Development of Hot Rolled High Carbon Steel Sheets with Excellent Formability

by Nobusuke Kariya, Nobuyuki Nakamura, Kazuhiro Seto, Takeshi Fujita, Yasushi Tanaka, Shunji Iizuka

Publisher - JFE Steel Corporation, Keihin, Japan

Category - General Novel

High carbon steel sheets used as automotive power train parts are formed in various complex shapes. Therefore, excellent formability is required for forming. In particular, stretch flange formability and high deformation capacity (local ductility) during heavy forming are essential. JFE Steel has developed formable high carbon steel sheets suitable for one piece forming of automotive power train parts. Newly developed hot rolled high carbon steel (Hyper-Burring SC) has excellent stretch flange formability due to fine and uniform dispersion of spheroidized cementites. The fine microstructure was realized by applying extremely rapid cooling after hot rolling. By controlling extremely rapid cooling, upper bainite and/or degenerate pearlite without proeutectoid ferrite and lamellar pearlite was formed, and the microstructure composed of fine and uniform dispersion of spheroidized cementites was formed after the subsequent spheroidizing. The hole-expanding ratio (HER) of the steel, which is an index of stretch flange formability, is superior to that of conventional high carbon steel sheets. The developed steel is expected to be widely used in automotive power train parts and has been applied, for example, to hub/drum parts of automatic transmission.

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