The present invention relates to a hot-rolled steel sheet.
Background Art
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 patts of automobiles are required to have
15 fatigue characteristics of a notch-free material and notch fatigue characteristics, 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-free material, it is effective to
makc the structure finer. For example, Patent Literatures 1 and 2 describe hotrolled
steel sheets that contain ultrafine ferrite grains with an average grain size of
less than 2 pm as hot-rolled, and contain bainite or the like as a secondary phase. It
is described that these steel sheets are excellent in ductility, toughness, fatigue
25 strength, and the like, and have low anisotropy in these 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, which is tile main phase, gradually decreases fiom the center of
30 the sheet thickness toward the surface layer and containing, in volume fraction, 5%
or more bainite or the like as a secondary phase. Furthern~ore, grain refining of a
martensite structure is also effective in itnproving fatigue characteristics. Patent
Literature 4 describes a machine structural steel pipe in which 80% or more in area
fraction of the ~nicrostructure is mastensite, the average block dian~etcr of the
martensitic structure is 3 CIIII or less, and the maximu~nb lock diameter is 1 to 3 times
5 the average block diameter, Patent Literature 4 also describes making the stsucture
of a slab before pipe-making into lo\ver bainite or martensite in hot rolling to
unifor~nly disperse carbon. However, although grain refining improves fatigue
characteristics of a notch-free material, it has no effect of delaying a crack
propagation rate, thus not contributing to an improvenlent in 11otc11 fatigue
10 characteristics.
[0004]
Regarding an improvement in notch fatigue characteristics, it has been
reported that it is effective to reduce a crack propagation rate by fornhg a composite
structure. In Patent Literature 5, hard bainite or martensite is dispersed in a
15 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.
Patent Literature
[OOOS]
Patent Literature 1 :
Patent Literature 2:
Patent Literature 3:
Patent Literature 4:
Patent Literature 5:
Patent Literature 6:
Patent Literature 7:
Citation List
Summary of invention
Technical Problem
[0006]
Patent Literature 5 does not describe a technique for improving press
formability, and does not pay pat-ticnlar attention to the hardness and shape of bainite
5 and martensite; hence, it seems that favorable press lormability is not achieved.
[0007]
Patent Literatures 6 and 7 lack consideration of ductility and workability
(e.g., hole expandability), which are needed in press for~ning.
[0008]
10 The present inventiot~h as 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 and having a tensile strength of 780 MPa or more.
Solution to Problem
15 [0009]
The present inventors have carried out extensive resexch 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, hy optimizing the
chen~ical colnposition and production conditions of a high-strength hot-rolled steel
20 sheet to control the microstructure of the steel sheet. The gist of the present
invention is as follows.
[OOlO]
(1)
A hot-rolled steel sheet having a chemical cotnposition consisting of, in
25 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 .ON,
V: 0 to 0.396,
Cr: 0 to 2.0%, and
the balance: iron and impurities, and
a microstn~ch~oref the hot-rolled steel sheet which contains bainite as the
15 main constituent, and contains hard phases constituted by martensite and/or austenite
in an amount of, in area fraction, eqnal to or more than 3% and less than 20%,
wherein 60% or Inore of the hard phases present in a sheet-thickness central
portion have an aspect ratio of 3 or more,
the hard phases present in the sheet-thickness central portion have a length
20 in a rolling direction of less than 20 pm, and
the sum of X-ray random intensity ratios of <011> orientation and
orientation as viewed fiom the rolling direction is 3.5 or more, and an X-ray random
intensity ratio of <001> orietitatioti as viewed from the rolling direction is 1.0 or less.
[OOII]
25 (2)
The hot-rolled steel sheet according to (I), containing, in mass%,
one or more selected from
Ti: equal to or rnore than (0.005 + 48/14m] + 48/32[S])% to equal to or less
than 0.3%, and
Nb: 0.01 to 0.3%,
where [N] indicates an N content (mass%) and [S] indicates an S content
(mass%).
[OO 121
(3)
The hot-rolled steel sheet according to (I), containing, it1 mass%,
one or tnore selected fro111
M~'0:. 0005 to 0.01%,
Ca: 0.0005 to 0.01%, and
REM: 0.0005 to 0.1%.
[00 131
10 (4)
The hot-rolled steel sheet according to (I), cotltainitig, in mass%,
B: 0.0002 to 0.01%.
[0014]
(5)
15 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 1.0%,
V: 0.01 to 0.3%, and
Cr: 0.01 to 2.0%.
[OOlS]
(6)
The hot-rolled steel sheet according to (I), comprising a hot-dip galvanized
25 layer or a galvannealed layer on its surface.
Advantageous Effects of Invention
[0016]
: According to the present invention, a hot-rolled steel sheet having excellent
30 fatigue characteristics in the rolling direction and workability atid having a tensile
strength of 780 Ml'a or nlore can be provided. The present iriverltion can be
suitably applied to steel sliccts with a slieet thickness of 8 tinn or less. A hot-rolled
steel sheet according to the present invention can prolong fatiguc life of suspension
parts and tlie like of a~~totilobilmea terials, thus significantly contributing to the
industry.
5
Brief Description of Drawings
[00 1 71
[FIG. I] FIG. 1 is a schematic diagram illustrating the shape and dimensions of test
pieces used for a fatigue test. FIG. ](a) is a plan view and a front view of a test
10 piece for measuring fatigue 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.
Description of Embodiments
[0018]
15 1. Microsttucture of hot-rolled steel sheet
1-1. Area fraction of eaclt phase constituting hot-rolled steel sheet
A hot-rolled steel sheet of the present invention is required to contain bainite
as tile main constituent, and contain hard pliases constituted by martensite and/or
austenite in an atnount of, in area fraction, equal to or more than 3% and less than
20 20%. When the microstmcture is a composite structure having hard phases as a
secondary phase in soft bainite serving as the main pliase, the tnain phase improves
ductility and the secondary phase improves strength, so that tlie steel sheet has a
favorable balance between strength and ductility. Containing bainite as the main
constituent means that bainite serving as the tilain phase in the hot-rolled steel sheet
25 has tlie highest area fraction. Convetitiotiall~~s,t eel sheets whose main pliase is
ferrite and secondary phase is the above-described hard phases have been widely
used. Particularly when a strength of 780 MPa or more in tensile strength is
required, bainite is used as the main phase in some cases. Furthernlore, hard phases
have an effect of obstructing fatigue crack propagation that occurs in a soft phase, to
30 reduce a fatigue crack propagation rate; therefore, a steel sheet having the abovedescribed
composite structure has excellent punching fatigue charactcristics. Hence,
a hot-rolled steel sheet of the present invention is made to have a ~~licrostructure
whose main cotlstituc~~ist bainite and in \vhich hard phases constituted by martensite
and/or austenite are distributed as a secondary phase. The area fiaction of bainite is
preferably 65 to 97%.
