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Daniel Schaeffler Daniel Schaeffler
President

Strong Choices Expand High-Strength Options

February 18, 2021


advanced-high-strength-steels-automotiveAutomotive manufacturers and their suppliers face increasing safety requirements combined with continued lightweighting pressure, driving them to use high-strength, formable and cost-effective sheet metals. Areas of the vehicle demanding the highest resistance to intrusion require use of the highest-strength products. Initially, the default option was a roll formed or tubular beam fabricated from martensitic steel. For the past two decades, coated press-hardening steels have been an option to reach the highest strength in components beyond the simple shapes formed from low-ductility martensitic steels. Domestic applications for 3rd-gen. advanced high-strength steels (AHSS) will expand in 2021, and beyond. And we’ve started to see the use of hot-formed aluminum parts, typically for higher-end vehicles. 

Martensite

Martensitic steels produced at the mill reach tensile strengths from 900 to 1700 MPa—depending on the grade, more carbon leads to higher strength. Hot-rolled martensitic steel requires a highly alloyed composition to gain the required hardenability; cold-rolled martensitic steel can use a leaner composition. Hot-dip galvanized or galvannealed coatings are not an option; only electrogalvanized or uncoated grades are available.

Along with the high tensile strength (TS) comes high yield strength (YS); a YS/TS ratio greater than 0.85 is common. The biggest challenge with these grades stems from their limited formability, typically less than 5-percent total elongation. Cold stamping of the lower-strength grades may be possible, but these steels typically are roll formed instead. Automotive applications for martensitic steels include bumper beams and door anti-intrusion beams.

Press-Hardening (Hot-Stamped) Steels

The hot-stamping process heats blanks made from press-hardening steels (PHS), and then forms and quenches them in the same press stroke. The process can form complex shapes because the steel is relatively soft when formed at high temperatures. Quenching under full ram load locks in the shape, greatly improving dimensional accuracy.

Carbon content and thermal cycle drive higher strength. The grade known as 22MnB5 has been the workhorse, producing parts with 1500-MPa TS.  Recent developments include 37MnB4, a grade that can achieve 2000-MPa TS. The 22 and 37 in these grade designations reflect carbon contents of 0.22 and 0.37 percent, respectively.  

In cold stamping, the tensile properties are set in the incoming sheet metal and change only from work hardening during forming the part to the desired shape. Conversely, during the hot-stamping process the quenching operation transforms the microstructure to martensite and provides the increased tensile properties. Process control is critical to achieve the targeted properties in what likely are safety-critical automotive parts.

Subjecting bare steel to the hot-stamping process results in the formation of a surface-scale layer. Shot blasting can remove the scale, but the uncoated steel remains at risk for rusting. As such, stampers can purchase PHS steels coated either with aluminum-silicon or zinc, with each presenting their own process-control issues. One such challenge: Aluminum-silicon and zinc melt at temperatures below that reached during hot stamping. 

3rd-Gen. AHSS

3rd-gen. AHSS grades exhibit improved ductility in cold-forming operations when compared to other steels at the same strength level. As such, they may offer a cold-forming alternative to press hardening steels in some applications. 

Examples include TRIP-assisted bainitic ferrite (TBF) and carbide-Free bainite (CFB), descriptions of essentially the same type of 3rd-gen. steel families. One grade may reach 1000-MPa TS with 13-percent elongation, while another might achieve 1200-MPa TS and 10-percent elongation.

The other family of commercialized 3rd-gen. steels is based on the quenching and partitioning process. Potential property combinations include 1000-MPa TS with 18-percent elongation, and 1200-MPa TS with 13-percent elongation.

Reaching these properties requires developing a specific balance of microstructural phases using complex annealing cycles. Only a handful of companies around the world have the equipment required to produce these cycles. While the annealing cycle for conventional grades is simply heat, hold and cool, AHSS grades require quench and hold cycles, and 3rd-gen. steels add to the complexity, since achieving the necessary microstructure in some grades requires quench, hold and reheat prior to a second quench.  

Process optimization continues, and yet another family of 3rd-gen. steels, medium-manganese steels, is being developed. Note that global standards for 3rd-gen. steels do not yet exist, with requirements contained within individual OEM standards.

Hot-Formed High-Strength Aluminum

Current AA7XXX alloys reach about 550-MPa TS after hot forming and aging. In addition to the higher alloy cost, offsetting some weight savings may be a thickness increase of 30 to 100 percent required to achieve structural performance similar to that of a steel-equivalent product. Development continues to create alloys and processes to reduce these cost penalties.

Another consideration in expanding the use of AA7XXX is recyclability.  Users of the more common sheet aluminum-grade families (AA5XXX and AA6XXX) segregate their scrap streams to maximize their value, since the alloying additions in one family are detrimental to the properties of the other. The 5XXX and 6XXX alloys have a maximum zinc content of around 0.25 percent, while the typical AA7XXX products require a minimum zinc content of more than 5 percent. Copper content also is markedly higher in the AA7XXX grades. Combining engineered scrap and offal from aluminum parts in a manner similar to the practices used when forming and processing steel parts leads to significant reductions in scrap value.

For more on hot forming of steel and aluminum, see the articles by Dr. Eren Billur in the August and October 2020 issues of MetalForming magazine. MF

Industry-Related Terms: Alloys, Ductility, Form, Forming, Hardenability, Layer, Martensite, Quenching, Ram, Scale, Scrap, Stroke, Tensile Strength, Thickness, Work Hardening
View Glossary of Metalforming Terms

 

See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Materials

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