Research On The Behavior Of Free Carbon Atoms On ULC-BH Steel Plate

To produce ULC-BH steels with stable bake hardening prosperity, the key is to stabilize the solute C content, so it is of great significance to master the behavior regularity of free carbon atoms under different processes. The thermodynamic calculation would be done in this research focusing on typical composition, meanwhile the free carbon atoms behavior and the regularity of mechanical properties were further studied. The main conclusions were as followed:The thermodynamic calculation model was based on the precipitations of Ti N, Ti C, Nb C, Nb N and Al N, which was suitable for both the austenite and ferrite phase. By studying the influence of different elements to the content of C, N, Nb or Ti, the model could help to estimate the content of the precipitation and the solute C in the austenite and ferrite phase.With the increasing content of C in steels, the solute C content was not related to the formation speed of Cottrell atmosphere, but showed quite a linear dependence on BH2 value. When the solute C content was 33×10-3% and 2.0×10-3%, the BH2 value was 72 MPa and 42 MPa respectively after atmosphere saturation. This result demonstrated that the maximum increment BH2=30MPa was not universal.Due to the specimens prestrained 1%, 2% and 5% respectively after aging treatment, as the dislocation density increases with increase in prestrain so the distance of carbon segregation to the dislocations became shorter when the solute C content was constant. Consequently, the speed of atmosphere formation became faster and the aging increment was smaller. Besides, aging temperature and aging time could both control the speed of atmosphere formation. On the condition of a higher aging temperature, the time of atmosphere formation became shorter, however, the BH2 value of different specimens were nearly same. Compared with the solute C content about 33×10-3% and2.0×10-3%, there was an obvious yield platform after artificial aging treatment when it was 33×10-3%.Therefore, in order to ensure the natural aging resistance, the solute C content should not be too high.Calculation model was studied and microcosmic mechanism was discussed, they could help to provide important theory and technology support for the production of ULC-BH steels.