Research Of Precipitate Behavior In One Micro-alloyed Low-carbon Steel After UFC

In view of the limited material resources and energy supply available together with serious enviorment impact concerns at the present time, the development of ultra-high strength steels has attracted a great deal of attentions from various countries in the world. As an exceedingly important mechanism of strengthening method except fine-grain strengthening, nano-precipitation strengthening is one of the significant developing directions for steel. Making a balance contribution to the tensile strength and yield strength, precipitation strengthening, which possess a lower brittle vector, has a little influence on the yield ratio of the steel. Relying on the National Natural Science Foundation of "Control of nano-cabide precipitates in low carbon steels under ultra fast cooling and comprehensive strengthening mechanism", the main content of this article is the precipitate behavior of nano-carbide in ferrite and bainite zone in low carbon steel under ultra fast cooling. Specific contents as follows:(1)The continuous cooling transformation behavior of experimental steel has been investigated by means of thermal simulation experiments. And the continuous cooling transformation curve and cooling transformation curve under ultra fast cooling conditions are obtained. The results showed that experimental steel has a wide bainite transformation area and the application of ultra fast cooling after deformation can lower ferrite transformation starting temperature. What’s more, the time-temperature-transformation (TTT) curve of experimental steel is obtained through JMatPro, which has important reference function to the isothermal quenching process formulation.(2)The precipitate absolutely dissolution temperatures of Nb(C,N), V(C,N), Ti(C,N) are1066℃,880℃,1429℃respectively through theoretical calculation. Precipitation temperature time (PTT) and nucleation rate-temperature (NrT) curves of Nb(C,N), V(C,N) are also obtained. Though the calculation of Thermo-Calc, the main precipitation in experimental steel is TiN in high-temperature region, NbC and VC in low-temperature region, composite precipitates existing at the same time. (3)The influence of isothermal temperature and isothermal time on the microstructure and precipitation behavior has been studied through isothermal quenching experiments. The volume fraction of ferrite and the density and volume fraction of precipitation whose size mainly distributes from2nm to5nm, increase with the extending of the isothermal time. It indicates that the nano-precipitations have an obvious influence on the microhardness of experimental steel through the microhardness and nanoindentation testing. The more the nano-precipitations are, the better the hardness is. Through calculating, the contributions of the precipitation strengthening to the yield strength are48.4MPa,65.8MPa and75.2MPa respectively after isothermal at600℃for300s,1200s and3600s. In this research, supersaturated precipitation and interface precipitation are the two main styles of precipitate in experimental steel, but interface precipitation just occurs in some ferrite. The structure of the experimental steel isothermal at550℃and500℃for different time is granular bainite and bainite ferrite. The nanoindentation testing shows that the longer the isothermal time is, the more homogeneous the distribution of the precipitation is.(4)The effects of final cooling temperatures and slow cooling rates on the microstructure and precipitation behavior of experimental steel after ultra fast cooling are studied by simulating process. The results show that the quantity of precipitation decreases with the lower of the final cooling temperature after UFC, but the most is620℃. The size of the precipitation, mainly distributing from2nm to5nm, decreases a little with lower final cooling temperature. The quantity of precipitation decreases with the lower slow cooling rates. The size of the grain decreases and the quantity of precipitation increases in the condition of60%deformation compared with the condition of without deformation.