Micro-alloyed steel

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. Therefore it is classed as a low-alloy highstrength steel within the carbon range. As a result of its strength,toughness,formability and weldability,the car body manufacturers are using this material to produce stronger,lighter-weight body structures. A typical composition utilized for a micro-alloyed high-strength steel (HSS) is as follows:
Percentage
Carbon (C)0.05–0.08
Manganese (Mn)0.80–1.00
Niobium (Nb)0.015–0.065
The percentage of niobium used depends on the minimum strength required.
Formable HSSs were developed to allow the automotive industry to design weight out of the car in support of fuel economy targets. A range of high- strength formable steels with good welding and painting characteristics have been developed. The steels are hot rolled for chassis and structural components,and cold reduced for body panels. Through carefully controlled composition and processing conditions,these steels achieve high strength in combination with good ductility to allow thinner gauges to be used:a reduction from 0.90mm down to 0.70mm is a general requirement.

There was a danger that the new HSS,thinner but just as strong,would lack the ductility whichallows it to be press formed into shape. This problem was overcome by the use of microalloying. In a metal crystal the atoms are in layers; when the crystals are stretched (as in forming),one layer of atoms slides over another.
The layers of atoms slide like playing cards in a pack,and in doing so are changing shape in a ductile way. The sliding can be controlled by adding elements to the steel such as niobium ortitanium. The element reacts with the carbon to produce fine particles which spread through the steel. The element controls the ductility and strengthens the steel,thereby improving the properties of the material.
These steels are low carbon,employing solution strengthening (cold reduced rephosphorized) or precipitation hardening (hot rolled and cold reduced micro-alloyed) elements to produce fine- grained steels which are suitable for welding by spot and MIG processes only.