Investigation On Strengthening-toughening Mechanism Of Advanced High-strength Steels

Demanding requirements of light-weight structure and emissions reduction drives continuously improvement in advanced high-strength steels (AHSS), which have played an irreplaceable role in modern industries. Among various types of AHSS, metastable austenitic stainless steel and high-strength low-alloy steel (HSLA) have exceptional combination of high ultimate tensile strength and ductility.Although the ultimate strength of metastable austenitic stainless steels can reach 1 GPa, the yielding strength is generally as low as ～300 MPa in the annealed state, that has limited their potential in industries. In this work, we report that a thermal mechanical treatment, i.e. warm rolling, can effectively enhance the yielding strength of the metastable austenitic stainless steel, while retaining both the formability and hardenability. It is shown that this evasion of strength-ductility trade-off is due to the decoupled strengthening and toughening mechanisms in metastable austenitic stainless steels. The strengthening effect originates from pre-stored dislocations by warm rolling while the toughening mechanism relies on the capability to nucleate new dislocations to carry plastic deformation, which are emitted from dispersed martensite. Following this guideline, other methods may be developed to enhance material performance in metal manufacturing. The underlying mechanism has been carefully studied by in situ transmission electron microscopy, revealing the various interactions between the microstructures and the martensite phase transformation.In addition, welding is necessary in building complex structures for HSLA steels. The microstructures of different regions in welded joints determine the strengthening-toughening characteristics of specimens. The correlation between the microstructures and the strengthening-toughening and fatigue properties of 10Ni5CrMoV steel welded joints has been systematically investigated. It is concluded that high strength steels are more sensitive to defects formed during welding. Defects with size less than 0.5 mm should be considered in safety assessment in welded joints of HSLA steels.