JPH03207551A - Stainless steel cast slab and mold for continuous casting - Google Patents

Abstract

PURPOSE:To remarkably reduce seam flaw on various kinds of hot rolled stainless steels by making center parts of both short sides the recessed shape inward in the range of the specific ratio to thickness of the cast slab over the whole area in casting direction. CONSTITUTION:In a mold 1 having rectangular shape in cross section, by bulging the center part 2 at both short sides inward to the cast slab side in the range of 3-15% the thickness of cast slab to be produced over the whole area in the casting direction, the cast slab having recessed shape inward at center part of the side face, is obtd. The recessed part of cast slab absorbs three dimensional width expansion (bulging) at the time of hot-rolling. (This phenomenon occurs, because such plastic deformation that center part in the plate thickness at rolled side face becomes projection in almost all case of hot rolling to the stainless steel plate by using the ordinary rectangular cast slab, is developed.) Therefore, over-hang quantity of the side face onto the horizontal face in the cast slab at the time of hot rolling can be reduced. By this method, the seam flaw in the hot rolling is reduced and trimming quantity for removing seam flaw before cold-rolling is reduced.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is particularly applicable to a material for a belt-shaped stainless hot rolled steel sheet.
The present invention relates to a stainless steel slab suitable for preventing the occurrence of surface flaws near the width end of a copper strip, and a mold for obtaining the same.

(Prior Art) Line-shaped surface flaws called seam flaws, which are parallel to the rolling direction, occur over a range of 10 to 20 mm on each side at both ends of the width of a hot-rolled stainless steel strip. These seam flaws occur over a much wider area than in ordinary steel and do not disappear even in the subsequent cold rolling process, so they are usually removed by trigging before cold rolling. Therefore, this results in a significant decrease in yield.

Various proposals have been made to prevent seam flaws.

For example, Japanese Patent Application Laid-Open No. 6 discloses a method of refining the crystal grains on the end face of the slab and preventing seam defects by rolling the slab in the width direction at a reduction rate of 15% or more before applying horizontal reduction to the slab.
It is disclosed in Japanese Patent No. 0-240301. Further, JP-A-60-33803 discloses a method for preventing seam defects by chamfering the shape of the corners of the slab to reduce the amount of wraparound of the end face toward the rolling surface side from the perspective of plastic working.

(Problem B to be Solved by the Invention) However, the method exemplified above is not necessarily a practical measure for preventing seam flaws in stainless steel. For example, if the former method (Japanese Patent Application Laid-Open No. 60240301) is attempted, a powerful width reduction device is required, which consumes a large amount of energy, and the rolling shape is also called dongbone after width reduction. A bulge at the width end is likely to occur and cause other scratches. In addition, the latter method (Japanese Unexamined Patent Publication No. 60-33803) requires a great deal of labor, and when applied to ferritic stainless steel, which has poor toughness, cracks due to thermal stress during machining occur. This has caused problems such as the occurrence of sludge defects that appear as sludge defects after hot rolling.

The object of the present invention is to solve these problems and provide a stainless steel slab that can prevent the occurrence of surface flaws as much as possible, and a continuous casting mold for obtaining the same.

(Means for solving the problem) The mechanism of occurrence of seam flaws is described in, for example, Japanese Patent Application Laid-Open No. 60-24030.
As stated in Publication No. 1, the following is often considered. First, horizontal reduction causes unevenness (wrinkles) on the side surface of the slab, which corresponds to the size of the crystal grains, and in some cases, cracks may occur. Furthermore, by applying horizontal reduction, the sides of the slab turn around to the horizontal plane (both front and back sides) due to three-dimensional deformation of the width edge, and the above wrinkles or cracks remain as linear flaws (seam flaws) at the width edge of the steel plate. do. In this way, the occurrence of seam defects is thought to be influenced by metallographic factors and plastic working factors in a complex manner.

The metallographic structure of common steel during hot stress U-working is generally austenite single phase, but stainless steel may have several types. Stainless steels include austenitic stainless steels such as SUS304, ferritic stainless steels such as SUS430, and martensitic stainless steels such as SUS410. The metal structure of martensitic and austenitic stainless steels during hot rolling is almost the same as ordinary steel, which is an austenitic structure, but ferritic stainless steels have a ferrite-austenite structure that contains around 30% austenite phase, like SUS430. SUS409 exhibiting a two-phase structure
Some exhibit a single-phase ferrite structure, such as In general, the ferrite phase is difficult to recrystallize during hot working, so
It is recognized that it is easy to carry over the coarse solidified structure during casting, and it is difficult to achieve a finer structure. It can be said that the influence greatly differs depending on the type of stainless steel mesh.

