids. When the hardness difference between adjacent metal structures is large, voids are likely to form at the interface between the metal structures. For example, voids are most likely to form at the interface between soft ferrite and hard martensite. In the steel of the present invention, in order to reduce the proportion of martensite adjacent to ferrite, granular bainite is controlled to be in contact with ferrite. This reduces the proportion of ferrite and martensite in contact, and improvement of hole expansion property can be expected. By setting the ratio of the number of martensite adjacent to ferrite to the number of metal structures adjacent to ferrite to 30% or less, the hole expansion property is significantly improved. Preferably, this number ratio is 28% or less, 25% or less, or 20% or less. In addition, the "number of metal structures adjacent to ferrite" means the number of all metal structures including martensite.

[0035] The difference between the maximum and minimum Vickers hardness values ​​at a load of 50 gf, measured at 30 μm intervals from a position 100 μm deep from the surface of the steel plate in the plate thickness direction to the center of the plate thickness, is 60 HV or less. The metal structure inside the steel plate is not uniformly distributed in the plate thickness direction, but is often distributed in bands with layers with a lot of hard structure and layers with little hard structure. This is due to the fact that the concentration of elements, especially manganese, is biased when measured along the plate thickness direction. If there is a bias in the element concentration in the plate thickness direction, which results in uneven distribution of the structure, a hardness difference is likely to occur between the soft layer and the hard layer, which becomes the starting point of voids and deteriorates the hole expandability.

[0036] The hardness is measured in the thickness direction, and the difference between the maximum and minimum values ​​is used as an index of the hardness variation of the band-shaped structure. By making the difference between the maximum and minimum values ​​60HV or less, the deterioration of the hole expandability can be suppressed and further, the deformability at the stress concentration portion can be improved. Preferably, the difference between the maximum and minimum values ​​of the hardness is 55Hv or less, 50Hv or less, or 30Hv or less.

[0037] The ratio HV30 / Hvi of the Vickers hardness HV30 at a depth of 30 μm from the surface of the steel plate in the plate thickness direction under a load of 0.29 N to the Vickers hardness Hvi at a load of 0.29 N at 1 / 4 of the plate thickness: preferably 0.8 or less By lowering the hardness of the steel sheet surface relative to the hardness of the inner layer, the bendability can be significantly improved. Therefore, in the steel sheet according to this embodiment, the average value of the Vickers hardness measured at 5 points at a depth of 30 μm from the surface of the steel sheet in the sheet thickness direction with a load of 0.29 N is defined as Hv30, and the average value of the Vickers hardness measured at 5 points at a position of 1 / 4 of the sheet thickness with a load of 0.29 N is defined as Hvi, and the ratio Hv30 / Hvi may be set to 0.8 or less. By setting Hv30 / Hvi to 0.8 or less, the bendability of the steel sheet can be further improved. Hv30 / Hvi may be set to 0.7 or less, 0.6 or less, or 0.5 or less.

[0038] Next, the identification of ferrite, granular bainite, martensite, upper bainite, lower bainite, pearlite, and retained austenite and the calculation of their volume fractions will be described.

[0039] Identification of each metal structure and calculation of the volume fraction can be performed by EBSD (Electron Back Scattering Diffraction), X-ray measurement, corrosion using Nital reagent or Lepera solution, and by observing a 100 μm × 100 μm region of the steel sheet cross section parallel to the rolling direction and perpendicular to the sheet surface at a magnification of 1,000 to 50,000 times using a scanning electron microscope. In addition, when measuring the volume fraction of each structure, the measurement points are three, and the average value is calculated.

[0040] The volume fraction of ferrite is determined by observing a 100 μm×100 μm region in the range of 1 / 8 to 3 / 8 of the plate thickness centered at 1 / 4 of the plate thickness in an electron channeling contrast image by a scanning electron microscope. The volume fraction of ferrite is determined by subtracting the volume fraction of granular bainite obtained by a method (EBSD) described later from the total area fraction of ferrite and granular bainite calculated by image analysis using the image analysis software Image J. The electron channeling contrast image is an image that displays the crystal orientation difference of crystal grains as a contrast difference, and the part of the image with uniform contrast (the part that does not include substructures such as blocks and packets, cementite, or residual austenite within the crystal grains and appears as a single uniform contrast) is ferrite. The means for distinguishing ferrite from granular bainite will be described later.

[0041] The volume fraction of retained austenite can be calculated by measuring the diffraction intensity using X-rays.

[0042] In measurements using X-rays, the specimen is mechanically and chemically polished to remove the area from the plate surface to a depth of 1 / 4, and MoKα radiation is used at the plate thickness position to calculate the fraction of retained austenite 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. The five-peak method is commonly used for calculation.

[0043] The volume fraction of martensite is determined by the following procedure. The observation surface of the sample is etched with Lepera solution, and a 100μm x 100μm area is observed with FE-SEM within the range of 1 / 8 to 3 / 8 of the plate thickness, centered at 1 / 4 of the plate thickness. Martensite and retained austenite are not corroded by Lepera corrosion, so the volume fraction of the uncorroded area is the total volume fraction of martensite and retained austenite. The volume fraction of martensite can be calculated by subtracting the volume fraction of retained austenite measured by X-ray from the volume fraction of this uncorroded area.

[0044] Martensite can be distinguished from other structures in electron channeling contrast images taken with a scanning electron microscope, where martensite is the region with high dislocation density and substructures such as blocks and packets within the crystal grains.

[0045] The identification of upper bainite, lower bainite, and tempered martensite is carried out as follows: The observation surface of the sample is corroded with Nital reagent, and a 100μm x 100μm area is observed with FE-SEM in the range of 1 / 8 to 3 / 8 of the plate thickness, with 1 / 4 of the plate thickness as the center. Upper bainite, lower bainite, and tempered martensite can be distinguished from the position and arrangement of cementite contained within the structure.

[0046] Upper bainite has cementite or retained austenite at the interface of lath-shaped bainitic ferrite. Lower bainite has cementite inside lath-shaped bainitic ferrite, has one type of crystal orientation relationship between bainitic ferrite and cementite, and has the same variant of cementite. Based on these characteristics, upper bainite and lower bainite can be identified.

[0047] Pearlite is identified by the following procedure. The observation surface of the sample is corroded with Nital reagent, and the area from 1 / 8 to 3 / 8 of the plate thickness, with 1 / 4 of the plate thickness at the center, is observed with an optical microscope. In the image observed with the optical microscope, the area with dark contrast is identified as pearlite, and the volume fraction of this area is calculated based on image analysis.

[0048] Granular bainite is an aggregate of multiple lath-shaped bainitic ferrites, and has a structure that looks like a single mass with a grain size of about 5 to 20 μm. This is because the recovery proceeds by heat treatment, and the crystal orientation difference at the interface between bainitic ferrite and bainitic ferrite becomes about 5° or less, and the interface appears to disappear. Granular bainite may contain retained austenite or martensite, but unlike upper bainite and lower bainite, granular bainite does not contain carbides inside. In addition, since granular bainite is formed in a high temperature region, recovery proceeds inside it. Therefore, although granular bainite has a dislocation substructure, the dislocation density is lower than that of upper bainite and lower bainite. Therefore, it is not possible to distinguish granular bainite from ferrite by conventional corrosion methods or secondary electron image observation using a scanning electron microscope.

[0049] As a result of the study by the present inventors, it was found that granular bainite has a minute crystal orientation difference within the crystal grains because it is composed of an aggregate of bainitic ferrite. By detecting the minute crystal orientation difference within the crystal grains, it is possible to distinguish between granular bainite and ferrite. Using EBSD, the range of 1 / 8 to 3 / 8 of the plate thickness centered at 1 / 4 of the plate thickness is observed at a step interval of 0.2 μm, and the value of grain average misorientation is calculated from the observation data. The observation is performed on a cross section parallel to the rolling direction and perpendicular to the plate surface.

[0050] The grain average misorientation value is the average of the misorientation between adjacent pixels in an area surrounded by grain boundaries that differ by 15° or more. This method makes it possible to detect minute crystal misorientation in bainitic ferrite.

[0051] The region where the grain average misorientation value is 0.5° or less contains ferrite and granular bainite. Therefore, the region where the grain average misorientation value is 0.5° or less, excluding the ferrite region where the electron channeling contrast image has a single uniform contrast, is the granular bainite. The area ratio calculated by image analysis is the volume ratio of granular bainite.

