What is seamless steel pipe?

What is seamless steel pipe?

Seamless steel pipes are made of a whole round steel perforated. Steel pipes with no welds on the surface are called seamless steel pipes. According to the production method, seamless steel pipes can be divided into hot-rolled seamless steel pipes, cold-rolled seamless steel pipes, cold-drawn seamless steel pipes, extruded seamless steel pipes, jacked pipes, etc. According to the cross-sectional shape, seamless steel pipes are divided into two types: round and special-shaped. Special-shaped pipes come in square, oval, triangle, hexagonal, melon-shaped, star-shaped, and finned pipes in various complex shapes. The maximum diameter is 900mm and the minimum diameter is 4mm. According to different uses, there are thick-walled seamless steel pipes and thin-walled seamless steel pipes. Seamless steel pipes are mainly used as petroleum geological drilling pipes, cracking pipes for petrochemical industry, boiler pipes, bearing pipes, and high-precision structural steel pipes for automobiles, tractors, and aviation.

In the field of industrial materials, seamless steel pipes are a testament to engineering excellence and offer countless applications in different fields. Known for their strength, durability and versatility, these pipes play a vital role in various industries such as oil and gas, construction, automotive and manufacturing. In this comprehensive exploration, we delve into the intricacies of seamless steel pipes, revealing their manufacturing processes, their advantages, applications and their significant impact on modern infrastructure.

Classification of seamless steel pipes

A steel pipe without seams along the perimeter of its cross-section. According to different production methods, they are divided into hot-rolled pipes, cold-rolled pipes, cold-drawn pipes, extruded pipes, pipe jacking, etc., all of which have their own process regulations.
The materials include ordinary and high-quality carbon structural steel (Q215-A ~ Q275-A and 10 ~ 50 steel), low alloy steel (09MnV, 16Mn, etc.), alloy steel, stainless acid-resistant steel, etc.
According to the purpose, it is divided into two categories: general purpose (for water transmission, gas pipelines and structural parts, mechanical parts) and special purpose (for boilers, geological exploration, bearings, acid resistance, etc.).

Seamless steel pipe production process

Manufacturing Process
The manufacturing process of seamless steel pipes is a complex yet fascinating journey that involves several stages, each contributing to the final product's quality and integrity. Let's break down the process into key steps:

Steel Billet Preparation:
The process begins with the selection of high-quality raw materials, typically in the form of solid round steel billets. These billets serve as the foundation for the seamless pipe and are carefully inspected to ensure they meet the required standards for composition and quality.
Heating and Piercing:
The selected billet undergoes intense heating to forge it into a malleable state. Once heated, the billet is pierced at its center using a mandrel mill or a piercing machine. This piercing creates a hollow tube, known as a shell, with the desired diameter.
Rotary Piercing:
The pierced billet is then elongated and reduced to achieve the desired outer diameter and wall thickness. Rotary piercing mills play a pivotal role in shaping the shell into a pipe with precise dimensions.
Mandrel Mill Process:
The hollow shell is elongated further using a mandrel mill, which employs a mandrel rod to control the internal diameter of the pipe. This process ensures uniformity in thickness and diameter along the length of the seamless pipe.
Stretch Reduction:
To refine the dimensions and enhance the pipe's properties, the pipe is subjected to a stretch reduction process. This involves elongating the pipe through a series of rollers to achieve the final size and wall thickness.
Annealing:
The seamless pipe undergoes annealing, a heat treatment process that imparts crucial properties such as hardness and toughness. Annealing also relieves internal stresses, ensuring the pipe's structural integrity.

Surface Treatment:

The final step involves surface treatment, which may include pickling, passivation, or coating, depending on the intended application. This treatment enhances corrosion resistance and protects the seamless pipe during transportation and storage.

①Main production processes of hot-rolled seamless steel pipes (△Main inspection processes):
Preparation and inspection of tube blank △ → heating of tube blank → perforation → pipe rolling → steel pipe reheating → diameter determination (reduction) → heat treatment △ → finished pipe straightening → finishing → inspection △ (non-destructive, physical and chemical, Taiwan inspection) → warehousing

②The main production processes of cold-rolled (drawn) seamless steel pipes:
Blank preparation→pickling and lubrication→cold rolling (drawing)→heat treatment→straightening→finishing→inspection

The production process of general seamless steel pipes can be divided into two types: cold drawing and hot rolling. The production process of cold-rolled seamless steel pipes is generally more complicated than hot rolling. The tube blanks must first be subjected to three-roll continuous rolling, and then extruded. After the sizing test, if there is no response crack on the surface, the round pipe will be cut by a cutting machine and cut into a blank of about one meter in length. Then enter the annealing process. Annealing requires pickling with acidic liquid. When pickling, pay attention to whether there is a large amount of bubbling on the surface. If there is a large amount of bubbling, it means that the quality of the steel pipe cannot meet the corresponding standards. In appearance, cold-rolled seamless steel pipes are shorter than hot-rolled seamless steel pipes. The wall thickness of cold-rolled seamless steel pipes is generally smaller than hot-rolled seamless steel pipes, but the surface looks brighter than thick-walled seamless steel pipes, and the surface is not too thick. It's quite rough, and there aren't many burrs in the diameter.

