Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
5~2
-- 2 --
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a high strenq-th steel plate
which is low in yield ratio and improved in strength-elongation
balance as well as in stretch flangeability, and a method for
manufacturing same. The high strength steel plate of the
present invention is particularly suitable for use as a material
for the wheel disc and/or wheel rim of a motor vehicle.
~2) Descri~tion of the Prior Art
Recently, various attempts are made in order to
improve the mileage of motor vehicles, including the reduc-tion
of the vehicle body weight which is considered to be most
effective. With regard to the reduction of the vehicle body
weight, there have thus far been made many proposals concerning
; the use of high strength steel plates and aluminum alloys along
with the reductions in size. Among these proposals, the reduc-
tion of the vehicle wheel weight is one of the most effective
means for improving the mileage, and the possibilities or
application of a hi~h strength steel plate to the wheel rim or
disc have been a subject of intensive ~tudies. The high
strength steel plates which have been proposed for this purpose
include the composite structure steel plate tdual phase steel
plate of ferrite ~ martensite, which i5 low in yield ratio
and has higher elongation as compared with the strength
showing excellent properties in formability and shape
1 fixability, However, the steel plate of this sort is inferior
in stretch flangeabi.lity so that, if applied to the ~ehicular
wheel disc or the like, it gives rise.to problems such as:
(1) the occurrence of cracking at an expanded hole
portion in the disc forming operation; or
(2) the occurrence of cracking at an expanded hole
por-tion in the fatigue test or in the running
test,
The present inventors have studiea in detail the
:10 relationship between the steel structure and stretch flange-
ability for the improvement thereof, and as a result found
that a steel plate of a ~errite + bainite structure is super-
ior to a dual phase steel plate of ferrite ~ martensite in
the stretch. flangea~ility, However, the steel plate of the
ferrite ~ ~ainite structure has a drawback that it is inferior
in the strength-elongation balance,
Further~ the wheel rim requires the resistance
weldability in addition to the stretch flangeability which
is required by the wheel disc, Another problem which is
encountered in applying a high strength steel plate to the
wheel rim is the cracking which takes place at a high rate
in the roll-forming operation subsequent to the 1ash butt
welding, the cracks occurring at a rate as high as about 50%
in the thermally affected zones in the forming stage~ Such
a high rate o cracking is detrimental to actual applications,
.~'~" ,
5~5~:
-- 4
1 SUMMARY OF THE INVENTION
The present invention aims at rational elimination
of the above-men-tioned drawbacks and problems of the prior
art. More specifically, ,it is a primary object of the present
invention to provide a hiyh stxength steel plate which has
a low yield ratio and good strength-elongation balance
characteristic to the dual phase steel plate of ferrite ~
martensite along with the excellent stretch flangeability
comparable to that of the ferrite + bainite steel, and a ~ethod
for manufacturing such high strength steel plates.
It is another object of the present invention to
provide a high strength steel plate which, besides the above-
mentioned properties, possesses excellent resistance weldability,
and a method for manufacturing same.
Still another object of the present inventicn is to
provide a high strength steel plate which is particularly
suitable for use as a material for wheel discs and/or wheel
rims of motor vehicles, and a method for manufacturing same.
According to one aspect of the present invention,
the above-mentioned objects are achieved by a high strength
steel plate which has low yield ratio and excellent strength-
elongation balance and stretch flangeability, the steel plate
containing 0O01 - 0.2% of C, 0.3 - 2.5% of Mn and 0.01 - 1.8%
of Si and having a tripple phase structure o polygonal
ferrite, bainite and martensite with a bainite areal rate o
5~
- 5-1 -
1 4-45~ and a martensite areal rate of 1 - 15%, the bainite
areal rate being greater than the martensite areal rate.'
According to another aspect of the present invention,
there is provided a method for producing a high strength
steel plate as staked above, which method comprisiny:
(1) subjecting a steel containing 0.01 - 0.02% of C, 0.03 -
2.5~ of Mn and 0.01 - 1.8% oE Si to a hot rolling-cooling
treatment selected from the group consisting of;
(i) hot rolling the steel at a fininshing roll
temperature above Ar3 point, followed by
coolin.g the hot rolled steel from the fini-
shing rolling temperature to a temperature
range between point Ar3 and point Ar~ at an
average cooling speed of 3 - 70C/sec,
(ii) hot rolling the steel at a finishing rolling
temperature above Ar3 pointj followed by
- cooliny the hot rolled ~teel from the finish-
ing rolling temperature to a temperature
range between point Ar3 and point Arl at an
average cooling speed of 3 ~ 70C/sec and
then by air cooling ar slow cool.ing for 2 -
20seconds from the temperature range of ~r3
to Arl, and
(iiij hot rolling the steel at a finishing rolling
temperature range of Ar3 to Arl, followed by
air cooling or slow cooling the hot rolled
steel for 2 - 20 ~econds from the temperature
range of Ar3 to Arl, thereafter
- 5-2 -
1 (2) cooling the steel thus subjected to hot rolling-
cooli.ng treatment to a temperature below 550C at an
average cooling speed not lower than 20C/sec, and
(3) taking up the cooled steel.
