Reconsidering the steel rebar market
Tim Parker and Paolo Berbotto discuss how changing perceptions of high-strength steel rebar behaviour during seismic events are suggesting significant changes for manufacturers and suppliers.
Traditional manufacturing of higher strength steel bar has been dominated by two approaches – micro alloying (MA) at the steelmaking stage, and heat treating the bar immediately after rolling by the quenched and self-tempered (QST) route. At a global level, steelmakers commonly offer a variety of products produced by combining the two processes.
Increasingly, efforts to define a larger part of the market for certified rebar as obtainable purely by MA have come up against concerns about the performance of heat treated bars in critical applications, especially of reinforcing bar in seismic conditions.
It may, therefore, be argued that integrated producers manufacturing their own billet, whether from virgin ore or re-melted scrap, may hold an advantage over re-rollers who will find it difficult to source alloyed billet.
It is too early to tell the extent of the shift in the two technological approaches’ relative market shares worldwide, but as it is China driving the change, this suggests there is capability for the impact to be strong. Considering the capabilities and market forces, Farnborough Engineering Consultants believes the strategic implications on the overall bar supply chain could be significant.
Difference in the processes?
The majority of modern reinforcing bar manufacturers are able to meet most market standards for the required combination of strength and ductility, which comes from MA or QST. MA uses the two advanced technologies of fine-grain and precipitation strengthening to improve strength. During steelmaking, micro alloying elements are added to the liquid steel, such as titanium, niobium – also known as columbium – and vanadium with a content ranging from 0.02% to 0.15%.
The alternative method favours additional heat treatment of low-carbon steel bar and removes the need for enhanced chemistry. QST is produced by rapid cooling of the outer layers of the bar by water quenching after completion of hot rolling. During this process, the surface layer of the bar is quenched into the hard martensite phase rim, while the core remains austenitic. The outer layers then undergo a tempering action as the residual greater heat in the core of the bar transfers its heat back to the surface, resulting in a tempered martensite grain structure around the periphery.
Finally, while free cooling, the austenitic core begins to change into ductile pearlite and ferrite due to its much slower cooling rate. The formation of this microstructure is dependent on the control parameters such as specific alloy chemistry, bar diameter, bar entry temperature to the rapid cooling system, duration of cooling, and cooling efficiency. Therefore, the layer typical of austenite below the quenched surface can transform completely or partially to bainite. The final microstructure of the rebar consists of a surface layer of tempered martensite, intermediate bainite layer and ductile core of ferrite and pearlite, which by the composite structural characteristics of the steel cross-section, is aimed at replicating the structural strengths achievable by the MA route.
In basic terms, the choice facing integrated rebar manufacturers has been that of additional variable costs (alloys) or an upfront capital cost for the heat treatment required to make QST product. QST implementation may also cause some down time in operations while the system is retrofitted.
Until recently, QST was favoured, perceived as a more cost-effective route to achieve strength and ductility, while encompassing re-rollers. However, the USA market has been holding out against the adoption of the QST process route, citing the ASTM A706 standard for weldable rebar’s requirement for a high tensile-to-yield strength ratio of at least 1.25 - unachievable by QST.
Minimum tensile-to-yield strength ratio has historically been a driving factor within markets where seismic-resistant properties are seen as especially important. The safety criterion has been critical to recent developments, which have seen a significant change in the market equilibrium in favour of greater use of MA, either alone or in conjunction with QST. This originates from the Asia-Pacific market, specifically China, where close attention to the process route has historically been a feature of some markets, such as New Zealand.
In response to increased earthquake severity, Chinese authorities introduced new regulations in November 2018. China accounts for a majority of world rebar volumes, and the new regulations caused a phase out of production – and by implication the domestic use – of the lowest yield strength mass volume products, including yield strengths of 235MPa and 335MPa. It also promoted the take-up of grades with yield strengths of 500MPa and 600MPa, but also minimum tensile-to-yield strength ratios of 1.25. Importantly, these grades have to be made by the MA route.
The China Iron & Steel Research Institute (CISRI) stated that the equivalent yield strength RRB500 grade made by QST is ‘not accepted by the Chinese building industry’. While this may not be fact, it indicates a major shift away from QST rebar in the world’s largest market, and RRB500 grade, and the lower yield strength RRB400 grade, both achieved by QST.
Drivers to change
Chinese authorities have to consider the impact earthquakes have on their country, therefore this is a major driver for change. But the reasons for favouring one rebar production route over another are likely to be more complex. For instance, China produced 55% of the world’s mined vanadium in 2018, and has the greatest estimated reserves of this metal.
However, this is not true for niobium, which China imports. By comparison, a relatively lower cost alloy that can be combined with vanadium to achieve the required properties, suggests that performance criteria have been key. Specifically, the following are the key advantages of the MA route over QST:
- Maximising tensile-to-yield strength ratio. This is a key criterion in terms of seismic safety. China’s CISRI finds that the highest performance is achievable by a vanadium-niobium combination, closely followed by reliance on vanadium as the key alloying element, both support achieving the desired ratio above 1.25, unlike QST. The UK’s statutory rebar certification authority, CARES, concurs with the CISRI’s view to an extent, noting that for MA-route rebar ‘the Rm/Re figure is particularly high for these steels, and they have a relatively high level of ductility’, whereas QST ‘has slightly lower levels elongation and Rm/Re’.
