Steel sheet and component including same

Abstract

Provided is a steel sheet comprising a base material steel sheet and a chemical conversion coating provided to the surface of the base material steel sheet, and characterized in that: the base material steel sheet has a chemical composition including, in terms of mass%, 0.10-1.00% Cr; the Cr maximum emission intensity in the chemical conversion coating measured by means of a high-frequency glow discharge optical emission spectroscopy device (GDS) satisfies the relationship expressed by Cr maximum emission intensity / Cr emission intensity of base material steel sheet < 1.10; and in an X-ray diffraction of the surface of the base material steel sheet, the half-value width of a ferrite phase peak present at 2θ = 45.6 ± 0.2° is 0.3° or more. Further 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, the dislocation density in the surface layer is 2×10 13 / m 2 Furthermore, Patent Document 2 describes a steel sheet for chemical conversion treatment characterized by the above-mentioned constitution, which can provide a steel sheet with excellent chemical conversion treatability regardless of the steel type, and more specifically, a steel sheet for which the dislocation density in the surface layer of the steel sheet is 2×10 13 / m 2 When this occurs, it is taught that "voids (areas where zinc phosphate crystals are not precipitated)" will no longer be observed on the chemically treated steel sheet, regardless of the type of steel.

[0005] JP 2003-277959 A JP 2007-254811 A

[0006] For example, Patent Document 2 teaches that Cr, which is added to steel to increase its tensile strength, significantly deteriorates its chemical conversion treatability and prolongs the time required for chemical conversion treatment, depending on the amount added. On the other hand, Cr is also 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 described in Patent Document 2, 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 areas," 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 a chemical conversion coating and the surface condition of a base steel sheet on which the chemical conversion coating is applied. As a result, the present inventors discovered that the chemical conversion treatability of a steel sheet can be sufficiently improved, and thereby the post-painting corrosion resistance of the steel sheet can be significantly improved, by controlling the Cr maximum emission intensity to satisfy a predetermined relationship when the chemical conversion coating is measured using a high-frequency glow discharge optical emission spectrometer (GDS), and by controlling the peak derived from the ferrite phase to satisfy predetermined characteristics in the X-ray diffraction of the surface of the base steel sheet on which the chemical conversion coating is applied, and thus completing the present invention.

[0009] The present invention has achieved the above-mentioned object 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, the maximum Cr emission intensity in the chemical conversion coating measured by a high-frequency glow discharge optical emission spectrometer (GDS) satisfies the relationship Cr maximum emission intensity / Cr emission intensity of base steel sheet < 1.10, and wherein, in an X-ray diffraction analysis of the surface of the base steel sheet, the half-width of a peak derived from a ferrite phase at 2θ = 45.6 ± 0.2° is 0.3° or more. (2) The steel sheet according to (1) above, wherein, in an X-ray diffraction analysis of the surface of the base steel sheet, the half-width of a peak derived from a ferrite phase at 2θ = 45.6 ± 0.2° is 0.5° or more. (3) The steel sheet according to (2) above, characterized in that in X-ray diffraction of the surface of the base steel sheet, the half-width of a peak derived from a ferrite phase present at 2θ = 45.6 ± 0.2° is 0.7° or more. (4) The steel sheet according to any one of (1) to (3) above, characterized in that the chemical composition includes, by mass%, Cr: 0.25 to 1.00%. (5) The steel sheet according to (4) above, characterized in that the chemical composition includes, by mass%, Cr: 0.50 to 1.00%. (6) The steel sheet according to any one of (1) to (5) above, characterized in that the chemical composition includes, by mass%, Ti: 0.02 to 1.00%. The steel sheet according to any one of (1) to (5) above, characterized in that the maximum Ti emission intensity in the chemical conversion coating measured by GDS satisfies the relationship Ti maximum emission intensity / Ti emission intensity of base steel sheet < 1.10. (7) The steel sheet according to any one of (1) to (6) above, characterized in that the base steel sheet has a Vickers hardness of 300 Hv or more. (8) A part, characterized in that it includes the steel sheet according to any one of (1) to (7) above.

[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 Plate> A steel plate according to an embodiment of the present invention comprises a base steel plate and a chemical conversion treatment film disposed on a surface of the base steel plate, wherein the base steel plate has a chemical composition including, by mass%, 0.10 to 1.00% Cr, the maximum Cr emission intensity of the chemical conversion treatment film measured with a high-frequency glow discharge optical emission spectrometer (GDS) satisfies the relationship: Cr maximum emission intensity / Cr emission intensity of base steel plate < 1.10, and in X-ray diffraction of the surface of the base steel plate, the half-width of a peak derived from a ferrite phase present at 2θ = 45.6 ± 0.2° is 0.3° or more.

