REFERENCE TO PATENTS, APPLICATIONS AND PUBLICATIONS PERTINENT TO THE INVENTION
[0001] As far as we know, there are available the following prior art documents pertinent
to the present invention:
(1) Japanese Patent Provisional Publication No. 62-142,726 dated June 26, 1987;
(2) Japanese Patent Provisional Publication No. 63-169,359 dated July 13, 1988; and
(3) Japanese Patent Provisional Publication No. 1-142,023 dated June 2, 1989.
[0002] The contents of the prior art disclosed in the above-mentioned prior art documents
will be discussed hereafter under the heading of the "BACKGROUND OF THE INVENTION."
FIELD OF THE INVENTION
[0003] The present invention relates to a wear-resistant steel having a high hardness in
an intermediate temperature region and a room-temperature region.
BACKGROUND OF THE INVENTION
[0005] A wear-resistant steel is used as a material for portions exposed to serious wear
in an industrial machine and a transportation machine such as a power shovel, a bulldozer,
a hopper or a bucket and parts thereof. Wear resistance of steel can be improved by
increasing hardness of the steel. A steel having a high hardness which contains carbon,
silicon and manganese in prescribed amounts and is additionally added with elements
to increase hardness, is therefore used as the wear-resistant steel mentioned above.
[0006] The following wear-resistant steels have so far been proposed as steels excellent
in wear resistance and satisfactory in weldability, toughness and workability:
(1) A wear-resistant steel sheet having an excellent weldability, disclosed in Japanese
Patent Provisional Publication No. 62-142,726 dated June 26, 1987, which consists
essentially of:
- carbon
- : from 0.10 to 0.19 wt.%,
- silicon
- : from 0.05 to 0.55 wt.%,
- manganese
- : from 0.90 to 1.60 wt.%,
and
- the balance being iron and incidental impurities;
- where, a carbon equivalent (C + 1/24 Si + 1/6 Nn + 1/40 Ni + 1/5 Cr + 1/4 Mo + 1/14
V) being within a range of from 0.35 to 0.44 wt.%
(hereinafter referred to as the "prior art 1").
The above-mentioned wear-resistant steel sheet of the prior art 1 may additionally
contain at least one of vanadium and niobium in an amount of up to 0.10 wt.%.
(2) A wear-resistant steel sheet having a high toughness, disclosed in Japanese Patent
Provisional Publication No. 63-169,359 dated July 13, 1988, which consists essentially
of:
- carbon
- : from 0.10 to 0.20 wt.%,
- silicon
- : from 0.03 to 0.75 wt.%,
- manganese
- : from 0.4 to 1.8 wt.%,
- phosphorus
- : up to 0.015 wt.%,
- sulfur
- : up to 0.002 wt.%,
- nitrogen
- : up to 0.0025 wt.%,
- sol. Al
- : from 0.001 to 0.080 wt.%,
- oxygen
- : up to 0.0020 wt.%,
and
the balance being iron and incidental impurities
(hereinafter referred to as the "prior art").
The above-mentioned wear-resistant steel sheet of the prior art 2 may additionally
contain at least one element selected from the group consisting of:
- copper
- : from 0.05 to 0.75 wt.%,
- nickel
- : from 0.05 to 1.50 wt.%,
- chromium
- : from 0.05 to 1.50 wt.%,
- molybdenum
- : from 0.01 to 0.75 wt.%,
and
- boron
- : from 0.0001 to 0.0025 wt.%.
(3) A wear-resistant steel sheet having an excellent bending workability, disclosed
in Japanese Patent Provisional Publication No. 1-142,023 dated June 2, 1989, which
consists essentially of:
- carbon
- : from 0.07 to 0.17 wt.%,
- silicon
- : from 0.05 to 0.55 wt.%,
- manganese
- : from 0.70 to 1.80 wt.%,
- vanadium
- : from 0.02 to 0.10 wt.%,
- boron
- : from 0.0003 to 0.0050 wt.%,
- aluminum
- : from 0.01 to 0.10 wt.%,
and
the balance being iron and incidental impurities
(hereinafter referred to as the "prior art 3").
The above-mentioned wear-resistant steel sheet of the prior art 3 may additionally
contain at least one element selected from the group consisting of:
- copper
- : from 0.05 to 0.30 wt.%,
- nickel
- : from 0.05 to 0.45 wt.%,
- chromium
- : from 0.05 to 0.20 wt.%,
and
- molybdenum
- : from 0.03 to 0.20 wt.%.
