Technical Field
[0001] The present invention relates to a high-intensity hot-rolled steel sheet having a
tensile strength (TS) of 540 to 780 MPa, only small variations in strength, and excellent
uniformity in strength between coils and within a coil, and being useful as a steel
sheet for automobiles and so forth, and to a method for manufacturing the same.
Background Art
[0002] From the viewpoint of global environmental protection, improvement in the fuel economy
of automobiles has recently been required to regulate the amount of CO
2 emission. In addition, it is also required to improve safety by focusing on collision
characteristics of automobile bodies in order to ensure the safety of passengers in
case of accident.
Thus, both the weight reduction and strengthening of automobile bodies are being actively
promoted. To simultaneously achieve the weight reduction and strengthening of automobile
bodies, an increase in the strength of a material for members and a reduction in weight
by reducing the thickness of sheets to the extent that rigidity is not impaired are
said to be effective. Nowadays, high-strength steel sheets are positively used for
automotive parts. The use of high-strength steel sheets results in a significant weight
reduction effect. Thus, in the motor vehicle industry, for example, there is a trend
toward the use of steel sheets as a structural member with a tensile strength (TS)
of 540 MPa or more.
[0003] Many automotive parts made from steel sheets are manufactured by press forming. Regarding
the formability of high-strength steel sheets, dimensional accuracy is important in
addition to prevention of cracking and wrinkling. In particular, the control of springback
is an important problem. Nowadays, new automobiles are developed very efficiently
by computer assisted engineering (CAE) can predict the amount of springback more accurately
by the input of the characteristics of the steel sheet. So, it is not necessary to
make many dies. Variations in the amount of springback cause a problem when parts
are connected to each other and thus should be reduced. If the steel sheets used for
automobile parts have wide variations in strength, CAE can not predict the amount
of springback. So, in particular, a high-strength steel sheet having only small variations
in strength and excellent uniformity in strength is required.
[0004] As a method for reducing variations in strength in a coil, Patent Document 1 (Japanese
Unexamined Patent Application Publication No.
4-289125) discloses the following method: In the case of hot-rolling Nb-containing low-manganese
steel (Mn: 0.5% or less), a rough-rolled sheet bar is temporarily wound into a coil.
Next, the sheet bar is joined to the preceding sheet bar while being unwound, and
then continuously finish-rolled to achieve uniformity in the strength of the high-strength
hot-rolled steel sheet in a coil. Patent Document 2 (Japanese Unexamined Patent Application
Publication No.
2002-322541) discloses a high-strength hot-rolled steel sheet with excellent uniformity in strength,
i.e., only small variations in strength, produced by the addition of both Ti and Mo
to form very fine precipitates uniformly dispersed therein.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 4-289125
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-322541
[0005] Furthermore,
JP 6 200351 A discloses a high strength hot rolled steel plate for working, having high strength
and superior stretch-flange formability. The steel has a composition consisting of,
by weight, 0.02-0.10% C, ≤2.0% Si, 0.5-2.0% Mn, ≤0.08% P, ≤0.006% S, ≤0.005% N, 0.01-0.1%
Al, 0.06-0.3% Ti, and the balance Fe with inevitable impurities and satisfying 0.50<(Ti-3.43N-1.5S)/4C
and further containing, if necessary, prescribed amounts of Nb, Mo, V, Zr, Cr, Ni,
and Ca. A structure, where the area ratio of low-temperature transformation products
and pearlite is regulated to ≤15% and TiC is dispersed in polygonal ferrite, is provided
to the steel, by which a hot rolled steel plate increased 111 strength as to have
≥ about 70kgf/mm
2 tensile strength and excellent in stretch-flange formability can be obtained. This
structure is obtained by finishing hot rolling at about 850-920 °C and then regulating
cooling velocity and coiling tempperature.
Disclosure of Invention
[0006] The foregoing related art, however, has problems described below.
[0007] The method described in Patent Document 1 has a problem in which when the sheet is
wound into a coil, the sheet is divided. Furthermore, the addition of Nb causes an
increase in cost, which is economically disadvantageous. In the steel sheet described
in Patent Document 2, which is a Ti system, it is necessary to add Mo, which is expensive,
thus causing an increase in cost. Moreover, in any of the patent documents, two-dimensional
uniformity in strength in the in-plane directions including both of the width direction
and the longitudinal direction of the coil is not taken into consideration. Disadvantageously,
even if the coiling temperature is uniformly controlled, the variations in the in-plane
strength of the coil are inevitably caused by different cooling histories for each
position in the wound coil.
[0008] In consideration of the above-described situation, it is an object of the present
invention to advantageously solve the foregoing problems and provide a high-strength
hot-rolled steel sheet having a tensile strength (TS) of 540 to 780 MPa, only small
variations in strength, and excellent uniformity in strength using a general-purpose
Ti-containing steel sheet, which is inexpensive.
