JP3533831B2 - Method for producing round billet for producing Cr-containing seamless steel pipe with good workability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a continuously cast round billet of Cr-containing steel, which is used for producing a seamless steel pipe, and more particularly to a center porosity (hereinafter simply referred to as porosity) and a solidification structure. The present invention relates to a method for producing a round billet which has good workability and is free from defects during punching of a Mannesmann by eliminating the defects to improve the internal quality.

[0002]

2. Description of the Related Art In the production of a seamless steel pipe, a round billet produced by slab-rolling a continuously cast slab or bloom is used, or is directly cast by continuous casting (reheating is not performed in the process of forming a billet. ) Mannestmann drilling, press drilling or extrusion drilling is performed using the manufactured round billet to form a hollow seamless steel pipe (base pipe), and thereafter,
The product is drawn by a rolling machine such as an elongator, a plug mill, a mandrel mill, etc., and finally made into a product with a constant size using a sizer or a stretch reducer.

[0003] In this case, a steel type such as carbon steel which is relatively easy to perform continuous casting and has good hot workability of its slab, can be perforated by using a round billet as cast. ,
A seamless steel pipe having good inner surface properties can be obtained.

On the other hand, in the as-cast condition, porosity and segregation are likely to occur in the shaft core, and the hot workability of Cr is poor.
In the manufacture of seamless steel pipes such as containing steel, generally,
A round billet from which porosity and segregation of the shaft core is removed by giving large processing by slabbing after continuous casting is used.

The main cause of the poor hot workability of round billets made of steel having a large amount of Cr is that the slab during continuous casting is accompanied by an increase in the content of Cr alloyed to improve corrosion resistance. This is because segregation and porosity are likely to occur in the shaft core part of. The cause of porosity, which has a particularly large effect on hot workability, is because the viscosity of molten steel is high, so that unlike the case of carbon steel, it is difficult to supply molten steel to the voids that occur during the final solidification stage of continuous casting. It is for the sake of benefit.

FIG. 7 is a diagram showing the relationship between the content of Cr in molten steel and the viscosity of molten steel. The viscosity increases as the amount of Cr in molten steel increases, and 13% (% by weight, the same applies below). The peak value is shown before and after. In addition, the occurrence of porosity is C
It is known that when the amount of r becomes 0.5% or more, it becomes a problem.

When manufacturing a seamless steel pipe using an as-cast round billet containing such defects inside, a compressing force, a shearing force, and a pulling force are applied during the Mannesmann drilling in the first stage of the mass production process. Since it is subjected to complicated working that complicates, porosity and segregation of the shaft core are the starting points, causing flaws on the inner surface of the pipe. As a result, the production cost rises due to a decrease in yield due to the generation of defective products and a decrease in efficiency due to the maintenance of defects.

As a technique for improving the internal quality of a round billet as it is continuously cast, a method in which an electromagnetic stirrer is installed outside the mold of a continuous casting machine and the molten steel in the mold is stirred is widely practiced. This is a technique of stirring molten steel in a mold with electromagnetic force to generate solidification nuclei and filling the central portion of the slab in the final solidified portion with equiaxed crystals to reduce porosity. However, this technology alone cannot completely prevent the occurrence of porosity.

Japanese Patent Publication No. 59-16862 discloses a technique for improving the internal quality by applying a slight reduction to a billet during continuous casting. This is a technique in which the billet in the final stage of solidification is reduced by a roll by the amount of solidification contraction to suppress the flow of concentrated molten steel and prevent center segregation.

"Materials and Process Magazine, Vol. 7, No. 1, 1"
Pp. 95, published in 1994 ”, shows an example of applying this technology to SUS410 steel, which performs two-stage reduction with the inside unsolidified, and the billet axis after reduction. The density of the part is increased to 7.7 g / cm ³ . However, the density without porosity is 7.8 g / c
It is 99% or less as compared with m ³ , and it cannot be sufficiently crimped. (Slight porosity can be seen in the shaft core even in the microphotographs shown.)

