Advances in lightweight steel technology: Ultra-high-strength steel, tailor blanks, and hydroforming

New technologies from Mubea and ThyssenKrupp

2016/08/15

Summary

  As advancements in reducing weight through utilization of CFRP and aluminum continue, weight reduction technologies that use steel materials are also steadily progressing. Based on the content of displays and workshops at the JSAE Exposition 2016 Yokohama, this report will introduce the latest trends for steel material weight reduction technologies, including hot formed steel, ultra-high-strength steel sheets, tailor rolled blanks, and hydroforming.

  Automakers are increasingly using hot-formed steel and ultra-high-strength steel sheets for the framework around the cabin. Since body side panels in particular are thick because of the need for high strength to restrain deformation during side collisions, there is a significant effect from the latest weight saving technologies in those areas. Toyota uses tailored ultra-high-strength steel blanks with reduced thickness in the lower sections for the A-pillar of the Toyota Prius to reduce overall weight.

  Mubea's tailor rolled blanks allow fine changes in sheet thickness by area when the material is created during roll forming. The blanks are utilized for the B-pillars of the VW Golf and Ford Focus among other uses.

  Moreover, ThyssenKrupp has developed steel sheets with high tensile strength and ductility as the sheets of the future. These enable energy absorption in areas around the cabin that restrain deformation. They also allow the use of deformation preventing steel sheets in areas like the engine compartment that primarily absorb energy. Additionally, this technology is expected to increase development freedom for autobody frames, contribute to improved safety performance in collisions, and reduce the overall weight of vehicles.

 

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Application of hot-formed parts and ultra-high-strength steel sheets to autobody frames

  The recent approach to autobody frame structures is to make it so engine compartment areas like the front side members have appropriate deformation during a frontal collision and are able to absorb energy. At the same time, components that make up the area around the cabin like the front pillars, B-pillars, and side sills are made to maintain a survival space for passengers by restraining deformation. The body side areas in particular need to restrain deformation as much as possible during a side collision. As a result, different properties are needed for the materials used in the respective frames. Body sides are made of steel materials with rigidity and high strength, and so it has become common recently for ultra-high-strength steel and hot-formed parts to be used.  Moreover, cold-formed parts are used for side members, which need to absorb impacts, and high ductility is valued over strength.

Body construction of the Audi A4
Body construction of the Audi A4
Source: Audi

  The body structure of the Audi A4 Sedan is pictured above. The engine compartment, which serves as a crushable zone, is made of aluminum and cold-formed steel parts, and hot-formed steel (highlighted in purple) is used heavily to form the frame surrounding the cabin. Hot-formed steel is used intensively to form A-pillars, B-pillars, and side sill inners in the body sides in particular.

Body construction of the Mercedes-Benz C-Class
Body construction of the Mercedes-Benz C-Class
Source: Mercedes-Benz

  The image above shows the body structure of a Mercedes Benz C-Class. Aluminum is used to form the engine compartment, while hot-formed steel and ultra-high-strength steel are used heavily to form the frame around the cabin.

Body construction of the Toyota Prius
Body construction of the Toyota Prius
Source: Toyota Motor Corporation

  The body structure of the Toyota Prius is shown above. In the same way as the Audi A4 and Mercedes Benz C-Class, hot-formed steel and 980 MPa-class ultra-high-strength steel is used in the frame around the cabin. The high-strength steel is used predominantly in body side members such as the A-pillar, B-pillar, and side sills in particular.

All-new Toyota Prius A-pillar
All-new Toyota Prius A-pillar
Lexus LS B-pillar
Lexus LS B-pillar

  The images above show a Toyota Prius A-pillar and Lexus LS B-pillar exhibited by Futaba Industrial Co., Ltd. at the JSAE Exposition 2016. Both of the pillars utilize tailored blank. Two different steel sheets are laser welded and press formed to make the parts. As can be noted in the images, both the A- and B-pillars have laser welds and sheet thickness varies above and below the weld points. The lower pillar section is subject to a lower strain as it has a large cross sectional area, and weight is reduced by making that section thinner. The sheet thickness of the Toyota Prius A-pillar is also reduced by using ultra-high-strength steel. Weight is reduced even further by using tailored blanks.



