The present invention relates to a hot-rolled steel sheet. The present
invention particularlyrelates to a high-strength hot-rolled steel sheet which is
preferable for car suspension members and the like and has excellent surface properties,
shape fixability, hole expansibility, and fatigue resistance.
Priority is claimed on Japanese Patent Application No. 2014-188845, filed in I
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Japan on September 17,2014, the content of which is incorporated herein by reference.
[Related Art] i
[0002]
In order to decrease the emission amount of carbonate gas exhausted from
cars, weight reduction of car bodies by using high-strength steel sheets is underway.
The above-described demand for high-strengthening also applies to structural members
or suspension members which account for approximately 20% of the body weight of
automobiles. High-strength hot-rolled steel sheets are also continuously applied to
these members.
[0003]
However, generally, the high-strengthening of steel sheets deteriorates their
material properties such as formability (workability). Therefore, it becomes a key
factor in the development of high-strength steel sheets to find a proper method for
high-strengthening without causing any deterioration of material properties.
Particularly, as properties required for steel sheets for structural members or
suspension members, workability and shape fixability during press forming and,
- 1 -
additionally, fatigue durability during use is important. It is important to balance a
high strength and the above-described properties at a high level.
[0004]
Furthermore, in addition to balancing the material properties of steel sheet at a
high level as described above, there is another demand in a variety of fields for
realizing a high added value as products from users' viewpoint. For example, for
steel sheets used for wheel disks, in order to cope with a necessity of a high
designability of aluminum wheels, there is a demand for designability (surface
properties) for steel sheet surfaces and burring properties (hole expansibility) favorable
enough to withstand working into complicated shapes.
[0005]
Generally, as high-strength hot-rolled steel sheet used as steel sheets for
suspension members, dual phase steel (DP steel) having a structure made of ferrite and
martensite is used.
[0006]
DP steel has excellent strength and elongation and, furthermore, also has
excellent fatigue resistance because of the presence of a hard layer. Therefore, DP
steel is suitable for hot-rolled steel sheets used for car suspension components.
However, DP steel generally contains a large amount of Si, which is a ferritestabilizing
element, in order to form a structure including ferrite as a primary body.
Therefore, DP steel is a kind of steel which is likely to form a defect called a Si scale
pattern on steel sheet surfaces. Therefore, DP steel has poor desiguability for steel
sheet surfaces imd is generally used for components that are placed inside cars and are
thus invisible.
Frnihermore, DP steel structure which includes both soft phase ferrite and
hard phase martensite, and thus deteriorates hole expansibility due to the difference in
hardness between these two phases. Therefore, at the moment, DP steel has a
problem in imparting a high added value as products which are demanded by users.
[0008]
There is a method for improving dcsignability for steel sheet surfaces. For
example, Patent Document 1 discloses a method in which descaling is carried out in a
state in which the temperature of a steel piece after rough-rolling is increased, thereby
manufacturing steel sheets having substantially no Si scale on the surface.
However, in the above-described method, there is a problem in that the
temperature after finish-rolling increases as the temperature of the steel piece after
rough-rolling increases, grain diameters are coarsened, and properties such as strength,
toughness, and fatigue properties deteriorate. In addition, there is still a possibility
that the Si scale pattern emerges after pickling even when there is no Si scale after
rolling since the Si scale pattern is generated in the following manner: Si scale is
generated, Si scale-generated portions deteriorate the degree of roughness of the
surface of a pickled steel sheet, and the shape emerges due to the difference in the
degree of roughness between the Si scale-generated portions and normal portions.
In consideration of what has been described above, in order to remove the Si
scale pattern on steel sheet surfaces and improve designability, it is necessary to
prevent the generation of Si scale. In the method of Patent Document 1, it is
considered that designability for steel sheet surfaces cannot be fully improved.
![0009]
There is a method for manufacturing DP steel having improved surface
properties of steel sheets by limiting the amount of Si added. For example, Patent
Document 2 discloses a method for manufacturing a high-strength thin steel sheet
having excellent workability and surface properties in which the equiaxial ferrite
volume percentage is 60% or more and the martensite volume percentage is 5% to 30%.
In the invention described in Patent Document 2, ferrite-generating elements
are limited. As a result, in the manufacturing method, cooling is initiated within two
seconds after completing the hot-rolling, and a steel sheet is cooled at 750°C to 600°C
at a cooling rate of 150°C/s or more, is held in a temperature range of 750°C to 600°C
for 2 to 15 seconds, is cooled at a cooling rate of 20°C/s or more, and is coiled at a
temperature of 400°C or lower. Therefore, in the method of Patent Document 2, the
driving force for the generation of ferrite is increased, and a large generation amount of
ferrite is ensured, thereby realizing both excellent surface properties and workability.
However, when the cooling rate after finish-rolling is l50°C/s or more, not
only ferritic transformation but also pearlitic transformation occur earlier. Therefore,
it becomes difficult to obtain a high ferrite fraction, and the fraction of hard phases
such as martensite or pearlite which deteriorate hole expansibility increases.
That is, in the method of Patent Document 2, DP steel having excellent
surface properties can be manufactured, but it is not possible to impart excellent hole
expansibility.
[0010]
Meanwhile, means for improving the hole expansibility of DP steel is known.
For example, Patent Document 3 discloses a method in which ferrite is sufficiently
generated, and a hard second phase (martensite) is finely dispersed a small fraction,
thereby manufacturing steel sheets having excellent elongation and hole expansibility.
However, in Patent Document 3, in order to sufficiently generate ferrite and
finely disperse a small fraction of martensite, the total amount of Si and AI, which are
- 4 -
feiTite-stabilizing elements, is set to 0.1% or more. Furthermore, in Patent Document
3, AI is used as a subsidiary element, and a large amount of Si is added. Therefore, Si
scale is generated on steel sheet surfaces, and the deterioration of designability is
expected.
