LOWERING OF THE YIELD RATIO IN SOLUTION H ARDENED INTERSTITIAL-FREE STEEL BY EMPLOYING TRANSFORMATION MICROSTRUCTURE

The employment of the transformation structure by the (alpha + gamma) intercritical annealing was investigated for achieving advantageous de crement of yield ratio (yield strength/tensile strength) in TS 440MPa grade extra low carbon IF steel with the solution hardening by prefere ntial P addition. The transformed microstructure obtained from dual ph ase annealing induced remarkable low yield ratio even in the high P-ad ded steel (0.10%P-1.5%Mn-0.5%Si). This phenomena, associated with decr ease of yield point elongation of as-annealed steel is attributable to the formation of the low temperature transformed phase, which is clas sified as the granular bainitic ferrite (alpha(B)) or bainitic ferrite (alpha(B)(o)) with high dislocation density. It can induce the much m ovable dislocation in ferrite matrix around the second phase. Low yiel d ratio was obtained even in the high Mn steel (0.04%P-2.2%Mn-0.4%Si). However, progress of gamma transformation exhibited considerable incr ease of both yield and tensile strengths predominantly, which is cause d by extremely high distribution of Mn in gamma phase. Addition of B a lso affects the yielding behavior in the intercritically annealed stee l. This can be explained by the contribution of B addition on the Mneq . formula.