Ultra Fine-Grained 6wt% Manganese TRIP Steel

Abstract

Ultra-fine grained TRIP steels (UFG-TRIP) containing 6wt%Mn were produced by intercritical annealing. An ultra-fine grained microstructure with a grain size less than 1μm was obtained. The formation mechanism of the high volume fraction of retained austenite was investigated by dilatometry, XRD and magnetic saturation. The fraction of retained austenite was strongly dependent on the annealing temperature. The tensile properties were also found to be strongly influenced by the annealing temperature with poorer mechanical properties being observed at higher annealing temperatures. It was found that the stabilization of the retained austenite was both a composition and size-effect, made possible by the grain refinement due to the reversely transformed martensite.

Austenite Stability Effects on Tensile Behavior of Manganese-Enriched-Austenite Transformation-Induced Plasticity Steel

“…[9][10][11][12][13][14][15][16][17][18] The observed tensile behavior is highly dependent on processing conditions and annealing temperature. Steels with radically different flow behavior, including discontinuous yielding, positive strain hardening, and serrated flow behavior at high strains have been observed.…”

“…Steels with radically different flow behavior, including discontinuous yielding, positive strain hardening, and serrated flow behavior at high strains have been observed. [9,[11][12][13][14][15][16][17][18] Differences in mechanical behavior may be explained in part by the dependence of the mechanical stability of austenite on variations in processing history.…”

“…Significant austenite fractions (20 to 40 pct) have been obtained in this way depending on processing and Mn content. [9][10][11][12][13][14][15][16][17][18] A methodology for identifying heat treatment conditions for optimal Mn enrichment has been developed based on an equilibrium thermodynamic analysis. [5,6] Predictions of austenite amount and composition as a function of annealing temperature were made using THERMO-CALC* software with the TCFE2 database for a 0.1-C 7.1-Mn 0.1-C steel, and the predicted austenite C, Mn, and Si contents are shown in Figure 1.…”

Comparative Investigation of Phase Transformation Behavior as a Function of Annealing Temperature and Cooling Rate of Two Medium‐Mn Steels

“…Since then, medium-Mn steels have gained increased attention. [16][17][18][19] These steel grades have an ultra-fine grained microstructure with a ferritic matrix and small dispersed retained austenite islands of about 30 vol%, which can be achieved via intercritical annealing. To ensure sufficient chemical stabilization of the austenite by C-and substantially Mn-enrichment, prolonged annealing times provided, e.g., by batch annealing are conducive.…”

“…[3][4][5][6][7][8] High strength levels combined with excellent formability offer the advantage of reduced steel sheet thickness and enhanced component complexity, both enabling car mass reduction. [9] Dual-phase (DP), TRIP, and complex-phase (CP) steels with R m  A 80 % 10.000-15.000 MPa% are commonly referred to as the first-generation AHSS.…”