Steel sheet and method for manufacturing same

Abstract

Provided is a steel sheet having excellent chemical conversion treatability and post-coating corrosion resistance even if a tramp element is contained. A steel sheet according to the present invention contains a specific amount of the tramp element. An integrated value Xa-b of the component concentration of an element X at a depth a-b (nm) by AES satisfies all of 3 × Sn0-3 / Sn3-10 ≤ 0.900, 2.00 ≤ Cr0-3 ≤ 12.00, and Cu0-3 ≤ 6.00.

Description

Steel plate and method for manufacturing the same

[0001] This invention relates to steel sheets and methods for manufacturing the same, and more particularly to steel sheets with excellent chemical treatment properties and corrosion resistance, and methods for manufacturing the same.

[0002] There are two methods for manufacturing steel plates: the blast furnace method, which uses blast furnace steel produced in a blast furnace using iron ore as the main raw material, and the electric arc furnace method, which uses electric arc furnace steel produced in an electric arc furnace using iron scrap as the main raw material. When steel is produced in a blast furnace, a large amount of coke is used, resulting in high emissions of carbon dioxide, a greenhouse gas, and raising concerns about its impact on global warming. On the other hand, when steel is produced in an electric arc furnace, although electrical energy is required to melt the iron scrap, it does not require the use of coke, which has the advantage of reducing carbon dioxide emissions.

[0003] In response to the recent rise in environmental awareness and the trend towards CO2 emission regulations, the production of steel sheets using the electric arc furnace method is being explored further. However, the iron scrap used as raw material contains trump elements such as Sn, Cu, Ni, Cr, and Mo, making it difficult to produce high-quality steel sheets. These trump elements can adversely affect the mechanical properties of the manufactured steel sheets and may also reduce their chemical treatment properties.

[0004] In response to this, technologies have been proposed to improve the chemical treatment properties of steel sheets containing trump elements. For example, Patent Document 1 describes a technology for obtaining a hot-rolled steel sheet with excellent chemical treatment properties by limiting the components in the steel and limiting the range in which the Ni content on the surface of the steel sheet is 0.5% by mass or more to 10-70%.

[0005] Furthermore, Patent Document 2 describes a technique for obtaining a steel sheet with excellent chemical conversion treatment properties by controlling the residue on the surface of the steel sheet to include microcathodes, and ensuring that these microcathodes contain at least 70 percent linear microcathodes. In Patent Document 2, the residue is defined as metal oxide particles and copper compound particles, and the microcathodes are defined as metal oxides or copper compounds with a higher potential than the base metal, and having a particle size of 2 μm or less.

[0006] International Publication No. 2021 / 157692, Japanese Patent Publication No. 2020-84325

[0007] On the other hand, in fields that use painted steel sheets, such as automobiles and construction, it is required that the paint film does not peel off even when subjected to external forces such as sliding and chipping when the steel sheet is exposed to the outdoors. Furthermore, it is required that the steel sheet does not corrode significantly from areas where scratches occur after painting. In other words, there is a growing need for steel sheets that do not compromise aesthetics and corrosion resistance even when used in harsh environments, especially cold-rolled steel sheets.

[0008] Thus, there is a need for cold-rolled steel sheets that maintain good chemical treatment properties even when containing trump elements, and that also have good corrosion resistance after painting.

[0009] However, Patent Documents 1 and 2 only consider technologies for improving the chemical treatment properties of steel sheets, and there is room for further consideration regarding improving the corrosion resistance of steel sheets after painting. In addition, Patent Documents 1 and 2 examine the chemical treatment properties of hot-rolled steel sheets, and do not examine improvements in the chemical treatment properties of cold-rolled steel sheets. Thus, Patent Documents 1 and 2 have not adequately considered improvements in chemical treatment properties and corrosion resistance after painting, particularly for steel sheets produced using the electric furnace method (cold-rolled steel sheets), and cannot satisfy the above-mentioned needs.

[0010] The present invention was made to improve upon the above-mentioned problems, and aims to provide a steel sheet that has excellent chemical treatment properties and excellent post-painting corrosion resistance, even when containing tramp elements, along with a method for manufacturing the same. Preferably, the aim is to provide a cold-rolled steel sheet that achieves both excellent chemical treatment properties and post-painting corrosion resistance, even when containing tramp elements, along with a method for manufacturing the same.

[0011] The inventors of the present invention have diligently studied to solve the above problems and have obtained the following findings regarding the effect of tramp elements on chemical conversion treatment properties and corrosion resistance after painting. <1> In steel sheets (cold-rolled steel sheets) containing tramp elements, the deposition of chemical conversion crystals is inhibited by the Sn-enriched layer formed on the surface. Therefore, good chemical conversion treatment properties can be obtained by removing the Sn-enriched layer. Annealing and pickling of cold-rolled steel sheets are effective in removing this Sn-enriched layer. <2> If the annealing is excessive, Cr is excessively concentrated on the surface of the steel sheet, which actually worsens the chemical conversion treatment properties. On the other hand, an appropriate amount of Cr concentrated on the surface of the steel sheet contributes to improving the corrosion resistance of the steel sheet after painting. Therefore, by adjusting the annealing and pickling conditions, the Cr content on the surface of the steel sheet can be controlled, and as a result, good chemical conversion treatment properties and corrosion resistance after painting can be obtained. <3> If the annealing and pickling are excessive, Cu is excessively concentrated on the surface of the steel sheet, which again worsens the chemical conversion treatment properties. Therefore, appropriate annealing and pickling conditions exist to suppress the impairing of chemical conversion treatment properties by Sn, Cr, and Cu.

[0012] This invention is based on the above findings, and its gist is as follows. [1] In mass percent, Cu: 0.01-0.50%, Ni: 0.01-1.00%, Sn: 0.001-0.100%, Cr: 0.02-0.30%, Mo: 0.001-1.000%, Zn: 0.500% or less, Pb: 0.500% or less, As: 0.500% or less, Sb: 0.500% or less, Bi: 0.500% or less, V: 0.500% or less, C: 0.50% or less, Si: 3.000% or less, Mn: 5.00% or less, B: 0.0100% or less, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, and N : Having a component composition containing 0.0100% or less, with the remainder being Fe and unavoidable impurities, and the integral value of the component concentration of element X in a region at depths a to b (unit: nm) from the surface, obtained by Auger electron spectroscopy (AES) on at least one surface of the steel plate. a-bA steel plate that satisfies the following equations (1) to (3) when the unit is at.% × nm. 3 × Sn 0-3 / Sn 3-10 ≦0.900...(1) 2.00≦Cr 0-3 ≦ 12.00...(2) Cu 0-3 ≤ 6.00 ... (3)

[0013] [2] The steel sheet according to [1] above, wherein a chemical conversion treatment coating is provided on the surface.

[0014] [3] A method for manufacturing a steel sheet, comprising, in order, an annealing step and a pickling step for a cold-rolled steel sheet having the component composition described in [1] above, wherein the annealing step is performed under conditions where the annealing temperature of the cold-rolled steel sheet is 700 to 900°C for a holding time of 50 to 400 seconds and the dew point during annealing is -40 to 0°C, and the pickling step is performed using a pickling bath with a temperature of 10 to 70°C and an acid concentration of 1 to 10% by mass, and the processing time for the cold-rolled steel sheet after annealing is 3 to 90 seconds.

[0015] [4] The method for manufacturing a steel sheet according to [3] above, wherein the acid component of the pickling bath is hydrochloric acid or sulfuric acid.

