HOT-ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING IT

Abstract

A hot-rolled steel sheet has, as its chemical composition, in % by mass: C: 0.01% to 0.30%; Si: 0.01% to 3.00%; Mn: 0.20% to 3.00%; P: 0.030% or less; S: 0.030% or less; Al: 0.001% to 2.000%; N: 0.0100% or less; and Ni: 0.02% to 0.50%, wherein among the measurement points where elemental analysis is performed in a 1 µm measurement step using an EPMA in a 250 µm × 250 µm region on a surface, the percentage of measurement points having a Ni content of 0.5% by mass or more is 10% to 70%.

Description

[1] The present invention relates to a hot-rolled steel sheet and to a method for manufacturing the hot-rolled steel sheet. Priority is claimed in Japanese patent application No. 2020-018844, filed on February 6, 2020, the contents of which are incorporated herein by reference. Background of the invention [2] In recent years, vehicle body weight reduction has been promoted through the use of high-strength steel sheets to reduce carbon dioxide (CO2) emissions. In addition to medium-strength steel sheets, high-strength steel sheets have also been frequently used in vehicle bodies to ensure occupant safety. More recently, the number of plug-in hybrid and electric vehicles is expected to increase due to stricter fuel consumption regulations and environmental regulations related to NOx and similar emissions. These next-generation vehicles require the installation of high-capacity batteries and further reductions in vehicle body weight. [3] To further reduce the weight of a vehicle body, it may be possible to replace the steel sheets with lightweight materials such as an aluminum alloy, a resin, CFRP and the like, or to further increase the strength of the steel sheets and, from the point of view of material cost and processing cost, it is realistic to use ultra high-strength steel sheets for mass-produced automobiles, excluding high-end cars. [4] For chassis components (e.g., lower control arms) where hot-rolled steel sheets are primarily used, high-strength steel sheets with a tensile strength of 540 MPa or more are applied to reduce weight. Even with high strength, stiffness may be insufficient if the sheet thickness is small. Therefore, changing the shape or structure of the components is considered a measure against insufficient stiffness, but this complicates the component shape or structure. Therefore, high-strength steel sheets used to reduce the weight of a vehicle body are required to have high strength and improved workability and fatigue properties. [5] For example, Patent Document 1 describes a method for manufacturing a hot-rolled steel sheet that has high strength and excellent surface properties, formability (ductility, flake properties), and notch fatigue properties. In Patent Document 1, to suppress a tiger-stripe-like scale pattern that impairs surface properties, the Si content is reduced, and a composite structure is transformed that includes precipitated Ti carbide-hardened polygonal ferrite. A 1% to 10% low-temperature transformed product is made to achieve high ductility and flake properties. [6] Furthermore, Patent Document 2 describes a high-strength hot-rolled steel sheet that has excellent ductility, fatigue, and corrosion resistance properties, and a method for manufacturing the high-strength hot-rolled steel sheet. In Patent Document 2, the Si content is reduced to suppress a tiger-stripe-like flaking pattern that impairs surface properties. Additionally, the Ti carbide size is controlled so that the mass of those with a circle equivalent grain size of 7 nm or more and 20 nm or less is 50% or more of the total Ti carbide mass, thereby improving fatigue properties. Furthermore, Patent Document 2 describes that the hot-rolled steel sheet has good chemical convertibility and resistance to post-coating corrosion. cu nonn / zznz / E / YiAi Previous technique document Patent document [7] Patent Document 1: PCT International Publication No. WO2014 / 051005 Patent Document 2: Unexamined Japanese Patent Application, First Publication No. 2016-204690 Description of the invention Problems that must be solved by the invention [8] The content of alloying elements is normally increased to obtain high strength and good workability and fatigue properties, including Patent Documents 1 and 2 described above. Even in a high-strength steel sheet of this type, which has a higher content of alloying elements, for example, in a case where a chemical conversion treatment such as zinc phosphate treatment is carried out under ideal operating conditions, chemical convertibility problems often do not occur. However, in industrial settings, in the chemical conversion treatment of vehicle components and the like, several components are continuously subjected to the chemical conversion treatment using the same chemical conversion fluid. In this case, the chemical conversion fluid gradually deteriorates, and the chemical conversion treatment may not be carried out under ideal operating conditions. The inventors have conducted studies and, as a result, have discovered that in the case of a high-strength steel sheet (for example, with a tensile strength of 490 MPa or more) that has a relatively large content of alloying elements being subjected to a chemical conversion treatment using a deteriorated chemical conversion treatment liquid, problems arise because the chemical convertibility is not necessarily sufficient, the transparency, which is a part where the base metal sheet is exposed to the surface of the steel sheet after the chemical conversion treatment is generated, and the adhesion between a lacquer and the steel sheet deteriorates when the surface of the steel sheet is coated with the lacquer. In the case where chemical convertibility is reduced due to the use of deteriorated chemical conversion treatment fluid, for example, measures are required to strictly manage the free acidity management value, such as using a large amount of an accelerator that increases chemical convertibility, among the operating conditions of the chemical conversion treatment, and this leads to an increase in manufacturing cost and a reduction in productivity.Therefore, in a case where good chemical convertibility can be obtained even in a high-strength steel sheet even in a case where the chemical conversion treatment fluid deteriorates and the chemical conversion treatment conditions vary, i.e., good post-coating corrosion resistance can be obtained under a wide range of chemical conversion treatment operating conditions, it is not necessary to strictly manage the chemical conversion treatment operating conditions, and it is possible to avoid an increase in manufacturing cost and a reduction in productivity. The present invention has been devised in view of the foregoing problems. An object of the present invention is to provide a hot-rolled steel sheet having excellent chemical convertibility and a method for manufacturing the hot-rolled steel sheet. cu ηοηη / ζζηζ / Ε / γίΛΐ Means to solve the problem [9] The inventors have conducted studies on the reason for the reduction in the chemical convertibility of a high-strength steel sheet depending on the conditions. As a result, it is believed that oxides of Si, Al, and the like, or concentrated layers of Mn, Cu, and the like, form on a surface of the high-strength steel sheet or in an area of ​​the surface layer near the surface even after pickling, and these inhibit Fe elution during the chemical conversion treatment, thus reducing chemical convertibility. The inventors have conducted further studies and, as a result, have found that partially concentrating (locally concentrating) Ni in the surface layer of the steel sheet accelerates Fe elution and improves chemical convertibility. The present invention has been completed based on the previous findings, and the essence thereof is the following hot-rolled steel sheet. (1) A hot-rolled steel sheet according to an aspect of the present invention, having, as a chemical composition, in % by mass: C: 0.01% to 0.30%; Si: 0.01% to 3.00%; Mn: 0.20% to 3.00%; P: 0.030% or less; S: 0.030% or less; Al: 0.001% to 2.000%; N: 0.0100% or less; Ni: 0.02% to 0.50%; Nb: 0% to 0.060%; V: 0% to 0.20%; Ti: 0% to 0.20%; Cu: 0% to 0.20%; Cr: 0% to 0.20%; Mo: 0% to 1.00%; B: 0% to 0.0020%; W: 0% to 0.50%; Mg: 0% to 0.010%; Ca: 0% to 0.0100%; REM: 0% to 0.0100%; Or: 0% to 0.0100%; Zr: 0% to 0.500%; Co: 0% to 0.500%; Zn: 0% to 0.500%; Sn: 0% to 0.500%; and a residue consisting of Fe and impurities, wherein among the measuring points where elemental analysis is performed in a 1 pm measuring step using an ERMA in a 250 pm x 250 pm region on a surface, a percentage of measuring points having a Ni content of 0.5% by mass or more is from 10% to 70%. (2) Hot-rolled steel sheet in accordance with (1), wherein the chemical composition may contain one or two or more elements selected from the group consisting of Nb: 0.003% to 0.060%, V: 0.01% to 0.20%, Ti: 0.01% to 0.20%, Cu: 0.01% to 0.20%, Cr: 0.01% to 0.20%, Mo: 0.01% to 1.00%, B: 0.0005% to 0.0020%, W: 0.01% to 0.50%, Mg: 0.001% to 0.010%, Ca: 0.0010% to 0.0100%, REM: 0.0010% to 0.0100%, and O: 0.0005% to 0.0100%. (3) Hot-rolled steel sheet in accordance with (2), wherein the chemical composition may contain Si: 0.50% to 3.00%. (4) Hot-rolled steel sheet in accordance with (2), wherein the chemical composition may contain Si: 0.01% or more and less than 0.50%, and Al: 0.050% to 2.000%. (5) Hot-rolled steel sheet in accordance with (2), wherein the chemical composition may contain Si: 0.01% or more and less than 0.50%, and Al: 0.001% or more and less than 0.050%, and a total of Si and Al may be 0.50% or more and less than 0.55%. (6) Hot-rolled steel sheet conforming to any of (3) to (5), wherein among the measuring points at which elemental analysis is performed on the surface, the percentage of measuring points having an O content of 0.5% by mass or more may be 30% or less. (7) Hot-rolled steel sheet conforming to any of (1) to (6), wherein the chemical composition may contain Cu: 0.01% to 0.20%, and Ni / Cu may be 0.50 or more. (8) Hot-rolled steel sheet conforming to any of (1) to (7), wherein between the measuring points at which elemental analysis is performed on the surface, the average interval between measuring points having a Ni content of 0.5% by mass or more may be from 3 to 10 pm. (9) Hot-rolled steel sheet conforming to any of (1) to (8), wherein the surface may have an oil film that prevents rusting. (10) Hot-rolled steel sheet conforming to any of (1) to (8), wherein the surface may have a chemical conversion film. (11) A method for manufacturing a hot-rolled steel sheet according to another aspect of the present invention, comprising: heating a piece of steel having the chemical composition according to (1) or (2) in a heating furnace; descaling the heated steel piece; and hot-rolling the steel piece after descaling to obtain a hot-rolled steel sheet, wherein in the heating, after the surface temperature of the steel piece reaches 1,100°C or more, the steel piece is held for 60 minutes or more under an atmosphere with an air ratio of 0.9 or more, and an extraction temperature of 1,180°C or higher, and in descaling, the steel part whose surface temperature is 1,170°C or higher is descaled at least once with an injection pressure of 5 to 50 MPa, and the surface temperature of the steel part is maintained at 100°C or higher for 20 to 240 seconds after descaling is completed. Effects of the invention

