Technical Field
[0001] The present disclosure relates to a high-strength hot-rolled steel sheet.
Background Art
[0002] As a strengthening method of increasing the strength of steel, (1) solid solution
strengthening by addition of elements such as C, Si, and Mn, (2) precipitation strengthening
using precipitates such as Ti and Nb, and (3) transformation hardening using a microstructure
as a continuous cooling transformation microstructure in which dislocation strengthening or
crystal fine grain strengthening is expressed are effective. In particular, members for
automobiles are being reduced in weight and improved in safety and durability, and there is a
demand for an increase in strength of a steel material as a material.
[0003] Solid solution strengthening has a smaller strength increasing effect than precipitation
strengthening and transformation hardening, and thus it is difficult to increase the strength
required of a material for an automobile member only by solid solution strengthening.
On the other hand, with regard to precipitation strengthening, technological
development for achieving high strength while maintaining excellent deformability of the
original uniform structure of a ferrite phase has started to be studied again in recent years. For
example, a method has been proposed in which carbide forming elements such as Ti, Nb, and
Mo are utilized to precipitate fine carbides to strengthen the ferrite structure (for example,
Patent Documents 1 to 3). In a structure having a relatively low dislocation density, which is
mainly composed of ferrite, fine carbides for improving strength are precipitated to increase
strength by precipitation strengthening.
[0004] According to these methods, it is necessary to form a ferrite structure transformed at a
relatively high temperature in order to develop precipitation strengthening. In order to develop
dislocation strengthening, it is necessary to perform phase transformation at a low temperature,
and thus it is difficult to develop both precipitation strengthening and dislocation strengthening.
[0005] On the other hand, there has been proposed a high-strength steel sheet having excellent
stretch flangeability, which includes an acicular ferrite structure transformed at a relatively low
temperature and has a structure in which fine carbides TiC and NbC are precipitated (for
example, Patent Document 4).
[0006] In general, it is known that precipitates are more likely to be nucleated in defects such
as dislocations and crystal grain boundaries than in portions without defects. Therefore,
2
conventionally, when the dislocation density is increased, it has been used for the purpose of
promoting precipitation on dislocations (for example, Patent Document 5).
[0007] Note that Non-Patent Document 1 proposes calculating the dislocation density using
strain of a crystal lattice obtained by measuring X-ray diffraction.
[0008] Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2003-89848
Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 2007-262487
Patent Document 3: Japanese Patent Application Laid-Open (JP-A) No. 2007-247046
Patent Document 4: Japanese Patent Application Laid-Open (JP-A) No. H7-11382
Patent Document 5: Japanese Patent Application Laid-Open (JP-A) No. 2013-133534
[0009] Non-Patent Document 1: G. K. Williamson and R. E. Smallman, "Dislocation densities
in some annealed and cold-worked metals from measurements on X-ray Debye-Scherrer
spectrum", Philosophical Magazine, Vol. 8, 1956, p. 34-46
SUMMARY OF INVENTION
Technical Problem
[0010] However, in Patent Documents 4 and 5, studies on utilization of both precipitation
strengthening and dislocation strengthening have not been sufficient. In order to increase the
strength of the precipitation-strengthened steel, generally, a method of increasing a precipitation
strengthening amount by increasing a content of an alloy element is considered. However, not
only the cost may increase, but also workability and the like may deteriorate, and an end face
of a hole formed by punching a steel sheet may be damaged, for example, peeled or turned up.
There has been room for examination for further increasing the strength while suppressing the
content of the alloy element.
[0011] Therefore, an object of the present disclosure is to provide a high-strength hot-rolled
steel sheet which suppresses damage to a punched edge of the steel sheet while suppressing a
content of an alloy element, and has a tensile strength of 850 MPa or more.
Solution to Problem
[0012] The present inventors aimed to obtain a large precipitation strengthening by
precipitating fine TiC precipitates after phase transformation while increasing the dislocation
density of a steel sheet by phase transformation to increase dislocation strengthening. Therefore,
the present inventors actively utilized bainitic ferrite having a high dislocation density for the
purpose of finely precipitating TiC precipitates after the bainitic ferrite is formed. However,
precipitation strengthening is not effectively exhibited when TiC precipitates are precipitated
on dislocations. Therefore, the present inventors aimed to efficiently exhibit dislocation
3
strengthening and precipitation strengthening by precipitating TiC precipitates on a matrix that
is not on the dislocations.
Then, the present inventors have found that it is possible to suppress a content of an
alloy element and to obtain high tensile strength while suppressing cost by efficiently
developing both dislocation strengthening due to a high dislocation density and precipitation
strengthening due to formation of a TiC precipitate in a matrix not on dislocations and
effectively utilizing the alloy element. Furthermore, the present inventors have found that a
decrease in workability due to the content of the alloy element is also suppressed, and that the
occurrence of damage on a punched edge of the steel sheet is suppressed.
[0013] The present disclosure has been made based on such findings, and the gist thereof is
as follows.
(1) A high-strength hot-rolled steel sheet having a chemical composition containing,
by mass:
C: from 0.030 to 0.250%;
Si: from 0.01 to 1.50%;
Mn: from 0.1 to 3.0%;
Ti: from 0.040 to 0.200%;
P: 0.100% or less;
S: 0.005% or less;
Al: 0.500% or less;
N: 0.0090% or less;
B: from 0 to 0.0030%;
a total of one or more of Nb, Mo and V: from 0 to 0.040%;
a total of one or more of Ca and REM: from 0 to 0.010%; and
a balance consisting of Fe and impurities, a mass ratio [Ti]/[C] of a Ti amount to a C
amount being from 0.16 to 3.00, and a product [Ti] × [C] of the Ti amount and the C amount
being from 0.0015 to 0.0160,
the high-strength hot-rolled steel sheet:
having a mean dislocation density of from 1 × 1014 to 1 × 1016 m-2; and
containing at least bainitic ferrite,
wherein a total area ratio of the bainitic ferrite and ferrite is 70% or more and less than
90%,
wherein a total area ratio of martensite and retained austenite is 5% or more and 30%
or less,
wherein, in ferrite crystal grains and in bainitic ferrite crystal grains, a mean number
4
density of TiC precipitates is from 1 × 1017 to 5 × 1018 [precipitates/cm3
],
wherein an amount of Ti present as a TiC precipitate precipitated in a matrix not on
dislocations is 30 mass% or more of a total amount of Ti in the steel sheet,
wherein a tensile strength is 850 MPa or more, and
wherein [Ti] and [C] represent the Ti amount and the C amount (mass%), respectively.
