DESCRIPTION
Title of Invention
HIGH-STRENGTH STEEL PLATE AND PRODUCING METHOD THEREOF
5
Technical Field
[0001]
The present invention relates to a high-strength steel plate for a structural
member used for construction machineries and industrial machineries, which is superior
10 in bndability, weldability and toughness, and has a yield strength of 885MPa or more
and a plate thickness of 6mm to 32mm, and a producing method thereof.
Background Art
[0002]
15 The construction machineries such as a crane, a concrete pumping truck and the
like tend to be grown in size with the more recent high-rise buildings. In order to
suppress-the weight increase due to thegrowthtr ize ofthe constructiorrmachineriesyarequest
of weight reduction of the structural member has risen, and a demand of the
high-strength steel which has a yield strength of 885MPa or more or has a tensile
20 strength of 950MPa or more tends to increase moreover. When the steel plate is applied
to boom materials and the like for the crane, severe bending is often given. For
example, the bending such as a severe bend radius of 2.Ot (it means that 180-degree
bending can be performed without cracking even if the bend radius is 2.0 times of the
plate thickness.) is often required. Moreover, the weldability which is necessary for a
25 manufacturing process and the toughness as the structural member are required for
2
important properties . Namely, there is a demand for the steel plate to have all of the
high-strength, excellent bendability, weldability and toughness.
[0003]
In regard to the high-strength steel plate which has the yield strength of 885MPa
5 grade (the tensile strength of approximately 950MPa or more ), although the high-strength
steel plate which has the tensile strength of 950MPa grade is disclosed in Patent
Document 1 for example, the steel plate is supposed to be a steel plate whose thickness is
thick relatively such as a penstock, the bendability is not taken into consideration
especially, and the steel plate lacks economic efficiency for the structural member used
10 for construction machineries and industrial machineries because the steel plate requires
the addition of a large amount of Ni to obtain its toughness.
[0004]
Although the Patent Document 2 relates to the producing method of the steel
plate which has the tensile strength of 950MPa or more for line pipes mainly, the
15 bendability is not taken into consideration and the producing method lacks for
productivity because the rolling must be conducted at lower temperature region such as
two phase region.
[0005]
Although the Patent Document 3 discloses the thick steel plate which has the
20 tensile strength of 95OMPa or more and is excellent in resistance to strain-aging
characteristics, the bendability is also not taken into consideration at all in the steel plate.
[0006]
Although the Patent Document 4 discloses the producing method of the steel
plate which has the tensile strength of 98OMPa or more without tempering, the steel plate
25 lacks economic efficiency because the steel plate needs a large amount of alloy elements
3
such as Mn to obtain the tensile strengthof 98OMPa by the condition of ultra low C
content of less than 0 .025%, and the bendability is also not taken into consideration.
[0007]
Although the Patent Document 5 relates to the steel which has the tensile
5 strength of 98OMPa grade with consideration for the bendability , improvement of the
bendability depends on grain refining . Thus, the steel lacks for productivity because
reheating and quenching are needed off line to control that the grain size becomes fine
and uniform, so that the steel cannot satisfy vigorous demand sufficiently . In addition,
it is not clear whether the bending such as the severe bend radius of 2.Ot is possible or not,
10 because only bending parameter is elongation.
[0008]
Although the Patent Document 6 relates to the hot rolled steel sheet which has
the tensile strength of 950MPa with consideration for the bendability and the weldability,
the steel sheet needs the addition of a large amount of Ti, the weldability seems to
15 deteriorate when it applies to a thick plate, and the steel sheet lacks economic efficiency
because the steel sheet requires Ni addition to compensate for a decrease in the toughness
caused by-the addition of alarge amount of Ti.
Citation List
20 Patent Document
[0009]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. H10-265893
[Patent Document 2] Japanese Unexamined Patent Application, First
25 Publication No. H8-269546
4
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2001-59142
[Patent Document 4] Japanese Unexamined Patent Application, First
Publication No. 2004-84019
5 [Patent Document 5] Japanese Unexamined Patent Application, First
Publication No. 2009-242832
[Patent Document 6] Japanese Unexamined Patent Application, First
Publication No. H5-23052
10 Summary of Invention
Technical Problem
[0010]
An object of the invention, in order to solve the above-mentioned problems, is to
provide a high-strength steel plate for a structural member used for construction
15 machineries and industrial machineries, which is superior in bendability, weldability and
toughness, and has a yield strength of 885MPa or more, a tensile strength of 950MPa to
I20&MPaand-a plate thickness of6mrn to 32mm, anda producing method thereof.
Solution to Problem
20 [0011]
The inventors made a thorough investigation of methods for greatly improving
bendability of a high-strength steel which has a yield strength of 885MPa or more and a
tensile strength of 950MPa to 1200MPa. As a result, it is found that the hardness of the
topmost surface of a steel plate has a huge impact. Namely, when the steel plate has a
25 softened layer with a certain thickness in the surface, it is possible to greatly improve the
5
bendability because cracks are prevented from initiating in the surface during bending
even if the strength of most parts of the steel plate is high. Since the bendability
deteriorates when the yield strength and the tensile strength exceed 1200MPa, it is
preferred that an upper limit of the yield strength and the tensile strength is 1200MPa.
5 As required, the upper limit of the yield strength and the tensile strength may be limited
to 1150MPa or 1100MPa. An upper limit of a plate thickness maybe limited to 25mm,
20mm, or 16mm because improvement of the bendability , strength, and weldability
becomes difficult with an increase in the plate thickness. A lower limit of the plate
thickness may be limited to 8mm or 10mm because it becomes difficult to secure a
10 starting temperature of cooling with a decrease in the plate thickness.
[0012]
Furthermore, itis found that microstructure of the steel plate is also important
and that there is high correlation between elongatedness of prior austenite grains along a
direction parallel to a bending direction (hereinafter, referred to as an aspect ratio) and
15 the bendability. Especially, the aspect ratio at a surface of the steel plate where a
deformation is large by bending, specifically an area from the surface to a depth of 1/4 of
the thickness (hereinafter , referred-to as a surface area front the thickness 1/41) and-aw
area from a depth of 3 /4 of the thickness to the other surface (hereinafter , referred to as a
backface-area from the thickness 3/4t), is important.