[0009] To overcome the foregoing problems, the inventors have conducted intensive studies
and have successfully provided a high-strength hot-rolled steel sheet having excellent
uniformity in strength and only small variations in strength over the entire area
of the hot-rolled steel sheet by controlling the chemical composition and the metal
microstructure of the steel sheet and the precipitation state of Ti that contributes
to precipitation strengthening, leading to the completion of the present invention.
[0010] The gist of a high-strength hot-rolled steel sheet according to the present invention
and a method for manufacturing the high-strength hot-rolled steel sheet are described
below, the steel sheet having only small variations in in-plane strength and excellent
uniformity in strength.
- [1] A high-strength hot-rolled steel sheet according to claim 1 includes, on a mass
percent basis, 0.05%-0.12% C, 0.5% or less Si, 0.8%-1.8% Mn, 0.030% or less P, 0.01%
or less S, 0.005%-0.1% Al, 0.01% or less N, 0.030%-0.080% Ti, the balance being Fe
and incidental impurities, and microstructures whose volume fraction of polygonal
ferrite is 70% or more, in which the amount of Ti present in a precipitate having
a size of less than 20 nm is 50% or more of the value of Ti* calculated using formula
(1) :
Ti* = [Ti] - 48/14 × [N] (1)
where [Ti] and [N] represent a Ti content (percent by mass) and a N content (percent
by mass), respectively, of the steel sheet.
- [2] A method for manufacturing a high-strength hot-rolled steel sheet according to
claim 2 includes the steps of heating a steel slab to 1150°C to 1300°C, the steel
slab containing, on a mass percent basis, 0.05%-0.12% C; 0.5% or less Si, 0.8%-1.8%
Mn, 0.030% or less P, 0.01% or less S, 0.005%-0.1% Al, 0.01% or less N, 0.030%-0.080%
Ti, and the balance being Fe and incidental impurities, subjecting the slab to finish
hot rolling at a finishing temperature of 800°C to 950°C, starting cooling at a cooling
rate of 20 °C/s or more within 2 seconds after the completion of the finish hot rolling,
stopping cooling at 650°C to 750°C, subsequently performing natural cooling for 2
seconds to 15 seconds, cooling the steel sheet at a cooling rate of less than 100
°C/s, and winding the steel sheet into a coil in the temperature range of 550°C to
650°C.
[0011] According to the present invention, it is possible to reduce variations in strength
in a coil of a high-strength hot-rolled steel sheet having a tensile strength (TS)
of 540 to 780 MPa, thereby achieving the stabilization of the shape fixability of
the steel sheet at the time of press forming and the strength and durability of a
part. This leads to improvement in reliability at the time of the production and use
of an automotive part. Furthermore, in the present invention, the above-mentioned
effect is provided without adding an expensive raw material such as Nb, thus reducing
the cost.
Brief Description of Drawings
[0012]
[Fig. 1] Fig. 1 shows the investigation results of the relationship between the volume
fraction of polygonal ferrite (%) and the tensile strength TS (MPa).
[Fig. 2] Fig. 2 shows the investigation results of the relationship between the proportion
of the amount of Ti contained in a precipitate having a size of less than 20 nm with
respect to Ti* and the tensile strength TS (MPa).
Best Mode for Carrying Out the Invention
[0013] The present invention will be described in detail below. 1) A method for evaluating
small variations in strength, i.e., uniformity in strength, according to the present
invention will be described.
[0014] An example of a target steel sheet is a coiled steel sheet having a weight of five
tons or more and a steel sheet width of 500 mm or more. In this case, in an as-hot-rolled
state, the innermost turn including the front end in the longitudinal direction, the
outermost turn including the rear end in the longitudinal direction, and regions extending
from both sides to 10 mm from both sides in the width direction are not evaluated.
Variations in the strength of the steel sheet are evaluated on the basis of tensile-strength
distribution obtained from two-dimensional measurement at at least 10 points in the
longitudinal direction and at least 5 points in the width direction. Furthermore,
the present invention covers a steel sheet having a tensile strength (TS) of 540 MPa
to 780 MPa. 2) The reason for the limitation of the chemical components (composition)
of steel according to the present invention will be described below.
[0015] The units of the content of each component in the steel composition are "percent
by mass" and are simply indicated by "%" unless otherwise specified.
C: 0.05% to 0.12%
[0016] C is an important element as well as Ti described below in the present invention.
C forms a carbide with Ti and is effective in increasing the strength of a steel sheet
by precipitation strengthening. In the present invention, the C content is preferably
0.05% or more and more preferably 0.06% or more from the viewpoint of precipitation
strengthening. A C content exceeding 0.12% liable to adversely affect satisfactory
elongation and flangeability. Thus, the upper limit of the C content is set to 0.12%
and preferably 0.10% or less.
Si: 0.5% or less
[0017] Si is effective in enhancing solid-solution strengthening and improving ductility.
To provide the effect described above, the Si content is effectively 0.01% or more.
A Si content exceeding 0.5% is liable to cause the occurrence of a surface defect
called red scale during hot rolling, which can reduce the quality of surface appearance
when a steel sheet is produced. Thus, the Si content is preferably 0.5% or less and
more preferably 0.3% or less.
