Production concepts of niobium microalloyed structural hollows by seamless pipe rolling

Seamless tubes are available with wall gages of up to 100 mm and outer diameters up to around 700 mm. Such tubes are typically used for oil country tubular goods, as well as for the automotive industry. Another large application area is that of structural hollows for buildings and construction applications. Due to weight and cost reduction efforts the demand for high strength seamless tubes is increasing. Many applications require high toughness in addition to high strength. The different rolling processes applied in production depend on wall gage and pipe diameter. The continuous mandrel mill is used to produce smaller gages and diameters; the plug mill covers medium gages and diameters; the pilger mill allows the production of larger diameters and heavy wall gage. In all these processes only a limited degree of thermomechanical rolling is possible. Therefore, strengthening and toughening by severe grain refinement, employing a conventional Nb-based microalloying concept are not easily achievable. Consequently, high strength and toughness seamless pipe is typically produced via a quench and temper process route. This route however is costly and constitutes a capacity bottleneck in the mill. An innovative low-C high-Nb alloy concept was identified to offer strength up to Grade X70 with very high toughness, off the rolling mill, ie. without a quench and temper treatment. The paper reveals the different functionalities of Nb in this concept based on the process of continuous mandrel mill rolling. In contrast the difficulties of obtaining a high strength and toughness combination with a more conventional alloy concept will be demonstrated. (AU) Copyright © 2015 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserve