[Name of Document] DESCRIPTION
[Title of the Invention] HIGH-STRENGTH HOT-ROLLED STEEL SHEET
HAVING EXCELLENT LOCAL DEFORMABILITY AND
MANUFACTURING METHOD THEREOF
[Technical Field]
[0001] The present invention relates to a high-strength hot-rolled steel
sheet having excellent local deformability for bending, stretch flanging,
burring, and the like, and a manufacturing method thereof.
This application is based upon and claims the benefit of priority of the
prior Japanese Patent Application No. 201 1-089250, filed on April 13, 201 1,
the entire contents of which are incorporated herein by reference.
[Background Art]
[0002] In order to abate emission of carbon dioxide gas from automobiles,
a reduction in weight of automobile vehicle bodies has been promoted by
using high-strength steel sheets. Further, in order also to secure the safety of
a passenger, a high-strength steel sheet has been increasingly used for an
automobile vehicle body in addition to a soft steel sheet.
[0003] In order to promote the reduction in weight of automobile vehicle
bodies from now on, a usage strength level of the high-strength steel sheet has
to be increased more than conventionally. In order to use the high-strength
steel sheet for an underbody part, for example, local deformability for burring
has to be improved.
[0004] However, when a steel sheet is increased in strength in general,
formability decreases, and as described in Non-Patent Document 1, uniform
elongation important for drawing and bulging decreases. In contrast to this,
in Non-Patent Document 2, there is disclosed a method of securing uniform
elongation even with the same strength by making a metal structure of a steel
sheet complex.
[0005] Meanwhile, there is also disclosed a metal structure control
method of a steel sheet that improves local deformability typified by bending,
hole expanding, and burring. Non-Patent Document 3 discloses that
controlling inclusions, making a structure uniform, and fbrther decreasing
hardness difference between structures are effective for improving bendability
and hole expandability. This is to improve the hole expandability by making
a structure uniform by structure control.
[0006] In order to attain achievement of strength and ductility,
Non-Patent Document 4 discloses a technique in which metal structure
control (precipitate control and transformation structure control) are
performed by cooling control after hot rolling, thereby obtaining
pro-eutectoid ferrite being a soft phase and bainite in terms of appropriate
fractions.
[0007] Meanwhile, Patent Document 1 discloses a method in which a
finishing temperature of hot rolling, a reduction ratio and a temperature range
of finish rolling are controlled, recrystallization of austenite is promoted,
development of a rolled texture is suppressed, and crystal orientations are
randomized, thereby improving strength, ductility, and hole expandability.
[Prior Art Document]
[Patent Document]
[0008] Patent Document 1: Japanese Laid-open Patent Publication No.
2009-26371 8
[Non-patent Document]
[0009] Non-Patent Document 1 : Kishida, Nippon Steel Technical Report
(1999) No. 371, p. 13
Non-Patent Document 2: 0. Matsumura et al., Trans. ISIJ (1987) vol.
27, p. 570
Non-Patent Document 3: Kato et al., Steelmaking Research (1 984) vol.
312, p. 41
Non-Patent Document 4: K. Sugimoto et al., ISIJ International (2000)
Vol. 40, p. 920
[Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[00 lo] The main factor of deterioration of local deformability is
"non-uniformity" of hardness difference between structures, non-metal
inclusions, a developed rolled texture, and the like. The most effective
factor among them is the "hardness difference between structures" disclosed
in Non-Patent Document 3. Besides, an effective controlling factor is the
"developed rolled texture" disclosed in Patent Document 1.
[0011] These factors are mixed in a complex manner, and the local
deformability of a steel sheet is determined. For maximizing an improved
margin of the local deformability by texture control, structure control is
performed in a combined manner, and it is necessary to eliminate the
"non-uniformity" ascribable to the "hardness difference between structures"
as mush as possible.
[0012] The present invention is to provide a high-strength hot-rolled steel
sheet having excellent local deformability capable of improving local ductility
of the high-strength steel sheet and also capable of improving anisotropy in
the steel sheet by turning a steel structure into a metal structure in which an
area ratio of bainite is 95% or more, together with controlling a texture, and a
manufacturing method thereof.
[Means for Solving the Problems]
[0013] According to the conventional knowledge, the improvements of
hole expandability, bendability, and the like have been performed by
controlling inclusions, making precipitates fine, structure homogenization,
turning structures into a single phase, a decrease in hardness difference
between structures, and the like. However, these are not sufficient, so that
an effect on anisotropy is concerned in a high-strength steel sheet to which Nb,
Ti, and the like are added. This causes problems that other forming elements
are sacrificed, the direction in which a material before forming is taken is
limited, and the like, and the use of the high-strength steel sheet is limited.
[00 141 Thus, the present inventors, in order to improve hole
expandability and bending workability of the high-strength steel sheet,
focused attention on the effect of a texture of the steel sheet and examined and
studied the effect in detail. As a result, it became clear that by controlling
intensities of orientations of a specific crystal orientation group, the local
deformability improves drastically without the elongation and strength
decreasing greatly.
[0015] The point where emphasis should be placed is that the present
inventors found out that the improved margin of the local deformability by the
texture control greatly relays on a steel structure, and the steel structure is
turned into a metal structure in which an area ratio of bainite is 95% or more,
thereby making it possible to maximize the improved margin of the local
deformability on the basis that the strength of the steel is secured.
[0016] Additionally, the present inventors found that in a structure in
which intensities of orientations of a specific crystal orientation group are
controlled, the size of crystal grains greatly affects the local ductility.
Generally, in a structure in which low-temperature generating phases (bainite,
martensite, and the like) are mixed, the definition of crystal grains is
extremely vague and quantification of them is difficult.
[0017] In contrast to this, the present inventors found it possible to solve
the problem of the quantification of crystal grains if a "grain unit" of crystal
grains is determined in the following manner.
[00 1 81 The "grain unit" of crystal grains determined in the present
invention is determined in the following manner in an analysis of orientations
of a steel sheet by an EBSP (Electron Back Scattering Pattern). That is, in
an analysis of orientations of a steel sheet by an EBSP, for example,
orientations are measured at 1500 magnifications with a measured step of 0.5
pm or less, and a position at which a misorientation between adjacent
measured points exceeds 15" is set to a boundary between crystal grains.
Then, a region surrounded with this boundary is determined to be the "grain
unit" of crystal grains.
[0019] With respect to crystal grains of the grain unit determined in this
manner, a circle-equivalent diameter d is obtained and the volume of crystal
grains of each grain unit is obtained by 4/3nd3. Then, a weighted mean of
the volume is calculated and a mean volume diameter (Mean Volume
Diameter) is obtained.
[0020] The present invention is made based on the above-described
knowledge and the gist thereof is as follows.
[002 11
[ll
A high-strength hot-rolled steel sheet having excellent local deformability
contains:
in mass%,
C: not less than 0.07% nor more than 0.20%;
Si: not less than 0.001% nor more than 2.5%;
Mn: not less than 0.01% nor more than 4.0%;
P: not less than 0.001% nor more than 0.15%;
S: not less than 0.0005% nor more than 0.03%;
Al: not less than 0.001% nor more than 2.0%;
N: not less than 0.0005% nor more than 0.01%;
0: not less than 0.0005% nor more than 0.01%; and
a balance being composed of iron and inevitable impurities, in which
an area ratio of bainite in a metal structure is 95% or more,
at a sheet thickness center portion being a range of 518 to 318 in sheet
thickness from the surface of the steel sheet, an average value of pole
densities of the { 100) <0 1 1> to (223 )< 1 10> orientation group represented by
respective crystal orientations of ( 100)<0 1 I>, { 1 1614 lo>, { 1 14}<1 lo>,
{113)<110>, (112)<110>, {335)<110>, and {223)<110> is 4.0 or less, and a
pole density of the {332)<113> crystal orientation is 5.0 or less, and
a mean volume diameter of crystal grains in the metal structure is 10 pm or
less.
P I
The high-strength hot-rolled steel sheet having excellent local deformability
according to [I], in which
to crystal grains of the bainite, a ratio of the crystal grains in which a ratio of
a length dL in a rolling direction to a length dt in a sheet thickness direction:
dL/dt is 3.0 or less is 50% or more.
II31
The high-strength hot-rolled steel sheet having excellent local deformability
according to [I], further contains:
one type or two or more types of
in mass%,
Ti: not less than 0.001% nor more than 0.20%,
Nb: not less than 0.001% nor more than 0.20%,
V: not less than 0.001% nor more than 1.0%, and
W: not less than 0.00 1% nor more than 1.0%.
[41
The high-strength hot-rolled steel sheet having excellent local deformability
according to [I], further contains:
one type or two or more types of
in mass%,
B: not less than 0.0001% nor more than 0.0050%,
Mo: not less than 0.001% nor more than 1.0%,
Cr: not less than 0.001% nor more than 2.0%,
Cu: not less than 0.001% nor more than 2.0%,
Ni: not less than 0.001% nor more than 2.0%,
Co: not less than 0.000 1 % nor more than 1.0%,
Sn: not less than 0.0001% nor more than 0.2%,
Zr: not less than 0.0001% nor more than 0.2%, and
As: not less than 0.000 1 % nor more than 0.50%.
1151
The high-strength hot-rolled steel sheet having excellent local deformability
according to [I], further contains:
one type or two or more types of
in mass%,
Mg: not less than 0.0001% nor more than 0.0 lo%,
REM: not less than 0.0001% nor more than O.l%, and
Ca: not less than 0.000 1% nor more than 0.0 10%.
