In this paper, resistance spot weldability of high‐Mn steels were investigated in order to get high reliability in welded joints of automotive components. Microstructural characterizations, cross‐tensile test (CTT), microhardness tests of spot welded parts were conducted. The effects of weld current on the microstructural characteristics, mechanical properties, and fracture modes were investigated using optical microscopy (OM) and scanning electron microscopy (SEM). The hardness in the weld nugget was observed to be lower than that in the base metal (BM). In CTT, the failure initiation was observed to occur at the boundary of the weld nugget. Also welding imperfections of welded parts were investigated. Liquation cracking in heat affected zone (HAZ), porosity, and shrinkage cavity were found most common welding defects in welded parts. Furthermore, the effects of welding imperfections on weld quality and failure criteria were identified and discussed.
There have been increasing demands for improving safety, fuel economy and dent resistance in the automotive industry. To meet these demands, a super‐formable high strength steel with excellent formability was investigated. The mechanical properties and precipitation behaviour of Ti‐Nb bearing Interstitial Free (IF) steels with various contents of Ti and P were studied. In addition, the effect of processing conditions such as the cold reduction ratio and the annealing temperature on the formability was analysed. Improved mean r‐value was obtained in Ti‐0.01Nb steels with Ti content ranging from 0.015% to 0.025%. This improvement was due to non‐scanvenging effect and FeTiP precipitates. Furthermore, with an increase in P content, the mean r‐value was increased because of grain refinement in the hot rolled strip.
The hot stamping process is an innovative forming method that could prevent the cracking of high strength steel sheets. The formability test of boron steel sheet using forming limit diagrams at elevated temperature is very complicated and time consuming job. In this paper, an alternative test method to evaluate the formability of boron steel in hot stamping has proposed. It measured the FLD0 instead of whole strain combinations of FLD with the tensile test machine and specially designed test rig. Test results shows that the proposed test method can simulate the plain strain condition fracture and can make the FLD of boron steel sheet at elevated temperature with less effort.
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