WHAT ARE ADVANCED HIGH STRENGTH STEELS?
Advanced high strength steels (AHSS) are stronger and more complex than the mild steels being used today. Understanding how these steels react during the forming process helps stamping and component manufacturers make better decisions regarding the selection of fabricating equipment. Failure to understand a steel’s global and local formability performance can result in lost production time, component rework and additional die repair.
Some of the key differences
between mild steel and advanced high strength steels are in their chemistry, thermal mechanical processing, and microstructures, all leading to profound differences in forming behavior. Mild steel consists mostly of a single
phase of ferrite, with various amounts of pearlite based on the percentage of carbon in the steel. They are characterized by low strength and excellent formability. Strength is achieved by a mechanism known as solid solution hardening. In comparison, high strength, low alloy steels (HSLA) achieve greater strength via higher concentration of carbon and the introduction of other alloying elements, such as manganese and silicon. Improvements in strength
are offset by reductions in formability.
THE CHEMICAL COMPOUNDS
Advanced high strength steels are primarily steels with a microstructure containing a phase or constituent in addition to ferrite in quantities sufficient to produce unique mechanical properties. These include martensite, bainite, austenite and/or retained austenite.
Some types of AHSS have a higher strain hardening capacity resulting in a strength-ductility balance superior to conventional steels. Other types have ultra-high yield and tensile strengths and show a bake hardening behavior. The various product families of AHSS are shown below in the Steel Strength – Ductility Diagram. Each ellipsoid represents the various strength and ductility combinations available within the product family.
Dual Phase steel is perhaps the most popular type of AHSS found today in body structures and components. These steels contain phases of both Ferrite and Martensite. The level of martensite and strength is directly related to the carbon concentration, but both phases are stable at room temperature via thermal-mechanical processing in the steel mill. The following micrograph and schematic show a dual phase microstructure.
Transformation Induced Plasticity or TRIP steels have multiple phases – Ferrite, Martensite, Retained Austenite and Bainite. During the work hardening of TRIP Steels, the retained austenite transforms to martensite, achieving very high strength in the final part. Formability decreases compared to a mild steel, but increases compared to HSLA steels.
AHSS IN PRACTICE
Product development in steel research centers has been aimed at creating stronger steels that allow vehicle crash regulations to be met, while also enabling thickness reduction for light-weighting. Weight reduction improves vehicle efficiency, important as vehicle manufacturers struggle to meet stiffening regulations for increasing fuel economy and reduced greenhouse gas emissions. At the same time, new steels must be formable to produce the complex geometries in today’s vehicle designs.
With AHSS, we find higher forming forces are required – this translates into:
- Higher press loads
- Higher blankholder and punch forces
- Accelerated die wear, requiring special die materials and surface treatments
- The potential for greater springback and part distortion
- Revised trim and cutting practices
- The use of temperature resistant lubricants
New analysis tools are being used to understand edge cracking, stretch bending limits, and instantaneous work hardening exponents. The result of the analysis has helped engineers understand the design limits of AHSS. Properly educating and training the individuals that will be required to perform process and die maintenance will greatly increase production robustness and manufacturing success.
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