Mn: 0.8% to 1.8%
[0018] Mn is effective in achieving higher strength and has the effect of reducing the transformation
point and the ferrite grain size. The Mn content needs to be 0.8% or more. More preferably,
the Mn content is set to 1.0% or more. A
[0019] Mn content exceeding 1.8% causes the formation of a low-temperature transformation
phase after hot rolling to reduce the ductility and is liable to make TiC precipitation
unstable. Thus, the upper limit of the Mn content is set to 1.8%.
P: 0.030% or less
[0020] P is an element effective for solid-solution strengthening. P also has the effect
of reducing the scale defect due to Si. An excessive P content more than 0.030%, however,
is liable to cause the segregation of P into grain boundaries and reduce toughness
and weldability. Thus, the upper limit of the P content is set to 0.030%.
S: 0.01% or less
[0021] S is an impurity and causes hot tearing. Furthermore, S is present in the form of
an inclusion in steel, deteriorating the various characteristics of a steel sheet.
Thus, the S content needs to be minimized. Specifically,
the S content is set to 0.01% because the S content is allowable to 0.01%.
Al: 0.005% to 0.1%
[0022] Al is useful as a deoxidizing element for steel. Al also has the effect of fixing
dissolved N present as an impurity, thereby improving resistance to room-temperature
aging. To provide the effect, the Al content needs to be 0.005% or more. An Al content
exceeding 0.5% leads to an increase in alloy cost and is liable to cause surface defects.
Thus, the upper limit of the Al content is set to 0.1%.
N: 0.01% or less
[0023] N is an element which degrades the resistance to room-temperature aging and in which
the N content is preferably minimized. A higher N content causes a reduction in resistance
to room-temperature aging. To fix dissolved N, it is necessary to perform the addition
of large amounts of Al and Ti. Thus, the N content is preferably minimized. The upper
limit of the N content is set to 0.01%.
Ti: 0.030% to 0.080%
[0024] Ti is an important element to strengthen steel by precipitation strengthening. In
the present invention, Ti contributes to precipitation strengthening by forming a
carbide with C.
[0025] That is, in order to produce a high-strength steel sheet having a tensile strength
TS of 540 MPa to 780 MPa, it is preferred to form fine precipitates each having a
size of less than 20 nm. Furthermore, it is important to increase the proportion of
the fine precipitates (each having a size of less than 20 nm). One of the reasons
for this may be that precipitates having a size of 20 nm or more are less likely to
provide the effect of suppressing dislocation migration and fail to sufficiently harden
polygonal ferrite, which can reduce strength. It is thus preferred that the precipitates
have a size of less than 20 nm. In the present invention, the fine precipitates containing
Ti and each having a size of less than 20 nm are formed by the addition of Ti and
C within the above ranges. In this specification, the precipitates containing Ti and
C are generically referred to as a "Ti-containing carbide". Examples of the Ti-containing
carbide include TiC and Ti
4C
2S
2. The carbide may further contain N as a component and may be precipitated in combination
with, for example, MnS.
[0026] In the high-strength steel sheet according to the present invention, it is observed
that the Ti-containing carbide is mainly precipitated in polygonal ferrite. This is
probably because supersaturated C is easily precipitated as a carbide in polygonal
ferrite because of a low solid-solubility limit of C in polygonal ferrite. The precipitates
allow soft polygonal ferrite to harden, thereby achieving a tensile strength (TS)
of 540 MPa to 780 MPa. Furthermore, Ti is readily bonded to dissolved N and thus an
element suitable for fixation of dissolved N. From that standpoint, the Ti content
is set to 0.030% or more.
However, an excessive addition of Ti only results in the formation of coarse undissolved
TiC or the like, which is a carbide of Ti but does not contribute to strength, and
is thus uneconomical, which is not preferred. The upper limit of the Ti content is
set to 0.080% from this viewpoint.
[0027] In the present invention, it is preferred that the composition of the balance other
than the components described above be substantially iron and incidental impurities.
3) The reason for the limitation of the steel microstructure of the steel sheet will
be described below. The steel sheet has microstructures whose the volume fraction
of polygonal ferrite is 70% or more, and the amount of Ti in a precipitate having
a size of less than 20 nm is 50% or more of the value of Ti* calculated using formula
(1).
[0028] The strength of the high-strength hot-rolled steel sheet according to the present
invention is determined by the superposition of the amounts of strengthening based
on three strengthening mechanisms, i.e., solid-solution strengthening, microstructural
strengthening, and precipitation strengthening, on the base strength of the steel
itself. The base strength is inherent strength of iron. The amount of solid-solution
strengthening is almost uniquely determined by a chemical composition. Thus, these
two strengthening mechanisms are negligibly involved in the variations in strength
in a coil. The strengthening mechanism that is the most closely related to the variations
in strength is precipitation strengthening, followed by microstructural strengthening.
