Classifications

• • 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
• • 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
  • B21B45/02—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 lubricating, cooling, or cleaning
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K7/00—Cutting, scarfing, or desurfacing by applying flames
  • B23K7/06—Machines, apparatus or equipment specially designed for scarfing or desurfacing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23P—METAL-WORKING NOT OTHERWISE PROVIDED FOR; COMBINED OPERATIONS; UNIVERSAL MACHINE TOOLS
  • B23P23/00—Machines or arrangements of machines for performing specified combinations of different metal-working operations not covered by a single other subclass
  • B23P23/04—Machines or arrangements of machines for performing specified combinations of different metal-working operations not covered by a single other subclass for both machining and other metal-working operations
• • 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • 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
  • B21B1/46—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 metal immediately subsequent to continuous casting
  • B21B1/466—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 metal immediately subsequent to continuous casting in a non-continuous process, i.e. the cast being cut before rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
  • B21B3/003—Rolling non-ferrous metals immediately subsequent to continuous casting, i.e. in-line rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
  • B21B3/02—Rolling special iron alloys, e.g. stainless steel
• • 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
  • B21B45/004—Heating the product
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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

Definitions

• This disclosure relates to hot rolling equipment and hot rolling methods.
• JP Patent Publication 7-242938 A describes a method for producing a hot-rolled steel sheet with excellent workability, which is characterized by coiling a steel containing, by weight, C: 0.01-0.20%, Si: 2.0% or less, Mn: 0.05-2.0%, and Ni: 0-0.50%, and further containing, as tramp elements, Cu: 0.01-0.50%, Sn: 0.001-0.050%, Pb: 0.001-0.010%, and As: 0.001-0.050%, after hot rolling, at 500°C or less.
• Japanese Patent Laid-Open Publication No. 6-297026 describes a method for preventing hot cracking in steel containing Cu and Sn, which is characterized by adding Si as an alloy component to steel containing Cu and Sn, then heating the steel to form scale, and then rolling the steel.
• Japanese Patent Application Laid-Open No. 2005-29886 describes a Cu-containing steel material that contains 0.06 mass% or more of Cu, and that has a solute concentration of at least one of Ti, Nb, and V of 0.01 mass% to 0.15 mass% and further contains at least one of the following: a P concentration of 0.01 mass% to 0.1 mass%, a REM concentration of 0.002 mass% to 0.15 mass%, and a S concentration of 0.01 mass% to 0.05 mass%.
• iron sources other than iron ore have come to be used in order to reduce carbon dioxide emissions in the steel manufacturing process.
• scrap iron when scrap iron is used as an iron source, surface defects due to the copper (Cu) contained in the scrap iron can occur during the steel manufacturing stage. For example, during the hot rolling process, red embrittlement cracking can occur due to the concentration of Cu on the surface of the steel, resulting in a decrease in the surface quality of the steel. For this reason, it is difficult to increase the proportion of scrap iron containing Cu used as an iron source, making it difficult to realize an environmentally friendly steel manufacturing process.
• Cu copper
• JP-A-7-242938, JP-A-6-297026, and JP-A-2005-29886 suppress red embrittlement cracking by adding alloys to steel.
• expensive Ni is added to the steel, which increases costs.
• JP-A-6-297026 Si is added to the steel material, which leads to a decrease in the workability of the steel material.
• the technology described in JP-A-6-297026 limits the range of use of the steel material, making it particularly difficult to use the steel material as exterior panels.
• the steel material is processed at a temperature range above the normal hot rolling temperature, which reduces the productivity of the steel material.
• JP 2005-29886 A elements that affect the material properties, such as Ti, V, and Nb, are added to the steel material. Furthermore, in the technology described in JP 2005-29886 A, expensive REM is added to the steel material, which increases costs. Thus, there is room for improvement in the technologies described in JP 7-242938 A, JP 6-297026 A, and JP 2005-29886 A.
• the purpose of this disclosure is to provide hot rolling equipment and a hot rolling method that can suppress red embrittlement cracking.
• the hot rolling equipment is a hot rolling equipment for hot rolling a steel material containing Cu, and includes a heating furnace for heating the steel material after casting to a predetermined temperature, a rough rolling device including a plurality of rolling mills for rough rolling the steel material heated in the heating furnace, a finishing rolling device for finishing the steel material rough rolled by the rough rolling device, and a removal device for removing a target area including cracks caused by a Cu-enriched layer that is present on the surface side of the steel material during or after the rough rolling and has a higher concentration of Cu components than the base material of the steel material, and the removal device is provided downstream of the rolling mill located most upstream of the plurality of rolling mills in the rough rolling device and upstream of the finishing rolling device in the conveying direction of the steel material.
• the hot rolling equipment according to the second aspect is the hot rolling equipment according to the first aspect, but is provided downstream of the removal device and upstream of the finishing rolling device in the conveying direction of the steel material, and is equipped with a post-removal descaling device that removes scale from the steel material using cooling water.
• the hot rolling equipment according to the third aspect is the hot rolling equipment according to the first or second aspect, and is provided with a rolling mill among the plurality of rolling mills in the rough rolling device that is located upstream of the removal device in the conveying direction of the steel material and adjacent to the removal device, and a pre-removal descaling device that is provided between the removal device and removes scale from the steel material using cooling water.
• the hot rolling equipment according to the fourth aspect is the hot rolling equipment according to any one of the first to third aspects, in which the removal device is provided between one of the plurality of rolling mills in the rough rolling device and another rolling mill adjacent to the one rolling mill.
• the hot rolling equipment according to the fifth aspect is the hot rolling equipment according to the fourth aspect, in which the removal device is provided after the rolling mill located most upstream of the multiple rolling mills in the rough rolling device in the conveying direction of the steel material.
• the hot rolling equipment according to the sixth aspect is the hot rolling equipment according to any one of the first to third aspects, in which the removal device is provided downstream of the rough rolling device and upstream of the finishing rolling device in the conveying direction of the steel material.
