Recently, Brother Dao's little sister Xiao Jiu was about to buy a car, and quietly asked Brother Dao a few questions.
To put it simply, yield strength is "how much force can be used to deform the metal", and what is tensile strengthPut simply, tensile strength is "how much force is used to break the metal".
In general, it is much easier to bend a steel plate than to break it, so the tensile strength of the same material is greater than the yield strength.
Suppose there are two cars, A and B, the tensile strength of the steel plate at the key position of car A is 1000MPa, and the yield strength of the steel plate at the key position of car B is 1000MPa, according to the above explanation, we can know that the tensile strength of the steel plate of car B must be much higher than 1000MPa. Then when these two cars are subjected to the same impact force of 1000MPa, the steel plate of car A will break, while the steel plate of car B will only be deformed, and will not cause fracture.
For automobiles, the occurrence of fracture is more harmful to passengers than the occurrence of deformation, so in terms of safety performance, the safety of car B is higher than that of car A.
In the process of automotive steel structure design, the yield strength index is indeed more important than the tensile strength index, but it does not mean that as long as the yield strength of the steel is selected, there is not only one kind of high-strength steel used in a car, different parts will use different steel grades, and different parts of the steel strength requirements are also different.
In automotive design and development, there are three main functional blocks that concern the yield strength and tensile strength of materials: stamping, safety, and durability. For stamping, automotive shock absorbers, spring seats, oil seal seats, bottom covers, oil cylinders, brackets, etc. all belong to automotive stamping modules. In general, the lower the yield strength of the material, the higher the tensile strength, the easier it is to form.
For safety, there are different requirements for different areas, and the energy absorption zone hopes to have a certain yield strength and tensile strength as high as possible, so that the material can absorb as much energy as possible during the deformation process. The passenger compartment wants the material to have as high yield strength as possible, which is roughly a trade-off between ensuring the safety of personnel and the effective safe space of the body. If it's too soft, there's no living space, and if it's too hard, people are just as likely to get hurt as they hit a wall.
For fatigue durability, within a certain yield strength range, the yield strength of the same type of steel grade is a little higher, and the durability life will be better.
In short, these two intensity indicators are complementary to each other, and one of them cannot be blindly pursued.
Maybe the old liberation truck was like this in the past, and I often heard the old drivers praise how thick the steel plate of the old liberation truck was. Most people still believe that the thicker the steel plate, the more resistant it is to impact. The thickness of the steel plate, the weight of the body and the stability of the chassis can ensure the stability and safety of drivingTherefore, "thin skin and thick skin" has also become one of the gimmicks for merchants to fool consumers.
In fact, the thicker the steel plate, the heavier the body, which does not improve performance, but only increases fuel consumption.
The stability of the car is mainly based on aerodynamics rather than its own weight, and under the premise of ensuring the strength, mode and safety performance of the car, the dead weight of the car can be reduced as much as possible, which can effectively reduce energy consumption and exhaust pollution. Studies have shown that a 100-kilogram reduction in vehicle weight can save 0.0 per 100 kilometers3-0.5l。It can also improve the dynamics and handling of the car, so that the vehicle can get better braking response and shorten the braking distance.
The main process in automobile production is stamping. Steel plates for automobile body are required to be easy to form, small rebound, and small wear and tear on the mold during the stamping process. The steel plate for automobiles has developed from mild steel with a yield strength of 350MPa, to duplex steel, phase transformation-induced plastic steel, multiphase steel, and martensitic steel, and the yield strength has increased all the way to 900MPa. The law of the whole development process is that the strength is getting higher and higher, and the martensite components in the metallographic structure are increasing, but at the same time, the machinability is getting worse and worse, the molding is becoming more and more difficult, and the stamping rebound is very large. At this time, the all-aluminum body structure appeared, and it was first used on the Audi A8 model.
Jaguar all-aluminium body.
However, the all-aluminum body structure is very costly, which limits large-scale application and can only be used in top-of-the-line models. As a result, people's eyes finally returned to steel.
To analyze the metallographic structure of the steel plate, it is necessary to have as much ferrite and austenite as possible in the steel plate if it is easy to form, but the strength of ferrite and austenite is very low;If you want high strength, you need to have as many martensite components as possible, but martensite is brittle and difficult to deform, and the technical development of automotive steel plates has fallen into a bottleneck for a while.
In fact, in addition to the deformation phase transformation of the steel plate, the martensitic structure in the automobile steel plate can also be obtained by heat treatment, so a new idea was born.
A series of new materials have been developed for steel plates, typically with a grade of 22MnB5 and the composition shown in the figure below. Since the main reinforcing element of these steel plates is boron, they are also known as boron steel.
The original metallographic structure of 22MnB5 is ferrite + pearlite, which is heated to about 900 degrees Celsius and converted into austenite, which is hydroformed and quenched in the mold to transform into 100% high-strength martensitic structure, with good molding performance, high strength after molding, and very superior performance.
This is what is often referred to as hot stamping steel in automotive brochures. The most prominent characteristics of hot-stamping steel are that it has a particularly high strength, a good formability, and almost no rebound. Theoretically, a car that uses hot stamping steel can reduce weight by 30% without using hot stamping steel at all, or increase strength by 30% with the same weight.
Almost all brands will use this indicator to say things. In particular, a number of third-tier small brands compare the amount of high-strength steel with each other, and it is difficult to distinguish between the real and the fake. But have you ever seen Land Rover, Hummer, Cadillac advertise how much hot stamping steel they use?
The sales guy is not wrong about one thing, if the whole vehicle uses hot stamping steel, it is really unaffordable. If you see a advertisement that 60% hot stamping steel is used, but the price is only a few hundred thousand, please take a detour.
In addition to being too expensive, the real reason is that good steel is used on the cutting edge, which is mainly used for anti-intrusion parts of the car body, and is now slowly starting to expand into energy-absorbing parts. Anti-intrusion parts are the structural parts that wrap the cockpit of the occupants of the car, mainly including A, B, C pillars, roof and bottom side beams, floor beams, front and rear body energy-absorbing areas, etc. These are often invisible areas, but they are critical areas that determine the safety of the vehicle. On the contrary, the body panels we usually see are made of low-strength steel sheets that are easy to form and inexpensive.
Strength diagram of steel used in the body of a certain brand.
When looking at the manufacturer's promotional materials, you should not mistake the slogan "Our body is made of 1000MP high-strength steel", and do not mistake it for thinking that the whole car is made of this kind of steel, in fact, only the rods of the four-door crash beam have reached this strength level.