3.4 Analysis of substance function and technical requirements on surface structure of
3.4.2 Application of Pretex® chrome coated working rolls in production of cold-
3.4.2.2 Key functionalities of the Pretex® functional chrome coating
current Pretex® process that are crucial for the different applications and need to be considered when reviewing alternatives. xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxx
The subsequent chapters are sub-divided into key process functionalities (see chapter 3.4.2.2.1) and key surface functionalities which in turn are differentiated between criteria related to the Pretex® surface topography (see chapter 3.4.2.2.2) and Pretex® coating durability and longevity (see chapter 3.4.2.2.3).
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52 3.4.2.2.1 Description of key process functionalities
Table 8: Key process functionalities of functional chrome plating using chromium trioxide
Key functionality Explanation Quantitative statement for alternative assessment Treatment time
(deposition rate)
Process requires short treatment time (high deposition rate) to ensure short maintenance cycles.
xxxxxxx min (xxxxxxxxx µm/min)
Suitability for heat-sensitive substrates
Coating application must be able at low process temperatures to ensure no material degradation.
Process temperature ≤ 400 °C
Accurateness to finished size (without
post-treatment)
Coating process must be highly adjustable to apply an application specific surface topography (number and diameter of spherical caps, etc.) and layer thickness
Process must be capable to deliver reproducible surface properties with
large components Process must be capable to plate working rolls with up to X m of working surface (part to be plated) and approx. X m overall length with diameters up to xxxx mm.
Barrel length: approx. xxxxx mm;
Barrel diameter: xxxxxxx mm Weight: xxxx t
Capability for component
refurbishment Process must be capable to refurbishment quantitative numbers of working rolls per year.
Refurbishment cycles: ≥ xxxxx rolls/year
Treatment time (deposition rate)
The overall treatment time and therefore the deposition rate of the functional chrome plating process is an important factor for Salzgitter. In general, turnover rates of working rolls used in cold-rolling processes are very high, typically in the range of thousands per year. Therefore, the refurbishment/maintenance cycle described in chapter 3.4.2.1 needs to be performed accordingly fast to maintain continuous production with as little as possible downtime.
Chromium trioxide based functional chrome plating in Salzgitter’s reactor plating systems enables high throughput, even in the case of large working rolls. The deposition rate of the metallic chrome coating is between xxxxxx µm per minute leading to a treatment time of xxxxxxx min per part.
Suitability for heat-sensitive substrates
Working rolls are made from heat sensitive base materials such as (chromium forged steel) and therefore must not be exposed to temperatures above 400 °C. Otherwise, the part of the roll (mostly outer surface area) exceeding 400 °C suffers internal stress leading to rapid material degradation.
Functional chrome plating using chromium trioxide is performed at xxx °C and therefore perfectly suitable for treating heat-sensitive materials.
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53 Accurateness to finished size (without post-treatment)
Functional chrome plating, in general, allows for either smooth or textured surfaces. In case of texturing, the substrate may be pre-treated, for example with shot blasting, before coating or a chrome surface may be etched after the coating process. Due to the pre- and post-processing procedures required in standard chrome plating, it is difficult to adjust surface modifications to the specific layer thickness needed.
The Pretex® functional chrome plating procedure is used for depositing a chrome layer and tailored texturing the surface of the substrate at the same time in a one-step process in a closed reactor. The chromium surface is exactly adjustable by topography (number and diameter of spherical caps etc.) and layer thickness. Both can be adjusted by the electrical field, which requires highly sophisticated and accurate parameters. Due to the accurately plated coating, neither a pre-treatment for texturing nor any post-treatment, like grinding, is needed, thus reducing energy and material consumption. All plating procedures yield accurate to finished size (layer thickness) functional chrome plating. By not requiring post-treatment the process reduces waste.
Exact reproduction of structure parameters (diameter, roughness)
Functional chrome plating using chromium trioxide is a highly reproducible process, i.e.
the applied coating on working rolls has only very little variances in surface structure parameters.
An alternative coating process must be capable to achieve high reproducibility with allowable tolerances of ± XxXX %.
Capability of plating large components
Chrome coated working rolls used for Salzgitter’s cold-rolling processes are large in size with up to X m in barrel length and a barrel diameter between xxxxxxxxxx mm. Depending on the size, the typical mass ranges between xXX t for a single working roll.
