Dual Phase Steels

Dual Phase Steels

 The term dual phase steels, or DP steels, refers to a class of high strength steels which is composed of two phases namely a purely ferrite matrix and a dispersed second phase of martensite (5 % to 30 %). In addition to martensite, small amounts of bainite and residual austenite may exist. DP steels were developed in the 1970s. The development was driven by the need for new high strength steels without reducing the formability or increasing costs.

DP steels starts as a low or carbon steel and is quenched from a temperature above A1 but below A3 on a continuous cooling transformation diagram. This results in a microstructure consisting of a ferrite matrix containing islands of martensite as the secondary phase (martensite increases the tensile strength). The desire to produce high strength steels with formability greater than micro alloyed HSLA (high strength low alloy) steel led the development of DP steel.

DP steels are low carbon micro alloyed steels. The steel microstructure consists of a very hard phase of martensite in a soft formable ferrite matrix (Fig 1). The soft ferrite phase is generally continuous, giving these steels excellent ductility. When these steels deform, strain is concentrated in the lower strength ferrite phase surrounding the islands of martensite, creating the unique high work hardening rate exhibited by these steels.

Microstructure of DP steel

Fig 1 Micro structure of DP steel

 The steel behave like composite materials where the ferrite matrix assures high cold formability, and the martensite is the strengthening element. The correct proportion between the two phases allows a continuous yield point, low yielding stress, and a high elongation value, a smooth flow stress curve with a high strain hardening coefficient, and better plasticity and formability. The microstructure of steel gives a good combination of high tensile strength, low yield-to-tensile strength ratio and very high initial work hardening rate with good elongation values. Dual phase is very formable, providing more flexibility in part design. The strength of the formed part is much higher than HSLA steel, especially at very low strain. The high initial work hardening rate and high tensile strength give DP steel a very high capacity to absorb energy, making these steels suitable for use in structural and reinforcement applications.

Moreover, DP steels exhibit a bake hardening (BH) effect, i.e. the yield strength increases upon aging at paint baking temperatures (170 deg C) after forming, giving rise to improved dent and crush resistance. The austenite to martensite phase transformation bears the main influence on the mechanical properties of DP steels. This phase transformation involves a volume expansion of 2 % to 4 %, causing an elastically and plastically deformed zone in the ferrite adjacent to martensite. Sometimes the martensite regions tend to percolate or appear in the form of elongated bands which is not desirable. Increasing the volume fraction of the hard second phase martensite generally increases the strength but sometimes reduces ductility

DP (ferrite plus martensite) steels are produced by controlled cooling from the austenite phase (in the case of hot strip products) or from the two phase ferrite plus austenite phase during an inter critical annealing treatment step (in the case of continuously annealed cold rolled and hot dip coated products) to transform some austenite to ferrite before a rapid cooling transforms the remaining austenite to martensite. Depending on the composition and process route, hot rolled steels requiring enhanced capability to resist stretching on a blanked edge (as typically measured by hole expansion capacity) can have a microstructure containing significant quantities of bainite.

Properties of DP steels

 The properties of DP steels are given below.

  • Composition – DP steel is formulated with a mixture of two substances. Ferrite matrix, a soft iron alloy, provides the soft phase. Martensite, a steel crystalline, provides the hard phase. In DP steels, carbon enables the formation of martensite at practical cooling rates by increasing the hardenability of the steel. Manganese, chromium, molybdenum, vanadium, and nickel, added individually or in combination, also help increase hardenability. Carbon also strengthens the martensite as a ferrite solute strengthener, as do silicon and phosphorus. These additions are carefully balanced, not only to produce unique mechanical properties, but also to maintain the generally good resistance spot welding capability. However, when welding the highest strength grade (DP 700/1000) to itself, the spot weldability may require adjustments to the welding practice.
  • Strength and weight – DP steel hardens rapidly during work. At strain levels of only 2 % to 3 %, strength increases from 150 MPa (mega Pascal) to 210 MPa. When compared to HSLA (high strength low alloy) steel, DP steel shows a weight reduction of upto 25 %.
  • Yield point elongation – It refers to the difference between the elongation at the start and at the finish of a material. Yielding is the area in which the strain occurs with no increase in the stress. DP steels exhibit virtually no yield point elongation.
  • Forming and welding – DP steels perform predictably in stamping, with a low incidence of instability and kinking. This is due to the fast hardening rate and absence of the yield point elongation. The steel also meets the requirements of automobile welding.
  • Forming limit (FLD) curves – A FLD curve is a plot of the maximum strain that the sheet metal can sustain. FLD curves in DP steels are calculated in the same manner as other steels. DP steel can withstand strain more effectively than HSLA steel.
  • Drawing – DP steels can be drawn on conventional tools, provided the settings are properly adjusted.
  • Spring back and bendability – Spring back is the elastic recovery that a sheet metal possesses. Because DP steel is highly stable with rapid hardening and no yield point elongation, spring back with this material is easier to control than in HSLA steel. The material is also highly bendable.
  • Bake hardening –DP steels have a bake hardening effect that is an important benefit compared to conventional higher strength steels. The bake hardening effect is the increase in yield strength resulting from elevated temperature aging (created by the curing temperature of paint bake ovens) after pre straining (generated by the work hardening due to deformation during stamping or other manufacturing process). The extent of the bake hardening effect in DP steels depends on the specific chemistry and thermal histories of the steels. Dual phase steels possess a high capacity for bake hardening, which increases the strength of the steel by approximately 35 MPa to 70 MPa.
  • Ultimate tensile strength (UTS) – The work hardening rate plus excellent elongation creates DP steels with much higher ultimate tensile strengths than conventional steels of similar yield strength.  UTS values of DP steels are in the range of 500 MPa to 1200 MPa.
  • Shelf life – DP steels do not age at room temperature.
  • Crash energy absorption – DP steels have higher capacity to absorb crash energy as compared to HSLA steels.

In short the main characteristics of DP steels are the following.

  • High yield strength due to stress focused on surrounding ferrite caused by volume growth and martensite formation
  • High UTS partly due to martensitic microstructure
  • High initial strain hardening rate
  • A high strain rate sensitivity (the faster it is crushed the more energy it absorbs)
  • The ratio of yield strength to UTS is around 0.5 to 0.6
  • Good uniform elongation
  • Good fatigue resistance

In the processing of the DP steels, the most important factors are composition, annealing temperature and time, as well as cooling rate. Alloying is determined by required mechanical properties and by annealing time. Annealing temperature and time affect the amount of austenite produced. The critical cooling rate is determined by formation of the austenitic region. The typical heat treatment of cold dual phase can be as follows.

  • Annealing at 800 deg C. At this temperature the structure contains carbon rich austenite and low carbon ferrite
  • Cooling of the steel strip by water quenching, resulting in the transformation of austenite into martensite. The ferrite phase retains a super saturation of carbon
  • Carbon is precipitation treated through an over ageing heat treatment
  • A final stage of stretch-leveling, which greatly improves the yield strength of DP steels.

Application of DP steels

DP steels are increasingly being used in many automobile applications. These steels are used in such applications as body panels, wheels, rocker reinforcements, bumpers, fasteners, shock towers and door intrusion beams.