Characterization of Inclusions in 3rd Generation Advanced High-Strength Steels
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AS the demands of improved passenger safety and reduced gas emissions increase for automobiles, the automotive industry calls for steels with high ductility and strength, as well as low density. To meet these requirements, the steel industry has developed advanced high-strength steels (AHSS). There are three generations of AHSS. The 1st generation of AHSS has considerably greater strength than conventional steel and contains ferrite base steels, such dual-phase (DP) and transformation-induced plasticity steels. However, the higher strength of 1st generation AHSS comes at the expense of good formability. This issue is solved by the development of 2nd generation AHSS, which contain steels, such as twinning-induced plasticity (TWIP) steel, having significantly improved strength and ductility. The high ductility is achieved by the addition of 15 to 30 pct Mn (austenite stabilizer), hence by attaining austenitic matrix. However, higher cost and processing problems, e.g., transverse cracking during continuous casting (CC) are associated with the use of high levels of Mn in the 2nd generation AHSS.[1–8]
MUHAMMAD NABEEL, MICHELIA ALBA, and NESLIHAN DOGAN are with McMaster University, 1280 Main Street West, Hamilton, ON, L8S 4L7 Canada. Contact e-mail: nabeelm@ mcmaster.ca ANDREY KARASEV and PA¨R G. JO¨NSSON are with the KTH Royal Institute of Technology, Brinellva¨gen 23, Stockholm, 10044 Sweden. Manuscript submitted December 25, 2018. Article published online May 30, 2019. 1674—VOLUME 50B, AUGUST 2019
The shortcomings of 2nd generation AHSS stimulated development of 3rd generation AHSS to obtain mechanical properties between the 1st and 2nd generations at lower cost than 2nd generation AHSS. The 3rd generation AHSS contain medium Mn content (up to 12 pct) and high Al (~ 3 pct) and Si (~ 3 pct) contents. The medium Mn contents serves to achieve retained austenite during cooling,[9] which enables high formability of steel. A higher Al and Si increases the stacking fault energy,[10] which significantly affects the deformation mechanism.[11] Due to the presence of a high number of alloying elements in AHSS, a higher number and complex chemistry of non-metallic inclusions can be anticipated compared to those in low-alloyed steels. Especially, given the fact that addition of Mn increases the solubility of N in molten iron,[12,13] combinations of nitride-oxide and nitride-sulfide inclusions are expected, which are not common in typical low-alloyed steels. The presence of Al2O3, AlN, MnO, and MnS and their combinations have been reported in different grades of AHSS.[1,11,14–21] Gigacher et al.[17] observed MnOAl2O3-AlN, MnO-Al2O3-MnS, MnO-Al2O3-AlN-MnS, and single AlN inclusions in Fe-(15 to 25 pct)Mn-3 pct Al-3 pct Si steel. Park et al.[18] investigated the characteristics of inclusions in Fe-(10 to 20 pct)Mn-(1 to 6 pct)Al alloys and classified the observed inclusions into seven types: Al2O3, AlN or AlON, MnAl2O4, Mn(S,Se), agglomerate Al2O3-Al(O)N, oxide core with Mn(S,Se) wrap, and Mn(S,Se) core with agglomeration of Al2O
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