Note: Descriptions are shown in the official language in which they were submitted.
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HIGH STRENGTH STEEL SHEET AND METHOD FOR MANUFACTURING SP,ME
FIELD OF THE INVENTION
The present invention relates to a high strength steel
sheet most suitable for automobile body, reinforcements, wheels,
and chassis parts and further for all kinds of machine structural
parts, and to a method for manufacturing same.
DESCRIPTION OF THE RELATED ARTS
For global environmental protection and further
improvement of the safety of passengers, automobile steel sheets
are studied to increase the strength and to decrease the thickness.
Since, however, increase in the strength of a material generally
decreases the press-formability, the widening of application
fields of high strength steel sheets faces an important issue
of increase in the formability.
A known response to the requirement is dual phase steel
sheet structured by ferrite and martensite as the main phases,
(the steel has several names of Dual Phase steel, DP steel, and
composite structural steel). Owing to low yield ratio,
(here_nafter referred to simply as YR), and high elongation, the
dual phase steel sheet.is superior in the press-formability such
as draw-forming property and surface precision after
press-forming (shape accuracy), thus the dual phase steel sheet
drew attention as an automobile material, and the development
thereof has been enhanced.
For example, the dual phase micro-structure in a hot-rolled
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steel sheet is achieved during the cooling step after hot-rolling
by transformation to pc>lygonal ferrite much enough to enrich a
solute element in the residual austenite, thus increasing in the
quench-hardenability due to the transformation to martensite.
The technology emphasizes the control of precipitated amount of
polygonal ferrite toform the micro-structure and to improve the
mechanical characteristics. Accordingly, various development
studies on the control of polygonal ferrite precipitation have
been given.
Patent Documents 1 through 11 disclose methods combining
with what is called the two-stage cooling process as a steel
composition design. The methods include the steps of: adding
large amount of ferrite-stabilizing elements represented by Si,
(and including P, Al, and the like) ; stopping cooling, in the
cooling step after hot-rolling, at near Al temperature where the
ferrite precipitation is accelerated; holding the temperature
for about 10 seconds; and applying cooling again.
Patent Documents 12 through 15 disclose manufacturing
methods to obtain desired steel sheet without adding the
ferrite-stabilizing element. That is, the methods adopt a
cooling-control pattern different from conventional method, for
example, dividing the rapid cooling after finish-rolling into
two st.ages.
Patent Documents 16 through 18 disclose methods to apply
rapid cooling immediately after the hot-rolling. In particular,
Patent Document 16 adopts the above-method for a low Si steel.
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Patent Document 1: JP-A-60-121225, (the term "JP-A" referred
to herein signifies "Japanese Patent Laid-Open Publication".)
Patent Document 2: JP-A-3-10049
Patent Document 3: JP-A-4-235219
Patent Document 4: JP-A-4-289126
Patent Document 5: JP-A-4-337026
Patent Document 6: JP-A-4-341523
Patent Document 7: JP-A-7-150294
Patent Document 8: JP-A-9-67641
Patent Document 9: JP-A-9-125194
Patent Document 10: JP-A-9-137249
Pa-ent Document 11: JP-A-10-195588
Patent Document 12: JP-A-54-065118
Patent Document 13: JP-A-56-136928
Patent Document 14: JP-A-3-126813
Patent Document 15: JP-A-4-276024
Patent Document 16: JP-A-2002-69534
Patent Document 17: JP-A-2001-192736
Patent Document 18: JP-A-2001-355023
Patent Documents 1 through 11, however, need to add excess
Si, P, and Al, though they show favorable mechanical
characteristics, thus they have problems of degradation in surface
property caused by red-scale formation, degradation in
coatability, and degradation in weldability. Consequently,
their applications are limited.
The steel sheets manufactured by the methods according
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to Patent Documents 12 through 15 contain small amount of Si,
P, and Al so that the cooling method in related art cannot fully
progress the transformation from austenite to ferrite on the
runout table after hot-rolling. As a result, the volume
percentage of polygonal ferrite becomes small, the volume
percentage of martensite becomes large, and the polygonal f errite
grains become coarse by the same reason, which fails to attain
adequate metallic micro-structure which is specified by the
present invention. Thus, the manufactured steel sheet shows
higher than 0.6 of YR in the mechanical characteristics, which
isaninferior characteristic. Toincrease the strain dispersion
and to improve the shape accuracy, YR is required to be 0.6 or
less.
