Metal treatment – Stock – Ferrous
Patent
1999-01-25
2000-03-14
Yee, Deborah
Metal treatment
Stock
Ferrous
420127, 420 84, C22C 3812, C22C 3860
Patent
active
060367903
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a microalloyed forging steel for machine structural use having a small deformation of a fracture surface when fracture-split and is generally applicable to steel blanks for machine structural use and machine parts which require a small deformation upon tensile and impact fracture.
BACKGROUND ART
The steels for machine structures, which are used to form parts of automobiles and industrial machinery, are usually supplied in the form of a straight bar or a coiled wire and are hot- or cold-worked to a desired shape, followed by various heat treatments, machining, etc., to provide a final part. When the processing from steel blanks to parts includes fracture-separation by cold tension, it is usually necessary to control the deformation upon fracture in order to ensure the required precision in the subsequent working step or to prevent occurrence of troubles in an automated working line.
Usual steel parts were conventionally formed by hot or cold forging, followed by quench-hardening and tempering to provide required strength and toughness. These days, microalloyed steels for hot forging (hereinafter simply referred to as "microalloyed forging steel"), which have the required strength in an as-forged state, are increasingly used. Replacing the quench-hardened and tempered steel with the microalloyed forging steel is advantageous because omission of heat treatment lowers the production cost and eliminates quenching distortion.
The forming method of microalloyed forging steel parts includes fracture-splitting by impact tension, working of required portions and then recoupling the fracture surfaces and is practically used typically for forming a connecting rod made, for example, of a steel having a relatively high carbon content such as Fe-0.72%C-0.22%Si-0.49%Mn0.062%S-0.04%V as described in "Fundamentals and Applications of Microalloying Forging Steels", (1996) 29 TMS.
The process of producing a connecting rod can be roughly summarized as hot-forging of a steel blank followed by air cooling, boring and drilling of a cap and a rod, mechanical splitting of a large end, recoupling of the fracture surfaces, bolting of the cap and the rod, and finish-machining.
This process is advantageous because relatively inexpensive steel blanks can be used and conventionally required high precision machining can also be omitted to reduce costs. However, the above-recited steel contains a large amount of carbon to enhance fracturability, and therefore, has a problem of low yield strength and fatigue strength as well as poor machinability.
Japanese Unexamined Patent Publication (Kokai) No. 8-291373 discloses a steel, for connecting rods, in which the carbon content is reduced from the above-recited steel while fracturability is ensured, and describes that the disclosed microalloyed steel for hot forging is "easy to fracture-separate and the fractured surface has a small deformation and is easily recoupled".
Japanese Unexamined Patent Publication (Kokai) No. 9-3589 discloses a low toughness microalloyed forging steel for connecting rods and describes that an increased N amount, in particular, provides a brittle fracture surface upon fracture-splitting and "the object is to provide a high strength, low toughness microalloyed forging steel which exhibits a flat, brittle fracture surface when fracture-split at room temperature".
However, the steels disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-291373 or Japanese Unexamined Patent Publication (Kokai) No. 9-3589 failed to provide a commercially acceptable fracturability.
DISCLOSURE OF THE INVENTION
The object of the present invention is to provide an inexpensive, medium carbon microalloyed forging steel for machine structural use having a small deformation when fractured in the state of as hot-worked by hot rolling, hot forging, etc. and being composed of a ferrite-pearlitic microstructure.
To reduce the deformation of a steel upon fracture, it is most effective to reduce the ductility of the steel. Several measur
REFERENCES:
patent: 5403410 (1995-04-01), Shikanai
patent: 5922145 (1999-07-01), Watari et al.
Hashiguchi Tetsuro
Kanisawa Hideo
Takada Hiromasa
Nippon Steel Corporation
Yee Deborah
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