Metals used in Vehicle Bodies
3.1 Know your Steels and their Yield Strength
Types of Steel |
Yield Strength |
Technical Information |
Mild Steel |
Less Than 220 (mPa) |
Wings |
Deformable Steel |
220N/mm |
Pedestrian Protection |
(SS) Soft Steel |
220 N/mm (MPa) |
Soft Steel |
ZSTE 260 |
260N/mm (MPa) |
Soft-Medium (HSS) |
ZSTE 300 |
300N/mm (MPa) |
Medium Strength (HSS) |
(DD) Deep Drawn |
440N/mm (MPa) |
Easily Formed (HSS) |
(IF) IF Steel |
440N/mm (MPa) |
High Strength Steel |
(IS) Isotropic Steel |
440N/mm (MPa) |
High Strength Steel 440 Max Steel |
(HSS) High Strength Steel |
220-550 N/mm (MPa) |
Medium Strength |
(DP) Dual Phase Steel |
450-1300N/mm (MPa) |
Many Crystalline Forms/Combinations |
(EHS) Extra High Strength |
550-800N/mm (MPa) |
Extra High Strength |
(UHS) Ultra High Strength Steel |
800-1300/mm (MPa) |
Excess Carbon for Hardness (Hardest Form) |
(Trip) Transformation Induced Plasticity |
500-800 (MPa) |
Carbon Silicon Aluminium Mix |
4.0 Qualities of Steel
Engineering materials are grouped into two main categories:
Ferrous Metals:
These are metals and alloys containing a high proportion of the element iron. e.g. mild steel
Non-ferrous Metals:
These materials refer to all the thirty eight remaining metals known to man.
Common non-ferrous metals are:
Properties of materials
The properties of materials are divided into three main categories:
Mechanical Properties
-Malleability
Chemical Properties
Physical Properties
Physical Properties
Tensile Strength
This is the ability of a material to withstand tensile (stretching) loads without breaking.
Compressive Strength
This is the ability of a material to withstand compressive (squeezing) loads without being crushed or broken.
Shear Strength
This is the ability of a material to withstand offset loads, (shearing actions)
Toughness (Impact Resistance)
This is the ability a natural material to withstand shatter – if a material shatters it is brittle (e.g. glass). toughness should not be confused with strength.
Elasticity
This is the ability a material to deform under load and return to its original size and shape when the load is removed.
Plasticity
This is the state of a material which has been loaded beyond the elastic state. This property is the exact opposite to elasticity. Under a load beyond that required to cause elastic deformation the material deforms permanently and will not return to its original size and shape when the load is removed.
Ductility
Is a particular form of the property of plasticity. The term is used when plastic deformation occurs as the result of applying a tensile load. A ductile material is required for cold pressing low-carbon steel sheets into motor car body panels.
Malleability
This is the term used when plastic deformation occurs as the result of applying a compressive load.
Hardness
This is defined as the ability of a material to withstand scratching (abrasion) or indentation by another hard body.
Rigidity (Stiffness)
This is measure of a material’s ability not to deflect under an applied load.
Electrical Conductivity
The term used to describe the resistance to flow of electrons (electric current through the material). A material that has the property of very good electric conductivity offers very little resistance to the flow of electric current e.g. copper.
Magnetic Properties
In the same manner that some materials can be good or bad conductors of electricity some materials can be good or bad conductors of magnetism – good magnetic conductors are the Ferro – magnetic materials which get their name from the fact that they are made from iron. Steel and associated alloying elements – magnetic materials can be sub- divided into two classes:
These retain their magnetism after the initial magnetising force has been removed.
These retain virtually no magnetism when the magnetising force is removed.
Thermal Conductivity
This is the ability of a material to transmit heat energy by conduction.
Fusibility
This is the ease with which materials will melt. Solder melts easily and therefore has the property of high fusibility. Material that will only melt at very high temperatures are said to have the property of low fusibility.
Temperature Stability
Substantial changes in temperature can have very significant effects of the structure and properties of materials.
