SM490 steel plate is a low-alloy high-strength structural steel defined by the Japanese Industrial Standard JIS G3106. The designation is simple to decode: SM identifies the weldable structural steel series and 490 indicates the minimum tensile strength class of 490 N/mm². Plates are delivered hot rolled or normalised, and the lean carbon-manganese composition keeps strength high while preserving the toughness, fatigue resistance and weldability that welded structures demand.
The grade is widely used in bridge construction, building frames, ship structures, rail vehicles, petroleum storage tanks, containers and other heavily welded fabrications. The sections below cover the six commonly ordered grades, the composition and mechanical property tables, delivery conditions such as Z-direction testing, international equivalents and typical applications.
Grade Structure of the JIS G3106 SM490 Family
JIS G3106 defines two strength levels inside the SM490 group. The standard grades SM490A, SM490B and SM490C share the same 490-610 N/mm² tensile range, while SM490YA and SM490YB raise the minimum yield point one step higher. The suffix letters indicate progressively stricter impact and carbon-control requirements rather than a change of tensile class:
SM490A - base grade with a minimum average Charpy absorbed energy of 27 J at 0 °C.
SM490B - tighter carbon control (C ≤ 0.18 %) with the same 27 J impact requirement.
SM490C - minimum average absorbed energy raised to 47 J at 0 °C together with a carbon equivalent limit of 0.44 %.
SM490YA and SM490YB - higher minimum yield point (365 N/mm² for t ≤ 16 mm) while retaining the 490-610 N/mm² tensile band.
Chemical Composition of SM490 Steel Plate
| Grade | Thickness (mm) | C max (%) | Si max (%) | Mn max (%) | P max (%) | S max (%) |
|---|---|---|---|---|---|---|
| SM490A | t ≤ 50 | 0.20 | 0.55 | 1.65 | 0.035 | 0.035 |
| SM490B | t ≤ 50 | 0.18 | 0.55 | 1.65 | 0.035 | 0.035 |
| SM490C | t ≤ 50 | 0.18 | 0.55 | 1.65 | 0.035 | 0.035 |
| SM490YA | t ≤ 50 | 0.20 | 0.55 | 1.65 | 0.035 | 0.035 |
| SM490YB | t ≤ 50 | 0.18 | 0.55 | 1.65 | 0.035 | 0.035 |
All figures are ladle or product-analysis maximums for the standard thickness range. Because manganese, phosphorus and sulphur are held low and the carbon level never exceeds 0.20 %, the steel remains readily weldable without the preheating penalties associated with quenched and tempered grades.
Mechanical Properties: Yield Point, Tensile Strength and Impact Energy
| Grade | Thickness (mm) | Yield point min (N/mm²) | Tensile strength (N/mm²) | Charpy absorbed energy (avg.) |
|---|---|---|---|---|
| SM490A / SM490B / SM490C | t ≤ 16 | 325 | 490 - 610 | 27 J at 0 °C (47 J for C) |
| SM490A / SM490B / SM490C | 16 < t ≤ 40 | 315 | 490 - 610 | 27 J at 0 °C (47 J for C) |
| SM490A / SM490B / SM490C | 40 < t ≤ 75 | 295 | 490 - 610 | 27 J at 0 °C (47 J for C) |
| SM490A / SM490B / SM490C | 75 < t ≤ 90 | 295 | 490 - 610 | 27 J at 0 °C (47 J for C) |
| SM490YA / SM490YB | t ≤ 16 | 365 | 490 - 610 | 27 J at 0 °C |
| SM490YA / SM490YB | 16 < t ≤ 40 | 355 | 490 - 610 | 27 J at 0 °C |
| SM490YA / SM490YB | 40 < t ≤ 75 | 335 | 490 - 610 | 27 J at 0 °C |
| SM490YA / SM490YB | 75 < t ≤ 90 | 325 | 490 - 610 | 27 J at 0 °C |
Minimum elongation ranges from roughly 17 % to 23 % depending on plate thickness and on whether a JIS No.1A or No.4 test piece is used, so the governing value should always be read together with the thickness and test-piece definition in the standard. Yield point falls gently as thickness increases, which is why thicker sections are designed against the lower tabulated values.
Delivery Condition, Thickness and Z-Direction Testing
SM490 plate is normally supplied in the as-rolled or normalised condition, and other delivery states can be agreed when a project requires a specific heat treatment or surface condition. Where welded joints are heavily restrained, plates can be ordered with a thickness-direction tensile test at Z15, Z25 or Z35 quality levels to demonstrate adequate through-thickness ductility and resistance to lamellar tearing.
Impact testing is performed on JIS test pieces at the specified temperature, and Charpy requirements can be verified against the certificate for each heat. For structures with fatigue-sensitive details, the combination of low carbon, fine grain practice and controlled sulphur keeps inclusion content low and improves performance in the weld heat-affected zone.
Equivalent Grades and Selection Guidance
EN 10025-2 S355JR, S355J0, S355J2 - the closest European grades in the 355 N/mm² yield class; S355J2 adds impact testing at -20 °C.
ASTM A572/A572M Grade 50 - the comparable North American low-alloy structural plate with a 345 N/mm² minimum yield point.
GB/T 1591 Q355B and Q355C - the Chinese low-alloy high-strength structural grades with matching strength levels.
Equivalence is always approximate: impact temperature, carbon equivalent limits, available thickness range and dimensional tolerances differ between standards. The reliable approach is to state the original JIS G3106 grade and thickness on the enquiry, then verify the mill test certificate against every ordered requirement.
Typical Applications
Bridge girders, box sections and orthotropic deck plates.
Building frames, columns and seismic-resistant welded nodes.
Ship hull structures, decks and superstructures.
Rail vehicle bodies, chassis and bogie components.
Storage tanks, pressure-resistant containment and containers.
Heavy machinery frames and industrial steelwork requiring weldability.
Frequently Asked Questions
Q: What does the 490 in SM490 steel plate mean?
The number indicates the minimum tensile strength class of 490 N/mm². Actual plates are delivered within the 490-610 N/mm² tensile band, with yield point between 295 and 365 N/mm² depending on grade and thickness.
Q: Which is stronger, SM490A or SM490YA?
Both share the 490-610 N/mm² tensile range, but SM490YA carries a higher minimum yield point - 365 N/mm² at t ≤ 16 mm against 325 N/mm² for SM490A. SM490YA is preferred where stiffness and load capacity drive the design.
Q: What is SM490 equivalent to in other standards?
The nearest equivalents are EN 10025-2 S355 quality grades, ASTM A572 Grade 50 and GB/T 1591 Q355B or Q355C. Impact temperature and carbon equivalent limits still differ, so the original grade should be specified when possible.
Q: Is SM490 steel plate weldable without preheating?
Thanks to the 0.20 % maximum carbon and 0.44 % carbon equivalent limit of the higher grades, SM490 welds readily with standard low-hydrogen processes. Thick sections and highly restrained joints still call for controlled heat input and, where the procedure requires it, light preheating.
Q: Why order SM490 plate with a Z-direction test?
Z15, Z25 and Z35 tests verify through-thickness ductility, which controls the risk of lamellar tearing in tee joints, corner joints and other connections that transmit load perpendicular to the plate surface.
Q: What delivery condition is standard for SM490 plate?
Hot rolled or normalised delivery is the norm, with plate thickness typically ranging from 6 mm to 90 mm. Other conditions and thicker sections can be produced against agreed technical terms.
