Hot-rolled steel sheet and method for manufacturing the same

Abstract

The purpose of the present invention is to provide: a hot-rolled steel sheet 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

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet with excellent scale adhesion properties, used in construction machinery, 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 with excellent scale adhesion properties, suitable as a material for parts of construction machinery that are subjected to laser cutting, and with small variations in scale adhesion properties in the width direction of the steel sheet, and a method for manufacturing the same. [Background technology]

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

[0003] Furthermore, the scale of hot-rolled steel sheets tends to delaminate more easily as the thickness of the sheet increases, due to the increased strain on the scale during deformation. As a result, delamination becomes more likely in highly processed forms such as bending and press forming. On the other hand, the demand for thicker black scale hot-rolled steel sheets has been increasing in recent years. For example, there is a strong demand for improved scale adhesion in hot-rolled steel sheets with thicknesses exceeding 5.0 mm.

[0004] Furthermore, variations in scale adhesion across the width of the steel sheet, and areas with poor adhesion, can cause processing problems. This is because, for example, the amount of thermal energy absorbed by the laser light on the surface of the steel sheet becomes unstable during laser cutting. As a result, the sheet must be used after removing areas with poor scale adhesion. Therefore, from the perspective 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] Conventionally, various proposals have been made to improve scale adhesion. For example, Patent Document 1 describes a steel material having a composition of C: 0.01~0.3%, Si: 0.20% or less, Mn: 0.01~2.0%, P: 0.10% or less, S: 0.10% or less, Al: 0.10% or less, Cr: 0.01~2.0% by mass%, with the remainder being Fe and unavoidable impurities. This material is subjected to rough rolling, descaling, and finish rolling at a finish rolling exit temperature of 800~950°C, satisfying the following equation (1), with an average cooling rate of 3°C / s or more and 80°C from the end of finish rolling to the start of winding. A hot-rolled steel sheet with excellent scale adhesion has been proposed, characterized by having a magnetite layer from the base metal side, magnetite grains and / or a eutectoid transformation structure of iron and magnetite in the upper layer of the magnetite layer, the average particle size of the magnetite grains and / or the average block size of the eutectoid transformation structure being 3 μm or more and 8 μm or less, and the mass fraction of wustite contained in the scale layer being 10% or less. |T2-T1|≦50℃ and |T3-T2|≦50℃···(1) However, in equation (1) above, T1: Temperature (°C) of the steel sheet after finish rolling, 30 m from the leading edge in the longitudinal direction and at the center in the width direction. T2: Temperature (°C) at the center of the longitudinal and widthwise directions of the steel sheet after finish rolling. T3: This is the temperature (°C) of the steel sheet after finish rolling, located 30 m from the tail end in the longitudinal direction and at the center in the width direction.

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

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

[0008] [Patent Document 1] Japanese Patent Publication No. 2019-183267 [Patent Document 2] Japanese Patent Publication No. 2004-027312 [Patent Document 3] Japanese Patent Publication No. 2012-148286 [Overview of the project] [Problems that the invention aims to solve]

[0009] The technology described in Patent Document 1 uses a steel material having a predetermined component composition and adjusts the finish rolling exit 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 metal side and / or the average block size of the eutectoid transformation structure of iron and magnetite in the scale layer. Furthermore, by controlling the longitudinal temperature of the steel sheet immediately after finish rolling, uniform improvement of scale adhesion in the longitudinal direction is achieved. However, there is no mention 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 tight-scale properties 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 steel sheet surface scale and the steel sheet base metal within a predetermined range. However, although the adhesion at the interface between the scale layer and the base metal is improved, there is a concern that the scale adhesion will become insufficient when the thickness of the hot-rolled steel sheet increases. Furthermore, there is no mention of a method for uniformly improving scale adhesion in the width direction.

[0011] Patent Document 3 discloses a method for manufacturing hot-rolled steel sheets, in which steel material is hot-rolled and wound into a coil. In this method, a roughly-rolled steel sheet is subjected to finish rolling at 850 to 1050°C, and then the finished-rolled hot-rolled steel sheet is wound into a coil at a winding temperature of 500 to 650°C while cooling both ends of the hot-rolled steel sheet so that the temperature at the ends falls below 480°C within 5 minutes of the start of winding. After that, the temperature at the ends is maintained below 480°C, and then, while still in the coil shape, it is slowly cooled from the point when the temperature at the ends falls below 400 to 480°C. This method proposes a hot-rolled steel sheet with particularly excellent scale adhesion at the edges of the hot-rolled coil. However, a scale structure suitable for improving adhesion in the central part in the width direction of the steel sheet is not specified. As a result, there is a concern that the scale adhesion may become insufficient when the thickness of the hot-rolled steel sheet increases. Furthermore, while adhesion at the very edge improves, there is a concern that adhesion within 200mm of the edge may not improve sufficiently.

