U.S. patent application number 12/532032 was filed with the patent office on 2010-06-10 for wear-resistant steel plate having excellent wear resistance at high temperatures and excellent bending workability and method for manufacturing the same.
Invention is credited to Tatsuya Kumagai, Naoki Saitoh.
Application Number | 20100139820 12/532032 |
Document ID | / |
Family ID | 40853104 |
Filed Date | 2010-06-10 |
United States Patent
Application |
20100139820 |
Kind Code |
A1 |
Kumagai; Tatsuya ; et
al. |
June 10, 2010 |
WEAR-RESISTANT STEEL PLATE HAVING EXCELLENT WEAR RESISTANCE AT HIGH
TEMPERATURES AND EXCELLENT BENDING WORKABILITY AND METHOD FOR
MANUFACTURING THE SAME
Abstract
This wear-resistant steel plate includes, in terms of mass %, C:
not less than 0.13% and not more than 0.18%, Si: not less than 0.5%
but less than 1.0%, Mn: not less than 0.2% and not more than 0.8%,
P: not more than 0.020%, S: not more than 0.010%, Cr: not less than
0.5% and not more than 2.0%, Mo: not less than 0.03% and not more
than 0.30%, Nb: more than 0.03% but not more than 0.10%, Al: not
less than 0.01% and not more than 0.20%, B: not less than 0.0005%
and not more than 0.0030%, and N: not more than 0.010%, with a
remainder being Fe and unavoidable impurities, wherein an element
composition is such that HI is 0.7 or greater and Ceq exceeds 0.50,
and an HB value (Brinell hardness) at 25.degree. C. is not less
than 360 and not more than 440.
Inventors: |
Kumagai; Tatsuya; (Tokyo,
JP) ; Saitoh; Naoki; (Tokyo, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
40853104 |
Appl. No.: |
12/532032 |
Filed: |
January 6, 2009 |
PCT Filed: |
January 6, 2009 |
PCT NO: |
PCT/JP2009/050024 |
371 Date: |
September 18, 2009 |
Current U.S.
Class: |
148/645 ;
148/330 |
Current CPC
Class: |
C21D 8/0263 20130101;
C22C 38/06 20130101; C22C 38/22 20130101; C22C 38/02 20130101; C22C
38/32 20130101; C22C 38/26 20130101; C22C 38/04 20130101; C21D
6/002 20130101 |
Class at
Publication: |
148/645 ;
148/330 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/00 20060101 C22C038/00; C22C 38/02 20060101
C22C038/02; C22C 38/32 20060101 C22C038/32; C22C 38/22 20060101
C22C038/22; C22C 38/42 20060101 C22C038/42; C22C 38/44 20060101
C22C038/44; C22C 38/54 20060101 C22C038/54 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 7, 2008 |
JP |
2008-000301 |
Oct 17, 2008 |
JP |
2008-268253 |
Claims
1. A wear-resistant steel plate having excellent wear resistance at
high temperatures and excellent bending workability, comprising, in
terms of mass %, C: not less than 0.13% and not more than 0.18%,
Si: not less than 0.5% but less than 1.0%, Mn: not less than 0.2%
and not more than 0.8%, P: not more than 0.020%, S: not more than
0.010%, Cr: not less than 0.5% and not more than 2.0%, Mo: not less
than 0.03% and not more than 0.30%, Nb: more than 0.03% but not
more than 0.10%, Al: not less than 0.01% and not more than 0.20%,
B: not less than 0.0005% and not more than 0.0030%, and N: not more
than 0.010%, with a remainder being Fe and unavoidable impurities,
wherein an element composition is such that HI defined below is 0.7
or greater and Ceq defined below exceeds 0.50, and an HB value
(Brinell hardness) at 25.degree. C. is not less than 360 and not
more than 440,
HI=[C]+0.59[Si]-0.58[Mn]+0.29[Cr]+0.39[Mo]+2.11([Nb]-0.02)-0.72[Ti]+0.56[-
V] Ceq=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14 wherein [C],
[Si], [Mn], [Ni], [Cr], [Mo], [Nb], [Ti] and [V] represent amounts
(mass %) of C, Si, Mn, Ni, Cr, Mo, Nb, Ti and V, respectively.
2. A wear-resistant steel plate having excellent wear resistance at
high temperatures and excellent bending workability according to
claim 1, wherein the steel plate further comprises, in terms of
mass %, one or more selected from the group consisting of Cu: not
less than 0.05% and not more than 1.5%, Ni: not less than 0.05% and
not more than 1.0%, Ti: not less than 0.003% and not more than
0.03%, and V: not less than 0.01% and not more than 0.20%.
3. A method for manufacturing a wear-resistant steel plate having
excellent wear resistance at high temperatures and excellent
bending workability, the method comprising: heating a slab having a
composition defined in claim 1 or 2 to a temperature of at least
1,200.degree. C., conducting hot rolling with a cumulative
reduction ratio of not less than 30% and not more than 65% at a
temperature of not more than 960.degree. C. and not less than
900.degree. C., finishing said hot rolling at a temperature of not
less than 900.degree. C.; and after completion of said hot rolling,
either immediately performing accelerated cooling to a temperature
of 200.degree. C. or lower such that a cooling rate within a plate
thickness center portion is at least 5.degree. C./s, or conducting
cooling to a temperature of 200.degree. C. or lower, subsequently
reheating to a temperature of not less than an Ac3 transformation
point, and then performing accelerated cooling to a temperature of
200.degree. C. or lower such that a cooling rate within a plate
thickness center portion is at least 5.degree. C./s.
Description
TECHNICAL FIELD
[0001] The present invention relates to a wear-resistant steel
plate having excellent wear resistance at high temperatures and
excellent bending workability that can be used in construction
machinery and industrial machinery, and also relates to a method
for manufacturing such a wear-resistant steel plate.