• the hot rolling equipment according to the seventh aspect is the hot rolling equipment according to any one of the first to sixth aspects, in which the removal device removes the target area from the steel material by machining, or removes the target area from the steel material by mechanical removal.
• the hot rolling equipment according to the eighth aspect is the hot rolling equipment according to any one of the first to seventh aspects, in which the heating furnace is provided separately from the production line of the casting machine that casts the steel material.
• the hot rolling equipment according to the ninth aspect is the hot rolling equipment according to any one of the first to seventh aspects, in which the heating furnace is provided on the production line of a casting machine that casts the steel material.
• the hot rolling method is a hot rolling method for hot rolling a steel material containing Cu, and includes a heating step in which the steel material after casting is heated to a predetermined temperature, a rough rolling step in which the steel material heated in the heating step is subjected to rough rolling multiple times, a finish rolling step in which the steel material rough rolled in the rough rolling step is subjected to finish rolling, and a removal step in which a target area that is present on the surface side of the steel material during or after the rough rolling and that contains cracks due to a Cu-enriched layer that has a higher concentration of Cu components than the base material of the steel material is removed, and the removal step is performed after the first rough rolling of the multiple rough rolling steps and before the finish rolling step.
• the hot rolling method according to the eleventh aspect is the hot rolling method according to the tenth aspect, further including a post-removal descaling step of removing scale from the steel material using cooling water after the removal step and before the finish rolling step.
• the hot rolling method according to the twelfth aspect is the hot rolling method according to the tenth or eleventh aspect, further including a pre-removal descaling step of removing scale from the steel material using cooling water after the rolling step immediately preceding the removing step among the multiple rough rolling steps and before the removing step.
• the hot rolling method according to the thirteenth aspect is a hot rolling method according to any one of the tenth to twelfth aspects, in which the removal step is performed when the surface temperature of the steel material during or after the rough rolling is 1000°C or less.
• the hot rolling method according to the 14th aspect is a hot rolling method according to any one of the 10th to 13th aspects, in which, in the removal step, when the target area is removed by machining, the conveying speed of the steel material during or after the rough rolling is 20 m/min or more.
• the hot rolling method according to the fifteenth aspect is a hot rolling method according to any one of the tenth to fourteenth aspects, in which the target area is an area of 1 mm or less in the thickness direction from the surface of the steel material during or after the rough rolling.
• this disclosure provides hot rolling equipment and a hot rolling method that can suppress red embrittlement cracking.
• FIG. 1 is a schematic diagram showing an example of a schematic configuration of hot rolling equipment according to a first embodiment of the present disclosure.
• FIG. 2 is a schematic diagram showing an example of a thermal history of the hot rolling equipment according to the first embodiment of the present disclosure.
• 1 is a cross-sectional view showing a steel material to be laser-cut by a laser-cutting device according to a first embodiment of the present disclosure.
• 1 is a schematic diagram showing an example of a schematic configuration of a cutting device according to a first embodiment of the present disclosure.
• FIG. 2 is a schematic diagram showing an example of a schematic configuration of a target region according to the first embodiment of the present disclosure.
• FIG. 3 is a flowchart showing an example of a hot rolling process according to the first embodiment of the present disclosure.
• FIG. 1 is a schematic diagram showing an example of a schematic configuration of hot rolling equipment according to a first embodiment of the present disclosure.
• FIG. 2 is a schematic diagram showing an example of a thermal history of the hot rolling
• FIG. 2 is a schematic diagram showing an example of a schematic configuration of a hot rolling apparatus and a laser cutting apparatus according to a first modified example.
• FIG. 13 is a schematic diagram showing an example of a schematic configuration of a hot rolling apparatus and a laser cutting apparatus according to a second modified example.
• FIG. 13 is a schematic diagram showing an example of a schematic configuration of a hot rolling apparatus and a laser cutting apparatus according to a second modified example.
• FIG. 13 is a schematic diagram showing an example of a schematic configuration of a hot rolling apparatus and a laser cutting apparatus according to a third modified example.
• FIG. 13 is a schematic diagram showing an example of a schematic configuration of a hot rolling apparatus and a laser cutting apparatus according to a fourth modified example.
• FIG. 13 is a schematic diagram showing an example of a schematic configuration of a hot rolling apparatus and a laser cutting apparatus according to a fourth modified example.
• FIG. 4 is a schematic diagram showing an example of a schematic configuration of hot rolling equipment according to a second embodiment of the present disclosure.
• 11 is a schematic diagram showing an example of a schematic configuration of a cutting device according to a second embodiment of the present disclosure.
• FIG. FIG. 13 is a schematic diagram showing an example of a schematic configuration of a hot rolling facility according to a third embodiment of the present disclosure.
• FIG. 13 is a schematic diagram showing an example of a thermal history of a hot rolling facility according to a third embodiment of the present disclosure.
• 1 is a graph showing the relationship between the Cu concentration in the steel plate and the depth of surface cracks in the steel plate for each of the examples and comparative examples.
• Fig. 1 is a diagram showing a schematic configuration of the hot rolling facility 10.
• hot rolling equipment 10 is equipment for hot rolling steel material 1.
• this hot rolling equipment 10 is provided separately from a production line of a continuous casting machine 11 that continuously casts steel material 1.
• Hot rolling equipment 10 includes a heating furnace 20, a laser cutting device 30, a descaling device 40, a rough rolling device 50, and a finish rolling device 60.
• steel material 1 may be cast individually, not limited to continuous casting.
• the heating furnace 20 heats the steel material 1 (e.g., slab 1A) obtained by cutting the steel material 1 discharged from the continuous casting machine 11 to a predetermined length, to a predetermined temperature.
• the "predetermined temperature” here is, for example, 1100°C or higher.