[0052] In the 100μm×100μm area at 1 / 4 of the plate thickness, the proportion of the metal structure adjacent to ferrite that is martensite is 30% or less Using EBSD, a 100 μm x 100 μm region at 1 / 4 of the plate thickness is observed at a step interval of 0.2 μm. The observation is performed on a cross section parallel to the rolling direction and perpendicular to the plate surface. The grain average misorientation value is calculated from the observation data. Next, the region where the grain average misorientation value is 0.5° or less is observed using an electron channeling contrast image to identify ferrite. Furthermore, martensite is identified among the metal structures adjacent to ferrite. The metal structure that is white in color and has a substructure visible inside is martensite. The number of metal structures adjacent to ferrite and the percentage of the number of martensites in that are calculated.

[0053] "The difference between the maximum and minimum Vickers hardness values ​​at a load of 50 gf obtained at 30 μm intervals from the surface of the steel plate to the center of the plate thickness, starting from a position 100 μm deep in the plate thickness direction from the surface of the steel plate" is determined as follows. First, starting from a position 100 μm deep in the plate thickness direction from the surface of the steel plate, the Vickers hardness is continuously measured at 30 μm intervals along the plate thickness direction with an indentation load of 50 gf. The difference between the maximum and minimum values ​​of the multiple hardness measurements obtained in this way is calculated. Note that the interval between each measurement point arranged in the plate thickness direction is preferably at least four times the distance of the indentation, if possible. In this specification, "at least four times the distance of the indentation" means a distance at least four times the length of the diagonal line of the rectangular opening of the indentation made by the diamond indenter when measuring the Vickers hardness.

[0054] Next, the reasons for limiting the composition of the steel sheet according to this embodiment will be described. In the following, % in the composition means mass %.

[0055] Ingredient composition C: 0.07% or more, 0.15% or less C is an element that ensures a predetermined amount of martensite and improves the strength of the steel sheet. If C is less than 0.07%, it is difficult to obtain a predetermined amount of martensite, and it is not possible to ensure a maximum tensile strength of 900 MPa or more. Therefore, C is set to 0.07% or more. C is preferably 0.09% or more, 0.10% or more, or 0.12% or more.

[0056] On the other hand, if the C content exceeds 0.15%, the formation of ferrite is suppressed and the elongation property is reduced, so the C content is set to 0.15% or less, and preferably 0.14% or less, 0.13% or less, or 0.12% or less.

[0057] Si+Al: 0.20% or more, 2.50% or less Silicon and aluminum are essential elements for obtaining a certain amount of granular bainite. Granular bainite is a metal structure in which dislocations present at the interface of bainitic ferrite are restored by heat treatment, resulting in the formation of bainitic ferrite in a block form.

[0058] Therefore, once cementite is formed at the interface of bainitic ferrite, granular bainite cannot be obtained.Si and Al are elements that act to suppress the formation of cementite.

[0059] If the Si+Al content (the sum of the Si content and the Al content) is less than 0.20%, the effect of suppressing the formation of cementite is not sufficiently obtained, and it becomes difficult to obtain granular bainite at a predetermined area ratio. Therefore, the Si+Al content is set to 0.20% or more. The Si+Al content is preferably 0.25% or more, 0.30% or more, or 0.40% or more.

[0060] On the other hand, if the Si+Al content exceeds 2.50%, the excessive addition of Si and / or Al causes slab cracking, so the Si+Al content is set to 2.50% or less. The Si+Al content is preferably set to 2.00% or less, 1.80% or less, or 1.60% or less.

[0061] Mn+Cr: 1.20% or more, 4.00% or less Mn and / or Cr are elements that contribute to improving strength, and also act to suppress ferritic transformation that occurs during heat treatment in continuous annealing equipment or continuous hot-dip galvanizing equipment.

[0062] If Mn+Cr (the sum of Mn content and Cr content) is less than 1.20%, the effect is not fully exerted, ferrite is generated in an amount exceeding the required area ratio, and a tensile strength of 900 MPa or more cannot be obtained. Therefore, Mn+Cr is set to 1.20% or more. Mn+Cr is preferably 1.30% or more, 1.40% or more, or 1.50% or more. Mn+Cr is more preferably 1.80% or more.

[0063] On the other hand, if Mn+Cr exceeds 4.00%, ferrite transformation is excessively suppressed, the required amount of ferrite cannot be secured, and elongation decreases. Therefore, Mn+Cr is set to 4.00% or less. Mn+Cr is preferably set to 3.00% or less, 2.70% or less, or 2.50% or less.

[0064] P: 0% or more, 0.040% or less P is an impurity element that segregates in the center of the steel plate thickness, impairs toughness, and embrittles welded parts. If P exceeds 0.040%, the strength of the welded parts and the hole expandability are significantly reduced. Therefore, P is set to 0.040% or less. P is preferably 0.030% or less, 0.020% or less, or 0.010% or less.

[0065] The less P, the better, and there is no particular lower limit. The P content may be 0%. On the other hand, if the P content of a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost increases significantly, which is economically disadvantageous. Therefore, the P lower limit may be set to 0.0001%, 0.0002%, or 0.0005%.

[0066] S: 0% or more, 0.010% or less S is an impurity element that impairs weldability and also impairs manufacturability during casting and hot rolling. S also forms coarse MnS and impairs hole expandability. If S exceeds 0.010%, the deterioration of weldability, manufacturability, and hole expandability becomes significant. Therefore, S is set to 0.010% or less. S is preferably 0.008% or less, 0.005% or less, or 0.004% or less.

[0067] The lower the S content, the better, and there is no particular lower limit. The S content may be 0%. On the other hand, if the S content in a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost increases significantly, which is economically disadvantageous. Therefore, the lower limit of S may be set to 0.0001%, 0.0002%, or 0.0005%.

[0068] N: 0% or more, 0.010% or less N is an element that forms coarse nitrides, impairs bendability and hole expandability, and causes blowholes during welding. If N exceeds 0.010%, the hole expandability decreases and the occurrence of blowholes becomes significant. Therefore, N is set to 0.010% or less. The N content may be 0.008% or less, 0.006% or less, or 0.005% or less.

[0069] The less N, the better, and there is no particular lower limit. The N content may be 0%. On the other hand, if the N content of a practical steel sheet is reduced to less than 0.0005%, the manufacturing cost increases significantly, which is economically disadvantageous. Therefore, the lower limit of N may be set to 0.0005%, 0.0008%, or 0.0010%.

[0070] O: 0% or more, 0.006% or less O is an element that forms coarse oxides, impairs bendability and hole expandability, and causes blowholes during welding. If O exceeds 0.006%, the hole expandability decreases and the occurrence of blowholes becomes significant. Therefore, O is set to 0.006% or less. The O content may be 0.005% or less, 0.004% or less, or 0.002% or less.

[0071] The less O, the better, and there is no particular lower limit. The O content may be 0%. On the other hand, if the O content in a practical steel sheet is reduced to less than 0.0005%, the manufacturing cost increases significantly, which is economically disadvantageous. Therefore, the lower limit of O may be set to 0.0005%, 0.0006%, or 0.0008%.

[0072] In addition to the above elements, the composition of the steel sheet according to this embodiment includes, for the purpose of improving characteristics, Mo: 0% or more, 0.50% or less, Ti: 0% or more, 0.20% or less, Nb: 0% or more, 0.20% or less, B: 0% or more, 0.010% or less, V: 0% or more, 0.50% or less, Cu: 0% or more, 1.00% or less, W: 0% or more, 0.1% or less, Ta: 0% or more, 0.1% or less, Ni: 0% or more, 1.00% or less. The steel sheet according to the present embodiment may contain one or more elements selected from the group consisting of Sn: 0% or more and 0.05% or less, Sb: 0% or more and 0.05% or less, As: 0% or more and 0.05% or less, Mg: 0% or more and 0.05% or less, Ca: 0% or more and 0.040% or less, Y: 0% or more and 0.05% or less, Zr: 0% or more and 0.05% or less, La: 0% or more and 0.05% or less, and Ce: 0% or more and 0.05% or less. In the steel sheet according to the present embodiment, these are optional elements. In the steel sheet according to the present embodiment, the content of these optional elements may be less than the lower limit (including 0%) shown below for each of these optional elements.

[0073] Mo: 0 to 0.50% Mo, like Cr, is an element that contributes to increasing the strength of steel sheets. This effect can be obtained even with a small amount. The Mo content may be 0%, but in order to obtain the above effect, the Mo content is preferably 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, if the Mo content exceeds 0.50%, coarse Mo carbides are formed, which may deteriorate the cold formability of the steel sheet. For this reason, the Mo content is preferably 0.50% or less, 0.40% or less, or 0.20% or less.

