ASTM A573/A573M is the specification for structural carbon-manganese-silicon steel plates supplied with improved toughness. Grade 58 is the lowest tensile band in that specification, and it is the grade normally selected when a structure needs moderate strength, good weldability and a certified notch toughness result rather than a high strength class. Typical uses are storage tanks, welded structural frames, bridge and building components, and general plate work where ordinary commercial quality plate would be too much of a risk.
Mechanical Requirements of Grade 58
| Property | Requirement |
|---|---|
| Yield strength (min.) | 32 ksi (approx. 220 MPa) |
| Tensile strength | 58 - 71 ksi (approx. 400 - 490 MPa) |
| Elongation, 8 in. (200 mm) gauge | ≥ 21 % |
| Elongation, 2 in. (50 mm) gauge | ≥ 24 % |
| Usable thickness | 3/16 in. (approx. 5 mm) to 6 in. (approx. 152 mm) |
| Weldability | Good; low carbon, no preheat needed in thin sections |
| Impact testing | Charpy V-notch at the temperature stated in the order |
The width of the tensile band, from 58,000 to 71,000 psi, exists so that one grade can cover a full range of plate thicknesses. Thicker plate in the same heat will test toward the lower part of the band, and the purchaser should not assume the upper end of the range.
Chemical Composition
| Element | Content (wt. %) | Function |
|---|---|---|
| Carbon (C) | ≤ 0.23 | Strength; limited to protect toughness and weldability |
| Manganese (Mn) | 0.60 - 0.90 | Strength and notch toughness |
| Silicon (Si) | 0.10 - 0.35 | Deoxidation |
| Phosphorus (P) | ≤ 0.035 | Impurity; restricted against embrittlement |
| Sulfur (S) | ≤ 0.04 | Impurity; restricted against hot shortness |
There is no deliberate alloy addition in this grade. The improved toughness relative to an ordinary structural plate comes from the combination of a controlled carbon content, a moderate manganese level and the fine-grain steelmaking practice required by the specification.
Why Improved Toughness Matters in Structural Plate
Structural failures in tanks and frames rarely start from a simple overload; they start from a notch combined with low energy absorption, which is exactly the condition that a toughness requirement is written to control. A welded frame has notches by definition at every weld toe, bolt hole and cope, so the ability of the plate to absorb energy before fracture is a genuine design parameter and not a formality. Ordering A573 Grade 58 with a stated Charpy test temperature gives the fabricator a certificated value instead of an assumption.
Equivalent Grades in Other Systems
Buyers who work across standards should treat equivalence carefully, because a similar designation does not mean identical limits. The nearest structural grades in other systems are the 235 to 275 MPa yield classes: GB/T 700 Q235B and GB/T 1591 Q355B, the S235 and S275 grades of EN 10025-2, and JIS G3101 SS400. These grades specify impact testing only when an additional requirement is ordered, whereas A573 Grade 58 carries the improved toughness attribute as part of the specification itself, which is the reason it is preferred for pressure-adjacent and safety-related plate work.
Fabrication and Inspection Notes
The low carbon content makes the grade easy to shear, flame cut, roll and weld with ordinary carbon steel consumables. Flame cutting of thick plate benefits from a warm edge or a controlled cutting speed, since a hard cut edge can otherwise form. Weld procedures are qualified to the applicable construction code, and the certificate for the plate should be checked against the heat number marking on each piece before fabrication begins. For orders where the toughness result is critical, the test temperature and the minimum absorbed energy relative to the plate thickness should both be stated.
Frequently Asked Questions
Q: What is ASTM A573 Grade 58?
A: It is the lowest tensile band of the ASTM A573/A573M specification for structural carbon-manganese-silicon plate with improved toughness, with a minimum yield strength of 32 ksi and a tensile range of 58,000 to 71,000 psi.
Q: What does the number 58 mean in Grade 58?
A: It refers to the minimum tensile strength of the grade in ksi, that is 58,000 psi or approximately 400 MPa. The companion grades A573 Grade 65 and Grade 70 are named in the same way.
Q: Is A573 Grade 58 suitable for low temperature service?
A: It is designed for atmospheric temperature duty. Where a low design temperature applies, the required Charpy test temperature and energy must be specified, and a plate grade intended for that temperature should be used.
Q: Can A573 Grade 58 be welded without preheat?
A: For thin and medium sections, preheat is usually unnecessary because the carbon and carbon equivalent are low. Thick sections, high restraint joints or cold workshop conditions may still call for a moderate preheat, and the welding procedure should state it.
Q: How does A573 Grade 58 differ from ASTM A36?
A: Both are carbon structural steels of a similar strength class, but A573 carries a defined toughness attribute and tighter chemistry control, so it is the better choice when the plate must perform under fatigue or dynamic loading.
Q: What should be checked on the material certificate?
A: Chemical composition, tensile and yield results, elongation on the correct gauge length, and the Charpy result if impact testing was ordered. Verify that the heat number matches the marking on each plate.
