Hot-rolled steel sheet and method for producing same

Abstract

The purpose of the present invention is to provide: a hot-rolled steel sheet having excellent scale adhesion, in particular, having small variations in scale adhesion in the width direction of a coil, even in a hot-rolled steel sheet having a larger plate thickness; and a method for producing the same.　The present invention provides a hot-rolled steel sheet having a prescribed component composition and having scale on the surface of the steel sheet, wherein: the scale in the width direction of the steel sheet has a structure including, in terms of area ratio, 20-60% of magnetite particles, at least 30% of an eutectoid transformation structure of iron and magnetite, where the magnetite comprises the magnetite particles and magnetite contained in the eutectoid transformation structure, and at most 15% of wustite, as well as at most 5% of hematite in terms of mass fraction; the average thickness of the scale in the width direction of the steel sheet is 5-20 μm; and the variation in the thickness of the scale in the width direction of the steel sheet is 4 μm or less.

Description

Hot-rolled steel sheet and manufacturing method thereof

[0001] The present invention relates to a hot-rolled steel sheet having excellent scale adhesion and a manufacturing method thereof, which is used for construction and industrial machinery, automobiles, home appliances, building materials, etc. The present invention particularly relates to a hot-rolled steel sheet having excellent scale adhesion and small variation in scale adhesion in the width direction of the steel sheet, which is suitable as a material for parts of construction and industrial machinery that are subjected to laser cutting, and a manufacturing method thereof.

[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, the scale on a hot-rolled steel sheet tends to peel off more easily as the thickness of the hot-rolled steel sheet increases, because the strain generated in the scale during deformation increases. As a result, the scale is more likely to peel off during forming with a large degree of processing, such as bending or press forming. Meanwhile, the need for thick, black-skinned hot-rolled steel sheets has been increasing in recent years. For example, there is a strong demand for improved scale adhesion even for hot-rolled steel sheets having a thickness of more than 5.0 mm.

[0004] Furthermore, the scale adhesion varies across the width of the steel sheet, and poor adhesion in some areas can cause processing problems. This is because, for example, the amount of thermal energy absorbed by the surface of the laser beam during laser cutting becomes unstable. Therefore, it becomes necessary to remove the areas with poor scale adhesion before use. Therefore, from the viewpoints of improving yield, workability, and appearance quality, there is a strong demand for hot-rolled steel sheets with uniform scale thickness across the width and excellent scale adhesion.

[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 width 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] JP 2019-183267 A JP 2004-027312 A JP 2012-148286 A

[0009] 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, no mention is made of a method for uniformly improving scale adhesion in the width direction.

[0010] 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 base steel of the steel sheet within a predetermined range. However, although the adhesion at the interface between the scale layer and the base steel is improved, there is a concern that the adhesion of the scale may be insufficient when the thickness of the hot-rolled steel sheet is increased. Furthermore, there is no mention of a method for uniformly improving the adhesion of the scale in the width direction.

[0011] The technology described in Patent Document 3 discloses a method for producing a hot-rolled steel sheet by hot-rolling a steel material and winding it into a coil. In this technology, a 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. This method proposes a hot-rolled steel sheet with excellent scale adhesion, particularly at the edge of the hot-rolled coil. However, no suitable scale structure is specified for improving adhesion in the center of the steel sheet in the width direction. As a result, there is a concern that scale adhesion may be insufficient if the thickness of the hot-rolled steel sheet is increased. Furthermore, although the adhesion at the very edge portion is improved, there is a concern that the adhesion within 200 mm from the edge is not sufficiently improved.

[0012] The present invention aims to solve the above-mentioned problems and to provide a hot-rolled steel sheet and a manufacturing method thereof that have excellent scale adhesion even in thicker hot-rolled steel sheets, with small variations in scale adhesion in the width direction of the steel sheet in particular, and improved laser cuttability.

