Strain Hardening Dependence on the Structure in Dual-Phase Steels

Herein, an extensive study is presented on the microstructure–tensile properties relationship in dual-phase (DP) steels. A series of ferrite-martensite DP steels with varied martensite volume fractions (Vm) from 0.17 to 0.86, microstructure morphologies (globular and elongated) and structure finenesses (ferrite grain sizes from 1.9 to 10.7 μm) are produced applying appropriate heat treatments. The tensile properties are studied, and the strain hardening behavior is analyzed in terms of Holloman, Crussard–Jaoul (C–J) and modified C–J approaches. The tensile curves reveal up to three strain hardening stages with the highest strain hardening exponent at the beginning of straining. Increasing Vm and refining the structure raises the number of strain hardening stages and improves the strain hardening capacity in the first stage (n1). For the DP steels with similar morphologies, the mean free path in ferrite (λf) is proposed to be the most significant microstructure factor affecting n1-value. The n1 of the elongated morphology shows stronger dependence on λf than the globular one. Finally, the DP steels are subjected to aging treatments, which lead to improved yield strength and total elongation, however, the strain hardening exponent decreases significantly.