The deformation of a material in the width direction during rolling is a problem that has been extensively studied both experimentally and theoretically. It is known that a double bulge or single halge side surface shape appears in the rolling of ordinary steel, but a method to prevent seam defects by applying pressure reduction at the time when the side surface shape is appropriate has been disclosed. It is disclosed in Publication No. 62-224406. However, in stainless steel, there are mesh types with different hot deformation resistances and various metallographic structures, so there are still many quantitative aspects of the three-dimensional deformation at the width ends of the slab that are unclear.

As is clear from the above, there is currently no method to theoretically clarify the shape of a mold that can produce stainless steel slabs with a shape that causes fewer seam flaws and a metal weave. It is no exaggeration. Therefore, in order to achieve the above object, the present inventor manufactured several types of stainless steel slabs using molds of various shapes,
Then, by performing hot rolling and experimentally reproducing the seam flaws, by looking at the results, he invented a stainless steel slab that could achieve the purpose and a mold for manufacturing this slab.

In other words, the stainless steel slab according to the present invention has a shape in which the center portions of both short sides are recessed inward within a range of 3 to 15% of the slab thickness over the entire casting direction. It is something to do.

Further, the continuous casting mold for stainless steel according to the present invention is a mold having a rectangular cross section, and the center portions of both short sides of the mold have a thickness of 3 to 3 mm, which is the thickness of the slab to be manufactured, over the entire casting direction. ~15
The gist is that the molding side bulges inward within a range of %.

In the present invention, the reason why the amount of inward bulge at the center of both short sides of the mold is set to 3 to 15% of the thickness of the slab to be manufactured is as follows.

That is, if it is less than 3%, as shown in FIG. 3, the effect of reducing seam flaws caused by the above-mentioned plastic deformation control will not be sufficient. On the other hand, if the bulge on the inner short side of the mold is further increased, the solidification rate of the stainless steel in this area will slow down (slow cooling), leading to coarsening of the solidified structure, which tends to promote the occurrence of seam defects. bring. Then, even after plastic deformation, the excessive dents cannot be absorbed by the hulling deformation and buckling occurs, leaving flaws such as splits in two at the end of the hot-rolled stainless steel sheet. Therefore, it is necessary that the depth of this flaw is not larger than the width of the seam flaw.

Therefore, if the amount of expansion of the inner short side of the mold exceeds 15% of the thickness of the slab, seam flaws and trimming due to the above flaws will increase, and this will not serve the purpose of the present invention. Therefore, the upper limit was set at 15%.

Incidentally, this mold may be used for continuous casting or steel ingot casting, and hot rolling is performed in the same direction as the casting direction. The material of the mold includes general materials such as cast steel or copper that can form such a mold shape. The slab of the present invention can be easily manufactured using the mold described above, but it is also possible to shape the slab after casting by machining or by forging. In general, the effect of preventing surface flaws is the same.

(Function) As in the present invention, by inflating the central part of the short side of the mold inward, a slab having a shape in which the central part of the side surface is concave is obtained, and the concave part of the slab is hot-rolled. (This phenomenon occurs in most cases when hot rolling stainless steel sheets using regular rectangular slabs occurs when the central part of the plate thickness on the rolling side is This is because it undergoes plastic deformation that becomes convex). Therefore, it is possible to reduce the amount that the side surface of the slab wraps around the horizontal surface during hot rolling.

(Example) Below are the results of experiments conducted to confirm the effects of the present invention.
The explanation will be based on the attached drawings.

The present inventor first developed a cast steel mold 1 having an inner cross-sectional dimension of approximately 52 mm x 104 mm and having various inner shapes.
was created. An example is shown in FIG.

As for the shape of the inner cross section, we considered a shape with a curved bulge 3 on the inside of the short side 2 as shown in FIG. 1, or a shape with a curved bulge on the outside (not shown).

Next, these molds 1 are made of SUS304 in a vacuum melting furnace.
A stainless steel slab was obtained by injecting molten metal at 1600°C with a combination of 430 and 409 stainless steel.

