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

 

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

 

Complex Phase

Ferrite-Bainite

1stGen AHSS, AHSS, Steel Grades

Ferrite-Bainite (FB) steels are hot rolled steels typically found in applications requiring improved edge stretch capability, balancing strength and formability.  The microstructure of FB steels contains the phases ferrite and bainite.  High elongation is associated with ferrite, and bainite is associated with good edge stretchability.  A fine grain size with a minimized hardness differences between the phases further enhance hole expansion performance.  These microstructural characteristics also leads to improved fatigue strength relative to the tensile strength.

FB steels have a fine microstructure of ferrite and bainite. Strengthening comes from by both grain refinement and second phase hardening with bainite. Relatively low hardness differences within a fine microstructure promotes good Stretch Flangable (SF) and high hole expansion (HHE) performance, both measures of local formability. Figure 1 shows a schematic Ferrite-Bainite steel microstructure, with a micrograph of FB 400Y540T shown in Figure 2. 

Figure 1:  Schematic Ferrite-Bainite steel microstructure.

Figure 1:  Schematic Ferrite-Bainite steel microstructure.

 

Figure 2: Micrograph of Ferrite-Bainite steel, HR400Y540T-FB

Figure 2: Micrograph of Ferrite-Bainite steel, HR400Y540T-FB.H-21

 

 

The primary advantage of FB steels over HSLA and DP steels is the improved stretchability of sheared edges as measured by the hole expansion test. Compared to HSLA steels with the same level of strength, FB steels also have a higher strain hardening exponent (n-value) and increased total elongation. Figure 3 compares FB 450/600 with HSLA 350/450 steel. Engineering and true stress-strain curves for FB steel grades are shown in Figure 4. 

Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and FB 450/600.

Figure 3: A comparison of stress strain curves for mild steel, HSLA 350/450, and FB 450/600.

 

Figure 4: Engineering stress-strain (left graphic) and true stress-strain (right graphic) curve for FB 450/600.T-10

Figure 4: Engineering stress-strain (left graphic) and true stress-strain (right graphic) curve for FB 450/600.T-10

 

Examples of typical automotive applications benefitting from these high strength highly formable grades include automotive chassis and suspension parts such as upper and lower control arms, longitudinal beams, seat cross members, rear twist beams, engine sub-frames and wheels.

Some of the specifications describing uncoated hot rolled 1st Generation ferrite-bainite (FB) steel are included below, with the grades typically listed in order of increasing minimum tensile strength and ductility.  Different specifications may exist which describe uncoated or coated versions of these grades.  Many automakers have proprietary specifications which encompass their requirements.

  • EN 10338, with the terms HDT450F and HDT580F D-18
  • VDA239-100, with the terms HR300Y450T-FB, HR440Y580T-FB, and HR600Y780T-FB V-3
  • JFS A2001, with the terms JSC440A and JSC590AJ-23