(2) The high-strength hot-rolled steel sheet according to (1), containing, by mass:
B: 0.0001% or more and less than 0.0005%.
(3) The high-strength hot-rolled steel sheet according to (1) or (2), containing, by mass:
the total of one or more of Nb, Mo, and V: from 0.01 to 0.040%.
(4) The high-strength hot-rolled steel sheet according to any one of (1) to (3),
containing, by mass:
the total of one or more of Ca and REM: from 0.0005 to 0.01%.
(5) The high-strength hot-rolled steel sheet according to any one of (1) to (4), wherein
the total area ratio of the bainitic ferrite and the ferrite is 80% or more and less than 90%.
(6) The high-strength hot-rolled steel sheet according to any one of (1) to (5), wherein
an area ratio of the bainitic ferrite is 50% or more and less than 90%.
Advantageous Effects of Invention
[0014] According to the present disclosure, it is possible to provide a high-strength hot-rolled
steel sheet which has high tensile strength while suppressing a content of an alloy element, and
in which damage to a punched edge of the steel sheet is less likely to occur during punching.
BRIEF DESCRIPTION OF DRAWINGS
[0015] Fig. 1A shows a schematic diagram of an arrangement of TiC precipitates on
dislocations.
Fig. 1B shows a schematic diagram of an arrangement of TiC precipitates on a matrix.
Fig. 2 is a diagram showing relationship between [Ti] × [C] and tensile strength,
between the case where a content of Ti present as a TiC precipitate precipitated in the matrix
that is not on the dislocations is 30 mass% or more and the case where the content of Ti is less
than 30% of a total Ti content of a steel sheet having a mean dislocation density in a range of
from 1 × 1014 to 1 × 1016 m-2.
DESCRIPTION OF EMBODIMENTS
[0016] Hereinafter, an exemplary embodiment of the present disclosure will be described in
detail.
5
[0017] In the present specification, the "%" indication of a content of each element of a
chemical composition means "mass %".
The content of each element of the chemical composition is sometimes referred to as
"element amount". For example, the content of C is sometimes expressed as C amount.
A numerical range indicated using “to” means a range including numerical values
described before and after “to” as a lower limit value and an upper limit value.
A numerical range when "greater than" or "less than" is attached to numerical values
described before and after "to" means a range not including these numerical values as a lower
limit value or an upper limit value.
In the numerical ranges according to stages herein, the upper limit value according to
one numerical range may be replaced with the upper limit value of any other numerical range
according to stages, and may be replaced with a value described in an Example. The lower
limit value according to one numerical range may be replaced with the lower limit value of any
other numerical range according to stages, and may be replaced with a value described in an
Example.
"0 to" as the content (%) means that the component is an optional component and need
not be contained.
The term “step” includes not only an independent step but also a step that cannot be
clearly distinguished from other steps as long as the intended purpose of step is achieved.
[0018]
A high-strength hot-rolled steel sheet according to the present embodiment (hereinafter
sometimes simply referred to as “steel sheet”):
has a predetermined chemical components, in which a mass ratio [Ti]/[C] of a Ti
content to a C content is from 0.16 to 3.00, and a product [Ti] × [C] of the Ti content and the C
content is from 0.0015 to 0.0160,
has a mean dislocation density of from 1 × 1014 to 1 × 1016 m-2; and
contains at least bainitic ferrite.
A total area ratio of the bainitic ferrite and ferrite is 70% or more and less than 90%.
A total area ratio of martensite and retained austenite is 5% or more and 30% or less.
In ferrite crystal grains and in bainitic ferrite crystal grains, a mean number density of
TiC precipitates is from 1 × 1017 to 5 × 1018 [precipitates/cm3
].
A content of Ti present as a TiC precipitate precipitated in a matrix not on dislocations
is 30 mass% or more of a total Ti content of the steel sheet.
A tensile strength is 850 MPa or more.
[Ti] and [C] represent the Ti amount and the C amount (mass%), respectively.
6
[0019] By virtue of the above configuration, the high-strength hot-rolled steel sheet according
to the present embodiment is a high-strength hot-rolled steel sheet having high tensile strength
and in which damage to a punched edge of the steel sheet is less likely to occur during punching.
The high-strength hot-rolled steel sheet according to the present embodiment has been found
by the following findings.
[0020] In order to improve the strength of the steel sheet, it is important to control an existence
state of Ti in the steel sheet. First, there are mainly three possible existence states in which Ti
exists as a solid solution, as a coarse TiN precipitate or a TiS precipitate, and as a TiC precipitate.
First, TiN precipitates or TiS precipitates have a very small solubility product in iron, are
precipitated even in a relatively high temperature austenite region, and become coarse, and thus
do not contribute to the strength of the steel sheet. An amount of TiN precipitates or TiS
precipitates precipitated is almost determined by contents of N and S in the steel sheet. Whether
residual Ti is precipitated as a TiC precipitate or remains as a solid solution atom greatly
changes due to the influence of thermomechanical treatment of the steel sheet. In the case of
Ti as a solid solution, Ti is uniformly present as a single atom in crystal grains, and the
strengthening mechanism of the steel sheet is a solid solution strengthening amount, but an
amount of increase in strength is small. On the other hand, when Ti is precipitated as a TiC
precipitate, the precipitation strengthening amount greatly changes depending on the number
density and size of the precipitate, and thus greatly affects the strength of the steel sheet.