That is, in the method of Patent Document 3, it is not possible to realize both
favorable hole expansibility and designahility for steel sheet surfaces.
[0011]
In addition, there is a method for improving the hole expansibility ofDP steel
with no need for ensuring the generation amount of ferrite by the addition of fe!Titestabilizing
elements. For example, Patent Document 4 discloses a method for
manufacturing DP steel having excellent hole expansibility by decreasing the
difference in hardness between two phases of ferrite and martensite.
Generally, as a method for decreasing the difference in hardness between two
phases of ferrite and martensite, strengthening of soft phases by means of the
precipitation strengthening of ferrite and softening of hard phases by means of the
tempering of martensite are known. However, in the former method, there is a
concern that shape fixability during press forming may be deteriorated in order to
increase yield strength. Regarding the latter method, it is difficult to carry out
tempering in the middle of the existing hot-rolling processes, and special devices such
as heating devices are separately required, and thus the latter method is poorly feasible
and is not desirable from the viewpoint of manufacturing efficiency and manufacturing
costs. In addition, even when special devices such as heating devices are installed, in
the latter method, there is a possibility that fatigue properties may be deteriorated due
to the softening of hard phases.
[0012]
- 5 -
11 d
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As described above, it has been difficult to manufacture hot-rolled steel sheets
having favorable hole expansibility and favorable designability for steel sheet surfaces
(excellent surface properties) by balancing high strength, shape fixability and fatigue
resistance at a high level.
[Prior Art Document]
[Patent Document]
[0013]
[Patent Document 1] Japanese Unexamined Patent Application Publication
No. 2006-152341
[Patent Document 2] Japanese Unexamined Patent Application Publication
No. 2005-240172
[Patent Document 3] Japanese Unexamined Patent Application Publication
No. 2013-019048
[Patent Document 4] Japanese Unexamined Patent Application Publication
No. 2001-303187
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0014]
The present invention has been made in consideration of the above-described
problems, and an object of the present invention is to provide a hot-rolled steel sheet
having excellent surface properties, shape fixability, hole expansibility, and fatigue
resistance.
[Means for Solving the Problem]
[0015]
The present inventors optimized the components and manufacturing
- 6 -
conditions of high-strength hot-rolled steel sheets and controlled the structures of steel
sheets. As a result of such efforts, the present inventors succeeded the manufacturing
of a high-strength hot-rolled steel sheet having no Si scale patterns on the surface,
having excellent fatigue resistance, and having excellent shape fixability and hole
expansibility.
Aspects of the present invention are as follows:
[0016]
1:'
1:!
[1] A hot-rolled steel sheet according to an aspect of the present invention
including:
by mass%,
C: 0.02% to 0.20%;
Si: more than 0% to 0. 15%;
Mn: 0.5% to 2.0%;
P: more than 0% to 0.10%;
S: more than 0% to 0.05%;
Cr: 0.05% to 0.5%;
Al: 0.01% to 0.5%;
N: more than 0% to 0.01 %;
Ti: 0% to 0.20%;
Nb: 0% to 0.1 0%;
Cu: 0% to 2.0%;
Ni: 0% to 2.0%;
Mo: 0% to 1.0%;
V: 0% to 0.3%;
Mg: 0% to O.ol %;
- 7 -
Ca: 0%to 0.01%;
REM: 0% to 0.1 %; and
B: 0% to 0.01 %,
with a remainder consisting of Fe and impurities, in which amounts of Cr and
Al added satisfy Expression (1) below,
wherein a metallographic structure has, by %by volume, a ferrite fraction of
more than 90% and 98% or less, a martensite fraction of2% to less than 10%, and,
furthermore, a fraction of a residual structure made of one or more of pearlite, bainite,
and residual austenite being less than 1%, the ferrite has an average circle-equivalent
diameter of 4 J.tm or more and a maximum circle-equivalent diameter of 30 J.tm or less,
and the martensite has an average circle-equivalent diameter of 10 ~tm or less and a
maximum circle-equivalent diameter of20 J.tm or less:
[Cr]x5+[Al]2:0.50 Expression (1)
here, in Expression (1), [Cr] represents an amount of Cr (mass%), and [AI]
represents an amount of AI (mass%).
[0017]
[2] The hot-rolled steel sheet according to [1 ], further inCluding:
by mass%, one or two of
Ti: 0.02% to 0.20%; and
Nb: 0.005% to 0.10%.
[3] The hot-rolled steel sheet according to [1] or [2], further including:
by mass%, one or more of
Cu: O.ol% to 2.0%;
Ni: 0.01% to 2.0%;
Mo: 0.01% to 1.0%; and
- 8 -
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V: 0.01% to 0.3%.
[0018]
[4] The hot-rolled steel sheet according to any one of [1] to [3], further
including:
by mass%, one or more of
Mg: 0.0005% to 0.01 %;
Ca: 0.0005% to 0.01 %; and
REM: 0.0005% to 0.1 %.
[5] The hot-rolled steel sheet according to any one of [1] to [3], further
including:
by mass%,
B: 0.0002% to 0.01 %.
[Effects of the Invention]
[0019]
According to the above-described aspect of the present invention, it is
possible to provide a hot-rolled steel sheet having no Si scale pattern on the surface,
that is, having excellent surface properties and having excellent fatigue resistance,
shape fixability, and hole expansibility.
[Brief Description of the Drawings]
[0020]
FIG. 1 is a graph showing a relationship between an amount of Cr and an
amonnt of Al for obtaining a desired microstructure specified by the present invention.
FIG. 2 is a schematic view showing the shape of a plane bending fatigue test
specimen used in the present example.
