Technical Field
[0001] The present invention relates to a high-strength steel plate having excellent weldability
and a method of manufacturing the same. In particular, the present invention relates
to a high-strength steel plate which is used as a structural member of a construction
machine or an industrial machine, has a yield strength of equal to or more than 885
MPa, a tensile strength of equal to or more than 950 MPa and equal to or less than
1130 MPa, and generally has a thickness of equal to or more than 6 mm and equal to
or less than 25 mm, and a method of manufacturing the same.
Priority is claimed on Japanese Patent Application No.
2010-248032, filed November 5, 2010, the content of which is incorporated herein by reference.
Background Art
[0002] Recently, there is a tendency to increase the size of construction machines such
as cranes and concrete pumping vehicles more and more with high-rise buildings. In
order to suppress an increase in weight with the increase in size of the construction
machine, needs for a lightweight structural member have been increased, and a demand
for so-called 100-kg/mm
2 steel class high-strength steel (for example, yield strength of equal to or more
than 885 MPa and tensile strength of equal to or more than 950 MPa) is tendency to
further increase. On the other hand, since a large additional amount of alloy elements
is added to the high-strength steel, preheating is generally performed in order to
avoid weld cracking at the time of performing a welding operation. However, there
has been a demand for steel which does not require preheating to perform a more effective
welding operation.
[0003] Since weld crack sensitivity is very largely influenced by diffusible hydrogen, it
is preferable that the diffusible hydrogen content in weld metal be suppressed to
be low. However, for example, in order to particularly suppress the diffusible hydrogen
content in a carbon dioxide arc welding operation which is widely used for welding
a structural member of a construction machine or an industrial machine, various management
including a management of lubricant oil and a cleaning of a groove surface of a welding
wire and the like, as well as a selection and a management of welding material, is
necessary so that hydrogen is not mixed in at the time of performing the welding operation,
and thereby a load in the operation is increased. Therefore, even when approximately
3.0 to 5.0 ml/100g of the diffusible hydrogen content in the carbon dioxide arc welding,
which is thought to be mixed in when welding operation management is slightly insufficient,
is contained in the steel, it is preferable for steel to have a sufficiently low crack
sensitivity in which cracking is not generated when welding is performed without preheating.
[0004] In a general strength standard of a 100-kg/mm
2 steel class steel plate, while yield strength is equal to or more than 885 MPa, and
an upper limit of yield strength is not present, tensile strength is, for example,
in a range of equal to or more than 950 MPa and equal to or less than 1130 MPa and
an upper limit of tensile strength is present. A steel plate used for a construction
machine or the like is usually bent and when the tensile strength of the steel plate
exceeds a specified upper limit, a load which is necessary for bending work is increased.
For this reason, it is necessary that the tensile strength of a steel plate not be
excessively increased in consideration of a case where work is limited due to facility
capacity.
[0005] For example, with respect to the high-strength steel plate having a yield strength
of 885 MPa-class, high tensile strength steel plates having a tensile strength of
950 MPa-class are disclosed in Patent Documents 1 and 2. However, these steel plates
are relatively thick and used for a penstock. Due to this, a large amount ofNi is
added to these steel plates as an essential element in order to secure toughness without
particularly considering bending workability and thereby, the steel plates are not
economical for use in a construction machine.
[0006] In Patent Document 3, high tensile strength steel having excellent weldability and
economic efficiency is disclosed. In the technology, a weld crack sensitivity index
Pcm is suppressed to be equal to or less than 0.29 so that weldability is secured.
However, the lowest crack stopping preheating temperature is 100°C in a y-groove weld
cracking test, and it is thought that weldability cannot be secured in welding without
preheating.
[0007] In Patent Document 4, a technology relating to high tensile strength steel having
excellent weldability and arrestability is disclosed. In the technology, it is necessary
to add Ni in order to secure toughness, and the steel plate is not economical for
use in a construction. In addition, while cracking is not generated in a y-groove
weld cracking test even without preheating, the diffusible hydrogen content is 1.2
ml/100g under the test conditions. Due to this, in this case, it is estimated that
a load is increased at the time of performing a welding operation to manage the diffusible
hydrogen content of weld metal.
[0008] In Patent Document 5, a technology relating to high tensile strength steel having
excellent weldability and HIC resistance is disclosed. In the technology, it is necessary
to add Ni and 0.6% or more of Mo in order to secure toughness, and the steel plate
is not economical for use in a construction machine. In addition, while cracking is
not generated in a y-groove weld cracking test even without preheating, the diffusible
hydrogen content is limited to 1.5 ml/100g under the test conditions. Due to this,
in this case, it is estimated that a load is increased at the time of performing a
welding operation to manage the diffusible hydrogen content in weld metal.
[0009] In Patent Document 6, a method in which a steel plate having a tensile strength exceeding
980 MPa is manufactured in a non-thermal refining manner is disclosed. In the method,
it is necessary to add a large amount of alloy elements such as 1.5% or more of Mn
in the steel in order to secure the tensile strength exceeding 980 MPa with a very
small amount of C which is equal to or less than 0.025%, and particularly, when the
amount of Mn is large, there is concern that the cracking sensitivity of a segregation
portion is degraded. However, there is no estimated value of weldability and excellent
weldability cannot be expected.
[0010] In Patent Document 7, a hot rolled steel sheet having a tensile strength of 950 MPa
or more in which bending workability and weldability are considered is disclosed.
Since it is necessary to add a large amount of Ti in the hot rolled steel sheet, it
is thought that weldability is degraded. In addition, since it is necessary to add
Ni in order to compensate for a decrease in toughness due to the addition of the large
amount of Ti, there is a problem in economic efficiency.
[0011] In Patent Document 8, a method of manufacturing a steel plate which is mainly used
for a line pipe and has a tensile strength of 950 MPa or more and excellent toughness
and weldability is disclosed. Since it is necessary that the amount of Mn is equal
to or more than 1.8%, there is concern that the cracking sensitivity of a segregation
portion is degraded, and since low temperature rolling is necessary in a ferrite-austenite
two-phase region, productivity is low.
Citation List
Patent Document
[0012]
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No.
H10-265893
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No.
H08-269542
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No.
H06-158160
[Patent Document 4] Japanese Unexamined Patent Application, First Publication No.
H11-36042
[Patent Document 5] Japanese Unexamined Patent Application, First Publication No.
H11-172365
[Patent Document 6] Japanese Unexamined Patent Application, First Publication No.
2004-84019
[Patent Document 7] Japanese Unexamined Patent Application, First Publication No.
