Technical Field
[000 I] The present disclosure relates to an abrasion-resistant steel.
Background Art
[0002] In general, the abrasion resistance of steels is correlated with the hardness. For
example, for abrasion-resistant steels used in industrial machines such as cutting edges of
industrial waste processing machines, a high hardness of from 360 to 550 in Brinell hardness
HB on the surface is required. In order to increase the hardness of a steel, it is effective to
make the metal structure into mmiensite by quenching, and conventionally, an
abrasion-resistant steel improved in hardenability by containing a variety of alloy elements
has been proposed (sec, for example, Patent Documents I to 4).
[0003] In recent years, with the increase in the size of industrial machine and the like, thick
abrasion-resistant steels are being required. For example, abrasion-resistant steel plates
having a plate thickness of about from 50 to I 00 mm are manufactured. From the viewpoint
of prolonging the life with respect to abrasion, abrasion-resistant steels having small
differences in hardness between surface layers and central portions in the plate thickness arc
being required. To satisfy such requirements, abrasion-resistant steels containing Nb and B,
and fmthcr one kind or more of Cu, Ni, Cr, Mo, V, and Ti have been proposed (for example,
see Patent Document 5).
[0004] Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2016-79459
Patent Document 2: JP-A No. 2014-194043
Patent Document 3: JP-A No. 2014-194042
Patent Document 4: JP-A No. 2012-214890
Patent Document 5: JP-A No. H09-118950
SUMMARY OF INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0005] Conventionally, for example. when manufacturing a thick abrasion-resistant steel, the
cooling rate at a central portion in the plate thickness becomes slow. Therefore, it is
necessary to contain a large content of expensive alloy elements such as Mo, Cr, Cu, and Ni to
secure hardenability, resulting in high cost. In such cases, B which remarkably improves the
hardenability of a steel in a trace content IS utilized as an extremely useful element for
I
~---!
r/
avoiding an increase in alloy cost.
[0006] Further, it is known that when B is added together with Mo, the hardenability is
remarkably improved. However, an effect corresponding to the contents of B and Mo may
not be obtained in some cases.
(0007] One aspect of the present disclosure is to provide an abrasion-resistant steel in which
the hardenability of B is effectively utilized, the plate thickness exceeds 50 mm, and the
hardness difference between a central portion in the plate thickness and the surface is small.
MEANS FOR SOLVING THE PROBLEMS
[0008] Means for solving the problem includes the following aspects.
[0009] An abrasion-resistant steel, including, by mass %:
more,
C: 0.10 to 0.40%,
Si: 0.05 to 0.50%,
Mn: 0.50 to 1.50%,
B: 0.0015 to 0.0050%,
Mo: 0.60 to 2.50%,
AI: 0 to 0.300%,
S: 0.0 I 0% or less,
P: 0.015% or less,
N: 0.0080% or less,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
Cu: 0 to 1.50%,
Ni: 0 to 2.00%,
Cr: 0 to 2.00%,
V: 0 to 0.20%,
Ca: 0 to 0.0 I 00%,
REM: 0 to 0.0 I 00%,
Mg: 0 to 0.0100%,
W: 0 to 2.00%, and
a balance: Fe and impurities, wherein:
contents (mass%) ofMo and B satisfy Mo x B > 0.00 I 0,
a mass fraction ofMo2FeB2 is tl·mn 0.0010 to 0.1000%,
an area 1'atio of martensite in a central portion in a thickness direction is 70% or
2
I
element.
Ceq obtained by the following (Formula I) is 0.80% or less, and
a plate thickness exceeds 50 mm;
Ceq= C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 (Formula I),
wherein, in (Formula 1), C, Mn, Cu, Ni, Cr, Mo, and V are contents (mass%) of each
<2> The abrasion-resistant steel according to , wherein a mass fi·action of
Fe23(C, B)6 is 0.0020% or less.
<3> The abrasion-resistant steel according to or <2>, wherein the contents
(mass%) ofMo and B satisfy Mo x B 2: 0.0015.
<4> The abrasion-resistant steel according to or <2>, wherein the contents
(mass%) ofMo and B satisfy Mo x B 2: 0.0020.
<5> The abrasion-resistant steel according to any one of to <4>, wherein a
content (mass%) ofMo satisfies from 0.70 to 2.50%.
