JPH0628786B2 - Large reduction method of slab in continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to unsolidification from the top portion, which is feared to occur when large reduction is performed in the vicinity of the solidification tip of a cast piece drawn from a continuous casting mold. The present invention relates to a reduction method capable of effectively preventing leakage of molten steel and ensuring sound quality.

(Prior Art) There are various methods for improving the internal quality of continuous cast slabs. Among them, it is effective to greatly reduce the vicinity of the solidification tip of the slab, and specifically A method of using and a method of using a forging die are known. In this regard, as literatures, for example, JP-A-61-14058, JP-A-61-4246,
JP-A 61-132247, JP-A 60-82257, JP-A 60-1
No. 21054 or JP-A No. 61-37356 is referred to.

(Problems to be solved by the invention) By the way, when performing the large reduction by applying the above-mentioned technology to the cast piece drawn out from the continuous casting mold, if the large reduction is continued even after the casting is finished, the unsolidified inside of the cast piece is not solidified. While molten steel may spurt from the top of the slab and damage the continuous casting machine, stopping the reduction at the end of casting to prevent such molten steel spurting will cause The internal quality could not be improved and the yield was forced to decline significantly.

Therefore, although a reduction is performed after the solidification of the slab is completed or under a condition close to the completion of the solidification, or at the end of the casting, a cold material is introduced to solidify the topmost portion of the slab, and then a trial is performed to perform the reduction. In the former case, since it is far from the proper rolling condition, the effect of improving center segregation etc. cannot be obtained, and in the latter case, the internal molten steel pushed up by the rolling breaks through the weak part of the solidified shell and leaks steel, or Since the solidified shell is deformed and the slab size becomes larger than the specifications of the continuous casting machine, there is a disadvantage that drawing cannot be performed.

In an attempt to counter this, the inventors previously proposed Japanese Patent Application No. 62-
In No. 328421, we proposed a reduction method using a steel leakage prevention metal frame. The above method is particularly advantageous for improving internal quality such as center segregation because it is possible to avoid scattering of molten steel from the top of the slab, which is caused by large reduction after the completion of supplying molten steel to the continuous casting mold. However, if the pulling out stop time when installing the steel frame for steel leakage prevention is not made as short as possible, solidification near the pressure reduction part is completed during the stop, and even if large reduction is restarted, the quality of the solidification part cannot be improved. there were.

It is an object of the present invention to provide a novel method capable of ensuring good quality even when there is no margin in such a metal frame installation time, that is, a slab drawing stop time.

(Means for Solving the Problem) The present invention applies a large reduction by a roll or a forging die to the vicinity of the solidification tip of a cast piece drawn from a continuous casting mold, and then withdraws the cast piece after completion of casting into the continuous casting mold. And once the large reduction is stopped, the steel frame for preventing steel leakage is inserted into the top part of the cast in the continuous casting mold, and then the continuous casting is continued in the continuous casting in which the large reduction is continued while pulling out the cast. It is characterized by compensating for the decrease in the unsolidified thickness in the reduction area due to the stoppage of the drawing of the slab by increasing the casting speed before the end of the casting into the mold than the speed at the time of steady large reduction. It is a method of large reduction of a slab in continuous casting.

(Operation) In the method of simply inserting the steel frame for preventing steel leakage (hereinafter simply referred to as the conventional method), if the drawing stop time of the slab exceeds a certain time, the unsolidified thickness of the slab at the rolling position is the minimum of the appropriate rolling conditions. Even if the casting speed is increased after the thickness becomes the thickness or less and the large reduction is restarted, the center segregation exceeds the upper limit value while recovering to the minimum thickness of the appropriate reduction condition, and therefore the slab in that area must be cut off. Becomes

Here, the upper limit of the stop time for ensuring the quality of the slab varies depending on the slab size, steel type, reduction amount, etc., and cannot be generally stated, but it is about 1 minute with a normal ^□ 300 mm bloom, It is generally not easy to stably install the metal frame in the.

Therefore, in the present invention, in order to increase the unsolidified thickness of the cast slab before drawing is stopped to the upper limit which is acceptable in terms of quality, the cast speed of the cast slab is made larger than the optimum cast speed in the steady state to extend the allowable stop time. The metal frame installation time was expanded to the range in which actual work was possible.

Specifically, withdrawal downtime slab during continuous casting _(t s), the unsolidified thickness (d) and the central segregation _{(C /} C o)
As shown in FIG. 1 which shows the relationship between the slab and the slab before the drawing is stopped, the drawing speed (v _c ) is increased from the steady optimum speed (v _o ) to (v ₀ ′) before the drawing is stopped. The unsolidified thickness (d) of the allowable limit (c
/ _Co ) 1 corresponding to the unsolidified thickness dou or less. That is, the minimum solidification thickness reduction due to the stoppage of the drawing of the slab is made larger than the solidification thickness dol necessary for improving the center segregation to extend the suspension time.

