Method for manufacturing hot rolled steel sheet

Abstract

To provide a method for manufacturing a hot rolled steel sheet, which can suppress surface cracking at the edge of the hot rolled steel sheet obtained by finish-rolling a slab roughly rolled to a slab thickness of 50 mm or more to a product plate thickness of 15 mm or more.SOLUTION: This method for manufacturing the hot rolled steel sheet, which finish-rolls a slab roughly rolled to a slab thickness of 50 mm or more through a plurality of rough-rolling paths to a hot rolled steel sheet having a product plate thickness of 15 mm or more, rapidly cools, in each rough-rolling path after a slab size becomes W / H≥10, a slab thickness becomes 100 mm or less, and at slab thickness average temperature on the width center over the full length of the slab becomes 1000°C or lower, the slab so as to lower the slab thickness average temperature on the width center to 15°C / s or higher, and roughly rolls the slab with a maximum value of a surface temperature at the slab edge set to 30°C or higher with respect to the surface temperature on the width center and with (ΔH / H) / (ΔW / W)≥30,1 mm≤ΔW≤10 mm.SELECTED DRAWING: Figure 11

Description

Technical Field

[0001] The present invention relates to a method for manufacturing an extra-thick hot-rolled steel sheet.

Background Art

[0002] Hot rolling generally refers to a process in which a rolled material with a rectangular cross-section continuously cast (hereinafter referred to as a "slab") is heated to several hundred to one thousand several hundred degrees Celsius in a heating furnace, and then rolled thinly by rough rolling and finish rolling, and wound into a coil shape.

[0003] FIG. 1 is an example of a conventionally commonly used hot rolling line 1. The slab 101 is heated by a heating furnace 3 and then rolled by a rough rolling mill 5 and a rough rolling mill 7 equipped with horizontal rolls, and then rolled by a finish rolling mill 9.

[0004] Rough rolling is generally performed six or more times by reciprocating rolling or one-way rolling. Although the hot rolling line 1 shown in FIG. 1 is equipped with two rough rolling mills 5 and 7, the number of units is not necessarily limited to this, and there may be three or more units. In addition, a vertical edger 11 is provided in the hot rolling line 1 as a device for pressing down the slab 101 in the width direction. Therefore, the slab 101 is subjected to pressing down in the slab thickness direction by the rough rolling mills 5 and 7 and pressing down in the width direction by the edger 11. Pressing down in the thickness direction is called horizontal mill pressing down, and pressing down in the width direction is called edger pressing down or width pressing down (as described in cited examples, etc.) in the present application. In addition, a sizing press 13 is installed immediately upstream of the rough rolling mills 5 and 7 for adjusting the slab width.

[0005] Furthermore, the hot rolling line 1 is provided with a hot scale breaker for primary scale removal (HSB15 shown in FIG. 1), a crop shear 19, and a finish rolling scale breaker for secondary scale removal (FSB17 shown in FIG. 1). The slab 101 heated in the heating furnace 3 is descaled by the HSB15 before being roughly rolled. Further, the roughly rolled slab 101 has its tip and tail ends cut by the crop shear 19, is descaled again by the FSB17, and is sent to the finishing rolling mill 9 before the finishing rolling mill 9. The steel sheet that has passed through the finishing rolling mill 9 is controlled-cooled by water-cooled or air-cooled cooling equipment (not shown) during conveyance on the run-out table 21 in order to control precipitates and transformation structures and obtain material properties such as the desired strength and elongation, and is then wound up by the coiler 23.

[0006] It is known that the hot-rolled steel sheet manufactured as described above is liable to generate linear minute surface defects called edge seams and hege defects on the edge portions on both ends in the width direction. An edge seam is a surface defect mainly generated by wrinkles associated with grain coarsening at the width direction ends in the manufacturing process of a stainless steel sheet wrapping around the steel sheet surface. On the other hand, a hege defect is a surface defect generated by the tension caused by the difference in metal flow in the rolling direction between the end and the center in the width direction of the slab. Since these surface defects also remain on the surface of the pipe manufactured in the subsequent process or the steel sheet after cold rolling, they cause quality degradation and yield loss. Therefore, techniques for manufacturing a hot-rolled steel sheet while suppressing surface defects have been proposed.

[0007] Patent Documents 1 to 4 disclose techniques for suppressing the generation of edge seams. The technique of Patent Document 1 is to reduce the thickness by 50% or more using only horizontal rolls in the first 3 passes of rough rolling, and to set the width reduction by the subsequent vertical rolls to 30 mm or less per pass. The technique of Patent Document 2 is to roll with a reduction ratio of 5% or less using vertical rolls before performing initial rolling with horizontal rolls, and to define the pass time until the subsequent initial rolling with horizontal rolls. The technique of Patent Document 3 is to perform width reduction rolling with a reduction ratio of 3% or more at least once before rough horizontal rolling. The technology of Patent Document 4 does not perform width reduction until the fourth pass before rough rolling, and performs width reduction using inclined vertical rolls in the subsequent passes.

[0008] In addition, Patent Document 5 discloses a technology for manufacturing a hot-rolled steel sheet without defects such as Hegel defects. The technology of Patent Document 5 heats the slab end before rough rolling, adjusts the temperature difference between the slab end temperature and the slab center temperature to be in the range of 20 to 200 °C during the first pass of rough rolling, and then performs hot rolling.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0010] Normally, the maximum plate thickness of the main hot-rolled steel sheet is about 6 mm. However, in the manufacturing process of an extremely thick hot-rolled steel sheet with a product plate thickness of 15 mm or more, rolling conditions are required to obtain the desired steel sheet properties. In particular, in order to ensure a predetermined toughness, it is necessary to set the total reduction ratio in the temperature range of 1000 °C or lower, which is the non-recrystallization temperature range, to 60% or more in rough rolling and finish rolling. Furthermore, since there is a limit to the reduction ratio in finish rolling, it is necessary to rapidly cool the slab in the subsequent passes of rough rolling and apply a reduction that satisfies the above conditions.

