[Name of Document] DESCRIPTION
[Title of the Invention] HOT-ROLLED STEEL SHEET AND
MANUFACTURING METHOD THEREOF
[Technical Field]
5 [0001] The present invention relates to a hot-rolled steel sheet and a
manufacturing method thereof. More specifically, the present invention
relates to a high-strength hot-rolled steel sheet excellent in stretch
flangeability and low-temperature toughness, and a manufacturing method
thereof.
10 [Background Art]
[0002] In order to suppress emission of carbon dioxide gas from an
automobile, reduction of weight of an automobile body is promoted by using
a high-strength steel sheet. Fusthel; in order to ensure safety of passengers,
the high-strength steel sheet has become widely used, in addition to a soft
15 steel sheet, for the automobile body. Furthermore, in order to promote
reduction of weight of the automobile body in future, it is necessary to
increase the strength level of the high-strength steel sheet more than before,
but the increase in strength of the steel sheet is generally accompanied by
deterioration of material properties such as formability (workability).
20 Therefore, how the strength is increased without deteriorating the material
properties is important in developing the high-strength steel sheet.
Particularly, a steel sheet used as a material of automobile members such as
an inner sheet member, a structure membel; and an underbody member is
required to have stretch flange workability, burring workability, ductility,
25 fatigue durability, impact resistance, col~osionre sistance, and so on according
to its usage. It is important how these material properties and high strength
propesty are ensured in a high-dimensional and well-balanced manner.
[0003] Further, the steel sheet used as the material of those members
needs to be improved also in low-temperature toughness so as to be resistant
to destruction even when being subjected to impact caused by collision or the
5 like after they are attached to the automobile as members after molding,
pal-titularly to secure the impact resistance in a cold district. This
low-temperature toughness is defined by vTrs (Charpy fiacture appearance
transition temperature) or the like. For this reason, it is also necessary to
consider the impact resistance itself of the above-described steel sheet.
10 That is, the steel sheet used as the material of parts including the
above-described members is required to have the low-temperature toughness
as a very important characteristic, in addition to excellent workability.
[0004] As for the improving method of the low-temperature toughness in
the high-strength steel sheet, its manufacturing methods are disclosed, for
15 example, in Patent Documents 1, 2, in which the low-temperature toughness
is improved by a method including a martensite phase adjusted in aspect ratio
as the main phase (Patent Document l), and a method of finely precipitating
carbide in ferrite having an average grain diameter of 5 to 10pm (Patent
Document 2).
20 However, in Patent Documents 1 and 2, nothing is mentioned about
the stretch flangeability and poor forming may be caused when applying the
steel sheet to a member that is to be subjected to burring. Further, also in a
steel pipe field and a thick plate field, there is knowledge about improvement
of the low-temperature toughness but the fomability as high as that of a thin
25 plate is not required, and there is similar concern.
[0005] As for the improving method of the stretch flangeability in the
high-strength steel sheet, a metal structure control method of a steel sheet for
improving local ductility is also disclosed, and that controlling inclusions,
making a single stlucture, and reducing the hardness difference among
structures are effective for the bendability and the stretch flangeability is
5 disclosed in Non-Patent Document 1. Further, a technique of improving the
strength, the ductility and the stretch flangeability by controlling the finishing
temperature of hot rolling, and the reduction ratio and the temperature range
of finish rolling, to promote the reclystallization of austenite, suppressing
development of a rolled texture, and randomizing the clystal orientations is
10 disclosed in Non-Patent Document 2.
It is conceivable to be able to improve the stretch flangeability by
uniformizing the metal structure and the rolled texture fsom Non-Patent
Documents 1, 2 in which, however, no consideration is made for
compatibility between the low-temperature toughness and the stretch
15 flangeability.
[0006] For compatibility between the stretch flangeability and the
low-temperature toughness is mentioned in Patent Document 3 which
discloses a technology of dispersing appropriate amounts of retained austenite
and bainite in a ferrite phase with controlled hardness and grain diameter.
20 However, it is a structure containing soft ferrite at 50% or more and is thus
difficult to respond to the demand for higher strength in recent years.
[Prior Ast Document]
[Patent Document]
[0007] Patent Document 1 : Japanese Laid-open Patent Publication No.
25 2011-52321
Patent Document 2: Japanese Laid-open Patent Publication No. 201 1-17044
Patent Document 3: Japanese Laid-open Patent Publication No. H7-252592
[Non-Patent Document]
[0008] Non-Patent Document 1: K. Sugimoto et al, "ISIJ International"
(2000) Vol. 40, p. 920
5 Non-Patent Document 2: Kishida, "Shinnittetsu giho" (1999) No. 371, p. 13
[Disclosure of the Invention]
[Problems to Be Solved by the Invention]
[0009] The present invention has been devised in consideration of the
above-described problems and its object is to provide a hot-rolled steel sheet,
10 in particular, a hot-rolled steel sheet having high strength and being excellent
in stretch flangeability and low-temperature toughness, and a manufacturing
method capable of stably manufacturing the steel sheet.
[Means for Solving the Problems]
[0010] The present inventors succeeded in manufacturing a steel sheet
15 excellent in stretch flangeability and low-temperature toughness by
optimizing the chemical colnposition and manufacturing conditions of a
high-strength hot-rolled steel sheet and controlling an texture and a
microstructure of the steel sheet. The gist thereof is as follows.
[0011] (1) A hot-rolled steel sheet including:
20 a chemical composition including: in mass%,
C: 0.01 to 0.2%;
Si: 0.001 to 2.5%;
Mn: 0.10 to 4.0%;
P: 0.10% or less;
S: 0.030% or less;
Al: 0.001 to 2.0%;
N: 0.01% or less;
Ti: (0.005 + 48114m + 48/32[S])% 5 Ti 5 0.3%;
Nb: 0 to 0.06%;
Cu: 0 to 1.2%;
Ni: 0 to 0.6%;
Mo: 0 tol%;
V: 0 to 0.2%;
Cr: 0 to 2%;
Mg: 0 to 0.01%;
Ca: 0 to 0.01%;
REM: 0 to 0.1%; and
B: 0 to 0.002%,
with a balance being composed of Fe and impurities;
an texture in which, at a central portion of a sheet thickness that is a
15 steel sheet poi-tion sectioned at a 318 thickness position and a 518 thickness
position of the sheet thickness from a surface of the steel sheet, an average
value of X-ray random intensity ratios of a group of {100}<011> to
{223}<110> orientations of a sheet plane is 6.5 or less and an X-ray random
intensity ratio of a {332)<113> ciystal orientation is 5.0 or less; and
20 a microst~ucture in which a total area ratio of tempered martensite,
martensite and lower bainite is more than 85 %, and an average crystal grain
diameter is 12.0 pm or less.
[0012] (2) The hot-rolled steel sheet according to (I), wherein
the chemical composition contains one or two or more selected from a
25 group consisting of: , in mass%,
Nb: 0.005 to 0.06%;
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%.
[0013] (3) The hot-rolled steel sheet according to (I) or (2), wherein the
chemical composition contains one or two or more selected fiom a group
consisting of: in mass%, Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%, and
REM: 0.0005 to 0.1%.
10 [0014] (4) The hot-rolled steel sheet according to any one of (1) to (3),
wherein the chemical composition contains, in mass%, B: 0.0002 to 0.002%.
[0015] (5) The hot-rolled steel sheet according to any one of (I) to (4),
including the microstructure in which assuming that an average value of
hardness is E (HVO.01) and a standard deviation is o (HVO.01) when
15 measuring the Vickers hardness at 100 points or more with a load of 0.098 N,
o (HVO.Ol)/E (HVO.01) is 0.08 or less.