[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 first conducted research focusing on the components contained in the 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, when the chemical conversion coating is measured using a high-frequency glow discharge optical emission spectrometer (GDS), the chemical conversion treatability of steel sheets can be improved by controlling the maximum Cr emission intensity so as to satisfy a predetermined relationship, more specifically, by controlling the maximum Cr emission intensity so as to satisfy the relationship of the following formula 1: maximum Cr emission intensity / Cr emission intensity of base steel sheet<1.10 ... formula 1

[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, 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 performing shot blasting 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 sufficiently remove Cr oxides formed on the surface of the steel sheet during steel sheet manufacturing and to inhibit or reduce the formation of new Cr oxides on the steel sheet surface in the subsequent water-rinsing step, and in connection with this, it is possible to control the maximum Cr luminescence intensity in the finally obtained chemical conversion coating so as to satisfy the relationship in the above-mentioned formula 1. As a result, the present inventors have discovered that the corrosion resistance of the steel sheet after painting can be improved.

[0015] If Cr oxides are formed on the steel sheet surface, naturally, Cr will also be incorporated into the chemical conversion coating after chemical conversion treatment. Therefore, the fact that the maximum Cr luminescence intensity in the chemical conversion coating satisfies the above formula 1, i.e., the fact that even the highest Cr luminescence intensity in the chemical conversion coating is controlled to less than 1.10 times the Cr luminescence intensity of the base steel sheet, suggests that the formation of Cr oxides on the steel sheet surface during steel sheet production is sufficiently suppressed or reduced. In other words, the fact that the maximum Cr luminescence intensity in the chemical conversion coating satisfies the above formula 1 suggests that the concentration of Cr on the steel sheet surface is sufficiently suppressed or reduced, and therefore suggests an improvement in the chemical treatability of the steel sheet.

[0016] However, subsequent studies by the present inventors revealed that even when the Cr maximum emission intensity of a chemical conversion coating satisfies the above formula 1, the chemical conversion treatability may not be sufficiently improved, and therefore the corrosion resistance after painting may not be sufficiently improved. Therefore, the present inventors then conducted further studies, focusing on the surface condition of the base steel sheet on which the chemical conversion coating is applied. As a result, the present inventors discovered that the chemical conversion treatability of a steel sheet can be further improved by controlling the peak derived from the ferrite phase in the X-ray diffraction of the surface of the base steel sheet to satisfy predetermined characteristics, more specifically, by controlling the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° in the X-ray diffraction of the surface of the base steel sheet to be 0.3° or more.

[0017] More specifically, if the lattice spacing of the ferrite phase were exactly the same, the resulting diffraction peak would have no width. Therefore, the half-width of the diffraction peak is considered to represent the degree of variation in the lattice spacing of the ferrite phase, i.e., the degree of strain in the ferrite phase. In relation to this, the inventors have found that in X-ray diffraction of the surface of a base steel sheet, the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° more clearly shows a correlation with the strain state of the base steel sheet surface, and the greater the strain introduced into the surface of the base steel sheet, the greater the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2°. Here, when the strain introduced into the surface of the base steel sheet increases, naturally, more dislocations are introduced into the surface of the base steel sheet, and subgrain boundaries may be formed due to dislocation rearrangement. While not intending to be bound by any particular theory, it is believed that corrosion generally occurs easily at linear defects such as dislocations and planar defects such as grain boundaries, and therefore, by introducing a large amount of strain into the surface of the base steel sheet to introduce dislocations or form subgrain boundaries, it is possible to promote the anodic dissolution (etching) of Fe during chemical conversion treatment. From this perspective, the present inventors conducted further studies focusing on the introduction of strain into the surface of the base steel sheet. As a result, the present inventors found that by appropriately controlling the shot blasting amount described above in relation to the removal of Cr oxides, it is possible not only to remove Cr oxides formed on the steel sheet surface but also to introduce sufficient strain into the steel sheet surface to promote the etching of Fe during chemical conversion treatment. More specifically, the inventors have found that by appropriately controlling the amount of shot blasting, it is possible to control the half-width of the peak derived from the ferrite phase present at 2θ = 45.6 ± 0.2° in the X-ray diffraction of the steel sheet surface to 0.3° or more, and that the strain state corresponding to this half-width improves the etching ability of Fe, thereby improving the chemical conversion treatability of the steel sheet.

[0018] According to the steel sheet according to the embodiment of the present invention, the phosphatability of the steel sheet can be further improved by a combination of the suppression or reduction of Cr oxide formation during steel sheet production (i.e., the effect of improving phosphatability based on the control of the above formula 1) and the control of the strain introduced into the base steel sheet surface (i.e., the effect of improving Fe etching ability based on the control of the half-width of the peak derived from the ferrite phase in X-ray diffraction). As a result, the corrosion resistance of the steel sheet after painting can be significantly improved. Here, even for steel sheets containing relatively high amounts of Cr, the phosphatability of the steel sheet can be further improved by appropriately controlling the Cr maximum emission intensity in the phosphatase coating and the half-width of a specific diffraction peak on the base steel sheet surface, thereby significantly improving the corrosion resistance of the steel sheet after painting. This fact was not previously known, and has now been discovered for the first time by the present inventors. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where excellent phosphatability and / or corrosion resistance after painting are required. Below, each component of the steel sheet according to the embodiment of the present invention will be described in more detail.

[0019] [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.

