[Name of Document] DESCRIPTION
[Title of the Invention] BAINITE-CONTAINING-TYPE HIGH-STRENGTH
HOT-ROLLED STEEL SHEET HAVES[G EXCELLENT ISOTROPIC
WORKABILITY AND MANUFACTUIONG METHOD THEREOF
5 [Technical Field]
[0001] The present invention relates to a bainite-containing-type
high-strength hot-rolled steel sheet having excellent isotropic workability and
a manufacturing method thereof.
This application is based upon and claims the benefit of priority of the
10 prior Japanese Patent Application No. 2011-079658, filed on March 31, 2011,
the entire contents of which are incorporated herein by reference.
[Background Art]
[0002] In recent years, for weight reduction in various members with the
aim of improving fuel efficiency of an automobile, a reduction in thickness by
15 achieving high strength of a steel sheet of iron alloy or the like and
application of light metal such as Al alloy have been promoted. However, as
compared to heavy metal such as steel, the light metal such as Al alloy has the
advantage of specific strength being high, but has the disadvantage of being
expensive significantly. Therefore, the application of light metal such as Al
20 alloy has been limited to special use. Thus, in order to promote the weight
reduction in various members more inexpensively and widely, the reduction
in thickness by achieving high strength of a steel sheet has been needed.
[0003] The achievement of high strength of a steel sheet causes
deterioration of material properties such as formability (workability) in
25 general. Therefore, how the achievement of high strength is attained without
deteriorating the material properties is important in developing a
high-strength steel sheet. Particularly, a steel sheet used as an automobile
member such as an inner sheet member, a structure member, or an underbody
member is required to have bendability, stretch flange workability, burring
workability, ductility, fatigue durability, impact resistance, corrosion
5 resistance, and so on according to its use. It is important how these material
properties and high strength property should be exhibited in a
high-dimensional and well-balanced manner,
[0004] Particularly, among automobile parts, a part obtained by working
a sheet material as a raw material and exhibiting a function as a rotor, such as
10 a drum or a carrier constituting an automatic transmission, for example, is an
important part serving as a mediator of transmitting engine output to an axle
shaft. Such a part exhibiting a function as a rotor is required to have
circularity as a shape and sheet thickness homogeneity in a circumferential
direction in order to decrease friction and the like. Further, for forming such
15 a part, forming methods such as burring, drawing, ironing, and bulging are
used, and a great emphasis is placed also on ultimate ductility typified by
local elongation.
[0005] Further, with regard to a steel sheet used for such a member, it is
necessary to improve a property that the steel sheet is formed and then is
20 attached to an automobile as a part and then the member is not easily broken
even when being subjected to impact caused by collision or the like. Further,
in order to secure the impact resistance in a cold district, it is also necessary to
improve low-temperature toughness. This low-temperature toughness is
defined by vTrs (a Charpy fracture appearance transition temperature), or the
25 like. For this reason, it is also necessary to consider the impact resistance
itself of the above-described steel member.
[0006] That is, a thin steel sheet for a part required to have sheet
thickness uniformity such as the above-described part is required to have, in
addition to excellent workability, plastic isotropy and low-temperature
toughness as very important properties.
5 [0007] In order to achieve the high strength property and the various
material properties such as formability in particular as above, in Patent
Document 1, for example, there has been disclosed a manufacturing method
of a steel sheet in which a steel structure is made of 90% or more of ferrite
and a balance of bainite, to thereby achieve high strength, ductility, and bore
10 expandability. However, with regard to a steel sheet manufactured by
applying the technique disclosed in Patent Document 1, the plastic isotropy is
not mentioned at all. On the condition that the steel sheet manufactured in
Patent Document 1 is applied to a part required to have circularity and sheet
thickness homogeneity in a circumferential direction, a decrease in output due
15 to false vibration and/or friction loss caused by an eccentricity of the part is
concerned.
[0008] Further, in Patent Documents 2 and 3, there has been disclosed a
technique of a high-tensile hot-rolled steel sheet to which high strength and
excellent stretch flange formability are provided by adding Mo and making
20 precipitates fine. However, a steel sheet to which the techniques disclosed in
Patent Documents 2 and 3 are applied is required to have 0.07% or more of
Mo being an expensive alloy element added thereto, and thus has a problem
that its manufacturing cost is high. Further, in the techniques disclosed in
Patent Documents 2 and 3 as well, the plastic isotropy is not mentioned at all.
25 On the condition that the techniques in Patent Documents 2 and 3 are also
applied to a part required to have circularity and sheet thickness homogeneity
in a circumferential direction, a decrease in output due to false vibration
and/or friction loss caused by an eccentricity of the part is concerned.
[0009] On the other hand, with regard to the plastic isotropy of the steel
sheet, namely a decrease in plastic anisotropy, in Patent Document 4, for
5 example, there has been disclosed a technique in which endless rolling and
lubricated rolling are combined, and thereby a texture of austenite in a shear
layer of a surface layer is regulated and in-plane anisotropy of an r value
(Lankford value) is decreased. However, in order to perform the lubricated
rolling with a small friction coefficient over an entire length of a coil, the
10 endless rolling is needed for preventing biting failure caused by slip between
a roll bite and a rolled sheet material during rolling. However, in order to
apply this technique, investment in facilities such as a rough bar joining
apparatus, a high-speed crop shear, and so on is needed and thus a burden is
large.
15 [0010] Further, in Patent Document 5, for example, there has been
disclosed a technique in which Zr, Ti, and Mo are compositely added and
finish rolling is finished at a high temperature of 950°C or higher, and thereby
strength of 780 MPa class or more is obtained, anisotropy of an r value is
small, and stretch flange formability and deep drawability are achieved.
20 However, 0.1% or more of Mo being an expensive alloy element is needed to
be added, and thus there is a problem that its manufacturing cost is high.
[0011] Further, a study of improving the low-temperature toughness of a
steel sheet has been advanced up to now, but a bainite-containing-type
high-strength hot-rolled steel sheet having excellent isotropic workability that
25 has high strength, exhibits plastic isotropy, improves hole expandability, and
further achieves also low-temperature toughness has not been disclosed in
«
Patent Documents 1 to 5.
[Prior Art Document]
[Patent Document]
[0012] Patent Document 1: Japanese Laid-open Patent Publication No.
5 H6-293910
Patent Document 2: Japanese Laid-open Patent Publication No. 2002-322540
Patent Document 3: Japanese Laid-open Patent Publication No. 2002-322541
Patent Document 4: Japanese Laid-open Patent Publication No. HI 0-183255
Patent Document 5: Japanese Laid-open Patent Publication No. 2006-124789
10 [Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[0013] The present invention has been invented in consideration of the
above-described problems, and has an object to provide a
bainite-containing-type high-strength hot-rolled steel sheet having excellent
15 isotropic workability that has high strength, is applicable to a member
required to have workability, hole expandability, bendability, strict sheet
thickness uniformity and circularity after working, and low-temperature
toughness, and has a steel sheet grade of 540 MPa class or more, and a
manufacturing method capable of manufacturing the steel sheet inexpensively
20 and stably.
[Means for Solving the Problems]
[0014] In order to solve the problems as described above, the present
inventors propose a bainite-containing-type high-strength hot-rolled steel
sheet having excellent isoti-opic workability and a manufacturing method
25 described below.
[0015] [1]
A bainite-containing-type high-strength hot-rolled steel sheet having excellent
isotropic workability, contains:
inmass%,
C: greater than 0.07 to 0.2%;
5 Si: 0.001 to 2.5%;
Mn: 0.01 to 4%;
P: 0.15% or less (not including 0%);
S: 0.03% or less (not including 0%));
N: 0.01% or less (not including 0%);
10 Al: 0.001 to 2%; and
a balance being composed of Fe and inevitable impurities, in which
an average value of pole densities of the {100}<011> to {223}<110>
orientation group represented by respective crystal orientations of
{100}<011>, {116}<110>, {114}<110>, {113}<110>, {112}<110>,
15 {335}<110>, and {223}<110> at a sheet thickness center portion being a
range of 5/8 to 3/8 in sheet thickness from the surface of the steel sheet is 4.0
or less, and a pole density of the {332}<113> crystal orientation is 4.8 or less,
an average crystal grain diameter is 10 |j,m or less and a Charpy fracture
appearance transition temperature vTrs is -20°C or lower, and
20 a microstructure is composed of 35% or less in a structural fraction of
pro-eutectoid ferrite and a balance of a low-temperature transformation
generating phase.
[2]
The bainite-containing-type high-strength hot-rolled steel sheet having
25 excellent isotropic workability according to [1], fiirther contains:
one type or two or more types of
^
in mass%,
Ti: 0.015 to 0.18%,
Nb: 0.005 to 0.06%,
Cu: 0.02 to 1.2%,
5 Ni: 0.01 to 0.6%,
Mo: 0.01 to 1%,
V: 0.01 to 0.2%, and
Cr: 0.01 to 2%.
[3]
10 The bainite-containing-type high-strength hot-rolled steel sheet having
excellent isotropic workability according to [1], further contains:
one type or two or more types of
in mass%,
Mg: 0.0005 to 0.01%,
15 Ca: 0.0005 to 0.01 %, and
REM: 0.0005 to 0.1%.
[4]
The bainite-containing-type high-strength hot-rolled steel sheet having
excellent isotropic workability according to [1], further contains:
20 in mass%,
B: 0.0002 to 0.002%.
[5]
A manufacturing method of a bainite-containing-type high-strength
hot-rolled steel sheet having excellent isotropic workability, includes:
25 on a steel billet containing:
in mass%.
C: greater than 0.07 to 0.2%;
Si: 0.001 to 2.5%;
Mn: 0.01 to 4%;
P: 0.15% or less (not including 0%);
5 S: 0.03% or less (not including 0%);
N: 0.01% or less (not including 0%);
Al: 0.001 to 2%; and
a balance being composed of Fe and inevitable impurities,
performing first hot rolling in which rolling at a reduction ratio of 40% or
10 more is performed one time or more in a temperature range of not lower than
1000°C nor higher than 1200°C;
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 Tl +
30°C nor higher than Tl + 200°C determined by Expression (1) below; and
15 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 a maimer that a waiting time
period t second satisfies Expression (2) below;
setting an average cooling rate in the primary cooling to 50°C/second or more
20 and performing the primary cooling in a manner that a temperature change is
in a range of not lower than 40°C npr higher than 140°C;
within three seconds after completion of the primary cooling, performing
secondary cooling in which cooling is performed at an average cooling rate of
15°C/second or more; and
25 after completion of the secondary cooling, performing air cooling for 1 to 20
seconds in a temperature region of lower than an Ar3 transformation point
temperature and an Arl transformation point temperature or higher and next
performing coiling at 450°C or higher and lower than 550°C.
