JPH06269804A - Manufacture of steel sheet having minimized surface flaw with hot rolling - Google Patents

Abstract

PURPOSE:To prevent the surface flaw that is formed at the time of hot rolling. CONSTITUTION:At the time of manufacturing a steel sheet by hot rolling, the generation of surface flaw at the time of hot rolling is decreased by making the slab thickness at outtermost edge part thicker than the slab thickness in the middle part of slab in the shape of a slab of base stock. Further, by taking the value of the ratio (k) (=DELTA/S) of the area DELTAS of a part bulged out from the rectangle to the area S of the rectangle when the slab is superposed on a perfect rectangle which is specified by the thickness in the middle part of slab and the width of slab as 0.003-0.03, the generation of surface flaw is decreased. In this way, the fine crack generated at the time of hot rolling is prevented and the yield of product is greatly improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to a method for producing a steel sheet having less surface flaws generated during hot rolling, that is, reducing surface flaws generated during hot rolling of a steel sheet by changing the cross-sectional shape of a slab. It is about how to make.

[0002]

2. Description of the Related Art Generally, surface flaws are likely to occur at the edges of hot-rolled steel sheets, and especially scales are generated inside the surface flaws caused by cracks and surface irregularities of the steel sheet, which may occur during pickling in the subsequent process. It is not removed sufficiently and becomes a serious surface defect after cold rolling, resulting in a decrease in product yield. In particular, surface defects called bald spots caused by microcracks during hot rolling enter the cold rolling process without being removed by the pickling process because the scale generated after cracking penetrates into the interior by rolling. If this occurs, linear defects that are long in the rolling direction become defects, and the yield reduction rate is particularly large. Further, a steel type in which a yield reduction due to these surface defects is a particular problem is a stainless steel which is likely to have a problem of product surface beauty and has high deformation resistance during hot rolling and poor hot workability.

Therefore, various techniques for reducing surface defects during hot rolling have hitherto been devised mainly for stainless steel. For example, JP-A-57-1615
No. 3 discloses a technique in which the components of an austenitic stainless steel are specified to secure hot workability and reduce the earing and bald spots of the steel. Japanese Unexamined Patent Publication (Kokai) No. 2-15806 discloses a technique for eliminating surface defects (pinholes) in stainless steel slabs by maintenance to eliminate the occurrence of bald spots. However, this technique cannot prevent minute cracks generated during hot rolling. Further, as a technique for reducing the occurrence of hot rolling flaws by devising a slab shape, Japanese Patent Laid-Open No. 58-58
No. 138502 and JP-A-3-207551. Both of them disclose a technique in which the central portion of the short side of the slab is recessed to reduce the edge seam flaw of the stainless steel. However, this technique cannot prevent the above-mentioned microcracking during hot rolling.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel sheet which has improved surface defects without a particular increase in process load in improving surface defects generated during hot rolling.

[0005]

In order to solve the above-mentioned problems, the present invention specifies the shape of a slab which is a material for hot rolling, and its skeleton is such that the shape of the material slab is opposite in its cross section. That is, the slab thickness at the outermost edge of the long side is made thicker than the slab thickness at the central portion and subjected to hot rolling. Further, in the cross section of the raw material slab, a rectangle defined by the length w of the opposite long side and the central portion slab thickness h _C is assumed, and when the rectangle is overlapped with the cross section of the slab, a portion protruding from the rectangle ( The protrusion portion is present in the slab edge portion, and the ratio k of the area ΔS of the protrusion portion to the rectangular area S (S = w × h _C ) (= ΔS / S) ) To 0.0
It is to be subjected to hot rolling with a value of 03 or more and 0.03 or less.