[10] In accordance with aspects of the present invention, it is possible to obtain a hot-rolled steel sheet having excellent chemical convertibility and a method for manufacturing the hot-rolled steel sheet. In the hot-rolled steel sheet of the present invention, even if the conditions of a chemical conversion treatment vary, a good chemical conversion film can be obtained. cu nonn / zznz / E / YiAi Brief description of the drawings

[11] Figure 1 is a diagram to illustrate a mechanism in which the formation of chemical conversion crystals is accelerated by Ni concentrated locally in a surface layer. Modalities of the invention

[12] A hot-rolled steel sheet in accordance with one embodiment of the present invention (hot-rolled steel sheet in accordance with this embodiment) will now be described. Hot-rolled steel sheet produced in accordance with this method has a predetermined chemical composition, and among the measuring points where elemental analysis is performed in a 1 pm measuring step using an ERMA in a 250 pm x 250 pm region on a surface, the percentage of measuring points having a Ni content of 0.5% by mass or more is from 10% to 70%. Hot-rolled steel sheet produced according to this method may have a chemically converted film and / or an electrodeposited coating on its surface. In addition, hot-rolled steel sheet produced according to this method may have an oil film on its surface that prevents oxidation.

[13] Chemical composition The reasons for limiting the chemical composition are described below. The percentage related to the chemical composition is by mass unless otherwise specified. Furthermore, in the following numerical limitation ranges with 'a', the values ​​at both ends are included as a lower and an upper limit. Numerical values ​​expressed as greater than or less than are not included in the numerical range.

[14] C: 0.01% to 0.30% Carbon (C) is an element that contributes to the high hardening of a steel sheet through structural reinforcement by producing a low-temperature transformed product, or through precipitation hardening by forming precipitates with titanium (Ti), niobium (Nb), and / or vanadium (V) if Ti, Nb, and / or V are present. If the carbon content is less than 0.01%, it is not possible to obtain the required strength of 300 MPa or more, 490 MPa or more, or even more preferably 540 MPa or more for the steel sheet. Therefore, the carbon content is 0.01% or more. The carbon content is preferably 0.03% or more, and very preferably 0.05% or more. If the carbon content exceeds 0.30%, the area ratio of the low-temperature processed product to the cementite, which are hard layers, increases, and workability decreases. Therefore, the carbon content should be 0.30% or less. Preferably, the carbon content should be 0.25% or less, and very preferably 0.20% or less.

[15] Yes: 0.01% to 3.00% Silicon (Si) is used as a strength-enhancing element and is important for ferrite formation. Si is also an effective deoxidizing agent. Therefore, the Si content is 0.01% or higher. If microstructure control is used to form ferrite, the Si content is preferably 0.50% or higher, and very preferably 0.80% or higher. If the silicon (Si) content increases, the ferrite temperature range expands towards the higher temperature side. Furthermore, regarding high-temperature steel oxidation, Si influences the growth rate and scale properties. During hot rolling, silicon in a steel sheet forms Fe₂SiO₄ on the surface. If the content is excessive, the Si concentrates on the surface, and this concentrated layer cannot be completely removed even after pickling, affecting chemical convertibility. Therefore, the Si content should be 3.00% or less. Preferably, it should be 2.50% or less, and very preferably 2.00% or less. The Si content can be less than 0.50% if microstructure control is not used for ferrite formation.

[16] Mn: 0.20% to 3.00% Manganese (Mn) contributes to the high hardening of steel sheets by reinforcing ferrite. Furthermore, with increasing Mn content, the austenite temperature range expands towards the lower temperature side, and the ferrite-austenite two-phase temperature range also expands. Additionally, Mn suppresses hot cracking caused by sulfur (S) by combining with sulfur (S) and fixing it as MnS. To achieve these effects, the Mn content is 0.20% or higher. For a preferred strength of 300 MPa or higher for the steel sheet, the Mn content is preferably 0.30% or higher. For a very preferably higher strength of 490 MPa or higher, the Mn content is very preferably 0.90% or higher.To obtain a strength, preferably still 540 MPa or more as the required strength for the steel sheet, the Mn content is preferably still 1.20% or more. If the Mn content exceeds 3.00%, manufacturing problems arise, such as cracking of the plates during casting. Therefore, the Mn content should be 3.00% or less. Ideally, the Mn content should be 2.50% or less, and very preferably Cu nonn / zznz / E / YiAi 2.00% or less.

[17] P: 0.030% or less The phosphorus (P) content is preferably low. If the P content exceeds 0.030%, P segregation leading to crystal granulation is significant, and consequently, local ductility deteriorates due to grain boundary embrittlement. Therefore, the P content is 0.030% or less. The P content is preferably 0.020% or less, and very preferably 0.015% or less. The phosphorus content can be 0%. However, if the phosphorus content is less than 0.005%, the cost increases significantly. Therefore, the lower limit for phosphorus content can be 0.005%.

[18] S: 0.030% or less The sulfur content is preferably low. If the sulfur content exceeds 0.030%, adverse effects on weldability, formability during casting or hot rolling, and hole expansion capacity increase. Therefore, the sulfur content is 0.030% or less. The sulfur content is preferably 0.015% or less, and very preferably 0.010% or less. The sulfur content can be 0%. However, if the sulfur content is less than 0.002%, the cost increases significantly. Therefore, the lower limit for sulfur content can be 0.002%.

[19] Al: 0.001% to 2.000% Aluminum (Al) is an element related to deoxidation and ferrite formation, similar to silicon (Si). Furthermore, increasing the Al content expands the ferrite temperature range towards the higher temperature side. Additionally, Al suppresses the formation of coarse cementite and contributes to improved hole expansion capacity. Therefore, the Al content is 0.001% or higher. Preferably, the Al content is 0.020% or higher, and very preferably 0.030% or higher. Moreover, if microstructure control is used to promote ferrite formation, the Al content is preferably 0.050% or higher. If the aluminum content exceeds 2,000%, the number of coarse aluminum-based inclusions increases. Furthermore, workability deteriorates, or surface defects develop. Additionally, a ladle nozzle is likely to become clogged during casting. Therefore, the aluminum content should be 2,000% or less. The preferred aluminum content is 1,200% or less, the most preferably 1,000% or less, and the most preferably still 0,400% or less. The aluminum content may be less than 0.050% if microstructure control for ferrite formation is not used.

[20] Ν: 0.0100% or less Nitrogen (N) reduces ductility if it remains in steel as nitrogen in solid solution. Furthermore, N combines with titanium (Ti) to form TiN. High N content leads to the precipitation of coarse TiN, impairing hole expansion capacity. Therefore, a low N content is preferable. If the N content exceeds 0.0100%, the adverse effects described above become noticeable. Therefore, the N content should be 0.0100% or less. The N content is preferably 0.0060% or less, and very preferably 0.0040% or less. The N content can be 0%. However, if the N content is less than 0.0010%, the cost increases significantly. Therefore, the lower limit for the N content can be 0.0010%.

[21] Ni: 0.02% to 0.50% Ni is the most important element for hot-rolled steel sheet produced according to this method. In the manufacture of hot-rolled steel sheet, primarily during the heating step of a steel workpiece in a heating furnace and the descaling step of removing scale from the heated steel workpiece, Ni is locally concentrated on the surface layer of the steel sheet near the interface between the steel sheet surface and the scale under specific operating conditions. If a chemical conversion treatment, such as zinc phosphate treatment, is applied to the surface of the steel sheet, a difference in ionization tendency occurs between the region where Ni is concentrated and the surrounding region where Ni is not concentrated.As a result, the Fe surrounding the locally concentrated Ni elutes to the surface of the steel sheet and acts as a precipitation nucleus for a chemical conversion film (chemical conversion coating). This film forms with small crystal sizes and does not generate transparency. In this way, it is possible to improve the adhesion between the lacquer and the steel sheet. If the Ni content is less than 0.02%, the aforementioned effects (transparency or increased size of the chemical conversion crystals) cannot be achieved; therefore, the Ni content should be 0.02% or higher. For example, if the Ni content is less than 0.02%, local Ni concentration does not occur. Consequently, iron elution in the chemical conversion bath is not accelerated, the size of the chemical conversion crystals increases, and coating adhesion deteriorates. The Ni content should preferably be 0.05% or higher. If the Ni content exceeds 0.50%, the Ni will cover the entire surface of the steel sheet (it is not a localized concentration), and therefore the aforementioned effects cannot be achieved. Furthermore, the cost increases. Therefore, the Ni content should be 0.50% or less. The Ni content should preferably be 0.45% or less, and very preferably 0.40% or less.

[22] Basically, hot-rolled steel sheet produced in accordance with this modality contains the elements described above with a remainder consisting of Fe and impurities. However, hot-rolled steel sheet may contain the following elements in quantities that will be described later. The following elements are optional and are not necessarily included.

