Note: Descriptions are shown in the official language in which they were submitted.
L78~L8~
Ti-tle of the Invention
A process for preparing aluminum-plated steel sheets
having low yield strength and high oxidation resistance
T hnical Field of the Invention
This invention relates to a process for preparing
molten-aluminum-plated steel sheets which have low yield
strength and exhibit low oxidation weight gain when
subjected to oxidation at high temperatures.
Back~round of the Inventio
Heretofore, molten-aluminum-plated steel sheets
(hereinafter simply referred to as "aluminum-plated steel
sheets") for use wherein heat resistance and corrosion
resistance are required have been mainly made of cold-
rolled sheets of low carbon rimmed steels. However, it
1.5 is well known that aluminum-plated steel sheets made of
rimmed steel substrate sheets incur degradation in quality
due to quench aging caused by rapid cooling at -the time of
plating, which hardens the material.
The main measures that can be employed to prevent
the above mentioned degradation are:
(1) To use substrate sheets made of a steel from which
C and N, which cause quench aging, have been removed as
completely as possible;
(2) To add a carbide-forming element such as Ti to fix
C and N in the substrate material, which cause quench
aging;
(3) To ef~ect over-aging of the aluminum-plated sheets
in which quench aging has occurred;
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and so forth.
S~owever, the measure (1) is not economical in the
case when steel sheets are plated by the in-line annealing
type hot dip plating apparatus with a non oxidizing furnace,
although it can be realized in the ordinary steel-making
process by employment of decarburizing annealing.
The measure (2) is economical per se, but when
it is applied to low carbon steels which are to be obtained
by the ordinary converter process only, a considerable amount
of Ti must be used and highly oxidizable Ti produces not a
small amount of oxide inclusion which results in degradation
of the surface quality of the product. Therefore, this
measure is not, in the f.inal analysis, either economically
or technically desirable.
The measure (3) is economical. But this treat-
ment may increase the Fe-Al alloy layer, which has poor
formability, even if the al~lminum pla-ting has been carried
out under the conditions that control formation of the Fe-Al
alloy layer. Therefore, by this treatment, although the
property of the substrate material is improved, the
formability of the aluminum-plated layer is impaired.
The assignee of this application previously
obtained a Japanese patent for an invention on a process
for preparing aluminum-plated steel sheets comprising:
?5 hot-rolling a steel essentially consisting of C: 0.001 -
0.020 %, Si: ' 0.05%, Mn: 0.05 - 0.40 ~0, Cr: 0.10 - 0.30 %,
effective Ti ttotal Ti less Ti in the oxide form): 0.03 -
0.40 ~ and that not less than 4 times (C ~ N) %, N: ~ o.oo6 %~
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o: c 0.020 ~, and the balance of Fe and inevitable
incidental impurities at a temperature not lower than
800C; cold-rolling the hot coil at a reduction rate of
40 ~ or more; annealing the rolled sheet at 800 - 950C,
and immersing the sheet in an Al-Si alloy bath (Si: ~ 10 %)
maintained at 640 - 700C for not more than 10 minutes
(Japanese Patent Publication No. 35532/76).
The aluminum-plated steel sheet obtained by
this process has a yield point strength of 13 - 17 kg/mm2
and an elongation of 44 - 47 % in the state of a cold-
rolled sheet of 0.8 mm thickness. Today, however, demand
for more easily formable materials is rising.
Disclosure o~ the Invention
We have studied ways fo]^ meeting this demand
and have arrived at the idea of employing the following
three measures to obtain improved aluminum-plated steel
sheets which have good formabilit~ with low yield strength
and are very low in the high temperature oxidation weight
gain.
(1) To lower the content of C and 0 in the
substrate steel sheet by employing vacuum degassing
treatment and preliminarily deoxidizing with Al in the
stage of steel-making and thus reducing the Ti con-tent which
is necessary to fix C and N and controlling formation of
oxides.
(2) To reduce the content of Mn which raises
yield strength and to add Cr which lowers yield strength.
(3) To promote agglomeration and growth of the
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precipitated Ti carbide by coiling the finished hot coil
at a temperature as high as 700C and thus to preventing
hardening by formation of carbide of the Ti added to the
substrate material. And to further promotion of agglomeration
of the Ti carbide by heating the cold-rolled sheet at a
temperature of 850C or more when it is passed through
an in-line annealing type hot dip plating apparatus with
a non-oxidizing furnace (which is commonly called NOF type
- plating apparatus in Japan).
Additionally, the invention of this application
is characterized in that high temperature oxidation
resistance of the material is improved by combined addition
of Ti and Cr. That is, oxidation weight gain at high
temperatures is remarkably reduced by decarburizing the
substrate material, and it is further improved by addition
of Ti. The reasons are~ (1) By decarburizing or addition
of Ti, cleanliness of the iron material is improved, and
Al of the aluminum layer easily diffuses into the Fe substrate
and an Al diffusion layer, having excellent high temperature
oxidation resistance is formed; and (2) The Ti in the material
diffuses toward the surface and forms a ~i-concentrated
layer under the Al diffusion layer when the material is
subjected -to high temperature, and thus prevents the further
diffusion of Al into the interior, thereby retarding decrease
in the Al concentration in the surface layerl and also fixes
oxygen which has penetrated into the iron.
