SN490B
Weldable 490 MPa-class hot-rolled structural steel for building construction · JIS G 3136:2022 — Rolled steels for building structure
SN490B is the B-quality, 490 MPa tensile-strength class in JIS G 3136, developed for welded building frames where controlled strength, weldability, toughness and predictable inelastic behavior matter. Unlike a general-purpose structural grade, it normally limits both maximum yield strength and yield ratio, helping preserve the intended hierarchy of yielding in seismic design. Requirements depend materially on thickness, product geometry and delivery condition; SN490B does not provide the mandatory through-thickness ductility and ultrasonic soundness associated with SN490C.
- 6–100 mmApplicable thickness · Applies to plate, strip, sections and flats; the relevant thickness definition varies with section geometry.
- 490–610 MPaTensile strength · Specified throughout the principal 6–100 mm thickness range.
- 325–445 MPa up to 40 mm; 295–415 MPa over 40–100 mmYield strength · The upper limit and yield-ratio control generally begin at 12 mm; special exceptions apply to thin-web H and CT sections.
- ≤80%Yield ratio · Normally applicable from 12 mm upward; certain H and CT sections with web thickness not exceeding 9 mm are permitted ≤85%.
- Average ≥27 J at 0 °CImpact toughness · Longitudinal V-notch Charpy requirement for thickness over 12 mm.
- No specified through-thickness tensile ductilityGrade limitation · For joints susceptible to lamellar tearing or demanding certified Z-direction performance, evaluate SN490C or another explicitly qualified product.
Overview
- Designation system
- JIS grade designation under JIS G 3136
- Product forms
- Steel plate, Steel strip, Structural sections, including H and CT sections, Flat steel
- Condition
- As rolled or as manufactured, Normalized by agreement; suffix N, Tempered by agreement; suffix T, Thermomechanically controlled; suffix TMC, Other agreed heat-treated condition, Ultrasonically tested plate or flat by agreement; suffix -UT
- Density
- 7850 kg/m³ (Representative density for carbon structural steel; JIS G 3136 does not specify density as a grade requirement.)
What the designation means
JIS G 3136 covers hot-rolled steels used as structural members in buildings. In SN490B, “490” identifies the tensile-strength class rather than a minimum yield strength, while “B” identifies the quality category. SN490B combines strength control, impact toughness and weldability provisions suited to welded building frames. The designation alone does not define a universal design stress; applicable design values must come from the governing building code and account for thickness and product form.
A central feature of SN B- and C-quality steels is control of excessive strength. For most thicknesses, JIS G 3136 specifies not only a minimum but also a maximum yield point or proof stress, together with a maximum yield ratio. These controls reduce the risk that unexpectedly high member strength will shift yielding into connections, columns or other locations that were not intended to form plastic hinges.
Product scope and delivery condition
| Item | Requirement or significance |
|---|---|
| Product forms | Steel plate, steel strip, structural sections and flat steel. |
| Applicable thickness | 6 mm to 100 mm inclusive. |
| Thickness for H and CT sections | The specified flange thickness t2 is used for principal grade-property classification; additional web-thickness exceptions affect yield limits. |
| Other sections | The applicable t, t1 or t2 dimension depends on the section type and the dimensional definitions in JIS G 3192. |
| Small sections and narrow flats | Separate mechanical-property provisions in Annex JA can apply to sections with a side below 40 mm and flats below 40 mm wide; they may also be invoked for certain slightly larger products. |
| Dimensional standards | JIS G 3192 for sections, JIS G 3193 for plate and strip, and JIS G 3194 for flats, subject to the specific modifications in JIS G 3136. |
| Condition | Designation treatment | Practical point |
|---|---|---|
| Normalized | N | Applied by purchaser–manufacturer agreement. |
| Tempered | T | Applied by agreement; confirm the complete manufacturing route rather than interpreting the suffix in isolation. |
| Thermomechanically controlled | TMC | Permitted for all grades by agreement; separate, lower Ceq and PCM limits are available for TMC plate. |
| Other appropriate heat treatment | Agreed symbol | Condition and symbol must be established between purchaser and manufacturer. |
| Ultrasonically tested plate or flat | -UT | For SN490B plate or flat 13–100 mm thick, ultrasonic requirements may be applied by agreement. The suffix follows any heat-treatment symbol, for example SN490BN-UT. |
Chemical composition and weldability controls
| Thickness | C max. | Si max. | Mn max. | P max. | S max. |
|---|---|---|---|---|---|
| 6–50 mm | 0.18% | 0.55% | 1.65% | 0.030% | 0.015% |
| >50–100 mm | 0.20% | 0.55% | 1.65% | 0.030% | 0.015% |
Other alloying elements may be added as necessary. Compliance therefore requires the weldability index and reported heat analysis to be checked, not just the five elements tabulated here.