Furthermore, it has been found that the position where the TiC precipitate precipitates affects
the strength of the steel material.
The present inventors paid attention to a position where a TiC precipitate (hereinafter,
also simply referred to as "precipitate") is formed.
As the position where the precipitates are formed, a case where the precipitates are
precipitated and formed at crystal grain boundaries, a case where the precipitates are
precipitated and formed on dislocations in crystal grains, and a case where the precipitates are
uniformly precipitated and formed in a matrix (hereinafter, also simply referred to as "matrix")
that is not on the dislocations in the crystal grains were considered. It is considered that normal
steel having a crystal grain size of several micrometers or more has a low density of crystal
grain boundaries, and precipitates at the crystal grain boundaries do not contribute to
strengthening. The precipitates have a property of being preferentially nucleated on
dislocations as compared with the matrix, but it is considered that whether the precipitates are
precipitated on dislocations or are uniformly precipitated in the matrix depends on the hot
rolling temperature and chemical composition, the degree of supercooling and the diffusion
length of precipitate-forming elements, the dislocation density, and the like.
7
Therefore, the present inventors considered that the position where TiC precipitates
are precipitated, the number density, the relationship between the contents of Ti and C in the
steel sheet, and the microstructure affect the strength of the steel sheet, and conducted studies.
[0021] The present inventors melted and hot-rolled a steel piece containing, in mass%, C:
from 0.030 to 0.250%, Si: from 0.01 to 1.50%, Mn: from 0.1 to 3.0%, Ti: from 0.040 to 0.200%,
P: from 0.100% or less, S: 0.005% or less, Al: 0.500% or less, N: 0.0090% or less, B: from 0
to 0.0030%, a total of one or two or more of Nb, Mo and V: from 0 to 0.040%, and a total of
one or two or more of Ca and REM: from 0 to 0.010%, the balance consisting of Fe and
impurities, to manufacture a steel sheet under various heat treatment conditions, and conducted
the following tests and studies.
[0022] The mean dislocation density of the obtained steel sheet was measured.
The present inventors determined that a large dislocation strengthening was obtained
when the mean dislocation density was in a range of from 1 × 1014 to 1 × 1016 m-2, and the
subsequent tests were performed on steel sheets having a mean dislocation density in the range
of from 1 × 1014 to 1 × 1016 m-2.
[0023] First, a test piece was taken from the steel sheet, and the tensile strength was measured.
[0024] Next, the microstructure was observed, the mean number density of the TiC
precipitates precipitated in the crystal grains was measured, and the formation position of the
TiC precipitates was observed.
[0025] For a steel sheet having a mean dislocation density in the range of from 1 × 1014 to 1
× 1016 m-2, relationship between [Ti] × [C] and the tensile strength when the Ti content is
denoted by [Ti] and the C content is denoted by [C] is shown in Fig. 2. Fig. 2 also shows
relationship of the number density of TiC precipitates and relationship between the case where
the content of Ti present as TiC precipitates precipitated in the matrix not on dislocations is 30
mass% or more and the case where the content of Ti is less than 30% of a total Ti content of the
steel sheet.
It is found that, in ferrite crystal grains and bainitic ferrite crystal grains, high strength
of 850 MPa or more as a target is obtained when the mean number density of TiC precipitates
is from 1 × 1017 to 5 × 1018 [precipitates/cm3
], and the content of Ti present as TiC precipitates
precipitated in the matrix not on dislocations is 30 mass% or more of the total Ti content of the
steel sheet. In addition, it was found that the value of [Ti] × [C] needs to be in the range of
from 0.0015 to 0.0160 in order to obtain the above structure.
The reason why the strength of the steel sheet becomes higher when the content of Ti
present as TiC precipitates precipitated in the matrix not on dislocations is high is considered
as follows. First, as the existence state of Ti other than the TiC precipitates precipitated in the
8
matrix, there are coarse TiN precipitates or coarse TiS precipitates described above, solid
solution Ti atoms, and TiC precipitates on dislocations. The coarse TiN precipitates or coarse
TiS precipitates and the solid solution Ti atoms provide a small strengthening amount for the
reasons described above. Next, when the TiC precipitates exist on dislocations, the dislocations
as obstacles and the TiC precipitates overlap with each other in position, so that the precipitates
are less likely to contribute as new obstacles to suppress an increase in the strengthening amount.
On the other hand, when the TiC precipitates are precipitated in the matrix, both dislocations
and the TiC precipitates effectively act as obstacles at the time of deformation, so that
precipitation strengthening can be more effectively utilized.
[0026] [Ti]× [C] is related to a temperature at which the TiC precipitates are completely
dissolved, that is, a lower limit temperature at which the TiC precipitates are not generated.
When the value of [Ti] × [C] is small, the lower limit temperature at which Ti and C are not
precipitated is low, and, when the value of [Ti] × [C] is large, the lower limit temperature at
which Ti and C are not precipitated is high.
As shown in Fig. 2, when the value of [Ti] × [C] was less than 0.0015, the content of
Ti present as TiC precipitates precipitated in the matrix could not be increased. The reason for
this is considered to be due to insufficient degree of supercooling in a cooling step. When the
value of [Ti] × [C] is small, the temperature at which TiC precipitates are precipitated is low,
and thus the degree of supercooling is lowered. It is considered that, when the degree of
supercooling is small, a driving force for precipitation is small, and a frequency of precipitation
on dislocations where the precipitates are more easily nucleated is high, so that the frequency
of precipitation of TiC in the matrix cannot be increased. It is considered that, when the value
of [Ti] × [C] is 0.0015 or more, the degree of supercooling of TiC precipitation increases, the
driving force for precipitation sufficiently increases, and precipitation occurs in the matrix in
addition to precipitation on dislocations.
On the other hand, even when the value of [Ti] × [C] exceeded 0.0160, and the ratio
of Ti present as a TiC precipitate precipitated in the matrix was increased, the strength decreased.
