ESSC & DUPLEX 2019

For mileage improvement and purification of exhaust gas, a high temperature exhaust gas and lightweight vehicle body are required. The heat-resistance of austenitic stainless steel is superior to ferritic stainless steel for the automotive exhaust manifold of a high-temperature exhaust gas engine and turbocharger systems at temperatures above 900°C. In turbocharger systems in particular, the replacement of austenitic cast steel or Ni-based alloy with austenitic stainless steel sheets may be effective from the viewpoint of lighter weight, lower cost, and higher turbocharger performance. Therefore, a new austenitic stainless steel sheet having heat resistance superior to conventional SUS310S (25%Cr-20%Ni) and SUSXM15J1 (19%Cr-13%Ni-3%Si) is necessary. In this study, in order to respond to the automotive exhaust manifold of a high-temperature exhaust gas engine and turbocharger systems, an excellent heat-resistant austenitic stainless steel sheet has been developed. By the fundamental researches of the effect of elements on high-temperature strength in austenitic stainless steel sheets, it was confirmed that the addition of N, C, and Mo was effective. Based on this result, 24%Cr-12%Ni-2%Si-0.6%Mo-0.1%C-0.2%N was selected as the optimum chemical composition for a new austenitic stainless steel sheet. The developed steel (NSSC®701) has higher strength than SUS310S and SUSXM15J1 between 700°C and 1000°C. At 900°C, 0.2% proof stress of the developed steels is 111MPa, and is approximately equal to Alloy 718 (Ni-based alloy), 1.8 times of SUS310S, 2.1 times of SUSXM15J1, and 4.3 times of SUS444 (19%Cr-2%Mo) with the highest high-temperature strength in ferritic grade. In addition, at 800°C and 900°C, the minimum creep strain rate of the developed steel was equal to or less than SUS310S and SUSXM15J1. Furthermore, regarding the high-temperature sliding property required for the internal parts of the turbocharger, the depth of the wear mark after pin on disk type friction and wear test of developed steel was less than half that of SUS310S and SUSXM15J1. In addition to the characteristics described above, the developed steel sheet shows superior characteristics in mechanical properties, high-temperature strength after aging, high-temperature high-cycle fatigue strength, and oxidation resistance in comparison with the conventional steel. Thanks to the various characteristics of the developed steel sheet, they can be used in the automotive exhaust system and turbocharger system parts.