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
  • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations

Definitions

• the present invention relates to a hot-rolled steel sheet with excellent scale adhesion, which is used in automobiles, home appliances, building materials, etc., and a manufacturing method thereof.
• 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 with excellent scale adhesion, consisting of adhesion at the interface between the scale and the base steel, and resistance to powder-like peeling, and a manufacturing method thereof.
• 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.
• powder-like scale peeling is a major cause of contamination of the processing line and deterioration of surface quality. Therefore, in order to improve the adhesion of the scale, it is necessary to suppress the peeling of powder-like scale.
• 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 unavoidable 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 temperature in the longitudinal direction of the steel plate immediately after finish rolling, uniform adhesion of the scale in the longitudinal direction is improved. However, no method is mentioned for suppressing the peeling of powdery scale.
• Patent Document 2 proposes a hot-rolled steel sheet with excellent tight-scaling properties, which is produced by hot-rolling steel to which prescribed 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 prescribed 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 surface or inside the scale, reducing adhesion.
• no method is mentioned for suppressing powder-like scale peeling.
• a steel slab having a predetermined composition is heated, then hot-rolled to a finish rolling exit 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.
• the area from the scale surface to a depth of at least 2 ⁇ m in the thickness direction is controlled so that it does not contain precipitated Fe, and there is a particular concern that powder-like scale peeling cannot be sufficiently suppressed.
• the present invention aims to solve the above problems and provide a hot-rolled steel sheet with excellent scale adhesion, consisting of adhesion between the scale and base steel interface and resistance to powder-like 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, and preferably more than 5.0 mm and not more than 20 mm.
• the inventors first investigated the cause of powder-like flaking of scale on conventional hot-rolled steel sheets. As a result, it became clear that the scale that flaks off in powder form is mainly composed of magnetite, and is caused by destruction inside the scale. In other words, by appropriately controlling the magnetite that forms on the surface layer of the scale, it is possible to suppress powder-like flaking.
• the scale formed during hot rolling is formed in the following order from the surface layer side at high temperatures: hematite, magnetite ( Fe3O4 ), and wustite (FeO).
• hematite magnetite
• Fe3O4 magnetite
• FeO wustite
• wustite 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 ).
• a eutectoid transformed structure consisting of magnetite and precipitated Fe (4FeO ⁇ Fe3O4 + Fe ).
• cracks were generated at the interface between magnetite formed at high temperatures and the eutectoid transformed structure, resulting in powder-like scale peeling.
• 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.
• the steel sheet surface is isolated from the atmosphere during coiling, and the temperature is held within a range in which wüstite is stably formed, thereby reducing some or all of the hematite and magnetite formed on the surface layer of the scale to wüstite.
• 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 layer side of the scale.
• the adhesion between the scale and the base steel interface is improved, and the amount of magnetite formed on the surface layer of the scale is reduced, thereby suppressing powder-like peeling.
• the composition further comprises, in mass%, Cu: 1.0% or less, Ni: 0.50% or less, The hot-rolled steel sheet according to [1], containing one or more of: [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.
• a method for producing a hot-rolled steel sheet comprising the steps of: rough hot rolling a steel material having the composition according to [1] or [2]; descaling the steel material; finish rolling the steel material at a finish rolling exit temperature of 800 to 950°C; cooling the steel material at a temperature range from the finish rolling exit temperature to 750°C at an average cooling rate of 5°C/s or more; cooling the steel material at a temperature range from 750°C to the start of coiling at an average cooling rate of 1°C/s or more and 30°C/s or less; coiling the steel material at a coiling temperature of 500 to 650°C; and holding the steel material at a temperature range of ⁇ 50°C or more and the coiling temperature or less for 100 minutes or more.
• a method for producing a hot-rolled steel sheet comprising the steps of: rough hot rolling a steel material having the composition described in [3], descaling the steel material, performing finish rolling at a finish rolling exit temperature of 800 to 950°C, cooling the steel material from the finish rolling exit temperature to 750°C at an average cooling rate of 5°C/s or more, cooling the steel material from 750°C to the start of coiling at an average cooling rate of 1°C/s or more and 30°C/s or less, coiling the steel material at a coiling temperature of 500 to 650°C, and holding the steel material in a temperature range of ⁇ 50°C or more and the coiling temperature or less for 100 minutes or more.
• the present invention makes it possible to easily and inexpensively manufacture hot-rolled steel sheets with excellent scale adhesion, which is of great industrial benefit.
