Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a process for
producing high strength cold rolled steel sheets having
improved paintability, weldability and workability. Steel
strips as well as cut steel sheets are collectively referred
to hereinafter as "sheet".
In recent years~ more and more high strength cold
rolled steel sheets have been used for automobile car bodies.
For tha production of high strength cold rolled
steel sheets, processes utilizing continuous annealing have
been predominantly adopted. However, in these production
processes, a large amount of elements such as C, Mn, Si, P
and Cr must be added to the steel for improving the strength.
When these elements, for example, C, Mn and Si, are
contained in the steel in a large amount, the weldability of
the steel is adversely affected by these elements, such that
the strength of the welded portion is deteriorated or the
fatigue of the welded portion is increased. Further, excessive
contents of these elements also produce undesirable effect on
the paintability (paint adhesion) of the steel when paint
~coating is applied to final products from the steel.
For all these reasons, demands have been increasingly
made for new processes which can produce high strength cold
rolled steel sheets having excellent workability with less
contents of these elements.
It is therefore an object of the present invention
to provide a process for producing high stren~th cold rolled
steel shee-ts which can meet the above demands.
The basic technical feature of the present invention
lies in that the adding of strength improving elements which
produce adverse effect on weldability and paintability is
lowered as much as possible, and the primary cooling after
~.~
soaking in a final continuous annealing is performed at a
controlled cooling rate.
~ In accordance with the invention, there is provided
a process for producing a high strength cold rolled steel sheet
having improved paintability and weldability, which comprises
the steps of:
hot rolling a steel containing 0.05 to 0.12% C,
no more than 1.2% Si, 0.7 to 1.5% Mn, 0.04 to 0.15% P, 0.01%
to 0.10% soluble Al, with thë balance being Fe and unavoidable
impurities,
; cold rolling the hot rolled steel sheet, and
subjecting the cold rolled steel sheet to a continuous
annealing by soaking at a temperature ranging from 730 to 850C
for a period ranging from 20 seconds to 2 minutes and rapidly
cooling the sheet at a cooling rate ranging from 30 to 300C/
second.
The process of the present invention enables to
overcome the various disadvantages of conventional processes
and at the same time to produce a high strength cold rolled
steel sheet having a tensile strength of 50 to 80 kg/mm and
yet well balanced between strength and ductility.
Carbon, silicon and manganese are effective to
strengthen the steel, and particularly carbon and manganese
must be contained in amounts not less than 0.05% C and 0.7%
Mn, respectively.
However, excessive contents of these elements produ-
ce adverse effects on weldability and paintability of the
resultant steel, and for this reason, the upper limits of
these elements should not exceed 0.12% C, 1.2% S~ and 1.5% Mn,
respectively.
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~4Z~6~3
Phosphorus is the most desirable element for giving
strength to the steel, because it can effectively give
strength to the steel without substantial adverse effects on
weldability and paintability, cont~ary to carbon, manganese and
silicon. However, phosphorus contents of less than 0.04% will
not produce the desired strength. Therefore, the phosphorus
content should be at least 0.04%. On the other hand, when the
` phosphorus content is excessive, the halance betwe4n strength
and ductility is deteriorated and at the same time the weld-
~` 10 ability is lowered, although the adverse effect is not so
great. Accordingly, the upper limit of the phosphorus content
should not exceed 0.15%.
Aluminum is necessary for the deoxidation of the
steel, and less than 0.01% Al is not enough for this purpose~
On the other hand, aluminum contents of no more than 0.10%
are enough for the purpose.
Regarding other elements which may be added, if
necessary, a smaller content is more preferable
Steel slabs having the chemical composition as
defined above are prepared by the conventional continuous
casting process or ingot-breaking process, and these steel
slabs are hot rolled, cold rolled and then subjected to a
continuous annealing treatment according to the present inven-
tion. Regarding the coiling temperature in the hot rolling
step, a higher temperature is better, and it is preferable to
adopt a coiling temperature of, for example, 650C or higher
for obtaining the desired strength-ductility balance.
