Development of microstructural banding in low-alloy steel with simulated Mn segregation
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MICROSTRUCTURAL banding in low-alloy steel is due to the segregation of substitutional alloying elements during dendritic solidification.[1–7] The addition of elements such as manganese, chromium, and molybdenum causes solidification to occur over a range of temperatures and compositions. Consequently, the dendrite cores solidify as relatively pure metal while the interdendritic spaces become enriched in solute. These high- and low-solute regions are elongated into parallel bands during rolling and forming operations.[8–11] Differences in austenite transformation behavior between bands lead to the formation of a laminated microstructure with discrete layers of martensite, bainite, ferrite, and pearlite.[11–17] Several investigations have shown Mn to be the alloying element most responsible for the development of microstructural banding in low-alloy steels. Voldrich[18] showed that carbon migrates from low- to high-Mn regions during cooling. Kirkaldy et al.[19] explained that this redistribution of carbon is due to the effect of substitutional alloying elements on the temperature at which austenite begins to transform to ferrite, i.e., the Ar3 temperature. During cooling, ferrite forms in bands with a high Ar3 temperature and rejects carbon into the austenite of adjacent low-Ar3 bands, resulting in the formation of carbon-rich and carbon-depleted layers. Grossterlinden et al.[13] used an electron microprobe analyzer to show that regions of carbon enrichment correspond to highMn locations. As stated earlier, the primary cause of banding is substitutional element concentration gradients. However, cooling rate, austenite grain size, and austenitizing temperature also influence the severity of microstructural banding. Thompson and Howell[14] investigated banding in 0.15 wt pct C, 1.40 TED F. MAJKA, Metallurgy Manager, is with Colfor Manufacturing, Inc., Minerva, OH 44657. Contact e-mail: [email protected] DAVID K. MATLOCK, Armco Foundation Fogarty Professor, and GEORGE KRAUSS, University Emeritus Professor, are with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Manuscript submitted October 22, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
wt pct Mn steel and concluded that the intensity of microstructural banding increases as the cooling rate decreases. The study showed that furnace cooling produces intense banding and air cooling yields poorly defined ferrite and pearlite bands. Increasing the cooling rate from the austenitic condition reduces the intensity of banding because it reduces the Ar3 temperature differences of the segregated bands. The authors also demonstrated that ferrite-pearlite banding is completely eliminated at cooling rates greater than 5 ⬚C/s. Ferrite-pearlite banding also disappears when the austenite grain size exceeds the spacing of the segregated bands by a factor of 2 or 3.[14,20] In this case, the influence of the grain boundary as a preferred site for nucleation dominates the effect of the compositional gradients. However, Samua
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