[GI
A manufacturing method of a high-strength hot-rolled steel sheet having
excellent local deformability, includes:
on a steel billet containing:
in mass%,
C: not less than 0.07% nor more than 0.20%;
Si: not less than 0.001% nor more than 2.5%;
Mn: not less than 0.01% nor more than 4.0%;
P: not less than 0.001% nor more than 0.15%;
S: not less than 0.0005% nor more than 0.03%;
Al: not less than 0.00 1% nor more than 2.0%;
N: not less than 0.0005% nor more than 0.01%;
0: not less than 0.0005% nor more than 0.01%; and
a balance being composed of iron and inevitable impurities,
performing first hot rolling in which rolling at a reduction ratio of 40% or
more is performed one time or more in a temperature range of not lower than
1000°C nor higher than 1200°C;
setting an austenite grain diameter to 200 pm or less by the first hot rolling;
performing second hot rolling in which rolling at 30% or more is performed
in one pass at least one time in a temperature region of not lower than a
temperature T1 + 30°C nor higher than T1 + 200°C determined by Expression
(1) below;
setting the total of reduction ratios in the second hot rolling to 50% or more;
performing final reduction at a reduction ratio of 30% or more in the second
hot rolling and then starting primary cooling in such a manner that a waiting
time t second satisfies Expression (2) below;
setting an average cooling rate in the primary cooling to 50°C/second or more
and performing the primary cooling in a manner that a temperature change is
in a range of not lower than 40°C nor higher than 140°C;
starting secondary cooling after completion of the primary cooling;
performing cooling down to a temperature region of not lower than Ae3 -
50°C nor higher than 700°C at an average cooling rate of lS°C/second or
more in the secondary cooling; and
performing coiling at higher than 350°C to 650°C.
T1 ("C) = 850 + 10 x (C +N) x Mn+ 350 x Nb+ 250 x Ti + 40 x B + 10 x
Cr+ 100 x Mo+ 100xV.-. (I)
t 5 2.5 x tl -.- (2)
Here, t 1 is obtained by Expression (3) below.
tl =O.OOl x ((Tf-Tl) x ~ 1 / 1 0 0-) 0~.1 09 x ((Tf-Tl) x P1/100)+3.1 ..- (3)
Here, in Expression (3) above, Tf represents the temperature of the steel billet
obtained after the final reduction at a reduction ratio of 30% or more, and P1
represents the reduction ratio of the final reduction at 30% or more.
171
The manufacturing method of the high-strength hot-rolled steel sheet having
excellent local deformability according to [6], in which
the total of reduction ratios in a temperature range of lower than TI + 30°C is
30% or less.
[81
The manufacturing method of the high-strength hot-rolled steel sheet having
excellent local deformability according to [6], in which
the waiting time t second hrther satisfies Expression (2a) below.
t < tl ..- (2a)
[91
The manufacturing method of the high-strength hot-rolled steel sheet having
excellent local deformability according to [6], in which
the waiting time t second hrther satisfies Expression (2b) below.
tl 5 t 5 tl x 2.5 (2b)
[lo1
The manufacturing method of the high-strength hot-rolled steel sheet having
excellent local deformability according to [6], in which
the primary cooling is started between rolling stands.
[Effect of the Invention]
[0022] According to the present invention, it is possible to provide a
high-strength hot-rolled steel sheet excellent in local deformability necessary
for bending, stretch flanging, burring, and the like and suitable for
manufacture of automobile parts and the like by controlling a texture and steel
structure of the steel sheet.
[0023]
[FIG. 11 FIG. 1 is a view showing the relationship between an average
value of pole densities of the { 100}<011> to {223)<110> orientation group
and a sheet thickness1 a minimum bend radius;
[FIG. 21 FIG. 2 is a view showing the relationship between a pole
density of the {332)<113> crystal orientation group and the sheet
thicknesslthe minimum bend radius;
[FIG. 31 FIG. 3 is a view showing the relationship between the number
of times of rolling at a reduction ratio of 40% or more in rough rolling and an
austenite grain diameter in the rough rolling;
[FIG. 41 FIG. 4 is a view showing the relationship between a reduction
ratio at T1 + 30 to T1 + 200°C and the average value of the pole densities of
the { 100) <0 1 1 > to (223 )<110> orientation group;
[FIG. 51 FIG. 5 is a view showing the relationship between the
reduction ratio at T1 + 30 to T1 + 200°C and the pole density of the
(332) <113> crystal orientation;
[FIG. 61 FIG. 6 is an explanatory view of a continuous hot rolling line;
[FIG. 71 FIG. 7 is a view showing the relationship between strength
and hole expandability of invention steels and comparative steels; and
[FIG. 81 FIG. 8 is a view showing the relationship between the strength
and bendability of the invention steels and the comparative steels.
[Mode for Carrying out the Invention]
[0024] Hereinafter, the contents of the present invention will be
explained.
[0025] (Crystal orientation)
There will be explained an average value of pole densities of the
{100)<011> to {223)<110> orientation group and a pole density of the
{332)<113> crystal orientation at a sheet thickness center portion being a
range of 518 to 318 in sheet thickness from a surface of a steel sheet.
[0026] In a high-strength hot-rolled steel sheet of the present invention,
(which will be sometimes called a "present invention steel sheet" hereinafter),
an average value of pole densities of the {100)<011> to {223)<110>
orientation group at a sheet thickness center portion being a range of 518 to
318 in sheet thickness from the surface of the steel sheet is a particularly
important characteristic value.
[0027] When X-ray diffraction is performed at the sheet thickness center
portion being the range of 518 to 318 in sheet thickness from the surface of the
steel sheet to obtain intensity ratios of respective orientations to a random
sample, as shown in FIG. 1, it is found that the average value of the pole
densities of the { 100) <0 1 1> to (223 )< 1 10> orientation group is less than 4.0
and a sheet thicknessla bend radius 2 1.5 that is required to work a
framework part is satisfied. Additionally, it is found that when a steel
structure is a metal structure in which an area ratio of bainite is 95% or more,
the sheet thicknesslthe bend radius 2 2.5 is satisfied.
[0028] When hole expandability and small limited bendability are
required, the average value of the pole densities of the {100)<011> to
(2231x1 10> orientation group is desirably less than 3.0.
[0029] When the above-described average value is 4.0 or more,
anisotropy of mechanical properties of the steel sheet becomes strong
extremely, and further local deformability in a specific direction is improved,
but a material in a direction different from the specific direction deteriorates
significantly, resulting in that it becomes impossible to satisfy the sheet
thicknesslthe bend radius 1.5. On the other hand, when the
above-described average value becomes less than 0.5, which is difficult to be
achieved in a current general continuous hot rolling process, the deterioration
of the local deformability is concerned.
[003 01 The {100)<011>, {116)<110>, {114)<110>, {113)<110>,
. {112)<110>, {335)<110>, and {223)<110>orientationsareincludedinthe
{ 100}<011> to (22314 10> orientation group.
[0031] The pole density is synonymous with an X-ray random intensity
ratio. The pole density (X-ray random intensity ratio) is a numerical value
obtained by measuring X-ray intensities of a standard sample not having
accumulation in a specific orientation and a test sample under the same
conditions by X-ray diffractometry or the like and dividing the obtained X-ray
intensity of the test sample by the X-ray intensity of the standard sample.
This pole density can be measured by any one of X-ray diffraction, an EBSP
(Electron Back Scattering Pattern) method, and an ECP (Electron Channeling
Pattern) method.
[0032] As for the pole density of the {100)<011> to {223)<110>
orientation group, for example, pole densities of respective orientations of
{100)<011>, {116)<110>, {114}<110>, {112)<110>, and {223)<110> are
obtained from a three-dimensional texture (ODF) calculated by a series
expansion method using a plurality (preferably three or more) of pole figures
out of pole figures of (1 101, {loo}, (21 11, and (3 10) measured by the
method, and these pole densities are arithmetically averaged, and thereby the
pole density of the above-described orientation group is obtained.
Incidentally, when it is impossible to obtain the intensities of all the
above-described orientations, the arithmetic average of the pole densities of
the respective orientations of { 100)<0 1 1>, { 1 16)<1 lo>, { 1 141-4 lo>,
{112)<110>, and {223)<110> may also be used as a substitute.
[0033] For example, for the pole density of each of the above-described
crystal orientations, each of intensities of (00 1)[1- 101, (1 16)[1- 101,
(1 14)[1-101, (1 13)[1-101, (1 12)[1-101, (335)[1-101, and (223)[1-101 at a $2 =
. 45" cross-section in the three-dimensional texture may be used as it is.
[0034] Due to the similar reason, the pole density of the {332)<113>
crystal orientation of the sheet plane at 518 to 318 in sheet thickness from the
surface of the steel sheet has to be 5.0 or less as shown in FIG 2. As long as
the above-described pole density is 5.0 or less, it is possible to satisfy the
sheet thickness/the bend radius L 1.5 that is required to work a framework
part. The above-described pole density is desirably 3.0 or less.
Additionally, it is found that when the structure of the present invention steel
sheet is a metal structure in which an area ratio of bainite is 95% or more, the
sheet thicknesslthe bend radius L 2.5 is satisfied.
