| |
An Introduction to High-Strength Low-Alloy Steel
Industry Needs.
Steels used for structural purposes must simultaneously satisfy several requirements at a competitive cost level. Several of the most important properties include:
1. Yield strength.
When an external load is applied, the structure must deform elastically, that is, without permanent deformation, so that when the load is removed, the structure returns to its initial shape. Usually the maximum design load is limited to a safety factor of between 0.5 and 0.8 percent of the yield strength. A standard structural steel grade, such as Grade A36 (C-Mn) has a yield strength of about 36,000 psi (200 MPa).
2. Ductility.
Ductility is a quantifiable property, normally determined by the amount of deformation prior to failure in a tensile test. If the yield strength is inadvertently exceeded, the ability of the steel to plastically deform without breaking is an important insurance policy to avoid catastrophic failure. While the structure may no longer be functional, more serious damage, and possible loss of life, is generally avoided.
3. Toughness.
Under conditions of high impact loading and low temperatures, cracks may form and propagate leading to brittle failure (little to no ductility), an event to be avoided if at
all possible.
4. Weldability.
For some structures, mechanical joining by bolts or rivets is adequate. However, many structures are assembled by welding and this requires that permanent damage is not incurred during the process. Welding is a form of heat treatment, and alloying and high cooling rates (e.g. by using thick sections which allow rapid heat extraction) are
generally deleterious.
Elastic modulus and Poisson's ratio are also inherent properties of structural steel and cannot be manipulated.
The Chemistry.
Most structural steel is a strong, durable, malleable alloy of iron, carbon and manganese, usually containing less than 0.35 percent carbon and up to approximately one percent of manganese. Generally referred to as low carbon-manganese (C-Mn) steel, the sections (beams, plates, etc.) are air cooled from the rolling mill or in some cases, cooled somewhat more rapidly by water sprays. Typically (C-Mn) actual yield strength is in the 36,000 to 50,000psi (200 to 350 MPa) range.
Alloys (typically chromium (Cr), molybdenum (Mo) and nickel (Ni) up to a total of less than 5%) may be added with the purpose of increasing response to cooling or quenching, followed by a reheating (tempering) thereby improving both strength and toughness. The resulting microstructure causes the metal to become harder and stronger but also less ductile and considerably more difficult to weld. This increased strength (commonly in the 100,000 to 150,000 psi [ 700 to 1,000 MPa ] range) is obtained at greater cost because of the extra treatment cycles involved.
There is a window in the property spectrum, between carbon steels up to 50,000 psi (300 MPA) and heat treated steels at 100,000 psi (700 MPa), for an intermediate set of properties which can be created by substituting small amounts of elements (usually in the 0.02 to 0.15 % range) such as vanadium (V), niobium (Nb) and/or titanium (Ti) in the chemistry, creating a ”microalloyed steel.” In these microalloyed steels, grain refining and precipitation strengthening are the primary mechanisms to increase yield strength while maintaining desired levels of ductility and weldability.
The History.
In all property demands in steel production, increased strength at the lowest cost is vital. Early high-strength, low-alloy steels (HSLA) steels were invented around 1929 as a means for reducing the weight of rail cars, especially coal cars. By increasing the strength of the steel, less steel could be used to carry the same loads. The strengthening mechanism involved dissolving copper and phosphorus in the iron base as two important alloys (solid solution hardening). Both have negatives but can be made to be adequately practical. The carbon (Cu) alloy is still an architectural material as in Corten steel because of its attractive brown patina. Corten steel has a strength limit of about 50,000 psi (350 MPa).
Two separate developments occurred in the early fifties. Norman Petch and Eric Hall independently discovered the effect of decreasing grain size in A36 on increasing yield strength. This has proven to be a powerful tool for providing "cheap" increased strength, and simultaneously making the material tougher, i.e. reducing the impact transition temperature where the mode of fracture changes from “ductile” at moderate temperatures to “brittle” at lower temperatures. The availability of fine grain steel is much greater today.
In the United States, steel researchers added vanadium (V) and/or niobium (Nb) to low carbon steel and were able to induce precipitation of carbonitrides to increase minimum yield strengths up to 50,000 psi (350 MPa). Because of lack of reproducibility, this avenue was not followed seriously until after 1970 when a new step in the steelmaking process called ladle metallurgy became practical. This resulted in new structural steels, such as ASTM A-572, which delivered the improved properties of yield strength, ductility, toughness and weldability.
