JP2013000785A - Method for manufacturing hollow steel slab for pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve solidified shell inner surface properties, such as flatness, clearance and internal cracks, when manufacturing a hollow steel slab for a seamless steel pipe by continuous casting.SOLUTION: A hollow cast slab having a vacuum hollow is formed by separating a molten core in a step of pulling up the cast slab to a 3/4 circumferential point in a curving type continuous casting method. The cast slab is extended, and next, when cutting the cast slab, a pair of wedge shaped teeth (the upperstream side is slant, and the downstream side is perpendicular) is pressed in the cast slab, and is cut off in such a manner as being bitten off. An upperstream side end is reduced, pressure-contacted, and closed, and the downstream side is opened, and the hollow steel slab with one end opened is obtained. Electromagnetical stirring is performed in a portion in which the molten core is separated in order to prevent clearance on the solidified shell inner surface. In order to press-bond the internal cracks to be harmless, the cast slab after extension is reduced by a rolling mill, and the steel slab after cutting is compressed in an axial direction. By using the steel slab for manufacturing the pipe, hole forming and rolling can be eliminated.

Description

  The present invention relates to a method for producing a hollow steel piece used for producing a seamless steel pipe.

  The seamless steel pipe manufacturing process consists of steel making to produce round or square steel slabs as raw materials, roll rolling with the steel slabs penetrating holes along the central axis, and then turning the rough pipes. Stretching, followed by constant diameter rolling to adjust the outer diameter and wall thickness of the stretched pipe. Each rolling is provided with an auxiliary rolling mill in addition to the main body forming machine, and a reheating process is inserted between the above processes as necessary.

  In the turning process, Mannesmann Piercer and press piercing mill are used as a turning machine, but the equipment becomes a big deal and requires a corresponding cost. In the turning process, strict processing is performed on the center of the material, so a high-quality material is also required.

If hollow steel slabs and rough pipes can be produced by continuous casting, the turning process is omitted, and a great economic effect can be obtained, but it has not been successful.
Prior Example 1: Non-Patent Document 1 details the result of trial production of a hollow circular section steel slab using a special continuous casting machine for experiments.

  The attempted continuous casting method (see FIG. 5) is as follows. In the curved continuous casting, the cross-sectional shape of the mold is a circle, and the drawing trajectory of the slab is made to circulate more than 1/2 turn beyond the lowest point of 1/4 turn. The casting condition is appropriately set to leave a molten core having a desired dimension at a half circumference portion. Next, when the slab is melted with a gas torch and a part of the slab opens, air enters. Stable until the melt core level is the same as the casting surface. After this level, it is pulled out as a hollow slab. Cut to an appropriate length to obtain a hollow steel piece with a circular cross section.

  In order to use the above-mentioned hollow steel piece as a pipe material, it becomes a problem whether the inner surface properties of the solidified shell can withstand the stretching process. In stainless steel (JIS: SUS304 equivalent), the inner surface properties were flat, but in high carbon steel, the problem of large irregularities was found. As a countermeasure, an electromagnetic stirrer was attached at about 150 ° from the casting surface, and the molten core was stirred. As a result, the inner surface of the solidified shell was flattened even in high carbon steel. It is clearly stated that good quality was obtained for stainless steel and low carbon steel in the trial production of steel pipes. There is no description about high carbon steel. It is estimated that there is a problem.

  Looking at the macro structure of the cross section of the slab, which is the above experimental data in detail, although it is not found in stainless steel, small crack-like voids are observed in the low carbon steel and high carbon steel just below the inner surface of the solidified shell. There is no explanation about this, but it is a phenomenon that cannot be ignored. The voids are considered to grow into shrinkage pores or porous in the normal continuous casting method in which the voids are solidified. The size of the void is 1 mm to several mm or less, but is oxidized by air during casting and cooling after casting. When reheating as a steel slab, it takes a long time, so it is obvious that oxidation proceeds further, and opens and expands during rolling, resulting in a flaw inside the steel pipe. Therefore, it is understood that not only flattening but also void disappearance is an important condition.

  Prior Example 2: Patent Document 1 discloses a novel method obtained by improving and developing the continuous casting method. In this method, the drawn slab is pulled up as a vacuum without opening, and the hollow slab is pressed down to form a solid material. In this method, even if a void defect or crack occurs immediately below the inner surface of the solidified shell, it is pressed and disappeared by reduction, and there is no problem. However, the hollow feature is lost with the realization.

