JPH11319904A - Method and device for hot rolling of thin steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of a high temp. rolling scale defect by controlling a surface temp. of a material to be rolled between stands of a finishing rolling will of hot rolling system to a specified value. SOLUTION: When a thin slab or a sheet bar after rough rolling is descaled and hot rolled by a finishing rolling mill consisting of plural stands 1, the surface temp. of a material 6 to be rolled is controlled to <=950 deg.C. The surface temp. of the material 6 to be rolled reaches the highest between a second stand F2 and a fourth stand F4, therefore, it is desirable to control the temp. to <=950 deg.C only in this range. A surface temp. of the material 6 to be rolled is measured by a thermometer 4 between the stands F2-F4. A water quantity and the pressure of a descaler 5 between stands of a high pressure water injection type are adjusted by a signal of the thermometer 4 through a control system, the surface temp. of the material 6 to be rolled is controlled to <=950 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolling method for a thin steel sheet, and more particularly, to a hot rolling method for suppressing the occurrence of scale defects caused by secondary scale generated after finish descaling, and an apparatus therefor. About.

[0002]

2. Description of the Related Art In general, a hot-rolled thin steel sheet is produced by directly heating a slab manufactured by a continuous casting machine or by reheating it in a heating furnace.
It is manufactured by rolling to a predetermined thickness by using a rough rolling mill and a finishing rolling mill including a plurality of stands, and winding it into a coil shape. Recently, a technique in which rough rolling is omitted using a thin slab has been put to practical use.

[0003] When hot rolling a slab, scales mainly composed of iron oxide formed on the surface of the material to be rolled are subjected to high-pressure water before rolling so that scale defects do not occur on the surface of the steel sheet as the final product. Descaling is performed by, for example, jetting. Descaling is completely done,
Since the material to be rolled is in a high temperature state, its surface is immediately oxidized and covered with scale.However, when the surface temperature of the material to be rolled is increased, the scale is vigorously generated, and the material is caught in the roll. High-temperature rollable scale defects called "scale-like scale" and "sand-like scale" occur.

[0004] In order to suppress the occurrence of such high-temperature rolling scale defects, for example, Japanese Patent Laid-Open Publication No. Hei 7-171610 discloses a method in which the scale thickness between stands of a finish rolling mill is estimated by calculation, and if necessary, the scale thickness between stands is determined. A method of descaling is described in JP-A-1-205810.
A method is disclosed in which the surface temperature of the material to be rolled after descaling and before finish rolling is controlled to 900 ° C. or less so that the temperature of the material to be rolled during rolling is not excessively increased.

[0005]

However, in the method described in Japanese Patent Application Laid-Open No. 7-171610, it is considered that the scale thickness between the stands of the finish rolling mill is predicted by calculation. In the method described in JP-A-1-205810, it is not possible to sufficiently suppress the occurrence of scale defects. There has been a problem that high-temperature rollability scale defects frequently occur when, for example, a high-speed (high-speed steel) roll having excellent wear resistance and surface roughness resistance is used as a work roll.

The present invention has been made to solve such a problem, and an object of the present invention is to provide a method and an apparatus for hot rolling a thin steel sheet which can reliably suppress the occurrence of scale defects at high-temperature rollability. .

[0007]

SUMMARY OF THE INVENTION The above object is achieved when a thin slab or a sheet bar after rough rolling is descaled and hot-rolled by a finishing mill comprising a plurality of stands.
The problem is solved by a hot rolling method for a thin steel sheet in which the surface temperature of the material to be rolled between the stands of the finishing mill is controlled to 950 ° C. or less.

[0008] Even if the present inventors set the surface temperature of the material to be rolled before finish rolling to 900 ° C. or less by the method described in Japanese Patent Application Laid-Open No. 1-205810, Investigating the cause of high-temperature rollability of scale defects when using high-speed rolls for work rolls. It has been clarified that this is because the surface temperature of the material to be rolled is greatly increased. Accordingly, in order to surely suppress the occurrence of high-temperature rollability scale defects, Japanese Patent Application Laid-Open No.
The method of controlling the surface temperature of the material to be rolled before finish rolling described in Japanese Patent No. 05810 is insufficient, and the surface temperature of the material to be rolled between the stands during the finish rolling must be controlled. I understand. Therefore, it was examined how to control the surface temperature of the material to be actually rolled between the stands.
It has been found that the temperature may be controlled to not more than ℃.

