JPH10113713A - Production of steel plate of controlled cooling - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a temp. distribution of in the steel plate width direction uniform when stopping controlled cooling. SOLUTION: When producing the steel plate of controlled cooling to cool down to a prescribed temp. on-line after hot rolling, descaling is conducted before start of controlled cooling while varying a colliding pressure of water at descaling in the steel plate width direction, a thickness distribution of a scale layer of front/rear faces of the steel plate before start of controlled cooling is regulated, successively, by conducting controlled cooling, a temp. distribution in the steel plate width direction at stopping of controlled cooling is made uniform.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for manufacturing a hot-rolled steel sheet, and more particularly to a technique for manufacturing a control-rolled steel sheet.

[0002]

2. Description of the Related Art For the purpose of improving the mechanical properties of hot-rolled steel sheets and reducing alloying elements, a controlled cooling method for cooling to a predetermined temperature online after completion of hot rolling has been put to practical use. However, when performing controlled cooling, at the steel sheet width direction end, since the cooling water flows in the width direction of the steel sheet, the steel sheet width direction end is strongly cooled, and the steel sheet width direction end is viewed from the side of the steel sheet. Cooling is also added, resulting in excessive cooling. As a result, a temperature difference occurs between the central portion and the end portion in the width direction of the steel sheet when the control cooling is stopped, and the temperature distribution in the width direction of the steel sheet becomes uneven.

[0003] The non-uniformity of the temperature distribution in the width direction of the steel sheet when the control cooling is stopped causes a residual strain after air cooling, and causes deformation when cutting the product. Non-uniform cooling also causes variations in mechanical properties. Therefore, uniform cooling in the width direction of the steel sheet in controlled cooling is extremely important. As a method of controlling cooling to uniformly cool the steel sheet in the width direction, a method of shielding cooling water at the steel sheet width direction end with a shielding plate in order to prevent excessive cooling at the steel sheet width direction end is disclosed in Japanese Patent Application Laid-Open No. 61-238413. No. 1993.

[0004]

However, there are cases where the above-described method cannot sufficiently prevent excessive cooling of the end portion in the width direction of the steel sheet. That is, when the shielding width is widened, the temperature drop amount at the end portion in the width direction of the steel sheet decreases, but the cooling stop temperature becomes higher than the target cooling stop temperature inside. Conversely, if the shielding width is reduced, the effect of preventing excessive cooling at the end portion in the width direction of the steel sheet decreases. As described above, it is difficult to uniformly cool the steel sheet in the width direction and make the temperature distribution uniform when the control cooling is stopped by the conventional technique.

The present invention has been made in view of the fact that the cooling rate of the steel sheet changes depending on the difference in the thickness of the scale layer on the surface of the steel sheet. It is an object of the present invention to provide a method for manufacturing a controlled cooling steel sheet in which the cooling response time at the start of controlled cooling is controlled by adjusting the temperature, and the temperature distribution in the steel sheet width direction when the controlled cooling is stopped is uniform.

[0006]

SUMMARY OF THE INVENTION The point of the invention is that when producing a controlled cooling steel sheet which is cooled to a predetermined temperature online after completion of hot rolling, the collision pressure of water during descaling in the width direction of the steel sheet before the start of the controlled cooling. To control the thickness distribution of the scale layer on the front and back surfaces of the steel sheet before starting the control cooling, and then perform the control cooling to make the temperature distribution in the width direction of the steel sheet uniform when the control cooling is stopped. It is characterized by the following.

[0007] The cooling rate of the steel sheet at the start of controlled cooling is high when the scale layer thickness is large, and is low when the scale layer thickness is small.
Therefore, the temperature of the steel sheet at the time of stopping the controlled cooling of the portion where the scale layer thickness is large is low, and conversely, the temperature of the steel sheet at the time of stopping the controlled cooling of the portion where the scale layer thickness is small becomes high. About this phenomenon,
It is disclosed by some of the inventors in Japanese Patent Application No. 3-248250.

[0008] The thickness of the scale layer formed on the surface of the steel sheet during hot rolling and cooling can be predicted from the air cooling time and the steel sheet surface temperature. No. -115061. The prediction model is as follows.

