U.S. patent application number 10/391732 was filed with the patent office on 2004-03-11 for high-hardness, high-toughness steels and crawler components, earth wear resistant components, fastening bolts, high-toughness gears, high-toughness, high contact pressure resistance gears, and wear resistant steel plates using the same.
This patent application is currently assigned to KOMATSU LTD.. Invention is credited to Takayama, Takemori.
Application Number | 20040047757 10/391732 |
Document ID | / |
Family ID | 29697641 |
Filed Date | 2004-03-11 |
United States Patent
Application |
20040047757 |
Kind Code |
A1 |
Takayama, Takemori |
March 11, 2004 |
High-hardness, high-toughness steels and crawler components, earth
wear resistant components, fastening bolts, high-toughness gears,
high-toughness, high contact pressure resistance gears, and wear
resistant steel plates using the same
Abstract
In order to provide a high-hardness, high-toughness steel, Si,
Al, Cr, Mo, V, W, Ni, and Co are more appropriately added so that
the steel can have an HRC hardness of 50 or higher and a Charpy
impact value of 5 kgf m/cm.sup.2 or more by tempering at a high
temperature of 600.degree. C. or higher. The steel is a martensite
steel containing at least C: 0.15 to 1.2% by weight and Si: 0.05 to
1.8% by weight, wherein Si is partially replaced by 0.15 to 1.6% by
weight of Al. The steel further contains Ni: 0.3 to 2.5% by weight;
Cr: 0.1 to 3.5% by weight; Mo: 0.1 to 1.7% by weight, wherein the
amount of Mo is not more than the upper limit determined by the
relation formula: Mo(% by weight)=1.7-0.5.times.(Si(% by
weight)+Al(% by weight)); one or both of V: 0.05 to 0.40% by weight
and W: 0.1 to 1.0% by weight; at least one alloying element of Mn,
Co, Cu, Ti, B, and Nb; inevitable impurities including P, S, N, and
O; and the balance consisting essentially of Fe.
Inventors: |
Takayama, Takemori; (Osaka,
JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
767 THIRD AVENUE
25TH FLOOR
NEW YORK
NY
10017-2023
US
|
Assignee: |
KOMATSU LTD.
Tokyo
JP
|
Family ID: |
29697641 |
Appl. No.: |
10/391732 |
Filed: |
March 19, 2003 |
Current U.S.
Class: |
420/107 ;
420/110 |
Current CPC
Class: |
C21D 9/42 20130101; C22C
38/24 20130101; C21D 2211/008 20130101; C22C 38/02 20130101; C21D
9/22 20130101; C22C 38/04 20130101; C21D 6/002 20130101; C22C 38/22
20130101; B32B 15/011 20130101; C21D 9/32 20130101; C21D 1/06
20130101; C22C 38/44 20130101 |
Class at
Publication: |
420/107 ;
420/110 |
International
Class: |
C22C 038/30; C22C
038/22 |
Foreign Application Data
Date |
Code |
Application Number |
May 10, 2002 |
JP |
2002-135274 |
Claims
What is claimed is:
1. A high-hardness, high-toughness steel containing at least C:
0.15 to 0.60% by weight, Si: 0.05 to 1.8% by weight, and Cr: 0.1 to
3.5% by weight, which comprises: Mo in an amount of 0.1 to 1.7% by
weight, wherein the amount of Mo is not more than the upper limit
determined by the relation formula: Mo(% by
weight)=1.7-0.5.times.Si(% by weight); one or both of V: 0.10 to
0.40% by weight and W: 0.1 to 1.0% by weight; at least one alloying
element of Mn, Ni, Co, Cu, Al, Ti, B, Nb, Zr, Ta, Hf, and Ca;
inevitable impurities including P, S, N, and O; and the balance
consisting essentially of Fe, wherein the steel is a quenched and
tempered martensite steel.
2. The steel according to claim 1, wherein Si is in an amount of
0.8 to 1.60% by weight, Cr is in an amount of at least 0.1% by
weight and less than 1.0% by weight, Mo is in an amount of 0.5 to
1.3% by weight, and B is added in an amount of 0.0005 to 0.005% by
weight.
3. The steel according to claim 1, wherein the amount of each
alloying element is adjusted to satisfy the relation formula:
26.2.ltoreq.5.8.times.(Si(% by weight)+Al(% by
weight))+2.8.times.Cr(% by weight)+11.times.Mo(% by
weight)+25.7.times.V(% by weight)+7.5.times.W(% by
weight).ltoreq.36.2.
4. The steel according to claim 1, wherein Co is added in an amount
of 1 to 20% by weight.
5. The steel according to claim 1, wherein the steel contains at
least one of Nb, Ti, Zr, Ta, and Hf in a total amount of 0.005 to
0.2% by weight.
6. The steel according to claim 1, wherein the steel provides an
HRC hardness of 50 to 60 and a Charpy impact value of at least 5
kgf m/cm.sup.2 after quenching and following tempering at a high
temperature of at least 600.degree. C.
7. The steel according to claim 1, wherein the steel is a tempered
martensite steel quenched and then tempered and has an HRC hardness
of at least 45 and a Charpy impact value that satisfies the
relation formula: log(Charpy impact value (kgf
m/cm.sup.2)).gtoreq.-0.0263.times.HRC+2.225 where its HRC hardness
is in the range from 45 to 55, or has an HRC hardness of at least
55 and a Charpy impact value of at least 6 kgf m/cm.sup.2.
8. A high-hardness, high-toughness steel containing at least C:
0.10 to 1.20% by weight and Si: 0.05 to 1.8% by weight, which
comprises: 0.15 to 1.6% by weight of Al partially replacing Si; 0.3
to 2.5% by weight of Ni; at least one alloying element of Mn, Cr,
Mo, V, W, Co, Cu, Ti, B, Nb, Zr, Ta, Hf, and Ca; inevitable
impurities including P, S, N, and O; and the balance consisting
essentially of Fe, wherein the steel is a quenched and tempered
martensite structure steel.
9. The steel according to claim 8, wherein the steel contains Cr in
an amount ranging from 0.1 to 3.5% by weight.
10. The steel according to claim 8, wherein Mo is added in an
amount of less than 1.7% by weight and the amount of Mo is not more
than the upper limit determined by the relation formula: Mo(% by
weight)=1.7-0.5.times.(- Si(% by weight)+Al(% by weight)) depending
on the amount of Si and Al.
11. The steel according to claim 8, wherein one or both of V: 0.05
to 0.40% by weight and W: 0.1 to 1.0% by weight are added to the
steel.
12. The steel according to claim 8, wherein the steel satisfies one
or both of limitation (1): Al is in an amount of 0.15 to 0.75% by
weight and Ni is in an amount of 0.3 to 2.0% by weight and
limitation (2): B is added in an amount of 0.0005 to 0.005% by
weight.
13. The steel according to claim 8, wherein the amount of each
alloying element is adjusted to satisfy the relation formula:
21.2.ltoreq.5.8.times.(Si(% by weight)+Al(% by
weight))+2.8.times.Cr(% by weight)+11.times.Mo(% by
weight)+25.7.times.V(% by weight)+7.5.times.W(% by
weight).ltoreq.41.2.
14. The steel according to claim 8, wherein Co is added in an
amount of 1 to 20% by weight.
15. The steel according to claim 8, wherein the steel contains at
least one of Nb, Ti, Zr, Ta, and Hf in a total amount of 0.005 to
0.2% by weight.
16. The steel according to claim 8, wherein the steel provides an
HRC hardness of 50 to 60 and a Charpy impact value of at least 5
kgf m/cm.sup.2 after quenching and following tempering at a high
temperature of at least 600.degree. C.
17. The steel according to claim 8, wherein the steel is a tempered
martensite steel quenched and then tempered and has an HRC hardness
of at least 45 and a Charpy impact value that satisfies the
relation formula: log(Charpy impact value (kgf
m/cm.sup.2)).gtoreq.-0.0263.times.HRC+2.225 where its HRC hardness
is in the range from 45 to 55, or has an HRC hardness of at least
55 and a Charpy impact value of at least 6 kgf m/cm.sup.2.
18. A high-hardness, high-toughness steel containing at least C:
0.25 to 0.55% by weight, Si: less than 0.8% by weight, and Cr: 3.5
to 5.5% by weight, which comprises: Mo in an amount of 0.3 to 1.0%
by weight; one or both of V: 0.10 to 0.40% by weight and W: 0.1 to
1.0% by weight; at least one alloying element of Mn, Ni, Co, Cu,
Al, B, Ti, Nb, Zr, Ta, Hf, and Ca; inevitable impurities including
P, S, N, and O; and the balance consisting essentially of Fe,
wherein the steel has a quenched and tempered martensite
structure.
19. The steel according to claim 18, wherein the steel contains Al
in an amount of 0.15 to 1.0% by weight and Ni in an amount of 0.3
to 2.5% by weight so that it has an improved resistance to temper
softening and an improved toughness.
20. The steel according to claim 19, wherein the amount of each
alloying element is adjusted to satisfy the relation formula:
21.2.ltoreq.3.times.(Si(% by weight)+Al(% by
weight))+2.8.times.Cr(% by weight)+11.times.Mo(% by
weight)+25.7.times.V(% by weight)+7.5.times.W(% by
weight).ltoreq.41.2.
21. The steel according to claim 18, wherein Co is added in an
amount of 1 to 20% by weight.
22. The steel according to claim 18, wherein the steel contains at
least one of Nb, Ti, Zr, Ta, and Hf in a total amount of 0.005 to
0.2% by weight.
23. The steel according to claim 18, wherein the steel provides an
HRC hardness of 50 to 60 and a Charpy impact value of at least 5
kgf m/cm.sup.2 after quenching and following tempering at a high
temperature of at least 600.degree. C.
24. The steel according to claim 18, wherein the steel is a
tempered martensite steel quenched and then tempered and has an HRC
hardness of at least 45 and a Charpy impact value that satisfies
the relation formula: log(Charpy impact value (kgf
m/cm.sup.2)).gtoreq.-0.0263.times.HRC+2.225 where its HRC hardness
is in the range from 45 to 55, or has an HRC hardness of at least
55 and a Charpy impact value of at least 6 kgf m/cm.sup.2.
25. A crawler component including a crawler bush, a crawler link, a
top or bottom tracker roller for a crawler, and a crawler shoe for
a crawler vehicle, which comprises the high-hardness,
high-toughness steel according to any one of claims 1 to 24,
wherein the steel is a quenched and tempered martensite steel
having an HRC hardness of at least 52 and a Charpy impact value of
at least 6 kgf m/cm.sup.2.
26. An earth wear-resistant component including a tunneling shank,
a tunneling disk cutter, a chisel tool, and a stirring blade for
soil improvement, which comprises the high-hardness, high-toughness
steel according to any one of claims 1 to 24, wherein the steel is
a quenched and tempered martensite steel having an HRC hardness of
at least 50 and a Charpy impact value of at least 8 kgf
m/cm.sup.2.
27. A fastening bolt for use in a construction machine, which
comprises the high-hardness, high-toughness steel according to any
one of claims 1 to 24, wherein the steel is a quenched and tempered
martensite steel having an HRC hardness of at least 40 and a Charpy
impact value that satisfies the relation formula: log(Charpy impact
value (kgf m/cm.sup.2)).gtoreq.-0.0263.times.HRC+2.225.
28. A high-toughness gear, which comprises the high-hardness,
high-toughness steel according to any one of claims 1 to 24,
wherein the steel is formed into a gear shape and then carburized,
quenched, and tempered to have a surface carbon concentration of
0.6 to 1.0% by weight, a surface carburizing depth of at least 0.4
mm, and an adjusted HRC hardness of 55 to 64 and to provide a
Charpy test piece with an equivalent depth and a Charpy impact
value of at least 8 kgf m/cm.sup.2.
29. A high-toughness, high contact pressure-resistance gear, which
comprises the high-hardness, high-toughness steel according to any
one of claims 1 to 24, wherein the steel is formed into a gear
shape and then carburized to have a surface carbon content of 0.8
to 1.3% by weight, temporarily cooled to the Al transformation
temperature or lower, and then heated again, quenched, and tempered
to have a surface carburized case depth of at least 0.4 mm, to
contain cementite particles with an average particle diameter of at
most 1 .mu.m dispersed in its quench-hardened case, to have an
adjusted HRC hardness of 59 to 65, and to provide a Charpy test
piece with its equivalent depth and a Charpy impact value of at
least 4 kgf m/cm.sup.2.