[0016] (6) The hot-rolled steel sheet according to any one of (1) to (5),
including mechanical properties that an r value (rC) in a direction
pe~pendiculart o a rolling direction is 0.70 or more, and an r value (1.30) in a
20 direction 30" from the rolling direction is 1.10 or less.
[0017] (7) The hot-rolled steel sheet according to any one of (1) to (6),
including mechanical propelties that an r value (rL) in a rolling direction is
0.70 or more and an r value (r60) in a direction 60' from the rolling direction
is 1.10 or less.
25 [0018] (8) The hot-rolled steel sheet according to any one of (1) to (7),
including a plating layer provided on the surface of the steel sheet.
[0019] (9) A manufacturing method of a hot-rolled steel sheet by
sequentially performing rough hot rolling, finish hot rolling, primary cooling
and secondaiy cooling on a slab including the chemical composition
according to any one of (1) to (7), and coiling a resultant slab into the
5 hot-rolled steel sheet, wherein:
the finish hot rolling is hot rolling in which with respect to a
temperature T1 defined in a following expression (I), a maximum reduction
ratio per pass in a first temperature region of (TI + 30)"C or higher and (TI +
200)"C or lower is 30% or more, a total reduction ratio in the first temperature
10 region is 50% or more, a total reduction ratio in a second temperature region
of TI°C or higher and lower than (TI + 30)"C is 0 to 30%, and the rolling is
completed in the first temperature region or the second temperature region;
the primary cooling is water cooling that satisfies a following
expression (2) and achieves a cooling amount of 40°C or higher and 140°C or
15 lower;
the secondaly cooling is water cooling that is stal-ted within thee
seconds after the primary cooling and performs cooling at an average cooling
rate of 30 "Clsec or higher; and
the coiling is to coil the slab at a temperature CT satisfying a
20 following expression (3),
TI ("C) = 850 + 10X(C + N)XMn + 350XNb + 250XTi + 40XB
+10XCr+ 100XMo+ 100XV ... (1)
1 5 tltl 5 2.5 . . . (2)
CT ("C) 5 max [Ms, 3501 ... (3)
25 tl = 0.001 X {(Tf - T l ) x ~ 1 / 1 0 0 }- ~0. 109X {(Tf - T1)XP1/100) +
3.1 . . . (4)
Ms("C)=561-474XC-33XMn- 17XNi-21XMo . .. (5)
where in the expression (1) and the expression (5), a symbol of each
element is a content (mass%) of the element in the steel,
in the expression (2), t is a time period (sec) fiom a final reduction in
5 the reduction in one pass at 30% or more in the first temperature region to
start of the primary cooling, and tl is a time period (sec) decided by the above
expression (4),
in the expression (3), max[ ] is a function of returning a maximum
value among arguments, and Ms is a temperature decided by the above
10 expression (5), and
in the expression (4), Tf and P1 are a steel sheet temperature and a
reduction ratio (%) in the final reduction in the reduction in one pass at 30%
or more in the first temperature region respectively.
[0020] (10) The manufacturing method of the hot-rolled steel sheet
15 according to (9), wherein the rough hot rolling achieves a maximum reduction
ratio per pass in a temperature region of 1000°C or higher and 1200°C or
lower of 40% or more, and an austenite average grain diameter of 200 pm or
less.
[0021] (11) The manufacturing method of the hot-rolled steel sheet
20 according to (9) or (lo), wherein a maximum heat generation due to plastic
deformation in a temperature region of (TI + 30)"C or higher and (TI +
150)"C or lower of the finish hot rolling is 18°C or lower.
[0022] (12) A manufacturing method of a hot-rolled steel sheet including:
performing a plating treatment on the surface of the hot-rolled steel sheet
25 obtained by the manufacturing method of the hot-rolled steel sheet according
to any one of (9) to (11).
[Effect of the Invention]
[0023] According to the present invention, it is possible to provide a
hot-rolled steel sheet, in particular, a high-strength steel sheet excellent in
stretch flangeability and low-temperature toughness. Use of the steel sheet
5 makes it possible to easily work the high-strength steel sheet and withstand
use in severe cold districts, thereby providing significant industrial
contribution.
[Mode for Carsying out the Invention]
[0024] Hereinafter, the content of the present invention will be explained
lo in detail.
[0025] Including an texture in which, at a central portion of a sheet
thickness that is a steel sheet poltion sectioned at a 318 thickness position and
a 518 thickness position of the sheet thickness from a surface of the steel sheet,
an average value of X-ray random intensity ratios of a group of {100}<011>
15 to {223}<110> orientations of a sheet plane is 6.5 or less and an X-ray
random intensity ratio of a {332}<113> crystal orientation is 5.0 or less:
The definitions of the X-ray random intensity ratios are particularly
impostant in the present invention.
The X-ray diffraction of the sheet plane is performed at the central
20 poltion of the sheet thickness that is the steel sheet portion sectioned at the
318 thickness position and the 518 thickness position of the sheet thickness
from the surface of the steel sheet, and the average value of X-ray random
intensity ratios of the group of the {100}<011> to {223}<110> orientations,
when the intensity ratios of orientations of a standard sample (random
25 sample) that has no specific crystal orientation but has random crystal
orientations are obtained, is set to 6.5 or less, thereby making it possible to
ensure excellent stretch flangeability satisfying a hole expansion ratio 2
140% and a tensile strength x hole expansion ratio 2 100000 MPa . % in a
material of a strength of 590 MPa level, a hole expansion ratio 2 90% and a
tensile strength x hole expansion ratio 2 70000 MPa % in a material of a
5 strength of 780 MPa level, and a hole expansion ratio 2 40% and a tensile
strength x hole expansion ratio 2 50000 MPa . % in a material of a strength
of 980 MPa level or more. Note that the average value of the X-ray random
intensity ratios of the group of the (1 00)<011> to (22314 lo> orientations of
the sheet plane is preferably 4.0 or less.
10 [0026] When the average value of the X-ray random intensity ratios of
the group of the {100)<011> to {223)<110> orientations is more than 6.5, the
anisotropy of the mechanical propel-ties of the steel sheet extremely increases,
so that the stretch flangeability in a specific direction improves but the stretch
flangeability in directions different thereftom significantly decreases,
15 resulting in difficulty in obtaining mechanical propelties satisfying the
aforementioned hole expandability. On the other hand, when the average
value of the X-ray random intensity ratios of the group of the {100)<011> to
{223)<110> orientations of the sheet plane becomes less than 0.5, which is
difficult to achieve in a current general continuous hot rolling process,
20 deterioration of the hole expandability is concerned. Accordingly, it is
preferable to set the average value of the X-ray random intensity ratios of the
group of the {100)<011> to {223)<110> orientations of the sheet plane to 0.5
or more.
[0027] Here, the average value of the X-ray random intensity ratios of the
25 group of the {100)<011> to {223)<110> orientations of the sheet plane is
obtained by arithmetically averaging the X-ray random intensity ratios of
{100)<011>, {116)<110>, {114}110, {113)<110>, {112)<110>,
(33514 lo>, and {223)<110> orientations.
[0028] The X-ray random intensity ratios of the orientations are measured
using an apparatus for X-ray diffraction, EBSD (Electron Back Scattering
5 Diffraction) or the like. It is only necessary to obtain from a
thee-dimensional texture calculated by a vector method on the basis of a
(110) pole figure, or from a three-dimensional texture calculated by a series
expansion method using a plurality (preferably thee or more) of pole figures
among {110), {loo), (2111, (3 10) pole figures.
