Title of the Invention] IIIGH-STRENGTH HOT-ROLLED STEEL SHEET HAVNG
EXCELLENT BAKING HARDENABILITY AND LOW TEMPERATURE
TOUGHNESS WIT14 MAXIMUM TENSILE STRENGTH OF 980 MPA OR MORE
5 [Technical Field]
[0001]
The present invention relates to a high-strength hot-rolled steel sheet having
excellent baking hardenability and low temperature toughness with a maximum tensile
strength of 980 MPa or more, and a method for producing such a high-strength hot-rolled
10 steel shect. The present invention relates to a steel sheet having excellent hardening
ability, after molding and coating-baking treatment, and excellent low temperature
toughness to be able to be used in extremely cold areas.
[Background Art]
[0002]
15 To reduce the exhaust amount of carbon dioxide gas from automobiles,
automobile bodies are being reduced in weight by using high-strength steel shects.
Furthermore, to secure the safety of drivers and passengers, in addition to soft steel sheets,
more and more high-strength steel sheets with a maximum tensile strenglh of 980 Ml'a or
more are becoming to be used for automobile bodies. To further reduce the weight of
20 automobile bodies, the strength of high-strength steel sheets during use has to be higher
than before. However, the incrcase in the strength of steel sheets typically leads to the
degradation of material characteristics such as formability (processability). Thus, it is a
lcey to the development of high-strength steel sheets how the strength is incrcascd without
the degradation of material characteristics.
25 [0003]
Steel sheets that are used for such members are required to have such a
performance that the members are unlikely to be damaged even when shocked by collision
or the like after steel sheets are molded and attached to automobiles as components. In
particular, in order to secure impact resistance in cold areas, low temperature toughness is
also demanded to be increased. The low temperature toughness is defined by vTrs
5 (Charpy fraction dislocation temperature), for example. For this reason, the impact
resistance of the above steel materials needs to be considered. In addition, high-strength
steel sheets are unlikely to be plastically deformed and will occur more easily; thus,
toughness is demanded as significant characteristics.
[0004]
10 As one of methods for increasing the strength of steel sheets without the
degradation in formability, there is a method of baking-hardening using coating-baking.
This method increases the strength of automobile members in the following manner:
through heat treatment at the timc of coating-baking treatment, dissolved C present in a
steel sheet concentrates at dislocations formed during molding or is precipitated as carbides.
15 Since hardening is performed after press formation in this method, there is no degradation
in press formability due to the increase in strength. Thus, this method is cxpccted to be
used for automobile structural members. As an index for evaluation of the baking
hardenability, there is known a testing method in which 2% prestrain is imparted at room
temperature and then heat treatment is performed at 170°C for 20 minutes to perform
20 evaluation at the time of rctensile testing.
[OOOS]
Both the dislocations formcd at the time of production and the dislocations formed
at the time oS press processing contribute to baking-hardcning; therefore, thc sum of them,
which is the dislocation density, and the amount of dissolved C in the steel sheet, are
25 important for the baking hardenability. An cxarnple of a steel sheet having excellent
halting hardenability while having a large amount of dissolved C is the steel sheet shown in
Patent Document 1 or 2. As a steel sheet that secures more excellent baking hardenability,
there is known a steel sheet including N in addition to dissolved C and having excellent
baking hardenability (Patent Documents 3 and 4).
Although the steel sheets shown in Patent Documents 1 to 4 can secure excellent
5 baking hardenability, these steel sheets are not suitable for production of high-strength
steel sheets with a maximum tensile strength of 980 or more that can contribute to high
strength of structural members and the reduction in the weight because the base phase
structure is a ferrite single phase.
[0006]
10 In contrast, being extremely hard, a martensite structure is typically used as a
main phase or the second phase in steel sheets having a strength as high as 980 MPa or
I more to increase the strength
I 1 However, since martensite includes an enormous number of dislocations, it has
I
been difficult to obtain excellent baking hardenability. This is because the dislocation
15 density is high compared to the amount of dissolved C in steel. In general, when the
amount of dissolved C is small compared to the dislocation density in a steel sheet, the
baking hardenability is degraded. Accordingly, when soft steel that does not include
many dislocations and steel of a martensite single phase are compared with each other, if
the amount of d~ssolvedC is the same, the baking hardenability of the martensite single
20 phasc is more degraded.
[0007]
Therefore, as steel sheets that were attempted to secure more excellent baking
hardenab~lity,t here are known steel sheets having higher strcngth by adding an element(s)
such as Cu, Mo, W, andlor the like to steel and precipitating carbides of these elements at
25 the time of baking-coating (Patent Documents 5 and 6). However, these steel sheets do
not have high econornic efficiency because the addition of expensive elements is necessary.
In addition, even though carbides of these elements are used, it has been still difficult to
secure the strength of 980 MPa or more.
[OOOS]
Meanwhile, as for a method for increasing the toughness of'a high-strength steel
5 sheet, for example, Patent Document 7 discloses a method for producing such a steel sheet.
There is lmown a method in which the aspect ratio of a martensite phase is adjusted the
martensite phase is used as a main phase (Patent Document 7).
In general, it is lcnown that the aspect ratio of martensite depeiids on the aspect
ratio of austenite grains before transformation. That is, martensite having a high aspect
10 ratio means martensite transformed from unrecrystallized austenite (austenite that is
extended by rolling), and martensite having a low aspect ratio means martensite
transformed from recrystallized austenite.
[0009]
From the above description, in order to reduce the aspect ratio of the steel sheet of
15 Patent Document 7, it is necessary to recrystallize austenite; in addition, in order to
recrystallize austenite, it is necessary to increase the temperature of final rolling.
Accordingly, the grain size of austenite and also the grain sizc of martensite have tended to
be large. In general, grain refining is known to be effcctive to increasc toughness. A
reduction in the aspect ratio can reduce factors that degrade toughness due to the shape, but
20 is accompanied the degradation of toughness due to coarse crystal grains; therefore, there
is a limit on the increase in toughness. In addition, Patent Document 7 mentions nothing
about the baking hardenability that a study of the present application has focused on, and
Patent Document 7 hardly secures sufficient baking hardenability.
[OO lo]
25 Fuitherinore, Patent Document 8 discloses that it is possible to increase the
strength and low temperature toughness by finely precipitating carbides in ferrite having an
average grain size of 5 to 10 pm. By precipitating dissolved C in steel as carbides
including Ti and the like, the strength ol the steel sheet is increased, so that it is considered
that the amount of dissolved C in steel is small and excellent baking hardenability is
unliltely to be obtained.
In this manner, it has been difficult for a high-strength steel sheet with 980 MPa or
more to have both excellent halting hardenability and excellent low temperature toughness.
