SM490B
Weldable 490 MPa-class carbon-manganese structural steel · JIS G 3106:2024 — Rolled steels for welded structure
SM490B is a non-proprietary JIS structural steel intended specifically for welded construction. It combines a specified tensile strength of 490–610 MPa with thickness-dependent yield strength and mandatory longitudinal Charpy V-notch toughness of at least 27 J at 0 °C for products over 12 mm thick. The B suffix identifies the toughness class, not a different tensile-strength level. Product form, thickness, delivery condition and test-piece rules must accompany the grade designation when it is specified or compared with another steel.
- JIS G 3106:2024Governing edition · Published February 20, 2024 and currently listed as active by the Japanese Standards Association.
- 490–610 MPaTensile strength · Specified across the normal thickness range.
- 325 to 275 MPaMinimum yield strength · Decreases in steps as product thickness increases from 16 mm or less to over 160–200 mm.
- 27 J minimum at 0 °CB-class toughness · Mean of three longitudinal Charpy V-notch specimens; required for product thickness over 12 mm.
- Up to 200 mm; flats up to 100 mmNormal thickness scope · SM490B plate may be supplied through 250 mm by purchaser-manufacturer agreement under the standard's special provisions.
- Substitution requires specification-level reviewNot automatically equivalent to S355 or Grade 50 · Yield strength, toughness, chemistry, product form, testing and delivery condition differ among standards.
Overview
- Designation system
- Japanese Industrial Standards (JIS)
- Product forms
- Steel plate and sheet, Strip in coil, Hot-rolled structural sections, Flat steel, Specially ordered hot-extruded sections
- Condition
- Condition without a heat-treatment suffix, Normalized by agreement: N, Tempered by agreement: T, Quenched and tempered: Q, Thermomechanically controlled by agreement: TMC
- Density
- 7850 kg/m³ (Nominal engineering value for carbon steel; density is not a defining JIS G 3106 acceptance requirement.)
What the designation means
SM490B belongs to the SM series of JIS G 3106 steels for welded structures such as bridges, ships, rolling stock, tanks, containers and other fabricated structures. “490” identifies the minimum tensile-strength class: the specified tensile range is 490–610 MPa. It does not mean 490 MPa minimum yield strength.
The suffix B is principally a toughness designation. SM490A, SM490B and SM490C share the same normal tensile and yield requirements, but A has no routine Charpy requirement, B requires 27 J at 0 °C, and C requires 47 J at 0 °C. Selecting among them is therefore primarily a fracture-toughness decision rather than a static-strength decision.
Product scope and thickness limits
| Product form | Applicable thickness or dimension | Important qualification |
|---|---|---|
| Plate, sheet, strip in coil and hot-rolled sections | Up to and including 200 mm | For sections, the controlling thickness is defined at the specified test-piece location. |
| Flat steel | Up to and including 100 mm | Narrow flats may be subject to the special tensile provisions for small products. |
| Steel plate supplied by special agreement | Over 200 up to and including 250 mm | Annex requirements apply; the normal chemistry and tensile tables must not simply be extrapolated. |
| Hot-extruded special sections | Thickness at least 5 mm; leg length or height up to 250 mm | Covered by a dedicated normative annex and supplied by purchaser-manufacturer agreement. |
The standard also contains special rules for sections with leg length below 70 mm and flats with width below 50 mm.
A certificate stating only “SM490B” is incomplete for engineering review. Product form, dimensions, heat-treatment or rolling condition, test results and applicable standard edition remain essential.
Chemical composition requirements
| Product thickness | C max. | Si max. | Mn max. | P max. | S max. |
|---|---|---|---|---|---|
| 50 mm or less | 0.18% | 0.55% | 1.65% | 0.035% | 0.035% |
| Over 50 through 200 mm | 0.20% | 0.55% | 1.65% | 0.035% | 0.035% |
These are maximum heat-analysis values. Elements not listed, or listed without limits, may be added as necessary; added alloying elements are to be reported.
