JPH11123437A - Manufacture of steel sheet having thin scale - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress growth of scale and to obtain a steel sheet having thin scale by executing hot finish rolling after holding the steel sheet containing a specified amount or more of Si at a temp. not lower than a specified temp., forming a fayalite layer on the surface and descaling the steel sheet just before a hot finishing mill. SOLUTION: The steel sheet containing >=0.1% Si is held at >=1173 deg.C, the fayalite (Fe2 SiO4 ) layer 6 is generated on the surface of the steel sheet (ground steel) 1 and the scale such as an FeO layer 7 and Fe3 O4 layer 8 is generated on that layer. Next, descaling is executed so that the fayalite layer 6 of 1 μm thick is left on the boundary of the ground steel of the steel sheet 1 and other scale layers are removed. The thickness of the remaining scale is affected by the collision pressure and colliding time of high-pressure water and stipulated by descaling index = (collision pressure) × (colliding time)<1/2> . The descaling index is selected taking the thickness of the fayalite layer 6 which is necessary to leave into consideration and satisfying collision pressure and colliding time of the high-pressure water are determined.

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 thin-scale steel sheet, and more particularly, to a method for producing a firelite (Fe ₂ Si).
By generating an O ₄ ) layer on the surface of the steel sheet, the descaling process immediately before hot finish rolling is facilitated, and the scale generated after the descaling process is suppressed to reduce the scale of the steel sheet surface to a thin scale. It relates to a manufacturing method.

[0002]

2. Description of the Related Art Hot-rolled steel sheets are subjected to rough rolling of a slab heated in a heating furnace, and then to final rolling by descaling with high-pressure water, cooling to a predetermined material on a run-out table, and then coiling. It is manufactured by winding up. Since all of these steps are performed in the atmosphere, a scale (secondary scale) is generated again on the hot steel sheet surface even if the descaling is performed immediately before the finish rolling.

FIG. 1 is a diagram showing the steps from descaling to winding before finish rolling of a hot rolling line. Generally, the scale generated on the surface of the steel sheet 1 as a material to be rolled is removed (descaled) by high-pressure water by the descaling device 2 before entering the finishing mill 3. The descaled steel sheet enters the finishing mill 3, is rolled to a predetermined thickness, is water-cooled by a cooling device 4 on a run-out table, and is cooled to a predetermined winding temperature.
And sent to the next process. The steel sheet manufactured in this manner has a scale of about 5 to 15 μ thickness usually formed on the surface of the steel sheet.

[0004] The hot-rolled steel sheet with the scale adhered not only has a poor appearance, but also undergoes processing such as bending and pressing, so that the scale peels off from the steel sheet surface. Can not do it. In addition, when subjected to cold rolling, there is a problem in that the scale is pressed or bitten on the surface of the steel sheet to generate surface flaws and deteriorate the surface quality. Under such circumstances, various techniques for producing a tight-scale hot-rolled steel sheet having excellent adhesion have been proposed.

[0005] As a conventional scale control method for producing a hot-rolled tight-scale steel sheet, a method in which the residence time in a high-temperature region after finish rolling is shortened by quenching, or an antioxidant is applied to the surface of the steel sheet. Alternatively, there is a method in which oxidation of the steel sheet surface is suppressed by contact with an inert gas such as nitrogen.

For example, a method belonging to the former is disclosed in
JP-A-190426 discloses that a hot rolled steel sheet rolled by a cooling device provided in a dummy
While quenching to near 0 ° C, hot-rolled steel sheet
A method for producing a hot-rolled steel sheet having good scale adhesion, which is wound in a temperature range of 50 to 600 ° C., has been proposed. As a method belonging to the latter, Japanese Patent Application Laid-Open No. 8-276201 discloses:
A method for producing a thin-scale hot-rolled steel strip by hot rolling while surrounding a traveling hot-rolled steel strip with a liquid and an inert gas, or a foam layer composed of a liquid and a reducing gas, while suppressing scale formation. A method for producing a thin-scale hot-rolled steel strip, characterized in that the thickness of the foam layer surrounding the hot-rolled steel strip is set to 50 mm or more from the upper surface and the lower surface of the traveling hot-rolled steel strip. I have.

