High-strength steel

High-strength steel 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) steel,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 crisis. Lighter car means 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 Vehicle Concepts (ULSB – ACV) group. This further research has led to the concept of 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 bought 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 1200MPa compared to LC steel which has a range of 140–180MPa. However, although the metal is stronger,it is not necessarily stiffer. That is,the body parts can not necessarily be made of thinner metal 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 other. 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 are also known as re-phosphorized – added phosphorous; isotropic – added silicone and bake hardened – strain 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 300MPa. 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 500MPa. 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.
Dual phase (DP) steel has a yield strength of between 500 and 1000MPa. 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.
Transformation induced plasticity (TRIP) steel has a yield strength of between 500 and 800MPa with greater figures attainable in some cases. TRIP steels may be alloyed with higher quantities of carbon and silicone and aluminium. The strength is triggered by work hardening by the stress induced during the stamping or forming process. That is, the retained austenite is transformed into martensite by the increasing strain during the stamping or other forming process.
Complex phase (CP) steel has a yield strength of 800–1200MPa. 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,screen pillars,cant rails and strengthening cross members on cars made after about 1990.
On new cars look out for panels with large swage lines,remember that the pressing process triggers the hardening of the metal and so makes the panel stiff both in shape and in microstructure. Damaged AHSS and HSS panels are not readily repairable as the impact changes the microstructure,making the metal harder. So,panel beating is not an option,replacement panels must be fitted in most cases.
The normal method of joining HSS and AHSS panels is by spot welding. The heat and pressure involved in the welding process changes the microstructure of the metal,so great care is needed in this process. Again,follow manufacturer’s instruction on these repairs. It is sensible to do tests before spot welding the new panels. You can use the undamaged sections of the panels which you have removed for test welds,changing weld time and current,then cutting through the welded area to check for penetration and adhesion.
Any form of applied heat to HSS and AHSS panels should be avoided,this includes trying to anneal or soften the panel for the purposes of straightening,heat shrinking or oxyacetylene cutting. Neither MIG plugging nor cold working are recommended. Remember that the nature of these processes will affect a change to the properties of the steel,and that the energy of the impact will have had the same effect on the panel.

Guide to spotting AHSS and HSS panels
•Look for reinforced areas such as door pillars and cross bracing areas,and where two or more panel parts over lap each other.
•Look for body panels with pronounced sharp swage lines.
•Feel for very thin panels.
•Listen for panels which when tapped gently give a crisp metallic ring.