[0002] The present application claims priority on Japanese Patent
Application No. 2008-000301, filed on Jan. 7, 2008, and Japanese
Patent Application No. 2008-268253, filed on Oct. 17, 2008, the
contents of which are incorporated herein by reference.
BACKGROUND ART
[0003] In construction machineries used for excavations within
mines and earthworks, many components require frequent regular
replacements due to ongoing wear. Among these types of components,
for steel materials, usage under conditions of high temperature
represents the most severe operating conditions. Because the
hardness of wear-resistant steel decreases with increasing
temperature, the wear of the steel tends to accelerate rapidly at a
temperature of not less than a certain value. This wear is
particularly marked for members such as bulldozer buckets in which
frictional heat is generated as a result of strong impacts, and
hoppers for sintered coke which are exposed to impacts with
high-temperature bodies. In these types of members, the temperature
of the surface of the steel plate that constitutes the member may
temporarily reach temperatures of approximately 300.degree. C. to
400.degree. C. Because frequent member exchange results in a
deterioration in the equipment operating efficiency, there is
considerable demand for a steel material (a wear-resistant steel)
that exhibits superior wear resistance even under these types of
conditions.
[0004] On the other hand, in order to enable application to various
shaped sites, or significantly reduce the number of welded
sections, favorable bending workability of the steel plate is often
very important for a wear-resistant steel.
[0005] Increasing of the hardness is effective in improving the
wear resistance. However, when a steel plate having high hardness
is subjected to bending, and particularly bending with a small bend
radius, the steel plate tends to be prone to breaking or cracking.
Moreover, if consideration is also given to factors such as the
value of the deformation resistance to bending and the degree of
spring-back, then having a high degree of hardness for a steel
plate is disadvantageous for achieving favorable bending
workability. In other words, the wear resistance and the bending
workability are generally mutually opposing properties. For
example, an HB500 class wear-resistant steel plate (with a Brinell
hardness at room temperature of approximately 450 to 550) exhibits
excellent wear resistance, but has relatively poor bending
workability. A steel having a lower degree of hardness such as an
HB400 class wear-resistant steel plate (with a Brinell hardness at
room temperature of approximately 360 to 440) can be subjected to
bending work comparatively easily, and can therefore be applied to
all manner of members that require favorable workability, but
cannot exhibit totally satisfactory wear resistance, particularly
in terms of the wear resistance under high-temperature
conditions.
[0006] Accordingly, imparting a wear-resistant steel having an
HB400 class room temperature hardness with favorable
high-temperature wear resistance properties could be said to be one
effective method of achieving a combination of favorable bending
workability and superior wear resistance at high temperatures.
[0007] A wear-resistant steel plate does not generally require a
particularly high toughness value, but must have a certain level of
toughness to ensure that the steel does not crack even when the
thickness of the steel plate decreases during use. In consideration
of use within cold regions, it is generally considered that the
Charpy absorption energy at -40.degree. C. should be not less than
27 J.
[0008] The inventors of the present invention have previously
disclosed, in Patent Document 1, a wear-resistant steel for
high-temperature applications having a Brinell hardness in the
order of HB500 class. The invention disclosed in this document was
designed with the high-temperature wear resistance as the
overriding priority, with no particular measures taken to improve
the bending workability, and therefore the steel is limited to
applications in which the bend radius is comparatively large.
[0009] Patent Document 2 relates to a wear-resistant steel for
intermediate and moderate temperatures that can be used in regions
of 300.degree. C. to 400.degree. C. This document gives no
consideration to toughness or workability, and no disclosure is
made regarding these properties; however, because the steel
includes an extremely high level of Si, it is thought that neither
the toughness nor the workability would be particularly
favorable.
[0010] Patent Document 3 relates to an HB400 class wear-resistant
steel having excellent bending workability, but absolutely no
consideration is given to the wear resistance under
high-temperature conditions.
[0011] In this manner, up until this point there have been no
suitable examples of HB400 class wear-resistant steels that exhibit
favorable bending workability as well as a high degree of wear
resistance under high-temperature conditions of 300.degree. C. to
400.degree. C.
[0012] Moreover, because a wear-resistant steel plate is a
consumable item, economy is also an important factor, and it is
desirable that the amount of expensive alloy elements added to the
steel is kept to a minimum.
[0013] Patent Document 1: Japanese Unexamined Patent Application,
First Publication No. 2001-49387
[0014] Patent Document 2: Japanese Unexamined Patent Application,
First Publication No. H03-243743
[0015] Patent Document 3: Japanese Unexamined Patent Application,
First Publication No. 2005-240135
DISCLOSURE OF INVENTION
Problems to be Solved by the Invention
[0016] The present invention aims to provide a wear-resistant steel
having a room temperature hardness in the order of HB400 class that
indicates favorable bending workability, has a high degree of wear
resistance even under high-temperature conditions of 300.degree. C.
to 400.degree. C., and is very economical.
Means to Solve the Problems
[0017] It is thought that in order to enhance the wear resistance
at high temperatures of 300.degree. C. to 400.degree. C., it is
important to maintain the hardness of the steel at these high
temperatures. On the other hand, the most economical way of
achieving a room temperature hardness of approximately HB400 is to
employ a martensite structure. However, a steel plate having a
martensite structure undergoes a large reduction in hardness as the
temperature is increased. Accordingly, with regard to steels
containing martensite structures (martensite steels) and having a
room temperature hardness of approximately HB400, methods of
improving the high-temperature wear resistance were investigated,
from the viewpoint of attempting to maintain the high-temperature
hardness at a level as high as possible.
