Study On Preparation Of Nano/micro-scale Ultrafine Grained Low Carbon Steel By Cold Rolling And Annealing

Grain refinement is an effective method to improve the mechanical properties of bulk metal structure materials,so the preparation of ultrafine grained metal materials which can improve both strength and plasticity has been concerned by researchers all over the world.In this paper,the ultrafine grained low carbon steel was prepared by changing the cold rolling reduction,annealing temperature and annealing time through phase change + cold rolling + annealing compound treatment route.By means of optical microscope and scanning electron microscope,the microstructure and fracture morphology of samples under different processing conditions were observed,the compositions of carbides in ultrafine grained steels were analyzed by X-ray diffraction,and the mechanical properties of the specimens were analyzed by tensile experiment and hardness test,the recrystallization activation energy of experimental steels was calculated according to microhardness measurement results.The following main conclusions are obtained:(1)When the nano/micro-scale grained steel is prerared by 50% cold rolling and annealing,recovery,recrystallization,and grain growth successively take place in the cold-rolled specimens with the increased annealing temperature and time,and the mechanical properties change.The sample annealed at 550 ? for 30 min shows a mixed microstructure composed of ultrafine nano-scale grains(the mean grain size of330 nm)and uniformly distributed nano-carbides and has a good combination of 867 MPa of tensile strength and 16.7% of the elongation.(2)According to the relationship between microhardness and recrystallization activation energy,the recrystallization activation energy of low carbon steels used in this experiment is determined to be 320.68 k J·mol-1.(3)When the cold rolling reduction increases to 70%,the storage capacity and nucleation points of the cold rolling samples increase with the increased reduction,the recrystallization start temperature decreases,and the recrystallization behavior is more complete under the same annealing process.With the increased annealing temperature and annealing time,the grain size of ferrite increase obviously,and the morphology of cementite changes from fine particles to block.When the annealing temperature and annealing time are 600? and 2min,ultrafine nano-scale grains(the mean grain size of 288 nm)which has a good combination of 753.8 MPa of tensile strength and 16.5% of the elongation are prepared.(4)Nano/micro-scale grained steels are prepared by cyclic annealing process,and the total deformation is increased by repeated cold rolling and annealing,but the further refinement of ultrafine grained ferrite is mainly determined by the second annealing temperature,annealing time and microstructure before the second annealing.Both the first and the second cold rolling reduction is 50%,annealing temperature is500 ?,550 ?,600 ?,650 ?,annealing time is 2min and 30 min respectively,the second annealing temperature is 600 ? and the annealing time is 10 min,ultrafine grained ferrite is not obviously refined,the precipitation of hard and brittle phase cementite increases and the size becomes larger,mechanical properties have not been significantly improved.