Description
Titlc of Invention
HOT-ROLLED STEEL SHEET
5
Technical Field
[OOOl]
The present invention relates to a hot-rolled steel sheet
10 Background Art
[0002]
Conventionally, for the purpose of reducing the weight of automobile bodies,
high-strength steel sheets have been often used for suspension parts or structural
parts of automobile bodies. Suspension parts of automobiles are required to have
15 fatigue characteristics of a notch-free material and notch fatigue eharacteristics, but
there has been a problem in that conventional high-strength steel sheets are
insufficient in such performance and do not allow parts to be reduced in sheet
thickness.
[0003]
20 To improve fatigue characteristics of a notch-fiee material, it is effective to
make the structure liner. For example, Patent Literatures 1 and 2 clcseribe hotrolled
steel sheets that contain ultrafine ferrite grains with an average grain size of
less than 2 pm as hot-rolled. It is described that these steel sheets are excellent in
ductility, toughness, fatigue strength, and the like, and have low anisotropy in these
25 characteristics. In addition, since a fatigue crack occurs from near the surface, it is
also effective to make the structure near the surface finer. Patent Literature 3
describes a hot-rolled steel sheet having a crystal grain size gradient structure in
which the average crystal grain size of polygonal ferrite gradually decreases from the
center of the sheet thickness toward the surface layer. Furthermore, grain refining
30 of a martensite strueturc is also effective in Improving fatigue characteristics.
Patcnt Literature 4 describcs a machine structural steel pipe in which 80% or more in
area fraction of the microstuucturc is martensite, the average block dianletcr of thc
martensitic structure is 3 pm or less, and thc maximum block diameter is 1 to 3 times
the average block diameter. Howeve& although grain refining improves fatigue
characteristics of a notch-fiee material, it has no elTec1 of delaying a crack
, 6 propagation rate, thus not contributing to an improvement in notch fatigue
characteristics.
[0004]
Regarding an improvement in notch fatigue characteristics, it has been
reported that it is effective to reduce a crack propagation rate by forming a composite
10 structure. In Patent Literature 5, hard bainite or martensite is dispersed in a
structure whose main phase is fine ferrite; thus, fatigue characteristics of a notch-free
material and notch fatigue characteristics are both achieved. It is reported in Patent
Literatures 6 and 7 that increasing an aspect ratio of martensite in a composite
structure reduces a crack propagation rate.
15
Citation List
Patent Literature
[0005]
Patent Literature 1 : JP Hll-92859A
20 Patent Literature 2: JP H11-152544A
Patent Literature 3: JP 2004-21 1199A
Patent Literature 4: JP 2010-70789A
Patent Literature 5: JP H04-337026A
Patent Literature 6: JP 2005-320619A
Patent Literature 7: JP H07-90478A
Summary of Invention
Technical Problem
[0006]
30 Patent Literature 5 does not describe a technique for improving press
forn~abilitya, nd docs not pay pai-t~culara ttention to the hardness and shape of bainite
and martensite; hencc, it seems that favorable press formability is not achieved.
100071
I'atent Literatures 6 and 7 lack consideration of' ductility and workability
(e.g., hole expandability), whlch are needed in press forming.
5 [0008]
The present invention has been made in order to solve such problems, and
aims to provide a hot-rolled steel sheet having excellent fatigue characteristics in the
rolling direction and workability
10 Solution to Problem
[0009]
The present inventors have carried out extensive research in order to achieve
the above object, and have succeeded in producing a steel sheet with excellent
fatigue characteristics in the rolling direction and workability, by optimizing the
15 chemical composition and production conditions of a high-strength hot-rolled steel
sheet to control the microstructure of the steel sheet. The gist of the present
invention is as follows.
[OO lo]
(1)
20 A hot-rolled steel sheet having a chemical composition consisting of, in
mass%,
C: 0.03 to 0.2%,
Mn: 0.1 to 3.0%,
P: 0.10% or less,
S: 0.03% or less,
A1 + Si: 0.2 to 3.0%,
N: more than 0% and equal to or less than 0.01%,
0: more than 0% and equal to or less than 0.01%,
Ti: 0 to 0.3%,
Nb: 0 to 0.3%,
Mg: 0 to 0.01%,
Ca: 0 to 0.01%,
REM: 0 to 0.1%,
B: 0 to 0.01%,
Cu: 0 to 2.0%,
Ni: 0 to 2.0%,
Mo: 0 to 1.0%,
V: 0 to 0.396,
Cr: 0 to 2.0%, and
the balance: iron and impurities,
10 wherein a microstructure of the hot-rolled steel sheet contains ferrite as the
main constituent, and contains hard phases constituted by martensite and/or austcnite
in an amount of, in area fraction, equal to or more than 3% and less than 20%,
hard phases with an aspect ratio of 3 or more accounts for 60% or more of
the hard phases present in a sheet-thickness central portion,
15 a length in a rolling direction of the hard phases present in the sheetthickness
central portion is less than 20 pm,
an average aspect ratio of ferrite grains is less than 5, and
the sum of X-ray random intensity ratios of <011> orientation and 111 1>
orientation as viewed from the rolling direction is 3.5 or more, and an X-ray random
20 intensity ratio of <001> orientation as viewed from the rolling direction is 1.0 or less.
[OOll]
(2)
The hot-rolled steel sheet according to (I), containing, in mass%,
one or more selected from
25 Ti: equal to or more than (0.005 + 48/14m] + 48132[S])% to equal to or less
than 0.3%, and
Nb: 0.01 to 0.3%,
where m] indicates an N content (mass%) and [S] indicates an S content
(mass%).
30 [0012]
(3)
The hot-rolled steel sheet according to (I), containing, in mass%,
one or more selected from
Mg: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%, and
REM: 0.0005 5 to 0.1%.
[0013]
(4)
The hot-rolled steel sheet according to (I), containing, in mass%,
B: 0.0002 to 0.01%.
10 [0014]
(5)
The hot-rolled steel sheet according to (I), containing, in mass%,
one or more selected from
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
Mo: 0.01 to LO%,
V: 0.01 to 0.3%, and
Cr: 0.01 to 2.0%.
[0015]
20 (6)
The hot-rolled steel sheet according to (I), comprising a hot-dip galvanized
layer or a galvannealed layer on its surface.
Advantageous Effects of Invention
25 [0016]
According to the present invention, a hot-rolled steel sheet having excellent
fatigue characteristics in the rolling direction and workability can be provided. The
present invention can be suitably applied to steel sheets with a sheet thickness of 8
mm or less. A hot-rolled steel sheet according to the present invention can prolong
30 fatigue life of suspension parts and the like of automobile mate~ials, thus
significantly contributing to tl~ein dustry.
Brief Description of Drawings
[0017]
[FIG 11 FIG 1 is a schematic diagram illustrating the shape and dimensions of tesl
5 pieces used for a fatigue test. FIG. l(a) is a plan view and a front view of a test
piece for measuring fatigue strength without a notch. FIG. I(b) is a plan view and a
front view of a test piece for measuring fatigue strength with a notch.
Description of Embodiments
10 [OOIS]
1. Microstructure of hot-rolled steel sheet
1-1. Area fraction of each phase constituting hot-rolled steel sheet
A hot-rolled steel sheet of the present invention is required to contain ferrite
as the main constituent, and contain hard phases constituted by martensite and/or
15 austenite in an amount of, in area fraction, equal to or more than 3% and less than
20%. When the microstructure is a composite structure having hard phases as a
secondary phase in ferrite serving as the main phase, ferrite improves ductility and
the hard phases improve strength, so that the steel sheet has a favorable balance
between strength and ductility. Furthermore, hard phases have an effect of
20 obstructing fatigue crack propagation in ferrite, to reduce a fatigue crack propagation
rate; therefore, a steel sheet having the above-described composite slructure has
excellent punching fatigue characteristics. Hence, a hot-rolled steel sheet of the
present invention is made to have a microstructure whose main constituent is ferrite
and in which hard phases constituted by martensite and/or austenite are distributed as
25 a secondary phase. Containing ferrite as the main constituent means that ferrite
serving as the main phase in the hot-rolled steel sheet has the highest area fraction.
The area fraction of ferrite is preferably 70 to 97%.
