JIS G3444 Carbon Steel Structure Pipe: A B2B Buyer's Guide to STK400, STK500 and Mill-Direct Sourcing from a Chinese Manufacturer

What JIS G3444 Actually Covers (and What It Doesn't)

JIS G 3444 is the Japanese Industrial Standard for "Carbon steel tubes for general structure." It applies to tubes used in civil engineering, construction, towers, scaffolding, foundation piles, and anti-landslide piles — explicitly non-pressure service. The current revision is JIS G 3444:2025, which tightened non-destructive testing requirements on welded joints and revised chemical composition limits for improved weldability. OD coverage typically runs from 21.7 mm up to 1016.0 mm; for foundation and anti-landslide piles, the standard applies primarily to OD 318.5 mm and below, while JIS A 5525 takes over above that threshold.

The crucial word in the title is "general structure." This is not a boiler tube, not a linepipe, not a heat-exchanger tube. If a buyer's RFQ asks for JIS G3444, they are buying load-bearing or framing members — columns, bracings, racks, fence posts, sign supports, transmission tower legs, scaffolding standards and ledgers, and the like. The standard sets mechanical properties and weldability expectations that make the tube safe to cut, drill, weld, and galvanize in a fabrication shop. It is one of the workhorse specifications behind Japan's domestic construction supply chain, and it is also widely used across Southeast Asia, the Middle East, and Africa where Japanese-funded or Japanese-spec projects are common.

The Two Grades You Will See 95% of the Time: STK400 and STK500

JIS G3444 defines a family of grades — STK290, STK400, STK490, STK500 — but in real procurement the conversation almost always narrows to two: STK400 for general structural framing, and STK500 where higher yield strength is required. STK290 and STK490 still exist in the standard; STK490 is the older 490 MPa class that overlaps with STK500 in many applications, and STK290 is the lightest grade typically reserved for fencing, light posts, and similar low-stress service.

Mechanically, the difference is significant:

The bend-test geometry and weld-tensile requirements are also grade-specific: STK400 is flattened to 2/3 D with a 90° bend over a 6D inside radius, while STK500 is flattened to 7/8 D under the same bend geometry. For an international buyer comparing JIS G3444 against a500 steel hollow sections (ASTM A500) or en10210 steel hollow sections (EN 10210), STK400 lines up roughly with A500 Grade B / S235JRH; STK500 lines up roughly with A500 Grade C / S355J0H. The mapping is not exact — carbon equivalent, impact requirements, and tolerance classes all differ — but it gives a procurement manager a reasonable starting point when an EPC specification allows cross-standard acceptance.

ERW vs Seamless for Structural Use: When It Matters, When It Doesn't

JIS G3444 allows both electric-resistance-welded (ERW) and seamless manufacturing routes, and for most general structural applications the choice has more to do with availability, OD range, and price than performance. ERW dominates the smaller OD range (typically up to around 660 mm) because the process produces a continuous, high-frequency induction-welded seam with consistent quality and tight dimensional control at a lower unit cost. Seamless becomes more common at larger OD or heavier wall, and is sometimes specified for cyclic-load or fatigue-sensitive members even at non-pressure service.

The welding process itself matters: a modern high-frequency ERW line produces a weld zone that, after proper post-weld heat treatment, can match the base metal in tensile strength — which is why the standard's "weld tensile" requirement is set equal to the parent-metal tensile for each grade. For galvanized structural tube, the ERW route is overwhelmingly dominant because galvanizing baths work best on a uniform wall thickness and a continuous weld seam without the geometric variations of a seamless hot-rolling process.

For projects that will see significant welding in the field — typical for tower fabrication or modular bridge components — the buyer should also pay attention to the carbon equivalent (CE) of the chosen grade. STK400 and STK500 are designed for good weldability, but at heavy wall thicknesses the CE can drift upward, and preheat procedures may be required. Ask the mill for the actual heat analysis CE value, not just the grade name.

Sizes, Tolerances, and Surface Condition: What to Write in the RFQ

A complete JIS G3444 RFQ should specify seven items, in this order: standard number (JIS G 3444), steel grade (STK400 / STK500), dimensions (OD × wall thickness × length), manufacturing route (ERW or seamless), end finish (plain end, threaded, or beveled), surface condition (black, hot-dip galvanized, or other coating), and quantity with delivery terms. Skipping any of these typically results in a quotation based on the mill's most-common stocking item, which may not match what the project actually needs.

Dimensional tolerances are where international projects most often run into trouble. JIS G3444's wall-thickness tolerance is typically ±10% under the standard's table, and OD tolerance tightens as the tube gets smaller. If a project is replacing an ASTM A500 or EN 10210 supply — which have their own tolerance classes — the buyer should confirm the mill is willing to certify to the tighter of the two specifications, and reflect that in the purchase order. For galvanized product, the OD is measured before galvanizing; the zinc coating adds 50–85 µm per side depending on the coating class.

On surface condition: black (as-rolled, with light oil) is the default for tube that will be painted, primed, or further fabricated. Hot-dip galvanized to JIS H 8641 or ASTM A123 is the dominant finish for outdoor structural service — towers, fencing, guard rails, sign supports, and the like. For coastal or highly corrosive atmospheres, additional coatings (3LPE, epoxy, powder over galv) can be specified, but most buyers keep JIS G3444 tube as black or HDG and let the project coating specification drive any further treatment.