H05-230529
[Patent Document 8] Japanese Unexamined Patent Application, First Publication No.
H08-269546
Summary of Invention
Technical Problem
[0013] An object of the present invention is to economically provide a high-strength steel
plate having excellent weldability which is used as a structural member of a construction
machine or an industrial machine, has a yield strength of equal to or more than 885
MPa, a tensile strength of equal to or more than 950 MPa and equal to or less than
1130 MPa, and generally has a thickness of equal to or more than 6 mm and equal to
or less than 25 mm, and a method of manufacturing the same.
Solution to Problem
[0014] The summary of the present invention is described as follows:
- (1) A high-strength steel plate according to an aspect of the present invention has
a chemical composition containing, % by mass: C: 0.05% or more and less than 0.10%;
Si: 0.20% or more and 0.50% or less; Mn: 0.20% or more and less than 1.20%; Cr: 0.20%
or more and 1.20% or less; Mo: 0.20% or more and 0.60% or less; Nb: 0.010% or more
and 0.050% or less; Ti: 0.005% or more and 0.030% or less; Al: 0.01 % or more and
0.10% or less; B: 0.0003% or more and 0.0030% or less; V: 0% or more and 0.10% or
less; Cu: 0% or more and 0.50% or less; and Ca: 0% or more and 0.0030% or less; and
limited to: Ni: 0.1% or less; P: 0.012% or less; S: 0.005% or less; and N: 0.0080%
or less; and a balance consisting of Fe and inevitable impurities, wherein Pcm defined
by a following (Formula 1) is equal to or less than 0.22%, A defined by a following
(Formula 2) is equal to or less than 2.0, a sum of a structural fraction of lower
a bainite and a structural fraction of a martensite is equal to or more than 90%,
the structural fraction of the lower bainite is equal to or more than 70%, an aspect
ratio of a prior austenite grain is equal to or more than 2, a yield strength is equal
to or more than 885 MPa, and a tensile strength is equal to or more than 950 MPa and
equal to or less than 1130 MPa.


where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are % by mass of C, Si,
Mn, Cu, Ni, Cr, Mo, V, and B in a chemical composition, respectively.
- (2) In the high-strength steel plate according to (1), a number density of a cementite
which is equal to or more than 50 nm may be equal to or less than 20 pieces/µm3.
- (3) In the high-strength steel plate according to (1) or (2), a thickness may be equal
to or more than 6 mm and equal to or less than 25 mm.
- (4) A method of manufacturing a high-strength steel plate according to an aspect of
the present invention includes heating a steel whose chemical composition contains,
% by mass: C: 0.05% or more and less than 0.10%; Si: 0.20% or more and 0.50% or less;
Mn: 0.20% or more and less than 1.20%; Cr: 0.20% or more and 1.20% or less; Mo: 0.20%
or more and 0.60% or less; Nb: 0.010% or more and 0.050% or less; Ti: 0.005% or more
and 0.030% or less; Al: 0.01% or more and 0.10% or less; B: 0.0003% or more and 0.0030%
or less; V: 0% or more and 0.10% or less; Cu: 0% or more and 0.50% or less; and Ca:
0% or more and 0.0030% or less, and limited to: Ni: 0.1% or less; P: 0.012% or less;
S: 0.005% or less; and N: 0.0080% or less; and a balance consisting of Fe and inevitable
impurities, in which Pcm defined by a following (Formula 3) is equal to or less than
0.22%, and A defined by a following (Formula 4) is equal to or less than 2.0, to 1100°C
or greater; performing hot rolling on the steel so that a cumulative rolling reduction
ratio in a non-recrystallization temperature region is equal to or more than 60%;
and performing on-line accelerated cooling on the steel from a temperature of equal
to or more than Ar3 to a temperature of equal to or less than 450°C and equal to or
more than 300°C at a cooling rate of equal to or more than 10°C/s and performing air
cooling after stopping the accelerated cooling.


where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are % by mass of C, Si,
Mn, Cu, Ni, Cr, Mo, V, and B in a chemical composition, respectively.
Advantageous Effects of Invention
[0015] According to the present invention, it is possible to economically provide a high-strength
steel plate having excellent weldability which is used as a structural member of a
construction machine or an industrial machine, has a yield strength of equal to or
more than 885 MPa, and generally has a thickness of equal to or more than 6 mm and
equal to or less than 25 mm.
Brief Description of Drawings
[0016]
FIG. 1 is a graph showing a relationship between Pcm and a crack stopping preheating
temperature in a y-groove weld cracking test.
FIG. 2 is a graph showing a relationship between a structural fraction of lower bainite
and a yield ratio.
FIG. 3 is a graph showing a relationship between an A value and a structural fraction
of lower bainite.
FIG. 4 is a flowchart schematically showing a method of manufacturing a high-strength
steel plate according to an embodiment of the present invention.
Description of Embodiments
[0017] In order to decrease weld crack sensitivity, it is known that it is effective to
decrease a weld crack sensitivity index Pcm. The inventors examined to what extent
the Pcm needs to be reduced not to generate weld cracking without preheating even
when approximately 3.0 to 5.0 ml/100g of the diffusible hydrogen content, which is
thought to be mixed when welding operation management was insufficient in carbon dioxide
arc welding, is contained in steel. A y-groove weld cracking test (a weld heat input
of 1.7 kJ/mm) specified by JIS Z3158 (1993) was performed on steel materials having
various chemical compositions with temperature and humidity being adjusted. All testing
materials had a thickness of 25 mm, and the test was necessarily performed on two
testing materials under the same condition. One of the two testing materials was used
as a testing material for analyzing hydrogen content, and immediately after the weld
cracking test, a sample was obtained from the testing material to measure a diffusible
hydrogen content using gas chromatography. A cracking presence evaluation test was
performed on the other testing material only when the diffusible hydrogen content
exceeded 5.0 ml/100g as a result of the analysis. The relationship between Pcm and
a cracking prevention preheating temperature of steel shown FIG. 1 was obtained from
the obtained result. That is, influences of the Pcm and preheating temperature of
the steel on the presence of cracking are shown in FIG. 1. From FIG. 1, it is found
that when the Pcm is decreased to equal to or less than 0.22% and the diffusible hydrogen
content is in a range of 5.1 to 6.0 ml/100g, cracking is not generated under the condition
of non-preheating (test temperature of 25°C).
