Steel sheet and method for manufacturing same

Abstract

Provided is a steel sheet having excellent chemical conversion treatment properties and high corrosion resistance, despite containing tramp elements. The steel sheet comprises prescribed elements at prescribed amounts, and if in a luminescence intensity profile of a wavelength indicating Ni, the profile being obtained by measuring the steel sheet in the depth direction from the surface of the steel sheet through glow discharge spectroscopy (GDS), the average intensity of Ni is denoted by Iave, the peak intensity of Ni is denoted by Imax, the depth (μm) indicating the Imax is denoted by Dmax, the Ni concentration (mass%) converted from the Imax is denoted by [Ni]max, and the depth (μm) indicating the intensity of (Imax - Iave) / 2 + Iave is denoted by D1 / 2, and furthermore, in the steel sheet, the Cu content (mass%) is denoted by [Cu], the Sn content (mass%) is denoted by [Sn], and the Cr content (mass%) is denoted by [Cr], the steel sheet satisfies expressions (1) to (3).　(1): 0.00 ≤ [Ni]max – 2.5[Cu] ≤ 6.00　(2): 0.020 ≤ D1 / 2　(3): 10[Sn] + 5[Cr] ≤ 1.81

Description

Steel plate and its manufacturing method

[0001] This invention relates to steel plates and methods for manufacturing the same.

[0002] Steel sheets are currently used in a wide range of fields, including automobiles, construction, and home appliances, and strong demand is expected to continue. There are two main methods for manufacturing steel sheets: the blast furnace method, which uses iron ore and coke to produce steel sheets in a blast furnace, and the electric arc furnace method, which uses scrap iron collected from the market. Traditionally, the blast furnace method, which offers superior cost advantages and can produce high-quality steel sheets, was the mainstream method of steel sheet manufacturing. However, with the recent increase in environmental awareness and CO2 emissions... ２ In response to the trend towards emission regulations, the production of steel sheets using the electric arc furnace method is also being considered. However, a challenge of the electric arc furnace method is that trump elements such as Cu, Ni, Sn, Cr, and Mo contained in iron scrap are mixed into the steel sheets produced by this method, degrading various properties. A particularly serious problem is the decline in the chemical treatment properties of the steel sheets.

[0003] In response to this, technologies have been proposed to improve the chemical treatment properties of steel sheets having trump elements. For example, Patent Document 1 describes a technology to obtain 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 steel sheet surface is 0.5% by mass to 10-70%. Patent Document 2 also describes a technology to obtain a steel sheet with excellent chemical treatment properties by having the residue on the steel sheet surface contain microcathodes, and by having the residue contain 70% or more linear microcathodes. In Patent Document 2, the residue refers to metal oxide particles and copper compound particles, and microcathodes refer to residues with a particle size of 2 μm or less among metal oxides or copper compounds that have a higher potential than the base metal.

[0004] WO2021 / 157692 Special Publication No. 2020-84325

[0005] However, Patent Documents 1 and 2 are technologies for improving the chemical conversion treatment properties of steel sheets, and do not aim to improve the corrosion resistance of the steel sheets themselves. In addition, regarding chemical conversion treatment properties, Patent Documents 1 and 2 do not consider chemical conversion inhibiting factors other than Cu, and in particular, the study on improving the chemical conversion treatment properties of steel sheets using the electric furnace method was insufficient.

[0006] The present invention aims to provide a steel sheet that has excellent chemical treatment properties and high corrosion resistance even while containing trump elements, along with a method for manufacturing the same.

[0007] The inventors of the present invention have diligently studied to solve the above problems and have obtained the following findings regarding chemical conversion treatment properties: <1> Chemical conversion treatment properties can be improved by defining the relationship between the amount of Ni on the surface of the steel sheet and the amount of Cu in the steel sheet. <2> Chemical conversion treatment properties can be improved by limiting the amount of Sn and Cr in the steel sheet.

[0008] Furthermore, the following findings were obtained regarding corrosion resistance. <3> Trump elements have the potential to improve the corrosion resistance of the steel sheet itself, and by defining the Ni distribution in the depth direction of the steel sheet, the corrosion resistance of the steel sheet itself can be improved. Here, the corrosion resistance of the steel sheet itself refers to the corrosion resistance that the steel sheet itself possesses, and is a characteristic that is evaluated separately from the influence of chemical treatment or surface treatment on corrosion resistance.

[0009] The present invention is based on the above findings, and its gist is as follows. [1] In mass%, Cu: 0.01 to 0.50%, Ni: 0.04 to 1.00%, Sn: 0.001 to 0.100%, Cr: 0.01 to 0.20%, Mo: 0.001 to 1.000%, Zn: 0 to 0.500%, Pb: 0 to 0.500%, As: 0 to 0.500%, Sb: 0 to 0.500%, Bi: 0 to 0.500%, V: 0 to 0.500%, C: 0 to 0.50%, Si: 0 to 3.000%, Mn: 0 to 5.00%, B: 0 to 0.0100%, P: 0 to 0.100%, S: 0 to 0.020%, Al: 0 to 0.100% and N: 0 to 0.0100% are contained, and the balance consists of Fe and unavoidable impurities, and is a steel sheet having a component composition, and by glow discharge optical emission spectrometry (GDS), the average intensity of Ni in the emission intensity profile of the wavelength indicating Ni obtained by measuring in the depth direction from the surface of the steel sheet is I ａｖｅ , the peak intensity of Ni is I ｍａｘ , the depth (μm) showing the I ｍａｘ is D ｍａｘ , the Ni concentration (mass%) converted from the I ｍａｘ is [Ni ｍａｘ , and when the Cu content (mass%) in the steel sheet is [Cu], the Sn content (mass%) is [Sn], and the Cr content (mass%) is [Cr], the steel sheet satisfying the following formulas (1) to (3). 0.00 ≦ [Ni ｍａｘ - 2.5[Cu] ≦ 6.00... (1) 0.020 ≦ D １／２ ​​​​​​​​... (2) 10[Sn] + 5[Cr] ≤ 1.81... (3). The steel sheet of [1], which is provided with a chemical conversion treatment film. [3] A method for manufacturing the steel sheet of [1], comprising: a heating step of heating a slab that satisfies the component composition of [1] and formula (3); a descaling step of descaling the heated slab; and a hot rolling step of performing hot rolling on the slab after descaling. The heating step includes: a first heat treatment of holding the slab at 1100 to 1300°C for 30 to 120 minutes in an atmospheric environment; and a second heat treatment of holding the slab at 1100 to 1300°C for 30 to 120 minutes in an environment controlled to a nitrogen atmosphere. A method for manufacturing a steel sheet.

