Technical Field
[0001] The present invention relates to steel plates. Specifically, the present invention
relates to a steel plate that is mainly used as materials for structures such as ships
buildings, bridges, and construction machinery, has a tensile strength of 490 MPa
to less than 650 MPa, and offers excellent fatigue properties.
Background Art
[0002] With upsizing, large structures such as ships, buildings, bridges, and construction
machinery require still better reliability of their structural components, because
such larger structures, if suffering from breakage, may receive larger damage. It
has been known that most of breakages in large structures is caused by fatigue fracture.
As possible solutions to this, various techniques for resisting fatigue fracture have
been developed. However, there are not a few cases where fatigue fracture causes breakage
of such a large structure, even now.
[0003] In general, portions of large structures susceptible to fatigue damage have been
protected from fatigue fracture by schemes such as designing of the portions to have
such a shape as to less cause stress concentration, and use of high-strength steel
plates. However, the resulting structures obtained according to the schemes cause
higher production cost due to a higher number of steps or due to the use of more expensive
steel plates. Accordingly, demands are made to provide a technique for allowing a
steel plate itself to have better fatigue properties. In general in steel plates,
it is known that fatigue strength, in particular fatigue limit, is in proportional
to tensile strength, and a steel plate having a high fatigue limit to tensile strength
ratio is considered as a steel plate having excellent fatigue properties, where the
fatigue limit to tensile strength ratio is calculated by dividing the fatigue limit
by the tensile strength.
[0004] Many researches have been performed so as to offer better fatigue properties. For
example, Nonpatent Literature (NPL) 1 presents how various influencing factors affect
fatigue strength. The literature mentions that solid-solution strengthening, precipitation
strengthening, grain refinement, and second phase strengthening contribute to better
fatigue properties, but dislocation hardening less contributes to better fatigue properties,
because the dislocation hardening is attended with increase in moving dislocations.
The fatigue fracture process can be divided into a process (1) and a process (2).
In the process (1), a load is repeatedly applied, and, finally, cracks are generated.
In the process (2), the generated cracks grow and lead to rupture (breakage). Of the
above-mentioned factors for better fatigue properties, solid-solution strengthening,
precipitation strengthening, and grain refinement are considered to be effective in
the process (1), because restrainment of accumulation of dislocation is effective
in this process. In contrast, grain refinement and second phase strengthening are
considered to be effective in the process (2), because elimination or minimization
of crack propagation is effective in this process.
[0005] Patent Literature (PTL) 1 proposes a technique in which the steel is controlled to
have a two-phase microstructure including fine ferrite and hard martensite, and the
difference in hardness between the two phases is specified to be 150 or more in terms
of Vickers hardness. The technique is intended to lower the crack propagation rate
and to contribute to a longer fatigue life after crack initiation.
[0006] PTL 2 proposes a technique for lowering the crack propagation rate by allowing the
steel to have a mixed microstructure including fine ferrite and bainite. This technique
is expected to offer longer fatigue life after crack initiation also in fatigue fracture.
However, the technique lacks consideration of fatigue properties before crack initiation
at all.
[0007] PTL 3 proposes a technique of allowing carbides to precipitate in ferrite phase so
as to offer higher fatigue strength. This literature, however, lacks description about
fatigue properties after crack initiation. In addition, the technique in PTL 3 targets
thin steel sheets and gives no consideration to toughness and other properties necessary
for large structures at all.
Citation List
Nonpatent Literature
Patent Literature
Summary of Invention
Technical Problem
[0010] The present invention has been made under these circumstances and has a main object
to provide a steel plate having excellent fatigue properties.
[0011] Other objects of the present invention will be apparent from descriptions in "Description
of Embodiments" below.
Solution to Problem
[0012] The present invention has achieved the objects and provides, according to a first
embodiment, a steel plate as follows. The steel plate contains C in a content of 0.02
to 0.10 mass percent, Mn in a content of 1.0 to 2.0 mass percent, Nb in a content
of greater than 0 mass percent to 0.05 mass percent, Ti in a content of greater than
0 mass percent to 0.05 mass percent, Al in a content of 0.01 to 0.06 mass percent,
and at least one element selected from the group consisting of Si in a content of
0.1 to 0.6 mass percent and Cu in a content of 0.1 to 0.6 mass percent, where the
total content of the at least one of Si and Cu is 0.3 mass percent or more, with the
remainder consisting of iron and inevitable impurities. The microstructure of the
surface layer of the steel plate includes at least one of ferrite and upper bainite
in a total fraction of 80 area percent or more. Grains of the at least one of ferrite
and upper bainite have an effective grain size of 10.0 µm or less. Of the microstructure
of the surface layer, grains of the remainder microstructure excluding the ferrite
and the upper bainite have an average equivalent circle diameter of 3.0 µm or less.
The steel plate has a dislocation density p of 2.5× 10
15 m
-1 or less, as determined by X-ray diffractometry.
[0013] As used herein, the term "average equivalent circle diameter" refers to an average
of "equivalent circle diameters" of grains of a phase in question, where the "equivalent
circle diameter" refers to the diameter of a grain of the phase in terms of a circle
having an equivalent area to the grain.
[0014] The fraction (proportion) of martensite-austenite constituent in the remainder microstructure
is preferably 5 area percent or less.
[0015] The steel plate according to the present invention preferably further contains at
least one selected from the group consisting of (a), (b), and (c) below. The steel
plate may have further improved property or properties depending on the type of an
element to be contained. Specifically, the steel plate preferably further contains
at least one selected from the group consisting of.
- (a) Ni in a content of greater than 0 mass percent to 0.6 mass percent, where the
ratio [Ni]/[Cu] of the Ni content [Ni] to the Cu content [Cu] is less than 1.2;
- (b) at least one element selected from the group consisting of V in a content of greater
than 0 mass percent to 0.5 mass percent, Cr in a content of greater than 0 mass percent
to 1.0 mass percent or less, and Mo in a content of greater than 0 mass percent to
0.5 mass percent; and
- (c) B in a content of greater than 0 mass percent to 0.005 mass percent.
[0016] The steel plate according to the present invention also preferably has a bainite
transformation start temperature Bs of 640°C or higher, where the bainite transformation
start temperature is calculated from the chemical composition according to Expression
(1):

where [C], [Mn], [Ni], [Cr], and [Mo] represent contents (in mass percent) respectively
of C, Mn, Ni, Cr, and Mo.
[0017] The present invention has archived the objects and provides, according to a second
embodiment, a steel plate as follows. This steel plate contains C in a content of
0.02 to 0.10 mass percent, Mn in a content of 1.0 to 2.0 mass percent, Nb in a content
of greater than 0 mass percent to 0.05 mass percent, Ti in a content of greater than
0 mass percent to 0.05 mass percent, Al in a content of 0.01 to 0.06 mass percent,
at least one element selected from the group consisting of Si in a content of 0.1
to 0.6 mass percent and Cu in a content of 0.1 to 0.6 mass percent, where the at least
one of Si and Cu is contained in a total content of 0.3 mass percent or more, and
B in a content of greater than 0 mass percent to 0.005 mass percent, with the remainder
consisting of iron and inevitable impurities. The microstructure of the surface layer
of the steel plate includes at least one of ferrite and upper bainite in a fraction
of 80 area percent or more. Grains of the at least one of ferrite and upper bainite
have an effective grain size of 10.0 µm or less. Of the microstructure of the surface
layer, grains of the remainder microstructure excluding the ferrite and the upper
bainite have an average equivalent circle diameter of 3.0 µm or less. The steel plate
has a dislocation density ρ of 2.5× 10
15 m
-1 or less as determined by X-ray diffractometry. A microstructure at a position at
a depth of one-fourth the thickness t of the steel plate from the surface along the
thickness direction includes upper bainite in a fraction of 80 area percent or more
in a longitudinal section in parallel with the rolling direction. Grains of the upper
bainite in the microstructure at the position at a depth of one-fourth the thickness
t have an effective grain size of 10.0 µm or less. Of the microstructure at the position
at a depth of one-fourth the thickness t, grains of the remainder microstructure excluding
the upper bainite have an average equivalent circle diameter of 3.0 µm or less.
[0018] Also in the steel plate according to the second embodiment, of the microstructure
of the surface layer, the remainder microstructure excluding the ferrite and the upper
bainite preferably has a fraction of martensite-austenite constituent of 5 area percent
or less.
[0019] The steel plate according to the second embodiment preferably further contains, in
chemical composition, at least one selected from the group consisting of (a) and (b)
below. The steel plate may have better property or properties according to the type
of an element to be contained. Specifically, the steel plate preferably further contains
at least one selected from the group consisting of
- (a) Ni in a content of greater than 0 mass percent to 0.6 mass percent, where the
ratio [Ni]/[Cu] of the Ni content [Ni] to the Cu content [Cu] is less than 1.2; and
- (b) at least one element selected from the group consisting of V in a content of greater
than 0 mass percent to 0.5 mass percent, Cr in a content of greater than 0 mass percent
to 1.0 mass percent or less, and Mo in a content of greater than 0 mass percent to
0.5 mass percent
[0020] The steel plate according to the second embodiment also preferably has a bainite
transformation start temperature Bs of 640°C or higher, where the bainite transformation
start temperature is calculated from the chemical composition according to Expression
(1):

where [C], [Mn], [Ni], [Cr], and [Mo] represent contents (in mass percent) respectively
of C, Mn, Ni, Cr, and Mo.
[0021] Assume that the microstructure at the position one fourth the thickness t of the
steel plate from the surface along the thickness direction meets the condition(s),
where the microstructure is in a longitudinal section in parallel with the rolling
direction. In this case, the microstructure at the position at a depth of one-fourth
the thickness t preferably includes grains each having a grain average misorientation
(GAM) of 1° or more in a fraction of 20 area percent to 80 area percent, where the
grain average misorientation is determined per one grain by electron backscatter pattern
(EBSP) analysis.
[0022] Assume that the microstructure at the position at a depth of one-fourth the thickness
t includes grains each having a GAM of 1° or more in a fraction of 20 area percent
to 80 area percent, where the grain average misorientation is determined per one grain
by EBSP analysis. In this case, the steel plate preferably meets conditions specified
by Expression (2) and Expression (3):

where [Si], [Mn], [Ni], [Cu], [Ti], [N], [Cr], [Nb], [C], and [Mo] represent contents
(in mass percent) respectively of Si, Mn, Ni, Cu, Ti, N, Cr, Nb, C, and Mo, and, when
at least one of the term {[Ti] - 3.4[N]} and the term {[Nb] - 7.7[C]} is negative,
the calculation according to the expression is performed as treating the at least
one term as "zero (0)".
