JPH08157948A - Production of hot rolled steel plate excellent in uniformity of color tone in plate width direction - Google Patents

Abstract

PURPOSE: To stably produce a hot rolled steel plate having uniform color tone in a plate-width direction and excellent in external appearance by controlling the coiling temp. of a hot rolled plate of low carbon steel to a specific temp. CONSTITUTION: A slab of steel, having a composition consisting of, by weight, 0.001-0.10% C, <0.07% Si, 0.02-0.60% Mn, <0.045% P, <0.045% S, <0.090% Al, <0.0100% N, and the balance Fe, is heated to 1000-1300 deg.C and hot-rolled, and the resultant hot rolled steel plate is coiled at a relatively low temp. not higher than 480 deg.C. Because of low coiling temp., adhesion between the steel plates forming a coil is reduced and the outside air is infiltrated into the plate width sensor part in the inner part of the coil and, as the result, the occurrence of nonuniformity in the thickness of an oxide film forming on the edge part and the center part of steel plate can be prevented. By this method, the hot rolled steel plate, free from nonuniformity of color tone in a steel plate width direction and excellent in external appearance, can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the appearance of hot-rolled steel sheets, and more particularly to a method for producing hot-rolled steel sheets having excellent color tone uniformity in the sheet width direction.

[0002]

2. Description of the Related Art Conventionally, low-carbon hot-rolled steel sheets have different color tones at the edge portion and width center portion of the steel sheet due to the difference in the oxygen supply amount in the coil state after hot rolling. Dark (dark blue), the rest of the width center is light (charcoal)
There was a problem. Such a change in color tone in the plate width direction poses a problem because the commercial value of the appearance is low. However, regarding the color tone uniformity of the steel sheet surface only in the state where only the low carbon steel is hot-rolled, there has been no conventional example showing a control method thereof.

As a similar matter, Japanese Patent Application Laid-Open No. 5-96319 discloses a method of manufacturing a thin scale steel sheet which realizes uniform cooling by performing running water cooling after finish rolling and equalizing water supply and drainage. ing. Further, in Japanese Unexamined Patent Publication (Kokai) No. 5-177233, a stepped table roller that can move in the axial direction is provided on the exit side after hot rolling, and an end surface of a hot rolled steel sheet is sandwiched between the steps.
A technique has been disclosed in which the thickness of the scale is made uniform by making the passing speed constant.

However, none of the inventions aims at uniformizing the scale thickness simply by uniform cooling, and does not mention the uniformity of color tone on the surface of the steel sheet.

[0005]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention According to the present invention, a non-uniform color tone change in the width direction of a low carbon hot-rolled steel sheet, which is a problem from the viewpoint of commercial value, can be obtained by adjusting the winding temperature during hot rolling to an appropriate temperature. It is an object of the present invention to provide a method for producing a hot-rolled steel sheet which is made uniform by the following and has excellent color tone uniformity.

[0006]

The present invention provides C:% by weight.
0.001-0.10%, Si: 0.07% or less, Mn: 0.02-0.60%,
1000-1300 steel slabs containing P: 0.045% or less, S: 0.045% or less, Al: 0.090% or less, N: 0.0100% or less.
It is heated in the temperature range of ℃, and the winding temperature during hot rolling is 480 ℃.
The method for producing a hot-rolled steel sheet having excellent color tone uniformity in the sheet width direction is characterized by the following.

[0007]

[Function] Among the hot-rolled steel sheets, those rolled up as a steel strip and held in the coil shape are caused by the difference in the oxygen (outside air) supply amount in the sheet width direction, and There is a difference in thickness. This oxide film layer thickness is evaluated as an oxide film layer thickness evaluation index. The edge of the steel sheet is in contact with the outside air even after winding, and a thick oxide film layer containing oxygen is formed on the surface layer (extreme surface layer) of the scale layer generated on the surface of the steel sheet during the coil cooling period after winding is completed. However, its thickness is shown to be about 6 by the oxide film layer thickness evaluation index. On the other hand, the width center portion of the steel sheet is shielded from the outside air inside the coil, so that the oxide film layer thickness evaluation index is about 2 to 3. The difference in the thickness of the oxide film layer of the outermost layer becomes the difference in the interference of light, which causes a change in the color tone of the steel sheet and causes a problem of nonuniformity.

The present invention solves the unevenness of the color tone of the steel sheet in the width direction by eliminating the difference in the oxygen supply amount in the width direction of the steel sheet. The present invention achieves uniform color tone in the width direction of a low carbon hot-rolled steel sheet, and the reasons for limiting the components and manufacturing conditions will be described below. C: C is added in an amount of 0.001% or more for the purpose of ensuring the desired strength, but if it exceeds 0.10%, elongation and toughness deteriorate, and it becomes difficult to mold the final product, resulting in 0.001 to 0.10%.
Must be within the range.

