Speaker
Description
The increasing use of steel scrap in modern steel production inevitably leads to elevated residual copper contents. Since there are currently no conventional metallurgical processing routes capable of efficiently removing copper from the melt, alternative alloy and process strategies are required to transform residual copper from a problematic impurity into a strengthening mechanism for high-performance steels. At the same time, the steel industry faces growing pressure to reduce energy consumption and CO₂ emissions associated with conventional heat treatment routes for high-strength components. In this context, the NitridAge concept was developed to investigate whether tailored hardness gradients between core and surface regions can be achieved within a single thermomechanical processing route while remaining economically feasible. For this purpose, a new alloy approach in combination with tailored thermomechanical-thermochemical processing was investigated.
The presented study investigates a modified low-alloyed Q&T steel 23MnB4 with intentionally increased Cu content designed for cold heading and cold extrusion operations. Instead of relying on conventional quenching and tempering, the proposed approach combines direct patenting, cold heading and precipitation hardening during gas nitriding. Particular attention was paid to the interaction between Cu precipitation, ferritic microstructure formation and nitriding response.
Experimental processing included laboratory melting, hot rolling, controlled cooling, subsequent cold deformation trials and simulation-supported nitriding process design. Microstructural characterization was carried out using light microscopy, SEM, hardness measurements and tensile testing. The influence of alloy composition on nitriding response, microstructural evolution and cold formability was also evaluated.
The results demonstrate that controlled cooling after adjusted hot rolling enables the formation of comparatively soft ferritic-pearlitic microstructures suitable for subsequent cold forging, despite elevated copper contents.
Subsequent nitriding results in a significant hardness increase at the surface and enables the development of adjustable hardness gradients between surface and core regions. Furthermore, the obtained results indicate that the combination of Cu-induced precipitation hardening and tailored nitriding treatments enables additional adjustment of surface and core properties, opening further possibilities for the development of energy-efficient cold-forged wear-resistant steel components with locally adapted mechanical performance.
Keywords: Cu-containing steels; recycled steel; gas nitriding; precipitation hardening; cold forging; hardness gradient; thermomechanical processing; cold formability; sustainable steel design; alloy design.