Classifications

• • 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
• • 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
  • 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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention relates to a hot-rolled steel sheet having an oxide scale on its surface, which is used in automobiles, home appliances, building materials, etc., and a manufacturing method thereof.
• the present invention relates to a hot-rolled steel sheet having excellent blackness and scale adhesion suitable for use as a material for building materials, truck frames, woks, etc., and having small variations in blackness and scale adhesion in the width direction of the steel sheet, and a manufacturing method thereof.
• Hot-rolled steel sheets are usually hot-rolled at high temperatures in an oxidizing atmosphere, and so scale (iron oxide) inevitably forms on the surface.
• black hot-rolled steel sheets are subjected to 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 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.
• 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 method in which a slab containing, by mass%, C: 0.02-0.20%, Mn: 0.1-2.0%, Si: 0.3% or less, P: 0.03% or less, S: 0.03% or less, Ni: 0.03-0.3%, Cu: 0.04-0.5%, Cr: 0.03-0.3%, the remainder being Fe and unavoidable impurities is heated to 1100°C or higher, hot rolling is completed in the temperature range of 800°C to 950°C, and the slab is rolled up at 400°C to 650°C. This results in a hot-rolled steel sheet with excellent tight-scaling properties, characterized by a surface roughness at the interface between the steel sheet surface scale and the steel sheet base metal of 300 or more irregularities of 0.5 ⁇ m or more per inch of length.
• Patent Document 3 also proposes a hot-rolled steel sheet having scale, the scale at a portion within 30 mm from the end face of the coil having a magnetite layer at an interface between the base steel and the scale that is in contact with the base steel by an area ratio of 90% or more, a layer of eutectoid structure of iron and magnetite on top of the magnetite layer in contact with the base steel, a layer of magnetite on top of the layer of eutectoid structure of iron and magnetite, a layer of hematite on top of the magnetite layer, the sum of the thicknesses of the magnetite layer on top of the layer of eutectoid structure of iron and magnetite and the hematite layer is 30% or less of the total thickness of the scale, and the difference between the thickness of the scale 30 mm from the end face of the coil and the thickness of the scale at the center of the coil is 2 ⁇ m or less.
• Patent Document 4 proposes a black hot-rolled steel sheet having excellent blackness, characterized in that the surface of the hot-rolled steel sheet contains C: 0.001 to 0.20 mass%, Si: 0.001 to 0.50 mass%, Mn: 0.05 to 2.0 mass%, P: 0.05 mass% or less, S: 0.05 mass% or less, and sol.Al: 0.01 to 0.10 mass%, with the balance being Fe and unavoidable impurities, has a scale having a thickness of more than 4 ⁇ m, 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 5 proposes a black-skinned hot-rolled steel sheet including a base hot-rolled steel sheet and a scale on the surface of the base hot-rolled steel sheet, the scale consisting of Fe 3 O 4 and Fe and having a thickness of 3.0 to 20 ⁇ m.
• the average grain size in the surface layer of the scale is 3.0 ⁇ m or less
• the Fe area ratio in the cross section of the scale is less than 1.0% in a region 0 to 1.0 ⁇ m from the outermost layer of the scale in the thickness direction of the scale.
• the Fe area ratio is 1.0% or more in a region 0 to 1.0 ⁇ m from the interface between the scale and the base hot-rolled steel sheet in the thickness direction of the scale.
• 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 mention is made of a method for uniformly improving scale adhesion in the width direction. Furthermore, no mention is made of a method for increasing the blackness of the scale.
• Patent Document 2 proposes a hot-rolled steel sheet with excellent tight-scaling properties, which is produced by hot-rolling steel to which predetermined amounts of Ni, Cu, and Cr have been added, and controlling the surface roughness of the interface between the steel sheet surface scale and the steel sheet base metal within a predetermined range.
• a method for increasing the blackness of the scale there is no mention of a method for uniformly improving the adhesion and blackness of the scale in the width direction.
• Patent Document 3 proposes a method for manufacturing hot-rolled steel sheet, in which steel material is hot-rolled and wound into a coil.
• the roughly-rolled steel sheet is then finish-rolled at 850-1050°C, and the finish-rolled hot-rolled steel sheet is then wound into a coil at a coiling temperature of 500-650°C, while both end faces of the hot-rolled steel sheet are cooled so that the temperature at the end faces becomes 480°C or less within 5 minutes from the start of winding.
