Method for producing hot-rolled steel sheet

Abstract

Provided is a method for producing a hot-rolled steel sheet having excellent chemical conversion treatment properties and post-coating corrosion resistance even when containing multiple types of tramp elements. The production method of the present invention is a method for producing a hot-rolled steel sheet having a predetermined chemical composition, the method sequentially comprising a hot rolling step and a pickling step for a steel slab, and satisfying a ratio of surface roughness of the hot-rolled steel sheet before and after pickling of Sa' / Sa ≦ 0.90.

Description

Method for manufacturing hot-rolled steel sheet

[0001] The present invention relates to a method for manufacturing a hot-rolled steel sheet, and particularly relates to a method for manufacturing a hot-rolled steel sheet excellent in chemical convertibility and post-painting corrosion resistance.

[0002] As methods for manufacturing steel sheets, there are a method using a blast furnace material produced in a blast furnace with iron ore as the main raw material (blast furnace method), and a method using an electric furnace material produced in an electric furnace with iron scrap as the main raw material (electric furnace method). When manufacturing by a blast furnace, a large amount of coke is used, so emissions of carbon dioxide, a greenhouse gas, are large, and there is concern about the impact on global warming. On the other hand, when manufacturing by an electric furnace, although electric energy for melting iron scrap is required, there is an advantage that it is not necessary to use coke, and carbon dioxide emissions can be suppressed.

[0003] In response to the recent increase in environmental awareness and the trend of CO₂ emission regulations, the manufacturing of steel sheets by the electric furnace method has also been progressing in research. However, since iron scrap used as a raw material contains tramp elements such as Sn, Cu, Ni, Cr and Mo, it is difficult to manufacture high-quality steel sheets. These tramp elements not only adversely affect the mechanical properties of the produced steel sheet, but may also reduce chemical convertibility.

[0004] Against this background, techniques for improving the chemical convertibility of steel sheets containing Ni, which is also cited as a tramp element, have been proposed. For example, Patent Document 1 describes a technique for obtaining a hot-rolled steel sheet excellent in chemical convertibility by limiting components in steel and limiting the range where the Ni content on the steel sheet surface is 0.5 mass% or more to 10 to 70%. In addition, Patent Document 2 describes a technique for improving not only zinc phosphate chemical convertibility but also Zr chemical convertibility in an ultra-high strength hot-rolled steel sheet having a tensile strength of 980 MPa or more, by controlling carbides and oxides in the surface layer of the steel sheet and further controlling the metal structure of the steel sheet.

[0005] On the other hand, techniques have also been proposed to ensure chemical conversion treatmentability by using pickling methods to remove scale. For example, Patent Document 3 describes how iron-based oxides, which are inevitably formed when removing scale, are removed after hot rolling by pickling with an acid solution with a reduced iron ion concentration. Thus, Patent Document 3 describes a technique to control the coverage rate of iron-based oxides on the surface of steel sheets and improve chemical conversion treatmentability.

[0006] International Publication No. 2021 / 157692, International Publication No. 2020 / 080553, Japanese Patent Publication No. 2016-29207

[0007] Regarding chemical conversion treatment methods, as mentioned above, in addition to conventional zinc phosphate conversion treatment, Zr conversion treatment is becoming increasingly widespread. Zr conversion treatment has environmental advantages such as not containing phosphoric acid and producing low sludge, and its future expansion is expected. Therefore, for steel sheets, good Zr conversion treatment properties are required in addition to good zinc phosphate conversion treatment properties.

[0008] In addition to chemical treatment properties, in fields using painted steel sheets such as automobiles and construction, it is required that the paint film does not peel off even when subjected to external forces such as sliding and chipping when the steel sheet is exposed to the outdoors. Furthermore, it is required that the steel sheet does not corrode significantly from areas where scratches occur after painting. In short, there is a growing need for steel sheets that do not compromise aesthetics or corrosion resistance even when used in harsh environments.

[0009] Thus, given the common technical understanding that trump elements reduce chemical conversion treatment properties, there is a need for hot-rolled steel sheets that maintain excellent chemical conversion treatment properties even when trump elements are present, and that also possess excellent post-painting corrosion resistance. Preferably, there is a need for hot-rolled steel sheets that maintain excellent zinc phosphate and Zr chemical conversion treatment properties even when multiple types of trump elements are present, and that also possess excellent post-painting corrosion resistance.

[0010] Furthermore, improvements in these chemical treatment properties and post-painting corrosion resistance are particularly needed for hot-rolled steel sheets other than high-tensile steel, which make up the majority of each product. Examples of hot-rolled steel sheets other than high-tensile steel are not particularly limited, but include, for example, hot-rolled steel sheets with a tensile strength of less than 490 MPa.

[0011] However, Patent Document 1 does not take into consideration the Zr chemical treatment properties, and there is room for further investigation regarding corrosion resistance after painting, so it cannot satisfy the above-mentioned needs. Furthermore, the pickling method for scale removal used in Patent Document 1 is within the scope of existing methods.

[0012] Regarding Patent Document 2, steel sheets manufactured by the electric furnace method generally contain Sn, Cu, Cr, etc., but Patent Document 2 does not contain these multiple elements simultaneously. Therefore, Patent Document 2 does not adequately consider trump elements, and there is room for further investigation regarding corrosion resistance after painting, thus failing to satisfy the above-mentioned needs. Furthermore, the technology in Patent Document 2 is limited to ultra-high-strength steel sheets, and no consideration is given to steel sheets with a tensile strength of less than 980 MPa.

[0013] Patent Document 3 does not take into consideration trump elements such as Sn, Cu, and Cr, and therefore cannot satisfy the above-mentioned needs.

[0014] The present invention aims to improve upon the above-mentioned problems and to provide a method for manufacturing hot-rolled steel sheets that have excellent chemical conversion treatment properties and corrosion resistance after painting, even when they contain multiple types of trump elements, such as electric furnace steel made from iron scrap. Preferably, the aim is to provide a method for manufacturing hot-rolled steel sheets that achieves both excellent zinc phosphate and Zr chemical conversion treatment properties and excellent corrosion resistance after painting, even when containing multiple types of trump elements. Furthermore, more preferably, the aim is to enable these excellent chemical conversion treatment properties and corrosion resistance after painting to be exhibited in hot-rolled steel sheets other than high-tensile steel.

[0015] To solve the above problems, the inventors diligently studied the effects of trump elements on chemical conversion treatment properties and corrosion resistance after painting, paying particular attention to pickling conditions. As a result, the following findings were obtained: <1> Hot-rolled steel sheets containing trump elements tend to have a higher surface roughness. Therefore, by controlling the pickling conditions after hot rolling to control the surface roughness of the steel sheet before and after pickling, good chemical conversion treatment properties and corrosion resistance after painting can be obtained. <2> If the pickling is excessive, the trump elements inside the steel sheet become concentrated on the surface, resulting in inferior chemical conversion treatment properties. Also, if the pickling is excessive, the surface roughness of the steel sheet after pickling cannot be suppressed, and corrosion resistance after painting is also inferior. For this reason, by devising the conditions of the pickling process, good zinc phosphate chemical conversion treatment properties, Zr chemical conversion treatment properties, and corrosion resistance after painting can be obtained. <3> In controlling the surface morphology of the steel plate, increasing the proportion of recesses on the steel plate surface (making the depth of the recesses in the height direction relatively longer) can distribute the stress between the steel plate and the coating film formed on it. As a result, paint peeling can be suppressed, and even better post-coating corrosion resistance can be obtained.

[0016] The present invention is based on the above findings, and its gist is as follows: [1] A method for manufacturing a hot-rolled steel sheet, comprising, in order, a hot-rolling step and a pickling step for a steel slab, wherein the hot-rolled steel sheet has the following composition by mass%, Cu: 0.02 to 0.40%, Ni: 0.01 to 0.40%, Sn: 0.001 to 0.050%, Cr: 0.01 to 0.20%, Mo: 0.001 to 0.500%, C: 0.50% or less, Si: 1.000% or less, Mn: 2.00% or less, Zn: 0.500% or less, Pb: 0.500% The following composition contains As: 0.500% or less, Sb: 0.500% or less, Bi: 0.500% or less, V: 0.500% or less, B: 0.0100% or less, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, and N: 0.0100% or less, with the remainder being Fe and unavoidable impurities, and in the pickling process, the surface roughness S of the hot-rolled steel sheet before pickling a The surface roughness S of the hot-rolled steel sheet after pickling is relative to the surface roughness S of the hot-rolled steel sheet after pickling. a 'But, Sa ′ / S a ≦ 0.90 ・・・(1) A method for producing a hot-rolled steel sheet, characterized by satisfying the following.

