Preparation, Microstructures And Properties Of Nanograined Steels By Plastic Deformation Of Martensite

The aim of this dissertation is to present the methods for the preparation of nanograined steels, one is martensite plastic deformation and subsequent recrystallization; other is martensite warm deformation. In addition, the warm deformation behavior and the relationship between the microstrcture and the property were characterized. This will provide a reference to fabrication of high-performane bulk nano-/ultrafine-grained steels. Nano-multilayer/submicro-grained steels were successfully prepared by using the heavy cold rolling, warm deformation and warm rolling. The warm compressive deformation behavior and the tensile properties were studied on Gleeble-3500 thermo-mechanical simulator. The microstructures was examined by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The feasibility of martensite warm rolling was also discussed.Experimental results show that nano-multilayer 15CrMnMoVA and Q235 steels can be prepared by the annealing of the lath martensite. Moreover, good combination of strength, plasticity and toughness can be achieved by modifying reduction and annealing parameter. For 15CrMnMoVA steel, parallel arrangemnt nanoscale lamella with average thickness of about 80 nm can be obtained in 15CrMnMoVA steel by quenching at 940°C, and cold rolling in reduction of 65%, and then annealing at 580°C for 90 min. The properties along rolling direction are as follows:σ_b=1581 MPa,σ_0.2=1567 MPa,δ=11.5%,α_k=55J·cm^-2, K_1C=110.7MPa·m^-1/2, and K_1SSC=94.6 MPa·m^-1/2 in 5%NaCl solution. For Q235 steel, nanograins with size of ²2.4 nm can be obtained by quenching at 940°C, and cold rolling in reduction of 93.6%, and then annealing at 350°C for 60 min, andσb reaches 1795 MPa. For 09MnNiDR steel, nano-/submicro-grained steel plate can be fabricated by warm rolling of martensite at 400°C and annealing. Nanograins with the average size of ³2.8 nm can be attained by annealing at 450°C for 60 min after the warm rolling. The tensile properties areσ_b=1430 MPa,σ_0.2=1008 MPa,δ=5.4%.The chemical composition of steels, especially Mo and V, has a great effect on thermal stability of the nanograined steels. The thermal stability is the highest for Cr-Mo-V steel, for which the lamella structures start to change when the temperature is up to 650°C; and the secondary for 09MnNiDR, for which the lamella structures start to change when the temperature is 500°C; the lowest for Q235 steel, for which the lamella structures start to change when the temperature reaches just to 450°C. Under the present experiment condition, the dependences of grain size on strength and hardness agree with Hall-Petch correlation. The corrosion resistance of the nano-multilayer steels is higher than that of the as received, which is mainly attributed to superhomogeneous and ultrfinement. The corrosion resistance of nano-multilayer Q235 steel annealed at 350°C for 480 min is better than that annealed under other parameters. The corrosion resistance decreases with annealing temperature increases.The main effect factors on flow behavior of warm deformation of martensitic 45 steel are chemical composition, deformation temperature, strain rate and strain, and the deformation temperature has the strongest effect on the flow stress. During deformation in the temperature ranged 550 to700°C, the flow stress of martensite is approximately equal to that of F+P, the working softening rate and strain rate sensitivity index for martensite are more than those for F+P. The appearance activation energies of the warm deformation of martensite and F+P are 370.3 and 355.6 kJ?mol-1, respectively. The warm deformation equations and the relationships between the deformation resistance and Z parameter of 45 steel with martensite and F+P were educed. The microstructure of martensitic 45 steel deformed in 550⁷00°C at 0.01 s-1 is composed of equiaxial ferritic grains with the mean size of 200nm¹.73μm and the granular cementite with the mean size of 32nm⁶2nm. The room-temperature tensile properties of warm deformed 45 steel with starting microstructure of martensite excel those of F+P.Based on actual ferrite rolling theory and the warm deformation behavior and microstructure evolution of martensitic and F+P microstructural medium carbon steel, it is demonstrated that the production of ultrafine grained steels by using warm deformation of martensite is feasible. The new route to prepare ultrafine grained steels by warm deformation of martensite was presented.