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
Microstructural Components
Steel grades are engineered to achieve specific properties and characteristics by the manipulation of mill processing parameters to achieve a targeted balance of microstructural components. Among the tools available to the steelmaker are alloy composition, rolling and processing temperatures, and cooling profile.
If steel is slowly cooled, only two components exist at room temperature: ferrite (abbreviated by the Greek letter α) and cementite (iron carbide, Fe3C). Alternating layers of ferrite and cementite appear under a microscope in a pattern similar to Mother-of-Pearl, leading to the term pearlite.
A steel alloy having approximately 0.80% carbon will contain only pearlite in the microstructure. Lower carbon levels create an alloy that combines ferrite and pearlite. Ferrite-pearlite microstructures form the basis of many C-Mn steels and some of the initial HSLA steels. At a given strength level, pearlite limits sheet formability.
At carbon levels below 0.008% or 80 ppm, only ferrite exists. Ferrite is low strength but very ductile, and is the microstructural phase in ultra-low carbon steels.
Additional phases are formed when the cooling profile can be changed. Some modern annealing furnaces are capable of controlling the cooling rate as well as holding at specific temperatures. This ability is a key facilitator in the production of most Advanced High Strength Steels. In addition to ferrite and pearlite, microstructural phases of bainite, austenite, and martensite can be produced, depending on the chemistry and the thermal cycle profile including quench rate and hold temperature.
Bainite is a phase that is associated with enhanced sheared edge ductility. Accelerated cooling in the hot mill run out table allows for the production of Ferrite-Bainite steels.
Austenite is not stable at room temperature under equilibrium conditions. An austenitic microstructure is retained at room temperature with the use of a combined chemistry and controlled thermal cycle. Deforming retained austenite is responsible for the TRIP effect.
Martensite is a very high strength phase, but has limited toughness. Steels with both ferrite and martensite in the microstructure are known as dual phase steels. A structure of 100% martensite can be produced directly at those sheet mills having equipment capable of achieving a minimum critical cooling rate. The ductility of this product is not sufficiently high for most stamping operations. However, sheet martensite is well suited for properly designed roll forming applications.
Martensite is the microstructural component of processed Press Hardening Steels. An elevated temperature and a more formable microstructure exist at the time of complex forming of these grades. Rapid cooling while the part is under full press load converts the microstructure to the high strength martensite.
