The Effect Of Nb On The Microstructure And Properties Of Cold Rolled And Annealed Mo Microalloyed Bainitic Steel

In order to meet the market demand for automotive cold-rolled steel and other products,two kinds of Fe-C-Si-Mn low carbon bainitic steels with different chemical components were designed and prepared based on the excellent mechanical properties and broad application prospects of bainitic steels.Steel A was added the element Mo,while steel B was complexly added the elements Nb and Mo.The two kinds of experimental steels were hot-rolled and cold-rolled with the same process,and annealed under the same annealing conditions.Microstructure analysis and mechanical properties test of the samples were conducted.The microstructure and properties of the two steels were compared and analyzed through the microscopic mechanism and strengthening mechanism.Combined with theoretical calculations,the effects of Nb on the microstructure and properties of cold rolled and annealed Mo microalloyed bainitic steel was studied.The results can provide the basis for the component design and optimization of cold-rolled microalloyed high strength steel.The results show that both the two low-carbon steels mainly consist of bainite,and the complex addition of Nb and Mo is conducive to improve the strength of steel during the hot rolling.The average grain size of the two experimental steels have no significant difference under the same conditions,and the recrystallization temperatures are similarly between 550-650 ?.However,no precipitates were observed in steel A,whereas,two types of precipitates,i.e.,Nb(C,N)and composite(Nb,Mo)(C,N),were observed in the steel B during the recrystallization.Thus the different strength of the two steel results in the different precipitation strengthening.Unlike the obvious grain refinement and precipitation strengthening in hot rolling,the increase in yield strength of Nb addition steels in cold rolling and annealing mainly results from the precipitation strengthening,while the effect of grain refinement strengthening can almost be ignored.