The functional chrome plating process performed by Salzgitter (reactor process) is capable of treating components in these dimensions. Thereby, high degrees of homogeneity of the chrome coating are achieved. The homogeneity of the chrome coating is a pre-requisite for further meeting process and product quality requirements.
Capability of component refurbishment (machinability)
During the overall lifetime of a working roll the chrome coating applied to the surface needs to be refurbished many times (approx. XXX refurbishment cycles until scrapping).
During the maintenance cycle approximately XXX mm of the outer surface material of the working roll is grinded down (up to XX µm are metallic chrome coating). With a total thickness of XX mm the outer, hardened part of the working roll can be re-used at least XXX times before it must be entirely scrapped and replaced. Therefore, the capability of component refurbishment is an important process criterion for alternative assessment.
Further, the machinability of the coating applied on working rolls is crucial since it affects the performance of maintenance/refurbishment cycle, i.e. the coating’s grindability and its re-application. The grinding process (pre-treatment) is crucial since after the re-application of the coating a specific surface roughness in combination with a certain peak count surface structures and waviness needs to be achieved. Potential alternatives need to be capable of suitable machining such as grinding. In summary, good machinability allows for lower treatment times, better handling of the working rolls in the roll grinding shop and easy to establish surface structures.
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54 3.4.2.2.2 Description of key functionalities related to Pretex® surface topography Table 9: Key functionalities related to Pretex® surface topography
Key functionality Explanation Quantitative statement for alternative assessment
‘Open’ and ‘closed’
hemispheric structures
Surface structure elements are of hemispheric shape and their adjustable distribution on the substrate surface is responsible for Pretex® key
functionalities. Thereby, ‘open’ and
‘closed’ structures refer to the count of hemispheric structure elements per area.
Average roughness value: elements retain liquids allowing for even formation of fluids.
Working surface needs to provide with optimal fluid guiding and wetting properties during cold-rolling to ensure high surface quality.
Embossing properties in cold-rolling process
Hemispherical surface elements must be transferred to sheet metal in cold-rolling process to yield calotte-like negative structures → textured sheet metal surface.
Adjustable impression of working roll surface texture up to 100 %.
Adjustable gliding and friction properties
Adjustment of Pretex® surface structure elements either yields good gliding properties due to reduced contact area or increased grip for e.g. feed rolls in metal rolling processes, depending on specifications set.
Plant setup is matched to friction coefficient between chrome coating and steel. Hydrophobic surface and interaction between roll surface and rolling medium is of high importance.
‘Open’ and ‘closed’ hemispheric structures
The Pretex® functional chrome plating procedure allows the generation of coatings with the same surface properties provided by ‘conventional’ hard chrome plating. In addition, a surface topography with unique properties is formed. The topography consists of hemispheric structures growing during the electroplating process (see Figure 26, right).
While the positive topography is called hemispherical, negative embossments, on e.g.
sheet metal, are so-called calotte-like topographies (Figure 26, right), established within a single process step without interruption.
After applying Pretex® coatings, no abrasive post-treatment procedures are necessary.
Height, diameter distribution and closeness of the hemispheric surface structures are adjustable yielding specific surface structure properties on working rolls required for different sheet metal products. The resulting topography is clearly the most important key functionality of the Pretex® procedure.
As mentioned in chapter 3.4.1.1, the structure forming chrome layer (‘structure layer') delivers the typical Pretex® topography. Structure elements that are rather widely distributed are called ‘open structure’ (Figure 26, left). Tightly distributed elements are called ‘closed structure’ (Figure 26, right). The density of the hemispheric structures is determined by the RPc value, indicating the count of structure peaks per centimeter area.
Open structure has values between XXXxxxXXX cm-1 and closed structures between xxxxxx cm-1.
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55 Figure 26: Graphical (above) and microscopic (below) illustration of open (left) and closed (right) Pretex® hemispheric structures
Fluid guiding and wetting properties
The hemispheric topography holds implications on the fluid guiding and wetting behavior of the working roll’s surface. The topography allows for the formation of a fluid film (e.g.
water, oil, paint or lubricant) which is evenly distributed on the structured surface (see Figure 27). This property is important for working roll surfaces as it influences the quality of the rolling process and thus the quality of the sheet metal product.
