Note: Descriptions are shown in the official language in which they were submitted.
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AA673-PCT
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DESCRIPTION
Title of Invention: High Yield Ratio Hot Rolled Steel
Sheet Which Has Excellent Low Temperature Impact Energy
Absorption and HAZ Softening Resistance and Method of
Production of Same
Technical Field
[0001] The present invention relates to maximum
tensile strength 600 MPa or more high yield ratio hot
rolled steel sheet which has an excellent low temperature
impact energy absorption and HAZ (heat affected zone)
softening resistance and a method of production of the
same. The steel sheet is suitable as a base material for
booms and frames of construction machinery and as a base
material for frames, members, etc. of trucks and cars
which are shaped mainly by bending and, further, as a
base material for line pipe.
Background Art
[0002] The frames of construction machinery and trucks
are assembled by shaping hot rolled steel sheet mainly by
bending and arc welding the shaped parts. Therefore, the
base material which is used for these parts is required
to have excellent bendability and arc weldability.
Further, construction machinery and trucks are sometimes
used in low temperature environments, so in particular
with frames for trucks etc., the properties of being
resistant to brittle fracture and of being able to
sufficiently absorb impact energy when impact is given,
even at a low temperature, are sought.
[0003] As steel sheet which has an excellent impact
energy absorption, there is the art disclosed in NPLT 1
and PLTs 1 to 2. However, these steel sheets contain
structures which include retained austenite or martensite
and further optimize the metal structures of the steel
sheets to achieve excellent impact properties. However,
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such structures of steel sheet had the problems of being
low in yield stress and having issues in bendability.
[0004] Further, PLT 3 discloses a method of producing
thin-gauge steel sheet which has a high impact energy
absorption ability at a high yield in a stable manner by
cold rolling. However, this method suffered from large
softening of the heat affected zone (HAZ) of the arc weld
zone and the inability to obtain a sufficient weld joint
strength and, further, was disadvantageous in terms of
production costs.
[0005] As a method of obtaining hot rolled steel sheet
which has excellent bendability and a high yield ratio,
for example, the method of dispersing Ti, Nb, and other
alloy carbides in the steel such as shown in PLTs 4 to 6
has been disclosed. However, steel sheet which utilizes
such precipitation strengthening sometimes suffers from a
large softening of the arc weld heat affected zone and a
drop in joint strength. Further, there were the problems
that sometimes brittle fracture occurred at a low
temperature and sometimes the amount of impact energy
absorption became small.
[0006] On the other hand, as art to suppress softening
of the weld heat affected zone, PLT 7 discloses the
method of compositely adding Mo and Nb or Ti, while PLT 8
discloses the method of optimizing the ingredients so as
to suppress HAZ softening even in precipitation
strengthened steel which contains Ti. However, with these
methods, there were the problems that sometimes brittle
fracture occurred at a low temperature and sometimes the
impact energy absorption amount became small.
[0007] PLT 9 .discloses the method of establishing
suitable rolling conditions from the rough rolling to
finish rolling of the steel slab and a suitable
subsequent cooling treatment so as to produce hot rolled
steel sheet for high strength electric resistance welded
steel pipe use which has excellent low temperature
toughness and weldability. This method controls the
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recrystallization in the rough rolling and finish rolling
of the steel slab to obtain a fine grain metal structure
and obtain steel sheet which has excellent low
temperature toughness, but does not intend control of the
size or distribution of alloy carbonitrides. As a result,
these are not optimized, so there was the problem of a
drop in the impact energy absorption.
[0008] PLT 10 discloses a method of establishing a
suitable rolling reduction rate and holding time in the
rough rolling process of a steel slab and suitable finish
rolling conditions so as to produce hot rolled high
strength steel sheet which has excellent toughness and
hydrogen cracking resistance. The object of the
optimization of the rough rolling process in this method
is the promotion of the recrystallization of steel, but
this does not intend control of the size or distribution
of alloy precipitates. As a result, these are not
optimized, so there was the problem of a drop in the
impact energy absorption. Regarding the finish rolling
conditions as well, with the method described in PLT 10,
there was the problem that it is not possible to control
the size or distribution of the alloy precipitates and
excellent impact absorption energy cannot be obtained.
[0009] PLT 11 discloses the art of suitably dispersing
precipitated particles in the weld heat affected zone so
as to obtain high strength hot rolled steel sheet which
has an excellent HAZ softening resistance. However, this
art disperses fine precipitates in the HAZ of the steel
sheet during arc welding, but the size of the
precipitated particles in the steel is not optimized, so
as a result there was the problem that the steel sheet
was not excellent in impact energy absorption.
Citations List
Patent Literature
[0010] PLT 1: Japanese Patent Publication No. 2007-
284776A
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PLT 2: Japanese Patent Publication No. 2005-290396A
PLT 3: Japanese Patent Publication No. 10-58004A
PLT 4: Japanese Patent Publication No. 2009-185361A
PLT 5: Japanese Patent Publication No. 2007-9322A
PLT 6: Japanese Patent Publication No. 2005-264239A
PLT 7: Japanese Patent Publication No. 2003-231941A
PLT 8: Japanese Patent Publication No. 2001-89816A
PLT 9: Japanese Patent Publication No. 2001-207220A
PLT 10: Japanese Patent Publication No. 10-298645A
PLT 11: Japanese Patent Publication No. 2008-280552A
Non-Patent Literature
[0011] NPLT 1: Nippon Steel Technical Reports, vol.
378 (2003), p. 2
Summary of Invention
Technical Problem
[0012] The present invention was made in consideration
of the above problems and has as its object the provision
of maximum tensile strength 600 MPa or more high yield
ratio hot rolled steel sheet which has both an excellent
low temperature impact energy absorption and HAZ
softening resistance and a method of production of the
same.
Solution to Problem
[0013] The inventors etc. investigated in detail the
factors influencing the HAZ softening and low temperature
impact energy absorption of steel sheet which contains Ti
and other alloy carbonitrides by which a high yield ratio
can be stably obtained. As a result, they discovered that
the amount of HAZ softening can be suppressed by
establishing suitable amounts of Ti, Nb, and Mn.
[0014] Further, the inventors etc. next intensively
studied the method of improving the low temperature
impact energy absorption and discovered for the first
time that by reducing the area percentage of pearlite in
the metal structure of the steel sheet and rather
eliminating as much as possible the retained austenite
,
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and martensite, which in the past had been considered
advantageous for improvement of the impact energy
absorption ability, and, further, by optimizing the
lattice matching with the matrix Fe and size of the alloy
carbonitrides which contain Ti and Nb which are dispersed
in the steel, in particular controlling the particle size
of alloy carbonitrides with incoherent interfaces, the
low temperature impact energy absorption, which was an
issue in precipitation strengthened steel, is improved.
[0015] In general, in precipitation strengthened steel
which contains Nb and Ti, the precipitates are controlled
so as to be present in a state of good lattice matching
having a specific crystal orientation in relation to the
matrix Fe, but this time the inventors etc. investigated
the relationship with the low temperature impact energy
absorption and as a result discovered that alloy
carbonitrides in the precipitated state with good lattice
matching with the matrix Fe tend not to become obstacles
to starting and propagation of cracks, while alloy
carbonitrides in an incoherent state with the matrix Fe
lower the low temperature impact energy absorption amount
even if relatively small in size. The mechanism by which
lattice matching of the alloy carbonitrides with the
matrix affects the low temperature impact energy
absorption amount is not certain, but it may be that if
the lattice matching of alloy carbonitrides and the
matrix Fe is poor, this becomes a starting point for
interfacial peeling or formation of voids and promotes
both ductile fracture and brittle fracture.