• the slab 1A is heated by the heating furnace 20 while being transported by the transport rolls 22. Note that, although slab 1A is given here as an example of the steel material 1, this is merely an example, and the steel material 1 may also be a billet.
• Steel material 1 is a steel material containing Cu, for example, a steel material containing 0.15 wt% (mass%) or more of Cu, and in particular, a steel material containing 0.25 wt% or more of Cu.
• the Cu content here means the average Cu content in the base material of steel material 1, which will be described later, or the average Cu content in the molten steel that becomes steel material 1.
• the Cu content of steel material 1 is, for example, 5.6 wt% or less.
• the descaling device 40 removes scale (i.e., oxide film) formed on both surfaces in the thickness direction of the slab 1A and on both surfaces (side end faces) in the width direction of the slab 1A before rough rolling by the rough rolling device 50.
• the descaling device 40 removes scale by spraying a fluid (e.g., cooling water) from a nozzle 42 onto the slab 1A. That is, the descaling device 40 sprays cooling water W, which is supplied from a cooling water tank (not shown) and has a predetermined water pressure using a pump (not shown), onto the surface of the slab 1A from the nozzle 42.
• a fluid e.g., cooling water
• the descaling device 40 removes scale from the surface of the slab 1A, and the temperature of the surface of the slab 1A is made less than 1100°C by heat removal by the cooling water.
• the temperature of the surface of the slab 1A e.g., the surface in the width direction of the slab 1A
• the descaling device 40 is measured by a radiation thermometer.
• the rough rolling device 50 performs rough rolling (hot rough rolling) on the slab 1A to thin the slab 1A to a predetermined thickness. Specifically, the rough rolling device 50 rolls the slab 1A, which has been rolled down in the width direction by the vertical rolls 24, from above and below to form a rough bar 1B.
• the rough rolling device 50 is equipped with four rolling mills 51, 52, 53, and 54. Each of the rolling mills 51 to 54 has multiple rolls that sandwich the slab 1A from both sides in the thickness direction.
• the rough rolling device 50 continuously rolls the slab 1A using the rolling mills 51 to 54.
• the slab 1A is thinned to a plate thickness of, for example, about 25 to 50 mm, and formed into a rough bar 1B.
• the slag cutting device 30 removes the surface portion (surface layer) of the steel material 1 by slag cutting the steel material 1 after the rough rolling process (i.e., during or after rough rolling).
• the details of the configuration of the slag cutting device 30 will be described later.
• the slag cutting device 30 is provided downstream of the rolling mill 51 located at the most upstream side in the conveying direction of the steel material 1 among the multiple rolling mills 51 to 54 equipped in the rough rolling device 50. Furthermore, the slag cutting device 30 is provided upstream of the finishing rolling device 60 in the conveying direction of the steel material 1.
• the temperature of the surface of the steel material 1 is set to 1000°C or less.
• removal is not limited to complete removal, but is a concept that includes a case where there is a lack of removal within a range generally acceptable in the technical field to which this disclosure belongs and does not go against the purpose of this disclosure.
• the slag cutting device 30 is an example of a removal device.
• the slag cutting device 30 is provided between the rolling mill 51 provided most upstream in the rough rolling device 50 and the rolling mill 52 provided in the second stage in the conveying direction of the steel material 1 (i.e., after the rolling mill 51 provided most upstream). In other words, the slag cutting device 30 is provided between the rolling mill 51 and the rolling mill 52 adjacent to the rolling mill 51.
• the finishing rolling device 60 further hot finish rolls the rough bar 1B to a predetermined thickness. Specifically, the finishing rolling device 60 finish rolls the rough bar 1B, which has been reheated by the rough bar heating device 62, to a plate thickness of about a few mm (e.g., 1 to 2 mm).
• the finishing rolling device 60 has multiple groups of finishing rolling rolls 64 across 6 to 7 stands. Each group of finishing rolling rolls 64 has multiple rolling rolls arranged in a straight line above and below.
• the finishing rolling device 60 gradually reduces the rough bar 1B by passing it through the gaps between the multiple groups of finishing rolling rolls 64.
• the steel strip 1C obtained by finish rolling the rough bar 1B to a predetermined plate thickness by this finishing rolling device 60 is sent to a cooling device 70. Note that, although an example of the finishing rolling device 60 having 6 to 7 stands is shown here, this is merely one example, and the finishing rolling device 60 may have, for example, a group of finishing rolls 64 with 5 stands.
• the cooling device 70 cools the steel strip 1C after finish rolling with cooling water W.
• the cooling device 70 is, for example, a pipe laminar nozzle type cooling device.
• the 1C cooled by the cooling device 70 is wound into a coil by the winding device 80 and transported from the hot rolling equipment 10 to the next process as a hot rolled coil 1D.
• FIG. 2 shows an example of the thermal history of the steel material 1 in a series of processes of continuous casting, hot rolling, and cold rolling.
• Cu remains in the steel material 1 without being oxidized, but in this temperature range, when the base material of the steel material 1 contains a relatively large amount of Cu (for example, when the average concentration of the Cu component in the base material is 0.15 wt % (mass %) or more), Cu is locally swept out of the iron oxide and infiltrates into the interface between the iron oxide and the base metal part of the steel material 1.
• the Cu swept out of the iron oxide mainly infiltrates into the grain boundaries of the surface layer of the base metal part of the steel material 1 and is concentrated.
• a region (hereinafter referred to as a "Cu-enriched layer") is formed in the surface layer (surface side) of the base metal part of steel material 1, in which the concentration of Cu is higher than the average concentration of Cu in the base material of steel material 1 (i.e., a Cu-enriched phenomenon occurs).
• the Cu-enriched layer refers to the region in the unoxidized base metal part of the steel material where, when a scale formed on the surface of the steel material due to oxidation, the concentration of Cu is higher than the average concentration of Cu in the base material of the steel material.