In the ordinary hot rolling of a thin steel sheet, the surface temperature of the material to be rolled is highest between the second and third stands and between the third and fourth stands of the finishing mill. It is more convenient if the surface temperature of the material to be rolled is controlled to 950 ° C. or lower.

In order to control the surface temperature of the material to be rolled to 950 ° C. or less, a control device for providing a thermometer and a high-pressure water jet type descaler between stands and controlling the conditions of the descaler in accordance with a signal from the thermometer is used. It can be performed by adjusting the amount of water and the pressure of the descaler while measuring the surface temperature of the material to be rolled using the finishing mill provided.

[0011]

FIG. 1 shows an embodiment of a finishing mill according to the present invention.

[0012] This finish rolling mill has a backup roll 2
And seven stand rolls F1 to F7 provided with work rolls 3. High-speed rolls are used as the work rolls of the stand 1 of F1 to F3. A thermometer 4 is provided between the stands 1 of F2 and F3 and between the stands 1 of F3 and F4.
And a greater number of inter-stand descalers 5 than before, and a control device (not shown) for controlling the conditions of the inter-stand descalers 5 in accordance with the signal of the thermometer 4 is provided.

Using this finishing mill, the surface temperature of the material 6 to be rolled is measured by the thermometer 4 between the stands 1 between F2 and F3 and between the stands 1 between F3 and F4, and the descaler 5 between the stands is controlled via the control device. If you adjust the amount of water and the pressure of
The surface temperature of the material 6 to be rolled during rolling can be controlled to 950 ° C. or less, and the occurrence of high-temperature rollability scale defects can be reliably suppressed.

(Embodiment) Using a finishing mill shown in FIG.
Between the stands of F2 and F3 and between the stands of F3 and F4, the surface temperature of the material to be rolled is changed by changing the conditions of the inter-stand descaler, and 12 kinds of mild steel plates having a thickness of 2 mm and a width of 1050 mm are manufactured. The relationship between the surface temperature and the presence or absence of high-temperature rollability scale defects was investigated.

The results are shown in Table 1. It can be seen that high-temperature rollability scale defects do not occur when the surface temperature of the material to be rolled is 950 ° C. or lower in any of the F2 and F3 stands and the F3 and F4 stands.

[0016]

[Table 1]

Further, when the surface temperature of the material to be rolled between the stands was controlled by the method of the present invention and when the surface temperature was not controlled, the rate of occurrence of monthly high-temperature rollable scale defects was investigated.
When controlled by the method of the present invention, the rate was 0.03%, and the occurrence rate of high-temperature-rollable scale defects was significantly reduced as compared with 2.70% when not controlled.

[0018]

Since the present invention is configured as described above, it is possible to provide a method and an apparatus for hot rolling a thin steel sheet capable of reliably suppressing the occurrence of scale defects at high-temperature rollability.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of a finishing mill according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Stand 2 Backup roll 3 Work roll 4 Thermometer 5 Descaler between stands 6 Rolled material

Claims (3)

[Claims] When a thin slab or a sheet bar after rough rolling is descaled and hot-rolled by a finishing mill comprising a plurality of stands, the surface temperature of a material to be rolled between the stands of the finishing mill is adjusted. A hot rolling method for a thin steel sheet controlled at 950 ° C. or lower. 2. The thin steel sheet according to claim 1, wherein the surface temperature of the material to be rolled between the second stand and the third stand and between the third stand and the fourth stand of the finishing mill is controlled to 950 ° C. or less. Hot rolling method. 3. A finishing mill comprising a thermometer and a high-pressure water jet type descaler between stands, and a control device for controlling conditions of the descaler in accordance with a signal from the thermometer.