JP2005262273A - Controlled cooling method for differential thickness steel plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for obtaining a differential thickness steel plate superior in flatness by means of a controlled cooling. <P>SOLUTION: When the differential thickness steel plate, whose thickness is different in a longitudinal direction, is hot rolled, and then, is subjected to the controlled cooling immediately, in a roller straightening machine installed on the entrance end of the controlled cooling equipment, the straightening is performed with a passing speed synchronized to that of the controlled cooling equipment, wherein the position of a roll in the straightening machine is adjusted in accordance with the thickness of the steel plate passing therein. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a controlled cooling method in which correction is performed before cooling when a differential thickness steel sheet having a thickness changed in the longitudinal direction of the steel sheet, particularly a thick differential thickness steel sheet, is uniformly cooled at high speed.

  So-called controlled cooling is performed in which a thick steel plate after hot rolling is immediately cooled to adjust the material. In particular, in a differential thickness steel plate used as a structural material for ships, bridges, and the like, it is effective to apply control cooling to the adjustment of the material. Here, the difference thickness steel plate is a thick steel plate in which portions having different thicknesses in the longitudinal direction continue through a taper portion in order to vary the strength mainly by the portion, or the thickness from the front end to the rear end of the steel plate. Tapered steel plates that gradually decrease (increase) are meant, and these are collectively called differential steel plates.

When performing controlled cooling after hot rolling of this differential thickness steel plate, it is important that the steel plate after cooling is flat. This is because if it is not flat, it will hinder the processing at the customer, etc., and the undulation of the steel sheet is required to be within an allowable range.
In order to ensure the flatness of the steel plate after controlled cooling, it is fundamental to achieve uniform cooling, but it is also important to flatten the steel plate in advance prior to this cooling.

As a method for ensuring the flatness before cooling, for example, in Patent Document 1, the correction reaction force acting on the leveling roll is kept constant, and the correction speed is adjusted based on the thickness of the tapered steel sheet. Secondly, in a method for manufacturing a differential thickness steel sheet hot, it is proposed to reheat the difference thickness forming portion (thin portion) between passes to obtain flatness.
JP-A-8-257637 JP-A-62-250004

  The method proposed in Patent Document 1 that keeps the correction reaction force acting on the leveling roll constant and adjusts the correction speed based on the thickness of the tapered steel plate is considered to surely provide a correction effect. However, when the correction is performed on the inlet side of the controlled cooling, the controlled cooling is continuously performed immediately after the correction. Therefore, the correction speed is a plate passing speed regulated by cooling. Therefore, it is actually difficult to change the correction speed according to the plate thickness in order to ensure the flatness of the steel plate, and this method is not suitable for the control coolant.

  In addition, the method of obtaining the flatness by reheating the difference thickness forming portion (thin portion) between passes when manufacturing the difference thickness steel plate hot in the proposal of Patent Document 2 is performed by reheating. Is not preferable because there is a possibility of change.

  Furthermore, these prior arts are merely descriptions relating to ensuring the flatness of the differential thickness steel plate, and are insufficient from the viewpoint of stably producing the differential thickness steel plate with excellent flatness by controlled cooling. This is because only the rolling method of the differential thickness steel sheet is considered, and the combination with the control cooling necessary for obtaining the required material with a small number of alloy elements is not considered.

As described above, in order to ensure the flatness of the differential thickness steel sheet and to achieve uniform material quality at a low cost, it is indispensable to apply controlled cooling to the differential thickness steel sheet. Both technologies have the problem that these cannot be realized.
Therefore, an object of the present invention is to propose a method for obtaining a differential thickness steel plate having excellent flatness by controlled cooling in order to solve the above-described problems.

That is, the present invention provides a roller straightening machine provided on the inlet side of the control cooling equipment when hot-rolling the differential thickness steel sheets having different thicknesses in the longitudinal direction and then immediately performing the control cooling. When performing correction at a plate feed speed synchronized with the plate feed speed, the thickness of the roll in the straightening machine is adjusted according to the thickness of the steel sheet passing through the straightening machine. This is a controlled cooling method.
The roller straightener is a device that performs correction by repeatedly bending in the longitudinal direction of the steel sheet, as represented by a leveler, from a plurality of rolls that are spaced apart in the longitudinal direction and the vertical direction. It is configured.

  According to the present invention, in the controlled cooling of the differential thickness steel sheet, the roll position in the roller straightening machine is changed according to the thickness of the steel sheet passing therethrough to perform appropriate correction, and thus the flatness after the controlled cooling is excellent. It becomes possible to manufacture the differential thickness steel plate stably.

Details of the present invention will be described below.
Now, it is inevitable that the rolled shape of the differential thickness steel plate is disturbed. In particular, with a difference thickness steel plate with a large difference in plate thickness, or with a unidirectional two-stage, convex or concave difference thickness steel plate whose cross-sectional shapes are shown in FIGS. Is difficult.

  Therefore, in order to ensure such a shape after the controlled cooling of the differential thickness steel plate, it is naturally required to cool at a high speed and uniformly in the controlled cooling. For example, when cooling is performed in a controlled cooling facility in which a plurality of pairs of upper and lower cooling nozzles are arranged and upper and lower draining roll pairs sandwiching the steel plate from above and below are arranged on the exit side of each cooling nozzle pair Furthermore, it is necessary to perform uniform cooling on the front and back surfaces of the steel plate.