[0029] The amount of strengthening by precipitation strengthening is determined by the size
and dispersion of precipitates (specifically, precipitate spacing). The dispersion
of precipitates can be expressed by the amount and size of the precipitates. Thus,
if the size and amount of the precipitates are determined, the amount of strengthening
by precipitation strengthening will be determined. Microstructural strengthening is
determined by the type of steel microstructure. The type of steel microstructure is
determined by a transformation-temperature range from austenite. If a chemical composition
and a steel microstructure are determined, the amount of strengthening will be determined.
4) Experimental facts used as the basis of the present invention will be described
below.
[0030] Steel A in which the amount of Ti added was 0.04% and steel B in which the amount
of Ti added was 0.06%, each of steel A and steel B having a basic chemical composition
of 0.08C-0.1Si-1.5Mn-0.011P-0.002S-0.017Al-0.005N, were ingoted in a laboratory into
cast strands. These cast strands were formed into sheet bars each having a thickness
of 25 mm by slabbing. Each of the sheet bars was heated to 1230°C, hot-rolled in five
passes so as to have a finishing temperature of 880°C, and water-cooled at a cooling
rate of 25 °C/s 1.7 seconds after finish rolling. At this time, the cooling stop temperature
was changed between 720°C and 520°C. After the water cooling, each sheet bar was subjected
to natural cooling for 10 seconds. Each sheet bar was inserted into an electric furnace
having a temperature of 500°C to 700°C and wound. The holding time in the furnace
was changed between 1 and 300 minutes. At this time, in the case where the difference
between the cooling stop temperature and the furnace temperature is 30°C or higher,
after the natural cooling, water cooling is performed at a cooling rate of 25 °C/s
in such a manner that the sheet bar has a temperature 30°C higher than the furnace
temperature. Hot-rolled steel sheets having different precipitation states of Ti and
different steel microstructures were manufactured by the method described above. The
hot-rolled steel strips were subjected to pickling and then temper rolling at an elongation
of 0.5%. Test pieces for a tensile test and analytical samples of precipitates were
taken.
[0031] Steel sheets in which the amount of Ti contained in precipitates having a size of
less than 20 nm was 50% or more of the amount of Ti* expressed as formula (1) described
below were selected from the hot-rolled steel sheets manufactured as described above.
Fig. 1 shows the investigation results of the relationship between the volume fraction
of polygonal ferrite (%) and the tensile strength TS (MPa). As shown in this figure,
the tensile strength TS tends to decrease as the volume fraction of polygonal ferrite
increases. At a volume fraction of polygonal ferrite of 70% or more, a change in TS
is small, and TS is stabilized.
[0032] For example, the volume fraction of polygonal ferrite can be determined as follows.
A portion of an L section (a section parallel to a rolling direction) of a steel sheet,
the portion excluding surface layers each having a thickness equal to 10% of the thickness
of the sheet, is etched with 5% nital. The microstructures of the etched portion are
photographed with a scanning electron microscope (SEM) at a magnification of 1000×.
Smooth ferrite crystal grains in which grain boundaries have a small step height of
less than 0.1 µm and corrosion marks are not left in the grains are defined as polygonal
ferrite. Polygonal ferrite is distinguished from other ferrite phases and different
transformation phases such as pearlite and bainite. These are color-coded with image-analysis
software. The area ratio is defined as the volume fraction of polygonal ferrite.
[0033] Similarly, steel sheets each having a volume fraction of polygonal ferrite of 70%
or more were selected from the hot-rolled steel sheets manufactured as described above.
Fig. 2 shows the investigation results of the relationship between the proportion
of the amount of Ti contained in a precipitate having a size of less than 20 nm with
respect to Ti* expressed as formula (1) described below and the tensile strength TS
(MPa). As described above, the precipitates each having a size of less than 20 nm
and contributing to precipitation strengthening are composed of added Ti. Thus, whether
Ti is efficiently precipitated as fine precipitates or not can be determined by the
grasp of the amount of Ti in the precipitate having a size of less than 20 nm. As
shown in this figure, TS tends to increase as the amount of Ti contained in the precipitate
having a size of less than 20 nm increases. In the case where the amount of Ti contained
in the precipitate is 50% or more of Ti*, a change in TS is small, and TS is stabilized.
[0034] From the above result, it is conceivable that in the case where the steel microstructures
are controlled so as to have a volume fraction of polygonal ferrite of 70% or more
and where the amount of Ti contained in the precipitate having a size of less than
20 nm is controlled in the range of 50% or more of Ti* expressed as formula (1) described
below, the resulting variations in strength are significantly small and practically
satisfactory even if inevitable variations in strength occur because the cooling histories
of the coil after winding are different for each position,
where [Ti] and [N] represent a Ti content (percent by mass) and a N content (percent
by mass), respectively, of the steel sheet.
[0035] Thus, in the case where requirements of the present invention are met, in other words,
in the case where the facts that a steel sheet has microstructures whose volume fraction
of polygonal ferrite is 70% or more and that the amount of Ti contained in a precipitate
having a size of less than 20 nm is 50% or more of Ti* expressed as formula (1) described
above are met, at any position of a steel sheet, even if the cooling histories of
a coil are different for each position, substantially the same amount of strengthening
is obtained at any position of the steel sheet. Thus, the steel sheet has only small
variation in strength and excellent uniformity in strength. 5) The amount of Ti contained
in a precipitate having a size of less than 20 nm can be measured by a method described
below.
