Ductility and strain-induced transformation in a high-strength transformation-induced plasticity-aided dual-phase steel
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I.
INTRODUCTION
F O R M A B L E high-strength dual-phase steel sheets I1,2,31 containing 10 to 20 vol pet retained austenite have been developed recently to produce various automotive structural parts, such as bumper reinforcements and door impact beams. These steels have good combinations of total elongation (TEL), above 30 pet, and tensile strengths (TSs), about 1000 MPa, at room temperature, m The increased elongations have been primarily ascribed to transformation-induced plasticity (TRIP) caused by the retained austenite. El-g1 According to works by Zackay et al. tSj and Tamura et al. t6J on TRIP steel.s such as Fe-Cr-Ni I5"61and Fe-Ni-C t61 metastable austenitic stainless steels, elongation enhanced considerably at the temperatures between Ms and Md. Therefore, similarly enhanced elongations may be attained in these TRIP-aided dual-phase steels by forming at a temperature above the Ms temperature. In the present article, the effects of testing temperature and strain rate on TELs and the other tensile properties were examined for the purpose of enhancing the TELs of a TRIP-aided dual-phase steel. Furthermore, the mechanism enhancing TELs was discussed through studies of the mechanical stability and strain-induced transformation behavior of retained austenite.
a 30-mm-thick slab. The chemical composition (weight percent) was 0.4 pet C, 1.5 pet Si, 1.5 pct Mn, 0.015 pct P, 0.0018 pet S, and 0.036 pet A1. The slab was reheated to 1200 ~ and hot-rolled to 3.2-mm thickness, finishing at 950 ~ and subsequently air cooling. After cold rolling the plates to 1.25 mm, JIS-13B-type tensile specimens (50-mm gage length by 12.5-mm width) were machined parallel to the rolling direction. All the specimens were intercritically annealed at 780 ~ for 1000 seconds, following by immersing in a salt bath at 400 ~ for 1000 seconds and then cooling in oil. Tensile tests were carried out on a hard-beam tensile testing machine over a temperature range between 27 ~ and 500 ~ and at mean strain rates from 2.8 x 10-5/s to 2.8 x 10-2/s (crosshead speed, 0.1 to 100 mm/min). Each specimen was directly heated using a pair of strip heaters (70 x 90 mm) during tension testing. Volume fractions of retained austenite were quantified by X-ray diffractometry using the five-peak method, tTl Scanning electron microscopy was used to examine the microstructures of etched specimens, and intercritically annealed or plastically strained thin-foil specimens were examined in a JEM-4000EX transmission electron microscope. IlL
E X P E R I M E N T A L RESULTS
A . Microstructure as Intercritically Annealed
II.
EXPERIMENTAL PROCEDURE
The steel used in this study was prepared as a vacuummelted 100-kg ingot followed by hot forging to produce KOH-ICHI SUGIMOTO, Associate Professor, is with the Department of Functional Machinery and Mechanics, Shinshu University Veda 386, Japan. MITSUYUKI KOBAYASHI, Professor, is with the Department of Mechanical Systems Engineering, Shinshu University, Nagano 380, Japan. SHUN-ICHI HASHIMOTO, Assistant General Manager, is w
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