EFFECT . OF MARTENSITE MORPHOLOGY ON THE MECHANICAL PROPERTIES OF DUAL PHASE STEEL

Abstract

An attempt has been made to develop dual phase (d-p) steels from cold-rolled plain carbon sheet steels. The plain carbon sheet steel is of 1 mm thickness and the chemical composition(wt%) is C-0.06, Si-0.04, Mn-0.5, S-0.03 and P-0.03 respectively. Attempt has been made to develop dual-phase microstructure from different paths in this work, which has not been attempted together yet. This sheet steels are given heat-treatment in four paths i.e. ARDP, ARyDP, NRDP and NR7DP to get ARDPs, ARyDPs, NRDPs and NRyDPs dual-phase steels. Intercritical heat-treatment is given at 740°C and holding it for 1, 2, and 3 min. respectively for each heat-treatment path followed by water quenching. The tensile samples have been cut from the sheet steel along the rolling direction with the help of die. Then these samples are used for initial heat-treatment i.e annealing, normalizing. These as-received and normalized samples are used for further heat-treatment to get dual-phase microstructure through different paths. TENSOMETER is used in the present investigation for tensile testing. The load versus elongation curves obtained from the test have been replotted into engineering stress-strain curves. Tensile properties i.e. yield strength, ultimate tensile strength, uniform elongation,and total elongation. SEM on tensile test fractured surfaces has been done to find out which type of fracture surface it is showing and the extent of ductile fracture. A good combination of strength and ductility of d-p steels has been observed in 2 min. generally, in all the four paths i.e. ARDP, ARyDP, NRDP and NR7DP. The best combination of strength and ductility has been found in the heat- treated cycle (path) ARDP2 where YS is 490 MPa, UTS is 652, and total % elongation is 16 percent.