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Aging of Carbon Steel Sheet and Strip – SAE J763 (1991)
This article provides a concise overview of the phenomenon of aging in hot-rolled, cold-rolled, and coated carbon steel sheet and strip, as outlined in SAE Information Report J763 (1991). Understanding aging is essential for recognizing and mitigating associated forming problems.
Understanding Aging in Carbon Steel
Aging refers to time-dependent changes in the physical and mechanical properties of low-carbon steel that adversely affect formability. These changes accelerate at elevated temperatures. Two primary forms are recognized:
- Strain Aging: Changes that occur after cold working. Rimmed steel is particularly susceptible.
- Quench Aging: Changes that occur after rapid cooling (typically a producer concern, not further addressed here).
Key Terminology: Temper rolling (or skin passing) is a light cold rolling operation to improve flatness, reduce stretcher strain tendency, and impart desired surface finish. It should not be confused with cold reduction, which involves substantial thickness reduction.
Effects of Strain Aging ⚠️
Strain aging of temper-rolled carbon steel can be identified by several easily recognizable effects that impact forming performance, especially for exposed parts. The table below summarizes these effects:
| Effect | Description |
|---|---|
| Surface Irregularities | Visible stretcher strains (Lüder’s lines) or fluting develop during forming, making parts unsuitable for exposed applications. |
| Ductility Reduction | The material becomes less able to deform plastically without fracturing. |
| Hardness Increase | Indentation resistance rises, often accompanied by a loss of workability. |
| Tensile Property Changes | Increase in yield point and yield point elongation; decrease in total elongation. |
These changes are particularly detrimental when forming complex shapes or requiring consistent material behavior over time.
Best Practices for Controlling Aging 🛠️
To minimize the adverse effects of strain aging during fabrication and in service, the following design and material selection practices are recommended:
- Use killed or stabilized steels (e.g., aluminum-killed) for exposed or critical formed parts.
- For unexposed parts, sheets that have not been temper passed may be used.
- Implement first-in, first-out (FIFO) inventory rotation to limit the time steel ages in storage.
- Apply temper rolling to improve flatness and reduce the tendency to stretcher strain, but recognize that the aging clock restarts after temper rolling.
- Consider that strain aging increases yield point and reduces ductility, which affects formability; adjust forming process parameters accordingly.
Common Mistake: Confusing temper rolling with cold reduction. Temper rolling imparts only a light reduction (typically <1% thickness change) and is not a substitute for cold reduction to achieve final thickness.
Frequently Asked Questions
What causes strain aging in carbon steel sheet and strip?
Strain aging occurs when interstitial atoms (primarily carbon and nitrogen) diffuse to dislocations introduced by cold working, pinning them and causing yield point return, increased hardness, and reduced ductility. This diffusion happens over time, accelerating at higher temperatures.
How can stretcher strains (Lüder’s lines) be prevented during forming?
Stretcher strains can be prevented by using non-aging or stabilized steels (e.g., killed steels with aluminum or titanium additions) that tie up interstitial elements. Additionally, temper rolling immediately before forming can temporarily suppress the yield point, but forming must occur before strain aging reoccurs. Proper inventory rotation also helps.
What is the difference between strain aging and quench aging?
Strain aging occurs after cold working, while quench aging occurs after rapid cooling from elevated temperatures. Quench aging is largely controlled by the steel producer and is not a typical concern for fabricators handling sheet and strip. Strain aging is the primary aging mode relevant to forming operations.
How does temper rolling affect aging behavior?
Temper rolling (skin passing) imparts a light cold reduction that introduces new dislocations and temporarily eliminates the yield point, making the steel more formable immediately after rolling. However, over time, strain aging will reoccur, potentially restoring the yield point and causing stretcher strains during later forming. Prompt forming after temper rolling is beneficial.