[0010] Here, the steel sheet of [1] can be, for example, a steel sheet manufactured using an electric furnace method. Further, the slab used in the manufacturing method of [3] can be, for example, a slab manufactured by an electric furnace method.

[0011] The steel sheet of the present invention has excellent chemical conversion treatability even if it has a Trump element and has high corrosion resistance.

[0012] It is a light emission intensity profile of the wavelength indicating Ni by glow discharge optical emission spectrometry (GDS) for the test material of No. 6 (example of the present invention) of the examples. It is a light emission intensity profile of the wavelength indicating Ni by GDS for the test material of No. 2 (comparative example) of the examples.

[0013] Hereinafter, embodiments of the steel sheet according to the present invention will be described. However, they are an example embodying the present invention and do not limit the configuration of the present invention. In the following description, the unit of the content of each element of the component composition is "mass%", and unless otherwise specified, it is simply indicated by "%". Further, regarding the numerical range, the notation "a to b" means a or more and b or less, including a and b, unless otherwise specified.

[0014] (Steel Sheet) First, the configuration of the steel sheet of the present invention will be described.

[0015] [Composition] The steel sheet of the present invention has a composition by mass% of Cu: 0.01 to 0.50%, Ni: 0.04 to 1.00%, Sn: 0.001 to 0.100%, Cr: 0.01 to 0.20%, Mo: 0.001 to 1.000%, Zn: 0 to 0.500%, Pb: 0 to 0.500%, As: 0 to 0.500%, Sb: 0 to 0.500%, Bi: 0 to 0.500%, V: 0 to 0.500%, C: 0 to 0.50%, Si: 0 to 3.000%, Mn: 0 to 5.00%, B: 0 to 0.0100%, P: 0 to 0.100%, S: 0 to 0.020%, Al: 0 to 0.100%, N: 0 to 0.0100%, and the balance consists of Fe and inevitable impurities. When the lower limit of the content is 0%, the content range includes 0%.

[0016] The steel sheet having the above composition can be a steel sheet manufactured by an electric furnace method, and its type is not particularly limited. Examples include general hot-rolled steel sheets or cold-rolled steel sheets such as extra-low carbon steel, and hot-rolled steel sheets or cold-rolled steel sheets such as high-tensile steel.

[0017] <Cu: 0.01 to 0.50%> Cu is an element that may be mixed as a tramp element, particularly in steel sheets manufactured by the electric furnace method. It is more likely to be mixed into the steel in a relatively large amount compared to other tramp element elements. In a steel sheet containing Cu, Cu is concentrated on the steel sheet surface, and the potential of this part is nobler than the surrounding area, suppressing the dissolution reaction of Fe in the chemical conversion treatment and causing a chemical conversion scale on the concentrated part. For these reasons, the Cu content is set to 0.50% or less. Also, although Cu is an element that contributes to an increase in the strength of the steel sheet, if it is too much, it becomes difficult to stabilize the mechanical properties in extra-low carbon steel, so it is preferably 0.30% or less. On the other hand, for economic reasons such as reducing Cu in iron scrap in the electric furnace method being costly, in addition to this, Cu is contained in bulk, which reduces the reactivity of the steel sheet itself and has the effect of improving the corrosion resistance of the steel sheet itself. For these reasons, the Cu content is set to 0.01% or more, preferably 0.05% or more, and more preferably 0.07% or more.

[0018] <Ni: 0.04-1.00%> 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 on the surface of the steel sheet as fine granules (for example, including the precipitation of Ni with an average particle size of less than 50 nm) forms microscopic cathodes, which promote the dissolution reaction of Fe around the Ni-enriched areas and improve the chemical conversion treatment properties by acting as crystal nuclei for the conversion crystals. Furthermore, a more preferable form is obtained when the state of Ni with an average particle size of less than 50 nm on the surface of the steel sheet is predominantly metallic, as this promotes cathode formation. In addition, Ni is a more noble metal than Fe, and has the effect of reducing the reactivity of the steel sheet itself and improving the corrosion resistance of the steel sheet itself. For these reasons, the Ni content should be 0.04% or more, preferably 0.20% or more. On the other hand, if the Ni content is too high, the concentrated areas on the surface change from fine granules to large lumps, degrading the chemical conversion treatment properties on the concentrated areas. Furthermore, since it is necessary to add Ni to the iron scrap in order to adjust the Ni content, the Ni content should be 1.00% or less, preferably 0.70% or less. Here, as a method for evaluating Ni precipitation with an average particle size of less than 50 nm on the surface of the steel plate, for example, a transmission electron microscope (TEM) can be used. Thin film samples are prepared from five arbitrary points on the surface of the steel plate using a focused ion beam (FIB), etc., and TEM images and Ni maps are obtained for each sample. The particle size of each is calculated and can be determined from the average of 10 fields of view, with 2 fields of view per sample. Typically, particles with a particle size of 1 nm or more are measured. In addition, the state of Ni on the surface of the steel plate can be evaluated from nanospectroscopy using X-ray photoelectron spectroscopy (XPS) or from the correspondence between Ni maps and O maps using energy-dispersive X-ray analysis (EDS) mounted on the TEM.

