JP4804995B2 - Manufacturing method of hot-rolled steel sheet - Google Patents

Description

  The present invention relates to a method for producing a hot-rolled steel sheet suitable for applications that require a beautiful surface, such as an automobile.

  Hot-rolled steel sheets used for automobiles and industrial machines are generally manufactured through a rough rolling process and a finish rolling process. FIG. 4 is a diagram schematically showing a conventional method for producing a hot-rolled steel sheet. 5 (a) to 5 (e) are cross-sectional views showing the state of the steel sheet in each step shown in FIG. 4 in the order of steps, FIG. 5 (a) after rough rolling, and FIG. 5 (b) after descaling. FIGS. 5C and 5D show the state of the product after the finish rolling, and FIG. 5E shows the state of the product after finish rolling. As shown in FIG. 4, in the manufacturing process of a hot-rolled steel sheet, first, a slab obtained by continuously casting a molten steel adjusted to a predetermined composition is rolled by a roughing mill 101, and then a plurality of rolling stands 102a to 102a. A hot rolling steel sheet 107 having a predetermined thickness is obtained by hot rolling with a finish rolling mill 103 constituted by 102 g.

  In such a conventional method for producing a hot-rolled steel sheet, as shown in FIG. 5 (a), a scale 109 made of oxide is generated on the surface of the steel sheet 108 in the rough-rolled steel sheet after the rough rolling. . Since the scale 109 causes wrinkles (scale wrinkles) in the subsequent finish rolling, in the conventional hot rolled steel sheet manufacturing method, the high pressure water 106 is directed from the nozzle 105 of the descaling device toward the rough rolled steel sheet 104. As shown in FIG.5 (b), after removing the scale of the surface of the steel plate 108, finish rolling is performed (for example, refer patent document 1 and 2).

For example, in the method for removing scale of a hot-rolled steel sheet described in Patent Document 1, when the pressure of water when spraying from the spray nozzle is P (kg / cm ² ), the collision pressure of water on the surface of the hot-rolled steel sheet The power consumption of the water supply pump is minimized by adjusting the flow rate of water and the distance between the tip of the injection nozzle and the steel plate so that the value is equal to or greater than A (kg / cm ² ) obtained by the following formula 1. To maintain and reduce operating costs.

  Moreover, in the rolling method of the hot-rolled steel sheet described in Patent Literature 2, the descaling apparatus and the first stage rolling stand of the finishing mill and / or the first stage rolling stand of the finishing mill and the first rolling stand. Generation of scale wrinkles is suppressed by forcibly cooling the center portion in the width direction of the hot-rolled steel sheet that is higher than the critical temperature for generating scale wrinkles between the second rolling stand.

Japanese Patent No. 2780600 Japanese Patent Publication No. 60-48241

  However, even if descaling is performed after rough rolling to remove the scale on the surface, the scale is generated again in the subsequent finish rolling process, and there is a problem that scale wrinkles are generated in the product. Specifically, as shown in FIG. 5C, the scale 109 generated by hot rolling in the first stage rolling stand (hereinafter also referred to as the first stand) 102a is shown in FIG. As shown, it grows between a first stand 102a and a second-stage rolling stand (hereinafter also referred to as a second stand) 102b. Then, the scale 109 on the grown surface is pushed into the steel plate during rolling in the second and subsequent rolling stands. As a result, as shown in FIG. 5 (e), a scale layer 111 having a non-uniform thickness is formed on the surface of the finished product (hot-rolled steel sheet), which is the cause of scale defects and poor appearance. Become.

  Fig.6 (a) is sectional drawing which shows the normal part of a hot-rolled steel plate, FIG.6 (b) is sectional drawing which shows the collar part of a hot-rolled steel plate. As shown in FIG. 6 (a), the scale layer 111 is uniformly formed on the surface of a normal hot-rolled steel sheet. However, as shown in FIG. The thickness of the scale layer 111 is thick, and there is also a part that is 1.3 times or more of the normal part.