Documentation a Reliable Mill Should Hand You With Every Shipment

The single biggest difference between a mill-direct structural tube supplier and a trading-company middleman is the paperwork. For every JIS G3444 shipment, a real mill should provide:

• Mill Test Certificate (MTC / MTR) to EN 10204 3.1, with heat number, chemical analysis, mechanical test results (tensile, yield, elongation, bend or flatten), and signature of the mill's QA manager.
• Heat number traceability — every tube bundle or piece should be marked with the same heat number that appears on the MTC. Without heat number traceability, post-shipment failure analysis is impossible.
• Dimensional inspection report covering OD, wall thickness, ovality, and length for the shipped lot, with sampling per the standard's table.
• NDT report for welded tube (eddy current or ultrasonic) confirming weld integrity per JIS G 3444:2025's enhanced NDT requirements.
• Coating certificate (if galvanized) confirming zinc-coating mass per JIS H 8641 or ASTM A123, with measured values.
• Country of origin and mill address clearly stated on the MTC and commercial invoice — required for customs clearance in most jurisdictions.

If a quotation arrives with a price that seems too good to be true and no mention of MTC, heat number, or NDT, the buyer is almost certainly talking to a trading company that has never seen the inside of the mill. The risk profile is fundamentally different.

How JIS G3444 Fits Inside a Mill-Direct Product Range

A mill-direct structural steel supplier will typically run JIS G3444 alongside the closely related specifications that an international project buyer is likely to also need: ASTM A500 for the Americas, EN 10210 for Europe and the Middle East, GB/T 8162 for China-domestic and Belt-and-Road projects, and JIS G3466 for square and rectangular hollow sections. The same production line that makes JIS G3444 STK400 round tube can usually also make A500 Grade B, S235JRH, and Q235B to GB/T 8162 with a grade change and a chemistry tweak. That is one of the practical advantages of working with a mill-direct source: a single purchase order can cover multiple standards, multiple grades, and multiple end finishes.

For an EPC contractor or steel structure fabricator, this matters because most modern projects are multi-standard. A transmission tower may have JIS G3444 STK400 main legs, ASTM A500 Gr.B cross-bracing, and EN 10210 S355J0H base plates — all from the same mill, on the same vessel, with consistent documentation. Bundling the supply also reduces the project risk of receiving mismatched material from three different sources with three different MTC formats.

In addition to the structural tube itself, a mill-direct supplier should be able to provide the full accessory package that goes with it: pipe fittings (BW, SW, threaded) for the connection details, pipe flanges and base plates for column-to-foundation connections, gasket, stud bolt & nut sets for any bolted connection detail, and — where the structural tube transitions to fluid service — industrial valves. This kind of single-source bundling is what makes a mill-direct relationship operationally different from a piece-by-piece spot purchase.

Where JIS G3444 Is Used in Real Projects

Five application families dominate JIS G3444 consumption in international projects:

1. Steel towers and masts. Power transmission towers, telecom monopoles and lattice masts, lighting masts, sign gantries. STK400 covers the majority of leg and bracing members; STK500 is used at the high-stress base and joint zones.
2. Scaffolding. Tube-and-clamp scaffolding systems, system scaffolding verticals and horizontals, access platforms. STK400 is the global default; galvanized finish is universal for safety and service life.
3. Building structure. Secondary framing for industrial buildings, canopies, mezzanines, roof trusses, warehouse racking uprights, fencing and gate frames.
4. Civil engineering. Foundation piles (where OD ≤ 318.5 mm; JIS A 5525 above that), anti-landslide piles, retaining structure members, drainage and stormwater structures.
5. Industrial equipment frames. Skid bases, conveyor galleries, machine guarding, material handling structure, port and terminal structural steel, mining support structure.

For any of these applications, the same caveat applies: JIS G3444 is non-pressure. If a project ever pushes pressure fluid or gas through the tube, the specification needs to change to JIS G 3454 (pressure service), JIS G 3456 (high-temperature), or JIS G 3460 (low-temperature). The grade name overlap with JIS G3444 is intentional, but the testing and inspection regime is fundamentally different.

What to Ask Before You Sign the PO

Six questions separate a reliable JIS G3444 supply from an unreliable one. Send these to the supplier before placing the order:

1. Can you provide MTC to EN 10204 3.1 with heat number for every shipped lot? If the answer is "we can provide a certificate" without specifying 3.1 and heat number, look elsewhere.
2. What is the actual heat-analysis carbon equivalent (CE) for the offered heat? Useful for field-welding procedure qualification, especially at heavy wall thickness.
3. What NDT method is applied to the ERW seam — eddy current, ultrasonic, or both — and what is the rejection threshold? Should match JIS G 3444:2025 requirements.
4. For galvanized product, what zinc-coating mass class (JIS H 8641 HDG class or ASTM A123 class) will be supplied, and can the actual measured value be reported on the MTC?
5. What is the dimensional tolerance you will certify to, and is it tighter than the JIS G3444 default if the project requires it?
6. What is the actual mill address and production line for this order, and can the buyer or its inspector visit the mill before shipment? A mill-direct supplier will answer all three without hesitation.