Quantitative evaluation of the contribution of carbide-free bainite, lath martensite, and retained austenite on the mechanical properties of C-Mn-Si high-strength steels

The third generation of advanced high-strength steels usually employs a multi-phase microstructure to obtain excellent comprehensive mechanical properties. Though the effects of phase morphology and content on the mechanical properties have been extensively studied, the quantitative contribution of the different phases has received little attention. In this study, the contributions of carbide-free bainite (CFB), lath martensite (LM) and retained austenite (RA) to the mechanical properties were quantitatively evaluated. A bainitic austempering treatment with different holding time followed by a quenching & partitioning treatment was designed for a CMn-Si steel. Different samples with almost the same content of retained austenite (RA), a sequentially higher content of CFB and a sequentially lower content of LM, i.e., B0-Q&P, B13-Q&P and B45-Q&P samples were obtained. With the increase of bainite content and decrease of martensite content, the yield strength and impact toughness gradually decrease, while the elongation gradually increases. The stresses at which the CFB, LM and RA begin to yield under strain were determined to be 728 +/- 78 MPa, 1300 +/- 70 MPa and 720 +/- 43 MPa, respectively. Strain hardening analysis of the samples showed that when the uniform plastic deformation is dominated by CFB, LM and film retained austenite (RAf), respectively, the corresponding strain hardening indexes are 0.27, 0.15 and 0.61. The yield strength of the B0-Q&P, B13-Q&P and B45-Q&P samples decreases and the elongation increases in order, which is mainly attributed to the increasing CFB content and decreasing LM content in turn. The impact toughness of the B0-Q&P, B13-Q&P and B45-Q&P samples decrease in order, which is mainly attributed to the successive increase of the unfavorable blocky retained austenite (RAb) for toughness.

Automated summary

In this study, the quantitative contribution of carbide-free bainite (CFB), lath martensite (LM) and retained austenite (RA) to the mechanical properties was evaluated. The increase of bainite content and decrease of martensite content lead to a decrease in yield strength and impact toughness, while an increase in elongation. The increasing content of CFB and decreasing content of LM mainly contribute to the observed changes in yield strength, elongation, and impact toughness.