INFLUENCE OF THERMAL TREATMENTS ON STRUCTURE AND PROPERTIES OF FERRITE - MARTENSITE STEEL

Abstract

The main purpose of this thesis is to study the effect of different heat treatments on structure and properties of ferrite-martensite (0.15 % C) dual phase (DP) steel. Two different kinds of DP steel were produced from the as received low carbon hot rolled steel with ferrite/pearlite microstructure using two different techniques, i.e., direct quenching (DQ) and intermediate quenching (IQ). DQ and IQ heat treatments included normalizing and hardening respectively of the as received steel at 950 oC then intercritical heat treatment between 740 and 800 oC then quenching in ice cooled water. The martensite volume fraction of the DQ dual phase steel increased from 16.3 % to 42.6 % when the intercritical heat treatment temperature increased from 740 oC to 800 oC. Hardness of DP steel was studied. It was found that hardness of either DQ or IQ dual phase steel (242 and 270 VH10 respectively) is higher than that of the as received steel (145 VH10). Tensile properties of DP steel were studied. Stress–strain curves of DQ and IQ dual phase steel showed a continuous yielding behavior unlike the normalized steel that showed a yield point. Besides continuous yielding, DQ and IQ dual phase steel have higher yield and ultimate tensile strength, initial rate of work hardening, but lower elongation and yield to ultimate tensile strength ratio than the normalized steel. Effect of testing temperature from 25 - 500 oC on the tensile properties of IQ -760 oC dual phase steel was studied. Stress – strain curves showed a continuous yielding behavior at all temperatures except at 300 - 400 oC due to dynamic strain aging happened. Effect of tempering from 100 to 500 oC on structure and properties of IQ-760 oC DP steel was studied. The effect of banded structure on room temperature impact toughness of DP steel was studied. It was found that the impact toughness of DQ and IQ dual phase steel both intercritically heat treated at 760 oC has greatly increased after the removal of banding to be 95 and 114 J respectively after being 10 and 25 J in the presence of banded structure.