Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
  • B21B1/24—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process
  • B21B1/26—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process by hot-rolling, e.g. Steckel hot mill
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B45/00—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
  • B21B45/04—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for de-scaling, e.g. by brushing
  • B21B45/08—Devices for surface or other treatment of work, specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills for de-scaling, e.g. by brushing hydraulically
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G3/00—Apparatus for cleaning or pickling metallic material
  • C23G3/02—Apparatus for cleaning or pickling metallic material for cleaning wires, strips, filaments continuously
  • C23G3/023—Apparatus for cleaning or pickling metallic material for cleaning wires, strips, filaments continuously by spraying

Definitions

• the present invention relates to a hot-rolled steel sheet with excellent scale adhesion that is used 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 that 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, and a method for manufacturing the same.
• Hot-rolled steel sheets are usually hot-rolled at high temperatures in an oxidizing atmosphere, so scale (iron oxide) inevitably forms on the surface.
• scale iron oxide
• processing 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 product.
• there is a demand for hot-rolled steel sheets with excellent adhesion of scale to the steel sheet surface and this demand is becoming stronger.
• 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 the adhesion of the scale, it is necessary to suppress flaky scale peeling.
• the thicker the hot-rolled steel plate the greater the distortion that occurs in the scale during deformation, and the more likely it is that the scale will peel off. Furthermore, the scale is more likely to peel off during large-scale processing such as bending and press forming. Meanwhile, the need for thick, black-skinned hot-rolled steel plate has been growing in recent years, and there is a strong demand for improved adhesion of the scale even for hot-rolled steel plate with a thickness of over 5.0 mm, for example.
• Patent Document 1 discloses a method for producing a steel material having a composition, by mass %, containing 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, by rough rolling, descaling, and then finish rolling at a finish rolling delivery temperature of 800 to 950°C that satisfies the following formula (1), and cooling at an average cooling rate from the end of finish rolling to the start of coiling of 3°C/s or more and 80°C or more.
• a hot-rolled steel sheet with excellent scale adhesion which is characterized in that it has a magnetite layer from the base steel side, and magnetite grains and/or an eutectoid transformed structure of iron and magnetite in an upper layer of the magnetite layer, the average grain size of the magnetite grains and/or the average block size of the eutectoid transformed structure is 3 ⁇ m or more and 8 ⁇ m or less, and the mass fraction of wustite contained in the scale layer is 10% or less.
• T1 Temperature (°C) of the steel plate after finish rolling at a point 30 m from the longitudinal end and at the center in the width direction
• T2 Temperature (°C) of the longitudinal center and width center of the steel plate after finish rolling
• T 3 The temperature (° C.) at a point 30 m from the longitudinal tail end and at the widthwise center of the steel plate after finish rolling.
• Patent Document 2 also proposes a hot-rolled steel sheet with excellent tight-scaling properties, which contains, by mass, C: 0.02-0.20%, Mn: 0.1-2.0%, Si: 0.3% or less, P: 0.03% or less, S: 0.03% or less, Ni: 0.03-0.3%, Cu: 0.04-0.5%, Cr: 0.03-0.3%, with the remainder being Fe and unavoidable impurities.
• the hot-rolling is then completed in the temperature range of 800°C to 950°C, and the steel sheet is rolled up at 400°C to 650°C.
• the surface roughness of the interface between the steel sheet surface scale and the steel sheet base metal is such that the number of irregularities of 0.5 ⁇ m or more per inch of length is 300 or more.
• Patent Document 3 proposes a black hot-rolled steel sheet having excellent blackness, characterized in that the hot-rolled steel sheet 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 has a scale having a thickness of more than 4 ⁇ m on the surface thereof, the scale has a composition containing 50% or more Fe 3 O 4 by volume, and does not contain precipitated Fe in a region from the scale surface to a depth of at least 2 ⁇ m in the thickness direction.
• Patent Document 1 a steel material having a predetermined composition is used, and the finish rolling exit temperature during hot rolling, the cooling rate after rolling, and the coiling temperature are adjusted. This optimizes the average particle size of magnetite particles in the upper layer of the magnetite layer on the base steel side in the scale layer and/or the average block size of the eutectoid transformation structure of iron and magnetite. Furthermore, by controlling the longitudinal temperature of the steel plate immediately after finish rolling, uniform improvement of scale adhesion in the longitudinal direction is achieved. However, no method is mentioned for suppressing flaky scale peeling.
