SM520B
High-strength weldable hot-rolled structural steel · JIS G 3106:2024 — Rolled steels for welded structure
SM520B is the 520 MPa tensile-strength-class, B-toughness grade in JIS G 3106 for welded structures. It combines minimum yield strength from 365 MPa at thin gauges to 325 MPa at 75–100 mm with 520–640 MPa tensile strength and a specified longitudinal Charpy V-notch average of at least 27 J at 0 °C for thickness over 12 mm. Product form, thickness, delivery condition and any agreed weldability controls must be stated when purchasing; the designation alone does not imply a particular rolling or heat-treatment route.
- JIS G 3106:2024Governing edition · Published February 20, 2024 and listed as the current edition by the Japanese Standards Association.
- 520–640 MPa tensile strengthStrength class · Minimum yield strength is thickness-dependent and ranges from 365 to 325 MPa through 100 mm.
- Average ≥27 J at 0 °CB-grade toughness · Longitudinal standard V-notch Charpy test for products over 12 mm thick.
- Generally ≤100 mmStandard thickness range · SM520B plate may be supplied over 100 mm to 150 mm only by purchaser–manufacturer agreement under Annex JA.
- C ≤0.20%, Si ≤0.55%, Mn ≤1.65%, P and S each ≤0.035%Heat-analysis limits through 100 mm · Other alloying elements may be added when necessary, so the full certified heat analysis matters.
- Not fixed by the grade symbolDelivery route · Normalizing, tempering, quenching and tempering, TMC processing or another agreed treatment may be used and identified by a suffix where applicable.
Overview
- Designation system
- Japanese Industrial Standard grade under JIS G 3106
- Product forms
- Hot-rolled plate and sheet, Hot-rolled strip in coil and cut lengths, Hot-rolled structural sections, Hot-rolled flat steel, Hot-extruded sections by agreement, Deformed flat steel by agreement
- Condition
- As rolled or producer-selected condition meeting the specification, Normalized by agreement (suffix N), Tempered by agreement (suffix T), Quenched and tempered (suffix Q), Thermo-mechanically controlled by agreement (suffix TMC), Other agreed heat-treated condition
- Density
- 7850 kg/m³ (Nominal engineering value for structural steel; density is not a grade requirement in JIS G 3106.)
What the designation means
JIS G 3106 covers highly weldable hot-rolled steels for welded structures such as bridges, ships, rolling stock, petroleum storage tanks and containers. Within that system, “SM” identifies steel for welded structures, “520” identifies the tensile-strength class, and “B” identifies the specified toughness level within this grade family. The number is not a minimum yield strength and should not be used as one in design calculations.
The B suffix is meaningful only in the context of JIS G 3106. For SM520B it requires 27 J at 0 °C; it should not be interpreted as a universal toughness designation across other standards.
Product form, thickness and delivery condition
| Product form | Applicable range or condition | Important qualification |
|---|---|---|
| Plate/sheet, strip in coil, sections and flat steel | Thickness ≤100 mm | This is the normal range in the main specification tables. |
| Plate only | Over 100 mm to 150 mm | Permitted by purchaser–manufacturer agreement; Annex JA chemistry and tensile requirements apply. |
| Hot-extruded sections | Thickness ≥5 mm; leg or height ≤250 mm | Applied by agreement under Annex JC; intended for special structural shapes. |
| Deformed flat steel | As agreed under Annex JD | Thickness is assessed according to the defined geometry and maximum-thickness rules. |
JIS G 3106 does not define SM520B by one mandatory metallurgical route. Steel may be supplied as rolled or may receive normalizing, tempering, quenching and tempering, thermo-mechanical control or another suitable agreed treatment. When a treatment symbol is used it is appended to the grade, for example SM520BN or SM520BTMC. Buyers should therefore treat the delivery condition as a controlled purchasing variable rather than infer it from the base designation.
Chemical composition and weldability controls
| Element | Requirement, mass % |
|---|---|
| Carbon, C | ≤0.20 |
| Silicon, Si | ≤0.55 |
| Manganese, Mn | ≤1.65 |
| Phosphorus, P | ≤0.035 |
| Sulfur, S | ≤0.035 |
These are heat-analysis limits. The standard permits other alloying elements to be added when necessary.
The basic five-element limits do not fully describe weldability because alloy additions outside the table are permitted. Review the complete certified heat analysis rather than estimating behavior from carbon content alone. Welding procedure qualification, consumable selection, hydrogen control, preheat and heat-input limits should be based on the actual composition, thickness, joint restraint, delivery condition and applicable fabrication code.
| Plate thickness | Maximum Ceq | Maximum PCM |
|---|---|---|
| ≤50 mm | 0.40% | 0.26% |
| >50 mm to 100 mm | 0.42% | 0.27% |
| >100 mm | By agreement | By agreement |
These limits apply to plate produced by thermo-mechanical control under an agreement. They are not automatic universal limits for every SM520B product.