EFFECTS OF THE INVENTION
[00 I 0] According to the disclosure, it is possible to provide an abrasion-resistant steel in
which the hardenability of B is effectively utilized, the plate thickness exceeds 50 mm, and
the hardness difference between a central portion in the plate thickness and the surface is
small. Accordingly, industrial contribution of the disclosure is extremely remarkable.
BRIEF DESCRIPTION OF DRAWINGS
[0011] Fig. is a view for explaining the hardenability of a conventional steel containing
Mo and B.
Fig. 2 is a view for explaining hardenability of a steel containing Mo and B.
Fig. 3 is a view showing an observation photograph of a central portion 111 the
thickness direction of an abrasion-resistant steel by an optical microscope.
DESCRIPTION OF EMBODIMENTS
[00 12] The abrasion-resistant steel which is an example of the disclosure below will be
described in detail.
In the disclosure, the percentage of the content of each element means % by mass
unless otherwise specified.
In the disclosure, a numerical range expressed using "from A to B" means a range
including numerical values A and Bas a lower limit value and an upper limit value.
[00 13] The abrasion-resistant steel of the disclosure has a predetermined chemical
composition, the contents (mass%) ofMo and B satisfy Mo x B > 0.0010, the mass fi"action
of Mo2FeB2 is fi·om 0.00 I 0 to 0.1000%, the area ratio of martensite in a central portion in the
3
thickness direction is 70% or more, Ceq obtained by the following (Formula I) is 0.80% or
less, and the plate thickness exceeds 50 mm.
[00 14] According to the above configuration of the abrasion-resistant steel of the disclosure,
the hardenability ofB is effectively utilized, and when the plate thickness exceeds 50 mm, the
difference in hardness between a central portion in the plate thickness and the surface
becomes small. The abrasion-resistant steel of the disclosure was discovered by the
following findings.
[00 15] The inventors have focused on the relationship between the contents of Mo and B
and the hardenability and studied abrasion-resistant steels with small change in hardness
depending on the cooling rate and a method of manufacturing the same. As a result, the
inventors obtained a finding that the hardenability is stably improved by increasing the
contents of B and Mo, by heating to an appropriate temperature and quenching, as compared
with conventional att. As a result of investigating the cause, it was found that a precipitation
Mo2FeB2 which has not been observed in low alloy steels is generated, thereby suppressing
reduction in hardenability. Specifically, it is as follows.
[00 16] In general, B is an element that improves hardenability even in a trace content, and
an effect is exhibited when the content is 0.0003% or more. Conventionally, Mo is known as
an element for improving hardenability by containing Mo together with B at the same time.
However, as shown in Fig. I, in the case of a steel containing less than 0.60% of Mo, it is
understood that when the content of B exceeds 15 ppm (0.0015 mass%), the hardenability
deteriorates greatly. The reason why the hardenability deteriorates is due to precipitation of
Fe23(C,B)6, and there was a problem that a stable hardenability is unable to be obtained unless
the content ofB is strictly controlled.
[0017] In order to more effectively utilize an effect of improving the hardenability due to
inclusion ofB, the inventors examined the relationship between the contents ofMo and Band
the hardenability. As a result, as shown in Fig. 2, it was found that when 0.60% or more of
Mo is contained, even when B is contained at 15 ppm or more, it is possible to stably obtain
high hardenability. \\Then a precipitate was analyzed by a transmission electron microscope,
it was found that Mo2FeB2 was precipitated.
[00 18] As shown in Fig. I, an effect of B is highest when the content is about 0.00 I 0%, and
the effect tends to deteriorate when the content is larger than or less than that. Therefore,
conventionally, B was contained in a content of about 0.00 I 0% in order to make the most of
the effect of B, and in such a case, as the thickness increases, the cooling rate decreases in a
central portion in the thickness, causing the hardness to decrease in the thickness direction
4
I
~
from the surface layer.
[0019] In order to fully utilize the hardcnability ofB, it is necessary to precipitate Mo2FeB2.
The inventors have fut1her studied and found that the contents of B and Mo need be 0.0015%
or more and 0.60% or more, respectively, as described above.
In addition to this, investigations were conducted from the thermodynamic point of
view on the condition that Mo2FeB2 is stably produced (mass fi·action: produced at fi·om
0.00 I 0 to 0.1 000%). As a result, the inventors have found that the product of Mo content
[Mo] (%) and B content [B] (%) is imp011ant and that the dependency of cooling rate on
hardenability decreases when [Mo] x [B] > 0.0010. Fut1her, it was found that the change in
the hardness in the thickness direction is fut1her suppressed as compared with the case where
the contents ofB and Mo are 0.0015% or more and 0.60% or more, respectively.
