JP2004082174A - Method for manufacturing seamless steel tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for stably manufacturing the seamless steel tube made of high-alloy steel without performing heavy investment of equipment in the manufacturing line of the seamless steel tube constituting of an inclined piercing mill and a mandrel mill. <P>SOLUTION: In the manufacturing line of the seamless steel tube constituting of the inclined piercing mill and the mandrel mill, a billet 1 on which machining for providing an axisymmetric recessed part 4 is preliminarily performed at the rear end part 3 of the billet before piercing is used. The recessed part 4 is machined so that the thickness in the rear end part 3 of the billet is ≥ 10 mm. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a seamless steel pipe, mainly a high alloy steel seamless steel pipe.
[0002]
[Prior art]
In the production of seamless steel pipes, the first processing process includes a drilling step for drilling holes in a billet heated to a high temperature. Drilling often uses the rotary forging effect, and one pass is performed by an inclined drilling machine using a barrel-shaped roll (Mannesmann drilling machine) or, more recently, a cross-piercing machine using a cone-type roll that is an extension of the conventional inclined drilling method. Is performed. The hollow shell after perforation is often subjected to elongation rolling by a plug mill for large diameter sizes and a mandrel mill for medium and small diameter sizes. In the case of a mandrel mill, a mandrel bar is inserted from the rear end side (BOT side) of the hollow shell after perforation, and elongation rolling using a caliber is performed by a plurality of tandem rolling mills. The types of mandrel mills are roughly classified into full float types and retained types. After completion of the mandrel mill rolling, all types pass through a step of pulling out a mandrel bar from a hollow shell.
[0003]
On the other hand, a demand for seamless steel pipes in recent years has led to a demand for steel pipes having higher strength and higher corrosion resistance as the use environment becomes severer. For example, steel pipes for boilers are required to have higher high-temperature strength as the operating temperature becomes higher. In order to meet this demand, it is inevitable to use high-alloy steel containing a large amount of alloying elements.
[0004]
However, it has been found that such a high alloy steel has a different metal flow during rolling than ordinary steel. That is, the high alloy steel has a smaller "lateral flow" or "widening" than the ordinary steel. Therefore, in a mandrel mill, ordinary steel is of a size that can be rolled without any problem, but in a high alloy steel, the gap between the mandrel bar and the mandrel bar is reduced, and the mandrel bar becomes difficult to come off after rolling. As a result, inner surface flaws are apt to occur, and in the worst case, the mandrel bar does not come off, leading to a stop of the production line (mandrel bar strip failure).
[0005]
In addition to installing a new mandrel bar puller with a large power, there are other ways to solve this problem:
[Countermeasure 1] A method of increasing the heating furnace extraction temperature in order to reduce the deformation resistance of the hollow shell when the mandrel bar is pulled out (that is, a method of reducing the tightening force on the mandrel bar).
[Countermeasure 2] A method of changing the processing distribution of drilling / drawing and increasing the amount of drilling to finish the hollow shell (for example, a method of reducing the amount of drilling with a mandrel mill by drilling thinner than before)
[Countermeasure 3] A method of shortening the time from the end of mandrel mill rolling to pulling out the mandrel bar and reducing the temperature drop of the hollow shell (this is also one method of reducing the tightening force on the mandrel bar)
And other measures.
[0006]
[Problems to be solved by the invention]
The method of heating the billet to a high temperature and raising the temperature of the hollow shell from the perforation to the time of pulling out the mandrel bar ([Measures 1]) not only increases the amount of scale in the heating furnace, but also increases the amount of scale. Not applicable to some high alloy steels. In general, drilling is a large process, so the heat generated during the process is large, and some high alloy steels undergo partial phase transformation during drilling. For example, in a normal 13Cr-based steel, processing from drilling to elongation is performed with a target of a single gamma phase. However, when the extraction temperature is high or the drilling schedule is severe, during drilling, the raw material temperature partially exceeds the γ → δ transformation temperature, and δ ferrite is partially generated on the inner surface side. When a hole is drilled in the two-phase region of γ and δ, the hot workability of the two is different, so that a lap-shaped inner surface flaw often occurs in the hollow shell after drilling. In addition, some high alloy steels show a property in which the ductility value sharply decreases at a certain temperature or higher, and it is not always advisable to increase the extraction temperature in order to pull out the mandrel bar.
[0007]
On the other hand, the method of changing the processing distribution of drilling and drawing from that of ordinary carbon steel, increasing the processing amount of drilling, and reducing the processing amount of drawing (above [Measure 2]) is a method of mandrel mill rolling. A gap can be secured between the bar and the hollow shell, which is considered to be effective in reducing the pulling force of the mandrel bar. However, if the amount of processing at the time of drilling is increased, that is, if a drilling schedule that makes the wall thinner is adopted, the hollow shell after drilling is inevitable due to inevitable asymmetric factors such as the temperature distribution of the billet and poor setting of the drilling machine itself. Becomes uneven, and the shape of the rear end of the hollow shell deteriorates significantly. For this reason, there is a risk that the mandrel bar cannot be charged into the hollow shell before the mandrel mill rolling (the rear end of the hollow shell partially extends, and the partially elongated portion is dripped with time. This will hinder the mandrel bar loading).
