High Strength Steels in Automobiles:

¶ … High Strength Steels in Automobiles:

The use of high strength steels by automakers has become the fastest growing automotive lightweight material in the automotive industry in the past two decades. The use of the high strength steels and ultra high strength steels has overtaken aluminum's growth rate by 13%. While the use of all grades of aluminum has grown by 143%, the use of high strength steel grades has increased by 162% since 1977. Though the growth of aluminum has been largely in the form of castings, particularly at the expense of cast iron, the high strength steels have replaced the older carbon steel grades.

Without sacrificing strength and performance, vehicle makers save weight by the use of thinner gauges through high strength steels. About 55% of today's vehicle mass includes up to 1800 pounds of steel which are in the form of sheet, bars and other steels. According to the American Metal Market, out of the 1500 pounds of this amount of steel sheet used in vehicles, 328 pounds are high strength steel sheets. On the other hand, the total amount of aluminum used in a vehicle is 236 pounds according to the American Metal Market ("Growth," 1999).

Throughout the history of automotive light weighting, the automakers have time after time relied on steel sheet as their choice of material for a long period of time. The automakers have also continued to accomplish their competitive goals through the improvement of their designs. Through the use of the ever increasing amounts of high strength steel sheet, the automakers also meet customers' needs for reasonably priced, fuel efficient vehicles which present greater safety and improved performance. By the application of advanced high strength steel sheet in designs, automakers are improving fuel efficiency and enhancing vehicle performance almost freely as compared using the more expensive aluminum materials.

What are High Strength Steels?

High strength steels are largely used because of their potential of cost reduction and weight savings while guaranteeing improved performance. When these high strength steels are integrated with suitable manufacturing techniques, they provide opportunities for manufacturing process consolidation, reduced product weight, enhanced crash performance and reduction of cost. The high strength steels are being utilized in more applications throughout different manufacturing industries as engineers are continually becoming acquainted with the various manufacturing techniques (Schaeffler, 2005).

In addition to yielding strengths greater than 275 MPa, High strength carbon and low-alloy steels have and can be divided into different categories with each having some basic variations in mechanical properties and available product forms. Notably, there is no common definition of high strength steel and different engineers define the steels depending on the strength groups of the steels. The yield strength and the strength groups are the basis and criterion with which the high strength steels are classified. These different classifications include:

Structural Carbon Steels:

This category includes mild steels, heat-treated carbon steels and hot-rolled carbon-manganese steels. Mild or low-carbon steels are not intentionally toughened by alloying elements other than carbon. These steels also contain some manganese for sulfur stabilization and silicon for de-oxidation. They are frequently used in the as-rolled, forged or annealed condition and are rarely quenched and tempered. The low-carbon mild steels, which are the largest category of mild steels, are used for forming and packaging. On the other hand, higher carbon and manganese mild steels have also been used for structural products like plate, sheet, bar, and structural segments.

Quenched and Tempered Low-Alloy Steel:

Steels which contain manganese, silicon or copper in quantities larger than the maximum limits of carbon steel or steels with specified ranges or minimums for any alloying additions are known as alloy steels. In addition to this low-alloy steels are those steels that contain alloy elements such as carbon with a total alloy content of about 8.0%. Most low-alloy steels are fit as engineering quenched and tempered steels and are usually heat-treated for engineering use. As compared to structural carbon steel, low-alloy steels with appropriate alloy compositions have greater harden-ability and can therefore provide high strength and good toughness in thicker segments through heat treatment. With their alloy components also providing advanced heat and corrosion resistance the quenched and tempered low-alloy structural steels are mainly available in the plate forms or bar products ("Cast Steel," n.d.).

Influence of High Strength Steels on Vehicle Design and Manufacture:

The use of high strength steels (HSS) in vehicle design and manufacture represent a new cutting edge in automotive design and manufacturing. As mentioned earlier, their properties of these steels allow thinner gauges to be applied in the entire auto body and thus rendering a stronger, lighter vehicle without major changes in cost structure. The contents of these steels continue to rise as they create a positive impact on fuel economy, emissions and safety which will in turn give OEMs the essential breathing room necessary for building of cars and trucks that still appeal to consumers.