[0018] In the related art, a 100-kg/mm
2 steel class steel plate is manufactured by a quenching and tempering process, and
generally contains tempered martensite as a main structure. However, it is not easy
to obtain the strength of 100-kg/mm
2 class steel in a case where a main structure is tempered martensite in a component
composition (chemical composition) that satisfies a low Pcm of equal to or less than
0.22%. A simple method for obtaining high strength with such low Pcm is that a martensite
structure is not subjected to tempering, that is, a martensite structure in an as-quenching
state is used. However, since the martensite structure in the as-quenching state has
many mobile dislocations, a yield ratio (yield strength/tensile strength) is low,
and when there is an attempt to secure the yield strength specified by the standard,
the tensile strength is forced to increase by all means. In the standard values of
the strength of 100-kg/mm
2 class steel according to the JIS standard, the yield strength is equal to or more
than 885 MPa and the tensile strength is equal to or more than 950 MPa and equal to
or less than 1130 MPa. When a target lower limit value of yield strength is set to
915 MPa, and a target upper limit value of tensile strength is set to 1100 MPa in
consideration of dispersions in quality (strength) in manufacturing with respect to
the above-described standard values, it is thought that the yield ratio (yield strength/tensile
strength) of equal to or more than 83% is a necessary condition. It is difficult to
obtain this yield ratio in the martensite structure in the as-quenching state. As
a result of variously examining the relationship between the structure and strength,
the inventors concluded that it is effective that, in order to obtain a high yield
ratio in the as-quenching state, the structure in the as-quenching state is controlled
to a structure which is mainly composed of lower bainite and the structural fraction
of the martensite is lowered.
[0019] In addition, the inventors specifically investigated to the relationship among a
structural fraction, strength and yield ratio of steel having various component compositions
in which C content is equal to or more than 0.05% and less than 0.10%, and Pcm is
equal to or less than 0.22%. As a result, first, in order to secure a yield strength
of equal to or more than 885 MPa, it becomes apparent that it is necessary that the
sum of the structural fraction of the lower bainite (lower bainite fraction) and the
structural fraction of the martensite (martensite fraction) be equal to or more than
90% (structural fractions of upper bainite and ferrite are less than 10%). Furthermore,
it was found that it is necessary for the steel plate structure to be a structure
which is mainly composed of lower bainite (lower bainite single phase structure or
a mixed structure of lower bainite and martensite) in order to satisfy a yield ratio
of equal to or more than 83%, specifically, the structural fraction of the lower bainite
included in the steel plate structure is equal to or more than 70% (FIG. 2). In addition,
in FIGS. 2 and 3 described later, a steel plate having a thickness of 6 to 25 mm in
which the sum of the fraction of lower bainite and the fraction of martensite is equal
to or more than 90% is used and the structure is controlled by stopping water cooling
at 300 to 450°C in the steel plate.
[0020] Next, the inventors examined a method of stably controlling the structure of the
steel plate in a structure which is mainly composed of lower bainite. For example,
although a cooling rate at the time of quenching is controlled to be in a predetermined
range and lower bainite can be obtained, a range of cooling rates to obtain lower
bainite is generally narrow and such controlling of the cooling rate is industrially
inadvisable. As a manufacturing process of stably and simply obtaining a structure
which is mainly composed of lower bainite, it is effective to stop water cooling at
an appropriate temperature in the middle of cooling, and slow down the cooling rate
by using air cooling thereafter, instead of accelerated cooling to room temperature
during quenching. When a water cooling stopping temperature (steel plate temperature
to transition from water cooling to air cooling) is lower than 300°C, the martensite
fraction is excessively high. Contrarily, when the water cooling stopping temperature
is higher than 450°C, upper bainite is easily formed. Therefore, it is desirable that
the water cooling stopping temperature be equal to or more than 300°C and equal to
or less than 450°C.
[0021] The inventors investigated in detail the relationship between the structural fraction
and strength of the steel in which the sum of the structural fraction of martensite
and the structural fraction of lower bainite is equal to or greater than 90% by manufacturing
the steel plate having a thickness of 6 to 25 mm under the condition in which the
water cooling stopping temperature is equal to or more than 300°C and equal to or
less than 450°C with respect to steel grades of various component compositions in
which C content is equal to or more than 0.05% and less than 0.10% and Pcm is equal
to or less than 0.22%.
As a result, since Mn and Ni have an effect of suppressing the lower bainite transformation,
particularly in a process of stopping the water cooling in the middle of the cooling,
it become apparent that Mn and Ni have a strong tendency to decrease the structural
fraction of the lower bainite, to increase the structural fraction of the martensite
when the water cooling stopping temperature is low, and to increase the structural
fraction of the upper bainite (upper bainite fraction) when the water cooling stopping
temperature is high. In addition, it is confirmed that Mo and V have a strong tendency
to suppress the formation of ferrite and upper bainite and to increase the structural
fraction of the lower bainite. Therefore, it is found that it is very effective to
suppress the content of Mn and Ni and increase the content of Mo and V in order to
stably obtain the structure which is mainly composed of lower bainite in the process
of stopping the water cooling in the middle of the cooling. Specifically, when A (A
value) defined by the following (Formula 6) is adjusted to be equal to or less than
2.0 in addition to the component composition conditions in which the C content is
equal to or more than 0.05% and less than 0.10%, and Pcm defined by the following
(Formula 5) is equal to or less than 0.22%, and the sum of the structural fraction
of the martensite and the structural fraction of the lower bainite is equal to or
more than 90%, it is found that the structure having lower bainite fraction of equal
to or more than 70% is reliably obtained (FIG. 3).
[0022] Since such a structure which is mainly composed of lower bainite is obtained and
the yield ratio is equal to or more than 83%, it is possible to stably satisfy the
lower limit of the yield strength (885 MPa) and the upper limit of the tensile strength
(1130 MPa) in consideration of a certain degree of dispersions in strength.

In the Formulas, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are respectively
% by mass of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B in the chemical composition.
[0023] Hereinafter, a high-strength steel plate according to an embodiment of the present
invention will be described in detail.
First, the reason to limit a component in steel of the present embodiment is described.
Hereinafter, "%" represents "% by mass".
C is an important element that has a significant effect on the strength of steel of
the present embodiment which has a structure which is mainly composed of lower bainite.
In order to obtain a yield strength of equal to or more than 885 MPa, it is necessary
for the C content to be equal to or more than 0.05%, and preferably equal to or more
than 0.055% or equal to or more than 0.060%. However, when the C content is equal
to or more than 0.10%, the tensile strength is excessively high. Therefore, the C
content is less than 0.10%, and desirably equal to or less than 0.095% and equal to
or less than 0.090%.