[Embodiments of the Invention]
- 9 -
[0021]
Hereinafter, a hot-rolled steel sheet according to an embodiment of the present
invention will be described.
First, the study results by the present inventors and new findings obtained
from the study results which lead to an idea of the present invention will be described.
[0022]
As a result of intensive studies, the present inventors found that, when the
amount of Si in steel is set to 0.15% or less (zero is not included), and, in the
metallographic structure, by % by volume, the ferrite fraction is set to more than 90%
and 98% or less, the martensite fraction is set to 2% or more and less than 1 0%, the
average circle-equivalent diameter and maximum circle-equivalent diameter of ferrite
are set to 4 )lm or more and 30 )lm or less respectively, and the average circleequivalent
diameter and maximum circle-equivalent diameter of martensite are set to
10 J.Lm or less and 20 J.Lm or less respectively, in hot-rolled steel sheets, it is possible to
ensure excellent surface properties not causing Si scale patterns on surfaces, excellent
fatigue resistance and shape fixability, favorable hole expansibility, and high strength.
[0023]
Next, the metallographic structure (microstructure) of a hot-rolled steel sheet
of the present embodiment will be described.
In the hot-rolled steel sheet according to the present embodiment, ferrite is
included as a primary phase, the volume percentage of ferrite is set to more than 90%
and 98% or less, and the average circle-equivalent diameter of ferrite is set to 4 J.Lm or
more. In such a case, favorable elongation, which is workability required during
press forming, is imparted, and the yield ratio is limited, whereby excellent shape
fixability can be obtained. In order to further improve elongation and shape fixability,
- 10 -
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it is preferable to set the volume percentage of ferrite to 92% or more and the average
circle-equivalent diameter to 6 Jill1 or more. Meanwhile, the upper limit of the
average circle-equivalent diameter of ferrite is not particularly limited, but is
preferably set to 15 J.tm or less from the viewpoint of hole expansibility.
In addition, when the maximum circle-equivalent diameter of ferrite is set to
more than 30 J.tm, it is not possible to ensure sufficient hole expansibility. Therefore,
the maximum circle-equivalent diameter of ferrite needs to be set to 30 Jill1 or less. In
order to further improve hole expansibility, the maximum circle-equivalent diameter of
ferrite is preferably set to 20 t-tm or less. Meanwhile, the lower limit of the maximum
circle-equivalent diameter of ferrite is not particularly limited, but is preferably set to
10 J.tm or more from the viewpoint of shape fixability.
[0024]
In the metallographic structure of the steel sheet according to the present
embodiment, in addition to the ferrite, martensite is included as a second phase, the
volume percentage of martensite is set to 2% or more and less than 10%, and the
average circle-equivalent diameter and maximum circle-equivalent diameter of
martensite are set to 1 0 t-tm or less and 20 t-tm or less respectively. In such a case, it is
possible to ensure excellent maximum tensile strength and hole expansibility, and,
furthermore, a high fatigue limit ratio.
Martensite is a hard metallographic structure and is effective for ensuring
strength. When the fraction of martensite is less than 2%, it is not possible to ensure
a sufficient maximum tensile strength. Therefore, the martensite fraction is set to 2%
or more and preferably set to 3% or more. However, when the martensite fraction is
l 0% or more, strain concentration caused by working occurs in the boundary between
hard martensite and soft metallographic structures, and it is not possible to ensure
- 11 -
sufficient hole expansibility. Therefore, the martensite fraction is set to less than 10%
and preferably set to 8% or less.
In addition, when the circle-equivalent diameter of martensite coarsens,
martensite fractures due to strain concentration, and hole expansibility are deteriorated.
Therefore, the average circle-equivalent diameter of martensite and the maximum
circle-equivalent diameter of martensite are set to l 0 f.Lm or less and 20 f.Lm or less
respectively. In order to further improve hole expansibility, it is preferable to set the
average circle-equivalent diameter of martensite to 5 f.Lm or less and the maximum
circle-equivalent diameter to 10 f.Lm or less. Meanwhile, the lower limits of the
average circle-equivalent diameter and maximum circle-equivalent diameter of
martensite are not particularly limited, but are preferably set to 2 f.Lm or more and 5 f.Lm
or more respectively from the viewpoint of ensuring strength or fatigue resistance.
[0025]
Furthermore, the hot-rolled steel sheet according to the present embodiment
may contain, as the metallographic structure of the remainder, a residual structure of
one or more of bainite, pearlite, and residual austenite as long as the total volume
percentage thereof is less than 1%. The fraction of the residual structure is preferably
low. When the volume percentage of the residual structure is 1% or more, strength
decreases, and fatigue durability deteriorates. Therefore, the volume percentage of
the residual structure needs to be limited to less than 1%. From the viewpoint of
ensuring strength or fatigue resistance, the volume percentage of the residual structure
maybeO%.
[0026]
Here, in the present embodiment, for the identification of ferrite, martensite,
and the residual structure which constitute the metallographic structure and the
- 12 -
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measurement of the area fractions and the circle-equivalent diameters, the reagent
disclosed in Japanese Unexamined Patent Application Publication No. S59-219473 is
used.
Regarding the measurement specimen, a sheet thickness cross-section parallel
to a rolling direction is sampled as an observed surface from a location at a point of 114
or 3/4 of the total width of the steel sheet. The observation surface is ground and is
etched with the reagent disclosed in Japanese Unexamined Patent Application
Publication No. S59-219473, and a location of 1/4 or 3/4 of the sheet thickness is
observed using an optical microscope, thereby carrying out image processing. The
area fractions of ferrite and martensite are measured in the above-described manner.
In the present embodiment, the average value of the area fractions measured at ten
visual fields in a 160 J.Lmx200 J.Lm region at a magnification of 500 times is used as the
area fraction of ferrite or martensite.