The composition is consistent with a weldable carbon-manganese structural steel, but the listed maxima do not define a unique metallurgical recipe. Producers may use microalloying, controlled rolling or heat treatment to achieve the required strength and toughness. Consequently, two compliant heats can have materially different carbon equivalent, hardenability and welding response.
The higher carbon limit above 50 mm recognizes the greater difficulty of obtaining strength through heavy section thickness. It should not be interpreted as assurance that all heavy SM490B has the same weldability as thinner product.
Mechanical properties and test conditions
| Thickness t | Minimum yield point or proof stress | Tensile strength |
|---|---|---|
| t ≤ 16 mm | 325 MPa | 490–610 MPa |
| 16 < t ≤ 40 mm | 315 MPa | 490–610 MPa |
| 40 < t ≤ 100 mm | 295 MPa | 490–610 MPa |
| 100 < t ≤ 160 mm | 285 MPa | 490–610 MPa |
| 160 < t ≤ 200 mm | 275 MPa | 490–610 MPa |
These values apply to the main product table. Small sections, narrow flats and plate over 200 mm are governed by special annex provisions.
| Thickness | Test piece | Minimum elongation |
|---|---|---|
| t ≤ 5 mm | JIS No. 5 | 22% |
| 5 < t ≤ 16 mm | JIS No. 1A | 17% |
| 16 < t ≤ 50 mm | JIS No. 1A | 21% |
| 40 < t ≤ 200 mm | JIS No. 4 | 23% |
The overlapping thickness ranges reflect test-piece applicability and product geometry; elongation values are not interchangeable without preserving the specified specimen. For No. 4 specimens over 100 mm thick, the tabulated value may be reduced by one percentage point for each 25 mm thickness increment or fraction, to a maximum reduction of three points.
The thickness effect is important in design and procurement. A 12 mm plate and a 120 mm plate may both be certified as SM490B and have the same tensile-strength range, yet their specified minimum yield strengths differ by 40 MPa. Design values must therefore be selected using the actual product thickness and the governing design code, not the grade name alone.
Impact toughness: the defining B-grade requirement
| Applicable thickness | Test temperature | Required absorbed energy | Specimen orientation |
|---|---|---|---|
| Over 12 mm | 0 °C | Mean of three specimens ≥ 27 J | V-notch specimen in the rolling direction |
Testing is performed in accordance with JIS Z 2242 using the applicable JIS G 3106 sampling provisions. Acceptance and retesting are governed through JIS G 0404.
The 27 J at 0 °C requirement is a specification classification point, not a universal lower service-temperature rating. Suitability for cold service also depends on section thickness, stress state, weld details, residual stress, loading rate, flaw population and applicable fracture-control rules. A lower test temperature or transverse testing may be agreed when the project requires greater assurance.
SM490B at 12 mm or less is not routinely Charpy-tested under the grade requirement. If toughness evidence is required at those thicknesses, it must be addressed explicitly in the purchase specification.
Delivery condition and weldability control
The unsuffixed designation does not establish one unique rolling or heat-treatment route. JIS G 3106 permits normalizing, tempering, quenching and tempering, thermomechanical control, or another agreed treatment as applicable. Agreed treatments are identified by suffixes such as N, T, Q or TMC. The condition must therefore be retained when reproducing a certificate or evaluating a replacement material.
| Plate thickness | Carbon equivalent Ceq, max. | Weld-crack sensitivity composition PCM, max. |
|---|---|---|
| 50 mm or less | 0.38% | 0.24% |
| Over 50 through 100 mm | 0.40% | 0.26% |
| Over 100 mm | By purchaser-manufacturer agreement | By purchaser-manufacturer agreement |
For TMC plate, PCM may be applied as the alternative to carbon equivalent by agreement. These TMC limits should not be presented as universal limits for every unsuffixed SM490B product.
Actual preheat, heat input, interpass temperature, consumable classification and hydrogen control cannot be selected from the grade name alone. They depend on the certified chemistry, combined thickness, restraint, process, consumable hydrogen level, ambient conditions and applicable welding code. Heavy unsuffixed plate should be reviewed using the mill analysis rather than assumed to have the lower TMC carbon-equivalent limits.