[0024] Since Si suppresses cementite from coarsening during slow cooling after stopping
water cooling in the process of stopping the water cooling in the middle of the cooling
described later, it is advantageous that Si content is high to obtain high strength.
For this reason, the Si content is equal to or more than 0.20% and desirably equal
to or more than 0.25% or equal to or more than 0.30%. However, when Si is excessively
added to steel, there is a concern that toughness thereof may be degraded, and the
upper limit of the Si content is 0.50%, and desirably 0.45% or 0.40%.
[0025] Mn is an element which is effective in improving strength by improving hardenability.
For this reason, Mn content is equal to or more than 0.20%, desirably equal to or
more than 0.30% or equal to or more than 0.50%. However, Mn has an effect of suppressing
lower bainite transformation, particularly in the process of stopping the water cooling
in the middle of the cooling, it becomes apparent that Mn has a strong tendency to
decrease the structural fraction of the lower bainite, to increase the structural
fraction of the martensite when the water cooling stopping temperature is low, and
to increase the upper bainite fraction when the water cooling stopping temperature
is high. In particular, when the Mn content is equal to or more than 1.20%, it is
difficult to obtain a yield ratio of equal to or more than 83%, and therefore the
Mn content is less than 1.20% and desirably 1.00% or equal to or less than 0.90%.
[0026] Since Cr is effective in improving strength by improving hardenability, Cr content
is equal to or more than 0.20% and desirably equal to or more than 0.25%, or equal
to or more than 0.30%. However, when Cr is excessively added to steel, weldability
is degraded and thereby, the Cr content is equal to or less than 1.20%, and desirably
equal to or less than 1.10% or equal to or less than 1.00%.
[0027] Mo is effective for stably forming lower bainite in the process of stopping the water
cooling in the middle of the cooling described later by suppressing the ferrite formation.
For this reason, it is necessary that the Mo content be equal to or more than 0.20%
and it is preferable to be equal to or more than 0.25% or equal to or more than 0.30%.
However, when a large amount of Mo is added to steel, weldability is deteriorated
and also, Mo is an expensive element. Therefore, the Mo content is equal to or less
than 0.60%, and desirably equal to or less than 0.58% or equal to or less than 0.55%.
[0028] Since Ni has an effect of suppressing lower bainite transformation similar to Mn,
particularly in the process of stopping the water cooling in the middle of the cooling,
Ni has a strong tendency to decrease the structural fraction of the lower bainite
and to increase the structural fraction of the martensite when the water cooling stopping
temperature is low, and to increase the upper bainite fraction when the water cooling
stopping temperature is high. For this reason, when Ni is added to steel, it is difficult
to obtain a yield strength of equal to or more than 83%. Therefore, Ni is not intentionally
added to steel, and Ni content is suppressed to be in a range in which Ni is inevitably
contained in the steel. Specifically, the upper limit of the Ni content is 0.1%, and
desirably 0.05% or 0.04%. The lower limit of the Ni content does not need to be particularly
limited and is 0%. In addition, when Cu is added to steel as a selective element,
Ni whose content is 0.5 times or more of Cu content may be added to steel while limiting
the Ni content to less than the above-described Ni content.
[0029] Nb forms fine carbide during rolling and widens a non-recrystallization temperature
region to enhance a controlled rolling effect so that Nb improves toughness by grain
refining. Therefore, Nb content is equal to or more than 0.010% and desirably equal
to or more than 0.015% or equal to or more than 0.020%. However, when Nb is excessively
added to steel, weldability is deteriorate and thereby, the Nb content is equal to
or less than 0.050%, and desirably equal to or less than 0.045% or equal to or less
than 0.040%.
[0030] In the present embodiment, B is used in order to secure suitable hardenability to
obtain the lower bainite structure. In order to obtain the suitable hardenability,
it is necessary to secure free B at the time of direct quenching. Since N forms BN
to decrease the free B, a suitable amount of Ti is added to steel to not form BN and
N is fixed as TiN.
[0031] Ti is contained in steel to fix N as TiN. That is, Ti content in the steel is equal
to or more than 0.005% and desirably 0.010% or equal to or more than 0.012%. However,
since an excessive addition of Ti degrades weldability in some cases, the upper limit
of the Ti content is 0.030%, and desirably 0.025% or 0.020%.
[0032] B has an effect for improving hardenability of steel, and it is necessary that B
content is equal to or more than 0.0003% to exhibit the effect, and preferably equal
to or more than 0.0005% or equal to or more than 0.0010%. However, when B whose content
exceeds 0.0030% is added to steel, weldability and toughness are degraded. Therefore,
the B content is equal to or less than 0.0030%, and desirably equal to or less than
0.0025% or equal to or less than 0.0020%.
[0033] When N is excessively contained in steel, BN is formed as described above so that
the hardenability improvement effect of B is inhibited and toughness is degraded.
Therefore, N content is suppressed to be equal to or less than 0.0080%, and desirably
equal to or less than 0.0060% or equal to or less than 0.0050%. In addition, since
N is inevitably contained in steel, the lower limit of the N content does not need
to be particularly limited, and is 0%.
[0034] Al is added to steel as a deoxidizing material, and Al content in the steel is generally
equal to or more than 0.01%. However, since an excessive addition of Al degrades toughness
in some cases, the upper limit of the Al content is 0.10%, and desirably 0.08% or
0.05%.
[0035] P is a harmful element that degrades toughness. Therefore, P content is suppressed
to be equal to or less than 0.012%, and desirably equal to or less than 0.010% or
equal to or less than 0.008%. In addition, since P is an inevitable impurity, the
lower limit of the P content does not need to be particularly limited and is 0%.
[0036] Since S is a harmful element that degrades bending workability by forming MnS, it
is desirable to decrease the S content as much as possible. Therefore, the S content
is suppressed to be equal to or less than 0.005%, and desirably equal to or less than
0.004% or equal to or less than 0.003%. In addition, since S is an inevitable impurity,
the lower limit of the S content does not need to be particularly limited and is 0%.
[0037] The elements described above are basic components (basic elements) of the steel according
to the present embodiment, and the chemical composition containing the basic elements
and composed of a balance Fe and inevitable impurities is a basic composition of the
present embodiment. However, in addition to the basic composition (instead of a part
of the balance Fe), the following elements (selective elements) may be further contained
in the present embodiment as needed. In addition, even when these selective elements
are inevitably mixed, the effect of the present embodiment is not impaired.
[0038] In other word, one or more kinds selected from V, Cu, and Ca can be added to the
steel as the selective element, in addition to the basic components.