• the Cu-enriched layer refers to the region from the interface between the scale formed on the surface of the steel material and the base metal part of the steel material, i.e., the surface of the base metal of the unoxidized steel material, toward the center of the thickness direction of the steel material, where concentrated Cu at the grain boundaries is no longer found.
• the "region where concentrated Cu at the grain boundaries is not found" here refers to the region where the concentration of Cu at the grain boundaries is equal to or less than the average concentration of the steel material + 1%.
• the base material of the steel material refers to the part of the steel material excluding the Cu-enriched layer.
• the conventional surface treatment method for example, surface defects such as iron oxide formed on the surface of slab 1A, inclusions and casting powder that have become embedded in the surface layer of slab 1A during casting, or air bubble marks on the surface layer of slab 1A are removed by a machine scarfer or the like.
• the removal of these surface defects is performed before the heating step in the hot rolling process (i.e., heating by the heating furnace 20). That is, the surface defects are removed from slab 1A before the hot rolling process.
• slab 1A is heated again to 1100°C or higher (e.g., about 1300°C), so scale is formed again on the surface of slab 1A.
• the Cu concentration phenomenon reoccurs in slab 1A, and red embrittlement cracking also reoccurs.
• the inventors therefore, after extensive research, came up with the idea of removing red embrittlement cracking caused by the Cu-enriched layer on the surface of slab 1A during the rough rolling process in the hot rolling process. That is, in the thermal history of steel material 1 shown in FIG. 2, after leaving the heating furnace 20, steel material 1 is cooled to 1000°C or less during the rough rolling process, and is not heated to 1100°C or more thereafter. Therefore, in processes after the rough rolling process, the Cu concentration phenomenon is suppressed, and red embrittlement cracking is also suppressed. Therefore, if red embrittlement cracking is removed from steel material 1 during or after the rough rolling process, it becomes unnecessary to remove red embrittlement cracking again from steel material 1 in subsequent processes.
• Red embrittlement due to Cu concentration mainly occurs at 1100°C to 1250°C.
• the upper limit of the temperature at which red embrittlement occurs increases as the Cu concentration in steel 1 increases.
• the higher the heating temperature of steel 1 the more the gamma (austenite) grain boundaries coarsen, and therefore the depth of red embrittlement cracking caused by Cu concentrated at the grain boundaries also increases.
• the higher the heating temperature of steel 1 the fewer the number of grain boundaries, and therefore the fewer the number of red embrittlement cracking in steel 1.
• the depth of penetration of Cu from the iron oxide into the base metal part of the steel material 1 is relatively shallow, at most about 50 ⁇ m.
• the reason for this is thought to be as follows. That is, when a crack that has occurred on the surface of the steel material 1 propagates, in the early stages of crack propagation, Cu in a liquid phase infiltrates the tip of the crack. This reduces the ductility of the grain boundaries, making it easier for the crack to propagate. On the other hand, as the crack propagates, the amount of Cu remaining at the grain boundaries increases. As a result, the amount of Cu present at the tip of the crack gradually decreases, and eventually the propagation of the crack stops. At this time, the depth of the surface crack is about the width of one austenite grain.
• the depth of the surface crack increases in the initial stage of the displacement, but as the displacement progresses, the depth of the surface crack does not increase, but the width of the surface crack increases. Therefore, in red embrittlement cracking due to the Cu concentration phenomenon, the progression depth of the surface crack is at most a few mm (e.g., 1 mm). Therefore, if a few mm (e.g., 1 mm) of the surface layer of the steel material 1 is removed after rolling, quality defects on the surface of the steel material 1 can be suppressed.
• the surface layer of the slab 1A is removed by the scarifying device 30.
• the scarifying device 30 has two scarfer units 31 that face both surfaces (top and bottom) in the thickness direction of the steel material 1, and two scarfer units 31 that face both surfaces (left and right) in the width direction (side end faces) of the steel material 1.
• the four scarfer units 31 have the same configuration. Therefore, the following will describe the scarfer unit 31 that faces one surface (top) in the thickness direction of the steel material 1.
• the scarfer unit 31 is equipped with a preheating gas jetting section 34 and a cutting oxygen jetting section 36.
• the preheating gas jetting section 34 jets preheating oxygen 32 and combustible gas 33.
• the cutting oxygen jetting section 36 jets cutting oxygen 37. From the lower part of the cutting oxygen jetting section 36, shielding gas 38 consisting of combustible gas is jetted together with the cutting oxygen 37.
• the steel material 1 is configured to be transported in the direction of arrow Y in FIG. 4.
• the flow of oxygen 37 for cutting ejected from the oxygen ejection section 36 for cutting is ejected further forward in the conveying direction Y of the steel material 1 than the flow of preheating oxygen 32 and combustible gas 33 ejected from the preheating gas ejection section 34.
• a preheating process is carried out.
• preheating oxygen 32 and combustible gas 33 are ejected from the preheating gas ejection portion 34 of the scarfer unit 31 toward the surface of the steel material 1, and the combustible gas 33 is burned.
• a part of the surface of the steel material 1 is melted by the heat of the burning combustible gas 33, forming a basin portion 1E.
• the length of the pool portion 1E formed on the surface of the steel material 1 along the conveying direction Y is, for example, in the range of approximately 20 mm to 30 mm.
• the cutting process is carried out.
• oxygen 37 for cutting is ejected from the oxygen ejection section 36 of the scarfer unit 31 toward the surface of the steel material 1, and the steel material 1 on which the basin section 1E has been formed is transported in the transport direction Y.
• shielding gas 38 is ejected from the oxygen ejection section 36 for cutting together with the oxygen 37 for cutting, and the oxygen 37 for cutting is protected by the shielding gas 38.
• the cutting process is an example of a removal process.
• the jet of oxygen for cutting 37 ejected from oxygen for cutting ejection portion 36 collides with the molten iron in the basin portion 1E of the steel material 1 being transported, causing an oxidation reaction between the molten iron and oxygen for cutting 37.