[0007] According to the above-mentioned prior arts 1 to 3, a wear-resistant steel having
a high room-temperature hardness is available in all cases. However, the prior arts
1 to 3 have the following problems: A wear-resistant steel is used also as a material
for a machine and parts thereof for treating slag at a temperature within an intermediate
temperature region of from about 300 to about 400°C in a slag yard. A wear-resistant
steel used as such a material should preferably have a Brinell hardness(HB) at a room-temperature
of at least 250, a Brinell hardness at a temperature of about 300°C of at least 90%
of its room-temperature Brinell hardness, and a Brinell hardness at a temperature
of about 400°C of at least 70% of its room-temperature Brinell hardness.
[0008] However, according to the wear-resistant steels of the prior arts 1 to 3, while it
is possible to improve wear resistance at a temperature within a room temperature
region, it is impossible to improve wear resistance at a temperature within an intermediate
temperature region of from about 300 to about 400°C. The wear-resistant steels of
the prior arts 1 to 3 are not satisfactory in terms of wear resistance when used as
a material for a machine and parts thereof employed at a temperature within an intermediate
temperature region.
[0009] With a view to improving wear resistance at a temperature within an intermediate
temperature region, a conceivable measure is to largely increase a room-temperature
hardness of steel, taking account of the decrease in hardness at a temperature within
an intermediate temperature region. When a room-temperature hardness of steel is increased
excessively, however, ductility, toughness, workability and weldability of the steel
are deteriorated.
[0010] Under such circumstances, there is a strong demand for the development of a wear-resistant
steel for the intermediate and room temperature service, which has a Brinell hardness
at a room-temperature of at least 250, and has a Brinell hardness at a temperature
of about 300°C of at least 90% of its room-temperature Brinell hardness, and a Brinell
hardness at a temperature of about 400°C of at least 70% of its room-temperature Brinell
hardness, the last two Brinell hardnesses being available without largely increasing
its room-temperature Brinell hardness, but such a wear-resistant steel for the intermediate
and room temperature service has not as yet been proposed.
SUMMARY OF THE INVENTION
[0012] An object of the present invention is therefore to provide a wear-resistant steel
for the intermediate and room temperature service, which has a Brinell hardness at
a room-temperature of at least 250, and has a Brinell hardness at a temperature of
300°C of at least 90% of its room-temperature Brinell hardness and a Brinell hardness
at a temperature of 400°C of at least 70% of its room-temperature Brinell hardness,
the last two Brinell hardnesses being available without largely increasing its room-temperature
Brinell hardness.
[0013] In accordance with one of the features of the present invention, there is provided
a wear-resistant steel for the intermediate and room temperature service, which has
a Brinell hardness at a room-temperature of at least 250, a Brinell hardness at a
temperature of 300°C of at least 90% of its room-temperature Brinell hardness, and
a Brinell hardness at a temperature of 400°C of at least 70% of its room-temperature
Brinell hardness, characterized by consisting essentially of:
- carbon
- : from 0.08 to 0.40 wt.%,
- silicon
- : from 0.8 to 2.5 wt.%,
- manganese
- : from 0.1 to 2.0 wt.%,
and
the balance being iron and incidental impurities.
[0014] The wear-resistant steel for the intermediate and room temperature service of the
present invention may additionally contain at least one element selected from the
group (A) consisting of:
(A)
- copper
- : from 0.1 to 2.0 wt.%,
- nickel
- : from 0.1 to 10.0 wt.%,
- chromium
- : from 0.1 to 3.0 wt.%,
- molybdenu
- m : from 0.1 to 3.0 wt.%,
and
- boron
- : from 0.0003 to 0.0100 wt.%.
The wear-resistant steel for the intermediate and room temperature service of the
present invention may additionally contain at least one element selected from the
group (B) consisting of:
(B)
- niobium
- : from 0.005 to 0.100 wt.%,
- vanadium
- : from 0.01 to 0.10 wt.%,
and
- titanium
- : from 0.005 to 0.100 wt.%.
[0015] Furthermore, the wear-resistant steel for the intermediate and room temperature service
of the present invention may additionally contain at least one element selected from
the above-mentioned group (A) and at least one element selected from the above-mentioned
group (B).
BRIEF DESCRIPTION OF THE DRAWING
[0016] Fig. 1 is a graph illustrating the relationship between a silicon content and a Brinell
hardness (HB) in a wear-resistant steel.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0017] From the above-mentioned point of view, extensive studies were carried out to develop
a wear-resistant steel for the intermediate and room temperature service having an
excellent wear resistance in an intermediate temperature region without largely increasing
its room-temperature hardness. As a result, findings were obtained that silicon contained
in steel had a function of increasing, for a certain range of the content thereof,
hardness of steel in an intermediate temperature region without increasing a room-temperature
hardness thereof.
