Nb Microalloyed High Strength Reinforcement Rolling Technology Research

500Mpa grade ribbed steel bars features with high strength, big security reserves, good seismic property, saving of steel products and convenient construction etc, so that it is more suitable for high-level、large span and seismic building structures, in addition, it is a more economical and efficient new building material. This paper aims at a steel plant in developing niobium microalloying and controlled cooling process development500MPa aseismatic reinforcement trial production process, appeared a few steel bars of elongation low and which have elongation instability phenomena. The objective of present thesis is to investigate the effect of soaking temperature and controlled cooling final temperature on the microstructure and properties of500MPa tested steel bar produced by the heat treatment and thermal simulation experiments. On the base of test researches, industrial trial was carried out, the microstructure and mechanical properties are investigated in this thesis. As follows are the conclusions:With increasing soaking temperature, the austenite grain size of tested steel bar increases, and coarse and asymmetric grains appear at1230℃. In order to make sure that Nb can dissolve adequately in austenite and austenite can be uniformity without apparent coarse grains, the soaking temperature for tested steel bar should be controlled from1200℃to1220℃The effect of final cooling temperature after rolling on the microstructure and phase transformation of tested steel bar has been investigated in this paper by means of thermo-simulated test. With Controlled cooling final temperature decreasing, the relative content of ferrite in core of tested steel bar gradually decreases, the relative content of pearlite gradually increases and the relative content of bainite gradually increases, as well as the ferrite grain size in core of sample gradually decreases. When controlled cooling final cooling temperature is680℃, ferrite grain size smaller, the microstructures in core of tested steel bar match great. Thereforce, the final cooling temperature of tested steel bar should be terminated in about680℃ Under the condition of industrial trial, the yield strength and tensile strength of the trial steel reach national standard requirements and have larger margin, all the mechanical properties of trial steel bar meet the seismic requirements of national standard.The quantitative calculation of various strengthening mechanisms on the yield strength of trial steel bar show that, fine-grain strengthening and transformation strengthening are the main contribution to strengthen and strength increments of them are186.5MPa and117.2MPa respectively...