  Prior Example 3: Patent Document 2 discloses a method for producing a steel piece in which a vacuum cavity is enclosed, as an application of the continuous casting method. In the case of this method, void defects are generated as in the first example. When both ends of the steel piece are cut and removed at room temperature, a hollow rough tube whose inner surface is not oxidized can be obtained. However, at the time of reheating, the voids are invaded by oxidation, and as described above, the gap is not suitable as a pipe material.

  By the way, in continuous casting, internal cracks due to thermal stress and bending stress in the solidified shell often occur. In many cases, there is no problem with pressure bonding in the next hot rolling process. However, in the case of hollow cast slabs, cracks on the inner surface of the shell will invade the oxidation reaction during reheating of the steel slab, and during rolling and drawing rolling. Since cracks expand, this defect is not allowed as a pipe material.

  From the above, in order to omit or reduce the roll rolling in the pipe making of continuously cast slab, 1) the steel slab is hollow and at least one end is open, and 2) the steel slab is turned. It can be seen that there are two conditions that the inner surface of the solidified shell has no material defects that hinder workability so that it can withstand rolling and stretching. It is more convenient if the outer diameter and inner diameter can be adjusted.

R. Tarmann, W. Poppmeier; Journal of Metals, April 1966, 453

Patent No. 2998737 Japanese Patent No. 3684731

The object of the present invention is to produce a hollow steel piece that can be used for a seamless steel pipe by continuous casting. Therefore, the problem to be solved is to improve the inner surface properties of the solidified shell to a level that can withstand turning and stretching. And
More specifically, in the process of forming the hollow cast slab, it prevents a small crack-like void generated on the inner surface of the shell when separating the solidified shell and the molten core, and various internal cracks connected to the solidified shell. It is to eliminate the adverse effects.

In order to solve the above problems, the following method is adopted.
The first invention is a method of cutting a hollow slab formed by the following continuous casting method into a steel slab, wherein a pair of wedge-shaped teeth arranged with the slab sandwiched between them are cut symmetrically on the slab. The tip of the slab is tapered by making the wedge shape of the wedge-shaped teeth cut by press-fitting and the wedge shape of the wedge is inclined with respect to the slab surface on the upstream side in the drawing direction and perpendicular on the downstream side. This is a method for producing a hollow steel piece, characterized in that the cavity is pressed and sealed, and the face to be cut is cut open in a vertical shape, thereby forming a hollow steel piece with one side opening.
It is a kind of curved continuous casting, and the slab drawing locus is composed of a 3/4 round curved part and a horizontal straight part following the curved part, and the three factors of mold cross-sectional dimension, curved radius and drawing speed By appropriately combining the slabs, the slabs are drawn over 1/2 lap while leaving the molten core inside the slabs, and further drawn over the molten steel height (about 1.4 m) equivalent to atmospheric pressure from the casting surface. Then, the molten core is separated from the solidified shell to form a hollow slab having a vacuum cavity, the slab is stretched horizontally at the uppermost 3/4 round point, and then the cavity of the slab is A continuous casting method in which a steel piece enclosing a cavity is obtained by partially closing and cutting.

  The contents described in the above “mark” are the method itself of the preceding example 2.

  In the second invention, the molten core including the meniscus is stirred by an electromagnetic stirring device provided immediately below the site where the vacuum cavity is formed, the inner surface of the solidified shell is flattened, and the high-concentration liquid phase at the solid-liquid boundary surface The hollow steel piece manufacturing method according to the first aspect of the present invention is characterized in that the formation of voids is prevented by scraping out the CO reaction to suppress the formation of voids.

  According to a third aspect of the present invention, the cross-sectional shape of the mold is a circle, an oval, a square or a polygon having a cross-sectional aspect ratio of 1 or more and 3 or less, and after the slab is stretched, a cavity is formed in the slab by at least one pair of rolls. The hollow steel slab manufacturing method according to the first or second aspect of the invention is characterized in that a reduction in the major axis direction is applied so as to remain, and a crack inside the solidified shell is pressure-bonded.

