Hot-rolled steel sheet and method for manufacturing the same

Abstract

The purpose of the present invention is to provide: a hot-rolled steel sheet with excellent uniformity in scale adhesion and blackness in the width direction of a hot-rolled coil; and a method for producing the hot-rolled steel sheet.　The hot-rolled steel sheet has a predetermined chemical composition and has mill scale on the surface thereof. The mill scale in the width direction of the steel sheet has a composition comprising, in terms of area ratio: 30% or more of magnetite particles; 20-60% of an eutectoid transformation structure of iron and magnetite, the magnetite is composed of the magnetite particles and magnetite included in the eutectoid transformation structure; 20% or less of wustite; and, in terms of mass fraction, 5% or less hematite. The area ratio of the precipitated Fe on the surface of the mill scale is 15% or less, and the average thickness of the mill scale in the width direction of the steel sheet is 3-20 μm.

Description

[Technical Field]

[0001] This invention relates to a hot-rolled steel sheet having oxide scale on its surface, used in automobiles, home appliances, building materials, etc., and a method for manufacturing the same. In particular, this invention relates to a hot-rolled steel sheet that is suitable as a material for building materials, truck frames, woks, etc., has excellent blackness and scale adhesion, and exhibits small variation in blackness and scale adhesion in the width direction of the steel sheet, and a method for manufacturing the same. [Background technology]

[0002] Hot-rolled steel sheets are typically hot-rolled at high temperatures and in an oxidizing atmosphere, inevitably causing scale (iron oxide) to form on their surface. When hot-rolled steel sheets with this scale still attached (hereinafter referred to as "black scale hot-rolled steel sheets") are subjected to processes such as temper rolling, bending, press forming, or laser cutting, some of the scale peels off. This results in processing defects, contamination of the processing line, and surface defects in the processed product. To avoid such problems, there is a growing demand for hot-rolled steel sheets with excellent scale adhesion to the steel sheet surface, and this demand is becoming increasingly strong.

[0003] Furthermore, in some regions such as Southeast Asia where black is culturally favored, having a beautiful black color adds further value to products such as building materials. For this reason, there is a growing demand for black-scale hot-rolled steel sheets with superior blackness.

[0004] Furthermore, there is variation in scale adhesion and blackness in the width direction of the steel sheet, and if there are areas with poor adhesion or blackness, those parts must be excluded from use. In particular, the scale tends to whiten and have poor adhesion at the edges of coils of black hot-rolled steel sheets. Therefore, from the viewpoint of improving yield, there is a strong demand for hot-rolled steel sheets with uniformly excellent scale adhesion and blackness in the width direction of the coil.

[0005] Conventionally, various proposals have been made to improve scale adhesion. For example, Patent Document 1 describes a steel material having a composition of C: 0.01~0.3%, Si: 0.20% or less, Mn: 0.01~2.0%, P: 0.10% or less, S: 0.10% or less, Al: 0.10% or less, Cr: 0.01~2.0% by mass%, with the remainder being Fe and unavoidable impurities. This material is subjected to rough rolling, descaling, and finish rolling at a finish rolling exit temperature of 800~950°C, satisfying the following equation (1), with an average cooling rate of 3°C / s or more and 80°C from the end of finish rolling to the start of winding. A hot-rolled steel sheet with excellent scale adhesion has been proposed, characterized by having a magnetite layer from the base metal side, magnetite grains and / or a eutectoid transformation structure of iron and magnetite in the upper layer of the magnetite layer, the average particle size of the magnetite grains and / or the average block size of the eutectoid transformation structure being 3 μm or more and 8 μm or less, and the mass fraction of wustite contained in the scale layer being 10% or less. |T2-T1|≦50℃ and |T3-T2|≦50℃···(1) However, in equation (1) above, T1: Temperature (°C) of the steel sheet after finish rolling, 30 m from the leading edge in the longitudinal direction and at the center in the width direction. T2: Temperature (°C) at the center of the longitudinal and widthwise directions of the steel sheet after finish rolling. T3: This is the temperature (°C) of the steel sheet after finish rolling, located 30 m from the tail end in the longitudinal direction and at the center in the width direction.

[0006] Furthermore, Patent Document 2 proposes a method in which a slab containing, by mass%, C: 0.02~0.20%, Mn: 0.1~2.0%, Si: 0.3% or less, P: 0.03% or less, S: 0.03% or less, Ni: 0.03~0.3%, Cu: 0.04~0.5%, and Cr: 0.03~0.3%, with the remainder being Fe and unavoidable impurities, is heated to 1100°C or higher, then hot-rolled in a temperature range of 800°C to 950°C, and then rolled up at 400°C to 650°C. This method proposes obtaining a hot-rolled steel sheet with excellent tight-scale properties, characterized by a surface roughness of 0.5 μm or more per inch in length, as the number of irregularities at the interface between the steel sheet surface scale and the steel sheet base metal being 300 or more.

[0007] Furthermore, Patent Document 3 proposes a hot-rolled steel sheet having scale, characterized in that the scale in a portion within 30 mm from the end face of the coil has a magnetite layer at the interface between the base metal and the scale, with an area ratio of 90% or more in contact with the base metal, an iron-magnetite eutectoid layer above the magnetite layer in contact with the base metal, a magnetite layer above the iron-magnetite eutectoid layer, a hematite layer above the magnetite layer, the sum of the thickness of the magnetite layer above the iron-magnetite eutectoid layer and the hematite layer is 30% or less of the total thickness of the scale, and the difference between the thickness of the scale 30 mm from the end face of the coil and the thickness of the scale in the center of the coil is 2 μm or less.

[0008] Furthermore, Patent Document 4 proposes a black scale hot-rolled steel sheet with excellent blackness, characterized in that the hot-rolled steel sheet has a composition of C: 0.001~0.20 mass%, Si: 0.001~0.50 mass%, Mn: 0.05~2.0 mass%, P: 0.05 mass% or less, S: 0.05 mass% or less, and sol.Al: 0.01~0.10 mass%, with the remainder being Fe and unavoidable impurities, and has a scale on its surface with a thickness of more than 4 μm, the scale has a composition containing 50% or more Fe3O4 by volume, and does not contain precipitated Fe in a region at least 2 μm deep in the thickness direction from the scale surface.

