Hot-rolled steel sheet and method for producing 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

Hot-rolled steel sheet and manufacturing method thereof

[0001] The present invention relates to a hot-rolled steel sheet having an oxide scale on its surface, which is used for automobiles, home appliances, building materials, etc., and a method for manufacturing the same. In particular, the present invention relates to a hot-rolled steel sheet that has excellent blackness and scale adhesion and small variations in blackness and scale adhesion in the width direction of the steel sheet, making it suitable for use as a material for building materials, truck frames, woks, etc., and a method for manufacturing the same.

[0002] Hot-rolled steel sheets are usually hot-rolled at high temperatures in an oxidizing atmosphere, which inevitably results in the formation of scale (iron oxides) on the surface. When hot-rolled steel sheets with this scale attached (hereinafter referred to as black hot-rolled steel sheets) are subjected to processes such as temper rolling, bending, press forming, and laser cutting, some of the scale peels off. This can result in processing defects, contamination of processing lines, and surface defects in processed products. To avoid such situations, there has been a demand for hot-rolled steel sheets with excellent scale adhesion on the steel sheet surface, and this demand is becoming increasingly stronger.

[0003] Furthermore, in some regions such as Southeast Asia, where black is culturally preferred, a beautiful black color adds further value to products such as building materials, and as a result, there is growing demand for black-skinned hot-rolled steel sheets with excellent blackness.

[0004] Furthermore, there is variation in the adhesion and blackness of the scale across the width of the steel sheet, and if there are areas with poor adhesion or blackness, those areas must be removed before use. In particular, the scale tends to whiten at the edge of the coil of a black-skinned hot-rolled steel sheet, resulting in poor adhesion. Therefore, from the perspective of improving yield, there is a strong demand for a hot-rolled steel sheet that has excellent, uniform scale adhesion and blackness across the width of the coil.

[0005] For example, Patent Document 1 discloses a steel material having a composition containing, by mass%, C: 0.01 to 0.3%, Si: 0.20% or less, Mn: 0.01 to 2.0%, P: 0.10% or less, S: 0.10% or less, Al: 0.10% or less, Cr: 0.01 to 2.0%, with the balance being Fe and unavoidable impurities, which is rough-rolled, descaled, and then finish-rolled at a finish-rolling outlet temperature of 800 to 950°C so as to satisfy the following formula (1): an average cooling rate from the end of finish rolling to the start of coiling: 3°C / s or more, and a cooling rate of 80°C / s or more. / s or less, and then coiled at a coiling temperature of 430 to 580°C, a hot-rolled steel sheet with excellent scale adhesion has been proposed, characterized in that it has a magnetite layer from the base steel side and, in an upper layer of the magnetite layer, magnetite grains and / or a eutectoid transformed structure of iron and magnetite, the average grain size of the magnetite grains and / or the average block size of the eutectoid transformed structure is 3 μm or more and 8 μm or less, and the mass fraction of wüstite contained in the scale layer is 10% or less. ２ -T １ |≦50°C and |T ３ -T ２ |≦50° C. (1) However, in the above formula (1), T １ T: Temperature (°C) at 30 m from the longitudinal tip and the widthwise center of the steel plate after finish rolling ２ T: Temperature (°C) at the center of the longitudinal direction and the center of the width direction of the steel plate after finish rolling ３ : Temperature (°C) at 30 m from the longitudinal tail end and at the widthwise center of the steel plate after finish rolling.

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

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

[0008] Furthermore, Patent Document 4 discloses a hot-rolled steel sheet having a composition of C: 0.001 to 0.20 mass%, Si: 0.001 to 0.50 mass%, Mn: 0.05 to 2.0 mass%, P: 0.05 mass% or less, S: 0.05 mass% or less, and sol. Al: 0.01 to 0.10 mass%, with the balance being Fe and unavoidable impurities, and having a scale having a thickness of more than 4 μm on the surface thereof, and the scale is composed of 50% or more Fe by volume. ３ O ４ and wherein the steel sheet does not contain precipitated Fe in a region extending from the scale surface to a depth of at least 2 μm in the thickness direction.

[0009] In addition, Patent Document 5 discloses a method for manufacturing a steel sheet using a hot-rolled steel sheet as a base material and Fe ３ O ４ and a scale on the surface of the base hot-rolled steel sheet, the scale consisting of Fe and having a thickness of 3.0 to 20 μm. The average grain 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 a region 0 to 1.0 μm from the outermost layer of the scale in the thickness direction of the scale. Furthermore, the Fe area ratio is 1.0% or more in a region 0 to 1.0 μm from the interface between the scale and the base hot-rolled steel sheet in the thickness direction of the scale.

[0010] JP 2019-183267 A JP 2004-027312 A JP 2012-148286 A JP 2004-043888 A International Publication No. 2018-186265

[0011] The technology described in Patent Document 1 uses a steel material having a predetermined chemical composition, and adjusts the finish rolling delivery temperature during hot rolling, the cooling rate after rolling, and the coiling temperature. This optimizes the average particle size of magnetite grains in the upper layer of the magnetite layer on the base steel side in the scale layer and / or the average block size of the eutectoid transformation structure of iron and magnetite. Furthermore, by controlling the temperature in the longitudinal direction of the steel sheet immediately after finish rolling, uniform scale adhesion in the longitudinal direction is improved. However, there is no mention of a method for uniformly improving scale adhesion in the width direction. Furthermore, there is no mention of a method for increasing the blackness of the scale.

