Hot-rolled steel sheet and method for manufacturing the same

Abstract

To provide a hot-rolled steel sheet that has excellent chemical treatment properties and good coating adhesion even when containing tramp elements. The sheet contains predetermined elements in predetermined amounts, and the Ni concentration, calculated from the peak intensity of Ni in the emission intensity profile at the wavelength indicating Ni, obtained by GDS measurement from the surface of the hot-rolled steel sheet in the depth direction, is [Ni] max Assuming that the hot-rolled steel sheet is defined as follows, and that the Cu content (mass%) in the hot-rolled steel sheet is defined as [Cu], the Sn content (mass%) as [Sn], the Cr content (mass%) as [Cr], and the Ni content (mass%) as [Ni], then 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)

Description

[Technical Field]

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

[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 to produce steel in an electric arc furnace. Traditionally, the blast furnace method has been the mainstream method for steel sheet manufacturing due to its cost advantages and ability to produce high-quality steel sheets. However, in response to the growing environmental awareness and CO2 emission regulations in recent years, the electric arc furnace method for steel sheet manufacturing 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 the scrap iron can be mixed into the steel sheet after electric arc furnace manufacturing, degrading various properties. A particularly serious problem is the reduced chemical treatmentability of hot-rolled steel sheets. When chemical treatmentability is reduced, problems arise such as a significant impairment of 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 containing trump elements. For example, Patent Document 1 describes a technology for obtaining a hot-rolled steel sheet with excellent chemical treatment properties by limiting the components in the steel and limiting the range in which the Ni content on the steel sheet surface is 0.5% by mass to 10-70%. Furthermore, Patent Document 2 describes a technique for obtaining a steel sheet with excellent chemical conversion treatment properties by ensuring that the residue on the surface of the steel sheet contains microcathodes, and that the microcathodes 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 5.0% or less in order to improve chemical treatmentability and corrosion resistance after painting. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] WO2021 / 157692 issue [Patent Document 2] Special Publication No. 2020-84325 [Patent Document 3] Japanese Patent Publication No. 2016-29207 [Overview of the project] [Problems that the invention aims to solve]

[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.

Means for Solving the Problem

[0010] As a result of intensive studies to solve the above problems, the present inventors obtained the following findings. <1> By defining the relationship between the amount of Ni on the steel sheet surface and the amount of Cu in the steel sheet, the chemical conversion treatment property can be improved. <2> By restricting the amounts of Sn and Cr in the steel sheet, the chemical conversion treatment property can be improved. <3> By controlling the ratio value of the amount of Cu to the amount of Ni in the steel sheet and the roughness of the steel sheet surface, the coating adhesion can be improved.

[0011] The present invention is based on the above findings, and the gist thereof 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%, and is a hot-rolled steel sheet having a component composition consisting of the balance Fe and inevitable impurities. 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) from the surface of the hot-rolled steel sheet in the depth direction, and is defined as [Ni] max Defined as, In the aforementioned hot-rolled steel sheet, The Cu content (mass%) is expressed as [Cu]. Sn content (mass%) is denoted as [Sn]. Cr content (mass%) is [Cr], When the Ni content (mass%) is defined as [Ni], The following equations (1) to (3) are satisfied, and A hot-rolled steel sheet having a surface roughness Sa of 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 hot-rolled steel sheets as described in [2][1], A heating step of heating a slab that satisfies the component composition, formula (2) and formula (3) described in [1], A descaling step is performed on the heated slab, A hot rolling process is performed on the slab after descaling. A method for manufacturing hot-rolled steel sheets, including The heating step 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 a controlled nitrogen atmosphere. The descaling process is performed under conditions of 30 to 120 MPa water pressure for 10 to 1800 seconds. A method for manufacturing hot-rolled steel sheets.

[0012] Here, the steel plate in [1] can be, for example, a steel plate manufactured using the electric furnace method. Furthermore, the slab used in the manufacturing method of [2] may be, for example, a slab manufactured by the electric furnace method. [Effects of the Invention]

[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. [Modes for carrying out the invention]

[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 explanation, the unit for the content of each element in the component composition is "mass%", and unless otherwise specified, it will simply be indicated as "%". Furthermore, regarding numerical ranges, the notation "a~b" means a value greater than or equal to b, and includes both a and b in the range, unless otherwise specified.

