KR20030039796A - Short side mould for preventing surface defect of slab when continuous casting - Google Patents

Abstract

PURPOSE: A short side mould for preventing surface defects neighboring corner of casting slab during continuous casting is provided to reduce surface cracks on the corner of casting slab by changing shape of a plane of the short side mould contacted with molten steel and extend life of the mould by reducing friction of the lower part of the mould. CONSTITUTION: The short side mould for preventing surface defects neighboring corner of casting slab during continuous casting is characterized in that a plane of the short side of the mould in continuous casting facility contacted with molten steel on the surface of molten metal is curved to satisfy the following expressions as it is being lowered along a length direction of the mould: £Expression 1 | L=2R*arcsin(W/2R), £Expression 2 |L'=Lt-S where R is radius of funnel changed in a casting direction, t is casting time, S is the amount of cooling contraction, and L is a circular arc length of the short side of the mould changed in a casting direction.

Description

SHORT SIDE MOULD FOR PREVENTING SURFACE DEFECT OF SLAB WHEN CONTINUOUS CASTING}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mold copper plate apparatus during continuous casting operation of steel, and to a short side mold for preventing surface defects near a corner of a cast during continuous casting to prevent surface cracks occurring around the corner of the cast.

In the continuous casting process, as shown in FIG. 1, after inserting a dummy bar 2 at the bottom of the mold 1, the molten steel 3 is cast from the tundish 4 through the immersion nozzle 5. If it flows into (1), it will solidify in the dummy bar (2), it will draw, and casting will start. During casting, casting is performed continuously while the initial solidification layer 6 is formed on the mold surface 1.

At this time, the solidification layer 6 in the mold (1) is moved down to take the heat more and more and the thickness increases and shrinks at the same time, in the case of a long side that varies depending on the casting width by changing the slope to combine the short side The shrinkage is compensated for, and in the case of the short side, the shrinkage is compensated by making the width of the short side mold slightly smaller toward the bottom.

However, the amount of shrinkage of the solidification layer 6 in the mold 1 depends on the degree of heat flux that loses heat from the mold 1, and the heat flux in the mold 1 is the lowest surface resistance of the mold 1 and the copper plate. Since the largest and decreases toward the bottom of the mold (1), the amount of shrinkage as shown in the case shown in Figure 2 is also reduced from the top to the bottom of the mold (1).

Therefore, in the upper part of the mold 1, the inclination of the mold 1 is smaller than the change of the shrinkage amount, so that the shrinkage amount cannot be compensated, so that an air gap is generated between the mold short side and the solidification layer 6 to promote the surface defect of the cast steel. In the lower part, the mold inclination is larger than the shrinkage amount, so that friction between the mold long side and the solidification layer 6 increases, and as shown in (a, b) of FIG. Causes severe wear.

4 (a, b) shows a mechanism for generating defects in which the corner portion is folded or torn because the amount of compensation of the mold is large or small in the cooling shrinkage of the cast steel.

On the other hand, it is a separate problem from the compensation of the cooling shrinkage, but the commonly used short side molds have a straight surface in contact with molten steel, and the short side molds and the long side molds make vertical contact with each other so that the corners of the cast parts are cooled in an enclosed form. This results in a strong cooling, and as a result, as shown in FIG. 5 (a), the left and right solidified shells are excessively contracted compared to the surroundings, resulting in air gaps, and unevenness due to iron static pressure caused by molten steel inside the solidified layer. Tensile stress is concentrated in the coagulation layer, causing surface cracks in the corners of the cast steel.

In addition, the non-uniform coagulation portion of the corner of the cast slab is reduced to the thickness before the completion of solidification of the slab when the non-coagulation light pressure operation to reduce the thickness as shown in Fig. 5 (b) protruding convex shell of the short side central convex There is a problem in that it is possible to cause a defect in the corners and short sides.

In order to solve this problem, the contact angle between the molten steel on the short side of the mold and the molten steel on the long side is made over 90˚, and the air shrinkage is made by compensating the cooling shrinkage of the solidified shell of the cast steel according to the distance from the hot water surface. You need to stop.