[0012] The technology described in Patent Document 2 proposes a hot-rolled steel sheet with excellent scale tightness by hot-rolling steel to which predetermined amounts of Ni, Cu, and Cr have been added and controlling the surface roughness of the interface between the surface scale of the steel sheet and the steel base metal within a predetermined range. However, there is no mention of a method for increasing the blackness of the scale. Furthermore, there is no mention of a method for uniformly improving the adhesion and blackness of the scale in the width direction.

[0013] The technology described in Patent Document 3 proposes a method for producing a hot-rolled steel sheet by hot-rolling a steel material and winding it into a coil. The rough-rolled steel sheet is subjected to finish rolling at 850 to 1050°C. The finish-rolled hot-rolled steel sheet is then coiled at a coiling temperature of 500 to 650°C. While the finish-rolled hot-rolled steel sheet is then cooled to a temperature of 480°C or less within 5 minutes of the start of coiling, both end faces of the hot-rolled steel sheet are then cooled so that the temperature at the end faces reaches 480°C or less within 5 minutes of the start of coiling. The end faces are then maintained at a temperature of 480°C or less, and the hot-rolled steel sheet is then slowly cooled while still in the coiled state from the point when the temperature at the end faces reaches 400 to 480°C, thereby achieving excellent scale adhesion in the width direction of the hot-rolled coil, particularly at the edges. However, no mention is made of a method for uniformly improving the blackness of the hot-rolled coil in the width direction.

[0014] The techniques described in Patent Documents 4 and 5 propose hot-rolled steel sheets that are excellent in blackness and scale adhesion, but they do not necessarily achieve uniform scale adhesion and blackness in the width direction of the hot-rolled coil. In particular, there has been a problem in that the scale becomes white at the edge portions of the coil and scale adhesion becomes poor.

[0015] The present invention aims to solve the above problems and to provide a hot-rolled steel sheet having excellent scale adhesion and uniform blackness in the width direction of the hot-rolled steel sheet, and a method for manufacturing the same.

[0016] The present inventors first investigated the reason why conventional hot-rolled steel sheets cannot obtain uniformly excellent scale adhesion and blackness in the width direction. The scale formed during hot rolling is composed of hematite (Fe) at high temperatures from the surface side of the scale. ２ O ３ ), magnetite (Fe ３ O ４ ), and wüstite (FeO). Of these, wüstite undergoes eutectoid transformation during cooling after coiling, resulting in the formation of an eutectoid transformed structure consisting of magnetite and precipitated Fe (4FeO → Fe ３ O ４+Fe). This eutectoid transformed structure consisting of magnetite and precipitated Fe has high consistency with the surrounding magnetite grains and base steel, and therefore contributes to improving scale adhesion. In addition, magnetite is an iron-based oxide that has a black color, and therefore contributes to the blackening of the surface of black hot-rolled steel sheet. On the other hand, it has been found that precipitated Fe formed with the eutectoid transformation and hematite formed on the surface of the scale at high temperatures during hot rolling are factors that inhibit the blackening of the surface.

[0017] In conventional techniques, the adhesion and blackness of the scale tended to be poor, particularly at the edges in the width direction. It was found that the decrease in adhesion at the edges was caused by the following: reoxidation by air entering through the edges after coiling excessively increases the amount of hematite on the scale surface and the magnetite layer consisting of columnar magnetite particles, reducing the proportion of eutectoid transformed structures and increasing the scale thickness at the edges. Furthermore, it was found that the decrease in blackness at the edges was caused by an excessive increase in the amount of hematite formed on the scale surface.

[0018] Furthermore, in conventional techniques, after hot-rolled coils are wound, the steel sheets adhere to each other at the center in the width direction and are isolated from the oxidizing atmosphere, which can reduce hematite and magnetite on the scale surface to wüstite and cause eutectoid transformation to occur on the scale surface. In this case, although the scale adhesion is improved, the amount of precipitated Fe on the scale surface increases, resulting in a problem of poor blackness.

[0019] Furthermore, when steel is manufactured under normal conditions, transformation expansion after coiling occurs unevenly along the length of the coil. This causes loosening of the hot-rolled coil, and air intrusion into the center of the width direction promotes reoxidation, reducing the amount of eutectoid transformation structure and increasing the amount of hematite. This can result in poor adhesion and blackness.

[0020] Therefore, the present inventors have conducted extensive research into means for solving the above problems and obtaining a hot-rolled steel sheet having uniformly excellent scale adhesion and blackness in the width direction of the coil, and have obtained the following findings.

[0021] (i) A steel material having a predetermined chemical composition is subjected to rough hot rolling, followed by descaling, and then finish rolling at a finish rolling outlet 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 that cause a decrease in adhesion. It also sufficiently generates magnetite particles in the surface layer that contribute to the blackening of the surface.

