Research On Rolling,Annealing Process,Microstructure And Properties Of Ultra-High Strength Q&P Steel

In this paper,two kinds of low-carbon Si-Mn-based alloys containing niobium and titanium were studied.The static CCT curves and recrystallization behavior of the investigated steel were measured by means of thermal expansion,transformation experiments,and thermal simulation experiments,respectively.The microstructure evolution of-the investigated steel during the hot rolling and conventional one-step Q&P process was revealed.Meantime,the new Q&P heat treatment process was studied.The structure of the investigated steel was further regulated and controlled by quenching in fully austenitized conditionsand tempering,which can improve its comprehensive mechanical properties.(1)The results show that the 0.2C-2Mn-1.5Si steel has Ac1 and Ac3 temperature of 700? and 882?,while 0.2C-2.5Mn-1.5Si steel has Aci temperature and Ac3 temperature of 687? and 870?The critical cooling rate for all-martensite of 0.2C-2Mn-1.5Si and 0.2C-2.5Mn-1.5Si are 10?/s and 5?/s,respectively.In addition,the hardenability of steel increase with increasing the content of Mn,while the temperature of Ac1 and Ac3 decrease with increasing the content of Mn.(2)During the hot rolling process,the microstructure of the 0.2C-2Mn-1.5Si steel changed f-rom martensite to bainite with the increasing of coiling temperature.The thickness of the oxide layer increased from 8.0?m to 15.5?m with the increasing of the rolling temperature.And the oxide layer near to the substrate was detected to be 2FeO·SiO2.(3)For 0.2C-2Mn-1.5Si steel,the volume fraction of ferrite decreased from 60%to 40%and then it remained stable when annealing time increases.The tempering effect increases and fresh martensite content decreased with the increasing partitioning time,The tensile strength of steel decreased from 1192MPa to 1022MP and the elongation increased from 20.4%to 25.1%.For 0.2C-2.5Mn-1.5Si steel,the proportion of ferrite decreased with the increasing of annealing time.And the residual austenite decreased from 12.8%to 7.3%,tensile strength increased from 972MPa to 1162MPa,elongation decreased from 22.6%to 17.5%with the increasing of annealing time.Moreover,the excessive fresh martensite began to be found and the elongation decreased significantly to 13.4%when the partitioning temperature higher than 400?,(4)For 0.2C-2Mn-1.5Si steel,it was studied at annealling temperatureof 850?and partitioning temperature of 320'C.The bainite transformation of the investigated steel increased and the carbon content of austenitic increased from 1.02wt.%to 1.13wt.%af-ter the quenching and one-step partitioning process.The yield strength and tensile strength of the investigated steel nearly remained at 604MPa and 1177MPa.respectively,while the elongation of the investigated steel increased from 20.4%to 24.1%.Additionally,the PSE of the investigated steel is higher than 28GPa·%.(5)For 0.2C-2Mn-1.5Si steel,it was studied at annealling temperature of 810?and partitioning temperature of 400v.After the quenching and one-step partitioning process,the organization of 0.2C-2Mn-1.5Si steel has undergone tremendous changes such as the massive martensite transforming into elongated bainite,isotactic ferrite transforming into strip-like ferrite.grain refinement,significant reduction of fresh martensite and the retained austenite volume fraction increasing from 11.7%to 19.6%.The yield strength of the investigated steel increased from 519MPa to 733MPa and the tensile strength increased from 986MPa to 1014MPa,while the elongation increased from 22.6%to 29.5%,Additionally the PSE is nearly 30GPa·%.