Modern high-strength low-alloy (HSLA) steels have to fulfill a combination of high strength and toughness. In general, these properties are contrary, due to the fact that a high strength normally decreases the toughness of the material. Nevertheless, a grain refinement increases both, the toughness and the strength of steels. For HSLA steels, thermomechanical rolling is the processing of choice to obtain a finegrained microstructure for industrially relevant applications. [1,2] Adding microalloying elements like niobium, titanium, or vanadium increases the temperature nonrecrystallization (T NR ), which is the temperature below which no complete static recrystallization between two rolling passes takes place. Higher rolling temperatures allow lower rolling forces but have the disadvantage of resulting in grain growth. Titanium as a microalloying element retards the grain coarsening because it leads to the formation of TiN precipitates. These TiN precipitates are stable at temperatures, where Nb is in solution and which often is the starting temperature of the rolling process. [3] The equilibrium temperature for Nb(C,N) in austenite was estimated by Irvine et al. [4,5] and is given in Equation (1). log ðm%NbÞ m%C þ 12 14Nb delays the recrystallization of austenite in two ways. On the one hand, by the solute-drag effect and on the other hand, by precipitates. The solute-drag effect retards the recrystallization when Nb is dissolved in the austenite but is less effective than Nb-enriched precipitates, e.g., carbides, nitrides, and carbonitrides

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Dynamic strain-induced transformation (DSIT) enables the formation of fine-grained ferritic microstructures, which are well suited for cold forming processes in the as-rolled condition. In this work, the formation mechanism, chemical composition, and crystallographic orientation of DSIT ferrite were investigated in a micro-alloyed steel and compared to pre-eutectoid ferrite. High-resolution techniques, such as scanning transmission electron microscopy and atom probe tomography (APT), were used for the investigations. To generate DSIT ferrite and pre-eutectoid ferrite, different experimental routes were applied using a compression deformation dilatometer. The results show a large number of NbC precipitates within DSIT ferrite, and show that the formation of DSIT ferrite is accompanied with C diffusion and the formation of retained austenite. APT measurements revealed that the C- and Mn concentration in DSIT ferrite is higher compared to pre-eutectoid ferrite. The crystallographic orientation of DSIT ferrite was examined using electron backscatter diffraction. The crystallographic orientation of DSIT ferrite after the deformation route revealed that the <111> plane normals are parallel to the compression direction with the <110> directions pointing towards the radial direction of the compressed sample. The results suggest that the formation of DSIT ferrite is a displacive mechanism, accompanied by C diffusion.

To achieve higher strength and good hardenability and at the same time use the positive effects of thermomechanical controlled processing, the concept of Nb and Ti microalloyed steels is increasingly used for high‐strength low‐alloy (HSLA) steels with higher C contents. Herein, how the addition of Ti affects the grain growth and static recrystallization behavior of a Nb microalloyed HSLA steel with a C content of 0.23 wt% is investigated. For this reason, alloys with varying Ti and constant Nb content are produced and investigated by means of annealing and double‐hit deformation experiments. Atom probe tomography measurements reveal that the Nb concentration in the matrix decreases with increasing Ti content. Therefore, the static recrystallization behavior is steadily inhibited with decreasing Ti content, as more dissolved Nb is available for the formation of strain‐induced NbC precipitates. The annealing experiments show that the combined addition of Ti and Nb is most effective against grain coarsening, regardless of whether the Ti content is 90 or 180 ppm. To use the positive properties of Ti against grain coarsening and Nb to inhibit recrystallization, a middle content must be chosen when alloying Ti to HSLA steels with higher C content.

This work aimed at developing a methodology for examining the naturally grown passive layer of a thickness of just a few nanometers of an austenitic CrMnN steel by means of atom probe tomography and gaining knowledge on the structure of this alloy’s passive layer. The sample surface was ground, polished, cleaned, degreased, electrolytically polished, and oxidized in air to produce a reproducible passive layer. The oxide layer was subsequently coated with a silver layer of a thickness of 3 μm. The silver layer protects the oxide layer during the preparation of the atom probe tips in the focused ion beam microscope and the alignment of the tip in the atom probe. The samples were measured in the atom probe’s pulsed-voltage mode. The findings show that an enrichment of oxygen, molybdenum, nitrogen, and chromium and a depletion of manganese, nickel, and iron occur in the area of the passive layer.