Figure 27: Fluid guiding and wetting properties on rolls Embossing properties in cold-rolling process
Pretex® coated working rolls are used to produce textured sheet metal products. For this the Pretex® structure on the surface of the working roll must be transferred to the sheet metal surface to yield the negative embossment, the so-called calotte-like topography on the metal surface which improve the sheet’s forming results. Additionally, sheet metal embossed with a Pretex® topography is an ideal basis for modern paint technology. The stochastic distribution of structure elements available from Pretex® coated products avoids the occurrence of Moiré effects and reduces the use of paint filler. This cuts-down
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56 subsequent process steps (e.g. filler drying) and therefore reduces energy consumption.
This is especially beneficial for Salzgitter’s customers from the automotive industry.
Additionally, the avoidance of paint filler reduces the usage of hazardous chemicals.
Figure 28: Embossing properties in cold-rolling of sheet metal Adjustable gliding and friction properties
Pretex® chrome plated rolls used in the metal feeding machinery meet the specific demands for friction and grip due to the hemispherical surface structures. The reduction of surface contact area of the roll and the sheet metal reduces the adhesion and avoids cold welding effects.
The improved gliding properties as a result of the reduced contact area derived from the hemispherical topography of Pretex® coated forming tools considerably extend the lifetime of these tools. As described above, feed rolls for sheet metal processing can be coated in such a way that the feeding behavior is improved by extended grip (grip effect).
In contrast, forming tools are provided with good gliding properties. Thereby, the adjustable topography allows for surface structures that provide good gliding properties or enhanced grip.
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57 3.4.2.2.3 Description of key functionalities related to Pretex® durability and longevity
Table 10: Key functionalities related to Pretex® durability and longevity
Key functionality Explanation Quantitative) statement for alternative assessment Surface hardness
Coating requires high surface hardness to withstand high mechanical pressures occurring during cold rolling processes.
>> 950 xxxxxxxxxx HV
Wear resistance
Coating requires high wear resistance to guarantee longevity of rolls used in cold-rolling processes. Durability and longevity are important for reducing maintenance costs.
Stable surface structure:
-no imprints on sheet metal product (strip cleanliness);
-roll defects must not be visible in optical inspection system.
Adhesion to substrate
Coating adhesion to the substrate is crucial for the longevity of rolls exposed to the abrasive application in cold-rolling and processes.
Excellent adhesion to the substrate for long time use.
Corrosion resistance
Coating requires good corrosion resistance properties to withstand oxidation aggravated by chemical and thermal environment occurring during metal processing.
DIN EN ISO 9227: xxxxxxxxxx h (depends on the aggressivity of the media used for cold rolling, e.g.
rolling emulsion)
Coating requires good resistance to rapid temperature changes occurring during cold-rolling processes.
Repetitive ultrashort surface temperature flashes ≤ xxxx °C
Tribological properties (coefficient of friction)
Coating requires optimal tribological properties (coefficient of friction) to ensure process safety.
Plant setup is matched to the friction coefficient between chrome coating and steel. Moreover, the
hydrophobic surface and the interaction between the roll surface and the cold-rolling medium is of high importance.
Surface morphology
Coating must have an optimal surface morphology to ensure corrosion
Depending on application a certain thickness of the coating is required for correct functionality during use. Layer thickness is often interlinked with corrosion resistance and hardness
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58 Surface hardness
Hardness is defined as the resistance of solid matter to various kinds of permanent shape changes when a force is applied. The total hardness of the product is the combined result of the substrate hardness and coating hardness. Salzgitter performs the measurement of the Vickers hardness (HV) of its metallic materials according to ISO 6507-1. This is the most common hardness test method and suitable to determine Salzgitter’s minimum requirements for chrome coated rolls of >> 950 xxxxxxxxxxxxx HV surface hardness. The coating requires high surface hardness to withstand high mechanical pressures occurring during cold rolling processes.
Wear resistance
Wear is the progressive metal loss from the surface of a solid compound triggered by mechanical impacts such as friction and side loads. The coating applied on working rolls require high wear resistance to guarantee longevity in Salzgitter’s cold rolling processes.
