Revolutionizing Rolled Materials: Endless Hot Rolled Coil (eHRC) from Arvedi ESP

Flat steel products have historically been divided into two main groups: hot-rolled coils (HRC) and cold-rolled coils (CRC). The invention of Arvedi Endless Strip Production (ESP) technology, which combines continuous casting and rolling of ­hot-rolled coils, introduced a new class: endless hot-rolled coils (eHRC). eHRC extends hot-rolled material into applications previously served only by cold-rolled products.

One of the first goals driving the development of the “endless” process in casting and rolling flat products was the need to produce thinner gauges as hot-rolled coils (HRC). Achieving stable production of gauges below 1.2 millimeters with a conventional hot rolling mill, which uses thick slabs, is challenging. The main factors limiting such production are:

1. The high reductions during rolling that affect mill stability, and
2. The difficulty of threading very thin strip heads into the finishing mill stands and transporting them to the downcoiler without a high cobble rate.

In response to these challenges, the “thin slab” design concept in coil-to-coil mode slightly reduced mill instabilities due to lower reductions required, but did not solve the “flying strip head issue.” By the late 1980s and early 1990s, the semi-endless process was introduced as a tentative solution.

This process fed the mill with very long, thin slabs, corresponding to several coils each, if the tunnel furnace between the caster and mill permitted. This configuration allowed the first coil from the slab to be rolled at a reasonable gauge to reach the downcoiler safely. The thickness of the following coils was progressively reduced before being divided into separate coils by the final shear installed just in front of the downcoiler.

The “semi-endless” process enabled the first commercial-scale production of coils below 1 millimeter, but it also had limitations. Only a minimal amount of thin-gauge coils could be produced in a “semi-endless” campaign. Moreover, the length of tunnel furnaces necessary to deliver the extensive “mother slabs” made the mill layout extremely long, increasing the gas consumption needed to reheat these huge tunnel furnaces. These limitations made the semi-endless mode an exception in hot strip mills.

The game changer was the development of the endless process, which, for the first time, enabled casting and rolling to be linked in a continuous sequence. Instead of cutting slabs after casting, the process keeps the material intact throughout rolling. The individual coils are separated only at the end of the rolling process, just before the down coiler.

The “flying head” problem was eliminated because only one strip head had to be threaded at the start of the casting sequence. The production of ultra-thin gauges could then continue uninterrupted for ­hundreds of kilometers.

The dawn of the new Endless Strip Production era began with the startup of the Arvedi ESP plant in ­Cremona, Italy, in 2009, in partnership with Primetals Technologies. The stable, industrial-scale production of massive quantities of thin and ultra-thin strips below 1 millimeter in thickness became a reality. The goal for the first ESP plant was to reach a thickness of 0.8 millimeters, a milestone achieved in 2010. However, this was just the beginning. Chinese ESP customers have already optimized their production lines, reaching a thickness of 0.7 millimeters and even a 0.6 millimeters minimum gauge, which was achieved in 2018 by Rizhao Steel Group Co., Ltd.

The advantages of this progress are evident. Many products that previously needed hot and cold rolling to reach the desired final thickness can now be produced directly as hot-rolled coils. Cold rolling cannot be entirely omitted because the final thickness must be smaller. Customers can still benefit from using thinner input material to decrease both CAPEX and OPEX in their cold rolling complexes.

The percentage of the market currently served by cold-rolled coils (CRC) that eHRC can target is significant, as seen in Figure 2. The thickness of the eHRC is a game changer, and the resulting geometrical tolerances and mechanical uniformity are equally outstanding.

Cold-Rolled Substitute Products

When cold rolling is omitted, the further downstream annealing process, which reduces internal tensions generated within the material by deformation at low temperature in cold rolling mills, can also be avoided. The correct comparison is between an eHRC, sold as pickled and oiled, versus an annealed CRC for a given material with a final thickness requested by the end-user. In most cases, according to the experience of Arvedi ESP plants worldwide, no skin-passing is required when working with Arvedi ESP. This process is only called for when the final use demands smoothing the coil surface roughness, which is provided by the pickling process.

Combining processes or even eliminating them leads to significant cost savings. Figure 3 illustrates this: the upper part compares the Arvedi ESP operational expenses (OPEX) with the conventional process. The lower part expands on this by showing how cold rolling and annealing steps can be eliminated.

The ESP concept was initially conceived as a cost-winning answer to the production of thin and ultra-thin gauges not reachable by conventional technologies and perfectly fitting for commodity applications. However, the outstanding quality performance of the coils produced in endless mode was demonstrated to be so evident that a new, very profitable market segment emerged. This market includes thicker end products from endless casting and rolling, including high-value-added grades with advanced mechanical properties.

Producers soon discovered that the uniformity during casting and rolling provided by the endless process represented an advantage for producing HSLA, AHSLA, API, and silicon steel. This is particularly important for markets where both geometrical properties and uniform microstructure are critical, such as automotive and electric applications.

