Diagnose And Prevent Microstructure Abnormity And Fluctuation In Mechanical Properties For Hot Rolled High Strength Steels

High strength low alloy steels have good characteristics includinglow cost and good toughness and good weldability.In order to acquire normalmicrostructure and stable mechanical properties.There is need to optimize theparameters of the controlled rolling and controlled cooling. The parameters of heatingprocess and TMCP process are studied on thermo-mechanical simulator.It is todiscuss the parameter on the effection of microstructure and mechanical propertiesand ensure the range of the parameters.The purpose is to provide the basis fordiagnosing and preventing microstructure abnormity and fluctuation in mechanicalproperties.The results show that the austenite grain growth is affected by the heatingtemperature and the holding time.The austenite grain size increases with theincreasing temperature and holding time.When the temperature is higher than1150℃,the austenite grain sizes are growing rapidly. When the temperature is lowerthan1150℃,the austenite grain sizes are growing gently and accelerating noticeably.In the research of the solid solution of alloying elements, the solid solution ofalloy elements increases with the increasing heating temperature and holding time.The solid solutions of Nb and Ti change from88%and9.1%to92.4%and13.6%.Deformation resistance decreases with the increasing deformation temperature.The relationship between deformation temperature and flow stress logarithm is linear.Deformation resistance increases with the increasing deformation strain rate. Therelationship between deformation strain rate logarithm and flow stress logarithm islinear.Recrystallization behavior can occur and achieve, which become more easilywith higher temperature and lower rate of deformation.It is not easy to occur withlower temperature and higher rate of deformation.With Matlab software, multivariatenon-linear simulation is carried out, and the mathematical model of the tested steel isobtained.Strain induced precipitation took place during holding process after hotdeformation, strain relaxation curves exhibit several short plateaus.It can get the PTT(Precipitation-Time-Temperature) curves by the stress-strain curves.The PTTcurvesexhibit typical “C” shape.The second phase can affect the recrystallizationbehavior.The inoculation time at920℃is the shortest.The second phase can separateout quickly.The static CCT curves is measured by the dilatometer on automatictransformation test recording device.By the CCT curve, the ferrite and pearliteproduct during high temperature transition region.The bainite products during thetemperature the transition region.The martensite products during the low temperatureregion.The phase transition point of ferrite and pearlite decreases with the decreasing cooling rate.The bainite phase transition point increases first and then decreases.Themartensite phase transition point decreases first and then be stable.Because the bainiteis the final microstructure,so the transformation temperature is during420℃to550℃.The cooling rate is during0.85℃to8.5℃.It can increase the strength of thesteel.The experiments test on the thermal simulation machine by changing theparameters.Only onsidering the cooling rate, it shows that the microstructure transfersfrom polygonal ferrite and pearlite to acicular ferrite and bainite. When onlyconsidering the finish rolling temperature, microstructure and mechanical propertiesare stable at920℃.In order to solve microstructure abnormity and fluctuation in mechanicalproperties,it can determine the appropriate range of process parameters.The heatingtemperature is1200℃.The roughing rolling temperature is1050℃.The finishingrolling temperature is920℃.The cooling rate is larger than5℃/s.It cools to thetemperature from450℃to500℃.