5.0 Qualities of Mild Steel
5.1 Plain Carbon Steels
Carbon steels and alloys can be heat treated in order to carry their mechanical properties.
The heat treatment processes appropriate to plain carbon steel are:
Annealing Processes
All annealing processes are concerned with rendering steel soft and ductile so that it can be cold – worked and or machined.
Normalising
This process resembles full annealing except that whilst in annealing the cooling rate is deliberately retarded, in normalising, the cooling rate is accelerated by taking the work from the furnace and allowing it to cool in free air.
Hardening
When plain carbon steel with a carbon content above 0.4% is cooled very rapidly from the appropriate temperature the steel becomes appreciably harder.
Tempering
Fully hardened plain carbon steel is brittle and hardening stresses are present. In such a condition it is of little practical use, and it has to be re-heated or tempered to relieve the stresses and reduce the brittleness.
Tempering or hardening
Is a process of improving the characteristics of a metal, especially steel. Tempering is carried out by heating the metal to a high temperature and then cooling it, usually by quenching it in oil or water.
Cold chisels, screwdrivers, springs and the jaws of stilsons are examples of tools which are tempered.
Annealing
Annealing is the treatment of a metal or alloy to reduce its brittleness and improve its ductility. Annealing is often referred to as the softening before work is continued, other wise it might fracture. Annealing is achieved by the application of heat. Copper pipes are annealed before spring bending. The pipe is heated to a dull red colour and then allowed to cool or quench in cold water.
5.2 Corrosion of metals
Factors governing corrosion
There are three ways in which metals corrode:
Dry Corrosion
Is the direct oxidation of metals which occurs when a freshly cut surface reacts with oxygen in the atmosphere.
Wet Corrosion
The oxidation of metals in the presence of air and moisture
The corrosion of metals by reaction with the dilute acids in rain due to the burning of fossil fuels (Acid Rain)
Galvanic Corrosion
This occurs when two dissimilar metals, such as Iron and Tin or Iron and Zinc are in intimate contact. They form a simple electrical cell in which rain, polluted with dilute atmospheric acids, acts as an electrolyte. An electric current is generated and circulates within the system. Corrosion occurs with one of the metals eaten away.
Anti –Corrosion Treatments
Pure zinc has a melting point of only 420ºC, and is the only commercial metal which can be refined by distillation. Pure zinc is relatively weak, but is widely used as a coating on steel to prevent corrosion by atmospheric attack and zinc – coated low – carbon steel sheet is known as galvanised iron. In the case of galvanised iron the zinc is more corrosion resistant than the steel and is less likely that it will be corroded.
Sacrificial Protection
This is the term used to describe the protection offered by Zinc to the mild steel. The automotive industry in seeking to provide extended warranties is turning increasingly to the use of Zinc coated steels. Different areas of a vehicle require different Zinc coatings and weights to meet appearance performance criteria.
6.1 Galvanised Steel
Zinc coated steel is available in two forms
The hot dipped product is generally used for under-body parts.
The electrolytic product is used for exposed body panels.
Both types offer barrier and corrosion protection. The electrolytic products are available in single and double sided coating.
Single Sided Zinc Coated Steel
Zinc is applied to one side of a steel sheet by either the hot dip or electrolytic process. Its uncoated side provides a surface for painting; therefore, it is used mainly for outer body panels. As the zinc coating is towards the inside of the car it protects against perforation corrosion.
One and Half Sided Zinc Coated Steel
This product is produced mainly by the Hot Dip process. One side of the sheet is coated with zinc and a thin layer of zinc-iron alloys is formed on the other side. It is primarily used for exposed panels where the zinc-iron layer is on the outside for cosmetic protection and the zinc side provides perforation protection.
Double – sided Zinc Coated Steel
This product is manufactured as the name suggests by applying zinc to both sides of the sheet with equal or different coating weights. By coating the steel with galvanised material it protects the surface from corrosion.
7.0 Qualities of Aluminium
7.1 Aluminium
For use as a constructional material pure aluminium lacks strength. For most engineering purposes, aluminium is alloyed in order to give a higher strength/weight ratio.