[OOI 91
The hard phases exert the effect of inhibiting fatigue crack propagation
30 when its area fraction is 3% or more. On the other hand, when the hard phases have
an area fraction of 20% or more, the hard phases serve as a starting point of a defect
called a void to reduce a holc expansion ratio, so that "(tensile strength (MPa)) x
(hole expansion ratio (%)) 2 35000n, which is required of suspension parts of
automobiles, is not satisfied. For this reason, hard phases constituted by martensite
or austenite are made to be present in an amount of equal to or more than 3% and
, 5 less than 20% in area fraction it1 a microstructure whose main constituent is ferrite.
The hard phases are preferably present in an amount of, in area fraction, 5% or more,
further preferably 7% or more.
[0020]
1-2. Aspect ratio of hard phase present in sheet-thickness central portion
10 Next, description is given on an aspect ratio of hard phases present in a
sheet-thickness central portion. When a punching fatigue test is performed in an
axial fatigue test, a fatigue crack occurs from the sheet-thickness central portion, and
the crack propagates in the sheet thickness direction, resulting in rupture. Here, to
inhibit the occurrence of a crack and initial propagation, the forms of the hard phases
15 in the sheet-thiclcness central portion are parlicularly important.
[0021]
An aspect ratio of a hard phase is defined by (the length of the major axis of
the hard phaselthe length of the minor axis of the hard phase). In a hot-rolled steel
sheet of the present invention, "the length of the major axis of the hard phase" is "the
20 length of the hard phase in the rolling direction of the steel sheet", and "the length of
the minor axis of the hard phase" is "the length of the hard phase in the thickness
direction of the steel sheet". With an increase in the aspect ratio of the hard phase,
the frequency at which a crack hits the hard phase, which obstructs fatigue crack
propagation, increases, and also the detouringlbranching distance of the crack
25 increases, which is effective in reducing a fatigue crack propagation rate. Here, a
hard phase whose aspect ratio is less than 3 has a small effect of inhibiting crack
propagation because the detouringibranching distance when a crack hits the hard
phase is small. Therefore, it is effective to increase hard phases whose aspect ratio
is 3 or more. For this reason, in the hot-rolled steel sheet of the present invention,
30 hard phases with an aspect ratio of 3 or more are made to account for 60% or more of
the hard phases present in the sheet-thickness central portion. The hard phases with
an aspect ratio of 3 or more preferably account Sbl. 80%) or more of the hard phase
prcsent in the sheet-thickness central portion.
[0022]
1-3. Length in rolling direction of hard phases prcsent in sheet-thickness central
, 5 portion
Description will be given on the length in the rolling direction ol the hard
phases present in the sheet-thickness central portion. When steel with a composite
structure containing ferrite and hard phase are deformed, the ferrite side, which is
soft, preferentially undergoes plastic deformation, and accordingly stress on the hard
10 phases increases with the deformation, so that great strain occurs at the interface
between ferrite and the hard phases.
[0023]
When the stress on the hard phases or the strain at the interfaces between
ferrite and the hard phases exceeds a certain degree, defects called voids occur in the
15 steel, and these voids connect to each other, resulting in rupture. A material in
which voids easily occur is vulnerable to local deformation and has low hole
expandability.
[0024]
When the hard phases extend in the rolling direction, stress and strain in
20 deformation are concentrated on the hard phases, causing voids to occur early; thus,
hole expandability is likely to deteriorate. In addition, in the sheet-thicltness central
portion, plastic constraint is stronger than in the surface layer portion, and thus voids
easily occur. Therefore, the length of the hard phases in the sheet-thickness central
portion is particularly important.
25 [0025]
According to the study by the present inventors, when the length in the
rolling direction of the hard phases prcsent in the sheet-thiclmess central portion is
controlled to less than 20 pm, the occurrence of voids can be inhibited, and "(tcnsile
strength (MPa)) x (hole expansion ratio (%)) > 35000", which is requircd of
30 suspe~isionp arts of automobiles, can be achieved. IIence, in a hot-rolled steel sheet
of the present invention, the length in the rolling direction of the hard phases present
in the sheet-thickness central portion is specified as less than 20 pn~. The length in
the rolling direction of the hard phases in the sheet-thickness central portion is
preferably less than 18 pm.
LOO261
5 The hard phases are constituted by martensite and/or austenite. That is,
there are the following thee forms: a hard phase containing only martensite, a hard
phase containing only austenite, and a hard phase containing both martensite and
austenite. The hard phase may be constituted by a single grain (martensite grain or
austenite grain), or a plurality of grains may aggregate to integrally constitute the
10 hard phase. Examples of a hard phase in which a plurality of grains aggregate
include an aggregate of a plurality of martensite grains, a plurality of austenite grains,
and an aggregate of one or a plurality of martensite grains and one or a plurality of
austenite grains.
[0027]
15 1-4. Aspect ratio of ferrite grain
Description will be given on the average aspect ratio of ferrite grains. The
aspect ratio of a ferrite grain is defined by (the length of the major axis of the ferrite
grain /the length of the minor axis of the ferrite grain). In a hot-rolled steel sheet of
the present invention, "the length of the major axis of the ferrite grain" is "the length
20 of the ferrite grain in the rolling direction of the steel sheet", and "the length of the
minor axis of the ferrite grain" is "the length of the ferrite grain in Llre thickness
direction of the steel sheet". In the case where finish rolling is completed in an
austenite region, the average aspect ratio of ferrite grains is less than 5. In contrast,
in the case where a final rolling temperature is low and rolling is performed in a
25 dual-phase region of austenite and ferrite, ferrite grains stretch in the rolling direction,
so that the average aspect ratio of the ferrite grains is 5 or more. When the average
aspect ratio of ferrite grains is 5 or more, the ferrite grains undergo work hardening,
which reduces the ductility of the steel sheet and does not allow (tensile strength
(MPa)) x (total elongation (%)) 2 18000 to be satisfied. Hence, in a hot-rolled steel
30 sheet of the present invention, the average aspect ratio of ferrite grains is made less
than 5.
100281
1-5. X-ray random intensity ratio
Description will be given on an X-ray random intensity ratio. Fatigue life
of a notch-free material is greatly influenced by life until occurrence of a fatigue
5 crack. The occurrence of a fatigue crack is known to proceed in a process of three
steps: (1) saturation of a dislocation structure, (2) fonnation of intrusion and
extrusion, and (3) formation of a fatigue crack.
[0029]
As a result of extensive studies, the present inventors have found that by
10 appropriately controlling crystal orientation in the stress loading direction in a fatigue
test to make the X-ray random intensity ratio satisfy a predetermined condition, it is
possible to delay (1) saturation of a dislocation structure of the process of three steps,
and improve fatigue life of a notch-free material in the rolling direction. A
mechanism for this is described below.
15 [0030]
A crystal structure of iron is a body-centered cubic @.c.c.) structure, and it
is said that 42 slip systems of {110)<111> system, {112}<111> system, and
{123}<111> system work. Notation of crystal orientation will be described later.
In polycrystals, deformability varies depending on crystal orientation, and the degree
20 of difficulty of deformation is decided by a Taylor factor. The Taylor factor is a
value defined by fornlula (F).
dCTi = Md& ... (F),
where Ti denotes the amount of slip of a slip system i, CT, denotes the total
sum of slip amounts of the whole slip that has worked, M denotes the Taylor factor,
25 and E denotes the whole plastic strain amount.
[0031]
With a decrease in the Taylor factor, the whole plastic strain amount
increases even when the total sum of slip amounts of the slip systems is small, which
allows plasticdeformation with s~nalle nergy. The Taylor factor changes depending
30 on crystal orientation with respect to the stress loading direction; thus, a crystal grain
in an orientation with a small Taylor factor is easily deformed, and a crystal grain in
an orientation with a large Taylor Sactor is difficult to deform.
PO321
Researchers have carried out calculation to reveal that, if the abovedescribed
42 slip systems are assumed in b.c.c. metal, values of Taylor factors when
5 <001> orientation, <011> orientation, and orientation are sub,jected to tensile
deformation are 2.1, 3.2, and 3.2, respectively, and <001> orientation is deformed
most easily, which leads to early formation of a dislocation structure. In contrast,
<011> orientation and orientation are difficult to deform, which leads to late
formation of a dislocation structure. That is, a crystal grain facing <001>
10 orientation with respect to the stress loading direction has a short fatigue crack
initiation life, and crystal grains facing <011> orientation and orientation have
a long fatigue crack initiation life.
