Steel sheet and component including same

Abstract

Provided is a steel sheet comprising a base steel sheet and a chemical conversion coating disposed on the surface of the base steel sheet. The steel sheet is characterized in that: the base steel sheet has a chemical composition containing, in mass%, 0.10-1.00% of Cr; and the S-maximum emission intensity and the Cr-maximum emission intensity of the chemical conversion coating as measured by a high-frequency glow discharge emission spectrometer (GDS) satisfy the relations of 1.1≤S-maximum emission intensity / S-emission intensity of base steel sheet≤20.0, and 1.1≤Cr-maximum emission intensity / Cr-emission intensity of base steel sheet≤5.0. Also provided is a component including the steel sheet.

Description

Steel plates and parts containing them

[0001] The present invention relates to a steel sheet and a part including the same.

[0002] It is known that improving the chemical conversion treatability of steel sheets is effective in improving the corrosion resistance of steel sheets after painting.

[0003] In this regard, Patent Document 1 discloses a method for manufacturing a hot-rolled steel sheet having a surface containing sulfides and a sulfur content of 0.05 mg / m per side. 2 Patent Document 1 also teaches that the above-mentioned configuration allows a chemical conversion coating that serves as a base for painting on the hot-rolled steel sheet to be formed uniformly and densely.

[0004] In Patent Document 2, a copper (Cu) content of 0.10 mass % or more and 0.50 mass % or less is contained, and the number of residual scales on the surface is 160,000 pieces / mm 2 Patent Document 2 teaches that, according to the above configuration, the particle size of the copper compound particles exposed on the steel sheet surface, which serves as the cathode point in chemical conversion treatment, is 2 μm or less, and the residual scale is reduced to a predetermined amount or less, thereby making it possible to provide a steel sheet with excellent chemical conversion treatability.

[0005] JP 2003-277959 A JP 2020-084238 A

[0006] Patent Document 2 teaches that electric furnace steel, which uses scrap iron as its raw material, contains many tramp elements such as copper (Cu), nickel (Ni), and tin (Sn) as well as scrap-derived elements such as chromium (Cr), making it difficult to use as automotive steel sheets. However, Patent Document 2 teaches that the steel sheets described therein can contain not only copper and copper compounds, but also carbon (C), silicon (Si), manganese (Mn), chromium (Cr), and the like. Among the above elements, Cr is an effective element for improving properties such as hole expandability, and is therefore sometimes added to steel sheets for automotive suspension components, for example. However, the addition of Cr can reduce the chemical conversion treatability of the steel sheet. Generally, reduced chemical conversion treatability can result in the formation of areas where the chemical conversion coating, known as "skid zones," is not formed, which can result in reduced corrosion resistance after painting.

[0007] Therefore, an object of the present invention is to provide a steel sheet containing Cr that can exhibit improved corrosion resistance after painting, and a part including the steel sheet.

[0008] In order to achieve the above object, the present inventors conducted research focusing on the components contained in chemical conversion coatings. As a result, the present inventors discovered that when measured with a high-frequency glow discharge optical emission spectrometer (GDS), the chemical conversion treatability of steel sheets can be improved by including S in addition to Cr and controlling the maximum emission intensities of these elements to satisfy a predetermined relationship, thereby significantly improving the corrosion resistance of the steel sheets after painting, and completed the present invention.

[0009] The present invention, which has achieved the above-mentioned object, is as follows: (1) A steel sheet comprising a base steel sheet and a chemical conversion coating disposed on a surface of the base steel sheet, wherein the base steel sheet has a chemical composition containing, by mass%, 0.10 to 1.00% Cr, and wherein the maximum S and Cr emission intensities in the chemical conversion coating measured by a high-frequency glow discharge optical emission spectrometer (GDS) satisfy the following relationships: 1.1≦S maximum emission intensity / S emission intensity of base steel sheet≦20.0 1.1≦Cr maximum emission intensity / Cr emission intensity of base steel sheet≦5.0 (2) The steel sheet according to (1) above, wherein the chemical composition contains, by mass%, 0.25 to 1.00% Cr, and wherein the maximum S and Cr emission intensities in the chemical conversion coating measured by GDS satisfy the following relationships: (3) The steel sheet according to (2) above, wherein the chemical composition includes, in mass %, 0.50 to 1.00% Cr, and the S and Cr maximum emission intensities in the chemical conversion coating measured by GDS satisfy the following relationships: 1.8≦S maximum emission intensity / S emission intensity of base steel sheet≦20.0 1.4≦Cr maximum emission intensity / Cr emission intensity of base steel sheet≦5.0 (4) The steel sheet according to any one of (1) to (3) above, wherein the base steel sheet has a Vickers hardness of 300 Hv or more. (5) A part comprising the steel sheet according to any one of (1) to (4).

[0010] According to the present invention, it is possible to provide a steel sheet containing Cr that can exhibit improved corrosion resistance after painting, and a part including the steel sheet.

[0011] <Steel Sheet> A steel sheet according to an embodiment of the present invention comprises a base steel sheet and a chemical conversion coating disposed on the surface of the base steel sheet, wherein the base steel sheet has a chemical composition containing, by mass %, 0.10 to 1.00% Cr, and wherein the maximum S and Cr emission intensities in the chemical conversion coating measured with a high-frequency glow discharge optical emission spectrometer (GDS) satisfy the following relationships: 1.1≦S maximum emission intensity / S emission intensity of base steel sheet≦20.0 1.1≦Cr maximum emission intensity / Cr emission intensity of base steel sheet≦5.0

[0012] Cr is an effective element for improving properties such as hole expandability, and is therefore sometimes added to steel sheets for automobile suspension parts, for example. However, as mentioned above, the addition of Cr can reduce the chemical treatability of the steel sheet. In particular, when the Cr content in the steel sheet is 0.10 mass% or more, such a reduction in chemical treatability can become significant. The inventors' recent studies have found that such a reduction in chemical treatability is due to Cr oxides formed on the steel sheet surface during the hot rolling process. More specifically, these Cr oxides can cause chemical conversion defects that inhibit the adhesion of the chemical conversion coating. Such chemical conversion defects lead to a reduction in corrosion resistance after painting, and therefore need to be addressed appropriately.