[Prior Art Documents]
[Patent Documents]
[0011]
10 [Patent Document 11 JP I-15-55586B
[Patent Document 21 JP 34047983
[Patent Document 31 JP 43629483
[Patent Document 41 JP 45248593
[Patent Document 51 JP 382271 1B
15 [Patent Document 61 JP 3860787B
[Patent Document 71 JP 201 1-52321A
[Patent Document 81 JP 20 11- 17044A
[Summary of the Inve~ltion]
[Problems to Be Solved by the Invention]
20 [0012]
The present invention has been made in view of thc abovc problems, and an object
of thc present invention is to provide a hot-rolled stccl sheet having excellent baking
hardenability and low temperature toughness with a maximum tensile strength of 980 MPa
or more, and a method for producing such a stcel sheet stably.
25 [Means for Solving the Problem(s)]
[OO 131
The present inventors have successfully produced a high-strength hot-rolled steel
sheet having excellelit baking hardenability and low tcmpcrature toughness with a
maximum tensile strength of 980 MPa or more, by optimizing the composition ol'the steel
sheet and conditions for producing the steel sheet and by controlling the structure of the
5 steel sheet. A summary of the steel sheet is as follows.
[0014]
(1)
A high-strength not-rolled steel sheet with a maximum tensilc strength of 980
MPa or more, the steel sheet having a composition consisting of, in mass%,
10 C: 0.01% to 0.2%,
Si: 0% to 2.5%,
Mn: 0% to 4.0%,
Al: 0% to 2.0%,
N: 0% to 0.01%,
Cu: 0% to 2.0%,
Ni: 0% to 2.0%,
Mo: 0% to 1.0%,
V: 0% to 0.3%,
Cr: 0% to 2.0%,
Mg: 0% to 0.01%,
Ca: 0% to 0.01%,
REM: 0% to 0.1%,
B: 0% to 0.01%,
P: less than or equal to 0.10%,
S: less than or equal to 0.03%,
0: less than or equal to 0.01%,
one or both of Ti and Nb: 0.01% to 0.30% in total, and
the balance being Fe and inevitable impurities,
wherein the steel sheet has a structure in which a total volume fraction of one or
both of tempered martensite and lower bainite is 90% or more, and a dislocation density in
5 the martensite and lower bainite is greater than or equal to 5x10'~( l/m2) and less than or
equal to 1x10'~(l /m2).
(2)
The high-strength hot-rolled steel sheet according to (I), wherein the one or both
of tempered martensite and lower bainite include 1x10~ (numbers/mm2) or more
10 iron-based carbides.
(3)
The high-strength hot-rolled steel sheet according to (I), wherein the onc or both
of tempered martensite and lower bainite have an effective crystal size of less than or equal
to 10 pm.
15 (4)
The high-strength hot-rolled steel sheet according to (I), including one or more of,
in mass%,
Cu: 0.01% Lo 2.0%,
Ni: 0.01% to 2.0%,
Mo: 0.01% to 1.0%,
V: 0.01% to 0.3%, and
Cr: 0.01% to 2.0%.
(5)
The high-strength hot-rolled steel shect according to (I), including one or more of,
25 in mass%,
Mg: 0.0005% to 0.01%,
Ca: 0.0005% to 0.01%, and
REM: 0.0005% to 0.1%.
(6)
The high-strength hot-rolled steel sheet according to (I), including, in mass%,
5 B: 0.0002% to 0.01%.
[0015]
(7)
A method for producing a high-strength hot-rolled steel sheet with a maximum
tensile strength of 980 MPa or more, the method including:
10 heating, optionally after cooling, a casting slab to a temperature of 1200°C or
more, the casing slab having a composition consisting of, in mass%,
C: 0.01% to 0.2%,
Si: 0% to 2.5%,
Mn: 0% to 4.0%,
15 Al: 0% to 2.0%,
N: 0% to 0.01%,
Cu: 0% to 2.0%,
Ni: 0% to 2.0%,
Mo: 0% to 1.0%,
20 V: 0% to 0.3%,
Cr: 0% to 2.0%,
Mg: 0% to 0.01%,
Ca: 0% to 0.01%,
REM: 0% to 0.1 %,
25 B: 0% to 0.01%,
P: less than or equal to 0.10%,
S: less than or equal to 0.03%,
0: lcss than or equal to 0.01%,
one or both of Ti and Nb: 0.01% to 0.30% in total, and
the balance being Fe and inevitable impurities;
5 completing hot rolling at a temperature of 900°C or more;
cooling the steel sheet at a cooling spced of 50°C/s or more on average from a
final rolling temperature to 400°C;
setting a cooling speed of not more than 5O0C/s at a temperalure of less than
400°C; and
10 coiling the steel sheet.
(8)
The method for producing a high-strength hot-rolled steel sheet according to (7),
further including:
performing galvanizing treatment or galvannealing treatment.
15 [Effects of the Invention]
[0016]
According to the present invention, it becomes possible to provide a high-strength
steel sheet having excellent baking hardenability and low temperature toughness with a
maximum tensile strength of 980 MPa or more. By use of this steel shcet, it becomes
20 easy to process the high-strength steel sheet, and also it becomes possible to use the
processed high-strength steel sheet with high durability in extremely cold areas; thus, the
industrial contribution of the high-strength stecl sheet is very remarltable.
[Mode(s) for Carrying out the Invention]
[0017]
The content of the present invention will be described below in detail.
According to the prescnt inventors' intensive study, a structure of a steel sheet has
a dislocation density of greater than or equal to 5x10'~( l/m2) and lcss than or equal to
1x10'~(l /m2), and includes one or both of tempered martensitc and lower bainite, each
including 1 XI o6 (numbers/mm2) or more iron-based carbides, in a total volume fraction of
90% or more. The present inventors have further found out that the effective crystal size
5 of tempered martensite and lower bainite is preferably 10 pm or less so that a high strength
of 980 MPa or more and excellent baking hardenability and low temperature toughness can
be secured. Here, the effective crystal size means a region surrounded by grain
boundaries having an orientation difference of 15" or more, which can be measured by
using EBSD, for example. Details thereof will be described later.
10 [0018]
[Microstructure of steel sheet]
First, a microstructure of a hot-rolled steel sheet according to the present invention
will be described.
In this steel sheet, the main phase is one or both of tempered martensite and lower
15 bainite in a total volume fraction of 90% or more, so that a maximum tensile strength of
980' MPa or more is secured. Accordingly, the main phase needs lo be one or both of
tempered martensite and lower bainite.
[OO 191
In the present invention, tempered martensite is the most important microstructure
20 to have a high strength, excellent baking hardenability, and excellent low temperature
toughness. Tempered marlensite is an aggregation of lath-shaped crystal grains including,
inside the lath, iron-based carbides having a major axis of 5 nm or more. In addition,
these carbides belong to a plurality of variants, in other words, a plurality of iron-based
carbides extending in differcnt directions.