[0020] The chemical conversion coating is not particularly limited as long as the maximum Cr emission intensity measured by GDS measurement satisfies the relationship of Equation 1 above, 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, the Cr content in the chemical conversion coating is limited within a range in which the maximum Cr emission intensity measured by GDS measurement satisfies the relationship of Equation 1 above. This makes it possible to achieve improved chemical treatability by suppressing or reducing the formation of Cr oxide during steel sheet production, regardless of the type of chemical conversion coating. For example, the chemical conversion coating may contain phosphoric acid or a phosphate salt, and the elements constituting the salt are elements of 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 may contain 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 chemical conversion coatings containing 50 mass% or more, preferably 90 mass% or more, of zinc phosphate. The chemical conversion coating may optionally contain other elements in addition to the above-mentioned phosphoric acid, zinc phosphate, zirconium, etc., such as at least one of Fe, Cr, Mn, Ti, Ni, Mg, Ca, and V, each in a maximum of 10 mass%, preferably 5 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 optical emission analyzer (ICP-AES measurement).

[0021] [Cr maximum emission intensity / Cr emission intensity of base steel sheet<1.10] In an embodiment of the present invention, when the steel sheet is measured using a high-frequency glow discharge optical emission spectrometer (GDS), the Cr maximum emission intensity of the chemical conversion coating satisfies the relationship of the following formula 1: Cr maximum emission intensity / Cr emission intensity of base steel sheet<1.10 ... formula 1

[0022] As described above, in the steel sheet according to the embodiment of the present invention, in connection with the suppression or reduction of the formation of Cr oxides on the steel sheet surface during steel sheet production, it is possible to control the chemical composition of the chemical conversion coating so that the maximum Cr luminescence intensity satisfies the relationship of the above formula 1. Therefore, conversely speaking, by controlling the chemical composition of the chemical conversion coating so that the maximum Cr luminescence intensity satisfies the relationship of the above formula 1, it is possible to obtain an effect of improving chemical conversion treatability based on the suppression or reduction of the formation of Cr oxides on the steel sheet surface during steel sheet production. As a result, it is possible to improve the corrosion resistance of the steel sheet after painting.

[0023] From the viewpoint of further improving the corrosion resistance of the steel sheet after painting, the smaller the value of the left side of the above formula 1, i.e., "Cr maximum emission intensity / Cr emission intensity of base steel sheet," the more preferable. For example, Cr maximum emission intensity / Cr emission intensity of base steel sheet may be 1.05 or less (i.e., Cr maximum emission intensity / Cr emission intensity of base steel sheet≦1.05, the same applies below), 1.00 or less, 0.95 or less, or 0.90 or less. There is no particular restriction on the lower limit, but for example, Cr maximum emission intensity / Cr emission intensity of base steel sheet may be 0.60 or more (i.e., 0.60≦Cr maximum emission intensity / Cr emission intensity of base steel sheet, the same applies below), 0.70 or more, 0.80 or more, or 0.85 or more.

[0024] [Measurement of Cr Maximum Emission Intensity by GDS / Cr Emission Intensity of Base Steel Sheet] The Cr maximum emission intensity by GDS / Cr emission intensity of base steel sheet is 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 intensity of the identified elements is 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.

[0025] When GDS measurement is performed in this manner from the steel sheet surface in the depth direction, the chemical conversion coating is determined to be a region where the emission intensity of the element that primarily constitutes the chemical conversion coating (for example, 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 (for example, 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). Next, for the chemical conversion coating determined in this manner, the maximum value of a graph obtained by smoothing the 10-point average of the Cr emission intensities obtained by GDS measurement in the depth direction is determined to be the "maximum Cr emission intensity." Meanwhile, the average Cr emission intensity is calculated within a depth range where the Cr emission intensity is sufficiently stable. For example, the average Cr emission intensity is calculated within a region 100 to 150 μm from the steel sheet side of the chemical conversion coating, and this is determined as the "Cr emission intensity of the base steel sheet." Finally, the value of "Cr maximum emission intensity / Cr emission intensity of the base steel sheet" is determined based on the "Cr maximum emission intensity" and "Cr emission intensity of the base steel sheet" obtained as described above. The depth in the GDS measurement is determined from the sputtering time. Specifically, the depth of the GDS mark (indentation) formed by the 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 per sputtering time from the steel sheet side of the chemical conversion coating is determined by multiplying the depth per sputtering time by the sputtering time required to measure the depth from the steel sheet side of the chemical conversion coating. The depth of the GDS mark 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 is measured in the diameter direction so as to cross the GDS mark (center of the circle ±0.5 mm).