  Moreover, when the water cooling apparatus is installed between the 1st stand and the 2nd stand of a finishing rolling process like the technique described in patent document 2, and the width direction center part of the steel plate is forcedly cooled, steel plate Although the surface of the glass is temporarily cooled, there is a problem that the temperature rises due to the recuperation and the scale is likely to be easily returned. Even if the thickness of the scale generated between the first stand and the second stand becomes uniform by this method, scale growth, fragmentation and embedding occur after the second stand. There is a problem that it cannot be completely prevented. Furthermore, in this method, since the outermost surface portion of the steel plate is forcibly cooled, there is also a problem that the characteristics of the product (hot rolled steel plate) after finish rolling are deteriorated.

  The present invention has been devised in view of the above-described problems, and provides a method for producing a hot-rolled steel sheet capable of improving the surface shape by suppressing the occurrence of scale wrinkles during hot rolling. For the purpose.

The method for producing a hot-rolled steel sheet according to the first invention of the present application includes a rough rolling step of roughly rolling a slab produced by continuous casting to obtain a rough rolled steel sheet, and a finish rolling mill comprising the rough rolled steel sheet comprising a plurality of rolling stands. A finish rolling step of hot rolling to obtain a hot-rolled steel plate, and in the finish rolling step, the water pressure is less than 7.8 MPa and the amount of water with respect to the steel plate after passing through the first stage rolling stand. Water is applied under conditions of 200 to 1500 liters / m ² · min, and at least the intermediate position between the first stage rolling stand and the second stage rolling stand is closer to the first stage rolling stand side. By covering the surface of the part with water, the scale thickness of the steel plate immediately before passing through the second stage rolling stand is set to 10 μm or less .

In the present invention, since the surface of the steel plate discharged from the first stage rolling stand is covered with water so that the surface of the steel plate is not directly exposed to the atmosphere, the growth of the scale formed on the surface of the steel plate is prevented. In addition to being suppressed, the reaction between C in steel and O in the atmosphere can be suppressed to prevent the generation of carbon dioxide. Thereby, since generation | occurrence | production of a scale parting and a swelling can be prevented, generation | occurrence | production of a scale flaw can be suppressed and a surface shape can be made favorable.

According to the present invention, the surface of the steel plate discharged from the first stage rolling stand is covered with water so that the surface of the steel plate is not directly exposed to the atmosphere, so that scale division and swelling are prevented. It is possible to produce a hot-rolled steel sheet having no surface defects and having a good surface shape.

  The best mode for carrying out the present invention will be described below with reference to the drawings. This inventor investigated the factor which the thickness nonuniformity generate | occur | produces in the hot-rolled steel plate after a finish rolling process, and acquired the knowledge shown below. 1A to 1C are cross-sectional views schematically showing changes in scale, and FIG. 1A is a diagram between a first stage rolling stand and a second stage rolling stand. FIG. 1B shows the state after the third stage rolling stand, and FIG. 1C shows the state after the third stage rolling stand. As shown in FIG. 1A, when the scale grows between the first stand and the second stand, a bulge 110 may occur on the surface of the steel plate. When the bulge 110 is generated on the surface in this way or the thickness of the scale exceeds a specific value, the scale 109 on the surface is divided by rolling on the second stand as shown in FIG. 1B. Further, as shown in FIG. 1 (c), after the third stage rolling stand (hereinafter also referred to as the third stand), the scale 109 is pushed into the steel plate 108, and the above-described FIG. 5 (e). A scale layer having a non-uniform thickness as shown in FIG.

