JP2002239612A - Method of manufacturing seamless tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a seamless tube made of high-alloy steel being a heavy-load material at a low cost with a manufacturing line for seamless tubes using an inclined piercing mill. SOLUTION: In the seamless tube manufacturing line having a heating furnace and the inclined piercing mill, after a base stock is machined into the shape of a hollow tube stock before charging it into the heating furnace and the base stock having the shape of the hollow tube stock is heated in the heating furnace so as not to exceed the zero-ductility temperature of the base stock, elongation rolling (expansion, thickness reduction, elongation) is performed with the inclined piercing mill.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a seamless pipe, mainly a high-alloy steel (high rolling load and power) high-load material.

[0002]

2. Description of the Related Art In the production of seamless steel pipes, there is a drilling step for making holes in a billet heated to a high temperature as a first processing process. Drilling often uses the rotary forging effect, and one pass is performed using 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. For the hollow shell after perforation, elongation rolling is performed by a plug mill for large diameter sizes and a mandrel mill for medium and small diameter sizes.

[0003] On the other hand, as a trend of seamless pipe users in recent years, steel pipes having higher strength and higher corrosion resistance have been desired with the severer use environment. In order to satisfy such requirements, it is inevitable to use a high alloy steel containing a large amount of alloying elements such as stainless steel.

However, since high-alloy steel has higher deformation resistance than ordinary steel, the power required at the time of drilling increases, and a drilling machine with a small motor capacity designed for ordinary steel has an overload. Cannot be produced. Although it is conceivable to newly introduce a drilling machine having a large motor capacity for manufacturing a high alloy steel seamless pipe, there is a problem that enormous cost is required for capital investment. Therefore, as a method other than newly installing a drilling machine having a large power, it is necessary to be able to process high alloy steel with the small drilling power of the existing drilling machine.

The following two methods can be considered as a method for reducing the piercing power of high alloy steel.

[0006] (A) Method of utilizing the reduction of deformation resistance at high temperature (high temperature heat extraction of billet) (B) Method of hot rolling a hollow shell formed by centrifugal casting or hollow continuous casting In the method A), the billet is heated to a high temperature.
Not only does the scale amount in the heating furnace increase, but it cannot be adopted in some high alloy steels. The reason for this is that there is a property that the ductility decreases rapidly in a high temperature range of 1000 ° C. or more, and the temperature of the rolled material exceeds the zero ductility temperature due to the processing heat at the time of drilling, and due to the shear strain peculiar to inclined rolling. This is because internal cracks occur.

Here, the "zero ductility temperature" will be described with reference to FIG. Fig. 4 shows the test temperature (° C) on the horizontal axis,
FIG. 4 is a characteristic diagram showing a change in ductility (drawing) in a high temperature region of SUS304, which is a typical austenitic stainless steel, with drawing (%) plotted on the vertical axis. About 12 in the figure
Although there is no change in ductility up to 80 ° C (characteristic line A),
If it exceeds about 1280 ° C., the ductility sharply decreases (characteristic line B). Such a temperature at which the ductility sharply decreases in the high temperature range is referred to as zero ductility temperature, and is hereinafter referred to as To. The zero ductility temperature To is different for each steel type.

The method (B) uses a hollow material instead of a solid material to reduce the amount of processing at the time of drilling, thereby reducing power and heat generation during tilt rolling, and has the problem of zero ductile cracking. It can be said that this is certainly an effective method.

[0009]

However, unlike high-grade steel, high-alloy steel is a so-called small lot, which is a small-sized product with a small production volume per lot. There is a problem that the production line cannot be adapted.

The method (B) using a hollow material obtained from a centrifugal casting method or a continuous casting method is suitable for mass production such as supplying billets of the same size. When preparing multiple sizes of outer diameters and wall thicknesses for the raw tubes, there is a problem that the manufacturing cost is significantly increased.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a seamless pipe for high-alloy steel, which is a high-load material, in a seamless pipe production line using an inclined piercing mill. An object is to provide a method that can be manufactured at low cost.

