DEVELOPMENT OF DEEP DRAWING CONTINUOUS ANNEALED I.F. STEELS FOR ENAMELLING
Renzo Valentini, Massimo De Sanctis, Adriano Solina
Dipartimento di Ingegneria Chimica, Chimica Industriale - Università di Pisa, Italy
Walter Sabba, Maurizio Mini - Ilva, Gruppo Riva, Italy

Abstract
The requirements for enamelling steels are more and more demanding in terms of drawing and enamelling attitude, and surface aspect. In order to satisfy these specifications continuous casting I.F. enamelling steels, treated through liquid steel degassing and processed in continuous annealing were developed.
This cycle guarantees significant performances and allows flexibility and fast production. Project and control of the process variables (chemical composition, thermomechanical treatments) rely on mathematical models which guarantee the product optimisation according to the market requirements.

Introduction
Traditional steels

More widespread use of continuous casting in the manufacture of steel products for enamelling presented again the fishscale problem.
As it is common knowledge fishscale is caused by hydrogen gas building up pressure at the interface between enamel and steel, resulting in the rupture of enamel [1].
The standard specifications to produce steels with sufficient resistance to fishscale are a high temperature strip coiling (> 720 °C) at the hot rolling end. This technique allows the carbide to increase (cementite); these carbides during cold reduction shatter and produce micro cracks in steel structure.
These traps influence hydrogen diffusivity and solubility in the steel in a significant way [2]. The presence of these micro-cracks leads to a degradation of mechanical properties of steel. To restore these properties it's necessary to use complex procedures in open-coil box annealing in decarburising atmosphere.
Lately several theoretical attempts of modelling the interactions of hydrogen atoms with lattice and interface heterogeneity have been developed and the optimal condition for manufacturing process phases, that have a great influence on these phenomena, have been found.
The more significant parameters in the rolling mill process are coiling temperature, cold reduction ratio and annealing temperature.

New enamelling steel - Quality and process
Recently to overcome problems due to Open Coil Annealing Process (e.g. long annealing time, non-homogeneity of mechanical performances along the strip, etc) several important steel makers have carried out important investments. The main ones are in hot rolling area and in cold rolling area (i.e. continuous annealing and pickling line coupled to tandem mill) to improve the performance of their plants.
The purpose of these investments was to produce thin sheet steels by on line processes with higher quality standards.
One of the main objectives in the research program in ILVA is the set up of the process to manufacture a new kind of steel: an Interstitial Free (I.F.) steel grade devoted to enamelling application. These steels have a low content of carbon and nitrogen stabilised by adding Ti.
Interstitial Free steel grades show higher performances in forming and drawing processes because of their low content in C.
The low C content is reached since the earlier steps of the manufacturing cycle by vacuum degassing (RH process). This allows using both a batch annealing and a continuous annealing as final annealing process.
A drawback is the lower surface adhesion in enamelling due to the presence of free residual Ti.
The steel fish-scaling resistance depends on the number and the nature of the traps able to capture hydrogen [3].
The capability to retain hydrogen is determined by the inclusional level, the second phases and more in general by the reticular defects. It is well known that the traps system may be created through the Ti compounds precipitation [4]. In particular, Ti compounds with C, N, S, i.e. Ti (C, N), TiS, Ti4C2S2.
The precipitation temperatures of these compounds are those typical of the hot rolling and coiling process (750 -1250 °C). The precipitates volumetric fraction and morphology depends on the strip chemical composition and thermomechanical treatment. In this research a numeric simulation program for the precipitates volumetric fractions calculation starting from the knowledge of the steel chemical composition has been worked out.

Experimental
Ilva carried out some experimental industrial casting. Appreciable variations to the concentration of the most relevant elements such as Si, Mn, S, Al, Ti and N have been brought (Table 1).
Hot rolling temperatures have been set in the range from 900 °C to 1250 °C; coils have been coiled at the hot rolling mill end at a temperature of 700 °C.
Then coils have been cold rolled in a tandem mill with a reduction ratio between 50% and 75% in order to have a final thickness in the range from 0,7 to 1,5 mm, to fulfil the market requirements.
Coils have been processed in continuous annealing line in Novi Ligure works adopting the cycle proper for deep drawing steel grade.