30. A high-toughness gear, which comprises the high-hardness,
high-toughness steel according to any one of claims 1 to 24,
wherein the steel is formed into a gear shape and then
induction-hardened and tempered to have an adjusted surface HRC
hardness of 52 to 64 and to provide a Charpy test piece having a
hardened depth equivalent to that of the steel and a Charpy impact
value of at least 5 kgf m/cm.sup.2.
31. A wear-resistant steel plate, which comprises the
high-hardness, high-toughness steel according to any one of claims
1 to 24, wherein the steel is quenched and tempered to have a high
tension of at least 50 kgf/mm.sup.2 and/or an adjusted HRC hardness
of at least 50, and weldable use of the steel plate includes a
bucket and a bulldozer blade.
32. An earth wear-resistant component for use in earth excavation
including a ripper point, an end bit, bucket tooth, an edge, and a
tunneling disk cutter, which comprises the high-hardness,
high-toughness steel according to any one of claims 1 to 24,
wherein the steel contains less than 3.5% by weight of Cr and
alloying elements each in a controlled amount that satisfies the
relation formula: 26.2.ltoreq.5.8.times.(Si(% by weight)+Al(% by
weight))+2.8.times.Cr(% by weight)+11.times.Mo(% by
weight)+25.7.times.V(% by weight)+7.5.times.W(% by
weight).ltoreq.41.2 so that the steel can have an HRC hardness of
at least 50 by tempering at 600.degree. C.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high-hardness,
high-toughness, wear-resistible steel for use in an excavating edge
member of a construction or earth work machine such as a hydraulic
excavator, a bulldozer, a wheel loader, a motor grader, an
underground piping burying machine, a soil-improvement machine, a
crusher for concrete, lumber, or the like, and a tunneling machine;
a crawler belt of a crawler vehicle; a reduction gear; and the
like, and relates to a member using such a steel.
BACKGROUND ART
[0002] Conventionally, examples of the wear-resistible steel widely
used in construction or earth work machines include SMnB, SCr,
SCrB, SCM, and SNCM medium carbon steels that are processed by
heat-treatment such as quenching and tempering. For example, the
components of a crawler vehicle belt such as a crawler bush, a
crawler shoe, a crawler link, a tracker roller, and a sprocket are
appropriately made wear-resistant based on the idea that toughness
can be established by reducing the carbon content. Excavating edge
members for use in cutting or excavating rock mass (such as a
ripper point, bucket tooth, and a cutting edge) are strongly needed
to have high performance. Therefore, such edge members have
improved in toughness so that cracking or breakage can be prevented
against more impact load and improved in wear resistance by high
hardening. In particular, the near-edge portion of the excavating
edge member is heated to about 600.degree. C. by severe friction
with rock mass. Therefore, high-hardness, high-toughness steels
with improved resistance to temper softening are often used for the
edge member.
[0003] The construction or earth work machine in operation
frequently goes over obstructions such as rock and other structures
and frequently swings to excavate the obstructions. The gears of
the reduction gears in the driving and swinging mechanisms can be
broken under impact load. Therefore, the gears are formed of
low-carbon case hardened steel carburized, quenched, and
tempered.
[0004] The crawler components of the construction or earth work
machine and the excavating edge members should satisfy both 1)
high-toughness for preventing cracking or breakage under impact
load and 2) high hardness for providing excellent wear resistance
against earth, rock, and the like. However, high toughness and high
hardness generally trade off with each other. Therefore, the
addition amount of carbon should be small in order to establish
toughness, the steel material (wear-resistible steel) for use
should contain hardenability-enhancing alloying elements each in an
appropriate amount, and the steel should be quenched and tempered
before used. However, such conditions can cause a problem of
insufficient wear resistance.
[0005] Toughness is important for the wear resistant members for
forming the crawler belt. Therefore, the carbon content of such
members is set at a low level, for example, as follows: 0.25 to
0.3% by weight in the crawler shoe, 0.3 to 0.35% by weight in the
tracker roller, 0.35 to 0.4% by weight in the crawler link, and
0.35% by weight in the sprocket. In addition, their quenched and
tempered hardness is adjusted to between HRC45 and HRC52. Under
such conditions, the members are often insufficient in wear
resistance and have a problem of the high cost of repairing the
crawler belt in the construction or earth work machine.
[0006] In terms of structure, wear resistance is important for the
crawler bush for forming the crawler belt. For example, therefore,
high-toughness SCM420 is carburized, quenched, and tempered to give
the crawler bush. However, its cost can be high, because the
carburizing process for forming a deep, very hard carburized case
takes a very long time period. In addition, it is susceptible to
damage, because the carburizing process can significantly reduce
the toughness.
[0007] In the process of a toughness-conscious member such as a
tunneling disk cutter or shank, for example, as disclosed in
Japanese Patent No. 3227730, a low carbon, high Ni steel (for
example, a steel corresponding to SNCM625 or SNCM630) is quenched
and tempered to show HRC45 or higher and a Charpy impact value of 5
kgf m/cm.sup.2 or more before used. However, its durability is
insufficient in both hardness and toughness, and its cost is
high.
[0008] The tooth plate of a jaw crusher for crushing rock and
concrete is also toughness-conscious and often uses Hadfield steel.
Such a plate also has a problem of insufficient wear
resistance.
[0009] In the process of a cutter (such as a soil cutter and a pin)
of a stirring machine for stirring and pulverizing earth such as a
soil-improvement machine and a tiller, the carbon content is
limited to 0.25 to 0.3% by weight, and the steel is quenched and
tempered to have an HRC level of 48 before used. Such a cutter also
has a problem of insufficient wear resistance.
[0010] When the construction or earth work machine such as the
bulldozer, the wheel loader, the hydraulic excavator, and the motor
grader is used to cut or excavate rock, the edge portion of the
excavating edge member (such as a ripper point, bucket tooth, a
cutting edge, and an end bit) or the tunneling disk cutter is
heated to a temperature of 300 to 600.degree. C. by severe friction
with the rock. In such a case, the initial hardness can
significantly be reduced so that the wear resistance can be
insufficient. The steel to be used should have not only high
toughness and high hardness but also sufficient resistance to
temper softening even in heating at about 600.degree. C. However,
it has been unclear how to appropriately add alloying elements (for
example, how to select alloying elements and how to determine the
addition amount) for improvement in the resistance to temper
softening. Therefore, excessive addition of the alloying elements
cannot be prevented so that the toughness can be reduced or the
cost can be high.
[0011] It has been very difficult for the wear-resistible steels
with various alloying elements to be free from "temper
brittleness", which develops by tempering at a temperature of 350
to 550.degree. C. after quenching. Therefore, such steels cannot
have sufficiently high hardness.
[0012] On the other hand, carbon steels show very little "temper
brittleness" even when tempered at a temperature of 350 to
550.degree. C. but can be insufficient in hardenability. Therefore,
high cleanliness steels made of high-alloy wear-resistible steels
with less P or S have been produced as less "temper brittleness"
steels. However, such steels can be expensive and therefore have a
problem with commercial availability.
[0013] For example, Japanese Patent Publication No. 55-12177 (1980)
discloses such a conventional wear-resistible steel, which contains
C: 0.25 to 0.40%, Si: 1.5 to 2.5%, Mn: 1.6% or less, Cr: 3.0 to
5.0%, and Mo: 0.5 to 1.2%. In such a wear-resistible steel,
however, the high content of Si, Cr, or Mo can lead to sharp
reduction in tempered hardness at 550.degree. C. or higher. Such a
steel is insufficient in wear resistance and uneconomical.
[0014] Japanese Patent Laid-Open No. 54-124816 (1979) discloses a
wear-resistible steel that contains C: 0.4 to 0.6%, Si: 0.8 to
1.7%, Mn: 0.4 to 0.8%, Cr: 0.6 to 2.0%, Mo: 0.1 to 0.8%, and Al:
0.2 to 1.0%. Japanese Patent Laid-Open No. 54-143715 (1979)
discloses a wear-resistible steel that contains C: 0.4 to 0.6%, Si:
0.8 to 1.7%, Mn: 0.4 to 0.8%, Cr: 0.6 to 2.0%, W: 0.1 to 0.5%, and
Al: 0.2 to 1.0%. However, these steels are insufficient in
resistance to temper softening and toughness.
[0015] Japanese Patent Laid-Open No. 59-107066 (1984) discloses a
wear-resistible steel that contains C: 0.4 to 0.6%, Si: 1.6 to
2.2%, Mn: 0.5% or less, Cr: 1.0 to 1.5%, Mo: 0.8 to 1.2%, V: 0.2 to
0.5%, and Ni: 1.0 to 2.0%. Such a wear-resistible steel is
insufficient in toughness as well as resistance to temper softening
because of the high content of Mo and V.
[0016] Japanese Patent Laid-Open No. 60-215743 (1985) discloses a
wear-resistible steel that contains C: 0.35 to 0.45%; Si: 0.6 to
1.5%; Mn: 1.8% or less; Cr: 2.5 to 4.5%; and Mo: 0.2 to 1.0%;
and/or at least one of V: 0.01 to 0.5%, Nb: 0.01 to 0.10%, and W:
0.01 to 0.5%; and Ti: 0.01 to 0.10%; and B: 0.0005 to 0.0030%. In
such a wear-resistible steel, a relatively high content of Cr can
reduce the Si-induced resistance to temper softening and always
reduces the toughness.
[0017] Japanese Patent Laid-Open No. 5-78781 (1993) discloses a
wear-resistible steel that contains C: 0.35 to 0.55%, Si: 0.5% or
less, Mn: 0.5% or less, P: 0.015% or less, S: 0.010% or less, Cr:
1.00 to 2.5%, Mo: 1.00 to 2.00%, V: 0.05 to 0.30%, B: 0.0003 to
0.0050%, Al: 0.005 to 0.10%, and Nb: 0.01 to 0.20%. In this steel,
the low Si-induced reduction in the resistance to temper softening
can be recovered by high Mo and V addition, but the recovered
resistance to temper softening is not sufficient. In order to
ensure the toughness, the content of P, S, and Mn is set low in the
steel so that the grain boundary can be strengthened. However, such
a steel is generally expensive and therefore has a problem with
commercial availability and can be insufficient even in
toughness.
[0018] In the driving and swinging mechanisms of the reduction
gears of the construction or earth work machine, the gears must be
prevented from fracturing due to impact load. For such a purpose,
the gears are formed of high-toughness, low-carbon case hardening
steels (with 0.1 to 0.25% by weight of C), carburized, quenched,
and tempered. However, the toughness can sharply decrease, as the
surface carburized case becomes deeper. Particularly, in such a
case, the gears cannot improve in toughness, if the carburized case
must be formed with a depth of 0.5 mm or more in terms of contact
pressure strength and dedendum bending fatigue strength. The
low-carbon case hardening steels have also been reduced in P and S
contents to have strengthened grain boundary, but such steels are
generally expensive and therefore have a problem with commercial
availability and can be insufficient even in toughness.
[0019] The present invention has been made in order to solve these
problems. It is therefore an object of the present invention to
provide a high-hardness, high-toughness steel that contains Si, Al,
Cr, Mo, V, W, Ni, and Co more appropriately added so as to have an
HRC hardness of 50 or higher and a Charpy impact value of 5 kgf
m/cm.sup.2 or more by tempering at a high temperature of
600.degree. C. or higher.
[0020] It is another object of the present invention to provide a
steel that contains Al and Ni in combination and therefore has high
toughness in spite of being formed of high-carbon, high-hardness,
quenched and tempered martensite steel.
[0021] It is yet another object of the present invention to provide
a variety of wear resistant members, gear members, and bolt members
that are each formed of the inventive steel and appropriately
heat-treated to show high hardness and high toughness.
SUMMARY OF THE INVENTION
[0022] The inventor has paid attention to a commonality between the
fact that the excavating edge member or the like is heated up to
600.degree. C. by friction and the process in which quenched steels
are tempered at 550.degree. C. or higher to recover the toughness
without temper brittleness. Thus, the inventor has investigated and
found appropriate carbon content for providing a sufficient
hardness of HRC45 or higher (preferably HRC50 or higher) even after
quenching and tempering at 550.degree. C. or higher, preferably
600.degree. C.; appropriate type and addition amount of alloying
elements (such as C, Si, Al, Cr, Mo, V, and W); and appropriate
technique in consideration of the interaction between alloying
elements as described below, so that unnecessary addition of the
alloying elements, which would otherwise cause brittleness and
deterioration, can be prevented, and economy can be achieved.
