JPH05318065A - Continuous casting method for slab by large unsolidified reduction - Google Patents

Abstract

PURPOSE:To reduce the center segregation of P and S in a cast slab by preventing accumulation of P and S pushed out with squeezing to the end part of unsolidified part in the cast slab in a diffusion range. CONSTITUTION:A continuous rolling reduction length LM to just before reaching to concd. permissible limit values of P and S range B, is set. When the rolling reduction length of the continuous squeezing by dies 4 reaches to the setting length LM the squeezing is stopped, and after passing through the diffusion range concentrating P and S without executing the rolling reduction by non- rolling reduction length LN, the squeezing is restarted. In such a way, the P and S concd. molten steel accumulated in the diffusion range B and becoming near the permissible limit values is confined in the nonrolling reduction part in the cast slab.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention continuously and largely reduces the unsolidified end of a slab drawn from a casting mold for continuous casting by using a forging device so that molten steel enriched with impurity solute elements can be spread in a diffusion region. The present invention relates to an improvement of a continuous casting method for cast slabs by large unpressurized large pressure for preventing center segregation of impurity solute elements by diffusing into molten steel.

[0002]

2. Description of the Related Art In a slab obtained by continuous casting of steel, so-called center segregation is inevitably formed, and in order to reduce or avoid adverse effects, it is being drawn and moved from a continuous casting mold through a secondary cooling zone. It is known that it is effective to apply a reduction in the thickness direction of the slab to the slab in the vicinity of the unsolidified end (crater end). For example, JP-A-61
-132247, in continuous casting of steel, an electromagnetic stirring action is applied to the unsolidified portion of the slab before the solidification completion point of the slab to equiax the unsolidified portion near the solidification completion point to solidify the slab. A continuous casting method is disclosed in which the end portion is locally pressed by a roll to forcibly form a solidification completion point.

Further, in Japanese Examined Patent Publication No. 3-66057, the unsolidified end portions of the cast pieces pulled out from the mold are brought close to each other in the process of expanding and contracting a hydraulic cylinder for driving a screw down by pressing down a pair of upper and lower molds through a lever. There is disclosed a slab forging device which continuously presses a slab in the thicknesswise direction at a central portion in the width direction by repeating separation. According to this forging device, the impurity solute elements (P, S) segregated at the unsolidified end of the cast piece are used.
For example, the concentrated molten steel of (1), etc. is largely reduced in the widthwise central portion by the amount of reduction exceeding the thickness of the unsolidified layer due to forging pressure, whereby the impurity solute elements segregated in the central portion are diffused into the molten steel in the diffusion region. Therefore, center segregation can be reduced.

[0004]

In the former technique, in which the roll is rolled down at the unsolidified end portion of the slab, the roll becomes a local load that comes into contact with the slab in a state close to line contact. Since there is a risk of internal cracking in the slab due to solidification shrinkage and bulging, the amount of reduction cannot be large, and there is a problem that center segregation cannot be prevented under light reduction.

Further, in the latter one in which the unsolidified end portion of the slab is largely reduced by using the die, the impurity solute element segregated in the central portion is diffused in the diffusion region, so that the central segregation can be prevented. When the large reduction of the unsolidified end portion is continuously performed, the impurity solute element of the concentrated molten steel that is forcibly discharged without solidifying at the unsolidified end portion is gradually accumulated in the diffusion region.

The impurity solute concentration in the diffusion region gradually increases as the length of the slab that greatly reduces the unsolidified end portion using such a die increases. As a result, it becomes difficult to produce a slab of stable quality,
In particular, the impurity solute elements having a small uniform dispersion coefficient such as P and S have a more remarkable concentration tendency.
There is a problem that internal cracks are likely to occur because the S concentration increases and only minute bulging occurs in the slab.

The present invention solves the above-mentioned problems of the prior art,
When using the mold to greatly reduce the unsolidified end of the slab,
It is an object of the present invention to provide a continuous casting method for cast slab by large unsolidified reduction, which can prevent impurity solute elements from accumulating in the diffusion region and adversely affecting the quality of the slab.

[0008]

Means for Solving the Problems In order to prevent accumulation due to impurity solute element diffusion in the diffusion region when the unsolidified end portion of the slab is largely reduced by forging, the diffusion region is appropriately timed. As a result of intensive research conducted by focusing on the fact that it is separated from other parts and blocking it from other parts, the present invention has been completed, and the gist of the present invention is as follows.

