Hot-rolled steel sheet and method for manufacturing same

Abstract

Provided is a hot-rolled steel sheet having excellent chemical conversion treatability and good coating film adhesion despite having tramp elements. This hot-rolled steel sheet contains predetermined amounts of predetermined elements, has a surface roughness Sa of 1.75 μm or more, and satisfies expressions (1) to (3) in which the concentration of Ni converted from the peak intensity of Ni in an emission intensity profile of a wavelength indicating Ni, obtained by measurement in the depth direction from the surface of the hot-rolled steel sheet by GDS, is defined as [Ni]max, and the content Cu (mass%), the content of Sn (mass%), the content of Cr (mass%), and the content of Ni (mass%) in the hot-rolled steel sheet are defined as [Cu], [Sn], [Cr], and [Ni], respectively.　(1): 0.00≤[Ni]max-2.5[Cu]≤6.00 (2): 10[Sn]+5[Cr]≤1.81 (3): 0.42≤[Ni] / [Cu]

Description

Hot-rolled steel sheet and method for manufacturing the same

[0001] This invention relates to hot-rolled steel sheets and methods for manufacturing the same.

[0002] Hot-rolled 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 hot-rolled steel sheets: the blast furnace method, which uses iron ore and coke to produce steel 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 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 electric arc furnace method for manufacturing steel sheets 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 after electric arc furnace manufacturing, degrading various properties. A particularly serious problem is the decline in the chemical treatment properties of hot-rolled steel sheets. When chemical treatment properties decline, problems arise such as a significant impediment to adhesion with the paint formed on the chemically treated surface in subsequent processes.

[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] Furthermore, Patent Document 3 proposes that for high-strength hot-rolled steel sheets containing 0.5 to 5.0% Si by mass, the average surface roughness Ra should be 2.0 μm or less and the average oxygen concentration on the surface should be 5.0% or less in order to improve chemical treatmentability and corrosion resistance after painting.

[0005] Japanese Patent Publication No. WO2021 / 157692, Japanese Patent Publication No. 2020-84325, Japanese Patent Publication No. 2016-29207

[0006] In fields that use painted steel sheets, such as automobiles and construction, there is a growing need for steel sheets that do not have their paint film peel off even when subjected to external forces such as sliding and chipping during outdoor exposure, thus preserving their aesthetics and corrosion resistance. This requires paint adhesion that exceeds current requirements. As mentioned above, trump elements reduce chemical conversion treatment properties, so there is a need for hot-rolled steel sheets that maintain chemical conversion treatment properties even with trump elements, and that also possess even better paint adhesion.

[0007] However, Patent Documents 1 and 2 do not take into consideration coating adhesion and therefore cannot satisfy the above-mentioned needs.

[0008] Furthermore, Patent Document 3 did not take into account the decrease in chemical conversion treatment properties due to the Trump element, and there was room for improvement in terms of chemical conversion treatment properties and coating adhesion.

[0009] Therefore, in view of the above issues, the present invention aims to provide a hot-rolled steel sheet that has excellent chemical treatment properties and good coating adhesion, even when having trump elements.

[0010] The inventors of the present invention have diligently studied to solve the above problems and have obtained the following findings: <1> 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, the chemical conversion treatment performance can be improved. <2> By limiting the amount of Sn and Cr in the steel sheet, the chemical conversion treatment performance can be improved. <3> By controlling the ratio of the amount of Cu to the amount of Ni in the steel sheet and the roughness of the steel sheet surface, the adhesion of the coating film can be improved.

[0011] 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: 0-0.500%, 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 A hot-rolled steel sheet having a component composition containing 0 to 0.0100% N, with the remainder being Fe and unavoidable impurities, wherein the Ni concentration is calculated from the peak intensity of Ni in the emission intensity profile at the wavelength indicating Ni, obtained by glow discharge emission spectroscopy (GDS) measured in the depth direction from the surface of the hot-rolled steel sheet, and is defined as [Ni]. ｍａｘ Assuming that the following are defined, and that the Cu content (mass%), Sn content (mass%), Cr content (mass%), and Ni content (mass%) in the hot-rolled steel sheet are defined as [Cu], the hot-rolled steel sheet satisfies the following equations (1) to (3), and the surface roughness Sa of the hot-rolled steel sheet is 1.75 μm or more: 0.00 ≤ [Ni] ｍａｘ-2.5[Cu] ≤ 6.00...(1) 10[Sn] + 5[Cr] ≤ 1.81...(2) 0.42 ≤ [Ni] / [Cu]...(3) A method for manufacturing a hot-rolled steel sheet as described in [2] and [1], comprising: a heating step of heating a slab satisfying the component composition, formula (2) and formula (3) described in [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, A method for manufacturing hot-rolled steel sheets, wherein the descaling process is performed under conditions of a water pressure of 30 to 120 MPa for 10 to 1800 seconds.