This is considered to be because the content concentrations of Ti and C are too high, so that the
temperature at which the TiC precipitates are completely dissolved becomes higher than the
temperature at which the TiC precipitates are solutionized in the austenite region, and a part of
TiC is already precipitated. The TiC precipitates in the austenite region are coarse and have a
low number density, and thus less contribute to precipitation strengthening. That is, it is
considered that, when the value of [Ti] × [C] is more than 0.0160, the concentrations of Ti and
C that generate fine precipitates contributing to precipitation strengthening cannot be increased,
and therefore that large tensile strength cannot be obtained. Furthermore, it is considered that,
9
as coarse TiC precipitates generated in the austenite region further grow during cooling, the
concentrations of Ti and C contributing to the generation of fine precipitates after phase
transformation may be lowered, or the number density may be lowered due to an increase in
size of TiC precipitates, and that the effect of increasing the strength is small.
[0027] In addition, it is considered that the content of the alloy element can be reduced and
the decrease in workability caused by the alloy element can be suppressed, by effectively
utilizing the alloy element through efficient development of both precipitation strengthening
and dislocation strengthening.
[0028] According to the above findings, the present inventors have found a high-strength hotrolled steel sheet which has high tensile strength while suppressing a content of an alloy element,
and in which damage to a punched edge of the steel sheet is less likely to occur during punching.
[0029] Hereinafter, details of the high-strength hot-rolled steel sheet according to the present
embodiment will be described.
[0030] (Chemical composition)
The chemical composition of the high-strength hot-rolled steel sheet according to the
present embodiment contains the following elements.
[0031] -Essential element-
C: from 0.030 to 0.250%
Carbon (C) is an important element that generates fine TiC precipitates and contributes
to precipitation strengthening, and is also a necessary element that segregates at crystal grain
boundaries to suppress the occurrence of damage to the punched edge of the steel sheet. An
amount of C required for exhibiting the effect is 0.030% or more, but, when the amount of C is
more than 0.250%, coarse cementite is generated, so that ductility, particularly, local ductility
is reduced. Therefore, the amount of C is from 0.030 to 0.250%, preferably from 0.040 to
0.150%.
[0032] Si: from 0.01 to 1.50%
Silicon (Si) is a deoxidizing element, and an amount of Si is 0.01% or more. Si is an
element that contributes to solid solution strengthening, but, when the amount of Si exceeds
1.50%, workability deteriorates. Therefore, an upper limit of the amount of Si is set to 1.50%.
Therefore, the amount of Si is from 0.01 to 1.50%, preferably from 0.02 to 1.30%.
[0033] Mn: from 0.1 to 3.0%
Manganese (Mn) is an element effective for deoxidation and desulfurization and also
contributes to solid solution strengthening, and therefore an amount of Mn is 0.1% or more.
From the viewpoint of reducing an area ratio of polygonal ferrite, the amount of Mn is
preferably 0.35% or more.
10
On the other hand, when the amount of Mn is more than 3.0%, segregation is likely to
occur, so that the workability deteriorates, and the cost is increased, which is not preferable.
Therefore, the amount of Mn is from 0.1 to 3.0%, preferably from 0.3 to 1.5%.
[0034] Ti: from 0.040 to 0.200%
Titanium (Ti) is an extremely important element that precipitates fine TiC precipitates
in grains of ferrite and bainitic ferrite and contributes to precipitation strengthening. An amount
of Ti is 0.040% or more because Ti precipitates in the matrix to increase the strength. On the
other hand, when the amount of Ti exceeds 0.200%, not only the cost increases, but also the
TiC precipitates tend to be coarsened, which makes manufacture difficult. In order to easily
achieve a suitable number density of TiC precipitates, the amount of Ti is preferably 0.150% or
less. Therefore, the amount of Ti is from 0.040 to 0.200%, preferably from 0.070 to 0.150%.
[0035] P: 0.100% or less
Phosphorus (P) is an impurity, and deteriorates workability and weldability. Therefore,
an amount of P is preferably as low as possible, and is limited to 0.100% or less. The amount
of P is preferably limited to 0.020% or less because P segregates at grain boundaries to decrease
ductility. However, from the viewpoint of the cost for removal of P, the amount of P is
preferably 0.005% or more.
[0036] S: 0.005% or less
Sulfur (S) is an impurity and particularly impairs hot workability. Therefore, an
amount of S is preferably as low as possible, and is limited to 0.005% or less. In order to
suppress a decrease in ductility due to an inclusion such as a sulfide, it is preferable to limit the
amount of S to 0.002% or less. However, from the viewpoint of the cost for removal of S, the
amount of S is preferably 0.0005% or more.
[0037] Al: 0.500% or less
Aluminum (Al) is a deoxidizing agent, and an amount of Al is 0.500% or less. When
Al is excessively contained, a nitride is formed and ductility is lowered. Thus, the amount of
Al is preferably limited to 0.150% or less. In order to sufficiently deoxidize molten steel, the
amount of Al is preferably 0.002% or more.
[0038] N: 0.0090% or less
Nitrogen (N) forms TiN, reduces the workability of steel and also leads to a reduction
in effective amount of Ti forming TiC precipitates. Therefore, an amount of N is preferably as
low as possible, and is limited to 0.0090% or less. However, from the viewpoint of the cost for
removal of N, the amount of N is preferably 0.0010% or more.
[0039] -Optional element-
The chemical composition of the high-strength hot-rolled steel sheet according to the
11
present embodiment may contain the following optional elements in addition to the essential
elements.
[0040] B: from 0 to 0.0030%
Boron (B) is an optional element that can be optionally contained in the steel sheet.
However, since it is an effective element that has an effect of suppressing phase transformation
and can increase an area ratio of bainitic ferrite while suppressing ferrite transformation as much
as possible under appropriate cooling step conditions, it is preferable to incorporate B as
necessary. Therefore, an amount of B is preferably 0.0001% or more.