Since V enhances a hardenability, has a precipitation strengthening effect of a tempered
martensite structure or a tempered bainite structure and is effective in improving
strength, V may be added as needed. However, since a large amount of V is added to
inhibit weldability in some cases, and V is an expensive element, a V content is equal
to or less than 0.10%, and desirably equal to or less than 0.090% or equal to or less
than 0.080%. In addition, in order to reduce the alloy cost, it is unnecessary to
intentionally add V to the steel, and the lower limit of the V content is 0%.
Cu is an element that improves strength by solid-solution strengthening, and Cu may
be added as needed. For example, Cu can be added to steel so that a Cu content is
equal to or more than 0.05%. However, when a large amount of Cu is added, the effect
of improving the strength by solid-solution strengthening reaches an upper limit.
Due to this, the Cu content is equal to or less than 0.50%, and desirably equal to
or less than 0.40% or equal to or less than 0.30%. Moreover, since Cu is an expensive
element, it is unnecessary to intentionally add Cu to the steel, and the lower limit
of the Cu content is 0% to reduce the alloy cost.
Ca has an effect of reducing a decrease in bending workability due to MnS by spheroidizing
a sulfide of a steel plate, and Ca may be added to steel as needed. In addition, Ca
is added to steel to achieve the object, and 0.0001% or more of Ca may be contained
in the steel. However, since a large amount of Ca is added to degrade weldability
in some cases, the upper limit of the Ca content is equal to or less than 0.0030%,
and desirably equal to or less than 0.0020% or equal to or less than 0.0010%. Furthermore,
it is unnecessary to intentionally add Ca to the steel, and the lower limit of the
Ca content is 0% to reduce alloy cost.
[0039] As described above, the high-strength steel plate of the present embodiment contains
the above-mentioned basic elements and has the chemical composition composed of the
balance Fe and inevitable impurities, or contains the above-mentioned basic elements,
one or more kinds selected from the above-mentioned selective element and has the
chemical composition composed of the balance Fe and inevitable impurities.
[0040] In addition to the condition of the respective element content ranges, the component
composition is adjusted so that the Pcm defined in the above (Formula 5) is equal
to or less than 0.22% in order to secure sufficient weldability as described above.
As described above, under the condition of Pcm of equal to or less than 0.22%, the
sum of the martensite fraction and the lower bainite fraction in the steel plate structure
is equal to or more than 90%, and the lower bainite fraction needs to be equal to
or more than 70% to satisfy a yield ratio of equal to or more than 83%. In order to
stably and easily obtain the structure which is mainly composed of lower bainite,
the component composition is adjusted so that A (A value) defined by the above (Formula
6) is equal to or less than 2.0.
Here, when V and Cu, which are selective elements, are not contained in the steel,
Pcm and "A" are respectively defined by the following (Formula 7) and (Formula 8).
The (Formula 7) and (Formula 8) correspond to the above (Formula 5) and (Formula 6)
respectively.

In the above (Formula 5) to (Formula 8), when each element (for example, V, Cu and
Ni) corresponding to each variable in the formulas is not contained in the steel,
the variable is substituted with 0.
A component composition satisfying the respective element content ranges and the conditions
of Pcm and A is the component composition of the present embodiment.
[0041] Next, the steel structure of the present embodiment will be described.
As described above, the sum of the martensite fraction and the lower bainite fraction
is equal to or more than 90%, and the lower bainite fraction needs to be equal to
or more than 70% to satisfy a yield ratio of equal to or more than 83% while weldability
required for general welding operation management is secured.
Here, in the lower bainite, a large amount of fine cementite is present at interfaces
between ferrite laths or inside ferrite lath. Since the fine cementite increases yield
strength and, particularly, cementite having a diameter (equivalent circle diameter)
of about 1 to 10 nm has a great yield strength improvement effect, it is desirable
that a large amount of the fine cementite be present. However, it is not easy to accurately
measure cementite with a size of several nm. Meanwhile, considering that a predetermined
amount of cementite is formed in the steel according to the manufacturing conditions
such as the C content, there is a tendency that the more fine cementite there is,
the less coarse cementite there is. Here, as a result of the detailed investigation
relating to the yield strength and the size and the number density of cementite, the
inventors found that specifically, the fact that the number density of the relatively
coarse cementite having a diameter (equivalent circle diameter) of equal to or more
than 50 nm is equal to or less than 20 pieces/µm
3 in the steel plate structure is a preferable condition to contain a large amount
of the fine cementite and remarkably improve the yield strength. It is possible to
easily achieve the yield ratio of equal to or more than 83% by containing a large
amount of the fine cementite in the steel plate structure. In addition, the lower
limit of the number density of the cementite is 0 pieces/µm
3.
In addition, a base material of a steel plate with a predetermined volume is eluted
by electrolysis using an extraction replica method to prepare a sample which is obtained
by extracting cementite, and the sample is observed by a transmission electron microscope
(TEM) to obtain the number (number density) of the cementite having an equivalent
circle diameter of equal to or more than 50 nm (cementite of equal to or more than
50 nm) per unit volume.
Furthermore, an aspect ratio of prior austenite (prior austenite grain) is equal to
or more than 2 as described later. The aspect ratio of prior austenite is a ratio
(axial ratio) of a long axis length to a short axis length of the prior austenite
and an average value of each axial ratio of each prior austenite grain. Therefore,
the lower limit of the aspect ratio is 1.
[0042] A method of manufacturing the high-strength steel plate according to an embodiment
of the present invention will be described in detail. The high-strength steel plate
was manufactured by the following method using a slab (steel) in which the component
composition in the steel is adjusted by addition or the like so as to satisfy the
component composition conditions of the present embodiment. FIG. 4 schematically shows
an outline the method of manufacturing the high-strength steel plate according to
the present embodiment.
In order to sufficiently solid-solute carbides or carbonitrides of alloy elements
such as Nb which enhances a controlled rolling effect and Mo which contributes to
hardenability in the steel, the slab is heated to a temperature (heating temperature)
of equal to or more than 1100°C (S1). While the upper limit of the heating temperature
is not particularly limited, it is preferable to be 1300 °C since productivity is
decreased or the grain diameter of the austenite at the time of heating is extremely
increased.
The heated slab is subjected to hot rolling to have a target thickness so that a cumulative
rolling reduction ratio in the non-recrystallization temperature region is equal to
or more than 60% (S2). The hot-rolled slab, that is, steel plate (steel) generally
has a thickness of 6 to 25 mm, and the thickness is not necessarily limited thereto.
