Hot-rolled steel sheet and manufacturing method thereof

Abstract

The purpose of the present invention is to provide: a hot-rolled steel sheet having excellent scale adhesion properties, especially flaky peeling resistance properties, even if the hot-rolled steel sheet has a larger sheet thickness; and a method for producing the same.　This hot-rolled steel sheet has a prescribed component composition and scale on the surface thereof, the scale having an average thickness of 20 μm or less. The scale has a structure that includes, in area ratio, at least 20% of magnetite, at least 40% of iron and magnetite eutectoid transformation structures, not more than 15% of wustite, and a total of at least 90% (including 100%) of magnetite, wustite, and iron and magnetite eutectoid transformation structures. The area percentage of precipitated Fe in a region from the most superficial layer of scale to 1 / 2 of the scale thickness in the thickness direction is at least 2%.

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet having excellent scale adhesion and used in automobiles, home appliances, building materials, etc., and a method for manufacturing the same. The present invention is particularly suitable as a material for parts of construction and industrial machinery that undergo processing such as temper rolling, bending, press forming, and laser cutting. The present invention relates to a hot-rolled steel sheet having excellent scale adhesion, consisting of adhesion between the scale and the base steel interface and resistance to powdery peeling, and a method for manufacturing the same. [Background technology]

[0002] Hot-rolled steel sheets are typically hot-rolled at high temperatures in an oxidizing atmosphere, which inevitably leads to the formation of scale (iron oxides) on the surface. When black-skinned hot-rolled steel sheets with this scale attached undergo 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 the processing line, and surface defects in the processed products. To avoid these problems, there is a growing demand for hot-rolled steel sheets with excellent scale adhesion to the steel surface, and this demand is growing stronger. In particular, powder-like scale peeling is a major cause of contamination of the processing line and deterioration of surface quality. Therefore, in order to improve scale adhesion, it is necessary to suppress powder-like scale peeling.

[0003] For example, Patent Document 1 discloses a method for producing a steel material having a composition containing, by mass%, 0.01 to 0.3% C, 0.20% or less Si, 0.01 to 2.0% Mn, 0.10% or less P, 0.10% or less S, 0.10% or less Al, 0.01 to 2.0% Cr, and the balance being Fe and unavoidable impurities, by rough rolling and descaling, followed by finish rolling at a finish rolling outlet temperature of 800 to 950°C that satisfies the following formula (1), and an average cooling rate from the end of finish rolling to the start of coiling of 3°C / s or more and 80°C / s or more. / s or less, and then coiling at a coiling temperature of 430 to 580°C. This provides a hot-rolled steel sheet with excellent scale adhesion, 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. |T2 - T1| ≦ 50℃ and |T3 - T2| ≦ 50℃ (1) However, in the above formula (1), T1: Temperature (°C) of the steel plate after finish rolling, 30 m from the longitudinal end and at the center in the width direction T2: Temperature (°C) at the center of the longitudinal direction and the center of the width direction of the steel plate after finish rolling T3: The temperature (°C) of the steel plate after finish rolling, 30 m from the longitudinal tail end and at the center in the width direction.

[0004] Patent Document 2 proposes a hot-rolled steel sheet with excellent scale-tightness, which is obtained by heating a slab containing, by mass, C: 0.02 to 0.20%, Mn: 0.1 to 2.0%, Si: 0.3% or less, P: 0.03% or less, S: 0.03% or less, Ni: 0.03 to 0.3%, Cu: 0.04 to 0.5%, and Cr: 0.03 to 0.3%, with the balance being Fe and unavoidable impurities, at 1100°C or higher, completing hot rolling in a temperature range of 800°C to 950°C, and coiling at 400°C to 650°C, resulting in a surface roughness at the interface between the steel sheet surface scale and the steel sheet base metal, with the number of irregularities of 0.5 μm or more per inch of length being 300 or more.

[0005] Patent Document 3 proposes a black hot-rolled steel sheet with excellent black color, which contains 0.001 to 0.20 mass% C, 0.001 to 0.50 mass% Si, 0.05 to 2.0 mass% Mn, 0.05 mass% or less P, 0.05 mass% or less S, and 0.01 to 0.10 mass% sol. Al, with the balance being Fe and unavoidable impurities, and which has a scale with a thickness of more than 4 μm on the surface of the hot-rolled steel sheet, the scale having a composition containing 50% or more Fe3O4 by volume, and which does not contain precipitated Fe in a region from the scale surface to a depth of at least 2 μm in the thickness direction. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6760425 [Patent Document 2] Patent No. 4153734 [Patent Document 3] Patent No. 4061996 Summary of the Invention [Problem to be solved by the invention]

[0007] 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 transformed 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 suppressing powdery scale peeling.

