U.S. patent application number 11/795192 was filed with the patent office on 2008-01-24 for steel for machine structural use with excellent strength, ductility, and toughness and method for producing the same.
Invention is credited to Kazukuni Hase, Tohru Hayashi, Hideto Kimura, Nobutaka Kurosawa, Keiichi Maruta, Takaaki Toyooka, Katsumi Yamada.
Application Number | 20080017283 11/795192 |
Document ID | / |
Family ID | 37604564 |
Filed Date | 2008-01-24 |
United States Patent
Application |
20080017283 |
Kind Code |
A1 |
Maruta; Keiichi ; et
al. |
January 24, 2008 |
Steel For Machine Structural Use With Excellent Strength,
Ductility, And Toughness And Method For Producing The Same
Abstract
A steel for machine structural use with a better
strength-ductility-toughness balance than maraging steel and
applications thereof are provided. The steel for machine structural
use with excellent strength, ductility, and toughness contains, in
percent by mass, more than 0.30% to 0.5% of carbon, 1.0% or less of
silicon, 1.5% or less of manganese, 0.025% or less of aluminum,
0.3% to 0.5% of molybdenum, and 0.0005% to 0.01% of boron, and the
balance is iron and incidental impurities. The steel has a
structure including at least 90% by volume of martensitic
structure. The martensitic structure includes blocks having a size
of 1.5 .mu.m or less. Dissolved boron is contained in an amount of
at least 0.0005% and is present at boundaries of prior austenite
grains in a concentration at least 1.5 times that in the prior
austenite grains.
Inventors: |
Maruta; Keiichi; (Okayama,
JP) ; Hayashi; Tohru; (Okayama, JP) ;
Kurosawa; Nobutaka; (Okayama, JP) ; Kimura;
Hideto; (Okayama, JP) ; Toyooka; Takaaki;
(Tokyo, JP) ; Hase; Kazukuni; (Okayama, JP)
; Yamada; Katsumi; (Kanagawa, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
220 Fifth Avenue
16TH Floor
NEW YORK
NY
10001-7708
US
|
Family ID: |
37604564 |
Appl. No.: |
11/795192 |
Filed: |
June 30, 2006 |
PCT Filed: |
June 30, 2006 |
PCT NO: |
PCT/JP06/13521 |
371 Date: |
July 12, 2007 |
Current U.S.
Class: |
148/579 ;
148/330 |
Current CPC
Class: |
C21D 9/46 20130101; C22C
38/00 20130101 |
Class at
Publication: |
148/579 ;
148/330 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2005 |
JP |
2005-195739 |
Nov 11, 2005 |
JP |
2005-326844 |
Mar 22, 2006 |
JP |
2006-079070 |
Claims
1. A steel for machine structural use with excellent strength,
ductility, and toughness, the steel comprising, in percent by mass,
more than 0.30% to 0.5% of carbon, 1.0% or less of silicon, 1.5% or
less of manganese, 0.025% or less of aluminum, 0.3% to 0.5% of
molybdenum, and 0.0005% to 0.01% of boron, the balance being iron
and incidental impurities, the steel having a tensile strength of
2,000 MPa or more and a total elongation of 10% or more.
2. The steel for machine structural use with excellent strength,
ductility, and toughness according to claim 1, the steel further
comprising, in percent by mass, at least one of 2.5% or less of
chromium, 1.0% or less of copper, 2.0% or less of nickel, and 0.5%
or less of vanadium.
3. The steel for machine structural use with excellent strength,
ductility, and toughness according to claim 1, the steel further
comprising, in percent by mass, at least one of 0.1% or less of
titanium and 0.1% or less of niobium.
4. A steel for machine structural use with excellent strength,
ductility, and toughness, the steel comprising, in percent by mass,
more than 0.30% to 0.5% of carbon, 1.0% or less of silicon, 1.5% or
less of manganese, 0.025% or less of aluminum, 0.3% to 0.5% of
molybdenum, and 0.0005% to 0.01% of boron, the balance being iron
and incidental impurities, the steel having a structure comprising
at least 90% by volume of martensitic structure, the martensitic
structure comprising blocks having a size of 1.5 .mu.m or less,
wherein dissolved boron is contained in an amount of at least
0.0005% and is present at boundaries of prior austenite grains in a
concentration at least 1.5 times that in the prior austenite
grains.
5. The steel for machine structural use with excellent strength,
ductility, and toughness according to claim 4, the steel further
comprising, in percent by mass, at least one of 2.5% or less of
chromium, 1.0% or less of copper, 2.0% or less of nickel, and 0.5%
or less of vanadium.
6. The steel for machine structural use with excellent strength,
ductility, and toughness according to claim 4, the steel further
comprising, in percent by mass, at least one of 0.1% or less of
titanium and 0.1% or less of niobium.
7. A steel sheet for machine structural use with excellent
strength, ductility, and toughness, the steel sheet comprising the
steel for machine structural use according to claim 1 and having a
thickness of 0.5 mm or less.