[0023] It has also been found that Al can significantly enhance the
resistance to temper softening and that the combined addition of Al
and Ni can provide significant toughness without the temper
brittleness even through tempering at low temperature. It has also
been found that the high-hardness, high-toughness tempered
martensite structure steel having a carbon content in the range of
up to 1.2% by weight and showing a Charpy impact value of 5 kgf
m/cm.sup.2 or more can be used to improve the wear resistance of
the various members.
[0024] In addition, Co, which can raise the magnetic transformation
temperature of quenched and tempered martensite, is added in an
appropriate amount so that high toughness is produced without high
temperature temper brittleness, and therefore the resistance to
high temperature temper softening further increases without
reduction in toughness.
[0025] In the present invention, the content (% by weight) of each
alloying constituent in the steel is defined for the reasons below.
As shown in Examples below, the reasons were found from the results
of analysis of hardness data on various wear resistant steels that
were tempered at a temperature of 200 to 700.degree. C.
[0026] C: 0.10 to 1.2% by Weight
[0027] Various quenched steels different in carbon content were
tempered at a low temperature of 200.degree. C. and then examined.
As a result, HRC45 or higher was achieved when the lower limit of
the carbon content was set at 0.15% by weight, and a Charpy impact
value of 5 kgf m/cm.sup.2 or more was achieved when the upper limit
of the carbon content was set at about 0.60% by weight. In
addition, the inventive steel that contains Al and Ni in
combination requires that the upper limit of the carbon content
should be 1.20% by weight, and considering that the inventive steel
should be applied to the carburized gears and the like, the lower
limit of the carbon content should be 0.1% by weight. In the
present invention, therefore, the carbon content should be from 0.1
to 1.2% by weight.
[0028] As mentioned above, the high-hardness, high-toughness steel
for use in the member for which resistance to temper softening is
important contains 0.25 to 0.55% by weight of carbon. On the other
hand, in the steels that contain little alloying element, the
carbon content has little effect on the hardness during tempering
at 400 to 600.degree. C. (a carbon content of 0.55% by weight or
more provides +2.5 of .DELTA.HRC at 500.degree. C. and +1.0 of
.DELTA.HRC at 600.degree. C.). Therefore, analysis was carried out
using, as the amount of resistance to temper softening (an increase
in hardness), the difference between the hardness obtained by
adding alloying elements and by tempering at 400 to 600.degree. C.
and the standard hardness of carbon steel. In order to obtain a
hardness of HRC45 or higher by tempering at 600.degree. C., the
carbon content is preferably 0.25% by weight or more.
[0029] As mentioned below, in order to increase the resistance to
temper softening, an alloying element such as Mo and V is
positively added to the high-hardness, high-toughness steel. In
such a case, a carbon content of 0.60% by weight or more is not
preferred. This is because such an increased carbon content can
reduce the solid solution amount of the alloying element which
would otherwise contribute to the resistance to temper softening in
the austenite phase area during heating for quenching; the
reduction in the role of the alloying element contributing to the
resistance to temper softening is not economical; and the carbide
in the tempered martensite increases in amount and size so that the
toughness is reduced.
[0030] The carbon element can significantly stabilize the
austenite. As mentioned below, therefore, the inventive steel that
contains a large amount of ferrite-stabilizing Si, Al, or Mo
preferably contains 0.10% by weight or more of carbon so that the
quenching temperature can be suppressed to 950.degree. C. or lower.
For the purpose of suppressing the addition amount of the
austenite-stabilizing element Mn, Ni, or Cr which can reduce the
quenching temperature, the carbon content is preferably 0.10% by
weight or more.
[0031] Si: 0.05 to 2.5% by Weight
[0032] Si is inevitably introduced by steel making and the Si
content is generally 0.05 to 0.3% by weight. In the present
invention, however, Si may be added in an amount of less than 2.5%
by weight, because it can suppress the precipitation of cementite,
contribute to the improvement in toughness by tempering at about
400.degree. C. or lower, and enhance the resistance to temper
softening. A Si addition amount of less than 0.3% by weight cannot
provide such significant effects. A Si addition amount of up to
about 4% by weight is known to enhance the resistance to temper
softening. However, the addition amount of Si should be determined
in such a range that Si can stabilize the .alpha.Fe phase to raise
the A3 transformation temperature and does not excessively raise
the quenching temperature. Therefore, in the case that the steel
with a carbon content from 0.1 to 0.35% by weight is used as a gear
member after carburized, quenched and tempered, the Si addition
amount is preferably suppressed to 2.5% by weight or less and is
more preferably 1.8% by weight in terms of the effect of Si
addition on Mo or V as described below.
[0033] In addition, it has been found that the maximum addition
amount of effective Mo (YMo % by weight) is preferably controlled
according to the formula YMo=1.7-0.5.times.Si % by weight (at
950.degree. C.) and the Mo addition amount is preferably set at the
effective addition amount or less depending on the Si addition
amount.
[0034] Si and V have also been found to interact with each other
similarly to Si and Mo. At a Si addition amount of 1.8% by weight
or more, the effective maximum addition amount of V (YV) was 0.15%
by weight at 925.degree. C., and at less than 1.8% by weight of Si,
it was 0.3% by weight, and at 950.degree. C., such maximum addition
amounts were 0.2% by weight and 0.4% by weight, respectively.
According to the present invention, the steel also contains Al,
which can stabilize the ferrite phase of the steel similarly to Si.
Therefore, the total addition amount of Al and Si should be 1.8% by
weight or less (Al+Si.ltoreq.1.8% by weight) so that an excessive
raise in the quenching temperature can be avoided.
[0035] The addition of Si can provide resistance to temper
softening at 400.degree. C. or higher, but such resistance is
significantly reduced (to about half at 600.degree. C.) when 3.5%
by weight or more of Cr coexists. This suggests that the combined
addition of Si and 3.5% by weight or more of Cr should not be
effective. This is owing to the reduction in the cementite
precipitation-suppressing effect of Si by the increase in the Cr
amount.
[0036] The coexisting Cr at a content of 3.5% by weight or more
also reduces the upper limit of the Mo addition amount to about
half, otherwise such Mo would be effective at enhancing the
resistance to temper softening. Therefore, it is apparent that Mo
in the effective addition amount or more can reduce the
toughness.
[0037] Al: 0.15 to 1.6% by Weight
[0038] Al has a very strong deoxidizing action. It is known that Al
reacts with nitrogen in the steel to form AlN and make the crystal
grains fine. Killed case hardened steels generally contain 0.005 to
0.05% by weight of Al. The solid Al dissolved in the steel has a
strong tendency to segregate at the grain boundary and functions to
strongly exclude, from the grain boundary, the impurity element
such as P and S, which can reduce the grain boundary strength, and
to strongly attract Ni, which can improve the grain boundary
toughness. According to the present invention, therefore, Al and Ni
are positively added at the same time so that the carbon content
can be high and the tempered martensite structure steel with a high
hardness of HRC45 or higher can drastically improve in toughness.
The positive addition of Al and Ni at the same time can also
prevent temper brittleness, which would otherwise be caused by
tempering at 350.degree. C. or higher. It is well known that the
drastic improvement in the toughness of the high-hardness tempered
martensite structure steel can lead to not only resistance to
impact load but also drastic prevention of intergranular fracture.
As described below, therefore, this performance can be used to
drastically improve the resistance to delayed fracture, for
example, in high-tensile bolts such as a crawler shoe bolt, or a
crawler bush or link of a crawler link assembly used after
press-fitting the crawler bush and pin.
[0039] It has been found that Mo and V as described below can
enhance the resistance to temper softening at a higher temperature
of 400.degree. C. or higher, and in contrast, Al and Si are
effective at enhancing the resistance to temper softening both at a
lower temperature of 400.degree. C. or lower and at a higher
temperature of 400.degree. C. or higher. Therefore, Al may be added
as needed with the relationship Al+Si.ltoreq.1.8% by weight
maintained. At an Al addition amount of 0.15% by weight or less,
improvement in toughness can be insufficient. At an Al addition
amount of 1.6% by weight or more, the A3 temperature can
excessively be raised, and the toughness improvement effect can be
saturated. Therefore, the Al addition amount should be from 0.15 to
1.6% by weight. Similarly to the above, in the case that the steel
is applied to the low carbon gear member, the Al addition amount is
preferably 1.2% by weight or less so that the .alpha.Fe phase can
be prevented from coexisting at a quenching temperature of
950.degree. C.
[0040] Al coexistent with Ni is preferred in terms of wear
resistance, because in such a case, the development of the age
hardening can further enhance the resistance to temper softening
(1Al-1Ni can produce +4 of .DELTA.HRC at 600.degree. C.) as
described below.
[0041] Ni: 0.3 to 2.5% by Weight
[0042] Ni can enhance hardenability and improve the toughness of
the tempered martensite steel. For example, SNCM case hardened
steel and AISI4340 high tension steel each contain 3.5% by weight
or less of Ni. Some quenched and tempered materials containing 2.5
to 4.0% by weight of Ni has been used as a shank steel product for
tunneling (Japanese Patent No. 3227730).
[0043] In the present invention, the combined addition of 0.3 to
2.0% by weight of Al and Ni is essential for more effective
contribution to toughness improvement, and therefore the lower
limit of the Ni addition amount should be 0.3% by weight. The upper
limit of the Ni addition amount is preferably 2.5% by weight,
because the combined addition of Ni and Al can enhance the
resistance to temper softening by the precipitation of NiAl
intermetallic compounds and improve the wear resistance, but
excessive addition can reduce the toughness and be economically
disadvantageous.
[0044] Al and Si strongly stabilize ferrite and these elements at a
high content can disadvantageously make the Ac3 temperature higher
so that a higher quenching temperature can be required. For
example, FIG. 1 shows that based on the effect of various alloying
elements on the A3 temperature line of Fe-3 wt % Si--C alloy,
alloyed steels with a lower carbon content (from 0.10% by weight)
can preferably be reduced in the heat treatment cost by the
quenching temperature-lowering effect of the Mn, Ni, or Cr
addition, and even at a C content of about 0.4% by weight, Mn or Ni
may preferably be added in an amount of about 1% by weight.
[0045] Mn: 0.3 to 3.0% by Weight
[0046] Mn has a significant desulfurizing action. Mn is effective
at improving the hardenability of steels. Similarly to Ni, Mn can
strongly stabilize the austenite phase of steel so that the A3
transformation temperature can be lowered and the quenching
temperature can effectively be reduced. The Mn element is also
effective at suppressing the raise in the A3 transformation
temperature by the addition of the ferrite-stabilizing element Al
or Si. In the present invention, therefore, the Si addition amount
should be 3.0% by weight or less, considering the effect of Mn, Ni,
Si, and Al on the eutectoid temperature and the approximate
relation: (Si+2.times.Al).apprxeq.(Ni+Mn). In such a addition
amount range, the quenching temperature is preferably suppressed
not to be 950.degree. C. or higher, and the old austenite crystal
grain is preferably suppressed in growth not to have an ASTM grain
size number of greater than 8, in terms of heat treatment cost.
[0047] Cr: 0.1 to 3.5% by Weight
[0048] Cr can improve the hardenability of steels and enhance the
resistance to temper softening. Its hardening effect is, however,
smaller than that of Mo, V, W, or the like. If its addition amount
is more than about 7.5 times as much as the coexisting carbon
content, the resistance to temper softening per Cr addition amount
(% by weight) at higher temperature can be reduced and the Si or
Al-induced resistance to temper softening at higher temperature can
also be reduced. Therefore, if the steel should benefit from the Si
or Al-induced resistance to temper softening, the maximum of the Cr
addition amount should be at most 7.5 times as much as the carbon
content. More specifically, the Cr addition amount should be 3.5 wt
% for 0.55 wt % of C, 2.9 wt % for 0.45 wt % of C, 2.3 wt % for
0.35 wt % of C, and 1.6 wt % for 0.25 wt % of C. If the Cr addition
amount should be more, the effective addition amount of Mo should
be reduced to about half in designing the alloy as described
below.
[0049] At a Cr addition amount of 5.5% by weight or more, the
Cr-induced resistance to temper softening can be reduced to about
half. Therefore, the following two methods can be used in designing
the alloy.