[0006]

The present invention will be described in detail below. The inventors of the present invention completed the present invention by meticulously investigating the relationship between the defect frequency generated during hot rolling and the shape of the raw material slab. First, FIG. 1 shows a slab shape according to the present invention. The figure shows the cross section of the slab. In the figure, w is the length of the opposite long side (that is, slab width), h _C is the average thickness of the slab center, and h _E is the thickness of the slab outermost edge. Are shown respectively. Further, a rectangle defined by the slab center thickness h _C and the slab width w is shown by a dotted line, and a portion protruding from this rectangle is shown by a slanted line in the drawing. The slab according to claim 1 of the present invention is a slab having a shape in which h _E > h _C. Further, the slab according to claim 2 has a portion protruding from the rectangle (hatched portion in the figure) at the slab edge portion, and a ratio k value (= ΔS) between the area ΔS of the protrusion and the rectangular area S. / S) is 0.
It is a slab having a shape of 003 or more and 0.03 or less. The edge portion and the central portion of the slab mean a range in which the slab width is roughly divided into three, as shown in the figure. It should be noted that the shaded area ΔS in the drawing is exaggerated for the sake of clear definition, and the k value is not related to the scope of the claims.

FIG. 2 shows a conventional slab shape.
(A) shows a cross section of a completely rectangular slab obtained when bulging does not occur during casting, and h and w in the drawing mean the slab thickness and the slab width, respectively. FIG. 6B shows a cross section of the slab when a recess is formed on the long side surface of the slab by bulging or the like during casting. The position where the recess is generated is usually formed in the range of 10 to 300 mm from the slab edge.
h represents the maximum recess depth, and Δw represents the distance from the slab edge to the maximum recess depth generation position.

The inventors of the present invention have investigated the relationship between the occurrence of hot rolling flaws and the shape of the slab, and as a result, have found that the recessed slab of FIG. 2 (b) has a hot rolling flaw rather than the perfect rectangular slab of FIG. 2 (a). We found that there were many outbreaks. Further, it has been found that the occurrence frequency is higher in the complete rectangular slab as it is closer to the slab edge, and in the concave part slab, the thermal defect is generated in the concave part with the occurrence frequency exceeding the rectangular slab. Further, it is recognized that the frequency of hot-rolling defects in the recessed slab depends on the maximum recess depth Δh. The larger Δh, the higher the frequency of defects. However, it has been found that when the slab thickness at the edge portion is thicker than the slab thickness at the central portion, the occurrence of hot-rolling defects is less than that of a complete rectangular slab.

Furthermore, the present inventors investigated the hot rolling flaw after the completion of one pass of the rough hot rolling in order to clarify the form of the flaw. As a result, it was found that all the flaws generated in the slab edge and the slab recess were cracks in the C direction (cracks in the direction perpendicular to the rolling direction). The size is a minute crack of 0.1 to 0.2 mm in the C direction and about 0.1 mm in depth, and it is stretched in the L direction (rolling direction) by the subsequent rolling process such as hot rolling and cold rolling, and finally. It was confirmed that the product plate will cause bald defects and the like that will seriously deteriorate the surface quality.

In view of the above facts, the inventors of the present invention have developed and devised a slab shape free from hot-rolling defects. That is,
The shape of the raw material slab is to be subjected to hot rolling by making the thickness of the slab at the outermost edge of the long sides facing each other in its cross section thicker than the thickness of the slab at the center. Further, in the cross section of the raw material slab, a rectangle defined by the length w of the opposite long side and the central portion slab thickness h _C is assumed, and the portion that protrudes from the rectangle when the rectangle and the slab cross section are overlapped. , The protruding portion exists at the slab edge portion,
Furthermore, the area ΔS of the protruding portion and the rectangular area S
The ratio k value (= ΔS / S) with (S = w × h _C ) is 0.00
It is to be subjected to hot rolling with a value of 3 or more and 0.03 or less.

