[Technical Field of the Invention]
[0001] The present invention relates to a high strength steel plate.
[Related Art]
[0002] The size of construction machines such as crane cars or industrial machines has been
increased with an increase in the height of buildings. However, in order to further
increase the size, it is necessary to reduce the weight of structural members of construction
machines or industrial machines. Accordingly, in order to reduce the weight of structural
members, a steel to be used for construction machines or industrial machines is required
to be high-strengthened.
[0003] However, in general, the total elongation is reduced in a case where the plate thickness
of a steel plate is limited while increasing the strength of the steel plate to suppress
the increase in the weight of members. For example, in a case where the plate thickness
is limited to 25 mm or less, it is difficult to secure a total elongation of 12% or
greater. In a case where the plate thickness is limited to 8 mm or less, it is more
difficult to secure the total elongation. In a case where the total elongation is
reduced, it is difficult to perform working. Accordingly, in a case where the steel
plate is used for members of construction machines or industrial machines, the steel
plate is required to have not only a strength, but also ductility such as a total
elongation. In addition, in a case where the steel plate is used as a structural member,
low temperature toughness is also required to prevent brittle fracture.
[0004] Based on such a background, a high strength steel plate having a tensile strength
of 780 MPa or greater, or further 950 MPa, and a method of manufacturing the high
strength steel plate are proposed.
[0005] For example, Patent Document 1 proposes a high strength steel plate having excellent
toughness which is obtained by hot-rolling and rapidly cooling a steel containing
an alloy added thererto and reducing the C content and to obtain appropriate hardenability,
and a method of manufacturing the steel plate.
[0006] However, the technology described in Patent Document 1 does not consider the workability
of the steel plate.
[0007] Patent Documents 2 to 4 propose a high strength hot rolled steel sheet which is manufactured
by coiling a steel in a coil after hot rolling as a steel sheet which is used for
construction machines or the like, and a method of manufacturing the hot rolled steel
sheet. Specifically, Patent Documents 2 to 4 disclose a method of manufacturing a
hot rolled steel sheet having a martensitic phase or a tempered martensitic phase
as a primary phase by performing hot rolling, rapid cooling to near a martensitic
transformation start temperature (Ms), holding for a predetermined period of time,
and coniling in a coil. However, in these methods, coiling in a coil is required,
and in the steel sheet obtained through these methods, a difference is generated between
characteristics in a rolling direction and characteristics in a direction perpendicular
to the rolling direction, and thus uniform characteristics are not obtained. In addition,
since a holding time in a temperature range in which a fine carbide is generated is
increased, the yield strength increases, and thus workability is reduced.
[0008] In conventional manufacturing of a high strength steel sheet, a heated slab was
hot-rolled and subjected to accelerated cooling to room temperature to transform the
microstructure to martensite, and then tempering (heat treatment) was performed to
increase ductility or toughness. In a case where the microstructure of the steel sheet
is transformed to martensite, the strength increases, and in order to secure ductility
or toughness, tempering is preferably performed after the accelerated cooling to transform
the microstructure to tempered martensite. However, in a case where the tempering
is omitted from the viewpoint of shortening the construction period or suppressing
an increase in the manufacturing cost, the microstructure is transformed to martensite,
and thus ductility or toughness is reduced although a high strength is obtained.
[0009] Patent Document 5 proposes a high strength steel plate in which a Mn content and
a Ni content are suppressed and a Mo content and a V content are increased to suppress
the formation of martensite and to provide a microstructure consisting mainly of lower
bainite, and a method of manufacturing the high strength steel plate.
[0010] However, since the technology described in Patent document 5 is based on the premise
that the microstructure is obtained by setting a cooling stop temperature to 300°C
to 450°C, a sufficient total elongation is not obtained. The inventors produced a
steel plate in accordance with the disclosure of Patent Document 5 and performed a
test, but a total elongation of 12% or greater was not obtained.
[0011] As described above, in a conventional high strength steel plate in which the plate
thickness is limited and the microstructure consists mainly of martensite, it is difficult
to secure ductility and toughness.
[0012] In addition, in a case where the steel plate is applied to the above-described structural
members, welding is generally performed. In welding, at a welding joint, a tensile
strength (joint strength) thereof is required to be not less than a value required
for a base metal in view of reliability of the structure. However, in a case where
a steel plate in which the main structure of the microstructure is martensite is welded,
a welding joint may have a lower tensile strength (joint strength) than a base metal
due to softening of a heat-affected zone, and a required value may not be satisfied.
[Prior Art Document]
[Patent Document]
[0013]
[Patent Document 1] Japanese Unexamined Patent Application, First Publication No.
2009-287081
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No.
2011-52320
[Patent Document 3] Japanese Unexamined Patent Application, First Publication No.
2011-52321
[Patent Document 4] Japanese Unexamined Patent Application, First Publication No.
2012-77336
[Patent Document 5] PCT International Publication No. WO2012/60405
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0014] The present invention is contrived in view of the circumstances, and an object thereof
is to provide a high strength steel plate which is preferably used for construction
machines or industrial machines and a method of manufacturing the high strength steel
plate. Specifically, an object of the invention is to provide a high strength steel
plate which has a plate thickness of 4.5 to 20 mm, a yield strength of 885 MPa or
greater, a tensile strength of 950 MPa or greater, a Charpy absorbed energy of 59
J/cm
2 or greater at -20°C, and a total elongation of 12% or greater, and in which the microstructure
consists mainly of martensite, and a tensile strength of a welding joint after welding
can be sufficiently secured, and a method of manufacturing the high strength steel
plate.
[Means for Solving the Problem]
[0015] The inventors examined the relationship between the ductility of a steel plate and
the accelerated cooling stop temperature. As a result, the inventors found that the
ductility is reduced in a case where the accelerated cooling stop temperature is 300°C
or higher or higher than a martensitic transformation completion temperature (Mf).
The inventors further proceeded the examination, and found that in a case where the
accelerated cooling is stopped at a temperature of 300°C or higher or a temperature
higher than Mf, untransformed austenite transforms to bainite in a microstructure,
voids starting from a coarse carbide (cementite) formed caused by the bainite are
excessively generated, and thus the ductility is reduced.
[0016] The inventors studied ways to suppress such a reduction in the ductility. As a result,
the inventors designed a component capable of increasing hardenability to suppress
the above-described bainitic transformation, and found new knowledge that in a case
where accelerated cooling to a temperature which is lower than 300°C and not higher
than Mf is performed after hot rolling, the microstructure can be allowed to consist
mainly of martensite, and thus the ductility of a high strength steel plate can be
secured.
[0017] The invention is contrived based on such knowledge, and the gist thereof is as follows.
[0018]
- (1) A high strength steel plate according to an embodiment of the invention consisits
of, as a chemical composition, by mass%: C: 0.050% to 0.100%, Si: 0% to 0.50%, Mn:
1.20% to 1.70%, P: 0.020% or less, S: 0.0050% or less, N: 0% to 0.0080%, B: 0.0003%
to 0.0030%, Ti: 0.003% to 0.030%, Nb: 0.003% to 0.050%, Cr: 0% to 2.00%, Mo: 0% to
0.90%, Al: 0% to 0.100%, Cu: 0% to 0.50%, Ni: 0% to 0.50%, V: 0% to 0.100%, W: 0%
to 0.50%, Ca: 0% to 0.0030%, Mg: 0% to 0.0030%, REM: 0% to 0.0030%, and a remainder
consisting of Fe and impurities, one or both of Cr and Mo is contained in an amount
of 0.20% or greater in total, the Cr content is 0.80% or less in a case where the
Mo content is greater than 0.50%, DI which is obtained by the Formula 1 is 2.0 to
7.8, Pcm which is obtained by the Formula 2 is 0.189% or greater, a microstructure
includes one or both of martensite and bainite such that a total area fraction thereof
is 99% or greater, an aspect ratio of prior austenite grains is 2.0 or greater, a
number fraction of cementite having a length of 1.0 µm or greater in a long axis direction
with respect to cementite having a length of 0.1 µm or greater in the long axis direction
is 5% or less, a plate thickness is 4.5 mm to 20 mm, a yield strength is 885 MPa or
greater, a tensile strength is 950 MPa or greater, a total elongation is 12% or greater,
and a Charpy absorbed energy at -20°C is 59 J/cm2.


