JIS G 3444 is a Japanese Industrial Standard that specifies carbon steel round (circular cross-section) tubes for general structural purposes. These tubes are widely used in building construction, bridges, steel towers, scaffolding, foundation piles, and other civil engineering and architectural applications. Key characteristics include adequate strength, good weldability, and reliable performance under static loading.
The standard is a single document, but its main technical provisions can be grouped as follows:
- Chemical composition requirements
- Mechanical properties
- Dimensional tolerances
- Inspection and testing methods
JIS G 3444 is essential for engineers and fabricators working with circular structural steel tubes in Japan or projects referencing Japanese standards, ensuring compliance with local regulations and clear specifications for yield strength, tensile strength, elongation, and quality control.
Scope of Application
JIS G 3444 applies to carbon steel circular (round) tubes for general structural purposes. It covers tubes manufactured by seamless, electric resistance welded (ERW), furnace welded (FW), and automatic arc welding processes. The standard is intended for structural frameworks, support members, trusses, and similar applications.
Product shape: The scope of JIS G 3444 is limited to circular tubes. Square and rectangular hollow sections are not covered by this standard. The Japanese Industrial Standard for square and rectangular structural tubes is JIS G 3466 (STKR).
Exclusions: The standard does not apply to tubes for pressure service, high-temperature applications, or fluid transport under pressure.
Temperature limitation: Static loading only; dynamic loads require additional verification; recommended operating range: -30°C to 120°C.
In summary, JIS G 3444 is the specification for round structural carbon steel tubes, while JIS G 3466 (STKR) is the complementary standard for square and rectangular structural hollow sections.
Classified Steels (Grades)
JIS G 3444 classifies steel tubes into several grades based on their tensile strength. The designation prefix "STK" stands for Steel Tube for Kōzō (structure in Japanese). The most common grades are:
STK290
low strength, used for light structures
01
STK400
general structural grade
02
STK490
higher strength for medium to heavy structures
03
STK500
high strength for demanding applications
04
STK540
highest strength grade
05
These grades are all non-alloy carbon steels. The number indicates the minimum tensile strength in N/mm² (MPa).
Designation System (Example: STK400)
The designation of a steel tube according to JIS G 3444 consists of:
- The letters STK
- A three-digit number indicating the minimum tensile strength in N/mm² (MPa)
For example: STK400
- STK – Steel tube for structural purposes
- 400 – Minimum tensile strength of 400 N/mm²
Unlike EN 10210, no letters for impact properties (e.g., J0, N, H) are used in JIS G 3444.
Chemical Composition Requirements (Ladle Analysis)
The following table summarises the maximum allowable chemical composition for the most common grades, according to JIS G 3444:
| Grade | C≤ | Si≤ | Mn≤ | P≤ | S≤ | Special Requirements |
| STK290 | – | – | – | 0.05 | 0.05 | – |
| STK400 | 0.25 | – | – | 0.04 | 0.04 | – |
| STK490 | 0.18 | 0.55 | 1.65 | 0.035 | 0.035 | Low-carbon design for weldability |
| STK500 | 0.24 | 0.35 | 0.30-1.30 | 0.04 | 0.04 | – |
| STK540 | 0.23 | 0.55 | 1.5 | 0.04 | 0.04 | Custom agreement for t>12.5mm |
Note: STK290 has no specified carbon, silicon, or manganese limits other than low carbon content. STK490 features low carbon (≤0.18%) for optimal weldability.
Mechanical Properties
| Symbol of Grade |
Tensile Strength N/mm2 |
Yield Point or Proof Stress N/mm2 |
Weld Tensile N/mm2 |
Flattening Resistance Strength |
Bendability | |
| Distance between Flat Plates(H) |
Bending Angle,a) |
Inside Radius |
||||
| Applicable Outside Diameter | ||||||
| All Outside Diameters |
All Outside Diameters |
All Outside Diameters |
All Outside Diameters |
50 mm or under | ||
| JIS G3444 STK 290 |
290 min. | - | 290 min. | 2/3D | 90° | 6D |
| JIS G3444 STK 400 |
400 min. | 235 min. | 400 min. | 2/3D | 90° | 6D |
| JIS G3444 STK 490 |
490 min. | 315min. | 490 min. | 7/8D | 90° | 6D |
| JIS G3444 STK 500 |
500 min. | 355 min. | 500 min. | 7/8D | 90° | 6D |
| JIS G3444 STK 540 |
540 min. | 390 min. | 540 min. | 7/8D | 90° | 6D |
| NOTE 1 In this table,D stands for the outside diameter of the tube.NOTE 2 1N/mm 2=1 MPa Note a) The bending angle shall be measured from the starting point of the bend. |
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- Yield strength is the lower yield point (or 0.2% proof stress for seamless tubes).
- Elongation is measured using a gauge length of 5.65√S₀ (proportional specimen) or 50 mm for standard test pieces.