[0074] Ti: 0 to 0.20% Ti is an important element for controlling the morphology of carbides. Ti can promote an increase in the strength of ferrite. Ti is also an element that may form coarse Ti oxides or TiN, thereby reducing the formability of the steel sheet. Therefore, from the viewpoint of ensuring the formability of the steel sheet, the Ti content is preferably as low as possible, and is preferably 0.20% or less, 0.10% or less, or 0.05% or less, and may be 0%. However, since reducing the Ti content to less than 0.001% leads to an excessive increase in refining costs, the lower limit of the Ti content may be 0.001%, 0.002%, or 0.003%.

[0075] Nb: 0 to 0.20% Nb, like Ti, is an element effective in controlling the morphology of carbides, and is also an element effective in refining the structure and improving the toughness of the steel sheet. This effect can be obtained even with a small amount. The Nb content may be 0%, but in order to obtain the above effect, it is preferable that the Nb content is 0.0001% or more, 0.0005% or more, or 0.0010% or more. However, if the Nb content is too high, a large number of fine and hard Nb carbides are precipitated, which increases the strength of the steel sheet and significantly deteriorates the ductility, and there is a risk that the formability of the steel sheet will decrease. For this reason, the Nb content is preferably 0.20% or less, 0.15% or less, or 0.10% or less.

[0076] B: 0~0.010% B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of low-temperature transformation structures such as bainite or martensite. In addition, B is an element that is beneficial for increasing the strength of steel. This effect can be obtained even with a small amount. The B content may be 0%, but in order to obtain the above effect, it is preferable that the B content is 0.0001% or more, 0.0005% or more, or 0.0010% or more. However, if the B content is too high, coarse B oxides are generated, and the B oxides become the origin of voids during press forming, which may reduce the formability of the steel sheet. For this reason, the B content is preferably 0.010% or less, 0.008% or less, or 0.005% or less. Note that it is necessary to pay close attention to the analysis in order to identify B of less than 0.0001%. If the B content is below the detection limit of the analysis device, the B content may be considered to be 0%.

[0077] V: 0~0.50% Like Ti and Nb, V is an element effective in controlling the morphology of carbides, and is also an element effective in refining the structure and improving the toughness of steel sheets. The V content may be 0%, but in order to obtain the above effects, the V content is preferably 0.001% or more, 0.005% or more, or 0.010% or more. However, if the V content is too high, a large number of fine V carbides are precipitated, which increases the strength of the steel material and reduces the ductility, and there is a risk of reducing the formability of the steel sheet. For this reason, the V content is preferably 0.50% or less, 0.40% or less, or 0.30% or less.

[0078] Cu: 0-1.00% Cu is an element that contributes to improving the strength of steel sheet. This effect can be obtained even with a small amount. The Cu content may be 0%, but in order to obtain the above effect, the Cu content is preferably 0.001% or more, 0.005% or more, or 0.010% or more. However, if the Cu content is too high, there is a risk of inducing red shortness and reducing productivity in hot rolling. For this reason, the Cu content is preferably 1.00% or less, 0.80% or less, or 0.50% or less.

[0079] W: 0~0.10% Like Nb and V, W is an element effective in controlling the morphology of carbides and improving the strength of steel sheets. The W content may be 0%, but in order to obtain the above effects, the W content is preferably 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if the W content is too high, a large number of fine W carbides are precipitated, which increases the strength of the steel sheet and reduces its ductility, and there is a risk of reducing the cold workability of the steel sheet. For this reason, the W content is preferably 0.10% or less, 0.08% or less, or 0.05% or less.

[0080] Ta: 0 to 0.10% Ta, like Nb, V, and W, is an element effective in controlling the morphology of carbides and improving the strength of steel sheets. The Ta content may be 0%, but in order to obtain the above effects, the Ta content is preferably 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Ta content is too high, a large number of fine Ta carbides are precipitated, which increases the strength of the steel sheet and reduces its ductility, and there is a risk of reducing the cold workability of the steel sheet. For this reason, the Ta content is preferably 0.10% or less, more preferably 0.02% or less, and even more preferably 0.010% or less.

[0081] Ni: 0 to 1.00% Ni is an element effective in improving the strength of steel sheet. The Ni content may be 0%, but in order to obtain the above effect, the Ni content is preferably 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Ni content is too high, the ductility of the steel sheet may decrease, which may lead to a decrease in formability. For this reason, the Ni content is preferably 1.00% or less, 0.80% or less, or 0.50% or less.

[0082] Sn: 0 to 0.050% Sn is an element that may be contained in a steel sheet when scrap is used as a raw material for the steel sheet. In addition, Sn may cause a decrease in cold formability of the steel sheet due to embrittlement of ferrite. Therefore, the lower the Sn content, the better. The Sn content is preferably 0.050% or less, more preferably 0.040% or less, or 0.030% or less, and may be 0%. However, since reducing the Sn content to less than 0.001% leads to an excessive increase in refining costs, the Sn content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[0083] Co: 0 to 0.50% Co, like Ni, is an element effective in improving the strength of steel sheet. The Co content may be 0%, but in order to obtain the above effect, the Co content is preferably 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Co content is too high, the ductility of the steel sheet may decrease, which may lead to a decrease in formability. For this reason, the Co content is preferably 0.50% or less, 0.40% or less, or 0.30% or less.

[0084] Sb: 0 to 0.050% Like Sn, Sb is an element that may be contained in a steel sheet when scrap is used as a raw material for the steel sheet. Sb may strongly segregate at grain boundaries, leading to embrittlement of the grain boundaries, reduced ductility, and reduced cold formability. For this reason, the lower the Sb content, the better. The Sb content is preferably 0.050% or less, more preferably 0.040% or less or 0.030% or less, and may be 0%. However, reducing the Sb content to less than 0.001% leads to an excessive increase in refining costs, so the Sb content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[0085] As: 0 to 0.050% Like Sn and Sb, As is an element that may be contained in a steel sheet when scrap is used as a raw material for the steel sheet. As is an element that strongly segregates at grain boundaries and may cause a decrease in cold formability. For this reason, the lower the As content, the better. The As content is preferably 0.050% or less, more preferably 0.040% or less or 0.030% 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 0.001% or more, 0.005% or more, or 0.010% or more.

[0086] Magnesium: 0 to 0.050% Mg 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. The Mg content may be 0%, but in order to obtain the above effect, the Mg content is preferably 0.0001% or more, 0.005% or more, or 0.010% or more. However, if the Mg content is too high, there is a risk of causing a decrease in cold formability due to the formation of coarse inclusions. For this reason, the Mg content is preferably 0.050% or less, and more preferably 0.040% or less or 0.030% or less.

[0087] Ca: 0 to 0.040% 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 0.001% or more, 0.005% or more, or 0.010% or more. However, if the Ca content is too high, coarse Ca oxides are generated, and the Ca oxides may become the starting point of crack generation during cold forming. For this reason, the Ca content is preferably 0.040% or less, and more preferably 0.030% or less or 0.020% or less.

[0088] Y: 0~0.050% Y, like Mg and Ca, 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, 0.005% or more, or 0.010% or more. However, if the Y content is too high, coarse Y oxides are generated, which may deteriorate the cold formability. For this reason, the Y content is preferably 0.050% or less, more preferably 0.040% or less, or 0.030% or less.

[0089] Zr: 0 to 0.050% Zr is an element that can control the morphology of sulfides with a small amount, similar to Mg, Ca, and Y. The Zr content may be 0%, but in order to obtain the above effects, the Zr content is preferably 0.001% or more, 0.005% or more, or 0.010% or more. However, if the Zr content is too high, coarse Zr oxides are generated, and cold formability may be reduced. For this reason, the Zr content is preferably 0.050% or less, and more preferably 0.040% or less or 0.030% or less.

[0090] La: 0 to 0.050% 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 effects, the La content is preferably 0.001% or more, 0.005% or more, or 0.010% or more. However, if the La content is too high, La oxides may be generated, which may deteriorate the cold formability. For this reason, the La content is preferably 0.050% or less, and more preferably 0.040% or less or 0.030% or less.

[0091] Ce: 0 to 0.050% Ce is an element that can control the morphology of sulfides with a small amount, similar to La. The Ce content may be 0%, but in order to obtain the above effect, the Ce content is preferably 0.001% or more, 0.005% or more, or 0.010% or more. However, Ce is an element that strongly segregates to grain boundaries, causing a decrease in the number ratio of grain boundary carbides, and if the number ratio of grain boundary carbides decreases, there is a risk that the formability of the steel sheet will decrease. For this reason, the Ce content is preferably 0.050% or less, more preferably 0.040% or less, or 0.030% or less.

