The innovative Minimill

WinLink – Flexible solutions for profitable direct rolling of bars

The conventional minimill is characterized by capacities ranging from 300,000 t/a to 600,000 t/a, use of locally available scrap, low impact on the electricity grid, a high degree of flexibility and low investment expenditures. However, the relatively long payback period of a standard minimill is a disadvantage for smaller production capacities. As an answer to this situation, Primetals Technologies has introduced WinLink (Figure 1). This directly linked caster-rolling mill solution reduces total final product costs in a highly competitive product-cost-based market by minimizing capital and operational expenditures (capex and opex), as seen in Figure 2. Savings are achieved through the use of innovative technologies. Thanks to the resulting cost advantages, WinLink production lines are even competitive starting at outputs of 300,000 t/a of rolled products for construction applications. With higher production capacities, the economy of scale is improved and profit margins increase. Payback time is reduced from seven years to less than five years compared to standard minimills of the same capacity. For example, a WinLink70 plant variant with an annual production of 450,000 t allows a 25% internal rate of return (IRR) to be obtained, which is the same as a larger conventional mini­­mill producing more than 700,000 t/a.

The WinLink concept

WinLink is based on the direct linking of a multi-strand billet caster and a rolling mill with high availability. Instead of a conventional billet reheating furnace, an advanced induction heating unit is employed. Liquid steel coming from an electric arc furnace or LD (BOF) converter is cast to billets and rolled into bars in a continuous, endless production line. While the typical application is for an output of between 300,000 t/a and 600,000 t/a of rebars, small flats and profiles may also be processed. Three different WinLink variants with different product dimensions and capacities have been developed: WinLink55, WinLink70 and WinLink85. Each of these variants can be augmented with the WinLinkFlex solution to enhance production capacities by an additional 30% (Table 1). In a WinLink line with a single high-speed billet caster, it is possible to produce from 45 t/h to up to 85 t/h. With the installation of an additional high-speed casting strand, the increased billet output can accommodate the production capacity of the meltshop. WinLink85 represents the most complete WinLink configuration. It includes a cantilever inline stand positioned between the continuous rolling mill and the hydraulic head-cropping shear (Figure 3). It reduces a rectangular 200 mm x 150 mm cast billet to a 160 mm x 160 mm billet, which is then fed to the continuous rolling mill. Line productivity can thus be increased by up to 600,000 t/a with this special mill stand. The WinLinkFlex minimill option represents a major step forward in the WinLink casting-rolling process. This variant allows a semi-endless production mode to be implemented for a productivity increase of 30% when compared to the endless production mode. Thanks to the high flexibility of the billet caster, billets can be cast in lengths between 18 m and 24 m. The billets are then hoisted from the line by means of a crane and quickly transferred to the rolling mill to increase total rolling productivity (Figure 4).

Decisive capex and opex advantages offered by WinLink

Continuous casting machine

The WinLink caster is equipped with two or more high-speed strands in order to directly feed the rolling mill in endless mode, or in semi-endless mode by applying WinLinkFlex technology. A multi-strand configuration significantly improves the economy of scale of the meltshop and provides better flexibility when faced with unpredictable market changes. While the primary casting strand is dedicated to the endless feeding of the rolling mill, the additional strand(s) can be used for the production of flexible-size billets, which are both finished and saleable. These billets may be rolled in other mills, rolled in the semi-endless mode by applying WinLinkFlex technology, or sold on the market. The unique WinLink caster configuration enables rolling productivity to be increased by up to 30%. To obtain the casting speeds required for direct feeding to the rolling mill under stable casting conditions, a state-of-the-art billet caster is required. Implemented technologies to enable high-speed casting include tundish flow control, mold-level control, high-speed Diamold tubes, Dynaflex hydraulic oscillators, as well as technologies for enhanced secondary cooling and continuous straightening.

