Steel is a group effort. The addition of alloying elements or additives heightens the properties of steel, improving its mechanical properties, weldability, and resistance to corrosion and reducing its production cost. In the case of microalloyed steels, adding very limited quantities of certain elements enhances strength substantially.
ASTM A572/A572M-21: Standard Specification for High-Strength Low-Alloy Columbium-Vanadium Structural Steel specifies five grades of high-strength low-alloy (HSLA) structural steel shapes, plates, sheet piling, and bars.
The Five Grades of ASTM A572-21
Grades 42 , 50 , and 55  are intended for riveted, bolted, or welded structures.
The other two grades specified by ASTM A572-21, grades 60  and 65  are intended for riveted or bolted construction of bridges. They are also used for riveted, bolted, or welded construction in other applications.
In its tables, ASTM A572-21 lays out the maximum product thickness or size, chemical requirements, and tensile requirements for each of the five grades.
Alloy Types in ASTM A572-21
The standard specification also notes the limitations of columbium, vanadium, titanium, and nitrogen, as well as combinations of these chemical elements.
Columbium, which commonly is known as niobium worldwide, is used, due to its ductility, in high-strength low-alloy steels. When niobium is added to steel, the overall structure is benefitted by the formation of new compounds that better the grain refining, retardation of recrystallization, and precipitation hardening of the steel. Overall, this increases the toughness, strength, and weldability of the microalloyed steel. Niobium microalloyed steels are used mostly for building automobiles and high-pressure gas transmission pipelines.
Almost all vanadium produced is used as a steel additive. With the addition of this element, steel becomes very tough, and its resulting qualities make it ideal for armor plate, axles, tools, piston rods, and crankshafts. Some vanadium makes steel shock- and vibration-resistant. Nuclear reactors generally have some vanadium alloy composition due to vanadium’s low neutron-absorbing abilities.
Titanium in low carbon steels helps to form a number of compounds that provide grain refinement, precipitation strengthening, and sulfide shape control. High-strength low-alloy steels containing titanium are used in the linepipe industry.
All steels contain some nitrogen. In high-strength low-alloy steels, nitrogen contributes to grain refinement and precipitation hardening. HSLA steels containing nitrogen are found in weldable reinforcing bars and, due to nitrogen’s positive influence on creep strength, stream and gas turbine components and boiler tubes.
Types by Alloy Content
For each of these elements, selection of type (1, 2, 3, or 5) is at the option of the producer, unless otherwise specified by the purchaser. The alloy content for each element is specified through heat and product analysis guidelines in ASTM A572-21.
Type 1 – columbium
Type 2 – vanadium
Type 3 – columbium, vanadium, and columbium plus vanadium
Type 5 – titanium, nitrogen, and vanadium
Adhering to these limitations allows the HSLA structural steel to acquire and attain its valued characteristics.
ASTM A572/A572M-21: Standard Specification for High-Strength Low-Alloy Columbium-Vanadium Structural Steel is available on the ANSI Webstore.