[0013] Therefore, the present inventors conducted studies focusing on the components contained in a chemical conversion coating in order to improve the chemical conversion treatability of steel sheets and thereby improve the corrosion resistance of the steel sheets after painting. As a result, the present inventors discovered that the chemical conversion treatability of steel sheets can be improved, and thereby the corrosion resistance of the steel sheets after painting can be significantly improved, by making the chemical conversion coating contain S in addition to Cr and controlling the maximum emission intensities of these elements to satisfy a predetermined relationship when measured with a high-frequency glow discharge optical emission spectrometer (GDS), more specifically, by controlling the maximum emission intensities of S and Cr in the chemical conversion coating to satisfy the relationships of the following Equations 1 and 2, respectively: 1.1≦S maximum emission intensity / S emission intensity of base steel sheet≦20.0 (Equation 1) 1.1≦Cr maximum emission intensity / Cr emission intensity of base steel sheet≦5.0 (Equation 2)

[0014] More specifically, in the production of steel sheets containing a relatively high amount of Cr, Cr oxides may form on the surface of the steel sheet during the hot rolling process. These oxides cannot be sufficiently removed by subsequent pickling, resulting in the concentration of Cr on the surface of the steel sheet. This concentration of Cr leads to a decrease in chemical conversion treatability, resulting in the formation of relatively large areas where the chemical conversion coating is not formed, known as "ske." On the other hand, areas where the chemical conversion coating is applied contain a relatively large amount of Cr. Therefore, when measured by GDS, the maximum chromium emission intensity of the chemical conversion coating is higher than the chromium emission intensity observed on the base steel sheet. Therefore, when such GDS measurement results are obtained, it is usually recognized that the chemical conversion coating is not uniformly applied to the entire steel sheet, and therefore the corrosion resistance of the steel sheet after painting is also expected to be reduced. However, the present inventors have discovered that, as will be described in detail later in connection with the manufacturing method, by supplying an accelerator that is a sulfur (S)-containing compound at a predetermined concentration and under predetermined conditions during pickling after the hot rolling step, and further performing the subsequent water-rinsing step with a rinsing solution having a relatively low, predetermined electrical conductivity, it is possible to cause the chemical conversion coating to contain S in addition to Cr, and to control the maximum emission intensities of S and Cr in the chemical conversion coating to satisfy the relationships in Equations 1 and 2, respectively. As a result, the present inventors have discovered that it is possible to improve the chemical treatability of the steel sheet, as well as to improve the corrosion resistance of the chemical conversion coating itself, thereby significantly improving the corrosion resistance of the steel sheet after painting.

[0015] Without intending to be bound by any particular theory, it is believed that using an accelerator that is an S-containing compound during pickling after the hot rolling process and then performing the subsequent water rinsing process with a rinse solution having a relatively low, predetermined electrical conductivity can suppress or reduce the formation of Cr oxides on the steel sheet surface and form an S—Cr layer in place of the Cr oxides. In relation to this, it is possible to suppress or reduce the occurrence of chemical conversion defects such as scale due to the formation of Cr oxides on the steel sheet surface during steel sheet production. Additionally, it is believed that forming an S—Cr layer on the steel sheet surface can improve the etching ability of Fe in the chemical conversion treatment solution during chemical conversion treatment, thereby enabling the formation of a uniform chemical conversion coating over the entire steel sheet. Furthermore, in relation to the formation of the S—Cr layer on the steel sheet surface, it is natural that S, in addition to Cr, is incorporated into the chemical conversion coating after chemical conversion treatment. Considering the fact that the corrosion resistance of steel sheets after painting was significantly improved by appropriately controlling the contents of these elements in the chemical conversion coating so that the relationship between Equations 1 and 2 above was satisfied, it is believed that effects other than the effect of suppressing or reducing Cr oxide formation during steel sheet production contribute to this improvement in corrosion resistance after painting. More specifically, it is believed that the corrosion resistance of the chemical conversion coating itself is improved when the chemical conversion coating contains appropriate amounts of S and Cr, i.e., amounts that satisfy the relationship between Equations 1 and 2 above. If the corrosion resistance of the chemical conversion coating is reduced, corrosion on the surface of the chemical conversion coating is more likely to progress, and this corrosion on the surface of the chemical conversion coating makes it more likely that the coating will peel off at the interface between the chemical conversion coating and the coating. In such cases, the corrosion resistance of the steel sheet after painting will naturally be reduced. However, according to the steel sheet according to the embodiment of the present invention, which includes a chemical conversion coating in which the S and Cr maximum emission intensities satisfy the relationships of the above formulas 1 and 2, respectively, in addition to the effect of improving chemical conversion treatability due to the suppression or reduction of the formation of Cr oxide during steel sheet production and the effect of improving Fe etching ability during chemical conversion treatment due to the formation of an S—Cr layer, it is possible to combine the effect of improving the corrosion resistance of the chemical conversion coating itself due to the inclusion of S and Cr, and it is believed that the combination of these three effects makes it possible to significantly improve the corrosion resistance of the steel sheet after painting.

[0016] Even in steel sheets containing a relatively large amount of Cr, the inclusion of an appropriate amount of S in addition to Cr in the chemical conversion coating can improve not only the chemical treatability but also the corrosion resistance of the chemical conversion coating itself, thereby significantly improving the corrosion resistance of the steel sheet after painting. This fact was not previously known, and has now been revealed for the first time by the present inventors. Therefore, steel sheets according to embodiments of the present invention are particularly useful in the automotive field, where excellent chemical treatability and / or corrosion resistance after painting are required. Each component of the steel sheet according to embodiments of the present invention will be described in more detail below.

[0017] [Chemical Conversion Coating] In an embodiment of the present invention, a chemical conversion coating is disposed on the surface of a base steel sheet, for example, on at least one surface, preferably both surfaces, of the base steel sheet. The thickness of the chemical conversion coating is not particularly limited, but may be, for example, 1.0 to 5.0 μm or 1.5 to 3.5 μm.

[0018] The chemical conversion coating is not particularly limited as long as it contains S and Cr in amounts that satisfy the relationship between Formulas 1 and 2, and may be any chemical conversion coating known to those skilled in the art. According to the steel sheet according to the embodiment of the present invention, by containing S and Cr in amounts that satisfy the relationship between Formulas 1 and 2, regardless of the type of chemical conversion coating, the steel sheet can significantly improve its post-painting corrosion resistance through a combination of the following: an improved chemical conversion treatability effect due to the suppression or reduction of Cr oxide formation during steel sheet production; an improved Fe etching effect due to the formation of an S—Cr layer during steel sheet production; and an improved corrosion resistance effect of the chemical conversion coating itself due to the inclusion of S and Cr. For example, the chemical conversion coating may contain phosphoric acid or a phosphate salt, and the elements that constitute the salt are elements from Groups 2 to 12 of the Periodic Table. More specifically, the chemical conversion coating may contain at least one of phosphoric acid, zinc phosphate, and zirconium, and particularly at least one of zinc phosphate and zirconium. Preferably, the chemical conversion coating contains zinc phosphate, consists essentially of zinc phosphate, consists of zinc phosphate, or consists of zinc phosphate. Specifically, examples include those consisting of 50% by mass or more, preferably 90% by mass or more, of zinc phosphate. The chemical conversion coating may optionally contain other elements in addition to the phosphoric acid, zinc phosphate, zirconium, etc., described above. For example, the chemical conversion coating may contain at least one of Fe, Cr, Mn, Ti, Ni, Mg, Ca, and V in an amount of up to 10% by mass, preferably up to 5% by mass. The chemical composition of the chemical conversion coating is determined by dissolving the coating in 5% dichromic acid and analyzing the dissolved components with an ICP atomic emission spectrometer (ICP-AES measurement).

