The Research On Development And Application Performance Of Ti-bearing Microalloyed Hot-rolled Enamel Steel And Magnetic Yoke Steel

In this paer,we study the development and applieation performanee of Ti-bearing mieroalloyed liot-rolled enamel stee and magnetic yoke steel These two kinds of steel were analzed using an automatic phase meter,thermnal simulaton testing machine,eieetronic tensile testing machine,optical microscope(OM),field emission scanning eleetron microsopy(SEM),electron baekscatter diffraction(EBSD),X ray diffraction(XKD),transmisson electron microsoopy(TEM)and other methodlogies,This research mainly includes the continuous cooling transformation(CCT)curve measurement of Ti-bearing microalloyed het-rolled enamel steel,the establishment of Johnson-Cook model,the effect of crystalization temperature pn the microstructures and properties of steel andenarnel coating,change of microstructure and properties at elevated temperature,and the mechanism and improvement of delamination fracture in magnetic yoke steel,the effect of Ti content on the mechanical and magnetic properties as well,The main results are listed as follows:[1)The CCT curve of Ti-bearing microalloyed hot-rolled enamel steel and corresponding microstructures,which were determined and analyzcd by thermal expansion method and metallographic method provided the theoretical baais for the design of rolling and cooling proeess.When the cooling rate was less than 0.5?/s,the ideal polygon ferrite and a small amount of pearlite could be obtained.Due to the small hardening tendency,the cold crack would not appear;in the matrix.(2)The isotermal compression test of Ti-bearing mieroalloyed hot-rolled enarnel steel was condueted under different processing conditions.A Johnson-Cook model for the high temperature flow strcss of RT360 steel was established.0=(115+190.28571)(1+0.041n?*)(1-T*0.94286).It could well predict the rheological belhavior of high temperatuee rolling stage from 800? to 1100?.The flow stress of RT360 steel increasedwith the increasing strain rate and it decreased with the increasing the processing temperature.Moreover,RT360 steel showed the typical dynamic recovery and recrystallization behavior undeer low strain of 0.01s-1 and 0.1s-1.(3)The effect of the ternperature of erystalization on the microstructures and meehanical' properties of enamel substrate RT330 was investigated.The experiment demonstrates that an increase in crystallization temperature results in increased grain size,which in turn causes a reduction in RT330 strength.However,second-phase particles generated at the bottom of dimples enhance the mechanical properties of the RT330 enamel substrate.Due to the interaction between the precipitation and the solid solution,we determine that the density of precipitations(TiN and TiC particles)reaches the maximum when crystallization treatment is at 850?.The maximum density of precipitations also maximizes the pinning effect of dislocations.Strengthening resulting from precipitations is significantly greater than the strength reduction caused by increased grain size.The crystallization at a temperature of 850? is optimum for treating RT330 enamel substrate as a means to improve the mechanical properties of the heat sink.(4)The effect of crystallization temperature on the microstructures and properties of the enamel coating was investigated.The results showed that the density of Ti(C,N)particles in RT360 substrate was maximized,due to the interaction between the precipitate and solid solution,when crystallization treatment was at 850?.At that temperature,the precipitate density optimized the hydrogen trapping sites,preventing the enamel coating from fish-scaling.Small Fe-rich islands were precipitated in the steel-enamel interface,forming anchor points due to Marangoni convection.Once the crystallization temperature exceeded 850?,sufficient anchor points were created,resulting in a good adherence interface.However,when crystallization treatment temperature reached 900?,excessive liquidity of the molten frit powders resulted in a nonuniform coating.The crystallization treatment of enanel-RT360 composite substrate at a temperature of 850? was optimum for improved coating of the heat sink.(5)The delamination fracture mechanism of Ti-bearing microalloyed magnetic yoke steel was investigated,while the design improvement measures were also proposed.The demonstrated that the delamination was a classic cleavage fracture.The phosphorus segregation bands in both grain boundaries and the interior of the grains increased the brittleness of the core of the cross section where the tensile fracture occurred,facilitating the cleavage fracture.The increased texture intensity on the {001} easy cleavage face also increased the likelihood of cleavage fracture at the half thickness of the cross section of the specimen.During tension test,the maximum stress concentration in the thickness direction occurred at the center of thickness.The delamination fracture propagated from and along that center area In order to eliminate the phosphorous segregation,the pouring temperature and casting speed in continucous casting stage should be reduced,and the eleetromagnetic stirning and soft reduction process in the slab casting also should be conducted.The optimization of rolling process could avoid the increased texture intensity on the {001}easy cleavage face in the center of the plate.(6)The effect of Ti content on the mechanical properties and magnetic properties of the Ti-bearing mieroalloyed magnetic yoke steel was studied Titanium in the test steel had strong effeet on the strengthening by the two kinds of mechanism of grain refinement and Qrowan bypassing mechanism,The higher the Ti content was,the more obvious the effect was.When the Ti content is greater than 0.16%,the Ti-bearing microalloyed magnetic yoke steel could mee the technical requirments of the standard.The iron less inereased with the inereasing Ti content Oneec Ti f content is less than 0.20%,the steel could meet the requirements of magnetic properties.To sum up,when the Ti content was between 0.16%and 0.20%Ti-bearing miicroalloyed magnetic yoke steel met both the mechanical and magnetic properties of the technical standards.