[0009] In addition, Patent Document 5 proposes a black skin hot-rolled steel sheet including a base metal hot-rolled steel sheet and a scale on the surface of the base metal hot-rolled steel sheet, which consists of Fe3O4 and Fe and has a thickness of 3.0 to 20 μm. The average particle size in the surface layer of the scale is 3.0 μm or less, and the Fe area ratio in the cross section of the scale is less than 1.0% in the region of 0 to 1.0 μm in the thickness direction of the scale from the outermost surface layer of the scale. Further, it is 1.0% or more in the region of 0 to 1.0 μm in the thickness direction of the scale from the interface between the scale and the base metal hot-rolled steel sheet.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0011] In the technology described in Patent Document 1, a steel material having a predetermined component composition is used, and the finish rolling outlet side temperature during hot rolling, the cooling rate after rolling, and the coiling temperature are adjusted. Thereby, the average particle size of magnetite grains in the upper layer of the magnetite layer on the substrate side in the scale layer and / or the average block size of the eutectoid transformation structure of iron and magnetite are optimized. Furthermore, by controlling the temperature in the longitudinal direction of the steel sheet immediately after finish rolling, improvement in the adhesion of the scale uniformly in the longitudinal direction is achieved. However, no method for uniformly improving the adhesion of the scale in the width direction is mentioned. Also, no method for increasing the blackness degree of the scale is mentioned.

[0012] In the technology described in Patent Document 2, a steel added with predetermined amounts of Ni, Cu, and Cr is hot-rolled, and a hot-rolled steel sheet excellent in tight scale property is proposed by controlling the surface roughness of the steel sheet surface scale and the steel sheet substrate interface within a predetermined range. However, no method for increasing the blackness degree of the scale is mentioned. Furthermore, no method for uniformly improving the adhesion and blackness degree of the scale in the width direction is mentioned.

[0013] In the technology described in Patent Document 3, a manufacturing method of a hot-rolled steel sheet manufactured by hot-rolling a steel material and winding it in a coil shape is proposed. The roughly rolled steel sheet is subjected to finish rolling at 850 to 1050 °C, and then, while winding the finish-rolled hot-rolled steel sheet in a coil shape at a coiling temperature of 500 to 650 °C, both end faces of the hot-rolled steel sheet are cooled so that the temperature at the end face becomes 480 °C or lower within 5 minutes from the start of coiling. Thereafter, the temperature at the end face is maintained at 480 °C or lower, and then, while remaining in a coil shape, slow cooling is performed from the time when the temperature at the end face is 400 to 480 °C, whereby excellent scale adhesion in the width direction of the hot-rolled coil, particularly at the edge portion, is obtained. However, no method for uniformly improving the blackness degree in the width direction with respect to the hot-rolled coil is mentioned.

[0014] While the technologies described in Patent Documents 4 and 5 propose hot-rolled steel sheets with excellent blackness and scale adhesion, uniform scale adhesion and blackness cannot always be obtained in the width direction of the hot-rolled coil. In particular, there was a problem in that the scale turned white and scale adhesion deteriorated at the edges of the coil.

[0015] The present invention aims to solve the above problems and provide a hot-rolled steel sheet and a method for manufacturing the same that exhibit uniformly excellent scale adhesion and blackness in the width direction of the hot-rolled steel sheet. [Means for solving the problem]

[0016] The inventors first investigated the reasons why conventional hot-rolled steel sheets do not exhibit uniformly excellent scale adhesion and blackness in the width direction. Scale generated during hot rolling is formed at high temperatures in the following order from the scale surface: hematite (Fe2O3), magnetite (Fe3O4), and wustite (FeO). Of these, wustite undergoes eutectoid transformation during cooling after coiling, creating a eutectoid transformation structure consisting of magnetite and precipitated Fe (4FeO → Fe3O4 + Fe). This eutectoid transformation structure, consisting of magnetite and precipitated Fe, contributes to improved scale adhesion because of its high compatibility with surrounding magnetite grains and the base metal. Furthermore, since magnetite is a black iron-based oxide, it contributes to the blackening of the surface of the black-scale hot-rolled steel sheet. On the other hand, the inventors found that precipitated Fe generated during eutectoid transformation and hematite generated on the scale surface at high temperatures during hot rolling are factors that inhibit surface blackening.

[0017] In conventional techniques, scale adhesion and blackness tended to be inferior, particularly at the edges in the width direction. It was found that the decrease in adhesion at the edges was due to the following: re-oxidation by air entering from the edges after coiling the scale leads to an excessive increase in the amount of hematite and magnetite layer consisting of columnar magnetite grains on the scale surface, which reduces the proportion of eutectoid transformation structure and increases the scale thickness at the edges. Furthermore, it was found that the decrease in blackness at the edges is due to an excessive increase in the amount of hematite generated on the scale surface.

[0018] Furthermore, in conventional techniques, after winding the hot-rolled coil, the steel sheets adhere closely to each other in the center in the width direction, shielding them from the oxidizing atmosphere. This can cause the hematite and magnetite on the scale surface to be reduced to wustite, and eutectoid transformation may proceed on the scale surface as well. In this case, although scale adhesion improves, the amount of Fe precipitated on the scale surface increases, resulting in a problem of inferior blackness.

[0019] Furthermore, under normal manufacturing conditions, transformation expansion after winding occurs unevenly along the longitudinal direction of the coil. This leads to loosening of the hot-rolled coil, and air intrusion in the center of the width direction promotes re-oxidation, resulting in a decrease in the amount of eutectoid transformation structure and an increase in the amount of hematite. It has been found that this can result in inferior adhesion and blackness.

[0020] Therefore, the present inventors diligently investigated means to solve the above problems and obtain a hot-rolled steel sheet having uniformly excellent scale adhesion and blackness in the width direction of the coil, and obtained the following findings.

[0021] (i) After hot rough rolling of a steel material having a predetermined component composition, descaling is performed, and finish rolling is carried out at a finish rolling exit temperature of 800 to 950°C, followed by cooling to the coiling temperature at a predetermined cooling rate. This appropriately controls the scale thickness and suppresses the occurrence of cracks in the scale, which can cause a decrease in adhesion. In addition, sufficient magnetite grains are generated on the surface, which contribute to the blackening of the surface.