A mathematical model has been developed devoted to precipitation prevision to simulate, by a numerical code, the behaviour of precipitates in these steels when continuous annealing processed.
The two following diagrams show the precipitate volumetric fraction curves vs.
temperature for two different typical I.F. enamelling steel chemical compositions, detailed in Table 2, the first relevant to "high nitrogen" steels, the second to "high sulphur" steels.

For which concerns the mathematical model utilized, the austenitic matrix is assumed to be thermodinamically balanced by the following compounds: Ti(CyN1-y), Ti4C2S2 and MnS, where y is the carbon molar fraction in the carbonitrides and 0≤y≤1.
The combination of the elementary equilibriums which takes into account the simultaneousness of these equilibriums, allows to write a system of 9 equations in 9 variables.
Experimental results moreover showed that in the Ti I.F. steels, due to the limited sulphur and carbon content, the developing of another phase containing sulphur, the Titanium Sulphate (TiS) is possible. Considering also this phase, the above mentioned model is extended and the resultant system is composed by 10
equations in 10 variables.
The solution of the non linear system of N equation in N variables, where N is a number varying according to the complexity of the same model, or the number of precipitate compounds which are considered in it, is faced writing:

The idea which is at the basis of the method selected for above metioned non linear systems solution calculation consists in the approximation of the F(x) function which describes the system (1) in a generic point x near to the point xK with the Taylor series development of the same function stopped at the first order, i.e.:

Where DF(xK) is defined Jacobian of the function F and is a matrix whose component of the i row and j column corresponds to the partial derivative of the i-th component Fi of the function F in respect of the j-th component xJ of the vector x, i.e.:
Assuming that the second term of the (2) is 0, it is possible to find a new point xK+1 through the relation:

where DF(xK)-1 is the inversed matrix of the Jacobian. Once found xK+1, the procedure is repeated in this point in order to calculate a new approximation xK+2 of the solution and so on.
The procedure convergence towards the correct system solution is controlled evaluating during the iterations the modulus of the vector F(xK): the search procedure ends when this one reaches a sufficiently low value.

figure 1 - Volumetric fraction vs. temperature for chemical composition 1

figure 2 - Volumetric fraction vs temperature for the chemical composition 2

Results
The thin sheet steels produced have been tested to check microstructure, mechanical characteristics and the behaviour in enamelling. Special attention has been paid to electrochemical hydrogen permeation tests to determinate the diffusion coefficient related to fish-scale susceptibility. These tests have been carried out with an innovative experimental apparatus based on a solid state sensor, suitable for I.F. steel grade, which cannot be satisfactory, tested with the traditional methods.
Then the manufactured experimental I.F. steels have been tested by enamel producers and bathtub producers to evaluate, from the end users point of view, the behaviour as far as adherence and drawability are concerned.
Fish-scale resistance, enamel adhesion and cold forming have been found to be satisfactory.

Microstructure
Optical and SEM analysis show a typical IF ferritic grain shape with the presence of a large number of Ti compounds (TiS, Ti4C2S2., Ti (C,N)) precipitates.
SEM analysis on the secondary phases pointed out that there is no appreciable differences between steels processed either at high or low coiling temperature. In the following figures the SEM photos of high Nitrogen and high Sulphur IF steels are shown.

figure 3 - SEM Photo of an high-Nitrogen IF steel. 500 x Nital attack

Mechanical properties
Mechanical properties, shown in Table 3, are comparable with the values of the best conventional Al-killed deep drawing steels.


figure 4 - Enlargement (5000x) of a Titanium nitride precipitate:
the microvoids are well visible

figure 5 - Enlargement (3500x) of a high Sulphur IF steel Titanium nitride precipitate: the distributed precipitates are well visible

The FLC have been determined at ILVA laboratory and show good deep drawing characteristics.
In figure 6 is reported a typical FLC for an IF steel among the experimental materials produced at ILVA works.

figure 6 - A typical LFC for an IF steel manufactured at ILVA works (two different thicknesses)

Electrochemical hydrogen permeation tests
Special attention has been paid to electrochemical hydrogen permeation tests to determinate the diffusion coefficient related to fish-scale susceptibility.
These tests have been carried out with an innovative experimental apparatus based on a solid state sensor, suitable for I.F. steel grade, which cannot be satisfactory, tested with the traditional methods.
The hydrogen permeation tests, show a hydrogen diffusion coefficient in the range from 0.8 to 1,2 10-10 m2/s.
As it is well known, D values minor than 1,5∗10-10 m2s guarantee good enamel performances as far as fish scale appearance is concerned [5].
Thereafter the permeation curves of two different IF steels, one for enamelling and one for other applications are shown (figure 7).