Further embodiments of the present invention
will become apparent from the particular description
of the invention which follows.
- BiRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention
: and many of the attendant advantages thereof will be
readily attained as the same becomes better understood by
~ reference to the following detailed description when taken
; in conjunc*ion with the accompanying drawings, in which
~0
5~
1 E~IGURE 1 is a diagram showing the relationship between
tensile s-trength and -total elongation and the relationship
between tensile strength and yie].d stress;
FIGURE 2 is a diagrarn showing -the relationship be-tween
tensile strength and hole expanding limit;
FIGURE 3 is a diagram showing the relationship between
martensite area rate and bainite area rate;
F-rGuRE 4 is a diagram showing the relationship between
the bainite area. rate and yield ratio and the relationship
between the bainite area rate and the hole expanding limit;
FIGURE 5 is a diagram showing the relationship between
the martensite area rate and hole expanding limit and the
relationship between the martensite area rate and the yield
ratio;
FIGURE 6 iS a dia~ram showing the relationship bet~een
the average diameter of martensite particle size and the hole
expanding limit;
FIGURE 7 is a diagram showing the hardness distribu-
tion in a weld portion after flash-hutt welding; and
FIGURE 8 is a diagram conceptionally showing the
method of the present invention.
DESCRIP'rION OF PREFERRED EMBODIMENTS
The term "bainite~' as used in the present invention
means mainly the bainitic ferrite but includes the bainite
which partly contains the so-called accicular fPrrite or a
5~S~
-- 7 --
1 a carhide~ The -term "mar-tensite" also includes partly -
retained austenite~
It is important in the present invention that the
composite structure consists of tripple phases of polygonal
ferrite, bainite and martensite. More particularly, as known
from FIGURE 1 depicting an embodiment which will be described
hereinlater, the yield ra-tio is minimum with the ferrite -
~martensite steel and maximum with the ferrite + bainite steel.
The yield ratio is lowered as the polygonal ferrite is intro-
duced into the ferrite ~ bainite steel, and it is furtherlowered to a value comparable to that of th`e ferrite ~
martensite steel when a small amount of martensite is intro-
duced to for a tripple--phase structure of polygonal ferrite +
bainite + martensite.
Similarly to the yield ratio, the tripple-phase steel
of polygonal ferrite + bainite + martensite shows a good
balance of strength-elongation which is akin to the value of
the ferrite + martensite steel, as shown in FIGURE 1.
With regard to the hole expansibility (the index of
the flanging extensibility), the steel of ferrite ~ martensite
is the worst and the ferrite + bainite steel is the best as
shown ln FIGURE 2. On the other hand, the tripple-phase
steel of polygonal ferrite + bainite + martensite shows a
high value of hole expansibility approximate to the value of
the ferrite + bainite steel.
Turning now to the fatigue strength, the tripple-phase
~ 5~5~
1 steel of polygonal ferrite ~ bainite ~ martensite shows a
value similar to the ferrite -~ hainite steel and is superior
to the ferrite + martensite steel.
As known from the foregoing date, the tripple-
phase steel of polygonal ferrite + bainite + martensite simul-
taneously possesses on].y the advantages of the ferrite +
martensite steel and the ferrite bainite steel, and is
excellent in a~l of the properties of the strength-elongation
balance, stretch flangeability and fatigue strength.
It is also known from these examples that, in the
tripple-phase s-teel structure according to the present
invention, the area rate of bainite should be in the range of
4 - 45% since a rate over 45% results in a drop in th~ effect
of lowering the yield ratio due to introduction of martensite
and a rate below 4% makes no dif~erence from the ferrite ~
martensite steel. The area rate of bainite is preferred to
be in the range of 6 ~ 35%.
More particularly, referring tc FIGURE 4 which shows
the relationship between the bainite area rate and the yield
ratio and the relationship between the bainita area rate and
the hole expansibility which is the index of the stretch
flangeability, plotting the pattern III of Example 4 ~denoted
by mark ~ and Example 5 (denoted by mark "O") which will
be described hereinlater. A material for ~he wheel disc and/or
wheel rim of the motor vehicle is required ~o have a stretch
flangeability hîgher than 150%, prefexably higher ~han 160~
5~
1 in the forming stage, with a yield ratio lower than 0.7 and
preferably lower than 0.6. As clear ~rom the experimental
data of FIGURE 4, intensive studies conducted by the present
inventors revealed the value of the bainite area rate suitable
for the yield ratio and the stretch flangeability which are
required for the material to be used for the vehicular wheel
disc and/or wheel rim. According to FIGURE 4, the bainite
area rate should be in the range of 4 - 45%.