[0008] 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 technology improves the adhesion at the interface between the scale layer and the base steel, there is a concern that scale spalling may occur on the surface or inside the scale, reducing adhesion. Furthermore, the technology does not mention a method for suppressing powdery scale spalling.

[0009] In the technology described in Patent Document 3, a steel slab having a predetermined chemical composition is heated, then hot-rolled to a finish rolling outlet temperature of 800°C or higher, and then cooled to 650°C or lower at a cooling rate of 50°C / s or higher, and coiled at 600°C or higher. This advantageously achieves blackening of the scale while maintaining its adhesion. However, to blacken the scale, the region from the scale surface to a depth of at least 2 μm in the thickness direction is controlled to contain no precipitated Fe, which raises concerns that powdery scale spalling may not be sufficiently suppressed.

[0010] The present invention aims to solve the above problems and to provide a hot-rolled steel sheet having excellent scale adhesion, including adhesion between the scale and the base steel and resistance to powdery peeling, and a manufacturing method thereof. The thickness of the hot-rolled steel sheet in the present invention is more than 2.0 mm and not more than 20 mm, preferably more than 5.0 mm and not more than 20 mm. [Means for solving the problem]

[0011] The present inventors first investigated the cause of powder-like spalling of scale on conventional hot-rolled steel sheets. As a result, it was revealed that the powder-like spalling scale is mainly composed of magnetite and is caused by fracture within the scale. In other words, by appropriately controlling the magnetite formed on the surface layer of the scale, it is possible to suppress powder-like spalling.

[0012] The scale that forms during hot rolling is formed in the following order from the surface at high temperatures: hematite, magnetite (Fe3O4), and wüstite (FeO). Of these, wüstite undergoes eutectoid transformation during cooling after coiling, resulting in the formation of a eutectoid-transformed structure consisting of magnetite and precipitated Fe (4FeO → Fe3O4 + Fe). With conventional technology, cracks occurred at the interface between the magnetite formed at high temperatures and the eutectoid-transformed structure, causing powdery scale peeling.

[0013] Therefore, the present inventors have conducted extensive research into means for solving the above problems and achieving excellent adhesion between the scale and the base steel and resistance to powdery peeling, and have arrived at 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, thereby appropriately controlling the scale thickness and suppressing the occurrence of cracks in the scale. (ii) After 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, the material is cooled at an average cooling rate of 1°C / s or more and 30°C / s or less in the temperature range from 750°C to the start of coiling, thereby suppressing excessive growth of scale at high temperatures. (iii) Coiling temperature: Coiling is performed at 500 to 650°C, and the temperature is held within a range of −50°C to the coiling temperature for 100 minutes or more. During coiling, the steel sheet surface is isolated from the atmosphere, and the temperature is held within a range where wüstite is stably formed, thereby reducing some or all of the hematite and magnetite formed on the surface of the scale to wüstite. After coiling, the steel sheet is held within a temperature range of −50°C to the coiling temperature for 100 minutes or more, whereby the eutectoid transformation of wüstite is sufficiently promoted, and an eutectoid transformation structure is also formed on the surface of the scale. As a result, adhesion between the scale and the base steel is improved, and the amount of magnetite formed on the surface of the scale is reduced, thereby suppressing powder-like peeling.

[0014] The present invention has been made based on the above findings, and specifically provides the following. [1] A hot-rolled 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, wherein the steel sheet has scale on its surface, the average thickness of the scale is 20 μm or less, and the scale has a structure containing, in area ratios, 20% or more of magnetite, 40% or more of a eutectoid transformed structure of iron and magnetite, and 15% or less of wustite, with the total of magnetite, wustite, and a eutectoid transformed structure of iron and magnetite being 90% or more (including 100%), and the area ratio of precipitated Fe in a region from the outermost layer of the scale to 1 / 2 of the scale thickness in the thickness direction is 2% or more. [2] The composition further comprises, in mass%, Cu: 1.0% or less, Ni: 0.50% or less, 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 method for producing a hot-rolled steel sheet, comprising rough hot rolling a steel material having the chemical composition according to [1] or [2], followed by descaling, finish rolling at a finish rolling outlet temperature of 800 to 950°C, cooling at an average cooling rate of 5°C / s or more in the temperature range from the finish rolling outlet temperature to 750°C, cooling at an average cooling rate of 1°C / s or more and 30°C / s or less in the temperature range from 750°C to the start of coiling, coiling at a coiling temperature of 500 to 650°C, and holding for 100 minutes or more in a temperature range of not less than -50°C of the coiling temperature and not more than the coiling temperature. [5] A method for producing a hot-rolled steel sheet, comprising rough hot rolling a steel material having the chemical composition described in [3], followed by descaling, and then finish rolling at a finish rolling outlet temperature of 800 to 950°C, cooling at an average cooling rate of 5°C / s or more in the temperature range from the finish rolling outlet temperature to 750°C, followed by cooling at an average cooling rate of 1°C / s or more and 30°C / s or less in the temperature range from 750°C to the start of coiling, coiling at a coiling temperature of 500 to 650°C, and holding for 100 minutes or more in a temperature range of not less than -50°C and not more than the coiling temperature. [Effects of the Invention]

