Heavy seamless tubes are produced by the Big Mannesmann Rolling Mill in Třinecké železárny plant. Their final microstructures and properties are not only given by steel chemistry, they are also largely influenced by cooling conditions after finish-rolling. Continuous cooling transformation (CCT) analysis of various steel grades has been increasingly an object of the researchers' attention. Nevertheless, the phase transformations are also affected by the initial microstructure (e.g. austenite grain size) and previous deformation (ratio of residual hardening and softening). For that reason the DCCT diagrams (including the influence of defined deformation e = 0.35) of the HSLA steel X70 were designed for two states of the initial microstructures. In rough outline, the tested steel has the following chemical composition: 0.16 C – 1.0 Mn – 0.2 Cr – 0.05 V – 0.03 Nb (in wt. %). Phase transformations in the course of cooling were studied by dilatometer tests (performed in the Gleeble 3800 hot deformation simulator, to wide cooling rates range, from 0.15 °C/s to 80 °C/s), metallographic analysis and hardness as well as micro-hardness measurement. Coarse gamma grains (G = 3) were produced by high-temperature preheating (1280 °C) of the dilatometer samples made from the continuously cast bloom with diameter of 400 mm. Fine initial microstructure (G = 11) was obtained by severe laboratory hot rolling of the cast material and normalizing. Lower temperature of austenitization (900 °C) has been set in the dilatometer to make such samples from the normalized material. Great differences were observed, some coarse-grain samples containing acicular ferrite, whereas mixture of martensite and austenite appeared in the fine-grain samples cooled with rates of at least 12 °C/s. Lower heating temperature substantially accelerated the gamma/alpha transformation at high cooling rates.Keywords: seamless tubes; HSLA steel; controlled rolling; continuous cooling transformation diagrams; microstructure.
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