5 [0019]
The hard phases exert the effect of inhibiting fatigue crack propagation
when their area fiaction is 3% or more. 011 the other hand, ~vlvhen 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 hole expansion ratio, so that "(tensile strength (MPa))
10 x (hole expansion ratio (%)) 2 35000", 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
less than 20% in area fraction in a microstruch~rew hose main constituent is bainite.
The hard phases are preferably present in at1 amount of, in area fraction, 5% or more,
15 fi~rthepr referably 7% or more.
[0020]
1-2. Aspect ratio of hard phase present in sheet-thickness central portion
Next, descriptiotl is give11 on ao aspect ratio of hard phases present in a
sheet-tl~ickness central portion. whet^ a punching fatigue test is performed in an
20 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
in the sheet-thickness central portion are particularly importat~t.
[0021]
25 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
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
30 direction of the steel sheet". With an incrcase i r ~th e aspect ratio of the hard phase,
the frequency at xv11ich a crack hits the hard phase, which obstructs fatigue crack
propagation, increases, and also the detouri~tg/brat~chi~di~sgta nce of the crack
increases, which is effective in reducing a fatigue crack propagation rate. Here, a
hard phase whose aspect ratio is less than 3 has a sti~alle ffect of inhibiting crack
propagation because the detouring/brancliitig distance whcn a crack hits the hard
5 phase is small. Therefore, it is effective to incrcase hard phases whose aspect ratio
is 3 or more. For this reason, in the hot-rolled stcel sheet of the present invention,
hard phases with an aspect ratio of 3 or more are made to account for 60% or more of
tlie hard phases present in the sheet-thickness central portion. The hard phases with
an aspect ratio of 3 or Inore preferably account for 80% or Inore of the hard phases
10 present in the sheet-thickness central portion.
[0022]
1-3. Length in rolling direction of hard phases present in sheet-thickness central
portion
Description will be given on the length in the rolling direction of the hard
15 phases present in the sheet-thickness central portion. When steel with a composite
st~ucturec ontaining bainite arid hard phases are deformed, tlie bainite side, which is
soft, preferentially undergoes plastic defortnation, and accordingly stress on the hard
phases increases with the defol-ilation, so that great strain occurs at the interface
between bainite and the hard phases.
20 [0023]
When the stress on the hard phases or the strain at the interfaces between
baitiite and the hard phases exceed a certain degree, defects called voids occur in the
steel, and these voids connect to each other, resulting in rupture. A material in
which voids easily occur is vt~ltierable to local deformation and has low hole
25 expandability.
100241
When the hard phases extend in the rolling direction, stress and strairi in
deformation are concentrated on the hard phase, causing voids to occur early; thus,
hole expandability is likely to deteriorate. In addition, in tlie sheet-thickness central
30 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.
[0025]
According to the study by the present inventors, when the length in thc
rolling direction of the hard phases present in the sheet-thickness central portion is
5 controlled to less than 20 pn~t,h e occurrcnce of voids can be inhibited, and "(tensile
strength (MPa)) x (hole expansion ratio (%)) 2 35000", which is required of
suspension parts of automobiles, can be achieved. Hence, in a hot-rolled steel sheet
of the present invention, the length in the rolling direction of the hard phase present
in the sheet-thickness central portion is specified as less than 20 ptn. The length in
10 the rolling direction of the hard phases in the sheet-thickness central portion is
preferably less than 18 p111.
[0026]
The hard phases are constituted by martensite andlor austenite. That is,
there are the following three forms: a hard phase containing only martensite, a hard
15 phase containing only austenite, and a hard phase containing both nlartensite 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
hard phase. Exanlples 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,
20 and an aggregate of one or a plurality of martensite grains and one or a plurality of
austenite grains.
[0027]
1-4. X-ray random intensity ratio
Description will be given on an X-ray random intensity ratio. Fatigue life
25 of a notch-free material is greatly influenced by life until occurrence of a fatigue
crack. The occurrence of a fatigue crack is known to proceed in a process of tlxee
steps: (1) saturation of a dislocation st~~~ctn(r2e), formation of intrusion and
extrusion, and (3) formation of a fatigue crack.
[0028]
30 As a result of extensive studies, thc present inventors have found that by
appropriately controlling crystal orientation in the stress loading direction in a fatigue
test to make the X-ray rand0111 intensity ratio satisfy a predetermined contlition, it is
possible to delay (I) saturation of a dislocatio~sl t~x~ctuorfe the process of three steps,
and improve fatigue lifc of a notch-free ~natcrial in the rolling direction. A
tnechanism for this is described below,
5 [0029]
A crystal structure of iron is a body-centered cubic (b.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 orierltatiori will be described later.
hl polycrystals, deformability varies depending on crystal orientation, and the degree
10 of difficulty of deformation is decided by a Taylor factor. The Taylor factor is a
value defined by formula (G).
dCTi = M ~..E. (G ),
where Ti denotes the amount of slip of a slip system i, Xi denotes the total
sutii of slip amounts of the whole slip that has worked, M denotes the Taylor factor,
15 and E denotes the whole plastic strain amount.
[0030]
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 plastic deformation with small energy. The Taylor factor changes depending
20 on crystal orientation with respect to the stress loading direction; thus, a crystal grain
in an.orietitation with a small Taylor factor is easily deformed, and a crystal gain in
an orientation with a large Taylor factor is difficult to defol-1x1.