• Patent Document 2 proposes a hot-rolled steel sheet with excellent tight-scaling properties by hot-rolling steel to which predetermined amounts of Ni, Cu, and Cr have been added, and controlling the surface roughness of the interface between the steel sheet surface scale and the steel sheet base steel within a predetermined range.
• the adhesion at the interface between the scale layer and the base steel is improved, there is concern that scale peeling may occur on the scale surface or inside the scale, reducing adhesion.
• the adhesion of the scale may decrease if the thickness of the hot-rolled steel sheet becomes greater.
• no method is mentioned for suppressing flaky scale peeling.
• Patent Document 3 a steel slab having a specified 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 adhesion of the scale.
• flaky scale peeling cannot be sufficiently suppressed.
• adhesion of the scale may decrease if the thickness of the hot-rolled steel sheet becomes larger.
• the present invention aims to solve the above problems and provide a hot-rolled steel sheet and its manufacturing method that has 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 leveler processing is performed.
• 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, and preferably more than 5.0 mm and not more than 25 mm.
• the inventors first investigated the cause of flake-like scale peeling on conventional hot-rolled steel sheets. As a result, it became clear that the scale that peels off in flakes peels off from the interface between the scale and the base steel. In other words, to suppress flake-like peeling, 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. In addition, 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.
• Scales formed during hot rolling are roughly divided into primary scales formed in slab heating furnaces and rough rolling processes, and secondary scales formed during finish rolling processes.
• primary scales are often completely removed by descaling before finish rolling, so the properties of the secondary scales have a large effect on the scale adhesion in the final product.
• the present inventors have conducted extensive research into means for obtaining excellent scale adhesion, particularly resistance to flaky peeling, even in hot-rolled steel sheets with larger plate thicknesses, and have obtained the following findings.
• a steel material having a prescribed composition is heated at a heating temperature of 1150°C or higher, scale is removed by descaling, and then 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 flaking, without completely removing the primary scale generated during rough rolling from the interface with the base steel.
• 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 peeling resistance.
• the steel sheet has a scale on the surface thereof, the average thickness of the scale is 25 ⁇ m or less, and the scale has, by area ratio, magnetite: 20% or more, eutectoid transformation structure of iron and magnetite: 30% or more, wustite: 15% or less, and a structure in which the sum of magnetite, wustite, and eutectoid transformation structure of iron and magnetite is 90% or more (including 100%), and the arithmetic average roughness of the interface between the scale and the base steel is 0.3 ⁇ m or more.
• a method for producing a hot-rolled steel sheet comprising heating a steel material having the composition according to [1] or [2] at a heating temperature of 1150°C or more, descaling, performing hot rough rolling and descaling with water injection at a pressure of 30 MPa or less, performing 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 a 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 a 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 more and the coiling temperature or less.
• the present invention makes it possible to easily and inexpensively manufacture hot-rolled steel sheets with excellent resistance to flaking of scale, which is of great industrial benefit.
• the present invention also makes it possible to prevent flaking of scale, which contributes greatly to improving the surface quality of products, preventing defective processing of products, and improving the working environment. It also solves problems such as the increase in thickness of hot-rolled steel sheets and the decrease in resistance to flaking of scale that accompanies processing under harsh conditions, particularly leveler processing.
• the hot-rolled steel sheet of the present invention contains the following composition. Note that “%”, which is the unit of content of the composition, means “mass %” unless otherwise specified.
• C 0.01-0.30% C is an element useful for ensuring strength. If its 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 100%, 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, which causes the scale to peel off during rolling and leads to scale defects. From the viewpoint of resistance to flaking of scale, it is preferably 0.20% or less.
• Si 0.50% or less
• Si is an element that acts as a deoxidizer, and in order to obtain this effect, it is preferable to contain 0.01% or more.
• the Si content exceeds 0.50%, Si is concentrated at the interface between the scale and the base steel, and a Si oxide layer is formed. At the interface between this Si oxide layer and the scale layer formed thereon, scale peeling is likely to occur. For this reason, the Si content is set to 0.50% or less. Preferably, it is 0.20% or less.
• P 0.10% or less
• P is an element that is desirably reduced as much as possible because it has a detrimental effect on grain boundary embrittlement.
• 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 in flake form. If the P content exceeds 0.10%, these detrimental effects become greater, so the P content is set to 0.10% or less, preferably 0.05% or less. Note that P does not have to be contained, but the P content is preferably 0.001% or more from the viewpoint of manufacturing costs.