[0017] In the present specification, the term "surface roughness S" used for a hot-rolled steel sheet before pickling a " and "surface roughness S" for a hot-rolled steel sheet after pickling a ′" is the arithmetic mean height of a surface defined in ISO 25178, and represents the average of the absolute values of the height differences of each point relative to the mean plane of the surface. The value of "S a ′ / S a " has no unit. "Surface roughness S a " and "surface roughness S a ′" can be measured by observing the surface of the steel sheet with a laser microscope to obtain surface irregularity data conforming to ISO 25178, and analyzing the surface irregularity data. Specific measurement conditions may follow the conditions described later for formula (1).

[0018] [2] In the pickling step, the peak height of the surface R relative to the valley depth of the surface R in the hot-rolled steel sheet after pickling v and the roughness skewness R p , respectively, satisfy: R sk / R p ≦ 1.00 ・・・(2) R v ≧ -0.20 ・・・(3) The method for producing a hot-rolled steel sheet according to [1] above, further satisfying: sk

[0019] In the present specification, "valley depth R v " is the maximum valley depth defined in JIS B 0601, and represents the maximum valley depth of the roughness curve at the reference length. Also, "peak height R p " is the maximum peak height defined in JIS B 0601, and represents the maximum peak height of the roughness curve at the reference length. The value of "R p / R v " has no unit. Furthermore, "roughness skewness R sk " is the degree of asymmetry (skewness) of the height distribution defined in JIS B 0601, and represents the skewness of the roughness curve at the dimensionless reference length. R sk ​When R is zero, the peaks (convex parts) and valleys (concave parts) follow a normal distribution, sk The larger the value, the greater the proportion of the surface occupied by recesses. sk The value of "" has no units. "Valley depth R v "Mountain height R p " and "roughness skewness R sk This involves observing the surface of the steel plate with a laser microscope to obtain surface roughness data in accordance with ISO 25178, and then determining the line roughness R in accordance with JIS B 0601 from the surface roughness data. v , R p , R sk Each of these can be measured by analyzing them. The specific measurement conditions for equations (2) and (3) can follow the conditions described later.

[0020] [3] The method for manufacturing a hot-rolled steel sheet according to [1] or [2] above, wherein the pickling step comprises, in order, a first pickling step, a drying step, and a second pickling step, wherein in the first pickling step, pickling is performed using a first pickling bath having a Ni ion concentration of 0.001% by mass or more, a hydrochloric acid concentration of 1.0 to 5.0% by mass, and a temperature of 70 to 90°C, under the conditions of a first pickling time of 40 to 60 seconds, in the drying step, drying is performed using cold air at a temperature of 0°C or lower, under the conditions of a drying time of 10 seconds or more, and in the second pickling step, pickling is performed using a second pickling bath having a sum of Cu ion concentration and Sn ion concentration of 1.0% by mass or less, a phosphoric acid concentration of 5.0% by mass or less, and a temperature of 10 to 30°C, under the conditions of a second pickling time of 30 to 60 seconds.

[0021] [4] The method for manufacturing a hot-rolled steel sheet as described in [3] above, wherein the pickling step is performed two or more times in succession.

[0022] According to the present invention, it is possible to provide a method for manufacturing hot-rolled steel sheets that have good chemical conversion treatment properties and corrosion resistance after painting, even when containing trump elements. More specifically, it is possible to provide a method for manufacturing hot-rolled steel sheets that achieves both excellent zinc phosphate and Zr chemical conversion treatment properties and excellent corrosion resistance after painting, even when containing predetermined amounts of multiple types of trump elements. Preferably, these excellent chemical conversion treatment properties and corrosion resistance after painting can be exhibited for hot-rolled steel sheets other than high-tensile steel.

[0023] The present invention will be described in detail below. The following description describes examples of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below. In the following description, the units for the content and concentration of each element in the component composition are "mass%", and unless otherwise specified, they are simply indicated as "%". Furthermore, regarding numerical ranges, the notation "~" includes the numerical values ​​before and after "~" as the lower and upper limits, respectively, unless otherwise specified. Additionally, if a unit is attached to only one of the numerical values ​​before or after "~", the same unit shall be attached to the other unless otherwise specified.

[0024] (Method for manufacturing hot-rolled steel sheets) The manufacturing method of the present invention is a method for manufacturing hot-rolled steel sheets containing predetermined amounts of Cu, Ni, Sn, Cr, and Mo, and comprises, in order, at least a hot-rolling step for a steel slab and a pickling step for the steel slab after hot-rolling (hot-rolled steel sheet before pickling). The manufacturing method of the present invention may optionally include other steps other than the hot-rolling step and the pickling step. In the manufacturing method of the present invention, the surface roughness S of the hot-rolled steel sheet before pickling a Surface roughness S of hot-rolled steel sheet after pickling a It is essential to perform an acid pickling process in order to suppress ' to a predetermined relationship. S before and after the acid pickling process a ' / S a Unless the chemical properties are suppressed to a predetermined level or lower, the resulting hot-rolled steel sheet will not be able to achieve both excellent chemical treatment properties and corrosion resistance after painting.

[0025] The hot-rolled steel sheets obtained by the manufacturing method of the present invention exhibit excellent chemical conversion treatment properties and corrosion resistance after painting. More specifically, they are excellent in zinc phosphate conversion treatment properties, Zr conversion treatment properties, and corrosion resistance after painting, making them applicable to a wide range of fields. In particular, they are useful as automotive steel sheets that require high levels of chemical conversion treatment properties and corrosion resistance. Furthermore, the hot-rolled steel sheets obtained by the manufacturing method of the present invention exhibit excellent chemical conversion treatment properties even when containing multiple types of trump elements, making them particularly useful for electric furnace materials made from iron scrap, thus offering exceptional industrial benefits in an environmentally conscious manner.

[0026] [Component Composition] First, the component composition of the hot-rolled steel sheet obtained by the manufacturing method of the present invention will be described. The hot-rolled steel sheet contains Cu: 0.02 to 0.40%, Ni: 0.01 to 0.40%, Sn: 0.001 to 0.050%, Cr: 0.01 to 0.20%, and Mo: 0.001 to 0.500%. Furthermore, the hot-rolled steel sheet may optionally contain, in amounts greater than 0%, one or more elements selected from the group consisting of C: 0.50% or less, Si: 1.000% or less, Mn: 2.00% or less, Zn: 0.500% or less, Pb: 0.500% or less, As: 0.500% or less, Sb: 0.500% or less, Bi: 0.500% or less, V: 0.500% or less, B: 0.0100% or less, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, and N: 0.0100% or less. The remainder of the component composition may consist of Fe and unavoidable impurities.

[0027] Cu: 0.02-0.40% Cu is an element that may 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 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 during chemical conversion treatment and causes chemical sizing on the Cu-concentrated area. For these reasons, the Cu content should be 0.40% 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. Furthermore, for example, in pickling, there is concern that the chemical conversion treatment properties and corrosion resistance after painting may deteriorate due to the concentration of Cu on the surface of the steel sheet. For this reason, it is preferable that the Cu content be 0.30% or less.

[0028] On the other hand, in the electric furnace method, for example, reducing the amount of Cu in iron scrap is cost-ineffective. In addition to these economic reasons, Cu, when included in the bulk (base metal), reduces the reactivity of the steel sheet itself and has the effect of improving the corrosion resistance of the steel sheet itself. For these reasons, the Cu content is set to 0.02% or more, preferably 0.03% or more, more preferably 0.05% or more, and even more preferably 0.07% or more.