[0019] [1.1≦S maximum emission intensity / S emission intensity of base steel sheet≦20.0] [1.1≦Cr maximum emission intensity / Cr emission intensity of base steel sheet≦5.0] In an embodiment of the present invention, when a steel sheet is measured using a high-frequency glow discharge optical emission spectrometer (GDS), the maximum emission intensities of S and Cr in the chemical conversion coating satisfy the relationships of the following formulas 1 and 2, respectively: 1.1≦S maximum emission intensity / S emission intensity of base steel sheet≦20.0 ... formula 1 1.1≦Cr maximum emission intensity / Cr emission intensity of base steel sheet≦5.0 ... formula 2

[0020] As described above, in the steel sheet according to the embodiment of the present invention, the formation of Cr oxides on the steel sheet surface during steel sheet production is suppressed or reduced, and an S—Cr layer is formed in place of the Cr oxides. This allows the chemical composition of the chemical conversion coating to be controlled so that the maximum S and Cr emission intensities satisfy the relationships in Equations 1 and 2, respectively. Conversely, by controlling the chemical composition of the chemical conversion coating so that S and Cr are contained in amounts such that the maximum S and Cr emission intensities satisfy the relationships in Equations 1 and 2, respectively, it is possible to obtain an improved chemical conversion treatability due to the suppression or reduction of Cr oxide formation during steel sheet production, an improved Fe etching ability due to the formation of an S—Cr layer during steel sheet production, and an improved corrosion resistance of the chemical conversion coating itself due to the inclusion of S and Cr. As a result, the combination of these effects makes it possible to significantly improve the corrosion resistance of the steel sheet after painting.

[0021] From the viewpoint of further improving the corrosion resistance of the steel sheet after painting, the larger the values ​​of the middle parts of the above formulas 1 and 2, i.e., "S maximum emission intensity / S emission intensity of base steel sheet" and "Cr maximum emission intensity / Cr emission intensity of base steel sheet," the more preferable. For example, the S maximum emission intensity / S emission intensity of base steel sheet may be 1.3 or more, 1.5 or more, 1.6 or more, 1.8 or more, 2.0 or more, 2.5 or more, 3.0 or more, 4.0 or more, 4.5 or more, or 5.0 or more. Similarly, the Cr maximum emission intensity / Cr emission intensity of base steel sheet may be 1.2 or more, 1.3 or more, 1.4 or more, 1.5 or more, 1.6 or more, 1.8 or more, 2.0 or more, 2.2 or more, 2.5 or more, or 2.8 or more.

[0022] Furthermore, even if excessive S is included in the chemical conversion coating, the effect saturates, and including more S than necessary in the chemical conversion coating increases production costs. Therefore, the S maximum emission intensity / S emission intensity of the base steel sheet is set to 20.0 or less, and may be, for example, 18.0 or less, 15.0 or less, 12.0 or less, 10.0 or less, or 8.0 or less. On the other hand, if excessive Cr is included in the chemical conversion coating, even when it is included together with S, the formation of Cr oxides during steel sheet production cannot be sufficiently suppressed or reduced, which may result in reduced chemical conversion treatability and, ultimately, reduced corrosion resistance after painting. Therefore, the Cr maximum emission intensity / Cr emission intensity of the base steel sheet is set to 5.0 or less, and may be, for example, 4.8 or less, 4.5 or less, 4.2 or less, 4.0 or less, 3.8 or less, 3.5 or less, or 3.2 or less.

[0023] [Measurement of S Maximum Emission Intensity / S Emission Intensity of Base Steel Sheet by GDS and Cr Maximum Emission Intensity / Cr Emission Intensity of Base Steel Sheet] The S Maximum Emission Intensity / S Emission Intensity of Base Steel Sheet by GDS and the Cr Maximum Emission Intensity / Cr Emission Intensity of Base Steel Sheet by GDS are measured as follows. First, using a high-frequency glow discharge optical emission spectrometer (e.g., manufactured by LECO Japan LLC, model number "GDS850A"), the surface of the steel sheet on which the chemical conversion coating is disposed is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma, and the steel sheet surface is analyzed in the depth direction while sputtering. Then, elements contained in the material are identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensities of the identified elements are measured. The measurement conditions are as follows: Ar gas pressure: 0.3 MPa, anode diameter: 4 mmφ, RF output: 30 W, measurement time: 200 to 1500 seconds.

[0024] When GDS measurement is performed in this manner from the steel sheet surface in the depth direction, the region in which the emission intensity of the element that primarily constitutes the chemical conversion coating (e.g., P when the chemical conversion coating primarily contains zinc phosphate, or Zr when the chemical conversion coating primarily contains zirconium) is 10 times or more the emission intensity of the corresponding element in the base steel sheet (e.g., P in the base steel sheet when the chemical conversion coating primarily contains zinc phosphate, or Zr in the base steel sheet when the chemical conversion coating primarily contains zirconium) is determined to be the chemical conversion coating. Next, for the chemical conversion coating determined in this manner, the maximum value of a graph in which the S emission intensity obtained by GDS measurement in the depth direction is smoothed as a 10-point average is determined as the "S maximum emission intensity." Similarly, for the above chemical conversion coating, the maximum value of a graph in which the Cr emission intensity obtained by GDS measurement in the depth direction is smoothed as a 10-point average is determined as the "Cr maximum emission intensity." On the other hand, the average value of the emission intensities of S and Cr is calculated within a depth range where the emission intensities are sufficiently stable. For example, the average value of the emission intensities of S and Cr is calculated within a region from 100 to 150 μm from the steel sheet side of the chemical conversion coating, and these are determined as the "S emission intensity of the base steel sheet" and the "Cr emission intensity of the base steel sheet," respectively. Finally, based on the "maximum S emission intensity," "maximum Cr emission intensity," "S emission intensity of the base steel sheet," and "Cr emission intensity of the base steel sheet" obtained as described above, the values ​​of "maximum S emission intensity / S emission intensity of the base steel sheet" and "maximum Cr emission intensity / Cr emission intensity of the base steel sheet" are determined. The depth in GDS measurement is determined from the sputtering time. Specifically, the depth of the GDS mark (dent) formed by GDS measurement is measured with a roughness meter, and the depth per sputtering time is calculated by dividing the depth of the GDS mark by the sputtering time. For example, the depth of the chemical conversion coating from the steel sheet side surface is determined by multiplying the depth per sputtering time by the sputtering time required to measure the depth from the steel sheet side surface of the chemical conversion coating. The depth of the GDS marks is measured using a roughness meter (for example, model number "SURFCOM TOUCH50" manufactured by ACCRETECH (Tokyo Seimitsu)), and the specific conditions are as follows.Measurement conditions: Measurement type: Cross-section measurement Evaluation length: 10 mm Shape removal: Both ends Calculation standard: JIS B 0601:2013 Reference height: -50 μm (when measuring 100 μm) Measurement location: The unevenness profile was measured in the diameter direction so as to cross the GDS mark (center of the circle ±0.5 mm).

