Cold stamping 1180-1470 MPa steel: what it takes

From the furnace to the cold press

For years, the strongest parts of a car body, such as pillars, roof rails and impact beams, were hot stamped: the blank is heated to around 900 °C, formed and quenched in the die. It works, but it needs a furnace over 30 m long and a lot of energy.

Steelmakers have since produced cold-formable grades at 1180 MPa and up to about 1470-1500 MPa. These parts are formed at room temperature on ordinary presses. The catch is that this steel behaves very differently in the die, and the problems that come with it are harder to see in a tryout.

The trade-off in one chart

Strength and elongation pull against each other. In one steelmaker's cold-rolled range, elongation falls from 17% at 980 MPa to 6% at the 1500 grade.

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The chart shows typical values from one steelmaker. A specification gives a range instead, and a delivered coil can fall anywhere inside it:

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The ranges are wide. A coil of the 1180 grade can have a yield strength anywhere from 900 to 1100 MPa, and that difference alone changes how much the part springs back. This is why forming simulation should use data measured on the actual material, not a catalogue value. Note that the minimum elongation here is measured on an 80 mm gauge (A80) and applies from 0.60 mm thickness, so it is not directly comparable with the chart.

In the chart, D marks a grade made for drawing and stretching, with good uniform elongation. B marks grades made for bending, which give up some of that for better local elongation and hole expansion, which matters at bends and cut edges. At this strength, choosing the grade by forming method is as important as choosing it by strength.

Two milestones worth knowing

• Nissan, 1.2 GPa (2013). Nissan developed a 1.2 GPa steel with enough elongation to cold press body parts, by controlling its structure at the sub-micron level. It first went into the Infiniti Q50, in the center pillar reinforcement and the roof rails.
• JFE Steel, 1.5 GPa. A fully martensitic steel made by water quenching at over 1,000 °C/s. Cold pressed, it replaced hot-stamped parts, first in a roof center reinforcement and then in many structural parts of the Toyota Prius. To control springback in long curved parts, JFE developed "stress reverse forming", which uses the Bauschinger effect to cut the stress that drives springback.

Four problems that get harder

1. Springback

Springback is the main challenge of cold stamping this steel. The higher the strength, the more elastic recovery after the die opens, and long curved parts twist and bow. Steelmakers recommend forming simulation to tune the part geometry, straight bend lines, engineered corner radii and draw beads. See What is springback and how is it reduced?

2. Cracks at bends and cut edges

With 6-8% elongation there is little margin. One 1500 MPa martensitic grade is guaranteed for bending at 4.0 times the sheet thickness and has a hole expansion ratio of about 40%. Tighter radii or stretched cut edges can crack where a forming limit diagram shows no problem. See stretch flange cracking and 3D-FLD.

3. Cracks that appear later

Above about 1000 MPa, steel becomes sensitive to hydrogen. Delayed cracking needs three things at once: a susceptible steel, hydrogen, and high stress. Sheared edges are especially vulnerable, and hydrogen can come from lubricants, pickling, electrogalvanizing or e-coat pretreatment. Tests such as SAE J3215 are used to rank materials. See delayed fracture prediction.

4. Press load and tool wear

Forming and trimming forces rise with strength. Some lines move to servo presses, and dies need wear-resistant materials and coatings.

Cold stamping or hot stamping?

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Neither route wins everywhere. Hot stamping still suits very complex shapes. Cold stamping wins on energy, cost and cycle time when the shape allows it and springback can be controlled.

Where simulation fits

Each problem above can be checked before the die is cut, but only with the right material model. At this strength, a simple isotropic hardening model is not enough for springback. The Bauschinger effect, the same effect JFE uses in stress reverse forming, has to be in the model, which is what Yoshida-Uemori provides.

Sources

• Kobe Steel: 780-1470 N/mm² grade cold-rolled steel sheet
• Nissan: Ultra high tensile strength steel with high formability
• worldsteel: JFE Steel water-quench 1.5 GPa steel and cold pressing technology
• AHSS Insights: Delayed cracking (hydrogen embrittlement)
• AHSS Insights: Considerations when deciding whether to cold form or hot form
• SSAB: Cold stamping 1500 and 1700 MPa automotive steels
• SSAB: Docol 1000CP (CR780Y980T-CP) data sheet
• SSAB: Docol 1200CP (CR900Y1180T-CP) data sheet
• SSAB: Docol 1300M (CR1030Y1300T-MS) data sheet
• SSAB: Docol 1500M (CR1220Y1500T-MS) data sheet