[0012] Here, the steel plate in [1] can be, for example, a steel plate manufactured using the electric furnace method. Also, the slab used in the manufacturing method in [2] can be, for example, a slab manufactured using the electric furnace method.

[0013] The hot-rolled steel sheet of the present invention exhibits excellent chemical treatment properties and good coating adhesion, even when it contains trump elements.

[0014] The following describes embodiments of the hot-rolled steel sheet according to the present invention, but these are merely examples that embody the present invention and do not limit the configuration of the present invention. In the following description, the unit of content of each element in the component composition is "mass%", and unless otherwise specified, it is simply indicated as "%". Also, regarding the numerical range, the notation "a to b" means a or more and b or less, and includes a and b in the range unless otherwise specified.

[0015] (Steel Plate) First, the structure of the steel plate of the present invention will be described.

[0016] [Composition] The hot-rolled steel sheet of the present invention has the following composition by mass%, Cu: 0.01-0.50%, Ni: 0.04-1.00%, Sn: 0.001-0.100%, Cr: 0.01-0.20%, Mo: 0.001-1.000%, Zn: 0-0.500%, Pb: 0-0.500%, As: 0-0.500%, Sb: 0-0.500%, Bi: 0 The composition contains ~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%, with the remainder being Fe and unavoidable impurities. If the lower limit of the content is 0%, the content range includes 0%.

[0017] The hot-rolled steel sheet having the above-mentioned component composition can be a steel sheet manufactured using the electric furnace method, and its type is not particularly limited. Examples include hot-rolled steel sheets with common component compositions such as ultra-low carbon steel, and hot-rolled steel sheets with component compositions such as high-tensile steel.

[0018] <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 causes chemical scaling on the 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 the steel sheet, if there is too much, it becomes difficult to stabilize the mechanical properties in ultra-low carbon steel, so it is preferable to keep it at 0.30% or less. On the other hand, for example, reducing Cu in iron scrap in the electric arc furnace method is disadvantageous in terms of cost, and in addition, Cu, when included in the bulk, 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 Cu content should be 0.01% or more, preferably 0.05% or more, and more preferably 0.07% or more.

[0019] <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 as fine granules on the surface of the steel sheet forms microscopic cathodes, which promote the dissolution reaction of Fe around the Ni-concentrated areas and improve the chemical conversion treatment properties by becoming a crystal nucleus for chemical conversion crystals. Furthermore, Ni is an element that is completely solid-soluble in Cu, and during the scale formation process in the hot rolling process, it concentrates together with Cu at the base metal-scale interface, forming surface irregularities and improving surface roughness. From the viewpoint of improving chemical conversion treatment properties and surface roughness, 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 may be 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.

[0020] <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.

[0021] <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.

[0022] <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.

[0023] <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.

[0024] <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.

[0025] <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.

[0026] <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.

[0027] <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.

[0028] <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.

[0029] <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.

[0030] <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. ２SiO ４ If Si remains, it will deteriorate the formation processability. Therefore, the Si content should be 3.000% or less, preferably 2.550% or less. The Si content may be 0%, but from the point of ensuring the above effects, it is preferably 0.013% or more.

[0031] <Mn: 0 to 5.00%> Mn is an element that can ensure a predetermined hardenability, suppress ferrite transformation, and ensure tempered martensite and bainite with a desired area ratio to guarantee strength. However, if Mn is too much, bainite transformation will be 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 point of ensuring the above effects, it is preferably 0.02% or more.