[0018] The present invention has been developed on the assumption
of high-temperature conditions of 300.degree. C. to 400.degree. C.,
and a temperature of 350.degree. C. was used as a representative
temperature for evaluating the properties of the steel. The wear
resistance at 350.degree. C. (350.degree. C. wear resistance) was
investigated for martensite steels having a variety of different
chemical compositions. These wear resistance evaluations were
conducted in the manner outlined below. Namely, the temperature of
the sample was controlled within a pin-on-disk wear testing
apparatus prescribed in ASTM G99-05, and wear testing was conducted
while the sample temperature was set to 350.degree. C.; thereby,
the amounts of wear for the test sample and for a standard sample
(SS400) were measured. The result for the SS400 as a standard was
used, and a 350.degree. C. wear resistance ratio was defined as
[amount of wear of SS400/amount of wear of test sample]. Thereby,
the 350.degree. C. wear resistance ratio was determined for the
sample. The larger the value for this wear resistance ratio
becomes, the more favorable the 350.degree. C. wear resistance
is.
[0019] FIG. 1 illustrates the relationship between the 350.degree.
C. wear resistance ratio and the added amount of Nb for a
martensite steel having a basic composition including 0.15% of C,
0.57% of Si, 0.41% of Mn, 1.37% of Cr, 0.08% of Mo, 0.012% of Ti,
0.0011% of B and 0.0032% of N, and having a variable amount of Nb.
When the added amount of Nb was within a range from 0 to 0.03%, the
350.degree. C. wear resistance ratio varies little, but once the
added amount of Nb exceeds 0.03%, the 350.degree. C. wear
resistance ratio increases significantly. Nb carbonitrides that
precipitate during rolling tend to inhibit recrystallization and
reduce the size of the steel microstructure, and therefore Nb is
usually added in an amount within a range from 0.01 to 0.02%.
However, Nb carbonitrides that precipitate during rolling have
almost no effect on the high-temperature hardness. On the other
hand, with regard to Nb that exists within the steel plate in a
solid solution state, when the temperature is within a range from
300.degree. C. to 400.degree. C., it still remains in a solid
solution state or it exists as extremely fine carbonitrides, and it
is surmised that either of these states will contribute to an
improvement in the high-temperature hardness. In other words, it is
thought that by adding Nb at an amount that vastly exceeds an
amount that precipitates during rolling, and then selecting
appropriate rolling and cooling conditions, the amount of solid
solution Nb within the steel plate can be increased, resulting in
an increase in the hardness when the steel plate is heated to
350.degree. C. and a resulting improvement in the 350.degree. C.
wear resistance.
[0020] The inventors of the present invention conducted detailed
investigations of the relationship between the steel alloy elements
and the 350.degree. C. wear resistance for a multitude of
martensite steels having an HB value at 25.degree. C. within a
range from 360 to 440. As a result, they derived a formula (I)
below for predicting the 350.degree. C. wear resistance ratio from
the chemical composition:
HI=[C]+0.59[Si]-0.58[Mn]+0.29[Cr]+0.39[Mo]+2.11([Nb]-0.02)-0.72[Ti]+0.56-
[V] (1)
[0021] wherein [C], [Si], [Mn], [Cr], [Mo], [Nb], [Ti] and [V]
represent the amounts (mass %) of C, Si, Mn, Cr, Mo, Nb, Ti and V,
respectively. In formula (I), the reason for subtracting 0.02 from
the Nb amount is to account for the amount of Nb that precipitates
during rolling.
[0022] FIG. 2 illustrates the relationship between HI and the
350.degree. C. wear resistance ratio of the martensite steel.
[0023] In the present invention, the target value for the
high-temperature wear resistance is set as a 350.degree. C. wear
resistance ratio of not less than 3.0, that is, an amount of
frictional wear that is 1/3 or less than that of SS400. From the
relationship illustrated in FIG. 2 it is clear that in order to
satisfy this target value, the HI value must be 0.7 or greater.
Moreover, if the HI value is 0.8 or higher, then the wear
resistance ratio is 4.0 or greater; therefore, even more favorable
wear resistance can be realized.
[0024] The formula (I) indicates that besides Nb, increasing the
added amounts of Si, Cr, Mo and V is also effective in improving
the 350.degree. C. wear resistance for a martensite steel.
[0025] Of these elements, both of Mo and V are elements that have
conventionally been added in large amounts to high-temperature
steels; however, because recent costs for these elements are
extremely high, the added amounts are preferably kept as small as
possible from the viewpoint of economic viability.
[0026] In contrast, Si and Cr are comparatively low-cost elements,
and are therefore advantageous in terms of improving the
350.degree. C. wear resistance. On the other hand, reducing the
amount of Mn is actually also advantageous in terms of achieving a
favorable 350.degree. C. wear resistance.
[0027] In order to ensure that martensite structures exist right
through to the center of the plate thickness, it is necessary to
ensure that the steel has satisfactory hardenability. Most
wear-resistant steel plate has a plate thickness of not more than
50 mm. If the value of Ceq in the following formula exceeds 0.50,
sufficient hardenability can be achieved to ensure that martensite
structures exist right through to the center of a steel plate
having a thickness of 50 mm.
Ceq=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14
[0028] wherein [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] represent
the amounts (mass %) of C, Si, Mn, Ni, Cr, Mo and V,
respectively.
[0029] In terms of toughness, appropriate upper limits must be
specified for the amounts of Si, P, S, Cr, Mo, Al, B and N in order
to ensure a Charpy absorption energy at -40.degree. C. of not less
than 27 J.
[0030] The present invention has been developed in light of the
above findings, and provides the aspects described below.
(1) A wear-resistant steel plate of the present invention having
excellent wear resistance at high temperatures and excellent
bending workability includes, in mass % values, C: not less than
0.13% and not more than 0.18%, Si: not less than 0.5% but less than
1.0%, Mn: not less than 0.2% and not more than 0.8%, P: not more
than 0.020%, S: not more than 0.010%, Cr: not less than 0.5% and
not more than 2.0%, Mo: not less than 0.03% and not more than
0.30%, Nb: more than 0.03% but not more than 0.10%, Al: not less
than 0.01% and not more than 0.20%, B: not less than 0.0005% and
not more than 0.0030%, and N: not more than 0.010%, with the
remainder being Fe and unavoidable impurities, wherein an element
composition is such that HI defined below is 0.7 or greater and Ceq
exceeds 0.50, and an HB value (Brinell hardness) at 25.degree. C.
is not less than 360 and not more than 440.