25 Thc structure of tempered martensite can be obtained by decreasing the cooling
speed at the time of cooling performed at a temperature of less than or equal to Ms point
(the temperature at which martensite transformation starts) or by making a martensite
structure and then tempering it at l0O0C to 600°C. In the present invention, precipitation
is controlled by cooling control at a temperature of less than 400°C.
[0020]
Lower bainite is also an aggregation of lath-shaped crystal grains including, inside
the lath, iron-based carbides having a major axis of 5 nm or more. In addition, these
carbides belong to a single variant, in other words, a group of iron-based carbides
extending in the same direction. Observation of the extending direction of carbides
makes it easier to discriminate between tempered martensite and lower bainite. Here, the
group of iron-based carbides extending in the same direction means that a difference in the
extension direction in the group of iron-based carbides is within 5".
[0021]
When the total volume fraction of one or both of te~npered martensite and lower
bainite is less than 90%, a high maximum tensile strength of 980 MPa or more cannot be
secured, and a maximum tensile strength of 980 MPa or more being one of requirements of
the present invention cannot be secured. Accordingly, the lower limit of the total volume
fraction of one or both of tempered martensite and lower bainite is 90%. On the other
hand, even when the total volume fraction is 10096, the high strength, excellent baking
hardcnability, and excellent low temperature toughness, which are effects of the present
invention, are shown.
[0022]
In the structure of the steel sheet, as another structure, one or more of ferrite, fresh
martensite, upper bainite, pearlite, and retained austenite may be contained in a total
voluine kaction of 10% or less as inevitable impurities.
I-Iere, fresh maitensite is defined as martensite that does not include carbides.
Although fresh martensite has high strength, the low temperature toughness is poor;
therefore, the volume fraction thereof needs to be limited to 10% or less. In addition, the
dislocation density is extremely high and the baking hardenability is poor. Accordingly,
the volume fraction thereof needs to be limited to 10% or less.
Retained austenite is transformed into fresh martensite when a steel material is
5 plastically deformed at the time of press-formation or when an automobile member is
plastically deformed at the time of collision, and thus, retained austenite has adverse effects
similar to those of fresh martensite described above. Accordingly, the volume fraction
needs to be limited to 10% or less.
[0023]
10 Upper bainite is an aggregation of lath-shaped crystal grains, and is an
aggregation of laths including carbides between laths. Carbides included between laths
serve as a starting point of fracture, and decreases the low temperature toughness. In
addition, since upper hainite is formed at higher temperatures than lower bainite, the
strength is low, and excessive formation thereof maltcs it difficult to secure a maximum
15 tensile strength of 980 MPa or more. This effect will become obvious if the volume
fraction of upper bainite exceeds lo%, and accordingly, the volume fraction thereof needs
to be limited to 10% or less.
[0024]
Ferrite means a bulk of crystal grains and a structure not including, inside the
20 structure, a lower structure such as a lath. Since ferrite is the softest structure and leads to
a reduction in strength, in order to secure a maximum tensile strength of 980 MPa or more,
it is necessary to have a limit being 10% or less. In addition, since ferrite is much softer
than tempered martensite or lower bainite, which is included in the main phase,
deformation concentrates at the interface between these structures to easily serve as a
25 starting point of a fracture, resulting in poor low temperature toughness. These effects
will become obvious if the volume fraction exceeds 10%; accordingly, the volume fiaction
thereof needs to be limited to 10% or less.
I'earlite leads 'to the decrease in strength and the degradation of low temperature
toughness, in the same manner as ferritc; accordingly, the volume fraction thereof nceds to
be limited to 10% or less.
5 [0025]
As for the steel sheet according to the present invention, which has the above
described structure, the identification of tcmpered martensite, fresh martensite, bainite,
ferrite, pearlite, austenite, and the balance included therein, the determination of existing
positions, and measurement of area fractions can be performed by corroding a cross section
10 in the steel sheet rolling direction or a cross section in a direction perpendicular to the
rolling direction using a nital reagent and a rcagent disclosed in JP S59-219473A, and then
observing the steel sheet by a scanning and transmission-type electron microscope at a
1000 to 100000 magnification.
The discrimination of thc structure is also possible by analysis of crystal
15 orientations by a FESEM-EBSP method or measurement of the hardness of a micro-region
such as micro-Vickers hardness measurement. For example, as described above,
tempered martensite, upper bainite, and lower bainite are different from each other in the
formation sites of carbides and relation of crystal orientations (extcl~ding directions).
Thus, by observing iron-based carbides in the inside of lath-shaped crystal grains by a
20 YE-SEM to examine extending direct~ons thereof, it is possible to easily discriminate
between bainite and tempered martensite.
t : [0026]
In the present invention, the volurne fractions of ferrite, pearlite, bainite, tempered
i
martensite, and fresh martensite are obtained in the following manner: samples are
25 extracted as observing surfaces by using cross sections in the sheet thicltness direction,
which is parallel to the rolling direction of the steel shcet; the observing surfaces are
polished and etched by nital, and a range of 118 to 318 thickness centering 114 of the sheet
thicltness is observed by a field emission scanning electron microscope (FE-SEM) to
measure area fractions as the volume fractions. The measurement is performed on ten
fields at a 5000 magnification for each sample, and an average is employed as the area
5 fractions.
[0027]
Since fresh martensite and retained austenite are not corroded sufficiently by nital
etching, in the observation by the FE-SEM, it is possible to clearly discriminate between
the above described structures (ferrite, bainitic ferrite, bainite, and tempered martensite).
10 Accordingly, it is possible to obtain the volume fraction of fresh martensite as a difference
between the area fiaction of an uncorroded region observed by the FE-SEM and the area
fraction of retained austenite measured by using X-rays.
[0028]
Thc dislocation density in the structure of one or both of tempered martensite and
15 lower bainite needs to be limited to 1x10'' (1/m2) or less. This is for obtaining excellent
baking hardenability. In general, the density of dislocations existing in tempered
martensite is high, so that excellent baking hardenability cannot be secured. Accordingly,
by controlling cooling conditions in hot rolling, in particular, by setting the cooling speed
at temperatures of less than 400°C to less than 50°C/s, excellent baking hardenability can
20 be obtained.
On the other hand, if the dislocation density is less than 5x10'' (l/m2), it will be
dif1;cult to secure a strength of 980 MPa or more, and accordingly, the lower limit of the
dislocation density is set to 5x10" (1/m2), desirably a value in a range from 8x10'~to
8x10'~(l /m2), more desirably a value in a range from 1x10't~o 5x10'~(l /m2).
25 [0029]
The dislocation density may be obtained by observation using X-rays or a
transmission-type electron microscope as long as the dislocation density can be measured.