As described above, the method for manufacturing
hot-rolled dual phase steel sheet according to the related art
adopts either the addition of ferrite-stabilizing element (Si,
P, Al, or the like) sacrificing the surface property and other
features or the sacriffication of mechanical characteristics.
Patent Documents 16 and 17, however, do not consider YR
and the metallic micro-structure to attain the YR.
Since Patent Document 18 is a technology to manufacture
a high Si steel, the surface property of the steel sheet becomes
poor. To improve the surface property, Si may be decreased. If,
however, the Si content is decreased, no adequate metallic
micro-structure is obtained, and the YR characteristic becomes
poor. Both the YR and the surface property cannot be satisfied
at a time.
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SUMMARY OF THE INVENTION
It is an object of the present invention to provide a high strength steel
sheet having
excellent formability (YR <_ 0.6) and excellent surface property through the
development of
a method for adequately controlling the metallic micro-structure and improving
the
mechanical characteristics ofthe steel sheet without adding excess ferrite-
stabilizing elements
(Si, P, and Al) which degrade the surface property, weldability, and the like,
and to provide
a method for manufacturing same.
The inventors of the present invention found a phenomenon which significantly
enhances the fine ferrite formation compared with conventional two-stage
cooling process,
even without adding excess ferrite-stabilization elements, by beginning the
ultra-rapid cooling
at 150 C/s or higher cooling rate within 2 seconds after the hot-rolling,
followed by holding
the temperatures between 750 C and 600 C for a specified period of time. The
inventors of
the present invention applied the finding to the manufacture of dual phase hot-
rolled high
strength steel sheet, and have perfected the present invention.
The present invention provides a high strength steel sheet consisting
essentially of:
0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01 % or
less S, 0.005%
or less N, 0.01 to 0.1 % Sol.Al, by mass, optionally containing at least one
element selected
from the group consisting of 0.01 to 0.3% Mo, 0.001 to 0.05% Nb, 0.001 to 0.1
% Ti, 0.0003
to 0.002% B, and 0.05 to 0.49% Cr, by mass, and the balance being Fe and
inevitable
impurities; and 60% or more polygonal ferrite by volume, and 5 to 30%
martensite by volume,
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wherein the polygonal ferrite has a mean grain size of 5 to 10 m, and the
high strength steel
sheet has a yield ratio of 0.6 or less.
The polygonal ferrite is preferably form 60 to 95% by volume.
The martensite is preferably from 10 to 20% by volume.
The high strength steel sheet preferably contains 0.01 to 0.5% Si by mass. The
Si
content is more preferably 0.25% or less by mass. Since Si has an effect to
increase the
strength, the Si content is preferably 0.01 % or more. The P content is
preferably from 0.020
to 0.06% by mass.
The high strength steel sheet preferably has a yield ratio of 0.6 or lower. If
the yield
ratio exceeds 0.6, the shape accuracy deteriorates during press-forming.
The present invention also provides a method for manufacturing high strength
steel
sheet comprising the steps of: casting a slab consisting essentially of 0.05
to 0.15% C, 0.5%
or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less
N, 0.01 to 0.1 %
Sol.Al, by mass, optionally containing at least one element selected from the
group consisting
of 0.01 to 0.3% Mo, 0.001 to 0.05% Nb, 0.001 to 0.1% Ti, 0.0003 to 0.002% B,
and 0.05 to
0.49% Cr, by mass, and the balance being Fe and inevitable impurities; hot-
rolling the cast
slab, directly or heating thereof, at a temperature of Ar3 point or more to
form a hot-rolled
steel sheet; a primary cooling step of cooling the hot-rolled steel sheet at a
cooling rate of
180 C/s or more to a temperature of from 750 C to 600 C, beginning cooling
thereof within
2 seconds after completing the hot-rolling; holding the cooled steel sheet at
a temperature
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between 750 C and 600 C for 2 to 15 seconds; a secondary cooling step of
cooling the
temperature-held steel sheet at a cooling rate of 20 C/s or more; and coiling
the cooled steel
sheet at a coiling temperature of 400 C or less.
The cooling rate in the primary cooling step is preferably in a range from 150
to
1000 C/s, and more preferably from 200 to 700 C/s.
The cooling rate in the secondary cooling step is preferably in a range from
20 to
1000 C/s.
The coiling temperature is preferably in a range from 0 C to 400 C.
The percentage for the ingredients of the steel, given in the description, is
% by mass.
