What Limits the Plasticity of DC02 Steel
DC02 is a low-carbon, aluminium-killed cold-rolled sheet grade intended for deep drawing, supplied under EN 10130 and the corresponding national standard GB/T 5213. In the as-cold-rolled condition its structure is heavily deformed: the grains are flattened along the rolling direction, dislocation density is high, and yield strength typically sits between 300 and 350 MPa with elongation below 20%. In that state the material cannot accommodate the wall thinning and stretch that a deep drawing operation demands. Plasticity has to be restored before the coil is delivered to the press.
Improvement therefore follows three routes working together. Annealing removes the stored energy of deformation and rebuilds a fine equiaxed grain structure. Chemistry control lowers the elements and phases that embrittle the matrix or act as crack initiation sites. Cleanliness and rolling practice keep the structure uniform so that strain is distributed instead of concentrating in a few weak bands. The sections below give the practical windows for each route.
Full Annealing: The Main Route to Plasticity
Batch or continuous full annealing, also called recrystallization annealing, is the single most effective method. The strip is heated above the ferrite recrystallization temperature but kept below the temperature at which a new austenite phase would form, held long enough for the new grains to grow and then cooled slowly.
| Parameter | Recommended value | Effect |
|---|---|---|
| Annealing temperature | 700–750 °C | Complete recrystallization of deformed ferrite |
| Holding time | 40–60 minutes | Uniform grain growth, stress relief |
| Cooling | Slow cooling to below 300 °C before removal | Prevents quenching stress and distortion |
| Resulting grain size | 15–30 µm, equiaxed | Balanced strength and formability |
| Dislocation density | Reduced by more than 80% | Removes cold work hardening |
| Elongation | From under 20% cold rolled to at least 34%, often 38–40% | Meets and exceeds the standard requirement |
| Yield strength | Falls from 300–350 MPa to 140–240 MPa | Much lower resistance to plastic deformation |
The temperature window matters in both directions. Below the recrystallization range the deformed structure survives and the strip stays hard and directional; well above it, grain growth becomes difficult to control and the sheet loses the fine uniform structure that gives consistent drawing behaviour.
Isothermal Annealing for Heavy Reduction Coils
When the cold rolling reduction ratio of DC02 exceeds about 70%, a single-temperature anneal risks abnormal grain growth, because the stored energy is high and a few grains can absorb their neighbours. A two-stage isothermal cycle solves this. The strip is first heated to about 750 °C and held for 30 minutes, then rapidly cooled to about 650 °C and held for a further 40 minutes before the final slow cool.
The advantage is a finer and more uniform grain size, normally 10–20 µm, instead of the coarse mixed structure left by a single-temperature treatment. Because the grain size distribution is narrower, the sheet deforms more evenly during drawing, the risk of localised thinning falls, and the scatter of mechanical properties along the coil is reduced.
Chemistry Control: Reducing Embrittling Elements and Hard Phases
Annealing alone cannot compensate for a composition that promotes brittleness. Four elements require attention, and the alloy addition of aluminium plays a positive role.
Carbon. Carbon strengthens the matrix, but above about 0.10% it forms carbides that raise hardness and lower ductility. The DC02 specification limits carbon to 0.10% maximum; production practice that keeps the level at 0.06–0.08% gives a further gain in formability.
Manganese. Excess manganese above roughly 0.45% combines with sulphur to form manganese sulphide inclusions, which are hard and brittle and become the origin of localised cracking during stamping. Holding manganese at or below 0.40% reduces the inclusion population and improves ductility.
Phosphorus and sulphur. Phosphorus causes cold brittleness by reducing ductility at low temperature, while sulphur forms low-melting-point sulphides that cause hot brittleness during high-temperature processing. The standard limits both to 0.035% maximum; tight control keeps phosphorus at or below 0.025% and sulphur at or below 0.020%, which removes most of the brittle inclusions that limit forming.
Aluminium. Aluminium is a strong deoxidiser and forms aluminium nitride particles that pin grain boundaries. The grade requires at least 0.020% aluminium, and an addition of 0.03–0.05% is enough to keep the annealed grain size at 20 µm or finer, giving a further 5–8% improvement in ductility.
Cleanliness and Structure Uniformity
Non-metallic inclusions are stress concentration points. Reducing them has a direct effect on how evenly the sheet deforms.
Ladle refining and vacuum degassing reduce oxide inclusions such as alumina and sulphide inclusions such as manganese sulphide to a total inclusion rating of 1.5 or better, evaluated by the procedure of GB/T 10561. A cleaner steel has fewer sites at which a crack can start.
Cold rolling should use many light passes with 15–20% reduction per pass rather than a few heavy passes. This limits localised grain elongation and the internal stress bands that form when deformation is uneven.
A light temper rolling pass of 1–3% after cold rolling removes flatness defects such as waviness and slightly deforms the surface grains, which makes recrystallization more uniform during the subsequent anneal.
Because DC02 is aluminium killed, the annealing cycle should be matched to the aluminium and nitrogen content so that the intended grain-pinning effect is obtained rather than a partially stabilised structure.
Combining the Three Levers
No single measure is sufficient on its own. A very clean steel with a poor annealing cycle remains hard, and a well-annealed coil produced from a high-phosphorus, inclusion-rich heat still cracks at the draw die. The working sequence is to fix cleanliness first, because it sets the ceiling on achievable ductility; then tighten the chemistry limits for carbon, manganese, phosphorus, sulphur and aluminium; then apply recrystallization or two-stage isothermal annealing with verified temperature uniformity; and finally confirm the outcome through tensile testing, grain size measurement and inclusion rating on a coil-by-coil basis. When elongation, grain size and inclusion rating are all within the combined target, the plastic behaviour of DC02 in the press becomes repeatable rather than grade-dependent.
FAQ
Q: What annealing cycle gives the best plasticity in DC02 steel?
Heat to 700–750 °C, hold for 40–60 minutes and cool slowly below 300 °C; elongation rises from under 20% cold rolled to at least 34%.
Q: When should isothermal annealing be used instead of full annealing?
When the cold rolling reduction exceeds about 70%, a two-stage cycle of 750 °C for 30 minutes followed by 650 °C for 40 minutes gives finer 10–20 µm grains.
Q: Which chemical elements most affect DC02 formability?
Carbon, manganese, phosphorus and sulphur should be kept low, with typical production targets of 0.06–0.08% carbon, 0.40% or less manganese, 0.025% phosphorus and 0.020% sulphur.
Q: How does aluminium addition improve ductility?
Aluminium forms nitride particles that pin grain boundaries, so a 0.03–0.05% addition keeps annealed grains at 20 µm or finer and adds 5–8% to ductility.
Q: Why does inclusion control matter for drawing?
Inclusions act as stress concentration points and start localised cracks, so a total inclusion rating of 1.5 or better per GB/T 10561 makes deformation more uniform.
Q: How does cold rolling practice support later annealing?
Many light passes of 15–20% reduction avoid stress bands, and a final temper rolling pass of 1–3% improves flatness and recrystallization uniformity.