[23] Cu: 0% to 0.20% Copper (Cu) is an element that contributes to increasing the strength of steel sheets. Therefore, copper may be present. To contribute to increased strength, the copper content is preferably 0.01% or more. The copper content is preferably 0.02% or more, and very preferably 0.04% or more. Copper (Cu) has a low melting point and concentrates at the interface between the inclusion and the base metal sheet through the austenite grain boundaries. If the Cu content is high, a concentrated Cu layer forms, reducing the treatment capacity of the zinc phosphate. If the Cu content exceeds 0.20%, a concentrated Cu layer covers the entire surface of the steel sheet, significantly impairing chemical convertibility. Therefore, the Cu content should be 0.20% or less. The Cu content is preferably 0.15% or less, and very preferably 0.10% or less. Furthermore, if the Ni / Cu ratio is < 0.50, a concentrated Cu layer is likely to form uniformly across the entire surface of the steel sheet, further compromising chemical convertibility. Therefore, Ni / Cu b 0.50 is preferable.

[24] Nb: 0% to 0.060% V: 0% to 0.20% Ti: 0% to 0.20% Cr: 0% to 0.20% Mo: 0% to 1.00% W: 0% to 0.50% Nitrogen (Nb), vanadium (V), titanium (Ti), chromium (Cr), molybdenum (Mo), and tungsten (W) are elements that increase the strength of steel sheets through precipitation hardening and / or solid solution hardening. Therefore, these elements may be present. If the aforementioned effects are achieved, the Nb content is preferably 0.003% or more, very preferably 0.005% or more, very preferably 0.010% or more, and very preferably 0.015% or more. Furthermore, the V content is preferably 0.01% or more. The Ti content is preferably 0.01% or more, very preferably 0.05% or more, very preferably 0.10% or more, and very preferably 0.15% or more. The Cr content is preferably 0.01% or more, very preferably 0.05% or more, and very preferably still 0.10% or more. The Mo content is preferably 0.01% or more, and very preferably 0.02% or more. The W content is preferably 0.01% or more, and very preferably 0.02% or more. If the Nb content exceeds 0.060%, the V content exceeds 0.20%, the Ti content exceeds 0.20%, the Cr content exceeds 0.20%, the Mo content exceeds 1.00%, and the W content exceeds 0.50%, the aforementioned effects saturate and economic efficiency decreases. Therefore, if Nb, V, Ti, Cr, Mo, and W are present, and the Nb content is 0.060% or less, the V content is 0.20% or less, the Ti content is 0.20% or less, the Cr content is 0.20% or less, the Mo content is 1.00% or less, and the W content is 0.50% or less, the economic efficiency is reduced. The Nb content is preferably 0.055% or less, and very preferably 0.050% or less. The V content is preferably 0.15% or less, and very preferably 0.08% or less. The Ti content is preferably 0.18% or less, and very preferably 0.17% or less. The Cr content is preferably 0.18% or less, and very preferably 0.15% or less. The Mo content is preferably 0.70% or less, and very preferably 0.05% or less. The W content is preferably 0.40% or less, and very preferably 0.03% or less.

[25] B: 0% to 0.0020% Boron (B) is an element that enhances hardenability, thereby increasing the fraction of a product phase transformed at low temperatures. Therefore, if the desired effect of enhanced hardenability is to be demonstrated, the product may contain 0.0005% or more of B. The B content is preferably 0.0010% or more, and preferably 0.0015% or more. If the B content exceeds 0.0020%, the effect becomes saturated, increasing the risk of cracking in the plates during a cooling step after continuous casting. Therefore, if B is to be contained, the B content should be 0.0020% or less. cu nonn / zznz / E / YiAi

[26] Mg: Ca: 0% to 0% to 0.010% 0.0100% REM: 0% to 0.0100% Mg, Ca and REM are elements that control the morphology of non-metallic inclusions that are origins of Fractures cause deterioration of workability and improve workability. Therefore, Mg, Ca, and REM may be included. If the above effects are achieved, the Mg content is preferably 0.001% or more, the Ca content is preferably 0.0010% or more, and the REM content is preferably 0.0010% or more. If the magnesium content exceeds 0.010%, the calcium content exceeds 0.0100%, and the REM content exceeds 0.0100%, the aforementioned effects become saturated and economic efficiency decreases. Therefore, in cases where magnesium, calcium, and REM are present, the magnesium content should be 0.010% or less, the calcium content 0.0100% or less, and the REM content 0.0100% or less. Ideally, the magnesium content should be 0.005% or less, the calcium content 0.0070% or less, and the REM content 0.0070% or less.

[27] O: 0% to 0.0100% Oxygen is an element that disperses a large quantity of fine oxides during the deoxidation of molten steel. Therefore, oxygen may be present. If the above effect is achieved, the oxygen content is preferably 0.0005% or more. The oxygen content is preferably 0.0010% or more, and very preferably 0.0020% or more. Oxygen (O) is an element that forms coarse oxides, which can cause fractures in steel if its content is too high, leading to brittle fractures or hydrogen-induced cracks. Therefore, the O content should be 0.0100% or less. From a weldability standpoint, the O content is preferably 0.0030% or less.

[28] Zr: 0% to 0.500% Co: 0% to 0.500% Zn: 0% to 0.500% Sn: 0% to 0.500% Even if Zr, Co, Zn, or Sn are present in an amount of 0.500% or less, the effects of the hot-rolled steel sheet produced in accordance with this method are not impaired. Therefore, one or more of Zr, Co, Zn, and Sn may be present in an amount of 0.500% or less, respectively.

[29] The amount of each element in the hot-rolled steel sheet in accordance with this modality (including a case in which a chemical conversion film or an anti-corrosive oil film is provided on the surface) is the average content over the full thickness of the sheet, obtained by ICP emission spectroscopic analysis using chips in accordance with JIS G 1201: 2014. The C and S content is obtained by a known high-frequency combustion method (combustion infrared absorption method). The O content is obtained using a known inert gas fusion non-dispersive infrared absorption method.

[30] Among the measurement points where elemental analysis is performed in a 1 pm measurement step using ERMA in a 250 pm * 250 pm region on the surface, the percentage of measurement points that have a Ni content of 0.5% by mass or more is from 10% to 70% The inventors have conducted studies on the reason for the reduction in the chemical convertibility of a high-strength steel sheet. As a result, it is believed that oxides of Si, Al, and similar elements, or concentrated layers of Mn, Cu, and similar elements, form on a surface or surface layer area of ​​the high-strength steel sheet even after pickling. These inhibit Fe elution during the chemical conversion treatment, thus reducing chemical convertibility, particularly when the chemical conversion treatment conditions have deteriorated due to variations during operation. In this regard, Ni concentrates partially (not across the entire surface) in the surface layer of the steel sheet to generate a potential difference between Ni and Fe, accelerating the elution of Fe around the Ni-concentrated layer. That is, Ni remains, and the surrounding regions elute and form precipitation nuclei of the chemical conversion film. Thus, a film forms in which the size of the chemical conversion crystals is small without causing transparency, and chemical convertibility is improved.For example, this is thought to be because, as shown in Figure 1, due to the concentrated layers 4 of Ni formed on the surface of a steel sheet (although Figure 1 shows a case where oxides of Si, Al and the like or concentrated layers 3 of Mn, Cu and the like remain, regardless of the presence or absence of the oxides or concentrated layers), a potential difference is generated between the locally concentrated Ni and a base metal sheet 1 on the surface, precipitation nuclei of chemical conversion crystals 5 crystallize from the parts where the potential difference is generated, and the formation of the chemical conversion crystals 5 is accelerated. The base metal sheet 1 refers to a part of the steel sheet that excludes the inclusions 2. Specifically, in a case where among the measurement points where elemental analysis is performed in a 1 pm measurement step using an ERMA in a 250 pm x 250 pm region on the surface, the percentage of measurement points that have a Ni content of 0.5% by mass or more is from 10% to 70%, chemical convertibility is improved. In a case where the percentage of measurement points that have a Ni content of 0.5% by mass or more is less than 10%, the effect of accelerating Fe elution is not sufficient and the chemical convertibility is not sufficiently improved. Furthermore, if the percentage of measurement points having a Ni content of 0.5% by mass or more is greater than 70%, the Ni exists almost uniformly on the surface of the steel sheet and the above effects cannot be sufficiently obtained. If the hot-rolled steel sheet produced according to this method has a chemically converted coating (including where an electrodeposition coating is provided by an additional electrodeposition coating), it may be difficult to perform elemental analysis on the surface of the hot-rolled steel sheet. In this case, where the elemental analysis is performed at a 1 pm measurement step using an ERMA in a rectangular region that is 10 pm in the thickness direction from the steel sheet surface and 500 pm in the width direction of the sheet in a cross-section in the thickness direction, the percentage of measurement points with a Ni content of 0.5% by mass or more is 10% to 70%. Among the measurement points where elemental analysis is performed at a measurement step of 1 pm using an ERMA in a 250 pm x 250 pm region on the surface, the percentage of measurement points having a Ni content of 0.5% by mass or more can be considered to be 10% to 70%. This is because Ni exhibits a substantially three-dimensional isotropic distribution over a 10 pm interval (surface layer area) in the direction of the sheet thickness from the surface.

[31] Measuring points having a Ni content of 0.5% by mass or more are preferably distributed in patches over the surface of the steel sheet. Specifically, the average interval between regions with a Ni content of 0.5% by mass or more is preferably 3 to 10 pm. If the average interval is less than 3 pm or greater than 10 pm, Fe elution around the concentrated Ni portion is less likely to be accelerated.