[0020] As described above, in the abrasion-resistant steel of the disclosure, an cftcct of
improving the hardenability of B can be effectively utilized. As a result, it was found that
when the plate thickness exceeds 50 mm, the hardness difference between a central p011ion in
the plate thickness and the surface becomes small.
[0021] As described above, conventionally, in a region where the Mo content is small (the
content ofMo is less than 0.60%), the hardenability improvement eftcct by B is very likely to
change due to a variation of B content (see Fig. I). Therefore, in order to enjoy a
hardenability improving effect by B, the content of B needs to be controlled within a narrow
range, resulting in a problem of high production load.
However, according to the abrasion-resistant steel of the disclosure, as shown in Fig.
2, by increasing the contents ofB and Mo, fi·om 0.0010 to 0.10% ofMo2FeB2 is precipitated.
As a result, it is possible to enjoy a hardenability improving effect by B in a wide range of B
content.
As described above, in the abrasion-resistant steel of the disclosure, the control range
of the B content can be relaxed and the manufacturing load can be reduced.
[0022] Hereinafte1~ the reasons for limiting the chemical composition of the
abrasion-resistant steel of the disclosure (hereinafter, also referred to as "chemical
composition of the disclosure") will be described.
[0023] The chemical composition of the disclosure contains C: 0. I 0 to 0.40%. Si: 0.05 to
0.50%, Mn: 0.50 to 1.50%, B: 0.0015 to 0.0050%, Mo: 0.60 to 2.50%, AI: 0 to 0.300%, S:
0.0 I 0% or less, P: 0.015% or less, N: 0.0080% or less, Ti: 0 to 0. I 00%, Nb: 0 to 0. I 00%, Cu:
0 to 1.50%, Ni: 0 to 2.00%, Cr: 0 to 2.00%, V: 0 to 0.20%, Ca: 0 to 0.0100%, REM: 0 to
0.0 I 00%, Mg: 0 to 0.0 I 00%, W: 0 to 2.00%, and the balance: Fe and impurities.
5
I
It is noted that AI, Ti, Nb, Cu, Ni, Cr, \~ Ca, REM, Mg, and Ware optional elements.
In other words, these elements may be not contained in the abrasion-resistant steel of the
disclosure, and the lower limit of the content of these elements is 0%.
[0024] [C: 0.10 to 0.40%]
C is the most eftective element to promote formation of matiensitc and increase the
hardness. In order to secure the hardness which is the controlling factor of the abrasion
resistance, the content of C is set to 0.10% or more. The content of C is preferably 0.13% or
more, and more preferably 0.15% or more. On the other hand, C is an element which
inhibits workability and weldability when contained in a large content. Therefore, the
content of C is set to 0.40% or less. The content of C is preferably 0.35% or less, and more
preferably 0.30% or less.
(0025] [Si: 0.05 to 0.50%]
Si is a deoxidizing element. In order to obtain an effect of deoxidation, the content
of Si is set to 0.05% or more. Si contributes to increasing the hardness by solid solution
strengthening. Therefore, the content of Si is preferably set to 0.10% or more. On the
other hand, when the content of Si exceeds 0.50%, the toughness remarkably decreases.
Therefore, the content of Si is set to 0.50% or less. The content of Si is preferably 0.40% or
less, and more preferably 0.30% or less.
[0026] [Mn: 0.50 to 1.50%]
Mn is an element contributing to the improvement of hardenability. In order to
promote formation of matiensite and ensure the hardness, the content ofMn is set to 0.50% or
more. The content of Mn is preferably 0.80% or more, and more preferably 1.00% or more.
On the other hand, excess Mn deteriorates the toughness. In patiicular, in the case of a thick
material whose plate thickness is enlarged, the influence becomes remarkable. Therefore,
the content ofMn is set to 1.50% or less. The content ofMn is preferably 1.40% or less, and
more preferably 1.30% or less.