Enamelling tests
No fish-scale defects have been found in the investigated steels. The enamel adherence, tested by EN 10209, is quite good and is a function of the Cu/P ratio.

figure 7 - Comparison between a typical IF non enamelling steel
and a IF enamelling steel

Discussion
The right development of I.F. steels for enamelling have to consider the main characteristics required and in particular good drawability, good fish-scaling resistance and good enamel adhesion.
Optimizing the performances can be obtained successfully through the utilisation of previsional models which, starting from the chemical composition and the thermomechanical treatment, allow the determination of the typology and the volumetric fraction of the Titanium precipitates.
As shown in figures 1 and 2, as a function of the selected chemical composition (high Sulphur or high Nitrogen content), the precipitation ofTitanium sulphides or carbosulphides, or alternatively Nitrogen nitrides and carbonitrides is favoured.
The metallurgical fish-scaling resistance mechanisms are different as a function of the precipitate type.
As shown in figure 5 (Titanium sulphide), for steel chemical compositions characterised by an high Sulphur content the formation of an high number of small traps distributed in the matrix is favoured.
Instead in case of steel chemical composition characterised by an high Nitrogen content, during continuous casting big Titanium nitride precipitates already develops. During the cold rolling microvoids develops (figure 4) which act as effective traps for the hydrogen [5].
The correct dosage of the Titanium content, in order to guarantee the Titanium precipitates formation to stabilize the steel has moreover a great importance, but excess of Titanium with consequent formation of enamel poor surface adhesion defect is to be avoided [6].
For which concerns mechanical properties, I.F. steels prove themselves as an excellent alternative for the production of bath tubes and other deep drawing parts [7].
In particular, high r (planar anisotropy) and high ultimate elongation values offer to the enamelled parts producers the possibility to develop models characterised by the most complex shapes.
In figure 8, obtained by plotting on the axis four characteristics values of the mechanical properties (yield strength, elongation, r, n) the better performances are shown, comparing an I.F. steel with a conventional steel, as yet happened in the automotive sector.

figure 8 - Comparison between mechanical properties of an IF enamelling steel and a conventional enamelling steel

Conclusions
The new I.F. steels open a new field of sheets for enamelling products. Within optimal chemical composition and termomechanical process, these sheet steels provide stable porcelain enamel adhesion, excellent mechanical properties and very good fish-scale resistance.
From these remarks it is possible to argue that the steel developed may be utilised in the near future to produce deep drawn enamelled parts (for instance bath tubes).
The experiences acquired on important sampling at industrial level, through a joint experimentation with enamellers and end product producers, give more than encouraging indications.
The experimentation showed the possibility to utilise these new steel grades for real applications in the near future.

References
1. P. De Gregorio, R.Valentini, A.Solina, F.Gastaldo, A. Buonpane: "Enamelling Steels and Total Quality of the Enamelled Products", XVI Enamellers' Congress, Stratford, 1992.
2. P. De Gregorio, R. Valentini, A. Solina, F.Gastaldo: La metallurgia italiana, vol.83, 1991,p.567.
3. R. Valentini, A. Solina, L. Paganini, P. De Gregorio: J.Material Science, vol.27, 1992, p.6579.
4. P. De Gregorio, R. Valentini, A. Buonpane: "Studio della smaltabilità di acciai da profondo stampaggio laminati a caldo", Conv.Ceca n.7210- KB/428-Rapp.finale-settembre 1993.
5. F. Gastaldo, G. Porcu: "Sviluppo applicativo degli acciai al Ti nella fabbricazione dei manufatti smaltati", convegno CISP, Noi Ligure, Luglio 1994.
6. C. Gazzo, F. Gastaldo, A. Solina, R.Valentini, G.Porcu , Lamiera, novembre 1995 p.114-120.
7. A. Van Cauter, J. Dilewijns, F. Horzenberger, B.C. De Cooman "The influence of the C/S ratio on the properties of Ti stabilised Enamelling Steels", 39th MWSP CONF.PROC.ISS, 1998 VOL. XXXV, p. 315-322.

 

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