Surface-treated steel sheets and parts containing the same

Abstract

Disclosed is a surface-treated steel sheet containing Ti and Al, and having improved post-paint corrosion resistance. The surface-treated steel sheet of this disclosure comprises a base steel sheet and a chemical conversion coating disposed on the surface of the base steel sheet. The chemical composition of the base steel sheet is, by mass%, Ti: 0.050 to 1.000% and Al: 0.050 to 2.000%. When elemental analysis of the surface-treated steel sheet of this disclosure is performed from the surface in the thickness direction by glow discharge emission spectrometry, the following relationships are satisfied: 1.1 ≤ I1 / I2 ≤ 20.0 and 1.1 ≤ I3 / I4 ≤ 5.0 (I1: maximum emission intensity for N in the chemical conversion coating, I2: average emission intensity for N in the base steel sheet, I3: maximum emission intensity for Ti in the chemical conversion coating, I4: average emission intensity for Ti in the base steel sheet).

Description

Technical Field

[0001] This application discloses a surface-treated steel sheet and components including the same.

Background Art

[0002] Techniques for improving the corrosion resistance after painting of a steel sheet by enhancing the chemical conversion treatment property of the steel sheet are known. For example, in Patent Document 1, pickling of a hot-rolled steel sheet is performed in a pickling solution containing a sulfur compound to remove the scale on the surface of the hot-rolled steel sheet and to cause the sulfur content per one side of the hot-rolled steel sheet to be 0.05 mg / m or more. Further, in Patent Document 2, after continuous annealing of a cold-rolled steel sheet, pickling of the cold-rolled steel sheet is performed in a pickling solution containing a sulfur compound to deposit sulfide of 0.5 to 100 mg / m in terms of S amount on the surface of the cold-rolled steel sheet. As described in Patent Documents 1 and 2, it is considered that by causing sulfur content to be present on the surface of the steel sheet, the chemical conversion treatment property of the steel sheet is enhanced and the corrosion resistance after painting of the steel sheet is improved. 2

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] When a large amount of titanium oxide is generated on the surface of the base steel sheet, the adhesion of the chemical conversion treatment film is inhibited due to the oxide, and as a result, the corrosion resistance after painting may decrease. This application discloses a surface-treated steel sheet having a base steel sheet containing Ti and Al and a chemical conversion treatment film disposed on the surface of the base steel sheet and having corrosion resistance after painting, and components including the same. [Means for solving the problem]

[0005] To solve the above problems, the inventors focused on the components contained in the chemical conversion coating and conducted an investigation. As a result, the inventors found that when the maximum luminescence intensity of Ti and N in the chemical conversion coating, as measured by a high-frequency glow discharge emission spectrometer (GDS), satisfies a predetermined relationship with the average luminescence intensity of Ti and N in the base steel sheet, the chemical conversion treatment properties are improved, and this significantly improves corrosion resistance after painting. Based on the above findings, this application discloses the following multiple embodiments as means for solving the above problems.

[0006] <Aspect 1> Surface-treated steel sheet, Base material steel plate, and A chemical conversion coating is placed on the surface of the base steel plate. Equipped with, The chemical composition of the aforementioned base steel sheet is, in mass%, Ti: 0.050~1.000%, and Al: 0.050~2.000%, Includes, When elemental analysis is performed from the surface of the surface-treated steel sheet in the thickness direction by glow discharge emission spectrometry, the following relationships (1) and (2): 1.1 ≤ I1 / I2 ≤ 20.0 (1) 1.1 ≤ I3 / I4 ≤ 5.0 (2) I1: Maximum luminescence intensity for N in the chemical conversion treated film. I2: Average luminescence intensity for N in the base steel plate. I3: Maximum luminescence intensity for Ti in the chemical conversion treated film. I4: Average luminescence intensity of Ti in the base steel sheet. The condition is met. Surface-treated steel sheet. <Aspect 2> A surface-treated steel sheet of embodiment 1, The following relationships (1-1) and (2-1): 1.5 ≤ I1 / I2 ≤ 20.0 (1-1) 1.3 ≦ I3 / I4 ≦ 5.0 (2-1) is satisfied, Surface-treated steel sheet. <Aspect 3> The surface-treated steel sheet of Aspect 1, The following relationships (1-2) and (2-2): 1.8 ≦ I1 / I2 ≦ 20.0 (1-2) 1.4 ≦ I3 / I4 ≦ 5.0 (2-2) is satisfied, Surface-treated steel sheet. <Aspect 4> The surface-treated steel sheet of any one of Aspects 1 to 3, The following relationship (3): 1.2 ≦ I5 / I6 ≦ 20.0 (3) I5: Maximum emission intensity of S in the chemical conversion coating I6: Average emission intensity of S in the base steel sheet is satisfied, Surface-treated steel sheet. <Aspect 5> The surface-treated steel sheet of any one of Aspects 1 to 4, The maximum concentration of Ni in the chemical conversion coating is 2.0 mass% or more, Surface-treated steel sheet. <Aspect 6> The surface-treated steel sheet of any one of Aspects 1 to 5, Having a Vickers hardness of 190 Hv or more, Surface-treated steel sheet. <Aspect 7> The surface-treated steel sheet of Aspect 6, Having a Vickers hardness of 300 Hv or more, Surface-treated steel sheet. <Aspect 8> The surface-treated steel sheet of any one of Aspects 1 to 7, The chemical composition of the base steel sheet is, in mass%, Ti: 0.080 to 0.300%, and Al: 0.080 to 0.500%, including, Surface-treated steel sheet. <Aspect 9> A surface-treated steel sheet according to any of embodiments 1 to 8, The chemical composition of the aforementioned base steel sheet is, in mass%, Ni: 0.040~1.000%, Cu: 0.040~1.000%, and Sn: 0.004~1.000%, Including one or more of the following: Surface-treated steel sheet. <Aspect 10> A surface-treated steel sheet according to embodiment 9, The chemical composition of the aforementioned base steel sheet is, in mass%, Ni: 0.040~1.000%, Cu: 0.040~1.000%, and Sn: 0.004~1.000%, including, Surface-treated steel sheet. <Aspect 11> Includes a surface-treated steel sheet from any of embodiments 1 to 10, parts. [Effects of the Invention]

[0007] The technology of this disclosure provides a surface-treated steel sheet having a base steel sheet containing Ti and Al and a chemical conversion coating disposed on the surface of the base steel sheet, and having corrosion resistance after painting, and a part containing the same. [Modes for carrying out the invention]

[0008] 1. Surface-treated steel sheet A surface-treated steel sheet according to one embodiment comprises a base steel sheet and a chemical conversion coating disposed on the surface of the base steel sheet. The chemical composition of the base steel sheet includes, by mass%, Ti: 0.050 to 1.000% and Al: 0.050 to 2.000%. In this embodiment, when elemental analysis is performed from the surface of the surface-treated steel sheet in the thickness direction by glow discharge emission spectrometry, the following relationships (1) and (2) are satisfied. 1.1 ≤ I1 / I2 ≤ 20.0 (1) 1.1 ≤ I3 / I4 ≤ 5.0 (2) I1: Maximum luminescence intensity for N in the chemical conversion treated film. I2: Average luminescence intensity for N in the base steel plate. I3: Maximum luminescence intensity for Ti in the chemical conversion treated film. I4: Average luminescence intensity of Ti in the base steel sheet.

[0009] Generally, when the chemical conversion treatment properties of a steel sheet deteriorate, areas where the chemical conversion coating is not formed, known as "skeleton," may occur, resulting in reduced corrosion resistance after painting. For example, if a large amount of Ti is present as an oxide on the surface of the steel sheet, it may inhibit the etching reaction on the surface during chemical conversion treatment, thus reducing the chemical conversion treatment properties of the steel sheet.

[0010] Therefore, the inventors focused on the components contained in the chemical conversion coating to improve the corrosion resistance of steel plates after painting. As a result, the inventors found that when the maximum luminescence intensity of Ti and N in the chemical conversion coating measured by glow discharge emission spectrometry and the average luminescence intensity of Ti and N in the base steel plate satisfy the above relationships (1) and (2), the chemical conversion treatment performance is improved, and as a result, the corrosion resistance after painting is significantly improved.