[32] The average interval between regions having a Ni content of 0.5% by mass or more is measured as follows. An average of intervals between adjacent measurement points having a Ni content of 0.5% by mass or more among the measurement points where elemental analysis is performed in a 1 pm measurement step using an ERMA in a 250 pm × 250 pm region on the surface of the hot-rolled steel sheet is defined as an average interval between regions having a Ni content of 0.5% by mass or more.

[33] In many cases, hot-rolled steel sheet is pickled before chemical conversion treatment, but in hot-rolled steel sheet produced in accordance with this method, Ni concentrates locally as described above even after pickling under normal pickling conditions (e.g., for 30 to 60 seconds using a 1 to 10 wt% hydrochloric acid solution at a temperature of 20°C to 95°C). Therefore, chemical convertibility is excellent even after pickling. Furthermore, hot-rolled steel sheet produced in accordance with this method may have an anti-corrosive oil film formed on the surface to prevent rust and the like after pickling and before the chemical conversion treatment begins.

[34] The measurement conditions for performing elemental analysis in a 1 pm measurement step using an ERMA in a 250 pm × 250 pm region on the surface and for obtaining the average interval between regions having a Ni content of 0.5% by mass or more are, for example, as follows. The measurement is performed using a JEOL Ltd. tungsten electron gun instrument (model number: JXA-8800RL) under conditions of an accelerating voltage of 15 kV, an irradiation current of 6 × 10⁻⁸ A, an irradiation time of 15 ms, and a beam diameter of 0.5 pm. The same conditions can also be applied in the case where the elemental analysis is performed in the cross section in the direction of the sheet thickness with a measurement step of 1 pm using an ERMA.

[35] The chemical convertibility enhancement effect on hot-rolled steel sheet in accordance with this method generated by the local concentration of Ni is effective for any steel sheet. However, in a steel sheet containing a large amount of Si and Al to increase strength or improve formability, a large amount of Si and Al oxides form on the surface of the steel sheet, and therefore chemical convertibility is reduced. Therefore, for example, 1) If the Si content is 0.50% or more, 2) if the Al content is 0.050% or more, even with a Si content of less than 0.50%, or 3) In the event that the total content of Si and Al is 0.50% or more, even with a Si content of less than 0.50% and an Al content of less than 0.050%, the effect of improving chemical convertibility is particularly large.

[36] Even if the hot-rolled steel sheet produced according to this method is subjected to chemical conversion treatment and electrodeposition coating, the previously described aspect in which Ni is locally concentrated rarely changes. That is, the distribution of regions with a Ni content of 0.5% by mass or less near the boundary between the chemical conversion film and the hot-rolled steel sheet (corresponding to the vicinity of the surface of the hot-rolled steel sheet that is the original sheet) in the hot-rolled steel sheet subjected to chemical conversion treatment is the same as on the surface of the hot-rolled steel sheet that is the original sheet. Therefore, the measurement result obtained by the following method can be considered as the percentage of measurement points having a Ni content of 0.5%.5% by mass or more (synonymous with the result of the measurement made on the surface of the hot-rolled steel sheet) on the surface of the hot-rolled steel sheet that is an original sheet before chemical conversion treatment. cu ηοηη / ζζηζ / Ε / γίΛΐ

[37] Among the measuring points where elemental analysis is performed, the percentage of measuring points that have an oxygen (0) content of 0.5% by mass or more is 30% or less In hot-rolled steel sheet produced in accordance with this method, among the measuring points at which elemental analysis is performed, the percentage of measuring points having an oxygen content of 0.5% by mass or more is preferably 30% or less. During elemental analysis, silicon and aluminum oxides form at measurement points with an oxygen content of 0.5% by mass or higher. The fact that the percentage of these measurement points is 30% or less indicates that the amount of silicon, aluminum, and similar oxides formed is small. These oxides reduce chemical convertibility by inhibiting the elution of iron during the chemical conversion treatment. Therefore, when the amount of oxides is small, a film forms in which the size of the chemical conversion crystals is small, without causing transparency, thus further improving chemical convertibility.

[38] In the event of elemental analysis, the ERMA analysis directed at an element having an atomic number equal to or greater than that of boron (B) is carried out in a 250 pm × 250 pm region in a 1 pm measurement step. The percentage of measurement points having a Ni content of 0.5% by mass or more when the total mass of the element having an atomic number equal to or greater than that of B is obtained is 100%. In the ERMA analysis performed on a steel sheet, if an anti-corrosion oil film forms on the surface, it is removed using a solvent such as acetone or alcohol so that the measurement can be taken directly on the surface. If scale forms, the measurement is taken after pickling under normal pickling conditions (e.g., for 30 to 60 seconds using a 1 to 10 wt% hydrochloric acid solution at a temperature of 20°C to 95°C). The ERMA analysis is performed using a JEOL Ltd. tungsten electron gun instrument (model number: JXA-8800RL) under conditions of an accelerating voltage of 15 kV, an irradiation current of 6 × 10⁸ A, an irradiation time of 15 ms, and a beam diameter of 0.5 pm.

[39] The structure (microstructure) of hot-rolled steel sheet produced according to this method is not limited. Regardless of the phase of the structure, the chemical convertibility improves with the local concentration of Ni. Furthermore, the chemical convertibility enhancement effect generated by the local concentration of Ni is significant in high-strength steel sheets containing a large amount of alloying elements. For example, the effect is clearly visible in hot-rolled steel sheets with a tensile strength of 300 MPa or more, significant in hot-rolled steel sheets with a tensile strength of 490 MPa or more, and even more pronounced in hot-rolled steel sheets with a tensile strength of 540 MPa or more. The thickness of the hot-rolled steel sheet in accordance with this method is not limited and is, for example, from 1.2 to 10.0 mm.

[40] A method for manufacturing hot-rolled steel sheet in accordance with this modality will now be described. Hot-rolled steel sheet in accordance with this method can be manufactured using a manufacturing method that has the following steps. (i) a heating step of heating a piece of steel in a heating furnace (ii) a descaling step of descaling the heated piece of steel (iii) a hot rolling step of hot rolling the piece of steel after the descaling step to obtain a hot-rolled steel sheet. The respective steps will be described.

[41] A casting step (steel part manufacturing step) that is performed prior to hot rolling is not particularly restricted. That is, after melting in a blast furnace, electric furnace, or similar, several secondary castings can be made to adjust the components as described above, and then the casting can be carried out by normal continuous casting or by an ingot method. Scrap metal can be used as raw material. cu nonn / zznz / E / YiAi

[42] Warm-up step Descaling step In the heating step, a steel piece, such as a plate, is heated in a heating furnace. Descaling then takes place before the process reaches the hot rolling step. The localized concentration of Ni is achieved primarily in the heating and descaling steps. Specifically, by accelerating the oxidation of a surface on the steel part during the heating step and selectively oxidizing the Fe, the Ni, which is less likely to oxidize than Fe, concentrates on the base metal side of the interface between the scale and the base metal. The preferentially formed oxides are then removed to some extent by descaling, and the steel part is held for a certain period of time or longer at a predetermined temperature range to further concentrate the Ni locally.

[43] In the heating step, after the surface temperature of the steel part reaches 1,100°C or more, the steel part is held for 60 minutes or more in an atmosphere with an air ratio of 0.9 or more, and the extraction temperature is 1180°C or higher. To form a sufficiently concentrated Ni layer on the surface layer in the heating furnace, it is necessary to accelerate the growth of scale on the steel part. If the air ratio in the heating furnace is less than 0.9, scale growth follows a parabolic curve, but it slows down for a limited time in the furnace. Therefore, it is not possible to form a layer with a sufficient Ni concentration at the interface between the scale and the base metal sheet. The air ratio can vary depending on the position in the heating furnace or changes over time during the heating period. It is preferable that the minimum air ratio at each position in the heating furnace during the heating period be 0.9 or higher, since the air ratio is 0.9 or higher when the steel part is heated. If the air ratio exceeds 1.5, the increased efficiency is accompanied by a greater amount of descaling, but heat loss also increases due to a higher volume of exhaust gas. This leads to a decrease in thermal efficiency and a rise in production costs. Therefore, the air ratio should preferably be 1.5 or less. The air ratio may vary depending on the position in the heating furnace or changes over time during the heating period. It is preferable that the maximum air ratio at each position in the heating furnace during the heating period be 1.5 or less, as this is the ideal ratio for heating the steel. Furthermore, if the retention time when the surface temperature of the steel part is 1,100°C or more is less than 60 minutes, the inclusion does not grow and a layer with sufficient Ni concentration cannot form at the interface between the inclusion and the base metal sheet. It is not preferable for the holding time to exceed 240 minutes, as this increases the amount of scaling and therefore reduces performance. Furthermore, the surface layer of the steel sheet becomes decarburized, and there is concern that the steel sheet's characteristics may deteriorate. The extraction temperature must be 1180°C or higher to ensure the surface temperature of the steel part during the descaling step (cu ηοηη / ζζηζ / E / γίΛΐ) performed after the heating step. If the time interval between the heating and descaling steps is long, the extraction temperature can be raised to 1200°C or higher to ensure the surface temperature of the steel part. In this mode, the extraction temperature is lower between a temperature calculated at a position 5 mm away from the upper surface of the steel piece in the direction of the thickness of the steel piece and a temperature calculated at a position 5 mm away from the lower surface of the steel piece in the direction of the thickness of the steel piece in a case where the calculation of heat transfer is performed by dividing the steel piece in the direction of the thickness by the atmospheric temperature of the heating furnace.

[44] In the descaling step, the steel part, which has a surface temperature of 1,170°C or higher, is descaled at least once with an injection pressure of 5 to 50 MPa. In addition, the surface temperature of the steel part is maintained at 100°C or higher for 20 to 240 seconds after the descaling is completed.