[0013] The present inventors first investigated the reason why uniform and excellent scale adhesion cannot be obtained in the width direction in conventional hot-rolled steel sheets. The scale formed during hot rolling is composed of hematite (Fe) from the surface side of the scale at high temperatures. 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). The eutectoid-transformed structure consisting of magnetite and precipitated Fe has particularly high compatibility with the surrounding magnetite grains and base steel, contributing to improved scale adhesion. With conventional technology, scale adhesion tended to be particularly poor within a range of 200 mm from the edge in the width direction. This is because the scale is reoxidized by air invading from the edge after coiling, resulting in an excessive increase in the amount of hematite on the scale surface and the magnetite layer near the scale surface, consisting of columnar magnetite grains. This has been shown to reduce the proportion of the eutectoid-transformed structure that contributes to improved scale adhesion, and to increase the scale thickness at the edge. Furthermore, it has been shown that within a range of 200 mm from the edge in the width direction, scale adhesion deteriorates at the edge and in the region 200 mm from the edge due to different mechanisms. Because the cooling rate is highest at the edge, wüstite remains in the final scale structure, further reducing adhesion. On the other hand, the cooling rate at the region 200 mm from the edge is slower due to exposure to air invading from the edge and recuperation from the center in the width direction. As a result, the amount of hematite and magnetite layers near the scale surface is greatest in the width direction, significantly reducing adhesion. In other words, it became clear that in order to obtain uniformly excellent scale adhesion in the width direction, it is necessary to improve scale adhesion at the edge and in the area 200 mm from the edge.

[0014] Furthermore, when manufactured under normal conditions, especially when the plate is thick, transformation expansion after coiling occurs unevenly along the length of the coil, which can cause loosening of the hot-rolled coil and lead to air intrusion in the center of the width direction, which can promote reoxidation and worsen adhesion.

[0015] Therefore, the present inventors have conducted extensive research into means for solving the above-mentioned problems and obtaining a hot-rolled steel sheet having excellent scale adhesion even in a thicker hot-rolled steel sheet, with small variations in scale adhesion, particularly in the width direction of the coil, and have obtained the following findings. (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, and then cooled 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 cause a decrease in adhesion. (ii) After the start of coiling, the entire coil is cooled at an average cooling rate of 0.5°C / s or more and 6.0°C / s or less, from the coiling temperature to a cooling stop temperature of 300°C or more and 450°C or less, where the temperature of the edge portion of the coil is from the coiling temperature. Here, the edge portion of the coil refers to the area within 200 mm from the edge in the width direction of the coil. This increases the rigidity of the coil and prevents loosening of the coil. As a result, the center of the coil width is isolated from the oxidizing atmosphere, suppressing reoxidation and achieving a sufficient eutectoid transformation structure suitable for improving adhesion. Furthermore, although it is difficult to completely isolate the coil's edge portions from the oxidizing atmosphere, cooling the edge portions suppresses reoxidation, and reheating from the center of the coil width promotes eutectoid transformation, ensuring excellent adhesion. This also suppresses variation in scale thickness across the width, improving scale adhesion uniformly across the width and improving laser cuttability.

[0016] The present invention has been made based on the above findings, and specifically provides the following: [1] A steel sheet having a composition containing, by 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. Al: 0.10% or less, N: 0.015% or less, with the balance being Fe and unavoidable impurities, and having scale on the surface of the steel sheet, the scale in the width direction of the steel sheet having, in area ratio, magnetite particles: 20% to 60%, iron and magnetite particles: 20% to 60%, [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 500°C or higher and 650°C or lower, and the entire coil is cooled at an average cooling rate of 0.5°C / s or higher and 6.0°C / s or lower from the start of coiling to a cooling stop temperature at which the temperature of the edge of the coil is from the coiling temperature to 300°C or higher and 450°C or lower. A method for producing a hot-rolled steel sheet.

[0017] According to the present invention, a hot-rolled steel sheet having excellent scale adhesion can be easily and inexpensively produced, which is of great industrial benefit. Furthermore, according to the present invention, the variation in scale adhesion in the width direction of the steel sheet can be reduced, which has the effect of significantly contributing to improving the surface quality of the product, improving the laser cuttability of the product, and improving the working environment. Furthermore, the present invention can also solve the problem of reduced scale adhesion that occurs with increasing thickness of the hot-rolled steel sheet.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] Si: 0.50% or less Si is an element that acts as a deoxidizer. Although it is not 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 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. For this reason, the Si content is set to 0.50% or less. Preferably, it is 0.20% or less.

[0023] 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. It is particularly effective in ensuring the strength of thick steel sheets, which are prone to strength loss after hot rolling. A content of less than 0.01% is ineffective. On the other hand, a content of more than 2.0% results in reduced toughness and the formation of Mn-based oxides at the interface between the scale and the base steel, resulting in reduced 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 contact of the steel sheet surface with an oxidizing atmosphere, even in the center of the coil width direction, leads to reoxidation, i.e., an increase in hematite and magnetite particles and a decrease in the eutectoid transformation structure, resulting in reduced 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%.