Tl (°C) = 850 + 10 X (C + N) X Mn + 350 X Nb + 250 X Ti + 40 X B + 10 X
Cr+lOOxMo+lOOxV -(1)
5 Here, C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the
element (mass%).
t ^ 2.5 X tl - (2)
Here, tl is obtained by Expression (3) below.
tl = 0.001 X ((Tf - Tl) X Pl/100)^ - 0.109 x ((Tf - Tl) x Pl/lOO) + 3.1 - (3)
10 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 PI
represents the reduction ratio of the final reduction at 30% or more.
[6]
The manufacturing method of the bainite-containing-type
15 high-strength hot-rolled steel sheet having excellent isotropic workability
according to [5], in which
the total of reduction ratios in a temperature range of lower than Tl + 30°C is
30% or less.
[V]
20 The manufacturing method of the bainite-containing-type
high-strength hot-rolled steel sheet having excellent isotropic workability
according to [5], in which
heat generation by working between respective passes in the temperature
region of not lower than Tl + 30°C nor higher than Tl + 200°C in the second
25 hot rolling is 18°C or lower.
[8]
10
The manufacturing method of the bainite-containing-type
high-strength hot-rolled steel sheet having excellent isotropic workability
according to [5], in which
the waiting time period t second fiirther satisfies Expression (4) below.
5 t to {223}<110> orientation group and
isotropy (l/|Ar|);
[FIG. 2] FIG 2 is a view showing the relationship between a pole density of
the {332}<113> crystal orientation and an isotropic index (l/|Ar|);
[FIG. 3] FIG 3 is a view showing the relationship between an average crystal
10 grain diameter ()J-m) and vTrs (°C); and
[FIG 4] FIG. 4 is an explanatory view of a continuous hot rolling line.
[Mode for Carrying out the Invention]
[0018] As an embodiment implementing the present invention, there will
be explained a bainite-containing-type high-strength hot-rolled steel sheet
15 having excellent isotropic workability, (which will be simply called a
"hot-rolled steel sheet" hereinafter), in detail. Incidentally, mass% related to
a chemical composition is simply described as %.
[0019] The present inventors earnestly studied the
bainite-containing-type high-strength hot-rolled steel sheet suitable for
20 application to a member required to have workability, hole expandability,
bendability, strict sheet thickness uniformity and circularity after working,
and low-temperature toughness, in terms of workability and fiirther
achievement of isotropy and low-temperature toughness. As a result, the
following new knowledge was obtained.
25 [0020] First, for obtaining the isotropy (decreasing anisotropy), formation
of a transformation texture fi"om non-recrystallized austenite, being the cause
12
of anisotropy, is avoided. In order to achieve it, it is necessary to promote
recrystallization of austenite after finish rolling. As its means, an optimum
rolling pass schedule in finish rolling and achievement of high temperature of
a rolling temperature are effective.
5 [0021] Next, for improving the low-temperature toughness, making
grains fine in each fi^acture of a brittle fi"acture, namely grain refining in each
microstructure is effective. For this, it is effective to increase nucleation
sites for a at the time of transformation of y to a, and it becomes necessary to
increase crystal grain boundaries of austenite that can be the nucleation sites
10 and dislocation density.
[0022] As its means, it becomes necessary to perform rolling at a y to a
transformation point temperature or higher and at as low a temperature as
possible, namely to make austenite remain non-recrystallized and in a state of
a non-recrystallization fraction being high, cause the y to a transformation.
15 This is because austenite grains after recrystallization grow quickly at a
recrystallization temperature, become coarse for an extremely short time, and
become coarse even in an a phase after the y to a transformation to thereby
cause significant toughness deterioration.
[0023] The present inventors invented an entirely new hot rolling method
20 capable of, on a higher level, balancing the isotropy and the low-temperature
toughness, which were considered difficult to be achieved because they
resulted in conditions opposite to each other by a normal hot rolling means.
[0024] First, as for the isotropy, the present inventors obtained the
following knowledge with regard to the relationship between isotropy and
25 texture.
[0025] In order to obtain the sheet thickness uniformity and circularity
13
that satisfy a part property in a state where the steel sheet remains worked
without being subjected to trimming and cutting processes, at least an
isotropic index (= l/|Ar|) is needed to be 3.5 or more.
[0026] Here, the isotropic index is obtained in a manner that the steel
5 sheet is worked into a No. 5 test piece described in JIS Z 2201 and the test
piece is subjected to a test by the method described in JIS Z 2241. l/|Ar|
being the isotropic index is defined as Ar = (rL - 2 x r45 + rC)/2, where plastic
strain ratios (r values: Lankford values) in a rolling direction, in a 45°
direction with respect to the rolling direction, and in a 90° direction with
10 respective to the rolling direction (sheet width direction) are defined as rL,
r45, and rC respectively.
[0027] (Crystal orientation)
As shown in FIG. 1, the isotropic index (= l/|Ar|) satisfies 3.5 or more
as long as an average value of pole densities of the {100}<011> to
15 {223}<110> orientation group represented by respective crystal orientations
of {100}<011>, {116}<110>, {114}<110>, {113}<110>, {112}<110>,
{335}<110>, and {223}<110> at a sheet thickness center portion being a
range of 5/8 to 3/8 in sheet thickness from the surface of the steel sheet is 4.0
or less. As long as the isotropic index is 6.0 or more desirably, the sheet
20 thickness uniformity and circularity that sufficiently satisfy the part property
in a state where the steel sheet remains worked can be obtained even though
variations in a coil are considered. Therefore, the average value of the pole
densities of the {100}<011> to {223}<110> orientation group is desirably 2.0
or less.
25 [0028] The pole density is synonymous with an X-ray random intensity
ratio. The pole density (X-ray random intensity ratio) is a numerical value
14
obtained by measuring X-ray intensities of a standard sample not having
concentration 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.
5 This pole density can be measured by any one of X-ray diffi-actometry, an
EBSP (Electron Back Scattering Pattern) method, and an EC? (Electron
Channeling Pattern) method.
[0029] As for the pole density of the {100}<011> to {223}<110>
orientation group, for example, pole densities of respective orientations of
10 {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 {110}, {100}, {211}, and {310} measured by the
method, and these pole densities are arithmetically averaged, and thereby the
15 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}<011>, {116}<110>, {114}<110>,
{112}<110>, and {223}<110> may also be used as a substitute.
20 [0030] For example, for the pole density of each of the above-described
crystal orientations, each of intensities of (001)[1-10], (116)[1-10],
(114)[1-10], (113)[1-10], (112)[1-10], (335)[1-10], and (223)[1-10] at a (1)2 =
45° cross-section in the three-dimensional texture may be used as it is.
[0031] Similarly, as shown in FIG 2, as long as the pole density of the
25 {332}<113> crystal orientation at the sheet thickness center portion being the
range of 5/8 to 3/8 in sheet thickness from the surface of the steel sheet is 4.8
15
or less, the isotropic index satisfies 3.5 or more. As long as the isotropic
index is 6.0 or more desirably, the sheet thickness uniformity and circularity
that sufficiently satisfy the part property in a state where the steel sheet
remains worked can be obtained even though variations in a coil are
5 considered. Therefore, the pole density of the {332}<113> crystal
orientation is desirably 3.0 or less.
[0032] With regard to the sample to be subjected to the X-ray
diflfractometry, EBSP method, or ECP method, the steel sheet is reduced in
thickness to a predetermined sheet thickness fi'om the surface by mechanical
10 polishing or the like. Next, strain is removed by chemical polishing,
electrolytic polishing, or the like, and the sample is manufactured in such a
manner that in the range of 5/8 to 3/8 in sheet thickness, an appropriate plane
becomes a measuring plane. For example, on a steel piece in a size of 30
mm(j) cut out from the position of 1/4 W or 3/4 W of the sheet width W,
15 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 dififractometry is
manufactured. With regard to the sheet width direction, the steel piece is
desirably taken from, of the steel sheet, the position of 1/4 or 3/4 from an end
20 portion.
[0033] 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 5/8 to 3/8 in sheet thickness fi*om the surface
of the steel sheet, but also at as many thickness positions as possible, and
25 thereby local ductile performance (local elongation) is fiirther improved.
However, the range of 5/8 to 3/8 from the surface of the steel sheet is
16
measured, to thereby make it possible to represent the material property of the
entire steel sheet generally. Thus, 5/8 to 3/8 of the sheet thickness is defined
as the measuring range.
[0034] Incidentally, the crystal orientation represented by {hkl}
5 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 so on are generic terms
10 for equivalent planes, and [hkl], (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 (111), (-111), (1-11), (H-l), (-1-11),
(-11-1), (1-1-1), and (-1-1-1) planes are equivalent to make it impossible to
make them different. In such a case, these orientations are generically
15 referred to as {111}. 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
20 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.
[0035] (Average crystal grain diameter)
Next, the present inventors examined the low-temperature toughness.
25 [0036] FIG. 3 shows the relationship between an average crystal grain
diameter and vTrs (a Charpy fracture appearance transition temperature). As
17
the average crystal grain diameter is smaller, vTrs becomes low in
temperature, and the toughness at low temperature is improved. As long as
the average crystal grain diameter is 10 \xm or less, vTrs becomes -20°C or
lower as a target, and thus the present invention is durable enough to be used
5 in a cold district.
[0037] Incidentally, the low-temperature toughness was evaluated by
vTrs (the Charpy fracture appearance transition temperature) obtained by a
V-notch Charpy impact test. In the V-notch Charpy impact test, a test piece
was made based on JISZ2202 and the test was performed according to the
10 contents defined in JISZ2242, and vTrs was measured.
[0038] Further, the low-temperature toughness is greatly affected by the
average crystal grain diameter of the structure, and thus the measurement of
the average crystal grain diameter in the sheet thickness center portion was
also performed. A microsample was cut out to have a crystal grain diameter
15 and microstructure thereof measured by using EBSP-OIM^^ (Electron Back
Scatter Diffraction Pattern-Orientation Image Microscopy). The
microsample was polished by using a colloidal silica abrasive for 30 to 60
minutes to be made and was subjected to an EBSP measurement under
measurement conditions of 400 magnifications, 160 p,m x 256 |j,m area, and a
20 measurement step of 0.5 |Lim.
[0039] The EBSP-OIM™ method is constituted by a device and software
that a highly inclined sample in a scanning electron microscope (SEM) is
irradiated with electron beams, a Kikuchi pattern formed by backscattering is
photographed by a high-sensitive camera and is image processed by a
25 computer, and thereby a crystal orientation at an irradiation point is measured
for a short time period.
«
18
[0040] In the EBSP method, it is possible to quantitatively analyze a
microstructure and a crystal orientation of a bulk sample surface. An
analysis area of the EBSP method is an area capable of being observed by the
SEM. It is possible to analyze the area with a minimum resolution of 20 nm
5 by the EBSP method, depending on the resolution of the SEM. The analysis
is performed by mapping an area to be analyzed to tens of thousands of
equally-spaced grid points. It is possible to see crystal orientation
distributions and sizes of crystal grains within the sample in a polycrystalline
material.
