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Kunming, China

Chen W.,Kunming University of Science and Technology | Yan X.-J.,WuKun Steel Co. | Shi Z.,Kunming University of Science and Technology
Zhongnan Daxue Xuebao (Ziran Kexue Ban)/Journal of Central South University (Science and Technology) | Year: 2012

HRB500E high-strength and seismic steel bars with Nb-microalloyed and controlled cooling process were produced in domestic steel plant. The mechanical properties, HV 5 hardness, microstructure for weldment, strength and fatigue strength of welding joint were researched by using metallographic microscopy, vickers hardness tester, flash-butt welding, fatigue testing machine and mechanical properties testing. The results show that strength changes between weldment and base metal are less than 5 MPa before and after welding, strength change of steel bars is little; fracture of weldment is away from welded joints with ductile fracture, welding properties is better; in the welding thermal cycles, core and outer layer of welding gap of joint, heat affected zone with coarse grain, heat-affected zone with fine grain are experienced austenite recrystallization, changes of microstructure and hardness are little; core and outer layer of mixed grain zone, base metal are experienced incomplete austenite recrystallization, core and outer microstructure of base metal is F+P+B and S, hardness of outer layer is 30 HV 5 higher than that of core, changes of microstructure and hardness are little; tensile breaking load of welded joints decreases from weld seam to mixed grain zone, tensile breaking load of welding seam and heat affected zone with coarse grain is higher than that of base metal; fatigue strength design of welded joint for rebars which adopts FAT75 fatigue design curve recommended by the International Institute of welding is safe. Source


Chen W.,Kunming University of Science and Technology | Chen W.,WuKun Steel Co. | Shi Z.,Kunming University of Science and Technology | Zhao Y.,WuKun Steel Co.
Zhongnan Daxue Xuebao (Ziran Kexue Ban)/Journal of Central South University (Science and Technology) | Year: 2011

Development of HRB500 anti-seismic rebars by Nb microalloyed and controlled cooling process was introduced. The microstructure, educt, mechanical properties, strengthening and toughening mechanism were researched and analyzed by means of metallographic microscopy, scanning electron microcopy, transmission electron microscopy, X-ray diffraction apparatus and tensile testing machine. The results show that fine-grain strengthening is the primary mechanism of strengthening and toughening for HRB500 rebars produced by Nb microalloyed and controlled cooling process, contribution of its strength is more than 40% of total strength; ferrite grain grade of rebars is more than 11.0, obvious effect of grain refinement is obtained. A large number of small and diffuse of Nb(CN) educts sized 10-20 nm are formed and distributed on ferrite matrix, grain boundary and dislocation lines, good effect of precipitation strengthening and grain refinement are received; impact transition of temperature reduces by 150°C by grain refinement, and higher strength for rebars is acquired. Meanwhile, rebars still have good plasticity and toughness. Source


Chen W.,Kunming University of Science and Technology | Chen W.,WuKun Steel Co. | Shi Z.,Kunming University of Science and Technology | Chen H.,WuKun Steel Co. | And 2 more authors.
Cailiao Rechuli Xuebao/Transactions of Materials and Heat Treatment | Year: 2013

In view of 500 MPa grade high-strength rebars V-Nb microalloyed by controlled cooling process in domestic steel plant, the effects of controlled cooling temperature after finishing rolling on microstructure, grain size, hardness and precipitates of the rebars were investigated by thermal simulation, SEM, hardness test, TEM and XRD. The results show that, when controlled cooling temperature is controlled in the range of 720-740°C, microstructure of the tested steel consists of ferrite, pearlite and a small amount of bainite (6%-15% mass factron), ratio and shape of phases in the microstructure is better, is favorable for improvement of strength, toughness and ductility of the rebars, the size grade of ferrite grain is in the range of 10.5-11.0, obvious effect of fine grain strengthening and anti-seismic performance are obtained; Vickers hardness of the tested steel is 208-213 HV, the corresponding tensile strength(Rm) increases from 700 MPa to 720 MPa. The size of a lot of small V (CN) and Nb (CN) precipitates is in the range of 5-30 nm, which distributes on grain boundary dislocation lines and within ferrite matrix, and the precipitate fraction is 70%. The excellent effect of precipitation strengthening and grain refinement are obtained for the steel by the controlled cooling process. Source


Chen W.,Kunming University of Science and Technology | Chen W.,WuKun Steel Co. | Shi Z.,Kunming University of Science and Technology | Zhao Y.,WuKun Steel Co. | And 2 more authors.
Advanced Materials Research | Year: 2011

500MPa high-strength seismic rebars was developed by nitrogen-rich & vanadium -microalloyed, controlled cooling technology in domestic steel plant of china, microalloy precipitates, microstructure, mechanical properties, welding performance and aging properties of rebars were researched by using metallographic microscopy, scaning electron microcopy, transmission electron microscopy, x-ray diffraction apparatus, flash-butt welding and mechanical properties testing. The results show that a large number of the dispersed V (C, N) precipitates are precipitated in the grain boundary, dislocation line and the ferrite matrix, precipitation size in the ferrite matrix is 10∼ 20nm, V(C,N) precipitates amount accounts for 67.54% of the total amount of vanadium in steel, effect of precipitation strengthening is remarkable; when termination temperature after controlled cooling for rebars is controlled at 710°C ∼750°C , the core microstructure is polygonal ferrite and pearlite, ferrite grain grade is 11.0, outer layer microstructure is acicular ferrite, pearlite and a small amount of bainite, good effects on fine-grain strengthening and seismic performance are obtained; rebars have low strain aging and good welding performance; production cost is reduced by RMB 100 Yuan per ton steel compared with VN micro-alloyed and hot-rolling process, benefit is obvious. © (2011) Trans Tech Publications. Source

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