In the Formulae 1 and 2, each of [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and
[B] represents a content of each element by mass%, and 0 is substituted in a case
where the element is not contained.
- (2) In the high strength steel plate according to (1), the microstructure may include
90% or greater of martensite in terms of area fraction.
- (3) In the high strength steel plate according to (1) or (2), the chemical composition
may include Cu: 0% to 0.25% by mass%.
- (4) In the high strength steel plate according to any one of (1) to (3), the chemical
composition may include Ni: 0% to 0.25% by mass%.
- (5) In the high strength steel plate according to any one of (1) to (4), the chemical
composition may include V: 0% to 0.050% by mass%.
- (6) In the high strength steel plate according to any one of (1) to (5), the chemical
composition may include W: 0% to 0.05% by mass%.
- (7) In the high strength steel plate according to any one of (1) to (6), the plate
thickness may be 4.5 mm to 15 mm.
- (8) In the high strength steel plate according to any one of (1) to (7), in a case
where the Mo content is represented by [Mo] and the Cr content is represented by [Cr],
[Mo]/[Cr] may be 0.20 or greater.
- (9) In the high strength steel plate according to (8), a Charpy absorbed energy at
-40°C may be 59 J/cm2 or greater.
- (10) In the high strength steel plate according to any one of (1) to (9), the Pcm
may be 0.196% or greater.
[Effects of the Invention]
[0019] According to the aspect of the invention, it is possible to provide a high strength
steel plate which has a yield strength of 885 MPa or greater, a tensile strength of
950 MPa or greater, and a total elongation of 12% or greater without containing a
large amount of expensive alloying elements. This steel plate exhibits excellent toughness
such that a Charpy absorbed energy at -20°C is 59 J/cm
2 or greater. In addition, by adjusting Pcm, that is a hardenability index, to be 0.189%
or greater, and preferably 0.196% or greater, a tensile strength of 950 MPa or greater
can be secured at a welding joint where a high strength steel plate according to the
invention is a base metal with a predetermined heat input or less in welding.
[0020] Furthermore, by controlling [Mo]/[Cr] that is a ratio of a Mo content [Mo] to a Cr
content [Cr], it is possible to provide a high strength steel plate having more excellent
toughness such that a Charpy absorbed energy at -40°C is 59 J/cm
2 or greater.
[0021] Accordingly, the invention can provide a high strength steel plate which is preferably
used for a structural member in construction machines or industrial machines to contribute
to an increase in the size or a reduction in the weight of the construction machines
or industrial machines without a significant increase in manufacturing cost, and thus
very significantly contributes to the industry.
[Brief Description of the Drawings]
[0022]
FIG. 1 is a diagram illustrating the relationship between: a total elongation; and
an accelerated cooling stop temperature Tcf, a hardenability index DI, and a C content.
FIG. 2 is a diagram showing the relationship between [Mo]/[Cr] and a Charpy absorbed
energy (vE-40) at -40°C.
FIG. 3 is a diagram showing the relationship between an accelerated cooling stop temperature
and a total elongation.
FIG. 4A is an SEM photograph showing the influence of an accelerated cooling stop
temperature on the shape of cementite in a case where the accelerated cooling stop
temperature is 290°C.
FIG. 4B is an SEM photograph showing the influence of an accelerated cooling stop
temperature on the shape of cementite in a case where the accelerated cooling stop
temperature is 400°C.
FIG. 5 is a photograph of a void generated from the vicinity of coarse cementite.
[Embodiments of the Invention]
[0023] Hereinafter, a high strength steel plate according to an embodiment of the invention
(hereinafter, may be referred to as a high strength steel plate according to this
embodiment) will be described in detail.
[0024] First, a chemical composition (components) of a high strength steel plate according
to this embodiment will be described. Hereinafter, the symbol % related to the content
means mass% unless otherwise noted.
(C: 0.050% to 0.100%)
[0025] C is a useful element for increasing a strength of steel, and is a very important
element for determining a total elongation of steel having a martensite structure.
In the high strength steel plate according to this embodiment, the C content is required
to be 0.050% or greater to obtain a sufficient strength. In order to further increase
the strength, the C content is preferably 0.060% or greater, 0.065% or greater, or
0.070% or greater. In a case where the C content is greater than 0.100%, the ductility
and toughness of steel deteriorate due to the generation of an excessive amount of
a carbide. Therefore, the C content is required to be 0.100% or less to obtain a good
total elongation and good toughness. In order to further improve the ductility, the
C content is preferably adjusted to be 0.095% or less, 0.090% or less, or 0.085% or
less.
(Si: 0.50% or less)
[0026] In a case where an excessive amount of Si is contained, the ductility or toughness
of steel is reduced. Therefore, the Si content is limited to 0.50% or less. It is
not necessary to particularly determine the lower limit of the Si content, and the
lower limit of the Si content is 0%. However, in a case where Si is used for deoxidation,
the Si content is preferably adjusted to be 0.03% or greater to obtain a sufficient
effect. In addition, Si is also an element which suppresses the generation of a carbide,
and in order to obtain this effect, the Si content is preferably adjusted to be 0.10%
or greater, and more preferably 0.20% or greater. In a case where it is not necessary
to obtain the effects, the upper limit of the Si content may be adjusted to be 0.45%,
0.40%, or 0.35%.
(Mn: 1.20% to 1.70%)
[0027] Mn is an important element for improving the hardenability of steel. The Mn content
is adjusted to be 1.20% or greater to obtain a high strength by increasing a martensite
area fraction in a microstructure. The Mn content is preferably adjusted to be greater
than 1.20%, 1.25% or greater, or 1.30% or greater, and more preferably 1.35% or greater
or 1.39% or greater. In a case where the Mn content is too high, ductility and toughness
may be reduced. Accordingly, the Mn content is adjusted to be 1.70% or less. More
preferably, the Mn content is adjusted to be 1.60% or less, 1.55% or less, or 1.50%
or less.
(P: 0.020% or less)
(S: 0.0050% or less)
[0028] P and S are elements inevitably contained as impurities in steel, and deteriorate
the toughness of steel. In addition, P and S are elements which deteriorate the toughness
of a heat-affected zone in a case where welding is performed. Therefore, the P content
is limited to 0.020% or less, and the S content is limited to 0.0050% or less. In
order to further improve the toughness, the P content may be adjusted to be 0.015%
or less, and the S content may be adjusted to be 0.0030% or less. Since the P content
and the S content are preferably low, and thus preferably reduced as much as possible.
Accordingly, it is not necessary to particularly determine the lower limits of the
P content and the S content, and the lower limits of the P content and the S content
are 0%. However, from the viewpoint of cost of dephosphorization or desulfurization,
the P content may be adjusted to be 0.001% or greater, and the S content may be adjusted
to be 0.0001% or greater.
(B: 0.0003% to 0.0030%)
[0029] B is an element which is segregated in the grain boundary to increase the hardenability
of steel, and is a useful element for exhibiting the effect even in a case where the
amount thereof is very small. In the high strength steel plate according to this embodiment,
the B content is adjusted to be 0.0003% or greater to increase martensite in a microstructure.
Preferably, the B content is adjusted to be 0.0005% or greater. In a case where the
B content is too high, the hardenability improving effect is saturated, and precipitates
such as a nitride or a carboboride are formed. Thus, ductility or toughness is reduced.
Therefore, the B content is adjusted to be 0.0030% or less. The B content is preferably
adjusted to be 0.0020% or less or 0.0015% or less.
(Ti: 0.003% to 0.030%)
[0030] Ti is an element which forms a nitride, and is an element which fixes N in steel
as TiN and suppresses the generation of BN. As described above, though B is an element
which increases hardenability, the effect of B is not obtained in a case where BN
is formed. In the high strength steel plate according to this embodiment, the Ti content
is required to be 0.003% or greater to secure hardenability by suppressing the formation
of BN. The Ti content is preferably adjusted to be 0.005% or greater, and more preferably
0.010% or greater. In a case where the Ti content is too high, TiN becomes coarse,
and thus ductility or toughness may be reduced. Accordingly, the Ti content is adjusted
to be 0.030% or less. The Ti content is preferably adjusted to be 0.020% or less.