• the heat of this oxidation reaction melts the surface of the steel material 1 one after another, and the surface of the steel material 1 is smelted.
• the surface of the steel material 1 is smelted by the heat of the oxidation reaction on the rear side of the basin portion 1E in the transport direction Y. In this way, the surface of the slab 1A after it has been heated in the heating furnace 20 is smelted by the smelting device 30.
• the cutting device 30 removes the target area T, which is an area including the cracks C caused by the Cu-enriched layer R.
• the target area T means a predetermined thickness area set from the surface of the steel material 1 toward the center in the thickness direction of the steel material 1 so as to include all of the Cu-enriched layer R, the cracks C caused by the Cu-enriched layer R, and the scale S.
• This target area T is a range of a predetermined distance from the surface of the steel material 1 (i.e., the surface of the scale S) in the thickness direction t of the steel material 1.
• the predetermined distance is, for example, greater than 0 mm and equal to or less than 3 mm (0 ⁇ distance ⁇ 3 mm), preferably greater than 0 mm and equal to or less than 2 mm (0 ⁇ distance ⁇ 2 mm), and more preferably greater than 0 mm and equal to or less than 1 mm (0 ⁇ distance ⁇ 1 mm).
• the thickness of the steel material 1 is, for example, about 25 to 50 mm. Therefore, by removing an area of 1 mm or less from the surface of the steel material 1 as the target area T in the thickness direction t of the slab 1A, the reduction in the thickness of the steel material 1 is reduced, and the yield of the slab 1A can be improved.
• h 6 ⁇ Vs -0.6 ...(1) h (mm): thickness of slab 1A, Vs (m/min): maximum conveying speed of slab 1A that can be continuously slab-scraped without interruption
• the faster the conveying speed of the slab 1A during cutting the thinner the cutting thickness of the slab 1A becomes.
• the faster the conveying speed of the slab 1A the thinner the cutting thickness of the slab 1A becomes.
• the depth of the surface cracks in the steel material 1 is considered to be about several mm (for example, 1 mm). Therefore, in order to keep the cutting thickness of the slab 1A at about 1 mm to 2 mm, it is preferable to set the conveying speed of the slab 1A to 20 m/min or more.
• the upper limit of the conveying speed of the slab 1A during cutting is appropriately set based on the operating conditions of the cutting device 30 and the thickness of the steel material 1 to be cut, and is set to, for example, 80 m/min or less.
• the machined device 30 also machines other surfaces in the thickness direction of the steel material 1, and both surfaces (side end faces) in the width direction of the steel material 1.
• the target area T including the Cu-enriched layer R is removed from the steel material 1 by melt cutting (removal), thereby removing the cracks C present at the grain boundaries GB. As a result, red embrittlement cracking on the surface of the steel material 1 is suppressed.
• FIG. 6 is a flow chart for explaining a part of the manufacturing process of the hot rolled steel sheet according to this embodiment.
• the heating furnace 20 heats the slab 1A after continuous casting to a predetermined temperature (e.g., about 1300°C).
• a predetermined temperature e.g., about 1300°C.
• step ST12 the slab 1A heated in step ST10 is descaled by the descaling device 40 using cooling water W.
• the surface temperature of the slab 1A drops to, for example, less than 1100°C.
• step ST14 the slab 1A descaled in step ST12 is rough rolled by the rough rolling device 50. As a result, the slab 1A is formed into a rough bar 1B having a predetermined thickness. After the rough rolling process of step ST14 is performed, the manufacturing process proceeds to step ST16.
• step ST16 the slab 1A roughly rolled in step ST14 is cut by the cutting device 30, and the target area T including the Cu-enriched layer is removed from the slab 1A.
• the cutting of the slab 1A by the cutting device 30 is performed after the first rough rolling of the slab 1A among multiple rough rollings (e.g., four times) of the slab 1A performed by the rough rolling device 50.
• the temperature of the surface of the slab 1A becomes 1000°C or less.
• step ST18 the rough rolling device 50 performs the remaining rough rolling on the slab 1A that was machined in step ST16. This forms the slab 1A into a rough bar 1B, and the rough rolling process ends. After the rough rolling process of step ST18 is performed, the manufacturing process proceeds to step ST20.
• step ST20 the rough bar 1B obtained in step ST18 is finish-rolled by the finishing rolling device 60. As a result, the rough bar 1B is formed into a steel strip 1C having a predetermined thickness. After the finishing rolling process of step ST20 is performed, the manufacturing process proceeds to step ST22.
• step ST22 the steel strip 1C obtained in step ST20 is wound by the winding device 80 and formed into a hot-rolled coil 1D. After the winding process of step ST22 is performed, part of the manufacturing process of the hot-rolled steel sheet according to this embodiment is completed.
• the slag cutting device 30 is provided downstream of the rolling device 51 located at the most upstream side in the conveying direction of the steel material 1 among the multiple rolling mills 51 to 54 in the rough rolling device 50, and upstream of the finishing rolling device 60.
• a predetermined temperature for example, 1300°C
• Cu in a liquid phase state is concentrated at the grain boundary GB near the surface of the steel material 1.
• the part of the steel material 1 where Cu is concentrated is less ductile than the base metal part of the steel material 1. For this reason, the occurrence of a Cu-concentrated layer is one cause of red embrittlement cracking.
• a slag cutting device 30 that removes surface cracks caused by the Cu-concentrated layer from the steel material 1 to be rough rolled. As a result, cracks that have occurred on the surface of the steel material 1 are removed during or after rough rolling. Therefore, according to this embodiment, the occurrence of red embrittlement cracking in steel material 1 is suppressed.
• the slag cutting device 30 is provided upstream of the rough rolling device 50 (e.g., upstream of the heating furnace 20) in the conveying direction of the steel material 1.
• the temperature of the steel material 1 remains high (e.g., 1100°C or higher)
• a Cu-enriched layer will form again in the steel material 1, causing red embrittlement cracking.