10 [0029] For example, for the X-ray random intensity ratio of each of the
above-described clystal orientations in the latter method, each of intensities of
(001)[1-1 01, (116)[1-1 01, (114)[1-1 01, (113)[1-1 01, (112)[1-1 01, (335)[1-1
01, (223)[1-1 01 at a $2 = 45" cross-section in the three-dimensional texture
may be used as it is. (1 with an upper bar indicating "minus 1" is expressed
15 withu-I".)
[0030] As described above, the average value of the X-ray random
intensity ratios of the group of the {100)<011> to {223)<1 lo> orientations of
the sheet plane means the arithmetic average of the X-ray random intensity
ratios of the above-described orientations, and may be replaced with the
20 arithmetic average of the X-ray random intensity ratios of the {100)<011>,
{116)<110>, {114)<110>, {112)<110>, and {223)<110> orientations when
it is impossible to obtain the X-ray random intensity ratios of all of the
above-described orientations.
[0031] Further, for the same reason, when the X-ray random intensity
25 ratio of the {332)<113> clystal orientation of the sheet plane is 5.0 or less
(desirably 3.0 or less) at the central portion of the sheet thickness that is the
steel sheet pol-tion sectioned at the 318 thickness position and the 518
thickness position of the sheet thickness from the surface of the steel sheet,
the tensile strength x hole expansion ratio 2 50000 that is required to work
an undesbody pait to be required immediately is satisfied. Fui-ther, the
5 above-described X-ray random intensity ratio of the {332)<113> clystal
orientation is preferable 3.0 or less.
When the above-described X-ray random intensity ratio of the
{332)<113> clystal orientation is more than 5.0, the anisotropy of the
mechanical propelties of the steel sheet extremely increases, so that the
10 stretch flangeability in a specific direction improves but the stretch
flangeability in directions different therefi-om significantly decreases to
decrease the hole expansion ratio. On the other hand, when the
above-described X-ray random intensity ratio of the {332)<113> clystal
orientation becomes less than 0.5, which is difficult to achieve in the current
15 general continuous hot rolling process, deterioration of the hole expandability
is concerned. Accordingly, it is preferable to set the above-described X-ray
random intensity ratio of the {332)<113> crystal orientation to 0.5 or more.
[0032] The reason why the above-described X-ray random intensity ratio
of the crystal orientation is impostant for improving the hole expandability is
20 not exactly clear but is presumed to be related to slip behavior of crystal in the
hole expansion working.
[0033] With regard to the sample to be subjected to the X-ray diffraction,
it is only necessaly to reduce the steel sheet in thickness to a predetermined
sheet thickness from the surface by mechanical polishing or the like, then
25 remove its strain by chemical polishing, electrolytic polishing or the like, and
at the same time, adjust the sample in accordance with the above-described
method so that an appropriate plane in the range of 318 to 518 of the sheet
thickness becomes a measuring plane, and then perform measurement.
[0034] As a matter of course, limitation of the above-described X-ray
intensity is satisfied not only in the vicinity of 112 of the sheet thickness, but
5 also in as many thicknesses as possible, whereby the hole expandability is
further improved. However, the central portion of the sheet thickness that is
the steel sheet portion sectioned at the 318 thickness position and the 518
thickness position of the sheet thickness from the surface of the steel sheet is
measured to thereby make it possible to generally represent the material
10 propesties of the entire steel sheet, and is therefore defined.
Incidentally, a crystal orientation represented by {hkl} means
that the normal direction to the sheet plane is parallel to and the rolling
direction is parallel to .
[0035] An r value (rC) in a direction perpendicular to the rolli~lgd irection
15 is 0.70 or more, and an r value (1-30) in a direction 30" from the rolling
direction is 1.10 or less:
Satisfying the following mechanical properties in addition to the
above-described texture makes it possible to ensure more excellent stretch
flangeability. Accordingly, it is preferable to satisfy the following
20 mechanical properties.
[0036] The r value (rC) in the direction perpendicular to the rolling
direction:
The rC is preferably 0.70 or more. Note that the upper limit of the r
value is not set in particular, but the rC set to 1.10 or less is preferable
25 because more excellent hole expandability can be obtained.
[0037] The r value (1-30) in the direction 30" from the rolling direction:
The r30 is preferably 1.10 or less. Note that the lower limit of the r
value in the direction is not set in particular, but the r30 set to 0.70 or more is
preferable because more excellent hole expandability can be obtained.
[0038] An r value (rL) in the rolling direction is 0.70 or more and an r
5 value (r60) in a direction 60' from the rolling direction is 1.10 or less:
Satisfying the following mechanical properties in addition to the
above-described texture makes it possible to ensure more excellent stretch
flangeability. Accordingly, it is preferable to satisfy the following
mechanical properties.
10 [0039] The r value (rL) in the rolling direction:
The rL is preferably 0.70 or more. Note that the upper limit of the rL
value is not set in particular, but the rL set to 1.10 or less is preferable because
more excellent hole expandability can be obtained.
[0040] The r value (r60) in the direction 60' from the rolling direction:
15 The r60 is preferably 1.10 or less. Note that the lower limit of the
r60 value is not set in particular, but the r60 set to 0.70 or more is preferable
because more excellent hole expandability can be obtained.
[0041] The above-described r values are each evaluated by a tensile test
using a JIS No. 5 tensile test piece. Tensile strain only has to be evaluated
20 usually in a range of 5 to 15% in the case of a high-strength steel sheet, and in
a range of uniform elongation.
[0042] A microstructure of the steel sheet:
First, the average clystal grain diameter and the identification method
of sttucture will be described.
25 In the present invention, average crystal grain diameter, fessite, and
retained austenite are defined using the EBSP-OIM (Electron Back Scatter
Diffsaction Pattern-Orientation Image Microscopy, trademark) method.
[0043] The EBSP-OIM method is constituted by a device and software of
irsadiating a highly inclined sample with electron beams in a scanning
electron microscope (SEM), photographing a Kikuchi pattern formed by
5 backscattering by a high-sensitive camera and subjecting it to computer
image-processing to thereby measure a crystal orientation at the irradiation
point within a short period of time. The EBSP method enables a quantitative
analysis of a fine structure and a clystal orientation of a bulk sample surface,
and can analyze them in an analysis area capable of being obse~ved by the
10 SEM with a resolution of 20 nm at a minimum though it depends on the
resolution of the SEM. The analysis is performed for several hours by
mapping an area to be analyzed for tens of thousands points in a grid state at
regular intervals.
In addition to that the phase can be identified from the structure of the
15 clystal orientation, it is possible to see the crystal orientation distribution and
the size of the clystal grain within the sample in a polycrystalline material.
It is possible to calculate a misorientation between adjacent measurement
points !kom measurement information, and the average value thereof is called
a KAM (Kernel Average Misorientation) value.
20 [0044] In the present invention, fsom an image obtained by mapping the
misorientation of the crystal grain defined as 15' being a thseshold value of a
high-angle tilt grain boundary generally recognized as a crystal grain
boundary, a grain is visualized to find an average crystal grain diameter.
Further, a stmcture in which an average of the KAM value in a crystal grain
25 surrounded by the high-angle tilt grain boundary of 15" is within 1" is defined
as fer-site. This is because the fel-site is a high-temperature transformation
phase and has small transformation strain. Ful-ther, a stlucture identified as
austenite by the EBSP method is defined as retained austenite.
