Effect of hot/warm roll-forming process on microstructural evolution and mechanical properties of local thickened U-rib for orthotropic steel deck

To improve the strength-toughness of traditional U-rib (TUR) and solve the problem of insufficient penetration between TUR and deckplate, a new local thickened U-rib (LTUR) has been proposed to improve the fatigue resistance of the weld joint under the premise of not increasing thickness and strength of the TUR material. And a hot/warm roll-forming process (RFP) adopting partially induction heating to 700-1000 °C was carried out to fabricate LTUR. The deformation behaviors in the forming process and microstructure of LTUR have been investigated. Mechanical properties and fracture mechanism of the LTUR after hot/warm RFP have been systematically discussed. Moreover, the results are compared with those obtained in cold RFP. Mechanical properties of the LTUR deformed above the critical transformation temperature (Ac3) show high performance characteristics with marked fatigue resistance and superior toughness. Upon increasing the heating temperature from 700 to 900 °C, the initial coarse ferrite-pearlite structure transform into equiaxed ultrafine ferrite (1-3 μm) and precipitates such as (Nb, Ti)(C, N) are uniformly distributed in the matrix. The average dislocation density of the specimens after hot roll-forming at heating temperature of 900 °C decreases dramatically compared with those of the specimens subjected to the cold RFP. Furthermore, a typical characteristic of ductile fracture mechanism and the high impact energy are more convinced that the specimens deformed above 900 °C have obtained an optimal combination of strength and toughness.