In order to solve this problem, Japanese Patent Laid-Open No. Hei 11-28550 has a shape where the surface in contact with the molten steel at the upper end of the short side mold has a shape of bending or bending in the width direction of the copper plate, and the thickness of the center portion of the short side mold in the width direction is lowered downward. In the lower part of the mold, the shape of the flat part without the concave surface of the center part was increased, thereby increasing the contact angle of the corner part and compensating the contraction amount of the long side.

However, the shape does not reflect the compensation for the shrinkage of the short side of the mold, and since the widthwise length of the short side solidification shell is kept constant in the longitudinal direction of the short side mold, the cooling shrinkage that occurs intensively under the hot water surface is reduced. There is a problem that cannot be compensated.

The present invention was created in order to solve the above problems, and is a short side mold in which the curvature radius increases toward the bottom of the mold while being in contact with the molten steel, and the widthwise cooling shrinkage and the mold in the width direction of the solidified shell of the short side portion of the cast during continuous casting. It is an object of the present invention to provide a short-side mold for preventing surface defects near the corners of a cast during continuous casting, which is able to prevent the occurrence of surface defects due to uneven solidification of the corner portion of the cast by matching the change in the length in the width direction.

1 is a conceptual diagram showing a general continuous casting process,

2 is a graph showing the distribution of cooling shrinkage according to the casting direction of a typical cast short side;

3 is an exemplary view showing a wear shape of a conventional short side mold;

4 is an exemplary view showing the quality defect of the cast steel cast by a conventional short-side mold,

Figure 5 is an exemplary view showing the shape change of the cast when subjected to uncoagulated low pressure of the cast cast by a conventional short-side mold,

6 is a graph showing a change in arc length and curvature radius in the longitudinal direction of the short-side mold according to the present invention;

7 and 8 are exemplary views showing a modification of the short side mold according to the present invention;

Figure 9 is a bar graph comparing the surface crack index of the slab corner portion by the present invention short side mold and the conventional short side mold.

Explanation of symbols on the main parts of the drawings

1: mold 2: dummy bar

3: molten steel 4: tundish

5: immersion nozzle 6: solidification layer

W: Casting thickness R: Radius of curvature of short-strained molten steel contact surface

E: Thickness of the part in contact with the long side in the short side mold

L: Distance from the surface of the mold to the specified position in the downward direction

Tm: Solidification temperature of molten steel Tout: Casting temperature at casting side

ℓ: length of the arc in contact with the molten steel in the short-sided mold

The above object of the present invention is that (1) ℓ = 2R * arcsin (W / 2R), (2) ℓ '= as the surface in contact with the molten steel in the hot water of the mold short side of the continuous casting equipment down along the longitudinal direction of the mold corner of cast iron during continuous casting formed to satisfy l t-S (where R is the funnel radius in the casting direction, t is the casting time, S is the amount of cooling shrinkage, and l is the arc length of the mold short edge in the casting direction). By providing a short-side mold for preventing near surface defects.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

When the steel solidifies and cools in the mold, the length decreases in proportion to the cooled temperature difference as follows.

Total cooling shrinkage of cast steel in mold = α * (Tm-Tout) * ℓ-- (Equation 1)

Where α is the coefficient of linear expansion of the steel (0.02 mm / m ° C), Tm and Tout are the solidification temperature of the steel at the water surface and the temperature of the solidification shell at the mold exit as shown in FIG. Indicates the direction length.

As shown in Fig. 2, the total shrinkage amount (S) of Equation 1 is large in the continuous casting mold and decreases toward the bottom of the mold, so the shrinkage in the longitudinal direction of the cast steel can be expressed by the following square root equation. have.

-(Equation 2)

Where S is the amount of shrinkage (mm), a is a constant and t is the casting time (seconds) from the mold bath surface.

That is, the coagulation layer contracts a lot at the top of the mold and becomes smaller toward the bottom.

