The present work deals with the dilatometric study of a hot-rolled 0.2C3Mn1.5Si lean medium Mn steel, mainly suitable for the quenching and partitioning (Q&P) heat treatment in both hot-rolled or cold-rolled condition, subjected to a variation of austenitization temperature. These investigations were performed in a temperature range of 800–1200 °C. In this context, the martensite transformation start temperature (Ms) was determined as a function of austenitization temperature and in turn obtained prior austenite grain size (PAGS). The results show rise in prior austenite grain size due to increasing austenitization temperature, resulting in elevated Ms temperatures. Measured dilatation curves were confronted with the metallographic analysis by means of scanning electron microscopy (SEM). The present paper also focuses on the construction of a continuous cooling transformation (CCT) and deformation continuous cooling transformation (DCCT) diagram of the investigated lean medium Mn steel in a range of cooling rates from 100 to 0.01 °C/s and their subsequent comparison. By comparing these two diagrams, we observed an overall shift of the DCCT diagram to shorter times compared to the CCT diagram, which represents an earlier formation of phase transformations with respect to the individual cooling rates. Moreover, the determination of individual phase fractions in the CCT and DCCT mode revealed that the growth stage of ferrite and bainite is decelerated by deformation, especially for intermediate cooling rates. Microstructural changes corresponding to cooling were also observed using SEM to provide more detailed investigation of the structure and present phases identification as a function of cooling rate. Moreover, the volume fractions obtained from the saturation magnetization method (SMM) are compared with data from X-ray diffraction (XRD) measurements. The discussion of the data suggests that magnetization measurements lead to more reliable results and a more sensitive detection of the retained austenite than XRD measurements. In that regard, the volume fraction of retained austenite increased with a decrease of cooling rate as a result of larger volume fraction of ferrite and bainite. The hardness of the samples subjected to the deformation was slightly higher compared to non-deformed samples. The reason for this was an evident grain refinement after deformation.

The automotive industry’s push for lightweight, high-strength materials has led to the advancement of third-generation advanced high-strength steels (AHSS). Known for their blend of ultra-high tensile strength and ductility, these steels are ideal for structural applications. However, their adoption has faced obstacles, notably due to Zinc (Zn)-assisted liquid metal embrittlement (LME), particularly in cases where Zn-based corrosion-resistant coatings are used during resistance spot welding (RSW). This study explores testing methodologies for evaluating LME susceptibility and examines the impact of silicon (Si) concentration, specifically between 0.5 and 1.4 wt.-%, on Zn-LME susceptibility in AHSS. This research introduces two primary testing approaches: spot welding and hot tensile tests, each designed to quantify LME behavior. Both methods show that higher Si levels correlate with increased LME sensibility, evidenced by greater ductility loss and the formation of longer critical cracks during welding. Thermodynamic modeling further demonstrates that Si affects phase stability in the Fe-Zn system, broadening the stability of liquid Zn. The findings highlight Si’s significant role in Zn-LME susceptibility and underscore the importance of robust testing methods to facilitate the safer application of AHSS in automotive manufacturing.

The impact of Si on Zn-induced liquid metal embrittlement (LME) in 3 ^rd generation advanced high strength steels (AHSS) during resistance spot welding has been widely studied, but the effect of Al is rather unknown. This study investigates the substitution of Al for Si by analyzing two steels with the fixed C and Mn-contents of 0.2 and 3 wt.-% respectively. Si and Al-contents are both set to 1.4 wt.-%. To minimize microstructural effects, all steels were quenched and tempered before electro-galvanizing. The effects of Si and Al were examined using hot tensile testing (600 – 900 °C, in 50 K steps) on a Gleeble 3800, resistance spot welding with prolonged welding times, thermodynamic calculations with Thermo-calc® and dilatometry. Results indicate that the use of either Si or Al increases the LME-susceptibility but substituting Al for Si significantly reduces Zn-induced LME-cracking. In hot tensile testing, higher testing temperatures generally increase the steel’s vulnerability to LME. But comparing both alloying elements to one another, Si causes a higher LME-susceptibility.