[0025] [Chemical Composition of Base Steel Sheet] In an embodiment of the present invention, the base steel sheet has a chemical composition containing, by mass%, 0.10 to 1.00% Cr. As described above, the present invention aims to provide a Cr-containing steel sheet that can exhibit improved corrosion resistance after painting. This objective is achieved by controlling the maximum S and Cr emission intensities in a chemical conversion coating, measured by GDS, to satisfy the relationships of Equation 1 and Equation 2, respectively. Therefore, the chemical composition of the base steel sheet is not particularly limited except for containing, by mass%, 0.10 to 1.00% Cr. Therefore, it is clear that elements other than Cr are not essential technical features for achieving the objective of the present invention. The chemical composition of the base steel sheet can contain, in addition to Cr, appropriate amounts of any alloying element commonly added in the technical field of the present invention. Hereinafter, the chemical composition of the base steel plate used in the steel plate according to the embodiment of the present invention will be described in detail. However, these descriptions are intended to merely exemplify preferred chemical compositions of the base steel plate for application in automotive steel plates and the like, and are not intended to limit the present invention to those using base steel plates having such specific chemical compositions.

[0026] In an embodiment of the present invention, for example, the base steel plate has, in mass %, C: 0.001 to 0.500%, Si: 0.01 to 3.00%, Mn: 0.10 to 3.00%, Al: 0.001 to 2.000%, Cr: 0.10 to 1.00%, P: 0.100% or less, S: 0.100% or less, N: 0.0100% or less, Ti: 0 to 0.150%, Nb: 0 to 0.150%, B: 0 to 0.0100%, Mo: 0 to 1.000%, Ni: 0 to 1.000%, Cu: 0 to 1.000%, Sn: 0 to 1.000%, V: 0 to 0.150%, W: 0 to 1.000%, It is preferable that the alloy has a chemical composition consisting of Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.010%, REM: 0 to 0.010%, As: 0 to 0.010%, Ir: 0 to 1.000%, and the balance: Fe and impurities. Each element will be described in more detail below.

[0027] [C: 0.001 to 0.500%] C is an element that inexpensively increases strength and is an important element for controlling the strength of steel. To fully obtain this effect, the C content is preferably 0.001% or more. The C content may be 0.005% or more, 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, 0.070% or more, 0.100% or more, or 0.150% or more. On the other hand, excessive C content may result in a decrease in elongation. For this reason, the C content is preferably 0.500% or less. The C content may be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, 0.250% or less, 0.200% or less, or 0.180% or less.

[0028] [Si: 0.01 to 3.00%] Si is an element that is effective in increasing strength as a solid solution strengthening element. To fully obtain this effect, the Si content is preferably 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, 0.30% or more, 0.50% or more, 0.80% or more, or 1.00% or more. On the other hand, excessive Si content may increase the steel strength but decrease the elongation. For this reason, the Si content is preferably 3.00% or less. The Si content may be 2.50% or less, 2.00% or less, 1.50% or less, or 1.20% or less.

[0029] [Mn: 0.10 to 3.00%] Mn is an element that improves the hardenability of steel and is effective in increasing strength. To fully obtain this effect, the Mn content is preferably 0.10% or more. The Mn content may be 0.50% or more, 1.00% or more, 1.30% or more, 1.50% or more, or 1.80% or more. On the other hand, excessive Mn content may increase the steel strength but reduce elongation. For this reason, the Mn content is preferably 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, or 2.00% or less.

[0030] [Al: 0.001 to 2.000%] Al acts as a deoxidizer for steel and has the effect of improving the soundness of steel. To fully obtain this effect, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, excessive Al content may generate coarse Al oxides, reducing the elongation of the steel sheet. For this reason, the Al content is preferably 2.000% or less. The Al content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0031] [Cr: 0.10 to 1.00%] Cr is an element that contributes to improving the hole expandability of steel sheets. It also enhances the hardenability of steel and contributes to improving its strength. To fully obtain these effects, the Cr content is set to 0.10% or more. The Cr content may be 0.12% or more, 0.15% or more, 0.18% or more, 0.20% or more, 0.25% or more, 0.30% or more, 0.40% or more, 0.50% or more, or 0.55% or more. On the other hand, even if Cr is contained in an excessive amount, the effect saturates, and adding more than necessary to the steel increases manufacturing costs. Furthermore, excessive Cr content may result in a decrease in chemical conversion treatability due to the formation of Cr oxides during steel sheet manufacturing. Therefore, the Cr content is set to 1.00% or less, and may be 0.90% or less, 0.80% or less, 0.75% or less, 0.70% or less, or 0.65% or less.

[0032] [P: 0.100% or less] P is an element that segregates at grain boundaries and promotes embrittlement of steel. Since a lower P content is preferable, ideally it is 0%. However, excessive reduction in the P content may result in a significant increase in costs. For this reason, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more. On the other hand, excessive P content may result in embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content is preferably 0.100% or less. The P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0033] [S: 0.100% or less] S is an element that generates non-metallic inclusions such as MnS in steel, resulting in a decrease in the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in the S content may result in a significant increase in costs. Therefore, the S content may be 0.0001% or more, 0.0002% or more, 0.0003% or more, 0.0004% or more, 0.0005% or more, 0.0006% or more, 0.0008% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive S content may cause cracks originating from non-metallic inclusions during cold forming. Therefore, the S content is preferably 0.100% or less. The S content may be 0.050% or less, 0.020% or less, or 0.010% or less.

[0034] [N: 0.0100% or less] N is an element that forms coarse nitrides in steel sheets and reduces the workability of the steel sheets. Since a lower N content is preferable, the ideal N content is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. Therefore, 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 may form coarse nitrides as described above, reducing the workability of the steel sheets. Therefore, the N content is preferably 0.0100% or less. The N content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0035] The base steel sheet preferably has the basic chemical composition described above. Furthermore, the base steel sheet may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0036] [Ti: 0 to 0.150%] [Nb: 0 to 0.150%] [V: 0 to 0.150%] Ti, Nb, and V form carbonitrides in steel and have the effect of improving the strength of the steel sheet through precipitation strengthening. The Ti, Nb, and V contents may be 0%, but to obtain this effect, the Ti, Nb, and V contents are preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, even if these elements are contained in excess, the effect saturates, and adding more than necessary to the steel increases manufacturing costs. Therefore, the Ti, Nb, and V contents are preferably 0.150% or less, and may be 0.120% or less, 0.100% or less, 0.080% or less, 0.050% or less, 0.020% or less, or 0.015% or less.

