Abstract
Two-step heat treatment with bainitic hold at 425 °C was applied to low alloyed high strength steel with partial substitution of silicon by aluminium. The steel was further micro-alloyed by niobium. Various soaking temperatures in the range of 750-1250 °C were used to analyse the effect of soaking on mechanical properties and microstructure of the steel. According to calculations in JMatPro, the temperatures below 900 °C should lie in an intercritical (tow phase) region, while the temperature of 900 °C should be the first one to ensure full austenitization of this steel. To dissolve different portion of niobium into solid solution, higher soaking temperatures were also used. Tensile strengths of 780-1069 MPa and total elongation of 22-46 % were achieved.
Highlights
Ongoing development of materials with increased tensile strength, ductility and formability are fuelled by the demands of an automotive and vehicle industry
TRIP steels belong to the first generation of advanced high strength steels (AHSS), using the principle described by Zackay [6] on a novel lean alloying concept
The most promising possibility seems to be partial substitution of silicon by aluminium, which has the same effect on carbide precipitation hindering and solid solution strengthening as silicon [13, 14]
Summary
Ongoing development of materials with increased tensile strength, ductility and formability are fuelled by the demands of an automotive and vehicle industry. The steel is alloyed by 1-2 % of manganese which increases stability of retained austenite and originally, between 1-2 % of silicon was used to prevent carbide precipitation during the treatment. As those high silicon steels possessed low surface quality, which presents problems during galvanising, attempts were made to find suitable substitution [12]. The effects of aluminium are weaker than those of silicon for the same contents of both elements This is the reason why a full substitution of silicon is not convenient, as it results in a decrease of tensile strengths [15,16,17,18]. The effect of soaking temperature on the final microstructure and mechanical properties of Al-Si-Mn-Nb TRIP steel
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