[0033]
The present inventors have found as a result of study that, when the sum of
15 X-ray random intensity ratios of <011> orientation and orientation as viewed
from the rolling direction is controlled to 3.5 or more and an X-ray random intensity
ratio of <001> orientation as viewed from the rolling direction is controlled to 1.0 or
less, fatigue characteristics in the rolling direction exhibit a favorable value of
(fatigue limit)/(tensile strength) of 0.55 or more. "Fatigue limit" here refers to
20 fatigue strength at 10 million cycles obtained with a notch-free fatigue test piece,
which is described later.
[0034]
On the basis of this finding, in a hot-rolled steel sheet of the present
invention, the sum of X-ray random intensity ratios of 101 1> orientation and 11 1 I>
25 orientation as viewed from the rolling direction is set to 3.5 or more, and an X-ray
random intensity ratio of <001> orientation as viewed from the rolling direction is
set to 1.0 or less. The sum of X-ray random intensity ratios of <011> orientation
and <11 I> orientation as viewed from the rolling direction is preferably 4.0 or more.
An X-ray random intensity ratio of <001> orientation as viewed from the rolling
30 direction is preferably 0.8 or less.
[0035]
1-6. Method for measuring n~icrostruciure of hot-rollcd steel sheet and X-ray
random intensity
(1) Method for measuring area fractions of ferrite and hard phascs
The area fractions of ferrite and hard phases constituted by martensitc
5 andlor austenite, which constitute the structure of a hot-rolled steel sheet of the
present invention as described above, are measured by using a sample obtained by
taking, as an observation surface, a cross-section perpendicular to the width direction
of the steel sheet. The observation surface of the sample is polished and subjected
to nital etching. Ranges of 114 thicltness (This means a position at 114 of the
10 thickness of the stecl sheet in the thickness direction of the steel sheet from the
surface of the steel sheet. The same applies io the following.), 318 thickness, and
112 thickness of the sheet thickness of the observation surface subjected to nital
etching are observed with a FE-SEM.
[0036]
15 For an observation target range of each sample, ten fields of view are
observed at a 1000-fold magnification, and in each field of view, the proportions of
areas occupied by ferrite and the hard phases are measured. The area of the hard
phases is the total area of mariensite and austenite. Then, the average values of the
proportions of areas occupied by ferrite and the hard phases of all fields of view are
20 obtained as the area ratios of ferrite and the hard phases.
[0037]
(2) Aspect ratio and length in rolling direction of hard phases present in sheetthickness
central portion, and aspect ratio of ferrite phase
The aspect ratio and the length in the rolling direction of ihe hard phases
25 present in the sheet-thickness central portion are determined regarding hard phases
located at 112 thickness of the sheet thickness in the above-described sample. 50 or
more hard phases located at 112 thickness of the sheet thickness in the abovedescribed
sample are obseryed using a FE-SEM, and the length in the steel sheet
rolling direction and the length in the steel sheet thickness direction of each hard
30 phase are measured. From the measurement results of these lengths, an aspect ratio
of each hard phase is calculated. The proportion of hard phases with an aspect ratio
of 3 or more among the observed hard phases is calculxted. In addition, the average
value of the lengths in thc rolling direction of the observed hard phascs is obtained as
the length in the rolling direction of the hard phases present in the sheet-thickness
central portion.
5 [0038]
The sheet-thickness central portion refers to a position at 112 of the
thickness of the steel sheet in the thickness direction of the steel sheet from the
surface of the steel sheet. For example, in the sheet-thickness central portion, any
50 hard phases in a field-of-view range of 50 pm x 200 pm are selected, and the
10 length in the rolling direction and the length in the steel sheet thickness direction of
each hard phase are measured. To increase measurement precision, instead of
selecting any 50 hard phases, the length in the rolling direction and the length in the
thickness direction may be measured for all of the hard phases in the field-of-view
range.
15 [0039]
The average aspect ratio of ferrite grains is determined regarding ferrite
grains located at 114 thickness to 112 thickness of the sheet thickness in the abovcdescribed
sample. 50 or more ferrite grains located at 114 thickness to 112 thickness
of the sheet thickness in the above-described sample are observed using a FE-SEM,
20 and the length in the steel sheet rolling direction and the length in the steel sheet
thickness direction of each ferrite grain are measured. From the incasurement
results of these lengths, an aspect ratio of each ferrite grain is calculated, and the
average value of the aspect ratios of the observed ferrite grains is obtained as the
average aspect ratio of the ferrite grains.
25 [0040]
(3) X-ray random intensity ratio
X-ray random intensity ratios of <001> orientation, <011> orientation, and
orientation as viewed from the rolling direction may be determined from an
inverse pole figure measured by X-ray diffraction. An X-ray random intensity ratio
30 is a value obtained in the following manner: X-ray intensities of a standard sample
without integration in a particular orientation and a sample are measured undcr the
samc conditions by X-ray diffraction or thc like, and the X-ray intensity of the
sample is divided by the X-ray intensity of the standard sample.
[0041]
Here, for a hot-rolled steel sheet, crystal orientation perpendicular to the
5 sheet surface is normally denoted by [hkl] or {hkl), and crystal orientation parallel to
the rolling direction is normally denoted by (uvw) or . (hkl) and
collectively refer to equivalent planes, and [hkl] and (uvw) refer to individual crystal
planes. In the present invention, which is targeted at a hot-rolled steel sheet whose
main constituent is ferrite with a b.c.c. structure, for example, (1 ll), (-1 1 I), (1-1 I),
10 (11-1) ( 1 1 1 ) ( - 1 1 ) (1-1-1) and (-1-1-1) planes are equivalent and
undistinguishable. In such a case, these orientations are collectively referred to as
. In crystallography, as for "-1 ", "-" is put above "1 " in the formal notation of
orientation, but here, "-1" is used for notation because of constraints in description.
[0042]
15 A sample for X-ray diffraction is fabricated in the following manner. A
rolling-direction cross-section (a cross-section perpendicular to the rolling direction)
of the steel sheet is polished by mechanical polishing, chemical polishing, or the like
and mirror-finished by buffing, and then strain is removed by electrolytic polishing,
chemical polishing, or the like. The range of X-ray diffraction is the whole sheet
20 thickness. If the whole sample cannot be measured at once, measurement may be
performed with the sheet thickness direction divided into several fields of view, and
the results may be averaged. If measurement by X-ray diffraction is difficult, a
statistically sufficient number of samples may be measured by an electron back
scattering pattern (EBSP) method or an electron channeling pattern (ECP) method,
25 and an X-ray diffraction random intensity ratio of each orientation may be
determined.
[0043]
2. Chemical composition of steel sheet
A hot-rolled steel sheet of the present invention has a chemical composition
30 containing the following elements. Hereinafter, the elements will be described
along with the reasons for confining the contents of the elements. In the description,
''0 h >> of the content of each element means "mass%".
100441
C: 0.03 to 0.2%
Carbon (C) is an importal-rt clement in the present invention. C generates
5 martensite and stabilizes austenite, thus contributing to strength improvement of the
hot-rolled steel sheet by structure strengthening, and also has an effect of inhibiting
crack propagation. Note that a C content less than 0.03% does not allow a
predetermined area fraction of the hard phases to be achieved; thus, an elfect of
improving punching fatigue characteristics is not exhibited. On the other hand, a C
10 content exceeding 0.2% leads to an excessive area haction of a low-temperature
transformation product constituting the hard phases serving as a secondary phase,
which reduces hole expandability. Accordingly, the C content is set to 0.03% to
0.2%. The lower limit of the C content is preferably 0.06%, and the upper limit is
preferably 0.18%.
15 [0045]
Mn: 0.1 to 3 .O%
Manganese (Mn) is contained for solid solution strengthening and also in
order to increase hardenability to generate martensite or austenite in the steel sheet
structure. A Mn content exceeding 3% saturates this effect. On the other hand, a
20 Mn content less than 0.1% makes it difficult to exert an effect of inhibiting
generation of pearlite and bainite during cooling. Accordingly, the Mn content is
set to 0.1 to 3.0%. The lower limit of the Mn content is preferably 0.3%, and the
upper limit is preferably 2.5%.
