Study On Microstructure Evolution And Mechanical Properties Of Microalloyed Steels Based On New Generation Thermo Mechanical Control Process

Microalloyed steels have been widely used in basic industries,such as oil,bridge,ship,etc.,due to the combination of high strength,excellent toughness and better weldability.The combination of strength and toughness is commonly obtained by grain refinement and precipitation hardening of Nb(C,N)and V(C,N).The recrystallization,transformation and precipitation behaviors can be accurately controlled using new generation thermo mechanical control process(NG-TMCP)with the key technology of ultra fast cooling(UFC).It is necessary to understand the principle of microalloy design under UFC.The determining role of UFC in controlling recrystallization,transformation and precipitation behaviors needs to be clarified.In this dissertation,effects of Nb and V on recrystallization,transformation and precipitation behaviors were systematically investigated.The determining role of UFC in controlling micro structure and mechanical properties of microalloyed steels was clarified.Influence of heat input on micro structure and mechanical properties of microalloyed steels at heat affected zone(HAZ)was analyzed.The reduction-manufacturing of Q460GJ was realized on the basis of UFC.The main research works are as follows:1.Effects of microalloying elements on recrystallization behaviors were systematically investigated,and the corresponding thermodynamic and kinetic models were established.The recrystallization behaviors of the 0.035Nb,0.05V,0.042Nb-0.05V and 0.035Nb-0.10V steels were investigated using single pass and double pass hot compression tests.The results show that both dynamic recrystallization activation energy and static recrystallization activation energy can be increased by Nb addition.The increment of dynamic recrystallization activation energy,?QDRX,is about 11.61 kJ·mol-1 when the mass fraction of Nb increases by 0.01%,and the increment of static recrystallization activation energy,?QSRX,is about 14.96 kJ·mol-1.The effect of V on the dynamic recrystallization activation energy can be ignored.However,the static recrystallization activation energy can be slightly increased by increasing V content.Moreover,the ?QSRX is about 1.19 kJ·mol-1 when the mass fraction of V increases by 0.01%.2.The continuous cooling transformation behaviors of the tested steels were systematically investigated.The effects of Nb and V on austenite transformation behaviors were clarified.The dynamic continuous cooling transformation(CCT)curves of the Nb-,V-and Nb-V-bearing steels were plotted.The results show that the Nb addition reduces ferrite transformation field,expands bainite transformation field,and increases hardenability.The Hmax(maximum hardness in theory)of the 0.05V steel can be increased by 55.6HV with the addition of 0.042%Nb.When the cooling rate is between 0.5?/s and 5?/s,there is nearly no effect of V on transformation behavior.When the cooling rate is greater than 10?/s,the V can promote bainite transformation.The Hmax of 0.035Nb steel can be increased by 5.7HV with the addition of 0.10%V.3.The strain-induced precipitation behavior of Nb carbonitride in austenite and the precipitation behavior of VC in ferrite were studied.The determining role of UFC in controlling precipitation behavior was clarified.The precipitation temperature time(PTT)curves of the Nb-,Nb-V-and V-bearing steels were plotted.The results show that "nose tip”temperature of the PPT curve is about 900? for the 0.035Nb steel.There is nearly no change in the "nose tip"temperature of the PPT curve by adding 0.10%V in the 0.035Nb steel.However,the precipitation driving force can be increased,and the PTT curve hence moves to the left.The "nose tip" temperature of the PPT curve is about 670? for the 0.05 V steel.The particle size is refined as the isothermal temperature decreases.The relatively frequency of particles with the diameter between 4 nm and 6 nm is up to 20%for the 0.05V steel isothermally held at 650? for 5s,resulting in greater precipitation hardening.4.On the basis of NG-TMCP,effects of cooling patterns on micro structure evolution and mechanical properties of microalloyed steel were investigated,and of the steels were developed by reducing microalloying elements.The effects of chemical compositions and process parameters on microstructure and mechanical properties evolution were studied under UFC conditions.The results show that the Nb-bearing steels possess excellent mechanical properties and the fine and polygonal ferrite is the predominant microstructure for the process of air cooled to 730-770? after finishing rolling,subsequently cooling to 580-620? using UFC.Moreover,the precipitation of nanometer-sized Nb(C,N)particles is sufficient.The yield strength of the 0.042Nb-0.05V steel can be increased by 100 MPa by comparison of the 0.05 V steel.The yield strength of the 0.035Nb-0.1 V steel can be increased by 50 MPa by comparison of the 0.035Nb steel,implying that the V has relatively poor strengthening effect.5.The weldability of 0.05V steel was investigated and the mechanism in the control of toughness at HAZ was defined.The effects of heat input on microstructure and toughness evolution of 0.05 V steel at HAZ were studied.The results show that the microstructures at HAZ become coarsen with the heat input increasing.When the heat input is less than 30kJ·cm-1,low temperature toughness of 0.05V steel is excellent.When the heat input is more than 30kJ·cm-1,low temperature toughness 0.05V steel is significantly reduced due to the formation of large blocks of M-A constituent.In addition,the composite oxide inclusions of Al with MnS can effectively promote acicular ferrite nucleation and improve the low temperature toughness at HAZ.6.Based on above mentioned results,it is realized that Q345GJ steel can be upgraded to Q460GJ steel in industry.The mechanical properties of upgraded steel can fully satisfy the standard requirements.