ZHANG Jing, ZHAO Zhi-bo, XIN Wen-bin, GU Jing-yuan, LUO Guo-ping, PENG Jun
The dynamic continuous cooling transformation curves of Nb-V-Ti-N microalloyed steel were studied by using Gleeble-3800 thermal-mechanical simulator, and the effect of cooling rate after recrystallization deformation on its microstructure and properties was investigated. The results show that when the cooling rate increases from 0.1℃/s to 30℃/s, the phase transformation of ferrite, pearlite, bainite and martensite occurs in the experimental steel, and the corresponding cooling rate range is 0.1-30℃/s, 0.1-8℃/s, 3-30℃/s and 15-30℃/s, respectively. With the increase of cooling rate, the transformation temperatures of ferrite, pearlite and bainite decrease from 785, 682 and 498℃ to 625, 553 and 415℃, respectively, while the transformation temperature of martensite increases from 342℃ to 365℃. When the cooling rate increases from 3℃/s to 8, 20 and 30℃/s, the proportion of large angle grain boundaries in the steel decreases from 0.734 to 0.509, the average effective grain size first decreases from 10.62 μm to 7.46 μm and then increase to 9.61 μm, and the minimum value is obtained at 20℃/s. The average KAM value first decreases slightly from 0.419° to 0.407° and then increases to 0.691 °, and the minimum value is obtained at 8℃/s. In addition, when the cooling rate increases from 0.1℃/s to 30℃/s, the microhardness of the experimental steel gradually increases from(197±23.3) HV0.1 to(316±11.8) HV0.1, and the yield strength increases from(475±67.1) MPa to(818±33.9) MPa. The analysis shows that in order to obtain a good strength-toughness match, the appropriate cooling rate of the experimental steel after recrystallization deformation is 8℃/s.