Study On The Structural Transformation Of High-Carbon Wire Rod 70 Steel By Cooling Control

With the development of the national economy and the increasing investment on infrastructure,the usage of high strength prestressed steel wire and steel strand increases dramatically.The hard wire rod,as the raw material,its microstructure and properties directly affect the post drawing process and deep processing.Therefore,it holds great practical significance to study the structural transformation by controlling cooling condition to improve the comprehensive performance of high quality hard wire.In this paper,the high-carbon wire rod 70 steel was taken as experimental material.Based on the actual production of a wire factory,this paper focused on the structural transformation,including sorbate content and pearlite sheet spacing under different cooling condition.The effect of cooling rate during continuous cooling on hardness,strength and plasticity were investigated.Under the experimental conditions of this paper,the main conclusions could be summarized as following:(1)The static and dynamic CCT curves were measured by thermal simulation test,metallographic analysis and hardness testing of steel.The effect of cooling rate on the CCT curves showed that the deformation accelerated the transformation of undercooled austenite,delayed delayed the martensite phase transition,and made the CCT curve move to the left.(2)The sorbitic content and the spacing of pearlite sheet were measured by metallographic and scanning electron microscope analysis.The sobitic content increased with increasing cooling rate and the maximum of 82.6%was obtained with the cooling rate of 10℃/s.With increasing cooling rate,the spacing of pearlite sheet decreased and its structure turns from lamellar to scatter.The calculation model of pearlite sheet spacing wasSp-1=3.09969+0.07984ΔT.(3)Under experimental condition,the optimum technological parameters were obtained as following:the phase transition temperature is 545℃-619℃,and the cooling rate is between 10/s and 12/s.According to this paper,the purpose of improving the comprehensive performance of the product has been achieved,which provides the theoretical basis for the industrial production.