Dual Phase Steels– From “Bumper Component Welding State-of-the-Art Survey” by D.W. Dickinson
AISI Bumper Project Group,Dec31, 2000
Dual Phase steels[1] are a group of steels, often based on the low alloy steel composition and rolling practices, which have been given a specific heat treatment which takes the material back into the dual phase (austenite plus ferrite) phase region (see figure) for a time sufficient to establish the two phases in equilibrium. On cooling, the austenite transforms to martensite islands in a matrix of low carbon ferrite. A typical microstructure is presented in figure. The ferrite promotes ductility while the martensite promotes strength.
There are a number of processing routes by which this dual phase structure can be attained. Some variation in the chemistry is required for these various routes as illustrated in this table. The techniques include box annealing, continuous annealing and strip produced directly from as rolled cooling.
In the box annealing processing technique, coils of strip are loaded into an annealing furnace and subjected to a thermal cycle of heating into the two phase region for a fairly long period of time as illustrated in this figure, after which, the coils are cooled to room temperature and shipped as dual phase product.
In the continuous annealed product, the coil is unwound and traversed through a continuous annealing furnace line, recoiled and cooled to room temperature. The temperature, line speed, run off table configuration and coiling conditions control the time temperature cycle, but it is a shorter time period than that for box annealing.
The as rolled dual phase materials[2] attain their properties directly from the hot rolling procedure by control of chemistry and rolling; and post-roll cooling and coiling techniques to attain the dual phase properties. As seen in this figure, lowering the coiling temperature sequentially produces a microstructure change ranging from ferrite-pearlite (P), to farrite-bainite-martensite (B), to ferrite martensite (M) as the coiling temperature is reduced. The tensile and ductility variations of this effect are seen in figure for a series of four as rolled dual phase alloys. Note that as the alloy content increases (moving from alloy 1 to 4 on this figure), the tensile strength increases and the nominal ductility decreases.
The properties of the dual phase steels in comparison to other steels is illustrated in figure. Note that they have higher tensile strengths than that of the structural quality solid solution hardened steels, and better ductility than that of the low alloy precipitation hardened (HSLA) type steels. Thus they find applications where both moderately high strength and good forming applications are required.
[1]ASM Handbook, Vol 1, 1990
[2]Hulka, K “Relationships between heat treatment conditions, microstructure and properties of Niobium microalloyed TRIP steel”, 41st MWSP Conf Proc. 1999