FIGURE S plots the relationship between the martensite
area rate and the stretch flangeability and the relationship
between the martensite area rate and the yield ratio in the
specimens 46, 47 and 53 to 57 which will be explained herein-
later. Similarly to the bainite area rate mentioned above,
the martensite area rate should be held in the range of 1 ~ 15%n
As ~lear from FIGURE 5, the yield ratio is increased if the
martensite area rate exceeds 1S% and it becomes di~ficult to--
attain hole expansibility greater than 150%. On the contrary,
with a martensite area rate smaller than 1%, the effect of
martensite introduction becomes smaller. The martensite area
rate should be in the range which guarantees a hole expansi-
bili-ty over 160% and a yield ratio below 0.6, namely, in the
range of 1 - 10%.
As seen in FIGUR~ 3 which plo-ts ~he correlation
between the martensite area rate and the bainite area rate in
relation with the hole expansibility, more particularly, the
pattern III of Example 4 (denoted by mark "~") and Example 5
-- 10 --
1 (denoted by mark "O") which will appear hereinlater, the bainite
area rate in the tripple-phase steel structure according to the
present invention should be greater than the martensite area
rate in order to secure hole expansibili.ty greater than 150%,
in additi.on to the above-defined conditions that the bainite
and martensite area rates should be respectively in the ranges
of 4 - 45% and 1 - 15~ The range which satisfies these
conditions is indicated by hatching in FIGURE 3.
FIGURE 6 is a plot of the relationship between the
particle size of the martensite and the hole expansibility,
more particularly, a plot of the results of experiments of
Table 20 using the composite steel structures of Table 19.
The hole e~pansibility is also dependent on the average dia-
meter of the martensite as shown in FIGURE 6. More specifi-
cally, this figure shows that the hole expansibility is further
improved by making the martensite finer even if the area
rates of the bainite and martensite are in the ab~ve~defined
ranges, namely, that the hole expansibility becomes greater
than 150~ when the average particle size of the martensite
is smaller than 6 microns. The hole expansibility is further
improved as the grain size is reduced to a value smaller than
5 microns and improved in a greater degree with a value smaller
than 4 microns.
It will bè appreciated from the foregoing description
that, with the tripple-phase steel structure acc~rding to the
present invention t it iS possible to guarantee a 1QW yield
s~s~
-
--ll--
l ratio along with excellent strength-elongation balance and
stre-tch flangeability by controlling the area rates of the
bainite and martensite, and that the stretch flangeability
can be further enhanced by making the mar-tensite finer.
As already s-tated hereinbeEore, one problem which
is encountered when applying a steel plate to a vehicular
wheel rim is the softening of the -thermally influenced por-
tions after the flash-butt welding, which occurs conspicuously
degree in the ferrite + martensite steel as a result of
decomposition of the martensite, the thermally influenced
portions giving rise to cracks in the subsequent roll-forming
stage in such a degree as to make the application of the
ferrite + martensite steel utterly difficult,. The softening
of the thermall~ influenced portion, however, is not observed
in the steel of the bainite structure, and the problem of
cracking in the cold-rolling (roll-forming) subsequent to
the welding operation is precluded, In order to overcome the
drawback of the ferrite + bainite structure steel, the present
inyentors conducted a comprehensi~e study varying the pro-
20 Portions of the ferrite + ~ainite + martensite structures in
relation with.the chemi.cal components involved, As a result,
it ha$ been found that, as shown in FIGUR~ 7, the steel of
; ferrite + martensite structure has an excellent softening resist-
ance similarly to the ferrite + bainite structure steel, even in
95~52
- 12 ~
1 the case of spot welding. It has also been revealed that a
slightly sof-tened state can be established by letting a
stable precipitant like NbC or the like exist in the thermally
affected zone or more positively leaving Nb in solid solu-
tion of after the hot-rolling for precipitating same in the
thermally affected zone, thereby preventing the initiation of
cracking from the thermally influenced portion in the succeed-
ing roll-forming operation.
The adjustment of the steel structure is attained b~
controlling the cooling condition during and after the hot-
rolling or by further controlling the annealing conditions
either (either continuous or batch annealing3 in the subsequent
stage, in connection with the chemical components which are
discûssed hereinafter.
The limitations of the respective chemical components
in the present invention are based on the following reasons.