In addition, similarly, the cross-sectional areas of grains of ferrite and
martensite are measured respectively by means of image processing, and, with an
assumption that all of the grains have a circular shape, the circle-equivalent diameter of
ferrite or martensite can be inversely computed from the areas. In the present
embodiment, the average value of all of the computed circle-equivalent diameters
measured at ten visual fields at a magnification of 500 times is used as the average
circle-equivalent diameter of ferrite or martensite. The largest one of all of the
computed circle-equivalent diameters is used as the maximum circle-equivalent
diameter of ferrite or martensite.
[0027]
Next, the reasons for limiting the chemical components of the hot-rolled steel
sheet of the present embodiment will be described. Meanwhile, "%" indicating the
- 13 -
amounts of the respective elements refers to "mass%".
[0028]

C is an element necessary to obtain the above-described desired
microstructure. However, when more than 0.20% of C is included, workability and
weldability deteriorate, and thus the amount of C is set to 0.20% or less. The more
preferred amount ofC is 0.15% or less. In addition, when the amount ofC is less
than 0.02%, the martensite fraction reaches less than 2%, and the strength decreases.
Therefore, the amount of C is set to 0.02% or more. The more prefened amount of C
is 0.03% or more.
[0029]

Si needs to be limited in order to prevent the properties of the steel sheet
surface from being deteriorated. When more than 0.15% of S is included, Si scale is
generated on the steel sheet surface during hot-rolling, and the properties of the pickled
steel sheet surface may be significantly deteriorated. Therefore, the amount of Si
needs to be set to 0.15% or less. The amount of Si is desirably limited to 0.10% or
less and more desirably limited to 0.08% or less. Meanwhile, the lower limit of the
amount of S is set to more than 0% since S inevitably intrudes into the steel sheet
during manufacturing.
[0030]

Mn is added to make the second phase structure of the steel sheet martensite
by means of quenching strengthening in addition to solid solution strengthening.
Even when more than 2.0% ofMn is added, this effect is saturated, and thus the upper
- 14 -
limit of the amount ofMn is set to 2.0%. On the other hand, when the amount ofMn
is less than 0.5%, an effect of suppressing pearlitic transformation or bainitic
transformation during cooling is not easily exhibited. Therefore, the amount of Mn is
0.5% or mqre and desirably 0.7% or more.
[0031]

P is an impurity included in hot metal, and the lower limit of the amount of P
is set to more than 0%. P is an element which segregates in grain boundaries and
degrades workability or fatigue properties as the amount of P increases. Therefore,
the amount ofP is desirably small. When more than 0.10% ofP is included, the
workability or fatigue properties and, furthermore, weldability are also adversely
affected. Therefore, the amount of P is limited to 0.10% or less and preferably
limited to 0.08% or less.
[0032]

S is an impurity included in hot metal, and the lower limit of the amount of S
is set to· more than 0%. S is an element which does not only cause cracking during
hot-rolling but also generates inclusions such as MuS, which deteriorates hole
expansibility, when the amount of S is too high. Therefore; the amount of S is
supposed to be extremely decreased. However, as long as the amount ofS is 0.05%
or less, the effects of the present invention are not impaired, and the amount of S is in
the allowable range, and thus the amount of S is limited to 0.05% or Jess. However,
in a case in which hole expansibility are further ensured, the amount of S is preferably
limited to 0.03% or less and more preferably limited to 0.01% or less.
[0033]
- 15 -

<[Cr] x5+[Al]2":0.50>
Cris required to obtain the above-described desired microstructure. The
inclusion of Cr suppresses the formation of iron-based carbides and thus suppresses
pearlitic transformation and bainitic transformation after ferritic transformation.
Furthermore, Cr enhances hardenability and thus enables martensitic transformation.
Therefore, Cr is an important element for balancing the strength, elongation, hole
expansibility, and fatigue properties of the steel sheet at a high level. These effects
cannot be obtained when the amount of Cr is less than 0.05%. On the other hand,
when the amount of Cr exceeds 0.5%, the effects are saturated. Therefore, the
amount of Cr is set to 0.05% or more and 0.5% or less. In order to further develop
the above-described effects, the an10unt of Cr is preferably set to 0.06% or more.
[0034]
Al accelerates ferritic transformation, furthermore, suppresses the formation
of coarse cementite, and improves workability. AI is required to impart excellent hole
expansibility and fatigue properties and, furthermore, shape fixability to the hot-rolled
steel sheet of the present embodiment. In addition, AI is also available as a
deoxidizing material. However, the excess addition of AI increases the number of Albased
coarse inclusions and causes the deterioration of hole expansibility and surface
damages. Therefore, the upper limit of the amount of AI is set to 0.5%. A preferred
amount of AI is 0.4% or less. On the other hand, when the amount of AI is less than
0.01 %, an effect of accelerating ferritic transformation cannot be obtained, and· thus the
amount of AI needs to be set to 0.01% or more. The more preferred amount of AI is
0.05% or more.
- 16 -
[0035)
Furthermore, in the hot-rolled steel sheet of the present embodiment, the
amount of Cr contributing to martensitic transformation and the amount of AI
accelerating ferritic transformation satisfy Expression (1) below. It is important for
the amounts of Cr and A1 to satisfy the expression since it becomes possible to
manufacture high-strength hot-rolled steel sheets having excellent fatigue resistance
and having excellent shape fixability and hole expansibility.
[0036)
FIG. 1 shows a relationship between the amount of Cr "mass%" and the
an1ount of AI "mass%" for obtaining the desired microstructure specified by the
present invention. In the graph of FIG. 1, "X" indicates comparative steel incapable
of obtruning the desired microstructure.
