Abstract: Steel has always been the main material of automobile manufacturing. The growing demand for environmental protection and energy conservation, tighter national policies on fuel consumption and higher safety requirements from consumers have forced automakers to seek stronger and lighter materials. In its report, the Center for Automotive Research (CAR) revealed that high-strength steel usage is expected to peak by 2020, accounting for nearly 15% of vehicle weight. By 2040, the share will gradually fall to nearly 5%, when other lightweight materials will also take a place in the automotive material sector.
Who wins the Battle of steel and Aluminum?
 
 
Steel has long been a major material in automobile manufacturing. The growing demand for environmental protection and energy conservation, tighter national policies on fuel consumption and higher safety requirements from consumers have forced automakers to seek stronger and lighter materials. In its report, the Center for Automotive Research (CAR) revealed that high-strength steel usage is expected to peak by 2020, accounting for nearly 15% of vehicle weight. By 2040, the share will gradually fall to nearly 5%, when other lightweight materials will also take a place in the automotive material sector.
 
 
 
Aluminum, which is less than half its weight and has excellent corrosion resistance compared with iron, was once a threat to auto steel. But because aluminum is relatively expensive and difficult to manufacture and maintain, many automakers prefer high-strength carbon steel instead of ordinary steel, creating a game between steel and aluminum. Wang Li, chief researcher of Baosteel Research Institute, Zhu Qiang, chair professor of Southern University of Science and Technology, Chen Shuming, professor of Jilin University, Zhang Haitao, researcher of Soochow University, and other industry experts discussed the "steel and aluminum war" in a roundtable discussion at the Automotive and Environment Forum recently held.
 
 
Steel has great application potential and cost advantage
 
 
With the continuous development of car steel, car steel is not a few decades ago many people in the impression of low carbon steel, now the car steel plate is becoming thinner, but the steel strength and corrosion resistance has been greatly improved. In order to meet the challenge of new materials, many iron and steel enterprises actively develop lightweight high-strength steel which can compete with other materials such as aluminum alloy. According to the data, the extra cost of high-strength steel for each vehicle is only 212 euros, which would save weight and fuel by about 5%.
 
 
What is the status quo and application potential of HSS in Chinese automobile market? Wang Li made an analysis of this. He said that the steel used in vehicles has been reduced in weight under continuous efforts. "A lot of technology has been used in this, and one of the contributions is high strength steel. Over the past 20 years, Baosteel has been participating in a program of the International Iron and Steel Association. If steel mills continue to use steel by developing new materials, how much potential does steel have? Through so many years of development, the last advice or technology to the automobile factory, one is to develop a variety of advanced high strength steel is still on the road, the second is to develop a lot of advanced manufacturing technology, while introducing the concept of the whole life cycle. For example, in a newly developed electric concept car, the body weight has been reduced by 40%. Its strength is higher with high-strength steel, 40% of which is over 1000 mpa, and only 5% is mild steel. Steel has great potential to improve strength."
 
 
"From the sales data of Baosteel, in 2017, China's self-owned brands of high-strength steel consumption accounted for 41%, European, Japanese, American, Korean and self-owned a total of more than 28 million cars, while Baosteel's materials are relatively higher grade models, our national average level will be a little lower than this level. According to our data last year, the application ratio of high-strength steel reached an average of 42-45%. This level should be relatively low, while that of foreign countries is 60-70%. This gap is our potential."
 
 
In the competition between aluminum and steel, the outstanding advantage of aluminum is its low density, and to achieve body weight reduction, the steel plate needs to be made thinner while maintaining the steel ratio. Whereas normal steel plates are usually 0.7 to 0.75mm thick, ultra-strength steel plates are now 0.65mm or thinner, with the new Opel Cefili engine cover plate being 0.6mm thick.
 
 
Wang Li said, "If the specific gravity of steel remains the same, the weight can only be reduced by thinning, but the density can be adjusted. Now we have a new idea to do, that is to adjust the density of steel. The advantage of aluminum is low density. To a certain extent, I can adjust my density by virtue of your advantages. We increased the elastic modulus of steel, and now the laboratory has done it. I would like to point out that since steel itself remains the same as the existing industrial foundation, there is still a lot of room for innovation. From this dimension, steel still has its vitality and market share. If the price of the car is more than 200,000 yuan, the choice of materials will be more, and the price of 100,000 yuan car is still mainly steel."
 
 
The cost problem has also become the main reason why other materials are difficult to replace steel. Chen Shuming said, "In the trend of automobile lightweight, although people are making lightweight materials, such as aluminum alloy, magnesium alloy and other lightweight composite materials, but high-strength steel is still in the main position, I think the most important factor is the cost, I believe that if the cost of carbon fiber down, it is possible that carbon fiber will replace this, it is not impossible. The key is now the cost is too high, steel now has a very big cost advantage."
 