The reasons for limiting the scope of the claims will be described below.
First, the invention according to claim 1 will be described. In the cross section of the raw material slab, the reason for making the outermost edge slab thickness of the opposing long side thicker than the central slab thickness is to reduce the occurrence of hot-defects to the same level as or more than that of the complete rectangular slab. . Here, the cross section of the slab means a plane perpendicular to the rolling direction when the slab is rolled, and does not necessarily have to be a plane perpendicular to the casting direction. The slab thickness at the outermost edge is the slab thickness at the slab end face as shown in FIG. 1, but may be the slab thickness at a position slightly inside the end face. What is the central slab thickness?
The slab thickness at the center of the slab width direction (long-side direction) may be sufficient, but in an actual slab, since there are minute irregularities, it is desirable that it is the average thickness of the entire central portion. Here, the central portion and the edge portion indicate a range in which the slab width (long side) is roughly divided into three, as shown in FIG. The difference between the thickness at the outermost edge of the slab and the average thickness at the center is not specified, but at least 1 mm or more is necessary. If this thickness difference becomes extremely large, disadvantages such as hot microcracking at the central portion rather than at the slab edge portion occur, so it is up to about 20% of the slab thickness. Further, in the present invention, the shape from the central portion of the slab to the outermost edge of the slab is not particularly specified, but it is desirable that the slab thickness changes smoothly in consideration of the gist of the present invention.

Next, the invention according to claim 2 will be described. The protruding portion (ΔS) defined in claims 2 to 1 exists in the slab edge portion, and the ratio k value (= ΔS / S) between the ΔS value and the virtual rectangular area S value (= h _C × w) is 0. .003
The reason why the range is set to 0.03 or less is to reduce the occurrence of hot-rolling defects more than in the case of a completely rectangular slab, or to eliminate it at all. The reason for setting the lower limit of the k value to 0.003 is that the effect of reducing the hot-rolling defect cannot be recognized below that. The reason for setting the upper limit of the k value to 0.03 is that if the k value is more than this, a flaw is likely to occur in the central portion and the original purpose cannot be achieved. Although the shape of the protruding portion is not particularly specified, it is desirable that the slab thickness at the protruding portion changes smoothly, and it is desirable that the slab thickness becomes thicker toward the outermost edge portion.

As means for increasing the thickness of the outermost edge of the slab to be thicker than the thickness of the central portion or for allowing the protruding portion to exist at the edge of the slab, the following can be considered. First, there is a method of thickening the slab edge portion during casting. For example, the length of the short side of the inner surface of the continuous casting mold is made larger than the length of the short side of the central portion of the long side. As shown in FIG. 3, both ends of the long side may be appropriately tapered. A predetermined slab can be obtained by casting using such a mold.

As another means, it is possible to improve the slab shape by rolling. For example, there is a method of using a caliber roll that can obtain a predetermined slab shape when slab-rolling a steel ingot. If the agglomeration step is omitted, the so-called dogbone shape should be formed by rolling with a vertical roll (for example, edger rolling) before the rough hot rolling with a horizontal roll to form the predetermined protruding portion. You can In addition, there is also a method of grinding / maintening the slab so that it has the shape of the present invention.

By the way, it is not always clear at present that the reason why the hot-defects can be improved or eliminated by making the thickness of the outermost edge of the slab thicker than the thickness of the central portion or the presence of the protruding portion is as follows. Can be considered. As described above, all the microcracks generated during the conventional rough hot rolling are cracked in the C direction, which indicates that tension in the rolling direction acts during rolling. It is considered that this rolling direction tension is due to the difference in metal flow between the slab center part and the edge part in the rolling direction. That is, the width of the edge portion is widened during rolling, and the amount of metal flow in the rolling direction is smaller than that in the central portion. As a result, the metal at the edge portion is dragged by the flow of the metal at the central portion, and tension in the rolling direction is generated at the edge portion. The above is considered to be the reason why even the conventional perfect rectangular slab had microcracks in the edge portion. See also FIG.
In the recessed slab shown in (b), it is considered that the tensile force in the rolling direction becomes high and defects are concentrated in the recessed area where the slab thickness is small and the metal flow in the rolling direction is small.
Therefore, the reason why the occurrence of hot-rolled defects in the slab shape according to the present invention is reduced is to increase the metal flow amount in the rolling direction of the edge portion to reduce the difference from the metal flow amount in the central portion,
It is presumed that the tension in the rolling direction at the edge portion was lowered to reduce the occurrence of hot rolling defects. That is, the method according to the present invention has a major feature that it is an invention that matches the metal flow during rolling, regardless of the steel type.