[0012] The present invention aims to solve the above problems and provide a hot-rolled steel sheet and a method for manufacturing the same that exhibits excellent scale adhesion even in hot-rolled steel sheets with greater thickness, particularly small variation in scale adhesion in the width direction of the steel sheet, and improved laser cutting performance. [Means for solving the problem]

[0013] First, the inventors investigated the reason why uniformly excellent scale adhesion cannot be obtained in the width direction of conventional hot-rolled steel sheets. The scale generated during hot rolling is formed in the order of hematite (Fe2O3), magnetite (Fe3O4), and wustite (FeO) from the scale surface side at high temperatures. Among these, when wustite undergoes eutectoid transformation during cooling after coiling, a eutectoid transformation structure composed of magnetite and precipitated Fe is formed (4FeO → Fe3O4 + Fe). The eutectoid transformation structure composed of magnetite and precipitated Fe has particularly high coherence with the surrounding magnetite grains and base iron, contributing to the improvement of scale adhesion. In the conventional technology, the scale adhesion was particularly inferior, especially in the range within 200 mm from the edge in the width direction. This is because the scale is re-oxidized by the air invading from the edge after coiling in a coil shape, resulting in an excessive increase in the amount of the magnetite layer near the scale surface composed of hematite and columnar magnetite grains on the scale surface. As a result, it became clear that the ratio of the eutectoid transformation structure that contributes to the improvement of scale adhesion decreases, and the scale thickness at the edge portion increases. Furthermore, it became clear that in the range within 200 mm from the edge in the width direction, the scale adhesion of the edge and the portion 200 mm from the edge decreases by different mechanisms. At the edge, the cooling rate is the highest, so wustite remains in the final scale structure, and the adhesion is more likely to decrease further. On the other hand, in the portion 200 mm from the edge, it is exposed to the air invading from the edge and the cooling rate decreases due to reheating from the center in the width direction. Therefore, the amount of generation of hematite and the magnetite layer near the scale surface is the largest in the width direction, and the adhesion significantly decreases. That is, it became clear that in order to obtain uniformly excellent scale adhesion in the width direction, it is necessary to improve the scale adhesion of the edge and the portion 200 mm from the edge.

[0014] Furthermore, when manufactured under normal conditions, especially when the plate thickness is thick, transformation expansion after coiling occurs non-uniformly in the longitudinal direction of the coil. As a result, it was found that there are cases where the hot-rolled coil unwinds, air invades even in the central portion in the width direction, re-oxidation progresses, and the adhesion deteriorates.

[0015] Therefore, the present inventors earnestly studied means for solving the above problems and obtaining a hot-rolled steel sheet having excellent scale adhesion even in a hot-rolled steel sheet with a larger plate thickness, and particularly having little variation in scale adhesion in the width direction of the coil, and obtained the following findings. (i) For a steel material having a predetermined component composition, descaling is performed after hot rough rolling, finish rolling is performed at a finish rolling exit side temperature of 800 to 950°C, and then cooling is performed at a predetermined cooling rate to the coiling temperature. Thereby, the scale thickness is appropriately controlled, and the generation of cracks in the scale, which causes a decrease in adhesion, is suppressed. (ii) After starting 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 while the temperature of the edge portion of the coil is brought from the coiling temperature to a cooling stop temperature of 300°C or more and 450°C or less. Here, the edge portion of the coil means a range within 200 mm from the edge in the width direction of the coil. Thereby, the rigidity of the coil is increased and coil loosening is prevented. As a result, in the center in the width direction, reoxidation can be suppressed by being blocked from the oxidizing atmosphere, and a eutectoid transformation structure suitable for improving adhesion can be sufficiently obtained. Further, although it is difficult to completely block the edge portion in the width direction of the coil from the oxidizing atmosphere, reoxidation is suppressed by cooling the edge portion, and eutectoid transformation proceeds by reheating from the center portion in the width direction, ensuring excellent adhesion. Also, thereby, variation in scale thickness in the width direction can be suppressed, and the adhesion of the scale is uniformly improved in the width direction, improving laser cutability.

[0016] The present invention has been made based on the above findings, and specifically provides the following. [1] The composition is such that, by mass%, it contains C: 0.01-0.30%, Si: 0.50% or less, Mn: 0.01-2.0%, P: 0.10% or less, S: 0.10% or less, sol.Al: 0.10% or less, and N: 0.015% or less, with the remainder being Fe and unavoidable impurities, and has scale on the surface of the steel sheet, and the scale in the width direction of the steel sheet is such that, by area percentage, magnetite grains: 20% to 60%, iron and magnetite A hot-rolled steel sheet having a structure in which eutectoid transformation structure: 30% or more, where magnetite consists of magnetite grains and magnetite contained in the eutectoid transformation structure, wustite: 15% or less, and hematite: 5% or less by mass fraction, 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 thickness of the scale in the width direction of the steel sheet is 4 μm or less. [2] The hot-rolled steel sheet according to [1], wherein the component composition further contains one or more of the following in mass%: Cu: 1.0% or less, Ni: 0.50% or less, and Cr: 2.0% or less. [3] The hot-rolled steel sheet according to [1] or [2], wherein the component composition further contains one or more of the following in mass%, in the following amounts: Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.01% or less, and Sb: 0.03% or less. A method for manufacturing a hot-rolled steel sheet, comprising: hot rough rolling of a steel material having the component composition described in any of [4][1] to [3], followed by descaling; finish rolling at a finish rolling exit temperature of 800°C to 950°C; cooling the temperature range from the finish rolling exit temperature to 750°C at an average cooling rate of 5°C / s or more; cooling the temperature range from 750°C to the start of winding at an average cooling rate of 1°C / s to 30°C / s; winding at a winding temperature of 500°C to 650°C; and cooling the entire coil at an average cooling rate of 0.5°C / s to 6.0°C / s from the start of winding until the temperature of the coil edge reaches a cooling stop temperature of 300°C to 450°C from the winding temperature. [Effects of the Invention]