8. A metal belt comprising the steel sheet according to claim 7,
the metal belt having an annular shape.
9. A method for producing a steel for machine structural use with
excellent strength, ductility, and toughness, the method comprising
quenching a steel material by heating at a rate of temperature rise
of 100.degree. C./s or more and tempering the steel material at
100.degree. C. to 400.degree. C., the steel material comprising, in
percent by mass, more than 0.30% to 0.5% of carbon, 1.0% or less of
silicon, 1.5% or less of manganese, 0.025% or less of aluminum,
0.3% to 0.5% of molybdenum, and 0.0005% to 0.01% of boron, the
balance being iron and incidental impurities.
10. The method for producing a steel for machine structural use
with excellent strength, ductility, and toughness according to
claim 9, wherein the steel material further comprises, in percent
by mass, at least one of 2.5% or less of chromium, 1.0% or less of
copper, 2.0% or less of nickel, and 0.5% or less of vanadium.
11. The method for producing a steel for machine structural use
with excellent strength, ductility, and toughness according to
claim 9, wherein the steel material further comprises, in percent
by mass, at least one of 0.1% or less of titanium and 0.1% or less
of niobium.
12. A method for producing a steel sheet for machine structural use
with excellent strength, ductility, and toughness, the method
comprising quenching a steel sheet by heating at a rate of
temperature rise of 100.degree. C./s or more and tempering the
steel sheet at 100.degree. C. to 400.degree. C., the steel sheet
comprising, in percent by mass, more than 0.30% to 0.5% of carbon,
1.0% or less of silicon, 1.5% or less of manganese, 0.025% or less
of aluminum, 0.3% to 0.5% of molybdenum, and 0.0005% to 0.01% of
boron, the balance being iron and incidental impurities, the steel
sheet having a thickness of 0.5 mm or less.
13. The method for producing a steel sheet for machine structural
use with excellent strength, ductility, and toughness according to
claim 12, wherein the steel sheet further comprises, in percent by
mass, at least one of 2.5% or less of chromium, 1.0% or less of
copper, 2.0% or less of nickel, and 0.5% or less of vanadium.
14. The method for producing a steel sheet for machine structural
use with excellent strength, ductility, and toughness according to
claim 12, wherein the steel sheet further comprises, in percent by
mass, at least one of 0.1% or less of titanium and 0.1% or less of
niobium.
15. A method for producing a metal belt, the method comprising
quenching a metal belt by heating at a rate of temperature rise of
100.degree. C./s or more and tempering the metal belt at
100.degree. C. to 400.degree. C., the metal belt comprising, in
percent by mass, more than 0.30% to 0.5% of carbon, 1.0% or less of
silicon, 1.5% or less of manganese, 0.025% or less of aluminum,
0.3% to 0.5% of molybdenum, and 0.0005% to 0.01% of boron, the
balance being iron and incidental impurities, the metal belt having
a thickness of 0.5 mm or less and having an annular shape.
16. The method for producing a metal belt according to claim 15,
wherein the metal belt further comprises, in percent by mass, at
least one of 2.5% or less of chromium, 1.0% or less of copper, 2.0%
or less of nickel, and 0.5% or less of vanadium.
17. The method for producing a metal belt according to claim 15,
wherein the metal belt further comprises, in percent by mass, at
least one of 0.1% or less of titanium and 0.1% or less of
niobium.
18. The steel for machine structural use with excellent strength,
ductility, and toughness according to claim 2, the steel further
comprising, in percent by mass, at least one of 0.1% or less of
titanium and 0.1% or less of niobium.
19. The steel for machine structural use with excellent strength,
ductility, and toughness according to claim 5, the steel further
comprising, in percent by mass, at least one of 0.1% or less of
titanium and 0.1% or less of niobium.
20. A steel sheet for machine structural use with excellent
strength, ductility, and toughness, the steel sheet comprising the
steel for machine structural use according to claim 4 and having a
thickness of 0.5 mm or less.
21. A metal belt comprising the steel sheet according to claim 20,
the metal belt having an annular shape.
22. The method for producing a steel for machine structural use
with excellent strength, ductility, and toughness according to
claim 10, wherein the steel material further comprises, in percent
by mass, at least one of 0.1% or less of titanium and 0.1% or less
of niobium.
23. The method for producing a steel sheet for machine structural
use with excellent strength, ductility, and toughness according to
claim 13, wherein the steel sheet further comprises, in percent by
mass, at least one of 0.1% or less of titanium and 0.1% or less of
niobium.
24. The method for producing a metal belt according to claim 16,
wherein the metal belt further comprises, in percent by mass, at
least one of 0.1% or less of titanium and 0.1% or less of niobium.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to steels for
machine structural use, including components of automobiles and
industrial machines. In particular, the present invention relates
to steels for machine structural use which have excellent strength,
ductility, and toughness and are particularly suitable for metal
belts, for example, used in continuously variable transmission
(hereinafter abbreviated as CVT), which are currently produced with
expensive steels such as maraging steel. The present invention also
relates to steel sheets for machine structural use and metal belts
produced with such steels.
