Ultrafine Grained Steels due to Super Short Interval Multi-Pass Rolling in Stable Austenite Region

Abstract

Ultrafine-grained steel sheets with the chemical composition of 0.15%C-0.74%Mn- 0.01%Si have been prepared using a laboratory rolling mill by Super Short Interval Multi-pass Rolling (SSMR) process, in which the inter-pass time is extremely shortened to enhance the cumulative strain. The SSMR process with a finish rolling around Ae3 leads to ultrafine equiaxed ferrite structure with 1μm in average grain size. In order to clarify the grain refinement mechanism in the SSMR process, the deformation substructure in deformed austenite was simulated using 70%Ni-30%Fe, which was a fcc alloy with equivalent stacking fault energy to C-Mn steels. TEM observations have shown that the dislocation substructure in the Ni-Fe alloy hot-rolled by SSMR process mainly consists of dislocation cells, of which size are refined to less than 1μm with shortening inter-pass time. It is concluded that the SSMR process can accumulate the deformation strain in austenite enough to densely nucleate ferrite inside austenite grains.

Grain Refinement of C–Mn Steel to 1 μm by Rapid Cooling and Short Interval Multi-pass Hot Rolling in Stable Austenite Region

“…The results obtained have been only partly published in some proceedings of international conferences. [16][17][18][19][20][21][22][23][24] Thus, in this article are given a thorough description of the explored grain refinement method and therein operating mechanisms. The investigated method is composed of short interval multi-pass hot rolling and subsequent ultra-fast cooling.…”

Refinement of Ferrite Grain Size near the Ultrafine Range by Multipass, Thermomechanical Compression

“…Considering the industrial feasibility, a reasonable interpass time (10 seconds) was used here, instead of super-short intervals (<1 second), as recommended previously. [8,9,27] The schedule used in MP-7 is aimed at precipitating out the Nb(C, N) during the roughing and intermediate pass for achieving better grain refinement after finishing passes. [10] E. Metallographic Analysis…”

“…[1,2] ous studies, where multiple passes were applied just above A r3 , [8,9,[25][26][27] or below A r3 , [28,29] without any high-temperature deformation. This schedule is aimed at understanding the grain refinement and interpass softening during multipass, isothermal deformation.…”

“…[1][2][3] Hence, for the industrial processing of UFF grained plate, the question arises of how to divide the entire deformation into a multipass schedule, restricting the deformation in each step within the feasible limit. Although some of the earlier studies [5][6][7][8][9][10] developed UFF grain structures through multipass rolling of thin strips (~2-mm thickness), that practice has hardly risen to the level of regular (industrial) production.…”

“…[1][2][3][4][5][6] In an industrial rolling mill, however, application of the aforementioned deformation conditions are difficult due to the requirement of excessive rolling load, precise temperature control, and water-cooling capability. [3][4][5][6][7][8][9][10] Industrial plate rolling is carried out in multiple light passes (less than 30 pct reduction per pass), which are separated by a short interpass time (5 to 30 seconds). [1][2][3] Hence, for the industrial processing of UFF grained plate, the question arises of how to divide the entire deformation into a multipass schedule, restricting the deformation in each step within the feasible limit.…”