Durability and longevity are important for reducing maintenance costs. Further, a stable surface structure ensures that no unwanted imprints occur on the surface of the sheet metal product.
There is no specific test in place at Salzgitter for measuring the wear resistances, but the surface structure must be stable enough to ensure no imprints (e.g. caused by scratches) on sheet metal products. This quality criterion is checked visually in optical inspection system and determines the time point when a working roll in a specific roll stand needs to be changed/refurbished.
Adhesion to substrate
Adhesion between the chrome coating and the roll body is of crucial importance for the longevity of working rolls. Only the excellent adhesion established by the functional chrome coating process is sufficient to use the working rolls in cold rolling processes for a long time. Beside the lower maintenance costs due to an extend in lifetime, the rolls need to be re-plated in longer time intervals which leads to a decrease in chemical consumption.
Adhesion is not determined directly by a specific measurement method but measured indirectly over roll lifetime in combination with quality of product.
Corrosion resistance
Corrosion describes the process of oxidation of a metallic material due to chemical reactions with its surroundings, especially under the effect of humidity and oxygen. In this context, the parameter corrosion resistance relates to the ability of a metal to withstand gradual destruction by chemical reaction with its environment. Components that inhibit corrosion can be categorized according to basic quality criteria which are inhibitive efficiency and versatility. Ideally, the component is compatible with subsequent layers and performs effectively on all major metal substrates. Furthermore, it needs to guarantee product stability (chemical and thermal) and reinforce the requested coating properties.
The minimum requirement for corrosion resistance of Salzgitter’s chrome coatings is xxxxxxxxxxxxxx h (depending on roll application) determined in the salt spray test according to DIN 50021 SS.
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59
Resistance to rapid temperature changes
The chrome coating is required to be resistant to rapid temperature changes. During cold-rolling, repetitive ultrashort (milliseconds) surface temperature flashes of up to XXX °C occur which weaken the coating’s internal structure and cause rapid degradation.
Functional chrome coatings ensure the resistance to these ultrashort flash temperatures.
Tribological properties (coefficient of friction)
Friction is the force resisting the relative motion of solid surfaces sliding against each other. In case of steel mills, the coefficient of friction of the tribological system in the respective rolling mill needs to be maintained in a suitable range enabling the rolling process without events of slipping and skidding. This is in particular important to cold-rolling processes with high rolling reduction (= reduction of strip thickness per roll stand). In comparison, temper rolling is less prone to slipping and skidding due to the lower degree of rolling reduction. Furthermore, the texture transfer in temper rolling is beneficial in terms of grip.
The coefficient of friction can be determined by tribological testing. It is a property of the entire tribological system, comprising the work roll surface in contact with the rolled metal strip, the lubricant and the type of process, which in turn includes a wide range of process parameters. Meaningful testing of the coefficient of friction can only be conducted under real rolling conditions using industrial rolling mills.
Surface morphology
Metallic chrome coatings on working rolls require the presence of a micro-cracked network within the deposited layer for the maintenance of lubrication. At the same time, no macro-cracks down to the substrate must be present in the coating layer. This would weaken the corrosion resistance properties as humidity would get in contact with the substrate (base material) which cause material degradation by oxidation.
In general, for working roll coatings the highest reachable count of cracks per centimeter is required. The minimum requirement for the density of microcracks is ccccccccccc cm-1. Layer thickness
The thickness of the chrome coating layer which needs to be applied on Salzgitter’s rolls is application dependent. Typical layer thicknesses of chrome coatings for Salzgitter’s applications vary between cccccccc µm.
There are several non-destructive methods available to determine the layer thickness, for example:
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60
• Magnetic method, ISO 2178 & ASTM D7091
• X-ray method, ISO 3497 & ASTM B 568
The layer thickness has a significant impact on the properties of the final metallic chrome coating. Thin deposits reduce the risk of cracks, whereas thick deposits increase wear resistance and corrosion performance. In general, functional chrome coatings require high degrees of hardness, which is dependent on the respective layer thickness. Additionally, the layer thickness of functional chrome coatings has a major influence on the corrosion resistance and resistance to chemicals properties.
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