[0029] Ni: 0.01-0.40% 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 Ni is concentrated as fine granules on the surface of the steel sheet, forming microscopic cathodes. This promotes the elution reaction of Fe around the Ni-enriched areas, which then act as nuclei for the formation of chemical conversion crystals, improving the chemical conversion processability. In addition, there are economic reasons, such as the cost disadvantage of reducing Ni in iron scrap in the electric arc furnace method. For these reasons, the Ni content is set to 0.01% or more, preferably 0.02% or more, more preferably 0.03% or more, and even more preferably 0.10% or more.

[0030] On the other hand, if the Ni content is high, the Ni-enriched areas on the steel sheet surface change from fine granules to large lumps, degrading the chemical treatment properties on the Ni-enriched areas. Furthermore, Ni is an element that is completely dissolved in Cu, and during the hot rolling process, it is concentrated together with Cu at the base metal-scale interface when scale is formed, forming surface irregularities. As a result, the surface roughness of the steel sheet increases, which tends to result in inferior corrosion resistance after painting. Moreover, it becomes necessary to add Ni to the iron scrap in order to adjust the Ni content. For this reason, the Ni content should be 0.40% or less, preferably 0.35% or less, and more preferably 0.30% or less.

[0031] Sn: 0.001-0.050% When Sn in the steel sheet dissolves during chemical conversion treatment, it reprecipitations as hydroxides, coating the surface of the steel sheet and significantly reducing the chemical conversion treatment performance. For this reason, the Sn content should be 0.050% or less, preferably 0.040% or less. On the other hand, in the electric furnace method, for example, reducing the Sn in the iron scrap is disadvantageous in terms of cost. For these economic reasons, the Sn content should be 0.001% or more, preferably 0.002% or more, and more preferably 0.005% or more.

[0032] 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. For this reason, the Cr content should be 0.20% or less, preferably 0.18% or less. On the other hand, for example, in the electric arc furnace method, reducing the Cr in iron scrap is disadvantageous in terms of cost, for economic reasons. For this reason, the Cr content should be 0.01% or more, preferably 0.05% or more.

[0033] Mo: 0.001 to 0.500%. Mo is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric arc furnace method. For example, in the electric arc furnace method, reducing the Mo content in iron scrap is cost-ineffective. In addition to these economic reasons, Mo is an element that increases the strength of steel sheets. From these viewpoints, the Mo content should be 0.001% or more, preferably 0.003% or more, and more preferably 0.005% or more. On the other hand, if the Mo content increases in order to increase the strength of the steel sheet, it becomes necessary to add Mo, which increases costs and is economically disadvantageous. For this reason, the Mo content should be 0.500% or less, preferably 0.400% or less.

[0034] C: 0.50% or less. C is an element that can be included from the viewpoint of improving the hardenability of steel sheets, improving strength by securing martensite, and controlling the volume fraction of residual γ to a desired range. However, if there is too much C, the area fraction of cementite increases and the workability decreases. For this reason, the C content should be 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less. The C content may be 0%, but from the viewpoint of ensuring the above effects, it is preferable to have 0.01% or more, and more preferably 0.02% or more.

[0035] Si: 1.000% or less. Si is an element that can be included to improve ferrite strength, suppress oxidation products in martensite and / or bainite, and stabilize residual γ to improve ductility. However, if there is too much Si, Fe2SiO4 formed on the surface of the steel sheet during hot rolling will remain, worsening the chemical conversion treatment properties. For this reason, the Si content should be 1.000% or less, preferably 0.900% or less, and more preferably 0.650% or less. The Si content may be 0%, but from the viewpoint of ensuring the above effects, it is preferable to have 0.013% or more.

[0036] Mn: 2.00% or less. Mn is an element that ensures the desired hardenability, suppresses ferrite transformation, and secures the desired area ratio of tempered martensite and / or bainite to guarantee strength. However, if there is too much Mn, the bainite transformation is significantly delayed, making it difficult to ensure high ductility. For this reason, the Mn content should be 2.00% or less, preferably 1.60% or less. The Mn content may be 0%, but from the viewpoint of ensuring the above effects, it is preferable to have 0.02% or more, and more preferably 0.16% or more.

[0037] Zn: 0.500% or less. Zn is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. To avoid impairing the properties of the present invention, the Zn content is 0.500% or less, preferably 0.450% or less, and more preferably 0.200% or less. The Zn content may be 0%, or for example, 0.001% or more.

[0038] Pb: 0.500% or less. Pb is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. Pb is also an element that has the effect of reducing segregation. In order to avoid impairing the properties of the present invention, the Pb content is 0.500% or less, preferably 0.450% or less, and more preferably 0.200% or less. The Pb content may be 0%, and can be, for example, 0.001% or more.

[0039] As: 0.500% or less. As is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. In order to avoid impairing the properties according to the present invention, the As content is 0.500% or less, preferably 0.450% or less, and more preferably 0.200% or less. The As content may be 0%, and can be, for example, 0.001% or more.

[0040] Sb: 0.500% or less. Sb is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. In order to avoid impairing the properties according to the present invention, the Sb content is 0.500% or less, preferably 0.450% or less, and more preferably 0.200% or less. The Sb content may be 0%, and can be, for example, 0.001% or more.

[0041] Bi: 0.500% or less. Bi is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. Bi is also an element that has the effect of reducing segregation. In order to avoid impairing the properties of the present invention, the Bi content is 0.500% or less, preferably 0.450% or less, and more preferably 0.200% or less. The Bi content may be 0%, and can be, for example, 0.001% or more.

[0042] V: 0.500% or less. V is an element that may be mixed in as a trump element, especially in steel sheets manufactured by the electric furnace method. V is also an element that increases the strength of steel sheets through precipitation strengthening. In order to avoid impairing the properties of the present invention, the V content is 0.500% or less, preferably 0.450% or less, and more preferably 0.300% or less. The V content may be 0%, and can be, for example, 0.003% or more, or 0.004% or more.

[0043] B: 0.0100% or less. B is an element that facilitates the formation of tempered martensite and / or bainite with a desired area ratio. It is also an element that can improve delayed fracture resistance due to the remaining solid solution of B. However, if there is too much B, it will lead to a significant decrease in hot rolling properties and cause surface defects. For this reason, the B content should be 0.0100% or less, preferably 0.0096% or less, and more preferably 0.0090% or less. The B content may be 0%, but in order to ensure the above effects, it is preferable to have 0.0001% or more, and more preferably 0.0002% or more.

[0044] P: 0.100% or less. P is an element that strengthens steel. However, if the P content is too high, it deteriorates the spot weldability. For this reason, the P content should be 0.100% or less, preferably 0.090% or less, and more preferably 0.075% or less. The P content may be 0%, for example, 0.001% or more, or 0.005% or more.

[0045] S: 0.020% or less. S is an element that has the effect of improving scale detachability during hot rolling and suppressing nitriding during annealing. However, if the S content is too high, it will cause deterioration of spot weldability and local elongation. For this reason, the S content should be 0.020% or less, preferably 0.019% or less, and more preferably 0.018% or less. The S content may be 0%, and from the viewpoint of suppressing scale detachment during hot rolling, it can be, for example, 0.001% or more, or 0.003% or more.

[0046] Al: 0.100% or less. Al is an element that can deoxidize and stabilize residual γ as a substitute for Si. However, if the Al content is too high, the strength of the material will decrease drastically. For this reason, the Al content should be 0.100% or less, preferably 0.080% or less. The Al content may be 0%, and from the viewpoint of stabilizing residual γ, it can be, for example, 0.002% or more, or 0.020% or more.

[0047] N: 0.0100% or less. N is an element that forms nitrides such as BN, AlN, and TiN in steel, and can reduce the hot rollability of steel and lower the surface quality. In addition, in steel containing B, there is the disadvantage of losing the effect of B through the formation of BN. For these reasons, the N content should be 0.0100% or less, preferably 0.0095% or less, and more preferably 0.0090% or less. The N content may be 0%, and from the viewpoint of cost increase due to N removal and ensuring strength through nitride formation, it can be, for example, 0.0002% or more, or 0.0005% or more.

[0048] Hot-rolled steel sheets have a composition consisting of the above elements, the remainder being Fe, and unavoidable impurities. Hot-rolled steel sheets having the above composition are assumed to contain multiple types, at least five types of trump elements, and can be steel sheets manufactured using the electric furnace method. The type of hot-rolled steel sheet is not particularly limited, but examples include hot-rolled steel sheets such as ultra-low carbon steel.

