Effects Of Initial Microstructure On Developing Martensitic And Duplex High Strength Steels

High strength steels(HSS)are complicated,highly developed materials,with precisely chosen chemical content and multiphase microstructures appearing from carefully controlled heating and cooling processes.Various strengthening procedures are selected to obtain a variety of toughness,ductility,strength and fatigue properties.Normally,a rise in strength conducts to decreased ductility or formability.The main structure of high strength steels,generally,is martensite and a combination of martensite and austenite for its transformation induced plasticity effect(TRIP).The study and development of these microstructures directly affects the final structure,which in turn influences the eventual mechanical properties of steel.The microstructure evolution kinetics can be quite different during heating and austenitization holding depending on different starting microstructures,where complicated microstructural evolution takes place,e.g.recrystallization,precipitation,austenite formation and so on,all of which can definitely affect the final microstructures and mechanical properties of the investigated steels.Thus it is necessary to investigate the effects of initial microstructures in order to optimize the state of austenite during full austenitization treatment,which is an important step for the subsequent heat treatment for obtaining high strength steels.The present thesis thus aims to reveal the relationship between initial microstructures and final mechanical properties through experimental investigations of two specific high strength steels,i.e.martensitic steel and austenite plus martensite duplex steel and give some suggestions on what initial microstructures may be chosen for desired mechanical properties.It will also be explained how the initial microstructures can affect the final microstructures and properties according to the experimental results.For martensitic steel,22MnB5 steels with modified compositions are used for model steels.Various initial microstructures were created by different annealing paths using as-received cold rolled steel sheets.The final microstructures and mechanical properties after full austenization and water quenching or die quenching for hot stamping process were compared and analyzed.The effects of Nb and V additions on the mechanical properties of 22MnB5 martensitic steel will also be studied by using two initial microstructures with and without Nb and V additions.Our results show that microstructure of recovered cold rolled ferrite dispersed with fine and dense cementite particles should be the most promising option as the initial microstructure for the development of high strength martensitic steel with excellent ductility.By optimizing the initial microstructure,the best properties reached are UTS=1875 MPa,TE=11.4%and UTSxTE greater than 21 GPa%for the martensitic steels.For the duplex(austenite and martensite)structure Ni steel(with 10 wt.%Ni)was selected as model steel and initial microstructures were created by warm rolling for the purpose of microstructure refinement and increasing yield strength.Excellent mechanical properties of UTS=1911 MPa,YS=956 MPa,TE=20.8%and UTSxTE>39.7GPa%,can be successfully obtained using proper initial microstructure and subsequent quenching and partitioning process.Details of the analysis of mechanical properties and the associated microstructures will be presented and discussed in the present thesis.