(Nb: 0.003% to 0.050%)
[0031] Nb is an element which significantly improves the hardenability of steel by being
contained together with B. In the high strength steel plate according to this embodiment,
the Nb content is adjusted to be 0.003% or greater to increase a martensite area fraction
in a microstructure. Nb is also an element which contributes to grain refining and
increases toughness by forming a fine nitride. In order to obtain this effect, the
Nb content is preferably adjusted to be 0.005% or greater. More preferably, the Nb
content is adjusted to be 0.010% or greater or 0.015% or greater. In a case where
the Nb content is too high, the nitride becomes coarse, and thus ductility or toughness
may be reduced. Accordingly, the Nb content is adjusted to be 0.050% or less. The
Nb content is preferably adjusted to be 0.040% or less, 0.035% or less, or 0.030%
or less.
(Cr: 2.00% or less)
(Mo: 0.90% or less)
(Total Content of One or Both of Cr and Mo is 0.20% or Greater, and In Case Where
Mo Content is Greater Than 0.50%, Cr Content is 0.80% or Less)
[0032] Cr and Mo are important elements for improving hardenability, and one or both of
Cr and Mo are contained. In the high strength steel plate according to this embodiment,
the total content of Cr and Mn is adjusted to be 0.20% or greater to increase a martensite
area fraction in a microstructure. The total content of Cr and Mn is preferably adjusted
to be 0.30% or greater, and more preferably 0.40% or greater. In consideration of
a case where either Cr or Mo is contained, the lower limits of the Cr content and
the Mo content are 0%. If necessary, the lower limit of the Cr content may be adjusted
to be 0.20% or 0.30%, and similarly, the lower limit of the Mo content may be adjusted
to be 0.20% or 0.30%.
[0033] In addition, in a case where the Cr content is greater than 2.00% or the Mo content
is greater than 0.90%, a fine carbide is generated, and thus ductility and toughness
are reduced. Therefore, the Cr content and the Mo content are adjusted to be 2.00%
or less and 0.90% or less, respectively. The Cr content is preferably adjusted to
be 1.50% or less or 1.00% or less, and more preferably 0.90% or less or 0.80%. The
Mo content is preferably adjusted to be 0.70% or less, and more preferably 0.60% or
less or 0.50%.
[0034] In a case where both Cr and Mo are contained, toughness is reduced in a case where
the content is too high. Accordingly, in a case where the Mo content is greater than
0.50%, the Cr content is required to be 0.80% or less. In this case, the Cr content
may be adjusted to be 0.70% or less. In a case where the Cr content is greater than
0.80%, the Mo content may be adjusted to be 0.50% or less, and in a case where the
Cr content is greater than 1.20%, the Mo content may be adjusted to be 0.40% or less.
The total content of Cr and Mo may be adjusted to be 2.50% or less, 2.00% or less,
1.50% or less, 1.30% or less, or 1.10% or less.
(N: 0.0080% or less)
[0035] N is inevitably contained as impurities. N forms BN and inhibits the hardenability
improving effect of B. Accordingly, the N content is limited to 0.0080% or less. The
N content is preferably limited to 0.0060% or less, and more preferably 0.0050% or
less. The N content is preferably reduced as much as possible, and the lower limit
thereof is 0%. However, from the viewpoint of cost of denitrification, the N content
may be adjusted to be 0.0001% or greater. The N content may be adjusted to be 0.0020%
or greater for refining of a microstructure by a nitride.
[0036] The above elements are contained as essential elements and impurities of the high
strength steel plate according to this embodiment, and basically, the high strength
steel plate according to this embodiment has components including the above-described
essential elements and a remainder consisting of Fe and impurities (the above-described
impurity elements and optional impurity elements other than the above-described impurity
elements). However, for deoxidation, an improvement in the strength and/or ductility,
refining of a microstructure, control of the form of a sulfide, or the like, the high
strength steel plate according to this embodiment may further contain, other than
the above-described components, one or more of 0.100% or less of Al, 0.50% or less
of Cu, 0.50% or less of Ni, 0.100% or less of V, 0.50% or less of W, 0.0030% or less
of Ca, 0.0030% or less of Mg, and 0.0030% or less of REM instead of a part of Fe.
Since these elements are not essential elements, the contents of the elements may
be 0%.
(Al: 0.100% or less)
[0037] Al is a deoxidizing element. In a case where Al is used for deoxidation, the Al content
is preferably adjusted to be 0.010% or greater to obtain a sufficient effect. In a
case where the Al content is too high, ductility or toughness is reduced due to the
formation of an oxide or a nitride. Therefore, the Al content is limited to 0.100%
or less even in a case where Al is contained. The Al content is preferably limited
to 0.080% or less, more preferably 0.050% or less, and even more preferably 0.030%
or less.
(Cu: 0.50% or less)
(Ni: 0.50% or less)
[0038] Cu and Ni are elements which improve the hardenability of steel. In a case where
a martensite area fraction in a microstructure is increased by increasing the hardenability,
the Cu content and the Ni content are preferably adjusted to be 0.10% or greater,
respectively. Since Cu and Ni are expensive elements, the Cu content and the Ni content
are preferably adjusted to be 0.50% or less, respectively, even in a case where Cu
and Ni are contained. The Cu content and the Ni content are more preferably adjusted
to be 0.40% or less, and even more preferably 0.30% or less, respectively.
(V: 0.100% or less)
[0039] V is an element which forms a carbide or a nitride. The V content is preferably adjusted
to be 0.005% or greater in a case where toughness is increased by grain refining by
a carbide or a nitride. In a case where the V content is too high, ductility or toughness
is reduced. However, since V has less adverse effects than Nb or Ti, the upper limit
of the V content is limited to 0.100% in a case where V is contained. The V content
is preferably adjusted to be 0.050% or less.
(W: 0.50% or less)
[0040] W is an element which improves the hardenability of steel. In order to obtain this
effect, the W content is preferably adjusted to be 0.05% or greater. In a case where
the W content is too high, weldability deteriorates. Therefore, the W content is adjusted
to be 0.50% or less or 0.30% or less even in a case where W is contained. If necessary,
the W content may be adjusted to be 0.02% or less or 0.01% or less.
(Ca: 0.0030% or less)
[0041] Ca is an element which controls the form of an oxide or a sulfide. In order to obtain
this effect, the Ca content is preferably adjusted to be 0.0001% or greater. The Ca
content is more preferably adjusted to be 0.0005% or greater, and even more preferably
0.0010% or greater. In a case where the Ca content is too high, the effect is saturated,
and ductility or toughness may be reduced due to the formation of inclusions. Therefore,
the Ca content is adjusted to be 0.0030% or less even in a case where Ca is contained.
(Mg: 0.0030% or less)
[0042] Mg is an element which acts to increase the toughness of steel by refining the microstructure.
In order to obtain this effect, the Mg content is preferably adjusted to be 0.0005%
or greater. In a case where the Mg content is too high, the effect is saturated, and
ductility or toughness may be reduced due to the formation of inclusions. Therefore,
the Mg content is adjusted to be 0.0030% or less even in a case where Mg is contained.
(REM: 0.0030% or less)
[0043] REM (rare earth metal) is an element which acts to increase the toughness of steel
by controlling the form of a sulfide, particularly, MnS. In order to obtain this effect,
the REM content is preferably adjusted to be 0.0001 % or greater. In a case where
the REM content is too high, inclusions including REM may become coarse, and thus
ductility or toughness may be reduced. Therefore, the REM content is adjusted to be
0.0030% even in a case where REM is contained.
[0044] Elements other than the above elements may be contained in a small amount within
a range not impairing the actions and effects.
[0045] In the high strength steel plate according to this embodiment, with the elements
adjusted to be within the above-described ranges, DI and Pcm, which are determined
by a chemical composition, are required to satisfy the following ranges, respectively.
(DI: 2.0 to 7.8)
[0046] DI is a hardenability index, and is obtained by (Formula 1). Here, each of [C], [Si],
[Mn], [Cu], [Ni], [Cr], and [Mo] in the formula represents a content (mass%) of each
element, and 0 is substituted in a case where the element is not contained.
[0047] As qualitatively shown in FIG. 1, in a case where a hardenability index DI is increased,
a reduction in the total elongation can be suppressed even in a case where an accelerated
cooling stop temperature Tcf is increased (that is, moved to the right in FIG. 1).
In a case where the accelerated cooling stop temperature Tcf is increased, an excessive
increase in the strength is suppressed, and toughness and/or ductility can be increased.