[0049] From the viewpoint of requiring high workability, hot-rolled steel sheets preferably have a tensile strength of less than 490 MPa, more preferably less than 400 MPa, and even more preferably less than 300 MPa.

[0050] The method for manufacturing hot-rolled steel sheets of the present invention is not particularly limited to sequentially performing a hot-rolling step and a pickling step under predetermined conditions on a steel slab, and any other steps may be further performed. Examples of other steps include a steel slab manufacturing step, a descaling step before pickling after hot-rolling, and a chemical conversion treatment step. Among each step, in the pickling step for removing scale formed in the hot-rolling step, it is important to perform the pickling so that the surface roughness of the steel sheet before and after pickling satisfies a predetermined relationship. In particular, when surface preparation to remove scale formed on the surface of a hot-rolled steel sheet containing multiple types of trump elements by pickling, it is important to suppress the concentration of trump elements such as Sn, Cr, and Cu into the surface layer region of the steel sheet. By performing the desired surface preparation with ingenious pickling conditions, the steel sheet can exhibit both excellent chemical conversion treatment properties and corrosion resistance after painting, even when containing a predetermined amount of trump elements.

[0051] [Steel Slab Manufacturing Process] An example of the steel slab manufacturing process is described below. For the manufacturing of steel slabs, molten steel derived from electric furnace materials using iron scrap as a raw material may be used, or molten steel derived from electric furnace materials may be mixed with molten steel derived from blast furnace materials. In addition, the components of the molten steel derived from blast furnace materials can be adjusted by conventional methods so that the resulting hot-rolled steel sheet has a predetermined component composition. In order for the hot-rolled steel sheet to have the predetermined component composition described above, it is preferable to obtain a steel slab having the above component composition using molten steel having the above component composition and following conventional methods. In other words, the steel slab has the following composition in mass percent: Cu: 0.02-0.40%, Ni: 0.01-0.40%, Sn: 0.001-0.050%, Cr: 0.01-0.20%, Mo: 0.001-0.500%, C: 0.50% or less, Si: 1.000% or less, Mn: 2.00% or less, Zn: 0.500% or less, Pb: 0.500% or less. Preferably, the composition contains As: 0.500% or less, Sb: 0.500% or less, Bi: 0.500% or less, V: 0.500% or less, B: 0.0100% or less, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, and N: 0.0100% or less, with the remainder being Fe and unavoidable impurities.

[0052] [Hot Rolling Process] An example of the hot rolling process is described below. In the hot rolling process, a steel slab, preferably having the above-described component composition, is heated and then rolled to obtain a hot-rolled steel sheet before pickling. The heating conditions for the steel slab and the hot rolling conditions can follow conventional methods.

[0053] [Descaling Process] An example of the descaling process is described below. The descaling process may be performed either during or after the hot rolling process. In order to remove the strong primary scale characteristic of Si-containing steel, it is preferable to perform a descaling treatment on the steel sheet surface by shot blasting, high-pressure water spraying, etc., before the finish rolling in the hot rolling process. By vigorously descaling the primary scale, it is possible to suppress surface irregularities of the hot-rolled steel sheet before pickling and to control the surface roughness more effectively. It is preferable to perform the finish rolling after the above descaling. The conditions for the finish rolling can be set as appropriate. However, the conditions for descaling are not limited as long as excessive surface roughness is not formed. An example of descaling is to spray high-pressure water at a spray pressure of 20 MPa from a distance of 100 mm at a spray angle of 30 degrees.

[0054] [Pickling Process] This section describes the pickling process for hot-rolled steel sheets obtained after hot-rolling into a steel slab, or after hot-rolling and descaling into a steel slab, before pickling. In this pickling process, the surface roughness S of the hot-rolled steel sheet before pickling is determined. a Surface roughness S of hot-rolled steel sheet after pickling a 'But, S a ' / S a It is essential to perform pickling so that the following condition is satisfied: ≤ 0.90 ... (1). If the above formula (1) is not satisfied during the pickling process, the resulting hot-rolled steel sheet after pickling will not be able to exhibit excellent chemical treatment properties and corrosion resistance after painting. In addition, during the pickling process, the surface valley depth R of the hot-rolled steel sheet after pickling is important. v Surface peak height R p , and roughness skewness R sk However, each of them is R p / R v ≦ 1.00...(2) R sk It is preferable to further satisfy ≥ -0.20 ... (3). By satisfying the above formulas (2) and (3) in the pickling process, the corrosion resistance and paint adhesion of the resulting hot-rolled steel sheet after pickling can be further improved.

[0055] To manufacture hot-rolled steel sheets while ensuring good chemical treatment properties and corrosion resistance after painting, the surface condition of the hot-rolled steel sheets during the manufacturing process is measured, and surface roughness parameters are formulated. The manufacturing method must satisfy at least formula (1) above, and preferably further satisfy formulas (2) to (3) above. These parameters follow formulas discovered by the inventors through experiments and can be measured and calculated using a laser microscope. The reasons for setting formulas (1) to (3) will now be explained.

[0056] The inventors of the present invention diligently investigated the causes of deterioration in chemical conversion treatment properties and post-painting corrosion resistance when manufacturing hot-rolled steel sheets having a component composition containing trump elements, and obtained the following findings: When removing scale from hot-rolled steel sheets by pickling before pickling, if the surface of the steel sheet is excessively dissolved, the trump elements, which are insoluble in acid, become concentrated on the surface of the steel sheet. As a result, when chemical conversion treatment is performed on the hot-rolled steel sheet, the reaction in the chemical conversion solution is hindered, resulting in inferior chemical conversion treatment properties. Hot-rolled steel sheets containing trump elements tend to have a higher surface roughness. The increased unevenness of the steel sheet surface prevents the chemical conversion crystals from covering the entire surface of the steel sheet, resulting in inferior chemical conversion treatment properties. In addition, the large unevenness of the steel sheet surface prevents the electrodeposition coating from forming uniformly, resulting in inferior post-painting corrosion resistance. <C> When the proportion of recesses in the uneven surface of a hot-rolled steel sheet decreases (the depth of the recesses in the height direction becomes relatively shallower, or the length of the recesses in the height direction becomes relatively shorter), stress between the steel sheet and the coating becomes concentrated in one area. As a result, this can lead to paint peeling, and the corrosion resistance after painting tends to be inferior.

[0057] Formula (1) In the events described in A and B above, the inventors found that when scale formed on the surface of a hot-rolled steel sheet is removed by pickling, many trump elements become concentrated on the steel sheet surface, degrading the chemical conversion treatment properties. The inventors also found that when scale is removed from a hot-rolled steel sheet by pickling, the post-paint corrosion resistance of the hot-rolled steel sheet after chemical conversion treatment may deteriorate. It was thought that these were due to the excessive dissolution of not only scale but also the steel sheet containing trump elements during the pickling of the hot-rolled steel sheet. Therefore, the inventors explored techniques to prevent excessive dissolution of the steel sheet during scale removal by pickling. As a result, it was found that by controlling the pickling process so that the surface roughness of the hot-rolled steel sheet after pickling is below a predetermined level compared to the surface roughness of the hot-rolled steel sheet before pickling, excessive dissolution of the steel sheet due to pickling can be prevented, and the chemical conversion treatment properties and post-paint corrosion resistance can be improved. From this finding, it is required that the surface of the hot-rolled steel sheet is not significantly roughened by the acid after pickling. The inventors of this invention obtained equation (1) by formulating this action. a ' / S a ≤ 0.90 ... (1)

[0058] S in equation (1) a and S a As mentioned above, both ' and ' refer to the surface roughness of the hot-rolled steel sheet, and S is the surface roughness of the hot-rolled steel sheet before pickling. a The surface roughness of the hot-rolled steel sheet after pickling is S a These roughness parameters can be measured using a laser microscope. Specifically, five fields of view are observed with a laser microscope set to 10x magnification, and the surface roughness S in each field of view is measured. a and S aEach can be calculated from the average value of '. A more specific measurement method using a laser microscope is described below. A non-contact surface shape measuring device (KEYENCE VK-X200) was used as the measuring device. Using a 10x lens, surface roughness data was acquired in accordance with ISO 25178 for each field of view of 1.8 mm × 2.5 mm. Subsequently, the entire measurement surface was analyzed under the conditions of no S filter (low-pass filter) and a 0.8 mm L filter (high-pass filter), and the surface roughness S in each field of view was determined. ａ and S a ' was obtained. And S was the average value for the 5 fields of view. a ' / S a The result was calculated.

