Steel supplies for ships

Materials World magazine
6 Jan 2020

Nam Technology Ltd Director, Norman McPherson FIMMM, discusses the UK steel industry supply chain and its relationship with naval vessel building.

When an order for ships for the UK Royal Navy is placed, there usually follows an element of euphoria from politicians, which may be related to retention of jobs and skills, or in some cases, misguided views about steel supply. However, in the case of the Type 26 frigate or the Type 31 frigate, the steel tonnage for one vessel would illicit a mild reaction, it being no more than 3,000 tonnes (t). In the most optimistic situation, there could be 6,000t being consumed in one year, due to build cycles. Compared with the annual crude steel production in the UK in 2018, 7.3 million tonnes, this is equivalent to around 0.1% of the crude steel production. These orders are never going to save the UK steel industry as it is attacked by lower cost products, many of which were standard output in the past.

In recent years, around 120,000t of steel were used on the HMS Queen Elizabeth aircraft carriers, yet even this tonnage is not significant. Within the overall tonnage of a vessel, a proportion of the steel is in the form of bar, mainly offset bulb plates, with a much smaller amount of flat bar and T bar.

The steel grade used for these vessels is generally Lloyds Grade DH36, a carbon-manganese-niobium steel. This is not seen as an issue, as this steel type is made in significant volumes worldwide. However, within most builds already referred to, small quantities of other grades such as A, AH36 and D are also used. The cost differential among these, with the possible exception of A grade, is not particularly great. However, it does make the bill of steel materials easier to operate.

Use of the material specification DH36 in these builds and other similar ones was stimulated by the demand to lightweight vessels. But this was not limited to naval vessels, with cruise ships being an interesting phenomenon where the upper decks use aluminium. Still, this move has yet to see adoption in the UK naval ship builds due to some firmly held views on its overall suitability.

UK ship orders

A number of announcements have been made in recent times about aeroplanes ordered for defence ships. In particular, the UK Ministry of Defence announced the Royal Navy’s Type 31 frigates will be built by Babcock at its Rosyth, UK, dockyard. The fleet of five ships have a total value of £1.25bln. The Arrowhead 140 design will be the UK Royal Navy’s newest class of warships, with the first example scheduled for launch in 2023.

The contract is said to have secured about 2,500 jobs, which is particularly advantageous following job cuts at the end of work on aircraft carriers HMS Queen Elizabeth and HMS Prince of Wales. According to Babcock, the programme will maximise a workforce of around 1,250 highly skilled roles in multiple locations throughout the UK, with around 150 new technical apprenticeships likely to be developed.

Steel sources

Currently, the steel supply for naval vessels is sourced from the reversing plate mills (RPM) mill process. The situation in the UK is that there is only one operating mill at Liberty Steel Dalzell Works in Motherwell, Scotland.

This is classed as a heavy plate and is probably not suited to rolling plate below 10mm thick. In addition, it is a relatively old mill and suffered from a lack of meaningful investment under previous owners.

However, in the frigate and destroyer sector, about 20-25% of the steel used is in the range of 4mm, 5mm and 6mm-thick. The 6mm has a number of options in Europe, but the former are either at or out of the operating limits on most light plate mills in Europe, with one or two exceptions. One method of supply is to use steel stockholders, but this adds cost to the materials, as, generally, such parties are very aware of what is happening in the marketplace, more so than steel procurement personnel in the construction companies.

Bulb bars can be sourced from within the UK, but some sizes are out of the UK mill range, as at the British Steel mill at Skinningrove in North East England where the current minimum bulb size is 16 x 7mm. Thin plate demands smaller bulb bars than that, and possible sources for small section bulb bars are Spain and Turkey.

One of the issues with the supply chain for thin plates is that they take longer to produce than those of standard thickness. This is an issue in buoyant market times when output can be ramped up by simply opting out of the thin plate market sector. Within Europe, there are a number of mills capable of supplying 5mm and 6mm thick plates, but there are probably only two mills with the capability to roll 4mm and 5mm-thick plates on a RPM. This is not a desirable situation.

Most mills and constructors use standard thicknesses where possible, as this allows easy replacement of material, so it is important that constructors are aware of the effect of deviating from the standard.

One example was a situation where an 8.5mm-thick plate had to be replaced at short notice. The grade was DH36, but an 8.5mm-thick plate was not going to appear on the rolling schedule until several weeks later. A 9mm-thick plate was available, but it went against the lightweighting policy.

Information sharing

A lack of communication between parties in the steel industry can exacerbate problems. At the moment, structural design, procurement and steelmills are all somewhat linked, while structural manufacture is separate. The problem with this route is that there can be elements of isolated thinking. It would be rare for structural design and steelmills to interact, despite what the steelmills have to offer in terms of product enhancement.

In addition, structural manufacturing can often be left isolated and so firms tend to be left to deal with issues that could have been resolved earlier. What is required is an overlapping structure with each group being involved with each other and in totality, sharing knowledge and accepting overall responsibility.

An ideal method would be to start with structural design, then procurement, steel mills, and finally structural manufacture. The consequences of actions at the design and procurement stages can have major effects on structural manufacture. In the case of procurement, the desire to save money and generate lowest price can lead to a situation of higher cost during construction. As an example, steel plate flatness at the lower end of the product mix is quite critical, as in built out of flatness leads to distortion during the cutting and welding processes. Good product flatness, although more expensive, minimises production costs.

Looking elsewhere

For some considerable time, RPM has been seen as the primary feedstock in shipbuilding, while some shipyards use hot strip mill (HSM) plate, which has been decoiled and levelled. The apparent drawbacks are that the narrower width available from HSM feedstock results in more seam welding which increases costs and the potential for thin plate distortion.

With the situation in RPM, the HSM process route has to be opened up for shipbuilding plate - the thinner gauges can be easily supplied, thickness control is good, and flatness has been reported to be better.

The issue about more welding with narrower plate has to be put into context and it is not every thin plate that needs to be maximum width. An exercise comparing the two sources would be welcome by the industry.

Although this is categorised as looking elsewhere, the steel supply for 4mm and 5mm-thick plate could be sourced from within the UK, from HSM. In addition, the 10mm and above could also be sourced within the UK. It seems that the options are valid, the tonnage is low, and the geometric product mix can be restricting.

Outsourced products

The placement of orders for naval vessels within the UK does not have a significant effect on the country’s steelmaking industry. Products such as bulb bars can be obtained from outside the country and meet the quality requirements and are cost competitive. It does, however, retain skills within UK shipbuilding.

Outsourcing of process steps in the local shipbuilding process is a viable option and has to be considered, despite the effect it may have on jobs. The tonnage levels now and those predicted in the future cannot be sustained with all the process steps involved. It is quite clear that the construction of naval ships will in no way sustain steelmaking in the UK.