JIS G3136 SN steels were developed specifically for seismic-resistant building construction in Japan. Their defining feature is not just a yield floor — it is a controlled yield ceiling and Fy/Fu ratio limit that ensures predictable energy absorption during an earthquake. ASTM A992 shares this seismic philosophy for wide-flange shapes. This guide compares SN400 and SN490 sub-grades against A992, explains why the upper yield limit exists, and identifies when SN grades are mandatory.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SN Grades
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G3136 | SN400A / SN400B / SN400C | Japan | Reference |
| JIS G3136 | SN490B / SN490C | Japan | Reference |
| ASTM A992/A992M | A992 | USA | ✅ Nearest Equivalent (seismic intent; W-shapes only) |
| EN 10025-2 | S355J2 / 1.0570 | Europe | ⚠️ Nearest Equivalent (no Fy/Fu control) |
Important scope difference: ASTM A992 is limited to W-shapes (wide-flange beams and columns). JIS G3136 SN grades apply to plates, sections, and bars. For plate and built-up section applications requiring seismic controls, SN grades have no direct ASTM equivalent — SN490B is the JIS specification of record.
2. Chemical Composition
SN490B — Most Common High-Strength SN Grade
| Element | SN490B | ASTM A992 | EN S355J2 |
|---|---|---|---|
| C | ≤ 0.18% | ≤ 0.23% | ≤ 0.22% |
| Si | ≤ 0.55% | ≤ 0.40% | ≤ 0.55% |
| Mn | ≤ 1.65% | ≤ 1.65% | ≤ 1.60% |
| P | ≤ 0.030% | ≤ 0.035% | ≤ 0.030% |
| S | ≤ 0.015% | ≤ 0.045% | ≤ 0.025% |
Sources: JIS G3136:2020, ASTM A992/A992M, EN 10025-2
SN490B sulfur limit (≤ 0.015%) is 3× tighter than A992 (≤ 0.045%). This low sulfur requirement suppresses MnS inclusion stringers, directly improving through-thickness ductility and lamellar tearing resistance — critical properties for column-beam moment connections subject to seismic loading.
3. Mechanical Properties
SN Sub-Grade Comparison
| Grade | Fy min (MPa) | Fy max (MPa) | Fu (MPa) | Fy/Fu max | Charpy | Z-direction |
|---|---|---|---|---|---|---|
| SN400A | ≥ 235 (34 ksi) | Not controlled | 400–510 (58–74 ksi) | Not controlled | None | None |
| SN400B | ≥ 235 (34 ksi) | ≤ 355 (51.5 ksi) | 400–510 (58–74 ksi) | ≤ 0.80 | ≥ 27 J at 0°C | None |
| SN400C | ≥ 235 (34 ksi) | ≤ 355 (51.5 ksi) | 400–510 (58–74 ksi) | ≤ 0.80 | ≥ 47 J at 0°C | Z25 |
| SN490B | ≥ 325 (47 ksi) | ≤ 445 (64.5 ksi) | 490–610 (71–88 ksi) | ≤ 0.80 | ≥ 47 J at 0°C | None |
| SN490C | ≥ 325 (47 ksi) | ≤ 445 (64.5 ksi) | 490–610 (71–88 ksi) | ≤ 0.80 | ≥ 47 J at 0°C | Z25 |
ASTM A992 Requirements
| Property | A992 Requirement |
|---|---|
| Fy minimum | 345 MPa (50 ksi) |
| Fy maximum | 450 MPa (65 ksi) |
| Fu minimum | 450 MPa (65 ksi) |
| Fu maximum | 620 MPa (90 ksi) |
| Fy/Fu ratio | ≤ 0.85 |
| Product form | W-shapes (wide flanges) only |
| Charpy impact | Not required |
Note: A992 Fy/Fu ≤ 0.85 is slightly less restrictive than SN490B’s ≤ 0.80. The SN limit is more conservative, ensuring greater strain-hardening reserve during seismic energy absorption.
4. Physical Properties
| Property | Value |
|---|---|
| Density | 7.85 g/cm³ (0.284 lb/in³) |
| Young’s Modulus | 200 GPa (29,000 ksi) |
| Thermal Conductivity | ~50 W/(m·K) (347 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 |
|---|---|---|---|
| As-rolled | Rolling finish ~900°C (1,652°F) | Air cool | Standard supply condition |
| Normalizing (optional) | 880–920°C (1,616–1,688°F) | Air cool | Grain refinement for improved toughness uniformity |
| Stress Relief (post-weld) | 550–620°C (1,022–1,148°F) | Furnace cool | Residual stress reduction in restrained connections |
SN steels are not heat treated for strength in service. For PWHT, keep temperature below 620°C (1,148°F) to preserve mechanical properties, particularly the controlled Fy/Fu ratio.
