PARTITIONING OF MICRO-ADDITIONS IN DUAL-PHASE STEELS AND STRUCTURE-PROPERTY CORRELATION

Four carbon (similar to 0.1%), manganese (similar to 1.5%) steels with or without titanium (similar to 0.1%) and boron(similar to 10 ppm) we re investigated. Plates (6 mm thick) with ferrite-pearlite microstruct ure were used as starting material and heated to 700 degrees C and 725 degrees C for intercritical annealing. The intercritical holding time was varied between 1 h and 15 h before air-cooling The plates. The st udy was primarily directed towards manganese partioning in dual phase steels in tile presence of other alloying elements. The product materi al consists mainly of ferrite and martensite phase. Extensive SEM-EDX analysis reveal that manganese could increase to as high as 6.5% in th e second phase, enhancing the hardenability of the steel. TEM studies show both lath and internally twinned martensite; the ferrite phase co ntains precipitate of alloy carbides. The tensile properties are bette r than the conventional Dual Phase Steels and there is essentially no transition in the fracture mode up to -30 degrees C.