[0035] When the pole density of the {332)<113> crystal orientation is
greater than 5.0, the anisotropy of the mechanical properties of the steel sheet
becomes strong extremely, and further the local deformability in a specific
direction is improved, but a material in a direction different from the specific
direction deteriorates significantly, resulting in that it becomes impossible to
satisfy the sheet thicknesslthe bend radius L 2.5. On the other hand, when
the above-described pole density becomes less than 0.5, which is difficult to
be achieved in a current general continuous hot rolling process, the
deterioration of the local deformability is concerned.
[0036] The reason why the pole densities of the crystal orientations are
important factors for shape freezing property at the time of bending working
is not necessarily obvious, but is inferentially related to slip behavior of
crystal at the time of bending deformation.
[0037] With regard to the sample to be subjected to the X-ray diffraction,
EBSP method, or ECP method, the steel sheet is reduced in thickness to a
predetermined sheet thickness from the surface by mechanical polishing or
the like. Next, strain is removed by chemical polishing, electrolytic
polishing, or the like, and the sample is fabricated in such a manner that in the
range of 5/8 to 318 in sheet thickness, an appropriate plane becomes a
measuring plane. For example, on a steel piece in a size of 30 mmo cut out
from the position of 1/4 W or 314 W of the sheet width W, grinding with fine
finishing (centerline average roughness Ra: 0.4a to 1.6a) is performed. Next,
by chemical polishing or electrolytic polishing, strain is removed, and the
sample to be subjected to the X-ray diffraction is fabricated. With regard to
the sheet width direction, the steel piece is desirably taken from, of the steel
sheet, the position of 114 or 314 from an end portion.
[003 81 As a matter of course, the pole density satisfies the
above-described pole density limited range not only at the sheet thickness
center portion being the range of 518 to 318 in sheet thickness from the surface
of the steel sheet, but also at as many thickness positions as possible, and
thereby local ductility performance (local elongation) is further improved.
However, the range of 518 to 318 from the surface of the steel sheet is
measured, to thereby make it possible to represent the material property of the
entire steel sheet generally. Thus, 518 to 3/8 of the sheet thickness is
prescribed as the measuring range.
[0039] Incidentally, the crystal orientation represented by {hkl)
means that the normal direction of the steel sheet plane is parallel to
and the rolling direction is parallel to . With regard to the crystal
orientation, normally, the orientation vertical to the sheet plane is represented
by [hkl] or {hkl) and the orientation parallel to the rolling direction is
represented by (uvw) or . {hkl) and are generic terms for
equivalent planes, and [hkl] and (uvw) each indicate an individual crystal
plane. That is, in the. present invention, a body-centered cubic structure is.
targeted, and thus, for example, the (1 1 I), (-1 1 I), (1 -1 I), (1 1 -I), (- 1-1 I),
(-1 1 -I), (1 -1 -I), and (-1-1-1) planes are equivalent to make it impossible to
make them different. In such a case, these orientations are generically
referred to as { 1 1 1 ). In an ODF representation, [hkl](uvw) is also used for
representing orientations of other low symmetric crystal structures, and thus it
is general to represent each orientation as [hkl](uvw), but in the present
invention, [hkl](uvw) and {hkl} are synonymous with each other.
The measurement of crystal orientation by an X ray is performed according to
the method described in, for example, Cullity, Elements of X-ray Diffraction,
new edition (published in 1986, translated by MATSUMURA, Gentaro,
published by AGNE Inc.) on pages 274 to 296.
[0040] (Mean volume diameter of crystal grains)
The present inventors earnestly examined texture control of a
hot-rolled steel sheet. As a result, it was found that under the condition that
a texture is controlled as described above, the effect of crystal grains in a
grain unit on the local ductility is extremely large and the crystal grains are
made fine, thereby making it possible to obtain drastic improvement of the
local ductility. Incidentally, as described above, the "grain unit" of the
crystal grains is determined in a manner that the position at which a
misorientation exceeds 15" is set as a boundary of crystal grains in an analysis
of orientations of the steel sheet by the EBSP.
[0041] As above, the reason why the local ductility improves is not
obvious. However, it is conceivably because when the texture of the steel
sheet is randomized and the crystal grains are made fine, local strain
concentration to occur in micron order is suppressed, homogenization of
deformation is increased, and strain is dispersed uniformly in micron order.
[0042] As there are more large crystal grains even though the number of
them is small, the deterioration of the local ductility becomes larger.
Therefore, the size of the crystal grains is not an ordinary mean diameter, and
a mean volume diameter defined as a weighted mean of volume is correlated
with the local ductility. In order to obtain an effect of improving the local
ductility, the mean volume diameter of the crystal grains needs to be 10 pm or
less. It is desirably 7 pm or less in order to secure the hole expandability at
a higher level.
[0043] (Equiaxial property of crystal grains)
As a result of hrther pursuit of the local ductility, the present
inventors found that when equiaxial property of the crystal grains is excellent
on the condition that the above-described texture and the size of the crystal
grains are satisfied, the local ductility improves. As an index indicating the
equiaxial property, a ratio of, of the crystal grains, a length dL in a rolling
direction to a length dt in a sheet thickness direction: dL/dt is employed.
Then, for the improvement of the local ductility, at least 50% or more of the
crystal grains excellent in equiaxial property in which dL/dt is 3.0 or less is
needed to all the bainite crystal grains. When the above-described crystal
grains excellent in equiaxial property are less than 50% to the bainite crystal
grains, the local ductility deteriorates.
[0044] (Chemical composition)
Next, there will be explained reasons for limiting a chemical
composition of the present invention steel sheet. Incidentally, % according
to the chemical composition means mass%.
[0045] C: not less than 0.07% nor more than 0.20%
C is an element increasing strength and-0.07 or more is needed. It is
preferably 0.08% or more. On the other hand, when C exceeds 0.20%,
weldability decreases, and workability deteriorates extremely due to an
increase in a hard structure, and thus the upper limit is set to 0.20%. When
C exceeds 0.10%, formability deteriorates, so that C is preferably 0.10% or
less.
[0046] Si: not less than 0.001% nor more than 2.5%
Si is an element effective for increasing mechanical strength of the
steel sheet, but when Si becomes greater than 2.5%, the workability
deteriorates and a surface flaw occurs, so that the upper limit is set to 2.5%.
When Si is large, a chemical conversion treatment property decreases, so that
it is preferably 1.0% or less. It is difficult to set Si to less than 0.001% in a
practical steel, so that the lower limit is set to 0.001%. It is preferably
0.01 % or more.
[0047] Mn: not less than 0.01% nor more than 4.0%
Mn is also an element effective for increasing the mechanical strength
of the steel sheet, but when Mn becomes greater than 4.0%, the workability
deteriorates, so that the upper limit is set to 4.0%. It is preferably 3.3% or
less. It is difficult to set Mn to less than 0.01% in a practical steel, so that
0.01% is set to the lower limit. It is preferably 0.07% or more.
[0048] When elements such as Ti that suppress occurrence of hot
cracking caused by S are not sufficiently added except Mn, the amount
satisfying MdS 2 20 in mass% is desirably added. Mn is an element that,
with an increase in the content, expands an austenite region temperature to a
low temperature side, improves hardenability, and facilitates formation of a
continuous cooling transformation structure having excellent burring
- .. workability. This effect is not easily exhibited when Mn is less than 1%, so
that 1% or more is desirably added.
[0049] P: not less than 0.00 1 % nor more than 0.15%
P is an impurity element and prevents deterioration of the workability
and cracking at the time of hot rolling or cold rolling, so that the upper limit is
set to 0.15%. It is preferably 0.10% or less, and is more preferably 0.05% or
less. It is difficult to decrease P to less than 0.001% in current general
refining (including secondary refining), so that the lower limit is set to
0.00 1 %.
[0050] S: not less than 0.0005% nor more than 0.03%
S is an impurity element and prevents deterioration of the workability
and cracking at the time of hot rolling or cold rolling, so that the upper limit is
set to 0.03%. It is preferably 0.01%, and is more preferably 0.005% or less.
It is difficult to decrease S to less than 0.0005% in current general refining
(including secondary refining), so that the lower limit is set to 0.0005%.
[0051] Al: not less than 0.001% nor more than 2.0%
For deoxidation, 0.001% or more of A1 is added. Further, A1
significantly increases a y to a transformation point, to thus be an effective
element when hot rolling at an Ar3 point or lower is directed in particular.
However, when it is too much, the weldability deteriorates, so that the upper
limit is set to 2.0%.
[0052] The Ar3 point is a temperature at which ferrite starts to precipitate
when alloy in an austenite single phase region is cooled. In the present
invention, the phrase of Ar3 point or higher is used in order to emphasize that
the structure is in an austenite single phase state.
[0053] When Si and A1 are contained excessively, cementite precipitation
during an overaging treatment is suppressed and the fraction of retained
austenite is likely to become too large, so that the total added amount of Si
and A1 is preferably less than 1 %.
[0054] N: not less than 0.0005% nor more than 0.01%
N is an impurity element and is set to 0.01% or less so as not to impair
the workability. It is preferably 0.005% or less. It is difficult to decrease N
to less than 0.0005% in current general refining (including secondary
refining), so that the lower limit is set to 0.0005%.
[0055] 0: not less than 0.0005% nor more than 0.01%
Similarly to N, 0 is an impurity element and is set to 0.01% or less so
as not to impair the workability. It is preferably 0.005% or less. It is
difficult to decrease 0 to less than 0.0005% in current general refining
(including secondary refining), so that the lower limit is set to 0.0005%.