20 [0013]
By controlling both the softened layer and the aspect ratio , the bendability of the
high-strength steel can be improved significantly . Specifically, in order for the steel
plate which has the yield strength of 885MPa or more and the plate thickness of 32mm or
less to satisfy the bendability such as the severe bend radius of 2.0t, the following control
25 is required.
6
[0014]
Fig. 1 is an example of the graph which indicates the relationship between
hardness and elongation by tensile tests in regard to various steel whose hardness are
almost uniform, and shows that the elongation of the steel is greatly dependent on the
5 hardness . In Fig. 1, as a test piece, No. 13B tensile test piece which is regulated in JIS
Z 2241 was used. Although Hv (Vickers hardness) of the high-strength steel which has
the yield strength of 885MPa or more exceeds approximately 300, the elongation is
improved over approximately 2% when Hv becomes from 300 to 250 or less. .
Specifically, when the steel plate has a softened layer where Hv is 250 or less and whose
10. thickness is 50 μm or more in the surface in the plate thickness, it is possible to greatly
improve the bendability of the whole steel plate because cracks are prevented from
initiating during bending by improvement of the elongation in the surface. However an
average hardness of whole thickness, namely the tensile strength, deteriorates when the
thickness of the softened layer increases. Thus the thickness of the softened layer is set
15 to 3% or less of the thickness of the steel plate.
[0015]
Although-the aspect ratio of the surfacnreafrom-the thickne 1/4t and the
baclcface area from the thickness 3/4t are important as described above, the aspect ratio at
the thickness 1/4t may be adopted as a parameter. In general rolling, the aspect ratio at
20 the thickness 1/4t and at the thickness 3/4t are almost the same, and the aspect ratio of the
surface area from the thickness 1/4t and the backface area from the thickness 3/4t
become larger than that. Therefore the aspect ratio of the surface area from the
thickness 1/4t and the backface area from the thickness 3/4t are regarded respectively as
the same level or more compared with the aspect ratio at the thickness 1/4t.
25 [0016]
7
Fig. 2 is a graph which indicates the investigation result of the relationship
between the thickness of the softened layer, the aspect ratio, and the bendability in regard
to various high-strength steel plates which have the yield strength of 885MPa or more,
the tensile strength of 950MPa to 1200MPa, and the plate thickness of 12mm to 32mm.
5 Evaluation of the bendability was conducted by JIS Z 2248 in which 180-degree bending
was conducted by the bend radius which was 2.0 times of the plate thickness (2.0t) using
No.1 test piece along a direction parallel to a final rolling direction (hereinafter, referred
to as a L direction) or perpendicular to the final rolling direction (hereinafter, referred to
as a T direction). In addition, it was judged as success when a fissure and any defect did
10 not occur on an outer side of a bend part after the bending. For evaluation of the aspect
ratio, a cross section at the thickness 1/4t which was parallel to the final rolling direction
in case of the bending to the L direction (hereinafter, referred to as a L section) or which
was perpendicular to the final rolling direction in case of the bending to the T direction
(hereinafter, referred to as a T section) was etched by picric acid to appear prior austenite
15 grain boundaries, five visual fields where each visual field was 200pmx2OOpm were
observed, and the aspect ratio was measured by image analysis. In addition, the
----microstructure of the steel of the present invention is-mainly-tempered marteusite-or
bainite, and the aspect ratio is an average aspect ratio of the prior austenite grain
boundaries.
20 [0017]
Fig. 2 shows that, when the steel plate includes a softened layer where Hv is 250
or less and whose thickness is 50 μm or more in the surface in the plate thickness and the
average aspect ratio of the prior austenite grains are 1.6 or more in the L section and the
T section at the thickness 1/4t respectively, the bending to both the L direction and the T
25 direction can be performed without cracking even if the bend radius is 2.Ot in regard to
8
the high-strength steel plate which has the yield strength of 885MPa or more and the
plate thielmess of 32mm or less.
[0018]
In order to increase the aspect ratio of the steel plates, a controlled rolling in a
5 non-recrystallization temperature range is effective. The aspect ratio in the L section
increases by a general rolling in one direction. However, the aspect ratio in the T
section does not increase. In order to increase the aspect ratio in both the L section and
the T section, it is effective to rotate 90-degree of rolling direction after a suitable broad
side pass rolling (the rolling to a direction perpendicular to the final rolling direction) in
10 the non-recrystallization temperature range and to roll to the final rolling direction, when
hot rolling is conducted.
[0019]
Fig. 3 indicates the relationship between the cumulative rolling reduction of the
rolling to the direction perpendicular to the final rolling direction and the average aspect
15 ratio in the T section in a temperature range of 1000°C or less (the non-recrystallization
temperature range) in regard to Nb added steel. In the same way, Fig. 4 indicates the
relationship between the cumutativcrojhng reduction of-the-rollingto the final rolling
direction and the average aspect ratio in the L section in the temperature range of 1000°C
or less. The aspect ratio was measured at the thickness 1/4t. In Fig. 3 and Fig. 4, the
20 steel plate for evaluation which includes C:0.11-0.16%, Si:0.27-0.33%, Mn:0.95-1.31%,
P:0.001-0.004%, 5:0.001-0.002%, Mo:0.16- 0.35%, AI:0.03-0.04%, Nb:0.016-0.033%,
Ti:0.012- 0.019%, B:0.0009-0.0019%, N:0.0033-0.0049%, and Pcm:0.24-0.29% as
chemical composition thereof and which has the plate thickness of 6mm to 32mm was
used.