The component C is an element which is essential for
maintaining the required streng~h and for forming the low
temperature transformation products like bainite and marten-
site but its content should be limited since a C-content in
excess of 0.2~ will cause a considerable deterioration in
ductility and impair the weldability (giving rise to a drop
in hardness of butt faces due to decarburization in the butt
welding, resulting in a large difference in hardness between
the weld line and the adjacent portions). In a case where
forming workability is required in particular, its content
- 13 -
1 should desirably be less than 0.09%. The lower limit of the
C-content should be 0.01 in order to secure the effects of
strengthening the structure and enhancing the hardenability.
The element Mn is necessary for improving the hardena-
bility and obtaining the desired structure. The improvemenk
in hardenability also contributes to the increase of streng-th
and to the enhancement of mechanical properties by stabilizing
the y-phase during the y-~ transformation (y-aus-tenite, a=
ferrite) after the hot-rolling. In order to secure these
effects, its content should be 0.3~ or more. However, with a
Mn content in excess of 2.5%, it causes a welding difficulty
and impairs the ductility ~elongation and stretch flangeability)
and the weldability in addition to a substantial increase of
the cost of the steel plate. Therefore, the upper limit should
be placed at 2.5%.
The element Si which is necessary for deoxidation of
the molten steel is also very effective as a substitutional
solid solu-tion hardening element. Therefore, it is essential
in order to obtain a steel plate with high strength and
ductility. Besides, it acts advantayeously to the formation
of clean polygonal ferrite. In the composite steel structure
as of the present invention, it accelera-tes the a-transforma-
tion during the y-a transformation after hot rolling and acts
to discharge the carbon in solid solution out of ~phase
and shift into y-phase. Consequently, it enhances the clean-
liness of a--phase and sta~ilizes y-phase by condensing carbon
5~L52
1 thereinto, thus facilitating the formation of a hard phase
which contributes to the improvement of the mechanical pro-
perties. In order to produce these effects while preventing
enbrittling oE the weld (an increase of transition tempera-
ture), the lower limit of the Si cont~nt should be placed at
0.01%. On the other hand, its upper limit should be placed
at 1.8% to prevent deteriorations of surface condition due
to production of oxidation scales.
According to the present invention, the following
elements may be included if desired in addition to the above-
mentioned elements.
The elements Cr, Cu, Ni and B are useul for improv-
ing the hardenability as well as for obtaining a desired
structure. The lower limits of their oontents should be
placed at a level which is sufficient enough for ~ecuring
these effects, while the upper limits should be placed at a
level whexe their effects are saturated and uneconomical.
More specifically, the steel plate according to the present
invention may contain at least one element selected from
the group consisting of 0.1-1.5% of Cr, 0.1 - 0.6~ of Cu,
0.1 - 1.0% of Ni and 0.0005 - 0O01% of B. Further, the element
Mo which also serves to improve the hardenability and produce
a desired structure similarly to the above-mentioned Cr, Cu,
Ni and B may ~lso be included in an amount of 0.01 - 0.2%
for the same reasons.
The elements Nb, V, Ti and Zr which serve to strengthen
9S~5Z
- 15 -
1 the precipitation are necessary not only for increasing the
strength but also for ~acilita-ting the forrnation of the
bainite structure by imposing an inflwence on the transform-
ing structure under coexistence with Mn or the like after
hot-rolling. Further, they make the structure finer and
serves to improve the stretch flangeability and to prevent
drops in hardness, improving not only the fatigue strength
of the parent plate but also the fatigue strength of the ~isc
as a whole. Moreover, they contribute to produce ~he hardena-
bility~-improving effec-t of B to a maximum degree. In order
to have these effects, it is necessary to include at least
one element selected from the group consisting of 0.01 - 0.16
of Nb, 0.0~ - 0.2~ of V, 0.01 - 0.1% of Ti and 0.02 - 0.2%
of Zr.
In addition, the component Nb particularly has an
influence on the transforming behaviors after the hot-rolling
and is most effective for the formation of the bainite struc-
ture. The elements Ti and Zr are further effective for con-
trolling the shape of the sulfide which is harmful to the
ductility, and the element V is effective for moderately
hardening the center portion of the weld (Hv ~ 25) as
compared with that of the parent material.
The rare earth metals (REM), Ca and Mg contribute
to the improvement of the ductility, particularly, the stretch
flangeability by controlling the shape of the sulfide. The
lower limits of their contents should be placed at a level
which is suEficient for producing that effect. The lower
- 16 -
1 limit is determined at a value at which the aimed effect becornes
saturated or uneconomical or in consideration of the content
which inversely impairs the cleanliness. More specifically,
the steel plate of the invention may include at least one
element selected from the group consisting of 0.005 - 0.1% of
a REM, 0.0005 - 0.01~ of Ca and 0.0005 - 0.01% of Mg. However,
it is desired that the total additive amount i5 not more than
about 0.1% since an excessive additive amount is rather
harmful to the cleanliness and lowers the ductility.