6. Machinability
- Machinability rating: approximately 65–70% relative to AISI 1212 baseline
- Standard HSS or carbide tooling for all operations
- Low sulfur content (≤ 0.015% for SN490B) reduces chip-breaking effectiveness compared to free-machining grades — expect longer, more continuous chips
- Drilling and milling: standard structural steel cutting parameters; reduce feed slightly vs SS400 for SN490B at low-S
- Flame cutting and plasma cutting are primary stock-removal methods for plate product
- Preheat to 50°C (122°F) before flame cutting for t > 25 mm (1 in)
7. Weldability
- Weldability: Excellent — low carbon (≤ 0.18% for SN490B) and low Ceq (< 0.45 typical) enable welding without preheat for most section sizes
- Preheat: Not required for t ≤ 25 mm (1 in) in ambient conditions; 50°C (122°F) recommended for t > 25 mm or restrained joints
- For seismic moment connections: low-hydrogen electrodes mandatory (E7018 class minimum; E8018 for SN490 matching strength)
- Low sulfur (≤ 0.015%) significantly reduces lamellar tearing risk in T-butt and cruciform joints
- SN_C grades (SN400C, SN490C) with Z25 requirement: Z-direction tensile test must be verified on plate prior to use in through-thickness stressed joints
- Welding Procedure Specifications should be qualified to JIS Z3040 or the applicable building standard
- Post-weld inspection: ultrasonic testing (UT) of moment connection welds is standard practice in seismic design
8. Common Mistakes
SN400A is the minimal sub-grade — it has no upper yield limit, no Fy/Fu ratio control, and no Charpy impact requirement. In practice, SN400A behaves like SM400A: a weldable structural steel with controlled carbon but none of the seismic-specific mechanical guarantees. For any connection in a seismically designed moment-resisting frame, SN400B is the minimum acceptable specification. Using SN400A in a seismic application technically complies with the grade name “SN” but violates the intent of JIS G3136 and Japan’s Building Standard Law seismic provisions (Notification No. 1025, 2000 revision).
A992 does not exist as a JIS specification. Japanese mills produce wide-flange sections to JIS G3192 dimensional standards and certify mechanical properties to JIS G3106 (SM grades) or JIS G3136 (SN grades). When a Japanese-sourced wide-flange section is needed for a seismically designed building, the correct specification is SN490B (for beam-column members) with cross-reference to A992 requirements noted for the design engineer’s review. Ordering “A992 equivalent” from a Japanese mill without specifying the JIS grade may result in SM490 material — which lacks the mandatory Fy/Fu ratio control of SN grades.
9. When to Choose SN Grades
✅ Choose SN grades when:
- ✅ Japan’s Building Standard Law seismic design provisions (Notification No. 1025, 2000 revision) apply — SN grades are explicitly referenced
- ✅ Column and beam connections in moment-resisting frames where yield overstrength must be controlled
- ✅ Any application requiring controlled Fy/Fu ratio ≤ 0.80 for predictable energy absorption during seismic events
- ✅ Connections where through-thickness Z-direction ductility is required — specify SN_C sub-grades (SN400C, SN490C)
- ✅ Cold-climate seismic applications where Charpy at 0°C is mandatory (SN400B/C, SN490B/C)
❌ Do not choose SN grades when:
- ❌ General non-seismic structural use — SS400 or SM400 is more cost-effective; SN grade premium is unnecessary
- ❌ Bridge applications — specify JIS G3106 SM grades (SM400, SM490, SM570) which are the correct bridge plate standards
- ❌ North American projects without equivalency documentation — specify ASTM A992 for W-shapes in AISC-governed seismic design
10. FAQ
Q: What is the key difference between SN400B and SM400B?
Both are JIS weldable structural steels with similar base strength (Fy ≥ 235 MPa, Fu 400–510 MPa) and carbon control. SN400B adds three requirements absent from SM400B: (1) an upper yield limit (Fy ≤ 355 MPa), (2) Fy/Fu ratio control (≤ 0.80), and (3) mandatory Charpy impact at 0°C (≥ 27 J). These features are specifically designed for energy-absorbing seismic connections where yield overstrength — actual yield significantly exceeding design yield — can overload non-yielding elements and connections. SM400B guarantees the steel is weldable; SN400B guarantees it will yield predictably in an earthquake.
Q: Is ASTM A992 the same as SN490B?
Similar seismic intent, different scope and requirements. Both control Fy/Fu ratio for seismic ductility — A992 at ≤ 0.85, SN490B at the stricter ≤ 0.80. A992 is limited to W-shapes (wide flanges), while SN490B applies to plates, shapes, and bars. SN490B has a significantly lower sulfur limit (≤ 0.015% vs A992’s ≤ 0.045%), providing better through-thickness ductility for column connection plates. SN490B also mandates Charpy impact (≥ 47 J at 0°C), while A992 has no Charpy requirement. For Japanese building construction, SN490B is the governing specification.
Q: Why does SN steel have an upper yield limit?
In seismic design, structural members are intended to yield and absorb earthquake energy in designated locations (plastic hinges). If the actual yield strength is significantly higher than the design yield — a condition called yield overstrength — the forces transferred to adjacent non-yielding elements (connections, column panels, foundations) exceed the design assumptions. This can cause brittle fracture at connections that were never intended to yield. The upper Fy limit and Fy/Fu ratio control in SN grades (B and C sub-grades) ensure the material yields at a predictable force level within the design intent. This is why SN400A (no upper yield control) is not acceptable for seismic moment connections, even though it carries the “SN” designation.
Q: When should I specify SN490C instead of SN490B?
SN490C adds Z25 through-thickness ductility testing to all SN490B requirements. Specify SN490C when the plate will be used in a joint where significant tensile stress acts in the through-thickness (Z) direction — most commonly in column flange-to-beam connection plates, column splice plates in thick sections, and diaphragm plates welded to box column walls. The Z25 requirement ensures the plate has ≥ 25% through-thickness reduction of area, resisting lamellar tearing under restraint.
Q: Is SN490B adequate for temperatures below 0°C?
SN490B mandates Charpy impact ≥ 47 J at 0°C (32°F). For applications in environments regularly below 0°C — northern Japan, cold storage facilities, or outdoor structures in cold climates — verify whether the 0°C test temperature provides adequate safety margin for the minimum service temperature. If service temperature reaches −10°C (14°F) or below, consult with the steel mill about lower-temperature Charpy test data, or consider specifying an EN S355J2 equivalent with −20°C (−4°F) Charpy certification.
comment