[003 11
Researchers have carried out calculation to reveal that, if the above-
25 described 42 slip systems are assumed in b.c.c. metal, values of Taylor factors when
<001> orientation, <011> orientation, and orientation are subjected to tensile
deformation are 2.1, 3.2, and 3.2, respectively, and <001> orientation is deformed
most easily, which leads to early fornlation of a dislocation stn~cture. In contrast,
(01 12 orientatioti and orientation are difficult to defonn, which leads to late
30 formation of a dislocation structure. That is, a clystal grain facing <001>
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 initiatioti life.
[0032]
The prcscnt inventors have found as a result of study that, when the sum of
5 X-ray random intensity ratios of orientation atid orientation as viewed
from the rolling dircctioli is co~~trollctod 3.5 or morc and an X-ray randonn 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 lit~~it)/(tensilset rength) of 0.55 or more. "Fatigue limit" here refers to
10 fatigue strength at 10 tnillion cycles obtained with a notch-free fatigue test piece,
which is described later.
[0033]
On the basis of this finding, in a hot-rolled steel sheet of the present
invention, the su~no f X-ray random intensity ratios of <011> orientation and
15 orientation as viewed from the rolling direction is set to 3.5 or more, and an X-ray
random intensity iatio 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 1> 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
20 direction is preferably 0.8 or less.
[0034]
1-5. Method for tneasuring microstructure of hot-rolled steel sheet and X-ray random
ititc~isity
(1) Method for tneasuririg area fractions of bainite arid hard phases
25 The area fractions of bainite and hard phases coristituted by martensite
andor 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
30 to tiital etching. Ranges of 1/4 thickness (This means a position at 1/4 of the
thickt~ess of the steel sheet in the thickness direction of the steel sheet from the
surface of the steel sheet. The sanle applies to the following.), 318 thickness, and
112 thickness of the shcct thickness of the obse~~~atsiuornfa ce subjected to nital
etching are observed \vith a FE-SEM.
[0035]
5 For an obse~vation target range of each sample, ten fields of view are
observed at a 1000-fold magnification, and in each field of vie\\: the proportions of
areas occupied by bainite and the hard phases are measured. The area of the hard
phases is the total area of martensite and austenite. Then, the average values of the
proportions of areas occupied by bainite and the hard phases of all fields of \ 'l'e w are
10 obtained as the area fractions of bainite and the bard phases. This method can be
used to measure the area fraction of ferrite or the like, as well as bainite, and
tnartensite and austenite (hard phases).
[0036]
(2) Aspect ratio and length in rolling direction of hard phases present in sheet-
15 thickness central portion
The aspect ratio and the length in the rolling direction of the hard phases
present in the sheet-thickness central pottion 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 above-
20 described sample are observed 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
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 at1 aspect ratio
of 3 or more among the observed hard phases is calculated. In addition, the average
25 value of the lengths in the rolling direction of the observed hard phases is obtained as
the length in the rolling direction of the hard phase present in the sheet-thickness
central portion.
[0037]
The sheet-thickness central portion refers to a position at 112 of the
30 thickness of the steel sheet in the thickness direction of the steel sheet from the
surface of the steel sheet. For exanlple, in the sheet-thickness central portion, any
50 hard phases in a field-of-view range of 50 ~ u nx 200 pm are selected, and the
length in the rollit~gd irection and the length in the steel sheet thickness direction of
each hard phase are measured. To increase ~ncasure~ncnptr ecision, instead of
selecting any 50 hard phases, the length in the rolling direction and the Iengtlt in the
5 thickness direction niay be ~neasured for all of the hard phases in the field-of-vicw
range.
[0038]
(3) X-ray random inte~lsityra tio
X-ray random intensity ratios of <001> orientation, <011> orientation, and
10 11 11> 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
is a value obtained in the following manner: X-ray intensities of a standard sample
without integration in a particular orientation and a satnple are measured under the
same conditions by X-ray difiaction or the like, and the X-ray intensity of the
15 sample is divided by the X-ray intensity of the standard sample.
[0039]
Here, for a hot-rolled steel sheet, crystal orientation perpendicular to the
sheet surface is nor~nallyd enoted by [hkl] or {hkl}, and crystal orientation parallel to
the rolling direction is nornlally denoted by (uvw) or . {hkl} and
20 collectively refer to equivalent planes, and [hkl] and (uvw) refer to individual crystal
planes. In the present invention, wvliich is targeted at a hot-rolled steel sheet whose
main constituent is bainite including ferrite with a b.c.c. structure, for example, (1 1 I),
(-Ill), (1-111, (11-I), (-1-11)> (-11-1), (1-1-11> and (-1-1-1) planes are equivalent and
undistinguishable. In such a case, these orientations are collectively referred to as
25 <111>. In crystallography, as for "-I", "-" is put above "1" in the for~t~naolt ation of
orientation, but here, "-1" is used for notation because of constraints in description.
[0040]
A sanlple for X-ray diffraction is fabricated in the following mantles. A
rolling-direction cross-section (a cross-section perpendicular to the rolling direction)
30 of the steel sheet is polished by mechanical polishing, chemical polishitig, or the like
and mirror-finished by buffing, and then strain is re~noved by electrolytic polishing,
chemical polishitlg, or the like. The range of X-ray diffraction is the whole shect
thickness. If the whole sample catmot be measured at once, measurement tnay be
performed with the sheet thickness direction divided into several fields of view, and
the results tnay be averaged. If measurement by X-ray diffraction is difficult, a
6 statistically suflicient nr~rnber of samples may be measured by an electron back
scattering pattern (EBSP) method or an electron chatmeling pattern (ECP) method,
and an X-ray diffraction random intensity ratio of each orientation may be
determined.
[0041]
10 2. Chemical composition of steel sheet
A hot-rolled steel sheet of the present invention has a cl~emicalc otnposition
containing the following elements. Hereinafter, the elements will be described
along with the reasons for contiming the contents of the elements. In the description,
"0h " of the content of each element means "mass%.
15 [0042]
C: 0.03 to 0.2%
Carbon (C) is an important element in the present invention. C generates
mat-tensite and stabilizes austenite, thus contributing to strength improvement of the
hot-rolled steel sheet by structure strengthening, and also has an effect of inhibiting
20 crack propagation. Note that a C content less than 0.03% does not allow a
predetermined volume fraction of the hard phases to be achieved; thus, an effect of
improving punching fatigue characteristics is not exhibited. On the other hand, a C
content exceeding 0.2% leads to an excessive area fraction of a low-temperature
transformation product constituting the hard phases serving as a secondary phase,
25 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%.