[0050] 1) A method in which the Cr addition amount is limited to
3.5% by weight or less and Si, Al, Mo, V, and W are controlled;
and
[0051] 2) A method in which the Cr addition amount is set in the
range of 3.5 to 5.5% by weight, and then the Si addition amount is
set at 0.5% by weight or less, and Mo is set in the effective
addition amount range (up to 1.0% by weight), and Al, V, and W are
controlled (a Mo addition amount of 1.0% by weight or more cannot
effectively contribute to the enhancement of the resistance to
temper softening).
[0052] In method 1), the maximum Cr addition amount is preferably
less than 3.5% by weight in terms of cost, and the minimum Cr
addition amount is preferably 0.1% by weight or more, because the
Cr-induced resistance to temper softening is not so high and the Cr
addition can enhance the hardenability of the steel. However, Cr
can significantly accelerate the cementite precipitation and
significantly increase the temper brittleness, which can generate
in tempering at about 350.degree. C. or higher. Therefore, the Cr
addition amount is preferably limited to less than 1% by weight, in
terms of toughness.
[0053] In method 2), for the steel to which Cr is added in an
amount of 3.5% by weight or more, Si is preferably added in a small
amount, and Mo is preferably added in an amount of the effective
addition amount or less, and V or W is preferably added to enhance
the resistance to temper softening. For example, in order to make
the 600.degree. C. tempered hardness comparable to that of hot work
tool steel SKD6 (C: 0.32 to 0.42%, Si: 0.8 to 1.2%, Mn: 0.5% or
less, Cr: 4.5 to 5.5%, Mo: 1.0 to 1.5%, and V: 0.3 to 0.5%), the
elements are preferably added as follows: Si: 0.5% by weight or
less, Cr: 3.5 to 5.5% by weight, Mo: 0.3 to 1.0% by weight, V: 0.2
to 0.4% by weight, and W: 0.1 to 0.8% by weight, and optionally Al:
0.15 to 0.6% by weight and Ni: 0.3 to 1.5% by weight.
[0054] Mo: 0.1 to 1.9% by Weight
[0055] Mo can improve the hardenability. Mo can also enhance the
toughness of the low-temperature tempered martensite steel and the
resistance to temper softening as mentioned above. Therefore, the
lower limit should be 0.1% by weight for the development of an
effective resistance to temper softening, and the upper limit
should be the maximum addition amount (YMo % by weight) up to which
Mo can be effective for the resistance to quench softening at
quenching temperature. Based on the relation to the solubility
limit of Mo carbide, the maximum addition amount should be as
follows: YMo=1.6 (at 900.degree. C.) in the absence of Si and Al,
and YMo=1.6 wt %-0.5.times.(Si wt %+Al wt %) in the presence of
both Si and Al. However, the constant 1.6 wt % can be altered
depending on the quenching temperature. The constant is
appropriately 1.6 at 900.degree. C., 1.9 at 950.degree. C., and 2.3
at 1000.degree. C. In terms of quenching equipment and the
productivity thereof and coarse crystal grain grown by heating, the
quenching temperature is preferably 950.degree. C. or lower, more
preferably 900.degree. C. or lower.
[0056] Concerning the contribution to the resistance to temper
softening at 600.degree. C., Si or Al can provide HRC=+5.8 per 1%
by weight, while Mo can provide HRC=+11. According to the above
relation, therefore, if Si or Al is effectively used as much as
possible, the Mo addition amount can apparently be reduced without
considerable reduction in the resistance to temper softening. In
the present invention, therefore, the maximum addition amount of Mo
is preferably less than 1% by weight in terms of economy.
[0057] As mentioned above, if Cr coexists at a content of 3.5% by
weight or more, the effective maximum addition amount (YMo % by
weight) is further reduced to about half. In the steel with a Cr
content of 3.5 to 5.5% by weight, therefore, the content of Mo is
preferably 1% by weight or less.
[0058] V: 0.05 to 0.4% by Weight
[0059] In contrast to Cr and Mo, V can significantly enhance the
resistance to temper softening in the tempering temperature range
of 600.degree. C. or higher and be effective at improving wear
resistance. In the present invention, therefore, either V or W (W
has a V-like action as described below) is an essential component.
However, the solid solubility of V carbide is low, and the V
carbide can precipitate into the austenite phase during heating at
quenching temperature so that the toughness can be reduced.
Therefore, the upper limit of the V addition amount is preferably
0.3% by weight. As described above, the upper limit can
appropriately be changed into 0.4% by weight at a quenching
temperature of 950.degree. C. and 0.5% by weight at 1000.degree.
C.
[0060] Where V coexists with Si and Al and (Si+Al) is 1.8% by
weight or more, the upper limit of the V addition amount should be
reduced to half, that is, 0.15, 0.2, or 0.25 wt % at each quenching
temperature. The quenching temperature is preferably 950.degree. C.
or lower in terms of quenching equipment and the productivity
thereof and coarse crystal grain grown by heating. Therefore, the
upper limit of the addition amount should be 0.4% by weight. More
preferably, the quenching temperature is 900.degree. C. or lower,
and therefore the upper limit of the addition amount is 0.3% by
weight. If Si coexists, a V addition amount of 1.0% by weight or
more can reduce the Si-induced resistance to temper softening, and
therefore the maximum addition amount is preferably 0.4% by weight
or less.
[0061] W: 0.1 to 1.0% by Weight
[0062] W does not produce resistance to temper softening as much as
Mo or V does. However, the resistance to temper softening produced
by W can be maximum at a temperature of 600 to 700.degree. C., and
the upper limit of the addition amount up to which W is effective
(YM) can be high. Therefore, either W or V is an essential
component. W can be effective in an addition amount of 0.1% by
weight or more. Similarly to V and Mo, the effective upper limit
depends on temperature and should be 0.8% by weight at 900.degree.
C., 1.7% by weight at 950.degree. C., and 2.5% by weight at
1000.degree. C. If W is added in an amount of 1% by weight or more,
the Mo-induced resistance to temper softening can significantly
reduced. W is more expensive than Mo. Therefore, the maximum
addition amount is preferably 1.0% by weight or less.
[0063] Co: 1 to 20% by Weight
[0064] It is well known that Co itself does not produce resistance
to temper softening. In the present invention, the addition of Co
can sharply raise the magnetic transformation temperature and
suppress the diffusion of other alloying elements. For example, Co
can be effective at raising the reaction temperature in the
formation of Si, Al, Cr, Mo, V, or W carbide which can produce the
resistance to temper softening. Co can exert age hardening together
with Si and Al, and coexisting Al can efficiently raise the Co
magnetic transformation temperature. In an appropriate addition
amount of Co, the magnetic transformation temperature increased by
18.degree. C. per 1% by weight, and such an effect was obtained up
to 10% by weight. In an amount of more than 10 to 20% by weight, an
increase of 10.degree. C. was obtained per 1% by weight. In an
amount of more than 20% by weight, the effect will be saturated,
and the cost can be too high. More efficiently, the usage of Co is
preferably 10% by weight or less.
[0065] B: 0.0005 to 0.0030% by Weight
[0066] B can significantly improve hardenability. In many cases,
the addition of B can be economically advantageous, because the
usage of the hardenability-enhancing alloying element such as Mn,
Cr, and Mo can be reduced. The addition amount of Cr, which tends
to cause high-temperature temper brittleness, can be reduced. In
the present invention, therefore, positive use of B is preferred.
The addition amount of B is appropriately from 0.0005 to 0.0030% by
weight, because it cannot be effective in an amount of less than
0.0005% by weight, and an amount of more than 0.0030% by weight is
known to produce BN precipitation that can reduce the
toughness.
[0067] B tends to segregate at the crystal grain boundary more
strongly than P or S. B tends to strongly allow S to segregate and
particularly, can strongly eliminate S from the grain boundary to
improve the grain boundary strength. Therefore, positive use of B
is preferred.
[0068] Zr, Nb, Ti: 0.005 to 0.20% by Weight
[0069] Zr, Nb, and Ti are known to make crystal grains fine and
added in a conventional amount range. However, an amount of more
than 0.2% by weight is known to increase the amount of carbide and
nitride precipitations and to be disadvantageous for toughness.
[0070] P, S: 0.03% by Weight
[0071] P and S are contained as inevitable impurities but important
because they are involved in the temper brittleness at a
temperature of 350 to 550.degree. C. The content of these elements
in high cleanliness steels is reduced as low as possible. In the
present invention, the maximum content of P or S may be more than
0.03% by weight, because high temperature tempering at 600.degree.
C. or higher can be used and/or the addition of Al and Ni can
prevent the temper brittleness. However, 0.03% by weight or less is
preferred in terms of stabilization of higher toughness, and 0.015%
by weight or less is more preferred, because such an amount
presents no cost problem with the conventional steel making
technique.
[0072] The present invention is based on the above discussion and
results. In a first aspect of the present invention, the
high-hardness, high-toughness steel contains Mo, V, and W in such
an appropriate addition amount that they can produce strong
resistance to temper softening while efficiently using Si-induced
resistance to temper softening. Such a steel is economical and
highly tough and wear-resistant.
[0073] According to the present invention, such a steel contains at
least C: 0.15 to 0.60% by weight, Si: 0.05 to 1.8% by weight, and
Cr: 0.1 to 3.5% by weight and is characterized by comprising Mo in
an amount of 0.1 to 1.7% by weight, wherein the addition amount of
Mo is not more than the upper limit determined by the relation
formula: the upper limit Mo(% by weight)=1.7-0.5.times.Si(% by
weight); one or both of V: 0.10 to 0.40% by weight and W: 0.1 to
1.0% by weight; at least one alloying element of Mn, Ni, Co, Cu,
Al, Ti, B, Nb, Zr, Ta, Hf, and Ca; inevitable impurities including
P, S, N, and O; and the balance consisting essentially of Fe,
wherein the steel is a tempered martensite steel.
[0074] In the present invention, in order to improve the toughness
of the higher-hardness steel tempered at 150.degree. C. or higher,
preferably, the Cr addition amount is limited to less than 1% by
weight, the amounts are limited as follows: Si: 0.8-1.6% by weight,
Cr: 0.1-1.0% by weight, and Mo: 0.5-1.3% by weight, and B is added
in an amount of 0.0005 to 0.005% by weight.
[0075] Preferably, the addition amount of each alloying element is
controlled to satisfy the relation formula:
26.2.ltoreq.5.8.times.(Si(% by weight)+Al(% by
weight))+2.8.times.Cr(% by weight)+11.times.Mo(% by
weight)+25.7.times.V(% by weight)+7.5.times.W(% by
weight).ltoreq.36.2, and quenching is carried out from a
temperature of 950.degree. C. or lower and then tempering process
is carried out at 600.degree. C. so that an HRC hardness of 50 to
60 is provided.
[0076] In a second aspect of the present invention, the
high-hardness, high-toughness steel contains at least 0.1 to 1.20%
by weight of carbon and 0.05 to 1.8% by weight of Si and is
characterized in that Si is partially replaced by 0.15 to 1.6% by
weight of Al, Ni is added to the steel in an amount of 0.3 to 2.5%
by weight, the steel contains at least one alloying element of Mn,
Cr, Mo, V, W, Co, Cu, Ti, B, Nb, Zr, Ta, Hf, and Ca; and inevitable
impurities including P, S, N, and O, the balance consists
essentially of Fe, and the steel has a quenched and tempered
martensite structure.
[0077] In the present invention, the Cr content is preferably in
the range of 0.1 to 3.5% by weight.
[0078] Mo may be added in an amount of less than 1.7% by weight and
up to its upper limit determined by the relation formula: the upper
limit Mo(% by weight)=1.7-0.5.times.(Si(% by weight)+Al(% by
weight)) depending on the addition amount of Si and Al so that a
high-toughness, wear-resistance steel having improved toughness can
be provided.
[0079] One or both of 0.05 to 0.40% by weight of V and 0.1 to 1.0%
by weight of W are preferably added so that the resistance to
temper softening can further be enhanced.
[0080] Preferably, Al is in an amount of 0.15 to 0.75% by weight in
order to prevent excessive Al addition and give an adequate
increase in the A3 transformation temperature, and one or both of
limiting the Ni amount to between 0.3 and 2.0% by weight and adding
0.0005 to 0.005% by weight of B for enhancement of hardenability at
low cost are made.
[0081] In order to ensure that the high-toughness, wear-resistance
steel with Al and Ni coexisting in the above content range has an
HRC hardness of 45 to 65 after tempering at 600.degree. C. or
higher, the addition amount of each alloying element is preferably
controlled to satisfy the relation formula:
21.2.ltoreq.5.8.times.(Si(% by weight)+Al(% by
weight))+2.8.times.Cr(% by weight)+11.times.Mo(% by
weight)+25.7.times.V(% by weight)+7.5.times.W(% by
weight).ltoreq.41.2.
