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High-strength martensitic steels with low Ms temperature

Bublikova D., Masek B., Jirkova H., Jenicek S., Ibrahim K., Cubrova J.

Archives of Materials Science and Engineering

2014

Vol. 65, nr 2

53--57

Purpose: Various novel heat treatments and thermomechanical treatment procedures have been developed recently in order to enhance mechanical properties of high-strength steels. Design/methodology/approach: The new heat treatment process was tested on newly designed low alloy AHSS steels alloyed with manganese, silicon, chromium and alternatively molybdenum and nickel. Two different cooling rates of 16°C/s and 1°C/s showed that over a wide interval of cooling rates a hardened structure based on martensite and retained austenite can be achieved with an average hardness from 538 to 716 HV10 and ultimate strength in the range between 1837 and 2354 MPa. Findings: The main aim of these steels was to decrease the temperature Mf to below 100°C by quenching in a water or oil bath and use common industrial techniques. Manganese alloying was used for the required decrease of temperatures Ms and Mf. Silicon and chromium were used to increase the ultimate strength, silicon was used to eliminate the carbide formation and to support sufficient oversaturation of martensite. Practical implications: The industrial application of this process is complicated by the need for quenching in the range between Ms and Mf. The Ms temperature normally lies in the 200-250 °C interval. This is why new steels with low Ms temperature were selected for the experimental programme. Thanks to this feature, the steels could be quenched in water or polymer-based baths. The primary alloying elements of these four selected steels were manganese, silicon, molybdenum and chromium. Originality/value: The experimental heat treatment led to microstructures of martensite with retained austenite in all steels. Their strengths were in the range of 1750-2400 MPa and their A5 mm elongation levels were up to 10%. The morphology and distribution of retained austenite dictate the resulting mechanical properties of the material.