Si: The upper limit of Si is set to 0.07%, because the addition of more than this causes the surface roughness to deteriorate due to the concentration of Si in the surface layer during heating, which adversely affects the surface color tone. Mn: Mn is added in 0.02% or more in order to secure strength and hot workability, but if it exceeds 0.60%, there are problems such as appearance of abnormal hardening structure due to center segregation and occurrence of welding cracks. It must be within the range of 0.02 to 0.60%.

The reason why P: P is set to 0.045% or less is that if it exceeds this value, it is concentrated at the scale interface, which causes poor appearance due to deterioration of scale adhesion and also causes embrittlement of grain boundaries. . S: 0.045% because S forms inclusions harmful to toughness.
Limited to: Al: The reason why Al is set to 0.090% or less is that workability is deteriorated due to the inclusion precipitation on the surface of the steel sheet and scale adhesion is deteriorated, which causes a poor appearance.

The reason why N: 0.0100% or less is that if it exceeds this value, the workability due to inclusions is significantly deteriorated. A steel slab having the above composition is heated to a temperature range of 1000 to 1300 ° C and hot rolled. This is because if the heating temperature is less than 1000 ° C, it is difficult to perform hot rolling thereafter, and if the heating temperature exceeds 1300 ° C, the workability of the steel sheet deteriorates due to the coarsening of γ grains.

The winding temperature is 480 ° C. or lower. Since the yield stress of steel sheet has temperature dependence, compared with the time of winding at over 480 ° C, which was conventionally performed for low carbon hot rolled steel sheet,
By winding at a lower temperature of 480 ° C. or less of the present invention, the yield stress of the steel sheet is wound in a higher region, so that the adhesion between the steel sheets during coiling is reduced, and even the center of the coil width inside the coil is reached. Oxygen (outside air) is supplied. As a result, the difference in the thickness of the surface oxide film layer formed at the edge portion and the center portion is reduced, so that it becomes possible to prevent uneven color tone in the steel sheet width direction.

[0013]

EXAMPLES Table 1 shows the experimental results on the components of the steels used in Examples and Comparative Examples, the hot rolling conditions, and the color tone uniformity of the edge portion and the center portion in the width direction of the hot rolled sheet. In each of Examples A to E, the winding temperature was 480 ° C. or less, and the color tone of the edge portion and the center portion showed good uniformity. Comparative Examples F to H are 480
It was wound at a temperature of over ℃, the edge part was dark blue and the center part was charcoal color, and a change in color tone was observed.

[0014]

[Table 1]

FIG. 1 shows an example of analysis of oxygen and iron components in the thickness direction of the steel sheet surface layer. As shown in FIG. 1, the low carbon hot rolled steel sheet has an oxide film layer having a higher oxygen concentration than the scale layer on the outer side (exterior surface layer) of the scale layer on the surface of the sheet. In Comparative Examples F to H shown in Table 1, the oxide film layer thickness of the edge portion (dark blue) is about 6 to 8, while the thickness of the center portion (charcoal color) is about 3. there were. However, as shown in FIG.
In B and E, both the edge and the center showed an oxide film layer thickness of about 6 to 8 and a good color tone uniformity (all dark blue).

FIG. 3 shows the relationship between the coiling temperature and the center part oxide film thickness evaluation index for steels of the same composition. Under the coiling conditions above 480 ° C., the center part oxide film was thin,
An evaluation index tended to be 4 or less, and color tone changes (non-uniformity) were observed at the edge and center portions. Winding temperature
By setting the temperature to 480 ° C or less, it is possible to set the oxide layer thickness index of the center part to 6 to 8 (similar to the edge part), and it is possible to manufacture a hot rolled steel sheet with excellent color tone uniformity. Becomes

[0017]

According to the present invention, the thickness of the oxide film layer in the center portion of the plate width is approximately the same as that of the edge portion (index 6).
To 8), and a hot-rolled steel sheet having excellent color tone uniformity between the edge portion and the center portion can be manufactured. INDUSTRIAL APPLICABILITY The present invention can improve the appearance without increasing the cost of the hot-rolled steel sheet, and has an effect of economically producing the hot-rolled steel sheet having high commercial value.

[Brief description of drawings]

FIG. 1 is a graph showing an example of analysis of oxygen and iron components in the plate thickness direction in the surface layer of a steel plate.

FIG. 2 is a graph showing changes in the surface oxide film layer thickness evaluation index depending on the plate width position.

FIG. 3 is a graph showing changes in a winding temperature and an oxide film layer thickness evaluation index in a plate width center portion.

Claims (1)

[Claims] 1. C: 0.001 to 0.10% by weight, Si: 0.07
%, Mn: 0.02 to 0.60%, P: 0.045% or less, S: 0.
A steel slab containing 045% or less, Al: 0.090% or less, and N: 0.0100% or less is heated in a temperature range of 1000 to 1300 ° C, and the winding temperature during hot rolling is set to 480 ° C or less. A method for producing a hot-rolled steel sheet having excellent color uniformity in the sheet width direction.