[0016] The inventors etc. engaged in extensive studies
on the process of production and ranges of ingredients
for realizing the above type of structure and as a result
completed maximum tensile strength 600 MPa or more hot
rolled steel sheet and plated steel sheet which achieve
both an HAZ softening resistance and low temperature
energy absorption and further are high in yield ratio and
excellent in bendability. That is, the gist of the
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present invention is as follows:
[0017] (1) High yield ratio hot rolled steel sheet
which has an excellent low temperature impact energy
absorption and HAZ softening resistance characterized by
comprising, by mass%,
C: 0.04 to 0.09%,
Si: 0.4% or less,
Mn: 1.2 to 2.0%,
P: 0.1% or less,
S: 0.02% or less,
Al: 1.0% or less,
Nb: 0.02 to 0.09%,
Ti: 0.02 to 0.07%, and
N: 0.005% or less,
a balance of Fe and unavoidable impurities,
where 2.0<Mn+8[%Ti]+12[%Nb]<2.6, and
having a metal structure which comprises an area
percentage of pearlite of 5% or less, a total area
percentage of martensite and retained austenite of 0.5%
or less, and a balance of both of ferrite and non-
tempered bainite, wherein an area percentage of bainite
is 10% or more,
having an average grain size of ferrite and bainite of
10 pm or less,
having an average grain size of alloy carbonitrides with
incoherent interfaces which contain Ti and Nb of 20 nm
or less,
having a yield ratio of 0.85 or more, and
having a maximum tensile strength of 600 MPa or more.
[0018] (2) The high yield ratio hot rolled steel sheet
which has an excellent low temperature impact energy absorption
and HAZ softening resistance according to (1), characterized
by further comprising, by mass%, V: 0.01 to 0.12%.
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[0019] (3) The high yield ratio hot rolled steel sheet
which has an excellent low temperature impact energy
absorption and HAZ softening resistance according to
(1) or (2), characterized by further comprising, by
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mass%, one or more of Cr, Cu, Ni, and Mo in a total of
0.02 to 2.0%.
[0020] (4) The high yield ratio hot rolled steel sheet
which has an excellent low temperature impact energy
absorption and HAZ softening resistance according to any
one of (1) to (3), characterized by further comprising,
by mass%, B: 0.0003 to 0.005%.
[0021] (5) The high yield ratio hot rolled steel sheet
which has an excellent low temperature impact energy
absorption and HAZ softening resistance according to any
one of (1) to (4), characterized by further comprising,
by mass%, one or more of Ca, Mg, La, and Ce in a total of
0.0003 to 0.01%.
[0022] (6) High yield ratio hot rolled steel sheet
which has an excellent low temperature impact energy
absorption and HAZ softening resistance characterized
that the high yield ratio hot rolled steel sheet
according to any one of (1) to (5) is plated or alloy
plated on a surface.
[0023] (7) A method of production of high yield ratio
hot rolled steel sheet which has an excellent low
temperature impact energy absorption and HAZ softening
resistance characterized by comprising, heating a steel
slab having a composition according to any one of (1) to
(5) to 1150 C or more, rough rolling the heated steel
slab, finishing the rough rolling at temperature between
1000 C to 1080 C, wherein a maximum rolling interval in
the rough rolling which is performed at 1150 C or less is
45 sec or less, after the rough rolling, holding the
steel slab for a holding time tl (sec) which satisfies
the following formula (1), then starting finish rolling,
performing finish rolling with a final rolling
temperature Tf which satisfies the following formula (2)
so as to obtain as steel sheet,
starting water cooling of the steel sheet within 3
seconds after the finish rolling, then cooling the steel
,
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,
sheet to temperature 700 C or less at a lowest cooling
rate of 8 C/sec or more, and coiling the steel sheet at
temperature between 530 C to 650 C.
1000x([96Ti]+[%Nb])>t1 .................. formula (1)
Tf>830+400([96Ti]+[%Nb]) ¨formula (2)
[0024] (8) The method of production of high yield
ratio hot rolled steel sheet according to (7)
characterized in that a final rolling temperature Tf
satisfies the following formula (3).
Tf>830+800([%Ti]+[%Nb]) ¨formula (3)
[0025] (9) A method of production of high yield ratio
hot rolled plated steel sheet which has an excellent low
temperature impact energy absorption and HAZ softening
resistance characterized by comprising, pickling the hot
rolled steel sheet which was obtained by the method of
production according to (7) or (8), heating steel sheet
at the Ac3 temperature or less, then dipping the steel
sheet in a plating bath to plate the surface of the steel
sheet.
[0026] (10) The method of production of high yield
ratio hot rolled plated steel sheet which has an
excellent low temperature impact energy absorption and
HAZ softening resistance according to (9) characterized
by further comprising alloying the plated steel sheet
after the plating.
Advantageous Effects of Invention
[0027] According to the hot rolled steel sheet of the
present invention, due to the above configuration, it is
possible to obtain high yield ratio hot rolled steel
sheet which has a maximum tensile strength of 600 MPa or
more and has excellent HAZ softening resistance and low
temperature energy absorption and further bendability.
With conventional steel sheet, there were the problems
that there were restrictions in use and operation at a
low temperature and a sufficient joint strength could not
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be obtained, but according to the hot rolled steel sheet
of the present invention, use in cold regions becomes
possible, increased strength enables the products to be
reduced in thickness, and the effect of reduction of
weight of construction machinery, automobiles, and trucks
can be expected.
[0028] Further, according to the method of production
of hot rolled steel sheet which has an excellent low
temperature impact energy absorption and a HAZ softening
resistance of the present invention, it becomes possible
to produce high yield ratio hot rolled steel sheet which
has a maximum tensile strength of 600 MPa or more and has
excellent HAZ softening resistance and low temperature
energy absorption and further bendability.
[0029] Note that, in the present invention, excellent
low temperature impact energy absorption means the impact
energy absorption in a Charpy impact test at -40 C is
70J/cm2 or more. Further, excellent HAZ softening
resistance means a difference AHV (=HVBm- HVHAz) of 40 or
less between the Vicker's hardness (HVBAz) of the softest
part of the weld heat affected zone (HAZ) and the
Vicker's hardness (HVBm) of the base material at the time
of arc welding by a weld current, voltage, and welding
speed selected to give good bead shape and by a weld heat
input of 10000J/cm or less. Further, "excellent
bendability" means an riira/t of 1.0 or less when the
thickness of the test piece in a 90 V bending test is "t"
and the limit radius of curvature where no cracks occur
is rlim.
Brief Description of Drawings
[0030] [FIG. 1] A graph which expresses the
relationship between Mn+8Ti+12Nb and vE-40 and AHV.
[FIG. 2] A graph which expresses the effect of the amount
of Ti+Nb on the relationship between the holding time tl
and vE_zio from the final rough rolling to the start of the
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finish rolling.
[FIG. 3] A graph which expresses the relationship of the
mass of Ti+Nb and Tf ( C) of the invention examples and
two types of comparative examples (A-7 and B-6) among the
types of steel which are shown in Table 2.
Description of Embodiments
[0031] Below, the present invention will be explained
in detail. First, the reasons for limiting the steel
ingredients of the high yield ratio hot rolled steel
sheet which has an excellent low temperature impact
energy absorption and HAZ softening resistance of the
present invention will be explained. Here, the "%" for
the ingredients means mass%.
[0032] "C: 0.04 to 0.09%"
If the amount of C is less than 0.04%, it is difficult to
secure a maximum tensile strength of 600 MPa or more. On
the other hand, if over 0.09%, the coarse and alloy
carbonitrides with incoherent interfaces which contain Ti
and Nb increase and the low temperature impact energy
absorption falls, so the content was limited to 0.04% to
0.09% in range.