10 [0041] In the present invention, fi-om an image mapped in a manner that
an orientation difference between crystal grains is defined as 15° being a
threshold value of a large angle tilt grain boundary recognized as a crystal
grain boundary generally, the crystal grains were visualized and the average
crystal grain diameter was obtained. Here, the "average crystal grain
15 diameter" is a value obtained by the EB SP-OIM™.
[0042] As described above, the present inventors revealed respective
requirements necessary for the steel sheet for obtaining the isotropy and the
low-temperature toughness.
[0043] The average crystal grain diameter directly related to the
20 low-temperature toughness becomes small as a finish rolling finishing
temperature is lower, and thus the low-temperature toughness is improved.
However, the average value of the pole densities of the {100}<011> to
{223}<110> orientation group at the sheet thickness center portion
corresponding to 5/8 to 3/8 fi*om the surface of the steel sheet and the pole
25 density of the {332}<113> crystal orientation, which are one of control
factors of the isotropy, are inversely correlated to the average crystal grain
19
diameter. That is, it is the relation in which when the average crystal grain
diameter is decreased in order to improve the low-temperature toughness, the
average value of the pole densities of the {100}<011> to {223}<110>
orientation group and the pole density of the {332}<113> crystal orientation
5 are increased and thus the isotropy deteriorates. The technique achieving the
isotropy and the low-temperature toughness has not been disclosed so far at
all.
[0044] The present inventors earnestly examined the
bainite-containing-type high-strength hot-rolled steel sheet suitable for
10 application to a member required to have workability, hole expandability,
bendability, strict sheet thickness uniformity and circularity after working,
and low-temperature toughness and allowing the isotropy and the
low-temperature toughness to be achieved and a manufacturing method
thereof As a result, the present inventors thought of a hot-rolled steel sheet
15 made of the following conditions and a manufacturing method thereof.
[0045] (Chemical composition)
First, there will be explained reasons for limiting a chemical
composition of the bainite-containing-type high-strength hot-rolled steel sheet
of the present invention, (which will be sometimes called a "present invention
20 hot-rolled steel sheet" hereinafter).
[0046] C: greater than 0.07 to 0.2%
C is an element contributing to increasing the strength of the steel, but
is also an element generating iron-based carbide such as cementite (FesC) to
be the starting point of cracking at the time of hole expansion. When C is
25 0.07% or less, it is not possible to obtain a strength improving effect by a
low-temperature transformation generating phase. On the other hand, when
20
C exceeds 0.2%, center segregation becomes noticeable and iron-based
carbide such as cementite (FcaC) to be the starting point of cracking in a
secondary shear surface at the time of punching is increased, resulting in that
a punching property deteriorates. Therefore, C is set to greater than 0.07 to
5 0.2%. When the balance between strength and ductility is considered, C is
desirably 0.15% or less.
[0047] Si: 0.001 to 2.5%
Si is an element contributing to increasing the strength of the steel and
also has a part as a deoxidizing material of molten steel, and thus is added
10 according to need. When Si is 0.001% or more, the above-described effect
is exhibited, but when Si exceeds 2.5%, a strength increasing effect is
saturated. Therefore, Si is set to 0.001 to 2.5%.
[0048] Further, when being greater than 0.1%, Si, with an increase in the
content, suppresses precipitation of iron-based carbide such as cementite and
15 contributes to improving the strength and to improving the hole expandability.
However, when Si exceeds 1.0%, an effect of suppressing the precipitation of
iron-based carbide is saturated. Therefore, Si is preferably greater than 0.1
to 1.0%.
[0049] Mn: 0.01 to 4%
20 Mn is an element contributing to improving the strength by
solid-solution strengthening and quenching strengthening and is added
according to need. When Mn is less than 0.01%, its addition effect cannot
be obtained, and when Mn exceeds 4%, on the other hand, the addition effect
is saturated, and thus Mn is set to 0.01 to 4%.
25 [0050] In order to suppress occurrence of hot cracking by S, when
elements other than Mn are not added sufficiently, the Mn amount allowing
1fl
21
the Mn amount (mass%) ([Mn]) and the S amount (mass%) ([S]) to satisfy
[Mn]/[S] ^ 20 is desirably added. Further, Mn is an element that, with an
increase in the content, expands an austenite region temperature to a low
temperature side, improves the hardenability, and facilitates formation of a
5 continuous cooling transformation structure having excellent burring. When
Mn is less than 1%, this effect is not easily exhibited, and thus Mn is
desirably 1% or more.
[0051] P: 0.15% or less
P is an impurity contained in molten iron, and is an element that is
10 segregated at grain boundaries and decreases the toughness. For this reason,
it is desirable as P is smaller, and when exceeding 0.15%, P adversely affects
the workability and weldability, and thus P is set to 0.15% or less.
Particularly, when the hole expandability and the weldability are considered,
P is desirably 0.02% or less. Incidentally, it is difficult to set P to 0% in
15 terms of operation, and thus 0% is not included.
[0052] S: 0.03% or less
S is an impurity contained in molten iron, and is an element that not
only causes cracking at the time of hot rolling but also generates an A-based
inclusion deteriorating the hole expandability. For this reason, S should be
20 decreased as much as possible, but as long as S is 0.03% or less, it falls within
an allowable range, and thus S is set to 0.03% or less. However, when the
hole expandability to such extent is needed, S is preferably 0.01% or less, and
is more preferably 0.005% or less. Incidentally, it is difficult to set S to 0%
in terms of operation, and thus 0% is not included.
25 [0053] Al: 0.001 to 2%
For molten steel deoxidation in a refining process of the steel, 0.001%
22
or more of Al is added, but the upper limit is set to 2% because an increase in
cost is caused. When Al is added in large amounts, the content of non-metal
inclusions is increased and the ductility and the toughness deteriorate, and
thus Al is desirably 0.06% or less. It is further desirably 0.04% or less.
5 [0054] Al is an element having a function of suppressing precipitation of
iron-based carbide such as cementite in the structure, similarly to Si. For
obtaining this function effect, Al is desirably 0.016% or more. It is further
desirably 0.016 to 0.04%.
[0055] N: 0.01% or less
10 N is an element that should be decreased as much as possible, but as
long as N is 0.01% or less, it falls within an allowable range. In terms of
aging resistance, however, N is desirably 0.005% or less. Incidentally, it is
difficult to set N to 0% in terms of operation, and thus 0% is not included.
[0056] The present invention hot-rolled steel sheet may also contain one
15 type or two or more types of Ti, Nb, Cu, Ni, Mo, V, and Cr according to need.
The present invention hot-rolled steel sheet may also fiirther contain one type
or two or more types of Mg, Ca, and REM.
[0057] Hereinafter, there will be explained reasons for limiting chemical
compositions of the above-described elements.
20 [0058] Ti, Nb, Cu, Ni, Mo, V, and Cr each are an element improving the
strength by precipitation strengthening or solid-solution strengthening, and
one type or two or more types of these elements may also be added.
[0059] However, when Ti, is less than 0.015%>, Nb is less than 0.005%,
Cu is less than 0.02%, Ni, is less than 0.0 P/o, Mo is less than 0.01%, V is less
25 than 0.01%, and Cr is less than 0.01%, their addition effects cannot be
obtained sufficiently.
23
[0060] On the other hand, when Ti is greater than 0.18%, Nb is greater
than 0.06%, Cu is greater than 1.2%, Ni is greater than 0.6%, Mo is greater
than 1%, V is greater than 0.2%, and Cr is greater than 2%, the addition
effects are saturated and economic efficiency decreases. Therefore, it is
5 desirable that Ti is 0.015 to 0.18%, Nb is 0.005 to 0.6%, Cu is 0.02 to 1.2%,
Ni is 0.01 to 0.6%, Mo is 0.01 to 1%, V is 0.01 to 0.2%, and Cr is 0.01 to 2%.
[0061] Mg, Ca, and REM (rare-earth element) each are an element that
controls the form of non-metal inclusions to be the starting point of fracture to
cause the deterioration of the workability and improves the workability, and
10 one type or two or more types of these elements may also be added. When
Mg, Ca, and REM are each less than 0.0005%, their addition effects are not
exhibited.
[0062] On the other hand, when Mg is greater than 0.01%), Ca is greater
than 0.01%, and REM is greater than 0.1%, the addition effects are saturated
15 and economic efficiency decreases. Therefore, it is desirable that Mg is
0.0005 to 0.01%, Ca is 0.0005 to 0.01%, and REM is 0.0005 to 0.1%.
[0063] Incidentally, the present invention hot-rolled steel sheet may also
contain 1% or less in total of one type or two or more types of Zr, Sn, Co, Zn,
and W within a range that does not impair the characteristics of the present
20 invention hot-rolled steel sheet. However, Sn is desirably 0.05% or less in
order to suppress occurrence of a flaw at the time of hot rolling.
[0064] B: 0.0002 to 0.002%
B is an element that increases the hardenability and increases a
structural fraction of the low-temperature transformation generating phase
25 being a hard phase and thus is added according to need. When B is less than
0.0002%, its addition effect cannot be obtained, and when B exceeds 0.002%,
24
on the other hand, the addition effect is saturated, and further there is a risk
that the recrystallization of austenite in hot rolling is suppressed and the y to a
transformation texture from non-recrystallized austenite is strengthened to
deteriorate the isotropy. Therefore, B is set to 0.0002 to 0.002%.
5 [0065] Further, B is also an element causing slab cracking in a cooling
process after continuous casting, and from this viewpoint, is desirably
0.0015% or less. It is desirably 0.001 to 0.0015%.
[0066] (Microstructure)
Next, there will be explained metallurgical factors such as a
10 microstructure of the present invention hot-rolled steel sheet in detail.
[0067] The microstructure of the present invention hot-rolled steel sheet
is composed of 35% or less in a structural fraction of pro-eutectoid ferrite and
a balance of the low-temperature transformation generating phase. The
low-temperature transformation generating phase means a continuous cooling
15 transformation structure, and is a structure recognized as bainite in general.
[0068] Generally, steel sheets having the same tensile strength are
compared, and then where a microstructure is an uniform structure occupied
by a structure such as the continuous cooling transformation structure, the
microstructure shows a tendency to be excellent in local elongation as is
20 typified by a hole expanding value, for example. Where the microstructure
is a composite structure composed of pro-eutectoid ferrite being a soft phase
and a hard low-temperature transformation generating phase (continuous
cooling transformation structure, including martensite in MA), the
microstructure shows a tendency to be excellent in uniform elongation that is
25 typified by a work hardening coefficient n value.
[0069] In the present invention hot-rolled steel sheet, the microstructure
n 25
is designed to be the composite structure composed of 35% or less in a
structural fraction of pro-eutectoid ferrite and a balance of the
low-temperature transformation generating phase in order to ultimately
balance the local elongation as is typified by the bendability and the uniform
5 elongation.