[0037] [B: 0 to 0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness. The B content may be 0%, but to achieve this effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if B is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0038] [Mo: 0 to 1.000%] [W: 0 to 1.000%] Mo and W are elements that improve the hardenability of steel and contribute to improving its strength. The Mo and W contents may be 0%, but to achieve these effects, the Mo and W contents are preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, even if these elements are contained in excess, the effects saturate, and including more than necessary in steel increases manufacturing costs. Therefore, the Mo and W contents are preferably 1.000% or less, and may be 0.800% or less, 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0039] [Ni: 0 to 1.000%] [Cu: 0 to 1.000%] Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening. The Ni and Cu contents may be 0%, but to achieve these effects, the Ni and Cu contents are preferably 0.010% or more, and may be 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, even if these elements are contained in excess, the effect saturates, and containing more than necessary in the steel increases manufacturing costs. Therefore, the Ni and Cu contents are preferably 1.000% or less, and may be 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less.

[0040] [Sn: 0 to 1.000%] Sn is an element effective in improving corrosion resistance. The Sn content may be 0%, but to obtain this effect, the Sn content is preferably 0.003% or more. The Sn content may be 0.004% or more, 0.006% or more, 0.008% or more, or 0.010% or more. On the other hand, even if excessive Sn is contained, the effect saturates, and including more Sn than necessary in steel increases manufacturing costs. Therefore, the Sn content is preferably 1.000% or less. The Sn content may be 0.800% or less, 0.600% or less, 0.400% or less, 0.200% or less, 0.100% or less, 0.080% or less, 0.050% or less, or 0.030% or less.

[0041] [Hf: 0-0.050%] [Mg: 0-0.050%] [Zr: 0-0.050%] [Ca: 0-0.010%] [REM: 0-0.010%] Hf, Mg, Zr, Ca, and REM are elements that can control the morphology of non-metallic inclusions. The Hf, Mg, Zr, Ca, and REM contents may be 0%, but to achieve these effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, or 0.001% or more. On the other hand, even if these elements are contained in excess, the effects saturate, and adding more than necessary to the steel sheet increases manufacturing costs. Therefore, the Hf, Mg, and Zr contents are preferably 0.050% or less, and may be 0.010% or less, 0.005% or less, or 0.003% or less. Similarly, the Ca and REM contents are each preferably 0.010% or less, and may be 0.005% or less or 0.003% or less.

[0042] [As: 0 to 0.010%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain this effect, the As content is preferably 0.001% or more. The As content may be 0.002% or more or 0.003% or more. On the other hand, even if an excessive amount of As is contained, the effect saturates, and containing more As than necessary in the steel sheet increases the manufacturing cost. Therefore, the As content is preferably 0.010% or less. The As content may be 0.008% or less or 0.005% or less.

[0043] [Ir: 0 to 1.000%] Ir is an element that segregates at prior austenite grain boundaries to increase the strength of the grain boundaries. The Ir content may be 0%, but to obtain this effect, the Ir content is preferably 0.001% or more. The Ir content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, even if an excessive amount of Ir is contained, the effect saturates, and adding more Ir than necessary to the steel material increases the manufacturing cost. Therefore, the Ir content is preferably 1.000% or less. The Ir content may be 0.500% or less, 0.100% or less, 0.030% or less, or 0.015% or less.

[0044] The remainder of the base steel plate other than the above elements consists of Fe and impurities. The impurities in the base steel plate are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the base steel plate is industrially manufactured.

[0045] The chemical composition of the base steel plate may be measured by a general analytical method. For example, the chemical composition of the base steel plate may be measured by first removing the chemical conversion coating in accordance with an appropriate method, such as JIS K 3151:1996, and then measuring the chips using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2022. Specifically, for example, a 35 mm square test piece is obtained from the base steel plate at a position 1 / 4 of the plate thickness, and the test piece is measured using a Shimadzu ICPS-8100 (measuring device) or the like under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.

[0046] [Thickness of Base Steel Plate] The thickness of the base steel plate is not particularly limited, but is generally 0.2 to 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 base steel plate may be, for example, 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0047] [Mechanical Properties] The steel sheet according to the embodiment of the present invention is not particularly limited, but for example, the base steel sheet may have a Vickers hardness of 100 Hv or more. The Vickers hardness may be 120 Hv or more, 150 Hv or more, 200 Hv or more, 250 Hv or more, 300 Hv or more, 350 Hv or more, 400 Hv or more, or 450 Hv or more. The upper limit is not particularly limited, but for example, the Vickers hardness may be 650 HV or less, 600 HV or less, 550 HV or less, or 500 HV or less. For example, by making the Vickers hardness of the base steel sheet 300 Hv or more, the steel sheet can be particularly suitable for use as a steel sheet for automobiles and building materials.

[0048] [Measurement of Vickers Hardness] Vickers hardness is determined in accordance with JIS Z 2244-1:2024 as follows. First, a test piece is cut out so that a cross section (thickness cross section) perpendicular to the surface can be observed from any position except the end of the base steel plate. The thickness cross section of the test piece is polished using silicon carbide paper of #600 to #1500, and then 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 diluted solution such as alcohol or pure water, and this thickness cross section is used as the measurement surface. Next, the Vickers hardness is measured using a micro Vickers hardness tester at a load of 1 kgf and at intervals of at least three times the indentation. Specifically, a total of 20 points are measured randomly at 1 / 4 of the plate thickness of the base steel plate, and the arithmetic average of these measurements is determined as the Vickers hardness of the base steel plate.

[0049] According to the steel sheet according to the embodiment of the present invention, as described above, even in steel sheets containing a relatively high amount of Cr, by adding an appropriate amount of S in addition to Cr to the chemical conversion coating, not only the chemical conversion treatability but also the corrosion resistance of the chemical conversion coating itself can be improved, thereby significantly improving the corrosion resistance of the steel sheet after painting. Therefore, the steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields requiring excellent corrosion resistance after painting, and is particularly useful for use in parts in the automotive field. In a preferred embodiment, an automobile part including the steel sheet according to the embodiment of the present invention is provided. Examples of automobile parts include suspension parts, frame parts, bumpers, and other structural and reinforcing parts that require strength, as well as exterior panel parts such as roofs, hoods, fenders, and doors that require high design quality. Because the steel sheet according to the embodiment of the present invention contains a relatively high amount of Cr, it can achieve properties such as improved hole expandability. Therefore, the steel sheet according to the embodiment of the present invention is useful for use as automobile suspension parts. Examples of automobile suspension parts include lower arms and trailing arms. All of the above-exemplified parts may contain the steel sheet according to the embodiment of the present invention in at least a part thereof, and therefore at least a part of these parts will satisfy the characteristics of the steel sheet described above. In parts of the steel sheet that do not come into direct contact with a die during forming such as press forming, or that come into direct contact with a die but are processed to a relatively small extent, the characteristics of the steel sheet do not change particularly before and after forming.

