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

• 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.
• a hot-rolled steel sheet with excellent scale adhesion has been proposed, 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 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 grains in the upper layer of the magnetite layer on the base steel side in the scale layer and/or the average block size of the eutectoid transformation structure of iron and magnetite. Furthermore, by controlling the longitudinal temperature of the steel plate immediately after finish rolling, uniform improvement of scale adhesion in the longitudinal direction is achieved. However, no method is mentioned for uniformly improving scale adhesion in the width direction.
• 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 steel within a predetermined range.
• the adhesion at the interface between the scale layer and the base steel is improved, there is a concern that the adhesion of the scale may become insufficient if the thickness of the hot-rolled steel sheet becomes greater.
• no method is mentioned for uniformly improving the adhesion of the scale in the width direction.
• Patent Document 3 discloses a method for manufacturing a hot-rolled steel sheet, which is manufactured by hot rolling a steel material and winding it into a coil.
• the roughly rolled steel sheet is subjected to finish rolling at 850 to 1050 ° C., and then the finish-rolled hot-rolled steel sheet is wound into a coil at a winding temperature of 500 to 650 ° C., while cooling both end faces of the hot-rolled steel sheet so that the temperature at the end faces becomes 480 ° C. or less within 5 minutes from the start of winding. Thereafter, the temperature at the end faces is maintained at 480 ° C.
• the present invention aims to solve the above problems and provide a hot-rolled steel sheet and a manufacturing method thereof that has excellent scale adhesion even in thicker hot-rolled steel sheets, with less variation in scale adhesion in the width direction of the steel sheet in particular, and improved laser cuttability.
• the present inventors first investigated the reason why the conventional hot-rolled steel sheet does not have uniformly excellent scale adhesion 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).
• the eutectoid transformation structure consisting of magnetite and precipitated Fe has particularly high consistency with the surrounding magnetite grains and base steel, and therefore contributes to improving scale adhesion.
• the scale adhesion tended to be particularly poor in the range within 200 mm from the edge in the width direction. This is because the amount of hematite on the scale surface and the magnetite layer near the scale surface, which consists of columnar magnetite particles, increases excessively due to the reoxidation of the scale by air entering from the edge after winding into a coil.
• the proportion of the eutectoid transformation structure that contributes to improving the scale adhesion decreases and the scale thickness at the edge increases.
• the adhesion of the scale decreases due to different mechanisms at the edge and the portion 200 mm from the edge. Since the cooling rate is the highest at the edge, wustite remains in the final scale structure, and the adhesion is likely to further decrease. On the other hand, at the portion 200 mm from the edge, the cooling rate is small due to exposure to air entering from the edge and reheating from the center portion in the width direction. Therefore, the amount of hematite and the magnetite layer near the scale surface is the largest in the width direction, and the adhesion is greatly decreased. In other words, it was revealed that in order to obtain uniformly excellent scale adhesion in the width direction, it is necessary to improve the scale adhesion at the edge and the portion 200 mm from the edge.
• a steel material having a predetermined 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, followed by cooling to the coiling temperature at a predetermined cooling rate. This allows the scale thickness to be appropriately controlled, and also suppresses the occurrence of cracks in the scale that cause a decrease in adhesion.
• the coil After starting winding, 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 450°C.
• the edge of the coil means a range within 200 mm from the edge in the width direction of the coil. This increases the rigidity of the coil and prevents loosening of the coil. As a result, the center in the width direction is isolated from the oxidizing atmosphere, thereby suppressing reoxidation, and a eutectic transformation structure suitable for improving adhesion can be sufficiently obtained.
• the edge is cooled to suppress reoxidation, and the eutectic transformation is promoted by reheating from the center in the width direction, thereby ensuring excellent adhesion.
• this also suppresses the variation in scale thickness in the width direction, and the adhesion of the scale is improved uniformly in the width direction, thereby improving the laser cuttability.
• the scale in the width direction of the steel sheet being, by area ratio, magnetite particles: 20% to 60%, iron and magnetite a eutectoid transformed structure of magnetite: 30% or more, where the magnetite is composed of the magnetite grains and magnetite contained in the eutectoid transformed structure, and has a structure in which wustite: 15% or less and hematite: 5% or less, by mass fraction; an average thickness of the scale in the width direction of the steel plate is 5 ⁇ m or more and 20 ⁇ m or less, and a variation in the thickness of the scale in the width direction of the steel plate is 4 ⁇ m or less.
• 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 500 ° C. or more and 650 ° C. or less.
• the entire coil is cooled at an average cooling rate of 0.5 ° C./s or more and 6.0 ° C./s or less.
• a method for producing a hot-rolled steel sheet 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 500 ° C. or more and 650 ° C. or less.
• 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 reduce the variation in scale adhesion across the width of the steel sheet, which has the effect of greatly contributing to improved surface quality of products, improved laser cuttability of products, and an improved working environment. Furthermore, it also makes it possible to solve issues such as the decrease in scale adhesion that accompanies an increase in the thickness of hot-rolled steel sheets.
• 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. It is not necessary to include Si, but in order to obtain this effect, it is preferable to include 0.01% or more. However, if the Si content exceeds 0.50%, Si will concentrate at the interface between the scale and the base steel, resulting in the formation of an Si oxide layer. 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. In particular, it has the effect of ensuring the strength of steel sheets that are thick and prone to strength loss after hot rolling. If the amount is less than 0.01%, the effect is small. On the other hand, if it is contained in an amount exceeding 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 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.