[0092] In the composition of the steel sheet according to the present embodiment, the balance excluding the above elements is Fe and impurities. The impurities are elements that are mixed in from the steel raw materials and / or during the steelmaking process, and the presence of which is permitted to the extent that it does not impair the properties of the steel sheet according to the present embodiment.

[0093] The steel sheet according to the present embodiment may have a plating layer. The plating layer of the steel sheet according to the present embodiment may be a hot-dip galvanized layer or a hot-dip zinc alloy plating layer (a plating layer composed of an alloy of zinc and additional elements such as Si and Al), or may be an alloyed hot-dip galvanized layer (alloyed plating layer) obtained by alloying these platings. The steel sheet according to the present embodiment may also have another plating layer (for example, an aluminum plating layer, etc.).

[0094] The hot-dip galvanized layer and the hot-dip zinc alloy plated layer preferably contain less than 7 mass% Fe, and the alloyed plated layer preferably contains 7 mass% to 15 mass% Fe. In the hot-dip galvanized layer, the hot-dip zinc alloy plated layer, and the alloyed plated layer, the components other than zinc and Fe are not particularly limited, and various configurations can be adopted within the usual range.

[0095] Next, a method for manufacturing a steel sheet according to this embodiment will be described.

[0096] The steel plate according to the present embodiment is manufactured as follows. A cast slab having the component composition of the steel plate according to the present embodiment is (a-1) Finish rolling is completed at 880°C or higher, (a-2) Cooling is performed at an average cooling rate of 20°C / sec or more from the finish rolling completion temperature to the coiling temperature, (a-3) hot rolling the steel sheet so that the coiling temperature is 680°C or less to obtain a hot-rolled steel sheet; (b) The hot-rolled steel sheet is pickled and then cold-rolled to obtain a cold-rolled steel sheet, and then (c) heating the cold-rolled steel sheet at a heating rate of 0.7°C / s to 10°C / s in the temperature range of 650°C or less, and switching the heating rate in the temperature range of 650°C to Ac1 point so that the heating rate in the temperature range of 650°C to Ac1 point (Ac1+20°C) is 0.6°C / s to 2.0°C / s; (d) Then, the material is heated to a maximum heating temperature of (Ac1+30°C) or more and 900°C or less, and the temperature is maintained in the above temperature range for 10 seconds or more and 500 seconds or less. (e) preferably, a cooling zone is provided in which the temperature is cooled to a range of 650° C. or more and 750° C. or less at a cooling rate of 0.5° C. / sec. or more and 20° C. / sec. or less; The average cooling rate between 580°C and 650°C is 5°C / sec or more. (f-1) Cool to a first retention temperature in the temperature range of 480°C to 580°C, and retain the temperature in this temperature range for 10 seconds to 100 seconds. (f-2) Dwell for 10 seconds or more and 500 seconds or less at a second dwell temperature in the temperature range of 500°C or more and 630°C or less, and then, if plating is to be performed, carry out step (g), or if plating is not to be performed, cool to room temperature. (g) In the case of plating, after step (f-2), the steel sheet is cooled to (zinc plating bath temperature -40) °C to (zinc plating bath temperature +50) °C and immersed in a hot-dip galvanizing bath to perform hot-dip galvanization, and if necessary, the hot-dip galvanized layer is subjected to an alloying treatment. (h) When tempering is performed, the steel sheet after retention, the steel sheet having hot-dip galvanizing or hot-dip zinc alloy plating, or the steel sheet having alloyed hot-dip galvanizing is tempered in a temperature range of 500°C or less. Tempering may be performed online or offline after continuous annealing. When tempering is performed online, the steel sheet may be cooled to a temperature range of room temperature or more and 300°C or less during the cooling process after step (f) or (g), and then heated to a temperature of 200°C or more and 400°C or less. The process conditions are described below.

[0097] (a-1) Finish rolling completion temperature: 880°C or higher The steel plate according to the present embodiment contains a relatively large amount of alloy elements in order to ensure a maximum tensile strength of 900 MPa or more. Therefore, it is necessary to increase the rolling load during hot rolling, so hot rolling is preferably performed at a high temperature. In addition, during hot rolling, the roughly rolled steel plate may be joined and hot rolling may be performed continuously.

[0098] Finish rolling is completed at 880°C or higher. If the finish rolling completion temperature range is less than 880°C, the rolling load becomes excessive, which may deteriorate productivity, so the temperature range is set to 880°C or higher. Furthermore, if the rolling is performed in the two-phase temperature range (austenite + ferrite), the shape after hot rolling deteriorates, making it impossible to pass the sheet through thereafter. There is no particular upper limit to the finish rolling completion temperature, but if the finish rolling completion temperature is 950°C or higher, productivity will deteriorate, so the upper limit of the finish rolling completion temperature is preferably set to 950°C.

[0099] (a-2) Average cooling rate from finish rolling completion temperature to coiling temperature: 20°C / sec or more Slabs produced by casting have center segregation and microsegregation. Microsegregation is elongated in the rolling direction by rolling, and enriched and depleted regions are formed in bands alternately along the plate thickness direction. Such band-like element segregation is particularly noticeable for manganese, and remains in the steel plate even after subsequent processes, affecting the structure formation during continuous annealing, increasing the non-uniformity in the plate thickness direction, and causing deterioration of formability.

[0100] When pearlite transformation progresses from the completion of finish rolling to coiling, band-like segregation of elements confirmed in measurements along the sheet thickness direction of the steel sheet is promoted. This is because manganese is concentrated in cementite in pearlite, and the manganese concentration difference confirmed in measurements along the sheet thickness direction becomes larger compared to when pearlite transformation does not progress. In order to prevent this, it is necessary to set the average cooling rate from the finish rolling completion temperature to the coiling temperature (the difference between the finish rolling completion temperature and the coiling temperature divided by the time required from the end of finish rolling to the start of coiling) to 20°C / sec or more. The average cooling rate from the finish rolling completion temperature to the coiling temperature is preferably 50°C / sec or more, more preferably 100°C / sec or more.

[0101] (a-3) Winding temperature: 680°C or less The hot-rolled steel sheet is coiled in a temperature range of 680°C or less. If the coiling temperature exceeds 680°C, pearlite transformation progresses, and as described above, the concentration of elements confirmed by measurement along the sheet thickness direction becomes large, and formability after continuous annealing deteriorates, so the coiling temperature range is set to 680°C or less. The coiling temperature is preferably 550°C or less, more preferably 450°C or less. There is no particular lower limit for the coiling temperature, but since it is technically difficult to coil at a temperature below room temperature, room temperature is the substantial lower limit.

[0102] (b) The hot-rolled steel sheet is pickled and then cold-rolled to obtain a cold-rolled steel sheet (hereinafter referred to as "steel sheet"). After hot rolling, the steel sheet is subjected to pickling and cold rolling. There are no particular restrictions on these steps. For example, pickling may be performed once, or may be performed multiple times as necessary. For cold rolling, the number of rolling passes and the reduction rate per pass may be appropriately set within a range in which a reduction rate of about 10% or more and 80% or less can be ensured.

[0103] To obtain a strength of 900 MPa or more, it is necessary to obtain a certain amount of hard structure. To achieve this, heating is performed by switching the heating rate as follows. (c1) Heating from room temperature to 650° C. at a heating rate of 0.7° C. / sec or more and preferably 10° C. / sec or less. (c2) The heating rate is switched within the temperature range of 650°C or higher and Ac1 point or lower. (c3) Heating is performed at a heating rate of 0.6°C / sec or more and 2°C / sec or less in the temperature range from Ac1 point to Ac1 point + 20°C.

[0104] The temperature range of Ac1 point or more and Ac1 point + 20°C is the time when austenite (γ structure) begins to form. If the heating rate in this temperature range is less than 0.6°C, austenite grain growth occurs preferentially in the part where Mn is segregated. Therefore, the austenite grains cannot be uniformly dispersed. On the other hand, if the heating rate in this temperature range exceeds 2°C / sec, the formation of austenite is delayed due to so-called overheating, and austenite is formed in a high temperature range. In this case, rapid austenite growth occurs in the part where Mn is segregated, and therefore the austenite grains cannot be uniformly dispersed. For the above reasons, it is necessary to heat the temperature range of Ac1 point or more and Ac1 point + 20°C at a heating rate of 0.6°C / sec or more and 2°C / sec or less.

