IEI, International Enamellers Institute

USINOR'S CONTRIBUTION TO ENAMELLING COST REDUCTIONS
Philippe Gousselot, Groupe Usinor, France
Christophe Perrot, Groupe Usinor, France

Introduction
Increasingly, our customers look to simplify their enamelling operations. The objectives are above all environmental and economical.
It is one of USINOR's main concerns to help them in their actions, particularly those creating value. How to simplify enamelling processes is one such line of action, and the objective of this presentation is to show, through examples of new products, USINOR's contribution in this field.
A step by step analysis of the enamelling processes shows, that to create value for our customers, we must provide products which:

Either eliminate intermediate stages during the normal production process

Or provide a technological breakthrough in relation to the normal production process, implying the simplification or the elimination of several stages.

figure 1 - The main customer processes

This is well illustrated by figure 2.

figure 2 - The impact of new products on the customer's production routing

A way of simplifying the production process is to eliminate the pre-treatment, and in particular, the degreasing stages. The use of steel coated with a dry film allows for this economy.
Forming is then carried out without further lubrication and the dry film, being compatible with the applied enamels, surface treatments may be eliminated.
The customer's gains are on two levels: no drawing compounds are required, and there are gains for disposal of surface treatment products. The latter implies an obvious environmental advantage, in that no effluent is produced.

Determining the value of the products developed and associated
specifications
Description of the method
The method consists in analysing requirements for each stage of a given part,
estimating the associated cost, and as a consequence, leading to the
establishment of the overall specification for the proposed solution.

Application of the method to a Hob Top coated in the 2C/1F
The example is that of a hob top, 0.80 mm thick, enamelled in 2C/1F.
Examining the production process provides the following answers concerning
the customer requirements, value and associated specifications:

The storage of oiled products may present a safety hazard to persons working in the area, thus requiring regular cleaning of the workshop. The new product must therefore present a dry surface.

The steel must not rust during storage. The product proposed must therefore have at least the same corrosion resistance as an oiled product.

The product must also eliminate the need for further oiling.

The product must also improve the drawability of the parts, with a reduction in the reject rate.

The parts not having been oiled during processing, the product applied must be compatible with the enamel, thus eliminating the need for degreasing.

figure 3 - The industrial chain, customer requirements, value and associated specifications

From this example, the conclusion is drawn that to simplify the processing of our products by customers, we must propose a product having a dry surface, a sufficient corrosion resistance, a guaranteed drawability and being compatible with enamelling without surface treatment.
In the case of the 2C/1F enamelled hob top, the value to the customer (his potential economy in using this type of product) has been estimated at several tens of euros/tuns of steel.
We will now present the solutions developed by USINOR satisfying this specification.

Some technical solutions for enamelling without degreasing
To produce a defect free enamelled surface with good adherence, it is required to control the enamel-steel reactions during the vitrification stage.
Daily experience shows to what point the overall process affects these reactions. Thus, the following causes are clearly identified;

The steel, through it's surface reactivity.

The ground coat by it's capacity to absorb the gazes produced on curing.

The pre-treatment, particularly the degreasing stage.

Firing and the furnace atmosphere humidity.

Powder enamelling is often performed on oiled steel, without degreasing. The obtained quality can be estimated satisfactory for oven cavities, but always unsatisfactory in the case of 2C/1F application. Most products used for protection (oils) lead to an abnormal reactivity Twix enamel and steel, creating defects.

The difficulty in producing steel capable of being enamelled without degreasing lies in being able to answer the following specifications:

Ease of forming,

Proven corrosion resistance,

Optimal enamelling capability,

Respect of the environment.

Selection of the protective products
Different product offering temporary protection have been tried and compared to the standard protective oils:

Surface to surface friction

Corrosion tests in humid atmosphere

Ground coat and 2C/1F enamelling

The perfect degreasing of these products, using standard spray products, has been established.
It means that these products are compatible with Direct-on White Enamelling; only the good friction behaviour is used in this case.

