Effects Of Controlled Rolling And Controlled Cooling On The Microstructure And Properties Of High-strength Ti-Zr Microalloyed Stee

Microalloying is a common and effective means to improve the comprehensive mechanical properties of steel materials.Compared with microalloying elements such as Nb,and V,Ti microalloying technology has very good economic value and application prospects.In order to improve the combination of Ti with O,N,S and reduce the consumption of Ti,stabilize the precipitation of Ti C,and play the role of fine grain strengthening and precipitation strengthening,adopts Ti-Zr composite microalloying combined with controlled rolling and cooling technology to research the effect of controlled rolling and controlled cooling process on the structrue as well as properties of Ti-Zr steel,and explore the preparation process of Ti-Zr microalloyed low carbon steel plate with good comprehensive mechanical properties.In this paper,the equilibrium solid solution product equations and interfacial energy equations for Zr C and Zr N in ferrite are derived by using relevant thermodynamic and kinetic theories,and the precipitation of(Ti,Zr)(C,N)in austenite and ferrite is calculated to provide theoretical guidance for the development of the controlled rolling and cooling process of Ti-Zr steel.The effect of final rolling temperature on the austenite structure and precipitation of Ti-Zr steel was investigated by thermal simulation experiments.The effect of post-rolling cooling method and coiling temperature on the final structure and mechanical properties of Ti-Zr steel was analyzed by laboratory rolling experiments,and the strengthening and toughening mechanism of high-strength and high-toughness Ti-Zr steel was discussed.The calculation results show that the addition of Zr at high temperature stage(above1000°C)will reduce the critical nucleation size of carbonitrides precipitating from austenite and increase the nucleation rate,so as to refine the precipitation in austenite and hinder the growth of austenite grain.In addition,Ti will solid solute in austenite with the addition of Zr(below 950 ℃),delay the nucleation of carbonitride particles in Ti microalloyed steel in austenite,promote the precipitation from ferrite at low temperature,and obtain more small-size precipitates.The results of thermal simulation experiments under different final rolling temperature show that in the temperature range of 850℃～950℃,the average austenite grain size of Ti steel and Ti-Zr steel first increases and then decreases,with the increase of final rolling temperature.And the addition of Zr can refine the austenite structure of Ti microalloyed steel during the controlled rolling process and delay and refine the deformation induced precipitation of Ti microalloyed steel during deformation.The experimental results of rolling steel show that isothermal treatment after rolling can significantly improve the strength of Ti-Zr steel,promote the precipitation of carbide and reduce the content of granular bainite(GB)and acicular ferrite(AF)in the matrix of Ti-Zr steel.In the temperature range of 600℃～700℃,the strength of Ti-Zr steel decreases with the increase of coiling temperature,the precipitation decreases,and the content of acicular ferrite in the matrix decreases and the content of polygonal ferrite(PF)gradually increases.The addition of Zr refines the ferrite organization and precipitation of Ti microalloyed steel,promotes the precipitation of fine carbon nitride in the ferrite,improves the strength of Ti microalloyed steel by combining precipitation strengthening with fine grain strengthening,and improves the toughness of Ti microalloyed steel by using the refining and spheroidizing effect of Zr on inclusions.When final rolling temperature of Ti-Zr steel is 850℃,the laminar flow is cooled to 600℃for 60 mins and cooled with the furnace,the high-strength and high toughness low-carbon steel plate with yield strength of more than 700 MPa,elongation of 20% and impact absorption energy of more than 140 J at-20℃ can be prepared.