• cracks that have occurred on the surface of the steel material 1 are removed during or after rough rolling, so the occurrence of red embrittlement cracking in the steel material 1 is suppressed.
• the slicing device 30 is provided downstream of the finishing rolling device 60 in the conveying direction of the steel material 1.
• the temperature of the steel material 1 has dropped sufficiently, so Cu concentration is unlikely to occur.
• the thickness of the steel material 1 (steel strip 1C) at the finish rolling stage is reduced compared to the thickness of the steel material 1 (slab 1A) up to the rough rolling stage. Therefore, if the surface cracks of the steel material 1 are removed at the finish rolling stage, the thickness of the steel material 1 will be further reduced, resulting in a decrease in the yield of the steel material 1.
• the cracks that have occurred on the surface of the steel material 1 are removed during or after rough rolling, so the decrease in the yield of the steel material 1 in the hot rolling process is suppressed.
• the slag cutting device 30 is provided between the rolling mill 51 and the rolling mill 52 adjacent to the rolling mill 51.
• the slag cutting device 30 is provided between the adjacent rolling mills 51 and 52 among the multiple rolling mills 51 to 54.
• the slag cutting device 30 is provided after the rolling mill 51, which is located most upstream in the conveying direction of the steel material 1, among the multiple rolling mills 51 to 54.
• the slag cutting device 30 is provided on the roll exit side of the rolling mill 51, which is located most upstream in the conveying direction of the steel material 1. This allows cracks that have occurred on the surface of the steel material 1 to be removed during rough rolling. Also, in the rough rolling process, surface cracks of the steel material 1 are removed when the steel material 1 is at its thickest. Therefore, according to this embodiment, the occurrence of red embrittlement cracking in the steel material 1 is suppressed, and a decrease in the yield of the steel material 1 in the hot rolling process is suppressed.
• the steel material 1 is subjected to sintering by the sintering device 30, thereby removing the target area T including the Cu-enriched layer from the steel material 1. Sintering is more productive than removal by cutting. Therefore, according to this embodiment, it is possible to suppress a decrease in productivity in the hot rolling equipment 10.
• the conveying speed of the steel material 1 is set to 20 m/min or more.
• the shave-cut thickness of the steel material 1 is approximately 1 mm. This removes surface cracks in the steel material 1. Therefore, according to this embodiment, the occurrence of red embrittlement cracking in the steel material 1 is suppressed.
• a region of 1 mm or less from the surface of the steel material 1 is removed in the thickness direction t of the steel material 1.
• Cracks caused by the Cu-enriched layer mainly occur in a region of 1 mm or less from the surface of the steel material 1 in the thickness direction t of the steel material 1.
• the progression depth of surface cracks in the steel material 1 is at most a few mm (for example, 1 mm). Therefore, removing a region of up to 1 mm from the surface of the steel material 1 in the thickness direction t of the steel material 1 contributes to removing the cracks. Therefore, according to this embodiment, the occurrence of red embrittlement cracks in the steel material 1 is suppressed.
• the slag cutting device 30 is provided between the second rolling mill 52 and the third rolling mill 53 adjacent to each other.
• the slag cutting device 30 is provided between the second rolling mill 52 and the third rolling mill 53 in the rough rolling device 50.
• the slag cutting device 30 is provided between the second rolling mill 52 and the third rolling mill 53
• the present disclosure is not limited to this example embodiment.
• the slag cutting device 30 may be provided between the third rolling mill 53 and the final stage rolling mill 54.
• the rough rolling device 50 includes four rolling mills has been given here, the present disclosure is not limited to this example embodiment.
• the rough rolling device 50 may include five or more rolling mills, or three or less rolling mills.
• the rough rolling device 50 is described by taking an example of the embodiment including the non-reverse type rolling mills 51 to 54, but the present disclosure is not limited to this example.
• the rough rolling device 50A is provided with a reverse type rolling mill 51A.
• the slag cutting device 30 is provided downstream of the reverse type rolling mill 51A in the conveying direction of the steel material 1.
• the steel material 1 is slag cut by the slag cutting device 30.
• the steel material 1 is moved in the opposite direction to the conveying direction, and the steel material 1 is rolled multiple times in the reverse type rolling mill 51A.
• the rough rolling device 50B includes a reversing rolling mill 51B, a non-reversing rolling mill 52B, and a tandem rolling mill 53B.
• the slag cutting device 30 is provided downstream of the reversing rolling mill 51B in the conveying direction of the steel material 1. That is, the slag cutting device 30 is provided between the reversing rolling mill 51B and the non-reversing rolling mill 52B. After the first rolling of the steel material 1 is performed in the reversing rolling mill 51B, the steel material 1 is slag cut by the slag cutting device 30.
• the steel material 1 is rolled multiple times in the reversing rolling mill 51B, the steel material 1 is rolled in the non-reversing rolling mill 52B and the tandem rolling mill 53B.
• the slag cutting device 30 is provided downstream of the reversing rolling mill 51B in the conveying direction of the steel material 1, but the present disclosure is not limited to this embodiment.
• the slag cutting device 30 may be provided between the non-reversing rolling mill 52B and the tandem rolling mill 53B.
• the cutting device 30 is provided after the rolling mill (reverse rolling mill 51B) located most upstream of the rough rolling devices 50 in the conveying direction of the steel material 1. This allows cracks that have occurred on the surface of the steel material 1 to be removed during rough rolling. Furthermore, in the rough rolling process, surface cracks of the steel material 1 are removed when the steel material 1 is at its thickest. Therefore, according to this modified example, the occurrence of red embrittlement cracks in the steel material 1 is suppressed, and a decrease in the yield of the steel material 1 in the hot rolling process is suppressed.