[0033] "Si: 0.4% or less"
If the amount of Si exceeds 0.4%, sometimes martensite or
retained austenite remains in the steel sheet structure
and the low temperature toughness and impact energy
absorption fall. For this reason, the suitable range was
made 0.4% or less. From the viewpoint of securing the
bendability, 0.2% or less is more preferable. The lower
limit of the amount of Si is not particularly set, but if
less than 0.001%, the production cost increases, so
0.001% is the substantive lower limit.
[0034] "Mn: 1.2 to 2.0%"
Mn is used to secure the strength of the matrix through
control of the metal structure of the steel. Further,
this is an element which contributes to the suppression
of HAZ softening of the weld zone. If less than 1.2%, the
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area percentage of the pearlite increases, the low
temperature impact energy absorption falls, and further
the amount of HAZ softening increases, so the strength of
the welded joint greatly falls compared with the strength
of the matrix. If over 2.0% is contained, sometimes hard
martensite is formed and the low temperature impact
energy absorption falls, so the suitable range is made
2.0% or less. From the viewpoint of securing the
bendability, the content is more preferably 1.8% or less.
[0035] "P: 0.1% or less"
P is used for securing the strength of steel. However, if
over 0.1% is included, the low temperature toughness
falls and, further, the low temperature impact energy
absorption cannot be obtained, so the suitable range is
made 0.1% or less. The lower limit is not particularly
set, but if less than 0.001%, the production cost
increases, so 0.001% is the substantive lower limit.
[0036] "S: 0.02% or less"
S is an element which affects the impact energy
absorption. If over 0.02% is included, even if
controlling the area percentage of the metal structure
and the average particle size of the alloy carbonitrides,
a low temperature impact energy absorption cannot be
obtained, so the suitable range is made 0.02% or less.
The lower limit is not particularly set, but if less than
0.0003%, the production cost increases, so 0.0003% is the
substantive lower limit.
[0037] "Al: 1.0% or less"
Al is used for deoxidation and control of the metal
structure of the steel sheet. If over 1.0%, the heat
affected zone in arc welding softens and a sufficient
welded joint strength cannot be obtained, so the suitable
range is made 1.0% or less. The lower limit is not
particularly set, but if less than 0.001%, the production
cost increases, so 0.001% is the substantive lower limit.
[0038] "Nb: 0.02 to 0.09%"
Nb is used as a precipitation strengthening element for
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adjusting the strength of the steel and is used for
suppressing softening of the weld HAZ. If less than
0.02%, no effect of suppression of softening of the weld
HAZ is seen, while if over 0.09%, coarse alloy
carbonitrides which contain incoherent precipitated Ti
and Nb increase and the low temperature impact energy
absorption becomes lower, so the content was limited to
0.02% to 0.09% in range.
[0039] "Ti: 0.02 to 0.07%"
Ti is used as a precipitation strengthening element for
adjusting the strength of the steel and is used for
suppressing softening of the weld HAZ. If less than
0.02%, obtaining the maximum tensile strength of 600 MPa
or more is difficult. Further, if over 0.07%, incoherent
precipitated coarse alloy carbonitrides which contain Ti
and Nb increase and the low temperature impact energy
absorption becomes lower, so the content is limited to
0.02% to 0.07% in range. To stably obtain a yield ratio
of 0.85 or more, 0.03% is preferably made the lower
limit.
[0040] "N: 0.005% or Less"
N contributes to the grain size of the metal structure of
the steel sheet through formation of nitrides. However,
if over 0.005%, the coarse and alloy carbonitrides with
incoherent interfaces which contain Ti and Nb increase
and the low temperature impact energy absorption becomes
lower, so the content was limited to 0.005% or less in
range. The lower limit is not particularly set, but if
less than 0.0003%, the production cost increases, so
0.0003% is the substantive lower limit.
[0041] "2.0Mn+8[%Ti]+12[%Nb]2.6"
"Mn+8[%Ti]+12[%Nb]" is the total of the ratios of
contribution of the different elements relating to the
low temperature impact energy absorption and the HZ
softening due to welding. As shown in FIG. 1, if plotting
the relationship of the indicator of impact energy
absorption of vE_40 and the indicator of HAZ softening of
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AHV for 11 types of steel differing in Ti and Nb, if the
value of this parameter is less than 2.0, a sufficient
HAZ softening resistance cannot be obtained (that is,
AHV>40) and obtaining a maximum tensile strength of 600
MPa or more becomes difficult, while if over 2.6, the
coarse and alloy carbonitrides with incoherent interfaces
which contain Ti and Nb increase and the low temperature
impact energy absorption becomes lower (that is, vE_
40<70J/cm2). For this reason, the suitable range was
limited to 2.0 to 2.6 in range.
[0042] In the present invention, as steel ingredients,
in addition to the above essential elements, it is also
possible to selectively include the following such
elements.
[0043] "V: 0.01 to 0.12%"
V may be used to adjust the strength of the steel.
However, if the content of V is less than 0.01%, there is
no such effect. Further, if over 0.12%, embrittlement
proceeds and the low temperature impact energy absorption
falls. For this reason, the suitable range was limited to
0.01 to 0.12%.
[0044] "One or More of Cr, Cu, Ni, and Mo in Total of
0.02 to 2.0%"
Cr, Cu, Ni, and Mo may be used to control the structure
of the steel. However, if the total content of the one or
more of these elements is less than 0.02%, there is no
above effect accompanying addition. Further, if over
2.0%, austenite is retained and the low temperature
impact energy absorption falls. For this reason, the
suitable range of the total of these elements was limited
to 0.02 to 2.0%.
[0045] "B: 0.0003 to 0.005%"
B may be used for control of the structure of the steel
sheet. However, if the amount of B is less than 0.0003%,
that effect is not exhibited. Further, if over 0.005%,
martensite is sometimes formed and the low temperature
impact energy absorption falls. For this reason, the
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suitable range was limited to 0.0003 to 0.005%.
[0046] "One or More of Ca, Mg, La, and Ce in a Total
of 0.0003 to 0.01%"
Ca, Mg, La, and Ce may be used for deoxidation of the
steel. However, if the total amount of the one or more of
these elements is less than 0.0003%, there is no such
effect, while if over 0.01%, brittle fracture occurs at a
low temperature and the impact energy absorption falls.
For this reason, the suitable range was limited to 0.0003
to 0.01%.
[0047] Note that the balance of the ingredients is Fe
and unavoidable impurities, but the steel ingredients in
the present embodiment are not particularly limited in
regard to other elements. Various elements may be
suitably included for adjusting the strength.
[0048] Next, the metal structure of the hot rolled
steel sheet of the present invention will be explained.
[0049] The hot rolled steel sheet of the present
invention may contain ferrite and bainite as main phases
and a balance of one or more of pearlite, martensite, and
retained austenite.
[0050] "Area Percentage of Pearlite"
In precipitation strengthened steel which contains Nb and
Ti, if the area percentage of pearlite exceeds 5%,
brittle fracture easily occurs at a low temperature and,
further, the impact energy absorption falls, so the upper
limit was made 5%. From the viewpoint of securing the
bendability, 3% or less is the preferable range. Note
that, the lower limit is not particularly set, but having
an area percentage of pearlite of close to zero is more
preferable in regard to the impact energy absorption.
[0051] "Total Area Percentage of Martensite and
Retained Austenite"
In precipitation strengthened steel which contains Nb and
Ti, if the total area percentage of martensite and
retained austenite exceeds 0.5%, brittle fracture easily
occurs at a low temperature and, further, the impact
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energy absorption falls. For this reason, the upper limit
of the total area percentage was made 0.5%. Note that,
the lower limit is not particularly set, but having a
total area percentage of martensite and retained
austenite of close to zero is more preferable in regard
to the impact energy absorption.
