Page 50 - MetalForming Magazine February 2023 - Metal Forming for the Automotive Industry

Talkin’ Bout My Generation
You’ve probably seen references to 3rd Gen steels—implying that there are 1st and 2nd Gen steels as well. This article explores their sim- ilarities, differences and evolution. The automotive industry drives much of this development, but the benefits offered by newer grades promote appli- cations in other industries.
Where it Began: 1st Gen Steels
During the era of tail fins and sock hops, automakers had only a handful of sheet-steel grades from which to choose. Lower-strength grades termed mild steels saw the most use. Metal- lurgists knew that adding more carbon and manganese boosted a steel’s strength, so these became the only available high-strength steels, although the tradeoff with higher strength was decreased ductility, toughness and weldability.
Vehicle designs at the time accom- modated the limited ductility with smaller, relatively flat body panels. As for the vehicle structure, engineers compensated for lower steel strength and ductility by increasing sheet thick- ness, without major concern for weight or safety.
In 1970, passage of the Federal Clean
Dr. Danny Schaeffler, with 30 years of materi- als and applications experience, is president of Engineering Quality Solutions (EQS) and chief content officer of 4M Partners. EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler is the metallurgy and forming technical editor of the AHSS Application Guidelines available from Worl- dAutoSteel at AHSSinsights.org.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
Air Act introduced environmental con- cerns to autobody suppliers. Also, that year the Highway Safety Act established the National Highway and Traffic Safety Administration, charged with helping to reduce deaths, injuries and econom- ic losses resulting from accidents. Fur- ther, the 1973 oil crisis triggered a focus on weight reduction and associated fuel efficiency. This interest spurred the development and commercializa- tion of steelmaking techniques, which impacted materials utilization on pas- senger vehicles.
By now, high-strength low-alloy steels had been in use for more than a decade in gas- and oil-pipeline appli- cations, but not until the 1980s did these grades become available at thick- nesses used in the auto industry. Com- pared to the carbon-manganese (CMn) steels of yesteryear, these grades did not suffer the decreased ductility and toughness that came with higher strength. Stronger stampings allowed for a thickness reduction and reduced part weight. Thinner parts meant pur- chasing less steel, partially offsetting any potential cost increase for these grades, which were considered advanced at the time.
The 1980s also saw the first appli- cations of bake-hardenable (BH) steels in Class A exposed body panels such as hoods, doors and decklids. These grades—lower strength when they arrive at the stamping plant—gain strength and dent resistance after form- ing and spending time in paint-curing ovens. This extra strength allows automakers to further reduce sheet thickness and reduce weight.
On the lower-strength side of the spectrum sit vacuum-degassed intersti- tial-free steels (IF or VDIF), highly formable grades capable of achieving surface quality comparable to a Class A exposed surface. These grades, also known as extra deep drawing steels
(EDDS), find use in fenders and quarter panels requiring more ductility than typ- ical BH applications. Body panels with highly curved surfaces provide automak- ers the flexibility to design aerodynamic shapes, helping to improve fuel economy and meet styling objectives.
Metallurgically, each of these steels has a microstructure primarily of ferrite. Strength increases through a combi- nation of smaller grains, carbide or nitride precipitation, and alloying ele- ments. Most grades can be produced via batch annealing (BA), but continu- ous annealing (CA) provides for faster processing and greater efficiency. The same principle applies to both: heat to a critical temperature, soak for long enough to ensure a uniform product then cool to ambient temperature. With CA, only a cross-section of the metal thickness along the coil width must be heated and then cooled at any one time, as opposed to the entire 20-ton coil simultaneously experiencing the same thermal profile. This allows for a reduc- tion in processing time from 9 days to 20 min. A new CA line could cost tens or even hundreds of millions of dollars to build, which limited their rollout.
Starting in the early 2000s and based on developments in Europe and Asia, steelmakers pushed the limits of their CA lines, increasing the cooling rates achievable on their existing equipment. The higher cooling rates allowed them to produce phases other than ferrite— martensite, bainite and retained austenite. The term retained austenite comes from austenite not being stable at ambient temperatures using low- carbon-steel compositions, but the higher cooling rate allows for this non- equilibrium phase to exist.
The ability to produce these addi- tional phases led to the development of new steel grades: dual-phase (DP), transformation-induced plasticity ( TRIP) and complex-phase (CP) steels,
Metal Matters By Daniel J. Schaeffler, Ph.D.
www.metalformingmagazine.com
MetalForming/February 2023 47