What Is SS400 Steel?
SS400 is a carbon structural steel grade defined in the Japanese Industrial Standard JIS G3101. In the designation, SS stands for general structural rolled steel and 400 indicates a minimum tensile strength of 400 MPa. SS400 steel tube is the same grade formed into circular, square or rectangular hollow sections and is one of the most economical structural tube materials available. Square and rectangular hollow sections made from SS400 are popular in building structures, machinery frames, conveyors and racking because they combine predictable strength with excellent weldability.
Chemical Composition
SS400 is essentially a plain carbon steel with small additions of manganese. The commonly published maximum limits for SS400 are listed below; the exact values for a specific product are governed by JIS G3101 and the supplier's mill certificate.
| Element | Maximum Limit (%) |
|---|---|
| Carbon (C) | 0.21 |
| Manganese (Mn) | 1.60 |
| Phosphorus (P) | 0.050 |
| Sulfur (S) | 0.050 |
Mechanical Properties
For flat products and structural sections up to 16 mm thickness, SS400 has a minimum yield point of 245 MPa and a tensile strength of 400 to 510 MPa. Elongation requirements depend on product form and thickness, typically ranging from 17% to 23% for plate and strip. For hollow sections, the mechanical properties are verified by tensile testing of the finished tube and are recorded on the test certificate. This strength level covers the majority of general structural applications, and the grade is not intended for pressure service or low-temperature duty.
Dimensions and Tolerances
SS400 square and rectangular tube is commercially available in a wide range of sizes. Common square tube side lengths range from 20 mm to 400 mm, with wall thicknesses from 1.5 mm to 16 mm. Dimensional tolerances on outside dimensions, wall thickness and straightness follow the applicable product standard or the agreed customer specification, and these tolerances should be confirmed at the ordering stage, especially for tube intended for automatic welding or precision framing.
Surface Treatments
Bare SS400 tube is prone to surface rust in outdoor or humid service, so protective treatments are normally specified. Hot-dip galvanizing provides the most durable protection for exterior structures and is the standard choice for outdoor frames and fencing. Painting and powder coating are used where appearance matters or where the environment is mildly corrosive. The choice of treatment should balance the service environment, expected life and budget.
Typical Applications
SS400 steel tube is used in steel building frames, roof trusses, factory structures, stadium space frames, machine bases, equipment supports, conveyor frames, racks, handrails and agricultural structures. For welded fabrications the grade welds easily without preheat in most sections, which keeps fabrication cost low.
Frequently Asked Questions
What does SS400 mean?
SS stands for structural steel and 400 for the minimum tensile strength of 400 MPa in the JIS G3101 designation system.
Is SS400 weldable?
Yes. SS400 is a low-carbon structural steel with good weldability using conventional methods such as MMA, MIG and TIG without preheat for most section sizes.
What is the yield strength of SS400?
The minimum yield point is 245 MPa for thicknesses up to 16 mm, with tensile strength of 400 to 510 MPa.
Can SS400 be used for pressure piping?
No. SS400 is a structural grade and is not listed in pressure piping standards. Use pressure grades such as JIS G3454 STPG370/410 or ASTM A106 for pressurised service.
What is the difference between SS400 and Q235?
Both are general structural carbon steels. SS400 is the Japanese JIS G3101 grade with 400 MPa minimum tensile strength, while Q235 is the Chinese GB/T 700 grade with 235 MPa minimum yield strength. They are often treated as approximately equivalent in simple structural use, but formal substitution should follow the project specification.