[0026] [Half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° in X-ray diffraction of the surface of the base steel sheet: 0.3° or more] In an embodiment of the present invention, in X-ray diffraction of the surface of the base steel sheet, the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° is controlled to be 0.3° or more. As described above, the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° is correlated with the strain state of the surface of the base steel sheet, and the larger the strain introduced into the surface of the base steel sheet, the larger the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2°. When the strain introduced into the surface of the base steel sheet becomes large, many dislocations are introduced into the surface of the base steel sheet, and subgrain boundaries may be formed due to rearrangement of dislocations. In an embodiment of the present invention, by controlling the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° to 0.3° or more, a large amount of strain can be introduced into the surface of the base steel sheet, thereby introducing dislocations that are prone to corrosion into the surface of the base steel sheet, and forming subgrain boundaries that are similarly prone to corrosion, thereby enabling the etching of Fe to be promoted during chemical conversion treatment. Therefore, according to an embodiment of the present invention, the combination of the effect of improving Fe etching ability based on such control of the half-width of the peak derived from the ferrite phase in X-ray diffraction and the effect of improving chemical conversion treatability based on the control of Equation 1 described above can further improve the chemical conversion treatability of the steel sheet, and as a result, the corrosion resistance of the steel sheet after painting can be significantly improved.

[0027] From the viewpoint of further improving the etching property of Fe during chemical conversion treatment and therefore more significantly improving the corrosion resistance of the steel sheet after painting, the larger the half-width of the peak derived from the ferrite phase present at 2θ = 45.6 ± 0.2°, the more preferable, and it may be, for example, 0.4° or more, 0.5° or more, 0.6° or more, 0.7° or more, 0.8° or more, 1.0° or more, 1.2° or more, 1.5° or more, 2.0° or more, or 3.0° or more. The upper limit is not particularly limited, and the half-width of the peak derived from the ferrite phase may be, for example, 12.0° or less, 10.0° or less, 8.0° or less, 6.0° or less, or 5.5° or less.

[0028] [Measurement of the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° in X-ray diffraction] There are three typical peaks observed in the X-ray diffraction chart of the ferrite phase. These are the (110) peak observed near 45 degrees, the (200) peak observed near 65 degrees, and the (211) peak observed near 82 degrees. Therefore, if a strong peak is observed near 45 degrees, it can be determined to be the (110) peak derived from the ferrite phase. The half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° in X-ray diffraction is measured as follows. First, if it is necessary to remove the chemical conversion coating to perform X-ray diffraction measurement, the chemical conversion coating is removed by an appropriate method. For example, when the chemical conversion coating is a zinc phosphate coating, the chemical conversion coating is removed from the steel sheet in accordance with JIS K 3151:1996, and then an X-ray diffraction pattern of the surface of the base steel sheet is obtained using an X-ray diffractometer (for example, "Ultima III" manufactured by Rigaku) ​​under the conditions described below. The width of the peak at half the height of the peak derived from the ferrite phase present at 2θ=45.6±0.2° is measured from the obtained X-ray diffraction pattern, thereby calculating the half-value width of the peak.

[0029]

[0030] [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 Cr maximum emission intensity of the chemical conversion coating measured by GDS to satisfy the relationship Cr maximum emission intensity / Cr emission intensity of the base steel sheet < 1.10, and by controlling the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° in X-ray diffraction of the surface of the base steel sheet to 0.3° or more. Therefore, the chemical composition of the base steel sheet is not particularly limited other than 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 objectives of the present invention. The chemical composition of the base steel sheet can include, in addition to Cr, appropriate amounts of any alloying elements 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.

[0031] 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 1.00%, 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.

[0032] [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.

[0033] [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.

[0034] [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.

[0035] [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.

[0036] [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.

[0037] [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.

[0038] [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, 0.0005% 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.

[0039] [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.

[0040] 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.

[0041] [Ti: 0 to 1.00%] Ti forms carbonitrides and the like in steel, improving the strength of the steel sheet through precipitation strengthening. While the Ti content may be 0%, to achieve this effect, the Ti content is preferably 0.001% or more, and may be 0.01% or more, 0.02% or more, 0.05% or more, 0.10% or more, or 0.12% or more. On the other hand, even if Ti is contained in an excessive amount, the effect saturates, and adding more Ti than necessary to the steel increases manufacturing costs. Therefore, the Ti content is preferably 1.00% or less, and may be 0.80% or less, 0.60% or less, 0.50% or less, 0.40% or less, 0.30% or less, 0.20% or less, or 0.15% or less.

[0042] [Nb: 0 to 0.150%] [V: 0 to 0.150%] Nb and V form carbonitrides in steel and have the effect of improving the strength of the steel sheet through precipitation strengthening. The Nb and V contents may be 0%, but to obtain such effects, the 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 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.

[0043] [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.

[0044] [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.

[0045] [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.

[0046] [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.

[0047] [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.

[0048] [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.

[0049] [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.

[0050] 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.

[0051] 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.

[0052] [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.

[0053] [Maximum Ti emission intensity / Ti emission intensity of base steel sheet<1.10] In a preferred embodiment of the present invention, when the steel sheet is measured using a high-frequency glow discharge optical emission spectrometer (GDS), the maximum Ti emission intensity of the chemical conversion coating satisfies the relationship of the following formula 2: Maximum Ti emission intensity / Ti emission intensity of base steel sheet<1.10 ... formula 2

[0054] In the production of steel sheets containing Ti, for example, steel sheets containing Ti in an amount of 0.02 to 1.00 mass%, as in the case of Cr, Ti oxides, nitrides, etc. may form on the surface of the steel sheet during the hot rolling process, resulting in a concentration of Ti on the surface of the steel sheet. Although the effect is smaller than in the case of Cr, such concentration of Ti may somewhat reduce the chemical conversion treatability. Therefore, from the viewpoint of further improving the chemical conversion treatability of steel sheets, it is preferable to sufficiently suppress or reduce the Ti oxides and nitrides formed on the steel sheet surface during steel sheet production, and as a result, it becomes possible to control the maximum Ti emission intensity in the finally obtained chemical conversion coating so as to satisfy the above formula 2.