[0024] 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. Preferably, it is set to 0.05% or less. P does not need to be contained, but from the viewpoint of production costs, the lower limit is preferably 0.001% or more.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] Next, the scale structure in the width direction of the steel plate of the present invention will be described. Note that the scale structure in the width direction of the steel plate means the scale structure in the center of the steel plate in the width direction and within 200 mm from the edge in the width direction. In practice, the scale structure is measured at the center of the steel plate in the width direction and at positions 5 mm and 200 mm from the edge in the width direction, and it is sufficient if the structure at each position is within the following range. Furthermore, if the structure at each measurement position is within the respective ranges below, the structure is considered to be uniform in the width direction.

[0041] Area ratio of magnetite particles: 20% or more and 60% or less 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 inside or adjacent to the eutectoid-transformed structure prior to the progress of 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, which is distinguishable from the magnetite contained in the eutectoid-transformed structure of iron and magnetite. Magnetite particles have higher ductility at room temperature than wüstite or hematite, and contribute to improving scale adhesion. In particular, magnetite seams have high compatibility with the base steel, and therefore contribute to improving scale adhesion by suppressing peeling from the scale-base steel interface. Since this effect cannot be fully achieved if the magnetite particles are less than 20%, the area ratio of magnetite particles is set to 20% or more. Preferably, it is 30% or more. On the other hand, if the magnetite particles are contained in an amount exceeding 60%, cracks will occur in the magnetite, resulting in poor scale adhesion. Therefore, the area ratio of the magnetite particles is set to 60% or less, preferably 50% or less.

[0042] Area ratio of eutectoid transformed structure of iron and magnetite: 30% or more 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 scale adhesion. If it is less than 30%, this effect cannot be sufficiently obtained, so the area ratio of the eutectoid transformed structure of iron and magnetite is set to 30% or more. It is preferably 35% or more, and more preferably 40% or more. There is no particular upper limit, but in order to obtain the combined effect of the magnetite particles and the effect of improving scale adhesion, the eutectoid transformed structure of iron and magnetite is preferably 80% or less, and more preferably 70% or less.

[0043] Area ratio of wüstite: 15% or less Wüstite is a stable phase at high temperatures, and most of it disappears due to 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 15% or less. The upper limit of the area ratio of wüstite is preferably 10% or less, more preferably 7% or less. The area ratio of wüstite may be 0%.

[0044] 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 is particularly likely to form at the edges in the coil width direction due to reoxidation of the scale caused by air entering from the edges after coiling. Hematite not only deteriorates scale adhesion but also causes surface defects such as red scale. For this reason, the hematite mass fraction is set to 5% or less.

[0045] The average thickness of the scale in the width direction of the steel sheet is 5 μm or more and 20 μm or less, and the variation in the scale thickness in the width direction of the steel sheet is 4 μm or less. An average scale thickness of less than 5 μm can cause processing problems because the amount of thermal energy of the laser beam absorbed by the steel sheet surface during laser cutting is insufficient. On the other hand, if the average scale thickness exceeds 20 μm, particularly in the case of a thick hot-rolled steel sheet, the strain applied to the scale surface during processing of the steel sheet increases, causing cracks in the scale and reducing scale adhesion. For this reason, the average scale thickness is set to 20 μm or less. Preferably, it is set to 18 μm or less, and more preferably, to 15 μm or less. Furthermore, in order to uniformly increase the adhesion of the scale in the width direction and obtain excellent laser cuttability, the average scale thickness in the width direction of the steel sheet is controlled to 5 μm or more and 20 μm or less, and the variation in the scale thickness in the width direction of the steel sheet is set to 4 μm or less. Preferably, the variation in the scale thickness in the width direction of the steel sheet is set to 3 μm or less. Here, the variation in scale thickness refers to the maximum difference in the average thickness of the scale measured at a plurality of measurement positions. The methods for measuring the plurality of measurement positions, the average scale thickness, and the variation in scale thickness will be described later.

[0046] 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.

[0047] The magnetite grains, the eutectoid transformed structure of iron and magnetite, and the area ratio of wüstite are measured by cutting out a cross section of the steel sheet perpendicular to the surface and parallel to the rolling direction, mirror-polishing it, and then observing it using a scanning electron microscope (SEM). The SEM field of view covers the entire scale thickness, from the scale surface to the interface between the scale and the steel sheet. Therefore, measurements can be made by observing a backscattered electron image of the cross section of the scale at a magnification that covers the entire scale thickness. In the backscattered electron image of the SEM, the magnetite grains are the darkest, the base steel is the brightest, and wüstite is the region that appears with intermediate contrast. Furthermore, the eutectoid transformed structure of iron and magnetite is the region where magnetite and iron are formed in layers.