[0011] To explain in more detail, in the manufacture of base steel sheets containing high levels of Ti and Al, Ti oxides are formed on the surface of the base steel sheet during the hot rolling process, and these oxides may not be completely removed even by subsequent pickling. When chemical conversion treatment is applied to a base steel sheet with Ti oxides on its surface, the chemical conversion treatment performance is reduced due to these oxides, and a relatively large area called "skew" is created where the chemical conversion treatment film has not formed. On the other hand, the areas where the chemical conversion treatment film is attached contain a relatively large amount of Ti, and therefore, when elemental analysis is performed by glow discharge emission spectrometry, the maximum emission intensity of Ti in the chemical conversion treatment film is higher than the average emission intensity of these elements observed in the base steel sheet. Therefore, when such elemental analysis results are obtained for a surface-treated steel sheet after chemical conversion treatment, it is usually considered that the chemical conversion treatment film is not uniformly attached to the entire surface of the base steel sheet, and thus the corrosion resistance of the surface-treated steel sheet after painting is expected to decrease. In response to this, the inventors have found that, as will be explained in detail later in relation to the manufacturing method, by supplying an inhibitor, which is a nitrogen (N)-containing compound, at a predetermined concentration and under predetermined conditions during pickling after the hot-rolling process, it is possible to incorporate N in addition to Ti into the chemical conversion treatment film, and to control the maximum luminescence intensity of N and Ti in the chemical conversion treatment film to satisfy the above relationships (1) and (2), respectively. As a result, the inventors have found that the chemical conversion treatment properties of the base steel sheet are improved, the corrosion resistance of the chemical conversion treatment film itself after the chemical conversion treatment is also improved, and the corrosion resistance of the surface-treated steel sheet after chemical conversion treatment is significantly improved.

[0012] While not intended to be bound by any particular theory, it is believed that using an inhibitor containing nitrogen during pickling after the hot rolling process suppresses or reduces the formation of titanium oxides on the surface of the base steel sheet, and simultaneously forms nitrogen-titanium bonds on the surface of the base steel sheet. As a result, chemical conversion defects such as sizing caused by the formation of titanium oxides on the surface of the base steel sheet are suppressed or reduced. In addition, the formation of nitrogen-titanium bonds on the surface of the base steel sheet improves the etching properties of Fe in the chemical conversion solution during the chemical conversion treatment, and as a result, the chemical conversion coating is formed uniformly across the entire surface of the base steel sheet. Furthermore, in relation to the formation of nitrogen-titanium bonds on the surface of the base steel sheet, nitrogen is incorporated in addition to titanium into the chemical conversion coating after the chemical conversion treatment. Considering the fact that the post-painting corrosion resistance of surface-treated steel sheets after chemical conversion treatment is significantly improved by appropriately controlling the content of these elements in the chemical conversion coating to satisfy the above relationships (1) and (2), it is thought that this improvement in post-painting corrosion resistance is due not only to the effect of suppressing or reducing the formation of Ti oxides during the manufacturing of the base steel sheet, but also to other effects. More specifically, it is thought that the corrosion resistance of the chemical conversion coating itself is improved by the chemical conversion coating containing an appropriate amount of N and Ti, i.e., an amount that satisfies the above relationships (1) and (2). If the corrosion resistance of the chemical conversion coating decreases, for example, the phenomenon of the coating peeling off at the interface between the chemical conversion coating and the coating film is more likely to occur. In such cases, the post-painting corrosion resistance of the surface-treated steel sheet will decrease. In contrast, with a surface-treated steel sheet having a chemical conversion coating that satisfies the above relationships (1) and (2), (A) Improved chemical treatment performance due to the suppression or reduction of Ti oxide formation during the manufacturing of the base steel sheet. (B) Effect of improving the etching properties of Fe during chemical conversion treatment based on the formation of N-Ti bonds, and (C)Corrosion resistance improvement effect of the chemical conversion treatment film itself based on the inclusion of N and Ti These effects are realized, and it is believed that the combination of these three effects makes it possible to significantly improve the corrosion resistance of surface-treated steel sheets after painting. Such surface-treated steel sheets are particularly useful as constituent materials for automotive parts that require excellent corrosion resistance after painting. The various components of a surface-treated steel sheet according to one embodiment will be described in more detail below.

[0013] 1.1 Types of chemical conversion coatings The chemical conversion coating is placed on the surface of the base steel sheet, for example, on at least one, preferably both, surfaces of the base steel sheet. The type of chemical conversion coating is not particularly limited as long as the above relationships (1) and (2) are satisfied, that is, it may be the same as known chemical conversion coatings in terms of components other than N and Ti. According to one embodiment of the surface-treated steel sheet, by having the chemical conversion coating contain N and Ti in amounts that satisfy the above relationships (1) and (2), regardless of the type of chemical conversion coating, it is possible to significantly improve the corrosion resistance of the surface-treated steel sheet after painting by combining the effect of improving chemical conversion treatmentability based on the suppression or reduction of Ti oxide formation during the manufacture of the base steel sheet, the effect of improving the etching properties of Fe based on the formation of N-Ti bonds during the manufacture of the base steel sheet, and the effect of improving the corrosion resistance of the chemical conversion coating itself based on the content of N and Ti. For example, the chemical conversion coating may contain at least one of phosphoric acid, zinc phosphate, and zirconium, and in particular may contain at least one of zinc phosphate and zirconium. In one embodiment, the chemical conversion coating may contain zinc phosphate, be essentially composed of zinc phosphate, be made of zinc phosphate, or be composed of zinc phosphate.

[0014] 1.2 Relationships (1) and (2) In this embodiment, when elemental analysis is performed from the surface of the surface-treated steel sheet in the thickness direction by glow discharge emission analysis, the following relationships (1) and (2) are satisfied. 1.1 ≤ I1 / I2 ≤ 20.0 (1) 1.1 ≤ I3 / I4 ≤ 5.0 (2) I1: Maximum luminescence intensity for N in the chemical conversion treated film. I2: Average luminescence intensity for N in the base steel plate. I3: Maximum luminescence intensity for Ti in the chemical conversion treated film. I4: Average luminescence intensity of Ti in the base steel sheet.

[0015] As described above, in the surface-treated steel sheet according to one embodiment, the formation of Ti oxides on the surface of the base steel sheet is suppressed or reduced during the manufacturing of the base steel sheet, and N-Ti bonds are formed on the surface of the base steel sheet. In relation to this, it is possible to control the chemical composition of the chemical conversion coating so that the maximum luminescence intensities of N and Ti satisfy the above relationships (1) and (2), respectively. Conversely, by controlling the chemical composition of the chemical conversion coating to contain N and Ti in amounts that satisfy the above relationships (1) and (2), it is possible to obtain an improved chemical conversion treatment effect based on the suppression or reduction of Ti oxide formation during the manufacturing of the base steel sheet, an improved etching effect of Fe based on the formation of N-Ti bonds during the manufacturing of the base steel sheet, and an improved corrosion resistance effect of the chemical conversion coating itself based on the content of N and Ti. As a result, the combination of these effects makes it possible to significantly improve the corrosion resistance of the surface-treated steel sheet after painting.

[0016] From the viewpoint of further improving the corrosion resistance of surface-treated steel sheets after painting, a larger value for I1 / I2 in the above relationship (1) is preferable. For example, I1 / I2 may be 1.2 or more, 1.3 or more, 1.4 or more, 1.5 or more, 1.6 or more, 1.7 or more, 1.8 or more, 1.9 or more, 2.0 or more, 2.1 or more, 2.2 or more, 2.3 or more, 2.4 or more, 2.5 or more, 2.6 or more, 2.7 or more, 2.8 or more, 2.9 or more, 3.0 or more, 3.5 or more, 4.0 or more, 4.5 or more, or 5.0 or more. Also, I3 / I4 may be 1.2 or more, 1.3 or more, 1.4 or more, 1.5 or more, 1.6 or more, 1.7 or more, 1.8 or more, 1.9 or more, or 2.0 or more. On the other hand, the effect saturates even if the chemical conversion coating contains an excessive amount of N. Furthermore, excessive nitrogen content in the chemical conversion coating can lead to increased manufacturing costs. In this regard, I1 / I2 should be 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. Also, if the chemical conversion coating contains excessive Ti, the corrosion resistance of the surface-treated steel sheet after painting may decrease, even if Ni is included along with Ti. Therefore, I3 / I4 should be 5.0 or less, and may be, for example, 4.5 or less, 4.0 or less, 3.5 or less, 3.0 or less, or 2.5 or less.

