Special Bar Quality Steel: What Does it Really Mean?
What makes special bar quality (SBQ) steel different, how is it manufactured, and what is its relationship to “steel cleanliness?”
In this article, I take a look at the fundamentals of SBQ steel and provide a short tutorial on clean steel production. It is the first in a series designed to familiarize steel buyers and end users with the specific attributes of this important material.
The information presented below is provided as a general instructional guide only and is not intended to be comprehensive or definitive. Please reach out to Jade Sterling Steel if you have questions about your steel project and its requirements.
SBQ Steel Meaning: What is special bar quality steel?
In short, SBQ steel is specially produced to more tightly controlled chemistries, as well as more exacting specifications for cleanliness, surface quality, internal soundness, microstructural homogeneity, and straightness.
However, it is important to note that the terms “special bar quality” and “SBQ” can be somewhat nebulous; they are both sometimes used interchangeably to represent a wide variety of higher-quality steel characteristics in carbon and alloy steel bars.
SBQ Steel Applications
Often used in forging, cold drawing, and machining, SBQ steel is commonly found in applications including but not limited to:
Gears and Shafts
Bearings
Hand Tools
Valves
Automotive / Off-Road Vehicle Parts
SBQ Steel Manufacturing and Steel Cleanliness
Generally, each steel mill will have specialized, “clean steel” melt practices for the production of SBQ material and employ special rolling/finishing practices for second-stage production. “Special Bar Quality” and/or SBQ designations will be cited on the material certification verifying that such practices were used in producing the steel.
What is clean steel?
Steel cleanliness can be conceptualized as a quantified metric for describing the size, distribution, shape, and quantity of processing contaminants from the steelmaking process. Most often referred to as “inclusions”, these contaminants can be of two types:
Indigenous: Inclusions produced during the melting process by reaction (oxidation, sulfidization). These inclusions are melt practice controlled.
Exogenous: Inclusions resulting from outside (typically oxidation) contamination during casting and rolling, and/or from exposure to atmospheric oxygen or mechanical entrainment during pouring and rolling.
SBQ-level steel cleanliness during steelmaking is controlled by employing specialized clean steel melt practices to neutralize free oxygen by “killing” and refining the liquid steel through the addition of deoxidizing agents such as Si, Al, and V. Additionally, the steel is refined using special “clean steel” slags to remove oxidation impurities. This is the purpose of the often specified “Ladle Refining” seen in many specification callouts.
Refining for Steel Cleanliness and Ladle Refining
In simple terms, refining can be thought of as a two-step process.
Step 1 involves deoxidizing the steel through additions of Si, Al, and or V. The schematic in Figure 1 provides a simple illustration of this mechanism. The objective is to remove the free oxygen from the molten steel. If it were to remain, the steel would be highly susceptible to both deleterious oxidation in later processes (casting), as well as porosity from the free oxygen. The process of removing the free oxygen is what is termed “killing,” and steel deoxidized in this manner is called “killed steel.”
Step 2 in refining is the removal of the oxidation product resulting from the killing process shown in Figure 1. This is what is referred to as “ladle refining.” The objective is to use a refining slag and inert gas stirring (typically argon) to “clean out” the resulting oxides through slag refining.
Figure 1: Simple description of what happens during steel deoxidation for Si / Al “killing”
From Figure 1, you can see the primary initial slag constituents being FeO and MnO. These are “bad” oxides, as they readily reduce and reform, continually feeding free oxygen back into the steel.
Figure Two: Simple schematic description of what happens during steel refining.
In Figure 2, this slag has been replaced with a refining (clean steel) slag containing high quantities of Ca, fluorspar, and Ca-Al. This chemically stabilizes the slag and allows it to more readily absorb the oxides produced from deoxidation, hence the often cited “ladle refined” quality designation.
The primary agents in the liquid steel to be refined are both oxides and sulfides. For simplicity, I
have not illustrated the sulfide reactions in Figure 2. Sulfur is reactive with oxygen, manganese, and even calcium. Its complex thermodynamics are beyond the scope of this discussion.
Characterizing Clean Steel: Clean Steel Standards and Ratings
Clean steel for SBQ can be characterized in a number of ways, using both qualitative and
quantitative testing metrics. These include (but may not be limited to):
ASTM / SAE Micro-inclusion ratings, used for micro inclusions (microscopic)
AMS 2301/2304 “Magnaflux” magnetic particle rating, used for macro inclusions (naked eye)
NDT Methods (Ultrasonic, Eddy current, dye penetrant), broad examination of both surface and internal cleanliness – usually at a macro scale.