• the temperature at the end faces is then maintained at 480°C or less, and the hot-rolled steel sheet is then slowly cooled while still in the coiled state from the point when the temperature at the end faces reaches 400-480°C, thereby obtaining excellent scale adhesion in the width direction of the hot-rolled coil, especially at the edges.
• a method for uniformly improving the blackness of the hot-rolled coil in the width direction is made.
• Patent Documents 4 and 5 propose hot-rolled steel sheets with excellent blackness and scale adhesion, but uniform scale adhesion and blackness are not necessarily obtained across the width of the hot-rolled coil. In particular, there is a problem in that the scale whitens at the edges of the coil and scale adhesion becomes poor.
• the present invention aims to solve the above problems and provide a hot-rolled steel sheet that has excellent scale adhesion and uniform blackness in the width direction of the hot-rolled steel sheet, and a manufacturing method thereof.
• the present inventors first investigated the reason why the conventional hot-rolled steel sheet does not have uniformly excellent scale adhesion and blackness in the width direction.
• the scale formed during hot rolling is formed in the order of hematite (Fe 2 O 3 ), magnetite (Fe 3 O 4 ), and wustite (FeO) from the scale surface side at high temperatures.
• wustite undergoes eutectoid transformation during cooling after coiling, and a eutectoid transformation structure consisting of magnetite and precipitated Fe is formed (4FeO ⁇ Fe 3 O 4 + Fe).
• This eutectoid transformation structure consisting of magnetite and precipitated Fe has high consistency with the surrounding magnetite grains and base steel, and therefore contributes to improving scale adhesion.
• magnetite is an iron-based oxide having a black color, and therefore contributes to blackening of the surface of the black-skinned hot-rolled steel sheet.
• precipitated Fe formed with eutectoid transformation and hematite formed on the scale surface at high temperatures during hot rolling are factors that inhibit the blackening of the surface.
• the adhesion and blackness of the scale tended to be poor, especially at the edges in the width direction. It was found that the decrease in adhesion at the edges is caused by the following. In other words, reoxidation by air entering from the edges after winding into a coil causes an excessive increase in the amount of hematite on the scale surface and a magnetite layer consisting of columnar magnetite grains, reducing the proportion of eutectoid transformation structures and increasing the scale thickness at the edges. It was also found that the decrease in blackness at the edges is caused by an excessive increase in the amount of hematite formed on the scale surface.
• the steel sheets come into close contact with each other at the center in the width direction and are isolated from the oxidizing atmosphere, which can cause the hematite and magnetite on the scale surface to be reduced to wüstite, and eutectoid transformation can occur on the scale surface as well.
• the scale adhesion improves, the amount of precipitated Fe on the scale surface increases, resulting in a problem of inferior blackness.
• the inventors therefore conducted extensive research into ways to solve the above problems and obtain hot-rolled steel sheets that have uniformly excellent scale adhesion and blackness across the width of the coil, and have come to the following conclusions.
• Winding temperature Winding at 450°C or higher and 600°C or lower appropriately controls the amount of eutectoid transformation structure that contributes to improved adhesion, and suppresses the formation of precipitated Fe on the steel sheet surface, which is a cause of whitening. Furthermore, after winding begins, the entire coil is cooled so that the temperature of the edge of the coil is cooled at an average cooling rate of 0.5°C/s or higher and 6.0°C/s or lower from the winding temperature to the cooling stop temperature of 300°C or higher and 430°C or lower. This reduces the temperature of the edge of the coil, increases the rigidity of the coil, and prevents loose winding.
• the edge of the coil means a range within 200 mm from the edge in the width direction of the coil.
• the center in the width direction is isolated from the oxidizing atmosphere, suppressing the formation of hematite due to reoxidation, and suppressing deterioration of adhesion and whitening.
• cooling the edges suppresses reoxidation, and reheating from the center in the width direction promotes eutectoid transformation, ensuring excellent adhesion and black color.
• the present invention has been made based on the above findings, and specifically provides the following. [1] In 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.
• the temperature range from the finish rolling exit temperature to 750 ° C. is cooled at an average cooling rate of 5 ° C./s or more, and then the temperature range from 750 ° C. to the start of coiling is cooled at an average cooling rate of 1 ° C./s or more and 30 ° C./s or less, and coiled at a coiling temperature of 450 ° C. or more and 600 ° C. or less.
