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Steel 101: Grades and repairability

Friday, April 28, 2017 - 07:00
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Materials have been a hot topic in the automotive industry over the last few years. As more vehicles roll off the assembly line with a greater mix of materials, the repair industry will be faced with the challenge of identifying the various materials, making sure their technicians have the right certifications and ensuring the shop has the right equipment to service the vehicle. One material growing at an unprecedented rate is advanced high-strength steel (AHSS). In fact, AHSS is the fastest growing material in automotive applications according to a 2015 report by Ducker Worldwide. Over the past three years, the amount of AHSS used each year in automotive applications has been 10 percent higher than forecasted.

Steel provides many benefits to both the automaker and the consumer, allowing automakers to maintain their existing manufacturing infrastructure and a lower cost of ownership to the consumer. Purchase price, insurance and repair are lower for steel-intensive vehicles than alternate material-intensive vehicles. As these new steels are introduced, the steel industry continues to work closely with the automotive repair industry to ensure proper technological knowledge is available. 

Why do automakers continue to use steel for their vehicles? Steel offers the best balance of performance, value and sustainability for body and chassis applications. In addition, the steel industry works closely with customers to develop innovative technology such as new steel grades and manufacturing technologies to provide solutions tailored for each application throughout the vehicle. There are more than 200 automotive steel grades featuring an array of properties, including formability up to 60 percent, and strengths from 200 MegaPascals (MPa) to 1900 MPa.

This chart shows the diversity of AHSS grades available today.

Automotive steels

Automotive steels are classified in several different ways. Common industry designations include conventional steels (interstitial-free and mild steels); high-strength steels (carbon-manganese, bake hardenable and high-strength, low-alloy steels); and AHSS (dual phase, transformation-induced plasticity, twinning-induced plasticity, ferritic-bainitic, complex phase and martensitic steels). Additional higher strength steels for the automotive market include hot-formed, post-forming heat-treated steels and steels designed for unique applications such as improved edge stretch and stretch bending.

The introduction of AHSS to light vehicle body structure applications can be challenging for organizations performing repair of vehicle structures. AHSS steels are typically produced by nontraditional thermal cycles and contain microstructural components whose mechanical properties can be altered by exposure to elevated temperatures. Typical repair practices such as welding or flame straightening could alter the microstructure and the AHSS mechanical behavior and affect the structural performance of AHSS components after repair.

There is also a need to evaluate the combination of heat application associated with welding during collision repair on the properties of high-strength steels and advanced high-strength steels. This is critical because if heat cannot be used for straightening operations, there will be an additional reliance on sectioning and welding techniques for repair of components made from AHSS materials.

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Conventional steels, interstitial-free and mild steels are widely produced, and with their exceptional formability (elongations of 30–60 percent), are typically used for complex shapes, including vehicle exterior painted surfaces, such as doors, fenders and deck lids. Conventional steels have an essentially ferritic microstructure.

Conventional and HSS have an essentailly single phase ferritic microstructure. Chemistry additions control grain size and strength.

High-strength steels (HSS) | high-strength low alloy (HSLA) steels are medium strength (approximately 400–800MPa) and used in various body structure, suspension and chassis parts, and wheels, where strength is needed for increased in-service load. HSLA steels are essentially single-phase ferritic microstructures strengthened primarily by the addition of micro-alloying elements.

Advanced high-strength steels (AHSS) are high-strength (generally greater than 500 MPa) and applied in the body structure, including beams and cross members, sill and pillar reinforcements and other energy-absorbing components. These steels provide the automotive design engineer with high value, lightweight solutions with the required stiffness (for improved ride and handling), crash energy management (to absorb front and rear crash energy) and strength (to provide anti-intrusion during side or roll-over accidents).

The principal difference between HSS and AHSS is the microstructure. HSS are single-phase ferritic steels with a potential for some pearlite in carbon–manganese grades. AHSS are primarily steels with a microstructure containing additional phases – for example martensite, bainite, austenite and/or retained austenite in quantities sufficient to produce unique mechanical properties. In addition to controlling the chemistry, AHSS require control of the cooling rate to help create the desired microstructure, either on the hot mill runout table (for hot-rolled products) or in the cooling section of the continuous annealing line furnace (continuously annealed or hot-dip coated products).

