Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Patent
1987-04-03
1989-02-07
Rosenberg, Peter D.
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
420 8, C22B 400
Patent
active
048029181
DESCRIPTION:
BRIEF SUMMARY
Technical Field
The present invention relates to a high-quality case hardened steel having excellent fatigue strength, durability life, and workability for use in machine structural components of vehicles, industrial machinery, and so on; and a method of producing said steel.
BACKGROUND ART
Machine structural components must satisfy various properties including those relating to fatigue strength, durability life, workability and the like. In particular, fatigue strength is becoming increasingly important with trends toward heavier loads and higher speed together with the requirements of higher performance in industrial machinery and vehicles. Studies are being made to develop a steel having higher fatigue strength durability life, workability, and the like.
In order to improve fatigue strength, a conventional method was propsed wherein an alloying element such as Ni or Mo is added in an appropriate amount to increase the material strength. In some applications, another conventional method was proposed which uses a special smelting method such as VAR or ESR so as to control the solid texture and to reduce the amount of non-metallic inclusions.
However, in the conventional method of simply adding the alloying element, a satisfactory long durability life cannot be obtained in some applications. Again, the latter method is costly and not suitable to mass-production.
DISCLOSURE OF INVENTION
The present invention has been made in consideration of this and is based on various studies made on the influence of various alloying elements on the fatigue strength of a resultant case hardened steel. Based on such studies, it was found that the cleanliness of the steel is very important, i.e., a mere trace of oxide and sulfide inclusions considerably reduces the fatigue strength, and that other impurities impair the fatigue strength.
Based on these findings, according to the present invention, an O content is set to be 0.0010% or less which is the minimum O content that can be achieved with the current vacuum degassing refinement technique, an S conzent is set to be 0.009% or less which is considerably smaller than that in conventional steel, and the amount of impurity element P is also set to be 0.012% or less, so as to greatly reduce the amount of non-metallic inclusions in the steel, thereby obtaining an excellent fatigue strength.
Since the steel according to the present invention has a very small amount of impurities, it has excellent cold workability.
In a method of manufacturing a highly pure, case hardened steel having low oxygen, sulfur, and phosphorus contents according to the present invention, good raw materials of steel are selected and, after oxidizing refinement in an electric furnace, are poured into a ladle. The smelted steel is subjected to dephosphorization during pouring into the ladle or thereafter.
The oxide slag on the smelted steel is absorbed by a vacuum slag cleaner. A highly basic slag having a basicity of 3 or more (a reducing slag having an excellent desulfurization property such that FeO +MnO .ltoreq.0.5% (by weight) and CaO/SiO.sub.2 /Al.sub.2 O.sub.3 =0.3 to 0.4) is prepared by electric heating. Reducing refinement is performed to reduce the amount of S to 0.009% by weight or less, the amount of 0 to 0.0020% by weight or less, and the low amount of P, while bath temperature is controlled, an inert gas is introduced through double porous bricks, under the presence of the highly basic slag, and the smelted steel is strongly agitated. Subsequently, vacuum degassing is performed by a circulating vacuum degassing apparatus such that vigorous circulating is performed during 2/3 of the total treatment time while weak circulating is performed during 1/3 of the total treatment time, thereby further reducing the amounts of O, N, and H. Reducing refinement is then performed by weakly agitating the smelted steel in a reducing atmosphere at a pressure higher than normal pressure to allow minute inclusions to float and be removed. Finally, sealed casting is performed to greatly reduce the O conte
REFERENCES:
patent: 3366471 (1968-01-01), Hill
patent: 4266974 (1981-05-01), Nitta
Eguchi Jun
Ooki Takao
Aichi Steel Works Limited
Rosenberg Peter D.
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