[0016] Here, the steel sheet in [1] can be, for example, a steel sheet manufactured using an electric furnace method, and can be a steel sheet containing multiple types of trump elements. The steel sheet in [1] can also be a cold-rolled steel sheet, can be a cold-rolled steel sheet manufactured using an electric furnace method, and can be a cold-rolled steel sheet containing multiple types of trump elements. Furthermore, the cold-rolled steel sheet used in the manufacturing method of [3] can be, for example, obtained from a slab manufactured using an electric furnace method, and can be obtained from a slab containing multiple types of trump elements.

[0017] According to the present invention, it is possible to provide a steel sheet that has good chemical treatment properties and corrosion resistance after painting, even when it contains predetermined amounts of multiple types of trump elements. Furthermore, according to the manufacturing method of the present invention, it is possible to manufacture a steel sheet that exhibits good chemical treatment properties and corrosion resistance after painting, even when it contains predetermined amounts of multiple types of trump elements.

[0018] This is an example of AES measurement results, and is a graph illustrating a case that does not satisfy formula (1). This is an example of AES measurement results, and is a graph illustrating a case that satisfies formula (1).

[0019] Hereinafter, the present invention will be specifically described. Note that the following description covers examples of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below. In the following description, the unit of content of each element in the component composition is "% by mass", and it is simply indicated by "%" unless otherwise specified. In addition, regarding numerical ranges, unless otherwise specified, the notation "~" means not less than the value described before "~" and not more than the value described after "~", and these endpoints are included in the range. Furthermore, when a unit is attached to only one of the numerical values described before and after "~", the same unit shall be attached to the other unless otherwise specified.

[0020] (Steel sheet) The steel sheet of the present invention contains predetermined amounts of Cu, Ni, Sn, Cr and Mo, and the contents of Sn, Cr and Cu over the surface layer region are each controlled within a predetermined range. By performing surface conditioning to control the content of these predetermined elements, even when the steel sheet contains a predetermined amount of tramp elements, the steel sheet can exhibit both excellent chemical convertibility and post-coating corrosion resistance. Since the steel sheet of the present invention is excellent in both chemical convertibility and post-coating corrosion resistance, it can be applied to a wide range of fields. In particular, it is useful as a steel sheet for automobiles, which requires high corrosion resistance for coated steel sheets. In addition, since the steel sheet of the present invention is excellent in chemical convertibility even when it contains tramp elements, it can be particularly usefully applied to electric furnace materials using iron scrap as a raw material, and exhibits exceptional industrially significant effects that are environmentally friendly. The steel sheet of the present invention can be favorably obtained, for example, according to the production method described later. More specifically, the contents of Sn, Cr and Cu in the surface layer region of the steel sheet can be controlled by annealing under predetermined conditions described later and surface conditioning by pickling.

[0021] The steel sheet may be a steel sheet produced by an electric furnace method, and the type thereof is not particularly limited. Examples of the steel sheet include common hot-rolled steel sheets or cold-rolled steel sheets such as ultra-low carbon steel, or hot-rolled steel sheets or cold-rolled steel sheets such as high-tensile steel. Among these, a cold-rolled steel sheet (including a cold-rolled steel sheet that has been subjected to steps such as annealing and pickling) is preferable from the viewpoint that the contents of Sn, Cr, and Cu in the surface layer region can be easily controlled within a predetermined range by the annealing step and pickling step described later. When the steel sheet is a cold-rolled steel sheet, the sheet thickness is usually 6 mm or less, preferably 3 mm or less, more preferably 1.5 mm or less, still more preferably 1.0 mm or less, and can usually be 0.5 mm or more.

[0022] The steel sheet of the present invention may further include a chemical conversion coating on at least one surface. Since the steel sheet of the present invention before the chemical conversion coating is formed has excellent chemical conversion treatability, it can be obtained as a steel sheet having a chemical conversion coating satisfactorily formed on the surface thereof.

[0023] First, the effect of the Sn-enriched layer formed on the surface of a steel sheet on chemical conversion treatability, which has been studied by the present inventors, will be described. The mechanism of the chemical conversion treatment reaction is as follows. When the steel sheet comes into contact with the chemical conversion treatment solution, iron is dissolved from the steel sheet, and as a counter reaction, reduction of hydrogen ions occurs in the chemical conversion treatment solution near the surface of the steel sheet. This is a reaction mechanism in which the pH at the interface of the steel sheet increases, and the chemical conversion coating precipitates on the surface of the steel sheet. Here, for example, when Sn is concentrated on the surface of the steel sheet, dissolution of iron is suppressed when the steel sheet comes into contact with the chemical conversion treatment solution. As a result, the transfer of electrons from the steel sheet to the chemical conversion treatment solution and the increase in pH at the steel sheet interface do not proceed normally. Thus, it is considered that the presence of the Sn-enriched layer formed on the surface of the steel sheet degrades the chemical conversion treatability.

[0024] Such deterioration of chemical conversion treatment properties due to Sn appears to occur mainly in zinc phosphate conversion treatment. Further investigation by the inventors revealed that the presence of Sn has a significant effect on nucleation, particularly in zinc phosphate conversion treatment, and that the presence of Sn greatly deteriorates the chemical conversion treatment properties of the steel sheet. In addition, it was found that the deterioration of the chemical conversion treatment properties of the steel sheet due to Sn prevents the electrodeposition coating that can be formed on the surface of the steel sheet from being carried out properly, resulting in inferior corrosion resistance after coating.

[0025] [Composition] The steel sheet of the present invention contains, by mass%, Cu: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Sn: 0.001 to 0.100%, Cr: 0.02 to 0.30%, and Mo: 0.001 to 1.000%. Furthermore, the steel sheet may optionally contain one or more elements selected from the group consisting of Zn: 0-0.500%, Pb: 0-0.500%, As: 0-0.500%, Sb: 0-0.500%, Bi: 0-0.500%, V: 0-0.500%, C: 0-0.50%, Si: 0-3.000%, Mn: 0-5.00%, B: 0-0.0100%, P: 0-0.100%, S: 0-0.020%, Al: 0-0.100%, and N: 0-0.0100%. The remainder of the component composition may consist of Fe and unavoidable impurities. When the lower limit of the above content is 0%, it includes cases where that element is not present.

[0026] Cu: 0.01-0.50% Cu is an element that is likely to be mixed in as a trump element, especially in steel sheets manufactured by the electric arc furnace method, and is likely to be mixed into the steel in a relatively large amount compared to other trump element elements. In steel sheets containing Cu, Cu is concentrated on the surface of the steel sheet, and the area in question has a higher potential than the surrounding area, which suppresses the dissolution reaction of Fe in the chemical conversion treatment, and chemical scaling occurs on the Cu-concentrated area. For these reasons, the Cu content should be 0.50% or less. In addition, although Cu is an element that contributes to increasing the strength of steel sheets, if there is too much, it becomes difficult to stabilize the mechanical properties of ultra-low carbon steel. Furthermore, for example, in pickling, there is concern about deterioration of chemical conversion treatment properties due to the concentration of Cu on the surface of the steel sheet. For this reason, it is preferable that the Cu content be 0.45% or less, and more preferably 0.30% or less. On the other hand, for example, in the electric arc furnace method, reducing the amount of Cu in iron scrap is disadvantageous in terms of cost. In addition to these economic reasons, the inclusion of Cu in the bulk (base metal) reduces the reactivity of the steel sheet itself, thereby improving the corrosion resistance of the steel sheet. For these reasons, the Cu content is set to 0.01% or more, preferably 0.02% or more, more preferably 0.05% or more, and even more preferably 0.07% or more.