 
In addition to the cost, in the range of strength to meet the demand, good and easy forming process is also the reason why steel is difficult to replace. "From a development point of view, automotive steel is not very strong, 1,000 mpa is enough. High-strength steel is now mainly strengthened by carbon, many have achieved 2200 mpa, but above 2200 mpa, there will be a sudden change, or 2200-2500 mpa by carbon reinforcement is basically impossible." Chen Shuming admitted, "I believe the steel will certainly have other materials to replace the carbon, the strength will be made higher and higher, but it is not necessarily used in the car, it may be used in other high-strength fields. For automobiles, we have a wide selection of steel under 1000 mpa, low cost, and the forming process is very good, so it will be difficult to replace steel for a while."
 
 
From the structural properties of the steel itself, it has good repairability. Zhu pointed out that steel itself carries phase transition, which has some advantages for some uses. "When it comes to automotive steel, because of the phase change, if you hit a hole, it's very easy to repair it, which is a little bit more difficult for composite or aluminum. For example, aluminum alloy composite materials, if a hole is broken, the basic repair is to replace the whole piece, the cost is high, which is the weakness of aluminum itself compared with steel."
 
Aluminum alloy development period encountered before the Wolf after the tiger
 
 
Statistics show that it takes 725kg of steel and cast iron and 350kg of stamped steel to make an average medium-sized car. By contrast, the weight of aluminium in a European car increased from 50kg in 1990 to 131.5kg in 2005, with most still being used in engine internals and cylinder blocks, and this proportion continues to rise. Aluminum is also popular in cars because it is half as heavy as iron, which is used to make steel, and it is more resistant to corrosion than steel.
 
 
At present, the use of all aluminum alloy body models have appeared in many. The Audi A8 has had an all-aluminum space frame body since its birth in 1994, and the Model S developed and manufactured by Tesla has an all-aluminum body. After Chery Jaguar Land Rover's all-aluminum body production line in Changshu, Jiangsu province, launched, the all-new Jaguar XFL, the first Chinese-made car, is made of 75 percent aluminum alloy. The Novelis RC5754 high-strength aluminum alloy is used in multiple body parts of the Jaguar XFL, with yield of 105-145 Mpa and tensile strength of 220 Mpa, providing excellent performance in terms of strength, corrosion resistance, connectivity and formability.
 
 
"There are more and more aluminum in cars, especially in the chassis, and also in the body, and now a lot of cars are going this way. The all-aluminum frame has some problems, but they are being solved." Zhang Haitao, a researcher at Soochow University, said, "Why use an all-aluminum frame? The first cost is relatively low, the cost of a small car may be thousands of yuan a frame, the most important is that the section design is very complex, and the bending and torsional stiffness of aluminum is better than that of steel."
 
 
In addition, aluminum has better resource recovery and longer life cycle than steel. "The recycling loss rate of aluminum is only 5 to 10 percent. If steel rusts, it is difficult to recycle it. In the long run, aluminum alloy has advantages. If aluminum is used on the wheel, now we have a consensus that aluminum alloy wheels are definitely better than steel, because steel is easy to rust, aluminum alloy scraping does not matter, this performance of steel is no way to compare, aluminum alloy composite performance in this respect has a unique advantage. In addition, the long life cycle is also important for the automotive industry. Every product is designed with long life cycle in mind, and aluminum is better positioned for that."
 
 
In terms of recycling, aluminum also does not have advantages. Zhu Qiang also pointed out that the composition of aluminum alloy is relatively complex, and there are also problems in recycling classification. "For example, for the die-cast frame, the plate two alloys can not be used together, must be separated, the connection of a great deal of effort, and the time to disassemble a great deal of effort, on the one hand, the recovery efficiency is not high, on the other hand, it is not easy to manage, this aspect of steel recovery is relatively simpler. In addition, there are a lot of issues involved in aluminum recycling, such as reducing usage, good aluminum is recycled and used for something that is not important, and something that is good ends up being of very low value."
 
 
In terms of material fatigue, aluminum is more risky than steel, and processing is limited. "The fatigue performance of key vehicle components is not only controlled by the properties of the material itself, but also by the material defects. Aluminum oxidation ability is very strong, these defects have a great impact on the fatigue performance of components, very easy to cause problems. Steel is not as oxidizing and its defects have a relatively low impact on fatigue performance." Zhu Qiang said, "You can't do complex parts just by forging. You have to process them after forging, otherwise it can't meet the needs of structural design. Generally speaking, there are two kinds of forging, or give up structural optimization, or reprocessing, but the fatigue performance of aluminum alloy will be reduced after the processed surface is damaged, and the cost will be increased. These are the problems that aluminum alloys have to overcome, and it is possible to replace steel."
 
