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When Safety Drives Metallurgy: Prospects for the Increased Use of Micro-Alloying in Rebar

Special Reports

by Chris Breckon, Managing Director, Farnborough Engineering Consultants (FEC).

Summary

Rebar manufacturers have for many years used a combination of two approaches to manufacturing higher-strength bar: one is using a combination of alloys (the “micro-alloying” route) at the steelmaking stage, the other is thermomechanically treating the steel once rolled to achieve similar properties by the Quenched and Self-Tempered bar route.

While some markets have shown a marked preference for one route over the other at a regulatory level, globally it has been common for steelmakers to offer a combination of products obtained by the two process routes.

Recently however, a push to define a larger part of the market for certified rebar as being obtainable only by micro-alloying has been gathering pace, amid concerns about the performance of thermomechanically treated bars in high-risk seismic environments.

That this change is originating in the world’s largest market, China, makes its potential impact all the greater. It is too early to conclude how great will be the shift in the two technological approaches’ relative market shares worldwide. However, our industryfocused consulting team believes that the strategic implications for the rebar supply chain could be significant

Micro-alloying versus Quench and Self-Temper

There are broadly speaking two approaches to the manufacture of modern reinforcing bar able to meet most market standards for the required combination of strength and ductility.

The first route consists of the enhanced use of key ferroalloys, including manganese, niobium (a.k.a. columbium) and above all vanadium at the raw steelmaking stage (micro-alloying, “MA” route).

The alternative is to forgo the enhanced chemistry in favour of additional thermomechanical treatment after the billet cast from raw steel is first rolled into bar. Rapid cooling of the outer layers of the bar by water quenching combined with the tempering action of the residual greater heat in the core of the bar results in Quenched and Self-Tempered (QST) bar, a product which by the composite structural characteristics of the steel cross-section is aimed at replicating the structural behaviours achievable by the MA route.

To an extent, the choice facing rebar manufacturers has been one between additional variable costs (the cost of the alloys for MA product) and an up-front additional capital cost (for the heat treatment required to make QST product).

Until recently, the contemporary trend appeared to be in the direction of QST, perceived as a more costeffective route to achieve the necessary combination of strength and ductility1 . The most prominent market holding out against the adoption of the QST process route was the USA, with a key reason identified as being the ASTM A706 standard for weldable rebar’s requirement for a high tensile to yield strength ratio of at least 1.25, not achievable by QST 2 .

A strong focus on a minimum tensile-to-yield strength ratio has historically been a feature of markets where seismic resistant properties are seen as especially important.

Table 1: Sample of worldwide rebar grades highlighting the changes in China

Source: FEC research MA: Micro Alloyed QST: Quench and Self Tempered (1): ChromX® was formerly known as MMFX2

(2): Source: https://www.cmc.com/en/americas/our-businesses/mill-products/chromx/high-strength

The above safety criterion has indeed been critical to the most recent developments, which have seen a significant change in the market equilibrium in favour of a return to greater use of micro-alloying, either on its own, or in conjunction with a degree of QST. This originates from the Asian market, and specifically China. Close attention to the process route used for rebar has historically been a feature of some markets in the Asia-Pacific, such as New Zealand, probably not coincidentally located around the highly seismic Pacific “ring of fire”3 .

Then in November 2018, China, accounting for a majority of world rebar volumes, introduced new regulations to phase out the production (and by implication the domestic use) of the lowest-yield strength mass volume products, including yield strengths of 235 MPa and 335 MPa, and to promote the take-up of the higher-strength HRB grades with yield strengths of 500 MPa and 600 MPa but also minimum tensile to yield strength ratios of 1.25. Importantly, these grades have to be made by the MA route.

According to a paper by the China Iron & Steel Research Institute (CISRI) the equivalent yield strength RRB500 grade made by QST is “not accepted by the Chinese building industry4 ”. While this may be a statement of the direction of travel rather than of fact (QST product has not yet disappeared from the Chinese market), it does suggest a major shift away from QST rebar in the world’s largest market.

Why the change?

The drivers for favouring one rebar production route over another can be complex. It may not be without relevance that China produced 55% of the world’s mined vanadium in 2018, and has the greatest estimated reserves of this metal5 .

However, China does need to import volumes of niobium, a relatively lower-cost alloy which can be combined with vanadium to achieve the required properties; this alone suggests that performance criteria have been key in driving the shift in China, as it is not without its costs for domestic steelmakers.

Specifically, the following are the key reported advantages of the MA route over QST:

1. Maximising tensile to yield strength ratio:

As noted above, 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 (see Figure 1 below).

The UK’s statutory rebar certification authority CARES concurs with the CISRI 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”6 .

Figure 1: Tensile to yield strength under MA and QST, according to CISRI

Source: CISRI, Influence of New Rebar Standard on Chinese Vanadium Market

2. Reheating above critical temperature:

It is suggested that a number of typical rebar treatment processes may heat the outer layers of the bar crosssection back above the tempering threshold, thus undoing the strengthening obtained through QST. These danger points can occur during welding, hot bending and potentially even hot dip galvanizing at around 450 C according to a papers published for the Philippine construction sector7 . Those specific restrictions which, as noted above, apply in the USA and New Zealand on the use of QST rebar in the presence of welding, certainly appear to share some of these concerns.

