Daniel Schaeffler Daniel Schaeffler
President

A Brief History of Steelmaking

August 31, 2020


Cast iron, wrought iron and steel have a history going back more than two millennia. Mass production of cheap steel began in the late 1800s with the invention of, and subsequent improvements to, the Bessemer converter. Used in the manufacture of rail steels, the process based on this converter produced steel of sufficient strength and durability to accommodate the heavier and faster railway engines and cars that developed during the rapid expansion of the railroad industry.  

US_Steelmaking_Process_Percentages_1950-2012The Bessemer process used the oxygen in blown air to remove impurities in molten pig iron. Unfortunately, air is comprised of 78-percent nitrogen, resulting in significant steel brittleness and which limited potential applications. Each 25-ton batch of molten steel, called a heat, took only 20 min. to produce, but that did not allow enough time to control the composition and remove unwanted elements such as phosphorus. Also, the Bessemer process did not accommodate the use of scrap metal as feedstock.

A Better Process Leads to the Steel Car

Open-hearth steelmaking replaced most Bessemer converters by 1900. This new process allowed for economical use of higher-phosphorus iron-ore deposits available in the United States, as well as the ability to use scrap metal as an input. The slower process, taking as long as 10 hr. per 200-ton heat, allowed for better control of composition and temperature, which in turn provided the ability to create new grades. These steels fueled the initial all-steel-body vehicle made for the Dodge Brothers Motor Co. by the Budd Co. in 1916, as well as the construction of skyscrapers and bridges, including the Empire State Building and the Golden Gate Bridge. More importantly to some, 1935 saw the introduction of the steel beer can by Pabst Brewing Co. Through the early 1960s, more than 80 percent of all steel produced in the United States came from the open-hearth process.

The mid-20th century welcomed the commercialization of oxygen steelmaking, which rapidly became the primary method to convert hot metal from a blast furnace into steel.  During the basic oxygen process (BOP), oxygen blown into a combination of molten pig iron and scrap metal refines the product into steel. Some steelmakers blow oxygen from the top onto the surface of the melt, in a basic oxygen furnace. Other options are the Q-BOP process, relying on bottom-blown oxygen steelmaking, or a combination of top and bottom blowing. In all cases, the refractories that line the vessel holding the molten steel are basic rather than acidic—hence the name.

This modern approach to steelmaking uses the same principles seen in the Bessemer process, but is substantially more efficient, with typical production rates of 250 tons every 45 min. A typical charge contains as much as 25-percent scrap metal, far beyond what the Bessemer Process allowed. Blowing commercially pure oxygen into the melt eliminates the detrimental nitrogen present in the air used back in Bessemer’s time. Oxygen steelmaking results in a low-residual, low-nitrogen product.

Welcome the Minimill

The growth of oxygen steelmaking starting in the 1960s coincided with developed countries generating more obsolete scrap following the post-WWII expansion period. Due to more available scrap combined with lower scrap-metal consumption in the oxygen steelmaking process as compared to the open-hearth process it replaced, excess scrap became readily available at relatively low cost. Favorable economics led to the growth of steelmaking in an electric-arc furnace (EAF). These scrap-based steelmaking facilities, called minimills, do not require as much start-up capital as required for the blast furnace/BOP integrated approach, nor do they require as large of a footprint. During the 1970s and 1980s, minimills began producing construction materials such as concrete-reinforcing bars. Whereas a blast furnace produces molten hot metal for the BOF shop continuously for years at a time, and even a brief shutdown incurs significant costs, minimills provide a significant flexibility advantage since operators can shut them down when a drop in orders makes it difficult to justify production. 

Starting in 1989, minimills expanded their ability to produce flat-rolled steel by introducing the compact strip production (CSP) process. CSP pairs EAF steelmaking with casting of 2- to 3-in.-thick slabs, immediately followed by hot rolling. In contrast to the integrated approach, where 9- to 10-in.-thick slabs are cooled and then reheated prior to hot rolling, CSP requires fewer steps and reduced energy requirements. The reduced thickness input into the hot-rolling mill enables the production of thin-gauge hot-rolled steels at thicknesses previously associated only with cold-rolled steels. Control of scrap quality and use of scrap substitutes, such as direct reduced iron, contributed to vastly improved product quality. Today, the CSP and BOF processes both produce high-quality world-class steels, with EAFs producing some 70 percent of the steel made in the United States, and BOFs responsible for the remainder. Note: EAF represents only about 30 percent of global steelmaking, owing (at least in part) to local scrap availability.

Molten-Steel Refinement

Whether melted using the BOP or EAF process, molten steel needs further refinement in composition and temperature prior to casting into slabs. This fine-tuning occurs during secondary steelmaking at the ladle-metallurgy station.  One type of composition adjustment happens during vacuum degassing, which achieves ultra-low carbon levels when carbon and oxygen combine and bubble out of the steel. Other ladle-metallurgy modifications include the removal of non-metallic inclusions, precise control of alloying elements and chemistry homogenization in the liquid heat.

Aluminum-Killed Steels

A bit of terminology history: Casting of all sheet steel currently is continuous, going directly from melt to slabs. Through the end of the 20th century, ingot casting remained an option at some companies. In ingot casting, excess oxygen dissolved in the melt led to continued reaction with carbon, resulting in carbon monoxide bubbling out of the still-liquid ingot, which changed the composition and created voids that remained in place during solidification.

Avoiding this bubbling required a deoxidation practice, by adding aluminum or silicon to the melt. Oxygen preferentially combines with both elements rather than with carbon, producing alumina or silica rather than bubbles of CO. Effective deoxidation resulted in little or no gas evolution during solidification, “killing” the steel since the melt lay quietly in the mold. This is the origination of the term aluminum-killed steels. MF

Industry-Related Terms: Bridges, Inclusions, Scrap, Surface, Thickness, Wrought
View Glossary of Metalforming Terms

 

See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Materials

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