• the present invention also makes it possible to prevent peeling at the interface between the scale and the base steel, which contributes greatly to improving the surface quality of products, preventing defective product processing, and improving the working environment. It also solves issues such as a decrease in resistance to powdery peeling.
• 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. A C content exceeding 0.30% generates CO gas 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. Therefore, 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 content 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.
• Mn 0.01-2.0%
• Mn is an element that not only renders solute S, which causes embrittlement during hot working, harmless as MnS, but also has the effect of improving strength. If the amount is less than 0.01%, the effect is small. On the other hand, if the Mn content exceeds 2.0%, it will lead to a decrease in toughness and will also form Mn-based oxides at the interface between the scale and the base steel, causing a decrease in the adhesion of the scale.
• the content is set to 0.01 to 2.0%, preferably 0.05% or more as the lower limit, and preferably 1.5% or less as the upper limit.
• P 0.10% or less
• P is an element that is desirably kept as low as possible because it has a detrimental effect on grain boundary embrittlement.
• P forms a very brittle oxide layer at the interface between the scale and the base steel, reducing the adhesion of the interface between the scale and the base steel. 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.
• S 0.10% or less
• S is an element that significantly deteriorates hot workability and toughness.
• S concentrates at the interface between the scale and the base steel, reducing the adhesion at the interface between the scale and the base steel. If the S content exceeds 0.10%, these adverse effects become greater, so the S content is set to 0.10% or less, and preferably to 0.05% or less.
• the S content is preferably set to 0.0001% 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. 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.
• 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 formation of irregularities at the interface between the scale and the base steel, and enhances adhesion at 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.
• 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 roughening of the interface between the scale and the base steel, and improves adhesion at the interface between the scale and the base steel. In order to obtain such effects, it is preferable to contain Ni at 0.01% or more. However, if the Ni content exceeds 0.50%, the above effects saturate, and there is a concern of an increase in costs. For this reason, when Ni is contained, it is set to 0.50% or less. Preferably, it is 0.40% or less.
• Cr 2.0% or less Cr has the effect of increasing strength, hardenability, and corrosion resistance.
• Cr concentrates at the interface between the scale and the base steel, and the scale bites into the base steel by making the interface uneven, and also has the effect of improving the adhesion of the interface between the scale and the base steel.
• the content exceeds 2.0%, the above effect saturates, so when Cr is contained, it is set to 2.0% or less.
• a more preferable lower limit is 0.07% or more.
• a more preferable upper limit is 1.0% or less.
• the most preferable lower limit is 0.12% or more.
• the most preferable upper limit 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.
• Mo 1.0% or less Mo has the effect of improving strength and hardenability and suppressing softening due to tempering. In order to obtain such effects, it is preferable to contain Mo at 0.1% or more. On the other hand, if it is contained in excess of 1.0%, the strength increases excessively and the toughness and formability deteriorate, so when Mo is contained, the amount is set to 1.0% 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.
• V 0.1% or less
• V is an element that improves the strength and toughness of the base material, and in order 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 cause a decrease in toughness. Therefore, when V 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 between the scale and the base steel interface. 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.
• Average thickness of scale is 20 ⁇ m or less If the average thickness of scale exceeds 20 ⁇ m, strain applied to the outermost layer of the scale and the interface between the scale and the base steel when processing the steel sheet becomes large, and the adhesion of the interface between the scale and the base steel and the resistance to powdery peeling decrease. For this reason, the average thickness of scale is set to 20 ⁇ m or less. It is preferably 18 ⁇ m or less, and more preferably 15 ⁇ m or less. There is no particular lower limit for the average thickness of scale, but from the viewpoint of ensuring the stability of the cut surface quality during processing such as laser cutting, it is preferably 3 ⁇ m or more. It is more preferably 5 ⁇ m or more.
• Magnetite 20% or more Magnetite has high compatibility with the base steel and contributes to improving the adhesion at the interface between the scale and the base steel. This effect cannot be fully obtained if the ratio is less than 20%, 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 amount of magnetite on the surface of the scale, which is the cause of powder-like peeling, increases, and resistance to powder-like peeling 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 transformation structure of iron and magnetite 40% or more
• the eutectoid transformation structure of iron and magnetite has high compatibility with magnetite and precipitated Fe and the base steel, and therefore contributes to improving the adhesion of the interface between the scale and the base steel.
• the surface layer side of the scale contains an eutectoid transformation structure of iron and magnetite
