EFFECT OF FINAL TEMPERATURE AFTER ULTRA-FAST COOLING ON MICROSTRUCTURAL EVOLUTION AND PRECIPITATION BEHAVIOR OF Nb-V-Ti BEARING LOW ALLOY STEEL
Xiaolin LI,Zhaodong WANG(),Xiangtao DENG,Yujia ZHANG,Chengshuai LEI,Guodong WANG
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819
Cite this article:
Xiaolin LI,Zhaodong WANG,Xiangtao DENG,Yujia ZHANG,Chengshuai LEI,Guodong WANG. EFFECT OF FINAL TEMPERATURE AFTER ULTRA-FAST COOLING ON MICROSTRUCTURAL EVOLUTION AND PRECIPITATION BEHAVIOR OF Nb-V-Ti BEARING LOW ALLOY STEEL. Acta Metall Sin, 2015, 51(7): 784-790.
High strength low-alloy (HSLA) steel has been widely used in buildings, bridges, ships and automobiles because of the remarkable high strength and forming property. Conventional HSLA steels are strengthened by a combination of grain refinement, solid-solution strengthening and precipitation hardening, and the contribution of precipitation hardening is considered to be minor, since many of the alloying elements are added to HSLA steels in the past basically for the strengthening of grain refinement. However, in recent research, yield strengths up to 780 MPa have been achieved in Ti and Mo bearing HSLA sheet steels by producing microstructures that consist of a ferritic matrix with nanometer-sized carbides, and the precipitation strengthening has been estimated to be approximately 300 MPa. Nowadays, thermo mechanical controll process (TMCP) is widely used to process HSLA steels, the final temperature of ultra-fast cooling (UFC) plays a decisive role for microstructure evolution and precipitation behavior, and finally determines the mechanical properties of the steels. In this work, the effects of final temperature after UFC on microstructural evolution, precipitation behavior and micro-hardness of Nb-V-Ti bearing low alloy steel were studied by using the thermal mechanical simulator, OM, HRTEM and micro-hardness instrument. The results showed that the microstructure and nucleation sites of micro-alloy carbides changed with final temperature after UFC. The microstructure changed from bainite to pearlite and ferrite and the nucleation sites changed from bainite to ferrite with final cooling temperature increasing. The number density of the precipitates in ferrite matrix was greater than that in bainite. Furthermore, the number density of the nanometer sized carbides got the maximum values at 620 ℃. The aspect ratios of the precipitates were close to 1, which meat that the precipitation morphology close to spherical. The sizes of the carbides were all less than 10 nm and became smaller with the decrease of final cooling temperature. Through the calculation by Orowan mechanism, the contributions of the precipitation strengthening to yield strength could reach 25.6% at the final cooling temperature of 620 ℃.
Fig.1 Curves of ultra-fast cooling (UFC) to different temperatures and dynamic continuous cooling transformation (CCT) (F—ferrite, P—pearlite, AF—acicular ferrite, B—bainite, M—martensite, CR—cooling rate)
Fig.2 OM images of experimental steel with final cooling temperature of 540 ℃ (a), 580 ℃ (b), 620 ℃ (c) and 660 ℃ (d) after UFC (AF—acicular ferrite, PF—polygonal ferrite)
Fig.3 Precipitation morphologies of experimental steel with final cooling temperatures of 540 ℃ (a), 580 ℃ (b), 620 ℃ (c) and 660 ℃ (d) after UFC
Fig.4 HRTEM images of nanometer-sized carbides in experimental steel with final cooling temperatures of 540 ℃ (a), 580 ℃ (b), 620 ℃ (c) and 660 ℃ (d) after UFC
Fig.5 Micro-hardness of experimental steel after UFC to different temperatures
Fig.6 Yield strength of matrix and the contribution of precipitation strength to yield strength of the experimental steel after UFC to different temperatures
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