Furthermore, Al is added in an amount of 0.005 - 0.6%
to serves as a deoxidi~er at the melting stage. If desired,
P may be added in a range which would not cause enbrittling
at the grain boundary. Similarly to Si, the element P is
a strong hardening component and has an effect of purifying the
ferrite, contributing to the improvement of elongation or
other properties. In order to have these effects, it should
be added in an amount of 0.03 - O~
The element S may be contained in a range which is
normally permitted for an impurity element, namely, in a range
]ess than 0.02%. An S-content less than 0.02% can produce
the effec~ of improving the formability; especially the stretsh
flangeability, and the ductility of the weld to a satisfac-
tory degree.
Now, the ba~ic concept of the method according to
the present invention is explained with reference to FIGU~E 8.
In FIGURE 8, plotted at ~ and ~ is a method according
~:~9~
- 17 -
1 to the in~ent.ion, in which a steel slab of a predeterrnined
composition is, after heat treatment at Tl, subjected to
continuous hot rolling, terminat.ing the hot rolling at a
temperature higher than the level T2 (corresponding to the
point Ar3). The rolled material is then cooled from 'che
finishing rolling temperature to a point between temperature
levels T2 and T3 (corresponding to the point Arl) at a
controlled cooling speed Cl. Thereafter, in I the material
is cooled to T4 (take-up temperature~ at the cooling speed
0 C2 and taken up at a tem.perature below T4. In the case of
the material is let for a while for air cooling or slow
cooling between the just-mentioned temperature levels T2 and
T3 and then immediately cooled off to the tempera-ture level
T4 (take-up temperature) at a cooling speed of C2 and taken
up at a temperature below the level T4.
In another method which is plotted.at ~ of
FIGURE 8, a steel slab o~ a predetermined composition is
subjected to continuous hot rolling after a heat treatment
at the temperature Tl, terminating the hot rolling at a point
between the temperature levels T2 and T3. Thereafter, the
material undergoes the air cooling or slow cooling, cooling
(at cooling speed C2) and take-up (at T4~ i.n ~he same manner
as in the above-mentioned method ~
In the method shown in FIGURE 8, the controlled
cooling (at C~ to a point between Ar3 and Arl in the pattern
the controlled cooling (at Cl) and the temperature and time
~ 5~
- 18 -
1 of the air or slow cooling in the pattern ~ , or the finishing
rolling to a point be-tween ~r3 and Arl and the temperature
or time of the following air or slow cooling in the pattern
are important as a preparatory stage for obtaining a
desired composite steel structure. The metallurgical mechanisms
which are involved in these three manufacturing patterns are
as follows.
The cooling stage from the hot rolliny finishing
temperature to the poînt in the range from Ar3 to Arl in the
pattern ~ is a region where mostly the polygonal ferrite
phase (a) and the austenite phase ~y) coexist, so that carbon
in solid solution of ~-phase is condensed into y-phase by
employing relatively slow cooling in that stage, thereby
stabilizing the ~-phase and improving the ductility through
purification of the ~-phase which contains less carbon in
solid sollltion~ Thereforej the average cooling speed Cl which
is employed for cooling to a temperature range from Ar3 to Ar
subsequent to the finish rolling at the tempexatuxe above the
point Ar3 should be 3-70C/sec. If the cooling speed Cl exceeds
70C/sec, it becomes diEficult to obtain an amount of the ~-phase
of a desired structure and to control the temperature appropri-
ately. On the contrary, with a cooling speed lower than 3C/sec,
there occurs Eerrite trans~ormation or pearlite transmation,
inviting a drop in productivity due to the lengthy cooliny
time. Therefore~ the cooling speed Cl is preferred to be in
the range of 3-30 CJs~c.
L5;~
-- 19 --
1 Similarly to Pattern ~, the aver~ge cooling speed Cl
from the rolling finish temperature to the temperature range of
Ar3 to Arl in Pattern ~ should be 3-70C/sec and is particularly
desired to be 20-70C/sec for the followiny reasons. Namely, al-
though the above mentioned metallurgical mechanisms take place
by effecting relatively slow cooling in Pattern ~ in cooling the
material from the rolling finish temperature to the level between
Ar3 and Arl, air cooling or slow cooling in Pattern ~ is e~fected
in the temperature range of Ar3 to Arl which is in the vicinity
of the ferrite transformation nose, in order to obtain the
mechanisms as mentioned above.