On the other hand, when the amount of B is more than 0.0030%, precipitates such as
BN are easily generated, and the effect is saturated. Thus, the amount of B is set to 0.0030%
or less. The amount of B is preferably 0.0020% or less. B has a very strong effect of
suppressing phase transformation, and the amount of B is more preferably less than 0.0005%
from the viewpoint of setting a total area ratio of bainitic ferrite and ferrite to 80% or more and
less than 90%.
[0041] Total of one or more of Nb, Mo, and V: from 0 to 0.040%
Niobium (Nb), molybdenum (Mo), and vanadium (V) are optional elements optionally
contained in the steel sheet. Nb, Mo, and V are elements that precipitate carbide in the ferrite
crystal grains similarly to Ti, but an alloy cost is high and a precipitation strengthening ability
is smaller than that of Ti. Therefore, one or more of Nb, Mo, and V may be contained, and a
total content thereof is set to from 0 to 0.040%.
On the other hand, Nb and V are elements effective for strengthening the steel sheet
by delaying recrystallization during hot rolling and refining crystal grains of the steel sheet. Mo
is an element for improving hardenability, and is also an effective element for increasing the
area ratio of bainitic ferrite while suppressing ferrite transformation as much as possible. In
order to sufficiently obtain these effects, a total content of Nb, Mo, and V is preferably 0.01%
or more.
In the steel sheet, these elements are combined with TiC precipitates and exist as (Ti,
M) C. Here, M is one or more of Nb, V, and Mo.
[0042] Total of one or more of Ca and REM: from 0 to 0.010%
Calcium (Ca) and REM are optional elements optionally contained in the steel sheet.
Ca and REM are elements having a function of controlling the form of inclusions which become
a starting point of fracture and cause deterioration of workability to detoxify the inclusions.
One or more of Ca and REM may be contained, and a total content thereof is set to
from 0 to 0.01% or less.
On the other hand, in order to sufficiently obtain the effect of controlling the form of
12
inclusions to detoxify the inclusions, the total content of one or more of calcium (Ca) and REM
is preferably 0.0005% or more.
Note that REM refers to a total of 17 elements of Sc, Y, and lanthanoids. A content of
the REM means a total content of at least one of these elements. In the case of lanthanoids,
lanthanoids are industrially added in the form of misch metal.
[0043] Balance: iron (Fe) and impurities
The impurities refer to components contained in a raw material or components mixed
in the course of manufacture and not intentionally incorporated in the steel sheet. Examples of
the impurities include nickel (Ni), copper (Cu), and tin (Sn), which may be mixed from scraps.
Contents of the impurities such as Ni, Cu, and Sn are each preferably 0.01% or less.
[0044] (Mass ratio [Ti]/[C] of Ti amount to C amount)
A mass ratio [Ti]/[C] of the Ti amount to the C amount is from 0.16 to 3.00.
It is important that the mass ratio [Ti]/[C] of the Ti amount to the C amount is 3.00 or
less. This value corresponds to a ratio of the numbers of Ti atoms/the numbers of C atoms of
about 0.75 or less in terms of the ratio of the number of atoms. In conventional precipitationstrengthened steel sheets, an excessive amount of Ti is incorporated relative to the amount of C
in order to precipitate TiC precipitates. However, in order to allow Ti to exist, in the steel sheet,
not as a solid solution Ti atom but as a TiC precipitate as much as possible and to effectively
contribute to precipitation strengthening, it is necessary to prevent the amount of Ti from being
excessive with respect to the amount of C. In addition, when the mass ratio [Ti]/[C] exceeds
3.00 and TiC precipitates are sufficiently precipitated, the amount of C segregated into crystal
grain boundaries is reduced, and the punched edge of the steel sheet is likely to be damaged. A
more preferable upper limit of the mass ratio [Ti]/[C] is 2.50 or less.
On the other hand, since the lower limit value of the Ti amount is 0.040% and the upper
limit value of the C amount is 0.250%, the lower limit value of the mass ratio [Ti]/[C] is 0.16
or more. A more preferable lower limit value of the mass ratio [Ti]/[C] is 0.46 or more.
[0045] (Product [Ti] × [C] of Ti amount and C amount)
A product [Ti] × [C] of the Ti amount and the C amount is from 0.0015 to 0.0160.
When [Ti] × [C] is less than 0.0015, the degree of supercooling for precipitation of TiC is
insufficient. Then, the content of Ti present as TiC precipitates precipitated in the matrix cannot
be increased, and the strength increasing effect is reduced. On the other hand, when [Ti] × [C]
is larger than 0.0160, the TiC precipitates cannot be completely dissolved in solutionization in
the austenite region, and a precipitation strengthening amount corresponding to the added
amount cannot be obtained in fine precipitation after phase transformation.
The product [Ti] × [C] of the Ti amount and the C amount is preferably from 0.0020
13
to 0.0150.
[0046] (Microstructure)
Next, the microstructure of the high-strength hot-rolled steel sheet according to the
present embodiment will be described.
[0047] -Total area ratio of bainitic ferrite and ferrite-
The high-strength hot-rolled steel sheet according to the present embodiment contains
at least bainitic ferrite. In addition, the total area ratio of bainitic ferrite and ferrite is 70% or
more with respect to the entire structure.
[0048] When the total area ratio of bainitic ferrite and ferrite is less than 70% with respect to
the entire structure, workability may deteriorate, and the punched edge may be damaged.
The total area ratio of bainitic ferrite and ferrite is more preferably 80% or more with
respect to the entire structure.
On the other hand, when the total area ratio of bainitic ferrite and ferrite is 90% or
more with respect to the entire structure, it is difficult to obtain high strength, and thus the total
area ratio of bainitic ferrite and ferrite is less than 90%. From the viewpoint of increasing the
strength of the steel sheet, the total area ratio of bainitic ferrite and ferrite is preferably 88% or
less, more preferably 86% or less, and still more preferably 85% or less.
[0049] -Area ratio of bainitic ferrite-
In the high-strength hot-rolled steel sheet according to the present embodiment, an area
ratio of bainitic ferrite with respect to the entire structure is preferably 50% or more, more
preferably 55% or more, and still more preferably 60% or more.