T = [a · τ · exp [b / (T + 273)]] ^C where t: thickness of scale layer τ: air cooling time T: steel sheet surface temperature a, b, c: constant

By combining this scale layer thickness prediction model with a rolling temperature prediction model using a well-known difference method, it is possible to simulate the thickness of the scale layer on the steel sheet surface after the completion of hot rolling.

Next, the thickness distribution of the scale layer on the front and back surfaces of the steel sheet before the start of the controlled cooling is adjusted according to the cooling water amount density in the controlled cooling. The thickness distribution of the scale layer is adjusted by changing the collision pressure of water during descaling to perform descaling. The change in the collision pressure of water in the width direction of the steel sheet during descaling is adjusted by adjusting the water pressure of each descaling header arranged in the width direction of the steel sheet, or each descaling header and the front and back surfaces of the steel sheet arranged in the width direction of the steel sheet. This is done by adjusting the distance between

As shown in FIG. 6, the thickness of the scale layer on the surface of the steel sheet after descaling becomes smaller as the collision pressure of water at the time of descaling becomes higher. On the other hand, if the collision pressure is reduced, the thickness of the scale layer after air cooling is increased. Thus, after hot rolling,
By changing the collision pressure of water in the steel sheet width direction and performing descaling, the scale layer thickness in the steel sheet width direction before the start of the controlled cooling can be adjusted.

Therefore, before the start of the controlled cooling, the front and back surfaces of the steel plate width direction ends to be strongly cooled are subjected to high descaling water collision pressure, while the front and back surfaces of the steel plate width direction central portion are subjected to low descaling water collision. By descaling with pressure and changing the scale thickness, it is possible to eliminate the temperature difference between the end and the center in the width direction of the steel sheet when the control cooling is stopped.

As described above, after the end of hot rolling and before the start of controlled cooling, descaling is performed using a descaling facility in which the collision pressure of water during descaling is variable in the width direction of the steel sheet. By adjusting the scale thickness distribution and then performing the controlled cooling, the temperature distribution in the steel sheet width direction when the controlled cooling is stopped can be made uniform.

[0015]

Embodiments of the present invention and comparative examples will be described below. Thickness 25mm, width 3000mm, length 10
After hot-rolling a 000 mm welded steel sheet at 870 ° C, as shown in Fig. 1, in addition to a descaling header with a uniform impact pressure in the sheet width direction, a width 200 mm capable of moving in the sheet width direction is applied.
An auxiliary descaling header of mm was provided, and at the time of descaling, the descaling was performed by increasing the collision pressure of water in the auxiliary descaling header. In other words, both ends of the descaling header where the collision pressure is uniform are shielded by shielding plates,
Between 1300mm width on both sides from the center in the width direction of the steel plate, the collision pressure is 3.5MPa with a uniform descaling header and the outer 200mm width (both ends of the steel plate) is 4.5MPa with an auxiliary descaling header. Descaling was performed. Thereafter, controlled cooling was started at 850 ° C, and stopped at 450 ° C. The water density during this time is 0.6m ³ / min · m ² .
FIG. 2 shows the temperature distribution in the width direction of the steel sheet when the control cooling is stopped at this time. The scale layer thickness at the end of hot rolling is
According to the prediction model, it was 30 μm, and the scale layer thickness after descaling was 25 μm at 3.5 MPa impact pressure and 8 μm at 4.5 MPa impact pressure.

The comparative example has a thickness of 25 mm and a width of 30 mm as described above.
After hot rolling a 00 mm, 10000 mm long welded structural steel sheet at 870 ° C, the steel was subjected to descaling at a water collision pressure of 4.5 MPa during descaling without changing the water collision pressure in the width direction of the steel sheet. After that, start controlled cooling from 850 ° C and 450 ° C
To stop controlled cooling. The water density during this time is 0.6m ³ / min
- a m ^2. At the time of controlled cooling, controlled cooling was performed under three conditions: no shield plate at both ends of the steel plate width, a shield plate width of 100 mm, and a shield plate width of 200 mm. The temperature distribution in the width direction of the steel sheet when the control cooling is stopped is shown in FIGS. The scale layer thickness at the end of hot rolling was 35 μm from the prediction model, and the scale layer thickness after descaling was 7 μm.