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

[0016] [Component composition] The hot-rolled steel sheet of the present invention has the following composition in 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 can be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method, and is likely to be mixed into the steel in relatively larger amounts than 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 during 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 steel sheets, 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, reducing the amount of Cu in iron scrap in processes such as electric furnace steelmaking is disadvantageous from an economic standpoint. In addition, the presence of Cu in the bulk reduces the reactivity of the steel sheet itself, thereby improving its corrosion resistance. For these reasons, the Cu content is set at 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 steel sheet surface forms microscopic cathodes, which promote the dissolution reaction of Fe around the Ni-concentrated areas and improve the chemical conversion treatment properties by acting as crystal nuclei for the 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 treatment properties on the concentrated areas. Furthermore, since it becomes necessary to add Ni to the iron scrap 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 reprecipitates as hydroxide, coating the surface of the steel sheet and significantly reducing the chemical conversion treatment properties. Therefore, the Sn content should be 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 should be 0.001% or more, preferably 0.005% or more.

[0021] <Cr:0.01~0.20%> Cr is an element that can 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, particularly 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 from an economic standpoint, and Mo is an element that increases the strength of steel sheets. Therefore, the Mo content is set to 0.001% or more, preferably 0.003% or more. 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. Therefore, 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%> Lead (Pb) is an element that may be mixed in as a trump element, particularly in steel sheets manufactured by the electric furnace method. Furthermore, Pb is an element that has the effect of reducing segregation. 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%, or for example, 0.001% or more.

[0025] <As:0~0.500%> As is an element that may be mixed in as a trump element, particularly in steel sheets manufactured by the electric furnace method. 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, particularly in steel sheets manufactured by the electric furnace method. To avoid impairing the properties of 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, particularly in steel sheets manufactured by the electric furnace method. Furthermore, Bi is an element that has the effect of reducing segregation. 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%, or for example, 0.001% or more.

[0028] <V:0~0.500%> V is an element that may be mixed in as a trump element, particularly 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%, or for example, 0.003% or more.

[0029] <C:0~0.50%> Carbon (C) is an element that can be included in steel plates to improve hardenability, enhance strength by ensuring martensite, and control 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, reducing workability. Therefore, the carbon content should be 0.50% or less, preferably 0.45% or less. The carbon content may be 0%, but it is preferable to have 0.01% or more to ensure the above effects.

[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, Fe2SiO4 formed on the surface of the steel sheet during hot rolling will remain, worsening the chemical conversion treatment properties. Therefore, the Si content should be 3.000% or less, preferably 2.550% or less. The Si content may be 0%, but it is preferable to have 0.013% or more to ensure the above effects.

[0031] <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.

[0032] <B:0~0.0100%> 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 lead to a significant decrease in hot ductility and cause surface defects, so the B content should be 0.0100% or less, preferably 0.0096% or less. The B content may be 0%, but in order to ensure the above effects, it is preferable to have a B content of 0.0001% or more.

[0033] <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.

[0034] <S:0~0.020%> S is an element that improves scale detachability during hot rolling and suppresses nitriding during annealing. However, if the S content is too high, it can degrade 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.

[0035] <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% 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.

[0036] <N:0~0.0100%> 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. Furthermore, in steel containing B, the formation of BN can negate the effects of B. 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, considering the cost increase due to N removal and the need to ensure strength through nitride formation.

[0037] The hot-rolled steel sheet of the present invention has a component composition consisting of the above components and the remainder being Fe and unavoidable impurities.

[0038] [Formula (1)~(3)] The hot-rolled steel sheet of the present invention is obtained by glow discharge emission spectroscopy (GDS), and the Ni concentration is calculated from the peak intensity of Ni in the emission intensity profile at the wavelength indicating Ni, which is measured in the depth direction from the surface of the hot-rolled steel sheet. max Defined as, In the aforementioned hot-rolled steel sheet, The Cu content (mass%) is expressed as [Cu]. Sn content (mass%) is denoted as [Sn]. Cr content (mass%) is [Cr], When the Ni content (mass%) is defined as [Ni], The following equations (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] Let's explain the reasoning behind the settings in equations (1) and (2).