[0045] Tempered martensite or lower bainite defined in the present
invention means a structure that transfoims from the austenite at an Ms point
s or lower when the Ms point is higher than 350°C, or at 350°C or lower when
the Ms point is 350°C or lower, and when the structure is observed under
TEM, cementite or metastable iron carbide precipitates in a mutli-variant state
in the same lath.
On the other hand, a structure in which cementite or metastable iron
10 carbide precipitates in a single-variant state in the same lath is defined as
upper bainite. It is conceivable that this is because the driving force for
precipitation of the cementite is lower than that of the tempered martensite or
the lower bainite.
Similarly, a structure, in which precipitation of cementite or
15 metastable iron carbide is not observed when the structure is observed under
TEM, is defined as maltensite.
Note that structural fiaction of them is obtained by taking TEM
photographs in 10 or more visual fields at 20000 magnifications and using the
point counting method.
20 [0046] Though, in the high-strength steel sheet, a single-phase or
dual-phase structure such as precipitation strengthened ferrite, bainite,
mastensite and the like is used to enhance its strength, the inventors found as a
result of an earnest study that when the stlucture is made to have a total area
ratio of the tempered maltensite, martensite and lower bainite of or more than
25 85 area% and an average clystal grain diameter of 12.0 pn or less, more
preferably, to have a hardness difference among the structures decreased to a
certain level or less, the stress concentration on the structure interface is
decreased to improve the stretch flangeability and the low-temperature
toughness. A structure having a sum of fractions of the tempered martensite
structure and the lower martensite of more than 85% has excellent balance
5 between strength and elongation and is thus more preferable. When the
average crystal grain diameter is more than 12.0 pm, it is difficult to ensure
excellent low-temperature toughness satisfying vTrs 5 - 40 "C.
Note that since there occurs no deterioration of the stretch
flangeability or the low-temperature toughness even if these stiuctures occupy
10 100% of the steel sheet, the upper limit of the structural fiaction is not
specified.
In the case of attaching importance to the improvement of ductility,
the ferrite may be contained at less than 15% in area ratio.
[0047] As for the hardness difference among stsuctures, assuming that an
15 average value of the hardness when measuring the Vickers hardness at 100
points or more using a micro Vickers with a load of 0.098 N (10gf) is E
(HVO.01) and a standard deviation of the hardness is o (HVO.Ol), it is
preferable to set o (HVO.Ol)/E (HVO.01) to 0.08 or less and contain the ferrite
at 5 area% or more, because excellent mechanical properties can be obtained
20 which achieve both the stretch flangeability and a total elongation satisfying a
tensile strength x hole expansion ratio 2 55000 MPa % and a tensile
strength x total elongation 1 14000 MPa . % and vTrs 2 - 40°C at a
tensile strength of 980 MPa level or more. Fui-ther, it is preferable to set the
above o (HVO.Ol)/E (HVO.01) to 0.06 or less because excellent mechanical
25 propelties can be obtained which achieve the stretch flangeability satisfying a
tensile strength x hole expansion ratio 2 60000 MPa . % and vTrs 5 -
40°C at a tensile strength of 980 MPa level or more. Setting the above o
(HVO.Ol)/E (HVO.01) to 0.08 or less decreases the fact that the interface
between the hard structure and the soft structure when observing the Charpy
fracture surface is the starting point of a crack, which can be presumed to be a
5 cause of the improvement of vTrs.
[0048] The lower limit of the o (HVO.Ol)iE (HVO.01) is not set in
pasticular, but is generally 0.03 or more.
[0049] A chemical composition of the steel sheet:
Next, the reason of limiting the chemical composition of the hot-rolled
10 steel sheet in the present invention will be described. Note that "%"
indicating the content means "mass%."
[0050] C: 0.01 to 0.2%
C (carbon) is an element having an action of improving the strength of
the steel sheet. When the C content is less than 0.01%, it is difficult to
15 obtain the effect by the above-described action. Accordingly, the C content
is set to 0.01% or more. On the other hand, when the C content is more than
0.2%, a decrease of ductility is caused, and the iron-based carbide such as
cementite (Fe3C) to be the starting point of cracking in a secondary shear
surface at the time of punching is increased to cause deterioration of the
20 stretch flangeability. Therefore, the C content is set to 0.2% or less.
[0051] Si: 0.001% to 2.5%
Si (silicon) is an element having an action of itnproving the strength of
the steel sheet and also perfor~ns a function as a deoxidizes of molten steel.
When the Si content is less than 0.001%, it is difficult to obtain the effect by
25 the above-described action. Therefore, the Si content is set to 0.001% or
tnore. Further, Si also has an action of suppressing the precipitation of the
iron-based carbide such as cementite and thereby improving the strength and
the hole expandability. From this viewpoint, the Si content is set to 0.1% or
more. On the other hand, even if the Si content is set to more than 2.5%, the
effect by the action of increasing the strength of the steel sheet is saturated.
5 Therefore, the Si content is set to 2.5% or less. Note that fsom the viewpoint
of effectively improving the strength and the hole expandability by
suppressing the precipitation of the iron-based carbide such as cementite, it is
preferable to set the Si content to 1.2% or less.
[0052] Mn: 0.10 to 4.0%
10 Mn (manganese) has an action of improving the strength of the steel
sheet by solid-solution strengthening and quench-hardening strengthening.
When the Mn content is less than 0.10%, it is difficult to obtain the effect by
the above-described action. Therefore, the Mn content is set to 0.10% or
more. Fui-thes, Mn has an action of expanding the austenite region
15 temperature to the low temperature side and thereby improving the
hardenability to facilitate formation of a low-temperature transformation
stiucture having an excellent burring property such as mastensite or lower
bainite. From this viewpoint, the Mn content is preferably set to 1% or more,
and more preferably 2% or more. Further, Mn also has an action of
20 suppressing occurrence of hot cracking caused by S. From this viewpoint, it
is preferable to contain the Mn amount ensuring that the Mn content ([Mn])
and the S content ([S]) satisfy [Mn]/[S] 2 20. On the other hand, even if
the Mn content is set to more than 4.0%, the effect by the action of improving
the strength of the steel sheet is saturated. Therefore, the Mn content is set
25 to 4.0% or less.
[0053] P: 0.10% or less
P (phosphorus) is an element generally contained as an impurity.
When the P content is more than 0.10%, P causes cracking in the hot rolling,
and is segregated at a grain boundary to decrease the low-temperature
toughness and also decrease the workability and the weldability. Therefore,
5 the P content is set to 0.10% or less. From the viewpoint of the hole
expandability and the weldability, the P content is preferably set to 0.02% or
less.
[0054] S: 0.030% or less
S (sulfur) is an element generally contained as an impurity. When
10 the S content is more than 0.030%, S causes cracking in the hot rolling, and
generates an A-based inclusion in the steel to deteriorate the hole
expandability. Therefore, the S content is set to 0.030% or less. From the
viewpoint of the hole expandability, the S content is preferably set to 0.010%
or less, and more preferably set to 0.005% or less.
15 [0055] Al: 0.001 to 2.0%
A1 (aluminum) has an action of deoxidizing molten steel in a refining
process of the steel to sound the steel. When the A1 content is less than
0.001%, it is difficult to obtain the effect by the above-described action.
Therefore, the A1 content is set to 0.001% or more. A1 ful-ther has, similarly
20 to Si, an action of suppressing the precipitation of the iron-based carbide such
as cementite and thereby improving the strength and hole expandability.
From this viewpoint, the A1 content is preferably set to 0.016% or more. On
the other hand, even if the A1 content is set to more than 2.0%, the effect by
the deoxidation action is saturated, resulting in economic disadvantage.
