Steel sheet, galvanized steel sheet, and component

Abstract

The objective of the present invention is to provide, with a novel configuration, a steel sheet and a galvanized steel sheet having excellent bendability, and a component including the same.　This steel sheet has a specific chemical composition, and is characterized in that, in glow discharge optical emission spectrometry from the surface of the steel sheet in the depth direction, the thickness of a region having a C concentration of 0.02% or more but less than 0.05% is 2.0 μm or more, and the crystal grain size of a ferrite phase within 2.0 μm of the surface layer of the steel sheet is 2.0 μm or less.

Description

Steel sheets, zinc-plated steel sheets and parts

[0001] This invention relates to steel sheets, zinc-plated steel sheets, and parts.

[0002] In recent years, there has been a trend towards increasing the strength of steel sheets used in various fields such as automobiles, home appliances, and building materials. For example, in the automotive sector, the use of thin, high-strength steel sheets is increasing in order to lighten vehicle bodies and parts and improve fuel efficiency. Such high-strength steel sheets used in automobile parts also require improved workability. Processing of high-strength steel sheets mainly involves bending, and good bendability is required.

[0003] Generally, when bending a steel sheet, tensile stress is applied in the circumferential direction to the surface layer on the outer circumference of the bend, while compressive stress is applied to the surface layer on the inner circumference of the bend. Therefore, the condition of the surface layer affects the bendability of a high-strength steel sheet. Accordingly, a technique is known to improve the bendability of a high-strength steel sheet by providing a soft layer on the surface layer, thereby mitigating the tensile and compressive stresses generated in the surface layer during bending.

[0004] For example, Patent Document 1 discloses a high-strength plated steel sheet having, in order from the interface between the steel sheet and the plating layer toward the steel sheet side, an internal oxide layer containing Si and / or Mn oxides, a soft layer containing the internal oxide layer, and a hard layer composed mainly of martensite and bainite, wherein the average depth T of the soft layer is 20 μm or more, and the average depth t of the internal oxide layer is 4 μm or more but less than T.

[0005] Furthermore, Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet in which the Vickers hardness at a position 5 μm from the surface in the thickness direction is 80% or less of the hardness at the halfway point in the thickness direction, and the hardness at a position 15 μm from the surface in the thickness direction is 90% or more of the Vickers hardness at the halfway point in the thickness direction.

[0006] Japanese Patent Publication No. 2015-34334, International Publication No. 2016 / 013145

[0007] In order to further reduce the weight and increase the strength of automotive parts, it is necessary to further improve the bendability of the steel sheets that make up automotive parts.

[0008] This invention has been made in view of the above circumstances, and aims to provide a steel sheet, a zinc-plated steel sheet, and a component with excellent bendability through a novel configuration.

[0009] The present invention includes at least the following embodiments.

[0010] (Aspect 1) A steel sheet, wherein the chemical composition of the steel sheet is, in mass%, C: 0.05 to 0.40%, Mn: 1.0 to 5.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0200% or less, Si: 0 to 0.65%, sol. It contains Al: 0-3.0%, O: 0-0.01%, B: 0-0.0100%, Ti: 0-0.1500%, Nb: 0-0.150%, V: 0-0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.00%, W: 0-1.00%, Ca: 0-0.1000%, Mg: 0-0.100%, Zr: 0-0.500%, Hf: 0-0.100%, Sn: 0-0.100%, As: 0-0.100%, and REM: 0-0.100%, with the remainder being Fe and impurities. A steel sheet characterized in that, as determined by glow discharge emission spectroscopy from the surface of the steel sheet toward the depth direction, the thickness of the region with a carbon concentration of 0.02% or more and less than 0.05% is 2.0 μm or more, and the crystal grain size of the ferrite phase within 2.0 μm of the surface layer of the steel sheet is 2.0 μm or less.

[0011] (Aspect 2) The chemical composition of the steel sheet is, in mass%, Si: 0.001 to 0.65%, sol. Al: 0.001-3.0%, O: 0.001-0.01%, B: 0.0001-0.0100%, Ti: 0.0001-0.1500%, Nb: 0.0001-0.150%, V: 0.001-0.150%, Cr: 0.001-2.00%, Ni: 0.001-2.00%, Cu: 0.001-2.00%, Mo: 0.001-1.00%, W: 0.0001-1.000%, Ca: 0.0001-0.1000%, Mg: 0.0001~0.100%, Zr: 0.001~0.500%, The steel sheet according to embodiment 1, characterized by containing one or more of the following: Hf: 0.0001 to 0.100%, Sn: 0.0001 to 0.100%, As: 0.0001 to 0.100%, and REM: 0.0001 to 0.100%.

[0012] (Aspect 3) The steel sheet according to aspect 1 or 2, characterized in that, as determined by the glow discharge emission spectroscopy analysis, the thickness of the region where the C concentration is 0.02% or more and less than 0.05% is 5.0 μm or more.

[0013] (Aspect 4) The steel sheet according to aspect 1 or 2, characterized in that, as determined by the glow discharge emission spectroscopy analysis, the thickness of the region where the C concentration is 0.02% or more and less than 0.05% is 10.0 μm or more.

[0014] (Aspect 5) The steel plate according to any one of the above aspects 1 to 4, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 360 Hv or more.

[0015] (Aspect 6) The steel plate according to any one of the above aspects 1 to 4, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 460 Hv or more.

[0016] (Aspect 7) The steel plate according to any one of the above aspects 1 to 4, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 550 Hv or more.

[0017] (Aspect 8) A zinc-plated steel sheet comprising: a steel sheet according to any one of aspects 1 to 7 above; and a Zn-containing plating layer disposed on at least a part of the surface of the steel sheet.

[0018] (Aspect 9) A component comprising a steel plate as described in any of the above aspects 1 to 7.

[0019] (Aspect 10) A component comprising the zinc-plated steel sheet described in Aspect 8 above.

[0020] According to the present invention, it is possible to provide steel sheets, zinc-plated steel sheets, and parts with excellent bendability.

[0021] Figure 1 is a graph showing the relationship between C concentration and depth in glow discharge emission spectroscopy analysis from the surface of a steel plate towards the depth direction.

[0022] To achieve the above objective, the inventors diligently studied means to improve the bendability of steel sheets. As a result, the inventors found that in steel sheets containing 1.0 to 5.0% Mn by mass, by decarburizing the surface layer of the steel sheet to a specific decarburized state and reducing the solid solution Mn concentration, the ferrite phase on the surface layer of the steel sheet is refined to a crystal grain size of 2.0 μm or less, resulting in excellent bendability. Furthermore, the inventors discovered a new method to promote decarburization of the surface layer of the steel sheet and reduce the solid solution Mn concentration by applying a specific strain to the surface layer of the steel sheet during manufacturing and then annealing the steel sheet under specific high dew point conditions.

[0023] The present invention was completed based on the above findings and includes the following embodiments.

[0024] Preferred embodiments of the steel sheet of the present invention will be described in detail below. In this specification, unless otherwise specified, numerical ranges enclosed by "~" include both a lower limit and an upper limit. However, if the lower limit and upper limit are described with numbers accompanied by "less than" or "greater than", they are not included in that numerical range.

[0025] <Steel Sheet> The steel sheet of one embodiment of the present invention has a chemical composition in mass percent of: C: 0.05 to 0.40%, Mn: 1.0 to 5.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0200% or less, Si: 0 to 0.65%, sol. It contains Al: 0-3.0%, O: 0-0.01%, B: 0-0.0100%, Ti: 0-0.1500%, Nb: 0-0.150%, V: 0-0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.00%, W: 0-1.00%, Ca: 0-0.1000%, Mg: 0-0.100%, Zr: 0-0.500%, Hf: 0-0.100%, Sn: 0-0.100%, As: 0-0.100%, and REM: 0-0.100%. It has a specific chemical composition in which the remainder consists of Fe and impurities.

[0026] Furthermore, the steel sheet of this embodiment has a characteristic structure in which the thickness of the region with a carbon concentration of 0.02% or more and less than 0.05% is 2.0 μm or more, as measured by glow discharge emission spectroscopy from the surface of the steel sheet in the depth direction, and the crystal grain size of the ferrite phase within 2.0 μm of the surface layer of the steel sheet is 2.0 μm or less.

