A closer look at low-carbon sheet steels

Simply put, steel is just iron plus no more than about 2.1 percent carbon. (If the steel has more than 2.1 percent carbon, it’s called cast iron.) With low-carbon steels, the carbon level tops out at around 0.1 percent to 0.2 percent.

There is not much else in steel, but what is there is important for the properties you want. For example, a key property sought after in steel is strength. For most grades, as the strength increases, the formability or ductility decreases. We know that pure elements are very soft. This is why wedding bands are not made from pure 24-carat gold. Impurities are added to give the ring some strength and hardness, reducing the designation to 18-carat gold or less. (This is not to be confused with my wife’s cubic zirconium “diamond.”) Pure iron is similarly soft. The impurities added to iron to change the properties include not only carbon (C), but also manganese (Mn), silicon (Si), phosphorus (P), titanium (Ti), and other elements, depending on the alloy and steel mill processing.

If you were to put on your magnifying glasses, you would see that at an atomic level, pure iron (or any other element) looks like a 3-D network of racked billiard balls all the same size. To make a steel alloy, some of the iron billiard balls are replaced with ones made of Mn, Si, P, Ti, or some other elements, which are similar but not identical in size to the iron balls. The disruption in the pure iron atomic network caused by these alloying additions is responsible for what is known as solid solution hardening. As the alloying increases, the straining in the atomic 3-D structure increases, requiring more force to deform the sheet metal, making it a higher-strength steel. At low levels of alloying additions, the atomic network is still mainly iron, and therefore is relatively soft.

Even though all the balls touch, small gaps, interstices, exist between the balls. Only small atoms, like carbon, can fit in these gaps. These small atoms are called interstitial elements.

In ultra-low-carbon (ULC) steel, the carbon level is typically less than 50 parts per million, which is the same as less than 0.005 percent. As a result, most of the interstices are not occupied, making it an interstitial-free (IF) steel. Conventional steel mill processing can get the carbon level down to about 0.02 percent of the overall mix. To get to these ultra-low carbon levels, molten steel is processed under a vacuum, which bubbles out carbon, hydrogen, and nitrogen. This is the origin of the term vacuum degassed interstitial-free (VD-IF) steel. Because this steel alloy is mainly iron and all pure elements are very formable, it is also referred to as extra-deep-drawing steels (EDDS). So although you hear about ULC, IF, VD-IF, and EDDS, they all mean the same thing: the most formable and dent-prone grade of steel.

While there are dozens of different grades of steel—and probably as many different ways to describe them—let’s highlight two of them: drawing steel (DS) and extra-deep-drawing steel (EDDS). A typical yield strength/elongation combination of 180 MPa/40 percent can be associated with DS, whereas EDDS can be 140 MPa / 44 percent. As you can see in Figure 1, the main chemistry difference is that DS has higher C and Mn, while EDDS also has a microalloying element to stabilize the carbon in the steel. In a ton of low-carbon DS produced by an integrated steel mill, 10.6 lbs. of impurities are found in the iron matrix, and a ton of EDDS has 8.3 lbs. of impurities. Either way, it’s mostly iron.

Quite likely you’ve been exposed to a slew of different acronyms when dealing with steel. Again, some of them mean the same thing. It is also important to realize that most specifications have chemistry requirements, with only suggestions of typical values of strength and elongation. If you order steel based only on chemistry, the steel provider cannot guarantee that you will get consistent forming properties in different shipments. Steel mill processing can result in significant changes in strength and elongation in coils that have the same chemistry.

Figure 1. These are typical compositions of drawing steels and extra-deep drawing steels produced by integrated steel mill processing. (A mini-mill starts with different feedstock, so the alloying levels are slightly different.)