JIS G3444

The Japanese Industrial Standard JIS G3444 is the standard for Carbon steel pipes used in general structural and low-pressure applications such as engineering, architecture, piles, construction, and other general purpose applications. Carbon steel pipes are also known as black steel pipes. The JIS G3444 standard covers the dimensions, tolerances, test methods, and materials for Carbon steel pipes used in general structural and low-pressure applications. The standard does not cover Carbon steel pipes used in high-pressure applications.

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Steel Grade of JIS G3444 Pipes

Classification and Symbols of JIS G3444 Steel Pipes

The JIS G3444 tubes shall be classified into five grades, and symbols of grade and manufacturing method shall be as given in below.

Symbol of Grade	Symbol of Manufacturing Method
	Tube Manufacturing Method	Finishing Method	Marking
STK 290	Seamless: S Electric resistance welding: E Butt welded: B Automatic arc welded: A	Hot finished: H Cold finished: C As electric resistance welded: G	Hot finished seamless steel tube: -S-H Cold finished seamless steel tube: -S-C As electric resistance welded steel tube: -E-G Hot finished ERW steel tube: -E-H Cold finished ERW steel tube: -E-C Butt welded steel tube: -B Automatic arc welded steel tube: -A
STK 400
STK 490
STK 500
STK 540
1. Dimensions shall be marked by outside diameter and wall thickness. 2. Manufacturer’s name or its identifying brand 3. Symbol indicating the type of coating (in the case of using coated steel sheet or coated steel strip). This abbreviation for this symbol shall be as agreed between the purchaser and the manufacturer.

Chemical Composition of JIS G3444

The tubes must be examined as per the standard, and the heat analysis results must match those in the table below. When the customer requests a product analysis, the tube must be tested as per the standard and the acquired product analysis values must meet the criteria in Table within the tolerances specified in Table of JIS G 0321.

Symbol of Grade	C	Si	Mn	P	S
JIS G3444 STK 290	–	–	–	0.050max.	0.050max.
JIS G3444 STK 400	0.25max.	–	–	0.040max.	0.040max.
JIS G3444 STK 490	0.18max.	0.55max.	1.65max.	0.035max.	0.035max.
JIS G3444 STK 500	0.24max.	0.35max.	0.30 to 1.30	0.040max.	0.040max.
JIS G3444 STK 540	0.23max.	0.55max.	1.50max.	0.040max.	0.040max.
Notes: 1. If necessary, other alloy elements than those given in this table may be added. 2. For STK540, the chemical composition of tubes of wall thickness over 12.5 mm may be agreed upon between the purchaser and the manufacturer.

Mechanical Properties of JIS G3444

- Tensile Strength and Yield Point or Proof Stress

To ensure the quality of steel tubes, they must be made with high-quality steel sheets or strips. These materials must then be tested to confirm their tensile strength, yield point, and proof stress. The tensile strength in the weld zone is also an important factor, and this must be confirmed as well. However, if the purchaser approves, the test for tensile strength in the weld may be omitted. All of these factors are important in ensuring that steel tubes are of the highest quality and will be able to withstand any stress or weight that is placed upon them.

Symbol of Grade	Tensile Strength N/mm2	Yield Point or Proof Stress N/mm2	Weld Tensile N/mm2	Flattening Resistance Strength	Bendability
	Tensile Strength N/mm2	Yield Point or Proof Stress N/mm2	Weld Tensile N/mm2	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.	315 min.	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 1 N/mm 2 = 1 MPa Note a) The bending angle shall be measured from the starting point of the bend.

- Flattening

The flattening test is a method of testing for flaws or cracks in seamless steel tubes, electric resistance welded steel tubes, or butt welded steel tubes. The test involves placing the tube between two plates and applying pressure to the top plate. The distance between the plates is regulated to ensure that the test is accurate. The flattening test is an important part of quality control for steel tubes, and it helps to ensure that the tubes are free from defects.

- Bendability

Tubes that are 50 mm or under in outside diameter can be specified for their bendability instead of their ability to be flattened. Other tubes must be tested for bendability by the standard, and the test piece must be free from flaws or cracks. The acceptable bend angle and an inside radius of bending are specified in the table above. This ensures that the tubes meet the necessary safety standards and can withstand the specified amount of pressure.

Dimensional Tolerances of JIS G3444

- Outside Diameter Tolerance a)

Classification	Outside Diameter b)	Tolerances on Outside Diameter 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 parts if the wall thickness can be confirmed 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 arc welded steel tubes exceeding 350mm in outside diameter shall be ±0.5%。

- Wall Thickness Tolerance a)

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 thickness of tubes exceeding 1016.0 mm in outside diameter may be agreed upon between the manufacturer and the purchaser.