[0036] After a sample is electrolyzed in an electrolytic solution by a predetermined amount,
the test piece is taken out of the electrolytic solution and immersed in a solution
having dispersibility. Then precipitates contained in this solution are filtered with
a filter having the pore size of 20 nm. Precipitates passing through the filter having
a pore size of 20 nm together with the filtrate each have a size of less than 20 nm.
After the filtration, the filtrate is appropriately analyzed by inductively-coupled-plasma
(ICP) emission spectroscopy, ICP mass spectrometry, atomic absorption spectrometry,
or the like to determine the amount of Ti in the precipitates having a size of less
than 20 nm. 6) An example of a preferred method for manufacturing a high-strength
hot-rolled steel sheet according to the present invention will be described below.
[0037] The composition of a steel slab used in the manufacturing method of the present invention
is the same as the composition of the steel sheet described above. Furthermore, the
reason for the limitation of the composition is the same as above. The high-strength
hot-rolled steel sheet of the present invention is manufactured through a hot-rolling
step of subjecting a raw material to rough hot rolling to form a hot-rolled steel
sheet, the raw material being the steel slab having a composition within the range
described above.
i) Heating Temperature: 1150°C to 1300°C
[0038] With respect to the heating temperature of a slab, the hot-rolled steel sheet is
preferably heated to 1150°C or higher in order that an undissolved Ti-containing carbide,
such as TiC, may not be present in a heating stage. This is because the presence of
the undissolved Ti-containing carbide adversely affects the tensile strength of a
hot-rolled steel sheet; hence, the absence of the undissolved Ti-containing carbide
is preferred. However, heating at excessively high temperatures causes problems, for
example, an increase in scale loss due to an increase in oxidation weight. Thus, the
upper limit of the heating temperature of the slab is preferably set to 1300°C.
[0039] The steel slab heated under the foregoing conditions is subjected to hot rolling
in which rough rolling and finish rolling are performed. Here, the steel slab is formed
into a sheet bar by the rough rolling. The conditions of the rough rolling need not
be particularly specified. The rough rolling may be performed according to a common
method. It is preferred to use what is called a sheet-bar heater from the viewpoints
of reducing the heating temperature of the slab and preventing problems during the
hot rolling.
[0040] Then the sheet bar is subjected to finish rolling to form a hot-rolled steel sheet.
ii) Finishing Temperature (FDT): 800°C to 950°C
[0041] A high finishing temperature results in coarse grains to reduce formability and is
liable to cause scale defects; hence, the finishing temperature is set to 950°C or
lower.
[0042] A finishing temperature of less than 800°C results in an increase in rolling force
to increase the rolling load and an increase in rolling reduction to develop an abnormal
texture in austenite non-recrystallization, which is not preferred from the viewpoint
of achieving uniform strength. Accordingly, the finishing temperature is set in the
range of 800°C to 950°C and preferably 840°C to 920°C.
[0043] To reduce the rolling force during the hot rolling, some or all passes of the finish
rolling may be replaced with lubrication rolling. The lubrication rolling is effective
from the viewpoint of improving uniformity in the shape of a steel sheet and uniformity
in strength. The coefficient of friction during the lubrication rolling is preferably
in the range of 0.10 to 0.25. Furthermore, a continuous rolling process is preferred
in which a preceding sheet bar and a succeeding sheet bar are joined to each other
and then the joined sheet bars are continuously finish-rolled. The use of the continuous
rolling process is desirable from the viewpoint of achieving the stable operation
of the hot rolling.
iii) Cooling (primary cooling) at a cooling rate of 20 °C/s or more within 2 seconds
after finish hot rolling
[0044] When a time exceeding 2 seconds elapses between the start of cooling and the completion
of the finish rolling, a strain accumulated during the finish rolling is relieved.
Thus, even if cooling control described below is performed, ferrite is not effectively
formed, failing to stably precipitate TiC. Furthermore, the same phenomenon is liable
to occur when the cooling rate is less than 20 °C/s.
iv) Stop of cooling in a temperature range of 650°C to 750°C and natural cooling step
for 2 seconds to 15 seconds
[0045] With respect to a temperature during natural cooling, in order to effectively precipitate
the Ti-containing carbide such as TiC in a short time required for the passage of
a steel sheet through a run-out table, it is necessary to hold the steel sheet for
a predetermined period of time in a temperature range where ferrite transformation
proceeds at a maximum. At a natural cooling (holding) temperature of less than 650°C,
it is impossible to ensure the amount of the Ti-containing carbide required for the
desired amount of strengthening because of a low growth rate of precipitates of the
Ti-containing carbide. At a natural cooling temperature exceeding 750°C, coarse Ti-containing
carbide grains are sparsely distributed because of the insufficient nucleation of
precipitates and a high growth rate, thereby reducing the strengthening ability. Accordingly,
the natural cooling temperature is set in the range of 650°C to 750°C.