HI=[C]+0.59[Si]-0.58[Mn]+0.29[Cr]+0.39[Mo]+2.11([Nb]-0.02)-0.72[Ti]+0.56-
[V]
Ceq=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14
[0031] wherein [C], [Si], [Mn], [Ni], [Cr], [Mo], [Nb], [Ti] and
[V] represent the amounts (mass %) of C, Si, Mn, Ni, Cr, Mo, Nb, Ti
and V, respectively.
(2) The wear-resistant steel plate having excellent wear resistance
at high temperatures and excellent bending workability according to
the aspect of the present invention disclosed in (1) above may
further include, in mass % values, one or more selected from the
group consisting of Cu: not less than 0.05% and not more than 1.5%,
Ni: not less than 0.05% and not more than 1.0%, Ti: not less than
0.003% and not more than 0.03%, and V: not less than 0.01% and not
more than 0.20%. (3) A method for manufacturing a wear-resistant
steel plate having excellent wear resistance at high temperatures
and excellent bending workability according to the present
invention includes: heating a slab having the composition disclosed
in (1) or (2) above to a temperature of at least 1,200.degree. C.,
conducting hot rolling with a cumulative reduction ratio of not
less than 30% and not more than 65% at a temperature of not more
than 960.degree. C. and not less than 900.degree. C., finishing the
hot rolling at a temperature of not less than 900.degree. C.; and
after completion of the hot rolling, either immediately performing
accelerated cooling to a temperature of 200.degree. C. or lower
such that a cooling rate within the center of the plate thickness
is at least 5.degree. C./s, or conducting cooling to a temperature
of 200.degree. C. or lower, subsequently reheating to a temperature
of not less than an Ac3 transformation point, and then performing
accelerated cooling to a temperature of 200.degree. C. or lower
such that a cooling rate within the center of the plate thickness
is at least 5.degree. C./s.
EFFECT OF THE INVENTION
[0032] According to the present invention, a wear-resistant steel
plate having a room temperature hardness in the order of HB400
class that indicates favorable bending workability, has a high
degree of wear resistance even under high-temperature conditions of
300.degree. C. to 400.degree. C., and is very economical can be
manufactured relatively easily.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a graph illustrating the relationship between the
added amount of Nb and the wear resistance at 350.degree. C.
[0034] FIG. 2 is a graph illustrating the relationship between the
HI value and the wear resistance at 350.degree. C.
BEST MODE FOR CARRYING OUT THE INVENTION
[0035] A more detailed description of the present invention is
presented below.
[0036] First is a description of the reasons for restricting each
of the steel components within the wear-resistant steel plate of
the present invention.
[0037] C is an important element in determining the hardness of the
martensite. In the present invention, in order to ensure that the
room temperature HB value within the plate thickness center portion
of a plate having a thickness of up to 50 mm is not less than 360
and not more than 440, the C content is set to not less than 0.13%
and not more than 0.18%.
[0038] Si is a particularly effective element for improving the
350.degree. C. wear resistance, and is also an inexpensive alloy
element. However, when a large amount of Si is added, reductions in
the toughness and the workability are caused. For these reasons,
the added amount of Si is set to not less than 0.50% but less than
1.0%. If particular emphasis is placed on the workability, then the
added amount of Si is preferably less than 0.8%.
[0039] Mn, by forming MnS, is essential for preventing a reduction
in the toughness and a deterioration in the bending workability
caused by grain boundary segregation of S, and is added in an
amount of not less than 0.2%. Since Mn enhances the hardenability,
it is preferable to add Mn in a large amount for the purpose of
ensuring more favorable room temperature hardness within the plate
thickness center portion of a plate having a thickness of up to 50
mm. However, on the other hand, Mn causes a reduction in the
high-temperature strength, and actually causes a decrease in the
350.degree. C. wear resistance. For this reason, the added amount
of Mn is preferably less than 0.5%. Even in terms of enhancing the
hardenability, the upper limit for the Mn content is 0.8%.
Accordingly, the added amount of Mn is set to not less than 0.2%
and not more than 0.8%, and is preferably not less than 0.2% but
less than 0.5%.
[0040] P is a harmful element that causes deterioration in the
bending workability and the toughness, and is incorporated as an
unavoidable impurity. Accordingly, the P content is suppressed to
not more than 0.020%. This amount is preferably 0.010% or lower.
The amount of P is preferably as low as possible in terms of the
bending workability and the toughness. However, since unavoidable
increases in the refining costs are required in order to reduce the
P content to less than 0.0005%, there is no necessity to limit the
P content to this type of extremely low level.
[0041] S is also a harmful element that causes deterioration in the
bending workability and the toughness, and is incorporated as an
unavoidable impurity. Accordingly, the S content is suppressed to
not more than 0.010%. This amount is preferably 0.005% or lower.
The amount of S is preferably as low as possible in terms of the
bending workability and the toughness. However, since unavoidable
increases in the refining costs are required in order to reduce the
S content to less than 0.0005%, there is no necessity to limit the
S content to this type of extremely low level.
[0042] Cr is effective in improving the hardenability and improving
the 350.degree. C. wear resistance, and is therefore added in an
amount of at least 0.5%. In order to obtain satisfactory
hardenability within the plate thickness center portion of a plate
having a thickness of up to 50 mm, the added amount of Cr is
preferably 1.0% or greater. However, excessive addition of Cr can
cause a reduction in the toughness, and therefore the Cr content is
limited to not more than 2.0%.