[0032] <B: 0 to 0.0100%> B is an element that makes it easier to generate tempered martensite and bainite with a predetermined area ratio, and can also improve the stress corrosion cracking resistance due to the remaining of dissolved B. However, if the content of B is too much, it will cause a significant decrease in hot ductility and surface defects. Therefore, the B content should be 0.0100% or less, preferably 0.0096% or less. The B content may be 0%, but from the point of ensuring the above effects, it is preferably 0.0001% or more.

[0033] <P: 0 to 0.100%> P is an element that strengthens steel. However, if the content of P is high, it will deteriorate the spot weldability. Therefore, the P content should be 0.100% or less, preferably 0.090% or less. The P content may be 0%, and for example, it can be 0.001% or more.

[0034] <S: 0 to 0.020%> S is an element that has the effect of improving scale peelability during hot rolling and the effect of suppressing nitridation during annealing. However, if the content of S is too high, it will cause deterioration of spot weldability and local elongation. Therefore, the S content is set to 0.020% or less, preferably 0.019% or less. The S content may be 0%, and from the viewpoint of improving scale peelability during hot rolling, for example, it can be set to 0.003% or more.

[0035] <Al: 0 to 0.100%> Al is an element that can stabilize residual γ as a deoxidizer or as a substitute for Si. However, if the content of Al is too high, the strength of the material will extremely decrease. Therefore, the Al content is set to 0.100% or less, preferably 0.080% or less. The Al content may be 0%, and from the viewpoint of stabilizing residual γ, for example, it can be set to 0.020% or more.

[0036] <N: 0 to 0.0100%> N is an element that forms nitrides such as BN, AlN, and TiN in steel, and is an element that can reduce the hot ductility of steel and deteriorate the surface quality. Also, in steel containing B, there is a disadvantage of disappearing the effect of B through the formation of BN. For these reasons, the N content is set to 0.0100% or less, preferably 0.0095% or less. The N content may be 0%, and from the viewpoints of cost increase due to denitrification and strength guarantee due to nitride formation, for example, it can be set to 0.0002% or more.

[0037] The hot-rolled steel sheet of the present invention has a component composition composed of the above components, the balance of Fe, and inevitable impurities.

[0038] [Formulas (1) to (3)] The hot-rolled steel sheet of the present invention is measured from the surface of the hot-rolled steel sheet in the depth direction by glow discharge optical emission spectrometry (GDS). In the emission intensity profile of the wavelength indicating Ni, the Ni concentration converted from the peak intensity of Ni is defined as [Ni] ｍａｘ and defined as follows: When the Cu content (mass%) in the hot-rolled steel sheet is [Cu], the Sn content (mass%) is [Sn], the Cr content (mass%) is [Cr], and the Ni content (mass%) is [Ni], the following formulas (1) to (3) are satisfied. 0.00 ≤ [Ni]ｍａｘ -2.5[Cu] ≤ 6.00...(1) 10[Sn] + 5[Cr] ≤ 1.81...(2) 0.42 ≤ [Ni] / [Cu]...(3)

[0039] The reasons for setting the formulas (1) to (2) will be described.

[0040] The inventors earnestly studied the causes of deteriorating chemical conversion properties of the steel sheet having the above component composition, and obtained the following findings. (A) Cu is concentrated relatively coarsely on the steel sheet surface to become a cathode point, inhibiting the precipitation of chemical conversion crystals on the concentrated portion. (B) Sn and Cr dissolved or precipitated in the steel, although they dissolve once during the chemical conversion treatment, cover the steel sheet surface by re-precipitation, suppressing the progress of the chemical conversion treatment reaction.

[0041] <Equation (1)>Regarding the event of (A) above, the inventors searched for a technique to improve the chemical conversion treatment property even for steel sheets with a high Cu content. When measuring the Ni distribution in the steel sheet depth direction, it was found that the higher the maximum concentration (mass%) of Ni observed near the steel sheet surface with respect to the Cu content (mass%) of the base material, the better the chemical conversion treatment property can be obtained. This finding is based on the fact that when the maximum concentration of Ni observed near the steel sheet surface becomes higher with respect to the Cu content of the base material, fine particles of Ni precipitate not only in the portion where Cu is not concentrated but also in the portion where Cu is concentrated. In the former case, Ni itself becomes the cathode point and the surrounding becomes the anode point, and the anode point acts as the crystal nucleus of the chemical conversion crystal, precipitating fine and uniform chemical conversion crystals. On the other hand, in the latter case, the portion where Cu is concentrated becomes the cathode point, and the Ni particles themselves act as the anode point, and the Ni particles themselves act as the crystal nucleus of the chemical conversion crystal, precipitating fine and uniform chemical conversion crystals. The inventors obtained Equation (1) by formulating these actions. 0.00 ≤ [Ni] ｍａｘ -2.5[Cu] ≤ 6.00...(1)

[0042] [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

[0043] 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.