The problems when this light reduction process is adopted are the deterioration of the billet shape caused by the reduction by the two rolls, and the occurrence of cracks near the solidification interface when the reduction amount is increased.

When the round billet is rolled by a pair of flat rolls, the sectional shape of the pressure lower portion is naturally flattened, and the flattened cross section leads to uneven thickness of the product. If the amount of reduction is increased in order to enhance the pressure bonding effect of porosity, the shape further moves away from the perfect circle, and it becomes difficult to roll and transport the round billet. Also, biting into the mill during drilling becomes unstable,
The rate of defects also increases.

In order to solve these problems, for example, Japanese Unexamined Patent Publication (Kokai) No. 7-108358 discloses a technique in which a mold having an elliptical cross section is used to manufacture a billet having an elliptical cross section, and the billet is pressed down in the major axis direction of the ellipse. Is proposed. According to this method, a billet having a cross section close to a perfect circle can be obtained.

However, if an oval mold is used,
Compared with the case of a perfect circle, the flow of molten metal during casting tends to be non-uniform, and fluctuations in the molten metal surface and entrainment of powder caused by this cause new defects.

A process aimed at improving the internal quality of a slab by subjecting a continuously cast slab to a large reduction is described in "Materials and Process Magazine, Vol. 7, No. 1, pp. 179, 1994", and It is disclosed in Kaihei 63-183765. This process is a method of applying a large reduction by a pair of molds during continuous casting (in-line reduction method), and since reheating is not necessary because the reduction is applied during continuous casting, the effect of eliminating the reduction of porosity is great, Equipment costs are high.

Incidentally, "Iron and Steel Magazine, Volume 60, No. 7, 8"
Page 75, 1974 "also shows a similar technique as an in-line reduction method, but this method is intended for blooms and billets having a rectangular cross section, and the Cr-containing steel that the present invention aims to achieve. There are many problems in applying to round billets.

Under such circumstances, in the production of a seamless steel pipe of Cr-containing steel, which is likely to cause flaws when Mannesmann piercing is performed using a round billet that is continuously cast (without reheating). It has been essential to use reheated and rolled round billets to improve internal quality. Then, in this direction, a method for avoiding the problem of the flaw generation during pipe manufacturing has been continuously sought.

For example, Japanese Unexamined Patent Publication (Kokai) No. 7-136702 discloses a method for producing a round billet for a seamless steel pipe of high Cr steel, which is produced by continuously casting a square-shaped bloom having a relatively large cross section, and then heating it. A method of applying a large reduction by slabbing is shown.

[0019]

[Problems to be Solved by the Invention] As described above, C
The method for improving the internal quality of the continuously cast round billet used for the Mannesmann drilling of r-containing steel is a method of performing a large reduction so as to give a plastic deformation during continuous casting, or a conventional method of heating-agglomeration. The former is typified by the in-line reduction method, but the facility cost is remarkably excessive, and the latter requires reheating, so the operating cost is high.

Under these circumstances, there is a demand for a simple and economical method for producing a continuously cast round billet with a small amount of internal scratches even when a seamless steel pipe is produced from a Cr-containing steel by the Mannesmann piercing method. Was there.

[0021]

In order to solve the above problems, the present inventors have various methods for applying a simple and effective reduction to a high temperature round billet immediately after continuous casting of Cr-containing steel. The present invention has been studied and completed. With the completion of the present invention, it is possible to obtain a round billet with good workability in which the porosity and solidification structure in the continuous cast slab are eliminated to improve the internal quality thereof and the occurrence of internal flaws during the Mannesmann drilling is small. It was

The first aspect of the present invention is a method for producing a round billet for producing a Cr-containing seamless steel pipe having good workability, which comprises the following steps.

Step of producing a round slab of Cr-containing steel by continuous casting Step of cutting the high temperature round slab after completion of solidification into a high temperature round billet by cutting at regular intervals in the length direction A step of applying a first reduction to the billet in the diameter direction to obtain a high temperature flattened round billet A step of applying a second reduction in the direction of maximum diameter to the flattened round billet The step of forming a high-temperature round billet having a reduced diameter. In the second invention, in the first invention, the first reduction of the step is a flat roll, a roll having a flat oval hole type, or a roll having an oval hole type. It is a method for producing a round billet for producing a Cr-containing seamless steel pipe having good workability, which is performed by using at least one of the above, and the second reduction of the step is performed by using a roll having a round hole type.

