Seamless steel pipe, hollow spring utilizing seamless steel pipe, and process for manufacturing the same

Abstract

There are provided: a seamless steel pipe formed from a cylindrical steel material billet through a hot isostatic extrusion step, wherein a depth of a contiguous flaw formed on an inner periphery surface and an outer periphery surface of the steel pipe is 50 μm or less; a hollow spring obtained by forming a hollow body in a shape of a coil or a bar or a bar with curved part from the seamless steel pipe made of spring steel and applying a surface treatment to the hollow body so that the hollow body has compressive residual stress; and a method for producing seamless steel pipe including: a billet molding step; a first heating step; a hot isostatic extrusion step; a second heating step; an extension step; a third heating step; and a pickling step.

Description

TECHNICAL FIELD[0001]The present invention relates to a cylindrical seamless steel pipe, to be used as a material for coiled spring and the like, and a method for producing the same.BACKGROUND ART[0002]Recently in automobile field, techniques of weight reduction have been developed: panels are more likely to be made of aluminum, and chassis and main bodies are improved to have high-tensile strength. Demand for weight reduction is now increasing in the current situation where international awareness of global warming caused by emission from vehicle becomes stronger, and reduction of CO2 contained in exhaust gas became a big issue.[0003]In the automobile field, techniques for reducing CO2 have been studied mainly towards the following two directions: a development of new power sources, such as hybrid engine, fuel cell motor system and battery motor system; and an improvement of existing engines.[0004]Major techniques for improving existing engines include “improvement of combustion me...

AI Technical Summary

Benefits of technology

[0036]The seamless steel pipe of the present invention has smooth outer periphery surface and inner periphery surface, since it is produced by hot isostatic extrusion processing. Therefore, there can be eliminated most parts that would otherwise become origin for fracture when the seamless steel pipe as a spring is continuously extended and contracted, and as a result, the seamless steel pipe exhibits excellent endurance, such as a long fatigue life.

[0037]In addition, in the seamless steel pipe of the present invention, weight reduction by approximately 30 to 40% can be attained as compared with a solid-core steel material having the same diameter.

[0038]The hollow spring of the present invention is obtained first as a seamless steel pipe with enhanced material cleanliness and surface properties by extruding a heated spring steel material with fluid pressure; and then as a hollow body in a shape of a coil or a bar or a bar with curved part from the seamless spring steel pipe by applying a surface treatment to the seamless spring steel pipe so that the seamless spring steel pipe has compressive residual stress. In the resultant coiled spring, a material strength of the hollow spring becomes high, which in turn suppresses permanent transformation (permanent set) and fracture of the spring even under high stress, and design stress can be made higher, to thereby obtain both weight reduction effect and raise of fatigue strength.

[0039]Effects of the present invention when applied to some automobile parts will be described below.

[0040]First, for enhancing engine performance, one technique is to improve output properties, including an increase of an engine revolution limit and a modification of a breathing port. With respect to the engine revolution limit, a movement of a valve operating system becomes a main factor that determines the limit. A rotational movement of a crankshaft transmitted through a cam is transformed into an up-and-down movement of the valve, and the up-and-down movement of the valve is pressed back by a valve spring. Therefore, the engine revolution limit is defined as a point at which the up-and-down movement of the valve spring cannot follow the rotational movement of the cam. In order to improve performance, it is necessary to increase spring load or reduce inertial mass of a reciprocally moving part, such as the valve including the spring. In order to meet this requirement, it is preferable to reduce a weight of the spring itself, to make the spring compact, and further, to maintain and improve spring load (for example, if a weight of the spring is reduced, a characteristic frequency of the valve spring increases, and accordingly, the engine revolution limit increases).

[0041]Due to the reduction of the inertial mass of the valve spring, a large improvement effect in fuel consumption is obtained. In general, an improvement of approximately 0.5 to 1.0% fuel consumption for every 20% reduction of the spring mass is obtained.

Problems solved by technology

Demand for weight reduction is now increasing in the current situation where international awareness of global warming caused by emission from vehicle becomes stronger, and reduction of CO2 contained in exhaust gas became a big issue.

However, in the method for producing seamless steel pipe in which Mannesmann-piercing is performed, there is a limitation on a minimum diameter of the rolling.

Therefore, in the production method according to the above-mentioned patent document, there arises a problem that production efficiency becomes extremely poor.

Grinding may result in generation of flaws, especially in the inner periphery surface of the seamless steel pipe.

In addition, in the production method disclosed in the above-mentioned patent document, the seamless steel pipe is produced by Mannesmann-piercing, and when a material with high hardness is used, tools may frequently be damaged.

Therefore, there was a problem that only a material with low hardness can be used, and thus only a seamless steel pipe with low endurance can be produced.

However, such a method is extremely poor in processing efficiency when producing a seamless steel pipe having a small diameter of, for example, approximately 10 mm, and thus feasibility is extremely low.

Though the above-mentioned patent document and the like proposed a production of seamless steel pipe, a seamless steel pipe having high hardness and a small diameter has not been produced in practice, due to the above-mentioned problems.

In a case of a hollow spring made of the conventional seamless steel pipe, there are disadvantages that a design stress should be made low and weight reduction effect is low, since a material strength of the seamless steel pipe itself is low and permanent transformation (permanent set) and fracture of the spring may occur under high stress.

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