1 .Introduetion In reeent界ars,manv researches have been earried on making ultra fine一grained steel<1,2>,noted as UFGS.By redueing the grain size of SM49O steel from 20拜m to 0.5户m,Nakaie云al.<司got a uFGs steel with褪oMPa strength and a duetile一to-brittle transition temper-ature(DBTT)below 77K.With relatively low earbon and low all叭several other 80OMPagrade steels had been obtained in laboratory through refining the grains<4>.It was demon-strated that the strength and toughness ean be enhaneed dramatieally饰getting mierongrains in steels,alld then these steels are 50 ealled innoVated steels,new general steels,orhigh performanee steels<1,2,5>. W匕ldability 15 imPortant to struetural steels.When ultra fine一grained steels werewelded,it was found that the eold eraeking tendeney was inhibited beeause of their lowearbon equlvalent,but the softening and deterioration of toughness in the heat affeetedzone(HAz)beeome signifieant<6,7>.A eonsiderable amount of work had been direeted to-wards identifying the mierostrueture charaeteristie of HAZ in ultra且ne一grained steels<8,9>.It was founded that the Primary austenite grains grew obviously in HAZ eoarse grain area,indueing the welding joint strength 1055. The present study was to investigate the welding HAZ mierostrueture transformationbehavior of a new kind of SOOMPa grade ultra fine struetured steel.Produeed by relaxation-preeipitation一eontrolling phase transformation technique(RPC),this steel has fine bainite239strueture and high strength with ultra low earbon and low all叮eontentllo>.Through di-latation analysis system,the Phase transformation of HAZ eoarse grain area was studied.VVith numerieal di瓜rential,the Peak transformation rate and its temPerature were ob-tained along with the start and finish Phase transformation temPeratures.The simulatedwelding mierostx·uetures of different eooling rates were earefully observed,and the austen-ite grain sizes were measured.The transformation behavior during welding Proeesswasdiseussed.It was envisaged that the researeh work would benefit a further understandingof the weldability of this SOOMPa grade ultra fine struetured steel.2 .ExPerimental忍.1 Materiais The SOOMPa grade ultra fine struetured steel was designed with much redueed earbonand other alloyixig elements.It was Proeessed using aeeeler就ed eooling in order to obtainthe neeessary refined bainite and ferrite mierostrueture in 12mm thickness steel Plates.The ehemieal eompositions(wt%)of this steel are as following:O.O41C,1.59Mn,0.008p,0 .0055,O.285Cu,O.26Ni,0.25Mo,0.O53Nb,0.O3OTi,0.0013B.Its mechanieal ProPerties:as15 825MPa:ab 15 85OMPa:占5 15 19%;沪15 68%;兀cv一Zooc are 86,77 and 68J.2.忍D以ato饥etr岁The dilatometer sPeeimens were subjeeted to a eontrolled thermal eyele under vaeuumin the THERMECAMSTOR一2 equiPment made by FUJI Eleetronie Industrial CorPora-tion.The diameter was deteeted without making eontaet with the sPeeimen by meansof a laser type displaeement meter and the transformation Part was measured from thevari就ion of exPansion.With a eomPuter system,the relative dilation,temPer就ure andtime were Pleked uP and reeorded during fthe whole Proeess,50 that the dilation eurveseould be numerieally differentiated to obtain information about the temPerature就whiehtransformation oeeurred and ther就e of transformation at different temPeratures. It must be emPhasized that the numerieal differential method used here was differentwith Thewhlis’work<11>.Beeause in the heating and eooling proeess,the temperatureswere eontrolled by adjusting the high frequeney induetion eleetrie eurrent and eoolinggas eurrent,the aetual temperatures at the speeimens were slightly undulated aroundthe Programming,these small undulations were usually within 1 eentigrade degree andhave little affeetion on the simulated thermal Proeess,but when these data were used innumerieal differential the ealeulated results will be unstable.In this work,the eurve ofdilation一time substitutes for that of dilation一temPerature,the numerieal diffe
More summaries about the MICROSTRUCTURE TRANSFORMATION IN THE WELDING HEAT AFFECTED ZONE OF 800MPa GRADE ULTRA FINE STRUCTURE