[0082] In a third aspect of the present invention, the
high-hardness, high-toughness steel contains at least C: 0.25 to
0.55% by weight, Si: less than 0.8% by weight, and Cr: 3.5 to 5.5%
by weight and is characterized in that Mo is added to the steel in
an amount of 0.3 to 1.0% by weight, one or both of V: 0.10 to 0.40%
by weight and W: 0.1 to 1.0% by weight are added to the steel, the
steel contains at least one alloying element of Mn, Ni, Co, Cu, Al,
B, Ti, Nb, Zr, Ta, Hf, and Ca; and inevitable impurities including
P, S, N, and O, the balance consists essentially of Fe, and the
steel is a tempered martensite steel.
[0083] In the present invention, to the steel containing 3.5 to
5.5% by weight of Cr, preferably, Al is added in an amount of 0.15
to 1.0% by weight, Ni is added in an amount of 0.3 to 2.5% by
weight, and Mo is added in an amount of 0.3 to 1.0% by weight.
[0084] Preferably, the addition amount of each alloying element is
controlled to satisfy the relation formula:
21.2.ltoreq.3.times.(Si(% by weight)+Al(% by
weight))+2.8.times.Cr(% by weight)+11.times.Mo(% by
weight)+25.7.times.V(% by weight)+7.5.times.W(% by
weight).ltoreq.41.2 in order to ensure an HRC hardness of 45 or
higher by tempering at 600.degree. C.
[0085] In each above aspect of the present invention, Co may be
added in an amount of 1 to 20% by weight so that the magnetic
transformation temperature of the tempered martensite can increase
by about 200.degree. C., the diffusion of alloys during the
tempering process can significantly be slowed down, and therefore
significant resistance to softening can be developed by tempering
even at 600.degree. C. or higher.
[0086] In each above aspect of the present invention, at least one
of Nb, Ti, Zr, Ta, and Hf is preferably added in a total amount of
0.005 to 0.2% by weight in order to produce fine crystal grains in
heating at high temperature for quenching.
[0087] In each above aspect of the present invention, the
high-hardness, high-toughness steel quenched and then tempered at a
high temperature of 600.degree. C. or higher can have an HRC
hardness of 50 or higher and a Charpy impact value of 5 kgf
m/cm.sup.2 or more.
[0088] In each above aspect of the present invention, the steel
quenched and then tempered at a temperature of 150.degree. C. or
higher can have an adjusted HRC hardness of 45 or higher and a
Charpy impact value that satisfies the relation formula: log(Charpy
impact value (kgf m/cm.sup.2)).gtoreq.-0.0263.times.HRC+2.225 where
its HRC hardness is in the range from 45 to 55, or have a HRC
hardness of 55 or higher and a Charpy impact value of 6 kgf
m/cm.sup.2 or more where its HRC hardness is 55 or higher.
[0089] The high-hardness, high-toughness steel according to each
above aspect of the present invention may be processed into a
crawler component such as a crawler bush, a crawler link, a top or
bottom tracker roller for a crawler, and a crawler shoe,
characterized by having a quenched and tempered HRC hardness of 52
or higher and a Charpy impact value of 6 kgf m/cm.sup.2 or more and
comprising a wear-resistant portion whose wear resistance is
increased to at least 1.2 times on average as much as that of a
conventional crawler component. Such a component can be obtained
through whole heating, quenching with a suitable cooling medium
such as water, an aqueous quenching solution, and a quenching oil,
and tempering at a temperature of 150.degree. C. to 400.degree. C.
However, it will be understood that the wear-resistant portion of
each component may be formed through high-frequency heating for
quenching and tempering.
[0090] The through-hardened crawler bush or link can maintain a
high HRC hardness of 55 or higher even at a tempering temperature
of 400.degree. C. or lower and show a very high Charpy impact
value, for example, even where it contains 0.6% by weight of
carbon, 1% by weight of Al and 1% by weight of Ni (see FIG. 11).
Therefore, it will be understood that the crawler bush and the
crawler link in the structure of the crawler bush press-fitted into
the crawler link can improve in the resistance to delayed fracture.
Therefore, the cost of heat-treating these components can
considerably be lower than that of heat-treating the crawler bush
produced by a conventional carburizing, quenching, and tempering
process or a conventional inside-outside diameter induction
hardening process. Such low-cost heat-treatment can be used in
place of deep induction hardening of the tread portion of the
crawler link to the bottom tracker roller after thermal refining of
the material. It will also be understood that chipping at both ends
of the crawler link which collide with the bottom tracker roller
can be prevented.
[0091] The high-hardness, high-toughness steel according to each
above aspect of the present invention may be processed into an
earth wear-resistant component such as a tunneling shank, a
tunneling disk cutter, a chisel tool, and a stirring blade for soil
improvement, characterized by having a quenched and tempered HRC
hardness of 50 or higher and a Charpy impact value of 8 kgf
m/cm.sup.2 or more, wherein the hardness and the Charpy impact
value are each increased to at least 1.2 times on average as much
as that of a conventional component. Such a component can be
obtained through whole heating, quenching with a suitable cooling
medium such as water, an aqueous quenching solution, and a
quenching oil, and tempering at a temperature of 150.degree. C. to
350.degree. C. However, it will be understood that the
wear-resistant portion of each component may be formed through
high-frequency heating for quenching and tempering. In terms of
cost, the addition amount of Ni is preferably 1% by weight.
[0092] The high-hardness, high-toughness steel according to each
above aspect of the present invention may be processed into a
fastening bolt for use in construction equipment such as a crawler
shoe bolt, a reduction gear, swing circle fixing bolts,
characterized by having a carbon content of 0.30% by weight or
more, a quenched and tempered HRC hardness of 40 or higher, and a
Charpy impact value that satisfies the relation formula: log(Charpy
impact value (kgf m/cm.sup.2)).gtoreq.-0.026- 3.times.HRC+2.225.
Such a component can be obtained through whole heating, quenching
with a suitable cooling medium such as water, an aqueous quenching
solution, and a quenching oil, and tempering at a temperature of
150.degree. C. to 500.degree. C. However, it will be understood
that the thread portion of the bolt may be induction-hardened and
tempered.
[0093] The high-hardness, high-toughness steel according to each
above aspect of the present invention may be formed into a gear
shape and then carburized, quenched, and tempered to result in a
gear or a high-toughness gear such as a crawler bush and a crawler
shoe bolt, characterized by having a surface carbon concentration
of 0.6 to 1.0% by weight, a surface carburizing depth of 0.4 mm or
more, and an adjusted HRC hardness of 55 to 64, and providing a
Charpy test piece with the equivalent depth and a Charpy impact
value of 5 kgf m/cm.sup.2 or more, preferably 8 kgf m/cm.sup.2 or
more.
[0094] The high-hardness, high-toughness steel according to each
above aspect of the present invention may be formed into a gear
shape, and then carburized to have a surface carbon content of 0.8
to 1.3% by weight, temporarily cooled down to the Al transformation
temperature or lower, and then heated again, quenched, and tempered
to result in a high-toughness, high contact pressure-resistance
gear, characterized by having a surface carburized case depth of
0.4 mm or more, containing cementite particles with an average
particle diameter of 1 .mu.m or less dispersed in its
quench-hardened case, having an adjusted HRC hardness of 59 to 65,
and providing a Charpy test piece with its equivalent depth and a
Charpy impact value of 4 kgf m/cm.sup.2 or more.
[0095] The high-hardness, high-toughness steel according to each
above aspect of the present invention may be formed into a gear
shape and then induction-hardened and tempered to result in a
high-toughness gear, characterized by having an adjusted surface
HRC hardness of 52 to 64 and providing a Charpy test piece with its
equivalent hardened depth and a Charpy impact value of 5 kgf
m/cm.sup.2 or more. The gear can be obtained through whole heating,
quenching with a suitable cooling medium such as water, an aqueous
quenching solution, and a quenching oil, and tempering at a
temperature of 150.degree. C. to 350.degree. C. However, it will be
understood that the gear tooth portion may be induction-hardened
and tempered to have a Charpy impact value of 5 kgf m/cm.sup.2 or
more.
[0096] The high-hardness, high-toughness steel according to each
above aspect of the present invention may quenched and tempered to
result in a wear-resistant steel plate, characterized by having a
high tension of 50 kgf/mm.sup.2 or higher and/or an adjusted HRC
hardness of 50 or higher and being weldable to a bucket, a
bulldozer blade, or the like for use. The wear-resistant steel
plate can improve in low-temperature cracking susceptibility in
welding to a bucket, a blade or the like of a construction or earth
work machine or improve in cracking susceptibility in
re-heating.
[0097] The steel according to each above aspect of the present
invention is applicable to excavating edge members that set
importance on the resistance to temper softening under frictional
heating. Therefore, the present invention is also directed to an
earth wear-resistant component for use in earth excavation such as
a ripper point, an end bit, bucket tooth, an edge, and a tunneling
disk cutter, characterized by comprising the high-hardness,
high-toughness steel which contains less than 3.5% by weight of Cr
and alloying elements each in a controlled addition amount that
satisfies the relation formula: 26.2.ltoreq.5.8.times.(Si(% by
weight)+Al(% by weight))+2.8.times.Cr(% by weight)+11.times.Mo(% by
weight)+25.7.times.V(% by weight)+7.5.times.W(% by
weight).ltoreq.41.2 so that the steel can have an HRC hardness of
50 or higher by tempering at 600.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0098] FIG. 1 is a graph showing the effect of various alloying
elements on the Ac3 temperature lines of Fe--Si based alloys;
[0099] FIG. 2 is a graph showing tempered hardness of various
wear-resistant steels;
[0100] FIG. 3 is a first graph showing the relationship between
hardness of various tempered steels and impact values thereof;
[0101] FIG. 4 is a second graph showing the relationship between
hardness of various tempered steels and impact values thereof;
[0102] FIG. 5 is a graph showing the results of preliminary
experiments on the relationship between the Charpy impact value and
the carbon content;
[0103] FIG. 6 is a graph showing found values (at a quenching
temperature of 870.degree. C.) and calculated values of tempered
hardness on products Nos. 1 to 9 for comparison;
[0104] FIG. 7 is a graph showing found values (at a quenching
temperature of 870.degree. C.) and calculated values of tempered
hardness on products Nos. 10 to 22 for comparison;
[0105] FIG. 8 is a graph showing found values (at a quenching
temperature of 900.degree. C.) and calculated values of tempered
hardness on products Nos. 23 to 29 for comparison;
[0106] FIG. 9 is a graph showing found values (at a quenching
temperature of 950.degree. C.) and calculated values of tempered
hardness on products Nos. 30 to 33 for comparison;
[0107] FIG. 10 is a graph showing found values (at a quenching
temperature of 900.degree. C.) and calculated values of tempered
hardness on products Nos. 34 to 38 for comparison;
[0108] FIG. 11 is a graph showing the relationship between the
tempered hardness and the Charpy impact value on products Nos. 47
to 49;
[0109] FIG. 12 is a graph showing the relationship between the
tempering temperature and the Charpy impact value on products Nos.
10, 12, and 47;
[0110] FIG. 13 is a graph showing conditions of carburizing and
quenching;
[0111] FIG. 14 is a diagram showing hardness distribution in
carburized, quenched, and tempered Charpy test pieces;
[0112] FIGS. 15(a) and 15(b) are micrographs each showing a surface
quench-hardened layer structure formed through the steps of
carburizing at 1000.degree. C. for 2 hours to provide a surface
carbon concentration of 1.1% by weight or 1.3% by weight, cooling
to room temperature, and re-heating at 850.degree. C. for 1 hour
for quenching and tempering; and
[0113] FIG. 16 is a graph showing the relationship between hardness
and wear ratio on various steels where the gouging wear amount of
the quenched and tempered S45C steel (Hv 500) is normalized as
1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0114] Referring to the drawings, examples of the high-hardness,
high-toughness steel according to the present invention are
described in the following.
EXAMPLE 1
[0115] Preliminary Research and Experiment
[0116] In Example 1, the handbook "Hagane no Netsu-Shori" (in
Japanese) (Heat-Treatment of Steels), revised 5th edition, edited
by The Iron and Steel Institute of Japan, published by MARUZEN CO.,
LTD, 1985 was referred to, and the relations between tempering
temperature and Rockwell hardness on various tough steels
(martensite steels) described therein were organized so that target
values for improvement of wear-resistant steels were investigated
in making the present invention.