25 Further, A1 may cause cracking in the hot rolling. Therefore, the A1 content
is set to 2.0% or less. From the viewpoint of suppressing generation of a
non-metal inclusion in the steel to improve the ductility and the
low-temnperature toughness, the A1 content is preferably set to 0.06% or less.
the A1 content is more preferably 0.04% or less.
[0056] N: 0.01% or less
5 N (nitrogen) is an element generally contained as an impurity. When
the N content is more than 0.01%, N causes cracking in the hot rolling, and
deteriorates the aging resistance. Therefore, the N content is set to 0.01% or
less. From the viewpoint of the aging resistance, the N content is preferably
0.005% or less.
10 [0057] Ti: (0.005 + 48114M + 48/32[S])% 5 Ti S 0.3%:
Ti (titanium) is an element having an action of improving the strength
of the steel sheet by precipitation strengthening or solid-solution
strengthening. When the Ti content is less than (0.005 + 48114M +
48/32[S])% that is decided by the N content M (unit: %) and the S content
15 [S] (unit: %), it is difficult to obtain the effect by the above-described action.
Therefore, the Ti content is set to (0.005 + 48114M + 48/32[S])% or more.
On the other hand, even if the Ti content is set to more than 0.3%, the effect
by the above-described action is saturated, resulting in economic
disadvantage. Therefore, the Ti content is set to 0.3% or less.
20 [0058] Nb, Cu, Ni, Mo, V, Cr:
Nb (niobium), Cu (copper), Ni (nickel), Mo (molybdenum), V
(vanadium) and Cr(chrornium) are elements each having an action of
improving the strength of the steel sheet by solid-solution strengthening or
quench-hardening strengthening. Therefore, one or two or more of the
25 elements can be appropriately contained as necessary. However, even if the
Nb content is set to more than 0.06%, the Cu content is set to more than 1.2%,
the Ni content is set to more than 0.6%, the Mo content is set to more than 1%,
the V content is set to more than 0.2%, and the Cr content is set to more than
2%, the effect by the above-described action is saturated, resulting in
economic disadvantage. Therefore, the Nb content is set to 0 to 0.06%, the
5 Cu content is set to 0 to 1.2%, the Ni content is set to 0 to 0.6%, the Mo
content is set to 0 to 1%, the V content is set to 0 to 0.2%, and the Cr content
is set to 0 to 2%. Note that to surely obtain the effect by the above-described
action, it is preferable to satisfy any one of Nb: 0.005% or more, Cu: 0.02%
or more, Ni: 0.01% or more, Mo: 0.01% or more, V: 0.01% or more, and Cr:
10 0.01% or more.
[0059] Mg, Ca, E M :
Mg (magnesium), Ca (calcium), and REM (rare-earth element) are
elements each having an action of controlling the form of the non-metal
inclusion being the starting point of fracture to cause deterioration of the
15 workability and thereby improving the workability. Therefore, one or two or
more of the elements tnay be appropriately contained as necessaly. However,
even if the Mg content is set to more than 0.01%, the Ca content is set to more
than 0.01%, and the REM content is set to more than 0.1%, the effect by the
above-described action is saturated, resulting in economic disadvantage.
20 Therefore, the Mg content is set to 0 to 0.01%, the Ca content is set to 0 to
0.01%, and the REM content is set to 0 to 0.1%. Note that to surely obtain
the effect by the above-described action, it is preferable to set the content of
any one of the elements Mg, Ca and REM to 0.0005% or more.
[0060] B:
25 B (boron) is an element that is segregated at the grain boundaly
similarly to C and has an action of increasing the grain boundary strength.
That is, B is segregated at the grain bounda~y as a solid-solution B similarly
to the solid-solution C and thereby effectively acts to realize prevention of the
kacture surface cracking. Further, even if C precipitates in the grain as
carbide to decrease the solid-solution C at the grain bounda~y, B is segregated
5 at the grain bounda~ya nd thereby can compensate for the decrease of C at the
grain bounda~y. Therefore, B may be appropriately contained as necessaly.
However, when the B content is set to more than 0.002%, rec~ystallizationo f
austenite in the hot rolling is excessively suppressed and a y to a
transformation texture from non-recrystallized austenite is strengthened to
10 deteriorate the isotropy. Therefore, the B content is set to 0 to 0.002% or
less. B is an element that may cause slab cracking in a cooling process after
continuous casting and, from this viewpoint, is preferably set to 0.0015% or
less. Note that to surely obtain the effect by the above-described action, the
B content is preferably set to 0.0002% or more. Further, B also has an
15 action of improving the hardenability, and facilitating formation of a
continuous cooling transformation structure being a microstructure that is
preferable for the burring property.
[0061] The balance is composed of iron (Fe) and impurities.
As the impurities, Zr, Sn, Co, Zn, W are contained in some cases, and
20 there is no problem as long as the total of the contents of these elements is 1%
or less.
[0062] Surface treatment:
A plating layer intended to improve co~~osiorens istance and so on is
provided on the surface of the above-described steel sheet to make a surface
25 treated steel sheet. The plating layer may be an electroplating layer or a
hot-dip plating layer. Examples of the electroplating layer include
electrogalvanizing, Zn-Ni alloy electroplating and so on. Examples of the
hot-dip plating layer include hot-dip galvanizing, alloying hot-dip galvanizing,
hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg
alloy plating, hot-dip Zn-Al-Mg-Si alloy plating and so on. A plating
5 adhesion amount is not limited in pasticular but may be similar to that in the
prior ai-t. Furthel; it is also possible to perform an appropriate conversion
treatment (for example, application and drying of a silicate-based
chromium-fsee conversion treatment liquid) after the plating to fusther
increase the col~osion resistance. Further, it is also possible to perform
10 organic coating forming, film laminating, and organic saltslinorganic salts
treatments.
100631 A manufacturing method of the hot-rolled steel sheet:
Next, the manufacturing method of the hot-rolled steel sheet of the
present invention will be described.
15 To realize the excellent stretch flangeability and low-temperature
toughness, it is important to form a predetermined texture and make a
structure mainly containing the tempered mastensite, mastensite and lower
bainite. Furthel; it is preferable that the hardness difference among
structures is small and the r value in each direction satisfies a predetermined
20 condition. The details of the manufacturing conditions for satisfying them
will be listed below.
[0064] The manufacturing method prior to the hot rolling is not
particularly limited. That is, it is only necessaly to perform, subsequent to
melting steel by a shaft furnace, an electric hrnace or the like, various kinds
25 of secondaly refining to adjust the steel so as to have the above-described
chemical composition, then cast it into a steel ingot or a slab by a method
such as normal continuous casting, casting by an ingot method, other thin slab
casting and so on. In the case of the continuous casting, the steel may be
cooled once to a low temperature and then reheated and subjected to hot
rolling, or a cast slab may be continuously hot-rolled. As a raw material,
5 scraps may be used.
[0065] The high-strength steel sheet excellent in sketch flangeability and
low-temperature brittleness of the present invention is obtained in the case of
satisfying the following requirements.
[0066] In order to set, to the above-described value ranges, the average
10 value of the X-ray random intensity ratios of the group of the {100)<011> to
{223)<110> orientations of the sheet plane and the X-ray random intensity
ratio of the {332}<113> clystal orientation, at the central poltion of the sheet
thickness located between the 518 and 318 thickness positions of the sheet
thickness from the surface of the steel sheet, in finish rolling after rough
15 rolling, on the basis of a TI temperature decided from the following
expression (1) from the steel sheet components,
T1 ("C)=850+ lOX(C+N)XMn+350XNb+250XTi+40XB +
10XCr+ 100XMo+ 100 X V ... (1)
working by heavy reduction rolling is performed at a large reduction ratio in a
20 first temperature region of (T1 + 30) "C or higher and (T1 + 200) "C or lower,
then reduction is not performed or working by soft reduction rolling is
performed at a small reduction ratio in a second temperature region of TI°C
or higher and lower than (TI + 30) "C, and the rolling is completed in the first
temperature region or the second temperature region, thereby ensuring local
25 deformability of a final product.
