Effect of Nb and Mo Addition on the Mechanical Properties of Hot Rolled TRIP-Aided Steel Sheets

摘要

In order to meet the automobile industry's need for weight reduction and safety improvements, the application of advanced high strength steel sheets such as TRIP (Transformation induced plasticity)-aided steel and DP (Dual Phase) steel with tensile strength of 780 and 980 MPa have been examined for suspensions and structural parts. It is well known that the higher the strength of steel sheets, the poorer the press formability Conventional HSLA steels could not satisfy the press formability for such high strength applications due to its low ductility. TRIP effect is the most successful mechanisms for increasing the strength of steels without deteriorating the strength and elongation balance. The formability of TRIP-aided steel primarily depends on the amount and stability of retained austenite. However the steel sheets containing high carbon and high alloying elements can not be applied for automobile parts due to their low weldability, low surface condition and high production cost. Hence, carbon content must be limited to less than 0.2% and Si, Mn and Al are limited to less than 2% respectively, which resulted in the difficulty to the production of 780 or 980 MPa steel. Although many research results on TRIP-aided steel produced by heat treatment after cold rolling have been reported, there were few reports on hot rolled TRIP-aided steel, and those that published were limited to relatively low strength levels.Based on the above mentioned back ground, the development of 780 Mpa grade TRIP-aided steel was carried out by taking into account of the addition of Nb and Mo and coiling conditions after hot rolling.