Deng Xiangtao

Precipitation Behavior of Nanometer‐Sized Carbides in A Nb‐Ti‐Bearing Low‐Carbon Steel

The microstructure, mechanical property and precipitation behavior in a low carbon Nb-Ti micro-alloyed steel were investigated using dilatometer, optical microscopy and transmission electron microscope. The results show that the microstructure of the experimental steel treated by isothermal quenching process mainly consists of ferrite and martensite. The volume fraction of ferrite increases with a decrease in the isothermal temperature. It is found that both of interphase precipitation and supersaturated precipitation would appear in the samples treated by isothermal quenching process. Along with the isothermal temperature decreasing, the precipitation state changes from interphase precipitation to supersaturated precipitation. The interphase precipitation of these carbides with different row spacing and different orientation in ferrite grains, is related to the mobility of interfaces during γ/α transformation based on ledge mechanism. In addition to {110}α plane suggested by the ledge mechanism, the planar sheets of interphase precipitation are also found to be parallel with{035}a planes. Moreover, the interphase precipitation carbides were identified to have a NaCl-type crystal structure with a lattice parameter of 0.432 nm and obey the Baker-Nutting (B-N) orientation relationship with respect to ferrite matrix. The contribution of the precipitation hardening to the yield strength of the experiment steel has been estimated to be ~337 MPa at 620 °C, based on Orowan mechanism.

Product Design and Production Practice of 700MPa High Strength Hot Rolled Strip for Auto Axle Tube

According to the technical specifications of 700MPa high strength automotive axle tube steel, a low cost of 0.07%C+1.5%Mn+0.05%Nb+0.10%Ti was designed. The high strength mainly relies on grain refinement strengthening and precipitation strengthening. The recrystallization, precipitation, and CCT curves of the 700MPa grade axle tube steel were studied in order to determine a reasonable TMCP process. By controlling the low level segregation band, low level of C and N content, 700MPa grade high strength automotive axle tube steel is successfully developed with excellent mechanical property, welding property, flattening and flaring property, torsion fatigue property, static torsional property and surface quality.