[0055] From the viewpoint of further improving the chemical conversion treatability of the steel sheet and therefore more significantly improving the corrosion resistance of the steel sheet after painting, the smaller the value of the left side of the above formula 2, i.e., "Ti maximum luminescence intensity / Ti luminescence intensity of base steel sheet," the more preferable. For example, Ti maximum luminescence intensity / Ti luminescence intensity of base steel sheet may be 1.08 or less (i.e., Ti maximum luminescence intensity / Ti luminescence intensity of base steel sheet≦1.08, the same applies below), 1.05 or less, 1.00 or less, or 0.95 or less. There is no particular lower limit, but for example, Ti maximum luminescence intensity / Ti luminescence intensity of base steel sheet may be 0.60 or more (i.e., 0.60≦Ti maximum luminescence intensity / Ti luminescence intensity of base steel sheet, the same applies below), 0.70 or more, 0.80 or more, or 0.90 or more.

[0056] [Measurement of Maximum Ti Emission Intensity by GDS / Ti Emission Intensity of Base Steel Sheet] The maximum Ti emission intensity by GDS / Ti emission intensity of base steel sheet is measured as follows. First, using a high-frequency glow discharge optical emission spectrometer (e.g., LECO Japan, G.K., Model No. "GDS850A"), the surface of the steel sheet on which the chemical conversion coating is applied is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma. The steel sheet surface is then sputtered and analyzed in the depth direction. The 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 intensity of the identified elements is estimated. Depth data can be estimated from the sputtering time. Specifically, the relationship between sputtering time and sputtering depth can be determined in advance using a standard sample, allowing the sputtering time to be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The obtained emission intensity is converted to mass % by creating a calibration curve. The measurement conditions are as follows. The calibration curve is corrected using a standard sample (Brammer, BSH-1B). Ar gas pressure: 0.3 MPa, anode diameter: 4 mmφ, RF output: 30 W, measurement time: 200 to 1500 seconds

[0057] When GDS measurement is performed in this manner from the steel sheet surface in the depth direction, the chemical conversion coating is determined to be a region where the emission intensity of the element that primarily constitutes the chemical conversion coating (for example, 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 (for example, 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). Next, for the chemical conversion coating determined in this manner, the maximum value in a graph obtained by smoothing the 10-point average of the Ti emission intensities obtained by GDS measurement in the depth direction is determined to be the "maximum Ti emission intensity." On the other hand, the average value of the luminescence intensity of Ti is calculated within a depth range where the luminescence intensity is sufficiently stable, for example, the average value of the luminescence intensity of Ti in a region from 150 to 200 μm from the surface of the chemical conversion coating on the steel sheet side is calculated, and this is determined as the “Ti luminescence intensity of the base steel sheet.” Finally, based on the “maximum Ti luminescence intensity” and “Ti luminescence intensity of the base steel sheet” obtained as described above, the value of “maximum Ti luminescence intensity / Ti luminescence intensity of the base steel sheet” is determined.

[0058] [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.

[0059] [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.

[0060] According to the steel sheet according to the embodiment of the present invention, as described above, even for steel sheets containing a relatively high amount of Cr, the chemical conversion treatability of the steel sheet can be further improved by appropriately controlling the Cr maximum emission intensity in the chemical conversion coating and the half-value width of a specific diffraction peak on the surface of the base steel sheet, 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.

[0061] <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.

[0062] The steel sheet according to the embodiment of the present invention can be manufactured by, for example, 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 shot blasting process in which the hot-rolled steel sheet is shot-blasted, a water-rinsing process in which the shot-blasted 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 an annealing process may be performed after the water-rinsing process. Each process will be described in detail below.

[0063] [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 method such as ordinary continuous casting or casting by an ingot method.

[0064] [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%.

[0065] [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 at a reduction rate of 0.6% or more, it is possible to accelerate the removal of Cr oxides and the like in the subsequent shot blasting process, compared to when skin-pass rolling is not performed. More specifically, by performing skin-pass rolling at a reduction rate of 0.6% or more, it is possible to introduce cracks into the scale formed on the steel sheet surface in the hot rolling process, more specifically, into the Fe scale and the subscale formed between the Fe scale and the steel sheet surface. Here, the subscale is mainly composed of oxides of Si, Cr, Ti, and the like. By introducing cracks not only into the Fe scale but also into the subscale formed directly below it, the Fe scale and subscale can be sufficiently and reliably crushed and removed by shot blasting in the subsequent shot blasting process, starting from the cracks. On the other hand, if skin-pass rolling is not performed or the reduction rate is less than 0.6%, and therefore cracks are not sufficiently introduced into the Fe scale and subscale, the Fe scale is removed by shot blasting in the subsequent shot blasting step, but the subscale formed directly below it is not necessarily sufficiently removed, and some of it may remain on the steel sheet surface. Without intending to be bound by any particular theory, this phenomenon is thought to be caused by the difference in intensity between the Fe scale and the subscale. In any case, if some of the subscale containing oxides of Cr, Ti, etc. remains, the values ​​of "maximum Cr luminescence intensity / Cr luminescence intensity of base steel sheet" and / or "maximum Ti luminescence intensity / Ti luminescence intensity of base steel sheet" may become somewhat high in the finally obtained chemical conversion coating.