[0117] As a result, as shown in FIG. 2, it has been found that SCr,
SCM, SNCM, and high-Si tough steels each containing 0.6% by weight
or less of carbon cannot have more than HRC45 by tempering at
600.degree. C. and that target values can be achieved by using SKD6
(0.4C-5Cr-1.3Mo-0.3V).
[0118] FIGS. 3 and 4 shows the relationship between the 150 to
700.degree. C. tempered hardness and the Charpy impact value on
steels including SUJ2 and SKH9. These graphs show that the Charpy
impact value can be 5 kgf m/cm.sup.2 or more where the HRC hardness
has an upper limit of about 56. FIG. 5 shows the result of
preliminary experiments in which steels each having the composition
as shown in Table 1 were quenched and then tempered at 200.degree.
C. for 2 hours, and the relationship between the Charpy impact
value and the carbon content was examined. The result shows that
there is almost no possibility that a carbon content of 0.55% or
more provides a Charpy impact value of 5 kgf m/cm.sup.2 or
more.
1TABLE 1 TPNo. C Si Mn P S Ni Cr Mo V W Ti B 1 1.06 0.3 0.39 0.014
0.011 0.06 1.36 0.05 2 0.94 0.28 1.01 0.01 0.014 0.05 0.59 0.04
0.56 3 0.88 0.22 1.72 0.011 0.007 0.06 0.1 0.22 4 0.76 0.21 0.3
0.007 0.005 1.59 0.07 0.02 5 0.59 1.56 0.78 0.009 0.012 0.05 0.29
0.4 0.15 6 0.58 1.2 0.73 0.014 0.014 0.07 0.66 0.45 Nb:0.08 7 0.55
0.29 0.35 0.015 0.016 3.02 1.36 0.33 8 0.49 0.3 0.33 0.016 0.008
3.04 1.23 0.34 9 0.36 0.25 1.43 0.021 0.019 0.05 0.12 0.05 10 0.32
0.35 0.77 0.022 0.027 0.09 0.99 0.18 11 0.25 0.31 0.56 0.015 0.005
1.83 0.62 0.23 Nb:0.05 12 0.22 0.26 0.8 0.012 0.005 0.06 0.93 0.41
13 0.18 0.24 0.71 0.018 0.018 0.06 0.94 0.18 14 0.4 2.36 0.38 0.009
0.018 0.05 0.9 0.01 15 0.28 1.84 0.72 0.028 0.031 0.13 2.11 0.4
0.03 0.05 0.0016 16 0.26 0.29 0.76 0.011 0.014 0.47 0.44 0.16
EXAMPLE 2
[0119] Preparation of Wear-Resistant Test Steels
[0120] Table 2 shows the compositions of the steels used. The
addition amount of each element was in the following range: C: 0.14
to 0.73% by weight, Si: at most 2.5% by weight, Mn: at most 1.3% by
weight, Cr: 0.3 to 8% weight, Mo: at most 4% by weight, V: at most
1% by weight, W: at 2% by weight, Al: at most 2% by weight, and Ni:
at most 2% by weight. Other elements such as Nb, B, and Ti were
added in a very small amount, and each level was selected
concerning the control of P and S and the like. The steels were
used in investigating the effect of each alloying element on the
resistance to temper softening (and the Charpy impact value). Each
ingot steel about 25 kg in weight was prepared using a
high-frequency smelter, formed into a round bar shape 32 mm in
diameter by hot forging, machined into a round bar 25 mm in
diameter, cut to have a suitable length, heat-treated in a certain
manner, and subjected to the experiments as shown below.
2TABLE 2 Quenching Hardness Hardness Temperature HRC Charpy Impact
Value HRC Charpy Impact Value TPNo. C Si Mn P S Ni Cr Mo V W B Nb
Al .degree. C. 200.degree. C. kg-m/cm.sup.2 600.degree. C.
kg-m/cm.sup.2 1 0.36 1.77 0.6 0.021 0.013 0.06 0.62 0.11 0.0026
0.015 870 51.20 3.10 36.40 8.20 2 0.35 1.68 1.21 -- -- -- 0.53 1.04
0.0026 0.031 .Arrow-up bold. 51.90 5.28 45.90 6.90 3 0.36 1.68 1.2
-- -- -- 0.51 1.05 0.11 0.0018 0.027 .Arrow-up bold. 51.50 6.10
48.60 7.12 4 0.31 0.24 1.23 -- -- -- 0.31 1.62 0.41 0.0021 0.033
.Arrow-up bold. 50.20 6.80 51.00 7.10 5 0.42 0.23 0.72 -- -- 1.51
0.71 1.32 0.31 0.041 .Arrow-up bold. 54.00 4.70 48.70 8.05 6 0.41
0.22 0.73 0.72 1.25 0.29 0.81 0.035 .Arrow-up bold. 55.00 7.45
53.70 8.39 7 0.41 0.21 0.58 1.59 0.36 1.28 .Arrow-up bold. 55.70
3.88 40.00 6.70 8 0.61 0.15 0.93 0.98 1.05 0.31 0.15 0.97 .Arrow-up
bold. 60.00 7.11 45.00 15.1 9 0.58 0.16 1.11 1.51 0.98 0.51 0.21
1.25 .Arrow-up bold. 59.30 6.89 50.20 13.6 10 0.45 1.45 0.46 0.01
0.006 1.49 0.52 0.14 0.0018 0.049 0.029 .Arrow-up bold. 56.50 4.23
46.70 6.12 11 0.49 1.45 0.46 0.009 0.008 1.01 1.03 0.15 0.0019
0.053 0.031 .Arrow-up bold. 56.20 4.44 49.50 5.98 12 0.47 0.31 0.46
0.009 0.008 2.01 1.03 0.15 0.0019 0.051 0.027 .Arrow-up bold. 57.00
3.93 47.20 5.40 13 0.48 0.31 0.46 0.009 0.008 1.5 1.58 0.15 0.002
0.051 0.029 .Arrow-up bold. 57.00 3.56 49.20 4.80 14 0.49 0.29 0.45
0.008 0.007 1.5 1.49 0.23 0.0019 0.049 0.031 .Arrow-up bold. 56.40
4.56 49.30 4.50 15 0.46 1.54 0.4 0.007 0.007 0.01 1 0.51 0.002
0.034 0.05 .Arrow-up bold. 57.00 3.93 40.40 5.70 16 0.39 0.93 1.02
0.015 0.008 0.08 0.97 0.95 0.5 0.022 980 52.60 4.00 48.00 5.22 17
0.43 0.26 0.44 0.014 0.011 0.08 1.01 0.48 0.001 0.034 0.037 870
55.20 35.60 18 0.47 0.25 0.4 0.006 0.009 0.01 1.01 1.05 0.0018
0.033 0.056 .Arrow-up bold. 56.30 3.93 41.30 7.13 19 0.44 1.44 0.44
0.012 0.006 0.05 0.98 0.49 0.0013 0.047 0.044 .Arrow-up bold. 57.30
4.27 40.40 6.88 20 0.45 0.24 0.4 0.012 0.007 0.03 1.02 0.48 0.31
0.0011 0.038 0.036 .Arrow-up bold. 55.30 3.83 40.40 6.89 21 0.45
1.46 0.39 0.011 0.009 0.06 0.96 0.98 0.001 0.044 0.036 .Arrow-up
bold. 56.80 4.10 41.80 5.90 22 0.41 0.25 0.35 0.01 0.006 0.03 1
0.49 0.0017 0.025 0.035 .Arrow-up bold. 54.10 34.90 23 0.37 1.01
1.02 0.008 0.009 -- 1.01 1.05 0.52 -- 980 53.40 50.40 24 0.47 0.63
0.52 -- -- -- 1.44 1.01 0.5 -- 900 56.80 50.40 25 0.49 1.57 0.55 --
-- -- 2.11 0.5 0.32 -- .Arrow-up bold. 57.30 50.00 26 0.47 0.61
0.51 -- -- -- 2.33 0.4 0.5 -- .Arrow-up bold. 56.10 4.80 49.00 5.57
27 0.58 0.62 0.51 -- -- -- 2.86 0.48 1.01 -- .Arrow-up bold. 59.80
50.40 28 0.57 0.64 0.52 -- -- -- 2.93 1.95 0.3 -- .Arrow-up bold.
59.20 3.50 55.00 4.86 29 0.63 0.99 0.51 -- -- -- 2.83 0.47 0.5 --
.Arrow-up bold. 60.80 3.81 50.30 5.23 30 0.51 1.74 0.42 -- -- --
2.6 1.04 0.62 -- 950 57.50 4.11 51.50 4.88 31 0.44 1.69 0.39 -- --
-- 3.3 2.52 0.52 -- .Arrow-up bold. 55.90 56.50 32 0.43 2.21 0.32
-- -- -- 4.05 0.92 -- .Arrow-up bold. 56.80 46.50 33 0.45 2.21 0.42
-- -- -- 4.35 2.44 0.51 -- .Arrow-up bold. 57.20 53.00 34 0.49 0.28
0.31 -- -- -- 4.9 2.86 1.1 1.95 -- 900 56.80 57.60 35 0.49 0.92
0.33 -- -- -- 4.82 2.82 1.05 2.02 -- .Arrow-up bold. 55.90 59.80 36
0.51 2.19 0.37 -- -- -- 4.69 2.88 1.05 2 -- .Arrow-up bold. 56.30
58.10 37 0.5 0.23 0.39 -- -- -- 4.91 3.88 1 0.05 -- .Arrow-up bold.
56.20 56.30 38 0.73 0.92 0.49 -- -- 1.06 8.15 1.09 0.8 -- .Arrow-up
bold. 57.50 45.00
EXAMPLE 3
[0121] Check Test for Resistance to Temper Softening (Effect of
Alloying Elements on Tempered Hardness)
[0122] In this example, each test piece 25 mm in diameter with the
composition as shown in Table 2 was heated at a temperature of
870.degree. C. to 980.degree. C. for 1 hour in N.sub.2 gas
atmosphere, then water-quenched, tempered at a temperature of 200
to 700.degree. C. for 2 hours, rapidly cooled in water, and
measured for hardness. The purpose of the test was to investigate
and analyze the effect of each alloying element on the resistance
to temper softening and to establish the way to design alloys that
can have a tempered HRC hardness of 45 or higher by tempering at
600.degree. C.
[0123] As a result of the check test, it has been found that the
effect of alloying elements on the hardness of the martensite steel
tempered at 600.degree. C. after quenched from 900.degree. C., for
example, can be calculated according to the following formula:
.DELTA.HRC=5.7.times.(Si(wt %)+Al(wt %))+2.8.times.Cr(wt
%)+11.times.Mo(wt %)+25.7.times.V(wt %)+7.5.times.W(wt %)
[0124] As described above, Mo, V, and W are accompanied by the
maximum addition amounts YMo, YV, and YW, respectively. If any of
these elements is added in an amount of more than the maximum
addition amount, the hardness of the alloyed steel is calculated by
using the maximum addition amount in place of the actual amount in
the formula.
[0125] FIGS. 6 to 10 each show the result of the measurement of
tempered hardness of different steel products (indicated by
"Found") shown in FIG. 2 and the result of the calculation of
tempered hardness based on the process in which the effect of each
alloying element on the tempered hardness is analyzed and
quantified (indicated by "Calculated"). These graphs show that
there is very good agreement between the calculated and found
values of the tempered hardness of each alloyed steel and that the
effect of each alloying element can almost reasonably be
quantified.
[0126] FIG. 6 shows the found values (at a quenching temperature of
870.degree. C.) and the calculated values of the tempered hardness
of products Nos. 1 to 9 for comparison. In the drawing, Nos. 1 to 3
show the effect of Mo or V addition on low Cr-high Si steels, Nos.
4 to 6 show the effect of V, Ni, and W, and Nos. 7 to 9 show the
effect of the combined addition of Al and Ni. The drawing shows
that Si, Mo, V, W, and Al each develop significant resistance to
temper softening even at low Cr content and that particularly, the
Al-induced resistance to temper softening is very well calculated
at the same degree of influence as Si, and Al develops the
resistance to temper softening by substantially the same mechanism
as Si.
