What Is SN490B HSLA Steel Plate?
SN490B is a high-strength low-alloy (HSLA) steel plate produced to JIS G3136, the Japanese standard that covers rolled steel for building structures. The grade is used for steel frames in high-rise buildings, industrial plants, stadiums and public facilities where strength, toughness and weldability have to be balanced in one material. The designation reads easily: SN identifies the building structural steel series, 490 is the minimum tensile strength in N/mm2, and B identifies the grade level inside the series.
Unlike general-purpose plate such as SS490, the SN series adds a defined yield strength range, a carbon equivalent limit for weldability and an impact toughness requirement. That combination is what makes SN490B HSLA steel plate a dedicated building-frame material rather than a plain structural plate.
Chemical Composition of SN490B Steel Plate
| Element | Maximum content, % |
|---|---|
| C | 0.14 |
| Si | 0.33 |
| Mn | 1.50 |
| P | 0.008 |
| S | 0.005 |
The very low phosphorus and sulphur limits shown above are the reason the grade keeps ductility and impact toughness even in thick sections. A tight carbon range and a controlled carbon equivalent keep the heat-affected zone soft enough for field welding, which is essential for bolted and welded building frames.
Mechanical Properties of SN490B Steel Plate
| Property | Value |
|---|---|
| Tensile strength | 490–610 MPa |
| Yield strength, thickness class I | ≥325 MPa |
| Elongation, thickness class I | ≥17% |
| Impact toughness | Charpy V-notch testing specified for the grade |
| Weldability | Suitable for conventional arc welding processes |
The specification also applies an upper limit to yield strength. That ceiling keeps the steel from becoming so strong relative to its tensile strength that energy absorption in a seismic event is lost, and it is one of the practical differences between the SN series and general structural grades.
SN490B and SN490C: Choosing the Right Grade
Both grades share the same chemical and tensile requirements, so the selection is normally decided by service temperature rather than by strength.
Grade B suits the majority of enclosed building frames and covered structures with moderate design temperatures.
Grade C is intended for lower service temperatures where higher impact toughness is required.
Because the SN series fixes both a minimum and a maximum yield strength and controls the carbon equivalent, it should not be treated as a simple substitute for a higher-strength general-purpose grade such as Q460; the two are governed by different standards and different design assumptions.
Plate thickness affects the guaranteed values. Confirm the thickness class used for the design before releasing a cutting list.
Welding, Cutting and Fabrication
Low carbon and low sulphur content allow manual metal arc, gas metal arc, flux cored and submerged arc welding without excessive preheating in normal plate thicknesses.
Heat input should be controlled in thick sections so that impact toughness in the heat-affected zone is preserved.
The plate cuts cleanly by plasma, laser and oxy-fuel processes, and it cold forms well for stiffener and gusset work.
Dimensional accuracy of the rolled surface supports straight saw-cut edges and tight fit-up for welded box sections and built-up girders.
Typical Applications
Seismic-resistant steel frames of high-rise office towers and residential buildings.
Landmark buildings, station halls and large-span roofs.
Industrial plant structures, pipe racks, trusses and crane support frames.
Nuclear power plant structures and heavyindustrial buildings requiring verified toughness.
Steel structure engineering and factory construction where weldability and a controlled yield range reduce fabrication risk.
FAQ
Q: What does the B in SN490B mean?
SN is the building structural steel series, 490 is the minimum tensile strength in N/mm2, and B is the grade level that carries the standard impact toughness requirement for normal service temperatures.
Q: Can SN490B plate be welded without preheating?
In common plate thicknesses with low carbon equivalent, preheating is often unnecessary, but thick sections, high restraint joints and cold workshop conditions still require a qualified welding procedure with controlled preheat and heat input.
Q: How is SN490B different from SS490 plate?
The SN series controls the yield strength range, carbon equivalent and impact toughness for building frames, while general-purpose grades such as SS490 mainly guarantee a minimum yield and tensile level for ordinary structural use.
Q: Is SN490B equivalent to Q460 steel?
No. They follow different standards and different design rules. SN490B fixes a narrower yield range plus toughness and carbon equivalent limits, so it should not be used as a direct replacement for a higher-strength general-purpose grade.
Q: What thickness range is normally produced?
SN490B plate is rolled across a wide range of thicknesses, from thin plate used for stiffeners to heavy plate for thick-column and beam connections. The guaranteed mechanical values depend on the thickness class, so the applicable class should be quoted with the order.
Q: Which industries use SN490B most?
Commercial and residential construction, industrial plant building, infrastructure and nuclear-related steel structures are the main users, because all of them need a material that combines high strength with predictable welding behaviour.