[0043] Mo improves the 350.degree. C. wear resistance, and adding a
small amount in the presence of Nb produces a large improvement in
the hardenability. For this reason, at least 0.03% of Mo must be
added. However, excessive addition of Mo can cause a reduction in
the toughness, and therefore the added amount of Mo has an upper
limit of 0.30%. Further, Mo has been extremely expensive in recent
years, and in terms of suppressing the alloy cost, the added amount
of Mo is preferably less than 0.10%.
[0044] Nb, due to its existence in a solid solution state within
the steel plate, is extremely effective in improving the
350.degree. C. wear resistance. The amount of Nb required to ensure
a satisfactory amount of solid solution Nb is an amount of greater
than 0.03%, and the amount is preferably 0.04% or greater. In the
present invention, because 0.13% or greater of C is included to
ensure a Brinell hardness at room temperature of not less than 360,
if the amount of Nb is too large, then Nb(CN) may not be
solid-solubilized completely during heating. This type of insoluble
Nb does not contribute to an improvement in the high-temperature
hardness, and may actually cause a reduction in the toughness. For
this reason, the added amount of Nb is not more than 0.10%, and is
preferably 0.08% or lower.
[0045] Al is added in an amount of not less than 0.01% as a
deoxidizing element or element for morphology control of
inclusions. Further, Al is also added in an amount of not less than
0.05% for the purpose of fixing N in order to ensure the necessary
amount of free B required to improve the hardenability. In either
case, excessive addition of Al can cause a reduction in the
toughness, and therefore the upper limit for the Al content is
0.20%, and preferably 0.10%.
[0046] B is an essential element that is extremely effective in
improving the hardenability. In order to ensure satisfactory
manifestation of this effect, at least 0.0005% of B is necessary.
However, if B is added in an amount exceeding 0.0030%, then the
weldability and the toughness of the steel may deteriorate, and
therefore the B content is set to not less than 0.0005% and not
more than 0.0030%.
[0047] If N is added in excess, N causes a reduction in the
toughness, and also forms BN; thereby, the effect of improving
hardenability that is provided by B is inhibited. As a result, the
N content is suppressed to not more than 0.010%. The N content is
preferably 0.006% or less. In terms of preventing any deterioration
in the toughness and avoiding BN formation, the amount of N is
preferably as low as possible. However, since unavoidable increases
in the refining costs are required in order to reduce the N content
to less than 0.001%, there is no necessity to limit the N content
to this type of extremely low level.
[0048] The above elements represent the basic components within the
steel of the present invention; however, one or more of the
elements Cu, Ni, V and Ti may also be added in addition to the
elements described above.
[0049] Cu is an element that is capable of improving the hardness
without reducing the toughness, and 0.05% or more of Cu may be
added for this purpose. However, if Cu is added in excess, then the
toughness may actually decrease, and therefore the added amount of
Cu is not more than 1.5%.
[0050] Ni is an element that is effective in improving the
toughness, and 0.05% or more of Ni may be added for this purpose.
However, because Ni is an expensive element, the amount added is
limited to not more than 1.0%.
[0051] V is an element that is effective in improving the
350.degree. C. wear resistance. An amount of 0.01% or more of V may
be added for this purpose. However, V is also an expensive element
and may cause a deterioration in the toughness if added in excess,
and therefore if added, the amount is limited to not more than
0.20%.
[0052] Ti may be added to fix N as TiN; thereby, the formation of
BN is prevented. As a result, the necessary amount of free B
required to improve the hardenability is ensured. An amount of
0.003% or more of Ti may be added for this purpose. However,
addition of Ti tends to cause a deterioration in the 350.degree. C.
wear resistance. Accordingly, the added amount of Ti is limited to
not more than 0.030%.
[0053] In addition to the restrictions on the component ranges
outlined above, as mentioned above, the element composition of the
present invention is also restricted so that the value of HI in
formula (1) is not less than 0.7, and the value of Ceq is greater
than 0.50. However, if the values of HI and Ceq are increased too
much, then the toughness may deteriorate, and therefore HI is
preferably not more than 1.2 and Ceq is preferably not more than
0.70.
[0054] Next is a description of a method for manufacturing the
wear-resistant steel plate of the present invention.
[0055] First, a slab having the steel component composition
described above is heated and subjected to hot rolling.
[0056] In the present invention, there are no particular
restrictions on the method used for manufacturing the slab prior to
the hot rolling. In other words, after melting in a blast furnace,
converter furnace or electric furnace or the like, a component
adjustment process can be conducted using any of the various
secondary refining techniques to achieve the targeted amount of
each element, and casting may then be conducted using a typical
continuous casting method, casting by an ingot method, or casting
by another method such as thin slab casting. Scrap metal may be
used as a raw material. In the case of a slab obtained by
continuous casting, the high-temperature cast slab may be fed
directly to the hot rolling apparatus, or may be cooled to room
temperature and then reheated in a furnace before undergoing hot
rolling. The components within the slab are the same as the
components within the wear-resistant steel plate of the present
invention described above.
[0057] In order to ensure satisfactory solid solubilization of Nb,
the heating temperature for the slab is 1,200.degree. C. or higher.
However, if a heating temperature is too high, coarsening of the
austenite structures occurs; thereby, a microstructure after hot
rolling does not become sufficiently fine and a deterioration in
the toughness is caused. Therefore, the heating temperature for the
slab is preferably not more than 1,350.degree. C.
[0058] During hot rolling, the cumulative reduction ratio is set to
not less than 30% and not more than 65% at a temperature of not
more than 960.degree. C. and not less than 900.degree. C. The
temperature and the reduction ratio are restricted to these ranges
so as to reduce the amount of Nb carbonitrides precipitated during
rolling to a requisite minimum which is necessary for ensuring
favorable grain refinement.
[0059] Further, in order to suppress unnecessary precipitation of
Nb carbonitrides and maximize the amount of solid solution Nb, the
hot rolling is preferably finished at a temperature of not less
than 900.degree. C. Furthermore, the hot rolling finishing
temperature must be not more than 960.degree. C.