[45] In the descaling step, the scale layer formed up to the heating step is removed. The scale layer exists in a state where an iron oxide and oxides of other elements are mixed. The scale layer is generally in a molten state at temperatures of 1,170°C or higher, but it is solidified and firm at temperatures below 1,170°C and is therefore difficult to remove by descaling. In particular, if the scale contains silicon, an oxide composed of Fe₂SiO₄ exists simultaneously with the iron oxide, enters between the iron oxides, and thus forms a firm scale after solidification. Therefore, in the method for manufacturing hot-rolled steel sheet according to this method, descaling is carried out at least once at a temperature of 1,170°C or higher.However, if the injection pressure during descaling is less than 5 MPa, the scale cannot be sufficiently removed. Furthermore, if the injection pressure during descaling exceeds 50 MPa, concentrated nickel near the interface is also removed during heating. Therefore, the injection pressure should be between 5 and 50 MPa. Descaling is preferably performed with an injection force of 50 to 700 MN / (ms) per unit time and unit width. The injection force per unit time and unit width is obtained from the product of a descaling pressure (MPa), a descaling time (seconds), and a sheet length (m) of the steel sheet being descaled.

[46] After descaling, the surface temperature of the steel part is maintained at 100°C or higher for 20 to 240 seconds after the descaling process is complete (primary descaling). By maintaining the temperature at 100°C or higher for 20 seconds or more, the surface of the steel sheet oxidizes again, and the Ni becomes even more concentrated at the interface. If the holding time at 100°C or higher is less than 20 seconds, the Ni concentration is insufficient. Therefore, the holding time should be 20 seconds or more. Ideally, the holding time should be 30 seconds or more. If the maintenance time after descaling exceeds 240 seconds, the scale thickness increases. Consequently, chemical convertibility and productivity decrease. Therefore, the maintenance time should be 240 seconds or less. The waiting time should preferably be 180 seconds or less. After the surface temperature of the steel piece is maintained at 100°C or more, the steel piece is rolled. After the surface temperature of the steel part has been maintained at 100°C or higher for 20 to 240 seconds following the completion of the descaling process, secondary descaling can be performed one or more times on the steel part in addition to the initial descaling (primary descaling). Secondary descaling removes the scale layer formed during maintenance. However, even if secondary descaling is performed in such a way that concentrated nickel is not removed, the injection pressure is 5 to 50 MPa, as in primary descaling. The surface temperature of the steel part before secondary descaling can be at or above 170°C, or below 170°C. The time to maintain the surface temperature of the steel part at 100°C or more from the completion of secondary descaling can be between 20 and 240 seconds, or it can be less than 20 seconds. If the time required to maintain the surface temperature of the steel part at 100°C or higher after the completion of secondary descaling exceeds 240 seconds, the scale thickness increases. Consequently, chemical convertibility and productivity decrease. As described above, with respect to secondary descaling, the temperature before descaling and the time to maintain the temperature at 100°C or higher after descaling are not limited. However, if secondary descaling is performed one or more times with a surface temperature of the steel part of 1170°C or higher, and the surface temperature of the steel part is maintained at 100°C or higher for 20 to 240 seconds after the completion of secondary descaling, the time to maintain the surface temperature of the steel part at 100°C or higher from the completion of primary descaling may be 20 seconds. In this way, regardless of whether only primary descaling is performed or both primary and secondary descaling are carried out, the surface temperature of the steel part can be maintained at 100°C or higher for a total of 20 seconds or more after the descaling process is complete. Given these characteristics, if multiple descaling processes followed by maintenance at 100°C or higher are performed, the maintenance time for any one or more maintenance cycles is preferably 20 seconds or more.

[47] cu nonn / zznz / E / YiAi Hot rolling pass The hot rolling conditions in the hot rolling step following the descaling step are not particularly restricted. These conditions can be adjusted according to the sheet thickness and the required mechanical properties. There are no restrictions on the cooling conditions after rolling. The steel part can be cooled to room temperature (down to 100°C or less). Alternatively, it can be coiled without cooling and air-cooled in a coiled state.

[48] ​​According to the above manufacturing method, it is possible to manufacture hot-rolled steel sheet in accordance with this method. cu ηοηη / ζζηζ / Ε / γίΛΐ Examples

[49] Hereafter, the present invention will be described in more detail with examples, but is not limited to these examples.

[50] Plates having a chemical composition shown in Tables 1A to 1C were heated under the heating conditions shown in Tables 2A to 2C, and were descaled under the descaling conditions shown in Tables 2A to 2C. Under the heating conditions, as described above, combustion control was carried out so that the minimum value of the air ratio at each position in a heating furnace and the maximum value of the air ratio at each position in the heating furnace were those shown in Tables 2A to 2C. As a descaling condition, primary descaling was only performed on steels 2, 34, 42 to 72, 75 to 82, and 86. Tables 2A to 2C show the surface temperature of the steel part before primary descaling, and the injection pressure and force per unit time and unit width during primary descaling. Additionally, the time required to maintain the surface temperature of the steel part at 100°C or higher after primary descaling is shown as a condition in Tables 2A to 2C. The minimum surface temperature of the steel part during the period from the completion of primary descaling until the steel part is rolled is described in Tables 2A to 2C. Steels 1, 3 to 33, 35 to 41, 73, 74, 83 to 85, 87, and 88 were subjected to primary descaling and then to secondary descaling. Tables 2A to 2C show the surface temperature of the steel part before primary descaling, the injection pressure and force per unit time and unit width in primary descaling, and the injection pressure and force per unit time and unit width in secondary descaling (for those subjected to secondary descaling, the pressure is described in the secondary descaling pressure column).In the case of secondary descaling, a longer time to maintain the surface temperature of the steel part after completion of primary descaling at 100°C or higher, and the time to maintain the surface temperature of the steel part after completion of secondary descaling at 100°C or higher, and a total holding time at 100°C or higher after descaling are shown as conditions in Tables 2A to 2C. Furthermore, with respect to descaling where the surface temperature of the steel part after completion of descaling was maintained at 100°C or higher for a longer time outside of primary and secondary descaling, the minimum surface temperature of the steel part during the period from the completion of the above descaling until the rolling of the steel part is described in Tables 2A to 2C. After descaling, finish rolling was performed at a finish rolling temperature set to 800°C or higher. After hot finish rolling, some of the steels were cooled to 100°C or lower, and others were coiled without cooling and air-cooled in a coiled state. cu nonn / zznz / E / YiAi

[51] Table IA cu ηοηη / ζζηζ / Ε / γίΛΐ Table IB CM iD Y Ni / Cu | o | 00Ί (si δ (XI O 42.00 | 66.00 | 24.50 1 oo íxí 2.00 | 2.00 | 2.00 | ο ο ίχί 2.00 | 2.00 | 2.00 I | οοζ 2.00 | 3.00 I 3.00 | 3.00 1 3.00 | 3.00 | 3.00 | 3.00 | 3.00 | 3.00 | I οοζ ο ο (XI (% en I masa) | Si + Al | δ 0.48 | in 0.29 | o o. o Csl o (xi 0.87 | 0.87 | 0.87 | 0.87 | 0.87 | 0.87 | 0.87 | 0.87 | 0.87 | 0.87 | 0.28 | CO (XI δ 0.28 1 0.28 | 0.28 | C0 (XI δ 0.28 | 0.28 | CO (XI δ C0 (XI δ 0.23 1 0.87 | 0.87 | Chemical composition (% in mass, the rest Fe and impurities) | c • - 5 o • • rij O 1 0.0021 | I 0.0025 | | REM | • • o 0.0035 | 0.0020 | o Σ 3 m • • • • • Σ • ώ 0.03 | • • δ ó δ ó δ o δ O δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ δ | 0.02 I δ 1 0.02 1 0.02 | | 0.02 I (XI ο δ 0.02 | | 0.02 | (XI δ (XI ο δ δ δ δ δ oo O o δ ο δ δ δ δ δ δ δ δ δ δ δ δ δ δ ο > z | 0.010 | | oloo | | 010Ο | | 0.012 II 0.019 | 1 0.019 1 0.019 1 | 0.019 | 0.019 I 1 0.019 I 1 0.019 | 0.019 | 1 0.019 I 0.019 z | 0.06 I 1 0.04 II 0.84 II 0.66 | Ζ0Ο I 0.02 II 0.02 | | 0.02 I (XI 1 0.02 I I 0.02 | | 0.02 I 0.02 I 0.02 I | 0.06 | 0.06 1 0.06 1 90'0 | 0.06 I 1 0.06 I 0.06 | 0.06 | 1 0.06 I 0.06 ο 1 0.02 I I 0.02 I z I 0.0020 | 1 0.0037 | | 0.0028 I 1 0.0020 | I 0.0034 | I 0.0052 | I 0.0032 I 0.0026 | | 0.0026 | I 0.0026 | | 0.0026 I 1 0.0026 | | 0.0026 | I 0.0026 | | 0.0026 I | 0.0026 I I 0.0026 | | 0.0055 I 0.0055 | 1 0.0055 1 | SS000 | 0.0055 I 1 0.0055 | 0.0055 | 1 0.0055 | 1 0.0055 | 0.0055 | 1 0.0028 1 1 0.0026 | I 0.0026 | < I 0.332 | 1 0.050 | | 0.036 | 1 0.282 | | 0.024 | I 0.256 | I 0.715 I I 0.030 | I 0.030 | I 0.030 | | 0.030 | 1 0.030 | 1 0.030 | I 0.030 | | 0.030 | | 0.030 I I 0.030 | | 0.268 | 0.268 | 1 0.268 1 0.268 | | 0.268 I 1 0.268 | 0.268 | | 0.268 | 1 0.268 | 0.268 | 1 0.026 1 | 0.030 | I 0.030 | ΙΛ | ιοοΌ | 1 0.007 | | 0.003 | 1 0.002 | | 0.009 | | 100'0 | I 0.007 I 0.012 | I 0.012 | I 0.012 | | 0.012 | 1 0.012 | 1 0.012 | I 0.012 | | 0.012 | | 0.012 I I 0.012 | | 0.006 | 0.006 | 1 0.006 1 0.006 | | 0.006 | | 0.006 | 0.006 | | 0.006 | | 0.006 | 0.006 | 0.003 1 | 0.012 | I 0.012 | 0- 0.008 I | 010Ό 0.007 | 0.013 I | ειο o 0.008 | 0.016 I | 010Ό | 010Ό | οιοο I οιοο | οιοο | οιοο | οιοο I οιοο | οιοο | οιοο | οιοο | οιοο 0.008 | 8000 0.008 1 8000 | 8000 0.008 | 800'0 0.008 | 0.008 | 0.008 1 0Ζ0Ό | 010Ό o.οιο | c Σ <n (XI 5 2 in ιη ιη ιη ιη ιη ιη ιη ιη (Ό Μ $ en $ rn ιη ιη ιη δ δ (XI o 5 o 00 o· I 0.84 | | *78 0 I 1 0.84 | | 0.84 I 1 0.84 | I 0.84 | I 0.84 | | 0.84 I | 0.84 I I 0.84 | δ Ο δ Q δ δ Q δ δ δ δ Q δ 1 0.20 1 | <78'0 | 1 0.84 | ω | 90'0 1 O· ó | 0.07 | I 0.05 I I 0.09 I 1 0.04 | δ I 0.05 I I 0.05 I 1 0.05 | | 0.05 I 1 0.05 I I 0.05 I I 0.05 | | 0.05 I | 0.05 I I 0.05 I | 90'0 1 0.06 1 0.06 1 0.06 | 0.06 I | 90'0 1 90'0 1 0.06 I 1 0.06 I 90'0 1 60'0 1 I 0.05 I 1 0.05 I Acero m CN en en en ID co Ό en £ co en £ o 5 Μ 5 ιη 5 5 5 ο ιη (XI ιη (Ό ιη ιη ιη ιη LD C0 ιη £ ο. cu נון / זחיז / ע / ילי