In order to keep a good balance among a strength, ductility, and toughness, DI is
preferably 2.0 or greater. DI is more preferably 3.0 or greater, and even more preferably
4.0 or greater. In a case where the hardenability excessively increases, the strength
excessively increases, and thus the toughness may be reduced. Accordingly, DI is preferably
7.8 or less. DI is more preferably 7.0 or less, and even more preferably 6.5 or less.

(Pcm: 0.189% or greater)
[0048] In general, a tensile strength (joint strength) of a welding joint is required to
be not less than a required tensile strength for a base metal provided in welding.
The inventors found that in a case where welding is performed on a steel plate in
which a main structure of a microstructure is martensite, a tensile strength (joint
strength) of a welding joint may be less than a tensile strength of a base metal due
to softening of a heat-affected zone. Accordingly, the inventors manufactured welding
joints using various high strength steel plates by changing weld heat input, and performed
a test. As a result, the inventors found that by increasing the hardenability of a
steel plate, specifically, by adjusting Pcm, which is obtained by (Formula 2), to
be 0.189% or greater, the softening of a heat-affected zone is suppressed, and in
a case where the welding is performed with 7.0 kJ/cm weld heat input that is the lower
limit value of a weld heat input range frequently applied to the manufacturing of
a structural member of construction machines or industrial machines, the tensile strength
of a welding joint can be 950 MPa or greater.

[0049] Each of [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] represents a content
(mass%) of each element, and 0 is substituted in a case where the element is not contained.
[0050] The inventors further conducted studies on the weld heat input and the strength of
the welding joint, and found that the strength of the welding joint can be evaluated
by JS which is calculated by (Formula a) using Pcm obtained by (Formula 2) and weld
heat input Hi [kJ/cm] from the component composition of the high strength steel plate
used in welding, and a joint strength of 950 MPa or greater can be secured at an actual
welding joint in a case where JS is 950 MPa or greater.

[0051] As can be seen from the above formula, it is found that the weld heat input is preferably
reduced as much as possible in order to secure the strength of the welding joint.
However, there is a lower limit in the weld heat input in order to secure the soundness
of the welding joint. It is not easy to reduce the weld heat input to be less than
7.0 kJ/cm in order to secure the welding productivity or the like in the manufacturing
of construction machines or industrial machines. In a case where the weld heat input
is 7.0 kJ/cm, Pcm necessary for adjusting JS to be 950 MPa or greater is 0.189% from
the above formula. That is, by adjusting Pcm to be 0.189% or greater, a joint strength
of 950 MPa or greater can be secured.
[0052] In addition, by adjusting Pcm to be 0.196% or greater, a joint strength of 950 MPa
or greater can be secured even in a case where the welding is performed with 10.0
kJ/cm weld heat input with which no special management is required. That is, by adjusting
Pcm to be 0.196% or greater, a welding joint strength of 950 MPa or greater can be
secured without special welding management.
[0053] Pcm may be adjusted to be 0.200% or greater, 0.205% or greater, 0.210% or greater,
or 0.215% or greater to secure the strength of the welding joint even with higher
weld heat input. It is preferable that the weld heat input be high since the number
of welding paths can be reduced, and thus the productivity can be improved. It is
not necessary to particularly determine the upper limit of Pcm, and the upper limit
may be 0.250% or less or 0.240% or less to prevent weld cracking or the like.
([Mo]/[Cr]: 0.20 or higher)
[0054] The inventors further proceeded the examination and studies of the influence of Cr
and Mo as hardenability increasing elements on toughness. As a result, the inventors
found that in a case where hardenability (DI) is constant, a ratio of Mo to Cr has
an influence on toughness. Specifically, it was found that in a case where a ratio
([Mo]/[Cr]) of the Mo content [Mo] to the Cr content [Cr] by mass% is high, a substructure
(packet or block) of the martensite is made fine, and as a result, toughness is improved.
For a further improvement of toughness, the ratio may be 0.40 or higher, 0.80 or higher,
or 1.00 or higher.
[0055] FIG. 2 is a diagram showing the relationship between [Mo]/[Cr] and a Charpy absorbed
energy at -40°C. In FIG. 2, the symbol "○" represents an actual measurement value,
and the symbol "●" represents an average of the actual measurement values.
[0056] As shown in FIG. 2, it is found that there is a tendency that the Charpy absorbed
energy at -40°C increases with an increase of [Mo]/[Cr], and in a case where [Mo]/[Cr]
is 0.20 or higher, the Charpy absorbed energy at -40°C is 59 J/cm
2 or greater. Accordingly, in a case where low temperature toughness is obtained, [Mo]/[Cr]
is preferably adjusted to be 0.20 or higher. Mo is an element which more easily forms
a fine carbide or cluster than Cr. Accordingly, in a case where the Mo content is
excessively higher than the Cr content, toughness may be reduced, and thus [Mo]/[Cr]
may be adjusted to be 2.00 or lower or 1.50 or lower.
[0057] The Charpy absorbed energy is measured by a Charpy test performed based on JIS Z
2242. A test piece collected from a thickness middle portion, in which the plate thickness
of the steel plate from which the test piece has been collected is 8 mm, and a longitudinal
direction is a rolling direction, is a sub-sized test piece of 10 mm×5 mm.
(Total Area Fraction of One or Both of Martensite and Bainite: 99% or greater and
Total elongation: 12% or greater)
[0058] The inventors conducted studies on the relationship between the hardenability and
the microstructure of a high strength steel plate, and a total elongation. As a result,
the inventors found that in a case where the hardenability is poor, the total elongation
is reduced, and a reduction in the total elongation, that is, a reduction in the ductility
occurs due to the generation of voids starting from a coarse carbide generated caused
by the bainite as shown in FIGS. 4A and 4B. In addition, the inventors obtained knowledge
that it is necessary to suppress the formation of bainite causing the formation of
coarse cementite in order to increase the ductility of a high strength steel plate.
In order to suppress the formation of bainite causing the formation of coarse cementite,
a structure mainly including martensite in which 90% or more of a microstructure is
martensite is preferably provided. In addition, in order to increase the strength
of a steel plate, the martensite area fraction in the microstructure is preferably
adjusted to be 90% or greater, more preferably 92% or greater, and even more preferably
94% or greater.
[0059] However, both the martensite and the bainite are continuous cooling transformation
structures, and it may be difficult to accurately distinguish the martensite and the
bainite by observing the microstructures. In such a case, it can be judged that the
formation of bainite causing the formation of coarse cementite is suppressed in a
case where the total area fraction of martensite and bainite is 99% or greater and
the total elongation is 12% or greater.
[0060] Accordingly, in the high strength steel plate according to this embodiment, the total
area fraction of one or both of martensite and bainite is adjusted to be 99% or greater,
and the total elongation as a structure index is adjusted to be 12% or greater. In
a case where it is possible to sufficiently distinguish the martensite and the bainite
by observing the microstructures, the martensite area fraction is preferably 90% or
greater.
[0061] In the high strength steel plate according to this embodiment, the martensite of
the microstructure is as quenched, and is different from tempered martensite obtained
after tempering treatment. Tempered martensite is not preferable since cementite grows
by tempering for a long period of time.
[0062] The remainder other than the above microstructures may include one or more of ferrite,
pearlite, and residual austenite.
[0063] The distinguishment of the microstructure and the measurement of the martensite area
fraction are performed using an optical microscope. Specifically, near a 1/4 t-portion
in a cross-section parallel to a rolling direction (a portion at a depth of 1/4 of
a plate thickness t from a steel plate surface in a plate thickness direction) is
subjected to nital etching, and two regions within a range of 120 µm×100 µm are photographed
at 500-fold magnification using an optical microscope to measure an area fraction
of a microstructure in which an acicular lath structure is developed. With respect
to the acicular structure, the cross-section of the steel plate is subjected to electrolytic
polishing, and then a portion near the 1/4 t-portion in the cross-section of the steel
plate is observed by a scanning electron microscope (SEM). Here, the magnification
is 5,000 times, and the photographing is performed within a range of 50 µm×40 µm.
In a case where a long axis direction of cementite is oriented in two or more directions
in the block, the acicular structure is defined as martensite, and an area fraction
of the region is obtained. The product of the acicular structure area fraction measured
using the optical microscope and the martensite area fraction measured using SEM is
an area fraction of the martensite structure of this steel.