[0017] According to the present invention, hot-rolled steel sheets with excellent scale adhesion can be manufactured easily and inexpensively, resulting in significant industrial benefits. Furthermore, the present invention can reduce variations in scale adhesion in the width direction of the steel sheet, thereby greatly contributing to improved surface quality of products, improved laser cutting performance of products, and an improved working environment. In addition, it can solve the problem of decreased scale adhesion that occurs with increasing thickness of hot-rolled steel sheets.

[0018] In this invention, the thickness of the hot-rolled steel sheet is greater than 2.0 mm and less than or equal to 25 mm, preferably greater than 5.0 mm and less than or equal to 25 mm. [Modes for carrying out the invention]

[0019] The hot-rolled steel sheet and its manufacturing method according to the present invention will be described in detail below. However, the present invention is not limited to the following embodiments. The following embodiments include those that are easily substituted or substantially identical to those of the present invention by those skilled in the art.

[0020] The hot-rolled steel sheet of the present invention contains the following component composition. Unless otherwise specified, the unit "%" used to indicate the content of the component composition means "mass%".

[0021] C: 0.01~0.30% Carbon (C) is a useful element for ensuring strength. If its amount is less than 0.01%, the effect of ensuring strength is small, so the amount of C should be 0.01% or more. If the amount of C exceeds 0.30%, CO gas is generated at the interface between the scale and the base metal, causing delamination of the scale-base metal interface during rolling and leading to scale defects, so the amount of C should be 0.30% or less. From the viewpoint of scale adhesion, it is preferably 0.20% or less.

[0022] Si:0.50% or less Si is an element that acts as a deoxidizing agent. While it is not strictly necessary to include Si, it is preferable to include 0.01% or more to obtain this effect. However, if the Si content exceeds 0.50%, Si will concentrate at the interface between the scale and the base metal, forming a Si oxide layer. Scale peeling is likely to occur at the interface between this Si oxide layer and the scale layer formed on top of it. For this reason, the Si content should be 0.50% or less. Preferably, it should be 0.20% or less.

[0023] Mn: 0.01~2.0% Mn is an element that not only neutralizes dissolved sulfur (S), which causes embrittlement during hot working, by converting it into MnS, but also has an effect on improving strength. In particular, it is effective in ensuring the strength of steel sheets, which tend to lose strength after hot rolling due to their large thickness. The effect is small if the amount is less than 0.01%. On the other hand, if the content exceeds 2.0%, it leads to a decrease in toughness and causes the formation of Mn-based oxides at the interface between the scale and the base metal, which causes a decrease in scale adhesion. In addition, the transformation after finish rolling is delayed, and the transformation is not completed by the time of winding, and the transformation proceeds partially and unevenly in the longitudinal direction after winding. As a result, the hot-rolled coil becomes loose after winding, and even in the center of the coil in the width direction, the steel sheet surface comes into contact with the oxidizing atmosphere, causing re-oxidation, i.e., an increase in hematite and magnetite grains and a decrease in the eutectoid transformation structure, leading to a deterioration of scale adhesion. For this reason, the amount of Mn should be 0.01 to 2.0%. The preferred lower limit is 0.05% or more. The preferred upper limit is 1.5% or less.

[0024] P:0.10% or less P is an element that should be kept as low as possible because it adversely affects grain boundary embrittlement. Furthermore, P forms a very brittle oxide layer at the interface between the scale and the base metal, reducing scale adhesion. These adverse effects become significant when the P content exceeds 0.10%, so it should be kept below 0.10%. Preferably, it should be below 0.05%. While P does not need to be included, from a manufacturing cost perspective, a lower limit of 0.001% or higher is preferable.

[0025] S: 0.10% or less S is an element that significantly degrades hot workability and toughness. Furthermore, S concentrates at the interface between the scale and the base metal, reducing scale adhesion. These adverse effects become significant when the S content exceeds 0.10%, so it should be kept below 0.10%. Preferably, it should be below 0.05%. While S does not need to be included, from a manufacturing cost perspective, a lower limit of 0.0001% or higher is preferable.

[0026] sol.Al: 0.10% or less sol.Al is an element that acts as a deoxidizing agent. The amount of sol.Al may be 0.00%, but it is preferable to contain 0.01% or more to obtain this effect. On the other hand, if the amount exceeds 0.10%, oxide-based inclusions increase and the cleanliness decreases. For this reason, the amount of sol.Al should be 0.10% or less. Preferably, it should be 0.06% or less.

[0027] N: 0.015% or less N is an element that forms nitrides such as BN, AlN, and TiN in steel, and it is an element that reduces the hot ductility of steel and degrades surface quality. Surface quality deteriorates significantly when the N content exceeds 0.015%. Therefore, the N content should be 0.015% or less. Preferably, the N content is 0.010% or less. Although it is not necessary to include N, it is preferable to have an N content of 0.0001% or more from the viewpoint of manufacturing cost. More preferably, the N content is 0.001% or more.

