Microstructural Changes of the Nanostructured Bainitic Steel Induced by Quasi-Static and Dynamic Deformation

Abstract

Changes in the microstructure of nanostructured bainitic steel induced by quasi-static and dynamic deformation have been shown in the article. The method of deformation and strain rate have important impact on the microstructure changes especially due to strain localization. Microstructure of nanostructured steel Fe-0.6%C-1.9Mn-1.8Si-1.3Cr-0.7Mo consists of nanometer size carbide-free bainite laths and 20-30% volume fraction of retained austenite. Quasi-static and dynamic (strain rate up to 2×10 2 s -1 ) compression tests were realized using Gleeble simulator. Dynamic deformation at the strain rate up to 9×10 3 s -1 was realized by the Split Hopkinson Pressure Bar method (SHPB). Moreover high energy firing tests of plates made of the nanostructured bainitic steel were carried out to produce dynamically deformed material for investigation. Adiabatic shear bands were found as a result of localization of deformation in dynamic compression tests and in firing tests. Microstructure of the bands was examined and hardness changes in the vicinity of the bands were determined. The TEM examination of the ASBs showed the change from the internal shear band structure to the matrix structure to be gradual. This study clearly resolved that the interior (core) of the band has an extremely fine grained structure with grain diameter ranging from 100 nm to 200 nm. Martensitic twins were found within the grains. No austenite and carbide reflections were detected in the diffraction patterns taken from the core of the band. Hardness of the core of the ASBs for examined variants of isothermal heat treatment was higher about 300 HV referring to steel matrix hardness.

Microstructure and Dynamic Mechanical Behavior of Thermomechanically Rolled 3Mn–Al and 5Mn–Al Sheet Steels

“…According to refs. [24,25], the formation of ASB is common during dynamic deformation. However, this is true only for steels with ultrahigh strength levels like nano-bainitic steels or martensitic steels (the steels with a strength level of 2000 MPa).…”

Controlling the Content and Morphology of Phase Constituents in Nanobainitic Steel Containing 0.6%C to Obtain the Required Ratio of Strength to Plasticity

“…Plates with thickness from 6 to 9 mm were hot rolled from the flat transfer bars. Based on examination results so far, and industrial trials of plates heat treatment, the difference between the Ms temperature and the IHT temperature should not exceed about +30 • C and period of time is in the range of 48-144 h [4][5][6][7][8]. The final heat treatment of the investigated steels consisted of austenitizing at 950 • C for 30 min, controlled air cooling, with a rate higher than 1 • C/s, to the temperature of IHT and direct holding at a temperature in the range of 200-225 • C. This was carried out in laboratory electric chamber furnaces.…”

“…Except for the size, the important difference between the two morphologies of retained austenite is the amount of carbon remaining in solid solution; the carbon content is higher in film-like than in blocky retained austenite, with crucial consequences for its mechanical stability [1,3]. Research on nanobainitic steels, focused on specific applications, has been carried out at Łukasiewicz-IMŻ [4][5][6][7][8]. Correlations between the chemical composition, microstructure, mechanical properties, production parameters, and functional properties are the subject of this research.…”