A third aspect of the present invention is the first aspect of the present invention, in which the roll having at least one of a flat surface, a flat oval hole type, or an oval hole type and a round hole type is used for the first step of the process. The reduction of the flat surface,
A method for producing a round billet for producing a Cr-containing seamless steel pipe having good workability, which is performed by at least one of a flat oval hole type or an oval hole type, and the second reduction of the step is performed by a round hole type.

The present invention is directed to a steel containing Cr. Steel containing no Cr has a low viscosity of molten steel and is unlikely to generate porosity. Further, since the segregation is small, the effect of using the method of the present invention is small. In addition, the above-mentioned Cr
The amount at which the adverse effect of is started to appear is 0.5%, and becomes the largest in the vicinity of 13% as shown in FIG.

Further, the effect of the present invention is most noticeable when the outer diameter of the round billet is 170 to 340 mmφ. If the outer diameter of the round billet is less than 170 mmφ,
Less porosity and less segregation. On the other hand, when the diameter exceeds 340 mmφ, the molten steel is likely to be replenished and the porosity is reduced.

The round billet manufacturing method of the present invention is made into a round billet without the conventional reheating-processing step, and a seamless steel pipe is manufactured by the Mannesmann drilling method using the round billet. It can be applied to Cr-containing steel. Further, the present invention can be applied to a Cr-containing steel that has been subjected to Mannesmann perforation by forming a round billet through a reheating-rolling process.

In the present invention, the round billet is subjected to the first and second rounds of double reduction by a relatively simple facility. The step of applying the reduction is after cutting the round cast into a round billet. In the first reduction and the second reduction, the directions of reduction are at right angles to each other, and the round billet is flattened in the first reduction to form a true circle reduced in diameter in the second reduction.

The rolls used for the first reduction are 1) flat rolls, 2) flat oval hole type rolls, or 3) oval hole type rolls. The billet is pressed by these rolls to flatten the billet having a perfect circular cross section. FIG. 2, FIG. 3, and FIG. 4 show schematic views of the rolls and the billet cross-sections pressed by the rolls.

In the second reduction, a round-hole type roll is used to apply a reduction in the direction of the maximum diameter to the round billet flattened in the first reduction, and the diameter is reduced again to obtain a true circular cross section. Round billet. FIG. 5 shows a schematic view of a cross section of the billet pressed by a round hole type roll.

When a roll having a flat oval or oval hole shape is used for the first rolling, the rolling is directed toward the center of the round billet, so that a compressive stress field is easily formed in the shaft core of the round billet. And the internal quality is improved. This effect is the second by the round hole type roll.
Expanded by the second reduction.

When a flat roll is used for the first reduction, the round hole type roll is used for the second reduction. Therefore, fine defects generated during the reduction by the flat roll are repaired. A round billet with excellent internal quality can be obtained.

The standard reduction ratio of the first reduction is 5 to 2
5%, that of the second reduction is in the range of 5 to 20%, and in the process of converting the round billet into a reduced diameter round billet,
A total reduction of 10 to 40% is applied by two reductions. The total area reduction rate (Ar) is calculated by the following equation (1).

Area reduction rate (Ar) = {(round billet diameter before first reduction) ²⁻ (round billet diameter after second reduction) ² } / (before first reduction) Round billet diameter) ²・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (1)

The surface reduction rate in the first and second reductions is calculated from the moving speed of the round billet before and after the reduction, but it is not always necessary to obtain a precise value.

In the present invention, the area reduction ratio (A
The reduction required area reduction rate (Ar ^* ) is defined as r). By performing a reduction of more than the required area reduction rate (Ar ^* ),
It is possible to virtually prevent the occurrence of flaws on the inner surface of the seamless steel pipe after the Mannesmann drilling.

