Standard Title & Scope 
JIS G3466: Carbon steel square and rectangular tubes for general structure.

Purpose: This Japanese Industrial Standard specifies requirements for welded carbon steel square, rectangular, and special-shaped flat-sided tubes (hollow structural sections - HSS) primarily intended for use in general welded, bolted, or riveted structures such as buildings, bridges, vehicles, ships, pylons, machinery frames, and other structural applications where mechanical properties are the primary design consideration. It does not cover tubes for pressure purposes, heat exchangers, or mechanical tubing.

Material & Grades 
Material: Carbon steel.

Grades: Defined by minimum yield strength. The primary grades are:

STKR400: Minimum Yield Strength of 400 MPa (N/mm²).

STKR490: Minimum Yield Strength of 490 MPa (N/mm²).

Grade Designation Explanation:

STK: Denotes "Steel Tube Kikaku" (Steel Tube Standard). 
R: Stands for "Ryouiki" (Yield Point/Strength). 
400 or 490: Represents the minimum yield strength in MPa. 
The "R" is crucial, distinguishing these structural grades from other STK grades (like STK400/490 without 'R') which might have different requirements or applications.

Manufacturing Process 
Tubes are manufactured by cold forming flat-rolled steel (hot-rolled or cold-rolled coil/sheet) into the desired square or rectangular shape.

The longitudinal seam is welded using techniques suitable for high-speed production, primarily Electric Resistance Welding (ERW) or sometimes high-frequency induction welding. Seamless tubes are not covered by this standard.

Chemical Composition  

Note: If necessary, Other alloy elements than those given in this table may be added. 
 

Mechanical Properties 

Elongation (A %): Minimum elongation values are specified based on the tube dimensions (wall thickness, width), generally ranging from around 15% to 30%. Higher elongation is typically required for thinner/larger sections.

Flattening Test: A critical test for welded tubes. A specified length is flattened between parallel plates to a distance H (calculated based on wall thickness t). The weld must be positioned at 90° or 0° to the direction of force. The tube must withstand this flattening without cracking or weld opening exceeding a specified limit. This tests weld integrity and ductility.

Dimensional Tolerances 
The standard defines strict tolerances for:

Outside Dimensions (Width, Height): e.g., ±0.5mm to ±1.5% depending on size.

Wall Thickness: e.g., ±10% of nominal thickness.

Corner Radius: Maximum limits specified.

Length: Tolerances for standard and precision-cut lengths.

Straightness: Maximum deviation per unit length.

Squareness (Perpendicularity of sides).

Mass: Tolerances on theoretical mass per meter are specified (typically around ±6% to ±10%).

Testing And Inspection 
Chemical Analysis: Performed on the base metal (usually melt analysis).

Mechanical Tests:

Tensile Test (mandatory for each lot/batch). 
Bend Test (mandatory for t ≤ 6mm). 
Weld Integrity:

Hydrostatic Test: Applied pressure specified (e.g., 5 MPa for 5 seconds) to check for leaks/seam integrity. OR 
Non-Destructive Testing (NDT): Electric test (like eddy current) or other approved methods (like ultrasonic) applied to the entire weld seam to detect imperfections. Manufacturers typically choose either hydrostatic or NDT. 
Visual Inspection: Surface condition (e.g., freedom from harmful defects like cracks, excessive scale, dents beyond tolerance).

Dimensional Inspection: Verification against specified tolerances.

Surface Condition 
Tubes are typically supplied as-produced ("black" or mill finish), which may have a light oxide layer (mill scale) or be slightly oiled for corrosion protection during transport/storage. Descaled (pickled) or galvanized tubes are not covered by the base JIS G 3466 standard (separate standards exist for coated tubes).

Typical Applications (General Structure) 
Building frames (columns, beams, trusses, purlins)

Machinery frames and guards

Support structures (racks, platforms, staging)

Agricultural equipment

Fencing and gates (structural components)

Material handling equipment

Furniture frames (industrial/commercial)

Signage structures

Automotive components (chassis parts, trailers)

Key Comparison (STKR400 Vs. STKR490) 
Strength: STKR490 offers higher yield (≥490 MPa) and tensile strength than STKR400 (≥400 MPa).

Formability/Weldability: STKR400 generally has slightly better formability and weldability due to potentially lower Carbon content and strength level.

Weight Saving Potential: STKR490 allows for potentially lighter structures due to its higher strength-to-weight ratio compared to STKR400 for the same load requirements.

Cost: STKR490 is typically more expensive than STKR400 due to the higher strength requirements and tighter chemical control.

In summary, JIS G 3466 STKR400 and STKR490 define cold-formed, welded carbon steel square and rectangular tubes used in non-pressure structural applications. They differ primarily in their minimum yield strength (400 MPa vs. 490 MPa), with corresponding differences in tensile strength, elongation, and chemical composition limits, allowing engineers to select the appropriate grade based on strength requirements, weight considerations, and fabrication needs.