Complex Phase
Complex Phase (CP) steels combine high strength with relatively high ductility. The microstructure of CP steels contains small amounts of martensite, retained austenite and pearlite within a ferrite/bainite matrix. A thermal cycle that retards recrystallization and promotes Titanium (Ti), Vanadium (V), or Niobium (Nb) carbo-nitrides precipitation results in extreme grain refinement. Minimizing retained austenite helps improve local formability, since forming steels with retained austenite induces the TRIP effect producing hard martensite.F-11
The balance of phases, and therefore the properties, results from the thermal cycle, which itself is a function of whether the product is hot rolled, cold rolled, or produced using a hot dip process. Citation P-18 indicates that galvannealed CP steels are characterized by low yield value and high ductility, whereas cold rolled CP steels are characterized by high yield value and good bendability. Typically these approaches require different melt chemistry, potentially resulting in different welding behavior.
CP steel microstructure is shown schematically in Figure 1, with the grain structure for hot rolled CP 800/1000 shown in Figure 2. The engineering stress-strain curves for mild steel, HSLA steel, and CP 1000/1200 steel are compared in Figure 3.
Figure 1: Schematic of a complex phase steel microstructure showing martensite and retained austenite in a ferrite-bainite matrix.
Figure 2: Micrograph of complex phase steel, HR800Y980T-CP.C-14
Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and CP 1000/1200.
DP and TRIP steels do not rely on precipitation hardening for strengthening, and as a result, the ferrite in these steels is relatively soft and ductile. In CP steels, carbo-nitride precipitation increases the ferrite strength. For this reason, CP steels show significantly higher yield strengths than DP steels at equal tensile strengths of 800 MPa and greater. Engineering and true stress-strain curves for CP steel grades are shown in Figure 4.
Figure 4: Engineering stress-strain (left graphic) and true stress-strain (right graphic) curves for a series of CP steel grades. Sheet thickness: CP650/850 = 1.5mm, CP 800/1000 = 0.8mm, CP 1000/1200 = 1.0mm, and Mild Steel = approx. 1.9mm.V-1
Examples of typical automotive applications benefitting from these high strength steels with good local formability include frame rails, frame rail and pillar reinforcements, transverse beams, fender and bumper beams, rocker panels, and tunnel stiffeners.
Some of the specifications describing uncoated cold rolled 1st Generation complex phase (CP) steel are included below, with the grades typically listed in order of increasing minimum tensile strength and ductility. Different specifications may exist which describe hot or cold rolled, uncoated or coated, or steels of different strengths. Many automakers have proprietary specifications which encompass their requirements.
- ASTM A1088, with the terms Complex phase (CP) steel Grades 600T/350Y, 780T/500Y, and 980T/700Y A-22
- EN 10338, with the terms HCT600C, HCT780C, and HCT980C D-18
- VDA239-100, with the terms CR570Y780T-CP, CR780Y980T-CP, and CR900Y1180T-CPV-3