[0022] (ii) Coiling temperature: By coiling at 450°C or higher and 600°C or lower, 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 causes whitening, is suppressed. Furthermore, after the start of coiling, the entire coil is cooled so that the temperature of the coil edge is cooled at an average cooling rate of 0.5°C / s or higher and 6.0°C / s or lower from the coiling temperature to a cooling stop temperature of 300°C or higher and 430°C or lower. This reduces the temperature of the coil edge, increases the rigidity of the coil, and prevents loosening of the coil. Here, the edge of the coil refers 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, thereby suppressing the formation of hematite due to reoxidation and suppressing deterioration of adhesion and whitening. Although it is difficult to completely isolate the widthwise edge portions of the coil from the oxidizing atmosphere, cooling the edge portions suppresses reoxidation, and reheating from the widthwise center portion promotes eutectoid transformation, ensuring excellent adhesion and black color.

[0023] The present invention was made based on the above findings, and specifically provides the following: [1] A steel sheet containing, in mass %, C: 0.01 to 0.30%, Si: 0.50% or less, Mn: 0.01 to 2.0%, P: 0.10% or less, S: 0.10% or less, sol. [2] A hot-rolled steel sheet according to [1], wherein the composition further contains, by mass%, one or more of 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 chemical composition further contains, in mass%, one or more of 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. [4] A steel material having the component composition according to any one of [1] to [3] is subjected to rough hot rolling, followed by descaling, and finish rolling at a finish rolling outlet temperature of 800°C or higher and 950°C or lower. The temperature range from the finish rolling outlet temperature to 750°C is cooled at an average cooling rate of 5°C / s or higher, and then the temperature range from 750°C to the start of coiling is cooled at an average cooling rate of 1°C / s or higher and 30°C / s or lower. The coiling temperature is 450°C or higher and 600°C or lower. After the start of coiling, the coil edge temperature is cooled at an average cooling rate of 0.5°C / s or higher and 6.0°C / s or lower from the coiling temperature to a cooling stop temperature of 300°C or higher and 430°C or lower. A method for producing a hot-rolled steel sheet.

[0024] According to the present invention, it is possible to easily and inexpensively produce a hot-rolled steel sheet having excellent scale adhesion and blackness, which is of great industrial benefit. Furthermore, according to the present invention, it is possible to reduce the variation in scale adhesion in the width direction of the coil, which has the effect of significantly contributing to improving the surface quality of products, preventing processing defects in products, and improving the working environment.

[0025] The thickness of the hot rolled steel sheet in the present invention is more than 2.0 mm and not more than 25 mm, and preferably more than 5.0 mm and not more than 25 mm.

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

[0027] The hot-rolled steel sheet of the present invention contains the following chemical composition: Note that "%", which is the unit of content of the chemical composition, means "mass %" unless otherwise specified.

[0028] C: 0.01 to 0.30% C is an element useful for ensuring strength. If the content is less than 0.01%, the effect of ensuring strength is small, so the C content is set to 0.01% or more. If the C content exceeds 0.30%, CO gas is generated at the interface between the scale and the base steel, causing peeling between the scale and the base steel interface during rolling and resulting in scale defects, so the C content is set to 0.30% or less. From the viewpoint of scale adhesion, the C content is preferably 0.20% or less.

[0029] Si: 0.50% or less Si is an element that acts as a deoxidizer. While Si does not necessarily need to be included, a content of 0.01% or more is preferable to achieve this effect. However, if the Si content exceeds 0.50%, Si concentrates at the interface between the scale and the base steel, forming a Si oxide layer. Scale spalling is likely to occur at the interface between this Si oxide layer and the scale layer formed thereon. Furthermore, descaling at high temperatures before finish rolling deteriorates, and the primary scale formed up to rough rolling is crushed during finish rolling, which makes so-called red scale more likely to form and results in poor blackness. For this reason, the Si content is set to 0.50% or less. Preferably, it is 0.20% or less.

[0030] Mn: 0.01 to 2.0% Mn is an element that neutralizes solute S, which causes embrittlement during hot working, by converting it to MnS and is also effective in improving strength. A content of less than 0.01% is ineffective. On the other hand, a content exceeding 2.0% results in a decrease in toughness and the formation of Mn-based oxides at the interface between the scale and the base steel, resulting in a decrease in scale adhesion. Furthermore, the transformation after finish rolling is delayed, and the transformation is not completed before coiling. After coiling, the transformation progresses unevenly both locally and longitudinally. This causes loosening of the hot-rolled coil after coiling, and the steel sheet surface comes into contact with an oxidizing atmosphere even in the center of the coil width direction, resulting in reoxidation, i.e., an increase in hematite and magnetite particles and a decrease in the eutectoid transformation structure, leading to a decrease in scale adhesion. For this reason, the Mn content is set to 0.01 to 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 is desirably kept as low as possible because it has a detrimental effect on grain boundary embrittlement. P also forms a very brittle oxide layer at the interface between the scale and the base steel, reducing the adhesion of the scale. If the P content exceeds 0.10%, these detrimental effects become greater, so the P content is set to 0.10% or less. The preferred upper limit is 0.05% or less. P does not need to be contained, but from the standpoint of production costs, the lower limit is preferably 0.001% or more.

