DESCRIPTION
HIGH-STRENGTH HOT-ROLLED STEEL SHEET EXCELLENT IN
HOLE EXPANDABILITY AND DUCTILITY AND
PRODUCTION METHOD THEREOF
Technical Field:
This invention relates to a high-strength hot-rolled
steel sheet, directed to automotive suspension components
mainly formed by press working, having a strength of at
least 980 N/mm2 at a sheet thickness of about 1.0 to about
6.0 mm and excellent in hole expandability and ductility,
and a production method of the steel sheet.
Background Art:
The needs for the reduction of the weight of a car
body, the integral molding of components and a reduction
in the production cost, through rationalization of a
production process, have been increased in recent years
as means for improving fuel efficiency to cope with the
environmental problems caused by automobiles, and the
development of high-strength hot-rolled steel sheets
having excellent press workability has been carried out.
Elongation and hole expandability are particularly
important in molding a hot-rolled steel sheet, and
Japanese Unexamined Patent Publication (Kokai) Nos. 6-
287685, 7-11382 and 6-200351 propose technologies that
improve the hole expandability by adjusting the addition
amounts of Ti, Nb and C and S to steel sheets having a
strength level of 590 to 780 N/mm2. However the
development of high-strength steel sheets exceeding 980
N/mm' is necessary to satisfy further needs for a
reduction in weight. Elongation and hole expandability
are deteriorated with an increase in the strength and the
hole expandability and ductility are contradictory, as is
well known in the art. It has therefore been difficult,
using the prior art technologies, to produce steel sheets
of the 980 N/mm2 level that are excellent in both
elongation and hole expandability.
Disclosure of the Invention:
To solve the problems of the prior art described
above, the invention contemplates to provide a highstrength
hot-rolled steel sheet that can prevent
deterioration of hole expandability and ductility with
the increase of strength above 980 N/mm2 and has high hole
expandability and high ductility even when its strength
is high, and a production method of such a steel sheet.
The high-strength steel sheet excellent in hole
expandability, ductility and ability of phosphate
coating, that is intended to solve the problems described
above, and its production method, are as follows.
(1) A high-strength hot-rolled steel sheet excellent in
hole expandability and ductility, containing in terms of
a mass%:
C: 0.01 to 0.09%,
Si: 0.05 to 1.5%,
Mn: 0.5 to 3.2%,
Al: 0.003 to 1.5%,
P: 0.03% or below,
S: 0.005% or below,
Ti: 0.10 to 0.25%,
Nb: 0.01 to 0.05%, and
the balance consisting of iron and unavoidable
impurities;
satisfying all of the following formulas <1> to <3>:
0.9 < 48/12 x C/Ti < 1.7 . . . <1>
50,227 x C - 4,479 x Mn > -9,860 . . . <2>
811 x C + 135 x Mn + 602 x Ti + 794 x Nb > 465
. . . <3>, and
having strength of at least 980 N/mm2.
(2) A high-strength hot-rolled steel sheet excellent in
hole expandability and ductility, containing in terms of
0.01 to 0.09%,
0.05 to 1.5%,
Mn: 0.5 to 3.2%,
Al: 0.003 to 1.5%,
P: 0.03% or below,
S: 0.005% or below,
Ti: 0.10 to 0.25%,
Nb: 0.01 to 0.05%,
at least one of
Mo: 0.05 to 0.40% and V: 0.001 to 0.10%, and
the balance consisting of iron and unavoidable
impurities;
satisfying all of the following formulas ' to <3>':
0.9 < 48/12 x C/Ti < 1.7 ... '
50,227 x C - 4,479 x (Mn + 0.57 x Mo + 1.08 x V) >
-9,860 . . . <2>'
811 x C + 135 x (Mn + 0.57 x Mo + 1.08 x V) + 602 x
Ti + 794 x Nb > 465 ... <3>!, and
having strength of at least 980 N/mm2.
(3) A high-strength hot-rolled steel sheet excellent in
hole expandability and ductility according to (1) or (2),
which further contains, in terms of mass%, 0.0005 to
0.01% of at least one of Ca, Zr and REM.
(4) A high-strength hot-rolled steel sheet excellent in
hole expandability and ductility according to any of (1)
through (3), which further contains, in terms of mass%,
0.0005 to 0.01% of Mg.
(5) A high-strength hot-rolled steel sheet excellent in
hole expandability and ductility according to any of (1)
through (4), which further contains, in terms of mass%,
at least one of:
Cu: 0.1 to 1.5% and
Ni: 0.1 to 1.0%.
(6) A production method of a high-strength hot-rolled
steel sheet excellent in hole expandability and ductility
according to any of (1) through (5), comprising the steps
of:
finishing hot rolling by setting a rolling finish
temperature to from an Ars transformation point to 950°C;
cooling the hot-rolled steel sheet to 650 to 800°C at
a cooling rate of at least 20°C/sec;
cooling then the steel sheet for 0.5 to 15 seconds;
further cooling the steel sheet to 300 to 600°C at a
cooling rate of at least 20°C/sec; and
coiling the steel sheet.
Brief Description of the Drawings:
Fig. 1 is a graph showing the effects, in a steel of
the invention, on elongation with respect to tensile
strength; and
Fig. 2 is a graph showing the effects, in the steel
of the invention, on an hole expansion ratio with respect
to tensile strength.
Best Mode for Carrying Out the Invention:
It is known, in high-strength steel sheets, that
elongation and hole expandability are deteriorated with
an increase in strength and the hole expandability and
ductility are contradictory. To solve the problem, the
inventors of the invention have conducted intensive
studies and have found that elongation and hole
expandability can be improved with high strength by
stipulating the ranges of C, Mn and Ti components. The
invention has thus been completed. In other words, the
inventors have derived relational formulas by clarifying
the influences of maximum utilization of precipitation
hardening of TiC and structure strengthening by Mn and C
on materials and have solved the problems described
above.
The reason for stipulation of each element of the
steel composition will be hereinafter explained.