[0064] In the above-described structure observation using a scanning electron microscope,
the orientation of the long axis direction of cementite in two or more directions
in the block may not be clearly distinguished. In this case, using an optical microscope,
the area fraction of a microstructure in which an acicular lath structure is developed
is defined as the total area fraction of martensite and bainite.
(Number Fraction of Cementite Having Length of 1.0 µm or Greater in Long Axis Direction
with respect to Cementite Having Length of 0.1 µm or Greater in Long Axis Direction:
5% or less)
[0065] As described above, in order to increase the ductility of a steel plate, it is important
to suppress the formation of bainite causing the formation of coarse cementite is
suppressed and to provide a microstructure mainly including martensite. However, in
order to further increase the ductility, it is effective to suppress the generation
of voids starting from a coarse carbide (particularly, cementite).
[0066] The inventors found that by controlling an accelerated cooling stop temperature,
the number fraction of the coarse carbide (particularly, cementite) having a length
of 1.0 µm or greater in a long axis direction can be reduced, and as a result, the
generation of voids can be suppressed and the total elongation can be improved. Specifically,
the inventors found that the total elongation can be improved by adjusting the number
fraction of cementite having a length of 1.0 µm or greater in a long axis direction
in cementite having a length of 0.1 µm or greater in a long axis direction to be 5%
or less.
[0067] As will be described later in detail, in the invention, by stopping accelerated cooling
at a temperature which is not higher than Mf and lower than 300°C, it is possible
to provide a structure mainly including martensite in which the generation of a coarse
carbide is suppressed. That is, the generation of voids starting from coarse cementite
having a length of 1.0 µm in a long axis direction can be suppressed by controlling
an accelerated cooling stop temperature.
[0068] The number density of cementite is measured using a scanning electron microscope
(SEM). Specifically, a cross-section of the steel plate is subjected to electrolytic
polishing, and then a portion near a 1/4 t-portion in the cross-section of the steel
plate is photographed within a range of 50 µm×40 µm at 5,000-fold magnification by
a scanning electron microscope (SEM). Based on the contrast in the obtained image,
the number of precipitates as cementite having an aspect ratio of 2.0 or greater and
a length of 0.1 µm or greater in a long axis direction is counted using image analysis
software. Similarly, the number of cementite having an aspect ratio of 2.0 or greater
and a length of 1.0 µm or greater in a long axis direction is counted. The obtained
number of precipitates of 1.0 µm or greater is divided by the number of cementite
of 0.1 µm or greater, and thus the number fraction (%) of cementite of 1.0 µm or greater
is obtained. The shape of the carbide is not particularly limited. However, in a case
where the carbide has an ellipsoidal shape, the "length in a long axis direction"
refers to a major axis.
(Aspect Ratio of Prior Austenite Grains is 2.0 or Greater)
[0069] In the high strength steel plate according to this embodiment, the aspect ratio of
prior austenite grains is adjusted to be 2.0 or greater. In a case where the aspect
ratio is less than 2.0, toughness may be reduced.
[0070] In a case where online accelerated cooling (direct quenching) is performed after
rolling in a non-recrystallization region, the aspect ratio of prior austenite grains
can be adjusted to be 2.0 or greater. In a case where quenching is performed after
rolling, cooling, and subsquent reheating, the structure worked by rolling is not
taken over, and the aspect ratio of prior austenite grains becomes less than 2.0.
[0071] The aspect ratio of prior austenite grains is measured by the following method. That
is, near a 1/4 t-portion in a cross-section parallel to a rolling direction, which
is positioned at a depth of 1/4 of a plate thickness t from a surface in a plate thickness
direction, is etched with nital, and two regions within a range of 120 µm×100 µm are
photographed at 500-fold magnification using an optical microscope. From the obtained
image, long axis lengths and short axis lengths of at least 50 prior austenite grains
are measured, and the long axis length is divided by the short axis length to obtain
an aspect ratio of each grain. An average of the aspect ratios of the prior austenite
grains is obtained.
[0072] Next, a plate thickness and mechanical properties of the high strength steel plate
according to this embodiment will be described.
(Plate thickness: 4.5 to 20 mm)
[0073] In general, a high strength steel plate which is used for cranes or the like has
a plate thickness of 4.5 to 20 mm. Therefore, the high strength steel plate according
to this embodiment has a plate thickness of 4.5 to 20 mm. However, the thickness is
preferably 4.5 to 15 mm in view of contribution to a reduction in the weight.
(Yield Strength: 885 MPa or greater)
(Tensile Strength: 950 MPa or greater)
[0074] In addition, high-strengthening is required to contribute to an increase in the size
or a reduction in the weight of construction machines or industrial machines. In order
to obtain a significantly economical effect, it is necessary to adjust the yield strength
to be 885 MPa or greater and to adjust the tensile strength to be 950 MPa or greater.
Although it is not necessary to particularly determine the upper limit of the yield
strength, and the upper limit may be 1,100 MPa or less. Although it is not necessary
to particularly determine the upper limit of the tensile strength, and the upper limit
may be 1,300 MPa or less or 1,250 MPa or less.
(Total elongation: 12% or greater)
[0075] In order to apply a high strength steel plate to members of construction machines
or industrial machines, workability such as bendability is required, and thus the
total elongation is adjusted to be 12% or greater. In addition, as described above,
the total elongation may be a structure index indicating whether the formation of
bainite, which causes the formation of coarse cementite is suppressed.
[0076] The yield strength, the tensile strength, and the total elongation are measured by
performing a tensile test based on JIS Z 2241. The value of the total elongation measured
by the tensile test depends on the shape of a test piece. The limit (12% or greater)
of the total elongation is a value in a case where a No. 5 test piece of JIS Z2241
(a flat test piece in which original gauge length is 50 mm, a parallel portion has
a width of 25 mm, and the thickness of the test piece is equal to that of the steel
plate) is used as a tensile test piece.
[0077] The elongation conversion formula based on a difference in the test piece shape is
also specified in ISO2566-1. Regarding the elongation of 12% in a No. 5 test piece
of JIS Z2241, in a case where a No. 13B test piece of JIS Z2241 (a flat test piece
in which original gauge length is 50 mm, a parallel portion has a width of 12.5 mm,
and the thickness of the test piece is equal to that of the steel plate) is used as
a tensile test piece, the elongation can be converted into 10.4%, and in a case where
a No. 13A test piece of JIS Z2241 (a flat test piece in which original gauge length
is 80 mm, a parallel portion has a width of 20 mm, and the thickness of the test piece
is equal to that of the steel plate) is used as a tensile test piece, the elongation
can be converted into 9.5%.
(Charpy Absorbed Energy at -20°C: 59 J/cm2 or greater)
[0078] In a case where construction machines or industrial machines are used in cold climates,
the high strength steel plate may be required to have low temperature toughness. Therefore,
the Charpy absorbed energy at -20°C is preferably 59 J/cm
2. More preferably, the Charpy absorbed energy at -40°C is preferably 59 J/cm
2 or greater.
[0079] The Charpy absorbed energy is measured by collecting a test piece in which a longitudinal
direction is a rolling direction from a thickness middle portion, and performing a
Charpy test based on JIS Z 2242 at -20°C or -40°C. It may be difficult to collect
a full-sized test piece of 10 mm×10 mm depending on the plate thickness of the steel
plate, and in such a case, a sub-sized test piece is used. The Charpy absorbed energy
is a value (J/cm
2) obtained by dividing the absorption energy by a cross-sectional area (cm
2) of the test piece in a bottom part of a V-notch. For example, in a case where a
full-sized test piece of 10 mm×10 mm is used, a measured Charpy absorbed energy value
(J) is divided by 1 cm×0.8 cm=0.8 cm2, and in a case where a sub-sized test piece
of 10 mm×5 mm is used, the measured Charpy absorbed energy value (J) is divided by
0.5 cm×0.8 cm=0.4 cm
2.
[0080] Next, a preferable method of manufacturing a high strength steel plate according
to this embodiment will be described.
[0081] The high strength steel plate according to this embodiment can be manufactured as
follows: a molten steel having a chemical composition within the above-described range
is melted in the usual manner, a slab obtained by casting the molten steel is heated
to perform hot rolling, accelerated cooling is performed, and after the accelerated
cooling is stopped, air cooling to room temperature is performed. In the manufacturing
of the high strength steel plate according to this embodiment, a heat treatment such
as tempering is not performed after the accelerated cooling is stopped or after the
air cooling to room temperature is performed. In a case where a treatment is performed,
martensite changes to tempered martensite. That is, the high strength steel plate
according to this embodiment is manufactured through non-heat treated manufacturing
steps in which a heat treatment is omitted to shorten the construction period or reduce
the manufacturing cost. The high strength steel plate according to this embodiment
manufactured through the non-heat treated manufacturing steps may be referred to as
a non-heat treated high strength steel plate.