  The fourth invention is characterized in that the steel slab obtained by cutting the slab is immediately compressed in the axial direction of the steel slab by pressing to press the crack inside the solidified shell. 3 It is a manufacturing method of the hollow steel piece described in invention.

  According to the method of the present invention, when a slab is first cut into a hollow steel slab, a new press-fitting and dividing method is adopted. Since the sealed state is maintained and the downstream side is opened, a hollow steel piece having one end opening can be easily obtained, and the first condition of the steel piece for pipe is solved.

  Secondly, the formation of small crack-like void defects that are likely to occur on the inner surface of the solidified shell in the process of forming a hollow is prevented by the following measures. That is, 1) the inner surface of the solidified shell is flattened by electromagnetic stirring directly below the liquid surface including the liquid surface on the cavity side, and 2) bubbles caused by the CO reaction by washing out the high-concentration liquid phase at the solid-liquid interface. It is supported by two effects of suppressing the occurrence.

Thirdly, internal cracks often occur in continuous cast slabs, but when the defects are close to the inner surface of the solidified shell, they expand into inner surface flaws in turning and rolling. According to the method of the present invention, the vacuum hollow cast slab is harmless by pressure-bonding the internal crack parallel to the slab axis by the side surface reduction of the cast slab immediately after stretching. Further, since the hollow slab is subjected to the axial compression process immediately after being cut, the crack surface perpendicular to the shaft is crimped and disappears. In addition, the second condition is solved as a pipe steel piece.
Due to the above effects, it is possible to use a hollow continuous cast steel slab for pipe making, omitting the rolling and rolling step, and reducing equipment costs and operating costs.

It is a schematic side view of the continuous casting apparatus which implements this invention. It is a figure explaining the method to cut | disconnect the vacuum hollow slab in this invention. The figure explaining the change of the internal crack by the pressing of a slab shows the cross section of a slab. The figure explaining the change of an internal crack by compression processing of a steel piece shows a longitudinal section of the steel piece. It is a reference figure of the continuous casting method in the prior example 1, and shows the position of electromagnetic stirring. An example of voids is shown in a cross-sectional photograph of a hollow slab from which the molten core has been separated.

  The manufacturing method of the hollow steel piece for pipes of this invention is demonstrated according to FIG. In FIG. 1, the entire structure is a kind of curved continuous casting, and the drawing locus of the slab is composed of a curved portion of 3/4 round and a subsequent horizontal straightened portion. The molten steel 3 supplied from the tundish 1 to the mold 2 is cooled in the mold 2 and continuously drawn at an appropriate speed by a pinch roll 5 while forming a solidified shell to form a slab 4. The slab 4 passes through the secondary cooling zone 6 and exceeds the ½ round point, and further reaches the molten steel height (about 1.4 m) Q point corresponding to atmospheric pressure from the casting surface to separate the molten core, A cavity 7 ′ is formed to form a vacuum hollow slab 7. The slab 7 is horizontally stretched by the straightening roll 8 at the uppermost 3/4 circumference.

Next, the slab 7 is cut into a predetermined length by a cutting machine 9. At the time of cutting, as shown in FIG. 2, a pair of wedge-shaped cutting teeth 21 are symmetrically press-fitted with the slab 7 sandwiched therebetween to bite them.
The surface 22 on the upstream side of the wedge-shaped tooth is inclined with respect to the slab axis, and the surface 23 on the downstream side is substantially perpendicular. As the press-fitting progresses, the solidified shell is inclined down on the upstream side, and the inner surfaces of the shells The vacuum cavity 7 'is sealed by pressure contact. After that, it is divided. On the downstream side, it is cut almost vertically and the end part does not roll down. Ventilation occurs when the tooth tip reaches the cavity without reaching the pressure contact. Thereafter, an opening 24 is formed and divided. The area around the opening of the cut surface is sagging. However, a hollow steel piece 10 with one end opening is obtained. Three processes of encapsulation, cutting and opening on the steel piece side can be simultaneously and easily performed by one machine.
The hollow steel piece 10 with the one end opening is subjected to drawing and rolling with a plug mill without the usual next step of turning.