[0027] Ni: 0.01-1.00% Ni is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric arc furnace method. In steel sheets containing Ni, the concentration of Ni as fine granules on the surface of the steel sheet forms microscopic cathodes, which promote the elution reaction of Fe around the Ni-enriched areas and improve the chemical conversion treatment properties by acting as crystal nuclei for the conversion crystals. In addition, there are economic reasons, such as the cost disadvantage of reducing Ni in iron scrap in the electric arc furnace method. For these reasons, the Ni content is set to 0.01% or more, preferably 0.03% or more, and more preferably 0.10% or more. On the other hand, if the Ni content is too high, the Ni-enriched areas on the surface of the steel sheet change from fine granules to large lumps, degrading the chemical conversion treatment properties on the Ni-enriched areas. Furthermore, it becomes necessary to add Ni to the iron scrap to adjust the Ni content. Therefore, the Ni content is 1.00% or less, preferably 0.85% or less, and more preferably 0.70% or less.

[0028] Sn: 0.001-0.100% Sn is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric arc furnace method. Sn forms a concentrated layer as an oxide on the surface of the steel sheet, inhibiting the reaction in the chemical conversion treatment solution, thus leading to a deterioration of the chemical conversion treatment properties. If the bulk of the steel sheet contains a large amount of Sn, a thick Sn concentrated layer is formed on the surface of the steel sheet, making it difficult to remove the Sn concentrated layer by, for example, pickling. In addition, when Sn in the steel sheet dissolves during the chemical conversion treatment, it re-deposits as hydroxide, coating the surface of the steel sheet and significantly reducing the chemical conversion treatment properties. For these reasons, the Sn content should be 0.100% or less, preferably 0.070% or less. On the other hand, reducing Sn in iron scrap is disadvantageous in terms of cost, for example, in the electric arc furnace method. For these economic reasons, the Sn content is set to 0.001% or more, preferably 0.004% or more, and more preferably 0.005% or more.

[0029] Cr: 0.02-0.30% Cr is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric arc furnace method. Similar to Sn, Cr dissolves in the steel sheet during chemical conversion treatment and then re-precipitations, coating the surface of the steel sheet and reducing the chemical conversion treatment properties. In addition, when Cr in the steel sheet is concentrated on the surface, for example by annealing, there is concern that the chemical conversion treatment properties may deteriorate due to the concentration of an excess amount of Cr. For these reasons, the Cr content should be 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less. On the other hand, for example, in the electric arc furnace method, reducing Cr in iron scrap is disadvantageous in terms of cost, which is an economic reason. In addition, the inclusion of a certain amount of Cr in the bulk of the steel sheet has the effect of reducing the reactivity of the steel sheet itself and improving the corrosion resistance of the steel sheet itself. In particular, it is expected that the corrosion resistance after painting will improve when an appropriate amount of Cr remains on the surface of the steel sheet. For these reasons, the Cr content should be 0.02% or more, preferably 0.03% or more, and more preferably 0.05% or more.

[0030] Mo: 0.001 to 1.000%. Mo is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric arc furnace method. For example, in the electric arc furnace method, reducing Mo in iron scrap is cost-ineffective. In addition to these economic reasons, Mo is an element that increases the strength of steel sheets. From these viewpoints, the Mo content should be 0.001% or more, preferably 0.003% or more. On the other hand, Mo has less influence on chemical treatment properties and corrosion resistance after painting compared to other trump elements, but if the Mo content is increased to increase the strength of the steel sheet, it becomes necessary to add Mo, which increases costs and is economically disadvantageous. For this reason, the Mo content should be 1.000% or less, preferably 0.800% or less, and more preferably 0.080% or less.

[0031] Zn: 0.500% or less. Zn is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. In order to avoid impairing the properties of the present invention, the Zn content is 0.500% or less, preferably 0.450% or less, and more preferably 0.200% or less. The Zn content may be 0%, for example, 0.001% or more, and 0.002% or more.

[0032] Pb: 0.500% or less. Pb is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. Pb is also an element that has the effect of reducing segregation. In order to avoid impairing the properties of the present invention, the Pb content is 0.500% or less, preferably 0.450% or less, and more preferably 0.200% or less. The Pb content may be 0%, for example, 0.001% or more, and 0.002% or more.

[0033] As: 0.500% or less. As is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. In order to avoid impairing the properties according to the present invention, the As content is 0.500% or less, preferably 0.450% or less, and more preferably 0.200% or less. The As content may be 0%, for example, 0.001% or more, and 0.002% or more.

[0034] Sb: 0.500% or less. Sb is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. In order to avoid impairing the properties according to the present invention, the Sb content is 0.500% or less, preferably 0.450% or less, and more preferably 0.200% or less. The Sb content may be 0%, for example, 0.001% or more, and 0.002% or more.

[0035] Bi: 0.500% or less. Bi is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. Bi is also an element that has the effect of reducing segregation. In order to avoid impairing the properties of the present invention, the Bi content is 0.500% or less, preferably 0.450% or less, and more preferably 0.200% or less. The Bi content may be 0%, and can be, for example, 0.001% or more, or 0.002% or more.

[0036] V: 0.500% or less. V is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. V is also an element that increases the strength of steel sheets through precipitation strengthening. In order to avoid impairing the properties of the present invention, the V content is 0.500% or less, preferably 0.450% or less, and more preferably 0.200% or less. The V content may be 0%, for example, 0.001% or more, and 0.003% or more.

[0037] C: 0.50% or less. C is an element that can be included from the viewpoint of improving the hardenability of steel sheets, improving strength by securing martensite, and controlling the volume fraction of residual γ to a desired range. However, if there is too much C, the area fraction of cementite increases and the workability decreases. For this reason, the C content should be 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less. The C content may be 0%, but from the viewpoint of ensuring the above effects, it is preferable to have 0.001% or more, and more preferably 0.01% or more.

[0038] Si: 3.000% or less. Si is an element that can be included to improve ferrite strength, suppress oxidation products in martensite and / or bainite, and stabilize residual γ to improve ductility. However, if there is too much Si, Fe2SiO4 formed on the surface of the steel sheet during hot rolling will remain, worsening the chemical conversion treatment properties. For this reason, the Si content should be 3.000% or less, preferably 2.550% or less, and more preferably 2.500% or less. The Si content may be 0%, but from the viewpoint of ensuring the above effects, it is preferable to have 0.001% or more, and more preferably 0.013% or more.

[0039] Mn: 5.00% or less. Mn is an element that ensures the desired hardenability, suppresses ferrite transformation, and secures the desired area ratio of tempered martensite and / or bainite to guarantee strength. However, if there is too much Mn, the bainite transformation is significantly delayed, making it difficult to ensure high ductility. For this reason, the Mn content is 5.00% or less, preferably 4.55% or less, and more preferably 4.50% or less. The Mn content may be 0%, but from the viewpoint of ensuring the above effects, it is preferable to have 0.02% or more, and more preferably 0.05% or more.

[0040] B: 0.0100% or less. B is an element that facilitates the formation of tempered martensite and / or bainite with a desired area ratio. It is also an element that can improve delayed fracture resistance due to the remaining solid solution of B. However, if there is too much B, it will lead to a significant decrease in hot rolling properties and cause surface defects. For this reason, the B content should be 0.0100% or less, preferably 0.0096% or less, and more preferably 0.0090% or less. The B content may be 0%, but from the viewpoint of ensuring the above effects, it is preferable to have 0.0001% or more.

