Production of Hot-Rolled Seamless Steel Pipes

The production process of seamless steel pipes can generally be divided into two types: cold drawing and hot rolling. Hot rolling, as the name suggests, involves rolling at high temperatures, resulting in low deformation resistance and allowing for large deformation amounts. Taking steel plate rolling as an example, the thickness of a continuously cast slab is generally around 230mm, while after rough rolling and finish rolling, the final thickness is 1-20mm. Simultaneously, due to the small width-to-thickness ratio of the steel plate, the dimensional accuracy requirements are relatively low, and shape problems are less likely to occur; the main focus is on controlling convexity.

 

 

The production process of cold-rolled seamless steel pipes is generally more complex than that of hot-rolled pipes. The billet first undergoes three-roll continuous rolling, followed by sizing testing. If no surface cracks are detected, the round pipe is cut into billets approximately one meter in length.

The hot-rolling process sequence is as follows:

 

Round billet → Heating → Piercing → Three-roll skew rolling, continuous rolling, or extrusion → Tube removal → Sizing (or reduction) → Cooling → Straightening → Hydrostatic test (or flaw detection) → Marking → Warehousing. Seamless steel pipes are made by piercing steel ingots or solid billets to form a rough tube, which is then hot-rolled, cold-rolled, or cold-drawn. The specifications of seamless steel pipes are expressed in millimeters as outer diameter * wall thickness.

 

Seamless steel pipes are divided into two categories: hot-rolled and cold-rolled (drawn) seamless steel pipes. Hot-rolled seamless steel pipes include general steel pipes, low and medium pressure boiler steel pipes, high pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, geological steel pipes, and other steel pipes. Cold-rolled seamless steel pipes are categorized into general steel pipes, low and medium pressure boiler steel pipes, high-pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, and other steel pipes. They also include carbon thin-walled steel pipes, alloy thin-walled steel pipes, stainless thin-walled steel pipes, and special-shaped steel pipes.

Hot-rolled seamless pipes generally have an outer diameter greater than 32mm and a wall thickness of 2.5-200mm. Cold-rolled seamless steel pipes can have an outer diameter down to 6mm and a wall thickness down to 0.25mm. Thin-walled pipes can have an outer diameter down to 5mm and a wall thickness less than 0.25mm. Cold-rolled pipes have higher dimensional accuracy than hot-rolled pipes.

 

Generally, seamless steel pipes are made from high-quality carbon structural steels such as 10, 20, 30, 35, and 45, low-alloy structural steels such as 16Mn and 5MnV, or alloy structural steels such as 40Cr, 30CrMnSi, 45Mn2, and 40MnB, through hot rolling or cold rolling. Seamless pipes made from low-carbon steels such as 10 and 20 are mainly used for fluid transportation pipelines.

 

Seamless pipes made of medium carbon steel, such as 45 and 40C, are used to manufacture mechanical parts, such as load-bearing components in automobiles and tractors. Generally, seamless steel pipes must guarantee strength and pass flattening tests. Hot-rolled steel pipes are delivered in either hot-rolled or heat-treated condition; cold-rolled pipes are delivered in either hot-rolled or heat-treated condition.