Typical grades of AHSS include:

Dual phase (DP) steels range in strength from 500MPa to 1200MPa and obtain their properties from the introduction of a martensitic phase into the ferrite microstructure. The ferrite phase provides excellent formability, while the martensitic phase provides the improved strength (higher ultimate tensile strength compared to conventional steel with similar yield strength). Dual phase steels are used increasingly in safety-critical auto body structural components because its higher ultimate tensile strength (UTS) provides much greater energy absorption over conventional HSS with the same yield strength (YS).

Dual Phase steel microstructure consists of a soft ferrite phase with a hard martensitic phase added to improve the strength. As the percentage of martensite increases the strength of the DP steel increases. Transformation Induced Plasticity steel microstructure consists of a soft ferrite phase with additional phases (martensite, austenite and bainite) to control strength and formability.

Transformation induced plasticity (TRIP) steels range have a similar range of strength as DP steels, 500MPa to 1200MPa, while providing improved formability . The improved formability is obtained with the introduction of additional phases (austenite and bainite) into the microstructure. These phases improve the work hardening properties of steel and provide additional energy absorption characteristics.

Ultra high-strength Steels (UHSS) are AHSS which are “ultra-high” in strength, greater than 980 MPa, and are used in areas where exceptional strength and anti-intrusion are needed, including the A-pillars, B-pillars, rockers and rails.

Examples of AHSS in 2017 vehicles 
Working directly with automakers on demonstration and enabling projects is helping automakers effectively implement new steel grades and technology for increased vehicle performance. As examples, new vehicle launches over the past three years have demonstrated rapid adoption of the latest steel and manufacturing innovations in these new designs. The 2017 North American Truck and Utility Vehicle of the year are two great examples of the use of AHSS in recent vehicle launches.

The rugged and aerodynamic body of the 2017 Honda Ridgeline uses a wide range of steel grades, including ultra-high strength hot-stamped steel to reduce weight in the vehicle body. These grades were chosen to achieve the best combination of strength, rigidity, dynamic performance and weight, thus enhancing the overall performance of the vehicle. Steel makes up 96 percent of the Ridgeline’s body, including 58 percent AHSS and 10 percent UHSS of the body structure. Advanced steel grades are also leveraged for increased safety, the new Ridgeline’s A-pillar stiffeners are made from 980 MPa ultra-high strength steel and the front door outer stiffener rings are made of 1500 MPa hot-stamped steel. The use of these steel grades helps better protect occupants in a frontal or side impact collision. The increased strength allows the components to be made lighter, which reduces overall vehicle weight for improved efficiency and dynamic performance.

2017 Honda Ridgeline body structure

The 2017 Chrysler Pacifica’s all-new body structure is comprised of 72 percent high-strength steel. It is approximately 250 pounds lighter, stiffer and more aerodynamic than the model it replaced. Much of the credit is a result of extensive use of hot-stamped, advanced high-strength steels, application of structural adhesives and an intense focus on mass optimization. It uses approximately 22 percent more high-strength steel than its predecessor, which contained 48 percent advanced high-strength steel for maximizing stiffness and strength while optimizing weight efficiency. Specific steel components contributing to the Pacifica’s lightweight suspension system include a thin-gauged steel front suspension cradle and rear suspension steel training arms. The trailing arm uses a “blade-style” design to ensure strength and durability without added mass. The Pacifica earned a 5-star safety rating from the U.S. National Highway Traffic Safety Administration.

2017 Chrysler Pacifica body structure

Conclusion

As the automotive industry works to meet increasingly stringent fuel economy and safety regulations, the North American steel industry is introducing new grades of AHSS. These grades provide strength, mass reduction and high-value, and help alleviate the impact on the environment. The variety of steel grades allows automakers to use the right grade in the right application for exceptional occupant protection, crash energy management and durability.

The trend of AHSS as the fastest growing lightweighting material exceeding industry forecasts, along with lower adoption of aluminum, is proof of the high value of steel to automakers and consumers alike. Clearly, steel will remain the material of choice in the automobile for years to come.

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