[0022] (ii) Winding temperature: By winding at a temperature of 450°C to 600°C, the amount of eutectoid transformation structure, which contributes to improved adhesion, is appropriately controlled, and the formation of precipitated Fe on the steel sheet surface, which is a cause of whitening, is suppressed. Furthermore, from the start of winding, the entire coil is cooled so that the temperature of the coil edges cools from the winding temperature to a cooling stop temperature of 300°C to 430°C at an average cooling rate of 0.5°C / s to 6.0°C / s. This lowers the temperature of the coil edges, increases the rigidity of the coil, and prevents loosening. Here, the coil edges refer to the area within 200 mm from the edge in the width direction of the coil. As a result, the center in the width direction is shielded from the oxidizing atmosphere, suppressing the formation of hematite associated with re-oxidation, thereby suppressing deterioration of adhesion and whitening. Furthermore, although complete isolation from the oxidizing atmosphere is difficult at the edges in the width direction of the coil, re-oxidation can be suppressed by cooling the edges, and the eutectoid transformation can be promoted by reheating from the center in the width direction, thereby ensuring excellent adhesion and black coloration.

[0023] This invention is based on the above findings and specifically provides the following: [1] A hot-rolled steel sheet having a composition by mass% containing C: 0.01~0.30%, Si: 0.50% or less, Mn: 0.01~2.0%, P: 0.10% or less, S: 0.10% or less, sol.Al: 0.10% or less, N: 0.015% or less, with the remainder being Fe and unavoidable impurities, having scale on the surface of the steel sheet, wherein the scale in the width direction of the steel sheet has, by area percentage, magnetite grains: 30% or more, eutectoid transformation structure of iron and magnetite: 20% or more and 60% or less, where the magnetite consists of the magnetite grains and magnetite contained in the eutectoid transformation structure, with wustite: 20% or less, and hematite: 5% or less by mass fraction, the precipitated Fe on the scale surface is 15% or less by area percentage, and the average thickness of the scale in the width direction of the steel sheet is 3 μm or more and 20 μm or less. [2] The hot-rolled steel sheet according to [1], wherein the component composition further contains one or more of the following in mass%: Cu: 1.0% or less, Ni: 0.50% or less, and Cr: 2.0% or less. [3] The hot-rolled steel sheet according to [1] or [2], wherein the component composition further contains one or more of the following in mass%, in the following amounts: Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.01% or less, and Sb: 0.03% or less. A method for manufacturing a hot-rolled steel sheet, comprising: hot rough rolling of a steel material having the component composition described in any of [4][1] to [3]; descaling; finish rolling at a finish rolling exit temperature of 800°C to 950°C; cooling from the finish rolling exit temperature to 750°C at an average cooling rate of 5°C / s or more; cooling from 750°C to the start of winding at an average cooling rate of 1°C / s to 30°C / s; winding at a winding temperature of 450°C to 600°C; and after the start of winding, cooling the entire coil at an average cooling rate of 0.5°C / s to 6.0°C / s so that the temperature of the coil edges cools from the winding temperature to a cooling stop temperature of 300°C to 430°C at an average cooling rate of 0.5°C / s or more. [Effects of the Invention]

[0024] According to the present invention, hot-rolled steel sheets with excellent scale adhesion and blackness can be manufactured easily and inexpensively, resulting in significant industrial benefits. Furthermore, according to the present invention, variations in scale adhesion can be reduced in the width direction of the coil, which greatly contributes to improving the surface quality of the product, preventing processing defects, and improving the working environment.

[0025] In this invention, the thickness of the hot-rolled steel sheet is greater than 2.0 mm and less than or equal to 25 mm, preferably greater than 5.0 mm and less than or equal to 25 mm. [Modes for carrying out the invention]

[0026] The hot-rolled steel sheet and its manufacturing method according to the present invention will be described in detail below. However, the present invention is not limited to the following embodiments. The following embodiments include those that are easily substituted or substantially identical to those of the present invention by those skilled in the art.

[0027] The hot-rolled steel sheet of the present invention contains the following component composition. Unless otherwise specified, the unit "%" used to indicate the content of the component composition means "mass%".

[0028] C: 0.01~0.30% Carbon (C) is a useful element for ensuring strength. If its amount is less than 0.01%, the effect of ensuring strength is small, so the amount of C should be 0.01% or more. If the amount of C exceeds 0.30%, CO gas is generated at the interface between the scale and the base metal, causing delamination of the scale-base metal interface during rolling and leading to scale defects, so the amount of C should be 0.30% or less. From the viewpoint of scale adhesion, it is preferably 0.20% or less.

[0029] Si:0.50% or less Si is an element that acts as a deoxidizing agent. While it is not strictly necessary to include Si, it is preferable to include 0.01% or more to obtain this effect. However, if the Si content exceeds 0.50%, Si becomes concentrated at the interface between the scale and the base metal, forming a Si oxide layer. Scale delamination is likely to occur at the interface between this Si oxide layer and the scale layer formed on top of it. Furthermore, descaling properties at high temperatures before finish rolling deteriorate, and the primary scale generated during rough rolling is pulverized during finish rolling, making so-called red scale more likely to form and resulting in inferior blackness. For this reason, the Si content should be 0.50% or less. Preferably, it should be 0.20% or less.

[0030] Mn: 0.01~2.0% Mn is an element that not only neutralizes dissolved sulfur (S), which causes embrittlement during hot working, by converting it into MnS, but also improves strength. Below 0.01%, the effect is small. On the other hand, above 2.0%, it leads to a decrease in toughness and forms Mn-based oxides at the interface between the scale and the base metal, causing a decrease in scale adhesion. Furthermore, the transformation after finish rolling is delayed, and the transformation is not completed by the time of winding, resulting in partial and non-uniform transformation in the longitudinal direction after winding. This causes loosening of the hot-rolled coil after winding, and even in the center of the coil's width direction, contact between the steel sheet surface and the oxidizing atmosphere leads to re-oxidation, i.e., an increase in hematite and magnetite grains and a decrease in the eutectoid transformation structure, leading to a deterioration of scale adhesion. Therefore, the Mn content should be between 0.01% and 2.0%. The preferred lower limit is 0.05% or more. The preferred upper limit is 1.5% or less.

