Ultra-Fine Grained Dual-Phase Steels


The Centre for Metallurgical Process Engineering, The University of British Columbia


This paper provides an overview on obtaining low-carbon ultra-fine grained dual-phase steels through rapid intercritical annealing of cold-rolled sheet as improved materials for automotive applications. A laboratory processing route was designed that involves cold-rolling of a tempered martensite structure followed by a
second tempering step to produce a fine grained aggregate of ferrite and carbides as the initial microstructure for rapid intercritical annealing. The intercritical annealing step was performed with heating and cooling rates of at least 100 °C/s and a holding time of 30 s. The intercritical temperature was selected to result in 20- 35% martensite in the final microstructures for C-Mn steels with carbon contents of 0.06, 0.12 and 0.17 wt%, respectively. The proposed processing routes produced an ultra-fine grained ferrite-martensite structure withgrain sizes of approximately 1 ?m for all three steels. The tensile strength of these ultra-fine grained dualphase steels can be increased by up to 200 MPa as compared to coarse-grained dual-phase steels while maintaining uniform elongation values. The rather narrow processing window necessary to obtain these properties was evaluated by determining the effect of intercritical annealing conditions on microstructure evolution. Further, the experimental results were confirmed with phase field simulations of austenite formation indicating that rapid heat treatment cycles are essential to obtain fine grained intercritical austenite that leads to martensite islands with sizes of 1 ?m and below in the final microstructure.


[1]  Tsipouridis P, Werner E, Krempaszky C, Tragl E. Formability of high strength dual-phase steels. Steel Res Int. 2006; 77:654-67.

[2]  Park JW, Kim JW,  Chung YH. Grain refinement of steel plate by continuous equal-channel angular process. Scr Mater. 2004; 51(2):181-84.

[3]  Andrade-Carozzo V, Jacques PJ. Interactions between recrystallization and phase transformations during annealing of cold rolled Nb-added TRIP-aided steels. Mater Sci Forum. 2007;539-543:4649-54.

[4]  Calcagonto M, Ponge D, Raabe D. Ultrafine grained ferrite/martensite dual steel fabricated by large strain warm deformation and subsequent intercritical annealing. ISIJ International. 2008;48:1096-101.

[5]  Azizi-Alizamani H, Militzer M, Poole WJ. Formation of ultrafine grained dual phase steels through rapid heating. ISIJ international. 2011;51:958-64.

[7]  Steinbach I, Pezzolla F, Nestler B, Seebelberg M, Prieler R, Schmitz GJ, Rezende JLL. A phase field concept for multiphase systems. Physica D. 1996;94:135-47.

[8] Azizi-Alizamani H, Militzer M. Phase field modeling of austenite formation from ultrafine ferrite-carbide aggregates in Fe-C. Int J Mat Res. 2010;101:534-41.

[9]  Savran V. Ph.D Thesis, Delf University of Technology, 2009.