Further, the area reduction rate which can reduce the occurrence rate of flaws to 10% or less at the time of punching Mannesmann is defined as the effective area reduction rate. If it is 10% or less, it can be made into a product by taking care of slight flaws. Needless to say, even if the reduction of the area reduction rate (Ar) is less than this, the corresponding effect can be obtained.

The required area reduction rate (Ar ^* ) and the effective area reduction rate (Are) are functions of the Cr content and the diameter of the round billet before the first reduction, and the following (2), (3)
It can be represented by a formula.

Required area reduction ratio (Ar ^* ) = f ₁ (Cr%, diameter of round billet before first reduction)・ ・ ・ (2) Effective area reduction (Are) = f ₂ (Cr%, diameter of round billet before first reduction) (3) Therefore, in the case of the area reduction rate (Ar) ≥ the required area reduction rate (Ar ^* ), there is substantially no flaw, and the area reduction rate (Ar) ≥ the effective area reduction rate ( In the case of Are), the occurrence of defects is 10% or less.

The required reduction rate (Ar ^* ) and effective area reduction rate (Are) are, as described above, a function of the Cr content in the steel and the diameter of the round billet before the first reduction. However, the Cr content is 0.5 to 15% and the round billet diameter is 170 to
In the range of 340 mmφ, it does not change significantly,
The required reduction rate (Ar ^* ) is about 20 to 30%, and the effective area reduction rate (Are) is about 10%.

In the rolling method of the present invention, as apparent from the shape of the roll, the strain applied to the round billet during rolling is more uniform than in the conventional light reduction method. Also. It also has an advantage that internal defects and deformation that cause cracking are less likely to occur during Mannesmann drilling inside the round billet.

[0042]

DETAILED DESCRIPTION OF THE INVENTION An embodiment of the present invention will be described with reference to FIG. The molten steel poured into the mold 1 becomes a round cast piece 2. When carrying out the present invention, as a general rule, molten steel is subjected to electromagnetic stirring in the mold 1 (not essential).
I do. This electromagnetic stirring process is performed in the same manner as a conventional device using a conventional device. As shown, the round slab 2 is bent from the vertical direction to the horizontal direction while it is solidifying. The drawing speed of the round cast slab 2 is controlled to a constant speed by the pinch roll 3.

In the case of the conventional process which does not perform reheating, after continuous casting, it is cut by the cutter 4 as it is, and the round billet 5 is cooled as it is, or by the pinch roll 3 as described above, the effect is not so great. After applying an unexpected reduction, it was cut into round billets 5 (in these cases, a considerable amount of flaws were generated at the time of punching the Mannesmann), or it was reduced by a continuous forging machine requiring a large capital investment. They were later cut into round billets, but for the reasons mentioned above, none of them are valid solutions.

In the present invention, as shown in the drawing, the round billet 2 is cut by the cutter 4 into the round billet 5, and the reduction for making the round billet 5 with a good internal quality in the high temperature state is carried out universally. First rolling stand 6 arranged in a form
It is performed by the continuous rolling mill 6 including the first and second rolling stands 62.

Normally, with respect to the speed of continuous casting,
Since the rolling speed is sufficiently high, a plurality of round cast pieces 2 (moving in parallel with each other) are simultaneously manufactured by a plurality of molds 1 and cut into a round billet 5. It is possible to implement the present invention by installing the number of continuous rolling mills 6 or less.

Further, it goes without saying that the round slab 2 is not necessarily bent and needs to be moved in the horizontal direction. It may be solidified in the vertical direction, cut, and further reduced.

In the embodiment of the invention described above, the reduction is applied by the two rolling stands.
(A) Flat surface, flat oval hole type, or
It is also possible to carry out at least one of the oval holes and (b) the round holes in one rolling stand having a pair of rolls provided in the cylinder length direction. Of course, in this case, it is necessary to rotate the round billet 1/4 in the circumferential direction (tilt) and reverse the conveying direction (reverse rolling) after the first reduction.

Each of the above-described two embodiments has advantages and disadvantages. Production efficiency (production amount), installation space,
Select an advantageous form in consideration of costs. For example, if there is only one rolling stand, the equipment cost will be low, but the production efficiency will be low.