[0015] According to the present invention, it is possible to easily and inexpensively produce a hot-rolled steel sheet having excellent scale adhesion, which is of great industrial benefit. Furthermore, according to the present invention, it is possible to prevent separation at the interface between the scale and the base steel, which has the effect of significantly contributing to improving the surface quality of products, preventing processing defects in products, and improving the working environment. Furthermore, it is possible to solve the problem of reduced resistance to powdery separation. DETAILED DESCRIPTION OF THE INVENTION

[0016] 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.

[0017] 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.

[0018] 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 at the interface between the scale and the base steel during rolling and resulting in scale defects, so the C content is set to 0.30% or less. From the viewpoint of adhesion at the interface between the scale and the base steel, the C content is preferably 0.20% or less.

[0019] Si:0.50% or less Silicon acts as a deoxidizer, and to achieve this effect, a silicon content of 0.01% or more is preferable. However, if the silicon content exceeds 0.50%, silicon concentrates at the interface between the scale and the base steel, forming a silicon oxide layer. At the interface between this silicon oxide layer and the scale layer formed thereon, scale spalling is likely to occur. For this reason, the silicon content is set to 0.50% or less, and preferably 0.20% or less.

[0020] Mn: 0.01 to 2.0% Mn is an element that not only neutralizes solute S, which causes embrittlement during hot working, as MnS, but also has the effect of improving strength. A content of less than 0.01% is ineffective, while a content exceeding 2.0% reduces toughness and forms Mn-based oxides at the interface between the scale and the base steel, causing a decrease in scale adhesion. For this reason, the Mn content is set to 0.01 to 2.0%. The preferred lower limit is 0.05% or more. The preferred upper limit is 1.5% or less.

[0021] P:0.10% or less P has a negative effect on grain boundary embrittlement, so it is desirable to minimize its content. Furthermore, P forms a very brittle oxide layer at the interface between the scale and the base steel, reducing the adhesion between the scale and the base steel. Since these negative effects become more pronounced when the P content exceeds 0.10%, the P content is set to 0.10% or less, preferably 0.05% or less. While P need not be included, the P content is preferably set to 0.001% or more from the viewpoint of manufacturing costs.

[0022] S: 0.10% or less S is an element that significantly deteriorates hot workability and toughness. Furthermore, S concentrates at the interface between the scale and the base steel, reducing the adhesion between the scale and the base steel. If the S content exceeds 0.10%, these adverse effects become significant, so the S content is set to 0.10% or less, preferably 0.05% or less. Although S may not be contained, it is preferable that the S content be 0.0001% or more from the viewpoint of manufacturing costs.

[0023] Sol.Al: 0.10% or less Sol-Al is an element that acts as a deoxidizer. The amount of sol-Al may be 0.00%, but to obtain this effect, it is preferable to contain 0.01% or more. On the other hand, if the amount exceeds 0.10%, oxide-based inclusions increase and cleanliness decreases. For this reason, the amount of sol-Al is set to 0.10% or less, preferably 0.06% or less.

[0024] 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 and surface quality of the steel. 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. Although N does not necessarily have to be contained, the N content is preferably 0.0001% or more from the viewpoint of manufacturing costs. More preferably, the N content is 0.001% or more.

[0025] 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 needed, to improve various properties.

[0026] Cu: 1.0% or less Cu is an element that concentrates at the interface between the scale and the base steel, promoting intergranular oxidation, and also promotes the formation of irregularities at the interface between the scale and the base steel, thereby improving 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 content exceeds 1.0%, molten Cu may penetrate 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.