[0105] In addition, in the temperature range from room temperature to 650°C, the heating rate is set to 0.7°C / sec or more and 10°C / sec or less. By setting the heating rate to 0.7°C / sec or more, the recrystallization of ferrite can be sufficiently promoted. However, if the heating rate is too high, it is expected that it will be difficult to set the heating rate in the temperature range from Ac1 point to Ac1 point + 20°C within the above range. Therefore, it is preferable that the heating rate until the temperature of the steel sheet reaches 650°C is 10°C / sec or less.

[0106] In addition, in order to set the heating rate in the temperature range of Ac1 point or more to Ac1 point + 20°C to 0.6°C / sec or less and 2°C / sec or less, the heating rate must be switched when the steel sheet temperature reaches a temperature range slightly lower than this temperature range. Specifically, the heating rate must be switched when the steel sheet temperature is in the temperature range of 650°C or more and Ac1 point or less. If the switching temperature is less than 650°C, unrecrystallized regions of ferrite grains remain, and the elongation of the finally obtained steel sheet decreases. On the other hand, if the switching temperature is more than Ac1 point or less, the above-mentioned overheating cannot be suppressed.

[0107] Note that the "heating rate" is a different concept from the "average heating rate." In the manufacturing method for a steel sheet according to this embodiment, the heating rate of the temperature of the steel sheet must always be within the above range in the temperature range of not less than Ac1 point and not more than Ac1+20°C.

[0108] (d) Maximum heating temperature: Ac1+30℃ or more, 900℃ or less, and holding time: 10 seconds or more, 500 seconds or less In order to obtain a strength of 900 MPa or more, it is necessary to obtain a predetermined amount of hard structure. If the maximum heating temperature is less than Ac1+30°C or the holding time is less than 10 seconds, the generation of austenite during annealing is suppressed, and the predetermined amount of hard structure cannot be obtained as the final structure, and the tensile strength of 900 MPa cannot be satisfied.

[0109] On the other hand, when the annealing temperature range exceeds 900°C or the holding time exceeds 500 seconds, the grain growth of austenite proceeds, the ferrite transformation is excessively suppressed, and the desired amount of ferrite cannot be obtained in the final structure, resulting in a decrease in ductility, so the annealing temperature range is set to 900°C or less. The annealing temperature range is preferably 850°C or less. The holding time means the time during which the temperature of the steel sheet is isothermally held at the maximum heating temperature.

[0110] The atmosphere in which the steel sheet is heated to the above temperature range is not particularly limited, but for example, the steel sheet may be heated in an atmosphere with a dew point of -15°C or more and 20°C or less. This promotes decarburization of the steel sheet surface, and further improves the bendability of the steel sheet. Furthermore, oxidation of easily oxidizable elements such as Si and Mn progresses, and therefore the coating adhesion can also be improved. Specifically, by setting the dew point when the steel sheet is heated to -15°C or more, it is possible to suppress the external oxidation state of Si and Mn while appropriately promoting decarburization, and thus it is possible to ensure even better coating adhesion. The dew point when the steel sheet is heated is more preferably -10°C or more. In addition, by setting the dew point when the steel sheet is heated to 20°C or less, it is possible to suppress the excessive progression of decarburization and the oxidation of the base steel sheet itself, which leads to the deterioration of the surface condition of the steel sheet. The dew point when the steel sheet is heated is preferably -24°C or less. There is no particular limitation on the means for controlling the atmosphere around the steel sheet. For example, the steel sheet may be heated and held at that temperature in a heating furnace and a soaking furnace of a continuous annealing line, and the atmosphere in these furnaces may be controlled within the above-mentioned ranges.

[0111] (e) After the steel sheet is held at the maximum heating temperature, the production line may be provided with a slow cooling zone in which the steel sheet is cooled to a temperature range of 650°C to 750°C at an average cooling rate of 0.5°C / sec to 20°C / sec. However, when the slow cooling zone is provided, the average cooling rate must be 0.5°C / sec or more. If the average cooling rate in the slow cooling zone is too small, the amount of ferrite becomes excessive, resulting in insufficient tensile strength. The average cooling rate is the average cooling rate in the temperature range from the maximum heating temperature to the steel sheet temperature at the exit of the slow cooling zone. It is the value obtained by dividing the difference between the maximum heating temperature and the steel sheet temperature at the exit of the slow cooling zone by the temperature at which the steel sheet temperature drops from the maximum heating temperature to the steel sheet temperature at the exit of the slow cooling zone. On the other hand, in the temperature range of 580° C. to 650° C., the average cooling rate is set to 5° C. / sec or more, regardless of the presence or absence of the slow cooling zone, which makes it possible to suppress the excessive formation of ferrite.

[0112] In the method for producing a steel sheet according to the present embodiment, it is necessary to generate a predetermined amount of granular bainite, and therefore it is necessary to control the temperature after annealing as follows. (f-1) Cool to a first retention temperature in the temperature range of 480°C to 580°C, and retain the temperature in this temperature range for 10 seconds to 100 seconds. (f-2) The material is allowed to dwell at a second dwell temperature in the temperature range of 500° C. to 630° C. for 10 seconds to 500 seconds, and then cooled to room temperature.

[0113] Annealing is performed at a maximum heating temperature of Ac1+30°C or more and 900°C or less. After completion of the temperature holding at this maximum heating temperature, the temperature of the steel sheet is first lowered to a temperature range of 480°C or more and 580°C or less. At this time, the steel sheet may be cooled via the above-mentioned slow cooling zone. Then, the temperature of the steel sheet is held at a first holding temperature in the temperature range of 480°C or more and 580°C or less for 10 seconds to 100 seconds. In the temperature range of 480°C or more and 580°C or less, no carbides are generated in the steel sheet, and bainitic ferrite is generated in the steel sheet. This can further promote the generation of granular bainite in the subsequent heat treatment.

[0114] Furthermore, the temperature of the steel sheet is held for 10 seconds or more and 100 seconds or less at a first holding temperature in the temperature range of 480°C to 580°C, and then the temperature of the steel sheet is set to a temperature range of 500°C to 630°C. If necessary, the steel sheet may be reheated. Then, the temperature of the steel sheet is held for 10 seconds or more and 500 seconds or less at a second holding temperature in the temperature range of 500°C to 630°C. This allows a recovery phenomenon to occur in the bainitic ferrite (a phenomenon in which atoms are diffused and lattice defects move by holding a metal at a high temperature, and the dislocation density inside the metal is reduced by annihilation of the lattice defects introduced into the bainitic ferrite during transformation). As a result, granular bainite is formed between the ferrite and martensite, and the ratio of the number of martensite adjacent to the ferrite can be reduced to 30% or less.

[0115] When the temperature holding time in the temperature range of 480° C. to 580° C. is insufficient, and when the temperature holding time in the temperature range of 500° C. to 630° C. is insufficient, granular bainite is not sufficiently generated. On the other hand, when the temperature holding time in the temperature range of 480° C. to 580° C. is excessive, and when the temperature holding time in the temperature range of 500° C. to 630° C. is excessive, granular bainite is excessively generated, and the tensile strength is significantly reduced.

[0116] (g) Plating and alloying processes If necessary, the steel sheet may be cooled to (zinc plating bath temperature -40) °C to (zinc plating bath temperature +50) °C and immersed in a hot-dip galvanizing bath or a hot-dip zinc alloy plating bath. After the hot-dip galvanizing or hot-dip zinc alloy plating bath is applied, the hot-dip galvanizing or hot-dip zinc alloy plating may be subjected to an alloying treatment. Note that the preferred conditions for the hot-dip galvanizing bath and the hot-dip zinc alloy plating bath are substantially the same, so only the hot-dip galvanizing bath will be mentioned below.

[0117] Cooling temperature range: (Zinc plating bath temperature -40)℃~(Zinc plating bath temperature +50)℃ In order to subject the steel sheet in which a predetermined amount of granular bainite has been generated by maintaining the steel sheet in step (f) to hot-dip galvanization, the steel sheet is cooled to a temperature range near the galvanization bath temperature, i.e., to a temperature range of (galvanization bath temperature -40) °C to (galvanization bath temperature +50) °C.

[0118] If the cooling temperature range is less than (galvanizing bath temperature - 40)°C, the temperature of the steel sheet will be too low compared to the temperature of the galvanizing bath, and the galvanizing bath temperature will drop due to the steel sheet entering the galvanizing bath, making it impossible to maintain an appropriate galvanizing bath temperature, so the cooling temperature range is set to (galvanizing bath temperature - 40)°C or higher, preferably (galvanizing bath temperature - 20)°C or higher.