As is clear from the graph of FIG. 1, when predetermined amounts or more of
Cr and AI are added so as to satisfy Expression (1) below, it is possible to increase the
average value of the circle-equivalent diameters of ferrite and, additionally, decrease
the circle-equivalent diameters of martensite, and thus the high-strength hot-rolled
steel sheet of the present embodiment having excellent shape fixability and hole
expansibility can be obtained. Meanwhile, in order to further develop this effect, the
left side ([Cr)x5+[Al]) of Expression (1) below is preferably set to 0.70 or more.
[Cr)x5+[Al]2:0.50 Expression (1)
[0037)
The reasons therefor are not absolutely clear; however, according to the
present inventors, are assumed as described below.
First, since the addition of the predetermined amount (being 0.01% to 0.5%
and satisfying Expression (1 )) of AI improves the transformation point, it is possible to
- 17 -
initiate ferritic transformation at a higher temperature. Therefore, ferrite grains grow,
the average value of the circle-equivalent diameters of the ferrite grains increases, and
the yield stress (0.2% proof stress) decreases. Therefore, the yield ratio decreases,
and the hot-rolled steel sheet has excellent shape fixability. Furthermore, due to the
improvement of the transformation point, the transformation is capable of initiating
before austenite coarsens by means of grain growth. Therefore, ferritic
transformation becomes possible at a larger number of nucleation sites, and residual
austenite after ferritic transformation fmely disperses. When the steel sheet is
quenched at this time, it is considered that martensite having a small circle-equivalent
diameter can be obtained. However, AI only has a weak effect of suppressing the
generation of iron-based carbides and allows the generation of pearlite or generates
bainite without being quenched. Therefore, a sufficient martensite fraction cannot be
obtained. Therefore, when Cr as well as AI is added as much as 0.05% to 0.5% and
Expression (1) is satisfied, as described above, the generation of iron-based carbides is
suppressed, and it is possible to enhance hardenability. That is, when the actions of
AI and Cr are combined together, martensite having a small circle-equivalent diameter
can be obtained, and hot-rolled steel sheets having favorable hole expansibility can be
obtained.
[0038]
Therefore, high-strength hot-rolled steel sheets having no Si scale pattern on
the surface, having excellent fatigue resistance, and having excellent shape fixability
and hole expansibility can be manufactured by adjusting the amounts of these two
elements. ; That is, in the present invention, it is important to satisfY Expression (1 ).
In addition, to DP steel of the related art, it is common to add Si, and Si is capable of
realizing the effects exhibited by AI and Cr. Therefore, in the related art, it is
- 18 -
considered to be impossible to confirm the above-described effect of the combined
addition of AI and Cr.
[0039]

N is an impurity element, and the lower limit of the amount ofN is set to
more than 0. When the amount ofN exceeds 0.01 %, coarse nitrides are formed, and
bendability or hole expansibility are deteriorated. Therefore, the upper limit of the
amount ofN is limited to 0.01% or less. In addition, when the amount ofN increases,
blow holes are generated during welding. Therefore, the amount ofN is preferably
decreased. The lower limit of the amount ofN is desirably small and is not
particularly specified. Since the amount ofN being set to less than 0.0005%
increases manufacturing costs, the amount ofN is preferably set to 0.0005% or more.
[0040]

The lower limit values of the amounts ofTi and Nb are 0%. Ti and Nb are
elements that form carbides and precipitation-strengthen ferrite. However, when
more than 0.10% ofNb is added, ferritic transformation is significantly delayed, and
elongation is deteriorated. Therefore, the upper limit of the amount ofNb is
preferably set to 0.10%. In addition, when more than 0.20% ofTi is added, ferrite is
excessively strengthened, and favorable elongation cannot be obtained. Therefore,
the upper limit of the amount ofTi is preferably set to 0.20%. Meanwhile, in order to
strengthen ferrite, 0.005% or more ofNb and 0.02% or more ofTi need to be added
respectively.
[0041]
- 19 -
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The lower limit values of the amounts of Cu, Ni, Mo, and V are 0%. Cu, Ni,
Mo, and V are elements having an effect of increasing the strength of hot -rolled steel
sheets by means of precipitation strengthening or solid solution strengthening, and any
one or more of these may be added. The above-described effect is saturated when the
amount of Cu is more than 2.0%, the amount ofNi is more than 2.0%, the amount.of
Mo is more than 1.0%, and the amount ofV is more than 0.3%, and thus the inclusion
of these elements of the above-described amounts is not preferred from the viewpoint
of manufacturing costs. Therefore, in a case in which Cu, Ni, Mo, and V are added as
necessary, the amount of Cu is preferably set to 2.0% or less, the amount ofNi is
preferably set to 2.0% or less, the amount ofMo is preferably set to 1.0% or less, and
the amount ofV is preferably set to 0.3% or less. Meanwhile, in a case in which Cu,
Ni, Mo, and V are added as necessary, when the amounts thereof are too small, it is not
possible to sufficiently obtain the above-described effect. Therefore, in a case in
which Cu, Ni, Mo, and V are added, the amount of Cu is preferably set to 0.01% or
more, the amount ofNi is preferably set to 0.01% or more, the amount of Mo is
preferably set to 0.01% or more, and the amount ofV is preferably set to 0.01% or
more.
[0042]

- 20 -
The lower limits of the amounts ofMg, Ca, and REM are 0%. Mg, Ca, and
REM (rare earth elements) are elements which serve as the starting point of fracture,
control the form of non-metallic inclusions causing the deterioration of workability,
and improve workability. However, the above-described effect is saturated when the
amount ofMg is more than 0.01 %, the amount of Ca is more than 0.01 %, and the
amount of REM is more than 0.1 %, and thus the inclusion of these elements of the
above-described amounts is not preferred from the viewpoint of manufacturing costs.
