Speaker
Description
The results of the development of normalized (P355NH) and quenched and tempered (P355QH) forged pipes (thickness from 20 mm to 70 mm) and P355QH connecting parts (thickness up to 420 mm) are described in this work. In the light of the target properties, concepts for proper steel design are discussed. A metallurgical analysis, including the role of chemical composition, forging and heat treatment conditions on the microstructure and mechanical properties, was performed.
The promising chemical compositions and process conditions were identified based on background knowledge and application of thermal-metallurgical models able to predict the fraction of microstructural constituents and hardness/strength of heat treated parts.
The model is an effective tool for defining the optimum chemical composition and heat treatment conditions to match the required tensile properties. In addition to virtual simulations, also laboratory tests and industrial trials were carried out to assess the effect of microstructure on the strength-impact toughness combination, starting from promising chemistries.
Low-carbon steels containing V (0.07-0.10%) and Nb (0.025-0.04%) can be safely austenitized up to 920 °C-940 °C to dissolve V rich precipitates, without formation of coarse austenite grains (> 30 μm) due to the pinning effect of Nb (C, N) fine particles which hinder grain boundary movement. In case of Nb low contents (< 0.025%), austenitizing temperatures of 860 °C to 890 °C are preferred.
Uniform and fine ferrite-pearlite and bainitic microstructures are fundamental to achieve strength combined with good impact toughness values, even after simulated post welding heat treatment. Examples of microstructures and mechanical properties of forged pipes and connecting parts are given.
This work was supported by the Italian Minister of Economic Development under Grant F-220024-00-X47 (INNOTECH Project).
| Speaker Company/University | Franchini Acciai SpA |
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