[0027] [8: 0.0015 to 0.0050%]
8 is the most important element that stably increases the hardenability of the
abrasion-resistant steel of the disclosure (reduces the dependency of the cooling rate). In
order to fully utilize an effect of improving the hardenability due to the inclusion of 8 by the
formation ofMo2Pc82, it is needed to make the content of8 0.0015% or more. The content
of 8 is preferably 0.0017% or more, and more preferably 0.0020% or more. On the other
hand, excess 8 coarsens a precipitate, which may impair the toughness. Therefore, the
content of B should be 0.0050% or less. The content of 8 is preferably 0.0045% or less or
G
I
0.0044% or less, and more preferably 0.0033% or less or 0.0026% or less.
[0028] [Mo: 0.60 to 2.50%]
Mo is an extremely impm1ant element for promoting the formation of Mo2FeB2
which contributes to improvement of hardenability by B. Mo is contained 0.60% or more in
order to stably utilize an effect of B. When the content ofMo is less than 0.60%, Mo2FeB2 is
not stably produced even when 0.0015% or more of B is contained. The content of Mo is
preferably 0.70% or more or 0.80% or more, and more preferably 0.90% or more or 1.00% or
more.
On the other hand, even when Mo is contained in a content exceeding 2.50%,
toughness is deteriorated due to coarsening of B precipitate (Mo2FeB2). Therefore, the Mo
content is set to 2.50% or less. However, Mo is an expensive element. Therefore, the
content ofMo is preferably 2.00% or less, and more preferably 1.50% or less.
[0029] Here, Mo is also an element that retards formation of precipitates (Fe23(C,B)6) that
impair an effect of improving the hardenability by B. Therefore, when the content ofMo is
within the above range, precipitation ofFe23(C,B)6, which causes decrease in the hardenability,
is easily suppressed.
[0030] [Mo x B > 0.0010]
In order to fully utilize the hardcnability of B, it is needed to precipitate a ce11ain
content of Mo2FeB2. Therefore, as described above, the contents of B and Mo need be
0.0015% or more and 0.6% or more, respectively.
In addition to this, the product of the Mo content (Mo) (%)and the B content [B] (%)
is important to ensure that Mo2FeB2 is stably produced (for example, produced with a mass
fraction of from 0.0010 to 0.100%). Therefore, [Mo] x [B] is set greater than 0.0010.
More preferably, [Mo] x [B] is 0.0012 or more, more preferably 0.0015 or more, particularly
preferably 0.0018 or more, and most preferably 0.0020 or more.
It is preferable that [Mo] x [B] is as large as possible. Therefore, it is set to 0.0125
or less which is obtained fi·om the upper limit of the contents ofMo and B. If necessary, the
upper limit of[Mo] x [B] may be 0.0100, 0.0070, or 0.0040.
[0031] Here, when Mo x B > 0.0010, in addition to stable formation of Mo2FeB2,
precipitation ofFe23(C,B)6 which impairs an effect of improving the hardenability by B is also
easily suppressed (for example, suppressed to a mass fraction of0.0020% or less).
[0032] Next, the content of AI, impurities S, P, and N used for deoxidation will be described.
[0033] [AI: 0 to 0.300%]
AI is a deoxidizing clement, and when the content thereof exceeds 0.300%, coarse
7
I
inclusions are formed to lower toughness. Therefore, the content of AI is set to 0.300% or
less. The content of AI is preferably 0.100% or less, and more preferably 0.070% or less.
On the other hand, deoxidation is also possible with elements other than AI, and the lower
limit may be 0%.
However, AI forms AIN, and is effective for suppressing formation of BN which
inhibits the hardenability. The finely precipitated AIN contributes to improvement of
toughness by refinement of crystal grain. In order to obtain such an effect, the content of AI
may be set to 0.0 I 0% or more or 0.030% or more.
[0034] (S: 0.010% or Less]
S is susceptible to grain boundary segregation and causes grain boundary cracking.
Therefore, the content of S is set to 0.0 I 0% or less. S is an element for forming MnS and
suppresses deterioration of toughness due to formation of coarse MnS. Therefore, the
content of S is preferably 0.005% or less. The content of S is more preferably 0.003% or
less. The content of S is desirably reduced as much as possible, and S may be permitted to
be contained 0.0001% or more in consideration of cost.
[0035] [P: 0.015% or Less]
P is a harmful clement causing grain boundary cracking and weld cracking.
Therefore, the content ofP is set to 0.015% or less. The content ofP is preferably 0.012% or
less. The content of P is desirably reduced as much as possible, and may be permitted to be
contained 0.00 I% or more in consideration of cost.