3. Corrosion risk:

Anecdotally, China’s new standards promoting MA over QST are also motivated by concerns that “has lower durability because it rusts easily and therefore poses a risk to building safety”8 .

How big could the market shift be?

The potential for changing specifications in the world’s largest market to spur changes further across the world market appears obvious, and FEC is aware of instances of interest in other regions on the part of certain producers of rebar who are considering product development in the direction of MA grades.

It is, however, too early to tell how large the shift will be, even in the Chinese market where it is originating. The change in China is still being implemented, and international customs codes do not allow the identification of rebar made by the MA route from rebar made by the QST route.

An indirect measurement could be obtained from growth in demand for vanadium, in ferro-vanadium form, as the alloy most directly promoted by the new specifications. Based on Chinese high-strength rebar market volumes and the expected content of vanadium, CISRI anticipated in late 2018 that around 10,000 t/y of new vanadium demand would be created in China, from a base of just over 70,000 t/y of consumption and over 90,000 t/y of production in 2018. In April 2019, the international vanadium association Vanitec commented “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 microalloying to produce high-strength rebar”9 .

However, after spiking upwards in late 2018, vanadium prices corrected sharply downwards in early 2019 (see Figure 2 below). A number of reasons have been advanced for this, prominent among these the reportedly patchy enforcement, and thus implementation, of the new Chinese rules enforcing the move away from QST towards MA product. It may be simply a matter of time before the effect becomes more clearly visible. Moreover, one of the factors depressing vanadium demand earlier this year was substitution in favour of niobium by Chinese mills hard pressed on costs to meet the new specifications. According to preliminary data, Chinese imports of niobium rose by 35% year-on-year in the first half of 2019 - a sign that the new rules are having an effect.

Figure 2: Vanadium prices’ volatility

Source: Australian Vanadium Ltd, May 2019 Corporate Presentation

What could the change mean for the steel industry?

The gathering momentum for a shift towards greater use of MA rebar has the potential to create winners and losers among the different market players. These include full-cycle steelmakers-rollers, who can control their own metallurgy and choose to go down the MA route, the rerollers, who do not have this option, and the wider community of market stakeholders who have a role in influencing standards.

It has always been an oversimplification to describe rebar as a “commodity” but 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 market.

If that happens, and if the average reroller should 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.

References

1, For example the CRM Group, an engineering concern backed among others by ArcelorMittal and Tata Steel, alone lists around 60 mostly European rebar production lines commissioned under its direct supervision and licensed to use its trademarked TEMPCORE process, which is one of the key QST processes in the market.

2, “Quench and Tempering: An Unfamiliar Process in North America”, 2010 AISTech Conference Proceedings, abstract accessible at http://digital.library.aist.org/pages/PR358-202.htm

3, Standard NZS 3101-1: Concrete structures standard - The design of concrete structures rules out product “manufactured by the in-line quenched and tempered process […] where welding, hot bending, or threading of bars occurs”. https://archive.org/stream/nzs.3101.1.2006/nzs.3101.1.200 6_djvu.txt

4, Influence of New Rebar Standard on Chinese Vanadium Market, CISRI, presented at China's 2019 4th International Vanadium Forum, Chengdu, Apr.12-14, 2019

5, USGS, Mineral Commodity Summary Vanadium 2019

6, The CARES Guide to Reinforcing Steels, Part 3 - Properties of Reinforcing Steels

7, The Qualified Use of QT/TMT Rebars in the NSCP 20156 and Why this is Necessary; and A Clear and Present Danger – the Use of QT or TMT Rebars in Seismic Zone 4 by Emilio M. Morales CE, Principal of EM2A Partners & Co, Quezon City, Philippines

8, FOCUS: Why China’s implementation of new rebar policy is failing to support vanadium prices, Metal Bulletin 20 December 2018

9, http://vanitec.org/latest-from-vanitec/article/implementation-of-chinese-rebar-standard-has-resulted-in-89-of-surveyed-reb/

Christopher James Breckon

CEng, MSc, BEng, MIMechE

Twenty five years of senior management experience in the metals sector from Board level engagement to project management and delivery of major contract assignments.

Successfully established Farnborough Engineering Consultants Limited (FEC) having previously worked at McLellan and Partners Ltd. providing independent consulting, engineering and project management advice and support.

Specialised in the provision of studies and support to project developers, financiers and operators in the iron and steel, ferro-alloys, aluminium, copper, silicon, power, glass and building materials sectors worldwide.

Management of assignments including market studies, client and lenders’ engineer roles on major industrial and infrastructure projects, independent technical expert services, due diligence reviews and bankable feasibility studies.

As a Project Manager for leading metallurgical plant suppliers, including Siemens MT, VAI, Kvaerner Metals and Davy International, gained direct experience of all project phases from proposal and supplier co–ordination through to commissioning and start–up. Proven track record for timely delivery of projects that meet or exceed their financial targets.

 

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