• 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 in order to obtain such an effect, it is preferable to contain 0.01% or more. On the other hand, if it is contained in excess of 0.10%, oxide-based inclusions increase and cleanliness decreases. For this reason, the amount of sol. Al is set to 0.10% or less, and preferably to 0.06% or less.
• N 0.015% or less
• N is an element that forms nitrides such as BN, AlN, and TiN in steel, and reduces the hot ductility of steel and the surface quality.
• the N content is set to 0.015% or less.
• the N content is preferably 0.010% or less. Note that N may not be contained, but 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.
• the hot-rolled steel sheet of the present invention may contain one or more of Cu: 1.0% or less, Ni: 0.50% or less, and Cr: 2.0% or less, as necessary, in order to improve various properties.
• Cu 1.0% or less
• Cu is an element that concentrates at the interface between the scale and the base steel to promote grain boundary oxidation, promotes the roughening of the interface between the scale and the base steel, and improves the adhesion of the interface between the scale and the base steel.
• it is preferable to contain 0.01% or more of Cu.
• it if it contains more than 1.0%, molten Cu will penetrate into the austenite grain boundaries of the base steel during heating, and there is a concern that the surface properties will deteriorate due to hot embrittlement. For this reason, if Cu is contained, it is set to 1.0% or less. Preferably, it is 0.8% or less.
• the present invention may further contain one or more of the following as necessary: 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.
• Nb 0.1% or less Nb is an element that improves the strength and toughness of the base material, and in order to obtain such an effect, it is preferable to contain 0.003% or more. On the other hand, if it is contained in excess of 0.1%, it may cause a decrease in toughness. Therefore, when Nb is contained, the amount is set to 0.1% or less.
• 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. In order to obtain these effects, it is preferable to contain Ti at 0.001% or more. On the other hand, if it is contained in an amount exceeding 0.03%, it may cause a decrease in toughness. Therefore, when Ti is contained, the amount is set to 0.03% or less.
• B 0.01% or less B is an element that enhances the hardenability of steel, and this effect can increase strength. In order to obtain this effect, it is preferable to contain 0.0005% or more of B. On the other hand, if the content exceeds 0.01%, this effect saturates, so if B is contained, the amount is set to 0.01% or less.
• 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. In order to obtain 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 liquid metal when the material is heated, corrodes the prior austenite grain boundaries, and reduces the adhesion of the interface between the scale and the base steel. For this reason, if Sb is contained, it is set to 0.03% or less.
• the remainder other than the above chemical components consists of Fe and unavoidable impurities.
• the allowable unavoidable impurities are O: 0.005% or less, Mg: 0.003% or less, Sn: 0.1% or less, and Ca: 0.01% or less.
• the optional elements contained below the lower limit do not impair the effects of the present invention. Therefore, when the optional elements are contained below the preferred lower limit, the optional elements are considered to be contained as unavoidable impurities.
• Magnetite 20% or more Magnetite has high compatibility with the base steel and contributes to improving the resistance to flaking of scale. If it is less than 20%, this effect cannot be obtained sufficiently, so the area ratio of magnetite is set to 20% or more. It is preferably 30% or more. There is no particular upper limit, but if the area ratio of magnetite exceeds 70%, the proportion of the eutectoid transformation structure of iron and magnetite that contributes to improving the resistance to flaking of scale decreases, and the resistance to flaking of scale may be impaired. For this reason, the area ratio of magnetite is preferably 70% or less. It is more preferably 60% or less. Note that the magnetite here can be distinguished from magnetite contained in the eutectoid transformation structure of iron and magnetite.
• Eutectoid transformed 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. If it is less than 30%, this effect cannot be sufficiently obtained, so the area ratio of the eutectoid transformed structure of iron and magnetite is set to 30% or more. It is preferably 40% or more, and more preferably 45% or more.
• Wustite 15% or less
• wustite may remain untransformed at room temperature.
• Wustite is more brittle than magnetite at room temperature, and has lower compatibility with the base steel compared to magnetite and 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.
• the area ratio of wustite is set to 15% or less. It is preferably 10% or less, and more preferably 7% or less.
• the area ratio of wustite may be 0%.
• a hematite layer may form in the outermost layer of the hot-rolled steel sheet.
• the formation of the hematite layer does not impair the effects of the present invention, so it is acceptable for the hematite layer to be present.
• the mass fraction of hematite is preferably 10% or less.
• the mass fraction of hematite can be regarded as an area fraction.