[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 the following in amounts of Cu: 1.0% or less, Ni: 0.50% or less, and Cr: 2.0% or less, as needed, in order to improve various properties.

[0029] Cu: 1.0% or less Cu is an element that concentrates at the interface between the scale and the base metal, promoting grain boundary oxidation, and also promotes the unevenness of the scale-base metal interface, thereby improving the adhesion between the scale and the base metal interface. To obtain these effects, it is preferable to contain 0.01% or more Cu. However, if the content exceeds 1.0%, molten Cu may penetrate into the austenite grain boundaries of the base metal during heating, raising concerns about deterioration of surface properties due to hot embrittlement. For this reason, if Cu is included, it should be 1.0% or less. Preferably, it is 0.8% or less.

[0030] Ni: 0.50% or less Like copper, nickel (Ni) is an element that concentrates at the interface between the scale and the base metal, promoting grain boundary oxidation and increasing the surface roughness of the interface, thereby improving adhesion between the scale and the base metal. To obtain these effects, it is preferable to contain 0.01% or more Ni. However, if the amount of Ni exceeds 0.50%, the aforementioned effects saturate, and an increase in cost is a concern. For this reason, if Ni is included, it should be 0.50% or less. Preferably, it is 0.40% or less.

[0031] Cr:2.0% or less Cr has the effect of increasing strength, hardenability, and corrosion resistance. In addition, Cr concentrates at the interface between the scale and the base metal, and the resulting unevenness of the interface allows the scale to penetrate into the base metal, thus improving the adhesion of the scale. To obtain these effects, it is preferable to contain 0.01% or more of Cr. On the other hand, if the content exceeds 2.0%, the above effect becomes saturated, so if Cr is included, it should be 2.0% or less. A more preferable lower limit is 0.07% or more, and even more preferable is 0.12% or more. A more preferable upper limit is 1.0% or less, and even more preferable is 0.8% or less.

[0032] In the present invention, if necessary, one or more of the following may be included: Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.01% or less, and Sb: 0.03% or less.

[0033] Mo: 1.0% or less Mo improves strength and hardenability and suppresses softening during tempering. To obtain these effects, it is preferable to contain 0.1% or more Mo. On the other hand, if the amount exceeds 1.0%, the strength may increase excessively, and toughness and formability may deteriorate. Therefore, when Mo is included, the amount should be 1.0% or less.

[0034] Nb: 0.1% or less Nb is an element that improves the strength and toughness of the base material. To obtain such effects, it is preferable to include 0.003% or more. On the other hand, including more than 0.1% may actually lead to a decrease in toughness. Therefore, when Nb is included, the amount should be 0.1% or less.

[0035] V: 0.1% or less V is an element that improves the strength and toughness of the base material. To obtain such effects, it is preferable to include 0.003% or more. On the other hand, including more than 0.1% may actually lead to a decrease in toughness. Therefore, when V is included, the amount should be 0.1% or less.

[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 heat-affected zone during welding. To obtain these effects, it is preferable to contain 0.001% or more Ti. On the other hand, if the content exceeds 0.03%, it may actually lead to a decrease in toughness. Therefore, if Ti is included, the amount should be 0.03% or less.

[0037] B: 0.01% or less B is an element that enhances the hardenability of steel. This effect can increase its strength. To obtain this effect, it is preferable to contain 0.0005% or more of B. On the other hand, if the amount exceeds 0.01%, this effect becomes saturated, so if B is included, the amount should be 0.01% or less.

[0038] Sb: 0.03% or less When the material is heated, Sb concentrates on the surface of the steel sheet, suppressing the decrease in carbon content on the steel sheet surface due to decarburization during heating. To obtain this effect, it is preferable to contain 0.001% or more Sb. On the other hand, if the content exceeds 0.03%, it may become a liquid metal when the material is heated, eroding the prior austenite grain boundaries and reducing the adhesion of scale. For this reason, if Sb is included, it should be 0.03% or less.

[0039] The remainder of the chemical components other than those mentioned above consists of Fe and unavoidable impurities. Acceptable unavoidable impurities include O: 0.005% or less, Mg: 0.003% or less, Sn: 0.1% or less, and Ca: 0.01% or less. Furthermore, any element mentioned above present below the preferred lower limit is also considered an unavoidable impurity.

[0040] Next, the scale structure in the width direction of the steel sheet of the present invention will be described. The scale structure in the width direction of the steel sheet refers to the scale structure in the center of the steel sheet in the width direction and within 200 mm from the edge in the width direction. In practice, the scale structure is measured in the center of the steel sheet in the width direction, at a position 5 mm from the edge in the width direction, and at a position 200 mm from the edge, 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 following range, the structure is considered to be uniform in the width direction.

