The Structure Design Of Gradient Mixed Grain Ferrum-based Alloy By Multiscale Computation

With the rapid development of ships,railways,automobiles and construction industry,steels are required to maintain high strength and good ductility.However,as the same time of improving the strength of steel,usually lose certain plasticity and toughness.With the introduction of the concept of making materials plain,the method to achieve a certain property by adding alloying elements is not the optimal method.Under this situation,a design of gradient mixed grain layered structure was proposed,and it was expected to gain a new material with high strength and good ductility through the microstructure design.First,in this paper,ER430 ferrite welding wire were used to weld on 6 mm thick 430steel plate.Cold rolling and heat treatment were carried out on the samples,and the Hall-Petch formula was fitted by metallographic analysis and tensile test.The values of?₀and k were 274 MPa and 589 MPa·?m^1/2 respectively.Secondly,with the orthogonal experimental design method,the finite element model of 0.5?m,2?m and 8?m with different grain size matching,modulation period and thickness ratio aiming at studying the mechanical properties was established.The"product of yield strength and elongation"was used to comprehensively evaluate the mechanical properties of the models.The results showed that the sensitivity of grain matching,thickness ratio and modulation period to product of yield strength and elongation was in the order of grain matching>thickness ratio>modulation period.The optimal grain size matching was 0.5?m and 2?m,when the aggregate thickness was 6mm,the modulation period was 3,and the thickness ratio was 4:1.The obtained gradient mixed grain layered structure stainless steel had the best properties with a yield strength of 978MPa,an elongation of 25.2%,a product of yield strength and elongation of 24645.6 MPa·%.At the same time,the strengthening mode of the gradient mixed grain layered structure was studied.The combination of fine grain strengthening and back stress strengthening was the conclusion.Finally,the tensile test simulations of the grain model with uniform size and gradient mixed grain layered structure were carried out in this paper.The strengthening mode was further studied.And the conclusion was also the combination of fine grain strengthening and back stress strengthening.In addition,it was inferred that the place where cracks were most likely to occur in the gradient mixed grain layered structure was at the interface between the large and small grain,and the most likely propagation path was the extended path of the plastic strain concentration region,and it was further predicted that crack propagation was limited in the small grain region in the“sandwich”model.Furthermore,multiple cracks may be generated in the small grain region,so that the strength and plasticity of the material were simultaneously improved.Besides,the fracture mode was transcrystalline fracture.