In the following section, we will review the ASTM/SAE micro-inclusion characterization, presenting what they are and how they are significant.
Example Clean Steel Evaluation
The typical micro-inclusion / cleanliness characterization consists of an examination of representative fields from a sample of a given steel heat, using a light microscope at with magnification of 100X. A popular method is specified under ASTM test E45. The method characterizes four different inclusion types:
Sulfides
Alumina (deoxidation product)
Silicates
Large Oxides (reoxidation contaminant)
The evaluation separates each inclusion types (designated by their respective letter) into “Thin” and “Heavy” categories, and reports the cleanliness of each type as an index number ranging between 0 and 4 (with 4 being the most severe). Therefore, a typical cleanliness report will contain a total of 8 numbers arranged as shown in the following example table (Table 1).
The number ratings are provided by cross reference with a standard provided in the ASME E45 specification, and more recently can be determined using computerized image analysis. Different quality levels and specifications will have different call-out maxima for these rating categories (for example ASTM A534).
For context, Figure 3 provides a concise visual representation of the four inclusion types, their source, and the relative impact of general steel quality. For example, some sulfide types are beneficial for steel machinability; Alumina (B) inclusions are detrimental to bearing rolling contact fatigue resistance.
Figure 3: Reference images for the 4 ASTM E45 inclusion types, possible sources and significance.
As a guide, Table 2 provides a general overview of what hypothetical rating ranges can mean for a steel’s quality attributes. Design specifications for steel parts are based on how such inclusions may affect both the processing and performance of said parts.
In general, the oxide and silicate inclusions represent harmful constituents, while some sulfides can be of benefit in certain applications (i.e. machinability). Jade Sterling Steel sales professionals, in consultation with the Metallurgical group, can assist with specific application and characterization questions.
Quantitative Inclusion Analysis: Forensics
In special circumstances, it can be important to evaluate specific inclusion occurrences for both process development/improvement, as well as failure analysis. In these cases, quantitative metallurgical techniques using a focused X-ray beam can be used in a scanning electron microscope to provide a breakdown of the inclusion constituents. Knowing what steel-making elements and/or additive constituents are present can aid in identifying more precisely the inclusion source.
For example, Na and K can be indicative of mold flux entrainment during casting. The image in Figure 4 shows an interesting group of different inclusion types, with their associated chemical compositions. More generally, quantitative analysis techniques can provide important troubleshooting and forensic information as to possible sources for “dirty” steel.
Figure 4: Examples of various inclusion types, showing appearance and chemical composition. (Ref: Y Ren et. al., Met Trans B, March 2014)
Learn More About Sourcing the Right SBQ Steel for Your Needs
In summary, micro-inclusion ratings are one of the metrics used to classify “Special Bar Quality” Steel. This tutorial was presented to familiarize Jade-Sterling Steel customers as to how steel cleanliness is obtained, and the meaning behind the inclusion rating metrics.
Future blog posts will discuss macro inclusions, soundness, and other SBQ parameters. The presentation here is solely an information tutorial, and not intended for design or specification purposes. Application engineering specialists should use this information as appropriate for their individual purposes.
Please reach out to our team if you have any questions or topics you would like to see covered in the future.
About the Author
Andrew Freborg
Andrew Freborg is the Metallurgy Director with Jade-Sterling. He is a licensed Professional Engineer in the State of Ohio, with 35 years of experience in process and product ferrous metallurgy. He has a BS in Metallurgical Engineering from the Illinois Institute of Technology and an MS in Mechanical Engineering from the University of Akron. His professional experience includes 6 years in automotive/aerospace steelmaking with Republic Engineered Steel (1989-1995), 21 years in steel processing research and development with DANTE Solutions (1996-2017), and 6 years in steel product metallurgy quality management. As a Senior Process Engineer within Republic’s primary steelmaking division, his career focused on the development of steel refining and casting practices to control cleanliness, as-cast bloom quality, and developed steel properties in subsequent processing. At DANTE Solutions, Mr. Freborg worked collaboratively in applying computational process modeling methods to address various processing challenges within the ferrous metals industry, including continuous cast mold design, thermal stress characterization, and ferrous alloy heat treatment. His principal focus was researching of quench hardening processes to improve the bending fatigue strength of helicopter transmission gears US Army AATD, and in support engineering with the Lockheed-Martin / Sikorsky’s FARDS program. Currently, Mr. Freborg oversees product metallurgy for Jade-Sterling.