[0082] Hereinafter, preferable conditions of each step will be described.
(Slab Heating Temperature: 1,100°C to 1,250°C)
[0083] The high strength steel plate according to this embodiment is required to contain
a predetermined amount of alloying elements to increase hardenability. Therefore,
a carbide or a nitride of alloying elements is generated in a slab provided in hot
rolling. In heating of the slab, it is necessary to decompose the carbide or the nitride
in order to form a solid solution in the steel, and the heating temperature is adjusted
to be 1,100°C or higher. In a case where the heating temperature of the slab is too
high, the grain size becomes coarse, and thus toughness may be reduced. Accordingly,
the heating temperature is adjusted to be 1,250°C or higher.
(Finishing Temperature: Ar3 (°C) or higher)
(Accelerated Cooling Start Temperature: Ar3 (°C) or higher)
[0084] A heated slab is hot-rolled. After the hot rolling, it is necessary to start accelerated
cooling at a temperature at which the microstructure is austenite in order to provide
a microstructure mainly including martensite through the accelerated cooling. Accordingly,
the hot rolling should be ended at a temperature at which the microstructure is austenite.
For this, the hot rolling finishing temperature is adjusted to be Ar3 (°C) or higher.
Ar3 (°C) is a temperature at which transformation from austenite to ferrite starts
during cooling, and can be obtained from thermal expansion behavior. In addition,
Ar3 (°C) can be simply obtained by (Formula b).

[0085] Here, each of [C], [Si], [Mn], [Ni], [Cu], [Cr], and [Mo] represents a content (mass%)
of each element, and 0 is substituted in a case where the element is not contained.
[0086] The hot rolling may be performed in the usual manner, and it is preferable that recrystallization
region rolling be performed with a cumulative rolling reduction of 50% to 80% within
a temperature range not lower than 1,050°C, or non-recrystallization region rolling
be performed with a cumulative rolling reduction of 50% to 90% within a temperature
range of Ar3 to 950°C.
(Cooling Rate of Accelerated Cooling: 30 to 200 °C/s)
[0087] In the accelerated cooling which is performed subsequently after the hot rolling,
martensite is formed. The cooling rate of the accelerated cooling is required to be
30 °C/s or higher to increase a martensite area fraction. A sufficient martensite
area fraction is not obtained when the cooling rate is lower than 30 °C/s. In order
to promote the martensitic transformation, the cooling rate is preferably increased,
but there is a restriction caused by a plate thickness or facility. Accordingly, the
upper limit may be 200 °C/s or lower. The cooling rate is calculated as follows: a
temperature variation of a steel plate surface after hot rolling is measured, and
a difference between a surface temperature before the water cooling is started and
a surface temperature immediately after the water cooling is stopped is divided by
a time required for cooling.
(Accelerated Cooling Stop Temperature: not higher than Mf (°C) and lower than 300°C)
[0088] The inventors conducted studies on the relationship between the hardenability and
the accelerated cooling stop temperature, and the microstructure and the total elongation.
Here, in a case where a steel plate is rapidly cooled after hot rolling, a martensitic
transformation start temperature Ms (°C) is obtained by (Formula 3). A martensitic
transformation end temperature Mf (°C) is lower than Ms (°C) by approximately 150°C,
and is obtained by (Formula 4). In (Formula 3), each of [C], [Mn], [V], [Cr], [Ni],
[Cu], [Mo], and [Al] represents a content (mass%) of each element, and 0 is substituted
in a case where the element is not contained.

[0089] In order to transform the microstructure to martensite, cooling to at least Ms (°C)
or lower is required, and in a case where cooling (rapid cooling) to Mf (°C) or lower
is performed, 90% or more of the microstructure transforms to martensite. However,
in a case where the cooling stop temperature is 300°C or higher, the cooling may become
unstable and a part of the microstructure transforms to not martensite but bainite.
Accordingly, the cooling stop temperature is adjusted to be not higher than Mf (°C)
and lower than 300°C.
[0090] As described above, the accelerated cooling stop temperature is very important, and
a precondition is set that the accelerated cooling is stopped at a temperature lower
than the martensitic transformation start temperature Ms (°C). In addition, in a case
where the accelerated cooling is performed to a temperature which is not higher than
the martensitic transformation completion temperature Mf (°C) and lower than 300°C,
the microstructure transforms to a structure mainly including martensite in which
the generation of a carbide is suppressed.
[0091] In a case where the accelerated cooling stop temperature is between Ms (°C) and Mf
(°C) (between Ms-Mf), the ductility of a high strength steel plate is influenced by
hardenability. That is, in a case where the hardenability is increased, the formation
of bainite is suppressed, and the generation of a coarse cementite-based carbide is
suppressed. Accordingly, the total elongation is improved, and the variation is also
reduced.
[0092] The relationship between the accelerated cooling stop temperature Tcf and Mf and
the total elongation, and the influence of DI and the C content on the total elongation
are qualitatively summarized as schematically shown in FIG. 1. Here, in FIG. 1, the
vertical axis represents a total elongation, the horizontal axis represents accelerated
cooling stop temperature Tcf, and DI represents a hardenability index which is obtained
by (Formula 1).
[0093] As shown in the graph of FIG. 1, in a case where the accelerated cooling stop temperature
Tcf is reduced, the martensitic transformation is promoted, and the formation of bainite
is suppressed. Accordingly, the total elongation is improved, and in a case where
Tcf is not higher than Mf, the total elongation becomes constant. In a case where
Tcf is not higher than Mf, the total elongation is almost decided by the C content,
and the total elongation is improved by reducing the C content.
[0094] In a case where the accelerated cooling stop temperature Tcf is between Ms and Mf,
the total elongation is improved together with a reduction of Tcf. However, in this
case, in a case where an alloying element is added to increase hardenability, DI is
increased, and thus the formation of bainite is suppressed. Accordingly, the generation
of a coarse carbide is suppressed, and thus the total elongation is improved.
[0095] The lower limit of the accelerated cooling stop temperature is not particularly limited,
and accelerated cooling may also be performed to room temperature. The accelerated
cooling stop temperature is preferably 100°C or higher to increase a yield strength
by, for example, the action of locking carbon atoms in the dislocation.
[0096] After the accelerated cooling is stopped, a heat treatment such as tempering is not
performed, and air cooling to room temperature is performed.
[Examples]
[0097] Hereinafter, examples of the invention will be described. Conditions in the following
examples are an example employed to confirm the feasibility and the effects of the
invention, and the invention is not limited to the example. In addition, without departing
from the gist of the invention, the invention can employ various conditions as long
as the object of the invention is achieved.
[0098] A slab obtained by steel making for chemical components (including a remainder consisting
of Fe and impurities) shown in Table 1 was formed into a steel plate having a plate
thickness of 4.5 to 20 mm under manufacturing conditions shown in Table 2. The "heating
temperature" represents a reheating temperature of the slab, the "rolling end temperature"
represents a temperature at which hot rolling is ended, the "water cooling start temperature"
represents a surface temperature of the steel plate when accelerated cooling (water
cooling) is started, the "cooling rate" represents an average cooling rate in a thickness
middle portion within a temperature range of Ar3 (°C) to an accelerated cooling stop
temperature, and the "water cooling stop temperature" represents a surface temperature
of the steel plate when water cooling is stopped. The surface temperature of the steel
plate was measured by a radiation-type thermometer, and the "cooling rate" was calculated
by obtaining a temperature of the thickness middle portion from the surface temperature
through thermal conductivity calculation. Tempering was not performed on any steel
plate.
[0099] The microstructure and the mechanical properties (yield strength, tensile strength,
total elongation, toughness, and joint strength) of the obtained steel plate were
evaluated.
[0100] The distinguishment of a microstructure and the measurement of a martensite area
fraction and a bainite area fraction were performed by the following method.
