A classy idea
A voestalpine proposal: Classification of steels based on further processing
voestalpine is proposing a more uniform nomenclature that is based on the further processing of the material and that would more effectively characterize material properties and optimum fields of application. At voestalpine, we like to constantly focus on customer benefit in every area of our business, and the naming of AHSS steels is no exception. What would the nomenclature for the classification of steels look like if it were to focus on the further processing of the material? Which parameters would be relevant to the system? voestalpine would like to open this up for discussion.
Current material classifications: Steel grades are classified based on their microstructure (DP), their alloy design (LA), special metallographic mechanisms (TRIP) or their manufacturing process (Q&P). These classification systems have grown historically, but they do not always characterize the needs of those who process the steels.
- The current classification does not do justice to characterization of the optimum application of the respective grade.
- Microstructure-based descriptions are very complicated for very fine-grained microstructures.
- In spite of their different microstructures, alloy designs or process routes, some steels exhibit very similar properties.
The norms and standards commonly specify only Rp0.2, Rm, Ag, A80, r and n values. Customer specifications often call for additional bending characteristics and hole expansion test values. These may be required for material certifications.
Practical experience calls for even more: We know from our experience, for example, that the description in the norms and standards is insufficient. It is important to note that dual-phase steels can be used optimally for deep drawing, whereas complex-phase steels are best suited to bending and edging processes.
The hole expansion value is usually a good indicator, though it must be noted that stamping can have an enormous effect on the result when specimens are being prepared. The divergence in the results between the various testing labs is very high, and the measurements require a great deal of time and effort. Additionally, the parameters measured in bending tests—such as the minimum radius or the bending angle at which a crack appears—are sometimes also used for the characterization of materials. Another perceived disadvantage is the fact that these parameters are dependent on the material hardening value determined in the tensile test. A further approach would be to use fracture mechanical characteristics. However, this would be at an enormous cost, and the values would not be practicable.
Global and local formability as an approach: Material hardening, and thus the n value as well as the uniform and total elongation, very well describes the resistance of a material to plastic instability and thus the material’s tendency toward the formation of diffuse or local necking. The material behavior described by these variables is often summarized under the phenomenon of global formability
Commonly used damage models and failure criteria provide very reliable descriptions of ductile failure. It takes a lot of effort, however, to carry out such testing. The current literature, e.g. Hance, B., “Advanced High Strength Steel (AHSS) Performance Levels,” SAE Technical Paper 2018-01-0629, 2018, shows that failure elongation determined from thickness and/or area reduction at fracture standard specimens or specially notched tensile test specimens also permits a reliable classification. The advantage of the variables determined from tensile test specimens lies in the comparatively small effort required (no additional test, no complex processing of specimens, simple geometry of the specimen). It is important to note that this type of testing has long been used to determine the ductility (z value) of round tensile test specimens. This material behavior is summarized under the phenomenon of local formability.
The proposal: Since material hardening and failure elongation sufficiently describe the material properties as best we know how at the current time, voestalpine Stahl GmbH would welcome a change to a new nomenclature system. A new system would take into account yield strength, tensile strength, material hardening (global formability) and failure elongation (local formability). These factors, including material behavior and the preferred field of application, would be included in the naming conventions.
The way forward: Although initial tests conducted in an effort to determine local formability based on failure elongation in standard tensile test specimens (according to EN/ASTM/JIS) or notched tensile test specimens have been very successful, some questions must still be answered, particularly with regard to standard tensile test specimens, for example:
- Failure elongation depends on the tensile state as well as the stress-strain analysis. This means that failure elongation depends heavily on the geometry of the tensile test specimen in certain width and thickness ranges.
- There is strong evidence that material hardening and perhaps the thickness of the specimen itself may have an effect on failure elongation.
In addition to incorporating these influential factors into future research and development projects, we must also devise a procedure by which we can determine the extent this failure elongation. The requirements range from cost-effective testing, low divergence in the results and achievement of high relevancy. From the current perspective, voestalpine prefers the use of broken standard tensile test specimens to determine the desired parameters. To determine the measured value, the most stable method seems to be the integrally calculated average reduction in thickness or the integrally calculated reduction in surface area.
After a measurement method has been selected, it would be necessary to characterize all AHSS steels and to define a standard containing the possible variables for local formability.
The new classification system: Based on practical experience and careful consideration of VDA 239-100, the naming convention could be as follows: CRnnnYmmmT-G1G2 (see Figure 1).
Note on G1 and G2: With respect to the voestalpine approach, the following descriptions under G1 are only to be viewed as proposals: drawing type (D) for materials with good global formability and thus good deep-drawing properties as well as flanging type (F) for materials with good local formability and thus high failure elongation, high hole expansion values and good bendability.
The descriptions “basic” and “superior” (perhaps “excellent” as well) under G2 are also only proposals for further discussion (see Figure 2).
The slide show highlights the areas covered by tests performed at voestalpine on ahss classic and ahss high-ductility steel grades and presents the systematic nomenclature that is based on global and local forming properties.
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