[0017] In one embodiment, a surface-treated steel sheet may satisfy the following relationships (1-1) and (2-1) when elemental analysis is performed from the surface in the thickness direction of the sheet by glow discharge emission analysis. 1.5 ≤ I1 / I2 ≤ 20.0 (1-1) 1.3 ≤ I3 / I4 ≤ 5.0 (2-1)

[0018] In one embodiment, a surface-treated steel sheet may satisfy the following relationships (1-2) and (2-2) when elemental analysis is performed from the surface in the thickness direction of the sheet by glow discharge emission analysis. 1.8 ≤ I1 / I2 ≤ 20.0 (1-2) 1.4 ≤ I3 / I4 ≤ 5.0 (2-2)

[0019] In one embodiment, a surface-treated steel sheet may satisfy the following relationships (1-3) and (2-3) when elemental analysis is performed from the surface in the thickness direction of the sheet by glow discharge emission analysis. 5.0 ≤ I1 / I2 ≤ 20.0 (1-3) 2.0 ≤ I3 / I4 ≤ 5.0 (2-3)

[0020] 1.3 Other Relationships As described above, in the surface-treated steel sheet according to one embodiment, in relation to suppressing or reducing the formation of Ti oxides on the surface of the base steel sheet during the manufacturing of the base steel sheet and forming N-Ti bonds on the surface of the base steel sheet, it is possible to control the chemical composition of the chemical conversion coating so that the maximum luminescence intensities of N and Ti satisfy the above relationships (1) and (2), respectively. On the other hand, in addition to controlling the chemical composition of the chemical conversion coating so that the maximum luminescence intensities of N and Ti satisfy the above relationships (1) and (2), respectively, it is considered that the corrosion resistance of the surface-treated steel sheet after painting will be further improved if the chemical composition of the chemical conversion coating is controlled to satisfy the following relationship.

[0021] 1.3.1 Relationships (3) In one embodiment, a surface-treated steel sheet may satisfy the following relationship (3) when elemental analysis is performed from the surface in the thickness direction of the sheet by glow discharge emission analysis. 1.2 ≤ I5 / I6 ≤ 20.0 (3) I5: Maximum luminescence intensity for S in the chemical conversion treated film. I6: Average luminescence intensity of S in the base steel plate.

[0022] While not intended to be bound by any particular theory, it is believed that by using an accelerator containing sulfur (S) during pickling after the hot rolling process, the formation of Ti oxides on the surface of the base steel sheet is suppressed or reduced, and S-Ti bonds are formed on the surface of the base steel sheet. As a result, chemical conversion defects such as sizing caused by the formation of Ti oxides on the surface of the base steel sheet are suppressed or reduced. In addition, it is believed that the formation of S-Ti bonds on the surface of the base steel sheet improves the etching properties of Fe in the chemical conversion solution during the chemical conversion treatment, and as a result, the chemical conversion coating is formed uniformly over the entire surface of the base steel sheet. Furthermore, in relation to the formation of the above S-Ti bonds on the surface of the base steel sheet, S is incorporated in addition to Ti into the chemical conversion coating after the chemical conversion treatment. It is believed that by appropriately controlling the content of these elements in the chemical conversion coating so as to satisfy the above relationship (3), the corrosion resistance of the chemical conversion coating itself is improved. As described above, it is believed that the corrosion resistance of surface-treated steel sheets after painting can be further improved by appropriately controlling the element content in the chemical conversion coating to satisfy the above relationships (1), (2), and (3).

[0023] In the above relationship (3), I5 / I6 may be 1.3 or greater, or 1.4 or greater, and may be 18.0 or less, 15.0 or less, 12.0 or less, 10.0 or less, 8.0 or less, 6.0 or less, 4.0 or less, or 2.0 or less.

[0024] 1.3.2 Relationships (4) In one embodiment, a surface-treated steel sheet may satisfy the following relationship (5) when elemental analysis is performed from the surface in the thickness direction of the sheet by glow discharge emission analysis. 0 ≤ I7 / I8 ≤ 10.0 (4) I7: Maximum luminescence intensity for Al in the chemical conversion treated film. I8: Average luminescence intensity of Al in the base steel sheet.

[0025] In the manufacture of base steel sheets containing large amounts of Ti and Al, not only Ti oxides but also Al oxides may form on the surface of the base steel sheet. Here, it is believed that the chemical treatment properties of the base steel sheet can be further improved by suppressing or reducing the formation of Al oxides on the surface of the base steel sheet. When the formation of Al oxides on the surface of the base steel sheet is suppressed or reduced, the maximum luminescence intensity for Al in the chemical treatment film after chemical treatment becomes smaller, that is, the I7 / I8 value becomes smaller. In other words, to improve the corrosion resistance of surface-treated steel sheets after painting, a smaller I7 / I8 value is preferable. In one embodiment, I7 / I8 may be 10.0 or less, 8.0 or less, 5.0 or less, 3.0 or less, 1.5 or less, 1.1 or less, or 1.0 or less. The lower limit is not particularly limited, but for example, I7 / I8 may be 0 or more, 0.1 or more, 0.2 or more, 0.3 or more, 0.4 or more, or 0.5 or more.

[0026] 1.4 Measurement conditions in glow discharge emission spectrometry The measurement conditions for glow discharge emission spectrometry are as follows. First, using a high-frequency glow discharge emission spectrometer (for example, LECO Japan LLC, model number "GDS850A"), the surface of a surface-treated steel sheet with a chemical conversion coating is placed in an Ar atmosphere, and a voltage is applied to generate a glow plasma. The surface of the surface-treated steel sheet is then sputtered and analyzed in the depth direction (thickness direction). The elements contained in the material are identified from the emission spectral wavelengths of the elements emitted when atoms are excited 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: 4mmφ RF output: 30W Measurement time: 200-1500 seconds

[0027] In this way, elemental analysis is performed from the surface of the surface-treated steel sheet in the depth direction (thickness direction), and the region in which the emission intensity of the element mainly constituting the chemical conversion coating (for example, P if the chemical conversion coating mainly contains zinc phosphate, or Zr if the chemical conversion coating mainly contains zirconium) is 10 times or more than the emission intensity of the corresponding element in the base steel sheet (for example, P in the base steel sheet if the chemical conversion coating mainly contains zinc phosphate, or Zr in the base steel sheet if the chemical conversion coating mainly contains zirconium) is determined as the region corresponding to the "chemical conversion coating". Next, in the region corresponding to the chemical conversion coating determined in this way, the graph of the emission intensity of N obtained by elemental analysis in the depth direction (thickness direction) is smoothed by averaging 10 points, and the maximum value of the graph after smoothing is determined as the "maximum emission intensity for N in the chemical conversion coating". Similarly, in the region corresponding to the chemical conversion coating described above, the graph of Ti emission intensity obtained by elemental analysis in the depth direction (plate thickness direction) is smoothed by averaging 10 points, and the maximum value of the graph after smoothing is determined as the "maximum emission intensity for Ti in the chemical conversion coating." On the other hand, the average values ​​of the emission intensities of N and Ti are calculated in the depth range in which the emission intensities are sufficiently stable. For example, the average values ​​of the emission intensities of N and Ti in the region from the interface between the region corresponding to the chemical conversion coating and the region corresponding to the base steel sheet to the base steel sheet side from 100 to 150 μm are calculated, and these are determined as the "average emission intensity for N in the base steel sheet" and the "average emission intensity for Ti in the base steel sheet," respectively. The "maximum luminescence intensity for S in the chemical conversion coating," the "average luminescence intensity for S in the base steel sheet," the "maximum luminescence intensity for Ni in the chemical conversion coating," the "average luminescence intensity for Ni in the base steel sheet," the "maximum luminescence intensity for Al in the chemical conversion coating," and the "average luminescence intensity for Al in the base steel sheet" are determined in the same manner as above, except for changing the element being analyzed.

[0028] In glow discharge emission spectrometry, the measurement depth is determined from the sputtering time. Specifically, the depth of the GDS mark (indentation) formed by the 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 base steel sheet side is determined by multiplying the depth per sputtering time by the sputtering time required to measure the depth of the chemical conversion coating from the base steel sheet side. The depth of the GDS mark is measured using a roughness meter (for example, ACCRETECH (Tokyo Seimitsu), model number "SURFCOM (registered trademark) TOUCH50"), and the specific conditions are as follows. Measurement conditions: Measurement type: Cross-sectional measurement Evaluation length: 10mm Shape removal at both ends Calculation standard JIS B 0601:2013 Reference height -50 μm (when measured at 100 μm) Measurement location: Measure the unevenness profile in the diametrical direction, crossing the GDS mark (center of the circle ±0.5mm).

[0029] 1.5 Maximum Ni concentration in chemical conversion treated coatings In one embodiment, the surface-treated steel sheet may have a maximum Ni concentration of 2.0% by mass or more in the chemical conversion coating.

