Carbon in Solution and the Charpy Impact Performance of Medium Mn Steels

ORIGINAL RESEARCH ARTICLE Carbon in Solution and the Charpy Impact Performance of Medium Mn Steels T.W.J. KWOK, F.F. WORSNOP, J.O. DOUGLAS, and D. DYE Carbon is a well known austenite stabiliser and can be used to alter the stacking fault energy and stability against martensitic transformation in medium Mn steels, producing a range of deformation mechanisms such as the Transformation Induced Plasticity (TRIP) or combined Twinning and Transformation Induced Plasticity (TWIP + TRIP) effects. However, the effect of C beyond quasi-static tensile behaviour is less well known. Therefore, two medium Mn steels with 0.2 and 0.5 wt pct C were designed to produce similar austenite fractions and stability and therefore tensile behaviour. These were processed to form lamellar and mixed equiaxed + lamellar microstructures. The low C steel had a corrected Charpy impact energy (KV10 ) of 320 J cm2 compared to 66 J cm2 in the high C steel despite both having a ductility of over 35 pct. Interface segregation, e.g., of tramp elements, was investigated as a potential cause and none was found. Only a small amount of Mn rejection from partitioning was observed at the interface. The fracture surfaces were investigated and the TRIP effect was found to occur more readily in the Low C Charpy specimen. Therefore it is concluded that the use of C to promote TWIP + TRIP behaviour should be avoided in alloy design but the Charpy impact performance can be understood purely in terms of C in solution. https://doi.org/10.1007/s11661-023-07157-y Ó The Author(s) 2023 I. INTRODUCTION MEDIUM Mn steels (4 to 12 wt pct Mn) are a relatively recent class of steels despite their conception in 1972.[1] Having been ‘‘rediscovered’’ as a leaner alternative to high Mn Twinning Induced Plasticity (TWIP) steels (16 to 30 wt pct Mn), medium Mn steels have been shown to exhibit several different plasticity enhancing mechanisms such as the Transformation Induced Plasticity (TRIP) effect[2,3] or a combined TWIP + TRIP effect.[4,5] Both mechanisms can be tailored through heat treatments and alloying to vary the strain hardening rate, leading to large elongations to failure of over 50 pct.[6,7] These tensile properties make medium Mn steels T.W.J. KWOK is with the Department of Materials, Royal School of Mines, Imperial College London, Prince Consort Road, London SW7 2BP, UK and also with the Singapore Institute of Manufacturing Technology (SIMTech), Agency for Science, Technology and Research (A*STAR), 5 Cleantech Loop, Cleantech Two Block B, Singapore 636732, Republic of Singapore. F.F. WORSNOP is with the Department of Materials, Royal School of Mines, Imperial College London and also with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139. J.O. DOUGLAS and D. DYE are with the Department of Materials, Royal School of Mines, Imperial College London. Contact e-mail: Manuscript submitted January 3, 2023; accepted July 27, 2023. Article published online August 10, 2023 4128—VOLUME 54A, OCTOBER 2023 very suitable materials for energy absorbing applications such as automotive crash pillars.[8,9] Current safety related automotive steels are designed to be either anti-intrusion or to crumple and absorb as much energy as possible in the event of a crash. Hot stamping or press hardening martensitic steels such as 22MnB5 are examples of anti-intrusion steels which were designed to be very strong and resist deformation.[10,11] Energy absorbing steels such as Dual Phase (DP) steels[12] are softer but significantly more ductile to allow the steel to crumple and fold, absorbing energy in the process. The opportunity for medium Mn steels, therefore, is to replace DP steels in the automotive Body in White (BIW)[8,9] as they have equivalent or better tensile properties and are also potentially cheaper due to the omission of expensive alloying elements such as Cr, Nb and V. The ability to exhibit the TWIP + TRIP effect upon deformation, therefore, was of considerable academic interest due to the prospect of activating two powerful plasticity enhancing mechanisms. Typically, TWIP + TRIP-type medium Mn steels do indeed exhibit larger elongations to failure compared to TRIP-type medium Mn steels ( 50 vs  25 pct).[4,13,14] The activation of the TWIP + TRIP effect depends on the control of Stacking Fault Energy (SFE) and stability against transformation of the austenite phase in medium Mn steels. In order to raise the SFE into the twinning regime, a large amount of C, typically more than 0.4 wt METALLURGICAL AND MATERIALS TRANSACTIONS A Table I. Composition of the Ingots Used to Produce High C and Low C Plate Steels in Mass Percent Obtained Using ICP; and IGF for Elements Marked with y High C Low C Mn Al Si Cy Ny Sy P Fe 4.35 6.30 3.03 2.17 1.46 0.99 0.491 0.223 0.003 0.004 0.002 0.001 < 0.005 < 0.005 bal. bal. pct, is needed while keeping the Mn content within the ‘‘medium’’ range of between 3 and 12 wt pct. However, our previous work[7] and the results by Lee et al.[4] showed that the strengthening effect from twinning was very small compared to the TRIP effect. It was therefore postulated that the large elongation in TWIP + TRIP-type medium Mn steels came from a very controlled TRIP effect due to the very stable and C-enriched austenite. Nevertheless, regardless of the strengthening contribution from TWIP or TRIP, TWIP + TRIP-type medium Mn steels still have higher strengths (due to the higher C content) and elongations than most TRIP-type medium Mn steels.[7] Since the energy absorbed during plastic deformation is equal to the area under a tensile curve, it should also follow that TWIP + TRIP-type medium Mn steels would be more suitable for energy absorbing applications than TRIP-type medium Mn steels. Furthermore, the TWIP effect was also shown to be active at high strain rates up to approximately 2000 s1 ,[15] while the TRIP effect is diminished at high strain rates due to adiabatic heating.[16] Therefore, it is possible that the TWIP effect might begin to play a significant role at higher strain rates. High strain rate tests such as the Hopkinson pressure bar test would be able to provide very useful information but are relatively difficult to perform.[15] Alternatively, Charpy V-notch tests can also provide some insights into the failure mechanisms, tear resistance, notch toughness and energy absorption at high strain rates of up to 103 s1 depending on the type of material.[17] In this study, the Charpy energies of two different medium Mn steels will be compared: a high C TWIP + TRIP-type medium Mn steel with a mixed equiaxed + lamellar microstructure, developed in previous work,[7] and a novel low C TRIP-type medium Mn steel with a fully lamellar microstructure. This study aims to identify and compare the failure mechanisms in both steels in order to guide future alloy design. II. EXPERIMENTAL Two steel ingots, High C and Low C, we (...truncated)