Length description and mechanical properties of large-diameter steel pipes

The main processing methods for large-diameter steel pipes are:

1. Forging steel: a pressure processing method that uses the reciprocating impact force of the forging hammer or the pressure of the press to change the blank into the shape and size we need.

2. Extrusion: It is a processing method in which the metal is placed in a closed extrusion cylinder, and pressure is applied at one end to extrude the metal from the specified die-hole to obtain a finished product with the same shape and size. It is mostly used to produce non-ferrous metal steel.

3. Rolling: A pressure processing method in which the steel metal billet passes through the gap (various shapes) of a pair of rotating rollers, and the material cross-section is reduced and the length is increased due to the compression of the rollers.

4. Drawing steel: It is a processing method in which the rolled metal billet (profile, tube, product, etc.) is pulled through the die hole to reduce the cross-section and increase the length. Most of them are used for cold processing. Large-diameter steel pipes are mainly completed by tension reduction and continuous rolling of hollow base materials without mandrels. The standard document for the production of large-diameter steel pipes shows that deviations are allowed when manufacturing large-diameter steel pipes:

5. Allowable length deviation: The allowable length deviation of the steel bar when delivered in fixed length shall not exceed +50mm.

6. Curvature and end: The bending of straight steel bars should not affect normal use, and the total curvature should not exceed 40% of the total length of the steel bar; the ends of the steel bars should be sheared straight, and local deformation should not affect use.

7. Length: Steel bars are usually delivered in fixed lengths, and the specific delivery length should be specified in the contract; when steel bars are delivered in coils, each coil should be a steel bar, and 5% of the coils in each batch are allowed to consist of two steel bars. The coil weight and coil diameter are determined by negotiation between the supply and demand parties.

Explanation of the length of large-diameter steel pipes:

1. Normal length (also known as non-fixed length): Any length within the length range specified in the standard and without a fixed length requirement is called normal length. For example, the standard for structural pipes stipulates: hot-rolled (extruded, expanded) steel pipes 3000mm~12000mm; cold-drawn (rolled) steel pipes 2000mm~10500mm.

2. Fixed length: The fixed length should be within the usual length range and is a fixed length required in the contract. However, it is unlikely to cut out the fixed length in actual operation, so the standard stipulates the allowable positive deviation value for the fixed length.

3. Multiple lengths: The multiple lengths should be within the usual length range, and the contract should indicate the single multiple lengths and the multiple of the total length (for example, 3000mm×3, that is, 3 times 3000mm, the total length is 9000mm). In actual operation, the allowable positive deviation of 20mm should be added to the total length, and the cut margin should be left for each single multiple length. If there is no multiple length deviation and cutting margin in the standard, it should be negotiated by the supply and demand parties and indicated in the contract. The double-length scale, like the fixed length, will greatly reduce the yield rate of the production enterprise. Therefore, it is reasonable for the production enterprise to raise the price, and the price increase is basically the same as the price increase of the fixed length.

4. Range length: The range length is within the usual length range. When the user requires a fixed range length, it must be stated in the contract.

Mechanical properties of large-diameter steel pipes:

1. Tensile strength: The stress (σ) obtained by dividing the large force (Fb) that the sample bears when it is broken during the stretching process by the original cross-sectional area (So) of the sample is called tensile strength (σb), and the unit is N/mm2 (MPa). It indicates the maximum ability of metal materials to resist damage under tension.

2. Yield point: For metal materials with yield phenomenon, the stress when the sample can continue to stretch without increasing force (maintaining constant) during the stretching process is called the yield point. If the force decreases, the upper and lower yield points should be distinguished. The unit of yield point is N/mm2 (MPa).

3. Elongation after fracture: In the tensile test, the percentage of the length increase of the gauge length after the specimen is broken to the original gauge length is called elongation. It is represented by σ and the unit is %.