JPS5881502A - Method for preventing formation of overlap flaw in bloomed slab - Google Patents

Abstract

PURPOSE:To prevent the formation of overlap flaws of bloomed slabs easily by curving the slopes in the projecting part of a square truncated pyramid shape formed on the bottom surface of a steel ingot toward the bottom surface side of the steel ingot and setting the contact angle between the slopes and the bottom surface of a steel plate and the height of the projecting part within a specified range. CONSTITUTION:In the stage of ingot making, a projecting part 5 of a square truncated pyramid shape or an elliptical truncated pyramid shape of which the slopes are increasingly wider toward the bottom surface side thereof is formed on the bottom surface of a steel ingot 4, whereby overlap flaws to be formed in the part of a bloomed slab corresponding to the bottom part of the steel ingot 4 is prevented. The respective slopes of said projecting part 5 are curved toward the diametrically opposed slope sides at about 5-200mm. raduis R of curvature, and the contact angle theta between the curved slopes thereof and the bottom surface on the projecting part 5 side of the ingot 4 is regulated to <=20 deg.. The height (h) of the part 5 from the bottom of the ingot 4 is set at 50-100mm.. Although the width in the bottom edge parts 6 around the part 5 has no direct influence upon the formation of overlap flaws, the width is preferably regulated to about 50-180mm. for convenience of ingot making work.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing folding defects that occur in a portion of a slab corresponding to the bottom of a steel ingot during blooming.

When manufacturing slabs for hot rolling by blooming a steel ingot obtained by the ingot making method, if the bottom of the steel ingot is flat, the end (tail) of slab 1 will have a fishtail as shown in Figure A1. 2
It is known that the mechanical pipe 3 is generated.

Such fishtail and mechanical pipe formation parts are cut off and removed as crop loss after blooming, so in order to improve the product yield, it is necessary to minimize their formation (
-It is desired to do something.

For this reason, suppression of fish sol and mechanical vibration generation at the slab end is set as L1.
5. During ingot making, a sloping surface or a protrusion 5 in the shape of a square truncated pyramid or an elliptical truncated pyramid is formed on a part of the bottom surface of the steel ingot 4. This is commonly done.

If such a convex part 5 is formed in a steel ingot, the formation of fishtails and mechanical pipes due to blooming will be reduced, but the bottom edge 6 around the convex part will not form with the convex part 5 during blooming. Starting from the boundary line 7 of , it is bent toward the convex portion 5 side, and a folding flaw 8 as shown in FIG. 3 is generated.

In the past, these folding defects were partially removed by cold scarfing after cooling the slab, but in many cases, like fishtails and overlaps, they had to be removed as crop losses.

Therefore, as a method for preventing folding defects, a method is used in which the width W of the bottom edge 6 around the convex portion is made zero.

However, since the convex part is formed by a well drilled on the F side of the surface plate on which the mold is placed during ingot making, in order to prevent folding defects using the -F distribution method, the mold size and well size are It is necessary to always prepare a large number of surface plates with matching surfaces and replace them every time the mold dimensions are changed. It is necessary to set it accurately as it will cause generation. Therefore, in the case of the above-mentioned method, there was a problem in that it required a lot of labor and time to replace the surface plate and set the mold.

In order to solve this problem, the present inventors have developed a method in which a convex part in the shape of a truncated quadrangular pyramid or a truncated elliptical pyramid is formed on a part of the bottom surface of a steel ingot, and a bottom edge remains around the convex part. The slope of the truncated pyramid-shaped or elliptical truncated-cone-shaped convex part is curved toward the bottom of the steel ingot, and the contact angle between the slope and the bottom of the steel ingot on the convex side and the height of the convex part from the bottom of the steel ingot are set within a certain range. If you do that,
It has been found that the radius of curvature of the slope and the bottom edge around the convex portion are related to the radius of curvature of the slope (and that folding cracks can be prevented from forming on the slab).

Table 1 shows that each slope on the bottom of the steel ingot 4 is curved toward the bottom of the steel ingot 4 and curved toward the opposite slope. This figure shows the state of folding defects generated in a slab when a steel ingot 7 having an elliptical truncated cone-shaped convex portion 7 is bloomed under conventional conditions. θ in the table is as shown in Figure 4,
It is the contact angle between the slope of the convex part and the bottom 1a of the convex part side steel ingot,
Further, h is the height of the convex portion from the bottom surface of the steel ingot. The width W of the bottom edge 6 around the convex portion (not shown in the table) is 50 to 150 mm, and the radius of curvature of the slope 1 (is 50 to 200 + III
+ is □.

