Alloys or metallic compositions – Ferrous – Chromium containing – but less than 9 percent
Patent
1995-08-09
1996-11-19
Yee, Deborah
Alloys or metallic compositions
Ferrous
Chromium containing, but less than 9 percent
148580, 148908, C22C 3846, C22C 3848, C21D 902
Patent
active
055759734
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a coil and leaf spring steel for suspending automobiles, and a manufacturing process therefor, in which the steel has superior mechanical properties and superior spring characteristics. Particularly the present invention relates to a high strength high toughness spring steel and a manufacturing process therefor, in which the decarburized layer formed during the heat treatment for improving the spring characteristics is significantly reduced, so that the fatigue properties and the sag resistance (deformation resistance) should be improved.
BACKGROUND OF THE INVENTION
Coming recently, as a measure against the environmental pollution and the atmospheric temperature rise, environmental projects relating to the combustion ratio are being worldwidely carried out. Accordingly, the automobile industries have been making efforts to reduce the weight of automobiles. In reducing the weight of automobiles, the automobile suspension spring is an important factor. However, the suspension spring has a considerable weight, and therefore it has become the object of the concern.
If the suspension spring is made to be light, the spring has to have a high stress capacity. The high stress capacity is achieved through the improvement of the fatigue strength and the sag resistance. If the fatigue strength is lowered, the fatigue life expectancy is shortened, thereby leading to an early breaking of the spring. If the sag resistance is small, the spring cannot support the automobile body, thereby leading to the contact of the automobile body with the bumper. Therefore, in order to overcome such problems, the sag resistance characteristics of the spring should be improved.
In accordance with such a trend, an Si added steel which has a superior sag resistance characteristics compared with the existing SAE6150 (Cr-V) alloy came to attract the attention of the industry. Of it, SAE9260 (1.8-2.2% Si, SUP7) has been developed into a superior material. However, it has the problems such as the shortening of the fatigue life expectancy due to the surface decarburization and the high cost for peeling the surface. In order to overcome such problems, SAE9254 was developed in which the Si content was reduced for preventing the surface decarburization without much aggravating the sag resistance, and in which Cr was added for reducing the decarburization. However, in view of the light weight of automobile which is presently emphasized, the sag resistance of SAE9254 is not satisfactory. Then a material (SRS60) having a more superior sag resistance property was proposed in Japanese Patent Publication No. Sho-57-27956, Sho-57-169062 and Sho-57-13148. This material is formed by adding a small amount of vanadium (V) into SAE9254 so as to improve the sag resistance property. This SRS60 achieved the improvements of the sag resistance property and strength, but it made no contribution to the improvement of the decarburization and the toughness.
Meanwhile, the stress capacity of springs has reached the limit owing to the development of the process and the improvement of the precision. Therefore the remaining task is to develop a high stress capacity spring material having a superior sag resistance capacity, a high elastic limit, a high fatigue strength and a high toughness even under a high stress.
In the case of the suspending spring, if the maximum design stress is improved from the conventional 110 kg/mm.sup.2 to 130 kg/mm.sup.2, the weight of the spring can be reduced by 25%.
Therefore, if the weight of spring is to be reduced, a spring material having a superior sag resistance characteristics is required. In this context, the addition of silicon is required, but in this case, the decarburization problem occurs during the hot rolling process and during the heat treatment. Further there is the problem that the high strength is accompanied by a low toughness. These problems have to be solved first of all.
The conventional techniques for inhibiting the decarburization are disclosed in Japanese P
REFERENCES:
patent: 4544406 (1985-10-01), Yamamoto et al.
patent: 4711675 (1987-12-01), Yamamoto et al.
patent: 4795609 (1989-01-01), Sakas et al.
patent: 5009843 (1991-04-01), Sugimoto et al.
patent: 5286312 (1994-02-01), Shimotsusa et al.
Bark Soo D.
Choi Hae C.
Choi Jong H.
Choi Jong K.
Kim Jang G.
Pohang Iron & Steel Co. Ltd.
Research Institute of Industrial Science & Technology
Yee Deborah
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