(0036] [N: 0.0080% or Less]
N is an element that forms a nitride, and when BN is generated, the hardenability
deteriorates. In order to suppress precipitation of BN, AI or Ti may be contained. Further,
when the content ofN exceeds 0.008%, coarse nitrides are formed, which causes deterioration
in toughness. Therefore, the content of N is set to 0.0080% or less. The content of N is
preferably 0.0070% or less, and more preferably 0.0060% or less. It is desirable that the
content of N be reduced as much as possible, and N may be permitted to be contained in a
content of 0.00 I 0% or more in consideration of cost.
(0037] Here, the abrasion-resistant steel of the disclosure may contain one kind, or two or
more kinds of Ti, Nb, Cu, Ni, Cr, V, and \V in order to improve abrasion resistance due to
formation of a precipitate or improvement of the hardcnability. The inclusion of these
elements is not indispensable. In other words, the lower limit of these contents is 0%.
(0038] (Ti: 0 to 0.1 00%]
Like AI, Ti is an element used for deoxidation and nitride formation. However,
8
when the content of Ti exceeds 0.1 00%, coarse TiN is formed and the toughness deteriorates.
Therefore, when Ti is contained, the content ofTi is set to 0.100% or less. The content ofTi
is preferably 0.050% or less, and more preferably 0.030% or less. In order to suppress
formation of BN and obtain an effect of improving the hardenability by B, the content of Ti is
preferably 0.0030% or more. The content of Ti is more preferably 0.0050% or more, and
still more preferably 0.0 I 00% or more. In order to suppress formation of BN by formation
of TiN, it is preferable to set Ti!N to 3.4 or more.
[0039] [Nb: 0 to 0.1 00%]
Nb is an element which forms carbides and nitrides and contributes to improvement
of toughness by refining the structure. However, even when Nb is excessively contained, an
effect thereof is saturated and the weldability is inhibited. Therefore, when Nb is contained,
the content ofNb is set to 0.100% or less. The content of Nb is preferably 0.050% or less.
On the other hand, in order to stably obtain the effect ofNb, the content ofNb is preferably
0.003% or more. The content of Nb is more preferably 0.005% or more, and still more
preferably 0.0 I 0% or more.
[0040] [Cu: 0 to 1.50%]
Cu is an effective element for increasing the hardness without deteriorating the
toughness. However, excess Cu causes hot cracking during manufacturing. 1l1erefore,
when Cu is contained, the content of Cu is set to 1.50% or less. The content of Cu is
preferably 1.00% or less, and more preferably 0.50% or less. On the other hand, in order to
stably obtain an effect of Cu, the content of Cu is preferably 0.05% or more. The content of
Cu is more preferably 0.10% or more.
[0041] [Ni: 0 to 2.00%]
Ni is effective for improving hardness and toughness. Howevet; excess Ni also
saturates an effect thereof, which raises the cost. Therefore, when Ni is contained, the
content ofNi is set to 2.00% or less. The content ofNi is preferably 1.00% or less, and more
preferably 0.80% or less or 0.50% or less. If necessary, the upper limit of the content ofNi
may be set to 0.40%, 0.25%, or 0.1 0%. On the other hand, in order to stably obtain the
effect of Ni, the content of Ni is preferably 0.05% or more. The content of Ni is more
preferably set to 0.1 0% or more.
[0042] [Cr: 0 to 2.00%]
Cr is an element for improving hardenability. However, when the content of Cr
exceeds 2.00%, toughness or weldability is impaired. Therefore, when Cr is contained, the
content of Cr is set to 2.00% or less. The content of Cr is preferably 1.50% or less, and more
9
I r" "
.
preferably 1.00% or less. In order to further improve weldability, Cr may be 0.60% or less
or 0.30% or less. On the other hand, in order to stably obtain the effect of Cr, the content of
Cr is preferably 0.10% or more. The content of Cr is more preferably 0.30% or more.
[0043] [V: 0 to 0.20%]
V is an element that forms a carbide and a nitride to refine the stoucture and to
improve hardenability. However, when the content of V exceeds 0.20%, toughness and
weldability are impaired. Therefore, when Vis contained, the content ofV is set to 0.20% or
less. The content of V is preferably 0.10% or less, and more preferably 0.05% or less. On
the other hand, in order to stably obtain the effect ofV, the content ofV is preferably 0.003%
or more. The content ofV is more preferably 0.0 I% or more.
