JPH091291A - Continuous casting method - Google Patents

Abstract

PURPOSE: To produce a cast slab having little center segregation and center porosity by discharging non-solidified remaining molten steel at the part of a specific value or higher of the vertical height, rolling the cast slab surface and press-sticking a discharged interface of the non-solidified remaining molten steel. CONSTITUTION: The vertical height H at the part, in which a solid phase ratio at the central part of the cast slab 8 is not exceeded to 0.3, is kept so as to become >=1.5m height from the molten steel surface level in a mold 1 and the cast slab 8 is continuously cast and drawn upward as an arc state. Then, the non-solidified remaining molten steel 11 at the part having >=1.5m vertical height H is discharged with a gravity. The cast slab 8 is rolled with rolling reduction rolls 5 arranged just after or after discharging to press stick the discharged interface of the non-solidified remaining molten steel developing a cavity 12. In the case the solid phase ratio exceeds 0.3, an apparent viscous resistance becomes large and the fluid discharge of the molten steel only with the gravity becomes difficult. The upper limit of the desirable vertical height is about 2.5m. In such a way, the continuous cast slab of the steel having little center segregation and center porosity can be cast.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method suitable for producing a slab of steel, alloy steel, stainless steel or the like having a significantly low center segregation or porosity.

[0002]

2. Description of the Related Art Center segregation and center porosity of a slab have been one of the major problems in continuous casting. For example, center segregation and center porosity of a bloom or billet slab for wire rods cause disconnection during wire drawing, and center porosity of the same slab for pipes causes inner surface flaws during pipe manufacturing. Further, the center segregation and center porosity of the slab slab for thick plate material cause hydrogen-induced cracking of the product.

As described above, the center segregation and center porosity of continuously cast slabs are directly connected to product defects, and a great deal of effort has been conventionally made to reduce them.

As a typical method for reducing these, there is an unsolidified light reduction method for cast pieces.

The center segregation and the center porosity are caused by solidification shrinkage at the final stage of solidification. In the non-solidification light reduction method, in order to compensate for this solidification shrinkage amount, a reduction corresponding to the shrinkage amount is applied from the surface of the slab. It is a thing.

On the other hand, Japanese Patent Laid-Open No. 59-39451 discloses the following method. That is, after pulling out the slab downward from the mold, before the unsolidified residual molten steel inside is completely solidified, guide the slab to the upper part of the molten steel surface level in the mold, and the crater end of the slab is cast. Located above the unsolidified residual molten steel surface level in the inside, and then crimping the vacuum cavity formed between the crater end and the unsolidified residual molten steel surface level in the slab, such as shrinkage holes and center segregation. It is intended to produce a cast product having no internal defects.

[0007]

In the conventional uncoagulated light reduction method, although center segregation and center porosity are considerably reduced, they are not completely eliminated, and this method has reached the technical limit. There is.

Although the method disclosed in Japanese Patent Laid-Open No. 59-39451 is a promising method, it has the following two problems.

In the method of simply locating the crater end of the slab above the level of the unsolidified residual molten steel surface in the slab and discharging the residual molten steel at the crater end by gravity to form a vacuum cavity, The molten steel exists between the solidified tissues at the crater end and is subjected to the action of the capillary force. Therefore, the molten steel may not be sufficiently fluidized and discharged, and may remain suspended between the solidified tissues. Since this residual molten steel is in the final stage of solidification, impure components are concentrated in this part, and if a reduction is applied to crimp the interface while the residual molten steel is suspended as described above, the residual molten steel is They accumulate and remain as a new granular segregation.

When the cast slab is pressed down and the vacuum cavity is pressure-bonded, the reduction does not work sufficiently due to the deformation resistance of the solidified shell in the portion perpendicular to the pressing surface. Further, the cast slab after the molten steel is discharged is cooled quickly, and accordingly the deformation resistance is increased.

The present invention is intended to solve the above problems, and an object of the present invention is to provide a continuous casting method for steel capable of producing a slab having a significantly reduced center segregation or porosity. is there.