  That is, when a steel sheet with a scale attached to the surface is rolled, the scale is also stretched as the steel sheet is stretched, but when the scale exceeds a certain thickness, a portion that cannot be stretched is generated and the scale is divided. For this reason, when the scale thickness is thick, even if the thickness is uniform, the scale is divided in the hot rolling at the next rolling stand, resulting in uneven thickness of the scale. Moreover, the limit thickness by which the scale produced | generated on the steel plate surface is parted by rolling changes with additive elements. It is presumed that this is because the adhesion between the steel sheet and the scale changes due to the generation of CO gas due to oxidation of C and the additive elements concentrated on the surface. Then, due to the change in adhesion, a circular bulge of about 0.2 to 30 mm is partially generated on the surface of the scale. Since the portion where the surface is swollen has a different ductility from the other portions, the thickness unevenness occurs in the hot rolling at the next rolling stand.

  Therefore, the present inventor conducted an intensive study in order to solve the above-described causes of unevenness of the two scale thicknesses, and as a result, by suppressing an increase in scale thickness that occurs between the first stand and the second stand. The present inventors have found that scale division occurring after the second stand can be prevented. In addition, the reaction between carbon in steel and oxygen in the atmosphere, which reduces the adhesion of the scale, is likely to occur immediately after hot rolling in the first stand, and in this part, contact between the steel plate surface and the atmosphere should be prevented. It was found that it can be suppressed.

  First, a method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention will be described. In the following description, mass% in the composition is simply described as%.

  FIG. 2 is a diagram schematically showing a rough rolling process and a finish rolling process in the method for producing a hot-rolled steel sheet of the present embodiment. 3 (a) to 3 (d) are sectional views showing the state of the steel plate in each step shown in FIG. 2 in the order of steps, and FIGS. 3 (a) to 3 (c) are views of the first stand and the second stand. Meanwhile, FIG. 3 (d) shows a state between the second stand and the third stand. In addition, in FIG. 2, components other than the work roll in the rough rolling mill 1 and each rolling stand 2a-2g are abbreviate | omitting illustration.

As shown in FIGS. 2 and 3, in the method for producing a hot-rolled steel sheet according to the present embodiment, first, a slab produced by continuous casting or the like and having a thickness of, for example, about 250 mm is heated in a heating furnace, and then roughened. Rough rolling is performed by the rolling mill 1 to obtain a rough rolled steel sheet 4 having a thickness of, for example, about 40 mm. Next, high pressure water 6 having a water pressure of 9.8 MPa (100 kgf / cm ² ) or more is sprayed from the nozzle 5 of the descaling device toward the rough rolled steel plate 4 to remove the scale generated on the surface of the rough rolled steel plate 4. .

Thereafter, the rough rolled steel sheet 4 is hot-rolled by a finish rolling mill 3 in which seven rolling stands 2a to 2g are arranged at substantially equal intervals in the rolling direction of the steel sheet, for example, hot rolled with a thickness of 2 to 5 mm. Get a steel plate. And in this finish rolling process, the nozzle 10 is arrange | positioned immediately after the 1st stand 2a, the water 11 is flowed out from this nozzle 10, and the surface of the steel plate after 1st stand 2a is coat | covered with the film | membrane of water. At that time, water is allowed to flow out under conditions of a water pressure of less than 7.8 MPa (80 kgf / cm ² ) and a water amount of 200 to 1500 liters / m ² · min.

If the surface temperature of the steel sheet is greatly lowered between the first and second stands, the workability and various physical properties in the subsequent hot rolling are reduced, so the reduction of the surface temperature of the steel sheet is within 20 ° C, preferably 10 ° C. However, if the water pressure flowing out from the nozzle 12 is 7.8 MPa or more, the water that collides with the steel sheet will rebound strongly, making it difficult to form a water film on the steel sheet surface. . Further, since water repelling occurs on the surface of the steel sheet, a water film cannot be formed on the surface of the steel sheet, and furthermore, water contacts locally to cause a temperature drop, leading to deterioration of the surface shape. Therefore, the effect of the present invention cannot be obtained when a method of injecting water at a high pressure from the nozzle 12 as in descaling is applied. Moreover, when the amount of water is less than 200 liters / m ² · min, it becomes difficult to form a water film on the steel sheet surface. On the other hand, when the amount of water exceeds 1500 liters / m ² · min, the effect is saturated.