Next, a solution for the material problem will be described.
First, the present inventor has grasped that a small void is generated on the inner surface of the solidified shell in the trial production of a hollow cast slab of high carbon steel. As described above, the inner surface property is an important problem for the steel piece for pipes, and the void is an inevitable problem. In the first example, there was no problem in the case of low carbon, and in the case of high carbon, the irregularities on the inner surface were severe, and as a countermeasure, as shown in FIG. is doing. However, the void problem is not conscious.

  FIG. 6 is a photograph showing the inner surface properties of a high carbon steel strip produced under the same conditions. Although it is flattened similarly by electromagnetic stirring, it can be seen that voids are generated due to the retention after stirring.

  In the present invention, as shown in FIG. 1, the electromagnetic stirrer 13 for rotating magnetic field is made downstream of the 190 ° portion, that is, including the liquid surface on the cavity side. As a result, not only the inner surface of the solidified shell is flattened in the high carbon steel, but also the CO reaction that induces voids by scraping out the high concentration liquid phase at the solid-liquid boundary. An agitation site is an important requirement for preventing voids.

Second, the cross-sectional shape of the resulting steel piece 10 is preferably a circle, but the cross-sectional shape of the mold is not only a circle but also an ellipse, square, rectangle, or polygon. A long octagon is desirable. The reason is described below.
In FIG. 1, the straightened hollow slab 7 is crushed by the crimping rolling mill 11 in the major axis direction of the cross section under the condition that the cavities do not disappear, and the crack is crimped and extinguished. In normal rolling, mild cracks are crimped and rendered harmless, but cracks in the vicinity of the inner surface tend to expand to scratches on the inner surface in lathe rolling or rough rolling, and the above crimping process is necessary.

  In order to make a circle under a single pressure, the aspect ratio of the mold cross section should be larger than 1. That is, an ellipse, a rectangle, and a long polygon are preferable. The long axis is crushed and guided to a circle. A circle requires two horizontal and vertical reductions. The reduction ratio is related to the crimping. In order to ensure the pressure bonding, the rolling reduction is preferably larger, for example, 20% or more. Conversely, light pressure promotes cracking. For this reason, the aspect ratio is preferably 1.2 or more. The reason for including 1.0 in the ratio is that the same effect can be obtained by performing two horizontal and vertical rollings in a circle.

  FIG. 3 illustrates a state in which a crack parallel to the slab shaft is crimped by pressing (dotted line indicates after crimping). The slab cross-sectional shape is a long octagon, and the hole shape is a circle. When an appropriate shape relationship is set, the rolling can be deformed under compressive stress in both the rolling direction and the width direction. That is, it is convenient for crimping / disappearance in the entire cross section. In the explanatory diagram, the reduction direction is vertical, but as shown in FIG.

If the aspect ratio is further increased, the casting efficiency is proportionally increased. However, in addition to the press rolling mill, a forming rolling mill that induces a circle is required. Appropriate selection should be made. The reason why the upper limit of the aspect ratio is 3 is a safe condition when priority is given to enhancing the casting efficiency over the number of rolling mills. If it is further increased, the burden of forming and rolling becomes excessive.

Thirdly, as shown in FIG. 4, the steel piece 10 obtained by cutting is immediately compressed in the axial direction by a press 12. The reason is that when the crack surface is parallel to the shaft, it is easily crimped by rolling, but may be incomplete when it is vertical. Axial compression is a solution to this problem. The necessary rolling force is equivalent to that of the cutting device, and the necessary rolling reduction is about 1% in principle, but several percent, for example, 3% is desirable due to variations in various factors.
During compression, a conical turning projection 42 is attached to the center of the press head 41 to correct the disordered opening shape of the opening-side cut surface. This facilitates the start of stretch rolling.
The cracks inside the slab are pressed by the above-described reduction and compression, so that they are harmless for slewing and stretching.

The change in the shape of the slab cut when pressing wedge-shaped teeth was verified using the plasticine model.
The shape of the test piece was an outer diameter of 50 mm, an inner diameter of 25 mm, and a length of 100 mm. The tooth opening angle was 45 ° on one side and 0 ° on the other side, and press-fitted with a vice. The inner surface on the inclined side was pressed by 15 mm press-fitting on one side, but the vertical side was not opened. When the press-fitting was further advanced, the tooth tip entered the cavity and opened and cut. The shape of the opening was considerably sag (in a state where the inner helicopter was pulled toward the center), and the shape became inconvenient for pushing the swirl plug. It became clear that correction of the inlet shape was necessary. This is the reason for inserting the conical protrusion in the compression process.

