SM400B
Weldable hot-rolled carbon-manganese structural steel · JIS G 3106:2024 — Rolled steels for welded structure
SM400B is the impact-tested quality of the JIS G 3106 400 MPa tensile-strength class for welded structures. It combines a specified carbon-manganese chemistry, 400–510 MPa tensile strength, thickness-dependent minimum yield strength, and—above 12 mm thickness—a minimum average Charpy V-notch energy of 27 J at 0 °C. Product form, thickness, test-piece type and delivery condition must remain attached to any property statement or purchase specification.
- 400–510 MPa tensile strengthStrength class · The “400” designation refers to the tensile-strength class, not a 400 MPa yield strength.
- 245 MPa at t ≤ 16 mmMinimum yield strength · Falls to 235 MPa above 16 through 40 mm, 215 MPa above 40 through 100 mm, 205 MPa above 100 through 160 mm, and 195 MPa above 160 through 200 mm.
- Average ≥27 J at 0 °CImpact quality · Longitudinal Charpy V-notch requirement for thickness above 12 mm.
- C ≤0.20% at t ≤50 mm; C ≤0.22% above 50 mmMain chemistry · Si ≤0.35%, Mn 0.60–1.50%, P and S each ≤0.035% for the normal thickness range.
- Plate, strip and sections ≤200 mm; flat steel ≤100 mmNormal thickness scope · SM400B plate above 200 through 250 mm is possible only by purchaser–manufacturer agreement under Annex JA.
- No numerical Ceq or PCM maximum is specified for ordinary SM400BImportant limitation · Welding procedure and preheat decisions should therefore use the actual heat analysis, section thickness, restraint and hydrogen-control conditions.
Overview
- Designation system
- Japanese Industrial Standards (JIS) grade designation
- Product forms
- Steel plate and sheet, Steel strip and cut lengths from strip, Hot-rolled structural sections, Flat steel, Hot-extruded special sections by agreement, Deformed flat steel by agreement
- Condition
- Delivery without a heat-treatment suffix, Normalized (N) by agreement, Tempered (T) by agreement, Quenched and tempered (Q) where applied, Thermomechanically controlled (TMC) by agreement, Other agreed heat-treated condition
- Density
- 7850 kg/m³ (Nominal carbon-steel density and JIS mass-calculation basis; not a grade-specific acceptance property.)
What the designation means
SM400B belongs to JIS G 3106, which covers highly weldable hot-rolled steels for welded structures such as bridges, ships, rolling stock, petroleum storage tanks and containers. These examples describe the standard’s scope; they do not by themselves establish suitability for a particular design code, pressure service, fatigue category or low-temperature duty.
Within the designation, SM identifies the JIS family for welded structures and 400 identifies the 400–510 MPa tensile-strength class. The B quality level adds specified chemistry and Charpy toughness. It should not be treated as merely another name for a general-purpose 400 MPa-class structural steel.
Applicable products and thicknesses
| Product form | Applicable thickness or dimension | Qualification |
|---|---|---|
| Plate/sheet, strip and hot-rolled sections | ≤200 mm | Main specification tables apply |
| Flat steel | ≤100 mm | Main specification tables apply |
| Plate above 200 through 250 mm | >200 to 250 mm | Only by purchaser–manufacturer agreement; Annex JA chemistry and tensile provisions apply |
| Hot-extruded special sections | Thickness ≥5 mm; side or height ≤250 mm | Annex JC, by agreement |
| Deformed flat steel | Product-specific dimensions | Annex JD, by agreement |
For sections, the applicable thickness is the specified local thickness at the relevant part of the section, as defined through JIS G 3192. Product-form rules must be checked rather than applying nominal plate thickness indiscriminately.
The standard permits special product routes and agreed extended thicknesses, but these are not automatically covered by the ordinary SM400B property table. The purchase order and inspection certificate should identify the applicable annex or agreement.
Chemical composition
| Thickness | C | Si | Mn | P | S |
|---|---|---|---|---|---|
| ≤50 mm | ≤0.20% | ≤0.35% | 0.60–1.50% | ≤0.035% | ≤0.035% |
| >50 to 200 mm | ≤0.22% | ≤0.35% | 0.60–1.50% | ≤0.035% | ≤0.035% |
These are heat-analysis limits. Elements not listed may be added when necessary; their presence can materially affect welding behavior even when the five tabulated elements comply.
| Thickness | C | Si | Mn | P | S |
|---|---|---|---|---|---|
| >200 to 250 mm | ≤0.22% | ≤0.35% | ≥0.60%; upper limit by agreement | ≤0.035% | ≤0.035% |
This Annex JA provision applies only when the extended plate thickness has been agreed between purchaser and manufacturer.
The composition is more tightly defined for welding than that of a general structural grade such as SS400. Nevertheless, compliance with the basic C-Si-Mn-P-S table is not a complete welding assessment. Mill additions of Cu, Ni, Cr, Mo, V, B or other elements may affect hardenability and should be considered from the actual inspection certificate.