[0046] A natural cooling time of less than 2 seconds results in an insufficient amount of
the Ti-containing carbide precipitated. It is thus difficult to ensure the amount
of strengthening required. A natural cooling time exceeding 15 seconds causes a reduction
in strengthening ability because coarse Ti-containing carbide grains are sparsely
distributed. Therefore, the natural cooling time is set in the range of 2 seconds
to 15 seconds.
v) Cooling (secondary cooling) at a cooling rate of 100 °C/s
[0047] In the case where the cooling rate subsequent to the natural cooling treatment is
100°C or more, the controllability of a coiling temperature is reduced, causing difficulty
in achieving stable strength. Thus, the cooling rate is set to less than 100 °C/s.
The lower limit of the cooling rate is not particularly limited but is 5 °C/s or more
from the viewpoint of inhibiting the coarsening of precipitates.
vi) Winding the steel sheet into a coil in a temperature range of 550°C to 650°C
[0048] In the case where the coiling temperature is less than 550°C, a portion that is not
transformed on the run-out table is formed as a low-temperature transformed phase
to cause variations in strength and a reduction in ductility. In the case where the
coiling temperature exceeds 650°C, the growth of the Ti-containing carbide such as
TiC proceeds after the completion of the winding, so that coarse Ti-containing carbide
grains are sparsely distributed, thereby reducing the strengthening ability. This
is also liable to cause variations in strength corresponding to cooling histories
after the winding. Thus, the coiling temperature is set in the range of 550°C to 650°C.
[0049] In the case where the variations in strength are taken into consideration in the
coil, the precipitation of the Ti-containing carbide such as TiC proceeds mainly in
a cooling stage after the completion of the winding; hence, it is desirable to take
into consideration of the cooling histories of the steel sheet after the winding.
In particular, the front end and the rear end of the coil are rapidly cooled, so that
the precipitation of the Ti-containing carbide does not sufficiently proceed, in some
cases. Thus, the temperatures of the front and rear ends of the coil are increased
with respect to the temperature of the inner portion of the coil other than the front
and rear ends, thereby further improving the variations in strength.
EXAMPLE 1
[0050] An example of the present invention will be described below.
[0051] Molten steels having compositions shown in Table 1 were made with a converter and
formed into slabs by a continuous casting process. These steel slabs were heated to
1250°C and rough-rolled into sheet bars. Then the resulting sheet bars were subjected
to a hot-rolling step in which finish rolling was performed under conditions shown
in Table 2, thereby forming hot-rolled steel sheets.
[0052] These hot-rolled steel sheets were subjected to pickling and temper rolling at an
elongation of 0.5%. Regions extending from both sides to 10 mm from both sides in
the width direction were removed by trimming. Various properties were evaluated. Steel
sheets were taken at positions at which the innermost turn including the front end
and the outermost turn including the rear end of the coil in the longitudinal direction
were cut. Furthermore, steel sheets were taken at 20 equally divided points of the
inner portion in the longitudinal direction. Test pieces for a tensile test and analytical
samples of precipitates were taken from both sides of each of the steel sheets in
the width direction and 8 equally divided points of each steel sheet in the width
direction.
[0053] The test pieces for a tensile test were taken in a direction (L direction) parallel
to a rolling direction and processed into JIS No. 5 test pieces. The tensile test
was performed according to the regulation of JIS Z 2241 at a crosshead speed of 10
mm/min to determine tensile strength (TS). Table 2 shows the investigation results
of tensile properties of the resulting hot-rolled steel sheets.
[0054] With respect to microstructures, a portion of an L section (a section parallel to
a rolling direction) of each of the steel sheets, the portion excluding surface layers
each having a thickness equal to 10% of the thickness of the sheet, was etched with
nital. The microstructures of the etched portion were identified with a scanning electron
microscope (SEM) at a magnification of 1000×. The volume fraction of polygonal ferrite
was measured by the method described above with image processing software.
[0055] The quantification of Ti in a precipitate having a size of less than 20 nm was performed
by a quantitative procedure described below.
[0056] The resulting hot-rolled steel sheets described above were cut into test pieces each
having an appropriate size. Each of the test pieces was subjected to constant-current
electrolysis in a 10% AA-containing electrolytic solution (10 vol% acetylacetone-1
mass% tetramethylammonium chloride-methanol) at a current density of 20 mA/cm
2 so as to be reduced in weight by about 0.2 g.
[0057] After electrolysis, each of the test pieces having surfaces to which precipitates
adhered was taken from the electrolytic solution and immersed in an aqueous solution
of sodium hexametaphosphate (500 mg/l) (hereinafter, referred to as an "SHMP aqueous
solution"). Ultrasonic vibration was applied thereto to separate the precipitates
from the test piece. The separated precipitates were collected in the SHMP aqueous
solution. The SHMP aqueous solution containing the precipitates was filtered with
a filter having a pore size of 20 nm. After the filtration, the resulting filtrate
was analyzed with an ICP emission spectrometer to measure the absolute quantity of
Ti in the filtrate. Then the absolute quantity of Ti was divided by an electrolysis
weight to obtain the amount of Ti (percent by mass) contained in the precipitates
each having a size of less than 20 nm. Note that the electrolysis weight was determined
by measuring the weight of the test piece after the separation of the precipitates
and subtracting the resulting weight from the weight of the test piece before electrolysis.