[0011] However, when the slab is rapidly cooled and roughly rolled during the production of an extremely thick hot-rolled steel sheet, as shown in Fig. 2, defects in the form of extending obliquely in the longitudinal direction (the slab conveyance direction) occur in the edge portions in the width direction of the hot-rolled steel sheet 111 (hereinafter referred to as "surface cracks 113"). Such surface cracks 113 cause a decrease in the operation rate due to coil scrap and defect rework during pipe manufacturing, which is the next process, and thus suppressing them has been an issue.

[0012] The technologies of Patent Documents 1 to 5 are effective as countermeasures for suppressing surface defects of hot-rolled steel sheets. For this reason, it was also considered whether it might be possible to suppress the surface cracks 113 that are the subject of the present application. However, the surface cracks 113 are different in form from surface defects such as edge seams and hege defects, and could not be suppressed by the technologies of Patent Documents 1 to 5.

[0013] The technologies of Patent Documents 1 to 4 suppress edge seams, which are the re-entrant of wrinkles due to grain coarsening at the ends, by defining the shape of the press pad and the reduction rate and reduction amount in the width direction of the slab in the stage before rough rolling. However, the surface cracks 113 of the hot-rolled steel sheet 111 could not be suppressed only by defining the reduction rate and reduction amount in the width direction during rough rolling.

[0014] Also, the technology of Patent Document 5 reduces hege defects by adjusting the temperature difference between the widthwise ends and the central part of the slab during the first rough rolling pass to a predetermined range (20 to 200°C) and adjusting the ratio of the deformation resistance of the slab ends to the central part of the slab. However, the surface cracks 113 of the hot-rolled steel sheet 111 could not be suppressed even when the temperature difference between the widthwise ends and the central part of the slab was adjusted to the above range during the first rough rolling pass.

[0015] The present invention has been made to solve the above problems, and an object thereof is to provide a method for manufacturing a hot-rolled steel sheet capable of suppressing surface cracks in the edge portions of a hot-rolled steel sheet having a product plate thickness of 15 mm or more.

Means for Solving the Problems

[0016] (1) The method for manufacturing a hot-rolled steel sheet according to the present invention is to perform rough rolling on a slab with a slab thickness of 50 mm or more at the end of rough rolling by a plurality of rough rolling passes, and finish rolling the rough-rolled slab to manufacture a hot-rolled steel sheet with a product sheet thickness of 15 mm or more. Among the plurality of rough rolling passes, after the dimensions of the slab satisfy W / H≥10, the slab thickness is 100 mm or less, and the average temperature of the slab thickness at the center in the width direction over the entire length of the slab becomes 1000°C or less, in each rough rolling pass, the slab is rapidly cooled so that the average temperature of the slab thickness at the center in the width direction of the slab decreases at 15°C / s or more, and the maximum value of the surface temperature in the edge portion within 300 mm from both ends in the width direction of the slab is 30°C or more higher than the surface temperature at the center in the width direction. (ΔH / H) / (ΔW / W)≥30, 1 mm≤ΔW≤10 mm, and the slab is rough-rolled in this way, which is characterized by this. However, W and H are, respectively, the slab width and slab thickness before rough rolling in each rough rolling pass. (ΔH / H) is the horizontal mill reduction ratio in each rough rolling pass. (ΔW / W) is the edge mill reduction ratio in each rough rolling pass. ΔH is the horizontal mill reduction amount in each rough rolling pass. ΔW is the edge mill reduction amount in each rough rolling pass.

[0017] (2) In the above (1), in each rough rolling pass for rapidly cooling the slab, the surface temperatures at the center in the width direction and the edge portion of the slab are measured, which is characterized by this.

Effects of the Invention

[0018] According to the present invention, it is possible to suppress the occurrence of cooling cracks in the edge portion due to the rapid cooling of the slab and the elongation of the cooling cracks due to rough rolling. As a result, it is possible to suppress surface cracks accompanied by dents in the edge portion of the hot-rolled steel sheet finished by finish rolling to a product plate thickness of 15 mm or more, and to manufacture a hot-rolled steel sheet having good surface properties.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4-1

Figure 4-2

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Mode for Carrying Out the Invention

[0020] The hot-rolled steel sheet targeted in the present invention, the mechanism of surface cracking occurring in the hot-rolled steel sheet, and the background leading to the present invention will be described.

[0021] <The hot-rolled steel sheet targeted in the present invention> Generally, in the steel sheet manufacturing line, after rough rolling, the slab thickness is 20 to 45 mm (hereinafter), the plate thickness after hot rolling is 1.6 to 4 mm (hereinafter), and the plate thickness after cold rolling is 0.6 to 1 mm (hereinafter). For example, for stainless steel, since it is necessary to set the slab temperature in the heating furnace to 1100 °C or higher for solid solution of alloy elements, it is common to rough roll the slab at 1350 to 1000 °C and set the slab thickness at the end of rough rolling to about 40 mm.