[0019] <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. When Sn in the steel sheet dissolves during chemical conversion treatment, it reprecipitations as hydroxide, coating the surface of the steel sheet and significantly reducing the chemical conversion treatment properties. Therefore, the Sn content is set to 0.100% or less, preferably 0.090% or less. Furthermore, for economic reasons, such as the cost disadvantage of reducing Sn in iron scrap in the electric arc furnace method, the Sn content is set to 0.001% or more, preferably 0.005% or more.

[0020] <Cr: 0.01-0.20%> 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 reprecipitations, coating the surface of the steel sheet and reducing the chemical conversion treatment properties. Therefore, the Cr content should be 0.20% or less, preferably 0.17% or less. Furthermore, for economic reasons, such as the cost disadvantage of reducing Cr in iron scrap in the electric arc furnace method, the Cr content should be 0.01% or more, preferably 0.03% or more.

[0021] <Mo: 0.001-1.000%> 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, reducing Mo in iron scrap in the electric arc furnace method is disadvantageous in terms of cost, and in addition, Mo is an element that increases the strength of steel sheets, so the Mo content is set to 0.001% or more, preferably 0.003% or more. Furthermore, although Mo is an element that increases the strength of steel sheets, if the Mo content is high, it becomes necessary to add Mo, which increases costs and is economically disadvantageous, so the Mo content is set to 1.000% or less, preferably 0.800% or less.

[0022] <Zn: 0-0.500%> Zn is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. To avoid impairing the properties according to the present invention, the Zn content is set to 0.500% or less, preferably 0.450% or less. The Zn content may be 0%, or for example, 0.001% or more.

[0023] <Pb: 0-0.500%> 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 set to 0.500% or less, preferably 0.450% or less. The Pb content may be 0%, and for example, it can be 0.001% or more.

[0024] <As: 0-0.500%> 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 of the present invention, the As content is set to 0.500% or less, preferably 0.450% or less. The As content may be 0%, or for example, 0.001% or more.

[0025] <Sb: 0-0.500%> 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 set to 0.500% or less, preferably 0.450% or less. The Sb content may be 0%, or for example, 0.001% or more.

[0026] <Bi: 0-0.500%> 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 set to 0.500% or less, preferably 0.450% or less. The Bi content may be 0%, and for example, it can be 0.001% or more.

[0027] <V: 0-0.500%> 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 set to 0.500% or less, preferably 0.450% or less. The V content may be 0%, and for example, it can be 0.003% or more.

[0028] <C: 0-0.50%> Carbon (C) is an element that can be included from the viewpoint of improving the hardenability of steel sheets, increasing strength by securing martensite, and controlling the volume fraction of retained austenite (retained γ) within a desired range. However, if there is too much carbon, the area fraction of cementite increases and the workability decreases, so the carbon content should be 0.50% or less, preferably 0.45% or less. The carbon content may be 0%, but from the viewpoint of ensuring the above effects, it is preferable to have 0.01% or more.

[0029] <Si: 0-3,000%> Si is an element that can be included to improve ferrite strength, suppress oxidation products in martensite and bainite, and stabilize residual γ to improve ductility. However, if there is too much Si, Fe will be formed on the surface of the steel sheet during hot rolling. ２ SiO ４ Since residual substances worsen the chemical conversion treatment properties, the Si content should be 3.000% or less, preferably 2.550% or less. The Si content may be 0%, but from the viewpoint of ensuring the above effects, it is preferable to have 0.013% or more.

[0030] <Mn: 0-5.00%> Mn is an element that ensures a predetermined hardenability, suppresses ferrite transformation, and secures tempered martensite or bainite with the desired area ratio to guarantee strength. However, if there is too much Mn, the bainite transformation is significantly delayed, making it difficult to ensure high ductility. Therefore, the Mn content should be 5.00% or less, preferably 4.55% 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.

[0031] <B: 0 to 0.0100%> B is an element that facilitates the formation of tempered martensite or bainite with a predetermined area ratio, and the residual solid solution of B can improve delayed fracture resistance. However, if the B content is too high, it will cause a significant decrease in hot ductility and result in surface defects, so the B content should be 0.0100% or less, preferably 0.0096% or less. The B content may be 0%, but from the viewpoint of ensuring the above effects, it is preferable to have a B content of 0.0001% or more.

[0032] <P: 0-0.100%> P is an element that strengthens steel. However, a high P content degrades spot weldability, so the P content should be 0.100% or less, preferably 0.090% or less. The P content may be 0%, or for example, 0.001% or more.

[0033] <S: 0-0.020%> 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, so the S content should be 0.020% or less, preferably 0.019% or less. The S content may be 0%, and from the viewpoint of improving scale detachability during hot rolling, it can be, for example, 0.003% or more.

[0034] <Al: 0-0.100%> Al is an element that can stabilize residual gamma as a deoxidant and substitute for Si. However, if the Al content is too high, the strength of the material will decrease drastically, so 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 gamma, it can be, for example, 0.020% or more.

[0035] <N: 0 to 0.0100%> N is an element that forms nitrides such as BN, AlN, and TiN in steel, and can reduce the hot ductility of steel and lower its 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. The N content may be 0%, and can be set to, for example, 0.0002% or more from the standpoint of cost increase due to N removal and ensuring strength through nitride formation.

[0036] The steel sheet in this invention has a component composition consisting of the above components, the remainder being Fe, and unavoidable impurities.

