JPH08150451A - Method for producing continuously cast slab for seamless steel pipe material - Google Patents

Abstract

(57) [Summary] [Objective] To provide a method for producing a continuously cast slab for a seamless steel pipe material. [Structure] By performing secondary cooling from immediately below the mold until the center of the slab begins to solidify while changing the cooling strength in the circumferential direction of the slab, asymmetric cooling is performed with respect to the center of the slab. After the solidification shell is generated, further secondary cooling at the final stage of solidification is continued until the central solid fraction of the liquid core reaches 0.8 or more after the center of the solidified liquid core starts to solidify. A method for producing a continuously cast slab for seamless steel pipe material. [Effect] By shifting the porosity outward from the center of the cross section of the slab and reducing its existing diameter,
It is possible to obtain a continuously cast slab for a seamless steel pipe material which does not cause defects on the inner surface of the pipe during piercing and rolling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to continuous casting for obtaining slabs such as square or round billets or blooms suitable as a material for seamless steel pipes such as carbon steel, low alloy steel, high alloy steel and stainless steel. Regarding the method.

[0002]

2. Description of the Related Art As one of the methods for producing a seamless steel pipe, a method of piercing and rolling a continuously cast slab having a round or square cross section by using a piercing machine such as a Mannesmann-mandrel mill system or a Mannesmann-plug mill system is carried out. Has been.

In these methods for producing a seamless steel pipe by the Mannesmann method, a raw material (corner or round billet) heated to a predetermined rolling temperature in a heating furnace is pierced and rolled by a piercing machine,
The hollow shell is expanded by a drawing and rolling machine such as a mandrel mill or a plug mill to reduce the wall thickness, and then the outer diameter is reduced by a drawing and rolling machine such as a stretch reducer or sizer to finish it into a steel tube.

[0004] In the 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.

By the way, the continuously cast slab has an axially discontinuous internal void at the center of the transverse section (section perpendicular to the drawing direction) of the slab corresponding to the final solidification position during casting. (Center Polo City) 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 cause a flaw on the inner surface of the pipe.

Further, even when a round billet is used after being subjected to a lumping process to press the internal voids,
The inner voids may not be sufficiently pressure-bonded to cause flaws on the inner surface of the pipe during piercing and rolling, and it is not possible to completely prevent the occurrence of inner surface flaws on the tube.

Japanese Unexamined Patent Publication (Kokai) No. 3-124352 proposes a "method for producing a continuously cast slab having excellent internal quality" capable of eliminating this internal void. In this method, a roll having a diameter of 2 to 5 times the thickness of the cast piece is used to roll down the unsolidified portion inside the cast piece, so that the internal voids are weakened by a weaker rolling force than the rolling force applied to the solidified cast piece. To reduce. By using this method, it is possible to expect a certain internal void reduction effect.

[0008]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In the method of manufacturing a continuously cast slab shown in Japanese Patent No. 352,
Depending on the size and temperature of the slab, it may not be possible to completely crimp the internal voids due to insufficient penetration of the rolling force into the interior.If the rolling force is further increased and a large reduction is applied, cracks may occur at the solidification interface. So-called "internal cracking" becomes a problem.

Further, in the case of a round billet, the roundness is inevitably impaired by the reduction, and the significance of the near net shape casting, which is the original aim of forming a round billet slab, is lost. .

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems found in the prior art and to provide a method for producing a continuously cast slab for a seamless steel pipe material which does not cause defects on the inner surface of the pipe during piercing and rolling.

[0011]

The gist of the present invention resides in the following method for producing a continuously cast slab for a seamless steel pipe material.

In continuous casting of a slab for a seamless steel pipe material, secondary cooling from immediately below the mold to before the center of the slab begins to solidify is performed by changing the cooling strength in the circumferential direction of the slab. By producing a solidified shell asymmetric with respect to the center of the slab, further secondary cooling at the end of solidification is performed, and after the center of the solidified liquid core begins to solidify, the center solid phase of the liquid core A method for producing a continuously cast slab for a seamless steel pipe material, which is continuously applied until the rate becomes 0.8 or more.