[0018] The present invention was made on the basis of the above-mentioned findings, and
the wear-resistant steel for the intermediate and room temperature service of the
present invention consists essentially of:
- carbon
- : from 0.08 to 0.40 wt.%,
- silicon
- : from 0.8 to 2.5 wt.%,
- manganese
- : from 0.1 to 2.0 wt.%,
and
the balance being iron and incidental impurities.
[0019] The wear-resistant steel for the intermediate and room temperature service of the
present invention may additionally contain at least one element selected from the
group (A) consisting of:
(A)
- copper
- : from 0.1 to 2.0 wt.%,
- nickel
- : from 0.1 to 10.0 wt.%,
- chromium
- : from 0.1 to 3.0 wt.%,
- molybdenum
- : from 0.1 to 3.0 wt.%,
- and boron
- : from 0.0003 to 0.0100 wt.%.
The wear-resistant steel for the intermediate and room temperature service of the
present invention may additionally contain at least one element selected from the
group (B) consisting of:
(B)
- niobium
- : from 0.005 to 0.100 wt.%,
- vanadium
- : from 0.01 to 0.10 wt.%,
and
- titanium
- : from 0.005 to 0.100 wt.%.
[0020] Furthermore, the wear-resistant steel for the intermediate and room temperature service
of the present invention may additionally contain at least one element selected from
the above-mentioned group (A) and at least one element selected from the above-mentioned
group (B).
[0021] The chemical composition of the wear-resistant steel for the intermediate and room
temperature service of the present invention is limited within a range as described
above for the following reasons.
(1) Carbon:
[0022] Carbon is an element which exerts an important effect on hardness of steel. However,
with a carbon content of under 0.08 wt.%, a Brinell hardness (HB) at a room-temperature
of at least 250 is not available. With a carbon content of over 0.40 wt.%, on the
other hand, a room-temperature Brinell hardness becomes excessively high to result
in deterioration of ductility, toughness, workability and weldability of steel. The
carbon content should therefore be limited within a range of from 0.08 to 0.40 wt.%.
(2) Silicon:
[0023] Silicon has a function of increasing hardness of steel in an intermediate temperature
region without increasing its room-temperature hardness. However, with a silicon content
of under 0.8 wt.%, a desired effect as mentioned above is not available.
[0024] The relationship between a silicon content and a Brinell hardness (HB) in a wear-resistant
steel was investigated. More particularly, for test pieces of a hardened wear-resistant
steel having a thickness of 20 mm, which contained 0.3 wt.% carbon, 0.7 wt.% manganese,
0.9 wt.% chromium and silicon in a certain amount, a Brinell hardness (HB) was measured
for each test piece at a room-temperature, 300°C, 400°C and 500°C with the silicon
content varying within a range of from about 0.4 to about 2.0 wt.%. The results are
shown in Fig. 1.
[0025] In Fig. 1, the mark "o" represents a Brinell hardness at a room-temperature of the
test piece; the mark "·" represents a Brinell hardness at a temperature of 300°C of
the test piece; the mark "Δ" represents a Brinell hardness at a temperature of 400°C
of the test piece; and the mark "▲" represents a Brinell hardness at a temperature
of 500°C of the test piece. As shown in Fig. 1, the test pieces showed a Brinell hardness
at a room-temperature of about 500 almost constantly irrespective of the increase
in the silicon content. The test pieces showed a Brinell hardness at a temperature
of 300°C of at least 450, i.e., about 90% of its room-temperature Brinell hardness
by increasing the silicon content to at least 0.8 wt.%. The test pieces showed a Brinell
hardness at a temperature of 400°C of at least 350, i.e., about 70% of its room-temperature
Brinell hardness by increasing the silicon content to at least 0.8 wt.%. The test
pieces showed a Brinell hardness at a temperature of 500°C also increased, though
on a relatively low level, by increasing the silicon content to at least 0.8 wt.%.
[0026] With a silicon content of over 2.5 wt.%, on the other hand, δ -ferrite is produced
in the steel structure, and this may cause degradation of a room-temperature hardness
of steel, and the manufacturing cost of steel becomes higher. The silicon content
should therefore be limited within a range of from 0.8 to 2.5 wt.%.
