The used HFW tube mills HFW welded steel pipe has been continuously developed with the competition with seamless steel pipe. For welded steel pipes to gradually replace seamless steel pipes, the key is to have high product quality and low production costs. Nowadays, with the advancement of steel smelting technology, hot-rolled steel strip technology and pipe-making technology, as well as the extension of process technology to specialization, differentiation and conservation, all these have created the improvement of the quality of HFW welded pipes and the reduction of costs. condition.

In order to make HFW welded pipe take advantage of uniform wall thickness, regular geometric size and low production cost, it can also overcome the inconsistency of mechanical properties and groove corrosion caused by the existence of welds, and enter the high level occupied by traditional seamless pipes. Oil and gas pipelines and oil well pipes and other fields. With the continuous progress and development of pipe making technology, the idea of seamless welded pipe was put forward. Compared with advanced foreign countries, the market share of HFW welded pipes produced by used HFW tube mills in my country is still very low, and seamless welded pipes will have a long way to go. The seamlessization of welded pipes mainly includes two connotations. One is geometric seamlessness, which uses mechanical methods and multiple hot scraping to remove burrs on the inner and outer welds of welded pipes; the other is physical seamlessness, which uses heat treatment and other processes to make the welds. , The metallographic structure and mechanical properties of the heat-affected zone (HAZ) tend to be consistent with the base material.

① Overall heating and tension reducing rolling of welded pipe

The thermal tension reducing process is actually normalizing + deforming heat treatment on the welded pipe. After heating the whole mother tube to 920～950℃, the weld seam and the metallographic structure of the tube body are completely austenitized, and then the tension reducer is used for rolling with large deformation in multiple passes. Generally, the final rolling temperature is not low. At 850°C, to meet the requirements of the welded pipe rolling, a fine and uniform grain structure will be formed inside.

After the welding seam undergoes phase transformation, it undergoes thermomechanical heat treatment with the base metal at the same time, and completes the recrystallization process under the same conditions. The internal stress brought by.

Compared with the welding seam heat treatment method, this technology makes the welded pipe the most seamless, and the technical indicators are also the most ideal. The main equipment is heating equipment (natural gas or intermediate frequency induction heating equipment), tension reducing machine and cooling bed, etc. This process has been valued and is gradually being promoted and applied for the production of high-grade HFW welded pipes.

②Overall heat treatment of welded pipe

The whole welded pipe is heated to the normalizing temperature to completely austenitize all the structures, and then air-cooled. This can basically eliminate the metallographic structure difference between the pipe body and the weld, and completely eliminate the internal stress of the welded pipe due to forming and welding, but there are still differences in the grain state of the weld and the pipe, which will bring about mechanics Insufficiency in performance and corrosion resistance.

In fact, the quenched and tempered heat treatment process of the used HFW tube mills welded pipe overall quenching + tempering (Q+T) is used, which can not only realize the seamlessness of the welded pipe, but also improve the material performance of the welded pipe.