Method for producing steel sheet

Abstract

To provide a method for producing a steel sheet by which the occurrence of ridging in the steel sheet can be more easily suppressed.SOLUTION: A hot-rolled steel sheet manufactured by hot-rolling equipment is temper-rolled at a draft of ≥ 1.0% by using dull rolls having a roughness Ra of ≥ 10 μm and a pitch of ≤ 6.0 25mm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a steel sheet. [Background technology]

[0002] In steel sheets that do not undergo phase transformation or only partially undergo phase transformation after casting, the coarse grains at the time of casting may be carried over to the hot-rolled steel sheet or product, causing defects called ridging or cracks and wrinkles during processing (hereinafter collectively referred to as "ridging").

[0003] For example, Patent Document 1 discloses a method for manufacturing steel sheets in which a continuously cast slab is heated and then width-pressed, followed by width rolling with vertical rolls and thickness rolling with horizontal rolls. In the technology described in Patent Document 1, an antioxidant containing a reducing agent is applied to the entire longitudinal length of the side surface of the slab before heating, and width pressing is performed using a press die with caliber grooves in which the spacing between the upper and lower side walls narrows toward the groove bottom. This completely prevents coarsening of crystal grains near the side surface of the slab during heating and suppresses the increase in the amount of edge seam defects that get into the slab when width-pressed using the press die. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-198601 Summary of the Invention [Problem to be solved by the invention]

[0005] In the technology described in Patent Document 1, it is necessary to apply an antioxidant to the side surface of a continuously cast slab before heating the slab in a heating furnace, and then descale the heated slab and then width press it. Since the technology described in Patent Document 1 requires many steps before rough rolling and finish rolling, it is desirable to be able to more simply suppress the occurrence of ridging in a steel sheet.

[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 method for manufacturing a steel sheet that can more easily suppress the occurrence of ridging in the steel sheet. [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 steel sheet, which comprises temper rolling a hot-rolled steel sheet produced by hot rolling equipment at a reduction rate of 1.0% or more using dull rolls having a surface roughness Ra of 10 μm or more and a pitch of 6.25 mm or less.

[0008] The chemical composition of the steel sheet may be, in mass %, C: 0.005% or less, Si: 2.5 to 5.0%, Al: 3% or less, Mn: 1.0% or less, S: 0.005% or less, P: 0.02% or less, Ti: 0.01% or less, and the balance: Fe and impurities.

[0009] Alternatively, the chemical composition of the steel sheet may be, in mass %, C: 0.12% or less, Si: 0.75% or less, Mn: 1.0% or less, S: 0.03% or less, P: 0.04% or less, Cr: 16 to 25%, and the balance: Fe and impurities. [Effects of the Invention]

[0010] As described above, according to the present invention, it is possible to more simply suppress the occurrence of ridging in a steel sheet. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is an explanatory diagram showing an example of a steel plate manufacturing facility according to an embodiment of the present invention. [Figure 2] FIG. 10 is a diagram for explaining how to determine the pitch of the roughness of the roll surface, showing an example of the distribution of the surface displacement of the roll. [Figure 3]3 is a diagram for explaining how to determine the pitch of the roughness of the roll surface, showing the results of frequency analysis of the distribution of the surface displacement of the roll shown in FIG. 2. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] 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.

[0013] [1. Equipment configuration] First, the configuration of a steel plate manufacturing facility according to one embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is an explanatory diagram showing an example of a steel plate manufacturing facility 1 according to this embodiment.

[0014] The manufacturing equipment 1 shown in FIG. 1 includes a hot rolling equipment 1A that rolls a heated slab to a predetermined thickness and winds it into a coil, and a temper rolling equipment 1B that temper rolls the hot-rolled steel sheet manufactured by the hot rolling equipment 1A.

[0015] [1-1. Hot rolling equipment] As shown in FIG. 1, the hot rolling equipment 1A includes, in order from the upstream side of the hot rolling equipment 1A, a rolling equipment 10, a runout table cooling zone 20, and a coiler 40.

[0016] The rolling equipment 10 is equipment that rolls slabs heated by a heating furnace (not shown) to form a strip-shaped steel plate. The configuration of the rolling equipment 10 is not particularly limited. For example, the rolling equipment 10 may be composed of only a finishing rolling mill as shown in FIG. 1, or may be composed of a roughing rolling mill and a finishing rolling mill. A descaling device that removes scale generated on the surface of the slab may be installed on the inlet side of the rolling equipment 10 (i.e., on the inlet side of the roughing rolling mill if the rolling equipment 10 is equipped with a roughing rolling mill and a finishing rolling mill, or on the inlet side of the finishing rolling mill if the rolling equipment 10 consists of a finishing rolling mill).

