23. - 25. 5. 2012, Brno, Czech Republic, EU EFFECT OF ADVANCED THERMOMECHANICAL TREATMENT ON MECHANICAL PROPERTIES OF LOW ALLOYED HIGH STRENGTH STEELS Hana JIRKOVÁ, Ludmila KUČEROVÁ, Vojtěch PRŮCHA, Bohuslav MAŠEK University of West Bohemia in Pilsen, Research Centre of Forming Technology – FORTECH, Univerzitní 22, 306 14 Pilsen, Czech Republic, h.jirkova@email.cz Abstract Specific treatment used for TRIP steels, as well as quenching and partitioning (Q-P) processing rely on producing multi-phase microstructures with retained austenite in low-alloyed steels in order to achieve excellent mechanical properties. It is namely the values of the product of strength and elongation that reach outstanding levels. The desired microstructure for exploiting the TRIP effect consists of carbide-free bainite, ferrite and a small amount of retained austenite. It can be produced by intercritical annealing. Depending on the chemical composition of the steel, this microstructure may exhibit strengths between 800 and 1000 MPa and elongations up to 30 %. Microstructures with martensite matrix produced by Q-P process contain retained austenite in the form of films between martensite laths. They show strengths of up to 2000 MPa and elongations of about 10 %. In order to achieve such properties, suitable combinations of alloying elements must be chosen to prevent carbide precipitation and to suppress competing reactions. Suitable heat treatment or thermomechanical treatment must be applied as well. Experimental schedules involving intercritical annealing and Q-P process that are described in this paper were carried out on two low-alloyed steels containing various levels of carbon, manganese and chromium. The intercritical annealing comprised a hold at 425 °C. Its purpose was for bainite to form and for retained austenite to stabilize. In the Q-P process, quenching was carried out below the M s temperature. Q-P- processed 42SiCr steel (0.4 %C) showed ultimate strength of 1900 MPa and an elongation above 15 %. In comparison, Q-P processed CMnSi TRIP steel (0.2 %C) showed a strength of 1175 MPa and elongation of A 5mm = 16 %. Heat treatment of 42SiCr steel simulating intercritical annealing produced atypical bainite- martensite-austenite microstructures with strengths above 1900 MPa and elongations of 7 %. The CMnSi TRIP steel contained ferrite-bainite structure with retained austenite and an elongation of 36 % combined with a strength of 835 MPa. Keywords: TRIP steel, Q-P process, thermomechanical treatment, high strength steels 1. INTRODUCTION In recent years, materials with microstructures of multiple components and with retained austenite have been proven to show an excellent combination of strength and ductility. There are several types of treatment relying on retained austenite as a means of improving plasticity of steels. The best-known ones include intercritical processing and Q-P processing. Both of them can be used successfully for treating low-alloyed steels. Intercritical annealing produces a TRIP-type microstructure consisting of ferrite, bainite and retained austenite, showing elongation of almost 40% and strength of 900 – 1000 MPa. Intercritical processing comprises five steps: rapid heating, the actual intercritical annealing, rapid cooling, a hold at the temperature of bainitic transformation, and cooling to room temperature [1-2]. The key aim of Q–P processing is to achieve high strength and to retain sufficient ductility. Strengths obtained through this process exceed 2000 MPa. The elongation is about 10-15% [3-5]. Q-P process comprises austenitizing, rapid cooling below M s , and a hold at a temperature below M s . During this hold, carbon partitions from oversaturated martensite and contributes to stabilization of retained austenite.