BACKGROUND ART
[0002] In the field of automobiles, higher fuel efficiency and
emission control have recently been demanded with the growing
awareness of environmental issues. Accordingly, developments have
been directed toward miniaturized, high-powered driving systems.
For example, the development of CVT is remarkable. Metals belts
used for CVT require high strength, high ductility, and high
toughness. Maraging steel is one of the steels currently used for
such applications. Techniques using maraging steel are disclosed
in, for example, Patent Documents 1 to 3. On the other hand,
techniques using metastable austenitic stainless steel are
disclosed in, for example, Patent Documents 4 and 5.
[0003] In general, however, alloying elements are added to
materials requiring higher strength, including the steels described
above. Maraging steel contains, for example, cobalt, molybdenum,
and chromium in addition to ten and several percent of nickel while
metastable austenitic stainless steel contains chromium and nickel
in amounts of ten and several percent. Such steels are
significantly costly, and the production thereof can be threatened
by the recent shortage of materials.
[0004] Patent Document 1: Japanese Unexamined Patent Application
Publication No. 2000-345302
[0005] Patent Document 2: Japanese Unexamined Patent Application
Publication No. 2002-38251
[0006] Patent Document 3: Japanese Unexamined Patent Application
Publication No. 2003-231921
[0007] Patent Document 4: Japanese Unexamined Patent Application
Publication No. 2002-53936
[0008] Patent Document 5: Japanese Unexamined Patent Application
Publication No. 2003-33803
DISCLOSURE OF INVENTION
[0009] In light of the above problems of the related art, an object
of the present invention is to provide a steel and steel sheet for
machine structural use which have high strength, high ductility,
and high toughness with a minimal increase in production costs, and
also provide a metal belt suitable as an endless metal belt for CVT
at low cost.
[0010] As a result of intensive studies to achieve the above
object, the inventors have found a solution to the above problems.
That is, the inventors have demonstrated that even a steel system
that does not contain such a large amount of nickel or chromium as
contained in maraging steel and austenitic stainless steel provides
a better balance between tensile strength and elongation and higher
toughness than maraging steel if the steel contains appropriate
amounts of molybdenum and boron and is quenched and tempered to
form a martensitic structure.
[0011] Further studies on detailed structures constituting the
martensitic structure (hereinafter referred to as substructures)
have found that an especially excellent strength-elongation balance
can be achieved by controlling blocks constituting the martensitic
structure to a predetermined size or less. These studies have also
found that excellent toughness can be ensure if dissolved boron is
contained in at least a predetermined amount and is present at
boundaries of prior austenite grains in a concentration at least
1.5 times that in the prior austenite grains.
[0012] The present invention, which has been completed depending on
the above findings, can be summarized as follows:
[0013] (1) A steel for machine structural use with excellent
strength, ductility, and toughness according to the present
invention contains, in percent by mass, more than 0.30% to 0.5% of
carbon, 1.0% or less of silicon, 1.5% or less of manganese, 0.025%
or less of aluminum, 0.3% to 0.5% of molybdenum, and 0.0005% to
0.01% of boron, and the balance is iron and incidental impurities.
The steel has a tensile strength of 2,000 MPa or more and a total
elongation of 10% or more.
[0014] (2) The steel according to Item (1) further contains, in
percent by mass, at least one of 2.5% or less of chromium, 1.0% or
less of copper, 2.0% or less of nickel, and 0.5% or less of
vanadium.
[0015] (3) The steel according to Item (1) or (2) further contains,
in percent by mass, at least one of 0.1% or less of titanium and
0.1% or less of niobium.
[0016] (4) A steel for machine structural use with excellent
strength, ductility, and toughness according to the present
invention contains, in percent by mass, more than 0.30% to 0.5% of
carbon, 1.0% or less of silicon, 1.5% or less of manganese, 0.025%
or less of aluminum, 0.3% to 0.5% of molybdenum, and 0.0005% to
0.01% of boron, and the balance is iron and incidental impurities.
The steel has a structure including at least 90% by volume of
martensitic structure. The martensitic structure includes blocks
having a size of 1.5 .mu.m or less. Dissolved boron is contained in
an amount of at least 0.0005% and is present at boundaries of prior
austenite grains in a concentration at least 1.5 times that in the
prior austenite grains.
[0017] (5) The steel according to Item (4) further contains, in
percent by mass, at least one of 2.5% or less of chromium, 1.0% or
less of copper, 2.0% or less of nickel, and 0.5% or less of
vanadium.
[0018] (6) The steel according to Item (4) or (5) further contains,
in percent by mass, at least one of 0.1% or less of titanium and
0.1% or less of niobium.
