The paper reveals information about crack behavior during hot rolling of heavy-gauge products. FEM-based mathematical modeling has been used to investigate the impact of crack shape and rolling parameters on the distribution of the principal tensile stress and the Cockroft-Latham fracture criterion value and on the crack closing capacity. The results clearly show that the crack shape has the most powerful influence on the observed quantities followed by the rolling geometry factors. To find a critical value of the LCK for a specific steel grade and an actual rolling process, a laboratory experiment has been conducted, in which problems related to propagation of initial cracks were detected. It consisted in rolling of wedge-shaped specimens with a notch, which was introduced for the purpose of raising the probability of crack occurrence. In order to find the Critical LCK value in the area of the notch, an additional computer simulation was performed. Results of these experiments led to the conclusion that the steel possesses excellent formability. The Critical LCK values were up to 300 MPa, which is, essentially, an order higher than in actual rolling. As the critical value of LCK depends, among others, on the metallurgical characteristics of the metal, the key to the issue is in the steel microstructure. The findings brought the conclusion that in this specific case the rolling itself affects the crack propagation to a small extent only. The heating process poses a much greater challenge, as it is the likely stage involving the steel overheating.

It is not realistic to optimize the roll pass design of profile rolling mills, which typically roll hundreds of profiles, using physical modelling or operational rolling. The use of reliable models of microstructure evolution is preferable here. Based on the mathematical equations describing the microstructure evolution during hot rolling, a modified microstructure evolution model was presented that better accounts for the influence of strain-induced precipitation (SIP) on the kinetics of static recrystallization. The time required for half of the structure to soften, t0.5, by static recrystallization was calculated separately for both situations in which strain-induced precipitation occurred or did not occur. On this basis, the resulting model was more sensitive to the description of grain coarsening in the high-rolling-temperature region, which is a consequence of the rapid progress of static recrystallization and the larger interpass times during rolling on cross-country and continuous mills. The modified model was verified using a plain strain compression test (PSCT) simulation of rolling a 100-mm-diameter round bar performed on the Hydrawedge II hot deformation simulator (HDS-20). Four variants of simulations were performed, differing in the rolling temperature in the last four passes. For comparison with the outputs of the modified model, an analysis of the austenite grain size after rolling was performed using optical metallography. For indirect comparison with the model outputs, the SIP initiation time was determined based on the NbX precipitate size distribution obtained by TEM. Using the PSCT and the outputs from the modified microstructure evolution model, it was found that during conventional rolling, strain-induced precipitation occurs after the last pass and thus does not affect the austenite grain size. By lowering the rolling temperature, it was possible to reduce the grain size by up to 56 μm, while increasing the mean flow stress by a maximum of 74%. The resulting grain size for all four modes was consistent with the operating results.

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Use of the protective steel capsules enabled to manage the laboratory hot flat rolling of the extremely brittle as-cast aluminide Fe-40at.%Al-Zr-B with the total height reduction of almost 70 %. The hot rolling parameters were optimized to obtain the best combination of deformation temperature (from 1160°C up to 1240°C) and rolling speed (from 0.14 m·s−1 to 0.53 m·s−1). The resistance against cracking and refinement of the highly heterogeneous cast microstructure were the main criteria. Both experiments and mathematical simulations based on FEM demonstrated that it is not possible to exploit enhanced plasticity of the investigated alloy at low strain rates in the hot rolling process. The heat flux from the sample to the working rolls is so intensive at low rolling speed that even the protective capsule does not prevent massive appearance of the surface transverse cracking. The homogeneity and size of product’s grain was influenced significantly by temperature of deformation, whereas the effect of rolling speed was relatively negligible. The optimal forming parameters were found as rolling temperature 1200°C and the rolling speed 0.35 m·s−1. The effective technology of the iron aluminide Fe-40at.% Al-Zr-B preparation by simple processes of melting, casting and hot rolling was thus established and optimized.