(3) Manganese:
[0027] Manganese has a function of improving hardenability of steel. However, with a manganese
content of under 0.1 wt.%, a desired effect as mentioned above is not available. With
a manganese content of over 2.0 wt.%, on the other hand, weldability of steel is degraded,
and the manufacturing cost of steel becomes higher. The manganese content should therefore
be limited within a range of from 0.1 to 2.0 wt.%.
(4) Copper:
[0028] Copper has a function of improving hardenability of steel. In the wear-resistant
steel of the present invention, therefore, copper is additionally added as required.
However, with a copper content of under 0.1 wt.%, a desired effect as mentioned above
is not available. With a copper content of over 2.0 wt.%, on the other hand, hot workability
of steel is degraded. The copper content should therefore be limited within a range
of from 0.1 to 2.0 wt.%.
(5) Nickel:
[0029] Nickel has a function of improving hardenability and low-temperature toughness of
steel. In the wear-resistant steel of the present invention, therefore, nickel is
additionally added as required. However, with a nickel content of under 0.1 wt.%,
a desired effect as mentioned above is not available. With a nickel content of over
10.0 wt.%, on the other hand, the manufacturing cost of steel becomes higher. The
nickel content should therefore be limited within a range of from 0.1 to 10.0 wt.%.
(6) Chromium:
[0030] Chromium has a function of improving hardenability of steel. In the wear-resistant
steel of the present invention, therefore, chromium is additionally added as required.
However, with a chromium content of under 0.1 wt.%, a desired effect as mentioned
above is not available. With a chromium content of over 3.0 wt.%, on the other hand,
weldability of steel is degraded, and the manufacturing cost of steel becomes higher.
The chromium content should therefore be limited within a range of from 0.1 to 3.0
wt.%.
(7) Molybdenum:
[0031] Similarly to chromium, molybdenum has a function of improving hardenability of steel.
In the wear-resistant steel of the present invention, therefore, molybdenum is additionally
added as required. However, with a molybdenum content of under 0.1 wt.%, a desired
effect as mentioned above is not available. With a molybdenum content of over 3.0
wt.%, on the other hand, weldability of steel is degraded and the manufacturing cost
of steel becomes higher. The molybdenum content should therefore be limited within
a range of from 0.1 to 3.0 wt.%.
(8) Boron:
[0032] Boron has a function of improving hardenability of steel with a slight content. In
the wear-resistant steel of the present invention, therefore, boron is additionally
added as required. However, with a boron content of under 0.0003 wt.%, a desired effect
as mentioned above is not available. With a boron content of over 0.0100 wt.%, on
the other hand, weldability and hardenability of steel are degraded. The boron content
should therefore be limited within a range of from 0.0003 to 0.0100 wt.%.
(9) Niobium:
[0033] Niobium has a function of improving hardness of steel through precipitation hardening.
In the wear-resistant steel of the present invention, therefore, niobium is additionally
added as required. However, with a niobium content of under 0.005 wt.%, a desired
effect as mentioned above is not available. With a niobium content of over 0.100 wt.%,
on the other hand, weldability of steel is degraded. The niobium content should therefore
be limited within a range of from 0.005 to 0.100 wt.%.
(10) Vanadium
[0034] Similarly to niobium, vanadium has a function of improving hardness of steel through
precipitation hardening. In the wear-resistant steel of the present invention, therefore,
vanadium is additionally added as required. However, with a vanadium content of under
0.01 wt.%, a desired effect as mentioned above is not available. With a vanadium content
of over 0.10 wt.%, on the other hand, weldability of steel is degraded. The vanadium
content should therefore be limited within a range of from 0.01 to 0.10 wt.%.
(11) Titanium:
[0035] Similarly to niobium, titanium has a function of improving hardness of steel through
precipitation hardening. In the wear-resistant steel of the present invention, therefore,
titanium is additionally added as required. However, with a titanium content of under
0.005 wt.%, a desired effect as mentioned above is not available. With a titanium
content of over 0.100 wt.%, weldability of steel is degraded. The titanium content
should therefore be limited within a range of from 0.005 to 0.100 wt.%.
[0036] In the present invention, for example, a slab of a wear-resistant steel having the
above-mentioned chemical composition may be hot-rolled to prepare a steel sheet, and
the thus prepared steel sheet may be subjected to heat treatments including a hardening
treatment, a tempering treatment, an ageing treatment and a stress relieving treatment.
Hardness and toughness of the steel sheet can further be improved by the application
of these heat treatments thereto.