Therefore, in a case in which Mg, Ca, and REM are added as necessary, the amount of
Mg is desirably set to 0.01% or less, the amount of Ca is desirably set to 0.0 l% or-less,
and the amount of REM is desirably set to 0.1% or less. Meanwhile, in order to
control the form of non-metallic inclusions and improve workability, 0.0005% or more
ofMg, 0.0005% or more of Ca., and 0.0005% or more of REM need to be added.
[0043]

The lower limit value of the amount ofB is 0%. In the present embodiment,
in addition to the above-described composition, B may be added in order for highstrengthening.
However, there are cases in which B deteriorates formability when
excessively included. Therefore, the upper limit of the amount ofB is preferably set
to 0.01 %. Meanwhile, in order to obtain the effect of high-strengthening, 0.0002% or
more B needs to be added.
[0044]
Meanwhile, in the present embodiment, the remainder other than the abovedescribed
elements is made of Fe and impurities. Examples of the impurities include
impurities included in raw materials such as mineral ores, scrap, and the like, and
impurities added during manufacturing steps.
- 21 -
In addition, as the impurity, for example, 0 forms non-metallic inclusions and
has an adverse influence on qualities, and thus the amount of 0 is desirably decreased
to 0.003% or less.
In addition, the present embodiment may contain a total of 1% of less of Zr,
Sn, Co, Zn, and W in addition to the above-described elements. However, Sn has a
concern of generating defects during hot-rolling, and thus, in the case of being
included, the amount of Sn is desirably 0.05% or less.
[0045]
Meanwhile, in the high-strength hot-rolled steel sheet of the present
embodiment, it is possible to improve corrosion resistance by providing a plated layer
such as a hot-dip galvanized layer obtained by a hot-dip galvanizing treatment or,
furthermore, a zinc alloy-plated (galvannealed) layer obtained by an alloying treatment
after a galvanizing treatment on the surface ofthe hot-rolled steel sheet described
above.
In addition, the plated layer does not need to be a pure zinc layer and may
contain elements such as Si, Mg, Zn, AI, Fe, Mn, Ca, and Zr so as to fmther improve
corrosion resistance. The provision of the above-described plated layer does not
impair the excellent fatigue resistance, shape fixability, and hole expansibility of the
hot-rolled steel sheet of the present embodiment.
[0046]
Furthermore, the hot -rolled steel sheet of the present embodiment may have
any of surface-treated layers obtained by the formation of an organic membrane, a film
lamination, an organic salt/inorganic salt treatment, a non-chromate treatment, or the
like. Even when these surface-treated layers are provided, the effects of the hot-rolled
steel sheet of the present embodiment can be sufficiently obtained without being
- 22 -
impaired.
[0047]
Next, a method for manufacturing the high-strength hot-rolled steel sheet of
the present embodiment described above will be described.
In order to realize hot-rolled steel sheets having excellent surface properties,
fatigue resistance and shape fixability, and favorable hole expansibility and high
strength, as described above, the metallographic structure is important. In the
metallographic structure, the ferrite fraction is set to more than 90% and 98% or less,
the martensite fraction is set to 2% to less than 10%, the fraction of the residual
structure made of one or more of pearlite, bainite, and residual austenite is set to less
than 1%, the average circle-equivalent diameter and maximum circle-equivalent
diameter of ferrite are set to 4 J.!m or more and 30 J.!m or less respectively, and the
average circle-equivalent diameter and maximum circle-equivalent diameter of
martensite are set to 10 ~-tm or less on an average and 20 J.tm or less respectively. The
details of manufacturing conditions for satisfying what has been described above at the
same time will be described.
[0048]
The manufacturing method preceding hot-rolling is not particularly limited.
That is, subsequent to melting using a blast furnace, an electric furnace, or the like, a
variety of secondary smelting processes are carried out so that the components are
adjusted as described above. Next, it is necessary to carry out ordinary continuous
casting, casting using an ingot method, and, additionally, casting using a method such
as thin slab casting. In the case of continuous casting, the steel sheet may be hotrolled
after being cooled to a low temperature and then heated again. An ingot may
be hot-rolled without being cooled to room temperature. Alternatively, a casting slab
- 23 -
may be continuously hot-rolled. Scrap may be used as a raw material as long as the
components can be controlled to be in the range of the present embodiment.
[0049]
The high-strength hot-rolled steel sheet of the present embodiment having
excellent surface properties, hole expansibility, and shape fixability and having
excellent fatigue resistance can be obtained in a case in which the following
requirements are satisfied.
[0050]
That is, in the manufacturing of the high-strength steel sheet, the steel sheet is
melted to the predetermined steel sheet components described above, and a casting
slab is cooled directly or after being cooled, and heating, thereby completing roughrolling.
For the obtained rough-rolled specimen, the end temperature of finish-rolling
is set to 800°C to 950°C, cooling is initiated within two seconds after completing the
finish-rolling, and the specimen is cooled to a first temperature range of 600°C to
750°C at an average cooling rate of 50°C/s to less than 150°C/s. After that, the
specimen is held in a second temperature range of the cooling end temperature or
lower and 550°C or higher for two seconds to 20 seconds in a state of the cooling rate
being 0°C/s to 1 0°C/s, then, is cooled from the cooling end temperature to 300°C at an
average cooling rate of 50°C/s or more, and is coiled at 300°C or lower. Therefore,
high-strength hot-rolled steel sheets having excellent surface properties, hole
expansibility, and shape fixability and having excellent fatigue resistance can be
manufactured.
[0051]
The finish-rolling end temperature needs to be set to 800°C to 950°C.