The former method requires a large amount of water for quenching, requires a large-scale cooling system, and has a disadvantage that the equipment cost increases. The latter method has a drawback that the running cost and the like are increased since an oxidation inhibitor and an inert gas are used.

[0008]

SUMMARY OF THE INVENTION Accordingly, the present invention provides a method for forming a steel sheet on a steel sheet surface after descaling without protecting the steel sheet surface with an inert gas such as nitrogen or using large-scale equipment. An object of the present invention is to provide a method for producing a thin-scale steel sheet while suppressing the growth of the scale.

[0009]

The inventor of the present invention has investigated the growth of scales generated after descaling immediately before finish rolling, and found that the desalting (Fe ₂ SiO ₄ ) layer was left on the steel sheet surface. When completed, it was found that subsequent scale formation could be suppressed.

The present invention has been completed based on this finding, and the gist of the present invention is as follows.

(1) A steel sheet containing 0.1% or more of Si is maintained at a temperature of 1173 ° C. or more, so that a steel sheet having a firelite (Fe ₂ SiO ₄ ) layer formed on its surface is hot-finished and rolled. A method for producing a thin-scale steel sheet, comprising hot-rolling after descaling immediately before.

(2) By scaling, the file
Alite (Fe_TwoSiO_Four) Fe formed on the layer O , F
e_ThreeO_FourRemoving the scale layer such as
(1) The method for producing a thin-scale steel sheet according to (1).

(3) The method for producing a thin-scale steel sheet according to the above (1) or (2), wherein descaling is further performed after hot finish rolling.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in detail.

In the prior art, a steel sheet having a high Si content is provided with a firelite layer (Fe ₂ Si
O ₄ ) was formed, and it was difficult to completely remove the firelite layer formed on the steel sheet surface, and it was considered that the descaling property was poor. However, in the case of a steel sheet having a small Si content, the scale generated after descaling immediately before finish rolling has a small scale of 6 to 7 μm, whereas the scale was reduced with the firelite layer remaining. It has been found that in a steel sheet, scale growth is suppressed and the scale thickness becomes 2 to 3 μm, and the scale becomes extremely thin as compared with the related art.

As described above, the reason why the growth of the scale is suppressed and the scale becomes thin can be considered to be that Si is concentrated at the interface of the steel sheet with the ground iron. That is, the amount of scale growth is limited by the diffusion rate in the scale, and the diffusion rate in the scale is FeO> Fe ₃ O ₄ > Fe ₂ O ₃ > SiO
_If Fe ₂ SiO ₄ corresponding to SiO ₂ having a low diffusion rate is present at the interface of the steel plate with the steel sheet, the growth of the scale is suppressed and the scale becomes thin.

According to the present invention, a firelite (Fe ₂ SiO ₄ ) layer is intentionally formed on the surface of a steel sheet, and the firelite layer is left in a descaling process to suppress scale growth after descaling immediately before finish rolling. Is what you do.

This will be described with reference to FIG.

FIG. 2A is a cross-sectional view of a steel sheet schematically showing a state in which the steel sheet is heated to generate a scale in which a firelite layer is present at the interface of the steel sheet with the ground iron. FIG.
(B) is a cross-sectional view of a steel sheet schematically showing a state in which the desalting is performed by leaving the firelite layer (about 1 μm) by descaling immediately before finish rolling.

As shown in FIG. 2 (a), a scale of a firelite (Fe ₂ SiO ₄ ) layer 6 is formed on the interface of the steel sheet 1 and a FeO layer 7 and a Fe ₃ O ₄ layer 8 are formed thereon. In order to do this, a steel sheet having a Si content of 0.1% or more is used, and this may be maintained at a high temperature of 1173 ° C. or more. That is, S
If the i content is less than 0.1%, the thickness of the firelite layer formed is too thin, and is not sufficient to suppress scale formation after descaling. When the heating temperature is 11
If the temperature is lower than 73 ° C., a firelite layer cannot be formed. For this reason, in the present invention, it is necessary to heat to a temperature of 1173 ° C. or more using a steel sheet having a Si content of 0.1% or more.

