Metallurgy of Ultra-Low Carbon Steels (DDS – EDDS)
ULC, IF, VD-IF, and EDDS are interchangeable terms that describe the most formable (high n-value) and lowest strength grade of steel.
Ultra-low-carbon (ULC) steels typically carbon levels less than 0.005%, or 50 parts per million. At these low alloying levels, the atomic structure is primarily iron, with unfilled spaces or gaps (called interstices) between the atoms – the origin of the term “interstitial-free” or IF. Molten steel needs an additional process prior to casting called vacuum degassing (VD) to reach these carbon levels. Because this steel alloy is mainly iron and all pure elements are very formable, it is also referred to as either deep drawing steel (DDS) or extra-deep-drawing steel (EDDS). Specifications which contain ULC grades are listed within the Mild Steels page.
Adding phosphorus to an IF grade increases the strength due to solid solution strengthening, precipitation of carbides and/or nitrides, and grain refinement. These higher strength IF-HS grades are widely used for both structural and closure applications. Work hardening from forming will increase panel strength, and is sometimes called a dent resistant steel grade. However, this alloying approach is not capable of producing a bake hardenable grade.
ULC, IF, VD-IF, and EDDS are interchangeable terms that describe the most formable (high n-value) and lowest strength grade of steel. Adding phosphorus, manganese, and/or silicon to these grades increases the strength due to solid solution strengthening, precipitation of carbides and/or nitrides, and grain refinement.
For most alloys, steelmaking practices attempt to reduce phosphorus to very low levels, since increased phosphorus content is sometimes associated with an increased risk of embrittlement. However, in the ladle metallurgy station after steelmaking, small controlled amounts of phosphorus are added back to the melt when certain grades are produced, leading to the term “rephosphorized.” Phosphorus is a potent solid solution strengthening element, where only small additions result in large increases in yield and tensile strength.
When phosphorus or other solid solution strengthening elements are used to increase the strength of interstitial-free steels, IF-HS (Interstitial-Free High Strength) steel is produced. Using phosphorus leads to the term IF-Rephosphorized steel, or IF-Rephos.
These alloys have composition controlled to improve r-value. In some products, small amounts of boron are added to counteract the embrittlement effects brought on by the phosphorus.
These higher strength IF-HS grades are widely used for both structural and closure applications. Work hardening from forming increases panel strength, which is why they may be described as dent resistant steels. However, this alloying approach is not capable of producing a bake hardenable grade.
Compared bake hardenable steels, carbon-manganese steels, and HSLA steels at similar strength levels, IF-HS grades are more formable, resulting from the ultra-low carbon chemistry and interstitial-free microstructure.
Some of the specifications describing uncoated cold rolled interstitial-free high strength (IF-HS) steel are included below, with the grades typically listed in order of increasing minimum yield strength and ductility. Different specifications may exist. Many automakers have proprietary specifications which encompass their requirements. Note that EN and VDA terminology is based on minimum yield strength, while JFS standard is based on minimum tensile strength.
- EN10268, with the terms HC180Y, HC220Y, and HC260Y D-17
- VDA239-100, with the terms CR160IF, CR180IF, CR210IF, and CR240IF V-3
- JFS A2001, with the terms JSC340P, JSC370P, JSC390P, and JSC440P J-23