Technical Field
[0001] This invention relates to a steel material which has improved fatigue crack driving
resistance and which is suitable for use in applications where a steel material is
expected to undergo repeated loads, such as boats and ships, marine structures, bridges,
buildings, tanks, and industrial or construction equipment, for example, and it also
pertains to a process for manufacturing the steel material.
Background Art
[0002] For steel materials which are used in boats and ships, marine structures, bridges,
buildings, tanks, and industrial or construction equipment, it is necessary to pay
attention to fatigue properties in order to guarantee their safety, since repeated
loads are often applied thereto. It is known that fatigue fracture of a steel material
is greatly influenced by environmental conditions and that when a steel material undergoes
repeated loads in a corrosive environment such as in seawater, its strength decreases.
[0003] The fatigue process of a steel material should be considered by dividing it into
two stages, i.e., generation of a crack in an area where stress concentration occurs
and subsequent growth of the crack, which are different in nature from each other.
In usual machine parts, generation of a macroscopic crack is considered to be the
working limit, so they are almost never designed to accept crack growth. However,
with a structure having a high redundancy, generation of a fatigue crack does not
soon lead to a fracture of the structure. Therefore, even if a fatigue crack is found
in a structure by a routine inspection before final fracture occurs, in the case where
the cracked portion is repaired or the crack does not grow during the service life
of the structure to a length sufficient to cause final fracture, the structure still
endures to use adequately in spite of the fatigue crack.
[0004] In a welded structure, since there are many toes of weld which become stress-concentrated
areas, it is nearly impossible from a technical standpoint and also not advisable
from an economical viewpoint to completely prevent the generation of fatigue cracks.
Therefore, it is important to retard crack growth as much as possible so as to greatly
extend the fatigue life (remaining life) from a state in which cracks exist.
[0005] With respect to a technique designed to retard fatigue crack growth and extend fatigue
life of a steel material, JP P05-185441A discloses an approach in which microcracks
are formed at the tip of a fatigue crack. However, the effectiveness of this approach
is limited to a crack in which the stress intensity factor range ΔK (the difference
between the maximum and minimum stress intensity factor) is small, that is, the case
where the crack is not long and the stress level is low. It is thought that this approach
is less effective for a crack having a medium level of ΔK range, which originates
from a weld and has a considerable length.
[0006] JP P04-337026A proposes a method for manufacturing a high-strength hot-rolled steel
plate having improved fatigue strength and fatigue crack propagating resistance in
which the phosphorus and copper contents are controlled such that the steel has dual
phases of ferrite with a grain size of 5 - 25 µm and a second phase comprising 10%
- 30% by volume. In that patent application, the fatigue crack propagating resistance
indicates the threshold stress intensity factor (ΔK
th) for fatigue crack growth. Thus, the proposed technique has an effect of increasing
the threshold stress intensity factor that is the lower limit for causing a fatigue
crack to grow but it is not effective in retarding fatigue crack growth.
[0007] Japanese Patent No. 2,692,134 discloses a steel plate having a fatigue crack growth
inhibiting effect which comprises a hard phase-forming matrix and a soft phase dispersed
in the matrix, the difference in hardness between the two phases being at least 150
in terms of Vickers hardness. However, that patent does not disclose the mechanical
properties of the steel. Moreover, that technique is applicable only in situations
in which the hard and soft phases of the structure are clearly distinguishable from
each other. In general, since the structure should be made fine in order to improve
the strength and toughness of the steel, the hard and soft phases are not always clearly
distinguishable from each other to such an extent that the difference in hardness
can be measured.
[0008] JP P2001-41868A discloses a method for assessing the fatigue crack growth rate of
a steel material containing at least 20% of bainite on the basis of the amount of
softening (cyclic softening parameter) determined when repeated loads are applied
to the steel with a controlled strain having a stress ratio of -4 to -0.25 and an
alternating waveform. According to that method, as long as a master curve showing
the correlation between the amount of cyclic softening and the fatigue crack growth
rate has been prepared, the fatigue crack growth rate can be assessed quickly and
efficiently from the amount of cyclic softening.
[0009] However, that method is merely an assessment method and it is not a measure which
is capable of providing an excellent steel which is worth while assessing by the method.
There is no disclosure as to whether the steel described therein has adequate strength,
toughness, and weldability as a structural steel.
[0010] It has been known that under conditions in which a cyclic strain is imposed, a steel
having a hardened structure becomes softer (i.e, shows cyclic softening), while an
annealed steel becomes harder. For steel materials showing cyclic softening, since
the nature of fatigue properties has not been elucidated, they do not have an established
criterion of industrial design. As a result, such steel materials have not been employed
in those applications which exploit their fatigue properties.
Disclosure of Invention
[0011] It is an object of the present invention to provide a structural steel material which
makes it possible to perform material design by using fatigue properties as a quantitative
parameter, particularly in a steel material showing cyclic softening, and a process
for manufacturing such a steel material.
[0012] The present invention provides a steel material having improved fatigue crack driving
resistance and a manufacturing method therefor on the basis of investigations on fatigue
properties of steel materials including softening behavior under conditions in which
a cyclic strain is imposed. The steel material also has strength, toughness, and weldability
which are optimal for structural steel for use in boats and ships, marine structures,
bridges, buildings, tanks, industrial or construction equipment, and the like.
