Method for manufacturing hot-rolled steel sheet and facility for manufacturing hot-rolled steel sheet

Abstract

To provide a manufacturing method of a hot-rolled steel sheet and a manufacturing facility of the hot-rolled steel sheet capable of preventing the occurrence of cracks in the steel sheet by cooling the surface of the steel sheet before rolling even when the concentration of Cu and Sn in a steel material is relatively high.SOLUTION: A hot-rolled steel sheet is manufactured by sequentially performing a heating process of heating a slab, a pre-descaling cooling process of cooling the heated slab before descaling, a descaling process of descaling the cooled slab, a rolling process of rolling the descaled slab, a run-out-table cooling process of cooling a belt-shaped steel sheet obtained by rolling the slab, and a coiling process of coiling the cooled steel sheet, and in the pre-descaling cooling process, the slab is cooled at a cooling rate of a surface of the slab of 1.5 °C / sec or more and 9.0 °C / sec or less for at least 10 seconds or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a hot-rolled steel sheet and a manufacturing facility for a hot-rolled steel sheet. [Background technology]

[0002] In order to improve the surface quality of steel sheets, it is common to remove scale formed on the surface of a slab before hot rolling. For example, Patent Document 1 discloses a technique for manufacturing hot-rolled steel sheets with excellent surface quality, in which a high-pressure water jet is sprayed onto the surface of the steel material during hot rolling to perform descaling and prevent the occurrence of scale defects and scale patterns. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 9-155436 Summary of the Invention [Problem to be solved by the invention]

[0004] Descaling before hot rolling is a process that must be carried out to prevent scale defects. In blast furnaces, converters, and electric furnaces (hereinafter referred to as "electric furnaces"), iron scrap, which is part of the raw material, is melted and refined to produce steel. The quality of iron scrap varies depending on its source, but from a cost perspective, there is an increasing need to use lower-quality iron scrap.

[0005] Low-grade scrap iron can contain impurities called tramp elements that cannot be removed during the refining process. These tramp elements can reduce the quality of steel. For example, Cu and Sn can cause embrittlement cracking on the surface of steel sheets. For this reason, it is generally necessary to keep the Cu and Sn content in steel to a low concentration. However, even if the scrap iron contains relatively high concentrations of Cu and Sn, if the cooling rate can be appropriately controlled to prevent embrittlement cracking on the surface of the steel sheet, it is possible to produce steel using lower-grade scrap iron.

[0006] Therefore, the present invention has been made in consideration of the above problems, and an object of the present invention is to provide a manufacturing method and manufacturing equipment for a hot-rolled steel sheet that can prevent cracks from occurring in the steel sheet by cooling the surface of the steel sheet before rolling even when the concentration of Cu and Sn in the steel material is relatively high. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, according to one aspect of the present invention, there is provided a method for producing a hot-rolled steel sheet, which produces a hot-rolled steel sheet by sequentially carrying out the following steps: a heating process for heating a slab; a pre-descaling cooling process for cooling the heated slab before descaling; a descaling process for descaling the cooled slab; a rolling process for rolling the slab after descaling; a runout table cooling process for cooling the steel sheet formed into a strip by rolling the slab; and a coiling process for winding the cooled steel sheet into a coil, and in the pre-descaling cooling process, the slab is cooled for at least 10 seconds or more at a cooling rate of the surface of the slab of 1.5°C / s or more and 9.0°C / s or less.

[0008] In the pre-descaling cooling step, the slabs may be water-cooled.

[0009] The above-mentioned method for producing a hot-rolled steel sheet may be carried out when the Cu content and Sn content of the hot-rolled steel sheet satisfy Cu+4Sn≧0.08 in mass %.

[0010] In addition, in order to solve the above-mentioned problems, according to another aspect of the present invention, there is provided a manufacturing facility for hot-rolled steel sheet, which comprises, from the upstream side of the manufacturing facility, a heating furnace, a pre-descaling cooling zone, a descaling device, rolling equipment, a runout table cooling zone, and a coil winder, and the pre-descaling cooling zone has a water cooling device that water-cools the surface of a slab heated by the heating furnace at a cooling rate of 1.5°C / s or more. [Effects of the Invention]

[0011] As described above, according to the present invention, even when the concentrations of Cu and Sn in the steel material are relatively high, the occurrence of cracks in the steel sheet can be prevented by cooling the surface of the steel sheet before rolling. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is an explanatory diagram illustrating an example of a manufacturing facility for a hot-rolled steel sheet according to an embodiment of the present invention. [Figure 2] 10 is a graph showing the temperature history calculation results of Test No. 5 of the example. [Figure 3] 10 is a graph showing the temperature history calculation results of Test No. 6 of the example. [Figure 4] 10 is a graph showing the temperature history calculation results of Test No. 7 of the example. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant explanations will be omitted.