[0046]
25 P: 0.10% or less
Phosphorus (P), which is an impurity contained in hot metal, is segregated
at a grain boundary and reduces low-temperature toughness along with an increase in
its content. Therefore, the P content is preferably as low as possible. A P contcnt
exceeding 0.10% adversely affects workability and weldability. Accordingly, the P
30 content is set to 0.10% or less. Particularly in terms of weldability, the uppel limit
of the P content is preferably 0.03'K.
100471
S: 0.03% or less
Sulfur (S) , which is an impurity containcd in hot metal, when contained too
much, causes a crack in hot rolling and also generates an inclusion, such as MnS,
5 which causes hole expandability to dcteriorate. Therefore, the S content should be
reduced as far as possible, whereas 0.03% or less is an allowable rangc.
Accordingly, the S content is set to 0.03% or less. Note that when a certain degree
of hole expandability is needed, the upper limit of the S content is preferably 0.01%,
furthcr preferably 0.005%.
10 [0048]
Si + Al: 0.2 to 3.0%
Silicon (Si) and aluminum (Al) are both important elements in the present
invention. Si and A1 has an effect of inhibiting {112)<111> slip in the iron, thus
delaying formation of a dislocation structure to improve fatigue crack initiation lifc.
15 This effect is obtained at a total content of Si and A1 (Si + Al) of 0.2% or more, and
is significant at 0.5% or more. Si + A1 exceeding 3.0% saturates the effect and
leads to poor economic efficiency. Accordingly, Si + A1 is set to 0.2 to 3.0%. The
lower limit of Si + A1 is preferably 0.5%. The A1 content in the present invention
refers to acid-soluble A1 (so-called "sol.Al"). Only one of Si and A1 may be
20 contained in an amount of 0.2 to 3.0%, or both Si and A1 may be contained in a total
amount 0f0.2 to 3.0%.
[0049]
N: more than 0% and equal to or less than 0.01%
Nitrogen (N), when present in steel as TiN, contributes to an improvement
25 in low-temperature toughness by making a crystal grain size finer in slab heating.
Therefore, N may be contained. Note that an N content more than 0.01% may
cause a blowhole to be formed in welding of a steel sheet to reduce joint strength of a
weld. Accordingly, the N content is set to 0.01% or less. On the other hand, an N
content less than 0.0001% is not preferable in terms of economic efficiency.
30 Therefore. the lower limit of the N content is preferably 0.0001% or more, further
preferably 0.0005%.
LO0501
0: more than 0% and equal to or less than 0.01%
Oxygen (0) forms oxide, which causes formability to deteriorate; hence, its
content needs to be suppressed. In particular, an 0 content cxcccding 0.01% lcads
5 to significant deterioration of formability. Accordingly, the 0 content is set to
0.01% or less. On the other hand, an 0 content less than 0.001% is not preferable
in terms of economic efficiency. Therefore, the lower limit of the 0 content is
preferahly 0.001% or more.
[005l]
10 Ti: 0 to 0.3%
Nb: 0 to 0.3%
Titanium (Ti) achieves both excellent low-temperature toughness and high
strength due to precipitation strengthening. Therefore, Ti may be contained as
necessary. Carhonitride of Ti or solid solution Ti delays grain growth in hot rolling,
15 which makes a grain size of the hot-rolled steel sheet finer and contributes to an
improvement in low-temperature toughness. However, a Ti content exceeding 0.3%
saturates this effect and leads to poor economic efficiency. Accordingly, the Ti
content is set to 0 to 0.3%. In addition, a Ti content less than (0.005 + 48/14m] +
48/32[S])% may be unable to provide this effect sufficiently. Hence, the Ti content
20 is preferably equal to or more than 0.005 + 48/14[N] + 48/32[S] (%) and equal lo or
less than 0.3%. Hcre, IN] and [S] denote the N content (%) and the S content (%),
respectively. Furthermore, a Ti conlent exceeding 0.15% may cause a tundish
nozzle to he clogged up easily in casting. Hence, the upper limit of the Ti content is
preferably 0.15%.
25 [0052]
Niobium (Nb) improves low-temperature toughness of a hot-rolled steel
sheet. Therefore, Nb may be contained as necessary. Carhonitride of Nb or solid
solution Nb delays grain growth in hot rolling, which makes a grain size of tile hotrolled
steel sheet finer and contributes to an improvement in low-temperature
30 toughness. However, a Nb content exceeding 0.3% saturates this effect and leads to
poor economic efficiency. Accordingly, the Nb content is set to 0 to 0.3%. In
addition, a Nh content less than 0.01% may be unable to provide this effect
suficiently. Therefore, the lower liillit of the Nb content is preferably 0.01%, and
the upper limit is preferably 0.1%.
lo0531
5 Mg: 0 to 0.01%
Ca: 0 to 0.01%
E M : 0 to 0.1%
Magnesium (Mg), calcium (Ca), and rare earth metal (REM) control the
form of a non-metallic inclusion, which serves as a starting point of breaking to
10 cause deterioration of workability, and thus improve workability. Therefore, one or
more of these elements may be contained as necessary. However, an Mg content
exceeding 0.01%, a Ca content exceeding 0.01%, or a FEM content exceeding 0.1%
saturates this effect and leads to poor economic efficiency. Accordingly, the Mg
content is set to 0 to 0.01%, the Ca content is set to0 to 0.01%, and the REM content
15 is set to 0 to 0.1%. When Mg, Ca, and REM are each contained in an amount of
0.0005% or more, the above effect is significantly exhibited. Therefore, the lower
limit of the Mg content is preferably 0.0005%, the lower liinit of the Ca content is
preferably 0.0005%, and the lower limit of the REM content is preferably 0.0005%.
Note that REM collectively refers to 17 elements in total, including Sc, Y, and
20 lanthanoid, and the REM content means the total amount of these elements.
[0054]
B: 0 to 0.01%
B is segregated at a grain boundary and increases grain boundary strength to
improve low-temperature toughness. Therefore, B may be contained in the steel
25 sheet as necessary. However, a B content exceeding 0.01% not only saturates this
effect but also leads to inferior economic efficiency. Accordingly, the B content is
set to 0 to 0.01%. The above effect is significant when the steel sheet has a B
content of 0.0002% or more. Therefore, the lower limit of the B content is
preferably 0.0002%, further preferably 0.0005%. The upper limit of the B content
30 is preferably 0.005%, further prefcrably 0.002%.
[0055]
Cu: 0 to 2.0%
Ni: 0 lo 2.0%
Mo: 0 to 1 .O%
V: 0 to 0.3%
5 Cr: 0 to 2.0%
Copper (Cu), nickel mi), molybdenum (Mo), vanadium (V), and chromium
(Cr) have an effect of improving the strength of a hot-rolled steel sheet by
precipitation strengthening or solid solution strengthening. Therefore, one or more
of these elements may he contained as necessary. However, a Cu content exceeding
10 2.0%, a Ni content exceeding 2.0%, a Mo content exceeding 1.0%, a V content
exceeding 0.3%, or a Cr content exceeding 2.0% saturates this effect and leads to
poor economic efficiency. Accordingly, the Cu content is set to 0 to 2.0%, the Ni
content is set to 0 to 2.0%, the Mo content is set to 0 to 1.0%, the V content is set to 0
to 0.3%, and the Cr content is set to 0 to 2.0%. When Cu, Ni, Mo, V, and Cu are
15 each contained in an amount of less than 0.01%, this effect is not provided
sufficiently. Therefore, the lower limit of the Cu content is preferably 0.01%,
further preferably 0.02%. The lower limit of the Ni content is preferably 0.01%, the
lower limit of the Mo content is preferably 0.01%, the lower limit of the V content is
preferably 0.01%, and the lower limit of the Cr content is preferably 0.01%. In
20 addition, the upper limit of the Cu content is preferably 1.2%, the upper limit of the
Ni content is preferably 0.6%, the upper limit of the Mo content is prelerably 0.7%,
the upper limit of the V content is preferably 0.2%, and the upper limit of the Cr
content is preferably 1.2%.
[0056]
25 Described above is a basic chemical composition of a hot-rolled steel sheet
of the present invention. The balance of the chemical composition of the hot-rolled
steel sheet of the present invention consists of iron and impurities. Impurities mean
components that are mixed in due to raw materials, such as ores or scrap, or other
factors when a steel material is produced industrially.