[0070] When pro-eutectoid ferrite is greater than 35%, the bendability
being an index of the local elongation decreases significantly, but the uniform
elongation is not so improved, and thus the balance between the local
elongation and the uniform elongation deteriorates. The lower limit of the
10 structural fraction of pro-eutectoid ferrite is not limited in particular, but when
the structural fraction is 5% or less, a decrease in the uniform elongation
becomes significant, and thus the structural fraction of pro-eutectoid ferrite is
preferably greater than 5%.
[0071] The continuous cooling transformation structure (Zw)
15 (low-temperature transformation generating phase) of the present invention
hot-rolled steel sheet is a microstructure defined as a transformation structure
positioned in the middle of a microstructure containing polygonal ferrite and
pearlite to be generated by a diffusive mechanism and martensite to be
generated by a non-difiusive shearing mechanism, as is described in The Iron
20 and Steel Institute of Japan, Society of basic research, Bainite Research
Committee/Edition; 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)
("reference literature").
25 [0072] That is, the continuous cooling transformation structure (Zw)
(low-temperature transformation generating phase) is defined as a
26
microstructure mainly composed of Bainitic ferrite (a^a), Granular bainitic
ferrite (ae), and Quasi-polygonal ferrite (ttq), 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
5 an optical microscopic observation structure.
[0073] Incidentally, similarly to polygonal ferrite (PF), an internal
structure of aq does not appear by etching, but a shape of ttq is acicular, and it
is definitely distinguished from PF. Here, of a targeted crystal grain, a
peripheral length is set to Iq and a circle-equivalent diameter is set to dq, and
10 then a grain having a ratio (Iq/dq) satisfying Iq/dq ^ 3.5 is aq.
[0074] The continuous cooling transformation structure (Zw)
(low-temperature transformation generating phase) of the present invention
hot-rolled steel sheet is a microstructure containing one type or two or more
types of a°B, OCB, and aq. Further, the continuous cooling transformation
15 structure (Zw) (low-temperature transformation generating phase) of the
present invention hot-rolled steel sheet may also further contain one of a
small amount of yr and MA, or both of them, in addition to one type or two or
more types of a°Bj OCB, and aq. Incidentally, the total content of yr and MA is
set to 3% or less in a structural fraction.
20 [0075] There is sometimes a case that the continuous cooling
transformation structure (Zw) (low-temperature transformation generating
phase) is not easily discerned by observation by optical microscope in etching
using a nital reagent. In such a case, it is discerned by using the
EBSP-OIM™. The EBSP-OIM™ (Electron Back Scatter Diffraction
25 Pattern-Orientation Image Microscopy) method is constituted by a device and
software in which a highly inclined sample in a scanning electron microscope
27
(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.
5 [0076] 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 by the EBSP method 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.
10 [0077] The analysis by the EBSP-OIM™ method is performed by
mapping an area to be analyzed to tens of thousands of equally-spaced grid
points. It is possible to see crystal orientation distributions and sizes of
crystal grains within the sample in a polycrystalline material. In the present
invention hot-rolled steel sheet, one discernible from a mapped image with an
15 orientation difference between packets defined as 15° may also be defined as
a grain diameter of the continuous cooling transformation structure (Zw)
(low-temperature transformation generating phase) for convenience. In this
case, a large angle tilt grain boundary having a crystal orientation difference
of 15° or more is defined as a grain boundary.
20 [0078] Further, the structural fraction of pro-eutectoid ferrite was
obtained by a Kernel Average Misorientation (KAM) method being equipped
with the EBSP-OIM™. The KAM method is that a calculation, in which
orientation differences among pixels of first approximations being adjacent
six pixels of a certain regular hexagon of measurement data, or second
25 approximations being 12 pixels positioned outside the six pixels, or third
approximations being 18 pixels positioned further outside the 12 pixels are
9
28
averaged and an obtained value is set to a value of the center pixel, is
performed with respect to each pixel.
[0079] This calculation is performed so as not to exceed a grain boundary,
thereby making it possible to create a map representing an orientation change
5 within a grain. That is, this map represents a distribution of strain based on a
local orientation change within a grain. Note that in the analysis, the
condition of which in the EBSP-OIM^^, the orientation difference among
adjacent pixels is calculated is set to the third approximation and one having
this orientation difference being 5° or less is displayed.
10 [0080] In examples of the present invention, the condition of which in the
EBSP-OIM (registered trademark), the orientation difference among adjacent
pixels is calculated is set to the third approximation and this orientation
difference is set to 5° or less, and the above-described orientation difference
third approximation is greater than 1°, which is defined as the continuous
15 cooling transformation structure (Zw) (low-temperature transformation
generating phase), and it is 1° or less, which is defined as ferrite. This is
because polygonal pro-eutectoid ferrite transformed at high temperature is
generated in a diffusion transformation, and thus a dislocation density is small
and strain within the grain is small, and thus, a difference within the grain in
20 the crystal orientation is small, and according to the results of various
examinations that have been performed so far by the present inventors, a
volume fraction of polygonal ferrite obtained by observation of optical
microscope and an area fraction of an area obtained by 1° or less of the
orientation difference third approximation measured by the KAM method
25 substantially agree with each other.
[0081] (Manufacturing method)
9 29
Next, there will be explained conditions of a manufacturing method of
the present invention hot-rolled steel sheet, (which will be called a "present
invention manufacturing method," hereinafter).
[0082] The present inventors explored hot rolling conditions making
5 austenite recrystallize sufficiently after finish rolling or during finish rolling
in order to secure the isotropy but suppressing grain growth of recrystallized
grains as much as possible and achieving the isotropy and the
low-temperature toughness.
[0083] First, in the present invention manufacturing method, a
10 manufacturing method of a steel billet to be performed prior to a hot rolling
process is not particularly limited. That is, in the manufacturing method of
the steel billet, subsequent to a melting process by a shaft fiimace, a steel
converter, an electric furnace, or the like, in various secondary refining
processes, a component adjustment is performed so as to be an aimed
15 chemical composition. Next, a casting process may also be performed by
normal continuous casting, or casting by an ingot method, or fiuther a method
such as thin slab casting.
[0084] Incidentally, a scrap may also be used for a raw material. Further,
when a slab is obtained by continuous casting, the slab may be directly
20 transferred to a hot rolling mill as it is in a high-temperature cast slab state, or
it may also be cooled to a room temperature and then reheated in a heating
fiimace, and then hot rolled.
- [0085] The slab obtained by the above-described manufacturing method
is heated in a slab heating process prior to the hot rolling process, but in the
25 present invention manufacturing method, a heating temperature is not
determined in particular. However, when the heating temperature is higher
^A
30
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 1150°C, operational efficiency deteriorates
significantly in terms of a schedule, and thus the heating temperature is
5 desirably 1150°C or higher.
[0086] Further, a heating time period 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 predetermined heating temperature, the
heating temperature is desirably maintained for 30 minutes or longer.
10 However, when the cast slab after being subjected to casting is directly
transferred to a hot rolling mill as it is in a high-temperature cast slab state to
be rolled, the heating time period is not limited to this.
[0087] (First hot rolling)
After the slab heating process, the slab extracted from the heating
15 furnace 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.
[0088] The rough rolling process (first hot rolling) is performed at a
temperature of not lower than 1000°C nor higher than 1200°C for reasons to
be explained below. When a rough rolling finishing temperature is lower
20 than 1000°C, reduction is performed in a state where the vicinity of a surface
layer of the rough bar is in a non-recrystallization temperature region, the
texture is developed, and the isotropy deteriorates. Further, hot deformation
resistance in the rough rolling increases, to thereby cause a risk that an
impediment is caused to the rough rolling operation.
25 [0089] On the other hand, when the rough rolling finishing temperature is
higher than 1200°C, the average crystal grain diameter is increased to
31
decrease the toughness. Further, a secondary scale to be generated during
the rough rolling grows too much, to thereby make it difficult to remove the
scale in descaling or finish rolling to be performed later. When the rough
rolling finishing temperature is higher than 1150°C, there is sometimes a case
5 that inclusions are drawn and the hole expandability deteriorates, and thus it is
desirably 1150°C or lower.
[0090] Further, in the rough rolling process (first hot rolling), in a
temperature range of not lower than 1000°C nor higher than 1200°C, rolling
at a reduction ratio of 40% or more is performed one time or more. When
10 the reduction ratio in the rough rolling process is less than 40%, the average
crystal grain diameter is increased and the toughness decreases. When the
reduction ratio is 40% or more, the crystal grain diameter becomes uniform
and small. On the other hand, when the reduction ratio is greater than 65%,
there is sometimes a case that inclusions are drawn and the hole expandability
15 deteriorates, and thus it is desirably 65% or less. Incidentally, in the rough
rolling, when the reduction ratio at a final stage and the reduction ratio at a
stage prior to the final stage are less than 20%, the average crystal grain
diameter is increased easily, and thus in the rough rolling, the reduction ratio
at the final stage and the reduction ratio at the stage prior to the final stage are
20 desirably 20% or more.
[0091] Incidentally, in terms of decreasing the average crystal grain
diameter of a final product, an austenite grain diameter after the rough rolling,
namely before the finish rolling is important and the austenite grain diameter
before the finish rolling is desirably small.
25 [0092] As long as the austenite grain diameter before the finish rolling is
200 )Lim or less, it is possible to greatly promote grain refining and
32
homogenizing. For efficiently obtaining this promoting effect, the austenite
grain diameter is desirably set to 100 |Lim or less. In order to achieve it, the
rolling at a reduction ratio of 40% or more is desirably performed two or
more times in the rough rolling process. However, when in the rough rolling
5 process, the rolling is performed greater than 10 times, there is a concern that
the temperature decreases or a scale is generated excessively.
[0093] In this manner, the austenite grain diameter before the finish
rolling is decreased, which is effective for promoting the recrystallization of
austenite in the finish rolling later. It is supposed that this is because an
10 austenite grain boundary after the rough rolling (namely before the finish
rolling) functions as one of recrystallization nuclei during the finish rolling,
[0094] The austenite grain diameter after the rough rolling is measured as
follows. That is, the steel billet (rough bar) after the rough rolling (before
being subjected to the finish rolling) is quenched as much as possible, and is
15 desirably cooled at a cooling rate of 10°C/second or more. The structure of
a cross section of the cooled steel billet is etched to make the austenite grain
boundaries appear, and the austenite grain boundaries are measured by an
optical microscope. On this occasion, at 50 magnifications or more, 20
visual fields or more are measured by image analysis or a point counting
20 method.
[0095] The rough bars obtained after the completion of the rough rolling
process may also be joined between the rough rolling process and a finish
rolling process to then have endless rolling such that the finish rolling process
is performed continuously performed thereon. On this occasion, the rough
25 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.
33
[0096] On the occasion of the hot rolling process, temperature variations
of the rough bar in a rolling direction, in a sheet width direction, and in a
sheet thickness direction are desirably controlled to be small. In this case,
according to need, a heating apparatus capable of controlling the temperature
5 variations of the rough bar in the rolling direction, in the sheet width direction,
and in the sheet thickness direction may be disposed between a roughing mill
in the rough rolling process and a finishing mill in the finish rolling process,
or between respective stands in the finish rolling process, and thereby the
rough bar may be heated.