[0067] That is, by the high reduction rolling in the first temperature
region of (T1 + 30)"C or higher and (TI + 200)"C or lower and the rolling
completion in the first temperature region, or by the high reduction rolling in
the first temperature region and the subsequent low reduction rolling in the
second temperature region of TI or higher and lower than (TI + 30)"C and
5 the rolling completion in the second temperature region, the average value of
the X-ray random intensity ratios of the group of the {100)<011> to
{223)<110> orientations of the sheet plane and the X-ray random intensity
ratio of the {332}<113> clystal orientation, at the central poltion of the sheet
thickness sectioned at the 518 thickness position and the 318 thickness position
10 of the sheet thickness fsom the surface of the steel sheet can be controlled as
are found in later-described Tables 2,3, whereby the hole expandability of the
final product is drastically improved.
[0068] The TI temperature itself can be obtained by the empirical
expression indicated in the above expression (I). The inventors
15 experimentally found fsom experiments that the recrystallization in the
austenite region of each steel is promoted on the basis of the TI temperature.
To obtain more excellent hole expandability, it is impostant to
accumulate strain by the heavy reduction in the first te~nperaturer egion, and it
is essential to set the maximum reduction ratio per pass in the first
20 temperature region to 30% or more, in other words, perform reduction in one
pass at a reduction ratio of 30% or more in the first temperature region at least
one or more times and set the total of reduction ratios to 50% or more.
Further~nore,i t is more preferable to set the total of reduction ratios to 70% or
more. On the other hand, setting the total of reduction ratios to more than
25 90% places a securement of temperature and a burden of excessive rolling,
and therefore it is preferable to set the total of reduction ratios to 90% or less.
[0069] Further, to promote uniform crystallization by releasing the
accumulated strain, it is necessary to suppress as much as possible the
working amount in the second temperature region of TI°C or higher and
lower than (TI + 30)"C, after the heavy reduction in the first temperature
5 region of (TI + 30)"C or higher and (TI + 200)"C or lower, and the total of
reduction ratios in the second temperature region of T1 "C or higher and lower
than (T1 + 30)OC is set to 0 to 30%. When the total of reduction ratios in the
second temperature region is more than 30%, the finally crystallized austenite
grain expands, and when the retention time period is short, recrystallization
10 does not sufficiently proceed, resulting in deterioration of the hole
expandability. Note that fsom the viewpoint of securing an excellent sheet
shape, it is desirable to set the reduction ratio to 10% or more, but in the case
of attaching more importance to the hole expandability, it is desirable to set
the reduction ratio to 0%, namely, not to perfol-m the low reduction rolling in
15 the second temperature region.
[0070] As described above, the manufacturing method of the present
invention is a method of controlling the texture of a product to improve its
hole expandability by uniformly and finely recrystallizing the austenite in the
finish rolling.
20 [0071] When .the rolling is perfolmed at a temperature lower than the
second temperature region or the rolling at the large reduction ratio is
performed in the second temperature region, the texture of the austenite grows
to make it difficult to obtain the above-described predetermined texture in the
finally obtained steel sheet. On the other hand, when the rolling is
25 completed at a temperature higher than the first temperature region or the
rolling at a small reduction ratio is perfolmed in the first temperature region,
coarsening and grain mixture become Inore likely to occur.
[0072] Note that as for whether the above-described defined rolling is
performed or not, the reduction ratio can be obtained by actual results or
calculation from the rolling load, sheet thickness measurement and the like,
5 and the temperature can be actually measured when an inter-stand
thermometer is installed or can be obtained by a calculation simulation in
consideration of heat generation by working from the line speed or the
reduction ratio or both of them.
[0073] The time period fiom final reduction in the reduction in one pass
10 at 30% or more in the first temperature region to the sta1-t of primary cooling
being water cooling greatly influences the stretch flangeability and the
low-temperature toughness.
The time period t (sec) from the final reduction pass in one pass at
30% or more in the first temperature region to the stal-t of the primary cooling
15 is set to satisfy the following expression (2) with respect to a steel sheet
temperature Tf ("C) and a reduction ratio P1 (%) in the final reduction in one
pass at 30% or more in the first temperature region.
When tltl is less than 1, the rec~ystallization is suppressed to fail to
obtain the predetermined texture, and when tltl is more than 2.5, coarsening
20 proceeds to significantly decrease the elongation and the low-temperature
brittleness.
1 2 tltl 5 2.5 . . . (2)
In the expression, tl is the time period (sec) decided by the following
expression (4).
25 tl = 0.001 X {(Tf - ~ 1 ) ~ ~ 1 / 1 0 -0 0}.1' 09X {(Tf - Tl)XP1/100} +
3.1 . . . (4)
[0074] A primary cooling amount that is the difference between the steel
sheet temperature at the start of cooling in the primary cooling and the steel
sheet temperature at the completion of the cooling (cooling temperature
change) is set to 40°C or higher and 140°C or lower. When the primary
5 cooling amount is lower than 40°C, it is difficult to suppress coarsening of the
austenite grain, resulting in deterioration of the low-temperature toughness.
On the other hand, when the primary cooling amount is more than 140°C, the
recrystallization becomes insufficient to make it difficult to obtain the
predetermined texture. Note that fiom the viewpoint of suppressing the
10 coarsening of the austenite grain, it is preferable to set the average cooling
rate in the primary cooling to 30 OC/sec or higher. It is unnecessaiy to limit
the upper limit of the average cooling rate in the primaly cooling in pal-ticular,
but it is preferable to set the average cooling rate to 2000 OC/sec or lower.
[0075] Cooling is started within three seconds after the primary cooling is
15 performed, to perform secondaly cooling of water-cooling at an average
cooling rate of 30 "Clsec or higher. Here, the secondary cooling means
water-cooling performed from the stai-t of the secondaiy cooling until the stai-t
of coiling, and the average cooling rate of the secondaiy cooling is the
average cooling rate in the water cooling and is calculated excluding the
20 period of suspending the water cooling in the case of suspending the water
cooling at the middle of the secondaiy cooling as described later.
From the completion of the primary cooling until the stai-t of the
secondary cooling, the steel sheet is kept in the high temperature region
because the water cooling is not performed. If the secondary cooling is
25 started after more than three seconds after the primary cooling is performed or
if the secondaiy cooling is perforined at an average cooling rate lower than 30
"Clsec within thee seconds after the pritnaly cooling is performed, the
structural fsaction of the high-temperature transformation phase such as ferrite,
pearlite, upper bainite becomes more than 15% during the secondary cooling
from the completion of the finish rolling until the start of coiling to fail to
5 obtain the desired structural fraction and the hardness difference among
stluctures, resulting in deterioration of the low-temperature toughness in
particular. The upper limit of the average cooling rate in the secondary
cooling is not particularly set, but a rate of 300 OCIsec or lower is the
adequate average cooling rate in terms of ability of the cooling facility.
10 [0076] In the case of attaching the importance to the improvement of
ductility and thus containing ferrite at 15% or less in area ratio, the water
cooling may be suspended in a range of 15 seconds or less in a temperature
region fiom 500°C to 800°C (two-phase region of ferrite and austenite) at the
middle of the second cooling.