[0119] On the other hand, if the cooling temperature range exceeds (galvanizing bath temperature + 50)°C, the steel sheet temperature becomes too high, and as the steel sheet enters the galvanizing bath, zinc and iron diffuse violently on the steel sheet surface, increasing the Fe concentration in the galvanized layer and causing embrittlement, so the cooling temperature range is set to (galvanizing bath temperature + 50)°C or lower, preferably (galvanizing bath temperature + 30)°C or lower.

[0120] The alloying treatment applied to the hot-dip galvanized layer is carried out by heating the hot-dip galvanized layer within a normal heating temperature range, that is, 300 to 550°C.

[0121] (h) After step (f) or step (g), the steel sheet may be tempered. The tempering step may be a step of holding or reheating at a predetermined temperature during final cooling to room temperature, or a step of reheating to a predetermined temperature after final cooling. The heating method in the tempering step is not particularly limited. However, from the viewpoint of suppressing a decrease in strength of the steel sheet, the holding temperature or heating temperature in the tempering step is preferably 500°C or less. Tempering may be performed online, or may be performed offline after continuous annealing. When tempering is performed online, the steel sheet may be cooled to a temperature range of room temperature to 300°C during cooling after step (f) or (g), and then heated to a temperature of 200°C to 400°C. EXAMPLES

[0122] (Experiment 1) Various steel plates were manufactured using various slabs having the chemical compositions shown in Tables 1-1 and 1-2 under various manufacturing conditions shown in Tables 2-1 to 3-2. In Tables 1-1 and 1-2, the contents of elements that were not added to the slabs are left blank. The units of the components of each slab are mass %, with the remainder being iron and impurities. The temperature holding time at the maximum heating temperature was within the range of 10 seconds to 500 seconds under all manufacturing conditions. In addition, the rolling reduction during cold rolling was within the range of 10 to 80% under all manufacturing conditions. The tempering temperature and tempering time under manufacturing conditions in which tempering was not performed are shown as blanks. If there is plating, it is either a hot-dip galvanized layer or a hot-dip zinc alloy plated layer. Regarding the steel sheets, values ​​outside the scope of the invention and values ​​that did not meet the pass / fail criteria are underlined. Regarding the manufacturing conditions, values ​​outside the range considered appropriate are underlined.

[0123] In Tables 2-1 and 2-2, the manufacturing conditions indicated as "without annealing zone" are those in which two-stage cooling was not performed after holding the temperature at the maximum heating temperature. In these manufacturing conditions, "average cooling rate up to 500°C" means the average cooling rate from the maximum heating temperature to the stationary temperature range (temperature at which the temperature stagnates). On the other hand, in Tables 2-1 and 2-2, the manufacturing conditions not indicated as "without annealing zone" are those in which two-stage cooling was performed after holding the temperature at the maximum heating temperature. In these manufacturing conditions, "average cooling rate in the annealing zone" means the average cooling rate from the maximum heating temperature to the cooling stop temperature in the annealing zone, and "average cooling rate up to 500°C" means the average cooling rate from the cooling stop temperature in the annealing zone to the stationary temperature range.

[0124] The metal structure at 1 / 4 of the plate thickness of these steel plates, the ratio of the number of martensite adjacent to ferrite to the number of metal structure adjacent to ferrite at 1 / 4 of the plate thickness (referred to as "the ratio of martensite in contact with ferrite" in the table), and the difference between the maximum and minimum Vickers hardness values ​​at a load of 50 gf measured at 30 μm intervals from a position 100 μm deep from the surface of the steel plate in the plate thickness direction to the center position of the plate thickness (referred to as "maximum difference in hardness in the plate thickness direction" in the table) were evaluated and shown in Tables 4-1 and 4-2. These evaluations were performed according to the methods described above.

[0125] In addition, the tensile strength (TS) and normal elongation (El 1 ), Processing part elongation (El 2) Hole expansion ratio (λ), α, and workability were evaluated and shown in Tables 5-1 and 5-2. The evaluation methods are as follows. The tensile strength of the steel plates was evaluated by taking JIS 5B test pieces from the steel plates so that the longitudinal direction was perpendicular to the rolling direction of the steel plates, and conducting tensile tests in accordance with JIS Z 2241: 2011. Steel plates with a tensile strength of 900 MPa or more were judged to pass the test in terms of tensile strength. Regarding the elongation of steel sheets, the normal elongation (El 1 ), and the elongation of the processed part (El 2 The normal elongation of the steel sheet, El 1 The evaluation of (total elongation El) was carried out in the same manner as the evaluation of tensile strength, by taking a JIS 5B test piece from the steel plate so that the longitudinal direction was perpendicular to the rolling direction of the steel plate, and conducting a tensile test in accordance with JIS Z 2241:2011. 1 Steel sheets having an elongation of 10.0% or more were judged to be acceptable. 2 The evaluation of elongation was performed by making a circular notch with a radius of R30 as shown in FIG. 3 in the longitudinal center 5 of a JIS5B test piece taken from the steel plate, and then performing a tensile test on this in the same manner as in the evaluation of normal elongation to measure the total elongation. 2 Normal elongation El 1 Divided by (El 2 / El 1 ) exceeding 0.280 was judged to pass the test with respect to deformability at the stress concentration area. The hole expandability (λ) of the steel sheets was evaluated by carrying out a hole expanding test in accordance with JIS Z 2256: 2010. Steel sheets with a hole expandability of 25.0% or more were judged to pass the hole expandability test. In the evaluation of the workability of steel sheets, steel sheets that satisfied the lower limit values ​​of the above elongation and hole expandability and further had an elongation x hole expandability (El x λ) of 300.0 or more were judged to have passed the workability test.

[0126] [Table 1-1]

[0127] [Table 1-2]

[0128] [Table 2-1]

[0129] [Table 2-2]

[0130] [Table 3-1]

[0131] [Table 3-2]

[0132] [Table 4-1]

[0133] [Table 4-2]

[0134] [Table 5-1]

[0135] [Table 5-2]

[0136] The examples in which the chemical composition and manufacturing conditions were appropriate were judged to have excellent tensile strength, elongation, and hole expansion properties, and to have high levels of both strength and formability. On the other hand, in the case of Comparative Example NN-32, the rolling load was too large and the shape of the hot-rolled sheet was poor, so the test after hot rolling was impossible. This is considered to be because the finish rolling completion temperature was too low. Comparative Example NN-33 had a shortage of martensite, while the ratio of martensite in contact with ferrite and the maximum difference in hardness in the thickness direction were excessive, resulting in insufficient elongation, hole expandability, workability, and deformability in the stress concentration area. This is presumably because the cooling rate from finish rolling to coiling was slow, which promoted the non-uniformity of the structure. In the comparative example NN-35, the ratio of martensite in contact with ferrite and the maximum difference in hardness in the sheet thickness direction were excessive, resulting in insufficient hole expandability, workability, and deformability in the stress concentration area. This is presumably because the high coiling temperature promoted the non-uniformity of the structure. In the comparative example NN-36, the ratio of martensite in contact with ferrite was excessive, resulting in insufficient hole expansion, workability, and deformability at the stress concentration area. This is presumably due to the slow heating rate before switching. In the comparative example NN-38, the amount of ferrite was excessive and other structures were insufficient, resulting in insufficient tensile strength. This is presumably due to the low maximum heating temperature. Comparative Example NN-39 had a small amount of ferrite and an excessive amount of martensite, resulting in impaired elongation and workability. This is presumably due to the high maximum heating temperature. In the comparative example NN-40, the amount of ferrite was excessive and the amount of martensite was insufficient, resulting in insufficient tensile strength. This is presumably due to the slow average cooling rate in the slow cooling zone. In Comparative Example NN-42, granular bainite and martensite were insufficient, and the proportion of martensite in contact with ferrite was excessive. As a result, the tensile strength, hole expandability, workability, and deformability at the stress concentration area were impaired. This is presumably because the retention time at 480 to 580 ° C was too long. In Comparative Example NN-44, granular bainite was not formed, martensite and bainite were excessive, and the proportion of martensite in contact with ferrite was excessive. As a result, the hole expansion property, workability, and deformability at the stress concentration part were impaired. This is presumably because the retention time between 480°C and 580°C was short. In Comparative Example NN-45, the amount of C was insufficient, and therefore martensite was insufficient, resulting in a loss of tensile strength. Comparative Example NN-46 contained excess C, which resulted in an excess amount of martensite, a deficiency in ferrite and granular bainite, and excess martensite, resulting in insufficient elongation, workability, and deformability in the stress concentration area. In Comparative Example NN-47, since the amount of Si+Al was insufficient, the amount of bainite was excessive while the amount of granular bainite was insufficient, resulting in insufficient hole expandability, workability, and deformability in the stress concentration area. In Comparative Example NN-48, the Si+Al content was excessive, so the steel sheet became embrittled and the elongation and workability were insufficient. In Comparative Example NN-49, since the Mn+Cr was insufficient, the amount of ferrite was excessive and the amount of martensite was insufficient, resulting in insufficient tensile strength. In Comparative Example NN-50, since Mn+Cr was excessive, coarse Mn oxides and sulfides were formed, and the hole expandability, workability, and deformability in the stress concentration area were deteriorated. In the comparative example NN-51, the ratio of martensite in contact with ferrite and the maximum difference in hardness in the plate thickness direction were excessive, resulting in insufficient workability and deformability in the stress concentration area. This is presumably due to an inappropriate finish rolling temperature. In the comparative example NN-52, the maximum difference in hardness in the plate thickness direction was excessive, and as a result, the deformability in the stress concentration area was insufficient. This is presumably because the heating rate was switched at a temperature range that was too low. In the comparative example NN-53, the maximum difference in hardness in the plate thickness direction was excessive, and as a result, the deformability in the stress concentration area was insufficient. This is presumably because the heating rate was switched at a temperature range that was too high. In the comparative example NN-54, the maximum difference in hardness in the plate thickness direction was excessive, and as a result, the deformability in the stress concentration area was insufficient. This is presumably because the heating rate in the temperature range of Ac1 to Ac1+20°C was too high. In the comparative example NN-55, the maximum difference in hardness in the plate thickness direction was excessive, and as a result, the deformability in the stress concentration area was insufficient. This is presumably because the heating rate in the temperature range of Ac1 to Ac1+20°C was too small. In Comparative Example NN-56, granular bainite was not generated, and the ratio of martensite in contact with ferrite was excessive, resulting in insufficient deformability in the stress concentration area. This is presumably due to insufficient retention time at 480 to 580°C. In Comparative Example NN-57, granular bainite was not generated, and the ratio of martensite in contact with ferrite was excessive, resulting in insufficient deformability in the stress concentration area. This is presumably due to insufficient retention time at 500 to 630°C. In Comparative Example NN-58, granular bainite was not generated, and the ratio of martensite in contact with ferrite was excessive, resulting in insufficient deformability in the stress concentration area. This is presumably due to excessive retention time at 500 to 630°C. In Comparative Example NN-59, granular bainite was not generated, and the ratio of martensite in contact with ferrite was excessive, resulting in insufficient deformability in the stress concentration area. This is presumably because no dwell time was provided in the temperature range of 480 to 580°C or 500 to 630°C.