In the high-strength hot-rolled steel sheet according to the present embodiment, the
area ratio of bainitic ferrite with respect to the entire structure is preferably less than 90%, more
preferably 88% or less, still more preferably 86% or less, and particularly preferably 85% or
less.
By setting the area ratio of bainitic ferrite within the above range, the dislocation
density of the steel sheet tends to fall within a desired range, and dislocation strengthening is
more efficiently developed. Therefore, the steel sheet has higher tensile strength and is less
likely to be damaged at the punched edge during punching, which is preferable.
[0050] -Area ratio of polygonal ferrite-
In the high-strength hot-rolled steel sheet according to the present embodiment, the
area ratio of polygonal ferrite with respect to the entire structure is preferably 0% or more and
40% or less, more preferably 0% or more and 35% or less, and still more preferably 0% or more
and 30% or less.
When the area ratio of polygonal ferrite is within the above range, a steel sheet having
14
higher tensile strength is obtained, which is preferable.
[0051] -Total area ratio of martensite and retained austenite-
The high-strength hot-rolled steel sheet according to the present embodiment contains
at least one of martensite or retained austenite.
A total area ratio of martensite and retained austenite is 5% or more with respect to the
entire structure. When the total area ratio of martensite and retained austenite with respect to
the entire structure is less than 5%, it is difficult to obtain high strength. Therefore, the total
area ratio of martensite and retained austenite is 5% or more.
On the other hand, when the total area ratio of martensite and retained austenite with
respect to the entire structure is more than 30%, the enrichment of carbon in martensite may be
insufficient, and the contribution to the improvement in strength may be weakened. Therefore,
the total area ratio of martensite and retained austenite is 30% or less.
The total area ratio of martensite and retained austenite with respect to the entire
structure is more preferably 20% or less from the viewpoint of suppressing damage to the
punched edge.
[0052] The observation of the microstructure is performed by mirror-polishing a sample,
subjecting the sample to nital etching, and observing the microstructure at a position of 1/4 of
a sheet thickness in a plate thickness direction from its surface with an optical microscope.
[0053] Here, the area ratio is measured by the following method.
First, a test piece cut out so as to obtain a cross section parallel to a rolling direction
and the sheet thickness direction of the steel sheet is mirror-polished, etched with a nital solution,
and a microstructure at a position of 1/4 of the sheet thickness is observed with an optical
microscope. Martensite, retained austenite, and pearlite are recognized, the area ratios of
martensite, retained austenite, and pearlite are measured by a point count method, and the total
area ratio of martensite and retained austenite is determined from the results. A value obtained
by subtracting the area ratios of martensite, retained austenite, and pearlite from 100% is
defined as the total area ratio of bainitic ferrite and ferrite.
Next, for the measurement of the area ratio of ferrite, a further electropolished test
piece is used. Subsequently, using the EBSP-OIMTM (Electron Back Scatter Diffraction
Pattern-Orientation Imaging Microscopy) method, EBSP measurement is performed under the
measurement conditions of a magnification of 2000 times, an area of 40 μm × 80 μm, and a
measurement step of 0.1 μm.
[0054] The EBSP-OIMTM method includes an apparatus and software to irradiate a highly
inclined sample with an electron beam in a scanning electron microscope (SEM), photograph a
Kikuchi pattern formed by backscattering with a highly sensitive camera, and perform computer
15
image processing to measure a crystal orientation at an irradiation point in a short time. In the
EBSP measurement, the crystal orientation on a surface of a bulk sample can be quantitatively
analyzed, and an analysis area is a region that can be observed by the SEM. Measurement is
performed over several hours, and regions to be analyzed are mapped at tens of thousands of
points in a grid shape at equal intervals, so that the crystal orientation distribution in the sample
can be known.
[0055] From the measurement results, the area ratio of ferrite is determined using the Kernel
Average Misorientation (KAM) method. The Kernel Average Misorientation (KAM) method
averages misorientation among six adjacent pixels of a certain pixel in the measurement data
and performs calculation for each pixel using the value as a value of the central pixel. By
performing this calculation so as not to exceed the crystal grain boundaries, it is possible to
create a map representing an orientation change in crystal grains. That is, this map represents
the distribution of strain based on a local orientation change in the crystal grains. Since ferrite
undergoes diffusional transformation and has small transformation strain, crystal grains in
which the average of misorientation between the six pixels and the central pixel is 1° or less as
determined by the KAM method are defined here as ferrite, and the area ratio thereof is
determined. The case where the misorientation between adjacent measurement points was 15°
or more was defined as crystal grain boundary.
The area ratio of bainitic ferrite with respect to the entire structure is calculated from
the difference between the total area ratio of bainitic ferrite and ferrite and the area ratio of
ferrite.
[0056] The area ratio of polygonal ferrite to the entire structure is measured as follows.
Polygonal ferrite is characterized by having a low dislocation density and a particularly
small misorientation over the entire region in the crystal grains. Therefore, in the present
embodiment, first, the average value x1 of the misorientation between the six pixels and the
central pixel as determined by the KAM method is obtained for each measurement point; further,
the average value x2 at all the measurement points in the crystal grains is obtained from the
average value x1 obtained at each measurement point; and the crystal grains in which the x2
value is 0.5° or less are defined as polygonal ferrite, and the area ratio thereof is determined.
In the ferrite, a region that is not determined to be polygonal ferrite is ferrite having a relatively
high dislocation density, such as acicular ferrite.
[0057] -Mean dislocation density-
The high-strength hot-rolled steel sheet according to the present embodiment has a
mean dislocation density of from 1 × 1014 to 1 × 1016 m-2.
When the mean dislocation density is 1 × 1014 m-2 or more, dislocation strengthening
16
is obtained.
On the other hand, when the mean dislocation density exceeds 1 × 1016 m-2,
recrystallization is likely to occur, and the strength is significantly reduced.
The mean dislocation density is more preferably from 2 × 1014 to 2 × 1015 m-2.