Fabrication and service implications
SM490B is intended for welding and normally offers a more controlled basis for welded fabrication than a general structural grade specified only by strength. This does not remove the need for qualified welding procedures, low-hydrogen practice where required, suitable edge preparation and control of fit-up and restraint. Arc strikes, excessive heat input, rapid cooling and repairs can still impair the heat-affected zone or introduce cracking.
Cold forming capability depends on thickness, bend radius, edge quality, rolling direction and actual strength. The tensile elongation requirement is not a forming guarantee. Tight-radius forming, forming transverse to elongated inclusions, or forming heavy plate should be qualified using producer recommendations and fabrication trials where consequences are significant.
SM490B is not a weathering or corrosion-resistant designation. Coating, galvanizing, corrosion allowance or another protection system must be selected for the exposure. Likewise, fatigue resistance of a welded structure is normally governed by weld geometry, defects, residual stress and detail category rather than by the 490 MPa base-metal tensile class.
Through-thickness ductility, lamellar-tearing resistance and ultrasonic quality are not inherent in the plain SM490B designation. Specify any Z-direction property or ultrasonic acceptance requirement separately.
Specifying and checking an order
| Item | Why it matters |
|---|---|
| Standard and edition | Requirements and annex provisions can change; state JIS G 3106:2024 unless another edition is contractually intended. |
| Full grade and condition | Record SM490B together with N, T, Q, TMC or another agreed suffix when applicable. |
| Product form and dimensions | Thickness determines yield strength, chemistry limits, impact-test applicability and available manufacturing routes. |
| Required toughness | The base grade gives 27 J at 0 °C only for thickness over 12 mm; colder, transverse or higher-energy testing must be specified. |
| Inspection document | Require traceable heat identity, chemistry, tensile results, Charpy results where applicable and the actual delivery condition. |
| Ceq or PCM reporting | Particularly important for welding-procedure and preheat assessment; confirm the formula and whether a contractual maximum applies. |
| Dimensional standard and tolerances | JIS G 3192, G 3193 or G 3194 applies according to product form; project tolerances may be more restrictive. |
| Additional quality requirements | Ultrasonic examination, through-thickness ductility, surface class, weld repair restrictions and special sampling are not created by the grade name alone. |
The manufacturer's inspection document should be checked against the ordered product rather than against a generic web table. Confirm the heat-treatment suffix, product thickness, yield requirement, tensile range, elongation specimen, impact-test temperature and orientation. For heavy plate supplied under the extended thickness agreement, verify the applicable annex values explicitly.
Grade selection and substitution boundaries
SM490B is appropriate when the project requires the SM490 static-strength level together with basic Charpy assurance at 0 °C. SM490A may be inadequate where impact toughness is required; SM490C offers higher specified absorbed energy but is not simply a universally superior substitute because form, thickness availability and project approvals still apply. SM490YB provides higher minimum yield strength and must not be confused with SM490B despite having the same tensile range and B-class impact requirement.
Cross-standard comparisons with EN S355J0, ASTM A572 Grade 50 or other nominal 355 MPa/Grade 50 steels are approximate. SM490B has lower specified yield strength than S355J0 or A572 Grade 50 in several thickness ranges, while its 0 °C Charpy requirement may be more explicit than that of a basic ASTM order. Chemistry, dimensions, sampling, certification and design-code acceptance must be reconciled before substitution.
Sources
- JIS G 3106:2024 — Rolled steels for welded structureJapanese Standards Association
- JIS G 3106:2024 English previewJapanese Standards Association
- Chapter 2, Metallic Materials — provisions based on JIS G 3106:2024Tokyo Metropolitan Government, Bureau of Construction
- Manufacturing Specifications — JIS G 3106 productsNippon Steel Structural Shapes Corporation
- Steel Construction Today & Tomorrow No. 49The Japan Iron and Steel Federation
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