[0041] Area ratio of magnetite grains: 20% to 60% The magnetite of the present invention consists of magnetite grains and magnetite contained in the eutectoid transformation structure. These are distinguishable structures as follows. The magnetite grains in the present invention include a magnetite layer near the scale surface consisting of columnar magnetite grains, and massive protereductive magnetite grains that are formed inside or adjacent to the eutectoid transformation structure prior to the progression of eutectoid transformation. Furthermore, a thin layer consisting of fine magnetite grains that forms at the scale-base metal interface, so-called magnetite seams, is included and is distinguished from the magnetite contained in the eutectoid transformation structure of iron and magnetite. Compared to wustite and hematite, magnetite grains have higher ductility at room temperature and contribute to improved scale adhesion. In particular, magnetite seams have high coherence with the base metal and contribute to improved scale adhesion by suppressing peeling from the scale-base metal interface. This effect cannot be sufficiently obtained if the amount of magnetite grains is less than 20%, so the area ratio of magnetite grains should be 20% or more. Preferably it is 30% or more. On the other hand, if magnetite grains make up more than 60%, cracks will form in the magnetite, resulting in poor scale adhesion. Therefore, the area ratio of magnetite grains should be 60% or less, preferably 50% or less.

[0042] Area ratio of eutectoid transformation structures of iron and magnetite: 30% or more The eutectoid transformation structure of iron and magnetite contributes to improved scale adhesion because of the high compatibility between magnetite, precipitated Fe, and the base iron. This effect is not sufficiently obtained if it is less than 30%, so the area ratio of the eutectoid transformation structure of iron and magnetite should be 30% or more. Preferably it is 35% or more, and more preferably 40% or more. There is no particular upper limit, but in order to obtain a combined effect with the effect of improving scale adhesion of magnetite grains, the eutectoid transformation structure of iron and magnetite is preferably 80% or less, and more preferably 70% or less.

[0043] Area ratio of Ustite: 15% or less Wustite is a phase that is stable at high temperatures and most of it disappears during cooling after winding due to eutectoid transformation. However, if the cooling rate is high, wustite may remain untransformed at room temperature. In particular, the cooling rate is relatively high near the edges in the width direction of the coil, so wustite is more likely to remain at room temperature there. At room temperature, wustite is more brittle than magnetite, and cracks can form in the scale, impairing the adhesion of the scale. For this reason, the area ratio of wustite should be 15% or less. The preferred upper limit for the area ratio of wustite is 10% or less, more preferably 7% or less. Note that the area ratio of wustite may be 0%.

[0044] Hematite mass fraction: 5% or less In addition to magnetite grains, eutectoid transformation structures of iron and magnetite, and wustite, hematite may form in layers on the surface of the scale. Hematite is particularly likely to form at the edges in the coil width direction, due to re-oxidation of the scale by air entering from the edges after winding. Hematite leads to a deterioration of scale adhesion and causes surface defects such as red scale. Therefore, the mass fraction of hematite should be kept below 5%.

[0045] The average scale thickness in the width direction of the steel plate is 5 μm or more and 20 μm or less, and the variation in scale thickness in the width direction of the steel plate is 4 μm or less. If the average thickness of the scale is less than 5 μm, it can cause processing problems. This is because the amount of thermal energy absorbed by the laser light on the surface of the steel plate during laser cutting is insufficient. On the other hand, if the average thickness of the scale exceeds 20 μm, especially when the thickness of the hot-rolled steel plate is large, the strain applied to the scale surface when processing the steel plate becomes large, causing cracks to form in the scale and reducing scale adhesion. For this reason, the average thickness of the scale should be 20 μm or less. Preferably, it should be 18 μm or less, and more preferably 15 μm or less. Furthermore, in order to uniformly improve the adhesion of the scale in the width direction and obtain excellent laser cutting performance, the average thickness of the scale in the width direction of the steel plate should be controlled to be between 5 μm and 20 μm, and the variation in scale thickness in the width direction of the steel plate should be 4 μm or less. Preferably, the variation in scale thickness in the width direction of the steel plate should be 3 μm or less. Here, the variation in scale thickness refers to the maximum difference between the average thickness of the scale measured at multiple measurement positions. The methods for measuring the average thickness of the scale at the multiple measurement locations, as well as the variation in scale thickness, will be described later.

[0046] Next, a method for measuring the scale structure and scale thickness of the hot-rolled steel sheet of the present invention will be described.

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

[0048] Hematite is formed very thinly on the surface of the scale and is likely to fall off during mirror polishing, so it is difficult to quantitatively evaluate it as an area ratio on SEM. Therefore, using an X-ray diffractometer and using CoK α as the radiation source, measure the integrated intensity of the diffraction peaks of each phase in the scale, and from the ratio of the integrated intensities of each phase in the standard sample and the test sample, the mass fraction can be obtained using the following formula (2). As the standard sample, a mixture of equal weights of Fe, FeO (wüstite), Fe2O3 (hematite), and Fe3O4 (magnetite) is used. Note that the mass fraction of hematite can be regarded as the area ratio. Mass fraction of hematite (Fe2O3) = (I Fe2O3 / R Fe2O3 ) × 100 / ((I Fe / R Fe ) + (I FeO / R FeO ) + (I Fe2O3 / R Fe2O3 ) + (I Fe3O4 / R Fe3O4 )) ···(2) However, in the above formula (2), I A : Integrated intensity of phase A in the test sample R A : Integrated intensity of phase A in the standard sample A: Fe, FeO, Fe2O3, or Fe3O4.