[0041] P: 0.100% or less. P is an element that strengthens steel. However, if the P content is too high, it deteriorates the spot weldability. For this reason, the P content should be 0.100% or less, preferably 0.090% or less, and more preferably 0.080% or less. The P content may be 0%, or for example, 0.001% or more.

[0042] S: 0.020% or less. S is an element that has the effect of improving scale detachability during hot rolling and suppressing nitriding during annealing. However, if the S content is too high, it will cause deterioration of spot weldability and local elongation. For this reason, the S content should be 0.020% or less, preferably 0.019% or less, and more preferably 0.018% or less. The S content may be 0%, and from the viewpoint of suppressing scale detachment during hot rolling, it can be, for example, 0.001% or more, or 0.003% or more.

[0043] Al: 0.100% or less. Al is an element that can deoxidize and stabilize residual γ as a substitute for Si. However, if the Al content is too high, the strength of the material will decrease drastically. For this reason, the Al content should be 0.100% by mass or less, preferably 0.080% or less. The Al content may be 0%, and from the viewpoint of stabilizing residual γ, it can be, for example, 0.001% or more, or 0.020% or more.

[0044] N: 0.0100% or less. N is an element that forms nitrides such as BN, AlN, and TiN in steel, and can reduce the hot rollability of steel and lower the surface quality. In addition, in steel containing B, there is the disadvantage of losing the effect of B through the formation of BN. For these reasons, the N content should be 0.0100% or less, preferably 0.0095% or less, and more preferably 0.0090% or less. The N content may be 0%, and from the viewpoint of cost increase due to N removal and ensuring strength through nitride formation, it can be, for example, 0.0002% or more, or 0.0005% or more.

[0045] The steel sheet has a component composition consisting of the above elements, the remainder being Fe, and unavoidable impurities.

[0046] [Surface layer structure] It is essential that, in at least one surface layer region of the steel sheet of the present invention, the contents of the elements Sn, Cr and Cu are controlled to predetermined amounts. According to studies conducted by the present inventors, by controlling the amounts of Sn, Cr and Cu in the surface layer region of the steel sheet so as to satisfy all of formulas (1) to (3), both chemical conversion treatability and coating corrosion resistance can be improved even for a steel sheet containing multiple types of tramp elements. Formulas (1) to (3) only need to be satisfied on one side of the steel sheet surface, but it is preferable that they are satisfied on both sides of the steel sheet surface.

[0047] The contents of Sn, Cr and Cu in the surface layer region of the steel sheet can be defined and confirmed from component concentration data versus depth obtained for each element by performing AES measurement on the surface (outermost surface) of the steel sheet. For example, to measure the component concentration of each element at a position 0 nm depth from the steel sheet surface, that is, on the steel sheet surface, AES measurement may be performed directly on the steel sheet surface. Further, to measure the component concentration of each element at a position 3 nm depth from the steel sheet surface, AES measurement may be performed on the steel sheet after sputtering the steel sheet surface to remove 3 nm in the depth direction (thickness direction). In this way, the component concentration of each element in a desired depth region can be measured.

[0048] Specifically, let X a-b be the integrated value of the component concentration of element X over a region from depth a (unit: nm) to depth b (unit: nm) inclusive, obtained by measurement in the depth direction from the steel sheet surface by the aforementioned AES, with the unit being at.%×nm, then: 3×Sn 0-3 / Sn 3-10 (unitless) ≦ 0.900 ···(1) 2.00 ≦ Cr 0-3 (unit: at.%×nm) ≦ 12.00 ···(2) Cu 0-3The following three conditions must all be met: (Unit: at.% × nm) ≤ 6.00 ... (3). Here, in this specification, the region in the depth direction starting from a depth of 0 nm from the surface of the steel plate (the surface of the steel plate itself) and ending at a depth of 3 nm from the surface may be referred to as the "surface region". Also, the region in the depth direction starting from a depth of 3 nm from the above surface and going to deeper depths may be referred to as the "internal region" for convenience. Furthermore, "at.%" refers to atomic percentage.

[0049] Figures 1 and 2 show examples of data obtained by measuring each element in the depth direction from the surface of the steel plate using AES. Figures 1 and 2 are graphs plotting the sputter depth (unit: nm), which represents the depth position of the steel plate, on the horizontal axis and the component concentration of each element (unit: at.%) on the vertical axis. Figure 1 is the measurement result at No. 1 in the example described later, and is an example to explain a comparative example that does not satisfy the conditions of the present invention. In the case of Figure 1, the integral value of the component concentration of Sn in the surface region at a depth of 0 to 3 nm is Sn. 0-3 And, the integral value of the Sn component concentration in the internal region at a depth of 3 to 10 nm. 3-10 Therefore, the value of the left side of equation (1) is 3 × Sn 0-3 / Sn 3-10 = 1.060. This value is greater than 0.900, so the condition of equation (1) is not satisfied. This is because Sn is present as a concentrated layer on the surface of the steel plate, 0-3 This is due to an increase in the value of . As a result, in Example No. 1 described later, the dissolution of the steel sheet was suppressed by the Sn-enriched layer during the chemical conversion treatment, resulting in poor chemical conversion treatment performance, and the evaluation of chemical conversion treatment performance was × (unsuitable). Furthermore, the corrosion resistance after painting was also × (unsuitable). Note that in the example in Figure 1, the value of Cr in formula (2) 0-3 , Cr 0-3 (at.% × nm) = 4.63, which is within the range of 2.00 to 12.00. Also, the value Cu in equation (3) 0-3 is Cu 0-3 (at.% × nm) = 1.45, which is within the range of 6.00 or less. Therefore, the specified conditions are satisfied for these Cr and Cu.

[0050] Figure 2 shows the measurement results at No. 6 in the embodiment described later, and is an example to illustrate an inventive example that satisfies the conditions of the present invention. In the case of Figure 2, the value on the left side of equation (1) is 3 × Sn 0-3 / Sn 3-10 = 0.758, satisfying the specified conditions. This is because the concentrated Sn on the surface of the steel plate was effectively removed by pickling, exposing a region with less Sn (Sn-deficient layer) on the surface of the steel plate. As a result, the Sn concentration near the surface of the steel plate decreased, and Sn 0-3 This is because it has become relatively smaller. Also, in the example in Figure 2, the value Cr in equation (2) 0-3 , Cr 0-3 (at.% × nm) = 7.69, which is within the range of 2.00 to 12.00. Furthermore, the value Cu in equation (3) 0-3 is Cu 0-3 (at.% × nm) = 2.90, which is within the range of 6.00 or less. Therefore, the specified conditions are satisfied for all of these Sn, Cr, and Cu. As a result, in Example No. 6 described later, the chemical conversion treatment properties were improved without impairing the corrosion resistance after painting, and both the chemical conversion treatment properties and the corrosion resistance after painting were evaluated as excellent (◎).

[0051] The reasons for formulating equations (1) to (3) are explained below.

