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Metallurgical

   

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Processing Solutions

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Spring'17 release of ME-databases

Transverse cracking on the surface of as-cast products is a long-standing problem in continuous casting of steel. Surface cracks occur mainly in the bending and straightening operations in a curved type continuous caster in a temperature range of 600°C to 1200°C. The reasons for this are either strain concentrations at film-like ferrite along austenite grain boundaries and/or precipitation of second phase particles. A high density of fine precipitates leads to (local) precipitation hardening with intensive stress concentrations, thus triggering the nucleation of wedge-type cavities as well as favouring interconnection of cavities surrounding the precipitates (Figure 1). This makes up the main mechanism of reduced ductility in the austenitic high-temperature region.

 

Figure1: Wedge-type cavities in microalloyed steel.

 

Experimentally, the probability that steel might suffer from transverse cracking in the course of the straightening operation during continuous casting is commonly assessed by hot tensile tests in a thermo-mechanical simulator (e.g. Gleeble). Thermokinetic simulations with MatCalc can predict the precipitation state, which corresponds to the particular heat treatment state. With this information, it is possible to estimate the susceptibility of transverse cracking. In the simulation, the complete thermo-mechanical history of the material, including the strain rate of deformation is considered.

The simulation starts with the cooling segment in the continuous casting process. Afterwards follows the heat treatment, which is performed on the Gleeble. The specimens are solution treated at 1320°C and, then, cooled down to test temperature, where tensile deformation is applied. The simulated strain rate is 1∙10-3s-1 and the simulation is stopped once the specimens have been fractured in the experiment. Figure 2 displays the calculated evolution of the mean particle radii and the number densities during the Gleeble heat treatment.

 
Figure2: Calculated evolution of the mean particle radius and the number density of (Ti,Nb),(C,N) precipitates.