[0032] S: 0.10% or less S is an element that significantly deteriorates hot workability and toughness. In addition, S concentrates at the interface between the scale and the base steel, reducing the adhesion of the scale. If the S content exceeds 0.10%, these adverse effects become significant, so the S content is set to 0.10% or less. Preferably, it is set to 0.05% or less. S does not need to be contained, but from the viewpoint of production costs, the lower limit is preferably 0.0001% or more.

[0033] Sol. Al: 0.10% or less Sol. Al is an element that acts as a deoxidizer. The sol. Al content may be 0.00%, but to obtain this effect, it is preferable to contain 0.01% or more. On the other hand, if it is contained in excess of 0.10%, oxide-based inclusions increase and cleanliness decreases. For this reason, the sol. Al content is set to 0.10% or less, preferably 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 reduces the hot ductility of steel and the surface quality. If the N content exceeds 0.015%, the surface quality deteriorates significantly. Therefore, the N content is set to 0.015% or less. The N content is preferably 0.010% or less. Note that N does not necessarily need to be contained, but from the viewpoint of manufacturing costs, the N content is preferably 0.0001% or more. 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 Cu: 1.0% or less, Ni: 0.50% or less, and Cr: 2.0% or less, as necessary, 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 steel to promote grain boundary oxidation, promotes the formation of irregularities at the interface between the scale and the base steel, and improves adhesion at the interface between the scale and the base steel. To achieve these effects, it is preferable to include 0.01% or more of Cu. However, if the Cu content exceeds 1.0%, molten Cu may penetrate into the austenite grain boundaries of the base steel during heating, which may cause 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 Cu, Ni is an element that concentrates at the interface between the scale and the base steel to promote grain boundary oxidation, promotes the formation of irregularities at the interface between the scale and the base steel, and improves adhesion at the interface between the scale and the base steel. To achieve these effects, it is preferable to contain 0.01% or more of Ni. However, if the Ni content exceeds 0.50%, the above effects saturate, and there is a concern that costs will increase. For this reason, if Ni is contained, 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. Furthermore, Cr concentrates at the interface between the scale and the base steel, roughening the interface so that the scale penetrates the base steel, thereby improving the adhesion of the scale. To achieve this effect, a Cr content of 0.01% or more is preferable. On the other hand, if the Cr content exceeds 2.0%, the above effect saturates, so if Cr is contained, the content is set to 2.0% or less. A more preferable lower limit is 0.07% or more, and even more preferably 0.12% or more. A more preferable upper limit is 1.0% or less, and even more preferably 0.8% or less.

[0039] In the present invention, if necessary, one or more of 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 may be further contained.

[0040] Mo: 1.0% or less Mo has the effect of improving strength and hardenability and suppressing softening associated with tempering. To obtain these effects, it is preferable to contain 0.1% or more of Mo. On the other hand, if the content exceeds 1.0%, the strength may increase excessively, and the toughness and formability may deteriorate. Therefore, if Mo is contained, the amount is set to 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 this effect, it is preferable to contain 0.003% or more. On the other hand, if it is contained in an amount exceeding 0.1%, it may actually result in a decrease in toughness. Therefore, if Nb is contained, the amount is set to 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 this effect, it is preferable to contain 0.003% or more. On the other hand, if it is contained in an amount exceeding 0.1%, it may actually result in a decrease in toughness. Therefore, when V is contained, the amount is set to 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 weld heat affected zone. To obtain these effects, it is preferable to contain 0.001% or more of Ti. On the other hand, if it is contained in an amount exceeding 0.03%, it may actually result in a decrease in toughness. Therefore, if Ti is contained, the amount is set to 0.03% or less.

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

[0045] Sb: 0.03% or less Sb concentrates in the surface layer of the steel sheet when the material is heated, and has the effect of suppressing a decrease in the C content in the surface layer of the steel sheet due to decarburization during heating. To achieve this effect, it is preferable to contain 0.001% or more of Sb. On the other hand, if the content exceeds 0.03%, Sb becomes a liquid metal when the material is heated, and may erode the prior austenite grain boundaries, reducing the adhesion of scale. For this reason, if Sb is contained, the content is set to 0.03% or less.

[0046] The balance other than the above chemical components consists of Fe and unavoidable impurities. The allowable unavoidable impurities are O: 0.005% or less, Mg: 0.003% or less, Sn: 0.1% or less, and Ca: 0.01% or less. Furthermore, even if the above optional elements are contained in amounts less than the preferred lower limit, they are also considered unavoidable impurities.

[0047] Next, the scale structure in the width direction of the hot-rolled steel sheet of the present invention will be described. Note that the scale structure in the width direction of the steel sheet refers to the scale structure at the center of the steel sheet in the width direction and at positions 5 mm from the edges in the width direction. It is sufficient if the structure at each position falls within the following ranges.