As a result of repeated experiments, the inventors of the present invention have found that the following problems can be solved by the following means.

By changing the cooling strength in the circumferential direction of the slab between immediately below the mold and before the center of the slab begins to solidify, an eccentric solidified shell asymmetric with respect to the center is generated. . The solidified shell asymmetrically generated in the initial stage of solidification grows in the circumferential direction to be non-uniform until the end,
The final solidification is eccentric from the center of the circular cross section. Due to the eccentricity and final solidification, voids (porosities) are generated at positions off the center of the slab.

Further, in the final stage of solidification, eccentricity is generated by continuously cooling the liquid core from the start of solidification until the solid fraction at the center of the liquid becomes 0.8 or more. The ongoing porosity is decreasing and becoming smaller.

A slab eccentric from the center of the slab and having a small porosity is piercing-rolled.

[0017]

1 and 2, a configuration example of a continuous casting apparatus for realizing the method of the present invention, the method of the present invention, and the operation and effect thereof will be described.

FIG. 1 is a vertical cross section in a side direction showing an example of a curved type continuous casting apparatus for round billets.

This apparatus cools the ladle 1 and the tundish 3 each equipped with a ladle nozzle 2 and a dipping nozzle 4, a mold 5 downstream of the tundish 3, and an initial stage of solidification of the slab 7 immediately below the mold 5. A secondary cooling spray zone 6, a final solidification secondary cooling spray zone 8 for cooling the final solidification stage of the cast slab 7, and a pinch roll 9 group are provided.

In FIG. 1, reference numeral 10 is molten steel.

The secondary cooling spray zone 6 immediately below the mold 5 is
By changing the cooling strength in the circumferential direction of the cast piece 7 immediately below the mold 5 and before the solidification start point of the center of the cast piece 7, the solidified shell asymmetrical with respect to the center of the cast piece 7.
This is a cooling device for producing 11. Hereinafter, this is referred to as an eccentric cooling spray zone 6.

FIG. 2 is an external view showing a structural example of the eccentric cooling spray zone 6. The eccentric cooling spray zone 6 is composed of a plurality of ring sprays 12 provided in multiple stages. The ring spray 12 surrounds a slab (round billet) 7, and the center of the ring spray 12 and the center of the slab 7 are separated. Arranged to match.

In the eccentric cooling spray zone 6, the following eccentric solidification cooling is applied. That is, from 1 directly below the mold 5
Up to the range of 1.5 m, the cooling strength of the entire circumference of the slab 7 is made uniform with a uniform water amount density for the purpose of preventing bulging.
This is because the solidified shell 11 of the slab 7 is thin in this range, and if it is cooled unevenly in the circumferential direction, the solidified shell 11 may break due to thermal stress. The desirable lower limit of the average cooling water amount density of the eccentric cooling spray zone 6 is 30 liters /
(min · m ² ). On the other hand, this water amount density may be high, so no upper limit is set, but a desirable upper limit in view of suppressing bending of the slab is 800 liters / (min · m ² ).

Between 1 to 1.5 m and 6 m immediately below the mold 5, the average cooling water amount density is reduced, and the average cooling water amount in the half round of the inner radius of the cast slab 7 out of the entire circumference of the slab 7 is reduced. The density is divided into about 100 to 60% of the whole, the average cooling water amount density of the half circumference on the side opposite to the inner radius is divided into about 0 to 40% of the whole, and the slab 7 is sprayed unevenly in the circumferential direction. This makes it possible to decenter the final solidification position of the slab. Desirable eccentricity range of solidified shell is 1% of slab diameter
That is all.

The length of the eccentric cooling spray zone 6 and the length for making the cooling strength of the entire circumference uniform, and the respective average cooling water amount densities vary depending on the slab diameter. Therefore, the number of ring sprays 12 may be determined in consideration of the maximum eccentric cooling effect. However, the spray density should be as uniform as possible in the slab withdrawal direction, so that the desirable range of the spacing of the ring spray 12 is about 50 to 800 mm.