Advantageous Effects of Invention
[0023] Advantageous effects of the present invention as disclosed in the description will
be briefly described as follows. Specifically, the present invention can practically
provide steel plates that have excellent fatigue properties.
Brief Description of Drawings
[0024]
Fig. 1 depicts schematic explanatory views of a steel plate according to the present
invention;
Fig. 2 depicts schematic explanatory views of a test specimen used in measurements
of fatigue properties;
Fig. 3 depicts schematic explanatory views of a compact tension test specimen used
in crack propagation rate measurement; and
Fig. 4 depicts conceptual diagrams illustrating grain boundaries, KAM, and GAM.
Description of Embodiments
[0025] The inventors of the present invention made investigations of the total life of steel
plates leading up to fatigue fracture and particularly of proportions of the prior-stage
life leading to crack initiation and of the subsequent-stage life from crack initiation
leading up to rupture. As a result, the inventors found that the prior-stage life
leading to crack initiation occupies about one half of the total life leading up to
fatigue fracture; and that the prior-stage life leading to crack initiation occupies
a larger proportion with a decreasing stress level and a resulting increasing total
life. The results indicate that increase of the total life leading up to fatigue fracture
requires not only better fatigue properties after crack initiation, but also better
fatigue properties leading to crack initiation. In particular, at around the fatigue
limit, increase in prior-stage life is considered to be effective because the prior-stage
life leading to crack initiation tends to occupy a larger proportion of the total
life.
[0026] The inventors made various investigations on conditions for a longer prior-stage
life. As a result, the inventors found that a steel plate as follows can have a longer
prior-stage life, and consequently have a longer total life leading up to fatigue
fracture. This steel plate is appropriately controlled in the chemical composition,
and conditions typically on the fraction(s) of principal phase(s), the effective grain
size of grains of the phase(s), the average equivalent circle diameter of the remainder
microstructure excluding the principal phase(s), and the dislocation density ρ as
determined by X-ray diffractometry. The present invention has been made based on these
findings.
First Embodiment
[0027] The first embodiment according to the present invention will be illustrated below.
The inventors made investigations on various steel plates about how elements to be
added affect the fatigue strength. Consequently, the inventors found that the addition
of Si and/or Cu significantly improves the fatigue strength. In general, moving dislocations,
which move by repeated stress, move irreversibly due typically to cross slip, and
this causes fatigue cracking. It is known that the dislocations form a cell structure
in this process. The inventors found that the addition of Si and/or Cu in a total
amount of 0.3 mass percent or more restrains the cell structure formation.
[0028] Detailed observations demonstrate that these elements do not form precipitates and
are not noticeably dissolved as solutes typically in carbides present in steel plates,
but are probably dissolved and present as solutes in the matrix. Specifically, it
is considered that a steel plate can have a longer fatigue life up to (prior to) crack
initiation by allowing these elements to be dissolved and present as solutes sufficiently
in the matrix to thereby restrain the irreversible movements of dislocations. The
inventors also found that other elements to be added, such as Mn and Cr, do not noticeably
offer the effect of restraining dislocations from forming a cell structure, but rather
lower the transformation temperature to cause the formation of lower bainite, which
has a high dislocation density; and that such other elements to be added do not so
much improve fatigue strength as compared with static strength.
[0029] In addition, the inventors prepared various steel plates under different rolling
conditions and made investigations on how the rolling conditions affect the mechanical
properties and fatigue strength of the resulting steel plates. As a result, the inventors
found that steel plates, when caused to have a higher dislocation density typically
by rapidly cooling the steel plates down to a low temperature, or by applying compression
reduction (performing rolling) at a temperature equal to or lower than the Ar
3 transformation temperature, have higher static strength such as yield stress and
tensile strength, but have not so higher fatigue strength as compared with the static
strength, and have a lower fatigue limit to tensile strength ratio.
[0030] This is probably because moving dislocations introduced upon steel plate production
serve as a dislocation hardening factor and contribute to higher static strength,
but cause cyclic softening during repeated deformation, namely, during a fatigue test,
and thereby less contribute to higher fatigue strength In particular, a steel plate
having a dislocation density ρ of greater than 2.5× 10
15 m
-1 as determined by X-ray diffractometry (XRD) undergoes significant improvement in
static strength due to dislocation hardening and tends to have a lower fatigue limit
to tensile strength ratio. The dislocation density ρ is preferably 2.0x 10
15 m
-1 or less, and more preferably 1.5× 10
15 m
-1 or less. The dislocation density ρ may be about 5.0× 10
13 m
-1 or more in terms of lower limit
[0031] Fig. 1 depicts schematic explanatory views of a steel plate according to the present
invention. Specifically, Figs. 1(a) and 1(b) are a schematic perspective view and
a schematic side view, respectively, of the steel plate according to the present invention.
Figs. 1(a) and 1(b) illustrates a rolling direction L, a transverse direction (width
direction) W, thickness direction D, a steel plate surface S1, and a section S2 in
the thickness direction D in parallel with the rolling direction L.
[0032] The inventors made investigations on various steel plates having different chemical
compositions and different microstructure morphologies to examine fatigue properties.
As a result, the inventors found that a steel plate is allowed to have excellent fatigue
properties by controlling the microstructure of a longitudinal section (namely, the
section S2 in Figs. 1(a) and 1(b)) as follows, where the longitudinal section is in
parallel with the rolling direction, and where the microstructure is in a surface
layer at a position adjacent to the steel plate surface S1, for example, in a surface
layer at a position about 1 to about 3 mm deep from the steel plate surface S1 in
the thickness direction. The surface layer herein is defined to be at a portion about
1 to about 3 mm deep from the steel plate surface, in order to evaluate a surface
layer of the steel plate itself, excluding a scale layer, because the steel plate
surface immediately after production may include the scale layer of about 0.1 to about
2 mm depth (thickness) when produced under some production conditions.
[0033] The ferrite and upper bainite are phases that are relatively resistant to introduction
of moving dislocations upon formation of the phases, as compared with other phases.
This restrains the fatigue limit to tensile strength ratio from decreasing and contributes
to longer life leading to crack initiation. To offer these advantageous effects, the
microstructure in the surface layer is controlled to include at least one of ferrite
and upper bainite in a total fraction of 80 area percent or more. The fraction of
the at least one of ferrite and upper bainite is preferably 85 area percent or more,
and more preferably 90 area percent or more. In terms of upper limit, the fraction
of the at least one of ferrite and upper bainite may be 100 area percent, but is typically
about 98 area percent or less.
[0034] The grains of ferrite and upper bainite in the surface layer, if coarsen, may often
undergo stress concentration, which causes cracks. When cracks after crack initiation
collide with grains, the cracks stop and bypass the grains, and this restrains crack
propagation. However, if the grains are coarse, cracks collide with the grains in
a relatively smaller frequency. Thus, it is considered that the fatigue crack propagation
in the surface layer is not sufficiently restrained. Based on these findings and considerations,
the effective grain size of grains is specified to be 10.0 µm or less, where the grains
herein are each defined as a region surrounded by high-angle grain boundaries with
a misorientation of 15° or more between adjacent grains of ferrite or upper bainite.
The term "effective grain size" refers to an average length of the grains in the thickness
direction.
[0035] The effective grain size of the grains of at least one of ferrite and upper bainite
is preferably 6 µm or less, and more preferably 5 µm or less. The lower limit of the
effective grain size of the grains of at least one of ferrite and upper bainite is
not limited, but is typically greater than about 2 µm.
[0036] The upper bainite phase can have a smaller size as compared with the ferrite phase,
but is attended with shear deformation upon transformation, to which moving dislocations
are readily introduced. In particular, bainitic transformation, when allowed to occur
at a low temperature, often gives a lower bainite phase containing a large amount
of moving dislocations. To restrain the lower bainite phase from forming, the bainite
transformation start temperature Bs is preferably controlled appropriately. From this
viewpoint, the bainite transformation start temperature Bs calculated according to
Expression (1) is preferably 640°C or higher, and more preferably 660°C or higher.
[0037] The remainder microstructure excluding the ferrite and the upper bainite in the surface
layer is controlled to have an average equivalent circle diameter of 3.0 µm or less.
The remainder microstructure is controlled to have an average equivalent circle diameter
of 3.0 µm or less, because the remainder microstructure, if having an average equivalent
circle diameter greater than 3.0 µm, may cause toughness and other properties to significantly
deteriorate. The average equivalent circle diameter of the remainder microstructure
is preferably 2.5 µm or less, and more preferably 2.0 µm or less in terms of upper
limit; and is preferably about 0.5 µm or more in terms of lower limit. The remainder
microstructure excluding the ferrite and the upper bainite in the surface layer basically
includes martensite, martensite-austenite constituent (MA), pearlite, and pseudo-pearlite.
[0038] Among them, the martensite-austenite constituent, which is formed typically in cooling
process after rolling, undergoes expansive transformation in its formation process,
introduces moving dislocations into the matrix, and causes the steel plate to have
a shorter life leading to crack initiation. To eliminate or minimize this, the proportion
of the martensite-austenite constituent in the remainder microstructure in the surface
layer is preferably controlled to be an area percent of 5% or less. The area percent
of the martensite-austenite constituent is preferably minimized, and is more preferably
3% or less, furthermore preferably 1% or less, and most preferably 0%.
[0039] Next, such a chemical composition of the steel plate according to the present invention
as to have better fatigue properties will be described. The steel plate according
to the present invention, as being incorporated with C, Mn, Nb, and other alloy elements
as appropriate, is allowed to surely have a fine ferrite phase and/or an upper bainite
phase. Simultaneously, the steel plate, as being controlled in elements to be added
such as Si and Cu as appropriate, restrains dislocations from forming a cell structure,
which causes fatigue crack initiation. Thus, the steel plate can have excellent fatigue
properties. From these viewpoints, the elements are controlled in the following manner.