[0052] "Metal Structure Which Has Balance of One or
Both of Ferrite And Bainite"
The area percentages of these are not particularly
limited, but from the viewpoint of securing bendability,
the bainite area percentage is preferably made 10% or
more.
[0053] "Average Grain Size of Ferrite and Bainite"
The average grain size of ferrite and bainite is a
correlative factor. If the average particle size is over
10 gm, even if controlling the average particle size of
the alloy carbonitrides which contain Nb and Ti,
sometimes the low temperature impact energy absorption
cannot be secured, so the upper limit was made 10 gm. 8
gm or less is a preferable condition enabling impact
energy absorption to be more stably secured. The lower
limit is not particularly set, but if the size is less
than 2 gm, the production cost greatly increases, so 2 gm
is the substantive lower limit.
[0054] In the present invention, the metal structure
of the steel sheet can be observed based on JIS G 0551 by
an optical microscope. The observed surface is obtained
by polishing the steel sheet, then etching it by a Nital
corrosive solution.
[0055] The area percentages of ferrite, bainite,
pearlite, and martensite can be measured by the point
count method or image analysis using structural
photographs obtained by an optical microscope or scan
type electron microscope (SEM). The area percentage of
retained austenite is measured by X-ray diffraction.
[0056] In the present invention, "bainite" includes
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upper bainite, lower bainite, and granular bainite.
Further, "pearlite" includes pearlite and pseudo
pearlite.
[0057] The grain size can be measured by observation
by an optical microscope or by crystal orientation
analysis by the EBSD method. Here, "the grain size" is
the average grain size "d" which is described in JIS G
0551.
[0058] "Average Particle Size of Alloy carbonitrides
with incoherent interfaces Which Contain Ti and Nb"
The particle size of alloy carbonitrides which contain Ti
and Nb and the lattice matching with the matrix structure
ferrite or bainite are important factors relating to the
low temperature impact energy absorption. In general, in
precipitation strengthened steel, it is known to cause
the precipitation of fine alloy carbonitrides with good
lattice matching with the matrix structure as fine
particles, but for improvement of the low temperature
toughness and improvement of the impact energy
absorption, it is important to control the alloy
carbonitride particles with poor lattice matching with
the matrix structure. If the average particle size of the
alloy carbonitrides with incoherent interfaces which
degrade the lattice matching is over 20 nm, the low
temperature impact energy absorption falls, so the
suitable range was limited to 20 nm or less. From the
viewpoint of obtaining a better impact energy absorption,
10 nm or less is the more preferable range. The lower
limit is not particularly set, but as a size enabling
analysis of the crystal orientation of the precipitate, 2
nm is the substantive lower limit.
[0059] Here, "alloy carbonitrides with incoherent
interfaces" means the state not coherent precipitated in
the matrix structure of ferrite or bainite and adjoining
ferrite and bainite not having the following crystal
orientation relationships (Baker-Nutting orientation
relationships):
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(100)MX//(100)Fe
(010)MX//(011)Fe
(001)MX//(0-11)Fe (Note: -1 is alternative notation for 1
with bar above it)
Here, M indicates Ti and Nb. The percentages occupied by
Ti and Nb are not an issue. Further, X indicates C and N.
The percentages occupied by C and N are not an issue.
When adding V or Mo, sometimes M contains V or Mo.
[0060] Note that, the alloy carbonitrides with
incoherent interfaces were analyzed for crystal
orientation and measured for average particle size using
a transmission type electron microscope (TEM). First, a
steel slab sample was rendered into a thin film of an
extent through which electron beams pass, the TEM was
used to analyze the crystal orientation between the
precipitate and the surrounding matrix phase Fe, then the
average particle size of 20 precipitates in order from
the largest diameter precipitates in the precipitates
which were judged to be incoherent precipitates was
measured. Here, the "particle size of a precipitate" is
measured as the equivalent circle diameter when assuming
a circle equivalent to the cross-sectional area of a
particle.
[0061] "Yield Ratio of 0.85 or More"
If the yield ratio is less than 0.85, sometimes the low
temperature impact energy absorption falls and the
bendability falls. For this reason, the lower limit of
the yield ratio was made 0.85.
[0062] Note that, in the present invention, riim/t was
used as the criteria for evaluation of the bendability.
Here, "t" is the thickness of the test piece and riim is
the limit radius of curvature at which no cracks occur in
a 900 V-bending test. An rLm/t of 1.0 or less was deemed
good bendability. 0.5 or less is the more preferable
range. The upper limit is not particularly set, but if
the value is over 1.1, the bendability may fall, so 1.1
or less is the more preferable range.
CA 02843588 2014-01-29
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[0063] "Maximum Tensile Strength of 600 MPa or More"
If the maximum tensile strength is less than 600 MPa, the
steel sheet does not contribute to reduction of weight of
the members of cars, trucks, construction machinery,
etc., so in the present invention, steel sheet of a
maximum tensile strength of 600 MPa or more is assumed.
[0064] Next, the method of production will be
explained in detail.
[0065] Before the hot rolling, it is necessary to heat
the steel slab of the ingredients which are prescribed in
the present invention to 1150 C or more to render the
alloy carbonitrides which are present in the steel slab a
solid solution state. If the heating temperature is less
than 1150 C, it becomes difficult to obtain a strength of
a maximum tensile strength 600 MPa or more. Further, the
coarse alloy carbonitrides do not sufficiently dissolve
and as a result coarse alloy carbonitrides remain, so the
low temperature impact energy absorption falls. For this
reason, the heating temperature of the steel slab was
limited to 1150 C or more. The upper limit is not
particularly set, but if over 1300 C, the effect becomes
saturated, so this is the substantive upper limit.
[0066] The above heated steel slab is rough rolled to
a rough bar. This rough rolling has to be completed
between 1000 C to 1080 C. If the finishing temperature is
less than 1000 C, coarse alloy carbonitrides precipitate
in the austenite and the low temperature impact energy
absorption falls, while if 1080 C or more, the austenite
grains become coarser, it is not possible to obtain an
average grain size of ferrite and bainite of 10 m or
less in the transformed structure after finish rolling,
cooling, and coiling, the low temperature toughness
deteriorates, and the impact energy absorption falls.
Further, in rough rolling performed at 1150 C or less, the
holding time between rolling reduction passes is an
important parameter which affects the average particle
CA 02843588 2014-01-29
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size of the incoherent alloy carbonitrides. In the method
of the present invention, the rough rolling is usually
performed by rolling 3 to 10 times or so, more preferably
rolling 5 to 10 times, but if the maximum holding time tO
between rolling passes performed at 1150 C or less is 45
sec or more, the alloy carbonitrides become coarser to an
extent affecting the impact energy absorption. For this
reason, the holding time between rolling reduction passes
was limited to within 45 seconds. Within 30 sec is more
preferable.
[0067] Next, the rough bar is finish rolled to obtain
a rolled material.
[0068] The time (tl) from after rough rolling finishes
to the start of the finish rolling is an important
parameter which affects the average particle size of the
alloy carbonitrides and the grain size of the ferrite and
bainite after transformation. As shown in FIG. 2, the
greater the total amount of Ti and Nb, the more the
holding time tl (arrow mark in figure) where the impact
energy absorption (vE_40) shifts from good (OK) to no good
(NG) increases. The holding time tl (sec) where the
absorption shifts from good (OK) to no good (NG)
substantially corresponds to 1000x([%Ti]+[%Nb]). In this
way, if the holding time tl (sec) from after the rough
rolling finishes to when the finish rolling starts is
1000x([%Ti]+[%Nb])sec or more, coarse alloy carbonitrides
precipitate in the austenite, the austenite crystal
grains become coarser, it is not possible to obtain an
average grain size of ferrite and bainite of 10 m or
less in the transformed structure after the finish
rolling, cooling, and coiling, the low temperature
toughness deteriorates, and the impact energy absorption
falls. 700x([%Ti]+[%Nb])>tlsec is the more preferable
range. Accordingly, the holding time tl (sec) was defined
by the following formula (1):
1000x([96Ti]+[%Nb])>t1 .............. formula (1)
CA 02843588 2014-01-29
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[0069] Further, in hot finish rolling, the final
rolling temperature Tf has an effect on the average
particle size of the alloy carbonitrides and the grain
size of ferrite and bainite after transformation, so is
an important condition in the present invention and
changes depending on the contents of Ti and Nb.
