THE DEVELOPMENT OF A NEW HOT ROLLED ENAMELLING STEEL FOR BOILERS: GRADE "SOLEMAIL 300"
Jacques Hernandez
Ronald Kefferstein
Thierry Malot
CRPC, Sollac Méditerranée, Fos sur Mer, Usinor Group /France

Abstract
A new grade of hot rolled steel for enamelling, the "Solemail 300" grade, has been developed at Sollac Méditerranée (Usinor Group) in order to satisfy boiler manufacturers. In addition to its excellent enamelling aptitude, this new grade is distinguished from conventional grades by its mechanical characteristics being guaranteed after drawing and enamel firing.

Introduction
Usinor, and in particular Sollac Méditerranée (Fos-sur-Mer plant, France), propose several grades of hot rolled steel for enamelling. One of the major applications relates to the manufacture of domestic boilers of which the interior is enamelled (single face enamelling).
Today, industrialists seek more and more the weight reduction of products in practically all sectors. Obviously, the field of boilers does not escape this! One solution consists of being able to propose robust and easily drawable grades,
available in a wide range of sizes and which allow, thanks to their mechanical properties, thickness reduction, whilst guaranteeing in-service mechanical characteristics. It is for this reason that the Hot Rolled Products Research Center (CRPC), located in the Sollac Méditerranée plant of Fos-sur-Mer has developed a new grade, "Solemail 300".

The new grade specifications
The boilers are tested under cyclic pressure from 12 to 15 Bars (1.2 to 1.5 MPa). This value, much higher than those met in service (6-7 bars), gives a safety margin even with respect to the phenomenon of fatigue. The most critical parts are the boiler end-caps. The boiler cylinders are formed by rolling and thus present very low states of deformation.
On the other hand, the end-caps, obtained by stamping, can present gradients of significant deformation and thickness according to the form or the material used.
Thus, starting from these purely mechanical criteria and while consulting our various boiler manufacturer customers, we have managed to define the specifications for the development of a " specialized " grade for the manufacture of such products.

This grade must be single-face enamellable with liquid or powder enamels, with excellent adhesion of the enamel layer and excellent resistance to fish-scaling, etc.
This grade must be available in a wide range of sizes, in particular for thicknesses of from 4 mm to 1.55 mm and even less.
This grade must be easily weldable to allow the various types of welding carried out to close the boiler cylinders, to assemble the end-caps and the various external connections. The control of the content of certain chemical elements is thus of primary importance (Phosphorus, Silicium...).
For various reasons (weldability, deep drawing, the requirements of certain German customers etc.), it was necessary to fix the maximum carbon content at 0.1%.
This grade must be easily drawable, taking into account the currently available tools of our customers and of the complex forms of certain end-caps; the mean mechanical characteristics targeted are 300 MPa for yield strength (Ys), 400 MPa for tensile strength (Ts) and 30% for elongation (E%).
Lastly, the most significant challenge of this specification is the guaranteeing of the mechanical properties after deformation and enamel firing. In particular, the yield strength (Ys) must be reserved (> 300 MPa) after elongation rising to 15% and a typical annealing of 10 minutes at 850 °C. This criterion makes it possible to avoid enamel flaking during pressure tests, in certain zones strongly deformed by the stamping.

The new grade properties and description
Chemical analysis choice
To develop this new grade, several industrial heats have been made in order to optimize the chemical composition and manufacturing conditions and in order to fulfil the requirements of our severe specifications.
Initially, the percentages of Carbon (C), Silicium (Si), Manganese (Mn) and Phosphorus (P) were fixed to satisfy the requirements for weldability and to avoid the crack type defects produced during the deep drawing of certain zones on the boiler end-caps.
In order to guarantee the enamelling aptitude of this grade and to create hydrogen traps, we chose Titanium (Ti) metallurgy. Indeed, it is well-known that certain Titanium precipitates trap the hydrogen released at the time of enamel firing and thus prevent the appearance of fish-scaling. We also chose to reduce the Sulphur content in order to decrease the number of (Ti, S) precipitates, such as Ti₄C₂S₂ . Our former studies clearly showed that this type of precipitate is less efficient in trapping hydrogen than the TiC carbides.
Lastly, let us say that the control of the Titanium, Sulphur (S) and Nitrogen (N) contents as well as the control of the precipitate formation during the manufacturing process will guarantee, not only the suitability for enamelling, but also mechanical properties and their conservation after shaping and enamel firing. The table below presents the chemical analysis of Solemail 300.

A thermodynamic calculation [(Ti) = (total Ti) - 3.42 (N) - 1.5 (S)] makes it possible to determine the effective Titanium level which will precipitate into TiC. This carbide is the principal element which guarantees the hardening of the grade, the conservation of mechanical properties after enamel baking as well as the aptitude to trap hydrogen during the enamelling process.
As an example, we have presented in the following table, the proportions of TiC and Ti₄C₂S₂ obtained for two chemical analyses very slightly different. This table shows in a very clear way the importance of the control of the chemical analysis at the time of manufacturing of this type of steel: in the example below, the analysis "B" will be harder than analysis "A" and will trap hydrogen much more easily. The relative proportions of Nitrogen (N), Sulphur (S) and Titanium (Ti) thus constitute a vital parameter.