Dimensional Tolerances
Tolerances on outside diameter, thickness, straightness, and mass are specified in JIS G 3444. Key typical tolerances include:
|
Parameter |
Tolerance (typical) |
|
Outside diameter (D) |
Varies by size; refer to JIS G 3444 tables |
|
Thickness (t) |
As specified per standard |
|
Straightness |
≤0.20% of total length |
| - Outside Diameter Tolerance a) | |||
| Classification | O.D. b) | Tolerances on O.D. c) |
Applicable Division |
| Class 1 | Under 50mm | ±0.5mm | Applied when there is no specification.Applicable to hot-finished seamless steel tubes. |
| 50mm or Over | ±1% | ||
| Class 2 | Under 50mm | ±0.25mm | Applied when there is a specification.Not applicable to hot-finished seamless steel tubes. |
| 50mm or Over | ±0.5% | ||
| Notes: a)For the local parts such as repaired pat if the wall thickness can be cofirmed to be within the tolerance in the table below,the outside diameter tolerance of this table need not be applied. b)The method for measuring the outside diameter of tubes exceeding 350mm in outside diameter may be determined by the circumferential length.The mutual conversion of outside diameter D and the circumferential length l shall be by the following formula: D = l / π Where D: outside diameter (mm) l: circumferential length(mm) π：3.1416 c) The tolerances on the outside diameter of ends of electric resistance welded and automatic arcweldedsteeltubes exceeding 350mm in outside diameter shall be ±0.5%. |
|||
| Classification | For Seamless Steel Tube | For Other than Seamless Steel Tube |
Applicable Division | ||
| Wall Thickness | Tolerance of Wall Thickness |
Wall Thickness |
Tolerance of Wall Thickness |
||
| Class 1 | Under 4mm | +0.6mm, -0.5mm |
Under 4mm | +0.6mm, -0.5mm |
Applied when there is no specification. Applicable to hot-finished seamless steel tubes. |
| 4mm or Over | +15%,-12.5% | 4mm or Over to and excl. 12mm |
+15%,-12.5% | ||
| 12mm or Over | +15%,-1.5mm | ||||
| Class 2 | Under 3mm | ±0.3mm | Under 3mm | ±0.3mm | Applied when there is a specification.Not applicable to hot-finished seamless steel tubes. |
| 3mm or Over | ±10% | 3mm or Over to and excl. 12mm |
±10% | ||
| 12mm or Over | +10%,-1.2mm | ||||
| Notes: a)The tolerances on the wall hickness of tubes exceeding 1016.0 mm in outside diameter may be agreed upon between the manufacturer and the purchaser. |
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*Note: Since JIS G 3444 covers only circular tubes, corner angle tolerances for square/rectangular shapes do not apply.*
| Nominal Size |
Outside Diameter |
Thickness (t) |
Weight | Cross Sectional Area |
Geometical Moment of rerte |
Modulus of Section |
Radius of Gyration |
| mm. | mm. | mm. | kg/m | cm² | cm⁴ | cm³ | cm |
| 15 | 21.7 | 2.00 | 0.972 | 1.238 | 0.607 | 0.560 | 0.700 |
| 20 | 27.2 | 2.00 | 1.24 | 1.583 | 1.26 | 0.930 | 0.890 |
| 2.30 | 1.41 | 1.799 | 1.41 | 1.03 | 0.880 | ||
| 25 | 34.0 | 2.30 | 1.80 | 2.291 | 2.89 | 1.70 | 1.12 |
| 32 | 42.7 | 2.30 | 2.29 | 2.919 | 5.97 | 2.80 | 1.43 |
| 2.50 | 2.48 | 3.157 | 6.40 | 3.00 | 1.42 | ||
| 40 | 48.6 | 2.30 | 2.63 | 3.345 | 8.99 | 3.70 | 1.64 |
| 2.50 | 2.84 | 3.621 | 9.65 | 3.97 | 1.63 | ||