[0030] While not intended to be bound by any particular theory, it is believed that introducing Ni ions into the pickling solution during the pickling process after the hot rolling process results in the formation of a very thin Ni plating on the surface of the base steel sheet (so-called displacement plating) by the Ni ions receiving electrons released when Fe on the surface of the base steel sheet dissolves into Fe ions due to the pickling solution. This formation of Ni plating, utilizing the difference in ionization tendencies between Fe ions and Ni ions, is thought to suppress or reduce the formation of Ti oxides on the surface of the base steel sheet, and to form Ni-Ti bonds on the surface of the base steel sheet. In addition, by carrying out the subsequent water washing process with a water washing solution having a relatively low predetermined electrical conductivity, it is thought that the Ni-Ti bonds formed on the surface of the base steel sheet are maintained on the surface of the base steel sheet. In other words, it is possible to suppress or reduce the occurrence of chemical conversion defects such as sizing caused by the formation of Ti oxides on the surface of the base steel sheet during the manufacturing of the base steel sheet. In addition, the formation of Ni-Ti bonds on the surface of the base steel sheet improves the etching properties of Fe in the chemical conversion solution during the chemical conversion treatment, and as a result, it is possible to uniformly form the chemical conversion coating over the entire steel sheet. Furthermore, in relation to the formation of the above Ni-Ti bonds on the surface of the base steel sheet, Ni is incorporated in addition to Ti into the chemical conversion coating after the chemical conversion treatment. In this respect, it is thought that the corrosion resistance of the surface-treated steel sheet after painting is further improved by controlling the maximum concentration of Ni in the chemical conversion coating to 2.0 mass% or more. The maximum concentration of Ni in the chemical conversion coating may be 2.2 mass% or more, 2.5 mass% or more, 2.7 mass% or more, 2.9 mass% or more, or 3.1 mass% or more, and may also be 5.0 mass% or less, 4.5 mass% or less, 4.0 mass% or less, or 3.5 mass% or less.

[0031] 1.6 Method for measuring the maximum Ni concentration in chemical conversion treated coatings The maximum concentration of Ni in a chemical conversion coating is measured by glow discharge emission spectrometry as follows. First, using a high-frequency glow discharge emission spectrometer (e.g., LECO Japan LLC, model number "GDS850A"), the surface of the surface-treated steel sheet is placed in an Ar atmosphere, and a voltage is applied to generate a glow plasma. The surface of the surface-treated steel sheet is then sputtered and analyzed in the depth direction (thickness direction). The elements contained in the material are identified from the emission spectral wavelengths of the elements emitted when atoms are excited in the glow plasma, and the emission intensity of the identified elements is measured. The measurement conditions are as described above. In this way, elemental analysis is performed from the surface of the surface-treated steel sheet in the depth direction (thickness direction), and the region corresponding to the "chemical conversion coating" is determined as described above. Next, in the region corresponding to the chemical conversion coating determined in this way, the graph of Ni emission intensity obtained by elemental analysis in the depth direction (thickness direction) is smoothed by averaging 10 points, and the maximum value of the graph after smoothing is determined as the "maximum emission intensity for Ni in the chemical conversion coating". The obtained "maximum luminescence intensity for Ni in the chemical conversion treated film" is converted to Ni concentration (mass%) to determine the "maximum Ni concentration in the chemical conversion treated film." Specifically, the relationship between luminescence intensity and Ni concentration can be determined in advance using a standard sample, allowing for the conversion of luminescence intensity to Ni concentration. The calibration curve used is the one installed on the "GDS850A" model manufactured by LECO Japan LLC. The standard sample "BS H-1B" manufactured by Brammer Standard Company can be used to calibrate the calibration curve.

[0032] 1.7 Chemical composition of base steel sheet As described above, for a base steel sheet containing Ti and Al, the corrosion resistance of the surface-treated steel sheet after painting can be improved by suppressing or reducing the formation of Ti oxides on its surface, forming N-Ti bonds on its surface, and controlling the amount of N and Ti in the chemical conversion coating after the chemical conversion treatment. In this embodiment, the chemical composition of the base steel sheet only needs to contain Ti: 0.050 to 1.000% and Al: 0.050 to 2.000% by mass%, and it is clear that elements other than Ti and Al are not essential technical features for solving the above problem. In other words, in this embodiment, the types and content of elements other than Ti and Al contained in the base steel sheet are not particularly limited. In addition to Ti and Al, the chemical composition of the base steel sheet may contain appropriate amounts of any alloying elements that are commonly added in this art. The chemical composition of the base steel sheet according to one embodiment will be described in detail below, but these descriptions are intended to be merely examples of preferred chemical compositions of base steel sheets for application in automotive steel sheets and the like.

[0033] The chemical composition of the base steel sheet according to one embodiment is, in mass%, C: 0.001~0.500%, Si: 0.01~3.00%, Mn: 0.10~3.00%, Ti: 0.050~1.000%, Al: 0.050~2.000%, P: 0.100% or less, S: 0.1000% or less, N: 0.0200% or less, O: 0.0100% or less, Nb: 0~0.150%, B: 0~0.0100%, Cr: 0~1.00%, Mo: 0~1.000%, Ni: 0~1,000%, Cu: 0~1.000%, Sn: 0~1.000%, V: 0~0.150%, W: 0~1.000%, Hf: 0~0.050%, Mg: 0~0.050%, Zr: 0~0.500%, Ca: 0~0.050%, REM: 0~0.010%, As: 0~0.100%, Ir: 0~1.000%, and Remainder: Fe and impurities It may consist of the following. Each element will be explained in more detail below.

[0034] [C:0.001~0.500%] Carbon (C) is an element that increases strength inexpensively and is an important element for controlling the strength of steel. To obtain this effect fully, it is preferable that the C content be 0.001% or more. The C content may also 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 lead to a decrease in elongation. For this reason, it is preferable that the C content be 0.500% or less. The C content may also 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.

[0035] [Si: 0.01~3.00%] Si is an effective element for increasing strength as a solid solution strengthening element. To obtain this effect fully, it is preferable that the Si content be 0.01% or more. The Si content may also 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, if the Si content is excessive, it may lead to a decrease in elongation along with an increase in steel strength. For this reason, it is preferable that the Si content be 3.00% or less. The Si content may also be 2.50% or less, 2.00% or less, 1.50% or less, or 1.20% or less.

[0036] [Mn: 0.10~3.00%] Mn is an element that enhances the hardenability of steel and is effective in increasing its strength. To fully obtain these effects, it is preferable that the Mn content be 0.10% or more. The Mn content may also 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, if the Mn content is excessive, it may lead to a decrease in elongation along with an increase in steel strength. For this reason, it is preferable that the Mn content be 3.00% or less. The Mn content may also be 2.80% or less, 2.50% or less, or 2.00% or less.

[0037] [Ti: 0.050~1.000%] Ti has the effect of improving the strength of steel sheets by forming carbonitrides in steel and strengthening them through precipitation. To obtain this effect sufficiently, the Ti content should be 0.050% or more. The Ti content may also be 0.060% or more, 0.080% or more, 0.100% or more, 0.120% or more, or 0.150% or more. On the other hand, if the Ti content is excessive, the effect will saturate, and including more Ti in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Ti content should be 1.000% or less. The Ti content may also be 0.800% or less, 0.600% or less, 0.500% or less, 0.400% or less, 0.300% or less, or 0.250% or less.

[0038] [Al:0.050~2.000%] Al acts as a deoxidizing agent for steel, has the effect of sounding down steel, and is also an effective element for increasing the strength of steel. To obtain these effects to the fullest, the Al content should be 0.050% or more. The Al content may be 0.060% or more, 0.080% or more, 0.100% or more, 0.150% or more, 0.200% or more, or 0.300% or more. On the other hand, if Al is present in excess, coarse Al oxide may be generated, which may reduce the elongation of the steel sheet. For this reason, the Al content should be 2.000% or less. The Al content may be 1.800% or less, 1.500% or less, 1.200% or less, 1.000% or less, 0.800% or less, 0.600% or less, or 0.500% or less.

[0039] [P:0.100% or less] P is an element that segregates at grain boundaries and promotes steel embrittlement. A lower P content is preferable, and ideally it should be 0%. However, excessive reduction of the P content can lead to a significant increase in cost. For this reason, the P content may be 0.0001% or more, or 0.001% or more, or 0.005% or more. On the other hand, excessive P content can lead to steel embrittlement due to grain boundary segregation, as described above. Therefore, it is preferable to have a P content of 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0040] [S:0.1000% or less] S is an element that generates nonmetallic inclusions such as MnS in steel, leading to a decrease in the ductility of steel parts. A lower S content is preferable, and ideally it should be 0%. However, excessive reduction of the S content can lead to a significant increase in cost. For this reason, the S content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more. On the other hand, excessive S content can lead to cracking during cold forming, originating from nonmetallic inclusions. Therefore, it is preferable to have an S content of 0.1000% or less. The S content may also be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0041] [N:0.0200% or less] N is an element that forms coarse nitrides in steel sheets, reducing their workability. A lower N content is preferable, ideally 0%. However, excessive reduction of the N content can lead to a significant increase in manufacturing costs. Therefore, the N content may be 0.0001% or higher, or 0.0005% or higher, or 0.0010% or higher. On the other hand, excessive N content can lead to the formation of coarse nitrides, as described above, reducing the workability of the steel sheet. Therefore, the N content is preferably 0.0200% or lower. The N content may also be 0.0100% or lower, 0.0080% or lower, 0.0060% or lower, or 0.0050% or lower.