10 [0097] As a system of the heating apparatus, various heating systems
such as gas heating, electrical heating, and induction heating are conceivable,
but as long as the heating system makes it possible to control the temperature
variations of the rough bar in the rolling direction, in the sheet width direction,
and in the sheet thickness direction to be small, any one of well-known
15 systems may also be used.
[0098] Incidentally, as the system of the heating apparatus, an induction
heating system having an industrially good temperature control response is
preferred. If among various induction heating systems, a plurality of
transverse-type induction heating apparatuses capable of being shifted in the
20 sheet width direction is installed, a temperature distribution in the sheet width
direction can be arbitrarily controlled according to the sheet width, and thus
the transverse-type induction heating apparatuses are more preferred.
Further, as the system of the heating apparatus, a heating apparatus
constituted by the combination of a transverse-type induction heating
25 apparatus and a solenoid-type induction heating apparatus that excels in
heating across the entire sheet width is the most preferred.
34
[0099] When the temperature is controlled using these heating
apparatuses, it sometimes becomes necessary to control an amount of heating
by the heating apparatus. In this case, the internal temperature of the rough
bar cannot be measured actually, and thus previously measured actual data
5 such as a charged slab temperature, a slab furnace existing time period, a
heating furnace atmospheric temperature, a heating furnace extraction
temperature, and fiirther a table roller transfer time period are used to estimate
temperature distributions in the rolling direction, in the sheet width direction,
and in the sheet thickness direction when the rough bar reaches the heating
10 apparatus, and then the amount of heating by the heating apparatus is
desirably controlled.
[0100] Incidentally, the control of the amount of heating by the induction
heating apparatus is controlled in the following manner, for example. A
characteristic of the induction heating apparatus (transverse-type induction
15 heating apparatus) is that when an alternating current is applied to a coil, a
magnetic field is generated in its inside. In an electric conductor positioned
in the magnetic field, an eddy current having an orientation opposite to the
current in the coil occurs in a circumferential direction perpendicular to a
magnetic flux by an electromagnetic induction effect, and by Joule heat of the
20 eddy current, the electric conductor is heated.
[0101] The eddy current occurs most strongly on the inner surface of the
coil and decreases exponentially toward the inside (this phenomenon is called
a skin effect). Thus, as a fi-equency is smaller, a current penetration depth is
increased and a heating pattern uniform in the thickness direction is obtained,
25 and conversely, as a frequency is larger, the current penetration depth is
decreased and a heating pattern that exhibits its peak at a surface layer and has
35
small overheating is obtained in the thickness direction.
[0102] Therefore, by the transverse-type induction heating apparatus, the
heating of the rough bar in the rolling direction and in the sheet width
direction can be performed in a conventional manner, and fiirther in terms of
5 the heating in the sheet thickness direction, by changing the frequency of the
transverse-type induction heating apparatus, the penetration depth is varied
and the heating temperature pattern in the sheet thickness direction is
controlled, to thereby make it possible to achieve uniformity of the
temperature distributions. Incidentally, a frequency-changeable-type
10 induction heating apparatus is preferably used in this case, but the frequency
may also be changed by adjusting a capacitor.
[0103] With regard to the control of the amount of heating by the
induction heating apparatus, a plurality of inductors having different
frequencies may be disposed and an allocation of an amount of heating by
15 each of the inductors may be changed so as to obtain the necessary heating
pattern in the thickness direction. With regard to the control of the amount
of heating by the induction heating apparatus, an air gap to a material to be
heated is changed and thereby the frequency changes, and thus by changing
the air gap, the desired frequency and heating pattern may also be obtained.
20 [0104] A maximum height Ry of the steel sheet surface (rough bar
surface) after the finish rolling is desirably 15 |am (15 |j,m Ry, 12.5 mm. In
12.5 mm) or less. This is clear because the fatigue strength of the hot-rolled
or pickled steel sheet is correlated to the maximum height Ry of the steel
sheet surface as is also described in Metal Material Fatigue Design Handbook,
25 edited by The Society of Materials Science, Japan, on page 84, for example.
[0105] In order to obtain this surface roughness, a condition of an impact
36
pressure P x a flow rate L ^ 0.003 of a high-pressure water onto the steel
sheet surface is desirably satisfied in descaling. Further, the subsequent
finish rolling is desirably performed within five seconds in order to prevent a
scale from being generated again after the descaling.
5 [0106] (Second hot rolling)
After the rough rolling process (first hot rolling) is completed, the
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 When the time between
10 the completion of the rough rolling process and the start of the finish rolling
process is longer than 150 seconds, the average crystal grain diameter is
increased to cause the decrease in vTrs.
[0107] In the finish rolling process (second hot rolling), a finish rolling
start temperature is set to 1000°C or higher When the finish rolling start
15 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.
[0108] Incidentally, the upper limit of the finish rolling start temperature
20 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 1150°C.
25 [0109] In the finish rolling, a temperature determined by the chemical
composition of the steel sheet is set to Tl, and in a temperature region of not
37
lower than Tl + 30°C nor higher than Tl + 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.
[0110] Here, Tl is the temperature calculated by Expression (1) below.
5 Tl (°C) = 850 + 10 X (C + N) X Mn + 350 X Nb + 250 X Ti + 40 X B +
lOxCr+lOOxMo+lOOxV -(1)
C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the
element (mass%).
[0111] Tl itself is obtained empirically. The present inventors learned
10 empirically by experiments that the recrystallization in an austenite region of
each steel is promoted on the basis of Tl.
[0112] When the total reduction ratio in the temperature region of not
lower than Tl + 30°C nor higher than Tl + 200°C is less than 50%, rolling
strain to be accumulated during the hot rolling is not sufficient and the
15 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
20 difficult to obtain the temperature region of Tl + 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.
[0113] In the finish rolling, in order to promote the uniform
recrystallization caused by releasing the accumulated strain, the rolling at
25 30% or more is performed in one pass at least one time at not lower than Tl +
30°C nor higher than Tl + 200°C.
38
[0114] Incidentally, in order to promote the uniform recrystallization, it is
necessary to suppress a working amount in a temperature region of lower than
Tl + 30°C as small as possible. In order to achieve it, the reduction ratio at
lower than Tl + 30°C is desirably 30% or less. In terms of sheet thickness
5 accuracy and sheet shape, 10% or less of the reduction ratio is desirable.
When the isotropy is fiirther obtained, the reduction ratio in the temperature
region of lower than Tl + 30°C is desirably 0%.
[0115] The finish rolling is desirably finished at Tl + 30°C or higher. In
the hot rolling at lower than Tl + 30°C, the granulated austenite grains that
10 are recrystalhzed once are elongated, thereby causing a risk that the isotropy
deteriorates.
[0116] (Primary cooling)
In the finish rolling, after the final reduction at a reduction ratio of
30% or more is performed, primary cooling is started in such a manner that a
15 waiting time period t second satisfies Expression (2) below,
t ^ 2.5 X tl - (2)
Here, tl is obtained by Expression (3) below.
tl = 0.001 X ((Tf - Tl) X Pl/100)^ - 0.109 x ((Tf - Tl) x Pl/100) + 3.1 - (3)
Here, in Expression (3) above, Tf represents the temperature of the steel billet
20 obtained after the final reduction at a reduction ratio of 30% or more, and PI
represents the reduction ratio of the final reduction at 30% or more.
[0117] 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
25 passes performed in the finish rolling. For example, when among the
rollings in a plurality of passes performed in the finish rolling, the reduction
39
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
5 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."
10 [0118] In the finish rolling, the waiting time period 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 fi-action and a coarse grain area ratio of the
steel sheet.
15 [0119] When the waiting time period t second exceeds tl x 2.5, the
recrystallization is already almost completed, but the crystal grains grow
significantly and grain coarsening advances, and thereby the r value and the
elongation are decreased.
[0120] The waiting time period t second fiirther satisfies Expression (4)
20 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 the fatigue property simultaneously.
t < t l - ( 4)
25 [0121] At the same time, the waiting time period t second fiirther satisfies
Expression (5) below, and thereby the recrystallization advances sufficiently
^ ^
40
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 ^ t ^ tl X 2.5 - (5)
5 [0122] The waiting time period t second satisfies Expression (5) above,
and thereby the average value of the pole densities of the {100}<011> to
{223}<110> orientation group shown in FIG. 1 becomes 2.0 or less and the
pole density of the {332}<113> crystal orientation shown in FIG 2 becomes
3.0 or less. Consequently, the isotropic index becomes 6.0 or more and the
10 sheet thickness uniformity and circularity that sufficiently satisfy the part
property in a state where the steel sheet remains worked are achieved.
[0123] Here, as shown in FIG. 4, 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
15 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 present invention
manufacturing method, in the rough rolling process (first hot rolling)
performed in the roughing mill 2, the rolling at a reduction ratio of 40% or
more is performed on the steel billet (slab) one time or more in the
20 temperature range of not lower than 1000°C nor higher than 1200°C.
[0124] 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
25 more is performed in one pass at least one time in the temperature region of
not lower than the temperature Tl + 30°C nor higher than Tl + 200°C.
41
Further, in the finishing mill 3, the total of the reduction ratios becomes 50%
or more.
[0125] 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
5 such a manner that the waiting time period t second satisfies Expression (2)
above or either Expressions (4) or (5) above. The start of this primary
cooling is performed by inter-stand cooling nozzles 10 disposed between the
respective the rolling stands 6 of the finishing mill 3, or cooling nozzles 11
disposed in the run-out-table 5.
10 [0126] 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. 4, 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
15 3 (on the right side in FIG. 4, on the downstream side of the rolling), the start
of the primary cooling is performed by the cooling nozzles 11 disposed in the
run-out-table 5, and thereby a case that the waiting time period t second does
not satisfy Expression (2) above or Expressions (4) and (5) above is
sometimes caused. In such a case, the primary cooling is started by the
20 inter-stand cooling nozzles 10 disposed between the respective the rolling
stands 6 of the finishing mill 3.
[0127] 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. 4, on the downstream side of
25 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
42
that the waiting time period t second can satisfy Expression (2) above or
Expressions (4) and (5) 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
5 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
the rolling stands 6 of the finishing mill 3.
[0128] Then, in this primary cooling, the cooling that at an average
cooling rate of 50°C/second or more, a temperature change (temperature
10 drop) becomes not lower than 40°C nor higher than 140°C is performed.
[0129] When the temperature change is lower than 40°C, the
recrystallized austenite grains grow and the low-temperature toughness
deteriorates. The temperature change is set to 40°C or higher, thereby
making it possible to suppress coarsening of the austenite grains. When the
15 temperature change is lower than 40°C, the effect cannot be obtained. On
the other hand, when the temperature change exceeds 140°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
20 thereby the elongation and local ductility also deteriorate. Further, when the
temperature change is higher than 140°C, an overshoot to/beyond an Ar3
transformation point temperature is likely to be caused. In the case, even by
the transformation from recrystallized austenite, as a result of sharpening
variant selection, the texture is formed and the isotropy decreases
25 consequently.