[0043]
Mn: 0.1 to 3.0%
30 Manganese (MI) is contained for solid solution strengthenil~g and also in
order to increase hardenability to generate martensite or austenite in the steel sheet
structure. A MII content exceeding 3% saturates this effect. On the othcr hand, a
MII content less than 0.1% rnakes it difficult to exert an effect of inhibiting
generation of pearlite during cooling. Accordingly, the Mn content is set to 0.1 to
3.0%. The lower limit ofthe MII content is preferably 0.3%, and the upper limit is
5 preferably 2.5%.
[0044]
P: 0.10% or less
Phospl~oius (P), which is an impurity contained in hot metal, is segregated
at a gain boundary and reduces low-tenlperature tougluiess along with an increase in
10 its content. Therefore, the P content is preferably as low as possible. A P content
exceeding 0.10% adversely affects workability and weldability. Accordingly, the P
content is set to 0.10% or less. Pat-ticularlp in tenns of weldability, the upper linlit
of the P content is preferably 0.03%.
[0045]
15 S: 0.03% or less
Sulhr (S) , which is an impurity contained in hot metal, when contained too
much, causes a crack in hot rolling and also generates an inclusion, such as MnS,
which causes hole expandability to deteriorate. Therefore, the S content should be
reduced as far as possible, whereas 0.03% or less is an allowable range.
20 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%,
fttrther preferably 0.005%.
[0046]
Si + Al: 0.2 to 3.0%
25 Silicon (Si) and aluminum (Al) are both itnportant elements in the present
invention. Si and Al has an effect of inhibiting {112)<111> slip in the iron, thus
delaying formation of a dislocation structure to improve fatigue crack initiation life.
This effect is obtained at a total content of Si and Al (Si + Al) of 0.2% or more, and
is significant at 0.5% or more. Si + Al exceeding 3.0% saturates the effect and
30 leads to poor economic cficiency. Accordingly, Si + Al is set to 0.2 to 3.0%. The
lower limit of Si + Al is preferably 0.5%. The A1 content in the present invention
refers to acid-solnble Ai (so-called "sol.Al"). Only one of Si and Al tnay be
contained in an amount of 0.2 to 3.0%, or both Si and A1 tnay be contained in a total
amount of 0.2 to 3.0%.
[0047]
5 N: more than 0% and equal to or lcss than 0.01%
Nitrogen (N), when present in steel as 'SiN, contributes to an improvenicnt
in lowv-teniperatnre toughness by making a crystal grain size iiner in slab beating.
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
10 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 tenns of econotnic efficiency.
Therefore, the lower limit of the N content is preferably 0.0001% or more, fitrther
preferably 0.0005%.
[0048]
15 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 exceeding 0.01% leads
to significant deterioration of formability. Accordingly, the 0 content is set to
0.01% or less. On the other hand, an O content less than 0.001% is not preferable
20 in terms of economic efficiency. Therefore, the lower litnit of the 0 content is
preferably 0.001% or tnore.
[0049]
Ti: 0 to 0.3%
Nb: 0 to 0.3%
25 Titanium (Ti) achieves both excellent low-temperature toughness and high
strength due to precipitation strengthening. Therefore, Ti may be contained as
necessary. Carbonitride of Ti or solid solution Ti delays grain growth in hot rolling,
which makes a grain size of the hot-rolled steel sheet finer and contributes to an
itnprovement in low-temperak~reto ughness. However, a Ti content exceeding 0.3%
30 saturates this effect and leads to poor economic efliciency. Accordingly, the Ti
content is set to 0 to 0.3%. In addition, a Ti content less than (0.005 + 48/14[N] +
48/32[S])% may be unable to provide this effcct snfficiently. Hence, thc Ti content
is preferably equal to or more than 0.005 -1- 48/14[N] + 48/32[S] (%) and equal to or
less than 0.3%. Here, [N] and [S] denote the N content (%) and the S content (%),
respectivelj~. Fut-tliemio~.e,a Ti content exceeding 0.15% may cause a tundish
5 nozzle to be clogged up easily in casting. I-Ietlce, the upper limit of the Ti content is
preferably 0.15%.
[OOSO]
Niobiutn (Nb) i~nproves low-temperature tougliness of a hot-rolled steel
sheet. Therefore, Nb may be contained as necessary. Carbonitride of Nb or solid
10 solution Nb delays grain growth in hot rolling, which makes a grain size of the hotrolled
steel sheet finer and contribntes to an inlprovetnent it1 low-temperature
toughness. However, a Nb content exceeding 0.3% saturates this effect and leads to
poor economic eEciency. Accordingly, tile Nb content is set to 0 to 0.3%. In
addition, a Nb content less than 0.01% tilay be unable to provide this effect
15 sufficiently. Therefore, the lowver limit of the Nb content is prefe~ably 0.01%, and
the upper limit is preferably 0.1 %.
[005 11
Mg: 0 to 0.01%
Ca: 0 to 0.01%
REM: 0 to 0.1%
Magnesium (Mg), calcium (Ca), and rare eal-th metal (REM)-control the
form of a non-metallic inclusion, which serves as a stat-ting point of breaking to
cause deterioration of workability, and thus improve workability. Therefore, one or
more of these elements may be contained as necessary. However, an Mg content
25 exceeding 0.01%, a Ca content exceeding 0.01%, or a REM 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
is set to 0 to 0.1%. When Mg, Ca, and REM are each contained in an anlount of
0.0005% or more, the above effect is significantly exhibited. Therefore, the lower
30 limit of the Mg content is preferably 0.0005%, the lower limit of the Ca content is
preferably 0.0005%, and the lower litnit of the KEM content is preferably 0.0005%.
Notc that REM collectively refers to 17 ele~lic~litns total, including Sc, Y, arid
laathanoid, and the REM content means tlie total amouut of these elements.
