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Welded roll forming system versus tube mills: What’s the difference?

Steel pipe rolling forming process is an elastic-plastic deformation process, the boundary conditions is very complicated and nonlinearity, so it is difficult to get the exact solution by using the analytical method and the traditional implicit static algorithm based on finite element method. The contact collide arithmetic based on explicit dynamic program LS-DYNA provides an effective way for accurate research on the elastic-plastic deformation of the hot-rolled seamless pipe rolling process. This paper takes the SMS MEER rolling mill of Φ 177PQF steel tube rolling units in Ansteel seamless steel pipe rolling mill as the research object, makes a coupled thermo-mechanical finite element method computation on steel pipe rolling process, gets the residual stress and strain change rule in the rolling process. Written the rolling process parametric programs by APDL, analyzed the influence of roller spacing and velocity parameters on residual stress and strain, which provides a reliable theory basis for improving the performance of hot rolling seamless steel pipe and the optimization of rolling technological parameters.

The welding of streel strip into a product, such as round tube or pipe, box frames, and structural members, is a multibillion-dollar industry that had its beginnings more than a century ago. Many items that we use today, like furniture tubing, oil pipe, and fuel lines, are manufactured on welded pipe cold rolling mill.

In the last 20 years OEMs have been asked to apply the principles learned from cold pilger to the much larger roll forming processing market. This has created another technology area today known as welded roll forming.

Companies may move from nonwelded open shapes to welded roll formed shapes for many reasons. Welded roll formed shapes have structural strength and integrity, help eliminate secondary operations in downstream manufacturing, and can offer savings in steel and construction labor. While there are benefits, there are also costs, which typically involve an investment in capital equipment and skilled people who know how to use the technology.

What are the differences between a tube mill and a welded roll form system? Which one should you consider? Can the same parts be run on both systems? Will the part quality be affected?

These questions, asked often within the industry, are plagued with many conditional aspects that cloud the final answer. Intricate details often are required to help machine builders focus on the best option.

Terminology

In the classic sense, a tube mill is a kind of welded roll form system; however, not every welded roll form system is a tube mill. Tube mills are welded roll form systems that have been fine-tuned to run a specific diameter range, typically at fast speeds (see Figure 1).

Diameter range is mentioned because a tube mill welds a round product. Progressive male and female roller dies (or rolls) shape the incoming strip for welding a specific diameter.

On all welded roll form systems, including tube mills, welding can occur with processes like high-frequency (HF) induction, HF contact, electric resistance, and laser beam welding. For the sake of this discussion, it is assumed that the correct welding process has been determined based on the specific material requirements.

Figure 2 shows the typical forming flower of a tube mill forming the strip into a round product just before welding. From there the round product can stay round or can be sized or reshaped into a square or rectangle.

In a tube mill, the forming machine forms the strip into a round, weldable product. The forming machine has two main parts: breakdown and fin-pass sections. Once welded, the tube can be left round, though it undergoes further forming to size it to a more accurate outside diameter. The sizing section shown in Figure 3 has round roll stands with specialty reshaping stands toward the exit and a double turks-head to finish. Used for straightening, a turks-head has two pairs of rolls, one arranged vertically and the other horizontally.

Defining a welded roll forming system is a little more difficult. Again, in the classical sense, a pipe hot rolling mill is one kind of a welded roll forming system. But if someone refers to a “welded roll forming system,” that person is probably not talking about a tube mill, but other roll forming machines able to form various, often highly complex shapes to within tight tolerances.

Much like forming on the tube mill, with breakdown and fin-pass sections, welded roll forming systems have a similar forming setup, with the fin-pass section occurring in the last few stands before welding (see Figure 4).
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