[0040] The inventors of the present invention diligently investigated the causes of deterioration in the chemical conversion properties of steel sheets having the above-mentioned component composition, and obtained the following findings. (A) The relatively coarse concentration of Cu on the steel plate surface creates cathode points, which inhibit the deposition of chemical conversion crystals on the concentrated areas. (B) Although Sn and Cr, which are dissolved or precipitated in the steel, dissolve during the chemical conversion treatment, they re-precipitation and coat the surface of the steel sheet, thereby suppressing the progress of the chemical conversion treatment reaction.

[0041] <Formula (1)> In response to the above-mentioned phenomenon (A), 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 increases with respect to the Cu content of the base material, fine particles of Ni precipitate not only in the portions where Cu is not concentrated but also in the portions where Cu is concentrated. In the former case, Ni itself becomes the cathode point and the surrounding area becomes the anode point, and the anode point acts as the crystal nucleus of the conversion crystal, thereby precipitating fine and uniform conversion crystals. On the other hand, in the latter case, the portion where Cu is concentrated becomes the cathode point, the Ni particles themselves act as the anode point, and the Ni particles themselves act as the crystal nucleus of the conversion crystal, thereby precipitating fine and uniform conversion crystals. The inventors of the present invention obtained Equation (1) by formulating these actions. 0.00 ≦ [Ni] max -2.5[Cu] ≦ 6.00···(1)

[0042] [Ni] in Equation (1) max (mass%) is determined by using the emission intensity profile of the wavelength indicating Ni measured in the depth direction from the surface of the steel sheet by glow discharge optical emission spectrometry (GDS). Specifically, the pre-sputtering time is set to 5 seconds, the output is set to 35 W, the gas replacement time is set to 35 seconds, and the pressure is set to 600 Pa. Data is collected from the surface of the steel sheet for 200 s at an interval of 10 ms (up to a depth of 13.5 μm from the surface) to obtain the emission intensity profile of the wavelength indicating Ni. The intensity between 180 and 200 s (corresponding to a depth between 12.2 and 13.5 μm from the surface) is I ave , and with the data after 400 ms (corresponding to a depth of 0.027 μm from the surface) from the start of measurement, the value (peak intensity) at which the Ni intensity becomes the maximum value is I max , taking the Ni content in the steel as [Ni] (mass%), [Ni] max is calculated by the following calculation. [Ni] max =I max ×[Ni] / I ave

[0043] The GDS analysis is not limited to the above measurement conditions, and conditions equivalent thereto can be adopted. The intensity (I aveThe 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, 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 selecting data from 400ms 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 to account for 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 equation (1) max A 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; therefore, it should be 0.00% or more, preferably 0.50% or more. On the other hand, if this value is too large, the surface of the steel sheet will be covered with coarse Ni, reducing the chemical conversion treatment performance; therefore, it should be 6.00% or less, preferably 5.50% or less.

[0045] <Formula (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 sheet, but they dissolve in the chemical conversion treatment solution during the chemical conversion treatment. However, as the chemical conversion treatment reaction progresses, when the pH rises on the surface of the steel sheet, the dissolved Sn and Cr precipitate as hydroxides with low solubility and coat the surface of the steel sheet like a film. Therefore, it is presumed that the chemical conversion treatment solution and the surface of the steel sheet 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 on the decrease in chemical conversion treatment performance is greater than that of Cr, and have obtained 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 during scale formation in the hot rolling process, is important. 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 areas may reduce the chemical conversion treatmentability, 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 treatmentability, and the amount discharged to the scale side is less than that of Cu. For example, when using the electric furnace method, both Cu and Ni are elements that cannot be avoided. Even in such cases, in order to effectively improve surface roughness while ensuring chemical conversion treatmentability, the inventors found that controlling the ratio of the amount of Cu to the amount of Ni in the steel sheet is effective, and obtained the formula (3). In the formula, [Ni] represents the Ni content (mass%) in the steel, and [Cu] represents 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, when 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 defining the value of Sa, which is the arithmetic mean height relative to the average surface, is effective among surface roughness parameters. 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 defining the value of Sa is more appropriate than defining the arithmetic mean height Ra relative 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. Therefore, Sa should be 1.75 μm or more, preferably 2.05 μm or more. Also, if Sa exceeds 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 in accordance with ISO 25178 using a laser microscope with a 10x objective lens, 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 present invention describes a method for manufacturing hot-rolled steel sheets. The hot-rolled steel sheet of the present invention comprises, in addition to the component composition described for the steel sheet, a heating step of heating a slab satisfying formulas (2) and (3), The descaling process involves descaling the heated slab, A hot rolling process is performed on the slab after descaling. A method for manufacturing steel plates, including The heating process includes a first heat treatment in which the slab is held at 1100-1300°C for 30-120 minutes in an atmospheric environment, and a second heat treatment in which, after the first heat treatment, the slab is held at 1100-1300°C for 30-120 minutes in a controlled nitrogen atmosphere. The descaling process is performed under conditions of 30 to 120 MPa water pressure for 10 to 1800 seconds. It is obtained by a method for manufacturing steel plates. After hot rolling, the steel sheet may be subjected to pickling and then chemical conversion treatment to obtain a chemically treated 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 a controlled nitrogen atmosphere.