[0031] P:0.10% or less P is an element that should be kept as low as possible because it negatively affects grain boundary embrittlement. Furthermore, P forms a very brittle oxide layer at the interface between the scale and the base metal, reducing scale adhesion. These negative effects become significant when the P content exceeds 0.10%, so it should be kept below 0.10%. A preferable upper limit is 0.05% or less. While P does not need to be included, a lower limit of 0.001% or more is preferable from a manufacturing cost perspective.

[0032] S: 0.10% or less S is an element that significantly degrades hot workability and toughness. Furthermore, S concentrates at the interface between the scale and the base metal, reducing scale adhesion. These adverse effects become significant when the S content exceeds 0.10%, so it should be kept below 0.10%. Preferably, it should be below 0.05%. While S does not need to be included, from a manufacturing cost perspective, a lower limit of 0.0001% or higher is preferable.

[0033] sol.Al: 0.10% or less sol.Al is an element that acts as a deoxidizing agent. The amount of sol.Al may be 0.00%, but it is preferable to contain 0.01% or more to obtain this effect. On the other hand, if the amount exceeds 0.10%, oxide-based inclusions increase and the cleanliness decreases. For this reason, the amount of sol.Al should be 0.10% or less. Preferably, it should be 0.06% or less.

[0034] N: 0.015% or less N is an element that forms nitrides such as BN, AlN, and TiN in steel, and it is an element that reduces the hot ductility of steel and degrades surface quality. Surface quality deteriorates significantly when the N content exceeds 0.015%. Therefore, the N content should be 0.015% or less. Preferably, the N content is 0.010% or less. Although it is not necessary to include N, it is preferable to have an N content of 0.0001% or more from the viewpoint of manufacturing cost. More preferably, the N content is 0.001% or more.

[0035] The above chemical components are essential components of the hot-rolled steel sheet of the present invention. In addition to the above chemical components, the hot-rolled steel sheet of the present invention may contain one or more of the following in amounts of Cu: 1.0% or less, Ni: 0.50% or less, and Cr: 2.0% or less, as needed, in order to improve various properties.

[0036] Cu: 1.0% or less Cu is an element that concentrates at the interface between the scale and the base metal, promoting grain boundary oxidation, and also promotes the unevenness of the scale-base metal interface, thereby improving the adhesion between the scale and the base metal interface. To obtain these effects, it is preferable to contain 0.01% or more Cu. However, if the content exceeds 1.0%, molten Cu may penetrate into the austenite grain boundaries of the base metal during heating, raising concerns about deterioration of surface properties due to hot embrittlement. For this reason, if Cu is included, it should be 1.0% or less. Preferably, it is 0.8% or less.

[0037] Ni: 0.50% or less Like copper, nickel (Ni) is an element that concentrates at the interface between the scale and the base metal, promoting grain boundary oxidation and increasing the surface roughness of the interface, thereby improving adhesion between the scale and the base metal. To obtain these effects, it is preferable to contain 0.01% or more Ni. However, if the amount of Ni exceeds 0.50%, the aforementioned effects saturate, and an increase in cost is a concern. For this reason, if Ni is included, it should be 0.50% or less. Preferably, it is 0.40% or less.

[0038] Cr:2.0% or less Cr has the effect of increasing strength, hardenability, and corrosion resistance. In addition, Cr concentrates at the interface between the scale and the base metal, and the resulting unevenness of the interface allows the scale to penetrate into the base metal, thus improving the adhesion of the scale. To obtain these effects, it is preferable to contain 0.01% or more of Cr. On the other hand, if the content exceeds 2.0%, the above effect becomes saturated, so if Cr is included, it should be 2.0% or less. A more preferable lower limit is 0.07% or more, and even more preferable is 0.12% or more. A more preferable upper limit is 1.0% or less, and even more preferable is 0.8% or less.

[0039] In the present invention, if necessary, one or more of the following may be included: Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.01% or less, and Sb: 0.03% or less.

[0040] Mo: 1.0% or less Mo improves strength and hardenability and suppresses softening during tempering. To obtain these effects, it is preferable to contain 0.1% or more Mo. On the other hand, if the amount exceeds 1.0%, the strength may increase excessively, and toughness and formability may deteriorate. Therefore, when Mo is included, the amount should be 1.0% or less.

[0041] Nb: 0.1% or less Nb is an element that improves the strength and toughness of the base material. To obtain such effects, it is preferable to include 0.003% or more. On the other hand, including more than 0.1% may actually lead to a decrease in toughness. Therefore, when Nb is included, the amount should be 0.1% or less.

[0042] V: 0.1% or less V is an element that improves the strength and toughness of the base material. To obtain such effects, it is preferable to include 0.003% or more. On the other hand, including more than 0.1% may actually lead to a decrease in toughness. Therefore, when V is included, the amount should be 0.1% or less.

[0043] Ti: 0.03% or less Ti is an element that improves the strength and toughness of the base material, and is also effective in ensuring toughness in the heat-affected zone during welding. To obtain these effects, it is preferable to contain 0.001% or more Ti. On the other hand, if the content exceeds 0.03%, it may actually lead to a decrease in toughness. Therefore, if Ti is included, the amount should be 0.03% or less.

[0044] B: 0.01% or less B is an element that enhances the hardenability of steel. This effect can increase its strength. To obtain this effect, it is preferable to contain 0.0005% or more of B. On the other hand, if the amount exceeds 0.01%, this effect becomes saturated, so if B is included, the amount should be 0.01% or less.

[0045] Sb: 0.03% or less When the material is heated, Sb concentrates on the surface of the steel sheet, suppressing the decrease in carbon content on the steel sheet surface due to decarburization during heating. To obtain this effect, it is preferable to contain 0.001% or more Sb. On the other hand, if the content exceeds 0.03%, it may become a liquid metal when the material is heated, eroding the prior austenite grain boundaries and reducing the adhesion of scale. For this reason, if Sb is included, it should be 0.03% or less.

