JPH06312248A - Slab and mold - Google Patents

Abstract

PURPOSE:To restrain the development of surface crack at the time of rolling a slab and to reduce the surface flaw on a product. CONSTITUTION:In this slab, the thickness in the spans of 1/6 width from both ends in the width direction of the slab is thicker than the average thickness in the span near the center of the width direction from the above spans. Further, the average thickness H<e> in the spans of 1/6 width from the both ends of the slab is made to be the thickness calculated in the following inequality by using the average thickness H<c> in the span near the center of the width direction from the above spans. H<e>=alpha(H<c>-h)+h. Wherein, 1.1<=alpha<=1.4, h: slab thickness after hot rolling. Further, a mold, to which related to the gap between the inner walls of the cross section thereof crossing at the right angle in the casting direction, the gap between the inner walls of the mold corresponding to the slab thickness in the spans from the both ends in the width direction to the 1/6 width of the cast slab is larger than the average value of the gap between the inner walls in the thickness direction in the span near the center of the width from the above spans, is used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a slab and a mold capable of reducing surface defects generated during hot rolling when manufacturing a slab to be subjected to hot rolling for manufacturing a steel sheet.

[0002]

2. Description of the Related Art Generally, surface defects are apt to occur at the edges of hot-rolled steel sheets, and surface defects called "scarring flaws" caused by micro-cracks during hot rolling are particularly If the produced scale bites into the inside by rolling and is subjected to the cold rolling step without being removed in the pickling step, it becomes a linear defect that is long in the rolling direction, and the yield reduction is particularly large.

Therefore, various techniques have been devised from the prior art in order to reduce surface defects during hot rolling. As a method of defining the component system of the material to be rolled, for example, JP-A-57-16153 for austenitic stainless steel and JP-A-3-294001 for steel containing Mn and S.
However, these limit the degree of freedom for the component system and are not general. In addition, Japanese Examined Patent Publication No. 55-50723, JP-A No. 2-15
No. 806 and the like disclose a technique for removing surface defects (pinholes) on a slab by caring for them to eliminate the occurrence of bald spots. However, in this technique, a process of care removal is indispensable, and if it is not sufficient, a slab surface defect that is a starting point of cracking will remain, and minute cracks will occur after hot rolling.

Further, as a technique for reducing the occurrence of flaws in a slab shape, JP-A-58-138502 and JP-A-3-2 are known.
No. 07551 is mentioned. Both of them disclose a technique of reducing the edge seam flaw of stainless steel by recessing the central part of the short side of the slab and suppressing the width spread (metal flow in the width direction) in the vicinity (nearest) of the short piece. However,
The effect of changing the shape of the C section of the slab short piece (cross section in the direction perpendicular to the casting) appears only in the vicinity of the edge, and it is not possible to deal with the case where microcracks on the surface during hot rolling occur in a relatively wide range.

Japanese Patent Publication No. 61-43133 is known as a conventional technique relating to the cross-sectional shape of a mold. However, this technology reduces cracks that occur during straightening by changing the cross-sectional shape of the mold (width of the upper and lower surfaces of the slab) and reducing the tensile stress that occurs on the upper surface of the slab when straightening the slab of a continuous casting machine. However, it cannot prevent cracking during hot rolling which is a post-process.

[0006]

DISCLOSURE OF THE INVENTION The present invention is capable of improving surface defects of a steel sheet in improving surface defects that occur during hot rolling without restricting the composition of the slab to be rolled or increasing the process load. It is intended to provide slabs and molds.

[0007]

In order to achieve the above-mentioned object, the present invention defines a width direction distribution of slab thickness (hereinafter abbreviated as thickness distribution) so that the rolling direction generated inside the slab during rolling. It is characterized in that fine cracks are prevented by adjusting the stress state and suppressing the generation of tension near the edges, which is the cause of cracking. The main points are (1) Figure 1
As shown in (a), the thickness of a section of 1/6 of the slab width from both ends in the width direction of the slab is made larger than the average thickness of the section closer to the center of the width, and
(2) an average thickness H ^e from both ends in the width direction of the slab of 1/6 of the slab width section is calculated by the following equation using from the average thickness H ^c of the width inboard section which It is to be a quantity. ^{H e = α (H c -h} ) + h where, 1.1 ≦ α ≦ 1.4, h is further slab thickness after hot rolling, (3) as shown in FIG. 1 (b), casting Regarding the inner wall gap of the mold in the thickness direction of the slab (hereinafter abbreviated as the inner wall gap) in the cross section orthogonal to the direction, the inner wall gap corresponding to the slab thickness in the section from both widthwise ends to 1/6 of the cast slab width is The mold is characterized in that it is larger than the average value of the inner wall gap in the section closer to the width center than this.

[0008]

The present invention will be described in detail below. The present inventors have completed the present invention by scrutinizing the relationship between the morphology / frequency of flaws generated during hot rolling and the thickness distribution of the slab to be rolled.