[53] Table 1C -7*Ni / Cu | 3.00 | 3.00 | 4.00 | 3.00 | O p \ \ \ \ \ I 00'9 | 3.00 | 1 3.00 1 3.00 | 3.00 | 3.00 I 3.00 | 3.00 | 3.00 | 2.00 | 6.00 | o 2.°0 o 3.00 | 2.00 o tn cTn cu w v_o <O -E < i / i 0.28 | 0.28 | 0.53 | lo cu m 0.23 | LO OI O 0.28 | 0.87 | 0.23 | p o I 3.06 I 0.28 | I 0.07 1 LO O co ó 0.28 I co OI o co o 0.28 | 2 0.87 | 0.87 | LO O 0.28 | 0.87 | o· LO O Composición química (% en masa, el resto Fe e impurezas) | c en 0.1112 | 0.2221 | «o 0.3821 | rb 0.4200 O - - REM | • - o • σ> Z 5 • • • • m 0.0017 | o Z • ώ • • □ 0.02 | 0.02 | po 0.02 | po 0.02 | po O o OI o ó 0.02 I 0.02 | 0.02 | 0.02 | 0.02 | ooooc > ó oo 0.02 | po 0.06 | i— o ó O ooo ó o ó ó ó o ó > • z 0.019 0.019 0.018 0.019 • 610'0 0.019 1 0.019 0.019 610Ό 0.019 0.018 0.019 z 0.06 | | 900 0.04 | 0.04 | 0.06 | 0.04 | 0.02 | | 90Ό 0.02 | 0.04 | 0.04 | I 90Ό | 0.06 | I 0.03 1 0.04 | I 90Ό I 90Ό | 90Ό 1 900 0.06 | 0.04 | 0.06 | 0.02 | OI oo CM OO 0.06 | 0.02 | 0.09 | z 0.0055 0.0055 0.0032 0.0032 0.0028 0.0028 0.0046 0.0051 0.0034 0.0028 0.0032 I 0.0030 1 0.0055 I 0.0016 1 0.0055 0.0055 0.0055 0.0055 0.0055 0.0055 0.0025 0.0031 0.0026 0.0028 0.0046 0.0055 9Ζ00Ό 0.0014 < 0.268 0.268 0.040 0.030 0.036 0.026 0.239 0.268 0.030 0.026 0.036 0.268 I 0.060 I 0.030 0.268 co CSI 0.268 0.268 0.268 0.035 1.050 0.030 0.031 0.239 0.268 0.030 0.024 σι 0.006 0.006 0.003 0.003 0.003 0.003 0.008 0.008 0.012 0.003 0.003 0.005 I 0.006 0.006 1 0.001 0.006 0.006 0.006 0.006 0.006 0.006 0.003 0.012 900'0 0.008 0.006 0.012 0.012 CL 0.008 | 0.008 | 0.012 | 0.012 | 0.007 | 0.020 | 0.008 | | 800'0 0.002 | 0.020 | 0.012 | I 0.006 I 0.008 | 1 800'0 | 0.008 | 0.008 | 0.008 I| 800'0 | 800'0 | 800'0 0.008 | 0.014 | | yes | yes 0.008 | 0.008 | | oioo co oo Z £ in ρ LO pp lo ñ 3 lo lo OI p CM OI p OI LO o LO o S 2.00 | in LO in £ i / ioooo ó 0.48 tsi 0.20 oooo 0.84 0.20 CO Ó 1S2 oooo 0.72 OO oooooooo en 0.05 0.84 0.84 oooo 0.84 LO ω 1 90Ό 0.06 | 0.08 | I 80'0 0.07 | To 60'0 0.03 | 1 90'0 1 90'0 0.09 | 1 80'0 0.23 I 0.06 | 0.05 I 0.06 | 0.06 | 0.06 I 0.06 | 1 90'0 I 90'0 0.07 | oo 0.05 | | 90'0 0.03 | 1 90Ό 0.05 | 0.05 | Acero 2 2 3 ID 2 2 co o £ LO C-* £ co o co E CM CO 2 ω ir» co o co £ co co. cu ηοηη / ζζηζ / Ε / γίΛΐ

[54] Table 2A Manufacturing method | Inlaid conditions | Minimum surface temperature of the steel part during the time period from inlaid to rolling (°C) g AAE = 3 £ § * 2 í£ 2 ® in 2 2 É § 9 A = £ i Total time to maintain surface temperature at 1,100°C or higher after completion of inlaid (sec) g R 2 TO in s S 3 ® CO un 2 2 £ 2 2 s co un 5 £ 2 íí 2 Longest time to maintain surface temperature at 1,100°C or higher after completing descaling (sec) in F £ g S 2 Ώ s 2 co 4 2 un un 2 en 2 2 2 2 2 5 co 4 co ex 2 « rsi Injection force during secondary descaling (MN / (ms)) eo 9 3 o co <N S s i S TO 3 co CN = — § K - 2 s Ξ X g θ s Presión de dcsincrustación secundaria (MPa) un in un un in un un un un un un un un un un un un un un un un un un 12 Fuerza de inyección durante la dcsincrustación primaria (MN / (ms)) 3 2 2 § 2 9 8 co -4 3 = 2 5 i S ® en | K CO g £ 3 S eo a 2 4 2 $ Presión de la dcsincrustación primaria (MPa) ir, un un in in un un in un un un un un un un un un un un un un un un un un un 12 Temperatura de la superficie de la pieza ce acero antes de la dcsincrustación primaria (’C) s 5 A 9 g £ 9 s 9 £ TO TO 8 g 8 £ 3 8 eo a 8 8 8 3 2 CO »· 8 g 8 o o O u Temperatura de extracción (’C) 2 1268 | O eo £ S to ® co un TO 1268 | A g 8 A 8 1248 | 2 2 X 8 2 3 8 g Tiempo de mantenimiento a 1,000'C or greater (min) £ CO £ 9 o~ 9 9 3 t 9 B g 2 £ o 2 s 2 2 9 2 2 = in TO A Maximum value of air ratio (-) (-) - A to A 2 2 o to AAA where A o in A 4 - in 1« * s = 2 A 2 £ 2 £ £ o 2 2 2 o,? cu ηοηη / ζζηζ / Ε / γίΛΐ

[55] Table 2B Manufacturing method | Descaling conditions | Minimum surface temperature of the steel part during the time period from descaling to rolling (°C) 5 *2 £ 5 2 = rgg = 2 g 2 s 2 = g = 2 g 1 2 Total time to maintain surface temperature at 1,100°C or higher after completion of descaling (sec) ES ssso >2 3 so & = = s co sss = ams? 3 Longest time to maintain surface temperature at 1,100°C or higher after completion of descaling (sec) s 5 s £ gggggs 3 sg § as S 3 s Pf £ mgs Injection force during secondary descaling (MN / (ms)| § S g £ § 2 sgs Secondary descaling pressure (MPa) £ un in in in un ir,Injection force during primary descaling (MN / (ms)) 5 5 2 5 5 £ £ g = g CO co gs £ » ñ co £ s = 2 Í2 Primary descaling pressure (MPa) (2 in in in in £ 9 2 g £ in in £ mi sa £ in in £ Surface temperature of the steel part before primary descaling (“O £ 2 3 £ £ 2 g 2 § aa co i £ 9 g £ 2 2 U6i § a co i £ s £ £ £ o S c 9 ω Extraction temperature ('C) £ >224 1262 1238 £ £ 0731 ggos £ g 3 g £ S 1260 g »· 2 Yes § gg Holding time at 1,000*0 or higher (min) 2 = 2 9 £ S £ g £ a 9 9 £ £ 5 SI ts a !n £ 2 £ g £ g Maximum air ratio value (-) □ 9 9 9 9 9 a 9 o Cl c M in 9 9 9 o in c, in in e. in 9 Minimum air ratio value (-) - - - a - 9 - - - 3 - 9 - - = - - ® - - og $ $ $ * ggg Yes g 3 g * $ * 2 S ñ 2 s 2 sss, cu ηοηη / ζζηζ / Ε / γίΛΐ

