| The microstructure of hot-rolled 14MnNb low carbon steel quenched by different processes in the two-phase region was investigated. By warm rolling of martensite-ferrite dual phase structure, bimodal sized ultrafine grained ferrite coupling with nano-scale carbides was obtained, in order to improve the plasticity of ultrafine grained low carbon steel.The recrystallization and tensile deformation behavior, and the relationship between structure and tensile mechanical properties were studied by using optical microscopy, scanning electron microscopy, transmission electron microscopy and tensile tests.Two quenching methods used in this study are as follows: (1) heating to full austenization temperature and then furnace cooling to region; (2) fully quenching and reheating to the two-phase region and brine cooling. These quenched steels were warmly rolled at 700 oC and 640 oC respectively.Results show that the microstructure of the tested steel after intercritical queching and warm rolling are composed of ultrafine bimodal-size ferrite grains coupling with nano-scale carbides, and the strength and elongation are improved. The size of ferritic grains in twice quenching sample is finer than that in quenching after furnance cooled sample. As the intercritical annealing temperature increasing, the volume fraction of martensite increases and the size of ferrite after warm rolling decreases . As the warm rolling temperature decreasing, the size of ferrite decreases.The results of the uniaxial tensile tests at room temperature show that the mechanical property of the twice quenching steel is superior to that of quenching after furnance cooling. The yield strength and tensile strength of the 700oC warmly rolled samples after completely quenched at 1000°C and intercritically quenched at 780 oC reach up to 622.7 MPa and 823.7 MPa respectively, and the uniform elongation and the total elongation are 19.1% and 10.6% respectively. |
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