The continuous annealing according to the present
invention comprises one of the following two heat cycles.
The first heat cycle consists of heating - soaking - rapid
cooling, and the second heat cycle consists of heating -
--3--
soaking - rapid cooling to overageing treatment tem~erature -
overageing treatment.
In the first heat cycle, a rapid heating is desirable
from the points of productivity, etc., and for this purpose,
a jet stream heating system is most preferable.
Regarding the soaking, it is performed at a tempera-
ture range of from 730 to 850C for a period ranging from 20
seconds to 2 minutes, because if the temperature is lower than
730C and the period is shorter than 20 seconds, no satisfacto-
ry recrystallization and grain growth can be achieved, hencethe desired ductility cannot be assured. On the other hand,
when the soaking temperature is higher than 850C, it is
excessively high and the strength-ductility ~alance is destroyed.
The soaking time may be longer, but a longer soaking time
requires a longer length of the furnace, which is economically
inattractive. For this xeason, the upper limit of the soaking
time is generally set at 2 minutes.
After the soaking has been completed~ the steel sheet
is subjected to a rapid cooling. The optimum cooling rate for
the rapid cooling ranges from 30 to 300C/second. At a cooling
rate below 30C/second,the amount of the alloying elements
mentioned hereinbefore must be increased in order to obtain
the desired strength of the resultant steel, however, the
increased amount of the alloying elements induces deteriora-
tion of weldability and paintability. On the other hand, when
the cooling rage is higher thar~ 300/second, the amount of
martensite formed by the rapid cooling increases, and deterio~
ration of the strength-ductility balance occurs. For all
these reasons, the cooling rate for the rapid cooling effec-ted
after the soaking has been completed is limited to the range of
from 30 to 300C/second In this way, in combination with the
--4--
- ~9 9L2~
specific steel composition set forth above, a high strength
cold rolled steel sheet well balanced between strength and
ductility and having excellent weldability and paintability
can be obtained most economicallyO
The cooling rate as defined above is difficult to
achieve by conventional cooling methods, such as water quench-
ing, gas screening and water jet, but can be easily achieved
by blowing a vapour-liquid mixture onto the steel sheet. The
advantage of this cooling method is that a uniform cooling
effect across the width of the steel strip can be obtained
in spite of a rapid cooling, so that the shape of the steel
strip can be improved and a uni~orm quality of the material
can be assured. A further advantage of this cooling method
is that it enables to control the cooling terminal point at
a desired strip temperature. This is very advantageous for
performing the second heat cycle.
~ In the application of the first type of heat cycle,
the steel sheet after the soaking is first slowly cooled from
the soaking temperature ranging from 730 to 850C to a tempe-
20 rature ranging from 720 to 650C at a cooling rate of no more
than 20C/second and then rapidly cooled at a cooling rate
ranging from 30 to 300C/second. In this way, a further impro-
ved strength~ductility balance can be obtained, although the
strength is slightly lowered.
The overageing treatment effected at a temperature
ranging from 500 to 300C in the second heat cycle is to precipi-
tate the carbon in solid solution in the steel, and to improvs
the ductility of the steel. According to this embodiment, the
rapid cooling effected at the cooling rate from 30 to 300C/
second after the soaking has been completed is terminated at
the overageing temperature, so that the steel sheet is succes-
~4~
sively subjected to the overageing treatment without thenecessity of reheating.
In this way, the heat cycle of cooling to room
temperature-reheating can be avoided so that the distribution
of carbon precipitates is markedly improved and fine precipi-
tates of carbon do not appear in the ferrite grains, but
relatively large precipitates of carbon appear in the grain
boundaries. Under this condition, a satisfactory ductility
can be obtained by the overageing treatment.
In the application of the second type of heat cycle,
the steel sheet is first slowly cooled from the soaking tempe-
rature ranging from 730 to 850C to a temperature ranging from
720 to 650C at a cooling rate of no more than 20~C/second and
then rapidly cooled to the overageing temperature ranging from
500 to 300C at a cooling rate ranging from 30 to 300C/second.
