Method for Producing Seamless Pipe

Abstract

Disclosed is a method for producing a seamless pipe by using a piercing mill which pierces and rolls a round billet heated to 1300° C. or lower, wherein the piercing mill is composed of a pair of skew rolls disposed to face each other around a pass line, a pair of guide devices disposed to face each other around the pass line, and a plug disposed along the pass line, between the pair of the skew rolls and also between the pair of the guide devices, wherein the piercing-rolling is performed under the conditions satisfying the following formulas (1) to (3) to prevent wrinkle flaws of the seamless pipe:−1.0<Δθ  (1)Δθ=θp−θr  (2)−0.37×Δθ+1.47≦Rn≦0.37×Δθ+2.67  (3)wherein the meanings of the individual symbols in the above-described formulas are as follows:θr: The half angle (°) between the pass line and the main roll face in the condition of the feed angle of the main rolls being zeroθp: The half angle (°) between the pass line and the reeling section of the plugRn: The number of times of the reeling of the plug

Description

[0001]The disclosure of International Application No. PCT / JP2009 / 053652 filed Feb. 27, 2009 including specification, drawings and claims is incorporated herein by reference in its entirety.TECHNICAL FIELD[0002]The present invention relates to a method for producing a seamless pipe.BACKGROUND ART[0003]As a method for producing a seamless pipe, the Mannesmann process is known in which a seamless pipe is obtained by subjecting a heated round billet to piercing-rolling with a piercing mill, then to elongation rolling with a mandrel mill, a plug mill or the like, and further to sizing with a sizing mill to yield the seamless pipe.[0004]The piercing mill is usually a piercing machine having rolling rolls consisting of a pair of barrel-shaped or cone-shaped main rolls (also referred to as skew rolls), guide devices such as a guide shoe, disk rolls, a roller-type guide or the like, and an inner surface regulating tool referred to as a plug.[0005]FIG. 1 is a schematic view illustrating an ex...

AI Technical Summary

Benefits of technology

[0065]According to the present invention, the properties and conditions of the inner surface of a hollow shell after piercing-rolling can be improved, and the wrinkle flaws in a seamless pipe which is obtained by performing elongation rolling and sizing after piercing-rolling can be prevented.

Problems solved by technology

During the piercing-rolling with the piercing mill, the asperities formed on the inner surface of the hollow shell are flattened out by the inner surface regulating tools in the lower process such as the plug of the elongator, the bar of the mandrel mill and the plug of a plug mill, and then the asperities develop into wrinkle flaws (eruption flaws).

In other words, the occurrence of the fine inner surface flaws in final products is attributed to the properties and conditions (roughness) of the inner surface of the hollow shell after the piercing with a piercing mill.

In particular, in the seamless steel pipes undergoing high pressures on the inner surfaces thereof, such as a fuel injection pipe, the occurrence of fine rice-grain-like flaws could lead to a serious accident through the blowout of the pipe initiated by such flaws.

When the inner diameter of the pipe is large, it is possible to mechanically remove the inner surface flaws with a grinder or the like, however, when the inner diameter of the pipe is small, it is difficult to completely remove the inner surface flaws.

Even if the removal of the inner surface flaws of a pipe having a small inner diameter is possible, the number of work steps is naturally increased, and problems from the viewpoint of a product may be left unsolved in such a way that the wall thickness of the flaw-removed portion is thin.

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