Influence of deformation induced ferrite, grain boundary sliding, and dynamic recrystallisation on hot ductility of 0·1–0·75%C steels

Abstract

The influence ofC on hot ductility in the temperature range 600-1000°C has been examinedfor three C contents (0·1,0'4, and 0·75 wt-%). Using a strain rate of 3 x 10-3 S-l, tensile specimens were heated to 1330°C before cooling to the test temperature. For the 0·4%C steel, two further strain rates of 3 x 10-2 and 3 x 10-4 S-l were examined. At the strain rate of 3 x 10-3 S-l, increasing the C content shifted the low ductility trough to lower temperatures in accordance with the trough being controlled by the y-a transformation. Thin films of the softer deformation induced ferrite formed around the y grain boundaries and allowed strain concentration to occur. Recovery to higher ductility at high temperatures occurred when these films could no longer form (i.e. above Ae3) and dynamic recrystallisation was possible. The thinfilms of deformation inducedferrite suppressed dynamic recrystallisation in these coarse grained steels when tested at low strain rates. Recovery of ductility at the low temperature side of the trough in the o·]%C steel corresponded to the presence of a large volume fraction offerrite, this being the more ductile phase. For the 0'4%C steel decreasing the strain rate to 3 x ]0-4 S-l resulted in a very wide trough -extended to both higher and lower temperatures compared with the other strain rates. The high temperature extension was due to grain boundary sliding· in the y. Recovery of the ductility only occurred when dynamic recrystallisation was possible and this occurred at high temperatures. At the low temperature end, thin films of deformation induced ferrite were present and recovery did not occur until the temperature was sufficiently low to prevent strain concentration from occurring at the boundaries. Of the two intergranular modes of failure grain boundary sliding produced superior ductility. At the higher strain rates there was less grain boundary sliding, which led to a lower temperature for dynamic recrystallisation. Higher strain rates also increased the rate of work hardening of deformation inducedferrite, reducing the strain concentration at the boundaries. Ductility started to recover immediately below Ae 3 , resulting in very narrow troughs. Finally, it was shown that the 2% strain that occurs during the straightening operation in continuous casting is sufficient to form deformation induced ferrite in steel containing 0·] % C.MSTj]809

Evaluation of the Hot Ductility of a C–Mn Steel Produced from Scrap Recycling

“…When the concentration of the strain in the ferrite film is the only embrittlement mechanism acting, the ductility rebounds at temperatures below the Ar 3 , when a higher amount of ferrite transforms homogeneously in the matrix, and at temperatures above the Ae 3 , since this is the highest temperature at which ferrite can form induced by deformation. 24,25) For the steels in Table 1, even though they are C-Mn steels, the rebound in the hot ductility at high temperatures takes place well above the calculated Ae 3 for any of the reheating conditions or in-situ melting. This means that other mechanisms appart from the concentration of the strain in the ferrite film are contributing to deteriorate the ductility of both steels in the austenite range.…”

Effect of High Temperature Deformation on the Hot Ductility of Nb Microalloyed Steel

“…[3] It is not surprising, therefore, that the HDH region has been observed to coincide with the onset of dynamic recrystallization in many studies. [41,56,57] However, there are numerous instances where dynamic recrystallization has been observed throughout ail, or part of the region of embrittlement. [3,54,56] Such results demonstrate that it is indeed grain boundary migration that is the important event, dynamic recrystallization being an effective means by which this can be achieved.…”

“…[39] It has been aIso shown that this ferrite film can be induced by deformation. [40][41][42][43][44][45] untransformed austenite…”

Assessment of Hot Ductility with Various Thermal Histories as an Alternative Method of in situ Solidification

“…1,12) Figure 8 shows the microstructures of a specimen after a tensile test at higher temperature. A fully recrystallized structure was revealed for specimen TH5 tested at 850°C (Fig.…”