In the present invention, by thin film observation using an electron microscope, the
dislocation density is measured. In the measurement, the film thickness of a
measurement region is measured and then the number of dislocations existing in the
5 volume is measured, so that the density is measured. The measurement is performed on
ten fields at a 10000 magnification for each sample to calculate the dislocation density.
[0030]
The one or both of tempered martensite and lower bainite according to the prcsent
invention desirably include 1 xloh (numbers/mm2) or more iron-based carbides. This is
10 for increasing the low temperature toughness of the base phase and for obtaining a balance
between the high strength and excellent low temperature toughness. That is, although
quenched martensite without any further treatment has a high strength, the toughness
thereof is poor and an improvement is needed. Accordingly, by precipitating 1x10'
(numbers1 mm2) or more iron-based carbides, the toughness of the main phase is improved.
15 [0031]
According to the present inventors' study on the relation between the low
temperature toughness and the number density of iron-based carbides, it has been revealed
that the excellent low temperature toughness can be secured by setting the number density
of carbides in one or both of tempered martensite and lower bainite to 1 x 1 0 ~
20 (numbers/mm2) or more. Accordingly, the number density of carbides in one or both of
tempered martensite and lower bainite is set to 1 x 1 o6 (numbers/mm2) or more, desirably
5x lo6 (numbers/mm2) or more, more desirably 1 x lo7 (numbers/mm2) or more.
In addition, the size of carbides precipitated tbrough the above treatment in the
prcsent invention is small, which is 300 nm or less, and most of the carbides are
25 precipitated in the laths of martdhsitc or bainite; accordingly, it is assumed that the low
temperature toughness is not degraded.
The number density of carbides is measured in the following manner: samples are
extracted as obscrving surfaces by using cross sections in the sheet thickncss direction,
which is parallel to the rolling direction of the stccl sheet; the obscrving surfaces arc
5 polished and etched by nital, and a range of 118 to 318 thickness centering 114 of the sheet
thickness is observed by a field emission scanning electron microscope (FE-SEM). The
measurement of thc number density of iron-based carbides is perlbrmed on ten fields at a
5000 magnification for each sample.
[0033]
10 In order to furthcr increase the low temperature toughness, one or both of
tempered martensitc and lowcr bainite are included as the main phase, and in addition, the
effective crystal sizc thercof is set to 10 pm or less. Effects of increasing the low
temperature toughncss become obvious by setting the effective crystal size to 10 pm or
less; accordingly, the effective crystal size is sct to 10 pm or less, desirably 8 pm or less.
15 The effective crystal size mentioned here means a region surrounded by grain boundaries
having a crystal orientation difference of 15" or more, which will be described later, and
corresponds to a block grain size in martensite or bainite.
[0034]
Next, methods for identifying an average crystal grain sizc and the structure will
20 be described. In the present invention, the average crystal grain size, fcrrite, and retained
austenite are defined by using an electron back scatter diffraction pattern-orientation image
microscopy (EBSP-OIMTM). The method of E B S P - 0 1 ~ 'is~ c onfigured by an apparatus
and software by which a highly inclined sample is irradiated with electron beams in a
scanning electron microscope (SEM), Kiltuchi patterns formcd by back scattering are
25 imaged by a high sensitivity camera, and computer image processing is performed to
measure the crystal orientation of the irradiation point in a short period of time. In the
EBSP method, it is possible to quantitatively analyze the microstructure and crystal
orientations on the surface of the bulk sample, the analysis area is a region that can be
observed by a SEM, and, depending on the resolution of the SEM, a resolution of a
minimum of 20 nm can be analyzed. In the present invention, from an image mapped by
5 defining the orientation difference in crystal grains as 15", which is the threshold of high
angle grain boundaries recognized commonly as crystal grain boundaries, grains are
visualized and the average crystal grain size is obtained.
[0035]
The aspect ratio of effective crystal grains (here, this means a region surrounded
10 by grain boundaries of 15" or more) of tempered martensite and bainite is desirably 2 or
less. Grains flattened in a specific direction have high anisotropy, and often have low
toughness because craclts propagate along grain boundaries at the time of Charpy testing.
Accordingly, it is necessary to malte the effective crystal grains as isometric as possible.
In the present invention, a cross section of the steel sheet in thc rolling direction is
15 observed, and a ratio (= LIT) of the length in the rolling direction (L) to the length in the
sheet thickness direction (T) was defined as the aspect ratio.
100361
[Chemical composition of steel sheet]
Next, rcasons for limits on the chemical composition of the high-strength
20 hot-rolled steel sheet according to the present invention will be described. Note that % as
the content means mass%.
C: 0.01% to 0.2%
C contributes to an increase in the strength of thc base material and improvement
in the baking hardenability, and also generates iron-based carbides such as cementite
25 (Fe3C), which serve as a starting point of breaking at the time of hole expansion. If the
content of C is less than 0.01%, the effect of increasing the strength as a result of structure
strengthening by a low temperature transformation generation phase cannot bc obtained.
If the content exceeds '0.2%, ductibility will be decreased and iron-based carbides such as
cementite (Fe3C), which serve as a starting point of breaking in a two-dimensional shear
plane at the time of punching process, will be increased, resulting in the degradation of
5 formability such as hole expandability. Therefore, the content of C is limited to the range
from 0.01% to 0.2%.
[0037]
Si: 0% to 2.5%
Si contributes to an increase in the strength of the base material and can be used as
10 a deoxidant of molten steel. Accordingly, preferably 0.001% or more Si is contained as
necessary. However, if the content exceeds 2.5%, the effect of contributing to the
increase in strength will be saturated; accordingly, the content of Si is limited to 2.5% or
less. In addition, when 0.1% or more Si is contained, as the content is increased, the
precipitation of iron-based carbides such as cementite is more suppressed in the material
15 structure, contributing to the increase in strength and hole expandability. If the content of
Si exceeds 2.5%, the effect of suppressing the precipitation of iron-based carbides will be
saturated. Therefore, the desirable range of the Si content is from 0.1% to 2.5%.
[0038]
Mn: 0% to 4%
20 Mn can be contained so that the steel sheet structure can have a main phase of one
or both of tempered martcnsite and lower bainite by, in addition to solution strengthening,
quenching-hardening. If the addition is performed such that the content of Mn exceeds
4%, this effect will be saturated. On the other hand, if the Mn content is less than 1%,
effccts of suppressing ferrite transformation and bainite transformation will not be shown
25 easily during cooling,. Accordingly, the content of Mn is desirably 1% or more, more
desirably from 1.4% to 3.0%
100391
One or both of Ti and Rb: 0.01% to 0.30% in total
Each of Ti and Nb is the most important constituent element in order to realize
both the excellent low temperature toughness and the high strength of 980 MPa or more.