30 [0057]
It has been confirmed that, as an element other than the above elements, one
or more of Lr, Sn, Co, Zn, and W may be contained in a total amount of 1% or lcss,
instead of part of iron, without impairment of the excellent fatigue characteristics in
the rolling direction and workability of the hot-rolled steel sheet of the present
invention. Aniong these elements, Sn may cause a flaw in hot rolling; hence, the
5 upper limit ofthe Sn content is preferably 0.05%.
[OOSS]
A hot-rolled steel sheet of the present invention having the above-described
structure and composition can have improved corrosion resistance by comprising, on
the surface, a hot-dip galvanized layer formed by hot dip galvanizing and a
10 galvannealed layer formed by alloying after plating. The plating layer is not limited
to pure zinc, and may contain elements such as Si, Mg, Al, Fe, Mn, Ca, and Zr for
further improved corrosion resistance. Comprising this plating layer does not
impair the excellent punching fatigue characteristics and workability of the hot-rolled
steel sheet of the present invention.
15 100591
Moreover, a hot-rolled steel sheet of the present invention may comprise a
surface-treating layer formed by any of formation of an organic film, film
laminating, organic saltslinorganic salts treatment, non-chromium treatment, and the
like, and still achieve an effect of the present invention.
20 [0060]
3. Method for producing hot-rolled steel sheet of the present invention
A method for producing a hot-rolled steel sheet is not particularly limited, as
long as a hot-rolled steel sheet having the aforementioned microstructure is obtained.
For example, a production method comprising the following steps [a] to [h] allows a
25 hot-rolled steel sheet of the present invention to be stably obtained. Hereinafter,
details of each step will be described as an example.
[0061]
[a] Slab casting step
There is no particular limitation on a method for produeing a slab prior to
30 hot rolling. That is, subsequent to production of ingot stcel using a blast furnace. an
electric furnace, or the like, various kinds of secondary snlelting may be performed
for adjustment to the above-described chemical composition, and then, a slab may be
casted by a normal method, such as continuous casting or thin slab casting. On this
occasion, scrap may be used as a raw matevial as long as a component range of the
present invention can be obtained by control.
5 [0062]
[b] Slab heating step
The casted slab is heated to a predetermined temperature for hot rolling. In
the case of continuous casting, the slab may be once cooled to a low temperature and
then heated again and subjected to hot rolling, or may be directly heated and
10 subjected to hot rolling subsequent to continuous casting without being cooled.
Heating time for the slab is equal to or more than time tl(s) specified in formula (A).
t, (s) = 1.4 x 10.~x Exp(3.2 x 104/(T, + 273)) ... (A),
where T1 ("C) is the average temperature of the slab in a soaking area.
[0063]
15 The heating time is thus specified for the following reason. In the structure
of the casted slab, segregation of Mn is present in the center of the slab. Therefore,
when the slab is not sufficiently heated, segregation of Mn remains in a sheetthickness
central portion of a hot-rolled steel sheet obtained by rolling. Since Mn
stabilizes austenite, a region in which austenite is likely to remain along the Mn
20 segregation occurs during cooling after the rolling. Consequently, martensite into
which austenite has been transformed at low temperature or remaining austenite is
likely to he present along the Mn segregation, which increases the length in the
rolling direction of the hard phases in the sheet-thickness central portion of the hotrolled
steel sheet.
25 [0064]
As a result of extensive studies, the present inventors have found that in
order to make the length in the rolling direction of the hard phases 20 pm or less, it is
necessary to set the heating time for the slab to time tl(s), specified in formula (A), or
more. Presumably, sufficiently long heating time for the slab promotes diffusion of
30 Mn, reduciug the length in the rolling direction of the hard phases. An effect of the
present invcntion is exerted even when the upper liliiit of the slab heating
temperature is not set, but cxccssivcly high heating temperature is not preferable in
terms of economic efficiency. IIence, the slab hcating temperature is preferably
lower than 1300°C. The lower limit of the slab heating temperature is preferably
1150°C. The heating time for the slah is not the elapsed time from heating start, but
5 time during which the slab is held at a predetermined heating temperature (e.g., a
temperature equal to or higher than 1150°C and lower than 1300°C).
[0065]
[c] Rough roiling step
After the slab heating step, a rough rolling step of hot rolling starts to be
10 performed without a wait on the slab extracted from a heating furnace; thus, a rough
bar is obtained. In the rough rolling step, the total reduction ratio during rough
rolling is set to 50% or more, and a slab surface layer is cooled to Ar3 transformation
point, expressed by formula (B) below, or lower twice or more, preferably three
times or more, during rough rolling. Specifically, the rough rolling step is
15 performed as multi-pass hot rolling, and a surface layer of the slah that has gone
through the previous pass is once cooled to Ar3 transformation point or lower and
then recuperated to a temperature higher than Ar3 transformation point. The slab
whose surface layer has been recuperated is rolled in a subsequent pass, and a surface
layer of the slab is cooled again to Arj transformation point or lower. This process
20 is repeated. The temperature of the slab surface layer in the present invention refers
to the temperature of the slab in a portion at 1 mm in the depth direction from the
slah surface, and can be estimated by thermal transfer calculation, for example.
Cooling the inside of the slab as well as the slab outermost surface to AQ
transformation point or lower increases the effect of recuperation.
25 Ar3(oC)=901-325~C+33~Si+287~P+40~Al-92x(Mn+Mo+
Cu) - 46 x Ni ... (B),
where each chemical symbol denotes the content (mass%) of the element.
[0066]
The rough rolling conditions are thus specified for the following reason.
30 To obtain a11 effect of the present invention of obtaining a hot-rolled steel sheet with
favorable fatigue characteristics in the rolling direction, it is essential that, in the hotrollcd
steel sheet, the sum of X-ray rando~nin tensity ratios of 4 1I > orientation and
orientation as viewed from the rolling dircction be 3.5 or more, and an X-ray
random intensity ratio of .<001> orientation as viewed from the rolling direction be
less than 1 .O. 'To thus control crystal orientation, it is important to make shear force
5 act on the steel shcet to develop 101 1> orientation and 11 1 I> orientation as strongly
as possible so that they reach a portion close to the center of the shcet thickness.
The influence of a structure formed by the action of shear force during rough rolling
is normally eliminated by recrystallization after rough rolling. The study by the
present inventors, however, has revealed that when the slab surface layer is once
10 cooled to Ar3 transformation point or lower during rough rolling, the structure during
rough rolling exerts a preferable influence on a final structure. A presumable
mechanism is described below.
[0067]
When sufficient shear force is applied during rough rolling and the slab
15 surface layer is once cooled to Ar3 transformation point or lower, the structure around
the surface layer is partly transformed from austenite to ferrite. At this time, ferrite
is influenced by the shear force during rough rolling;-thus, as viewed from the rolling
direction, <11 1> orientation and <011> orientation increase and <001> orientation
decreases.
20 [0068]
Ferrite in the surface layer is recuperated and reversely transformed to
austenite by the next pass. On this occasion, austenite that has undergone reverse
transformation has an orientation having a certain orientation relationship with the
crystal orientation of ferrite before transformation. When the surface layer
25 austenite after reverse transformation is further subjected to rough rolling and cooled
again to Ari transformation point or lower, the surface layer structure is partly
transformed from austenite to ferrite again. The crystal orientation of austenite
before transformation is influenced by the crystal orientation of former ferrite; thus,
orientation and <011> orientation of ferrite after transformation further
30 inerease as compared with after the previous pass.
COO691
In this manner, during rough rolling, applying sufl~cienls hear l01.ci~n each
pass and transforming a surface layer by cooling to Am transformation point or lower
are repeated; thus, near the surface layer, orientation and <011> orientation
increase and <001> orientation decreases. To sufficiently exert this effect, it is
5 necessary to set the reduction ratio during rough rolling to 50% or more to apply
sufficient shear force; hence, in this step, the slab surface layer is cooled to Ar3
transformation point or lower twice or more, preferably three times or more.
[0070]
[dl Finish rolling step
10 In a finish rolling step following the rough rolling step, two passes or more
of rolling in which a shape ratio X, determined from formula (C) below, is 2.3 or
more are performed at a slab surface layer temperature of 1100°C or lower to make
the total reduction ratio 40% or more.
