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.

Effects of TMCP Schedule on Precipitation,Microstructure and Properties of Ti-microalloyed High Strength Steel

Using the similar compositions of the Ti-microalloyed high-strength steels produced by the thin-slab casting process of compact strip production（CSP）,four thermo-mechanical control processes（TMCP）after the simulated thickslab casting,i.e.the two hot rolling routes and the two cooling processes,were designed,aiming at achieving the same mechanical properties as the thin strip products.The final microstructures after the four TMCP processes were examined by optical microscope（OM）,scanning electron microscope（SEM）and transmission electron microscope（TEM）.The tensile properties and Charpy impact energy were measured correspondingly.Strain-induced TiC precipitation was found in the two-stage rolling route with the finish rolling temperature at low levels,leading to grain refinement due to the pinning effect during austenite recrystallization.Precipitation hardening in ferrite was observed when a period of isothermal holding was applied after hot rolling.It could be concluded that both finish rolling temperature and the subsequent isothermal holding temperature were crucial for the achieved strength level due to the combined effect of grain refinement and precipitation hardening.At the same time,it was found that the isothermal holding led to poor impact toughness because of remarkable precipitation hardening.Therefore,it was suggested that the precipitation kinetics of titanium carbides in both austenite and ferrite should be investigated in future.

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.