[0027] (Effects) As described above, the steel sheet of this embodiment has a specific chemical composition and a unique decarburization state in which the thickness of the region with a C concentration of 0.02% or more and less than 0.05% is 2.0 μm or more, as determined by glow discharge emission spectroscopy (GDS analysis). This unique decarburization state is obtained, as will be described later, by applying a specific strain to the surface layer of the steel sheet during the manufacturing process and further annealing the steel sheet under specific high dew point conditions, thereby promoting the internal oxidation of Mn and suppressing the external oxidation of Mn on the surface of the steel sheet, that is, promoting the decarburization of the surface layer of the steel sheet. At this time, as the internal oxidation of Mn is promoted, the solid solution Mn concentration in the ferrite phase within 2.0 μm of the surface layer of the steel sheet is reduced, and the ferrite phase is refined to a grain size of 2.0 μm or less. Generally, decarburization and the decrease in solid solution Mn concentration are independent of each other, but in this invention, the decarburization of the surface layer of the steel sheet is promoted by the above-described method. As described above, the refinement of the ferrite phase enables uniform deformation on the steel surface, contributing to improved bendability. Thus, the steel sheet of this embodiment exhibits excellent bendability due to the promotion of decarburization on the surface of the steel sheet.

[0028] The components of the steel sheet of this embodiment will be described in detail below with reference to the drawings. Here, Figure 1 is a graph showing the relationship between C concentration and depth in GDS analysis from the surface of the steel sheet in the depth direction. In Figure 1, the graph labeled "with shot blasting" represents a steel sheet that has been subjected to a specific strain on the surface layer of the steel sheet by shot blasting during manufacturing, and then annealed under specific high dew point conditions, i.e., the steel sheet of this embodiment. On the other hand, the graph labeled "without shot blasting" represents a steel sheet that has been annealed without shot blasting during manufacturing.

[0029] [Thickness of the region with a C concentration of 0.02% or more and less than 0.05% in GDS analysis: 2.0 μm or more] As shown in Figure 1, the steel plate of this embodiment has the characteristic that, in GDS analysis from the surface of the steel plate (base steel plate) toward the depth direction, the thickness of the region with a C concentration of 0.02% or more and less than 0.05% is 2.0 μm or more. Here, the thickness of the region with a C concentration of 0.02% or more and less than 0.05% refers to the thickness of a continuous section toward the depth direction where the C concentration is 0.02% or more and less than 0.05% (the difference between the start depth and the end depth of the section). Hereinafter, such a characteristic may simply be referred to as "characteristics related to C concentration". Note that in the graph labeled "SB present" shown in Figure 1, the thickness of the region with a C concentration of 0.02% or more and less than 0.05% is approximately 20 μm.

[0030] The characteristic that the thickness of the region with a carbon concentration of 0.02% or more but less than 0.05% is 2.0 μm or more means that the surface layer of the steel sheet has been decarburized to the extent necessary to exhibit excellent bendability.

[0031] The thickness of the region with a C concentration of 0.02% or more and less than 0.05% is preferably 5.0 μm or more, and more preferably 10.0 μm or more. Furthermore, there is no particular upper limit to the thickness of the region with a C concentration of 0.02% or more and less than 0.05%, but for example, it is 40.0 μm.

[0032] Herein, in this specification, "steel plate surface layer" refers to the region near the outermost surface of the steel plate, and specifically, in the thickness direction of the steel plate, it means the region from the outermost surface of the steel plate to a depth of 5.0 μm.

[0033] Furthermore, regarding the outermost surface of the steel plate, which serves as the reference for the depth position of the steel plate, this specification defines the 0 μm position as the depth position where the Fe emission intensity reaches 0.7 times or more the internal Fe emission intensity in GDS analysis, and this 0 μm position is considered the outermost surface of the steel plate. The internal Fe emission intensity is the Fe emission intensity in a sufficiently deep region of the steel plate. This region is a region where there is almost no change in Fe concentration in the depth direction, and is a region that is judged to be "steel" according to common technical knowledge. The internal Fe emission intensity can be, for example, the Fe emission intensity at a sputtering time of 1000 seconds.

[0034] It should be noted that the "steel sheet" covered by this invention may be a "base steel sheet" of a steel sheet having some kind of coating on its surface, such as a plated steel sheet. In such cases, the outermost surface of the steel sheet that serves as the reference for the depth position of the steel sheet is the outermost surface of the base steel sheet (for example, the interface between the steel sheet and the plating layer). In such cases as well, the outermost surface is the depth position where the Fe emission intensity in the GDS analysis reaches 0.7 times or more the Fe emission intensity inside, i.e., the 0 μm position.

[0035] The GDS analysis should be conducted as follows.

[0036] (GDS Analysis Method) Glow discharge optical emission spectrometry (GD-OES) of elements on the surface of steel sheets is performed according to the method specified in JIS K0144:2018 "Surface Chemical Analysis - General Rules for Glow Discharge Optical Emission Spectroscopy." Specifically, using a glow discharge optical emission spectrometer, the surface of the steel sheet to be measured is placed in an Ar atmosphere, and a voltage is applied to generate a glow plasma. The steel sheet surface is then sputtered and analyzed in the depth direction. The element-specific emission spectral wavelengths emitted when atoms are excited in the glow plasma are used to identify the element contained in the steel sheet, namely C (carbon), and the emission intensity of the identified element is estimated.

[0037] The depth data can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth in advance using a standard sample, the sputtering time can be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the steel plate surface.

[0038] The C concentration is determined by converting the emission intensity of C obtained as described above into % by mass. Specifically, the emission intensity can be converted into the C concentration by obtaining the relationship between emission intensity and C concentration in advance using a standard sample. A calibration curve installed in a glow discharge optical emission spectrometer (model number "GDS850A") manufactured by LECO Japan G.K. can be used. For calibration of the calibration curve, "BS H-1B", which is a standard sample manufactured by Brammer Standard Company, can be used.

[0039] GDS analysis is performed at five measurement points separated from each other by 5 mm or more on the surface of the steel sheet, and the arithmetic mean value thereof is employed.

[0040] As the glow discharge optical emission spectrometer used for GDS analysis, a commercially available analyzer can be used. In the present embodiment, a glow discharge optical emission spectrometer (model number "GDS850A") manufactured by LECO Japan G.K. is used. The measurement conditions are as follows. The detection pitch is 0.1 seconds. Background is removed from the obtained data. Thereafter, as the average value of C emission intensity in the surface layer of the steel sheet, the arithmetic average value of C emission intensity in the surface layer of the steel sheet is employed. On the other hand, as the C emission intensity in the depth direction in the surface layer of the steel sheet, a moving average of a total of 11 points including the center point for each depth plus 5 points before and after the center point is employed. Anode diameter: 4 mmφ RF output: 30 W Measurement time: 200 to 1500 seconds

[0041] [Crystal grain size of ferrite phase within 2.0 µm of the steel sheet surface layer: 2.0 µm or less] As described above, the steel sheet of the present embodiment is characterized in that the crystal grain size of the ferrite phase within 2.0 µm of the steel sheet surface layer is 2.0 µm or less. Hereinafter, such a feature may be simply referred to as "the feature related to the crystal grain size of the ferrite phase".

[0042] The feature that the crystal grain size of the ferrite phase within 2.0 µm of the steel sheet surface layer is 2.0 µm or less means that internal oxidation of Mn during annealing is promoted, whereby the solid solution Mn concentration in the ferrite phase within 2.0 µm of the steel sheet surface layer is sufficiently reduced, and the ferrite phase is refined to a crystal grain size of 2.0 µm or less. This feature also means that the steel sheet surface layer has been decarburized to an extent necessary for exhibiting excellent bendability. Note that the sufficient reduction of the solid solution Mn concentration means that the solid solution Mn concentration is reduced to 1.0% or less. The solid solution Mn concentration may be 0.8% or less, 0.6% or less, 0.4% or less, or 0.2% or less.

[0043] In this specification, the region within 2.0 µm from the steel sheet surface layer means the region from the outermost surface of the steel sheet (in the case of a plated steel sheet, the interface between the base steel sheet and the plating layer) to the depth position of 2.0 µm on the steel sheet surface layer.

[0044] The steel sheet of the present embodiment can exhibit excellent bendability because it has the above-mentioned feature relating to C concentration and the feature relating to the crystal grain size of the ferrite phase, that is, decarburization of the steel sheet surface layer is promoted and the solid solution Mn concentration is sufficiently reduced.

[0045] For the steel sheet of the present embodiment, from the viewpoint of more reliably obtaining excellent bendability, the crystal grain size of the ferrite phase within 2.0 µm of the steel sheet surface layer is preferably 1.5 µm or less, and more preferably 1.0 µm or less.

[0046] The crystal grain size of the ferrite phase within 2.0 µm of the steel sheet surface layer can be determined according to the following measurement method.

