Continuous Improvement: Roof bolts – Increasing circularity in specialized applications

InfraBuild’s steel manufacturing business supplies steel bar products to customers, specialising in the provision ground support solutions to the mining and civil construction sectors. 

These customers transform the bar to make roof bolts used for ground stabilisation in underground mining and tunnelling. Along with a two-pack grouting system, the bolts are inserted into regularly spaced pre-drilled holes providing support for the roof and walls, providing a safer work environment. This material has demanding and stringent mechanical properties to ensure the performance of the finished bolt. 

The traditional HSAC840 roof bolt grade has previously been manufactured using blast furnace manufacturing techniques with a low recycled steel content. In December 2019, InfraBuild commenced a project to manufacture this grade at its Sydney Steel Mill, using an electric arc furnace (EAF) manufacturing process. This created an opportunity to significantly increase the recycled scrap steel content and utilise virtually 100 per cent scrap metal ferrous feed.  

The first stage of the project involved several trial heats to perfect the chemistry of the steel. The second stage involved working with the UNSW to compare the stress corrosion cracking resistance of the EAF produced steel to the blast furnace produced steel. Stress corrosion cracking can lead to premature failure of bolts in some situations leading to an increased risk for workers in the underground environment. The trials demonstrated that the new EAF manufactured product resulted in a finished product which performs at a level which is at least the equivalent of the blast furnace produced version.

However, there remained the question whether the Sydney Steel Mill’s caster could produce the new roof bolt in commercial quantities.  More trial heats were conducted with gradual improvements each time until the caster team cracked the code – producing a seven-heat sequence and then a ten-heat sequence with no castability issues.  

The EAF manufactured product is now being produced and supplied to the market in significant volumes, effectively replacing the blast furnace alternative.

InfraBuild is pleased to have transformed this product to an EAF manufactured feed, increasing the amount of recycled content in the finished product, whilst delivering on our customers’ stringent and exacting requirements for the mechanical properties of the product.