[0012]

SUMMARY OF THE INVENTION A seamless pipe manufacturing method according to the present invention is a method for manufacturing a seamless pipe in a seamless pipe manufacturing line having a heating furnace and an inclined piercing and rolling mill before charging the material into the heating furnace. After machining into the shape of a tube and heating the material in the shape of a hollow shell in a heating furnace so as not to exceed the zero ductility temperature of the material, elongation rolling (expansion, wall thinning, elongation) by the inclined piercing mill. It is characterized by doing.

As described above, in order to reduce the power during inclined piercing and rolling of a high-alloy steel with a high load, it is most convenient to use a hollow material instead of a solid billet as a material before working. The most suitable method is a method in which a hollow material having a large outer diameter and a large thickness is formed by machining from a cold billet.

[0014] It is needless to say that the accuracy of the outer diameter, the inner diameter, and the wall thickness can be obtained by machining, so that the wall accuracy of the hollow shell after the piercing and rolling is improved. The machining method is not particularly limited as long as it can perform boring.

The billet before working may be manufactured by any of centrifugal casting, solid continuous casting, hollow continuous casting, and bulk rolling. Furthermore, the cross-sectional shape of the billet before processing is not particularly limited, and may be, for example, a round cross section or a square cross section.

Next, the inner surface cracks generated during inclined piercing rolling can be solved by controlling the temperature of the material to be rolled after piercing rolling to be equal to or lower than the zero ductility temperature To. The temperature of the material to be rolled during piercing and rolling to be controlled depends on the main parameters such as the steel type, the temperature of the material to be rolled before rolling, and the conditions of inclined piercing and rolling (dimensions of hollow shell before and after processing, processing speed, etc.). Therefore, it can be obtained in advance using an analysis method such as a finite element method using computer simulation.

By the way, since the degree of working of the inclined piercing rolling is excessively large and the heat generated by the working becomes extremely large, the temperature of the raw material may be significantly increased during working depending on the type of steel. For example, when a solid material of stainless steel is pierced and rolled, the temperature of the material is increased by about 100 ° C. as compared to before the working. Therefore, even if the heating furnace extraction temperature is set to be equal to or lower than the zero ductility temperature To, the high alloy steel may exceed the zero ductility temperature To at the time of piercing rolling, and may cause internal surface cracking.

However, in the method of the present invention, the hollow shell having a predetermined thickness is machined before piercing and rolling to reduce the heat generated during the piercing and rolling, and the temperature of the material is reduced to zero ductility during the working. Since the temperature does not exceed To, the occurrence of inner surface cracks is suppressed.

Since it is generally impossible to greatly change the heating furnace extraction temperature, the heating furnace extraction temperature is not changed for small lot materials, and the hole is drilled by adjusting the dimensions of the hollow shell during machining. It is desirable to control the temperature of the material to be rolled after rolling.

[0020]

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a diagram schematically showing the appearance of a material to be rolled before and after machining. In the drawing, reference numeral 2A denotes a solid billet having a round cross section manufactured by a continuous casting method, reference numeral 2B denotes a billet having a hollow round cross section manufactured by a centrifugal casting method, and reference numeral 2C.
Indicates a billet having a square cross section manufactured by a slab rolling method. In the present invention, the billet 2A, 2B, 2C having such various cross-sectional shapes is used as a raw material to form a hollow shell 2D having a desired size by machining, heated to a predetermined temperature, and then pierced and rolled.

The outer diameter D and the thickness t of the hollow shell 2D are determined by the material conditions (material, temperature before rolling, etc.) and processing conditions (hollow shell size after piercing and rolling, processing speed, processing temperature, etc.).
Can be calculated in advance based on That is, based on the performance and characteristics (equipment parameters) inherent to the existing heating furnace and the inclined piercing mill at the seamless pipe production line, and the zero ductility temperature To and the size of the material (material parameters), the hollow shell is used. Is obtained. Further, based on the outer diameter D and the wall thickness t, the inner diameter d of the hollow shell is obtained.
Is found. If the temperature of the hollow shell during processing is lower than the zero ductility temperature To, no inner surface cracks occur during piercing and rolling.