[0022] On the other hand, the hot-rolled steel sheet targeted in the present application is used as a material for line pipes, and as the characteristics of the final product, it is required to be extremely thick (product plate thickness 15 to 30 mm), high strength, and high toughness. In the present application, to make the product plate thickness 15 to 30 mm, the slab thickness at the end of rough rolling is set to 50 mm or more and 65 mm or less. Also, high strength means a tensile strength of 400 MPa or more, and high toughness means 200 J or more at 0 °C in the Charpy impact test. In the present application, the plate width of the hot-rolled steel sheet is 1000 to 2300 mm.

[0023] The tensile strength can be measured, for example, by taking a JIS No. 5 test piece (test piece length: 500 mm) from the 1 / 4 point in the plate width direction of the hot-rolled steel sheet and performing a tensile test in accordance with JIS Z 2242. By this test, not only the tensile strength: TS (MPa), but also the 0.2% proof stress: YS (MPa) and elongation: El (%) can be obtained. Toughness can be measured, for example, by a Charpy impact test in accordance with JIS Z 2242. In this test, a V-notch test piece (width: 10 mm × height: 10 mm × length: 55 mm, V-notch angle 45°, notch bottom width 8 mm, notch root radius 0.25 mm) is taken from a point 1 / 4 in the plate width direction, and the impact absorption energy is measured at a predetermined test temperature (for example, 0°C).

[0024] Note that the slab thickness at the time of extraction from the heating furnace is set based on, for example, the possibility of ensuring the solidification time during continuous casting, the reduction ratio of the slab in the rough rolling pass, the product plate thickness of the produced hot-rolled steel sheet, and the mechanical properties (strength and toughness), etc., and is, for example, 220 to 260 mm.

[0025] Then, in order to obtain a hot-rolled steel sheet with a predetermined product plate thickness and mechanical properties (strength, toughness), after the slab thickness becomes 100 mm or less and the average temperature of the slab thickness at the center in the width direction over the entire length of the slab becomes 1000°C or less in the rough rolling pass, the slab is rapidly cooled at a cooling rate of 15°C / s or more.

[0026] Surface defects (edge seams, hege defects) that appear in general hot-rolled steel sheets (for example, stainless steel sheets and silicon steel sheets) are generated, as described above, by a phenomenon in which the corner portions of the slab wrap around onto the surface as they are subjected to edge rolling reduction and horizontal mill rolling reduction. And the edge seam and the hege defect have a linear form along the edge of the hot-rolled steel sheet.

[0027] On the other hand, the surface crack 113 targeted in the present application randomly occurs longitudinally within about 50 mm from both ends in the width direction in the form of a dent as shown in FIG. 2. Further, the surface crack 113 has a form in which the dent extends in the longitudinal and width directions of the slab and has an angle of perpendicular to about 30° with respect to the longitudinal direction, rather than being parallel.

[0028] Thus, since the surface crack 113 clearly differs in form from the edge seam and Hegel flaw, it is considered that the generation mechanism is also different. Therefore, as described above, with the conventional techniques for suppressing the edge seam and Hegel flaw, it was not possible to suppress or prevent the surface crack 113.

[0029] <Mechanism of Surface Crack Occurring in Hot-Rolled Steel Sheet> The inventor earnestly studied the mechanism in order to suppress the surface crack 113 in the edge portion of the hot-rolled steel sheet 111.

[0030] Figure 3 is a graph of the time history of the slab thickness and slab temperature when rapid cooling was performed after the 6th pass of rough rolling in which the slab thickness became 100 mm or less and the average slab thickness temperature became 1000 °C or less. In Figure 3, all the temperature histories indicate the values at the center of the slab width. The surface temperature is the measured value by a near-infrared camera. The average slab thickness temperature and the slab thickness center temperature are the results by simulation, and are values calculated using the slab thickness, the surface temperature of the slab, and the cooling conditions, etc. as input conditions.

[0031] As shown in Figure 3, the surface temperature of the slab has dropped below the transformation point of 600 °C in the 6th pass. From this, it can be seen that the surface of the slab is at a temperature at which grain boundary ferrite precipitates and a two-phase structure is formed in the 6th pass of rapid cooling.

[0032] The surface temperature of the slab shown in Figure 3 is that at the center in the width direction, but the edge portion of the slab is likely to have a lower temperature than the central portion in the width direction due to heat extraction from the end face. Therefore, when rapid cooling is performed during rough rolling, as shown in Figure 3, it is considered that the surface temperature of the edge portion is likely to be below the transformation point. From the above, in the latter-stage passes of rough rolling (the 6th pass and subsequent passes in Figure 3) where rapid cooling is required, it is important to control the rolling conditions for obtaining predetermined mechanical properties and dimensions also in the edge portion.

[0033] The mechanism of surface crack 113 occurring in the edge portion of hot-rolled steel sheet 111 with a two-phase structure on the surface of the slab due to rapid cooling during rough rolling will be described based on FIGS. 4-1 and 4-2.

[0034] FIG. 4-1(a) is a diagram showing (i) the distribution of the α-phase and γ-phase on the surface of slab 101 where grain boundary ferrite has precipitated to form a two-phase structure, and (ii) the cross-section of cracks (cooling cracks 103) generated in the grain boundary ferrite. Further, FIG. 4-1(b) is a diagram showing that cooling crack 103 remains as surface crack 113 in hot-rolled steel sheet 111 after being rolled.

[0035] When slab 101 with a surface having a two-phase structure of the α-phase and γ-phase due to rapid cooling is roughly rolled, as shown in FIG. 4-1(a)(i), strain concentrates in the soft grain boundary ferrite (α-phase), and cracks (cooling cracks 103) are generated. Then, the cooling cracks 103 generated in slab 101 are elongated by subsequent rough rolling (horizontal roll reduction and edger reduction), and remain as surface cracks 113 in hot-rolled steel sheet 111 as shown in FIG. 4-1(b).