[0059] According to the inventors' studies, S in formula (1) a ' / S a By controlling the value of to 0.90 or less, the surface roughness of the hot-rolled steel sheet after pickling is sufficiently smooth compared to before pickling, indicating that scale removal is performed well under appropriate pickling conditions. Furthermore, the chemical conversion treatment properties and post-painting corrosion resistance of the hot-rolled steel sheet after pickling can be improved. A smoother surface of the hot-rolled steel sheet after pickling indicates that more appropriate scale removal has been performed by pickling, suppressing the concentration of tramp elements on the surface and improving chemical conversion treatment properties. In addition, by preventing excessive irregularities from occurring on the surface of the hot-rolled steel sheet after pickling, the chemical conversion crystals cover the entire steel sheet, and the subsequent coating film is formed uniformly, improving post-painting corrosion resistance. Therefore, S a ' / S a It must be 0.90 or less, preferably 0.88 or less, and more preferably 0.85 or less. Also, from the viewpoint of surface appearance, S a ' / S a It is preferable that it be 0.50 or higher, and it can be 0.57 or higher.

[0060] Here, the inventors further investigated pickling conditions that are effective in controlling the surface roughness as described above. They found that the concentration of tramp elements directly below the scale in the depth direction due to pickling, coupled with the difficulty in controlling the pickling intensity in a single pickling bath, which can lead to excessive dissolution of the steel sheet directly below the scale, were considered to be factors contributing to the deterioration of surface roughness. They found that dividing the pickling process for scale removal into two stages, lowering the acid concentration during pickling, and introducing a drying process at sub-zero temperatures were effective in improving the surface roughness of the hot-rolled steel sheet as described above. This is thought to be because reducing the removal of scale by pickling to the absolute minimum suppresses the dissolution of the base material, the steel sheet, and drying in a sub-zero environment suppresses the formation of new scale and prevents an increase in surface irregularities of the steel sheet caused by the residue after pickling.

[0061] If the pickling strength is too high, excessive dissolution of the steel sheet will occur, resulting in a higher surface roughness of the hot-rolled steel sheet after pickling. As a result, the trump elements dissolved from the steel sheet become concentrated on the surface of the hot-rolled steel sheet, making it impossible to achieve excellent chemical conversion treatment properties and corrosion resistance after painting. Also, if the residue after pickling is not removed from the surface of the hot-rolled steel sheet, some parts of the steel sheet surface will be excessively pickled, increasing the surface roughness of the hot-rolled steel sheet. As a result, the trump elements become concentrated on some parts of the steel sheet surface, causing localized areas of chemical conversion defects. On the other hand, if the pickling is too weak, scale will remain on the surface of the hot-rolled steel sheet, resulting in inferior chemical conversion treatment properties. In this case, because scale remains, the surface roughness of the hot-rolled steel sheet does not change significantly before and after pickling, S a ' / S a It is not possible to reduce the value below a predetermined level. In this way, by appropriately adjusting the pickling conditions, the surface roughness of the hot-rolled steel sheet before and after pickling can be controlled more effectively, and better chemical treatment properties and corrosion resistance after painting can be ensured.

[0062] Formula (2) Furthermore, as a result of diligent research, the inventors have found that in order to further improve chemical treatmentability and corrosion resistance after painting, it is important to consider not only the phenomenon in B above, but also the phenomenon in C. Because hot-rolled steel sheets contain trump element components, the surface irregularities of the steel sheet tend to increase with pickling. In particular, the larger the gap between the peak height and the valley depth in the surface irregularities of the steel sheet, specifically the greater the degree of peak height relative to valley depth, the worse the chemical treatmentability becomes. Also, the larger the degree of peak height relative to valley depth, the less uniform the electrodeposition coating after chemical treatment will be formed, and as a result, the corrosion resistance after painting will also be worse.

[0063] For example, when removing scale formed on the surface of a hot-rolled steel sheet by pickling, using an acid solution with strong pickling ability can lead to excessive pickling of the steel sheet directly beneath the scale after the scale has been removed, further increasing the surface irregularities of the steel sheet. In particular, tramp elements tend to concentrate in various places directly beneath the scale. In areas where tramp elements are concentrated, the steel sheet does not dissolve because it is nobler than the surrounding area, but in areas where tramp elements are thin, the steel sheet is more likely to dissolve, causing irregularities to form on the surface of the hot-rolled steel sheet after pickling. According to the inventors' research, tramp elements tend to concentrate more in the convex parts of the steel sheet surface, and the chemical conversion treatment performance is particularly poor in these convex parts, resulting in uneven electrodeposition coating.

[0064] The inventors considered that while the inclusion of trump element components inevitably increases surface irregularities in the steel sheet, the gap between peak height and valley depth on the steel sheet surface could be improved by further optimizing the pickling conditions. Therefore, to ensure even better chemical treatment properties and uniformity of electrodeposition coating by controlling the surface irregularities of the steel sheet, particularly the gap between peak height and valley depth (the degree of peak height relative to valley depth), they obtained equation (2), which formalizes this effect. p / R v ≤ 1.00 ... (2)

[0065] R in equation (2) p and R vThe value of can be measured using a laser microscope. Specifically, five fields of view are observed with the laser microscope set to a magnification of 10x, and three lines are drawn randomly in each field of view. Then, the line roughness R obtained from each line is measured. p and R v Each can be calculated as an average value. More specifically, a non-contact surface shape measuring device (KEYENCE VK-X200) was used as the laser microscope measurement device. Then, using a 10x lens, surface roughness data was acquired in accordance with ISO 25178 for each field of view of 1.8 mm x 2.5 mm. Subsequently, the acquired surface roughness data was analyzed across the entire measurement surface under the conditions of no S filter (low-pass filter) and a 0.8 mm L filter (high-pass filter), thereby determining the peak height R, which is the line roughness parameter for each field of view in accordance with JIS B 0601. p and valley depth R v We obtained the following results for each of the 15 R values ​​obtained from the 5 fields of view. p and R v From each mean, R p / R v The result was calculated.

[0066] R in equation (2) p / R v By further controlling the value to 1.00 or less, it is shown that in the surface shape of the hot-rolled steel sheet after pickling, the degree of height (depth) of the convex and concave portions is equivalent in terms of maximum value comparison, or the concave portions are larger than the convex portions. At least the convex portions are not larger than the concave portions, and furthermore, the greater the degree of concave portions, the more the anchoring effect between the steel sheet and the coating film works. This improves paint adhesion and further improves corrosion resistance after painting. Therefore, R p / R v It is preferably 1.00 or less, more preferably 0.96 or less, and even more preferably 0.90 or less. Also, R p / R v If the radius becomes too small, small, deep holes will form on the smooth surface of the steel plate, which may impair the appearance of the steel plate. Therefore, from the viewpoint of surface appearance, R p / R vIt is preferable that it be 0.50 or higher, and it can be 0.59 or higher.