[0052]
B: 0 to 0.01%
B is segregated at a grain boundary aud increases grain boundary strength to
improve low-temperature toughness. Therefore, B tilay be contained in the stccl
sheet as necessary. However, a B content exceeding 0.01% riot only saturates this
effect hut also leads to inferior eco~lomice fficiency. Accordingly, the B content is
set to 0 to 0.01%. The above effect is significant when the steel sheet has a B
10 content of 0.0002% or more. Therefore, the lower limit of the B content is
preferably 0.0002%, fi~r-thepr referably 0.0005%. The upper limit of the B content
is preferably 0.005%, further preferably 0.002%.
[0053]
Cu: 0 to 2.0%
Ni: 0 to 2.0%
Mo: 0 to 1.0%
V: 0 to 0.3%
Cr: 0 to 2.0%
Copper (Cu), nickel (Ni), molybdenum (Mo), vanadium (V), and chromium
20 (Cr) have an effect of improving the strength of a hot-rolled steel sheet by
precipitation strengthening or solid solution strengthening. Therefore, one or Inore
of these elements may be contained as necessary. Howevel; a Cu content exceeding
2.0%, a Ni content exceeding 2.0%, a Mo content exceeding 1.0'36, a V content
exceeding 0.3%, or a Cr content exceeding 2.0% saturates this effect and leads to
25 poor economic efficiency. Accordingly, the Cu content is set to 0 to 2.0%, tlie Ni
content is set to 0 to 2.0%, the Mo content is set to 0 to 1.0%, the V coritent is set to 0
to 0.3%, and the Cr cotitent is set to 0 to 2.0%. When Cu, Ni, Mo, V, and Cu are
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%,
30 fiuther preferably 0.02%. The lower limit of the Ni content is preferably 0.01%, the
lo\ver liniit of the Mo co~ltc~ilst p referably 0.01%, the lotver limit of tlie 17 coritent is
preferably 0.01%, and the lower lin~ito f the Cr content is prcfcrably 0.01%. In
addition, the upper lin~ito f the Cu content is preferably 1.2%, the upper limit of the
Ni content is preferably O.6%, the upper limit of the Mo content is preferably 0.7%,
the upper lirnit of the V content is preferably 0.2%, and the upper limit of the Cr
5 content is preferably 1.2%.
[0054]
Described above is a basic che~nicalc on~positiono f a hot-rolled steel sheet
of the present invention. The balance of the chcmical co~npositiono f the hot-rolled
steel sheet of the present invention consists of iron and impurities. Iinpurities mean
10 conlponents that are mixed in due to raw tnaterials, such as ores or scrap, or other
factors when a steel tnaterial is produced industrially.
[OOSS]
It has been confinned that, as an element other than the above elements, one
or more of Zr, Sn, Co, Zn, and W may be contained in a total amount of 1% or less,
15 instead of part of iron, withont in~painnento f the excellent fatigue characteristics in
the rolling direction and workability and the tensile strength of 780 MPa or more of
the hot-rolled steel sheet of the present invention. Among these elements, Sn may
cause a flaw in hot rolling; hence, the upper limit of the SII content is preferably
0.05%.
20 [0056]
A hot-rolled steel sheet of the present invention having the above-described
structure and co~npositionc an have improved corrosion resistance by comprising, on
the surface, a hot-dip galvanized layer formed by hot dip galvanizing and a
galvannealed layer formed by alloying after plating. The plating layer is not limited
25 to pure zinc, and may contain elernents such as Si, Mg, Al, Fe, Mn, Ca, and Zr for
fin-ther 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.
[0057]
30 Moreover, a hot-rolled steel sheet of thc present invention may con~prisea
surface-treating layer fonned by any of formation of an organic film, film
laminating, organic salts/inorganic salts treatment, non-clirontiom treattnent, and the
like, and still achieve an effect of the present invention.
[OOSS]
3. Method for producing hot-rolled steel sheet of the present inventior~
5 A method for producing a hot-rolled steel sheet is not particularly limited, as
long as a hot-rolled steel sheet having the aforementioned n~icrostructurei s obtained.
For example, a production method comprising the l'ollowing steps [a] to [h] allows a
hot-rolled steel sheet of tlie present invention to be obtained stably. I-Iereinafter,
details of each step will be described as an example.
10 [0059]
[a] Slab casting step
There is no particular limitation on a niethod for producing a slab prior to
hot rolling. That is, subsequent to production of ingot steel using a blast furnace, an
electric furnace, or the like, various kinds of secondary smelting may be performed
15 for adjustment to the above-described cliemical coniposition, atid 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 material as long as a compone~itr ange of the
present invention can be obtained by control.
[0060]
20 [b] Slab beating step
The casted slab is heated to a predetermined temperature for hot rolling. In
the case of continuous casting, tlie slab may be once cooled to a low temperature atid
then heated again and subjected to hot rolling, or may be directly heated and
subjected to hot rolling subsequent to continuous casting without being cooled.
25 Heating time for the slab is equal to or more than time tl(s) specified in formula (A).
tl (s) = 1.4 x 10.~x Exp(3.2 x 10"/(~1+ 273)) ... (A),
where TI ("C) is the average temperature of the slab in a soaking area.
[0061]
The heating time is thus specified for the followitig reason. In thestructure
30 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 \~~11icahu stenite is likely to remain along tlie Mn
segregation occurs during cooling after the rolling. Consequently, martensite into
which austenite has been transforn~ed at low temperature or remaining austenite is
5 likely to be present along the Mn segregation, \vhicIi increases the length in the
rolling direction of the hard phases in the sheet-thickness central portion of tlie liotrolled
steel sheet.
[0062]
As a result of extensive studies, the present inventors have found that in
10 order to make the length in the rolling direction of the hard phases 20 ptn 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
Mn, reducing the length in the rolling direction of the hard phases. An effect of the
present invention is exerted ellen when the upper litnit of the slab heating
15 temperature is not set, but excessively high heating temperature is not preferable in
terms of economic efficiency. Hence, the slab heating temperature is preferably
lower than 1300°C. The lower limit of the slab heating temperature is preferably
1150°C. The heating time for the slab is not the elapsed time fiom heating start, but
time during which the slab is held at a predetermined heating teniperature (e.g., a
20 temperature equal to or higher than 1 150°C and lower than 1300°C).