[0046] The remainder of the chemical components other than those mentioned above consists of Fe and unavoidable impurities. Acceptable unavoidable impurities include O: 0.005% or less, Mg: 0.003% or less, Sn: 0.1% or less, and Ca: 0.01% or less. Furthermore, any element mentioned above present below the preferred lower limit is also considered an unavoidable impurity.

[0047] Next, the scale structure in the width direction of the hot-rolled steel sheet of the present invention will be described. The scale structure in the width direction of the steel sheet refers to the scale structure in the center of the steel sheet in the width direction and at a position 5 mm from the edge in the width direction. The structure at each position should be within the following range.

[0048] Area ratio of magnetite grains: 30% or more The magnetite of the present invention consists of magnetite grains and magnetite contained in the eutectoid transformation structure. These structures are distinguishable as follows. The magnetite grains in the present invention include a magnetite layer near the scale surface consisting of columnar magnetite grains, and massive proeutectoid magnetite grains that are formed inside or adjacent to the eutectoid transformation structure prior to the progression of eutectoid transformation. Furthermore, a thin layer consisting of fine magnetite grains that forms at the scale-base metal interface, so-called magnetite seams, are included and are distinguished from the magnetite contained in the eutectoid transformation structure of iron and magnetite. Compared to wustite and hematite, magnetite grains have high ductility at room temperature and contribute to improved scale adhesion. In particular, magnetite seams have high coherence with the base metal and contribute to improved scale adhesion by suppressing peeling from the scale-base metal interface. In addition, the magnetite layer near the scale surface consisting of columnar magnetite grains is a structure that contributes to blackening. If the magnetite grain content is less than 30%, these effects cannot be fully obtained; therefore, the area ratio of magnetite should be 30% or more, preferably 40% or more. On the other hand, if the magnetite grain content exceeds 80%, cracks may form in the magnetite, resulting in inferior scale adhesion. For this reason, the area ratio of magnetite grains should preferably be 80% or less, and more preferably 70% or less.

[0049] Area ratio of eutectoid transformation structures of iron and magnetite: 20% to 60% The eutectoid transformation structure of iron and magnetite contributes to improved scale adhesion because of the high compatibility between magnetite, precipitated Fe, and the base iron. This effect is not sufficiently obtained if the area ratio is less than 20%. For this reason, the area ratio of the eutectoid transformation structure of iron and magnetite should be 20% or more, preferably 25% or more, and more preferably 30% or more. On the other hand, if the eutectoid transformation structure of iron and magnetite exceeds 60%, precipitated Fe derived from the eutectoid transformation will also be generated near the scale surface, which causes whitening. For this reason, the eutectoid transformation structure of iron and magnetite should be 60% or less, preferably 55% or less, and more preferably 50% or less.

[0050] Area ratio of Ustite: 20% or less Wustite is a phase that is stable at high temperatures and most of it disappears during cooling after winding due to eutectoid transformation. However, if the cooling rate is high, wustite may remain untransformed at room temperature. In particular, the cooling rate is relatively high near the edges in the width direction of the coil, so wustite is more likely to remain at room temperature there. At room temperature, wustite is more brittle than magnetite, and cracks can form in the scale, impairing the adhesion of the scale. For this reason, the area ratio of wustite should be 20% or less, preferably 15% or less, and more preferably 10% or less. The area ratio of wustite may be 0%.

[0051] Hematite mass fraction: 5% or less In addition to magnetite grains, eutectoid transformation structures of iron and magnetite, and wustite, hematite may form in layers on the scale surface. Hematite can lead to poor adhesion and whitening, as well as causing surface defects such as red scale. For this reason, the mass fraction of hematite should be kept below 5%.

[0052] Area ratio of deposited Fe on the scale surface: 15% or less Precipitated Fe on the scale surface inhibits the blackening of the scale surface. Therefore, if its area ratio exceeds 15%, sufficient blackness cannot be achieved. Accordingly, the area ratio of precipitated Fe on the scale surface should be 15% or less. Alternatively, the area ratio of precipitated Fe on the scale surface may be 0%.

[0053] The average scale thickness in the width direction of the steel plate is between 3 μm and 20 μm. If the average thickness of the scale is less than 3 μm, the color tone of the eutectoid transformation structure formed on the base metal side of the scale, as well as the luster and color tone of the base metal, affect the color tone of the surface layer, causing it to appear white. For this reason, the average thickness of the scale should be 3 μm or more, preferably 4 μm or more. On the other hand, if the average thickness of the scale exceeds 20 μm, the strain applied to the scale surface when processing the steel plate increases, causing cracks to form in the scale and reducing scale adhesion. For this reason, the average thickness of the scale should be 20 μm or less, preferably 18 μm or less, and more preferably 15 μm or less.

[0054] Next, a method for measuring the scale structure and scale thickness of the hot-rolled steel sheet of the present invention will be described.

[0055] Scale structure measurements are performed at the center and edges of the steel sheet in the width direction. Specifically, the center of the hot-rolled coil in the width direction and a point 5 mm from the edge in the width direction are used as observation points for evaluation.

[0056] The area ratios of magnetite grains, the eutectoid transformation structure of iron and magnetite, and wustite can be measured by cutting out a cross-section of the steel sheet thickness perpendicular to the surface and parallel to the rolling direction from the observation position, and then mirror-polishing it. Subsequently, the backscattered electron image of the scale cross-section can be observed using a scanning electron microscope (SEM). The SEM field of view should be the entire scale thickness, from the scale surface to the interface between the scale and the steel sheet. Therefore, the backscattered electron image of the scale cross-section should be observed at a magnification that allows the entire scale thickness to fit within the field of view. In the SEM backscattered electron image, magnetite is the darkest, the base iron and precipitated Fe in the eutectoid transformation structure are the brightest, and wustite is the region with intermediate contrast. The eutectoid transformation structure of iron and magnetite is the region where magnetite and iron are formed in layers.