[0036] FIGS. 4-2(a) to (c) are diagrams for explaining the deformation (elongation) of cooling crack 103 accompanying the rough rolling of slab 101. In each of FIGS. 4-2(a) to (c), (i) is a macro view of the deformation of the upper corner portion in a cross-section perpendicular to the longitudinal direction of slab 101, and (ii) is a micro view of the cross-section of the upper corner portion of slab 101.

[0037] Viewed macroscopically, when rapidly cooled slab 101 is edger-reduced, as shown in FIG. 4-2(a)(i), cracks (cooling cracks 103) are generated on the surface of the edge portion. When further edger-reduced, as shown in FIG. 4-2(b)(i), slab 101 bulges in the front and back surface directions at the edge portion and has a so-called dock bone shape when viewed in cross-section. After that, when horizontally mill-reduced, as shown in FIG. 4-2(c)(i), the dock bone shape is crushed and flattened.

[0038] On a microscopic scale, when the slab 101 is further edger-reduced, the cooling crack 103 generated on the surface of the edge portion shown in Fig. 4-2(a)(ii) shears and expands as the dock bone shape is formed as shown in Fig. 4-2(b)(ii). Subsequently, when the slab 101 is horizontally mill-reduced, as shown in Fig. 4-2(c)(ii), the cooling crack 103 becomes shallower and deforms into a form that extends obliquely with respect to the longitudinal direction. The thus-deformed cooling crack 103 remains as a surface crack 113 at the edge portion of the hot-rolled steel sheet 111 after finish rolling (see Fig. 2). As described above, it can be explained that the surface crack 113 is caused by the cooling crack 103 that was generated and elongated by the rough rolling of the slab 101 before finish rolling.

[0039] <Background of the Invention> Subsequently, based on the above findings regarding the mechanism of the surface crack 113 occurring in the hot-rolled steel sheet 111 shown in Fig. 2, the inventor studied a method for suppressing the occurrence of the surface crack 113.

[0040] The inventor focused on the fact that the surface crack 113 is caused by the cooling crack 103 that occurred and elongated in the edge portion of the slab 101 that was rapidly cooled and then rough-rolled. Therefore, the inventor studied methods for suppressing the generation and elongation of the cooling crack during the rough rolling process.

[0041] Regarding the suppression of the generation of the cooling crack 103, the inventor recalled that the surface temperature of the edge portion should be made higher than that at the center in the width direction so that the edge portion does not drop excessively due to rapid cooling. The inventor changed the surface temperatures of the edge portion and the center in the width direction in each rough rolling pass of rapid cooling and investigated the surface temperature conditions that can suppress the generation of the cooling crack 103. As a result, it was considered that the generation of the cooling crack 103 in the edge portion can be suppressed by making the maximum value of the surface temperature within 300 mm from the edge in the width direction 30°C or more higher than the temperature at the center in the width direction.

[0042] Next, the inventor studied the suppression of the elongation of the cooling crack 103. FIG. 5 is a graph showing (a) the amount of horizontal mill reduction and the horizontal mill reduction ratio in each rough rolling pass, and (b) the amount of edger reduction and the edger reduction ratio. In FIGS. 5(a) and (b), the bar graph shows the amount of reduction, and the line graph shows the reduction ratio. The rough rolling of the slab 101 is performed according to the amount of horizontal mill reduction (and the horizontal mill reduction ratio) and the amount of edger reduction (and the edger reduction ratio) in each rough rolling pass, as illustrated in FIG. 5.

[0043] The amount of edger reduction is set to be substantially equal to the spread in the slab width direction due to the horizontal mill reduction. Also, in the latter passes of the rough rolling, since the slab thickness becomes thinner and a dog-bone shape is formed at the edge portion, the edger reduction efficiency decreases. For this reason, a large amount of edger reduction is set in the latter rough rolling passes.

[0044] In order for the hot-rolled steel sheet to satisfy a predetermined strength (such as tensile strength) and toughness, in the latter passes of the rough rolling, it is necessary to rapidly cool the slab and set the horizontal mill reduction ratio so that the total reduction ratio in the rough rolling and the finish rolling is 60% or more. Among such constraints, the slab thickness and slab width schedule of the rough rolling passes need to be determined so as to suppress the elongation of the cooling cracks generated by the rapid cooling of the slab.

[0045] As described above, the horizontal mill reduction makes the cooling crack 103 shallow, while the edger reduction makes the cooling crack 103 deep. Therefore, the inventor considered that the horizontal mill reduction ratio ΔH / H and the edger reduction ratio ΔW / W in each rough rolling pass may be appropriately set to suppress the elongation of the cooling crack 103. Based on this idea, the inventor conceived of defining the ratio (ΔH / H) / (ΔW / W) of the horizontal mill reduction ratio ΔH / H and the edger reduction ratio ΔW / W in the rough rolling pass where rapid cooling is performed as an index for suppressing the elongation of the cooling crack 103. In the present application, the ratio thus defined (= (ΔH / H) / (ΔW / W)) is referred to as the cooling crack index.

[0046] Here, in the horizontal mill reduction rate ΔH / H, ΔH is the difference between the slab thickness on the rough rolling inlet side and the slab thickness on the rough rolling outlet side for each rough rolling pass, and H is the slab thickness on the rough rolling inlet side for each rough rolling pass. Also, as shown in FIG. 6, in the edger reduction rate ΔW / W, ΔW is the difference (W1 - W2) between the slab width W1 on the rough rolling inlet side and the slab width W2 on the rough rolling outlet side for each rough rolling pass, and W is the slab width on the rough rolling inlet side for each rough rolling pass.

