Research Of Microstucture And Precipitate Control In Nb-Ti Micro-alloyed Low-carbon Steel

In high-strength low-carbon microalloyed steels, micro-alloyed carbides in ferrite or bainite have significant precipitation strengthening effect. The new generation TMCP, with ultra fast cooling (UFC) technology as the core, has obvious advantages in precipitate control, but the past studies are mostly based on conventional cooling methods. Thus, the research of precipitation principle under the UFC condition has great necessity. In this paper, the equilibrium precipitates of micro-alloying elements in Nb-Ti steel and Ti steel were calculated, the continuous cooling transformation process was investigated, and the effects of UFC and the cooling process after UFC on the structure performance and precipitation behavior were study. The main contents are as follows:(1) By the Thermo-Calc thermodynamic calculation software, the equilibrium precipitates of micro-alloying elements, as well as the element contents of the precipitates, with the change of temperature were calculated. The results showed that the precipitates at each temperature have a coexist feature that precipitates mostly have a main precipitate with two elements and a complex precipitate with the other elements. With the decrease of temperature, the main precipitate in Nb-Ti steel is TiN, NbC or VC in order, and the complex precipitate is (Nb, V, Ti) (CN).(2) By the thermal simulation machine, the effect of cooling rate, Nb and deformation on the austenite phase transformation behavior during its continuous cooling process was investigated. The results showed that experimental steel has a wide bainite transformation area; Nb makes the overall CCT curve moved to the right, which greatly reduces the eutectoid ferrite and pearlite transformation aera, while expands the acicular ferrite and granular bainite transformation area. Thus the Nb-alloyed steel can get more acicular ferrite at a lower cooling rate; Austenite deformation has the ferrite and pearlite transformation promoted, but has a dual effect on bainite transformation, that is, bainite transformation is suppressed in low cooling rate and promoted in high cooling rate.(3) By the thermal simulation machine, the effects of isothermal temperature and isothermal time of the isothermal and quenching process under the UFC condition on phase transformation and precipitation behavior in Nb-Ti steel were investigated. The results isothermal at ferrite transformation aera showed that when isothermal at 660℃ the transformed martensite distributes more uniformly and the precipitates have a larger quantity, therefore a higher hardness is possessed. When isothermal at 660℃for 60min, the steel has a high hardness and strength, the hardness reaches HV285, while the converted tensile strength reaches 980MPa.(4) The results isothermal at bainite transformation aera showed that with the increase of time, the amount of M/A islands in granular bainite transformed at 580℃ decreases and the mean diameter reduces gradually, thus the hardness of granular bainite decreases and the toughness is promoted corresponding. The nano precipitates have a larger quantity isothermal at 580℃. When isothermal for 60min, the steel possesses a good strength and toughness, the hardness reaches HV245, the converted tensile strength reaches 850MPa and the M/A has a mean diameter of 0.29μm.(5) By simulating isothermal and quenching process under conventional cooling rate, the effect of UFC on the microstructure and precipitate was studied in comparison. The results showed that the UFC makes the width of bainite lath obtained by isothermal at 500℃ reduced by 100-200nm which is conductive to the microstructure refinement; makes the M/A in granular bainite obtained by isothermal at 580℃ with a larger amount and a smaller mean diameter, which is conductive to the toughness of granular bainite. The UFC is also conductive to the precipitation, therefore the hardness and strength is improved with the converted tensile strength increased by 120MPa to the largest extent.(6) By simulating the slow cooling process under UFC, the effect of cooling rate and cooling temperature on microstructure and precipitation in Ti steel was investigated. The results showed that the lower the cooling temperature is, the faster the grain refinement with the increase of cooling rate is, the higher the dislocation density in ferrite matrix is, the smaller the average size of precipitates is, the larger the amount of precipitate is, and so the higher the the hardness and strength of structure is.