[0063]
[c] Rough rolling step
After the slab heating step, a rough rolling step of hot rolling starts to be
perfornled without a wait on the slab extracted from a heating filmace; thus, a rough
25 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 transfornlation
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
perfornled as nlulti-pass hot rolling, and a surface layer of the slab that has gone
30 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 rec~~pcratcisd r olled in a subsequent pass, and a surface
layer of the slab is cooled again to Ar3 transfor~i~atiopno int or lowver. This process
is repeated. The temperature of the slab surface layer in the present invention refcrs
to the temperature of the slab in a portion at 1 mm in the depth direction from the
5 slab surface, and can be estimated by thennal transfer calculation, for csample.
Cooling the inside of the slab as well as the slab outermost surface to Ar3
transfor~nationp oint or lower increases the effect of recuperation.
Ar3 ("C) = 901 - 325 x C + 33 x Si t 287 x P t 40 x A1 - 92 x (Mil + Mo +
Cu) - 46 x Ni ... (B),
10 where each chemical symbol denotes the content (mass%) of the element.
[0064]
The rough rolling conditions are thus specified for the following reason.
To obtain an effect of the present invention of obtaining a hot-rolled steel sheet with
favorable fatigue cl~aracteristicsin the rolling direction, it is essetltial that, in the hot-
15 rolled steel sheet, the sum of X-ray random intensity ratios of <011> orientation and
orientation as viewed from the rolling direction be
less than 1 .O. To thus control crystal orientation, it is important to make shear force
act on the steel sheet to develop <011> orientation atid <11 I> orientation as strongly
20 as possible so that they reach a portion close to the center of the sheet 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, ho\vevel; has revealed that when the slab surface layer is- once
cooled to Ar3 transformation point or lower during rough rolling, the structure during
25 rough rolling exerts a preferable influence on a final structure. A presumable
mechanism is described below.
[0065]
When suficicnt shear force is applied during rough rolling and the slab
surface layer is once cooled to & transformation point or lower, the structure around
30 the surface layer is partly trausformed fro111 austenite to ferrite. At this time, ferrite
is influenced by the shear force during rough rolling; thus, as viewed from the rolling
direction, 1 1 1 I> orientation and <01 I> orientation increase and <001> orientation
decreases.
Ferrite in the surface layer is recuperated and reversely transfo~.med to
6 austenite by the next pass. On this occasion, austenitc that has undergone reverse
transformation has an orientation having a certain orientation relationship wit11 the
crystal orientation of ferrite before transforniation. When the surface layer
austenite after reverse transfor~liationis further subjected to rough rolling and cooled
again to Ar3 transformation point or lower, the surface layer st~ucture is partly
10 transformed from austenite to ferrite again. The crystal orientation of austenite
before transfornlation is influenced by the clystal orientation of former femite; thus,
orientation and <011> orientation of fenite after transforrnation fi~rther
increase as conlpared with after the previous pass.
[0067]
15 In this manner, during rough rolling, applying sufficient shear force in each
pass and transforming a surface layer by cooling to AT3 transformation point or lower
are repeated; thus, near the surface layer, orientatiotl and 101 1> orientation
increase and <001> orientation decreases. To sufficiently exert this effect, it is
necessary to set the reduction ratio during rough rolling to 50% or Inore to apply
20 sufficient shear force; hence, in this step, the slab surface layer is cooled to AT3
transformation point or lower twice or more, preferably three times or more.
[0068]
[dl Finish rolling step
In a finish rolling step follo\ving the rough rolling step, two passes or more
25 of rolling in which a shape ratio X, determined from fornula (C) below, is 2.3 or
more are perfor~ned at a slab surface layer temperature of 1100°C or l0\\7er to make
the total reduction ratio 40% or more.
[Math. 11
where L denotes tlie diameter of a rolling niill roll, hi, denotes the sheet
thickness on tlie rolling mill roll entty side, and h,,,l denotes thc sheet thickness on
the rolling mill roll exit side.
5 [0069]
The present inventors have found that, to make shear force of hot rolling act
on the steel sheet deeply by rolling at 1 100°C or lowvel; it is necessary to satisfy a
sliape ratio X, specified in formula (C), of 2.3 or tnore in at least twvo passes of the
total nu~nbero f passes of hot rolling. As expressed by forniulae (C1) to (C3) below,
10 the shape ratio X is a ratio between contact arc length Id and average sheet thickness
h,,, of the rollit~gm ill roll and the steel sheet.
x = ld/hn, ... (Cl)
Id = (L x (hin - ho,,t)/2)'R ... (C2)
h," = (hin + lbud/2 ... (C3)
15 [0070]
Even if the shape ratio X determined from formula (C) is 2.3 or more, one
pass of rolling is not enough for sufficient introduction depth of shear strain. When
the introduction depth of shear strain is insufficient, alignment of ferrite in
orientation and <011> orientation as viewed from tlie rolling direction is weak,
20 which results in a reduction in fatigue characteristics in the rolling direction.
Accordingly, the number of passes in which the shape ratio X is 2.3 or more is set to
two or more.
[0071]
The nutnber of passes of rolling in the finish rolling step is preferably as
25 large as possible. When the number of passes is three or more, the sliape ratio X
may be set to 2.3 or more it1 all passes. To increase the thickness of a shear layel;
the value of the shape ratio X is preferably as large as possible. The value of the
shape ratio X is preferably 2.5 or more, further preferably 3.0 or more.
[0072]
When rolling in \vhich the shape ratio X is 2.3 or more is performed at high
tetllperah~re, subsequent recrystallizatioll may brcak textures increasing Young's
modulus. Hence, rolling in a~hichth e tlumbcr of passes in which the shape ratio X
6 is set to 2.3 or tnore is confined is performed in a state where the slab surfacc layer
temperature is 1100°C or lower. In addition, a larger amount of introduction of
shear strain leads to further develop~l~eonft crystal grains in oriel~tationa ttd
<011> orientation as viewed from the rolling direction, \vl~ich improve fatigue
characteristics in the rolling direction of the steel sheet. This effect is significant
10 when the total reduction ratio at llOO°C or lower is 40% or more; hence, the total
reduction ratio at 1100°C or lower is set to 40% or more.
[0073]
Reduction in the final pass of finish rolling is performed at equal to or
higher than (Tz - 100) "C and lower than (TI + 20) OC, preferably equal to or higher
15 than (T2 - 100) OC and lower than T2 (OC), and the reduction ratio is set to equal to or
tnore than 3% and less than 40%. The reduction ratio is preferably equal to or more
than 10% and less than 40%. T;! is a temperature specified in formula (D) below.