[Math. 11
where L denotes the diameter of a rolling mill roll, hi, denotes the sheet
thickness on the rolling mill roll entry side, and h,,, denotes the sheet tllickness on
the rolling mill roll exit side.
[0071]
20 The present inventors have found that, to make shear force of hot rolling act
on the steel sheet deeply by rolling at ll0O0C or lower, it is necessary to satisfy a
shape ratio X, specified in formula (C), of 2.3 or more in at least two passes of the
total number of passes of hot rolling. As expressed by formulae (Cl) to (C3) below,
the shape ratio X is a ratio between contact arc length Id and average sheet thickness
25 h, of the rolling mill roll and the steel sheet.
X= ld/hm ... (Cl)
Id = (L x (h,,, - h,,,,)/2)In ... (C2)
h,,, = (h,,, + h0,,t)/2 ... (C3)
[0072]
Evcn if the shape ratio X determined from formula (C) is 2.3 or more, one
pass of rolling 1s not enough for suffic~entin troduction depth of shear strain. When
5 the introduction depth of shear strain is insufficient, alignment of ferrite in
orientation and <011> orientation as viewed from the rolling direction is weak,
which results in a reduction in fatigue characteristics in thc rolling direction.
Accordingly, the number of passes in which the shape ratio X is 2.3 or more is set to
two or morc.
10 [0073]
The number of passes of rolling in the finish rolling step is preferably as
large as possible. When the number of passes is three or more, the shape ratio X
may be set to 2.3 or more in all passes. To increase the thickness of a shear layer,
the value of the shape ratio X is preferably as large as possible. The value of the
15 shape ratio X is preferably 2.5 or more, further preferably 3.0 or more.
[0074]
When rolling in which the shape ratio X is 2.3 or more is performed at high
temperature, subsequent recrystallization may break textures increasing Young's
modulus. Hence, rolling in which the number of passes in which the shape ratio X
20 is set to 2.3 or more is confined is performed in a state where ihe slab surface layer
temperature is 1100°C or lower. In addition, a larger amount of introduction of
shear strain leads to further developn~ent of crystal grains in orientation and
<011> orientation as viewed from the rolling direction, which improve fatigue
characteristics in the rolling direction of the steel sheet. This effect is significant
25 when the total reduction ratio at ll0OoC or lower is 40% or more; hence, the total
reduction ratio at 1100°C or lower is set to 40% or more.
[0075]
Reduction in the final pass of finish rolling is performed at equal to or
higher than (T2 - 100) OC and lower than (Tl + 20) "C, preferably equal to or higher
30 than (T2 - 100) "C and lower than T2 (OC), and the reduction ratio is set to equal to or
more than 3% and less than 40%. ll1e reduction ratio is preferably equal to or more
than 10% and less Lhan 40%. Tz is a temperature spccificd in ftil.mula (D) below.
T z ( " C ) = 8 7 0 + 1 0 x ( C + N ) x M n + 3 5 0 x N b i - 2 5 0 x T i i + 4 0 x B + 1 0 x
Cr+100xMo+l00xV ...( D),
where each chemical symbol denotes the content (mass%) of the element.
5 LO0761
Reduction conditions in this final pass are very important in controlling an
aspect ratio of the hard phases in the sheet-thickness central portion. Performing
rolling in a temperature range of equal to or higher than (T2 - 100) "C and lower than
(T2 + 20) "C increases the aspect ratio of the hard phases in the sheet-thickness
10 central portion, presumably because rolling in a state where recrystallization is
inhibited increases the aspect ratio of austenite, and the shapes are inherited by the
hard phases. To exert this effect of increasing the aspect ratio of the hard phases, it
is necessary to set the reduction ratio in the final reduction to 3% or more. Rolling
with a reduction ratio of40% or more places a great burden on a rolling mill; hence,
15 a reduction ratio equal to or more than 3% and less than 40% is preferable.
[0077]
If reduction in the final pass is performed in a temperature range of lower
than (T2 - 100) 'C, rolling proceeds in a dual-phase region of ferrite and austenite,
and ferrite undergoes work hardening, which results in a reduction in the ductility of
20 the steel sheet. If reduction in the final pass is performed in a temperature range of
(Tz + 20) "C or higher, the aspect ratio of the hard phases in the sl~cet-thickness
central portion is small. This is presumably because promotion of recrystallization
of austenite. resulting in reduced aspect ratio of austenite, influences the forms of the
hard phases. Hence, reduction in the final pass is performed in a temperature range
25 of equal to or higher than (T2 - 100) OC and lower than (T2 + 20) OC. Reduction
under these conditions makes the aspect ratio of the hard phases 3 or more.
[0078]
[el First cooling step
In a first cooling step following the finish rolling step, the average cooling
30 rate from the final reduction temperature of finish rolling to 750°C is set to 60°C/s or
more, becausc a cooling rate less than 60°C/s may cause the length in the sheet
thickness direction of the hard phases in the sheet-thickness central portion to be 20
pm or more. Although the cause of the coi~elation bctwcen cooling rate and the
length in the sheet thickness direction of the hard phases is uncertain, there is a
possibility that a cooling rate of 6O0C/s or more inalces it difficult for dislocation
5 introduced in thc final reduction of finish rolling to recover, and the dislocation
works as the core of ferrite transformation; thus, untransformed austcnite in the
sheet-thickness central portion is divided by ferrite, which results in a reduction in
the length in the sheet thickness direction of the hard phases.
[0079]
I0 In the field of steel plates, there has been an example aiming at inhibition of
fatigue crack propagation by control of an aspect ratio of hard phases, but no
document has reported achievement of both the inhibition of fatigue crack
propagation and workability such as hole expandability, probably for the following
reasons. In the field of steel plates, rolling strain does not easily reach a plate-
15 thickness central portion. In addition, because of thick plate thickness, cooling rate
in the plate-thickness central portion is not enough, so that recovery of dislocation
proceeds; consequently, the core of ferrite transformation cannot be introduced
sufficiently, which prevents a reduction in the length of the hard phases.
[0080]
20 [fl Soaking step
In a soaking step following the first cooling step, the steel shect is held for 5
s or more in a temperature range of equal to or higher than 600°C and lower than
750°C. The soaking step is essential to obtaining a microstructure whose main
constituent is ferrite. The holding lime is set to 5 s or more, because holding time
25 of 5 s or less does not allow ferrite to serve as the main constituent of the
microstructure or makes an area fraction of the hard phases 20% or more, which
reduces ductility and a hole expansion ratio.
[0081]
[g] Second cooling step
30 In a second cooling step following the soaking step, with regard to a
temperature T3("C) specified in formula (E) below, the average cooling rate in a
temperature range of equal to or higher than T3("C) and lower than 600°C is set to
5O0C/s or more. The average cooling rate is set to 50°C/s or more because an
average cooling rate of less than 5O0C/s leads to generation of bainite and pcarlite in
the structure, making it difficult to obta~ue nough fraction of the hard phases, which
5 causes notch fatigue characteristics to detcriorate.
T3 ("C) = 561 - 474 x C - 33 x Mn - 17 x Ni - 17 x Cr - 21 x Mo ... (E),
where each chemical symbol denotes the content (mass%) of the element.
[0082]
[h] Winding step
10 The steel sheet is wound after the second cooling step. The te~nperatureo f
the steel sheet in winding (winding temperature) is set to T3("C), specified in formula
(E), or iower. Winding at a high temperature exceeding T3("C) leads to generation
of bainite and pearlite in the structure, making it difficult to obtain enough fraction of
the hard phases, which causes punching fatigue characteristics to deteriorate.
15 [0083]
Through the production steps described above, a hot-rolled steel sheet of the
present invention is produced.
[0084]
After the completion of all of the steps [a] to [h], for the purpose of
20 correcting the shape of the steel sheet or of improving ductility by introducing
mobile dislocation, or the like, skin pass rolling in which a reduction ratio is equal to
or more than 0.1% and equal to or less than 2% is preferably performed. In
addition, after the completion of all of the steps, for the purpose of removing scales
attached on the surface of the obtained hot-rolled steel sheet, pickling may be
25 performed on the obtained hot-rolled steel sheet as necessary. Furthermore, after
pickling, skin pass rolling or cold rolling in which a reduction ratio is 10% or less
may be performed on the obtained hot-rolled steel sheet in-line or off-line.