[0060] After the hot rolling, accelerated cooling is conducted to
obtain martensite structures, either by performing direct quenching
or by reheating the rolled steel and then performing quenching.
[0061] In the case of direct quenching, after completion of the hot
rolling, the rolled plate is immediately subjected to accelerated
cooling to a temperature of 200.degree. C. or lower at a cooling
rate of at least 5.degree. C./s (the cooling rate within the center
of the plate thickness).
[0062] In the case of reheating and quenching, after completion of
the hot rolling, the rolled plate is cooled once to a temperature
of 200.degree. C. or lower (the cooling rate is arbitrary),
subsequently reheated to a temperature of not less than the Ac3
transformation point, and then subjected to accelerated cooling to
a temperature of 200.degree. C. or lower such that the cooling rate
within the center of the plate thickness is at least 5.degree.
C./s.
[0063] During the accelerated cooling conducted immediately after
completion of the hot rolling in the case of direct quenching, or
the accelerated cooling conducted after reheating in the case of
reheating and quenching, the cooling rate increases as the
thickness of the steel plate decreases. In the present invention,
the target plate thickness is typically assumed to be approximately
within a range from 4.5 mm to 50 mm. The cooling rate for a plate
having a thickness of 4.5 mm may be extremely high; however, there
are no particular problems associated with such a high rate, and no
upper limit is specified for the cooling rate.
[0064] A tempering heat treatment is not particularly necessary;
however, a heat treatment at a temperature of not more than
300.degree. C. does not cause the properties of the steel plate to
depart from the scope of the present invention.
EXAMPLES
[0065] Steels A to AI having the compositions shown in Tables 1 and
2 were melted to obtain slabs. The obtained slabs were heated to a
temperature of at least 1,230.degree. C., and then were subjected
to processes under the manufacturing conditions shown in Tables 3
and 4 to manufacture steel plates having plate thicknesses ranging
from 6 to 45 mm (each of the Steels No. 1 to 17 represents an
example of the present invention, whereas each of the Steels No. 18
to 44 represents a comparative example).
TABLE-US-00001 TABLE 1 (Mass %) C Si Mn P S Cu Ni Cr Mo Al Nb Ti V
B N Ceq HI Component of A 0.144 0.74 0.41 0.003 0.002 1.31 0.09
0.08 0.052 0.0014 0.0027 0.53 0.83 inventive steel B 0.161 0.71
0.32 0.003 0.002 1.25 0.06 0.04 0.072 0.014 0.0009 0.0045 0.51 0.88
C 0.154 0.69 0.29 0.004 0.002 1.68 0.04 0.08 0.043 0.0024 0.0028
0.58 0.94 D 0.147 0.74 0.38 0.002 0.002 1.35 0.08 0.03 0.061 0.009
0.0011 0.0035 0.53 0.87 E 0.165 0.79 0.69 0.003 0.001 1.03 0.27
0.05 0.085 0.012 0.0010 0.0033 0.59 0.76 F 0.149 0.73 0.38 0.005
0.001 1.22 0.15 0.04 0.039 0.012 0.0013 0.0031 0.52 0.80 G 0.147
0.94 0.42 0.003 0.002 1.25 0.08 0.04 0.042 0.009 0.0014 0.0031 0.53
0.89 H 0.163 0.74 0.44 0.003 0.003 0.89 0.24 0.03 0.059 0.013 0.040
0.0012 0.0041 0.51 0.79 I 0.137 0.68 0.42 0.004 0.003 0.28 1.32
0.08 0.09 0.063 0.0021 0.0029 0.52 0.80 J 0.177 0.74 0.42 0.003
0.002 1.15 0.08 0.08 0.049 0.050 0.0019 0.0031 0.53 0.82 K 0.149
0.72 0.37 0.003 0.002 0.41 1.32 0.06 0.07 0.053 0.0008 0.0028 0.53
0.84 L 0.148 0.69 0.37 0.004 0.002 0.35 0.26 1.51 0.07 0.02 0.063
0.021 0.0012 0.0051 0.56 0.88
TABLE-US-00002 TABLE 2 (Mass %) C Si Mn P S Cu Ni Cr Mo Al Nb Ti V
B N Ceq HI Component of M 0.111 0.75 0.42 0.004 0.001 1.25 0.08
0.05 0.060 0.021 0.0013 0.0038 0.48 0.77 comparative steel N 0.217
0.64 0.34 0.009 0.002 1.19 0.09 0.07 0.051 0.0014 0.0038 0.56 0.84
O 0.142 0.35 0.42 0.002 0.001 1.32 0.13 0.02 0.059 0.014 0.0009
0.0041 0.52 0.61 P 0.142 1.37 0.40 0.009 0.002 1.12 0.08 0.05 0.053
0.0008 0.0029 0.51 1.14 Q 0.167 0.61 0.08 0.002 0.001 1.41 0.07
0.06 0.041 0.0011 0.0037 0.51 0.96 R 0.148 0.77 0.91 0.006 0.003
1.46 0.15 0.06 0.054 0.0011 0.0041 0.66 0.63 S 0.154 0.68 0.38
0.037 0.001 1.54 0.07 0.04 0.065 0.013 0.0010 0.0046 0.57 0.89 T
0.152 0.69 0.39 0.005 0.015 1.55 0.07 0.04 0.066 0.013 0.0009
0.0050 0.57 0.90 U 0.152 0.69 0.43 0.004 0.001 1.90 1.37 0.12 0.08
0.069 0.0012 0.0035 0.56 0.86 V 0.172 0.75 0.32 0.006 0.001 0.42
0.26 0.02 0.060 0.008 0.0011 0.0034 0.41 0.73 W 0.144 0.65 0.39
0.008 0.001 2.50 0.07 0.06 0.055 0.0013 0.0041 0.75 1.13 X 0.166
0.69 0.42 0.006 0.001 1.45 0.01 0.06 0.047 0.0009 0.0039 0.56 0.81