[0019] (7) A steel sheet for machine structural use with excellent
strength, ductility, and toughness according to the present
invention is formed of the steel for machine structural use
according to one of Items (1) to (6) and has a thickness of 0.5 mm
or less.
[0020] (8) A metal belt according to the present invention is
formed of the steel sheet according to Item (7) and has an annular
shape.
[0021] (9) A method for producing a steel for machine structural
use with excellent strength, ductility, and toughness according to
the present invention includes quenching a steel material by
heating at a rate of temperature rise of 100.degree. C./s or more
and tempering the steel material at 100.degree. C. to 400.degree.
C. The steel material contains, in percent by mass, more than 0.30%
to 0.5% of carbon, 1.0% or less of silicon, 1.5% or less of
manganese, 0.025% or less of aluminum, 0.3% to 0.5% of molybdenum,
and 0.0005% to 0.01% of boron, and the balance is iron and
incidental impurities.
[0022] (10) In Item (9), the steel material further contains, in
percent by mass, at least one of 2.5% or less of chromium, 1.0% or
less of copper, 2.0% or less of nickel, and 0.5% or less of
vanadium.
[0023] (11) In Item (9) or (10), the steel material further
contains, in percent by mass, at least one of 0.1% or less of
titanium and 0.1% or less of niobium.
[0024] (12) A method for producing a steel sheet for machine
structural use with excellent strength, ductility, and toughness
according to the present invention includes quenching a steel sheet
by heating at a rate of temperature rise of 100.degree. C./s or
more and tempering the steel sheet at 100.degree. C. to 400.degree.
C. The steel sheet contains, in percent by mass, more than 0.30% to
0.5% of carbon, 1.0% or less of silicon, 1.5% or less of manganese,
0.025% or less of aluminum, 0.3% to 0.5% of molybdenum, and 0.0005%
to 0.01% of boron, and the balance is iron and incidental
impurities. The steel sheet has a thickness of 0.5 mm or less.
[0025] (13) In Item (12), the steel sheet further contains, in
percent by mass, at least one of 2.5% or less of chromium, 1.0% or
less of copper, 2.0% or less of nickel, and 0.5% or less of
vanadium.
[0026] (14) In Item (12) or (13), the steel sheet further contains,
in percent by mass, at least one of 0.1% or less of titanium and
0.1% or less of niobium.
[0027] (15) A method for producing a metal belt according to the
present invention includes quenching a metal belt by heating at a
rate of temperature rise of 100.degree. C./s or more and tempering
the metal belt at 100.degree. C. to 400.degree. C. The metal belt
contains, in percent by mass, more than 0.30% to 0.5% of carbon,
1.0% or less of silicon, 1.5% or less of manganese, 0.025% or less
of aluminum, 0.3% to 0.5% of molybdenum, and 0.0005% to 0.01% of
boron, and the balance is iron and incidental impurities. The metal
belt has a thickness of 0.5 mm or less and has an annular
shape.
[0028] (16) In Item (15), the metal belt further contains, in
percent by mass, at least one of 2.5% or less of chromium, 1.0% or
less of copper, 2.0% or less of nickel, and 0.5% or less of
vanadium.
[0029] (17) In Item (15) or (16), the metal belt further contains,
in percent by mass, at least one of 0.1% or less of titanium and
0.1% or less of niobium.
[0030] The present invention can provide a steel for machine
structural use which has excellent strength, ductility, and
toughness without containing large quantities of expensive alloying
elements, a metal sheet for machine structural use produced with
the steel, and a metal belt produced with the metal sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a diagram illustrating a method for a fatigue
evaluation test with an endless metal belt.
BEST MODE FOR CARRYING OUT THE INVENTION
[0032] The composition, structure, strength, and elongation of the
steel according to the present invention will now be described in
detail.
[0033] 1. Composition
[0034] The reasons for the specified composition will be described.
The contents (%) of the individual elements in the composition are
all expressed in percent by mass.
[0035] Carbon: more than 0.30% to 0.5%
[0036] Carbon is an element essential for ensuring required
strength and toughness. If the carbon content is not more than
0.30%, a predetermined strength is difficult to ensure. The upper
limit is 0.5% because more than 0.5% of carbon decreases ductility
and toughness and promotes formation of huge carbide grains in the
structure of the steel, which significantly degrade fatigue
properties.
[0037] Silicon: 1.0% or less
[0038] The steel can contain silicon because it serves as a
deoxidant during the production of the steel. The upper limit is
1.0% because more than 1.0% of silicon significantly decreases the
ductility of the steel.
[0039] Manganese: 1.5% or less
[0040] The steel can contain manganese because it serves as a
deoxidant during the production of the steel. The upper limit is
1.5% because more than 1.5% of manganese significantly decreases
the ductility of the steel.
[0041] Aluminum: 0.025% or less
[0042] Aluminum is an element effective for deoxidation. In
addition, aluminum is an element effective to maintain strength and
toughness because it inhibits growth of austenite grains during
quenching. The aluminum content, however, is limited to the above
range because an aluminum content exceeding 0.025% results in a
saturated effect and the disadvantage of increased costs.