Application of tailor rolled blanks

  The tailor rolled blanks are an advanced version of the tailored blanks mentioned earlier. Instead of joining sheets with different thicknesses by laser welding, the sheet is rolled to the exact thicknesses required before it is press formed.

Ford Focus B-pillar (lower section)
Ford Focus B-pillar (lower section)
Ford Focus B-pillar (upper section)
Ford Focus B-pillar (upper section)

  The image above shows a Ford Focus B-pillar that was exhibited by Mubea at the JSAE Exposition 2016. The thickness varies finely from bottom to top (1.35mm to 2.30mm to 2.10mm to 2.40mm to 2.70mm to 2.40mm to 2.10mm). B-pillars are critical safety components that retain space for passengers during a side crash, and deformation must be kept to a minimum. At the same time, from the perspective of weight reduction it is desirable to reduce sheet thickness that is greater than necessary. Because of this, the lower area, which makes up a large cross-section, is reduced to 1.35 mm, and the middle area, which absorbs the largest impacts including from the door hinges, is made 2.7 mm. When pillars are made from conventional tailored blanks, steel sheets of different thicknesses are joined by welding, and as a result the areas where the thickness changes have a terraced form. In contrast to this, tailor rolled blank materials change gradually with 1/100 the inclination. Because of this, unlike laser welded tailor blank material, there are no seams, and changes are gradual and there is no work hardening. Tailor rolled blanks also protects against concentrated internal stress, which prevents cracks that develop in those areas during side collisions.

Application of tailor rolled blanks to the Ford Focus B-pillar
Application of tailor rolled blanks to the Ford Focus B-pillar
Source: Mubea

  Mubea's documentation provides details on a comparison of a conventional B-pillar structure, and one that uses tailor rolled blanks with the Ford Focus in the picture described above. In the case of a conventional structure the B-pillar reinforcement must be composed of 2 parts with respective thicknesses of 2.0 mm and 1.4 mm. Components with tailor rolled blank material that have the same strong rigidity are made as one parts that changes thickness from 1.35 to 2.7 mm depending on the area, which results in fewer components overall. Therefore, weight can be reduced 1.3 kg, or 10%.

  As shown in the bottom left of the Ford Focus material cutting image above, the two B-pillars are placed vertically in opposite directions, which raises the yield from the steel panel. The B-pillar is slim at the top and becomes wider at the bottom, and placing the two parts in opposite directions as shown rather than in the same orientation minimizes material waste. Moreover, the elaborate design of tailor rolled blank material makes it possible to invert the cutting layout vertically by making it so that among the eight changes in thickness, the dimensions of the seven levels on the upper part of the B-pillar (there are 13 spaces, including those where there are gradual changes) have upper and lower symmetry. This indicates a meticulous pursuit not just of weight reduction, but cost reduction as well.

Tailor rolled blank solution by Mubea
Tailor rolled blank solution by Mubea
Source: Mubea

  The image above shows the manufacturing process for Mubea's tailor rolled blank material. By controlling the position of the blue roller, the company says that the thickness of the sheet can be freely changed through pressure from flexible rolling. This enables the separate creation of areas with uniform thickness and others with gradual changes. Finely optimizing sheet thickness allows strong rigidity to be maintained, while at the same time reducing weight and cost. These materials have been utilized for the VW Golf, and are gaining popularity with mostly U.S. and European OEMs.



Application of tailor rolled blanks to hydroforming

Application of tailor rolled blanks to hydroforming
Application of tailor rolled blanks to hydroforming
Source: Mubea

  This section will introduce an application example of hydroformed steel tubes that use tailor rolled blanks. A steel tube material is made by welding a sheet with the thickness changed by area with the flexible rolling process described above. The tube material is then made into a product by bending it with hydroforming. The image above shows a longitudinal frame of the rear suspension member used in the Audi A4, Q5, A6, and A8.