[0042] [O:0.0100% or less] O is an element that, when mixed in during the manufacturing process, forms coarse inclusions and reduces the workability of steel sheets. A lower O content is preferable, and ideally it should be 0%. However, excessive reduction of the O content can lead to a significant increase in manufacturing costs. For this reason, the O content may be 0.0001% or more, or 0.0005% or more, or 0.0010% or more. On the other hand, excessive O content can form coarse inclusions as described above, reducing the workability of steel sheets. Therefore, it is preferable to have an O content of 0.0100% or less. The O content may also be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0043] The preferred basic chemical composition of the base steel sheet is as described above. Furthermore, the base steel sheet may optionally contain at least one of the following elements.

[0044] [Nb: 0~0.150%] [V: 0~0.150%] Nb and V have the effect of improving the strength of steel sheets by forming carbonitrides in the steel and strengthening through precipitation. The Nb and V content may be 0%, but in order to obtain such an effect, the Nb and V content is 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, if these elements are included in excess, the effect will saturate, and including them in the steel more than necessary will lead to an increase in manufacturing costs. Therefore, the Nb and V content is 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.

[0045] [B: 0~0.0100%] B improves low-temperature toughness by segregating at grain boundaries and increasing grain boundary strength. The B content may be 0%, but to obtain this effect, it is preferable that the B content be 0.0001% or more. The B content may also be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the B content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the B content be 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0046] [Cr: 0~1.00%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength. While the Cr content may be 0%, it is preferable that the Cr content be 0.001% or more to obtain such effects. The Cr content may be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, if the Cr content is excessive, the effect will saturate, and including more Cr in the steel than necessary will lead to increased manufacturing costs. Therefore, the Cr content is preferably 1.00% or less, and may be 0.80% or less, 0.50% or less, 0.30% or less, 0.20% or less, or 0.10% or less.

[0047] [Mo: 0~1.000%] [W: 0~1.000%] Mo and W are elements that enhance the hardenability of steel and contribute to improving its strength. While the Mo and W content may be 0%, to obtain such effects, it is preferable that the Mo and W content be 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if these elements are included in excess, the effect will saturate, and including them in the steel more than necessary will lead to increased manufacturing costs. Therefore, it is preferable that the Mo and W content be 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.

[0048] [Ni: 0~1.000%] [Cu: 0~1.000%] Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening. The Ni and Cu content may be 0%, but in order to obtain such effects sufficiently, the Ni and Cu content is preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.060% or more, 0.080% or more, 0.100% or more, 0.150% or more, or 0.200% or more. On the other hand, if these elements are included in excess, the effect will saturate, and including them in the steel more than necessary will lead to an increase in manufacturing costs. Therefore, the Ni and Cu content is 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.

[0049] [Sn: 0~1.000%] Sn is an effective element for improving corrosion resistance. While the Sn content may be 0%, it is preferable that the Sn content be 0.001% or more to obtain a sufficient effect. The Sn content may also be 0.003% or more, 0.004% or more, 0.006% or more, 0.008% or more, or 0.010% or more. On the other hand, if the Sn content is excessive, the effect will saturate, and including more Sn in the steel than necessary will lead to an increase in manufacturing costs. Therefore, it is preferable that the Sn content be 1.000% or less. The Sn content may also 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.

[0050] [Hf: 0~0.050%] [Mg: 0~0.050%] [Zr:0~0.500%] [Ca: 0~0.050%] [REM: 0~0.010%] Hf, Mg, Zr, Ca, and REM are elements that can control the morphology of nonmetallic inclusions. The content of Hf, Mg, Zr, Ca, and REM may be 0%, but to obtain such an effect, the content of each of these elements is preferably 0.0001% or more, and may be 0.0005% or more, or 0.001% or more. On the other hand, if these elements are included in excess, the effect will saturate, and including them in the steel sheet more than necessary will lead to an increase in manufacturing costs. Therefore, the content of Hf, Mg, and Ca is preferably 0.050% or less, and may be 0.010% or less, 0.005% or less, or 0.003% or less, respectively. Similarly, the Zr content is preferably 0.500% or less, and may be 0.100% or less, 0.050% or less, or 0.010% or less. Similarly, the REM content is preferably 0.010% or less, and may be 0.005% or less, or 0.003% or less. In this specification, REM refers to the collective term for 17 elements, including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0051] [As: 0~0.100%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain such an effect, it is preferable that the As content be 0.001% or more. The As content may also be 0.002% or more, or 0.003% or more. On the other hand, if the As content is excessive, the effect will saturate, and including more As than necessary in the steel sheet will lead to an increase in manufacturing costs. Therefore, it is preferable that the As content be 0.100% or less. The As content may also be 0.050% or less, 0.010% or less, 0.008% or less, or 0.005% or less.

[0052] [Ir: 0~1.000%] Ir is an element that segregates at prior austenite grain boundaries, increasing their strength. While the Ir content may be 0%, it is preferable that the Ir content be 0.001% or higher to obtain this effect. The Ir content may also be 0.003% or higher, 0.005% or higher, or 0.010% or higher. On the other hand, excessive Ir content leads to saturation of the effect, and including more Ir than necessary in the steel increases manufacturing costs. Therefore, it is preferable that the Ir content be 1.000% or lower. The Ir content may also be 0.500% or lower, 0.100% or lower, 0.030% or lower, or 0.015% or lower.

[0053] [Remaining components: Fe and impurities] In the base steel sheet, the remainder other than the above-mentioned elements consists of Fe and impurities. Impurities in the base steel sheet refer to 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 sheet is manufactured industrially, as well as components that are included to the extent that they do not affect the effects of the present invention.

[0054] 1.8 Method for measuring the chemical composition of base steel sheet The chemical composition of the base steel sheet can be measured using general analytical methods. For example, the chemical composition of the base steel sheet can be determined by first removing the chemical conversion coating using an appropriate method, such as in accordance with JIS K 3151:1996, and then measuring the chips using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece can be obtained from the 1 / 4 position of the base steel sheet thickness, and the composition can be determined by measuring it using a Shimadzu ICPS-8100 or similar (measuring device) under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0055] 1.9 Thickness of the base steel plate The thickness of the base steel sheet is not particularly limited and may be, for example, 0.2 to 8.0 mm. In one embodiment, the sheet 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, and may be 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0056] 1.10 Mechanical properties of surface-treated steel sheets The surface-treated steel sheet according to this embodiment is not particularly limited, but may have a Vickers hardness of, for example, 100 Hv or more. The Vickers hardness may be 120 Hv or more, 150 Hv or more, 190 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.

[0057] 1.11 Method for measuring Vickers hardness The Vickers hardness of surface-treated steel sheets is determined as follows: First, a test piece is cut from any position on the surface-treated steel sheet, excluding the edges, so that a cross-section perpendicular to the surface (thickness cross-section) can be observed. The thickness cross-section of the test piece is polished using #600 to #1500 silicon carbide sandpaper, 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. This thickness cross-section is used as the measurement surface. Next, the Vickers hardness is measured using a micro-Vickers hardness tester with a load of 1 kgf at intervals of at least three times the indentation length. Specifically, a total of 20 measurements are taken randomly around the 1 / 4 position of the thickness of the surface-treated steel sheet, and the arithmetic mean of these measurements is determined as the Vickers hardness of the surface-treated steel sheet.

[0058] 2. Applications (parts) Parts including the surface-treated steel sheet according to this embodiment have excellent post-painting corrosion resistance. The surface-treated steel sheet according to this embodiment is useful for use as a component material for parts requiring post-painting corrosion resistance, particularly automotive parts. In a preferred embodiment, an automotive part including the surface-treated steel sheet according to this embodiment is provided. Examples of automotive parts include undercarriage parts, frame parts, bumpers, other structural and reinforcing parts requiring strength, and exterior parts such as roofs, hoods, fenders, and doors that require high aesthetic appeal. These parts only need to include the surface-treated steel sheet according to this embodiment in at least a portion of them, and therefore at least a portion of these parts will satisfy the characteristics of the surface-treated steel sheet described above. In forming processes such as press forming, the characteristics of the surface-treated steel sheet do not change particularly before and after forming in areas that do not directly contact the mold or, even if they do, areas that directly contact the mold but undergo relatively little processing.

[0059] 3. Method for manufacturing surface-treated steel sheets A preferred manufacturing method for the surface-treated steel sheet according to this embodiment will be described below. The following description is intended to illustrate a characteristic method for manufacturing the surface-treated steel sheet according to one embodiment, and is not intended to limit the surface-treated steel sheet to those manufactured by the manufacturing method described below.