T2("C)=870+10x(C+N)xMn+350~Nb+2S0xTi+40xB+10x
Cr + 100 x Mo + 100 x V ... (D),
20 \vhere each chemical sylnbol denotes the content (mass%) of the element.
[0074]
Reduction conditions in this final pass are very impo~-tanitn controlling an
aspect ratio of the hard phases in the sheet-thickness central portion. Perfornling
rollu~gin a temperature range of equal to or higher than (T* - 100) OC and lower than
25 (T2 + 20) "C increases the aspect ratio of the hard phases in the sheet-thickness
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. Rolli~lg
30 with a reduction ratio of 40% or nlore places a geat burden on a rollit~gm ill; hence,
a reduction ratio equal to or tilore than 3% and less than 40% is preferable.
[0075]
If reduction in the linal pass is pcrfortned in a temperature ratlgc of lower
than (Tz - 100) "C, rolling proceeds in a dual-phase region of ferrite and austenite.
Thus, of ferrite is promoted by strain-induced transfortnation, so that
5 bainite does not serve as the main constituent of the tiiicrostructure. Moreovel; in
this case, generated ferrite is utuecrystallizcd ferrite with low ductility, and thus the
steel sheet has low ductility and does not satisfy (tensile strength (MPa)) x (total
elongation (%)) ? 18000. If reduction in the final pass is perforonned in a
temperature ratige of (Tz + 20) "C or higher, the aspect ratio of the hard phases in the
10 sheet-thickness central portion is stnall. This is presuti~ably because protnotion of
reclystallization of austenite, resulting in reduced aspect ratio of austenite, influences
the fortn of the hard phases. Hence, reduction it1 the final pass is perfomled in a
tenlperature range of equal to or higher than (TI - 100) OC and lower than (T2 +
20) "C. Reduction under these conditions makes the aspect ratio of the hard phases
15 3 or more.
[0076]
[el First cooling step
In a first cooling step following the finish rolling step, the average cooling
rate from the final reduction temperature of finish rolling to 750°C is set to 60°C/s or
20 tnore, because a cooling rate less than 60°C/s may cause the length in the sheet
thickness direction of the hard phases in the sheet-tliickness central portion to be 20
pm or more. Although the cause of the correlation between 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 60°C/s or more tnakes it difficult for dislocation
25 introduced in the final reduction of finish rolling to recover, and the dislocation
works as the core of ferrite transfortnation; thus, untransfor~ned austetiite in the
sheet-thickness central portiot~ is divided by ferrite, which results in a reduction in
the length in the sheet thickness direction of the hard phases.
[0077]
30 In the field of steel plates, there has been an example aiming at ithibition of
fatiguc crack propagation by cotitrol of an aspect ratio of hard phases, but no
document has reported achievement of hot11 the inhibition of fatigue crack
propagation and \vorkability such as hole expandabilit): probably for the followirlg
reasons. In the field of steel plates, rolling strain does not easily reach a platethickness
central portion. In addition, because of thick plate thickness, cooling rate
5 in the plate-thickness central portion is not enough, so that recovery of dislocation
proceeds; consequently, the core of ferritc transfort~iation catulot be introduced
sufficiently, which prevents a reduction in the length ofthe hard phases.
[0078]
[fl Second cooling step
10 In a second cooling step following the first cooling step, with regard to a
tetliperature T3(oC) specified in fortnula (E) below, the average cooling rate in a
temperature range of equal to or higher than T3("C) and lower that1 750°C is set to
5O0C/s or more for the following reason. An average coolitlg rate of less than
5O0C/s leads to large ferrite transformation quantity during cooling, so that bainite
15 does not sellre as the main constituent bf the microstructure; thus, the hot-rolled steel
sheet cannot have a tensile strength of 780 MPa or more.
T3("C)=830-270xC-90xMn-37xNi-70xCr-83xMo.. .(E ),
xvhere each chemical symbol denotes the content (mass%) of the element.
When the content or an element in formula (E) is zero, zero is substituted.
20 [0079]
[g] Soaking step
In a soaking step follox\~ing the second cooling step, with regard to the
temperature T3rC) specified in formula (E) and a temperature T4cC) specified in
formula (F) below, the steel sheet is held for 5 s or more in a temperature range of
25 equal to or higher than T3("C) and lower than T4("C). The soaking step is essential
to making bainite serve as the main constituent of the n~icrostructure. The holding
time is set to 5 s or more because holdit~gt ime of 5 s or less makes an area fraction
of the hard phases 20% or more, whicli reduces ductility and a hole expansion ratio.
T4("C)=561-474xC-33xMn-17xNi-17xCr-21 xMo ...( F),
30 where each chenlical symbol denotes the content (mass%) of the element.
When the content of an elenlent in formula (F) is zero, zero is substituted.
[008O]
[h] Winding step
The steel sheet is wound after the soaking stcp. The temperature of the
steel sheet in winding (winding temperature) is set to &("C), specified in fonnula (F),
5 or lower. Winding at a high telnperaturc exceeding T4rC) leads to an cxccssive
~~olwnfrea ction of bainite in the structure, making it difficult to obtain enough
fraction of the hard phases, which causes punching fatigue characteristics to
deteriorate.
[0081]
10 Through the production steps described above, a hot-rolled steel sheet of the
present invention is produced.
[0082]
After the conlpletion of all of the steps [a] to [I]], for the purpose of
correcting the shape of the steel sheet, or of improving ductility by introducitlg
15 mobile dislocation, or the like, skin pass rolling in which a reduction ratio is equal to
or nlore 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, picklir~g nlay be
performed on the obtained hot-rolled steel sheet as necessary. Furthern~ore, after
20 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.
[0083]
A hot-rolled steel sheet or the present invelltion is produced through, in
addition to the rolling steps, continuous casting, pickling, and the like, which are
25 normal hot-rolling steps; hoxvever, even if produced with the steps partly skipped, the
hot-rolled steel sheet can have excellent fatigue in the rolling direction characteristics
and workability, which are effects of the present invention.
