CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2019/009811, filed Mar. 11, 2019 which claims priority to Japanese Patent Application No. 2018-065875, filed Mar. 29, 2018, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention is a technology relating to a hollow stabilizer.

Herein, in this specification, the Vickers hardness is expressed by average hardness in the thickness direction. “High strength” indicates a case where the strength is 400 HV or more and preferably 450 HV or more in terms of the Vickers hardness.

BACKGROUND OF THE INVENTION

In usual, an automobile has a stabilizer in order to reduce the rolling of an automobile body in cornering or to keep the traveling stability in high speed traveling. Recently, it is common to use a hollow stabilizer containing a steel pipe as the stabilizer in order to reduce the weight of the automobile body.

Such a hollow stabilizer commonly contains a seamless steel pipe or an electric resistance welded steel pipe produced by electric resistance welding as a raw material. Such a steel pipe is molded into a target stabilizer shape by cold rolling, and then subjected to thermal refining treatment, such as quenching or quenching and tempering, to be made into a product. In particular, the electric resistance welded steel pipe is relatively inexpensive and excellent in dimensional accuracy, and therefore has been widely used as a raw material for producing the hollow stabilizer.

As the inventions of conventional hollow stabilizers, the inventions described in PTLS 1 and 2 are mentioned, for example.

More specifically, PTL 1 describes a production method for a hollow stabilizer excellent in fatigue resistance.

According to PTL 1, a welded steel pipe having a composition containing, in terms of % by mass, C: 0.2 to 0.38%, Si: 0.35% or less, Mn: 0.3 to 1.5%, Al: 0.1% or less, Ti: 0.005 to 0.1%, and B: 0.0005 to 0.005% is used as a raw material and the raw material is subjected to heat treatment including heating the raw material to a temperature in the range of 800 to 1000° C., and then subjected to reduction rolling with a cumulative reduction ratio: 40% or more at a rolling temperature: 600 to 850° C. to be made into a steel pipe for producing a hollow stabilizer. Thereafter, according to PTL 1, the steel pipe is sequentially subjected to a molding step of molding the steel pipe into a stabilizer shape by cold bending and a heat treatment step of applying quenching treatment and tempering treatment to the steel pipe to form a hollow stabilizer. PTL 1 describes that the production method can improve the fatigue resistance by an inexpensive method.

According to PTL 2, a steel sheet having a composition containing, in terms of % by mass, C: 0.15 to 0.40%, Si: 0.05 to 0.50%, Mn: 0.30 to 2.00%, Al: 0.01 to 0.10%, Ti: 0.001 to 0.04%, B: 0.0005 to 0.0050%, and N: 0.0010 to 0.0100% and containing Ti, N to satisfy (N/14)<(Ti/47.9) is made into a substantially cylindrical-shaped open pipe, end portions of the open pipe are butted against each other, and then electric resistance welding is performed to the end portions while adjusting heat input such that the bond width is 30 to 65 μm by high frequency resistance welding to form an electric resistance welded pipe. Then, according to PTL 2, the electric resistance welded pipe is heated to a temperature equal to or higher than a Ac3transformation point, and then subjected to stretch reducing with a draft equal to or higher than (1-25/Bond width before stretch reducing (μm)×100% in terms of an outer diameter ratio, so that the bond width is set to 25 or less. PTL 2 describes producing an electric resistance welded steel pipe for heat treatment excellent in flatness by the treatment above. PTL 2 describes that the produced electric resistance welded steel pipe is suitable for the use of being subjected to quenching treatment, e.g., a hollow stabilizer, and that the reduced-carbon layer width of an electric resistance weld zone is narrow, and therefore, even when quenching treatment by rapid and short-time heating is applied thereto, a reduction in the quenching hardness of the electric resistance weld zone can be suppressed, so that a hollow stabilizer excellent in durability can be obtained.

PATENT LITERATURE

PTL 1: JP 2005-076047 A

PTL 2: JP 2008-208417 A

SUMMARY OF THE INVENTION

When hot reducing work is applied to a steel pipe as in the technologies described in PTLS 1 and 2, irregularities accompanying pipe inner surface reducing are generated in the steel pipe. Particularly in the electric resistance weld zone, bead cutting treatment is applied to an internal weld zone in a base pipe stage before the hot reducing work so that an internal weld bead cut portion is formed. The weld bead cut shape in the base pipe stage is affected by a difference between the hardness of a weld bond zone and the hardness of a peripheral portion thereof. Therefore, the shape of the internal weld bead cut portion is deformed into a complicated three-peak shape by the hot reducing work. Then, the present inventors have conducted an examination to obtain a finding that a portion having the three-peak shape of the internal weld bead cut portion serves as a stress concentration portion when a torsional stress is given to a stabilizer to cause a reduction in the fatigue characteristics depending on the situation. The reduction in the fatigue characteristics due to the stress concentration becomes remarkable particularly in the case of a high strength stabilizer.