[0053] 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.

[0054] In the first heat treatment, if the holding temperature is below 1100°C, a sufficient amount of Ni cannot be deposited on the steel sheet surface, resulting in a decrease in chemical conversion treatment properties and surface roughness. Therefore, the holding temperature should be 1100°C or higher. Furthermore, if the holding temperature exceeds 1300°C, excessive scale growth occurs, and the Ni that had been deposited on the steel sheet surface migrates into the scale, making it impossible to deposit a sufficient amount of Ni on the steel sheet surface. 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 precipitated 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 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 steel sheet becomes excessive, and the amount of Ni precipitated on the steel sheet surface decreases, resulting in a decrease in chemical conversion treatment performance. 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. 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 the slab may be cooled after the first heat treatment, reheated, and then the second heat treatment may be performed. The cooling after the first heat treatment may end at a temperature of 700°C or lower, for example, to room temperature.

[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 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, thus reducing the 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 precipitated 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. Regarding the descaling temperature, it is preferable to set it to 1300°C or lower, more preferably 1250°C or lower, in order to suppress the movement of Ni from the base metal to the scale. Furthermore, from the viewpoint of the need to efficiently remove the formed scale, it is preferable to set it to 1100°C or higher, more preferably 1150°C or higher. 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 carried out under conditions of 30-120 MPa water pressure for 10-1800 seconds. A water pressure of 50 MPa or higher is preferable, as it facilitates the removal of scale without residue. Furthermore, a water pressure of 100 MPa or lower is preferable. The descaling time is preferably 30 seconds or more, as this allows for easy removal of scale without any residue. Furthermore, the descaling time is preferably 6000 seconds or less. Other descaling conditions are not particularly limited and conventional ones 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 plate thickness and properties. The cooling after hot rolling is also not particularly limited.

[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. [Examples]

[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 consecutively, 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 using a water pressure of 20-130 MPa for 5-1900 seconds to remove scale. After descaling, hot finishing rolling was performed to control the plate thickness to 3.2 mmt to prepare the test specimen.

[0065] GDS analysis was performed on the test material under the following conditions. Using a GDS analyzer (GD-Profiler2, Horiba, Ltd.), the emission intensity profile for Ni was obtained under the following conditions. Pre-sputtering time: 5 seconds High-frequency output: 35W Gas replacement time: 35 seconds Pressure: 600 Pa Measurement time: 200s (Interval: 10ms) After the measurement was completed, the depth of the sputter marks was measured using a non-contact surface shape measuring device (VK-X200 manufactured by KEYENCE). The sputtering velocity can be calculated by dividing the depth of the sputtered mark by the measurement time (200 s = 20,000 ms), and this can be converted to the depth corresponding to each time interval.

[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 value was defined as the average value of the luminescence intensity over a period of 180-200 seconds (corresponding to a depth of 12.2-13.5 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 measuring device manufactured by Keyence Corporation (KEYENCE VK-X200), surface topography 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 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.

