Metal treatment – Stock – Ferrous
Patent
1995-01-05
1997-08-26
Yee, Deborah
Metal treatment
Stock
Ferrous
148649, 148653, 148654, 148328, C22C 3824, C21D 800
Patent
active
056606486
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION
Among steel materials which are worked to produce machine parts such as automobile parts and industrial machine parts, the present invention relates to a microalloyed steel for hot forging from which machine parts are prepared by working, for example, hot forging or hot rolling (as-hot worked in some cases), and subsequently by aging in some cases, a process for producing a microalloyed hot forging using said steel, and a microalloyed hot forging.
PRIOR ART
Many automobile parts and industrial machine parts are prepared by hot working a steel bar, and quenching and tempering the worked products to refine the structure and enhance the strength and toughness, and the parts thus obtained are used. However, machine parts which are prepared without quenching and tempering treatment to reduce the cost, namely parts prepared from microalloyed steel for hot forging (referred to as a microalloyed steel hereinafter) have become increasing common in recent years. Reduced fuel consumption of automobiles has been required in recent years, to protect the global environment. One effective method for reducing fuel consumption is decreasing the weight of vehicles. Accordingly, improving the strength of automobile parts and thus making the parts small and light are important goals.
To particularly strengthen steel parts, the steel parts must have a bainite or martensite structure. Various kinds of inventions have been disclosed relating to microalloyed steels of bainite structures. Japanese Patent Kokai Publication NO. 1-177339 discloses a microalloyed steel which can be used in an as-air cooled state after hot forging. Since bainitic steel has a disadvantage that it has a low yield strength, the steel has been conventionally toughened by further aging. For example, Japanese Patent Kokai Publication No. 2-25516 discloses a method comprising aging a bainitic steel at a temperature of 200.degree. to 600.degree. C. after forging to toughen the steel.
However, demand for automobiles of low fuel consumption has become increasingly strong, and further strengthened and toughened parts therefor are required.
SUMMARY OF THE INVENTION
As a result of investigating the improvement of the tensile strength of conventional steel parts used as driving-related parts of automobiles, the present inventors have discovered that a microalloyed steel with a bainite structure which mostly has heretofore had a tensile strength of approximately up to 1,000 MPa can be relatively easily improved to have a tensile strength of at least 1,000 MPa by increasing alloying elements therein, and thus they have achieved the present invention.
Furthermore, it has been difficult to impart a toughness necessary for driving-related parts of automobiles as well as a tensile strength of at least 1,000 MPa to the bainitic microalloyed steel. Still furthermore, the bainitic microalloyed steel has a problem that it has a low yield ratio.
An object of the present invention is to provide a steel material for a hot forged microalloyed bainitic steel parts which has in an as-hot forged state a tensile strength exceeding 1,000 MPa and a high toughness and realizes a high yield strength, namely a microalloyed steel for hot forging, a process for producing a microalloyed hot forging and the hot forging.
The subject matter of the present invention is as described below.
(1) A microalloyed steel for hot forging 0.90 to 3.00% of Si, from 1.20 to 3.00% of Mn, from 0.10 to 0.50% of Cr, from 0.03 to 0.10% of S, from 0.05 to 0.50% of V, from 0.0080 to 0.0200% of N and the balance Fe and unavoidable impurities, formula
(2) A microalloyed steel for hot forging according to (1), wherein said microalloyed steel further comprises one or two selected from the group of from 0.005 to 0.050% of Al and from 0.002 to 0.050% of Ti.
(3) A microalloyed steel for hot forging of Mo and from 0.01 to 0.50% of Nb in addition to the components according to (1) and the balance Fe and unavoidable impurities, (%V+%Nb) formula
(4) The microalloyed steel for hot forgi
REFERENCES:
patent: 4409026 (1983-10-01), Yamada et al.
patent: 5282906 (1994-02-01), Heitmann et al.
Koyasu Yoshiro
Mori Motohide
Suzuki Masami
Takada Hiromasa
Nippon Steel Corporation
Yee Deborah
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