A NEW PROCESS OF DIRECT ENAMELING ON VARIOUS STEEL GRADES COATED WITH ELECTRO-DEPOSITED NICKEL-MOLYBDENUM ALLOY
Fumiaki Sato, Yoshihiro Johno, Yoshikazu Ko
[Tokan Material Technology Co., Ltd. (TOMATEC); the former Ferro Enamels (Japan) Ltd.] 1-27, Oyodo Kita 2-chome, kita ku, Osaka-shi, Osaka 531-8526 Japan Hitoshi Kuroda, Masao Komai, Takahiro Hayashida (Toyo Kohan Co., Ltd.) 1296-1, Higashitoyoi, Kudamatsu-shi, Yamaguchi-ken 744-8611, Japan

Introduction
In Japan we are mostly running ultra-low-carbon, high-oxygen continuous casting steel for direct enameling. However, this grade of steel has poor in drawing properties, as shown in Fig.1.
Although titanium-stabilized ultra-low-carbon steel has excellent drawing properties and is finding use in deep drawn bathtubs or other products using the two-coat/two-fire process, it is unfavorable for direct enameling due to black specks and inconsistent adherence.
There has been great demand for a new process to enameling directly on steel without conventional complex preparation, and a new direct-on process for both titanium-stabilized ultralow- carbon steel and also aluminum-killed low carbon steel without any defects.
Steel coated with electro-deposited nickel-molybdenum alloy, developed by Tokan Material Technology Co., Ltd. and Toyo Kohan Co., Ltd. is now moving out from laboratory to pilot production lines.
Fig. 2 (a), (b) show special sheet steel coated with electro-deposited nickel-molybdenum alloy. Fig. 3 shows a SEM Photograph of the developed steel surface.

Steel grades in this development
High-oxygen steel, titanium-stabilized steel, and aluminum-killed steel, all with low carbon from the continuous casting processes were used in this new development.
Table 1. shows the thickness and typical chemical analysis of the steels used.

Production procedure of electro-deposited nickel-molybdenum alloy sheet steel
Various cold rolled sheet steels are alkaline degreased and acid etched lightly before electrodeposition. Electro-deposited of Nickel-Molybdenum is applied in the prescribed condition. Metal alloy weight is 1.5 to 20.0 g/m² of Nickel and 0.4 to 7.0 g/m² of Molybdenum.
Heat-treatment is performed after nickel-molybdenum alloy plating in reducing atmosphere.

Enameling results on the newly developed steel
Normal titanium white enamel was applied and fired directly on the newly developed steel as usual.
Surface texture and adherence were checked on specimens, which have good textures with no black specks and no fish scaling, in addition to excellent adherence as evaluated with the PEI adherence meter.
Table 2. shows Enameling Result

Fig. 4 shows surface texture and adherence on an enameled specimen of high-oxygen steel coated with electro-deposited nickel-molybdenum.
Fig. 5 shows the new titanium-stabilized steel.
Fig. 6 shows the new aluminum-killed steel.
Fig. 7 (a), (b) and (c) shows the interlayer between the enamel layer and the new titaniumstabilized
steel substrate; the image on the (b), (c) are highly magnified. The interface is rough, possibly contributing to the adherence observed.
Fig. 8 (a) and (b) show the adherence surface after removing the enamel layer from a specimen by immersion in 40 % NaOH solution for 48 h. Many micro protrusions consisting of iron, nickel and molybdenum remained on the steel surface, which was confirmed by EDX analysis.
Fig. 9 (a) shows the relation among PEI adherence (%), enamel thickness and the number of times of white cover coat and fire. The adherence is stable. Fig. 9 (b) shows an enameled specimen of one-coat/ one-fire (120 μm) and Fig. 9 (c) shows an enameled specimen of two-coat/two-fire (240 μm). Fig.10 (a), (b), (c), (d), (e), (f) and (g) show the typical enameled products manufactured with newly developed steel based on titanium-stabilized steel. Fig.10 (h) shows oven square plate applied by enamel powder in electrostatic spraying system to the new titanium-stabilized steel.
Enamel texture and adherence after firing are excellent.
Fig.11 (a), (b), (c), (d) and (e) show the adherence test results with the Impact tester according to ISO 4532, in which N means a load in Newton.
Fig.12 (a) shows a plate part on the top of gas cook stove. Fig.12 (b) and (c) show the interfacial zones of sections taken from a flat area of the plate and a pressed area. Both the pressed (deep drawn) area and the flat area have excellent adherence.
Fig.13 (a) and (b) show the finish of direct enamel on the spot-welded area, where (a) is front side, (b) is back side. There are no difficulties at the spot welding, and enamel texture and adherence are excellent.
Fig.14 (a), (b), (c) show kerosene stove with spot welded area. Enamel texture and adherence are good at the spot welded area.