[0014] [1. Equipment configuration] First, the configuration of a manufacturing facility for a hot-rolled steel sheet according to an embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is an explanatory diagram showing an example of a manufacturing facility 1 for a hot-rolled steel sheet according to this embodiment.

[0015] The manufacturing facility 1 shown in Fig. 1 is a facility that rolls a heated slab to a predetermined plate thickness and winds it into a coil. As shown in Fig. 1, the manufacturing facility 1 for hot-rolled steel sheets according to this embodiment includes, in order from the upstream side of the manufacturing facility 1, a heating furnace 10, a pre-descaling cooling zone 20, a descaling device 30, a rolling facility 40, a runout table cooling zone 50, and a coiler 70.

[0016] The heating furnace 10 heats the slab to a predetermined temperature. The slab heated by the heating furnace 10 is cooled in the pre-descaling cooling zone 20, and then descaled by the descaling device 30 to remove scale formed on the surface of the slab.

[0017] The pre-descaling cooling zone 20 may be, for example, a water-cooling zone equipped with a water-cooling device 25 that sprays water onto the slab and water-cools the slab using the water-cooling device 25, or a cooling zone that cools the slab by convective heat conduction using gas containing air. The pre-descaling cooling zone 20 according to this embodiment has the cooling capacity to cool the surface of the slab at a cooling rate of 1.5°C / s or more. This allows the slab to be appropriately slowly cooled before descaling, as described below.

[0018] The pre-descaling cooling zone 20 may be located downstream of the heating furnace 10 in the sheet passing direction and upstream of the descaling device 30 in the sheet passing direction, but is preferably installed near the descaling device 30. For example, the pre-descaling cooling zone 20 may be located within a range of up to 5 m upstream of the descaling device 30 in the sheet passing direction. By locating the pre-descaling cooling zone 20 near the descaling device 30, the slab cooled in the pre-descaling cooling zone 20 can be descaled by the descaling device 30 before it recuperates.

[0019] The descaling device 30 is installed on the inlet side of the rolling mill on the most upstream side in the sheet passing direction in the rolling equipment 40. The slab from which surface scale has been removed by the descaling device 30 is rolled in the rolling equipment 40. The configuration of the rolling equipment 40 is not particularly limited. For example, the rolling equipment 40 may be composed of only a finishing rolling mill, or may be composed of a roughing rolling mill and a finishing rolling mill.

[0020] The steel sheet formed into a strip by rolling a slab in the rolling equipment 40 is cooled in the runout table cooling zone 50. The runout table cooling zone 50 has a runout table configured by arranging a plurality of table rolls 51 at predetermined intervals in the sheet passing direction, and the steel sheet rolled in the rolling equipment 40 is cooled while being transported on the runout table. The runout table cooling zone 50 may include a water cooler 53 that water-cools the steel sheet being transported on the runout table, and an edge mask 55 installed between adjacent table rolls 51 corresponding to the water cooler 53.

[0021] The steel sheet that has passed through the runout table cooling zone 50 and cooled is guided by pinch rolls 60 to a coiler 70, which is a coil winding machine, and wound into a coil shape by a mandrel 75 at a predetermined winding temperature to become coil C.

[0022] [2. Manufacturing method of hot-rolled steel sheets] In the method for manufacturing a hot-rolled steel sheet according to this embodiment, a manufacturing facility 1 shown in Fig. 1 is used, for example, to sequentially perform a heating process, a pre-descaling cooling process, a descaling process, a rolling process, a runout table cooling process, and a coil winding process on a slab to manufacture a hot-rolled steel sheet. When the rolling facility 40 is composed of a roughing mill and a finishing mill, the roughing rolling process and the finishing rolling process are performed in that order in the rolling process. In this case, a pre-descaling cooling zone 20 is provided before the descaling device 30 installed upstream of the roughing mill, similar to the manufacturing facility 1 shown in Fig. 1.