[0008]
The method of pulling out the mandrel bar immediately after the end of the mandrel mill rolling ([Measures 3]) requires high-speed transport equipment on the rear side of the mandrel mill and requires a large capital investment.
[0009]
The present invention has been made in order to solve the above-described problems.In a seamless steel pipe production line including an inclined piercing mill and a mandrel mill, a high alloy steel It is an object of the present invention to provide a method for stably producing a seamless steel pipe.
[0010]
[Means for Solving the Problems]
The method for manufacturing a seamless steel pipe according to the present invention is a method for manufacturing a seamless steel pipe including an inclined piercing mill and a mandrel mill. It is characterized by using a billet that has been applied.
[0011]
As described above, in order to avoid trouble when pulling out the mandrel bar without making a large capital investment, it is effective to make a thin hole and improve the shape of the rear end of the hollow shell. . For this purpose, the simplest method is to remove the rear end portion of the billet shaft center removed by the drilling plug at the time of drilling by machining in advance. By performing such a pre-process on the billet, the rear end shape of the hollow shell after perforation is improved. That is, the influence of the inevitable asymmetry factor at the time of perforation can be reduced.
[0012]
The method of machining the billet rear end is not particularly limited as long as it is machined so as to be axially symmetric with respect to the billet axis. When working with a cold billet, cutting is desirable, and when working with a hot billet, forging is desirable in terms of yield. Since the shape of the billet rear end before drilling depends on the size of the billet used and the shape of the drilling plug, it cannot be limited, but the temperature drop of the billet from the heating furnace to the drilling machine, and the time required for drawing the mandrel mill In order to reduce the temperature drop of the hollow shell, it is important to have a wall thickness of at least 10 mm or more. If the temperature drop is large, the deformation resistance of the material increases, which causes the material to break during processing and causes roll flaws.
[0013]
Although the cost is slightly increased due to the increased number of steps in the manufacturing process, the high alloy steel pipe is insignificant compared to the high cost and the interruption of the manufacturing line due to the defective mandrel bar strip.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
FIGS. 1, 2 and 3 show the sectional shapes of the rear end of the billet before perforation used in the present invention. In these figures, reference numeral 1 denotes a billet, and in each of the examples, a round billet of SUS16 manufactured by a continuous casting method is used. 2 is a billet front end, 3 is a billet rear end.
These billets 1 were machined by a large lathe in the state of cold pieces so that the minimum thickness t became 10 mm or more. Reference numeral 4 denotes a machined concave portion, and the concave portion 4 has a shape symmetric with respect to the billet axis. The depth of the recess 4 is about 300 mm. Further, the cross-sectional shape of the concave portion 4 is arbitrary as illustrated, and is not particularly limited.
[0015]
As described above, the billet 1 in which the concave portion 4 has been formed in the rear end portion 3 of the billet in advance is heated to a predetermined temperature in a heating furnace and extracted, and then the inclined piercing and rolling is set so that the wall thickness after piercing is 12 mm. Perforated with a machine. In each case shown in the figures, the shape of the rear end of the hollow shell after perforation was good, and it was confirmed that the charging of the mandrel bar in the next step and the elongation rolling in the mandrel mill had no problem in the production. Further, after the rolling by the stretch reducer, the product was inspected for dimensions by a refining line, and as a result, a product having no inner surface flaw was produced.
[0016]
【The invention's effect】
As described above, according to the present invention, a billet that has been subjected to processing having an axially symmetric concave portion is used at the rear end of the billet before drilling, so that the wall thickness after drilling can be reduced. Even in alloy steel pipes, a seamless steel pipe having no inner surface flaws can be stably manufactured without requiring special equipment investment.
[Brief description of the drawings]
FIG. 1 is a diagram showing a cross-sectional shape (part 1) of a rear end portion of a billet before perforation used in the present invention.
FIG. 2 is a view showing a sectional shape (part 2) of a rear end portion of a billet before perforation used in the present invention.
FIG. 3 is a view showing a sectional shape (part 3) of a rear end portion of a billet before perforation used in the present invention.
[Explanation of symbols]
1 billet 2 billet tip 3 billet rear end 4 recess

Claims (2)

In a seamless steel pipe production line comprising an inclined piercing rolling mill and a mandrel mill, a seamless billet characterized by using a billet which has been subjected to processing having an axially symmetric concave portion at the rear end of the billet before piercing. Manufacturing method of steel pipe. The method for manufacturing a seamless steel pipe according to claim 1, wherein the concave portion is processed so that a thickness of a billet rear end portion is 10 mm or more.

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