| Manufacturing condition | Thickness | Ceq maximum |
|---|---|---|
| Products other than TMC plate | ≤40 mm | 0.44% |
| Products other than TMC plate | >40–100 mm | 0.46% |
| TMC plate, when agreed | ≤50 mm | 0.38% |
| TMC plate, when agreed | >50–100 mm | 0.40% |
The reduced TMC limits apply specifically to thermomechanically controlled plate and require purchaser–manufacturer agreement.
| Manufacturing condition | Thickness | PCM maximum |
|---|---|---|
| Products other than TMC plate | 6–100 mm | 0.29% |
| TMC plate | ≤50 mm | 0.24% |
| TMC plate | >50–100 mm | 0.26% |
Do not apply the lower TMC-plate limits to conventional sections, flats or non-TMC plate.
Ceq and PCM are specification controls, not complete welding procedures. Preheat, consumable classification, diffusible hydrogen, heat input, interpass temperature and inspection must be established for the actual thickness, restraint, joint geometry and service requirements.
Strength and ductility requirements
| Applicable thickness | Yield point or proof stress | Tensile strength | Yield ratio |
|---|---|---|---|
| 6 to <12 mm | ≥325 MPa | 490–610 MPa | Not specified |
| 12–16 mm | 325–445 MPa | 490–610 MPa | ≤80% |
| >16–40 mm | 325–445 MPa | 490–610 MPa | ≤80% |
| >40–100 mm | 295–415 MPa | 490–610 MPa | ≤80% |
For H and CT sections whose defined web thickness t1 is 9 mm or less, the yield-strength upper limit is not applied and the maximum yield ratio is 85%.
| Thickness | JIS tensile specimen | Minimum elongation |
|---|---|---|
| 6–16 mm | No. 1A | 17% |
| >16–40 mm | No. 1A | 21% |
| >40–50 mm | No. 1A | 21% |
| >40–50 mm | No. 4 | 23% |
| >50–100 mm | No. 4 | 23% |
The specimen designation and thickness range are integral to the requirement; elongation values from different specimen geometries should not be compared directly.
The reduction in specified yield strength above 40 mm is important in design and purchasing. A blanket description of SN490B as “325 MPa yield steel” is incorrect for products over 40 mm thick. Conversely, the tensile range remains 490–610 MPa, so the yield-ratio requirement continues to limit the relationship between actual yield and tensile strengths.
Toughness, through-thickness behavior and internal soundness
| Applicability | Temperature | Requirement | Specimen and orientation |
|---|---|---|---|
| SN490B over 12 mm thick | 0 °C | Average of three specimens ≥27 J | V-notch, longitudinal to rolling direction |
A lower test temperature may replace 0 °C by agreement. Transverse testing may also be agreed; with purchaser approval, it can replace the longitudinal test.