C is limited to 0.01 to 0.09%. C is an element
necessary for precipitating carbides and securing the
strength. When the C content is less than 0.01%, a
desired strength cannot be secured easily. When the C
content exceeds 0.09%, the effect of increasing the
strength disappears and, moreover, ductility is
deteriorated. Therefore, the upper limit is set to
0.09%. Preferably, C is 0.07% or smaller because it is
the element that invites deterioration of hole
expandability.
Si is an element that improves strength by solid
solution hardening, promotes ferrite formation by
suppressing the formation of detrimental carbides, is
important for improving elongation and can satisfy both
strength and ductility. To acquire such effects, at
least 0.05% of Si must be added. When the addition
amount increases, however, a de-scaling property
resulting from Si scales and the ability of phosphate
coating drop. Therefore, the upper limit is set to 1.5%.
Incidentally, the range of Si is preferably from 0.9 to
1.3% to simultaneously satisfy the hole expandability and
ductility.
Mn is one of the important elements in the
invention. Though Mn is necessary for securing strength,
it deteriorates elongation. Therefore, the Mn content is
as small as possible as long as the strength can be
secured. Particularly when a large amount of Mn beyond
3.2% is added, micro segregation and macro segregation
are more likely to occur and the hole expandability is
remarkably deteriorated. Therefore, the upper limit is
set to 3.2%. Particularly when elongation is of
importance, the Mn content is preferably 3.0% or below.
On the other hand, Mn has a function of making S that is
detrimental for the hole expandability harmless as Mn£A
To obtain such an effect, at least 0.5% of Mn must be
added.
Al is effective as a deoxidizer, suppresses the
formation of detrimental carbides and promotes the
ferrite formation in the same way as Si and improves
elongation, so that both strength and ductility can be
satisfied. When used as the deoxidizer, at least 0.003%
of Al must be added. When the Al content exceeds 1.5%,
on the other hand, the ductility improvement effect is
saturated. Therefore, the upper limit is set to 1.5%.
Because the addition of a large amount of Al lowers
cleanness of the steel, the Al content is preferably 0.5%
or below.
P undergoes solid solution in a ferrite and lowers
ductility. Therefore, its content is limited to 0.03% or
below.
S forms MnS, operates as the starting point of
destruction and remarkably lowers hole expandability as
well as ductility. Therefore, its content is limited to
0.005% or below.
Ti is one of the most important elements in the
invention and is effective for securing strength through
precipitation of TiC. Degradation of elongation by Ti is
smaller than Mn and, Ti is used effectively. To obtain
this effect, at least 0.10% of Ti must be added. When a
large amount of Ti is added, on the other hand,
precipitation of TiC proceeds during heating for hot
rolling and Ti does not contribute any longer to the
strength. Therefore, the upper limit is set to 0.25% at
the upper limit of the existing heating temperature.
Nb is an element effective for securing the strength
through NbC precipitation in the same way as the addition
of Ti. Because degradation of elongation is less in
comparison with Mn, Nb is used effectively. To obtain
this effect, at least 0.01% of Nb must be added.
However, because the addition effect is saturated even
when 0.05% or more of Nb is added, the upper limit is set
to 0.05%.
Mo is an element that contributes to the improvement
of strength in the same way as Mn but lowers elongation.
Therefore, its addition amount is preferably small as
long as the strength can be secured. Particularly, when
the Mo content exceeds 0.40%, the drop of ductility
becomes great and the upper limit is therefore set to
0.40%. When Mo is added as a partial substitute for Mn,
it; can mitigate Mn segregation. To obtain this effect,
at least 0.05% of Mo must be added.
V is an element that contributes to the improvement of strength in the same way as Mo and Mn but deteriorates elongation. Therefore, the addition amount of V is preferably small as long as the strength can be secured. Further, when the V content exceeds 0.10%, cracking is likely to occur during casting. Therefore, the upper limit is set to 0.10%. V can mitigate Mn segregation when added as a partial substitute for Mn. To obtain this effect, at least 0.001% of V must be added.
Ca, Zr and REM are effective elements for controlling the form of sulfide type inclusions and improving the hole expandability. To render this controlling effect useful, at least 0.0005% of at least one kind of Ca, Zr and REM is preferably added. On the other hand, the addition of a greater amount invites coarsening of the sulfide type inclusions, deteriorates cleanness, lowers ductility and invites the cost of production. Therefore, the upper limit is set to 0.01%.
When added, Mg combines with oxygen and forms oxides. The.inventors of this invention have found that refinement of MgO or composite oxides of Al203, Si02, MnO and Ti2O3 containing MgO formed at this time lets them have smaller sizes as individual oxides and have a uniform dispersion state. Though not yet clarified, these oxides finely dispersed in the steel form fine voids at the time of punching, contribute to the dispersion of the stress and suppress the stress concentration to thereby suppress the occurrence of coarse cracks and to improve the hole expandability. However, the effect of Mg is not sufficient when its content is less than 0.0005%. When the content exceeds 0.01%, the improvement effect is saturated and the production cost increases. Therefore, the upper limit is set to 0.01%.
Cu and Ni are the elements that improve hardenability. These elements are effective for securing

