Continuous Cooling Transformation of Under-Cooled Austenite of SXQ500/550DZ35 Hydropower Steel

Zhenglei Tang,Yuezhang Lu,Ran Guo,Fuming Wang,Yang Zhang,Zhen Liu

doi:10.3390/met11101562

Abstract

The expansion curves of the continuous cooling transformation of undercooled austenite of SXQ500/550DZ35 hydropower steel at different heating temperatures and cooling rates were measured by use of a DIL805A dilatometer. Combined with metallography and Vickers hardness measurement, the continuous cooling transformation diagrams (CCT) of the studied steel under two different states were determined. The results show that in the first group of tests, after the hot-rolled specimens were austenitized at 920 °C, when the cooling rate was below 1 °C·s−1, the microstructure was composed of ferrite (F), pearlite (P) and bainite (B). With the cooling rates between 1 °C·s−1 and 5 °C·s−1, the microstructure was mainly bainite, and martensite (M) formed as the cooling rate reached 5 °C·s−1. When the cooling rate was up to 10 °C·s−1, the microstructure was completely martensite and the hardness value increased significantly. In the second group of tests, after the hot-rolled specimens were quenched at 920 °C and then heated at an intercritical temperature of 830 °C, in comparison with the first group of tests, and except for additional undissolved ferrites in each cooling rate range, the other microstructure types were basically the same. Due to the existence of undissolved ferrite, the microstructures of the specimens heated at intercritical temperatures were much finer, and the toughness values at low temperatures were better.

Highlights

SXQ500/550DZ35 hydropower steel plate is a kind of low-carbon, low-alloy, highstrength steel with quenched and tempered processes, which is usually designed to be used for the seat ring of hydropower equipment
It was shown that the microstructure resulting from the intercritical process can be effectively refined with a few ferrite grains formed, resulting in a good match between the strength and toughness of steels, which is beneficial to industrial production
It was shown that the microstructure resulting from the intercritical pro2coefs1s3 can be effectively refined with a few ferrite grains formed, resulting in a good match between the strength and toughness of steels, which is beneficial to industrial production

Summary

Introduction

SXQ500/550DZ35 hydropower steel plate is a kind of low-carbon, low-alloy, highstrength steel with quenched and tempered processes, which is usually designed to be used for the seat ring of hydropower equipment The microstructure of this steel plate is designed to be low bainite, which requires high strength, good weldability, and high low-temperature toughness. The traditional heat treatment process for ultra-thickness steel plate is still the quenching and tempering processes, which can obtain the required strength, but the obtained low temperature toughness is usually not high enough and cannot meet the user’s requirements [3–6]. It was shown that the microstructure resulting from the intercritical process can be effectively refined with a few ferrite grains formed, resulting in a good match between the strength and toughness of steels, which is beneficial to industrial production. This work intends to design the combination of a low-carbon bainite micrositnrtuecntdusretoanddesiingtnertchreitcicoaml baninnaetaiolinngo,f wa hloicwh-cwarilbloandobpaitnaitdeumctiiclreopsthrauscetuarnedanredfiinnetdermcriitcircoa-l satnruncetaulrinegto, wenhhicahncweitlhl eadstorpent gathduacntdiletopuhgahsneeasns dofrsetfienelesd. microstructure to enhance the strenAgtchoanntidnutoouugshcnoeoslsinogf strteaenlssf.ormation diagram of the undercooled austenite of steels (CCTA) ccaonnqtiuniuteoiunstucoitoivlienlgy tsrhaonwsfothrme matiicornosdtiraugcrtaumreotfypthee, turnandsefrocromolaetdioanutsitmeneitaenodftsrtaeneslsf(oCrCmTe)dcapnhaqsueitaeminotuunitti,vwelhyicshhonwotthoenlmy ipcrroosvtirduecstuarethteyopree,ttircaanl sbfaosrims afotirotnhteimsueitaanbdletrhaenasttfroeramtmedenpthparsoeceasms oofusntte,ewlsh, bicuht nalostoopnllayyspraonviimdepsoartathnetorroelteicinalsbtuadsiisesfoarbtohuet sthueitadbelveehloepa-t mtreenattmofenntepwrogcreasdseosfosftseteeles,lsbauntdalnsoewplhayeastatnreiamtmpoernttanptrorocelessiens.studies about the developmentRoefsenaerwchgarnaddedseovfesltoepemlseanntdonf euwltrhae-tahtitcrkenaetmssesntet eplrpoclaetsessesS.XQ500/550DZ35 have not yet beReensecaormchplaentedddaenvdelfoepwmsetnutdoiefsualtbroau-tthpichkanseestsrastnesefloprmlataetsioSnXtQh5er0m0/o5d5y0nDaZm3i5cshaavnednkoi-t nyeetticbseehnavceombepelnetreedpoanrtdedfeswo sfatur.dTiehsearebfoouret,pthhaissewtorarknswfoirllmsatutidony tthheerCmCoTdydniaamgriacms sanodf tssksasuiuiunninrinidgtegteaallatbepebin-l-clrpepdesohhtpthpeaeaeacrscsrohveehtp,ne,ineuueoobsrnlnleotooddiegegfeneysyrrt-ro-hpcpecfeoaopatororohealalrxeememtdpdeeeeedxatatrpeueiusrmesrossrstteieffefmnaononrirtreti.aetniiTelnntahtastaneetlenderrrscdecettrrlefhistioettheiilrwcceseiaan,wiillttntheahaittrnneihcdsrnnrcidiweeftrfaiiaifeoctllfiairirecnnelkragngzelwtonoozntciffoleoclSnS,oosXXedolt,QiQuelndicd5n5gie0yd0gc00riitrna/d/ha5tgi5tee5ne5ts0gCsh0,D,DeCtttZhhhmZT3eee3i5ndncm5risppaositctegrrserooetreorlvavuslsmisi.tcdd.rtsiuiunnocrgge-f

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Results

Conclusion