[0023] In the heating process, the slab is heated to a predetermined temperature in a heating furnace 10. After the heating process, in a pre-descaling cooling process, the heated slab is cooled in a pre-descaling cooling zone 20. Then, in the descaling process, the slab cooled in the pre-descaling cooling zone 20 is descaled in a descaling device 30 to remove scale formed on the slab surface. After the descaling process, in the rolling process, the descaled slab is rolled to a predetermined thickness in a rolling facility 40 to form a strip-shaped steel plate. In the runout table cooling process, the steel plate is cooled by passing through a runout table cooling zone 50, and then in the coiling process, it is guided by a pinch roll 60 to a coiler 70 and wound into a coil around a mandrel 75 at a predetermined coiling temperature.

[0024] In the method for producing a hot-rolled steel sheet according to this embodiment, the slab is cooled for at least 10 seconds in the pre-descaling cooling step at a cooling rate of the surface of the slab of 1.5°C / sec to 9°C / sec. This prevents embrittlement cracking from occurring on the surface of the steel sheet, even when the steel sheet is made from iron scrap containing relatively high concentrations of Cu and Sn, which are tramp elements that can cause deterioration in the quality of steel, as part of its raw materials.

[0025] The tramp elements Cu and Sn are less susceptible to oxidation than Fe. For this reason, even if iron scrap containing Cu and Sn is melted and refined in a blast furnace, converter, or electric furnace, Cu and Sn are difficult to remove. Steel produced by melting and refining iron scrap in a blast furnace, converter, or electric furnace is cooled and solidified in a mold in a continuous casting facility, for example, and the solidified parts are gradually pulled out to become a continuous steel product. When the steel product (slab) is heated in a heating furnace, Fe oxidizes, but Cu and Sn do not, but instead concentrate and accumulate at the interface.

[0026] Scale forms on the surface of slabs heated in a heating furnace. To remove this scale, high-pressure water is sprayed onto the surface of the slab in a descaling device. At this time, the surface of the slab is cooled and shrinks due to the high-pressure water sprayed for descaling. On the other hand, the center of the slab is difficult to cool, so a temperature difference occurs between the surface and center of the slab, resulting in thermal stress. When the temperature difference between the surface and center of the slab becomes large, the tensile stress generated on the surface by the thermal stress increases, making the surface more susceptible to cracking. In particular, slabs that use iron scrap containing relatively high concentrations of Cu and Sn as part of their raw materials are more susceptible to surface cracking.

[0027] Thus, the cracks that occur on the surface of the slab due to descaling are caused by the thermal stress that occurs due to descaling. Therefore, in the method for producing a hot-rolled steel sheet according to this embodiment, after the slab is heated in a heating furnace and before descaling, which is performed before rolling, the slab is cooled at a cooling rate that is lower than that of the subsequent descaling step (i.e., a pre-descaling cooling step is performed), thereby suppressing the tensile stress that occurs on the surface of the slab and preventing cracks from occurring.

[0028] The slab can be cooled in the pre-descaling cooling zone at a cooling rate of 1.5°C / sec to 9.0°C / sec for at least 10 seconds to adequately relieve the tensile stress on the slab surface. The cooling time is determined by the slab threading speed and the length of the pre-descaling cooling zone.

[0029] If the cooling rate of the slab surface is less than 1.5°C / s, the surface temperature of the slab does not drop significantly before descaling, even if the cooling time is 10 seconds, causing sudden thermal stresses to occur due to descaling, resulting in surface cracks in the slab. On the other hand, if the cooling rate of the slab surface is greater than 9.0°C / s, the surface of the slab cools rapidly before descaling, causing large thermal stresses to occur, resulting in surface cracks in the slab.

[0030] Therefore, in the pre-descaling cooling zone, the slab surface is cooled at a rate of 1.5°C / sec to 9.0°C / sec for at least 10 seconds, allowing the slab to be slowly cooled before descaling. This reduces the temperature difference between the surface and center of the slab caused by descaling, and appropriately relieves the tensile stress that occurs on the surface of the slab. As a result, surface cracking caused by descaling can be suppressed, even for slabs made from a portion of iron scrap containing relatively high concentrations of Cu and Sn.