5 Carbonitrides thereof or dissolved Ti and Nb delay the growth of grains at thc time of hot
rolling, thercby contributing to refinement of the grain size of a hot rolled sheet and the
increase in the low temperature toughness. Dissolved N is imporrant because dissolved N
promotes the growth of grains. At the same time, 'Ti is particularly important because Ti
can exist as TiN to contribute to the increase in the low temperature toughness through the
10 refinement of the grain size at the time of heating the slab. In order to obtain a grain size
of the hot rolled sheet being 10 pm or less, 0.01% or more Ti and Nb, alone or in
combination, needs to be contained. If the total content of Ti and Nb exceeds 0.30%, the
above effect will be saturated and the econon~ic efficiency will be lowered. Therefore,
the content of Ti and Nb in total is desirably the range from 0.02% to 0.25%, more
15 desirably the range from 0.04% to 0.20%.
[0040]
Al: 0% 2.0%
A1 may be contained because A1 suppresses the formation of coarse cementite and
increases the low temperature toughness. In addition, A1 can be used as a deoxidant.
20 However, excessive A1 will increase the number of Al-based coarse inclusions, resulting in
the degradation of hole expandability and surface scratches. Therefore, the upper limit of
the A1 content is 2.0%, desirably 1.5%. Since it is diflicult to contain 0.001% or less Al,
this is a substantial lower limit.
[0041]
25 N: 0% to 0.01%
N may be contained because N increases the baking hardenability. Ilowever, N
might lead to the formation of blowholes at the time of welding, which might decrease the
strength of joints of welded parts. Accordingly, the content of N needs to be 0.01% or
less. On the other hand, the content of N being 0.0005% or less is not economically
efficient, and therefore, the content of N is desirably 0.0005% or more.
5 [0042]
The above elements are the basic chemical composition of the hot rolled steel
sheet according to the present invention, and the following composition may be further
contained.
One or more of Cu, Ni, Mo, V, and Cr may be contained because these elements
10 suppress ferrite transformation at the time of cooling and change the steel sheet structure
into one or both of a tempered martensite structure and a lower bainite structure. In
addition, one or more of these elements may be contained because these elements have an
effect of increasing the strength of the hot rolled steel sheet by precipitation strengthening
or solution strengthening. However, if the content of each of Cu, Ni, Mo, V, and Cu is
15 less than 0.01%, the above effects will not be shown sufficiently. In addition, if the
content of Cu exceeds 2.0%, the content of Ni exceeds 2.0%, the content of Mo exceeds
1.0%, the content of V exceeds 0.3%, and the content of Cr exceeds 2.0%, the above
effects will be saturated and the economic efficiency will be lowered. Therefore, it is
desirable that, in a case where one or more of Cu, Ni, Mo, V, and Cr are contained as
20 necessary, the contents of Cu, Ni, Mo, V, and Cr range from 0.01% to 2.0%, from 0.01% to
2.0%, from 0.01% to 1.0%, from 0.01% to 0.3%, and from 0.01% to 2.0%, respectively.
[0043]
One or more of Mg, Ca, and REM (rare earth metal) may be contained becausc
these elements control the form ol' non-metal inclusions serving as a starting point of
25 fracture and a factor of the degradation of processability so as to increase processability.
When the total content of Ca, REM, and Mg is 0.0005%, the effects will be obvious.
Accordingly, in a case where one or more of these elements are contained, the total content
thereof needs to be 0.0005% or more. In addition, if the content of Mg exceeds 0.01%,
the content of Ca exceeds 0.01%, and the content of REM exceeds 0.1%, the above effects
will be saturated and the economic efficiency is lowered. Therefore, it is desirable that
5 the content of Mg, the content of Ca, and the content of REM range from 0.0005% to
0.01%, from 0.0005% to 0.01%, and from 0.0005% to 0.1%, respectively.
[0044]
B contributes to the change of the steel sheet structure into one or both of a
tempered martensitc structure and a lower bainite structure by delaying ferrite
10 transformation. In addition, in the same manner as C, by segregating B in the grain
boundaries to incrcase the grain boundary strength, the low temperature toughness is
increased. Thus, B may be contained in the steel sheet. However, this effect becomes
obvious when the content of B in the steel sheet is 0.0002% or more; accordingly, the
lower limit thereof is desirably 0.0002%. On the other hand, if the content of B exceeds
15 0.01%, the effect is saturated and the economic efficiency is lowered; accordingly, the
upper limit is 0.01%. The content of B is desirably in the range from 0.0005% to 0.005%,
more desirably from 0.0007% to 0.0030%.
[0045]
As for the other elements, even when one or more of Zr, Sn, Co, Zn, and W are
20 contained in a total content of 1% or less, the effects of the present invention are confirmed
to not be damaged. Among these elements, Sn might generate scratches at the time of
hot-rolling; accordingly, the content thereof is desirably 0.05% or less.
[0046]
In the present invent~ont,h e composition other than the above is Fe, but inevitable
25 impurities that are mixed from raw materials for melting such as scraps or refractories are
acceptable. Typical impurities arc as follows.
[0047]
P: 0.10% or less
P, which is an impurity contained in molten pig iron, is segregated in the grain
boundaries, and as the content thereof is increased, the low temperature toughness is
5 decreased more. Accordingly, the content of P is desirably as low as possible, and is
0.10% or less because the content being more than 0.10% will adversely affect the
processability and weldability. In particular, considering weldability, the content of P is
desirably 0.03% or less. The lower the content ofP is, the more preferable it is; however,
a reduction more than necessary will burden a steelmaliing process with a heavy load.
10 Accordingly, the lower limit of the content of P may be 0.001%.
[0048]
S: 0.03% or less
S is also an impurity contained in molten pig iron. If the content of S is too high,
breaking will be generated at the time of hot rolling, and also inclusions such as MnS,
15 which degrades hole expandability, will be generated. Accordingly, the contcnt of S
should be as low as possible, and 0.03% or lcss is within an acceptable range. Therefore,
the content of S is 0.03% or less. Note that, in a case where a certain hole expandability
is necessary, the content of S is preferably 0.01% or less, more preferably 0.005% or less.
The lower the content of S is, the more preferable it is; however, a reduction more than
20 necessary will burden a stcclmaliing process with a heavy load. Accordingly, the lower
limit of the content of S may be 0.0001%.
[0049]
0: 0.01% or lcss
Too much 0 generates coarse oxides serving as a starting point of fracture in steel
25 and causes brittle fracturc or hydrogen induced cracking, so that the content of 0 is 0.01 or
less. For on-site weldability, the content of 0 is desirably 0.03% or less. The content of
0 may be 0.0005% or more because 0 disperses a large number of fine oxides at the time
of deoxidation of molten steel.