For the above-mentioned unsolidified thickness range dol to dou, for example, the optimum conditions are obtained in advance by an experiment as shown in FIGS. 2 to 5, and the optimum conditions are used as a reference, and correction is made in consideration of the actual rolling reduction. It can be easily decided.

Here, FIG. 2 shows the ratio (δ / d) of the reduction amount (δ) in the forging process to the unsolidified thickness (d) of the central portion of the slab and the solid fraction (f of the central portion of the slab at the reduction position. _s ), the preferred range for reducing the center segregation is shown in FIG.
Pressure from the relationship between the solid fraction at the center of the slab at the pressing position upon the pressure and (f _s) and slab thickness center of carbon segregation ratio (C / C _o) at /d≧0.5 conditions The preferred range for avoiding internal cracking and negative segregation caused by
The figure shows the thickness (D) and unsolidified thickness (D) of the slab obtained by continuous casting when the slab was reduced at δ / d = 0.5 or more.
The preferred range in which the center segregation can be reduced from the situation of the center segregation (C / C _o ) in FIG. 5 is shown in FIG. 5, and FIG. 5 shows the electromagnetic stirring in the relationship between the cast piece thickness (D) before the reduction and the unsolidified thickness (d). A suitable range effective in reducing the center segregation is obtained from each of the appropriate ranges.

By the way, FIG. 6 shows the relationship between the slab withdrawal stop time, the unsolidified thickness and the center segregation in the conventional method. In the conventional method, as described above, pull the stop time (t _s) is unsolidified thickness (d) becomes less than the minimum thickness of the proper pressure conditions in the pressure region exceeds a certain time, even if a subsequent v _{c =} v ₁ The range of the shaded area in the figure, that is, the length l _c needs to be cut off, which deteriorates the yield.

In the present invention, depending on the casting conditions and the stopping time, it becomes difficult to keep the unsolidified thickness (d) before and after stopping the withdrawal of the cast piece within a predetermined range (dou to dol) as shown in FIG. 7, Although a defective part l _c ′ may occur, the amount of cut-off in the region accelerated before the pulling out can be reduced as compared with the conventional method, which is advantageous in improving the yield.

(Example) When continuously casting a slab having a thickness of 270 mm and a width of 340 mm (steel type S53C), a forging process with a reduction amount of 70 mm was performed under the conditions shown in Table 1 near the completion of solidification of the slab to prevent leakage of molten steel. The presence / absence, center segregation, and cut length (I _c ) of defective cast pieces were investigated. The results are also shown in Table-1.

From the above results, it was confirmed that according to the present invention, the center segregation was remarkably improved and no defective cast piece was generated.

(Effects of the Invention) According to this invention, the following effects can be expected.

1) Molten steel does not leak even after the completion of casting, and the molten steel can be rolled over almost the entire length in the vicinity of the optimum rolling condition, so that the quality of the cast piece at the top is improved and the yield of the cast piece is greatly improved.

2) It can be carried out only by controlling the withdrawal speed of the slab, so it is inexpensive, and it can be applied to conventional continuous casting machines without any problems, so the range of application is wide.

[Brief description of drawings]

FIG. 1 is an explanatory view of the reduction procedure according to the present invention, FIG. 2 is a graph showing the relationship between the reduction amount and the unsolidified thickness and the solid fraction in the center of the slab, and FIG. Phase ratio relationship graphs, FIGS. 4 and 5 are relationship graphs of cast slab thickness and unsolidified thickness before reduction, FIG. 6 is an explanatory view of the reduction procedure according to the conventional method, and FIG. 7 is the present invention. FIG. 8 is another explanatory drawing of the rolling-down procedure according to FIG.

Claims (1)

[Claims] 1. A large reduction by a roll or a forging die is applied to the vicinity of the solidified tip of a cast piece drawn from a continuous casting mold, and after the casting into the continuous casting mold is completed, the withdrawal and the large reduction of the casting piece are temporarily stopped. Then, in the continuous casting mold, insert the steel frame for steel leakage prevention into the top of the cast piece, then pull out the cast piece again and continue the large reduction while continuing the large reduction. The previous casting speed,
A large reduction method for a cast slab in continuous casting, which is characterized by compensating for a decrease in the unsolidified thickness in the reduction region due to stoppage of drawing of the slab by increasing the speed at a constant large reduction.