[0056] In the present invention steel sheet, it is also possible that one type
or two or more types of Ti, Nb, V, and W islare added, to thereby generate
fine carbonitride, and strength improvement is achieved by precipitation
strengthening.
[0057] Ti: not less than 0.001% nor more than 0.20%
Nb: not less than 0.00 1 % nor more than 0.20%
V: not less than 0.00 1 % nor more than 1 .O%
W: not less than 0.001% nor more than 1 .O%
In order to obtain an effect of improving the strength by the
precipitation strengthening in a manner to add one type or two or more types
of Ti, Nb, V, and W, it is necessary to add 0.001% or more of each of Ti, Nb,
V, and W. Ti, Nb, V, and W are each preferably 0.01% or more. However,
even when they are added excessively, the effect of increasing the strength is
only saturated, so that the upper limits of Ti and Nb are each set to. 0.20%,.and
the upper limits of V and W are each set to 1.0%. Ti and Nn are each
preferably not less than 0.01% nor more than 0.1%, and V and W are each
preferably not less than 0.01% nor more than 0.6%.
[0058] In the present invention steel sheet, in order to secure the strength
by increasing the hardenability of the structure to perform second phase
control, one type or two or more types of B, Mo, Cr, Cu, Ni, Co, Sn, Zr, and
As may also be added.
[0059] B: not less than 0.0001% nor more than 0.0050%
Mo: not less than 0.001% nor more than 1 .O%
Cr, Cu, Ni: not less than 0.001% nor more than 2.0%
Co: not less than 0.0001% nor more than 1 .O%
Sn, Zr: not less than 0.000 1% nor more than 0.2%
As: not less than 0.000 1% nor more than 0.50%
In order to obtain the effect of improving the strength by the second
phase control, it is necessary to add 0.0001% or more of B, 0.001% or more
of each of Mo, Cr, Ni, and Cu, and 0.0001% or more of each of Co, Sn, Zr,
and As. B is preferably 0.001% or more, Mo, Cr, Ni, and Cu are each
preferably 0.005% or more, and Co, Sn, Zr, and As are each preferably
0.001% or more.
[0060] However, when they are added excessively, the workability is
deteriorated, so that the upper limit of B is set to 0.0050%, the upper limit of
Mo is set to 1.0%, the upper limit of each of Cr, Cu, and Ni is set to 2.0%, the
upper limit of Co is set to 1.0%, the upper limit of each of Sn and Zr is set to
0.2%, and the upper limit of As is set to 0.50%.
[0061] In the present invention steel sheet, in order to improve local
formability, one type or two or more types of Mg, REM, and Ca may also.be
further added.
[0062] Mg: not less than 0.000 1 % nor more than 0.01 0%
REM: not less than 0.0001 % nor more than 0.1%
Ca: not less than 0.000 1 % nor more than 0.0 10%
Mg, REM, and Ca are important elements to be added for making
inclusions harmless. In order to obtain an effect of making inclusions
harmless, 0.0001% or more of each of Mg, REM, and Ca is added.
[0063] Mg, REM, and Ca are each preferably 0.001% or more. On the
other hand, when they are added excessively, cleanliness of the steel is
deteriorated, so that Mg is set to 0.010% or less, REM is set to 0.1% or less,
and Ca is set to 0.010% or less.
[0064] (Metal structure)
Next, there will be explained a metal structure of the present invention
steel sheet.
[0065] The structure of the present invention steel sheet is a metal
structure in which an area ratio of bainite is 95% or more, and is preferably a
bainite single phase structure. The steel structure is turned into the metal
structure in which an area ratio of bainite is 95% or more (including a bainite
single phase), thereby making it possible to achieve the strength and the hole
expandability.
[0066] Further, above-described structure is generated by transformation
at relatively high temperature, to thus have no necessity to be cooled down to
low temperature when being manufactured, and is a preferred structure also in
terms of material stability and productivity.
[0067] As the balance, 5% or less of pro-eutectoid ferrite, pearlite,
martensite, and retained austenite is allowed. Pro-eutectoid ferrite has no
problem as long as it is precipitation-strengthened sufficiently, but
pro-eutectoid ferrite sometimes becomes soft depending on the chemical
composition, and further when the area ratio becomes greater than 5%, the
hole expandability slightly decreases due to hardness difference from bainite.
[0068] When an area ratio of pearlite becomes greater than 5%, the
strength andlor the workability sometimes deteriorateldeteriorates. When an
area ratio of martensite becomes greater than 1% or an area ratio of retained
austenite to be martensite by strain-induced transformation becomes greater
than 5%, an interface between bainite and a structure harder than bainite
becomes a starting point of cracking and the hole expandability deteriorates.
As long as the area ratio of bainite is set to 95% or more, the area ratio of
pro-eutectoid ferrite, pearlite, martensite, and retained austenite being the
balance becomes 5% or less, so that the balance of the strength and the hole
expandability can be well maintained. However, the area ratio of martensite
needs to be set to less than 1%.
[0069] Bainite in the present invention steel sheet is a microstructure
defined as a continuous cooling transformation structure (Zw) positioned at
an intermediate stage between a microstructure containing polygonal ferrite
and pearlite to be generated by a diffusive mechanism and martensite to be
generated by a non-diffusive shearing mechanism, as is described in The Iron
and Steel Institute of Japan, Society of basic research, Bainite Research
CornmitteelEdition; Recent Research on Bainitic Microstructures and
Transformation Behavior of Low Carbon Steels - Final Report of Bainite
Research Committee (in 1994, The Iron and Steel Institute of Japan).
[0070] That is, the continuous cooling transformation structure (Zw) is
defined as a microstructure mainly composed of Bainitic ferrite (aoB),
Granular bainitic ferrite (a*), and Quasi-polygonal ferrite (a,), and further
containing a small amount of retained austenite (yr) and Martensite-austenite
(MA) as is described in the above-described reference literature on pages 125
to 127 as an optical microscopic observation structure.
[007 11 Incidentally, similarly to polygonal ferrite (PF), an internal
structure of a, does not appear by etching, but a shape of a, is acicular, and it
is definitely distinguished from PF. Here, on the condition that of a targeted
crystal grain, a peripheral length is set to lq and a circle-equivalent diameter is
set to dq, a grain having a ratio (lqldq) of them satisfying lqldq 2 3.5 is a,.
[0072] The continuous cooling transformation structure (Zw) of the
present invention steel sheet is defined as a microstructure containing one
type or two or more types of aoBa,s , a,, y, and MA. Incidentally, the total
content of y, and MA being small in amount is set to 3% or less.
[0073] There is sometimes a case that the continuous cooling
transformation structure (Zw) is not easily discerned even when it is etched
using a nital reagent to be observed by an optical microscope. In such a case,
it is discerned by using the E B S P - 0 1 ~ ~T~h.e E B S P - 0 1 ~(E~le~ctr on
Back Scatter Difiaction Pattern-Orientation Image Microscopy) is
constituted by a device and software in which a highly inclined sample in a
scanning electron microscope SEM (Scanning Electron Microscope) is
irradiated with electron beams, a Kikuchi pattern formed by backscattering is
photographed by a high-sensitive camera and is image processed by a
computer, and thereby a crystal orientation at an irradiation point is measured
for a short time period.
[0074] In the EBSP method, it is possible to quantitatively analyze a
microstructure and a crystal orientation of a-bulk sample surface. As long as
an area to be analyzed is within an area capable of being observed by the
SEM, it is possible to analyze the area with a minimum resolution of 20 nm,
depending on the resolution of the SEM. The analysis by the EBSP-OIMTM
is performed by mapping an area to be analyzed to tens of thousands of
equally-spaced grid points.
[0075] It is possible to see crystal orientation distributions and sizes of
crystal grains within the sample in a polycrystalline material. In the present
invention, one discernible from a mapped image with a misorientation
between packets defined as 15O may also be defined as the continuous cooling
transformation structure (Zw) for convenience.
[0076] The structural fraction of pro-eutectoid ferrite was obtained by a
Kernel Average Misorientation (KAM) method being equipped with the
E B S P - 0 1 ~ ~ ~T.h e KAM method is that a calculation, in which
misorientations among pixels of adjacent six pixels (first approximations) of a
certain regular hexagon of measurement data, or 12 pixels (second
approximations) positioned outside the six pixels, or 18 pixels (third
approximations) positioned further outside the 12 pixels are averaged and an
obtained value is set to a value of the center pixel, is performed with respect
to each pixel.
[0077] The above-described calculation is performed so as not to exceed
a grain boundary, thereby making it possible to create a map representing an
orientation change within a grain. That is, the created map represents a
distribution of strain based on a local orientation change within a grain.
Note that the analysis condition in the present invention is set to the third
approximation of which in the EBSP-OIMTM, the misorientation among
adjacent pixels is calculated, and one having-. this misorientation being 5" or
less is displayed.
[0078] In the present invention steel sheet, pro-eutectoid ferrite is defined
as a microstructure up to a planar fi-action of pixels of which the
misorientation among adjacent pixels is calculated to be lo or less in the third
approximation. Polygonal pro-eutectoid ferrite transformed at high
temperature is generated in a diffision transformation, and thus a dislocation
density is small and strain within the grain is small, and thus, a difference
within the grain in the crystal orientation is small.
[0079] Then, according to the results of various examinations that have
been performed so far by the present inventors, it was possible to confirm that
a volume fraction of polygonal ferrite obtained by observation of optical
microscope and an area fraction of an area obtained by lo of the third
approximation of the misorientation measured by the KAM method
substantially agree with each other. Therefore, pro-eutectoid ferrite in the
present invention steel sheet is defined as described above.