The secondary cooling by the eccentric cooling spray zone 6 is stopped before the center portion of the slab liquid core begins to solidify, specifically, before the central solid fraction reaches 0.2. However, it is desirable that the spray stop position be before the time when the central solid fraction at the time when the slab surface temperature becomes maximum due to the heat recovery after the stop becomes 0.2. This is because porosity starts to be generated from the time when the central solid fraction of the slab liquid core becomes 0.2, and when the surface temperature is rising due to recuperation during this time, tensile stress is generated inside the slab. This is because there is a risk of working and increasing porosity.

Further, from the eccentric cooling spray zone 6, a secondary cooling spray zone at the end of solidification (hereinafter,
A cooling spray zone at the end of solidification) 8 is provided. This cooling device is not particularly limited, but may have the same structure as the eccentric cooling spray zone 6 shown in FIG. This cooling is performed in order to apply heat shrinkage due to cooling from the outer surface of the slab 7 and suppress porosity which is generated inside the slab due to solidification shrinkage.

The starting position of the final solidification spray is most effective when the central solid fraction of the slab 7 is in the range of 0.2 to 0.3. The reason for this is that the fluidity of the molten steel deteriorates from this point, and even if negative pressure due to solidification shrinkage occurs, the molten steel does not flow sufficiently and porosity is generated because it becomes difficult to fill the shrinkage holes. This is because heat is applied to the slab to compensate for solidification shrinkage inside.

Further, from the time when the center of the uncoagulated liquid core begins to solidify (final solidification stage), the thermal contraction of the slab suppresses the generation of porosity and reduces the spread of porosity in order to suppress the spread of porosity. In the terminal cooling, it is necessary to continuously cool the liquid core until the solid fraction reaches 0.8. The portion with a solid fraction of 0.8 or less is fragile without strength in the slab, and easily breaks when tensile stress occurs in the central part due to reheat due to cooling stop at a solid fraction of less than this, Instead, the porosity will expand.

The range of the average cooling water amount density of the cooling spray zone 8 at the final stage of solidification is preferably 25 to 100 liters / (min · m ² ). If it exceeds 100 liters / (min · m ² ), cooling of the surface of the slab will proceed rapidly to a low temperature state, the cooling rate will rather decrease and tensile stress will occur inside the slab, leading to expansion of porosity. On the other hand, 25 liters / (m
If it is less than in · m ² ), the shrinkage effect on the surface of the slab is small, and it becomes impossible to sufficiently suppress the generation of porosity.

As the cooling medium, ordinary cooling water or a cooling medium in place of it, air mist spray, or the like can be used with any cooling spray.

By the above method, the following effects (1) to (4) can be obtained.

(1) Before the center of the slab begins to solidify immediately below the mold, the cooling strength is changed in the circumferential direction of the slab to form a solidified shell asymmetric with respect to the center. , Furthermore, when this solidified shell is finally eccentric from the center of the slab and is finally solidified, the porosity is generated by shifting from the center of the slab. (2) If the porosity is small, leave it as it is or with a predetermined size. If the tube is processed by piercing and rolling after being processed into the billet, the porosity does not come out to the inner surface of the tube and is in the wall thickness of the tube, and it is completely crimped during processing and does not become a flaw on the inner surface of the tube.

(3) Generally, porosity has a spread in the radial direction of the slab. When this spread is large, the pipe inner surface is exposed and oxidized during pipe production to generate scale, which is caught during processing and becomes an inner surface flaw.

However, by newly cooling the surface of the slab from the final solidification stage (when the center of the liquid core begins to solidify), the generation of porosity due to heat shrinkage of the slab is suppressed, and the spread of porosity is expanded. Can be reduced.

(4) It is possible to reduce the porosity to a small extent and to reduce the defects on the inner surface of the pipe due to the reason (2) above.

[0037]

EXAMPLES In the example of the present invention, a slab was manufactured under the following conditions by using the curved type continuous casting apparatus for billets shown in FIGS. 1 and 2.