[0066] Therefore, from the viewpoint of further improving the chemical conversion treatability of the steel sheet by sufficiently reducing the values ​​of "maximum Cr luminescence intensity / Cr luminescence intensity of base steel sheet" and "maximum Ti luminescence intensity / Ti luminescence intensity of base steel sheet" and therefore more significantly improving the corrosion resistance of the steel sheet after painting, it is preferable to carry out skin-pass rolling at a reduction rate of 0.6% or more, and more preferably at a reduction rate of 1.0% or more.

[0067] [Shot blasting 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 shot blasted at 10 to 1000 kg / m 2 The shot blasting treatment is carried out with a shot amount of 10 to 1000 kg / m. 2 By controlling the shot blasting rate to 10 to 1000 kg / m, it is possible not only to remove scale formed on the steel sheet surface, more specifically, subscale containing Fe scale and Cr oxides, but also to introduce strain into the steel sheet surface sufficient to promote etching of Fe during chemical conversion treatment. 2 By controlling the strain to this value, the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° in the X-ray diffraction of the steel sheet surface can be controlled to 0.3° or more, and the strain state corresponding to this half-width improves the etching ability of Fe during chemical conversion treatment, thereby improving the chemical conversion treatability of the steel sheet. Therefore, according to this production method, the chemical conversion treatability of the steel sheet can be further improved by combining the suppression or reduction of Cr oxide formation during steel sheet production, i.e., the effect of improving chemical conversion treatability based on the control of the following formula 1, with the control of the strain introduced into the base steel sheet surface, i.e., the effect of improving Fe etching ability based on the control of the half-width of the peak derived from the ferrite phase in the X-ray diffraction, and as a result, the corrosion resistance of the steel sheet after painting can be significantly improved. Cr maximum emission intensity / Cr emission intensity of base steel sheet < 1.10 ... formula 1

[0068] Without shot blasting, it is not possible to introduce strain sufficient to promote Fe etching during chemical conversion treatment into the steel sheet surface, and it is also not possible to sufficiently remove Fe scale and subscale containing Cr oxides formed on the steel sheet surface during steel sheet production. In relation to this, it is not possible to control the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° in the X-ray diffraction of the base steel sheet surface to 0.3° or more, and it is also not possible to control the maximum Cr emission intensity of the final chemical conversion coating to satisfy the relationship in Equation 1 above. As a result, the chemical conversion treatability of the steel sheet is reduced, and therefore the desired post-painting corrosion resistance cannot be obtained. Therefore, the shot blasting step is an essential step in this manufacturing method. In order to remove Fe scale and subscale containing Cr oxides and introduce strain sufficient to promote Fe etching during chemical conversion treatment into the steel sheet surface, the shot dose in the shot blasting step is set to 10 kg / m. 2 From the viewpoint of further enhancing the effect of improving the etching property of Fe during the chemical conversion treatment, the larger the amount of shots in the shot blasting step, the more preferable it is, for example, 50 kg / m 2 or more than 100 kg / m 2 It is preferable that the resistance is 200 kg / m or more. 2 Although there is no particular upper limit, if the blasting amount is too large, the Fe scale and subscale can be sufficiently removed and sufficient strain can be introduced into the steel sheet surface to promote etching of Fe, but the surface quality of the base steel sheet may deteriorate. Therefore, the blasting amount is preferably 1000 kg / m or more. 2 Preferably, it is 800 kg / m or less. 2 or less than 500 kg / m 2 The following is the result.

[0069] [Rinsing Process] The shot-blasted hot-rolled steel sheet is then rinsed in the rinsing process. This rinsing process is performed 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 rinsing solution used in the rinsing process has a relatively high electrical conductivity, a redox reaction may occur on the steel sheet surface from which Cr oxides and the like have been removed in the previous shot blasting process, resulting in the formation of a relatively large amount of new Cr oxides. 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 1 above. In such cases, this can be said to indirectly indicate the presence of areas where the chemical conversion coating is not formed, known as "whiteout," and as a result, the corrosion resistance of the final steel sheet after painting will be reduced. 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 left side of the above formula 1, the lower the electrical conductivity of the washing solution, the more preferable it is, specifically, 10 mS / m or less.