Tensile properties
| Thickness | Minimum yield point or 0.2% proof stress | Tensile strength |
|---|---|---|
| ≤16 mm | 365 MPa | 520–640 MPa |
| >16 mm to 40 mm | 355 MPa | 520–640 MPa |
| >40 mm to 75 mm | 335 MPa | 520–640 MPa |
| >75 mm to 100 mm | 325 MPa | 520–640 MPa |
Unless otherwise specified, upper yield strength ReH is used when a yield point appears; otherwise Rp0.2 is measured.
| Thickness | JIS tensile specimen | Minimum elongation |
|---|---|---|
| ≤5 mm | No. 5 | 19% |
| >5 mm to 16 mm | No. 1A | 15% |
| >16 mm to 40 mm | No. 1A | 19% |
| >40 mm to 50 mm | No. 1A or No. 4 | 19% or 21%, respectively |
| >50 mm to 100 mm | No. 4 | 21% |
Elongation values cannot be compared without retaining the specified specimen geometry.
| Thickness | Minimum yield | Tensile strength | Elongation |
|---|---|---|---|
| >100 mm to 150 mm | 315 MPa | 520–640 MPa | No. 4 specimen: nominally 21%, subject to the specified thickness deduction |
For each additional 25 mm of thickness, or fraction thereof, over 100 mm, one percentage point is deducted from the tabulated elongation, to a maximum deduction of three points.
Small sections and narrow flats may fall under the separate Annex JB tensile provisions because their available specimen geometry differs from that used for larger products.
Impact toughness requirement
| Applicability | Test temperature | Minimum absorbed energy | Specimen and orientation |
|---|---|---|---|
| Product thickness >12 mm | 0 °C | Average of three specimens ≥27 J | JIS standard V-notch specimen, longitudinal to rolling direction |
The notch-length direction is perpendicular to the rolling surface. Acceptance of the three-specimen set is assessed under JIS G 0404.
The standard permits testing at a lower temperature by agreement, but this must be explicitly ordered. The normal 27 J at 0 °C requirement is not evidence of suitability for every low-temperature, high-restraint, fatigue-critical or fracture-critical application. Such service may require a lower test temperature, greater absorbed energy, transverse testing, additional fracture-mechanics criteria or project-specific toughness qualification.
Specifying, inspecting and fabricating SM520B
| Item | Why it matters |
|---|---|
| Standard and edition | State JIS G 3106:2024 where the current requirements are intended. |
| Grade and condition | Specify SM520B plus any required N, T, Q, TMC or other agreed condition. |
| Product form and dimensions | Properties, thickness limits and dimensional standards depend on whether the product is plate, coil, section or flat. |
| Impact testing | State any lower temperature, transverse orientation or enhanced energy requirement beyond the normal B-grade test. |
| Weldability data | For TMC plate, state whether Ceq or PCM limits are required; request full heat analysis needed for calculation. |
| Nondestructive testing | Ultrasonic or other NDT is not an automatic SM520B requirement and needs an agreed method, sampling plan and acceptance level. |
| Inspection document | JIS G 3106 calls for an inspection document; unless otherwise ordered, the referenced default is a JIS G 0415 inspection certificate 3.1. |
| Dimensional tolerances | Sections, plate/strip and flats are controlled through JIS G 3192, G 3193 and G 3194, respectively. |
For welding, do not assume that all heats or delivery routes will behave identically. Confirm the treatment suffix and certificate chemistry, then qualify the welding procedure for the governing construction code. Low-hydrogen practice becomes increasingly important with thicker material, high restraint and higher calculated Ceq or PCM. Any post-weld heat treatment, hot forming or substantial thermal straightening should be reviewed against the original delivery condition because it can alter strength and toughness developed by controlled rolling or heat treatment.
A mill certificate showing the correct grade symbol is necessary but not sufficient for critical work. Verify product form, thickness, heat-treatment suffix, heat number traceability, full chemistry where relevant, tensile results, Charpy temperature and orientation, and any separately ordered NDT.
Substitution and comparison
SM520B should not be replaced solely by matching nominal yield or tensile strength. A substitution review must compare the governing product specification, thickness-dependent strength, toughness temperature and orientation, chemistry and weldability controls, delivery condition, dimensions and tolerances, inspection certification and project approval requirements. Grades listed as counterparts in commercial tables are normally approximate comparisons rather than interchangeable specifications.
Sources
- JIS G 3106:2024 — Rolled steels for welded structureJapanese Standards Association
- JIS G 3106:2024 English previewJapanese Standards Association
- JIS G 3106 revision technical draftJapan Iron and Steel Federation
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