The present work describes the analysis of carbo‐nitride precipitation kinetics in tempered martensite of Nb–Ti‐microalloyed steel with a carbon content of 0.3 wt%. Based on the information obtained from transmission electron microscopy and scanning electron microscopy, a computational simulation procedure is developed within the software package MatCalc, which is capable of describing the experimental results in terms of the number density, composition, and type of precipitate phases. No explicit fitting parameters are used in the computer simulation. The input data is entirely based on independent physical or microstructural parameters. To determine the chemical composition and type of precipitates, energy dispersive X‐ray spectroscopy and selected area electron diffraction are utilized. The simulation results and the experimentally obtained information are in good agreement.

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This work presents different modeling approaches for the description of the microstructure evolution in a typical C–Mn micro‐alloyed steel for pipes and tubes in the oil field industry. A comparison between the continuous‐cooling‐transformation (CCT) diagram of the material with no deformation and the DCCT diagram after single‐ and multiple‐step deformation is presented. The experiments are performed on a high‐speed deformation dilatometer. The start of austenite decomposition is simulated with the software JMatPro, whereas the bainite and martensite start temperature are additionally evaluated with empirical formulae available in the literature. The precipitation state of the micro‐alloying elements is investigated by transmission and scanning electron microscopy and compared with calculated ones by using the software package MatCalc. Our simulation results are directly applicable to the optimization of microstructure and properties in the production of steels for oil country tubular goods.

Residual stresses in quenched seamless steel tubes highly depend on the cooling conditions to which the tubes have been subjected. The design aspect of how to use controlled cooling strategies in multiphase steel tubes to achieve certain residual stress and phase configurations is discussed. In an experimentally validated finite element (FE) model considering a coupled evolution of martensite and bainite, three cooling strategies are tested for a low-alloyed 0.25 wt.% C steel tube. The strategies are (i) external cooling only, (ii) internal and external cooling for low residual stresses in a mainly martensitic tube, and (iii) internal and external cooling with low cooling rate for a mainly bainitic tube. The strategies represent design cases, where low residual stresses with different phase compositions are provoked, in order to show the potential of numerical analysis for residual stress and property design. It can be concluded that, for the investigated steel class, intense external cooling leads to a characteristic residual stress profile regardless of the dimension. A combination of external and internal cooling allows a more flexible design of residual stress and phase distribution by choosing different cooling parameters (i.e., water amount and cooling times). In general, lower cooling rates lead to lower thermal misfit strains, and thus less plasticity and lower residual stresses.

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