[0049]

EXAMPLE FIG. 6 shows the relationship between the area reduction rate (Ar) in the case of producing a 170 mmφ billet for mannesmann drilling of steel containing 13% Cr and the occurrence rate of flaws on the inner surface of the pipe after mannesmann drilling. Show.

The effective area reduction rate at which the defect occurrence rate is 10% or less is 10%. It can be seen that the required area reduction ratio (Ar ^* ) at which the maintenance of the flaw is substantially unnecessary after the Mannesmann punching (a slight flaw remains to an acceptable level) is 30%.
When the area reduction rate (Ar) is 5%, the defect occurrence rate is 50%, and when the reduction is not performed (Comparative Example), it is 100%.
All the values were about the same when the Cr content was 0.5 to 15% and the billet diameter was 170 to 340 mmφ.

[0051]

As a result of the completion of the present invention, it is possible to manufacture a seamless steel pipe by a continuous casting-pipe manufacturing process (direct casting of billet) using a round billet of as-continuously cast Cr-containing steel. Significantly, it is possible to manufacture a high-value-added high-alloy steel pipe with low internal cost and high efficiency by adding a low-cost device without major modification of the conventional device.

[Brief description of drawings]

FIG. 1 is a schematic view of a continuous casting device (including a continuous rolling mill) for carrying out the present invention.

FIG. 2 shows a state in which the flat roll is first pressed.

FIG. 3 shows a state in which the flat oval hole type roll is first pressed.

FIG. 4 shows a state in which the oval roll is used for the first rolling.

FIG. 5 shows a state in which the round-hole type roll is in a second rolling operation.

FIG. 6 is a graph showing the relationship between the area reduction rate and the rate of occurrence of flaws on the inner surface of a steel pipe in Examples and Comparative Examples of the present invention.

FIG. 7 is a diagram showing the relationship between the Cr content and the viscosity of molten steel.

[Explanation of symbols]

1. Mold for continuous casting machine 2 ... Round slab 3 ... Pinch roll 4 ... Cutter 5 ... Round billet 51 .. Flat part of round billet 52 ··· Reduced diameter of round billet 6 ... Continuous rolling mill 61 ... No. 1 rolling stand 62 ... Second rolling stand 71 ... Flat roll 72 ... Flat oval hole type roll 73 ... Oval hole type roll 74 ... Round hole type roll

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junichi Shoda 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Rikyo Nakagome 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Inventor Hidenori Anai 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (56) Reference JP-A-8-117814 (JP, A) JP-A-58-188502 ( JP, A) JP 58-125301 (JP, A) JP 7-136702 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B21B 1/00-1/46 B21B 27/02 B22D 11/00-11/22 B22J 1/02

Claims (3)

(57) [Claims] 1. A method for producing a round billet for producing a Cr-containing seamless steel pipe having good workability, which comprises the following steps. Step of producing a round slab of Cr-containing steel by continuous casting The high temperature round slab after completion of solidification is cut at a constant interval in the length direction to form a high temperature round billet. A step of performing a first reduction by applying a reduction in a diameter direction to obtain a high temperature flattened round billet A second reduction is performed by applying a reduction in the direction of the maximum diameter to the flattened round billet, and a diameter reduction is performed. The process of making a high temperature round billet. 2. The first reduction of the step is performed by using at least one of a flat roll, a roll having a flat oval hole type, or a roll having an oval hole type, and a second reduction of the step. The method for producing a round billet for producing a Cr-containing seamless steel pipe having good workability according to claim 1, wherein the step is performed by using a roll having a round hole shape. 3. A roll provided with at least one of a flat surface, a flat oval hole type, or an oval hole type, and a round hole type is used for flattening the first reduction of the step of claim 1. 2. A seamless steel pipe having good workability for Cr-containing steel according to claim 1, wherein at least one of a flat surface, a flat oval hole type, and an oval hole type is used, and the second reduction of the step is performed by a round hole type. Manufacturing method of round billet for manufacturing.