| 2.80 | 3.16 | 4.029 | 10.6 | 4.36 | 1.62 | ||
| 3.20 | 3.58 | 4.564 | 11.8 | 4.86 | 1.61 | ||
| 50 | 60.5 | 2.30 | 3.30 | 4.205 | 17.8 | 5.90 | 2.06 |
| 3.20 | 4.52 | 5.760 | 23.7 | 7.84 | 2.03 | ||
| 4.00 | 5.57 | 7.100 | 28.5 | 9.41 | 2.00 | ||
| 65 | 76.3 | 2.80 | 5.08 | 6.465 | 43.7 | 11.5 | 2.60 |
| 3.20 | 5.77 | 7.349 | 49.2 | 12.9 | 2.59 | ||
| 4.00 | 7.13 | 9.085 | 59.5 | 15.6 | 2.58 | ||
| 80 | 89.1 | 2.80 | 5.96 | 7.591 | 70.7 | 15.9 | 3.05 |
| 3.20 | 6.78 | 8.636 | 79.8 | 17.9 | 3.04 | ||
| 90 | 101.6 | 3.20 | 7.76 | 9.892 | 120 | 23.6 | 3.48 |
| 4.00 | 9.63 | 12.26 | 146 | 28.8 | 3.45 | ||
| 5.00 | 11.9 | 15.17 | 177 | 34.9 | 3.42 | ||
| 100 | 114.3 | 3.20 | 8.77 | 11.17 | 172 | 30.2 | 3.93 |
| 3.50 | 9.56 | 12.18 | 187 | 32.7 | 3.92 | ||
| 4.50 | 12.2 | 15.52 | 234 | 41.0 | 3.89 | ||
| 125 | 139.8 | 3.60 | 12.1 | 15.40 | 357 | 51.1 | 4.82 |
| 4.00 | 13.4 | 17.07 | 394 | 56.3 | 4.80 | ||
| 4.50 | 15.0 | 19.13 | 438 | 62.7 | 4.79 | ||
| 6.00 | 19.8 | 25.22 | 566 | 80.9 | 4.74 | ||
| 150 | 165.2 | 4.50 | 17.8 | 22.72 | 734 | 88.9 | 5.68 |
| 5.00 | 19.8 | 25.16 | 808 | 97.8 | 5.67 | ||
| 6.00 | 23.6 | 30.01 | 952 | 115 | 5.63 | ||
| 7.10 | 27.7 | 35.26 | 1100 | 134 | 5.60 |
| Nominal Size |
Outside Diameter |
Thickness (t) |
Weight | Cross Sectional Area |
Geometical Moment of rerte |
Modulus of Section |
Radius of Gyration |
| mm. | mm. | mm. | kg/m | cm² | cm⁴ | cm³ | cm |
| 175 | 190.7 | 4.50 | 20.7 | 26.32 | 1140 | 120 | 6.59 |
| 5.30 | 24.2 | 30.87 | 1330 | 139 | 6.56 | ||
| 6.00 | 27.3 | 34.82 | 1490 | 156 | 6.53 | ||
| 7.00 | 31.7 | 40.40 | 1710 | 179 | 6.50 | ||
| 8.20 | 36.9 | 47.01 | 1960 | 206 | 6.46 | ||
| 200 | 216.3 | 4.50 | 23.5 | 29.94 | 1680 | 155 | 7.49 |
| 5.80 | 30.1 | 38.36 | 2130 | 197 | 7.45 | ||
| 6.00 | 31.1 | 39.64 | 2190 | 203 | 7.44 | ||
| 7.00 | 36.1 | 46.03 | 2520 | 233 | 7.40 | ||
| 8.00 | 41.1 | 52.35 | 2840 | 263 | 7.37 | ||
| 8.20 | 42.1 | 53.61 | 2910 | 269 | 7.36 | ||
| 250 | 267.4 | 6.00 | 38.7 | 49.27 | 4210 | 315 | 9.24 |
| 6.60 | 42.4 | 54.08 | 4600 | 344 | 9.22 | ||
| 7.00 | 45.0 | 57.26 | 4860 | 363 | 9.21 | ||
| 8.00 | 51.2 | 65.19 | 5490 | 411 | 9.18 | ||
| 9.00 | 57.3 | 73.06 | 6110 | 457 | 9.14 | ||
| 9.30 | 59.2 | 75.41 | 6290 | 470 | 9.13 | ||
| 300 | 318.5 | 6.00 | 46.2 | 58.91 | 7190 | 452 | 11.1 |
| 6.90 | 53.0 | 67.55 | 8200 | 515 | 11.0 | ||
| 8.00 | 61.3 | 78.04 | 9410 | 591 | 11.0 | ||
| 9.00 | 68.7 | 87.51 | 10500 | 659 | 10.9 | ||
| 10.30 | 78.3 | 99.73 | 11900 | 744 | 10.9 | ||
| 350 | 355.6 | 6.40 | 55.1 | 70.21 | 10700 | 602 | 12.3 |
| 7.90 | 67.7 | 86.29 | 13000 | 734 | 12.3 | ||
| 9.00 | 76.9 | 98.00 | 14700 | 828 | 12.3 | ||
| 9.50 | 81.1 | 103.3 | 15500 | 871 | 12.2 | ||
| 12.00 | 102 | 129.5 | 19100 | 1080 | 12.2 | ||
| 12.70 | 107 | 136.8 | 20100 | 1130 | 12.1 | ||
| 400 | 406.4 | 7.90 | 77.6 | 98.90 | 19600 | 967 | 14.1 |
| 9.00 | 88.2 | 112.4 | 22200 | 1090 | 14.1 | ||
| 9.50 | 93.0 | 118.5 | 23300 | 1150 | 14.0 | ||
| 12.00 | 117 | 148.7 | 28900 | 1420 | 14.0 | ||
| 12.70 | 123 | 157.1 | 30500 | 1500 | 13.9 |
For high-quality carbon steel tubes manufactured to JIS G 3444 and other international standards, please contact your trusted steel supplier--LEFIN STEEL.