Study On Fabrication, Refinement Behavior And Mechanical Property Of Carbide/Ultrafine-Grained Ferrite Multiphase Steels

Advanced ultrafine-grained multiphase steel has high strength,high toughness(ductility)and high wear resistance.Because of its excellent comprehensive mechanical properties,it is extensively utilized in the fields of advanced manufacturing equipments,heavy-load-bearing mechanical components,high-speed rail transport,automobile industry,military weapons and aerospace,and so on.Grain refinement is the most effective method to simultaneously improve the strength and toughness of metal materials.The effective grain size of the metal materials can be refined into submicron-scale(less than 1.0 μm)or even nano-scale(less than 100 nm),using the conventional severe plastic deformation(SPD)method,such as high pressure torsion(HPT),equal channel angular pressing(ECAP).However,SPD processing method has high requirement for manufacturing modls and dimention limititions of the prepared materials.For steels with high-deformation resistance,it is difficult to meet the actual demand of large-scale production.Rolling is a method widely method used in production of large-scale bulk metal materials.In this study,the carbide/ultrafine-grained ferrite multiphase steels were prepared by rolling under relatively small equivalent strain assisted by granular-carbides without SPD.The effects of rolling process,carbide morphology and volume fraction and on the microstructure evolution,ferrite refinement behavior and mechanical properties of the cold-rolled multiphase steels were systematically studied,and the grain refinement mechanism assisted by granular carbide was revealed.Based on these results,a new type of high-strength multiphase alloy steel with granular-and lamellar-carbide/ultrafine-grained ferrite was developed.Combined with large deformation warm rolling,the pearlitic transformation from ultra-fine metastable austenite was further studied.The main results are as follows:The effect of carbide morphology and volume fraction on microstructure refinement of cold-rolled carbide /ferreite steels were systematically studied.The results show that the existence of granular carbides can significantly promote the refinement of ferrite matrix in the multiphase steel during cold rolling.After a 94% cold rolling,the proportion of high angle grain boundaries of the ferrite matrix increases with the increase of volume fraction of granular carbides in the experemental pure Fe,AISI 1020 steel,AISI 1045 steel and AISI 1065 steel with same initial ferrite grain size(15.0±3.0 μm).The ultrafine-grained ferrite with an average grain size of 530 nm was obtained in the AISI 1065 steel containing 16.5 vol.% granular carbides.Based on Fe-0.8C hypereutectoid steel,the effects of different proportions of granular and lamellar carbides on the refinement of ferrite in cold-rolled multiphase steel were studied.It was found that introducing granular carbides into lamellar pearlite can substantially promote the refinement of ferrite grains during cold rolling.After cold rolling with 94% reduction,the average grain size of ferrite can be refined to 380 nm for the multiphase steel containing 2.9 vol.% granular and 17.1 vol.% lamellar carbides(Fe3C).M(?)ssbauer spectroscopy and X-ray diffraction analyses revealed that carbides decompose during cold rolling and the supersaturated carbon concentration in ferrite reaches 0.81 at.%.The results of theoretical multiscale simulation model of carbon supersaturation indicate that the number of carbon atoms segregated on the dislocations per unit length is 2.57 C-atoms/nm,which is much higher than that of lamellar pearlite steel without granular carbides.Based on the above results,a method named as "granular carbide-assisted grain refinement" to prepare ultrafine carbide/ferrite multiphase structure was proposed.Instead of severe plastic deformation(SPD),by controlling the volume fraction and morphology of carbides in advance,the ultrafine grained ferrite(less than 1.0 μm)can be produced through cold rolling at relatively low equivalent strains.As a consequence,a new "sandwich" carbide/ferrite multiphase steel(Fe-0.69C-1.0Si-0.72Mn-2.41Cr-1.45Ni-0.61 Mo,wt.%)with hard granular carbide(M23C6 type)and lamellar carbide(M3C type)was developed.The microstructure evolution and mechanical properties upon different cold rolling deformations was systematically studied.The results show that with the increase of deformation,the ferrite phase in the multiphase steel presents a typical three-stage characteristics during cold rolling deformation assisted by gruanular carbides.The first stage is the rapid multiplication of smallangle grain boundaries when the cold rolling deformation is less than 30%.The second stage is that the large angle grain boundaries are formed and gradually increase when the cold rolling deformation is 30%~60%.The third stage is rapid multiplication of large angle grain boundaries and ferrite grains are significantly refined when the cold rolling deformation is 70% ~90%.The average grain size of ferrite is refined to 650 nm after 70% cold rolling reduction.And after 90% cold rolling reduction,the ferrite grain was further refined to 240 nm,and t the yield and tensile strengths of this multiphase steel reached 1710 MPa and 2081 MPa,respectively.The analysis showed that the precipitate strengthening,grain refinement strengthening and solid solution strengthening are the main strengthening mechanisms.The carbide/ultrafine ferrite multiphase steel with ultrahigh strength and high ductility was successfully prepared by heavily warm-rolling and the subsequent heat-treatment control.Morover,the transformation behavior of lamellar/granular carbides and ultrafine ferrite from ultrafine metastable austenite was studied.The results show that after 90% warm-rolling deformation,quenching and tempering at 200℃ for 1 h,the average grain size of ferrite in the steel is 650 nm.The yield strength and tensile strength are significantly increased from 299 MPa and 559 MPa for hot-rolling steel to 1905 MPa and 2163 MPa for heavy warm-rolling steel respectively,and the total elongation still maintains 11.7%.Grain refinement strengthening and precipation strengthening are the main strengthening mechanisms.The results of eutectoid transformation from metastable austenite show that compared with the eutectoid transformation of coarse austenite,the high-density dislocation introduced by warm-rolling can significantly promote the pearlite transformation kinetics of metastable austenite.The nucleation rate of pearlite transformation of ultrafine metastable austenite is significantly increased,and the incubation period is significantly reduced.For the newly developed multiphase steel,through 90% warm rolling and the eutectoid transformation of metastable austenite with high density defects,the preparation of granular and lamellar pearlite ultrafine structure was realized.The diffusion and segregation of carbon on the high-density dislocations during warm-rolling processing are the main results of the carbide morphology change.It is found that the lamellar pearlite transformation can still occur even when the prior austenite grain size is refined to 680 nm.