[0037] [Formulas (1) to (3)] The steel sheet of the present invention is obtained by glow discharge emission spectroscopy (GDS) and the average intensity of Ni in the emission intensity profile of the wavelength showing Ni is I ａｖｅ , the peak intensity of Ni is I ｍａｘ , the aforementioned I ｍａｘ The depth (μm) that indicates this is D ｍａｘ , the aforementioned I ｍａｘ The Ni concentration (mass%) converted from [Ni] ｍａｘ , (I ｍａｘ -I ａｖｅ ) / 2+I ａｖｅ The depth (μm) that indicates the intensity is D １／２ Assuming that the Cu content (mass%), Sn content (mass%), and Cr content (mass%) in the steel plate are defined as [Cu], then the following equations (1) to (3) are satisfied: 0.00 ≤ [Ni] ｍａｘ -2.5[Cu] ≦ 6.00...(1) 0.020 ≦ D １／２ ... (2) 10[Sn] + 5[Cr] ≤ 1.81 ... (3) Equations (1) and (2) only need to be satisfied by either steel plate surface, but it is preferable that both steel plate surfaces satisfy equations (1) and (2).

[0038] The reasons for setting equations (1) to (3) will be explained.

[0039] The inventors of the present invention diligently investigated the causes of deterioration in the chemical conversion properties of steel sheets having the above component composition and obtained the following findings: (A) Cu becomes relatively coarsely concentrated on the surface of the steel sheet, forming cathode points that inhibit the precipitation of chemical conversion crystals on the concentrated areas. (B) Sn and Cr, which are dissolved or precipitated in the steel, dissolve during the chemical conversion treatment, but re-precipitation coats the surface of the steel sheet, thereby suppressing the progress of the chemical conversion treatment reaction.

[0040] <Equation (1)> In response to the phenomenon described in (A) above, the inventors explored techniques to improve the chemical conversion treatment properties of steel sheets with a high Cu content. They found that when measuring the Ni distribution in the depth direction of the steel sheet, the higher the maximum concentration (mass%) of Ni observed near the surface of the steel sheet relative to the Cu content (mass%) of the base material, the better the chemical conversion treatment properties that can be obtained. This finding is based on the fact that when the maximum concentration of Ni observed near the surface of the steel sheet is high relative to the Cu content of the base material, fine Ni particles precipitate not only in areas where Cu is not concentrated, but also in areas where Cu is concentrated. In the former case, Ni itself acts as the cathode point, and its surroundings act as the anode point, and the anode point acts as the crystal nucleus of the chemical conversion crystal, causing the deposition of fine and uniform chemical conversion crystals. On the other hand, in the latter case, the Cu-enriched area acts as the cathode point, and the Ni particle itself acts as the anode point, and the Ni particle itself acts as the crystal nucleus of the chemical conversion crystal, causing the deposition of fine and uniform chemical conversion crystals. The inventors formulated these effects and obtained equation (1): 0.00 ≤ [Ni] ｍａｘ -2.5 [Cu] ≦ 6.00...(1)

[0041] [Ni] in equation (1) ｍａｘThe (mass %) is determined using the emission intensity profile of the Ni wavelength measured in the depth direction from the surface of the steel plate by glow discharge emission spectroscopy (GDS). Specifically, the pre-sputtering time is set to 5 seconds, the output to 35 W, the gas replacement time to 35 seconds, and the pressure to 600 Pa. Data is collected from the steel plate surface at 10 ms intervals for 200 s (down to a depth of 13.5 μm from the surface) to obtain the emission intensity profile of the Ni wavelength. The intensity between 180 and 200 s (corresponding to a depth of 12.2 to 13.5 μm from the surface) is used as I ａｖｅ Furthermore, the value (peak intensity) at which the Ni intensity reaches its maximum value in the data from 400 ms after the start of measurement (corresponding to a depth of 0.027 μm from the surface) is defined as I ｍａｘ The Ni content in the steel is defined as [Ni] (mass%), and the following calculation is performed to determine [Ni] ｍａｘ Calculate [Ni]. ｍａｘ = I ｍａｘ ×[Ni] / I ａｖｅ

[0042] GDS analysis is not limited to the above measurement conditions, and similar conditions can be adopted. Strength (I) corresponding to the Ni content in the steel plate ａｖｅ The intensity is the average value of the intensity over a section of at least 1.3 μm measured at a depth of 12 μm or more from the surface, for example, the length of the section can be 1.3 to 3.0 μm. The measurement position can be, for example, a position from a depth of 12 μm or more from the surface to the center of the plate thickness. In the above measurement conditions, the reason for using data from 400 ms after the start of measurement is to remove noise from the measured values ​​near the surface of the steel plate. The data to be removed considering noise should be set appropriately depending on the measurement conditions and measuring equipment, but it is preferable to remove data in the range of less than 0.025 μm from the surface, for example, data in the range of less than 0.027 μm from the surface should be removed.

[0043] [Ni] obtained ｍａｘ Then, using the Cu content in the steel, [Cu] (mass%), the [Ni] in formula (1) ｍａｘA value of -2.5 [Cu] can be calculated. The smaller this value, the lower the amount of Ni on the surface of the steel sheet, so it should be 0.00% or more, preferably 0.50% or more. Also, if this value is too large, the surface of the steel sheet will be covered with coarse Ni, reducing the chemical conversion treatment properties, so it should be 6.00% or less, preferably 5.50% or less.