In the high-strength hot-rolled steel sheet ofthe present embodiment, hole
- 24 -
expansibility are enhanced when the ferrite fraction in the structure is set to more than
90% to 98%. However, in a case in which the finish-rolling end temperature exceeds
950°C, ferritic transformation is delayed, and a ferrite fraction of more than 90%
cannot be ensured. In addition, in a case in which the finish-rolling end temperature I
is lower than 800°C, transformation occurs in the middle of rolling, and an
inhomogeneous structure is formed. As a result, it becomes difficult for the steel
sheet to have favorable hole expansibility. Therefore, the finish-rolling end
temperature is set to 800°C to 950°C. Preferably, the finish-rolling end temperature is
set to 820°C to 930°C.
[0052]
Cooling is initiated within two seconds after completing the finish-rolling, and
specimen is cooled to the first temperature range of 600°C to 750°C at an average
cooling rate of 50°C/s to less than 150°C/s. After that, the specimen is held in a
second temperature range of the cooling end temperature or lower and 550°C or higher
for two seconds to 20 seconds in a state of the cooling rate being 0°C/s to l0°C/s.
[0053]
In a case in which longer than two seconds elapses after completing the
finish-rolling to the initiation of the cooling and/or a case in which the average cooling
rate to the first temperature range is less than 50°C/s, austenite grain diameters before
transformation are coarsened. Therefore, it is ncit possible to set the circle-equivalent
diameter of martensite to 1 0 tJ.m or less on average and 20 tJ.m or less at maximum.
Additionally, since ferritic transformation is delayed, it becomes difficult to ensure a
ferrite fraction of more than 90%. Therefore, cooling is initiated within two: seconds
after completing the finish-rolling, and the average cooling rate to the first temperature
range is set to 50°C/s or more. Preferably, the average cooling rate is set to 70°C/s or
- 25 -
more. On the other hand, when the average cooling rate to the first temperature range
is set to 150°C/s or more, pearlitic transformation occurs earlier, and thus it is not
possible to ensure a ferrite fraction of more than 90%. As a result, it becomes
i
' i:
difficult to manufacture hot -rolled steel sheets having favorable hole expansibility. i'
I :I r!
I: Therefore, the average cooling rate to the first temperature range is set to less than
150°C/s and preferably set to 130°C/s or less. I ~
[0054]
In addition, in a case in which the upper limit temperature of the first
temperature range is higher than 750°C and/or a case in which the holding time
(cooling time) in the second temperature range is shorter than two seconds as well, it is
not possible to ensure a ferrite fraction of more than 90%. Therefore, the first
temperature range is set to 750°C or lower, and the holding time in the second
temperature range is set to two seconds or longer. A preferred upper limit
temperature is 720°C or lower, and the holding time is five seconds or longer.
However, when the holding time exceeds 20 seconds, pearlite is generated, and thus it
is not possible to ensure a martensite fraction of 2% or more. Therefore, the holding
time in the second temperature range is set to 20 seconds or shorter and preferably set
to 15 seconds or shorter.
[0055]
Furthermore, in a case in which the lower limit temperature of the first
temperature range is lower than 600°C, it is not possible to set the circle-equivalent
diameter of ferrite to 4 J.U11 or more on average and 30 !J.m or less on maximum, and
high-strength hot -rolled steel sheets having excellent shape fixability cannot be
manufactured. Therefore, the lower limit of the first temperature range is set to
600°C or higher. A preferred .lower limit temperature of the first temperature range is
- 26 -
650°C or higher.
[0056]
As described above, it is important that cooling is initiated within two seconds
after completing the finish-rolling, the specimen is cooled to the first temperature
range of 600°C to 750°C at an cooling rate of 50°C/s to less than 150°C/s, furthermore,
after that, the specimen is held in the second temperature range of the cooling end
temperature or lower and 550°C or higher for two seconds to 20 seconds in a state of
the cooling rate being 0°C/s to 10°C/s.
[0057]
Next, after being held (cooled) in the second temperature range, the specimen
is cooled from the holding (cooling) end temperature to 300°C at an average cooling
rate of 50°C/s or more. When the average cooling rate from the second temperature
range holding (cooling) end temperature to 300°C is less than 50°C/s, bainitic
transformation cannot be avoided, it is not possible to ensure a martensite fraction of
2% or more, and excellent fatigue properties cannot be obtained. Preferably, the
· average cooling rate from the holding (cooling) end temperature to 300°C is 60°C/s or
more. Meanwhile, the upper limit of the average cooling rate from the holding
(cooling) end temperature to 300°C is not particularly limited, but is preferably set to
1 00°C/s or less from the viewpoint of avoiding the introduction of strain into ferrite.
[0058]
Coiling after the cooling of the hot-rolled steel sheet needs to be carried out at
300°C or lower. This is because it is necessary to transform the secondary phase in
the metallographic structure to martensite. Since bainite is generated at a coiling
temperature of higher than 300°C, it is not possible to ensure 2% or more of martensite,
and excellent fatigue properties cannot be obtained. Preferably, the coiling
- 27 -
------- ~-:-.:.-c·--~ ~-: -.
temperature is set to 270°C or lower.
[0059]
As a result, the high-strength hot-rolled steel sheet of the present embodiment
can be manufactured.
Meanwhile, for the purpose of correction of the steel sheet shape or
improvement in ductility by the introduction of moving dislocations, it is desirable to
carry out skin-pass rolling with a reduction of 0.1% to 2% after the end of all steps.
In addition, after the end of all steps, pickling may be carried out on the
obtained hot-rolled steel sheet as necessary for the purpose of removing scale attached
to the surface of the obtained hot-rolled steel sheet. Furthermore, after the pickling,
skin-pass or cold-rolling may be carried out on the obtained hot-rolled steel sheet inline
or off-line with a reduction of 10% or less.
