Effect Of Cerium On Microstructures And Mechanical Properties In Heat-affected Zone Of High-strength Steel Applied To Construction Machinery

As thick steel plates have been much more widely used in marine engineering,bridage engineering,nuclear power,wind power,military industry,buildings,major technical equipment manufacturing and other fields,a higher welding performance of thick steel plates are also required.In order to improve the microstructures and mechanical properties in coarse-grained heat-affected zone(CGHAZ),oxide metallurgy was proposed by Japanese researchers.By adding elements such as Ti,Ca,Mg,Zr and RE into steel,fine dispersed oxides of these elements with highmelting point are formed to act nucleation core for intragranular acicular ferrite formation,which achieves great effects on improving toughness of CGHAZ in steels with low carbon equivalents such as EH36 ship building steel.However,for high carbon equivalent high strength steel,bainite and martensite transformation occurred in CGHAZ,which limited the welding performance improvement through inducing acicular ferrite formation.Moreover,based on the influence of rare earth on bainite/martensite transformation behavior,grain boundary and phase boundary characteristics,second phases and austenite grains,it is beinifical to improve the weldability of high carbon equivalent steel through rare earth addition.Thus,in the present study,an 800-MPa high-strength low-alloy steel for construction machinery was chosen and the behaviors of second phases,microstructures and mechanical properties during welding thermal cycle were investigated,to clarify the effect of Ce on welding performance of high strength steel applied to construction machinery.Rare earth Ce can modify the inclusions in steel and refine them.For the steel without Ce addition,the typical inclusions were Al2O3 inclusion.When the Ce content was 0.0065 wt%,the typical inclusions were transformed to CeAlO3 and Ce2O2S.Further increasing Ce content to 0.0110 wt%and 0.0670 wt%,the typical inclusions were Ce2O2S.With increasing Ce content,the inclusions in steel was modifiyed as Al2O3→CeAl11O18→CeAlO3→Ce2O3→Ce2O2S.After Ce addition,angular and large-sized Al2O3 inclusion were transformed to spherical rare earth inclusion,which was benifical to limit the formation of cracks in CGHAZ.Moreover,lots of nano-sized rare earth inclusions were observed after Ce addition,attributing to inhibit austenite grains growth in CGHAZ.Due to a high melting point and formation temperature,rare earth inculsion could keep stably in the welding process.The continuous cooling transformation(CCT)curves of the steel indicated that when the cooling rate increased from 0.5℃/s to 50℃/s,the microstructure gradually transformed from granular bainite and upper bainite to lower bainite and martensite.Bainite and martensite transformation were occurred in the CGHAZ,thus a mixed lower baintei and martensite could be controlled to be formed in CGHAZ through suitable welding process.Ce addition decreased the macro and micro segregation of main alloy elements in ingot,improving the distribution uniformity of carbides.During the process of welding thermal cycle,when the temperature rised above 1100℃,M7C3 and MC carbides basically dissolved.Nb(C,N)and Ti(C,N)in the steel began to coarsen,and lots of Nb(C,N)dissolved gradually,corresponding to obviously growth of austnite grains in CGHAZ.Then,with cooled to room temperature,only part of the coarsened Ti(C,N)about a hundred of nanometers were observed in CGHAZ,and M7C3,MC and Nb(C,N)did not reprecipitate in the welding cooling stage.Ce addition refnined carbonitrides in steel,which provided a siginificant inhibiting effect on grain growth before their dissolution in CGHAZ.With th increase of welding heat input,the toughness of CGHAZ decreased obviously.When the heat input was 20 kJ/cm,the microstructures in CGHAZ were composed of martentsite and lower bianite.When the heat input increased to 40 kJ/cm,the microstructures in CGHAZ were transformed into upper bainite and granular bainite.Ce addition effectively improved the impact toughness of the steel.Compared to Ce-free steel,when the Ce content was 0.0065 wt%,the absorbed impact energies were increased 59.2%in base metal and 48.2%in CGHAZ.By refining inclusiosn to limit crack formation,inhibiting grain growth and increasing propagation of high-angle grain boundaries in CGHAZ,rare earth Ce obviously improved the welding performance of 800 MPa high-strength low-alloy steel.