Next, the resulting amount of Ti (percent by mass) contained in the precipitates each
having a size of less than 20 nm was divided by Ti* calculated by substituting the
Ti content and the N content shown in Table 1 in formula (1), thereby determining
the proportion (%) of the amount of Ti contained in the precipitates each having a
size of less than 20 nm.
Table 1
Symbol |
Chemical component (% by mass) |
|
T (°C) |
Remarks |
C |
Si |
Mn |
P |
S |
Al |
N |
Ti |
Ti* |
|
A |
0.071 |
0.01 |
1.35 |
0.008 |
0.005 |
0.034 |
0.0035 |
0.035 |
0.023 |
678 |
Adaptation example |
B |
0.075 |
0.01 |
1.30 |
0.008 |
0.003 |
0.032 |
0.0032 |
0.045 |
0.034 |
679 |
Adaptation example |
C |
0.082 |
0.01 |
1.25 |
0.008 |
0.004 |
0.040 |
0.0030 |
0.058 |
0.048 |
678 |
Adaptation example |
D |
0.090 |
0.01 |
1.35 |
0.010 |
0.005 |
0.034 |
0.0015 |
0.05 |
0.045 |
676 |
Adaptation example |
E |
0.085 |
0.01 |
1.40 |
0.008 |
0.005 |
0.034 |
0.0020 |
0.032 |
0.025 |
678 |
Adaptation example |
F |
0.078 |
0.01 |
1.65 |
0.008 |
0.003 |
0.035 |
0.0015 |
0.042 |
0.037 |
682 |
Adaptation example |
G |
0.079 |
0.25 |
1.35 |
0.008 |
0.005 |
0.035 |
0.0030 |
0.034 |
0.024 |
685 |
Adaptation example |
H |
0.081 |
0.01 |
0.50 |
0.008 |
0.003 |
0.036 |
0.0032 |
0.042 |
0.031 |
672 |
Comparative example |
I |
0.040 |
0.01 |
1.35 |
0.009 |
0.002 |
0.034 |
0.0032 |
0.045 |
0.034 |
670 |
Comparative example |
J |
0.095 |
0.01 |
1.35 |
0.008 |
0.005 |
0.034 |
0.0032 |
0.025 |
0.014 |
675 |
Comparative example |
K |
0.082 |
0.01 |
1.35 |
0.008 |
0.005 |
0.036 |
0.0033 |
0.1 |
0.089 |
679 |
Comparative example |
Table 2
Steel sheet No. |
Steel No. |
Thickness mm |
Heating temperature °C |
Finishing temperature (FDT) °C |
Cooling start times |
Primary cooling rate °C/s |
Natural cooling temperature °C |
Natural cooling time s |
Secondary cooling rate °C/s |
Coiling temperature (CT) after finish hot rolling °C |
Volume fraction of polygonal ferrite % |
Amount of Ti present in precipitate with size of less than 20 nm % by mass |
Proportion of amount of Ti contained in precipitate with size of less than 20 nm % |
TS MPa |
Proportion of compliant steel microstructure % |
Proportion of compliant TS % |
ΔTS MPa |
Remarks |
1 |
|
6.0 |
1220 |
880 |
1.7 |
25 |
700 |
10 |
25 |
600 |
91 |
0.019 |
82 |
623 |
100 |
100 |
44 |
Inventive example |
2 |
|
2.6 |
1220 |
880 |
0.8 |
55 |
700 |
7 |
55 |
600 |
92 |
0.018 |
79 |
603 |
100 |
100 |
33 |
Inventive example |
3 |
|
6.0 |
1100 |
880 |
1.7 |
25 |
700 |
10 |
25 |
600 |
87 |
0.007 |
32 |
582 |
56 |
92 |
64 |
Comparative example |
4 |
|
6.0 |
1220 |
1000 |
1.7 |
25 |
700 |
10 |
25 |
600 |
63 |
0.012 |
53 |
604 |
6 |
100 |
68 |
Comparative example |
5 |
A |
6.0 |
1220 |
880 |
3.4 |
25 |
700 |
10 |
25 |
600 |
97 |
0.009 |
38 |
597 |
5 |
97 |
51 |
Comparative example |
6 |
|
6.0 |
1210 |
880 |
1.7 |
15 |
700 |
10 |
15 |
600 |
86 |
0.010 |
45 |
605 |
7 |
95 |
61 |
Comparative example |
7 |
|
6.0 |
1210 |
880 |
1.7 |
25 |
760 |
10 |
25 |
600 |
95 |
0.009 |
39 |
602 |
5 |
85 |
73 |
Comparative example |
8 |
|
6.0 |
1210 |
880 |
1.7 |
25 |
700 |
1 |
25 |
600 |
53 |
0.011 |
49 |
619 |
0 |
89 |
53 |
Comparative example |
9 |
|
6.0 |
1220 |
880 |
1.7 |
25 |
700 |
10 |
25 |
670 |
90 |
0.007 |
31 |
579 |
3 |
57 |
55 |
Comparative example |
10 |
|
6.0 |
1220 |
880 |
1.7 |
25 |
700 |
10 |
25 |
540 |
42 |
0.009 |
40 |
624 |
0 |
72 |
51 |
Comparative example |
11 |
B |
4.5 |
1220 |
880 |
1.4 |
35 |
700 |
10 |
35 |
600 |
84 |
0.025 |
73 |
