Investigation On Isothermal Precipitation And Strengthening Effect Of Nano Carbide In Titanium Micro-alloyed Steel

The ti-microalloyed high strength steel is widely used,mainly because adding a small amount of the titanium element in the low carbon steel can play a significant strengthening effect.At present,the physical and metallurgical characteristics of the ti-microalloyed steel have been systematically studied.However,due to the simultaneous occurrence of the undercooled austenite transformation and the nano carbide precipitation in the isothermal process,both of them will affect the yield strength of the steel.In order to clarify the law of isothermal precipitation and precipitation strengthening effect of nano carbides in the ti-microalloyed steel,the low carbon steel with the same composition and the ti-microalloyed steel were compared.The main conclusions are as follows:(1)The yield strength numerical values of the experimental steel at 600 ? and 700 ? were studied by the isothermal compression method.The yield strength of the ti-microalloyed steel under the same process is always higher than that of the low carbon steel.At the same temperature,with the extension of isothermal time,the yield strength numerical value of the two experimental steels first increases rapidly to a certain value and then tends to be flat.The strength curve of the ti-microalloyed steel increases slowly at 700 ? and then decreases after reaching the peak,which reflects the change of precipitation strengthening effect.The isothermal treatment at 700 ? promotes grain growth,and the isothermal treatment at 600 ? is more beneficial to obtain the significant precipitation strengthening effect.(2)The isothermal transformation behavior of experimental steel was studied by an isothermal quenching experiment.The results show that the diffusion rate of titanium steel is less than 700 ? and the transformation time is about 300 s at 600 ?.The undercooling degree of the isothermal transformation at 600 ? is large,and the energy storage period is needed.In the middle stage(5-20 s),the phase transition rate is the fastest.The results show that the diffusion rate at 700 ? is fast,the yield strength at the early stage of the phase transformation(15-30 s)increases rapidly,and the phase transformation rate is also fast.The microstructures change during the isothermal transformation at 600 ? and are quite complex.There are various forms of ferrite appearing such as the quasi polygonal ferrite and the acicular ferrite.However,most of the matrix structure in the process of 700 ? phase transformation is polygonal ferrite.(3)Through the analysis of the difference between the low carbon steel and the ti-microalloyed steel and the Vickers hardness distribution,it is found that the yield strength of the interphase precipitation at 700 ? in the early stage(15-30 s)increases obviously.In the middle stage of the interphase precipitation(30-65 s),the precipitation rate slows down,and the strengthening effect reaches the best at about 65 s,and the precipitation strengthening effect is105 MPa.At the later stage of the interphase precipitation(65-120 s),the amount of precipitation is very small,and Ti C particles will grow up,which is not conducive to the increase of the yield strength.The dispersion precipitation will occur in the isothermal time after the phase separation,but the yield strength increment is small.In the middle stage(5-20 s)of the interphase precipitation at 600 ?,the precipitation rate is fast and the amount is large.In the later stage(20-300 s),the element diffusion becomes slow and the nucleation points are basically saturated.At the end of the interphase precipitation,the dispersive precipitation occurs in the isothermal process of 300-2000 s,accompanied by the considerable strengthening effect.The effect of isothermal precipitation strengthening is the best at about 1000 s,and the contribution is 170 MPa.(4)The fine grain strengthening and the precipitation strengthening are two main strengthening mechanisms in the ti-microalloyed steel.In the different isothermal processes,the yield strength of the ti-microalloyed steel obtained by the isothermal treatment at 700 ? for 65 s is higher than that obtained by the non-isothermal treatment.The yield strength of the ti-microalloyed steel obtained by the isothermal treatment at 600 ? for 1000 s is 228 MPa higher than that obtained by the non-isothermal treatment.The yield strength of the ti-microalloyed steel is always higher than that of low carbon steel under the same process,which is mainly attributed to the remarkable precipitation strengthening effect of the Ti C precipitated during the isothermal process of titanium steel.The isothermal phase transformation has a significant influence on the precipitation behavior.During the isothermal transformation process at 600 ? and 700 ?,the phase transformation and the precipitation of the ti-microalloyed steel are basically completed before the middle stage.The precipitation is more abundant under the 600 ? process,which is due to the long phase transformation time.The yield strength increases significantly with more nucleation points.The precipitation rate is faster under the isothermal process of 700 ?,which is due to the short phase transition time,and the coarsening of particles decreases the yield strength.The isothermal precipitation includes the interphase precipitation and the dispersion precipitation.The speed of the interphase precipitation is different in the process of the phase transformation at two temperatures.