First, the inventors of the present invention have scrutinized the occurrence of hot rolling flaws in slabs. As a result, as shown in FIG. 2, it was found that all the flaws that were generated were C-direction cracks (cracks in the direction perpendicular to the rolling direction), and the frequency of occurrence was higher the closer to the slab edge. In addition, the size of the cracks is 0.1 to 2 mm in the C direction and 0.1 mm in depth, and the cracks are concentrated and appear after the first pass of rough hot rolling, and the subsequent rolling process such as hot rolling or cold rolling. It was confirmed that the resulting product was stretched in the L direction (rolling direction), resulting in a bald flaw or the like that fatally deteriorates the surface quality of the final product plate.

The present inventors have paid attention to the above experimental facts, particularly that all microcracks are cracked in the C direction, and the tension in the rolling direction generated inside the slab during rolling causes cracking, that is, the main cause of surface flaws on the product. Presumed to be. The generation of the tension in the rolling direction can be roughly explained by the difference in the metal flow in the rolling direction between the central portion of the slab width and the edge portion. That is, the width of the slab edge portion is easily widened during rolling, the amount of metal flow in the rolling direction is smaller than that in the central portion, and the material of the edge portion is relatively stretched by the material of the central portion. Tension in the rolling direction is generated in the part.

However, with such a qualitative interpretation, detailed examination of the flaw generation mechanism and evaluation of the flaw prevention measures as in the present invention cannot be achieved. Therefore, the present inventors analyzed the stress in the rolling direction by the three-dimensional rigid-plastic finite element method and obtained the following findings.

As shown in FIG. 3, the stress in the rolling direction (positive tension) generated inside the slab during rolling is distributed in a substantially parabolic shape in the width direction and changes from compression to tension from the width center to the edge. To do. The area where tension is generated is about 1 from the edge.
It is a / 6 width portion, and becomes the maximum near the edge. Also,
If the so-called slab size (plate thickness / plate width ≧ about 0.1), this tendency does not change. Of these findings, the width of the tension generation region is understood as follows. In single-stand rolling of a slab, the total stress in the rolling direction in the width direction is zero, that is, the average value of the stress in the rolling direction in the width direction is zero.
Here, if the distribution of stress in the rolling direction is a parabola (quadratic curve), the position in the width direction that gives the average value is 1 / (2
^{.3 1/2} ) width, that is, (3-3 ^1/2 ) / 6 from the edge =
1.268 / 6 wide point, about 1 from the above edge
The result that tension is generated in the / 6 width portion is understood.

This rolling-direction stress distribution corresponds well to the flaw generation situation shown in FIG. 2, and indicates that flaws (cracks) are concentrated and generated at a portion where high tension is generated in the rolling direction. This means that if the tension in the rolling direction inside the slab that occurs near the edge during rolling can be reduced, the occurrence of flaws can be essentially suppressed.

The distribution of the stress in the rolling direction in the width direction can be understood macroscopically from the difference in the drawing in the rolling direction accompanying the metal flow in the width direction. In other words, the slab thickness near the edge is increased. It is considered that the tension generated at the edge portion can be reduced by giving such a thickness distribution. However, it is not clear what kind of slab thickness distribution can effectively suppress the tension generation.

Therefore, the present inventors examined this thickness distribution by the above-mentioned three-dimensional finite element method. as a result,
As shown by the broken line in FIG. 4, when the range of increasing the thickness is unnecessarily widened, the tension of the edge portion decreases, but the reaction (as described above, the sum of the rolling direction stress in the width direction should be zero). The tension increase near the width center becomes remarkable as the tension decrease at the edge part and the tension increase at the width center part of the same amount.) It was first found that the range from the edge to ⅙ of the slab width (hereinafter, abbreviated as edge ⅙ width portion) is suitable as the range in which is increased. Further, as shown in FIG. 5, the effect of reducing the tension does not depend so much on the pattern of the thickness distribution of the edge ⅙ width portion, and is almost determined by the average thickness, and as shown in FIG. edge 1/6 the average thickness H ^e width portion is marked in the width center portion as long as it is an amount that is calculated by the following formula using from the average thickness H ^c of the slab width central portion of the width inboard this It was found that the tension of the edge 1/6 width portion can be reduced without increasing the tension.

[0016] ^{H e = α (H c -h} ) + h where, 1.1 ≦ α ≦ 1.4, h is about setting a range of alpha slab thickness after hot rolling, the edge is less than 1.1 The effect of preventing the generation of microcracks due to the reduction of the tension of the part is not sufficient, and if it exceeds 1.4, the tension in the width center part increases, and the generation of microcracks in the width center part becomes a problem. is there.