[0013] A steel material having improved fatigue crack driving resistance according to the
present invention has a cyclic softening parameter of at least 0.65 and at most 0.95,
the cyclic softening parameter being represented by the ratio (σ
15/σ
1) of the stress at the maximum strain in the first cycle (σ
1) to that in the 15th cycle (σ
15) measured when a waveform of incremental and decremental cyclic loads is applied
15 times with a maximum tensile and compressive strain of ±0.012, a frequency of 0.5
Hz, and the number of waves to the maximum strain being 12.
[0014] The steel material preferably has a composition by mass% which comprises C: 0.02
- 0.20%, Si: at most 0.60%, Mn: 0.50 - 2.0%, Al: 0.003 - 0.10%, and optionally one
or more elements selected from (A) one or more of Cu: at most 1.5%, Ni: at most 1.5%,
Cr: at most 1.20%, Mo: at most 1.0%, and V: at most 0.10%, (B) one or two of Nb: at
most 0.10% and Ti: at most 0.10%, (C) B: 0.0003 - 0.0020%, and (D) Ca: 0.0005 - 0.010%,
and which has a value of carbon equivalent, Ceq, represented by the following formula
of from 0.28 - 0.65:
Ceq (%) = C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14.
[0015] The steel material having improved fatigue crack driving resistance according to
the present invention can be manufactured by any of the following processes:
(a) a process comprising subjecting a hot-rolled steel material of the above-described
composition to heat treatment one or more times, the heat treatment comprising reheating
to a temperature above the Ac1 point followed by cooling to 550°C or below at a cooling rate of at least 5°C/s;
(b) a process comprising cooling a hot-rolled steel material of the above-described
composition from a temperature range of at least (Ar3 point - 100)°C and at most (Ar3 point + 150)°C to 550°C or below at a cooling rate of at least 5°C/s; and
(c) a process comprising cooling a hot-rolled steel material of the above-described
composition from a temperature range of at least (Ar3 point - 100)°C and at most (Ar3 point + 150)°C to 550°C or below at a cooling rate of at least 5°C/s and subsequently
subjecting it to heat treatment one or more times, the heat treatment comprising reheating
to a temperature above the Ac1 point followed by cooling to 550°C or below at a cooling rate of at least 5°C/s.
[0016] In any of the above-described processes, a final heat treatment may be performed
by tempering with heating to a temperature below the Ac
1 point.
Brief Description of Drawings
[0017]
Figure 1 is a diagram showing an incremental and decremental strain waveform;
Figure 2 is graph showing the cyclic softening parameter and the fatigue crack growth
rate; and
Figure 3 is a graph showing the amount of strain at the tip of a fatigue crack determined
by the finite element method.
Detailed Description of the Invention
[0018] Now the present invention will be described in detail. In the following description,
all percentages are mass% unless otherwise indicated.
[0019] The present invention relates to assessment of a steel material with respect to the
degree of softening under conditions that a cyclic strain is imposed, so a steel material
to which the present invention pertains has a structure which becomes softer when
subjected to cyclic strain (i.e., a hardened structure).
[0020] A steel material having a hardened structure becomes softer by loading with a cyclic
strain to give a cyclic softening parameter σ
15/σ
1. It has been found that the value of the cyclic softening parameter σ
15/σ
1 has a correlation with the fatigue crack growth rate (da/dN) in a stress intensity
factor range (ΔK) of 20 MPa·m
0.5 which is ordinarily used and thus can be used for assessment of fatigue crack growth
rate.
[0021] If a steel material has a cyclic softening parameter of less than 0.65, it will have
a low crack growth rate, but its toughness and weldability will be deteriorated and
its use as a structural steel will be significantly limited. On the other hand, if
it has a cyclic softening parameter of greater than 0.95, not only does the crack
growth rate become high, but the strength is decreased. Thus, the cyclic softening
parameter is made at least 0.65 and at most 0.95. It is preferably at least 0.70 and
at most 0.90.
[0022] The cyclic softening parameter σ
15/σ
1 used herein will be described.
[0023] The waveform of the strain which is imposed on a steel material to determine the
cyclic softening parameter is an alternating waveform in which tensile and compressive
loads are applied alternatingly in order to assess the amount of cyclic softening
of the steel material. The waveform is an incremental and decremental waveform with
a frequency of 0.5 Hz and a strain range of 0.024 during strain incrementation (a
maximum tensile and compressive strain of ±0.012). The frequency of 0.5 Hz was selected
in view of suppressing internal heat build-up. The strain range of 0.024 was selected
since the stress intensity factor range (ΔK) which is ordinarily employed is 20 MPa·m
0.5. In the incremental stage, the strain reaches a maximum value in 12 waves, and in
the decremental stage, it returns to zero in 12 waves. A combination of a single incremental
stage and a single decremental stage constitutes a set, which is hereafter referred
to as a "block".
[0024] Figure 1 shows a diagram of strain waveform in which the abscissa is time (sec) and
the ordinate is the amount of strain. Only the first and second blocks are shown in
the figure, but the number of blocks is fifteen or the incremental and decremetal
strain waveform is repeated 15 times. Fifteen repetitions for the block were selected
since it is thought that the softening effect attained by cyclic strain will almost
saturate by 15 blocks.