[0048] Area ratio of magnetite particles: 30% or more The magnetite of the present invention consists of magnetite particles and magnetite contained in the eutectoid-transformed structure. These are structures that can be distinguished as follows. The magnetite particles in the present invention include a magnetite layer near the scale surface consisting of columnar magnetite particles, and massive pro-eutectoid magnetite particles that form within or adjacent to the eutectoid-transformed structure prior to the progression of the eutectoid transformation. Furthermore, they also include a thin layer consisting of fine magnetite particles that forms at the scale-base steel interface, known as a magnetite seam, and are distinguished from magnetite contained in the eutectoid-transformed structure of iron and magnetite. Magnetite particles have higher ductility at room temperature than wüstite or hematite, contributing to improved scale adhesion. In particular, the magnetite seam has high compatibility with the base steel, and therefore contributes to improved scale adhesion by suppressing peeling from the scale-base steel interface. Furthermore, the magnetite layer near the scale surface consisting of columnar magnetite particles is a structure that contributes to blackening. If the magnetite particles are less than 30%, these effects cannot be sufficiently obtained, so the area ratio of magnetite is set to 30% or more, preferably 40% or more. On the other hand, if the magnetite particles are contained in an amount exceeding 80%, cracks may occur in the magnetite, resulting in poor scale adhesion. For this reason, the area ratio of magnetite particles is preferably 80% or less, more preferably 70% or less.

[0049] Area ratio of eutectoid-transformed structure of iron and magnetite: 20% or more and 60% or less The eutectoid-transformed structure of iron and magnetite has high compatibility between magnetite and precipitated Fe and the base steel, and therefore contributes to improving the adhesion of scale. If it is less than 20%, this effect cannot be sufficiently obtained. Therefore, the area ratio of the eutectoid-transformed structure of iron and magnetite is set to 20% or more, preferably 25% or more, and more preferably 30% or more. On the other hand, if the eutectoid-transformed structure of iron and magnetite exceeds 60%, precipitated Fe derived from the eutectoid transformation is also generated near the scale surface, which causes whitening. Therefore, the eutectoid-transformed structure of iron and magnetite is set to 60% or less, preferably 55% or less, and more preferably 50% or less.

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

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

[0052] Area ratio of precipitated Fe on the scale surface: 15% or less Precipitated Fe on the scale surface is a factor that inhibits the blackening of the scale surface. Therefore, if the area ratio exceeds 15%, sufficient blackness cannot be obtained. Therefore, the area ratio of precipitated Fe on the scale surface is set to 15% or less. Furthermore, the area ratio of precipitated Fe on the scale surface may be 0%.

[0053] The average thickness of the scale in the width direction of the steel sheet is 3 μm or more and 20 μm or less. If the average thickness of the scale is less than 3 μm, the color tone of the eutectoid transformation structure formed on the base steel side of the scale and the luster and color tone of the base steel affect the color tone of the surface layer, causing it to appear white. Therefore, the average thickness of the scale is 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 the steel sheet is processed increases, causing cracks to occur in the scale, resulting in a decrease in scale adhesion. Therefore, the average thickness of the scale is 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 a hot-rolled steel sheet according to the present invention will be described.

[0055] The scale structure is measured at the center and edge portions in the width direction of the steel sheet. That is, the center portion in the width direction of the hot rolled coil and portions 5 mm from the edge in the width direction are used as observation positions for evaluation.

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

[0057] Hematite is formed very thinly on the surface of the scale and is easily removed during mirror polishing, making it difficult to quantitatively evaluate it as an area ratio on an SEM. α The integrated intensity of the diffraction peak of each phase in the scale is measured using a radiation source. The mass fraction can be calculated from the ratio of the integrated intensity of each phase in the standard sample and the test sample using the following formula (2). The standard samples were Fe, FeO (wustite), and Fe ２ O ３ (hematite), Fe ３ O ４ The mass fraction of hematite can be regarded as the area fraction. ２ O ３ ) mass fraction = (I Ｆｅ２Ｏ３ / R Ｆｅ２Ｏ３ ) × 100 / ((I Ｆｅ / R Ｆｅ ) + (IＦｅＯ / R ＦｅＯ ) + (I Ｆｅ２Ｏ３ / R Ｆｅ２Ｏ３ ) + (I Ｆｅ３Ｏ４ / R Ｆｅ３Ｏ４ )) ... (2) However, in the above formula (2), 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, Fe ２ O ３ , or Fe ３ O ４ is.

[0058] The area ratio of precipitated Fe on the scale surface can be measured by observing a backscattered electron image of the surface at the above observation position using an SEM at a magnification of 3000x, and determining the area ratio of precipitated Fe that appears with the brightest contrast by image analysis.

[0059] The average thickness of scale in the width direction of the coil can be measured as follows. For example, thickness cross sections perpendicular to the steel sheet surface and parallel to the rolling direction are cut out from the width center of the hot-rolled steel sheet and from a portion 5 mm from the width edge of the hot-rolled steel sheet, and mirror-polished. The scale thickness is then measured at three locations using an SEM, and the thickness can be determined by averaging these measurements. Note that the average thickness of scale in the width direction of 3 μm to 20 μm in the present invention means that the average thickness of scale is 3 μm to 20 μm in the width center of the hot-rolled steel sheet and from a portion 5 mm from the width edge.