Conclusions
Special electro-deposited nickel-molybdenum on ultra-low-carbon high-oxygen steel, titaniumstabilized steel, and low carbon aluminum-killed steel makes it possible to enamel directly on steel without conventional direct-on pre-treatment.
The enameled products on the newly developed steels have excellent texture with no black specks, pinholes, or fish scale. This new development may also make it possible to enamel directly even on steel grades that have been unusable to this point for the direct-on process due to defects on the finish.
Steel coated with special electro-deposited nickel-molybdenum has metallic gloss without any smut, so there are no problems in applying the cover coat enamel with dipping and flow coating. The cover coat can be directly applied on the newly developed steel prepared with the no pickle/no nickel process.
The new steel simplifies the direct-on enameling process, because there is no need for complex metal preparation or expensive electrostatic application systems.
This new process is the economical and environmentally friendly process.

Fig. 1 Poor drawing properties.

Fig. 2 (a) Coil steel coated with electro-deposited nickel-molybdenum.

Fig. 2 (b) Sheet steel coated with electro-deposited nickel-molybdenum.

Fig. 3 SEM photograph of the developed sheet steel surface.

Fig. 4 An enameled specimen on high-oxygen steel coated with electro-deposited Nickel-Molybdenum.

Fig. 5 An enameled specimen on titanium-stabilized steel coated with electro-deposited Nickel-Molybdenum.

Fig. 6 An enameled specimen on aluminum-killed steel coated with electro-deposited Nickel-Molybdenum.

Fig. 7 (a) The interlayer between the enamel layer and the titanium-stabilized steel substrate.

Fig. 7 (b) A highly magnified zone of Fig. 7 (a).

Fig.7 (c) A highly magnified zone of Fig.7 (b).

Fig. 8 (a) The adherence surface after removing enamel layer from a specimen.

Fig. 8 (b) A highly magnified surface of Fig. 8 (a).

Fig. 9 (a) PEI Adherence (%) - Enamel thickness (μm-The number of times of white cover coat and fire.

Fig. 9 (b) An enameled specimen one-coat / one -fire (120 μm).

Fig. 9 (c) An enameled specimen two-coat / two- fire (240 μm). Fig.10 (a) Saucepan. Fig.10 (b) Canister.

Fig.10 (a) Saucepan.	Fig.10 (b) Canister.
Fig.10 (c) Canister.	Fig. 10 (d) Plate part of gas cook stove.

Fig. 10 (e) Top plate of kerosene stove.

Fig. 10 (f) Range top plate.

Fig. 10 (g) Bottom plate of kitchen cabinet.

Fig. 10 (h) Oven square plate enameled by powder coating.

Fig.10 (h) Oven square plate (underside) enameled by powder coating.

Fig.11 (a) Top plate of gas cook stove.

Fig.11 (b) A backside of Fig. 9 (a).

Fig. 11 (c) Canister.	Fig. 11 (d) Saucepan.
Fig. 11 (e) Turntable of range.

Fig.12 (a) Plate part of gas cook stove.

Fig. 12 (b) The interfacial zone of section at a flat a area of Fig. 10 (a).

Fig. 12 (c) The interfacial zone of section at a pressed (deep drawn) area of Fig. 10 (a).

Fig. 13 (a) The finish of direct enamel on.

Fig. 13 (b) A backside of Fig. 11 (a).

Fig. 14 (a) Kerosene stove with spot welded area.

Fig. 14 (b) Enameled Kerosene stove.

Fig. 14 (c) Magnified insides of Fig. 14 (a), (b).