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Stronger and Lighter
That’s the claim of steelmakers as they roll out new generations of higher-strength steels. The toughness and ever-improving formability of these alloys allow creativity in part design and a decrease in steel usage, ultimately leading to reduced part and end-product weight and cost.
By Louis A. Kren, Senior Editor
In recent years, suppliers have unleashed newer classes of steels, each said to be stronger than the last. You can expect to pay extra for the raw materials, and challenges in tooling and forming may mean slightly longer lead times and higher manufacturing costs. But if your application requires strength, durability and weight reduction, new higher-strength steel blends fit the bill. Because less material is needed to provide the same or improved part strength, the upfront costs can be negated in the end.
Driven by Automotive
Higher-strength steels and their new cousins, advanced-high-strength steels (AHSS), have proven their worth in automotive applications where designers face constant corporate, government and consumer challenges to provide improved rider safety without stressing the wallet at the gas pump. For example, General Motors is beginning to apply AHSS in refreshed designs to improve vehicle frontal-crash, side-impact and roof-crush performance. Typical AHSS automotive applications include motor compartments, longitudinal rails, rocker inners and B-pillar reinforcements.
GM and other automakers have partnered with steelmakers and associations such as the American Iron & Steel Institute (AISI) to examine just how higher-strength steels can improve vehicle safety while reducing vehicle weight. Visitors to AISI’s www.autosteel.org can learn about advancements in steels and how those are parlayed into automotive design and production.
What Does “Higher-Strength” Mean?
Higher-strength steels, available as coil, sheet or blanks, are characterized by a low carbon content (below 0.25 percent) while medium- and high-carbon steels can contain as much as 1 percent carbon. The lower carbon content of higher-strength steel enables improved formability and weldability. In addition, such steel often employs ingredients to provide increased corrosion resistance or certain mechanical properties. The relative term “high strength” typically refers to steel that has a yield strength of at least 40 ksi. High-strength low-alloy (HSLA) formulations often offer yield strengths above 80 ksi, while AHSS grades exhibit yield strengths surpassing 200 ksi.
Yield strength is an important consideration in material selection. It refers to the maximum amount of stress that can be subjected upon a material without causing plastic deformation. That is, once stress surpasses a material’s yield strength, the material changes shape permanently. Obviously, a material with a higher yield strength requires more force to form it, necessitating the use of stronger tooling and forming machinery with higher capacities.
But the same metallurgical factors that increase yield strength and give steel its advantages also decrease its stretchability, making it difficult to form as compared to standard-strength grades. That said, the resulting part is better able to hold its shape during stress, such as in a vehicle crash. This is the major reason why high-strength steels find increasing use in applications that require rigidity and strength under load. Because high-strength steel is inherently stronger, it can provide similar structural integrity as common carbon steel in lower quantities. This brings weight and cost advantages. In addition, providers have made strides in increasing the ability to form these super steels.
A Host of Advantages
The application of higher-strength steels is increasing in more and more parts for a wider range of industries, reports Dr. Stuart Keeler, author of MetalForming magazine’s Science of Forming column. The usual reasons for increasing yield strength, he reiterates, are greater structural loads, longer fatigue life and maximum energy absorption.
Two lesser known reasons for utilizing higher-yield-strength steel are increased dent resistance and reduced in-plant handling damage. When part styling demands flatter sections with less curvature or sweep, panel denting occurs more frequently during end-customer use. Dent resistance becomes important as weight-reduction programs are accomplished by thickness reduction of the steel sheet. Higher-strength steels provide such resistance while meeting weight-reduction goals.
The trend toward larger panels with flatter sections also leads to increased handling and possible damage in press and assembly shops. For example, an automotive outer hood is prone to denting, bending, twisting and other forms of unwanted deformation until it is mated to the inner panel with welded edges and bonded to inner panel supports. For these applications, the material’s increased yield strength can reduce this in-plant damage prior to assembly.
While the automotive industry has been a leader in increased dent resistance and reduced in-plant handling damage, producers of appliances, architectural building panels, consumer items and agricultural equipment now require increased resistance to denting and other damage, opening another avenue for higher-strength steels.
Breaking Down the Grades
Here’s a rundown of general high-strength-steel grades and their applications.
High-Strength and HSLA: These grades have yield strengths greater than 40 ksi, and are furnished in nearly all standard forms. They may be purchased as cold-rolled strip to thicknesses just above 0.006 in. if improved thickness control or surface finish is paramount. HSLA processing methods are available that enhance toughness while improving yield strength. Typically, HSLA steel is supplied based on the mechanical properties required by the customer, leaving decisions as to the exact makeup of the steel, and its alloying elements, with the provider.
Confused by Steel Grades?
A few decades ago, we had three main sources of music—radio (AM/FM), 78-rpm vinyl records and reel-to-reel tape recorders. At the same time, the main types of formable steel were commercial quality (CQ), draw quality (DQ) rimmed and aluminum-killed draw quality (AKDQ).
Today music comes in records, CD, DVD, cassettes and many other mix-and-match options. Fortunately, or unfortunately, today also brings many grades and subgrades of steel as compositions and processing are tailored more and more to meet specific customer needs.
Is there a science behind the various grades of steel? Yes, but it is often well hidden beneath different names of steel. These names may be related to steel-making practices (AKDQ), metallurgical structures (dual-phase), application (enameling iron), company names (XYZ Steel NR2442A) or just plain hype (Superduperform). Notice that none of the above names give a clue about the mechanical properties of the steel—the keys to assessing relative formability, meeting application specifications or assuring in-service performance.
A good parts supplier can help you navigate the nomenclature and ensure use of the appropriate material for your work.
--Stuart Keeler, PhD., Metalforming columnist
Advanced High-Strength: These are causing a buzz, especially in automotive circles. Vehicle designers, spurred on by coming government requirements and vehicle-owner demands for increased safety and better gas mileage, slowly have been incorporating these advanced steels into new-car programs. Expect inclusion of AHSS in vehicles to soar in coming years as engineers, OEMs and parts suppliers realize just what the material can do. Curt Horvath, a development engineer at GM’s Materials and Appearance Center, in a presentation at AISI’s 2004 Great Designs in Steel Seminar, reported that dual-phase AHSS will comprise 35 percent of the material in a typical GM passenger-vehicle structure within the next five to 10 years, up from zero percent only a few years ago. And beyond ten years out, that number should increase to nearly 50 percent.
AHSS offer high formability considering their tensile strength—they are easier to form than HSLA steels at similar initial yield strengths, but produce higher final-part strength. In addition, they provide higher energy absorption than HSLA steel. Here are two types finding increased use in automotive applications:
- Dual-phase: These steels often are used in automotive structural applications as replacements for HSLA grades, due to their capacity to absorb crash energy and resist fatigue. Parts produced via dual-phase steel include front and rear rails, crush cans, rocker reinforcements, B- and C-pillar reinforcements, crossmembers, bumpers and door intrusion beams.
- Transformation-induced plasticity (TRIP): TRIP steels, the newest development in AHSS, exhibit better ductibility at a given strength level than other AHSS, enabling higher formability. This results from the transformation of retained austenite, a ductile high-temperature phase of iron, to martensite, a tougher phase, during deformation. That means that final parts are stronger than the initial material used to form them. This characteristic allows production of more complicated parts than is typical with other higher-strength grades.
For more on what you should know about material selection, pick up a copy of Design Guildelines, published by the Precision Metalforming Association, Independence, OH. Other information for this article was provided by Dr. Stuart Keeler; Metals Handbook, Second Edition, published by ASM International; and US Steel Corp.