SN400B
Hot-rolled carbon-manganese structural steel for building construction · JIS G 3136:2022 — Rolled steels for building structure
SN400B is the 400 MPa tensile-strength-class B grade in JIS G 3136, intended for welded, general building structural members where controlled strength, ductility and base-metal toughness are important. Unlike ordinary general structural steel, it controls the upper as well as lower yield strength at most thicknesses, limits yield ratio, specifies weldability through carbon equivalent or an agreed Pcm limit, and requires 0 °C Charpy testing above 12 mm thickness. It does not provide the guaranteed through-thickness ductility of SN400C.
- 6–100 mmApplicable thickness · Applies to plate, strip, sections and flat steel; the relevant thickness definition for sections depends on section geometry.
- 400–510 MPaTensile strength · Specified throughout the applicable thickness range.
- 235–355 MPa up to 40 mm; 215–335 MPa above 40 mmControlled yielding · Upper yield limits and an 80% maximum yield ratio apply from 12 mm, subject to specific H- and CT-section exceptions.
- ≥27 J at 0 °CBase-metal toughness · Average Charpy V-notch absorbed energy for thickness over 12 mm, normally using longitudinal specimens.
- Ceq ≤0.36%Weldability control · For non-TMC SN400B; Pcm ≤0.26% may be used instead by purchaser–supplier agreement.
- SN400B-UTOptional internal-soundness grade · For plate and flat steel 13–100 mm, ultrasonic examination to JIS G 0901 Class Y may be agreed.
Overview
- Designation system
- JIS grade designation
- Product forms
- Steel plate, Steel strip, Hot-rolled structural sections, Flat steel
- Condition
- Hot-rolled/as supplied, Normalized, suffix N, when applicable, Tempered, suffix T, when applicable, Thermo-mechanically controlled, suffix TMC, when applicable, Other agreed heat-treatment condition
- Density
- 7.85 g/cm³ (Conventional nominal density for carbon steel; not a defining requirement of JIS G 3136.)
What the designation defines
JIS G 3136 covers hot-rolled steels specifically for structural members of buildings. SN400B is one of five grades in the standard. “400” identifies the tensile-strength class: the specified tensile range begins at 400 MPa. The suffix B is a performance and intended-use category, not a higher strength level than A or a lower strength level than C.
The B category is the usual SN400 grade for welded primary structural members, including members expected to develop inelastic behavior. It combines weldability and toughness requirements with limits on yield-strength variability. The resulting controls support building designs in which the location and sequence of yielding matter.
SN400B is not a plate-only designation. It may cover plate, strip, structural sections and flat steel, but availability, test-piece geometry and the applicable definition of thickness differ by product form.
Chemical composition and weldability controls
| Nominal thickness | C, max. | Si, max. | Mn | P, max. | S, max. |
|---|---|---|---|---|---|
| 6–50 mm | 0.20% | 0.35% | 0.60–1.50% | 0.030% | 0.015% |
| >50–100 mm | 0.22% | 0.35% | 0.60–1.50% | 0.030% | 0.015% |
These are heat-analysis requirements. Alloying elements not listed may be added when necessary; added elements are to be reported as required by the standard.
| Control route | Maximum | Application |
|---|---|---|
| Carbon equivalent, Ceq | 0.36% | Standard weldability index for SN400B up to 100 mm |
| Weld-cracking susceptibility composition, Pcm | 0.26% | May replace Ceq by agreement between purchaser and supplier |
The applicable route should be clear in the order and inspection documentation; Ceq and Pcm are not interchangeable numerical scales.
The specified Ceq or Pcm ceiling is a material acceptance requirement, not a complete welding procedure. Preheat, heat input, consumable selection and hydrogen control still depend on the actual mill-sheet chemistry, product thickness, joint restraint, welding process and project requirements.
Strength and ductility requirements
| Thickness | Yield point or proof stress | Tensile strength | Maximum yield ratio |
|---|---|---|---|
| 6 to <12 mm | ≥235 MPa | 400–510 MPa | Not specified |
| 12–16 mm | 235–355 MPa | 400–510 MPa | 80% |
| >16–40 mm | 235–355 MPa | 400–510 MPa | 80% |
| >40–100 mm | 215–335 MPa | 400–510 MPa | 80% |
For H-sections and CT-sections with web thickness t1 ≤9 mm, the yield-strength upper limit does not apply and the maximum yield ratio is 85%.
| Thickness | JIS tensile specimen | Minimum elongation |
|---|---|---|
| 6–16 mm | No. 1A | 18% |
| >16–40 mm | No. 1A | 22% |
| >40–50 mm | No. 1A | 22% |
| >40–50 mm | No. 4 | 24% |
| >50–100 mm | No. 4 | 24% |
JIS G 3136 Annex JA gives different specimen and elongation requirements for sections with a side below 40 mm and flats below 40 mm wide; it may also be applied to certain slightly larger products.
The upper yield-strength limit is as important as the minimum. A material substantially stronger in yield than assumed can shift yielding into connections, welds or other unintended locations. The yield-ratio limit preserves a defined margin between first yielding and ultimate tensile strength, supporting stable plastic deformation rather than treating excess strength as automatically beneficial.
