JPH11156407A - Manufacture of hot rolled steel sheet excellent in surface quality - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a hot rolled steel sheet of high Si excellent in surface quality with less red scale flaw. SOLUTION: In advance of hot rolling of a stock slab of a steel sheet containing >=0.1% Si, a surface temperature T ( deg.C) at the time the rolled stock is taken out of a heating furnace and a time t (s) up to the first descaling by high pressure water after the taking-out are to satisfy a relation: 1170 <=T-3.01 t, and the energy force of the high pressure water on the sheet surface is to be >=120 kJ/m<2> .

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 having a high surface quality and capable of preventing scale flaws of a high Si content steel sheet in which scale flaws are likely to be a problem.

[0002]

2. Description of the Related Art Conventionally, high-strength hot-rolled steel sheets having good formability have been developed for automobile wheels and the like. In order to improve formability and increase strength, Si addition is preferred. However, if the Si content is high, surface scale flaws called so-called red scales or island scales are likely to be generated, and this has led to a reduction in the yield of automotive wheel materials that emphasize appearance. The cause of the above-mentioned scale flaws in the high Si content steel is that the steel and Si oxide firelite (Fe ₂ SiO ₄ ) are generated during the heating, and the local oxidation along the grain boundaries of the steel is caused. This is because firelite melts due to its low melting point, and the scale is hardly peeled off.

[0003] As measures against the red scale, literature (Ishii et al .:
Materials and Processes As introduced in CAMP-ISIJ Vol. 6 (1993) p. 1340) and the like, there are a method of heating steel at a higher temperature than normal and a method of heating steel at a lower temperature. The former is a method in which scale formation is actively promoted by heating at a high temperature, and the scale formation amount is made larger than the local oxidation depth of firelite, thereby facilitating scale peeling.

[0004] The latter is a method of preventing red scale by heating at a low temperature so as not to cause local oxidation. However, the former lacks certainty as a measure against red scale, in addition to wear of refractory of heating furnaces, increase in heating energy cost. In the latter case, since the rolling is performed at a low temperature, the sheetability deteriorates to cause a trouble, or the rolling load becomes excessive and the thickness accuracy may not be obtained.

[0005] Under such a background, Japanese Patent Laid-Open No. 5-131 is disclosed.
No. 202 discloses a method of reducing the width of a slab with a vertical scale breaker, brushing the surface with a rotating brush, and then blowing high-pressure water to enhance descaling. JP-A-63-68214 discloses a method in which the pressure of high-pressure water at the time of finish rolling is higher than a specific value.

[0006]

However, the above-mentioned Japanese Patent Application Laid-Open
The technology disclosed in JP-A-131202 has a short rotating brush life and has a problem when applied to actual production. Also,
The technique disclosed in Japanese Patent Application Laid-Open No. 63-68214 is not practical because the equipment cost for strengthening high-pressure water descaling becomes enormous. An object of the present invention is to solve the problems in the prior art and to provide a method for producing a hot-rolled steel sheet having few scale flaws and excellent surface quality.

[0007]

The present inventors have studied the descaling behavior of Si-containing steel in detail, and as a result,
(a) to (e) were obtained.

[0008] (a) Observation of the state of peeling of the scale due to high-pressure water injection shows that there are scale flaws due to poor peeling of the upper primary scale (thickness of 1 to 2 mm) and the firelite layer at the interface between the primary scale and the steel plate. It can be classified as peeling failure.

(B) not less than the eutectic point of firelite 11
By performing descaling with high-pressure water at 70 ° C or higher, the firelite layer can be almost completely removed. Steel plate temperature is 117
If the temperature is lower than 0 ° C., the firelite layer at the interface between the scale and the steel sheet cannot be removed even if the energy per unit area of the high-pressure water is increased.

(C) Unless the energy per unit area of the high-pressure water exceeds a certain value, the firelite layer cannot be peeled off. (d) If the firelite is removed in the first descaling, the scale formed on the firelite layer can be easily removed in the subsequent high-pressure water descaling. (e) Therefore, after exiting the furnace, treating the first descaling at a high temperature is an effective means of preventing red scale.

