Abstract
This article introduces the microstructural and mechanical properties of low and medium-carbon advanced martensitic steels (AMSs) subjected to heat-treatment, hot- and warm- working, and/or case-hardening processes. The AMSs developed for sheet and wire rod products have a tensile strength higher than 1.5 GPa, good cold-formability, superior toughness and fatigue strength, and delayed fracture strength due to a mixture of martensite and retained austenite, compared with the conventional martensitic steels. In addition, the hot- and warm-stamping and forging contribute to enhance the mechanical properties of the AMSs due to grain refining and the improvement of retained austenite characteristics. The case-hardening process (fine particle peening and vacuum carburization) is effective to further increase the fatigue strength.
Highlights
The strain-induced transformation of austenite to martensite enhances the ductility of austenitic steels such as Fe-Ni, Fe-Ni-C, and Fe-Cr-Ni steels
The advanced martensitic steels (AMSs) sheets can be produced by lower heat-treatment temperature than the third-generation AHSSs (Type A) with bainitic ferrite/martensite (BF/M) structure matrix
As they possess the same excellent cold formability as the third-generation AHSSs (Type A), some applications to the automotive frame members can be expected in the future [153,154,155]
Summary
The strain-induced transformation of austenite to martensite enhances the ductility of austenitic steels such as Fe-Ni, Fe-Ni-C, and Fe-Cr-Ni steels. Third-generation AHSS (Type A): TRIP-aided bainitic ferrite (TBF) steel [9,10,29,30,31,32,33], one-step and two-step quenched and partitioned (Q&P) steels [7,8,9,25,34,35,36,37,38,39,40,41], carbidefree bainitic (CFB) steel [42,43,44,45,46,47,48,49], and duplex type medium manganese (D-MMn) steel [9,25,50,51,52,53,54,55,56,57]. The improvement mechanisms of the mechanical properties are detailed by relating to the matrix structure, the strain-induced transformation behavior of metastable retained austenite, and/or the martensite/austenite constituent (MA phase)
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