[0130] When the average cooling rate in the primary cooling is less than
43
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.
5 [0131] Further, in order to suppress the grain growth and obtain the 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.
[0132] A rolling ratio (the reduction ratio) can be obtained by actual
10 performances or calculation from the rolling load, sheet thickness
measurement, or/and the like. The temperature of the steel billet during the
rolling can be obtained by actual measurement 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, or/and
15 like, or can be obtained by the both methods.
[0133] Further, as has been explained previously, in order to promote the
uniform recrystallization, the working amount in the temperature region of
lower than Tl + 30°C is desirably as small as possible and the reduction ratio
in the temperature region of lower than Tl + 30°C is desirably 30% or less.
20 For example, in the event that in the finishing mill 3 on the continuous hot
rolling line 1 shown in FIG. 4, 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. 4, on
the upstream side of the rolling), the steel sheet is in the temperature region of
not lower than Tl + 30°C nor higher than Tl + 200°C, and in passing through
25 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).
44
the steel sheet is in the temperature region of lower than Tl + 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
5 or is performed, the reduction ratio at lower than Tl + 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 Tl + 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 Tl + 30°C is desirably 0%.
10 [0134] In the present invention manufacturing method, 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
15 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.
20 [0135] Further, within three seconds after the completion of the primary
cooling, secondary cooling in which cooling is performed at an average
cooling rate of 15°C/second or more is performed. When the time period to
the start of the secondary cooling exceeds three seconds, pearlite
transformation occurs and the targeted microstructure cannot be obtained.
25 [0136] When the average cooling rate of the secondary cooling is less
than 15°C/second, as expected, the pearlite transformation occurs and the
45
targeted microstructure cannot be obtained. Even though the upper limit of
the average cooling rate of the secondary cooling is not limited in particular,
the effect of the present invention can be obtained, but when warpage of the
steel sheet due to thermal strain is considered, the average cooling rate is
5 desirably 300°C/second or less.
[0137] The average cooling rate is not less than 15°C/second nor more
than 50°C/second, which is a region allowing stable manufacturing. Further,
as will be shown in examples, the region of 30°C/second or less is a region
allowing more stable manufacturing.
10 [0138] Next, air cooling is performed for 1 to 20 seconds in a temperature
region of lower than the Ar3 transformation point temperature and an Arl
transformation point temperature or higher. This air cooling is performed in
the temperature region of lower than the Ar3 transformation point temperature
and the Arl transformation point temperature or higher (a
15 ferrite-austenite-two-phase temperature region) in order to promote the ferrite
transformation. When the air cooling is performed for less than one second,
the ferrite transformation in the two-phase region is not sufficient and thus the
sufficient uniform elongation cannot be obtained, and when the air cooling is
performed for greater than 20 seconds, on the other hand, the pearlite
20 transformation occurs and the targeted microstructure cannot be obtained.
[0139] The temperature region where the air cooling is performed for 1 to
20 seconds is desirably not lower than the Arl transformation point
temperature nor higher than 860°C in order to easily promote the ferrite
transformation. A holding time period (an air cooling time period) for 1 to
25 20 seconds is desirably for 1 to 10 seconds in order not to decrease the
productivity extremely.
46
[0140] The Ar3 transformation point temperature can be easily calculated
by the following calculation expression (a relational expression with the
chemical composition), for example. When the Si content (mass%) is set to
[Si], the Cr content (mass%) is set to [Cr], the Cu content (mass%) is set to
5 [Cu], the Mo content (mass%) is set to [Mo], and the Ni content (mass%) is
set to [Ni], the Ar3 transformation point temperature can be defined by
Expression (6) below.
Ar3 = 910 - 310 X [C] + 25 x [Si] - 80 x [Mneq] - (6)
[0141] When B is not added, [Mneq] is defined by Expression (7) below.
10 [Mneq] = [Mn] + [Cr] + [Cu] + [Mo] + ([Ni]/2) + 10([Nb] - 0.02) -
(7)
[0142] When B is added, [Mneq] is defined by Expression (8) below.
[Mneq] = [Mn] + [Cr] + [Cu] + [Mo] + ([Ni]/2) + 10([Nb] - 0.02) + 1
... (8)
15 [0143] Subsequently, in a coiling process, a coiling temperature is set to
not lower than 450°C nor higher than 550°C. When the coiling temperature
is higher than 550°C, after the coiling, tempering in a hard phase occurs and
the strength decreases. On the other hand, when the coiling temperature is
lower than 450°C, during cooling after the coiling, non-transformed austenite
20 is stabilized, and in a product steel sheet, retained austenite is contained and
martensite is generated, and thereby the hole expandability decreases.
[0144] Incidentally, with the aim of achieving the improvement of the
ductility by correction of the steel sheet shape and/or introduction of mobile
dislocation, skin pass rolling at a reduction ratio of not less than 0.1% nor
25 more than 2% is desirably performed after the completion of all the processes.
[0145] Further, after the completion of all the processes, pickling may
47
also be performed with the aim of removing the scale adhering to the surface
of the obtained hot-rolled steel sheet. After the pickling, on the hot-rolled
steel sheet, skin pass or cold rolling at a reduction ratio of 10% or less may
4ilso be performed inline or offline.
5 [0146] On the present invention hot-rolled steel sheet, a heat treatment
may also be performed on a hot dipping line after the casting, after the hot
rolling, or after the cooling, and fiirther on the heat-treated hot-rolled steel
sheet, a surface treatment may also be performed separately. On the hot
dipping line, plating is performed, and thereby the corrosion resistance of the
10 hot-rolled steel sheet is improved.
[0147] When galvanizing is performed on the pickled hot-rolled steel
sheet, after the hot-rolled steel sheet is dipped in a galvanizing bath to then be
pulled up, an alloying treatment may also be performed on the hot-rolled steel
sheet according to need. By performing the alloying treatment, in addition
15 to the improvement of the corrosion resistance, welding resistance against
various weldings such as spot welding is improved.
Example
[0148] Next, examples of the present invention will be explained, but
conditions of the examples are condition examples employed for confirming
20 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 fi-om the spirit of the invention.
[0149] (Example 1)
25 Cast billets A to P having chemical compositions shown in Table 1
were each melted in a steel converter in a secondary refining process to be
48
subjected to continuous casting and then were directly transferred or reheated
to be subjected to rough rolling. In the subsequent finish rolling, they were
each reduced to a sheet thickness of 2.0 to 3.6 mm and were subjected to
cooling by inter-stand cooling of a finishing mill or on a run-out-table and
5 then were coiled, and hot-rolled steel sheets were manufactured.
Manufacturing conditions are shown in Table 2.
[0150] Incidentally, the balance of the chemical composition shown in
Table 1 is composed of Fe and inevitable impurities, and each underline in
Table 1 and Table 2 indicates that the value is outside the range of the present
10 invention or outside the preferable range of the present invention.
[0151] [Table 1]
STEEL
A
B
C
D
E
F
G
H
I
J
K
L
M
N
0
P
Q
CHEMICAL COMPOSITION (UNIT: MASS%)
C
0.070
0.071
0.067
0.036
0.043
0.042
0.089
0.180
0.022
0.004
0.230
0.091
0.100
0.081
0.090
0.087
0.084
SI
1.20
1.17
0.14
0.94
098
0.73
0.91
0.03
0.05
0.12
0.18
0.02
0.03
0.01
0.02
0.02
0.02
Mn
2.51
2.46
1.98
1.34
0.98
1.04
1.20
0.72
1.12
1.61
0.74
1.50
1.45
1.51
1.55
1.52
1.49
P
0.016
0.011
0.007
0.008
0.010
0.011
0.008
0.017
0.009
0080
0.017
0.007
0.008
0.010
0.011
0.008
0.007
S
0.003
0.002
0.001
0.001
0.001
0.001
0.001
0.004
0.004
0.002
0.002
0.001
0.001
0.001
0.001
0001
0001
Al
0.023
0.029
0.011
0.020
0.036
0.024
0.033
0.011
0.025
0.041
0.005
0.011
0.020
0.036
0.024
0.033
0.03!