[0127] From the comparison between the calculations based on the
analysis and the actual measurements, hardening caused by the aging
effect of NiAl based intermetallic compounds has been demonstrated
in the case that Al coexists with Ni (.DELTA.HRC=+4 by 1Al+1Ni).
However, at a quenching temperature of 870.degree. C., Mo in No. 2,
Mo and V in No. 4, V in No. 5, and V and W in No. 6 are each
slightly in excess of the above-mentioned effective addition
amount. In such cases, for example, the resistance to temper
softening is found to be more effectively developed where the steel
is tempered after quenched from 950.degree. C. (for example, in the
effective Mo amount: YMo=1.3-0.5.times.(Si+Al)).
[0128] FIG. 7 shows the found values (at a quenching temperature of
870.degree. C.) and the calculated values of the tempered hardness
of products Nos. 10 to 22 for comparison. In the drawing, product
No. 16 undergoes the process in which quenching is performed at a
higher temperature of 980.degree. C. and then tempering is
performed, and it shows that the process of dissolving 0.5% by
weight of V into the alloy and then tempering effectively
contributes to the resistance to temper softening.
[0129] FIG. 8 shows the found values (at a quenching temperature of
900.degree. C.) and the calculated values of the tempered hardness
of products Nos. 23 to 29 for comparison. This shows the results of
the investigation of the relationship between high content of Cr
and Mo and V addition and demonstrates that Mo and V forms the
above-described relationship even in the case that Cr coexists in
an amount of about 3% by weight. In No. 28, Mo is added in large
amount, but the effective Mo amount should be about 1.0% by weight
in consideration of the amount of coexisting Si and the high carbon
content (and the precipitation of Mo carbide into the austenite)
(.DELTA.YMo=0.15). Therefore, it has been found that the product is
highly hardened to have HRC55, which is equivalent to or higher
than the 600.degree. C. tempered hardness of hot work tool steel
SKD6, for example. In contrast to SKD6 which is practically used
after the process of quenching at 1000 to 1050.degree. C. and
tempering at 550 to 600.degree. C. for adjusting HRC to 53 or
lower, products Nos. 23 to 29 according to the present invention is
apparently useful as a hot work tool steel. These products have Cr
and Mo contents reduced to less than 3% and less than 1% by weight,
respectively, and are quenched at a reduced temperature of
900.degree. C. and therefore more economical. The carbon content of
these steels is limited to at most 0.55% by weight. However, it
will be understood that the carbon content is more preferably 0.45%
by weight or less in terms of the content range of SKD6.
[0130] FIG. 9 shows the found values (at a quenching temperature of
950.degree. C.) and the calculated values of the tempered hardness
of products Nos. 30 to 33 for comparison. The drawing shows the
effect of higher Cr content than those in FIG. 8. For example, as
shown by the comparison between Nos. 31 and 33, the result of
analysis shows that the effect of Cr on the resistance to temper
softening drastically decreases as the Cr content becomes about
3.5% by weight or more and that the steel with a Cr content of more
than 3.5% by weight is drastically reduced in the Si-induced
resistance to temper softening. Therefore, it is apparent that the
Cr usage is preferably limited to 3.5% by weight or less so that Cr
can be effective at developing the resistance to temper
softening.
[0131] If Cr is added in large amount as in Nos. 29 to 33, the
mechanism of the Cr-induced resistance to temper softening becomes
more significant as the carbon content becomes smaller compared
with the Cr addition amount. From the result of the analysis, it
has been found that as describe above, the addition of Cr in an
amount of more than about 7.5 times as much as the carbon content
reduces the resistance per Cr addition amount.
[0132] FIG. 10 shows the found values (at a quenching temperature
of 900.degree. C.) and the calculated values of the tempered
hardness of products Nos. 34 to 38 for comparison. The drawing
shows the effect of Mo, W, and Si in a high Cr content range. It
has been found, from the result, that the maximum effective
addition amount of W is about 1.0% by weight at 900.degree. C. and
that the addition of W in an amount of more than 1% by weight
drastically reduces the maximum effective addition amount of Mo,
and therefore the addition amount of W is desirably not more than
1% by weight. It has also been found that 6% by weight or more of
Cr further drastically reduces the Cr-induced resistance to temper
softening (see No. 38).
[0133] It is apparent, from the above, that in developing the
high-toughness, wear-resistance steels, the addition of the
alloying element in an amount of more than the maximum addition
amount as described above is not only uneconomical but also makes
almost no contribution to wear resistance and can reduce the
toughness. It is also apparent that the contribution of the
alloying element such as Si, Al, and Cr to the resistance to temper
softening depends on the addition amount range, and therefore such
an element is preferably added so as to be more effective in
consideration of economy.
[0134] In order to provide the martensite steel with an HRC
hardness of 45 or higher by tempering at 600.degree. C., the
alloying elements should be used in such a combination that the
following formula is satisfied.
21.1.ltoreq.5.7.times.(Si(wt %)+Al(wt %))+2.8.times.Cr(wt
%)+11.times.Mo(wt %)+25.7.times.V(wt %)+7.5.times.W(wt %)
[0135] In order to produce high-toughness, wear-resistance steels,
limitations are preferably placed on the upper limit of the
hardness obtained by tempering at 600.degree. C., and, for example,
such hardness is preferably set at HRC55 or lower in the case that
SKD6 or the like is referred to as a conventional standard.
However, the combined addition of Al and Ni can increase such an
upper limit of the hardness to HRC65, and therefore the addition
amount range of the alloying element is preferably in accordance
with the following formula:
21.2.ltoreq.5.8.times.(Si(wt %)+Al(wt %))+2.8.times.Cr(wt
%)+11.times.Mo(wt %)+25.7.times.V(wt %)+7.5.times.W(wt
%).ltoreq.41.2
EXAMPLE 4
[0136] Results of Impact Test
[0137] Table 2 also shows the results of the 2U Charpy impact test
(using JIS (Japanese Industrial Standards) No. 3 test piece) in
which the steels tempered at 200.degree. C. or 600.degree. C. for 2
hours were examined. Table 3 also shows the results on additional
materials according to the present invention and comparative
materials. It has been found, from the results, that among the
materials tempered at 200.degree. C. or 600.degree. C. for 2
hours,
[0138] 1) the materials each with about 1% by weight or less of Cr
and with Mo positively added;
[0139] 2) the materials with 0.81% by weight or less of W added;
and
[0140] 3) the materials with Al and Ni added in combination have
high-toughness. It has also been found that thanks to the combined
addition of Al and Ni, the upper limit of the carbon content can be
about 1.2% by weight in the tempered martensite structure steel
showing such toughness as a Charpy impact value of 5 kgf m/cm.sup.2
or more.
3TABLE 3 Quenching TPNo. C Si Mn P S Ni Cr Mo V W B Nb Al .degree.
C. 39 0.15 0.07 0.5 0.012 0.009 0.09 1.01 0.26 1.07 870 40 0.14
0.07 1.19 0.014 0.008 1.00 1.01 0.25 1.06 870 41 0.22 0.22 0.82
0.014 0.012 0.13 1.15 0.15 0.031 870 42 0.46 0.82 1.21 0.021 0.011
-- 0.59 0.52 0.16 0.82 0.029 900 43 0.46 0.65 0.51 0.013 -- -- 2.52
0.42 0.31 0.51 0.034 900 44 0.43 0.63 0.78 0.015 -- 1.01 2.41 0.48
0.25 0.31 0.81 900 45 1.03 -- -- -- -- -- 1.56 -- 850 46 0.35 0.21
1.2 0.011 0.012 -- 0.11 0.0012 0.029 850 47 0.6 0.25 0.93 0.016
0.008 0.98 1.04 0.35 0.97 850 48 0.63 0.24 0.97 0.013 0.008 1.02
1.03 0.34 0.48 850 49 1.09 0.15 0.76 0.018 0.013 0.97 1.31 0.15
0.52 870 Hardness Charpy Impact Value Hardness Charpy Impact Value
TPNo. 200.degree. C. kg-m/cm.sup.2 600.degree. C. kg-m/cm.sup.2 39
42.80 6.70 Al Steel 40 43.10 28.00 AlNi Steel 41 47.20 8.20 SCM420H
42 56.40 6.82 46.40 8.51 43 56.10 6.91 50.80 9.81 44 55.20 7.83
47.30 14.7 45 61.30 0.79 36.70 3.79 SUJ2 46 50.50 7.70 SMnB435H 47
60.1 12.1 47.5 15.2 (500.degree. C.) 48 60.7 11.8 49 63.1 6.21
[0141] FIG. 11 shows the relationship between the hardness and the
Charpy impact value on products Nos. 47, 48, and 49 each quenched
from the temperature as shown in the table and tempered at each
temperature of 200 to 500.degree. C. for 3 hours. No temper
brittleness-induced decrease in the impact value was observed
between low-temperature tempered HRC60 and 500.degree. C. tempered
HRC47, and particularly, the toughness was rapidly recovered by
tempering at 200.degree. C.
[0142] FIG. 12 shows the relationship between each tempering
temperature and the Charpy impact value on product No. 47 in Table
3 and products Nos. 10 and 12 in Table 2. It is apparent, from the
drawing, that product No. 12 is rapidly embrittled by tempering at
200.degree. C. or higher, but product No. 10 with a high Si content
retains the toughness at 350.degree. C. or lower and is
significantly embrittled at 500.degree. C. and recovers the
toughness at 600.degree. C. It is also apparent that product No. 47
with Al and Ni added in combination shows no temper embrittlement
but very high toughness.
[0143] The example of the low-carbon Al--Ni steel (No. 40) is found
to show excellent toughness even in the tempering process at a low
temperature of 200.degree. C. It is also apparent, from the
comparison with the results on the steels in Table 3 (Nos. 39, 41,
45, and 46), that the Al--Ni steel has a very high toughness in a
wide range of carbon content and a wide range of hardness and that
a suitable carbon content is preferably from 0.15 to 1.20% by
weight. Therefore, it has been found that the alloy is preferably
designed in such a manner that HRC45 or higher is established by
tempering at 600.degree. C. while wear resistance is retained at
HRC45 or higher.
EXAMPLE 5
[0144] Results of Impact Test on Carburized, Quenched and Tempered
Steels
[0145] Steels Nos. 39, 40, and 41 in Table 3 were each normalized
at 980.degree. C. and then formed into a test piece shape for the
Charpy impact test, giving test pieces for use in this example. The
carburizing, quenching and tempering process was carried out as
shown in FIG. 13. The carbon potential was set so as to provide
0.85% by weight of carbon at 930.degree. C. The time period for
carburization diffusion was set at 5 hours so as to provide a
carburized case depth of 0.8 to 1.2 mm. The tempering was carried
out at 180.degree. C. for 3 hours.
[0146] In FIG. 14 showing the distribution of the hardness in the
test pieces carburized, quenched, and tempered, the hardness of the
surface carburized case is presented as a Vickers hardness from
Hv750 to Hv800 (corresponding to HRC62 to HRC64), and the Charpy
impact values of the respective test pieces are as follows: No. 39:
1.74 kgf m/cm.sup.2; No. 40: 11.9 kgf m/cm.sup.2; and No. 41: 1.24
kgf m/cm.sup.2. As a result of the observation of the structure,
products Nos. 39 and 40 was found to keep the .alpha.Fe phase
remaining. Therefore, a high temperature of 910.degree. C. was
substituted for 850.degree. C. in the tempering process after
carburizing, and the test pieces were subjected to the Charpy
impact test. As a result, drastic improvement was achieved as
follows: No. 39: 2.52 kgf m/cm.sup.2 and No. 40: 22.6 kgf
m/cm.sup.2. In particular, the impact value of product No. 40 is
close to that of the low-carbon base material as shown in Table
3.
[0147] It is apparent, from the results, that they are highly
preferable steels to form the gear of the reduction gear or the
slewing gear in the construction or earth work machine, which tends
to receive impact load. It is also apparent that they are
preferable steels to form the crawler bush and the like, which is
used after the process of carburizing, quenching, and
tempering.
[0148] FIGS. 15(a) and 15(b) are photographs each showing the
structure with a depth of 0.2 mm from the surface of the Charpy
test piece, which was prepared by carburizing product No. 40 at
1000.degree. C. so as to provide a surface carbon content of 1.1%
by weight (a) or 1.3% by weight (b), temporarily cooling it to the
A1 temperature or less, re-heating it at 870.degree. C. for
quenching, and tempering it at 200.degree. C. for 3 hours. In each
structure, cementite particles with an average particle diameter of
1 .mu.m or less are almost homogeneously dispersed, and the surface
carburized case has an HRC hardness of 62. Conventionally, gear
members having a surface carburized case structure in which lots of
fine cementite particles are dispersed have been excellent in
contact pressure resistance and expected to form a more compact
gear for reduction gears, but have been very poor in toughness.