• the temperature of the edge portion of the coil is cooled at an average cooling rate of 0.5 ° C./s or more and 6.0 ° C./s or less from the coiling temperature to a cooling stop temperature of 300 ° C. to 430 ° C. or less.
• a method for producing a hot-rolled steel sheet comprising: cooling the entire coil.
• the present invention makes it possible to easily and inexpensively manufacture hot-rolled steel sheets with excellent scale adhesion and blackness, which is of great industrial benefit.
• the present invention can reduce the variation in scale adhesion across the width of the coil, which has the effect of greatly contributing to improving the surface quality of products, preventing product processing defects, and improving the working environment.
• the thickness of the hot-rolled steel sheet in this 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 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 the amount is less than 0.01%, the effect of ensuring strength is small, so the C amount is set to 0.01% or more. If the C content exceeds 0.30%, CO gas is generated at the interface between the scale and the base steel, causing peeling of the interface between the scale and the base steel during rolling, which causes scale defects, so the C amount is set to 0.30% or less. From the viewpoint of scale adhesion, the C amount is preferably 0.20% or less.
• Si 0.50% or less
• Si is an element that acts as a deoxidizer. Although it is not necessary to include Si, it is preferable to include 0.01% or more in order to obtain such an effect. However, if the Si content exceeds 0.50%, Si is concentrated at the interface between the scale and the base steel, and an 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. Furthermore, the descaling property at high temperatures before finish rolling is deteriorated, and the primary scale generated up to rough rolling is crushed in finish rolling, so that so-called red scale is likely to occur, and the blackness becomes inferior. For this reason, the Si content is 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 content exceeds 2.0%, it leads to a decrease in toughness and forms Mn-based oxides at the interface between the scale and the base steel, causing a decrease in the adhesion of the scale. In addition, the transformation after finish rolling is delayed, and the transformation is not completed before coiling, and the transformation progresses partially and unevenly in the longitudinal direction after coiling.
• the Mn amount is set to 0.01 to 2.0%.
• the preferable lower limit is 0.05% or more.
• the preferable upper limit is 1.5% 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 adhesion of the scale. If the P content exceeds 0.10%, these detrimental effects become greater, so the P content is set to 0.10% or less.
• the preferred upper limit is 0.05% or less. P does not need to be contained, but the lower limit 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 of the scale. If the S content exceeds 0.10%, these adverse effects become greater, so the S content is set to 0.10% or less.
• the S content is set to 0.05% or less.
• S may not be contained, but the lower limit is preferably 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 improves 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 enhances 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 scale.
• it is contained in excess of 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, and even more preferable is 0.12% or more.
• a more preferable upper limit is 1.0% or less, and even more preferable is 0.8% or less.
• 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 an effect, 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 may increase excessively and the toughness and formability may deteriorate. For this reason, 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. 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 an amount exceeding 0.1%, it may lead to 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. 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 an amount exceeding 0.1%, it may lead to 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 actually lead to 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 has the effect of increasing the hardenability of steel. This effect can increase the 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 of the steel sheet when the material is heated, and has the effect of suppressing the decrease in the C content in the surface layer of the steel sheet due to decarburization during heating. In order to obtain such an 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, and may erode the prior austenite grain boundaries and reduce the adhesion of scale. 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.
• any of the above optional elements are contained in amounts less than the preferred lower limit, they are also considered to be unavoidable impurities.
• the scale structure in the width direction of the hot-rolled steel sheet of the present invention refers to the scale structure in the center of the steel sheet in the width direction and at a position 5 mm from the edge in the width direction. It is sufficient if the structure at each position is within the following range.
• the magnetite of the present invention is composed of magnetite particles and magnetite contained in the eutectoid transformation structure. These are structures that can be distinguished as follows.
• the magnetite particles in the present invention include a magnetite layer near the scale surface consisting of columnar magnetite particles, and massive pro-eutectoid magnetite particles that are formed inside or adjacent to the eutectoid transformation structure prior to the progress of eutectoid transformation.
• a thin layer consisting of fine magnetite particles formed at the scale-base steel interface, so-called magnetite seam is included, and is distinguished from magnetite contained in the eutectoid transformation structure of iron and magnetite.
• Magnetite particles have higher ductility at room temperature than wustite and hematite, and contribute to improving the adhesion of the scale.
• the magnetite seam has high compatibility with the base steel, and contributes to improving the adhesion of the scale by suppressing peeling from the scale-base steel interface.