[0037] Now, the wear-resistant steel of the present invention is described more in detail
by means of examples while comparing with a wear-resistant steel for comparison outside
the scope of the present invention.
EXAMPLES
[0038] Ingots of the wear-resistant steel of the present invention having the chemical compositions
within the scope of the present invention as shown in Table 1, and ingots of a wear-resistant
steel for comparison having the chemical compositions outside the scope of the present
invention as shown also in Table 1, were melted in a melting furnace, and then cast
into slabs. The resultant slabs were then hot-rolled to prepare samples of the wear-resistant
steel of the present invention (hereinafter referred to as the "samples of the invention")
Nos. 1 to 13 having a thickness of 15 mm, and samples of the wear-resistant steel
for comparison outside the scope of the present invention (hereinafter referred to
as the "samples for comparison") Nos. 1 to 4 also having a thickness of 15 mm.
[0039] The samples of the invention Nos. 1 to 4 and 6 to 13, and the samples for comparison
Nos. 1 to 3 were subjected to any one of the following heat treatments as shown in
the column of "heat treatment" in Table 1. The sample of the invention No. 5 and the
sample for comparison No. 4 were maintained in the as-rolled state without being subjected
to any heat treatment.
(1) A sample is hardened by heating the sample to a temperature of 900°C and then
water-quenching the heated sample from the above-mentioned temperature (hereinafter,
this heat treatment being referred to as the "RQ");
(2) A sample is subjected to the above-mentioned RQ, and then tempered at a temperature
as shown in the parentheses in the column of "heat treatment" in Table 1 (hereinafter,
this heat treatment being referred to as the "RQT");
(3) A sample is directly hardened by immediately water-quenching the sample from the
hot-rolling finishing temperature (hereinafter, this heat treatment being referred
to as the "DQ"); and
(4) A sample is subjected to the above-mentioned DQ, and then tempered at a temperature
as shown in the parentheses in the column of "heat treatment" in Table 1 (hereinafter,
this heat treatment being referred to as the "DQT").
[0040] Then, for each of the samples of the invention Nos. 1 to 13 and the samples for comparison
Nos. 1 to 4, a Brinell hardness (HB) at a room-temperature, a Brinell hardness at
a temperature of 300°C and a Brinell hardness at a temperature of 400°C were investigated.
The results are shown in Table 2. In the column of "Brinell hardness (HB)" in Table
2, the values of Brinell hardness shown in the subcolumns of "at 300°C" and "at 400°C"
were obtained by converting the values measured in the tensile test, although the
values of Brinell hardness shown in the subcolumn of "at room-temperature" were obtained
by means of the Brinell test. Each value of percentages shown in the parentheses in
the subcolumns of "at 300°C" and "at 400°C" presents a ratio of each value of Brinell
hardnesses at temperatures of 300°C and 400°C to a value of its Brinell hardness at
a room-temperature.

[0041] As is clear from Tables 1 and 2, each of the samples for comparison Nos. 1 to 3,
which have a low silicon content outside the scope of the present invention, has a
Brinell hardness at a temperature of 300°C within a range of from 83 to 85% of its
room-temperature Brinell hardness, and a Brinell hardness at a temperature of 400°C
within a range of from 65 to 68% of its room-temperature Brinell hardness, both of
which are lower than the target values in the present invention. The sample for comparison
No. 4, which has a low carbon content outside the scope of the present invention,
has a room-temperature Brinell hardness of 150, which is far lower than the target
value in the present invention.
[0042] Each of the samples of the invention Nos. 1 to 13 has, in contrast, a Brinell hardness
at a room-temperature within a range of from 304 to 522, which is higher than the
target value in the present invention, and has a Brinell hardness at a temperature
of 300°C of at least 90% of its room-temperature Brinell hardness, which is the target
value in the present invention, and has a Brinell hardness at a temperature 400°C
of at least 70% of its room-temperature Brinell hardness, which is the target value
in the present invention. Thus, each of the samples of the invention Nos. 1 to 13
has an excellent wear resistance in the intermediate temperature region without largely
increasing its room-temperature hardness.
[0043] According to the present invention, as described above in detail, it is possible
to obtain a wear-resistant steel for the intermediate and room temperature service,
which has a Brinell hardness at a room-temperature of at least 250, and has a Brinell
hardness at a temperature of 300°C of at least 90% of its room-temperature Brinell
hardness, and a Brinell hardness at a temperature of 400°C of at least 70% of its
room-temperature Brinell hardness, the last two Brinell hardnesses being available
without largely increasing its room-temperature Brinell hardness, thus providing industrially
useful effects.