Method of plating steel

Upon forming a plating coat on steel works, the works are held in a fluorine- or fluoride-containing gas atmosphere in a heated condition to thereby remove oxidized scales such as SiOx, MnOx and the like on the surface layer of the work and to form a fluoride layer to prevent forming new oxidized scale. The fluoride layer is easily eliminated by an action of added flux to a plating bath or by heating the fluorinated steel works in an atmosphere containing hydrogen prior to dipping in the plating bath to activate the steel surface. The activated steel surface is improved in compatibility with the plating bath and the like to realize a good quality plating coat formation.

TECHNICAL FIELD 
This invention relates to a method of plating steel. 
PRIOR ART 
A hot Zinc dipping and a hot Aluminum dipping are applied to various uses 
since a plated coat of them has an excellent rust preventive property 
and,since those methods require a low cost. Platings such as the hot zn 
dipping and the hot Aluminum dipping are conducted by precleaning the 
steel works, such as cleaning with acid, then heating them at 650.degree. 
to 800.degree. C. in an annealing process in a reduction atmosphere, 
cooling them to the vicinity of the temperature of a plating bath and then 
dipping them in a plating bath to form a plating coat. In such a plating 
process, in order to obtain a good quality plating coat, not only enough 
precleaning of the steel articles is required, but also it is preferable 
that oxidized scales such as SiOx, MnOx on the steel work surface layer 
are not generated in an annealing process under an atmosphere of 
reduction. Recently, a lot of Si is contained in the steel works since 
continuous casting material is used and a high strength material is 
required. This Si content in the steel work is changed to an oxidized 
scale in the annealing process in said reduction atmosphere . This 
presents an obstacle in forming a plating coat by causing problems such as 
uneven plating and bad appearance. 
In order to prevent generating the oxidized scale, it is necessary not to 
change quality of Si and the like to the oxidized scale in the annealing 
process in an reduction atmosphere. Generally, however, in a conventional 
continuous treatment process, a reduction atmosphere is formed with a 
mixed gas composed of N.sub.2 and H.sub.2 and the above-mentioned steel 
materials are heated and cooled in this atmosphere. Since formation of an 
oxide of Si or Mn requires low energy, an oxide scale is easily formed. 
Therefore, it is impossible to restrain the above oxidation completely 
even if the reduction property is increased and even if the heating 
temperature is lowered. For example, in the hot Zinc dipping, a steel 
material containing not less than 0.5% of Si is deteriorated in plating 
quality since wettability of galvanizing bath is prevented due to the 
above oxide scale formation. In the hot Aluminum dipping, the above 
problem is also caused in the case having not less than 2% content of Si. 
OBJECT OF THE INVENTION 
Accordingly, this invention has an object to provide a method of plating 
steel in which a high quality plating coat is easily formed. 
SUMMARY OF THE INVENTION 
In order to accomplish the above-mentioned object, the method of plating 
according to the invention comprises holding the steel works or articles 
in an atmosphere of fluorine- or fluoride-containing gas and then forming 
a plating coat on the fluorided works. 
In this invention, prior to forming a plating coat on the steels works or 
articles, the steel works are held in an atmosphere of fluorine- or 
fluoride-containing gas in a heated condition, whereby an oxidized scale 
such as SiOx and MnOx on the steel work surface is removed and at the same 
time a fluoride layer is formed so that the surface of the steel works is 
covered with the fluoride layer. The fluoride layer generally comprises a 
thin film having thickness of about several tens to several thousands.ANG. 
and is stable at 300.degree. C. to 600.degree. C. and prevents newly 
forming an oxidized scale. Since the fluoride layer is more porous 
compared with the oxide layer formed at the same temperatures and has rich 
sublimation, it is easily removed with the action of an added flux such as 
chloride and/or A1 in the plating bath of the next step. As for the 
removal of the fluorinated layer, the steel works with the fluoride layer 
formed thereon are introduced into an atmosphere of an inert gas 
containing hydrogen or an atmosphere containing moisture (water) and 
heated to remove the layer, prior to dipping in the above-mentioned 
plating bath. Since the steel surface after removing the fluoride layer is 
activated, its conformability with a plating bath and the like is improved 
to realize formation of a good quality plating coat. 
This invention is described in detail below. 
In this invention, steel works are charged into a heating and cooling 
furnace and held in an atmosphere of fluorine- or fluoride-containing gas 
in a heated condition, and then a plating coat is formed thereon. 
Subjects to be plated are steel works. The steel works include various 
steel materials such as a carbon steel or stainless steel. The shape and 
the like of the steel works are not limited particularly. It may be in a 
shape of plate or sheet, coil, and screw which is processed. The steel 
materials used in this invention include not only a single material but 
also alloy which is composed of iron as a main composition and other 
metallic materials. 