[0043] Molybdenum: 0.3% to 0.5%
[0044] Molybdenum, a particularly important element in the present
invention, increases strength and toughness without significantly
decreasing ductility. Addition of 0.3% or more of molybdenum is
required to realize its effect. The upper limit is 0.5% because
addition of more than 0.5% of manganese does not contribute to a
further increase in strength and toughness and results in increased
costs. Also, the addition of an excessive amount of manganese
decreases ductility.
[0045] Boron: 0.0005% to 0.01%
[0046] Boron is a useful element effective to improve quenching
properties and provides grain boundary strengthening, which
contributes to an increase in the strength of the entire steel. A
boron content of 0.0005% or more is required to realize its effect.
The boron content, however, is limited to the above range because a
boron content exceeding 0.01% results in a saturated effect.
[0047] The above elements are basic components of the steel
according to the present invention. In addition to these
components, the steel can optionally contain the following
elements:
[0048] Chromium: 2.5% or less
[0049] Chromium is effective to improve quenching properties and is
useful to ensure hardening depth. An excessive content of chromium,
however, promotes formation of carbide residues by a carbide
stabilization effect, thus decreasing strength. Thus, a minimal
content of chromium is preferred, although a chromium content of up
to 2.5% is acceptable. A chromium content of 0.2% or more is
preferred to realize the effect of improving quenching
properties.
[0050] Copper: 1.0% or less
[0051] Copper is effective to improve quenching properties, and
also increases strength when dissolved in ferrite. If the copper
content exceeds 1.0%, the steel can be cracked during hot rolling.
Thus, the copper content is limited to the above range. A copper
content of 0.2% or more is preferred to realize the effect of
improving quenching properties and strength.
[0052] Nickel: 2.0% or less
[0053] Nickel is effective to improve quenching properties, and
also contributes to an increase in strength and toughness because
it can inhibit growth of carbides and formation of carbide films at
grain boundaries to increase grain boundary strength. However,
nickel is an extremely expensive element, and addition of more than
2.0% of nickel significantly increases steel costs. Hence, the
nickel content is preferably limited to 2.0% or less. A nickel
content of 0.5% or more is preferred to realize the effect of
improving quenching properties, strength, and toughness.
[0054] Vanadium: 0.5% or less
[0055] Vanadium can be expected to serve as a strengthening element
by combining with carbon in the steel. Vanadium also has the effect
of increasing softening resistance in tempering, thus contributing
to increased strength. The vanadium content is limited to the above
range because a vanadium content exceeding 0.5% results in a
saturated effect. A vanadium-content of 0.1% or more is preferred
to realize the effect of increasing strength.
[0056] The steel according to the present invention can further
contain at least one of the following components:
[0057] Titanium: 0.1% or less
[0058] Titanium combines with nitrogen, contained as an incidental
impurity, to prevent formation of BN. This avoids attenuation of
the effect of boron, that is, improving quenching properties. The
titanium content is preferably limited to 0.1% or less because a
titanium content exceeding 0.1% results in formation of a large
quantity of TiN, which decreases strength and fatigue strength. A
titanium content of 0.005% or more is preferred to realize its
effect.
[0059] Niobium: 0.1% or less
[0060] Niobium has the effect of improving quenching properties and
also serves as a precipitation strengthening element to contribute
to increased strength and toughness. The niobium content is
preferably limited to 0.1% or less because a niobium content
exceeding 0.1% results in a saturated effect. A niobium content of
0.005% or more is preferred to realize its effect.
[0061] The balance of the steel, other than the elements described
above, is iron and incidental impurities. Typical incidental
impurities include sulfur, phosphorus, nitrogen, and oxygen, which
can be contained in amounts of up to 0.05%, up to 0.05%, up to
0.01%, and up to 0.01%, respectively.
[0062] 2. Structure
[0063] While the preferred ranges of composition have been
described above, only the limitation of the composition to the
above ranges is insufficient in the present invention, and the
structure of the steel must also be controlled as follows:
[0064] Steel structure: at least 90% by volume of martensitic
structure
[0065] Martensite is a structure essential to achieve strength. The
steel according to the present invention provides excellent
properties if it contains at least 90% by volume of martensitic
structure. Accordingly, the volume percentage of martensitic
structure is limited to the above range. If the volume percentage
of martensite falls below 90%, the steel contains excessive
quantities of untransformed phases such as residual austenitic
phase and precipitates such as carbides, which do not contribute to
increased strength. This makes it difficult to achieve high
strength, namely, 2,000 MPa or more.
