Study on Isothermal Precipitation Behavior of Nano‐Scale (Nb,Ti)C in Ferrite/Bainite in 780 MPa Grade Ultra‐High Strength Steel

Abstract

In order to precisely control the nano-scale (Nb,Ti)C precipitate in hot-rolled 780 MPa Nb-Ti microalloying C-Mn steel, isothermal precipitation behavior of nano-scale (Nb,Ti)C precipitate in the ultra-high strength steel was investigated by the thermal simulation experiments. The results indicated that defects of deformed supercooled austenite became the preferential nucleation sites of nano-scale (Nb,Ti)C precipitate and ferrite, so there was a competition mechanism for austenitic defects between ferritic transformation and precipitate nucleation. Bainitic transformation could effectively freeze austenitic defects, and additional defects are formed because of volume expansion in bainitic transformation process, so bainitic transformation could promote precipitate nucleation. However, precipitate was impacted by both nucleation driving force and atom diffusibility, so the peak temperature of nano-scale (Nb,Ti)C precipitate was 5508C. On the basis of the above theoretical results, hot rolling experiments results showed that when the coiling temperature was 5508C, the yield strength and tensile strength were 710 and 790 MPa, respectively, and the microstructure of hot-rolled steels was mainly bainitic ferrite, and a large number of <10 nm nano-scale (Nb,Ti)C precipitates were obtained. Precipitation strengthening contribution to reached 325 MPa.

Effect of microalloying with molybdenum and boron on the microstructure and mechanical properties of ultra-low-C Ti bearing steel

“…The heavy deformation in the austenitic non-recrystallization zone encourages the precipitation of TiC in the ferrite transformation zone. Previous studies suggest that TiC is formed rather than TiN in the temperature range of 500-650°C, and the precipitation can be completed in 3600 s [7,24,25]. A high density of 3-5 nm precipitates were observed in Ti steel, Ti-Mo steel, and Ti-B steel, with stoichiometric compositon of TiC, (Ti,Mo)C, and TiC, respectively.…”

“…A high density of dislocations and deformation bands were generated during heavy deformation in the austenitic non-crystallization zone, which provided additional sites for nucleation of ferrite and precipitates, resulting in the refinement of substructure and dispersion of precipitates. Cooling rate greater than 15°C/s after deformation inhibits the nucleation of coarse polygonal ferrite and large-sized precipitates during the cooling process [24][25][26], and nanoscale TiC are precipitated during near isothermal holding of the coiling process.…”

Microstructural Characteristics with Various Finish Rolling Temperature and Low Temperature Toughness in Hot Rolled Nb–Ti Ferritic Steel

“…Additionally, the isothermal precipitation behaviors of (Nb,Ti)C in ferrite and bainite phases were also explored. [15][16][17] Finish rolling temperature plays a more critical role in grain refinement and precipitation strengthening, compared with the coiling temperature which influences just precipitation strengthening. 5,18) Besides finish rolling temperature, cooling rate after hot rolling also significantly influences grain size and precipitation behavior, and ultra fast cooling (UFC) technology is used to obtained higher strength steel.…”

The Impact of Isothermal Treatment on the Microstructural Evolution and the Precipitation Behavior in High Strength Linepipe Steel

“…It is not a long isothermal process for the surface and near surface position of the steel coil. When the coiling temperature for a HSLA steel is 550 • C, a large number of <10 nm nano-scale (Nb,Ti)C precipitates are obtained [21]. The precipitation kinetics of (Nb,Ti)C particles in ferrites depends on the isothermal temperature.…”