Abstract
The effects of heat treatment and forming conditions on retained austenite characteristics and ductility of 0.1-0.6C-1.5Si-1.5Mn, mass%, ultra high-strength TRIP-aided sheet steels with bainitic ferrite matrix were investigated. These steels possessed large total elongations of about 20-25% in a tensile strength ranging from 700 to 1 300 MPa when austempered at temperatures above martensite-start temperature (MS). The total elongations were enhanced by warm forming at two temperatures, TP1 and TP2. The first peak forming temperatures TP1s were between 0°C and 75°C and were nearly constant regardless of carbon content of the steels. This was associated with the strain-induced martensite transformation of a large amount of metastable retained austenite which suppressed a rapid fall of strain-hardening rate in an early strain range to resultantly increase the uniform and total elongations. On the other hand, the second peak forming temperatures TP2s were between 200 and 300°C and further large total elongations beyond 30% were achieved in high carbon steels (0.4% C and 0.6% C steels) with tensile strength of 1 300-1 500MPa. The large improvement was controlled by both the strain-induced bainite transformation and dynamic strain aging.
Original language | English |
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Pages (from-to) | 1608-1614 |
Number of pages | 7 |
Journal | Isij International |
Volume | 44 |
Issue number | 9 |
DOIs | |
Publication status | Published - 2004 |
Externally published | Yes |
Keywords
- Bainitic ferrite
- Ductility
- Retained austenite
- TRIP
- Ultra high-strength steel
- Warm forming
ASJC Scopus subject areas
- Mechanics of Materials
- Mechanical Engineering
- Metals and Alloys
- Materials Chemistry