Microalloyed steel
Encyclopedia
Microalloyed steel is a type of alloy steel that contains small amounts of alloy
ing elements (0.05 to 0.15%). Standard alloying elements include: niobium
, vanadium
, titanium
, molybdenum
, zirconium
, boron
, and rare-earth metals. They are used to refine the grain microstructure
and/or facilitate precipitation hardening.
These steels lie, in terms of performance and cost, between carbon steel and low alloy steel
. Yield strength is between 500 and 750 MPa (72,518.9 and 108,778.3 psi) without heat treatment
. Weldability
is good, and can even be improved by reducing carbon content while maintaining strength. Fatigue life and wear resistance are superior to similar heat treated steels. The disadvantages are that ductility
and toughness
are not as good as quenched and tempered (Q&T) steels. They must also be heated hot enough for the all of the alloys to be in solution; after forming the material must be quickly cooled to 540 to 600 °C (1,004 to 1,112 F).
Cold worked microalloyed steels do not require as much cold working to achieve the same strength as other carbon steel; this also leads to greater ductility. Hot worked
microalloyed steels can be used from the air-cooled state. If controlled cooling is used, the material can produce mechanical properties similar to Q&T steels. Machinability
is better than Q&T steels because of their more uniform hardness and their ferrite
-pearlite
microstructure.
Because microalloyed steels are not quenched and tempered they are not susceptible to quench cracking, nor do they need to be straightened or stress relieved. However, because of this they are through hardened and do not have a softer and tougher core like quench and tempered steels.
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
ing elements (0.05 to 0.15%). Standard alloying elements include: niobium
Niobium
Niobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
, vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
, titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
, zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
, boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
, and rare-earth metals. They are used to refine the grain microstructure
Microstructure
Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
and/or facilitate precipitation hardening.
These steels lie, in terms of performance and cost, between carbon steel and low alloy steel
Low alloy steel
Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low-alloy steels and high-alloy steels...
. Yield strength is between 500 and 750 MPa (72,518.9 and 108,778.3 psi) without heat treatment
Heat treatment
Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...
. Weldability
Weldability
The weldability, also known as joinability, of a material refers to its ability to be welded. Many metals and thermoplastics can be welded, but some are easier to weld than others...
is good, and can even be improved by reducing carbon content while maintaining strength. Fatigue life and wear resistance are superior to similar heat treated steels. The disadvantages are that ductility
Ductility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...
and toughness
Toughness
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...
are not as good as quenched and tempered (Q&T) steels. They must also be heated hot enough for the all of the alloys to be in solution; after forming the material must be quickly cooled to 540 to 600 °C (1,004 to 1,112 F).
Cold worked microalloyed steels do not require as much cold working to achieve the same strength as other carbon steel; this also leads to greater ductility. Hot worked
Hot working
Hot working refers to processes where metals are plastically deformed above their recrystallization temperature. Being above the recrystallization temperature allows the material to recrystallize during deformation. This is important because recrystallization keeps the materials from strain...
microalloyed steels can be used from the air-cooled state. If controlled cooling is used, the material can produce mechanical properties similar to Q&T steels. Machinability
Machinability
The term machinability refers to the ease with which a metal can be machined to an acceptable surface finish. Materials with good machinability require little power to cut, can be cut quickly, easily obtain a good finish, and do not wear the tooling much; such materials are said to be free machining...
is better than Q&T steels because of their more uniform hardness and their ferrite
Ferrite (iron)
Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material...
-pearlite
Pearlite
Pearlite is often said to be a two-phased, lamellar structure composed of alternating layers of alpha-ferrite and cementite that occurs in some steels and cast irons...
microstructure.
Because microalloyed steels are not quenched and tempered they are not susceptible to quench cracking, nor do they need to be straightened or stress relieved. However, because of this they are through hardened and do not have a softer and tougher core like quench and tempered steels.