- Reheating above critical temperature. It is suggested that a number of typical processes applied to rebar by users may heat the outer layers of the bar cross-section above the tempering threshold, undoing the strengthening obtained through QST. These danger points can occur during welding, hot bending and potentially even hot dip galvanising at around 450°C.
- Corrosion risk. Anecdotally, China’s new standards promoting MA over QST are also motivated by concerns that QST rebar ‘has lower durability because it rusts easily and therefore poses a risk to building safety’.
Too early to scale the shift
The potential for changing specifications in the world’s largest market to spur progress further across the world market appears obvious. An indicator can be seen from growth in demand for ferro-vanadium. CISRI anticipated in late 2018 that around 10,000 tonnes per year (t/y) of new vanadium demand would be created in China, from a consumption of approximately 70,000t/y and over 90,000t/y of production in 2018. In April 2019, the international vanadium association, Vanitec, stated that, ‘the [new Chinese] standard has been active for over four months and the results thus far are impressive’, citing a survey of ‘nearly 200 Chinese rebar producers showing that 89% of the producers use vanadium micro-alloying to produce high-strength rebar’.
However, following a sharp upwards trending in late 2018, vanadium prices corrected in early 2019. A number of reasons have been theorised, including patchy enforcement, and thus implementation, of the new Chinese rules for the move away from QST.
Change for the steel industry?
Increasing momentum for a shift towards greater use of MA rebar has the potential to be divisive among market players. Full-cycle steelmakers-rollers, that can control their own metallurgy and choose to go down the MA route, the re-rollers, dependant on procured billet, and the wider community of market stakeholders all have a role in influencing standards. As a result, a greater gap may be opening up between two market tiers, one specifying a level of performance that comes to be increasingly seen as the preserve of MA material, and the rest of the commodity market.
If that happens, and if re-rollers find it challenging to reliably source MA-grade merchant billet, an expanded upper-quality tier of the rebar sector could be numbered among those speciality steel markets, such as that for oil and gas pipe, which strategically tend to favour vertically integrated producers.
Technical implications of MA
Micro alloying is not a simple matter of adding vanadium to the melt in a secondary steelmaking ladle. The chemistry required relies on many variables, not least the cost of alloys. Mini-mills taking scrap as a feedstock already have a cost advantage, as there is likely to be residual levels of alloys in the scrap, often not accounted for in their pricing. Judicious scrap purchasing can offset some of the cost of adding alloys – this can put blast furnace and direct reduced iron producers at a disadvantage when convention suggests that full backward integration to ore would have the benefit of purity of materials.
Ferro-manganese is one of the lowest cost alloys to bring benefits with yield stress, but a level of grain refinement is required to give the high tensile-to-yield strength ratio that some standards are demanding. Minimising alloying costs will be a crucial part of being an efficient and profitable rebar producer, and optimising equipment and practices to keep the alloying to a minimum will be necessary.
The quantities of alloys used are small, however, their cost is substantial. Typical quantities of vanadium used are around 0.1% whereas the cost of ferro vanadium (FeV) is around US$35/kg. Achieving 0.1% vanadium in the cast would require the addition of around 2kg of FeV for every tonne of steel at an additional cost of US$70/t.
Implications for investment
Mill operators that have purchased equipment to produce rebar at commodity grade achieved using QST may find their mills significantly overloaded when rolling MA steels. Lower cost alloying may include additions such as silicon, which would benefit from submerged pouring at the continuous caster – this would be difficult on the standard 130mm square billet. New mills equipped with a 150mm square billet caster and additional stands would be at an advantage if they could use lower cost additions to achieve the requirements, thereby minimising the expensive alloys such as FeV, chromium and nitrogen vanadium alloys (NitroVan).
Strength and ductility can be achieved with grain refinement without the outer periphery tempered martensitic structure associated with QST. This grain refinement can be achieved with thermo-mechanical rolling, where the finishing reductions take place at lower temperatures and the grain refinement occurs throughout the whole cross section. European equipment manufacturer Danieli recently claimed it had received a 1.2 million tonne per year bar mill from a Chinese steel company capable of rolling 8-40mm rebar to HRBF 400E – 500E all from 165mm square billet. The specification is indicative of a mill capable of rolling ultra fine grain steel to achieve the properties required by the new Chinese rebar standard.
Grain refinement of metals is the only strengthening mechanism that simultaneously enhances the toughness of a material. Advanced thermomechanical processing (ATMP) routes in commercial large-scale rolling mills can produce ultra-fine grained (UFG) materials with a ferrite grain size of around 1μm. The implications are that while existing QST mills can produce MA steels, purpose-built MA mills may be required to stay competitive.
At first glance, the gathering momentum for a changing standard seemed to benefit steelmakers fully in control of their supply chain economics, but the availability and cost of straight alloy procurement is costly. At the other end of the scale, the re-roller may be outpriced by billet suppliers. It may just be the clear mini mill operator that wins this round.