Microstructure and Properties of a Low Carbon Ti-V Microalloyed Steel

Abstract

Due to the largely inhomogeneous deformation among constituent phases, the advanced high-strength multi-phase steels are always facing challenges when applied to automotive parts where local formability is critically required. In this work, two characteristic microstructures were produced from a low carbon Ti-V microalloyed steel by varying the cooling path. In the ferrite single-phase microstructure resulted from “ultra-fast cooling (UFC) + furnace-cooling (FC)”, the hole-expanding ratio of 200% and tensile strength of 647 MPa were achieved. In the ferrite-bainite-martensite (F+B+M) multi-phase microstructure produced by “UFC + air-cooling (AC) + UFC”, the ferrite has been strengthened by Ti-V carbides to promote the strain partitioning, which resulted in the tensile strength of ≥780MPa, a moderate elongation and hole-expanding ratio of 93%. The strengthening contributions of Ti-V carbides were calculated to be 126MPa and 149MPa in the ferrite single-phase and F+B+M multi-phase microstructure, respectively.

Keywords

orowan strengthening / ultra-fast cooling / high-strength / hole-expanding ratio / Ti-V carbide

References

[1] WorldAutoSteel. FutureSteelVehicle Overview Report[EB/OL]. www.worldautosteel.org

[2] Mazinani M, Poole WJ. Effect of Martensite Plasticity on the Deformation Behavior of a Low–Carbon Dual–Phase Steel[J]. Metall. Mater. Trans. A, 2007, 38: 328-339.

[3] Dykeman J, Hoydick D, Link T, et al. Material Property and Formability Characterization of Various Types of High Strength Dual Phase Steel[J]. SAE Tech. Paper, 2009

[4] Seto K, Funakawa Y, Kaneko S. Hot Rolled High Strength Steels for Suspension and Chassis Parts “NANOHITEN” and “BHT Steel”[J]. JFE Tech. Rep., 2007, 10: 19-25.

[5] Caballero FG, Allain S, Cornide J, et al. Design of Cold Rolled and Continuous Annealed Carbide–free Bainitic Steels for Automotive Application [J]. Mater. Design, 2013, 49: 667-680.

[6] Cai MH, Ding H, Lee YK, et al. Effects of Si on Microstructural Evolution and Mechanical Properties of Hot–rolled Ferrite and Bainite Dual–phase Steels[J]. ISIJ Int., 2011, 51: 476-481.

[7] Choi SH, Kim EY, Kim SI. The Micromechanical Deformation Behaviors of Hot–rolled 590FB Steel during Hole–expansion Test[J]. Int. J. Plasticity, 2014, 58: 184-200.

[8] Funakawa Y, Shiozaki T, Tomita K, et al. Development of High Strength Hot–rolled Sheet Steel Consisting of Ferrite and Nanometer–sized Carbides[J]. ISIJ Int., 2004, 44: 1 945-1 951.

[9] Diaz–Fuentes M, Iza–Mendia A, Gutierrez I. Analysis of Different Acicular Ferrite Microstructures in Low–Carbon Steels by Electron Backscattered Diffraction Study of Their Toughness Behavior[J]. Metall. Mater. Trans. A, 2003, 34: 2 505-2 516.

[10] Takahashi M, Kawano O, Hayashida T, et al. High Strength Hot–rolled Steel Sheets for Automobiles[J]. Nippon Steel Tech. Rep., 2003, 88: 8-12.

[11] Yen HW, Chen PY, Huang CY, et al. Interphase Precipitation of Nanometer–sized Carbides in a Titanium–Molybdenum–Bearing Low–carbon Steel[J]. Acta Mater., 2011, 59: 6 264-6 274.

[12] Zurob HS, Hutchinson CR, Béché A, et al. A Transition from Local Equilibrium to Paraequilibrium Kinetics for Ferrite Growth in Fe–CMn: A Possible Role of Interfacial Segregation[J]. Acta Mater., 2008, 56: 2 203-2 211.

[13] Campos SS, Morales EV, Kestenbach HJ. On Strengthening Mechanisms in Commercial Nb–Ti Hot Strip Steels[J]. Metall. Mater. Trans. A, 2001, 32: 1 245-1 248.

[14] Yong QL. Secondary Phases in Steels[M]. Beijing: Metall. Industry Press, 2006 44-47.