[0044] <Equation (2)> Furthermore, in response to the phenomenon described in (A) above, the inventors explored methods to further improve the chemical conversion treatment properties and found that when the Ni distribution in the depth direction of the steel sheet was measured, the wider the Ni distribution in the depth direction, the less the chemical conversion treatment properties deteriorated even in the later stages of the chemical conversion treatment process. This is presumed to be because the widespread presence of Ni in the depth direction increases the amount of Ni precipitated in the depth direction, and even if Ni in the surface layer is removed by etching during the chemical conversion treatment, new Ni particles are supplied, thus maintaining good chemical conversion treatment properties even in the later stages of the chemical conversion treatment process. In addition, a wide distribution of Ni in the depth direction results in a large number of noble Ni particles remaining on the surface of the steel sheet even after the chemical conversion treatment, which can significantly reduce the reactivity of the steel sheet surface and is expected to impart the property of improved corrosion resistance to the steel sheet itself. In contrast, conventional methods that merely precipitate fine Ni, oxide particles, or copper compound particles on the surface of steel sheets are insufficient for achieving higher levels of chemical conversion performance. These particles are removed by the etching reaction of Fe during the chemical conversion treatment process, thus only providing a chemical conversion improvement effect in the initial stages of the treatment. The inventors formulated these effects to obtain equation (2): 0.020 ≤ D １／２ ... (2)

[0045] D in equation (2) １／２ This is determined using glow discharge emission spectroscopy (GDS). Specifically, an emission intensity profile at the wavelength showing Ni is obtained under the measurement conditions described for equation (1), and in the same manner, I ａｖｅ , and I ｍａｘ After determining this, the strength (I) corresponding to the Ni content in the steel plate is calculated as follows: ａｖｅ Based on this, I, whose peak intensity is half that of Ni, １／２ Calculate I１／２ = (I ｍａｘ -I ａｖｅ ) / 2+I ａｖｅ

[0046] (I ｍａｘ -I ａｖｅ ) / 2+I ａｖｅ D is the depth (μm) that indicates the intensity. １／２ It is calculated as follows: Check the data for the emission intensity profile of the wavelength showing Ni, and first, after the depth is 400 ms or more from the start of the measurement and the peak has been measured, I １／２ The GDS measurement time until it reaches t １／２ Confirm the (ms) value. After the GDS measurement is completed, measure the sputter mark depth (μm) of the steel plate with a laser microscope, and divide by the time required for the GDS measurement, 200,000 ms (200 s), to determine the depth per unit measurement time. １／２ By taking the product with, D １／２ Calculate D obtained in this way. １／２ This can be used in equation (2).

[0047] GDS analysis is not limited to the above measurement conditions, and similar conditions can be adopted.

[0048] D １／２ The smaller the value of, the lower the supply of Ni particles in the later stages of the chemical treatment, so it should be 0.020 μm or more, preferably 0.050 μm or more, and more preferably 0.080 μm or more. Also, D １／２ If the value is too large, it will be necessary to add a large amount of Ni to increase the Ni content, which may increase costs. Therefore, a value of 0.500 μm or less is preferred, and more preferably 0.200 μm or less.

[0049] <Equation (3)> As a result of diligent research, the inventors have found that considering the phenomenon described in (B) above is also important for improving chemical conversion treatment performance. Sn and Cr exist either as solid solutions in the steel or precipitated on the surface of the steel plate, but they dissolve in the chemical conversion treatment solution during chemical conversion treatment. However, as the chemical conversion treatment reaction progresses, if the pH rises on the surface of the steel plate, the dissolved Sn and Cr precipitate as hydroxides with low solubility and coat the surface of the steel plate like a film. Therefore, it is presumed that the chemical conversion treatment solution and the surface of the steel plate are physically separated, leading to a decrease in chemical conversion treatment performance. In response to this mechanism, the inventors have determined that it is necessary to limit the amount of Sn and Cr in order to improve chemical conversion treatment performance, and that the influence of Sn is greater than that of Cr in reducing chemical conversion treatment performance, and have obtained the formula (3). In the formula, [Sn] is the Sn content (mass%) in the steel, and [Cr] is the Cr content (mass%) in the steel. 10[Sn]+5[Cr]≦1.81...(3)

[0050] The larger the value of 10[Sn] + 5[Cr] in formula (3), the lower the chemical conversion treatment performance due to the above mechanism. Therefore, it should be 1.81% or less, preferably 1.70% or less. The above value can be 0.06% or more, preferably 0.10% or more, and more preferably 0.20% or more.

[0051] (Manufacturing Method) The manufacturing method for the steel sheet of the present invention will now be described. The steel sheet of the present invention is obtained by a manufacturing method for a steel sheet that includes a heating step of heating a slab that satisfies the component composition and formula (3) described for the steel sheet, a descaling step of performing descaling on the heated slab, and a hot rolling step of performing hot rolling on the slab after descaling, wherein the heating step includes a first heat treatment of holding the slab at 1100 to 1300°C for 30 to 120 minutes in an atmospheric environment, and a second heat treatment of holding the slab at 1100 to 1300°C for 30 to 120 minutes in an environment controlled to a nitrogen atmosphere after the first heat treatment. After hot rolling, it may be subjected to pickling and then chemical conversion treatment to obtain a chemically converted steel sheet.

[0052] [Heating Process] The method for manufacturing the slab subjected to the heating process is not particularly limited, but slabs manufactured by the electric furnace method are advantageous in that they can be easily adjusted to satisfy a predetermined component composition and formula (3). Scrap can be used as the raw material.

[0053] To obtain the steel sheet of the present invention, control of the holding temperature, holding time, and atmosphere during the slab heating process is important. Specifically, the heating process includes a first heat treatment in which the slab is held at 1100 to 1300°C for 30 to 120 minutes in an atmospheric environment, and a second heat treatment in which, after the first heat treatment, the slab is held at 1100 to 1300°C for 30 to 120 minutes in an environment controlled to a nitrogen atmosphere.

[0054] The first heat treatment is a process aimed at promoting scale formation on the slab surface and is carried out in an atmospheric environment. As the scale grows, Ni tends to migrate towards the scale, thus promoting the deposition of Ni on the steel plate surface.