Induction furnace

After cooling the cast strand to the temperature required to guarantee solidification of the core, the temperature of the strand is uniformly equalized for rolling in a compact induction furnace with a high energy-transfer capacity. The IGBT (insulated-gate bipolar transistor)-controlled induction furnace is designed to provide up to 200°C of temperature increase. Its installed power ranges from about 2,000 kW to 4,000 kW with efficiency of more than 75%. The specific consumption of the IGBT induction furnace ranges from 15 kWh/t to 45 kWh/t, versus 180 kWh/t to 200 kWh/t for a gas-fired furnace with conventional hot charging. CO₂ emissions are thus significantly reduced.

WinLink configuration with inline cantilever stand
WinLinkFlex variant with billet-transfer system

Rolling stands

The housingless and double-support design of Red Ring stands has been implemented in more than 6,000 individual installations. This mill stand – currently in its fifth generation – is renowned for its high rigidity, durability and ease of maintenance (Figure 5). For a rebar mix with diameters ranging from 8 mm to 40 mm, the WinLink mill includes 18 to 20 Red Ring stands. Smaller dimensions are multi-slit (up to 4x) to increase their productivity with finishing speeds between a maximum of 22 m/s in a high-speed delivery system and 15 m/s in a setup with an apron delivery table. Intermediate and finishing stands are equipped with a quick-change system to minimize change time. To prolong production campaigns, the finishing stands are outfitted with carbide rolling rings mounted on shafts. Groove change is achieved by a fast guide-changing system that is integrated in a motorized rest bar. Stands and guides are completely preset and adjusted offline, while both stand and drive components use bearings with an extended lifetime.

Power Slitter

The Power Slitter (Figure 6) is equipped with slitting discs driven by motors with variable current control. It provides highly accurate and consistent slitting cuts of the preformed strand that promotes stable working conditions. The Power Slitter represents an important advantage over the traditional cutting operation, which is done by means of slitting guides that offer no control, require very precise alignment with the rolling stand, and have to be positioned as close to the rolling roll as possible. Besides offering higher speeds and better accuracy, slitting discs also have a lifetime up to ten times longer than slitting guides.

Red Ring stands equipped with a quick-change system
Power Slitter

Quenching and tempering

The nozzles used in the quenching-tempering system are characterized by their high cooling capacity and efficiency. This contributes to a compact equipment arrangement; increased lifetime of pumps, valves and other critical components; and reduced water consumption. In comparison to other systems, the lower water flow reduces the tendency for cobbles, which improves the reliability, availability and output of the line. When required, water boxes can be easily removed from the line and replaced by means of a transverse exchange system – instead of having to be hoisted out with a crane. This minimizes exchange times.

Finishing equipment

Bar hot dividing is optimized so that only multiples of commercial product lengths are sent to the cooling bed, while shorter lengths are removed by a dedicated scrapping unit. Following cooling, cold commercial cutting is done by a static shear. Alternatively, hot commercial cutting may be considered. In this case, a high-speed, hot-cutting station is positioned at the entry section of a shorter cooling bed. A fully automated bar-counting station is installed between the cooling bed and the bundling station, which is designed for high production rates and boasts consistent accuracy exceeding 99.9%.

Automation and condition monitoring

Advanced automation systems are applied to monitor and control the entire production process and to ensure that the required quality demands are met. Steel grades and products are carefully tracked throughout the entire line, all the way to final product dispatch. The latest condition-monitoring systems are also employed to allow real-time monitoring of the main equipment parameters so that symptoms of developing failures may be detected early on and corrective actions quickly implemented. Mill operators are assisted with predictive maintenance techniques to maximize equipment lifetime. Condition monitoring thus contributes to cost-efficient production, full utilization of wear parts and reduced plant downtime.

Decisive economical and production benefits

In conventional minimills, the production of smaller quantities of standard carbon-steel grades for the construction industry and infrastructure applications is not always attractive from a financial standpoint. In comparison, WinLink offers attractive process variants with decisive economical and production benefits for bar producers. These include the reduction of the total final product cost, low investment expenditures, decreased transformation costs, major energy savings, higher mill yield, smaller space requirements and lower CO₂ emissions. Furthermore, WinLink additionally offers the potential to exploit the capacity of a full-size meltshop by balancing production between rolled products and saleable billets.