[0080] (Manufacturing method)
Next, there will be explained a manufacturing method of the present
invention steel sheet. In order to achieve excellent local deformability, it is
important to form a texture having required pole densities and to manufacture
a steel sheet satisf4ring conditions according to making crystal grains fine and
equiaxial property and homogenization of crystal grains. Details of
manufacturing conditions for satisfying these conditions at the same time will
be explained below.
[0081] A manufacturing method prior to hot rolling is not limited in
particular. Subsequent to melting by a shaft furnace, an electric furnace, or
the like, secondary refining may be variously performed, and next casting
may be performed by normal continuous casting, or casting by an ingot
method, or fbrther a casting method such as thin slab casting. In the case of
continuous casting, it is possible that a cast slab is once cooled down to low
temperature and thereafter is reheated to then be subjected to hot rolling, or it
is also possible that a cast slab is subjected to hot rolling continuously. A
scrap may also be used for a raw material.
[0082] The slab obtained by the above-described manufacturing method
is heated in a slab heating process prior to a hot rolling process, but in the
manufacturing method of the present invention, a heating temperature is not
determined in particular. However, when the heating temperature is higher
than 1260°C, a yield decreases due to scale off, and thus the heating
temperature is preferably 1260°C or lower. On the other hand, when the
heating temperature is lower than 11 50°C, operational efficiency deteriorates
significantly in terms of a schedule, and thus the heating temperature is
desirably 1 1 5 0°C or higher.
[0083] Further, a heating time in the slab heating process is not
determined in particular, but in terms of avoiding central segregation and the
like, after the temperature reaches a required heating temperature, the heating
temperature is desirably maintained for 30 minutes or longer. However,
when the cast slab after being subjected to casting is directly transferred as it
is in a high-temperature cast slab state to be rolled, the heating time is not
limited to this.
[0084] (First hot rolling)
After the slab heating process, the slab extracted f?om a heating
hrnace is subjected to a rough rolling process being first hot rolling to be
rough rolled without a wait, and thereby a rough bar is obtained.. In the
high-strength steel sheet having excellent local deformability of the present
invention, an austenite grain diameter after the rough rolling, namely before
finish rolling is important. The austenite grain diameter before the finish
rolling is desirably small, and the austenite grain diameter of 200 pm or less
greatly contributes to making crystal grains fine and homogenization of a
main phase.
[0085] In order to obtain the austenite grain diameter of 200 pm or less
before the finish rolling, as shown in FIG. 3, in rough rolling in a temperature
region of not lower than 1000°C nor higher than 1200°C, it is necessary to
perform rolling at least one time or more at a reduction ratio of 40% or more.
[0086] As the reduction ratio is larger and the number of times of
reduction at a large reduction ratio is larger, fine grains can be obtained. The
austenite grain diameter is desirably set to 100 pm or less, and in order to
achieve it, rolling at 40% or more is desirably performed two times or more.
However, when in the rough rolling, the reduction is greater than 70% and
rolling is performed greater than 10 times, there is a concern that the
temperature decreases or a scale is generated excessively.
[0087] In this manner, the decrease in the austenite grain diameter before
the finish rolling is effective for the improvement of the local deformability
through control of recrystallization promotion of austenite in the finish rolling
later, making crystal grains fine, and making crystal grains equiaxial in a final
structure.
[0088] It is supposed that this is because an austenite grain boundary after
the rough rolling (namely before the finish rolling) functions as one of
recrystallization nuclei during the finish rolling. The confirmation of the
austenite grain diameter after the rough rolling is performed in a manner that
a steel sheet piece before being subjected to the finish rolling is quenched as
much as possible, and is cooled at a cooling rate of 10°C/second or more, for
example, and a cross section of the steel sheet piece is etched to make
austenite grain boundaries appear, and the austenite grain boundaries are
observed by an optical microscope. On this occasion, at 50 or more
magnifications, 20 visual fields or more are observed and confirmed by image
analysis or a point counting method.
[0089] (Second hot rolling)
After the rough rolling process (first hot rolling) is completed, a finish
rolling process being second hot rolling is started. The time between the
completion of the rough rolling process and the start of the finish rolling
process is desirably set to 150 seconds or shorter.
[0090] In the finish rolling process (second hot rolling), a finish rolling
start temperature is desirably set to 1000°C or higher. When the finish
rolling start temperature is lower than 1000°C, at each finish rolling pass, the
temperature of the rolling to be applied to the rough bar to be rolled is
decreased, the reduction is performed in a non-recrystallization temperature
region, the texture develops, and thus the isotropy deteriorates.
[0091] Incidentally, the upper limit of the finish rolling start temperature
is not limited in particular. However, when it is 1150°C or higher, a blister
to be the starting point of a scaly spindle-shaped scale defect is likely to occur
between a steel sheet base iron and a surface scale before the finish rolling
and between passes, and thus the finish rolling start temperature is desirably
lower than 1 150°C.
COO921 In the finish rolling, a temperature determined by the chemical
composition of the steel sheet is set to T1, and in a temperature region-of not .
lower than TI + 30°C nor higher than TI + 200°C, the rolling at 30% or more
is performed in one pass at least one time. Further, in the finish rolling, the
total of the reduction ratios is set to 50% or more. By satisfying this
condition, at the sheet thickness center portion being the range of 518 to 318 in
sheet thickness from the surface of the steel sheet, the average value of the
pole densities of the { 100) <0 1 1> to (223 ) < 1 10> orientation group becomes
less than 4.0 and the pole density of the {332)<113> crystal orientation
becomes 5.0 or less.
[0093] Here, T1 is the temperature calculated by Expression (1) below.
T1 ("C)=850+10x(C+N)xMn+350xNb+250xTi+40xB+
10 x Cr+ 100 x Mo+ 100 x V -.. (1)
C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the
element (mass%).
[0094] FIG. 4 and FIG. 5 each show the relationship between each
reduction ratio in the temperature region and each pole density of the
orientation. As shown in FIG. 4 and FIG. 5, heavy reduction in the
temperature region of not lower than T1 + 30°C nor higher than T1 + 200°C
and light reduction at TI or higher and lower than T1 + 30°C thereafter
control the average value of the pole densities of the {100)<011> to
(2231-4 10> orientation group and the pole density of the {332)<113> crystal
orientation at the sheet thickness center portion being the range of 518 to 318
in sheet thickness from the surface of the steel sheet, and thereby the local
deformability of the final product is improved drastically, as shown also in
Tables 2 and 3 (see paragraphs in Example).
[0095] T1 itself is obtained empirically. The present inventors learned
empirically that .the recrystallization in an austenite region of the steel is
promoted on the basis of TI. In order to obtain better local deformability, it
is important to accumulate strain by the heavy reduction, and the total of the
reduction ratios of 50% or more is essential.
[0096] When the total reduction ratio in the temperature region of not
lower than T1 + 30°C nor higher than T1 + 200°C is less than 50%, rolling
strain to be accumulated during the hot rolling is not sufficient and the
recrystallization of austenite does not advance sufficiently. Therefore, the
texture develops and the isotropy deteriorates. When the total reduction
ratio is 70% or more, the sufficient isotropy can be obtained even though
variations ascribable to temperature fluctuation or the like are considered.
On the other hand, when the total reduction ratio exceeds 90%, it becomes
difficult to obtain the temperature region of T1 + 200°C or lower due to heat
generation by working, and further a rolling load increases to cause a risk that
the rolling becomes difficult to be performed.
[0097] In the finish rolling, in order to promote the uniform
recrystallization caused by releasing the accumulated strain, the rolling at
30% or more is performed in one pass at least one time at not lower than T1 +
30°C nor higher than T1 + 200°C.
[0098] Incidentally, in order to promote the uniform recrystallization, it is
necessary to suppress a working amount in a temperature region of lower than
T1 + 30°C as small as possible. In order to achieve it, the reduction ratio at
lower than T1 + 30°C is desirably 30% or less. In terms of sheet thickness
accuracy and sheet shape, the reduction ratio of 10% or less is desirable.
When the isotropy is further obtained, the reduction ratio in the temperature
region of lower than Tl + 30°C is desirably 0%.
[0099] The finish rolling is desirably finished at TI + 30°C or higher. In
the hot rolling at lower than T1 + 30°C, the granulated austenite grains that
are recrystallized once are elongated, thereby causing a risk that the isotropy
deteriorates.
[0100] That is, in the manufacturing method of the present invention, in
the finish rolling, by recrystallizing austenite uniformly and. finely, the texture
of the product is controlled and the local deformability such as the hole
expandability and the bendability is improved.
[0 10 11 A rolling ratio can be obtained by actual performances or
calculation fiom the rolling load, sheet thickness measurement, orland the like.
The temperature can be actually measured by a thermometer between stands,
or can be obtained by calculation simulation considering the heat generation
by working fiom a line speed, the reduction ratio, orland like. Thereby, it is
possible to easily confirm whether or not the rolling prescribed in the present
invention is performed.
[0102] When the hot rolling is finished at Ar3 or lower, the hot rolling
becomes two-phase region rolling of austenite and ferrite, and accumulation
to the { 100)<011> to (223 ) orientation group becomes strong. As a
result, the local deformability deteriorates significantly.