[0047] (Method for measuring crystal grain size of ferrite phase) First, a sample is collected from the steel sheet to be measured, with the thickness cross-section of the steel sheet as the observation surface. Next, after mechanically polishing the observation surface of the sample to obtain a mirror finish, electrolytic polishing is performed. In one or more observation visual fields within the range from the surface to the depth position of 2.0 µm on the observation surface, a total of 2.0 × 10 －９ m ２The above area is subjected to crystal structure and orientation analysis using the SEM-EBSD method to identify the ferrite phase. Next, the sample is thinned using FIB processing in a FIB-SEM combined system (model "NB5000", manufactured by Hitachi High-Tech Corporation). The acceleration voltage during FIB processing is set to 40-5kV. However, voltages below 10kV are considered finishing processes. In addition, the mesh used is made of molybdenum, and the protective film is made of carbon. Then, five fields of view of the thinned sample are observed using a spherical aberration-corrected transmission electron microscope (Cs-TEM, model "JEM-ARM200F NEOARM", manufactured by JEOL Ltd.). In each observed field of view, up to 15 straight lines are drawn in two mutually orthogonal directions, the depth direction and the vertical direction, with a total length of at least 150 μm, and the crystal grain size of the ferrite is determined by the line segment method. More specifically, in accordance with JIS G 0551:2020, the average line segment length per crystal grain is calculated from the length of a line segment drawn perpendicular to the depth direction and the number of intersections with grain boundaries. This measures the grain size of the ferrite grains forming the ferrite phase, and the number-average value D is determined. The same measurement is performed for five fields of view, and the average grain size of the ferrite phase is determined by arithmetic mean of the above number-average values ​​D for the five fields of view. This average grain size is determined as the grain size of the ferrite phase. The acceleration voltage during observation with Cs-TEM is set to 200 kV.

[0048] In order for a steel sheet to possess both the aforementioned characteristics regarding C concentration and the characteristics regarding the grain size of the ferrite phase, a specific strain should be applied to the surface layer of the steel sheet during the manufacturing process, and then the steel sheet should be annealed under specific high dew point conditions to promote decarburization of the surface layer and reduce the solid solution Mn concentration. Specifically, this is done as follows.

[0049] First, a specific large strain is imparted to the surface of the steel sheet by applying a shot blast treatment to the surface of the steel sheet after the hot rolling process (hereinafter sometimes referred to as "hot-rolled black steel") or after pickling.

[0050] Next, in the cold rolling process, the convex parts of the uneven structure on the surface of the steel sheet are flattened, thereby introducing the aforementioned large strain into the outermost layer of the steel sheet (the range from the outermost surface to a depth of 4 μm or less).

[0051] Cold-rolled steel sheets with such specific strains are annealed under specific high dew point conditions. As the temperature rises during annealing, recrystallization occurs in the outermost layer where the specific strains were introduced, forming a fine ferrite phase. Furthermore, internal oxides of Mn are formed at the grain boundaries of this ferrite phase.

[0052] As the temperature rises further above the A1 transformation point, the fine ferrite phase transforms into the austenite phase, and the internal oxides pin the growth of austenite grains. Decarburization also begins above the A1 transformation point, but the formation of the internal Mn oxides mentioned above reduces the amount of metallic Mn in the solid solution (i.e., a Mn-depleted layer is formed), suppressing the external oxidation of Mn on the outermost surface of the steel. As a result, decarburization progresses further, that is, the inhibition of decarburization is suppressed.

[0053] In this way, by promoting decarburization of the steel sheet surface and reducing the solid solution Mn concentration, it is possible to combine the characteristics related to the C concentration and the characteristics related to the grain size of the ferrite phase described above. The specific manufacturing method and conditions for the steel sheet of this embodiment will be described later.

[0054] [Chemical Composition] Next, the chemical composition of the steel sheet of this embodiment will be described in detail. As described above, the steel sheet of this embodiment has the following composition in mass%, C: 0.05 to 0.40%, Mn: 1.0 to 5.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0200% or less, Si: 0 to 0.65%, sol. It contains Al: 0-3.0%, O: 0-0.01%, B: 0-0.0100%, Ti: 0-0.1500%, Nb: 0-0.150%, V: 0-0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.00%, W: 0-1.00%, Ca: 0-0.1000%, Mg: 0-0.100%, Zr: 0-0.500%, Hf: 0-0.100%, Sn: 0-0.100%, As: 0-0.100%, and REM: 0-0.100%. It has a specific chemical composition in which the remainder consists of Fe and impurities.

[0055] The following provides a more detailed explanation of each of these elements.

[0056] [C: 0.05-0.40%] Carbon (C) is an important element for controlling the strength of steel. To ensure the strength of the steel, the C content should be 0.05% or more. To prevent the C concentration on the surface of the steel plate from becoming too high, the C content should be 0.40% or less. The C content may be 0.08% or more, 0.10% or more, or 0.15% or more. The C content may be 0.37% or less, 0.35% or less, or 0.30% or less.

[0057] [Mn: 1.0-5.0%] Mn is an effective element for forming a hard structure and improving the strength of steel. Considering the strength of the steel, the Mn content should be 1.0% or more. Also, considering the decrease in workability such as bendability due to Mn segregation, the Mn content should be 5.0% or less. The Mn content may be 1.5% or more, 2.0% or more, or 2.5% or more. The Mn content may be 4.5% or less, 4.0% or less, or 3.5% or less.

[0058] [P: 0.0300% or less] P is an impurity commonly found in steel. P is an element that segregates at grain boundaries and promotes embrittlement of steel. A lower P content is preferable, so ideally it should be 0%. However, excessive reduction of the P content can lead to a significant increase in cost. For this reason, the P content may be 0.0001% or more, or 0.001% or more, or 0.005% or more. On the other hand, excessive P content can lead to embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content should be 0.0300% or less. The P content may also be 0.0250% or less, 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0059] [S: 0.0300% or less] S is an impurity commonly found in steel. S is an element that generates nonmetallic inclusions such as MnS in steel, leading to a decrease in the ductility of steel parts. A lower S content is preferable, so ideally it should be 0%. However, excessive reduction of the S content can lead to a significant increase in cost. For this reason, the S content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, excessive S content can lead to a decrease in weldability and a decrease in workability such as bendability due to an increase in MnS precipitation. Therefore, the S content should be 0.0300% or less. The S content may be 0.0250% or less, 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0060] [N: 0.0200% or less] N is an impurity commonly found in steel. N forms coarse nitrides in steel, reducing the workability, such as bendability, and weldability of steel sheets. A lower N content is preferable, ideally it should be 0%. However, excessive reduction of the N content can lead to a significant increase in manufacturing costs. For this reason, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content can form coarse nitrides, reducing workability, such as bendability, and weldability. Therefore, the N content should be 0.0200% or less. The N content may also be 0.0150% or less, 0.0100% or less, or 0.0080% or less.

[0061] [Si: 0-0.65%] Si is an element that promotes ferrite stabilization and decarburization. When Si is present, decarburization proceeds easily on the surface of the steel sheet, and the stabilization of the ferrite on the surface of the steel sheet improves its bendability. These effects of including Si can also be obtained by including sol. Al, so Si is not an essential element in the steel sheet of the present invention. Therefore, the Si content may be 0%, but in order to sufficiently obtain the above effects, the Si content may be 0.001% or more. On the other hand, if Si is included in excess, external oxidation will proceed during high dew point annealing, and oxides (scale) will be formed on the surface of the steel sheet, which may suppress decarburization on the outermost surface and reduce the above effects. For this reason, the Si content should be 0.65% or less. The Si content may be 0.01% or more, 0.05% or more, or 0.10% or more. The Si content may be 0.60% or less, 0.55% or less, or 0.50% or less.

[0062] [sol. Al: 0-3.0%] Al is an element that promotes ferrite stabilization and decarburization by being dissolved in steel. Here, sol. Al means Al ２ O ３ This refers to acid-soluble Al, which is not an oxide and is soluble in acid. The above effects obtained by including sol. Al can also be obtained by including Si, so sol. Al is not an essential element in the steel sheet of the present invention. Therefore, the sol. Al content may be 0%, but in order to sufficiently obtain the above effects, the sol. Al content may be 0% or more. On the other hand, if sol. Al is included in excess, external oxidation will progress during high dew point annealing, and oxides (scale) will be formed on the surface of the steel sheet, which may suppress decarburization at the outermost surface and reduce the above effects. For this reason, the sol. Al content should be 3.0% or less. The sol. Al content may be 0.01% or more, 0.05% or more, 0.1% or more, 0.3% or more, or 0.5% or more. The sol. Al content may be 2.0% or less, 1.5% or less, or 1.0% or less.