[0025] The ratio σ
15/σ
1 is defined as a cyclic softening parameter, where σ
1 is the stress corresponding to the maximum strain in the first block and σ
15 is the stress corresponding to the maximum strain in the 15th block.
[0026] Figure 2 is a graph showing the relationship between the cyclic softening parameter
defined above and the fatigue crack growth rate. As described above, a certain correlation
can be observed between these parameters.
[0027] The mechanism thereof is considered to be as follows.
[0028] Application of a cyclic load with alternating tensile and compressive or positive
and negative forces to a steel material causes inversion movement of dislocations
at the tip of a fatigue crack, and the dislocations move or disappear to cause the
material to soften. The softening relaxes the strain at the tip of the fatigue crack,
thereby decreasing the driving force of fatigue crack growth.
[0029] The strain relaxation phenomenon at the tip of a fatigue crack was analyzed by the
finite element method. The mechanical properties of the steel which underwent cyclic
softening (cyclically softened material) were divided into analysis elements to describe
a softened zone around a fatigue crack, and a model to which a load is applied such
that the stress intensity factor range at the crack tip or front is 20 MPa·m
0.5 was presumed.
[0030] Figure 3 shows a result of the analysis in terms of the strain at the tip of a fatigue
crack of a homogeneous steel material compared to that of a cyclically softened material.
It was confirmed that the strain of a cyclically softened material at the tip of a
fatigue crack is smaller than that of a homogeneous material. Thus, in a cyclically
softened steel material, it is thought that relaxation of strain imposed on the crack
tip contributes to suppression of fatigue crack growth.
[0031] A target in the present invention is that a steel material has a fatigue crack growth
rate (da/dN) of at most 4.0×10
-5 mm/cycle with a stress intensity factor range of 20 MPa·m
0.5 in a fatigue test in air.
[0032] In a preferred embodiment, a steel material according to the present invention has
the chemical composition described above for the following reasons.
Carbon: 0.02 - 0.20%
Carbon (C) is an element which is effective in order to provide a structural steel
with strength. With a carbon content of less than 0.02%, it is difficult to achieve
the strengthening effect. On the other hand, a carbon content of greater than 0.20%
decreases the weldability of a steel and makes it difficult to process the steel by
welding, thereby limiting its working range as a structural steel. In order to attain
a high strength with good weldability, the carbon content is preferably in the range
of 0.04 - 0.15%.
Silicon: at most 0.60%
Silicon (Si) has a deoxidizing effect. However, an Si content of greater than 0.60%
deteriorates the toughness of a steel. Preferably, the Si content is 0.05 - 0.5%.
Manganese: 0.50 - 2.0%
Manganese (Mn) is also an element which has an effect of providing a steel with
strength. With an Mn content of less than 0.50%, its effect is not sufficient. An
Mn content of greater than 2.0% deteriorates the toughness of a steel. Preferably,
the Mn content is 0.70 - 1.8%.
Aluminum: 0.003 - 0.10%
[0033] Aluminum (Al) has a deoxidizing effect. With an Al content of less than 0.003%, its
effect is not sufficient. An Al content of greater than 0.10% deteriorates the toughness
of a steel. Preferably, the Al content is 0.010 - 0.050%.
[0034] The steel material according to the present invention may further contain at least
one element selected from the following groups, in addition to the above-described
elements:
(A) at least one of Cu, Ni, Cr, Mo, and V;
(B) Nb and/or Ti;
(C) B; and
(D) Ca.
Copper: at most 1.5%
[0035] Copper (Cu) is an element which is effective in order to improve the strength and
corrosion resistance of a steel, but a Cu content of greater than 1.5% causes the
steel to have a deteriorated toughness. A preferable content of Cu, when added, is
0.10 - 1.0%.
Nickel: at most 1.5%
Nickel (Ni) is an element which is effective in order to improve the strength and
toughness of a steel. However, when Ni is added in an amount of greater than 1.5%,
not only are these effects saturated, but costs are increased. A preferable content
of Ni, when added, is 0.050 - 1.3%.
Chromium: at most 1.2%
Like Cu, chromium (Cr) is an element which is effective in order to improve the
strength and corrosion resistance of a steel. However, a Cr content of greater than
1.5% causes the steel to have a deteriorated toughness. A preferable content of Cr,
when added, is 0.10 - 1.0%.
Molybdenum: at most 1.0%
Molybdenum (Mo) is an element which is effective in order to improve the hardenability
and strength of a steel. However, when the Mo content is greater than 1.0%, not only
is the toughness deteriorated, but costs are increased. A preferable content of Mo,
when added, is 0.050 - 0.80%.
Vanadium: at most 0.10%
Vanadium (V) has an effect of increasing the strength of a steel and may be added
in order to secure a high strength as a structural steel. However, the addition of
V in an amount of greater than 0.10% deteriorates the toughness of the steel. A preferable
content of V, when added, is 0.010 - 0.080%.
Niobium; at most 0.10%
Niobium (Nb) is an element which is effective in order to improve the toughness
of a steel. However, the addition of Nb in an amount of greater than 0.10% results
in a decrease in toughness to the contrary. A preferable content of Nb, when added,
is 0.020 - 0.050%.
Titanium: at most 0.10%
Like Nb, titanium (Ti) is also an element which has an effect of improving the
toughness of a steel. However, to the contrary, the addition of Ti in an amount of
greater than 0.10% results in a decrease in toughness. A preferable content of Ti,
when added, is 0.010 - 0.050%.
