Structural nonuniformity: Impact on the cold resistance of low carbon microalloyed steel for building cоnstructions

Abstract

The nonuniformity of macro- and microstructure of low-carbon microalloyed steel pipes across their wall thickness was estimated comprehensively using optical, scanning and transmission electron microscopy. The texture of the steel was studied by X-ray diffraction analysis. Both the actual and prior austenite structures were analyzed. The possible impact of all peculiarities of the structure, namely, substructure, texture and nonmetallic inclusion as well on the cold resistance of the steel was considered and discussed. The cold resistance of steel was estimated based on the ductile-brittle transition temperature (DBTT) determined in using Сharpy impact tests performed in a wide temperature range (from ambient and subzero temperatures). Correlations between the structural features of samples, which were cut from the near-surface areas of steel pipes and at the pipe-wall midthickness, and the DBTT were found. The high nonuniformity of ductility was demonstrated, which can be characterized by the fact that the DBTT of midthickness areas of pipe wall exceeds that of near-surface areas by 30°. The lower DBTT (the higher cold resistance) corresponds to the steel characterized by thin (less than 20 ​μm) fragmented lath-like bainite bands.

Keywords

• Low carbon microalloyed steel
• Ductile-brittle transition temperature
• Impact toughness
• Bainite bands
• Fracture surface