| The present study investigated the effect of N addition on the strength characteristics of 0.1%C-1%Mn-0.5%Mo-0.02%Ti fire-resistant steel for construction with low yield ratio. 0.1%C-1%Mn-0.5%Mo-0.02%Ti(A) and 0.1%C-1%Mn-0.5%Mo-0.02%Ti-0.015%N(B) specimens showed bainite phase in matrix of ferrite and pearlite phase, the addition of N suppressed the transformation of pearlite and accelerated transformation to bainite. Yield strength(Y.S) ratio, yield and tensile strength of B specimen was higher value than those of A specimen, but yield ratio(Y.R) value was lower. In the two specimens treated by air cooling or quenching and tempering after rolling, yield strength and Y.S ratio decreased slowly with increasing test temperature, and rapidly decreased at over 600℃. But Y.S ratio at 600℃ was higher than 70%. In case of specimen tempered after quenching, yield and tensile strength were higher than that of air cooling specimen but Y.S ratio was lower. With increasing finish rolling temperature, volume fraction of ferrite and pearlite phase was decreased, and volume fraction of bainite and Y.S ratio was rapidly increased, while Y.R value was unchanged. In two specimens, Y.S ratio was strongly controlled by volume fraction of second phase, but Y.R value and yield strength were affected by other factors than the volume fraction of second phase. |
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