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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 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
Conventional HSS, Steel Grades
Carbon-Manganese Steels (CMn) are a lower cost approach to reach up to approximately 280MPa yield strength, but are limited in ductility, toughness and welding.
Increasing carbon and manganese, along with alloying with other elements like chromium and silicon, will increase strength, but have the same challenges as CMn steels with higher cost. An example is AISI/SAE 4130, a chromium-molybdenum (chromoly) medium carbon alloy steel. A wide range of properties are available, depending on the heat treatment of formed components. Welding conditions must be carefully controlled.
The 1980s saw the commercialization of high-strength low-alloy (HSLA) steels. In contrast with alloy steels, HSLA steels achieved higher strength with a much lower alloy content. Lower carbon content and lower alloying content leads to increased ductility, toughness, and weldability compared with grades achieving their strength from only solid solution strengthening like CMn steels or from alloying like AISI/SAE 4130. Lower alloying and elimination of post-forming heat treatment makes HSLA steels an economical approach for many applications.
This steelmaking approach allows for the production of sheet steels with yield strength levels now approaching 800 MPa. HSLA steels increase strength primarily by micro-alloying elements contributing to fine carbide precipitation, substitutional and interstitial strengthening, and grain-size refinement. HSLA steels are found in many body-in-white and underbody structural applications where strength is needed for increased in-service loads.
These steels may be referred to as microalloyed steels, since the carbide precipitation and grain-size refinement is achieved with only 0.05% to 0.10% of titanium, vanadium, and niobium, added alone or in combination with each other.
HSLA steels have a microstructure that is mostly precipitation-strengthened ferrite, with the amount of other constituents like pearlite and bainite being a function of the targeted strength level. More information about microstructural components is available here.
Some of the specifications describing uncoated cold rolled high strength low alloy (HSLA) steel are included below, with the grades typically listed in order of increasing minimum yield 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. Note that ASTM, EN and VDA terminology is based on minimum yield strength, while JIS and JFS standards are based on minimum tensile strength. Also note that JIS G3135 does not explicitly state that these grades must be supplied with an HSLA chemistry. A C-Mn approach is satisfactory as long as the mechanical property criteria are satisfied.
- ASTM A1008M, with the terms HSLAS 45[310], 50[340], 55[380], 60[410], 65[450], and 70[480] along with HSLAS-F 50 [340], 60 [410], Grade 70 [480] and 80 [550]A-25
- EN10268, with the terms HC260LA, HC300LA, HC340LA, HC380LA, HC420LA, HC460LA, and HC500LAD-5
- JIS G3135, with the terms SPFC340, SPFC370, SPFC390, SPFC440, SPFC490, SPFC540, and SPFC590J-3
- JFS A2001, with the terms JSC440R and JSC590RJ-23
- VDA239-100, with the terms CR210LA, CR240LA, CR270LA, CR300LA, CR340LA, CR380LA, CR420LA, and CR460LAV-3