the second phase percentage and the strength when added
particularly at the point at which a cooling rate is low
so as to control the texture. To make this effect
useful, at least 0.1% of Cu or at least 0.1% of Ni is
preferably added. However, the addition of these
elements in greater amounts promotes degradation of
ductility. Therefore, the upper limit of Cu is 1.5% and
1.0% for Ni.
The steel does not come off from the range of the
invention even when it contains, as unavoidable impurity
elements, not greater than 0.01% of N, less than 0.1% of
Cu, less than 0.1% of Ni, not greater than 0.3% of Cr,
less than 0.05% of Mo, not greater than 0.05% of Co, not
greater than 0.05% of Zn, not greater than 0.05% of Sn,
not greater than 0.02% of Na and not greater than 0.0005%
of B, for example.
As a result of intensive studies for solving the
problems described above, the inventors of this invention
have found that elongation and the hole expandability can
be improved, with high strength, by stipulating the
ranges of C, Mn and Ti components. In other words, the
present inventors have derived the following three
relational formulas by clarifying the influences of
maximum utilization of TiC precipitation hardening and
texture strengthening by Mn and C on the materials. The
relational formulas will be hereinafter explained.
When the addition amount of C is smaller than that
of Ti, solid solution Ti increases and deteriorates
elongation. Therefore, the relation 0.9 < 48/12 x C/Ti
is stipulated. On the other hand, when the C content is
excessively greater than the Ti content, TiC precipitates
during heating for hot rolling and the increase of the
strength cannot be obtained. In addition, the hole
expandability is deteriorated due to the increase of the
C content in the second phase. Therefore, the relation
48/12 x C/Ti < 1.7 is set. In other words, the following
formula <1> must be satisfied. Particularly when the
hole expandability is important, the relation 1.0 < 48/12 x C/Ti <1.3 is preferably satisfied.
0.9 < 48/12 x C/Ti < 1.7 ... <1>
The formation of ferrite is suppressed with the increase of the addition amount of Mn. Consequently, the second phase percentage increases and the strength can be secured more easily but the drop of elongation occurs. C improves elongation, though the hole expandability
drops, by hardening the second phase. Therefore, to
secure elongation required for at least 980 N/mm2, the following
formula <2> must be satisfied:
50,227 x C - 4,479 x Mn > -9,860 . . . <2>
Since the effect of each of Mo and V is determined by its atomic equivalent at this time, the formula <2> changes to <2>' under the condition in which Mo or V is added:
50,227 x C - 4,479 x (Mn + 0.57 x Mo + 1.08 x V)>-9.860
. . . <2>'
To secure workability, the two formulas described above must be satisfied. It is relatively easy in the steel sheets of a 780 N/mm2 level to satisfy these two formulas while securing the strength. To secure the strength exceeding 980 N/mm2, however, it is unavoidable to add C that deteriorates the hole expandability and Mn that deteriorates elongation. Therefore, to secure the strength exceeding 980 N/mm2, it is necessary to adjust the components so as to satisfy the range of the following formula <3> while satisfying the two formulas described above:
811 x C + 135 x Mn + 602 x Ti + 794 x Nb > 465
. . . <3>
As the effect of each of Mo and V is determined by its atomic equivalent at this time, the formula <3> changes to <3>' under the condition in which Mo or V is added:
811 x C + 135 x (Mn + 0.57 x Mo + 1.08 x V)
+ 602 x Ti + 794 x Nb > 465