C: 0.02 to 0.10 mass percent
Carbon (C) is important to allow the steel plate to have strength at certain level
[0040] To this end, the carbon content is specified to be 0.02 mass percent or more. The
carbon content is preferably 0.03 mass percent or more, and more preferably 0.04 mass
percent or more. In contrast, the steel plate, if containing carbon in an excessively
high content, may have excessively high strength to fail to have desired tensile strength.
In addition, this steel plate, when undergoing accelerated cooling, may have excessive
hardenability, have a large dislocation density ρ, and offer lower fatigue properties.
To eliminate or minimize these, the carbon content is controlled to be 0.10 mass percent
or less, and is preferably 0.08 mass percent or less, and more preferably 0.06 mass
percent or less.
Mn: 1.0 to 2.0 mass percent
[0041] Manganese (Mn) has a significance to ensure hardenability so as to give a fine microstructure.
To effectively offer this activity, the Mn content is specified to be 1.0 mass percent
or more. The Mn content is preferably 1.2 mass percent or more, and more preferably
1.4 mass percent or more. However, the steel plate, if containing Mn in an excessively
high content, may have excessive hardenability, have a higher dislocation density
p, and fail to have sufficient fatigue properties. To eliminate or minimize these,
the Mn content is controlled to be 2.0 mass percent or less, and is preferably 1.8
mass percent or less, and more preferably 1.6 mass percent or less.
Nb: greater than 0 mass percent to 0.05 mass percent
[0042] Niobium (Nb) is effective for better hardenability and for a finer microstructure.
To effectively offer these activities, the Nb content is preferably controlled to
be 0.01 mass percent or more, and more preferably 0.02 mass percent or more. However,
the steel plate, if containing Nb in an excessively high content, may have excessive
hardenability and fail to have desired fatigue properties. To eliminate or minimize
this, the Nb content is controlled to be 0.05 mass percent or less, and is preferably
0.04 mass percent or less, and more preferably 0.03 mass percent or less.
Ti: greater than 0 mass percent to 0.05 mass percent
[0043] Titanium (Ti) effectively contributes to better hardenability and, simultaneously,
forms TiN to allow the heat-affected zone upon welding to have a finer microstructure
and to restrain reduction in toughness. To offer these activities, the Ti content
is preferably 0.01 mass percent or more, and more preferably 0.02 mass percent or
more. However, Ti, if contained in an excessively high content, may form coarse TiN
particles and may cause the steel plate to have properties such as toughness at lower
levels. To eliminate or minimize this, the Ti content is controlled to be 0.05 mass
percent or less, and is preferably 0.04 mass percent or less, and more preferably
0.03 mass percent or less.
Al: 0.01 to 0.06 mass percent
[0044] Aluminum (Al) is useful for deoxidation and, if contained in a content less than
0.01 mass percent, may fail to offer effective deoxidation. The Al content is preferably
0.02 mass percent or more, and more preferably 0.03 mass percent or more. However,
the steel plate, if containing Al in an excessively high content, may have excessive
hardenability, have a higher dislocation density p, and fail to offer desired fatigue
properties. To eliminate or minimize these, the Al content is controlled to be 0.06
mass percent or less, and is preferably 0.05 mass percent or less, and more preferably
0.04 mass percent or less.
At least one of Si in a content of 0.1 to 0.6 mass percent and Cu in a content of
0.1 to 0.6 mass percent
[0045] Silicon (Si) contributes to solid-solution strengthening to a large extent and is
necessary for ensuring the strength of the base metal. Simultaneously, this element
restrains the movements of dislocations and restrains the cell structure formation.
To effectively offer these activities, the Si content is specified to be 0.1 mass
percent or more. The Si content is preferably 0.2 mass percent or more, and more preferably
0.3 mass percent or more. However, the steel plate, if containing Si in an excessively
high content, may include the remainder microstructure formed in excess and coarsely
and may suffer from reduction in other properties such as toughness. To eliminate
or minimize this, the Si content is controlled to be 0.6 mass percent or less, and
is preferably 0.55 mass percent or less, and more preferably 0.5 mass percent or less.
[0046] Copper (Cu) restrains the cross slip of dislocations and effectively restrains the
cell structure formation. To effectively offer the activities, the Cu content is specified
to be 0.1 mass percent or more. The Cu content is preferably 0.2 mass percent or more,
and more preferably 0.3 mass percent or more. However, the steel plate, if containing
Cu in an excessively high content, may not only have excessive hardenability, but
also become susceptible typically to cracking upon hot working. To eliminate or minimize
these, the Cu content is controlled to be 0.6 mass percent or less, and is preferably
0.55 mass percent or less, and more preferably 0.5 mass percent or less.
[0047] Si and Cu can offer the common activity of restraining cell structure formation
of dislocations. From this viewpoint, the steel plate may contain each of these elements
alone or in combination. The effect of restraining cell structure formation of dislocations
by Si and Cu is effectively offered when the total content of Si and Cu ([Si] + [Cu])
is 0.3 mass percent or more. The total content is preferably 0.4 mass percent or more.
A preferred upper limit of the total content ([Si] + [Cu]) is the total of the preferred
upper limits of the two elements.
[0048] The steel plate according to the present invention includes the elements as mentioned
above as a basic composition, with the remainder consisting of approximately iron.
However, it is naturally accepted that inevitable impurities such as P, S, and N are
contained in the steel The inevitable impurities are brought into the steel in circumstances
of raw materials, facility materials, and production equipment. It is also effective
that the steel plate according to the present invention positively contains one or
more of elements below. The steel plate can have a still better property or properties
according to the type(s) of element(s) to be contained.
Ni: greater than 0 mass percent to 0.6 mass percent
[0049] Nickel (Ni) effectively contributes to better hardenability and contributes to a
finer microstructure. Simultaneously, this element effectively restrains cracking
upon hot working, where the cracking may more readily occur by the addition of Cu.
To effectively offer these effects, Ni is preferably contained in a content of 0.1
mass percent or more, and more preferably 0.2 mass percent or more. However, the steel
plate, if containing Ni in an excessively high content, may have excessive hardenability,
have an excessively high dislocation density p, and thereby fail to have desired fatigue
properties. To eliminate or minimize these, the Ni content is preferably controlled
to be 0.6 mass percent or less, more preferably 0.5 mass percent or less, and furthermore
preferably 0.4 mass percent or less.
[0050] The steel plate, if having an excessively high Ni content [Ni] with respect to the
Cu content [Cu], may hardly enjoy the effect of restraining cell structure formation
of dislocations by Cu. To eliminate or minimize this, the ratio ([Ni]/[Cu]) of the
Ni content [Ni] to the Cu content [Cu] is preferably controlled to be less than 1.2,
and more preferably 1.1 or less. The ratio ([Ni]/[Cu]) may be about 0.5 or more in
terms of lower limit.
[0051] At least one element selected from the group consisting of V in a content of greater
than 0 mass percent to 0.5 mass percent, Cr in a content of greater than 0 mass percent
to 1.0 mass percent or less, and Mo in a content of greater than 0 mass percent to
0.5 mass percent
[0052] Vanadium (V), chromium (Cr), and molybdenum (Mo) effectively allow the steel plate
to have better hardenability and to have a finer microstructure. To offer these activities,
the steel plate preferably contains each of V in a content of 0.01 mass percent or
more, Cr in a content of 0.1 mass percent or more, and Mo in a content of 0.01 mass
percent or more alone or in combination. However, the steel plate, if containing at
least one of these elements in an excessively high content, may have excessive hardenability,
have an excessively high dislocation density p, and fail to have desired fatigue properties.
To eliminate or minimize these, the contents of V, Cr, and Mo are preferably controlled
to be respectively 0.5 mass percent or less, 1.0 mass percent or less, and 0.5 mass
percent or less. The contents of V, Cr, and Mo are more preferably controlled to be
respectively 0.4 mass percent or less, 0.8 mass percent or less, and 0.4 mass percent
or less.
B: greater than 0 mass percent to 0.005 mass percent
[0053] Boron (B) contributes to better hardenability and, in particular, restrains a coarse
ferrite phase from forming, and thereby allows a fine upper bainite phase to form
more readily. To offer these effects, the boron content is preferably controlled to
be 0.0005 mass percent or more, and more preferably 0.001 mass percent or more. However,
the steel plate, if containing boron in an excessively high content, may have excessive
hardenability, have an excessively high dislocation density p, and fail to have desired
fatigue properties. To eliminate or minimize these, the boron content is preferably
controlled to be 0.005 mass percent or less, and more preferably 0.004 mass percent
or less.
[0054] The steel plate according to the present invention not limited, but having an excessively
smell thickness, may less offer longer crack propagation life. From this viewpoint,
the steel plate has a thickness of preferably 6 mm or more, and more preferably 10
mm or more.
[0055] The steel plate according to the present invention meets the conditions (requirements)
and is not limited in production method. However, it is preferred to control production
conditions as mentioned below, so as to give the microstructure morphology for better
fatigue properties. The production conditions are conditions in a series of production
process for the steel plate using a slab, such as a slab, having a chemical composition
within the ranges. In the production process, a steel is made via ingot making and
casting, and is subjected to hot rolling. Specifically, the production conditions
include the heating temperature before hot rolling; the cumulative compression reduction
in the entire hot rolling process; the finish-rolling temperature; the average cooling
rate from the finish-rolling temperature or 800°C, whichever is lower, down to 600°C;
and the cooling stop temperature.
[0056]
Temperature of heating before hot rolling: 1000°C to 1200°C
Cumulative compression reduction in entire hot rolling process: 70% or more
Finish-rolling temperature: from the Aratransformation temperature to (the Ar3 transformation temperature + 150°C)
Average cooling rate from the finish-rolling temperature or 800°C, whichever is lower,
down to 600°C: 15°C/second or less
Cooling stop temperature: 500°C or higher
[0057] Before the hot rolling, the slab is preferably heated up to a temperature range of
1000°C to 1200°C, and more preferably up to 1050°C or higher. The heating is preferably
performed up to a temperature range of 1000°C or higher so as to eliminate or minimize
coarsening of grains and to still ensure a cumulative compression reduction in hot
rolling of 70% or more, as mentioned below. However, the heating, if performed up
to an excessively high temperature of higher than 1200°C, may fail to contribute to
refinement (size reduction) of the microstructure, even when sufficient compression
reduction is applied. To eliminate or minimize this, the heating temperature is preferably
controlled to be 1200°C or lower, and more preferably 1150°C or lower.