[0044] The abrasion-resistant steel of the disclosure may contain one or both of Ca and
REM (Rare-Earth Metal) to control the morphology of an inclusion by forming an oxide or a
sulfide. The inclusion of these elements is not indispensable, and the lower limit of these
contents is all 0%.
[0045] [Ca: 0 to 0.0100%]
Excess Ca coarsens an inclusion, which inhibits the toughness. Therefore, when Ca
is contained, the content of Ca is set to 0.0 I 00% or less. The content of Ca is preferably
0.008% or less, and more preferably 0.0060% or less. On the other hand, in order to stably
obtain the effect thereof, the content of Ca is preferably 0.0003% or more. The content of
Ca is more preferably 0.0005% or more, and stillmore preferably 0.0010% or more.
[0046] [REM: 0 to 0.0100%]
Like Ca, excess REM coarsens an inclusion, which inhibits the toughness.
Therefore, when REM is contained, the content of REM is set to 0.0 I 00% or less. The
content of REM is preferably 0.0080% or less, and more preferably 0.0060% or less. On the
other hand, in order to stably obtain the effect thereof, the content of REM is preferably
0.0003% or more. The content of REM is more preferably 0.0005% or more, and stillmore
preferably 0.00 I 0% or more.
[0047] Here, REM means a rare earth element, which is a generic term for 17 kinds of
clements consisting of Sc (scandium), Y (yttrium), La (lanthanum), Ce (cerium), Pr
(praseodymium), Nd (neodymium). Pm (promethium), Sm (samarium), Eu (europium), Gd
(gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb
(ytterbium), and Lu (lutetium).
The REM content refers to the total content of the above 17 kinds of elements.
[0048) The abrasion-resistant steel of the disclosure may contain Mg having the same effect
10
as Ca and REM in place ofCa and REM or together with one of or both Ca and REM.
[0049] [Mg: 0 to 0.0 I 00%]
Like Ca, excess Mg coarsens an inclusion, which inhibits the toughness. Therefore,
when Mg is contained, the content of Mg is set to 0.0 l 00% or less. The content of Mg is
preferably 0.0050% or less, and more preferably 0.0030% or less. The content of Mg is not
indispensable, and the lower limit of the content is 0%. On the other hand, in order to stably
obtain the effect, the content of Mg is preferably 0.0003% or more. The content of Mg is
more preferably 0.0005% or more, and still more preferably 0.00 l 0% or more.
[0050] The abrasion-resistant steel of the disclosure may contain W in order to improve the
abrasion resistance by improving the hardenability.
[0051] [W: 0 to 2.00%]
W is an element that improves hardenability. However, when the content of W
exceeds 2.00%, toughness or weldability is impaired. Therefore, when W is contained, the
content of W is set to 2.00% or less. The content of W is preferably 1.50% or less, and more
preferably 1.00% or less. The inclusion ofW is not indispensable, and the lower limit of the
content is 0%. On the other hand, in order to stably obtain an effect of W, the content of W
is preferably 0.10% or more. The content of W is more preferably 0.30% or more. W is an
expensive element, and the upper limit of the content may be 0.30%, 0.1 0%, or 0.02%.
[0052] In the abrasion-resistant steel of the disclosure, the composition other than the above
composition of steel are Fe and impurities.
Here, the impurities are composition that are mixed by a variety of factors in a
manufacturing process, such as raw materials such as ores and scraps \\1Jen a thick steel plate
is industrially produced, and mean those acceptable within a range not adversely affecting the
disclosure. However, in the disclosure, it is necessary to prescribe the upper limit for P, S,
and N among the impurities as described above.
[0053] Examples of impurities include at least one kind of Sn, Sn, As, and Pb. Each
content ofSn, Sn, As, and Pb is preferably fi·om 0 to 0.10%. If necessary, the upper limit of
the individual content of these elements may be 0.05% or 0.0 I%. The lower limit of the
content of these elements is 0%.
