Reinterpretation and demonstration of deformation ageing in cold-formable steels

Many processing companies use some grades of steel for cold forming. Due to ageing, the formability of unalloyed mild steel sheets deteriorates over time; their formability is not uniform, and surface defects appear, which in many practical applications leads to scrap. This can be particularly important for processing companies when selecting machine settings, as there may be differences in the properties of materials from different manufacturers and the properties of materials of different ages from the same manufacturer. We investigated the ageing behaviour of DC01 steel grade by testing many samples from the third day after production up to one year. Various methods were used to detect ageing, such as tensile tests at different test speeds and supplemented with a digital image correlation (DIC) system, thermo-electric power measurement, deep drawing cup tests, and Nakajima tests. We showed that a reinterpretation of the definition of ageing is necessary. Standard tensile testing can no longer detect the ageing tendency of steels manufactured using the latest technology. However, when supplemented with DIC, these tests show ageing with high sensitivity. Among the used methods the applicability of this technique in industrial material testinglaboratories is the most recommended.