Microstructure And Mechanical Properties Of Heterostructured Low Carbon Dual-phase Steels With High Tensile Strength

Steels are the most widely used structural materials since the inception of the industrial age.Strong steels with low cost are highly desirable for large-scale industrial applications.How to improve the comprehensive mechanical properties of low carbon and low alloy steel has always been a great challenge for the industry and researchers.Recently,heterostructured materials has attracted much attention due to their unprecedented strength and ductility.Importantly,they can be produced by current industrial facilities at large scale and low cost.Due to the heterogeneous zones with dramatic variations in mechanical and/or physical properties,the mechanical behavior and deformation mechanism of the heterostructured materials show many characteristics different from that of homogeneous materials,which makes their superior strength-ductility synergy than conventional materials.Therefore,utilizing and tuning the microstructural heterogeneity in steels is a potential method to improve mechanical properties.The present work mainly focuses on the low-carbon and low-alloy steel.We tune the microstructural heterogeneity and design the microstructure of heterostructured low carbon steels according to the idea of heterostructured materials,in order to obtain the heterostructured low carbon steels with excellent mechanical properties.Firstly,according the ideal microstructural model of heterostructured materials,we prepared heterostructured DP steels with high martensite content via flexible plastic deformation and annealing treatment for lowcarbon steels with different initial microstructure.And the in-depth physical mechanisms of strengthening and ductilization of heterostructured DP steels were studied.In addition,the concept of heterostructured materials is also applied to the preparation of ultra-strong lowcarbon steel.We produced the nano-lamellar DP steel with an average lamellar thickness of17.8 nm,whose yield strength and tensile strength can reach up to 2.05 GPa and 2.15 GPa,respectively.Finally,we provide two viable methods for the nano-lamellar steel,which can not only remain the ultra-high strength but also improve the ductility.The main contributions of this dissertation are as following:(1)The heterostructured DP steel with high martensite content(> 60%)was produced by intercritically annealing of the cold-rolled steel with microstructure of ferrite and martensite.Compared with the cold-rolled steel which has a limited strength and ductility of 1.06 GPa and1.3%,the heterostructured DP steels show superior mechanical properties.Typically,the DP steels annealed at 780 ℃ and 820 ℃ show an improving of ductility to 7.6% and 4%,respectively,and the UTS is also improved to 1.51 GPa and 1.74 GPa,respectively.The experiments show that both HDI stress and effective stress increases with martensite content,which results in significant HDI hardening and dislocation hardening in heterostructured DP steels.Thus,the heterostructured DP steels show significant improvement in both strength and ductility.(2)Laminate low-carbon steels with ferrite and spheroidized cementite were produced by warm rolling at 600 ℃.Subsequent intercritical annealing with short duration was conducted to prepare lamellar DP steels with high martensite content.With higher degree of warm rolling,ferrite grains are more refined and carbide spheroidization is more significant,thus generating more austenite nucleation sites to enhance austenite formation.The 40% warm-rolled sample annealed at 840 ℃ presents an impressive mechanical property with UTS of 1559 MPa and UE of 7.1 % among the laminate DP steels.The excellent strength-ductility synergy is contributed to its proposed laminate heterostructure feature,in which minor laminate ferrite grains are embedded in the hard martensite matrix.With higher martensite content,higher magnitude of Bauschinger effect and HDI stress were found in the laminate HMDP steels.It is mainly resulted from the development of meso-scale internal stresses distribution which is induced by the mechanical heterogeneity between ferrite and martensite.(3)The ultra-strong steel with nano-lamellar structure was produced by improving the mechanical compatibility between constituent phases and cooperating with the interstitial atoms in steels which helps with stabilizing the nanostructure.The present work produced the nanolamellar steel with an average lamellar thickness of 17.8 nm,which has ultra-high strength of2.15 GPa through the two-step processing.This result creates the record of grain refinement and strengthening of low carbon steels.Under the same rolling reduction of 90%,warm rolling shows more effective grain refinement than rolling at room temperature(the average lamellar thicknesses are 17.8 and 54.6 nm,respectively).This processing strategy provides two unreported mechanisms:(i)improving deformation compatibility of dual-phase heterostructure by adjusting warm rolling temperature to make martensite bear more plastic strain and(ii)segregating carbon atoms to lamellar boundaries to stabilize the nano-lamellae.(4)In view of nano-lamellar low-carbon DP steel obtained by warm rolling at 300 ℃,two methods to maintain the ultra-high strength and improve the ductility of low carbon steel were explored.The first scheme is to explore the appropriate heat treatment process to prepare heterostructured DP steels with superior strength and ductility.Among the heterostructured DP steels prepared by warm rolling and intercritically annealing,the 80% warm rolled sample annealed at 760 ℃ for 15min(80%-760DP)have a typical structure of equiaxed ferrite embedded in the fine martensite matrix.The 80%-760 DP steel has high tensile strength of 2.0GPa and uniform elongation of 5.2%.Compared with other samples,the higher strength contrast between hetero-domains and the ideal microstructure of the soft domain completely constrained by the surrounding hard matrix in 80%-760 DP steel make the strain partition more significant at the initial deformation stage,resulting in stronger HDI hardening and excellent mechanical properties.The second scheme is to transform the nano-lamellar DP structure into ultra-fine lath martensitic structure by proper heat treatment.Ultra-high strength lath martensite was prepared by heat treatment at 870 ℃ with different annealing times for the nano-lamellar steel.Particularly,the tensile strength of 2.0 GPa can be maintained in the sample annealed at 870 ℃for 15 min,and the uniform elongation and total elongation can be increased to 4.6% and 8.8%,respectively.Compared with the martensite obtained by direct quenching,the lath martensite from warm rolling and annealing has higher strength.This is mainly because that the prior austenite grains can be refined by heat treatment of nano-lamellar DP structure,and then the internal boundary structures such as packet and lath can be further refined.In addition,the dislocation density increases with the decrease of prior austenite grain size.According to the mixture rule,the boundary strengthening caused by finer lath size and the dislocation strengthening caused by higher dislocation density make the lath martensite structure from nano-lamellar DP structure has higher strength.