Next, a case of manufacturing a high alloy steel seamless pipe will be described with reference to FIG.

The solid billet having a round cross section is drawn from the continuous casting mold, cooled, and cut into a round solid solid billet 2A having a predetermined length (step S1). The material of the round billet solid billet 2A is austenitic stainless steel (SUS316)
And its outer diameter is, for example, 180 mm.

The heating furnace extraction temperature of the hollow shell is set from the temperature characteristic data unique to the existing heating furnace (step S2). The outer diameter D of the hollow shell based on the set heating furnace extraction temperature, material conditions (material, temperature before rolling, etc.) and processing conditions (hollow shell size after piercing and rolling, processing speed, processing temperature, etc.) And the thickness t are calculated in advance (step S
3). For example, the outer diameter D during machining, the inner diameter d, and
The wall thickness t was determined.

The billet 2A is carried into the inclined piercing and rolling line (step S4), and the inner and outer surfaces are machine-cut to form the hollow shell 2A.
D (Step S5). Note that the mechanical cutting is not necessarily performed in the inclined piercing and rolling line, but may be performed in another line or another factory. For example, a large lathe is used for the mechanical cutting.

Next, the hollow shell 2D is charged into a heating furnace and heated (step S6). Hollow shell 2 with temperature sensor
The temperature of D is measured, and the measured temperature is lowered to a temperature lower than To within a range not exceeding the zero ductility temperature To, for example, 1
After heating to 250 ° C., the hollow shell 2D is extracted from the heating furnace (step S7).

The hollow shell 2D is mounted on the inclined piercing mill shown in FIG.
And a seamless pipe 2E having a desired size was obtained using the pair of rolling rolls 5a and 5b, the perforated plug 6, and the pair of guide shoes 8 (step S8).

When a large-diameter pipe is manufactured, the raw material is processed in the order of heating furnace, piercer, elongator, plug mill, reeler, and sizer. Incidentally, in the above embodiment, the perforated plug 6 is used like an elongator.

No cracks were found on the inner surface of any of the seamless pipes 2E during rolling, and the power of these inclined piercing mills was within the range of the equipment specifications. As a result of dimensional inspection of the product at the refinement line, it was possible to produce a high alloy steel seamless pipe comparable to ordinary steel.

[0031]

Since the present invention is constructed as described above, a seamless pipe of high alloy steel having no inner surface defects can be manufactured with a high yield.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an external shape of a material to be rolled (a billet) before and after processing.

FIG. 2A is a schematic partial cross-sectional view showing a tilted piercing and rolling mill viewed from a side, and FIG. 2B is a schematic cross-sectional view showing the tilted piercing and rolling mill viewed from a pass line direction.

FIG. 3 is a process chart showing a method for manufacturing a seamless pipe according to an embodiment of the present invention.

FIG. 4 is a characteristic diagram showing the temperature dependence of drawing.

[Explanation of symbols]

2A: round section billet manufactured by continuous casting method (before machining), 2B: hollow round section billet manufactured by centrifugal casting method (before machining), 2C: square section billet manufactured by lumping and rolling method (machine) Before processing), 2D: billet with round hollow section after machining, 5a, 5b: rolling roll, 6: perforated plug, 8: guide shoe.

Continuing on the front page (72) Inventor Tatsuro Katsumura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Tatsuharu Oda 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan (72) Inventor Motoharu Yamazaki 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd.

Claims (3)

[Claims] 1. In a seamless pipe production line having a heating furnace and an inclined piercing mill, a raw material is machined into a hollow shell shape before charging into the heating furnace, and the hollow shell shape material is removed. After heating in a heating furnace so as not to exceed the zero ductility temperature of the material, elongation rolling (expansion,
A method for producing a seamless pipe, characterized by thinning and stretching. 2. The method for manufacturing a seamless pipe according to claim 1, wherein said material is made of a steel type whose hot ductility sharply decreases at 1000 ° C. or higher. 3. The method according to claim 1, wherein the material is manufactured by using any one of a centrifugal casting method, a hollow continuous casting method, a solid continuous casting method, and a bulk rolling method. The method for producing a seamless tube according to any one of the above.