Isothermal transformation and precipitation behaviors of titanium microalloyed steels

The microstructure transformation and precipitation behavior of nano-carbides in Ti microalloyed steel during isothermal process were studied by a compression test on a Gleeble 3800 thermomechanical simulator and analyzed by optical microscopy,transmission electron microscopy and other methods.The results show thatγ→αphase transformation and TiC precipitation take place in Ti microalloyed steel during the isothermal process,and time–temperature–transformation curve and precipitation–time–temperature(PTT)curve are all of“C”-type.During the isothermal process,the interphase precipitation of TiC mostly occurs at the period of the phase transformation,and the random precipitation of TiC mostly occurs on the ferrite after the phase transformation.The increment in yield strength at the initial stage of isothermal transformation mainly comes from phase transformation strengthening.With the increase in isothermal time,the precipitation hardening effect becomes more important for nucleation and growth of titanium carbides and eventually reaches the maximum value at the precipitation finished point of the PTT curve.

Effect of strain-induced precipitation on microstructure and properties of titanium micro-alloyed steels

The strain-induced precipitation behavior of titanium micro-alloyed steel was examined through the stress relaxation method.In addition,the relationship between strain-induced precipitation and isothermal precipitation was explored.The findings revealed that the strain-induced precipitation and recrystallization processes of titanium micro-alloyed steel coexist and compete at the same time.The results also showed that the recrystallization process was inhibited with straininduced precipitation.Moreover,a large amount of nano-sized TiC particles precipitated in Ti micro-alloyed steel.Notably,the strain-induced precipitated TiC had a size of 10 nm and isothermally precipitated TiC had a size of 3–6 nm.Additionally,there was a clear competitive relationship between strain-induced precipitation and isothermal precipitation.The findings also showed that strain-induced precipitation had an obvious effect on the refinement of austenite although the effect was not obvious on the increase in the yield strength.Furthermore,isothermal treatment was shown to be more advantageous than strain-induced precipitation.Finally,the major increase in the yield strength was mainly attributed to the precipitation strengthening of nano-sized TiC during isothermal precipitation.