Table 1 (Note) ○Folding flaws/yellow ~ △Mild folding flaws × Folding consternation From Table 1, the formation of folding flaws is determined by the contact angle θ and the height h of the convex part from the bottom of the steel ingot. If θ is 20 degrees or less and h is 50 to 100 degrees, folding defects will not occur.

On the other hand, even if θ is 2011 or less, if h is out of the above range, not only will it not be possible to prevent folding defects, but the proportion of overlamp portions and fishsol portions will increase, resulting in a bottoming out of the yield. In other words, when h is set to 50, the number of oat wraps in the center of the slab increases, and when h exceeds 100, it becomes convex (5).

It becomes a fishtail.

In the case of the present invention, as long as the contact angle θ and the height h of the protrusion from the bottom of the steel ingot have the above relationship, the width W of the bottom edge around the protrusion + the radius of curvature R of the body surface is equal to 1 no. However, it is preferable to control these values within a certain range for the purpose of agglomeration work.

In other words, if the width W is less than 50 mm, it takes time to align the well position of the surface plate and the mold when setting the mold on the surface plate during ingot preparation, resulting in a decrease in work efficiency, and conversely, the width W is less than 180 mm. (If the well diameter of the surface plate is too small, the surface plate may be melted and damaged when molten steel is injected using the top pouring method, and the contact angle θ may differ from the set value.) It is preferable that the width W is 50 to 180 m+.

Regarding the radius of curvature R, if R is too large, the bottom area of the surface plate well will decrease, causing melting damage to the surface plate and causing scratches on the surface of the steel ingot due to splash generation. From this point of view, it is preferable that R be in the range of 50 to 200.

Table 2 shows the occurrence of folding defects at the slab end and the crop loss rate when steel ingots made in the same mold (6) by the method of the present invention and the conventional method were bloomed under the same conditions. be.

No. 2 Clothing (Note) The dimensions of the steel ingot, as shown in Figure 5, indicate the dimensions of the following parts.

(a) a and b are the middle of the long side and the middle of the short side of the bottom of the steel ingot, respectively.

(b) C and d are the long side [1'' and short side 1] of the convex portion at the base on the bottom side of the steel ingot, respectively.

(c) C' and d' are the middle of the long side and the middle of the short side of the tip of the convex portion, respectively.

) Wl and W2 are the bottom edge around the protrusion on the bottom of the steel ingot, Wl is the long side IIJ, and W2 is the short side l].

0→ θ1 and θ2 are the contact angles between the steel ingot bottom 11 and the slope of the convex part, θ1 is the contact angle on the long side of the steel ingot bottom, and θ2 is the contact angle on the short side.

(c) h is the height of the convex part from the bottom of the steel ingot.

As described above, according to the present invention, it is possible to prevent the formation of folding defects during blooming rolling under the same agglomeration conditions as conventional ones,
The crop loss rate is also reduced and the blooming yield can be improved.

[Brief explanation of the drawing]

FIG. 1 shows the shape of the end portion of the slab, (5) is a plan view showing the middle direction, and (B) is a side view showing the thickness direction. FIG. 2 does not show the conventional steel ingot shape; (A) is a front view showing the long side, and (r3) is a side view showing the short side. FIG. 3 shows an end portion of a slab obtained by blooming a steel ingot with a convex portion formed on the bottom surface, in which (A) is a plan view showing the width direction, and (B) is a side view showing the thickness direction. No. 1
Figure (・ Shows the bottom of the steel ingot used in civil engineering inventions,
) is a front view showing the long side 0I11, and (13) is a 1'' side view showing the short side. Fig. 5 shows the parts of the steel ingot used in the examples, and (A) shows the bottom side. (B) is a front view showing the long side side, and (B) is a side view showing the short side side. 1...Slab, 2...Fishtail, 3...
Mechanical pipe, 4... Steel ingot, 5... Convex part, 6...
・Bottom edge around the convex part, I... Boundary line, 8... Folding flaw, W... Width of bottom edge, θ... Convex slope and convex part 1
itlI, i1! l Contact angle with the bottom of the ingot, R... Radius of curvature of the slope, 11... Height of the convex part from the bottom of the steel ingot

Claims (1)

[Claims] During ingot making, a convex part in the shape of a truncated cone or truncated elliptical cone is formed on the bottom of the steel ingot on an inclined plane or as it goes towards the vertical side. In a method of eliminating folding defects that occur in a portion corresponding to the bottom of the lump, each slope of the convex portion is curved toward the opposite slope side, and the curved slope and the bottom of the steel lump on the side of the convex portion are curved. ] The contact angle (θ) with m is 20 degrees or less, and the height g (11) of the convex part from the bottom of the steel ingot is 50 to 100 m. How to prevent it.