[0101] A cross-section of a steel plate was subjected to mirror polishing, and then near
a 1/4 t-portion in the cross-section parallel to a rolling direction was subjected
to nital etching. Two regions within a range of 120 µm×100 µm were photographed at
500-fold magnification using an optical microscope to measure an area fraction of
a microstructure in which an acicular lath structure was developed. Regarding the
acicular structure, the cross-section of the steel plate was subjected to electrolytic
polishing, and then a portion near the 1/4 t-portion in the cross-section of the steel
plate was observed by a scanning electron microscope (SEM). Here, the magnification
was 5,000 times, and the photographing was performed within a range of 50 µm×40 µm.
In a case where a long axis direction of cementite was oriented in two or more directions
in the block, the acicular structure was defined as martensite, and an area fraction
of the region was obtained. The product of the acicular structure area fraction measured
using the optical microscope and the martensite area fraction measured using SEM was
an area fraction of the martensite structure of this steel. An acicular structure
other than the martensite was determined as bainite.
[0102] In the above-described structure observation using a scanning electron microscope,
in a case where it was not possible to clearly distinguish whether or not the long
axis direction of cementite is oriented in two or more directions in the block, the
area fraction of a microstructure in which an acicular lath structure was developed
was defined as the total area fraction of martensite and bainite using an optical
microscope.
[0103] The total area fraction of martensite and bainite of 99% or greater, or in a case
where it was possible to clearly distinguish the martensite, the martensite area fraction
of 90% or greater was set as a target value.
[0104] A microstructure (remainder) other than the "martensite and bainite" described in
Table 3 includes one or more of ferrite, pearlite, bainite, and residual austenite.
[0105] Furthermore, a cross-section of the steel plate was subjected to electrolytic polishing,
and then a portion near a 1/4 t-portion in the cross-section of the steel plate was
observed by a scanning electron microscope (SEM) to measure the number density of
cementite. Specifically, a cross-section of the steel plate was subjected to electrolytic
polishing, and then a portion near a 1/4 t-portion in the cross-section of the steel
plate was photographed within a range of 50 µm×40 µm at 5,000-fold magnification by
a scanning electron microscope (SEM). Based on the contrast in the obtained image,
the number of precipitates as cementite having an aspect ratio of 2.0 or greater and
a length of 0.1 µm or greater in a long axis direction was counted using image analysis
software. Similarly, the number of cementite having an aspect ratio of 2.0 or greater
and a length of 1.0 µm or greater in a long axis direction was counted. The obtained
number of precipitates of 1.0 µm or greater was divided by the number of cementite
of 0.1 µm or greater, and thus the number fraction (%) of cementite of 1.0 µm or greater
was obtained. In a case where the number fraction of cementite of 1.0 µm or greater
was 5% or less, this was judged as a good result.
[0106] The aspect ratio of prior austenite grains was measured. Specifically, near a 1/4
t-portion in a cross-section parallel to a rolling direction was etched with nital,
and two regions within a range of 120 µm×100 µm were photographed at 500-fold magnification
using an optical microscope. From the obtained image, long axis lengths and short
axis lengths of at least 50 prior austenite grains were measured, and the long axis
length was divided by the short axis length to obtain an aspect ratio of each grain.
An average of the aspect ratios of the prior austenite grains was obtained and defined
as an aspect ratio of the prior austenite grains. In a case where the aspect ratio
of the prior austenite grains was 2.0 or greater, this was judged as a good result.
[0107] A test piece (overall thickness) was collected from the steel plate, and a tensile
strength, a yield strength, and a total elongation were measured based on JIS Z 2241.
In addition, a Charpy absorbed energy at -20°C and a Charpy absorbed energy at -40
°C were measured based on JIS Z 2242. The tensile test piece is a No. 5 test piece
(overall thickness) collected such that a longitudinal direction is perpendicular
to the rolling direction, and the yield strength is 0.2% proof stress. The Charpy
test piece is a sub-sized test piece of 10 mm×5 mm collected from a thickness middle
portion such that a longitudinal direction is the rolling direction.
[0108] The mechanical properties were evaluated to be good in a case where the yield strength
was 885 MPa or greater, the tensile strength was 950 MPa or greater, the total elongation
was 12% or greater, and the absorbed energy value at -20°C (vE
-20) was 59 J/cm
2 or greater as a result of the tests.
[0109] Welding joints were produced using steel plates having good mechanical properties
(steel plate Nos. 1 to 16) and a steel plate No. 32 in which Pcm was less than 0.189%.
[0110] The welding method was MAG welding, and the weld heat input was 7.0 kJ/cm or 10.0
kJ/cm. In a case where the heat input was 7.0 kJ/cm, the welding conditions were set
as follows: a current of 280 A, a voltage of 27 V, and a welding rate of 65 cm/min.
In a case where the heat input was 10.0 kJ/cm, the welding conditions were set as
follows: a current of 305 A, a voltage of 29 V, and a welding rate of 53 cm/min.
[0111] The tensile strength (joint strength) of the welding joint was evaluated by a tensile
test specified in JIS Z 3121, and evaluated to be good in a case where the tensile
strength was 950 MPa or greater.
[0112] The evaluation results are shown in Table 3. In Table 3, the underlined numerical
values indicate that the values are out of the range of the invention, or target properties
are not obtained.
[0113] Steel plate Nos. 1 to 16 are invention examples, and an excellent strength, excellent
ductility, and excellent toughness are obtained. In addition, the joint strength is
950 MPa or greater. In an example in which Mo/Cr is 0.20 or higher, excellent toughness
is obtained even at a test temperature of -40°C.
[0114] Steel plate Nos. 17 to 35 are comparative examples, and one or more of the yield
strength, the tensile strength, the total elongation, and vE
-20 does not satisfy a target value.
[0115] Since each of steel plate Nos. 17, 26, and 29 had a low C or Mn content, the strength
thereof was reduced. The fraction of martensite was also insufficient in the steel
plate Nos. 26 and 29.
[0116] A steel plate No. 20 had a low Mn content and had low hardenability. Accordingly,
ferrite and bainite were formed other than martensite, and thus the amount of martensite
formed did not satisfy the range of the invention. As a result, the strength was significantly
reduced.
[0117] In each of steel plate Nos. 18, 19, 21, 22, 23, 27, 28, and 30, the C content, the
Si content, the Mn content, the Cr content, or the Mo content was too high, and thus
ductility and toughness were low.
[0118] In a steel plate No. 24, worked ferrite was formed other than martensite due to a
low rolling end temperature and a low water cooling start temperature. Thus, the fraction
of martensite did not satisfy the range of the invention, and as a result, the strength
was low.
[0119] In a steel plate No. 33, worked ferrite was formed other than martensite due to a
low water cooling start temperature. Thus, the fraction of martensite did not satisfy
the range of the invention, and as a result, the strength was low.
[0120] In steel plate Nos. 25 and 34, untransformed austenite transformed to bainite due
to a high water cooling stop temperature, and thus the fraction of martensite was
low. In addition, the total elongation was low due to the excessive generation of
voids starting from a coarse carbide (cementite) formed caused by the bainite. In
addition, the steel plate No. 34 had a low yield strength.
[0121] A steel plate No. 31 had a high Cr content and a high Mo content, and DI was too
high. Accordingly, the toughness and the total elongation were low.
[0122] In a steel plate No. 32, Pcm was low, and thus the joint strength was less than 950
MPa.
[0123] In a steel plate No. 35, the aspect ratio of prior austenite grains was less than
2.0 with a low rolling reduction in a non-recrystallization region. Thus, the toughness
was low.