Next, as shown in FIG. 2B, the descaling condition immediately before the finish rolling is controlled so that the firelite layer 6 having a thickness of about 1 μm remains at the interface of the steel plate 1 with the ground iron, and the other scale layers are removed. Descale to remove.
The descaling condition can be determined by selecting the condition of the high-pressure water for descaling. That is, it has been found that the thickness of the remaining scale is affected by the collision pressure and the collision time of the high-pressure water and can be defined by (collision pressure) × (collision time) ^1/2 . In the present invention, this is defined as the Deske index (collision pressure × time ^1/2 ). That is, the skew index = (collision pressure) × (collision time) ^1/2
Holds.

FIG. 3 is a graph showing the relationship between the Deske index (kgf / cm ² · msec ^1/2 ) and the thickness of the remaining scale (μm) for a steel sheet having a 1.0% Si content.

As shown in FIG. 3, the Deske index (collision pressure ×
As time ^1/2 ) increases, the remaining scale thickness (μ
m) is reduced, the descaling condition in the present invention is to select a deske index in consideration of the thickness of the firelite layer necessary for remaining, and to select a high-pressure water satisfying the selected deske index. The collision pressure and the collision time may be determined. When the steel sheet shown in FIG. 3 is targeted, it is necessary to select a Deske index of at least 60 or more in order to leave 1 μm or more of the firelite layer. S in steel sheet
If the i content increases, the firelite layer formed on the surface of the steel sheet becomes thicker, so that the deske index as a descaling condition needs to be increased. Therefore, even when the Si content in the steel sheet is different, the selection of the Deske index is
What is necessary is just to select the Deske index which can leave the firelite layer by the descaling. After the finish rolling, the scale on the steel sheet surface may be removed by descaling as necessary.

[0024]

EXAMPLE A high burring steel sheet having a Si content of 0.6% was heated to 1264 ° C. to perform descaling.

The descaling condition is a pressure of 300 kgf.
/ Cm ² , a flow rate of 200 l / min, and a Deske index of 51.
As a result of performing the test at a setting of 3, a firelite (Fe ₂ SiO ₄ ) layer of about 1 μm remained on the steel sheet surface. The descaled steel sheet was finish-rolled and the scale thickness was measured after cooling.
m thin scale had formed. When a tape peeling test was performed on the adhesion of the thin scale, good results were obtained.

According to the present invention, a firelite layer is formed on the surface of a steel sheet containing Si, and descaling is performed immediately before the finish rolling to leave the firelite layer by descaling. It was confirmed that the presence of the compound can suppress scale growth after descaling.

[0027]

The thin-scale steel sheet can be manufactured at low cost by using the conventional equipment as it is simply by controlling the Si content in the steel sheet and managing the heating and descaling conditions.

[Brief description of the drawings]

FIG. 1 is a view showing a process from descaling to winding up before finish rolling of a hot rolling line.

FIG. 2 (a) is a cross-sectional view of a steel sheet schematically showing a state in which a steel sheet is heated to generate a scale in which a firelite layer is present at an interface between a steel sheet and a steel sheet, and (b) is finish rolling. Immediately before, by descaling, the firelite layer (about 1μ)
It is sectional drawing of the steel plate which shows the state which performed descaling except m) typically.

FIG. 3 is a diagram showing the relationship between the Deske Index (kgf / cm ² · msec ^1/2 ) and the thickness of the remaining scale (μm) for a steel sheet having a 1.0% Si content.

[Explanation of symbols]

1 steel (the base steel) 2 descaling device 3 hot finish rolling mill 4 cooling device 5 coiler 6 fayalite layer 7 FeO layer 8 Fe ₃ O ₄ layer

Claims (3)

[Claims] 1. A steel sheet containing 0.1% or more of Si:
By maintaining the temperature at 173 ° C. or higher, a steel sheet having a firelite (Fe ₂ SiO ₄ ) layer formed on its surface is descaled immediately before a hot finish rolling mill, and then subjected to hot finish rolling. Manufacturing method of scale steel sheet. (2) Fire scaling is performed by descaling.
G (Fe_TwoSiO_Four) Fe formed on the layer O , Fe_ThreeO_Four
2. The method according to claim 1, wherein the scale layer is removed.
Method for manufacturing thin-scale steel sheet. 3. The method for producing a thin scale steel sheet according to claim 1, wherein descaling is further performed after hot finish rolling.