[0050]
The high-strength hot-rolled steel sheet according to the prescnt invention, which
5 has the above described structure and chemical composition, can have high corrosion
resistance by including, on a surface thereof, a hot dip galvanized layer formed by hot dip
galvanizing treatment and a galvannealed layer formed by galvannealing treatment
(galvannealing treatment means treatment using a hot-dip plating process and an alloying
process). Note that the plated layer is not limited to pure zinc, and any of the elements
10 such as Si, Mg, Zn, Al, Fe, Mn, Ca, and Zr may be added so as to further increase the
coi~osion resistance. Inclusion of such a plated layer does not damage the excellent
halting hardenability and low temperature toughness of the present invention.
Alternatively, the effects of the present invention can be shown by including a
surface-treating layer formed by any of the following: formation of an organic film, film
15 laminating, organic salts/inorganic salts treatment, non-chromium treatment, and the like.
LO05 11
[Method for producing steel sheet]
Next, a melhod for producing the steel sheet according to the present invention
will be described.
20 In order to achieve the excellent baking hardenability and low temperature
toughness, it is important that the dislocation density is 1x10" (l/ln2) or less, the number
of iron-based carbides is 1 x106 (numbers/mm2) or more, and the total content of one or
both of tempered martensite and lower bainitc, each of which has a grain size of 10 pm or
less, is 90% or more. Details of production conditions for satisfying all of the above
25 conditions will be described below.
[0052]
There is no particular limitation on the production method before hot rolling.
That is, subsequently to melting in a blast furnace, electric furnace, or the like, secondary
refining is performed in a various manner so that the composition is adjusted to be the
above composition, followed by casting by normal continuous casting, an ingot method,
5 thin slab casting, or the like.
In a case of continuous casting, cooling may be performed to make the
temperature low and then reheating may be performed before hot rolling, an ingot may be
hot-rolled without cooliug to room temperature, or a casting slab may be hot-rolled
continuously. As long as the composition can be controlled within the range according to
10 the present invention, scraps may be used as a raw material.
lo0531
The high-strength steel sheet according to the present invention is obtained when
the following requirements are satisfied.
To produce the high-strength steel sheet, mclting is performed to obtain a
15 predetermined steel sheet composition, and then optionally after cooling, a casting slab is
heated to a temperature of 1200°C or more, hot-rolling is completed at a temperature of
900°C or more, the steel sheet is cooled at a cooling speed of 50°Cis or more on average
from a final rolling Lemperature to 400°C and the steel sheet is coiled a1 a temperature of
less than 400°C and a cooling speed of not more than 5O0Cis. In this manner, it is
20 possible to produce a high-strength hot-rolled steel sheet having excellent baking
hardenability and low temperature toughness with a maximum tensile strength of 980 MPa
or more.
[00541
The temperature for heating the slab in hot rolling needs to be 1200°C or more.
25 In the steel sheet according to the present invention, austenite grains are prevented from
being coarse by using dissolved Ti and Nb, and accordingly, it is necessary to dissolve
NbC and Tic that have been precipitated at the time of casting. If the temperature for
heating the slab is less than 120OoC, carbides of Nb and Ti will take a long time to be
melted, and thus the crystal grain size will not be refined thereafter and the effect of
increasing the low temperature toughness caused by the refinement will not be shown.
5 Therefore, the temperature for heating the slab needs to be 1200°C or more. The effect of
the present invention can be shown even without any particular upper limit on the
temperature for heating the slab; however, excessively high temperature for heating is not
economically efficient. Therefore, the upper limit on the temperature for heating the stab
is desirably less than 1300°C.
10 [0055]
The final rolling temperature needs to bc 900°C or more. Large numbcrs of Ti
and Nb are added to the steel sheet according to the present invention in order to refine the
grain size of austenite. Accordingly, if the final rolling is performed in a temperature
range of less than 900°C, austenite will be unliltely to be recrystallized and grains
15 extending in the rolling direction will be generated, easily causing the degradation of
toughness. Furthermore, when unrecrystallized austenite is transformed into martensite
or bainite, dislocations accumulated in austenite are inherited to martensite or bainite, so
that the dislocation density in the steel sheet cannot be within the rangc regulated in the
present invention, resulting in the degradation of baking hardenability. Therefore, the
20 final rolling temperature is 900°C or more.
[0056]
It is necessary to perform cooling at an average cooling speed of 5OoC1s or more
from the final rolling temperature to 400°C. If' the cooling speed is less than 5O0C/s,
rerrite will be formed halfway on the cooling, and it will become difficult to make the
25 volume ratio of the main phase, one or both of tempered martensite and lower bainite, be
90% or more. Accordingly, the avcrage coolillg speed needs to be 5O0C/s or more.
However, if ferrite is not formed during the cooling process, air cooling may be performed
at temperatures from tlie final rolling temperature to 400°C.
[0057]
Note that it is preferable to set the cooling speed from a Bs point to the
5 temperature at which the lower bainite is generated (hereinafter referred to as lower bainite
generating temperature) to 50°C/s or more. This is for avoiding the formation of upper
bainite. If the cooling speed from the Bs point to the lower bainite gencrating
temperature is less than 50°C/s, the upper bainite will be generated; furthermore, fresh
martensite (martensite having a high dislocation density) will be generated between laths
10 of bainite, or retained austenite (will be transformed into martensite having a high
dislocation density at the time of processing) will exist, resulting in the dcgradation of
baking hardenability and low temperature toughness. Note that the Bs point is the
temperature at which upper bainite is started to be generated, the temperature being defined
depending on the composition, and is 550°C for convenience. Although also defined
15 depending on the composition, the lower bainite generating temperature is 400°C for
convenience. From the final rolling temperature to 400°C, the average cooling speed is
set to 50°C/s or more, and the cooling speed especially from 550°C to 400°C is set to
50°C/s or more.
Note that setting the average cooling speed to 50°C/s or more from the final
20 rolling temperature to 400°C includes the case wherc the cooling speed is set to 50°C/s or
more from the final rolling tcmperaturc to 550°C and the cooling speed is set to less than
50°C/s from 550°C to 400°C. However, under this condition, upper bainite is easily
generated, and greater than 10% upper bainite might be partially generated. Accordingly,
it is preferable to set the cooling speed to 5O0C/s or more from 550°C to 400°C.
25 [0058]
The maximum cooling speed at temperatures of less than 400°C needs to be less
than 5O0C/s. This is for making a main phase of one or both of tempered martensite and
lower bainite in which the dislocation density and the number density of iron-based
carbides are set to within the above range. If the maximum cooling speed is SO0C/s or
more, the iron-based carbides and the dislocation density will not be within the above
range, and excellent baking hardenability and toughness are not obtained. Thus, the
maximum cooling speed needs to be less than SOoC/s.