These advantages, coupled with reductions in cost and environmental impact, have always been part of the DNA of the Arvedi ESP concept. Above all, Arvedi ESP can cover the full range of products currently produced by conventional hot strip mills. It offers greater efficiency, quality, and reliability. The process is capable of producing thin and ultra-thin gauges, as well as all typical coil thicknesses made by conventional plants: up to one inch, or 25.4 millimeters.

This includes hard-to-produce grades, such as peritectic steel, which must be cast at low speeds to ensure quality. Thanks to specific solutions developed for Arvedi ESP, the caster and mill can be uncoupled. This flexibility allows them to operate in non-endless mode when needed.

This evolution in Arvedi ESP has been made possible by:

• Increasing slab thickness, allowing significant ­reduction rates for thicker strips
• Developing optimized roughing mill configurations with three or even four roughing mill stands
• Introducing multi-mode induction heaters capable of efficiently controlling heat transfer to thin and thick transfer bars, making endless operation ­energetically convenient for thicker gauges
• Implementing patented cooling and power cooling features for each thickness and steel grade
• Using advanced rolling strategies to optimize the number of finishing stands involved in rolling
• Developing powerful shears to cut thicker strips in endless mode.

Over the years, a diverse range of eHRC grades has been developed to meet the needs of various markets. HSLA (High Strength Low Alloy) components, which ensure lightness and robustness for vehicle structures, have seen impressive development in recent years for the automotive market. With typical thicknesses between one and two millimeters, eHRC with strengths up to 700 MPa are ideal for such applications.

These automotive applications constituted almost 30% of the market served by the Acciaieria Arvedi plant in Italy, the cradle of ESP, out of 42% of HSLA, with thicknesses ranging from one to over eight millimeters. These materials are used for structural components (e.g., car bodies), mechanical components (e.g., distribution shafts), and precision tubes.

API Pipe Grades

• 2-step rolling configuration enables perfect control over the temperature profile.
• Thermomechanical rolling and “pancaking” achieve the desired fine-grain structure.
• Reduced alloying element content lowers the ­Niobium needed for grain structure refinement.
• Production capability: API X70 grades up to 25.4 mm in endless mode

Advanced High Strength Steels (AHSS)

• Temperature profile control ensures the required ­multiphase structure.
• Produce DP (Dual Phase) and TRIP (Transformation Induced Plasticity)
• Produce up to 12.7 mm thickness in ­endless mode

Weather-Resistant Steels

• Produce CORTEN grades
• 1.4 mm thickness for container manufacturing

This ongoing journey of refining technology and ­operational expertise stems from over 15 years of ­collaboration with industry partners. Producers have manufactured more than 110 million tons of coils on ESP lines in endless mode. These achievements ­showcase the transformative potential of Arvedi ESP.

From its inception, Arvedi ESP has evolved far beyond its original role in commodity steel markets, emerging as a standard for producing diverse flat steel products. As the global focus on sustainability intensifies, transforming the steel industry into an environmentally responsible sector has become essential.

For customers in demanding markets like automotive, meeting expectations for sustainable steel is now a top priority. Arvedi ESP addresses this requirement with groundbreaking efficiency. It achieves remarkable reductions in energy use and greenhouse gas emissions by eliminating fossil fuels and leveraging the thermal energy in cast strands.

Carbon Footprint and Energy Efficiency

• Directly produces less than 1 kg of CO₂ per ton of eHRC Scope 1 emissions.
• 115 kWh/t for 120 mm thick cast strand down to 2 mm low-carbon steel in endless mode, reducing Scope 2 emissions.
• No fuel required thanks to the absence of a tunnel furnace, fully exploiting the thermal energy of the cast strand.

While residual elements like copper, tin, antimony, lead, and phosphorus are essential considerations for new plants designed with electric arc furnace (EAF) meltshops and increased scrap rates, Arvedi ESP manages tramp elements with ease. In conventional tunnel furnace setups, slabs are exposed to prolonged high temperatures, leading to surface cracks (hot shortness) due to the copper content. With rapid processing time of just one minute, the Arvedi ESP tolerates nearly double the copper content without compromising quality. Moreover, eliminating a tunnel furnace means a scrap-based electric arc furnace meltshop combined with an Arvedi ESP is an ideal, sustainable combination.

Endless hot-rolled coils (eHRC) have emerged as more than just a product of the advanced production method. They represent a distinct product class with unique characteristics. Successful products with their specific properties, such as the Arvtech® steel family (from Arvedi Steel), RE® (Rizhao Endless), DE® (Dingsheng Endless), and TE® (Taihang Endless) from Rizhao Steel, Fujian Dingsheng Steel, and Taihang Steel, respectively, showcase the metallurgical precision and uniformity of eHRC. This makes them a viable alternative to CRC in various applications. With faster production cycles and reduced environmental impact, the Arvedi ESP redefines the standards of rolled steel.

The Arvedi ESP process represents a paradigm shift in steel production, combining economic efficiency, sustainability, and superior product quality. Seamlessly integrating advanced technologies opens new possibilities for the steel industry, meeting the demands of modern applications while reducing environmental impact. For engineers and industry stakeholders, eHRC is not merely an improvement but a transformation, setting the stage for the future of rolled materials and leading the next generation of sustainable steel solutions.