Nucor Steel has an extensive special bar quality (SBQ) steel program that dates back to the conversion of its plants in South Carolina, Tennessee and Nebraska to handle the product decades ago. In 2002, the company purchased an idled plant in Tennessee that was dedicated to SBQ production. Since then, Nucor has allocated staff and investment assets toward the continued development of its SBQ product lines.
Nucor Steel is North America’s most diversified steel and steel-products company, as well as one its largest suppliers of special bar quality (SBQ) steel. The company is committed to serving its various customers by catering to their specific, application-driven SBQ requirements. The company’s Engineered Bar group is comprised of facilities located in Darlington, S.C., Memphis, Tenn., and Norfolk, Neb. These facilities produce finished and semi-finished SBQ steel products ranging from 20.125-inch-diameter as-cast bloom down to 7/32-inch wire rod.
Over its 65-year history, Nucor has developed a reputation as a leader in quality by satisfying the stringent demands of customers. The Darlington and Norfolk divisions began operations in 1968 and 1972, respectively. While not originally built to service SBQ demand, both locations have successfully made this transition through significant upgrades. These have facilitated Nucor’s growth into the demanding SBQ market.
In 2002, the company purchased a relatively new (though idled in 1996) SBQ mill in Memphis, Tenn., When rebuilt and restarted in 2008, it became Nucor’s first dedicated SBQ mill. The modernization included installation of a new melt shop, continuous caster, rolling mill and finishing operation totaling about $520 million.
The steep learning curve of SBQ has led to collaborative customer relationships and extensive technical trials developing Nucor’s abilities to support the demands of SBQ. The company emphasizes partnering with customers to understand their needs and develop the best solution for their application.
Investments in Staff and Assets
Nucor’s technical staff of metallurgical engineers and quality-control teammates works closely with the technical-services and sales teams to quickly turn customers’ feedback and requests into process and product improvements. This results in a tailored experience for each customer, product and application. Ultimately, the company’s most important assets are its people and culture. Each Nucor teammate is dedicated to taking care of customers by striving to be the safest, highest-quality, lowest-cost and most productive steel and steel-products company in the world.
Investments totaling close to $1 billion have been made throughout the SBQ divisions to meet Nucor’s aim of reliably suppling forgers with premier SBQ steel. The benefits of these investments include improvements relating to control over chemical properties, inclusion engineering, product size diversity, dimensional tolerance, surface quality and inline nondestructive testing. Additionally, the company’s financial strength translates to supply-chain stability during times of economic uncertainty.
In 2015, Nucor Steel Memphis added an additional ladle-metallurgy station and a fourth strand to its existing continuous caster. Consistent forging response from batch to batch is enabled by precise chemistry control and consistent physical properties. The added ladle-metallurgy furnace (LMF) allows the Memphis operation the residence time to meet these quality demands. Together, the $51 million investment allows Nucor to meet the quality and quantity demands of a growing industry.
Steel is hot rolled to desired shape.
One of the challenges of SBQ is the necessity to control the population, distribution and chemical makeup of steel inclusions. This may range from no nonmetallic inclusions for bearing quality to significant amounts of intentional manganese sulfides for machinability. This is referred to as inclusion engineering.
An example of utilizing inclusion engineering is the crankshaft of a modern high-performance engine. This component is expected to withstand hundreds of millions of revolutions during the lifetime of the engine while also being economical to manufacture. The near-net shape of a forged crankshaft requires a significant degree of finish-machining prior to installation. Reduction in total machining leads to saved labor and tooling costs.
In this case, an engineered solution Nucor uses is the introduction of sulfur-cored wire to a steel melt to exploit the effects of manganese-sulfide inclusions. This specific inclusion works by providing a means for machining chips to break off at the cutting head, thus reducing tool contact time. Secondarily, it forms a lubricating layer on the tool that can further extend tool life.
However, this process requires a delicate balance. If too many inclusions are formed or if they are too large, the resulting product can have unfavorable fatigue properties. If there are too few inclusions, the part will not meet the desired machining properties.
Just as chemical control and inclusion engineering provide extra value to customers, Nucor’s extensive size range of semi-finished and finished SBQ material can meet virtually any forging need. The company’s facility has the largest cross-section rounds caster in the U.S., producing up to a 20.125-inch-diameter as-cast product down to 8.875-inch diameter. Customers appreciate the inherent quality advantages of a bloom cast in a round mold, which eliminates the challenges associated with internal and surface defects possible on the corners of square or rectangular billets. The sound internal quality, dimensional tolerance and surface finish are achieved through expert control and manipulation of casting parameters and liquid-steel quality. Consequently, semi-finished product has adequate quality and is often used directly in forging applications.
Though melting and casting parameters are significant to the internal quality of a forging, the performance of Nucor’s rolling mills are critical to near-net forging and cannot be overlooked. Hot working refines the microstructure of the steel by breaking up the grains during elongation to form a homogenized and smaller grain structure, which in turn improves mechanical properties. The degree to which hot working affects the final properties is proportional to the amount of work done.
Depending on the style and design of the forging process, poor dimensional control of material can result in poor quality for the finished forging. Nucor understands that the importance of dimensional control is critical for its forging partners.
During stock preparation, mults are typically cut to a set length based on a targeted theoretical weight. If the dimensions of the bar are not consistent, a mult weighing more than theoretical will require additional machining. Undersized stock can lead to insufficient die fill, on the other hand, resulting in a scrapped part. Round bars exceeding the ovality tolerance or out-of-square bars create issues for the forger. The unexpected distribution of out-of-dimension material in the die can result in unfavorable flow patterns, the development of weld lines in inappropriate locations and areas with too much or too little material.
Finished rolled bar
Nucor typically performs nondestructive testing (NDT) on its finished product. Inline ultrasonic testing is performed on rolled bar up to 8.75 inches in diameter. This not only ensures optimum quality but is utilized as a process feedback mechanism for continuous improvements. Based on customer and product specifications, Nucor also has the capability to test material in additional offline NDT. This consists of both ultrasonic and thermographic inspection.
The offline ultrasonic testing consists of an identical machine to that of the inline testing setup, but it is run at a slower pace with higher sensitivity. Offline inspection is a more focused near-surface and core flaw-identification technique. Thermographic surface inspection works by lightly heating the surface of the material and then imaging the resulting piece as it cools. Surface imperfections will cool differently than the surrounding areas. This map can be used to grade bar, or it can be used as a guide for surface cleanup. These investments total roughly $30 million and constitute an integral piece of Nucor’s quality system.
Nucor is committed to continue its growth as a valued and reliable SBQ supplier to its current and future forging customers. The company intends to do this through continuous improvement, close customer relationships, careful investment, advanced technical development and commercial excellence.