SM570 (JIS G3106) is Japan’s highest-strength weldable structural plate steel, achieving 460 MPa (67 ksi) minimum yield through thermomechanical control process (TMCP) rolling with micro-alloy additions. Its closest ASTM counterpart is A572 Gr.65, though SM570 consistently exceeds that grade’s yield floor. This guide covers equivalent grades, HSLA composition, mechanical properties, TMCP welding constraints, and selection guidance for high-rise buildings, long-span bridges, and offshore structures.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SM570
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G3106 | SM570 | Japan | Reference |
| ASTM A572/A572M | A572 Gr.65 | USA | ✅ Nearest Exact (Fy = 450 MPa / 65 ksi) |
| EN 10025-3 | S460N / 1.8901 | Europe | ⚠️ Nearest Equivalent |
| EN 10025-4 | S460M | Europe | ⚠️ Nearest Equivalent (thermomechanically rolled) |
| ISO 630-2 | S460 | International | ⚠️ Nearest Equivalent |
SM570 yield (≥ 460 MPa / 66.7 ksi for t ≤ 13 mm) slightly exceeds A572 Gr.65 (≥ 450 MPa / 65 ksi), making SM570 a conservative structural upgrade where Gr.65 is specified.
2. Chemical Composition
| Element | SM570 | ASTM A572 Gr.65 | EN S460N |
|---|---|---|---|
| C | ≤ 0.18% | ≤ 0.23% | ≤ 0.20% |
| Si | ≤ 0.55% | ≤ 0.40% | ≤ 0.60% |
| Mn | ≤ 1.70% | ≤ 1.65% | ≤ 1.70% |
| P | ≤ 0.035% | ≤ 0.040% | ≤ 0.030% |
| S | ≤ 0.035% | ≤ 0.050% | ≤ 0.030% |
| Nb + V + Ti | Micro-alloy additions (HSLA strategy) | Cb or V addition | Nb, V, Ti permitted |
Sources: JIS G3106:2020, ASTM A572/A572M, EN 10025-3
SM570 achieves its strength primarily through TMCP (thermomechanical control process) rolling combined with micro-alloy additions (Nb, V, Ti). This low-carbon, micro-alloyed approach delivers high yield with good weldability — but places strict constraints on welding heat input.
3. Mechanical Properties
Tensile and Yield Properties by Thickness
| Property | SM570 | ASTM A572 Gr.65 | EN S460N |
|---|---|---|---|
| Tensile Strength | ≥ 570 MPa (≥ 82.7 ksi) | ≥ 550 MPa (≥ 80 ksi) | 550–720 MPa (80–104 ksi) |
| Yield Point (t ≤ 13 mm / 0.51 in) | ≥ 460 MPa (≥ 66.7 ksi) | ≥ 450 MPa (≥ 65 ksi) | ≥ 460 MPa (≥ 66.7 ksi) |
| Yield Point (13–32 mm / 0.51–1.26 in) | ≥ 440 MPa (≥ 63.8 ksi) | ≥ 450 MPa (≥ 65 ksi) | ≥ 440 MPa (≥ 63.8 ksi) |
| Yield Point (32–50 mm / 1.26–1.97 in) | ≥ 420 MPa (≥ 60.9 ksi) | ≥ 450 MPa (≥ 65 ksi) | ≥ 420 MPa (≥ 60.9 ksi) |
| Elongation (GL = 5.65√A) | ≥ 19% | ≥ 15% | ≥ 17% |
| Charpy Impact | ≥ 47 J at 0°C (≥ 35 ft·lbf at 32°F) | Not required | ≥ 27 J at −20°C |
Note: SM570 yield decreases with increasing thickness. For sections 32–50 mm (1.26–1.97 in), yield drops to ≥ 420 MPa (60.9 ksi). Verify the applicable thickness tier against A572 Gr.65’s flat 450 MPa for sections in the 32–50 mm range.
4. Physical Properties
| Property | Value |
|---|---|
| Density | 7.85 g/cm³ (0.284 lb/in³) |
| Young’s Modulus | 200 GPa (29,000 ksi) |
| Thermal Conductivity | ~48 W/(m·K) (333 BTU·in/(hr·ft²·°F)) |
| Thermal Expansion (20–100°C) | 11.7 × 10⁻⁶ /°C (6.5 × 10⁻⁶ /°F) |
| Specific Heat (approx.) | ~490 J/(kg·K) (0.117 BTU/(lb·°F)) |
5. Heat Treatment Conditions
| Process | Temperature | Cooling | Purpose |
|---|---|---|---|
| TMCP Rolling (mill process) | Controlled rolling finish ~800–900°C (1,472–1,652°F) | Accelerated cooling | Delivers specified yield via grain refinement + micro-alloy precipitation |
| Normalizing (alternative delivery) | 900–950°C (1,652–1,742°F) | Air cool | Used when TMCP is not specified; slightly lower yield potential |
| Stress Relief (post-weld) | 550–620°C (1,022–1,148°F) | Furnace cool | Residual stress reduction; must stay below 620°C to preserve TMCP properties |
Post-weld heat treatment above 620°C (1,148°F) risks over-tempering the TMCP microstructure and reducing yield strength below the specified minimum. If PWHT is required for a SM570 weldment, restrict temperature to ≤ 620°C and confirm with the steel mill’s technical guidance.
