SM490B and SM490C are Japanese structural steel grades covered by JIS G3106, the standard for hot rolled steels for welded structure. Both sit in the 490 MPa tensile class and both are used for bridges, building frames, ships and general welded fabrication, so the two designations are frequently treated as alternatives. The suffix letter is the only difference in the grade label, yet it controls two things that matter in design and fabrication: the maximum carbon content and the absorbed energy required in the Charpy V-notch impact test. SM490C demands higher impact energy than SM490B on the same test temperature. This article explains composition, mechanical properties, impact requirements and the practical selection rules for each grade.
What SM490B and SM490C Are Under JIS G3106
JIS G3106 defines hot rolled steel plates, sheets and strips for welded structures with a tensile strength of 490 MPa class. The base designation SM490 is followed by a letter that increases in toughness requirement: the A grade carries no mandatory impact test, the B grade requires a specified absorbed energy, and the C grade requires a higher absorbed energy with a tighter carbon limit. All three are delivered as-rolled, in a controlled rolled condition, or normalized when ordered that way, and they are routinely supplied with a mill certificate that reports the chemical analysis and mechanical test results of the heat and the plate. SM490B and SM490C are available as plate in thicknesses from about 6 mm to 200 mm, and both are readily welded by shielded metal arc, submerged arc and gas shielded processes with appropriate consumables and preheat.
Chemical Composition Compared
The element limits are almost identical, with carbon as the decisive difference: the C grade holds carbon to a lower maximum, which improves weldability and notch toughness in the heat affected zone.
| Element, max % | SM490B | SM490C |
|---|---|---|
| Carbon (C) | 0.20 | 0.18 |
| Silicon (Si) | 0.55 | 0.55 |
| Manganese (Mn) | 1.65 | 1.65 |
| Phosphorus (P) | 0.035 | 0.035 |
| Sulfur (S) | 0.035 | 0.035 |
Mechanical Properties Compared
Strength requirements are identical for the two grades, so the choice between them is never driven by yield or tensile level.
| Thickness and property | SM490B | SM490C |
|---|---|---|
| Thickness 16 mm and under: yield min (MPa) | 325 | 325 |
| Thickness 16 mm and under: tensile (MPa) | 490-610 | 490-610 |
| Thickness 16 mm and under: elongation min (%) | 17 | 17 |
| Thickness 16 mm to 90 mm: yield min (MPa) | 295-315 | 295-315 |
| Thickness 16 mm to 90 mm: tensile (MPa) | 490-610 | 490-610 |
| Thickness 16 mm to 90 mm: elongation min (%) | 23 | 23 |
Impact Toughness: The Real Difference Between B and C
JIS G3106 tests Charpy V-notch specimens taken in the rolling direction at 0 °C, and it is the required absorbed energy that separates the two grades. SM490B must average a minimum of 27 J per set of three specimens, while SM490C must average a minimum of 47 J at the same 0 °C test temperature, with each individual value above a stated fraction of the average. That is a substantially larger energy absorption, and it is achieved through the lower carbon limit together with tighter process control and, where required, a normalized delivery condition. In practice, the 47 J requirement of SM490C gives a wider safety margin against brittle fracture in thick plate, in low ambient temperatures, in highly restrained welded joints and in structures subject to dynamic or fatigue loading such as bridge girders, crane structures and ship hulls. Where a project code demands documented toughness at 0 °C with an absorbed energy above the basic level, SM490B cannot be substituted by declaration alone, because the test acceptance threshold is a property of the specified grade.
Applications, Grade Selection and International Equivalents
SM490B: building frames, general welded beams and columns, storage tanks, industrial plant structures and standard civil engineering work where the basic 27 J impact requirement at 0 °C is adequate.
SM490C: bridge main girders, crane and port machinery structures, ship structural members, pressure retaining welded frames and any thick or cold region structure where higher notch toughness is specified.
Both grades: weldability is good at the specified carbon levels; preheat and interpass control should follow the fabrication specification, and the impact test temperature written in the purchase order must be reproduced on the mill certificate.
Selection rule: fix the required absorbed energy first, then choose the lowest grade that satisfies it, since the C grade carries a price premium for the extra testing and tighter chemistry control.
European designations: the EN 10025-2 S355 grades are close in yield level but apply their own impact classes, so cross reference is approximate.
USA: ASTM A572 Grade 50 offers a comparable 345 MPa minimum yield with no mandatory impact energy for the base grade.
China: Q355B and Q355C under GB/T 1591 are the closest strength equivalents, with impact energy requirements rising from B to C quality in the same way.
Frequently Asked Questions
Q: What is the main difference between SM490B and SM490C?
Impact toughness. SM490B requires a minimum absorbed energy of 27 J and SM490C requires 47 J in the Charpy V-notch test at 0 °C, and SM490C also holds carbon to a lower maximum of 0.18%.
Q: Do SM490B and SM490C have different yield strength?
No. Both grades specify a minimum yield of 325 MPa for thickness up to 16 mm and a tensile range of 490-610 MPa, so the selection is based on toughness rather than strength.
Q: At what temperature is SM490 tested for impact?
JIS G3106 specifies the Charpy V-notch test at 0 °C for the B and C grades, on longitudinal specimens, with 27 J required for B and 47 J for C.
Q: Does SM490A need an impact test?
No mandatory impact test is required for the A grade, which is why it is used for less critical welded structures and why B and C carry the test requirements.
Q: Can SM490B be used instead of SM490C?
Only where the design code accepts the lower absorbed energy. If the specification calls for 47 J at 0 °C, the plate must be certified as SM490C.
Q: What is SM490C equivalent to in European standards?
The closest European grades are the EN 10025-2 S355 series, which share a similar yield level but define their own impact classes, so equivalence must be confirmed against the project specification.
Q: Is SM490 supplied normalized?
It can be supplied as-rolled or in a controlled rolled condition as standard, and normalized plate can be ordered where the specification or the impact requirement makes it necessary.