According to the present invention, the unsolidified end portion of the slab drawn from the casting mold for continuous casting is continuously subjected to a large pressure reduction by using a forging device so that the molten steel enriched with the impurity solute element is introduced into the molten steel in the diffusion region. In a continuous casting method of a slab by large unpressurized pressure to prevent central segregation of impurity solute elements, impurity solute elements are diffused and accumulated in the molten steel in the diffusion region, and the concentration limit of impurity solute elements can be accumulated. Set the continuous forging slab length until just before reaching the value, when the slab length under large pressure by continuous forging pressure reaches the set length, the forging pressure is stopped and the impurity solute element concentrated molten steel accumulated in the diffusion region Contain without passing through, then contain the concentrated molten steel in which the impurity solute element accumulated in the diffusion region in the slab has reached near the allowable limit value by restarting the large reduction due to the forging pressure of the slab Characterized by Coagulation is a continuous casting method of the slab by a large pressure.

Further, in the continuous casting method for cast slabs by the above-mentioned unsolidified large reduction, if the length of the cast slabs which are not reduced by stopping the forging pressure is set to be equal to or longer than the length of the diffusion region of the impurity-solute element concentrated molten steel, It is preferable that the length of the cast piece, which is safe and is largely reduced by the forging pressure, is determined so that the impurity solute element having a remarkable concentration tendency is equal to or less than the allowable limit value.

[0011]

The present invention will be described in detail below with reference to the drawings. FIG. 1 schematically shows a continuous casting facility equipped with a continuous forging device, in which a slab 2 is supported by a support roll 3 while being pulled out from a mold 1 by using a pinch roll 5 and arranged in the continuous forging device. A slab 2 with a pair of upper and lower molds 4
In the state where the unsolidified end portion (crater end) is continuously forged, the concentrated molten steel A in which the impurity solute elements such as P and S, which have an adverse effect on the center segregation, are concentrated, depends on the mold 4. It is pushed up to the diffusion region B on the upstream side by a large pressure reduction, while the solidified shell of the slab 2 is continuously crimped to become a sound slab with reduced center segregation.

However, such a slab 2 is formed by using the mold 3.
When the large-scale reduction of the non-solidified end portion is continuously performed, the concentrated molten steel of the impurity solute element discharged without solidification is accumulated in the diffusion region B due to the forging pressure, and the continuous reduction length of the slab 2 is increased. As described above, the concentration thereof increases.

Therefore, according to the present invention, the impurity solute elements such as P and S are diffused and accumulated in the molten steel in the diffusion region B, and the continuous forging pressure length of the cast slab 2 until just before the concentration limit value of the impurity solute elements is reached. Set L _M. Then, while the reduction length is measured by the measure roll 6, the continuous reduction length L _M for the cast piece 2 by the die 4 is reached, or immediately before that, the forging pressure by the die 4 is once stopped and the non-forging pressure is passed. That is, the forging pressure is stopped until the non-forging pressure length L _{N including} the diffusion region (length 1) concentrated near the concentration allowable limit value due to the accumulation of impurity solute elements passes through the die 4.

When the slab portion having the non-forging pressure length L _N has passed, when the large reduction due to the forging pressure of the slab is restarted by using the mold 4, the concentration of the impurity solute element becomes close to the concentration allowable limit value. The molten steel can be contained in the slab 2 having a non-forging length L _N. Therefore, the impurity solute element concentration in the diffusion region B is low when the large reduction due to the forging pressure of the cast piece 2 is restarted by the die 4, and the large reduction due to the continuous forging pressure is reached until the continuous forging pressure length L _M is reached again. It can be carried out.

When a cast piece is continuously subjected to a large reduction by forging pressure using a mold under constant casting conditions, the reduction length of the cast piece due to the forging pressure and the segregation of P and S at the center of the cast piece after the forging pressure The degrees are shown in FIGS. 2 and 3. 2 and 3
As shown in, within the range of the reduction length shown in both figures,
The segregation degree of P and S increases almost linearly with the reduction length. 2 and 3, the allowable limit value of the segregation degree of the slab is C / Co (P) for P and C / C for S.
o (S), and the reduction lengths required to keep this value below the permissible limit value are L _P and L _S , respectively, and the reduction length L of the slab is L.
Is represented by the ratio L / L _S with L _p or L _S.

[0016] Thus L _p ≧ L L _S at the time of the _S, L
_{When p} ≥ L _S , the maximum length L that continuously reduces L _p
The reduction length as _M reaches this maximum length (L _M ), or the reduction is temporarily stopped immediately before reaching the maximum length, and the non-rolling length L _{N of} not less than the length l of the diffusion region B of the concentrated molten steel is L _N. As described above, the slab portion is formed, and re-rolling by the forging pressure is started again.

