Mild steel is used for the outer panels where complex shape is required for styling and other large area panels such as floor sections, wheel arches inner sections etc. High strength steels (HSS) are used strategically for occupant safety. Often due to the intrinsic strength of the material they cannot be formed easily in complex shapes as required by styling. Instead they perform their safety critical role sandwiched in between and behind the visible styled panels. High-strength steel with yield strengths above 300 Mpa / c 40,000 pounds per square inch now comprise about one-fifth of the total ferrous content of new cars, and their use is growing.

Introduction

Steel is plentiful as a resource. It can be produced from the raw ingredients, Iron Ores or Oxides and converted to iron in a process known as smelting or produced from re-cycled scrap and re-melted in a furnace. In some cases the molten metal is poured into moulds to form castings.

In other cases, the molten metal is poured into moulds to form billets which in turn are physically wrought by machines into the required steel product e.g. it might be drawn into wire, rolled into girders or passed through a succession of high-pressure rollers to form sheet on a large roll called a coil. This is the stuff from which car bodywork is made.

Traditionally steel is a material used to make things where weight and corrosion are not problem factors.

The steel produced in this way has a very wide tolerance of physical characteristics, due to its raw ingredient being a wide mix of mainly uncontrolled metal waste and impurity. Most of the impurity is evaporated off as a result of the high melting temperatures and charred remnants / cinders float to the surface to be removed as slag.

The controls for changing the properties of these steels might be temperature, gas injection into the crucible of molten metal e.g. carbon dioxide and cooling / quenching cycles. Similarly a metallurgist (metals chemist) can prescribe the addition of certain other elements e.g. silicon, chrome, aluminium, carbon etc to change the structure and therefore the performance of the end product.

Steels therefore, can be developed for many different functions depending upon its eventual use as a finished product.

So with metals for manufacturing the vehicle body, the metallurgist can control the chemistry and process to give specific characteristics, for example.

So steel is the material of choice because, as stated earlier it is "Traditionally a material used where weight and corrosion are not problem factors".

For the purposes of this web document it isn't necessary to explore the metallurgy of steels used in vehicle body construction, but it is important to be aware of the trends in construction techniques of the BIW (Body In White) and the materials used for giving guidance on welding techniques.

The car body manufacturer has learned to change over the last few decades. Weight has become a problem and so has Corrosion.

Corrosion

Corrosion has fundamentally been controlled by steel coating technologies such as Zinc Electro coating, Galvannealing and Hot dipping of steel coils prior to pressing Corrosion is further controlled by sealing and painting operations including wax injection processes.

Weight

We want, Comfort = increased torsional rigidity = better suspension = more weight.

We want, Luxury e.g power windows = increased weight

We Don't want, Weight = less fuel efficiency = environmental problem = cost of ownership.

But Cars just got heavier. For example

Golf Mk 1 750 -850 kilos
Golf Mk 2 845 - 985 kilos
Golf Mk 3 960 - 1380 Kilos
Golf Mk 4 1155 - 1477 Kilos
Golf Mk 5 1155 - 1590 Kilos

Data courtesy of CAB

Why? Because as consumers we want more and more out of our driving experience, e.g power steering, wider body, air conditioning and a stronger safer car. As the bodies became heavier, manufacturers looked for other, lighter materials from which to make the components.

How has the car manufacturer made weight savings in the overall build weight?

Substitution of steel for aluminium and plastic as a material for making all car components has resulted in today's average car having double the amount of nonferrous materials than it did thirty tears ago, meaning that, iron and steel declined from about three-quarters of a car's total mass to about two-thirds.

Why not build car bodies from lighter metals or plastic?

Steel remains the metal of choice for the major part of the construction of the BIW. Factors such as formability, material cost, strength to weight ratio, recyclablility, resistance to bending once formed, torsional resistance are all contributing factors, generally beating other materials such as plastics and aluminium.

However, the use of plastic and other lightweight materials is not uncommon when used strategically for weight saving and production cost factors e.g. crash beams and bumper covers.

Most of the iron and steel (ferrous) fraction is mild steel, a highly formable, steel grade with the relatively low yield strength of 20,000 to 23,000 pounds per square inch (psi) / 130 - 160 Megapascals (Mpa).