[0103] In order to make the crystal grains fine and suppress elongated
grains, a maximum working heat generation amount at the time of the
reduction at not lower than T1 + 30°C nor higher than TI + 200°C, namely a
temperature increased margin by the reduction is desirably suppressed to
18°C or less. For achieving this, inter-stand cooling or the like is desirably
applied.
[0 1041 (Primary cooling)
After the final reduction at a reduction ratio of 30% or more is
performed in the finish rolling, primary cooling is started in such a manner
that a waiting time t second satisfies Expression (2) below.
t 5 2.5 x tl ..- (2)
Here, tl is obtained by Expression (3) below.
tl = 0.001 x ((Tf - Tl) x ~11100-) 0~.1 09 x ((Tf - Tl) x P11100) + 3.1 -.-( 3)
Here, in Expression (3) above, Tf represents the temperature of a steel billet
obtained after the final reduction at a reduction ratio of 30% or more, and P1
represents the reduction ratio of the final reduction at 30% or more.
[0105] Incidentally, the "final reduction at a reduction ratio of 30% or
more" indicates the rolling performed finally among the rollings whose
reduction ratio becomes 30% or more out of the rollings in a plurality of
passes performed in the finish rolling. For example, when among the
rollings in a plurality of passes performed in the finish rolling, the reduction
ratio of the rolling performed at the final stage is 30% or more, the rolling
performed at the final stage is the "final reduction at a reduction ratio of 30%
or more." Further, when among the rollings in a plurality of passes
performed in the finish rolling, the reduction ratio of the rolling performed
prior to the final stage is 30% or more and after the rolling performed prior to
the final stage (rolling at a reduction ratio of 30% or more) is performed, the
rolling whose reduction ratio becomes 30% or more is not performed, the
rolling performed prior to the final stage (rolling at a reduction ratio of 30%
or more) is the "final reduction at a reduction ratio of 30% or more."
[0106] In the finish rolling, the waiting time t second until the primary
cooling is started after the final reduction at a reduction ratio of 30% or more
is performed greatly affects the austenite grain diameter. That is, it greatly
affects an equiaxed grain fraction and a coarse -grain area ratio of the steel
sheet.
[O107] When the waiting time t exceeds tl x 2.5, the recrystallization is
already almost completed, but the crystal grains grow significantly and grain
coarsening advances, and thereby an r value and the elongation are decreased.
[0108] The waiting time t second further satisfies Expression (2a) below,
thereby making it possible to preferentially suppress the growth of the crystal
grains. Consequently, even though the recrystallization does not advance
sufficiently, it is possible to sufficiently improve the elongation of the steel
sheet and to improve fatigue property simultaneously.
t < tl ..- (2a)
[0 1 091 At the same time, the waiting time t second further satisfies
Expression (2b) below, and thereby the recrystallization advances sufficiently
and the crystal orientations are randomized. Therefore, it is possible to
sufficiently improve the elongation of the steel sheet and to greatly improve
the isotropy simultaneously.
tl S t 5 tl x 2.5 ... (2b)
[0110] Here, as shown in FIG. 6, on a continuous hot rolling line 1, the
steel billet (slab) heated to a predetermined temperature in the heating furnace
is rolled in a roughing mill 2 and in a finishing mill 3 sequentially to be a
hot-rolled steel sheet 4 having a predetermined thickness, and the hot-rolled
steel sheet 4 is carried out onto a run-out-table 5. In the manufacturing
method of the present invention, in the rough rolling process (first hot rolling)
performed in the roughing mill 2, the rolling at a reduction ratio of 20% or
more is performed on the steel billet (slab) one time or more in the
temperature range of not lower than 1000°C nor higher than 1200°C.
.. [0111] The rough bar rolled to a predetermined thickness in the roughing
mill 2 in this manner is next finish rolled (is subjected to the second hot
rolling) through a plurality of rolling stands 6 of the finishing mill 3 to be the
hot-rolled steel sheet 4. Then, in the finishing mill 3, the rolling at 30% or
more is performed in one pass at least one time in the temperature region of
not lower than the temperature T1 + 30°C nor higher than T1 + 200°C.
Further, in the finishing mill 3, the total of the reduction ratios becomes 50%
or more.
[O 1 121 Further, in the finish rolling process, after the final reduction at a
reduction ratio of 30% or more is performed, the primary cooling is started in
such a manner that the waiting time t second satisfies Expression (2) above or
either Expression (2a) or (2b) above. The start of this primary cooling is
performed by inter-stand cooling nozzles 10 disposed between the respective
two of the rolling stands 6 of the finishing mill 3, or cooling nozzles 11
disposed in the run-out-table 5.
[0113] For example, when the final reduction at a reduction ratio of 30%
or more is performed only at the rolling stand 6 disposed at the front stage of
the finishing mill 3 (on the left side in FIG. 6, on the upstream side of the
rolling) and the rolling whose reduction ratio becomes 30% or more is not
performed at the rolling stand 6 disposed at the rear stage of the finishing mill
3 (on the right side in FIG. 6, on the downstream side of the rolling), if the
start of the primary cooling is performed by the cooling nozzles 11 disposed
in the run-out-table 5, a case that the waiting time t second does not satis@
Expression (2) above or Expressions (2a) and (2b) above is sometimes caused.
In such a case, the primary cooling is started by the inter-stand cooling
nozzles 10 disposed between the respective two of the rolling stands 6 of the
finishing mill 3.
[0114] Further, for example, when the final reduction at a reduction ratio
of 30% or more is performed at the rolling stand 6 disposed at the rear stage
of the finishing mill 3 (on the right side in FIG. 6, on the downstream side of
the rolling), even though the start of the primary cooling is performed by the
cooling nozzles 11 disposed in the run-out-table 5, there is sometimes a case
that the waiting time t second can satisfy Expression (2) above or Expressions
(2a) and (2b) above. In such a case, the primary cooling may also be started
by the cooling nozzles 11 disposed in the run-out-table 5. Needless to say,
as long as the performance of the final reduction at a reduction ratio of 30%
or more is completed, the primary cooling may also be started by the
inter-stand cooling nozzles 10 disposed between the respective two of the
rolling stands 6 of the finishing mill 3.
[0115] Then, in this primary cooling, the cooling that at an average
cooling rate of 50°C/second or more, a temperature change (temperature
drop) becomes not less than 40°C nor more than 140°C is performed.
[O 1 161 When the temperature change is less than 40°C, the recrystallized
austenite grains grow and low-temperature toughness deteriorates. The
temperature change is set to 40°C or more, thereby making it possible to
suppress coarsening of the austenite grains. When the temperature change is
less than 40°C, the effect cannot be obtained. On the other hand, when the
temperature change exceeds 1 40°C, the recrystallization becomes insufficient
to make it difficult to obtain a targeted random texture. Further, a ferrite
phase effective for the elongation is also not obtained easily and the hardness
of a ferrite phase becomes high, and thereby the elongation and the local
deformability also deteriorate. Further, when the temperature change is
greater than- 140°C, an overshoot to/beyond an Ar3 transformation point
temperature is likely to be caused. In the case, even by the transformation
fiom recrystallized austenite, as a result of sharpening of variant selection, the
texture is formed and the isotropy decreases consequently.
[O 1 171 When the average cooling rate in the primary cooling is less than
50°C/second, as expected, the recrystallized austenite grains grow and the
low-temperature toughness deteriorates. The upper limit of the average
cooling rate is not determined in particular, but in terms of the steel sheet
shape, 200°C/second or less is considered to be proper.
[0118] Further, in order to suppress the grain growth and obtain more
excellent low-temperature toughness, a cooling device between passes or the
like is desirably used to bring the heat generation by working between the
respective stands of the finish rolling to 18°C or lower.
[0119] The rolling ratio (reduction ratio) can be obtained by actual
performances or calculation fiom the rolling load, sheet thickness
measurement, orland the like. The temperature of the steel billet during the
rolling can be actually measured by a thermometer being disposed between
the stands, or can be obtained by simulation by considering the heat
generation by working from a line speed, the reduction ratio, orland like, or
can be obtained by the both methods.
[0120] Further, as has been explained previously, in order to promote the
uniform recrystallization, the working amount in the temperature region of
lower than TI + 30°C is desirably as small as possible and the reduction ratio
in the temperature region of lower than TI + 30°C is desirably 30% or less.
For example, in the event that in the finishing mill 3 on the continuous hot
rolling line 1 shown in FIG. 6, in passing through one or two or more of the
rolling stands 6 disposed on the front stage side (on the left side in FIG. 6, on
the upstream side of the rolling), the steel sheet is in the temperature region of
not lower than TI + 30°C nor higher than T1 + 200°C, and in passing through
one or two or more of the rolling stands 6 disposed on the subsequent rear
stage side (on the right side in FIG. 6, on the downstream side of the rolling),
the steel sheet is in the temperature region of lower than TI + 30°C, when the
steel sheet passes through one or two or more of the rolling stands 6 disposed
on the subsequent rear stage side (on the right side in FIG. 4, on the
downstream side of the rolling), even though the reduction is not performed
or is performed, the reduction ratio at lower than T1 + 30°C is desirably 30%
or less in total. In terms of the sheet thickness accuracy and the sheet shape,
the reduction ratio at lower than T1 + 30°C is desirably a reduction ratio of
10% or less in total. When the isotropy is further obtained, the reduction
ratio in the temperature region of lower than TI + 30°C is desirably 0%.