N
0.0026
0.0040
0.0O46
0.0028
0.0034
0.0041
0.0038
0.0035
0.0047
0.0027
0.0051
0.0046
0.0028
0.0034
0.0041
0.0038
0.0039
Ti
0.144
0.179
0.091
0.126
0.099
0.035
0.000
0.025
0.102
0.025
0000
0.026
0.020
0.022
0.024
0.023
0.000
Nb
0.020
0.017
0.038
0.041
0.000
0.019
0.000
0.000
0.000
0.025
0.000
0.000
0.000
0.000
0.011
0.000
0.000
Cu
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.06
0.00
0.00
0.00
0.00
000
Ni
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.03
0.03
0.00
0.00
0.00
0.00
Mo
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.48
0.00
0.00
0.00
V
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.10
0.00
0.00
Cr
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.91
0.00
B
0.0014
0.0000
0.0000
0.0000
0.0009
0.0000
0.0000
0.0000
0.0011
0.0011
0.0000
0.0000
0.0000
0.0010
0.0000
0.0000
0.0015
MR
0.0022
0.0000
0.0019
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
00000
00000
Ca
0.0000
0.0024
0.0000
0.0000
0.0021
0.0000
0.0022
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
Rem
0.0000
0.0000
0.0000
0.0000
0.0000
0.0018
0.0000
0.0000
0.0020
0.0020
0.0020
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
OTHERS
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
00000
NOTE
PRESENT INVENTION
PRESENT INVENTION
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
PRESENT INVENTION
PRESENT INVENTION
COMPARATIVE STEEL
COMPARATIVE STEEL
COMPARATIVE STEEL
PRESENT INVENTION
PRESENT INVENTION
PRESENT INVENTION
PRESENT INVENTION
PRESENT INVENTION
PRESENT INVENTION
4^
B 50
[0152] [Table 2]
INVENTION
PRESENT
COMPAHATTVE
COMPARATIVE
RXAMPIJi
PRESENT
INVENnoN
COMPARATIVE
EXAMPLE
PRESENT
COMPAHATTVE
CO(*ARATIVE
EXAMPLE
COWARATIVE
EXAMPLE
COM>ARATIVE
COM*ARATIVE
EXAMPLE
COKfARATIVE
COWAHATTVE
COMPARATIVE
COSfARATIVE
COMPARATIVE
EXAMPLE
COM>ARATTVE
COMPARATIVE
EXAMPLE
COliffARATlVE
COKTARATIVE
COMPARATIVE
COMPAHATTVE
HIESENT
INVENnON
COMPARATIVE
EXAMPLE
COMPAHATTVE
COMPARATIVE
PRESENT
PRESENT
PRESENT
INVENnoN
PRESENT
PRESENT
INVENTION
PRESENT
INVKNnON
NUMBER
1
2
3
4
5
6
7
8
9
10
II
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
li«TALLLR01CAL FACTORS
COMPONENT
A
B
C
D
E
F
0
0
G
G
0
G
G
G
G
G
0
G
G
G
G
G
G
G
G
G
H
I
J
K
L
M
N
0
P
Q
Art TRANSFORMATION
POINT
TEKFERATLRE
859
723
720
798
779
833
82S
825
825
825
825
825
825
825
825
825
825
825
825
825
825
825
825
825
825
825
813
751
699
800
772
779
662
766
705
701
TI
895
903
887
896
875
866
851
851
851
851
851
851
851
851
851
851
851
851
851
851
851
851
851
851
851
851
858
876
865
852
858
856
905
871
866
851
MANUFACTURINO CONDITIOIC
HEATING TEMPERATURE
HEAT!t*3
TEM*ERATIIRE
1260
1260
1230
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1200
1180
1180
1180
1180
1180
1180
HOLDfNO
TO*
raRHJD
45
45
45
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
90
90
90
90
90
90
ROLOHROU-INO
NUMBER OF TIMES
OF REDUCTION
AT lOeCC OR mOHER
ATWWORMDRE
2
2
REDUCTTON
RATIO
ORHIOHER
45/45
45/45
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
25/25/25
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
50
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
40/40/40
TIME PERIOD
TO START
OF FINISH
ROLUNO
(KC)
60
60
60
90
90
90
90
90
180
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
FINISH ROU
TOTAL
REDUCTION
RATIO
90
90
93
89
89
89
89
89
89
15
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
89
TT
t;)
990
990
980
990
970
960
950
950
950
950
850
1050
950
950
950
950
950
950
950
950
950
950
950
950
950
950
980
960
950
940
960
950
940
950
940
940
PI
40
40
35
32
32
32
32
32
32
32
32
32
22
32
32
32
32
32
32
32
32
32
32
32
32
32
35
32
32
32
32
32
32
32
32
32
IN3
MAXDtUMWORKINO
HEAT OBJURATION
TEMPERATURE
15
12
15
12
12
12
12
12
12
12
12
12
12
2S
12
12
12
12
12
12
12
12
12
12
12
]2
15
12
12
12
12
12
12
12
12
12
COOUNO
tl
(MC)
0.40
0.51
0.62
0.73
071
0.72
0.65
0.65
0.65
0.65
3.14
0.21
-
0.65
0.65
0.65
0.65
0.65
0.65
0.65
0.65
0.65
0.65
0.65
0.65
0.65
0.27
0.89
0.88
0.82
0.61
0.73
2.00
0.99
1.08
0.81
t'1.5
1.00
1.28
1.55
1.83
1.79
1.81
1.63
1.63
1.63
1.63
7.85
0.53
-
1-63
1.63
1.63
1.63
1.63
1.63
1.63
1.63
1.63
1.63
1.63
1.63
1.63
0.66
2.22
2.19
2.05
1.52
1.83
5.00
2.47
2.71
2.03
TIME PERIOD
T« START
OF PRIMARY
COOLING
(IB)
1.0
1.0
0.8
0.9
0.9 •
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
LB
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.6
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.7
„,
2.5
2.0
1.3
1.2
1.3
1.2
1.4
1.4
1.4
1.4
0.3
4.2
-
1.4
2J.
1.4
1.4
1.4
1.4
1.4
1.4
1.4
1.4
1.4
1.4
1.4
2.3
1.0
1.0
I.I
1.5
1.2
0.5
0.9
0.8
0.9
PRIMARY
COOLING
135
60
65
60
60
60
45
60
60
60
60
60
60
60
60
i
60
60
60
60
60
60
60
60
60
60
65
60
60
60
60
60
60
60
60
60
PRIMARY
COOLINO
lEMPERATURE
CHANGE
fCl
90
90
110
70
70
70
70
70
70
70
70
70
70
70
70
70
2H
2m
70
70
70
70
70
70
70
70
no
70
70
70
70
70
70
70
70
70
TIKE PERIOD
TO START
OF SECOf©ARV
(«c)
1.5
2.5
1.0
16
1.6
1.6
1.6
1.6
1.6
1.6
1.6
1.6
1.6
1.6
1.6
1.6
1.6
1.6
10.0
1.6
1.6
1.6
1.6
1.6
1.6
1.6
1.0
1.0
1.0.
1.0
1.0
1.0
1.0
1.0
1.0
1.0
SECONDARY
COOLING
RATE
fC/.«)
30
30
40
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
i
25
25
25
25
25
25
20
20
20
20
20
20
20
20
20
20
AffiCOOLBO
TEMPERATURE
RBOION
660
660
680
680
670
690
700
700
700
700
700
700
700
700
700
700
700
700
700
700
84fl
JSfl
700
700
700
670
670
670
670
670
670
670
670
670
670
AKCOOLmO
HOLDING
TIME PERIOD
2
8
5
5
2
2
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
2S
4
4
2
2
2
2
10
10
10
10
10
10
COBJNG
TEWERATURE
470
470
470
470
470
470
500
500
500
500
500
500
500
500
500
500
500
500
500
500 •
500
500
500
500
100
650
530
530
530
530
470
470
470
470
470
470
tyi
»WII»lMI»ll|l|WII,WWI»J.t..|.|UUIIMI]l,imTO {223)<110>
ORIENTATION GROUP
1.7
1.7
1.8
1.7
20
20
20
1.7
3.1
42
5A
1.7
5.3
1.7
1.7
1.8
1.8
iA
1.9
2.0
20
20
20
20
2.0
2.0
18
20
20
26
20
1.9
3.9
3.6
3.7
3.3
POLE DENSITY OF
{332}<113>
CRYSTAL ORIENTATION
2.5
25
2.6
25
29
29
29
25
42
ifl
i 5
25
ifi
25
25
26
26
12
28
29
29
29
29
29
29
29
26
29
29
3.7
29
28
28
26
26
28
MECHANICAL PROPERTIES
TENSILE TEST
YP
SlPa)
906
857
677
700
716
412
475
484
490
482
475
458
477
480
478
481
471
468
420
418
409
581
601
390
370
400
548
396
355
986
588
570
592
585
578
603
TS
Itffs)
998
1015
744
261
770
28S
577
580
588
581
575
560
577
571
585
579
577
566
521
P,<^
m
644
650
495
622
an
655
522
462
1126
711
702
720
700
695
732
El
(%)
15
14
11
ID
9
28
30
28
27
28
28
29
28
28
26
27
27
28
24
25
26
i;
14
27
28
26
26
30
35
i
24
25
24
25
25
23
ISOTROPY
\/\Ar125
125
92
125
65
65
65
125
3.8
12
11
125
3.0
125
125
92
92
IC
75
65
55
65
65
65
65
65
92
65
65
43
65
75
48
46
47
43
HOLE
EXPANSION
X
(%)
71
75
71
70
70
68
131
125
123
88
87
132
85
136
135
130
133
8$
67
65
66
76
75
69
41
69
141
122
140
22
105
97
101
96
93
91
BENDABIUTY
MINIMUM
BEND RADIUS
06
05
06
0.8
08
lA
02
01
0.1
02
02
01
0.1
02
02
01
02
02
lA
U
Li
0.9
08
Li
11
IS
01
LL
01
0.8
01
008
01
O07
0.1
01
TOUGHNESS
vTrs
-58
-48
-48
-68
J l
-48
-25
JJ.
-20
-68
-125
=5
-80
=12
6
a
-17
-80
-40
-48
-40
-48
-40
-40
-40
-40
-48
-40
=5
:24
-80
-58
-93
-127
-80
-80
i
^i^^ll^^^p|w^!l^f!l|l^^>)lpl51W^pp^;«ll^pli
58
[0167] "STRUCTURAL FRACTION" is the area fraction of each
structure measured by a point counting method from an optical microscope
structure. "AVERAGE CRYSTAL GRAIN DIAMETER" is the average
crystal grain diameter measured by the EBSP-OIM™^.
5 [0168] "AVERAGE VALUE OF X-RAY RANDOM INTENSITIES OF
{100}<011> TO {223}<110> ORIENTATION GROUP" is the pole density
of the {100}<011> to {223}<110> orientation group parallel to the rolled
plane. "POLE DENSITY OF {332}<113> CRYSTAL ORIENTATION" is
the pole density of the {332}<113> crystal orientation parallel to the rolled
10 plane.
[0169] "TENSILE TEST" indicates the result obtained after a tensile test
being performed on a C-direction JIS No. 5 test piece. "YP" indicates the
yield point, "TS" indicates the tensile strength, and "EL" indicates the
elongation.
15 [0170] "ISOTROPY" indicates the inverse number of |Ar| as an index.
"HOLE EXPANSION X" indicates the result obtained by the hole expanding
test method described in JFS T 1001-1996. "BENDABILITY (MINIMUM
BEND RADIUS)" indicates the result obtained by performing a test using a
No. 1 test piece (t x 40 mm W x 80 mm L), at a pressing jig speed of 0.1
20 m/second, in accordance with the pressing bend method (roller bend method)
described in JIS Z 2248. YP ^ 320 MPa, Ts ^ 540MPa,El ^ 18%, X
^ 70%, and the minimum bend radius ^ 1 mm were accepted.
[0171] Incidentally, a length L between supporting points is L = 2r + 3t,
where the sheet thickness is set to t (mm) and the inside radius of a tip of the
25 pressing jig is set to r (mm).
[0172] In this method, a bending angle was set up to 170°, and thereafter
i 59
an interposed object having a thickness twice as large as the radius of the
pressing jig was used, the test piece was pressed against the interposed object
to be wound therearound, and with a bending angle of 180°, cracking in the
outside of a bent portion was observed visually.
5 [0173] "MINIMUM BEND RADIUS" is one that the test is performed by
decreasing the inside radius r (mm) until cracking occurs and the minimum
inside radius r (mm) that does not cause cracking is divided by the sheet
thickness t (mm) to be made dimensionless by r/t. "MINIMUM BEND
RADIUS" becomes the smallest in the case of close-contact bending that is
10 performed without the interposed object, and in the case, "MINIMUM BEND
RADIUS" is zero. Incidentally, a bending direction was set at 45° from the
rolling direction. "TOUGHNESS" is indicated by the transition temperature
obtained by a subsize V-notch Charpy test.
[0174] The invention examples correspond to the nine examples of Steel
15 numbers 1, 2, 7, 27, and 31 to 35. In these invention examples of Steel
numbers, the high-strength steel sheet in which the texture of the steel sheet
having a required chemical composition is obtained, the average value of the
pole densities of the {100}<011> to {223}<110> orientation group of the
sheet plane at a sheet thickness of 5/8 to 3/8 from the surface of the steel sheet
20 is at least 4.0 or less, the pole density of the {332}<113> crystal orientation is
4.8 or less, and the average crystal grain diameter at the sheet thickness center
is 9 |a,m or less, the microstructure is composed of pro-eutectoid ferrite in a
structural fraction of 35% or less at the sheet thickness center and the
low-temperature transformation generating phase, and the tensile strength is
25 540 MPa class or more is obtained.