The standard Charpy requirement is a grade qualification at one temperature and orientation. It is not by itself a fracture-mechanics assessment or proof of suitability for every low-temperature detail, high strain-rate event, thick welded joint or fatigue-sensitive service. Projects needing performance below 0 °C or in the transverse direction should state the required test temperature, orientation and acceptance criteria explicitly.
| Characteristic | SN490B status | Consequence |
|---|---|---|
| Through-thickness tensile reduction of area | Not specified | The B designation does not certify Z-direction ductility. |
| Ultrasonic testing of plate and flat 13–100 mm | Optional by agreement | When ordered as -UT, acceptance is to JIS G 0901 grade Y under JIS G 3136. |
| Automatic ultrasonic requirement | No | Do not infer internal-soundness certification from the plain SN490B designation. |
| Lamellar-tearing-sensitive details | Requires engineering review | Where significant through-thickness weld shrinkage stress exists, SN490C or another product with specified through-thickness properties may be more appropriate. |
Fabrication implications
SN490B is intended for welded construction and has tighter weldability controls than many general structural steels, but sound fabrication still depends on procedure qualification and control of hydrogen and restraint. Mill certificates should be used to calculate or verify the applicable Ceq or PCM. Thick, highly restrained joints may require preheat even when the grade complies comfortably with its composition limit.
Flame cutting and welding can locally harden the heat-affected zone or degrade toughness if heat input and cooling are uncontrolled. Repair welding, tack welding, arc strikes and attachment details should be managed under the applicable fabrication standard. Matching only the nominal tensile class of the consumable is insufficient; deposited-metal strength, toughness, hydrogen level and positional requirements must also be considered.
Cold forming capacity cannot be inferred solely from the tabulated tensile elongation. Bend radius, edge condition, rolling direction, thickness and strain ageing can govern performance. Hot forming or post-fabrication heat treatment can alter a normalized or TMC microstructure and should not be performed without confirming that final properties and certification remain acceptable.
Specifying and purchasing SN490B
| Item | Why it matters |
|---|---|
| Full standard and edition | Specify JIS G 3136:2022 rather than SN490B alone. |
| Product form and dimensions | Property ranges, tolerances and test sampling depend on plate, strip, flat or section geometry. |
| Delivery condition | State as required: conventional, N, T, TMC or another agreed treatment. |
| Weldability basis | Identify whether Ceq or PCM is to govern; TMC plate limits require the appropriate agreement. |
| Impact testing additions | State any lower temperature or transverse-orientation requirement. |
| Ultrasonic examination | For plate or flat 13–100 mm, order SN490B-UT when JIS G 0901 grade Y acceptance is required. |
| Inspection document | Unless otherwise specified, JIS G 3136 calls for a JIS G 0415 inspection certificate 3.1. |
| Project-specific testing | Add through-thickness, weldability, fracture-toughness or other tests only when justified and clearly defined. |
| Traceability | Require grade, heat, dimensions, condition and any -UT suffix to remain traceable through cutting and fabrication. |
The inspection certificate should be checked for actual heat analysis, applicable Ceq or PCM constituents, yield strength, tensile strength, elongation, yield ratio where required, Charpy results where applicable, heat-treatment condition and any agreed ultrasonic result. Acceptance should use the thickness and product-form rules applicable to the delivered item, not a generic SN490B data table.
JIS G 3136 applies a plate and strip thickness tolerance with a generally limited negative deviation of 0.3 mm; positive tolerance varies with nominal thickness and width. Confirm the complete dimensional requirement where minimum delivered thickness or mass is commercially important.
Substitution and cross-standard use
SN490B should not be replaced solely on the basis of a similar minimum tensile or yield strength. Its controlled maximum yield strength, yield ratio, thickness-dependent strength, Charpy requirement, weldability indices and JIS dimensional rules are part of the grade definition. Commonly compared grades may lack one or more of these controls or apply different sampling, orientations and thickness breakpoints.
For substitution, compare the governing product specifications and project requirements rather than commercial equivalence tables. The review should cover chemistry, Ceq or PCM, actual and specified strength ranges, yield ratio, impact temperature and direction, through-thickness properties, ultrasonic acceptance, delivery condition, dimensions, certification and legal approval under the applicable building code.
Sources
- JIS G 3136:2022 — Rolled steels for building structure: official standard recordJapanese Standards Association
- JIS G 3136:2022 — Official English previewJapanese Standards Association
- JIS G 3136 revision draft and technical tablesThe Japan Iron and Steel Federation
- Steel Construction Today & Tomorrow No. 49The Japan Iron and Steel Federation
- JIS G 3136 manufacturing specifications for structural sectionsNippon Steel Structural Shapes Corporation
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