Impact toughness and through-thickness performance
| Applicable thickness | Test temperature | Minimum absorbed energy | Specimen and orientation |
|---|---|---|---|
| >12 mm | 0 °C | 27 J average of three specimens | Standard V-notch; longitudinal to rolling direction |
A lower test temperature or transverse orientation may be agreed. The acceptance of the three-specimen set follows JIS G 0404.
The Charpy requirement applies to the parent steel and should not be interpreted as qualification of weld metal or heat-affected-zone toughness. Welding procedure qualification and any project-specific fracture-toughness requirements remain separate matters.
SN400B has no mandatory through-thickness tensile reduction-of-area requirement. Where substantial through-thickness tensile stress or lamellar-tearing risk exists—such as heavily restrained diaphragm or box-column details—SN400C or another specifically qualified Z-quality product may be required.
Ultrasonic examination and the -UT suffix
For SN400B plate and flat steel from 13 through 100 mm, the purchaser and supplier may agree to ultrasonic examination under JIS G 0901. Material accepted to Class Y is designated SN400B-UT, with any heat-treatment suffix positioned before “-UT,” for example SN400BN-UT.
SN400B-UT is still SN400B. Ultrasonic acceptance improves assurance against specified internal discontinuities, but it does not add the guaranteed through-thickness ductility of SN400C and is not an automatic substitute for that grade.
Product condition, dimensions and testing
The standard permits normalizing or tempering when needed and allows thermo-mechanical control or other agreed heat treatment. Applicable suffixes include N, T and TMC. The condition shown on the material marking and inspection certificate must match the ordered condition; heat-treated material is tested by heat-treatment lot as prescribed by the standard.
General dimensional rules are referenced to JIS G 3192 for sections, JIS G 3193 for plate and strip, and JIS G 3194 for flat steel. JIS G 3136 then imposes its own thickness tolerances. Consequently, a purchase order should identify both the product form and nominal dimensions rather than relying on the grade designation alone.
For section products, thickness classification is not simply the largest dimension. JIS G 3136 identifies the controlling thickness by section type—for example, flange thickness t2 for H- and CT-sections and web thickness t1 for I-sections and channels. This affects the applicable strength range, test requirements and interpretation of certificates.
Fabrication implications
SN400B is intended for welded construction, but weldability remains procedure-dependent. The actual heat analysis should be used when establishing welding controls. Thick, highly restrained joints, low ambient temperature, high hydrogen input or large carbon-equivalent values can require measures beyond those used for thinner or lightly restrained joints.
Thermal cutting, cold forming, flame straightening and post-weld heating can alter local properties. JIS grade compliance does not by itself approve a particular fabrication cycle. Project specifications, qualified procedures and producer guidance should be checked where severe forming, high heat input or significant thermal treatment is planned.
Corrosion resistance is that of ordinary unalloyed structural steel. SN400B is not weathering steel and normally requires a suitable coating, fire-protection system or other environmental protection appropriate to the building and exposure.
Ordering and certificate review
| Item | What should be stated or checked |
|---|---|
| Standard and grade | JIS G 3136:2022, SN400B, including required adoption or project edition |
| Product form | Plate, strip, section type or flat steel |
| Dimensions | Nominal thickness and other dimensions; identify the controlling section thickness where relevant |
| Delivery condition | Base grade or required suffix such as N, T or TMC |
| Weldability route | Ceq requirement or agreed Pcm alternative |
| Impact testing | Standard 0 °C longitudinal test or any agreed lower temperature/transverse test |
| Internal soundness | Whether SN400B-UT is required for eligible plate or flat steel |
| Inspection document | JIS G 0415 inspection certificate 3.1 is the standard default unless otherwise specified |
| Project additions | Any supplementary toughness, weldability, surface, dimensional, traceability or fabrication requirements |
Certificate review should verify grade and suffixes, heat or inspection number, dimensions, heat analysis, Ceq or Pcm and all elements used in its calculation, tensile results for the correct thickness category, yield ratio where applicable, elongation specimen, Charpy results above 12 mm and UT results when “-UT” was ordered.
Substitution boundaries
Matching a nominal 400 MPa tensile class is not enough to establish interchangeability. A substitute must be checked for product form, thickness range, yield minimum and maximum, tensile range, yield ratio, elongation, Charpy test temperature and orientation, chemistry, weldability index, dimensional tolerances, internal-soundness requirements, delivery condition and certification.
SS400, SM400B, ASTM A36 and superficially similar EN grades are not automatically equivalent to SN400B. Some may provide comparable minimum strength or toughness in selected dimensions, but they do not necessarily reproduce the SN400B combination of upper yield control, yield-ratio limit, JIS weldability controls and building-specific dimensional requirements.
Sources
- JIS G 3136:2022 — Rolled steels for building structure: standard recordJapanese Standards Association
- JIS G 3136:2022 — English previewJapanese Standards Association
- Draft revision text for JIS G 3136Japan Iron and Steel Federation
- Characteristics of structural steels for building constructionJFE Steel Corporation
- Manufacturing specifications for JIS G 3136 structural sectionsNippon Steel Structural Shapes Corporation
- Structural ShapesNippon Steel Corporation
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