[0017] The steel sheet formed into a strip shape by rolling a slab in the rolling equipment 10 is cooled in the runout table cooling zone 20. The runout table cooling zone 20 has a runout table configured by arranging a plurality of table rolls 21 at predetermined intervals in the sheet passing direction, and the steel sheet rolled in the rolling equipment 10 is cooled while being transported on the runout table. The runout table cooling zone 20 may include a water cooler 23 that water-cools the steel sheet being transported on the runout table, and an edge mask 25 installed between adjacent table rolls 21 corresponding to the water cooler 23.

[0018] The steel sheet that has passed through the runout table cooling zone 20 and cooled is guided by pinch rolls 30 to a coiler 40, which is a coil winding machine, and wound into a coil shape by a mandrel 45 at a predetermined winding temperature to become coil C.

[0019] [1-2. Temper rolling equipment] The temper rolling equipment 1B includes, for example, a payoff reel 51, a skin pass mill 53, and a tension reel 55, as shown in FIG.

[0020] The payoff reel 51 pays out the coil C produced in the hot rolling equipment 1A and sends the steel plate to the skin pass mill 53. The skin pass mill 53 reduces the steel plate. The skin pass mill 53 has a pair of work rolls 53a, 53b and a pair of backup rolls 53c, 53d. The steel plate reduced by the skin pass mill 53 is wound into a coil by a tension reel 55.

[0021] The skin pass mill 53 according to this embodiment uses dull rolls with roughened roll surfaces as the work rolls 53a and 53b.

[0022] Rolling in the skin-pass mill 53 develops a shear texture in the surface layer of the steel sheet. This shear texture changes depending on the friction coefficient between the surfaces of the work rolls 53a and 53b and the steel sheet surface. For example, when rolling with bright rolls, the strength of this shear texture changes depending on the friction coefficient. On the other hand, it has been found that when rolling with rolls that have geometrical irregularities such as dull rolls (i.e., roughened rolls), the shear texture in the surface layer of the steel sheet is disrupted in response to these irregularities, resulting in refinement of the crystal grains.

[0023] Therefore, in the steel sheet manufacturing equipment 1 according to this embodiment, dull rolls are used as the work rolls 53a, 53b of the skin pass mill 53 of the temper rolling equipment 1B, thereby disrupting the shear texture in the surface layer of the steel sheet during temper rolling and preventing coarse grains from remaining in the surface layer of the steel sheet after temper rolling, thereby suppressing the occurrence of ridging in the steel sheet.

[0024] More specifically, the work rolls 53a and 53b are dull rolls having a surface roughness Ra of 10 μm or more and a pitch of 6.25 mm or less, and the rolling reduction of the skin pass mill 53 is 1.0% or more.

[0025] The surface roughness Ra is the arithmetic mean roughness specified in JIS B 0601. The roughness pitch may be determined, for example, by frequency analysis of the distribution of surface displacement in a predetermined direction (e.g., the roll barrel length direction) of the dull roll. More specifically, first, the surface displacement of the roll at each position in the predetermined direction (e.g., the roll barrel length direction) of the dull roll is measured to obtain a surface displacement distribution such as that shown in FIG. 2. The surface displacement distribution is then subjected to frequency analysis using a Fourier transform or the like, thereby identifying the roughness pitch of the dull roll surface. For example, frequency analysis of the surface displacement distribution shown in FIG. 2 results in the relationship between pitch and amplitude as shown in FIG. 3. In the example shown in FIG. 3, the pitch P0 with the largest amplitude is defined as the roughness pitch of the dull roll.

[0026] If the surface roughness Ra of the work rolls 53a, 53b is less than 10 μm, the shear texture in the surface layer of the steel sheet cannot be adequately separated, and coarse grains remain in the surface layer of the steel sheet after temper rolling. Furthermore, if the pitch of the roughness of the roll surfaces of the work rolls 53a, 53b is greater than 6.25 mm, the interval at which the texture is separated becomes large, and coarse grains remain in the surface layer of the steel sheet after temper rolling. Furthermore, when temper rolling is performed using dull rolls with a surface roughness Ra of 10 μm or more and a pitch of 6.25 mm or less, a rolling reduction of 1.0% or more is required to reliably separate the shear texture in the surface layer of the steel sheet. This rolling reduction is greater than that used in general temper rolling. By performing temper rolling under these conditions, the shear texture in the surface layer of the steel sheet can be separated, and ridging in the steel sheet can be suppressed.

