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 25 μm or less. The scale has a structure that includes, in area ratio, at least 20% of magnetite, at least 30% of iron and magnetite eutectoid transformation structures, not more than 15% of wüstite, and a total of at least 90% (including 100%) of magnetite, wüstite, and iron and magnetite eutectoid transformation structures. The arithmetic average roughness of the interface between the scale and ground iron is at least 0.3 μm.

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet with excellent scale adhesion suitable for use in automobiles, home appliances, building materials, etc., and a method for manufacturing the same.The present invention relates to a hot-rolled steel sheet with excellent scale adhesion suitable as a material for parts of construction and industrial machinery that are subjected to processing such as temper rolling, bending, press forming, and laser cutting, 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 poor processing, contamination of the processing line, and surface defects in the processed products. To avoid these problems, hot-rolled steel sheets with excellent surface scale adhesion are in demand, and this demand is becoming increasingly strong. In particular, flaky scale peeling is a major cause of contamination of the processing line, deterioration of surface quality, and deterioration of laser cuttability. Therefore, in order to improve scale adhesion, it is necessary to suppress flaky scale peeling.

[0003] Furthermore, the scale on hot-rolled steel sheets tends to peel off more easily as the thickness of the hot-rolled steel sheet increases, because the strain generated in the scale during deformation increases. Furthermore, the scale is more likely to peel off during high-process forming, such as bending and press forming. Meanwhile, the need for thicker, black-skinned hot-rolled steel sheets has been increasing in recent years, and there is a strong demand for improved scale adhesion, even for hot-rolled steel sheets with a thickness of, for example, more than 5.0 mm.

[0004] Conventionally, various proposals have been made for improving the adhesion of scale. For example, Patent Document 1 discloses a steel material having a composition containing, by mass%, C: 0.01 to 0.3%, Si: 0.20% or less, Mn: 0.01 to 2.0%, P: 0.10% or less, S: 0.10% or less, Al: 0.10% or less, Cr: 0.01 to 2.0%, with the balance being Fe and inevitable impurities, which is subjected to rough rolling, descaling, and 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 to 80°C / s. / 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.

[0005] 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.

[0006] 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]

[0007] [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]

[0008] 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 flaky scale peeling.

[0009] The technology described in Patent Document 2 proposes a hot-rolled steel sheet with excellent scale tightness by hot-rolling steel to which predetermined amounts of Ni, Cu, and Cr have been added and controlling the surface roughness of the interface between the surface scale of the steel sheet and the steel base steel within a predetermined range. However, although the adhesion at the interface between the scale layer and the steel base steel is improved, there is a concern that scale spalling may occur on the surface or inside the scale, reducing adhesion. Furthermore, there is a concern that the adhesion of the scale may decrease as the thickness of the hot-rolled steel sheet increases. Furthermore, there is no mention of a method for suppressing flake-like scale spalling.

[0010] 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 the scale adhesion. However, there is a concern that flaky scale peeling may not be sufficiently suppressed. Furthermore, there is a concern that the scale adhesion may decrease if the thickness of the hot-rolled steel sheet increases.

[0011] The present invention aims to solve the above problems and to provide a hot-rolled steel sheet having excellent scale adhesion, particularly resistance to flake-like peeling, even when the hot-rolled steel sheet has a larger plate thickness or is processed under severe conditions, particularly when leveling is performed, and a manufacturing method thereof. The plate thickness of the hot-rolled steel sheet in the present invention is more than 2.0 mm and not more than 25 mm, preferably more than 5.0 mm and not more than 25 mm. [Means for solving the problem]

[0012] The inventors first investigated the cause of flake-like scale spalling on conventional hot-rolled steel sheets. As a result, they found that the flake-like scale spalls from the interface between the scale and the base steel. That is, to suppress flake-like spalling, it is important to appropriately control the interface properties between the scale and the base steel and increase the contact area between the scale and the base steel. Furthermore, because the chemical composition of the base steel also affects the contact area between the scale and the base steel, it is also necessary to appropriately control the chemical composition of the base steel.