The term "high strength steel sheet" referred to herein signifies a steel
sheet having
more than 590 MPa of tensile strength (TS), which TS values are suitable for
machine
structural parts.
The present invention provides high strength steel sheet having excellent
formability
and surface property. The steel sheet manufactured by the present invention
has low YR (0.6
or less with high strength, high ductility, excellent press-formability,
excellent surface
property, and excellent
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spot weldability, thus the steel sheet can readily be applied
to the automobile parts and machine structural parts. Since the
high strength steel sheet can be manufactured by the conventional
processfor manufacturing mild steel sheet, and since the attained
performancethereofisfavorable without addingspecialelements,
the manufacturing cost can be decreased. Accordingly, the high
strenc-th steel sheet according to the present invention is highly
expected in practical uses in the future, and is expected to
contribute to the conservation of global environment by the weight
reduct:ion of automobile and to the social development through
the improvement of safety of automobile.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a graph showing the relation between the yield
ratio (YR) and the primary cooling rate.
EMBODIMENTS OF THE INVENTION
The high strength steel sheet according to the present
invention specif ies the composition as below, specif ies the volume
percentage of polygonal ferrite to 60% or more, specifies the
volume percentage of martensite to a range from 5 to 30%, and
specifies the mean grain size of polygonal ferrite to a range
from 5 to 10 um. These specifications are the most important
conditions of the present invention. With the composition and
micro-structure specified above, the high strength steel sheet
havincexcellentformability and surface property can be obtained.
The high strength steel sheet can be manufactured by the sequential
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steps of: hot-rolling the steel at Ar3 pint or higher temperature;
beginning cooling of the steel sheet within 2 seconds after the
completion of hot-rolling;cooling the steel sheet to temperatures
between 7 50 C and 600 C at cooling rates of 150 C/s or more; holding
the temperature of steel sheet in a range from 750 C to 600 C
for 2 to 15 seconds; cooling the steel sheet at cooling rates
of 20 C/s or more; and coiling the steel sheet at 400 C or below.
In the manufacturing method, the beginning of cooling within 2
secondsafter completing the hot-rolling,the ultra-rapid cooling
at 150 C/s or higher cooling rate, and the holding in a temperature
range from 750 C to 600 C are also critical conditions of the
present invention.
The present invention is described in more detail in the
following.
First, the reason of specifying the chemical composition
of the steel sheet according to the present invention is described.
C: 0.05 to 0.15%
Carbon is an important element to strengthen the
marterisitic phase. To attain satisfactory strength, the C
content needs to be 0.050 or more. If, however, the C content
exceeds 0. 15%, austenite stabilizes, and the dual phase formation
becomes difficult, which degrades the ductility. Accordingly,
the C content is specified to a range from 0.05% to 0.15%.
Regarding the spot weldability, the C content below 0.07% may
degrade the tensile shear strength. If the C content exceeds
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0.10%, the cross tension strength may decrease. Therefore, the
C content is preferably in a range from 0.07 to 0.10%.
Si: 0.5% or less
Silicon degrades the surface property by red scale and
also degrades the coatability and weldability. If the Si content
exceeds 0.5%, the bad influence of Si becomes significant.
Consequently, the Si content is specified to 0.5% or less. If
the application of steel sheet emphasizes the surface property,
the Si content is preferably 0.25% or less. Since Si has an effect
to increase the strength, the Si content is preferably 0. 01 % or
more.
Mn: 1.00 to 2.00%
Manganese plays an important role for forming dual phase
micro-structure by suppressing the pearlite formation during
cooling after hot-rolling. If the Mn content is less than 1. 00%,
however, the effect is not sufficient, and pearlite is formed
to increase YR, thus degrading the press-formability. If the
Mn content exceeds 2.00%, austenite excessively stabilizes to
prevent the formation of polygonal ferrite. Therefore, the Mn
content is specified to a range from 1. 00 to 2. 00%. Furthermore,
the Mn content below 1.30% may decrease the strength so that the
Mn content is preferably 1.30% or more. When the Mn content
exceeds 1. 80%, the elorigation may degrade so that the Mn content
is preferably 1.80% or less.
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P: 0.09% or less
When the P content exceeds 0.09%, the elongation is
significantly degraded. Accordingly, the P content is specified
to 0. 09 % or less. If the P content exceeds 0. 06%, the toughness
at welded section degrades to decrease the joint strength in some
cases. Therefore, the P content is preferably 0.06% or less.
