| 500MPa grade high-strength and anti-seismic rebar features with good seismic properties,high strength,saving steel consumption,convenient construction and etc,so that it is more suitable for large span,high-rise and seismic building structures,which is a economical and efficient new building material.At present,500MPa grade high-strength and anti-seismic rebar has become a high performance,high quality and upgrading steel product,which was speeded up and encouraged to develop in China.500MPa high-strength and anti-seismic rebar is industrialized produced by using V microalloyed technology in China,which features with some disadvantages such as high production cost and the unstable seismic performance of small specification rebar,which restrict the popularization and application of the product.Recently,in order to reduce the production cost of rebar,500MPa high strength rebar by microalloyed combined with controlled cooling technology was developed and studied by some steel mills and scholars,and achieved some results,but some aspects such as the chemical composition design,the heating system,controlled cooling technology,anti-seismic performance,strengthening and toughening coupling mechanism,are the lack of the further research.500MPa high-strength rebars was industrialized produced by microalloyed combined with controlled cooling technology in a domestic iron and steel Co.,Ltd,the rebars features with some disadvantages such as unstable seismic performance,low elongation.Based on the above problems,effects of three different Nitrogen-rich microalloyed and controlled cooling technology on the microstructure and mechanical properties of 500MPa high-strength rebars were designed and studied in this paper,the optimal production technology which is Nitrogen-rich,V-Nb microalloyed and controlled cooling technology was put forward,on the basis of this technology,the further study was carried out.Effects of billet heating system and controlled cooling technology(controlled cooling temperature,cooling speed)on the microstructure and mechanical properties of 500MPa high-strength rebars were systematically studied by the application of heat-treatment and thermal simulation experiment,the reasonable heating system and the key parameters of controlled cooling process were obtained.On the basis of experimental study,the industrialization production technology of 500MPa high-strength and anti-seismic rebars by using Nitrogen-rich,V-Nb microalloyed and controlled cooling technology was developed,the mass and stabilization production were achieved.Effects of Nitrogen-rich microalloyed and controlled cooling technology on the microstructure and mechanical properties of 500MPa high-strength rebars showed that,the amount of strength surplus for rebars by Nitrogen-rich,V-Nb microalloyed and controlled cooling technology is greater than 35MPa,ratio of tensile strength and yield strength is greater than 1.28,break elongation is higher than 20.0%,uniform elongation is higher than 12.0%,strength and toughness,and anti-seismic performance of rebars is optimal.Microstructure of rebars is consisted of ferrite,pearlite and a small amount of bainite(6.7wt%),the ferrite grain is greater than 10.0 grade,ratio and shape of microstructure is better,is helpful to improve the comprehensive performance of rebars.A lot of small V(CN)and Nb(CN)precipitates are formed on ferrite matrix,the better effect of precipitation strengtheness are obtained.Effects of heating system on the microstructure of 500MPa high-strength rebars showed that,the soaking temperature of tested steel is controlled at 1070-1180℃,the austenite grain size of steel increases with the increasing of temperature.When temperature increases to 1180℃,the austenitic grain unusually grows up and the size of grain is uneven.The soaking temperature of tested steel is controlled at 1070-1160℃,and their relation can be expressed as follows:LnD=-0.14.7-1/10-4K-1+34.75.The reasonable heat soaking temperature should be controlled at 1120-1160℃ for V-Nb microalloyed 500MPa high-strength rebars.Effects of controlled cooling system on the microstructure and phase transformation of V-Nb microalloyed 500MPa high-strength rebars showed that,the cooling rate is less than 50℃/s,the bainite content in core of tested steel increases from 5%to 10%with the increasing of temperature,the distribution and shape of microstructure is better;the cooling rate is higher than 50℃/s,the bainite content in core of tested steel increases from 10%to 50%with the increasing of temperature,plenty of granular bainite are produced,the distribution and shape of microstructure is bad,is not helpful to improve the strength and toughness of rebars.When the cooling rate is less than 50℃/s,ferrite grain size of the tested steel decreases rapidly with the increase of cooling rate;the cooling rate is controlled at 50℃/s,size of ferrite grain is 6.7um and grade of ferrite grain is 11.0 grade,obvious effect of fine grain strengthening are obtained;the cooling rate is higher than 50℃/s,ferrite grain size of the tested steel decreases slowly with the increase of cooling rate.when controlled cooling final temperature after rolling is controlled at 720～740℃,ratio and shape of microstructure is better,meanwhile,a lot of small V(CN)and Nb(CN)precipitates sized from 2nm to 20 nm are formed and distributed on ferrite matrix,grain boundary and dislocation lines,and precipitate fraction is 70%,the better effect of precipitation strengtheness are obtained.Based on the experimental results and production practice in a domestic iron and steel Co.,Ltd,chemical composition control,steel-making and steel-rolling key control technology of 500MPa high-strength rebars by using Nitrogen-rich,V-Nb microalloyed and controlled cooling technology were designed and developed,industrialization integrated manufacturing technology was developed,mass and stabilization production of product was achieved.The microstructure and mechanical properties,strengthening and toughening mechanism of industrial trial rebars showed that,500MPa high-strength rebars by using Nitrogen-rich,V-Nb microalloyed and controlled cooling technology features with some advantages such as better anti-seismic performance,low strain aging,high and low temperature tensile properties and good welding performance,the comprehensive performance of rebars is excellent;microstructure of rebars is consisted of ferrite,pearlite and bainite(6.0-1 0.0wt%),a small amount of bainite on refining ferrite matrix is helpful for the improvement of anti-seismic performance of rebars;the main strengthening way to rebars have multiphase microstructure strengthening,fine-grain strengthening and precipitation strengthening,rate of strength contribution is 26%,35%,and 20%respectively;the fraction of V(CN)and Nb(CN)precipitates is more than 70%,the effect of precipitation strengthening is remarkable;ductile-brittle transit temperature of rebars is lower than-30℃,high strength and good plasticity-toughness are obtained.500MPa high-strength and anti-seismic rebars was produced by using Nitrogen-rich,V-Nb microalloyed and controlled cooling technology in a domestic iron and steel Co.,Ltd,the industrialization production of low cost and large scale were achieved,the performance of rebars is excellent,anti-seismic qualification rate of rebars is more than 99.45%,compared with the traditional VN microalloying technology,production cost is reduce 49 yuan per ton,market competitiveness of product is significantly increased,promoted the large-scale industrial production and application of 500MPa high-strength and anti-seismic rebars,speeded up the upgrading of construction steel. |
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