[0027] 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 should be 0.40% or less.

[0028] Cr:2.0% or less Cr has the effect of increasing strength, hardenability, and corrosion resistance. Cr also concentrates at the interface between the scale and the base steel, roughening the interface so that the scale penetrates the base steel, improving adhesion at the interface between the scale and the base steel. To achieve this effect, a Cr content of 0.01% or more is preferable. However, if the Cr content exceeds 2.0%, the above effect saturates, so if Cr is contained, the content should be 2.0% or less. A more preferable lower limit is 0.07% or more. A more preferable upper limit is 1.0% or less. A most preferable lower limit is 0.12% or more. A most preferable upper limit is 0.8% or less.

[0029] 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.

[0030] Mo: 1.0% or less Mo improves strength and hardenability and suppresses softening caused by tempering. To achieve 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 increases excessively and the toughness and formability deteriorate. Therefore, if Mo is contained, the amount is set to 1.0% or less.

[0031] Nb: 0.1% or less Nb is an element that improves the strength and toughness of the base material, and 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 cause a decrease in toughness. Therefore, if Nb is contained, the amount is set to 0.1% or less.

[0032] V: 0.1% or less V is an element that improves the strength and toughness of the base material, and 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 cause a decrease in toughness. Therefore, if V is contained, the amount is set to 0.1% or less.

[0033] 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 achieve these effects, it is preferable to contain 0.001% or more of Ti. On the other hand, if the content exceeds 0.03%, there is a risk that the toughness will be reduced. Therefore, if Ti is contained, the amount is set to 0.03% or less.

[0034] B: 0.01% or less B is an element that improves the hardenability of steel, and this effect can increase strength. To obtain this effect, it is preferable to contain 0.0005% or more of B. However, if the content exceeds 0.01%, this effect saturates, so if B is contained, the amount should be 0.01% or less.

[0035] Sb: 0.03% or less Sb concentrates in the surface layer when the material is heated, and has the effect of suppressing the decrease in the C content in the surface layer 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%, it becomes a liquid metal when the material is heated, corroding the prior austenite grain boundaries and reducing the adhesion between the scale and the base steel interface. For this reason, if Sb is contained, it should be 0.03% or less.

[0036] The balance other than the above chemical components consists of Fe and unavoidable impurities, of which the allowable contents are O: 0.005% or less, Mg: 0.003% or less, Sn: 0.1% or less, and Ca: 0.01% or less.

[0037] When the optional components are contained in amounts less than the preferred lower limit, the optional elements contained in amounts less than the preferred lower limit do not impair the effects of the present invention. Therefore, when the optional elements are contained in amounts less than the preferred lower limit, the optional elements are considered to be contained as inevitable impurities.

[0038] Next, the scale structure of the hot-rolled steel sheet of the present invention will be described.

[0039] Average scale thickness is 20 μm or less If the average thickness of the scale exceeds 20 μm, strain is increased at the outermost layer of the scale and at the interface between the scale and the base steel when the steel sheet is processed, resulting in reduced adhesion at the interface between the scale and the base steel and reduced resistance to powdery peeling. Therefore, the average thickness of the scale is set to 20 μm or less. It is preferably 18 μm or less, and more preferably 15 μm or less. There is no particular lower limit for the average thickness of the scale, but from the viewpoint of ensuring stable cut surface quality during processing such as laser cutting, it is preferably 3 μm or more. It is more preferably 5 μm or more.

[0040] Magnetite: 20% or more Magnetite has high compatibility with the base steel and contributes to improving adhesion at the interface between the scale and the base steel. If the area ratio is less than 20%, this effect cannot be fully achieved, so the area ratio of magnetite is set to 20% or more. Preferably, it is 30% or more. There is no particular upper limit, but if the area ratio of magnetite exceeds 70%, the amount of magnetite in the surface layer of the scale, which is the cause of powder-like spalling, increases, and resistance to powder-like spalling may be impaired. For this reason, the area ratio of magnetite is preferably 70% or less. More preferably, it is 60% or less. Note that this magnetite can be distinguished from the magnetite contained in the eutectoid transformation structure of iron and magnetite.