[0012]

DISCLOSURE OF THE INVENTION The inventors of the present invention have found that casting conditions for effectively discharging unsolidified residual molten steel in a slab by gravity, in particular, solid phase condition of the slab at the discharging portion and vertical direction. The present invention was made by clarifying the height position condition.

The gist of the present invention resides in the following continuous casting method.

A continuous casting method in which a slab is cast upward in an arc shape, the unsolidified residual molten steel in the slab is discharged by gravity, and then the unsolidified pressure is applied, which is drawn out and discharged under the following conditions. A continuous casting method characterized by performing reduction and pressure bonding.

When the slab is continuously drawn out in the range from the vertical direction to the horizontal direction and gravity is not used for discharging, the vertical height of the portion where the solid fraction of the central portion of the slab does not exceed 0.3. Is continuously drawn from the molten steel in the mold at a level of 1.5 m or more.

The unsolidified residual molten steel in the cast slab having a vertical height of 1.5 m or more is continuously discharged by gravity.

The surface of the slab is pressed by a pressing device provided immediately after the discharging or after that, and the discharging interface of the unsolidified residual molten steel inside the solidified shell is continuously crimped.

This method is preferably applied to the production of billets or blooms of steel or alloy steel or stainless steel.

Desirable as the rolling down device is a device equipped with a convex rolling down roll.

[0020]

The construction of a continuous casting machine for carrying out the method of the present invention and the method of the present invention will be described with reference to FIG. Fig. 1 (a)
FIG. 3 is a vertical cross-sectional view of a curved continuous casting machine and a slab in a lateral direction. This apparatus includes at least a mold 1, a dipping nozzle 2 connected to a tundish (not shown), a secondary cooling spray 3 in the casting direction, a group of guide rolls 4, a pair of reduction rolls 5, a group of straightening rolls 6 and a pair of rolls. It is provided with a pinch roll 7 and is a curved type having an arc shape as illustrated. FIG.
In FIG. 8, reference numeral 8 is a slab, 9 is molten steel, 10 is a solidified shell, and 12 is a cavity. That is, the cast piece 8 shown in FIG. 1 shows a state in which the unsolidified molten steel due to gravity is discharged and the cavity 12 is formed.

As the reduction roll 5, two or more pairs of rolls may be used. The desirable arc range is about 2 to 10 mR, and the slab has a cross-sectional shape of 100 to 350 mm × 100 to 4
The billet or bloom is about 50 mm square.
The secondary cooling spray 3 has a secondary cooling zone of just below the mold 1.
It is provided so as to have a length of about 8 to 5 m, water, air mist or the like is used as a cooling medium, and a desirable secondary cooling specific water amount range is 0.4 to 2 liters / kg-steel. It is desirable that the straightening rolls 6 are provided at a position where the drawing speed direction of the cast slab 8 is parallel to the molten steel surface in the mold (3/4 circumferential position of the cast slab 8 at the level above the mold 1). The casting speed Vc of the slab 8 is controlled by the pinch roll 7, and the desirable casting speed Vc is in the range of 0.5 to 4 m / min.

In the method of the present invention, the cast piece 8 is cast in an upward arc shape as shown in FIG. Continuous casting and continuous drawing are performed while maintaining the vertical height H of the portion not exceeding 0.3 so as to be higher than the molten steel surface level in the mold 1 by 1.5 m or more.

The term "central solid fraction" as used herein is defined by a normal continuous casting method, in which a slab is continuously extracted vertically or obliquely downward or horizontally, and is not discharged by gravity. Is the solid fraction of the central part in the case of. The portion where the solid fraction of the central portion does not exceed 0.3 can be determined as one point under certain conditions.

Then, the unsolidified residual molten steel in the slab 8 at the portion where the vertical height H is 1.5 m or more is continuously discharged by gravity. This will be described with reference to FIGS. 1 (b) and 1 (c).