[0049] Also, the average thickness of the scale is measured by the following procedure. For example, cut out a plate thickness cross-section perpendicular to the steel plate surface and parallel to the rolling direction from the center of the width direction of the hot-rolled steel plate, a portion 200 mm from the edge in the width direction of the hot-rolled steel plate, and a portion 5 mm from the edge in the width direction of the hot-rolled steel plate, and mirror-polish it. Then, measure the scale thickness three times each with SEM and average them to obtain the scale thickness at each width position. Note that the average thickness of the scale in the width direction of the present invention means that the average thickness of the scale at each width position is 5 μm or more and 20 μm or less. Furthermore, the variation in scale thickness in the width direction can be determined by subtracting the minimum value from the maximum value of the average scale thickness in the center of the width direction of the hot-rolled steel sheet, the portion 5 mm from the edge in the width direction, and the portion 200 mm from the edge in the width direction.

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

[0051] In this invention, the temperature specified in each step refers to the surface temperature of the slab (steel slab) or steel plate, and can be measured with a radiation thermometer or the like. Unless otherwise specified, the average cooling rate is "(cooling start temperature - cooling stop temperature) / cooling time".

[0052] In the present invention, the method for manufacturing the steel material having the above-described component composition is not particularly limited, and any commonly used method can be applied. For example, it is desirable to melt molten steel having the above-described component composition in a converter or electric furnace, and then use a casting method such as continuous casting to produce steel material such as a slab. There is no problem in using the ingot-part rolling method. Normally, the steel material is heated and then hot-rolled. This heating should be sufficient to achieve solid solution formation, preferably above the Ac3 point. Specifically, the normal slab heating temperature range of 1060°C to 1300°C is suitable. In the case of slabs manufactured by continuous casting, direct rolling may be applied, either as is or while being held to suppress temperature drop.

[0053] The hot rolling process consists of rough rolling and finish rolling. Rough rolling only requires the formation of sheet bars of predetermined dimensions; the conditions for rough rolling are not particularly limited. Before rough rolling, it is preferable to remove the scale generated by slab heating by descaling. Furthermore, since finish rolling is performed at a predetermined temperature, the rolled material may be heated during the process using heating means such as a sheet bar heater. After rough rolling and before finish rolling, the scale generated on the sheet bar surface is removed at the entrance of the rolling mill by descaling using high-pressure water or the like.

[0054] Next, finish rolling is performed. If the finish rolling entry temperature exceeds 1100°C, the scale thickness increases, and the adhesion of the scale may decrease. On the other hand, if the finish rolling entry temperature is below 950°C, it can lead to a significant increase in rolling load, which can reduce productivity. Furthermore, as the product sheet thickness increases, the finish rolling entry thickness also increases. For example, if the product sheet thickness exceeds 5.0 mm, a long time is required before the start of finish rolling, which can lead to a decrease in productivity. Therefore, the finish rolling entry temperature is preferably 1100°C or lower, and more preferably 1050°C or lower. In addition, it is preferable that the lower limit of the finish rolling entry temperature be 950°C or higher.

[0055] Finishing rolling temperature: 800°C to 950°C If the finishing rolling temperature is below 800°C, cracks will occur due to a decrease in scale ductility. These cracks promote the re-oxidation of the scale, generating hematite and causing a decrease in scale adhesion. In addition, the scale structure becomes finer and the hardness of the scale itself increases, further reducing scale adhesion. On the other hand, if the finishing rolling temperature exceeds 950°C, excessive scale growth increases the scale thickness and reduces scale adhesion. In addition, the grain size of each phase in the scale structure increases, further reducing scale adhesion. Therefore, the finishing rolling temperature should be between 800°C and 950°C. The preferred lower limit is 820°C or higher. The preferred upper limit is 930°C or lower.

[0056] Cooling is performed at an average cooling rate of 5°C / s or higher over a temperature range from the finish rolling temperature up to 750°C. Since scale grows faster at higher temperatures, rapid cooling of the high-temperature area immediately after finish rolling is necessary to suppress the decrease in 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, the scale will grow excessively, causing a decrease in scale adhesion. For this reason, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C should be 5°C / s or higher. Preferably, it should be 7°C / s or higher. On the other hand, if the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C exceeds 80°C / s, the scale structure may become finer, which may reduce scale adhesion. In addition, the ductility of the scale may decrease, causing cracks to form, and these cracks may promote the re-oxidation of the scale, generating hematite and causing a decrease in scale adhesion. For this reason, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is preferably 80°C / s or lower. More preferably, it is 50°C / s or lower.

[0057] Cooling from 750°C to the start of winding at an average cooling rate of 1°C / s to 30°C / s. In the temperature range from 750°C to the start of winding, scale growth is relatively slower than in the high-temperature range immediately after finish rolling, but it is necessary to suppress the decrease in scale adhesion due to excessive scale growth. If the average cooling rate in the temperature range from 750°C to the start of winding is less than 1°C / s, the scale will grow excessively, causing a decrease in scale adhesion. For this reason, the average cooling rate in the temperature range from 750°C to the start of winding should be 1°C / s or more. Preferably, it should be 3°C / s or more. On the other hand, if the average cooling rate in the temperature range from 750°C to the start of winding exceeds 30°C / s, cracks will occur in the scale due to the refinement of the scale structure and the increase in the stress difference with the base metal. These cracks promote the re-oxidation of the scale, generating hematite and reducing scale adhesion. For this reason, the average cooling rate in the temperature range from 750°C to the start of winding should be 30°C / s or less. Preferably, it should be 20°C / s or less.

