Abstract
The employment of the transformation structure by the (α+γ) intercritical annealing was investigated for achieving advantageous decrement of yield ratio (yield strength/tensile strength) in TS 440MPa grade extra low carbon IF steel with the solution hardening by preferential P addition. The transformed microstructure obtained from dual phase annealing induced remarkable low yield ratio even in the high P-added steel (0.10%P-1.5%Mn-0.5%Si). This phenomena, associated with decrease of yield point elongation of as-annealed steel is attributable to the formation of the low temperature transformed phase, which is classified as the granular bainitic ferrite (αB) or bainitic ferrite (αOB) with high dislocation density. It can induce the much movable dislocation in ferrite matrix around the second phase. Low yield ratio was obtained even in the high Mn steel (0.04%P-2.2%Mn-0.4%Si). However, progress of γ transformation exhibited considerable increase of both yield and tensile strengths predominantly, which is caused by extremely high distribution of Mn in γ phase. Addition of B also affects the yielding behavior in the intercritically annealed steel. This can be explained by the contribution of B addition on the Mneq. formula.