[0063] [O: 0-0.01%] O is an element that is mixed in during the manufacturing process and may be included as needed. Since O is not an essential element in the steel sheet of the present invention, the lower limit of the O content is 0%. The O content may be 0.001% or more, 0.003% or more, or 0.005% or more. On the other hand, from the viewpoint of suppressing the formation of coarse oxides and ensuring the ductility and formability of the steel sheet, the O content should be 0.01% or less. The O content may be 0.009% or less, 0.008% or less, or 0.007% or less.

[0064] [B: 0 to 0.0100%] B is an element that enhances hardenability and contributes to improved strength, as well as strengthening grain boundaries by segregating at them and improving toughness. By segregating on the surface of the steel sheet in a solid solution state, B can suppress the penetration of Zn into the grain boundaries and improve LME resistance. B is an element that may be included as needed and is not an essential element in the steel sheet of the present invention, so the lower limit of the B content is 0%. In order to fully obtain each of the above effects, the B content may be 0.0001% or more. Also, from the viewpoint of ensuring sufficient toughness, the B content should be 0.0100% or less. The B content may be 0.0010% or more, 0.0020% or more, 0.0030% or more, or 0.0040% or more. The B content may be 0.0080% or less, 0.0070% or less, or 0.0060% or less.

[0065] [Ti: 0 to 0.1500%] Ti is an element that precipitates as TiC during the cooling of steel and contributes to improving its strength. Ti is an element that may be included as needed and is not an essential element in the steel sheet of the present invention, so the lower limit of the Ti content is 0%. In order to fully obtain the above effects, the Ti content may be 0.0001% or more. On the other hand, if Ti is included in excess, coarse TiN may be generated, which may impair toughness, so the Ti content should be 0.1500% or less. The Ti content may be 0.0002% or more, 0.0003% or more, 0.0004% or more, or 0.0005% or more. The Ti content may be 0.1200% or less, 0.1000% or less, or 0.0800% or less.

[0066] [Nb: 0-0.150%] Nb is an element that enhances the hardenability of steel and contributes to improving its strength. Nb is an element that may be included as needed, and is not an essential element in the steel sheet of the present invention, so the lower limit of the Nb content is 0%. In order to fully obtain the above effects, the Nb content may be 0.0001% or more. Also, from the viewpoint of ensuring sufficient toughness, the Nb content should be 0.150% or less. The Nb content may be 0.001% or more, 0.010% or more, 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, the Nb content may be 0.120% or less, 0.100% or less, or 0.080% or less.

[0067] [V: 0-0.150%] V is an element that enhances hardenability and contributes to improved strength. V is an element that may be included as needed and is not an essential element in the steel sheet of the present invention, so the lower limit of the V content is 0%. In order to fully obtain the above effects, the V content may be 0.001% or more. Also, from the viewpoint of ensuring sufficient toughness, the V content should be 0.150% or less. The V content may be 0.005% or more, 0.010% or more, 0.030% or more, or 0.050% or more. The V content may be 0.120% or less, 0.100% or less, or 0.080% or less.

[0068] [Cr: 0-2.00%] Cr is an effective element for increasing the hardenability and strength of steel. Cr is an element that may be included as needed, and is not an essential element in the steel sheet of the present invention; therefore, the lower limit of the Cr content is 0%. To fully obtain the above effects, the Cr content may be 0.001% or more. On the other hand, if Cr is included in excess, a large amount of Cr carbide may be formed, impairing the hardenability; therefore, the Cr content should be 2.00% or less. The Cr content may be 0.01% or more, 0.05% or more, or 0.10% or more. The Cr content may be 1.80% or less, 1.50% or less, 1.20% or less, or 1.00% or less.

[0069] [Ni: 0-2.00%] Ni is an effective element for increasing the hardenability and strength of steel. Ni is an element that may be included as needed and is not an essential element in the steel sheet of the present invention, so the lower limit of the Ni content is 0%. In order to fully obtain the above effects, the Ni content may be 0.001% or more. On the other hand, excessive addition of Ni increases costs, so the Ni content should be 2.00% or less. The Ni content may be 0.005% or more, 0.01% or more, 0.05% or more, or 0.10% or more. The Ni content may be 1.80% or less, 1.50% or less, 1.20% or less, or 1.00% or less.

[0070] [Cu: 0-2.00%] Cu is an effective element for increasing the hardenability and strength of steel. Cu is an element that may be included as needed, and is not an essential element in the steel sheet of the present invention, so the lower limit of the Cu content is 0%. In order to fully obtain the above effects, the Cu content may be 0.001% or more. On the other hand, in order to suppress the reduction of toughness and cracking of the slab after casting, the Cu content should be 2.00% or less. The Cu content may be 0.005% or more, 0.01% or more, 0.05% or more, or 0.10% or more. The Cu content may be 1.80% or less, 1.50% or less, 1.20% or less, or 1.00% or less.

[0071] [Mo: 0-1.00%] Mo is an element that enhances the hardenability of steel and contributes to improving its strength. Mo is an element that may be included as needed and is not an essential element in the steel sheet of the present invention, so the lower limit of the Mo content is 0%. In order to fully obtain the above effects, the Mo content may be 0.001% or more. Also, from the viewpoint of ensuring sufficient toughness, the Mo content should be 1.00% or less. The Mo content may be 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, the Mo content may be 0.80% or less, 0.70% or less, 0.60% or less, or 0.50% or less.

[0072] [W: 0-1.000%] W is an element effective in increasing the hardenability and strength of steel. W is an element that may be included as needed, and is not an essential element in the steel sheet of the present invention, so the lower limit of the W content is 0%. In order to fully obtain the above effects, the W content may be 0.0001% or more. On the other hand, in order to suppress the decrease in toughness, the W content should be 1.000% or less. The W content may be 0.0005% or more, 0.001% or more, 0.005% or more, or 0.010% or more. The W content may be 0.800% or less, 0.700% or less, 0.600% or less, or 0.500% or less.

[0073] [Ca: 0 to 0.1000%] Ca is an element that contributes to controlling the morphology of nonmetallic inclusions, particularly to the fine dispersion of nonmetallic inclusions, and thereby enhances toughness. Ca is an element that may be included as needed, and is not an essential element in the steel sheet of the present invention, so the lower limit of the Ca content is 0%. In order to fully obtain the above effects, the Ca content may be 0.0001% or more. On the other hand, if Ca is included in excess, deterioration of surface properties may become apparent, so the Ca content should be 0.1000% or less. The Ca content may be 0.0002% or more, 0.0003% or more, 0.0004% or more, or 0.0005% or more. The Ca content may be 0.0800% or less, 0.0700% or less, 0.0600% or less, or 0.0500% or less.

[0074] [Mg: 0-0.100%] Mg is an element that contributes to controlling the morphology of nonmetallic inclusions, particularly to the fine dispersion of nonmetallic inclusions, and thereby enhances toughness. Mg is an element that may be included as needed, and is not an essential element in the steel sheet of the present invention, so the lower limit of the Mg content is 0%. In order to fully obtain the above effects, the Mg content may be 0.0001% or more. On the other hand, if Mg is included in excess, deterioration of surface properties may become apparent, so the Mg content should be 0.100% or less. The Mg content may be 0.0005% or more, 0.001% or more, 0.005% or more, or 0.010% or more. The Mg content may be 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[0075] [Zr: 0-0.500%] Zr is an element that contributes to controlling the morphology of nonmetallic inclusions, particularly to the fine dispersion of nonmetallic inclusions, thereby enhancing toughness. Zr is an element that may be included as needed, and is not an essential element in the steel sheet of the present invention, so the lower limit of the Zr content is 0%. In order to fully obtain the above effects, the Zr content may be 0.001% or more. On the other hand, if Zr is included in excess, deterioration of surface properties may become apparent, so the Zr content should be 0.500% or less. The Zr content may be 0.003% or more, 0.005% or more, 0.008% or more, or 0.010% or more. The Zr content may be 0.450% or less, 0.400% or less, 0.350% or less, or 0.300% or less.

[0076] [Hf: 0-0.100%] Hf is an element that contributes to controlling the morphology of nonmetallic inclusions, particularly to the fine dispersion of nonmetallic inclusions, and thereby enhances toughness. Hf is an element that may be included as needed, and is not an essential element in the steel sheet of the present invention, so the lower limit of the Hf content is 0%. In order to fully obtain the above effects, the Hf content may be 0.0001% or more. On the other hand, if Hf is included in excess, deterioration of surface properties may become apparent, so the Hf content should be 0.100% or less. The Hf content may be 0.0005% or more, 0.001% or more, 0.005% or more, or 0.010% or more. The Hf content may be 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[0077] [Sn: 0-0.100%] [As: 0-0.100%] Sn and As are elements that are effective in improving corrosion resistance. Sn and As are elements that may be included as needed, and are not essential elements in the steel sheet of the present invention, so the lower limit of the Sn and As content is 0% each. The Sn and As content may be 0.0001% or more, 0.0005% or more, and 0.001% or more, respectively. On the other hand, if Sn and As are included in excess, the above effect will saturate, and including more than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, the Sn and As content should be 0.100% or less each. The Sn and As content may be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less, respectively.