Aluminium is approximately one – third the weight of steel. In the modern motor body vehicle the saving of weight is its most important advantage and although on average the panel thickness used is approximately double that of steel, a considerable weight saving can be achieved.
The Oxide film which forms on the surface of the metal is only 0.002cm thick and is transparent. However impurities in the atmosphere turn it to various shades of grey, if this film is broken it will reform providing complete protection for the metal.
Aluminium alloys can be formed into four groups:
Alloys which are not heated:
Alloys which are heat treated:
Non Heat treatable alloys
Wrought, including pure aluminium, gain in strength by cold working such as rolling, pressing, beating and any similar type of process.
Heat Treatable Alloys
These are strengthened by controlled heating and cooling followed by ageing at either room temperature or at 100 - 200ºC.
The most commonly used elements in aluminium alloys are Cooper, Manganese, Silicon, Magnesium and Zinc.
Alloy Steels
Plain carbon steel contain up to 1.0% manganese. Consequently a steel is not classed as an alloy steel unless it contain more than 1.0% manganese or deliberate additions or other elements.
Alloy Groups
Alloy can be divided into two main groups:
Nickel Steels
Nickel increases the strength of steel by dissolving in the ferrite. Its main effect, however, is to increase roughness by limiting grain-growth during heat treatment. Nickel steels are always low-carbon steels or alternatively medium-carbon steels with very small amounts of nickel.
Main Uses: Crankshafts – axles, crown wheels, camshafts
Chromium Steel
When chromium is added to steel the hardness of the steel is increased. However, the main disadvantage of chromium is unless care is taken to limit both temperature and time of heat treatment brittleness may arise. Low – chromium steels also have a good resistance to wear.
Main uses: Ball and roller bearings, spanners, connection rods and steering arms.
Nickel – Chromium Steels
These steels suffer from a defect known ‘as brittleness’ and for this reason straight Nickel – Chromium steels and have been almost entirely replaced by Nickel – Chromium – Molybdenum steels.
Nickel – Chromium Steels
These steels suffer from a defect known as ‘temper brittleness’ and for this reason straight nickel – chromium steels have been almost entirely replaced by nickel – chromium – molybdenum steels.
Nickel – Chromium – Molybdenum Steels
The addition of 0.3% molybdenum to steel eliminates the defect known of temper brittle’s which nickel – chromium steels suffer from.
Uses: Differential shafts, crankshafts and other highly stressed parts.
Manganese Steels
Steel contain some manganese but it is only when the manganese content exceeds 1.0% that it is regarded as an alloying element. Manganese increases the strength and toughness of a steel but less effectively than nickel. Low – manganese steels are used as substitutes for other more expensive, low-alloy steels.
T.I.G welding was developed for welding magnesium alloys. Other processes are unsatisfactory because of the reactivity of molten magnesium.
Tool Steels
The primary requirement of a tool steel is that it will have considerable hardness and wear-resistance combined with reasonable mechanical strength and toughness. Tool steels which work at high speeds are generally alloy steels containing one or more of the following elements – chromium, tungsten, molybdenum or vanadium.
High - Speed Steel
High speed steel, as we know it was first shown to the public in 1900. The maximum cutting efficiency is attained with a composition of 18% tungsten, 4% chromium, 1% vanadium and 0.75% carbon. Since molybdenum is now cheaper than tungsten many modern high steel contain large amounts of molybdenum to replace much of the tungsten.
8.0 Qualities of Stainless Steel
The rust-resisting property of high-chromium steel was discovered in the early years of the last century.
Chromium imparts the stainless properties to these steels by coating the surface with a thin but extremely dense film of chromium oxide, which effectively protects the surface from further attack.
There are two main types of stainless steel:
Straight chromium steels contain 13% or more of chromium. These steels provided they contain sufficient carbon, can be heated to give a hard structure. Stainless cutlery steel is of this type. Some of these steels however contain little or no carbon and are pressed and deep drawn to produce such articles as kitchen sinks, beer barrels, refrigerator parts.
