HIGH-STRENGTH LOW-CARBON SHEET STEEL BY THERMOMECHANICAL TREATMENT .1. STRENGTHENING MECHANISMS

Considerable attention has been focused recently on the development of higher strength automobile stampings. Traditionally increasing the strength of a stamping has been accomplished by utilizing a higher strength material with its attendant problems in springback, die wear and press loads. An alternative approach to high strength stampings based on thermomechanical (TMT) processing of a low carbon steel is described. With this approach, the contributions to strength due to work hardening during stamping and age hardening after stamping are incorporated into the overall processing scheme. The TMT process consists of heating a low carbon steel to the two phase (α + γ) region and quenching to produce a dispersion of martensite in a ferrite matrix. A second, low temperature heat treatment is then employed to improve the total elongation to acceptable levels. The processed steel is then stamped and subsequently aged at either ambient or elevated temperature to develop final strength. The relative contributions of the individual operations of the TMT process to final strength are discussed and the required heat treatment parameters are evaluated in terms of the operative strengthening mechanisms. © 1979 American Society for Metals and the Metallurgical Society of AIME.