JPH10128510A - Method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a continuously cast billet for seamless steel pipe base stock without causing any surface flaw in the pipe by cooling an upper half periphery and a lower half periphery of the cast billet differently from each other in cooling rate and executing the electromagnetic stirring after intermediate part of the secondary cooling zone. SOLUTION: Molten steel 3 poured into a mold 4 from a ladle 1 through a tundish 2 is cooled in the mold 4, and while forming solidified shell 7, the molten steel 3 is stirred by the electromagnetic stirring device 5 in the mold, and crystal nuclei being equiaxed crystal source are separated and deposited downward. The cast billet 8 is cooled just below the mold in the secondary cooling zone 6, and while gradually growing the solidified shell 7, the solidification is progressed. Then while drawing out as the round cast billet by pinch rolls 11 by applying the electromagnetic force near the intermediate part of the secondary cooling zone, the solidification is completed. In the range from >2m to 20m position just below the mold, the upper half periphery and the lower half periphery are cooled differently from each other in the cooling water quantity. In such a case, e.g. the sprayed water density at the upper half periphery in the peripheral direction of the cast billet is defined as 10l/m<2> .min and the sprayed water density at the lower part periphery in the peripheral direction of the cast billet is defined as 50l/m<2> .min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting steel for obtaining an optimum round billet as a material for a seamless steel pipe such as high alloy steel and stainless steel.

[0002]

2. Description of the Related Art As one method of manufacturing a seamless steel pipe, a method of manufacturing a continuously cast slab having a round cross section using a drilling machine such as a Mannesmann mandrel mill system or a Mannesmann plug mill system has been performed. The method of manufacturing a seamless steel pipe by the Mannesmann method is that a material (round billet) heated to a predetermined rolling temperature in a heating furnace is pierced and rolled by a piercing machine, and then the hollow shell is drawn and rolled by a mandrel mill or a plug mill. After reducing the wall thickness by a pipe mill, the outer diameter is reduced by a rolling mill such as a stretch reducer or a sizer to finish the product steel pipe.

[0003] In a seamless steel pipe, since the inner part of the material used is the inner surface of the pipe, soundness is required not only on the outer surface of the material but also on the inner part.

[0004] By the way, a continuously cast slab has an axially discontinuous internal void at the center of the cross section (cross section perpendicular to the drawing direction) of the slab corresponding to the final solidification position during casting. Exists. This internal void may not be sufficiently pressed during piercing and rolling, and may be exposed on the inner surface of the pipe to become a flaw on the inner surface of the pipe.

Japanese Patent Laid-Open Publication No. 8-52555 (hereinafter referred to as Reference 1) discloses a method for preventing the occurrence of flaws on the inner surface of a pipe due to the internal space. This method shifts the final solidification position of the slab outward from the center of the slab by changing the cooling strength in the circumferential direction of the slab in the secondary cooling zone in the continuous casting method of the square or round billet.

[0006]

The method for producing a continuous cast slab disclosed in Reference 1 is caused by internal voids by shifting the final solidification position from the center of the slab by a maximum of 3% of the slab diameter. The occurrence of flaws on the inner surface of the pipe is prevented. The reason why the maximum eccentricity is set to 3% in the cited reference 1 is that if the eccentricity is further increased, the difference between the upper and lower cooling rates must be increased, and as a result, the slab is bent. .

SUMMARY OF THE INVENTION It is an object of the present invention to provide a continuous casting method for steel which solves the conventional problems and does not cause bending of the cast slab after casting and does not cause any flaws in the pipe inner surface after piercing and rolling.

[0008]

In order to solve the above-mentioned problems, a continuous casting method of steel is a method of drawing a slab with a round mold and continuously casting molten steel. In the secondary cooling zone during the cooling, the water density of the cooling water in the slab circumferential direction is increased by increasing the lower half circumference from the upper half circumference and cooled, and electromagnetic stirring is performed from the middle part of the secondary cooling zone and thereafter, The final solidification position is eccentric from the slab center.

Generally, in continuous casting, the slab that has left the mold is cooled by spray water or cooling, and the heat corresponding to the latent heat of solidification is removed from the surface of the slab to solidify the molten steel and develop a solidified shell. Solidifies sequentially in the axial direction. At this time, if the cooling of the slab is uniform in the circumferential direction, the speed of development of the solidified shell is almost the same over the entire circumference of the slab, and the center of the slab cross section is the final solidification position.