• the magnetite layer near the scale surface consisting of columnar magnetite particles is a structure that contributes to blackening. If the magnetite particles are less than 30%, these effects cannot be obtained sufficiently, so the area ratio of magnetite is set to 30% or more, preferably 40% or more. On the other hand, if the magnetite particles are contained in excess of 80%, cracks may occur in the magnetite, resulting in poor scale adhesion. For this reason, the area ratio of magnetite particles is preferably 80% or less, more preferably 70% or less.
• Area ratio of eutectoid transformation structure of iron and magnetite 20% or more and 60% or less
• the eutectoid transformation structure of iron and magnetite has high compatibility with magnetite and precipitated Fe and base steel, and therefore contributes to improving the adhesion of scale. If it is less than 20%, this effect cannot be obtained sufficiently. Therefore, the area ratio of the eutectoid transformation structure of iron and magnetite is set to 20% or more, preferably 25% or more, and more preferably 30% or more.
• the eutectoid transformation structure of iron and magnetite exceeds 60%, precipitated Fe derived from the eutectoid transformation is also generated near the scale surface, which causes whitening. Therefore, the eutectoid transformation structure of iron and magnetite is set to 60% or less, preferably 55% or less, and more preferably 50% or less.
• Mass fraction of hematite 5% or less
• hematite may form in layers on the surface of the scale. Hematite leads to deterioration of adhesion and whitening, and also causes surface defects such as red scale. For this reason, the mass fraction of hematite is set to 5% or less.
• Area ratio of precipitated Fe on the scale surface 15% or less Precipitated Fe on the scale surface is a factor that inhibits the blackening of the scale surface. Therefore, if the area ratio exceeds 15%, sufficient blackness cannot be obtained. Therefore, the area ratio of precipitated Fe on the scale surface is set to 15% or less. Also, the area ratio of precipitated Fe on the scale surface may be 0%.
• the average thickness of the scale in the width direction of the steel sheet is 3 ⁇ m or more and 20 ⁇ m or less.
• the average thickness of the scale is 3 ⁇ m or more, preferably 4 ⁇ m or more.
• the average thickness of the scale exceeds 20 ⁇ m, the strain applied to the scale surface when processing the steel sheet becomes large, and cracks are generated in the scale, which reduces the scale adhesion. Therefore, the average thickness of the scale is 20 ⁇ m or less, preferably 18 ⁇ m or less, more preferably 15 ⁇ m or less.
• the scale structure is measured at the center and edge of the steel plate in the width direction.
• the observation positions for evaluation are the center of the hot rolled coil in the width direction and a position 5 mm from the edge in the width direction.
• the area ratio of magnetite grains, the eutectoid transformation structure of iron and magnetite, and wustite is measured by cutting out a cross section of the plate thickness perpendicular to the steel plate surface from the observation position and parallel to the rolling direction as the observation surface and mirror polishing it. Then, it can be measured by observing the backscattered electron image of the cross section of the scale using a scanning electron microscope (SEM).
• SEM scanning electron microscope
• the field of view of the SEM is the entire scale thickness from the scale surface to the interface between the scale and the steel plate, which is the observation target. Therefore, the backscattered electron image of the scale cross section is observed at an observation magnification that fits the entire scale thickness within the field of view.
• magnetite is the darkest region
• the base steel and precipitated Fe in the eutectoid transformation structure are the brightest
• wustite is the region that appears with intermediate contrast.
• the eutectoid transformation structure of iron and magnetite is the region where magnetite and iron are formed in layers.
• Hematite is formed very thinly on the scale surface and is easily removed during mirror polishing, making it difficult to quantitatively evaluate it as an area fraction on a SEM. Therefore, an X-ray diffractometer is used to measure the integrated intensity of the diffraction peaks of each phase in the scale using a CoK ⁇ source. From the ratio of the integrated intensities of each phase in the standard sample and the test sample, the mass fraction can be calculated using the following formula (2). As the standard sample, an equal weight mixture of Fe, FeO (wustite), Fe 2 O 3 (hematite), and Fe 3 O 4 (magnetite) is used. The mass fraction of hematite can be regarded as an area fraction.
• 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, Fe 2 O 3 , or Fe 3 O 4 .
• the area ratio of precipitated Fe on the scale surface can be measured by observing the backscattered electron image of the surface at the above observation position using an SEM at a magnification of 3000x, and determining the area ratio of precipitated Fe that appears with the brightest contrast through image analysis.