Tensile properties
| Applicable thickness | Minimum yield or proof stress |
|---|---|
| ≤16 mm | 245 MPa |
| >16 to 40 mm | 235 MPa |
| >40 to 100 mm | 215 MPa |
| >100 to 160 mm | 205 MPa |
| >160 to 200 mm | 195 MPa |
| >200 to 250 mm agreed plate | 195 MPa |
Unless otherwise specified, the upper yield point is used. Where a yield point does not appear, 0.2% proof stress is measured.
| Tensile strength (MPa) | Minimum | Maximum |
|---|---|---|
| Specified tensile-strength range | 400 | 510 |
Applies across the listed normal and agreed extra-thick SM400B ranges.
| Thickness | JIS tensile specimen | Minimum elongation |
|---|---|---|
| ≤5 mm | No. 5 | 23% |
| >5 to 16 mm | No. 1A | 18% |
| >16 to 40 mm | No. 1A | 22% |
| >40 to 50 mm | No. 1A | 22% |
| >40 to 50 mm | No. 4 | 24% |
| >50 to 200 mm | No. 4 | 24% |
For thickness above 100 mm, the No. 4 elongation requirement is reduced by one percentage point for each additional 25 mm or fraction thereof, to a maximum reduction of three points. Small sections and narrow flats can be governed by Annex JB instead.
Yield and elongation values cannot be quoted responsibly without thickness and specimen information. A certificate showing 215 MPa minimum yield for a thick plate and one showing 245 MPa for thin plate may both comply fully with SM400B.
Impact toughness and the significance of B quality
| Applicability | Test temperature | Minimum absorbed energy | Specimen orientation |
|---|---|---|---|
| SM400B products with thickness >12 mm | 0 °C | Average of three specimens ≥27 J | Longitudinal to rolling direction; V-notch normal to the rolled surface |
A lower test temperature may replace 0 °C by agreement. Transverse testing may also be agreed, but orientation must be stated because longitudinal and transverse results are not interchangeable.
The impact requirement is the principal practical distinction between SM400B and SM400A. It is a modest baseline toughness guarantee rather than a universal low-temperature service rating. Fracture-critical work, thick welded joints, high restraint, fatigue-sensitive details or service below the specified test temperature may require a lower test temperature, higher absorbed energy, transverse testing, fracture-mechanics data or another grade.
For thickness of 12 mm or less, the standard does not impose the SM400B Charpy requirement. If toughness is needed in thin material, it must be addressed explicitly in the purchase specification.
Welding significance
SM400B is intended for welded construction, and its carbon-manganese chemistry is more controlled than that of general structural steel. This does not eliminate hydrogen cracking, heat-affected-zone hardening, lamellar tearing or toughness concerns. Welding consumable selection, heat input, preheat/interpass temperature, hydrogen control and joint preparation remain matters for the applicable welding code and qualified procedure.
For fabrication planning, request the complete heat analysis rather than calculating weldability from the five mandatory composition entries alone. Preheat cannot be selected from the grade name alone.
Delivery condition and processing
JIS G 3106 permits normalizing, tempering, quenching and tempering, thermomechanical control and other agreed treatments where needed. Treatment symbols are appended to the grade when applicable—for example, an N or TMC suffix. The same base designation can therefore cover products made through different rolling and heat-treatment routes, provided the applicable requirements are met.
Hot forming, stress relieving or other fabrication heat treatment can alter strength and toughness, particularly for thermomechanically controlled material. If significant post-delivery heating is planned, its temperature-time cycle and the mill’s delivery route should be reviewed before material approval. The designation alone does not guarantee retention of properties after arbitrary heat treatment.
JIS G 3106 does not make SM400B corrosion-resistant. Atmospheric or immersion corrosion protection must be supplied by coating, corrosion allowance, environmental control or selection of a corrosion-resistant grade, as appropriate to the design.
Ordering and certification
| Item | Why it matters |
|---|---|
| Standard edition and grade | Specify JIS G 3106:2024, SM400B rather than only “400-grade structural steel”. |
| Product form and dimensions | Controls the applicable dimensional standard, thickness range and tensile requirements. |
| Delivery or heat-treatment condition | Identify any required N, T, Q, TMC or other agreed condition. |
| Impact test details | For enhanced requirements, state temperature, energy, orientation and applicable thickness rather than relying only on the B suffix. |
| Inspection document | JIS G 3106 provides a JIS G 0415 inspection certificate 3.1 unless otherwise specified. |
| Full heat analysis and weldability value | Request all intentional alloy additions and Ceq or PCM when needed for welding engineering. |
| Dimensional tolerances | JIS G 3192 applies to sections, JIS G 3193 to plate/sheet/strip and JIS G 3194 to flats; tighter tolerances require explicit agreement. |
| Supplementary examination | Ultrasonic testing, through-thickness properties, surface class or special NDT acceptance criteria are not created merely by specifying SM400B. |
The certificate should be checked against the ordered product rather than against a generic SM400B summary. Confirm heat number, dimensions, product form, delivery condition, heat analysis, thickness-dependent yield requirement, tensile strength, elongation specimen and result, and—where thickness exceeds 12 mm—the Charpy temperature, orientation and three-specimen result.
Substitution and design cautions
SM400B should not be substituted solely by matching nominal yield or tensile strength. A valid comparison must include product standard and edition, product form, thickness, heat treatment, chemistry and weldability, Charpy temperature and orientation, tensile specimen, dimensional tolerances, inspection documentation and any project-specific testing.
ASTM A36 and European S275 grades are sometimes associated with SM400B in commercial comparison tables because their strength levels overlap. They are not identical designations: chemistry, impact-test basis, thickness schedules, specimen rules and certification requirements differ. Any proposed replacement requires engineering and contractual review.
Sources
- JIS G 3106:2024 — Rolled steels for welded structureJapanese Standards Association
- JIS G 3106:2024 English previewJapanese Standards Association
- JIS G 3106 revision draft and technical tablesThe Japan Iron and Steel Federation
- Manufacturing specifications — JIS G 3106 rolled steels for welded structuresNippon Steel Structural Shapes Corporation
- Basic knowledge of iron and steelThe Japan Iron and Steel Federation
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