Dual Phase Steels OverviewEvan Sanders

Dual Phase Steels

● Microstructure○ 75-85 vol% ferrite○ Remainder mixture of martensite, lower bainite,

retained austenite○ Usually consists of more than 2 phases

● Essentially just a low carbon steel thermomechanically processed for better formability than ferrite-pearlite steels of similar tensile strength

Stress-Strain Behavior

● Characteristically different from HSLA (High Strength Low Alloy) or plain carbon steels○ Continuous Stress-Strain curve with no yield point

elongation○ Work harden rapidly at low strains○ Low yield strength○ High UTS○ Strength-Ductility data falls on separate curve

Development

● Ferrite-Martensite steels developed by British Iron and Steel Research Association (BISRA, UK) and Inland Steel Corporation (ISC, US) in mid 1960s○ Focus was for producing steels with tinplate○ Neither group focused on improved formability

● Development for formability triggered in 1970s by conflicting demands in automotive industry for decreased weight for fuel economy and increased weight to meet safety standards○ Matsuoka & Yammamori, and Hayami and

Furukawa from Japan and M.S. Rashid from the US.

Processing Methods

● Before processing, starting steel consists of a ferrite matrix with grain boundary iron carbides and small islands of pearlite

● 3 types of processing methods to produce dual phase steel○ Continuous annealed○ Batch annealed○ As-rolled

Continuous annealed method

● Rapid heating above the critical temperature● Short time holding at that temperature● Cooling below the martensitic start

temperature● Some processes also include a short time

tempering above 500 degrees Celsius● Rate of heating is far less critical than the

heating temperature● Faster cooling required for steels with lower

hardenability

Batch annealed

● Used with high alloy content and high hardenability

● Very slow cooling (days)

As-Rolled

● Steel composition chosen such that 80-90% of the steel is transformed to ferrite after the final roll pass in normal conventional hot rolling and before entering the coiler

● Remaining 10-20% does not transform until slow cooling in the coiler

● This method possible with steels that express certain characteristics in their continuous cooling transformation diagrams

Deformation behavior

● Typically stress strain behavior is not satisfied for dual phase steels

● 2 proposed methods for changes in deformation behavior○ n i(j)=[log(σj)-log(σj-1)]/ [log(εj)-log(εj-1)]○ σ=σo+Bε^m○ Where σ is the true stress, σo us the true yield

stress, B and m are constants, and j=1 to L, where L is the number of segments in the curve

Deformation behavior (cont)

● The shear and volume change accompanying the austenite to martensite transformation upon cooling from above the critical temperature produce numerous free mobile dislocations in the surrounding ferrite matrix○ Upon application of the load, free dislocations move

with stresses much less than that required to move restrained dislocations as commonly found in ferrite-pearlite steels, so dual phase steels yield plastic flow at lower stresses of equivalent tensile strength

○ Magnitude of work hardening in dual phase steels at low strains too large to be explained by dislocation interactions alone

Deformation behavior (cont)

● Martensite is the principal load bearing constituent○ Volume percent of martensite and steel strength are

linearly related○ Carbon content is also important though, and

separate linear relationships exist○ Martensite strength can be increased by decreasing

its particle size

Transformation Mechanisms

● Continuous annealed○ Upon heating the steel above the critical

temperature,islands of carbon-rich, nonequilibrium austenite form at the carbide locations.■ Heating temp determines volume fraction of

austenite and carbon content that can exist○ Carbon migration

Transformation Mechanisms (cont)

● Batch annealed○ Similar to those observed during continuous

annealing■ However, grain size and substructure are

characteristic of slower cooling rates