• the average thickness of the scale in the width direction of the coil can be measured as follows. For example, a cross section perpendicular to the steel sheet surface and parallel to the rolling direction is cut out from the width center of the hot-rolled steel sheet and from a portion 5 mm from the width edge of the hot-rolled steel sheet, and mirror-polished. The scale thickness is then measured at three locations using an SEM, and the thickness can be determined by averaging these measurements. Note that the average thickness of the scale in the width direction of the present invention, 3 ⁇ m to 20 ⁇ m, means that the average thickness of the scale in the width direction is 3 ⁇ m to 20 ⁇ m in each of the width center and the portion 5 mm from the width edge of the hot-rolled steel sheet.
• 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 does not need to be 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 as long as the material is sufficiently dissolved, and is preferably heated to the Ac 3 point or higher.
• the usual slab heating temperature range of 1060°C to 1300°C is appropriate.
• direct rolling may be applied in which the material is rolled as it is or while being held in order to suppress 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 high water pressure or the like at the entry side of the 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 scale may decrease. On the other hand, if the finish rolling entry temperature is less than 950°C, a significant increase in the rolling load may occur, which may decrease productivity. Also, as the product thickness increases, the finish rolling entry thickness 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 less, and more preferably 1050°C or less. Also, it is preferable that the lower limit of the finish rolling entry temperature is 950°C or more.
• Finish rolling exit temperature 800°C or more and 950°C or less
• finish rolling exit temperature 800°C or more and 950°C or less
• the finish rolling exit temperature is set to 800°C or more and 950°C or less.
• the preferable lower limit is 820°C or more.
• the preferable upper limit is 930°C or less.
• 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 Scale grows faster in the high temperature region. Therefore, in order to suppress the deterioration of scale adhesion due to excessive growth of scale, it is necessary to cool the high temperature region immediately after 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 the deterioration of scale adhesion. Therefore, the average cooling rate in the temperature range from the finish rolling exit temperature to 750°C is 5°C/s or more, 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 may become finer and the scale adhesion may decrease.
• cracks are generated due to the decrease in ductility of the scale, and these cracks promote the reoxidation of the scale to generate hematite. As a result, it may cause the deterioration of the adhesion and blackness 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 the deterioration of scale adhesion 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 scale adhesion. 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. 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. As a result, cracks are generated in the scale, and these cracks promote the reoxidation of the scale. As a result, hematite is generated, causing deterioration of the adhesion and blackness of the scale. For this reason, the average cooling rate in the temperature range from 750° C. to the start of coiling is set to 30° C./s or less, and preferably 20° C./s or less.
• Coiling temperature 450°C to 600°C
• the steel sheet is coiled at a coiling temperature of 450°C to 600°C. If the coiling temperature is less than 450°C, the eutectoid transformation from wustite does not occur sufficiently after coiling, and excessive wustite remains at room temperature. As a result, wustite has a brittle characteristic at room temperature, and the adhesion of the scale decreases. If the coiling temperature exceeds 600°C, the scale grows excessively, and the adhesion of the scale decreases.
• the coiling temperature is set to 450°C to 600°C.
• the preferable lower limit is 470°C or more.
• the preferable upper limit is 580°C or less.
• the entire coil After the start of winding, the entire coil is cooled so that the temperature of the edge part of the coil is cooled at an average cooling rate of 0.5°C/s to 6.0°C/s from the winding temperature to the cooling stop temperature of 300°C to 430°C.
• the entire coil After the start of winding, the entire coil is cooled so that the temperature of the edge part of the coil is cooled at an average cooling rate of 0.5°C/s to 6.0°C/s from the winding temperature to the cooling stop temperature of 300°C to 430°C. This reduces the temperature of the edge part of the coil, increases the rigidity of the coil, and prevents loosening of the coil.
• the entire coil means both edge parts and the plate surface of the coil.
• Both edge parts of the coil mean a range of 200 mm or less in the width direction of the coil from both edges in the width direction of the coil.
• the plate surface of the coil means the surface of the coil in the range other than both ends.
• the cooling of the edge portion suppresses reoxidation, and the reheating from the center portion in the width direction promotes eutectoid transformation, thereby ensuring excellent adhesion and blackness.
• the temperature of the edge portion can be measured, for example, by a radiation thermometer.