[0058] Winding temperature: 500℃ or more and 650℃ or less After the cooling described above, the steel sheet is wound at a winding temperature of 500°C to 650°C. If the winding temperature is below 500°C, the eutectoid transformation from wustite does not occur sufficiently after winding, and an excessive amount of wustite remains at room temperature. As a result, since wustite is brittle at room temperature, the adhesion of the scale decreases. If the winding temperature exceeds 650°C, the scale grows excessively, reducing the adhesion of the scale. In addition, in the central part in the width direction, which is shielded from the oxidizing atmosphere after winding, the hematite and magnetite on the scale surface are reduced to wustite, and a sufficient amount of magnetite grains cannot be obtained. Therefore, the winding temperature should be between 500°C and 650°C. The preferred lower limit is 530°C or higher. The preferred upper limit is 630°C or lower.

[0059] After winding begins, the entire coil is cooled so that the temperature of the coil edges cools from the winding temperature to a cooling stop temperature of 300°C to 450°C, at an average cooling rate of 0.5°C / s to 6.0°C / s. After winding begins, the entire coil is cooled so that the temperature of the coil edges cools from the winding temperature to a cooling stop temperature of 300°C to 450°C at an average cooling rate of 0.5°C / s to 6.0°C / s. This lowers the temperature of the coil edges, increases the rigidity of the coil, and prevents winding loosening. Here, the entire coil refers to both edges and the surface of the coil. Both edges of the coil refer to the area within 200 mm in the width direction of the coil from both edges in the width direction of the coil. The surface of the coil refers to the surface of the coil excluding both ends. As a result, re-oxidation can be suppressed in the center of the coil in the width direction by being shielded from the oxidizing atmosphere, and a sufficiently eutectoid transformation structure suitable for improved adhesion can be obtained. Furthermore, although complete shielding from the oxidizing atmosphere is difficult at the edges in the width direction of the coil, re-oxidation can be suppressed by cooling the edges, and eutectoid transformation can be promoted by reheating from the center in the width direction, ensuring excellent adhesion. Furthermore, this suppresses variations in scale thickness in the width direction, and the uniform improvement of scale adhesion in the width direction improves laser cutting performance. The temperature of the edge can be measured, for example, by a radiation thermometer. The temperature of the plate surface is not specified because it is not possible to measure the temperature of the plate surface at a specific longitudinal position during winding, but the above effects can be obtained by cooling the entire coil using the same cooling method as the edge. If the cooling stop temperature at the edge of the coil exceeds 450°C, the above effects cannot be fully obtained. Also, if the cooling stop temperature at the edge of the coil is less than 300°C, the edge becomes overcooled, and a large amount of wustite remains in the scale at the edge, leading to a decrease in adhesion. For this reason, the cooling stop temperature at the edge of the coil should be between 300°C and 450°C. Preferably, it should be between 320°C and 430°C. Also, if the average cooling rate at the edge is less than 0.5°C / s, the above effects cannot be fully obtained. If the average cooling rate at the edge exceeds 6.0°C / s, cracks will form in the scale due to the refinement of the scale structure and the increased stress difference with the base metal. These cracks promote the re-oxidation of the scale, leading to the formation of hematite and a decrease in scale adhesion.Therefore, the average cooling rate at the edges should be between 0.5°C / s and 6.0°C / s. Preferably, it should be between 1.0°C / s and 5.0°C / s. There are no specific requirements for the method of cooling the coil, but it is preferable to cool it using a cooling device that sprays water onto the surface and both edges of the coil while winding it in a winding machine.

[0060] Furthermore, it is preferable to place the cooled coil in a coil box or cover it to promote the eutectoid transformation from wustite and suppress oxidation of the outermost periphery and edges.

[0061] Furthermore, in the case of hot-rolled steel sheets wound into a coil, the steel sheets may be deformed using a roller leveler or tension leveler to perform shape correction treatment. For example, in the case of a hot-rolled steel sheet with a thickness of 12 mm, two upper rolls and three lower rolls with a diameter of φ250 mm are arranged, and shape correction treatment is performed under the condition of a pressing amount of 2 mm. [Examples]

[0062] The following describes embodiments of the present invention.

[0063] Steel with the composition shown in Table 1 was melted and cast to produce steel materials. These steel materials were hot-rolled under the conditions shown in Table 2 to produce hot-rolled coils with thicknesses of 3 to 23 mm. The obtained hot-rolled coils were shaped by leveling and then cut to predetermined lengths to produce hot-rolled sheets. Test specimens were taken from the center and edges in the width direction of the obtained hot-rolled sheets, and the scale structure, scale thickness, adhesion, and laser cutability were evaluated using the following method. The evaluation results are shown in Table 3.