Boron: 0.00030 - 0.020%
Boron (B) is an element which is effective in improving the hardenability of a
steel and controlling the ferrite content thereof. These effects are not exhibited
sufficiently with a B content of less than 0.00030%. On the other hand, a B content
of greater than 000020% causes the toughness of the steel to deteriorate. A preferable
content of B, when added, is 0.00080 - 0.0015%.
Calcium: 0.00050 - 0.010%
Calcium (Ca) is an element which has an effect of spheroidizing non-metallic inclusions
and improving the toughness of a steel. Such an effect is not achieved with a Ca content
of less than 0.00050%. When the Ca content is greater than 0.010%, a large amount
of inclusions such as CaO and CaS are formed, leading to a deterioration in toughness.
A preferable content of Ca, when added, is 0.0010 - 0.0050%.
Carbon equivalent (Ceq): 0.28 - 0.65%
Ceq (%) = C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14
[0036] The carbon equivalent, Ceq, which has the above formula, is an index to assess the
hardenability and weldability of a steel material and is widely used. However, Ceq
has been used merely as an index to obtain a steel material having desired mechanical
properties and weldability, and there has been found no research which investigates
Ceq in relation to fatigue crack driving resistance.
[0037] In order to suppress fatigue crack growth and satisfy the strength properties generally
desired for a structural steel, i.e., a tensile strength, TS, of at least 500 N/mm
2 and Charpy absorbed energy value at 0°C, vE
0, of at least 27 J, a steel material should have a fine (not coarsened) microstructure.
The present inventors have found that with a steel having a fine microstructure, the
value of Ceq has a relation not only to the mechanical properties and weldability
of the steel, but to the fatigue crack growth rate thereof.
[0038] Namely, with a steel having a value of Ceq of less than 0.28%, not only is its strength
low, but the fatigue crack growth is not sufficiently suppressed due to its low level
of cyclic softening. On the other hand, with a steel having a value of Ceq of greater
than 0.65%, although the fatigue crack growth of the steel is suppressed, its weldability
is deteriorated, and it is difficult to work the steel material by welding, leading
to a severe limitation in its use.
[0039] Next, a method of manufacturing a steel material according to the present invention
will be described.
[0040] A mass of a steel having a composition as described above which has been prepared
by continuous casting, for example, is hot-rolled. The resulting hot-rolled steel
plate is then cooled at a controlled cooling rate and/or subjected to heat treatment
so as to obtain a steel material having a cyclic softening parameter adjusted to at
least 0.65 and at most 0.95.
[0041] The hot rolling process is not critical and may be performed in a conventional manner.
A steel material according to the present invention is generally in the form of a
hot-rolled plate, but it may be in another form such as shape steel, bar, pipe, or
the like depending on the use thereof. Thus, the hot-rolling process may be replaced
by other hot-working process.
[0042] Following the hot rolling, heat treatment is performed by reheating to a temperature
above the Ac
1 point followed by rapid cooling, or the steel plate as hot-rolled is cooled rapidly,
or a combination of these is employed, as described below, thereby resulting in the
formation of a steel material having a hardened structure in which the cyclic softening
parameter is between 0.65 and 0.95. Thereafter, the steel material may be further
subjected to tempering as described below.
Heat Treatment (Reheating and Rapid Cooling):
[0043] The temperature at which the hot-rolled steel material is reheated is above the Ac
1 point of the steel. If the temperature for reheating is less than the Ac
1 point, no austenite transformation occurs, so a steel material having the desired
cyclic softening parameter cannot be obtained and the steel material has deteriorated
fatigue crack growth properties. A preferable temperature range for reheating is between
100°C and 300°C above the Ac
1 point.
[0044] Following reheating, rapid cooling is performed at a cooling rate of at least 5°C/s.
If the cooling rate is less than 5°C/s, cooling is so slow that the resulting steel
material has an increased fatigue crack growth rate and a decreased strength and toughness.
The cooling rate after reheating is preferably at least 10°C/s. Although there is
no particular upper limit on the cooling rate, it depends on the size of the steel
material (the thickness if it is a steel plate). For example, it is possible for a
steel plate having a thickness of 10 mm or smaller to achieve a cooling rate of 50°C
or higher.
[0045] The temperature at which the rapid cooling is stopped is 550°C or lower. If the cooling
is stopped at a temperature above 550°C, the steel material has an increased fatigue
crack growth rate and hence deteriorated cyclic softening properties. A preferable
temperature at which the rapid cooling is stopped is 450°C or below.
[0046] The above-described heat treatment by reheating and subsequent rapid cooling may
be performed two or more times as required. In order to maintain the hardened structure
thus formed, after the heat treatment, the steel material is not subjected to additional
heat treatment except for the after-mentioned tempering.
Rapid Cooling Following Hot Rolling:
[0047] A steel material having a cyclic softening parameter of 0.65 - 0.95 can be produced
merely by rapidly cooling the steel material as hot-rolled from a temperature range
of from (Ar
3 point - 100)°C to (Ar
3 point + 150)°C.
[0048] The cooling rate just after hot rolling is at least 5°C/s and the temperature at
which the rapid cooling is stopped is 550°C or below for the same reasons described
above for cooling after reheating.