[0060] The surface-treated steel sheet according to this embodiment is, for example, A casting process in which molten steel with adjusted chemical composition is cast to form a steel billet. Hot rolling process, in which steel billets are hot-rolled to obtain hot-rolled steel sheets. Preferably, a skin pass rolling step is performed in which the obtained hot-rolled steel sheet is subjected to skin pass rolling. Pickling process for pickling hot rolled steel sheets, Preferably, an alkali treatment step in which the pickled hot-rolled steel sheet is treated with alkali, and A chemical conversion treatment process in which a chemical conversion treatment film is formed on the surface of the obtained base steel sheet. It can be manufactured by performing the following steps. The manufacturing method when hot-rolled steel sheet is used as the base steel sheet will be described in detail below, but the base steel sheet includes not only hot-rolled steel sheet but also cold-rolled steel sheet. Therefore, when cold-rolled steel sheet is used as the base steel sheet, for example, the cold-rolling step and further the annealing step may be performed after the pickling step or alkali treatment step described above. Each step will be described in detail below.

[0061] 3.1 Casting Process The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then steel billets may be cast using methods such as conventional continuous casting or ingot casting.

[0062] 3.2 Hot rolling process Hot-rolled steel sheets can be obtained by hot-rolling cast steel billets. The hot-rolling process is carried out by hot-rolling the cast steel billet either directly or after it has been cooled and then reheated. When reheating is performed, the heating temperature of the steel billet may be, for example, 1100 to 1250°C. In the hot-rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction ratio of each rolling step can be appropriately determined according to the desired metal structure and sheet thickness. For example, the finishing temperature of the finish rolling may be 900 to 1050°C, and the reduction ratio of the finish rolling may be 10 to 50%.

[0063] 3.3 Skin Pass Rolling Process The obtained hot-rolled steel sheet is preferably subjected to skin pass rolling in the next skin pass process. For example, by performing skin pass rolling with a reduction ratio of 0.6% or more, inhibitors introduced in the next pickling process can be efficiently supplied to the base steel sheet, and as a result, the formation of N-Ti bonds on the surface of the steel sheet can be further promoted. 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 surface of the steel sheet in the hot-rolling process. In this case, the pickling solution introduced in the next pickling process, more specifically the pickling solution containing an inhibitor which is an N-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 the N of the inhibitor contained in the pickling solution and the Ti on the surface of the steel sheet can be promoted, and the formation of N-Ti bonds on the surface of the steel sheet can be further promoted. By further promoting the formation of N-Ti bonds on the surface of the steel sheet, the above-mentioned I1 / I2 values ​​can be increased in the final chemical conversion coating. In this case, the effects related to the simultaneous inclusion of N and Ti in the chemical conversion coating, namely the improvement in chemical conversion treatment performance based on the suppression or reduction of Ti oxide formation, the improvement in the etching properties of Fe during chemical conversion treatment based on the formation of N-Ti bonds, and the improvement in the corrosion resistance of the chemical conversion coating itself based on the inclusion of N and Ti, can be obtained at a very high level. As a result, it becomes possible to more significantly improve the corrosion resistance of steel plates after painting.

[0064] 3.4 Pickling process Hot-rolled steel sheets obtained in the hot-rolling process or after the skin-pass process are pickled in the following pickling process. In this manufacturing method, it is important that the pickling solution used in the pickling process contains an inhibitor, which is an N-containing compound, at a concentration of 10 to 1500 ppm, and that the hot-rolled steel sheets pass through the pickling solution at a line speed of 0.10 to 10.00 m / s. By passing through the pickling solution at a certain line speed, the reaction between N in the inhibitor contained in the pickling solution and Ti on the surface of the steel sheet can be promoted. As a result, the formation of N-Ti bonds on the surface of the hot-rolled steel sheet is promoted, thereby making it possible to sufficiently suppress or reduce the formation of Ti oxides. If the line speed is less than 0.10 m / s, the formation of N-Ti bonds on the surface of the hot-rolled steel sheet will be insufficient, and the formation of Ti oxides will not be sufficiently suppressed or reduced. As a result, the I3 / I4 value in the final chemical conversion coating may become greater than 5.0, and the above relationship (2) may not be satisfied. In this case, the chemical conversion treatment performance is reduced due to the formation of Ti oxides during steel sheet manufacturing, resulting in a decrease in the corrosion resistance of the final steel sheet after painting. From the viewpoint of further improving corrosion resistance after painting, the line speed is preferably 1.00 m / s or higher, and more preferably 1.20 m / s or higher or 1.50 m / s or higher.

[0065] On the other hand, if the inhibitor concentration in the pickling solution is less than 10 ppm and / or the line speed is greater than 10.00 m / s, the N component cannot be sufficiently adsorbed onto the surface of the hot-rolled steel sheet, making it impossible to promote the formation of N-Ti bonds. As a result, the I1 / I2 value in the final chemical conversion coating may be less than 1.1, failing to satisfy the above relationship (1). In this case, the above effects related to the inclusion of N in the chemical conversion coating, namely the improvement in chemical conversion treatment performance based on the suppression or reduction of Ti oxide formation, the improvement in Fe etching performance during chemical conversion treatment based on the formation of N-Ti bonds, and the improvement in the corrosion resistance of the chemical conversion coating itself based on the inclusion of N and Ti, cannot be fully obtained, and as a result, the corrosion resistance of the final steel sheet after painting will be reduced.

[0066] Furthermore, excessively high levels of inhibitors in the pickling solution lead to saturation of the effect, and including more inhibitors than necessary in the pickling solution increases manufacturing costs. Therefore, it is preferable that the concentration of inhibitors in the pickling solution be 1500 ppm or less. Examples of N-containing inhibitors include thiocyanic acid, ammonium thiosulfate, aniline (C6H5-NH2), amines (R-NH2, R1-NH-R2, R1-N(-R2)(-R3)), and polyamines (H2N-C n H 2n It is preferable that the inhibitor is one or more selected from (NH2), etc. The concentration of the inhibitor contained in the pickling solution is preferably 200 to 1500 ppm, more preferably 400 to 1500 ppm.

[0067] The pickling solution used in the pickling process may contain an accelerator that is a sulfur-containing compound. For example, by containing an accelerator that is a sulfur-containing compound in the pickling solution at a concentration of 10 to 1500 ppm, and controlling the hot-rolled steel sheet to pass through the pickling solution at a line speed of 0.10 to 10.00 m / s as described above, the reaction between the sulfur in the accelerator contained in the pickling solution and the titanium on the surface of the steel sheet can be promoted. As a result, the formation of sulfur-tinted steel bonds on the surface of the hot-rolled steel sheet can be promoted, thereby sufficiently suppressing or reducing the formation of titanium oxides. The accelerator that is a sulfur-containing compound is preferably one or more selected from, for example, thioglycolic acid (mercaptoacetic acid), thiosulfuric acid, thiocyanic acid, thiocarboxylic acid, thiourea, sodium thiosulfate, etc. The concentration of the accelerator contained in the pickling solution is preferably 200 to 1500 ppm, more preferably 400 to 1500 ppm.

[0068] The pickling solution used in the pickling process may contain a Ni-containing compound that serves as a Ni ion source. For example, by containing a Ni-containing compound in the pickling solution at a concentration of 10 to 1500 ppm in terms of Ni ions, and controlling the hot-rolled steel sheet to pass through the pickling solution at a line speed of 0.10 to 10.00 m / s as described above, the reaction between the Ni ions contained in the pickling solution and the Ti on the surface of the steel sheet can be promoted. As a result, the formation of Ni-Ti bonds on the surface of the hot-rolled steel sheet can be promoted, thereby sufficiently suppressing or reducing the formation of Ti oxides. The Ni-containing compound that serves as a Ni ion source is preferably one or more selected from, for example, nickel chloride, nickel sulfate, nickel sulfide, nickel nitrate, nickel carbonate, etc. For example, from the viewpoint of acid wastewater treatment, if the pickling bath is a hydrochloric acid bath, it is reasonable for the Ni-containing compound to be nickel chloride, and similarly, if the pickling bath is a sulfuric acid bath, it is reasonable for the Ni-containing compound to be nickel sulfate. The concentration of Ni ions in the pickling solution is preferably 200 to 1500 ppm, more preferably 400 to 1500 ppm.

[0069] The pickling process is not particularly limited in terms of conditions other than the inhibitor and line speed mentioned above, and should be carried out under conditions suitable for removing scale formed on the surface of the steel sheet using commonly used pickling solutions, such as hydrochloric acid solutions or sulfuric acid solutions containing inhibitors.