[0058] The cumulative compression reduction in the entire hot rolling process is preferably
70% or more, and more preferably 75% or more. To reduce the microstructure size, in
particular to reduce the effective grain size, sufficient compression reduction is
to be applied in the non-recrystallization temperature range.
[0059] In addition, the finish-rolling temperature is preferably controlled within the range
of the Ar
3 transformation temperature to the (Ar
3 transformation temperature + 150°C) so as to ensure desired fine microstructure and
to still restrain excessive dislocations from being introduced into the microstructure
after rolling (as-rolled microstructure). The finish-rolling temperature is more preferably
controlled within the range of (the Ar
3 transformation temperature + 20°C) to (the Ar
3 transformation temperature + 100°C).
[0060] The "cumulative compression reduction" is a value calculated according to Expression
(4):

where to represents the slab rolling start thickness (in millimeter (mm)) when the
temperature at a position 3 mm deep from the surface falls within the rolling temperature
range; t
1 represents the rolling finish thickness of the slab (in mm) when the temperature
at a position 3 mm deep from the surface falls within the rolling temperature range;
and t
2 represents the thickness of the slab, such as a slab, before the rolling.
[0061] The Ar
3 transformation temperature employed herein is a value determined according to Expression
(5):

where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], and [Nb] represent contents (in mass
percent) respectively of C, Si, Mn, Cu, Ni, Cr, Mo, and Nb.
[0062] After hot rolling finish, cooling is preferably performed at an average cooling rate
of 15°C/second or less from the finish-rolling temperature or 800°C, whichever is
lower, down to 600°C or lower. The cooling, if performed at an average cooling rate
greater than 15°C/second, may cause the microstructure transformation to complete
at an approximately low temperature unless a process such as isothermal holding is
performed. This causes excessive introduction of dislocations and fails to give desired
fatigue properties. The average cooling rate is more preferably 10°C/second or less.
[0063] The cooling at the average cooling rate may be stopped at a temperature (namely,
cooling stop temperature) of 500°C or higher. This restrains coarse ferrite phase
formation and ensures a fine ferrite or upper bainite phase. The cooling, if stopped
at a temperature lower than 500°C, may cause the transformation to complete at a low
temperature, cause excessive dislocation to be introduces, and fail to give desired
fatigue properties.
[0064] The temperature range within which cooling is performed at the average cooling rate
is from 800°C down to 600°C when the finish-rolling temperature is higher than 800°C;
and is from the finish-rolling temperature down to 600°C when the finish-rolling temperature
is lower than 800°C. The average cooling rate in terms of lower limit is preferably
3.0°C/second or more, from the viewpoint of microstructure control in the steel plate,
as mentioned below.
Second Embodiment
[0065] Next, the second embodiment according to the present invention will be illustrated.
Steel plates for use in large structures also require lower crack propagation rate,
namely, better crack propagation properties (better crack propagation resistance).
This is because, when the crack propagation rate is low, even in case of fatigue cracking
generation, the damaged portion can be found and repaired before the crack leads to
nature.
[0066] The inventors performed crack propagation tests and microstructure observations on
various steel plates. As a result, the inventors found that control of a microstructure
morphology at a specific position, in addition to the microstructure control in the
first embodiment, allows a steel plate to have not only excellent fatigue properties,
but also excellent crack propagation properties. The microstructure to be controlled
herein is a microstructure at a position at a depth of one-fourth the thickness t
of the steel plate from the surface along the thickness direction and in a longitudinal
section in parallel with the rolling direction, as illustrated in Fig. 1(b). The position
at a depth of one-fourth the thickness t is selected herein for evaluations at an
average position in the interior of the steel plate in the thickness direction. The
longitudinal section that is in parallel with the rolling direction and at the position
at a depth of one-fourth the thickness t is basically a region on a line, but the
actual microstructure observation is performed in a region with a certain spread around
the position (see after-mentioned experimental examples).
[0067] Specifically, the microstructure at the position at a depth of one-fourth the thickness
t of the steel plate is preferably controlled to include upper bainite in a fraction
of 80 area percent or more, to have an effective grains size of grains of the upper
bainite of 10.0 µm or less, and to have an average equivalent circle diameter of the
remainder microstructure excluding the upper bainite of 3.0 µm or less. To mainly
include upper bainite in the microstructure in the interior thereof, the steel plate
has to contain boron (B).
[0068] The upper bainite phase is a phase that allows fine grain boundaries to be uniformly
dispersed in the microstructure. This can restrain crack propagation. To offer this
effect, the microstructure at the position at a depth of one-fourth the thickness
t of the steel plate preferably includes upper bainite in a fraction of 80 area percent
or more. The upper bainite fraction in the interior of the steel plate is more preferably
85 area percent or more, and furthermore preferably 90 area percent or more. The upper
bainite fraction in the interior of the steel plate in terms of upper limit may be
100 area percent, but is typically about 98 area percent or less.
[0069] The grain size of upper bainite at the position at a depth of one-fourth the thickness
t of the steel plate affects fatigue crack propagation properties and, if the steel
plate has a larger grain size of upper bainite (includes coarse upper bainite grains),
may fail to sufficiently restrain crack propagation. To eliminate or minimize this,
grains preferably have an average length in the thickness direction, namely, an effective
grain size of 10.0 µm or less, where the grains are each defined as a region surrounded
by high-angle grain boundaries having a misorientation between adjacent upper bainite
grains of 15° or more. The lower limit of the grain size is not specified, because
the smaller the grain size is, the better crack propagation properties are. The effective
grain size is more preferably 8 µm or less, and furthermore preferably 7 µm or less.
[0070] The remainder microstructure excluding the upper bainite is preferably controlled
to have an average equivalent circle diameter of 3.0 µm or less. This is because the
remainder microstructure, if having an average equivalent circle diameter greater
than 3.0 µm, may cause the steel plate to suffer from significant reduction in other
properties such as toughness. The remainder microstructure basically includes martensite
and MA, as with the surface layer. These hard phases as the remainder microstructure
can contribute to lower crack propagation rate. The average equivalent circle diameter
in terms of lower limit is about 0.5 µm or more.
[0071] In addition, the inventors focused attention on kernel average misorientation (KAM)
and grain average misorientation (GAM) in grains at the position at a depth of one-fourth
the thickness t of the steel plate. The KAM is a misorientation between each measurement
point and an adjacent point in one grain. The GAM is an average of KAMs in the one
grain.
[0072] Figs. 4(a), 4(b), and 4(c) are conceptual diagrams respectively of grain boundaries,
KAM, and GAM. The hexagons in Figs. 4(a), 4(b), and 4(c) represent EBSP measurement
points. The periphery of a region indicated with a thick line in Fig. 4(a) is a high-angle
grain boundary having a misorientation of 15° or more, and a region surrounded by
the periphery is defined as a "grain".
[0073] The KAM is an average of misorientations in the one grain. Fig. 4(b) schematically
illustrates how to determine the KAM. For example, the measurement point 1 is in contact
with three measurement points in one grain, i.e., n=3. The KAM as the average of misorientations
(numerical values in squares) between the measurement points is calculated as 0.5.
The KAM may be calculated acoording to the expression:

[0074] The GAM is an average of KAMs in one grain. Fig. 4(c) schematically illustrates hot
to determine the GAM. In an example in Fig. 4(c), there are nine measurement points,
i.e., m = 9, and the GAM as an average of KAMs in one grain is calculated as 0.64.
The GAM may be calculated according to the Expression

[0075] The crack propagation may be restrained when both grains each having a large misorientation
in the drain, and grains each having a small misorientation in the grain are appropriately
dispersed in the microstructure. To effectively restrain the crack propagation, grains
having a GAM greater than 1° are preferably present in a proportion of 20% or more,
more preferably 30% or more, and furthermore preferably 40% or more, of the entire
microstructure. In contrast, grains having a large grain misorientation, if present
in an excessively large proportion, may weaken the effect of restraining the crack
propagation by the presence of grains having different misorientations as a mixture.
To eliminate or minimize this, the proportion in area percent in terms of upper limit
is preferably 80% or less, more preferably 70% or less, and furthermore preferably
60% or less.
[0076] The control of the proportion of grains having a GAM greater than 1° in the interior
of the steel plate within the range specified in the present invention may be performed
typically, but not limitatively, by allowing the upper bainite to be a mixed microstructure
of bainitic ferrite and granular bainitic ferrite, where the bainitic ferrite has
a large misorientation, and the granular bainitic ferrite has a small misorientation.
[0077] To give such a mixed microstructure of bainitic ferrite and granular bainitic ferrite,
it is preferred to control the steel plate to meet conditions specified by Expression
(2) and Expression (3), in addition to allow the steel plate to have a chemical composition
within the range specified in the present invention and to control the production
method of the steel plate in a manner mentioned later. Expressions (2) and (3) are
expressed as follows:

where [Si], [Mn], [Ni], [Cu], [Ti], [N], [Cr], [Nb], [C], and [Mo] represent contents
(in mass percent) respectively of Si, Mn, Ni, Cu, Ti, N, Cr, Nb, C, and Mo, and, when
at least one of the term {[Ti] - 3.4[N]} and the term {[Nb] - 7.7[C]} is negative,
the calculation according to the expression is performed as treating the at least
one term as "zero (0)".
[0078] The elements indicated in Expression (2) are elements having low carbide formation
ability. With an increasing left-side value of Expression (2), all the transformation
curves of ferrite, bainitic ferrite, and granular bainitic ferrite shift to a longer
time side. Specifically, bainitic ferrite and granular bainitic ferrite are more easily
form with an increasing left-side value of Expression (2), assuming that the cooling
rates are identical.
[0079] In contrast, the elements indicated in Expression (3) are elements having high carbide
forming ability. With an increasing left-side value of Expression (3), only the transformation
curves of ferrite and granular bainitic ferrite shift toward a longer time side, but
the transformation curve of bainitic ferrite changes (shifts) little. Specifically,
with an increasing left-side value of Expression (3), bainitic ferrite, which has
a large misorientation, is more readily formed, as compared with granular bainitic
ferrite.
[0080] The control of the left-side values of Expression (2) and Expression (3) allows the
microstructure to be a mixed microstructure of bainitic ferrite and granular bainitic
ferrite and to include these phases in appropriate proportions.
