GB/T 31315-2014Cold drawn or cold rolled precision weld steel tubes for mechanical structures (English PDF)

5.1 Outside diameter, inside diameter and wall thickness

5.1.1 Steel pipes are usually delivered in outside diameter and wall thickness.

When the buyer requires delivery in outside diameter and inside diameter OR inside diameter and wall thickness, it should be indicated in the contract.

5.2 Length

5.2.1 Normal length

The normal length of steel tube is 3000 mm ~ 12000 mm.

5.2.2 Fixed length and double length

According to the requirements of the buyer, after negotiation of the seller and the buyer and specified in the contract, steel tubes can be delivered in fixed length or double length.

5.2.3 Other requirements

According to the requirements of the buyer, after negotiation of the seller and the buyer and specified in the contract, steel tubes of other length and tolerance can be supplied.

5.3 Degree of curvature

5.3.1 The degree of curvature should not exceed 1.5 mm/m.

5.4 Head shape

Both ends of the steel tube should be perpendicular to the tube axis. The incision burr should be removed.

5.5 Non-roundness and uneven wall thickness

The non-roundness and uneven wall thickness of steel tube should not be greater than 80% of the outside diameter tolerance and wall thickness tolerance respectively.

5.6 Delivery weight

Steel tubes can be delivered in actual weight or theoretical weight. The theoretical weight per meter of steel tube shall be in accordance with GB/T 17395 (the density of the steel is 7.85 kg/dm3).

6.1 Steel designation and chemical composition

6.2 Manufacturing method

6.2.1 Steel smelting method

The steel should be the fully killed steel which uses electric furnace or oxygen converter smelting.

6.2.2 Manufacturing method of steel tube

The steel tube should be made by cold drawing or cold rolling or other suitable methods via welding tube.

6.3 Delivery state

The steel tube shall be delivered in either of the states listed in Table 3.

6.5 Process performance

6.5.1 Flattening test

Steel tubes with a wall thickness to outer diameter ratio of less than 15% in the +A and +N states shall be subjected to flattening test. The distance H between plates after specimen flattening is calculated according to equation (1).

6.7 Surface roughness

When the buyer has requirements for surface roughness of steel tube, it should be indicated in the contract. The parameters of surface roughness shall be determined by the contour arithmetic mean deviation Ra specified in GB/T 1031.

Its surface roughness value and the sampling length at the time of measurement shall be determined by the seller and the buyer.

6.8 Surface quality

6.8.1 The inner and outer surfaces of the steel tube should be smooth. The depth of the local pits, scratches and small scratches should not exceed 0.08 mm. The actual wall thickness of these defects shall be not less than the minimum value allowed for wall thickness deviation.

cold rolled state are allowed to have phosphate and lubricant adhesion layers from the manufacturing process.

6.8.4 The inner and outer surfaces of steel tubes delivered by heat treatment state are allowed to have oxide film layer which does not affect surface examination but not loose oxide skin.

8.1 Inspection and acceptance

The inspection and acceptance of the steel tube shall be carried out by the quality and technical supervision department of the seller.

8.2 Batching rules

The steel tubes shall be inspected and accepted in batches. Each batch should consist of steel tubes of same designation, of same furnace number, of same specification and of same heat treatment system (furnace number) or of same delivery state. The quantity of each batch of steel tubes shall not exceed the following provisions.

8.3 Re-inspection and determination rules

The re-inspection and determination rules of steel tubes should comply with the provisions of GB/T 2102.