When a high-strength hot-rolled steel sheet is
produced by hot rolling, the finish rolling end
temperature must be higher than the Arj transformation
point to suppress the formation of ferrite and to improve
the hole expandability. When the temperature is raised
excessively, however, the drop of the strength and
ductility occurs owing to coarsening of the texture.
Therefore, the finish rolling end temperature must be not
higher than 950°C.
To acquire the high hole expandability, it is
important to rapidly cool the steel sheet immediately
after the end of the rolling and the cooling rate must be
at least 20°C/sec. When the cooling rate is less than
20°C/sec, it becomes difficult to suppress the formation
of carbides that are detrimental to the hole
expandability.
Rapid cooling of the steel sheet is thereafter
stopped once and air cooling is applied in the invention.
This is important to increase the occupying ratio of
ferrite by precipitating it and to improve ductility.
However, pearlite, that is detrimental to the hole
expandability, occurs from an early stage when the air
cooling start temperature is less than 650°C. When the
air cooling start temperature exceeds 800°C, on the other
hand, the formation of ferrite is slow. Therefore, not
only the air cooling effect cannot be obtained easily but
the formation of pearlite is likely to occur during
subsequent cooling. For this reason, the air cooling
start temperature is from 650 to 800°C. The increase of
ferrite is saturated even when the air cooling time is
longer than 15 seconds and loads are applied to
subsequent cooling rate and control of a coiling
temperature. Therefore, the air cooling time is not
longer than 15 seconds. When the cooling time is less
than 0.5 seconds, the formation of ferrite is not
sufficient: and the effect of improvement of elongationcannot be obtained. The steel sheet is again cooled rapidly after air cooling and the cooling rate must be at
least 20°C/sec, too. This is because, detrimental pearlite is likely to be formed when the cooling rate is less than 20°C/sec.
The stop temperature of this rapid cooling, that is, the coiling temperature, is set to 300 to 600°C. This is because, martensite, that is detrimental to the hole expandability, occurs when the coiling temperature is less than 300°C. When the coiling temperature exceeds 600°C, on the other hand, pearlite and cementite that are detrimental to the hole expandability, are more easily formed.
A high-strength hot-rolled steel sheet excellent in workability and having a strength of higher than 980 N/mm2 can be produced by combining the components and the rolling condition described above. When surface treatment (for example, zinc coating) is applied to the surface of the steel sheet according to the invention, such a steel sheet has the effects of the invention and does not leave the scope of the invention. Examples:
Next, the invention will be explained with reference to examples thereof.
Steels having components tabulated in Table 1 and Table 2 (continuing Table 1) are molten and continuously cast into slabs in a customary manner. Symbols A to Z represent the steels having the components of the invention. Steel having a symbol a has a Mn addition amount outside the range of the invention. Similarly, steel b and steel d have a Ti addition amount and a C addition amount outside the ranges of the invention, respectively. Further, steel having a symbol c has values of formulas <1> and <3> outside the range of the invention. These steels are heated at a temperature higher than 1,250°C in a heating furnace and are hot