As a means for increasing the slab thickness distribution in the edge portion and decreasing it in the width central portion, the mold shown in FIG. 1 (b) is preferable. That is, according to the slab and apparatus of the present invention, it is possible to effectively reduce the rolling direction stress (tension) directly related to the occurrence of surface cracks in the slab due to hot rolling, which is the starting point of product defects. In the present invention, the shape of the thickness distribution from the central portion of the slab width to the slab outermost edge is not particularly specified, but when the thickness distribution is discontinuous, additional shear deformation is concentrated near the discontinuity point. However, since there is a concern that defects may be caused due to this, it is desirable to have a smooth thickness distribution.

Other means for obtaining the slab of the present invention include a method of cutting a rectangular slab and a method of imparting a thickness distribution by edging rolling using a vertical roll. However, both require additional steps, and the former causes a reduction in yield and work efficiency, and the latter causes a new problem of cracking due to the rolling direction tension in the width center portion that occurs during edging rolling. .

[0019]

Embodiments will be described in detail below with reference to embodiments. Table 1
The stainless steel having the components shown in 1 was melted in a converter, and a slab having a slab width center thickness of 165 mm and a slab width of 1250 mm was continuously cast. When casting a slab, two types of thickness distributions (uniform distribution and distribution according to the present invention) shown in FIG. 7 are used.
In order to provide the inner wall gap of 165 mm in the entire width, the inner wall gap increases parabolically on both ends from the width center ± 410 mm point compared to the flat type and +5 mm at both ends.
2) with a profile that expands the inner wall gap
Different types of molds were used. The casting direction of the mold is tapered in consideration of solidification shrinkage, and the above-mentioned cross-sectional shape is the size of the mold outlet. The cross-sectional shape of the slab after casting is within ± 1 mm of the dimension shown in Fig. 7, and the difference in slab thickness at both ends of the slab when the flat type and the edge inner wall gap widening type are used is 4. It was 5 mm. Rolled diameter 120
Using a 0 mm roll, the rolling temperature was set to 1100 ° C. and the rolling reduction at the width center was set to 10%.

FIG. 8 shows the widthwise distribution of the occurrence frequency of slab surface cracks after rolling. As is apparent from the figure, when the mold for expanding the inner wall gap of the edge portion to which the method of the present invention is applied is used, the frequency of occurrence of cracks near the edge is drastically reduced, and the yield is significantly improved.

[0021]

[Table 1]

[0022]

As described above in detail, according to the present invention, when producing a slab to be subjected to hot rolling for producing a steel sheet, by setting an appropriate slab thickness distribution and casting, The surface flaws of the product after hot rolling can be significantly reduced, and the product yield can be improved, which is a great advantage in terms of industry.

[Brief description of drawings]

FIG. 1 (a) is an example of a slab thickness distribution according to claims 1 and 2 of the present invention. (B) A schematic view showing one mode of a mold (claim 3) for realizing the thickness distribution according to claims 1 and 2.

FIG. 2 is a schematic diagram showing a distribution state of cracks on the surface of a slab after rolling by a conventional method.

FIG. 3 is a diagram showing an example of an analysis result by a three-dimensional rigid-plastic finite element method regarding a widthwise distribution of rolling direction stress generated inside a slab during rolling by a conventional method.

FIG. 4 is a diagram showing an example of results of analysis performed to obtain knowledge about influences of a range in which the slab thickness is changed, a thickness distribution pattern near an edge, and an average thickness when the present invention is performed.

FIG. 5 is a diagram showing an example of a result of analysis performed to obtain knowledge about influences of a range in which a slab thickness is changed, a thickness distribution pattern near an edge, and an average thickness when the present invention is performed.

FIG. 6 is a diagram showing an example of results of analysis performed to obtain knowledge about influences of a range in which a slab thickness is changed, a thickness distribution pattern near an edge, and an average thickness when the present invention is performed.

FIG. 7 is a diagram showing a slab thickness distribution set in the example.

FIG. 8 shows a comparison between the conventional method and the method of the present invention regarding the occurrence frequency of slab surface cracks after rolling in the examples.

Claims (3)

[Claims] 1. In a slab to be subjected to hot rolling for producing a steel sheet, a thickness of a section of 1/6 of the slab width from both ends in the width direction of the slab is in a section closer to the center of the width. A slab characterized by being larger than the average thickness. 2. A steel slab is subjected to hot rolling for the production of an average thickness H ^e 1/6 section of the slab width from both ends in the width direction of the slab, which than the width near the center A slab characterized by a quantity calculated by the following formula using the average thickness H ^c of the section. ^{H e = α (H c -h} ) + h where, 1.1 ≦ α ≦ 1.4, h : slab thickness after hot rolling 3. In the inner wall gap of the mold cross section orthogonal to the casting direction, the inner wall gap corresponding to the slab thickness in the section from both widthwise ends to 1/6 of the casting slab width is a section closer to the center of the width. The mold is characterized in that it is larger than the average value of the inner wall gap in the thickness direction of the mold.