[Table 2]
Steel plate No. |
Ar3 °C |
Ms °C |
Mf °C |
Heating Temperature °C |
Rolling End Temperature °C |
Water Cooling Start Temperature °C |
Cooling Rate °C/s |
Water Cooling Stop Temperature °C |
1 |
727 |
459 |
309 |
1230 |
860 |
770 |
97 |
284 |
2 |
732 |
466 |
316 |
1230 |
859 |
765 |
108 |
234 |
3 |
715 |
449 |
299 |
1230 |
859 |
773 |
95 |
217 |
4 |
722 |
455 |
305 |
1230 |
859 |
760 |
97 |
265 |
5 |
717 |
450 |
300 |
1230 |
860 |
787 |
87 |
281 |
6 |
707 |
443 |
293 |
1230 |
860 |
803 |
149 |
190 |
7 |
715 |
451 |
301 |
1230 |
912 |
730 |
140 |
230 |
8 |
702 |
443 |
293 |
1230 |
860 |
792 |
117 |
272 |
9 |
705 |
434 |
284 |
1230 |
860 |
805 |
130 |
281 |
10 |
723 |
461 |
311 |
1230 |
860 |
769 |
106 |
294 |
11 |
717 |
459 |
309 |
1230 |
859 |
764 |
105 |
246 |
12 |
720 |
459 |
309 |
1230 |
859 |
750 |
97 |
270 |
13 |
701 |
443 |
293 |
1230 |
860 |
768 |
84 |
282 |
14 |
687 |
439 |
289 |
1230 |
860 |
793 |
79 |
276 |
15 |
711 |
453 |
303 |
1230 |
924 |
810 |
124 |
281 |
16 |
734 |
457 |
307 |
1230 |
860 |
756 |
90 |
273 |
17 |
737 |
468 |
318 |
1230 |
855 |
790 |
112 |
295 |
18 |
705 |
439 |
289 |
1230 |
860 |
730 |
103 |
132 |
19 |
738 |
458 |
308 |
1230 |
860 |
804 |
113 |
307 |
20 |
810 |
506 |
356 |
1230 |
857 |
821 |
104 |
256 |
21 |
660 |
416 |
266 |
1230 |
843 |
752 |
98 |
201 |
22 |
713 |
443 |
293 |
1230 |
824 |
764 |
97 |
224 |
23 |
693 |
444 |
294 |
1230 |
846 |
734 |
104 |
189 |
24 |
724 |
457 |
307 |
1230 |
700 |
662 |
112 |
123 |
25 |
722 |
455 |
305 |
1230 |
859 |
765 |
104 |
393 |
26 |
715 |
465 |
315 |
1230 |
855 |
790 |
103 |
291 |
27 |
705 |
443 |
293 |
1230 |
860 |
742 |
97 |
140 |
28 |
742 |
465 |
315 |
1230 |
860 |
815 |
122 |
278 |
29 |
757 |
477 |
327 |
1230 |
885 |
780 |
94 |
289 |
30 |
665 |
424 |
274 |
1230 |
836 |
790 |
98 |
242 |
31 |
689 |
431 |
281 |
1230 |
860 |
805 |
130 |
270 |
32 |
734 |
468 |
318 |
1230 |
846 |
741 |
89 |
290 |
33 |
719 |
455 |
305 |
1230 |
720 |
640 |
96 |
143 |
34 |
728 |
461 |
311 |
1230 |
846 |
768 |
94 |
435 |
35 |
708 |
452 |
302 |
1230 |
1000 |
900 |
98 |
210 |
[Table 3]
Steel plate No. |
Plate thickness (mm) |
Martensite +Bainite % |
Martensite % |
Aspect Ratio of Prior Austenite Grains |
Number Fraction of Cementite of 1 µm or Greater % |
Yield Strength MPa |
Tensile Strength MPa |
Total elongation % |
vE-20 J/cm2 |
vE-40 J/cm2 |
Weld Heat Input kJ/cm |
Joint Strength MPa |
Remarks |
1 |
8 |
100 |
- |
3.6 |
3 |
895 |
1072 |
14 |
331 |
255 |
10.0 |
1015 |
|
2 |
8 |
100 |
94 |
3.7 |
1 |
895 |
1070 |
15 |
270 |
250 |
7.0 |
960 |
|
3 |
8 |
100 |
95 |
4.0 |
0 |
895 |
1133 |
15 |
270 |
232 |
10.0 |
1135 |
|
4 |
8 |
100 |
- |
3.5 |
2 |
986 |
1145 |
14 |
293 |
252 |
10.0 |
1089 |
|
5 |
8 |
100 |
92 |
3.0 |
1 |
991 |
1183 |
13 |
243 |
76 |
10.0 |
1160 |
|
6 |
6 |
100 |
96 |
2.8 |
0 |
937 |
1140 |
13 |
233 |
87 |
10.0 |
1134 |
|
7 |
20 |
99 |
93 |
4.0 |
0 |
1040 |
1214 |
15 |
268 |
85 |
10.0 |
1156 |
|
8 |
8 |
100 |
98 |
3.8 |
1 |
992 |
1174 |
13 |
223 |
77 |
10.0 |
1184 |
Invention |
9 |
8 |
100 |
- |
4.0 |
2 |
919 |
1131 |
13 |
90 |
11 |
10.0 |
1146 |
Examples |
10 |
4.5 |
100 |
91 |
2.8 |
1 |
921 |
1053 |
15 |
246 |
155 |
7.0 |
1052 |
|
11 |
8 |
99 |
92 |
4.1 |
0 |
905 |
1034 |
15 |
230 |
219 |
10.0 |
1051 |
|
12 |
8 |
100 |
92 |
3.6 |
1 |
1010 |
1250 |
13 |
209 |
130 |
10.0 |
1096 |
|
13 |
8 |
100 |
94 |
2.8 |
1 |
987 |
1240 |
14 |
193 |
114 |
10.0 |
1251 |
|
14 |
8 |
100 |
93 |
4.1 |
1 |
984 |
1184 |
14 |
230 |
205 |
10.0 |
1194 |
|
15 |
8 |
99 |
91 |
4.0 |
0 |
1014 |
1254 |
14 |
168 |
98 |
10.0 |
1096 |
|
16 |
8 |
100 |
92 |
3.5 |
0 |
1025 |
1264 |
13 |
120 |
95 |
10.0 |
1183 |
|
17 |
8 |
100 |
- |
3.5 |
1 |
823 |
1002 |
18 |
323 |
261 |
|
- |
Comparative Example |
18 |
8 |
100 |
98 |
3.2 |
2 |
1010 |
1279 |
11 |
55 |
44 |
|
- |
19 |
8 |
100 |
- |
3.8 |
2 |
961 |
1097 |
11 |
56 |
43 |
|
- |
20 |
8 |
74 |
64 |
4.0 |
29 |
731 |
912 |
21 |
343 |
255 |
|
- |
21 |
6 |
99 |
- |
3.5 |
1 |
981 |
1153 |
11 |
32 |
6 |
|
- |
22 |
6 |
100 |
91 |
4.3 |
0 |
896 |
1076 |
11 |
46 |
13 |
|
- |
23 |
6 |
100 |
91 |
2.9 |
0 |
997 |
1209 |
11 |
58 |
46 |
|
- |
|
24 |
8 |
82 |
72 |
2.3 |
30 |
778 |
901 |
15 |
280 |
246 |
|
- |
25 |
8 |
91 |
- |
3.3 |
9 |
889 |
1014 |
9 |
270 |
216 |
|
- |
26 |
8 |
86 |
- |
4.0 |
26 |
783 |
940 |
18 |
338 |
330 |
|
- |
27 |
8 |
100 |
98 |
3.8 |
3 |
1045 |
1359 |
10 |
43 |
13 |
|
- |
28 |
8 |
100 |
- |
3.0 |
2 |
896 |
1122 |
11 |
42 |
14 |
|
- |
29 |
8 |
82 |
72 |
2.8 |
34 |
745 |
945 |
14 |
330 |
209 |
|
- |
30 |
8 |
99 |
- |
4.0 |
1 |
941 |
1201 |
11 |
34 |
8 |
|
- |
31 |
8 |
100 |
91 |
2.9 |
1 |
991 |
1203 |
11 |
45 |
11 |
|
- |
32 |
8 |
100 |
91 |
3.9 |
1 |
887 |
984 |
14 |
248 |
193 |
7.0 |
927 |
33 |
8 |
78 |
68 |
2.2 |
32 |
764 |
894 |
16 |
334 |
293 |
|
- |
34 |
8 |
92 |
- |
3.6 |
11 |
867 |
1004 |
10 |
309 |
234 |
|
- |
35 |
8 |
100 |
93 |
1.4 |
3 |
921 |
1186 |
13 |
46 |
16 |
|
- |
The underlines mean that the values are out of the range of the invention, or target
properties are not obtained. |
[Industrial Applicability]
[0124] According to the invention, it is possible to provide a high strength steel plate
which has a yield strength of 885 MPa or greater, a tensile strength of 950 MPa or
greater, and a total elongation of 12% or greater without containing a large amount
of expensive alloying elements. In addition, this steel plate exhibits excellent toughness
such that a Charpy absorbed energy at -20°C is 59 J/cm
2 or greater. Accordingly, the invention is useful for industry.