Here, cooling at temperatures of less than 400°C and a cooling speed of not more
than 5O0C/s is achieved by air cooling, for example. The cooling here not only means
cooling but also includes coiling the steel sheet in isothermal holding, that is, coiling at
temperatures of less than 400°C. Furthermore, the cooling speed is controlled in this
temperature range in order that the dislocation density and the number density of
iron-based carbides in the steel sheet structure are controlled. Thus, afler cooling is
performed such that the temperature becomes the temperature at which martensite
transformation starts (Ms point) or less, even when the temperature is increased and
reheating is performed, it is still possible to obtain a maximum tensile strength of 980 MPa
or more, excellent baking hardenability, and excellent toughness, which are the effects of
the present invention.
[0059]
In general, ferrite transformation needs to be suppressed to obtain martensite, and
cooling at SO0C/s or more is said to be necessary. In addition, at low temperatures,
dislocations occur from a temperature range called film boiling range in which the heat
transfer coeficient is relatively low and cooling is difficult, to a temperature range called
nucleate boiling temperature range in which the heat transfer coefficient is high and
cooling is easy. In a case where the cooling is stopped at a temperature range of less than
40OoC, the coiling temperature is likely to vary, and accordingly, the material quality
varies. Thus, typically, the coiling temperature has often been set to temperatures greater
than 400°C or to room temperature.
As a result, it'is assumed that it has not been found out in the related art that the
coiling at temperatures of less than 400°C or the decrease in cooling speed can lead to a
maximum tensile strength of 980 MPa or more, excellent baking hardenability, and
5 excellent temperature toughness.
[0060]
Note that, in order to increase ductility by the correction of thc steel sheet and
formation of movable dislocations, after all the steps are finished, skin-pass rolling is
desirably performed at a reduction of from 0.1% to 2%. In addition, aficr all the steps are
10 finished, in order to remove scales attached onto the surface of the thus obtained hot-rolled
steel sheet, the hot-rolled steel sheet may be pickled as necessary. Furthermore, after
pickling, the resulting hot-rolled steel sheet may be subjected to skin-pass or cold rolling at
a reduction of 10% or less in an in-line or off-line manner.
[0061]
15 The steel sheet of the present invention is produced through continuous casting,
rough rolling, final rolling, or pickling, which are a typical hot-rolling process; however,
even when part of them is omitted in the production, the effects of the present invention,
which are a maximu~n tensile strength of 980 MPa or more, excellent baiting hardenability,
and excellent low temperature toughness, can be secured.
20 In addition, after thc hot-rolled steel sheet is produced, even when hcat bcatment
is performed in a temperature range from 100°C to 600°C in an in-line or off-line manner
in order to precipitate carbides, the eSfects of the present invention, which are excellent
baking hardenability, excellent low temperature toughness, and a maximum tensile strength
of 980 MPa or more, can be secured.
25 [0062]
The steel sheet having a maximum tensile strength of 980 Ml'a or more in the
present invention means a steel sheet having 980 MPa or more maximum tensile stress
measured by tensile testing in conformity to JIS Z 2241 using JIS No. 5 test piece that is
cut out in a direction perpendicular to the rolling direction of hot rolling.
The steel sheet having excellent baking hardenability in the present invention
5 means a steel sheet having 60 MPa or more, desirably 80 MPa or more, difference in yield
strength at the time of retensile testing after 2% tensile prestrain is imparted, followed by
heat treatment at 170°C for 20 minutes. The above difference corresponds to baking
hardenability (BH) measured in conformity with coating-baking-hardening testing methods
described in an appendix of JIS G 3 135.
10 The steel sheet having excellent toughness at low ternpcratures in the present
invention means a steel sheet having -40°C fraction dislocation tcmpcrature (vTrs)
measured by Charpy testing conducted in conformity with JIS Z 2242. In the present
invention, since the target steel sheet is mainly used for automobile application, the
thickness is typically about 3 mm. Thus, the surface of the hot-rolled steel sheet is
15 grinded and the steel sheet is processed into a 2.5-mm sub-size test piece.
[Examples]
[0063]
The technical content of the present invention will be described by taking
Examples of the present invention.
20 As Examples, inventive steels A to S satisfying the conditions of the present
invention and comparative steels a to k, component compositions of which are shown in
Table 1, and results of studies thereof will be described.
After these steels were casted, directly the steels were heated to a temperature
range oT from 1030°C to 130OoC, or the steels were cooled to room temperature and then
25 reheated to this tcmperaturc range. Then, hot rolling was perfomled under conditions
shown in 'l'ables 2-1 and 2-2, final rolling was performcd at temperatures of from 760°C to
1030°C, and cooling and coiling were performed under conditions shown in Tables 2-1 and
2-2. Thus, hot-rolled'steel sheets having a thickness of 3.2 mm wcre produced. Then,
pickling was performed and 5% skin-pass rolling was performed.
[0064]
5 Various test pieces were cut out from the thus obtained hot-rolled steel sheets to
perform material quality testing and structure observation.
Tensile testing was conducted by cutting out JIS No. 5 test pieces in a direction
perpendicular to the rolling direction, in conformity with JIS Z 2242.
The baking hardenability was measured by cutting out JIS No. 5 test pieces in a
10 direction perpendicular to the rolling direction, in conformity with a
coating-halting-hardening testing method described in an appendix of JIS G 3135. The
prestrain was 2% and the heat treatment conditions were 170°C x 20 minutes.
Charpy testing was conducted in conformity with J1S Z 2242, and fracture
dislocation temperatures were measured. Since each of the steel sheets of the present
15 invention had a thickness of less than 10 mm, both surfaces of the hot-rolled steel sheet
were grinded to be 2.5 mm in thickness, and then the Charpy testing was conducted.
Some of the steel sheets were obtained as hot-dip-galvanized steel sheet (GI) and
galvanncaled steel sheet (GA) by heating the hot-rolled stcel sheet to 660°C to 720°C, and
performing hot dip galvanizing treatment or plating treatment followed by alloying heat
20 treatment at 540°C to 580°C, so that the material quality testing was conducted.
Micro-structure observation was performed by the above method, and each
structure was measured for volume fraction, dislocation density, the number density of
iron-based carbides, effective crystal size, and aspect ratio.
[0065]
Tables 3-1 and 3-2 show the results.
It is clear that only the steels satisfying the conditions of thc present invention had
a maximum tensile strength of 980 MPa or more, excellent baking hardenability, and
excellent low temperature toughness.
In contrast, steels A-3, B-4, E-4, 5-4, M-4, and S-4 were not able to have the
structure fraction and eEective crystal size within the range of the present invention, and
5 had lower strength and poor low temperature toughness because carbides of Ti and Nb that
were precipitated at thc time of casting are unlikely to be dissolved due to the temperature
for heating the slab being lcss than 120OoC, even though the other hot-rolling conditions
were within the range of the present invention.
Steels A-4, B-5, J-5, M-5, and S-5 were formed at too low final rolling
10 temperature, so that rolling was performed in a range of unrecrystallized austenite.
