Precipitation behavior and strengthening mechanism of Q420 grade hot rolled V microalloyed ultra large H-beam

This study investigates the precipitation characteristics, precipitation kinetics, and strengthening mechanisms of Q420 grade hot-rolled ultra-large H-beams, focusing on three representative sites: the web center (W), flange quarter (F), and R-angle (R). Experimental results reveal nano-scale V(C, N) precipitation at all three sites, primarily featuring random and interphase precipitation, along with a small amount of austenite region precipitates and fibrous precipitates (FB). The average particle diameters are similar across sites: 8.2 +/- 0.1 nm (W), 8.8 +/- 0.2 nm (F), and 8.1 +/- 0.1 nm (R). Calculations of precipitation-time-temperature (PTT) and nucleationrate-temperature (NrT) curves indicate that precipitates nucleate uniformly during cooling, consistent with transmission electron microscope (TEM) observations, which reveal minimal precipitation at grain boundaries (GB) and dislocations. Strengthening contributions from precipitation are calculated at 87 MPa for W, 50 MPa for F, and 56 MPa for R. The low contributions can be attributed to the presence of numerous FB and precipitationfree zones (PFZ) at these sites. Among the three sites, W exhibits the highest strengthening due to its smallest interphase precipitation spacing. The F site, characterized by the highest dislocation density, has the largest average precipitate size and least interphase precipitation, resulting in the lowest contribution to precipitation strengthening.