Effect Of Cooling Rate After Rolling On Microstructure And Properties Of Nitrogen-Micro Alloyed HRB400Steel

With the constinuous deepening of industrialization and urbanization in China, as well as experiencing a transformation period that strength of reinforced bar products are upgrading from low to high, HRB400will gradually occupy the construction market in the next few years or even more. The application of Nitrogen, acting as a kind of strengthening element in HRB400production, improves quality grade by a sharp reduction of the alloy consumption. Meanwhile, it could contribute to the construction of a resource-saving and environment-friendly society. The main research object in this paper is nitrogen-microalloyed HRB400steel. In order to clarify the reinforcement of second phase particle, precipitate action are studied under different cooling rate using the MATLAB calculation and transmission electron microscopy (TEM); the relationship between microstructure and mechanical properties is explained with observation and analysis using optical microscope (OM) and scanning electron microscope (SEM), and the comparison with ordinary HRB400production is carried out.The following results are obtained:1. There are two types of the second phase particles in N-microalloyed HRB400steel:square particles which are carbon vanadium nitride and elliptic particles (or irregular shape) which are (V, Ti)(C, N). With the increase of cooling rate, the volume fraction of precipitates first increases and then decreases, but the diameter of the deposition only decreases.2. The main microstructure of N-microalloyed HRB400steel is Ferrite and Pearlite, and a certain amount of acicular ferrite. The pearlite content of N-microalloyed HRB400steel basically remains between35%～45%and the pearlite content experienced a slide trend with the decreasing of cooling rate.3. When the self-tempering temperature is below800℃, the cooling rate is the primary contributing factor that affects the ferrite grain size; when the temperature is higher than850℃, the main reason affecting the ferrite grain size is microalloying elements. The average grain size of ferrite in N-microalloyed HRB400steel is lower than that in the ordinary production at the same conditions.4. When the self-tempering temperature is680℃, the average lamella spacing of pearlite in N-microalloyed HRB400steel is168nm; when the self-tempering temperature is725℃～800℃and850℃～950℃, the average lamellar spacing of pearlite increased from140nm to200nm or so, and there is a proportional relationship in the temperature range.