The gist of the present invention obtained from the above-mentioned findings is as follows: prior to hot rolling of a steel slab containing 0.1% or more of Si, a surface temperature T (° C.) at the time of extraction with a heating furnace;
Time t until the first high-pressure water descaling after extraction
And (s) satisfying the following relationship: 1170 ≦ T−3.01t, and producing high-pressure water energy of 120 kJ / m ² or more on the surface of the steel sheet. It is in.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors first studied the required energy of high-pressure water. High Si steel (Si content 0.50
(0.60%) was heated in a test furnace while changing the temperature, and was injected while changing the energy of high-pressure water. The energy of high-pressure water varies depending on the discharge pressure, flow rate, nozzle shape, nozzle height, steel sheet speed, etc., but the energy applied per unit area of the material to be descaled (pressure x flow rate / (steel sheet speed / spray width)) ) Shows the relationship between the specimen temperature and the energy of high-pressure water required to maintain the surface quality.

The energy E of high-pressure water on the steel sheet surface
(Energy per unit area (J / m ² ) is expressed by E = P × Q / (V × 2 × H × tan (θ / 2)) where the nozzle outlet discharge pressure P: (Pa) Flow rate Q: (m ³ / s) Nozzle opening angle θ: (°) Nozzle height H: (m) Steel sheet moving speed V: (m / s).

FIG. 1 shows the relationship between the energy of high-pressure water, the temperature of a test piece, and the surface quality after rolling. As can be seen from the figure, in order to completely perform descaling, the specimen temperature is at least 1170 ° C., the energy of high-pressure water is at least 12
0 kJ / m ² is required.

Further, the scale on the surface of the slab has a low thermal conductivity, so that the temperature rapidly drops immediately after leaving the heating furnace.
FIG. 2 shows the relationship between the elapsed time after the heating furnace extraction and the temperature drop. Since the temperature at the time of descaling is required to be 1170 ° C. or more, the relationship between the surface temperature T (° C.) at the time of heating and the elapsed time after the furnace extraction needs to satisfy the following equation.

1170 ≦ T−3.01t

[0017]

EXAMPLE A test piece 1 of high Si steel (Si = 0.7%) was prepared using a test rolling mill corresponding to a quarter of the actual machine shown in FIG.
(Width 70 mm x thickness 30 mm x length 200 mm).
The conditions of the heating furnace extraction surface temperature (3 levels) and the time from the extraction of the heating furnace 2 to the start of the descaling by the first descaling device 3 (4 levels) were changed to prepare a total of 12 test pieces. Next, the first rolling mill 4 rolled to a thickness of 20 mm, and waited for 40 seconds immediately before the second rolling mill group 6. Further, high-pressure water descaling is performed by the second descaling device 5, and each of the rolling mills in the second rolling mill group 6 has 13 mm,
It was rolled to 8 mm and 5 mm, and was water-cooled by a cooling device 7 to 500 to 580 ° C. The state of red scale formation of the test specimen after rolling was observed. Table 1 shows the temperature of the test piece at the start of the first descaling and the evaluation of the steel sheet surface after rolling.

[0018]

[Table 1]

When the heating temperature was high and the time until descaling was short, no red scale was formed. Red scale was formed at lower heating temperatures and longer times to descaling.

[0020]

According to the present invention, when producing a hot-rolled steel sheet having a high Si content, a hot-rolled steel sheet having few red scale flaws and excellent surface quality can be produced.

[Brief description of the drawings]

Figure 1: Energy per unit area of material to be treated and descaling state of descaling high-pressure water (surface evaluation after rolling)
6 is a graph showing the relationship of FIG.

FIG. 2 is a graph showing the relationship between the elapsed time after extraction from a heating furnace and the amount of decrease in surface temperature.

FIG. 3 is a schematic side view of a test rolling mill.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Test piece 2 Heating furnace 3 1st descaling device 4 1st rolling mill 5 2nd descaling device 6 2nd rolling mill group 7 Cooling device

Claims (1)

[Claims] 1. Prior to hot rolling of a steel slab containing 0.1% or more of Si, the surface temperature T (° C.) at the time of extraction in a heating furnace and the temperature after the extraction up to the first descaling with high-pressure water are described. Production of a hot-rolled steel sheet having excellent surface quality, wherein the time t (s) satisfies the relationship of 1170 ≦ T−3.01t, and the energy of high-pressure water is 120 kJ / m ² or more at the steel sheet surface. Method.