Effect Of Controlled Cooling Process On Microstructure And Properties Of Nb Microalloyed High Strength Reinforcement

There is a widely application to improve comprehensive mechanical properties of rebar by controlled rolling and cooling combined with microalloying. The cooling process lead to the transformation of austenite into different microstructure, which ultimately determine the final properties of product. Therefore, a intensive study on the influence of controlled cooling process on microstructure and properties is significantly important to design appropriate thermomechanical processing parameters.In the present work, controlled rolling and cooling process of three Nb microalloyed steels (0.014%,0.027%,0.036%) were simulated on a Gleeble-3800 thermomechanical simulator. Austenite continuous cooling transformation of three Nb microalloyed steels at different cooling rates and final cooling temperatures are investigated by morphology observation (optical microscopy, scanning electron microscopy, transmission electron microscopy), electron back scattered diffraction technique, hardness examine combine with theoretical analyze. The microstructure and mechanical properties after transformation have been analyzed and the evolution characteristics of microstructure of Nb microallyed steel are summarized, in addition, provides a guidence for developing production process parameters of rebar.The study of the effect of cooling rate on microstructure and mechanical properties of Nb-microalloyed steels suggested that as cooling rate increases from 0.5 to 30℃/s, the microstructure change from polygonal ferrite and pearlite at low cooling rates to acicular ferrite, degenerated pearlite and granular bainite at intermediate cooling rate, and then to lath bainite at high cooling rate. The amount and the size of M/A constituent decrease and the distribution of M/A constituent becomes more dispersive while increasing the cooling rate. The ferrite grain size decrease and the hardness of steel increase with increasing cooling rate and Nb content. Inverse pole figure, miscorientation angle map and effective grain size were examined by the electron back scattered diffraction(EBSD) technique, it was found that the proportion of high angle grain boundaries decreases and the proportion of low angle grain boundaries increases as increasing Nb content and cooling rate, this result lead to enhanced strength and weak toughness of Nb microalloyed steel.The study of the effect of final cooling temperature on microstructure and mechanical properties of Nb-microalloyed steels indicated microstructure after transformation are strongly affected by final cooling temperature and Nb content. As a given Nb content, acicular ferrite and granular bainite increase and pearlite decrease with increasing final cooling temperature. An addition of Nb is sufficient to promote the formation of acicular ferrite and bainite, especially for granular bainite. When the final cooling temperature is 600℃, the type of bainite is mainly lath bainite for 0.014Nb steel, however, for 0.027Nb steel and 0.036Nb steel, the type of bainite is granular bainite+ lath bainite and lath bainite, respectively. The amount of M/A constituent decrease and the morphology becomes from coarse island to fine lath-type with increasing Nb content and decreasing final cooling temperature, ferrite grain size decrease, when the final cooling temperature is higher than 680℃, Nb has a significant effect on grain size. The carbon extraction replica suggested the precipitation was carbide and/or nitride of niobium precipitation and morphology include spherical, cuboidal and very fine. The volume fraction of precipitation gradually increases with increasing Nb content and decreasing final cooling temperature.