rolled into hot-rolled steel sheets having a sheet
thickness of 2.6 to 3.2 mm. The hot rolling condition is
Table 2 (continuing Table 1)
(Table Removed)

* Ar3 = 900 - 510C + 28Si - 50Mn + 229Ti
An underline indicates that the steel is outside the range of the invention.
Table 3

(Table Removed)
An underline indicates that the steel is outside the range of the invention.

Table 4 (continuing Table 3)

(Table Removed)
An underline indicates that the steel is outside the range of the invention.
C3 has
a coiling temperature outside the range of the invention.
Similarly, J2 has an air cooling start temperature
outside the range of the invention, P3 has a finish
temperature outside the range of the invention and S3 has
a coiling temperature outside the range of the invention.
Each of the resulting hot-rolled steel sheets is
subjected to a tensile test by using a JIS No. 5 test
piece and a hole expansion test. As for the hole
expandability, a hole expansion ratio A, = (d-d0) /d x 100
is evaluated.
The ratio is obtained from a hole diameter (d)
formed when a crack perforates through the sheet
thickness while expanding a punched hole having a
diameter of 10 mm using a 60 conical punch and an initial
hole diameter (d0: 10 mm).
Table 3 and Table 4 (continuing Table 3) tabulate
the tensile strength TS, elongation El and the hole
expansion ratio X. of each test piece. Fig. 1 shows the
relation between the strength and elongation and Fig. 2
shows the relation between the strength and the hole
expansion ratio. It can be understood that the steels of
the invention have a higher elongation or a better hole
expansion ratio than Comparative Steels. It can thus be
understood that the steel sheets according to the
invention have both an excellent hole expansion ratio and
good duct ility.
Table 1
(Table Removed)
Ar3 = 900 - 510C + 28Si - 50Mn + 229Ti
An underline indicates that the steel is outside the range of the invention.
Table 2 (continuing Table 1)
(Table Removed)
Ar3 = 900 - 510C + 28Si - 50Mn + 229Ti
An underline indicates that the steel is outside the range of the invention.
Table 3
(Table Removed)

An underline indicates that the steel is outside the range of the invention.
Table 4 (continuing Table 3;
(Table Removed)

An underline indicates that the steel is outside the range of the invention.
Industrial Applicability:
As described above in detail, the invention can
economically provide a high-strength hot-rolled steel
sheet having a tensile strength of at least 980 N/mm2 and
satisfying both an hole expandability and ductility.
Therefore, the invention is suitable as a high-strength
hot-rolled steel sheet having high workability. The
high-strength hot-rolled steel sheet according to the
invention can reduce the weight of a car body, can
achieve integral molding of components and
rationalization of a production process, can improve a
fuel efficiency and can reduce the production cost.
Therefore, the invention has large industrial value.

We claim:
1. A high-strength hot-rolled steel sheet excellent in hole expandability and
ductility, consisting essentially of, in terms of a mass%:
C: 0.01 to 0.09%,
Si: 0.05 to 1.5%,
Mn: 0.5 to 3.2%,
Al: 0.003 to 1.5%,
P: 0.03% or below, S: 0.005% or below,
Ti: 0.10 to 0.25%,
Nb:0.01 to 0.05%, and
the balance consisting of iron and unavoidable impurities, wherein the steel of
said steel sheet substantially contains no Mg and satisfying all of the following
conditions <1> to <3>:
0.9 5 48/12xC/Ti < 1.7 . . . <1>
50,227xC-4479xMn>-9860 . . . <2>
811xC+135xMn+602xTi+794xNb>465 . . . <3>, and having strength of at least
980N/mm2-
optionally, comprising at least one of Mo: 0.05 to 0.40% and V: 0.001 to
0.10%, and the balance consisting of iron and unavoidable impurities, wherein
the steel of said steel sheet substantially contains no Mg and satisfying all of
the following conditions <1>' to <3>':
0.9 s 48/12xC/Ti < 1.7 . . . <1>'
50,227xC-4479x (Mn+0.57xMo+1.08xV) >-9860 . . . <2>'
81lxC+135x (Mn+0.57xMo+1.08xV) +602xTi+794xNb>465
. . . <3>',
and having strength of at least 980 N/mm2 2. A high-strength hot-rolled steel sheet excellent in hole expandability, and
ductility as claimed in claim 1 , which optionally contains, in terms of
mass%, 0.0005 to 0.01% of at least one of Ca, Zr and REM.
3. A high-strength hot-rolled steel sheet excellent in hole expandability, and
ductility as claimed in any of claims 1 to 2, which optionally contains, in
terms of mass%, at least one or more of;
Cu: 0.1 to 1.5% and Ni: 0.1 to 1.0%
4. A production method of a high strength hot rolled steel sheet excellent in
hole expandability and ductility as claimed in any one of claims 1 to 3,
comprising the steps of:
finishing hot rolling by setting a rolling end temperature to from an Ar3
transformation point to 950°C;
cooling a hot rolled steel sheet to 550 to 800°C at a cooling rate of at
least 20°C/sec;
air cooling then the steel sheet for 0.5 to 15 seconds;
further cooling the steel sheet to 300 to 600°C at a cooling rate of at least
20°C/sec; and
cooling the steel sheet.