Furthermore, the P content of 0. 020 % ormore enhances the formation
of polygonal ferrite to decrease YR. Thus the P content is
preferably 0.020% or more.
S: 0.01% or less
Sulfur is an impurity in the crude steel and degrades the
formability and weldability of steel sheet as the base material.
Accordingly, it is preferred to remove or reduce S in the steel
making process as far as possible. Since, however, excess
reduction of S increases the refining cost, the S content is
specified to 0. 01% or less, which level brings the S substantially
harmless.
N: 0.005% or less
Nitrogen is an impurity in the crude steel and degrades
theformability of steel sheet as the basematerial. Accordingly,
it is preferred to remove or reduce N in the steel making process
as far as possible. Since, however, excess reduction of N
increases the refining cost, the N content is specified to 0. 005 %
or less, which level brings the N substantially harmless.
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Sol.Al: 0.01 to 0.1%
Aluminum is added for deoxidization and for precipitating
N as AIN. If the Al content is less than 0.01%, the effect of
deoxidization and denitrification becomes insufficient. If the
Al content exceeds 0.1%, the effect of Al addition saturates,
which is uneconomical. Consequently, the Sol.Al content is
specified to a range from 0.01 to 0.1%.
The steel according to the present invention attains the
desired characteristics by the addition of above essential
elemer_ts. Adding to the essential elements, however, the steel
according to the present invention may further include one or
more element of Mo, Nb, Ti, B, and Cr at need for further increasing
the st.rength. In that case, the respective contents of below
0.01%, 0.001%, 0.001%, 0.0003%, and 0.05% cannot give the
satisfactory effect of addition. If the content of Mo, Nb, Ti,
and B exceeds 0.3%, 0.05%, 0.1%, and 0.002%, respectively, the
formation of dual phase micro-structure is hindered and the
precipitation hardening becomes excessive so that the mechanical
characteristics degrade (YR increases or elongation decreases).
If the Cr content exceeds 0.49%, the performance of chemical
conversion treatment degrades. When these element are added,
there-fore, the Mo content is specified to a range from 0.01 to
0. 3 0, Nb from 0. 001 to 0. 05 0, Ti from 0. 001 to 0. 1%, B from 0. 0003
to 0.002%, and Cr from 0.05 to 0.49%.
The balance of the above composition is Fe and inevitable
impurities. Regarding the inevitable impurities, for example,
0 is preferably specified to 0.003% or less because 0 forms a
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non-metallic inclusion to degrade the quality. According to the
present invention, the steel may further include trace elements
which do not harm the function and use of the present invention,
namely Ni, V, Cu, Sb, Sn, Mg, and REM within a range of 0.1% or
less.
Secondly, the reason to specify the metallic
micro-structure according to the present invention is described
below.
The volume percentage of polygonal ferrite is specified
to 60% or more. The volume percentage of polygonal ferrite is
a critical condition to achieve the low YR characteristic which
is a feature of the present invention. To attain 0.6 or lower
YR, the volume percentage of polygonal ferrite is required to
become 60% ormore. The polygonal ferrite is found in the ferritic
phase, and is distinguished from the acicular ferrite in the
morphology, and is limited to the one having 5 or lower ratio
of maximum diameter to minimum diameter of the ferritic crystal
grain.
The volume percentage of martensite is specified to a range
from 5 to 30%. Similar with the volume percentage of polygonal
ferrite, the volume percentage of martensite is an important
condition of the present invention because the volume percentage
thereof influences the strength, the ductility, and the low YR
characteristic. If the volume percentage of martensite is less
than 5%, the strength becomes low, and no low YR characteristic
is attained. If the volume percentage of martensite exceeds 30%,
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the ductility degrades. Therefore, the volume percentage of
martensite is specified to a range from 5 to 30%. To attain better
low YR characteristic, the volume percentage of martensite is
preferablyin a rangefroml0to20%. The residualmicro-structure
contains acicular ferrite, bainite, pearlite, and the like. The
volume percentage of residual micro-structure is, however, not
specifically limited because the respective volume percentages
of polygonal ferrite and martensite within the above-specified
range assure the effect of the present invention.