[0027] The steel plate manufacturing equipment 1 according to this embodiment has been described above.

[0028] [2. Manufacturing method of hot-rolled steel sheets] In the method for manufacturing a steel sheet according to this embodiment, the hot-rolled steel sheet is temper-rolled at a reduction ratio of 1.0% or more using dull rolls having a surface roughness Ra of 10 μm or more and a pitch of 6.25 mm or less.

[0029] The method for manufacturing a steel sheet according to this embodiment is carried out using, for example, manufacturing equipment 1 shown in FIG. 1 . In hot rolling equipment 1A, a slab heated in a heating furnace (not shown) is rolled by rolling equipment 10 to form a strip-shaped steel sheet. The strip-shaped steel sheet is cooled in a runout table cooling zone 20, then guided by pinch rolls 30 to a coiler 40, and wound into a coil shape by a mandrel 45 at a predetermined coiling temperature to form a coil C. Next, in temper rolling equipment 1B, the coil C manufactured by the hot rolling equipment 1A is paid off by a payoff reel 51, and the hot-rolled steel sheet is sent to a skin-pass mill 53. The sent-out steel sheet is reduced in the skin-pass mill 53 and then wound into a coil shape by a tension reel 55.

[0030] In this way, by temper rolling the hot-rolled steel sheet at a reduction ratio of 1.0% or more using dull rolls having a surface roughness Ra of 10 μm or more and a pitch of 6.25 mm or less, the shear texture in the surface layer of the steel sheet can be disrupted, and the occurrence of ridging in the steel sheet can be suppressed.

[0031] The method for manufacturing a steel sheet according to this embodiment is particularly effective when applied to the production of a steel sheet that does not undergo phase transformation or undergoes only partial phase transformation after casting. Here, a steel sheet that does not undergo phase transformation or undergoes only partial phase transformation after casting refers to a steel sheet whose transformation rate after casting is 50% or less in any temperature range after casting. Note that this transformation rate is defined as the area ratio of the transformed phase in the observation field when observing the cross-sectional microstructure of a steel material when rapidly cooled during transformation.

[0032] An example of a steel sheet having a transformation rate of 50% or less after casting is a steel sheet having a chemical composition, in mass%, of C: 0.005% or less, Si: 2.5 to 5.0%, Al: 3% or less, Mn: 1.0% or less, S: 0.005% or less, P: 0.02% or less, Ti: 0.01% or less, and the balance being Fe and impurities.

[0033] In such steel sheets, C (carbon) is an essential element for increasing the strength of the steel sheet, but an excessive C content deteriorates the material properties, so the C content is set to 0.005% or less.

[0034] Silicon (Si) is a solid solution strengthening element and is effective in improving the material properties of steel sheets. However, excessive Si content degrades formability. Therefore, the Si content is set to 2.5% to 5.0%.

[0035] Al (aluminum) is an effective element for deoxidizing steel. However, excessive content of Al deteriorates formability. Therefore, the Al content is set to 3.0% or less.

[0036] Manganese (Mn) is a powerful austenite stabilizing element, but excessive Mn content forms MnS, which deteriorates the material. Therefore, the Mn content should be 1.0% or less.

[0037] S (sulfur) deteriorates the formability of steel sheet and also forms MnS, which deteriorates the material quality, so the S content is preferably low. The S content is set to 0.005% or less, which is the range in which formability is not significantly deteriorated.

[0038] P (phosphorus) embrittles steel sheets, so a low P content is desirable. The P content is set to 0.02% or less, which is the range in which the embrittlement of steel sheets is not significant.

[0039] Titanium (Ti) is an element that is mixed into steel as an impurity, but if it is contained in excess, it will deteriorate the formability of the steel sheet and will form TiC and TiN, which will deteriorate the material quality. Therefore, the Ti content is set to 0.01% or less.

[0040] Nb (niobium) is a carbide-forming element and is effective in increasing the strength of steel sheets, so it may be added as needed. However, if it is added in excess, the effect saturates and costs increase. Therefore, the Nb content may be 0.001% or less. Furthermore, N (nitrogen) is an element contained as an impurity, but a low content is desirable in order to increase formability and improve material properties. The N content may be 0.002% or less, as this is the range in which formability is not significantly deteriorated.

[0041] Alternatively, an example of a steel sheet having a transformation rate of 50% or less after casting is a steel sheet having a chemical composition, in mass%, of C: 0.12% or less, Si: 0.75% or less, Mn: 1.0% or less, S: 0.03% or less, P: 0.04% or less, Cr: 16 to 25%, and the balance: Fe and impurities.

