Pressure marking (or pressure mottling) is an uneven gloss pattern on the surface of a prepainted coil.  It is generally caused by dissimilarity in the gloss and in the roughness between the top and bottom surfaces of a coil.  These gloss differences are transferred with time and pressure to either surface.  Pressure marking is not a degradation of the paint surface except in extreme conditions.  The irregular pattern is typically temporary and will dissipate with time and exposure to heat or ambient temperature.

In the mechanism by which pressure marking occurs, the gloss components, which are under normal suspension within the topcoat and are random in directionality, are suppressed or flattened within the topcoat under the pressure of coiling. 

Coil coaters use various best practices related to the backer specification, rewind tension and coiling temperature in order to minimize pressure marking.

Storage stain on galvanized sheet steel is a corrosion product that is typically white, but which can also take the form of grey or black deposits on the surface. Since the most common form of discolouration is white in appearance, storage stain is often called “white rust.” It can occur when sheets of galvanized steel that are in contact with each other (in a coil or stacked in lifts/bundles) get wet, either by direct water impingement, or condensation between the surfaces, causing the zinc to react with moisture in the absence of free air circulation.

Passivation coatings have been in use for many years and are very effective in minimizing the tendency for corrosion when the sheets get wet in coil or bundle form. However, they do not eliminate its occurrence if the product is subject to very adverse conditions.  Steel sheet producers use the term “passivation treatment” or “chemical treatment” interchangeably for this surface application.

White rust is characterized by a mirror image appearance in between laps/sheets at the point at which the material became wet.

Steel is an alloy of iron and other elements.  Some elements are intentionally added to iron for the purpose of attaining certain specific properties and characteristics.  Other elements are present incidentally and cannot be easily removed.  Such elements are referred to as “trace” or “residual” elements. 

Many product specifications have mandatory requirements for reporting certain elements and these vary.  Most mills routinely provide heat analysis which includes the elements below.  Although it is possible to analyze for other elements this is most often not practical or necessary unless they are additions (e.g. Pb – Lead, Sb – Antimony or Co - Cobalt).

C – Carbon

Mn – Manganese

P – Phosphorous

S – Sulphur

Si – Silicon

Cu – Copper

Ni – Nickel

Cr – Chromium

Mo – Molybdenum

V – Vanadium

Cb (Nb) – Columbium (Niobium)

Ti – Titanium

Al – Aluminum

N – Nitrogen

B - Boron

Sn - Tin

Ca – Calcium

There are thousands of steel alloys and their categorization is complex and varies by governing body.  Most, however, can be broadly grouped into Plain Carbon Steel, Ultra Low Carbon (ULC) Steel, High Strength Low Alloy (HSLA) Steel, Alloy Steel, High Alloy Steel (including Stainless Steel and Tool Steel) and Electrical Steel.  Advanced High Strength Steel (AHSS) is the newest classification of steels. 

Alloying elements often serve different purposes in different steels.  For example, Manganese contributes to steel’s strength and hardness in the as rolled condition but another important characteristic is its ability to increase hardenability which is critical in heat treating. 

The effect of alloying elements on steel properties is a huge subject.  The following is a very cursory summary of the influence of the above elements in common flat rolled products.  More information may be found on the websites of governing bodies and materials information societies such as ASM International.

Carbon is the principal hardening element in steel.  Hardness and strength increase proportionally as Carbon content is increased up to about 0.85%.  Carbon has a negative effect on ductility, weldability and toughness.  Carbon range in ULC Steel is usually 0.002 – 0.007%.  The minimum level of Carbon in Plain Carbon Steel and HSLA is 0.02%.  Plain Carbon Steel grades go up to 0.95%, HSLA Steels to 0.13%.

Manganese is present in all commercial steels as an addition and contributes significantly to steel’s strength and hardness in much the same manner but to a lesser degree than carbon. Manganese improves cold temperature impact toughness.  Increasing the Manganese content decreases ductility and weldability.  The typical Manganese content is 0.20 – 2.00%.

Phosphorus is most often a residual but it can be an addition.  As an addition it increases hardness and tensile strength. It is detrimental to ductility, weldability and toughness.  Phosphorus is also used in re-phosphorized high strength steel for automotive body panels. Typical amounts as a residual are less than 0.020%.

Sulphur is present in raw materials used in iron making.  The steelmaking process is designed to remove it as it is almost always a detrimental impurity. A typical amount in commercial steel is 0.012%, and 0.005% in formable HSLA.

Silicon can be an addition or a residual.  As an addition it has the effect of increasing strength but to a lesser extent than Manganese. A typical minimum addition is 0.10%.  For post galvanizing applications the desired residual maximum is 0.04%.

Copper, Nickel, Chromium (Chrome), Molybdenum (Moly) and Tin are the most commonly found residuals in steel. The amount in which they are present is controlled by scrap management in the steelmaking process. Typically the specified maximum residual quantities are 0.20%, 0.20%, 0.15% and 0.06% respectively for Copper Nickel, Chromium and Molybdenum but the acceptable limits depend mainly on product requirements.  Copper, Nickel, Chromium and Molybdenum, when they are additions, have very specific enhancing effects on steel.  A Tin residual maximum is not usually specified but its content in steel is normally kept to 0.03% or less due to its detrimental characteristics.

