Dual phase steels (1)


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Dual phase steels (1)

  1. 1. Dual Phase Steels Overview Evan Sanders
  2. 2. 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
  3. 3. 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
  4. 4. 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
  5. 5. 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
  6. 6. 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
  7. 7. Batch annealed● Used with high alloy content and high hardenability● Very slow cooling (days)
  8. 8. 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
  9. 9. 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
  10. 10. 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
  11. 11. 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
  12. 12. 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
  13. 13. Transformation Mechanisms (cont)● Batch annealed ○ Similar to those observed during continuous annealing ■ However, grain size and substructure are characteristic of slower cooling rates