[0044] [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.

[0045] <Equation (2)> As a result of diligent research, the inventors have found that it is important to consider the phenomenon described in (B) above in order to improve the chemical conversion treatment performance. Sn and Cr exist either as solid solutions in the steel or as precipitates on the surface of the steel plate, but they dissolve in the chemical conversion treatment solution during the 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 the decrease in chemical conversion treatment performance, and have obtained the formula (2). 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...(2)

[0046] The larger the value of 10[Sn] + 5[Cr] in formula (2), 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.

[0047] <Formula (3)> The inventors conducted various investigations focusing on surface roughness to improve coating adhesion and confirmed that the enrichment of Cu and Ni among the trump elements at the base metal-scale interface is important during scale formation in the hot rolling process. It is presumed that the enriched areas of Cu and Ni physically expand the base metal-scale interface, forming irregularities and contributing to the improvement of the surface roughness of the hot-rolled steel sheet. However, it was found that increasing the amount of Cu to increase the volume of the enriched area may reduce the chemical conversion treatment properties, and that Cu is easily discharged to the scale side, thus having a limited effect on surface roughness. In contrast, as mentioned above, Ni has a limited effect on reducing chemical conversion treatment properties, and the amount discharged to the scale side is less than that of Cu. For example, when using the electric furnace method, the inclusion of both Cu and Ni is unavoidable. However, even in such cases, in order to ensure chemical conversion treatment while effectively improving surface roughness, the inventors have found that controlling the ratio of Cu content to Ni content in the steel sheet is effective, and have obtained the formula (3). In the formula, [Ni] is the Ni content (mass%) in the steel, and [Cu] is the Cu content (mass%) in the steel. 0.42 ≤ [Ni] / [Cu] ... (3)

[0048] The value of [Ni] / [Cu] in formula (3) should be 0.42 or higher, preferably 0.70 or higher, from the viewpoint of coating adhesion. Within this range, the surface roughness Sa can be easily controlled to the range described later. Furthermore, if the value of [Ni] / [Cu] is 3.50 or lower, the need for Ni addition or component adjustment is less likely to occur, making it easier to avoid increased costs and economic disadvantages. Therefore, the value of [Ni] / [Cu] should preferably be 3.50 or lower, and more preferably 2.0 or lower. The value of [Ni] / [Cu] preferably satisfies the following formula (4): 0.42 ≤ [Ni] / [Cu] ≤ 3.50 ... (4)

[0049] [Surface Roughness] The inventors further investigated conditions that contribute to improving coating adhesion and confirmed that specifying the value of Sa, which is the arithmetic mean height with respect to the average surface, is effective among surface roughness values. It is presumed that when the value of Sa is above a certain level, the protrusions bite into the coating, creating an anchoring effect and improving adhesion. Furthermore, in actual usage environments, external forces that peel off the coating occur in various directions in two dimensions, so it is more appropriate to specify the value of Sa than the arithmetic mean height Ra with respect to the average line. If Sa is less than 1.75 μm, the above-mentioned anchoring effect is insufficient, and the effect of improving coating adhesion cannot be obtained. For this reason, Sa should be 1.75 μm or more, preferably 2.05 μm or more. Also, if Sa is greater than 5.00 μm, the electrodeposited coating may become thin at the tips of the protrusions, potentially reducing corrosion resistance. From this point of view, Sa is preferably 5.00 μm or less, and more preferably 3.00 μm or less. Furthermore, Sa is obtained by acquiring surface topography data using a laser microscope with a 10x objective lens in accordance with ISO 25178, and then analyzing the entire measurement surface under the conditions of no S filter (low-pass filter) and a 0.8 mm L filter (high-pass filter).

