Study On Controlling Of Microstructure And Performance Of Banintic Forging Steels

Microalloyed medium-carbon forging steels,which is also often called non-quenched&tempered(NQ&T)steels in China and Japan,have been widely used for the production of machinery components,such as automobile forging parts,primarily due to their advantages of energy saving,environmental protection and lower cost.However,conventional ferritic-pearlitic NQ&T steels can hardly meet the mechanical properties requirements for safety components due to their low strength and low toughness.Therefore,it is necessary to research and develop bainitic forging steels with high strength and high toughness.In the present work,three bainitic forging steels20Mn2Si Cr S(MB4),20Mn2Si Cr VS(MB5)and 20Mn2Si Cr VTi S(MB6)with different microalloying elements were designed.The effects of V and Ti microalloying and controlled forging on the microstructure and mechanical properties evolution of the tested steels were systematically investigated,and the influences of post-forging tempering treatment on further enhancing the mechanical properties of MB6 steel were explored.Finally,the susceptibility to hydrogen embrittlement(HE)of MB6 steel was explored by slow strain rate tensile(SSRT)test,thermal desorption spectrometry(TDS)analysis and hydrogen permeability test.One commercial steel 42Cr Mo with quenched and tempered microstructure was used for comparison.The main results are listed as following:The effects of V and Ti microalloying on the microstructure and mechanical properties evolution of the three tested steels were investigated.The results show that compared with base steel MB4,the A_c1and A_c3temperatures of MB5 steel increase notably by?30?when adding 0.13 wt.%V.The addition of V in MB5 steel inhibits the formation of pro-eutectoid ferrite and promotes bainitic formation.The cooling rate range of obtaining fully bainitic microstructure(>90 vol.%bainite)is broadened from0.83-1.48?/s to 0.43-1.82?/s when microalloying with 0.13 wt.%V.Both MB4 and MB5 steels exhibit primarily granular bainite microstructure after post-forging continuous cooling.The microstructure(especially for the M/A constituents)of MB5steel is obviously refined compared with that of MB4 steel.The addition of a small amount of Ti in MB5 steel can further inhibit the formation of pro-eutectoid ferrite,and promote the formation of lath-like bainite.Different from MB5 steel,MB6 steel primarily exhibits lath-like lower bainite after post-forging continuous cooling.The additions of V and V+Ti could enhance the comprehensive mechanical properties of the tested steels,especially for strength improvement.The ultimate tensile strength(UTS)of MB5 steel microalloyed with V reaches 1100 MPa grade,and the UTS of MB6 steel microalloyed with V+Ti reaches 1300 MPa grade.The results of physical-chemical phase analysis reveal that?8.5%of the total added V is in the V(C,N)precipitates in MB5 steel.As for MB6 steel,almost all the added Ti is in the(V,Ti)(C,N)precipitates while only?7%of the added V is in the precipitates.The calculated precipitation strengthening values of M(C,N)of MB5 and MB6 steels are?40 MPa and?45 MPa,respectively.The effect of controlled forging on the microstructure and mechanical properties of MB6 steel was studied.The results show that lath-like lower bainitic microstructure could be obtained through controlling finish-forging temperature and post-forging cooling rate.Correspondingly,the UTS level and impact energy of the tested steel is up to 1400 MPa and 64 J,respectively.Except for the yield strength(YS)and the yield strength ratio(YS/UTS),MB6 steel exhibits identical or even higher mechanical properties than conventional quenched and tempered 42Cr Mo steel.Instrumental impact tests reveal that the crack initiation energy,propagation energy and the ability of arresting crack growth of forging steel with lath-like lower bainitic are significantly higher than those of the forging steel with primarily granular bainite microstructure.The reduction or even elimination of large blocky M/A,the increase in high-angle boundaries fraction and microstructure refinement are primarily responsible for this enhancement.It is thus suggested that suitable controlled forging is a promising strategy to regulate and control the microstructure and mechanical properties of bainitic forging steel.The tempering behavior of MB6 steel subjected to controlled forging was investigated.It was found that with the increase in tempering temperature(T_temp),the UTS decreases gradually from 1418 MPa of the as-forged specimen to 1094 MPa of the specimen tempered at 500?,while the YS increases gradually with increasing T_tempat first,reaching a peak at 400?,then it decreases with further increasing T_temp.As a result,the yield strength ratio increases continuously from 0.73 in the as-forged state to0.93 of the specimen tempered at 500?.Unlike those of the strengths,the impact energy increases at first,then it decreases and reaching a valley at 400?,and finally it increases notably again at T_tempof 500?,an increase of about 27%than that of the as-forged one.It is concluded that suitable tempering treatment after forging can obtain better strength and toughness balance of the tested bainitic forging steel,and thus help to expand its application range.The results of the study of the HE susceptibility of MB6 steel with granular bainitic and lath-like lower bainitic microstructures show that the hydrogen diffusion coefficient of the as-forged granular bainitic sample is significantly lower than that of the as-forged lath-like lower bainitic sample,and the hydrogen content of the former after electrochemical hydrogenation is significantly higher than that of the latter.Although the UTS level of the as-forged lath-like lower bainitic sample is significantly higher than that of the as-forged granular bainitic sample,the HE susceptibility indexes(HEI)of the two samples are basically the same,and it is significantly lower than the 42Cr Mo steel with similar UTS level.The hydrogen diffusion coefficient and HEI of the sample tempered at 500?are significantly lower than those of the as-forged sample,but their HEI values are still slightly higher than that of quenched and tempered 42Cr Mo steel with similar UTS level.The microstructure change and the decrease in the strength level of the as-tempered samples are the main reasons for the obvious reduction of HE susceptibility.