FIG. 1 (b) is a side sectional vertical view of the main part of the cast slab showing a state before discharge. In the figure, reference numeral 11 is the unsolidified residual molten steel in the slab in the portion where the solid fraction of the central part of the slab 8 does not exceed 0.3. As shown in the figure, the vertical height H of the unsolidified residual molten steel 11 is maintained to be higher than the molten steel surface level in the mold by 1.5 m or more, and the unsolidified residual molten steel 11 is moved to the lower molten steel 9 by gravity. Discharge continuously.

FIG. 1 (c) is a vertical cross-sectional view in the side direction of the main part of the cast slab showing a state after discharge. Cavity 1 inside after discharging
A cast piece 8 having 2 is obtained.

Next, at least a pair of reduction rolls 5 provided immediately after or after the discharge, apply a reduction to the surface of the slab 8 to discharge the unsolidified residual molten steel 11 and then the cavity 12
The discharge interface of the unsolidified residual molten steel inside the solidified shell 10 in which is formed is continuously crimped.

The portion where the unsolidified residual molten steel in the slab is continuously discharged by gravity is defined as the portion where the solid fraction of the central portion of the slab does not exceed 0.3, and the vertical height H of the portion is set within the mold. The reason why the molten metal level is maintained at 1.5 m or higher is as follows.

In steel, alloy steel or stainless steel, when the solid fraction exceeds 0.3, the apparent viscosity rapidly increases and the fluidity becomes extremely poor. In such a solidified state, the molten steel flow does not easily occur even when a force is externally forcibly applied by the unsolidified light reduction, let alone the gravity, the molten steel flow discharge does not occur.

However, when the central solid fraction is 0.3 or less, the apparent viscous resistance including the solid phase is small, so that it is not much different from the viscosity of the liquid simple substance, and therefore the central solid fraction is 0.
By setting the vertical height H of the portion not exceeding 3 to a height of 1.5 m or more from the molten metal level in the mold, it is possible to discharge the unsolidified residual molten steel by gravity. This is because the molten steel density of steel or alloy steel or stainless steel is 7 to 7.3 g / cm ³ , and the atmospheric pressure of the molten steel surface level in the mold (1033 g / cm
^This is because a molten steel head superior to ² ) can be obtained.

In this way, the cast slab 8 having the cavity 12 inside can be obtained, but in order to be more reliable, the portion where the solid fraction of the central portion does not exceed 0 should be the molten steel surface in the mold. It is recommended that the height is 1.5m or more from the level. The upper limit of the desirable vertical height H is about 2.5 m.

The position to which the pressure reduction is applied may be immediately after the discharge or until the entrance side of the straightening roll 6 thereafter, but the former is preferable. This is because, in the latter case, the temperature of the solidified shell 10 is drastically lowered by discharging, so that the pressure bonding property at the discharging interface may be deteriorated.

As a device for rolling down the cavity 12 thus obtained, it is extremely effective to use a convex roll having a convex portion as shown in FIG. 2 as a rolling down roll.
FIG. 2 is a view showing a cross section taken along the line AA in the case where the reduction roll 5 in FIG. 1 is a convex roll. With the convex-type reduction roll 13 having the protrusions 14 as shown in the figure, it is possible to apply the reduction to only the cavity 12 without reducing the solidification shell 10 of the cast piece 8 other than the protrusions 14. In this case, the final cross-sectional shape of the cast piece has a concave portion depending on the reduction by the convex portion 14, but it may be appropriately shaped by reduction in a rolling process after continuous casting. The desirable shape of the convex portion 14 is about 5 to 40 mm in height and about 20 to 100 mm in width.

The maximum pressing reaction force of the pressing roll 5 and the convex pressing roll 13 is in the range of about 20 to 80 tons, and the maximum roll interval (in the thickness direction of the slab) is reduced by 10 to 80 m.
m.

[0035]

【Example】

(Invention Example 1) A curved continuous casting machine having the structure shown in FIG.
(3 mR) was cast under the following conditions, and the center segregation and center porosity of the slab were investigated.