[0050] <Method for manufacturing steel sheet> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, but is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0051] The steel sheet according to the embodiment of the present invention can be manufactured by, for example, performing a casting process in which molten steel having an adjusted chemical composition is cast to form a steel billet, a hot rolling process in which the steel billet is hot-rolled to obtain a hot-rolled steel sheet, an optional skin-pass rolling process in which the obtained hot-rolled steel sheet is skin-pass rolled, a pickling process in which the hot-rolled steel sheet is pickled, a water-rinsing process in which the pickled hot-rolled steel sheet is rinsed with water, and a chemical conversion treatment process in which a chemical conversion coating is formed on the surface of the base steel sheet. Below, a manufacturing method in which a hot-rolled steel sheet is used as the base steel sheet will be specifically described, but the base steel sheet according to the embodiment of the present invention encompasses not only a hot-rolled steel sheet but also a cold-rolled steel sheet. Therefore, when a cold-rolled steel sheet is used as the base steel sheet, for example, a cold-rolling process and / or an annealing process may be performed after the water-rinsing process. Each process will be described in detail below.

[0052] [Casting Step] The conditions for the casting step are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0053] [Hot Rolling Process] A hot-rolled steel plate can be obtained by hot-rolling a cast steel slab. The hot rolling process is carried out by reheating the cast steel slab directly or after cooling it once, followed by hot rolling. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling step can be appropriately determined depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[0054] [Skin-pass Rolling Process] The obtained hot-rolled steel sheet may optionally be subjected to skin-pass rolling in the subsequent skin-pass process. By performing skin-pass rolling with a reduction ratio of 0.6% or more, the accelerator introduced in the subsequent pickling process can be efficiently supplied to the base steel sheet, thereby further promoting the formation of an S—Cr layer on the steel sheet surface. More specifically, by performing skin-pass rolling with a reduction ratio of 0.6% or more, cracks can be introduced into the scale formed on the steel sheet surface in the hot rolling process. In this case, the pickling solution introduced in the subsequent pickling process, more specifically, the pickling solution containing an accelerator that is an S-containing compound, can be efficiently supplied to the surface of the base steel sheet through the cracks in the scale. As a result, the reaction between S in the accelerator contained in the pickling solution and Cr on the steel sheet surface can be promoted, thereby further promoting the formation of an S—Cr layer on the steel sheet surface. By further promoting the formation of an S—Cr layer on the steel sheet surface, it is possible to increase both the value of “S maximum luminescence intensity / S luminescence intensity of base steel sheet” and the value of “Cr maximum luminescence intensity / Cr luminescence intensity of base steel sheet” in the final chemical conversion coating. In this case, the effects associated with the simultaneous inclusion of S and Cr in the chemical conversion coating, namely, the effect of improving chemical conversion treatability due to the suppression or reduction of Cr oxide formation, the effect of improving Fe etching during chemical conversion due to the formation of an S—Cr layer, and the effect of improving the corrosion resistance of the chemical conversion coating itself due to the inclusion of S and Cr, can be achieved at very high levels. As a result, it is possible to more significantly improve the corrosion resistance of the steel sheet after painting.

[0055] [Pickling Process] The hot-rolled steel sheet obtained in the hot rolling process or the hot-rolled steel sheet after the optional skin-pass process is pickled in the subsequent pickling process. In this manufacturing method, it is important that the pickling solution used in the pickling process contains an accelerator, which is a S-containing compound, at a concentration of 10 to 1500 ppm and that the hot-rolled steel sheet is controlled to pass through the pickling solution at a line speed of 0.10 to 10.00 m / s. Passing the hot-rolled steel sheet through the pickling solution at a certain line speed promotes the pickling of the hot-rolled steel sheet and also promotes the reaction between S in the accelerator contained in the pickling solution and Cr on the steel sheet surface. As a result, the formation of an S—Cr layer on the surface of the hot-rolled steel sheet is promoted, thereby making it possible to sufficiently suppress or reduce the formation of Cr oxides. If the line speed is less than 0.10 m / s, the formation of the S—Cr layer on the surface of the hot-rolled steel sheet will be insufficient, and the formation of Cr oxides will not be sufficiently suppressed or reduced. As a result, the value of "Cr maximum luminescence intensity / Cr luminescence intensity of base steel sheet" in the finally obtained chemical conversion coating may become larger than 5.0, making it impossible to satisfy the relationship in formula 2 below. In this case, the formation of Cr oxide during steel sheet production reduces chemical conversion treatability, and the corrosion resistance of the finally obtained steel sheet after painting decreases. From the viewpoint of further improving corrosion resistance after painting, the line speed is preferably 1.00 m / s or more, and more preferably 1.20 m / s or more. 1.1≦Cr maximum luminescence intensity / Cr luminescence intensity of base steel sheet≦5.0 ... formula 2

[0056] On the other hand, if the concentration of the accelerator contained in the pickling solution is less than 10 ppm and / or the line speed exceeds 10.00 m / s, the S component cannot be sufficiently adsorbed on the surface of the hot-rolled steel sheet, making it difficult to form an S-Cr layer, and the value of "S maximum emission intensity / S emission intensity of base steel sheet" in the finally obtained chemical conversion coating may be less than 1.1, making it impossible to satisfy the relationship in formula 1 below. In this case, the above-mentioned effects associated with the inclusion of S in the chemical conversion coating, namely, the effect of improving chemical conversion treatability due to the suppression or reduction of Cr oxide formation, the effect of improving Fe etching ability during chemical conversion due to the formation of an S-Cr layer, and the effect of improving the corrosion resistance of the chemical conversion coating itself due to the inclusion of S and Cr, cannot be fully obtained, resulting in a decrease in the corrosion resistance after painting of the finally obtained steel sheet. 1.1≦S maximum emission intensity / S emission intensity of base steel sheet≦20.0 ... formula 1

[0057] Furthermore, even if an excessive amount of accelerator is added to the pickling solution, the effect will saturate, and adding more accelerator than necessary to the pickling solution will increase production costs. Therefore, the concentration of accelerator contained in the pickling solution is preferably 1500 ppm or less. The accelerator is not particularly limited as long as it is an S-containing compound, but is preferably an S-containing organic compound such as thioglycolic acid, thiosulfuric acid, thiocyanic acid, thiocarboxylic acid, thiourea, sodium thiosulfate, and / or ammonium thiosulfate. The concentration of accelerator contained in the pickling solution is preferably 150 to 1500 ppm, more preferably 200 to 1500 ppm.

[0058] The conditions for the pickling step are not particularly limited other than the above-mentioned accelerator and line speed, and the step may be carried out under conditions suitable for removing scale formed on the surface of the steel sheet using a commonly used pickling solution, for example, a hydrochloric acid solution containing an inhibitor that suppresses corrosion of the base steel sheet.