15 [0077] Here, the suspension of the water cooling is performed to proceed
the ferrite transformation in the two-phase region. When the suspension
time of the water cooling is more than 15 seconds, the ferrite area ratio
becomes more than 15% to increase the hardness difference among structures,
resulting in deterioration of the stretch flangeability and the low-temperature
20 toughness in some cases. Therefore, in the case of suspending the water
cooling at the middle of the secondaly cooling, it is desirable to set the time
period to 15 seconds or less. Further, it is desirable to set the temperature
region where the water cooling is suspended to 500°C or higher and 800°C or
lower to easily proceed the ferrite transfor~nation, and set the time period
25 when the water cooling is suspended to 1 second or more. Note that fiom
the viewpoint of productivity, it is more desirable to set the time period for
suspending the water cooling to 10 seconds or less.
[0078] After the above-described secondary cooling is performed, coiling
is performed at a coiling temperature CT ("C) satisfying the following
expression (3). When the steel sheet is coiled at a temperature higher than
5 the right side in the following expression (3), the structural fraction of the
high-temperature transformation phase such as ferrite, pearlite, upper bainite
becomes 15% or more to fail to obtain the desired structural fiaction and
hardness difference among structures, resulting in deterioration of the stretch
flangeability and the low-temperature toughness. It is desirable to coil the
10 steel sheet at a temperature lower than 300°C in the case of satisfying vTrs
5 - 40, and achieving a hole expansion ratio 2 140% and a tensile
strength x hole expansion ratio 2 100000 m a . % in a material of a strength
of 590 MPa level, achieving a hole expansion ratio 2 90% and a tensile
strength x hole expansion ratio 2 70000 MPa % in a material of a strength
15 of 780 MPa level, and achieving a hole expansion ratio 2 40% and a tensile
strength x hole expansion ratio 2 50000 MPa . % in a material of a strength
of 980 MPa level or more.
CT ("C) 5 max [Ms, 3501 .. . (3)
In the expression, Ms is decided fiom the following expression (5),
20 and the symbol of each element in the following expression (5) indicates the
content (mass%) of the element in the steel.
Ms("C)=561-474 X C-33 X Mn-17 X Ni-21 X Mo
.. . (5)
[0079] Note that to satisfy the above-described suitable values of rC, r30,
25 the austenite grain diameter after the rough hot rolling, namely, before the
finish hot rolling is important, and the austenite grain diameter before the
finish hot rolling is desirably small. Concretely, by setting the average grain
diameter (circle-equivalent average diameter) of the austenite to 200 p m or
less, the above-described suitable values can be obtained.
[0080] Then, to set the austenite average grain diameter to 200 pm or less
5 before the finish hot rolling, it is only necessary to set the maximum reduction
ratio per pass in a temperature region of 1000°C or higher and 1200°C or
lower in the rough hot rolling to 40% or more, in other words, to perform the
reduction in one pass at a reduction ratio of 40% or more at least one or more
times.
10 Therefore, the rough hot rolling preferably achieves a maximum
reduction ratio per pass in the temperature region of 1000°C or higher and
1200°C or lower of 40% or more, and an austenite average grain diameter of
200 pm or less.
[0081] Note that as the reduction ratio is larger or the number of times of
15 reduction is larger, the austenite grain can be made finer. Further, it is
preferable to set the austenite average grain diameter to 100 pm or lower, and
to this end, it is desirable to perform the reduction in one pass at a reduction
ratio of 40% or more two or more times. However, the rough hot rolling
more than 10 passes may decrease the temperature and excessively generate
20 scale, and the reduction in one pass at a reduction ratio more than 70% may
draw the inclusion to cause deterioration of the hole expandability.
Therefore, it is desirable to perform the reduction in one pass at a reduction
ratio of 40% or more 10 passes or less, and set the maximum reduction ratio
to 70% or less.
25 [0082] By making the austenite grain diameter before the finish hot
rolling smaller, the rec~ystallization of austenite in the finish hot rolling
process is promoted to realize the improvement of the hole expandability
achieved by setting the rC value and the r30 value to the suitable values. It
is presumed that the austenite grain boundaly after the rough hot rolling
(namely, before the finish hot rolling) functions as one recrystallization
5 nucleus in the finish hot rolling.
[OO83] Here, the confirmation of the austenite grain diameter after the
rough hot rolling is performed by cooling as quickly as possible a sheet piece
before it is subjected to the finish hot-rolling, concretely, by cooling the sheet
piece at a cooling rate of 10 "Clsec or higher, then etching the structure in the
10 cross section of the sheet piece to expose the austenite grain boundaiy, and
then performing measurement with an optical microscope. In this event, the
measurement is performed in 20 or more visual fields at 50 or more
magnifications by the image analysis or the point counting method.
[0084] Further, to satisfy the above-described suitable ranges for the rL in
15 the rolling direction and for the r60 in the direction 60" from the rolling
direction, it is desirable to suppress the maximum heat generation due to
plastic deformation in a temperature region of (T1 + 30)"C or higher and (T1
+ 150)"C or lower being the first temperature region, namely, a temperature
increased margin (OC) of the steel sheet by reduction to 18°C or lower. To
20 suppress the maximum heat generation due to plastic deformation as
described above, it is desirable to use inter-stand cooling.
[0085] Note that for the purpose of improving the ductility by cossection
of the steel sheet shape or introduction of mobile dislocation, it is desirable to
perform skin pass rolling being soR reduction at a reduction ratio of 0.1% or
25 more and 2% or less after the completion of all of processes. Further, after
the completion of all of processes, for the purpose of removing the scale
adhering to the surface of the obtained hot-rolled steel sheet, pickling may be
performed for the obtained hot-rolled steel sheet as necessary. After
performing the pickling, skin pass or cold rolling at a reduction ratio of 10%
or less may be performed inline or offline for the obtained hot-rolled steel
5 sheet.
Furthermore, a plating layer may be provided on the surface of the
steel sheet as necessaly to make a surface treated steel sheet. The plating
layer may be an electroplating layer or a hot-dip plating layer, and the
treatment method may be realized by a normal method.'
lo Examples
[0086] Next, the technical content of the present invention will be
explained taking examples of the present invention.
The examples were studied using adaptable steels satisfying claims of
the present invention being steels A to P and comparative steels being steels a
15 to e, which have chemical compositions listed in Table 1.
[0087] These steels were kept as they were or once cooled to room
temperature after casting, then reheated to a temperature range of 900°C to
1300°C, then subjected to the hot rolling under the conditions listed in Table
2-1 and Table 2-2, cooled under the conditions listed in Table 2-1 and Table
20 2-2 to form hot-rolled steel sheets with a thickness of 2.3 to 3.4 mm. Thus
obtained hot-rolled steel sheets were subjected to pickling, then subjected to
skin pass rolling at a reduction ratio of 0.5%, subjected to hot-dip galvanizing
treatment and htther alloying treatment with past of them, and provided for
material quality evaluation. Note that alphabet characters attached to the
25 heads of test numbers in Table 2-1, Table 2-2, Table 3-1 and Table 3-2
indicate the steel types in Table 1.
[0088] The chemical components in each steel are listed in Table 1, and
manufacturing conditions for each hot-rolled steel sheet are listed in Table 2-1
and Table 2-2. Further, the steel structure, grain diameter and mechanical
properties (r value in each direction, tensile strength TS, elongation EL, hole
5 expansion ratio I., brittleness ductility transition temperature vTrs) of each
hot-rolled steel sheet are listed in Table 3-1 and Table 3-2.