By making the surface in contact with the molten steel of the short side shape, and changing the radius of curvature (R) of the curved surface as shown in Figure 6 (a) it is possible to adjust the length (ℓ) of the solidified shell of the short side.

As shown in FIG. 2, the amount of shrinkage that is continuously contracted while the slab descends downward from the bath surface can be compensated by the shape of the mold as shown in FIG. 6 (b).

ℓ = 2R * arcsin (W / 2R) -------- (Equation 3)

ℓ ₂ = ℓt-S -------- (Equation 4)

Using the equations (2) and (3), the length of the lower arc (l ₂ ) is gradually subtracted from the length of the upper arc (l ₁ ) by the amount of cooling shrinkage (s). If 4) is satisfied, the cooling shrinkage can be compensated by the change in the arc length of the mold, thereby preventing the air gap between the solidified shell and the mold.

Here, R is a funnel radius that changes in the casting direction, t is casting time, and S is cooling shrinkage.

Figure 7 (a ~ d) is a modification of the present invention, after processing the surface in contact with the molten steel in the upper surface of the short-side mold to the curvature radius R1 and then gradually curvature radius to satisfy the above formula (4) Increasingly, the exit of the mold is a mold that compensates the amount of cooling shrinkage at the short side of the slab by machining into a curved surface of curvature radius R2.

Since the mold can be designed in various shapes of the short side, it is advantageous to largely adjust the contact angle of the corner portion between the short side mold and the long side mold in order to alleviate the strong cooling effect of the slab corner portion.

8 is another modified example of the present invention, after processing the surface in contact with the molten steel in the upper surface of the short-sided mold to the curvature radius R1 and gradually increasing the curvature radius to satisfy the above formula (4) to the exit of the mold As the mold is processed to be flat to compensate for the amount of cooling shrinkage in the short side of the slab, the radius of curvature is limited. However, since the slab has a rectangular cross section, it is easy to predict and adjust the width in the rolling process.

Figure 9 is a graph showing the results applied to the short side for casting 75mm thin slab.

As shown in the drawing, the surface contacting the molten steel of the upper side of the short side mold was machined to a radius of 120 mm, and the radius increased toward the bottom, and the mold of the shape as shown in FIG. 7 reaching 152 mm from the mold exit side was applied. The surface defect index multiplied by frequency and intensity was significantly reduced to 0.37 compared to the previous 0.68.

The reduction of surface cracks in the corners of cast steel is due to the fact that shrinkage generated during cooling after solidification of the short sides and corners of cast steel is compensated by mold to prevent air gaps and thus uneven solidification. have.

As described in detail above, the present invention is to compensate for the non-linear change in the cooling shrinkage of the short side portion of the cast during the continuous casting by changing the shape of the surface in contact with the molten steel of the short side mold between the solidified shell and the mold There is an effect that can reduce the surface crack occurrence of the corner portion of the cast by preventing the air gap of the cast, and additionally increase the life of the mold by reducing the friction of the lower part of the mold.

Claims (3)

Short side mold for preventing surface defects near the corners of the cast during continuous casting, characterized in that the surface in contact with the molten steel in the hot water of the mold short side of the continuous casting equipment is lowered along the longitudinal direction of the mold to satisfy the following equation. . (1) ℓ = 2R * arcsin (W / 2R) (2) ℓ '= ℓt-S Where R is the funnel radius that changes in the casting direction, t is the casting time, S is the amount of cooling shrinkage, and l is the arc length of the mold short side that changes in the casting direction. The method of claim 1, wherein the short side mold is formed by processing a surface of the upper surface of the mold in contact with the molten steel with a radius of curvature R1, and gradually increasing the radius of curvature so as to satisfy Equation (1) and (2). Short-sided mold for preventing surface defects near the corners of the cast during continuous casting, characterized in that it has another radius of curvature R2. The method of claim 1, wherein the short side mold is processed to a surface of curvature radius R1 in contact with the molten steel at its upper surface, and gradually increasing the radius of curvature so as to satisfy Equation (1) and (2). Short side mold for preventing surface defects near the corners of the cast during continuous casting characterized in that it is formed to have a flat surface.