[0059] [Rinsing Process] The pickled hot-rolled steel sheet is then rinsed in the subsequent rinsing process. This rinsing process is carried out by rinsing the steel sheet with a rinsing solution having an electrical conductivity of 20 mS / m or less, for example, rinsing water having an electrical conductivity of 20 mS / m or less. Rinsing water having an electrical conductivity of 20 mS / m or less can be obtained, for example, by using an ion exchange membrane. If the electrical conductivity of the rinsing solution used in the rinsing process is relatively high, the S—Cr layer formed on the steel sheet surface in the previous pickling process may at least partially separate from the steel sheet surface during rinsing, resulting in a redox reaction on the steel sheet surface, resulting in the formation of a relatively large amount of Cr oxide. In such cases, the value of “Cr maximum luminescence intensity / Cr luminescence intensity of base steel sheet” in the final chemical conversion coating will naturally be greater than 5.0, making it impossible to satisfy the relationship in Equation 2 above. In such cases, this indirectly indicates the presence of a region where the chemical conversion coating, known as “skid,” is not formed, resulting in a decrease in the corrosion resistance of the final steel sheet after painting. Therefore, washing with a washing solution having a lower electrical conductivity, specifically a washing solution having an electrical conductivity of 20 mS / m or less, is very important in terms of suppressing the formation of Cr oxides. From the viewpoint of further suppressing the formation of Cr oxides and therefore further reducing the value of the middle part of the above formula 2, the lower the electrical conductivity of the washing solution, the more preferable it is, specifically 10 mS / m or less.

[0060] [Chemical Conversion Treatment Step] Finally, in the chemical conversion treatment step, a chemical conversion coating is formed on the surface of the base steel sheet, for example, on at least one surface, preferably both surfaces, of the base steel sheet. The type and treatment conditions of the chemical conversion treatment are not particularly limited as long as the chemical conversion coating finally obtained satisfies the relationships of the above formulas 1 and 2. For example, the chemical conversion treatment can be carried out under any conditions suitable for forming a chemical conversion coating that contains at least one of phosphoric acid, zinc phosphate, and zirconium and satisfies the relationships of the above formulas 1 and 2.

[0061] According to this manufacturing method, in steel sheets for which improving chemical conversion treatability and, therefore, post-painting corrosion resistance is difficult due to a relatively high Cr content, by using a pickling solution containing an accelerator that is an S-containing compound at a concentration of 10 to 1500 ppm in the pickling step, controlling the line speed of the hot-rolled steel sheet through the pickling solution to be 0.10 to 10.00 m / s, and further performing the subsequent water-rinsing step with a water-rinsing solution having an electrical conductivity of 20 mS / m or less, it is possible to suppress or reduce the formation of Cr oxides on the steel sheet surface and form an S—Cr layer in place of the Cr oxides. In relation to the formation of the S—Cr layer, it is possible to obtain a chemical conversion coating that satisfies the relationship of Equation 1 and Equation 2 above after the subsequent chemical conversion treatment step. Therefore, with steel sheets manufactured according to this manufacturing method, it is possible to significantly improve the corrosion resistance of steel sheets after painting by combining the effect of improving chemical conversion treatability due to the suppression or reduction of Cr oxide formation during steel sheet manufacturing, the effect of improving Fe etching ability due to the formation of an S—Cr layer during steel sheet manufacturing, and the effect of improving the corrosion resistance of the chemical conversion coating itself due to the inclusion of S and Cr. Therefore, steel sheets manufactured by this manufacturing method can contribute to industrial development by extending the life of steel sheets used for automobiles and building materials.

[0062] The steel sheet according to the embodiment of the present invention may be used as the various automotive parts described above, for example, after a coating film is optionally formed on the surface thereof. Whether or not an automotive part having a coating film includes the steel sheet according to the embodiment of the present invention can be determined by removing the coating film from a sample taken from the automotive part. In this case, the location where the sample is taken and the coating film removal process are as follows.

[0063] [Sample collection locations] Samples will be collected from automotive parts, avoiding the following locations (i) to (iv): (i) Locations within 20 mm from the toe of spot welds and locations within 20 mm from the toe of the bead of arc / laser welds (ii) Processed areas with a curvature radius of less than 15 mm, and locations within 5 mm from such processed areas (iii) Edges within 5 mm from the cut end surface of the part (iv) Locations within 5 mm from locations where red rust is visible

[0064] [Paint Removal Process] The paint film is removed from a sample cut from an automobile body under the following conditions to expose the steel sheet. A paint remover (Neo River #160, manufactured by Sansai Kako Co., Ltd.) is applied to the surface at room temperature and allowed to stand for approximately 5 minutes. The paint film is then removed by rubbing with a hard sponge or similar (e.g., Kanefiel, manufactured by AION Co., Ltd.). The sample is then rinsed with water and dried. The remaining paint film is then confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after rinsing and drying. In the element distribution image obtained by EPMA, regions with a carbon concentration of 10% by mass or more are identified, and if the area ratio of such regions is 5% or more, it is determined that the paint film has not been sufficiently removed. To measure the area ratio of regions with a carbon concentration of 10% by mass or more, first obtain an element distribution image of carbon using an EPMA with a carbon concentration range of 10 to 30%. The obtained element distribution image is then subjected to image processing to measure the area ratio. Image analysis software "ImageJ" was used for image processing. The C element distribution image was then loaded into ImageJ, and binarized using "Make Binary" in "Binary" under "Process" so that areas with a C concentration of 10% by mass or more were displayed as black and areas with a C concentration of less than 10% by mass were displayed as white. After binarization, "Measure" under "Analyze" was used to read the value for "Area fraction" in "Results," and this value was determined as the area fraction of areas with a C concentration of 10% by mass or more. If peeling of the coating film was insufficient, removal of the coating film was repeated until the area fraction of areas with a C concentration of 10% by mass or more became less than 5%.

[0065] The present invention will be described in more detail below with reference to examples, but the following examples are merely illustrative of the present invention and are not intended to limit the present invention in any way. It goes without saying that the present invention can be modified as desired without departing from the gist of the present invention.

[0066] In the following examples, steel sheets according to the embodiments of the present invention were produced under various conditions, and the properties of the produced steel sheets were investigated.

[0067] First, molten steel was cast by continuous casting to form a steel billet having the chemical composition shown in Table 1. The steel billet was cooled once, reheated to 1200°C, and hot-rolled. Hot rolling was performed by rough rolling and finish rolling. The finish rolling temperature was 900 to 1050°C, and the finish rolling reduction was 30%. Next, the obtained hot-rolled steel sheet was appropriately subjected to skin-pass rolling at the reduction shown in Table 2, and then pickled. Pickling was performed using a pickling solution containing 10% hydrochloric acid as an acid, 0.04% IBIT 710K manufactured by Asahi Chemical Industry Co., Ltd. as an inhibitor to suppress corrosion of the base steel sheet, and thioglycolic acid as an accelerator at the concentrations shown in Table 2. The hot-rolled steel sheet was controlled to pass through the pickling solution at the line speed shown in Table 2. The temperature of the pickling solution during pickling was 85°C.

[0068] Next, the pickled hot-rolled steel sheet was rinsed with rinse water having the electrical conductivity shown in Table 2 to obtain a hot-rolled steel sheet (base steel sheet) having a thickness of 3.0 mm. Finally, a 50 mm x 50 mm sample of the obtained base steel sheet was subjected to a zinc phosphate treatment (SD5350 system: standard manufactured by Nippon Paint Industrial Coatings Co., Ltd.) as a chemical conversion treatment under the following conditions: Degreasing: Immersion in a degreaser (Fine Cleaner E2032A / B) at 40°C for 2 minutes, followed by water rinsing; Surface conditioning: Immersion in a surface conditioner (Preparen X) at room temperature for 30 seconds; Chemical conversion treatment: Immersion in a zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes, followed by water rinsing and drying.

