What Is JIS G3444?
JIS G3444 is the Japanese Industrial Standard for carbon steel tubes for general structural purposes. It is the specification that most structural steel buyers refer to when they search for a 'standard JIS G3444 pipe': a carbon steel tube, supplied welded or seamless, with guaranteed chemical composition and mechanical properties for load-carrying structures rather than for pressure or fluid service.
The tubes covered by JIS G3444 are used in civil engineering and building structures - steel towers, scaffolding, footing piles, foundation piles, piles for landslide suppression - as well as in machinery frames, racks, handrails, temporary works and similar members where predictable yield strength and weldability matter more than pressure integrity.
Standard Scope and Applicability
The standard applies to carbon steel tubes with an outside diameter generally from 21.7 mm up to 1016.0 mm. Two application limits are written into the scope and are frequently missed:
For foundation piles, JIS G3444 covers only welded tubes with an outside diameter under 318.5 mm.
For piles for landslide suppression, it covers seamless tubes and welded tubes with an outside diameter under 318.5 mm; welded piles of 318.5 mm or larger fall under JIS A 5525, and centrifugally cast tubes are governed by JIS G5201.
Both ERW (electric resistance welded) and seamless manufacturing routes are permitted, so a JIS G3444 order can be fulfilled either way unless the purchaser states a preference.
Grade Designations and Typical Applications
JIS G3444 designates grades by tensile strength level. STK stands for Steel Tube for general structure (K for the Japanese word for structure), and the number indicates the minimum tensile strength in N/mm2.
| Grade | Former designation | Minimum tensile strength | Typical applications |
|---|---|---|---|
| STK400 | STK41 | 400 N/mm2 | General structures, scaffolding, temporary works, handrails, small machine frames |
| STK490 | STK50 | 490 N/mm2 | Higher-load members, towers, trusses, larger machine frames |
| STK540 | STK55 | 540 N/mm2 | Heavy machinery, crane structures, high-load columns and lattice members |
Chemical Composition (Maximum, %)
The following ladle-analysis limits are those of the current JIS G3444 edition. The values are the ceilings a mill must hold; actual heats are usually below them.
| Grade | C | Si | Mn | P | S |
|---|---|---|---|---|---|
| STK400 | 0.25 | 0.35 | 1.30 | 0.040 | 0.040 |
| STK490 | 0.18 | 0.55 | 1.65 | 0.035 | 0.035 |
| STK540 | 0.23 | 0.55 | 1.50 | 0.040 | 0.040 |
The STK490 grade keeps carbon low and compensates with manganese, which improves weldability in thick sections. STK540 balances carbon and manganese for strength while staying within limits that permit conventional arc welding with standard electrodes.
Mechanical Properties
Yield strength is specified in two wall-thickness brackets; tensile strength and elongation are common to all thicknesses. Elongation values below relate to the No. 11 proportional test piece; slightly reduced minima apply to small-diameter tubes per the standard's test-piece rules.
| Grade | Yield point min (wall t up to 16 mm) | Yield point min (wall t over 16 mm) | Tensile strength | Elongation min (%) |
|---|---|---|---|---|
| STK400 | 245 N/mm2 | 235 N/mm2 | 400-510 N/mm2 | 23 |
| STK490 | 325 N/mm2 | 315 N/mm2 | 490-610 N/mm2 | 20 |
| STK540 | 400 N/mm2 | 390 N/mm2 | 540-660 N/mm2 | 16 |
These figures make STK400 the workhorse for scaffolding and general building works, STK490 the usual choice for structural frames and towers, and STK540 the grade for heavily loaded members where the extra 40-95 N/mm2 of yield over STK490 pays for itself in section weight.
Manufacturing, Testing and Inspection
Welded tubes are produced by high-frequency electric resistance welding from hot-rolled strip or sheet; the weld zone is normally post-treated to remove the external bead, and the seam is verified by eddy-current or ultrasonic testing.
