JP2000158111A - Continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a billet of a good transverse sectional shape by preventing a generation of center porosity at a thickness center part of the billet wherein a rolling draft and thickness of the unsolidified part are specified so as to roll the billet inclusive of an un-solidified part. SOLUTION: Rolling draft and thickness of the unsolidified part are set within a range shown by the formulae R/L=0.12δ+0.08 and L<=100, wherein R is a rolling draft mm, L is a thickness mm of the unsolidified part whose solid phase rate is 0.99 or less at a rolling start time at a rolling position, δis a steel solid shrinkage rate %. A solid shell 5 is formed within a mold when a molten steel 3 is poured into a mold 2 from an immersion nozzle, the solid shell 5 gradually increases its thickness while passing through a guide roll 4, becoming a billet 7 having an unsolidified part 6, and is rolled by an unsolidified rolling roll 9 and a mold rolling roll device 10 so as to be pulled up with a pinch roll 11. While the unsolidified part including billet is rolled so as to satisfy the formulae, an upper limit value of R/L is preferably a degree of 2 for restricting an enlargement of a facility and, a degree of 20 mm is effective as a lower limit of the unsolidified part thickness L.

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 for carbon steel, stainless steel, high alloy steel and the like, and more particularly, to a cast slab having no center porosity at the center of thickness and having a good cross-sectional shape. It relates to the continuous casting method obtained.

[0002]

2. Description of the Related Art Internal defects called center porosity (hereinafter simply referred to as porosity) are apt to occur in a continuous cast slab of steel in the center of thickness. Even if such a slab is rolled, a product with good internal quality cannot be obtained.

[0003] For example, the porosity of a slab for producing a hard steel wire remains as a defect at the center of the wire after hot rolling. When such a wire is cold-drawn, a disconnection accident called a "cuppy break" may occur. Also,
Even when such a slab is hot rolled and processed into a steel bar,
The porosity of the slab remains as a defect in the center of the steel bar. When this steel bar is cold-extruded, a defect called a chevron crack may occur.

When a seamless pipe is manufactured by the Mannesmann method from a continuous casting step to a rolling step or a forging step, if the slab has porosity, the seamless pipe may have internal flaws. Furthermore, the porosity of the slab for manufacturing a thick plate may remain on the thick plate and cause a decrease in mechanical properties and the like.

[0005] Such porosity at the center of the thickness of the slab is caused by the fact that the molten steel does not easily flow in the final solidified portion, and the solidification is completed without the molten steel being replenished in a narrow gap caused by volume shrinkage during the solidification. I do.

As a measure for preventing the occurrence of porosity, there is a method of lightly reducing the unsolidified portion of a slab. In the unsolidified portion of the cast slab, the roll is rolled down by the roll by the cast slab thickness corresponding to the solidification shrinkage. However, under light pressure that compensates for coagulation contraction,
The effect of preventing porosity is small.

Japanese Patent Application Laid-Open No. 3-124352 discloses a method in which a roll having a diameter of 2 to 5 times the thickness of a slab is used to reduce the final solidified portion by a solidification shrinkage or more to prevent porosity and the like. Has been presented. But with this method,
Depending on the cross-sectional shape of the slab, the cross-sectional shape may be deteriorated by rolling down, or the occurrence of porosity may not be prevented depending on the steel.

[0008]

SUMMARY OF THE INVENTION The present invention prevents the occurrence of center porosity at the center of the thickness of a slab in continuous casting of carbon steel, stainless steel, high alloy steel, etc. An object of the present invention is to provide a continuous casting method capable of obtaining a good cast slab.

[0009]

The gist of the present invention resides in a continuous casting method shown in the following (1) and (2).

(1) A continuous casting method of steel in which a continuous cast slab including an unsolidified portion is reduced under conditions represented by the following formulas (A) and (B).

R / L ≧ 0.12δ + 0.08 (A) L ≦ 100 (B) where, R: reduction amount (mm) L: solid phase ratio at the reduction position at the start of reduction 0 Thickness of unsolidified portion of 0.9 or less (mm) δ: solidification shrinkage ratio of steel (%) (2) A continuous casting method for steel in which the slab is further reduced after the reduction described in (1) above.