Since the air cooling or slow cooling is effected
in the vicinity of the ferrite transformation nose, a
predetermined amount of ferrite can be ob-tained .in a short
time period due to the accelerated ferrite transformation,
and carbon in solid solution of the ferrite phase which has
! transformed during the slow cooling is condensed into the
austenite phase. As a rPsult, the amount of carbon in solid
solution of the ferrite phase is reduced, enhancing the purity
and ductility. Namely, as the above-mentioned mechanisms
are obtained by the air cooling or slow cooling in the temperature
range of Ar3 to Arl, in Pattern ~ , it-is desirable to ~ffect
the cooling from the hot rolling finish temperature to the
temperature range Ar3 to Arl at as high a speed as possible,
in contrast to Pattern ~. On the other hand, the austenite
phase with an increased am~unt of carbon is stabilized to
facilitate the formation of the low temperature transforma-
tion products in the s~sequent cooling stage. The time
. .
52
- 20 -
l period of the air or slow cooling should not be too short in
order to obtain the desired amount of ferrite nor too lony
to avoid the ferrite transformation in the entire steel
structure or the pearlite transforrnation. Further, since
it is limited by the length of the run-out table, the time
period of the air cooling or slow cooling should be 2 - 20
seconds.
Now, as shown in Pattern ~ , the controlled cooling
~Cl) is not required when the finish rolling is carried out in
the temperature range of Ar3 to Arl. In this instance, the
fixtish rolling temperature is preferred to be higher than
710C. Although the air cooling or 510w cooling from the
finish rolling temperature is effected after termination of
the finish rol~ing in Pattern ~ , the metallurgical mechanisms
and the time of air cooling or slow cooling are the same as
in Pattern ~ .
The cooling at C2 subseguent to the cooling of
Patterns ~ to ~ is intended for converting the austenite
into a hard low-temperature transformation product and should
be e~ected at an average cooling speed (C2) higher than
20C/sec, preferably in the range o~ 30 - 70C/sec. With a
lower cooling speed C2, the austenite is transformed into
pearlite in part or in its entirety. On the con~rary, a
cooling speed higher than the above-defined range results
- in a lower strength-ductility balance and a higher yield
ratio.
~S^ll 5~
- 21 -
1 The reason why a hard phase is formed at such a
relatively low cooling speed C2 is the enhancemen-t of stability
of the austenite which takes place in the meantime, so that
the transitional stage from the finishing rolliny temperature
to the initiation of the C2 cooling is important. Further,
the fact that the hard phase is mainly made of the bainite
phase permits the cooling at a relatively low speed C~ and
contributes to the improvement of the strength-ductility
balance:
Thereafter, the steel plate is wound up at a predeter-
mined temperature and this take-up temperature (T4) constitutes
another important point of the present invention. More
particularly, it is desirable to effect the cooling to room
temperature in order to form martensite, which however in
turn brings about various defects such as a drop in the
strength~ductility balance due to the existence of carbon
in solid solution remaining in the course of cooling to the
tempexature T~ or a rise of the yield ratio, coupled with the
above-mentioned shortcomings of the ferrite ~ martensite
steel~ Therefore, it is preferred to wind up at A temperature
above 300C for obtaining the desired structureO The upper
limit of the take-up temperature T4 should be place at 550C
since pearlite transformation takes place at temperatures
above 550C unle5s -the alloy elements are added in large
quantities.
1 The hot-rolled steel plate which is obtained by the
above-described me-thod is of a tripple phase structure con-
sisting of polygonal ferri-te, bainite (i.e., the carbide-
including bainite and ~ainitic ferrite) and martensite (partly
containing retained austenite) and particularly has bainite
phase areas at a rate of 4 - 45% and martensite areas at a
rate of 1 15%, the area rate of the bainite phase being
greater than that of the martensite phase. This steel plate
is in any of the properties of yield ratio, strength-elonga-
tion balance, stretch flangeability and resistance weldability.
The range of the chemical composition of the steel
plate which is intended by the method of the invention, the
preferred ranges of the area rates of its bainite and
- martensite phases and the particle size of the martensite
are same as defined hereinbefore.
Hereafter, the steel structure and its manufacturing
method of the invention are illustrated more particularly by
way cf a nun~er of examples.
Example 1
The materials of the chemical compositions shown in
Table 1 were each melted in a vacuum melter, roughly rolled
into a 30 mm thick slab and then into a 4 mm thick plate by
3-pass hot rolling.
The materials which were cooled to room temperature
by air cooling subsequent to the hot rolling were heated to
a temperature in the range of 7B0 - 950C for 5 - 10 minutes
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1 and then cooled under different conditions to ob~,ain specimens
of different structures.