632 |
100 |
100 |
40 |
Inventive example |
12 |
1.6 |
1220 |
880 |
0.6 |
100 |
700 |
5 |
80 |
600 |
84 |
0.026 |
75 |
621 |
100 |
100 |
39 |
Inventive example |
13 |
4.5 |
1220 |
880 |
1.4 |
35 |
640 |
10 |
35 |
600 |
56 |
0.015 |
44 |
653 |
0 |
93 |
75 |
Comparative example |
14 |
4.5 |
1220 |
880 |
1.4 |
35 |
700 |
30 |
35 |
600 |
85 |
0.010 |
30 |
602 |
3 |
83 |
68 |
Comparative example |
15 |
C |
3.2 |
1230 |
880 |
0.9 |
50 |
700 |
6 |
50 |
600 |
95 |
0.036 |
76 |
665 |
100 |
100 |
33 |
Inventive example |
16 |
D |
6.0 |
1220 |
880 |
1.7 |
25 |
700 |
10 |
25 |
600 |
97 |
0.030 |
67 |
658 |
100 |
100 |
49 |
Inventive example |
17 |
E |
6.0 |
1210 |
880 |
1.7 |
25 |
700 |
10 |
25 |
600 |
86 |
0.016 |
65 |
600 |
100 |
100 |
36 |
Inventive example |
18 |
F |
6.0 |
1230 |
870 |
1.7 |
25 |
700 |
10 |
25 |
600 |
97 |
0.027 |
72 |
617 |
100 |
100 |
43 |
Inventive example |
19 |
G |
4.5 |
1230 |
880 |
1.4 |
35 |
700 |
10 |
35 |
600 |
83 |
0.017 |
72 |
645 |
100 |
100 |
40 |
Inventive example |
20 |
H |
6.0 |
1230 |
890 |
1.7 |
25 |
700 |
10 |
25 |
600 |
90 |
0.013 |
43 |
526 |
4 |
0 |
46 |
Comparative example |
21 |
I |
6.0 |
1230 |
890 |
1.7 |
25 |
700 |
10 |
25 |
600 |
94 |
0.013 |
37 |
504 |
3 |
0 |
62 |
Comparative example |
22 |
J |
6.0 |
1230 |
870 |
1.7 |
25 |
700 |
10 |
25 |
600 |
91 |
0.005 |
37 |
536 |
5 |
0 |
33 |
Comparative example |
23 |
K |
6.0 |
1220 |
870 |
1.7 |
25 |
700 |
10 |
25 |
600 |
59 |
0.059 |
67 |
792 |
6 |
100 |
64 |
Comparative example |
[0058] In the results shown in Table 2, values of the proportion of the volume fraction
of polygonal ferrite, the amount of Ti contained in precipitates each having a size
of less than 20 nm with respect to Ti* expressed as formula (1), and the tensile strength
TS are defined as representative values at a middle portion in the longitudinal and
transverse directions. The proportion of compliant steel microstructures is defined
as the proportion of points where both requirements of the volume fraction of polygonal
ferrite and the proportion of the amount of Ti in the precipitates each having a size
of less than 20 nm are satisfied to 189 measurement points. The proportion of compliant
TS is defined as the proportion of points where TS is 540 MPa or more to 189 measurement
points. ΔTS is a value obtained by determining the standard deviation σ of TS values
at 189 measurement points and multiplying the standard deviation σ by 4.
[0059] As is clear from the investigation results shown in Table 2, in any inventive example,
the steel sheet having satisfactory uniformity in strength is manufactured, in which
the steel sheet has a TS of 540 MPa or more, which is high strength, and the variations
in strength (ΔTS) in the coil in the in-plane direction are 50 MPa or less.
Industrial Applicability
[0060] According to the present invention, it is possible to stably manufacture a hot-rolled
steel sheet having a tensile strength (TS) of 540 MPa or more and only small variations
in strength at low cost, which provides a marked, industrially beneficial effect.
For example, the use of a high-strength hot-rolled steel sheet of the present invention
for automotive parts reduces variations in the amount of springback after formation
using the high-tensile steel sheet and variations in collision characteristics, thus
making it possible to design automobile bodies with higher accuracy and to contribute
sufficiently to the collision safety and weight reduction of automobile bodies.