[0175] The comparative examples of the steel sheet other than the
60
above-described examples each fall outside the range of the present invention
due to the following reasons.
[0176] With regard to Steel numbers 3 to 5, the C content is outside the
range of the present invention, and thus the microstructure is outside the range
5 of the present invention and the elongation is poor. With regard to Steel
number 6, the C content is outside the range of the present invention, and thus
the microstructure is outside the range of the present invention and the
bendability is poor.
[0177] With regard to Steel number 8, the number of times of the
10 reduction at 1000°C or higher at 35% or more in the rough rolling is outside
the range of the present invention, and thus the average crystal grain diameter
is outside the range of the present invention and the toughness is poor. With
regard to Steel number 9, the time period to the start of the finish rolling is
long, the average crystal grain diameter is outside the range of the present
15 invention, and the toughness is poor.
[0178] With regard to Steel number 10, the average value of the pole
densities of the {100}<011> to {223}<110> orientation group and the pole
density of the {332}<113> crystal orientation are both outside the range of the
present invention and the isotropy is low.
20 [0179] With regard to Steel number 11, the value of Tf is outside the
range of the present invention, and thus the average value of the pole densities
of the {100}<011> to {223}<110> orientation group and the pole density of
the {332}<113> crystal orientation are both outside the range of the present
invention and the isotropy is low.
25 [0180] With regard to Steel number 12, the value of Tf is outside the
range of the present invention, and thus the average crystal grain diameter is
61
outside the range of the present invention and the toughness is poor. With
regard to Steel number 13, the value of PI is outside the range of the present
invention and at each of the rolling stands Fl to F7 in the finish rolling, the
reduction at a reduction ratio of 30% or more was not performed, and thus the
5 average value of the pole densities of the {100}<011> to ,{223}<110>
orientation group and the pole density of the {332}<113> crystal orientation
are both outside the range of the present invention and the isotropy is low.
[0181] With regard to Steel number 14, the maximum working heat
generation temperature is outside the range of the present invention, and thus
10 the average crystal grain diameter is outside the range of the present invention
and the toughness is poor. With regard to Steel number 15, the time period
to the primary cooling is outside the range of the present invention, and thus
the average crystal grain diameter is outside the range of the present invention
and the toughness is poor. With regard to Steel number 16, the primary
15 cooling rate is outside the range of the present invention, and thus the average
crystal grain diameter is outside the range of the present invention and the
toughness is poor.
[0182] With regard to Steel number 17, the primary cooling temperature
change is outside the range of the present invention, and thus average crystal
20 grain diameter is outside the range of the present invention and the toughness
is poor. With regard to Steel number 18, the primary cooling temperature
change is outside the range of the present invention, and thus the average
value of the pole densities of the {100}<011> to {223}<110> orientation
group and the pole density of the {332}<113> crystal orientation are both
25 outside the range of the present invention and the isotropy is low.
[0183] With regard to Steel number 19, the time period to the secondary
i 62
cooling is outside the range of the present invention, and thus the
microstructure is outside the range of the present invention, the strength is low,
and the bendability is poor. With regard to Steel number 20, the secondary
cooling rate is outside the range of the present invention, and thus the
5 microstructure is outside the range of the present invention, the strength is low,
and the bendability is poor.
[0184] With regard to Steel number 21, the air cooling temperature region
is outside the range of the present invention, and thus the microstructure is
outside the range of the present invention, the strength is low, and the
10 bendability is poor.
[0185] With regard to Steel number 22, the air cooling temperature region
is outside the range of the manufacturing method of the hot-rolled steel sheet
of the present invention, and thus the microstructure is outside the range of
the present invention and the elongation is poor. With regard to Steel
15 number 23, the air cooling temperature holding time period is outside the
range of the present invention, and thus the microstructure is outside the range
of the present invention and the elongation is poor. With regard to Steel
number 24, the air cooling temperature holding time period is outside the
range of the present invention, and thus the microstructure is outside the range
20 of the present invention, the strength is low, and the bendability is poor.
[0186] With regard to Steel number 25, the coiling temperature is outside
the range of the present invention, and thus the microstructure is outside the
range of the present invention and the bendability is poor. With regard to
Steel number 26, the coiling temperature is outside the range of the present
25 invention, and thus the microstructure is outside the range of the present
invention, the strength is low, and the bendability is poor.
i 63
[0187] With regard to Steel number 28, the C content is outside the range
of the present invention, and thus the microstructure is outside the range of
the present invention, the strength is low, and the bendability is poor. With
regard to Steel number 29, the C content is outside the range of the present
5 invention, and thus the microstructure is outside the range of the present
invention, the strength is low, and the bendability is poor. With regard to
Steel number 30, the C content is outside the range of the present invention,
and thus the microstructure is outside the range of the present invention and
the elongation is poor.
10 [Industrial Applicability]
[0188] As has been described previously, according to the present
invention, it is possible to easily provide a steel sheet applicable to a member
required to have workability, hole expandability, bendability, strict sheet
thickness uniformity and circularity after working, and low-temperature
15 toughness (an inner sheet member, a structure member, an underbody member,
an automobile member such as a transmission, and members for shipbuilding,
construction, bridges, offshore structures, pressure vessels, line pipes, and
machine parts, and so on). Further, according to the present invention, it is
possible to manufacture a high-strength steel sheet having excellent
20 low-temperature toughness and 540 MPa class or more inexpensively, ^ d
stably. Thus, the present invention is the invention having high industrial
value.
[Explanation of Codes]
[0189] 1 continuous hot rolling line
25 2 roughing mill
3 finishing mill
«
64
4
5
6
10
11
hot-rolled steel sheet
run-out-table
rolling stand
inter-stand cooling nozzle
cooling nozzle 11

65
[Name of Document] What is claimed is
[Claim 1] A bainite-containing-type high-strength hot-rolled steel sheet
having excellent isotropic workability, comprising:
in mass%,
5 C: greater than 0.07 to 0.2%;
Si: 0.001 to 2.5%;
Mn: 0.01 to 4%;
P: 0.15% or less (not including 0%);
S: 0.03% or less (not including 0%);
10 N: 0.01 % or less (not including 0%);
Al: 0.001 to 2%; and
a balance being composed of Fe and inevitable impurities, wherein
an average value of pole densities of the {100}<011> to {223}<110>
orientation group represented by respective crystal orientations of
15 {100}<011>, {116}<110>, {114}<110>, {113}<110>, {112}<110>,
{335}<110>, and {223}<110> at a sheet thickness center portion being a
range of 5/8 to 3/8 in sheet thickness from the surface of the steel sheet is 4.0
or less, and a pole density of the {332}<113> crystal orientation is 4.8 or less,
an average crystal grain diameter is 10 |Lim or less and a Charpy fracture
20 appearance transition temperature vTrs is -20°C or lower, and
a microstructure is composed of 35% or less in a structural fraction of
pro-eutectoid ferrite and a balance of a low-temperature transformation
generating phase.
[Claim 2] The bainite-containing-type high-strength hot-rolled steel sheet
25 having excellent isotropic workability according to claim 1, frirther
comprising:
w 66
one type or two or more types of
in mass%,
Ti: 0.015 to 0.18%,
Nb: 0.005 to 0.06%,
5 Cu: 0.02 to 1.2%,
Ni: 0.01 to 0.6%,
Mo: 0.01 to 1%,
V: 0.01 to 0.2%, and
Cr: 0.01 to 2%.
10 [Claim 3] The bainite-containing-type high-strength hot-rolled steel sheet
having excellent isotropic workability according to claim 1, further
comprising:
one type or two or more types of
in mass%,
15 Mg: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%, and
REM: 0.0005 to 0.1%.
[Claim 4] The bainite-containing-type high-strength hot-rolled steel sheet
having excellent isotropic workability according to claim 1, fUrther
20 comprising:
in mass%,
B: 0.0002 to 0.002%.
[Claim 5] A manufacturing method of a bainite-containing-type
high-strength hot-rolled steel sheet having excellent isotropic workability,
25 comprising:
on a steel billet containing:
^
67
in mass%,
C: greater than 0.07 to 0.2%;
Si: 0.001 to 2.5%;
Mn: 0.01 to 4%;
5 P: 0.15% or less (not including 0%));
S: 0.03% or less (not including 0%);
N: 0.01%) or less (not including 0%);
Al: 0.001 to 2%; and
a balance being composed of Fe and inevitable impurities,
10 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;
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 Tl +
15 30°C nor higher than Tl + 200°C determined by Expression (1) below; and
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 a manner that a waiting time
period t second satisfies Expression (2) below;
20 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;
within three seconds after completion of the primary cooling, performing
secondary cooling in which cooling is performed at an average cooling rate of
25 15°C/second or more; and
after completion of the secondary cooling, performing air cooling for 1 to 20
68
seconds in a temperature region of lower than an Ar3 transformation point
temperature and an Arl transformation point temperature or higher and next
performing coiling at 450°C or higher and lower than 550°C.
Tl (°C) = 850 + 10 X (C + N) X Mn + 350 X Nb + 250 X Ti + 40 X B + 10 X
5 Cr+lOOxMo+lOOxV -(1)
Here, C, N, Mn, Nb, Ti, B, Cr, Mo, and V each represent the content of the
element (mass%).
t ^ 2.5 X tl - (2)
Here, tl is obtained by Expression (3) below.
10 tl = 0.001 X ((Tf - Tl) X Pl/100)^ - 0.109 x ((Tf - Tl) x Pl/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 PI
represents the reduction ratio of the final reduction at 30% or more.
[Claim 6] The manufacturing method of the bainite-containing-type
15 high-strength hot-rolled steel sheet having excellent isotropic workability
according to claim 5, wherein
the total of reduction ratios in a temperature range of lower than Tl + 30°C is
30% or less.
[Claim 7] The manufacturing method of the bainite-containing-type
20 high-strength hot-rolled steel sheet having excellent isotropic workability
according to claim 5, wherein
heat generation by working between respective passes in the temperature
region of not lower than Tl + 30°C nor higher than Tl + 200°C in the second
hot rolling is 18°C or lower.
25 [Claim 8] The manufacturing method of the bainite-containing-type
high-strength hot-rolled steel sheet having excellent isotropic workability
69
according to claim 5, wherein
the waiting time period t second further satisfies Expression (4) below.
t < t l - ( 4 )
[Claim 9] The manufacturing method of the bainite-containing-type
5 high-strength hot-rolled steel sheet having excellent isotropic workability
according to claim 5, wherein
the waiting time period t second further satisfies Expression (5) below.
tl ^ t ^ tl x 2 . 5 - ( 5)
[Claim 10] The manufacturing method of the bainite-containing-type
10 high-strength hot-rolled steel sheet having excellent isotropic workability
according to claim 5, wherein
the primary cooling is started between rolling stands.