[0041] Eutectoid transformation structure of iron and magnetite: 40% or more The eutectoid-transformed structure of iron and magnetite contributes to improving adhesion at the interface between the scale and the base steel because magnetite and precipitated Fe are highly compatible with the base steel. Furthermore, when the surface layer of the scale contains a eutectoid-transformed structure of iron and magnetite, the amount of magnetite on the surface of the scale, which is the cause of powder-like spalling, is reduced, thereby contributing to improving resistance to powder-like spalling. This effect cannot be fully achieved if the area ratio is less than 40%, so the area ratio of the eutectoid-transformed structure of iron and magnetite is set to 40% or more. It is preferably 45% or more, and more preferably 50% or more.

[0042] Wustite: 15% or less In addition to magnetite and the eutectoid-transformed structure of iron and magnetite, wüstite may remain untransformed at room temperature. Wüstite is more brittle than magnetite at room temperature, and cracks may form in the scale, impairing the scale's resistance to powdery peeling. Furthermore, wüstite has lower compatibility with the base steel compared to magnetite or the eutectoid-transformed structure of iron and magnetite, which can impair the adhesion at the interface between the scale and the base steel. For this reason, the area ratio of wüstite is set to 15% or less. Preferably, it is 10% or less, and more preferably, it is 7% or less. The area ratio of wüstite may be 0%.

[0043] In addition to wüstite, a hematite layer may be formed in the outermost layer of the hot-rolled steel sheet. However, the formation of the hematite layer does not impair the effects of the present invention, and therefore it is acceptable to include the hematite layer. Since the hematite layer causes surface defects such as red scale, the mass fraction of hematite is preferably 10% or less. The mass fraction of hematite can be considered as an area fraction.

[0044] Total of magnetite, wustite, and eutectoid transformation structure of iron and magnetite: 90% or more (including 100%) If a large amount of phases other than hematite, such as magnetite, wustite, or the eutectoid-transformed structure of iron and magnetite is contained, the total fraction of magnetite, wustite, or the eutectoid-transformed structure of iron and magnetite may be less than 90%. In such cases, this can cause surface defects and reduce the adhesion and powder-like spalling resistance of the interface between the scale and the base steel. From the perspective of ensuring surface quality, the desired adhesion and powder-like spalling resistance of the interface between the scale and the base steel, the total fraction of magnetite, wustite, or the eutectoid-transformed structure of iron and magnetite is set to 90% or more.

[0045] The remaining structure other than magnetite, wustite, and the eutectoid transformation structure of iron and magnetite includes, in addition to hematite, non-ferrous oxides such as Si-based oxides and Cr-based oxides, and the effects of the present invention are not impaired as long as the total area ratio of these is 10% or less. Furthermore, these remaining structures can be measured by X-ray diffraction analysis in the same manner as the method for measuring the mass fraction of hematite described below.

[0046] Precipitated Fe in the region from the outermost layer of the scale to 1 / 2 of the scale thickness in the thickness direction: 2% or more By temperature control, as described below, part or all of the hematite and magnetite formed on the surface of the scale at high temperatures is reduced to wüstite. After coiling, the temperature is held within a range of −50°C or higher from the coiling temperature to the coiling temperature for 100 minutes or longer to sufficiently promote the eutectoid transformation of wüstite, thereby forming an eutectoid-transformed structure on the surface side of the scale. As a result, adhesion between the scale and the base steel is improved, and the amount of magnetite formed on the surface of the scale is reduced, thereby suppressing powder-like spalling. The amount of the eutectoid-transformed structure on the surface side can be expressed as the area ratio of precipitated Fe in the region from the outermost layer of the scale to half the scale thickness in the thickness direction. From the viewpoint of suppressing powder-like spalling, the precipitated Fe in the region from the outermost layer of the scale to half the scale thickness in the thickness direction is set to 2% or more, preferably 3% or more. There is no particular upper limit for the precipitated Fe in the region from the outermost layer of the scale to half the scale thickness in the thickness direction, but it is preferably 50% or less.

[0047] Next, a method for measuring the scale structure of a hot-rolled steel sheet according to the present invention will be described.

[0048] The area ratios of magnetite, the eutectoid structure of iron and magnetite, wüstite, and precipitated iron are measured by cutting a cross section of the steel sheet perpendicular to the surface and parallel to the rolling direction and mirror-polishing it. Then, using a scanning electron microscope (SEM) to observe the backscattered electron image of the cross section of the scale at 3000x magnification, the magnetite appears as the darkest region, the base steel and precipitated iron appear as the brightest, and wüstite appears as an intermediate region. The average thickness of the scale is calculated by measuring the scale thickness at three random locations using the SEM and averaging the results. The area ratios of magnetite, wüstite, and the eutectoid structure of iron and magnetite are calculated based on the total area of the oxide scale taken as 100%. If there are voids in the oxide scale, they are excluded from the calculation of the area ratio.