[0013] Scale formed during hot rolling is roughly divided into primary scale formed in a slab heating furnace or during rough rolling, and secondary scale formed during finish rolling. In conventional techniques for thin steel sheets, primary scale is often completely removed by descaling before finish rolling, so the properties of the secondary scale have a significant impact on scale adhesion in the final product. Based on these findings, the present inventors have conducted extensive research into means for achieving excellent scale adhesion, particularly resistance to flake-like peeling, even in hot-rolled steel sheets with greater plate thicknesses, and have obtained the following findings. (i) A steel material having a specified chemical composition is heated at a heating temperature of 1150°C or higher, and scale is removed by descaling. After that, descaling is performed by rough hot rolling and water injection at a pressure of 30 MPa or less. This results in an uneven scale / base steel interface in the final product that is suitable for improving resistance to flake-like spalling, without completely removing the primary scale formed during rough rolling from the interface with the base steel. (ii) After rough hot rolling, finish rolling is performed at a finish rolling outlet temperature of 800 to 950°C, thereby appropriately controlling the scale thickness while maintaining an uneven interface between the primary scale and the base steel, and suppressing the occurrence of cracks in the scale that cause a decrease in flake-like peeling resistance. (iii) 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. This prevents a decrease in resistance to flake peeling due to excessive growth of scale at high temperatures. (iv) Coiling temperature: Coiling is performed at 500 to 630°C, and the temperature is held within a range of not less than −50°C of the coiling temperature and not more than the coiling temperature for 100 minutes. In this case, by holding the coiling temperature within a range of not less than −50°C of the coiling temperature and not more than the coiling temperature for 100 minutes after coiling, the eutectoid transformation of wüstite is sufficiently advanced, thereby improving the resistance to flake peeling of the scale.

[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, having scale on the steel sheet surface, the average thickness of the scale being 25 μm or less, the scale having a structure containing, in area ratios, 20% or more of magnetite, 30% 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 arithmetic mean roughness of the interface between the scale and the base steel being 0.3 μm 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 heating a steel material having the chemical composition according to [1] or [2] at a heating temperature of 1150°C or higher, descaling, followed by rough hot rolling and descaling with water injection at a pressure of 30 MPa or less, finish rolling at a finish rolling outlet temperature of 800 to 950°C, cooling at an average cooling rate of 5°C / s or higher in the temperature range from the finish rolling outlet temperature to 750°C, cooling at an average cooling rate of 1°C / s or higher and 30°C / s or lower in the temperature range from 750°C to the start of coiling, coiling at a coiling temperature of 500 to 630°C, and holding for 100 minutes or more in a temperature range of -50°C or higher and lower than the coiling temperature. [5] A method for producing a hot-rolled steel sheet, comprising heating a steel material having the chemical composition described in [3] at a heating temperature of 1150°C or higher, descaling, followed by rough hot rolling and descaling with water injection at a pressure of 30 MPa or less, finish rolling at a finish rolling outlet temperature of 800 to 950°C, cooling at an average cooling rate of 5°C / s or higher in the temperature range from the finish rolling outlet temperature to 750°C, cooling at an average cooling rate of 1°C / s or higher and 30°C / s or lower in the temperature range from 750°C to the start of coiling, coiling at a coiling temperature of 500 to 630°C, and holding for 100 minutes or more in a temperature range of -50°C or higher and lower than the coiling temperature. [Effects of the Invention]

[0015] According to the present invention, a hot-rolled steel sheet having excellent resistance to scale flaking can be easily and inexpensively produced, which is of great industrial benefit. Furthermore, according to the present invention, scale flaking can be prevented, 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 can also solve problems such as a decrease in scale flaking resistance that occurs with increasing the thickness of hot-rolled steel sheets and processing under harsh conditions, particularly leveling processing. 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, reducing the contact area between the scale and the base steel, causing scale spalling during rolling and resulting in scale defects, so the C content is set to 0.30% or less. From the viewpoint of resistance to scale spalling in the form of flakes, 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, reducing the resistance of the scale to spalling. 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 resistance of the scale to flaking. Since these adverse 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 resistance of the scale to flaking. 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, the S content is preferably set to 0.0001% or more from the viewpoint of production 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. Furthermore, 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. While N need not be included, 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 the 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 Cu. However, if the Cu 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 contained, 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 scale and base steel, roughening the interface so that the scale penetrates into the base steel, improving the adhesion at the interface between the scale and 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 a decrease in the amount of C in the surface layer during heating. To achieve this effect, it is preferable to include 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 at the interface between the scale and the base steel. For this reason, if Sb is included, 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 25 μm or less If the average thickness of the scale exceeds 25 μm, the strain applied to the outermost layer of the scale when the steel sheet is processed increases, causing cracks to form in the scale, which reduces the adhesion between the scale and the base steel interface. For this reason, the average thickness of the scale is set to 25 μm or less. It is preferably 20 μm or less, and more preferably 18 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 the resistance to scale flaking. If the area ratio is less than 20%, this effect cannot be sufficiently obtained, 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 proportion of the eutectoid transformed structure of iron and magnetite, which contributes to improving the resistance to scale flaking, decreases, and the resistance to scale flaking 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 the magnetite here can be distinguished from the magnetite contained in the eutectoid transformed structure of iron and magnetite.