[0064] [Table 1]

[0065] [Table 2]

[0066] [Table 3]

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

[0068] The mass fraction of hematite was determined using an X-ray diffractometer, CoK α The integrated intensity of the diffraction peaks of each phase in the scale was measured using a radiation source. The mass fraction was determined from the ratio of the integrated intensities of each phase in the standard sample (an equal-weight mixture of Fe, FeO (wustite), Fe2O3 (hematite), and Fe3O4 (magnetite)) and the test sample using the following equation (2). Mass fraction of hematite (Fe2O3) = (I Fe2O3 / R Fe2O3 ) × 100 / ((I Fe / R Fe )+(I FeO / R FeO )+(I Fe2O3 / R Fe2O3 )+(I Fe3O4 / R Fe3O4 )) ···(2) However, in equation (2) above, I A Integrated intensity of phase A in the test sample R A Integrated intensity of phase A in the standard sample A: Fe, FeO, Fe2O3, or Fe3O4.

[0069] The average scale thickness was determined by cutting out thickness sections perpendicular to the steel sheet surface and parallel to the rolling direction from the center of the width direction of the obtained hot-rolled sheet, 200 mm from the edge in the width direction, and 5 mm from the edge in the width direction. After mirror polishing, the scale thickness at each width position was determined by measuring the scale thickness at three arbitrary locations using SEM and averaging the results.

[0070] Scale adhesion was evaluated by taking test specimens from the center of the width direction of the hot-rolled sheet after leveling, 200 mm from the edge in the width direction, and 5 mm from the edge in the width direction. Tape was applied to the surface of the steel sheet and the scale was peeled off. The presence or absence of exposed base metal on the surface of the steel sheet and the amount of scale attached to the tape were then evaluated. Specifically, tape was applied to the surface of the steel sheet, the peeled tape was attached to a transparent sheet, scanned, and the amount of scale peeled off was measured by image processing. If the area ratio of scale attached to the peeled tape was less than 10%, the scale adhesion was considered excellent and was indicated with ○ in Table 3. On the other hand, if the area ratio of scale attached to the peeled tape was 10% or more, the scale adhesion was considered poor and was indicated with × in Table 3.

[0071] Laser cutting performance was evaluated by laser cutting a hot-rolled sheet after leveling using an Amada Machinery ENSIS3015AJ laser cutting machine and fiber laser oscillator, cutting in a straight line parallel to the width direction. If cutting was not possible, or if dross, droplets, or notches were observed on the cut surface and a stable cut surface could not be obtained, the laser cutting performance was considered poor and was marked with × in Table 3. On the other hand, if none of these occurred and a stable cut surface was obtained in the width direction, the laser cutting performance was considered excellent and was marked with ○ in Table 3. 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 surface of the steel sheet.

[0072] The example of the present invention shown in Table 3 exhibited excellent uniform scale adhesion in all sections of the hot-rolled sheet, including the center in the width direction, 5 mm from the edge in the width direction, and 200 mm from the edge in the width direction, as well as excellent laser cutability. In contrast, the comparative example showed inferior adhesion or laser cutability at any of these width positions.

Claims

1. In mass percent, C: 0.01-0.30%, Si: 0.50% or less, Mn: 0.01-2.0%, P: 0.10% or less, S: 0.10% or less, Sol.Al: 0.10% or less N: Contains 0.015% or less, The composition consists of Fe and unavoidable impurities. The steel plate has scale on its surface. The scale in the width direction of the steel plate is, in terms of area ratio, Magnetite particles: 20% to 60% Eutectoid transformation structure of iron and magnetite: 30% or more. Here, the magnetite consists of the magnetite grains and the magnetite contained in the eutectoid transformation structure. Ustite: Less than 15%, The structure has a mass fraction of hematite of 5% or less. The average thickness of the scale in the width direction of the steel plate is 5 μm or more and 20 μm or less. Furthermore, a hot-rolled steel sheet in which the variation in the thickness of the scale in the width direction of the steel sheet is 4 μm or less.

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

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

4. A method for manufacturing a hot-rolled steel sheet according to claim 1 or 2, wherein a steel material having the above-mentioned component composition is used. After hot rough rolling, descaling is performed. Finish rolling exit temperature: Finish rolling is performed at a temperature of 800°C to 950°C. After cooling the temperature range from the aforementioned finish rolling temperature to 750°C at an average cooling rate of 5°C / s or more, The temperature range from 750°C to the start of winding is cooled at an average cooling rate of 1°C / s to 30°C / s. Winding temperature: Winding at 500°C to 650°C. A method for manufacturing a hot-rolled steel sheet, comprising cooling both edges of the coil and the surface of the coil so that, from the start of winding until the temperature of the coil edge is cooled at an average cooling rate of 0.5°C / s to 6.0°C / s, from the winding temperature to a cooling stop temperature of 300°C to 450°C.

5. A method for manufacturing a hot-rolled steel sheet according to claim 3, wherein a steel material having the above-mentioned component composition is used. After hot rough rolling, descaling is performed. Finish rolling exit temperature: Finish rolling is performed at a temperature of 800°C to 950°C. After cooling the temperature range from the aforementioned finish rolling temperature to 750°C at an average cooling rate of 5°C / s or more, The temperature range from 750°C to the start of winding is cooled at an average cooling rate of 1°C / s to 30°C / s. Winding temperature: Winding at 500°C to 650°C. A method for manufacturing a hot-rolled steel sheet, comprising cooling both edges of the coil and the surface of the coil so that, from the start of winding until the temperature of the coil edge is cooled at an average cooling rate of 0.5°C / s to 6.0°C / s, from the winding temperature to a cooling stop temperature of 300°C to 450°C.

Images