Evaluation of Toughness Characteristics of API Grade Pipeline Steel Produced on Compact Strip Production (CSP) Line

As concerns for environmental impact of oil and gas transmission pipelines and overall public safety of the transmission pipeline systems are raised in the public domain, development of optimum toughness characteristics are a key attribute. Toughness performance as measured by charpy impact testing or drop weight tear testing (DWTT) is heavily influenced first by the average transformed grain size and more importantly the cross sectional uniformity/distribution. In addition, the crystallographic texture created can further improve or detract from the toughness performance. The final transformed cross sectional grain size along with the uniformity/distribution is heavily influenced by the available total metallurgical reduction ratio, microalloy design, proper generation of the various recrystallization behavior types during rolling, critical per pass reductions and the final post rolling cooling rate. The final crystallographic texture is influenced by final rolling temperature, cooling rate and final cooling stop temperature. When the final cross sectional grain size and overall uniformity/distribution are marginal for optimum toughness, the addition of favorable crystallographic textures can enhance the toughness performance. The challenge of producing optimum toughness on a thin slab caster, <100 mm thickness, is well known due to the available metallurgical reduction ratios. Typically, for API grade steels, a metallurgical reduction ratio ≥7:1 is required in order to achieve optimum toughness. However, in a thin slab caster the maximum metallurgical reduction ratios possible can be between 5:1 and 7:1 depending on the final thickness. Nucor Steel Gallatin has been working to optimize the overall toughness of API X-grades for transmission pipeline steels in thicknesses up to 12.7 mm using their thin slab Compact Strip Production (CSP) production facility. By utilizing a proper understanding of reducing the as-cast thin slab, along with the key alloy/process attributes and recrystallization behavior kinetics during the rolling process to optimize the final transformed cross sectional grain size and more importantly the uniformity/distribution, a high level of toughness performance can be realized. In addition, a further understanding of the contribution of specific crystallographic textures can further improve the toughness performance of these grades. This paper will discuss alloy/process parameters that have been studied and optimized to improve the low temperature toughness of API steels. In addition, toughness performance and metallographic characterization of different processing parameters will be presented.