The internal voids of the slab are generated in the final solidification stage where the fluidity of the molten steel is reduced. If the cooling in the circumferential direction of the slab is uniform, the inner space of the slab is almost at the axial center (at right angles to the drawing direction). At the center of the cross section in the direction).

According to the present invention, an internal void generated at a final solidification position of a continuous cast slab is shifted from an axial center of a slab cross section,
For the purpose of improving the inner quality of the seamless steel pipe and preventing bending after casting, a great effect is obtained by combining the following two means.

By changing the distribution of the cooling water amount in the circumferential direction of the slab in the secondary cooling zone, the cooling intensity in the circumferential direction is changed. As a result, the growth of the solidified shell develops preferentially in the semi-peripheral part having the higher cooling strength than in the remaining semi-peripheral part having the lower cooling strength.

Further, when electromagnetic stirring is performed after the middle part of the secondary cooling zone, the tip of the solidified shell at that location is washed by stirring the molten steel, and the crystal at the tip of the solidified shell is liberated, and on the upper surface side of the slab, The crystals fall and fall down,
The crystals are deposited on the solidified shell on the underside of the slab.
As a result, the growth of the solidified shell on the upper surface side of the slab is delayed, and the growth of the solidified shell on the lower surface side of the slab is promoted, so that the thickness of the solidified shell on the lower surface side of the slab becomes larger than the upper surface side.

The reason why the electromagnetic stirring is performed after the middle part of the secondary cooling zone is that if the electromagnetic stirring is performed before the middle part of the secondary cooling zone, the superheat degree of the molten steel in that part is high. This is because the amount of the crystals released from the solidified shell tip redissolved and deposited on the solidified shell on the lower side of the slab is reduced, and the effect of electromagnetic stirring cannot be sufficiently expected.

Thus, in addition to the differential cooling in the secondary cooling zone,
The solidification speed of the slab is significantly different between the upper and lower parts by causing the crystal movement and accumulation phenomenon to act synergistically by performing electromagnetic stirring after the middle part of the secondary cooling zone, and the final solidification position is eccentric from the axis. As a result, internal voids are formed at positions off the center of the slab.

Further, by applying an electromagnetic stirring force in the secondary cooling zone, a sufficient equiaxed zone is formed in the final solidified portion, and the internal voids can be reduced in size and dispersed.

[0017]

Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a conceptual diagram of a curved continuous casting machine for producing a round slab according to an embodiment of the present invention.

In the drawings, reference numeral 1 denotes a ladle, 2 denotes a tundish, 3 denotes molten steel, 4 denotes a mold, 5 denotes an electromagnetic stirring device for the mold, 6
Is a secondary cooling zone, 7 is a solidified shell, 8 is a slab, 9 is an electromagnetic stirring device for the secondary cooling zone, 10 is a spray, and 11 is a pinch roll.

With the curved continuous caster for producing round slabs shown in FIG.
A round billet having a diameter of 210 mm was continuously cast from a% Cr steel. Here, the molten steel 3 injected into the mold 4 from the ladle 1 via the tundish 2 is cooled by the mold 4 and solidified shell 7.
The molten steel 3 inside the mold is stirred by the electromagnetic stirring device 5 of the mold to separate the crystal nuclei serving as the equiaxed crystal source and precipitate downward (to the side of the secondary cooling zone).

After the molten steel 3 whose center is unsolidified exits the mold 4, the slab 8 is cooled from immediately below the mold in the secondary cooling zone 6 and solidification proceeds while the solidified shell 7 gradually grows, and the solidification proceeds. A magnetic stir force is applied near the intermediate portion to cause the
Thus, the solidification is completed while being drawn out as a round slab.

The secondary cooling zone 6 was provided up to a position 20 m below the mold. The spray 10 in the secondary cooling zone 6 is a scare type water spray, which surrounds the slab 8 as shown in FIG. 2 and has four nozzles in the circumferential direction and 200 m in the casting direction.
Nozzles were arranged at m intervals. Each spray 10 provided 10 sprays. Each spray 10 was configured such that the amount of spray water could be changed on the upper surface side and the lower surface side.