~ hus according -to this invention a process for
preparing molten-aluminum-plated steel sheets having low
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yield streng-th and high resistance to high-temperature
oxidation comprising: producing a steel the chemical
composition of which essentially consisting of C: 0.001 -
0.020 %, Mn: 0.05 - 0.30 ~, Cr: 0.05 - S%, Al: 0.01 -
0.10 %, Ti: 0.10 - 0.50 % and that not less than 10 times
the precentage of C, the balance consisting of Fe and
inevitable incidental impurities, by the ordinary
converter-refining and vacuum degassing; making it into
a slab by the ordinary casting and slabbing or the ordinary
continuous casting; continuously hot-rolling said slab
coiling it at a temperature not lower than 700C; cold-
rolling the resulting hot coil after the ordinary pickling
treatment; heating the cold-rolled sheet at a temperature
not lower than 850C; and plating it with molten aluminum
by means of an in-lining annealing type hot dip plating
apparatus with a non-oxidizing furnace is provided.
In t`he pre~erred embodiment, the C content is
0cOOl - 0.010 %, the Mn conten-t is 0.05 - 0.20 %, the
Cr content is 0.07 - 0.45 %, the Al content is 0.02 -
0.05 %, and the Ti content is 0.15 - 0.40 % and not less
than 20 times the C content.
In the more preferred embodiment, the C content
is 0.001 - 0u007 %, the Mn content is 0.10 - 0,17 %, the
Cr content is 0.07 - 0.42 ~0, the Al content is 0.03 - 0.041 %,
and the Ti content is 0.19 - 0.23 % and not less than 30
times the C content, the hot coil coiling temperature is
720 - 730C and the heating temperature at plating is
860 - 900C.
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The reasons for the numerical limitations
defined in the main claim are as follows.
The lower the carbon content is, the more the
effect of quench aging is reduced. Therefore, i-t is
desirable to reduce the C content as much as possible.
But it is not easy to reduce the C content to less than
0.001 % even by the modern steel-making process in which
vacuum degassing is employed. Even if it can be achieved,
it is not an economical operation. Therefore, the lower
limit of the C content is defined as 0.001 %. The reason
why its upper limit is defined as 0.020 % is that if the
C content is over this limit, the amount of Ti to be added
for prevention of the undesirable effect of C inducing
quench aging must be uneconomically increased.
The reason for defining the Mn content as 0 05 -
0.30 % is that it is difficult to obtain a steel the Mn
content of which is less than 0.05 % by the ordinary steel-
makin~ process and when the Mn content exceeds 0.30 %, the
steel becomes hard and, as a consequence, has high yield
strength.
The reason for defining the Cr content as 0.05 -
0.50 % is that Cr in the amount of less than 0.05 % does
not give sufficient effect in reducing yield ratio, and
on the other hand, more than 0.50 % of Cr also reduces
said effect.
Al is used for deoxidation of the mol-ten steel
and, especially in this invention, it plays an important
role as the preliminary deoxidation material which prevents
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was-teful use of Ti. From this point of view, the lower
limit of the Al content is defined as 0.01 %. However,
if Al is added in an amount over 0.10 %, the surface
properties and formability of the resulting steel sheet
are impaired.
The reason why the Ti content is defined as
0.10 - 0.50 % and 10 times the C content is as follows.
If the Ti content is less than 0.10 %, the effect of
improving high temperature oxidation resistance as represented
by oxidation weight gain is not sufficient, although the
yield strength is rather low, On the other hand, if the
Ti content is in excess of 0.50 %, the material becomes
hard and loses its low yield strength characteristic,
although the oxidation weight gain becomes smaller. If the
Ti content is less than 10 times the C content, the fixation
o~`C with Ti is not sufficient, resulting in a rise in the
yield strength and an increase in the oxidation weight gain,
which eliminate the charac-teristics of this invention~
In the process of this invention, Si, P and S
as the inevitable impurities can be present to the extent
that is ordinary and common in steels of this kind. Nitrogen
and O can, without any inconveniences, be present at the
levels usually attained by the vacuum degassing process.
The reason why the coiling temperature is defined
as not lower than 700C is that at temperature lower than
this, softening of the material owing to agglomeration and
growth of the Ti precipitate, which has been formed by fixation
of C with Ti, is not sufficient, and thus one of the features
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of this invention is lost. Further it is required that
in the aluminum plating line the cold-rolled sheet must
be heated at not lower than 850C before entering the
plating bath in order to give the cold-rolled sheet an
annealing effect so that the Ti precipitate further
agglomerates to larger particles and thus softens the
material.
