Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Patent
1993-08-26
1995-07-11
Yee, Deborah
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C21D 800
Patent
active
054317533
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an austenitic high manganese steel which is used in fields requiring a high formability such as automobile steel sheet, electronic panel sheet, and the like. Particularly the present invention relates to an austenitic high manganese steel having a good formability, high strengths and superior weldability.
BACKGROUND OF THE INVENTION
In the application field of steel, those which require best formability are automobile steel sheets, and electronic panel sheets.
Particularly, in the automobile industry, the discharge of carbon dioxide is more strictly regulated coming recently for alleviating the air pollution. In accordance with this trend, there has been demanded a high strength steel sheet which has a good formability, as well as improving the combustion rate of the fuel, and reducing the weight of the automobile.
Conventionally, as the automobile steel sheet, a extra low carbon steel in which the matrix structure is a ferrite has been used for assuring the formability (U.S. Pat. Nos. 4,950,025, 4,830,686 and 5,078,809).
However, in the case where the extra low carbon steel is used for the automobile steel sheet, although the formability is superior, the tensile strength is lowered to 28-38 kg/mm.sup.2. Consequently the weight of the automobile cannot be reduced, and the safety of the automobile is lowered, thereby jeopardizing the lives of passengers.
The extra low carbon steel having the fenite matrix ferrite can include up to 0.005% of carbon, and the solubility limit for impurities is very low. If carbon and other impurities are added in excess of the solubility limit, then carbides and oxides are formed, with the result that particular textures cannot be developed during cold rolling and annealing processes, thereby degrading the formability.
Thus, in the case of the conventional automobile steel sheet having the fenite matrix, the addition of carbon is reduced to about 0.003%, as well as reducing other impurities to extremely small amounts for enhancing the formability. Consequently, there are accompanied difficulties such that special treatment such as degassing treatment has to be carried out in the steel making process, and that particular textures have to be developed during cold rolling and annealing processes.
Further, a multi-phase steel in which the low strengths of the extra low carbon steel are improved is disclosed in U.S. Pat. No. 4,854,976. In this steel, Si, Mn, P, Al and B are added in large amounts to form a bainite structure and retained austenite structure of less than 8%, thereby increasing the tensile strength to 50-70 kg/mm.sup.2. However, due to the difference of the deformation capabilities between the bainite structure and the retained austenite structure, the formability is lowered, and therefore, this material is limitedly used in automobile parts which do not require a high formability.
Meanwhile, the steel sheet which is used as the external panel of electronic apparatus has to be non-magnetic material which is not influenced by magnetic fields, as well as being high in its strengths and formability. Therefore, austenitic stainless steel is mainly used for this purpose, but this steel contains expensive nickel to about 8%, while its magnetic susceptibility becomes unstable due to strain-induced .alpha.'-martensites during its manufacturing process.
The present inventors have been engaged for many years in studying on how to overcome the disadvantages of the conventional automobile steel sheet and the electronic steel sheet, and have successfully developed an austenitic high manganese steel having superior formability and strengths.
So far, no case has been found in which a high manganese steel is used to attempt providing good formability and high strength.
Currently, the high manganese steel is used in nuclear fusion reactor, in magnetic floating rail for the purpose of preventing electrostatic charges, and as non-magnetic structural material for transformers (Japanese Patent Laying-opening No. Sho-63-35758, 64-
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patent: 4847046 (1989-07-01), Kim et al.
patent: 4854976 (1989-08-01), Era et al.
patent: 4956025 (1990-09-01), Koyama et al.
patent: 4968357 (1990-11-01), Liu
patent: 5078809 (1992-01-01), Kinoshita et al.
Chang Rae W.
Han Jae K.
Kim Tai W.
Kim Young G.
Pohang Iron & Steel Co. Ltd.
Research Institute of Industrial Science & Techology
Yee Deborah
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