Steel wire for concrete reinforcement

Soft steel wire rod is not always better

Case description:

There is a brand new single steel wire drawing machine to produce wire for Card clothing using soft steel wire rod .

The company uses two types of wire rod:

  • First: Stelmor treated steel wire rod ASTM 1080, 5.5 mm. HCl pickled, phosphated and borated. For a final diameter of 1.33 mm (0.0524 inches). Drawn in 13 dies.
  • Second: Spheroidized steel wire, alloyed ASTM 1080 modified Mn-Cr-W carbon (0.88%C), wire rod diameter: 5.5 mm. HCl pickled, phosphated and borated. Final diameter 1.18 mm (0.0465 inches). Drawn in 13 dies.

The final speed for both steels were 25 m/sec (4 921 ft/min).

In spite that they were cleaning the wire rod by acid method, the lay out is ready to fit the mechanical descaling and coating systems.

The pickling tanks are small compared to the coil size, so they must cut by half each coil in order to clean the steel. It is needed much more time to prepare the coil before the cleaning operation, besides in the cutting operation, the steel wire rod got tangled easily.

The new plant is about 300 meters from pickling line.

Management Low efficiency related problems in steel wire drawing:

The new plant were employing an old pointer machine. Its range of pointing is from ⅜” (9.5 mm) to 0.078” (1.6 mm). The pointing machine were not able to point the three last dies.

Because this, the pointing operation had to be done by grinding with the grinder of the welding machine. The time lost were huge. At least 30 minutes to get the wire in the next capstan and 90 minutes to get to the last capstan.

Due to wire breaks or die changes, the efficiency was 75% maximum in one 12-hour shift.

Low efficiency process related problems:

They were planning to produce, 1.18 mm (0.0465”) modified 1080 (0.88%C) low alloy Mn-Cr-W.

As they were expecting wire breaks on a single wire drawing operation from 5.5 to 1.18 mm, they thought that a softer steel would be much easier to draw, so they design the process as follow:

  • Spheroidized wire rod, pickled and phosphated
  • First breaking draw from 5.5 mm to 3 mm (0.118”) in 5 dies.
  • Spheroidizing
  • Pickle and phosphate
  • Second draw to final diameter in 8 dies
  • Patenting and in-line cleaning
  • Flat rolling
  • In line cutting the card profile, quenching and tempering

In this case, the solution is not to have a spheroidized structure for being able to draw this alleged difficult steel grade. 

Low efficiency process related problems:

 That the alloy elements hardens the as rolled steel is a common thought. The only main element that hardens on quenching and tempering the steel is carbon, so the Mn, Cr and W, in this case are added for other purposes:

  • Modified carbon: To have enough availability of carbon to form carbides without losing hardening properties.
  • Mn: Increases the wearing resistance
  • Cr: The main alloy element since it:
    • Increases the corrosion resistance
    • Rises  hardenability (Keep high hardness on tempering).
    • Growth abrasion and wearing resistance
  • W: In this case the addition of Chromium is essential for the final properties, W is needed to avoid the chromium to form non active carbides. The function of tungsten is to form tungsten carbides (during the final heat treatment) instead of Chromium 

 

Conclussion:

The modified 1080 MnCrW, for wire drawing do not need special treatment such as spheroidized structure.

In fact, the spheroidized structure is very harmful for this large draft process since the globules do not deform and act as a non-metallic inclusions leading to wire breaks in the 8th or 9th die.

With a proper steel, it is easy to draw from 5.5 to 1.18 mm (0.218” to 0.0465”) in 13 dies at 25 m/sec., avoiding one drawing operation and therefore saving time.

Changing the process would lead to a at least 100 % increase over the original process (spheroidized wire rod) since the right process will save a complete wire drawing operation on the same machine

 

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