Research Of Microstructure And Properties In Ultra-fine Bainite Steel

Ultra-fine bainite steel is an advanced high strength steel with good application prospect by virtue of its high strength, high plasticity and good toughness according to bainite microstructure, grain refining and phase transformation induced plasticity. In order to improve strength, elongation and strength-ductility of ultra-fine bainite steel, three methods, such as pulsed magnetic field treatment, adjusting content of carbon and chromium, and controlling the austenitizing condition, are used to control morphology and content of retained austenite, and the carbon content in retained austenite. The main results are as follows:By researching the microstructure and properties of ultra-fine bainite steel under the pulsed magnetic field treatment, it is found that pulse magnetic field accelerates the bainite transformation and reduces the amount of residual austenite when the experimental steel is austenitized at 900 ℃ for 30 mins, then quick cooled to 300℃, and added 1.5T pulsed magnetic field for 0.5h during the incubative period of bainite transformation. However, pulsed magnetic field does not obviously accelerate the bainite transformation when the experimental steel is homogenised at 1250℃for 48 hours, then is austenitized at 950℃ for 0.5h, and added different intensity pulsed magnetic field during the incubative period and transformation starting period of bainite transformation.The research results of microstructure and properties of different carbon content of ultra-fine bainite steel show that: when the carbon content is 0.64%, the thickness of ferrite strip and retained austenite film are larger, the quantity of ultra-fine bainite is little, and the content of retained austenite is 30.25%. When the content of carbon is 0.82%, it is easy to form ultra-fine bainite structure, the size of retained austenite film and blocks become thinner, and the content of retained austenite is 20.25%. When the carbon content is up to 0.93%, the formation of retained austenite block becomes easy, and the content of retained austenite is 31.51%. Tensile strength(1760MPa) and elongation(8.8%) of 0.81%C steel are lower than the tensile strength(2230MPa) and elongation(10%) of 0.64%C steel and 0.93%C steel. By studing the microstructure and properties of ultra-fine bainite steel with Cr or without Cr, it is found that, compared with the 0%Cr ultra-fine bainite steel, for the 1%Cr ultra-fine bainite steel, the bainitic ferrite lath becomes thinner, the content of retained austenite is reduced, the tensile strength, elongation, and strength-ductility are increased obviously.By studing the microstructure and properties of ultra-fine bainite steels under different austenitizing condition, it is found that the quality of retained austenite in the ultra-fine bainite structure is 17.38%, 21.79% and 22.68% when the experimental steel is austenitized at 900℃, 950℃ and 1000℃for 20 minutes. For the ultra-fine bainite steel austenitized at 900 ℃and 950℃, the tensile strength and elongation of the experimental steel are identical. But for the experimental steel austenitized at 1000℃, the tensile strength reduces, and the elongation increases. When the experimental steel is austenitized at 950℃for 10 mins, 20 mins and 30 mins, the quality of retained austenite in the ultra-fine bainite structure is 14.69%、21.79% and 18.19%, respectively. When the austenitizing time is 10 mins, the elongation(6.3%) of the sample is the highes. When the austenitizing time is 20 mins, the tensile strength(1887MPa) of the sample is the highest. When the austenitizing time is 30 mins, both the tensile strength(1887MPa) and elongation(5.7%) of the sample are lower. Compared with the heating rate of 0.20℃/s, the sample is heated at the rates of 0.27℃/s and 0.49℃/s, the TRIP effect is easier to happen during the tensile test of ultra-fine bainite steel. When the heating rate is 0.27℃/s, the largest thickness of retained austenite film with continous distribution reaches 1.1μm, the tensile strength, elongation and the strength-ductility of sample is 1855 MPa, 21.2% and 39326MPa·%. In the experiment range, the favourate conditions of TRIP effect are: the retained austenite film distributes continuously, the quanity of retained austenite ranges from 20% to 25%, the content of carbon in the retained austenite is almost 1.24% or so, and the thickness of retained austenite film is 0.5μm-1.1μm.