25 [0020]
9
These figures show that, when the cumulative rolling reduction is 25% or more
in the temperature range of 1000°C or less, the average aspect ratio in both the T section
and the L section are 1.6 or more. Therefore, the broad side pass rolling in which the
cumulative rolling reduction is 25% or more is conducted in the temperature range of
5 1000°C or less, the rolling direction rotates 90-degree, and subsequently the rolling in
which the cumulative rolling reduction is 25% or more to the final rolling direction is
conducted additionally as a specific rolling method. In order to obtain the preferable
aspect ratio by the method, Nb which lias the effect of extension of the
non-recrystallization temperature range must be added.
10 [0021]
The weldability and toughness deteriorate with an increase in alloy elements to
obtain high strength. The inventors made an investigation of the relation between a
weld cracking parameter Pcm and a preheating temperature by conducting Y-groove weld
cracking test which is regulated in JIS Z 3158 in regard to various steel plates which have
15 the yield strength of 885MPa or more, the tensile strength of 950MPa to 1200MPa and
the plate thickness of 25mm to 32mm. The result is indicated in Fig. 5. In order to
seduce workload of welding, it is preferable that the preheating temperature is as low as
possible. Thus, a target is that a preheating temperature to arrest cracking, namely a
preheating temperature where a root crack ratio becomes zero, is 50°C or less in case of
20 the plate thickness of 25mm to 32mm. Fig. 5 shows that Pcm is 0.29% or less in order
that the root crack ratio becomes zero completely at the preheating temperature of 50°C,
which results in an upper limit of the amount of alloy elements as the target.
[0022]
For the toughness, a target is that an average value of absorbed energy of impact
25 tests at -40°C by using JIS Z 2242 v-notch test piece is 33J/cm2 or more which is
)o
considered to be enough as a structural member. That is obtained by limitation of an
upper limit of the amount of alloy elements which deteriorate the toughness and by
selection of a suitable tempering temperature.
[0023]
5 An aspect of the present invention employs the following.
(1) A high-strength steel plate comprising as a chemical composition, by
mass %, C : 0.10% to 0.18%, Si : 0.20% to 0.80%, Mn : 0.20% to 1.60%, Mo : 0.10% to
0.60%, Nb : 0.010% to 0.050%, Ti : 0.005% to 0.030%, Al : 0.01% to 0.10%, B :
0.0003% to 0.0030%, P : 0.012% or less, S : 0.005% or less, N : 0.0060% or less, and a
10 ba[anceconsisting of iron and unavoidable impurities, wherein, Peat defined by a
following (Equation 1) is 0.29% or less; the high-strength steel plate includes a softened
layer with Hv of 250 or less and with a thickness of 50 μm or more and of not greater
than 3% of a plate thickness of the steel plate in a surface of the steel plate; average
aspect ratio of prior austenite grains are 1.6 or more respectively in a cross section
15 parallel to a final rolling direction and a cross section perpendicular to the final rolling
direction at a position of a depth of 1/4 of the plate thickness from a surface; a yield
strengthisl85MPa or more; and-an-average value of absorbed energyafimpact tests at-
-40°C is 33J/em2 or more.
Pcm-[C]+ [Si]/30+ [Mn]/20+ [Cu]/20+ [Ni]/60+ [Cr]/20+ [Mo]/1 s+ [V]/10+5
20 [B] • . • (Equation 1)
where the [C], [Si] ,[Mn], [Cu], [Ni], [Cr], [Me], [V],and [B] are the amounts of
C, Si, Mn, Cu, Ni, Cr, Mo, V, and B, by mass %, respectively.
(2) The high-strength steel plate according to (1), further including, by mass%,
at least one of Cu:0.01% to 1.00%, Ni:0.01% to 0.25%, Cr:0.01 % to 2.00%, V:0.005% to
25 0.080%, and Ca:0.0001% to 0.0030%.
11
(3) The high-strength steel plate according to (1) or (2), wherein the plate
thickness is 6mm to 32mm, and a tensile strength is 950MPa to 1200MPa.
(4) A producing method of a high-strength steel plate, the method including,
heating a slab satisfying the chemical composition according to (1) or (2) at a
5 temperature of 1250°C to 1350°C for 3 hours or more under an atmosphere having an
oxygen concentration is 3% or more, reheating the slab at a temperature of 1100°C or
more after cooling or continuously, hot rolling to obtain a steel plate with a plate
thickness of 6mm to 32mm, wherein a rolling in which a cumulative rolling reduction is
25% or more is conducted to a direction perpendicular to a final rolling direction in a
10 temperature range of 1000°C or less, the direction of the rolling rotates 90-degree, and a
final rolling in which the cumulative rolling reduction is 25% or more is conducted,
water cooling after the hot rolling from a temperature of Ar3 or more to a temperature of
300°C or less, and tempering at a temperature of 400°C or more.
15 Advantageous Effects of Invention
[0026]
According to the present invention, it is possible to provide, by a low cost, a
high-strength steel plate for a structural member used for construction machineries and
industrial machineries, which is superior in bendability, weldability and toughness, and
20 has a yield strength of 885MPa or more, a tensile strength of 950MPa to 1200MPa, an
average value of absorbed energy of impact tests at -40°C of 33J/cm2 or more, and a plate
thickness of 6mm to 32mm.
Brief Description of Drawings
25 [0027]
12
Fig. 1 is the graph which indicates the relationship between hardness and
elongation by tensile tests in regard to steel whose hardness is almost uniform.
Fig. 2 is the graph which indicates the relationship between thickness of
softened layer, aspect ratio, and bendability.
5 Fig. 3 is the graph which indicates the relationship between cumulative rolling
reduction of rolling to direction perpendicular to final rolling direction and average
aspect ratio in T section in the temperature range of 1000°C or less.
Fig. 4 is the graph.which indicates the relationship between'cumulative rolling
reduction of rolling to final rolling direction and average aspect ratio in L section in the
10 temperature range of 1000°C or less.
Fig. 5 is the graph which indicates the relationship between Pcm and preheating
temperature to arrest cracking of Y-groove weld cracking tests.
Description of Embodiments
15 [0028]
Hereinafter, the present invention will be described in detail.
reasurrforlimitatton of a -chemical compo-sitiowof the present inverition-will
be described.