[0049] If the temperature at which the rapid cooling is started is lower than (Ar
3 point - 100)°C, the cyclic softening properties and strength are deteriorated. On
the other hand, if it is higher than (Ar
3 point + 150)°C, the austenite grains in the steel are coarsened, thereby deteriorating
the toughness. A preferable temperature at which rapid cooling is started is in the
range of from (Ar
3 point - 50)°C to (Ar
3 point + 100)°C.
[0050] After the hot-rolled steel plate is rapidly cooled in the above-described manner,
the aforementioned heat treatment by reheating and subsequent rapid cooling may be
performed one or more times.
Tempering:
[0051] The steel material which has been rapid cooled after hot rolling and/or reheating
in the above-described manner may be finally subjected to tempering. Particularly,
when the rapid cooling is performed at a higher cooling rate (e.g., 50°C or higher),
it is preferable to perform tempering, since a significant improvement in toughness
can be attained by tempering. The tempering temperature is below the Ac
1 point of the steel. If it is higher than the Ac
1 point, tempering causes austenite transformation, thereby deteriorating the cyclic
softening properties and decreasing the strength and toughness. The tempering temperature
is preferably 550°C or below.
[0052] A steel material according to the present invention has a cyclic softening parameter
in the range of from 0.65 to 0.95. It should be understood by those skilled in the
art from the foregoing description that the cyclic softening parameter can be adjusted
by the conditions for heat treatment and cooling and/or Ceq.
[0053] In accordance with the present invention, it is possible to quantitatively assess
the fatigue properties of a steel material and hence to perform material design using
the assessment. It also becomes feasible to provide a steel material having improved
fatigue crack driving resistance by use of the material design. The steel material
exhibits excellent properties even in an environment containing chlorine or chloride
ions. Therefore, it is suitable for use in various structures including boats and
ships, marine structures, bridges, buildings, tanks, and industrial or construction
equipment.
Examples
(Example 1)
[0054] Steels each having a composition and a value of Ceq as shown in Table 1 were prepared
by melting in a test furnace in a conventional manner. Table 1 also shows the values
of the Ar
3 and Ac
1 points of each steel.
[0055] Each steel was made in the form of a slab 150 mm in thickness by normal hot forging.
The slab was heated to 1150°C and hot-rolled to produce a steel plate having a thickness
shown in Table 1.
[0056] These steel sheets were subjected to the following procedures.
[0057] For Steels 1, 2, and 13 which had a thickness of 10 mm or smaller, each hot-rolled
steel plate was subjected to heat treatment once by reheating to 200°C above its Ac
1 point followed by cooling to room temperature at a cooling rate of 60°C/s, and finally
tempering was performed at 400°C.
[0058] For Steels 3 - 12 and 14 - 28, immediately after hot-rolling, each steel plate was
cooled to 450°C starting from the temperature that was equal to 50°C above its Ar3
point. The cooling rate was 30°C/s for a plate thickness of 15 mm, 20°C/s for a plate
thickness of 25 mm, 10°C/s for a plate thickness of 40 mm, or 5 - 8°C/s for a plate
thickness of 50 mm. Tempering was not performed.
[0059] Appropriate test pieces or specimens were taken from these steel plates so as to
evaluate a central portion along the thickness of each plate and were used for a cyclic
softening test, a fatigue crack growth test, a tensile test, and a Charpy impact test.
[0060] The cyclic softening test was conducted in air at room temperature with a purely
alternating strain waveform using a test bar having a diameter of 6 - 8 mm and a parallel
length of 15 mm, which had been taken from a central portion of the steel plate in
its thickness direction in such a manner that the longitudinal direction of the test
bar was coincident with the rolling direction. An extensiometer having a gauge length
of 12.5 mm was attached to the parallel zone of the test bar, and an axial force was
applied to the test bar while controlling the strain by use of the extensiometer as
a sensor. The testing machine used was an electro-hydraulic closed-loop fatigue testing
machine, and the strain waveform was an incremental step waveform of the incremental
and decremental type. The waveform had a frequency of 0.5 Hz and a strain range of
0.024, and the strain reached the maximum strain in 12 waves during a strain incremental
stage and returned to zero in 12 waves in a strain decremental stage (see Figure 1).
[0061] As described previously, a set of such an incremental and a decremental stage is
referred to as a "block". The stress corresponding to the maximum strain in the first
block was measured and taken as σ
1, while that corresponding to the maximum strain in the 15th block was measured and
taken as σ
15, and the ratio σ
15/σ
1 was determined as a cyclic softening parameter.
[0062] In the fatigue crack growth test, a CT (compact) specimens was taken in such a manner
that the crack growing direction was perpendicular to the rolling direction of the
plate. The test was performed in air at room temperature under loading conditions
of a frequency of 25 Hz and stress ratio (minimum stress/maximum stress) of 0.1 in
accordance with ASTM Specifications (E647).
[0063] The fatigue crack growth rate was determined to be the growth rate at the point that
the stress intensity factor range ΔK at the crack tip was 20 MPa·m
0.5. In view of the fatigue crack growth rate of conventional materials which is within
the range of 5 to 6×10
-5 mm/cycle, the target fatigue crack growth rate was set at 4.0×10
-5 mm/cycle or lower.