[0054] [Ceq: 0.80% or Less]
The carbon equivalent (Ceq) is an index of hardenability, and it is preferably as large
as possible in order to reduce the change in hardness in the thickness direction of the
abrasion-resistant steel. However, the increase in Ceq means an increase in the content of an
alloy. Therefore, Ceq should be limited as much as possible from the viewpoint of reducing
II
I r"
.
the alloy cost. The higher the carbon equivalent, the higher the susceptibility to low
temperature cracking after welding, and therefore, it is needed to increase a preheating
temperature during welding of a steel. In the disclosure, in order to reduce the alloy cost and
set the preheating temperature to 200°C or less, Ceq is set to 0.80% or less. Ceq is
preferably 0. 75% or less, and more preferably 0. 70% or less. On the other hand, Ceq is
preferably 0.50% or more in order to effectively suppress the change in the hardness in the
thickness direction of the abrasion-resistant steel. Ceq is more preferably set to 0.60% or
more.
[0055] Here, Ceq is expressed by the following (Formula I).
Ceq= C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 (Formula 1)
where C, Mn, Cu, Ni, Cr, Mo, and V are the content (mass %) of each element, and
when the element is not contained, the value is set to 0. According to the disclosure, it is
possible to suppress the increase in the content of alloy by improving steel hardenability or
the like by controlling the content of each element contained in a steel within the range
individually explained above and limiting the relationship of C, Mn, Cu, Ni, Cr, Mo, and V
within the above range.
[0056] Next, the metal sttucture of the abrasion-resistant steel of the disclosure will be
described.
[0057] In order to ensure abrasion resistance, it is preferable that the metal stmcture is hard
mattensite. In particular, from the viewpoint of prolonging the life span, it is impmtant to
prevent the hardness from decreasing in the thickness direction fi·om the surface layer. In
relation to this, in the abrasion-resistant steel of the disclosure, the area ratio ofmattensite in a
central portion in the thickness direction is high, and therefore, it is possible to secure
abrasion resistance for a long time. The metal structure (the balance) other than the
mattensite is not particularly limited, and one kind, or two or more kinds of ferrite, pearlite
and bainite may be used.
[0058] [Area Ratio ofMattensite in Central Pmtion in Thickness Direction: 70% or More]
Specifically, in the abrasion-resistant steel of the disclosure, the area ratio of
martensite in a central portion in the thickness direction is set to 70% or more. The area
ratio is preferably as high as 80% or more or 90% or more, and more preferably almost 100%.
Therefore, according to the abrasion-resistant steel of the disclosure, the abrasion resistance
can be secured over a long period of time.
[0059] Here, "a central portion in the thickness direction" means a range of 0.5 mm (or I
mm thick) from the center in the thickness direction ofthe abrasion-resistant steel.
12
•• I
Specifically, "a central portion in the thickness direction" refers to a range of0.5 mm
from the center in the thickness direction when the abrasion-resistant steel is a steel plate, in
the plate thickness direction in the case of a steel tube, or in the flange thickness direction in
the case of a section steel.
[0060] The area ratio of martensite is the area ratio when a cross section cut along the
thickness direction is observed. In the disclosure, martensite is a lath structure. The lath
structure is an extremely fine structure (elongated structure with a thickness of about from 0.1
to 0.2 [.1111), and as observed with an optical microscope, it is observed as a structure showing
parallel black-and-white contrast as shown in Fig. 3.
[0061] Specifically, the area ratio of mat1ensite is measured by the following method.
A sample cut along the thickness direction is obtained fi·om an abrasion-resistant
steel to be measured. Polishing and nital etching are performed on a cut surface of the
sample. Next, on the cut surface of the sample, a region located in a central pmtion in the
thickness direction is obse1ved with an optical microscope (it is preferable to set an
observation field of view to an area of200 Jlm x 200 [.lm or more). Next, the lath structure is
detennined in the observation field of view. Then, the area ratio of the lath structure
dete1mined with respect to the observation field of view is obtained as the area ratio of
ma11ensite.
[0062] However, even 111 cases in which the area ratio of ma11ensite (lath ma11ensite
structure) is 70% or more, when the Vickers hardness at a central po11ion in the thickness
direction (HVI 0 of JIS Z2244 (2009)) is less than 70% of the hardness of martensite obtained
fi·om the content of C according to the following Formula 2, obse1vation with an electron
microscope is carried out. As a result, the area where cementite has been observed is
determined as bainite and is excluded, and the area ratio of ma11ensite is obtained. In other
words, when the Vickers hardness HVIO in a central pmtion in the thickness direction is HM
x 0.70 or more, it- is only necessary to determine the area ratio of mat1ensite by an optical
microscope and obse1vation by an electron microscope is unnecessary.