[0066] Martensitic structure: the structure includes blocks having
a size of 1.5 .mu.m or less
[0067] A finer martensitic structure is preferred in terms of, for
example, fatigue resistance. The martensitic structure, which is a
typical structure transformed from austenite, has complicated
substructures divided generally into the following structure units:
martensite laths, the smallest unit, which have only slight
differences in crystal orientation from adjacent ones and thus have
no dominant effect on mechanical properties; blocks, groups of
adjacent laths with substantially equivalent crystal planes and
orientations, several blocks being contained in each austenite
grain before transformation; and packets, groups of blocks with
equivalent crystal planes but different growth directions.
Formation of a finer martensitic structure substantially means
formation of smaller structure units. Most effectively, this can be
achieved by forming finer blocks. Martensite laths in blocks can be
assumed as substantially continuous structures with low-angle tilt
grain boundaries. On the other hand, the sizes of blocks, packets,
and austenite grains before transformation probably have a direct
effect on the mechanical properties of the material because of
high-angle tilt grain boundaries. The size of blocks can be
evaluated by, for example, orientation imaging microscopy or
transmission electron microscopy (TEM). Although packets are
another substructure unit of martensitic structure, the size of
packets is preferably controlled with a structure unit smaller than
packets with high-angle tilt grain boundaries, namely, blocks. It
is not practical in actual processes to check all products for the
size of austenitic structure before transformation before final
heat treatment. Hence, the size of blocks in the martensitic
structure should be controlled because they constitute a
substructure which can readily be evaluated for final products
(particularly, after the final heat treatment) and which affects
the mechanical properties of the material. The steel according to
the present invention provides a particularly excellent
strength-ductility balance and toughness if the blocks have an
average size of 1.5 .mu.m or less. The term "size" used herein
means an average grain size generally used for evaluation of steel
structure. For example, an average grain size determined by an
intercept method can be used.
[0068] Distribution of dissolved boron: dissolved boron is
contained in the steel in an amount of at least 0.0005% and is
present at boundaries of prior austenite grains in a concentration
at least 1.5 times that in the grains after, for example,
quenching
[0069] The steel according to the present invention provides stable
mechanical properties if the distribution of dissolved boron is
controlled as described below. In the present invention, as
described above, the content of boron is specified for obtaining
improved quenching properties and grain boundary strengthening. A
sufficient amount of dissolved boron and the distribution thereof
is very important to realize the effect of boron. The amount of
boron dissolved in the steel is decreased with, for example,
formation of BN and M.sub.23(C,B).sub.6 (where M is a metal
element). Addition of an element that combines readily with
nitrogen, such as titanium, is effective to inhibit the formation
of BN. In a carbon-rich steel system, however, the added element is
dissolved in carbides by substitution and thus fails to provide the
expected effect. Accordingly, sufficient dissolution in the .gamma.
range is essential. In addition, the dissolved boron is preferably
present mainly at boundaries of prior austenite grains. Grain
boundary strength, which significantly affects mechanical
properties including strength, elongation, and toughness, is
increased if the dissolved boron is present mainly at boundaries of
prior austenite grains with a difference in concentration from the
interior of the grains (i.e., grain boundary segregation). The
grain boundary segregation of the dissolved boron will prevent
grain boundary segregation of phosphorus, which could cause grain
boundary embrittlement. The studies by the inventors have confirmed
that the steel can more reliably achieve stable toughness if the
dissolved boron is contained in an amount of at least 0.0005% after
final heat treatment, such as high-frequency heating quenching or
low-temperature tempering at 400.degree. C. or less, and is present
at boundaries of prior austenite grains in a concentration 1.5
times that in the prior austenite grains after the final heat
treatment.
[0070] The amount of dissolved boron can be determined by
subtracting the amount of precipitated boron from the amount of
boron added. The amount of precipitated boron is determined by
extracting and separating boron-containing precipitates occurring
as oxides, nitrides, carbides, or intermetallic compounds through
electrolysis, for example, and directly measuring the content of
boron in the precipitates. The concentration distribution of
dissolved boron in and at boundaries of prior austenite grains can
be determined by, for example, secondary ion mass spectrometry
(SIMS) to confirm that the ionic strength of the boundaries of the
prior austenite grains is at least 1.5 times that of the interior
of the grains if the grains have a grain size of 10 .mu.m or more.
Other effective means for high-sensitivity detection include
determination of an electron energy loss spectrum (EELS) of grain
boundaries by TEM and .alpha.-ray track etching (ATE), in which a
film is irradiated with .alpha. rays emitted from a boron isotope
with a mass number of 10 (B.sub.10) by radioactivation of a sample
in, for example, a nuclear reactor, although SIMS is the most
suitable in terms of detection sensitivity and quantitative
determination for a trace amount of boron. As described above,
grain boundary embrittlement can be avoided if the dissolved boron
is contained in an amount of at least 0.0005% and is present mainly
at boundaries of prior austenite grains.
