JPH0222424A - Roll cooling method for hoop - Google Patents

Abstract

PURPOSE:To control cooling speed of a hoop without lowering line speed by passing the hoop through plural cooling rolls and changing pushing rate or roll interval of the cooling rolls in accordance with thickness of the hoop. CONSTITUTION:The hoop is continuously passed through plural cooling rolls. In the roll cooling method for the hoop, the pushing rate of the cooling rolls and/or roll interval are changed so that the cooling speed of the steel strip becomes to the preset optimum cooling speed. By this method, contacting angle and non-contacting length between the steel strip and the cooling roll are changed. Further, by this method, the cooling speed of the hoop can be controlled without lowering the line speed.

Description

[Detailed description of the invention] [Industrial application field]

This invention relates to a method for cooling a high-temperature steel strip during continuous annealing of the steel strip, etc. When the steel strip is cooled by a plurality of internally cooled rolls, the indentation amount and roll spacing of the rolls are changed to cool the steel strip. This invention relates to a roll cooling method for steel strip that controls the cooling rate.

[Conventional technology]

The roll cooling method, which is used as a method for cooling high-temperature steel strip in continuous annealing furnaces, is a method in which the steel strip is cooled by passing it through multiple cooling rolls arranged in a staggered manner, and the equipment cost is low. It has the characteristics of low energy consumption and good surface quality of the steel strip. By the way, in continuous annealing furnaces for steel plates, the cooling rate has a great effect on the material quality of the product, so in addition to equipment costs and energy consumption, the cooling rate of the steel strip is also one of the important management items. For example, in the case of cold-rolled steel sheets for drawing, if the heat pattern is such that the secondary cooling end temperature and overaging temperature are almost the same, a cooling rate of about 80°C/sec is required from the viewpoint of aging resistance and elongation. Appropriate. However, in the current roll cooling method, it has been impossible to control the cooling rate of the steel strip for the reasons described below. That is, heat transfer from the steel strip to the roll in the roll cooling method is heat transfer from solid to solid, but the heat transfer coefficient does not change much even if various conditions are changed, so it is almost constant. Since it is not possible to change the surface roughness due to problems such as occurrence of scratches on the steel strip, the heat transfer coefficient is a constant that cannot be changed. Further, the cooling rate of the steel strip while in contact with the roll is proportional to the heat transfer coefficient and the difference between the steel strip temperature and the roll surface temperature, and is inversely proportional to the plate thickness. Therefore, in order to control the temperature of the steel strip in contact with the rolls for any given steel strip temperature and thickness, the heat transfer coefficient or the roll surface temperature must be controlled. In addition, even if the temperature of the cooling medium (generally water) is changed, the difference between the roll surface temperature and the steel strip temperature does not change much, and the difference between the roll inner surface and the cooling medium is a constant that cannot be controlled in practice. For reasons such as the fact that even if the heat transfer coefficient is changed, the effect on the roll surface temperature is small, the cooling rate of the steel strip cannot be sufficiently controlled by controlling the roll surface temperature. For the above reasons, it is extremely difficult to greatly change the cooling rate of the steel strip by controlling the roll surface temperature, or to obtain the same cooling rate from thin to thick materials. Therefore, as a method for controlling the cooling rate of the steel strip, a method was first proposed in which the cooling rate of the steel strip is controlled by changing the moving length of the steel strip by changing the pushing amount of the cooling roll. (Japanese Unexamined Patent Publication No. 166830, 1983)
. In this method, the length of movement of the steel strip from the cooling start point to the cooling end point is changed by changing the vertical position of the rolls while keeping the total contact circumferential length between the cooling roll and the steel strip constant.
This method has the advantage that the average cooling rate of the steel strip can be changed without changing the line speed or the amount of temperature drop of the steel strip, and the material quality of the product can be improved without changing production costs. [Problems to be Solved by the Invention] However, the above-mentioned method of varying the average cooling rate of the steel strip by changing the indentation amount of the cooling roll cannot achieve the same cooling rate from thin to thick plates. If you try to do so, the disadvantage is that you will be forced to reduce production. The reason is explained below. As shown in Figure 3, the above method is a method in which the total contact circumference length is adjusted by changing the wrapping angle θ of the steel strip (2) around each roll by moving the cooling roll (1) up and down. ,
In this case, the cooling rate of the roll alone Ec, the roll contact time t
The contact circumference 21 and roll contact angle (wrapping angle) θ per 10-ru are determined from the following equations (1) to (4).・・・・・・・・・(1) t = T, -T2 ・・・・・・・・・(2) E, α, heat transfer coefficient (K cat/m2-h −'C )T
1: Cooling start temperature ('C) T2: Cooling end temperature ('C) T3: Roll surface temperature ('C) h, plate thickness (mm) CP: Specific heat (Kcal/-・℃) ■, line speed (m /min) N: Number of rolls D = Roll diameter (m) Note that the temperature drop in the non-contact area is small and can be ignored. Furthermore, the non-contact long history can be determined from the following equations (5) and (6) based on the roll contact angle θ and the roll spacing. O = Yu One death, 7 cities: No-contact time. From the above relational expression, for example, if a roll with a large heat transfer coefficient α is designed to achieve an appropriate cooling rate for a thick plate, the average cooling rate Ec' will be large for a thin plate. . For this reason, current methods include changing the temperature and flow rate of the cooling medium to change the heat transfer coefficient α, and spraying opposed gas jets on the opposite side of the surface that comes into contact with the rolls. There is a limit to this, and the average cooling rate must be adjusted by lowering the line speed V. Therefore, production volume is forced to decrease. This invention was made in view of the problems of the conventional technology, and its purpose is to reduce the line speed and control the cooling rate for a wide range of plate thickness changes from thin plates to thick plates. This is an attempt to propose a method that can be used without any problems.