Furthermore, with an increase in the strength of the stabilizer and with a reduction in the pipe thickness, a stress on the pipe outer surface side and a stress on the pipe inner surface side become closer to each other with respect to the stress generated when the stabilizer is twisted. Therefore, the stabilizer is likely to be affected by the surface properties or the surface shape of the pipe inner surface side, which increases the probability of the occurrence of a crack with the pipe inner surface side as the starting point. The occurrence of a crack is a major issue of a pipe produced by the hot reducing work. Cold drawing (cold drawing work) smoothes the inner surface shape but involves an increase in the production cost.

Aspects of the present invention have been made focusing on the above-described points. It is an object according to aspects of the present invention to provide an electric resistance welded steel pipe for producing a hollow stabilizer excellent in fatigue resistance and having high strength and a hollow stabilizer excellent in fatigue resistance and having high strength.

The present inventors have examined the influence of the shape of an internal weld bead zone on the fatigue characteristics of a high strength hollow stabilizer. As a result, the present inventors have obtained a finding that it is important, with respect to the fatigue, to control a depth H of a thin portion (trough portion) of a three-peak shape and an angle θ formed by the thin portion (trough portion) and right and left peak portions containing central thick portions of the three-peak shape in the internal weld bead zone within a specific range.

Aspects of the present invention have been accomplished by further adding an examination based on the finding.

In order to solve the problems, in an electric resistance welded steel pipe for producing a hollow stabilizer of one aspect of the present invention, an internal weld bead cut portion has a three-peak shape, a depth H of a trough portion of the three-peak shape is 0.3 mm or less, and an angle θ formed by a central portion in the circumferential direction of the trough portion and the top of right and left peak portions located on both the right and left sides of the trough portion is 160° or more and less than 180°.

One aspect of the present invention is a hollow stabilizer containing the electric resistance welded steel pipe for producing a hollow stabilizer of one aspect described above and having Vickers hardness of 400 HV or more and less than 580 HV.

A production method for a hollow stabilizer of one aspect of the present invention includes applying cold bending to the electric resistance welded steel pipe for producing a hollow stabilizer of one aspect described above, and then applying heat treatment including quenching treatment or quenching and tempering treatment thereto so that the hardness after the heat treatment is set to 400 HV or more and less than 580 HV in terms of the Vickers hardness.

A production method for an electric resistance welded steel pipe for producing a hollow stabilizer of one aspect of the present invention includes a first step of molding a steel sheet into a cylindrical shape by cold molding to form an open pipe, butting end portions in the width direction of the open pipe against each other, and performing electric resistance welding to the end portions to form an electric resistance welded pipe, a second step of heating the electric resistance welded pipe to a temperature of 850° C. or more and 1000° C. or less, and then applying hot stretch reducing thereto at a rolling temperature: 650° C. or more and 1000° C. or less and at a cumulative reduction ratio: 30% or more and 90% or less, and a third step of adjusting an internal weld bead cut portion having an outline shape of a three-peak shape along the circumferential direction in which a trough portion with a reduced thickness is formed on each of both the right and left sides across a weld zone such that a depth H of the trough portion is 0.3 mm or less and an angle θ formed by a central portion in the circumferential direction of the trough portion and the top of right and left peak portions located on both the right and left sides of the trough portion is 160° or more and less than 180°.

Aspects of the present invention can provide an electric resistance welded steel pipe suitable for the production of a hollow stabilizer excellent in fatigue resistance. In particular, the electric resistance welded steel pipe is suitable for a high strength hollow stabilizer excellent in fatigue resistance.

For example, aspects of the present invention can simply produce a high strength hollow stabilizer having hardness of 400 HV or more and holding excellent corrosion fatigue resistance, and thus can exhibit remarkable industrial effects. Moreover, aspects of the present invention can also exhibit an effect that, even when the hardness is further increased, e.g., hardness of 450 HV or more, a reduction in fatigue characteristics is not observed and a contribution to a further thickness reduction of a stabilizer can be made, for example.