[0085]
A hot-rolled steel sheet of the present invention is produced through, in
30 addition to the rolling steps specified in the present invention, continuous casting,
pickling, and the like, which are nomial hot-rolling steps. Even if produced with
the steps other than thosc spccificd in thc picsent invention paltly slzipped, the hotrollcd
steel sheet can have excellent fatigue characteristics in thc lolling direction
and workability, which are effects of the present invention.
LO0861
5 Moreover, even if, after the hot-rolled steel sheet is once produced, hcat
treatment is performed on-line or off-line in a temperature range of 100 to 600°C for
the purpose of improving ductility, the hot-rolled steel sheet can have excellent
fatigue characteristics in the rolling direction and workability, which are effects of
the present invention.
10 [0087]
The hot-rolled steel sheet produced through the above steps may be
subjected to an additional step, such as performing hot dip galvanizing or alloyed hot
dip galvanizing, or performing surface treatment by formation of an organic film,
film laminating, organic saltslinorganic salts treatment, non-chromium treatment, and
15 the like.
[0088]
4. Method for evaluating characteristics of hot-rolled steel sheet
(1) Tensile strength characteristics
Of mechanical properties of a hot-rolled steel sheet, tensile strength
20 characteristics (tensile strength and total elongation) are evaluated in conformance
with JIS Z 2241 20 11. A test piece is No. 5 test piece of JIS Z 224 1 201 1, taken
from a 1/4W (This means a position at 114 of the width of the steel sheet in the width
direction of the steel sheet from the end portion in the width direction of the steel
sheet. The same applies to the following.) or 3/4W position of the sheet width of
25 the steel sheet with the rolling direction serving as the longitudinal direction.
[0089]
(2) Hole expansion ratio
A hole expansion ratio of a hot-rolled steel sheet is evaluated by a hole
expansion test in conformance with a test method described in the Japan Iron and
30 Steel Federation Standard JFS T 1001-1996. A test piece is taken from a position
similar to that of the tensile test piecc, and is provided with a punching hole by a
cylindrical punch. A steel sheet with exccllcnt worlcability in the present invention
refers to a steel sheet that satisfies (tensile strength (MPa)) x (total elongation (%)) 2
18000 and (tensile strength (MPa)) x (hole expansion ratio (%)) 1 35000.
[0090]
5 (3) Fatigue characteristics
FIG. 1 is a schematic diagram illustrating the shape and dimensions of test
pieces used for a fatigue test. FIG. l(a) is a plan view and a front view of a test
piece for measuring htigue strength without a notch. FIG. l(b) is a plan view and a
front view of a test piece for measuring fatigue strength with a notch.
10 [0091]
To evaluate fatigue characteristics in the rolling direction of a hot-rolled
steel sheet, test pieces with the shape and dimensions illustrated in FIG. 1 are used.
Each test piece is taken from a position similar to that of the tensile test piece with
the rolling diection serving as the longitudinal direction. The test piece illustrated
15 in FIG. ](a) is a test piece for obtaining fatigue strength without a notch. The test
piece illustrated in FIG. I(b) is a punched test piece for obtaining fatigue strength of
a notched material, and is provided with a punching hole 1 by a cylindrical punch
like the hole expansion test piece in order to allow evaluation close to fatigue
characteristics evaluation in actual use of automobile parts. A punching clearance is
20 set to 10%. Both fatigue test pieces are subjected to grinding for three triangle
finish (expressed by surface roughness finish symbols) from the outernlost layer to a
depth of approximately 0.05 mm.
[0092]
Using these test pieces, a stress controlled tensile-tensile fatigue test is
25 performed under conditions of a stress ratio R of 0.1 and a frequency of 15 to 25 I3z.
A steel sheet with excellent fatigue characteristics in the rolling direction in the
present invention refers to a steel sheet whose value (fatigue limit ratio) obtained by
dividing fatigue strength at 10 million cycles obtaincd with the notch-free fatigue test
piece by tensile strength obtained in the tensile test is 0.55 or more, and whose value
30 (punching fatigue limit ratio) obtaincd by dividing fatigue strength at 10 million
cycles obtained in the punching fatigue test by tensile strength obtained in the tensile
test is 0.30 or more
[0093]
Hereinafter, the present invention will be described mole specifically in
Examples. Note that the present invention is not limited by the following Examples.
5
[Examples]
[0094]
Molten steel having chemical compositions shown in Table 1 was produced.
100951
[Table 11
10096)
According to Table 1, chemical compositions of steels A to I were within a
chemical composition range specified in the present invention. Meanwhile, steel
"a" had too low a C content, steel "b" had too high a C contcnt, steel "c" had Loo
5 high a P content, and steel "d" had too high a S content. The underlines indicate
component amounts falling outside the invention range.
100971
Using the molten steel with the chemical compositions of steels A to J and
steels "a" to "d", hot-rolled steel sheets were produced by the above-described steps
10 [a] to [h]. Each step was performed under conditions shown in Tables 2 and 3. In
step [dl, rolling at llOO°C or lower was performed in six passes of PI to P6. Steels
A to J and steels "a" to "d" shown in Tables 2 and 3 correspond to the molten steel
with the chemical compositions shown in Table 1, and indicate the used molten steel.
As TI ("C), the average temperature of a soaking area of a heating furnace was
1.5 measured as the average temperatue of the slab in a soaking area. P 1 to P6 indicate
first to sixth passes in the finish rolling step.
Table 2 Production canditions
Shaoe ratio X in eaoh rolline
100991
[Table 31
Table 3 Production conditions
10 loo]
Regarding the pioduced hot-rolled steel sheets, arca fractions of fcrritc, hard
phases (inartensite and austenite), and the other structure were dcteiinincd, and the
shapes of ferrite grains and the hard phases and an X-ray random intensity ratio were
5 measured. In addition, tensile strength characteristics, a hole expansion ratlo, and
fatigue characteristics were measured. As conditions for measuring these
charactcristics, the above-described measurement conditions were used. Fatigue
test pieces with the shape and dimensions illustrated in FIG. 1 were used, and the
thickness of each test piece was set to 3 mm. Tables 4 and 5 show the measurement
10 results of the characteristics. Steel grades of the hot-rolled steel sheets were a hotrolled
steel sheet without plating (HR), a hot-dip galvanized steel sheet without
alloying after plating (GI), and an alloyed hot-dip galvanized steel sheet (GA).
7, r--.
Table 4 Properties +$ 2
,
0.70 1 Comparative ifeel
[i: 2
0 .....
P
d
---
E-2
E-3
E-4
E-5
-
C-7
C-8
D- 1
E- i
Sum af X-ray random
intensity ratios of
orientation as
iiewed from roiling
direclian
HR
HR
GA
HR
Steel Remarks
HR
GA
HR
HR
Ferrite
volume
(")
Volume
haction
itrurture !\I
Steel grade*
88.1
88.5
92.0
88.4
Length in raiiing
direction of hard
in sheet.
central portion
!#m)
Proportion of hard
phase with srpert
of more
in hard in
sheet-thickness
centrai portion (b!
90.4
93.8
88.0
89.5
Sum of rulvme
fraotions of
martenrite and
avrieniie (4)
l i . 9
13.3
6.0
13.6
Aversge
aspeot rstio
of
grains
Name Of
other
5fructurc
3.9
1.4
12.0
10.5
Sum of X-ray random
intensihi ratios of (01 1)
orientation and
orientation as "ie,ved
from rolling direction
891 25.5
919j 17.8
91.21 12.7
951 12.8
bainite
bainite
e2.7
2.5 1 4.52 1 070 / Developed steel
4.0 1 3.26 / 103 / Cam~arativc steel
3.6 / 5.99 / 0.55 1 Developed steel
5.7
5
89.3
88
83.7
83.9
13.5
9.3
10.7
12.5
0.64 I Developed steel
3.7 1 5.04 1 0.88 I Camparative steel
0.58 Develapad sk$e!-
0.65 Developed steel
[0 1031
5 As shown in Tables 2 to 5, steels A-1, B-1, C-I, C-3, C-5, C-7, D-1, E-1, E-
3, E-5, E-7, E-9, E-10, E-13, E-14, E-17, E-18, l?-l, G-l, H-l, 1-1, and J-l are
examples each having a chemical composition and microstructure of steel satisfying
those specified in the present invention. Meanwhile, steels C-2, C-4, C-6, C-8, E-2,
E-4, E-6, E-8, E-11, E-12, E-15, E-16, a-1, b-I, c-1, and d-1 a e examples each
having a chemical cornpos~tion and microstructure of stccl not satisfying thosc
specified in the presenl invention. The "other structure" of each of C-6 to C-8 was
hainite.
