Design of novel high strength bainitic steels: Part 1

Abstract

Mixed microstructures consisting of ®ne plates of upper bainitic ferrite separated by thin ®lms of stable retained austenite have seen many applications in recent years. There may also be some martensite present, although carbides are avoided by the judicious use of silicon as an alloying element. The essential principles governing the optimisation of such microstructures are well established, particularly that large regions of unstable high carbon retained austenite must be avoided. With careful design, impressive combinations of strength and toughness have been reported for high silicon bainitic steels. The aim of the present work was to ascertain how far these concepts could be extended to achieve unprecedented combinations of strength and toughness in bulk samples subjected to continuous cooling transformation, consistent with certain hardenability and processing requirements. Thus, this paper (part 1 of a two part study) deals with the design, using phase transformation theory, of a series of bainitic alloys, given a set of industrial constraints. Part 2 of the study concerns the experimental veri®cation of the design process.MST/4644

Tensile Ductility of Nanostructured Bainitic Steels: Influence of Retained Austenite Stability

“…Bainitic microstructures formed at low temperatures (350 • C or less) have received a considerable amount of attention in the recent years [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. These microstructures are obtained in relatively high carbon steels (0.6-1.2 wt %, although the concept can be extended to lower carbon contents) through isothermal transformation over durations ranging from 10 to over 100 h [5,11,15].…”

Mechanical Properties of a Bainitic Steel Producible by Hot Rolling

“…A carbide-free bainitic (CFB) or bainitic ferrite microstructure is known to enhance the ductility of steels due to the absence of brittle cementite particles. In addition, a CFB microstructure can guarantee also an ultra high strength due to the strengthening from ultrafine (< 100 nm) bainitic ferrite sheaves as well as high dislocation density inherent to the bainitic phase transformation [1][2][3]. On the other hand, retained austenite, when present in sufficient amount, is known to enhance the ductility in advanced high strength steels (AHSS) through transformation-induced plasticity (TRIP) effect [4][5][6].…”

Carbide-Free Bainite: Compromise between Rate of Transformation and Properties

“…The first consists of steels whose properties are achieved by continuous cooling transformation. [14][15][16][17][18][19][20][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][23][24][25][26][27][28][29][30][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.…”