Cold drawn seamless tube manufacturing process

1.What is cold drawn seamless pipe?

Cold-rolled seamless pipes refer to steel plates or steel strips processed into various types of steel through cold drawing, cold bending, cold drawing and other cold processing at normal temperature. The use of precision seamless pipes to manufacture mechanical structures or hydraulic equipment can greatly save machining hours, improve material utilization, and at the same time help improve product quality.

2. Production process of cold drawn seamless pipes

The production process of cold-drawn seamless pipes mainly includes raw material preparation, billet heating, perforation, rolling, drawing, annealing, pickling, testing and other steps. The following is a detailed introduction:

(1) Raw material preparation

The main raw materials of seamless steel pipes are pig iron, scrap steel and alloy elements with high carbon content. At this stage, various raw materials need to be accurately weighed and appropriate amounts of alloy elements added.

(2) Blank heating

The selected steel billet or steel plate is put into the furnace for heat treatment to achieve a certain degree of plasticity and forgeability.

(3) Perforation

The purpose of perforating the heated steel billet or steel plate is to discharge the internal gas of the steel billet or steel plate and provide conditions for the subsequent rolling and drawing steps.

(4)Rolling

After the piercing is completed, the steel billet or steel plate is put into the rolling mill for preliminary rolling. The purpose is to reduce the cross section of the steel billet or steel plate and provide conditions for the subsequent drawing step.

(5)Drawing

The billet after initial rolling is put into a drawing machine for drawing processing to further reduce the cross-section of the billet and at the same time improve the internal structure and performance of the billet.

(6) Annealing

After drawing is completed, annealing treatment is performed to make the structure and performance of the seamless steel pipe more stable and uniform.

(7) Pickling

In order to remove the oxide scale and rust layer on the surface of the steel pipe, pickling is performed.

(8)Detection

Finally, the seamless steel pipe is comprehensively tested, including appearance quality, mechanical properties, chemical composition, dimensional deviation, etc.

Each of the above steps requires precise control to ensure that the quality of seamless steel pipes meets standard requirements.

3. Cold rolled seamless pipe vs hot rolled seamless pipe

(1) Technology

① Cold-rolled shaped steel allows local buckling of the section, so that the bearing capacity of the rod after buckling can be fully utilized; while hot-rolled shaped steel does not allow local buckling of the section.

② The causes of residual stress in hot-rolled steel and cold-rolled steel are different, so the distribution on the cross-section is also very different. The residual stress distribution on the cold-formed thin-walled steel section is curved, while the residual stress distribution on the hot-rolled steel or welded steel section is film-type.

③The free torsional stiffness of hot-rolled steel is higher than that of cold-rolled steel, so the torsional performance of hot-rolled steel is better than that of cold-rolled steel.

(2) Advantages and disadvantages

①Cold drawn seamless pipe

Advantages: fast forming speed, high output, and does not damage the coating. It can be made into a variety of cross-sectional forms to adapt to the needs of use conditions; cold rolling can cause large plastic deformation of steel, thereby improving the yield of the steel. point.

shortcoming:

a. Although there is no hot plastic compression during the forming process, there are still residual stresses in the section, which will inevitably affect the overall and local buckling characteristics of the steel;

b. Cold-rolled steel sections generally have open sections, which results in lower free torsional stiffness of the section. It is prone to torsion when subjected to bending and torsional buckling when subjected to pressure, and its torsional resistance is poor;

c. The wall thickness of cold-rolled formed steel is small, and there is no thickening at the corners where the plates are connected, so the ability to withstand localized concentrated loads is weak.

②Hot-rolled seamless pipe

Hot-rolled seamless pipes are compared to cold-rolled seamless pipes. Cold-rolled seamless pipes are rolled below the recrystallization temperature, while hot-rolled seamless pipes are rolled above the recrystallization temperature.

advantage:

It can destroy the casting structure of the steel ingot, refine the grains of the steel, and eliminate defects in the microstructure, thereby making the steel structure dense and improving the mechanical properties. This improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to a certain extent; bubbles, cracks and looseness formed during pouring can also be welded under the action of high temperature and pressure.

shortcoming:

a. Stratification phenomenon

After hot rolling, the non-metallic inclusions (mainly sulfides, oxides, and silicates) inside the steel are pressed into thin sheets, resulting in delamination (sandwiching). Delamination greatly deteriorates the tensile properties of the steel along the thickness direction and may cause interlaminar tearing as the weld shrinks. The local strain induced by weld shrinkage often reaches several times the yield point strain, which is much larger than the strain caused by load;

b. Residual stress caused by uneven cooling

Residual stress is the internal self-balanced stress in the absence of external force. Hot-rolled steel sections of various sections have such residual stress. Generally, the larger the cross-section size of the section steel, the greater the residual stress. Although residual stress is self-balanced, it still has a certain impact on the performance of steel components under the action of external forces. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc.

c. Difficult to control thickness and side width

We are familiar with thermal expansion and contraction. Even if the length and thickness are up to standard after hot rolling at the beginning, there will still be a certain negative difference after cooling. The wider the side width and the thicker the thickness, the more obvious this negative difference will be. Therefore, for large-sized steel, the side width, thickness, length, angle, and edge lines of the steel cannot be too precise.

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