[0052] The inventors diligently investigated the causes of deterioration in the chemical conversion treatment properties and post-painting corrosion resistance of steel sheets having the above-mentioned component composition. They obtained the following findings: In steel sheets containing tramp elements, especially cold-rolled steel sheets, the Sn-enriched layer formed in the surface region of the steel sheet suppresses the dissolution of the steel sheet into the chemical conversion treatment solution, thereby inhibiting the precipitation of chemical conversion crystals. If the amount of Cr dissolved or precipitated in the surface region of the steel sheet is in excess of a predetermined value, the Cr will coat the surface of the steel sheet during the chemical conversion treatment, inhibiting the precipitation of chemical conversion crystals in the same way as Sn. On the other hand, by including Cr in the surface region of the steel sheet above a predetermined value, good post-painting corrosion resistance of the steel sheet can be ensured. <C> If the amount of Cu dissolved or precipitated in the surface region of the steel sheet is in excess of a predetermined value, chemical conversion scaling will occur on the Cu-enriched area during the chemical conversion treatment. Chemical conversion scaling is a state of chemical conversion treatment failure in which chemical conversion crystals are not precipitated on the surface of the steel sheet.

[0053] Formula (1) In response to the phenomenon described in A above, the inventors explored techniques to improve chemical conversion treatment properties even when a Sn-enriched layer is formed on the surface of a steel sheet (cold-rolled steel sheet) containing trump elements. As a result, it was found that chemical conversion treatment properties can be improved by cleaning the surface of the steel sheet by pickling under appropriate conditions. This means that the pickling removes the Sn-enriched layer formed on the surface of the steel sheet, exposing the Sn-deficient region (Sn-scarce region) directly below the depth direction of the Sn-enriched layer to the surface of the steel sheet. In other words, it was found that good chemical conversion treatment properties can be obtained by making the Sn concentration in the surface region of the steel sheet sufficiently lower than the Sn concentration in the internal region of the steel sheet. Furthermore, it was found that good chemical conversion treatment properties lead to the formation of a good chemical conversion film, and as a result, the corrosion resistance after painting when a coating is applied on top of it can also be improved. Here, "Sn-deficient region" refers to a region in which Sn is present in the in-plane direction with a content lower than the average Sn content contained in the entire steel sheet, or a region in which Sn is absent in the in-plane direction. This "Sn-deficient region" can be identified, for example, by XPS; energy-dispersive X-ray spectroscopy (EDX) during transmission electron microscopy (TEM) observation in the cross-sectional direction.

[0054] To expose the Sn-deficient region on the steel sheet surface, it is crucial to first anneal the steel sheet (cold-rolled steel sheet) under specified conditions for a certain period of time. When the steel sheet is annealed under specified conditions for a certain period of time, the Sn dissolved in the base metal diffuses to the surface of the steel sheet and becomes concentrated, allowing for the intentional formation of an oxide Sn film on the surface of the steel sheet (cold-rolled annealed sheet). At this time, the movement of Sn from the base metal to the oxide Sn film on the surface creates an Sn-deficient region between the surface and the interior of the steel sheet. In the unpickled state of the steel sheet (cold-rolled annealed sheet), the presence of a Sn-concentrated layer on the surface results in inferior chemical conversion treatment properties. However, by removing the intentionally formed Sn-concentrated layer on the surface of the steel sheet through subsequent pickling under specified conditions, an improvement in chemical conversion treatment properties can be obtained. In other words, to ensure excellent chemical conversion treatment properties, it is important to control the Sn concentration in the surface region, extending from the surface of the steel sheet towards the depth. The inventors formulated these effects based on experimental data, obtaining equation (1). 3 × Sn 0-3 / Sn 3-10 ≤ 0.900 ... (1)

[0055] As shown in the figure above, Sn in equation (1) 0-3 (at.% × nm) is obtained by integrating the quantitative Sn concentration measured in the surface region of the steel plate from 0 to 3 nm in depth from the surface using Auger electron spectroscopy (AES). Similarly, the Sn in equation (1) 3-10 (at.% × nm) is obtained by integrating the quantitative Sn concentration measured by AES in the internal region of the steel plate at a depth of 3 to 10 nm from the surface.

[0056] The specific measurement method for AES is described below. The measurement device used was the PHI710 manufactured by ULVAC-FI. The measurement conditions were a vacuum level of 7.0E-7Pa (7.0 × 10⁻¹⁰) inside the device. -7The electron gun acceleration voltage was set to 10 kV and the current to 10 nA. Ar sputtering was performed from the surface of the steel plate at a pitch of 1 nm in the depth direction, and the distribution of component concentrations from the surface of the steel plate to the depth direction was measured for each element. The component concentration of each element was quantified using the average matrix relative sensitivity coefficient. The peaks and energy bands used for quantification were Sn: 425–442 eV, Cr: 523–533 eV, Cu: 910–925 eV. This procedure was repeated until the sputtering depth was 10 nm. After that, the measurement results were graphed with the sputtering depth (nm) on the horizontal axis and the component concentration for each element (at.%) on the vertical axis (see also Figures 1 and 2). From the obtained graph, the component concentration over a specified depth region for each element was calculated as the area, thereby determining the integral concentration value Sn 0-3 Sn 3-10 , Cr 0-3 and Cu 0-3 This can be determined. When performing AES analysis, the measurement conditions are not limited to those described above, and similar conditions can be adopted.

[0057] 3 × Sn in equation (1) 0-3 / Sn 3-10 By suppressing the value (unitless) to 0.900 or less, it means that the Sn concentration in the surface region of the steel plate is controlled to be lower to a predetermined extent than the Sn concentration in the internal region of the steel plate. In this way, in order to achieve improved chemical treatment properties and corrosion resistance after painting, 3 × Sn 0-3 / Sn 3-10 The value of must be 0.900 or less, preferably 0.800 or less. On the other hand, the surface of the steel plate may be optionally plated according to the desired properties. In this case, from the viewpoint of improving the plating properties, and especially from the viewpoint of improving the hot-dip plating properties, 3×Sn 0-3 / Sn 3-10 The value of is preferably 0.400 or higher, more preferably 0.500 or higher, and even more preferably 0.650 or higher.

[0058] Formula (2) As a result of diligent research, the inventors have also found that it is important to consider the phenomenon described in B above in order to improve corrosion resistance after painting. That is, Cr contained in steel is dissolved or precipitated on the surface of the steel sheet by heat treatment, etc. When an appropriate amount of Cr is present in the surface region of the steel sheet, the corrosion resistance performance of Cr is exhibited, and the corrosion resistance of the steel sheet after painting can be improved. If the amount of Cr in the surface region of the steel sheet is less than a predetermined range, the corrosion resistance performance of Cr cannot be exhibited, and corrosion resistance after painting cannot be ensured. On the other hand, if the amount of Cr in the surface region of the steel sheet is excessive, it coats the surface of the steel sheet during chemical conversion treatment and inhibits the precipitation of chemical conversion crystals.

[0059] Furthermore, annealing and pickling conditions are crucial for concentrating a predetermined amount of Cr on the steel sheet surface. First, by performing annealing under appropriate conditions, an appropriate amount of Cr can diffuse and concentrate on the steel sheet surface, contributing to improved corrosion resistance. One of the important conditions during annealing is controlling the dew point, which can alter the diffusion of Cr. By controlling it towards a higher dew point, it becomes easier to oxidize more Cr. As a result, the diffusion of Cr toward the surface of the steel sheet becomes more active, and Cr can be more concentrated in the surface region of the steel sheet. On the other hand, if the dew point during annealing is too high, the oxidation of Si and Mn will also be promoted, and there is a risk that oxides that cannot be completely removed in the subsequent pickling process will be formed.

