Effect of Carbon and Nitrogen on <i>Md</i>₃₀ in Metastable Austenitic Stainless Steel

Abstract

Md 30 is defined as the temperature at which 50 vol.% of α'-martensite is formed at a true tensile strain of 0.3 in metastable austenitic steels. The effect of C concentration on Md 30 is known to be identical to that of N, as shown by Nohara's equation. However, we found that Md 30 of C-added steel is lower than that of N-added steel, which indicates that the effect of C concentration on the mechanical stability of austenite is more significant than that of N. In addition, the relationship between Md 30 and C and N concentration is not linear. The effect of C and N concentration on Md 30 is higher at lower C and N concentration (< 0.1%). As this effect was not considered in the previous study, the austenite-stabilizing effects of these elements were underestimated. Therefore, in this study, new equations were proposed to accurately estimate Md 30 of an Fe-Cr-Ni alloy system. The modified Md 30 equation is shown below:

Effect of Carbon and Nitrogen on Work-hardening Behavior in Metastable Austenitic Stainless Steel

“…A comparison between C-and N-added steels reveals that the volume fraction of DIM in C-added steels is lower than that in N-added steels, meaning that the mechanical stabilizing effect of C is higher than that of N, as reported in a previous study. [4][5][6] In these steels, ε-martensite with HCP structure is also formed as an intermediate phase of the γ→α' transformation, but we confirmed that the amount was small (5 vol.% at most) and the ε→α' transformation was completed at 70% cold rolling for all steels. 4) Figure 2 shows phase + image quality (IQ) maps (a)-(c) and crystallographic orientation maps extracted only DIM (BCC) (d)-(f) in 20% cold-rolled Base (a) (d), 0.1C (b) (e), and 0.1N steel (c) (f).…”

“…In the 0.1C and 0.1N steels, DIM grains are aligned along straight lines within austenite grains, indicating that the DIM was nucleated in plate-like structures including deformation twins and ε-martensite developed in austenite. [4][5][6][14][15][16] The block size of DIM is fine, similar to that of Base steel. The effects of C and N on the mechanical stability of austenite, the transformation mechanism, and the crystallographic characteristics of DIM have already been studied in detail from the previous reports.…”

“…The effects of C and N on the mechanical stability of austenite, the transformation mechanism, and the crystallographic characteristics of DIM have already been studied in detail from the previous reports. [4][5][6] Figure 3(a) shows the changes in the hardness as a function of thickness reduction by cold rolling for Base, 0.1C, and 0.1N steels. The hardness of each steel increases with the reduction in the thickness, and the work hardening of 0.1C and 0.1N steels are larger than that of Base steel despite of the smaller amount of DIM in 0.1C and 0.1N steels.…”

“…In the above viewpoint, there is a difference in the effects of C and N; C is more effective in increasing the mechanical stabilization of austenite than N, [4][5][6] whereas it is generally accepted that C is more effective in increasing the hardness of athermal martensite formed by quenching than N. [7][8][9][10] Therefore, the effects of C and N on the work-hardening behavior of metastable austenitic steel are expected to be different; nevertheless, no systematic investigation has been done.…”

Morphology Dependence on Mechanical Stability of Second-phase Austenite in Martensitic Steels

“…The mechanical stability of austenite has been the subject of many studies. In addition to the chemical composition, [5][6][7] crystallographic orientation, 8) and size [9][10][11] of austenite, various factors, such as the type of matrix phase 12) and the strength difference between matrix and austenite, 13) have been reported to affect austenite mechanical stability. In the austempering, which is a general heat treatment to obtain retained austenite in low-alloy carbon steel, carbon (C) is enriched in the untransformed austenite through bainitic transformation, low-alloy TRIP assisted steel.…”