18/8 chromium/nickel steels contain as the name suggests 18% chromium and 8% nickel the remaining being iron. This type of steel cannot be hardened (except by cold working). It is used in chemical plant and food processing where the combination of corrosion resistance at elevated temperatures, high strength and non-toxicity are very valuable properties.
These steels suffer from a defect known as weld decay – this is caused when during welding to the chromium is depleted in the region close to the weld, causing corrosion to occur in this area.
Weld decay can be overcome if the steel contains the additional element titanium, niobium or molybdenum.
Micro – Alloyed Steel (HSS):
This steel is basically a carbon – manganese steel having a low carbon content but with the addition of micro-alloying elements such as niobium and titanium.
A typical composition used for HSS is as follows:
Carbon 0.05 - 0.08%
Manganese 0.80 - 1.00%
Niobium 0.015 – 0.065%
Note: The % of niobium used depends on the minimum strength required.
HSS is classed as low-alloy high strength steel with a yield strength between 220 and 550 MPa. Car manufacturers are using this material to produce stronger, lighter-weight body structures because of its strength, toughness, formability and weldability.
8.1 High Strength Steels
High strength steels have been introduced into automotive production slowly, only because of the need for specialised press tools to form body panels from this stronger material. The die tools need to be harder than for normal low carbon (LC) steels and the presses need to be stronger and more accurate. HSS came about because of the need to make vehicles lighter following the 1970 fuel crises. Lighter cars mean better fuel economy. This lead to American car makers forming the Ultra Light Steel Auto Body (ULSB) group and the Ultra Light Steel Body – Advanced high – strength steel (AHSS) as the materials have been developed and understood.
Cost
Steel costs about one-fifth of the price of aluminium when brought in the quantities needed by a car maker. Also the iron and steel industry has hundreds of years of practical experience in shaping and forming steel compared to the other materials which could be used to make vehicle bodies.
Properties of HSS
High - strength steel has a yield strength ranging from 300 to 1200 MPa compared to LC steel which has a range of 140 – 180 MPa. However, although the metal is stronger, it is not necessarily stiffer. That is, the body parts can not necessarily be made thinner as they are likely to sag. If you look at the swage lines on the latest vehicles, you will see that many panels are stiffened by the use of swaging. The current modern shapes are to allow the usage of thinner sheet steel which is lighter and of course cheaper. Oddly however, the new vehicles are not lighter in weight, this is because of the addition of electrical body controls such as electric windows and seats. HSS is not as easy to form as LC steel; also some types of HSS can be drawn better than others. Generally the extra strength of HSS is brought about by changes in the steel microstructure during the steel processing. The following paragraphs discuss the different types of HSS and AHSS steels.
HSS also known as re-phosphorized – added phosphorous; isotropic – added silicone and bake hardened – age hardened. The two most common types used in vehicle body construction are MSLA and HSLA.
Medium – strength low alloy steel has a yield strength of between 180 and 300 MPa. This steel is made by dissolving more phosphorous or manganese alloy into the molten steel during manufacture.
High – strength low alloy steel has a yield strength of between 250 and 500 MPa. This is made by adding small amounts of titanium or niobium to the molten steel which produces a fine dispersion of carbide particles.
Advanced high-strength steel types are aimed at producing steel with suitable mechanical properties for the forming of vehicle body parts, usually through the hydro forming process using water pressure to mould the metal over the die.
Duel phase (DP) steel has a yield strength of between 500 and 1000 MPa. It is made by adding carbon to enable the formation of (hard) martensite in a more ductile ferrite matrix. Manganese chromium, vanadium or nickel may also be added. The DP steel may have its strength triggered by either bake hardening or work hardening when it is stressed under the stamping or other forming process.
Complex phase (CP) steel has a yield strength of 800 – 1200 MPa. CP steel has a very fine microstructure using the same alloying elements as in DP or TRIP steel with the possible additions of niobium, titanium and /or vanadium. Again the high strength is triggered by applied strain.
