Defects
Steel defects are imperfections that can affect a material’s surface, internal quality, dimensions, mechanical properties, or performance. They can develop at different stages, from casting to processing to storage. Understanding where defects originate and how they affect the material can help manufacturers identify problems earlier and make better decisions about material selection and processing.
Many defects begin during steel production, where chemistry, inclusions, casting conditions, and rolling practices can influence the finished material. Others may develop during downstream processing or appear as mechanical issues when the steel is processed or put into service.
Steel Making & Defects
Watch the previously recorded webinar, “Steel Making & Defects”, taught by our very own, Allison Lucak, PE. Learn how defects can develop throughout the steel making process and what they can mean for material performance.
Why do blast furnaces use torpedo cars instead of some other means of transporting the pig iron to the next step?
Torpedo cars are lined with refractory bricks to keep the pig iron at temperature while it’s being delivered to the steelmaking facilities. This allows the torpedo cars to act as a buffer in case the molten pig iron needs to hang out for a bit. An example of this: Cleveland Cliffs Riverdale has a caster but no steelmaking capabilities, and they rely on torpedo cars to bring steel from Indiana Harbor. 12 miles!
Design-wise, torpedo cars are actually quite large, and they are carried via a rail system at the steel mill. The ends are able to rotate, which allows the torpedo car to turn on its side to empty.
Torpedo cars were engineered to withstand the monumental task of transporting extremely heavy loads of molten metal across potentially significant distances.
What is the size of a heat of steel?
Are slivers more common in certain grades or all grades?
What impacts the most on slivers process variables related to casting or reheating in the hot rolling? Hot rolling load have any influence on slivers?
Slivers are a casting defect, so hot rolling only affects how they appear (linear and elongated). The hot rolling equivalent to slivers could be scabs, where something is getting pressed into the surface at the hot strip mill.
As briefly touched on in the presentation, casting is an extremely delicate and complicated process. I’m sure there are process variables that increase the propensity for slivers, but as I am not a casting expert,I can’t thoroughly answer your question.
I can say that mold powder getting pressed and entrapped in the surface of the slab is a common cause of slivers, and that sometimes they occur on slabs very early in a casting sequence.
How often does the water need to be replaced at a tandem mill or is it able to continue to be reused indefinitely?
Water management is dependent on the steel mill – some take, use, clean, and then return the water.
Other mills recycle their water by running it through cooling towers, cleaning baths, and then reusing it.
The only water lost during recycling is lost through steam.
Steel mills can use in excess of millions of gallons per hour!
What kind of steel is considered soft steel?
When using a coil that is galvanized at the mill, then slit at the processor, is there a way to prevent rusting?
Are you able to discuss laser quality steel and how the manufacturing process may be different?
Why do you use cardboard strips interlaced in the coil when slitting heavy gauge coils?
Is residual stress a rejectable condition?
Can the crossbow be created by the decoiling process? If it is created during the decoiling It is an elastic or plastic defect?
Typically, lines that decoil do not pull with enough tension to induce crossbow into the material.
In order to induce plastic deformation, the equipment needs to exceed the steel’s yield strength, so inducing shape is grade dependent. Any shape you do see is plastic deformation – elastic deformation is usually tough to detect with the naked eye.
If you’re uncoiling a coil with crossbow, it’s likely already in the coil.
Is there a standard amount of camber that is “acceptable” or is any amount of camber unacceptable?
What are the numbers in the denomination of the CE equation?
The CE equation gives an estimation for how likely the material is to form martensite. Not only is carbon the largest factor, but the iron-carbon relationship is well understood. Adding in a bunch of other elements greatly complicates things.
The idea behind the CE equation is to take all of the other elements and relate them to a carbon percentage to determine, as a whole, how easily the chemistry will harden. Chromium, molybdenum,and vanadium play a greater role in hardenability than nickel and copper, so their contribution is larger (a small denominator).
There are a few different version of carbon equivalent equations out there with varying degrees of specificity. The one above is good for general use.
How does Steel Warehouse monitor surface quality defects when processing a coil? Cameras?
The mechanical properties are the same in the width of the plate, but they might be different in the length? Is this correct?
The mechanical properties are determined by chemistry and cooling. The coil should have the same chemistry, but slight cooling differences are unavoidable. The very outer and inner laps cool faster,along with the outside edges of the coil.
Because of this, we crop the OD and ID and we do not test for mechanical properties within a few inches of the mill coil edge. Apart from the edges, the width of a plate should have less variation than can be possible down the length of the coil.
Approximately what percentage of the defects mentioned in the presentation are only problematic cosmetically? Meaning, are a significant number of these defects problematic in that the steel wouldn’t hold up in what it’s needed to do (i.e., the bridge could collapse, or the lawn mower would rip apart)?
Most part design calculates the stress the part will see in service. Stress is force/area.
The majority of the defects in the presentation are surface defects. If a surface defect is severe enough, it could affect the area portion of the force/area equation.
Say you found a really gross sliver that was determined to be 0.020” deep on 0.25” material. The thickness is effectively decreasing to 0.23”, raising the amount of stress the material undergoes.
Given factors of safety, is extremely unlikely that surface defects could compromise the integrity of the part. Huge, substantial defects might, but it’s rare these defects even make it out of the steel mill.
That leaves inclusions and rust.
Inclusions can be concerning because you can’t see them. 100 years ago inclusions were unpredictable and probably caused a few, if not a lot, of catastrophic failures. Today, steelmakers use methods to control inclusions, and the “clean” steel products are both predictable and higher performing.
Rust seems like a benign defect because everyone knows what it is. It’s familiar! The problem with rust
is that it gets worse over time, and will eventually affect the structural integrity of the steel. There are
grades of steel specifically produced to corrode at a slower rate – they’re called weathering steels – but without surface protection, rusting is unavoidable. The one positive aspect of corrosion is that it’s
predictable: parts in service that rust have a lifespan during which they can continue to rust safely (for
example, bridges).