[0057] Hematite forms in a very thin layer on the scale surface and is easily removed during mirror polishing, making it difficult to quantitatively evaluate its area percentage on SEM. Therefore, using an X-ray diffractometer, CoK α The integrated intensity of the diffraction peaks of each phase in the scale is measured using a radiation source. From the ratio of the integrated intensities of each phase in the standard sample and the sample under test, the mass fraction can be determined using the following equation (2). As the standard sample, an equal weight mixture of Fe, FeO (wustite), Fe2O3 (hematite), and Fe3O4 (magnetite) is used. Note that the mass fraction of hematite can be considered as the area fraction. Mass fraction of hematite (Fe2O3) = (I Fe2O3 / R Fe2O3 ) × 100 / ((I Fe / R Fe) + (I FeO / R FeO ) + (I Fe2O3 / R Fe2O3 ) + (I Fe3O4 / R Fe3O4 )) ··· (2) However, in the above formula (2), I A : The integrated intensity of phase A in the test sample R A : The integrated intensity of phase A in the standard sample A: It is Fe, FeO, Fe2O3, or Fe3O4.

[0058] The area ratio of the deposited Fe on the scale surface can be measured by observing the backscattered electron image of the surface at the above observation position using SEM at a magnification of 3000 times and obtaining the area ratio of the deposited Fe that appears with the highest contrast by image analysis.

[0059] Also, the average thickness of the scale in the width direction of the coil can be measured as follows. For example, a plate thickness cross-section perpendicular to the steel plate surface and parallel to the rolling direction is cut out from the central part in the width direction of the hot-rolled steel plate and a portion 5 mm from the edge in the width direction of the hot-rolled steel plate, and mirror-polished. Then, the scale thickness is measured three times each using SEM, and the average is obtained by averaging these values. Note that the average thickness of the scale in the width direction of the present invention being 3 μm or more and 20 μm or less means that the average thickness of the scale is 3 μm or more and 20 μm or less in each of the central part in the width direction of the hot-rolled steel plate and the portion 5 mm from the edge in the width direction.

[0060] Next, the manufacturing method of the hot-rolled steel plate of the present invention will be described.

[0061] Note that the temperature specified in each step in the present invention refers to the surface temperature of the slab (steel slab) or the steel plate, and can be measured by a radiation thermometer or the like. Also, unless otherwise specified, the average cooling rate is "(cooling start temperature - cooling stop temperature) / cooling time".

[0062] In the present invention, the method for manufacturing the steel material having the above-described component composition is not particularly limited, and any commonly used method can be applied. For example, it is desirable to melt molten steel having the above-described component composition in a converter or electric furnace, and then use a casting method such as continuous casting to produce steel material such as a slab. There is no problem in using the ingot-part rolling method. Normally, the steel material is heated and then hot-rolled. This heating should be sufficient to achieve solid solution formation, preferably above the Ac3 point. Specifically, the normal slab heating temperature range of 1060°C to 1300°C is suitable. In the case of slabs manufactured by continuous casting, direct rolling may be applied, either as is or while being held to suppress temperature drop.

[0063] The hot rolling process consists of rough rolling and finish rolling. Rough rolling only requires the formation of sheet bars of a predetermined size; the conditions for rough rolling are not particularly limited. Furthermore, since finish rolling is performed at a predetermined temperature, the rolled material may be heated during the process using heating means such as a sheet bar heater. Before rough rolling and finish rolling, scale formed on the sheet bar surface is removed at the entrance of the rolling mill by descaling using high-pressure water or the like.

[0064] Next, finish rolling is performed. If the finish rolling entry temperature exceeds 1100°C, the scale thickness increases, and the adhesion of the scale may decrease. On the other hand, if the finish rolling entry temperature is below 950°C, it can lead to a significant increase in rolling load, which can reduce productivity. Furthermore, as the product sheet thickness increases, the finish rolling entry thickness also increases. For example, if the product sheet thickness exceeds 5.0 mm, a long time is required before the start of finish rolling, which can lead to a decrease in productivity. Therefore, the finish rolling entry temperature is preferably 1100°C or lower, and more preferably 1050°C or lower. In addition, it is preferable that the lower limit of the finish rolling entry temperature be 950°C or higher.

[0065] Finishing rolling temperature: 800°C to 950°C If the finishing rolling temperature is below 800°C, cracks will occur due to a decrease in scale ductility. These cracks promote the re-oxidation of the scale, generating hematite, which causes deterioration in scale adhesion and blackness. In addition, the scale thickness decreases, and sufficient blackness cannot be obtained. Furthermore, the scale structure becomes finer, and the hardness of the scale itself increases, reducing scale adhesion. On the other hand, if the finishing rolling temperature exceeds 950°C, the scale thickness increases due to excessive scale growth, and scale adhesion decreases. In addition, the grain size of each phase in the scale structure increases, further reducing scale adhesion. Therefore, the finishing rolling temperature should be between 800°C and 950°C. The preferred lower limit is 820°C or higher. The preferred upper limit is 930°C or lower.

[0066] Cooling is performed at an average cooling rate of 5°C / s or higher over a temperature range from the finish rolling temperature up to 750°C. Scale grows faster at higher temperatures. Therefore, to suppress the decrease in scale adhesion due to excessive scale growth, it is necessary to cool the high-temperature area immediately after finish rolling quickly. If the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is less than 5°C / s, the scale will grow excessively, causing a decrease in scale adhesion. For this reason, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C should be 5°C / s or higher, preferably 7°C / s or higher. On the other hand, if the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C exceeds 80°C / s, the scale structure may become finer, which may reduce scale adhesion. In addition, the ductility of the scale will decrease, causing cracks to form, and these cracks will promote the re-oxidation of the scale, leading to the formation of hematite. As a result, this may cause deterioration in scale adhesion and blackness. For this reason, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C should preferably be 80°C / s or less, and more preferably 50°C / s or less.

