Complex Phase (CP) Steels

CP steels typify the transition to steel with very high ultimate tensile strengths. The microstructure of CP steels contains small amounts of martensite, retained austenite and pearlite within the ferrite/bainite matrix. An extreme grain refinement is created by retarded recrystallization or precipitation of microalloying elements like Ti or Nb. Figure 2-10 shows the grain structure for hot rolled CP 800/1000.

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 Figure 2-10: Photomicrograph of CP 800/1000

In comparison to DP steels, CP steels show significantly higher yield strengths at equal tensile strengths of 800 MPa and greater. CP steels are characterized by high energy absorption, high residual deformation capacity and good hole expansion. Engineering and true stress-strain curves for CP steel grades are located in Figure 2-11 in the Advanced High-Strength Steels Application Guidelines.

Current production grades of CP steels and example automotive applications:

CP 500/800

CP 600/900

CP 680/780                Frame rails, chassis components, transverse beams

CP 750/900                B-pillar reinforcements, tunnel stiffener

CP 800/1000              Rear suspension brackets, fender beam

CP 850/1180

CP1000/1200              Rear frame rail reinforcements, rocker outer

CP1050/1470              Rocker panels, bumper beams