[0042] In such steel sheets, C (carbon) is an essential element for increasing the strength of the steel sheet, but an excessive C content deteriorates formability, so the C content is set to 0.12% or less.

[0043] Silicon (Si) is a solid solution strengthening element and is effective in increasing the strength of steel sheets. However, excessive Si content degrades formability. Therefore, the Si content is set to 0.75% or less.

[0044] Manganese (Mn) is a powerful austenite stabilizing element, but excessive content reduces formability. Therefore, the Mn content is set to 1.0% or less.

[0045] S (sulfur) deteriorates the formability of steel sheets, so a low S content is desirable. The S content is set to 0.03% or less, which is the range in which formability is not significantly deteriorated.

[0046] P (phosphorus) embrittles steel sheets, so the lower the content, the better. The P content is set to 0.04% or less, as this range does not significantly embrittle the steel sheets.

[0047] Cr (chromium) is an element that is effective in improving the corrosion resistance of steel sheets. However, excessive Cr content can degrade manufacturability. Therefore, the Cr content is set to 16% or more and 25% or less.

[0048] Niobium (Nb) is a carbide-forming element and is effective in increasing the strength of steel sheets, so it may be added as needed. However, if it is added in excess, the effect saturates and costs increase. Therefore, the Nb content may be 0.001% or less.

[0049] Aluminum (Al) is an effective element for deoxidizing steel and may be included as needed. However, excessive inclusion of Al changes the hardenability. Therefore, the Al content may be 1.0% or less. Mo (molybdenum) is an effective element for improving the corrosion resistance of steel sheet and may be included as needed. However, excessive inclusion of Mo may reduce toughness, so the Mo content may be 2% or less. Vanadium (V) forms carbonitrides and has the effect of precipitation strengthening the steel, so may be included as needed. On the other hand, excessive inclusion of V significantly reduces toughness due to precipitation strengthening. Therefore, the V content may be 0.02% or less.

[0050] In steel sheets with such chemical compositions, no or only partial phase transformation occurs after casting, and therefore there is a high possibility that the coarse grains formed during casting will be carried over to the hot-rolled steel sheet or the product. By manufacturing these steel sheets using the steel sheet manufacturing method according to this embodiment, the shear texture in the surface layer of the steel sheet is disrupted, the coarse grains formed during casting can be subdivided, and the occurrence of ridging in the steel sheet can be suppressed. [Example]

[0051] [Verification 1] Several slabs with the chemical compositions (steel types A1 to E1) shown in Table 1 below were prepared and rolled to a thickness of 1.8 mm using test hot rolling equipment. These hot-rolled steel sheets were then temper rolled using a single-stand skin-pass mill as shown in Figure 1 under the conditions (processes No. 1 to 5) shown in Table 2 below. The diameter of the work rolls of the skin-pass mill was kept constant at 750 mm.

[0052] [Table 1]

[0053] [Table 2]

[0054] For steel grades A1 to D1, regardless of the conditions of process Nos. 1 to 5 shown in Table 2, temper rolling resulted in problems other than ridging, and none of the produced steel sheets were suitable for use as products. For steel grade A1, the C content exceeded 0.005%, causing the material to harden excessively, resulting in poor formability. For steel grade B1, the Mn content exceeded 1.0%, causing the material to harden excessively, resulting in poor formability. For steel grade C1, the Si content exceeded 5.0%, causing surface defects. For steel grade D1, temper rolling was possible, but the Al content exceeded 3.0%, causing clogging of the steelmaking nozzle, which affected operations in order to clear the clogging.

[0055] When temper rolling was performed on steel grade E1 under the conditions of process No. 1, ridging occurred in the product because the surface roughness Ra of the skin-pass mill work roll was 9 mm. When temper rolling was performed on steel grade E1 under the conditions of process No. 3, the roughness pitch of the skin-pass mill work roll was 6.50 mm, which was too large, causing ridging in the product. When temper rolling was performed on steel grade E1 under the conditions of process No. 4, the reduction rate in the skin-pass mill was 0.8%, which was too small, causing ridging in the product.

[0056] In contrast, when temper rolling was performed on steel grade E1 under the conditions of process No. 2 or process No. 5, no ridging occurred in the product and a good appearance was obtained.

[0057] Steel types A1 to E1 all had an S content of 0.005% or less, a P content of 0.02% or less, a Ti content of 0.01% or less, and a N content of 0.02% or less. However, if any of these components exceeded the specified value, none of the steels were suitable for use as products. Specifically, if the S content exceeded 0.005% or the P content exceeded 0.02%, the material became embrittled. If the Ti content exceeded 0.01%, the material became too hard and its formability deteriorated. If the N content exceeded 0.02%, surface cracks occurred in the steel sheet.

