Page 41 - MetalForming February 2016

Questions Answered Regarding Advanced High-Strength Steels
By Stuart Keeler
manufacturers change the atomic cell structure by replacing iron atoms with atoms of other elements such as potas- sium, titanium and silicon.
The simplest AHSS grade, dual- phase (DP) steel, exhibits a microstruc- ture of approximately 10 percent high- strength martensite, present as islands as the second phase within the ferrite microstructure. The martensitic phase helps to increase the general strength of the steel. Also, during deformation of DP steel, the boundaries between fer- rite and martensite phases undergo a mixing that creates an increase in workhardening and instantaneous n- values.
• How does DP 350/600 differ from HSLA 350/450?
Fifty years ago, high-strength low- alloy (HSLA) steels became avail- able. And with yield strengths ranging from 30 to 80 ksi, they still find plenty of use today. Meanwhile, met- allurgists and mechanical engineers around the world have remained hard at work creating another class of steels known as advanced high-strength steels (AHSS). These alloys are more complex than HSLA steels in composi- tion, processing and application.
Here, I’m pleased to answer some of the questions being asked about AHSS grades by designers, buyers, press and repair shops, and others affected by these new steels.
• What factors drive AHSS research around the world?
The main driver is the North Amer- ican automotive industry, working to increase fuel efficiency as required by new legislation. The primary technique is lightweighting, which seeks to reduce the thickness of stamped parts. Con- sequently, the strength of the steel must be increased to balance the thickness reduction. Unfortunately, reduced thickness and increased steel strength cause a reduction in stretchability.
• Are the AHSS grades simply an extension of HSLA steels?
Stuart Keeler (Keeler Technologies LLC) is known worldwide for his discovery of forming limit diagrams, development of circle-grid analysis and implementation of other press-shop analysis
tools. Keeler’s metalform- ing experience includes 24 years at National
Steel Corporation and 12 years at The Budd Com- pany Technical Center, enabling him to bring a very diverse background to this column and to the sem- inars he teaches for PMA.
Keeler Technologies LLC P.O. Box 283
Grosse Ile, MI 48138 keeltech@comcast.net
No. HSLA and AHSS exhibit major differences in their microstructures and different microstructures create different properties. HSLA steels start with a specific ladle of molten metal that solidifies into a microstructure called ferrite phase. Meanwhile, AHSS grades comprise a mixed microstruc- ture that includes one or more phases other than ferrite.
As steel solidifies, and depending on a number of variables, the grain size within the microstructure can become large or small. A relatively small grain size makes for stronger steel, because smaller grains have a larger ratio of strong grain boundaries to weaker inner grain volumes. To increase the strength of HSLA steels,
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