[0049] The mass fraction of hematite was measured using an X-ray diffractometer. αThe integrated intensity of the diffraction peaks of each phase in the scale is measured using a radiation source. The ratio of the integrated intensity of each phase in the test sample to that in the standard sample (a mixture of equal weights of Fe, FeO (wustite), Fe2O3 (hematite), and Fe3O4 (magnetite)) can be calculated using the following formula (2). Mass fraction of phase A = (I A / R A )×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. The mass fraction of hematite detected using an X-ray diffractometer is regarded as an area fraction and subtracted from the total area of the oxide scale described above, which is 100%, and the area fractions of magnetite, eutectoid transformation structure, and wüstite are calculated from the remaining area.

[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 using a radiation thermometer, etc. Unless otherwise specified, the average cooling rate is ((cooling start temperature - cooling stop temperature) / cooling time).

[0052] In the present invention, the method for producing the steel material having the above-mentioned composition does not need to be particularly limited, and any commonly used method can be used. For example, it is desirable to melt molten steel having the above-mentioned composition in a converter, electric furnace, or the like, and then produce a steel material such as a slab by a casting method such as continuous casting. There is no problem with using an ingot-blooming rolling method. Usually, the steel material is heated and then hot-rolled. This heating is sufficient as long as sufficient solid solution is achieved, and the temperature is preferably heated to the Ac3 point or higher. Specifically, a temperature range of 1060°C to 1300°C, which is the normal slab heating temperature range, is appropriate. In the case of a slab produced by a continuous casting method, direct rolling may be used, in which the slab is rolled as is or after being held to prevent 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 a sheet bar of the specified dimensions is obtained in the rough rolling. Furthermore, in order to perform the finish rolling at a specified temperature, the material to be rolled may be heated during the process using a heating means such as a sheet bar heater. Before the finish rolling, scale formed on the surface of the sheet bar is removed by conventional descaling using high water pressure or the like at the entry side of the finish rolling mill.

[0054] Next, finish rolling is performed. If the finish rolling entry temperature exceeds 1100°C, the scale thickness increases, which may reduce the adhesion at the interface between the scale and the base steel and the resistance to powdery peeling of the scale. On the other hand, if the finish rolling entry temperature is 950°C or lower, the rolling load increases significantly, which may reduce 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 finish rolling can begin, which may reduce productivity. Therefore, the finish rolling entry temperature is preferably 1100°C or lower, more preferably 1050°C or lower. Furthermore, the lower limit of the finish rolling entry temperature is preferably 950°C or higher.

[0055] Finishing rolling outlet temperature: 800 to 950°C If the finish rolling exit temperature is less than 800°C, cracks occur due to a decrease in scale ductility. This reduces the adhesion between the scale and the base steel, and these cracks promote the reoxidation of the scale, generating hematite, which reduces the scale's resistance to powder spalling. Furthermore, the scale structure becomes finer, and the hardness of the scale itself increases, reducing the adhesion between the scale and the base steel and the resistance to powder spalling. On the other hand, if the finish rolling exit temperature exceeds 950°C, excessive scale growth increases the scale thickness, reducing the adhesion between the scale and the base steel and the resistance to powder spalling. Furthermore, the grain size of each phase in the scale structure increases, reducing the scale's resistance to powder spalling. Therefore, the finish rolling exit temperature is set to 800 to 950°C. The preferred lower limit is 820°C or higher. The preferred upper limit is 930°C or lower.

[0056] Cooling from the finish rolling exit temperature to 750°C at an average cooling rate of 5°C / s or more Because scale grows more rapidly in high-temperature regions, rapid cooling of the high-temperature region immediately after finish rolling is necessary to prevent excessive scale growth from deteriorating the adhesion between the scale and the base steel and the scale's resistance to powder spalling. 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, resulting in a decrease in the adhesion between the scale and the base steel and a decrease in the scale's resistance to powder spalling. Therefore, the average cooling rate in the temperature range from the finish-rolling exit temperature to 750°C should be 5°C / s or higher, preferably 7°C / s or higher. On the other hand, if the average cooling rate in the temperature range from the finish-rolling exit temperature to 750°C exceeds 80°C / s, the scale structure may become finer, resulting in a decrease in scale adhesion. Furthermore, a decrease in scale ductility can cause cracks to form, which can promote the reoxidation of the scale, resulting in the formation of hematite, which can lead to a decrease in the scale's resistance to powder spalling. 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 less, and more preferably 50°C / s or less.