[56] Table 2C Minimum surface temperature of the steel part during the time period from descaling to rolling ('C) £ s 5 S 3 5 9 2 ® 2 Ξ 2 = = S 1204 | = Total time to maintain surface temperature at 1,100'C or higher after completing descaling (sec) g 5 s 5 sososgo ír 3 £ CM CO ss 5 s <D Tiempo más largo para mantener la temperatura de superficie a 1,1 DO’C o mayor después de completar la desincrustación (seg) s ñ £ £ ® * ΓΜ m $ s £ 2 a Condic Fuerza de inyección durante la desincrustación secundaria (MN / (ms)) s S S 5 2 2 •o Q Presión de dcsincrustación secundaria (MPa) £ 12 i SI 12 2 2 2 O o •Q Fuerza de inyección durante la dcsincrustación primaria (MN / (ms)) § g: co o s S co s ® § ξ S ξ CO O i 5 co g: K s S S 2 s 2 o « ίο Σ ’ Sí Λ “ ¡ñ £ ú.s ¿ - 2 2 2 2 2 2 2 2 2 2 2 2 2 2 = = 2 2 a 12 12 12 2 2 = = Temperature of the surface of the maple stone before primary desincrustation (°C) co co s 1247 | § S i S w CM « £ s K 5 s ® 2 £ ra de ('C) Extracción temperature a £ * ΙΛ ia K s CO £ S s E Time of maintenance at 1,000'C or higher (min) aa 2 i 2 ® S ¢0 <N s? ® ® ® s Ξ S a 3 Condición' Valor máximo de la relación de aire (-) π - - - - -- - A - - - - - ‘Ί - - 2 Valor mínimo de la relación de aire (-) 2 5 - - - 2! 2í □ s 2! 2? = 2í 1 2 2 5 ® 5 2 «2 £ * 2 S 2 3 2 £ S. cu ηοηη / ζζηζ / Ε / γίΛΐ

[57] The hot-rolled steel sheet obtained was pickled under conditions of 30 to 60 seconds using a 1 to 10 wt% (wt%) hydrochloric acid solution at a temperature of 20°C to 95°C, and the element-centered ERMA analysis having an atomic number equal to or greater than that of B was performed on a 250 pm × 250 pm region on the surface after pickling in a 1 pm measurement step under the above conditions to obtain the percentage of measurement points having a Ni content of 0.5% by mass or more and the percentage of measurement points having an oxygen content of 0.5% by mass or more when the total mass of the element having an atomic number equal to or greater than that of B was 100%, and to obtain the average interval between regions having a Ni content of 0.5% by mass or more. The results are shown in the surface structure column in Tables 3A to 3C. In Tables 3A to 3C, an average interval of < 1 (pm) between regions where the Ni content of the pickled surface is 0.5% by mass or more shows that the average interval is smaller than the measurement step and cannot be measured.

[58] In addition, the tensile strength of the hot-rolled steel sheet obtained was evaluated. Tensile strength (TS) was measured in accordance with JIS Z 2241: 2011 using a JIS Z Test Piece No. 5 2241: 2011 collected with a direction (sheet width direction) orthogonal to the rolling direction as a longitudinal direction, at a position either W / 4 or 3W / 4 away from one end of the steel sheet in the sheet width direction where W is the sheet width. The result of the tensile strength (TS) is shown in Tables 3A to 3C together with the thickness of the hot-rolled steel sheet.

[59] Furthermore, the hot-rolled steel sheet obtained was pickled under the pickling conditions described above. Then, under the following conditions, assuming that a chemical conversion treatment fluid deteriorated due to continuous use or similar factors, a chemical conversion treatment was performed on the pickled hot-rolled steel sheet as described above, and the chemical convertibility was evaluated. The effects on the present steel sheet can be shown independently of a zinc phosphate-based chemical conversion treatment fluid, and, for example, the evaluation was carried out under the following conditions. (1) Degreasing Treatment: Chemical product manufactured by Nippon Paint Holdings Co. Ltd.: SD400 Chemical product temperature: 42 Chemical spray time on the surface of the test piece: 120 seconds (2) Surface adjustment treatment: Chemical product manufactured by Nippon Paint Holdings Co., Ltd.: 5N-10 Chemical immersion time: 20 seconds (3) Chemical conversion treatment: Chemical product manufactured by Nippon Paint Holdings Co., Ltd.: SURFDINE DP4000 Chemical product temperature (chemical conversion bath temperature): 35°C Bathing time: 60 seconds Free Acidity: 0.5 pt Total Acidity (TA): 25 pt Accelerator: 2.0 points (4) Water wash treatment: City water (sprinkler) City water temperature: 25°C Wash time with water: 30 seconds (5) Wash treatment with pure water: Deionized water (spray) Temperature of deionized water: 25°C Washing time with pure water: 30 seconds Here, free acidity is defined such that if 3 drops of bromophenol blue are added to 10 mL of a chemical conversion treatment liquid and a neutralization titration is carried out using 0.1 N sodium hydroxide until the color changes from yellowish-green to bluish-green, 1 mL of the 0.1 N sodium hydroxide required for this case is indicated as 1 pt. Furthermore, total acidity is defined such that if 3 drops of phenolphthalein are added to 10 mL of a chemical conversion treatment liquid and a neutralization titration is carried out using 0.1 N sodium hydroxide until the color changes from colorless to pink, 1 mL of the 0.1 N sodium hydroxide required for this case is indicated as 1 pt. The chemical convertibility improvement effect on the present steel sheet can also occur with chemical conversion treatment fluids from other model numbers or other companies, regardless of whether the chemical conversion treatment fluid is used under the chemical conversion treatment conditions shown above. In a case where no transparency was observed and the size of the chemical conversion crystals was 10 µm or less as a result of the chemical conversion treatment, the steel sheet was considered to have excellent chemical convertibility. This is because: regardless of whether the chemical conversion treatment was performed, the adhesion between the steel sheet and the lacquer is reduced when the base metal sheet is exposed, i.e., transparency is generated; and the coating's adhesion is reduced due to the cohesive fracture of the zinc phosphate film itself if the size of the chemical conversion crystals after the chemical conversion treatment is greater than 10 µm. If the size of the chemical conversion crystals is 10 pm or less, the adhesion between the lacquer and the steel sheet and the corrosion resistance after peeling the coating film are improved, and if the size of the chemical conversion crystals is 5 pm or less, the adhesion between the lacquer and the steel sheet and the corrosion resistance after peeling the coating film are further improved.In the examples, steel sheets in which no transparency was generated and the size of the chemical conversion crystals was 5 pm or less were evaluated as A (Examples of the invention), steel sheets in which no transparency was generated and the size of the chemical conversion crystals was greater than 5 pm and equal to or less than 10 pm were evaluated as B (Examples of the invention), and steel sheets in which transparency was generated or the size of the chemical conversion crystals was greater than 10 pm even without generating transparency were evaluated as C (Comparative examples).

[60] Although not shown in the tables, prior to the evaluation of chemical convertibility, the ERMA analysis directed to an element having an atomic number equal to or greater than that of B was carried out in a measurement step of 1 pm in a rectangular region of 10 pm in the direction of the sheet thickness from the surface of the steel sheet χ 500 pm in the direction of the sheet width in a cross section of the hot-rolled steel sheet subjected to the chemical conversion treatment in the direction of the sheet thickness to obtain the percentage of measurement points that had a Ni content of 0.5% by mass or more when the total mass of the element having an atomic number equal to or greater than that of B was 100%, and the result of the same was the same as the percentage of measurement points that had a Ni content of 0.5% by mass or more, measured in a 250 pm x 250 pm region on the surface after pickling and before chemical conversion treatment.