Accordingly, the dislocation density in the hot-rolled sheet became too high and the baking
hardenability became poor, and in addition, the grains were extended in the rolling
direction and the aspect ratio was high. Therefore, the stecls A-4, B-5, 5-5, M-5, and S-5
had a h~ghas pect ratio and poor toughness.
15 [0066]
Steels A-5, B-6, J-6, M-6, and S-6 were formcd at a cooling speed of less than
50°C/s from the final rolling temperature to 400°C, so that a large amount of ferrite was
formed during cooling. Accordingly, high strength was hardly securcd and the interface
between ferrite and martensite served as a starting point of fracture. Therefore, the steels
20 A-5, B-6, J-6, M-6, and S-6 had poor low temperature toughness.
Steels A-6, B-7, 3-7, M-7, and S-7 were formed at a maximum cooling speed of
50°C/s or more at temperatures of lcss than 40OoC, so that the dislocation density in
martensite became high and the baking hardenability became poor. In addition, the
plecipitat~on amount of carbides was insufficient, and therefore the steels A-6, B-7, 5-7,
25 M-7, and S-7 had poor low temperature toughncss
Note that, in the steel B-3 in Examples, in a case where the cooling speed was set
to 45°C/s from 550°C to 400°C, the average cooling speed was 8O0C/s from 95OoC, which
is the final rolling temperature, to 400°C. Therefore, the average cooling speed of 50°C
or more was satisfied; however, the steel sheet structure included 10% or more upper
bainite partially, and the material quality thereof varied.
5 [0067]
A steel A-7 was formed at a coiling temperature as high as 480°C, so that the steel
sheet structure became an upper bainite structure. Accordingly, a maximum tensile
strength of 980 MPa or more was hardly obtained and coarse iron-based carbides
precipitated between laths existing in the upper bainite structure served as a starting point
10 of fracture. Therefore, the steel A-7 had poor low temperature toughness.
Steels B-8, 5-8, and M-8 were formed at coiling temperatures as high as from
580°C to 620°C, so that the steel sheet structure became a mixed structure of ferrite and
pearlite including carbides of Ti and Nb. Accordingly, most of C in the steel sheet was
precipitated as carbides, and a sufficient amount of dissolved C was not secured.
15 Therefore, the steels B-8,543, and M-8 had poor baking hardenability.
LO0681
In addition, as shown in steels A-8, A-9, B-9, B-10, E-6, E-7, J-9, J-10, M-9,
M-10, S-9 and S-10, even when galvannealing treatment or galvarmealing treatment is
performed, the material quality of the present invention can be secured.
20 In contrast, the steels a to k whose steel sheet components were not within the
range of the present invention were not able to have a maximum tensile strength of 980
MPa or more, excellent baiting hardenability, and excellcnt low temperature toughncss, as
defined in the present invention.
100691
25 [Table 11
[Table 2-1 ]
Ranges beyond the present invention are underlined.
['rable 2-21
Ranges beyond the present invention are underlined.
[0071]
[Table 3-11
3 $
Q
rc
W
rC, u
HR represents hot-rolled steel sheet, G1 represents hot-dipgalvanired steel sheet, GA represents galvannealed steel sheet. Ranger beyond the present invention are underlined.

[Name of Document] CLAIMS
[Claim 11
A high-strength hot-rolled steel shect with a maximum tensile strength of 980
MPa or more, the steel sheet having a composition consisting of, in mass%.
5 C: 0.01% to 0.2%,
Si: 0% to 2.5%,
Mn: 0% to 4.0%,
Al: 0% to 2.0%,
N: 0% to 0.01%,
Cu: 0% to 2.0%,
Ni: 0% to 2.0%,
Mo: 0% to 1.0%,
V: 0% to 0.3%,
Cr: 0% to 2.0%,
Mg: 0% to 0.01%,
Ca: 0% to 0.01%,
REM: 0% to 0.1%,
B: 0% to 0.01%,
P: less than or equal to 0.10%,
S: less than or equal to 0.03%,
0: less than or equal to 0.01%,
one or both of Ti and Nb: 0.01% to 0.30% in total, and
the balance being Fe and inevitable impurities,
wherein the steel sheet has a structure in which a total volume fraction of one or
25 both of tempered martensite and lower bainite is 90% or morc, and a dislocation density in
the martensite and lower bainite is greater than or cqual to 5x10'~(l /m2) and less than or
equal to 1x10'~(l /m2).
[Claim 2)
The high-strength hot-rolled steel sheet according to claim 1, wherein the one or
both of tempered martensite and lower bainite include 1x10~(n umbers/mm2) or more
5 iron-based carbides.
[Claim 31
The high-strength hot-rolled steel sheet according to claim 1, wherein the one or
both of tempered martensite and lower bainite have an effective crystal size of less than or
equal to 10 pm.
10 [Claim 41
The high-strength hot-rolled steel sheet according to claim 1, comprising one or
more of, in mass%,
Cu: 0.01% to 2.0%,
Ni: 0.01% to 2.0%,
Mo: 0.01% to 1.0%,
V: 0.01% to 0.3%, and
Cr: 0.01% to 2.0%.
[Claim 51
The high-strength hot-rolled stcel sheet according to claim 1, comprising one or
20 more of, in mass%,
Mg: 0.0005% to 0.01%,
Ca: 0.0005% to 0.01%, and
REM: 0.0005% to 0.1%.
[Claim 61
25 The high-strenbah hot-rolled steel sheet according to claim 1, comprising, in
mass%,
B: 0.0002% to 0.01%.
[Claim 71
A method for producing a high-strength hot-rolled steel sheet with a maximum
tensile strength of 980 MPa or more, the method comprising:
5 heating, optionally after cooling, a casting slab to a temperature of 1200°C or
more, the casing slab having a composition consisting of, in mass%.
C: 0.01% to 0.2%,
Si: 0% to 2.5%,
Mn: 0% to 4.0%,
Al: 0% to 2.0%,
N: 0% to 0.01%,
Cu: 0% to 2.0%,
Ni: 0% to 2.0%,
Mo: 0% to 1.0%,
V: 0% to 0.3%,
Cr: 0% to 2.0%,
Mg: 0% to 0.01%,
Ca: 0% to 0.01%,
REM: 0% to 0.1%,
B: 0% to 0.01%,
P: less than or equal to 0.10%,
S: less than or equal Lo 0.03%,
0: less than or equal to 0.01%,
one or both of 'Ti and Nb: 0.01% to 0.30% in total, and
the balance being Fe and inevitable impurities;
completing hot rolling at a temperature of 900°C or more;
cooling the steel sheet at a cooling speed of 50°C/s or more on average from a
final rolling temperature to 400°C;
setting a cooling speed of not more than 5O0C/s at a temperature of less than
400°C; and
5 coiling the steel sheet
[Claim 81
The method for producing a high-strength hot-rolled steel sheet according to claim
7, further comprising:
performing galvanizing treatment or galvannealing treatment.