[0031] The method for producing a hot-rolled steel sheet according to this embodiment is particularly effective when iron scrap containing relatively high concentrations of Cu and Sn is used as part of the raw material. Specifically, the method for producing a hot-rolled steel sheet according to this embodiment is preferably applied when the Cu and Sn contents of the hot-rolled steel sheet satisfy Cu + 4Sn ≥ 0.08 by mass. A higher Cu content increases the likelihood of surface defects (Cu defects). Furthermore, steel sheets containing Sn together with Cu are more susceptible to surface defects. If the Cu and Sn contents of the hot-rolled steel sheet satisfy Cu + 4Sn < 0.08 by mass, surface cracks in the slab due to descaling do not occur even without applying the method for producing a hot-rolled steel sheet according to this embodiment. However, if Cu + 4Sn ≥ 0.08, applying the method for producing a hot-rolled steel sheet according to this embodiment appropriately controls the cooling rate of the slab surface before rolling, thereby suppressing the occurrence of surface cracks in the slab due to descaling. [Example]

[0032] Hot-rolled steel sheets were produced in a thin slab continuous line consisting, in order from upstream to downstream, of a continuous casting facility, a tunnel furnace, a pre-descaling cooling zone, a descaling device, a finishing rolling mill, a runout table cooling zone, and a coiler. The pre-descaling cooling zone, located between the tunnel furnace (heating furnace) and the descaling device, was 2.23 m long and consisted of a water cooling device installed from a position 3.6 m downstream in the sheet threading direction from the tunnel furnace to just before the descaling device. The finishing rolling mill was a rolling facility consisting of six rolling stands.

[0033] Tests (Test Nos. 1 to 12) shown in Table 3 below were conducted on this thin slab continuous line. In each test, a 50 mm thick slab with the chemical composition (steel types A to D) shown in Table 1 below was cast, heated to 1150°C in a tunnel furnace, and then cooled in a pre-descaling cooling zone using the process (Process Nos. 1 to 6) shown in Table 2 below. Immediately after the pre-descaling cooling zone, scale formed on the surface of the slab was removed using a descaling device, and the slab was rolled to a thickness of 3 mm using a finishing mill. After rolling, the steel plate was cooled in a runout table cooling zone and wound into a coil using a coiler.

[0034] [Table 1]

[0035] [Table 2]

[0036] After cooling the coils produced in each test to room temperature, the front and back surfaces of the steel sheets were inspected for defects on an inspection line. If defects were found on the front or back surface of the steel sheet, the Cu concentration around the defect was investigated to determine whether it was a Cu defect. To determine whether a defect was a Cu defect, an EPMA (Electron Probe Micro Analyzer) was used to investigate the Cu concentration at the crack interface, and if Cu enrichment was found, the defect was determined to be a real Cu defect. The evaluation results for the coils produced in each test are shown in Table 3 below. In this verification, the threshold for determining Cu enrichment was set to 10 times the Cu concentration of the base material. However, because whether a certain Cu enriched area becomes a defect varies depending on the rolling conditions, the threshold for determining Cu enrichment can be determined appropriately.

[0037] [Table 3]

[0038] As shown in Table 3, in Test Nos. 1 to 4, no Cu defects occurred, regardless of the cooling conditions before descaling shown in Table 2. This is because the hot-rolled steel sheet was produced using Steel Type A, which has low Cu and Sn concentrations and satisfies Cu + 4Sn < 0.08 in mass %. Therefore, it can be seen that the method for producing a hot-rolled steel sheet according to the present invention does not cause Cu defects, even when producing a hot-rolled steel sheet of a steel type that satisfies Cu + 4Sn < 0.08 in mass %.

[0039] In Tests Nos. 5 to 8, hot-rolled steel sheets were manufactured using steel type B, which satisfied the condition Cu + 4Sn ≥ 0.08 by mass. In Test No. 5, the cooling rate in the cooling zone before descaling was too slow, and it is presumed that the subsequent descaling caused sudden thermal stress on the surface of the slab, resulting in Cu defects.

[0040] The temperature history calculation results for Tests Nos. 5 to 7 are shown in Figures 2 to 4. In Figures 2 to 4, the horizontal axis represents time from the start of casting, and the vertical axis represents the slab temperature. Figures 2 to 4 also show the slab temperature from 600 seconds after the start of casting, when the cast slab was heated in the tunnel furnace. Specifically, the cast slab was heated in the tunnel furnace until 641 seconds after the start of casting (heating process), air-cooled from 641 to 660 seconds (air-cooling during transport from the tunnel furnace to the pre-descaling cooling zone), and cooled in the pre-descaling cooling zone from 660 to 680 seconds (pre-descaling cooling process). The slab's surface was then descaled from 680 to 730 seconds (descaling process), and air-cooled from 730 to 750 seconds (air-cooling during transport from the descaling device to the finishing mill), after which the first rolling pass was initiated by the finishing mill.