[61] A SEM was used to observe transparency. Specifically, the presence or absence of transparency was investigated by confirming whether there is a surface where the base metal sheet is exposed in a 250 pm × 250 pm region in each of the three fields of view on both sides of the steel sheet after the chemical conversion treatment by SEM. Similarly, with respect to the size of the chemical conversion crystals, the grain diameters (diameters) of the chemical conversion crystals were obtained in a 250 pm * 250 pm region in the SEM observation performed as described above, and the average of the grain diameters (diameters) of chemical conversion crystals was defined as the size of the chemical conversion crystals. The worst result among those obtained in 6 visual fields observed is shown in property column 10 of the product treated by chemical conversion in Tables 3A to 3C. Table 3A σι Observations Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Comparative example | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Comparative example | Evaluation « < < < < < < < < < < < < < < < < < < < < < o < < < < < < < o Properties of product treated by chemical conversion Chemical conversion crystal sizes (µm) in in in CM un cu in in.Presence or absence of transparency Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence Absence TS (MPa) O co co 2 o 5 en o co 2 un 00 co un $ cu un co Ό co r- 5 $ s 2 Steel thickness (mm) N a C4 en sq esi £ CM cu o DO -o co co aa C^ cu a = | Surface structure | Average interval between regions where the Ni content is 0.5% by mass or more (um) or a Ό oo - a or a -o CO a or Ό co Ό a a o· co Percentage of measurement points at which the oxygen content of the pickled surface is 0.5% by mass or more (%) a in £ 2 ooaso Csl a Ό a or CU o <5 in θ' cu cu co CU •o CU <2 cu s cu Percentage of measurement points at which the Ni content of the pickled surface is 0.5% by mass or more (%) CM s <2 co CJ 5 ss CU CO £ •O co en s un en o un co un O un en co Steel - CO -o co o 2 2 2 un 2 2: o F5 CU s¡ K £ co a. with ηοηη / ζζηζ / Ε / γίΛΐ Table 3B in LO Observations Comparative Example | Comparative Example | Comparative Example | Comparative Example | Comparative Example | Comparative Example | Comparative Example | Comparative Example | Comparative Example | Comparative Example | Comparative Example | Comparative Example | Example of the Invention | Example of the Invention 1 Comparative Example | Comparative Example | Comparative Example | Comparative Example | Comparative Example | Comparative Example | Comparative Example 1 Comparative Example | Example of the Invention | Example of the Invention | Comparative Example | Comparative Example | Comparative Example | Comparative Example | Example of the Invention | Example of the Invention | Evaluation oooooo < < ooo < < o < < Properties of product treated by conversion | chemical | Chemical conversion crystal sizes (um) or co 2 £ - £2 == $2 - un un O.£2 2 £2 a a or in in 2 - - - a Presence or absence of transparency Presence | Presence | Absence | Absence | Presence | Presence | Presence | Presence | Presence | Presence | Presence | Presence | Absence | 1 Absence 1 Presence | Presence | Presence | Presence | Presence | Presence | I Presence 1 Presence | Absence | Absence | Presence | Presence | Presence | Absence | Absence | Absence | TS (MPa) 543 2 2 in ID co sos 2 CSI co a 542 is a 9¿S co a or a 3 a § or o- CSI a a a a is a a co a co a co a 495 542 is a Steel thickness (mm) N <sj a CSÍ en es CSÍ CM es CN es es co co co co co co Ό co = co Estructura de superficie Intervalo promedio entre regiones donde el contenido de Ni es 0.5% by mass or more (· / .) 2 so O un VI CSI VI <2 o co O - aas un r- E rt un un s S2 E S2 rt rt percentage of measurement points at which the oxygen content of the pickled surface is 0.5% by mass or more (%) O en co P s co O CSI s £ CSI 3 £ s 3 a es 3 s S o co CSI percentage of measurement points at which the Ni content of the pickled surface is 0.5% by mass or more (· / .) O. oa un a um ol OH CO SI s rt rt rt cd col μ sa rt o. col col = Steel m co un ín όση O 5 CSI 5 un 5 5 co 5 o un uñ CSI un co un un un un s uñ co un. cu ηοηη / ζζηζ / Ε / γίΛΐ

[64] Table 3C Observations Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention 1 Example of the invention | Example of the invention | Example of the invention | Comparative example | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Example of the invention | Comparative example | Comparative example | Example of the invention | Comparative example | Example of the invention | Example of the invention | Example of the invention 1 Example of the invention | Example of the invention | Evaluation < < < < < < < < « CO co < ​​< < « « CO moo co o co < ​​< < < Properties of product treated by conversion | chemical | Chemical conversion crystal sizes (µm) in tN co in co o- o in co in ID co - - m O co •o in Presence or absence of transparencyAbsence | Absence | Absence | Absence | Absence | Absence | Absence | 1 Absence 1 Absence | Absence | Absence | Presence | Absence | Absence | Absence | Absence | Absence | Absence | Absence | Absence | Presence | Presence | Absence | Absence | Absence | Absence | 1 Absence | Absence | TS (MPa) 2 Z9S 492 S £ § 469 486 | or 782 | 402 Z62 or co or co omo Ό CO Ό s £ £ o co o co m Steel thickness (mm) 2 2 N 3 co co o» Coi in 3 o in A in Surface structure Average interval between regions where the Ni content is 0.5% by mass or more (µm) m pl - 2 O - Ό Csj p 0 - » oo» Percentage of demethylation points at which the oxygen content of the pickled surface is 0.5% by mass or more (%) 00 CM mo S2 co in 2 θ' Oí K¡ ir> - - mm Oí co O. m CM CN s 2 o in Percentage of measurement points at which the Ni content of the pickled surface is 0.5% by mass or more (%) Ξ 00 CN in Ό £ 00 CM - s in Ό S cN OI Si s »· in Steel 3 2 5 5 £ £ P £ little co £ £ 2 co £ £ co co cu ηοηη / ζζηζ / Ε / γίΛΐ

[65] As shown in Tables 1A to 1C and 3A to 3C, in all Examples of the invention having a chemical composition within the range of the present invention, in which the percentage of measurement points in which the surface Ni content was 0.5% by mass or more was from 10% to 70%, no transparency was generated, the size of the chemically converted crystals was 10 pm or less, and the chemical convertibility was excellent. In the comparative examples where one or more of the chemical compositions and the percentage of measurement points where the surface Ni content was 0.5% by mass or more was outside the range of the present invention, transparency was generated or the size of the chemical conversion crystals was greater than 10 pm, so the chemical convertibility was not sufficient. cu ηοηη / ζζηζ / Ε / γίΛΐ Industrial applicability

[66] According to the present invention, it is possible to obtain a hot-rolled steel sheet having excellent chemical convertibility and a method for manufacturing the hot-rolled steel sheet. In the hot-rolled steel sheet of the present invention, even if the conditions of a chemical conversion treatment vary, a good chemical conversion film can be obtained. Consequently, the present invention has high industrial applicability. Brief description of the reference symbols

[67] 1: base metal sheet 2: inlay 3: Oxides of Si, Al and the like, or concentrated layers of Mn, Cu and the like 4: concentrated Ni layer

Claims

1. A hot-rolled steel sheet comprising, as chemical composition, in % by mass: C: 0.01% to 0.30%; Si: 0.01% to 3.00%; Mn: 0.20% to 3.00%; P: 0.030% or less; S: 0.030% or less; Al: 0.001% to 2.000%; N: 0.0100% or less; Ni: 0.02% to 0.50%; Nb: 0% to 0.060%; V: 0% to 0.20%; Ti: 0% to 0.20%; Cu: 0% to 0.20%; Cr: 0% to 0.20%; Mo: 0% to 1.00%; B: 0% to 0.0020%; W: 0% to 0.50%; Mg: 0% to 0.010%; Ca: 0% to 0.0100%; REM: 0% to 0.0100%; O: 0% to 0.0100%; Zr: 0% to 0.500%; Co: 0% to 0.500%; Zn: 0% to 0.500%; Sn: 0% to 0.500%; and a remainder consisting of Fe and impurities, wherein among the measurement points at which elemental analysis is performed in a 1 pm measurement step using an ERMA in a 250 pm x 250 pm region on a surface, a percentage of measurement points having a Ni content of 0.5% by mass or more is 10% to 70%.

2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition contains one or two or more elements selected from the group consisting of Cu, Nb, V: 0.003% to 0.060%, 0.01% to 0.20%, Ti: 0.01% to 0.20%, Cu: 0.01% to 0.20%, Cr: 0.01% to 0.20%, Mo: 0.01% to 1.00%, B: 0.0005% to 0.0020%, W: 0.01% to 0.50%, Mg: 0.001% to 0.010%, Ca: 0.0010% to 0.0100%, REM 0.0010% to 0.0100%, OR: 0.0005% to 0.0100%.

3. The hot-rolled steel sheet according to claim 2, wherein the chemical composition contains Si: 0.50% to 3.00%.

4. The hot-rolled steel sheet according to claim 2, wherein the chemical composition contains Si: 0.01% or more and less than 0.50%, and Al: 0.050% to 2.000%.

5. The hot-rolled steel sheet according to claim 2, wherein the chemical composition contains Si: 0.01% or more and less than 0.50%, and Al: 0.001% or more and less than 0.050%, and a total of Si and Al is 0.50% or more and less than 0.55%.

6. The hot-rolled steel sheet according to any of claims 3 to 5, wherein among the measuring points at which elemental analysis is performed on the surface, a percentage of measuring points having an O content of 0.5% by mass or more is 30% or less.

7. The hot-rolled steel sheet according to any of claims 1 to 6, wherein the chemical composition contains Cu: 0.01% to 0.20%, and Ni / Cu is 0.50 or more.

8. The hot-rolled steel sheet according to any of claims 1 to 7, wherein, between the measuring points at which elemental analysis is performed on the surface, an average interval between the measuring points having a Ni content of 0.5% by mass or more is 3 to 10 pm.

9. The hot-rolled steel sheet according to any of claims 1 to 8, wherein the surface has an oil film that prevents oxidation.

10. The hot-rolled steel sheet according to any of claims 1 to 8, wherein the surface has a chemical conversion film.

11. A method for manufacturing a hot-rolled steel sheet comprising: heating a piece of steel having the chemical composition according to claim 1 or 2 in a heating furnace; descaling the heated steel piece; and hot-rolling the steel piece after pickling to obtain a hot-rolled steel sheet, wherein in the heating, after the surface temperature of the steel piece reaches 1100°C or more, the steel piece is held for 60 minutes or more in an atmosphere with an air ratio of 0.9 or more, and an extraction temperature of 1,180°C or 5 higher, and in descaling, the steel part whose surface temperature is equal to or greater than 170°C is descaled at least once with an injection pressure of 5 to 50 MPa, and 10 the surface temperature of the steel part is maintained at 100°C or more for 20 to 240 seconds after descaling is completed.