For further improving the balance between the strength
and the ductility, or the product of strength and elongation,
the mean grain size of polygonal ferrite is preferably specified
to a range from 5 to 10 p m. Generally, the elongation in tensile
test i_s expressed by the sum of uniform elongation and local
elongation. If the grain size of polygonal ferrite is less than
5gm, the uniform elongation may decrease in some cases. If
the grain size of polygonal ferrite exceeds 10 m, the local
elongation degrades, though the value of local elongation is
withiriallowable range. Presumable reason of the phenomenon is
the following. For a dual phase steel, if the grains become coarse,
the deformation becomes nonuniform so that stress intensifies
into a certain section, which enhances the generation of
micro-cracks.
The method for manufacturing high strength steel sheet
having excellent formability and surface property according to
the present invention is described in the following.
The high strength steel sheet according to the present
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invention is manufactured by the steps of : casting a slab prepared
to havethe chemical composition given above; applying hot-rolling
to the slab, directly or heating thereof, at Ar3 point or higher
temperature; beginning cooling the slab within 2 seconds after
completing the hot-rolling to temperatures ranging from 750 C
to 600 C at cooling rates of 150 C/s or more; holding the cooled
slab at temperatures between 750 C and 600 C for 2 to 15 seconds;
applying cooling to the temperature-held slab at cooling rates
of 20 C/s or more; and coiling the cooled slab at temperatures
of 400 C or below.
The method for casting the slab is not specifically limited.
For the case of continuous casting, hot-rolling may be done
directly or may be done after reheating after cooling.
The hot-rolling is conducted at Ar3 point or higher
temperature. If the hot-rolling is done below the Ar3 point,
the hot-rolling proceeds in the dual phase region of ferrite and
auster.Lite, which hinders the formation of polygonal ferrite,
increases YR, and decreases the ductility.
After completing the hot-rolling, the cooling begins
within 2 seconds to cool the steel to a temperature range from
750 C to 600 C, which is the holding temperature range, at cooling
rates of 150 C/s or more. The primary cooling which is given
immediately after the hot-rolling is the most important condition
to attain the effect of the present invention, (the effect of
low YR attained bytheenhancementofpolygonalferriteformation)
With thus specified primary cooling and conduction of immediate
rapid cooling, the holding step at temperatures of from 750 C
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to 600 C succeeding to the primary cooling allows the fine
transformed polygonal ferrite to be drastically enhanced. If
the period between the completion of hot-rolling and the beginning
of cooling exceeds 2 seconds, ferrite is irregularly formed in
the austenite grain boundaries to hinder the transformation to
polygonal ferrite during the holding step after the cooling. If
the cooling rateislessthan150 C/s,theirregular precipitation
of ferrite in the austenite grain boundaries during the cooling
step cannot be prevented, which hinders the transformation to
polygonal ferrite during the holding step after cooling. For
further increasing the effect, the primary cooling rate is
preferably 200 C/s or more. If the primary cooling rate exceeds
1000 C:/s,the metallicmicro-structure becomesnonuniform within
the sheet thickness range, and the mechanical characteristics
may degrade. Accordingly, the primary cooling rate is preferably
1000 C:/s or less, and more preferably 700 C/s or less.
After completing the primary cooling, the steel is held
to a temperature range from 750 C to 600 C for 2 to 15 seconds.
If the temperature range for holding the steel is above 750 C,
the driving force of ferrite transformation becomes small, and
no transformation enhancement effect is attained. If the
temperature range therefor is below 600 C, the ferrite
transformation which is controlled by the diffusion of Fe atoms
delays, and satisfactory polygonal ferrite formation cannot be
attained. If the holding time is less than 2 seconds, the ferrite
transformation time is not sufficient, which fails to attain the
low YF: characteristic. If the holding time exceeds 15 seconds,
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the pearlite formation begins to degrade the mechanical
characteristics.
After holding the steel sheet, the secondary cooling is
conducted at cooling rates of 20 C/s or more, and the coiling
of the steel sheet is done at temperatures of 400 C or below.
The cooling rate in the secondary cooling is required to be 20 C/s
or more to suppress the formation of pearlite and bainite during
cooling. If the secondary cooling rate exceeds 1000 C/s, the
metallic micro-structure becomes nonuniform within the sheet
thickness range, and the mechanical characteristics may degrade.
Therefore, the secondary cooling rate is preferably 1000 C/s or
less.
The coiling temperature is required to be 400 C or below
to pre'vent the formation of pearlite and bainite after coiling,
to form martensite, and. to attain the target of 0. 6 or lower YR.
