from the conferences organized by TANGER Ltd.
The steel with 0.3% C – 1.2% Mn – 0.8% Cr – 0.2% Mo is intended for the production of seamless pipes in the Mannesmann method, i.e. by piercing of the round billet by skew rolling and subsequent longitudinal rolling on a pilgrim mill. The heating and finishing temperatures are relatively high and it is therefore necessary to know the continuous cooling transformation (CCT) diagrams in a wider range of temperature conditions. At a heating rate of 0.167 °C∙s-1, the temperature Ac3 = 824 °C was determined dilatometrically for the investigated steel. Subsequently, two different austenitizing temperatures were chosen for dilatometric tests. Continuous cooling transformation diagrams with previous deformation (DCCT) were constructed after the compressive true strain of 0.35 with the strain rate of 1 s-1, carried out at the appropriate austenitizing temperature of 860 °C and/or 1,000 °C. Dilatometric curves were obtained after cooling at constant rates in the range of 0.1 – 35 °C/s. Their mathematical processing was combined with metallographic analysis and hardness measurement of selected samples. After cooling at rates lower than about 0.8°C∙s-1, the structure consisted of ferrite, bainite and pearlite in varying proportions. Higher cooling rates led to an increasing share of martensite. In the high-temperature DCCT diagram, bainite plays a more significant role, and both the Ferrite-start and Pearlite-start curves are shifted towards longer times. This can be explained by the coarser initial austenitic grains after heating the samples to a temperature of 1,000 °C.
Keywords: Low-aloy steel, dilatometry, DCCT diagram, microstructure© This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.