[0060] Next, a method for producing a hot-rolled steel sheet according to the present invention will be described.

[0061] The temperature specified in each step in the present invention refers to the surface temperature of the slab (steel slab) or steel plate, and can be measured with a radiation thermometer, etc. Unless otherwise specified, the average cooling rate is defined as "(cooling start temperature - cooling stop temperature) / cooling time".

[0062] In the present invention, the method for producing a steel material having the above-mentioned composition does not need to be particularly limited, and any commonly used method can be applied. For example, it is desirable to produce molten steel having the above-mentioned composition in a converter or electric furnace, and then produce a steel material such as a slab by a casting method such as a continuous casting method. However, there is no problem even if an ingot-blooming and blooming rolling method is used. Usually, the steel material is heated and then hot-rolled. This heating is sufficient as long as sufficient solid solution is achieved, and preferably, Ac ３ Specifically, a temperature range of 1060°C to 1300°C, which is the normal slab heating temperature range, is appropriate. In the case of a slab produced by a continuous casting method, direct rolling may be applied in which the slab is rolled as is or while being held in order to suppress a temperature drop.

[0063] The hot rolling process consists of rough rolling and finish rolling. The conditions for rough rolling need not be particularly limited as long as a sheet bar of the specified dimensions is obtained during rough rolling. Furthermore, in order to perform finish rolling at a specified temperature, the material to be rolled may be heated midway using a heating means such as a sheet bar heater. Before rough rolling and finish rolling, scale formed on the surface of the sheet bar is removed by descaling using high water pressure or the like at the entry side of the rolling mill.

[0064] Next, finish rolling is performed. If the finish rolling entry temperature exceeds 1100°C, the thickness of the scale increases and the adhesion of the scale may decrease. On the other hand, if the finish rolling entry temperature is less than 950°C, the rolling load may increase significantly, which may decrease productivity. Furthermore, as the product thickness increases, the thickness at the finish rolling entry becomes thicker. For example, if the product thickness exceeds 5.0 mm, a long time is required before the start of finish rolling, which may decrease productivity. Therefore, the finish rolling entry temperature is preferably 1100°C or less, and more preferably 1050°C or less. Furthermore, the lower limit of the finish rolling entry temperature is preferably 950°C or more.

[0065] Finish rolling outlet temperature: 800°C or higher and 950°C or lower. If the finish rolling outlet temperature is lower than 800°C, cracks occur due to a decrease in scale ductility. These cracks promote reoxidation of the scale, producing hematite, which causes deterioration of scale adhesion and blackness. Furthermore, the scale thickness decreases, making it difficult to obtain sufficient blackness. Furthermore, the scale structure becomes finer, and the hardness of the scale itself increases, resulting in a decrease in scale adhesion. On the other hand, if the finish rolling outlet temperature exceeds 950°C, excessive scale growth increases the scale thickness, resulting in a decrease in scale adhesion. Furthermore, the grain size of each phase in the scale structure increases, resulting in a decrease in scale adhesion. Therefore, the finish rolling outlet temperature is set to 800°C or higher and 950°C or lower. The preferred lower limit is 820°C or higher. The preferred upper limit is 930°C or lower.

[0066] Cooling at an average cooling rate of 5°C / s or more in the temperature range from the finish rolling exit temperature to 750°C: Scale grows faster in high-temperature regions. Therefore, to prevent a decrease in scale adhesion due to excessive scale growth, it is necessary to rapidly cool the high-temperature region immediately after finish rolling. If the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is less than 5°C / s, scale grows excessively, causing a decrease in scale adhesion. Therefore, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is set to 5°C / s or more, preferably 7°C / s or more. 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 becomes finer, which may result in a decrease in scale adhesion. Furthermore, a decrease in scale ductility causes cracks, and these cracks promote the reoxidation of the scale, resulting in the formation of hematite. This may result in a decrease in scale adhesion and blackness. Therefore, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is preferably 80°C / s or less, and more preferably 50°C / s or less.

[0067] Cooling in the temperature range from 750°C to the start of coiling at an average cooling rate of 1°C / s or more and 30°C / s or less Although scale growth is relatively slower in the temperature range from 750°C to the start of coiling than in the high-temperature region immediately after finish rolling, it is necessary to suppress deterioration of scale adhesion due to excessive scale growth. If the average cooling rate in the temperature range from 750°C to the start of coiling is less than 1°C / s, scale grows excessively, causing deterioration of scale adhesion. For this reason, the average cooling rate in the temperature range from 750°C to the start of coiling is set to 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 coiling exceeds 30°C / s, the scale structure becomes finer and the stress difference with the base steel becomes larger. This causes cracks to form in the scale, and these cracks promote reoxidation of the scale. This causes hematite to form, causing deterioration of scale adhesion and blackness. Therefore, the average cooling rate in the temperature range from 750°C to the start of coiling is set to 30°C / s or less, and preferably 20°C / s or less.