[0041] In Test No. 5, as shown in Figure 2, the central temperature of the slab remained almost unchanged from the time the slab was heated in the tunnel furnace until the descaling of the surface of the slab was completed by the descaling device. On the other hand, the surface temperature of the slab dropped by several tens of degrees Celsius after the slab was heated in the tunnel furnace, during air cooling during transport and the cooling process before descaling. However, the temperature difference with the central temperature of the slab when it was rapidly cooled in the descaling process became large. As a result, sudden thermal stress occurred on the surface of the slab.

[0042] On the other hand, in Test No. 6, the slab was slowly cooled in the pre-descaling cooling zone under the pre-descaling cooling conditions of Process No. 2, and in Test No. 7, the slab was slowly cooled in the pre-descaling cooling zone under the pre-descaling cooling conditions of Process No. 3. As a result, as shown in Figures 3 and 4, the surface of the slab was cooled more than in Test No. 5, and the center of the slab was also cooled. This reduced the temperature difference between the surface and center temperatures of the slab that occurs during the descaling process, and alleviated the tensile stress that occurs on the surface, which is thought to be why no cracks occurred on the surface of the slab in Tests No. 6 and No. 7.

[0043] In addition, in Test No. 8, the cooling rate in the pre-descaling cooling zone was too fast, which caused the surface of the slab to cool rapidly in the pre-descaling cooling zone, resulting in large thermal stress and leading to cracks.

[0044] In Test No. 9, hot-rolled steel sheets were produced using steel type C, and in Test No. 10, hot-rolled steel sheets were produced using steel type D. Steel types C and D had Cu and Sn contents that satisfied the Cu + 4Sn ≥ 0.08 mass %, meaning that their Cu and Sn concentrations were relatively high. However, by cooling the slab under appropriate pre-descaling cooling conditions, such as in Process No. 2, and then performing descaling, it was possible to reduce the thermal stress generated on the slab surface and avoid the occurrence of cracks on the slab surface even after descaling.

[0045] In Test No. 11, the cooling time in the cooling zone before descaling was insufficient at less than 10 seconds, resulting in the occurrence of Cu defects. In contrast, in Test No. 12, the cooling time in the cooling zone before descaling was 10 seconds, resulting in no Cu defects and a good appearance.

[0046] Furthermore, when the heating temperature in the tunnel furnace was 850 to 1180°C, the slab thickness was 20 to 300 mm, and the final plate thickness by the finishing mill was in the range of 1.2 to 20 mm, it was confirmed that no Cu defects occurred and a good appearance was obtained, as in Test Nos. 6, 7, 9, 10, and 12 above.

[0047] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Explanation of symbols]

[0048] 1. Hot-rolled steel sheet manufacturing equipment 10 Furnace 20 Pre-descaling cooling zone 25 Water cooling system 30 Descaling device 40 Rolling Equipment 50 Runout Table Cooling Zone 51 Table Roll 53 Water cooling system 55 Edge Mask 60 pinch rolls 70 Coiler 75 mandrel

Claims

1. A method for manufacturing a hot-rolled steel sheet, a heating step of heating the slab; a pre-descaling cooling step of cooling the heated slab before descaling; a descaling step of descaling the cooled slab; a rolling step of rolling the descaled slab; a runout table cooling process in which the slab is rolled to cool the steel plate into a strip shape; a coil winding step of winding the cooled steel sheet into a coil; The above steps are carried out in order to manufacture a hot-rolled steel sheet. In the pre-descaling cooling step, the slab is cooled at a cooling rate of the surface of the slab of 1.5°C / sec or more and 9.0°C / sec or less for at least 10 seconds.

2. The method for producing a hot-rolled steel sheet according to claim 1 , wherein the pre-descaling cooling step involves water-cooling the slab.

3. The method for producing a hot-rolled steel sheet according to claim 1 or 2, wherein the Cu content and Sn content of the hot-rolled steel sheet satisfy Cu + 4Sn ≥ 0.08 in mass%.

4. A manufacturing facility for hot-rolled steel sheets, The manufacturing equipment includes, in order from the upstream side, a heating furnace, a pre-descaling cooling zone, a descaling device, a rolling facility, a runout table cooling zone, and a coil winding machine; The pre-descaling cooling zone is a hot-rolled steel sheet manufacturing facility having a water cooling device that water-cools the surface of the slab heated by the heating furnace at a cooling rate of 1.5°C / s or more.

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