Mechanical properties obtained
The chemical analysis specifications, and in particular the N/Ti/S proportions, the manufacturing conditions (casting, slab re-heating, hot-rolling temperature, coiling temperature etc.) make it possible to obtain mechanical characteristics in conformity with the initial specifications.
The two tables, below, show the mean mechanical characteristics obtained for two populations of hot-rolled coils (thicknesses lower or higher than 2.5 mm). These characteristics were measured on ISO 20x80 transversal test pieces.

Properties after drawing and enamelling
The major benefit of this new grade lies in the conservation of mechanical characteristics after forming and enamel baking.
Indeed, for certain conventional grades, some adhesion defects in the enamel layer (enamel flakes) can appear during the pressurization test at from 12 to 15 Bars, carried out on the assembled and enamelled boilers.
This defect, well-known by manufacturers, often appears in the areas most heavily stamped, like fixing bosses.
In these areas, deformation of from 10 to 15% can be reached during stamping.
With conventional steels whose mechanical characteristics are not guaranteed after elongation and annealing, small deformations appear during this type of test and lead to the rupture of the enamel/steel interface, as illustrated in figure 1.

figure 1 - Enamel flaking on a boiler end-cap after the pressurization test Conventional grade

It is thus advisable to check the mechanical characteristics obtained after various types of elongation (5 to 15 %) the and simulation of enamel baking at 850 °C for 10 minutes.
The following tables present the yield strength obtained for a conventional steel and for two populations of the new grade Solemail 300.

figures 2 and 3 - Traction curves obtained on as- rolled products and on 10 % elongated and baked products (thickness: 1.8 mm)

Thanks to the process of hot rolling and the particular Titanium metallurgy, this new grade, for thicknesses lower than 2.5 mm, sees its mechanical characteristics maintained after elongation and annealing. For higher thicknesses, mechanical characteristics are lower in the as-rolled state (Ys of about 290 MPa) but increase considerably after elongation and annealing.

Suitability for enamelling
Comparison tests of sensitivity to fish-scaling were carried out between various conventional grades and the new Solemail 300 grade. They were carried out by the application of "sensitive" enamels, intended to reveal the defect. After enamel baking and accelerated ageing, the Solemail 300 has shown a much higher resistance to the defect than the other grades.

figures 4 and 5 - Appearance of enamelled samples after fish-scaling test

Enamel adhesion was measured under the conditions of standard EN 10209, appendix D, after the application of a boiler type liquid enamel. All the samples tested showed an adhesion of level 1 (very good).
These results were confirmed by micrographic observations with a Scanning Electron Microscope (SEM), where one can note that there is a very strong reactivity between the metal and the enamel layer (see figures 6 and 7).
Within the framework of a working group carried out in collaboration with the CISP (working group WG7), tensile tests on enamelled samples of this grade (enamelled with powder or liquid boiler enamels, applied to various thicknesses) also show that the cracking of enamel occurs beyond a yield stress of 300 MPa and for elongation higher than 0.18%.
Up to this limit, the adhesion of the various enamels tested was excellent.

figures 6 and 7 - Enamel adhesion on the Solemail 300 grade

Metallurgic description
Micro-alloying elements such as Titanium exert a strong influence on the mechanical properties of steel. To obtain the desired mechanical characteristics it is necessary to control:

To satisfy the needs of the boiler market and to obtain mechanical property conservation after elongations of up to 15% and enamel baking at 850 °C, it is necessary to control the precipitation from slab re-heating up to coil cooling, by favouring the fine and coherent precipitation of TiC to the detriment of Ti₄C₂S₂ precipitates which are coarser and less hardener.
This precipitation takes place during hot rolling (precipitation induced by the deformation) or at the time of the phase shift at high temperature (precipitation induced by the phase transformation).
The following figure (figure 8) presents the influence of different manufacturing conditions on carbide precipitation for the same steel chemical composition.
It is by controlling the density and the nature of these precipitates, that it is possible to conserve the mechanical properties after shaping and enamel firing.

figure 8 - Influence of different manufacturing conditions on carbide precipitation in the Solemail 300

Conclusion
The new grade Solemail 300, developed by Sollac Méditerranée (Usinor Group, Fos/Mer plant) is already used by several boiler manufacturers in France and Italy.
It has many advantages and should, in short time, replace the conventional enamelling grades. Solemail 300 is easily drawable and weldable. Its conservation of mechanical properties after various elongations (up to 15%) and enamel firing at 850 °C, constitutes a major asset and makes it possible to reduce boiler thicknesses, whilst guaranteeing the in-service mechanical resistance and during pressure tests. A yield strength of 300 MPa is systematically guaranteed after elongation and firing. Lastly, this nuance, thanks to its fine TiC precipitates, presents an excellent suitability for enamelling.