Furthermore, to prevent the fluctuations of strength within the
coil, the coiling temperature is preferably 300 C or below, and
more preferably 200 C or below. If the coiling temperature
becomes below 0 C, the cooling by water becomes difficult so that
the coiling temperature is preferably 0 C or above.
To thus obtained high strength steel sheet according to
the present invention, a skin pass rolling may further be applied
for shape-correction. In addition, various surface treatments
such as hot-dip galvanization and electro-galvanization may be
applied to the high strength steel sheet according to the present
invention as the base material.
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Example 1
Slabs having respective chemical compositions given in
Table 1 were prepared by continuous casting. They were cooled,
thenheatedtotemperatures from1100 Cto 1300 C, andwere treated
by final rolling at temperatures in a range from Ar3 point to
850 C to obtain steel sheets having thicknesses of from 1.6 to
3.2 mm. Within 1 second after completing the final rolling,
cooling began on the steel sheets to conduct the primary cooling
to a temperature range from 680 C to 720 C at cooling rates from
300 to 500 C/s. After that, the steel sheets were held at the
temperature range for 7 to 12 seconds. Then, the steel sheets
were cooled at cooling rates from 25 to 30 C/s, and were coiled
at350 Corlower temperature to obtain the respective hot-rolled
steel sheets. As for Steel No. 4, however, the temperature to
stop the primary cooling was 550 C, and Steel No. 5 was coiled
at 450 C, thus adjusting the micro-structure thereof given in
Table 1. The percentage of polygonal ferrite and the percentage
of martensite were det.ermined by observing the cross section
vertical to the sheet width direction and by measuring the area
percentage of each phase. Regarding the grain size of polygonal
ferrite, the segmental. method was applied to measure the
above-described cross sectional micro-structure to derive an
average value of the value in the rolling direction and the value
in the sheet thickness direction.
CA 02493523 2005-01-21
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2 a O) () N 2 07 ~ N C7 0) G) ~ 0) N G) G) N=~ N a) ~ G) () a) N
~ m ~ a a am C.C.C.C. n a C. a m- C. C. C.C.C.C. a m c a~ 2 C.C.C. C.
0 c a E E E ~ E E E E E E E ~v E m E E ~c E E ~o E E E
o-m co ca o-m aca w m m a~e m m m am am m am m aca m m
y E x x x E x E x x x x E x x x x E x E x x E x x E x x x
H ~ o w w w o w o w w w w o w w LU w o w o w LU o w w p w w w
V U U U U U U U
CA 02493523 2005-01-21
- 20 -
For each of thus obtained hot-rolled steel sheets, the
mechanical characteristics, the surface property, and the spot
weldability were evaluated. The result is given in Table 2. The
evaluation methods are the following. The mechanical
characteristics were determined by the test per JIS Z2241 with
a JIS No. 5 Tensile Test sample (prepared by cutting the steel
sheet lateral to the rolling direction) . The surface property
was determined by visual observation in terms of presence/absence
of red scale. Regardir.ig the spot weldability, spot-welding was
given under a condition to form a nugget having the size of (5
x sheet thickness (mm) ), and then the peal test using a chisel
was applied to break the sheet to observe the fracture mode.
Fracture on main portion of the sheet was evaluated to 0, and
fracture on welded section was evaluated to X.
CA 02493523 2005-01-21
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Table 2
Mechanical characteristics Surface Spot
Classification Steel No.
YP(MPa) TS(MPa) EI(%) YR property weldability
Comparative 1 390 520 36.5 0.75 0 0
Example
Example 2 354 610 31.1 0.58 0 0
Example 3 352 640 29.7 0.55 0 0
Comparative 4 390 600 31.7 0.65 0 0
Example
Comparative 5 428 570 33.3 0.75 0 0
Example
Example 6 345 650 29.2 0.53 0 0
Example 7 373 690 27.5 0.54 0 0
Example 8 462 810 23.5 0.57 0 0
Comparative 9 683 1050 18.1 0.65 0 x
Example
Example 10 369 670 28.4 0.55 0 0
Example 11 336 590 32.2 0.57 0 0
Example 12 352 640 29.7 0.55 A 0
Comparative 13 369 670 28.4 0.55 x 0
Example
Comparative 14 451 530 35.8 0.85 IL 0
Example
Example 15 383 710 26.8 0.54 0 0
Comparative 16 723 850 22.4 0.85 0 0
Example
Example 17 385 700 27.1 0.55 0 0
Comparative 18 468 780 24.4 0.60 0 x
Example
Example 19 398 675 28.1 0.59 0 0
Example 20 389 695 27.3 0.56 0 0
Table 2 shows that all the steels according to the present
invention, (Example steels), have excellent mechanical
characteristics (YR < 0. 6) , and give favorable surface property
and weldability. Steel Nos. 12 and 17 gave somewhat degraded
CA 02493523 2005-01-21
- 22 -
surface property owing to slightly high Si content. However,
Steel Nos. 12 and 17 were judged to be at a level of raising no
significant problem in practical use.