In the manufacturing method according to the present invention, the surface temperature distribution in the width direction of the steel sheet when the control cooling is stopped is, as shown in FIG. The scale layer thickness at the end of the steel sheet width is reduced and the thickness of the scale layer at the center in the sheet width direction is adjusted thicker, so that the surface temperature distribution in the steel sheet width direction is almost uniform and the steel sheet width direction The temperature deviation between the end and the center is 23 ° C.

On the other hand, in the comparative example, since the thickness of the scale layer before the start of the controlled cooling is constant in the width direction of the steel sheet, FIG. As described above, the temperature deviation between the end portion and the center portion in the width direction of the steel sheet when the control cooling is stopped is 72 ° C. Further, when the width of the shielding plate at both ends of the steel sheet width is 100 mm during the controlled cooling, as shown in FIG. 4, the temperature deviation between the end and the central part in the width direction of the steel sheet when the controlled cooling is stopped is 48 ° C. In addition, when the width of the shielding plate at both ends of the steel sheet width is 200 mm during the controlled cooling, as shown in FIG.
℃, but the temperature inside the steel sheet width end is
The large width of the shielding plate hinders cooling of that part, and is about 15 ° C higher than the target controlled cooling stop temperature of 450 ° C. As described above, it is difficult to uniformly control the temperature distribution in the width direction of the steel sheet when the controlled cooling is stopped by the method using the shielding plate during the controlled cooling. In addition, the steel plate surface temperature at the time of control cooling stop was measured with the infrared radiation thermometer.

In the above-described embodiment, an example using the auxiliary descaling header has been described. However, by subdividing the descaling header and changing the collision pressure of water at the time of descaling, the control cooling stop is stopped. Can improve the uniformity of the temperature distribution in the width direction of the steel sheet. Naturally, even after the end of hot rolling, even for a steel sheet having a temperature deviation on the surface, before the start of control cooling, the temperature distribution on the steel sheet surface when the control cooling is stopped depends on the surface temperature deviation. By changing the collision pressure of water at the time of descaling and adjusting the thickness of the scale layer so as to be uniform, the temperature distribution on the surface of the steel sheet when the control cooling is stopped can be made uniform.

[0020]

As is apparent from the above description,
According to the present invention, it is possible to control the cooling response time of the steel sheet at the start of control cooling to adjust the scale layer thickness distribution on the front and back surfaces of the steel sheet before the start of control cooling, and to control the steel sheet width direction at the time of control cooling stop. The temperature distribution can be made uniform.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a descaling facility in which the collision pressure of water during descaling is changed using an auxiliary descaling header.

FIG. 2 is a diagram showing a surface temperature distribution in a width direction of a steel sheet when controlled cooling is stopped in the manufacturing method according to the present invention.

FIG. 3 is a diagram illustrating a surface temperature distribution in a width direction of a steel sheet when control cooling is stopped when there is no shielding plate at both ends of the steel sheet in a comparative example.

FIG. 4 shows a comparative example in which the shielding plate width at both ends of the steel plate is 100.
FIG. 4 is a diagram showing a surface temperature distribution in the width direction of the steel sheet when control cooling is stopped when the value is mm.

FIG. 5: The width of the shielding plate at both ends of the steel plate in the comparative example is 200
FIG. 4 is a diagram showing a surface temperature distribution in the width direction of the steel sheet when control cooling is stopped when the value is mm.

FIG. 6 is a view showing a result of performing descaling using a descaling facility after completion of hot rolling, and then air cooling, and measuring the thickness of the scale layer on the steel sheet surface after air cooling.

Claims (1)

[Claims] In the production of a controlled cooling steel sheet which is cooled to a predetermined temperature online after the end of hot rolling, descaling is performed by changing the collision pressure of water at the time of descaling in the width direction of the steel sheet before the start of controlled cooling, A controlled cooling steel sheet characterized by adjusting the thickness distribution of the scale layer on the front and back surfaces of the steel sheet before starting the control cooling, and then performing the control cooling to make the temperature distribution in the width direction of the steel sheet uniform when the control cooling is stopped. Manufacturing method.