[0060] Furthermore, regarding the pickling conditions after annealing described above, in order to properly utilize the corrosion resistance performance of Cr, it is necessary to first sufficiently remove the aforementioned Sn-enriched layer. If the pickling is insufficient and the Sn-enriched layer cannot be completely removed, Cr will be embedded below the depth of the Sn-enriched layer remaining on the outermost surface of the steel sheet, and the corrosion resistance-improving effect of Cr will not be fully realized. As a result, the effect of improving corrosion resistance after painting will not be realized. On the other hand, if pickling is performed excessively, Cr will be removed from the surface region of the steel sheet along with the Sn-enriched layer, and the effect of improving corrosion resistance will not be obtained. In this way, by optimizing the annealing and pickling conditions, the amount of Cr in the surface region of the steel sheet can be controlled, and effects on not only chemical conversion treatment but also corrosion resistance after painting can be ensured. The inventors formulated these effects based on experimental data and obtained equation (2). 2.00 ≤ Cr 0-3 (at.%×nm) ≦ 12.00...(2)

[0061] As shown in the figure and above, Cr in equation (2) 0-3 (at.% × nm) is obtained by integrating the quantitative value of Cr concentration measured in the surface region of the steel plate from 0 to 3 nm in depth from the surface using AES. The measurement conditions and calculation method are as described above. Cr in Equation (2) 0-3 The smaller the value of Cr, the lower the corrosion resistance after painting due to the above mechanism. 0-3 The value of is set to 2.00 or higher, preferably 5.00 or higher. On the other hand, Cr in formula (2) 0-3 The larger the value of , the lower the chemical treatment performance due to the above mechanism. Therefore, Cr 0-3 The value of is set to 12.00 or less, preferably 10.00 or less.

[0062] Formula (3) In response to the phenomenon described in C above, the inventors have found that by controlling the amount of Cu in the surface region of the steel sheet containing the trump element, even better chemical conversion treatment performance can be ensured. When there is an excess of Cu near the surface of the steel sheet, it leads to a deterioration in chemical conversion treatment performance. In particular, on the Cu-enriched areas in the surface region of the steel sheet, the dissolution of the steel sheet into the chemical conversion treatment solution does not occur normally, and the formation of nuclei for chemical conversion crystal grains does not occur normally. As a result, coarsening of chemical conversion crystal grains and / or chemical scaling are observed, especially on these Cu-enriched areas. Furthermore, this deterioration in chemical conversion treatment performance makes it difficult to form a good chemical conversion coating, and as a result, the corrosion resistance after painting when a coating is applied on top of it also deteriorates.

[0063] Furthermore, further optimization of pickling conditions is crucial to control the amount of Cu in the surface region of the steel sheet. As mentioned above, pickling can improve the chemical conversion treatment properties of Sn and Cr. On the other hand, since Cu is poorly soluble in acids such as hydrochloric acid and sulfuric acid, pickling causes Cu to concentrate and accumulate on the surface of the steel sheet. In particular, the greater the amount of steel sheet dissolved during pickling, the more Cu in the bulk (base metal) accumulates on the surface. This means that over-pickling further concentrates Cu near the surface of the steel sheet, resulting in inferior chemical conversion treatment properties. In other words, by further optimizing the pickling conditions, the amount of Cu in the surface region of the steel sheet can be suppressed, and the improvement effect of pickling on chemical conversion treatment properties can be maximized. The inventors formulated these effects based on experimental data and obtained equation (3). Cu 0-3 (at.%×nm) ≦ 6.00...(3)

[0064] As shown in the figure above, Cu in equation (3) 0-3 (at.% × nm) is obtained by integrating the quantitative value of Cu concentration measured in the surface region of the steel plate from 0 to 3 nm in depth from the surface using AES. The measurement conditions and calculation method are as described above. Cu in equation (3) 0-3The value of is set to 6.00 or less, preferably 5.30 or less, and more preferably 5.00 or less, from the viewpoint of improving the precipitation of chemical crystal grains during chemical treatment and from the viewpoint of improving corrosion resistance after painting. On the other hand, from the viewpoint of improving corrosion resistance, Cu 0-3 The value of is preferably 0.30 or higher, more preferably 0.50 or higher, and even more preferably 0.80 or higher.

[0065] (Method for Manufacturing Steel Sheets) The present invention provides a method for manufacturing steel sheets, in which a cold-rolled steel sheet containing predetermined amounts of Cu, Ni, Sn, Cr, and Mo is subjected to at least an annealing process under predetermined conditions and a pickling process under predetermined conditions, in sequence. By controlling the composition of the cold-rolled steel sheet and the conditions of the annealing and pickling processes within predetermined ranges, it is possible to manufacture a steel sheet that exhibits both excellent chemical conversion treatment properties and excellent post-painting corrosion resistance, even when containing predetermined amounts of multiple trump elements. The manufacturing method of the present invention yields a steel sheet with excellent chemical conversion treatment properties and post-painting corrosion resistance, making it applicable to a wide range of fields. In particular, it is useful as a method for manufacturing automotive steel sheets, which are used in various harsh environments after painting. Furthermore, since the manufacturing method of the present invention yields a steel sheet with excellent chemical conversion treatment properties and post-painting corrosion resistance even when containing trump elements, it can be particularly usefully applied to electric furnace methods using iron scrap as raw material, providing exceptional industrial benefits in an environmentally conscious manner. Furthermore, according to the manufacturing method of the present invention, for example, the steel sheet of the present invention described above can be obtained in good condition.

[0066] In addition to the steps described above, the manufacturing method of the present invention may optionally further include other steps. Other steps are not particularly limited and include, for example, a steel slab manufacturing step, a hot rolling step for the steel slab, a pickling step for the hot-rolled steel sheet, a cold rolling step for the hot-rolled steel sheet, and a rust-preventive oil application step for the cold-rolled steel sheet.

[0067] [Preparation of Steel Slabs] An example of the steel slab preparation process is described below. For the preparation of steel slabs, molten steel derived from electric furnace materials using iron scrap as a raw material may be used, or molten steel derived from electric furnace materials may be mixed with molten steel derived from blast furnace materials. Alternatively, the components of the molten steel derived from blast furnace materials can be adjusted by conventional methods to achieve a predetermined component composition. In this way, using molten steel with a component composition containing the above-mentioned component composition, a steel slab having a component composition containing at least Cu: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Sn: 0.001 to 0.100%, Cr: 0.02 to 0.30%, and Mo: 0.001 to 1.000% can be obtained by conventional methods.

[0068] [Hot Rolling] An example of the hot rolling process is described below. In the hot rolling process, a steel slab having the above-mentioned component composition is heated and then rolled to obtain a hot-rolled steel sheet. The heating conditions for the steel slab and the hot rolling conditions can follow conventional methods.

[0069] [Pickling of Hot-Rolled Steel Sheets] An example of a pickling process that can be carried out after hot rolling is described. In this pickling process, the obtained hot-rolled steel sheet is pickled to remove the oxide scale present on the surface of the hot-rolled steel sheet. The pickling conditions can follow conventional methods.

