| The paper studied the superplastic behavior﹑performance optimization andengineering applications of Mn-Si-Cr series ultrahigh strength steels systematically.The superplasticity of Mn-Si-Cr series ultrahigh strength steels was evaluated bythe strain rate sensitivity index (i.e. m value) and elongation obtained through smallspecimen thermomechanical testing and plate-shape tensile testing respectively. In orderto study the effect of microstructure evolution on superplastic characteristics duringsmall specimen thermomechanical testing, a "double m value" method was firstly putforward by introducing the strain rate change method at the initial and later stages ofwarm deformation.The paper successfully developed three technologies to obtain equiaxed and fine(ferrite+spherical carbides) duplex microstructures in Mn-Si-Cr series medium andhigh carbon steels. The microstructure evolution during the technologies andsuperplastic deformation was intensively studied. It was found that appropriateMn-Si-Cr alloying and technology improved the superplasticity of ultrahigh strengthsteels effectively. The Mn-Si-Cr series ultrahigh carbon steel could exhibit superplasticcharacteristics and high elongations (~900%at1023K). Based on repeated lab tests, wepresented a new concept of "superplastic potential" and explained the realizationmechanism of the "potential" during superplastic deformation of Mn-Si-Cr seriesultrahigh strength steels. Study also showed that a dynamic equilibrium of grain sizecould be reached at a low strain (~0.7) during warm deformation of martensite and thestable second phase in this equilibrium suppressed the growth of matrix grains. Duringwarm deformation at different temperatures, the dynamic equilibrium microstructureswith different morphologies (e.g. equiaxed and fine (ferrite+spherical carbides) duplexmicrostructures and (austenite+ferrite) duplex microstructures) were obtained. Theaustenite in the (austenite+ferrite) duplex microstructures was formed bystrain-induced transformation and could transform to martensite during cooling afterwarm deformation. As a result, an ultrahigh strength was ensured. Based on this, wepresented an idea for designing relatively simple superplastic deformation schemes.Through Mn-Si-Cr alloying, the hardenability of the designed ultrahigh strength steels was greatly improved. The hardness difference between the surface and center of1600MPa oil-quenched articulated shafts of hydraulic support in a diameter of170mmis only HRC5. The granular structure of Mn-Si-Cr series ultrahigh strength steels doesnot exhibit temper brittleness. Therefore, the tempering used for the performanceoptimization can be simplified to a great extent. In addition, combined with Vmicroalloying, Mn-Si-Cr series ultrahigh strength finished rolled rebars with relativelylow yield ratio were developed.
|
PDF Full Text Request