Fluorine- or fluoride-containing gas used for making a fluorine- or 
fluoride containing gas atmosphere means a gas in which fluorine-source 
component comprising either a single member selected from NF.sub.3, 
BF.sub.3, CF.sub.4, HF, SF.sub.6, F.sub.2, C.sub.2 F.sub.6, WF.sub.6, 
CHF.sub.3, SiF.sub.4, and the like or a mixture thereof is contained in an 
inert gas such as N.sub.2. In addition to the above-mentioned gasses, as 
the fluorine- or fluoride-containing gas, F.sub.2 which is formed by 
decomposing compounds which include F such as BF.sub.3, CF.sub.4, HF, 
SF.sub.6, C.sub.2 F.sub.6, WF.sub.6, CHF.sub.3, SiF.sub.4, with a 
thermal-cracking apparatus may be used. When the compound such as BF.sub.3 
to be cracked is used, a cracker is disposed in front of or in the 
vicinity of the heating and cooling furnace to thermal-crack the compound. 
Then the cracked and produced F.sub.2 is mixed with N.sub.2 and the 
mixture is introduced into the furnace. Fluorine- or fluoride-containing 
gas used in this invention include F.sub.2 produced by cracking as 
discussed above. 
Among the fluorine source components mentioned above, NF.sub.3 is the most 
suited for practical use since it is superior in safety, reactivity, 
controlability, and ease of handling. A mixed gas composed of, for 
example, 1% NF.sub.3, gas+5% F.sub.2 +94% N.sub.2 (by weight) is also 
practically used. In such a fluorine- or fluoride-containing gas, 
concentration of a fluorine component such as NF, and the like is set 1 to 
15%, preferably 2 to 7% by weight (weight standard: hereinafter the same) 
from the view point of efficiency. The atmosphere of fluorine- or 
fluoride-containing gas is formed by filling such a gas or a mixture in a 
predetermined space. 
According to the invention, the steel material is plated, for example, as 
follows. A steel material heated at 650.degree. C. to 750.degree. C. is 
introduced to a cooling step and then introduced into an atmosphere of 
fluorine- or fluoride-containing gas and held at 300.degree. C. to 
600.degree. C. therein for one to eight minutes, preferably two to three 
minutes. By holding the steel in this atmosphere, oxides such as SiOx,. 
and MnOx on the surface of the steel material are eliminated and a 
fluoride layer is formed thereon. Then the steel material with the 
fluoride film formed on its surface is dipped in a plating bath such as a 
Zinc plating bath or an Aluminum plating bath or the like to form a 
plating coat. In this case, the fluorinated steel works are not dipped in 
the plating bath directly, but the works may be contacted with a mixed gas 
composed of hydrogen and nitrogen (H.sub.2 ; 90 to 10%+N.sub.2 ; the rest) 
to remove the fluoride layer and then they are dipped in the plating bath. 
In a batch of process, the amount of NF.sub.3 to obtain a good quality 
plating coat is about 50g to 700g per ton of the steel material.

EXAMPLE 1 
An experimental heat treatment furnace 1 as shown in FIG. 1 was used. In 
the figure, reference numeral 20 is a furnace body having adiabatic walls, 
21 is a heater disposed by burying in a peripheral direction of the 
furnace body 20 and 22 is a slidable door disposed at the bottom of the 
furnace body 20 and the door 22 slides in right and left direction as seen 
in FIG. 1. At a ceiling of the furnace body 20 is connected a conduit 23 
for introducing a gas into the furnace 20. The reference numeral 24 
indicates steel articles to be treated. Under the furnace 20 is disposed 
Zinc pot furnace 25 which is partitioned by the slidable door 22. In the 
Zinc pot furnace 25, an induction coil 26 is disposed by burying in 
surrounding walls as shown in FIG. 2, and Zinc liquid is filled therein 
having temperature of 480.degree.C. 
In this furnace, a few samples of steel works (cold-rolled steel material: 
1% Si, 0.2% Mn) were heated to 750.degree. C. in an N.sub.2 atmosphere, 
then cooled to lower the temperature to 350.degree.C., then the samples 
were blown a fluoride containing gas (N.sub.2 +5%NF.sub.3) in the furnace 
for one minute and held therein for two to five minutes. After that, a 
N.sub.2 gas was blown thereinto, temperature was raised to 500.degree. C. 
in an N.sub.2 atmosphere and then one of the samples was taken out of the 
furnace for an elemental analysis. The rest of the samples were 
transferred into the Zinc pot furnace 25 by opening the slidable door 22 
and dipped in a zinc bath to galvanize them with Zn. After galvanizing 
with Zn, they were taken out of the furnace 25 and at the same time blown 
thereon with N.sub.2 gas, then cooled and dried, whereby galvanized 
samples were obtained. After supplying the fluoride-containing gas 
composed of N.sub.2 +5% NF.sub.3 into the heat treatment furnace and 
holding the samples for two to five minutes therein, a mixed gas composed 
of N.sub.2 +25% H.sub.2 was supplied therein and under the atmosphere the 
samples were heated to 500.degree. C. and held for ten minutes. Then the 
samples were transferred into the Zinc pot furnace 25 to galvanize them. 