Seamless tubes are hot-finished from solid billets by rotary piercing and elongation rolling.
Routine tests per JIS G3444 include the tensile test, flattening test, bend test, and a hydrostatic test - or an eddy-current test in lieu of hydrostatic where agreed - plus dimensional and visual checks.
Each heat is released with a mill test certificate stating the chemical analysis and mechanical test results; the tube is marked with the grade, outside diameter and wall thickness, the manufacturer's name or mark, and the heat or lot identification.
Equivalents and Related Standards
JIS G3444 grades are frequently cross-referenced to European and American structural tube standards. The table below is an indicative engineering cross-reference, not a one-to-one substitution; confirm acceptability against the project specification and the governing design code before replacing one standard with another.
| JIS G3444 | EN 10219 / EN 10210 | ASTM A500 | Approximate GB counterpart |
|---|---|---|---|
| STK400 | S235JR | Gr B | Q235B |
| STK490 | S275JR | Gr C | Q275 |
| STK540 | S355JR | - | Q355B |
For square and rectangular hollow sections the parallel Japanese standard is JIS G3466 (STKR400, STKR490, STKR540). Do not confuse JIS G3444 with JIS G3445, which covers carbon steel tubes for machine structures (STKM grades), or with the pressure-service standards JIS G3454 and JIS G3461.
What Buyers Should Specify on a JIS G3444 Order
Standard and edition (for example JIS G3444:2021), grade, outside diameter, wall thickness, and length (fixed or random).
Manufacturing route if the project requires it: welded (ERW) or seamless.
Surface condition: JIS G3444 is a black-pipe standard; for outdoor structures specify hot-dip galvanizing as an additional treatment (commonly to JIS H8641 or ASTM A123), which is applied after fabrication of the member where possible.
End finish (plain or beveled), and whether the pipe must be delivered cut to exact scaffold lengths.
Documentation: mill test certificate per heat, and third-party inspection if the contract requires it.
Packaging and bundling for container shipment, including end caps to protect threads or bevels.
Our in-house QC verifies every heat against the chemical and mechanical limits above before release, and retains test records for traceability on request.
Frequently Asked Questions
Is JIS G3444 pipe welded or seamless?
Both. JIS G3444 covers electric-resistance-welded tubes and seamless tubes of carbon steel. The purchaser should state the preferred route because the two forms have different availability, price and weld-seam characteristics.
What does STK mean in JIS G3444?
STK is the standard's grade prefix for carbon steel tubes for general structure, and the number is the minimum tensile strength in N/mm2: STK400, STK490 and STK540 for the three current grades.
What is JIS G3444 equivalent to in European or American standards?
Indicatively, STK400 corresponds to EN 10219/10210 S235JR and ASTM A500 Gr B; STK490 to S275JR and ASTM A500 Gr C; STK540 to S355JR. Treat the comparison as an engineering cross-reference and verify against the design specification.
What size range does JIS G3444 cover?
Outside diameters from 21.7 mm to 1016.0 mm, with wall thicknesses per the standard's size table. A very common stocked size is 48.6 mm outside diameter with 2.4 mm or 3.2 mm wall, used for scaffolding and temporary works.
Does JIS G3444 include galvanized pipe?
No. The standard covers black carbon steel tubes. Galvanizing is a separate treatment normally specified to JIS H8641 or ASTM A123, and it is often applied to JIS G3444 scaffold and fence tubes for outdoor exposure.
Can JIS G3444 pipe be used for pressure or boiler service?
No. JIS G3444 is a structural standard; it has no pressure rating. For pressure piping, boiler tubes or machine-structure applications, select the appropriate standard such as JIS G3454, JIS G3455, JIS G3461, or ASTM A106/A53, which define the required hydrostatic and elevated-temperature properties.
How is JIS G3444 pipe marked and documented?
Tubes are marked with the grade, size, manufacturer's name or mark and heat identification. Each delivery is accompanied by a mill test certificate with the chemical analysis and mechanical test results for the heats supplied.