• Total of magnetite, wustite, and eutectoid transformation structure of iron and magnetite 90% or more (including 100%)
• the total fraction of magnetite, wustite, and the eutectoid transformed structure of iron and magnetite is set to 90% or more.
• the remaining structures other than magnetite, wustite, and the eutectoid transformation structure of iron and magnetite include non-ferrous oxides such as Si-based oxides and Cr-based oxides in addition to hematite, but as long as the total area ratio of these is 10% or less, the effect of the present invention is not impaired.
• these remaining structures can be measured by X-ray diffraction analysis in the same manner as the measurement method for hematite described below.
• Arithmetic mean roughness of the interface between the scale and the base steel is 0.3 ⁇ m or more
• the arithmetic mean roughness of the interface between the scale and the base steel is set to 0.3 ⁇ m or more, preferably 0.4 ⁇ m or more.
• the arithmetic mean roughness of the interface between the scale and the base steel is preferably 5 ⁇ m or less.
• the area ratios of magnetite, eutectoid transformation structure of iron and magnetite, wustite, and precipitated Fe are measured by cutting a cross section of the steel plate perpendicular to the surface and parallel to the rolling direction, and mirror-polishing it. After that, a scanning electron microscope (SEM) is used to observe the backscattered electron image of the cross section of the scale at a magnification of 3000 times. In the backscattered electron image of the SEM, magnetite is the darkest area, the base steel and precipitated Fe are the brightest, and wustite is the area that appears with intermediate contrast.
• the average thickness of the scale is calculated by measuring the scale thickness at three arbitrary positions with the SEM and averaging the results.
• the mass fraction of hematite detected using an X-ray diffraction device is regarded as an area fraction and is subtracted from the total area of the above-mentioned oxide scale, which is 100%, and the area fractions of magnetite, eutectoid transformation structure, and wustite are calculated from the remaining areas.
• Heating temperature 1150°C or higher
• the above steel material is heated at a heating temperature of 1150°C or higher. This heating is performed for the purpose of sufficiently solidifying the steel base material and roughening the interface between the primary scale and the base steel. If the heating temperature is less than 1150°C, the roughening of the interface does not proceed sufficiently, so the heating temperature is set to 1150°C or higher. It is preferably 1180°C or higher. It is also preferably 1300°C or lower. In the case of a slab produced by a continuous casting method, direct rolling may be applied in which the slab is rolled as is or while being held in order to suppress a decrease in temperature.
• the hot rolling process consists of rough rolling and finish rolling.
• Rough rolling only requires that a sheet bar of the specified dimensions be obtained, and there is no need to limit the conditions for rough rolling.
• Scale formed during heating of the steel material is descaled by water injection before rough rolling. This is because the scale formed during heating is very thick, which reduces the resistance of the scale to flake-like peeling in the final product and causes surface defects. Descaling at this time requires only that the thick scale can be removed, and any conventionally known method may be used.
• the material to be rolled may be heated midway using a heating means such as a sheet bar heater.
• Descaling by water injection at a pressure of 30 MPa or less Before finish rolling, descaling is performed at the inlet side of the finish rolling mill by water injection at a pressure of 30 MPa or less. This allows the primary scale generated during rough rolling to be completely removed from the interface with the base steel, and an uneven interface between the scale and the base steel that is suitable for improving the flake-like peeling resistance in the final product to be obtained. If the water injection pressure exceeds 30 MPa, the scale is completely removed from the interface, resulting in insufficient roughness at the interface between the scale and the base steel, and the adhesion at the interface between the scale and the base steel may decrease. Therefore, the water injection pressure is set to 30 MPa or less, and preferably 25 MPa or less. In addition, from the viewpoint of preventing an excessive increase in scale thickness, the water injection pressure is preferably 5 MPa or more, and more preferably 10 MPa or more.
• Finish rolling exit 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 occur, which leads to a decrease in the resistance of the scale to flake-like peeling. In addition, the scale structure becomes finer, and the hardness of the scale itself increases, causing a decrease in adhesion at the interface between the scale and the base steel. On the other hand, if the finish rolling exit temperature exceeds 950°C, the scale grows excessively, increasing the scale thickness and decreasing 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.
• 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 Since scale grows faster in high temperature regions, in order to suppress a decrease in the scale's resistance to flake-like peeling due to excessive growth of scale, it is necessary to quickly cool the high temperature region immediately after finish rolling. If the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is less than 5°C/s, the scale grows excessively, causing a decrease in the 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. It is preferably 7°C/s or more.
• the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C exceeds 80°C/s, the scale structure becomes finer, which may cause a decrease in the adhesion at the interface between the scale and the base steel.