[0047] Then, as shown in FIG. 7, the inventor set the cooling crack index (ΔH / H) / (ΔW / W) for each rough rolling pass and investigated the presence or absence of surface cracks 113 in the hot-rolled steel sheet 111. In this investigation, the rapid cooling of the slab 101 was performed for the rough rolling passes (the 8th pass to the 10th pass) after the slab thickness became 100 mm or less and the average temperature of the slab thickness at the center in the width direction became 1000 °C or less. Regarding the cooling rate of the rapid cooling, it was set so that the average temperature of the slab thickness at the center in the slab width direction decreased at 15 °C / s or more. In FIG. 7(a), the cooling crack index was set to less than 30, and in FIG. 7(b), the index was set to 30 or more.

[0048] When the cooling crack index was less than 30, surface cracks 113 were observed, whereas when the cooling crack index was 30 or more, surface cracks 113 were not observed. From this result, it was found that by setting the cooling crack index to 30 or more [(ΔH / H) / (ΔW / W) ≧ 30] in each rough rolling pass where rapid cooling is performed, the elongation of the cooling crack 103 can be prevented and the occurrence of surface cracks 113 in the hot-rolled steel sheet 111 can be suppressed.

[0049] Furthermore, the inventor examined the appropriate edger reduction amount ΔW for setting the cooling crack index to 30 or more. As a result, it was found that the edger reduction amount ΔW should be 1 mm or more and 10 mm or less.

[0050] The present invention has been made based on the above examination results, and its specific configuration is as follows.

[0051] <Method for manufacturing hot-rolled steel sheet> The method for manufacturing a hot-rolled steel sheet according to this embodiment manufactures a hot-rolled steel sheet with a slab thickness of 50 mm or more at the end of rough rolling and a finished product thickness of 15 mm or more by finish rolling.

[0052] Among a plurality of rough rolling passes, after the slab dimensions become W / H≥10, the slab thickness becomes 100 mm or less, and the average temperature of the slab thickness at the center in the width direction over the entire length of the slab becomes 1000 °C or less, in each rough rolling pass, the slab is rapidly cooled so that the average temperature of the slab thickness at the center in the width direction of the slab decreases at 15 °C / s or more. Furthermore, in each of the above rough rolling passes, along with the rapid cooling of the slab, the maximum value of the surface temperature at the edge portion within 300 mm from both ends in the width direction of the slab is 30 °C or more with respect to the surface temperature at the center in the width direction, and furthermore, rough rolling is performed with (ΔH / H) / (ΔW / W)≥30 and 1 mm≤ΔW≤10 mm. However, W and H are the slab width and slab thickness before rough rolling in each rough rolling pass, respectively (ΔH / H) is the horizontal mill reduction ratio in each rough rolling pass (ΔW / W) is the edge rolling reduction ratio in each rough rolling pass ΔH is the horizontal reduction amount in each rough rolling pass ΔW is the edge rolling reduction amount in each rough rolling pass

[0053] In the present invention, the rapid cooling of the slab means reducing the average temperature of the slab thickness at the center in the width direction of the slab at a cooling rate of 15 °C / s or more. As a means for rapid cooling, it is possible to exemplify injecting cooling water onto both the upper surface and the lower surface of the slab. The means for rapid cooling may be any means that can achieve the above cooling rate, such as injecting cooling water only onto the upper surface or the lower surface of the slab, injecting mist containing a large amount of water vapor, or using a combination of mist and cooling water.

[0054] The reasons for setting the rolling reduction conditions in the rough rolling pass for rapid cooling to (ΔH / H) / (ΔW / W)≥30 and 1 mm≤ΔW≤10 mm are as follows. If the cooling crack index (ΔH / H) / (ΔW / W) is less than 30, or the edger reduction ΔW is more than 10 mm, the elongation of cooling cracks cannot be suppressed. If the edger reduction ΔW is less than 1 mm (for example, 0 mm), the dimensional accuracy of the slab width cannot be ensured. This is because the slab becomes edge-free and the slab width is determined only by the horizontal mill reduction. In addition, setting ΔW to less than 1 mm is not a practically meaningful setting considering setting errors, etc., so it is necessary to avoid it.

[0055] Also, the reason for setting the rough rolling under the rolling reduction conditions of (ΔH / H) / (ΔW / W) ≥ 30 and 1 mm ≤ ΔW ≤ 10 mm as the rough rolling pass after (1) the slab dimensions are W / H ≥ 10, (2) the slab thickness is 100 mm or less, and (3) the average temperature of the slab thickness at the center in the width direction over the entire length of the slab is 1000 °C or less is as follows.

[0056] (1) When a slab with W / H ≥ 10 is rapidly cooled and rough rolled, a dog-bone shape is likely to be formed. And when cooling cracks occur in a slab with a dog-bone shape, the cooling cracks are likely to elongate due to further rough rolling (see Fig. 4-2 described above). Therefore, when the slab dimensions are W / H ≥ 10, by rough rolling under the above rolling reduction conditions, the elongation of cooling cracks can be suppressed.

[0057] (2) The reason for setting the slab thickness to 100 mm or less is that the rolling reduction rate of the slab to be rapidly cooled and rough rolled is specified in order to ensure toughness.

[0058] (3) The reason for setting the average temperature of the slab thickness to 1000 °C or less is that the hot-rolled steel sheet targeted in the present application is a material for line pipes, and extremely thick (15 - 30 mm) and high toughness are required. Note that the average slab thickness temperature may be calculated using a temperature model that can be estimated based on the surface temperature of the slab or the like. The temperature model performs a simulation by providing the slab thickness, the surface temperature of the slab, the cooling conditions, and the like. In such a temperature model, the average slab thickness temperature can be obtained, for example, by calculating the temperature distribution in the slab thickness direction using the difference method.