[0041] Eutectoid transformation structure of iron and magnetite: 30% or more The eutectoid-transformed structure of iron and magnetite has high compatibility with magnetite and precipitated Fe and the base steel, and therefore contributes to improving the adhesion at the interface between the scale and the base steel. This effect cannot be fully achieved if the area ratio is less than 30%, so the area ratio of the eutectoid-transformed structure of iron and magnetite is set to 30% or more. It is preferably 40% or more, and more preferably 45% 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 in addition, it has lower compatibility with the base steel compared to magnetite or the eutectoid-transformed structure of iron and magnetite, which can cause a loss of 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. It is preferably 10% or less, and more preferably 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 magnetite, wustite, and the eutectoid transformed structure of iron and magnetite is contained, such as hematite, and the total fraction of magnetite, wustite, and the eutectoid transformed structure of iron and magnetite is less than 90%, it will cause surface defects and reduce resistance to flaking.From the viewpoint of ensuring surface quality and the specified resistance to flaking, the total fraction of magnetite, wustite, and 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 measurement method for hematite described below.

[0046] The arithmetic mean roughness of the interface between the scale and the base steel is 0.3 μm or more By ensuring that the arithmetic mean roughness of the interface between the scale and the steel substrate is 0.3 μm or more, it is possible to suppress flaking, which is thought to be caused by peeling at the interface. If the arithmetic mean roughness of the interface between the scale and the steel substrate is less than 0.3 μm, this effect is insufficient and resistance to flaking becomes poor. Therefore, the arithmetic mean roughness of the interface between the scale and the steel substrate is set to 0.3 μm or more, and preferably to 0.4 μm or more. Furthermore, although there is no particular upper limit, from the viewpoint of ensuring stable cut surface quality during processing such as laser cutting, the arithmetic mean roughness of the interface between the scale and the steel substrate is preferably 5 μm 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. α The 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] The arithmetic mean roughness of the interface between the scale and the base steel can be determined by removing the scale from the steel sheet surface by pickling, measuring the three-dimensional shape of the base steel surface using a laser microscope, and extracting five roughness curves parallel to the rolling direction with a reference length of 100 μm. The arithmetic mean roughness of these sections, determined in accordance with JIS B 0633:2001, can then be calculated by averaging the arithmetic mean roughnesses.

[0051] Next, a method for producing a hot-rolled steel sheet according to the present invention will be described.

[0052] 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).

[0053] 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 applied. For example, it is preferable to produce molten steel having the above-mentioned composition in a converter, electric furnace, or the like, and then form it into a steel material such as a slab by a casting method such as a continuous casting method. However, there is no problem in using an ingot casting-blooming rolling method.

[0054] Heating temperature: 1150℃ or more The steel material is heated at a heating temperature of 1150°C or higher. This heating is performed to achieve sufficient solid solution of the steel base material and to roughen the interface between the primary scale and the base steel. If the heating temperature is lower than 1150°C, the roughening of the interface will not progress sufficiently, so the heating temperature is set to 1150°C or higher. Preferably, it is 1180°C or higher. Also, it is preferably 1300°C or lower. In the case of slabs produced by continuous casting, direct rolling may be applied, in which the slab is rolled as is or while being held to prevent a temperature drop.

[0055] 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 they produce a sheet bar of the specified dimensions. Before rough rolling, descaling is performed by water injection to remove scale formed during heating of the steel material. This is because the scale formed during heating is very thick, reducing the resistance to flake-like peeling of the scale in the final product and causing surface defects. Descaling at this time can be performed by any conventionally known method, as long as it can remove the thick scale. Furthermore, in order to perform 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.

[0056] Pressure: Descaling with water injection under 30 MPa Before finish rolling, descaling is performed at the entry side of the finish rolling mill by water injection at a pressure of 30 MPa or less. This prevents the primary scale formed during rough rolling from being completely removed from the interface with the base steel, resulting in an uneven interface between the scale and the base steel that is suitable for improving resistance to flake-like peeling in the final product. If the water injection pressure exceeds 30 MPa, the scale will be completely removed from the interface, resulting in insufficient roughness at the interface between the scale and the base steel, which may reduce the adhesion at the interface between the scale and the base steel. Therefore, the water injection pressure is set to 30 MPa or less, preferably 25 MPa or less. Furthermore, from the viewpoint of preventing an excessive increase in scale thickness, the water injection pressure is preferably 5 MPa or more, more preferably 10 MPa or more.

[0057] Next, finish rolling is performed. If the finish rolling entry temperature exceeds 1100°C, the thickness of the scale increases, which may reduce the adhesion at the interface between the scale and the base steel. On the other hand, if the finish rolling entry temperature is 950°C or lower, a significant increase in the rolling load may occur, 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 the start of finish rolling, 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.