[0069]

[0070]

[0071] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0072] [Evaluation of corrosion resistance after painting] Electrodeposition coating (Powernics Excel 1200: manufactured by Nippon Paint Industrial Coatings Co., Ltd.) was performed on steel sheet samples with a chemical conversion coating at an electrodeposition temperature of 30°C and a film thickness of 13 μm, followed by a baking treatment at 170°C for 30 minutes. Next, a saltwater immersion test (SDT) was performed on the electrodeposition-coated samples. Specifically, the electrodeposition-coated samples were immersed in a 5% NaCl aqueous solution at 50°C for 1000 hours. After the SDT test, the removed samples were dried, and then a tape peeling test was performed on one side of the sample. The peeled tape was scanned, and the area ratio of the coating peeled was calculated by binarization using the image analysis software "ImageJ." The corrosion resistance after painting was evaluated as follows. AAA: Peeled area rate less than 5% AA: Peeled area rate 5 to less than 10% A: Peeled area rate 10 to 15% B: Peeled area rate more than 15%

[0073] Steel sheets that were rated AAA, AA, or A for corrosion resistance after painting were evaluated as Cr-containing steel sheets that can exhibit improved corrosion resistance after painting. The results are shown in Table 2. In Table 2, "Ss / Sb" means the value of "maximum S emission intensity / S emission intensity of base steel sheet" according to GDS, and similarly, "Crs / Crb" means the value of "maximum Cr emission intensity / Cr emission intensity of base steel sheet" according to GDS.

[0074] Referring to Table 2, in Comparative Examples 37 and 40, the concentration of the accelerator contained in the pickling solution in the pickling step was low, which is thought to have prevented sufficient adsorption of the S component on the surface of the hot-rolled steel sheet, making it difficult to form an S—Cr layer. As a result, the value of “S maximum luminescence intensity / S luminescence intensity of base steel sheet” in the finally obtained chemical conversion coating was less than 1.1, and the corrosion resistance after painting was reduced. In Comparative Example 38, the line speed in the pickling step was low, which is thought to have prevented sufficient suppression or reduction of the formation of Cr oxides on the surface of the hot-rolled steel sheet. As a result, the value of “Cr maximum luminescence intensity / Cr luminescence intensity of base steel sheet” in the finally obtained chemical conversion coating was greater than 5.0, and the corrosion resistance after painting was reduced. In Comparative Examples 39 and 41, the washing water used in the water-rinsing step had high electrical conductivity, which is thought to have caused the S—Cr layer formed on the steel sheet surface in the previous pickling step to at least partially separate from the steel sheet surface during water-rinsing, resulting in an oxidation-reduction reaction on the steel sheet surface, resulting in the formation of a relatively large amount of Cr oxide. As a result, the value of "maximum Cr luminescence intensity / Cr luminescence intensity of base steel sheet" in the finally obtained chemical conversion coating was greater than 5.0, and the corrosion resistance after painting was reduced.

[0075] In contrast, in all of the steel sheets according to the examples, the pickling step used a pickling solution containing an accelerator that is an S-containing compound at a concentration of 10 to 1500 ppm, the hot-rolled steel sheet was controlled to pass through the pickling solution at a line speed of 0.10 to 10.00 m / s, and the subsequent water-rinsing step was performed using a water-rinsing solution having an electrical conductivity of 20 mS / m or less. This is thought to have suppressed or reduced the formation of Cr oxides on the steel sheet surface and formed an S-Cr layer in place of the Cr oxides. As a result, the maximum S and Cr luminescence intensities in the finally obtained chemical conversion coating were controlled to satisfy the relationships of the following formulas 1 and 2, respectively, and the corrosion resistance after painting was significantly improved. 1.1≦S maximum luminescence intensity / S luminescence intensity of base steel sheet≦20.0 (Equation 1) 1.1≦Cr maximum luminescence intensity / Cr luminescence intensity of base steel sheet≦5.0 (Equation 2)

[0076] In particular, in Examples 3, 4, 11, 18, 30, and 31 in which the Cr content was 0.25 mass% or more, the accelerator concentration was 150 ppm or more, the line speed was 1.00 m / s or more, and the electrical conductivity of the wash water was 10 mS / m or less, the value of "S maximum luminescence intensity / S luminescence intensity of base steel sheet" in the above formula 1 was 1.5 or more, and the value of "Cr maximum luminescence intensity / Cr luminescence intensity of base steel sheet" in the above formula 2 was 1.3 or more, and as a result, the evaluation of corrosion resistance after painting was AA, and the corrosion resistance after painting was further improved. Furthermore, in Examples 5 to 9, 12 to 16, 19 to 27, and 32 to 36, in which the Cr content was 0.50 mass% or more, the accelerator concentration was 200 ppm or more, the line speed was 1.20 m / s or more, and the electrical conductivity of the wash water was 10 mS / m or less, the value of "S maximum emission intensity / S emission intensity of base steel sheet" in the above formula 1 was 1.8 or more, and the value of "Cr maximum emission intensity / Cr emission intensity of base steel sheet" in the above formula 2 was 1.4 or more, and as a result, the evaluation of corrosion resistance after painting was AAA, and corrosion resistance after painting was further improved.

Claims

1. A steel sheet comprising a base steel sheet and a chemical conversion coating disposed on the surface of the base steel sheet, wherein the base steel sheet has a chemical composition containing, by mass%, 0.10 to 1.00% Cr, and wherein the maximum S and Cr emission intensities in the chemical conversion coating measured by a high-frequency glow discharge optical emission spectrometer (GDS) satisfy the following relationships: 1.1≦S maximum emission intensity / S emission intensity of base steel sheet≦20.0 1.1≦Cr maximum emission intensity / Cr emission intensity of base steel sheet≦5.0 2. The steel sheet according to claim 1, characterized in that the chemical composition contains, in mass %, 0.25 to 1.00% Cr, and the maximum S and Cr emission intensities in the chemical conversion coating measured by GDS satisfy the following relationships: 1.5≦maximum S emission intensity / S emission intensity of base steel sheet≦20.0 1.3≦maximum Cr emission intensity / Cr emission intensity of base steel sheet≦5.0 3. The steel sheet according to claim 2, characterized in that the chemical composition contains, in mass %, 0.50 to 1.00% Cr, and the maximum S and Cr emission intensities in the chemical conversion coating measured by GDS satisfy the following relationships: 1.8≦maximum S emission intensity / S emission intensity of base steel sheet≦20.0 1.4≦maximum Cr emission intensity / Cr emission intensity of base steel sheet≦5.0 4. The steel plate according to any one of claims 1 to 3, wherein the base steel plate has a Vickers hardness of 300 Hv or more.

5. A part, characterized in that it comprises a steel sheet according to any one of claims 1 to 4.

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