Special Bar Quality Steels
Special Bar Quality Steels
Special bar quality (SBQ) steel is the steel which is used for a class of long products engineered for very tough application. The higher the load of application, the more special is the SBQ steel. The term ‘special bar quality’ is a term predominantly being used in North American steel industry. In Europe, SBQ steels are normally known as ‘engineering steels’. Both of these terms refer to steel types as well as to bar products, and as they are frequently with associated with the steel grades and they refer to billet and slab as well as bar. The SBQ steel term is used to describe steel long products having properties for more demanding processing or end-use applications which cannot be met by merchant bar quality (MBQ) steel grades.
SBQ steels represent a wide variety of higher quality plain carbon and alloy steel bars, which are developed for challenging and high stress applications. These steels stand out different and tall among a diversity of grades which are in use in various demanding processes. SBQ steels, which are being offered today by the manufacturers, are in the form of stainless steel bar, steel round bar, tubes and other products of high demand and of relevance in the manufacturing sector. These steels are mostly used for application in hot and cold forging, machining, and cold drawing industries for the production of automotive parts, bearings, gears, hand tools, electric motor shafts, valves, and other vital products which are to be resistant to the metal fatigue through prolonged and continuous use as moving parts in machineries / automobiles.
The specification of SBQ steels can range from plain carbon and carbon-manganese steels to alloy and stainless steels containing different percentages of aluminum, nickel, chromium and other alloying elements. The chemistry of these steels is more complex than MBQ steels. These steels can have several grades. SBQ steels can have products of shapes which can be classified into round, flat, square, and hexagonal bars. The products can be in the form of hot rolled steel bars, or cold finished steel bars.
SBQ steels are at the higher end of the quality spectrum in both metallurgical consistency and dimensional accuracy. Some end uses of these steels can need superior surface quality, special chemical restrictions, metallurgical characteristics, heat treatment, or surface finishes. The internal quality of these steels is very important since machining and bending activities are involved during the processing of these steels in subsequent stages.
Heterogeneity in bulk steel is inherent in the manufacturing processes of the steel and SBQ steel has to have minimal heterogeneity which is acceptable to the steel user. The heterogeneities in steel include non-metallic inclusions, macro-segregation, micro segregation, micro-voids, or discontinuities which are normally generated during the casting of the liquid steel and which are not healed during the subsequent hot deformation processes. There is a quantified and assured acceptance level of the heterogeneity in the steel in terms of the quality of the steel bars in case of SBQ steels. In terms of application, SBQ steel is to perform in a highly loaded condition which is possible because of lower level of defect initiation sites. Several safety critical high warranty prone components, where strength, fatigue life, and durability requirements are there, fall in this category. The components which are subjected to rotation, twisting, and bending type of load application require SBQ steels.
SBQ steel has emerged as the necessary requirement in various applications like those defined by high speed machining, high stress, and high tensile strength among other finer attributes of engineering and metallurgical application. SBQ steel bars have enhanced tolerance for higher stress application. SBQ steels normally contain higher quantity of alloying elements than MBQ steel grades of steel bars, and are produced with more precise dimension and chemistry.
For comparison, MBQ steels are manufactured to specified sizes with appropriate chemical limits for meeting a set of properties, where the end use is non-critical. MBQ steel bars are being produced as rounds, squares, angles, channels and beams etc. The bars go through the standard production process without any treatment given to the billets and with the chemical analysis ranges staying within the standard limits for carbon, manganese, phosphorus and sulphur. The bars have liberal tolerance controls. The surface and core defects are wide and not well quantified. The use of MBQ bars can involve mild bending, hot forming, punching and welding and in applications where additional machining requirements are negligible. The quality norms in terms of internal porosity, surface seams are liberal. MBQ steel bars are rarely used in applications where heat treatment of steel is needed. Fig 1 shows the comparison of MBQ steels and SBQ steels.
Fig 1 Comparison of merchant bar quality and special bar quality steels
SBQ steels are produced with more precise dimensions and chemistry. The products are to be produced to satisfy the customer’s specific requirements, including those related to chemical composition, malleability, hardness, ductility, and steel surface condition. These steels are normally machined, forged, or cold drawn during subsequent processing. Hence these steels are required to have superior mechanical properties and defect free surface.
SBQ steels are at the higher end of the quality scale both in terms of dimensional accuracy and metallurgical consistency. Some of the end users of SBQ steels can need superior surface quality, or special chemical restrictions, metallurgical characteristics, heat treatment or surface finishes. The internal quality of these steels is very important since machining and bending activities are involved during the processing of these steels in subsequent stages. In order to achieve the desired metallurgical quality, the steel has to be particularly clean. This makes the production route of these steels, a bit complex. Steel with high cleanliness is, achieved through the integrated steel making route equipped with a secondary steel treatment process. In case of steelmaking through electric arc furnace, the inputs are to be cleaner iron bearing inputs and the steel is to be treated in a ladle refining furnace before casting. Internal metallurgical quality and consistency is, achieved by a meaningful reduction during the rolling process. These steels are, produced either in fixed cut lengths or in coil form.