6. Machinability
- Machinability rating: approximately 60–65% relative to AISI 1212 baseline (higher strength = more cutting resistance)
- Carbide tooling recommended for turning and milling operations; HSS suitable only at reduced speeds
- Higher yield and micro-alloy content increase tool wear compared to SM400/SS400
- Flame cutting and plasma cutting are the standard stock-removal methods for plate fabrication
- Preheat to 50°C (122°F) minimum before flame cutting for thicknesses above 25 mm (1 in)
- Grinding: standard practice for weld preparation and surface finishing
7. Weldability
- Weldability: Good for an HSLA grade — low carbon content (≤ 0.18%) keeps Ceq manageable despite higher strength
- Preheat: 50–100°C (122–212°F) for t > 25 mm (1 in); not required for thinner sections in ambient conditions
- Low-hydrogen electrodes mandatory (E8018 class or equivalent for matching strength; E7018 acceptable for under-matching filler)
- Heat input control is critical: limit welding heat input to ≤ 50 kJ/cm (typical; confirm with mill WPS documentation) to prevent HAZ softening that destroys TMCP-delivered strength
- Interpass temperature: maintain below 250°C (482°F) to avoid excessive heat accumulation
- Preheat and interpass temperature monitoring especially important in multi-pass welds on thick sections
- For seismic moment connections, consult the steel mill’s approved Welding Procedure Specification (WPS)
8. Common Mistakes
SM570 achieves its yield strength through TMCP rolling and micro-alloy precipitation hardening — mechanisms that are thermally sensitive. Welding procedures designed for SM400 or SS400 typically allow high heat input (often 60–80 kJ/cm or more) to maximize productivity. Applied to SM570, this excess heat softens the heat-affected zone, potentially dropping HAZ yield below 400 MPa — well under the 460 MPa design basis. SM570 requires heat-input-controlled WPS developed and validated specifically for the grade. Never carry over welding procedures from lower-strength grades without engineering review.
SM570 has a minimum yield of 460 MPa (66.7 ksi) for thin sections, while SM490 yields at ≥ 325 MPa (47 ksi) — a 41% difference. Engineers substituting SM490 for SM570 (typically for cost or availability) without structural recalculation under-deliver on column or beam capacity by a significant margin. Conversely, substituting SM570 for SM490 increases connection forces (due to higher actual yield) that non-yielding elements must resist. Neither substitution is safe without full structural re-analysis.
9. When to Choose SM570
✅ Choose SM570 when:
- ✅ High-rise building columns and beams in Japan’s seismic zones where higher yield reduces section weight
- ✅ Long-span bridges where weight reduction through higher yield is critical to span geometry
- ✅ Offshore structures requiring combined high strength and verified Charpy impact toughness
- ✅ Applications where A572 Gr.65 is specified and Japanese mill supply is preferred
- ✅ Projects where Charpy at 0°C (32°F) is a design requirement — SM570 mandates ≥ 47 J
❌ Do not choose SM570 when:
- ❌ Welding heat input cannot be controlled — the TMCP strength advantage is destroyed by high heat input welding
- ❌ Sections exceed 100 mm (3.9 in) — through-thickness properties require additional Z-direction testing not standard in SM570
- ❌ SM490 yield is sufficient — SM570 carries a cost premium and stricter fabrication requirements
- ❌ North American code compliance is required without re-certification — SM570 is JIS-certified; ASTM A572 Gr.65 is required for AISC-governed projects unless equivalency is documented
10. FAQ
Q: Does SM570 require special welding procedures?
Yes. SM570’s strength comes from TMCP rolling and micro-alloy precipitation hardening — thermally sensitive mechanisms. Welding heat input must typically be limited to ≤ 50 kJ/cm to prevent HAZ softening. Use low-hydrogen consumables (E8018 class for matching strength, E7018 for under-matching), control interpass temperature below 250°C (482°F), and use a WPS developed specifically for SM570. Obtain and follow the steel mill’s welding procedure documentation for the specific plate thickness.
Q: What is the closest EN grade to SM570?
EN S460N (normalized, yield ≥ 460 MPa) or S460M (thermomechanically rolled, yield ≥ 460 MPa) are the closest equivalents. EN S460N has a similar yield and tensile range, but the Charpy requirement is at −20°C (−4°F) versus SM570’s 0°C (32°F). EN S460M more closely mirrors SM570’s TMCP production method and has similar heat-input sensitivity in welding.
Q: Can SM570 replace A572 Gr.65 in a North American project?
Structurally, SM570 yield (≥ 460 MPa / 66.7 ksi for t ≤ 13 mm) meets or exceeds A572 Gr.65 (≥ 450 MPa / 65 ksi). However, North American structural codes (AISC, AASHTO) require ASTM-certified materials. SM570 carries JIS certification only. Using SM570 on an ASTM-governed project requires a formal equivalency determination — typically involving independent mechanical testing, chemistry review, and engineer-of-record approval. Plan for this documentation lead time when considering a substitution.
Q: How does SM570 perform in cold climates?
SM570 mandates Charpy impact testing at 0°C (32°F) with a minimum of 47 J (35 ft·lbf) — the same requirement as SM400C and SM490C. For applications at temperatures below 0°C, verify whether 0°C Charpy is sufficient for the design temperature, or specify an additional low-temperature Charpy test. EN S460N’s −20°C (−4°F) Charpy requirement may be more appropriate for arctic or sub-arctic service.
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