Carbide-Free Bainite: Compromise between Rate of Transformation and Properties
- PDF / 449,268 Bytes
- 7 Pages / 593.972 x 792 pts Page_size
- 119 Downloads / 220 Views
INTRODUCTION
STRONG steels based on tempered martensitic microstructures continue to serve well in situations requiring a good combination of strength and toughness.[1–5] Bainitic steels in this category have not in the past been competitive even though they have the potential of achieving the final structure by continuous cooling transformation, primarily because of the uncontrolled presence of cementite particles. The problem was not resolved by suppressing cementite precipitation using solutes such as silicon, until the realization that the thermodynamic limit to the extent of transformation that can be achieved also needed to be engineered during alloy design.[6,7] This latter effect helps avoid large residues of untransformed austenite, which can then go on to produce martensite, which cracks.[6–8] Excluding the transformation induced plasticity (TRIP)-assisted steels where the dominant phase is allotriomorphic ferrite,[9–13] there are therefore two classes of strong steels that have been developed based on a microstructure consisting of bainitic ferrite, carbon-enriched retained austenite, and perhaps some martensite. The first consists of steels whose properties are achieved by continuous cooling transformation.[14–21] The second is where the hardenability is controlled so that the transformation temperature is dramatically suppressed, giving slender plates of bainitic ferrite, under isothermal conditions.[22–31] The transformation time in the latter case, however, can be greater than 10 days at temperatures as low as 125 C, making the process suitable for large components; strength values in excess of 2000 MPa can routinely be achieved. SANGEETA KHARE, Researcher, and H.K.D.H. BHADESHIA, Director and Professor, are with the Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea. Contact e-mail: [email protected] KYOOYOUNG LEE, Senior Researcher, is with the Automotive Steel Products Research Group, Technical Research Laboratories, POSCO, Jeonnam 545-090, Republic of Korea. Manuscript submitted March 16, 2009. Article published online February 6, 2010 922—VOLUME 41A, APRIL 2010
There is an industrial requirement for the mass production of strong steel sheet a few millimeters thick using a continuous process that is rapid and in which the transformation must occur over a narrow temperature range around 350 C, making the process essentially isothermal. The transformation temperature is fixed by the production process and the transformation time must be limited to 10 minutes. The ultimate tensile strength should be approximately 1400 MPa with a ductility of about 20 pct. The purpose of the present work was to see if such a steel can be achieved based on the carbide-free bainitic microstructure described previously.
II.
FEASIBILITY
The short isothermal transformation time is the vital design criterion, and to assess whether this can be achieved, data were collected from reported isothermal transformation experiments on carbi
Data Loading...