[0137] (Experiment 2) Various steel plates were manufactured using various slabs having the chemical compositions shown in Tables 6-1 and 6-2 under various manufacturing conditions shown in Tables 7-1 to 8-2. In Tables 6-1 and 6-2, the contents of elements that were not added to the slabs are shown as blanks. The units of the components of each slab are mass %, with the remainder being iron and impurities. The temperature holding time at the maximum heating temperature was within the range of 10 seconds to 500 seconds under all manufacturing conditions. In addition, the rolling reduction during cold rolling was within the range of 10 to 80% under all manufacturing conditions. The tempering temperature and tempering time under manufacturing conditions in which tempering was not performed are shown as blanks. If there is plating, it is either a hot-dip galvanized layer or a hot-dip zinc alloy plated layer. Regarding the steel sheets, values ​​outside the scope of the invention and values ​​that did not meet the pass / fail criteria are underlined. Regarding the manufacturing conditions, values ​​outside the range considered appropriate are underlined.

[0138] In Tables 7-1 and 7-2, the manufacturing conditions indicated as "without annealing zone" are those in which two-stage cooling was not performed after holding the temperature at the maximum heating temperature. In these manufacturing conditions, "average cooling rate up to 500°C" means the average cooling rate from the maximum heating temperature to the stationary temperature range (temperature at which the temperature stagnates). On the other hand, in Tables 7-1 and 7-2, the manufacturing conditions not indicated as "without annealing zone" are those in which two-stage cooling was performed after holding the temperature at the maximum heating temperature. In these manufacturing conditions, "average cooling rate in the annealing zone" means the average cooling rate from the maximum heating temperature to the cooling stop temperature in the annealing zone, and "average cooling rate up to 500°C" means the average cooling rate from the cooling stop temperature in the annealing zone to the stationary temperature range.

[0139] The metal structure at 1 / 4 of the plate thickness of these steel plates, the ratio of the number of martensite adjacent to ferrite to the number of metal structure adjacent to ferrite at 1 / 4 of the plate thickness (referred to as "the ratio of martensite in contact with ferrite" in the table), and the difference between the maximum and minimum Vickers hardness values ​​at a load of 50 gf measured at 30 μm intervals from a position 100 μm deep from the surface of the steel plate in the plate thickness direction to the center position of the plate thickness (referred to as "maximum difference in hardness in the plate thickness direction" in the table) were evaluated and shown in Tables 9-1 and 9-2. These evaluations were performed according to the methods described above.

[0140] Furthermore, the tensile strength (TS), elongation (El), hole expandability (λ), α, and formability of these steel sheets were evaluated and are shown in Tables 10-1 and 10-2. The evaluation methods are as follows. The tensile strength of the steel plates was evaluated by taking JIS 5B test pieces from the steel plates so that the longitudinal direction was perpendicular to the rolling direction of the steel plates, and conducting tensile tests in accordance with JIS Z 2241: 2011. Steel plates with a tensile strength of 900 MPa or more were judged to pass the test in terms of tensile strength. Regarding the elongation of steel sheets, the normal elongation (El 1 ), and the elongation of the processed part (El 2 The normal elongation of the steel sheet, El 1 The evaluation of (total elongation El) was carried out in the same manner as the evaluation of tensile strength, by taking a JIS 5B test piece from the steel plate so that the longitudinal direction was perpendicular to the rolling direction of the steel plate, and conducting a tensile test in accordance with JIS Z 2241:2011. 1 Steel sheets having an elongation of 10.0% or more were judged to be acceptable. 2 The evaluation of elongation was performed by making a circular notch with a radius of R30 in the center of the longitudinal direction of a JIS5B test piece taken from the steel plate as shown in Figure 3, and then performing a tensile test on this as in the evaluation of normal elongation to measure the total elongation. 2 Normal elongation El 1 Divided by (El 2 / El 1 ) exceeding 0.280 was judged to pass the test with respect to deformability at the stress concentration area. The hole expandability (λ) of the steel sheets was evaluated by carrying out a hole expanding test in accordance with JIS Z 2256: 2010. Steel sheets with a hole expandability of 25.0% or more were judged to pass the hole expandability test. In the evaluation of the workability of steel sheets, steel sheets that satisfied the lower limit values ​​of the above elongation and hole expandability and further had an elongation x hole expandability (El x λ) of 300.0 or more were judged to have passed the workability test. The limit bending angle α is defined as the bending angle at the maximum load obtained by a bending test in accordance with VDA-238-100. The bending threshold satisfies the following formula, which is obtained using the thickness t of the steel plate. This threshold is necessary to avoid cracks due to bending deformation during a collision. Steel plates with a bending threshold of 0 or more were judged to pass the test in terms of bendability. (Bending threshold) = α - (7.96 × t 2 -38.4×t+120)≧0

[0141] [Table 6-1]

[0142] [Table 6-2]

[0143] [Table 7-1]

[0144] [Table 7-2]

[0145] [Table 8-1]

[0146] [Table 8-2]

[0147] [Table 9-1]

[0148] [Table 9-2]

[0149] [Table 10-1]

[0150] [Table 10-2]

