| Ultra-high strength steel(UHSS)can reduce steel consumption and improve the safety coefficient so significantly that it is widely used in various industrial fields.However,the traditional UHSS is difficult to be smelt as results of a high alloying addition and complex compositions.The subsequent manufacture processes,which include casting,forging,and quenching and tempering treatment,are much complicated.Accordingly,the strength is sometimes difficult to match the toughness,and eventually the steels exhibit high strength and low toughness.Also,lengthy production process,high energy consumption,high cost,small batch production are unfavorable factors for the UHSS.It is urgent to provide a new reduced composition and development of a more efficient process for UHSS production.This dissertation focuses on the exploitation of a nano-sized low-alloy bainitic UHSS via a compact hot rolled process.Based on the design of alloy composition for the novel UHSS,we extensively studied its continuous cooling transformation(CCT)and hot deformation behavior.The dynamic and static recrystallization behavior and austenite refinement were investigated in detail.Furthermore,rolling schedules with variable reductions and cooling modes were conducted to evaluate the influences of distinctive process and parameters.The mechanism and ideal process for producing B/M duplex phase UHSS using different controlled rolling and cooling process are proposed.The main research procedures and conclusions are summarized as below:(1)For the composition of bainitic steel,parameters of an original bainitic steel,including temperature,equilibrium,and alloy composition,were calculated and optimized to design a novel reduction chemical composition.The alloy content was investigated as a function of the mechanical properties to find the most desirable composition.It is clearly demonstrated that when the cooling rate reachs to 10 ?/s,the strengthening effects of optimized alloy elements is most effective in steel.(2)The continuous cooling transformation behavior was revealed in order to explore the influences of the deformation,the cooling rate and other factors on the phase transformation.The results showed that,with increased cooling rate,polygonal ferrite(PB)and granular bainitic(GB)transform to lathe bainite(LB)and lath martensite(LM),and then transform to lath martensite(LM),lathe bainite(LB)and residual austenite(RA)in turn.The deformation of the non-recrystallization zone promoted the phase transformation of ???,and the strain has an inhibitory action on ??GB transformation.The element C in the deformation induced ferrite(DIF)diffuses from a interface to y phase,which would enhance the stability of y phase and delay the ??GB transition.With the further increased cooling rate,the diffusion paths of C would decrease and the PF transition would be restrained.When the cooling rate achieved 5 ?/s,it was the same phase mechanism for strain induced transformation and undeformed static transformation.(3)Single pass compression experiment combined with metallographic analysis,the Zener-Hollomon equation of hot deformation,then the hot deformation equation and deformation resistance model were calculated and established.The P-J method was used to determine the critical strain of dynamic recrystallization,and the critical strain model for dynamic recrystallization was constituted.Single pass metallographic observation was implemented to reveal the influences of the deformation parameters and the interval pass time(tip)on the austenite structure evolution during the dynamic and static recrystallization.Explorations have shown that,at higher deformation temperature of 1050 ?,the dynamic recrystallization occurred with the strain rate between 0.01 s-1,and 5 s-1,while it took place in a fairly large temperature range of 850 ? to 1050 ? with a lower strain rate of 0.01 s-1.(4)A double pass compression experiment combined with metallographic analysis was employed to study the austenite static recrystallization behavior.The critical temperature of static recrystallization in the experimental steel was determined.Using the offset method with different compensations,the effects of interval pass time on static recrystallization were explored,and this allowed the calculation of the activation energy of static recrystallization in experimental steel.The optimal single pass static recrystallization time was used to establish the effect of the pass interval time on the austenite structure transformation in static recrystallization.The results showed that the softening way was mainly static recovery at a pass interval time less than 5 s.Furthermore,when it increased to 10 s,the static recrystallization occurred on the triple grain boundaries.When it further increased to 50 s,austenitic overcame pinning of alloy precipitate and began fully initiate and growth up.The rolling interval time was not beneficial above 100 s.Otherwise;it is distinctly possible to give rise to the static recrystallized austenite growth up rapidly.(5)The regulation method for the microstructure and mechanical properties of UHSS using conventional rolling equipment was explored by studying the effects of different reduction schedules and cooling processes on the mechanical properties and microstructures of rolled products.Then,a study was made on the influences of processing parameters on the microstructure and mechanical properties.Based on the above research,these studies revealed the influences mechanism of different controlled rolling and cooling processes on the B/M complex phase UHSS.The results indicated that,adopting the one process of CR+AC,CR+CC and CR+DQ,the general iron and steel manufacturing enterprise could produce 1600 MPa grade nanoscale bainitic UHSS with good strength and toughness. |
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