Cost effective low temperature bainite
Final Report Abstract
Three alloying strategies aimed at producing low cost, fast transforming, high strength fine bainitic microstructures were investigated under the scope of this project. These strategies are a low Mn strategy (0.3 wt.% Mn), a high Al-strategy (3 and 5 wt.%), and a strategy of combining the low carbon (0.23 and 0.3 wt.) and high Al (0.7 to 3 wt.%). Within the first and the second strategies the carbon varies from 0.54 to 0.95 wt.%. The Al-content within the first strategy is kept at 0.95 wt.%. All strategies dispensed with Co, Cr and Ni for cost-effectiveness purpose. Fast transformation times ranging from 650 s to 4600 s were obtained for all studied conditions. The highest mechanical strengths were obtained by the low Mn alloying strategy, with tensile strengths ranging from 1900 - 2140 MPa at elongation values of ~10 %. Respecting similar C contents, increasing the Al content within the second strategy lowers the transformation times compared to the alloys of the first strategy. Nevertheless, because of the high Al-content of the second strategy, more carbon is required to maintain sufficiently low martensite start temperatures (Ms). This mandatory carbon increase led to an overall increase in the transformation times within this strategy. On the other hand, increasing Al to 3 wt.% led to a drop in the maximum tensile strength from 2140 to 2000 MPa, while the elongation values remain around 10 %, which is observed for the low Mn strategy. Further increasing Al to 5 wt.% yielded brief incubation periods and shorter transformation times (1000 – 1800 s) despite the high C contents utilized (0.67 – 0.94 wt.%). However, this came at the cost of a deterioration of the strengthductility balance, with strength values ranging from 1330 – 1845 MPa and ductility of ~5 %. Increasing the Al content within the third strategy from 0.7 to 2.8 wt.% (~ 0.3 wt.% C and ~ 2.9 wt.% Mn) lowers the transformation time from 3000 to 2000 s, respectively, at a cost of a reduction in tensile strength and elongation% from 1330 to 1270 MPa and from 13.5 to 7.7%, respectively. The introduction of δ-ferrite to the microstructure of the 3 wt.% Al alloy (~ 0.23 wt.% C and ~ 4.2 wt.% Mn) increased elongation% up to 16% and reduced the tensile strength to 1105 MPa. Continuously cooling of this steel-group at a rate of 0.3 Ks-1 yielded almost fully martensitic structures, with bainite partly forming only in the alloys with higher Al. Continuous cooling at a rate of 0.03 Ks-1 increased the tensile strength by about 100 MPa at similar elongation%. Lowering the cooling rate down to 0.003 Ks^-1 yielded similar properties as isothermal treatment because most of the transformation is concluded near the starting temperature. Problems were encountered with the ThermoCalc when attempting to calculate the A3 temperatures of the higher Al alloys. The differences between the calculated and the experimental values required for full austenitization reached up to ~170 °C and in some cases the alloys were not fully austenitizable (contrary to ThermoCalc’s calculations). Consequently, two alloys (3 wt.% Al of the second strategy and 5 wt.% Al of the third one) were austenitized to maintain around 10 vol.% δ-ferrite in their microstructure prior to austempering. The presence of δ-ferrite in the microstructure was found to lower the UTS, improve the ductility, while deteriorating the impact properties of the alloy. For both the low and high C groups, the impact properties increased with the increase of Al to 2 – 3 wt.% followed by a drop as the Al was increased further. Upon impact testing of select alloys from (- 160 to 50 °C), it was observed that changing the Al content has little to no effect on changing the impact transition temperature. The effect of generating deformation induced ferrite (DIF) in the microstructure of the low carbon group alloys prior to the bainitic transformation was inconclusive, improving the uniform elongation in some cases while deteriorating the strength-ductility balance in others. Additionally, select alloys were subjected to bake hardening investigations to help expand a newly developed module in MatCalc software into the field of fine bainitic microstructures. The results of strain ageing experiments reveal high bake hardening strengths ranging from 130 – 240 MPa after short ageing times of less than 20 min. The developed model was able to successfully predict the bake hardening behaviour of the alloys investigated.
Publications
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(2020): High-Strength Low-Cost Nano-Bainitic Steel. In: J. Mater. Eng. Perform. 29 (4), pp. 2418–2427
Akram, Mohamad; Palkowski, Heinz; Soliman, Mohamed
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(2021): Nano-bainite Generated in Low and Medium Carbon Steels via an Economical Alloying Strategy. In: Steel Res. Int.
Akram, Mohamad; Palkowski, Heinz; Soliman, Mohamed
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(2021): Nano-Bainitic Steels: Acceleration of Transformation by High Aluminum Addition and Its Effect on Their Mechanical Properties. In: Metals 11 (8), 1210
Akram, Mohamad; Soliman, Mohamed; Palkowski, Heinz