• the slag cutting device 30 is provided downstream of the rough rolling device 50 in the conveying direction of the steel material 1. Furthermore, the slag cutting device 30 is provided upstream of the finish rolling device 60 in the conveying direction of the steel material 1. Specifically, the slag cutting device 30 is provided between the exit side of the rough rolling device 50 and the entry side of the finish rolling device 60. Thus, in the third modified example, the slag cutting device 30 is provided after the rough rolling device 50 and before the finish rolling device 60 in the conveying direction of the steel material 1. As a result, cracks that have occurred on the surface of the steel material 1 are removed after the rough rolling process.
• the temperature of the steel material 1 after the rough rolling process has sufficiently decreased (for example, 1000°C or less)
• a Cu-enriched layer is unlikely to form, and the re-occurrence of surface cracks in the steel material 1 is suppressed. Therefore, according to this modified example, the occurrence of red embrittlement cracks in the steel material 1 is suppressed.
• the steel material 1 is melt-cut before finish rolling, which improves the yield in the hot rolling process compared to when the steel material 1, whose plate thickness has been reduced by finish rolling, is melt-cut.
• the slag cutting device 30 is provided downstream of the rough rolling device 50 and upstream of the finish rolling device 60 in the conveying direction of the steel material 1.
• a pre-removal descaling device 40A is provided downstream of the rough rolling device 50 and upstream of the slag cutting device 30 in the conveying direction of the steel material 1.
• the pre-removal descaling device 40A is provided between the most downstream rolling mill 54 of the multiple rolling mills 51-54 in the rough rolling device 50 in the conveying direction of the steel material 1 and the slag cutting device 30.
• the basic configuration of this pre-removal descaling device 40A is the same as that of the descaling device 40.
• the pre-removal descaling device 40A removes scale formed on the surface of the steel material 1 before (just before) the steel material 1 is scalded by the scalding device 30.
• the pre-removal descaling process is performed after the rough rolling process and before the scalding process (removal process).
• the cutting device 30 when the steel material 1 is being cut by the cutting device 30, if scale or the like is attached to the surface of the steel material 1, the combustion reaction between the cutting oxygen 37 (see FIG. 4) sprayed from the cutting device 30 and the steel material 1 is inhibited, and the cutting of the steel material 1 may be interrupted.
• the cutting thickness of the steel material 1 by the cutting device 30 is kept at around 1 mm to 2 mm, the cutting of the steel material 1 by the cutting device 30 is likely to be interrupted.
• the pre-removal descaling device 40A removes scale adhering to the surface of the steel material 1. This makes it easier to stabilize the combustion reaction between the melt-cutting oxygen 37 (see Figure 4) sprayed from the melt-cutting device 30 and the steel material 1. Therefore, interruptions in the melt-cutting of the steel material 1 by the melt-cutting device 30 can be suppressed.
• the pre-removal descaling device 40A also removes scale adhering to the surface of the steel material 1 by spraying cooling water W onto the surface of the steel material 1. At this time, the surface of the steel material 1 is cooled by the cooling water W, which further suppresses the re-generation of a Cu-enriched layer on the steel material 1. Therefore, the occurrence of red embrittlement cracking in the steel material 1 is suppressed. Furthermore, in the fourth modified example, the pre-removal descaling device 40A cools the surface of the steel material 1 to less than 1100°C. Therefore, the re-generation of a Cu-enriched layer on the steel material 1 is further suppressed.
• a post-removal descaling device 40B is provided downstream of the cutting device 30 and upstream of the finishing rolling device 60 in the conveying direction of the steel material 1.
• the basic configuration of this post-removal descaling device 40B is the same as that of the descaling device 40.
• the post-removal descaling device 40B removes scale and the like adhering to the surface of the steel material 1 after the surface of the steel material 1 has been shave-cut by the shave-cutting device 30.
• the post-removal descaling process is performed after the shave-cutting process (removal process) and before the finish rolling process.
• the post-removal descaling device 40B to remove scale and other materials adhering to the surface of the steel material 1, the quality of the hot finish rolling of the steel material 1 by the finishing rolling device 60 can be improved.
• the post-removal descaling device 40B removes scale adhering to the surface of the steel material 1 by spraying cooling water W onto the surface of the steel material 1. At this time, the surface of the steel material 1 is cooled by the cooling water W, which further suppresses the re-generation of a Cu-enriched layer on the steel material 1. Furthermore, in the fourth modified example, the surface of the steel material 1 is cooled to less than 1100°C by the post-removal descaling device 40B. Therefore, the re-generation of a Cu-enriched layer on the steel material 1 is further suppressed. As a result, the occurrence of red-hot embrittlement cracking in the steel material 1 is suppressed.
• the pre-removal descaling process, the laser cutting process, and the post-removal descaling process are performed after the rough rolling process.
• the pre-removal descaling process, the laser cutting process, and the post-removal descaling process may be performed during the rough rolling process.
• a pre-removal descaling device 40A, a laser cutting device 30, and a post-removal descaling device 40B are provided between adjacent rolling mills 51, 52, 53, and 54.
• the pre-removal descaling device 40A, the slicing device 30, and the post-removal descaling device 40B are provided between the rolling mills 51 and 52, between the rolling mills 52 and 53, or between the rolling mills 53 and 54.
• a pre-removal descaling process is performed before the cutting process (removal process), and a post-removal descaling process is performed after the cutting process (removal process).
• a pre-removal descaling process is performed before the cutting process (removal process)
• a post-removal descaling process is performed after the cutting process (removal process).
• at least one of the pre-removal descaling process and the post-removal descaling process can be omitted.
• the target region T including the Cu-enriched layer is removed from the steel material 1 by the laser cutting device 30, but the present disclosure is not limited to this example.
• the target region T is removed from the steel material 1 by a cutting device 90 instead of the laser cutting device 30.
• the cutting device 90 is an example of a removal device.
• the hot rolling equipment 10 is provided with a cutting device 90 instead of the slag cutting device 30.
• the cutting device 90 removes the surface portion of the steel material 1 by cutting the steel material 1 discharged from the heating furnace 20.