[0064] The tensile test was performed with a No. 4 test piece specified in JIS Z2201 (1998),
which was taken from a central portion of the steel plate in its thickness direction
in such a manner that the longitudinal direction of the test piece was perpendicular
to the rolling direction of the plate.
[0065] The Charpy impact test was carried out with a V-notched impact test piece specified
in JIS Z2202 (1998), which was taken from a central portion of the steel plate in
its thickness direction in such a manner that the longitudinal direction of the test
piece was coincident with the rolling direction of the plate. The test was repeated
three times at each test temperature, and the brittle-ductile fracture transition
temperature (vTrs) was determined.
[0066] The results of these tests are shown in Table 2. In the table, the marks "○", "Δ",
and "×" in the tensile test and the Charpy impact test indicate the following.
[0067] In the tensile test, the mark "○" indicates that the tensile strength is 500 MPa
or higher and the mark "Δ" indicates that it is lower than 500 MPa.
[0068] In the Charpy impact test, the mark "○" indicates that the brittle-ductile fracture
transition temperature (vTrs) is -20°C or lower, the mark "Δ" indicates that it is
higher than -20°C and up to 0°C, and the mark " × " indicates that it is higher than
0°C. When the value of vTrs of a material is -20°C or lower, the absorbed energy is,
on the average, at least 150 J at 0°C and at least 100 J at -20°C.
Table 2
Steel
No. |
Cyclic softening parameter
σ15/σ1 |
Fatigue crack
growth rate
(x10-5mm/cycle) |
Tensile
Test |
Charpy
impact
test |
1 |
0.935 |
3.84 |
○ |
○ |
2 |
0.927 |
3.77 |
○ |
○ |
3 |
0.889 |
3.16 |
○ |
○ |
4 |
0.863 |
2.87 |
○ |
○ |
5 |
0.820 |
1.92 |
○ |
○ |
6 |
0.845 |
2.38 |
○ |
○ |
7 |
0.811 |
1.72 |
○ |
○ |
8 |
0.931 |
3.88 |
○ |
○ |
9 |
0.881 |
2.99 |
○ |
○ |
10 |
0.657 |
0.42 |
○ |
○ |
11 |
0.738 |
1.12 |
○ |
○ |
12 |
0.712 |
0.83 |
○ |
○ |
13 |
0.869 |
3.64 |
Δ |
○ |
14 |
0.624 |
0.21 |
○ |
Δ |
15 |
0.980 |
4.95 |
Δ |
○ |
16 |
0.788 |
1.34 |
○ |
Δ |
17 |
0.921 |
3.54 |
○ |
Δ |
18 |
1.092 |
5.95 |
Δ |
○ |
19 |
0.776 |
1.68 |
○ |
Δ |
20 |
0.882 |
3.25 |
○ |
Δ |
21 |
0.886 |
3.22 |
○ |
Δ |
22 |
0.925 |
3.57 |
○ |
× |
23 |
0.775 |
1.58 |
○ |
Δ |
24 |
0.764 |
1.60 |
○ |
Δ |
25 |
1.120 |
6.32 |
○ |
× |
26 |
0.930 |
3.78 |
○ |
Δ |
27 |
1.052 |
5.67 |
○ |
Δ |
28 |
0.934 |
3.76 |
○ |
Δ |
[0069] As is shown in Table 2, Steels Nos. 15, 18, 25, and 27 had a cyclic softening parameter
which is beyond the range defined in the present invention, and they had a large fatigue
crack growth rate in air. Steel No. 14 had an excessively small cyclic softening parameter
and hence poor impact properties. Steels Nos. 13, 16, 17, 19 - 24, 26, and 28 had
a good cyclic softening parameter and also a good fatigue crack growth rate, but their
strength or impact properties were slightly poor.
(Example 2)
[0070] Using Steel No. 4 in Example 1 (Ac
1 point: 688°C, Ar
3 point: 774°C) in which all of the cyclic softening parameter, fatigue crack growth
rate, tensile strength, and vTrs reached their respective target values, the effects
of manufacturing conditions were surveyed.
[0071] A 150 mm-thick steel slab was heated to 1150°C and hot-rolled to produce a 25 mm-thick
steel plate. The cooling stage following hot rolling and/or heat treatment (reheating
and cooling) after completion of hot rolling were performed under the conditions shown
in Table 3. The same tests as described in Example 1 were carried out using the respective
test pieces for these tests taken from the resulting steel plates. Table 4 shows the
cyclic softening parameter, fatigue crack growth rate, tensile strength, and Charpy
impact test result (vTrs).