Note that the tensile test conformed to JIS Z 2241, and the hole
expansion test conformed to The Japan Iron and Steel Federation Standard
JFS T1001. The X-ray random intensity ratio was measured at a pitch of 0.5
10 pm at the central portion of the sheet thickness between the 318 to 518
thickness positions of the sheet thickness from the surface of the steel sheet in
the cross sections parallel to the rolling direction and the sheet thickness
direction using the above-described EBSD. Further, the r value in each
direction was measured by the above-described method. The Vickers
15 hardness was measured at a load of 0.098 N (10 gf) using the micro Vickers
testes. The Charpy test was performed conforming to JIS Z 2242 with the
steel sheet processed into a 2.5 mm sub-size test piece.
[0089] From the evaluation results indicated in Table 3-1 and Table 3-2,
only the steel sheets satisfying the conditions defined in the present invention
20 have excellent stretch flangeability and low-temperature toughness.
[0090] [Table 11

[0091] [Table 2-11

[0092] [Table 2-21
5

[0093] [Table 3-11

[0094] [Table 3-21

[Name of Document] Claims
[Claim 11 A hot-rolled steel sheet comprising:
a chemical composition comprising: in mass%,
C: 0.01 to 0.2%;
Si: 0.001 to 2.5%;
Mn: 0.10 to 4.0%;
P: 0.10% or less;
S: 0.030% or less;
Al: 0.001 to 2.0%;
N: 0.01% or less;
Ti: (0.005 + 48/14[N] + 48/32[S])% 5 Ti 5 0.3%;
Nb: 0 to 0.06%;
Cu: 0 to 1.2%;
Ni: 0 to 0.6%;
Mo: 0 tol%;
V: 0 to 0.2%;
Cr: 0 to 2%;
Mg: 0 to 0.01%;
Ca: 0 to 0.01%;
20 REM: 0 to 0.1%; and
B: 0 to 0.002%,
with a balance being composed of Fe and impurities;
an texture in which, at a central portion of a sheet thickness that is a
steel sheet portion sectioned at a 318 thickness position and a 518 thickness
25 position of the sheet thickness fsom a surface of the steel sheet, an average
value of X-ray random intensity ratios of a group of {100)<011> to
{223}<110> orientations of a sheet plane is 6.5 or less and an X-ray random
intensity ratio of a {332)<113> crystal orientation is 5.0 or less; and
a microstlucture in which a total area ratio of tempered maitensite,
maitensite and lower bainite is more than 85 %, and an average crystal grain
5 diameter is 12.0 pm or less.
[Claim 21 The hot-rolled steel sheet according to claim 1, wherein
the chemical composition contains one or two or more selected fiom a
group consisting of: , in mass%,
Nb: 0.005 to 0.06%;
10 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%.
15 [Claim 31 The hot-rolled steel sheet according to claim 1 or claim 2,
wherein the chemical composition contains one or two or more selected from
a group consisting of: in mass%, Mg: 0.0005 to 0.01%, Ca: 0.0005 to 0.01%,
and REM: 0.0005 to 0.1%.
[Claim 41 The hot-rolled steel sheet according to any one of claims 1 to 3,
20 wherein the chemical composition contains, in mass%, B: 0.0002 to 0.002%.
[Claim 51 The hot-rolled steel sheet according to any one of claims 1 to 4,
comprising the microstructure in which assuming that an average value of
hardness is E (HVO.01) and a standard deviation is o (HVO.01) when
measuring the Vickers hardness at 100 points or more with a load of 0.098 N,
25 o (HVO.Ol)/E (HVO.01) is 0.08 or less.
[Claim 61 The hot-rolled steel sheet according to any one of claims 1 to 5,
comprising mechanical properties that an r value (rC) in a direction
perpendicular to a rolling direction is 0.70 or more, and an r value (1.30) in a
direction 30" fiom the rolling direction is 1.10 or less.
[Claim 71 The hot-rolled steel sheet according to any one of claims 1 to 6,
5 comprising mechanical properties that an r value (rL) in a rolling direction is
0.70 or more and an r value (r60) in a direction 60" from the rolling direction
is 1.10 or less.
[Claim 81 The hot-rolled steel sheet according to any one of claims 1 to 7,
comprising a plating layer provided on the surface of the steel sheet.
10 [Claim 91 A manufacturing method of a hot-rolled steel sheet by
sequentially performing rough hot rolling, finish hot rolling, primary cooling
and seconda~y cooling on a slab comprising the chemical composition
according to any one of claims 1 to 7, and coiling a resultant slab into the
hot-rolled steel sheet, wherein:
15 the finish hot rolling is hot rolling in which with respect to a
temperature T1 defined in a following expression (I), a maximum reduction
ratio per pass in a first temperature region of (TI + 30)OC or higher and (T1 +
200)"C or lower is 30% or more, a total reduction ratio in the first temperature
region is 50% or more, a total reduction ratio in a second temperature region
20 of Tl°C or higher and lower than (T1 + 30)"C is 0 to 30%, and the rolling is
completed in the first temperature region or the second temperature region;
the primary cooling is water cooling that satisfies a following
expression (2) and achieves a cooling amount of 40°C or higher and 140°C or
lower;
25 the secondary cooling is water cooling that is started within three
seconds after the primary cooling and performs cooling at an average cooling
rate of 30 "Clsec or higher; and
the coiling is to coil the slab at a temperature CT satisfying a
following expression (3),
T1 ("C)=850+10X(C+N)XMn+350XNb+250XTi+40XB+
5 lOXCr+ lOOXMo+ l00XV ... (1)
1 5 tftl 5 2.5 . . . (2)
CT ("C) 5 max [Ms, 3501 ... (3)
tl = 0.001 X {(Tf - ~ 1 ) ~ ~ 1 / 1-0 00.1}09~X {(Tf - Tl)XP1/100} +
3.1 . . . (4)
Ms(OC)=561-474XC-33XMn-17XNi-21XMo
... (5)
where in the expression (I) and the expression (5), a symbol of each
element is a content (mass%) of the element in the steel,
in the expression (2), t is a time period (sec) fsom a final reduction in
15 the reduction in one pass at 30% or more in the first temperature region to
sta1-t of the primary cooling, and tl is a time period (sec) decided by the above
expression (4),
in the expression (3), max[ ] is a function of returning a maximum
value among arguments, and Ms is a temperature decided by the above
20 expression (5), and
in the expression (4), Tf and P1 are a steel sheet temperature and a
reduction ratio (%) in the final reduction in the reduction in one pass at 30%
or more in the first temperature region respectively.
[Claim 101 The manufacturing method of the hot-rolled steel sheet
25 according to claim 9, wherein the rough hot rolling achieves a maximum
reduction ratio per pass in a temperature region of 1000°C or higher and
1200°C or lower of 40% or more, and an austenite average grain diameter of
200 pm or less.
[Claim 111 The manufacturing method of the hot-rolled steel sheet
according to claim 9 or claim 10, wherein a maximum heat generation due to
5 plastic deformation in a temperature region of (T1 + 30)"C or higher and (TI
+ 150)"C or lower of the finish hot rolling is 18°C or lower.
[Claim 121 A manufacturing method of a hot-rolled steel sheet
comprising: performing a plating treatment on the surface of the hot-rolled
steel sheet obtained by the manufacturing method of the hot-rolled steel sheet
10 according to any one of claims 9 to 11.
Dated this July 23,2014
[RANJNA MEI-ITA-DUTT]
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT[S]