[Means to solve the problem]

When cooling a steel strip by continuously passing it through multiple cooling rolls placed at appropriate intervals, rolls with a large heat transfer coefficient α are used to cool the thick plate at an appropriate cooling rate. If the plate is thin, the average cooling rate Ec'
becomes large. There is a limit to the change in cooling rate caused by changing the plate thickness by changing the amount of push of the cooling roll or adjusting the temperature and flow rate of the cooling water, but increasing the distance between the rolls will increase the non-contact time of the steel strip. Therefore, the average cooling rate E of the thin material is
This invention was discovered based on this knowledge, and its gist is to reduce the cooling rate of the steel strip to a predetermined optimum cooling rate.
This is a method of controlling the cooling rate of the steel strip by changing either or both the pushing amount of the cooling roll and the roll interval.

[For production]

The roll cooling method of the present invention will be explained with reference to FIG. In Figure 1, three cooling rolls (1-1) with the same radius r
(1-2) (1-3) are arranged at a roll interval, and the steel strip (2) is wound around and in contact with these three cooling rolls while moving in the direction of the arrow. Among the three cooling rolls, the cooling rolls at both ends (1-1)
(1-3) has only the roll spacing variable, and the central cooling roll (1-2) has variable roll spacing and roll pushing amount. In order to increase the non-contact time in order to lower the average cooling rate Ec' of the thin material, either the non-contact circumference 2 in the equation (8) is increased or the line speed V is decreased. However, reducing line speed leads to a decrease in productivity, so
In this invention, a method of increasing the contact circumference is adopted. Since it is necessary to increase the distance between the rolls in order to increase the non-contact circumference, this invention adopts a method of widening the distance between the rolls as a means of lowering the average cooling rate Ec' of the thin material. That is, when roll-cooling a thin article, the cooling roll (1) is moved horizontally as shown by the broken line in FIG. 1 to widen the distance between the rolls. By widening the roll interval, the non-contact circumference 2 of the steel strip (2) becomes longer, so the steel strip (2)
The average cooling rate Ec' of can be lowered. In addition, in the case of a thick steel strip, the cooling roll (1) is similarly moved horizontally as shown by the solid line in the figure to narrow the distance between the rolls. By narrowing the roll spacing, steel strip (
Since the contact angle θ of 2) becomes smaller and the contact circumference r11 becomes longer, the average cooling rate can be increased. Therefore, by changing the distance between the rolls, it is possible to cool the rolls from thin to thick materials at the same line speed. The pushing amount of the cooling roll (1) is changed as appropriate depending on the plate thickness and the roll spacing. In the present invention, as a method for varying the pushing amount of the cooling roll and the roll interval, for example, a cylinder type can be adopted. [Example] Using the experimental layout shown in Fig. 2, which consists of five cooling rolls of 300 mm diameter, plate thicknesses of 1.6 mm and 0.8 mm were prepared.
Table 1 shows the average cooling rate when the strip was cooled under the conditions shown in Table 1, in comparison with a conventional method in which the roll interval was constant and only the contact angle θ was changed. In Figure 2, the solid line indicates the roll position for a steel strip with a thickness of 1.6 mm, and the broken line indicates the roll position for a steel strip with a thickness of 0.8 mm, and the roll spacing is 0.34 mm and 0.44 m, respectively.
It was set to m. In addition, in the conventional method, the roll spacing was set to 0.34 mm (-
set). As is clear from Table 1, even for thin steel strips, an appropriate average cooling rate was obtained by changing the roll spacing.

【Effect of the invention】

As explained above, according to the method of the present invention, by changing the roll interval, an appropriate cooling rate can be obtained even for thin materials at the maximum line speed. It is possible to cool a wide range of sheet thicknesses at high speed, and it is highly effective in the primary cooling treatment of continuous annealing lines for cold-rolled steel sheets for drawing.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the roll cooling method of this invention, FIG. 2 is a schematic diagram showing a roll cooling device in an embodiment of this invention, and FIG. 3 is a diagram showing the roll cooling method previously proposed by this inventor. It is a schematic diagram. 1...Cooling roll 2...Steel strip-14'
;

Claims (1)

[Claims] In a roll cooling method for steel strip, in which the steel strip is cooled by passing it continuously through multiple cooling rolls, the amount of pushing in of the cooling rolls and the roll spacing are adjusted so that the cooling rate of the steel strip reaches a predetermined optimum cooling rate. A method for cooling a roll of steel strip, characterized by changing one or both of them to control the cooling rate of the steel strip.