Martensitic steel: excellent formability-to-strength ratio

What is the definition of martensitic steel?

When austenite iron is very rapidly quenched, the austenite is transformed into a very hard crystalline microstructure called “martensite.”

The martensitic transformation begins when the austenite reaches the martensite start temperature (M_s) and continues until the lower transformation temperature (M_t) is reached. In general, the higher the percentage of conversion to martensite, the stronger the resulting martensitic steel. These steels undergo post-quench martensitic tempering to improve their ductility, so that even MS steels with very high strength levels can be cold-formed using processes like cold-roll forming.

To produce cold-rolled martensitic steel (aka MS or MART), a cold-rolled steel strip is fed into a continuous annealing line (CAL). In the CAL, the material is annealed, water-quenched, and tempered. The austenite formed in the CAL, when rapidly quenched, is transformed into martensite. The CAL’s tempering operation provides the ultra-high strength steel with improved ductility.

Docol^® martensitic grades have less than 2% alloying elements, providing good weldability.

• Extremely high yield strength (R_p0,2) and tensile strength (R_m).
• Superior energy absorption and impact strength for anti-intrusion zones.
• Ideal for vehicle lightweighting.
• Cold formability in bending, stamping, and roll-forming — including 3D forming.
• Martensitic steel is significantly more cost-effective than aluminum: using optimized design profiles, MS steels can deliver high-strength performance that is similar to aluminum and at the same weight.

With the highest strength of our cold-formable steels, Docol martensitic steel enables excellent lightweighting and increased crash performance:

• EV battery protection
• Sill (rocker) reinforcements
• Door/side intrusion beams
• Bumpers
• Roof reinforcements
• Seat structures

Cold-rolled Docol 1700M steel from SSAB is an eco-friendly, pound-for-pound alternative to aluminum for lightweight components used in passive safety systems. Due to Docol martensitic steel’s ductility, critical safety components are now being fabricated using cost- and energy-efficient cold forming processes.

For example, Shape Corp. uses 3D roll-forming to create lighter, more compact, and stronger A-pillars and roof rail tubes in Docol martensitic 1700MPa. Designed for the Ford 2020 Explorer and 2020 Escape, these key safety components increase driver visibility, interior space, and air bag placement optimization.

Docol 1700 martensitic is the material of choice for ultra-high strength steel (UHSS) applications, including side impact beams, bumpers, structural components, and EV battery enclosures. Docol design specialists can provide stimulated crash performance data on several optimized 1700MPa profiles, as well as simulations for many of our other martensitic grades: Docol 900M, 1100M, 1200M, 1300M, 1400M and 1500M.

• Excellent cold formability for bending, roll-forming, and stamping at room temperatures.
• Quite good drawability and good edge-stretching and hole-expansion properties.
• Good weldability, including resistance spot welding and laser welding.
• Docol^® martensitic steel grades are highly consistent — in their mechanical properties — avoiding stresses that can lead to unwanted or unexpected springback.

• Superior fatigue properties for all vehicles: BEV, ICE or hybrid.
• Seven high-strength grades: 900MPa, 1100MPa, 1200MPa, 1300MPa, 1400MPa, 1500MPa and 1700MPa.
• Grades are available to meet OEM-specific standards and certifications.
• Electrogalvanized (EG) corrosion resistance available on request.
• Global availability and support. All grades listed are now commercially available.

Safety engineers: Create new anti-intrusion zones (e.g., for BEV battery packs) and optimize the space utilization of existing zones (e.g., passenger compartments).

Lightweighting engineers: Leverage the extreme steel strength and aluminum-like weight of martensitic steel.

Production engineers: Use familiar cold forming operations (bending, stamping, roll-forming) and exciting new operations, like 3D roll-forming, on a UHSS steel.

Procurement officers: Realize major cost reductions — with no performance penalty — by replacing aluminum with Docol martensitic steel. (And reduce your carbon footprint at the same time.)

Sustainability officers: Lower the embedded CO₂ in your materials and improve your life cycle assessment with our “future-proofed” Docol martensitic steels.

Electrogalvanized martensitic steel

Docol electrogalvanized MS steels provide corrosion resistance using a uniform, high quality, protective coating, which can be applied as a very thin coat, if desired. Docol EG-coated martensitic grades can be readily formed and joined, with excellent resistance spot welding (RSW) properties.

Docol EZ (or ZE) martensitic automotive steels are available from SSAB in the following strengths: Docol 900M, 1100M, 1200M, 1300M, 1400M and 1500M. With SSAB’s long and extensive experience in making electrogalvanized martensitic steels, car OEMs can have any of their concerns regarding hydrogen embrittlement (HE), also known as hydrogen-induced cracking (HIC), fully addressed.

High yield strength martensite for crash performance

World Auto Steel points out a correlation between sheet steel yield strength and the 3-point bending deformation (crash behavior) of hat-shaped parts:
“CR1200Y1470T-MS has similar performance to hot-stamped PHS-CR1800T-MB and PHS-CR1900T-MB at the same thickness, and exceeds the frequently used PHS-CR1500T-MB. For this reason, there may be the potential to reduce costs and even weight with a cold-stamping approach, providing appropriate press, process, and die designs are used.” World Auto Steel’s Metallurgy of Martensitic Steels.

With our Trial Material Stock Center, you can get AHSS samples for most of our commercially available steels – as well as samples for our newest but not yet commercially released grades. Unique to the industry, we ship coils, sheets and custom lengths of any size. In 1 to 2 weeks, your material is ready for dispatch to anywhere in the world.