[O 12 11 In the manufacturing method of the present invention, a rolling
speed is not limited in particular. However, when the rolling speed on the
final stand side of the finish rolling is less than 400 mpm, y grains grow to be
coarse, regions in which ferrite can precipitate for obtaining the ductility are
decreased, and thus the ductility is likely to deteriorate. Even though the
upper limit of the rolling speed is not limited in particular, the effect of the
present invention can be obtained, but it is actual that the rolling speed is
1800 mpm or less due to facility restriction. Therefore, in the finish rolling
process, the rolling speed is desirably not less than 400 mpm nor more than
1800 mpm.
[O 1221 (Secondary cooling)
In the present invention steel sheet, cooling control after the
above-described primary cooling also. becomes important in order to form a
required steel structure. In order to suppress ferrite transformation and turn
the metal structure into 95% or more of bainite in an area ratio, a cooling rate
in a temperature region of not lower than Ae3 - 50°C nor higher than 700°C,
being a temperature region near the nose of the ferrite transformation, is
important.
[0123] When the cooling rate in this temperature region is slow, there is
sometimes a case that the area ratio of pro-eutectoid ferrite exceeds 5%, so
that it is necessary to set an average cooling rate to 15"C/second or more. In
order to securely suppress the area ratio of pro-eutectoid ferrite to 5% or less,
the average cooling rate is preferably 20°C/second or more, and is more
preferably 30°C/second or more.
[0 1241 Ae3 ["C] can be calculated by Expression (4) below by the
contents of C, Mn, Si, Cu, Ni, Cr, and Mo [mass%]. The calculation is
performed with the element that is not contained set as 0%.
Ae3 = 91 1 - 239C - 36Mn + 40Si - 28Cu - 20Ni - 12Cr + 63Mo ... (4)
[O 1251 (Coiling)
In the present invention, a coiling temperature is also important and
needs to be set to higher than 350°C to 650°C. When the coiling
temperature exceeds 650°C, the area ratio of ferrite structure increases,
thereby making it impossible to bring the area ratio of bainite to 95% or more.
In order to securely bring the area ratio of bainite to 95% or more, the coiling
temperature is referably set to 600°P C or lower.
[0 1261 When the coiling temperature is 350°C or lower, martensite
increases and the hole expandability deteriorates, so that the lower limit of the
coiling temperature is set to higher than 350°C. In order to securely
suppress generation of martensite, the coiling temperature is preferably 400°C
or higher.
[0127] In the hot rolling, it is also possible that sheet bars are bonded
after the rough rolling to be subjected to the finish rolling continuously. On
this occasion, the rough bars may also be coiled into a coil shape once, stored
in a cover having a heat insulating function according to need, and uncoiled
again to be joined. On the hot-rolled steel sheet, skin pass rolling may also
be performed according to need. The skin pass rolling has an effect of
preventing stretcher strain to occur at the time of working and forming and
has an effect of correcting the shape.
[0128] The present invention steel sheet can be applied not only to
bending working but also to combined forming mainly composed of bending
working such as bending, bulging, and drawing. Even when a surface
treatment is performed on the present invention steel sheet, the effect of
improving the local deformability does not disappear, so that even when
electroplating, hot dipping, deposition plating, organic coating film forming,
film laminating, organic saltslinorganic salts treatment, non-chromium
treatment, or the like is performed, the effect of the present invention can be
obtained.
Example
[0 1 291 Next, examples of the present invention will be explained.
Incidentally, conditions of the examples are condition examples employed for
confirming the applicability and effects of the present invention, and the
present invention is not limited to these condition examples. The present
invention can employ various conditions as long as the object of the present
invention is achieved without departing from the spirit of the invention.
- - Chemical compositions of respective steels used in, examples are shown in
Table 1. Respective manufacturing conditions are shown in Table 2 and
Table 3. Further, structural constitutions and mechanical properties of
respective steel types under the manufacturing conditions in Table 2 are
shown in Table 4. Structural constitutions and mechanical properties of
respective steel types under the manufacturing conditions in Table 3 are
shown in Table 5. Incidentally, each underline in Tables indicates that a
numeral value is outside the range of the present invention or is outside the
range of a preferred range of the present invention.
[O 1301 There will be explained results of examinations using invention
steels A to T having the chemical compositions shown in Table 1 and
similarly using comparative steels a to h. Incidentally, in Table 1, each
numerical value of the chemical compositions means mass%.
[013 11 These steels were cast and then as they were, or were reheated
after once being cooled down to room temperature and were heated to a
temperature region of 1000°C to 1300°C, and then were subjected to hot
rolling under the conditions shown in Table 2 and Table 3, and hot-rolled steel
sheets each having a thickness of 2 to 5 mm were obtained, and next were
cooled on a run-out-table, coiled, pickled, and were subjected to material
evaluation. Incidentally, in Table 2 and Table 3, English letters A to T and
English letters a to i that are added to the steel types indicate to be the
respective components of Steels A to T and a to i in Table 1.
[0132] In the hot rolling, first, in rough rolling being first hot rolling,
rolling was performed one time or more at a reduction ratio of 40% or more in
a temperature region of not lower than 1000°C nor higher than 1200°C.
However, with respect to Steel types E2, H3, and 52 in Table 2, and Steel
types E2', H3', and 52' in Table 3, in the rough rolling, the rolling at a
reduction ratio of 40% or more in one pass was not performed. The number
of times of reduction and each reduction ratio (%) in the rough rolling, and an
austenite grain diameter (pm) after the rough rolling (before finish rolling) are
shown in Table 2 and Table 3.
[0 1331 After the rough rolling was finished, the finish rolling being
second hot rolling was performed. In the finish rolling, rolling at a reduction
ratio of 30% or more was performed in one pass at least one time in a
temperature region of not lower than T1 + 30°C nor higher than T1 + 200°C,
and in a temperature range of lower than T1 + 30°C, the total reduction ratio
was set to 30% or less. Incidentally, in the finish rolling, rolling at a
reduction ratio of 30% or more in one pass was performed in a final pass in
the temperature region of not lower than T1 + 30°C nor higher than T1 +
200°C.
[0134] However, with respect to Steel types G2, H4, and M3 in Table 2
and Steel types G2', H4', and M3' in Table 3, the rolling at a reduction ratio
of 30% or more was not performed in the temperature region of not lower
than TI + 30°C nor higher than TI + 200°C. Further, with regard to Steel
types C2, F3, and H6 in Table 2 and Steel types C2', F3', and H6' in Table 3,
the total reduction ratio in the temperature range of lower than T1 + 30°C was
greater than 30%.
[0135] Further, in the finish rolling, the total reduction ratio was set to
50% or more. However, with regard to Steel types G2, H4, and M3 in Table
2 and Steel types G2', H4', and M3' in Table 3, the total reduction ratio was
less than 50%.
[O 1361 Table 2 and Table 3 show, in the finish rolling, the reduction ratio
(Oh)in the final pass in the temperature region of not lower than T1 -t 30°C
nor higher than T1 + 200°C and the reduction ratio in a pass at one stage
earlier than the final pass (reduction ratio in a pass before the final) (%).
Further, Table 2 and Table 3 show, in the finish rolling, the total reduction
ratio (%) in the temperature region of not lower than TI + 30°C nor higher
than T1 + 200°C and a temperature Tf after the reduction in the final pass in
the temperature region of not lower than T1 + 30°C nor higher than T1 +
200°C. Incidentally, the reduction ratio (%) in the final pass in the
temperature region of not lower than TI + 30°C nor higher than T1 + 200°C
in the finish rolling is particularly important, to thus be shown in Table 2 and
Table 3 as P 1.
[0137] After the final reduction at a reduction ratio of 30% or more was
performed in the finish rolling, primary cooling was started before a waiting
time t second exceeding 2.5 x t 1. In the primary cooling, an average cooling
rate was set to 50°C/second or more. Further, a temperature change (a
cooled temperature amount) in the primary cooling was set to fall within a
range of not less than 40°C nor more than 140°C.
[0138] Under the manufacturing conditions shown in Table 2, after the
final reduction at a reduction ratio of 30% or more was performed in the
finish rolling, the primary cooling was started before the waiting time t
second exceeding tl (t < tl). On the other hand, under the manufacturing
conditions shown in Table 3, after the final reduction at a reduction ratio of
30% or more was performed in the finish rolling, the primary cooling was
started before the waiting time t second exceeding a range of tl or longer to
2.5 x tl (tl I t 5 tl x 2.5). Incidentally, ['I (dash) was added to each
reference numeral of the steel types following the manufacturing conditions
shown in-Table 3 in order to distinguish the ranges of the ~aitingti me t ..
second.
[0139] However, with respect to Steel types H8', K2', and N2' shown in
Table 3, the primary cooling was started after the waiting time t second
exceeded 2.5 x tl since the final reduction at a reduction ratio of 30% or more
in the finish rolling. With regard to Steel type M2 in Table 2 and Steel type
M2' in Table 3, the temperature change (cooled temperature amount) in the
primary cooling was less than 40°C, and with regard to Steel type H10 in
Table 2 and Steel type H10' in Table 3, the temperature change (cooled
temperature amount) in the primary cooling was greater than 140°C. With
regard to Steel type H11 in Table 2 and Steel type HI1 ' in Table 3, the average
cooling rate in the primary cooling was less than 50°C/second.
[0 1401 Table 2 and Table 3 show t 1 (second) and 2.5 x t 1 (second) of the
respective steel types. Further, Table 2 and Table 3 show the waiting time t
(second) from completion of the final reduction at a reduction ratio of 30% or
more to start of the primary cooling, t/t 1, the average cooling rate ("Clsecond)
in the primary cooling, and the temperature change (cooled temperature
amount) (OC).
