Abstract:We study here the underlying factors that govern the stability of austenite in a medium Mn (Fe–0.18C–11Mn–3.8Al) (wt-%) steel. In this regard, a novel heat treatment involving intercritical quenching and tempering was designed to obtain high total elongation (TEL) and high ultimate tensile strength (UTS) in the cold-rolled steel. And the UTS and TEL approached 920–1150 MPa and 35–65%, respectively. The product of TEL and UTS (PSE) exceeded 40 GPa%, with a maximum value of 60 GPa%. A detailed analysis of micros… Show more
“…In most medium Mn steels in the literature, the phases and compositions obtained during intercritical annealing are retained to room temperature. [21,[24][25][26] From Figure 2, it can be seen that the slight differences in composition between the 400 g and 5 kg ingot did not result in a significant change in the expected phase fractions at the chosen IA temperature of 750 C. Thermo-Calc also predicted that d-ferrite should not be expected below 1370 C, but it was clearly observed in the as-cast micrographs of both 400 g and 5 kg ingots in Figure 3. From Figure 3, the as-cast micrographs showed d-ferrite dendrite cores and an austenite/martensite matrix.…”
Section: A As-cast and Homogenized Microstructurementioning
Two ingots weighing 400 g and 5 kg with nominal compositions of Fe–8Mn–4Al–2Si–0.5C–0.07V–0.05Sn were produced to investigate the effect of processing variables on microstructure development. The larger casting has a cooling rate more representative of commercial production and provides an understanding of the potential challenges arising from casting-related segregation during efforts to scale up medium Mn steels, while the smaller casting has a high cooling rate and different segregation pattern. Sections from both ingots were homogenized at 1250 $$^{\circ} $$
∘
C for various times to study the degree of chemical homogeneity and $$\delta $$
δ
-ferrite dissolution. Within 2 hours, the Mn segregation range (max–min) decreased from 8.0 to 1.7 wt pct in the 400 g ingot and from 6.2 to 1.5 wt pct in the 5 kg ingot. Some $$\delta $$
δ
-ferrite also remained untransformed after 2 hours in both ingots but with the 5 kg ingot showing nearly three times more than the 400 g ingot. Micress modeling was carried out, and good agreement was seen between predicted and measured segregation levels and distribution. After thermomechanical processing, it was found that the coarse untransformed $$\delta $$
δ
-ferrite in the 5 kg ingot turned into coarse $$\delta $$
δ
-ferrite stringers in the finished product, resulting in a slight decrease in yield strength. Nevertheless, rolled strips from both ingots showed $$>900$$
>
900
MPa yield strength, $$>1100$$
>
1100
MPa tensile strength, and $$>40$$
>
40
pct elongation with $$<10$$
<
10
pct difference in strength and no change in ductility when compared to a fully homogenized sample.
“…In most medium Mn steels in the literature, the phases and compositions obtained during intercritical annealing are retained to room temperature. [21,[24][25][26] From Figure 2, it can be seen that the slight differences in composition between the 400 g and 5 kg ingot did not result in a significant change in the expected phase fractions at the chosen IA temperature of 750 C. Thermo-Calc also predicted that d-ferrite should not be expected below 1370 C, but it was clearly observed in the as-cast micrographs of both 400 g and 5 kg ingots in Figure 3. From Figure 3, the as-cast micrographs showed d-ferrite dendrite cores and an austenite/martensite matrix.…”
Section: A As-cast and Homogenized Microstructurementioning
Two ingots weighing 400 g and 5 kg with nominal compositions of Fe–8Mn–4Al–2Si–0.5C–0.07V–0.05Sn were produced to investigate the effect of processing variables on microstructure development. The larger casting has a cooling rate more representative of commercial production and provides an understanding of the potential challenges arising from casting-related segregation during efforts to scale up medium Mn steels, while the smaller casting has a high cooling rate and different segregation pattern. Sections from both ingots were homogenized at 1250 $$^{\circ} $$
∘
C for various times to study the degree of chemical homogeneity and $$\delta $$
δ
-ferrite dissolution. Within 2 hours, the Mn segregation range (max–min) decreased from 8.0 to 1.7 wt pct in the 400 g ingot and from 6.2 to 1.5 wt pct in the 5 kg ingot. Some $$\delta $$
δ
-ferrite also remained untransformed after 2 hours in both ingots but with the 5 kg ingot showing nearly three times more than the 400 g ingot. Micress modeling was carried out, and good agreement was seen between predicted and measured segregation levels and distribution. After thermomechanical processing, it was found that the coarse untransformed $$\delta $$
δ
-ferrite in the 5 kg ingot turned into coarse $$\delta $$
δ
-ferrite stringers in the finished product, resulting in a slight decrease in yield strength. Nevertheless, rolled strips from both ingots showed $$>900$$
>
900
MPa yield strength, $$>1100$$
>
1100
MPa tensile strength, and $$>40$$
>
40
pct elongation with $$<10$$
<
10
pct difference in strength and no change in ductility when compared to a fully homogenized sample.