[0078] [REM: 0-0.100%] REM (rare earth elements) are elements that contribute to controlling the morphology of nonmetallic inclusions, particularly to the fine dispersion of nonmetallic inclusions, and thereby enhance toughness. REM is an element that may be included as needed, and is not an essential element in the steel sheet of the present invention, so the lower limit of the REM content is 0%. In order to fully obtain the above effects, the REM content may be 0.0001% or more. On the other hand, if REM is included in excess, deterioration of surface properties may become apparent, so the REM content should be 0.100% or less. The REM content may be 0.0005% or more, 0.001% or more, 0.005% or more, or 0.010% or more. The REM content may be 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less. REM stands for Rare Earth Metal, and refers to the two elements Sr and Y, and the 15 elements belonging to the lanthanide series.

[0079] In the steel sheet of the present invention, the remainder of the elements other than those mentioned above consists of Fe and impurities. Here, impurities are components that are inevitably mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap. The impurities can be included in a range that does not adversely affect the bendability of the steel sheet of the present invention, that is, within a range that allows the bendability required for the steel sheet of the present invention to be obtained.

[0080] Furthermore, regarding optional components, the chemical composition of the steel sheet is, in mass%, Si: 0.001-0.65%, sol. Al: 0.001-3.0%, O: 0.001-0.01%, B: 0.0001-0.0100%, Ti: 0.0001-0.1500%, Nb: 0.0001-0.150%, V: 0.001-0.150%, Cr: 0.001-2.00%, Ni: 0.001-2.00%, Cu: 0.001-2.00%, Mo: 0.001-1.00%, W: 0.0001-1.000%, Ca: 0.0001-0.1000%, Mg: 0.0001~0.100%, Zr: 0.001~0.500%, It may contain one or more of the following: Hf: 0.0001 to 0.100%, Sn: 0.0001 to 0.100%, As: 0.0001 to 0.100%, and REM: 0.0001 to 0.100%.

[0081] The chemical composition of steel sheets can be measured using general analytical methods. For example, the chemical composition of steel sheets can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2022. Specifically, a test piece with a width and length of 35 mm square (for example, a 35 mm square test piece with a sample thickness of 3 / 4 the thickness of the steel sheet) is obtained, machined so that the analysis surface is located 1 / 4 of the sheet thickness away from the surface of the steel sheet and parallel to the surface of the steel sheet. The chemical composition can then be determined by measuring it using a Shimadzu ICPS-8100 or similar (measuring device) under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method. The amount of sol (Al) can be measured by atomic absorption spectrometry in accordance with JIS G1257-10-2:2013. The amount of oxygen (O) can be measured using inert gas fusion-nondispersive infrared absorption spectroscopy.

[0082] The thickness of the steel plate is not particularly limited, but generally it is between 0.2 and 8.0 mm. For example, the thickness may be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, 1.6 mm or more, or 2.0 mm or more. Similarly, the thickness of the steel plate may be 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0083] [Vickers Hardness] In the steel plate of this embodiment, the hardness of the steel plate is not particularly limited, but for example, the Vickers hardness of the steel plate at a depth of 1 / 4 of the plate thickness may be 360 ​​Hv or more. Generally, as the strength and hardness of the steel plate increase, it becomes more difficult to improve its bendability, but the present invention is particularly advantageous in that even with such a high-strength steel plate, it can exhibit excellent bendability as described above. The Vickers hardness of the steel plate at a depth of 1 / 4 of the plate thickness may be 460 Hv or more or 550 Hv or more. The upper limit of the Vickers hardness is not particularly limited, but for example, the Vickers hardness of the steel plate at a depth of 1 / 4 of the plate thickness may be 950 Hv or less, 900 Hv or less, 850 Hv or less, or 800 Hv or less.

[0084] The Vickers hardness of steel plates can be measured in accordance with JIS Z 2244-1:2024 as follows: First, a test piece is cut from any position on the steel plate, excluding the edges, so that a cross-section perpendicular to the surface (a cross-section along the thickness direction) can be observed. The thickness cross-section of the cut test piece is polished using silicon carbide sandpaper of #600 to #1500 grit. Next, the thickness cross-section of the test piece is polished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water, and this thickness cross-section is used as the measurement surface. Then, using a micro-Vickers hardness tester, the Vickers hardness of the test piece is measured at intervals of at least three times the indentation depth with a load of 1 kgf. Specifically, a total of 20 points are measured randomly at a depth of 1 / 4 of the thickness of the test piece, and the arithmetic mean of these measurements is adopted as the Vickers hardness of the steel plate.

[0085] (Plating layer) Furthermore, the steel sheet of this embodiment may or may not have a plating layer. In other words, the steel sheet of this embodiment may be a plated steel sheet or an unplated steel sheet.

[0086] If the steel sheet in this embodiment is a plated steel sheet, the plating layer may be formed on only one side of the base steel sheet surface, or on both sides. It may also be formed on only a portion of the sheet surface. The plating layer may be alloyed.

[0087] The chemical composition of the plating layer is not particularly limited, but examples include a zinc-based plating layer containing zinc (Zn). In other words, the steel sheet of this embodiment may be a plated steel sheet in which a zinc-based plating layer is disposed on at least a part of the base steel sheet.

[0088] The zinc-based plating layer is not particularly limited as long as it is a plating layer containing Zn, but examples include hot-dip galvanizing such as Zn-0.2%Al(GI) and hot-dip Zn-Al-Mg alloy plating such as Zn-1.5%Al-1.5%Mg. Specific examples of zinc-based plating layers include Zn-0.2%Al(GI), Zn-(0.3-1.5)%Al, Zn-4.5%Al, Zn-0.09%Al-10%Fe(GA), Zn-1.5%Al-1.5%Mg, Zn-6.0%Al-3.0%Mg, Zn-12.0%Al-6.0%Mg, Zn-20.0%Al-8.0%Mg, Zn-11%Al-3%Mg-0.2%Si, Zn-11%Ni, and Zn-15%Mg. Such zinc-based plating layers may contain elements such as Si, Ca, and Fe in amounts of 1.0% or less each.

[0089] The chemical composition of the plating layer can be determined by dissolving the plating layer in an acidic solution to which an inhibitor that suppresses corrosion of the steel sheet has been added, and then measuring the resulting solution by ICP (inductively coupled plasma) emission spectroscopy. As the acidic solution to which the inhibitor has been added to dissolve the plating layer, for example, a 10% hydrochloric acid solution to which 0.06% by mass of inhibitor (manufactured by Asahi Chemical Industries, Ltd., Ibit 710K) has been added can be used.

[0090] The thickness of the plating layer is not particularly limited, but may be, for example, 3 to 50 μm. The amount of plating layer applied is also not particularly limited, but may be, for example, 10 to 170 g / m² per side. ２This may be the case. The amount of plating layer can be determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel plate, and measuring the weight change before and after dissolution of the plating layer.

[0091] <Parts> As described above, the steel sheet of this embodiment is a steel sheet with excellent bendability. Therefore, the steel sheet of this embodiment is useful as a raw material for parts that require excellent bendability. In particular, the steel sheet of this embodiment is useful as a raw material for automobile parts.

[0092] In other words, one embodiment of the present invention is a part that includes the steel sheet or plated steel sheet of the above embodiment. Furthermore, examples of parts include the above-mentioned automobile parts. Specific examples of automobile parts include the frame parts and bumpers of an automobile, as well as other structural and reinforcing parts that require strength. Further specific examples of automobile parts include exterior parts such as roofs, hoods, fenders, and doors that require high aesthetic appeal. These parts only need to include the steel sheet or plated steel sheet of the above embodiment in at least a portion of the part. Therefore, these parts have the characteristics of the steel sheet or plated steel sheet of the above embodiment in at least a portion of the part. Note that in parts of the steel sheet that do not directly come into contact with the mold during forming such as press forming, or that are in direct contact with the mold but undergo relatively little processing, the characteristics of the steel sheet do not change particularly before and after forming.

[0093] When taking samples from automotive parts for various measurements and analyses, the following locations (i) to (iv) should be avoided: (i) Welded areas: within 20 mm of the toe of spot welds, and within 20 mm of the toe of arc / laser welds (ii) Machined areas: machined areas with a radius of curvature of less than 15 mm, and within 5 mm of the above machined areas (iii) Ends: ends within 5 mm of the cut end face of the part (iv) Red rust: within 5 mm of areas where red rust is visible

[0094] When taking samples from a coil for various measurements and analyses, the outermost part may have a changed surface condition. Therefore, samples should be taken from the third turn onwards from the outside of the coil, avoiding the end 100 mm away.

