SM400A
Hot-rolled carbon-manganese structural steel for welded structures · JIS G 3106:2024 — Rolled steels for welded structure
SM400A is the basic 400 MPa tensile-strength grade in JIS G 3106 for welded structures. It combines thickness-dependent minimum yield strength with controlled carbon, phosphorus and sulfur, but the A subgrade has no mandatory Charpy impact-energy requirement. It is therefore appropriate only where the design and purchasing specification do not require guaranteed notch toughness, through-thickness properties or building-specific controls beyond JIS G 3106.
- 400–510 MPaTensile-strength range · Standard requirement for the normal thickness range and agreed extra-thick plate range.
- 245 MPa at t ≤ 16 mmMinimum yield strength · Falls progressively with thickness, reaching 195 MPa above 160 mm.
- Up to 200 mmStandard thickness range · SM400A plate/sheet may be supplied above 200 mm through 450 mm by purchaser-manufacturer agreement under the normative annex.
- Not specified for SM400AImpact toughness · The A suffix must not be interpreted as a guaranteed Charpy toughness class.
- 0.23% max at t ≤ 50 mmCarbon limit · The heat-analysis limit increases to 0.25% above 50 mm.
- Grade alone is insufficientCritical purchasing distinction · Product form, thickness, delivery condition, dimensional standard, inspection document and any toughness or weldability requirements must also be stated.
Overview
- Designation system
- Japanese Industrial Standards (JIS) grade designation
- Product forms
- Plate and sheet, Strip in coil and cut lengths, Hot-rolled sections, Flats, Hot-extruded sections by agreement, Deformed flats under the applicable annex
- Condition
- Hot-rolled delivery without a heat-treatment suffix, Normalized (N) by agreement, Tempered (T) by agreement, Quenched and tempered (Q), where applicable, Thermo-mechanically controlled (TMC) by agreement, Other agreed heat-treatment condition
- Density
- 7850 kg/m³ (Representative density used for carbon-steel mass calculations; not a grade-specific requirement of JIS G 3106.)
What the designation means
SM400A is one member of the SM series specified by JIS G 3106 for highly weldable hot-rolled steels used in welded structures such as bridges, ships, rolling stock, petroleum storage tanks and containers. “SM” identifies steel for welded structures, “400” identifies the 400 MPa minimum tensile-strength class, and “A” identifies the least toughness-controlled subgrade within the SM400 group.
The number 400 is not the minimum yield strength. SM400A has a specified tensile range of 400–510 MPa, while minimum yield strength depends on product thickness.
Product scope, thickness and delivery condition
| Product or condition | Applicability | Important qualification |
|---|---|---|
| Plate/sheet, strip in coil, sections and flats | Normal SM400A scope | Applicable thickness is normally up to 200 mm. |
| Extra-thick plate/sheet | Over 200 mm through 450 mm | Requires agreement between purchaser and manufacturer; Annex JA chemistry and tensile requirements apply. |
| Hot-extruded sections | Permitted by agreement | Subject to the separate quality provisions for hot-extruded sections. |
| Deformed flats | Covered by JIS G 3106:2024 | Subject to the specific requirements of the applicable normative annex. |
| Heat-treated or TMC product | Permitted where specified or agreed | The relevant suffix, such as N or TMC, is added to the grade designation. Processing route must not be assumed from “SM400A” alone. |
Mechanical requirements are tied to the thickness at the test-piece location. For sections, this may not be the nominal overall section dimension. Small sections and narrow flats are subject to special tensile provisions, so the main plate-style property table should not automatically be applied to every section size.
Specified chemical composition
| Thickness | C, max. | Si | Mn | P, max. | S, max. |
|---|---|---|---|---|---|
| t ≤ 50 mm | 0.23% | No numerical limit | At least 2.5 × C | 0.035% | 0.035% |
| 50 < t ≤ 200 mm | 0.25% | No numerical limit | At least 2.5 × C | 0.035% | 0.035% |
| 200 < t ≤ 450 mm plate/sheet by agreement | 0.25% | No numerical limit | At least 2.5 × C | 0.035% | 0.035% |
Composition values are heat-analysis requirements. Elements without listed limits, and elements not shown in the table, may be added as necessary. Consequently, the grade symbol does not define one fixed chemical recipe.
SM400A has less restrictive carbon and manganese controls than SM400B or SM400C and does not carry a general grade-wide maximum carbon-equivalent or weld-crack-sensitivity value. For welding procedure development—particularly with thick material, high restraint, high heat input or hydrogen-sensitive consumables—the actual certified heat chemistry should be obtained and used in the applicable weldability calculation.