[0070] It was learned that if the final rolling
temperature Tf is 830+400x([%Ti]+[%Nb]) or less, coarse
alloy carbonitrides with no lattice matching with the
matrix precipitate and the low temperature impact energy
absorption falls. Therefore, the final rolling
temperature Tf is set so as to satisfy the following
formula (2).
Tf>830+400([%Ti]+[%Nb]) ¨.formula (2)
This relationship (2) is found from the relationship of
the type of steel of Table 2 explained later and the
final rolling temperature Tf. FIG. 3 shows the
relationship between the mass% of Ti+Nb and Tf ( C) of an
invention example and comparative example (A-7 and B-6)
in the types of steel which are shown in Table 2. Here,
it is learned that the case where the coefficient "a" of
the part "a([%Ti]+[%Nb])" is.made 400, that is, formula
(2), is the boundary at which the -40 C impact absorption
energy vE_40 becomes 70J/cm2 or more.
[0071] When the coefficient "a" is 800, that is, when
Tf>830+800([%Ti]+[%Nb]) ¨.formula (3),
compared with when the coefficient "a" is 400, the -40 C
impact absorption energy vE_40 shifts somewhat from the
boundary of 70J/cm2 or more. However, in the region where
the coefficient "a" is 400 to 800, the wait time until
the start of finish rolling becomes longer and the
possibility of alloy carbonitrides starting to
precipitate becomes higher, so the Tf is preferably
controlled based on the formula (3) where the coefficient
"a" is 800.
[0072] The upper limit of the final rolling
CA 02843588 2014-01-29
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temperature Tf is not particularly set, but the grain
size of the ferrite and bainite tends to become coarser,
so 970 C or less is more preferable.
[0073] Right after the final rolling, the rolled
material is water cooled. The time from when the final
rolling finishes to the start of air cooling has an
effect on the low temperature base material toughness and
impact energy absorption through the 7-particle size and
average particle size of the alloy carbonitrides. If the
air-cooling time right after the final rolling exceeds 3
sec, the impact energy absorption tends to fall, so the
water cooling is started within 3 seconds. The lower
limit is not particularly set, but in general facilities
is substantially 0.2 sec or more.
[0074] After the air cooling right after the final
rolling, the rolled material is cooled to obtain the hot
rolled steel sheet. This cooling is an important process
for controlling the metal structure. The cooling is
performed down to 700 C or less by the lowest cooling rate
of 8 C/sec or more.
[0075] If the stop temperature of the cooling exceeds
700 C, alloy carbonitrides easily precipitate coarsely at
the grain boundaries, pearlite easily forms, the grain
size of the ferrite becomes larger, and the low
temperature impact energy absorption falls. On the other
hand, when the lowest cooling rate down to 700 C is less
than 8 C/sec, the alloy carbonitrides easily precipitate
coarsely at the grain boundaries, pearlite easily forms,
the grain size of the ferrite becomes larger, and the low
temperature impact energy absorption falls.
[0076] Here, a lowest cooling rate 8 C/sec or more
means that the cooling rate between temperatures from the
air-cooling finishing temperature to 700 C never becomes
lower than 8 C/sec. For this reason, for example, this
means air cooling is not performed in this temperature
CA 02843588 2014-01-29
= - 22 -
range. In this way, in the present invention, air-cooling
is not performed in the middle of the cooling process
using water cooling unlike in the past.
[0077] The cooling stop temperature is more preferably
680 C or less, while the lowest cooling rate is more
preferably 15 C/sec or more. The upper limit of the lowest
cooling rate is not particularly set, but if the rate is
over 80 C/sec, uniform cooling in the hot rolled coil
becomes difficult and the fluctuations in strength in the
coil become greater. For this reason, 80 C/sec or less is
preferable.
[0078] Next, the cooled hot rolled steel sheet is
coiled up. The coiling temperature is made 530 to 650 C.
If the coiling temperature is less than 530 C, sometimes
martensite or retained austenite forms and the drop in
low temperature toughness and drop in impact energy
absorption become remarkable. Further, if over 650 C, the
area percentage of the pearlite becomes greater and the
drop in low temperature toughness and drop in impact
energy absorption become remarkable.
[0079] The thus obtained hot rolled steel sheet may
also be reheated (annealed). In this case, if the
temperature of the reheating exceeds the Ac3 temperature,
coarse alloy carbonitrides precipitate and the low
temperature impact energy absorption falls. For this
reason, the suitable range of the reheating temperature
is limited to the Ac3 temperature or less. The heating
method is not particularly designated and may be a method
using furnace heating, induction heating, ohmic heating,
high frequency heating, etc.
[0080] The heating time is not particularly
determined, but if the heating and holding time at 550 C
or more exceeds 30 minutes, to obtain a 590 MPa or more
tensile strength, the highest heating temperature is
preferably made 700 C or less.
[0081] Note that, the reheating (annealing) may be
CA 02843588 2014-01-29
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performed after coiling the hot rolled steel sheet and
before the temperature falls to room temperature.
[0082] Skin pass rolling or leveler rolling is
effective for correcting the shape, aging, and improving
the fatigue characteristics, so may be performed after
pickling or before pickling. If performing skin pass
rolling, the upper limit of the rolling rate is
preferably made 3%. This is because if over 3%, the
shapeability of the steel sheet is impaired. Further,
pickling may be performed in accordance with the
objective.
[0083] Next, the hot dipped galvanized steel sheet and
method of production of the same of the present invention
will be explained.
[0084] The hot dipped galvanized steel sheet of the
present invention is the above-mentioned hot rolled steel
sheet of the present invention on the surface of which a
plating layer or alloy plating layer is provided.
[0085] The hot rolled steel sheet which was obtained
by the above-mentioned method was pickled, then a
continuous galvanization facility or continuous annealing
and galvanization facility was used to heat the steel
sheet and hot dip coat it to form a plating layer on the
surface of the hot rolled steel sheet.
[0086] If the heating temperature of the steel sheet
exceeds the Ac3 temperature, a drop in the tensile
strength of the steel sheet and a drop in the low
temperature impact energy absorption occur, so the
suitable range of the heating temperature is limited to
the Ac3 temperature or less. The closer the heating
temperature to Ac3, the more rapidly the tensile strength
falls. The base materials greatly fluctuate in grade, so
Ac3-30 C or less is the more preferable range of heating
temperature.
[0087] Further, after the hot dip coating,
galvannealization may be performed to obtain a hot dip
galvannealed layer.
CA 02843588 2014-01-29
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[0088] Note that, the plating type is not limited to
galvanization. It may also be other plating so long as
the upper limit of the heating temperature is the Ac3b
temperature.
[0089] Further, in the present invention, the method
of production preceding the hot rolling is not
particularly limited. That is, a blast furnace,
converter, electric furnace, etc. may be used for
melting, then various types of secondary refining may be
used to adjust the ingredients to give the targeted
contents of ingredients. Next, the steel may be cast by
any method such as normal continuous casting, casting by
the ingot method, or also thin slab casting etc. For the
feed material, scrap may also be used. In the cast of a
slab which is obtained by continuous casting, the high
temperature cast slab may be directly sent as is to the
hot rolling mill or may be cooled down to room
temperature, then reheated at a heating furnace and then
hot rolled.