[O 1041
5 For all of the hot-rolled steel sheets of the present invention exanlples, such
as steel A-1, the area fraction of the hard phases, the proportion of the hard phases
with an aspect ratio of 3 or more in the hard phases present in the sheet-thickness
central portion, the length in the rolling direction of the hard phascs present in the
sheet-thickness central portion, the average aspect ratio of ferrite grains, and the X-
10 ray random intensity ratios all satisfied those specified in the present invention.
Furthermore, all of the hot-rolled steel sheets of the present invention examples
satisfied (tensile strength (MPa)) x (total elongation (%)) t 18000 and (tensile
strength (MPa)) x (hole expansion ratio (%)) 2 35000, and exhibited a fatigue limit
of 0.55 or more and a punching fatigue limit of 0.30 or more.
15 [0105]
For steel C-2 as a comparative example, the average cooling rate from the
final reduction temperature to 750°C in step [el was 43'CIs, which is too low.
Therefore, the length in the rolling direction of the hard phases in the sheet-thickness
central portion was as long as 22.9 pm, and (tensile strength (MPa)) x (hole
20 expansion ratio (%)) 1 35000 was not satisfied.
[0106]
For steel C-4, the holding time in a temperature range of equal to or higher
than 600°C and lower than 750°C in step [fJ was 3.1 s, which is too short; thus, the
area fraction of the hard phases was as high as 83.0%, ferrite not serving as the main
25 constituent of the microstructure. Therefore, low ductility was exhibited, and
(tensile strength (MPa)) x (total elongation (%)) > 18000 was not satisfied.
[0107]
For steel C-6, the average cooling rate in a temperature range of equal to or
higher than T3("C) and lower than 600°C in step [g] was too low. For steel C-8, the
30 winding temperature in step [h] was 513"C, which is higher than T3 (494OC).
Therefore, bainite was generated in the structure of the hot-rolled steel sheet, and the
area fiaction of the had phases was as low as less than 3%. Consequentl~: the
punching fatigue limit ratio in the rolling direction was as low as less than 0.3.
[0108]
For steel E-2; the slab heating time in step [b] was 1168 s, \which is shorler
5 than time ti (1244 s) specified in formula (A). Therefore, the length in the rolling
dircction of the hard phases in the sheet-thickness central portion was as long as 25.5
pm, and (tensile strength (MPa)) x (hole expansion ratio (%)) 2 35000 was not
satisfied.
[O 1 091
10 For steel E-4, the total reduction ratio during rough rolling in stcp [c] was as
low as 46%. For steel E-6, the number of times of cooling the slab surface layer to
Ar3 trailsformation point or lower during rough rolling in step [c] was only once.
For steel E-8, of the six rolling passes in step [dl, only one pass satisfied a shape ratio
X of 2.3 or more. For steel E-11, the reduction ratio of rolling at llOO°C or lower
15 in step [dl was as low as 35%. Therefore, in these steels, the sum of X-ray random
intensity ratios of <011> orientation and orientation as viewed from the
rolling direction was as low as less than 3.5, and an X-ray random intensity ratio of
<001> orientation as viewed from the rolling direction was more than 1.0.
Consequently, these steels all exhibited a fatigue limit ratio in the rolling direction as
20 low as less than 0.55.
[OllO]
For steel E-12, the reduction temperature in the final pass of finish rolling in
step [dl was 762"C, which is lower than T2 (877'C), specified in formula (D), by
more than 100°C. Therefore, the average aspect ratio of ferrite grains was as large
25 as 6.3, and in the tensile test, ferrite grains underwent work hardening to reduce the
ductility of the steel sheet. Consequently, (tensile strength (MPa)) x (total
elongation (%)) 2 18000 was not satisfied.
[Olll]
For steel E-15, the reduction temperature in the final pass of finish rolling in
30 step [dl was 913"C, which is higher than T2 (877"C), specified in formula (D), by
more than 20°C. For stecl E-16, the reduction ratio in the final pass of finish rolling
in stcp [dl was as low as 2%. Thercforc, for hot11 examples, the proportion ol' the
hard phases with an aspect ratio of 3 or more in the hard phases prcscnt in the sheetthickness
central poilion was as low as less than 60%, and the punching fatigue limit
ratio in the rolling dircction was as low as less than 0.3.
5 [0112]
For steel a-I, the C content was 0.018%, which is too low. Therefore, the
punching fatigue limit ratio in the rolling direction was as low as less than 0.3.
[0113]
For steel b-1, the C content was 0.254%, which is too high. For steel d-I,
10 the S content was 0.0361%, which is too high. Therefore, both examplcs exhibited
low hole expandability and did not satisfy (tensile strength (MPa)) x (hole expansion
ratio (%)) 2 35000.
[0114]
For steel c-1, the P content was 0.155%, which is too high. Therefore, low
15 workability was exhibited, and neither (tensile strength (MPa)) x (total elongation
(%)) ? 18000 nor (tensile strength (Ma)) x (hole expansion ratio (%)) 2 35000 was
satisfied.
Industrial Applicability
20 [0115]
According to the present invention, a hot-rolled steel sheet having excellent
fatigue characteristics in the rolling direction and workability can be provided. The
present invention can be suitably applied to steel sheets with a sheet thickness of 8
mm or less. A hot-rolled steel sheet according to the present invention can prolong
25 fatigue life of suspension parts and the like of automobile materials, thus
significantly contributing to the industry.
Reference Signs List
[01 161
30 1 punching hole of fatigue test piece
CLAIMS
Claim 1
A hot-rollcd steel sheet having a chemical cornposition consisting of, in
mass%,
5 C: 0.03 to 0.2%,
Mn: 0.1 to 3.0%,
P: 0.10% or less,
S: 0.03% or less,
A1 + Si: 0.2 to 3.0%,
N: more than 0% and equal to or less than 0.01%,
0: more than 0% and equal to or less than 0.01%,
Ti: 0 to 0.3%,
Nb: 0 to 0.3%,
Mg: 0 to 0.01%,
Ca: 0 to 0.01%,
REM: 0 to 0.1%,
B: 0 to 0.01%,
Cu: 0 to 2.0%.
Ni: 0 to 2.0%,
Mo: 0 to 1.0%,
V: 0 to 0.396,
Cr: 0 to 2.0%, and
the balance: iron and impurities,
wherein a microstructure of the hot-rolled steel sheet contains ferrite as the
25 main constituent, and contains hard phases constituted by martensite and/or austenite
in an amount of, in area fraction, equal to or more than 3% and less than 20%,
hard phases with an aspect ratio of 3 or more accounts for 60% or more of
the hard phases present in a sheet-thickness central portion,
a length in a rolling direction of the hard phases present in the sheet-
30 thickness central pofiion is less than 20 hm,
an average aspect ratio of ferrite grains is less than 5, and
the sun1 of X-ray random intensity ratios oS orientation and
oricntation as viewed from the rolling direction is 3.5 or morc, and an X-ray random
intensity ratio of <001> orientation as viewed from the rolling direction is 1.0 or lcss.
5 Claim 2
The hot-rolled stcel sheet according to claim 1, comprising, in mass%,
one or more selected from
Ti: equal to or more than (0.005 + 48114m] + 48/32[S])% to equal to or less
than 0.3%, and
10 Nb: 0.01 to 0.3%,
where [N] indicates an N content (mass%) and [S] indicates an S content
(mass%).
Claim 3
15 The hot-rolled steel sheet according to claim 1, comprising, in mass%,
one or more selected from
Mg: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%, and
REM: 0.0005 to 0.1%.
20
Claim 4
The hot-rolled steel sheet according to claim 1, comprising, in mass%,
B: 0.0002 to 0.01%.
25 Claim 5
The hot-rolled steel sheet according to claim 1, comprising, in mass%,
one or more selected from
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
Mo: 0.01 to 1.0%,
V: 0.01 to 0.3'4, and
Cr: 0.01 to 2.0%.
Claim 6
The hot-rolled steel sheet according lo claim I, comprising a hot-dip
5 galvanized layer or a galvannealed layer on its surface.