  Further, it is desirable that the surface of the steel plate is covered with water immediately after passing through the first stand 2a. Specifically, the steel plate surface is covered with water within 0.2 seconds after passing through the first stand 2a. It is desirable to do. When the time from when the steel sheet is discharged to when it is covered with water exceeds 0.2 seconds, the adhesion of the scale on the steel sheet surface is reduced, which promotes scale separation.

  In the method for producing a hot-rolled steel sheet according to the present embodiment, the surface of the steel sheet does not need to be covered with water in the entire period from the first stand 2a to the second stand 2b. What is necessary is just to be covered with water from immediately after 2a to the middle position between the first stand 2a and the second stand 2b. This is because the portion on the second stand 2b side from the intermediate position is less likely to cause scale division than the portion on the first stand 2b side.

  The hot-rolled steel sheet thus finish-rolled is cooled by a cooling device in a state of being placed on a hot run table, and then wound in a coil shape by a winding device.

  In the method for manufacturing a hot-rolled steel sheet according to the present embodiment, a water film is formed on the surface of the steel sheet 8 after passing through the first stand 2a to prevent contact between the steel sheet 8 and the atmosphere. As shown in (d) to (d), even after hot rolling in the second stand 2b, the thickness of the scale 9 is almost the same as that after hot rolling in the first stand 2a. Thereby, generation | occurrence | production of the scale parting at the time of the hot rolling after the 3rd stand 2c can be suppressed. Moreover, since the reaction between C in the steel plate 8 and O in the atmosphere is prevented, it is possible to prevent a decrease in adhesion between the first stand 2a and the second stand 2b. As a result, generation of scale wrinkles during hot rolling can be suppressed, so that a hot-rolled steel sheet having a good surface shape can be obtained.

  In the method for producing a hot-rolled steel sheet according to the present embodiment, for example, the steel composition exceeds C: 0% and is 0.02% or less, Si: 0.001 to 0.9%, Mn: 0.1 to 2 0.5%, P: more than 0% to 0.007% or less, S: more than 0% to 0.004% or less, Al: 0.01 to 0.1% and N: more than 0% to 0.01% or less And, if necessary, at least one selected from the group consisting of Cr: 0.02-0.5%, Cu: 0.02-1.6 and Ni: 0.02-1% In the case of a low C steel plate with a small amount of C in the steel, such as a steel plate containing elements and the balance being Fe and unavoidable impurities, the second stand 2b shown in FIG. The thickness of the scale 9 of the steel sheet hot-rolled by the stand 2b is desirably 25 μm or less.

Further, for example, the steel composition is C: 0.04 to 0.35%, Si: 0.02 to 0.9%, Mn: 0.1 to 2.5%, P: more than 0% and 0.007% Hereinafter, S: more than 0% and 0.004% or less, Al: 0.01 to 0.1%, N: more than 0% and 0.01% or less, and Cr: 0.02 to 0.15% Further, if necessary, it contains at least one element selected from the group consisting of Cu: 0.02 to 1.6 and Ni: 0.02 to 1%, the balance being Fe and inevitable impurities In the case of a steel sheet to which an element that has an effect of improving the adhesion of the scale is added, such as a steel sheet, swelling is likely to occur as compared with the low C steel described above. It is desirable to narrow the thickness tolerance. Specifically, in the steel plate having such a composition, it is desirable that the thickness of the scale 9 immediately before the second stand 2b is 20 μm or less. For this purpose, the water pressure flowing out from the nozzle 12 is less than 5 MPa, the amount of water Is preferably 1000 to 1500 liters / m ² · min.