“…Microstructurally, this means being able to continuously form small isolated grains of -martensite [134]. In medium Mn steels, the austenite stability can also be ‘tuned’ to produce a range of strain hardening rates by adjusting the IA parameters [72,135]. Figure 9. Tensile curves from different steels exhibiting different types of plasticity-enhancing mechanisms.…”
Medium Mn steels are an emerging class of 3rd generation advanced high-strength steels. These steels have received significant attention due to their high strengths, large ductilities and also lower cost compared to their predecessor high Mn Twinning Induced Plasticity (TWIP) steels. Additionally, medium Mn steels have been found to exhibit TWIP and/or Transformation Induced Plasticity (TRIP) effects which can be harnessed to give a high strain hardening rate. Many thermomechanical processing concepts in the literature have been developed, producing multiple microstructure types with differentmechanical properties. The present review therefore aims to summarise the current knowledge of medium Mn steel alloy design especially on the processing, microstructure and property relationships in medium Mn steels. It complements the review of Sun et al. [Physical metallurgy of medium-Mn advanced high-strength steels, Int Mater Rev. 2023.], written independently and in parallel, which focusses more on the phase interfaces and thermodynamics.
“…Thermo-Calc is often used during the alloy design stage to give an indication of phase fractions and approximate compositions of each phase, especially within the intercritical temperature regime. In most medium Mn steels in the literature, the phases and compositions obtained during intercritical annealing are retained to room temperature [21,[24][25][26]. From Figure 2, it can be seen that the slight differences in composition between the 400 g and 5 kg ingot did not result in a significant change in the expected phase fractions at the chosen IA temperature of 750 • C. Thermo-Calc also predicted that δ-ferrite should not be expected below 1370 • C but it was clearly observed in the as-cast micrographs of both 400 g and 5 kg ingots in Figure 3.…”
Section: As-cast and Homogenised Microstructurementioning
Two ingots weighing 400 g and 5 kg with nominal compositions of Fe-8Mn-4Al-2Si-0.5C-0.07V-0.05Sn were produced to investigate the effect of processing variables on microstructure development. The larger casting has a cooling rate more representative of commercial production and provides an understanding of the potential challenges arising from casting-related segregation during efforts to scale up medium Mn steels, whilst the smaller casting has a high cooling rate and different segregation pattern. Sections from both ingots were homogenised at 1250 • C for various times to study the degree of chemical homogeneity and δ-ferrite dissolution. Within 2 h, the Mn segregation range (max − min) decreased from 8.0 to 1.7 wt% in the 400 g ingot and from 6.2 to 1.5 wt% in the 5 kg ingot. Some δ-ferrite also remained untransformed after 2 h in both ingots but with the 5 kg ingot showing nearly three times more than the 400 g ingot. Micress modelling was carried out and good agreement was seen between predicted and measured segregation levels and distribution. After thermomechanical processing, it was found that the coarse untransformed δ-ferrite in the 5 kg ingot turned into coarse δ-ferrite stringers in the finished product, resulting in a slight decrease in yield strength. Nevertheless, rolled strips from both ingots showed >900 MPa yield strength, >1100 MPa tensile strength and >40% elongation with <10% difference in strength and no change in ductility when compared to a fully homogenised sample.
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