Like any other newly introduced material, high strength steels have exceptional characteristics which require keen attention in vehicle design and manufacturing. The industry leaders in automotive engineering and the steel industry continue to team up while gaining valuable experience in applying high strength steels in volume production.

The motivation behind the development of high strength steels for more than two decades now is largely because of the pressure to reduce fuel consumption in vehicle design. While other engineers went to the lab to examine materials and manufacturing in search of lightweight, the 1973 fuel crunch sent some engineers to the drawing board to build smaller cars. The application of high strength steel in key structural areas on vehicles was realized by the 1980s. These steels were first applied in safety-critical applications and then dent resistance areas with the result being the reduction of the vehicle weight ("ULSAB," n.d. ).

The worldwide steel industry has since taken high strength steel applications to the next level with the introduction of Ultra Light Steel Auto Body project (ULSAB). From its ability to carry a great deal of stress and peak faster than other materials throughout high energy loading such as a car crash, the appeal to apply high-strength steels has increased tremendously. Consequently, when working with high strength steels, engineers rely on several guidelines which include:

Stamping:

One of the most critical guidelines for manufacturers is the need for careful calculations on the limit geometry, precise control over the stroke and good definition of the strain. In the development stage, manufacturers usually start with the most decisive pieces of a component (or subassembly) and thereafter make changes to the less important parts. It's also important for them to key in on the functional necessities of a subassembly and work out details with the vendors. In working with the high strength steels, flexibility is always a major asset and the manufacturers should be prepared to take actions to compensate for changes that will occur in development and tryout the changes.

Welding:

For the engineers in vehicle design and manufacture, it's necessary for them to start by identifying weld gap tolerance requirements. They should also identify main tolerances during the formation of control roll and be prepared to create space in the process to allow for weld gap variability. Prior to the welding, some parts may need cleaning due to the various effects on materials and processes caused by different gas shields. The overall structural performance can be optimized by the number of innovative welding technologies and weld designs.

Roll Forming:

The high strength steels need more stands on the roll former and using the minimum allowable bend radius for the material while positioning any holes away from the radius helps to attain better tolerances. Setting up a forceful maintenance schedule and using the suitable lubrication in the process helps to maintain a program for the roll-forming line.

Working high strength steel can at times be a thorough sensitive process with the differences in the steel often threatening to throw off the balance of the process which results in less uniform parts. To achieve this, manufacturers in the automotive industry are now utilizing e computer tools and analysis. The automotive industry has also placed lots of pressure on the steel industry to reduce the difference in mechanical properties.

When it comes to working with these high strength steels, springback is considered to be one of the more notorious problems. Springback's magnitude basically depends on the steel's yield strength and sheet thickness with most of the grades of automotive steel exhibiting some degree of springback. The engineers in the automotive industry always plan for the springback by over bending parts.

High Strength Steels and Motorsport:

Unlike in the road vehicles' design and manufacture which uses carbon steel tools, the motorsport industry uses the high speed still. This type of steel is used in high speed applications for instance in things like drill bits and power saw blades. Though carbon steel tools particularly bits and blades are still commonly used, high speed steel is a replacement for carbon steel tools. As compared to carbon steel, the development of high speed steel has many advantages over carbon and is therefore more popular in high speed applications (Black, 2010).

This type of steel works due to several very important factors such as the type of metals used, high treatment temperatures and its ability to melt metals. Being an alloy that combines several metals, high speed steel can be kept hard even under extreme temperatures because the steel can provide heat resistance. The steel also remains hard even under intensive high temperatures due to the high temperature treatment.

This steel is an alloy which combines several metals such as tungsten, molybdenum, chromium, cobalt and others. The most commonly used type of steel in high speed steel products is tungsten. Nonetheless, various types and designations of high speed steel exist with each of these steels having their own special combinations. Due to its ability to melt metals, high speed steel is extremely significant. The melting of metals can even happen from friction resulting from drilling and sawing.