[0070] [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 finally obtained chemical conversion treatment coating satisfies the relationship of the above formula 1 and / or formula 2. For example, the chemical conversion treatment can be carried out under any conditions suitable for forming a chemical conversion treatment coating that contains at least one of phosphoric acid, zinc phosphate, and zirconium and satisfies the relationship of the above formula 1 and / or formula 2.

[0071] According to this manufacturing method, in the case of steel sheets for which it is difficult to improve chemical conversion treatability and therefore corrosion resistance after painting due to the relatively high Cr content, it is possible to improve the corrosion resistance of steel sheets by a shot blasting process of 10 to 1000 kg / m 2

[0043] By performing shot blasting with a shot volume of 0.01g / cm² and then 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, thereby obtaining a chemical conversion coating that satisfies the relationship in Equation 1, and to introduce strain into the steel sheet surface sufficient to promote Fe etching during chemical conversion. Therefore, with a steel sheet manufactured according to this manufacturing method, the chemical conversion treatability of the steel sheet can be further improved by combining the suppression or reduction of Cr oxide formation during steel sheet manufacturing (i.e., the effect of improving chemical conversion treatability based on the control of Equation 1) with the control of strain introduced into the base steel sheet surface (i.e., the effect of improving Fe etching based on the control of the half-width of the ferrite phase-derived peak in X-ray diffraction), thereby significantly improving the corrosion resistance of the steel sheet after painting. Therefore, a steel sheet manufactured by this manufacturing method can contribute to industrial development by extending the service life of steel sheets for automobiles and building materials.

[0072] 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.

[0073] [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

[0074] [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%.

[0075] [Chemical Conversion Coating Removal Process] If the chemical conversion coating needs to be removed from a sample cut from an automobile body and the paint removed in order to perform X-ray diffraction measurement, the chemical conversion coating is removed using an appropriate method. For example, if the chemical conversion coating is a zinc phosphate coating, the chemical conversion coating is removed from the sample in accordance with JIS K 3151:1996. Specifically, the sample is immersed in a 5% chromic acid solution heated to 75°C for 15 minutes to remove the chemical conversion coating. The sample is then rinsed and dried. The remaining state of chemical conversion crystals is 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 P concentration of 5% by mass or more are identified, and if the area ratio of these regions is 5% or more, it is determined that the chemical conversion coating has not been sufficiently removed. To measure the area ratio of regions with a P concentration of 5% by mass or more, an element distribution image of P is first obtained using EPMA with a P concentration range of 5 to 10%. Next, the area ratio is measured by image processing the obtained element distribution image. Image analysis software "ImageJ" is used for image processing. After loading the P element distribution image into ImageJ, it is binarized in "Make Binary" under "Binary" in "Process" so that areas with a P concentration of 5% by mass or more are displayed as black and areas with a P concentration of less than 5% by mass are displayed as white. After binarization, "Measure" under "Analyze" is used to read the value of "Area fraction" in "Results," and this value is determined as the area ratio of areas with a P concentration of 5% by mass or more. If peeling of the chemical conversion coating is insufficient, removal of the chemical conversion coating is repeated until the area ratio of areas with a P concentration of 5% by mass or more becomes less than 5%.

[0076] 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.

[0077] 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.

[0078] First, molten steel was cast by a continuous casting method to form a steel slab having the chemical composition shown in Table 2. The steel slab was once cooled, reheated to 1200°C, and hot rolled. Hot rolling was performed by rough rolling and finish rolling, with the finish rolling ending at a temperature of 900 to 1050°C and a finish rolling reduction of 30%. Next, the obtained hot-rolled steel sheet was appropriately subjected to skin-pass rolling at the reduction shown in Table 3, and then subjected to shot blasting. Shot blasting was performed using a shot material (TSH30 manufactured by IKK Shot Co., Ltd.) at the shot amount shown in Table 3.

[0079] Next, the shot-blasted hot-rolled steel sheet was rinsed with rinse water having the electrical conductivity shown in Table 3 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.

[0080]

[0081]

[0082] [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 10 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 paint 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%

[0083] 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 3. In Table 3, "Crs / Crb" means the value of "maximum Cr luminescence intensity / Cr luminescence intensity of base steel sheet" according to GDS, and similarly, "Tis / Tib" means the value of "maximum Ti luminescence intensity / Ti luminescence intensity of base steel sheet" according to GDS.

[0084] Referring to Table 3, in Comparative Examples 38 and 40, shot blasting was performed in the shot blasting step, but the amount of shot material projected was small. Therefore, although the formation of Cr oxides on the steel sheet surface could be suppressed or reduced, sufficient strain could not be introduced into the surface of the hot-rolled steel sheet. As a result, although the Cr maximum emission intensity in the finally obtained chemical conversion coating could be controlled to satisfy the relationship in the following formula 1, the half-value width of the peak derived from the ferrite phase present at 2θ = 45.6 ± 0.2° in the X-ray diffraction of the surface of the base steel sheet was less than 0.3°, and the corrosion resistance after painting was reduced. Cr maximum emission intensity / Cr emission intensity of base steel sheet < 1.10 ... formula 1

[0085] In Comparative Examples 39 and 41, the washing water used in the washing step had high electrical conductivity, which is thought to have caused an oxidation-reduction reaction to occur on the steel sheet surface from which Cr oxides and the like had been removed in the previous shot blasting step, resulting in the formation of relatively large amounts of new Cr oxides. 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 1.10 or more, and the corrosion resistance after painting was reduced.