Strengthening by cold deformation was always an option but the modest strength increase was always offset by a reduction in ductility.
Since then, there has been continuing refinement in the use of vanadium in HSLA steels. The recognition of the importance of nitrogen in the vanadium precipitation strengthening mechanism has enhanced the cost effectiveness of vanadium additives. This was highlighted in the 90’s with the advent of thin slab casting with electric furnaces. The inherent high-nitrogen steels from the electric furnaces were synergistic with the use of vanadium, which today has resulted in greatly expanding the use of HSLA-V Steels.
The Role of Vanadium.
Today, the common microalloying elements used in HSLA steels are vanadium, niobium and to a lesser extent, titanium. Vanadium is one of the most abundant, easily accessible and widely distributed metals in the earth's crust. Iron ores containing vanadium are largely available by open pit mining and can be economically smelted and converted into ferrovanadium. However, in the United States, the primary source of vanadium is through recovery from the pentoxide in spent catalyst from oil refining operations. These catalysts, along with other vanadium-bearing "waste" materials, are processed for recycling by several companies who, in turn, supply ferrovanadium alloys. The environmental benefits of recycled vanadium are worth noting. Each year six million pounds of vanadium are recycled from spent catalysts. This reduces the need to mine for vanadium minerals, which reduces the energy consumption and subsequent pollution from mining.
The use of recycled vanadium also reduces the energy requirements normally associated with processing ores, eliminating or reducing the need for land filling these "wastes," and ensures a domestic supply of vanadium for U.S. steel producers.
High-strength, low-alloy steel microalloyed with vanadium or HSLA-V Steel is intended to represent those steel grades where a small addition of vanadium (generally less than 0.15%) provides enhanced strength over standard low C-Mn steels, while meeting or exceeding all requirements for ductility, weldability and toughness. These steels are normally supplied in the as-rolled or as-forged condition, eliminating the need for subsequent heat treatments. Eliminating heat treating negates the need for higher alloy contents of Cr, Ni and Mo (hence "Low Alloy"), and provides significant energy savings. Compared with niobium or titanium, vanadium steels offer certain process advantages and cost reductions in continuous casting and hot rolling.
During continuous casting, vanadium steels have a reduced tendency for cracking when straightening the cast slab or billet compared to niobium steels. This characteristic of vanadium reduces scrap and/or rework costs.
The higher solubility of vanadium carbonitride particles means that lower reheat temperatures can be used prior to rolling or forging, saving energy costs. Since HSLA-V Steels are able to keep the vanadium carbonitrides in solution during rolling, and the vanadium in solution does not retard the recrystallization process, the grain structure can actually be refined during the rolling process. In this practice, refined austenite grains are achieved by repeated recrystallization after each roll pass, resulting in a continuous reduction of the austenite grain size. This practice of refining the austenite grain size is called "recrystallization controlled rolling." These even smaller austenite grains ultimately transform into small ferrite grains during cooling after the rolling process, resulting in optimum mechanical properties.
Overall, the role of vanadium as an alloy is preferred because of its ease of use compared to other alloy systems. This includes improved castability and greater solubility resulting in lower heating costs, ease of rolling because of lower rolling force requirements and the predictability of strengthening as a result of the proportional relationship between the vanadium additions and final strength.
HSLA-V Steel. Strategic Steel for Industry
HSLA-V Steel can be a strong candidate virtually anywhere strength and reduced weight are important, along with such important benefits as ductility, total elongation and weldability. These properties provide improved competitiveness in engineering applications and components in bridges, buildings, pipes and tubing and vehicles. In commercial design throughout the private sector, HSLA-V Steel is a valuable asset. These steels are also becoming increasingly important and could be crucial in selected military applications, where reducing the weight of such essentials as vehicles, weapon systems and bridges useful in rapid deployment is a top priority.
HSLA-V Steel is being used in a wide range of applications where factors including material reliability, environmental issues, fabrication costs and design challenges come into play.
HSLA-V Steel is available from virtually all domestic structural steel manufacturers in a variety of product forms, such as, plate, sheet, roll, angles, bar and beams.
|
|
 |