Table 3
Mark |
Manufacturing conditions |
A |
Reheating to 900°C and then cooling to RT at a rate of 20°C/s |
B |
Reheating to 900°C, then cooling to RT at a rate of 20°C/s, and tempering with heating
to 500 °C |
C |
Reheating to 900°C, then cooling to RT at a rate of 20°C/s, reheat-int to 750°C, and
cooling to RT at a rate of 15°C/s |
D |
Reheating to 900°C, then cooling to RT at a rate of 20°C/s, reheat-int to 780°C, cooling
to RT at a rate of 20°C/s, and tempering with heating to 450°C |
E |
Following hot rolling, cooling from 830°C to 500°C at a rate of 20°C/s |
F |
Following hot rolling, cooling from 800 °C to 450°C at a rate of 25°C/s and then tempering
with heating to 400°C |
G |
Following hot rolling, cooling from 750°C to RT at a rate of 30°C/s |
H |
Following hot rolling, cooling from 800°C to RT at a rate of 30°C/s and then reheating
to 750°C and cooling to RT at a rate of 25°C/s |
I |
Reheating to 650°C and then cooling to RT at a rate of 20°C/s |
J |
Reheating to 900°C, then cooling to RT at a rate of 3°C/s, and tempering with heating
to 450°C |
K |
Reheating to 900°C, then cooling to RT at a rate of 20°C/s, reheat-int to 660°C, and
cooling to RT at a rate of 15°C/s |
L |
Reheating to 900°C, then cooling to RT at a rate of 20°C/s, reheat-int to 780°C, cooling
to RT at a rate of 3°C/s, and tempering with heating to 400°C |
M |
Following hot rolling, cooling from 830°C to 600°C at a rate of 20°C/s |
N |
Following hot rolling, cooling from 800°C to 450°C at a rate of 25°C/s and then tempering
with heating to 700°C |
O |
Following hot rolling, cooling from 650°C to RT at a rate of 30°C/s |
P |
Following hot rolling, cooling from 750°C to RT at a rate of 30°C/s and then reheating
to 650°C and cooling to RT at a rate of 25°C/s |
Table 4
Manufacturing
conditions |
Cyclic softening parameter
σ15/σ1 |
Fatigue crack
growth rate
(x10-5mm/cycle) |
Tensile
Test |
Charpy
impact
test |
A |
0.852 |
2.42 |
○ |
○ |
B |
0.911 |
3.21 |
○ |
○ |
C |
0. 831 |
1.98 |
○ |
○ |
D |
0.912 |
3.05 |
○ |
○ |
E |
0.875 |
2.61 |
○ |
○ |
F |
0.891 |
2.97 |
○ |
○ |
G |
0.860 |
2.85 |
○ |
○ |
H |
0.855 |
2.47 |
○ |
○ |
I |
1.105 |
6.24 |
Δ |
× |
J |
1.011 |
5.03 |
Δ |
Δ |
K |
0.976 |
4.98 |
○ |
× |
L |
0.987 |
5.01 |
Δ |
× |
M |
0.979 |
4.95 |
○ |
○ |
N |
0.981 |
4.96 |
Δ |
× |
O |
0.993 |
5.02 |
Δ |
○ |
P |
0.984 |
4.88 |
○ |
× |
[0072] As can be seem from Table 4, Steel Plates A to H in which the manufacturing conditions
fell within the range defined in the present invention had a cyclic softening parameter
which was in the proper range of 0.65 - 0.95 and a fatigue crack growth rate which
was below the target maximum value of 4.0×10
-5 mm/cycle. Their tensile strength and impact properties were also good.
[0073] In contrast, Steel Plates I to P in which the manufacturing conditions were outside
the range defined in the present invention had a cyclic softening parameter of greater
than 0.95, and their fatigue crack growth rate did not reach the target value. In
addition, except for Steel Plate M, they were not adequate at least one of tensile
strength and impact strength.
(Example 3)
[0074] A corrosion fatigue crack growth test and a corrosion fatigue were performed on nine
steels in Table 2, i.e., Steels Nos. 3, 4, 5, 7, 12, 15, 18, 25, and 27.
[0075] The corrosion fatigue crack growth test was conducted in seawater at room temperature.
The test pieces used in this test were CT specimens having the same shape as described
in Example 1. The difference from the fatigue crack growth test in air was that the
cycle rate was 0.17 Hz in order to match with the cycle of waves in the sea. The stress
ratio was set at 0.1, which was the same as in the fatigue crack growth test in air.
[0076] The corrosion fatigue test was performed in five environments: sea water at room
temperature, seawater at 60°C, an aqueous saturated chlorine solution at room temperature,
a 1% saline solution (aqueous sodium chloride solution) at room temperature, and a
3% saline solution at room temperature. The seawater used herein means artificial
seawater prescribed in ASTM specifications. Room temperature indicates that the test
was conducted without temperature control, while 60°C indicates that the temperature
was controlled so as to maintain that temperature by means of a thermostat. The two
saline environments were prepared in order to show the effect of sodium chloride alone
on corrosion fatigue strength, and the 3% saline environment corresponds to a seawater
environment containing about 3.5% sodium chloride.
[0077] With respect to the environment of seawater at room temperature which was selected
as the standard testing environment for a corrosion fatigue test, the test was also
performed in different load modes or with a test piece having a differently worked
test surface to evaluate the effects of these parameters.
[0078] Like a corrosion fatigue crack growth rate, corrosion fatigue strength also greatly
depends on the cycle rate, and the lower the cycle rate, the more significantly the
corrosion fatigue strength decreases. Therefore, the cycle rate of repeated loads
in the corrosion fatigue test was the same rate of 0.17 Hz in all the test runs so
as to match with the cycle of wave loads in the sea. The stress ratio was set at 0.1,
which is the standard value most widely employed in a fatigue test.
[0079] The load modes used in the corrosion fatigue test were three modes of axial force,
bending, and torsion, in which axial force load was the standard load mode.