Y 0.172 0.77 0.44 0.007 0.002 1.37 0.56 0.06 0.051 0.0009 0.0041
0.69 1.05 Z 0.155 0.80 0.38 0.005 0.002 1.38 0.08 0.23 0.053 0.0012
0.0042 0.55 0.91 AA 0.143 0.74 0.39 0.003 0.001 1.27 0.15 0.05
0.013 0.0013 0.0029 0.53 0.77 AB 0.143 0.62 0.41 0.007 0.003 1.50
0.09 0.06 0.135 0.0014 0.0420 0.56 0.98 AC 0.159 0.66 0.42 0.006
0.002 1.34 0.08 0.05 0.064 0.060 0.0012 0.0038 0.54 0.77 AD 0.159
0.73 0.40 0.004 0.001 1.25 0.11 0.06 0.071 0.240 0.0013 0.0040 0.55
1.01 AE 0.157 0.66 0.42 0.004 0.003 1.36 0.14 0.04 0.049 0.015
0.0001 0.0037 0.56 0.80 AF 0.152 0.63 0.40 0.005 0.003 1.25 0.09
0.07 0.055 0.0055 0.0040 0.52 0.76 AG 0.153 0.63 0.39 0.005 0.001
1.29 0.12 0.03 0.045 0.017 0.0013 0.0125 0.53 0.76 AH 0.151 0.57
0.44 0.005 0.003 1.25 0.07 0.06 0.039 0.0011 0.0037 0.52 0.66 AI
0.148 0.61 0.37 0.005 0.002 1.09 0.08 0.06 0.062 0.0012 0.0037 0.47
0.73
TABLE-US-00003 TABLE 3 Cumulative Hot Cooling reduction ratio
rolling rate Re- Cooling Accelerated plate Heating at not more than
finishing after Ac3 heating rate cooling Steel Steel thick- temper-
960.degree. C. and not temper- completion temper- temper- after
finishing compo- plate ness Manufacturing ature less than
900.degree. C. ature of rolling ature ature reheating temperature
nent No. (mm) method (.degree. C.) (%) (.degree. C.) (.degree.
C./s) (.degree. C.) (.degree. C.) (.degree. C./s) (.degree. C.)
Inven- A 1 40 Direct 1260 52 923 11 -- -- -- <50 tive quenching
steel A 2 20 Reheating and 1260 43 907 0.5 910 930 27 <50
quenching B 3 40 Direct 1260 51 911 12 -- -- -- 110 quenching B 4
30 Direct 1260 60 912 18 -- -- -- <50 quenching C 5 45 Reheating
and 1260 50 908 0.2 893 930 8 <50 quenching D 6 40 Direct 1260
45 916 10 -- -- -- <50 quenching D 7 20 Direct 1260 48 909 31 --
-- -- <50 quenching E 8 40 Direct 1260 50 926 10 -- -- -- <50
quenching F 9 45 Direct 1260 55 910 9 -- -- -- 90 quenching G 10 40
Direct 1260 54 918 10 -- -- -- <50 quenching H 11 40 Direct 1260
44 912 10 -- -- -- <50 quenching I 12 10 Reheating and 1260 49
905 0.9 909 930 55 <50 quenching I 13 40 Direct 1260 52 911 11
-- -- -- <50 quenching J 14 32 Reheating and 1230 47 915 0.25
907 930 18 <50 quenching K 15 6 Reheating and 1230 51 909 1.6
901 930 108 <50 quenching K 16 16 Reheating and 1230 48 915 0.6
901 930 41 <50 quenching L 17 40 Direct 1260 50 921 10 -- -- --
<50 quenching Com- M 18 32 Direct 1260 55 914 12 -- -- -- <50
par- quenching ative N 19 40 Direct 1260 52 921 25 -- -- -- <50
steel quenching O 20 40 Direct 1260 55 916 12 -- -- -- 100
quenching P 21 40 Direct 1260 53 915 11 -- -- -- <50 quenching Q
22 25 Direct 1260 50 910 22 -- -- -- <50 quenching
TABLE-US-00004 TABLE 4 Cumulative Hot Accelerated reduction ratio
at rolling Cooling rate Re- Cooling cooling Plate Heating not more
than finishing after Ac3 heating rate finishing Steel Steel thick-
temper- 960.degree. C. and not temper- completion temper- temper-
after temper- compo- plate ness Manufacturing ature less than
900.degree. C. ature of rolling ature ature reheating ature nent
No. (mm) method (.degree. C.) (%) (.degree. C.) (.degree. C./s)
(.degree. C.) (.degree. C.) (.degree. C./s) (.degree. C.) Com- R 23
40 Direct quenching 1260 44 914 10 -- -- -- <50 par- S 24 40
Direct quenching 1260 52 910 10 -- -- -- <50 ative T 25 40
Direct quenching 1260 51 918 10 -- -- -- <50 steel U 26 25
Direct quenching 1260 52 920 24 -- -- -- 120 V 27 40 Direct
quenching 1260 55 909 12 -- -- -- <50 W 28 40 Direct quenching
1260 44 923 12 -- -- -- 70 X 29 40 Direct quenching 1260 50 911 10
-- -- -- <50 Y 30 40 Direct quenching 1260 52 920 11 -- -- --
<50 Z 31 40 Direct quenching 1260 49 910 11 -- -- -- 90 AA 32 40
Direct quenching 1260 48 916 11 -- -- -- 110 AB 33 40 Direct
quenching 1260 50 921 12 -- -- -- <50 AC 34 40 Direct quenching
1260 49 913 13 -- -- -- <50 AD 35 40 Direct quenching 1260 50
913 12 -- -- -- <50 AE 36 40 Direct quenching 1260 48 912 10 --
-- -- <50 AF 37 40 Direct quenching 1260 55 913 11 -- -- --
<50 AG 38 40 Direct quenching 1260 52 915 11 -- -- -- <50 AH
39 40 Direct quenching 1260 52 920 10 -- -- -- <50 AI 40 40
Direct quenching 1260 52 914 10 -- -- -- <50 A 41 40 Direct
quenching 1150 46 915 13 -- -- -- <50 A 42 40 Direct quenching
1260 15 925 12 -- -- -- <50 A 43 40 Direct quenching 1260 75 905
10 -- -- -- <50 A 44 40 Direct quenching 1260 51 905 0.4 -- --
-- --
[0066] Each of these steel plates was evaluated for room
temperature hardness, wear resistance at 350.degree. C., bending
workability, and toughness.