Examples
[0090] Below, examples will be used to further explain
the present invention.
[0091] Steels A to AC which have the chemical
ingredients which are shown in Table 1 were produced by
the following method. First, the steels were cast to
prepare steel slabs, then the steel slabs were reheated
and rough rolled to rough bars under the hot rolling
conditions and annealing and plating conditions which are
shown in Table 2-1 and Table 2-2. Next, the rough bars
were finish rolled to obtain 4 mm thick rolled materials,
then these were cooled and taken up as hot rolled steel
sheet.
[0092] Table 1
Steel No. C Si _ Mn P s Al Ti Nb N Mn+8T1+12Nb
Ac3 Others Remarks
A 0.04 0.3 1.7 0.01 0.001 0.05 0.03 0.05 0.002
2.5 853 Inv. steel
B 0.05 0.3 1.5 0.01 0.001 0.8 0.07 0.04
0.003 2.5 900 Inv. steel
C 0.08 0.03 1.2 0.02 0.002 0.03 0.06 0.04
0.003 2.2 857 Inv. steel
D 0.06 0.03 1.4 0.01 0.003 0.03 0.05 0.04
0.002 2.3 848 Ca: 0.0015 Inv. steel
E 0.04 0.3 1.8 0.01 . 0.003 0.03 0.06
0.05 0.003 2.9 861 Comp. steel
F 0.09 0.03 1.3 0.01 0.005 0.03 0.03 0.02
0.002 1.8 832 Comp. steel
G 0.02 0.03 - 1.5 0.01 0.003 0.04 0.05
0.03 0.002 2.3 866 Comp. steel
H 0.10 0.03 1.3 0.01 0.003 0.04 0.03 0.04
0.002 2.0 829 Comp. steel
I 0.05 0.5 1.3 0.01 0.003 0.04 0.03 0.04 0.002
2.0 869 Comp. steel
J 0.05 0.03 1.0 0.01 0.003 0.04 0.03 0.07
0.003 2.1 , 857 Comp. steel
K 0.05 0.03 2.1 0.01 0.003 0.04 0.04 0.04
0.003 2.9 828 Comp. steel
L 0.05 0.03 1.3 0.08 0.003 0.04 0.04 0.04
0.003 2.1 901 Inv. steel
M 0.05 0.03 1.3 0.12 0.003 0.04 0.04 0.04
0.003 21 929 Comp. steel 0
N 0.05 0.03 1.3 0.01 0.015 0.04 0.04 0.04
0.003 2.1 852 Inv. steel
O 0.05 0.03 1.3 = 0.01 0.022 0.04 0.04
0.04 0.003 2.1 852 Comp. steel 2
P 0.05 = 0.03 1.3 0.01 0.003 1.3 0.04 0.04 0.003
2.1 902 Comp. steel m
Fl.
Q 0.05 0.03 1.3 0.01 0.003 0.04 0.005 0.05
0.003 1.9 838 Comp. steel w
in
R 0.05 0.03 1.3 0.01 0.003 0.04 0.09 0.06
0.003 2.7 872 Comp. steel m
S 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.003
0.003 1.7 852 Comp. steel m
T 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.10
0.003 2.8 852 Comp. steel K.)
o
U 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.04
0.006 2.1 852 Comp. steel. I H
Fi.
(1)
V 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.04
0.003 2.1 858 V: 0.06 Inv. steel
W 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.04
0.003 2.1 848 Cr: 0.3, Cu: 0.05, Ni: 0.05
Inv. steel N) H
(II
i
X 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.04
0.003 2.1 851 Mo: 0.3, B: 0.002 Inv.
steel K.)
Y 0.05 0.03 1.3 0.01 0.003 0.04 0.04 0.04
0.003 2.1 852 Ce: 0.002, La: 0.001 Inv. steel
I m
Z 0.05 0.03 1.3 0.01 0.003 0.04 0,04 0.04
0.003 2.1 842 Mg: 0.002, Cu: 0.5 Inv. steel
AA 0.04 0.3 1.9 0.01 0.001 0.05 0.02 0.02 0.002
2.3 842 Inv. steel
AB 0.04 0.3 2.1 0.01 0.001 0.05 0.02 0.02 0.002
2.5 836 Comp. steel
AC 0.04 0.3 1.8 0.01 0.001 0.05 0.01 0.003 0.002
1.9 841 Comp. steel
[0093] Table 2
Table 2-1
SRT RFT tO tl Tf t2 CRmin SCT CT Max. annealing
Plating type
Remarks
( C) ( C) (sec) (sec) ( C) (sec) ( C/s) ( C) ( C)
temp. ( C)
A-1 1230 1020 25 50 900 2 25 680 600
Inv. ex.
A-2 1130 1000 25 50 900 2 25 680 600
A-3 1230 960 25 50 900 2 25 680 600
_
A-4 1230 1100 25 50 900 2 25 680 600
A-5 1230 1020 25 20 900 2 25 680 600
Inv. ex.
A-B 1230 1020 25 120 900 2 25 680 600
-
A-7 1230 1020 25 50 860 2 25 680 600
A-8 1230 1020 25 50 900 6 12 680 600
A-9 1230 1020 25 50 900 2 15 700 640
Inv. ex.
A-10 1230 1020 25 50 900 2 5 680 600
n
A-11 1230 1020 25 50 900 2 20 720 680
o
A-12 1230 1020 25 50 900 2 25 560 520
K.)
A-13 1230 1020 25 50 900 2 25 610 550 680
Galvanization Inv. ex. _ m
Fl.
A-14 1230 1020 25 50 900 2 30 580 530 680
Galvannealization Inv. ex. w
in
A-15 1230 1020 25 50 900 2 25 680 600 880
Galvanization m
A-16 1230 1020 50 50 900 2 25 680 600
m
K.)
A-17 1230 1020 70 50 900 2 25 680 600
o
A-18 1230 1020 120 50 900 2 25 680 600
I H
FP
(1)
B-1 1250 1040 25 60 880 2 50 650 570
Inv. ex.
B-2 1250 1000 25 60 880 2 50 650 570
Inv. ex. NJ H
al
I
B-3 1250 970 25 120 880 2 50 650 570
K.)
ko
B-4 1250 1100 25 60 880 2 50 650 570
I
B-5 1250 1040 25 60 880 2 50 650 570
Inv. ex.
B-6 1250 1040 25 60 850 2 50 650 570
B-7 1250 1040 25 60 880 6 10 650570
_ B-8 1250 1040 25 60 880 2 5 650 - 570
B-9, 1250 1040 25 60 880 2 50 680
620 Inv. ex.
B-10 1250 1040 25 60 880 2 50 710 660
B-11 1250 1040 25 60 880 2 50 510 480
B-12 1250 1040 50 60 880 2 50 650 570
B-13 1250 1040 120 60 880 2 50 650 _ 570
C-1 1250 1040 25 45 880 2 50 570 600 ,
Inv. ex.
C-2 1250 1040 25 45 880 2 50 670 600 730
Galvanization Inv. ex. -
,
[0094] Table 2-2
D-1 1259 1040 25 45 889 2 50 670 600
Inv. ex.
E-1 1250 1040 25 60 880 2 50 670 600
F-1 1250 1040 25 60 880 2 50 670 600
G-1 1250 1040 25 60 880 2 50 670 600
H-1 1250 1040 25 60 880 2 50 679 600
1-1 1250 1040 25 60 880 2 50 670 600
J-1 1250 1040 25 60 880 2 50 670 600
K-1 1250 1040 25 60 880 2 50 670 600
L-1 1250 1040 25 45 880 2 50 670 600
Inv. ex.