[0080] A plate test piece was used in the axial force load mode and bending load mode of
the corrosion fatigue test. The width of the test piece was 80 mm in the grip portions
and 25 mm in the test portion, the width decreasing smoothly in a curve of R100 from
the grip portions to the test portion. The thickness of the test piece was 12 mm in
the grip portions and 6 mm in the test portion, the thickness decreasing smoothly
in a curve of R40 from the grip portions to the test portion.
[0081] The test pieces used in the torsional load mode were in the shape of an axially symmetrical
round bar which had a diameter of 12 mm in the grip portions and 6 mm in the test
portion.
[0082] The test surface was worked by machining, plasma arc cutting, or laser cutting, with
machining being the standard.
[0083] When the corrosion fatigue strength was evaluated by causing a fatigue crack on a
machined surface, the finish machining of a test surface was performed in such a way
that the maximum height of surface roughness was between 1.6 and 6.3 µM along a length
of 8 mm.
[0084] In the case of evaluating the corrosion fatigue strength originating at a plasma
arc-cut surface, the plasma arc cutting technique was applied when the flat surface
of a test piece was cut out. In order to ensure that a fatigue crack originated at
the plasma arc-cut surface, the corners in the cross section of the test piece had
been chamfered with a curve of R1 with a baby grinder. The plasma arc cutting was
performed under the following conditions:
Conditions for plasma arc cutting:
[0085]
electric current: 240 A, voltage: 110 V, cutting speed: 1000
mm/minute, electrodes: tungsten electrodes, gas: mixed H2-N2-Ar gas.
[0086] When the corrosion fatigue strength was evaluated on a laser-cut surface, the cut
surface was prepared by laser cutting under the following conditions. Also in this
case, in order to ensure that a fatigue crack originated at the cut surface, the same
chamfering as in the plasma arc-cut surface was employed.
Conditions for laser cutting:
[0087]
CO2 laser, output: 40 kW (continuous), position: lateral, cutting speed: 2.5 m/min,
focal distance: 381 mm (parabolic condensing), defocusing amount: +8 mm.
[0088] In the corrosion fatigue test, for each test run, i.e., for each combination of a
steel and a testing condition, 6 to 8 test pieces were tested to prepare an S-N curve
(fatigue strength diagram), on which the corrosion fatigue strength was determined
to be the fatigue strength Δσ (stress range: maximum stress minus minimum stress)
at a finite life of 1 × 10
6 cycles at fatigue fracture.
[0089] The results of the corrosion fatigue crack growth test and the corrosion fatigue
test are shown in Tables 5 and 6, respectively. For reference, the values of cyclic
softening parameter and fatigue crack growth rate in air shown in Table 2 are also
included in Table 5.
Table 5
Steel
No. |
Cyclic
softening
parameter
σ15/σ1 |
Fatigue crack growth
rate (x10-5 mm/cycle) |
Environmental
acceleration of
fatigue crack
growth rate |
Remarkks |
|
|
Air |
Seawater |
|
|
3 |
0.889 |
3.16 |
17.38 |
5.5 |
Inventive |
4 |
0.863 |
2.87 |
17.79 |
6.2 |
" |
5 |
0.820 |
1.92 |
9.22 |
4.8 |
" |
7 |
0.811 |
1.72 |
9.98 |
5.8 |
" |
12 |
0.712 |
0.83 |
4.23 |
5.1 |
" |
15 |
0.980* |
4.95 |
24.26 |
4.9 |
Comparative |
18 |
1.092* |
5.95 |
36.30 |
6.1 |
" |
25 |
1.120* |
6.32 |
34.76 |
5.5 |
" |
27 |
1.052* |
5.67 |
33.45 |
5.9 |
" |
*Outside the range defined herein. |

[0090] As can be seen from Table 5, although the growth rate of corrosion fatigue cracks
(fatigue crack growth rate in seawater) was higher than the fatigue crack growth rate
in air for all the steel materials, the degree of acceleration of growth rate of corrosion
fatigue cracks relative to the fatigue crack growth rate in air (i.e., environmental
acceleration of fatigue crack growth rate) was almost constant and scarcely depended
on the steel type. Thus, it was confirmed that the growth rate of corrosion fatigue
cracks of a steel can be suppressed by suppressing its fatigue crack growth rate in
air.
[0091] From the results of the corrosion fatigue test in Table 6, it can be seen that the
axial force fatigue crack strength originating at the machined surface in seawater
at room temperature was over 400 MPa and was thus excellent for Steels Nos. 3, 4,
5, and 7 according to the present invention, but it was as low as 310 MPa for Steels
Nos. 15, 18, 25, and 27 which were comparative examples having a cyclic softening
parameter of greater than 0.95. Comparing those steels having the full data (No. 3
and 25), the corrosion fatigue strength of the steel according to the present invention
(No. 3) was significantly superior to that of the comparative steel (No. 25) in all
the testing environments, in all the test surfaces, and in all the load modes.
[0092] Upon observation of the fracture surface and its neighborhood of each test piece
after the corrosion fatigue test, no apparent differences in the shape and size of
corrosion pits were found between the steels according to the present invention and
the comparative steels. However, by measuring the micro hardness in the vicinity of
the bottom of a corrosion pit, all the steels according to the present invention had
a lower micro hardness than the comparative steels. It is thought that due to its
cyclic softening properties, a steel according to the present invention has a decreased
hardness, which favorably affects the formation of fatigue cracks in a corrosive environment.