JPH06190426A - Manufacture of hot rolled steel sheet - Google Patents

Abstract

PURPOSE:To manufacture a hot rolled steel sheet whose scale is hard to peel and yield of which is high by water-cooling a steel sheet to be rolled to a specified temp. before the outlet side of a final finishing mill and coiling at a pecified temp. after that. CONSTITUTION:The stand of the finishing mill is composed of seven stands. Plural spray nozzles 3 whose discharge pressure is about 40kg/cm<2> are provided between a 6th stand 1b and a 7th stand 1a. The steel sheet 2 to be rolled is cooled with water to at least 600 C from a temp. in the neighborhood of the Ar3 point before the outlet side of the final finishing mill with these spray nozzles. After that, it is coiled in the temp. range of 550-600 deg.C. In this way, scale is not peeled, so the environment is not contaminated, also yield is improved and the hot rolled steel sheet excellent in material and coiled shape is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel sheet which produces a scale having good adhesion and a small thickness.

[0002]

2. Description of the Related Art As a method for producing a hot-rolled steel sheet in which the amount of scale deposited on the surface of the hot-rolled steel sheet is extremely small, there is a technique disclosed in Japanese Patent Publication No. 4-31762. A method of manufacturing a hot rolled steel sheet based on this technique, as shown in FIG.
A method of rapidly cooling the hot-rolled steel sheet 22 immediately after coming out of the finishing rolling mill 21 to 500 ° C. or less at a cooling rate of 200 ° C./sec or more using a plurality of spray nozzles 23 provided at the rear of the finishing rolling mill 21. is there.

[0003]

[Problems to be Solved by the Invention] Japanese Patent Publication No. 4-31762
In order to reduce the surface scale of hot-rolled sheet, No. 2) When wustite formed on the surface of the steel sheet transforms into magnetite (about 500 ° C), the scale hardly grows. B) The scale becomes thicker in proportion to the time from the exit side of the finish rolling mill to 500 ° C or less. The idea is to suppress the absolute amount of scale.

However, by quenching to 500 ° C. or less, there are problems that a steel sheet having too high strength is formed due to transformation strengthening and that the coiling shape becomes defective due to low temperature coiling. An object of the present invention is to provide a method for manufacturing a hot-rolled steel sheet that prevents scale peeling and solves the above problems.

[0005]

According to the present invention, the adhesion of magnetite is good as shown in FIG. 2, and the scale exfoliation has a slight thickness when most of the scale is composed of magnetite. It is focused on that there is no problem.

As a result of studying the relationship between scale thickness, constituent elements and scale peeling by various experiments, if the water cooling stop temperature at the exit side of the finish rolling mill is at least 600 ° C. or lower than the temperature near the Ar ₃ point. When the scale is mostly made of magnetite, the problem due to scale exfoliation does not occur, and the longer the temperature is kept in the range of 550 to 600 ° C, the more the transformation to magnetite progresses based on the chemical reaction of chemical formula 1. , The adhesion of the scale is improved, which is 6
After the quenching was stopped at 00 ° C., it was understood that no problem occurs in terms of peelability if the time required from the exit of the final finishing mill to the winding operation is about. It was also confirmed that when the coiling was carried out in the range of 550 to 600 ° C., problems in the material and defects in the coil winding shape could be solved.

In the method for manufacturing a hot rolled steel sheet according to the present invention, the hot rolled steel sheet is rapidly cooled to at least 600 ° C. or less by the exit side of the finish rolling mill, and the rapidly cooled hot rolled steel sheet is wound in the temperature range of 550 to 600 ° C. .

[0008]

According to the present invention, the scale is mostly composed of magnetite by rapidly cooling it to at least 600 ° C. by the exit side of the finish rolling mill and further winding it at 550 to 600 ° C., and its thickness also causes scale peeling. It is possible to obtain a hot-rolled steel sheet that is suppressed to the extent that it does not occur, and that has a good material and winding shape.

The rolled hot-rolled steel sheet is 550-
Since it is wound at a temperature of 600 ° C., unlike the case of low-temperature winding at 500 ° C. or less, there is no problem with the material and the winding shape of the coil does not deteriorate.

[0010]

EXAMPLE A method for manufacturing a hot rolled steel sheet according to an example of the present invention will be described.
It will be described with reference to FIG. In the present manufacturing method, when the stand of the finish rolling mill is composed of seven stands and the seventh stand 1a is a dummy stand, the sixth stand 1b is used.
And the seventh stand 1a are provided with a plurality of spray nozzles 3 having a discharge pressure of about 40 kg / cm ² , and the hot rolled steel sheet 2 is 600
It is designed to be rapidly cooled to around ℃. In this case, the cooling water flows to the downstream side of the seventh stand 1a and the roll of the finishing mill of the seventh stand 1a is brought into contact with the hot rolled steel plate 2 so that the hot rolled steel plate 2 is not cooled more than necessary, I try to drain it. Not shown, 7th stand 1
A thermometer is arranged on the downstream side of a, the surface temperature of the hot-rolled steel sheet 2 is measured, the discharge amount of cooling water is controlled in accordance with the temperature, and the surface temperature of the hot-rolled steel sheet 2 is adjusted to 550 to 600. The temperature is kept in the range of ° C.

When the surface temperature of the hot-rolled steel sheet 2 is kept in this temperature range, wustite (FeO) is 2 as a scale,
Magnetite (Fe ₃ O ₄ ) is 93, Hematite (Fe ₂
O ₃ ) is produced at a rate of 5. Of these, the most produced magnetite is the thinnest. It is qualitatively known that the thin scale has good adhesion, but when the actual adhesion is measured, as shown in the graph of Fig. 2, the adhesion of magnetite is 10 times that of wustite and 4 times that of hematite. It can be seen that the hot-rolled steel sheet produced according to the present invention is less likely to have scales peeled off and has a good yield.

[0012]

According to the present invention, it is possible to manufacture a hot-rolled steel sheet in which scale does not easily peel off and the yield is good.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.

FIG. 2 is a graph showing the adhesion of the scale.

FIG. 3 is an explanatory view showing a method for manufacturing a conventional hot-rolled steel sheet.

[Explanation of Codes] 1a 7th stand of finish rolling mill 1b 6th stand of finish rolling mill 2 Hot rolled steel plate 3 Spray nozzle

Claims (1)

[Claims] 1. During hot rolling, the rolled steel sheet is at least 6 from the temperature near the Ar ₃ point to the exit side of the final finishing mill.
A method for producing a hot-rolled steel sheet, comprising water cooling to 00 ° C., and then winding in a temperature range of 550 to 600 ° C.