[0070] 3.5 Alkali treatment process The pickled hot-rolled steel sheet is preferably subjected to alkali treatment in the following alkali treatment step. In steel sheets containing relatively large amounts of Ti and Al, not only Ti oxides but also Al oxides may form on the surface of the steel sheet during manufacturing. Although such Al oxides can be removed to some extent by the above pickling treatment, they can be dissolved relatively easily in an alkaline solution. Therefore, in a preferred embodiment of this manufacturing method, the hot-rolled steel sheet after the pickling step is subjected to alkali treatment using an alkaline solution under conditions of pH 10 to 14 and immersion time of 0.5 to 5.0 seconds, thereby sufficiently removing or reducing any Al oxides that may be present on the surface of the steel sheet. By sufficiently removing or reducing Al oxides on the surface of the steel sheet, the chemical conversion treatment properties of the steel sheet can be further improved, and the maximum Al emission intensity in the final chemical conversion treatment film can be reduced, for example, so that the maximum Al emission intensity satisfies the above relationship (5). Therefore, by performing an alkaline treatment process in addition to the pickling process, the chemical conversion treatment properties of the steel sheet can be further improved compared to the case of pickling alone, and as a result, the corrosion resistance of the steel sheet after painting can be significantly improved. The alkaline solution is not particularly limited, but may be an aqueous solution of sodium hydroxide (NaOH), for example.

[0071] In the alkali treatment process, the hot-rolled steel sheet is washed with water after the alkali treatment. The washing is not particularly limited and should be carried out under conditions appropriate for washing away any excess alkali solution present on the surface of the steel sheet.

[0072] 3.6 Chemical treatment process Finally, in the chemical conversion treatment process, a chemical conversion coating is formed on the surface of the base steel sheet, for example, on at least one, preferably both, surfaces of the base steel sheet. The type and conditions of the chemical conversion treatment are not particularly limited, as long as the final chemical conversion coating satisfies relationships (1) and (2) above. For example, the chemical conversion treatment can be carried out under any conditions suitable for forming a chemical conversion coating that includes at least one of phosphoric acid, zinc phosphate, and zirconium and satisfies relationships (1) and (2).

[0073] According to this manufacturing method, as described above, in the pickling process, a pickling solution containing an inhibitor, which is an N-containing compound, at a concentration of 10 to 1500 ppm is used, and 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. This suppresses or reduces the formation of Ti oxides on the surface of the base steel sheet and forms N-Ti bonds on the surface of the base steel sheet. In relation to the formation of N-Ti bonds, it becomes possible to obtain a chemical conversion treatment film that satisfies relationships (1) and (2) after the subsequent chemical conversion treatment process. Therefore, with a surface-treated steel sheet manufactured according to this manufacturing method, the post-painting corrosion resistance of the surface-treated steel sheet can be significantly improved by combining the effect of improved chemical conversion treatment properties based on the suppression or reduction of Ti oxide formation during the manufacturing of the base steel sheet, the effect of improved Fe etching properties based on the formation of N-Ti bonds during the manufacturing of the base steel sheet, and the effect of improved corrosion resistance of the chemical conversion treatment film itself based on the content of N and Ti. Therefore, by using surface-treated steel sheets manufactured by this method as constituent materials for various parts such as automobile parts, it becomes possible to extend the lifespan of those parts, thereby contributing to the development of industry.

[0074] 4. Supplement The surface-treated steel sheet according to this embodiment can be used, for example, as one of the various automobile parts described above, after a coating film is arbitrarily formed on its surface. Whether or not an automobile part having a coating film includes the surface-treated steel sheet according to this embodiment can be determined by removing the coating film from a sample taken from the automobile part. The sample collection location and coating film removal process in this case are as follows.

[0075] 4.1 Sample collection site When collecting samples from automotive parts, avoid the following locations (i) to (iv). (i) Within 20 mm of the toe of a spot weld, and within 20 mm of the toe of the bead of an arc / laser weld. (ii) Machining area with a radius of curvature of less than 15 mm, and areas within 5 mm of said machining area (iii) Ends within 5 mm from the cut end face of the part (iv) Areas within 5 mm of the area where red rust is visible to the naked eye

[0076] 4.2 Paint film removal process For samples cut from automotive parts, the coating is removed under the following conditions to expose the steel plate. A coating remover (Neoriver #160, manufactured by Sansai Chemical Co., Ltd.) is applied to the surface at room temperature and left to stand for about 5 minutes. Then, the coating is removed by rubbing with a hard sponge or similar material (e.g., Kanefeel, manufactured by AION Co., Ltd.). After that, the sample is washed with water and dried. At this time, the remaining state of the coating is confirmed by SEM-EPMA measurement of the sample surface (100 μm square, 5 fields of view) after washing and drying. In the elemental distribution image obtained by EPMA, regions with a C concentration of 10 mass% or more are identified, and if the area ratio of these regions is 5% or more, it is judged that the coating has not been removed sufficiently. To measure the area ratio of regions with a C concentration of 10 mass% or more, first, an elemental distribution image of C is obtained in EPMA with the C concentration range set to 10-30%. The specific measurement conditions for EPMA are as follows. Equipment: JEOL Ltd. JXA-8230 Electron Probe Microanalyzer Acceleration voltage: 15kV Irradiation current: 0.05μA Surface analysis:WDS Analysis interval: 300 μm or longer Area ratio: Average value of 5 fields of view Next, the area fraction is measured by image processing of the obtained elemental distribution image. The image analysis software "ImageJ" is used for image processing. After loading the elemental distribution image of C into ImageJ, the image is binarized using "Make Binary" in "Process" > "Binary" so that areas with a C concentration of 10 mass% or more are displayed in black and areas with a C concentration of less than 10 mass% are displayed in white. After binarization, the value of "Area fraction" in "Results" is read using "Measure" in "Analyze" and this value is determined as the area fraction of the areas with a C concentration of 10 mass% or more. If the paint film is not sufficiently removed, the removal of the paint film is repeated until the area fraction of the areas with a C concentration of 10 mass% or more is less than 5%. [Examples]

[0077] The present invention will be described in more detail below with reference to examples, but these examples are merely examples of the present invention, and the present invention is not limited in any way to these examples. Needless to say, the present invention can be modified as desired without departing from the spirit of the invention.

[0078] 1. Manufacturing of surface-treated steel sheets First, molten steel was cast using a continuous casting method to form steel billets having the chemical composition shown in Table 1. After the steel billets were cooled, they were reheated to 1200°C and hot-rolled. Hot rolling was carried out by rough rolling and finish rolling, with the final temperature of the finish rolling being 900-1050°C and the reduction ratio of the finish rolling being 30%.

[0079] [Table 1]

[0080] Next, the obtained hot-rolled steel sheets were subjected to skin-pass rolling at the reduction ratios shown in Table 2, followed by pickling. The pickling was carried out using a pickling solution containing 10% hydrochloric acid as the acid, and further containing Asahi Chemical's Ibit 710K as an inhibitor to suppress corrosion of the base steel sheet, thioglycolic acid (mercaptoacetic acid) as an accelerator, and nickel chloride as a Ni-containing compound, at the concentrations shown in Table 2. The hot-rolled steel sheets were 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.

[0081] Next, the pickled hot-rolled steel sheet was subjected to alkaline treatment using an alkaline solution, specifically an aqueous NaOH solution, under the pH and immersion time conditions shown in Table 2, followed by washing with water to obtain a hot-rolled steel sheet (base steel sheet) with a thickness of 3.0 mm. Finally, a sample of the obtained base steel sheet measuring 150 mm × 70 mm × 3.0 mm was subjected to zinc phosphate treatment (according to specifications of Nippon Parkerizing Co., Ltd.) as a chemical conversion treatment under the following conditions. Degreasing: Immerse in a degreasing agent (Fine Cleaner E2001) at 40°C for 2 minutes, then rinse with water. Surface preparation: Immerse in a surface preparation agent (Preparen Z) at room temperature for 30 seconds. Chemical treatment: Immerse in zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes, then rinse with water and dry.

[0082] 2. Evaluation of surface-treated steel sheets The properties of the surface-treated steel sheets obtained were measured and evaluated by the following method.

[0083] 2.1 Corrosion resistance after painting (coating adhesion) Samples of surface-treated steel sheets with a chemical conversion coating were subjected to electrodeposition coating (Power Float 1200: manufactured by Nippon Paint Industrial Coatings Co., Ltd.) at an electrodeposition temperature of 30°C and a film thickness of 15 μm, followed by baking at 170°C for 30 minutes. After baking, the samples were subjected to 56 cycles of a combined cycle corrosion test in accordance with JASO M609:2024(A), and the blister width of the coating film at the end faces of the long sides of the samples was measured. The measurement method and evaluation criteria for the blister width are as follows. (1) The area from one end of the longer side (length 150 mm) of the sample to 45 mm is defined as "Range A," the area from the other end to 45 mm is defined as "Range B," and the 60 mm area between Range A and Range B is defined as "Range C." (2) Of the 60 mm range C described above, the range from one end to 20 mm is defined as "range C1", the range from the other end to 20 mm is defined as "range C2", and the 20 mm range between range C1 and range C2 is defined as "range C3". (3) For each of the ranges C1, C2, and C3, measure the maximum blister width of the coating in a direction parallel to the coating surface and perpendicular to the longer side. (4) For each of the two longer sides, find three maximum bulge widths as described in (1) to (3) above. (5) The arithmetic mean of the six maximum swelling amounts obtained in (4) above is considered to be the "sampling amount of the sample". (6) The degree of swelling of the sample obtained in (5) above shall be evaluated according to the following evaluation criteria. AAA: Sample swelling width is less than 3mm AA: The swelling width of the sample is 3 mm or more and less than 6 mm A: The swelling width of the sample is 6 mm or more and less than 9 mm B: The swelling width of the sample is 9 mm or more

[0084] When the above evaluations are AAA, AA, and A, they were evaluated as surface-treated steel sheets containing Ti and Al, having improved corrosion resistance after painting. The results are shown in Table 2.