In contrast, Steel No. 1 which is a comparative example
had low C content, outside the range of the present invention,
so that the hardness cf martensite was unsatisfactory, which
increased the YR value. Steel Nos. 4 and 5 had the volume
percentage of polygonal ferrite or the volume percentage of
martensite outside the range of the present invention so that
they failed to form favorable dual phase micro-structure and they
gave high YR value. Steel No. 9 had large C content outside the
range of the present invention so that the ferrite formation
delaye-dto fail in attaining favorable dual phasemicro-structure,
and resulted in high YR value, as well as degrading the spot
weldability. Steel No. 13 had large Si content outside the range
of the present invention so that red scale was generated to give
poor surface property. Steel No. 14 had small Mn content outside
the range of the present invention so that the austenite became
instable and the pearlite was generated, thus giving high YR value.
Steel No. 16 had large Mn content outside the range of the present
invention so that the amount of formed polygonal ferrite became
small, and the YR value became high. Steel No. 18 had large P
content outside the range of the present invention so that the
spot weldability signi.ficantly degraded.
CA 02493523 2005-01-21
- 23 -
Example 2
Some of the slabs having the respective chemical
compositions given in Ta.blelwere treated by hot-rolling, cooling,
and coiling under the respective manufacturing conditions given
in Table3, thus obtained the respective hot-rolled steel sheets.
CA 02493523 2005-01-21
-24-
U
C 7 N M v a fD I- 01 (p 1- V h IC) OD 0 N a) M N N M 0
= CD 1- N v M U) c0 CD er Co L[) L[) V _ p7 h CO ~ 1[') Lo
O~ M M M ~ M N N M N ~ M M N N N C') y M v7
d
a
E
a~
m
Qi yLC) CO N LL') 0 -q 0 0 N N O (D M 0 O LO LO M O 0
p C o v N M v v v N N LO N N M ~ V M C') N N M M M N
U
p O
cl) U
G CD
~ N a N N 0 h 0 OD O O M O N ~ O N O M I-
Ss
d
00 0o h 1- tD a0 O 0 0 0 0 lC N u) (O p~ fp N eY M N
a o M M 0 0 O 0 M W LC) U) M [O N v Lf) M M CD
t~ h IO 1- I- h (O f0 0o h 0 a0 M fD tD N. CO h f0 tD fD fD N.
E
a~
N
Z` m U
~p C o O 0 0 0 0 O O 0 0 0 0 0 0 0 0 0 0 0 O O 0 0
E O CD m M M O~ N LO v O M ~ O C CO p) N 0 M 1- 0 tG Oo
U R N LO N N II C M N LO N K) N 0 LO ~C) LO M ~ LO ~
0- ~
a
y
CD
E
o=
p N tn
p
U CD
a~
a
~
v, CC o o 0 0 o O o 0 o_ o O 0 0 O o 0 0 O o O O o
y p Zo M tf N M LL') V M N M M N V N M -7 t[) M N M M M
a0 Oo W GO aD tD c0 00 c0 aD c0 c0 aD 00 a0 aD a0 c0 cD aD a0 0o
E
N
p C
CM
c~ O o 0 0 0 o O O o 0 0 0 0 o 0 0 0 0 0 o O o
O LO M Lf7 LC) LL') O O N LO t0 O M 6o M O CO V 0 tC) N
N N ~ N N ~ y N ~ ~ N N ~
a) Qe N N N 0
= E ~
a)
N Z N N N N N N N N CD CO fp fD (D (D (O fD f0
I ¾
U w LL (D = z 0 0. C'3 cn r> n'1 c/)
u) a) a) a) aD a~ m a>
~ - a) a> g ' =~
a~ w d a) m m a u m a) W 2 a> 2 m a> a) 2! , - d a~ m a? 2 2 T
'~'~ m a 'a afliu - ~- a n a~- a a n~- m- a n a a a a~ adt01 E E uu uu
a>I
y y U lU W W W U aD W W W0IU Uw w IU UW W W W 0
CA 02493523 2005-01-21
- 25 -
For each of thus obtained hot-rolled steel sheets, the
mechanical characteristics, the surface property, and the spot
weldability were evaluated. The result is given in Table 4. The
evaluation methods were the same with those in Example 1.