The delivery state of hot-rolled seamless steel pipes is generally in a hot-rolled state after heat treatment. After passing the quality inspection, the hot-rolled seamless steel pipe must undergo strict manual selection by the staff. After the quality inspection, the surface must be oiled, followed by multiple cold-drawing experiments. After the hot-rolling treatment, a perforation experiment must be performed. , if the diameter of the perforation is too large, it needs to be straightened and corrected. After straightening, it is transferred by the transmission device to the flaw detection machine for flaw detection experiments. Finally, it is labeled, arranged in specifications, and placed in the warehouse.
Round tube blank → heating → piercing → three-roll cross rolling, continuous rolling or extrusion → tube removal → sizing (or diameter reduction) → cooling → straightening → hydraulic test (or flaw detection) → marking → warehousing seamless steel pipe It is made of steel ingots or solid tube blanks through perforation and then hot-rolled, cold-rolled or cold-drawn. The specifications of seamless steel pipes are expressed in terms of outer diameter * wall thickness in millimeters.

The outer diameter of hot-rolled seamless steel pipes is generally greater than 32mm and the wall thickness is 2.5-200mm. The outer diameter of cold-rolled seamless steel pipes can be up to 6mm and the wall thickness can be up to 0.25mm. The outer diameter of thin-walled pipes can be up to 5mm and the wall thickness is less than 0.25mm. Rolling has higher dimensional accuracy than hot rolling.

General seamless steel pipes are made of hot-rolled or cold-rolled high-quality carbon structural steels such as 10, 20, 30, 35, 45, 16Mn, 5MnV and other low-alloy structural steels, or 40Cr, 30CrMnSi, 45Mn2, 40MnB and other composite steels. Seamless pipes made of 10 and 20 grade low carbon steel are mainly used for fluid transportation pipelines. Seamless tubes made of medium carbon steel such as 45 and 40Cr are used to manufacture mechanical parts, such as stressed parts of automobiles and tractors. Generally, seamless steel pipes must ensure strength and flattening tests. Hot-rolled steel pipes are delivered in a hot-rolled or heat-treated state; cold-rolled steel pipes are delivered in a heat-treated state.

Hot rolling, as the name suggests, has high temperature, so the deformation resistance is small and large deformation can be achieved. Take the rolling of steel plates as an example. Generally, the thickness of continuous casting billet is about 230mm, and after rough rolling and finish rolling, the final thickness is 1~20mm. At the same time, due to the small width-to-thickness ratio of the steel plate, the dimensional accuracy requirements are relatively low, and plate shape problems are not prone to occur. The main focus is to control the crown. For those with structural requirements, it is generally achieved through controlled rolling and cooling, that is, controlling the opening rolling temperature and final rolling temperature of finishing rolling. Round tube blank → heating → perforation → heading → annealing → pickling → oiling (copper plating) → Multi-pass cold drawing (cold rolling) → billet tube → heat treatment → straightening → hydraulic test (flaw detection) → marking → storage.

Seamless steel pipe uses

Seamless steel pipes are widely used.
1. General-purpose seamless steel pipes are rolled from ordinary carbon structural steel, low alloy structural steel or alloy structural steel. They have the largest output and are mainly used as pipes or structural parts for transporting fluids.

2. Available in three categories according to different uses:
a. Supplied according to chemical composition and mechanical properties;
b. Supplied according to mechanical properties;
c. Supplied according to hydraulic pressure test. If the steel pipes supplied according to categories a and b are used to withstand liquid pressure, they must also undergo a hydrostatic test.

3、Special-purpose seamless pipes include seamless pipes for boilers, chemical and electric power, geological seamless pipes, and petroleum seamless pipes. Seamless steel pipes have hollow sections and are widely used as pipelines for transporting fluids, such as pipelines for transporting oil, natural gas, coal gas, water and certain solid materials. Compared with solid steel materials such as round steel, steel pipes are lighter in weight when the bending and torsional strength is the same, and they are an economical cross-section steel material. It is widely used in the manufacture of structural parts and mechanical parts, such as oil drill pipes, automobile transmission shafts, bicycle frames, and steel scaffolding used in construction. The use of steel pipes to make ring parts can improve material utilization, simplify the manufacturing process, and save materials and processing. Steel pipes have been widely used for manufacturing.