[0068] Coiling temperature: 450°C or higher and 600°C or lower After the above cooling, the steel sheet is coiled at a coiling temperature of 450°C or higher and 600°C or lower. If the coiling temperature is lower than 450°C, the eutectoid transformation from wüstite does not occur sufficiently after coiling, and an excessive amount of wüstite remains at room temperature. As a result, wüstite is brittle at room temperature, which reduces the scale adhesion. If the coiling temperature exceeds 600°C, the scale grows excessively, which reduces the scale adhesion. Furthermore, in the widthwise central portion, which is isolated from the oxidizing atmosphere after coiling, hematite and magnetite in the surface layer are reduced to wüstite. The eutectoid transformation of this wüstite causes Fe to precipitate on the steel sheet surface, which inhibits blackening. Furthermore, at the widthwise edge portions, the formation of hematite due to reoxidation is promoted, which causes whitening. Therefore, the coiling temperature is set to 450°C or higher and 600°C or lower. The preferred lower limit is 470°C or higher. The preferred upper limit is 580°C or lower.

[0069] After the start of coiling, the entire coil is cooled so that the temperature of the coil edge portions is cooled from the coiling 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 the start of coiling, the entire coil is cooled so that the temperature of the coil edge portions is cooled from the coiling 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 reduces the temperature of the coil edge portions, increases the rigidity of the coil, and prevents loosening of the coil. Here, "the entire coil" refers to both edge portions and the plate surface of the coil. "Both edge portions of the coil" refers to a range within 200 mm in the width direction of the coil from both edges in the width direction of the coil. Furthermore, "plate surface of the coil" refers to the surface of the coil in a range other than both end portions. As a result, the widthwise center portion of the coil is isolated from the oxidizing atmosphere, thereby suppressing hematite formation due to reoxidation, suppressing whitening, and sufficiently obtaining a eutectoid transformation structure suitable for improving adhesion. Furthermore, although it is difficult to completely isolate the widthwise edge portions of the coil from the oxidizing atmosphere, cooling the edge portions suppresses reoxidation, and reheating from the widthwise center promotes eutectoid transformation, ensuring excellent adhesion and blackness. The temperature of the edge portions can be measured, for example, using a radiation thermometer. The temperature of the sheet surface is not specified because it is not possible to measure the temperature of the sheet surface at a specific longitudinal position during coiling. However, the above-mentioned effect can be achieved by cooling the entire coil using the same cooling method as the edge portions. If the cooling stop temperature at the edge portions of the coil exceeds 430°C, the above-mentioned effect cannot be fully achieved. Furthermore, if the cooling stop temperature at the edge portions of the coil is less than 300°C, the edge portions will be supercooled, leaving a large amount of wüstite in the scale at the edge portions, resulting in reduced adhesion. Therefore, the cooling stop temperature at the edge portions of the coil is set to 300°C or higher and 430°C or lower. The preferred lower limit is 320°C or higher. The preferred upper limit is 400°C or lower. Furthermore, if the average cooling rate at the edge portion is less than 0.5° C. / s, the above effect cannot be sufficiently obtained.If the average cooling rate at the edge portion 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 steel. These cracks promote the reoxidation of the scale, resulting in the formation of hematite, which reduces the adhesion and blackness of the scale. For this reason, the average cooling rate at the edge portion is set to 0.5°C / s or more and 6.0°C / s or less, and preferably 1.0°C / s or more and 5.0°C / s or less. There are no particular restrictions on the cooling method for the coil, but it is preferable to cool the coil using a cooling device that sprays water onto the plate surface and both edges of the coil while winding it in a winding machine, for example.

[0070] It is preferable that the cooled coil is placed in a coil box or covered to promote the eutectoid transformation from wustite and to suppress oxidation of the outermost periphery and edges.

[0071] Furthermore, the hot-rolled steel sheet wound into a coil may be subjected to shape correction treatment by deforming the steel sheet using a roller leveler, tension leveler, or the like.

[0072] Examples of the present invention will be described below.

[0073] Steels having the compositions shown in Table 1 were melted and cast to prepare steel materials. These steel materials were hot-rolled under the conditions shown in Table 2 to prepare black-skinned hot-rolled coils having a thickness of 2 mm. Test specimens were taken from various positions in the width direction of the obtained hot-rolled coils, and the scale structure, scale thickness, adhesion, and blackness were evaluated by the following methods.

[0074]

[0075]

[0076] The area ratios of magnetite grains, the eutectoid transformed structure of iron and magnetite, and wüstite were measured as follows. A cross section of the sheet thickness perpendicular to the steel sheet surface and parallel to the rolling direction was cut out as an observation surface from the center of the hot-rolled coil in the width direction and from a portion 5 mm from the edge in the width direction, and mirror-polished. Then, a backscattered electron image of the cross section of the scale was observed using an SEM at a magnification of 3000x to measure. In the backscattered electron image of the SEM, magnetite is the darkest region, the base steel and precipitated Fe in the eutectoid transformed structure are the brightest, and wüstite is the region that appears with intermediate contrast. The eutectoid transformed structure of iron and magnetite is the region where magnetite and iron are formed in layers.