[0058] A method of measuring the mean dislocation density is as follows.
For the measurement of the mean dislocation density, X-ray diffraction is used, and
measurement is made by mirror-polishing a sample so that a surface at a position of 1/4 of the
sheet thickness is horizontal to the sheet surface (rolled surface).
From the strain measured by the X-ray diffraction, a mean dislocation density ρ is
determined by the following equation described in Non-Patent Document 1.
Equation: ρ = 14.4 ε2
/b2
wherein ε is a strain obtained from the X-ray diffraction measurement, and b is a
Burgers vector (0.25 nm).
[0059] -Mean number density of TiC precipitate in crystal grain-
In the high-strength hot-rolled steel sheet according to the present embodiment, the
mean number density of TiC precipitates is from 1 × 1017 to 5 × 1018 [precipitates/cm3
] in the
ferrite crystal grains and in the bainitic ferrite crystal grains.
The mean number density of the TiC precipitates precipitated in the crystal grains is
preferably high in order to utilize precipitation strengthening. Therefore, in order to obtain
dislocation strengthening and precipitation strengthening to achieve a tensile strength of 850
MPa or more, the mean number density of TiC precipitates in the ferrite crystal grains and the
bainitic ferrite crystal grains is from 1 × 1017 to 5 × 1018 [precipitates/cm3
], and preferably from
2 × 1017 [precipitates/cm3
] to 5 × 1018 [precipitates/cm3
].
[0060] The mean number density of TiC precipitates is measured by a three-dimensional atom
probe measurement method as follows.
First, a needle-shaped sample is prepared from a sample to be measured by a cutting
and electropolishing method, using a focused ion beam working method together with an
electropolishing method as necessary, and three-dimensional atom probe measurement is
performed on the needle-shaped sample. In the three-dimensional atom probe measurement,
integrated data is reconstructed to obtain an actual atom distribution image in a real space.
[0061] Then, the formation position of the TiC precipitates in the needle-shaped sample is
confirmed, and the number density of the TiC precipitates precipitated in the crystal grains in
the ferrite crystal grains and the bainitic ferrite crystal grains is determined from the volume of
the entire stereoscopic distribution image including the TiC precipitates and the number of the
TiC precipitates. An average value obtained by performing this operation five times is defined
17
as " mean number density of TiC precipitates precipitated in the crystal grains".
[0062] An average diameter of the TiC precipitates precipitated in the crystal grains is
preferably 0.8 nm or more from the viewpoint of increasing the precipitation strengthening
amount. On the other hand, when the average diameter is too large, the mean number density
tends to decrease, and the precipitation strengthening amount decreases, which is not preferable.
However, since it is essential that the mean number density be within the above range in order
to increase the precipitation strengthening amount, the upper limit of the average diameter is
not defined.
[0063] The average diameter of the TiC precipitates precipitated in the crystal grains is a
diameter (spherical equivalent diameter) calculated, on the assumption that the TiC precipitates
are spherical, from the number of constituent atoms of the observed TiC precipitates and the
lattice constant of TiC. The diameters of 30 or more TiC precipitates are arbitrarily measured,
and an average value thereof is determined.
[0064] -Amount of Ti existing as TiC precipitates precipitated in matrix -
In the high-strength hot-rolled steel sheet according to the present embodiment, the
amount of Ti present as TiC precipitates precipitated in the matrix not on dislocations (that is,
the amount of Ti contained in the TiC precipitates) is 30 mass% or more of the total amount of
Ti in the steel sheet.
By setting the amount of Ti present as TiC precipitates precipitated in the matrix that
is not on dislocations to 30 mass% or more of the total amount of Ti in the steel sheet, the ratio
of TiC precipitates precipitated in the matrix can be increased, both precipitation strengthening
and dislocation strengthening can be greatly developed, and a steel sheet having high tensile
strength can be obtained while reducing the amount of Ti.
It is more preferable that the amount of Ti present as TiC precipitates precipitated in
the matrix not on dislocations be 40% or more of the total amount of Ti in the steel sheet.
On the other hand, the amount of Ti present as TiC precipitates precipitated in the
matrix not on dislocations is preferably as high as possible, but it is difficult to prevent
coarsening of the precipitates in terms of the manufacturing process. Thus, the amount of Ti is
preferably 90 mass% or less of the total amount of Ti in the steel sheet.
[0065] The amount of Ti present as TiC precipitates precipitated in the matrix not on
dislocations is measured by the three-dimensional atom probe measurement method as follows.
First, the three-dimensional atom probe measurement is performed in the same
procedure as the method of measuring the mean number density described above, and the
formation position of the TiC precipitate is confirmed.
From the steric configuration of the TiC precipitates, when the TiC precipitates are
18
arranged in a row, the TiC precipitates are determined to be those precipitated on dislocations,
and, when the TiC precipitates are arranged independently, the TiC precipitates are determined
to be those precipitated in the matrix that is not on the dislocations.
Fig. 1A shows a schematic diagram of an arrangement of TiC precipitates precipitated
on dislocations, and Fig. 1B shows a schematic diagram of an arrangement of TiC precipitates
precipitated in the matrix not on the dislocations. In addition, there is also a case where both
of (A) TiC precipitates precipitated on dislocations and (B) TiC precipitates precipitated in a
matrix not on the dislocations are included in the same crystal grain, and thus it is determined
to which of (A) and (B) each precipitate corresponds. The amount of Ti present as TiC
precipitate precipitated in the matrix not on dislocations (mass ratio with respect to the total
amount of Ti in the steel sheet) was calculated from the volume of the entire stereoscopic
distribution image of the TiC precipitates, the number of Ti atoms constituting the TiC
precipitates precipitated in the matrix not on dislocations, and the Ti content of the steel sheet.
In the tables and figures, this Ti amount is referred to as " matrix-precipitated Ti ratio".