From the results of the Charpy impact test (No. 40: 4 to 6 kgf
m/cm.sup.2 and No. 41: 0.7 to 1.0 kgf m/cm.sup.2), however, it has
been found that the steel can improve in toughness by adding Al and
Ni in combination according to the present invention and that such
a steel with cementite particles dispersed can be used to form a
high contact pressure-resistant gear.
EXAMPLE 6
[0149] First Example of Application of High-Hardness,
High-Toughness Steels
[0150] In this example, wear resistance data of some conventional
wear resistant components for construction or earth work machinery,
which are potential use of the above high-hardness, high-toughness
steels, are organized to show the effect and the advantage of the
present invention. Table 4 shows typical components and their
carbon content and their quenched and tempered hardness, and
tempering parameters calculated from typical alloying constituents.
Many of these components are designed to satisfy both high
toughness and high hardness and therefore commonly contain 0.25 to
0.40% by weight of carbon. Such components are rarely used at a
hardness of HRC52 or higher and therefore apparently insufficient
in wear resistance. It is apparent that the bucket tooth, the
ripper point, the end bit, and the cutting edges, which are
frequently used in excavating rock and therefore need resistance to
temper softening, can be insufficient in wear resistance, because
the constituents are controlled to provide the tempering parameter
between 10 and 22, and the hardness after tempering at 600.degree.
C. is as low as between HRC33 and HRC46.
4TABLE 4 Carbon 600.degree. C. Content Tempering % by Hardness Heat
Components Manufacturer Parameter* Weight HRC Treatment Ingredients
Bucket Tooth A 13.8 0.33C 48.about.52 Through-hard
1.75Si0.6Cr0.1MoB A 4.0 0.33C 48.about.52 .Arrow-up bold. 1Mn1CrB B
12.0 0.35C 48.about.52 .Arrow-up bold. 1Si1.3Mn0.8Cr0.04Mo0.15V C
15.7 0.3C 48.about.52 .Arrow-up bold. 1.7Si1.2Mn1.35Cr0.2Mo D 12.5
0.26C 48.about.52 .Arrow-up bold. 1.2Si1.2Cr0.25Mo E 7.7 0.35C
48.about.52 .Arrow-up bold. 1.2Mn0.5Cr E 10.1 0.26C 45.about.49
.Arrow-up bold. 0.7Si1.2Ni0.7Cr0.35Mo Ripper Point F 21.6 0.30
48.about.52 Trough-hard 1.6Si2Cr0.35Mo0.1V A 22.5 0.44C 48.about.52
.Arrow-up bold. 0.4Mn2Cr1Mo0.15V A 13.8 0.33C 48.about.52 .Arrow-up
bold. 1.75Si0.6Cr0.1MoB Cutting Edge F 4.5 0.3C 45.about.49
Through-hard Edge A 13.8 0.33C 45.about.49 .Arrow-up bold.
1.75Si0.6Cr0.1MoB A 15.6 0.40C 45.about.49 .Arrow-up bold.
2.2Si0.4Mn1Cr End Bit F 22.9 0.3C 45.about.49 Through-hard A 6.3
0.3C 45.about.49 .Arrow-up bold. 0.6Si1.2Mn0.4Cr0.15Mo Segment F
2.7 0.33C 45.about.49 Through-hard 1.3Mn0.2Cr Teeth A 2.5 0.35C
48.about.52 .Arrow-up bold. Bottom F 3.7 0.35C 48.about.52
Through-hard 1.2Mn0.5Cr Tracker Roller A 4.2 0.35C 48.about.52
.Arrow-up bold. 1.2Mn0.5Cr0.1MoB Crawler Link F 3.5 0.35C
50.about.52 Induction 1.2Mn0.5Cr A 9.8 0.4C 50.about.54 .Arrow-up
bold. 0.4Mn1Cr0.5MoB Crawler Shoe F 3.9 0.25C 47.about.49
Through-hard 1Mn0.5Cr A 7.7 0.3C 48.about.52 .Arrow-up bold.
0.7Si0.6Mn1.2CrB Crawler Shoe Bolt A 1.9 0.35C 38.about.42
Through-hard S35BC Disc Cutter A 5.8 0.4C 50.about.54 Through-hard
1.8Ni0.8Cr0.2Mo Tool Bit A 5.0 0.4C 45.about.50 Through-hard
3Ni0.5Cr0.25Mo Shank G 14.0 0.3C 45 .Arrow-up bold. 3Ni3Cr0.5Mo
Soil Cutter A 3.9 0.27C 48.about.50 Through-hard 1.4Mn0.5Cr0.05MoB
Bucket A 3.9 0.27C 48.about.50 Through-hard 1.4Mn0.5Cr0.05MoB
Wear-resistible Steel Plate *Tempering Parameter = 5.8 .times.
(Si(% by weight) + Al(% by weight)) + 2.8 .times. Cr(% by weight) +
11 .times. Mo(% by weight) + 25.7 .times. V(% by weight) + 7.5
.times. W(% by weight) **600.degree. C. Tempered Hardness (HRC) =
23.6 + Tempering Parameter
[0151] Such steels include no case where Al and Ni are added in
combination in order to provide high hardness and high toughness.
Considering the results of the Charpy impact values in the above
examples, such steels still have problems of cracking, chipping,
and fracturing due to insufficient toughness.
[0152] The applicant has data concerning the relationship between
the hardness of various steels and the gouging wear resistance,
wherein the wear resistance (W: wear amount) of the quenched and
tempered steel with a Vickers hardness of Hv500 is normalized as 1,
and reduction in hardness by friction heating is not significant.
Such wear resistance is approximately calculated according to the
formula: W.times.(Hv).sup.2=250000 (see FIG. 16). It is apparent,
from this result, that if the components with the toughness
unchanged can have the average hardness increased from HRC50
(Hv513) to HRC55 (Hv600), their wear resistance would significantly
increase by about 20% or more. Therefore, it is apparent that for
example, the high-hardness, high-toughness steel with Al and Ni
added in combination can be used and appropriately heat-treated to
form a significantly improved-wear resistance crawler link, crawler
shoe, crawler bush, bucket tooth, cutting edge, end bit, segment
teeth, bottom tracker roller, tunneling tool bit, shank, disk
cutter, chisel tool, or soil cutter for earth stirring in a
soil-improvement machine, each having a hardness of HRC55 or higher
and a Charpy impact value of 5 kgf m/cm.sup.2 or more.
[0153] It is also apparent that the bucket tooth, ripper point, end
bit, and cutting edges, which are frequently used in excavating
rock and need resistance to temper softening, can be prevented from
cracking, chipping, or fracturing by using the above tempering
parameters, appropriately adding the alloying elements so as to
provide a hardness of HRC45 or higher, preferably HRC50 or higher
by tempering at 600.degree. C., and enhancing the toughness by the
combined addition of Al and Ni.
[0154] The results of the impact test in Examples 4 and 5 suggest
that the improvement in toughness by the combined addition of Al
and Ni should lead to reinforcement of the grain boundary (old
austenite grain boundary) and be very effective at improving the
crawler shoe bolt, which would otherwise have a problem with
resistance to delayed fracture. It is known that the delayed
fracture frequently occurs in the bolts using quenched and tempered
steels with a hardness of HRC40 or higher. The delayed fracture
also tends to occur in steels that are significant in temper
brittleness. Therefore, conventional bolts are often made of boron
steels (corresponding to S35C in Table 4) which contain alloying
elements in a small amount and B so as to have enhanced
hardenability. However, such steels have a Charpy impact value of
about 7 to 11 kgf m/cm.sup.2 at HRC40. Such a value is not
satisfying in comparison with the improved Charpy impact value of
the steel with Al and Ni added in combination according to the
present invention. Therefore, it is apparent that a higher tension
bolt can be produced by using the steel that is reduced in temper
brittleness and significantly improves in the grain boundary
strength to satisfy HRC 41 or more and the relation formula:
log(Charpy impact value (kgf
m/cm.sup.2)).gtoreq.-0.0263.times.HRC+2.225 according to the
present invention.
[0155] Example 4 and FIG. 11 show that product No. 40 in Table 3
ensures a high Charpy impact value at a hardness of HRC60. Product
No. 40 has a very high toughness (see Example 5) in contrast to the
carburized, quenched and tempered SCM420H product, which is
supposed to form gears. Therefore, it is apparent that the
inventive steel with Al and Ni added in combination and 0.45 to
1.2% by weight of carbon can be used and worked into a gear shape
and then induction-quenched and tempered or subjected to known
appropriate quenching and tempering to form a gear with a surface
hardness of HRC55 or higher at lower cost than that for
conventional carburized, quenched and tempered gears. It will be
understood that the surface hardness is preferably HRC58 or higher
in terms of improvement in contact pressure resistance strength of
the gear and that the cementite particles with an average particle
diameter of 1 .mu.m or less are preferably dispersed in the surface
layer.
[0156] The results of the impact test in Examples 4 and 5 show that
the quenched and tempered steel and the carburized, quenched and
tempered steel each with a high carbon content of up to 1.2% by
weight have a Charpy impact value of 5 kgf m/cm.sup.2 or more. It
is therefore apparent that such steels are also applicable to a
tap, a press die, a chisel, a shirring shear blade, a saw blade, a
cutter, and the like.
EXAMPLE 7
[0157] Second Example of Application of High-Hardness,
High-Toughness Steels
[0158] This example focuses on the fact that low temperature
cracking or re-heating cracking of high tension steel plates or
wear resistant steel plates in the construction or earth work
machine, which are potential use of the above high-hardness,
high-toughness steels, is attributed to the welding heat-induced
embrittlement of old austenite grain boundary in the base metal.
This example shows that the inventive high-hardness, high-toughness
steels are effective at preventing such weld cracking.
[0159] When higher tension welding steel plates or wear resistant
welding plates are developed, it is important to show the way to
prevent the low temperature cracking in the base metal after
welding, and therefore specifications of such steel plates are
provided based on their chemical constituents. In manufacturing
high tension steels with a tension of 50 kgf/mm.sup.2 or higher and
manufacturing wear resistant welding steels using the same, the
applicant controls the steel constituents to set the Nippon Steel
Corporation's specification PH at 1 or less. PH is calculated from
the content (% by weight) of the respective elements according to
the following formula:
PH=C+Mn/10+Cr/15+Mo/6+3V+40P+100B
[0160] From this relation formula, the amount of P is limited to
0.01% by weight or less because P can significantly cause grain
boundary embrittlement and significantly facilitate the weld
cracking, the amount of B is carefully controlled in order to
ensure the hardenability of the steels, and the carbon content is
limited to between 0.1 and 0.3% by weight.
[0161] For example, the bucket wear-resistible steel plate in Table
4, which is to be fillet-welded to the bottom of the bucket, has a
limited carbon content of 0.3% by weight or less and comprises
controlled constituents according to the above to be free from weld
cracking. Therefore, such a steel can be insufficient in wear
resistance. In this example, two types of quenched and tempered
steel plates: Fe-0.45C-0.21Si-1.2Mn-0.5Ni-0.15Cr-0.018P-0.0011B
(PH=1.41) and this plus 0.26% by weight of Al (PH=1.48) (each about
HRC54 in hardness, 15 mm in thickness, 70 mm in width, and 600 mm
in length) were each fillet-welded to the bucket bottom at two
corners in the longitudinal direction at room temperature under
CO.sub.2, and weld cracking was examined. As a result, two out of
five Al-free steel plates generated cracks, but 20 Al-added steel
plates had no crack.
[0162] After each welded bucket was heated at 500.degree. C. for 30
minutes for the purpose of removing the residual stress, it was
air-cooled to room temperature and examined for cracking at the
weld. As a result, the remaining three Al-free steel plates all
generated cracks, but the Al-added steel plates had no crack.
Therefore, it has been found that the high tension steel plate with
0.15% by weight or more of Al and 0.3% by weight or more of Ni
added in combination will have a PH upper limit of 1.4 to 1.48 and
that in the case that the P content is limited to 0.02% by weight
or less, the addition amount of carbon will be up to about 0.6% by
weight and therefore the carbon content will appropriately be from
0.1 to 0.6% by weight.
* * * * *