[0059] Further, the cooling rate of the slab 101 can be obtained, for example, from the time history of the average slab thickness temperature at the center in the width direction of the longitudinal end portion (a position about 1 m from the leading end).

[0060] As described above, according to the present embodiment, the occurrence and elongation of the cooling crack 103 can be suppressed in each rough rolling pass of rapid cooling. Thereby, the surface crack 113 accompanied by the dent in the edge portion can be suppressed, and the hot-rolled steel sheet 111 having a product plate thickness of 15 mm or more and a good surface property can be manufactured.

[0061] Note that in each rough rolling pass of rapid cooling, it is preferable to measure the surface temperatures of the center in the width direction and the edge portion of the slab 101. Thereby, it can be confirmed that the maximum value of the surface temperature in the edge portion is 30°C or more higher than the surface temperature in the center in the width direction, and the surface crack 113 can be surely prevented even when the capacity of the equipment changes due to aging deterioration.

[0062] The surface temperatures of the center in the width direction and the edge portion of the slab 101 may be measured by photographing the entire width of the slab 101 with a near-infrared camera 27 installed, for example, on the outlet side of the rough rolling mills 5 and 7, particularly on the outlet side of the rough mill cooling equipment 25, as shown in FIG. 8. The near-infrared camera 27 can measure a temperature range of 800°C or more and 1000°C or less, and preferably can set the measurement temperature range. The near-infrared camera 27 is not limited to photographing the entire width of the slab 101 as described above, and may measure the surface temperatures of the center portion and the edge portion in the width direction of the slab 101 respectively.

[0063] The slab thickness, slab width schedule, and temperature schedule may be determined by inputting the specifications regarding the slab thickness, slab width, and temperature at the time of passing through each rough rolling pass into the process computer and obtaining the equipment settings such as the opening of the lower-level equipment side (horizontal mill, edger, etc.). At this time, the edger reduction amount for each rough rolling pass may be set individually.

[0064] Furthermore, the supercooling of the edge part due to rapid cooling may be suppressed using, for example, an edge masking device 29 as shown in FIG. 9. The edge masking device 29 shown in FIG. 9 prevents cooling water from being directly applied to the edge part by shielding the cooling water sprayed from the rough mill cooling equipment 25 at the edge part with a shielding plate 31. The arrangement of the shielding plate 31 may be, for example, within a range that can cover the cooling water injection nozzles installed up to 300 mm in the width direction from the edge.

[0065] FIG. 10 shows an example of measuring the surface temperature of the rapidly cooled slab 101. FIG. 10(a) is a near-infrared image of the slab surface captured by the near-infrared camera 27, and the gray-scale shading represents the high and low surface temperatures. It can be seen that the edge part has a lighter gray-scale shading and a higher temperature than the center in the width direction. FIGS. 10(b) and (c) show the surface temperature distribution in the slab width direction obtained from the near-infrared image of the slab surface. FIG. 10(b) shows the result when cooling water was not directly sprayed on the edge part by the edge masking device 29 (FIG. 9). FIG. 10(c) shows the result when the entire width of the slab 101 was directly sprayed with cooling water for cooling without using the edge masking device 29.

[0066] When the edge masking device 29 was used, the temperature difference between the maximum value of the surface temperature at the edge part of the slab 101 and the surface temperature at the center in the width direction was approximately 50°C (FIG. 10(b)). In this case, no surface cracks 113 were observed at the edge part of the hot-rolled steel sheet 111 finished by finish rolling to a product plate thickness of 15 mm or more as shown in FIG. 2.

[0067] On the other hand, when the edge masking device 29 was not used, the temperature difference between the maximum surface temperature at the edge portion of the slab 101 and the surface temperature at the center in the width direction was about 25°C (Fig. 10(c)). And as shown in Fig. 2, surface cracks 113 were observed at the edge portion of the hot-rolled steel sheet 111.

[0068] In this way, by not directly injecting cooling water onto the edge portion, it becomes possible to make the maximum surface temperature at the edge portion 30°C or more higher than that at the center in the width direction. Thereby, the generation of cooling cracks 103 can be suppressed, and the surface cracks 113 at the edge portion of the hot-rolled steel sheet 111 can be suppressed.

[0069] Note that the means for making the surface temperature at the edge portion higher than that at the center in the width direction is not limited to using the edge masking device 29. For example, an edge heating device that heats the edge portion with a heater may be used.

[0070] In the present invention, the edge portion that specifies the maximum value in the width direction of the surface temperature of the slab 101 is a portion within 300 mm from both ends in the width direction. As shown in Figs. 10(b) and (c), the surface temperature of the slab reaches its maximum value at a position 100 mm to 200 mm inside from the edge in any case, and rapidly decreases outside thereof. Therefore, if the edge portion is set within 300 mm, the maximum value of the surface temperature can be included.

[0071] The position where the surface temperature shows the maximum value may be approximately at the center of the slab width when no special cooling is performed. However, when the maximum value of the surface temperature exists at the edge portion, it becomes 100 to 200 mm from the edge as described above. The portion closer to the center in the width direction becomes substantially uniform in temperature if the cooling conditions are the same in the width direction. This is because the slab thickness is constant and the heat extraction from the slab surface is substantially uniform. In other words, the profile of the surface temperature distribution is substantially determined by the shape of the slab and does not take local minimum or maximum values, so the surface temperature becomes maximum at a position 100 to 200 mm inside from the edge.