[0050] (Manufacturing Method) The manufacturing method for the hot-rolled steel sheet of the present invention will now be described. The hot-rolled steel sheet of the present invention is obtained by a manufacturing method for a steel sheet, comprising: a heating step of heating a slab satisfying formulas (2) and (3) in addition to the component composition 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 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 a nitrogen atmosphere after the first heat treatment, and the descaling step is performed under conditions of a water pressure of 30 to 120 MPa for 10 to 1800 seconds. After hot rolling, it may be subjected to pickling and then chemical conversion treatment to obtain a chemically converted steel sheet.

[0051] [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 formulas (2) and (3). Scrap can be used as the raw material.

[0052] To obtain the hot-rolled 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.

[0053] The first heat treatment is a process aimed at promoting scale formation on the slab surface and is performed 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.

[0054] 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 a decrease in chemical conversion treatment properties and surface roughness. 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.

[0055] 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, resulting in a decrease in chemical conversion treatment properties and surface roughness. 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. Preferably, the holding time is 45 minutes or more, and also 100 minutes or less.

[0056] 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 deposited on the surface due to heating into the steel sheet. This process ensures that the uneven surface of the base metal-scale interface formed by the first heat treatment remains electrically noble due to the diffusion of Ni, thereby preventing the dissolution of the protrusions during the pickling process.

[0057] In the second heat treatment, if the holding temperature is below 1100°C, the diffusion of Ni on the uneven surface decreases, which promotes the dissolution of the protrusions during pickling and reduces the surface roughness. 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 sheet becomes excessive, and the amount of Ni precipitated on the surface of the steel sheet decreases, thus reducing the chemical conversion treatment properties. 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 temperatures for the first heat treatment and 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.

[0058] In the second heat treatment, if the holding time at the holding temperature is less than 30 minutes, the diffusion of Ni on the uneven surface decreases, which promotes the dissolution of the protrusions during pickling and reduces the surface roughness. 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, resulting in a decrease in chemical conversion treatment performance. 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.

[0059] 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.

[0060] [Descaling Process] In the descaling process, descaling is performed on the heated slab. 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.

[0061] Descaling can be performed by spraying high-pressure water. To control the surface roughness Sa of the hot-rolled steel sheet within a desired range, the descaling process is performed under conditions of a water pressure of 30 to 120 MPa for 10 to 1800 seconds. A water pressure of 50 MPa or higher is preferable, as it facilitates the removal of scale without residue. A water pressure of 100 MPa or lower is also preferable. A descaling time of 30 seconds or more is preferable, as it facilitates the removal of scale without residue. A descaling time of 6000 seconds or lower is also preferable. Other descaling conditions are not particularly limited, and conventional conditions can be used.

[0062] [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.

[0063] The obtained hot-rolled steel sheet can be used to manufacture a steel sheet with a chemical conversion coating (chemical conversion treated steel sheet). A chemical conversion treated steel sheet can be obtained by pickling the hot-rolled steel sheet of the present invention and then performing a chemical conversion treatment. The methods and conditions for pickling and chemical conversion treatment are not particularly limited, and conventional methods can be used. Furthermore, the obtained hot-rolled steel sheet can be used to manufacture a steel sheet with a plating layer (plated steel sheet). A plated steel sheet can be obtained by pickling and then performing a plating treatment. The methods and conditions for plating treatment are not particularly limited, and conventional methods can be used.

[0064] [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 listed in Table 1 under a nitrogen or air atmosphere by indirect heating, and in the second heat treatment, the slabs were held at the time and temperature listed in Table 1 under a nitrogen or air atmosphere by indirect heating. 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. Subsequently, descaling was performed at a water pressure of 20 to 130 MPa for 5 to 1900 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.

[0065] 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 sputtered 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 sputtered marks by the measurement time (200 s = 20000 ms), and this can be converted to the depth corresponding to each time.

[0066] Based on the obtained emission intensity profile, [Ni] ｍａｘ Find the ([Ni] in equation (1) ｍａｘ The value (mass %) of -2.5 [Cu] was calculated. In the calculation, I ａｖｅ This was defined as the average value of the luminescence intensity between 180 and 200 seconds (corresponding to a depth of 12.2 to 13.5 μm from the surface).