[0095] <Method for Manufacturing Steel Sheets> Next, a preferred method for manufacturing the steel sheet according to one embodiment of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing the steel sheet of this embodiment and is not intended to limit the steel sheet to those manufactured by the manufacturing method described below.

[0096] The steel sheet of this embodiment can be manufactured by a manufacturing method that includes a casting step of casting molten steel with an adjusted chemical composition to form a steel billet, a hot rolling step of hot rolling the steel billet to obtain a hot-rolled steel sheet, a shot blasting step of applying shot blasting treatment to the surface of the hot-rolled steel sheet (hot-rolled black steel) after the hot rolling step or the hot-rolled steel sheet after pickling, a cold rolling step of cold rolling the hot-rolled steel sheet after shot blasting to obtain a cold-rolled steel sheet, and an annealing step of annealing the cold-rolled steel sheet after the cold rolling step under specific high dew point conditions. Optionally, a pickling step of pickling the hot-rolled steel sheet after the hot rolling step, a plating step of applying plating treatment to the annealed steel sheet (base steel sheet), etc., may also be performed.

[0097] The following will explain in detail the preferred conditions for these processes.

[0098] [Casting Process] In the steel plate manufacturing method of this embodiment, the casting process is a process of forming a steel billet by casting molten steel with an adjusted chemical composition. The conditions of the casting process are not particularly limited. For example, the casting process may involve melting in a blast furnace or electric furnace, followed by various secondary smelting processes, and then casting by methods such as conventional continuous casting or ingot casting.

[0099] [Hot Rolling Process] The hot rolling process is a process of obtaining hot-rolled steel sheets by hot rolling a steel billet. The hot rolling process is carried out by hot rolling the cast steel billet either directly or after it has been cooled and then reheated. When reheating is performed, the heating temperature of the steel billet may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction ratio of each rolling can be appropriately determined according to the desired metal structure and sheet thickness. For example, the finishing temperature of the finish rolling may be 900 to 1050°C, and the reduction ratio of the finish rolling may be 10 to 50%.

[0100] The hot-rolled steel sheet, after finish rolling, is wound at a predetermined winding temperature. The winding of the hot-rolled steel sheet is carried out at a winding temperature of 500°C or higher. The winding temperature may be 520°C or higher or 550°C or higher. The winding temperature may be 600°C or lower or 580°C or lower.

[0101] [Pickling Process] The pickling process is a process of pickling hot-rolled steel sheets after the hot-rolling process. In the pickling process, the hot-rolled steel sheets are pickled to remove surface oxides and internal oxides. The conditions for the pickling process are not particularly limited, and it is sufficient to carry it out under conditions appropriate for removing surface oxides and internal oxides using a commonly used pickling solution, for example, a hydrochloric acid solution of a predetermined concentration containing an inhibitor that suppresses corrosion of the steel sheet. Pickling may be carried out in one step, or it may be carried out in multiple steps to ensure that surface oxides and internal oxides are completely removed.

[0102] Since the pickling process is not essential for obtaining the steel sheet of the present invention, the hot-rolled steel sheet after the hot-rolling process may be subjected to the subsequent shot-blasting process without going through the pickling process.

[0103] [Shot Blasting Process] The shot blasting process is a process of applying shot blasting to the surface of a hot-rolled steel sheet (hot-rolled mill scale material) after the hot-rolling process or after pickling. Shot blasting is a process in which spherical projectiles are projected onto the surface of the hot-rolled steel sheet. By applying such shot blasting to the surface of the steel sheet, a specific large strain can be imparted to the surface of the steel sheet. At this time, the surface roughness Ra of the steel sheet after shot blasting will be 0.8 μm or more. This surface roughness Ra may be 1.4 μm or more or 2.0 μm or more. Also, the surface roughness Ra of the steel sheet may be 5.0 μm or less. In this specification, surface roughness Ra means arithmetic mean roughness Ra. The surface roughness Ra of the steel sheet is measured in accordance with JIS B 0651:2022. Specifically, the surface roughness Ra is measured over a length of 0.4 mm at a position at least 10 mm away from the edge of the steel plate sample to be measured, using a needle-type roughness meter (needle tip diameter 2 μm) conforming to JIS B 0651:2022. The measurement is performed three times for each sample, and the average of the arithmetic mean roughness Ra obtained from the three measurements is calculated. The calculated average value is taken as the surface roughness Ra of the steel plate.

[0104] The abrasive material used in shot blasting is not particularly limited, but for example, steel balls (shots) with a central particle size of 40 to 450 μm can be used. Examples of such steel balls include WINOA IKK JAPAN's TSH30. The amount of abrasive material projected is, for example, 5 to 400 kg / m. ２ The projection rate is preferably 60 kg / m². ２ The above, and more preferably 100 kg / m ２ That concludes the explanation. The projection speed of the projection material is 5 kg / second or more, preferably 30 kg / second or more, and more preferably 60 kg / second or more.

[0105] [Cold Rolling Process] The cold rolling process is a process in which hot-rolled steel sheets, after shot blasting, are cold-rolled to obtain cold-rolled steel sheets. In the cold rolling process, the convex parts of the uneven structure on the surface of the steel sheet formed by shot blasting are crushed and flattened, thereby introducing the aforementioned large strain into the outermost layer of the steel sheet (the range from the outermost surface to a depth of 4 μm or less). At this time, the surface roughness of the steel sheet becomes smaller than the surface roughness after shot blasting. The surface roughness Ra of the steel sheet after the cold rolling process is 0.1 to 2.0 μm. The change in surface roughness Ra of the steel sheet before and after the cold rolling process is, for example, 0.8 μm or more, and may be 1.4 μm or more or 2.5 μm or more. In particular, by setting the projection speed of the abrasive material in the shot blasting process to 30 kg / second or more, and the change in surface roughness Ra before and after the cold rolling process to 1.4 μm or more, the thickness of the region with a C concentration of 0.02% or more and less than 0.05% can be set to 5.0 μm or more. Furthermore, by setting the projection speed of the abrasive material in the shot blasting process to 60 kg / second or more, and the change in surface roughness Ra before and after the cold rolling process to 2.5 μm or more, the thickness of the region with a C concentration of 0.02% or more and less than 0.05% can be set to 10.0 μm or more.

[0106] In the cold rolling process, the reduction ratio can be appropriately determined according to the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold rolling process, the material may be cooled to room temperature by air cooling, for example.

[0107] [Annealing Process] The annealing process is a process in which the cold-rolled steel sheet, that is, the cold-rolled steel sheet into which the above-mentioned specific strain has been introduced, is annealed under specific high dew point conditions.

[0108] In the annealing process, first, as the temperature rises, recrystallization occurs in the outermost layer where the specific strain described above has been introduced, forming a fine ferrite phase. Furthermore, internal oxides of Mn are formed at the grain boundaries of this ferrite phase. Then, as the temperature rises further and exceeds the A1 transformation point, the fine ferrite phase transforms into an austenite phase, and the internal oxides pin the growth of austenite grains. Decarburization also begins above the A1 transformation point, but the formation of the internal oxides of Mn reduces the amount of metallic Mn in solid solution (i.e., a Mn-depleted layer is formed), suppressing external oxidation of Mn on the outermost surface of the steel. As a result, decarburization progresses further, that is, the inhibition of decarburization is suppressed.

[0109] In this way, the annealing process promotes decarburization of the steel sheet surface layer, thereby reducing the solid solution Mn concentration. As a result, the steel sheet after the annealing process possesses both the above-mentioned characteristics regarding C concentration (the thickness of the region with a C concentration of 0.02% or more and less than 0.05% is 2.0 μm or more) and the characteristics regarding the crystal grain size of the ferrite phase (the crystal grain size of the ferrite phase within 2.0 μm of the steel sheet surface layer is 2.0 μm or less).

[0110] The specific conditions for the annealing process are as follows: First, the steel sheet is heated from room temperature to the holding temperature in an atmosphere with a dew point of -20°C to 20°C. The holding temperature is in the range of 750°C to 900°C. The sheet is held at this holding temperature for 40 to 300 seconds. The holding time is preferably 80 to 200 seconds, and more preferably 100 to 200 seconds.

[0111] The atmosphere during the annealing process may be a reducing atmosphere, more specifically, a reducing atmosphere containing nitrogen and hydrogen. Examples of a reducing atmosphere include one containing 1 to 10 volume percent hydrogen (for example, 2 volume percent hydrogen and nitrogen in balance).

