According to the manufacturing method, the steel pipe can be divided into two categories: seamless steel pipe and welded seam steel pipe. Among them, ERW steel pipe is the main variety of welded steel pipe. Today, we mainly talk about two kinds of steel pipes used as raw materials for petroleum casing pipe: seamless casing and ERW casing.
Seamless casing-casing made of seamless steel pipe as raw material;
Seamless steel tubes refer to steel tubes manufactured by four methods: hot rolling, cold rolling, hot drawing, and cold drawing. There are no welds on the pipe body itself.
ERW casing-casing made of electric welded pipe as raw material
ERW (Electric Resistant Weld) steel pipe refers to a straight seam welded pipe manufactured through a high-frequency resistance welding process. The raw material steel plate (coil) of ERW welded pipe is made of low-carbon microalloy steel rolled by TMCP (Thermo-Mechanical Control Process).
1. Outer diameter tolerance
Seamless steel pipe: Hot-rolled forming process is used, and the sizing is completed at about 8000C. The raw material composition, cooling condition and cooling state of the roller have a great influence on its outer diameter, so the outer diameter control is difficult to be accurate and fluctuates The range is larger.
ERW steel pipe: adopts cold-formed forming and sizing by 0.6% reduction. The process temperature is basically constant at room temperature, so the outer diameter control is accurate and the fluctuation range is small, which is helpful to eliminate black leather buckles;
2. Wall thickness tolerance
Seamless steel pipe: It is produced by round steel perforation, and the wall thickness deviation is large. The subsequent hot rolling can partially eliminate the unevenness of the wall thickness, but the most advanced unit can only be controlled within ±5~10%t.
ERW steel pipe: using hot-rolled coil as raw material, the thickness tolerance of modern hot-rolling can be controlled within 0.05mm.
The outer surface defects of the blanks used in seamless steel pipes cannot be eliminated by the hot rolling process, and the defects can only be polished off after the finished product is finished; the spiral path left after perforation can only be partially eliminated in the wall reduction process.
ERW steel pipe uses hot-rolled coils as raw materials. The surface quality of the coils is the surface quality of ERW steel pipes. The surface quality of hot-rolled coils is easy to control and high in quality. Therefore, the surface quality of ERW steel pipes is much better than seamless steel pipes.
Seamless steel pipe: hot-rolled forming process is adopted. The raw material composition of the steel pipe, the cooling condition and the cooling state of the roll have a great influence on its outer diameter, so it is difficult to control the outer diameter accurately and the fluctuation range is large.
ERW steel pipe: adopts cold bending forming, so the outer diameter control is accurate and the fluctuation range is small.
5. Tensile test
The tensile performance indexes of seamless steel pipes and ERW steel pipes conform to API standards, but the strength of seamless steel pipes is generally at the upper limit and the plasticity is at the lower limit. In comparison, the strength index of ERW steel pipes is in the best state, and the plasticity index is 33.3% higher than the standard. The reason is that the performance of hot-rolled coils, the raw material of ERW steel pipes, is guaranteed by means of microalloying smelting, out-of-furnace refining, and controlled cold and controlled rolling; seamless steel pipes mainly rely on means to increase carbon content, which is difficult to ensure strength and plasticity. Reasonable match.
The raw material of ERW steel pipe, hot-rolled coil, has extremely high precision during the rolling process, which can ensure the uniform performance of all parts of the coil.
7. Grain size
The raw material of ERW steel pipe-hot-rolled coils are made of wide and thick continuous casting billets, with a thicker fine-grained surface solidification layer, no columnar crystal regions, shrinkage and porosity, small composition deviation, and compact structure; in the subsequent rolling process The application of controlled cold and controlled rolling technology further ensures the grain size of raw materials.
8. Anti-collapse test
ERW steel pipe is due to the characteristics of its raw materials and pipe making process. Its wall thickness uniformity and ellipticity are far superior to seamless steel pipes, which is the main reason why the collapse resistance performance is higher than that of seamless steel pipes.
9. Impact test
Since the impact toughness of the base material of ERW steel pipe is several times that of seamless steel pipe, the impact toughness of the weld is the key to ERW steel pipe. By controlling the impurity content of raw materials, the height and direction of slitting burrs, the shape of forming edges, the welding angle, and the welding speed , Heating power and frequency, welding extrusion volume, intermediate frequency withdrawal temperature and depth, air cooling section length and other process parameters ensure that the impact energy of the weld reaches more than 60% of the base metal. If further optimized, the weld impact energy can be close to the base Material to achieve seamless performance.
10. Blasting test
The blasting test performance of ERW steel pipe is much higher than the standard requirement, mainly due to the high uniformity of wall thickness and uniform outer diameter of ERW steel pipe.