[0077] Hematite was analyzed using an X-ray diffractometer. α The integrated intensity of the diffraction peaks of each phase in the scale was measured using a radiation source. ２ O ３ (hematite), Fe ３ O ４ The mass fraction was calculated using the following formula (2) from the ratio of the integrated intensity of each phase in the test sample to that in the mixture of equal weights of hematite (Fe ２ O ３ ) mass fraction = (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 : Integrated intensity of phase A in the test sample R Ａ : Integrated intensity of phase A in the standard sample A: Fe, FeO, Fe ２ O ３ , or Fe ３ O ４ is.

[0078] The area ratio of precipitated Fe on the outermost surface of the steel sheet was measured by observing backscattered electron images of the scale surface at the center of the width direction of the hot-rolled coil and at a position 5 mm from the edge in the width direction using an SEM at a magnification of 3000. The area ratio of precipitated Fe that appeared with the brightest contrast was determined by image analysis.

[0079] The average thickness of the scale in the width direction of the coil was determined by cutting out thickness sections of the hot-rolled coil from the width center and from a portion 5 mm from the width edge, perpendicular to the steel sheet surface and parallel to the rolling direction, and then mirror-polishing the sections. After that, the scale thickness was measured at three locations using an SEM and the average was calculated.

[0080] Scale adhesion was evaluated by taking 30 mm × 100 mm test pieces from the widthwise center of the hot-rolled coil and from a section 5 mm from the widthwise edge, with the longitudinal direction parallel to the rolling direction. Next, a bending test using the push-bending method (JIS Z 2248) was performed. Cellophane tape was applied to the surface of the test piece on the outer side of the bend, which was then peeled off. The amount of scale adhered to the tape was measured by image analysis. The bending test conditions were a ratio r / t of the push-ring tip radius r to the test piece thickness t of 4, and a bending angle of 180°. The evaluation criteria were as follows, with ◯ and △ representing excellent adhesion: ◯: Almost no peeling was observed. (The area ratio of the scaled area was less than 10%). △: Some peeling was observed, but no practical problems were encountered. (The area ratio of the scaled area was 10% or more but less than 50%). ×: Peeling was observed in many areas, resulting in practical problems. (The area ratio of the scaled portion is 50% or more) The blackness was measured using a spectrophotometer (CM-700d manufactured by KONICA MINOLTA) after degreasing the steel sheet surface in the width direction center of the hot rolled coil and in the area 5 mm from the edge in the width direction with alcohol. ＊ a ＊ b ＊ L of the color system (JIS Z 8729) ＊ The value was determined by measuring the L ＊The smaller the value, the closer to black the color is. The viewing angle during measurement was 10°, and the main light source was auxiliary illuminant D65 (daylight, color temperature 6504K), and the measurement was performed in specular light removal mode. ＊ The value was considered to be excellent in blackness.

[0081] The examples of the present invention shown in Table 2 were uniformly excellent in scale adhesion and blackness both in the widthwise center of the hot-rolled coil and in the region 5 mm from the widthwise edge, whereas the comparative examples were poor in adhesion or blackness at either or both widthwise positions.

Claims

1. In mass %, C: 0.01 to 0.30%, Si: 0.50% or less, Mn: 0.01 to 2.0%, P: 0.10% or less, S: 0.10% or less, sol.

1. A hot-rolled steel sheet having a component composition containing Al: 0.10% or less, N: 0.015% or less, with the balance being Fe and unavoidable impurities, having scale on a surface of the steel sheet, the scale in the width direction of the steel sheet having, in area ratios, magnetite grains: 30% or more, a eutectoid transformed structure of iron and magnetite: 20% to 60%, wherein the magnetite consists of the magnetite grains and magnetite contained in the eutectoid transformed structure, wustite: 20% or less, and a structure having, in mass fractions, hematite: 5% or less, an area ratio of precipitated Fe on a surface of the scale being 15% or less, and an average thickness of the scale in the width direction of the steel sheet being 3 μm to 20 μm.

2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition further contains, in mass%, one or more of the following: Cu: 1.0% or less; Ni: 0.50% or less; Cr: 2.0% or less.

3. The hot-rolled steel sheet according to claim 1 or 2, wherein the chemical composition further contains, in mass%, one or more of the following: 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.

4. A method for manufacturing hot rolled steel sheet, comprising the steps of: rough hot rolling a steel material having a chemical composition as defined in any one of claims 1 to 3, followed by descaling; finish rolling at a finish rolling exit temperature of 800°C or more and 950°C or less; cooling in the temperature range from the finish rolling exit temperature to 750°C at an average cooling rate of 5°C / s or more; cooling in the temperature range from 750°C to the start of coiling at an average cooling rate of 1°C / s or more and 30°C / s or less; coiling at a coiling temperature of 450°C or more and 600°C or less; and cooling the entire coil so that the temperature of the edge of the coil is cooled at an average cooling rate of 0.5°C / s or more and 6.0°C / s or less from the start of coiling to a cooling stop temperature of 300°C to 430°C or less from the coiling temperature.

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