[0084]
Moreover, even if, afler the hot-rolled steel sheet is once produced, heat
30 treatlilent is perfornled on-liue or off-line in a tenlperature range of 100 to 600°C for
the purpose of improving ductility, the hot-rolled steel sheet can have excelle~lt
latigue characteristics in the rolling directioti and \vorkability, which are erects of
the present invention.
[0085]
The hot-rolled steel sheet produced through the above steps may be
5 subjected to an additional step, such as performing hot dip galvanizing or alloyed hot
dip galvanizing, or performing surface treatn~ent by fos~nation of an organic film,
film laminating, organic saltslinorganic salts treatment, non-chro~niutnt reatment, and
the like.
[0086]
10 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
characteristics (tensile strength and total elongation) are evaluated in conformance
with JIS Z 2241 2011. A test piece is No. 5 test piece of JIS Z 2241 2011, taken
15 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
the steel sheet wit11 the rolling direction serving as the longitudinal direction.
[0087]
20 (2) Hole expansion ratio
A hole expansion ratio of a hot-rolled steel sheet is evaluated by a hole
expansion test in confor~~lancwei th a test method described in the Japan Iron and
Steel Federation Standard JFS T 1001-1996. A test piece is taken from a position
similar to that of the tensile test piece, and is provided with a punching hole by a
25 cylindrical punch. A steel sheet with excellent workability 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 (%)) 135000.
[0088]
(3) Fatigue characteristics
30 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 measi~ring fatigue strength without a notch. FIG. I(b) is a plan view and a
h n t view of a test picce for measuring fatigue strength \vith a aotch.
[OOS9]
To evaluate fatigue characteristics in the rolling directio~l of a hot-1.olletl
5 steel sheet, test pieces with the shape and dimensions illustrated in FIG. I are used.
Each test piece is taken from a position sinlifar to that of the tensile test piece ~vitli
the rolling direction sel-ving as the longitudinal direction. The test piece illustrated
in FIG. l(a) is a test piece for obtaining fatigue strength without a notch. The test
piece illustrated in FIG. l(b) is a punched test piece for obtaining fatigue strength of
10 a notched material, and is provided with a punchiug 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
set to 10%. Both fatigue test pieces are subjected to grinding for thee triangle
finish (expressed by surface roughness finish symbols) ftom the outermost layer to a
15 depth of approximately 0.05 mm.
[0090]
Using these test pieces, a stress controlled tensile-tensile fatigue test is
performed under conditions of a stress ratio R of 0.1 and a frequency of 15 to 25 Hz.
A steel sheet with excellent fatigue characteristics in the rolling direction in the
20 present invention refers to a steel sleet whose value (fatigue limit ratio) obtained by
dividing fatigue strength at 10 rnilliot~c ycles obtained with the notch-flee fatigue test
piece by tensile strength obtained in the tensile test is 0.55 or more, and whose value
(punching fatigue litnit ratio) obtained by dividing fatigue strength at 10 million
cycles obtained in the punching fatigue test by tensile strength obtained in the tensile
25 test is 0.30 or more.
[0091]
I-Iereinafter, the presetit invention will be described tnore specifically in
Exanlples. Note that the present invention is not limited by the followi~lgE xamples.
30 [Examples]
100921
31/47
Molten steel having chemical co~ilpositionss hown in Table 1 was produced.
[0093]
[Table 11
LO0941
According to Table 1, cl~etiiicalc ompositions of steels A to I were within a
chemical cotnposition range specified in the present invention. Mean~vliile, steel
"a" had too low a C content, steel "b" had too high a C contetit, stecl "c" had too
6 high a P content, steel "d" had too high a S content, arid steel "en had too low a total
content of Si and Al. The underlines indicate component amounts falling outside
the inventio~ria nge.
[0095]
Using the molten steel with the chemical compositions of steels A to H arid
10 steels "a" to "e", hot-rolled steel sheets were produced by the above-described steps
[a] to [h]. Each step was performed under conditions s l ~ o ~inn tT ables 2 and 3. In
step [dl, rolli~lga t 1100°C or lower was perfornled in six passes of PI to Pb. Steels
A to H and steels "a" to "e" sho~vi~t1i T ables 2 and 3 correspottd to the molten steel
with the chemical con~positionss hown it1 Table 1, and indicate the used molten steel.
15 As TI PC), the average tempetature of a soaking area of a heating furnace was
ttleasured as the average tentperature of the slab in a soaking area. PI to P6 indicate
first to sixth passes in the finish rolling step.

CLAIMS
Claim 1
A hot-rolled steel sheet having a chemical composition 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: Inore 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 0.3%,
Cr: 0 to 2.0%, and
the balance: iron and impurities, and
a microstrt~cture of the hot-rolled steel sheet which contains bainite as the
25 main constituent, and contains hard phases constituted by martensite andor austenite
in an amount of, in area fraction, equal to or more than 3% and less than 20%,
wherein 60% or more of the hard phases present in a sheet-tliickness central
portion have an aspect ratio of 3 or more,
the hard phases present in the sheet-thickness central portion have a length
30 in a rolling direction of less than 20 pm, and
the sum of X-ray randotn intensity ratios of <011> orientation and
orientation as viewed from thc rolling direction is 3.5 or more, and an X-ray rand0111
intensity ratio of <001> orientation as viewed from the rolling direction is 1.0 or less.
Clainl 2
5 The hot-rolled steel sheet according to claitn 1, comprising, in mass%,
one or Inore selected froni
Ti: equal to or Inore than (0.005 + 48/34p] + 48/32[S])% to equal to or less
than 0.3%, and
Nb: 0.01 to 0.3%,
10 \vliere indicates an N content (mass%) and [S] indicates an S content
(mass%).
Claim 3
The hot-rolled steel sheet according to claim 1, comprising, in mass%,
one or more selected fiom
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 clairn 1, having a chemical
composition comprising, in mass%,
B: 0.0002 to 0.01%.
The hot-rolled steel sheet according to clainl 1, comprising, in inass%,
one or tilore selected fiom
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
Ma: 0.01 to 1.0%,
V: 0.01 to 0.3%, and
Cr: 0.01 to 2.0%.
Claiin 6
The hot-rolled steel sheet according to claim 1, con~prising a hot-dip
5 galvanized layer or a galvannealed layer on its surface.