Development of cold rolled high strength low-alloy steel

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Development of cold rolled high strength low-alloy steel - 420LA for automotive structure

Wang, T.H. / Ko, H.H. / Tu, T.N.

For weight lightening, energy saving and safety requirements, China Steel Co. (CSC) has developed the highest grade of automotive structural use HSLA (High Strength Low Alloy Steel)-420LA. The strength enhanced by solid solution strengthening, grain refining by Thermomechanical Controlling Process (TMCP) during hot rolling, and appropriate annealing design. C, Mn, P, Si solid solutions provide the basic strength of HSLA. Complex precipitates and grain refining further improved the mechanical property. With the control of rough and fine rolling temperature above and below T(r) (Recrystallization stop temperature) respectively during hot rolling, grain refining effect and (Ti-Nb-V) (C, N) precipitates which induced by massive strain increase the mechanical strength and exhibit good elongation. According to the experiment results, increasing TiN ratio by combined Ti-Nb-V addition reaches the best mechanical performance. Besides, the annealing temperature is designed to be lower for thicker steel strips and the products with longitudinal direction mechanical property assurance, avoiding lower cold reduction may decrease the strength by larger ferrite grain size after annealing. In addition, Ca-Si addition during secondary refining improved the cleanliness by reducing the amount of elongated-type MnS precipitates. With the metallurgical design, the average YS, EL% are 440MPa, 23-25%, and toughness (TS x EL%) is also promoted, which lead to a steel grade that can absorb more impact energy and better resistance against crash.