[0067] Equation (3) In relation to the phenomenon described in C above, the inventors conducted various further investigations focusing on the shape of the surface roughness of the hot-rolled steel sheet to further improve the adhesion of the coating film. They found that the ratio of the area occupied by recesses to the area occupied by convex parts on the surface of the steel sheet is important. The presence of irregularities on the surface of the hot-rolled steel sheet ensures an anchoring effect between the steel sheet and the coating film. In this case, as the proportion of recesses on the underlying steel sheet increases, the anchoring effect becomes stronger, and the adhesion between the steel sheet and the coating film is further improved. Specifically, the larger the proportion of recesses on the steel sheet side compared to the convex parts, the larger the volume of paint that penetrates into the recesses. Then, when shear stress is applied to the hot-rolled steel sheet after painting, the stress per unit area borne by the painted part becomes smaller, and coating peeling is suppressed. By suppressing coating peeling, even better post-painting corrosion resistance can be ensured even when external forces are applied. The inventors obtained Equation (3) which formalizes this effect. R sk ≥ -0.20 ... (3)

[0068] R in equation (3) sk The value can be measured using a laser microscope. Specifically, five fields of view are observed with the laser microscope set to a magnification of 10x, and three lines are drawn randomly in each field of view. The value can then be obtained as the average value of the line roughness skewness obtained from each line. More specifically, a non-contact surface shape measuring device (KEYENCE, VK-X200) was used as the laser microscope measuring device. Using a 10x lens, surface roughness data was acquired in accordance with ISO 25178 for each 1.8 mm × 2.5 mm field of view. Subsequently, the acquired surface roughness data was analyzed across the entire measurement surface under the conditions of no S filter (low-pass filter) and a 0.8 mm L filter (high-pass filter), thereby obtaining the line roughness skewness for each field of view in accordance with JIS B 0601. The average value of the total of 15 lines obtained from the five fields of view was then used to obtain R sk The result was calculated.

[0069] R in equation (3)sk By further controlling the value to be -0.20 or more, it is shown that in the surface profile of the hot-rolled steel sheet after pickling, the proportion of recessed areas relative to protruding areas becomes sufficiently large from the viewpoint of post-coating corrosion resistance. The larger the proportion of these recessed areas, the more the post-coating corrosion resistance improves for the reasons described above. Therefore, R sk is preferably -0.20 or more, more preferably -0.19 or more, still more preferably -0.10 or more, and even more preferably R sk has a positive value (greater than 0), and even more preferably 0.05 or more. From the viewpoint of sliding properties, R sk is preferably 0.50 or less, and may be 0.18 or less.

[0070] In the pickling step, it is preferable to devise individual process conditions and perform a first pickling step, a drying step, and a second pickling step in this order. By removing scale by dividing pickling into multiple passes with drying in between, fine adjustment of pickling conditions becomes possible, and the aforementioned surface roughness state can be more favorably achieved in the hot-rolled steel sheet after pickling. Hereinafter, the first pickling step, the drying step, and the second pickling step are described in detail.

[0071] [First Pickling Step] In the first pickling step, scale on the surface can be removed using a first pickling bath for a hot-rolled steel sheet after hot rolling and before pickling, more preferably for a hot-rolled steel sheet after a hot rolling step including the aforementioned descaling of primary scale and before pickling. The acid solution for the first pickling bath is preferably a hydrochloric acid solution.

[0072] Hydrochloric Acid Concentration in the First Pickling Bath If the hydrochloric acid concentration in the first pickling bath exceeds 5.0% by mass, the scale removal becomes excessively strong, potentially causing dissolution by pickling even to the steel sheet (base metal) directly beneath the scale. As a result, the surface of the hot-rolled steel sheet after pickling tends to become rough. Furthermore, it becomes difficult to increase the degree to which depressions occupy the surface shape of the hot-rolled steel sheet after pickling. On the other hand, if the hydrochloric acid concentration in the first pickling bath is less than 1.0% by mass, the scale cannot be sufficiently removed, and it may be difficult to control the surface roughness of the hot-rolled steel sheet after pickling to a predetermined level. Furthermore, scale removal takes a long time, reducing manufacturability. For these reasons, the hydrochloric acid concentration in the first pickling bath is preferably 1.0% or higher, preferably 5.0% or lower, more preferably in the range of 1.0% to 5.0%, and even more preferably in the range of 1.5% to 5.0%.

[0073] Temperature of the first pickling bath If the temperature of the first pickling bath is below 70°C, the scale removal capacity may be insufficient, and it may not be possible to control the surface roughness of the hot-rolled steel sheet after pickling to below a predetermined level. Also, scale removal will take a long time. On the other hand, if the temperature of the first pickling bath is above 90°C, scale removal will be excessive, and there is a possibility that the base metal will dissolve. As a result, the surface of the hot-rolled steel sheet after pickling tends to be rough. Also, it is difficult to increase the degree of concavity in the surface shape of the hot-rolled steel sheet after pickling. Furthermore, the volatilization of the acid solution will increase costs and make management difficult. For this reason, the temperature of the first pickling bath is preferably 70°C or higher, preferably 90°C or lower, and more preferably in the range of 70°C to 90°C.

[0074] Ni ion concentration in the first pickling bath: In a typical pickling line, Ni ions are not present in the pickling solution when it is prepared as a pickling bath. However, Ni, one of the trump element components, is expected to have the effect of improving chemical conversion treatment properties. By applying this effect and intentionally including Ni ions in the pickling solution, it is expected that the Ni component will be re-deposited on the surface of the hot-rolled steel sheet during the pickling process.

[0075] Even more surprisingly, our inventors' studies have shown that including a certain amount of Ni ions in the first pickling bath makes it easier to satisfy the above-mentioned equations (1) and (3) relating to surface roughness than when no Ni ions are included at all. This is thought to be because Ni may precipitate when the steel sheet is pickled in a pickling bath containing Ni ions, and the precipitated Ni has a favorable effect on the surface shape and reactivity of the steel sheet.

[0076] Specifically, the Ni ion concentration in the first pickling bath is preferably 0.001% or higher by mass, more preferably 0.002% or higher, and even more preferably 0.003% or higher. On the other hand, from an operational management standpoint, the Ni ion concentration in the first pickling bath is preferably 1.0% or lower by mass, and can be 0.020% or lower. Note that the Ni ion concentration in the first pickling bath naturally increases when hot-rolled steel sheets containing trump elements are continuously pickled. Therefore, it is sufficient to raise the Ni ion concentration to within the above range when a new first pickling bath is prepared.

[0077] The first pickling time is preferably 40 seconds or more, more preferably 45 seconds or more, preferably 60 seconds or less, more preferably 40 to 60 seconds, and even more preferably 45 to 60 seconds. If the first pickling time is 40 seconds or more, it is generally sufficient to remove scale. On the other hand, if the first pickling time exceeds 60 seconds, the surface of the steel sheet may be excessively pickled, which may lead to an increase in tramp elements. In addition, the surface of the hot-rolled steel sheet after pickling becomes rough, and it becomes difficult to increase the degree of concavity in the surface shape. Furthermore, problems such as a decrease in yield and accelerated deterioration of the pickling bath may occur. The first pickling time is the time during which the dissolution reaction proceeds while the hot-rolled steel sheet and the first pickling bath are in contact. The first pickling time can be, for example, the time during which the hot-rolled steel sheet is immersed in the first pickling bath.

[0078] [Drying Step] After the first pickling step, the surface of the hot-rolled steel sheet can be dried in a sub-zero environment to effectively remove any residue from the steel sheet surface. Drying temperature It is preferable to dry for 10 seconds or more using sub-zero cold air with a drying temperature of 0°C or below. If the temperature of the gas applied to the surface of the hot-rolled steel sheet (drying temperature) is higher than 0°C, oxidation of the surface of the hot-rolled steel sheet may progress during this drying step. This makes it difficult to control the surface roughness of the hot-rolled steel sheet after pickling to be smooth, and it is difficult to increase the degree of depressions in the surface irregularities. A drying temperature of -5°C or below is more preferable. Furthermore, from the viewpoint of operational management, -20°C or above is preferable.

[0079] If the drying time, specifically the time for applying cold air to the surface of the hot-rolled steel sheet, is less than 10 seconds, the residue on the surface of the steel sheet cannot be completely removed, resulting in uneven pickling and an increase in surface irregularities. On the other hand, from a yield perspective, it is desirable to keep the drying time to 30 seconds or less.

[0080] In the drying step, "cold air" refers to any gas with a desired flow rate and a temperature of 0°C or below. Preferred gases include air and non-oxidizing gases such as Ar and N2. From the viewpoint of suppressing oxidation on the steel sheet surface, drying with a non-oxidizing gas is more preferable. On the other hand, from the viewpoint of cost, drying with air is more preferable. Using air at 0°C or below as the cold air in the drying step is even more preferable because it reduces costs without promoting oxidation on the steel sheet surface.

[0081] [Second Pickling Step] In the second pickling step, the hot-rolled steel sheet after the first pickling step and drying step can be treated with a second pickling bath to remove any trace amounts of scale and / or oxides remaining on the surface after drying. The acid solution used as the second pickling bath is preferably a phosphoric acid solution.

