Ultra-fast Cooling Technolygy And Strength-toughness Behavior Of A Low-carbon Bainite Steel With Ultra-high Strength

In recent yerars, the steel plates with excellent combination of ultra-high strength and tounghness are urgent needed due to increasing demands for larger scale structure and construction machinery, and the employed conditions requirement increasingly severe. In this paper, a steel plate over100kg grade and high tougnness under NG-TMCP (New Generation Thermo-mechanical control process) can be obtained in laboratory. Base on analysis of the fine microstructure, the precipitation and the second phase particle in different TMCP conditions, the optimize process for lager industrialized production was determined.The main works involved as follows:(1) When single press test is taken, then cooling with UFC, the microstructure mainly fine lath bainite which contains interlath retained austenite films, and amount of substructure distrubition in bainite packet parallelly. While cooling with common TMCP, the microstructure compsed of lath bainite, upper baninite and a portion of grain bainite. The thickness of bainite lath increased and the M/A island presence on the matrix.At relatively higher reheating rate and lower tempering temperature, abundantly M(C, N) particles homogeneously distributed over the lath interior and boundaries. While with the reheating rate decreasing, the tempering time and tempreture increasing, the M(C, N) particles mainly detected on lath boundaries coarsen gradually. The quantity and the size of M/A island all decreased. The width of bainite lath increased.(2) The result of controlled rolling and controlled cooling in experiment suggest that: Using UFC process, the110kg grade steel exhibited yield strength and tensile strength of1020MPa and1180MPa respectively. The120kg grade steel has a yield strength and tensile strength of1073MPa and1296MPa respectively. The steel plates have both excellent tounghness and tensile strength. It can meet the requirements of WELDOX960E and WELDOX1030E high strength steel of CEN. The Cu-LCB steel has a higher yiled strength by 50～100MPa than the stsndard requirement of HSLA-100, an elongation of16.84%and the impact energy can achive at153J at-90℃.(3) The optimum processing parameters for industrial production are:(a) the reheating temperature is1200℃;(b) two stages controlled rolling is adopted, in which the first stage rolling started at1050～1100℃and the second stage rolling started at:(c)110kg grade steel:800～860℃, finish cooling temperature is200～420℃, cooling rate is40～70℃/s;110kg grade steel:800～860℃, finish cooling temperature is300～420℃, cooling rate is50～65℃/s; Cu-LCB:800～850℃finish cooling temperature is300～350℃, cooling rate is50～75℃/s.(4) Different temperature aging test were carred out to study the aging temperature on the microstructure and mechanical properties. The results show that the yield strength and the hardness all achived the maximum peak, and precipitates(ε-Cu, M(C, N)) are finely and homogeneously distributed by aging at500℃.While the yield strength and the hardness decrease dramatically and precipitates are coarsened with aging temperature increasing.650℃was selected as a proper aging temperature.