Research On The Flux-cored Wire For Low-alloy High-strength Weathering Steel And Microstructure And Properties Of The Welded Joint

Weathering steel,because of its good bearing capacity and atmosphere corrosion resistance,is largely applied for manufacturing car body and bogie of railway vehicle.With the gradual development of high-speed,heavy-load and safe railway transportation,strength and atmosphere corrosion resistance of low-alloy weathering steel for railway vehicle have been improved continually.Weathering steel with yield strength 450MPa has been widely used.Welding is the important technology in railway vehicle manufacture.At present,shielded metal arc welding and solid wire gas metal arc welding are the main process for high-strength low-alloy weathering steel in our country.There is no matching flux-cored wire for 450MPa-leve weathering steel.High efficient productivity and automatical welding can be reliazed with flux-cored wire arc welding.And it also can be adjusted properties of weld metal by reasonably varing component of flux-cored wire.Therefore,developing flux-cored wire for 450MPa-level high-strength low-alloy weathering steel,welding structure with high strength,toughness,fatigue strength and atmosphere corrosion resistance matching to the steel can be obtained,which has important theoretical significance and engineering application value.In this paper,"slag-gas dual protection"and"comprehensive deoxidation"are adopted to purify weld metal and refine grain to improve strength and toughness.Component of flux-cored wire is adjusted to make transition smooth between weld metal and base metal to lower stress concentrate and enhance fatigue strength.Meanwhile,by increasing right amount of Ni element,reducing C element and S element and refining grain,the atmosphere corrosion resistance matching to base metal can be obstained.Based on the mechanism above,a gas-shielding flux-cored wire for 450MPa-level high-strength low-alloy weathering steel is successfully developed.The component of flux-cored wire（mass percent）is,rutile（Ti O₂）25～40%,quartz sand（Si O₂）3～7%,magnesite（Mg O）1～3%,aluminosilicate（K₂O·Al₂O₃·6Si O₂）3～12%,sodium fluoride（Na F）1～4%,sodium oxide（Na₂O）1～3%,low-carbon ferromanganese 5～15%,ferrosilicon 3～7%,Ni2.5～5.5%,Cr 2.5～7%,Cu 1～3%,ferrotitanium 1.5～5.5%,ferromolybdenum and ferrovanadium≤3%and the rest of iron powder.During welding,it is easy to ignite arc.The arc is steady and weld appearance is good.It can achieve flat welding,horizontal position welding and vertical position welding.The slag can be easily got out and welding fume is a little.The welding procedure evaluation of flux-core wire indicates that it is can reach the welding requirement.Microstructure of depoisted metal of flux-cored wire is analyzed via optical microscope（OM）,scanning electron microscope（SEM）and transmission electron microscope（TEM）.The depoisted metal of flux-cored wire is mainly consisted of fine-grain acilular ferrite,with some granular bainite and a small amount grain boundry ferrite,side plate ferrite and bulk ferrite in it.SEM analysis demonstrates that there is large amount distribution of nonmetal oxide inclusion Ti-Mn-Si-Al-O in the depoisted metal.These oxide inclusions act as the core for acicular ferrite nucleation.TEM analysis demonstrates that there is high density dislocation in both acicular ferrite and bainite of deposited metal.Mechnical properties of deposited metal of flux-cored wire are studied by tensile test,low-temperature impact test and rotary bending fatigue test.The yield strength,tensile strength,low-temperature impact toughness at-40℃and fatigue strength of flux-cored wire deposited metal achieve 575MPa,620Mpa,96J and 351Mpa,which are 1.28,1.13,1.6 and2.2 times of the target value of design respectively.Low carbon design and ferromanganese addition for desulfulration can decrease harmful inclusion and purification deposited metal.Ni element can lower ductile/brittle transition temperature.Ti element can combine with O element and other element to form sufficient inclusion（dispersion distribution）for acicular ferrite nucleation.Adding Mo and V elements further refines grain size of deposited metal through precipitation hardening.The acicular ferrite and granular bainite are distributed crossly,which can inhibite grain boundary crack formation and improve slip formation resistance.The grain boundary resistance is enhanced to prevent crack extension.Therefor low-temperature impact toughness and fatigue strehgth can be obstained.By means of cyclic immersion corrosion test and electrochemical test,the corrosion behavior of depoisted metal of flux-cored wire in simulating industrial environment is studied.The morphology,constructure and phase composition of corrosion products are analyzed via SEM and X-ray diffractometer.The results show that after cyclic immersion corrosion,the corrosion production of depoisted metal of flux-cored wire is compact and integrated with substrate.In the preliminary corrosion period,the rust layer is mainly composed of Fe₃O₄ andα-Fe OOH.While in the later corrosion period,γ-Fe OOH andα-Fe OOH are the main phase in rust layer.In electrochemical test,the open circuit potential and linear polarization resistance of bare steel sample and sample with rust layer after cyclic immersion corrosion are both high than that of base metal.The fitting results of Tafle curves show that the free-cirrosion potential is higher than that of base metal and the free-corrosion current is lower than that of base metal.The fitting and calculating results of AC electrochemical impedance spectroscopy show that the corrosion current density is lower than that of base metal.Comparing with base metal,the more content of Ni element in deposited metal of flux-cored wire can shift the corrosion potential towards positive and lest content C element and S element can lower the corrosion rate.Due to Cu element and Cr element local enrichment between the inner rust layer and the substrate,the corrosion products of deposited metal of flux-cored wire is promoted to the thermodynamical steady phaseα-Fe OOH,which can make the the rust layer more even,compact and closely to substrate.Furmore,the deposited metal of flux-cored wire is mainly composed of fine-grain acicular ferrite,which makes the anode current can be beared by more grain boundary and the anode corrosion current density is lower.In the preliminary and later corrosion period in simulating industrial environment,the corrosion resistance of deposited metal of flux-cored wire is better than that of base metal.The effect of different heat input on microstructure and properties of joint with flux-cored wire arc welding are studied.The results show that,under the lower heat input of0.74KJ/mm and middle heat input of 0.96KJ/mm,the microstructure in surface weld is mainly consisted of fine-grain acilular ferrite,with some granular bainite and a small amount grain boundry ferrite and side plate ferrite.The microstructure in center of weld metal is uniform grain-refining ferrite.The microstructure of coarse grained zone in welding heat affected zone is composed of lath bainite and granular bainite and fine grained region is uniform ferrite.The yield strength,tensile strength,low-temperature impact toughness at-40℃,fatigue strength and corrosion resistance are matched with base metal.Under the higher heat input of1.19KJ/mm,because of alloy element burning loss,oxygen content in weld metal increasing and cooling speed decreasing,the microstructure of weld metal and heat affected zone become coarser.Embrittlement is occurred in weld metal and the corrosion resistance in simulating industrial environment is lower.Adopting heat input of 0.96KJ/mm during flux-cored arc welding can obtain excellent microstructure,mechanical properties and corrosion resistance.Meanehile,the productivity can be appropriately increased.