[0055] In the first heat treatment, if the holding temperature is below 1100°C, a sufficient amount of Ni cannot be deposited on the surface of the steel sheet, resulting in reduced chemical conversion treatment performance. Therefore, the holding temperature should be 1100°C or higher. Also, if the holding temperature exceeds 1300°C, excessive scale growth occurs, and the Ni that had been deposited on the surface of the steel sheet migrates into the scale, making it impossible to deposit a sufficient amount of Ni on the surface of the steel sheet. Therefore, the holding temperature should be 1300°C or lower. Preferably, the holding temperature is 1150°C or higher, and also 1250°C or lower.

[0056] In the first heat treatment, if the holding time at the holding temperature is less than 30 minutes, a sufficient amount of Ni cannot be deposited on the surface of the steel sheet, and the chemical conversion treatment performance decreases. Therefore, the holding time should be 30 minutes or more. Also, if the holding time exceeds 120 minutes, excessive scale growth occurs, and the Ni that had been deposited on the surface of the steel sheet migrates into the scale, making it impossible to deposit a sufficient amount of Ni on the surface of the steel sheet. Therefore, the holding time should be 120 minutes or less. The holding temperature is preferably 45 minutes or more, and also 100 minutes or less.

[0057] The second heat treatment involves replacing the heat treatment atmosphere from an atmospheric environment to a nitrogen atmosphere to stop scale growth and to diffuse some of the Ni precipitated on the surface due to heating into the steel sheet. This process ensures that a sufficient amount of Ni exists even at a depth of 0.020 μm or more from the surface of the steel sheet. Therefore, even if the surface of the steel sheet dissolves due to etching during the chemical conversion treatment, a sufficient supply of Ni particles that improve the chemical conversion treatment properties can be provided. Similarly, since a sufficient amount of Ni particles are present even after the chemical conversion treatment, the corrosion resistance of the steel sheet itself can be improved.

[0058] In the second heat treatment, if the holding temperature is below 1100°C, the amount of Ni supplied in the depth direction decreases, reducing the effect of improving chemical conversion treatmentability and corrosion resistance. Therefore, the holding temperature should be 1100°C or higher. Also, if the holding temperature exceeds 1300°C, the movement of Ni into the interior of the steel plate becomes excessive, and the amount of Ni precipitated on the surface of the steel plate decreases, thus reducing chemical conversion treatmentability. Therefore, the holding temperature should be 1300°C or lower. The holding temperature is preferably 1150°C or higher, and also 1250°C or lower. The holding temperature of the first heat treatment and the holding temperature of the second heat treatment may be the same or different. The first and second heat treatments may be performed consecutively, or after the first heat treatment, the slab may be cooled and then reheated to perform the second heat treatment. The cooling after the first heat treatment can end at a temperature of 700°C or lower, and may be cooled to room temperature, for example.

[0059] In the second heat treatment, if the holding time at the holding temperature is less than 30 minutes, the amount of Ni supplied in the depth direction decreases, reducing the effect of improving chemical conversion treatmentability and corrosion resistance. Therefore, the holding time should be 30 minutes or more. Also, if the holding time exceeds 120 minutes, the movement of Ni into the interior of the steel sheet becomes excessive, and the amount of Ni precipitated on the surface of the steel sheet decreases, thus reducing chemical conversion treatmentability. Therefore, the holding time should be 120 minutes or less. The holding temperature is preferably 45 minutes or more, and also 100 minutes or less.

[0060] The second heat treatment shall be carried out under a nitrogen atmosphere. A nitrogen atmosphere means a nitrogen partial pressure of 0.99 atmospheres or higher. If the nitrogen partial pressure is less than 0.99 atmospheres, scale growth is promoted, Ni deposited on the steel sheet surface migrates to the scale side, and the amount of Ni on the steel sheet surface decreases, thus reducing the chemical conversion treatment performance. Furthermore, since the second heat treatment involves replacing the atmosphere with a nitrogen atmosphere, it is preferable to carry it out using a heating method that allows for atmosphere control, such as indirect heating or induction heating.

[0061] [Descaling Process] In the descaling process, descaling is performed on the heated slab. The descaling method and conditions are not particularly limited, and conventional methods can be used. The descaling temperature is preferably 1300°C or lower, more preferably 1250°C or lower, from the viewpoint of suppressing the movement of Ni from the base metal to the scale, and preferably 1100°C or higher, more preferably 1150°C or higher, from the viewpoint of the need to efficiently remove the formed scale. Here, "descaling temperature" refers to the surface temperature of the slab at the start of descaling.

[0062] Descaling can be performed by injecting high-pressure water. Regarding the impact pressure of the high-pressure water, from the standpoint of removing scale without any residue, the water pressure is preferably 30 MPa or higher, more preferably 50 MPa or higher. Furthermore, from the standpoint of controlling surface irregularities, the water pressure is preferably 120 MPa or lower, more preferably 100 MPa or lower. Other descaling conditions are not particularly limited, and conventional conditions can be used.

[0063] [Hot Rolling Process] In the hot rolling process, the slab after descaling is hot-rolled to obtain a hot-rolled steel sheet. It is preferable that the time between the completion of descaling and the start of hot rolling be as short as possible. The conditions for hot rolling are not particularly limited and can be adjusted as appropriate according to the desired sheet thickness and properties. There are also no particular limitations on cooling after hot rolling.

[0064] The resulting steel sheet may be pickled and then subjected to a chemical conversion treatment. The methods and conditions for pickling and chemical conversion treatment are not particularly limited, and conventional methods can be used. The steel sheet of the present invention may be a steel sheet with a chemical conversion coating (chemical conversion treated steel sheet). Furthermore, after pickling, a plating treatment may be performed. The methods and conditions for the plating treatment are not particularly limited, and conventional methods can be used. The steel sheet of the present invention may be a steel sheet with a plating layer (plated steel sheet).

