JIS Steel Grades — SS400, SM490, SN490 Japanese Standard Guide

Japanese Industrial Standards (JIS) define the structural steel grades used in Japan and across East and Southeast Asia. The JIS steel grade system is fundamentally different from both ASTM and EN designations: it uses a prefix that indicates the steel's intended application, not just its yield strength. Understanding these prefixes is essential for selecting the correct grade for welded structures, seismic building frames, or general fabrication.

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JIS Steel Grade Families at a Glance

JIS structural steel is organized into specification families, each with a distinct prefix that defines the material's purpose and quality level:

The key distinction: SS grades are for general fabrication and non-welded applications. SM grades add guaranteed weldability and notch toughness. SN grades add seismic ductility and through-thickness requirements for building moment frames.

JIS G3101 — SS Series: Rolled Steel for General Structure

SS grades are the most basic and least expensive Japanese structural steels. They are not intended for welded primary structural members.

Critical limitation: SS400 has no guaranteed weldability, no Charpy impact requirement, and no restrictions on carbon equivalent. Welding SS400 is possible with appropriate procedures, but the lack of chemical controls means heat-affected zone (HAZ) properties are unpredictable. For any welded structural connection, use SM or SN grades instead.

SS400 is the most widely used Japanese structural steel globally — it is the default "mild steel" specification across much of Asia. The Fy = 245 MPa (t ≤ 16 mm) is approximately 36 ksi, equivalent to ASTM A36.

Thickness effect: Like all hot-rolled steels, SS400 yield strength decreases with increasing thickness:

JIS G3106 — SM Series: Weldable Structural Steel

SM grades are designed for welded structures. Each grade has a subgrade suffix (A, B, or C) indicating increasingly stringent toughness requirements.

SM Subgrade Meanings (A, B, C)

The progression from A to B to C represents a systematic improvement in both chemistry control and toughness. SM490B is the workhorse welded structural steel in Japan — equivalent to EN S355J0 and ASTM A572 Grade 50. SM490C provides 47 J of Charpy energy (vs 27 J for B) and tighter carbon equivalent control, making it the preferred grade for dynamically loaded welded structures.

SM490Y: The "Y" suffix indicates a yield point that is one grade higher than the standard SM490 while maintaining the same tensile strength range (490–610 MPa). This is achieved through microalloying (niobium, vanadium), which increases yield strength without proportionally increasing tensile strength. SM490YA is commonly used for column members where the higher yield (365 MPa vs 325 MPa for SM490A) reduces section weight.

SM vs SS — A Critical Distinction

Engineers specifying steel for projects in Japan or Asia must be aware that SS400 is often the default grade offered by fabricators because it is cheapest. The specifier must explicitly call for SM400B or SM490B when welding is required. A specification that simply says "SS400" will result in steel with no guaranteed weldability being delivered to the site.

JIS G3136 — SN Series: Earthquake-Resistant Building Frame Steel

SN grades are specifically designed for seismic-resistant steel building frames and were introduced after the 1995 Kobe earthquake exposed vulnerabilities in conventionally specified SM-grade steel. SN grades impose additional requirements beyond strength and toughness:

What Makes SN Steel Different from SM Steel?

1. Upper yield point limit (Fy/Fu ratio): SN grades impose a maximum yield-to-tensile ratio of 0.80, compared to 0.85 for ASTM A992. This is stricter than any ASTM or EN specification. The purpose is to ensure that plastic hinges form and rotate before fracture — critical for seismic energy dissipation. A low Fy/Fu ratio provides plastic deformation capacity after yield. SN490B, with Fy <= 325 MPa and Fu >= 490 MPa, achieves Fy/Fu <= 0.66 — well below the 0.80 maximum — providing exceptional ductility.

2. Upper yield point cap: Unlike SM grades (which only specify minimum yield), SN grades specify both minimum and maximum yield strength. For SN490B, Fy must be between 325 and 445 MPa. This prevents "over-strength" steel (which can shift plastic hinges to unintended locations) from being supplied. If a mill delivers SM490 with an actual Fy of 420 MPa, it meets SM490 requirements — but if it were SN490, it would be rejected because the yield exceeds 445 MPa.

3. Charpy at 0 degrees C minimum: All SN grades require Charpy V-notch testing at 0 degrees C with minimum 27 J — equivalent to SM subgrade B. SN400A is the only SN grade that may waive Charpy testing for secondary members.

4. Through-thickness properties: SN grades intended for column applications (where the beam flange pulls through the column flange at a moment connection) require through-thickness tensile testing per JIS G3199. The minimum reduction of area in the Z-direction is 25%, preventing lamellar tearing at restrained welded moment connections.

SN Subgrade Meanings

SN490C is the highest-specification Japanese building frame steel: guaranteed Fy range (325–445 MPa), Charpy at 0 degrees C, carbon equivalent limit, and through-thickness ductility verified. This is the grade used for the column sections in Japan's high-rise seismic-resistant buildings.