• the details of the cutting device 90 will be described later.
• the cutting device 90 is provided downstream of the rolling mill 51 located at the most upstream side in the conveying direction of the steel material 1 among the multiple rolling mills 51 to 54 in the rough rolling device 50.
• the cutting device 90 is provided upstream of the finishing rolling device 60 in the conveying direction of the steel material 1. More specifically, the cutting device 90 is provided between the rolling mill 51 located at the most upstream side in the rough rolling device 50 and the rolling mill 52 located in the second stage in the conveying direction of the steel material 1. In other words, the cutting device 90 is provided between the rolling mill 51 and the rolling mill 52 adjacent to the rolling mill 51.
• the cutting device 90 is equipped with a rotary cutting tool 92.
• the rotary cutting tool 92 has a holder 94 and a number of cutting blades 96 attached to the outer periphery of the holder 94.
• the rotary cutting tool 92 rotates by receiving power from a power source (not shown).
• the rotating cutting blades 96 come into contact with the surface of the steel material 1, cutting the surface of the steel material 1.
• the cutting device 90 cuts the surface of the steel material 1, thereby removing the target area T on the surface of the steel material 1 after it has been heated in the heating furnace 20.
• the present disclosure is not limited to this example embodiment.
• one surface of the steel material 1 may be cut by multiple rotary cutting tools 92.
• the cutting device 90 may be provided with rotary cutting tools 92 at positions facing both surfaces of the steel material 1 in the thickness direction and both surfaces (side end faces) of the steel material 1 in the width direction.
• the target area T including the Cu-enriched layer is removed from the steel material 1 by the cutting device 90.
• Mechanical removal such as cutting is less likely to cause deterioration of the surface properties of the steel material 1 (e.g., surface roughness, etc.) compared to removal by surface cutting. Therefore, according to this embodiment, deterioration of the surface properties of the steel material 1 during the hot rolling process can be suppressed.
• the removal device is not limited to the cutting device 90, and may be, for example, a mechanical removal method such as grinding using a grinding wheel device.
• the heating furnace 20 is provided separately from the production line of the continuous casting machine 11 that continuously casts the steel material 1
• the present disclosure is not limited to this example.
• the heating furnace 102 is provided on the production line of the continuous casting machine that continuously casts the steel material 1.
• the continuous casting machine 11 continuously casts a slab 1A that is thinner (for example, 50 mm to 100 mm) than the continuous casting machine 11 in the first embodiment.
• a hot rolling facility 100 is provided on the production line of this continuous casting machine 11.
• the production line of the hot rolling facility 100 is continuous with the production line of the continuous casting machine 11.
• This hot rolling facility 100 includes a heating furnace 102, a descaling device 40, a rough rolling device 50, a pre-removal descaling device 40A, a slicing device 30, a post-removal descaling device 40B, a rough bar heating furnace 104, and a finishing rolling device 60.
• the rough rolling device 50 of the third embodiment has two rolling mills 51, 52, as an example.
• the heating furnace 102 is, for example, a table heating furnace.
• the heating furnace 102 is also provided upstream of the rough rolling device 50 in the conveying direction of the steel material 1.
• This heating furnace 102 heats the steel material 1 (for example, slab 1A) obtained by cutting the steel material 1 discharged from the continuous casting machine 11 to a predetermined temperature (1100°C or higher).
• the rough bar heating furnace 104 is, for example, a table heating furnace.
• the rough bar heating furnace 104 is provided downstream of the rough rolling device 50 and upstream of the finish rolling device 60 in the conveying direction of the steel material 1. This rough bar heating furnace 104 heats the steel material 1 that has been rough rolled by the rough rolling device 50 to a predetermined temperature (less than 1100°C).
• FIG. 15 shows an example of the thermal history of the steel material 1 in a series of processes of continuous casting, hot rolling, and cold rolling in the third embodiment.
• the temperature of the steel material 1 is about 1500°C.
• scale is generated on the surface of the steel material 1.
• the steel material 1 becomes 1000°C or lower during the rough rolling process, and is not heated to 1100°C or higher thereafter. Therefore, in the third embodiment, the same effect as in the first embodiment can be obtained.
• a number of steel plates (steel material 1) with different Cu contents were prepared.
• the steel plates had the same C: 0.2 wt%, Si: 0.01-0.25 wt%, Mn: 0.5 wt%, T-Al: 0.03%, and the remainder Fe, and the Cu contents were 0.2 wt%, 0.5 wt%, 1 wt%, and 1.5 wt%, respectively.
• each steel plate was heated and held at 1100°C or higher (1200°C), it was stretched at a strain rate of 5/s to a cross-sectional area reduction rate of 33%, causing cracks in each steel plate.
• 1 mm of the surface of each steel plate was removed by sintering.
• the steel plates according to the comparative example were the same as the steel plates according to the embodiment, except that the surface was not sintered.
• Each steel plate according to the embodiment and comparative example was cut, and the depth of the surface cracks of each steel plate was measured within the field of view by observing the cut surface. The average depth of the multiple surface cracks measured was then calculated for each steel plate in the examples and comparative examples.
• the hot rolling facility, wherein the removal device is provided downstream of a rolling mill that is located most upstream in a conveying direction among the plurality of rolling mills in the roughing rolling device and upstream of the finishing rolling device.
• a hot rolling method for hot rolling a steel material containing Cu comprising: a heating step of heating the steel material after continuous casting to a predetermined temperature; a rough rolling step of performing rough rolling a plurality of times on the steel material heated in the heating step; a finish rolling process for performing finish rolling on the steel material that has been rough rolled in the rough rolling process; A removal process of removing a target area including a crack caused by a Cu-enriched layer that is present on the surface side of the steel material during or after rough rolling and has a higher concentration of Cu component than in the base material,
• the hot rolling method wherein the removing step is performed after a first of the plurality of rough rolling steps and before the finish rolling step.

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