Complex Phase

Complex Phase

1stGen AHSS, AHSS, Steel Grades

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 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 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.

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-3

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

 

Whereas Dual Phase and TRIP steels excel at stretch forming and deep drawing, they are not as good in bending and cut-edge stretching. For applications needing these characteristics, complex phase steels are a good choice.

Table 1 compares galvannealed Dual Phase and Complex Phase steels having similar tensile strength and elongation as measured in a tensile test.  The characteristics with the biggest difference are the yield strength and the hole expansion ratio which is a measure of cut-edge stretchability.

In the VDA-238 bending test, 980 DP exhibited a significant fracture with a bending angle of 67°.  The 980 CP showed no fracture at 80°, and was capable of bending to over 100° before fracturing, Figure 5.N-33  The galvannealed 980 CP suppressed crack generation in axial crushing deformation to a much greater degree than the 980 DP.

Figure 5. Bendability of 980 DP (sample A) and 980 CP (sample B). N-33

Figure 5. Bendability of 980 DP (sample A) and 980 CP (sample B).N-33

 

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

 

Microstructural Components

Microstructural Components

Steel grades are engineered to achieve specific properties and characteristics by the manipulation of mill processing parameters to achieve a targeted balance of microstructural components. Among the tools available to the steelmaker are alloy composition, rolling and processing temperatures, and cooling profile.

If steel is slowly cooled, only two components exist at room temperature: ferrite (abbreviated by the Greek letter α) and cementite (iron carbide, Fe3C). Alternating layers of ferrite and cementite appear under a microscope in a pattern similar to Mother-of-Pearl, leading to the term pearlite.

A steel alloy having approximately 0.80% carbon will contain only pearlite in the microstructure. Lower carbon levels create an alloy that combines ferrite and pearlite. Ferrite-pearlite microstructures form the basis of many C-Mn steels and some of the initial HSLA steels. At a given strength level, pearlite limits sheet formability.

At carbon levels below 0.008% or 80 ppm, only ferrite exists. Ferrite is low strength but very ductile, and is the microstructural phase in ultra-low carbon steels.

Additional phases are formed when the cooling profile can be changed. Some modern annealing furnaces are capable of controlling the cooling rate as well as holding at specific temperatures. This ability is a key facilitator in the production of most Advanced High Strength Steels. In addition to ferrite and pearlite, microstructural phases of bainite, austenite, and martensite can be produced, depending on the chemistry and the thermal cycle profile including quench rate and hold temperature.

Bainite is a phase that is associated with enhanced sheared edge ductility. Accelerated cooling in the hot mill run out table allows for the production of Ferrite-Bainite steels.

Austenite is not stable at room temperature under equilibrium conditions. An austenitic microstructure is retained at room temperature with the use of a combined chemistry and controlled thermal cycle. Deforming retained austenite is responsible for the TRIP effect.

Martensite is a very high strength phase, but has limited toughness.  Steels with both ferrite and martensite in the microstructure are known as dual phase steels.  A structure of 100% martensite can be produced directly at those sheet mills having equipment capable of achieving a minimum critical cooling rate. The ductility of this product is not sufficiently high for most stamping operations. However, sheet martensite is well suited for properly designed roll forming applications.

Martensite is the microstructural component of processed Press Hardening Steels. An elevated temperature and a more formable microstructure exist at the time of complex forming of these grades. Rapid cooling while the part is under full press load converts the microstructure to the high strength martensite.