[0112] The steel sheet after the annealing process may optionally be subjected to a plating process.

[0113] [Plating Process] The plating process involves applying a plating treatment to the surface of the annealed steel sheet (base steel sheet). This plating process forms a plating layer on at least a portion of the surface of the annealed steel sheet. In other words, a plated steel sheet is obtained.

[0114] The plating process should be carried out according to methods known to those skilled in the art. The conditions for the plating process should be set appropriately, taking into consideration the chemical composition, thickness, and amount of the plating layer. The type and chemical composition of the plating layer are as described above.

[0115] For example, a hot-dip galvanized steel sheet may be obtained by immersing an annealed steel sheet in a molten zinc bath having an Al concentration of 0.155% by mass or more and less than 0.190% by mass for 1.0 to 15.0 seconds. The amount of Al in the plating film can be adjusted by the composition of the molten zinc bath, the bath temperature, and the immersion time in the molten zinc bath.

[0116] Furthermore, the plated steel sheet obtained by the plating process may optionally be subjected to an alloying process. The alloying process may be carried out according to methods known to those skilled in the art. The conditions for the alloying process may be set appropriately considering the chemical composition, thickness, and amount of the plated layer. For example, the alloying process may be carried out by holding the plate in a temperature range of 500 to 600°C for 5 to 30 seconds.

[0117] As described above, the steel sheet of this embodiment can be obtained. In particular, by performing shot blasting on the hot-rolled steel sheet after the hot-rolling process at a projection speed of 5 kg / second or more, and then controlling the change in surface roughness before and after the cold-rolling process to 0.8 μm or more, a specific strain can be applied to the surface layer of the steel sheet. Furthermore, by annealing the steel sheet under specific high dew point conditions, namely, holding it in an atmosphere with a dew point of -20°C to 20°C at a holding temperature of 750°C to 900°C for 40 to 300 seconds, the internal oxidation of Mn can be promoted and the external oxidation of Mn on the surface of the steel sheet can be suppressed (i.e., decarburization of the surface layer of the steel sheet can be promoted). The steel sheet obtained in this way has a specific chemical composition as described above, and has a characteristic structure in which, as determined by GDS analysis, the thickness of the region with a C concentration of 0.02% or more and less than 0.05% is 2.0 μm or more, and the crystal grain size of the ferrite phase within 2.0 μm of the surface layer of the steel sheet is 2.0 μm or less. In other words, as described above, the steel sheet of this embodiment exhibits excellent bendability because the decarburization of the surface layer of the steel sheet is promoted, and the solid solution Mn concentration is reduced.

[0118] As described above, the steel sheet of this embodiment preferably has a thickness of 5.0 μm or more in the region where the C concentration is 0.02% or more and less than 0.05%. A steel sheet that satisfies these characteristics can be obtained by further employing manufacturing conditions in which the projection speed of the abrasive material in the shot blasting process is 30 kg / second or more, and the change in surface roughness Ra before and after the cold rolling process is 1.4 μm or more.

[0119] Similarly, in this embodiment, it is more preferable that the thickness of the region where the C concentration is 0.02% or more and less than 0.05% is 10.0 μm or more. A steel sheet that satisfies these characteristics can be obtained by further employing manufacturing conditions in which the projection speed of the abrasive material in the shot blasting process is 60 kg / second or more, and the change in surface roughness Ra before and after the cold rolling process is 2.5 μm or more.

[0120] In addition to the above-described steps, the steel plate manufacturing method of this embodiment may further include any additional processing steps.

[0121] As described above, the steel sheet of the present invention has excellent bendability and can therefore be suitably used not only in the automotive field but also in a wide range of fields such as home appliances and building materials. It can be particularly suitably used as an automotive part.

[0122] The present invention is not limited to the embodiments described above or the following examples, and can be appropriately combined, substituted, or modified without departing from the purpose and spirit of the present invention.

[0123] The present invention will be described in more detail below with reference to examples, but the following examples are merely examples of the present invention, and the present invention is not limited in any way to these examples.

[0124] In the following examples, steel sheets according to embodiments of the present invention and steel sheets serving as comparative examples of the present invention were manufactured under various conditions, and the properties of the obtained steel sheets were investigated.

[0125] (Production of steel sheet of Test No. 1) First, molten steel was cast by a continuous casting method to form a steel slab having the chemical composition shown in Test No. 1 of Table 1 below. After this steel slab was once cooled, it was reheated to 1200°C, subjected to hot rolling, and coiled at a coiling temperature of 520°C or higher. Hot rolling was carried out by performing rough rolling and finish rolling. The finishing temperature of finish rolling was 900 to 1050°C, and the rolling reduction of finish rolling was 30%.

[0126] Next, shot blasting treatment was performed on the surface of the obtained hot-rolled steel sheet (hot-rolled black scale material). Specifically, a projection material (TSH30 from WINOA IKK JAPAN) was projected onto the surface of the hot-rolled steel sheet at a projection rate of 5 kg / sec. The surface roughness Ra of the steel sheet surface after the shot blasting treatment was 1.2 μm.

[0127] The hot-rolled steel sheet after the shot blasting treatment was cold-rolled at a rolling reduction of 50% to obtain a cold-rolled steel sheet having a sheet thickness of 1.6 mm. The surface roughness Ra of the steel sheet surface after cold rolling was 0.4 μm, and the amount of change in surface roughness Ra before and after cold rolling was 0.8 μm.

[0128] The obtained cold-rolled steel sheet was heated from room temperature to a holding temperature of 800°C in an atmosphere with a dew point of -5°C, and held at this holding temperature for 40 seconds, whereby the cold-rolled steel sheet was subjected to annealing treatment.

[0129] In this way, the steel sheet (non-plated steel sheet) of Test No. 1, which is an example of the present invention, was obtained.

[0130] (Production of steel sheet of Test No. 2) A steel sheet (plated steel sheet) of Test No. 2, which is an example of the present invention, was obtained in the same manner as the steel sheet of Test No. 1, except that the chemical composition was changed to that shown in Test No. 2 of Table 1 below, the projection rate of the shot blasting treatment was changed to 30 kg / sec, and further a plating treatment was performed.

[0131] In the plating treatment for the steel sheet of Test No. 2, the annealed steel sheet was immersed in a hot-dip galvanizing bath at 450°C for 3 seconds, then pulled out at 100 mm / sec, and N ２ the coating weight was adjusted to 50 g / m by wiping gas ２ to obtain the steel sheet of Test No. 2 (Zn-0.2% Al (GI)).

[0132] (Manufacturing of steel sheets for Tests No. 3-5, 7-19, 21-28, and 35-37) Except for changing the chemical composition to that shown in Table 1 below, and changing the shot blasting speed, annealing conditions, and plating type to those shown in Table 2 below, steel sheets (plated steel sheets) for Tests No. 3-5, 7-19, 21-28, and 35-37, which are examples of the present invention, were obtained in the same manner as in Test No. 2.

[0133] The types of plating shown in Table 2 are as follows: a: Unplated (i.e., no plating treatment applied) b: Hot-dip galvanized (Zn-0.2%Al(GI)) c1: Hot-dip Zn-Al-Mg alloy plating (Zn-1.5%Al-1.5%Mg) c2: Hot-dip Zn-Al-Mg alloy plating (Zn-6.0%Al-3.0%Mg) c3: Hot-dip Zn-Al-Mg alloy plating (Zn-12.0%Al-6.0%Mg) c4: Hot-dip Zn-Al-Mg alloy plating (Zn-20.0%Al-8.0%Mg)

[0134] (Manufacturing of steel plates for Test No. 6 and 20) Except for changing the chemical composition to that shown in Table 1 below and changing the projection speed and annealing conditions for the shot blasting treatment to those shown in Table 2 below, steel plates (unplated steel plates) for Test No. 6 and 20, which are examples of the present invention, were obtained in the same manner as in Test No. 1.

[0135] (Manufacturing of steel sheets for Tests No. 29-34 and 38-42) Except for changing the chemical composition to that shown in Table 1 below, and changing the projection speed and annealing conditions for the shot blasting treatment to those shown in Table 2 below, comparative examples of steel sheets (plated steel sheets) for Tests No. 29-34 and 38-42 were obtained in the same manner as in Test No. 2.

[0136] Analysis of the chemical composition of each steel sheet obtained in Tests No. 1 to 42 revealed that each was similar to the chemical composition of the steel billet before hot rolling. Furthermore, in the chemical composition of each alloyed hot-dip galvanized steel sheet in Tests No. 1 to 42, the oxygen content was 0.01% or less in all cases.