[0082] Phosphoric Acid Concentration in the Second Pickling Bath If the phosphoric acid concentration in the second pickling bath exceeds 5.0% by mass, scale removal becomes excessively strong, potentially causing dissolution by pickling even to the steel sheet (base metal) directly beneath the scale. As a result, the surface of the hot-rolled steel sheet after pickling tends to become rough. Furthermore, it becomes difficult to increase the degree to which depressions occupy the surface shape of the hot-rolled steel sheet after pickling. In addition, the concentration of phosphorus on the steel sheet surface due to the concentration of phosphoric acid may result in inferior Zr conversion treatment performance. On the other hand, if the phosphoric acid concentration in the second pickling bath is 1.0% or higher by mass, it is easier to completely remove trace amounts of scale and oxides remaining on the steel sheet surface. For this reason, the phosphoric acid concentration in the second pickling bath is preferably 1.0% or higher, more preferably 3.0% or higher, preferably 5.0% or lower, more preferably in the range of 1.0% to 5.0%, and even more preferably in the range of 3.0% to 5.0%.

[0083] Temperature of the second pickling bath If the temperature of the second pickling bath is below 10°C, the scale removal capacity may be insufficient, and there is a risk that residual trace amounts of scale and oxides cannot be completely removed. Furthermore, it may not be possible to control the surface roughness of the hot-rolled steel sheet after pickling to below a predetermined level. On the other hand, if the temperature of the second pickling bath is above 30°C, the pickling capacity may be excessively activated, and the pickling may extend to the area of ​​the steel sheet directly below the scale, causing dissolution. As a result, the surface of the hot-rolled steel sheet after pickling tends to become rough. In addition, it is difficult to increase the degree to which depressions occupy the surface shape of the hot-rolled steel sheet after pickling. For these reasons, the temperature of the second pickling bath is preferably 10°C or higher, more preferably 15°C or higher, preferably 30°C or lower, more preferably 25°C or lower, more preferably in the range of 10 to 30°C, and even more preferably in the range of 15 to 25°C.

[0084] Second pickling time The second pickling time is preferably 30 seconds or more, more preferably 40 seconds or more, preferably 60 seconds or less, more preferably 30 to 60 seconds, and even more preferably 40 to 60 seconds. If the second pickling time is 30 seconds or more, it is likely to be sufficient to remove even the remaining scale. On the other hand, if the second pickling time exceeds 60 seconds, the surface of the steel sheet may be excessively pickled, which may lead to the concentration of tramp elements. In addition, the surface of the hot-rolled steel sheet becomes rough after pickling, and it is difficult to increase the degree of concavity in the surface shape. Furthermore, the concentration of phosphorus components on the surface of the steel sheet due to the concentration of phosphoric acid may result in inferior Zr conversion treatment performance. The second pickling time is the time during which the dissolution reaction proceeds while the hot-rolled steel sheet and the second pickling bath are in contact. The second pickling time can be, for example, the time during which the hot-rolled steel sheet is immersed in the second pickling bath.

[0085] In steel sheets containing trump elements, the concentration of Cu ions and Sn ions in the second pickling bath can lead to a decrease in chemical conversion treatment performance due to the concentration of Sn and / or Cu on the surface of the steel sheet. Therefore, a method that does not concentrate Sn and Cu in the steel sheet during pickling is preferred. In the second pickling bath, as pickling continues, metal ions from the steel sheet components dissolve into the phosphoric acid solution, which is the pickling solution, and Sn ions and / or Cu ions from the trump elements gradually become present. If the Sn ions and / or Cu ions dissolved in the second pickling bath re-deposit on the surface of the steel sheet, it may lead to a decrease in chemical conversion treatment performance. Therefore, even when pickling is performed continuously, it is preferable to keep the amount of Sn ions and Cu ions in the second pickling bath as low as possible.

[0086] Even more surprisingly, our studies have shown that when the concentration of Cu ions and / or Sn ions in the second pickling bath is suppressed, the above-mentioned equations (1) to (3) relating to surface roughness are more likely to be satisfied. This is thought to be because if a large amount of Cu and Sn, which are nobler than steel sheets, is dissolved in the pickling bath, Cu and / or Sn may precipitate on the surface of the steel sheet in the pickling bath, potentially increasing the surface roughness. In particular, in a phosphoric acid solution with relatively low pickling power, if a higher concentration of Cu and / or Sn ions than the above-mentioned Ni ions is present, it is presumed that the rate of Cu and Sn precipitation will increase, making it difficult to control the surface roughness.

[0087] To ensure superior chemical treatment properties and corrosion resistance after painting, it is preferable, more preferable, that the sum of the concentrations of Sn ions and Cu ions in the second pickling bath be 1.0% or less by mass, 0.9% or less, even more preferable, 0.5% or less, and even more preferable, 0.1% or less. The sum of the concentrations of Sn ions and Cu ions in the second pickling bath can be 0% by mass, or 0.005% or more. If the Sn ion concentration and Cu ion concentration in the second pickling bath increase due to the dissolution of Sn and Cu from the hot-rolled steel sheet, the sum of the Sn ion concentration and Cu ion concentration can be reduced by replacing a portion of the phosphoric acid solution with fresh solution.

[0088] In hot-rolled steel sheet manufacturing lines, long lengths of hot-rolled steel sheets obtained after the hot-rolling process are sometimes transported, and the pickling process is continuously applied to these long lengths of hot-rolled steel sheets. When the pickling process is applied continuously in this manner, as mentioned above, ions of trump elements such as Cu and Sn derived from the base metal dissolve in the pickling bath, impairing the chemical conversion treatment properties and post-painting corrosion resistance of the hot-rolled steel sheet after pickling. However, if the pickling process, including the first pickling step, drying step, and second pickling step, is performed continuously two or more times with the process conditions controlled as described above, the total concentration of Cu and Sn ions in the second pickling bath can be kept low while continuing the pickling process. This allows for the continuous application of superior chemical conversion treatment properties and post-painting corrosion resistance.

[0089] Furthermore, in a continuous production line for hot-rolled steel sheets, Ni ions are usually not present in the pickling bath during the initial stages of line operation. However, if the pickling process is performed two or more times consecutively, and the process includes a first pickling step, a drying step, and a second pickling step with controlled process conditions as described above, Ni ions will be present in the first pickling bath during the initial stages of the line, further improving the chemical conversion treatment properties and post-painting corrosion resistance due to the Ni component. Thus, to further improve chemical conversion treatment properties and post-painting corrosion resistance, it is preferable to use a process that includes a first pickling step, a drying step, and a second pickling step with controlled process conditions, especially when the pickling process is performed two or more times consecutively.

[0090] [Chemical Conversion Treatment Process] Following the pickling process, the hot-rolled steel sheet may be subjected to a chemical conversion treatment to obtain a hot-rolled steel sheet with a chemical conversion coating. The chemical conversion treatment conditions can follow conventional methods. The hot-rolled steel sheet obtained in this invention has excellent chemical conversion treatment properties, so whether it is zinc phosphate chemical conversion treatment or Zr chemical conversion treatment, it can be a hot-rolled steel sheet with a well-formed chemical conversion coating. Furthermore, the hot-rolled steel sheet obtained in this invention also has excellent corrosion resistance after painting, so even if, for example, painting is performed after this chemical conversion treatment process to obtain a hot-rolled steel sheet with a coating, its corrosion resistance will remain excellent.

[0091] The present invention will be described in more detail below based on examples. However, the present invention is not limited to these examples. A steel slab having the component composition shown in Table 1 was heated and subjected to a hot rolling process. The remainder other than the elements shown in Table 1 is Fe and unavoidable impurities. In the hot rolling process, the steel slab was heated to 1250°C, then rough rolling was performed, followed by descaling, and finish rolling was performed at 910°C. After that, the steel sheet was wound at a winding temperature of 425°C and cooled to room temperature. In this way, a hot-rolled steel sheet (steel slab after hot rolling) before pickling was obtained.