[0057] Cooling from 750°C to the start of coiling at an average cooling rate of 1°C / s to 30°C / s In the temperature range from 750°C to the start of coiling, scale growth is relatively slower than in the high-temperature region immediately after finish rolling. However, it is necessary to prevent excessive scale growth from deteriorating the adhesion between the scale and the base steel and the resistance to scale powder spalling. If the average cooling rate in the temperature range from 750°C to the start of coiling is less than 1°C / s, the scale will grow excessively, causing a decrease in the adhesion between the scale and the base steel and a decrease in the resistance to scale powder spalling. 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. This will cause cracks to form in the scale, reducing the adhesion between the scale and the base steel. Furthermore, these cracks will promote the reoxidation of the scale, resulting in the formation of hematite and a decrease in the resistance to scale powder spalling. For this reason, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is set to 30°C / s or less, and preferably 20°C / s or less.

[0058] Winding temperature: 500~650℃ After the cooling process, the steel sheet is coiled at a coiling temperature of 500 to 650°C. By insulating the steel sheet surface from the atmosphere during coiling and maintaining it within a temperature range where wüstite is stably formed, some or all of the hematite and magnetite formed on the surface of the scale are reduced to wüstite. After coiling, the steel sheet is maintained at a temperature range of −50°C or higher and lower than the coiling temperature for 100 minutes or longer, which allows the eutectoid transformation of wüstite to proceed sufficiently, resulting in the formation of an eutectoid-transformed structure on the surface of the scale. This improves the adhesion between the scale and the base steel, and reduces the amount of magnetite formed on the surface of the scale, thereby suppressing powdery spalling. If the coiling temperature is lower than 500°C, the eutectoid transformation of wüstite does not occur sufficiently, leaving excessive wüstite remaining at room temperature. As a result, the adhesion between the scale and the base steel and the resistance of the scale to powdery spalling are reduced. Furthermore, the reduction of hematite and magnetite in the scale surface layer to wüstite does not proceed sufficiently, resulting in insufficient eutectoid transformation of the scale surface layer and reduced powder spalling resistance. If the coiling temperature exceeds 650°C, the scale grows excessively, reducing the adhesion at the interface between the scale and the base steel and the scale's powder spalling resistance. Therefore, the coiling temperature is set to 500°C or higher and 650°C or lower, with a preferred lower limit of 530°C or higher. A preferred upper limit is 630°C or lower.

[0059] Maintain a temperature range of -50°C or higher below the winding temperature for 100 minutes or more After coiling, the eutectoid transformation of wüstite is sufficiently promoted by holding the temperature range from the coiling temperature −50°C to the coiling temperature for 100 minutes or more. If the holding time in the temperature range from the coiling temperature −50°C to the coiling temperature is less than 100 minutes, the eutectoid transformation structure formed on the surface layer of the scale becomes insufficient, and resistance to powdery spalling decreases. Furthermore, if the coiling temperature is held at less than −50°C, the eutectoid transformation of wüstite does not occur sufficiently, and excessive wüstite remains at room temperature. As a result, the adhesion at the interface between the scale and the base steel and the resistance to powdery spalling of the scale decrease. Therefore, the holding time in the temperature range from the coiling temperature −50°C to the coiling temperature is set to 100 minutes or more, preferably 120 minutes or more. Furthermore, if the holding time is long, internal oxidation at the interface between the scale and the base steel may proceed excessively, resulting in a decrease in scale adhesion. Therefore, the holding time in the temperature range from the coiling temperature −50°C to the coiling temperature is preferably 300 minutes or less.

[0060] It is preferable that the coil after winding be placed in a coil box or be covered to prevent 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, tension leveler, etc. For example, for a hot-rolled steel sheet with a thickness of 12 mm, shape correction processing is performed by arranging two upper rolls and three lower rolls with a diameter of 250 mm and applying a pressing amount of 2 mm. [Example]

[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 6 to 18 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 each portion of the obtained hot-rolled sheets, and the scale structure and scale adhesion were evaluated using the methods described below.

[0064] [Table 1]

[0065] [Table 2]

[0066] The area ratios of magnetite, the eutectoid structure of iron and magnetite, wüstite, and precipitated Fe were measured by cutting a cross section of the steel sheet perpendicular to the surface and parallel to the rolling direction and mirror-polishing it. Then, a scanning electron microscope (SEM) was used to observe the backscattered electron image of the cross section of the scale at a magnification of 3000x. In the backscattered electron image of the SEM, magnetite appears as the darkest region, the base steel and precipitated Fe appear as the brightest region, and wüstite appears as an intermediate region. The average thickness of the scale was calculated by measuring the scale thickness at three random locations using the SEM and averaging the results.