Different types of steels have been used:

SOLFER; decarburised steel

SOLEMA; 2C/1F steel

DC04EK; conventional 2C/2F enamelling steel

The enamelling modes examined are:

pickle free ground mass

2C/1F Powder/Powder

2C/2F Wet/Powder; in this case the ground coat is "activated" to favour wetting of the substrate by the enamel.

The acceptance criteria after curing are: adherence less than or equal to 3 (EN10209) and a surface free of defects such as pin holes, orange peel, bubbles, etc.
Some products above all tested feature a low friction coefficient. We selected a dry film, which gives a very low friction coefficient, as shown below.

figure 4 - Dry film friction coefficient curves

To perform wet enamelling, it is necessary to add a tensio-active element to the enamel and so obtain a quality enamelled product. This has allowed us to succeed 2C/1F Wet/powder coatings.

Results on enamelling SOLFER without degreasing
The corresponding results below clearly show:

That it is impossible to produce a quality product in the presence of standard protective oils.

Total absence of surface defects with powder coatings.

2C/1F is borderline.

Powder coats require a slight increase in firing temperature to optimize adhesion.

Enamelling results on other steel grades

SOLEMA feature good results in 2C/1F,

DC04EK produces good powder coated surfaces with a slight tendency to orange peel. No pinholes or bubbles are observed.

Industrial production of dry film coated steels
Two types of coating applications have been used:

Roll coat on a coil coating line,

Spray coating and drying.

We have checked that the drying stage does not decrease the characteristics required:

friction and drawability,

Enamelling capability.

Roll coat on a coil coating line
Several trials have been carried out. It has allowed us to validate the laboratory and customer trials, as shown below.
The first trials were made on the oven bottom pan, powder coated. The laboratory results are as follow:

A deposited coating weight of 0.5 g/m2/side, leads an excellent behaviour during friction tests for blank holder pressures less than 60MPa. Higher pressures may produce slight pollution in the tools,

The enamelling without degreasing succeeded perfectly. The surface and adherence obtained were optimal.

The trials in customers enamelling shop have shown:

Very good corrosion resistance in coil form and drawn parts.

Production output for drawing without polluting the tools or damaging the steel surface. Realisation of hundreds pieces has shown no degradation of stamping conditions.

A total absence of enamelling defects.

The second trial was done with a coating weight of 1.5 g/m²/side.
Enamelling without degreasing featured some defects originating from particles sticking to the parts during the transfer stages.
However, no defects were obtained from causes linked to poor control over the surface reactivity.
We therefore decided to limit the dry film coating weight to 0.8 g/m²/side.

Spray coating and drying
The product is applied by a set of sprays, the coating thickness evened by passing thro' pressure rolls, after which the product is dried on exiting.
One of this trial's objectives was to determine the coating weight required to enable the steel to be used after being subjected to normal storage conditions and to forming without added lubricant and without breakage.
The coating weight obtained - 0.3 to 0.4 g/m2/side - was insufficient to avoid some breaks in the corners.
However, and logically given the low coating weight, the enamelling without degreasing was good.
Furthermore, no trace of rusting was seen after 3 months of coil storage.
These different trials have allowed us to determine the main product and process parameters required to produce a steel capable of being enamelled without degreasing.

Conclusions and the future
The enamelling world is in constant evolution and seeks the best compromise in terms of quality/cost.
The USINOR approach we have set out is but an example seeking to simplify the enamelling process by the suppression of the costly intermediate stages, together with the respect of the environment.
Enamelling without degreasing thus seems to be a relevant response to the daily concerns of our customers.
Industrial productions in our customer's workshops have shown that large-scale application presents no difficulties.
Today our staff takes the development of this new product to our clients. It can be tested and eventually adapted to specific process, like enamelling, in term of application and kind of enamel used.