JIS G3106 SM490YB is a low-alloy, high-strength structural steel plate specified by the Japanese Industrial Standards for welded structures. The SM prefix identifies structural steel for welding, 490 indicates the minimum tensile strength class, and the trailing YB marks a yield-controlled grade with a restricted carbon content. The result is a plate that combines a predictable strength level with the weldability and formability demanded by heavy fabrication shops.
Two variants exist in the same tensile class. SM490YA controls yield strength at a lower level, while SM490YB raises the specified yield and tightens the carbon limit so that toughness in the heat-affected zone remains stable after multi-pass welding. SM490YB plates are commonly supplied in the as-rolled or controlled-rolled condition in thicknesses from 6 mm to 160 mm, with wider plate widths for large welded panels and reduced weld seams.
The grade is not intended for pressure containment or low-temperature service; for those duties, plates with guaranteed impact energy at reduced temperature should be selected instead.
Chemical Composition of JIS G3106 SM490YB
Typical ladle analysis limits for the YB grade are shown below. The carbon ceiling is lower than that of the YA variant, which is the main metallurgical reason for its improved weldability.
| Element | Maximum, % |
|---|---|
| C | 0.18 |
| Si | 0.55 |
| Mn | 1.60 |
| P | 0.035 |
| S | 0.035 |
Alloying additions are balanced rather than maximised. Manganese provides solid-solution strengthening, silicon supports deoxidation, and phosphorus and sulfur are held low to limit segregation and lamellar tearing in thick welded joints. Carbon equivalent, calculated as Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, should be agreed between mill and fabricator for sections above 50 mm because it governs cold-cracking risk and the preheat level required before welding.
Mechanical Properties by Thickness
Yield strength falls as plate thickness increases, which reflects the slower cooling rate and coarser microstructure of heavy sections. Tensile strength remains in the 490 to 610 MPa band across the full thickness range.
| Thickness, mm | Yield strength, min MPa | Tensile strength, MPa |
|---|---|---|
| Up to 16 | 365 | 490 to 610 |
| Over 16 to 40 | 355 | 490 to 610 |
| Over 40 to 75 | 335 | 490 to 610 |
| Over 75 to 100 | 325 | 490 to 610 |
| Over 100 to 160 | 325 | 490 to 610 |
Elongation is normally guaranteed at a minimum of 17 to 21 percent depending on thickness and test direction. Because the grade is strength-controlled rather than toughness-controlled, impact testing is specified only when the end user requires it, for example for bridge components exposed to low service temperatures.
Welding and Fabrication Behaviour
Carbon and carbon equivalent are held low enough for shielded metal arc, submerged arc, gas metal arc and flux-cored welding without special consumables.
Preheat of 50 to 100 degrees Celsius is recommended for thicknesses above 40 mm or when ambient temperature is below 5 degrees Celsius.
Cold forming such as bending, flanging and roll forming is performed readily; minimum bend radii of about 1.5 times thickness are typical for transverse bends.
Flame cutting leaves a clean edge, and machined edges are stable thanks to uniform hardness through the thickness.
Post-weld heat treatment is not normally required for thicknesses below 40 mm.
Applications of SM490YB Plate
Frames, columns and support systems of high-rise buildings, including welded box columns and built-up beams.
Bridge girders, piers and deck plates where fatigue resistance and consistent yield strength matter.
Chassis, body frames and structural members of trucks, trailers and rail vehicles.
Hulls, deck structures and internal framing of general cargo vessels and barges.
Storage tanks, silos, hoppers and container frames for industrial plants.
Supply Range, Tolerances and Inspection
Plates are supplied to JIS G3106 with standard edge condition, either as-rolled edges or sheared and gas-cut edges, and with allowable thickness tolerances as agreed. Mill test certificates are issued to EN 10204 3.1 for each heat and plate, recording chemical analysis, tensile results and, where ordered, impact energy. Additional options include ultrasonic examination, charpy V-notch impact testing, Z-direction properties for thick tee joints, and surface treatment by shot blasting and priming.
Frequently Asked Questions
Q: Is SM490YB the same as SM490YA?
No. They share the same 490 MPa tensile class, but SM490YB specifies a higher yield strength and a lower maximum carbon content than SM490YA.
Q: What is the maximum thickness available?
SM490YB plate is regularly produced up to 160 mm; heavier sections are possible by special agreement with the mill.
Q: Does SM490YB require impact testing?
Impact testing is not mandatory for the base grade, but it can be ordered with a specified test temperature and minimum energy for low-temperature applications.
Q: How does SM490YB compare with S355J2?
Both are weldable structural grades with similar yield strength. SM490YB follows Japanese standards, while S355J2 is specified under EN 10025-2 with guaranteed impact energy at minus 20 degrees Celsius.
Q: What preheat is needed before welding?
As a rule, 50 to 100 degrees Celsius is applied for thicknesses above 40 mm or for cold workshop conditions, and the exact level should follow the qualified welding procedure.
Q: Can SM490YB be used for pressure vessels?
It is not a pressure vessel grade. Boiler and pressure vessel service requires plates produced to dedicated pressure equipment standards with certified toughness.