Further, for example, the steel composition is C: 0.04 to 0.35%, Si: 0.001 to 2.5%, Mn: 0.1 to 2.5%, P: more than 0% and 0.05% Hereinafter, S: more than 0% and not more than 0.02%, Al: 0.01 to 0.1% and N: more than 0% and not more than 0.01%, and if necessary, Cr: 0.00%. Contains at least one element selected from the group consisting of 02 to 0.5%, Cu: 0.02 to 1.6, and Ni: 0.02 to 1%, with the balance being Fe and inevitable impurities In the case of a general steel plate, scale splitting and swelling are likely to occur as compared with the low C steel plate and the adhesion improving element-containing steel plate described above, so the allowable range of the thickness of the scale 9 immediately before the second stand 2b is further increased. It is desirable to make it narrower. Specifically, in the steel plate having such a composition, it is desirable that the thickness of the scale 9 immediately before the second stand 2b is 10 μm or less. For this purpose, the water pressure flowing out from the nozzle 12 is less than 5 MPa, the amount of water Is preferably 1000 to 1500 liters / m ² · min.

In the method for producing a hot-rolled steel sheet according to the present embodiment, water 11 flows out from the nozzle 10 and the steel sheet surface is covered with a water film. However, the present invention is not limited to this, and instead of water. Further, the surface of the steel sheet may be covered with a gas having an oxygen content of 0.1% by volume or less, such as Ar gas and N ₂ gas. In that case, the oxygen content is less than 5 MPa of gas of 0.1 volume% or less, and the flow rate is 100-1500 liters / m ² · min. When the pressure of the gas flowing out from the nozzle 12 is 5 MPa or more, the effect is saturated. Moreover, when the gas flow rate is less than 100 liter / m ² · min, the effect of preventing the growth of the scale by preventing the oxidation of the steel sheet surface cannot be obtained. On the other hand, when the gas flow rate exceeds 1500 liters / m ² · min, the effect is saturated. Further, when a gas having an oxygen content of 0.1% by volume or less is used, it is desirable that the surface of the steel plate is covered with the gas immediately after passing through the first stand 2a, as in the case of water. It is desirable to coat the steel sheet surface with a gas having an oxygen content of 0.1% by volume or less within 0.2 seconds after passing through the first stand 2a. In addition, when the gas used instead of water is a gas having an oxygen content of more than 0.1% by volume, the effect of suppressing oxidation on the steel sheet surface is reduced, which is not preferable.

  Moreover, in the manufacturing method of the hot-rolled steel sheet of the embodiment, the case where hot rolling is performed by a finishing mill provided with seven rolling stands has been described, but the present invention is not limited to this, and finishing is performed. The rolling mill may be provided with two or more rolling stands.

Examples of the present invention will be described below. First, as an example of the present invention, a hot-rolled steel sheet was produced by the manufacturing method of the first embodiment described above. Specifically, a slab having a composition shown in Table 1 below and having a thickness of 250 mm was heated to 1220 ° C., and then roughly rolled to a thickness of 35 mm by a rough rolling machine to prepare a rough rolled steel sheet. Next, water is sprayed toward the rough rolled steel sheet at a pressure of 13.6 MPa to remove the scale formed on the surface, and then the steel sheet is immediately passed between 7 rolling stands and hot rolled. Rolling was performed to produce a hot-rolled steel sheet having a thickness of 3.2 mm. At that time, in the finish rolling process, a nozzle is arranged immediately after the first stand, and within 0.2 seconds after the steel plate passes through the first stand, the water pressure from the nozzle is 2.9 MPa and the water amount is 1000 liters / m ² · min. Water was applied and the surface of the steel sheet was covered with a water film (Condition I). In addition, as a comparative example of the present invention, a slab having a composition shown in Table 1 below and having a thickness of 250 mm is roughly rolled by the same method and conditions as in the above-described example, and then water is not applied after discharging the first stand. It was finish-rolled by a conventional method to produce a hot-rolled steel sheet having a thickness of 3.2 mm (Condition II). Further, as a comparative example, in the finish rolling process, a nozzle is arranged immediately after the first stand, and within 0.2 seconds after the steel plate passes through the first stand, the water pressure from the nozzle is 9.8 MPa, the water amount is 1000 liters / m ² · min. A hot rolled steel sheet having a thickness of 3.2 mm was prepared in the same manner as in the above-described example except that water was applied under the above conditions and the surface of the steel sheet was covered with a water film (Condition III). The balance in the steel composition shown in Table 1 below is Fe and inevitable impurities. Moreover, the underline in the following Table 1 shows that it is outside the scope of the present invention.