In this way a further improved strength-ductility balance can
be obtained, although the strength is slightly lowered.
As described above, the two types of heat cycle can
be adopted in the present invention. When more importance is
placed on the low yield ratio (yield point/tension strength),
the first type of heat cycle is used and, when more importance
is placed on the strength-ductility balance, the second type
of heat cycle is performed.
With respect to the overageing treatment time, it
may be the same as conventionally used and a period ranging
from 30 seconds to 5 minutes is appropriate.
The present invention will now be illustrated by
the following non limiting examples.
Steel slabs having compositions as shown in Table 1
were prepared and hot rolled into hot rolled steel strips of
2.5 mm in thickness, and then cold rolled into cold rolled
-6-
~L2~
steel strips of 0.7 mm in thickness~ The cold rolled steel
strips thus obtained were subjected to a continuous annealing
under conditions involving various cooling rates with or
without overageing treatment, as shown in Table 20
In the cases where the overageing treat~ent was
performed in one group, the cooling after the soaking was
effected to xoom temperature, and after reheating the over-
ageing treatment was performed; in the other group, the cooling
: after the soaking was terminated at the overageing temperature
and the overageing treatment was done without reheating there-
before. The weldability and ~aintability of the resultant
steel sheets are shown in Table 1.
In Table 1, steels A and B have a similar chemical
composition, but the cooling terminal point was controlled and
the overageing treatment (e~ was performed in the case of
steel A, and the overageing treatment (d) was done after re-
heating in the case of steel B. As clearly shown by comparing
steel A with steel B, steel A has a lower yield point and a
larger elongation thàn steel B. This indicates that the cool-
ing terminal point control is more advantageous.
In the cases of steels C and D, the composition was
adjusted so as to produce a tensile strength of 60 kg/mm2
respectively when cooled at a rate of 30C/ second (b) and cooled
by immersion in water (h); steel C is shown to have a better
yield point and elongation than steel D.
In the cases of steels E and F, the composition was
adjusted so as to produce a tensile strength of 70 kg/mm2
respectively when slowly cooled at 10C/second (a) and rapidly
cooled at 100C/second (c).
As an increased amount of alloying elements was
required under the continuous annealing condition (a), both
20~
paintability and weldability are poor.
Lastly in the cases of steels G and H, steel G wasrapidly cooled at 300c/~econd and steel ~ was rapidly cooled
at 350C~second Sg). As shown, the elongation is low in steel
H due to the excessively high cooling rate.
Steels I and J represent embodiments of the present
invention in which the steel sheets after the soaking were
slowly cooled to the rapid cooling starting temperature of
690C at a cooling rate of 10C/ second; the resultant steel
sheets of I and J show better ductility and improved strength-
ductility balance as compared with steels A and E which also
represent embodiments of the present invention, although the
strength is slightly lowered.
As will be well understood from the foregoing embodi~
ments A, C, E, G, I and J of the present invention, a cold
rolled steel sheet well balanced between strength and ductility
and having excellent paintability and weldability can be
obtained in accordance with the present invention.
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Continuous Annealing Conditions
: I
.
Soaking : . ITerminal I Overageing Conditions
' Conditions j Coolln,g I'i'emperat~re~ I ,
Temp (C) I Cn/dltln)Slof Cboling ¦Reheating I Temp.~C)xTime ,
x Time(sec.)l sec- j (C) l l (sec.)l
_ I ! I '
a ~ 10 Room _ _
Temparature~
':~, c . 100 ~
1 d . 100 ¦ Yes 1 400 x 60
. l
e ~ 750 x 60 100 400 no 400 x 60 .
.~ f 300 Room _ _
. 350 Temperature _
~ Water 20 _ _
? h Immersion
?
Mbre ~han _
~ 690C/10
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Less than Temperature
. 690C/100
~ ,
Mbre than
~` i 690C/10 400 no 400 x 60
Less than
6900C/100 ~
.
.
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