Steel type: 0.8% C steel (0.8% C-0.2% S
i-0.02% P-0.01% S) Temperature: Molten steel superheat degree in tundish 40 ° C Slab size: 150 mm square (billet) Secondary cooling spray: Spray belt length is 1 m directly under the mold, water spray system secondary cooling ratio The amount of water is 0.8 liter / kg-steel Rolling roll: Convex type (diameter of convex part is 190 mm, diameter other than convex part is 150 mm, convex part length is 70 mm, total length is 200 mm) Maximum pressing reaction force of pressing roll: 50 tons ( By providing a distance piece, the maximum roll interval can be reduced by 3
0 mm) Casting speed (Vc): 2 m / min A straightening roll was provided at the upper 3/4 circumference of the mold, and a slab was continuously drawn at the above casting speed by a pinch roll.
As a result, the position of the portion where the solid fraction was 0.3 in the center of the slab was set to 1.5 m in height from the molten steel surface level in the mold.

(Inventive Example 2) As a reduction roll, a diameter of 150
Using a flat roll of mm, the other conditions were the same as those of Example 1 of the present invention, and the cast piece was rolled down.

(Comparative Example) The reduction roll was the same as in Inventive Example 2, the casting speed was reduced to 1.7 m / min, and the crater end portion (central solid phase) was placed at a position 1.5 m from the molten steel surface level in the mold. The ratio was 0.8). The other conditions were the same as those of Example 1 of the present invention.

In each of the tests, a center segregation and center porosity were investigated by cutting out a 2 m long sample from a cast piece of a stationary part after casting and observing the center longitudinal joint surface. Table 1 summarizes these results.

[0040]

[Table 1]

As shown in Table 1, in Example 1 of the present invention, neither center segregation nor center porosity was found. In the invention example 2, the reduction did not proceed sufficiently and the center porosity remained a little. However, regarding the center segregation, the discharge of the unsolidified residual molten steel from the solid fraction of the central portion of 0.3 proceeded sufficiently, resulting in 0.

On the other hand, in the comparative example, since the unsolidified residual molten steel was discharged from the crater end portion, a part of the segregated molten steel remained suspended, resulting in granular segregation. Further, as in the case of Example 2 of the present invention, the center porosity remained because the reduction was insufficient.

[0043]

According to the method of the present invention, it is possible to manufacture a continuously cast slab of steel which is remarkably low in center segregation and center porosity.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration example of a continuous casting machine for carrying out the method of the present invention. (a) is a vertical cross-sectional view in a lateral direction of a curved continuous casting machine and a cast piece. (b) and (c) are vertical cross-sectional views in the side surface direction of the main part of the cast slab showing the states before and after discharging.

FIG. 2 is a view showing a cross section taken along line AA in the case where the reduction roll in FIG. 1 is a convex roll.

[Explanation of symbols]

1: Mold, 2: Immersion nozzle, 3: Secondary cooling spray, 4: Guide roll, 5: Reduction roll, 6: Straightening roll, 7: Pinch roll,
8: cast slab, 9: molten steel, 10: solidified shell, 1
1: Unsolidified residual molten steel in the slab where the solid fraction of the central part does not exceed 0.3, 12: Cavity, 13: Convex rolling roll,
14: convex

Claims (1)

[Claims] 1. A continuous casting method in which a slab is cast upward in an arc shape, the unsolidified residual molten steel in the slab is discharged by gravity, and then the unsolidified pressure is applied to the slab. The vertical height of the part where the solid fraction of the central part of the slab does not exceed 0.3, when it is assumed that the slab is continuously drawn out in the horizontal direction and is not discharged by gravity, is calculated from the molten steel surface level in the mold. 1.
The unsolidified residual molten steel in the slab having the vertical height of 1.5 m or more was continuously discharged by gravity by continuously pulling it out while maintaining it at 5 m or more, and provided immediately after or after that. A continuous casting method characterized by applying a reduction to the surface of a slab with a reduction device to continuously press the discharge interface of the unsolidified residual molten steel inside the solidified shell.