Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-07-13
2001-03-20
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C420S121000, C420S126000
Reexamination Certificate
active
06203630
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to machinery structural parts which are formed by a cold working and are to be produced by strengthening through an induction quenching, the parts being required to have high rolling fatigue strength and torsion fatigue strength, for example, in an outer race for a joint of constant velocity, and a steel for induction quenching to be used thereto.
2. Description of the Related Art
In general, as machinery structural parts required to have the high fatigue strength such as the outer race for joints of constant velocity, medium carbon steels containing C: 0.40 to 0.60% are used. These steels are formed through a cold forging, and then increased in surface hardness by the induction quenching treatment so as to enhance the rolling fatigue strength and the torsion fatigue strength. These machinery structural parts have recently been demanded for further improving higher strengthening and cold workability because of making light weight.
Since the medium carbon steel is generally poor in the cold workability, many techniques for improving it have been developed. For example, JP-A-62-23929 and JP-A-62-196327 disclose technologies that Si and Mn in steel are limited, deoxidation and denitrification are carried out by Al and Ti, a fine amount of B is added to guarantee a high hardenability with the amount of small alloying addition, and temperature conditions of hot rolling or finish rolling temperature are controlled for improving the cold workability.
JP-A-2-129341 discloses a method for improving the cold workability of steel by limiting amounts of Si and Mn, decreasing alloying elements by adding Al, Ti and B as the above two examples, and limiting upper limits of N, S and O.
On the other hand, enhancing of strength, in particular improvement of fatigue strength mainly depend upon hardening in a skin portion by the induction quenching and compressive residual stress generated thereby, and efforts are directed to adjusting of chemical compositions in steel for efficiently demonstrating effects by the induction quenching. In parts requiring the rolling fatigue strength as outer races for joints of constant velocity, it is desirable that the hardness of the rolling face is high, but if the hardness is enhanceed, notch sensibility is increased resulting to invite a lowering of the fatigue strength, and so the enhancing of hardness is limited.
It is known that, in a hard steel, non-metallic inclusions in steel serve as sources of stress concentration and lowers the fatigue strength of steel. JP-A-2-129341 discloses a method of limiting an upper limit of O content to 0.0020%, taking prevention of deterioration of rolling fatigue life into consideration, and limiting an upper limit of Ti content to 0.05%, paying attention to prevention of forming large nitrides harmful to the rolling fatigue life.
The fatigue strength may be enhanced to a certain extent by providing methods of adjusting chemical compositions in steel as mentioned above, however, it has been difficult to decrease dispersions of the fatigue strength, in particular dispersions of the rolling fatigue life.
SUMMARY OF THE INVENTION
In view of such circumstances, it is accordingly an object of the invention to provide a steel suited to the induction quenching, having an excellent cold workability, high rolling fatigue strength and torsion fatigue strength, and less dispersions of the fatigue strength as well as machinery structural parts.
In the steel for the induction quenching, having the excellent cold workability, rolling fatigue strength and torsion fatigue strength and the machinery structural parts, mainly the contents of Si and Mn are limited for enhancing the cold workability, and B is added at a proper content for compensating the lowering of induction hardenability. For enhancing the effect of the B addition, the contents of O and N are limited, and Al and Ti of appropriate contents are added for carrying out deoxidation and denitrification. Cr, Ni and Mo may be added at small contents for compensating the hardenability of steel and increasing toughness of the same. Further, if decreasing contents of S, O and N forming non-metallic inclusions, and controlling sizes of formed non-metallic inclusions, the fatigue strength of the induction-quenched steel is improved and the dispersion thereof are lowered.
According to the present invention, the steel for high frequency induction quenching having excellent cold workability, rolling fatigue strength and torsion fatigue strength
(1) contains by mass %
C: 0.45 to 0.60%,
Si: 0.01 to 0.15%,
Mn: 0.20 to 0.60%,
S: 0.012% or lower,
Al: 0.015 to 0.040%,
Ti: 0.005 to 0.050%,
B: 0.0005 to 0.0050%,
N: 0.010% or lower,
O: 0.0010% or lower, and
balance being Fe and unavoidable impurities,
wherein maximum sizes of contained non-metallic inclusions are, in terms of equivalent circular diameters, 15 &mgr;m or less in oxide based non-metallic inclusions, 5 &mgr;m or less in nitride based non-metallic inclusions, and 15 &mgr;m or less in sulfide based non-metallic inclusions respectively, and the numbers of the non-metallic inclusions of the equivalent circular diameters being 1 &mgr;m or more are 6 or less per 1 mm
2
in the oxide based inclusions, 10 or less per 1 mm
2
in the nitride based non-metallic inclusions, and 5 or less per 1 mm
2
in the sulfide based non-metallic inclusions.
(2) The steel further contains, in addition to (1), at least one of
Cr: 1.00% or lower,
Mo: 0.50% or lower, and
Ni: 1.50% or lower.
The inventive machinery structural parts have the excellent cold workability, rolling fatigue strength and torsion fatigue strength:
(3) comprises the steel for high frequency induction quenching as set forth in any one of (1) and (2).
DETAILED DESCRIPTION OF THE INVENTION
Further reference will be made to reasons for limiting the containing percentage of the chemical composition in the steel for induction quenching, having the excellent cold workability, rolling fatigue strength and torsion fatigue strength.
C: 0.45 to 0.60%
C is a necessary element for raising the quenched hardness and securing the strength of the machinery structural parts. It is therefore necessary to contain C at least 0.45%. But if excessively containing, since the cold workability and machinability are spoiled and quenching cracks might be caused when the induction quenching is performed, the upper limit of C is determined to be 0.60%.
Si: 0.01 to 0.15%
Si is added as a deoxidizing agent when melting a steel, and for exhibiting the addition effect, Si should be added at least 0.01%. But if the content is as an ordinary deoxidizing agent, it deteriorates the cold workability of steel, and for enhancing the cold workability, the upper limit is determined to be 0.15%. Preferably, Si is contained in the range of 0.05 to 0.10%.
Mn: 0.20 to 0.60%
Mn serves as a deoxidizing agent when melting a steel and enhances a hardenability of steel. For exhibiting these effects, Mn should be added at least 0.20%. But if excessively containing, since the cold workability and machinability are spoiled, the upper limit of Mn is set to be 0.60%. Preferably, Mn is contained in the range of 0.20 to 0.50%.
S: 0.012% or lower
S forms sulfide based non-metallic inclusions (JIS: A1 based inclusions) in steel and damages the cold workability and decreases the fatigue strength. So the less, the more desirable, but if it is too low, since the machinability decreases, S may be contained in the range of 0.012% or lower. Preferably, S is contained in the range of 0.010% or lower.
Al: 0.015 to 0.040%
Al is a strong deoxidizing element and prevents crystal grain of steel from coarsening. For obtaining these effects, Al of 0.015% or higher is contained. But since Al forms Al2O3 as one of oxide based non-metallic inclusions and injures the fatigue strength of steel, the upper limit of Al is set to be 0.040%. Preferably, Al is contained in the range of 0.020% to 0.035%.
Ti: 0.005 to 0.050%
Ti of 0.005% or higher is added for improving the hardenability of ste
Kurebayashi Yutaka
Matsumura Yasushi
Nakamura Sadayuki
Bacon & Thomas PLLC
Daido Tokushuko Kabushiki Kaisha
Yee Deborah
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