[0086] In Comparative Example 42, the shot blasting treatment was not performed in the shot blasting step, and therefore the formation of Cr oxides on the steel sheet surface could not be suppressed or reduced, and sufficient strain could not be introduced into 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 1.10 or more, and further, in the X-ray diffraction of the surface of the base steel sheet, the half-value width of the peak derived from the ferrite phase present at 2θ = 45.6 ± 0.2° was less than 0.3°, and the corrosion resistance after painting was reduced.

[0087] In contrast, in the steel plates according to all the examples, the shot blasting process was carried out at a pressure of 10 to 1000 kg / m 2 and the subsequent rinsing step was carried out with a rinsing solution having an electrical conductivity of 20 mS / m or less, which is believed to have suppressed or reduced the formation of Cr oxides on the steel sheet surface and introduced strain into the steel sheet surface sufficient to promote Fe etching during chemical conversion treatment. As a result, the maximum Cr emission intensity in the finally obtained chemical conversion treatment film could be controlled to satisfy the relationship of Equation 1 above, and further, the half-width of the peak derived from the ferrite phase present at 2θ = 45.6 ± 0.2° in the X-ray diffraction of the surface of the base steel sheet could be controlled to 0.3° or more, thereby significantly improving the corrosion resistance after painting.

[0088] In particular, the shot amount in the shot blasting process is 100 kg / m 2It is believed that in Examples 2, 11, and 18, where the reduction ratio was 0.5%, the half-width of the peak derived from the ferrite phase at 2θ = 45.6 ± 0.2° in the X-ray diffraction of the surface of the base steel sheet was 0.5°, and as a result, the corrosion resistance after painting was evaluated as AA, further improving the corrosion resistance after painting. Similarly, in Examples 3, 12, 19, and 31, where skin-pass rolling was performed at a reduction ratio of 0.6%, it is believed that in the subsequent shot blasting process, not only Fe scale but also subscale containing Ti oxides and nitrides was sufficiently removed. In relation to this, the maximum Ti emission intensity of the finally obtained chemical conversion coating was able to be controlled to satisfy the relationship of the following formula 2, and as a result, the corrosion resistance after painting was evaluated as AA, further improving the corrosion resistance after painting. Ti maximum emission intensity / Ti emission intensity of base steel sheet < 1.10 ... formula 2

[0089] Skin pass rolling is carried out at a reduction rate of 0.6% or more, and the shot amount in the shot blasting process is 200 kg / m 2 In the above Examples 4 to 9, 13 to 16, 20 to 28, and 32 to 37, the maximum Ti emission intensity in the finally obtained chemical conversion coating could be controlled to satisfy the relationship of the above formula 2, and furthermore, in the X-ray diffraction of the surface of the base steel sheet, the half-value width of the peak derived from the ferrite phase present at 2θ = 45.6 ± 0.2° was 0.7° or more, and as a result, the corrosion resistance after painting was rated AAA, and the corrosion resistance after painting could be further improved.

Claims

1. A steel plate comprising a base steel plate and a chemical conversion coating disposed on the surface of the base steel plate, wherein the base steel plate has a chemical composition containing, by mass, 0.10 to 1.00% Cr, the maximum Cr emission intensity in the chemical conversion coating measured using a high-frequency glow discharge optical emission spectrometer (GDS) satisfies the relationship Cr maximum emission intensity / Cr emission intensity of base steel plate < 1.10, and in X-ray diffraction of the surface of the base steel plate, the half-width of a peak derived from a ferrite phase present at 2θ = 45.6 ± 0.2° is 0.3° or more.

2. The steel sheet according to claim 1, characterized in that in X-ray diffraction of the surface of the base steel sheet, the half-value width of the peak at 2θ = 45.6 ± 0.2° derived from the ferrite phase is 0.5° or more.

3. The steel sheet according to claim 2, wherein in X-ray diffraction of the surface of the base steel sheet, the half-value width of the peak at 2θ = 45.6 ± 0.2° derived from the ferrite phase is 0.7° or more.

4. A steel plate according to any one of claims 1 to 3, characterized in that the chemical composition contains, in mass %, Cr: 0.25 to 1.00%.

5. The steel sheet according to claim 4, characterized in that the chemical composition contains, in mass %, Cr: 0.50 to 1.00%.

6. The steel sheet according to any one of claims 1 to 5, characterized in that the chemical composition contains, in mass %, 0.02 to 1.00% Ti, and the maximum Ti luminescence intensity in the chemical conversion coating measured by GDS satisfies the relationship: maximum Ti luminescence intensity / Ti luminescence intensity of base steel sheet < 1.

10.

7. The steel plate according to any one of claims 1 to 6, wherein the base steel plate has a Vickers hardness of 300 Hv or more.

8. A part, characterized in that it comprises a steel sheet according to any one of claims 1 to 7.

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