[0048] 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, and the mass fraction can be calculated from the ratio of the integrated intensity of each phase in the standard sample to that in the test sample using the following formula (2). 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 ) + (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.

[0049] The average scale thickness is measured by the following procedure. For example, thickness cross sections perpendicular to the steel sheet surface and parallel to the rolling direction are cut out from the widthwise center of the hot-rolled steel sheet, a portion 200 mm from the widthwise edge of the hot-rolled steel sheet, and a portion 5 mm from the widthwise edge of the hot-rolled steel sheet, and mirror-polished. The scale thickness is then measured at three locations using an SEM, and the average is used to determine the scale thickness at each width position. Note that the average scale thickness in the width direction in this invention means that the average scale thickness at each width position is 5 μm or more and 20 μm or less. The variation in the widthwise scale thickness can be determined by subtracting the minimum value from the maximum value of the average scale thickness at the widthwise center of the hot-rolled steel sheet, a portion 5 mm from the widthwise edge, and a portion 200 mm from the widthwise edge.

[0050] Next, a method for producing a hot-rolled steel sheet according to the present invention will be described.

[0051] 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".

[0052] 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 normal slab heating temperature range of 1060°C to 1300°C 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.

[0053] The hot rolling process consists of rough rolling and finish rolling. The rough rolling conditions do not need to be particularly limited as long as the rough rolling can produce a sheet bar of the specified dimensions. Before rough rolling, it is preferable to remove scale formed by slab heating by descaling. 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. After rough rolling, before 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.

[0054] 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 finish rolling entry thickness also increases. 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, more preferably 1050°C or less. Furthermore, the lower limit of the finish rolling entry temperature is preferably 950°C or more.

[0055] 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 a decrease in scale adhesion. In addition, 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. In addition, 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.

[0056] 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. Because scale grows faster in high-temperature regions, rapid cooling of the high-temperature region immediately after finish rolling is necessary to prevent deterioration of scale adhesion due to excessive scale growth. If the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is less than 5°C / s, excessive scale growth occurs, resulting in deterioration of scale adhesion. For this reason, 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 deterioration of scale adhesion. Furthermore, cracks occur due to a decrease in scale ductility, and these cracks promote reoxidation of the scale, resulting in the formation of hematite, which may result in deterioration of scale adhesion. Therefore, the average cooling rate in the temperature range from the finish rolling delivery temperature to 750°C is preferably 80°C / s or less, more preferably 50°C / s or less.

[0057] 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 in 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 will grow excessively, causing deterioration in 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 will become finer and the stress difference with the base steel will increase, causing cracks to form in the scale. These cracks promote reoxidation of the scale, which produces hematite and reduces scale adhesion. For this reason, the average cooling rate in the temperature range from 750°C to the start of coiling is set to 30°C / s or less. Preferably, it is 20° C. / s or less.

[0058] Coiling temperature: 500°C or higher and 650°C or lower After the above cooling, the steel sheet is coiled at a coiling temperature of 500°C or higher and 650°C or lower. If the coiling temperature is lower than 500°C, 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, and scale adhesion decreases. If the coiling temperature exceeds 650°C, scale grows excessively, and scale adhesion decreases. Furthermore, in the width direction central portion that is isolated from the oxidizing atmosphere after coiling, hematite and magnetite on the scale surface are reduced to wüstite, and a sufficient amount of magnetite particles cannot be obtained. Therefore, the coiling temperature is set to 500°C or higher and 650°C or lower. A preferable lower limit is 530°C or higher. A preferable upper limit is 630°C or lower.

[0059] 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 450°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 450°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 reoxidation and enabling a sufficient eutectoid transformation structure suitable for improving adhesion to be obtained. Furthermore, although it is difficult to completely isolate the widthwise edge portions of the coil from the oxidizing atmosphere, cooling the edges suppresses reoxidation, and reheating from the widthwise center promotes eutectoid transformation, ensuring excellent adhesion. This also suppresses variation in scale thickness across the width, improving scale adhesion uniformity across the width and improving laser cuttability. 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 for the edge portions. If the cooling stop temperature at the edge portions of the coil exceeds 450°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 overcooled, leaving a large amount of wüstite in the scale at the edge portions, resulting in reduced adhesion. For this reason, the cooling stop temperature at the edge portions of the coil is set to 300°C or higher and 450°C or lower, preferably 320°C or higher and 430°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 and a decrease in scale adhesion. 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 coil cooling method, 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.

[0060] 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.

