Thermodynamic Analysis and Effect of Precipitates on Hot Ductility of Nb-Ti Microalloying Ultra-Low Carbon Steel

• Published in: Teshugang
• Main Authors: Zeng Yanan, Sun Yanhui, Ma Zhifei, Xu Rui, Ai Xi
• Format: Article
• Language: Chinese
• Published: Editorial Office of Special Steel 2014-09-01
• Online Access: http://www.specialsteeljournal.com/thesisDetails?columnId=129792286&Fpath=home&index=0

Based on double sub-lattice model, a compound thermodynamic model of (Nb_xTi_1-x) (C_yN_1-y) is established ,and the mol fraction, atom site ratio and precipitation sequence of each ingredient in precipitated phase of Nb-Ti microalloying ultra-low carbon steel (/% ：0.02C, 0.12Si, 1.70Mn, 0.012P, 0.004S, 0.101Nb, 0.009Ti, 0.010Als) at 1023-1623 K have been calculated. The effect of precipitates on hot ductility of 230 mm casting slab of steel is studied and the established thermodynamic model is verified with test by using Gleeble thermal simulator, transmission electron microscope and energy dispersive spectrometer. Results show that at 1523 K the solute mol fraction of Nb and Ti in steel are respectively 5.4 x 10^-4and 3.87 x 10^-5with temperature decreasing to 1023 K the solute content of Nb and Ti tends to zero. With decreasing temperature the Ti and N atom site ratio gradually decreases while the Nb and C atom site ratio gradually increases, the sequence of evolution of precipitates is Nb_0.315Ti_0.685C_0.02N_0.98(Nb_xTi_1-x) (C_yN_1-y) , Nb_0.85Ti_0.15C_0.71N_0.29the calculated values are coordinate with experimental results. With precipitates size less than 60nm and number of particles more than 5 per/μm²the hot ductility o£ casting slab decreases obviously； at 1241 K the tensile strength of steel is 63.8 MPa to easily form cracks； meanwhile it is found at fracture of Gleeble specimen that Al, Si, Mn, Nb and Ti concentrate at grain boundaries, and the caves caused by carbo-nitride lead to formation of cracks at action of stress. Therefore during casting process the straightening temperature of casting slab should be ≥ 1241 K.