Table 4
Volume Volume Grain size of Mechanical characteristics
Classification Symbol percentage of percentage of polygonal ferrite
polygonal ferrite martensite (%) ( )U m) YP TS El YR
(%) (MPa) (MPa) (%)
Example A 75 10 8 341 620 30.6 0.55
Example B 75 10 9 342 610 31.1 0.56
Example C 80 10 10 354 610 31.1 0.58
Comparative D 50 10 13 447 630 30.2 0.71
example
Comparative E 40 5 15 488 650 29.2 0.75
example
Exampie F 80 10 9 365 640 29.7 0.57
Example G 75 10 7 330 600 31.7 0.55
Example H 80 10 6 369 670 28.4 0.55
Comparative I 30 15 10 490 680 27.9 0.72
example
Example J 85 12 8 381 680 27.9 0.56
Example K 90 10 7 336 590 32.2 0.57
Example L 80 10 5 364 650 29.2 0.
56
Comparative M 35 5 4 446 550 34.5 0.81
example
Comparative N 45 10 4 441 630 30.2 0.70
example
Example 0 85 13 9 389 670 28.4 0.58
Example P 80 10 8 358 640 29.7 0.56
Comparative Q 80 0 12 451 530 35.8 0.85
example
Comparative R 80 3 11 498 560 33.9 0.89
example
Example S 85 8 9 369 670 28.4 0.55
Example T 75 18 8 412 710 26.8 0.58
Example U 85 15 5 366 690 27.5 0.53
Example V 85 15 8 385 700 27.1 0.55
Comparative W 75 0 9 475 650 29.2 0.73
example
CA 02493523 2005-01-21
- 26 -
Table 4 shows that all the steels according to the present
invention, (Example steels), have excellent mechanical
charac.teristics (YR <- 0.6). All the Example steels showed
favora.ble surface property and spot weldability within the range
of Example 2.
In contrast, Symbol D which is a comparative example had
a long period between the completion of rolling and the beginning
of primary cooling, outside the range of the present invention,
thus ferrite wasformed:irregularly before beginning the cooling,
which resultedin unf avorable dual phase micro-structure and high
YR value. Symbol E had low primary cooling rate outside the range
of the present invention so that ferrite was formed irregularly
before beginning the cooling, which resulted in unfavorable dual
phase micro-structure and high YR value. Symbol I had high
temperature of stopping the primary cooling outside the range
of the present invention so that the formation of ferrite during
the succeeding holding step became insufficient, which resulted
in unfavorable dual phase micro-structure and high YR value.
Symbol M had low temperature of stopping the primary cooling
outside the range of the present invention so that the formation
of feri-ite during the succeeding holding step became insufficient,
which resulted in unfavorable dual phase micro-structure and high
YR value. SymbolN hadinsufficient holding timeafter the primary
cooling outside the range of the present invention so that the
formation of ferrite became insufficient, which resulted in
unfavorabledualphasemicro- structureand high YR value. Symbol
Q had long holding time after the primary cooling outside the
CA 02493523 2005-01-21
- 27 -
range of the present invention so that pearlite was formed during
holdirig step, which resulted in unfavorable dual phase
micro--structure and high YR value. Symbol R had low secondary
coolirig rate outside the range of the present invention so that
bainite was formed during cooling step, which resulted in
unfavorable dual phase micro-structure and high YR value. Symbol
W had :7igh coiling temperature outside the range of the present
invention so that bainite was formed after coiling, which resulted
in unfavorable dual phase micro-structure and high YR value.
Figure 1 shows the relation between YR and the primary
coolirig rate for Steel No. 2. The figure shows that favorable
characteristics giving low YR value is attained at 150 C/s or
higher primary cooling rate, which is the range of the present
invention. Symbol D failed to attain favorable result because
the time before the primary cooling was 5 seconds, which is outside
the range of the present invention.
Since the steel sheet according to the present invention
has excellentpress-for.mability and excellent surface property,
the steel is also applicable to formed parts which emphasize the
appearance.