  Hydroforming is a die forming process for shaping hollow steel tubes into complex shapes that also makes it possible to change the cross sectional shape. A hollow steel tube is placed in a female die and fluid is injected into it. High pressure is applied to the fluid while the tube is pressed with the die to form it. This method is commonly used to make suspension member frames.

  In the Audi example presented, the hydroformed frame has a variable sheet thickness ranging from 1.8mm to 3.6mm. Whereas conventional structures have a constant thickness, hydroformed frames practically realize frames where the thickness is reduced by up to one half depending on the area.

  Including 2WD and 4WD specifications, Audi has 6 varieties of suspension members for the A4, Q5, A6, and A8. However, only one kind of hydroforming shaping die is needed thanks to tailor rolled blank material sheet thickness variation. When hydroforming and using steel tubes with different thicknesses, even with the differences in dimension from the sheet thickness variation in the interior, the same hydroforming die can be used if the exterior is the same. Because of this, 6 different components with their respective specifications can be formed with one die.

Application of hydroforming to parts shared by four Audi models
Application of hydroforming to parts shared by four Audi models
Source: Mubea


The future of hot forming and cold forming techniques

  A technical lecture was given by ThyssenKrupp on the future of hot forming and cold forming. The details are summarized below.

Application of hot forming and cold forming to autobody frame structures
Application of hot forming and cold forming to autobody frame structures
Source: ThyssenKrupp

  The present usage of hot formed and cold formed parts is illustrated in the image above. Cold-formed parts are used mainly in the engine compartment to absorb impact energy during a frontal collision. Hot-formed parts are used in the frame around the passenger's cabin to retain a space for passengers. They are used especially in the body sides to minimize the structural deformation during a side collision.

Thyssenkrupp's development goals for steel sheets
ThyssenKrupp's development goals for steel sheets
Source: ThyssenKrupp

  The company has a medium- to long-range goal to develop new steel grades and technologies for automotive use. Conventional hot forming steel has high tensile strength but its ductility is at a level that does not meet expectations. However, ThyssenKrupp is currently developing a new manganese (aluminum) steel with high tensile strength and ductility. The company is also developing a third-generation cold forming steel that has both high-dimension tensile strength and ductility.

Thyssenkrupp's new-generation hot-forming and cold-forming steels
ThyssenKrupp's new-generation hot forming and cold forming steels
Source: ThyssenKrupp

  The image above shows an example of the weight reduction when a hot formed steel sheet is utilized for a B-pillar. As noted previously, B-pillars play an important role in reducing deformation during a side collision to maintain space for passengers, and this is why high strength is required. It is necessary to make panels thicker to increase strength, but the adoption of high strength steel sheets makes to possible to make them thinner and reduce weight. By changing the B-pillar material from 2.0 mm 800 MPa-class cold formed dual phase steel to 1,500 MPa-class manganese-boron steel, ThyssenKrupp was able to reduce weight by 15%.

 Moreover, the right side of the image shows the characteristics of cold-formed and hot-formed steel. TriBond (indicated in orange) is 3-layer composite material made with high manganese boron type steel by ThyssenKrupp that has better crash performance than cold-formed steel material (indicated in blue) while at the same time featuring part complexity (workability). TriBond has a three layer structure with 1,500 MPa-class manganese boron steel sandwiched between 500 MPa-class manganese boron steel. This gives it both high tensile strength and flexural properties.

 

Application of Thyssenkrupp's new-generation hot-forming and cold-forming steels
Application of ThyssenKrupp's next-generation hot-forming and cold-forming steels
Source: ThyssenKrupp

  When the new hot-forming and cold-forming steel grades ThyssenKrupp is aiming for become a reality, they will enable steel sheets that serve that have both high tensile strength for resisting deformation and high ductility to absorb impact energy. This will result in energy being absorbed in areas around the cabin where deformation must be kept to a minimum, and also resist deformation in the engine compartment and other crushable zones that primarily absorb energy. The increased freedom of development for autobody frame structures will also make further contribution to improving crash safety performance and reducing vehicle weight.

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