[0065] [Preparation of Test Specimens] Using slabs with the same composition as the steel components listed in Table 1, the slabs underwent a first heat treatment and a second heat treatment using the manufacturing conditions described in Table 2. In the first heat treatment, the slabs were held at the time and temperature described in Table 2 by indirect heating in a nitrogen or air atmosphere, and in the second heat treatment, the slabs were held at the time and temperature described in Table 2 by indirect heating in a nitrogen or air atmosphere. The first and second heat treatments were performed continuously, except where indicated as "cooling followed by reheating." In the case of "cooling followed by reheating," after the first heat treatment, the slab was cooled to 500°C, and then heated to the holding temperature for the second heat treatment. After that, descaling was performed with a water pressure of 60 MPa for 300 seconds to remove scale. After descaling, hot finishing rolling was performed to control the plate thickness to 3.2 mmt to prepare the test specimens.

[0066] GDS analysis was performed on the test material under the following conditions. Using a GDS analyzer (GD-Profiler 2, Horiba, Ltd.), the emission intensity profile indicating Ni was obtained under the following conditions: Pre-sputtering time: 5 seconds High-frequency output: 35 W Gas replacement time: 35 seconds Pressure: 600 Pa Measurement time: 200 s (interval: 10 ms) After the measurement was completed, the depth of the sputtering marks was measured using a non-contact surface shape measuring device (VK-X200, KEYENCE). The sputtering rate can be calculated by dividing the depth of the sputtering marks by the measurement time (200 s = 20000 ms), and this can be converted to the depth corresponding to each time.

[0067] Based on the obtained emission intensity profile, [Ni] ｍａｘ Find the ([Ni] in equation (1) ｍａｘThe value (mass %) of -2.5 [Cu] was calculated. In the calculation, I ａｖｅ This was the average value of the emission intensity between 180 and 200 s (corresponding to the interval of 12.2 to 13.5 μm depth from the surface). Furthermore, based on the obtained emission intensity profile, D in equation (2) was calculated. １／２ The (μm) value was calculated.

[0068] The results are shown in Table 2. Figures 1 and 2 show the emission intensity profiles of Ni at different wavelengths for No. 6 (the present invention example) and No. 2 (the comparative example). The horizontal axis represents the measurement time.

[0069] The test specimens were sheared to a size of 150 x 70 mm. The processed specimens were then pickled in a 5% hydrochloric acid + 0.03% inhibitor (Super Hyvilon AS-31F, manufactured by Sugimura Chemical Co., Ltd.) at 90°C for 60 seconds, followed by chemical conversion treatment under the conditions shown below. The chemical conversion treatment was carried out in the order of 1. to 5. below.

[0070] <Chemical Treatment> 1. Degreasing agent: Nippon Paint Surf Cleaner EC90M / L Bath temperature: 45℃ Treatment time: 120s Degreasing method: Spray

[0071] 2. Washing water quality: Ion-exchanged water Treatment time: 30s Washing method: Spray

[0072] 3. Surface conditioning agent: Surffine 5N-10 manufactured by Nippon Paint Co., Ltd. Bath temperature: Room temperature Treatment time: 30 seconds Surface conditioning method: Immersion

[0073] 4. Chemical treatment agent: Surfdyne EC1000R-1 manufactured by Nippon Paint Co., Ltd. Bath temperature: 38°C Treatment time: 90 s Chemical treatment method: Immersion Total acidity (TA): 22 pts Free acidity (FA): 0.8 pts Accelerator (AC): 3.0 pts

[0074] 5. Washing water quality: Ion-exchanged water Treatment time: 30s Washing method: Spray

[0075] A portion of the samples after chemical conversion treatment were subjected to electrodeposition coating for corrosion resistance testing. Electrodeposition coating was performed using Kansai Paint's GT150V to achieve a coating thickness of 15 μm, followed by baking at 170°C for 20 minutes.

[0076] [Evaluation Method] <Chemical Scale> Chemical scale refers to a state in which no chemical crystals have precipitated on the steel plate. Therefore, in order to quantitatively evaluate the chemical scale, the following method was used to evaluate the chemical scale.

[0077] After chemical conversion treatment, a scanning electron microscope (SEM) was used to image 10 fields (1 field: approximately 254 μm × 190 μm) of the sample. The SEM was set to backscattered electron mode, applied voltage of 15 kV, and magnification of 2000x. The image data for each field was then divided into 11 equal parts by vertical and horizontal lines, and it was checked whether the base metal was exposed or whether chemical crystals had precipitated at the intersections of the vertical and horizontal lines. Out of 1000 measurement points (intersections of vertical and horizontal lines in the entire field), if the base metal was exposed at fewer than 5 points, it was judged as A; if it was between 5 and 10 points, it was judged as B; and if it was 11 or more points, it was judged as C. A and B were considered acceptable. The results are shown in Table 3.

[0078] <Chemical Conversion Grain Size> Chemical conversion grain size refers to the particle size of chemical conversion crystals precipitated on the steel plate, and was evaluated using the following method.

[0079] Ten fields (approximately 64 μm x 48 μm per field) were imaged from the chemically treated samples using a scanning electron microscope (SEM) in backscattered electron imaging mode, with an applied voltage of 15 kV and a magnification of 2000x. The image data from each field was then divided into 11 equal parts by vertical and horizontal lines, and the major axis of the chemical crystals at the intersections of these lines was measured. The average of these measured major axes was then used as the particle size of the chemical crystals. A size of 7 μm or less was considered suitable, while a size greater than 7 μm was considered unsuitable. If no chemical crystals were present at the intersections, they were not included in the calculation of the average value. The size of the chemical crystals measured was defined as having a minor axis of 0.5 μm or larger. The results are shown in Table 3.