Hereinafter, an example of specific conditions in the case of producing a hollow steel piece for a medium diameter pipe by the method of the present invention will be described.
Mold section dimension: Octagon 164mm x 256mm, C = 57mm
Slab cross-sectional dimension: Octagon 160mm × 250mm, C = 56mm
Curvature radius R: 3.9m
Effective machine length L: π × R + 1.4 = 13.5
Coagulation constant k: 23 mm / √min
Solidified shell thickness d: 40 mm
Solidified shell thickness ratio α: 40/160/2 = 0.5
Single weight w: 200kg / m
Solidification time t: 3 min
Drawing speed V: 4.5 m / min
Casting efficiency p: 54t / h
Electromagnetic stirring part: 190 ° ~ 200 °
Pressure reduction ratio h: 0.30
Wedge tooth opening angle: 45 ° upstream, 0-20 downstream (curved surface)
Compression reduction ratio l: 0.03
Steel piece cross-sectional dimensions: Circle, outer diameter 175 mmφ, inner diameter 80 mmφ

  FIG. 6 is a photograph showing the properties of the cross section of the slab from which the molten core was separated in continuous casting of a 160 mm square slab of high carbon steel. The state is 0.5 minutes after the completion of electromagnetic stirring, and voids are observed on the inner surface of the solidified shell. The effect of separating while stirring can be read.

  The method for producing a hollow steel piece by continuous casting according to the present invention can be implemented by partially modifying an existing continuous casting machine of the same type, and can expand the types of products that can be produced at an existing factory.

1: Tundish 2: Mold 3: Molten steel 4: Cast slab 5: Pinch roll 6: Secondary cooling zone 7: Hollow cast slab 7 ': Vacuum cavity Q: Cavity side liquid level 8: Straight roll 9: Cutting machine 10: One end open hollow steel piece 11: crimping mill 12: press 13: electromagnetic stirring device 21: wedge-shaped tooth 22: inclined surface 23: vertical surface 24: opening 41: press head 42: conical protrusion

Claims (4)

In the method of cutting a hollow slab formed by the following continuous casting method into a steel slab, a pair of wedge-shaped teeth arranged with the slab sandwiched between them are cut into the slab symmetrically and bitten. In addition, the wedge shape of the wedge-shaped teeth is inclined with respect to the slab surface on the upstream side in the drawing direction and perpendicular to the slab surface, so that the tip shape of the slab is deformed into a tapered shape and the cavity is pressure-welded. A method for producing a hollow steel piece, characterized in that it is sealed and the cut face is cut open in a vertical shape, thereby forming a hollow steel piece with one side opening.
It is a kind of curved continuous casting, and the slab drawing locus is composed of a 3/4 round curved part and a horizontal straight part following the curved part, and the three factors of mold cross-sectional dimension, curved radius and drawing speed By appropriately combining the slabs, the slabs are drawn over 1/2 lap while leaving the molten core inside the slabs, and further drawn over the molten steel height (about 1.4 m) equivalent to atmospheric pressure from the casting surface. Then, the molten core is separated from the solidified shell to form a hollow slab having a vacuum cavity, the slab is stretched horizontally at the uppermost 3/4 round point, and then the cavity of the slab is A continuous casting method in which a steel piece enclosing a cavity is obtained by partially closing and cutting.   The melt core including the meniscus is stirred by an electromagnetic stirrer provided immediately below the site where the vacuum cavity is formed, the solidified shell front surface is flattened, and the high-concentration liquid phase at the solid-liquid interface is scraped to perform the CO reaction. The method for producing a hollow steel piece according to claim 1, wherein the formation of voids is suppressed.   The mold cross-sectional shape is a circle, oval, square, or polygon having a cross-sectional aspect ratio of 1 or more and 3 or less, and the slab is stretched, and then the long axis direction is left in the slab by at least one pair of rolls. The method for producing a hollow steel piece according to claim 1 or 2, wherein the crack in the solidified shell is pressure-bonded by pressure reduction.   The steel slab obtained by cutting the slab is immediately compressed in the axial direction of the steel slab by pressing to press the crack inside the solidified shell. A method for producing a hollow steel piece.