[0067] Cooling from 750°C to the start of winding at an average cooling rate of 1°C / s to 30°C / s. In the temperature range from 750°C to the start of winding, scale growth is relatively slower than in the high-temperature range immediately after finish rolling, but it is necessary to suppress the decrease in scale adhesion due to excessive scale growth. If the average cooling rate in the temperature range from 750°C to the start of winding is less than 1°C / s, the scale will grow excessively, causing a decrease in scale adhesion. For this reason, the average cooling rate in the temperature range from 750°C to the start of winding should be 1°C / s or more, preferably 3°C / s or more. On the other hand, if the average cooling rate in the temperature range from 750°C to the start of winding exceeds 30°C / s, the scale structure becomes finer and the stress difference with the base metal increases. This causes cracks to form in the scale, and these cracks promote the re-oxidation of the scale. This generates hematite, causing deterioration in scale adhesion and blackness. For this reason, the average cooling rate in the temperature range from 750°C to the start of winding should be 30°C / s or less, preferably 20°C / s or less.

[0068] Winding temperature: 450℃ or higher and 600℃ or lower After the cooling described above, the steel sheet is wound at a winding temperature of 450°C to 600°C. If the winding temperature is below 450°C, the eutectoid transformation from wustite does not occur sufficiently after winding, and an excessive amount of wustite remains at room temperature. As a result, since wustite is brittle at room temperature, the adhesion of scale decreases. If the winding temperature exceeds 600°C, the scale grows excessively, reducing the adhesion of scale. In addition, in the central part in the width direction, which is shielded from the oxidizing atmosphere after winding, the hematite and magnetite on the surface are reduced to wustite. This wustite undergoes eutectoid transformation, causing Fe to precipitate on the surface of the steel sheet, which inhibits blackening. Furthermore, at the edges in the width direction, the formation of hematite by re-oxidation is promoted, causing whitening. Therefore, the winding temperature should be 450°C to 600°C. The preferred lower limit is 470°C or higher. The preferred upper limit is 580°C or lower.

[0069] After winding begins, the entire coil is cooled so that the temperature of the coil edges cools from the winding temperature to a cooling stop temperature of 300°C to 430°C, at an average cooling rate of 0.5°C / s to 6.0°C / s.After winding begins, the entire coil is cooled so that the temperature of the coil edges cools from the winding temperature to a cooling stop temperature of 300°C to 430°C at an average cooling rate of 0.5°C / s to 6.0°C / s. This lowers the temperature of the coil edges, increases the rigidity of the coil, and prevents winding loosening. Here, the entire coil refers to both edges and the surface of the coil. Both edges of the coil refer to the area within 200 mm in the width direction of the coil from both edges in the width direction of the coil. The surface of the coil refers to the surface of the coil excluding both ends. As a result, the central part of the coil in the width direction is shielded from the oxidizing atmosphere, suppressing hematite formation associated with re-oxidation, suppressing whitening, and allowing for the acquisition of a sufficiently eutectoid transformation structure suitable for improved adhesion. Furthermore, although complete shielding from the oxidizing atmosphere is difficult at the edges in the width direction of the coil, cooling the edges suppresses re-oxidation, and reheating from the central part in the width direction promotes eutectoid transformation, ensuring excellent adhesion and blackness. The temperature of the edge can be measured, for example, by a radiation thermometer. The temperature of the plate surface is not specified because it is not possible to measure the temperature of the plate surface at a specific longitudinal position during winding. However, the above effects can be obtained by cooling the entire coil using the same cooling method as the edge. If the cooling stop temperature at the edge of the coil exceeds 430°C, the above effects will not be sufficiently obtained. Also, if the cooling stop temperature at the edge of the coil is less than 300°C, the edge will be overcooled, resulting in a large amount of wustite remaining in the scale at the edge and a decrease in adhesion. For this reason, the cooling stop temperature at the edge of the coil should be between 300°C and 430°C. A preferred lower limit is 320°C or higher. A preferred upper limit is 400°C or lower. Also, if the average cooling rate at the edge is less than 0.5°C / s, the above effects will not be sufficiently obtained. If the average cooling rate at the edge exceeds 6.0°C / s, cracks will occur in the scale due to the refinement of the scale structure and the large stress difference with the base metal. These cracks promote the re-oxidation of the scale, leading to the formation of hematite, which reduces the scale's adhesion and blackness.Therefore, the average cooling rate at the edges should be between 0.5°C / s and 6.0°C / s. Preferably, it should be between 1.0°C / s and 5.0°C / s. There are no specific requirements for the method of cooling the coil, but it is preferable to cool it using a cooling device that sprays water onto the surface and both edges of the coil while winding it in a winding machine.

[0070] Furthermore, it is preferable to place the cooled coil in a coil box or cover it to promote the eutectoid transformation from wustite and suppress oxidation of the outermost periphery and edges.

[0071] Furthermore, in the case of hot-rolled steel sheets wound into a coil shape, the steel sheets may be deformed using a roller leveler, tension leveler, or the like to perform a shape correction treatment. [Examples]

[0072] The following describes embodiments of the present invention.

[0073] Steel with the composition shown in Table 1 was melted and cast to obtain steel materials. These steel materials were hot-rolled under the conditions shown in Table 2 to obtain 2 mm thick black-scale hot-rolled coils. Test specimens were taken at various positions in the width direction of the obtained hot-rolled coils, and the scale structure, scale thickness, adhesion, and blackness were evaluated using the following method.

[0074] [Table 1]

[0075] [Table 2]

[0076] The area ratios of magnetite grains, the eutectoid transformation structure of iron and magnetite, and wustite were measured as follows. A cross-section of the hot-rolled coil was cut from the center in the width direction and 5 mm from the edge in the width direction, perpendicular to the steel plate surface and parallel to the rolling direction, and used as the observation surface. This section was then mirror-polished. The measurements were then taken by observing the backscattered electron image of the scale cross-section using a scanning electron microscope (SEM) at an observation magnification of 3000x. In the SEM backscattered electron image, magnetite was the darkest, the base iron and precipitated Fe in the eutectoid transformation structure were the brightest, and wustite was visible in an area with intermediate contrast. The eutectoid transformation structure of iron and magnetite is a region where magnetite and iron are formed in layers.

