Manufacturing process of cold rolled steel pipe
Cold rolling, done often at room temperature, has the added effect of work hardening and strengthening the material thus further improving the material’s mechanical properties. It also improves the surface finish and holds tighter tolerances. However, room temperature steel is less malleable than hot steel, so cold rolling cannot reduce the thickness of a work piece as much as hot rolling in a single pass. Commonly cold-rolled products often include similar hot rolled products like sheets and bars, but are usually smaller.
Heat treatment is an important part in cold drawing and cold rolling of steel pipes. During the production process, heat treatment of steel pipes can be classified into tube billets heat treatment, intermediate tube heat treatment and finished tube heat treatment according to the process. Methods, purposes and scope of application of heat treatment for steel pipes are as the following table.
|Heat treatment||Heating temperature||Purposes||Suitable steel||Scope of application|
From 20 to 40℃
|Refining grains, eliminating internal stress, network and Widmannstatten structures and reducing hardnes||Hyposteel||Heat treatment of tube billets and finished pipes|
From Ac1 to Ac3
From780 to 800℃
From Ac1 to Acm
From750 to 770℃
Improving dispersion of pearlite, eliminating internal stress, preventing decarburization and adopting slow cooling rate can obtain spherical pearlite
||For hyper-eutectoid steel||Intermediate heat treatment, finished pipe heat treatment and spheroidizing heat treatment|
From 600 to 700℃
Eliminating internal stress and work hardening, reducing hardness and restoring plasticity
|All kinds of carbon steel and low alloy steel||Intermediate heat treatment|
From 30 to 50℃
Improving the machinability of mild steel and mechanical properties, refining austenite grains and eliminating net carbide
||Hyposteel||Finished pipe heat treatment|
|Quenching||Eliminating work hardening and internal stress||Intermediate heat treatment and finished pipe heat treatment for stainless steel pipes|
|Quenching and high temperature tempering||Normalizing and high temperature tempering and annealing and tempering||Eliminating internal stress, improving toughness of the material and obtaining high strength and hardness||Low alloy structural steel||
Finished pipe heat treatment
What are the purpose and methods of pickling in the processing of cold-rolled steel pipes?
The oxide scale on the inner and outer surfaces of pipe billets should be removed before the cold drawn and cold roll processes. Methods of removing the oxide scale include mechanical methods such as shot blasting and polishing, chemical methods and electrochemical methods. The pickling method is used for removing the oxide scale on the surface of cold drawn and cold roll steel pipes. Pickling methods include sulfuric acid pickling, hydrochloric acid pickling, mixed acid pickling, hydrofluoric acid pickling and alkali-acid pickling. Sulfuric acid pickling is one of the most widely used methods. Sulfuric acid pickling is usually adopted for carbon steel and low alloy structural steel which don’t have special requirements.
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry (www.ugsteelmill.com)