[0060]
In addition, after the coiling, a galvanizing treatment may be carried out as
necessary. For example, a hot-dip galvanized layer obtained by a hot-dip galvanizing
treatment or, furthermore, a zinc alloy-plated (galvannealed) layer obtained by an
alloying treatment after a galvanizing treatment may be formed.
[0061]
Furthermore, a surface-treated layer obtained by the formation of an organic
membrane, a film lamination, an organic salt/inorganic salt treatment, a non-chromate
treatment, or the like may be formed on the surface of the hot-rolled steel sheet.
[Example]
[0062]
Hereinafter, the technical contents of the present invention will be further
described with reference to examples of the present invention. Meanwhile,
- 28 -
I,
I!
I'
'I
[i
II
conditions in the examples described below are examples of conditions for confirming
the feasibility and effects of the present invention. The present invention is not
limited to these condition examples. In addition, the present invention is allowed to
employ a variety of conditions within the scope of the gist of the present invention as
long as the object of the present invention is achieved.
[0063]
As the examples, study results obtained using steels A to I shown in Table 1
which satisfy the component composition of the present invention (invention steels)
and steels a to f which do not satisfY the component composition of the present
invention (comparative steels) will be described.
[0064]
All of the invention steels and the comparative steels were cast, then, were
reheated immediately or after being cooled to room temperature, and were roughly
rolled. After that, the obtained rough-rolled specimens were hot-rolled under
conditions shown in Table 2 and were cooled, air-cooled, and coiled under conditions
shown in Table 2, thereby producing hot-rolled steel sheets all having a sheet thickness
of3.4mm.
Meanwhile, on some of the hot-rolled steel sheets, skin-pass rolling was
carried out before pickling with a reduction in a range of0.3% to 2.0%.
[0065]
After that, for the obtained steel sheets A-1 to I-1 and a-1 to f-1, the following
properties were evaluated.
[0066]
JIS No. 5 test specimens were cut out in a direction perpendicular to the
rolling direction, tensile tests were carried out according to JIS Z 2241, and the yield
- 29 -
stress (YP), the maximum tensile strength (TS), and the yield ratio (YR) were obtained.
Meanwhile, test specimens having a maximum tensile strength of 590 MPa or more in
the tensile test were evaluated as having "high strength". In addition, test specimens
having a yield ratio of 80% or less were evaluated as "having excellent shape
fixability".
[0067]
The hole expansion value (!c) was measured using the hole expansion test
method described in Japan Iron and Steel Federation Standard JFS TlOOl-1996.
Meanwhile, test specimens having a hole expansion value lc of 80% or higher were .
evaluated as having "excellent hole expansibility".
[0068]
The fatigue limit ratio was computed as a value obtained by carrying out a
completely-reversed plane bending fatigue test on a plane bending fatigue test
specimen and dividing the fatigue strength at the 2x 106 cycle by the maximum tensile
strength TS of the steel sheet. As the plane bending fatigue test specimen, a specimen
having a length of98 mm, a width of38 mm, a minimum cross-sectional portion with
a width of 20 mm, notches with a radius of curvature of 30 mm, and a sheet thickness t
remaining unchanged after rolling as shown in FIG. 2 was used.
Meanwhile, test specimens having a fatigue limit ratio of 0.45 or more were
evaluated as "having excellent fatigue resistance".
[0069]
In addition, in order to evaluate the surface properties of the steel sheets,
whether or not Si scale pattern were formed on the steel sheet surface was visually
observed.
[0070]
- 30 -
In addition, regarding the formability (workability) of the hot-rolled steel
sheet according to the present invention, test specimens having an elongation (El)
obtained from the tensile test of 24% or higher were evaluated as having excellent
formability.
[0071]
Regarding some of the hot-rolled steel sheets shown in Table 3, the hot-rolled
steel sheets were heated to 660°C to 720°C and were subjected to a hot-dip galvanizing
treatment so as to produce hot-dip galvanized steel sheets (GI), and then material tests
were carried out. Alternatively, an alloying thermal treatment was carried out at
540°C to 580°C after the hot-dip galvanizing treatment so as to produce galvannealed
steel sheets (GA), and then material tests were carried out. "HR" in Table 3 indicates
hot-rolling which had not been subjected to a plating treatment.
[0072]
Microstructural observation was carried out using the above-described method,
and the volume percentages (fractions) of the respective structures, and the average
circle-equivalent diameters and maximum circle-equivalent diameters of ferrite and
martensite were measured. Meanwhile, the "residual structure fraction" in the table
indicates the volume percentage of the structure made of one or more of pearlite,
bainite, and residual austenite. In addition, regarding the "residual structure fraction"
in the table, the expression of":0.50 Expression ( 1)
here, in Expression (1), [Cr] represents an amount ofCr (mass%), and [AI]
represents an amount of Al (mass%).
2. The hot-rolled steel sheet according to Claim 1, further comprising:
by mass%, one or two of
Ti: 0.02% to 0.20%; and
Nb: 0.005% to 0.10%.
3. The hot-rolled steel sheet according to Claim 1 or 2, further comprising:
by mass%, one or more of
Cu: 0.01% to 2.0%;
Ni: 0,01% to 2.0%;
Mo: 0.01% to 1.0%; and
V: 0,01% to 0.3%.
4. The hot-rolled steel sheet according to any one of Claims 1 to 3, further
comprising:
by mass%, one or more of
Mg: 0.0005% to O.oi %;
Ca: 0.0005% to 0.01 %; and
REM: 0.0005% to 0.1 %.
5. The hot-rolled steel sheet according to any one of Claims 1 to 4, further
comprising:
by mass%,
B: 0.0002% to 0.01 % .