In the secondary cooling zone 6, after being uniformly cooled from below the mold to 2m, the upper half of the slab in the circumferential direction of the slab (from the non-reference side or the top side of the continuous casting machine) from 2m to 20m.
And the lower half circumference (reference side or ground side of the continuous casting machine) was cooled with different water density.

In the embodiment, the slab 8 was uniformly cooled over the entire circumference of the secondary cooling zone 6 in a range from immediately below the mold to a position 2 m from the secondary cooling zone 6 with the same amount of four spray waters in the circumferential direction.

The reason is that if the solidified shell 7 is cooled unevenly in the thin portion immediately below the mold, tensile stress is generated in the circumferential direction of the slab, which may cause the solidified shell to break.

In this range, the water density is 50 l / m ² ·
Cooled in min. As described above, by uniformly cooling the entire circumference of the slab 8 from the area immediately below the mold to 2 m relatively strongly, the strength of the solidified shell is controlled so that the solidified shell does not break at the time of subsequent uneven cooling. Was secured. The uniform cooling length immediately below the mold can be changed slightly depending on the cross-sectional size of the slab.

In the range from more than 2 m immediately below the mold to a position of 20 m below, the upper half circumference (the opposite side of the continuous casting machine or the top side) and the lower half circumference (the reference side or the ground side of the continuous casting machine) in the circumferential direction of the slab. Uneven cooling was performed with different water density. In this case, the water density of the upper half of the slab circumferential direction is 10 l / m ² · min, the water density of the lower half of the slab circumferential direction is 50 l / m ² · min (Example 1), and 75 l / m ² · min ( Example 2), 100 l
/ M ² · min (Example 3).

The round billet produced by the continuous casting machine was heated at 1200 ° C. in a heating furnace and then pierced and rolled by a piercing machine to produce a hollow shell 14 having an outer diameter of 180 mm and a wall thickness of 8 mm.
FIG. 3 shows the pipe production state. In the drawing, reference numeral 12 denotes a pair of drum-shaped rolls which are vertically inclined and are pierced and rolled by the upper and lower rolls and a plug 13 between the upper and lower rolls to rotate and advance a solid round billet 14 in a spiral shape. The hollow shell 15 is finished. At this time, solid round billet 1
The internal space 16 that is shifted outward from the center of 4 is not exposed on the inner surface of the hollow shell 15 and is within the thickness of the tube, and the internal space 16 is pressed by rolling during rolling.

With respect to the obtained hollow shell, cracks in the middle part of the pipe wall and flaws on the inner surface of the pipe were examined using an ultrasonic flaw detector.
The result is shown in FIG. 4 as the total number of flaws generated for each billet.

[0030]

【Example】

(Example 1) In the range from more than 2 m immediately below the mold to the position of 20 m, the water density in the upper half of the slab was 10 l / m ² · min, and the water density in the lower half of the slab was 50 l / m ² · min. FIG. 5 shows the deviation (referred to as the eccentricity of the internal void) from the center of the position where the internal void is generated as viewed from the cross section of the slab of Example 1.
Shown in In Example 1, the position at which the internal voids were generated was 12.6 mm away from the center of the slab toward the upper half circumference. The eccentricity of the internal void was 6%.

(Example 2) The water density of the upper half of the slab is 10 l / m ^{2 in} the range from more than 2 m immediately below the mold to the position of ²⁰ m.
Min, the water density of the lower half of the slab is 75 l / m ²
FIG. 3 shows the deviation (referred to as eccentricity of the internal gap) from the center of the position where the internal gap is generated as viewed from the cross section of the slab of Example 1 in which n was unevenly cooled. In Example 2, the position at which the internal void was generated was a position 14.7 mm away from the center of the slab toward the upper half circumference. The eccentricity of the internal void was 7%.

(Example 3) In a range from more than 2 m immediately below the mold to a position of 20 m below, the water density of the upper half circumference of the slab is 10 l / m ^2.
· Min, the water density of the lower half of the slab is 100 l / m ² · m
FIG. 3 shows the deviation (referred to as eccentricity of the internal void) from the center of the position where the internal void was generated as viewed from the cross section of the slab of Example 3 in which the cooling was performed non-uniformly. In the third embodiment, the position at which the internal void is generated is a position 16.8 mm away from the center of the slab toward the upper half circumference. The eccentricity of the internal void was 8%.