Carbon (C) is an important element which influences greatly strength of the steel
20 of the present invention which has mainly tempered martensite structure or tempered
bainite structure. The addition of 0.10% or more is necessary to obtain the yield
strength of 885MPa or more, but the addition over 0.18% results in a decrease in
bendability, so that an upper limit is 0.18%. In order to obtain the strength stably, a
lower limit of C content maybe 0.11% or 0.12%. For improvement of weldability, the
25 upper limit of C content may be 0.17%, 0.16%, or 0.15%.
13
[0029]
Silicon (Si) has improvement effect of balance between the strength and
toughness, which derives from preventing cementite from coarsening during tempering
for martensite structure. Si content is 0.2% or more because the cementite coarsens
5 easily in the martensite structure if Si content is below 0.2%. Preferably Si content is
0.25% or more. On the other hand, an upper limit is 0.80% because the toughness may
deteriorate when Si is added in surplus. For improvement of the toughness, the upper
limit of Si content may be limited to 0.55%, 0.40%, or 0.34%.
[0030]
10 Manganese (Mn) is an element which improves hardenability and the strength,
so that the addition is 0.20% or more. However the addition over 1.60% may result in a
decrease in the bendability, which is caused by acceleration of segregation and formation
of coarse MnS, so that an upper limit is 1.60%. Preferably Mn content is 1.40% or
more. For improvement of the strength, a lower limit of Mn content may be limited to
15 0.45%, 0.70%, 0.85%, or 0.95%. For improvement of the bendability, the upper limit of
Mn content may be limited to 1.40%, 1.30%, or 1.25 %.
[0031] --
Molybdenum (Mo) improves the hardenability by small amount of the addition
comparatively and is exceedingly effective in strength improvement of the tempered
20 martensite structure or the tempered bainite structure by precipitation strengthening, so
that the addition of 0.10% or more is necessary. However excessive addition may
deteriorate the weldability, and Mo is an expensive element, so that the addition is 0.60%
or less. For improvement of the strength, a lower limit of Mo content may be limited to
0.17%, 0.25%, or 0.30%. For improvement of the weldability, an upper limit of Mo
25 content may be limited to 0.55%, 0.50%, or 0.45%.
14
[0032]
Niobium (Nb) is a necessary element to form fine carbides during rolling, to
broaden the non-recrystallization temperature range, to improve controlled rolling effect,
and to increase an aspect ratio of austenite. In addition, Nb is effective in the strength
5 improvement of the tempered martensite structure or the tempered bainite structure by
the precipitation strengthening. In order that the non-recrystallization temperature range
is 1000°C or less, Nb content is 0.010% or more. However excessive addition may
deteriorate the weldability, the addition is 0.050% or less. For improvement of the
strength and grain refining, a lower limit of Nb content may be 0.014% or 0.018%. For
10 improvement of the weldability, an upper limit of Nb content may be 0.040%, 0.035%, or
0.031%.
[0033]
In the present invention, Boron (B) is utilized to obtain sufficient hardenability,
and thus it is necessary to secure free B atdirect quenching. Titanium (Ti) is added in
15 order to fix Nitrogen (N) as TiN, since N reduce the free B by forming BN.
[0034]
Ti is added 0.005%0 oorr more t the above mentionn purpose, but excessive
addition may deteriorate the weldability, so that an upper limit is 0.030%. For fixing N
certainly as TiN, a lower limit of Ti content maybe 0.008%, 0.010%, or 0.012%. For
20 improvement of the weldability, the upper limit of Ti content may be 0.026% or 0.020%.
[0035]
Aluminum (Al) is added 0.01% or more as deoxidizing agent, but excessive
addition may deteriorate the toughness, so that an upper limit is 0.10%. A lower limit
of Al content maybe 0.02% or 0.03% to deoxidize certainly. For improvement of the
25 toughness, the upper limit of Al content may be 0.07% or 0.05%.
15
[0036]
Boron (B) should be added 0.0003% or more to obtain improvement effect of
the hardenability and the strength, the addition over 0.0030% may deteriorate the
weldability and the toughness, so that B content is 0.0003% or more and 0.0030% or less.
5 For improvement of the strength, a lower limit of B content may be 0.0005%, 0.0008%,
or 0.0010%. For improvement of the weldability and the toughness, an upper limit of B
content may be 0.0025%, 0.0022%, or 0.0019%.
[0037]
Phosphorus (P) is an unavoidable impurity and harmful element which decreases
10 the toughness. Thus, P content is limited to 0.012% or less. For improvement of the
toughness, P content may be 0.010% or less, 0.008% or less, or 0.005% or less.
[0038]
Sulfur (S) is an unavoidable impurity and harmful element which decreases the
bendability by formation of MnS, so that it is preferable that S is reduced as much as
15 possible. Thus, S content is limited to 0.005% or less. For improvement of the
bendability; S content may be 0.004% or less, 0.003% or less, or 0.002% or less.
[0039]
Nitrogen (N) deteriorates the improvement effect of the hardenability of B and
decreases the toughness by forming BN as described above, when N content is excessive,
20 so that N content is suppressed to be 0.006% or less. In order to obtain the
improvement effect of the hardenability of B certainly, an upper limit of N content may
be 0.0050% or 0.0041 %. Since Nis merged as an unavoidable impurity during
production of steel, a lower limit is not specified in particular.
[0040]
25 Although the above is base component for the steel of the present invention,
16
furthermore at least one of Cu, Ni, Cr, V, and Ca can be added in addition to the above
components in the present invention. Although these components have improvement
effect of the following properties, it is preferable that these components are not added in
order to increase economic efficiency which is required of the steel for the construction
5 and industrial machineries.
[0041]
Copper (Cu) is an element which can improve the strength by solute
strengthening without a decrease in the toughness, so that the addition may be 0.01% or
more. However the improvement effect of the strength saturates even if Cu is added
10 excessively and excessive addition may deteriorate the weldability, so that the addition is
1.00% or less. For the increase in economic efficiency, an upper limit of Cu content
may be limited to 0.55%, 0.35%, or 0.25%.
