Optimisation of ballistic properties of nanostructured bainitic steel plates for container armour system

Abstract: An innovative armour system containing plates made of nanostructured bainite steel is under development to improve operating properties of a light armoured observation-protective container (LAOC) providing the 2nd protection level according to STANAG 4569. Armour system solution of the container has been modified because the new plates have different mechanical and technological properties than currently used armour plates. The paper presents results of investigation of material characteristics and firing tests of plates made of nanostructured steel in the semi-industrial scale. To optimise parameters of heat treatment of the plates the kinetics of phase transformation was examined, and mechanical properties were measured, and changes in microstructure in the area of projectile interaction at firing tests were analysed for different variants of treatment. The reported investigations precede the industrial scale production process for plates made of the new grade steel designed for the armour system of LAOC. Optimisation of mechanical properties of the plates has been carried out by selection of such production parameters as chemical composition of the steel, hot working, interprocess heat treatment and temperature and time of final heat treatment (isothermal annealing). Effect of primary segregation of alloying and residual elements on protective properties of the plates is indicated. As a result of applying the annealing temperature in the range of 210-225°C (Ms temperature is ca. 200°C at cooling rate 1-2°C/s) and time in the range of 120-70 hours respectively, the following properties have been achieved: YS0.2 (yield strength) 1400-1500 MPa; UTS (ultimate tensile strength) 2000-2150 MPa; TE (total elongation) 10-13%, impact toughness KV at room temperature 10-16 J and hardness 590-610 HV (53-54 HRC). Microstructure of the plates consists of carbide free lathy nanobainite and 10-21% (volume fraction) of retained austenite. Plates of thickness 6.3 and 7.5 mm and ammunition type of 5.56x45 mm M193 and 7.62x51 mm API BZ have been used in the firing tests. Based on results of firing tests and microstructure examination in the area of projectile interaction the phenomena have been identified indicating high efficiency of ballistic protection of investigated plates, manifested by high ability to absorption and dissipation of projectile energy without susceptibility to cracking. Based on the results of mechanical properties measurements and firing tests a chemical composition of the nanostructured bainitic steel adjusted for industrial production of plates of thickness in the range of 6-8 mm has been developed. An analysis of container armour system mass reduction possibility, basing on results of investigation of the nano-structured bainite steel plates made in the semi-industrial scale, has been carried out. Basing on the analysis of the container construction and operational conditions some sensitive areas of ballistic protection have been specified to be subjected to firing tests. Ballistic examination will be carried out using the container segments representing the selected sensitive areas.