• the magnetite on the surface layer of the scale which is the cause of powdery spalling, is reduced, thereby contributing to improving the resistance to powdery spalling.
• This effect cannot be sufficiently obtained if it is less than 40%, so the area ratio of the eutectoid transformation structure of iron and magnetite is set to 40% or more. It is preferably 45% or more, and more preferably 50% or more.
• Wustite 15% or less
• wustite may remain untransformed at room temperature. Wustite is more brittle than magnetite at room temperature, and cracks in the scale impair the powder-like peeling resistance of the scale.
• wustite has a lower compatibility with the base steel compared to magnetite and the eutectoid transformed structure of iron and magnetite, which causes the adhesion of the interface between the scale and the base steel to be impaired.
• 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 may be less than 90%. In such a case, it may cause surface defects and may reduce the adhesion and powdery peeling resistance of the interface between the scale and the base steel.
• the predetermined adhesion and powdery peeling resistance of the interface between the scale and the base steel, 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 method for measuring the mass fraction of hematite described below.
• temperature control described later part or all of the hematite and magnetite formed on the surface layer of the scale at high temperatures is reduced to wustite.
• the temperature is held in the coiling temperature range of -50°C or more and the coiling temperature or less for 100 minutes or more to sufficiently advance the eutectoid transformation of wustite, so that an eutectoid transformed structure is also formed on the surface layer side of the scale.
• the amount of the eutectoid transformed structure on the surface layer side can be expressed by the area ratio of precipitated Fe in the region from the outermost layer of the scale to 1/2 of the scale thickness in the thickness direction.
• precipitated Fe in the region from the outermost layer of the scale to 1/2 of the scale thickness in the thickness direction is set to 2% or more. It is preferably 3% or more.
• the amount of precipitated Fe in the region extending from the outermost layer of the scale to half the thickness of the scale in the thickness direction but it is preferably 50% 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 area ratios of magnetite, wustite, and eutectoid transformation structure of iron and magnetite are values when the area of the entire oxide scale is taken as 100%, and if there are voids in the oxide scale, they are excluded from the calculation of the area ratio.
• 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.
• the temperature specified in each process in this invention refers to the surface temperature of the slab (steel slab) or steel plate, and can be measured with a radiation thermometer or the like. Furthermore, unless otherwise specified, the average cooling rate is ((cooling start temperature - cooling stop temperature) / cooling time).
• the method for producing the steel material having the above-mentioned composition is not particularly limited, and any commonly used method can be applied.
• a casting method such as a continuous casting method.
• the steel material is heated and then hot rolled. This heating is sufficient to achieve a sufficient solid solution, and is preferably heated to the Ac 3 point or higher.
• a normal slab heating temperature range of 1060°C to 1300°C is appropriate.
• direct rolling may be applied in which the slab is rolled as it is or while being held for the purpose of suppressing a temperature drop.
• the hot rolling process consists of rough rolling and finish rolling. There is no need to limit the conditions for rough rolling as long as the rough rolling can produce a sheet bar of the specified dimensions.
• the material to be rolled may be heated midway using a heating means such as a sheet bar heater.
• a heating means such as a sheet bar heater.
• scale formed on the surface of the sheet bar is removed by descaling using conventional high water pressure at the entry side of the finish rolling mill.
• finish rolling is performed. If the finish rolling entry temperature exceeds 1100°C, the thickness of the scale increases, and the adhesion of the interface between the scale and the base steel and the resistance of the scale to powder-like peeling may decrease. On the other hand, if the finish rolling entry temperature is 950°C or lower, the rolling load increases significantly, which may decrease productivity. In addition, 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 decrease productivity. Therefore, the finish rolling entry temperature is preferably 1100°C or lower, and more preferably 1050°C or lower. In addition, the lower limit of the finish rolling entry temperature is preferably 950°C or higher.
• Finish rolling exit temperature 800 to 950°C If the finish rolling exit temperature is less than 800°C, cracks are generated due to a decrease in the ductility of the scale. This reduces the adhesion between the scale and the base steel, and the cracks promote the reoxidation of the scale to generate hematite, which causes a decrease in the powdery spalling resistance of the scale. In addition, the scale structure becomes finer, and the hardness of the scale itself increases, reducing the adhesion and powdery spalling resistance of the scale and the base steel.
• the finish rolling exit temperature exceeds 950°C
• the scale grows excessively, increasing the thickness of the scale, reducing the adhesion between the scale and the base steel, and reducing the powdery spalling resistance of the scale.