Here, when the cumulative rolling reduction ratio in the non-recrystallization temperature
region is equal to or more than 60%, it is possible to introduce sufficient working
strain and to appropriately control strength properties of the steel plate. In addition,
the non-recrystallization temperature region is a temperature region of equal to or
more than Ar3 and equal to or less than 960°C, in which recrystallization (reduction
of working strain) after rolling can be prevented. Moreover, the Ar3 (Ar3 transformation
point) is a temperature in which the ferrite transformation is started at the time
of cooling and can be measured by a hot working simulator manufactured by Fuji Electronic
Industrial Co., Ltd (THERMECMASTOR-Z). In the Ar3 measurement, after the steel (sample)
is heated up to 1200°C, retained for 10 minutes and cooled at 2.5°C/minute, a volume
change at the time of cooling is measured to determine Ar3 on the basis of the volume
change. The cumulative rolling reduction ratio in the non-recrystallization temperature
region is less than 100%.
On-line accelerated cooling (water cooling) is performed on the steel plate (steel)
obtained by the hot rolling after the hot rolling from the temperature of equal to
or more than Ar3 (water cooling starting temperature). Hardenability can be increased
by performing the on-line accelerated cooling, which is advantageous to decrease Pcm.
The reason that the accelerated cooling starting temperature is set to the temperature
of equal to or more than Ar3 is that ferrite or upper bainite is formed and the strength
of the steel plate is significantly degraded when the accelerated cooling is started
from the temperature of less than Ar3. After starting the accelerated cooling, the
accelerated cooling is stopped at a temperature of equal to or more than 300°C and
equal to or less than 450°C (water cooling stopping temperature), and air cooling
is performed (S3). When the water cooling stopping temperature exceeds 450°C, upper
bainite is easily formed and there is a tendency to decrease the yield strength and
the tensile strength. In addition, when the water cooling stopping temperature is
less than 300°C, the structural fraction of martensite is increased and the yield
ratio is decreased so that it is difficult for the lower limit of the yield strength
and the upper limit of the tensile strength to be compatible. Here, the accelerated
cooling (water cooling) is cooling in which an average cooling rate in 1/4t parts
of the steel plate is equal to or more than 10°C/s in a temperature region which is
equal to or more than the cooling stopping temperature and equal to or less than Ar3,
and the upper limit of the average cooling rate of the accelerated cooling is not
particularly limited. In addition, the air cooling (retained in the atmosphere) is
cooling in which an average cooling rate in 1/4t parts of the steel plate is equal
to or less than 1°C/s in a temperature region which is equal to or more than the room
temperature and less than the cooling stopping temperature, and the lower limit of
the average cooling rate of the air cooling is not particularly limited. The 1/4t
parts of the steel plate are a portion which is distant from a surface of the steel
plate in a thickness center (depth) direction by a distance of 1/4 of the thickness,
and the cooling rate of the 1/4t parts is obtained from temperature change obtained
by performing a thermal analysis. By the air cooling after the accelerated cooling,
70% or more of lower bainite can be obtained and sufficiently fine cementite can be
secured. In this case, the number density of relatively coarse cementite of equal
to or more than 50 nm is equal to or less than 20 pieces/µm
3 with respect to the most of the obtained steel plates.
In the steel plate manufactured by the present embodiment, the sum of the lower bainite
fraction and the martensite fraction is equal to or more than 90%, the lower bainite
fraction is equal to or more than 70%, and the aspect ratio of the prior austenite
is equal to or more than 2 as a property the structure of the steel plate manufactured
by the on-line accelerated cooling. In addition, it is possible to achieve the yield
strength of equal to or more than 885 MPa and the tensile strength of equal to or
more than 950 MPa and equal to or less than 1130 MPa without performing tempering
in the present embodiment.
On the other hand, when the steel plate is subjected to reheating and quenching after
finishing cooling without performing the on-line accelerated cooling, the aspect ratio
of the prior austenite in the steel plate is less than 2.0. In this case, since tempering
is necessary to secure the yield ratio, the number of processes and process time are
increased and industrially, cost is increased.
In addition, when the steel plate is wound after the accelerated cooling and left
in a coil shape, the cooling rate in the time of air cooling is significantly decreased,
and the number density of relatively coarse cementite of equal to or more than 50
nm exceeds 20 pieces/µm
2. For this reason, it is not desirable that the coil-shaped steel plate be subjected
to air cooling after the accelerated cooling and it is desirable that the steel plates
be left to be air-cooled without overlapping each other until the temperature of the
steel plate is equal to or less than 250°C. That is, until the temperature of the
steel plate is equal to or less than 250°C, it is desirable that the steel plates
be not overlapped over each other (for example, so that the surfaces of the steel
plates can be in contact with air) and be air-cooled. After the temperature of the
steel plate reaches equal to or less than 250°C, the steel plates may be air-cooled
in an overlapped manner.
Moreover, after the hot rolling, when the steel plate obtained by performing the accelerated
cooling is tempered at a high temperature, the cementite tends to be coarse so that
it is difficult to secure the sufficiently fine cementite.
Examples
[0043] Steel composition Nos. A to AP having component compositions shown in Tables 1 and
2 were smelted to obtain slabs and using the slabs, steel plates with numbers 1 to
55 having thickness of 6 to 25 mm were manufactured according to manufacturing conditions
in Tables 3 and 4. In Tables 1 and 2, when Cu, Ni, V and Ca are not intentionally
added to the steel, the amounts of these chemical components are provided with parentheses.
In addition, In Tables 3 and 4, after the accelerated cooling (water cooling) was
stopped, the steel plates were not wound and were air-cooled one by one, until the
temperature of the steel plate is 250°C.
For the steel plates Nos. 1 to 55, the structural fractions of lower bainite and martensite,
the number (number density) of cementite of equal to or more than 50 nm, the aspect
ratio of prior austenite, the diffusible hydrogen content of weld metal in a y-groove
weld cracking test, were measured by the following method, and the yield strength,
tensile strength, weldability and toughness were evaluated. Tables 5 and 6 show structures
and properties of the steel plates obtained from these measurements and evaluations.