The present inventors conducted a rolling test of a slab including an unsolidified portion using several types of steels of different materials, and investigated the occurrence of porosity. As a result, the above equations (A) and (B) were obtained. If the pressure is reduced to satisfy the condition expressed by
It was found that porosity can be prevented.

FIG. 2 shows the relationship between the ratio R / L of the reduction amount R to the thickness L of the unsolidified portion and the solidification shrinkage ratio δ which can prevent the occurrence of porosity when the slab including the unsolidified portion is reduced. FIG. This figure shows a steel containing 0.5% by weight of C (hereinafter referred to as 0.5% C steel) and a steel containing 1.0% by weight of C (hereinafter referred to as 1.0% C steel). ), An austenitic stainless steel containing 18% by weight of Cr and 8% by weight of Ni (hereinafter referred to as SUS304), and a Cr steel containing 13% by weight of Cr (hereinafter referred to as 13% Cr steel). This shows the result of rolling down a slab including an unsolidified portion.

As can be seen from this figure, there is a clear relationship between R / L, the solidification shrinkage of steel and the occurrence of porosity. That is, if R / L is equal to or more than the value of (0.12δ + 0.08) on the right side of the expression (A) including the solidification shrinkage δ (%) of steel, occurrence of porosity can be prevented. Here, the solidification shrinkage δ is a value specific to steel.

The solidification shrinkage δ (%) inherent to the steel is the volume shrinkage when the liquid steel is solidified to a solid phase, and is generally about 3 to 5%. I have. However, since detailed data on the volumetric shrinkage rates for various steels has not been published, the solidification shrinkage rates of the above-mentioned four types of steels were determined by the following method.

That is, the volume of the liquid phase of steel per unit weight can be determined, for example, by referring to the technical data “Metal” (vol. 67, 1).
997, no. 11, p20), using a static drop method, which is a method for measuring the density of a molten metal, by converting the measured density per unit weight into the volume of the liquid phase of steel. The volume of the solid phase immediately after solidification was measured in the same manner. From the measured liquid and solid volumes,
The coagulation shrinkage δ (%) was determined.

The solidification shrinkage δ of the four steels tested was about 2.5-6.5%, as shown in FIG.

The thickness L (mm) of the unsolidified portion at the start of the reduction at the reduction position is the thickness of the portion having a solid fraction of 0.99 or less. The upper limit is 100 as shown in the equation (B).
mm. When L exceeds 100 mm, for example, even if the value of R / L is the same value of 0.8, the thickness of the unsolidified portion remaining after rolling is smaller than that in the case where the thickness L of the unsolidified portion is small, 100 mm or less (Meaning a thickness obtained by subtracting the value of R from the value of L). Therefore, porosity may be generated before the center of the slab is completely solidified after the reduction. Further, the equipment becomes large.

In the method of the present invention for rolling down a slab including an unsolidified portion, the slab undergoes large deformation. Therefore, when casting billets including blooms and round slabs for the production of wire rods, steel bars, pipes, etc., the reduced slabs,
Further reduction is desirable. For example, when bloom is used as a material for hot rolling for wire rod and steel bar production,
If the bloom is greatly deformed due to unsolidification pressure, it is difficult to perform subsequent hot rolling. Therefore, it is effective to further reduce the slab for the purpose of forming the slab deformed by the unsolidification reduction into an appropriate shape.

[0020]

FIG. 1 is a view showing an example of a continuous casting apparatus for carrying out the method of the present invention. When the molten steel 3 is injected from the immersion nozzle 1 into the mold 2, a solidified shell 5 is formed in the mold, and the solidified shell 5 gradually increases in thickness while passing through the guide roll 4. And the unsolidified part 6
Slab 7 which has an unsolidified pressing roll 9
After the coagulation, it is reduced by the reduction roll device 10 and pulled out by the pinch roll 11.

The method of the present invention is particularly suitable for the casting of blooms used in the production of wires, steel bars, seamless pipes, etc., billets including round shapes, and slabs having a rectangular cross section used in the production of thick plates. I have.

FIG. 3 is a diagram for explaining the method of the present invention by way of an example of casting a slab having a round cross section. FIG.
(A) is a longitudinal cross-sectional view showing a state where a casting slab is reduced by a reduction roll. FIG. 3B is a graph showing I- in FIG.
It is a cross-sectional view of the slab in the I 'line.