Comparative Example 1 Comparative Example 1 is a conventional technique, in which the slabs were uniformly cooled by making the spray water amount density in the secondary cooling zone uniform. That is, a cast slab was produced without applying any electromagnetic stirring in the mold or electromagnetic stirring in the secondary cooling. In this case, the cooling from the position immediately below the mold to the position 2 m is uniformly cooled under the same spraying conditions as in the embodiment of the present invention. The water density is 50 l without any difference in the water density between the half circumference and the lower half.
/ M ² · min, the slab was uniformly cooled. The eccentricity of the internal void was 0%.

Comparative Example 2 In Comparative Example 2, a slab was produced using the technique of Reference Example 1 without performing electromagnetic stirring in the secondary cooling zone. In this case, the cooling from the position immediately below the mold to the position 2 m is uniformly cooled under the same spraying conditions as in the embodiment of the present invention. Differentiate the water density between the half lap and the lower lap,
The water density in the upper half of the slab circumferential direction is 30 l / m ²
min, the water density in the lower half of the slab circumferential direction is 50
The cast slab was subjected to differential cooling under the condition of 1 / m ² · min.

As shown in FIG. 4, the internal voids 16 of the embodiment of the present invention and the comparative example 2 are of the same size. Is located 4.2 mm off the upper half circumferential side of the slab. The eccentricity of the internal void was 2.1%.

As shown in FIG. 4, in the case of Comparative Example 1, 36 flaws were generated. On the other hand, Comparative Example 2
In the case where the eccentricity of the internal voids is 2.0%, the number of occurrences of flaws on the inner surface of the pipe is reduced by half, but not completely.

On the other hand, when the eccentricity of the internal gap is set to 5% or more according to the embodiment of the present invention (in FIGS. 4 and 5, the eccentricity is 6.0%, 7.0%, and 8.0%). To indicate)
In each case, there was almost no difference in the number of generated flaws, indicating that the reduction effect was remarkable. In addition, as is clear from FIG. 4, there is no difference in the effect of reducing flaws at the eccentricity of 7.0% and 8.0%, and it is understood that the eccentricity is saturated at 8%.

[0038]

According to the present invention, differential cooling between the upper half circumference and the lower half circumference in the slab circumferential direction and electromagnetic stirring are performed in the middle part of the secondary cooling zone and thereafter, and the internal void generated at the final solidification position of the continuous cast slab. Is greatly shifted outward from the center of the cross section of the slab, so that it is possible to provide a continuous cast slab for a seamless steel pipe material that does not cause any flaws on the inner surface of the pipe during piercing and rolling.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a curved continuous casting machine for producing a round billet according to an embodiment of the present invention.

FIG. 2 is a schematic sectional view showing a state of spraying of a secondary cooling zone spraying device according to an embodiment of the present invention, and FIG. 2 (a) is a diagram showing a state of spraying from immediately below a mold to 2m of a secondary cooling zone.
(B) is a figure which shows the situation of the spray from over 2 m of a secondary cooling zone to the end of a secondary cooling zone.

FIG. 3 is an explanatory view showing a processing state when piercing and rolling a round billet continuously cast by eccentric solidification according to an embodiment of the present invention.

FIG. 4 is a diagram showing the state of occurrence of flaws in hollow shells manufactured according to the example of the present invention and the comparative example.

FIG. 5 is an explanatory diagram showing a state of internal voids of a slab cast according to an example and a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ladle 2 Tundish 3 Molten steel 4 Mold 5 Mold electromagnetic stirring device 6 Secondary cooling zone 7 Solidified shell 8 Cast piece 9 Electromagnetic stirring device for secondary cooling zone 10 Spray 11 Pinch roll 12 Hour roll 13 Plug 14 Solid round Billet 15 Hollow shell 16 Internal gap

Claims (1)

[Claims] In a method for continuously casting molten steel into a round-section slab, the water density of cooling water in the circumferential direction of the slab is increased from the upper half circumference in a secondary cooling zone from immediately below the mold until the slab completely solidifies. A continuous casting method for steel, comprising enlarging a lower half circumference, performing electromagnetic stirring in an intermediate portion of a secondary cooling zone and thereafter, and decentering a final solidification position of a slab from a center of the slab.