The same galvanized samples were given as the above-mentioned. 
EXAMPLE 2 
Steel samples were heated to 750.degree. C. in an N.sub.2 atmosphere and 
then cooled to 500.degree.C. At this temperature the samples were held in 
an atmosphere of fluorine- or fluoride-containing gas having same 
composition as in the Example 1. Except this, galvanized samples were made 
as well as in the Example 1. 
EXAMPLE 3 
A mixed gas composed of 1% NF.sub.3 +5% F.sub.2 +94% N.sub.2 was used as 
the fluorine- or fluoride containing gas. Except that, galvanized samples 
were made as well as in the Example 1. 
COMATIVE EXAMPLE 
The same samples used in the Example 1 were used and they were heated at 
750.degree. C. in an N.sub.2 +50% H.sub.2 atmosphere, then cooled to 
500.degree. C. and dipped in a zincking bath as well as in the Example 1 
without fluorination,. Except this, galvanized samples were obtained as 
well as in the Example 1. 
An elemental analysis in a depth direction in the outermost surface layer 
part was conducted by SIMS (secondary ion mass spectrometer) against an 
ungalvanized sample (what are in a step just before galvanizing) obtained 
in the Examples 1, 2 and the Comparative Example. The results were shown 
in FIGS. 3, 4 and 5. In these drawings, straight line A shows Fe, straight 
line B; Si, straight line C; Mn. The FIG. 3 corresponds to the Example 1, 
the FIG. 4 to the Example 2 and the FIG. 5 to the Comparative Example. 
That is, "concentration" of Si and/or Mn (Concentrating degree of Si 
and/or Mn is relatively high.) was not seen in the outermost outside 
surface layer part in the Examples 1 and 2 as clearly seen in FIGS. 3 and 
4. On the contrary, in the Comparative Example shown in FIG. 5, oxidation 
of the most outside surface layer occurred to form oxides such as SiOx and 
MnOx on the surface layer, whereby it was seen that concentration of Si 
and Mn in the surface layer becomes relatively high, we define this 
condition as "concentration." 
A bending part was examined by conducting an adhesion bending test for 
examining adhesiveness of a plating coat on each sample formed in the 
Examples and the Comparative Examples. As a result, blister of the plating 
coat was not seen on the bending part in the Examples 1 and 2, while that 
of the plating coat was largely caused on the bending part in the 
Comparative Example and bad adhesiveness of the plating coat was observed. 
The sample obtained in the Example 3 showed the same structure and the 
same plating coat properties as those obtained in the Example 1. 
It is obvious from the above-mentioned results that it is possible, 
according to the invention, to conduct a high quality plating such as a 
hot zinc dipping of high Si containing steel materials to which it was, in 
particular, hard to plate conventionally, and that it is also possible to 
produce a steel sheet or a steel plate which needs high strength. In 
addition, an economical merit is given by-productively, which allows to 
decrease largely or to cut completely H.sub.2 gas for reduction used in a 
conventional annealing process. In the Examples, an experimental furnace 
apparatus was used for plating, but it is possible to carry out by 
utilizing a ready-made equipment by improving a cooling step equipment 
partially for an actual continuous hot zinc dipping line. 
Effect of the Invention 
As mentioned above, in the method of plating steel according to the 
invention, since a plating Coat is formed on the steel surface after 
holding the steel material in an atmosphere of fluorine- or 
fluoride-containing gas in a heated condition, an oxide layer of the 
surface is removed and a fluoride layer is formed thereon to cover and 
protect the surface. The steel material with the fluoride film formed is 
dipped in a plating bath directly or introduced in an inert gas atmosphere 
containing hydrogen and heated to thereby decompose and remove the 
fluoride layer, and then dipped in the plating bath. In case of dipping in 
the plating bath directly, the fluoride film is decomposed and removed by 
an action of an added flux such as Chloride and A1 added to the plating 
bath. In case of dipping in the plating bath after introducing the steel 
work into the hydrogen-containing inert gas atmosphere, since the fluoride 
layer is removed prior to dipping in the plating bath, the plating 
treatment can be done swiftly. In this way the fluoride layer is removed 
and an activated steel surface is exposed after being removed. Therefore, 
the plating coat is well-adhered and formed preferably on the activated 
steel surface to give a high quality plating.