JIS G 3474 STKT590 High Tensile Strength Steel Tubes for Tower Structural Purposes

Material

High-tensile structural steel tubes designated under JIS G 3474 (“High strength steel tubes for steel tower”), grade STKT590. These tubes are manufactured primarily for use in transmission towers or other steel tower structures, offering elevated strength compared with standard structural steel tubes. 

Specification

• Standard: JIS G 3474

• Grade: STKT590

• Tube type: typically longitudinally welded tubes (electric-resistance welded or automatic arc welded) for steel towers 

• Applicable outside diameter range (typical): 139.8 mm to 1,117.6 mm in standard JIS specification. 

• Thickness: as per user agreement; for instance standard clause may allow up to ~25 mm thickness for the grade in table for “25 mm or less” in some editions. 

Key Features

• High tensile strength: minimum tensile strength of 590 MPa and up to ~740 MPa. 

• Good yield strength: minimum proof/yield strength of 440 MPa. 

• Controlled chemical composition enabling reliable welding performance and structural stability (see next section).

• Carbon equivalent limit (Cₑq ≤ 0.40 %) per JIS standard, promoting weldability. 

• Designed for tower structural usage — makes it ideal for lattice towers, transmission towers, guyed masts, etc. 

Our Advantages

As a factory-specialist producer of STKT590 tubes, we address key customer concerns from a manufacturing perspective:

• Straightness and dimensional precision: Through continuous welded tube production with calibrated rolling, we ensure tube straightness within tight tolerances and flat end faces perpendicular to the axis, meeting the JIS requirement that “the tube shall be practically straight and both ends shall be square to the axis”. 

• Strict thickness and diameter tolerances: We adhere to the dimensional tolerances in the JIS standard (for outer diameter and wall thickness) and maintain internal production records—this reduces misalignment and improves fit-up on tower site. For example, for thickness ≥12 mm the tolerance is +15 % / −1.5 mm (for certain sizes) under JIS G 3474. 

• Reliable mechanical properties: Our internal QA ensures the minimum tensile strength (590–740 MPa) and minimum yield strength (≥440 MPa) are consistently met or exceeded, verified by coupon testing. This ensures that the tube can carry high loads as required in tower structures. 

• Controlled chemical composition and weldability: By limiting C ≤0.12 %, Si ≤0.40 %, Mn ≤2.00 %, P ≤0.030 %, S ≤0.030 %, Nb+V ≤0.15%, and B ≤0.0002 % in line with JIS G 3474 table 2, we ensure low inclusion levels, stable microstructure, and better weldability. 

• Tailored for tower fabrication: We cooperate with tower manufacturers to supply cut lengths, chamfered ends, weld bevels, or combinations of diameters and thicknesses per site drawings, facilitating efficient erection on site.

Chemical Composition

* Note: STKT520 is included for illustrative purposes though detailed composition may not be publicly documented in the same way.

Mechanical Properties

Here is a table showing mechanical property minima for STKT590 plus comparable grades.

Applicable Standards

• JIS G 3474: High strength steel tubes for steel tower (Japan) 

• Execution manufacturing methods per standard: manufacture by electric-resistance welding or automatic arc welding (straight seam) using fine-grain killed steel; no heat treatment typically required. 

• Chemical analysis by JIS G 0320, product analysis by JIS G 0321. 

Applications

General Application Area

These tubes are used in tower structural applications, especially in transmission towers, power line towers, communications towers, wind-turbine tower components, or other lattice-structure towers requiring high strength and welded tubular members.

Specific Use Cases

• Fabrication of lattice tower legs (diagonal and vertical members) using STKT590 tubes to reduce weight and improve structural capacity compared to conventional tubes.

• Horizontal members/bracing tubes in transmission towers where high tensile strength allows smaller section sizes and better economy.

• Pre-fabricated welded sections for guyed masts or telecom towers where the tube must meet strict dimensional and straightness requirements.

• In wind-tower applications (lower sections) where high strength tubes enable thinner walls and larger diameter for improved stiffness and structural efficiency.