Ready-use low-carbon steel mechanical component for plastic deformation and method for making same

Abstract

A ready-for-use low-carbon steel mechanical component, like wheel swivel joints of terrestrial vehicles, pins, shafts, suspension bars, links . . . with elevated characteristics obtained by a hot or cold plastic transformation of a laminated long steel product (wire or rod) without any further heat treatment, the chemical composition of said steel complies with the following analysis, given in percentages by weight, based on the iron: C≦0.15%; 0.04%≦Nb≦0.10%; 0.001%≦B≦0.005%; 0.15%≦Mo≦0.35%; 1.3%≦Mn≦2.0%; 0.15%≦Si≦1.30%; 0.01%≦Al≦0.08% and N≦0.015% with Ti≧3.5×% N and said long steel product is obtained from a semi-finished product coming from the continuous casting and hot-rolled in the austenitic range to obtain a bainitic or essentially bainitic structure, and worked by a cold or hot plastic transformation into its final shape, exhibiting a tensile strength at break greater than 800 MPa. The invention is particularly directed to applications of stamping or cold forging or hot forging. But, it also applies to other applications of plastic deformation, such as wire drawing, deep drawing, stamping, etc . . . .

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The invention relates to low-carbon steel mechanical components with elevated characteristics, such as wheel swivel joints of terrestrial vehicles, pins, shafts, suspension bars, links, or other ready-for-use analogous mechanical components obtained by plastic deformation of a long steel product (wire, rod . . . ). [0003] 2. Background Art [0004] It is known that steels for plastic deformation must exhibit characteristics of both deformability and strenght. Thus, during the manufacture of mechanical components for which some of them are intended, they have to be able to endure major changes in shape without rupture, and at times exhibit good mechanical properties in the finished product. In fact, in certain cases, the required properties of the components obtained from these steels are close to those of Class 10.9 according to Specification ISO 898, i.e. a minimum tensile strength of 1000 MPa and a minimum yield str...

AI Technical Summary

Benefits of technology

[0012] More specifically, the object of the invention is to develop a grade of low-carbon steel specifically for the manufacture of mechanical components possessing a bainitic or essentially bainitic structure, which can be already obtained with low cooling rates at the core—as low as 1° C. / s—and offering not only good deformation properties, but also a good machinability for the manufacture of components by cold or hot deformation, without a heat treatment after forming, said grade having high mechanical properties such that said components will meet the quality requirements of Classes 8.8 to 12.9 of ISO specification 898.

[0052] Consequently, the invention opens up production for the line of hot-rolled long products of large diameters, destined for stamping workshops or cold forges, and for those allotted to hot forges, it saves an additional final heat treatment of quenching / tempering. To clarify still further, it should be noted that for typical as-rolling conditions, the limits of diameters are about 20 to 25 mm for the grades according to the invention.

Problems solved by technology

Thus, during the manufacture of mechanical components for which some of them are intended, they have to be able to endure major changes in shape without rupture, and at times exhibit good mechanical properties in the finished product.

All these various heat treatments require procedures, which although understood, are still costly and which do not always meet the objectives.

In any respect, they increase production time and costs.

Nevertheless, due to the level of cooling capabilities generally available on a hot rolling mill, these grades only allow an essentially bainitic structure to be obtained with rolled wires or rods of relatively small diameter, in fact rarely exceeding 8 mm.

Above this, one obtains a degraded bainite or bainite associated with ferrite, leading to a drastic reduction in mechanical properties of the rolled products.

Furthermore, since the structure is not well controlled, there is a risk of a wide dispersion of the mechanical properties within the same reel of wires or between several reels of wire or between several rods or in the same rod at the end of the hot rolling.

Similar problems have been encountered with hot-forging steel grades, where the thickness of the forged component often imposes severe constraints on cooling in order to meet the cooling rate at the core, required for obtaining the aimed bulk bainitic structure.

In addition, as the periphery of the component is inevitably cooled much more vigorously than the core, internal stresses result which can lead to unacceptable permanent deformations.

These constraints imposed on the cooling are so severe for the presently known grades of steel used, that the bainitic structure can not always be obtained directly in the conditions of the rolling process, nor even after forging, with the result that numerous mechanical components must be subjected to a heat treatment after forming.