[0067] The room temperature hardness was evaluated by using a
Brinell hardness test method (JIS Z 2243) to measure the hardness
at 25.degree. C. The target value for the room temperature hardness
was a value of not less than HB360 and not more than HB440.
[0068] As described above, the wear resistance was evaluated by
conducting wear testing using a pin-on-disk wear testing apparatus
prescribed in ASTM G99-05 with the temperature of the sample held
at 350.degree. C., and then determining a wear resistance ratio
relative to a SS400 standard sample (amount of wear of SS400/amount
of wear of test sample). The target value for the wear resistance
was a wear resistance ratio of 3.0 or greater.
[0069] Evaluation of the bending workability was conducted in the
following manner. Namely, using the method prescribed in JIS Z
2248, a JIS No. 1 test piece was subjected to a bend test to
180.degree. in the C-direction at a bend radius of four times the
plate thickness (4t), and after the bend test, the external
appearance of the curved portion of the test piece was examined.
The steel plate was deemed to have passed if no cracking or other
defects were observed on the outside of the curved portion.
[0070] Evaluation of the toughness was conducted in the manner
described below. Namely, a No. 4 Charpy test piece prescribed in
JIS Z 2201 was sampled from the center of the plate thickness in a
direction orthogonal to the rolling direction, an impact test was
performed at -40.degree. C., and the absorption energy was
measured. Three test pieces were subjected to impact tests at
-40.degree. C., and the average value for the absorption energy was
determined. The target value for the toughness was an average value
of not less than 27 J.
[0071] The results obtained are tabled in Tables 5 and 6.
[0072] In Tables 1 to 6, underlined numerical values represent
component values outside the ranges specified by the present
invention, or unsatisfactory temperature conditions or
properties.
TABLE-US-00005 TABLE 5 Steel 25.degree. C. Brinell 350.degree. C.
wear 4t Absorption plate hardness resistance bend energy at No.
(HB10/3000) ratio test -40.degree. C. (J) Inventive 1 378 3.59 Pass
59 steel 2 399 3.96 Pass 61 3 393 3.79 Pass 53 4 406 4.12 Pass 45 5
391 3.82 Pass 65 6 376 3.56 Pass 57 7 395 4.06 Pass 51 8 379 3.88
Pass 52 9 382 3.65 Pass 63 10 384 3.77 Pass 64 11 377 3.47 Pass 61
12 372 3.50 Pass 54 13 396 3.76 Pass 46 14 401 3.92 Pass 57 15 381
3.82 Pass 69 16 401 4.11 Pass 55 17 375 3.74 Pass 57 Comparative 18
348 2.92 Pass 76 steel 19 420 3.79 Fail 37 20 363 2.47 Pass 61 21
387 4.09 Fail 20 22 386 3.88 Fail 54
TABLE-US-00006 TABLE 6 Steel 25.degree. C. Brinell 350.degree. C.
wear 4t Absorption plate hardness resistance bend energy at No.
(HB10/3000) ratio test -40.degree. C. (J) Comparative 23 373 2.62
Pass 59 steel 24 395 3.76 Fail 16 25 392 3.82 Fail 18 26 386 3.47
Pass 22 27 347 2.74 Pass 50 28 413 3.82 Pass 18 29 364 2.89 Pass 57
30 399 4.31 Pass 20 31 384 3.71 Pass 18 32 368 2.80 Pass 31 33 351
3.19 Pass 45 34 363 3.08 Pass 21 35 371 3.85 Pass 19 36 315 1.79
Pass 81 37 380 3.50 Pass 19 38 379 3.53 Pass 15 39 372 2.80 Pass 45
40 349 3.20 Pass 58 41 372 2.62 Pass 30 42 387 3.62 Fail 20 43 361
2.53 Pass 65 44 312 1.64 Pass 100
[0073] In Steel No. 1 to 17 that represent examples of the present
invention in Table 5, all of the values for the above-mentioned
room temperature hardness, 350.degree. C. wear resistance, bending
workability, and toughness satisfied the respective target values.
In contrast, in Steel No. 18 to 40 of the comparative examples, in
which the steel composition departed from the chemical composition
range specified in the present invention, even though manufacture
of the steel was conducted using the method of the present
invention, at least one of the room temperature hardness, the
350.degree. C. wear resistance, the bending workability or the
toughness did not satisfy the target value. Moreover, in Steel No.
41 to 44, in which the steel composition satisfied the range
specified in the present invention, but the manufacturing method
departed from the method prescribed in the present invention, at
least one of the room temperature hardness, the 350.degree. C. wear
resistance, the bending workability or the toughness failed to
satisfy the target value.
INDUSTRIAL APPLICABILITY
[0074] According to the present invention, a wear-resistant steel
plate having an HB400 class room temperature hardness, that
indicates favorable bending workability, has a high degree of wear
resistance even under high-temperature conditions of 300.degree. C.
to 400.degree. C., and is very economical can be manufactured
relatively easily. As a result, the present invention can be used
favorably for construction machinery and industrial machinery
members that require superior wear resistance under
high-temperature conditions, such as bulldozer buckets in which
frictional heat is generated as a result of strong impacts, and
hoppers for sintered coke which are exposed to impacts with
high-temperature bodies.
* * * * *