• the temperature of the sheet surface is not specified because it is not possible to measure the temperature of the sheet surface at a specific longitudinal position during winding, but the above effect can be obtained by cooling the entire coil using the same cooling method as the edge portion. If the cooling stop temperature at the edge portion of the coil exceeds 430°C, the above effect cannot be obtained sufficiently.
• the cooling stop temperature at the edge portion of the coil is set to 300°C or more and 430°C or less.
• the preferable lower limit is 320°C or more.
• the preferable upper limit is 400°C or less.
• the average cooling rate at the edge portion is less than 0.5°C/s, the above effect cannot be obtained sufficiently. If the average cooling rate at the edge exceeds 6.0°C/s, cracks will occur in the scale due to the refinement of the scale structure and the large stress difference with the base steel.
• the average cooling rate at the edge is set to 0.5°C/s or more and 6.0°C/s or less, and preferably 1.0°C/s or more and 5.0°C/s or less.
• a cooling device that sprays water onto the plate surface and both edges of the coil while winding the coil in a winding machine, for example.
• the coil After cooling, it is preferable to place the coil in a coil box or to cover the coil to promote the eutectoid transformation from wüstite and to suppress oxidation of the outermost periphery and edges.
• the hot-rolled steel sheet wound into a coil may be deformed using a roller leveler, tension leveler, etc. to perform shape correction processing.
• the area ratio of magnetite grains, the eutectoid transformation structure of iron and magnetite, and wustite were measured as follows. A cross section of the plate thickness perpendicular to the steel plate surface and parallel to the rolling direction was cut out from the center of the hot-rolled coil in the width direction and from a portion 5 mm from the edge in the width direction as the observation surface, and mirror-polished. After that, a backscattered electron image of the cross section of the scale was observed using an SEM at a magnification of 3000x to measure.
• magnetite In the backscattered electron image of the SEM, magnetite is the darkest region, the precipitated Fe in the base steel and the eutectoid transformation structure is the brightest, and wustite is the region that appears with intermediate contrast.
• the eutectoid transformation structure of iron and magnetite is the region where magnetite and iron are formed in layers.
• hematite an X-ray diffractometer was used to measure the integrated intensity of the diffraction peaks of each phase in the scale using a CoK ⁇ radiation source.
• the mass fraction was calculated using the following formula ( 2 ) from 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)).
• 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, Fe 2 O 3 , or Fe 3 O 4 .
• the area ratio of precipitated Fe on the outermost surface of the steel sheet was measured by observing backscattered electron images of the scale surface at the center of the width direction of the hot-rolled coil and at a magnification of 3000x using an SEM.
• the area ratio of precipitated Fe that was visible with the brightest contrast was determined by image analysis.
• the average thickness of the scale in the width direction of the coil was determined by cutting out a cross section of the sheet thickness perpendicular to the steel sheet surface and parallel to the rolling direction from the center of the width direction of the hot-rolled coil and from a portion 5 mm from the edge in the width direction. After mirror polishing, the scale thickness was measured at three points on each sheet using an SEM and the average was calculated.
• the adhesion of the scale was evaluated by taking 30 mm x 100 mm test pieces from the widthwise center of the hot-rolled coil and from a portion 5 mm from the widthwise edge so that the longitudinal direction was parallel to the rolling direction.
• a bending test by the push bending method JIS Z 22448 was performed, and then cellophane tape was applied to the surface of the test piece on the outer side of the bend and peeled off, and the amount of scale attached to the tape was measured by image analysis.
• the bending test conditions were a ratio r/t of the push fitting tip radius r to the test piece sheet thickness t of 4, and a bending angle of 180°.
• the evaluation criteria were as follows, with ⁇ and ⁇ being excellent in adhesion.
• the blackness was obtained by measuring the L * value of the L * a * b * color system (JIS Z 8729), which is generally considered to be close to human vision. The smaller the L * value, the closer to black it is.
• the viewing angle during measurement was 10°, and the main light source was auxiliary illuminant D65 (daylight, color temperature 6504K), and the measurement was performed in the regular reflection removal mode. An L * value of 40 or less was considered to be excellent in blackness.
• the examples of the present invention shown in Table 2 had uniformly excellent scale adhesion and blackness both in the widthwise center of the hot-rolled coil and in the area 5 mm from the widthwise edge, whereas the comparative examples had poor adhesion or blackness at either or both widthwise positions.

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