[0042]
Nickel (Ni) has improvement effect of the hardenability and the toughness, so
15 that the addition maybe 0.01% or more. However Ni is an expensive element and a
softened layer required for the present invention may not be appropriately obtained by
suppression of decarbDnizing reactionwherrNi is added excessively, so-that the addition
maybe 0.25% or less. As required, the addition maybe 0.20% or less or 0.15% or less.
[0043]
20 Chromium (Cr) improves the hardenability and is effective in strength
improvement, so that the addition may be 0.01% or more. However excessive addition
may deteriorate the weldability, so that the addition is 2.00% or less. For the increase in
economic efficiency, an upper limit of Cr content may be limited to 1.00%, 0.55%, or
0.35%.
25 [0044]
17
Vanadium (V) also improves the hardenability and is effective in strength
improvement and is effective in the strength improvement of the tempered martensite
structure or the tempered bainite structure by the precipitation strengthening , so that the
addition maybe 0.005% or more. However the addition of a large amount of V may
5 deteriorate the weldability and V is an expensive element, so that the addition is 0.080%
or less. For the increase in economic efficiency , an upper limit of V content maybe
limited to 0.07%, 0.04%, or 0.03%.
[0045]
Calcium (Ca) has alleviation effect of influence of harmful MnS for the
10 wndability by spheroidizing of sulfides of a steel plate, so that the addition may be
0.0001% or more. However the addition of a large amount of Ca may deteriorate the
weldability, so that an upper limit of the addition is 0.0030% or less. For the increase in
economic efficiency, the upper limit of Ca content may be limited to 0.0020%, 0.0015%,
or 0.0010%.
15 [0046]
In addition to the limitation of the above chemical composition, as described
above, the-ehenriealtomposition in the-present invention also is limited such that Pcmdefined
by a following (Equation 1) is 0.29% or less in order to obtain the weldability
and the strength. For improvement of the weldability, an upper limit of Pcm may be
20 limited to 0.28%, 0.27%, or 0.26%. For improvement of the strength, a lower limit of
Pcm may be limited to 0.22%, 0.23%, or 0.24%.
Pcm=[C]+ [Si]/30+ [Mn]/20+ [Cu]/20+ [Ni]/60+ [Cr]/20+ [Mo]/15+ [V]/10+5
[B] .''(Equation 1)
Moreover, it is preferred that the chemical composition is limited such that a
25 Ceq defined by a following (Equation 2) is from 0.38% to 0.60%. For improvement of
18
the weldability, an upper limit of the Ceq may be limited to 0 .55%, 0.52%, or 0.49%.
For improvement of the strength, a lower limit of the Ceq may be limited to 0.40%,
0.42%, or 0.44%.
Ceq=[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14 • • . (Equation 2)
5 where the [C], [Si] ,[Mn], [Cu], [Ni], [Cr], [Mo], [V],and [B] are the amounts of C, Si,
Mn, Cu, Ni, Cr, Me, V, and B, by mass %, respectively.
[0047]
Next, a preferable producing method of a high-strength steel plate of the present
invention is described below. At first, a slab satisfying the chemical composition as
10 described above is heated at a temperature of 1250°C to 1350°C for 3 hours or more
under an atmosphere having an oxygen concentration is 3% or more. This is to form a
decarburized layer whose thickness is thick relatively in a surface of the slab, and is to
reserve the decarburized layer with the thickness of 50μm or more in the surface of the
steel plate when the slab is hot-rolled to the steel plate with the thickness of 32nim or less
15 finally, in order to form a softened layer in a surface of the steel plate. The decarburized
layer which has enough thickness of 50μm or more is not obtained when the oxygen
concentration is below 3% and the heating temperature is below 1250°C. When the
heating temperature is over 1350°C, the decarburized layer becomes too thick and an
average hardness of whole thickness or strength by tensile tests may deteriorate, so that
20 an upper limit of the heating temperature is 1350°C. At this time, since the thickness of
the decarburized layer of the steel plate becomes thin relatively if a thickness of the slab
is thick, it is preferable that the thickness of the slab for heating is not over 350mm. In
order to obtain the thickness of the decarburized layer of the steel plate sufficiently, the
thickness of the slab may be 300mm or less or 250mm or less. In order to prevent a
19
ratio of the decarburized layer in the steel plate from increasing, the thickness of the slab
may be 50mm or more, 80mm or more, or 100mm or more.
When heating time is long excessively, the decarburized layer also becomes too thick and
the average hardness of whole thickness or the strength by tensile tests may deteriorate,
5 so that it is preferable that the heating time is not over 30 hours.
[0048]
The slab just after heating may be cooled once or be dealt with continuously to
next process which is.reheating process of 1100°C or more, depending on the timing of
the following process. Although a condition of cooling is not limited especially as
10 material in case of cooling, cooling may be conducted after hot rolling or forging till an
adequate thickness as to the final plate thickness of the steel plate preliminarily. The
slab is reheated at 1100°C or more in order that Nb is dissolved as solid solution
sufficiently, and is hot-rolled to the steel plate with a plate thickness of 6mm to 32mm.
[0049]
15 A rolling in which a cumulative rolling reduction is 25% or more is conducted in
a temperature range of 1000°C or less, a direction of the rolling rotates 90-degree, and a
final rolling in which the cumulative rolling reduction is 25% or more is conducted.
According to the necessity, the direction of the rolling rotates 90-degree after this. As a
result of the hot rolling, average aspect ratio are 1.6 or more respectively in the T section
20 and the L section and the softened layer with a thickness of 50μm or more is included in
the surface of the steel plate, and thus a bending to both the L direction and the T
direction can be performed without cracking even if a bend radius of the bending is 2.Ot
of the steel plate thickness in regard to the high-strength steel plate which has a yield
strength of 885MPa or more and the plate thickness of 32mm or less. For improvement
25 of bendability furthermore, at least one of the cumulative rolling reduction of the
20
directions may be 30% or more and 35% or more.