[0072] In the rough rolling pass, a temperature drop of about 30°C occurs from the tip to the tail end in the longitudinal direction of the slab. However, in the rough rolling pass with rapid cooling, if the maximum value of the surface temperature at the edge part at the tip of the slab is 30°C or more higher than the surface temperature at the center in the width direction, this temperature difference can be ensured over the entire length in the longitudinal direction.

Example

[0073] In order to verify the effects of the present invention, the following experiments were conducted. In the experiment, first, a slab 101 with a slab thickness of 250 mm was heated in a heating furnace 3 and rolled to a slab thickness of 50 - 55 mm at the end of rough rolling through a plurality of rough rolling passes. Then, the rough-rolled slab 101 was rolled into a hot-rolled steel sheet 111 with a plate thickness of 15.8 mm - 25.0 mm and a plate width of 2000 mm. The target lower limits of the tensile strength and the impact absorption energy at 0°C of the hot-rolled steel sheet were set to 400 MPa and 200 J, respectively.

[0074] As the slab 101, one having a component composition containing, by mass%, C: 0.03 - 0.2%, Si: 0.02 - 0.2%, and optionally containing Mn: 0.2 - 1.10%, Cr: 1.0 - 8.0%, Nb: 0.005 - 0.030%, with the balance being Fe excluding inevitable impurities, was used.

[0075] As specific examples of the manufactured hot-rolled steel sheet 111, the component compositions of the hot-rolled steel sheet A and the hot-rolled steel sheet B are shown below by mass%. (Hot-rolled steel sheet A) C: 0.117%, Si: 0.01%, Mn: 0.57%, P: 0.016%, S: 0.004%, N: 0%, Cr: 0.038%, Nb: 0% (Hot-rolled steel sheet B) C: 0.142%, Si: 0.2%, Mn: 0.7%, P: 0.022%, S: 0.004%, N: 0.0022%, Cr: 0.033%, Nb: 0%

[0076] The temperature conditions in the experiment were a heating temperature (SRT) of 1150 - 1250°C by the heating furnace 3, a rough rolling outlet side temperature (RDT) of 900 - 1000°C, and a finish rolling outlet side temperature (FDT) of 500 - 600°C.

[0077] In order to suppress the occurrence and elongation of the cooling crack 103 at the edge portion during rough rolling of the slab 101 (Fig. 4-2), the following conditions a to c were defined. Condition a: Maintain a maximum temperature difference of 30°C or more (ΔT ≧ 30°C) between the maximum surface temperature at the edge portion within 300 mm from the edge and the center in the width direction. Condition b: The cooling crack index is 30 or more ((ΔH / H) / (ΔW / W) ≧ 30). Condition c: The edge rolling reduction amount in each rough rolling pass is 1 mm or more and 10 mm or less (1 mm ≦ ΔW ≦ 10 mm). Then, rough rolling was performed with various changes in each setting of conditions a to c, and the presence or absence of surface cracks 113 (Fig. 2) at the edge portion of the hot-rolled steel sheet 111 was investigated.

[0078] Table 1 shows the combinations (No. 1 to No. 24) in which each setting of conditions a to c was changed, the rough thickness, the presence or absence of surface cracks 113 at the edge portion of the hot-rolled steel sheet 111, and the product thickness. The rough thickness (mm) in the table indicates the slab thickness at the end of rough rolling, and the product thickness indicates the sheet thickness of the hot-rolled steel sheet after finishing rolling.

[0079]

Table 1

[0080] No. 1 to No. 10 satisfy all of condition a (temperature difference), condition b (cooling crack index), and condition c (edge rolling reduction amount). On the other hand, No. 11 to No. 15 satisfy condition a (temperature difference), but do not satisfy condition b (cooling crack index) and condition c (edge rolling reduction amount). Also, No. 16 to No. 20 satisfy condition b (cooling crack index) and condition c (edge rolling reduction amount), but do not satisfy condition a (temperature difference). Furthermore, No. 21 to No. 24 satisfy condition b (cooling crack index), but do not satisfy condition a (temperature difference) and condition c (edge rolling reduction amount).

[0081] For Nos. 1 to 10 that satisfy all of the above conditions a to c, surface cracks 113 did not occur in the edge portions of the hot-rolled steel sheet 111.

[0082] On the other hand, for Nos. 11 to 15 that do not satisfy condition b (cooling crack index) and condition c (edge rolling reduction), the cooling cracks 103 generated in the edge portions of the slab 101 elongated, and surface cracks 113 occurred in the edge portions of the hot-rolled steel sheet 111.

[0083] Also, for Nos. 16 to 20 that do not satisfy condition a (temperature difference), a large number of cooling cracks 103 occurred in the edge portions of the slab 101, and surface cracks 113 occurred in the edge portions of the hot-rolled steel sheet 111.

[0084] Furthermore, for Nos. 21 to 24 that do not satisfy condition a (temperature difference) and condition c (edge rolling reduction), it was not possible to suppress the occurrence and elongation of the cooling cracks 103 in the edge portions of the slab 101, resulting in surface cracks 113 remaining in the edge portions of the hot-rolled steel sheet 111.

[0085] As shown in the previous FIGS. 4-1 and FIGS. 4-2(a) and (b), when the edge rolling reduction ΔW is large, the edge portion swells and a dock bone shape is likely to be formed, so the cooling cracks 103 are likely to elongate.

[0086] In this experiment, as shown in Table 1, for Nos. 1 to 10 that satisfy the cooling crack index of (ΔH / H) / (ΔW / W) ≧ 30 and the edge rolling reduction of mm ≦ ΔW ≦ 10 mm, no occurrence of surface cracks was observed.

