Lower Strength Steels, Steel Grades
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.
Lower Strength Steels, Steel Grades
Metallurgy of Mild Steels
Mild steels are low carbon steels with no alloying elements added for substantial strengthening, and for that reason are characterized by relatively lower yield strength. Typically, mild steels have less than 0.10% carbon.
These steels have a microstructure that is primarily ferrite. The amount of pearlite in the microstructure is a function of the amount of carbon in the steel, with lower carbon resulting in a lower fraction of pearlite. More information about microstructural components is available here.
Ultra-low carbon steels are a type of mild steel. These grades are typically the lowest yield strength and highest ductility available. Generally, these steels have less than 0.005% carbon, or less than 50 ppm C. More information is contained on the page for Ultra-Low Carbon steels.
Different names may describe mild steels, but these differences came from steel mill production techniques that are no longer in use. Since all sheet steels in use today are continuously cast, there are no significant differences between these terms:
- Drawing Quality (DQ)
- Drawing Steel (DS)
- Aluminum Killed Drawing Quality (AKDQ)
- Drawing Quality Aluminum Killed (DQAK)
- Drawing Quality Special Killed (DQSK)
Mild steels are described in OEM, regional or global specifications with different syntax. While certain grade definitions between these specifications may be similar, the user is cautioned against using conversion charts without first confirming aspects which might be different, such as minimum, maximum, or typical values of chemical or mechanical properties. For example, ASTM specifications which cover lower strength steels list only typical values for tensile properties, but these are non-mandatory and the user may receive product outside the ranges shown. Additionally, JIS specifications which cover lower strength steels do not have minimum yield strength requirements, and the minimum elongation varies by thickness.
Some of the specifications describing uncoated cold rolled mild steel are included below, with the grades typically listed in order of increasing ductility. Different specifications may exist which describe hot or cold rolled, uncoated or coated, or steels of different strengths. Many automakers have proprietary specifications which encompass their requirements.
- ASTM A1008M, with the terms CS, DS, DDS, and EDDSA-25
- EN10130, with the terms DC01, DC03, DC04, DC05, and DC06D-3
- JIS G3141, with the terms SPCC, SPCD, SPCE, SPCF, and SPCGJ-2
- JFS A2001, with the terms JSC270C, JSC270D, JSC270E, JSC270F, and JSC260GJ-23
- VDA239-100, with the terms CR1, CR2, CR3, CR4, and CR5V-3
Conventional HSS, Steel Grades
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