[0050]
In order to maintain the aspect ratio of microstructure introduced by the hot
rolling, it is necessary that water cooling is conducted on line from a temperature of Ar3
5 or more to a temperature of 300°C or less following on the hot rolling. The water
cooling on line means that the water cooling is conducted following on the hot rolling by
water-cooling equipment which is arranged on the same line with rolling machine,
namely, direct quenching is conducted. A purpose of the direct quenching is to obtain
martensite or bainite as the microstructure in order to acquire high-strength. In addition,
10 the direct quenching has an advantage for productivity since the direct quenching does
not need quenching off line, in other words, quenching after reheating. In order to
maintain the aspect ratio of the microstructure introduced by the hot rolling, it is
preferable that an onset temperature of water cooling is high, and thus the onset
temperature of water cooling may be 750°C or more, 780°C of more, or 800°C or more
15 in so far as the onset temperature of water cooling is Ar3 or more.
[0051]
Since the martensite structure as quenched or the bainite structure as quenched
have high density of moving dislocation, the yield strength becomes very low as
compared with the tensile strength. Tempering at a temperature of 400°C or more is
20 conducted after the quenching to obtain tempered martensite structure or tempered
bainite structure, and thus the high strength such as the yield strength of 885MPa or more
and superior toughness are acquired at the same time. The reason why the tempering
temperature is 400°C or more is to avoid an embrittlement range of 300°C to 400°C and
to obtain precipitation strengthening sufficiently by fine carbides such as Mo. Time of
21
tempering may be 15min. or more. For improvement of the strength, an upper limit of
the tempering temperature may be 600°C, 570°C, and 550°C. To sufficiently obtain the
precipitation strengthening, a lower limit of the tempering temperature may be 425°C,
450°C, and 475°C.
5 [0052]
In addition, although it is possible that the heating at the high temperature for
the long time to form the decarburized layer is heating for the hot rolling of the steel plate,
in the case, properties of the steel plate may deteriorate because coarsening of austenite
during the heating at the high temperature for the long time leads to coarsening of grain
10 size of theaustenite after the rolling, so that it is preferable that slab soaking which is
conducted for reduction of segregation is utilized.
Example
[0053]
15 Slabs were made by casting from steels whose chemical composition were A to
Al shown in Table 1, and steel plates with a plate thickness of 6mm to 32mm were
produced by production conditions which were shown in Table 2 as examples Ito 18 and
comparative examples 19 to 49 respectively. All steel plates were cooled once after the
heating at a temperature of 1250°C to 1350°C for 3 hours or more (Decarburizing heat
20 treatment in Table 2). After that, the steel plates were reheated at 1100°C or more
(Heating temperature of Hot rolling and accelerated cooling in Table 2). Ar3 were
measured values which were measured by gauging thermal expansion of test pieces
which were sampled from the steel plates during cooling under a condition of 5°C/min
following on heating at 1100°C.
22
In regard to the steel plates, a thickness of a softened layer which has Hv of 250 or less in
a surface of the steel plate and an aspect ratio of austenite grains of the L direction and
the T direction were measured , and a yield strength, a tensile strength, weld cracking,
bendability, and toughness were evaluated.
5
[Tablel]
[0054]
[Tablet]
[0055]
[0056]
10 The thickness of the softened layer which has Hv of 25 0 or less in the surface of
the steel plate was measured by carrying out micro-Vickers hardness test (HvO.1) which
is regulated in JIS Z 2244 in an interval of 10μm, and a ratio of the softened layer in the
surface to the plate thickness was calculated.
The aspect ratio of prior austenite grains of the L direction and the T direction
15 was measured by image analysis of five visual fields at the thickness 1 /4t where each
visual field was 200μmx200μm and prior austenite grain boundaries were etched by
picric acid.
[0057]
The yield strength and the tensile strength were measured by tensile test which is
20 regulated in JIS Z 2241 and by using a tensile test piece of I A which is regulated in JIS Z
2241. The yield strength of 885MPa or more was considered to be good.
[0058]
The bendability was evaluated by JIS Z 2248 in which 180-degree bending was
conducted by the bend radius which was 2.0 times of the plate thickness (2.0t) using
25 No.1 test piece along the L direction and the T direction. The case in which a crack, a
23
scar, and any defect did not occur on an outer side of a bend part after the bending in the
both directions was considered to be good.
[0059]
The weld cracking was evaluated by Y-groove weld cracking test which is
5 regulated in JIS Z 3158. Condition of welding was heat input of 15kJ/em in CO2
welding, and the plate thickness of the steel plate which was used for the evaluation was
25mm and 32mm. Asa result of the test, a root crack ratio of zero at a preheating
temperature of 50°C was considered to be good. In regard to the steel plate of example
1, 4, and 8 which have the thickness of 12mm and 6mm, Y-groove weld cracking test was
10 omitted because weldability was considered to be as the same as example 2, 5, and 6
which have the same chemical composition.
[0060]
The toughness was evaluated by an average value of absorbed energy of impact
tests at -40°C by using 3 pieces of JIS Z 2242 v-notch test piece which was sampled
15 perpendicular to rolling direction from a central part of the plate thickness, and a target
was 33J/cm2 or more. In regard to the steel plate with the thickness of 6mm, a charily
-specimen o subsize of 5mm was used,-and the-target was value-o the absorbed-energy
of 33J/cm2 or more.
[0061]
20 In Table 1, underlined chemical composition and underlined Pcm mean that the
value is out of range of the present invention. In Tablet, underlined values mean that
the production condition is out of range of the present invention or property is
unsatisfactory.
[0062]
25 In regard to examples 1 to 18 in Tables, all examples include the softened layer
24
with Hv of 250 or less and with the thickness of 50 μm or more and of not greater than
3% of the steel plate thickness in the surface of the steel plate, satisfy that average aspect
ratio of the prior austenite grains of the L direction and the T direction are 1.6 or more
respectively at the thickness 1/4t, and satisfy the target of the yield strength, the
5 bendability, the weld cracking, and the toughness.