M-1 1250 1040 25 45 880 2 50 670 600
N-1 1250 1040 25 45 880 2 50 670 600
Inv. ex.
-
0-1 1250 1040 25 60 880 2 50 670 600
P-1 1250 1040 25 60 880 2 50 670 600
Q-1 1250 1040 25 60 880 2 50 670 600
n
R-1 1250 1040 25 60 889 2 50 670 600
S-1 1250 1040 25 60 880 2 50 670 600
o
K.)
T-1 1250 _ 1040 25 60 880 2_ 50 670 600
m
Fl.
U-1 1250 1040 25 60 880 2 50 670 600
w
V-1 1250 _ 1040 25 50 880 2 50 670 600
Inv. ex. in
m
W-1 1250 1040 25 50 880 2 - 50 670 600
Inv. ex. m
X-1 1250 1040 25 50 880 2 50 670 600
Inv. ex. 1\-)
o
Y-1 1250 1040 25 50 880 2 50 670 600
Inv. ex. I H
FP
Z-1 1250 1040 25 50 880 2 50 670 600
Inv. ex. 1
AA-1 1250 1040 25 50 860 2 50 670 600
Inv. ex. rv 0
AB-1 1250 1040 25 50 889 2 50 670 600
1
K.)
AC-1 1250 1040 25 50 880 _ 2 50 670 600
I m
SRT: Slab heating temperature
RFT: Rough rolling finishing temperature
tO: Rolling time at rough rolling performed at 11500C or less
tl: Time from end of rough rolling to start of finish rolling
Tf: Final finish rolling temperature
t2: Air cooling time after final finish rolling
CRmin: Minimum cooling rate during CFT from after air cooling
SCT: Water cooling stop temperature
CT: Coiling temperature
CA 02843588 2014-01-29
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,
[0095] In Table 1, the chemical compositions are given
by mass%. Further, in Table 1, Ac3( C) is the value which
is calculated by the following formula:
Ac3=910-210[%C]+45[%Si]-
30[%Mn]+700[%P]+40[%A1]+400[%Ti]+32[%Mo]-11[%Cr]-20[%Cu]-
15[%Ni]
wherein, %C, %Si, %Mn, %P, %Al, %Ti, %Mo, %Cr, %Cu, and
%Ni respectively indicate the contents in steel of C, Si,
Mn, P, Al, Ti, Mo, Cr, Cu, and Ni.
[0096] In Table 1, the chemical compositions of the
steels correspond to the chemical compositions of the
steels of the steel numbers in Table 2 with the same
alphabet letters as the steel numbers.
[0097] In Table 2, "SRT" indicates the slab reheating
temperature ( C). "RFT" indicates the rough rolling finish
temperature ( C). "t0" indicates the maximum holding time
(sec) between rough rolling operations performed at 1150 C
or less. "tl" indicates the time (sec) from the end of
the rough rolling to the start of the finish rolling.
"Tf" indicates the final finish rolling temperature ( C).
"t2" shows the air cooling time right after the last
finish rolling (sec). "CRmin" indicates the minimum
cooling rate in the SCT after air cooling ( C/sec). "SCT"
indicates the water cooling stop temperature ( C). "CT"
indicates the coiling temperature ( C).
[0098] The Steels A-12 to A-14 and C-2 are hot dipped
galvanized steel sheets which were produced by pickling
the hot rolled steel sheets, then annealing them on a
continuous annealing and galvanization line at the
annealing temperatures which are shown in Table 2, then
galvanizing them.
[0099] Note that, the galvanization dipping
temperature was made 450 C while, for galvannealing
treatment, the alloying temperature was made 500 C.
[0100] First, the metal structures and alloy
CA 02843588 2014-01-29
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,
carbonitrides of the prepared steel sheet were examined.
[0101] The metal structure of the steel sheet, as
explained above, was observed based on JIS G 0551 for the
L-cross-section by an optical microscope. Further, the
area percentages of the different structures were
measured by the point count method or image analysis
using structural photographs at regions of 1/4t thickness
of the L-cross-section (position of 1/4t from surface of
steel sheet when sheet thickness is "t"). The grain sizes
of the ferrite and bainite were measured by calculating
the nominal particle size based on JIS G 0552.
[0102] The alloy carbonitrides with incoherent
interfaces which contain Ti and NB were analyzed for
crystal orientation and measured for average particle
size by rendering the steel slab sample into a thin film
of an extent through which electron beams pass and using
a transmission type electron microscope (TEM). 20 or more
alloy carbonitride particles were examined.
[0103] Next, to measure the amount of softening of the
weld heat affected zone (HAZ), arc welding was used to
prepared a lap joint. The welding was performed in an
atmosphere of CO2: 100% with a heat input of about 5000 to
8000J/cm in range. After welding, the cross-section was
polished and the base material and the weld heat affected
zone (HAZ) were tested for Vicker's hardness aiming at 0
or less softening. The above measurement results are
shown in Table 3. Note that, in Table 3, "F" indicates
ferrite, "B" indicates bainite, "A" indicates retained
austenite, "M" indicates martensite, and "P" indicates
pearlite, "d(F, B)" indicates the average grain size ( m)
of ferrite and bainite, "dmcm" indicates the average
particle size (nm) of alloy carbonitrides with incoherent
interfaces, and "AHV" indicates the difference between
HVBm and HVHAz when the Vicker's hardness of the softest
part of the weld heat affected zone is HVHAz and the
Vicker's hardness of the base material is HVBN=
.,..
[0104] Table 3
Table 3-1
YP TS Metal structure percentage (%)
Bend-
Steel No. (MPa) (MPa) F+B M A
ability
El (%) YR d(r,B)
dmcN AHV vE-40 Remarks
P
A-1 600 640 25 0.94 98 2 8
12 20 120 VG Inv. ex.
A-2 550 590 27 0.93 98 2 9
25 18 60 VG
A-3 590 530 25 0.94 98_ 2 8
22 20 60 VG
A-4 600 645 25 0.93 98 2 14
14 18 55 VG
A-5 600 640 25 0.94 98, 2 7
13 19 110 VG Inv. ex.
A-6 600 640 25 0.94 100 12
18 20 60 VG
A-7 590 630 25 0.94 94 6 8
21 22 65 VG
_ A-8 595 635 25 0.94 98 2 10
21 20 65 VG
A-9 580 620 26 D 94 97 3 9
15 18 100 VG Inv. ex.
A-10 570 610 27 0.93 94 6 11
15 18 65 VG 0
A-11 555 600 27 0.93 93 7 13
12 17 55 VG
A-12 490 575 29 0.85 99 1 7
12 17 64 VG o
K.)
A-13 640 650 24 0.98 98 2 8
13 24
_
110 VG Inv. ex. op
Fl.
A-14 600 610 25 0.98 100 7
13 20 120 VG Inv. ex.
L61.)
A-15 500 550 26 0.91 100 8
20 _ 16 50 VG op
A-16 600 635 25 0.94 98 2 8
1320
_
60 VG op
A-17 590 630 25 0.94 98. - 2
8 21 18
55
VG K)
o
A-18 590 625 25 0.94 982 8
27 18 50 VG 1 H
_ _
FP
1
8-1 630 630 24 0.93 99 1 8
1421
_
100 VG Inv. ex.
8-2 630 630 24 0.93 99- _ 1
8 15 22 85 VG Inv. ex. H
CD
1
8-3 610 665 25 0.92 99 1 8
22 21 50 VG K.)
8-4 625 675 24 0.93 100 12
13 21 65 VG I ko
B-5 630 680 24 0.93 100, 8
15 25 90 VG Inv. ex.
