What properties should steel for body armor have?
The key requirement for the material of an armor plate is the ability to withstand the impact of a certain type of ammunition without penetrating and hitting a person. Among steels, such ballistic resistance is provided by high-strength low-alloy and alloy grades. But the specific level of properties already depends on the protection class of the armor plate, the chemical composition and processing parameters of the alloy.
For example, let’s take the 4th protection class. Practice has shown that effective protection in the eight-millimeter thicknesses of steel, which, in addition to carbon (0.27-0.45%), contains additives of chromium, manganese, nickel, molybdenum, boron and other alloying elements. Such steels are necessarily subjected to heat treatment, which provides obtaining high complex of strength properties (yield strength at least 1000 MPa, temporary resistance up to 2000 MPa and more) and high hardness at least HB 470.
At the same time the material must have sufficient plasticity and impact toughness to resist the development of brittle cracks. The relative elongation after heat treatment must be not less than 7-10%, and the impact force must be not less than 10 Joules.
Armor plate must be solid and homogeneous in structure and properties, without welds and stress concentrators.
What heat treatment is used in manufacturing of steel plates for body armor?
Heat treatment regimes depend on chemical composition of steel and required set of mechanical properties. The basic state of supply of steel plates for manufacturing bulletproof vest level 5, as well as the finished steel armor elements, is after quenching and low tempering.
There are also exceptions: some grades (e.g. Armox Advance from SSAB) are supplied in quenched condition without tempering, while some others are medium tempered at higher temperatures after quenching. Do not subject the inserts to any high temperature stresses after hardening as this could cause a total or partial loss of performance properties.
What grades of steel are suitable for armor plates?
Such materials should be divided into two groups. The first is represented by steel sheets that already have the required set of mechanical characteristics. Such rolled steel is cut and subjected to bending, giving the necessary dimensions and shape of the plate, after which they are ready for use.
Among such steels, the following ballistic steel grades from the leading world producers are worth mentioning: Swebor Armor 500, Swebor Armor 550, Swebor Armor 560, Swebor Armor 600 from the Swedish company SWEBOR; Miilux Protection 600, Miilux Protection 500 from Finnish Miilux Oy; Mars 650, Mars 600, Mars 500 from Belgian ArcelorMittal Industeel and other analogs.
This group also includes various industrial wear-resistant steels, which are also available in hardened condition and have similar chemical composition and level of properties (for example, some of the Hardox grades from SSAB).