[0080] <Amount of chemical deposition> The amount of chemical deposition is the weight per unit area of ​​chemical crystals deposited on the steel plate, and was evaluated using the following method.

[0081] For the measurement of chemical deposition deposition, quantitative analysis was performed using X-ray fluorescence analysis. Specifically, ultra-low carbon mild steel was prepared by first treating it under different chemical deposition treatment conditions, and the P intensity was measured for each ultra-low carbon mild steel by X-ray fluorescence analysis. Furthermore, the chemical deposition crystals were removed from each ultra-low carbon mild steel using chromic acid, and the deposition amount was calculated from the weight difference before and after removal. A calibration curve for X-ray fluorescence analysis was created using these values. Using this calibration curve, the sample after chemical deposition treatment was analyzed by X-ray fluorescence analysis, and the deposition amount deposited on the steel plate surface was calculated. This deposition amount was 2.0 g / m². ２ Based on the above, we determined that 2.0 g / m² is suitable. ２ Values ​​below a certain threshold were deemed unsuitable. The results are shown in Table 3.

[0082] <Bare Corrosion Resistance> For the bare corrosion resistance test, a 120 x 50 mm evaluation surface was prepared by chemical treatment using the method described above, followed by sealing the edges and back surface with waterproof tape. The sample was subjected to a combined cycle test (JASO M609) for 120 cycles. After the test, rust was removed using an aqueous hydrochloric acid solution with an inhibitor added, in accordance with ISO 8407 C. 3.1, and the maximum corrosion depth at 50 points on the evaluation surface was measured using a micrometer. A value of less than 1.5 mm was judged as A, 1.5 to 2.0 mm as B, and greater than 2.0 mm as C. A and B were considered acceptable. The results are shown in Table 3.

[0083] <Corrosion Resistance After Painting> For the corrosion resistance test after painting, samples treated with chemical conversion and electrodeposition coating using the method described above were used. The samples were cross-cut using an NT cutter S or A type (manufactured by Nippon Transfer Paper Co., Ltd.) under the conditions of a length of 90 mm, a crossing angle of 45°, and a load of 300 g. After cutting, the end faces and back faces of the samples were sealed with waterproof tape and subjected to a salt spray test (JIS Z 2371) for 720 hours. After the test, the coating blistering formed around the cross-cut area of ​​the samples was checked, and half of the widest blister width on both sides of the cross-cut was measured. If this measurement was less than 1.5 mm, it was judged as A; if it was between 1.5 and 2.0 mm, it was judged as B; and if it was greater than 2.0 mm, it was judged as C. A and B were considered acceptable. The results are shown in Table 3.

[0084]

[0085]

[0086]

[0087] The results in Tables 1-3 show that the present invention example has superior chemical treatment properties and high corrosion resistance compared to the comparative example.

[0088] According to the present invention, a steel sheet having excellent chemical treatment properties and high corrosion resistance can be provided along with a method for manufacturing the same. The steel sheet of the present invention is applicable to various uses such as automotive steel sheets and has high industrial utility.

Claims

1. A steel sheet having a component composition containing, in mass%, Cu: 0.01 to 0.50%, Ni: 0.04 to 1.00%, Sn: 0.001 to 0.100%, Cr: 0.01 to 0.20%, Mo: 0.001 to 1.000%, Zn: 0 to 0.500%, Pb: 0 to 0.500%, As: 0 to 0.500%, Sb: 0 to 0.500%, Bi: 0 to 0.500%, V: 0 to 0.500%, C: 0 to 0.50%, Si: 0 to 3.000%, Mn: 0 to 5.00%, B: 0 to 0.0100%, P: 0 to 0.100%, S: 0 to 0.020%, Al: 0 to 0.100% and N: 0 to 0.0100%, with the balance being Fe and unavoidable impurities, and in the emission intensity profile of the wavelength indicating Ni obtained by measuring in the depth direction from the surface of the steel sheet by glow discharge optical emission spectrometry (GDS), the average intensity of Ni is I ａｖｅ , ｍａｘ , ａｖｅ , ｍａｘ , ｍａｘ , ｍａｘ , ｍａｘ , １／２ , ｍａｘ , １／２ , ｍａｘ , the peak intensity of Ni is I ｍａｘ , the depth (μm) indicating the I ｍａｘ is D ｍａｘ , the Ni concentration (mass%) converted from the I ｍａｘ is [Ni] ｍａｘ , when the Cu content (mass%) in the steel sheet is defined as [Cu], the Sn content (mass%) is defined as [Sn], and the Cr content (mass%) is defined as [Cr], a steel sheet satisfying the following formulas (1) to (3). 0.00 ≦ [Ni] ｍａｘ - 2.5[Cu] ≦ 6.00... (1) 0.020 ≦ D １／２ ... (2) 10[Sn] + 5[Cr] ≦ 1.81... (3)​​​​​​​​ 2. The steel sheet according to claim 1, wherein the steel sheet is a steel sheet having a chemical conversion treatment coating.

3. A method for manufacturing a steel sheet according to claim 1, comprising: a heating step of heating a slab satisfying the component composition and formula (3) described in claim 1; a descaling step of performing descaling on the heated slab; and a hot rolling step of performing hot rolling on the slab after descaling, wherein the heating step comprises a first heat treatment of holding the slab at 1100 to 1300°C for 30 to 120 minutes in an atmospheric environment; and a second heat treatment of holding the slab at 1100 to 1300°C for 30 to 120 minutes in an environment controlled to a nitrogen atmosphere after the first heat treatment.

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