In FIG. 3 (a), a cast piece 7 having an unsolidified portion 6 is formed by a pair of upper and lower surfaces having a hollow shape.
Is reduced by When rolling down, the thickness L of the unsolidified portion
(Mm) using an unsolidified rolling roll to reduce the slab
(Mm). The position indicated by reference numeral 12 is a solidification interface having a solid phase ratio of 0.99. The solid phase ratio is obtained by analyzing the temperature at each position of the slab by heat transfer solidification analysis, and the liquidus temperature unique to the steel is obtained. And the solidus temperature.

Since the slab shown in FIG. 3B has a round shape, the thickness L of the unsolidified portion means the diameter of the unsolidified portion. If the cross section is a square or rectangular slab other than round,
The thickness L of the unsolidified portion is the thickness of the unsolidified portion of the slab, that is, the thickness between solidified interfaces having a solid fraction of 0.99 or less on the long side in the case of a rectangle.

In the method of the present invention, the slab including the unsolidified portion is reduced to satisfy the expression (A). Where R / L
The value of the upper limit of is not particularly limited, but if the value of R / L is 1 or more, it will be reduced to the thickness of the unsolidified portion or more,
The equipment becomes larger. Therefore, the upper limit of R / L is desirably about 2 in order to suppress an increase in the size of the equipment.

In the method of the present invention, the slab including the unsolidified portion is reduced to satisfy the formula (B). here,
The lower limit of the thickness L of the unsolidified portion is not particularly limited, but if it is too small, the effect of the reduction cannot be sufficiently obtained. Therefore, the lower limit of L is desirably about 20 mm.

The unsolidified rolling roll may have at least one pair of rolls, and may have two or more pairs of rolls. FIG.
The unsolidified pressing roll 9 shown in (a) is a pair of upper and lower horizontal rolls. In the case of a slab for manufacturing a thick plate, this horizontal roll method is suitable in terms of equipment arrangement. In the case of a billet including a bloom or a round slab for manufacturing a wire, a steel bar, a seamless pipe, or the like, a pair of vertical rolls may be used in addition to the horizontal roll system. Further, the surface shape of the unsolidified pressing roll 9 is preferably a flat shape in the case of a slab other than a billet of a round slab, but is preferably a hole shape in the case of a billet of a round slab. .

When the slab including the unsolidified portion is reduced and then the slab is further reduced to form the slab into an appropriate shape,
The thickness center of the slab may be completely solidified,
An unsolidified portion may remain. When the unsolidified portion remains, the thickness of the unsolidified portion at the start of the reduction is preferably about 20 mm or less. When it exceeds 20 mm, porosity may be generated in the cast slab after molding.

In order to obtain such a reduction effect for forming a slab, it is preferable to use at least one pair of rolls. FIG. 1 shows a molding press-down roll device 10 including a pair of vertical rolls 10-1 and a pair of horizontal rolls 10-2 on the downstream side in the casting direction. Whether the first pressing roll is vertical or horizontal may be selected depending on whether the last unsolidified pressing roll is horizontal or vertical. That is,
If the last unsolidified reduction roll is horizontal, the first forming reduction roll is preferably vertical. The setting position of the forming reduction roll is provided on the downstream side in the casting direction of the unsolidified reduction roll. However, if the distance from the unsolidified rolling roll is too large, the slab temperature will be too low.
m is desirable.

When the method of the present invention for rolling down a slab including an unsolidified portion is carried out, it can be carried out in the following procedure. That is, the solidification shrinkage δ inherent to steel is determined by the above-described static drip method or the like before casting. In casting, when casting conditions such as a casting speed and a condition for secondary cooling of a slab are determined, a thickness L (mm) of an unsolidified portion of the slab at a rolling-down position is obtained. Therefore, the reduction amount R (mm) of the unsolidified portion may be selected so that R / L satisfies the expression (A).

[0031]

EXAMPLE A continuous casting apparatus having the structure shown in FIG.
4 were continuously cast.

