Microstructure, precipitation and mechanical properties of a titanium-tungsten alloyed hot rolled high strength steel

Abstract

A new titanium-tungsten-bearing hot rolled high strength bainitic steel was developed through the design of chemical composition and rolling processing. The chemical composition of 0.04C-0.1Ti-0.4W (wt%) was determined to make full use of alloying elements through considering the atomic ratio of elements. The rolling condition in the region through austenite recrystallization to austenite non-recrystallization region was adopted to realize a homogenous and fine microstructure. The effects of coiling temperature (CT) on the microstructure, precipitation and mechanical properties of the steel were investigated. Results showed that the average effective grain sizes of bainite ferrite were measured to be 2.2 µm, 1.8 µm and 2.4 µm for the sample of CTs 550 °C, 600 °C and 650 °C, respectively. The precipitates formed during coiling process were identified as nanoscale NaCl-type (TiW)C carbides that contain a high level of Ti and W. As CT increases, the precipitation amount of nanoscale precipitate increases. At the CT of 600 °C, an optimal combination of strength and ductility was achieved (yield strength: 733 MPa; ultimate tensile strength: 806 MPa; uniform elongation: 14.9%; total elongation: 24.4%). In addition, due to re-precipitation of nanoscale carbides, the strength of the steels coiled at 600 °C was obviously increased after tempering at 650 °C for 15 × 103 s holding, exhibiting superior ageing strengthening effect.