[0067] 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. The integrated intensity was calculated using the ratio of the integrated intensity of each phase in the test sample to that in the standard sample (a mixture of equal weights of Fe, FeO (wustite), Fe2O3 (hematite), and Fe3O4 (magnetite)) using the following formula (2). Mass fraction of phase A = (I A / R A )×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.

[0068] In the present invention, scale adhesion was evaluated by evaluating the adhesion of the interface between the scale and the steel substrate and the resistance to powder peeling. Specifically, test specimens were taken from the hot-rolled sheet after leveling, and tape was applied to the steel sheet surface to peel the scale. Evaluation was based on the amount of exposed steel substrate on the steel sheet surface and the amount of scale adhered to the tape. The peeled tape was then attached to a transparent sheet, scanned, and the amount of peeled scale was measured using image processing. Regarding the exposed steel substrate, images of the steel sheet surface were taken with a camera, and the area ratio of the exposed steel substrate, i.e., the proportion of the area of the exposed steel substrate to the area of the peeled tape, was measured. When the area ratio of the exposed steel substrate was less than 10% and the area ratio of the scale adhered to the peeled tape was less than 10%, the interfacial adhesion and resistance to powder peeling were deemed excellent, and this was marked with a circle in Table 2. On the other hand, when the area ratio of the exposed steel substrate on the steel sheet surface was 10% or more, the interfacial adhesion between the scale and the steel substrate was deemed poor, and this was marked with an X in Table 2. Furthermore, when the area ratio of scale adhering to the peeled tape was 10% or more, it was determined that the resistance to powder peeling was poor, and this was recorded as x in Table 2.

[0069] The examples of the present invention shown in Table 2 were excellent in interfacial adhesion between the scale and the base steel and in resistance to powdery spalling of the scale, whereas the comparative examples were poor in interfacial adhesion between the scale and the base steel and / or resistance to powdery spalling of the scale.

Claims

1. In mass%, C: 0.01-0.30%, Si: 0.50% or less, Mn: 0.01-2.0%, P: 0.10% or less, S: 0.10% or less, sol. Al: 0.10% or less, N: 0.015% or less, with the balance being Fe and unavoidable impurities; The steel sheet has scale on the surface, The average thickness of the scale is 20 μm or less, The scale is an area ratio, Magnetite: 20% or more, Eutectoid transformation structure of iron and magnetite: 40% or more, Wustite: 15% or less, The structure has a total of 90% or more (including 100%) of magnetite, wustite, and eutectoid transformation structures of iron and magnetite, A hot-rolled steel sheet, wherein the area ratio of precipitated Fe in a region extending from the outermost surface layer of the scale to half the thickness of the scale in the thickness direction is 2% or more.

2. The component composition further comprises, in mass%, Cu: 1.0% or less, Ni: 0.50% or less, The hot-rolled steel sheet according to claim 1, further comprising at least one of the following: Cr: 2.0% or less.

3. The component composition further comprises, 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, The hot-rolled steel sheet according to claim 1 or 2, comprising one or more of the following:

4. The method for producing a hot-rolled steel sheet according to claim 1 or 2, wherein a steel material having the above-mentioned composition is After hot rough rolling, descaling is performed. Finish rolling is performed at a temperature on the delivery side of the finish rolling mill at 800 to 950°C. After cooling in the temperature range from the finish rolling delivery 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 coiling at an average cooling rate of 1°C / s or more and 30°C / s or less, Winding temperature: Winding at 500 to 650°C, Maintaining the temperature range of -50°C or higher and lower than the winding temperature for 100 minutes or more; A manufacturing method for hot-rolled steel sheets.

5. The method for producing a hot-rolled steel sheet according to claim 3, further comprising the steps of: After hot rough rolling, descaling is performed. Finish rolling is performed at a temperature on the delivery side of the finish rolling mill at 800 to 950°C. After cooling in the temperature range from the finish rolling delivery 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 coiling at an average cooling rate of 1°C / s or more and 30°C / s or less, Winding temperature: Winding at 500 to 650°C, Maintaining the temperature range of -50°C or higher and lower than the winding temperature for 100 minutes or more; A manufacturing method for hot-rolled steel sheets.

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