JIS vs ASTM vs EN — Steel Grade Equivalents

Welding SS400, SM490, and SN490 — Practical Guide

Welding SS400

SS400 can be welded with standard E49XX electrodes (equivalent to AWS E70XX), but the results are unpredictable because chemical composition is not controlled. Weld procedure qualification (WPQ) is essential — do not assume that a procedure qualified on one heat of SS400 will produce acceptable results on another heat. For critical welds, specifiers should either: (1) upgrade to SM400B (adds 10–15% cost but guarantees weldability), or (2) require chemical analysis on each heat of SS400 and reject heats exceeding CE = 0.40%.

Welding SM490B

SM490B is fully weldable with standard procedures:

• Electrodes: E49XX (AWS E70XX equivalent) for general applications. For welds that must match base metal tensile strength (490 MPa minimum), use E55XX (AWS E80XX equivalent).
• Preheat: For thicknesses over 38 mm, preheat to 38–65 degrees C when ambient temperature is above 0 degrees C. For thicknesses over 38 mm below 0 degrees C ambient, preheat to 65–95 degrees C.
• Heat input: 1.5–3.5 kJ/mm for SMAW; 1.5–5.0 kJ/mm for SAW and FCAW. Higher heat inputs (>5.0 kJ/mm) risk grain coarsening in the HAZ with associated toughness reduction.

Welding SN490B/C

SN490 welding follows the same procedures as SM490 but with the additional requirement that the weld metal must meet the same Fy upper limit as the base metal. This means filler metals that produce excessively high-strength weld deposits (>490 MPa yield) are not permitted if the connection is designed to yield the base metal first. For moment frame connections under AISC 358-type prequalified procedures, use undermatching electrodes (e.g., E49XX for SN490 base metal) to ensure the plastic hinge forms in the beam, not in the weld.

JIS Chemical Composition — Typical Ranges

The carbon equivalent (CE) limits for SM and SN grades are critical for weldability. SM400B and SM490B share the same CE limit (0.38%), which is comparable to A992 (CE ~0.38–0.44). SM520C has the strictest CE limit (0.36%) among the SM/SN grades, reflecting its use in fatigue-sensitive welded bridges. SM570 and SN490B/C have higher CE limits (0.44%) due to the higher strength requirements — preheat is typically required for these grades when thickness exceeds 25 mm.

Frequently Asked Questions

What does SS400 mean?

SS = Steel Structure (general structural steel). 400 = minimum tensile strength in MPa (400–510 MPa range). The yield strength is 245 MPa (t ≤ 16 mm). SS400 is the most common structural steel in Asia — equivalent to ASTM A36 in concept — but it is not intended for welded primary members because chemical composition and carbon equivalent are not controlled. For welded structures, use SM400B or SM490B instead.

Can I weld SS400?

Technically yes, but it is not recommended for primary structural welds. The chemical composition is uncontrolled — one heat may weld beautifully (CE ~0.30%), while the next heat may crack extensively (CE ~0.52%). If SS400 must be welded, implement a receiving inspection program that measures carbon equivalent on every heat and rejects heats exceeding CE = 0.40%. Better practice: specify SM400B, which guarantees weldability for approximately 10–15% cost premium.

What is the difference between SM490 and SN490?

SM490 is for general welded structures (bridges, cranes, industrial buildings). It guarantees minimum yield (325 MPa) and tensile (490–610 MPa) but places no upper limit on yield strength. SN490 is for seismic-resistant building frames with strict upper yield limits (Fy <= 445 MPa for SN490B) and Fy/Fu ratio control (<= 0.80). SN490 also requires Charpy testing at 0 degrees C and may require through-thickness (Z-quality) testing. The cost premium for SN490B over SM490B is approximately 8–12%.

What Japanese steel is equivalent to A992?

SN490B is the closest Japanese equivalent to ASTM A992, though the match is not exact. Both specify Fy/Fu ratio control (A992: max 0.85; SN490B: max 0.80 — stricter). A992 has Fy = 50 ksi (345 MPa) minimum; SN490B has Fy = 325–445 MPa (47–65 ksi). The JIS grade is slightly lower minimum yield but imposes a ceiling on yield strength that A992 does not. For typical building applications where yield above 50 ksi is acceptable, A992 is the more practical US specification and SN490B is the equivalent Japanese specification.

Is SM520 stronger than SM490?

Yes — SM520 has higher tensile strength (520–640 MPa vs 490–610 MPa for SM490) and slightly higher yield (365 MPa vs 325 MPa for SM490). The yield strength is identical to SM490Y (365 MPa), but SM520 provides 5% higher tensile strength. For beam design where deflection governs (stiffness-critical, not strength-critical), the 365 MPa yield of SM520 provides no benefit over SM490Y — the extra cost (approximately 5–8%) is not justified. SM520 is most valuable for tension members and for welded connections where the higher tensile strength increases bolt bearing and tear-out resistance.