[0061] Furthermore, the hot-rolled steel sheet wound into a coil may be subjected to shape correction processing by deforming the steel sheet using a roller leveler, a tension leveler, etc. For example, for a hot-rolled steel sheet having a thickness of 12 mm, shape correction processing is performed by arranging two upper rolls and three lower rolls each having a diameter of 250 mm, with a pressing depth of 2 mm.

[0062] Examples of the present invention will be described below.

[0063] 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 form hot-rolled coils with thicknesses of 3 to 23 mm. The obtained hot-rolled coils were subjected to shape correction using a leveler, and then cut to a predetermined length to obtain hot-rolled sheets. Test specimens were taken from the widthwise center and widthwise edge portions of the obtained hot-rolled sheets, and the scale structure, scale thickness, adhesion, and laser cuttability were evaluated using the methods described below. The evaluation results are shown in Table 3.

[0064]

[0065]

[0066]

[0067] The area ratios of magnetite grains, the eutectoid transformed structure of iron and magnetite, and wüstite were measured by cutting out a cross section of the steel sheet perpendicular to the surface and parallel to the rolling direction, mirror-polishing it, and then observing a backscattered electron image of the cross section of the scale using an SEM at a magnification of 3000. In the backscattered electron image of the SEM, the magnetite grains are the darkest region, the base steel is the brightest, and wüstite appears with intermediate contrast, and the eutectoid transformed structure of iron and magnetite is the region where magnetite and iron are formed in layers.

[0068] The mass fraction of hematite was measured 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.

[0069] The average thickness of the scale was measured by cutting out thickness sections of the hot-rolled sheet perpendicular to the steel sheet surface and parallel to the rolling direction from the widthwise center, a section 200 mm from the widthwise edge, and a section 5 mm from the widthwise edge. After mirror polishing, the scale thickness was measured at three arbitrary positions using an SEM and averaged to determine the scale thickness at each widthwise position.

[0070] Scale adhesion was evaluated by collecting test specimens from the widthwise center of the hot-rolled sheet after leveling, from a portion 200 mm from the widthwise edge, and from a portion 5 mm from the widthwise edge. Tape was then applied to the surface of the steel sheet to peel off the scale. The scale adhesion was then evaluated based on whether or not the base steel was exposed on the steel sheet surface and the amount of scale adhered to the tape. That is, tape was applied to the surface of the steel sheet, and the peeled tape was attached to a transparent sheet, after which the image was scanned and the amount of peeled scale was measured by image processing. When the area ratio of the scale adhered to the peeled tape was less than 10%, the scale adhesion was deemed excellent and was marked with a circle in Table 3. On the other hand, when the area ratio of the scale adhered to the peeled tape was 10% or more, the scale adhesion was deemed poor and was marked with an X in Table 3.

[0071] For laser cuttability, the hot-rolled sheet after leveling was subjected to linear laser cutting parallel to the width direction using an ENSIS3015AJ laser cutting machine manufactured by AMADA MACHINERY CO., LTD. and a fiber laser oscillator. When cutting was impossible, or when adhesion of dross or droplets or the occurrence of notches was observed on the cut surface and a stable cut surface was not obtained, the laser cuttability was judged to be poor and this was recorded as × in Table 3. On the other hand, when neither of these occurrences occurred and a stable cut surface was obtained in the width direction, the laser cuttability was judged to be excellent and this was recorded as ○ in Table 3. Note that oxygen was used as the assist gas during laser cutting, the cutting speed was 1500 mm / min, the laser output was 3 kW, and the focal position was 3.0 mm from the steel sheet surface.

[0072] The examples of the present invention shown in Table 3 had uniformly excellent adhesion to scale and excellent laser cuttability in all of the widthwise center, the portion 5 mm from the widthwise edge, and the portion 200 mm from the widthwise edge of the hot-rolled sheet. In contrast, the comparative examples had poor adhesion or laser cuttability at all widthwise positions.

Claims

1. A steel sheet having a composition containing, by 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.Al: 0.10% or less, N: 0.015% or less, with the balance being Fe and unavoidable impurities, having scale on the surface of the steel sheet, the scale in the width direction of the steel sheet having, by area ratio, magnetite particles: 20% to 60% and a eutectoid transformed structure of iron and magnetite: 30% or more, wherein the magnetite is composed of the magnetite particles and magnetite contained in the eutectoid transformed structure, wustite: 15% or less, and hematite: 5% or less, the average thickness of the scale in the width direction of the steel sheet being 5 μm to 20 μm, and the variation in thickness of the scale in the width direction of the steel plate is 4 μm or less.

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 producing 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 500°C or more and 650°C or less; and cooling the entire coil 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 or more and 450°C or less, where the temperature of the edge of the coil is from the coiling temperature.

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