[0087] FIG. 11 is a graph plotting the relationship between the presence or absence of surface cracks 113 in the edge portions of the hot-rolled steel sheet 111 with various settings of conditions a to c during rough rolling. In this graph, the horizontal axis is the temperature difference ΔT, and the vertical axis is the cooling crack index (ΔH / H) / (ΔW / W). From FIG. 11, it can be seen that the surface cracks 113 do not occur in the region where the temperature difference is 30°C or more and the cooling crack index is 30 or more.

[0088] Table 2 shows the results of tensile tests and impact tests conducted on each of hot-rolled steel sheet A and hot-rolled steel sheet B, and the measured tensile strength and impact absorption energy. Further, Fig. 12 is a graph showing the average, maximum, and minimum of the tensile strength and impact absorption energy shown in Table 1. As described above, the tensile test and the impact test were carried out in accordance with the provisions of JIS Z 2242, and in both tests, the sampling position of the test piece was near the edge part (a position corresponding to 1 / 4 of the plate width).

Table 2

[0089] The average tensile strength of hot-rolled steel sheet A was 480 MPa (maximum: 484 MPa, minimum: 474 MPa), and the average tensile strength of hot-rolled steel sheet B was 579 MPa (maximum: 594 MPa, minimum: 565 MPa), both of which were 400 MPa or more as the target. Also, the average impact absorption energy of hot-rolled steel sheet A was 269 J (maximum: 283 J, minimum: 258 J), and the average impact absorption energy of hot-rolled steel sheet B was 261 J (maximum: 282 J, minimum: 251 J), both of which were 200 J or more as the target.

[0090] As described above, according to the present invention, it has been demonstrated that when manufacturing the hot-rolled steel sheet 111 rolled to a product plate thickness of 15.8 mm to 25.0 mm, surface cracking 113 at the edge part can be suppressed. Further, it has been shown that the hot-rolled steel sheet manufactured according to the present invention has high strength (tensile strength of 400 MPa or more) and high toughness (impact absorption energy at 0 °C in the Charpy impact test of 200 J or more).

[0091] In the above embodiment, the upper limit of the product plate thickness was 25.0 mm, but in the case of manufacturing a hot-rolled steel sheet with a product plate thickness of 30 mm according to the present invention, since the total rolling reduction rate of rough rolling and finish rolling can be set to 60% or more, it is possible to suppress surface cracking.

[0092] In addition, the tensile strength and toughness (impact energy absorption) of the hot-rolled steel sheets shown in Table 2 and FIG. 12 were measured using test pieces collected from the vicinity of the edge portion of a hot-rolled steel sheet in which the edge portion of the slab was rapidly cooled to a temperature 30°C or higher than the center in the width direction in the latter pass of rough rolling. And both the measured tensile strength and toughness are the target values (400 MPa or more, 200 J or more). From these results, it is considered that the tensile strength (400 MPa or more) and toughness (200 J or more) are ensured even in the central portion in the width direction where the surface temperature is lower and the cooling rate is higher than that of the edge portion.

[0093] This example has been described by exemplifying the case of manufacturing hot-rolled steel sheets A and B having the above-described component compositions. However, the present invention is not limited to the manufacture of hot-rolled steel sheets having these component compositions, and can be applied to the manufacture of hot-rolled steel sheets having high strength (tensile strength of 400 MPa or more) and high toughness (impact energy absorption of 200 J or more in the Charpy impact test at 0°C).

Description of Signs

[0094] 1 Hot rolling line 3 Heating furnace 5 Rough rolling mill 7 Rough rolling mill 9 Finish rolling mill 11 Edger 13 Sizing press 15 HSB 17 FSB 19 Crop shear 21 Run-out table 23 Coiler 25 Rough mill cooling equipment 27 Near-infrared camera 29 Edge masking device 31 Shielding plate 101 Slab 103 Cooling crack 111 Hot-rolled steel sheet 113 Surface crack

Claims

1. A method for manufacturing a hot-rolled steel sheet, which comprises rough rolling a slab having a slab thickness of 50 mm or more at the end of rough rolling by a plurality of rough rolling passes, and finish rolling the rough-rolled slab to produce a hot-rolled steel sheet having a product sheet thickness of 15 mm or more, among the plurality of rough rolling passes, in each rough rolling pass after the dimensions of the slab become W / H≥10, the slab thickness becomes 100 mm or less, and the average temperature of the slab thickness at the center in the width direction over the entire length of the slab becomes 1000°C or less, rapidly cooling the slab so that the average temperature of the slab thickness at the center in the width direction of the slab decreases at 15°C / s or more, and the maximum value of the surface temperature at the edge portion within 300 mm from both ends in the width direction of the slab is 30°C or more higher than the surface temperature at the center in the width direction, (ΔH / H) / (ΔW / W)≥30, 1 mm≤ΔW≤10 mm, and rough rolling the slab as such, characterized in that it is a method for manufacturing a hot-rolled steel sheet. However, W and H are, respectively, the slab width and slab thickness before rough rolling in each rough rolling pass (ΔH / H) is the horizontal mill reduction ratio in each rough rolling pass (ΔW / W) is the edger reduction ratio in each rough rolling pass ΔH is the horizontal mill reduction amount in each rough rolling pass ΔW is the edger reduction amount in each rough rolling pass

2. In each rough rolling pass for rapidly cooling the slab, measuring the surface temperatures of the center in the width direction and the edge portion of the slab, characterized in that it is the method for manufacturing a hot-rolled steel sheet according to Claim 1.

Images