• 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.
• 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.
• 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.
• the scale structure in the width direction of the steel plate refers to the scale structure in the width direction center of the steel plate and within a range of 200 mm from the width direction edge.
• the scale structure is measured at the width direction center of the steel plate and at positions 5 mm and 200 mm from the width direction edge, and it is sufficient if the structure at each position is within the following range.
• the structure at each measurement position is within the respective ranges below, the structure is considered to be uniform in the width direction.
• the average thickness of the scale in the width direction of the steel sheet is 5 ⁇ m or more and 20 ⁇ m or less, and the variation in the thickness of the scale in the width direction of the steel sheet is 4 ⁇ m or less. If the average thickness of the scale is less than 5 ⁇ m, it causes processing troubles. This is because the amount of thermal energy of the laser light absorbed by the steel sheet surface during processing by laser cutting is insufficient. On the other hand, if the average thickness of the scale exceeds 20 ⁇ m, particularly when the thickness of the hot-rolled steel sheet is large, the strain applied to the scale surface during processing of the steel sheet becomes large, and cracks are generated in the scale, resulting in a decrease in scale adhesion.
• the magnetite grains, the eutectoid transformation structure of iron and magnetite, and the area ratio of wustite are observed using a scanning electron microscope (SEM) after cutting out a cross section of the sheet perpendicular to the surface of the steel sheet and parallel to the rolling direction, mirror-polishing it, and then observing it.
• SEM scanning electron microscope
• the field of view of the SEM covers the entire scale thickness from the scale surface to the interface between the scale and the steel sheet. Therefore, it can be measured by observing the backscattered electron image of the scale cross section at an observation magnification that covers the entire scale thickness.
• the hot rolling process consists of rough rolling and finish rolling.
• Rough rolling only needs to produce a sheet bar of the specified dimensions, and there is no need to limit the conditions for rough rolling.
• the material to be rolled may be heated midway using 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.
• the cooling stop temperature at the edge portion of the coil is set to 300°C or more and 450°C or less. It is preferably 320°C or more and 430°C or less.
• the average cooling rate at the edge is less than 0.5°C/s, the above effect cannot be sufficiently obtained.
• the average cooling rate at the edge exceeds 6.0°C/s, cracks occur in the scale due to the refinement of the scale structure and the increase in the stress difference with the base steel.
• 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 grains, the eutectoid transformation structure of iron and magnetite, and wustite were measured by cutting out a cross section of the steel plate perpendicular to the surface and parallel to the rolling direction, mirror-polishing it, and then observing the backscattered electron image of the cross section of the scale using an SEM at a magnification of 3000x.
• the magnetite grains are the darkest area
• the base steel is the brightest
• wustite is the area that appears with intermediate contrast
• the eutectoid transformation structure of iron and magnetite is the area where magnetite and iron are formed in layers.
• the mass fraction of hematite was determined by measuring the integrated intensity of the diffraction peaks of each phase in the scale using an X-ray diffractometer with 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 average thickness of the scale was measured by cutting out a cross section of the hot-rolled sheet perpendicular to the steel sheet surface and parallel to the rolling direction from the widthwise center, a portion 200 mm from the widthwise edge, and a portion 5 mm from the widthwise edge. After mirror polishing, the scale thickness was measured at three arbitrary positions using an SEM, and the scale thickness at each widthwise position was determined by averaging.
• test pieces were taken from the widthwise center of the hot-rolled sheet after leveling, from a portion 200 mm from the widthwise edge, and from a portion 5 mm from the widthwise edge, and tape was applied to the surface of the steel sheet to peel off the scale.
• the scale was then evaluated based on whether or not the base steel was exposed on the steel sheet surface and the amount of scale attached to the tape. That is, tape was applied to the surface of the steel sheet, and the peeled tape was attached to a transparent sheet, after which it was scanned and the amount of peeled scale was measured by image processing. If the area ratio of the scale attached to the peeled tape was less than 10%, the scale adhesion was deemed excellent and this was recorded as ⁇ in Table 3. On the other hand, if the area ratio of the scale attached to the peeled tape was 10% or more, the scale adhesion was deemed poor and this was recorded as ⁇ in Table 3.
• Laser cuttability was evaluated by laser cutting a straight line parallel to the width direction of the hot-rolled sheet after leveling using an ENSIS3015AJ laser cutting machine manufactured by Amada Machinery and a fiber laser oscillator. If cutting was impossible, or if dross or droplets or notches were found on the cut surface and a stable cut surface was not obtained, the laser cuttability was deemed poor and this was recorded as x in Table 3. On the other hand, if neither of these occurred and a stable cut surface was obtained in the width direction, the laser cuttability was deemed excellent and this was recorded as o in Table 3. Note that oxygen was used as the assist gas during laser cutting, the cutting speed was 1500 mm/min, the laser output was 3 kW, and the focal position was 3.0 mm from the steel sheet surface.

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 (1)

Applications Claiming Priority (2)

Publications (1)

Family

ID=96138215

Family Applications (1)

Country Status (2)

Citations (7)

• 2024
  • 2024-11-22 JP JP2025515633A patent/JPWO2025134686A1/ja active Pending
  • 2024-11-22 WO PCT/JP2024/041514 patent/WO2025134686A1/ja active Pending

Patent Citations (7)

Also Published As

Similar Documents

Legal Events