[0058] Finishing rolling outlet temperature: 800 to 950°C If the finish rolling exit temperature is less than 800°C, the ductility of the scale decreases, causing cracks to form, which reduces the scale's resistance to flake-like spalling. Furthermore, the scale structure becomes finer, and the hardness of the scale itself increases, reducing the adhesion at the interface between the scale and the base steel. On the other hand, if the finish rolling exit temperature exceeds 950°C, excessive growth of the scale increases the scale thickness, reducing the adhesion at the interface between the scale and the base steel. 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.

[0059] 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 a decrease in scale resistance 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 adhesion at the interface between the scale and the base steel. For this reason, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is set to 5°C / s or more, preferably 7°C / s or more. On the other hand, if the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C exceeds 80°C / s, the scale structure may become finer, which may cause a decrease in adhesion at the interface between the scale and the base steel. Furthermore, a decrease in scale ductility may cause cracks to form, which may cause a decrease in adhesion at the interface between the scale and the base steel. 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, more preferably 50°C / s or less.

[0060] 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 deterioration in the interfacial adhesion between the scale and the base steel due to excessive scale growth. If the average cooling rate in the temperature range from 750°C to the start of coiling is less than 1°C / s, scale will grow excessively, causing a decrease in the interfacial adhesion between the scale and the base steel. 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 may cause cracks to form in the scale, reducing the interfacial adhesion between the scale and the base steel. 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, preferably 20°C / s or less.

[0061] Winding temperature: 500~630℃ After the above cooling, the steel sheet is coiled at a coiling temperature of 500 to 630°C. If the coiling temperature is less than 500°C, the eutectoid transformation of wüstite does not occur sufficiently, and an excessive amount of wüstite remains at room temperature. As a result, wüstite is brittle at room temperature, and the resistance to flake-like spalling of the scale decreases. If the coiling temperature exceeds 630°C, the scale grows excessively, reducing the adhesion at the interface between the scale and the base steel. Therefore, the coiling temperature is set to 500°C or higher and 630°C or lower, with a preferred lower limit being 530°C or higher. Also, a preferred upper limit is 620°C or lower.

[0062] 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, or if the coiling temperature is held at a temperature less than −50° C., the eutectoid transformation of wüstite does not occur sufficiently, and excessive wüstite remains at room temperature. As a result, wüstite is brittle at room temperature, and the scale flake spallation resistance decreases. 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 scale / base steel interface may proceed excessively, resulting in a decrease in the scale flake spallation resistance. Therefore, the holding time in the temperature range from the coiling temperature −50° C. to the coiling temperature is preferably 300 minutes or less.

[0063] 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.

[0064] 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]

[0065] Examples of the present invention will be described below.

[0066] 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 adhesion were evaluated using the methods described below.

[0067] [Table 1]

[0068] [Table 2]

[0069] 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.

[0070] 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, and 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)) was used to calculate the intensity using the following equation (2). Mass fraction of phase A = (I A / R A )×100 / ((IFe / 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.

[0071] The arithmetic mean roughness of the interface between the scale and the base steel was determined by removing the scale from the steel sheet surface by pickling, measuring the three-dimensional shape of the base steel surface using a laser microscope, extracting five roughness curves parallel to the rolling direction with a reference length of 100 μm, and averaging the arithmetic mean roughness of these sections.

[0072] The scale adhesion in the present invention was evaluated by flake peeling resistance. Specifically, 30 mm × 100 mm test pieces were taken from the leveled hot-rolled sheet so that the longitudinal direction was parallel to the rolling direction. Then, they were bent using the push-bending method in accordance with JIS Z 2248. Tape was applied to the steel sheet surface on the outer side of the bend to peel off the scale. The bending conditions were a ratio r / t (r = radius of the pusher tip r to the thickness t of the test piece) of 4 and a bending angle of 180°. Images of the steel sheet surface where the tape had been peeled were taken with a camera, and the area ratio of exposed steel substrate, i.e., the proportion of the area of the exposed steel substrate to the area of the peeled tape, was measured from the images. When the area ratio of exposed steel substrate was less than 10%, the test piece was deemed to have excellent flake peeling resistance, and this was marked with a circle in Table 2. On the other hand, when the area ratio of exposed steel substrate was 10% or more, the test piece was deemed to have poor flake peeling resistance, and this was marked with an X in Table 2.

[0073] The examples of the present invention shown in Table 2 were excellent in resistance to flaking of scale, whereas the comparative examples were poor in resistance to flaking of 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 25 μm or less, The scale is an area ratio, Magnetite: 20% or more, Eutectoid transformation structure of iron and magnetite: 30% 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, The hot-rolled steel sheet has an arithmetic mean roughness of the interface between the scale and the base steel of 0.3 μm 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 heating at a temperature of 1150°C or higher and descaling, Hot rough rolling and pressure: Descaling with water injection at 30 MPa or less; 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 630°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 heating at a temperature of 1150°C or higher and descaling, Hot rough rolling and pressure: Descaling with water injection at 30 MPa or less; 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 630°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.

Images