[0070] [Cold Rolling] An example of the cold rolling process is described below. In the cold rolling process, a cold-rolled steel sheet with a desired thickness can be obtained by further rolling the hot-rolled steel sheet. The thickness of the cold-rolled steel sheet is not particularly limited, but is usually 6 mm or less, preferably 3 mm or less, more preferably 1.5 mm or less, even more preferably 1.0 mm or less, and can be 0.5 mm or more. The rolling rate in cold rolling is preferably 30% or more, and more preferably 40% or more, with a cumulative rolling rate of 30% or more. During hot rolling and pickling of hot-rolled steel sheets, Sn may become concentrated on the surface of the steel sheet, and the chemical conversion treatment properties are further impaired in steel sheets with concentrated Sn on the surface. Therefore, by thinning the Sn-concentrated layer through cold rolling, the adverse effect of Sn on the chemical conversion treatment properties can be further reduced. As a result, the improvement effect of the chemical conversion treatment properties by subsequent annealing and pickling treatments of the cold-rolled steel sheet can be further enhanced. If the cumulative rolling rate is less than 30%, a relatively large amount of the Sn-enriched layer remains, and the improvement effect on chemical conversion treatment properties may not be enhanced. On the other hand, there is no upper limit to the cumulative rolling rate from the viewpoint of improving chemical conversion treatment properties, but from the viewpoint of cold rolling load and material properties, it is preferable to keep the cumulative rolling rate at 85% or less. Otherwise, the manufacturing method of ordinary cold-rolled steel sheets can be followed.

[0071] [Annealing of Cold-Rolled Steel Sheets] In the annealing process for cold-rolled steel sheets, the annealing temperature, holding time, and dew point must be within a predetermined range. By performing annealing after cold rolling, Sn dissolved in the base metal diffuses to the surface of the steel sheet and becomes concentrated, allowing for the intentional formation of an oxide Sn film in the surface region of the steel sheet. At this time, Sn moves from the base metal to the oxide Sn film in the surface region of the steel sheet, creating an Sn-deficient region on the surface of the base metal. As a result, when pickling is performed in the next step, an Sn distribution in the depth direction can be formed, which improves the chemical treatment properties. In addition, as described above, annealing under predetermined conditions allows for appropriate diffusion of Cr to the surface of the steel sheet, thereby improving corrosion resistance after painting. Annealing of cold-rolled steel sheets is preferably performed in a non-oxidizing atmosphere for iron in order to prevent oxidation of the steel sheet surface.

[0072] The annealing temperature of cold-rolled steel sheets must be between 700°C and 900°C. If the annealing temperature is below 700°C, the concentration, diffusion, and migration of Sn on the steel sheet surface will be insufficient, preventing the formation of an Sn-deficient region between the steel sheet surface and the interior, and thus the improvement in chemical conversion treatment properties due to subsequent pickling cannot be expected. Furthermore, the concentration of Cr on the steel sheet surface will also be insufficient, and the improvement in corrosion resistance after painting cannot be expected. On the other hand, if the annealing temperature exceeds 900°C, the material properties of the steel sheet may deteriorate. In addition, the concentration of Sn on the steel sheet surface will be excessive, and the Sn-concentrated layer cannot be sufficiently removed by subsequent pickling, so the improvement in chemical conversion treatment properties cannot be expected. The annealing temperature is preferably 710°C or higher, more preferably 720°C or higher, preferably 880°C or lower, and more preferably 850°C or lower.

[0073] The holding time within the above range of annealing temperature must be 50 seconds or more, preferably 80 seconds or more, and more preferably 150 seconds or more. If the holding time is less than 50 seconds, the concentration, diffusion, and migration of Sn are insufficient, and the chemical conversion treatment properties are not improved. Also, the concentration of Cr on the steel sheet surface is insufficient, and the corrosion resistance after painting is not improved. On the other hand, the holding time within the above range of annealing temperature must be 400 seconds or less, preferably 350 seconds or less, and more preferably 250 seconds or less. If the holding time exceeds 400 seconds, Sn and Cu may be excessively concentrated on the steel sheet surface, and in addition, Cr may also be excessively concentrated, so the chemical conversion treatment properties decrease. Furthermore, from the viewpoint of productivity, exceeding 400 seconds is undesirable because the manufacturability decreases.

[0074] When annealing at the above-mentioned temperature and holding time, the annealing dew point must be between -40°C and 0°C. Preferably, the dew point is above -30°C, preferably below -5°C, and more preferably between -30°C and -5°C. If the annealing dew point is below -40°C, the concentration, diffusion, and migration of Sn will be insufficient, and the chemical conversion treatment properties will not improve. Also, the concentration of Cr on the steel sheet surface will be insufficient, and the corrosion resistance after painting will not improve. On the other hand, if the annealing dew point exceeds 0°C, Sn, Cr, and Cu will be excessively concentrated on the steel sheet surface, reducing the chemical conversion treatment properties. In addition, oxides of Si and Mn will be concentrated on the steel sheet surface, and the improvement effect on chemical conversion treatment properties by pickling cannot be expected.

[0075] [Pickling of Cold-Rolled Steel Sheets] In the pickling process for cold-rolled steel sheets after annealing, it is necessary to keep the temperature and acid concentration of the pickling bath within a predetermined range, and the pickling treatment time within a predetermined range. By pickling the cold-rolled steel sheet after annealing, the Sn-enriched layer formed near the surface of the steel sheet by annealing is removed, exposing the Sn-deficient region to the surface region of the steel sheet and improving the chemical conversion treatment properties. Furthermore, by following appropriate pickling conditions, it is possible to suppress the amount of Cu in the surface region of the steel sheet while leaving an appropriate amount of Cr in the surface region. This makes it possible to achieve both excellent chemical conversion treatment properties and paint corrosion resistance of the steel sheet.

[0076] The temperature of the pickling bath used during pickling must be between 10°C and 70°C, preferably above 20°C, preferably below 60°C, and more preferably between 20°C and 60°C. If the bath temperature falls below 10°C, the effect of removing the Sn-enriched layer cannot be sufficiently obtained, and good chemical conversion treatment performance cannot be ensured. If the bath temperature exceeds 70°C, excessive dissolution of the steel sheet will cause Cu enrichment on the surface of the steel sheet, resulting in a decrease in chemical conversion treatment performance.

[0077] The acid concentration in the acidic bath must be between 1% and 10% by mass. If the acid concentration is below 1%, the effect of removing the Sn-enriched layer will not be sufficient, and good chemical conversion treatment performance will not be ensured. If the acid concentration exceeds 10%, excessive dissolution of the steel plate will occur, leading to Cu enrichment on the steel plate surface, thus reducing chemical conversion treatment performance. The acid concentration in the pickling bath is preferably 5% or more, and preferably 8% or less.

[0078] The pickling treatment time must be between 3 seconds (s) and 90 seconds (s), preferably 5 seconds or more, preferably 60 seconds or less, and more preferably 5 to 60 seconds. If the treatment time is less than 3 seconds, the effect of removing the Sn-enriched layer is not sufficiently obtained, and good chemical conversion treatment performance cannot be ensured. If the treatment time exceeds 90 seconds, excessive dissolution of the steel sheet causes Cu enrichment on the steel sheet surface, reducing the chemical conversion treatment performance. Furthermore, exceeding 90 seconds is undesirable because it reduces productivity. The pickling treatment time refers to the time during which the dissolution reaction of the steel sheet surface is carried out by contact between the steel sheet surface and the pickling bath, and can be, for example, the immersion time of the steel sheet in the pickling bath.

[0079] The acid component of the pickling bath is preferably hydrochloric acid or sulfuric acid. This is because using a pickling bath with hydrochloric acid or sulfuric acid as the acid component makes it easier to expose a Sn-deficient layer on the steel sheet surface, in which the Cr and Cu content is suppressed to the desired extent, while effectively removing the Sn-enriched layer.