[0085] 2.2 Elemental analysis by glow discharge optical emission spectrometry Elemental analysis was performed on the surface-treated steel sheet from the surface in the plate thickness direction by glow discharge optical emission spectrometry. The results are shown in Table 2. In Table 2, "I1", "I2", "I3", "I4", "I5", "I6", "I7", and "I8" are the emission intensities when elemental analysis is performed on the surface-treated steel sheet from the surface in the plate thickness direction by glow discharge optical emission spectrometry, and mean the following. Also, "Ni maximum concentration" means the maximum concentration of Ni in the chemical conversion coating film measured by glow discharge optical emission spectrometry. The measurement conditions of glow discharge optical emission spectrometry are as described in the embodiments of this specification. Also, in Table 2, "G" regarding "I7 / I8" means satisfying the relationship of 1.1 < I7 / I8 ≤ 10.0, and "VG" means satisfying the relationship of 0 ≤ I7 / I8 ≤ 1.1. I1: Maximum emission intensity for N in the chemical conversion coating film I2: Average emission intensity for N in the base steel sheet I3: Maximum emission intensity for Ti in the chemical conversion coating film I4: Average emission intensity for Ti in the base steel sheet I5: Maximum emission intensity for S in the chemical conversion coating film I6: Average emission intensity for S in the base steel sheet I7: Maximum emission intensity for Al in the chemical conversion coating film I8: Average emission intensity for Al in the base steel sheet

[0086]

Table 2

[0087] 3. Evaluation Results Referring to Table 2, in Comparative Examples 36 and 38, it is believed that the concentration of inhibitors, which are N-containing compounds, in the pickling solution was low during the pickling process, resulting in insufficient adsorption of N components onto the surface of the hot-rolled steel sheet and thus inability to promote the formation of N-Ti bonds. As a result, the "I1 / I2" value in the final chemical conversion treated film was less than 1.1, leading to decreased corrosion resistance after painting. In Comparative Example 37, it is believed that the low line speed during the pickling process resulted in insufficient formation of N-Ti bonds on the surface of the hot-rolled steel sheet, and thus inability to sufficiently suppress or reduce the formation of Ti oxides. As a result, the "I3 / I4" value in the final chemical conversion treated film was greater than 5.0, leading to decreased corrosion resistance after painting.

[0088] In contrast, in Examples 1 to 35, by using an acid pickling solution containing an inhibitor, which is an N-containing compound, at a concentration of 10 to 1500 ppm during the pickling process, and by controlling the hot-rolled steel sheet to pass through the acid pickling solution at a line speed of 0.10 to 10.00 m / s, it is believed that the formation of Ti oxides on the surface of the steel sheet was suppressed or reduced, and N-Ti bonds were formed on the surface of the steel sheet instead of the Ti oxides. As a result, the "I1 / I2" and "I3 / I4" ratios in the final chemical conversion coating fell within the predetermined range, and the corrosion resistance after painting was significantly improved. Among Examples 1 to 35, Examples 5 to 9, 13 to 17, 22 to 26, and 31 to 35, which underwent a skin pass rolling process with a reduction ratio of 0.6% or higher, showed an I1 / I2 ratio of 1.8 or higher, resulting in better corrosion resistance after painting. Furthermore, Examples 6 to 9, 14 to 17, 23 to 26, and 32 to 35, which underwent a skin pass rolling process with a reduction ratio of 0.6% or higher, a pickling process using a pickling solution with an N compound concentration of 400 ppm or higher at a line speed of 1.50 m / s or higher, and an alkali treatment process, showed an even larger I1 / I2 ratio, resulting in even better corrosion resistance after painting.

[0089] Furthermore, the Preparen Z used as a surface conditioning agent before chemical conversion treatment in the above examples contains Ti colloid. Generally, surface conditioning before chemical conversion treatment includes surface conditioning using Ti colloid and surface conditioning using zinc particles, and it is known that surface conditioning using zinc particles is more advantageous for chemical conversion treatment. However, as shown in these examples, according to the technology of this disclosure, even if surface conditioning using Ti colloid, which is not as advantageous as surface conditioning using zinc particles, is performed, the corrosion resistance after coating can be sufficiently improved.

[0090] Based on the results shown in Examples 1 to 35, surface-treated steel sheets that satisfy the following requirements (A) to (C) can be said to have excellent corrosion resistance after painting.

[0091] (A) The surface-treated steel sheet comprises a base steel sheet and a chemical conversion coating disposed on the surface of the base steel sheet. (B) The chemical composition of the base steel sheet is, in mass%, Ti: 0.050 to 1.000% and Al: 0.050 to 2.000%. (C) When elemental analysis is performed from the surface of the surface-treated steel sheet in the thickness direction by glow discharge emission analysis, the following relationships (1) and (2) are satisfied. 1.1 ≤ I1 / I2 ≤ 20.0 (1) 1.1 ≤ I3 / I4 ≤ 5.0 (2) I1: Maximum luminescence intensity for N in the chemical conversion treated film. I2: Average luminescence intensity for N in the base steel plate. I3: Maximum luminescence intensity for Ti in the chemical conversion treated film. I4: Average luminescence intensity of Ti in the base steel sheet.

Claims

1. Surface-treated steel sheet, Base material steel plate, and A chemical conversion coating is placed on the surface of the base steel plate. Equipped with, The chemical composition of the aforementioned base steel sheet is, in mass%, Ti: 0.050 to 1.000%, and Al: 0.050-2.000%, Includes, When elemental analysis is performed from the surface of the surface-treated steel sheet in the thickness direction by glow discharge emission spectrometry, the following relationships (1) and (2): 1.1≦I １ / I ２ ≦20.0 (1) 1.1≦I ３ / I ４ ≦5.0 (2) I １ : Maximum luminescence intensity for N in the chemical conversion treated film I ２ : Average luminescence intensity for N in the base steel sheet I ３ : Maximum luminescence intensity for Ti in the chemical conversion treated film I ４ : Average luminescence intensity for Ti in the base steel sheet The condition is met. Surface-treated steel sheet.

2. A surface-treated steel sheet according to claim 1, The following relationships (1-1) and (2-1): 1.5≦I １ / I ２ ≦20.0 (1-1) 1.3≦I ３ / I ４ ≦5.0 (2-1) The condition is met. Surface-treated steel sheet.

3. A surface-treated steel sheet according to claim 1, The following relationships (1-2) and (2-2): 1.8≦I １ / I ２ ≦20.0 (1-2) 1.4≦I ３ / I ４ ≦5.0 (2-2) The condition is met. Surface-treated steel sheet.

4. A surface-treated steel sheet according to any one of claims 1 to 3, The following relationship (3): 1.2≦I ５ / I ６ ≦20.0 (3) I ５ : Maximum luminescence intensity for S in the chemical conversion treated film I ６ : Average luminescence intensity of S in the base steel sheet The condition is met. Surface-treated steel sheet.

5. A surface-treated steel sheet according to any one of claims 1 to 3, The maximum concentration of Ni in the aforementioned chemical conversion treatment film is 2.0% by mass or more. Surface-treated steel sheet.

6. A surface-treated steel sheet according to any one of claims 1 to 3, Having a Vickers hardness of 190 Hv or higher, Surface-treated steel sheet.

7. A surface-treated steel sheet according to claim 6, Having a Vickers hardness of 300 Hv or more, Surface-treated steel sheet.

8. A surface-treated steel sheet according to any one of claims 1 to 3, The chemical composition of the aforementioned base steel sheet is, in mass%, Ti: 0.080–0.300%, and Al: 0.080-0.500%, including, Surface-treated steel sheet.

9. A surface-treated steel sheet according to any one of claims 1 to 3, The chemical composition of the aforementioned base steel sheet is, in mass%, Ni: 0.040-1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004-1.000%, Including one or more of the following: Surface-treated steel sheet.

10. A surface-treated steel sheet according to claim 9, The chemical composition of the aforementioned base steel sheet is, in mass%, Ni: 0.040-1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004-1.000%, including, Surface-treated steel sheet.

11. Includes a surface-treated steel sheet according to any one of claims 1 to 3, parts.

Images