[0092] Next, the hot-rolled steel sheet obtained before pickling was subjected to a pickling process. In the pickling process, the prepared sample (hot-rolled steel sheet before pickling) was cut into small pieces measuring 70 mm x 150 mm to serve as test specimens. The first pickling step, drying step, and second pickling step were performed sequentially on these test specimens under the conditions shown in Table 2. In each pickling step, the test specimen was immersed in each experimental pickling tank, which served as the pickling bath. Each pickling time is the time the test specimen was immersed in each pickling bath. In the drying step after pickling, the test specimen was inserted from the top of a space with the drying temperature shown in Table 2 (a freezer depending on the drying temperature), and cold or warm air, which is air at the drying temperature shown in Table 2, was blown onto both sides of the test specimen. After the drying step, the second pickling step was performed, in which the steel sheet as a test specimen was washed with pure water and dried. This drying after washing was carried out by blowing room temperature air (dry air). In this way, a hot-rolled steel sheet was obtained. Furthermore, the obtained hot-rolled steel sheet had a component composition equivalent to that of the steel slab used.

[0093] The hot-rolled steel sheets manufactured as described above were evaluated for their chemical treatment properties and corrosion resistance after painting as follows. The evaluation tests were conducted by taking test pieces (sometimes simply referred to as steel sheets) from the hot-rolled steel sheets.

[0094] [Chemical Conversion Treatment Properties] Zinc phosphate chemical conversion treatment was performed using commercially available zinc phosphate chemical conversion agents (surface modifier: Preparen X, chemical conversion agent: Palbond SX35, both manufactured by Nippon Parkerizing Co., Ltd.). After degreasing the steel plate, zinc phosphate chemical conversion treatment was performed at a treatment temperature of 35°C for a treatment time of 90 seconds to coat the surface of the steel plate with zinc phosphate. The zinc phosphate coverage rate on the surface of the steel plate after chemical conversion treatment was calculated as follows. The surface of the steel plate after chemical conversion treatment was observed using a scanning electron microscope (SEM) at a field of view of 1000x magnification, and the area of ​​the zinc phosphate crystal coating area and the exposed base metal area were determined separately, and the area ratio of the zinc phosphate crystal coating area to the total area was determined. The same measurement was performed for 10 fields of view, and the average value of the area ratio of the zinc phosphate crystal coating area obtained for each was used as the zinc phosphate coverage rate (area %) to evaluate the chemical conversion treatment properties. The zinc phosphate coating rate was judged as follows: less than 80% was marked with × (unsuitable), 80% to less than 98% was marked with ○ (good), and 98% or more was marked with ◎ (excellent). ○ and ◎ were considered acceptable. The results are shown in Table 3.

[0095] A commercially available Zr conversion treatment agent (Palceed 1500, manufactured by Nippon Parkerizing Co., Ltd.) was used. The conversion treatment was performed at a temperature of 40°C for 90 seconds, and the amount of zirconium deposited on the surface of the steel sheet was measured. The amount of Zr deposited was measured using an X-ray fluorescence analyzer. A calibration curve showing the relationship between the amount of Zr deposited and X-ray intensity was created using test pieces with known Zr deposited amounts, and the amount of Zr deposited on the surface of the target steel sheet was determined using the calibration curve method. The Zr deposited amount was 15 mg / m². 2 Less than 15 mg / m² is marked with an asterisk (×) (unsuitable). 2 30mg / m or more 2 Less than 30 mg / m² is marked with ○ (good), and 30 mg / m² is marked with ○. 2 The above were judged as ◎ (Excellent). 〇 and ◎ were considered passing grades. The results are shown in Table 3.

[0096] [Corrosion Resistance After Painting] A portion of the steel plate that had undergone phosphorylation treatment using the method described above was further subjected to electrodeposition coating in order to conduct a corrosion resistance test. For electrodeposition coating, Kansai Paint Co., Ltd.'s GT150V was used, and the coating was baked at 170°C for 20 minutes to achieve a coating film thickness of 15 μm. Subsequently, cross-cuts were made on the surface of the steel plate using an NT cutter S or A type (manufactured by Nippon Transfer Paper Co., Ltd.) with a length of 90 mm, a crossing angle of 45°, and a load of 300 g, to prepare test specimens for the post-paint corrosion resistance test. After sealing the ends and back surfaces of these test specimens with waterproof tape, they were subjected to a salt spray test (JIS Z 2371) for 720 hours. After the test, the blistering of the coating film formed around the cross-cut area of ​​the test specimen was observed, and the value of half the widest blister width on both sides of the cross-cut was measured and used to evaluate the post-paint corrosion resistance. The measured values ​​were then judged as follows: 1.5 mm or less was marked ◎ (Excellent), between 1.5 mm and less than 2.0 mm was marked ○ (Good), and 2.0 mm or more was marked × (Unacceptable). ○ and ◎ were considered acceptable. The results are shown in Table 3.

[0097]

[0098]

[0099]

[0100] The results in Tables 1-3 show that the present invention example has superior chemical conversion treatment properties and post-painting corrosion resistance compared to the comparative example. More specifically, the present invention example has superior zinc phosphate chemical conversion treatment properties, Zr chemical conversion treatment properties, and post-painting corrosion resistance compared to the comparative example. Thus, the manufacturing method of the present invention can provide hot-rolled steel sheets with excellent chemical conversion treatment properties and post-painting corrosion resistance, despite containing trump elements.

[0101] According to the present invention, it is possible to provide a hot-rolled steel sheet that achieves both excellent chemical conversion treatment properties and corrosion resistance after painting. The hot-rolled steel sheet obtained by the manufacturing method of the present invention, even when containing trump elements, exhibits excellent zinc phosphate conversion treatment properties, Zr conversion treatment properties, and corrosion resistance after painting, making it applicable to various uses such as automotive steel sheets, and is environmentally friendly and highly industrially useful.

Claims

1. A method for manufacturing a hot-rolled steel sheet, comprising, in order, a hot-rolling step and a pickling step for a steel slab, wherein the hot-rolled steel sheet has the following composition by mass%: Cu: 0.02 to 0.40%, Ni: 0.01 to 0.40%, Sn: 0.001 to 0.050%, Cr: 0.01 to 0.20%, Mo: 0.001 to 0.500%, C: 0.50% or less, Si: 1.000% or less, Mn: 2.00% or less, Zn: 0.500% or less, Pb: 0.500% The following composition contains As: 0.500% or less, Sb: 0.500% or less, Bi: 0.500% or less, V: 0.500% or less, B: 0.0100% or less, P: 0.100% or less, S: 0.020% or less, Al: 0.100% or less, and N: 0.0100% or less, with the remainder being Fe and unavoidable impurities, and in the pickling process, the surface roughness S of the hot-rolled steel sheet before pickling a The surface roughness S of the hot-rolled steel sheet after pickling is relative to the surface roughness S of the hot-rolled steel sheet after pickling. a 'But, S a ' / S a A method for manufacturing hot-rolled steel sheets, characterized in that it satisfies ≤ 0.90 ... (1).

2. In the pickling step, in the hot-rolled steel sheet after the pickling, the surface valley depth R v relative to the surface peak height R p and the roughness skewness R sk respectively satisfy: R p / R v ≦ 1.00 ・・・(2) and R sk ≧ -0.20 ・・・(3), the method for producing a hot-rolled steel sheet according to claim 1, which further satisfies the above.

3. The method for manufacturing a hot-rolled steel sheet according to claim 1 or 2, wherein the pickling step comprises, in order, a first pickling step, a drying step, and a second pickling step, wherein in the first pickling step, pickling is performed using a first pickling bath having a Ni ion concentration of 0.001% by mass or more, a hydrochloric acid concentration of 1.0 to 5.0% by mass, and a temperature of 70 to 90°C, and the first pickling time is 40 to 60 seconds, wherein in the drying step, drying is performed using cold air at a temperature of 0°C or lower, and the drying time is 10 seconds or more, and in the second pickling step, pickling is performed using a second pickling bath having a sum of Cu ion concentration and Sn ion concentration of 1.0% by mass or less, a phosphoric acid concentration of 5.0% by mass or less, and a temperature of 10 to 30°C, and the second pickling time is 30 to 60 seconds.

4. The method for manufacturing a hot-rolled steel sheet according to claim 3, wherein the pickling step is performed two or more times in succession.

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