Straight seam steel pipe strength and steel grade expression method

Straight seam steel pipe is a steel pipe in which the weld seam is parallel to the longitudinal direction of the steel pipe. Usually divided into metric electric welded steel pipes, electric welded thin-walled pipes, transformer cooling oil pipes, etc. The production process of straight seam welded pipe is simple, the production efficiency is high, the cost is low, and the development is rapid. The strength of spiral welded pipes is generally higher than that of straight seam welded pipes. Welded pipes with larger diameters can be produced from narrower billets, and welded pipes with different diameters can also be produced from billets of the same width. However, compared with straight seam pipes of the same length, the weld length is increased by 30~100%, and the production speed is lower.

Straight seam high-frequency welded pipe (ERW) is divided into two forms: induction welding and contact welding according to different welding methods. It uses hot-rolled wide steel coils as raw materials and undergoes pre-bending, continuous forming, welding, heat treatment, sizing, straightening, Cutting, and other processes, compared with spiral welded pipes, has the advantages of shorter welds, high dimensional accuracy, uniform wall thickness, good surface, and high-pressure resistance. However, the disadvantage is that it can only produce small and medium-diameter thin-walled pipes, and gray spots are easily produced at the welds. Unfused, grooved corrosion defects. At present, the most widely used fields are city gas, crude oil, refined oil transportation, etc.

Straight seam submerged arc welded pipe (LSAW) is produced by using a single medium-thick plate as raw material, pressing (rolling) the steel plate into a tube blank in a mold or forming machine, and enlarging the diameter using double-sided submerged arc welding. Its finished product specifications range is wide, the toughness, plasticity, uniformity, and density of the weld are good, and it has the advantages of large pipe diameter, thick pipe wall, high-pressure resistance, low-temperature resistance, and strong corrosion resistance. When constructing high-strength, high-toughness, long-distance oil and gas pipelines, most of the steel pipes required are large-diameter, thick-walled straight seam submerged arc welded pipes. According to API standards, in large oil and gas transmission pipelines, longitudinal submerged arc welded pipes are suitable pipe types when passing through Class 1 and Class 2 areas such as alpine zones, seabed, and densely populated urban areas. According to different molding methods, it can be divided into:

UOE welded pipe: After the single steel plate is pre-bent at the edge, it undergoes U forming, O forming, internal welding, external welding, mechanical cold expansion, and other processes;

JCOE welded pipe: that is, pre-welding, forming, and cold expansion after welding according to "J-C-O-E";

HME welded pipe: formed by mandrel rolling method according to "C-C-O", followed by cold diameter expansion after welding.

How to express the steel grade of large diameter straight seam steel pipe:

1. Carbon structural steel

① It consists of Q + number + grade symbol + deoxidation method symbol. Its steel number is prefixed with "Q", which represents the yield point of the steel. The following number represents the yield point value in MPa. For example, Q235 represents a carbon structural steel with a yield point (σs) of 235MPa.

②The steel number can be marked with a symbol indicating the grade and deoxidation method. The grade symbols are A, B, C, and D respectively. The deoxidation method symbol F represents boiling steel, B represents semi-killed steel, Z represents killed steel, and TZ represents special killed steel. The symbols for killed steel do not need to be marked, that is, ZT and TZ do not need to be marked. For example, Q235-AF represents boiling steel.

③Special-purpose carbon steel, such as bridge steel, marine steel, etc., basically uses the representation method of carbon structural steel, but a letter indicating the purpose is appended to the steel number.

2. Carbon structural steel

①The two digits at the beginning of the steel number indicate the carbon content of the steel, expressed as a few ten-thousandths of the average carbon content. For example, for steel with an average carbon content of 0.45%, the steel number is "45". It is not a sequential number, so it cannot be read as 45 gauge steel.

② For carbon structural steel with high manganese content, the manganese element should be marked, such as 50Mn.

③ Boiling steel, semi-killed steel, and special-purpose carbon structural steel should be specially marked after the steel number. For example, semi-killed steel with an average carbon content of 0.1% has a steel number of 10b.

3. carbon tool steel

①The steel number is marked with "T" to avoid confusion with other steel types.

②The number in the steel number indicates the carbon content, expressed as a few thousandths of the average carbon content. For example, "T8" indicates a uniform carbon content of 0.8%.

③ For those with higher manganese content, "Mn" will be marked after the steel number, such as "T8Mn".

④The phosphorus and sulfur content of carbon tool steel is lower than that of ordinary carbon tool steel. The letter "A" is added after the steel number to indicate the difference, such as "T8MnA".