However,

Strength

Where vehicles are designed for strength, the aim is primarily to try to handle crash loads, using higher-strength steels in crash-sensitive parts such as impact beams, bumper bars, sill sections, and B-pillar reinforcements. The different design requirements of the various zones of the vehicle require the designer to choose a steel characteristic to match. As a result, High-strength steels come in a spectrum of strength levels.

Steel is a metal alloy, the major component of which is iron with a carbon content of between 0.02% and 1.7% by weight. Depending on the requirements the steel maker can use many alloying materials but carbon is the most cost effective. Varying the recipe and the processes controls the steel characteristics such as tensile strength, ductility, hardness etc

The higher the tensile strength (Rm) -

the lower the breaking elongation (A)

Steel Strength

HSS: High Strength Steels tensile strength Rm= 300 MPA - 450 MPa / c 44 - 65,000 psi

VHSS: Very High Strength Steels tensile strength Rm= 450 MPA -1000 MPa / c 65 - 145,000 psi

UHSS: Ultra High Strength Steels tensile strength Rm > 1.000 MPa / c 145,000 psi plus

Medium-strength low-alloy (MSLA) steels, which have yield strengths from 170 - 280 Mpa / c 25,000 and 40,000 pounds per square inch, MSLA steels have the same carbon levels as mild steels, but are strengthened by dissolving more phosphorus or manganese alloy ingredients into the melt during manufacturing. The alloy additions make MSLA somewhat more costly than milder grades.

High-strength low-alloy (HSLA) steel varieties, with yield strengths in the range of 280 - 520 Mpa / c 40,000 to 75,000 pounds per square inch, are hardened by the addition of small amounts of titanium or niobium, which produces fine dispersions of carbide particles. Farther up the strength ladder are dual-phase steels, which feature yield strengths from 520 - 1100 Mpa / c 75,000 to 150,000 pounds per square inch.

Dual-phase steels, typically mostly ferrite and some martensite (two iron-alloy compositions), are produced by alloying iron with magnesium and silicon, followed by special processing. Dual-phase steels have relatively high formability, the downside is there is a lot of alloy in these grades, making them more costly and more difficult to weld and zinc-coat for corrosion protection.

Ultra High Strength (UHSS) steels grades featuring even higher-strength levels (800 - 1500 Mpa / c 115,000 to 215,000 pounds per square inch), such as the fully martensitic varieties, are available, but their limited formability means the use of special processing methods such as hot stamping.

The objective of decreasing the weight, whilst improving passenger security can also be attributed to the wider application of higher-strength steels, which can often perform the same structural function with less metal.

How does this translate to a Passenger Vehicle Body where the designers have used current material technology to enhance occupant security.

Occupant security

OK. Why not build the BIW (Body In White / Bodyshell) solely from high strength steels?

Cost is a major factor here but the real reason is one of strategic passenger safety. Designing a Bodyshell (BIW) solely from HSS is counterproductive.

The value of HSS in car construction is that occupants are kept safer due to the increased compression resistance or resistance to deformation of certain components so that in the event of collision the forces of decellaration reduce the effect of abrupt braking on the human body. Imagine a car which did not absorb these forces, all the energy of a sudden stop would be felt on the occupant's body parts.

High strength steels have a major and critical role to play in vehicle construction for passenger protection.

Passenger Protection

Side Impact

Side Impact protection

Of significant importance is side impact protection (SIP). This is the most vulnerable form of collision for an occupant. SRS features alone will not prevent injury. is insufficient distance between an object such as a telegraph pole and the human shoulder for decellaration so a steel is chosen which has even higher resistance to stretching and compression. This is designed not to give way to impact forces but instead to spread the load across and into the whole bodyshell.

Therefore, the whole body side (including the portals and doors) is regarded as a safety wall. The heavier the car the more strength is needed from the steel for SIP.

To achieve the same performance from mild steel would require very thick sheet steels and heavy fabrications, the forfeit being an increase in weight and loss of style.

Front and Rear Impact

However, the car body is designed to decelerate very rapidly in frontal impacts and absorb the impact energy before it can transmit this instant extra high load onto the occupants. Imagine if the BIW remained intact and strong. The occupants alone would absorb the impact energy.

Euro Ncap ratings are important to car manufacturers and testing is getting more stringent with tests in the future possibly reaching speeds up to 70 kph.

For reference, look up Euro N Cap, http://en.wikipedia.org/wiki/Euro_NCAP

Similar factors are designed-in for the rear impacts.