[0081] The left-side value of Expression (2) and the left-side value of Expression (3)
may be adjusted as appropriate in consideration of the proportion of grains having
a GAM greater than 1°, are not limited, but are preferably 40 or more and 2 or less,
respectively. The left-side value of Expression (2) is more preferably 45 or more,
and furthermore preferably 50 or more. The left-side value of Expression (3) is more
preferably 1.5 or less, and furthermore preferably 1.0 or less. The upper limits of
the left-side values of Expression (2) and Expression (3) are inevitably determined
by the ranges of contents of the elements.
[0082] To ensure the microstructure morphology as mentioned above, the steel plate is controlled
to meet the conditions for the control of the microstructure morphology in the surface
layer, and, in addition, the cumulative compression reduction and the compression
reduction in a non-recrystallization temperature range upon hot rolling are preferably
controlled as follows.
[0083] Cumulative compression reduction in the entire hot rolling process: 80% or more Compression
reduction in the non-recrystallization temperature range: less than 85%
[0084] Average cooling rate from the finish-rolling temperature or 800°C, whichever is lower,
down to 600°C or lower: 3.0°C/second or more
[0085] To control the upper bainite in the interior of the steel plate to have an effective
grain size of 10.0 µm or less, the cumulative compression reduction during the hot
rolling process may be increased and is preferably 80% or more. The hot rolling, if
performed at an insufficient cumulative compression reduction, may allow the surface
layer microstructure to be fine, but may fail to allow the microstructure in the interior
of the steel plate to be sufficiently fine and fail to sufficiently reduce the crack
propagation rate. The cumulative compression reduction is more preferably 85% or more.
[0086] To surely give an upper bainite fraction of 80 area percent or more at the position
at a depth of one-fourth the thickness t, the cooling after the hot rolling is preferably
performed at an average cooling rate of 3.0°C/second or more, and more preferably
5°C/second or more, where the cooling is from the finish-rolling temperature or 800°C,
whichever is lower, down to 600°C or lower.
[0087] The hot rolling, if performed at an excessively high compression reduction in the
non-recrystallization temperature range, may cause ferrite nucleation sites to increase
in number, cause ferritic transformation to readily occur, and cause the upper bainite
fraction to decrease. This fail to give sufficient effects of lowering the crack propagation
rate. Accordingly, excessive compression reduction in the non-recrystallization temperature
range is to be avoided so as to surely give an upper bainite fraction in the interior
of the steel plate of 80 area percent or more. From this viewpoint, the cumulative
compression reduction in the non-recrystallization temperature range may be controlled
to be preferably less than 85%, and more preferably 80% or less.
[0089] The present invention will be illustrated in further detail with preference to several
examples (experimental examples) below. It should be noted, however, that the examples
are by no means intended to limit the scope of the invention; that various changes
and modifications can naturally be made therein without deviating from the spirit
and scope of the invention as described herein; and all such changes and modifications
should be considered to be within the scope of the invention.
Experimental Example 1
[0090] Ingots of steels having chemical compositions corresponding to Steels A to W as given
in Table 1 were made via melting and casting according to a common ingot-making technique,
subjected to hot rolling under conditions of rolling condition types "a" to "l" given
in Table 2, and yielded steel plates having a thickness of 20 mm. In Table 1, an element
indicated with "-" was not added; and the symbol "[Si]+[Cu]" refers to the total content
of Si and Cu. The Ar
3 transformation temperatures given in Table 1 are values determined according to Expression
(5). In Table 2, the term "entire hot rolling process cumulative compression reduction"
refers to the cumulative compression reduction in the entire hot rolling process.
[Table 1]
Steel |
Chemical composition (in mass percent) with the remainder consisting of iron and inevitable
impurities |
Bs |
[Si]+[Cu] |
[Ni]/[Cu] |
Ar3 transformation temperature |
Left-side value of Expression (2) |
Left-side value of Expression (3) |
C |
Si |
Mn |
Al |
Cu |
Ni |
Cr |
Mn |
V |
Nb |
Ti |
B |
N |
A |
0.05 |
0.31 |
1.56 |
0.03 |
0.31 |
0.30 |
- |
- |
- |
0.019 |
0.015 |
0.0012 |
0.0044 |
665 |
0.62 |
0.97 |
738 |
49 |
0.00 |
B |
0.05 |
0.53 |
1.52 |
0.04 |
- |
0.33 |
- |
- |
- |
0.020 |
0.015 |
0.0008 |
0.0043 |
667 |
0.53 |
- |
782 |
52 |
0.01 |
C |
0.06 |
0.25 |
1.59 |
0.03 |
0.33 |
0.16 |
- |
- |
- |
0.020 |
0.015 |
0.0009 |
0.0040 |
665 |
0.58 |
0.48 |
731 |
45 |
0.03 |
D |
0.05 |
- |
1.55 |
0.03 |
0.54 |
0.51 |
- |
- |
- |
0.019 |
0.013 |
0.0010 |
0.0052 |
658 |
0.54 |
0.94 |
715 |
45 |
0.00 |
E |
0.05 |
0.38 |
1.81 |
0.04 |
0.24 |
0.20 |
- |
- |
- |
0.018 |
0.021 |
0.0009 |
0.0048 |
646 |
0.62 |
0.83 |
728 |
53 |
0.10 |
F |
0.04 |
0.41 |
1.61 |
0.03 |
0.32 |
- |
- |
- |
- |
0.020 |
0.016 |
0.0009 |
0.0042 |
674 |
0.73 |
0.00 |
742 |
48 |
0.04 |
G |
0.07 |
0.44 |
1.55 |
0.03 |
0.33 |
0.29 |
- |
- |
- |
0.025 |
0.016 |
- |
0.0043 |
661 |
0.77 |
0.88 |
727 |
54 |
0.03 |
H |
0.04 |
0.42 |
1.55 |
0.03 |
0.32 |
0.32 |
0.2 |
- |
- |
0.019 |
0.018 |
0.0015 |
0.0047 |
654 |
0.74 |
1.00 |
741 |
53 |
3.84 |
I |
0.04 |
0.41 |
1.57 |
0.03 |
0.32 |
0.31 |
- |
0.02 |
- |
0.023 |
0.019 |
0.0009 |
0.0047 |
665 |
0.73 |
0.97 |
735 |
53 |
0.26 |
J |
0.05 |
0.39 |
1.52 |
0.03 |
0.35 |
0.30 |
- |
- |
0.02 |
0.023 |
0.017 |
0.0008 |
0.0042 |
669 |
0.74 |
0.86 |
734 |
52 |
0.06 |
K |
0.01 |
0.41 |
1.55 |
0.03 |
0.31 |
0.25 |
- |
- |
- |
0.019 |
0.016 |
0.0009 |
0.0043 |
679 |
0.72 |
0.81 |
761 |
51 |
0.03 |
L |
0.15 |
0.42 |
1.55 |
0.03 |
0.32 |
0.18 |
- |
- |
- |
0.020 |
0.016 |
0.0007 |
0.0043 |
643 |
0.74 |
0.56 |
718 |
51 |
0.03 |
M |
0.05 |
0.10 |
1.61 |
0.03 |
- |
- |
- |
- |
- |
0.021 |
0.018 |
0.0013 |
0.0044 |
672 |
0.10 |
- |
748 |
32 |
0.06 |
N |
0.05 |
1.00 |
1.55 |
0.03 |
0.31 |
0.23 |
- |
- |
- |
0.020 |
0.017 |
0.0014 |
0.0042 |
668 |
1.31 |
0.74 |
755 |
72 |
0.06 |
O |
0.03 |
0.34 |
2.12 |
0.03 |
0.29 |
0.18 |
- |
- |
- |
0.022 |
0.017 |
0.0013 |
0.0049 |
624 |
0.63 |
0.62 |
700 |
58 |
0.01 |
P |
0.05 |
0.35 |
0.55 |
0.03 |
0.33 |
0.32 |
- |
- |
- |
0.020 |
0.019 |
0.0013 |
0.0052 |
755 |
0.68 |
0.97 |
811 |
33 |
0.03 |
O |
0.05 |
0.52 |
1.57 |
0.03 |
1.50 |
- |
- |
- |
- |
0.018 |
0.016 |
0.0012 |
0.0045 |
675 |
2.02 |
0.00 |
682 |
70 |
0.01 |
R |
0.03 |
0.21 |
1.51 |
0.03 |
0.25 |
1.20 |
- |
- |
- |
0.019 |
0.019 |
0.0010 |
0.0051 |
642 |
0.46 |
4.80 |
733 |
59 |
0.03 |
S |
0.06 |
0.41 |
1.56 |
0.03 |
0.30 |
0.15 |
1.5 |
- |
- |
0.020 |
0.020 |
0.0008 |
0.0045 |
563 |
0.71 |
0.50 |
726 |
50 |
28.60 |
T |
0.04 |
0.42 |
1.52 |
0.03 |
0.29 |
0.28 |
- |
1.00 |
- |
0.021 |
0.018 |
0.0009 |
0.0045 |
589 |
0.71 |
0.97 |
739 |
51 |
10.06 |
U |
0.03 |
0.41 |
1.55 |
0.03 |
0.31 |
0.22 |
- |
- |
1.00 |
0.025 |
0.018 |
0.0009 |
0.0044 |
674 |
0.72 |
0.71 |
737 |
51 |
0.06 |
V |
0.08 |
0.39 |
1.82 |
0.03 |
0.31 |
0.19 |
0.8 |
0.30 |
- |
0.022 |
0.019 |
0.0008 |
0.0046 |
557 |
0.70 |
0.61 |
702 |
55 |
18.27 |
W |
0.05 |
0.15 |
1.10 |
0.03 |
0.16 |
0.05 |
- |
- |
- |
0.019 |
0.015 |
0.0012 |
0.0044 |
665 |
0.31 |
0.97 |
738 |
28 |
0.00 |
[Table 2]
Rolling condition type |
Heating temperature (°C) |
Cumulative compression reduction in entire hot rolling process (%) |
Cumulative compression reduction in non-recrystallization range (%) |
Finish-rolling temperature (°C) |
Average cooling rate down to 600°C (°C/sec) |
Cooling stop temperature (°C) |
a |
1150 |
90 |
75 |
820 |
5 |
583 |
b |
1150 |
85 |
75 |
800 |
12 |
580 |
c |
1100 |
80 |
75 |
820 |
AC* |
570 |
d |
1175 |
80 |
75 |
870 |
3.1 |
600 |
e |
1150 |
75 |
75 |
750 |
8.1 |
580 |
f |
1150 |
80 |
75 |
820 |
5.1 |
520 |
g |
1250 |
90 |
75 |
820 |
5 |
580 |
h |
1150 |
50 |
75 |
820 |
5 |
580 |
i |
1150 |
90 |
75 |
700 |
5 |
580 |
j |
1150 |
90 |
75 |
820 |
30 |
580 |
k |
1150 |
90 |
75 |
800 |
5 |
150 |
l |
1150 |
90 |
95 |
820 |
5 |
580 |
* AC: air cooling at an average cooling rate of 0.8°C/sec |
[0091] The steel plates were each subjected to measurements of the microstructure and effective
grain size of the steel plate, the size of the remainder microstructure as a second
phase, the tensile strength, the fatigue properties, and the dislocation density ρ,
according to procedures as follows. Test specimens in all the measurements were sampled
so that the measurement position be a position 3 mm deep from the steel plate surface.