 
In the automobile chassis material, aluminum has replaced part of steel before, but in recent years with the progress of steel technology, chassis steel has introduced a new solution. Zhu Qiang said, "Now in terms of steel chassis, we have developed several technologies. One is the arm. We are now at 780 mpa. Now we can make steel triangle arm. There is also a link between the two wheels that is very heavy, and now we have developed a new technology that reduces the weight by 40 percent, and by using the coating, the corrosion problem is solved, and the steel itself is improving. "Now that steel and aluminum are competing and reinforcing each other, there are more options for auto companies and they are growing out of that."
 
 
In fact, automotive aluminum has entered the stage of the Wolf behind the tiger. In front of steel manufacturers through continuous improvement of performance, now steel without nickel can achieve rust, while behind the magnesium alloy, carbon fiber and other materials with the cost reduction and performance improvement, have formed an impact on the aluminum market. Zhu Qiang pointed out, "If you want to do well, aluminum can only develop rapidly, because steel has been done for so many years to replace it has been difficult, aluminum must be industrialized as soon as possible, so as not to be easily replaced by later comers, the current challenges and opportunities for automotive aluminum are coexisting."
 
 
Steel - aluminum hybrid body structure is the trend
 
 
A growing number of automotive engineers are focusing more on the mix of lightweight materials, not just the specific ratio of steel to aluminum, but the right mix of materials. The new Audi A8, unveiled at the Frankfurt Motor Show last year, has reformed and upgraded Audi's all-aluminum space frame body structure technology. It has abandoned the all-aluminum body, which Audi is always proud of, and reduced the proportion of aluminum alloy to 58%. In addition to high-strength steel, more composite materials have been added to the body material, but the body weight has increased by nearly 51 kg compared with the current model. The current A8 model has gained weight from 236kg to 282kg.
 
 
The new generation Audi A8 uses aluminum alloy material to build the overall frame of the body. In order to ensure the structural strength, aluminum castings are used in the key connecting parts and aluminum sheet metal parts are used on the body surface. In the cage structure of the body and cabin, a lot of hot formed ultra high strength alloy steel is used, far more than the existing A8 high strength steel only used in the B-pillar, the high strength steel material has five times more stiffness and 40 percent less weight than the steel of 20 years ago. Magnesium alloy is added to the body structure and CFRP carbon fiber composite is used at the rear of the carriage, reducing the weight of the body from details such as the rear siding.
 
 
"In the future, aluminum will be used more and more in the whole body of the car, there will be a lot of hybrid body, for example, Audi A8 all-aluminum body also started to do hybrid body, and now many domestic car companies are following suit. The main problem in the connection of steel and aluminum is corrosion resistance, by gluing, by tethering without welding. Steel for the upper body, aluminum for the lower body, in the form of tethering. For example, the window frames of BAIC vehicles are made of steel above and aluminum below. This combination of mixed applications is increasing. It's not that steel is bad, but I think steel and aluminum together have a better future." Zhang Haitao said.
 
 
Wang Li also pointed out that there was a competition between steel and aluminum as early as the 1940s, and after years of development, there is now some consensus on the materials used in cars, that is, the right materials used in the right place. And steel itself is developing rapidly, with both competition and cooperation. And this competition is more beneficial to the development of automobile enterprises, because the existence of competition automobile enterprises can have more choices. Looking ahead, new energy vehicles may have higher requirements for lightweight.
 
 
"Independent brands must have a lightweight strategy. If the steel is well used, its potential is not small. It is easy to achieve 10% weight reduction of the body in white by using the proportion and type of high-strength steel used in joint venture brands. "If we use some new technologies and processes, it is possible to reduce the weight by more than 20 percent. We have analyzed many of our own brand models, this area of potential is still very big, the gap is our motivation."
 
 
Compared with the traditional body, the structure of the steel and aluminum mixed body is designed according to the corresponding ratio of different properties of materials. While improving the safety, the overall body is stronger and lighter, convenient for modular design, and the body has more space to configure complex technology and more diversified products. In addition to joint-venture brands, many homegrown brands such as SAIC Motor, BAIC and BYD are increasingly using a hybrid steel and aluminum body structure. With the development of domestic automobile manufacturing industry, while steel and aluminum compete on the same stage to push forward the innovation of automobile materials, how to correctly apply a variety of materials such as high-strength steel, aluminum alloy, magnesium alloy and carbon fiber composite materials in the vehicle in the future will become the most important topic of automotive lightweight.