[0151] The examples in which the chemical composition and manufacturing conditions were appropriate were excellent in tensile strength, elongation, and hole expansion, and were judged to have both strength and formability at high levels. In addition, the examples except for example NA-39 were also excellent in limit bending angle. Example NA-39 had a Hv30 / Hvi that was not within the preferred range, so the evaluation result of the limit bending angle was not within the preferred range, but since the other evaluation results were excellent, it was judged to be a steel plate that had high levels of strength, formability, and deformability at the stress concentration part. On the other hand, in the case of Comparative Example NA-32, in addition to the excessive rolling load, the hot-rolled sheet had a defective shape, and therefore the test after hot rolling was impossible. This is believed to be because the finish rolling completion temperature was too low. Comparative Example NA-33 had a shortage of martensite, while the ratio of martensite in contact with ferrite and the maximum difference in hardness in the sheet thickness direction were excessive, resulting in insufficient elongation, hole expandability, workability, and deformability in the stress concentration area, and the bending threshold was also less than 0. This is presumably because the cooling rate from finish rolling to coiling was slow, which promoted non-uniformity in the structure. In Comparative Example NA-36, the ratio of martensite in contact with ferrite and the maximum difference in hardness in the sheet thickness direction were excessive, resulting in insufficient hole expandability, workability, and deformability in the stress concentration area, and the bending threshold was also less than 0. This is presumably because the high coiling temperature promoted non-uniformity in the structure. In Comparative Example NA-37, the ratio of martensite in contact with ferrite was excessive, resulting in insufficient hole expandability, workability, and deformability in the stress concentration area, and the bending threshold was also less than 0. This is presumably due to the slow heating rate before switching. In the comparative example NA-41, the amount of ferrite was excessive and other structures were insufficient, resulting in insufficient tensile strength. This is presumably due to the low maximum heating temperature. Comparative Example NA-42 had a small amount of ferrite and an excessive amount of martensite, resulting in impaired elongation and workability. This is presumably due to the high maximum heating temperature. In Comparative Example NA-46, the amount of ferrite was excessive and the amount of martensite was insufficient, resulting in impaired workability. This is presumably due to an insufficient cooling rate at 580°C or higher. In Comparative Example NA-48, granular bainite was not generated, martensite was insufficient, while bainite was excessive, and the proportion of martensite in contact with ferrite was excessive. As a result, the hole expandability, workability, and deformability in the stress concentration area were impaired, and the bending threshold was less than 0. This is presumably because the dwell time between 480°C and 580°C was short. In Comparative Example NA-49, the amount of granular bainite was excessive and the amount of martensite was insufficient, resulting in insufficient tensile strength. This is presumably because the retention time at 480°C to 580°C was too long. In the comparative example NA-50, the amount of C was insufficient, and therefore martensite was insufficient, resulting in a loss of tensile strength. Comparative Example NA-51 had an excess of C, which resulted in an excess of martensite and a deficiency in other structures, resulting in insufficient elongation, workability, and deformability in the stress concentration area. In Comparative Example NA-52, since the amount of Si+Al was insufficient, the amount of granular bainite was insufficient while the amount of bainite was excessive, resulting in insufficient workability and deformability in the stress concentration area. In Comparative Example NA-53, the Si+Al content was excessive, so the steel sheet became embrittled and the elongation and workability were insufficient. In Comparative Example NA-54, since the Mn+Cr was insufficient, the amount of ferrite was excessive while the martensite was insufficient, resulting in insufficient tensile strength. In Comparative Example NA-55, since Mn+Cr was excessive, coarse Mn oxides and sulfides were formed, and the hole expandability, workability, and deformability in the stress concentration area were deteriorated. In the comparative example NA-56, the maximum difference in hardness in the plate thickness direction was excessive, resulting in insufficient deformability in the stress concentration area. This is presumably because the heating rate was switched at a temperature range that was too low. In the comparative example NA-57, the maximum difference in hardness in the plate thickness direction was excessive, resulting in insufficient deformability in the stress concentration area. This is presumably because the heating rate was switched at a temperature range that was too high. In the comparative example NA-58, the maximum difference in hardness in the plate thickness direction was excessive, resulting in insufficient deformability in the stress concentration area. This is presumably because the heating rate in the temperature range of Ac1 to Ac1+20°C was too high. In the comparative example NA-59, the maximum difference in hardness in the plate thickness direction was excessive, and as a result, the deformability in the stress concentration area was insufficient. This is presumably because the heating rate in the temperature range of Ac1 to Ac1+20°C was too small. In Comparative Example NA-60, granular bainite was not generated, and the ratio of martensite in contact with ferrite was excessive, resulting in insufficient deformability in the stress concentration area. This is presumably due to insufficient retention time at 480 to 580°C. In Comparative Example NA-61, granular bainite was not generated, and the ratio of martensite in contact with ferrite was excessive, resulting in insufficient deformability in the stress concentration area. This is presumably due to insufficient retention time at 500 to 630°C. In Comparative Example NA-62, granular bainite was not generated, and the ratio of martensite in contact with ferrite was excessive, resulting in insufficient deformability in the stress concentration area. This is presumably due to excessive retention time at 500 to 630°C. In Comparative Example NA-63, granular bainite was not generated, and the ratio of martensite in contact with ferrite was excessive, resulting in insufficient deformability in the stress concentration area. This is presumably because no dwell time was provided in the temperature range of 480 to 580°C or 500 to 630°C. [Industrial Applicability]

[0152] As described above, the present invention can provide a steel sheet having an excellent formability and a maximum tensile strength of 900 MPa or more suitable for use as a structural member of an automobile, etc., and a manufacturing method thereof. The steel sheet of the present invention is particularly excellent in collision safety, and therefore the present invention has a high applicability in the automobile industry and the steel sheet manufacturing and processing industry. [Explanation of symbols]

[0153] 1 Prior austenite grain boundary 2 Bainitic ferrite 3 Granular bainite 5. Carbide 6 Upper Bainite 7 Lower Bainite

Claims

1. The chemical composition, in mass%, is C: 0.07% or more, 0.15% or less, Si+Al: 0.20% or more, 2.50% or less, Mn+Cr: 1.20% or more, 4.00% or less, P: 0% or more, 0.040% or less, S: 0% or more, 0.010% or less, N: 0% or more, 0.010% or less, O: 0% or more, 0.006% or less, Mo: 0% or more, 0.50% or less, Ti: 0% or more, 0.20% or less, Nb: 0% or more, 0.20% or less, B: 0% or more, 0.010% or less, V: 0% or more, 0.50% or less, Cu: 0% or more, 1.00% or less, W: 0% or more, 0.10% or less, Ta: 0% or more, 0.10% or less, Ni: 0% or more, 1.00% or less, Sn: 0% or more, 0.050% or less, Co: 0% or more, 0.50% or less, Sb: 0% or more, 0.050% or less, As: 0% or more, 0.050% or less, Mg: 0% or more, 0.050% or less, Ca: 0% or more, 0.040% or less, Y: 0% or more, 0.050% or less, Zr: 0% or more, 0.050% or less, La: 0% or more and 0.050% or less, and Ce: 0% or more and 0.050% or less; The balance is Fe and impurities, The metal structure at 1 / 4 of the plate thickness is, in terms of volume fraction, Ferrite: 10% or more and less than 50% Granular bainite: 5% or more and less than 40%; Martensite: 30% or more and 55% or less, Upper bainite and lower bainite: less than 30% in total; Perlite: less than 10%; and Retained austenite: less than 5%; In the plate thickness 1 / 4 portion, the ratio of the number of the martensite adjacent to the ferrite to the number of the metal structures adjacent to the ferrite is 30% or less, A steel plate in which the difference between the maximum and minimum Vickers hardness values ​​at a load of 50 gf measured at 30 μm intervals from a position 100 μm deep from the surface of the steel plate in the plate thickness direction to the center position of the plate thickness is 60 HV or less.

2. The chemical composition, in mass%, Mo: 0.01% or more, 0.50% or less, Ti: 0.001% or more, 0.20% or less, Nb: 0.0001% or more, 0.20% or less, B: 0.0001% or more, 0.010% or less, V: 0.001% or more, 0.50% or less, Cu: 0.001% or more, 1.00% or less, W: 0.001% or more, 0.10% or less, Ta: 0.001% or more, 0.10% or less, Ni: 0.001% or more, 1.00% or less, Sn: 0.001% or more, 0.050% or less, Co: 0.001% or more, 0.50% or less, Sb: 0.001% or more, 0.050% or less, As: 0.001% or more, 0.050% or less, Mg: 0.0001% or more, 0.050% or less, Ca: 0.001% or more, 0.040% or less, Y: 0.001% or more, 0.050% or less, Zr: 0.001% or more, 0.050% or less, La: 0.001% or more and 0.050% or less, and Ce: 0.001% or more, 0.050% or less The steel sheet according to claim 1, further comprising one or more selected from the group consisting of:

3. a ratio Hv30 / Hvi of a Vickers hardness Hv30 at a position 30 μm deep from the surface of the steel plate in the plate thickness direction under a load of 0.29 N to a Vickers hardness Hvi at a 1 / 4 portion of the plate thickness under a load of 0.29 N is 0.8 or less; Tensile strength is 900 MPa or more The steel sheet according to claim 1 or 2.

4. The steel sheet according to any one of claims 1 to 3, characterized in that the surface has a hot-dip galvanized layer or a hot-dip zinc alloy plated layer.

5. The steel sheet according to any one of claims 1 to 3, characterized in that the surface has a galvannealed layer.

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