Steel type: 13% Cr steel Diameter of slab: 190 mm Steady casting speed: 2.5 m / min Length of eccentric cooling spray zone: 6 m Number of ring sprays: 10 from 1 m below the mold to 1-6 m 50 pieces Ring interval: 500 mm Cooling medium: Water Water amount density: 300 liters / (m from directly below the mold to 1 m
Uniform cooling with in · m ² ) Up to 1 to 6 m, the inner radius of the cast slab is 120 liters / (min · m ² ), and the opposite half is 20 liters / (min · m ² ). Band (ring spray) Position: 20-25m from molten metal meniscus in mold Cooling starts when central solid fraction is 0.2 Ring spray number: 51 Ring interval: 100mm Cooling medium: Water Water density: 50 liters / (min -M ² ) As Comparative Example 1, the secondary cooling was carried out evenly in the circumferential direction of the slab only within the range of 1 m from just below the mold (this corresponds to the conventional ordinary casting method). 1m directly under
The uniform cooling up to and including the eccentric cooling spray were used. As Comparative Example 3, only uniform cooling up to 1 m directly under the mold and the final cooling solidification cooling spray were used.

The slab thus obtained was heated to 1200 ° C. in a heating furnace and then pierced and rolled by a piercing machine to manufacture a hollow shell having an outer diameter of 63 mm and a wall thickness of 5 mm.

FIG. 3 is a vertical cross-sectional view in the side direction for explaining the porosity existing in the round slab and the condition of piercing and rolling. 13 is a plug, 14 is a round slab, 15 is porosity, 16 is a hollow shell, and 17 is a drum-shaped roll. FIG. 4 shows a line a- in FIG.
It is a figure explaining the cross section of the round cast piece in a ', the amount of eccentricity of a solidification shell, and porosity existence diameter.

Samples were cut out from the obtained slab and a part of the raw pipe, and the eccentricity of the solidified shell, the diameter of porosity and the number of inner surface defects after pipe making were examined. Table 1 shows the results.

[0042]

[Table 1]

As shown in Table 1, in the examples of the present invention, the amount of eccentricity was 10 mm (about 5.3% of the slab diameter), and the porosity existing diameter was as small as 15 mm. As shown in FIG. 3, since the tip of the plug 13 is placed in the center of the round slab 14 during piercing and rolling, the plug 13 does not have the porosity 15 present. It is crimped in the thick wall. As a result, no flaws on the inner surface of the pipe were produced after the pipe was made.

On the other hand, in the case of each of the comparative examples, a portion where porosity exists was applied to the plug, and a flaw on the inner surface of the pipe was generated.

[0045]

According to the method of the present invention, the porosity generated at the final solidification position of the continuously cast slab is shifted outward from the center of the transverse section of the slab, and the existing diameter is reduced. It is possible to obtain a continuously cast slab for a seamless steel pipe material which does not cause defects on the inner surface of the pipe during piercing and rolling.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view in a side direction showing an example of a curved continuous casting device for round billets for realizing the method of the present invention.

FIG. 2 is an external view showing a structural example of an eccentric cooling spray zone.

FIG. 3 is a vertical cross-sectional view in a side direction for explaining the porosity present in a round cast piece and the situation of piercing and rolling.

FIG. 4 is a diagram illustrating a cross section of a round cast piece along line aa ′ in FIG. 3, an eccentric amount of a solidified shell, and a porosity existing diameter.

[Explanation of symbols]

1: Ladle, 2: Ladle nozzle, 3: Tundish, 4: Immersion nozzle, 5: Mold, 6: Secondary cooling spray zone (eccentric cooling spray zone), 7: Cast slab, 8: Secondary cooling spray at the end of solidification Band, 9: Pinch roll, 10: Molten steel, 11: Solidified shell, 12: Ring spray, 13: Plug, 14: Round slab, 15: Porosity, 16: Hollow tube, 17: Drum roll

Claims (1)

[Claims] 1. In continuous casting of a slab for a seamless steel pipe material, secondary cooling is performed immediately below the mold until before the center of the slab begins to solidify, and the cooling strength is varied in the circumferential direction of the slab. After the solidification shell that is asymmetric with respect to the center of the slab is generated, the secondary cooling at the end of solidification is further performed and the center of the liquid core is started after the center of the solidified liquid core begins to solidify. A method for producing a continuously cast slab for a seamless steel pipe material, which is continuously applied until the solid phase ratio becomes 0.8 or more.