[0063] Vickers hardness of martensite (Vickers hardness when I 00% of metal structure IS
martensite) HM can be obtained by the following Formula. Here, Cis the content ofC (unit:
mass%).
Hlvf = 884C (I - 0.3C2
) + 294 (Formula 2)
[0064] [Mass Fraction ofMo2FeB2 : 0.0010 to 0.1000%]
In the present disclosure, precipitation of Mo2FeB2 IS important for effectively
utilizing the hardenability of B. Therefore, the mass fraction ofi'Vfo2FeB2 is set to 0.0010%
13
or more. The mass fraction of Mo2FeB2 is preferably 0.0015% or more, more preferably
0.0020% or more, and particularly preferably 0.0040% or more. However, also when
Mo2FcB2 is excessively precipitated, the effect of B is impaired, and therefore, the mass
tl·action ofMo2FeB2 is set to 0.1000% or less.
[0065] [Mass Fraction ofFen{C, B)6: 0.0020% or Less]
In contrast, Fe23(C,B)6 is a precipitate which impairs an effect of B. Therefore, it is
preferable to suppress precipitation ofFe23(C,B)6. Therefore, the mass fraction ofFe23(C,B)6
is preferably 0.0030% or less. The mass tl·action ofFe23(C,B)6 is preferably 0.0020% or less,
more preferably 0.0015% or less, and particularly preferably 0.00 I 0% or less. The lower
limit of the mass fraction ofFe23(C,B)6 is not specified, and may be 0%.
[0066] Here, the content of precipitation of Mo2FeB2 and Fe23(C,B)6 is determined by an
extraction residue analysis method. In the extraction residue analysis method, a steel is
dissolved by electrolysis in a non-aqueous solvent (acetylacetone-methanol solution or the
like) to dissolve a parent phase, and the residue (a precipitate and an inclusion) is extracted
with a filter having a pore size (diameter) of 0.2 J.Ull and separated. The mass fraction of
compounds contained in the residue after separation can be determined by identification by
X-ray diffraction method and measuring the content of each element by chemical analysis.
[0067] Next, a method of manufacturing the abrasion-resistant steel of the disclosure will be
described.
[0068] The abrasion-resistant steel of the disclosure is manufactured by hot working a steel
piece into a desired shape, cooling to room temperature, reheating, and quenching the piece.
[0069] In the manufacturing of the abrasion-resistant steel of the disclosure, the reheating
temperature of quenching is impottant to generate Mo2FeB2. \Vhen the temperature exceeds
I, I 00°C, Mo2FeB2 solid-dissolves, the reheating temperature is set to I, I 00°C or lower. The
reheating temperature is preferably I ,000°C or less. The reheating temperature is set to Ac3
or higher. Although Ac3 may be used from known calculations or measured values, Ac3 may
be calculated from the content of each element, for example, using the following Formula.
Here, C, Si, Mn, Ni, and Cr arc the content (unit: mass%) of each element.

CLAIMS
I. An abrasion-resistant steel, comprising, by mass%:
C: 0.1 0 to 0.40%,
Si: 0.05 to 0.50%,
Mn: 0.50 to 1.50%,
B: 0.0015 to 0.0050%,
Mo: 0.60 to 2.50%,
AI: 0 to 0.300%,
S: 0.0 I 0% or less,
P: 0.0 15% or less,
N: 0.0080% or less,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
Cu: 0 to 1.50%,
Ni: 0 to 2.00%,
Cr: 0 to 2.00%,
V: 0 to 0.20%,
Ca: 0 to 0.0!00%,
REM: 0 to 0.0100%,
Mg: 0 to 0.0100%,
W: 0 to 2.00%, and
a balance: Fe and impurities, wherein:
contents (mass%) ofMo and B satisfy Mo x B > 0.0010,
a mass fi·action ofMo1FeB2 is fi·om O.OO!Oto 0.1000%,
an area ratio of mmiensite in a central portion in a thickness direction is 70% m
Ceq obtained by the following (Formula I) is 0.80% or less, and
a plate thickness exceeds 50 mm;
Ceq= C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 (Formula I),
wherein, in (Formula l ), C, Mn, Cu, Ni, Cr. l'vlo, and V are contents (mass%) of each
2. The abrasion-resistant steel according to claim l, wherein a mass fraction of
Fe23(C, B)6 is 0.0020% or less.
3. The abrasion-resistant steel according to claim I or 2, wherein the contents