[0058] Furthermore, when the Si content was less than 2.5%, no coarse grains remained in the hot-rolled steel sheet, and therefore ridging did not occur in the product, regardless of the surface roughness of the work rolls of the skin-pass mill.

[0059] [Verification 2] Several slabs with the chemical compositions (steel types A2 to G2) shown in Table 3 below were prepared and rolled to a thickness of 3.2 mm using a test hot rolling facility. These hot-rolled steel sheets were then temper rolled using a single-stand skin-pass mill as shown in Figure 1 under the conditions (processes No. 1 to 5) shown in Table 2 above. The roll diameter of the skin-pass mill was kept constant at 750 mm.

[0060] [Table 3]

[0061] For steel grades A2 to E2, regardless of the conditions of process Nos. 1 to 5 shown in Table 2, temper rolling resulted in problems other than ridging, and none of the produced steel sheets were suitable for use as products. For steel grade A2, the C content exceeded 0.13%, causing the material to become overhardened and resulting in poor formability. For steel grade B2, the Si content exceeded 0.75%, causing surface defects. For steel grade C2, the Mn content exceeded 1.0%, causing the material to become overhardened and resulting in poor formability. For steel grade D2, the Cr content was less than 16%, resulting in problems with corrosion resistance. For steel grade E2, the Cr content exceeded 25%, causing the material to become overhardened and resulting in poor formability. Furthermore, the use of a large amount of expensive Cr resulted in excessive manufacturing costs.

[0062] When temper rolling was performed on steel grades F2 and G2 under the conditions of process No. 1, ridging occurred in the product because the surface roughness Ra of the skin-pass mill work roll was 9 mm. Furthermore, when temper rolling was performed on steel grades F2 and G2 under the conditions of process No. 3, the roughness pitch of the skin-pass mill work roll was 6.50 mm, which was too large, causing ridging in the product. Furthermore, when temper rolling was performed on steel grades F2 and G2 under the conditions of process No. 4, the reduction rate in the skin-pass mill was 0.8%, which was too small, causing ridging in the product.

[0063] In contrast, when temper rolling was performed on steel types F2 and G2 under the conditions of process No. 2 or process No. 5, no ridging occurred in the products and a good appearance was obtained.

[0064] Steel grades A2 to G2 all contained 0.03% or less of S and 0.04% or less of P, but if the content of either of these elements exceeded the specified value, none of them were suitable for use as products. Specifically, if the S content exceeded 0.03% or the P content exceeded 0.04%, the material became embrittled. If the Mo content exceeded 2%, the material became too hard, causing problems with formability, and the large amount of expensive Mo used resulted in excessive manufacturing costs. If the V content exceeded 0.02%, excessive precipitates formed, causing the material to become too hard.

[0065] In addition, the Al content of steel types A2 to G2 was 1.0% or less in all cases. However, when the Al content exceeded 1.0%, although temper rolling was possible, clogging occurred in the steelmaking nozzle, and the need to clear the clogging affected operations.

[0066] When the Cr content was less than 16% and more than 25%, no ridging occurred in the product, regardless of the surface roughness of the work roll of the skin-pass mill.

[0067] 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]

[0068] 1. Steel plate manufacturing equipment 1A Hot Rolling Equipment 1B Temper rolling equipment 10 Rolling equipment 20 Runout Table Cooling Zone 21 Table Roll 23 Water cooling system 25 Edge Mask 30 pinch rolls 40 Coiler 45 mandrel 51 Payoff Reel 53 Skin pass mill 53a, 53b Work rolls 53c, 53d backup roll 55 Tension reel

Claims

1. A method for producing a steel sheet, comprising temper rolling a hot-rolled steel sheet produced by hot rolling equipment at a rolling reduction of 1.0% or more using dull rolls having a surface roughness Ra of 10 μm or more and a pitch of 6.25 mm or less.

2. The steel plate has a chemical composition, in mass%, C: 0.005% or less, Si: 2.5-5.0%, Al: 3% or less, Mn: 1.0% or less, S: 0.005% or less, P: 0.02% or less, Ti: 0.01% or less, The method for producing a steel sheet according to claim 1, wherein the balance is Fe and impurities.

3. The steel plate has a chemical composition, in mass%, C: 0.12% or less, Si: 0.75% or less Mn: 1.0% or less, S: 0.03% or less, P: 0.04% or less, Cr: 16-25%, The method for producing a steel sheet according to claim 1, wherein the balance is Fe and impurities.

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