[0044]

[0045]

[0046]
[Table 3]
|
Steel Sheet No. |
Steel Composition No. |
Hot Rolling and Accelerated Cooling |
Rolling Heating Temperature (°C) |
Thickness (mm) |
Rolling Reduction in Non-Recrystallization Temperature Region (%) |
Water Cooling Start Temperature (°C) |
Water Cooling Stopping Temperature (°C) |
Cooling Rate in Accelerated Cooling (°C/s) |
Example |
1 |
A |
1180 |
25 |
64 |
804 |
325 |
42 |
2 |
A |
1170 |
6 |
70 |
761 |
420 |
105 |
3 |
B |
1150 |
25 |
66 |
819 |
360 |
40 |
4 |
B |
1175 |
9 |
62 |
750 |
425 |
71 |
5 |
C |
1200 |
25 |
64 |
794 |
330 |
38 |
6 |
D |
1165 |
25 |
63 |
820 |
370 |
40 |
7 |
D |
1130 |
12 |
61 |
767 |
410 |
66 |
8 |
E |
1150 |
25 |
64 |
809 |
335 |
41 |
9 |
E |
1135 |
12 |
66 |
764 |
395 |
67 |
10 |
F |
1160 |
25 |
66 |
798 |
405 |
40 |
11 |
G |
1155 |
25 |
64 |
802 |
375 |
44 |
12 |
H |
1135 |
25 |
70 |
799 |
360 |
37 |
13 |
I |
1125 |
25 |
64 |
779 |
375 |
38 |
14 |
J |
1180 |
25 |
65 |
794 |
410 |
37 |
15 |
K |
1150 |
25 |
67 |
804 |
350 |
38 |
16 |
L |
1160 |
25 |
66 |
787 |
320 |
41 |
17 |
M |
1180 |
25 |
66 |
784 |
335 |
37 |
18 |
M |
1145 |
16 |
69 |
765 |
365 |
58 |
Comparative Example |
19 |
N |
1170 |
25 |
67 |
820 |
350 |
43 |
20 |
O |
1175 |
25 |
68 |
782 |
360 |
40 |
21 |
P |
1150 |
25 |
64 |
820 |
380 |
40 |
22 |
Q |
1150 |
25 |
66 |
816 |
370 |
37 |
23 |
R |
1150 |
25 |
66 |
810 |
375 |
42 |
24 |
S |
1165 |
25 |
65 |
789 |
315 |
38 |
25 |
S |
1165 |
25 |
68 |
792 |
435 |
43 |
26 |
T |
1160 |
25 |
67 |
804 |
345 |
39 |
27 |
U |
1160 |
25 |
62 |
798 |
330 |
40 |
28 |
V |
1150 |
25 |
65 |
802 |
430 |
40 |
[0047]
[Table 4]
|
Steel Sheet No. |
Steel Composition No. |
Hot Rolling and Accelerated Cooling |
Rolling Heating Temperature |
Thickness |
Rolling Reduction in Non-Recrystallization Temperature Region |
Water Cooling Start Temperature |
Cooling Stopping Temperature |
Cooling Rate in Accelerated Cooling |
(°C) |
(mm) |
(%) |
(°C) |
(°C) |
(°C/s) |
Comparative Example |
29 |
V |
1150 |
25 |
66 |
797 |
320 |
43 |
30 |
W |
1150 |
25 |
66 |
785 |
410 |
41 |
31 |
X |
1140 |
25 |
67 |
799 |
375 |
37 |
32 |
Y |
1155 |
25 |
66 |
805 |
370 |
42 |
33 |
Z |
1180 |
25 |
65 |
801 |
375 |
38 |
34 |
AA |
1175 |
25 |
66 |
798 |
350 |
40 |
35 |
AB |
1160 |
25 |
64 |
788 |
370 |
35 |
36 |
AC |
1165 |
25 |
66 |
813 |
380 |
43 |
37 |
AD |
1160 |
25 |
66 |
778 |
340 |
39 |
38 |
AE |
1160 |
25 |
68 |
800 |
375 |
41 |
39 |
AF |
1165 |
25 |
65 |
779 |
350 |
43 |
40 |
AG |
1160 |
25 |
67 |
804 |
360 |
41 |
41 |
AH |
1175 |
25 |
66 |
799 |
345 |
37 |
42 |
AI |
1165 |
25 |
64 |
802 |
350 |
39 |
43 |
AJ |
1145 |
25 |
66 |
789 |
355 |
40 |
44 |
AK |
1155 |
25 |
66 |
800 |
405 |
40 |
45 |
AL |
1125 |
25 |
65 |
785 |
380 |
37 |
46 |
AM |
1160 |
25 |
67 |
805 |
440 |
39 |
47 |
AN |
1160 |
25 |
66 |
799 |
325 |
38 |
48 |
AO |
1130 |
25 |
66 |
784 |
435 |
40 |
49 |
AP |
1130 |
25 |
67 |
769 |
330 |
38 |
50 |
D |
1050 |
25 |
65 |
799 |
355 |
35 |
51 |
B |
1170 |
25 |
63 |
680 |
360 |
30 |
52 |
A |
1150 |
25 |
68 |
801 |
495 |
13 |
53 |
A |
1140 |
25 |
64 |
798 |
150 |
41 |
54 |
A |
1150 |
25 |
62 |
** |
** |
** |
55 |
A |
1170 |
25 |
64 |
802 |
333 |
7 |
After rolling and air cooling, steel sheets are reheated to 930°C and cooled from
810°C to 350°C at a cooling rate of 40°C/s. |
[0048]

[0049]

[0050] After a cross-section of the steel plate had been subjected to mirror polishing,
the cross-section of the steel plate was subjected to nital corrosion and the vicinity
of the 1/4t parts of the cross-section of the steel plate was observed with a scanning
electron microscope (SEM). Here, a magnification was 3000 times and 15 fields of view
in a range of 25 x 20 µm were selected. Areas of lower bainite and martensite were
measured from images obtained from the observation to calculate the respective structural
fractions (area ratios). In addition, in the same manner as that of the images, the
long axis length and the short axis length of the prior austenite were measured, and
an aspect ratio was obtained by dividing the long axis length by the short axis length
from an image obtained by observing a cross-section which is parallel to a rolling
direction (longitudinal direction) of the steel plate in the vicinity of the 1/4t
parts (L-shaped cross-section, a cross-section perpendicular to a thickness center
direction). Moreover, a base material of a steel plate with a predetermined volume
from the steel plates Nos. 1 to 55 was eluted by electrolysis using the extraction
replica method to prepare a sample which was obtained by extracting cementite, and
the sample was observed by a transmission electron microscope (TEM) to obtain the
number density of the cementite having an equivalent circle diameter of equal to or
more than 50 nm. In the number measurement, while precipitate other than the cementite
was distinguished by EDX, precipitate of equal to or more than 50 nm, other than the
cementite was rarely present in the steel plates Nos. 1 to 55.