[0063]
On the other hand , in regard to comparative examples 19 to 37 which do not
satisfy the chemical composition of the present invention as shown by underline in the
Tables, although the producing method satisfy the production condition of the present
10 invention, these comparative examples do not satisfy at least one of the yield strength,
the bendability, the weld cracking, and the toughness.
Comparative example 38 which satisfies the chemical composition but does not
satisfy Pcm is that the weld cracking is rejected.
[0064]
15 Comparative example 39 in which the heating temperature of decarburization is
low, comparative example 41 in which the heating time of the decarburization is short,
and comparative example 42 in which the oxygen concentration infurnace during the
decarburization is low are that the thickness of the softened layer in the surface is thin,
though the chemical composition and Pcm of these comparative examples are in the
20 range of the present invention. Thus, the bendability of these comparative examples is
rejected. Comparative example 40 in which the heating temperature of decarburization
is excessively high is that the ratio of the softened layer in the surface is high, so that the
yield strength is low. Comparative example 43 in which the heating temperature of the
hot rolling is low is that grain refining of austenite is insufficient because Nb is not
25 dissolved as solid solution, so that the bendability is rejected. Comparative example 44
25
in which the cumulative rolling reduction of a direction perpendicular to a final rolling
direction at 1000°C or less is low and comparative example 45 in which the cumulative
rolling reduction of the final rolling direction at 1000°C or less is low are that the aspect
ratio of the L direction and the T direction is low respectively, so that the bendability is
5 rejected. Comparative example 46 in which the onset temperature of water cooling is
low and comparative example 47 in which the end temperature of the water cooling is
low are that required hardened structure is not obtained, so that the yield strength is low.
Comparative example 48 in which the tempering is not conducted is that the yield
strength is low. Comparative example 49 in which the tempering temperature is low
10 is that the toughness is low.
TABLE. 1
mass 1%1
Steel
Composition
'
51 '3
f
P S q
q
3
j Cu Ni Cr MO Al Nb V Ti Ca, 9 E
1
Ce¢4 Peat*
Ara
temperature
(L)
A 0. 934 '• 0.36 L 39 0.004 0.002 1 55 0. 02 0.02P 0.616 0.0011 0.0042 0.516 0.256 892
6 0. 178 0. 31 6.48 0.003 0.001 0.56 0.08 0.016 0.026 0. 0069 0.0031 0.409 0.252 715
C 0. 159 0.31
- ,
1.26 0.009 9. O91 0- 35 0.04 0.025 6.818 0. 0011 0, 0038 0.479 1264 692
D 0. 129 C 29 .54 0.004 0.602 0.3". 0. 04 0. 037 0. 009 7, 0 009 0.0032 0.488 0.843 684
E 9 0. 112 0. 8 1. a7 O. fl02 0.002 C_ 93 9: 99 0.68 0. 014 0. 920 ? 0.0012 3 0.0041 0. 538 0, 245 689
F 0, 135 0. 36 1.92 0. {103 0.0 l 0. 77 9. 25 0. 08 0.022 0.613 0-.0015 1 0 0033 9.537 0, 261 008
e G 9 9. E24 0. 30 9.78 0.003 6. 091 0, 31 0. 19 1. 15 0.17 0 04 0-022 0.012 0.0009 0.0042 0. 549 0.270 719
E1 0. 138 0.28 0.89 10.009 0.002 i 0.89 0.28 0.42 0. 15 0.635 C. 008 0. 0052 0.11042 0.533 0. L61 708
I 0. 145 9. 28 1.22 0.005 0.002 4.43 0.03 0.626 0. 062 0.6I3 . 0.0011 0.0034 .472 0.256 689
1 0.133 9.33 1. 36 0.00 `33 0.00 t 0 31 0 34 6.03 0- 013' 3 012 0.0009 0.0024 0. 0036 0.826
.
0262 699
8 0. 137 9.29 1.05 0.003 0.01)1 .22 0.64 0 29 0.03 0.020 0.609 70610 0.0 337 0.530 f 0.239 765
L 0. 143 0.31 736 0.003 0.003 0. 25 0.55 37.23 0.04 1 O.9E5 0-01 0..0015 0- 0038 0.5411- 0.28? 695 NJ 9. €68 0.34 0. 0 0.003 0- 002 0.49 6.93 0.031 0.017 0.0012 0- 0018 0.0041 0.468 0. 76 700
N 0.129 0 a` 1 1 15 0.00 5 0. 002 0.52 23 0. 34 0. 04 0.013 0. 011 0.00 1 6 0.0029 0.433 0.204 705
0 0: 189 0.32 0.72 0.004 M02 0.35 139 0.04 0.018 0.018 0.0 112 6, 0 331 0.400 0.265. 711
P 1 0.29 1.55 0.004 6.002 0.38 0.C3 0.012 0.615 0.6120 0.0033 0.440 0.254 692
Q - 0- 20 5. 0.28 0.80. 0.005 0.901 01 28 0.05 0.012 6.014 0.0012 0.0035 1 398 0.27? 709
it 0: 155 9,11 1.49 1004 0.001 9.31 0.08 0.015 0.020 O.U014 0.0029 0.484 0.257 683
S 1 0. 132 4.33 8.005 0.003 0. 33 0. 04 0- 013 0,. 220 1 0.0009 0.0034 0.474 0.255 705
T IF 172 0.38 _ 0.004 9. 903. 0.5. 0. C4 '. . 0.020 0. 0 42 0.0011 0.0940 0.334 0.239 731
m U 0 0.136 9.32 it 0.003 6. 002 0.28 0.64 0.016 9.6f3 0.0012 0.0033 0.658 9.278 686
Y 0.149 9.31 t. 33 '..=1 8.602 9.31 4.03. 0.015 0-015 0.0014 00034 0.481 0, 264 693
6 IM 0.28 1.25 0.005 LM 1 0.32 0. 04 10115 A 914 0.0070 0.0033 0.438 0.266 696
R a 0.. 148 0.3;11 1.53 0.050 0. 901 0.04 0. 014 0-018 10008 0.0032 0.427 6. 242 691
> Y 0. 137 0 31 1. 11 0. 004 0.002 0.05 1 014 0.013 0. Q012 10041 0.515 0. 257 695