Steel Plate Surface Layer Microstructure
[0092] A sample was cut out at a position 3 mm deep from the steel plate surface so as to
expose a plane in parallel with the rolling direction of the steel plate and in perpendicular
to the steel plate surface. This was polished using wet emery papers of #150 to #1000
and was then polished to a mirror-smooth state using a diamond abrasive as an abrasive.
The mirror-smooth test specimen was etched with 2% nitric acid-ethanol solution, i.e.,
Nital solution, the etched test specimen was observed in three view fields in an observation
area of 3.71× 10
-2 mm
2 at 400-fold magnification, images of which were taken and analyzed using an image
analyzing software Image Pro Plus ver. 4.0 supplied by Media Cybernetics so as to
fractionate phases in the microstructure. Values in the three view fields were averaged,
and the average was defined as the area percentages of the individual phases. The
observation area was such that one view field in a size of 166 µm in the thickness
direction and 222.74 µm in the rolling direction was defined around the position 3
mm deep from the steel plate surface.
Ferrite and Upper Bainite Affective Grain Sizes in Surface Layer
[0093] The effective grain size of ferrite and/or upper bainite was analyzed at a position
3 mm deep from the steel plate surface in a longitudinal section in parallel with
the rolling direction of the steel plate. The measurement was performed by scanning
electron microscope (SEM)-electron backscatter pattern analysis (EBSP). Specifically,
a grain size was measured, where the "grain" is defined as a region surrounded by
a high-angle grain boundary having a misorientation between adjacent grains of 15°
or more. The measurement was performed using an EBSP system (trade name OIM) supplied
by TEX SEM Laboratories in combination with a SEM, in a measurement area of 200 µm
by 200 µm at a measurement step (interval) of 0.5 µm. A measurement point having a
confidence index of less than 0.1 was excluded from the analysis object, where the
confidence index indicates the reliability of a measurement orientation. The cut lengths
of the grain boundaries thus determined were measured at 100 points in the thickness
direction, and an average of the cut lengths was defined as the effective grain size.
However, a measurement with an effective grain size of 2.0 µm or less was determined
as a measurement noise and excluded. The observation area was determined as a region
around the position 3 mm deep from the steel plate surface with a spread of 100 µm
on both sides in the thickness direction.
Remainder Microstructure Size
[0094] The size of the remainder microstructure excluding the ferrite and the upper bainite
was determined in the following manner. A sample was cut out at a position 3 mm deep
from the steel plate surface so as to expose a plane in parallel with the rolling
direction of the steel plate and in perpendicular to the steel plate surface. This
was polished using wet emery papers of #150 to #1000 and was then polished to a mirror-smooth
state using a diamond abrasive as an abrasive. The mirror-smooth test specimen was
etched with 2% nitric acid-ethanol solution, i.e., Nital solution, the etched test
specimen was observed in an observation area of 3.71× 10
-2 mm
2 at 400-fold magnification. The observation area was determined as a region around
the position 3 mm deep from the steel plate surface with a spread of 100 µm on both
sides in the thickness direction. Images of the observed test specimen were taken
and analyzed using the image analyzing software, an area per one grain of the remainder
microstructure was calculated, and the equivalent circle diameter of grains of the
remainder microstructure was determined from the calculated area. In this experimental
example, measurements in three view fields were averaged, and the average was defined
as the equivalent circle diameter.
[0095] Of the remainder microstructure, the area percentage of MA was determined in the
following manner. The mirror-smooth test specimen after polishing to a mirrorsmooth
state was etched with a LePera etchant and observed in an observation area of 3.71×10
-2 mm
2 at 400-fold magnification A phase corroded to white was defined as the MA, images
of which were taken and analyzed using the image analyzing software to fractionate
phases. Measurements in five view fields were averaged, and the average was defined
as the area percentage of MA. The LePera etchant was a 5:6:1 mixture of a solution
A, a solution B, and ethanol The solution A was a solution of 3 g of picric acid in
100 ml of ethanol. The solution B was a solution of 1 g of sodium disulfite in 100
ml of distilled water.
Tensile Strength
[0096] The tensile strength TS was measured by sampling a tensile test specimen having a
thickness of 4 mm and a gauge length of 35 mm from each steel plate at a position
2 to 6 mm deep from the steel plate surface, and subjecting the test specimen to a
tensile test according to JIS Z 2241:2011.
Fatigue Properties
[0097] The fatigue properties were determined in the following manner. A steel plate sample
having a thickness of 4 mm was cut out from each steel plate at a position 2 to 6
mm deep from the steel plate surface, from which a test specimen as illustrated in
Fig. 2 was prepared. The test specimen surface was polished with emery papers up to
#1200 to eliminate or minimize influence of surface conditions. The resulting test
specimen was subjected to a fatigue test using a servo-electric hydraulic fatigue
tester supplied by INSTRON Co., Ltd. under conditions as follows.
Testing environment: |
room temperature, in the air |
Control type: |
load control |
Control waveform: |
sinusoidal wave |
Stress ratio R: |
-1 |
Testing frequency: |
20 Hz |
Number of cycles to complete testing: |
5000000 |
[0098] The fatigue properties are affected by the tensile strength TS. To eliminate or minimize
the influence of the tensile strength, a 5000000-cycle fatigue limit to tensile strength
ratio was determined, and a sample, when having a 5000000-cycle fatigue limit to tensile
strength ratio of greater than 0.51, was accepted herein. The 5000000-cycle fatigue
limit to tensile strength ratio is a value determined by dividing a 5000000-cycle
fatigue limit by the tensile strength TS. The 5000000-cycle fatigue limit was determined
in the following manner. Each test specimen was subjected to a fatigue test at such
a stress amplitude as to give a stress amplitude σa to tensile strength TS ratio (σa/TS)
of 0.51, and whether the test specimen underwent rupture upon the 5000000th cycles
was examined.
Dislocation Density ρ
[0099] The dislocation density ρ was determined by subjecting each sample to X-ray diffractometry
to determine a half peak width (full-width at half maximum) of α-Fe, and calculating
the dislocation density from the half peak width. The measurement conditions and principle
will be illustrated below. An analyzer used herein was an X-ray diffractometer RAD-RU300
(trade name, supplied by Rigaku Corporation), with a cobalt tube as a target. The
half peak width was calculated from the results of X-ray diffractometry via peak fitting,
based on which the dislocation density ρ was calculated according to Expression (6):

where ε represents the strain; and b represents the Burgers vector (= 0.25×10
-9 m).
[0100] The strain ε is a value calculated according to the HaII method based on Expression
(7) and Expression (8):

where β represents the true half peak width (in radian (rad)); θ represents the Bragg
angle (in degree (°)); λ represents the wavelength (in nanometer (nm)) of incident
X ray; D represents the crystal size (in nm); β
m represents the measured half peak width; and β
s represents the half peak width (apparatus constant) of a sample with no strain. The
true half peak width β was calculated from β
m and β
s according to Expression (8), and this was substituted into Expression (7), based
on which βcos θ/λ - sin θ/λ was plotted. Three points, i.e., (110), (211), and (220)
points were fitted by the method of least squares. The strain ε was calculated from
the slope (2ε) of the fitting line and was substituted into Expression (6) to calculate
the dislocation density p.
[0101] Table 3 presents, of each steel plate, the microstructure, the effective grain size,
the remainder microstructure size, the tensile strength TS, the fatigue properties,
and the dislocation density p.