The results of the microscopic observatio~ and
measurement of the resective specimens are shown in Table 2
along wi-th the results of measurement of the mechanical
properties. Wit,h regard to the results of measurement, the
relations of the tensile strength with the yield stress,
total elongation and hole expansibility are plotted in FIGURES
l~and 2. As stated hereinbefore, the specimen Nos. 1 to 7
which have a steel structure according to the present inven-
tion show excellent properties in the yield ratio, strength-
~longation balance and stretch flangeability.
Table 3 shows the properties and the results of
fatigue test of the specimens which were same as the
specimen No. 6 in the chemical composition of the starting
material but formed into different structures by varying the,
heating and cooling conditions. As seen therefrom, the
steel according to the invention shows an excellent property
also in the fatigue strength.
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1 Nextly, a steel plate was produced Erom a specimen
which was obtained by adding 0.1~ of Ce to Specimen No. 2,
hot-rolling, cooling and taking up the specimen under the
conditions under the conditions shown in Table 4. The thus
obtained steel plate was subjected to a wheel disc forming
test of actual scale ~n = 20). Table 5 below shows the
results of the m.icroscopic observation and measurement, along
with the results of measurement of mechanical properties and
the rate of defective wheel discs in forming operation. As
seen therefrom, the steel according to the present invention
is extremely low in the rate of defective wheel discs.
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1 Example 2:
The materials of the chemical compositions shown in
Table 5 were each melted in a vacuum melter, roughly rolled
into a 30 ~n thick slab and then into a 4 l~m thick plate by
3-pass hot rollingO
The materials which were cooled to room temperature
by air cooling were heated at the temperatures shown in
Table 7 for 5 minutes and then cooled under diferent condi-
tions to obtain specimens of different structures.
The results of microscopic observation and measure-
ment of the thus obtained specimens are shown in Tables 7 and
8, along with the results of measurement o~ mechanical pro-
perties. As mentioned hereinbefore, the steel specimens 20 to
22 according to the present invention were invariably low
in the yield ratio and excellent in strength-elongation
balance and stretch flangeability.
In this Example, it is particularly to be noted
that the components which play a main role in the steel
structure of the invention are Si and Mn. Namely, in a case
where the Si and Mn components are in the above-defined
ranges and the ratio o~ Si/Mn is greater than 1, the yield
ratio, strength-elongation balance and hole expansibility
of the tripple phase steel structure of the invention are
improved to a considerable degree. The rea~ons for this
phenomenon is not known at the present stage but are
considered as ~ollows.
~5~
1 (1) The amount and length of the sulfide inclusions
are increased by the addition Si. In view of the larye
content of Mn, the ratio of Si/Mn is desired to be greater
than 1Ø
(2) The component Si has a higher hardening effect
than Mn and lowers the stacking fault energy, delaying the
formation of cells. As a result, the cell size becomes
finer to permit higher elongation and reduction.
(3) The component Si accelerates the condensation
of C from ferrite to y-phase, thereby purifying the ferrite
and makiny a large difference in hardness between the ferrite
and the low temperature transformation product to permit a
high elongation.
.
Table 6 - Chemical Compositions (%)
_ _._ _
Spe imen C Si Mn S Other elements
_ .
0.06 1.3 1.1 0.007
21 0.06 1.3 1.0 0.006 Nb 0.02, V 0.03, Cr 0.6
22 0.07 1.3 1.0 0.004 Cr 0.85 REM 0.08
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- 33 -
1 Example 3:
The materials of the chemical compositions shown
in Table 9 were each melted in a vacuum melter and roughly
rolled into a 30 mm thick slab and then into a 3.4 mm thick
steel plate by 3-pass hot rolling.
Further, the materials which were cooled to room
temperature by air cooling were quickly hea-ted to 950C and,
after retaining that temperature for several minutes, they
were cooled under different conditions to obtain specimens
of desired st~uctures.
The conditions of -the heat treatment and the results
of the measurement of the microscopic structures are shown
in Table 1OD
Table 9 ~ Chemical Compositions
¦ P No ¦ C Si Mn S ¦Other elements
23 0.05 0.4 1.5 0.005 Cr 0.8
Inven-
24 0.04 0.5 1.6 0.003 Cr 0.5 Ce 0.003 tion
Nb 0.02
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1 A material containing 0.04% of Ce in addition to -the
composition of the specimen No. 23 and a material of the sarne
composition as the specimen No. 24 were prepared by melting
the materials on the spot, followed by blooming and hot
rolling, and cooled and wound up under the conditions shown
in Table 11. The resulting steel plate specimens were formed
into ordinary wheel rims of an actual si~e by flush-butt
welding and rolling forming. The microscopic structures,
mechanical properties and wheel rim formability of the respec-
tive steel plates are shown in Table 12.