,
B-6 620 670 24 0.93 100 8
21 24 60 VG
B-7 620 670 24 0.93 100 10
23 26 60 VG
B-8 515 665 24 0.92 100 10
216
_ 2
65 VG
B-9 650 680 -__ 24 0.96 97. 3 = 9
14 22
_
80 VG Inv. ex.
B-10 600 640 25 0.94 94 6 12
23 35 55 VG _
B-11 480 580 27 0.83 98 2 8
12 65 G
8-12 625 675 24 0.93 99_ _ 1 9
14 22 55 VG _
B-13 620 670 - 24 0.93 99 _ 1 9
14 24 50 VG
C-1 560 620 27 0.90 98 2 9
12 36 80 VG Inv. ex.
C-2 585 600 25 0.98 98 2 9
14 33 70 VG Inv. ex.
,..
[0105] Table 3-2
D-1 605 695 25 0.87 98 2 8 15
30 85 VG Inv. ex.
E-1 620 685 24 0.91 98 2 7 14
8 65 VG
F-1 570 595 23 0.96 98 2 8 15
52 65 VG
Q-1 545 580 28 0.94 100 10 13
44 75 VG
H-1 590 720 24 0.82 97 3 10 15
41 65 P
1-1 595 715 24 0.83 97 2 1 8 15
42 60 P
J-1 615 690 24 0.89 96 6 8 22
33 55 VG
K-1 605 720 24 0.84 98 2 7 21
6 60 P
L-1 625 680 26 0.92 98 2 9 14
38 80 VG Inv. ex.
M-1 665 700 24 0.95 98 2 8 14
37 40 G
N-1 595 640 25 0.93 98 2 9 12
33 75 G Inv. ex.
0-1 600 640 25 0.94 98 2 8 13
34 45 P
P-1 570 620 27 0.96 98 2 10 13
48 95 VG
Q-1 540 595 28 0.91 98 2 8 12
43 110 VG n
R-1 720 780 21 0.92 98 2 9 21
37 45 VG
S-1 615 640 26 0.96 98 2 8 13
56 90 VG o
n)
T-1 680 720 23 0.94 97 2 8 22
22 65 VG m
Fl.
U-1 655 700 24 0.94 98 2 8 21
34 60 VG w
in
V-1 665 700 24 0.95 98 2 8 15
36 80 VG Inv. ex. m
W-1 625 675 24 0.93 98 2 7 14
34 90 VG Inv. ex. m
X-1 620 670 24 0.93 100 8 15
34 100 VG Inv. ex. n)
o
_
Y-1 630 680 24 0.93 100 7 15
35 90 VG Inv. ex. 1 H
FP
O
Z-1 650 700 24 0.93 100 8 15
36 100 VG Inv. ex.
AA-1 555 635 26 0.87 100 8 13
24 100 G Inv. ex. W H
_
I¨` I
AB-1 525 630 25 0.83 98 2 8 11
42 65 G n)
-
AC-1 555 580 28 0.96 100 7 11
41 120 VG I ko
d(F,B): Average grain size of ferrite and bainite ( m)
dmcN: Average particle diameter of incoherent alloy carbonitrides
AHV: HAZ softening of arc weld zone (HV)
vE-40: Charpy impact energy absorption at -40 C (J/cm2)
CA 02843588 2014-01-29
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[0106] Next, the steel sheet was evaluated for
strength properties, low temperature impact energy
absorption, and bendability.
[0107] The steel sheets were evaluated for strength
properties by the following method. First, the test
material was worked to a No. 5 test piece described in
JIS Z 2201. Further, this No. 5 test piece was subjected
to a tensile test in accordance with the method described
in JIS Z 2241 and the maximum tensile strength (TS),
yield strength (YS), and elongation (EI) were found.
[0108] The low temperature impact energy absorption
was evaluated by a Charpy impact test. Based on JIS Z
2202, a thickness 3 mm 2 mmV-notch test piece was
prepared. The test piece was cooled to -40 C, then a
Charpy impact test was performed and the impact energy
absorption (J/cm2) was measured.
[0109] The bending test was performed by the V-block
method of JIS Z 224 (bending angle: 90'). The thickness of
the test piece was "t". The limit bending radius riim with
no cracks was measured.
[0110] The above measurement results are shown in
Table 3. Note that, as explained above, in Table 3, "vE_
40" is the Charpy impact absorption value (J/cm2), while
"riiin/t" is the value of the limit bending radius rlim
divided by the sheet thickness. An rlindt of 0.5 or less
is ranked as "VG "(very good), over 0.5 to 1.0 or less in
range is ranked as "G" (good), and over 1.0 is ranked as
"P" (poor).
[0111] The Steel A-2 had a slab heating temperature
outside of the suitable range, so is a comparative
example where then tensile strength was less than 600 MPa
and the low temperature impact energy absorption was low.
[0112] The Steels A-3 to A-4 and the Steels B-3 to B-4
had rough rolling finish temperatures outside of the
suitable range, so are comparative examples where the low
temperature impact energy absorptions were low.
[0113] The Steel A-6 and the Steel B-3 had times from
CA 02843588 2014-01-29
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the end of rough rolling to the start of finish rolling
outside of the suitable range, so are comparative
examples where the low temperature impact energy
absorptions were low.
[0114] The Steels A-7 to A-8, the Steel A-10, and the
Steels B-6 to B-8 had conditions of finish rolling and
cooling conditions after finish rolling outside of the
suitable range, so are comparative examples where the low
temperature impact energy absorptions were low.
[0115] The Steel A-11 and the Steel B-10 had water
cooling finish temperatures after finish rolling and
coiling temperatures of the hot rolled steel sheets
outside of the suitable range, so are comparative
examples where the low temperature impact energy
absorptions were low.
[0116] The Steel A-12 and the Steel B-11 had coiling
temperatures of the hot rolled steel sheets outside of
the suitable range, so are comparative examples where the
tensile strengths were less than 600 MPa and the low
temperature impact energy absorptions were low.
[0117] The Steel A-15 had an annealing temperature of
the Ac3 temperature or more, so is a comparative example
where the low temperature impact energy absorption was
low.
[0118] The Steels F-1, Q-1, S-1, AB-1, and AC-1 had
values of amounts of Mn, amounts of Ti, and amounts of Nb
outside of the suitable range, so are comparative
examples where the amounts of softening of the HAZ were
large. Among these, the Steels F-1, Q-1, and AC-1 had
tensile strengths of less than 600 MPa.
[0119] The Steel G-1 had an amount of C outside of the
suitable range, so is a comparative example where the
strength was less than 600 MPa and the amount of
softening of the HAZ was large.
[0120] The Steels H-1, I-1, K-1, and AB-1 had amounts
of C, amounts of Si, and amounts of Mn outside the
suitable ranges, so are comparative examples where
CA 02843588 2014-01-29
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,
martensite or retained austenite was present, the low
temperature impact energy absorption was low, and further
the bendability was poor. The Steel J-1 had an amount of
Mn outside of the suitable range, so is a comparative
example where pearlite was present and the low
temperature impact energy absorption was low.
[0121] The Steels M-1 and 0-1 had amounts of S and
amounts of P which were excessive, so are comparative
examples where the low temperature impact energy
absorptions were low.
[0122] The Steels E-1, R-1, T-1, and U-1 had amounts
of Ti, amounts of Nb, and amounts of N outside the
suitable ranges, so are comparative examples where coarse
alloy carbonitrides were present and the low temperature
impact energy absorptions were low.
[0123] The Steel P-1 had an excessive amount of Al, so
is a comparative example with softening of the HAZ.
[0124] As opposed to this, the invention examples all
had yield ratios of 0.85 or more, maximum tensile
strengths of 600 MPa or more, and excellent low
temperature impact energy absorption and HAZ softening
resistance.
