The production process for zinc strip pipes has been designed to be “highly efficient and low-cost.” The core process is divided into four main steps, which differ significantly from the conventional hot-dip galvanized steel pipes:
1. Preparation of the base material: Pre-galvanized steel strips are selected.
Base material type: Hot-dip galvanized steel strip is continuously certified (thickness: 0.8 mm – 3.0 mm). A layer of pure zinc (partially containing a zinc-iron alloy layer) with a thickness ranging from 10 micrometers to 50 micrometers has been pre-formed on the surface of the steel strip. The quality of the zinc coating shall comply with the standard GB/T 2518-2019 «Continuous Hot-Dip Galvanized Steel Sheets and Strips Coated with Zinc-Alloy».
Steel Strip Selection: Choose the steel strip material based on the application of the steel pipe (mostly low-carbon steels Q195 and Q235). Low-carbon steel boasts excellent weldability and ductility, making it suitable for rolling into a circular shape. At the same time, select the zinc coating thickness according to the service environment (typically 10 to 20 micrometers under normal conditions, and 30 to 50 micrometers in cases of mild corrosion).
2. Forming and Welding: High-Frequency Welding for Pipe Production
Rolling into a circular shape: Galvanized steel strips are gradually rolled into a circular tubular shape by a continuous rolling mill. The rolling process requires precise control of curvature to ensure that the roundness of the steel pipe meets the specifications (and to avoid an elliptical shape).
High-Frequency Welding: This process employs high-frequency induction welding technology (frequency range: 100 kHz – 500 kHz), in which the heat generated by the high-frequency current melts the edges of the joint areas of the steel strip ends, eliminating the need for additional welding wires and enabling direct welding into the pipes. The welding speed is remarkably high—reaching up to 10–30 meters per minute—and the weld joint features a dense fusion zone, ensuring a tight and leak-proof performance of the pipeline.
Online leveling: After welding, the steel pipe passes through a leveling machine to correct weld distortions (such as bending and elliptical deviation), ensuring the straightness of the steel pipe (bending degree per meter ≤ 1 mm).
3. Cutting and trimming: to the desired length
Cutting to a fixed length: According to customer requirements (common lengths are 6 meters, 9 meters, and 12 meters), use a flying saw machine to cut the steel pipe online. The cut surface must be smooth and free of burrs (to avoid scratching sealing parts during installation).
Weld Joint Finishing: For certain high-end products, the weld joint area will be polished to remove any excess height of the weld joint, resulting in a smoother pipe surface. At the same time, the quality of the weld joint will be inspected (ensuring there are no false welds or cracks).
4. Quality inspection: Monitoring key indicators
Visual inspection: Check whether the zinc coating on the surface of the steel pipe is uniform (with no exposed iron or black spots), whether the weld area is continuously connected (with no missing welds), and whether the surface of the cut is smooth.
Dimensional Inspection: Use calipers to measure the outer diameter, wall thickness, and length. The deviations must comply with the GB/T 3091-2015 standard “Welded Steel Pipes for Low-Pressure Fluid Transport” (e.g., outer diameter deviation ±0.5 mm, wall thickness deviation ±0.3 mm).
Sealing Performance Test: Conduct a hydrostatic test on the pipeline (test pressure ≥ 0.6 MPa), maintaining it for 10 to 30 seconds without leakage to ensure the sealing performance of the weld joint.