STRUCTURE PROPERTY CORRELATION IN DUAL-PHASE STEELS

Abstract

Dual-phase (d-p) steels are characterised by a micro structure consisting of a dispersion of about 20% martensite in a ductile, ferrite matrix. Dual phase steels. have good combination of strength and ductility like composites and formability approaching that of plain carbon steels. A lot of work have been done on alloyed d-p steels but very limited on plain carbon d-p steels. In present work mechanical properties of plain carbon d-p steels produced from cold-rolled plain carbon steel sheets by different intercritical heat treatment cycles are correlated to their microstructure. For this, three plain carbon steels sheets having 0.075% carbon (steel II, 0.095% carbon (steel IT) and 0.2% carbon (steel III) are used. D-p steels have been produced from cold rolled steel sheet of above mentioned composition by intercritical annealing at 74000 for a series of intercritical annealing time i.e. 0.5 mints, 1 mints, 1.5 mints, 2 mints, 3 mints, 4 mints, 5 mints, 7 mints, 8 mints, 10 mints, 15 mints, and 30 mints followed by water quenching. Ductile fracture have been observed in all the tensile test specimens of d-p steel I, steel II and stell III. Martensite volume fraction (MVF) has been found to increase with increase in intercritical annealing time for all the three d-p steels samples. Further, with increase in carbon content, in the steel MVF has found to increase. For the heat treatment cycle 740°C, 0.5 min, WQ, d-p steel-I has MVF d-p steel -II has MVF 11% and d-p steel-III has MVF l4%. With increase in intercritical annealing time, strength and ductility have been found to decrease. Sincerely, strength and ductility have also been found to decrease with increase in MVF. Variation in hardness values with time and MVF has followed the same trend as UTS. The best mechanical properties have been for heat treatment cycle 740°C, 0.5 min, WQ. At this steel-I shows UTS of 560 MPa with uniform elongation (ductility) 16%, steel—II shows UTS of 595 MPa with uniform elongation 14% and steel-III shows UTS of 628 MPa with uniform elongation 13%. Steel-I shows optimum mechanical properties among all the samples for heat treatment cycle 740°C 0.5 mints WQ. The above results have been discussed on the basis of recrystallisation of cold-worked ferrite grains and kinetics of austenite formation from cold worked pearlite and ferrite mixture.