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1 Further, the results of examination of the hardness
distribution afker flush-butt welding of these hot-rolled
steel plates are shown in FIGURE 7. It will be seen there-
from that the softening occurs in a considerable degree in
the heat affected zone of the F ~ M steel structure (Specimen
No. I) owing to the decomposition of the second phase marten-
site.
On the other hand, in the F + B + M steel structure
according to the present invention (Specimen Nos. II and III),
the softening is leassened conspicuously and, in the presence
of NbC, a slight degree of hardening is observed in the heat
affected zone instead of softening. Consequently, there is
no possibility of the breakage initiating from the heat
affected zone in the roll-forming stage of the F + B ~ M
steel structuxe subsequent to the flash-butt welding. This
can be confirmed by reference to Table 2 which shows the rate
of defective wheel rims.
Example 4:
Slabs of 30mm in thickness were obtained by melting
in a high frequency vacuum melter the steel materials of
different compositions as shown in Table 13, followed by
forgeing and rough rolling. After heating to 1200C, the
slabs were finished into 3~2 mm thick steel plates by 3-pass
hot rolling employing a variety of tempera~ures above the
point Arl and then taken up at different tempera~ures below
600C. Table 14 shows the conditions of hot rolling of these
- 39 -
l steel plates along with the results of the observation of
microscopic structures. In Table 15, there are sho~n the
mechanical proper-ties of the steel plates of Table 14 and
the values of tenacity and variations in hardness after
flash-butt welding under the following conditions.
Welding Conditions:
Flash margin: 3 mm
Flash time: 3 seconds
Upset margin: 3 mm
Upset time: 2/60 seconds
Upset speed: 150 mm/sec
Blank size: 30mm(w) x 75mm(Q) x 3.2mm(t)
In Table 15, "Y.P./T.S." ~yield ratio) is used as
an index for judging ~he Eormability and a lower value means
a higher shape fixability or workability. On the other hand,
"YPE" (yield poi~t elonyatiDn) indicates the presence or
absence o~ wrinkles in those portions which are subjected to
tensile stress by working, and the value of the yield point
elongation should be as small as possible in order to prevent
~he wrinkling.
The term "TSXEQ" (strength-elongation balance)
indicates the balance between the strength and ductility,
and a higher value of TSXEQ means a better balance. JThe hole
expansibility (~) is an index of the stretch flangeability
and a higher value reflects a better stretch flangeability.
- 40 -
1 With regard to the flash butt weldiny, the terrn "vEs"
(upper shelf energy) and "vTrs" ~charpy V-notch transition
temperature) are indexes of the weld tenacity~ which is better
when higher in the value of vEs and lower in the value of
vTrs. The symbolic expression "~Hv" indicates the hardness
of the weld bounding portion - the hardness of the parent
material, and "aHv" the hardness of the welding heak affected
~one - the hardness of the parent material. The value of
"~Hv" should not be too high since otherwise cracking occurs
during the roll-forming operation due to a drop in ductility.
On the other hand, a disjoint takes place if the value of
'~Hv" is low
As seen from Table 15, the steel plates produced by
the method of the present invention are-low in the yield
ratio with no yield point elongation and have good strength-
elongation balance. Besides, the are excellent in the stretch
flangeability as well as in the tenacity of the weld, showing
a smaller increase of the hardness of the weld bounding portion
and a smaler dxop in the hardness of the weld heat affected
zone, thus as a whole exhibiting excellent resistance welda-
bility.
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1 Example 5:
The s-teel material of the composition shown in Table
16 was melted in a high frequency vacuum melter and hot-rolled
in the same manner as in Example 4, varying the cooliny
speed and -the taXe-up temperature to obtain intended steel
structures. Table 17 below shows the conditions of the hot
rolling and the results of the observation of microscopic
structures of the hot rolled steel plates, while Table 18
shows their mechanical structures.
Table 16 - Chemical Composition-(wt~)
_ _
Steel C Si Mn p S Cr Al Others
_
G 0.10 0.2 1.3 0.008 0.005 0.7 0.025 Ce 0.007
.
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1 It will be also obvious from the foregoing results
that the steel plates produced by the method of the present
invention are low in the vield ratio and conspicuously improved
in strength-elongation balance as well as in stretch flangea-
bility.
Example 6:
The steel specimens of the compositions shown in
Table 19 were prepared, employing the conditions of heat
treatments shown also in Table 19. The mlcroscopic structu.res
and mechanical properties of the resulting steel plates shown
in Table 20. As clear therefrom, the steel plates produced
by the method of the invention all show a 150~ or higher hole
expanding limit.
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