Modeling Of Austenite-ferrite Transformation During Hot Rolling And Heat Treatment Processes Of Enameling Steels

The microstructural transformation and carbon redistribution of enameling steels during hot rolling process and heat treatment process are studied by combining experiments and numerical simulations.Based on the experimental results,a two-dimensional(2D)cellular automaton(CA)model coupled with thermodynamic data and finite element simulation is proposed.The CA model includes the continuous nucleation of austenite(?)and ferrite(a),phase transformations driven by the local carbon concentration,strain energy,and interfacial mobility,the carbon partitioning at the austenite(?)/ferrite(?)interface and carbon diffusions in the a and ? phases.The proposed CA model is applied to simulate the phase transformations during the dynamic strain induced transformation(DSIT)rolling and continuous rolling processes,as well as during the heat treatments of intercritical annealing at 815 ?,subsequent continuous cooling at different cooling rates,and tempering at different temperatures.In addition,the effect of holding time on the microstructure and mechanical properties of the cold-rolled and hot-rolled steels are studied by means of heat treatment experiments.The simulation results of hot rolling processes show that with decreasing temperature before Pass 4 during the DSIT rolling,the increase of ferrite volume fraction is slow at first,and then it accelerates and slows down again in the final stage.During the rolling process of Pass 4,the temperature increases rapidly and the rolling force applied to the steel sheet is large.The simulated microstructure changes little due to the short rolling time,while the carbon depletion regions near the a/y interface are observed.After rolling,with the temperature decreasing,the ??? reverse transformation takes place until the transformation rate decreases to zero,and the normal ??? transformation starts afterwards.Accordingly,the volume fraction of the a phase decreases at first and then increases as the normal ??? begins.The simulated microstructure after DSIT rolling consists of fine ferrite grains and the dispersive lump phase enriched with C,which distributes at the a/a grain boundaries.During Pass 7 of the continuous rolling process,??? phase transformation proceeds.The increment rate of ferrite volume fraction is slow at first,then enhances gradually,and is slow down again in the final stage.The simulated microstructure after the continuous rolling mainly consists of the ferrite grains with relative large sizes and small amount of coarse lump phase enriched with carbon.The simulation results of heat treatment processes show that during intercritical annealing at 815 ?,the transformation mode of ??? changes from the interface controlled to diffusion controlled,presenting a mixed-mode.After holding for 300 s at 815?,the C concentrations in both a and y phases reach their respective equilibrium values.After subsequent cooling to room temperature,the carbon distribution is more uniform when cooled at 1.5?/s than that cooled at 5.0 ?/s.After the additional tempering at different temperatures for 5 min for the sample cooled at 5.0 ?/s,the carbon distribution becomes uniform gradually with increasing tempering temperature.The simulation results can be used to understand the mechanisms of the relevant experimental phenomena.The experimental results of holding different times show that for the cold-rolled Sample 2 with relative low levels of elements carbon,manganese,and silicon,the yield strengths of the samples after annealed at different temperatures and then air cooled to room temperature are lower than that of the as-rolled sample,and decrease with the increase of annealing temperature.Holding time has nearly no effect on the yield strength.Regarding the cold-rolled Sample 18 with relative higher levels of the elements carbon,manganese,silicon,the yield strengths after annealing at different temperatures and then sand cooled to room temperature are higher than that of the air cooled and as-rolled samples.The yield strengths of Sample 18 with 10 min holding time are higher than that of 5 min holding time for 46 MPa at 760 ? and 27 MPa at 840 ?,respectively.For the heat treatment of annealing at 760?870 ?and subsequent air cooled to room temperature,the yield strengths of the samples holding for 10 min are about 15 MPa higher than that of the samples holding for 5 min.For the hot-rolled Sample 17 having the same composition with Sample 18,holding time has less effect on the yield strength when annealed at 700?815? and then air cooled.The yield strengths of the samples annealed at 840 0C and 870 0C and air cooled increase about 25 MPa with longer holding time.