[0071] 3. Strength and Elongation
[0072] Tensile strength: 2,000 Mpa or more; total elongation: 10%
or more
[0073] The strength and total elongation of the steel according to
the present invention are limited to the above ranges because the
steel requires at least such strength and ductility levels to
achieve properties comparable to those of maraging steel, a
currently expensive steel intended to be replaced with the steel
according to the present invention. If the steel has the
composition and structure described above, it can achieve a tensile
strength of 2,000 MPa or more, a total elongation of 10% or more,
and high toughness. The studies by the inventors have also
demonstrated that a metal belt for CVT produced with a steel having
the above composition, a tensile strength of 2,000 MPa or more, and
a total elongation of 10% or more has durability comparable to that
of a metal belt produced with a conventional maraging steel.
[0074] Next, a method for producing a steel for machine structural
use according to the present invention will be described. The steel
is produced by quenching and tempering a steel material having the
above composition. The rate of temperature rise in quenching and
tempering temperature, which are important in the present
invention, must be controlled as follows:
[0075] Rate of temperature rise in heating for quenching:
100.degree. C./s or more
[0076] If the rate of temperature rise in heating for quenching
falls below 100.degree. C./s, the blocks of martensitic structure
grow to a size exceeding 1.5 .mu.m. In this case, the steel cannot
have a good strength-ductility balance. Hence, the rate of
temperature rise in heating for quenching must be 100.degree. C./s
or more.
[0077] Tempering temperature: 100.degree. C. to 400.degree. C.
[0078] If the tempering temperature falls within the range of
100.degree. C. to 400.degree. C., the boron contained in the steel
is concentrated at the grain boundaries without diffusion or
precipitation, thus contributing to grain boundary strengthening.
If the tempering temperature is 400.degree. C. or less, the steel
maintains its high strength, high ductility, and high toughness in
synergy with a fine grain effect. Excessive tempering temperatures
result in decreased strength and decreased concentration of boron
at the grain boundaries, thus significantly decreasing toughness.
From this viewpoint, the tempering temperature must be 400.degree.
C. or less. If the tempering temperature is less than 100.degree.
C., the steel exhibits insufficient elongation and fails to provide
a total elongation of 10% or more. Accordingly, the tempering
temperature should fall within the range of 100.degree. C. to
400.degree. C.
[0079] The steel material used can be one prepared by subjecting a
steel ingot with the above composition to hot or cold working, such
as rolling or forging. The steel ingot with the above composition
can be one produced by converter melting or vacuum melting. In
particular, if the steel material used is a steel sheet, a steel
ingot or a continuously cast slab is subjected to hot rolling with
heating, scale removal by pickling, and cold rolling to produce a
steel sheet with a predetermined thickness. If a metal belt is to
be produced with the steel sheet, the sheet is cold-rolled to a
thickness of 0.5 mm or less, is cut into a predetermined width and
length, and is formed into an annular shape to produce a metal
belt.
[0080] The above steel material (including a steel sheet and a
metal belt) is subjected to quenching and tempering to form a
martensitic structure. The heating means used for these treatments
can be high-frequency heating, furnace heating, infrared heating,
or electrical heating.
[0081] The steel thus produced (including a steel sheet and a metal
belt) has a strength-ductility balance comparable to that of
maraging steel despite low production costs and can be used for
automobile parts requiring high strength, high ductility, and high
toughness. In particular, a metal belt produced with the steel is
suitable for use as an endless metal belt for CVT, which is
currently produced with maraging steel.
EXAMPLES
Example 1
[0082] Examples will now be described.
[0083] Steels shown in Table 1 were produced by vacuum melting.
These steels were heated to 1,100.degree. C. and were hot-rolled
into sheets with a thickness of 3 mm. These sheets were pickled to
remove surface scale and were cold-rolled. The rolling was repeated
many times. After the sheets were rolled to a thickness of 0.8 mm,
they were annealed to remove work strain and were further
cold-rolled to a final thickness of 0.4 mm. These materials were
subjected to heat treatment and evaluation described below.
[0084] The structures of the steels, which are to be subjected to
high-frequency heating quenching, after the final heat treatment
are expected to contain only a martensitic phase formed by
transformation from the austenite temperature range, an
untransformed ferrite phase that can result from insufficient
heating, and undissolved inclusions and precipitates such as
carbides. These phases can be discriminated by developing the
structures by nital etching, one of the generally used methods, and
observing them using an optical microscope. Accordingly, the volume
percentage of martensitic structure was determined by the method
described below. The above materials were cut to a size of 20
mm.times.20 mm, were heated to 920.degree. C. by high-frequency
heating, were quickly quenched, and were tempered at 170.degree. C.
for 20 minutes to prepare samples. The surfaces of the samples were
etched with nital and were observed using an optical microscope to
determine the area percentage of the region of phases other than
martensitic phase which were discriminated by optical microscopy
(i.e., untransformed ferrite phase and undissolved inclusions and
precipitates such as carbides). The volume percentage of
martensitic phase in the examples was determined by converting the
area percentage of the region of the phases other than the
martensitic phase to volume percentage and subtracting it from
100%. In the invention examples, the martensitic phase accounted
for most of the structure because the temperature for
high-frequency quenching was 920.degree. C., which falls within the
austenite range.