Defects and Solutions of Seamless Steel Pipes

1. Seamless steel pipe folding

It exists on the outer or inner surface of seamless steel pipes in a straight or spiral, continuous or discontinuous state. The main reason for folding is that the quality of the pipe material is poor, there are folds in the pipe itself, or there are inclusions, serious scratches and cracks on the surface, there are edges and corners in the grinding area, and folding occurs after stretching after drawing. In order to avoid folding, the quality of pipe materials should be improved and attention should be paid to inspection and grinding.

2. Seamless steel pipe cracks

It refers to the small cracks distributed in straight lines or spirals on the inner and outer surfaces of seamless steel pipes, with a depth of 1 mm or more, and there are continuous and discontinuous cracks. According to the analysis of Shandong Jute Pipe Industry, the reasons are: there are subcutaneous bubbles and subcutaneous inclusions in the hot-rolled tube blanks, there are cracks or deep pits on the steel tube before drawing, and during the hot rolling or cold drawing operations, Longitudinal scratches or scratches have occurred. Preventing the formation of cracks also lies in improving the quality of pipe blanks and strengthening the inspection and grinding of pipe materials. At the same time, pitting, scratches and scratches on the pipe during the cold drawing production process should be avoided.

3. Seamless steel pipe scratches

The defect is characterized by longitudinal linear scratches of varying lengths on the inner and outer surfaces of the seamless pipe, mostly in the form of grooves, but they may also be raised stripes. The main reasons for scratches (scratches) are: there are internal scratches on the pipe material, which cannot be eliminated during drawing, oxide iron scale remains on the steel pipe and the mold sticks to the steel, the strength and hardness of the mold are insufficient or uneven, and the mold Chips and wear occur, the hammer head is defective, the sharp edges of the transition part of the hammer head damage the mold, etc. In order to prevent scratches and scratches, the quality of each preparation process before drawing should be improved, and molds with high strength, hardness, and good smoothness should be used.

4. Seamless steel pipe pits

This is one of the most common surface defects of seamless pipes. Local depressions of varying areas are distributed on the surface of the steel pipe. Some are periodic and some are irregular. The pits are caused by oxide scale or other hard dirt being pressed into the surface of the steel pipe during the drawing or straightening process, or the warping that originally existed on the surface of the steel pipe is peeled off. Shandong Jute Pipe Industry pointed out that the measures to prevent pits are to carefully inspect the pipe material and remove defects such as warped skin, keep the work site, tools and lubricants clean, and prevent oxide scale and dirt from falling on the surface of the steel pipe.
When there are round or sharp spiral marks on the outer surface of the seamless steel pipe, the straightening machine should be checked. Due to the incorrect position and incorrect angle of the straightening roller, the steel pipe rubs the convex shoulder on the edge of the straightening roller during straightening, there are worn grooves on the straightening roller, and excessive bending at both ends of the steel pipe may appear on the outer surface of the steel pipe. Dimples.

5. Seamless steel pipe pitted surface

It is characterized by small, point-like pits appearing on the surface of the steel pipe. The main reason for pitting is pitting corrosion during pickling. After annealing, the oxide scale is too thick and straightened and then pressed into the surface of the steel pipe to form pitting. The steel pipe is stored in a humid environment, and even pitting corrosion occurs after water enters. After the rust spots are removed, Forming pockmarks.

6. Seamless steel pipe spots

Since the seamless steel pipe comes into contact with water (or water vapor) during processing or storage and is not treated in time (such as drying), the traces left after the water evaporates are called water spots. If the seamless steel pipe has been in contact with water for a long time, or the water contains corrosive substances Rust spots will form when exposed to substances. Water spots generally have no depth, while rust spots often corrode to the surface of the steel pipe, and the pitted surface will still remain after removal.

7. Seamless steel pipe warping

Its characteristic is that the inner and outer surfaces of seamless steel pipes have thin flakes that are partially separated from the metal matrix. Some of them are lumpy and discontinuous. They may or may not take root on the pipe wall, but they cannot peel off naturally. The reasons include poor steel quality and the existence of subcutaneous bubbles, which are exposed after cold drawing; the warped skin produced during hot rolling was not removed from the tube blank until cold drawing; the original deep and angular transverse pits on the steel tube were After pulling, the skin becomes warped.

8. Seamless steel pipe scarring

Defects such as folding and peeling on the inner and outer surfaces of the steel pipe were not removed in time, and thin flakes that could not be separated from the metal matrix were formed during subsequent processing. Some of them were lumpy and discontinuous. They may or may not take root on the pipe wall, but they cannot peel off naturally. , called scarring. Surface defects must be carefully checked and removed in time to eliminate scarring.