In the process of this invention, the composition
of the substrate steel is different from that of the substrates
steel of the method of the aforementioned Japanese Patent
Publication r~O. 3~532/76 only in that the steel of this
invention contains Al added intentionally. But the procedure
is completely different from -that of said invention.
Description of Embodiments of the Inven-tion
Now the invention is ex]?lained in detail specifi-
cally on the basis o~ the~results of experiments,
Steel samples, the compositions of which are
indicated in Table 1, were prepared by converter refining
and vacuum degassing. The ingots were subjected to slabbing
and continuous hot-rolling and the rolled products were
coiled at varied temperatures as indicated therein and
hot coils of 2.5 mm thickness were obtained. After ordinary
pickling, the hot coils were made into cold-rolled sheets of
0.8 mm thickness. The thus prepared cold-rolled sheets were
heated at varied temperatures and plated with aluminum (60g/m2)
by means of an in-line annealing type hot dip plating apparatus
with a non-oxidi~ing furnace (practically, a modified Sendzimir
apparatus), under the ordinary conditions. The aluminum-plated
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sheets were subjected to the material test and oxidationtest. The results (mechanical properties and oxidation
weight gains) are shown in the same table. The material
tests were carried out with specimens prepared according
to JIS (Japanese Industrial Standards) Z-2201 No. 5 Cllt
in the direction of rolling, and the oxidation test with
five repeated runs of a cycle of holding the samples at
830C in -the atmosphere for 48 hours and cooling to room
temperature.
In Table 1, samples D, E and F are within the
composition range defined in this invention and F is within
the scope of the invention in the heating temperature before
the plating. This table shows that materials excellent both
in formability and oxidation resistance are obtained only
when all three fac-tors of the composition of -the substrate
s-teel, the coiling temperature and the plating temperature
satisfy the conditions defined in this invention.
When sample A and B are compared, it is learned
that decrease in the ~ content contributes to lowering of
the yield strength. Comparison of B and C shows that
addition of Ti contributes to lowering of the yield
strength, too. Comparison of C and F shows tha-t combined
addition of Ti and Cr also contributes to lowering of the
yield strength.
2~ Table 2 shows mechanical properties of the
aluminum-plated steel sheet samples which were prepared
by obtaining the steels with the compositions indicated
therein and treating them in the same way as above.
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According to -this table, it is learned that Cr is effective
for lowering the yield strength when contained in the amount
of about 0.05 - 5 %
Table 3 shows mechanical properties and oxidation
weight gains of the aluminum-plated steel sheet samples
which were prepared by obtaining the steels the compositions
of which are indicated therein and treating them in the same
way as above. According to this table, it is learned that
when the amount of the added Ti is 0.1 % or more and that
more than 10 times the amount of C, the effects of lowering
the yield strength and oxidation weight gain are achieved.
But as seen in sample G, when the Ti content exceeds 0.5 %,
the yield strength again rises.
Table 4-2 shows the results of the same tes-ts as
above carried out with respect to -the samples prepared in
the same way as above from steels with -the compositions
indicated in Table ~-1. Accordln~ to -this -table, it is
learned that when the coiling temperature is lower than 700C,
the softening of the material is not sufficient, and when the
heating temperature before plating is 850C or higher, the
softening is remarkable.
Preferred embodiments of this invention are described
below.
Example 1
Ingots, the compositions of whlch are indicated
in Table 5 as sample No. 1 and No. 2, were obtained by
vacuum-degassing molten steel prepared by an LD converter
so as to reduce the contents of C and 0, and thereafter
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adjusting the composition by addition of ferro alloys such as
ferrochromium, ferrotitanium, ferromanganese, etc. The ingots
were made into slabs and the slabs were hot-rolled into
2.5 mm thick hot coils under the conditions indicated in
Table 6. After pickling, the hot coils were cold-rolled
into 0.8 mm thick sheets. The cold~rolled sheets were
plated with aluminum by means of an in-line annealing type
hot dip plating apparatus with a non-oxidizing furnace (a
modified Sendzimir apparatus) under the conditions indicated -
Table 6. The mechanical properties and oxidation weightgain of the thus obtained aluminum plated steel sheets as
tested in the same manner as above are shown in Table 6.
Both sheets exhibit excellent properties --- low yield
strength and high oxidation resistance at high temperatures.
1~ Example 2
Slabs, the compositions of which are indicated
in Table 5 as sample No. 3 and No~ L~, were obtained by
continuous cas-ting after smelting in the same way as in
Example 1. The slabs were made into aluminum-plated steel
sheets in the same way as described in Example 1. Mechanical
properties and the test results of the thus obtained steel
sheets as tested in the same way as in Example 1 are shown
in Table 6. These sheets have excellent characteristics
comparable to those of the sheets of Example 1.
2~ Industrial ApPlicability
The products of this invention are suitable for
manufacturing parts with complicated shapes used at high
temperatures such as exhaust gas treating apparatuses for
internal combustion engines.
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