Tensile properties
| Nominal thickness at test location | Minimum yield point or proof stress |
|---|---|
| t ≤ 16 mm | 245 MPa |
| 16 < t ≤ 40 mm | 235 MPa |
| 40 < t ≤ 75 mm | 215 MPa |
| 75 < t ≤ 100 mm | 215 MPa |
| 100 < t ≤ 160 mm | 205 MPa |
| 160 < t ≤ 200 mm | 195 MPa |
| 200 < t ≤ 450 mm plate/sheet by agreement | 195 MPa |
| Thickness range (MPa) | t ≤ 200 mm | 200 < t ≤ 450 mm plate/sheet by agreement |
|---|---|---|
| Specified tensile strength | 400–510 | 400–510 |
| Thickness | JIS tensile test piece | Minimum elongation |
|---|---|---|
| t ≤ 5 mm | No. 5 | 23% |
| 5 < t ≤ 16 mm | No. 1A | 18% |
| 16 < t ≤ 50 mm | No. 1A | 22% |
| t > 40 mm | No. 4 | 24% |
For No. 4 specimens from products over 100 mm thick, the tabulated elongation is reduced by one percentage point for each 25 mm increment, or fraction thereof, above 100 mm, with a maximum reduction of three points. Extra-thick plate above 200 mm uses a No. 4 specimen and has a minimum elongation of 21%. Specimen selection and the product-specific provisions must be retained when comparing test certificates.
The principal limitation: no specified notch toughness
JIS G 3106 does not specify Charpy absorbed energy for SM400A. Passing the chemistry and tensile requirements therefore provides no standard assurance of notch toughness at 0 °C or any other temperature. This matters for dynamically loaded structures, thick welded joints, high restraint, fatigue-sensitive details, low-temperature service and locations where brittle-fracture consequences are significant.
| Grade | Tensile-strength class | Standard Charpy requirement for products over 12 mm | Practical interpretation |
|---|---|---|---|
| SM400A | 400–510 MPa | None | Use only where specified toughness is not required. |
| SM400B | 400–510 MPa | Mean 27 J minimum at 0 °C, longitudinal V-notch | Basic toughness-controlled option. |
| SM400C | 400–510 MPa | Mean 47 J minimum at 0 °C, longitudinal V-notch | Higher specified toughness and tighter carbon control. |
SM400B or SM400C should not be selected solely by informal grade hierarchy. Required test temperature, specimen orientation, minimum energy, thickness, sampling and project fracture-control rules must be checked explicitly.
Welding and fabrication implications
SM400A is intended for welded construction and generally offers better composition control for welding than a general structural grade such as SS400. That intent does not eliminate the need for a qualified welding procedure. Preheat and heat-input decisions depend on the actual composition, thickness, joint restraint, hydrogen level, process, consumable strength, ambient temperature and applicable fabrication code.
Cold forming and thermal cutting should be evaluated using the actual product thickness and delivery condition. Heavy cold deformation can reduce local ductility and toughness, while flame-cut edges and welded attachments may become critical crack-initiation locations. If the design relies on forming capability, impact toughness after forming, through-thickness ductility, maximum hardness or post-weld heat treatment response, those requirements must be added separately; they are not inherent guarantees of SM400A.
A mill certificate showing low actual carbon equivalent is useful production information, but it does not convert SM400A into SM400B or SM400C because the specified impact testing and acceptance requirements remain different.
Ordering and inspection checklist
| Item | Why it matters |
|---|---|
| Full designation and edition | Specify SM400A to JIS G 3106:2024 unless a different contractual edition applies. |
| Product form and dimensions | The standard covers several forms, and mechanical testing, tolerances and available manufacturing routes differ. |
| Delivery condition | State hot-rolled, N, TMC or other required condition rather than assuming the production route. |
| Thickness range | Yield strength changes with thickness; plate above 200 mm requires explicit agreement. |
| Dimensional and mass tolerances | Confirm the applicable companion standard: JIS G 3192 for sections, JIS G 3193 for plate/sheet/strip, or JIS G 3194 for flats, as relevant. |
| Inspection document | Require traceable heat analysis and tensile results in the certificate format needed by the project. |
| Impact testing, if required | SM400A contains no standard Charpy guarantee; temperature, orientation, energy and sampling must be stated separately or a suitable subgrade selected. |
| Weldability data | Request reporting of all alloying elements needed to calculate the carbon equivalent or weld-crack-sensitivity parameter used by the fabrication specification. |
| Ultrasonic or through-thickness requirements | These are not automatic SM400A requirements and must be ordered separately where lamellar tearing or internal soundness is critical. |
| Surface condition and repair restrictions | State project limits for defects, grinding and weld repair where the general JIS appearance provisions are insufficient. |
Acceptance should compare the certificate with the ordered product form, thickness and condition—not merely confirm that “SM400A” appears on the document. Heat identity, dimensions, mechanical-test applicability, analysis basis and any supplementary tests should remain traceable to the delivered material.
Use and substitution boundaries
SM400A is suitable as a general weldable structural steel where its thickness-dependent strength and absence of mandatory notch-toughness testing are acceptable. It is not, by designation alone, a pressure-vessel steel, low-temperature steel, weathering steel, seismic building steel or guaranteed through-thickness steel.
Cross-standard tables sometimes associate SM400A with ASTM A36, EN structural grades or other national 400 MPa-class steels. Such associations are approximate only. Chemistry, yield-strength basis, thickness effects, impact requirements, product forms, testing frequency, tolerances and certification differ. Substitution requires comparison of the complete governing specifications and project requirements.
Sources
- JIS G 3106:2024 — Rolled steels for welded structureJapanese Standards Association
- JIS G 3106:2024 English previewJapanese Standards Association
- Hot Rolled Steel Sheet — excerpts from Japanese Industrial StandardsJFE Steel Corporation
- Steel Plates Product CatalogueNippon Steel Corporation
- Ordinary Steel Products Order Size Table — JIS G 3106The Japan Iron and Steel Federation
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