The Four Families a Vehicle Programme Chooses Between
Automotive steel is not a single product. Body engineering selects from four broad families, each covering a different balance between formability, strength and cost. Mild deep drawing grades form the outer panels and complex inner panels, bake hardening grades start soft enough to form and gain strength during the paint cure, high strength low alloy grades provide a higher yield strength for structural members with a predictable forming behaviour, and dual phase grades combine a soft ferrite matrix with hard martensite islands to give high tensile strength while retaining enough elongation to be pressed into shape. Grade definitions and mechanical windows follow VDA 239-100 and EN 10338 in Europe, GB/T 20564 in China, JIS G3135 for the SPFC series in Japan, and ASTM A1008 or A1011 for high strength low alloy sheet in North America.
Mechanical Windows by Grade Family
| Family | Example grade | Yield strength | Tensile strength | Elongation A80 |
|---|---|---|---|---|
| Mild deep drawing | DC04 to EN 10130 | 210 MPa max | 270-350 MPa | 38% min |
| Bake hardening | CR180BH to VDA 239-100 | 180-240 MPa as delivered | 290-370 MPa | 32% min |
| High strength low alloy | CR300LA to VDA 239-100 | 300-380 MPa | 380-480 MPa | 21% min |
| Dual phase | CR440Y780T-DP | 440-550 MPa | 780 MPa min | 14% min |
The table shows the trade the designer is making. Moving from a 210 MPa mild grade to a 780 MPa dual phase grade multiplies the tensile strength by more than two while cutting elongation from 38% to 14%. In practice that allows a component to be gauged down, which is the whole purpose of the higher strength families: for equal load capacity a dual phase part can often be produced 15 to 25% thinner than the mild steel version, and the reduction in mass is what justifies the higher price per tonne. The limiting factor is usually stiffness rather than strength, so the substitution is applied to parts where the load path, not the panel geometry, governs.
Bake Hardening and the Paint Cure
Bake hardening grades rely on a controlled amount of solute carbon that stays in solution after annealing. The steel is soft when the panel is pressed, which keeps the drawing operation inside the forming limit, and during the paint bake cycle the solute carbon precipitates onto dislocations and raises the yield strength. A typical cycle of about 20 minutes at 170 °C to 180 °C produces an increase of at least 30 MPa in the yield strength measured after the bake, so the finished component is stronger than the delivered coil. Exposed panels benefit most because they are formed from a soft grade and then gain dent resistance in the paint shop at no extra forming risk.
Hot Rolled and Coated Automotive Product
Chassis, suspension arms, wheels and structural members are made from hot rolled material rather than cold rolled sheet. EN 10025-2 covers the plain structural grades such as S355, while EN 10149-2 covers the high yield strength hot rolled steels for cold forming, with grades such as S500MC and S700MC delivering minimum yield values of 500 MPa and 700 MPa in the micro-alloyed condition. Corrosion protection for these parts is provided by hot-dip galvanizing, galvannealing, electro-zinc coating under ASTM A879, or by the paint film itself where the part is inside the body shell. For exposed panels the substrate surface quality and the roughness range are specified as tightly as the mechanical values, because any surface defect shows through the topcoat at a wet film thickness of little over 100 µm.
Forming Limits and Verification
Automotive grades are qualified with more than a tensile test. The plastic strain ratio r and the strain hardening exponent n describe how the sheet resists thinning during a draw, and the forming limit curve derived from those values predicts where a press will split. Hole expansion ratio under ISO 16630 is measured on dual phase and other high strength grades because their high tensile strength comes with a reduced ability to stretch a cut edge, and a flange that performs well in mild steel can crack in a 780 MPa grade at the same hole clearance. Springback increases with yield strength and is compensated in the die geometry. Production verification covers coil-to-coil mechanical testing, thickness profile across the width, flatness, surface roughness and a surface inspection against a defined defect catalogue, with the results reported on a certificate to EN 10204 3.1 for each coil and heat.
An order should identify the standard and grade, the delivery condition such as annealed and temper rolled or hot rolled pickled, the surface class, the roughness range, the thickness and its tolerance, the coil or blank size and the oil film specification. Blanks for tailor welded assemblies must be supplied with the welding parameters and the certified compatibility of the different gauges, since a weld between grades of very different strength moves the failure location. Where the part will be formed before painting, specify the forming lubricant on the coil, because a mill-applied oil designed for transit will not carry an aggressive draw. Documentation for a vehicle programme additionally includes the material declaration, the chemical analysis for restricted substances, and a dimensional inspection report for every lot supplied.
Frequently Asked Questions
Q: What is the difference between mild and bake hardening steel?
A: Both are low carbon cold rolled grades, but bake hardening steel contains solute carbon that precipitates during the paint cure and raises yield strength by at least 30 MPa. Mild steel gains no useful strength from the same cycle.
Q: Why do dual phase grades have a higher tensile strength than HSLA grades?
A: Dual phase steels combine a ductile ferrite matrix with hard martensite islands, which distributes strain and delays necking. On the same tensile level a dual phase grade offers better formability than a micro-alloyed HSLA grade.
Q: How much gauge reduction do high strength steels allow?
A: For equal load capacity a substituted part can often be 15 to 25% thinner, but the saving is limited by panel stiffness and by the hole expansion behaviour of the higher strength grade in any flanged or pierced feature.
Q: Which standard covers automotive sheet mechanical properties?
A: VDA 239-100 and EN 10338 are the main references for cold rolled cold forming grades, GB/T 20564 covers the Chinese equivalents, and EN 10149-2 covers high yield hot rolled grades for cold forming.
Q: Does the zinc coating affect forming?
A: It deforms with the steel at normal radii. Where very tight radii or high strains are combined with a heavy coating, lubricant selection and die radius are adjusted, because coating pick-up on the die changes friction over a production run.
