Complex phase (CP) steels for the highest hole expansion ratios
Complex phase steels have the highest — up to 100% — hole expansion ratios available from advanced high strength steels. Their high HER values provide excellent cold-forming properties for punched and stretched edges/flanges, as well as deep-drawn shapes. CP steels’ superior yield strengths are used for high crash-energy absorption, while their elevated UTS levels enable thinner walls for significant automobile weight reductions.
CP steel basics
Complex phase steels have a bainite-ferrite microstructure with small amounts of martensite and retained austenite and pearlite. The thermal cycles used to produce CP retards recrystallization while promoting different carbo-nitride precipitations, resulting in extreme grain refinements for excellent ductility, even for high (e.g., 1200 MPa or 174 ksi) tensile strength grades.
The carbo-nitride precipitations also increase complex phase steel’s ferrite strength; as a result, CP steels have higher yield strengths than dual phase (DP) steels at equivalent tensile strength levels. Complex phase steels’ high yield strengths are used by body-in-white designers for high crash-energy absorption for safety components, such as in frame rails, reinforcements, beams, and more.
Steel properties
- Up to 100% hole expansion ratios — significantly reduce or eliminate micro cracks and burrs on cut/sheared edges.
- Greatly increases local formability, providing ductility for deep-drawn components with complex geometries.
- Excellent for energy-efficient cold-forming and fineblanking of auto parts with complex, punched edges.
- Improved cleanliness and grain refinement provides tight radii bendability and stretch flangeability even at very high yield strengths.
- Excellent true fracture strain (local formability).
- High crash-energy absorption.
- Good fatigue resistance and durability.
New Docol® hot-rolled 800 MPa HER steels with up to 100% HER
To stretch or form cut edges that are crack-free can be a major challenge when working with advanced-high strength steels. To reduce the risk of cracked (raised) edges, SSAB is introducing two new versions of hot-rolled 800 megapascal steels with extremely high edge ductility.
Many of these steel properties should contradict one another: formability vs. high strength, high edge ductility vs. good crash performance. But these steels deliver all of these properties – and, especially, they deliver very high edge ductility.
These new high edge ductility AHSSs can step in as "problem-solvers". Or, they can be used on auto parts where you want higher confidence that you won't have cracked edges.