  Furthermore, securing the shape before cooling is also important. For this reason, as shown in FIG. 2, a differential thickness steel plate (hereinafter simply referred to as a steel plate) 2 exiting the hot rolling mill 1 is introduced into the controlled cooling facility 3. Prior to this, it is necessary to make the shape of the steel plate 2 flat by using the roller straightening machine 4 on the entry side of the equipment 3. In addition, in FIG. 2, the code | symbol 5 is a straightening machine which further arranges the shape of the steel plate 2 after control cooling, and can also be further corrected as needed.

Therefore, in the straightening machine 4 described above, it is important to adjust the position of the roll according to the thickness of the steel plate 2 through which the straightening machine passes.
That is, for example, as shown in FIG. 3, the straightening machine 4 includes a plurality of seven rolls 4 a to 4 g in the illustrated example so as to sandwich the sheet passing lines of the steel plate 2 alternately, and these rolls 4 a to 4 g. The shape of the steel plate 2 can be corrected by repeatedly bending the steel plate 2 through the group.

  When the straightening machine 4 having such a configuration is fed and corrected at a sheet feeding speed synchronized with the sheet feeding speed in the control cooling equipment 3 in the subsequent process, as shown in FIG. Accordingly, by continuously adjusting the positions of the rolls 4a to 4c arranged on the upper and lower sides, the same bending strain is given regardless of the plate thickness variation, and the steel plate before control cooling is surely corrected. This is the greatest feature of the present invention.

  In the example of FIG. 3, the case of the differential thickness steel plate 2 having a two-stage difference thickness portion (tapered portion) in the longitudinal direction is shown. However, as shown in FIG. In the taper portion T1 where the thickness changes from the thick portion to the thin portion after the positions of the rolls 4a to 4c are sequentially shifted upward and held as they are in the parallel portion, the taper portion T1 changes to the thin portion. As shown in the figure (b), the position is shifted downward, the roll position is controlled following the change in the steel plate thickness, and a certain bending strain is applied. In particular, since the thin part is often stretched in the rolled shape of the differential thickness steel sheet, correction of the thin part is indispensable in order to ensure the steel sheet shape over the entire length.

  Here, in order to ensure the flat shape of the steel sheet, it is desirable to set the roll indentation amount to 6 mm to 2 mm, and it is desirable to change these roll indentation amount setting values depending on the shape, thickness and width of the steel sheet. In addition, the roll push-in amount in the straightening machine is defined by the plate thickness minus (the difference in height between the lower end of the adjacent upper roll and the upper end of the lower roll). Further, a flat steel plate is obtained usually by decreasing the roll pressing amount from upstream to downstream so that the bending strain gradually decreases.

  Incidentally, the adjustment of the roll push-in amount in the roller straightening machine is performed by moving the positions of the rolls arranged on the upper side of the sheet passing line, that is, rolls 4a to 4c in FIG.

  In the control cooling line shown in FIG. 2, the steel plate 2A having the shape and dimensions shown in FIG. 4 and the steel plate 2B having the shapes and dimensions shown in FIG. Controlled cooling including lateral correction was applied. In Tables 1 and 2, the upper roll position is the average of the heights based on the pass lines of a plurality of upper rolls, and the cooling start and stop temperatures are the length of the steel sheet surface temperature on the cooling equipment entry side. It is a longitudinal direction average value of a direction average value and a steel plate surface temperature on the cooling facility delivery side.

Subsequently, about the steel plate after each control cooling, the longitudinal direction temperature deviation which is the difference of the maximum temperature in the longitudinal direction of the width direction center part temperature and minimum temperature was investigated. As shown in Table 1, the results of the survey show that in the method of adjusting the upper roll position of the straightening machine, uniform cooling was achieved with a temperature deviation in the longitudinal direction of 10 ° C. According to the law, the value was as large as 40 ° C. That is, according to the invention method, since the upper roll position of the straightening machine is continuously adjusted according to the thickness of the plate steel plate, it is possible to correct the entire length of the steel plate, so that uniform cooling can be realized. On the other hand, in the conventional method, since only the thickest part was corrected, uniform cooling could not be realized due to a shape defect in the thin part.
The above evaluation results are the same in the case of the steel plate 2B shown in Table 2.

FIG. 6 shows a comparison of the cold straightening rate between the inventive method and the conventional method. The target material was a unidirectional steel plate for shipbuilding shown in FIG. 4, and this was corrected prior to controlled cooling under the conditions shown in Table 1. With the conventional method, the correction rate was about 65%, but with the invention method, a reduction of about 35% was achieved.
Here, the cold straightening rate is the ratio of the number of steel sheets that have been straightened with a cold straightening machine for a shape defect.

Thus, the method of the present invention can obtain excellent flatness as compared with the conventional method.
Further, as a matter of course, the steel plate produced by the method of the present invention has a good value with less variation in the plate than the steel plate produced by the conventional method.

It is a figure which shows the cross-sectional shape of various difference thickness steel plates. It is a figure which shows a control cooling line. It is a figure which shows the correction machine arrange | positioned at the entrance side of control cooling equipment. It is a figure which shows the shape and dimension of a difference thickness steel plate. It is a figure which shows the shape and dimension of a difference thickness steel plate. It is a figure which shows a cold correction rate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Hot rolling mill 2 Steel plate 3 Control cooling equipment 4 Straightening machine 4a-4g Roll

Claims (1)

  When performing controlled cooling immediately after hot-rolling the differential thickness steel sheets having different thicknesses in the longitudinal direction, the roller straightening machine provided on the inlet side of the controlled cooling equipment is synchronized with the feeding speed in the controlled cooling equipment. A method for controlling and cooling a differential thickness steel sheet, comprising: adjusting the position of a roll in the straightening machine according to the thickness of the steel sheet passing through the straightening machine when performing straightening at a plate speed.

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