[O 14 11 After the primary cooling, secondary cooling was started. In this
secondary cooling, cooling was performed down to a temperature region of
not lower than Ae3 -50°C nor higher than 700°C at an average cooling rate of
15"C/second or more. However, with regard to Steel types A2, G3, H2, 12,
and L2 in Table 2 and Steel types A2', G3', H2', I2', and L2' in Table 3, the
average cooling rate in the secondary cooling was less than 15"C/second.
Table 2 and Table 3 show, in the secondary cooling, the average cooling rate
to the temperature region of not lower than Ae3 -50°C nor higher than 700°C
of the respective steel types.
[O142] Thereafter, coiling was performed at higher than 350°C to 650°C,
and hot-rolled original sheets each having a thickness of 2 to 5 mm were
obtained. However, with regard to Steel types B2, D2, and H9 in Table 2
and Steel types B2', D2', and H9' in Table 3, a coiling temperature was higher
than 650°C. With regard to Steel type N2' in Table 3, the coiling
temperature was 350°C or lower. Table 2 and Table 3 show the coiling
temperature ("C) of the respective steel types.
[0143] Table 4 and Table 5 show an area ratio (structural fraction) (%) of
bainite, pearlite, pro-eutectoid ferrite, martensite, and retained austenite in a
metal structure of the respective steel types. Incidentally, Table 4 shows the
structural constitutions and the mechanical properties of the steel types
following the manufacturing conditions in Table 2. Further, Table 5 shows
the structural constitutions and the mechanical properties of the steel types
following the manufacturing conditions in Table 3. Incidentally, with regard
to the structural fraction in Table 4 and Table 5, B means bainite, P means
pearlite, F means pro-eutectoid ferrite, M means martensite, and rA means
retained austenite. Table 4 and Table 5 show, of the respective steel types,
an average value of pole densities of the {100)<011> to {223)<110>
orientation group, a pole density of the {332)<113> crystal orientation, a
mean volume diameter of crystal grains (size of a grain unit) (pm), and a ratio
of crystal grains having dL/dt of 3.0 or less (equiaxed grain ratio) (%).
Further, Table 4 and Table 5 show, of the respective steel types, tensile
strength TS (MPa), an elongation percentage El (%), a hole expansion ratio h
(%) as an index of the local deforrnability, and a limit bend radius by 60"
V-shape bending (a sheet thicknessla minimum bend radius). In a bending
test, C-direction bending (C-bending) was performed. Incidentally, a tensile
test and a bending test were based on JIS Z 2241 and Z 2248 (a V block 90"
bending test). A hole expansion test was based on the Japan Iron and Steel
Federation standard JFS T1001. The pole density of each of the crystal
orientations was measured using the previously described EBSP at a 0.5 pm
pitch on a 318 to 518 region at sheet thickness of a cross section parallel to the
rolling direction.
[0144] As a preferred index of the local deformability, TS 2 440 MPa,
El 2 15%, h 2 90%, and the sheet thicknesslthe bend radius > 2.3 were
set to be satisfied. It is found that only ones satisfling the prescriptions of
the present invention can have both the excellent hole expandability and
bendability as shown in FIG. 7 and FIG. 8.
[0 1451
[Table 11

[Name of Document] Claims
[Claim 1] A high-strength hot-rolled steel sheet having excellent local
deformability comprising:
in mass%,
C: not less than 0.07% nor more than 0.20%;
Si: not less than 0.001% nor more than 2.5%;
Mn: not less than 0.01% nor more than 4.0%;
P: not less than 0.001% nor more than 0.15%;
S: not less than 0.0005% nor more than 0.03%;
Al: not less than 0.00 1 % nor more than 2.0%;
N: not less than 0.0005% nor more than 0.01%;
0: not less than 0.0005% nor more than 0.01%; and
a balance being composed of iron and inevitable impurities, wherein
an area ratio of bainite in a metal structure is 95% or more,
at a sheet thickness center portion being a range of 518 to 318 in sheet
thickness from the surface of the steel sheet, an average value of pole
densities of the { 100) <0 1 1> to (223 ) <110> orientation group represented by
respective crystal orientations of { 100)<0 1 1 >, { 1 16) < 1 lo>, { 1 14) <1 lo>,
{113)<110>, {112)<110>, {335)<110>, and {223)<110> is 4.0 or less, and a
pole density of the (3321x1 13> crystal orientation is 5.0 or less, and
a mean volume diameter of crystal grains in the metal structure is 10 pm or
less.
[Claim 2] The high-strength hot-rolled steel sheet having excellent local
deformability according to claim 1, wherein
to crystal grains of the bainite, a ratio of the crystal grains in which a ratio of
a length dL in a rolling direction to a length dt in a sheet thickness direction:
dL/dt is 3.0 or less is 50% or more.
[Claim 3] The high-strength hot-rolled steel sheet having excellent local
deformability according to claim 1, further comprising:
one type or two or more types of
in mass%,
Ti: not less than 0.001% nor more than 0.20%,
Nb: not less than 0.001% nor more than 0.20%,
V: not less than 0.001% nor more than 1.0%, and
W: not less than 0.00 1% nor more than 1.0%.
[Claim 4] The high-strength hot-rolled steel sheet having excellent local
deformability according to claim 1, further comprising:
one type or two or more types of
in mass%,
B: not less than 0.0001% nor more than 0.0050%,
Mo: not less than 0.00 1 % nor more than 1.0%,
Cr: not less than 0.001% nor more than 2.0%,
Cu: not less than 0.00 1% nor more than 2.0%,
Ni: not less than 0.001% nor more than 2.0%,
Co: not less than 0.0001% nor more than 1.0%,
Sn: not less than 0.0001% nor more than 0.2%,
Zr: not less than 0.000 1% nor more than 0.2%, and
As: not less than 0.000 1 % nor more than 0.50%.
[Claim 5] The high-strength hot-rolled steel sheet having excellent local
deformability according to claim 1, further comprising:
one type or two or more types of
in mass%,
Mg: not less than 0.0001% nor more than 0.01 0%,
REM: not less than 0.0001% nor more than O.1%, and
Ca: not less than 0.0001% nor more than 0.010%.
[Claim 61 A manufacturing method of a high-strength hot-rolled steel
sheet having excellent local defo&ability, comprising:
on a steel billet containing:
in mass%,
C: not less than 0.07% nor more than 0.20%;
Si: not less than 0.001% nor more than 2.5%;
Mn: not less than 0.01% nor more than 4.0%;
P: not less than 0.001% nor more than 0.15%;
S: not less than 0.0005% nor more than 0.03%;
Al: not less than 0.001% nor more than 2.0%;
N: not less than 0.0005% nor more than 0.01%;
0: not less than 0.0005% nor more than 0.0 1 %; and
a balance being composed of iron and inevitable impurities,
performing first hot rolling in which rolling at a reduction ratio of 40% or
more is performed one time or more in a temperature range of not lower than
1000°C nor higher than 1200°C;
setting an austenite grain diameter to 200 pm or less by the first hot rolling;
performing second hot rolling in which rolling at 30% or more is performed
in one pass at least one time in a temperature region of not lower than a
temperature TI + 30°C nor higher than T1 + 200°C determined by Expression
(1) below;
setting the total of reduction ratios in the second hot rolling to 50% or more;
performing final reduction at a reduction ratio of 30% or more in the second
hot rolling and then starting primary cooling in such a manner that a waiting
time t second satisfies Expression (2) below;
setting an average cooling rate in the primary cooling to 50°C/second or more
and performing the primary cooling in a manner that a temperature change is
in a range of not lower than 40°C nor higher than 140°C;
starting secondary cooling after completion of the primary cooling;
performing cooling down to a temperature region of not lower than Ae3 -
50°C nor higher than 700°C at an average cooling rate of 15"C/second or
more in the secondary cooling; and
performing coiling at higher than 350°C to 650°C.
T1 ("C) = 850 + 10 x (C +N) x Mn+ 350 x Nb + 250 x Ti + 40 x B + 10 x
Cr+ 100 x Mo+ 100 x V-.. (1)
t 5 2.5 x tl (2)
Here, tl is obtained by Expression (3) below.
tl = 0.001 x ((Tf - Tl) x ~11100)' - 0.109 x ((Tf - Tl) x P11100) + 3.1 --.( 3)
Here, in Expression (3) above, Tf represents the temperature of the steel billet
obtained after the final reduction at a reduction ratio of 30% or more, and P1
represents the reduction ratio of the final reduction at 30% or more.
[Claim 7] The manufacturing method of the high-strength hot-rolled steel
sheet having excellent local deformability according to claim 6, wherein
the total of reduction ratios in a temperature range of lower than T1 + 30°C is
30% or less.
[Claim 8] The manufacturing method of the high-strength hot-rolled steel
sheet having excellent local deformability according to claim 6, wherein
the waiting time t second further satisfies Expression (2a) below.
t < tl .-. (2a)
[Claim 9] The manufacturing method of the high-strength hot-rolled steel
sheet having excellent local deformability according to claim 6, wherein
the waiting time t second further satisfies Expression (2b) below.
tl 5 t 5 tl x 2.5 ... (2b)
[Claim 10] The manufacturing method of the high-strength hot-rolled steel
sheet having excellent local deformability according to claim 6, wherein
the primary. cooling is started between rolling stands.