[0067] The surface roughness Sa of the test material was measured under the following conditions. Using a non-contact surface shape analyzer manufactured by Keyence Corporation (KEYENCE VK-X200), surface topographic data was acquired in accordance with ISO 25178 using a 10x lens. The surface roughness Sa was then obtained by analyzing the entire measurement surface under the conditions of no S filter (low-pass filter) and a 0.8 mm L filter (high-pass filter). The results are shown in Table 2.

[0068] 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 s, 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.

[0069] <Chemical Treatment> 1. Degreasing agent: Nippon Paint Surf Cleaner EC90M / L Bath temperature: 45℃ Treatment time: 120s Degreasing method: Spray

[0070] 2. Washing water quality: Ion-exchanged water Treatment time: 30s Washing method: Spray

[0071] 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

[0072] 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

[0073] 5. Washing water quality: Ion-exchanged water Treatment time: 30s Washing method: Spray

[0074] A portion of the samples after chemical treatment were subjected to electrodeposition coating for corrosion resistance testing. Electrodeposition coating was performed using Kansai Paint's GT150V, with a coating thickness of 15 μm, and then baked at 170°C for 20 minutes.

[0075] [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 perform a quantitative evaluation of chemical scale, the following method was used to evaluate the chemical scale.

[0076] 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.

[0077] <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.

[0078] 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 to be measured was defined as having a minor axis of 0.5 μm or larger. The results are shown in Table 3.

[0079] <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.

[0080] 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.

[0081] <Coating Adhesion> For the coating adhesion test, samples were treated with chemical conversion and electrodeposition coating using the method described above, then immersed in a hot water bath adjusted to 60°C ± 2°C for 30 days, and then removed from the hot water bath and dried. Using an NT cutter S or A type (manufactured by Nippon Transfer Paper Co., Ltd.), a grid pattern was cut with sufficient force to reach the substrate, forming 100 squares of 2 mm each. Cellophane adhesive tape with a width of 24 mm as specified in JIS Z 1522 was applied to the formed squares by rubbing it with the pad of a finger to avoid leaving air bubbles, and then instantly peeled off perpendicular to the sample surface. After peeling, a new tape was applied perpendicular to the previous application direction and the peeling operation was repeated. Subsequently, the grid pattern was photographed, and the peeled area relative to the total area of ​​the squares was calculated by binarization using the image analysis software ImageJ. If the calculated peeled area ratio was less than 5%, it was judged as A; if it was between 5% and 15%, it was judged as B; and if it was greater than 15%, it was judged as C. A and B were considered acceptable. The results are shown in Table 3.

[0082]

[0083]

[0084]

[0085] The results in Tables 1-3 show that the present invention example has superior chemical conversion treatment properties and coating adhesion compared to the comparative example.

[0086] According to the present invention, a hot-rolled steel sheet with excellent chemical treatment properties and good coating adhesion can be provided along with a method for manufacturing the same. The hot-rolled steel sheet of the present invention can be applied to various uses such as automotive steel sheets and has high industrial utility.

Claims

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: 0-0.500%, 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%, A hot-rolled steel sheet having a component composition containing Al: 0 to 0.100% and N: 0 to 0.0100%, with the remainder being Fe and unavoidable impurities, wherein the Ni concentration is calculated from the peak intensity of Ni in the emission intensity profile at the wavelength indicating Ni, obtained by glow discharge emission spectroscopy (GDS) measured in the depth direction from the surface of the hot-rolled steel sheet, and is defined as [Ni]. ｍａｘ Assuming that the following are defined, and that the Cu content (mass%), Sn content (mass%), Cr content (mass%), and Ni content (mass%) in the hot-rolled steel sheet are defined as [Cu], the hot-rolled steel sheet satisfies the following equations (1) to (3), and the surface roughness Sa of the hot-rolled steel sheet is 1.75 μm or more: 0.00 ≤ [Ni] ｍａｘ -2.5[Cu]≦6.00...(1) 10[Sn]+5[Cr]≦1.81...(2) 0.42≦[Ni] / [Cu]...(3) 2. A method for manufacturing a hot-rolled steel sheet according to claim 1, comprising: a heating step of heating a slab satisfying the component composition, formula (2) and formula (3) according to 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 a nitrogen atmosphere after the first heat treatment, and the descaling step is performed under conditions of a water pressure of 30 to 120 MPa for 10 to 1800 seconds.

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