[0077] Hematite was analyzed using an X-ray diffractometer, CoK α The integrated intensity of the diffraction peaks of each phase in the scale was measured using a radiation source. The mass fraction was determined from the ratio of the integrated intensities of each phase in the standard sample (an equal-weight mixture of Fe, FeO (wustite), Fe2O3 (hematite), and Fe3O4 (magnetite)) and the test sample using the following equation (2). Mass fraction of hematite (Fe2O3) = (I Fe2O3 / R Fe2O3 ) × 100 / ((I Fe / R Fe )+(I FeO / R FeO )+(I Fe2O3 / R Fe2O3 )+(I Fe3O4 / R Fe3O4 )) ···(2) However, in equation (2) above, I A Integrated intensity of phase A in the test sample R A Integrated intensity of phase A in the standard sample A: Fe, FeO, Fe2O3, or Fe3O4.

[0078] The area ratio of deposited Fe on the outermost surface of the steel sheet was determined by observing backscattered electron images of the scale surface in the center of the width direction of the hot-rolled coil and 5 mm from the edge in the width direction using a scanning electron microscope (SEM) at a magnification of 3000x. The area ratio of deposited Fe visible with the brightest contrast was determined by image analysis.

[0079] Furthermore, the average scale thickness in the width direction of the coil was determined by cutting out thickness sections perpendicular to the steel plate surface and parallel to the rolling direction from the center of the width direction of the hot-rolled coil and from 5 mm from the edge in the width direction. After mirror polishing, the scale thickness was measured at three locations using SEM and the average of these measurements was used to determine the average scale thickness.

[0080] Scale adhesion was evaluated by taking 30mm x 100mm test specimens from the center of the hot-rolled coil in the width direction and from 5mm from the edge in the width direction, with the longitudinal direction parallel to the rolling direction. Next, a bending test using the press-bend method (JIS Z 2248) was performed, and then cellophane tape was applied to the surface of the outer part of the bent specimen and peeled off. The amount of scale adhering to the tape was evaluated by measuring the amount by image analysis. The conditions for the bending test were a ratio of r / t (radius r at the tip of the press tool to the thickness t of the specimen) of 4, and a bending angle of 180°. The evaluation criteria were as follows, with ○ and △ indicating excellent adhesion. ○: Almost no peeling observed. (Scale adhesion area ratio is less than 10%) △: Although some peeling is observed, it does not pose a practical problem. (Scale adhesion area ratio is 10% or more but less than 50%) ×: Peeling is observed in many areas, posing a practical problem. (Scale adhesion area ratio is 50% or more) Blackness was measured using a spectrophotometer (KONICA MINOLTA CM-700d) after degreasing the steel sheet surface with alcohol at the center of the width direction of the hot-rolled coil and at a point 5 mm from the edge in the width direction. The blackness was then measured using L, which is generally considered to be close to human vision. * a * b * L of the color system (JIS Z 8729) * The value was determined by measurement.* A smaller value indicates a color closer to black. The viewing angle during measurement was 10°, and the primary light source was an auxiliary illuminant D65 (daylight, color temperature 6504K). Measurements were taken in specular reflection rejection mode. L values ​​below 40. * The value was considered to indicate superior blackness.

[0081] The example of the present invention shown in Table 2 exhibited excellent uniform scale adhesion and blackness in both the widthwise center of the hot-rolled coil and the portion 5 mm from the widthwise edge, whereas the comparative example showed inferior adhesion or blackness at either or both widthwise positions.

Claims

1. In mass percent, C: 0.01-0.30%, Si: 0.50% or less, Mn: 0.01-2.0%, P: 0.10% or less, S: 0.10% or less, Sol. Al: 0.10% or less. N: Contains 0.015% or less, The composition consists of Fe and unavoidable impurities. The steel plate has scale on its surface. The scale in the width direction of the steel plate is In terms of area ratio, Magnetite particles: 30% or more Eutectoid transformation structure of iron and magnetite: 20% to 60% Here, the magnetite consists of the magnetite grains and the magnetite contained in the eutectoid transformation structure. Ustite: Less than 20%, The structure has a mass fraction of hematite of 5% or less. The amount of Fe deposited on the surface of the scale is 15% or less by area ratio. A hot-rolled steel sheet in which the average thickness of the scale in the width direction of the steel sheet is 3 μm or more and 20 μm or less.

2. The aforementioned component composition is further expressed in mass%, Cu: 1.0% or less, Ni: 0.50% or less, Cr: 2.0% or less, A hot-rolled steel sheet according to claim 1, containing one or more of the following.

3. The aforementioned component composition is further expressed in mass%, Mo: 1.0% or less Nb: 0.1% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.01% or less, Sb: 0.03% or less A hot-rolled steel sheet according to claim 1 or 2, containing one or more of the following.

4. A method for manufacturing a hot-rolled steel sheet according to claim 1 or 2, wherein a steel material having the above-mentioned component composition is used. After hot rough rolling, descaling is performed. Finish rolling exit temperature: Finish rolling is performed at a temperature of 800°C to 950°C. After cooling the temperature range from the aforementioned finish rolling temperature to 750°C at an average cooling rate of 5°C / s or more, The temperature range from 750°C to the start of winding is cooled at an average cooling rate of 1°C / s to 30°C / s. Winding temperature: Winding at 450°C to 600°C. A method for manufacturing a hot-rolled steel sheet, comprising cooling both edges of the coil and the surface of the coil from the start of winding until the temperature of the coil edge is cooled from the winding temperature to a cooling stop temperature of 300°C to 430°C, at an average cooling rate of 0.5°C / s to 6.0°C / s, thereby cooling the entire coil.

5. A method for manufacturing a hot-rolled steel sheet according to claim 3, wherein a steel material having the above-mentioned component composition is used. After hot rough rolling, descaling is performed. Finish rolling exit temperature: Finish rolling is performed at a temperature of 800°C to 950°C. After cooling the temperature range from the aforementioned finish rolling temperature to 750°C at an average cooling rate of 5°C / s or more, The temperature range from 750°C to the start of winding is cooled at an average cooling rate of 1°C / s to 30°C / s. Winding temperature: Winding at 450°C to 600°C. A method for manufacturing a hot-rolled steel sheet, comprising cooling both edges of the coil and the surface of the coil from the start of winding until the temperature of the coil edge is cooled from the winding temperature to a cooling stop temperature of 300°C to 430°C, at an average cooling rate of 0.5°C / s to 6.0°C / s, thereby cooling the entire coil.

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