The slab 2 is cut to a predetermined length by the cutting torch 7, and the continuous rolling length L _M of the slab 2 from the start of the rolling by the die 4 forging the slab using the major roll 6 is started. And the non-rolled length L _N is measured, and the slab 2 is cut by the cutting torch 7 so as to have the non-rolled length L _{N including the} diffusion region B, and this portion is scraped or treated as a downgraded product. To do.

[0018]

[Embodiment] The thickness which becomes the material for carbon steel S53C for machine structure by the continuous casting equipment of the curved type one-point straightening method shown in FIG.
A continuously cast slab having a size of 400 mm and a width of 560 mm was manufactured according to the method of the present invention and a conventional method was carried out for comparison.

The casting conditions of the present invention are as follows. Casting speed: 0.51 m / min Forging die installation position: 26 m from the meniscus of the mold Reduction amount: 110 mm Casting length: 80 m L _p = 80 m: C / Co (P) = 1.0 L _S = 90 m: C / Co ( S) = 1.0 Set rolling length for continuous forging pressure L _M : 40 m Set length for non-rolling L _N : 2.5 m Segregation degree of P and S in the center of the slab obtained by continuous casting under the above conditions Are shown in Table 1.

[0020]

[Table 1]

As shown in Table 1, in the conventional method, the impurity solute elements P and S continue to be accumulated in the diffusion region, and the segregation degree of P and S in the center of the slab increases as the rolling length of the slab increases. At the position where the distance from the bottom of the cast slab is 70 m, the segregation degree of both P and S is 1.1, which exceeds the allowable limit value.

On the other hand, in the present invention, when the set rolling length L _M = 40 m by the continuous forging pressure is reached, the set non-rolling length L _N is reached.
= 2.5, the molten steel enriched with the impurity elements P and S is confined and blocked, so the segregation degree of P and S is stabilized and the permitted enrichment limit values C / Co (P) and C / Co (S) are 1.0. It is possible to suppress the following, and it is possible to manufacture a slab of good quality with less center segregation.

[0023]

As described above, according to the present invention, the concentrated molten steel in which the impurity solute element accumulated in the diffusion region in the cast piece is close to the permissible limit value is confined in the cast piece and shut off. When the reduction is restarted, it is possible to restart without being affected by the influence, and it is possible to stably produce a good quality slab with less center segregation.

[Brief description of drawings]

FIG. 1 is a schematic view showing a continuous casting facility equipped with a continuous forging device.

FIG. 2 is a diagram showing the relationship between the reduction length (L / L _p ) of a slab and the segregation degree of P.

FIG. 3 is a diagram showing the relationship between the reduction length (L / L _s ) of the cast slab and the segregation degree of S.

[Explanation of symbols]

 1 Mold 2 Cast Piece 3 Support Roll 4 Mold 5 Pinch Roll 6 Measure Roll 7 Cutting Torch

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazukazu Mizoda 1-chome, Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture (no address) Inside the Mizushima Works, Kawasaki Steel Co., Ltd. (72) Toshisetsu Matsukawa Mizushima Kawasaki, Kurashiki City, Okayama Prefecture Tsudori 1-chome (No house number) Kawasaki Steel Co., Ltd. Mizushima Works

Claims (3)

[Claims] 1. An unsolidified end portion of a slab extracted from a casting mold for continuous casting is continuously and largely reduced by using a forging device.
In the continuous casting method of the cast piece by unsolidified large pressure to diffuse the concentrated solute of the impurity solute element into the molten steel of the diffusion region and prevent the center segregation of the impurity solute element, the impurity solute element in the molten steel of the diffusion region is Set the continuous forging pressure slab length until just before reaching the concentration limit of impurity solute elements accumulated, and stop the forging pressure when the large slab length under continuous forging pressure reaches the set length. Then, the impurity solute element concentrated molten steel accumulated in the diffusion region is passed without being reduced, and thereafter, the impurity solute element accumulated in the diffusion region in the slab is restarted by restarting the large reduction due to the forging pressure of the slab. Is a continuous casting method for cast slabs by large unsolidified reduction, which is characterized by containing concentrated molten steel that has reached near an allowable limit value. 2. The casting according to claim 1, wherein the length of the slab that is not reduced by stopping the forging is equal to or longer than the length of the diffusion region of the impurity-solute element-concentrated molten steel. Continuous piece casting method. 3. The cast product by large unpressurization according to claim 1, wherein the length of the cast product which is largely reduced by the forging pressure is determined so that the impurity solute element having a remarkable concentration tendency becomes equal to or less than the allowable limit value. Continuous casting method.