[Table 3]
Test number |
Steel |
Rolling condition type |
Tensile strength TS (MPa) |
Microstructure* of steel plate surface layer |
Total of ferrite and upper bainite in steel plate surface layer (area percent) |
Effective grain size (µm) |
Remainder microstructure size (µm) |
MA fraction in steel plate surface layer (area percent) |
ρ [× 101.m-1] |
Fatigue properties |
1 |
A |
a |
581 |
F+B |
95 |
5.5 |
1.1 |
1.1 |
1.2 |
no rupture |
2 |
B |
a |
597 |
F+B |
90 |
7.2 |
1.5 |
1.3 |
1.0 |
no rupture |
3 |
C |
a |
529 |
F+B |
94 |
4.9 |
0.8 |
0.7 |
1.3 |
no rupture |
4 |
D |
a |
630 |
F+B |
93 |
4.6 |
0.9 |
0.1 |
1.7 |
no rupture |
5 |
E |
a |
559 |
F+B |
92 |
6.2 |
1.0 |
0.5 |
0.9 |
no rupture |
6 |
F |
a |
540 |
F+B |
94 |
5.8 |
1.0 |
0.5 |
1.0 |
no rupture |
7 |
G |
a |
536 |
F+B |
94 |
5.7 |
1.4 |
0.7 |
0.8 |
no rupture |
8 |
H |
a |
610 |
F+B |
90 |
4.4 |
0.9 |
0.6 |
1.7 |
no rupture |
9 |
I |
a |
621 |
F+B |
92 |
4.3 |
0.8 |
0.2 |
1.6 |
no rupture |
10 |
J |
a |
617 |
F+B |
95 |
4.5 |
1.0 |
0.3 |
1.6 |
no rupture |
11 |
A |
b |
627 |
F+B |
91 |
4.3 |
0.7 |
1.1 |
1.7 |
no rupture |
12 |
A |
c |
555 |
F |
96 |
6.1 |
1.6 |
0.1 |
0.8 |
no rupture |
13 |
A |
d |
571 |
F+B |
96 |
5.6 |
0.7 |
1.8 |
1.2 |
no rupture |
14 |
A |
e |
589 |
F+B |
92 |
5.8 |
1.1 |
12 |
1.0 |
no rupture |
15 |
A |
f |
629 |
F+B |
93 |
4.5 |
0.9 |
0.4 |
1.8 |
no rupture |
16 |
A |
l |
638 |
F+B |
98 |
3.5 |
0.6 |
0.3 |
1.8 |
no rupture |
17 |
W |
b |
621 |
F+B |
90 |
4.4 |
1.2 |
0.4 |
1.2 |
no rupture |
18 |
K |
c |
410 |
F |
91 |
- |
- |
0.1 |
- |
- |
19 |
L |
a |
791 |
B |
38 |
- |
- |
0.4 |
- |
- |
20 |
M |
a |
581 |
F+B |
92 |
4.8 |
1.4 |
0.5 |
1.7 |
rupture |
21 |
N |
a |
624 |
F+B |
81 |
6.0 |
4.2 |
2.1 |
1.7 |
rupture |
22 |
O |
a |
701 |
B |
88 |
4.4 |
0.8 |
1.1 |
3.7 |
rupture |
23 |
P |
a |
462 |
F |
83 |
10.2 |
1.4 |
0.1 |
1.8 |
rupture |
24 |
Q |
a |
616 |
F+B |
83 |
6.2 |
1.0 |
1.3 |
5.0 |
rupture |
25 |
R |
a |
631 |
F+B |
87 |
5.8 |
1.1 |
1.4 |
4.8 |
rupture |
26 |
S |
a |
647 |
F+B |
85 |
4.5 |
0.8 |
1.1 |
3.8 |
rupture |
27 |
T |
a |
638 |
F+B |
84 |
4.7 |
0.8 |
1.0 |
3.7 |
rupture |
28 |
U |
a |
625 |
F+B |
85 |
5.3 |
1.1 |
1.1 |
4.4 |
rupture |
29 |
V |
a |
625 |
F+B |
84 |
5.0 |
1.0 |
0.9 |
3.8 |
rupture |
30 |
A |
g |
587 |
B |
91 |
10.2 |
0.9 |
5.3 |
1.1 |
rupture |
31 |
A |
h |
591 |
F+B |
90 |
10.3 |
0.3 |
0.2 |
1.1 |
rupture |
32 |
A |
i |
612 |
F |
83 |
5.3 |
1.3 |
0.3 |
2.9 |
rupture |
33 |
A |
j |
659 |
B |
96 |
4.9 |
0.8 |
1.5 |
3.1 |
rupture |
34 |
A |
k |
643 |
B |
95 |
5.1 |
0.7 |
0.2 |
2.7 |
rupture |
* F represents ferrite, B represents bainite, and M represents martensite, where,
when the microstructure is indicated as "B", the remainder microstructure is martensite. |
[0102] These results indicate as follows. Specifically, Test Nos. 1 to 17 were produced
under appropriately controlled conditions using steels having appropriately controlled
chemical compositions, met the conditions in the surface layer specified in the present
invention, and offered excellent fatigue properties.
[0103] In Test Nos.18 to 34 were samples failing to meet at least one of the conditions
specified in the present invention, and each had poor fatigue properties. Among them,
Test No. 18 employed a steel plate derived from Steel K having a low carbon content
and failed to have a tensile strength TS at the predetermined level. Accordingly,
other properties than the microstructure were not evaluated in this sample. Test No.
19 employed a steel plate derived from Steel L having an excessively high carbon content
and had an excessively high tensile strength TS. Accordingly, other properties than
the microstructure were not evaluated in this sample.
[0104] Test No. 20 employed a steel plate derived from Steel M not meeting the condition
that "the total content of Si and Cu is 0.3% or more", failed to restrain cell structure
formation of dislocations, and had inferior fatigue properties. Test No. 21 employed
a steel plate derived from Steel N having an excessively high Si content, had an excessively
large size of the remainder microstructure, and had inferior fatigue properties.
[0105] Test No. 22 employed a steel plate derived from Steel O having an excessively high
Mn content, had a high tensile strength TS and a high dislocation density ρ, and offered
inferior fatigue properties. Test No. 23 employed a steel plate derived from Steel
P having an excessively low Mn content, failed to have a tensile strength TS at the
predetermined level, had an excessively large effective grain size, and offered inferior
fatigue properties.
[0106] Test No. 24 employed a steel plate derived from Steel Q having an excessively high
Cu content, had an excessively high dislocation density ρ, and offered inferior fatigue
properties. Test No. 25 employed a steel plate derived from Steel R having an excessively
high Ni content and not meeting the condition: [Ni]/[Cu] < 1.2. This sample had an
excessively high dislocation density ρ, and offered inferior fatigue properties.
[0107] Test No. 26 employed a steel plate derived from Steel S having an excessively high
Cr content, had an excessively high dislocation density ρ, and offered inferior fatigue
properties. Test No. 27 employed a steel plate derived from Steel T having an excessively
high Mo content, had an excessively high dislocation density ρ, and offered inferior
fatigue properties.
[0108] Test No. 28 employed a steel plate derived from Steel U having an excessively high
V content, had an excessively high dislocation density ρ, and offered interior fatigue
properties. Test No. 29 employed a steel plate derived from Steel V having a bainite
transformation start temperature Bs lower than 640°C, had an excessively high dislocation
density ρ, and offered inferior fatigue properties.
[0109] Test No. 30 was a sample produced via rolling under conditions of the type g with
an excessively high heating temperature in hot rolling, had an excessively large effective
grain size, and offered inferior fatigue properties. Test No. 31 was a sample produced
via rolling under conditions of the type h with an excessively low cumulative compression
reduction in hot rolling, had an excessively large effective grain size, and offered
inferior fatigue properties.
[0110] Test No. 32 was a sample produced via rolling under conditions of the type i with
an excessively low finish-rolling temperature, had an excessively high dislocation
density p, and offered inferior fatigue properties. Test No. 33 was a sample produced
via rolling under conditions of the type j with an excessively high average cooling
rate down to 600°C, had an excessively high dislocation density p, and offered inferior
fatigue properties. Test No. 34 was a sample produced via rolling under conditions
of the type k with an excessively low cooling stop temperature, had an excessively
high dislocation density p, and offered inferior fatigue properties.
Experimental Example 2
[0111] The steel plates of Test Nos.1 to 17 given in Table 3 were each subjected to evaluations
of the fraction and effective grain size of upper bainite in the interior of the steel
plate, namely, at the position at a depth of one-fourth the thickness t of the steel
plate; and the size of the remainder microstructure as a second phase, according to
procedures similar to those in Experimental Example 1. Test specimens were sampled
by procedures similar to the above procedures, except for sampling them at a position
at a depth of one-fourth the thickness t of the steel plate. These steel plates were
also subjected to measurements of the portion of grains having a GAM of 1° or more,
and the crack propagation rate by methods as follows.
Measurement of Proportion of Grains Having a GAM of 1° or more
[0112] The proportion of grains each having a GAM of 1° or more at the position at a depth
of one-fourth the thickness t of the steel plate was measured by SEM-EBSP. Specifically,
the grain size was measured while defining the "grain" as a region surrounded by a
high-angle grain boundary having a misorientation between adjacent grains of 15° or
more. The measurement was performed using the EBSP equipment (trade name OIM) supplied
by TEX SEM Laboratories in combination with a SEM. The measurement was performed in
a measurement area of 200 µm by 200 µm at a measurement step (interval) of 0.5 µm.
The measurement area was a region around the position at a depth of one-fourth the
thickness t of the steel plate with a 100 µm spread on both sides in the thickness
direction. A measurement point having a confidence index CI of less than 0.1 was excluded
from the analysis object, where the confidence index indicates the reliability of
a measurement orientation. The GAM as an average of KAMs in one grain was determined,
where each KAM represents the misorientation between a measurement point and an adjacent
point. Thus, grains each having a GAM of 1° or more were identified. The term "grain"
herein refers to a grain into which high strain is introduced. The measurement was
performed in three view fields per one steel type, and an average of area fractions
of grains having a GAM of 1° or more was calculated.
Crack Propagation Rate Measurement
[0113] The crack propagation rate was measured by preparing a compact tension test specimen,
subjecting the compact tension test specimen to a fatigue crack propagation test in
accordance with American Society for Testing Materials (ASTM) standard E647 using
a servo-electric hydraulic fatigue tester under conditions as follows. The compact
tension test specimen was sampled at the position at a depth of one-fourth the thickness
t of the steel plate and had dimensions illustrated in Fig. 3. The crack length was
determined using the compliance method.
Testing environment: |
room temperature, in the air |
Control type: |
load control |
Control waveform: |
sinusoidal wave |
Stress ratio R: |
-1 |
Testing frequency: |
5 to 20 Hz |
[0114] In this test, a value in a stable growth region at ΔK of 20 (MPa m
1/2), in which Paris' law holds, was used as a representative value for evaluation, where
ΔK is specified by Expression (9). A sample having a crack propagation rate of 5.0×
10
-5 mm/cycle or less at ΔK of 20 (MPa m
1/2) was evaluated as having excellent crack propagation properties. Expression (9) is
expressed as follows:

where "a" represents the crack length (in millimeter (mm)); n represents the number
of cycles (in cycle); and C and p are constants independently determined by conditions
such as materials and load.
[0115] Results of these are presented in Table 4. In the "evaluation" in Table 4, a sample
having a crack propagation rate of 5.0× 10
-5 mm/cycle or less was evaluated as having excellent fatigue crack propagation properties
and is indicated with "A"; and a sample having a crack propagation rate of 4.0× 10
-5 mm/cycle or less was evaluated as having still further excellent fatigue crack propagation
properties and is indicated with "AA".