Moreover, Ar3 (Ar3 transformation point) was measured by a hot working simulator manufactured
by Fuji Electronic Industrial Co., Ltd (THERMECMASTOR-Z), and in the Ar3 measurement,
after the steel (sample) was heated up to 1200°C, retained for 10 minutes and cooled
at 2.5°C/minute, a volume change at the time of cooling was measured to determine
Ar3 on the basis of the volume change.
[0051] In addition, the yield strength and the tensile strength were measured by acquiring
1A-type specimens for a tensile test specified in JIS Z 2201 (1998) from the steel
plates Nos. 1 to 55 according to a tensile test specified in JIS Z 2241 (1998). As
a result of the tensile test, when the yield strength is equal to or more than 885
MPa, and the tensile strength is equal to or more than 950 MPa and equal to or less
than 1130 MPa, the yield strength and the tensile strength of the steel plate were
respectively evaluated as "Pass".
[0052] A y-groove weld cracking test specified by JIS Z 3158 (1993) was performed on the
steel plates Nos. 1 to 55 to evaluate weldability. In the y-groove weld cracking test,
temperature and humidity were adjusted to perform carbon dioxide arc welding at a
heat input of 15 kJ/cm, and the steel plate provided for the evaluation had a thickness
of 25 mm. As a result of the test, in a case that a root crack ratio was 0 without
preheating (room temperature 25°C), the weldability of the steel plate was evaluated
as "Pass". In addition, since it is considered that the steel plates Nos. 2, 4, 7,
9 and 18 having a thickness of 6 to 16 mm have the same weldability as that of the
steel plates Nos. 1, 3, 6, 8 and 17 having similar components, the y-groove weld cracking
test was omitted for the steel plates Nos. 2, 4, 7, 9 and 18.
In addition, in the y-groove weld cracking test, each of two testing materials was
subjected to welding in which the same conditions such as temperature, humidity and
a heat input were set, and one of the two testing materials was sampled immediately
after the welding so that the diffusible hydrogen content of the weld metal was measured
using the gas chromatography method specified by JIS Z 3118 (2007). As a result of
the analysis, only when the diffusible hydrogen content exceeded 5.0 ml/100g, the
other testing material was subjected to the evaluation test of weldability (presence
of cracking).
[0053] 4-type Charpy specimens specified in JIS Z 2201 (1998) were sampled at a right angle
with respect to the rolling direction from the thickness center portion, and an absorbed
energy of a Charpy impact test was measured at -40°C to evaluate toughness on the
basis of an average value (vE-40) of the absorbed energies of 3 specimens so that
27 J was set to a target value of toughness. In addition, a 5 mm subsize Charpy specimen
was sampled for the steel plate having a thickness of 6 mm and 9 mm, and an absorbed
energy value of equal to or more than 27 J per 1 cm
2 was set to a target value of toughness.
[0054] In addition, chemical component amounts, Pcm values, and A values underlined in Tables
1 and 2 do not satisfy the conditions of the present invention. In the same manner,
numerical values underlined in Tables 3 and 4 represent values that do not satisfy
the manufacturing conditions of the present invention. Numerical values underlined
in Tables 5 to 6 represent values that do not satisfy the steel plate structure of
the present invention or have insufficient properties.
In all the steel plates Nos. 1 to 18 in Table 2, the sum of the lower bainite fraction
and the martensite fraction (lower bainite fraction + martensite fraction) is equal
to or more than 90%, the lower bainite fraction is equal to or more than 70%, and
the yield strength, tensile strength, yield ratio, weldability and toughness satisfied
the target value. Here, in the y-groove weld cracking test which was performed to
evaluate the weldability, since the diffusible hydrogen content in the weld metal
was in a range of 5.1 to 6.0 ml/100g, it was confirmed that weld cracking was not
generated in the range. Therefore, in the carbon dioxide arc welding, when the diffusible
hydrogen content is 3.0 to 5.0 ml/100g which is thought to be mixed when the welding
operation management was slightly insufficient, the diffusible hydrogen content is
lower than the diffusible hydrogen content in the range so that it can be considered
that the weld cracking is not generated. Here, when tempering was performed on the
steel plates Nos. 1 to 18 at 500°C, the number density of the relatively coarse cementite
of equal to or more than 50 nm was increased and the weld strength was degraded in
comparison with a case where tempering was not performed. In addition, for example,
in the manufacturing conditions of the steel plate No. 7, when the cumulative rolling
reduction ratio in the non-recrystallization temperature region was changed to less
than 60%, any of the strength properties (for example, toughness) was degraded in
comparison with the steel plate No. 7 because of not being capable of introducing
sufficient working strain in the steel.
Contrarily, while the steel plates Nos. 19 to 42 in which each chemical component
amount underlined in Tables 1 and 2 does not satisfy the conditions of the present
invention satisfy the manufacturing conditions of the present invention, one or more
of the yield strength, tensile strength, weldability, and toughness do not reach the
target value.
[0055] In the steel plates Nos. 43 to 49, each chemical component amount satisfied the conditions
of the present invention. However, in the steel plates Nos. 43 to 45 in which the
Pcm value does not satisfy the conditions of the present invention, weldability was
failed. In the same manner, in the steel plates Nos. 46 to 47 in which the A value
does not satisfy the conditions of the present invention, the yield strength was failed.
In addition, in the steel plates Nos. 48 and 49 in which any one of Pcm and A values
does not satisfy the conditions of the present invention, the weldability and the
yield strength were failed.
[0056] In the steel plates Nos. 50 to 55, each chemical component amount and values of Pcm
and A satisfied the conditions of the present invention. However, in the steel plates
Nos. 50 to 55, any one of manufacturing conditions did not satisfy the conditions
of the present invention. For this reason, in the steel plates Nos. 50 to 55, the
structure condition of the steel plate (one or more of lower bainite + martensite
fractions and the lower bainite fractions) did not satisfy the conditions of the present
invention and one or more of the yield strength, tensile strength and toughness were
also failed.
In addition, in the steel plate No. 54, after the slab was rolled to manufacture a
steel plate and air cooling was performed on the steel plate, the steel plate was
reheated to 930°C, and cooled in a temperature region which is from 810°C to 350°C
at a cooling rate of 40°C/s. Therefore, for example, manufacturing cost was increased
in the steel plate No. 54 in comparison with the steel plate No. 52.
Industrial Applicability
[0057] It is possible to economically provide a high-strength steel plate which has a yield
strength of equal to or more than 885 MPa, and a tensile strength of equal to or more
than 950 MPa and equal to or less than 1130 MPa, and a method of manufacturing the
same.