The hot ductility of Ti-Nb microalloyed steel has been investigated to evaluate the sensitivity to surface crack formation during the continuous casting process. Tensile samples were subjected to different thermal treatments and were tested at deformation temperatures ranging from 650°C to 1000°C using a strain rate of 10-3s-1. It has been found, that the investigated steel evinced poor ductility over almost the whole testing temperature range characterized by marked grain boundary cracking, irrespective of which thermal cycle has been utilized or whether the samples have been melted or only reheated. Microstructural examinations and supplementary thermo-kinetic computer simulations revealed distinct Ti-Nb precipitation throughout the microstructure being responsible for the deteriorated materials hot ductility.

Crack formation on the surface of continuously cast microalloyed steel can cause severe production problems. In the present investigation, the impact of the thermal history of "Surface Structure Control Cooling" (SSCC) on the ductility behavior of Ti-killed microalloyed steel is examined with the goal of preventing transverse cracks. The SSCC heat treatment consists of a double phase transformation as a result of intensive cooling and reheating. Metallography and transmission electron microscopy analysis are carried out to describe the evolution of microstructure and precipitation state. In addition, the precipitate phase fraction, mean radius, and number density are simulated with the thermokinetic software MatCalc and compared with experimental results. The results of the computer simulations are in good agreement with experimental observation and a clear correlation between precipitation and reduced ductility is observed.

For understanding and describing the materials behaviour during phase transformations, dilatometer measurements are commonly used in the scientific community. Generally, standard CCT diagrams are generated by means of dilatometer experiments. However, in many cases, real weld cycles are more suitable for studying the materials behaviour instead of using CCT diagrams. In this regard we talk about so-called in-situ diagrams.In this work, two different devices have been used to obtain CCT as well as in-situ diagrams of a low alloyed steel 10GN2MFA. The goal was to compare the results obtained on the one hand by using a widely used standard dilatometer (Bähr DIL-805A/D) and on the other hand by the usage of a thermomechanical simulator called Gleeble®3800. The paper should finally show advantages as well as disadvantages of using different devices and to keep in mind that the optimal measurement method depends on different parameters. Realized experiments also have revealed the range of operational and diagnostic potential of used devices.

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