[Table 4]
Test number |
Steel |
Rolling condition type |
Microstructure* in the interior of steel plate |
Upper bainite fraction in the interior of steel plate (area percent) |
Upper bainite effective grain size in the interior of steel plate (µm) |
Size of remainder microstructure excluding upper bainite in the interior of steel
plate (µm) |
Fraction of grains having GAM of 1° or more (area percent) |
Crack propagation rate (× 10-5 mm/cycle) |
Evaluation |
1 |
A |
a |
F+B |
91 |
6.7 |
1.5 |
55 |
3.2 |
AA |
2 |
B |
a |
F+B |
87 |
7.7 |
2.1 |
65 |
3.4 |
AA |
3 |
C |
a |
F+B |
90 |
6.9 |
1.0 |
53 |
3.9 |
AA |
4 |
D |
a |
F+B |
88 |
6.1 |
1.1 |
55 |
3.8 |
AA |
5 |
E |
a |
F+B |
88 |
7.3 |
1.3 |
61 |
3.5 |
AA |
6 |
F |
a |
F+B |
88 |
6.4 |
1.4 |
58 |
3.0 |
AA |
7 |
G |
a |
F+B |
55 |
6.6 |
1.7 |
7 |
5.8 |
- |
8 |
H |
a |
F+B |
83 |
5.9 |
1.5 |
83 |
4.1 |
A |
9 |
I |
a |
F+B |
90 |
5.8 |
1.4 |
83 |
4.2 |
A |
10 |
J |
a |
F+B |
93 |
5.2 |
1.6 |
62 |
3.1 |
AA |
11 |
A |
b |
F+B |
87 |
5.5 |
1.3 |
54 |
3.8 |
AA |
12 |
A |
c |
F |
0 |
11.2 |
2.0 |
2 |
7.2 |
- |
13 |
A |
d |
F+B |
93 |
7.2 |
1.3 |
45 |
3.3 |
AA |
14 |
A |
e |
F+B |
82 |
10.5 |
1.4 |
66 |
5.9 |
- |
15 |
A |
f |
F+B |
87 |
5.6 |
1.3 |
70 |
3.5 |
AA |
16 |
A |
l |
F+B |
27 |
5.6 |
1.3 |
10 |
5.2 |
- |
17 |
W |
b |
F+B |
81 |
6.8 |
2.1 |
15 |
4.2 |
A |
* F represents ferrite, B represents bainite, and M represents martensite. |
[0116] These results indicate and demonstrate as follows. Specifically, Test Nos.1 to 6,
10, 11,13, and 15 employed steels having appropriately controlled chemical compositions
and were produced under appropriately controlled production conditions, met the preferred
conditions in the interior of the steel plate, had a crack propagation rate of 40×
10
-5 mm/cycle or less, and had still further excellent fatigue crack propagation properties.
1. A steel plate comprising:
C in a content of 0.02 to 0.10 mass percent;
Mn in a content of 1.0 to 2.0 mass percent;
Nb in a content of greater than 0 mass percent to 0.05 mass percent;
Ti in a content of greater than 0 mass percent to 0.05 mass percent;
Al in a content of 0.01 to 0.06 mass percent; and
at least one element selected from the group consisting of
Si in a content of 0.1 to 0.6 mass percent; and
Cu in a content of 0.1 to 0.6 mass percent;
where a total content of the at least one of Si and Cu is 0.3 mass percent or more,
with the remainder consisting of iron and inevitable impurities,
wherein a microstructure of a surface layer of the steel plate comprises at least
one of ferrite and upper bainite in a total fraction of 80 area percent or more,
wherein grains of the at least one of ferrite and upper bainite have an effective
grain size of 10.0 µm or less,
wherein, of the microstructure of the surface layer, grains of a remainder microstructure
excluding the ferrite and the upper bainite have an average equivalent circle diameter
of 3.0 µm or less, and
wherein the steel plate has a dislocation density ρ of 2.5× 1015 m-1 or less as determined by X-ray diffractometry.
2. The steel plate according to claim 1,
wherein, of the microstructure of the surface layer, the remainder microstructure
excluding the ferrite and the upper bainite has a fraction of martensite-austenite
constituent of 5 area percent or less.
3. The steel plate according to claim 1,
further comprising, in chemical composition, at least one selected from the group
consisting of
(a) Ni in a content of greater than 0 mass percent to 0.6 mass percent, where a ratio
[Ni]/[Cu] of the Ni content [Ni] to the Cu content [Cu] is less than 1.2;
(b) at least one element selected from the group consisting of
V in a content of greater than 0 mass percent to 0.5 mass percent;
Cr in a content of greater than 0 mass percent to 1.0 mass percent or less; and
Mo in a content of greater than 0 mass percent to 0.5 mass percent; and (c) B in a
content of greater than 0 mass percent to 0.005 mass percent.
4. The steel plate according to claim 3,
wherein the remainder microstructure has a fraction of martensite-austenite constituent
of 5 area percent or less.
5. The steel plate according to any one of claims 1 to 4,
wherein the steel plate has a bainite transformation start temperature Bs of 640°C
or higher, where the bainite transformation start temperature is calculated from the
chemical composition according to Expression (1):

wherein [C], [Mn], [Ni], [Cr], and [Mo] represent contents (in mass percent) respectively
of C, Mn, Ni, Cr, and Mo.
6. A steel plate comprising:
C in a content of 0.02 to 0.10 mass percent;
Mn in a content of 1.0 to 2.0 mass percent;
Nb in a content of greater than 0 mass percent to 0.05 mass percent;
Ti in a content of greater than 0 mass percent to 0.05 mass percent;
Al in a content of 0.01 to 0.06 mass percent;
at least one element selected from the group consisting of
Si in a content of 0.1 to 0.6 mass percent; and
Cu in a content of 0.1 to 0.6 mass percent,
where a total content of the at least one of Si and Cu is 0.3 mass percent or more;
and
B in a content of greater than 0 mass percent to 0.005 mass percent,
with the remainder consisting of iron and inevitable impurities,
wherein a microstructure of a surface layer of the steel plate comprises at least
one of ferrite and upper bainite in a total fraction of 80 area percent or more,
wherein grains of the at least one of ferrite and upper bainite have an effective
grain size of 10.0 µm or less,
wherein, of the microstructure of the surface layer, grains of a remainder microstructure
excluding the ferrite and the upper bainite have an average equivalent circle diameter
of 3.0 µm or less,
wherein the steel plate has a dislocation density ρ of 2.5× 1015 m-1 or less as determined by X-ray diffractometry,
wherein a microstructure at a position at a depth of one-fourth the thickness t of
the steel plate from the surface along a thickness direction comprises upper bainite
in a fraction of 80 area percent or more in a longitudinal section in parallel with
a rolling direction,
wherein grains of the upper bainite in the microstructure at the position at a depth
of one-fourth the thickness t have an effective grain size of 10.0 µm or less, and
wherein, of the microstructure at the position at a depth of one-fourth the thickness
t, grains of the remainder microstructure excluding the upper bainite have an average
equivalent circle diameter of 3.0 µm or less.
7. The steel plate according to claim 6,
wherein, of the microstructure of the surface layer, the remainder microstructure
excluding the ferrite and the upper bainite has a fraction of martensite-austenite
constituent of 5 area percent or less.
8. The steel plate according to claim 6,
further comprising, in chemical composition, at least one selected from the group
consisting of
(a) Ni in a content of greater than 0 mass percent to 0.6 mass percent, where a ratio
[Ni]/[Cu] of the Ni content [Ni] to the Cu content [Cu] is less than 12; and
(b) at least one element selected from the group consisting of:
V in a content of greater than 0 mass percent to 0.5 mass percent;
Cr in a content of greater than 0 mass percent to 1.0 mass percent or less; and
Mo in a content of greater than 0 mass percent to 0.5 mass percent,
9. The steel plate according to claim 8,
wherein, of the microstructure of the surface layer, the remainder microstructure
excluding the ferrite and the upper bainite has a fraction of martensite-austenite
constituent of 5 area percent or less.
10. The steel plate according to claim 6,
wherein the microstructure at the position at a depth of one-fourth the thickness
t comprises grains each having a grain average misorientation (GAM) of 1° or more
in a fraction of 20 area percent to 80 area percent, where the grain average disorientation
is determined per one grain by electron backscatter pattern observation.
11. The steel plate according to claim 10,
wherein the steel plate further meets conditions specified by Expression (2) and Expression
(3):

wherein [Si], [Mn], [Ni], [Cu], [Ti], [N], [Cr], [Nb], [C], and [Mo] represent contents
(in mass percent) respectively of Si, Mn, Ni, Cu, Ti, N, Cr, Nb, C, and Mo, and, when
at least one of the term {[Ti] - 3.4[N]} and the term {[Nb] - 7.7[C]} is negative,
calculation according to the expression is performed as treating the at least one
term as "zero (0)".
12. The steel plate according to claim 10,
further comprising, in chemical composition, at least one selected from the group
consisting of
(a) Ni in a content of greater than 0 mass percent to 0.6 mass percent, where a ratio
[Ni]/[Cu] of the Ni content [Ni] to the Cu content [Cu] is less than 1.2; and
(b) at least one element selected from the group consisting of
V in a content of greater than 0 mass percent to 0.5 mass percent;
Cr in a content of greater than 0 mass percent to 1.0 mass percent or less; and
Mo in a content of greater than 0 mass percent to 0.5 mass percent
13. The steel plate according to claim 12,
wherein, of the microstructure of the surface layer, the remainder microstructure
excluding the ferrite and the upper bainite has a fraction of martensite-austenite
constituent of 5 area percent or less.
14. The steel plate according to claim 11,
further comprising, in chemical composition, at least one selected from the group
consisting of
(a) Ni in a content of greater than 0 mass percent to 0.6 mass percent, where a ratio
[Ni]/[Cu] of the Ni content [Ni] to the Cu content [Cu] is less than 1.2; and
(b) at least one element selected from the group consisting of:
V in a content of greater than 0 mass percent to 0.5 mass percent;
Cr in a content of greater than 0 mass percent to 1.0 mass percent or less; and
Mo in a content of greater than 0 mass percent to 0.5 mass percent.
15. The steel plate according to claim 14,
wherein, of the microstructure of the surface layer, the remainder microstructure
excluding the ferrite and the upper bainite has a fraction of martensite-austenite
constituent of 5 area percent or less.
16. The steel plate according to any one of claims 6 to 15,
wherein the steel plate has a bainite transformation start temperature Bs of 640°C
or higher, where the bainite transformation start temperature is calculated from the
chemical composition according to Expression (1):

wherein [C], [Mn], [Ni], [Cr], and [Mo] represent contents (in mass percent) respectively
of C, Mn, Ni, Cr, and Mo.