[0080] [Rust-preventive oil application] Cold-rolled steel sheets (cold-rolled and annealed sheets) after annealing and pickling may be coated with rust-preventive oil. This prevents the formation of a new Sn-enriched layer, such as a Sn oxide film, on the surface of the steel sheet from which the Sn-enriched layer has been removed by pickling. From this viewpoint, it is preferable to carry out the rust-preventive oil application process in a deoxygenated atmosphere.

[0081] A chemical conversion treatment process can be applied to at least one surface of the steel sheet obtained in this manner, according to a conventional method. Since the steel sheet obtained according to the above manufacturing method has excellent chemical conversion treatment properties, a good chemical conversion coating can be formed even with a conventional chemical conversion treatment.

[0082] The present invention will be described in more detail below based on examples. However, the present invention is not limited to these examples. A steel slab having the component composition shown in Table 1 was heated and hot-rolled, and the resulting hot-rolled steel sheet was pickled. The remainder other than the elements shown in Table 1 is Fe and unavoidable impurities.

[0083] Next, the hot-rolled steel sheet after pickling was cold-rolled at a cumulative rolling rate of 70% to obtain a cold-rolled steel sheet with a thickness of 0.8 mm. Then, the obtained cold-rolled steel sheet was annealed under various conditions shown in Table 2, with the annealing temperature, holding time, and dew point during annealing being changed. The annealing was carried out in a nitrogen atmosphere (non-oxidizing atmosphere) containing 5 vol.% hydrogen.

[0084] After annealing, cold-rolled steel sheets were cleaned and surface-conditioned by pickling with hydrochloric acid or sulfuric acid. Various pickling baths with different bath temperatures and acid concentrations, as shown in Table 2, were prepared. A stirrer was placed at the bottom of the bath container, and the pickling bath was stirred at a speed of 300 rpm while the cold-rolled steel sheets were immersed in the bath for various immersion times, as shown in Table 2. In this way, the amount of dissolution on the steel sheet surface during pickling was controlled, and the degree of surface conditioning was varied.

[0085] To prevent surface oxidation, rust-preventive oil was applied to the cold-rolled steel sheet within six hours of pickling, after surface cleaning by pickling.

[0086] The steel sheets manufactured as described above were evaluated for their chemical treatment properties and corrosion resistance after painting as follows. In Tables 1-3, the same No. corresponds to the same sample. Furthermore, the final steel sheets had the same component composition as shown in Table 1.

[0087] [Chemical Conversion Treatment Performance] Commercially available zinc phosphate chemical conversion treatment agents (surface modifier: Preparen X, chemical conversion treatment agent: Palbond SX35, both manufactured by Nippon Parkerizing Co., Ltd.) were used. After degreasing the steel plate, zinc phosphate chemical conversion treatment was performed at a treatment temperature of 35°C for a treatment time of 90 seconds to coat the surface of the steel plate with zinc phosphate. The zinc phosphate coverage rate on the surface of the steel plate after chemical conversion treatment was calculated as follows. The surface of the steel plate after chemical conversion treatment was observed using a scanning electron microscope (SEM) at a field of view of 1000x magnification, and the area of ​​the zinc phosphate crystal coating area and the exposed base metal area were determined separately, and the area ratio of the zinc phosphate crystal coating area to the total area was determined. The same measurement was performed for 10 fields of view, and the average value of the area ratio of the zinc phosphate crystal coating area obtained for each was used as the zinc phosphate coverage rate (area %) to evaluate the chemical conversion treatment performance. The zinc phosphate coverage rate was judged as follows: less than 80% was marked with × (unsuitable), 80% to less than 98% was marked with ○ (good), and 98% or more was marked with ◎ (excellent). ○ and ◎ were considered acceptable. The results are shown in Table 3.

[0088] [Corrosion Resistance After Painting] Electrodeposition coating was performed on a portion of the steel plate samples that had undergone chemical conversion treatment using the method described above, in order to conduct a corrosion resistance test. Electrodeposition coating was carried out using Kansai Paint Co., Ltd.'s GT150V, by baking at 170°C for 20 minutes to achieve a coating film thickness of 15 μm. Subsequently, cross-cuts were performed on the surface of the steel plate samples using an NT Cutter S or A type (manufactured by Nippon Transfer Paper Co., Ltd.) under conditions of a length of 90 mm, a crossing angle of 45°, and a load of 300 g, to prepare test specimens for the post-paint corrosion resistance test. After sealing the ends and back surfaces of these test specimens with waterproof tape, they were subjected to a salt spray test (JIS Z 2371) for 720 hours. After the test, the blistering of the coating film formed around the cross-cut areas of the test specimens was observed, and the value of half the widest blister width on both sides of the cross-cut was measured and used to evaluate the post-paint corrosion resistance. The measured values ​​were then judged as follows: 1.5 mm or less was marked ◎ (Excellent), between 1.5 mm and less than 2.0 mm was marked ○ (Good), and 2.0 mm or more was marked × (Unacceptable). ○ and ◎ were considered acceptable. The results are shown in Table 3.

[0089]

[0090]

[0091]

[0092] The results in Tables 1-3 show that the present invention example has superior chemical treatment properties and corrosion resistance after painting compared to the comparative example. Thus, the steel sheet of the present invention and the steel sheet obtained according to the manufacturing method of the present invention exhibit excellent chemical treatment properties and corrosion resistance after painting, even when containing multiple types of trump elements.

[0093] According to the present invention, a steel sheet that achieves both excellent chemical treatment properties and corrosion resistance after painting can be provided, along with a method for manufacturing the same. The steel sheet and its manufacturing method according to the present invention are applicable to various uses such as automotive steel sheets, are environmentally friendly, and have high industrial utility.

Claims

1. In mass percent, Cu: 0.01-0.50%, Ni: 0.01-1.00%, Sn: 0.001-0.100%, Cr: 0.02-0.30%, Mo: 0.001-1.000%, Zn: 0.500% or less, Pb: 0.500% or less, As: 0.500% or less, Sb: 0.500% or less, Bi: 0.500% or less, V: 0.500% or less, C: 0.50% or less, Si: 3.000% or less, Mn: 5.00% or less, B: 0.0100% or less, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, and N : Having a component composition consisting of 0.0100% or less, with the remainder being Fe and unavoidable impurities, and the integral value of the component concentration of element X in the region at depths a to b (nm) from the surface, obtained by Auger electron spectroscopy on at least one surface of the steel plate, is X a-b A steel plate that satisfies the following equations (1) to (3) when (at.% × nm). 3 × Sn 0-3 / Sn 3-10 ≦0.900...(1) 2.00≦Cr 0-3 ≦ 12.00...(2) Cu 0-3 ≤ 6.00 ... (3) 2. The steel sheet according to claim 1, wherein a chemical conversion treatment coating is provided on the surface.

3. A method for manufacturing a steel sheet, comprising, in order, an annealing step and a pickling step for a cold-rolled steel sheet having the component composition described in claim 1, wherein the annealing step is performed under conditions where the annealing temperature of the cold-rolled steel sheet is 700 to 900°C for a holding time of 50 to 400 seconds and the dew point during annealing is -40 to 0°C, and the pickling step is performed using a pickling bath with a temperature of 10 to 70°C and an acid concentration of 1 to 10% by mass, and the processing time for the cold-rolled steel sheet after annealing is 3 to 90 seconds.

4. The method for producing a steel sheet according to claim 3, wherein the acid component of the pickling bath is hydrochloric acid or sulfuric acid.

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