Q&P PROCESS IN PRESS-HARDENING OF 42SICR STEEL

Abstract

One of today’s advanced heat treatment routes for high-strength steels is the Q&P process which delivers high ultimate strengths combined with good ductility. The resulting microstructure is a combination of martensite and small fractions of bainite and retained austenite. Retained austenite has the form of thin needles adjacent to martensite laths. The use of this process in industrial practice is complicated by the need for holding at the partitioning temperature when retained austenite becomes stabilized by carbon migration from super-saturated martensite. Engineers therefore seek process routes in which interrupting the cooling process at a particular temperature and holding at that temperature do not pose technological problems. One of the available options is press hardening, often incorporated in the treatment of car body parts. 42SiCr steel, which is alloyed with manganese, silicon and chromium, was Q&P-processed using experimental sequences with various quenching and partitioning temperatures. The soaking time and temperature and cooling rates were identical to the parameters used in real-world processes. Correctly-chosen parameters led to martensitic-bainitic microstructures with a portion of retained austenite, ultimate strength of around 2000 MPa and A20 elongation of more than 10%.