• the grain size of each phase in the scale structure increases, reducing the powdery spalling resistance of the scale. 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 the high temperature range, in order to suppress a decrease in the adhesion of the interface between the scale and the base steel and in the powdery peeling resistance of the scale due to excessive growth of the scale, it is necessary to cool the high temperature range immediately after the finish rolling quickly. 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 of the interface between the scale and the base steel and in the powdery peeling resistance of the scale.
• 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 decrease the adhesion of the scale.
• cracks are generated due to the decrease in ductility of the scale, and these cracks promote the reoxidation of the scale, which generates hematite and may cause a decrease in the powdery spalling resistance of the scale.
• 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, the growth of scale is relatively slower than in the high temperature region immediately after finish rolling, but it is necessary to suppress a decrease in the adhesion of the interface between the scale and the base steel and in the powdery peeling resistance of the scale 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 adhesion of the interface between the scale and the base steel and in the powdery peeling resistance of the scale.
• the average cooling rate in the temperature range from 750°C to the start of coiling is set to 1°C/s or more. It is preferably 3°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 causes cracks in the scale, which reduces the adhesion between the scale and the base steel interface, and these cracks promote reoxidation of the scale, which produces hematite and reduces the powder-like peeling resistance of the scale.
• 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.
• Winding temperature 500-650°C
• the steel sheet is coiled at a coiling temperature of 500 to 650°C.
• the coiling isolates the surface of the steel sheet from the atmosphere and keeps it within the temperature range where wustite is stably formed, so that the scale formed on the surface layer is prevented from forming.
• the temperature is kept in the range of -50°C to the coiling temperature for 100 minutes or more to fully induce the eutectoid transformation of wustite.
• the adhesion between the scale and the base steel is improved, and the amount of magnetite produced on the scale surface is reduced, resulting in a powder-like structure.
• 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. The preferred upper limit is 630° C. or less.
• the eutectoid transformation of wustite is sufficiently promoted by holding for 100 minutes or more in a temperature range of from ⁇ 50° C. to the coiling temperature. If the holding time in the temperature range of from ⁇ 50° C. to the coiling temperature is less than 100 minutes, the eutectoid transformation structure formed on the surface layer side of the scale becomes insufficient, and the powdery peeling resistance decreases. Furthermore, 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.
• the holding time in the temperature range of from ⁇ 50° C. to the coiling temperature is set to 100 minutes or more. It is 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, which may cause a decrease in scale adhesion, and therefore the holding time in the temperature range of not less than the coiling temperature -50°C and not more than the coiling temperature is preferably 300 minutes or less.
• the coil After winding, it is preferable to place the coil in a coil box or to cover the coil to prevent oxidation of the outermost circumference and edges.
• 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 adhesion of the scale was evaluated by the adhesion of the interface between the scale and the base steel and the resistance to powdery peeling. That is, a test piece was taken from the hot-rolled sheet after leveling, and the scale was peeled off by attaching tape to the surface of the steel sheet. The amount of exposed base steel on the steel sheet surface and the amount of scale attached to the tape were evaluated. The peeled tape was attached to a transparent sheet, then scanned and the amount of peeled scale was measured by image processing.
• the area ratio of the exposed part of the base steel that is, the ratio of the area of the part where the base steel was exposed to the area of the part where the tape was peeled off, was measured from the obtained image.
• the area ratio of the exposed part of the base steel was less than 10% and the area ratio of the scale attached to the peeled tape was less than 10%, the interface adhesion and resistance to powdery peeling were considered to be excellent, and this was marked with ⁇ in Table 2.
• the area ratio of the exposed part of the base steel on the steel sheet surface was 10% or more, the interface adhesion between the scale and the base steel was considered to be poor, and this was marked with ⁇ in Table 2.
• the area ratio of scale adhering to the peeled tape is 10% or more, the resistance to powdery peeling is deemed poor, and this is indicated as x in Table 2.
• the examples of the present invention shown in Table 2 had excellent interfacial adhesion between the scale and the base steel and resistance to powdery peeling of the scale, whereas the comparative examples had poor interfacial adhesion between the scale and the base steel and/or resistance to powdery peeling of the scale.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

Priority Applications (3)

Applications Claiming Priority (2)

Publications (1)

Family

ID=92904797

Family Applications (1)

Country Status (4)

Cited By (2)

Citations (6)

Family Cites Families (2)

• 2024
  • 2024-01-16 JP JP2024571291A patent/JP7722607B2/ja active Active
  • 2024-01-16 KR KR1020257031415A patent/KR20250150132A/ko active Pending
  • 2024-01-16 WO PCT/JP2024/000926 patent/WO2024202398A1/ja not_active Ceased
  • 2024-01-16 CN CN202480019961.9A patent/CN120917168A/zh active Pending

Patent Citations (6)

Also Published As

Similar Documents

Legal Events