  And about each hot-rolled steel plate of the Example and comparative example produced with the above-mentioned method, the scale thickness just before the 2nd stand 2b, the presence or absence of a swelling, and the presence or absence of the scale flaw in the manufactured hot-rolled steel plate were confirmed. . The results are summarized in Table 2 below. In Table 2 below, the case where blisters or scale wrinkles occurred is indicated by ×, and the case where these did not occur is indicated by ○.

  As shown in Table 2 above, Example No. in which water was applied to the steel sheet immediately after passing through the first stand under the conditions (Condition I) within the scope of the present invention. 1-No. All the hot rolled steel sheets of No. 3 did not swell or scale during hot rolling, and had a good surface shape. On the other hand, Comparative Example No. 2 was prepared without applying water to the steel plate immediately after passing through the first stand (Condition II). All the 4-6 hot-rolled steel sheets had a scale thickness exceeding 10 μm, and swollen and scale wrinkles occurred during hot rolling. Moreover, the steel plate immediately after passing through the first stand was sprayed with water at a water pressure exceeding the range of the present invention (Condition III). 7-No. The hot rolled steel sheet of No. 9 had a scale thickness of 10 μm or less, but swollen and scale wrinkles occurred. 1-No. The surface shape was inferior to the hot-rolled steel plate No. 3.

(A)-(c) is sectional drawing which shows the change of a scale typically, (a) is between a 1st rolling stand and a 2nd rolling stand, (b) is 2nd Between the stage rolling stand and the third stage rolling stand, (c) shows the state after the third stage rolling stand. It is a figure which shows typically the rough rolling process and the finish rolling process in the manufacturing method of the hot rolled sheet steel of embodiment of this invention. (A)-(d) is sectional drawing which shows the state of the steel plate in each process shown in FIG. 2 in order of a process, (a)-(c) is between a 1st stand and a 2nd stand, (d) is. The state between a 2nd stand and a 3rd stand is shown. It is a figure which shows the manufacturing method of the conventional hot-rolled steel sheet typically, (A)-(e) is sectional drawing which shows the state of the steel plate in each process shown in FIG. 4 in order of a process. (A) is sectional drawing which shows the normal part of a hot-rolled steel plate, (b) is sectional drawing which shows the collar part of a hot-rolled steel plate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,101 Rough rolling mill 2a-2g, 102a-102g Rolling stand 3,103 Finish rolling mill 4,104 Rough rolled steel plate 5,10,105 Nozzle 6,106 High pressure water 7,107 Hot rolled steel plate 8,108 Steel plate 9, 109 Scale 11 Water 110 Swell 111 Scale layer

Claims (2)

A rough rolling step of roughly rolling a slab produced by continuous casting to obtain a rough rolled steel sheet,
A finish rolling step of obtaining the hot-rolled steel sheet by hot rolling with a finish rolling mill composed of a plurality of rolling stands.
In the finish rolling step, the steel plate after passing through the first stage rolling stand is subjected to water under conditions of a water pressure of less than 7.8 MPa and a water amount of 200 to 1500 liters / m ² · min. By covering the surface of the portion closer to the first-stage rolling stand than the intermediate position between the first-stage rolling stand and the second-stage rolling stand with water, the second-stage rolling stand A method for producing a hot-rolled steel sheet, wherein the steel sheet has a scale thickness of 10 μm or less immediately before passing . The method for producing a hot-rolled steel sheet according to claim 1, wherein the steel sheet is coated within 0.2 seconds after passing through the first stage rolling stand.

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