[0016]

Embodiments will be described in detail below with reference to embodiments. Table 1
The stainless steel having the components shown in 1) was melted according to the usual melting method, and the slab center thickness was 165 mm and the slab width was 125 mm.
Slabs of 0 mm and 1000 mm were cast. There are three types of molds used in casting. For casting a 1000 mm width slab, an A mold with a completely rectangular inner surface is used, and for casting a 1250 mm width slab, a B mold with a completely rectangular inner surface and a C shape having the shape shown in FIG. A template was used. The short side length L _E of the edge portion of the C mold was 10 mm longer than the short side length L _C of the central portion. The drawing speed during continuous casting was set to two levels, high speed casting and low speed casting. A part of the obtained slab was subjected to hot rolling in the same shape, and a part of the slab was maintained and subjected to hot rolling while changing its shape. All hot-rolled coils were made into a cold-rolled coil having a thickness of 1.5 mm through a normal pickling and cold-rolling process. The cold rolled coil was rewound to determine the number of flaws generated per 1 m in the rolling direction, which was taken as the flaw occurrence frequency.

Table 2 summarizes the above process conditions, the slab shape and the defect occurrence frequency. W in the table is the slab width, h
_C is the average thickness of the slab center, h _E is the slab thickness at the slab outermost edge (slab end face), Δh is the maximum recess depth when the slab has a recess (depth from the horizontal plane of the slab center) ), Δw is the distance from the slab outermost edge (slab end face) to the maximum recess depth generation position, and ΔS is the portion that protrudes from the virtual rectangle when the virtual slab (defined by w and h _C ) and the material slab are overlapped. , K means the ratio (ΔS / S) between the ΔS value and the virtual rectangular area S (= h _C × w).

As is clear from Table 2, it is recognized that the occurrence of surface defects is smaller when the slab according to the method of the present invention is used as compared with the slab according to the conventional method. The effect of the present invention is clear by comparing the defect occurrence frequency of the slab code 10 that is determined to be a substantially complete rectangular slab with the defect occurrence frequency of the slab of the present invention. Particularly, the slab according to the present invention as defined in claim 2 (slab code: 3, 4, 9, 11, 12, 1)
The defect occurrence frequency in the case of manufacturing 3) is extremely low, and the yield improving effect is remarkably large.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

As described in detail above, the effects of the present invention are
When a steel sheet is manufactured by hot rolling, hot rolling from a slab having a predetermined slab shape can significantly reduce the surface defects of the product and improve the product yield, which is a great industrial benefit.

[Brief description of drawings]

1 shows a cross-sectional shape of a slab according to the invention. Here, the cross section means a plane perpendicular to the rolling direction when the slab is rolled.

FIG. 2 is a view showing a conventional slab shape, in which (a) shows a cross section of a completely rectangular slab obtained when bulging does not occur during casting, and (b) shows slab length due to bulging during casting. The slab cross section when a concave part is formed in the side surface is shown.

FIG. 3 shows a cross section of a continuous casting mold for obtaining the slab shape of the present invention. The cross section here means a plane perpendicular to the casting direction.

Claims (2)

[Claims] 1. When manufacturing a steel sheet by hot rolling,
A method for producing a steel sheet with less surface defects by hot rolling, characterized in that the shape of a raw material slab is set such that the slab thickness at the outermost edge portions of the long sides facing each other in its cross section is made larger than the slab thickness at the central portion. 2. When manufacturing a steel sheet by hot rolling,
The shape of the material slab is assumed to be a short shape defined by the length w of the long side and the central portion slab thickness h _C that face each other in the cross section, and when this rectangle and the slab cross section are overlapped, it protrudes from the rectangle. A portion (hereinafter referred to as a protruding portion) is obtained, and the protruding portion exists at the slab edge portion, and the area ΔS of the protruding portion and the rectangular area S (S = w)
The method for producing a steel sheet with few surface defects by hot rolling, characterized in that the ratio k value (= ΔS / S) with respect to × h _C ) is set in the range of 0.003 or more and 0.03 or less.