Gerdau Modernizes Mill to Serve the North American SBQ Steel Market
Gerdau recently invested more than $150 million in its Monroe, Mich., mill to improve its efficiency of operation and enhance its SBQ product offerings. The push toward cleaner steel has been supported through this modernization and by melt-practice enhancements and strict process controls to produce bearing-quality steels in its continuous caster.
Due to the increased need for forged parts with improved performance and better fatigue life, special-bar-quality (SBQ) steel producers are being held to higher expectations than ever before. While the automotive industry is driving this need for increased performance, markets including agricultural and construction equipment, industrial and energy, and other transportation markets have this same expectation.
Gerdau, a leading supplier of SBQ steel, has prepared to better serve the forging industry through modernizing its processes and equipment, finding ways to improve steel cleanliness and remaining intensely focused on forgers.
Modernizing the Monroe Mill
Driven by the changing characteristics for forged applications, Gerdau recently invested $155.6 million on state-of-the-art manufacturing equipment and process-control improvements at its SBQ mill in Monroe, Mich.
The expansion began in late 2012 with melt-shop improvements, which included the commissioning of a new larger-section (210-mm square), four-strand continuous caster equipped with the latest online automated process controls to provide high-quality blooms. The larger section gives the final product a higher reduction ratio, which greatly enhances overall internal quality.
The new caster also features a hydraulic oscillator, which allows oscillation to be adjusted to fit the characteristic needs of each unique grade of steel being produced. The line includes automated slag carryover detection and control from ladle to tundish coupled with a larger tundish for added flotation time. Argon shrouding and a one-piece submerged ceramic nozzle from tundish to mold prevent re-oxidation during casting and enhance steel cleanliness. Closed-loop flow control from tundish to mold and automatic mold-powder monitoring and additions also improve surface quality.
Advancements continued into 2014 with the commissioning of a new twin-tank vacuum degasser to support Monroe’s high-production environment. This gave the mill enhanced capabilities for lower-torr vacuum pressure and increased time under vacuum. In addition, the unit is equipped with automated cored alloy-wire feeding. This advanced equipment and capability results in exceptional cleanliness and precise chemistry control.
In 2015, a new walking-beam reheat furnace and six additional rolling-mill stands were installed to process the larger 210-mm blooms. The walking-beam furnace improves surface quality and lowers decarburization through better temperature and soaking-time control. A new pass design was developed to optimize reduction between stands in an oval to round configuration for a gradual reduction at each stand. This greater reduction ratio of rolled bars results in improved surface- and internal-quality characteristics.
As a complement to the enhancements made in the melt shop and rolling mill, the latest technology for value-added straightening and inspection capabilities was added in a new facility, which was designed to optimize process layout and product flow. The facility houses a new 10-roll high-speed automatic straightener and NDT capabilities, including ultrasonic testing with phased-array technology and magnetic-flux-leakage surface inspection and 100% in-line X-ray grade verification, allowing for greater discernment of quality attributes.
All of these equipment and process enhancements result in higher-quality engineered steel bars that support the forgers’ need to meet the challenging performance expectations of the end users. These improvements also expand the array of applications that can be supported by forged SBQ steels.
Advancements in Steel Cleanliness
The global push for energy efficiency and stricter CO2 emission regulations has significantly increased the importance of clean steels in the automotive industry. The need for high-strength steel parts with lightweight design and increased fatigue life and performance is more important than ever. Bearing steels used in safety-critical parts – including wheel hubs, constant-velocity joints (CVJs), transmission and differential gears, steering racks and pinions, and others – are clear examples.
Steel micro-cleanliness is critical to meet these needs and requires strict adherence to reliable melt-shop practices. Gerdau’s team of R&D, process and product metallurgists has made significant strides in sustainable steel cleanliness through enhanced melt-practice developments and strict process controls to produce bearing-quality steels in its continuous caster. Improved clean-steel practices have been developed to reduce the size and frequency of inclusions. These improvements have been validated using the latest automated SEM, extreme value analysis (EVA) and ultrasonic immersion testing equipment with low-, medium- and high-frequency capability. In addition, constant development of optimal refractories and fluxes are required to enhance clean-steel capabilities.
The latest generation of wheel hubs and spindles is a perfect example. This highly stressed application has evolved over the last few years into today’s wheel-bearing assembly. The Gen III design unit, which is permanently joined during the manufacturing process, consists of a medium-carbon steel spindle and outer ring. Any surface or subsurface nonmetallic inclusions could cause spalling and eventual failure. This occurs because the localized stresses are sufficient to cause micro-cracking around the inclusion, which propagates into larger, more severe cracking and eventually results in a spall failure. Modern clean-steel technologies mitigate these risks.
Another example where clean steels are necessary is for high-quality transmission gears. These components are mostly low-carbon, fine-grained, carburized parts where steel cleanliness is highly correlated to fatigue life. Due to high carburizing temperatures, grain coarsening may become an issue.
High levels of contact stress make the gear teeth susceptible to premature failure. Gerdau has engineered special grades focusing on higher cleanliness while maintaining the fine grain structure. These grades save time and costs in heat treating while providing higher fatigue life to the gears.
Commitment to Forgers
Gerdau has been focused on supporting the forging industry for more than 40 years and will continue to expand its capabilities to ensure that North American forging remains globally competitive. With state-of-the-art equipment and a team of experienced professionals who are committed to providing excellent service and quality, Gerdau will remain a partner to the forging industry and is prepared to support the challenges in this ever-changing environment.
For more information: Co-author Patrick Doyle is chief metallurgist at Gerdau and may be reached at firstname.lastname@example.org or 734-384-6605. Co-author Eduardo Scheid is R&D tech. specialist at Gerdau and may be reached at email@example.com or 479-653-2696. Co-author Denise de Oliveira is also an R&D tech specialist and may be reached at firstname.lastname@example.org or 734-384-6726.