Vanadium, Columbium and Titanium are strengthening elements that are added to steel singly or in combination.  In very small quantities they can have a very significant effect hence they are termed micro-alloys.  Typical amounts are 0.01 to 0.10%.  In Ultra Low Carbon Steel Titanium and Columbium are added as “stabilizing” agents (meaning that they combine with the Carbon and Nitrogen remaining in the liquid steel after vacuum degassing).  The end result is superior formability and surface quality.

Aluminum is used primarily as a deoxidizing agent in steelmaking, combining with oxygen in the steel to form aluminum oxides which can float out in the slag.  Typically 0.01% is considered the minimum required for “Aluminum killed steel”. Aluminum acts as a grain refiner during hot rolling by combining with Nitrogen to produce aluminum-nitride precipitates.  In downstream processing aluminum-nitride precipitates can be controlled to affect coil properties.

Nitrogen can enter steel as an impurity or as an intentional addition.  Typically the residual levels are below 0.0100 (100 ppm).

Boron is most commonly added to steel to increase its hardenability but in low carbon steels it can be added to tie up Nitrogen and help reduce the Yield Point Elongation thus minimizing coil breaks. At the same time, when processed appropriately, the product will have excellent formability.  For this purpose it is added in amounts up to approximately 0.009%.  As a residual in steel it is usually less than 0.0005%.

Calcium is added to steel for sulphide shape control in order to enhance formability (it combines with Sulphur to form round inclusions). It is commonly used in HSLA steels especially at the higher strength levels.  A typical addition is 0.003%.

“Skin Lamination” associated with entrapped mold powder in sheet steel

“Skin lamination” is a term often used to describe surface imperfections of various types including that associated with entrapped mold powder.

Mold powder is an inherent part of the casting process; a consumable used to lubricate the mold and to protect the molten steel from the atmosphere.  Mold powder that is entrapped in the solidifying steel shell can be rolled out into a thin layer close beneath, or just at, the surface of the hot rolled sheet.  It sometimes becomes obvious only during forming.

Steelmakers endeavor, through careful control of the casting process, to keep the mold conditions as “quiet” as possible to minimize mold powder entrapment.  This includes maintaining precise control of the liquid steel level in the mold, and controlling steel flow in the mold. The process is monitored using an “automatic mold level control system” to eliminate the subjectivity associated with humans.  Detection of events by mold level measurement enables a decision-making process on acceptance for an order, which may lead to slab conditioning. 

• Coil Breaks are creases which appear as lines transverse to the rolling direction and generally extend across the width of the steel.
• If a coil is sheeted and used in flat form the condition is considered unsightly.
• Coil Breaks are purely cosmetic.They are non-injurious and will not affect formability.Coil Breaks will not affect the integrity of formed parts.The presence of Coil Breaks is a clear indication of very formable and ductile material.

What causes Coil Breaks?

• Coil Breaks are caused by the phenomenon of “yield point elongation” or “elongation at the yield point” which is inherent in low carbon steel.
• When a tension test specimen is subjected to load there is an initial range of loading in which no permanent deformation occurs, i.e. if the load is removed at any value within this range the specimen will return completely to its original dimensions (elastic range).
• As the tensile load on the specimen is increased through the elastic range a stress will be reached at which the specimen will begin to deform in a plastic manner, i.e. it will undergo a permanent set which is not recoverable upon release of the load.
• Some materials, with increasing stress, show a gradual departure from elastic behaviour.Many steels, however, exhibit an abrupt yielding and show an increase in strain without any appreciable increase of stress when yielding occurs.Such materials are said to have a yield point.The amount of extension which occurs between the initial yield point and the point at which the load begins to rise steadily again is called the “yield point elongation”.
• When hot rolled coils are uncoiled for further processing, such as pickling, slitting, cutting to length or temper rolling there are strains associated with the uncoiling.Localized yielding occurs at the point of unbending when the strain exceeds the yield point of the material.This is what Coil Breaks are.The strain is a function of the ratio of the thickness of the material to the diameter of the coil and it is greatest at the wraps closest to the inside diameter and so Coil Breaks are most prevalent there.
• Since the mechanism of formation of Coil Breaks is dependent on the yield point it is clear that materials with higher yield strength (e.g. HSLA) are far less susceptible to Coil Breaks.
• Other factors that affect coil breaks are:
  • Tension
  • Shape
  • Length of time allowed for cooling after hot rolling (longer is better)
• Coil Breaks will further occur at other points of bending, such as roller leveling.Roller leveler breaks are said to be controlled Coil Breaks in that they are caused by bending the sheet over regularly spaced rolls.

   

  Eliminating Coil Breaks

• Modification to chemistry with Boron to change the characteristics of yielding behaviour can minimize Coil Breaks.

• Skin rolling or temper rolling can be used to alleviate coil breaks in two ways.A small cold reduction is applied to create numerous nucleating points for Coil Breaks that are so small so as not to be obvious.In addition, pre-existing Coil Breaks are flattened and masked.
• In order to guarantee freedom from Coil Breaks on hot rolled steel the product must be skin rolled or temper rolled.
• Coil Breaks are not usually seen on fully processed cold rolled sheet because the processing includes temper rolling after annealing.Similarly coated products are either tension leveled or temper rolled in line.