[0069] <Chemical treatment> 1. Degreasing Chemicals: Nippon Paint Surf Cleaner EC90M / L Bath temperature: 45℃ Processing time: 120s Degreasing method: spray

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

[0071] 3.Surface adjustment Chemical: Surf Fine 5N-10 manufactured by Nippon Paint Co., Ltd. Bath temperature: room temperature Processing time: 30s Surface conditioning method: immersion

[0072] 4. Chemical treatment Chemical: Surfdyne EC1000R-1 manufactured by Nippon Paint Co., Ltd. Bath temperature: 38℃ Processing time: 90s Chemical treatment method: Immersion Total acidity (TA): 22pt Free acidity (FA): 0.8pt Accelerator (AC): 3.0pt

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

[0074] 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, with a coating thickness of 15 μm, and was 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 quantitatively evaluate chemical scale, the following method was used to evaluate the chemical scale.

[0076] After chemical conversion treatment, 10 fields of view (1 field of view: approximately 254 μm × 190 μm) were imaged using a scanning electron microscope (SEM) in backscattered electron mode, with an applied voltage of 15 kV and a magnification of 2000x. The image data for each field of view was then divided into 11 equal parts by vertical and horizontal lines, and the exposure of the base metal or the deposition of chemical crystals was checked at the intersections of these lines. Out of 1000 measurement points (intersections of vertical and horizontal lines across the entire field of view), fewer than 5 points where base metal exposure was confirmed were classified as A, 5 to 10 points as B, and 11 or more points as C. A and B were considered acceptable. The results are shown in Table 3.

[0077] <Chemical particle size> The particle size of the chemical conversion crystals precipitated on the steel plate was evaluated using the following method.

[0078] Ten fields (approximately 64 μm × 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 chemically converted 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 chemically converted 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 chemically converted 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 deposits> The amount of chemical deposition deposited is the weight per unit area of ​​chemical deposition crystals deposited on the steel plate, and was evaluated using the following method.

[0080] For the measurement of chemical conversion adhesion, quantitative analysis was performed using X-ray fluorescence analysis. Specifically, ultra-low carbon mild steel was prepared by first treating it under different chemical conversion conditions, and the P intensity was measured for each ultra-low carbon mild steel by X-ray fluorescence analysis. Furthermore, the chemical conversion crystals were removed from each ultra-low carbon mild steel using chromic acid, and the adhesion 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 the calibration curve, the samples after chemical conversion treatment were analyzed by X-ray fluorescence, and the amount of deposit deposited on the steel plate surface was calculated. This deposit 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 warm water bath adjusted to 60°C ± 2°C for 30 days. After being removed from the warm water bath and dried, the samples were used. 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] [Table 1] TIFF0007878616000002.tif233144TIFF0007878616000003.tif233126

[0083] [Table 2] TIFF0007878616000005.tif233160TIFF0007878616000006.tif233140

[0084] [Table 3] TIFF0007878616000008.tif213170TIFF0007878616000009.tif184170

[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. [Industrial applicability]

[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 percent, Cu: 0.01 to 0.50%, Ni: 0.04-1.00%, Sn: 0.001 to 0.100%, Cr: 0.01-0.20%, 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: 0-0.500%, V: 0 to 0.500%, C: 0-0.50%, Si: 0-3.000%, Mn: 0 to 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 composition in which N: 0 to 0.0100% is present, with the remainder being Fe and unavoidable impurities. 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) from the surface of the hot-rolled steel sheet in the depth direction, and is defined as [Ni]. ｍａｘ Defined as, In the aforementioned hot-rolled steel sheet, The Cu content (mass%) is denoted as [Cu]. The Sn content (mass%) is indicated as [Sn]. Cr content (mass%) is [Cr], When the Ni content (mass%) is defined as [Ni], The following equations (1) to (3) are satisfied, and A hot-rolled steel sheet having a surface roughness Sa of 1.75 μm or more. 0.00 ≦ [N+] ｍａｘ -2.5[C] ≦ 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, A heating step of heating a slab satisfying the component composition, formula (2) and formula (3) described in claim 1, A descaling step is performed on the heated slab, A hot rolling process is performed on the slab after descaling. A method for manufacturing hot-rolled steel sheets, including The heating step 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 a controlled nitrogen atmosphere. The descaling process is performed under conditions of a water pressure of 30 to 120 MPa for 10 to 1800 seconds. A method for manufacturing hot-rolled steel sheets.

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