GB/T 40316-2021Welded high strength shaped and round steel tubes for automobile structure (English PDF)

4.1 Naming method of steel grade

Steel grades are named according to the specified minimum upper yield strength values, which are divided into 355, 380, 420, 460, 500, 550, 600, 650, 700.

4.2 Classification and code of steel tube precision level

4.2.1 The steel tubes are classified and coded according to the precision level of the outer diameter (side length) as follows:

a) Ordinary level: PD (P A/B).a;

b) Higher-level: PD (P A/B).b;

c) Advanced level: PD(P A/B).c.

4.2.2 The steel tubes are classified and coded according to the wall thickness precision level as follows:

a) Ordinary level: PT.a;

b) Advanced level: PT.c.

4.2.3 The steel tubes are classified and coded according to the precision level of bending degree are as follows:

a) Ordinary level: PS.a;

b) Higher-level: PS.b;

c) Advanced level: PS.c.

4.2.4 The steel tubes are classified and coded according to the prevision level of side roughness:

a) Ordinary level: PF.a;

b) Advanced level: PF.c.

6.1 Dimensions and allowable deviation

6.7.1 Delivery weight

Steel tubes are delivered according to actual weight. If it is stated in the contract, the steel tubes can also be delivered according to the theoretical weight.

6.7.2 Calculation of theoretical weight

With the density of steel of 7.85kg/dm³, the calculation of the theoretical weight per unit length of round steel tubes shall comply with the provisions of GB/T 21835; and the theoretical weight per unit length of the shaped steel tubes shall comply with the provisions of GB/T 6726.

6.7.3 Allowable deviation of weight

The allowable deviation between the actual weight and the theoretical weight of the delivered steel tube is ±5%. According to the request of the purchaser, after negotiation between the supplier and the purchaser and indicated in the contract, other allowable deviations of weight can be specified.

7.1 Designation and chemical composition of steel

7.1.1 The steel grade and chemical composition (melting analysis) of the steel shall comply with the provisions of Table 8. The supplier and the purchaser shall determine the steel designation through negotiation on the steel grade when ordering. According to the requirements of the buyer, after negotiation between the supplier and the purchaser and indicated in the contract, steel tubes of other strength grades and/or chemical compositions can be supplied.

7.1.2 The allowable deviation of the chemical composition of the finished steel tube shall comply with the provisions of GB/T 222.

7.6.1 Flattening

7.6.1.1 The round steel tube shall be subjected to a flattening test. The length of the flattened sample shall be no less than 63.5mm. The weld-seams of the two samples shall be located at 90° and 0° from the direction of force. The sample shall be pressed until the distance between the two plates is 2/3 of the outer diameter of the steel tube. After the test, the sample should not show cracks or clefts.

7.6.1.2 Shaped steel tubes shall be subjected to flattening test. The length of the flattened sample shall be no less than 63.5mm, and the test method shall comply with the provisions of Appendix C. The sample shall be pressed until the distance between the two plates is 2/3 of the vertical height (C) or the diagonal height (D). After the test, the sample shall not show cracks or clefts.

7.6.2 Bending

For round steel tubes with an outer diameter no greater than 60.3mm, a bending test can be taken to replace the flattening test. There shall be no filler during the bending test, the radius of the bending centre (bending die) is 6 times the outer diameter of the steel tube; the bending angle is 90°; and the weld-seam is located at the maximum tensile deformation of the bending. After the test, the sample shall not show cracks or clefts.

7.6.3 Flaring

According to the request of the purchaser, after negotiation between the supplier and the purchaser, and indicated in the contract, the round steel tube shall be subjected to the flaring test. The top centre taper of the flaring test is one of 30°, 45° or 60°; and the flaring rate of the outer diameter of the sample shall be 6%. After the test, the sample shall not show cracks or clefts.

7.7 Non-destructive testing

7.7.1 Round steel tubes shall be subjected to non-destructive testing according to one of the following methods:

a) Carry out ultrasonic testing according to the provisions of GB/T 5777-2019, and the acceptance level is U4;

b) Carry out eddy current testing according to the provisions off GB/T 7735-2016, and the acceptance level is E4H or E4.

7.7.2 According to the request of the purchaser, after negotiation between the supplier and the purchaser, and indicated in the contract, the shaped steel tube shall be subjected to the magnetic flux leakage test. During the magnetic flux leakage test, the acceptance level shall comply with the provisions of F4 in GB/T 12606-2016.

7.8 Surface quality

7.8.1 Surface defects

There shall be no visible cracks, scars, folding, delamination, lap welding and other defects on the inner and outer surfaces of the steel tube. These deficiencies shall be completely removed; the depth of removal shall not exceed the lower deviation of the wall thickness; and the actual wall thickness of the removal location shall be no less than the minimum allowable wall thickness.

7.8.2 Weld-seam

7.8.2.1 The burrs on the outer weld seam of the steel tube shall be removed and flattened.

7.8.2.2 According to the request of the purchaser, after negotiation between the supplier and the purchaser, and specified in the contract, round steel tubes with an outer diameter greater than 25mm or shaped steel tubes with a circumference greater than 100mm can be delivered without internal burrs.

7.8.2.3 When steel tubes are delivered with internal burrs removed, the reinforcement of the inner weld-seams shall comply with the provisions of Table 11; and after removing the inner weld-seam, the remaining wall thickness of the steel tubes shall be no less than the minimum allowable wall thickness. When the purchaser does not specify the reinforcement precision level of the inner weld-seam of the steel tube in the contract, it shall comply with the provisions of the ordinary level.

B.1 Torsion value

Due to manufacturing reasons, the steel tube rotates into a spiral around its longitudinal axis and produces a torsion angle. The maximum deviation distance per unit length is called the torsion value, and the unit is millimetres per meter (mm/m).

B.2 Principle

If the steel tube is twisted, within a unit length, one end of its side will deviate from the plane formed by the other three end points, and this deviation distance is the torsion value.

B.3 Measurement procedures

Take two steel plates whose side length is greater than that of the steel tube under test; and fix them on a standard flat plate with a parallel spacing of 1m, which is taken as a simple measuring tool for measuring the torsion value of the steel tube. When measuring, cover the measuring tool on the surface of the steel tube to be measured (the direction of the measuring tool is consistent with the direction of the steel tube); so that one end of the measuring tool is close to the steel tube plane. If there is torsion, the other end of the measuring tool forms an angle with the steel tube plane; use a feeler gauge to measure the maximum distance H of this angle, the value of which is the torsion value of the steel tube. The angle alpha can also be measured with a protractor, and the torsion value H is calculated according to Formula (B.1). The schematic diagram of the torsion value determination is shown in Figure B.1.

Where: H - torsion value, in mm/m; alpha - torsion angle per meter, in °; A - side length of the steel tube, in mm; R - outer arc radius, in mm. […]