• the ductility of the scale decreases, which may cause cracks to occur and cause a decrease in the adhesion at the interface between the scale and the base steel.
• 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.
• Cooling in the temperature range from 750°C to the start of coiling at an average cooling rate of 1°C/s or more and 30°C/s or less In the temperature range from 750°C to the start of coiling, scale grows relatively slower than in the high temperature region immediately after finish rolling, but it is necessary to suppress a decrease in the interface adhesion between the scale and the base steel due to excessive growth of scale. 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 grows excessively, causing a decrease in the interface 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.
• the average cooling rate in the temperature range from 750°C to the start of coiling exceeds 30°C/s, the scale structure becomes fine and the stress difference with the base steel becomes large. This may cause cracks to occur in the scale, and the interface adhesion between the scale and the base steel may decrease. 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. The rate is preferably 20° C./s or less.
• Winding temperature 500-630°C
• 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 wustite does not occur sufficiently, and an excessive amount of wustite remains at room temperature. As a result, wustite is brittle at room temperature, and the resistance to flake-like peeling of the scale is reduced. If the coiling temperature exceeds 630°C, the scale grows excessively, and the adhesion at the interface between the scale and the base steel is weakened. 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 and a preferred upper limit being 620° C. or lower.
• the eutectoid transformation of wustite is sufficiently promoted by holding for 100 minutes or more in a temperature range of ⁇ 50° C. to the coiling temperature. If the holding time in the temperature range of ⁇ 50° C. to the coiling temperature is less than 100 minutes or if the coiling temperature is held at less than ⁇ 50° C., the eutectoid transformation of wustite does not occur sufficiently and an excessive amount of wustite remains at room temperature. As a result, wustite is brittle at room temperature, and the flake spalling resistance of the scale is reduced.
• the holding time in the temperature range of ⁇ 50° C. to the coiling temperature is set to 100 minutes or more. It is preferably 120 minutes or more.
• the holding time in the temperature range of ⁇ 50° C. to the coiling temperature is preferably 300 minutes or less.
• the hot-rolled steel sheet wound into a coil may be deformed and subjected to shape correction processing using a roller leveler, tension leveler, etc.
• a roller leveler For example, for a hot-rolled steel sheet with a thickness of 12 mm, two upper rolls with a diameter of 250 mm and three lower rolls are arranged, and shape correction processing is performed under the condition of a pressing amount of 2 mm.
• the area ratios of magnetite, eutectoid transformation structure of iron and magnetite, wustite, and precipitated Fe were measured by cutting a cross section of the steel plate 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 is the darkest area, the base steel and precipitated Fe are the brightest, and wustite is the area that appears with intermediate contrast.
• the average thickness of the scale was determined by measuring the scale thickness at three random positions using the SEM and averaging the results.
• the mass fraction of hematite was determined using an X-ray diffractometer with a CoK ⁇ radiation source to measure the integrated intensity of the diffraction peak of each phase in the scale, and from the ratio of the integrated intensity of each phase in the test sample to that in a standard sample (a mixture of equal weights of Fe, FeO (wustite), Fe2O3 (hematite), and Fe3O4 (magnetite)), using the following formula ( 2 ).
• the arithmetic mean roughness of the interface between the scale and the base steel was determined by first removing the scale from the steel plate surface by pickling, then 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 ⁇ m. The arithmetic mean roughness of these sections was then averaged to determine the roughness.
• the adhesion of the scale was evaluated by the resistance to flaking. That is, a 30 mm x 100 mm test piece was taken from the hot-rolled sheet after leveling so that the longitudinal direction was parallel to the rolling direction, and then bending was performed by the push bending method in accordance with JIS Z 2248. Tape was applied to the steel sheet surface on the outside of the bend to peel off the scale.
• the conditions for bending were a ratio r/t of the radius r of the push fitting tip to the thickness t of the test piece of 4, and a bending angle of 180°.
• An image of the steel sheet surface in the area where the tape was peeled off was taken with a camera, and the area ratio of the exposed steel sheet, that is, the ratio of the area of the exposed steel sheet to the area of the peeled off tape, was measured from the obtained image.
• the area ratio of the exposed steel sheet was less than 10%, it was determined that the resistance to flaking was excellent, and this was indicated as ⁇ in Table 2.
• the area ratio of the exposed steel sheet was 10% or more, it was determined that the resistance to flaking was poor, and this was indicated as ⁇ in Table 2.

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