Applications of AHSS
TRIP and CP steel is ideal for use in crash zones. It is excellent for absorbing energy during impact. CP steel is often used for ‘A’ and ‘B’ posts and bumper attachments. Increasingly AHSS steel is used for strengthening members to which other steel panels are welded, in other words a steel composite structure.
Repair of HSS and AHSS Panels
One of the problems is that it is not possible to recognize HSS and AHSS panels by sight. Therefore it is essential to follow the guidelines offered by the vehicle manufacturer on recommended repair methods. As a general guide, look out for parts such as ‘A’, ‘B’ and ‘C’ posts.
Boron Steels
High strength steel such as boron steel is used for the chassis sections, ‘A’ posts, ‘B’ posts and sills. Boron is the family name of a large group of high strength sheet steels (UHSS) which are continually under development. Boron steels have been developed to improve harden ability during heat treatment by the deliberate addition of boron to a range of medium carbon steels. They posses harden ability equivalent to that of much higher carbon steel and more expensive low alloy steels. In general it is used to provide extra strength in the sill area, ‘B’ post, chassis areas, rear cross members and roll over bars. It is difficult to repair and certain repair methods should only be employed.
However, whichever new car you are working with now or in the future it s very likely that you will find some baron steel somewhere. The car manufacturers without exception are all looking at inverter welders as a means to welding this new steel. It is the process that is used in their factories, so it logical that the same process should be now used in the bodyshops.
The steels are sensitive to excess temperatures, not only for welding but for corrosion protection (see MIG Welding/Brazing for temperatures) if there is excess heat the steel will have a risk of
fracture at the edge of the weld – not always immediately, it can be up to 2 to 3 years later. On the rear of the weld you may see small crystallization, as there is no protection and as the weld metal gets exposed to the atmosphere it will crystallise. The rule applies to spot welding and MIG welding.
Never use the MIG Braze plug method on 8mm, 10mm or 12mm holes as it will break, you must always cut a slot in the metal then seam braze using the MIG brazing process.
Boron steel contains a very small amount of boron, typically 0.001-0.004 percent. This steel is used by many of the major manufacturers.
Up until recently, the repair methods for vehicles with this kind of steel (EHS and UHS) have been very difficult. One of the problems being the removal of spot welds from the damaged section of the car. Now milling machines type spot weld cutters are available.
8.2 Identification of Various Steels
It is very hard to find out what steel is being used in different areas, with all the confusion around what vehicles include which steels, there is now a quick and easy solution for identifying the steel you are dealing with:
1. Get manufacturers data
2. Get thatchem data
3. Get a meter to test it
8.3 Manufacturing Methods of Steel
Sheet products are first cast by the semi-continuous casting process, then scalped to remove surface roughness and preheated in readiness for hot rolling. They are first reduced to the thickness of plate and then to sheet if this is required. Hot rolling is followed by cold rolling, which imparts finish and temper in bringing the metal to the gauge required. Material is supplied in the annealed (soft condition) and in at least three degrees of hardness, H1, H2 and H3 (in ascending order of hardness).
Summary
Most modern vehicles are constructed from a number of different steels, partly to obtain an optimised body (collision safety, rigidity, fuel economy, etc.) and partly to make them light and as easy to repair as possible.
Steels are divided into four groups according to their tensile and yield strength, that is to say the force necessary to bring about plastic deformation of the material.
Yield and tensile summary:
SS |
Soft Steel |
Maximum yield point of 220 MPa |
HS |
High Strength Steel |
Steel with a yield point 220-450 MPa |
EHS |
Extra High Strength Steel |
Steel with a yield point 450-800 MPa |
UHS |
Ultra High Strength Steel |
Steel with yield point 800-1400 MPa |
When welding on these high strength steels follow manufacturers recommendations. The sectioning and butt welding of EHS and UHS is not recommended as extreme heat alters the characteristics of the metal.
High strength steels are extensively used in automotive body structure where durability is a requirement. Due to the low-carbon and low-alloy content, these grades offer sufficient formability at the strength levels and have good weldability. The use of high strength steel saves weight because vehicle makers can use thinner gauge metal without sacrificing strength and performance.
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