Nonmetallic inclusions, microstructure and mechanical properties of low-carbon boron microalloyed pipe steel

The paper describes the effect of boron, manganese, and sulfur in low-carbon pipe steel on the composition and size of nonmetallic inclusions, microstructure, and mechanical properties. Generalization of the results of the study of nonmetallic inclusions in low-carbon pipe steel microalloyed with boron made it possible to identify three main groups of nonmetallic inclusions in the studied samples of hot-rolled products: oxide-sulfide and oxysulfide, oxide, and sulfide inclusions. In the rolled product sample [B] = 0 %, nonmetallic inclusions of no more than 4 μm in size are represented as single inclusions or sulfide films, as well as silicate inclusions. Samples of rolled products with boron are characte­rized mainly by small rounded nonmetallic inclusions: silicate inclusions and complex inclusions consisting of oxide phase of the Al₂O₃ – MgO system in a shell of manganese and calcium sulfides. When boron is introduced in an amount of 0.006 – 0.011 %, the structure changes from ferrite-pearlite to ferrite-bainite, with a decrease in the average size of the ferritic grain; at 0.011 %, from 8.7 to 6.8 μm. Microalloying of low–carbon pipe steel with boron in the amount of 0.006 – 0.011 %, containing 1.4 – 1.6 % Mn and 0.003 – 0.011 % S, due to the predominant formation of a finely dispersed ferrite-bainite structure and rounded nonmetallic inclusions of no more than 5.0 μm in size, provides high strength properties and, as a result, the production of hot-rolled metal with a thickness of 10 mm of X80 strength category without heat treatment. Economically manganese alloyed pipe steel containing 0.006 % B, 1.4 % Mn and 0.003 % S has the best mechanical properties, high processability and good weldability, with a carbon equivalent of С_eq = 0.30 % at a rate of not more than 0.45 %, a crack resistance coefficient of Р_см = 0.17 % at a rate of not more than 0.25 % and a ratio of σт /σв = 0.85 at a rate of not more than 0.90.

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