[0066] The "TiC precipitates" include not only carbides but also carbonitrides in which
nitrogen is mixed in carbides. The “TiC precipitates” also include precipitates in which one or
more of Nb, Mo, and V are dissolved as a solid solution in the TiC precipitates ((Ti, M) C
precipitates [M represents one or more of Nb, V, and Mo]).
[0067] -Tensile strength-
A tensile strength of the high-strength hot-rolled steel sheet according to the present
embodiment is 850 MPa or more.
The tensile strength of the high-strength hot-rolled steel sheet according to the present
embodiment is 860 MPa or more.
However, from the viewpoint of preventing deterioration in workability, the tensile
strength of the high-strength hot-rolled steel sheet according to the present embodiment may
be, for example, 1050 MPa or less.
[0068] The tensile strength is measured as follows.
First, a No. 5 test piece is taken from the steel sheet in accordance with JIS Z 2201:1998.
Subsequently, a tensile test is performed in accordance with JIS Z 2241:2011, and the tensile
strength is measured.
[0069] (Manufacture method)
Next, an example of a method of manufacturing the high-strength hot-rolled steel sheet
according to the present embodiment will be described.
The method of manufacturing the high-strength hot-rolled steel sheet according to the
present embodiment includes, for example, a hot rolling step of heating a steel piece that
19
satisfies the chemical composition of the high-strength hot-rolled steel sheet according to the
present embodiment for hot rolling thereof to obtain a steel sheet; a cooling step of cooling the
steel sheet obtained through the hot rolling step; and a winding step of winding the cooled steel
sheet.
[0070] (Hot rolling step)
In the hot rolling step, a steel piece that satisfies the chemical composition of the highstrength hot-rolled steel sheet according to the present embodiment is subjected to, for example,
hot rolling through rough rolling and finish rolling to obtain a hot-rolled steel sheet.
As the steel piece, a steel piece obtained by melting and casting steel by a conventional
method is used. The steel piece is preferably manufactured by a continuous casting facility
from the viewpoint of productivity.
[0071] A heating temperature in the hot rolling is preferably 1200°C or higher, and more
preferably 1220°C or higher in order to sufficiently decompose and dissolve Ti and carbon in
the steel sheet. On the other hand, it is not economically preferable to set the heating
temperature to an excessively high temperature, and thus it is preferable to set the heating
temperature to 1300°C or lower.
After casting, the steel piece may be cooled to 1200°C or lower and then heated to a
temperature of 1200°C or higher to start rolling. When a steel piece cooled to 1200°C or lower
is used, it is preferable to heat the steel piece to a temperature of 1200°C or higher and hold the
steel piece for 1 hour or more.
[0072] A final working temperature FT [°C] of hot rolling is preferably 920°C or higher, and
more preferably 940°C or higher. This is intended to suppress the generation of coarse TiC
precipitates in austenite and to promote the recovery of dislocations by working to suppress the
nucleation of polygonal ferrite during cooling. The final working temperature FT [°C] of hot
rolling is more preferably 950°C or higher in order to suppress precipitation of TiC precipitates
at a high temperature. Here, in order to suppress nucleation of polygonal ferrite, the final
working temperature FT [°C] is more preferably 940°C or higher, but may be 920°C or higher
and lower than 940°C when the Mn amount is 0.35% or more.
However, from the viewpoint of suppressing the occurrence of scale defects, the final
working temperature FT [°C] is preferably 1050°C or lower.
The final working temperature FT indicates a temperature at which the hot-rolled
rolled sheet is discharged from the final stand.
[0073] (Cooling step)
In the cooling step, the hot-rolled steel sheet is subjected to primary cooling, secondary
cooling, and tertiary cooling.

CLAIMS
1. A high-strength hot-rolled steel sheet having a chemical composition comprising,
by mass:
C: from 0.030 to 0.250%;
Si: from 0.01 to 1.50%;
Mn: from 0.1 to 3.0%;
Ti: from 0.040 to 0.200%;
P: 0.100% or less;
S: 0.005% or less;
Al: 0.500% or less;
N: 0.0090% or less;
B: from 0 to 0.0030%;
a total of one or more of Nb, Mo and V: from 0 to 0.040%;
a total of one or more of Ca and REM: from 0 to 0.010%; and
a balance consisting of Fe and impurities, a mass ratio [Ti]/[C] of a Ti amount to a C
amount being from 0.16 to 3.00, and a product [Ti] × [C] of the Ti amount and the C amount
being from 0.0015 to 0.0160,
the high-strength hot-rolled steel sheet:
having a mean dislocation density of from 1 × 1014 to 1 × 1016 m-2; and
comprising at least bainitic ferrite,
wherein a total area ratio of the bainitic ferrite and ferrite is 70% or more and less than
90%,
wherein a total area ratio of martensite and retained austenite is 5% or more and 30%
or less,
wherein, in ferrite crystal grains and in bainitic ferrite crystal grains, a mean number
density of TiC precipitates is from 1 × 1017 to 5 × 1018 [precipitates/cm3
],
wherein an amount of Ti present as a TiC precipitate precipitated in a matrix not on
dislocations is 30 mass% or more of a total amount of Ti in the steel sheet,
wherein a tensile strength is 850 MPa or more, and
wherein [Ti] and [C] represent the Ti amount and the C amount (mass%), respectively.
2. The high-strength hot-rolled steel sheet according to claim 1, comprising, by mass:
B: 0.0001% or more and less than 0.0005%.
34
3. The high-strength hot-rolled steel sheet according to claim 1 or 2, comprising, by
mass:
the total of one or more of Nb, Mo, and V: from 0.01 to 0.040%.
4. The high-strength hot-rolled steel sheet according to any one of claims 1 to 3,
comprising, by mass:
the total of one or more of Ca and REM: from 0.0005 to 0.01%.
5. The high-strength hot-rolled steel sheet according to any one of claims 1 to 4,
wherein the total area ratio of the bainitic ferrite and the ferrite is 80% or more and less than
90%.
6. The high-strength hot-rolled steel sheet according to any one of claims 1 to 5,
wherein an area ratio of the bainitic ferrite is 50% or more and less than 90%.