1 0. 150 0. 34 1. 12 0. 005 6.602 6.37 0.618 0. 016 0.0051 0.0029 1 0.453 0, 250 699
AA Q 147 0.28 1.24 0.966 O.001 '. 0.42 ? 0.150 iL 412 0.0012 0_0035 0.470 0.252 693
A$ 0. 148. 0.30 L 33 0.004 0.003 '., 6.33 1 0: 07 -U01 I 0. 1€12 6.00E I 0. 0041 9- 46S 9.252 701
s AC 0. 1+12 L.33 1.40 0.003 0 462 0.28 0.05 LLM 0. Ott 0.0015 0.0032 0.458 0.249 692
AD 0.144 0.31 1.34 0.005 0.002 1- 28. 0.03 0.012 0.0013 0.0041 0.456 0.247 1 694
Al 7 9. 155 0.29 1. 16 0.004 0. 001 50.40 0.64 0. 0 t$ ^ 0.0011 1 0. 0035 1 484 0.255 596
A6 0. 144 0.29 1.37 0.005 0.003 132 0.03 0. 912 4.011 D. e 0.0034 0.484 U. 2441 692
AG 0.133 6.32 . 1.22 0.608 0'. 602 0.30 . 005 0.915 9.012 1 0. UO 0.00391 0.425 0.247] 706
AN 0. 139 0. 30 1.38 1 0.694 0. 001 0.40 0.08 0.010 0.013 0.00!'-9 D- 0.482 0. 11 591
Al 9. 374 11.39 1.54 0, 003 G. 003 0.57 0. 65 0, 414 0 014 0. 0910 0.0028 O. 574 - 676
4Ce4=C+Si/24+Un/6112i/40+Cr/5+Mo74+V/14
+4Pea'_=C+Si/30+4n / 26+Cu / 20+P11/60fCr /20+1tn/l5+V/10
TABLE. 2
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a:Steel Plate
b:Steel Comopsition
e:Decarburizing heat treatment
d:Heating temperature of Decarhurization ('C)
e:Heating time of Decarburization (hr)
f:Oxygen concentration in furnace during Decarburization (%)
g:Hot rolling and accelerated cooling
k:Plate thickness after final rolling (mm)
I:Onset temperature of water cooling ('C)
m:End temperature of water cooling (°C)
n:Tempering temperature (°C)
o:Properties of Steel Plate
p:Yield strength (MPa)
h:Heating temperature of Hot rolling (°C) - q:Teusilc strongth (.MPa)
C:Aspeot ratio of T direction
u:Aspeel ratio of L direction
v:Bendability of T direction
w:Bendability of I, direction
x:Peldability
y:Toughness vF -'10 (J/cm2)
i:Cumulative rolling reduction of direction perpendicular to final rolling at 1000' C or less (%) r:Thickness of softened layer of 11v250 or less in surface (μm)
j:Cumulative rolling reduction of final rolling direction at 1000° C or Less (%) s:Thickness ratio of softened layer of 11v250 or less in surface (%)
26

CLAIMS
What is claimed is:
1. A high-strength steel plate comprising as a chemical composition, by mass %,
5 C : 0. 10% to 0. 18%,
Si : 0.20% to 0.80%,
Mn : 0.20% to 1.60%,
Mo : 0.10% to 0.60%,
O
Nb : 0.010% to 0.050%,
10 Ti 0.005% to 0.030%,
Al : 0.01% to 0.10%,
B : 0.0003% to 0.0030%,
P : 0.012% or less,
S : 0.005% or less,
15 N : 0.0060% or less, and
a balance consisting of iron and unavoidable impurities,
wherein, Pcn2 tlefined-byafoilowing (Equation 1) is Or 29% or less;
the high-strength steel plate includes a softened layer with Hv of 250 or less and
with a thickness of 50 μm or more and of not greater than 3% of a plate thickness of the
20 steel plate in a surface of the steel plate;
average aspect ratio of prior austenite grains are 1.6 or more respectively in a
cross section parallel to a final rolling direction and a cross section perpendicular to the
final rolling direction at a position of a depth of 1/4 of the plate thickness from a surface;
a yield strength is 885MPa or more; and
25 an average value of absorbed energy of impact tests at -40°C is 33J/cm2 or more.
2
Pcm=[C]+ [Si]/30+ [Mn]/20+ [Cu]/20+ [Ni]/60+ [Cr]/20+ [Mo]/15+ [V]/10+5
[B] ... (Equation 1)
where the [C],•[Si] ,[Mn], [Cu], [Ni], [Cr], [Mo], [V],and [B] are the amounts of
C, Si, Mn, Cu, Ni, Cr, Mo, V, and B, by mass %, respectively.
5
2. The high-strength steel plate according to claim 1, further comprising, by mass%,
at least one of
Cu:0.01% to 1.00%,
Ni:0.01 % to 0.25%,
10 Cr:0.01% to 2.00%,
V:0.005% to 0.080%, and
Ca:0.0001 % to 0.0030%.
3. The high- strength steel plate according to claim 1 or 2, wherein the plate
15 thickness is 6mm to 32mm, and a tensile strength is 950MPa to 1200MPa.
4. A producing method of a high-strength steel plate, the method comprising,
heating a slab satisfying the chemical composition according to claim 1 or 2 at a
temperature of 1250°C to 1350°C for 3 hours or more under an atmosphere having an
20 oxygen concentration is 3% or more,
reheating the slab at a temperature of 1100°C or more after cooling or
continuously,
hot rolling to obtain a steel plate with a plate thickness of 6mm to 32mm,
wherein a rolling in which a cumulative rolling reduction is 25% or more is conducted to
211
2
a direction perpendicular to a final rolling direction in a temperature range of 1000°C or
less, the direction of the rolling rotates 90-degree, and a final rolling in which the
cumulative rolling reduction is 25% or more is conducted,
water cooling after the hot rolling from a temperature of Ar3 or more to a
5 temperature of 300°C or less, and
tempering at a temperature of 400°C or more.