SBQ steels are produced to meet its tough application requirements. Stress tolerance of the steel is enhanced which provide higher level of consistency and integrity. For achieving the desired level of performance, chemical composition and cleanliness of the steel are critical requirements for getting the desired mechanical properties. While the chemistry can be obtained as per the alloy design principles, the achievement of cleanliness levels is critical meeting the service requirements of the SBQ steels. Steel cleanliness has a direct impact on the dynamic properties such as fatigue life of a component. The SBQ steel bars are being produced using processes which enhance quality parameters needed for the end use and as per the customer specifications. The quality requirements of the SBQ bars include lowest levels of chemical segregation, inclusion rating, and internal and surface defects.
The SBQ alloy steels meet critical levels of hardness and / or hardenability, strength, ductility, toughness, fatigue resistance, fracture resistance, wear resistance, machinability, and formability. Few grades are applied for high temperature applications which demand high temperature strength, thermal fatigue, creep, and oxidation etc.
Speaking from the perspective of a metallurgist, SBQ steels show very high stress resilience and associated metal fatigue tolerance. These attributes define precisely the SBQ steels and make them apt for versatile applications where authentic efficiency parameters are strictly demanded. However, to ensure such high characteristics, the manufacturing of SBQ steels is needed to be guarded throughout the entire production so that there are no discrepancies. The most fundamental parameters which are controlled include the chemical composition and purity quotient of the steel and the mechanical properties like hardening which is achieved through thermal treatment processes like, annealing, normalizing, and quenching and tempering etc. Impurities in the steel can drastically bring down the fatigue life cycle duration of the component while cleaner steels have very high metal fatigue tolerance even under continuous run conditions. The purity is also directly related to the thermally induced hardness (stress) resilience attributes, since it can be affected if impurities are present in the steel body. Products made from SBQ steels thus guarantee the best results when converted into components of practical use.
SBQ steels are required when certain input steel quality is, demanded for a metallurgical process such as hot forging, cold forging, machining, and heat treatment etc. Use of MBQ steel bars can lead to failure in achieving the target properties in application. Use of MBQ steel bars can also cause excessive defect during the processing which can lead to component failure. The process of manufacturing SBQ steel removes excessive pipe formation, chemical in-homogeneity, surface defects, and internal core defects. The SBQ steel bars are inspected and conditioned at every stage of the production process. The frequency and the degree of surface defects is influenced by chemical composition, type of steel, and bar size. Resulphurized grades, certain low carbon killed steels, and boron treated steels are most susceptible to the surface defects.
Some end uses or production procedures during the subsequent processing can necessitate one or more of extra requirements. These requirements include special hardenability, internal soundness, non-metallic inclusion rating, and surface condition, microstructure and mechanical properties. The quality specification for steel bars to which only one of these special requirements is applied is the ‘restrictive requirements’ of the quality. The restrictive requirements can be a single special restriction or more. Multiple restrictive requirement SBQ bars are those to which two or more restrictive requirements are required to be applied.
ASTM standard A 576 -90 b covers specification for plain carbon SBQ steels for applications which includes forging, heat treating, cold drawing, machining and many structural uses. This specification has got 53 grades under the category of non-resulphurized steels, 15 grades under the category of resulphurized steels, and 5 grades under rephosphorized and resulphurized steel category. The range of carbon content in these steels varies from grade to grade with minimum of 0.08 % and a maximum of 0.76 % while the manganese content in these steels ranges from a minimum of 0.30 % to a maximum of 1.65 %. Sulphur content in the 15 grades of the resulphurized steels also have different ranges of sulphur with a minimum of 0.08 % to a maximum of 0.33 %. Similar variations are there in the case of resulphurized and rephosphorized steels.
Uses of SBQ steels
SBQ steels are used in safety critical applications. These steels are also used for a wide range of industrial applications. The petroleum industry uses them for the valves and pipeline couplings for oil extraction. In the hand tools sector, these steels are used for hammers, screw-drivers and wrenches, among others. SBQ steels are also used for wind turbine components, bearings and seamless tubes.
SBQ steel bars are used in transportation sector such as automobiles, railways, marine etc., energy sector such as thermal power, hydro power, nuclear power, and wind power etc., strategic defence, nuclear and aerospace sectors, oil and gas industry, tool and die industry, mining industry, and industrial machinery etc.
Most SBQ steel producers seek ties with the automobile manufacturers as the automobile industry is the key driver of the SBQ steel grades. The main application areas for SBQ steels in the automotive industry are engine, transmission, steering, suspension and hydraulic components etc. (Fig 2). SBQ steels are also used for bolts, springs, cable wires, chains, tyre beads and many similar applications in the automobile industry.
Fig 1 Use of SBQ steels in automobiles