[0137]

[0138] For each of the steel sheets obtained in Test Nos. 1 to 42 as described above, the thickness of the region where the carbon concentration was 0.02% or more and less than 0.05% by GDS analysis, the solid solution Mn concentration of the ferrite phase within 2.0 μm of the steel sheet surface, the crystal grain size of the ferrite phase, and the Vickers hardness were measured. Furthermore, the bendability of each steel sheet in Test Nos. 1 to 42 was evaluated according to the evaluation method described below. The measurement results and evaluation results are shown in Table 2 below.

[0139] Note that the underlines next to the various values ​​in Tables 1 and 2 below indicate that they are outside the range of steel plates covered by the present invention. Similarly, manufacturing conditions that deviate from the manufacturing conditions envisioned by the inventors are underlined, but this does not limit the steel plates covered by the present invention.

[0140] <Evaluation of Bendability> The bendability of the steel sheet will be evaluated by performing a bending test in accordance with the VDA standard (VDA 238-100:2020-07) specified by the German Association of the Automotive Industry, and the maximum bending angle obtained from this bending test will be evaluated based on the evaluation criteria below. The conditions for the bending test will be as follows.

[0141] Specimen dimensions: 60 mm x 30 mm Specimen thickness: 1.6 mm Bending edge: Parallel to the width direction of the plate Test method: Roll support, punch pressing Roll diameter: φ30 mm Punch shape: Tip radius = 0.4 mm Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm Pressing speed: 20 mm / min Testing machine: SHIMADZU AUTOGRAPH 20kN

[0142] (Evaluation Criteria) AAA: Maximum bending angle is 30° or more AA: Maximum bending angle is 25° or more A: Maximum bending angle is 20° or more B: Maximum bending angle is less than 20°

[0143] The evaluation criteria are as follows: a rating of A or higher (i.e., ratings A, AA, and AAA) indicates excellent bendability, while a rating of B indicates poor bendability. A rating of AAA indicates extremely excellent bendability.

[0144]

[0145] Steel sheets No. 1-28 and 35-37 are examples of the present invention, and as shown in Table 2, all of them exhibited excellent bendability. On the other hand, steel sheets No. 29-34 and 38-42, obtained under manufacturing conditions that did not yield the steel sheets of the present invention, are comparative examples. Their chemical composition, the thickness of the region in the steel sheet surface structure with a carbon concentration of 0.02% or more and less than 0.05%, and the crystal grain size of the ferrite phase were all outside the range of the present invention, and all of them exhibited inferior bendability.

[0146] In test No. 29, the high Si content and the low dew point and short holding time during annealing likely prevented sufficient internal oxidation of Mn during annealing, resulting in the formation of external Mn oxides on the steel sheet surface. In other words, decarburization was not sufficiently promoted on the surface of the steel sheet, and the concentration of solid-solution Mn could not be reduced. As a result, the bendability was likely poor.

[0147] In test No. 30, the high Si content and excessively high dew point during annealing likely prevented sufficient internal oxidation of Mn during the annealing process, resulting in the formation of external Mn oxides on the steel sheet surface. In other words, decarburization was not sufficiently promoted on the surface of the steel sheet, and the concentration of solid-solution Mn could not be reduced. This likely resulted in poor bendability.

[0148] In Test No. 31, the high Si content and low holding temperature during annealing likely prevented sufficient internal oxidation of Mn during the annealing process, resulting in the formation of external Mn oxides on the steel sheet surface. This suggests that decarburization was not sufficiently promoted on the surface of the steel sheet, and the concentration of solid-solution Mn could not be reduced. Consequently, the resulting material exhibited poor bendability.

[0149] In test No. 32, the high Si content and short holding time during annealing likely prevented sufficient internal oxidation of Mn during annealing, resulting in insufficient decarburization on the steel sheet surface. Consequently, the resulting material exhibited poor bendability.

[0150] In test No. 33, the high Si content and low projection speed during shot blasting likely prevented sufficient internal oxidation of Mn during annealing, resulting in insufficient decarburization on the steel sheet surface. Consequently, the resulting material exhibited poor bendability.

[0151] In test No. 34, since shot blasting was not performed, it is thought that the internal oxidation of Mn during annealing was not sufficiently promoted, and decarburization on the surface of the steel sheet was not sufficiently promoted. As a result, the bendability was inferior.

[0152] In Test No. 38, the annealing was performed at a low dew point, which likely prevented sufficient internal oxidation of Mn during annealing, resulting in the formation of external Mn oxides on the steel sheet surface. In other words, decarburization was not sufficiently promoted on the surface of the steel sheet, and the concentration of solid-solution Mn could not be reduced. As a result, the bendability was likely inferior.

[0153] In test No. 39, the annealing was performed at an excessively high dew point, which is thought to have prevented sufficient internal oxidation of Mn during annealing, resulting in the formation of external Mn oxides on the steel sheet surface. In other words, decarburization was not sufficiently promoted on the surface of the steel sheet, and the concentration of solid-solution Mn could not be reduced. As a result, the bendability was likely to be poor.

[0154] In test No. 40, the annealing was performed at a low holding temperature, which likely resulted in insufficient internal oxidation of Mn during annealing, leading to the formation of external Mn oxides on the steel sheet surface. In other words, decarburization was not sufficiently promoted on the surface of the steel sheet, and the concentration of solid-solution Mn could not be reduced. As a result, the bendability was likely inferior.

[0155] In Test No. 41, the short holding time during annealing likely prevented sufficient internal oxidation of Mn during the annealing process, resulting in insufficient decarburization on the steel sheet surface. Consequently, the resulting material exhibited poor bendability.

[0156] In test No. 42, the shot blasting treatment was performed at a low projection speed, which is thought to have prevented sufficient internal oxidation of Mn during annealing, and thus insufficient decarburization on the surface of the steel sheet. As a result, the bendability was likely inferior.

Claims

1. A steel plate, wherein the chemical composition of the steel plate is, in mass%, C: 0.05-0.40%, Mn: 1.0-5.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0200% or less, Si: 0-0.65%, sol. It contains Al: 0-3.0%, O: 0-0.01%, B: 0-0.0100%, Ti: 0-0.1500%, Nb: 0-0.150%, V: 0-0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.00%, W: 0-1.00%, Ca: 0-0.1000%, Mg: 0-0.100%, Zr: 0-0.500%, Hf: 0-0.100%, Sn: 0-0.100%, As: 0-0.100%, and REM: 0-0.100%, with the remainder being Fe and impurities. A steel sheet characterized in that, as determined by glow discharge emission spectroscopy from the surface of the steel sheet toward the depth direction, the thickness of the region with a C concentration of 0.02% or more and less than 0.05% is 2.0 μm or more, and the crystal grain size of the ferrite phase within 2.0 μm of the surface layer of the steel sheet is 2.0 μm or less.

2. The chemical composition of the steel sheet is, in mass%, Si: 0.001 to 0.65%, sol. Al: 0.001-3.0%, O: 0.001-0.01%, B: 0.0001-0.0100%, Ti: 0.0001-0.1500%, Nb: 0.0001-0.150%, V: 0.001-0.150%, Cr: 0.001-2.00%, Ni: 0.001-2.00%, Cu: 0.001-2.00%, Mo: 0.001-1.00%, W: 0.0001-1.000%, Ca: 0.0001-0.1000%, Mg: 0.0001~0.100%, Zr: 0.001~0.500%, The steel sheet according to claim 1, characterized by containing one or more of the following: Hf: 0.0001 to 0.100%, Sn: 0.0001 to 0.100%, As: 0.0001 to 0.100%, and REM: 0.0001 to 0.100%.

3. The steel sheet according to claim 1 or 2, characterized in that, as determined by glow discharge emission spectroscopy, the thickness of the region where the C concentration is 0.02% or more and less than 0.05% is 5.0 μm or more.

4. The steel sheet according to claim 1 or 2, characterized in that, as determined by glow discharge emission spectroscopy, the thickness of the region where the C concentration is 0.02% or more and less than 0.05% is 10.0 μm or more.

5. The steel plate according to claim 1 or 2, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 360 Hv or more.

6. The steel plate according to claim 1 or 2, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 460 Hv or more.

7. The steel plate according to claim 1 or 2, characterized in that the Vickers hardness at a depth of 1 / 4 of the plate thickness of the steel plate is 550 Hv or more.

8. A zinc-plated steel sheet comprising: a steel sheet according to claim 1 or 2; and a Zn-containing plating layer disposed on at least a portion of the surface of the steel sheet.

9. A component comprising the steel plate described in claim 1 or 2.

10. A component comprising the zinc-plated steel sheet described in claim 8.

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