In order to perform hot rolling to reproduce seam flaws, the black scale of the above slab was removed by grinding, and a sheet of approximately 50 mm x 1
A rolled test piece having dimensions of 00 mm x 150 rra was used. For hot rolling, the diameter is 500 mm, 35
Two horizontal reduction bath schedules were carried out on a rolling mill with 0 mm and 200 mm rolls.

The schedule is as follows.

(a) 50 →44 →34 →23 →14.5 →
9.5 →7.0 →5.0(b)5 0→39.5→
26 → 16.5 → 11, 3 → 7.9 → 6.3 → 5.0 This rolling schedule is approximately 1/1 of the rolling schedule in a continuous hot rolling mill that produces actual stainless hot rolled strips.
It is of 4 scales.

The heating conditions and rolling temperature conditions were within the range normally used for the stainless steel type. That is, SUS3
04 is 1250°C, SUS430 is 1200°C, S
USl09 was heated to 1.50°C, and the rolling start temperature was the same as the heating temperature (1150°C for SUS304, 1150°C for SUS4
30 is 1125°C, SUS409 is 1100°C, etc.)
The width of seam flaws generated at both ends (four locations) of the front and back surfaces of the hot-rolled stainless steel sheet was measured, and the "width of seam flaws" was evaluated by half the sum of the widths. FIG. 3 shows an example of the results organized by mold shape. This shows the change in the width of seam flaw occurrence when the height of the bulging portion of the mold of the system shown in FIG. 1 was varied. Under the conditions indicated by the whiskers on the upper right side of the marks indicating data points in the figure, a significant two-piece flaw occurred on the side surface of the steel plate. From FIG. 3, compared to the width of seam flaw occurrence in stainless steel when a conventional rectangular mold is used (a value corresponding to 10 mm of short side bulge on the horizontal axis in FIG. 3), the present invention In other words, ``a mold characterized by having a shape in which the center portions of the left and right short sides of the inner side bulge inward by 3 to 15% relative to the casting thickness in a vertical cross section in the longitudinal direction of casting, or a mold cast using such a mold. It is clear that the width of seam flaws is significantly reduced when using stainless steel slabs. In addition, seam flaws with a width approximately four times the width of the seam flaws corresponding to the rectangular mold shown in Figure 3 have occurred in the actual hot-rolled stainless steel strip, and the similarity law is almost the same between this experiment and the seam flaw width. It can be said that it is imposing (slab thickness: 200 ribs, roll diameter: 1100-1400
m+n). From these facts, it is presumed that the effectiveness of the present invention is widespread.

Note that the bulging portion provided at the center of the short side of the mold is not limited to the shape shown in FIG. 1, but may of course have the shape shown in FIGS. 2(a) to (C). .

(Effects of the Invention) As explained above, the present invention is a mold having a rectangular cross section, and the center portions of both short sides are in the range of 3 to 15% of the thickness of the cast billet over the entire region in the casting direction. 11 The mold bulges inward and the stainless steel slab cast by this mold can be used to easily and inexpensively eliminate seam defects in hot-rolled stainless steel sheets of various steel types. The amount of trimming required to remove seam flaws before cold rolling can be significantly reduced.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the shape of a mold according to an embodiment of the present invention.
(a) is a front view, (b) is a side view, Fig. 2 (a) (b)
) (C) is a diagram showing an example of a shape that can be applied as the shape of the inner short side of the cross section in the casting direction of the mold according to the present invention, and FIG. 3 is a graph showing the relationship between the seam flaw generation width and the mold shape. 1 is the mold, 2 is the short side, and 3 is the curved protrusion. 1 2 Figure 2 J4 1F Figure 3 Arrow 1, j. <'3 Minonohira Mi) Nibunzai Suukichi 1Y (
H) Yazumasa, J. Kurami f (mn)

Claims (2)

[Claims] (1) A stainless steel slab characterized by having a shape in which the center portions of both short sides are recessed inward within a range of 3 to 15% of the thickness of the slab over the entire casting direction. (2) A mold with a rectangular cross section used when manufacturing the slab according to claim 1, and the thickness of the slab to be manufactured at the center part of both short sides of the mold over the entire casting direction. 3-15% of the size
A stainless steel continuous casting mold characterized by an inward bulge on the casting side within a range of .