[0032]

[Table 1]

A round slab having a diameter of 230 mm and a round cross section was cast, and the slab including the unsolidified portion was rolled down and the slab was pressed to form a slab having a diameter of 190 mm. The unsolidified rolling roll is set at a position 20 m from the molten steel meniscus, and a flat roll is used.
The rolling was performed with one pair of horizontal rolls. The rolling for forming is performed by using a roll provided with a press-down surface of a hole type, and a pair of vertical rolls installed at 26 m from the molten steel meniscus, and a pair of upper and lower rolls installed immediately downstream of the vertical roll in the casting direction. Of two horizontal rolls. The area of the secondary cooling of the slab surface was set to 6 m from the lower end of the mold, and cooling was performed with a specific water volume of 0.2 L / kg · steel.

Table 2 shows casting conditions and test results.

[0035]

[Table 2]

The casting speed was 1.7 or 2.2 m / min. In addition, the thickness of the unsolidified portion was determined by calculation based on heat transfer solidification analysis of the slab temperature, and was confirmed by adding Fe-S to molten steel and measuring it. The solidification shrinkage of the steel was determined by a static drop method before the casting test.

From the slab obtained by rolling down after complete solidification,
Ten cross-sectional samples were taken at 100 mm intervals in the casting direction.

The porosity was examined by visually observing the number and shape of porosity in the cross-sectional sample and measuring the size. The total porosity area was a value obtained by approximating the shape to a circle or an ellipse, obtaining one porosity area from the measured dimensions, and summing them, and the average value of 10 samples was obtained. The ratio of the total porosity area to the area of the slab cross section was evaluated as the porosity area ratio.

The circular deviation rate (%) is obtained by calculating the center of gravity of the cross section of the slab, measuring the distance from the center of gravity to the outer surface at a 30 ° pitch in the circumferential direction, and calculating the difference from the target circle radius to be obtained. It was determined by averaging the results of three samples, defined as the ratio divided by the radius of the circle. The permissible circular deviation rate for billets used in hot tube rolling is usually within about 3%.

Test No. of the present invention example 1 to No. In 6,
According to the solidification shrinkage ratio of the steel, the slab including the unsolidified portion was reduced within the range of the R / L value specified in the present invention. Each of the porosity area ratios was 0.01%, which was good. The circular deviation rate was good at 3% or less.

Test No. of Comparative Example 7-No. In 10,
The test was performed by excluding the lower limit of the R / L value specified in the present invention.
The porosity area ratio was as high as 1.5 to 1.7%, and the porosity was high. This is because the effect of the reduction was not obtained because the slab including the unsolidified portion was not reduced by a reduction amount corresponding to the solidification shrinkage of the steel.

[0042]

According to the method of the present invention, in continuous casting of carbon steel, stainless steel, high alloy steel, etc., a slab having no center porosity at the center of the thickness of the slab and having a good cross-sectional shape can be obtained. Obtainable.

[Brief description of the drawings]

FIG. 1 is a view showing one example of a continuous casting apparatus for carrying out a method of the present invention.

FIG. 2 is a graph showing the relationship between the occurrence of porosity, the ratio R / L of the reduction amount R to the thickness L of the unsolidified portion, and the coagulation contraction rate δ.

FIG. 3 is a view for explaining the method of the present invention by way of an example of casting a slab having a round cross section.

[Explanation of symbols]

1: Immersion nozzle 2: Mold
3: Molten steel 4: Guide roll 5: Solidified shell
6: unsolidified portion 7: cast piece 8: casting direction 9: unsolidified reduction roll 10: forming reduction roll device 10-1: vertical roll 10-2: horizontal roll 11: pinch roll 12: solid phase ratio 0.99
Solidification interface L: Thickness of unsolidified part R: Reduction amount

Claims (2)

[Claims] 1. A continuous casting method for steel, comprising reducing a continuous cast slab including an unsolidified portion under conditions represented by the following formulas (A) and (B). R / L ≧ 0.12δ + 0.08 (A) L ≦ 100 (B) where R: reduction amount (mm) L: solid phase ratio 0.99 or less at the start of reduction at the reduction position Thickness of unsolidified part of steel (mm) δ: solidification shrinkage of steel (%) 2. A continuous casting method for steel, wherein the slab is further reduced after the reduction according to claim 1.