[0085] Blocks, one of the substructures of martensitic structure,
were evaluated by the method described below. The above materials
were cut into samples with a size of 20 mm.times.20 mm. These
samples were heated to 920.degree. C. by high-frequency heating,
were quickly quenched, and were tempered at 170.degree. C. for 20
minutes. Subsequently, the samples were cut into samples for
microscopy with a size of 10 mm.times.10 mm. These samples were
evaluated for blocks by orientation imaging microscopy.
[0086] Crystal orientation information was obtained at a total of
about 11,000 points in two fields of view of 10 .mu.m.times.10
.mu.m regions on each sample. In each field of view, the boundaries
of closed regions of the same colors were recognized as blocks. The
size of the blocks in the field of view was determined by the same
intercept method as generally used for determination of average
grain size. The simple arithmetic average of all measurements of
the fields of view was determined as the average size of the blocks
of the material.
[0087] The content of dissolved boron in each steel was determined
by subtracting the amount of precipitated boron from the amount of
boron added. The amount of precipitated boron was determined by
electrolytic extraction analysis. First, the above materials were
cut into samples with a size of 30 mm.times.30 mm. These samples
were heated to 920.degree. C. by high-frequency heating, were
quickly quenched, and were tempered at 170.degree. C. for 20
minutes. Subsequently, 1 g of each tempered sample was electrolyzed
in a 10% acetylacetone electrolytic solution, and electrolysis
residues were filtered out to determine the amount of precipitated
boron.
[0088] The concentration distribution of dissolved boron in each
sample was measured by the method described below. The samples with
a size of 10 mm.times.10 mm used in the evaluation of block size
were mirror-polished again for concentration distribution
measurement by SIMS. In the measurement by SIMS, the primary ions
O.sub.2.sup.+ were used to obtain two fields of view of ion images
of the secondary ions BO.sub.2.sup.- with a mass number of 43 from
regions with a field stop of 150 .mu.m (in diameter). Average
secondary ion strengths at boundaries of grains and in the interior
of the grains in each field of view were determined, and the ratio
therebetween was determined. Finally, the arithmetic average of the
ion strength ratios of the two fields of view was determined as the
concentration distribution ratio of the sample.
[0089] Boundaries of prior austenite grains were inspected as
follows. The samples with a size of 10 mm.times.10 mm used in the
measurement of concentration distribution of dissolved boron were
used as samples for microscopy. L-shaped cross sections, parallel
to the rolling direction, of the samples used in the measurement of
concentration distribution of dissolved boron were mirror-polished
and were exposed to an etchant, to develop boundaries of prior
austenite grains. The etchant was prepared by dissolving 50 g of
picric acid in 500 g of water and adding 11 g of sodium
dodecylbenzenesulfonate, 1 g of ferrous chloride, and 1.5 g of
oxalic acid to the picric acid aqueous solution. The boundaries of
prior austenite grains were inspected using an optical microscope
at a magnification of .times.1,000.
[0090] The materials were cut into tensile test pieces (JIS No. 5)
by electrical discharge machining. The test pieces were heated to
920.degree. C. by high-frequency heating, were quickly quenched,
and were tempered at 170.degree. C. for 20 minutes. The test pieces
were subjected to a tensile test.
[0091] Similarly, a maraging steel (Fe--18Ni--10Co--5Mo--0.4Ti) was
processed until cold rolling and was cut into a test piece with the
same shape as above. The test piece was heated to 820.degree. C.,
was quenched by air cooling, and was subjected to aging by heating
being to 520.degree. C.
[0092] In evaluation of toughness, unlike the above, the steels
were hot-rolled to a thickness of 15 mm and were cut into charpy
test pieces with U-notches extending in the C direction of the
rolled sheets. The test pieces were heated to 920.degree. C. by
high-frequency heating, were quickly quenched, and were tempered at
170.degree. C. for 30 minutes. The test pieces were subjected to a
charpy test, which was conducted under two different conditions,
namely, test temperatures of -40.degree. C. and 40.degree. C., and
the measured absorption energies were compared.
[0093] Table 1 shows measurements of the volume percentage of
martensitic structure, tensile strength, total elongation, and
toughness. According to Table 1, the steels within the scope of the
present invention had a better strength-ductility balance than the
maraging steel and also had high toughness.
Example 2
[0094] The effect of structure was examined. All the test methods
used were the same as those used in Example 1 except that the
high-frequency heating was performed at varying temperatures to
examine the effect of the volume percentage of martensite.
[0095] In comparative examples, for example, the amount of
untransformed ferrite phase was increased by lowering the heating
temperature. As a result, the volume percentage of martensite fell
below 90%. The test results are shown in Table 2, which shows that
the formation of less than 90% by volume of martensitic structure
resulted in significantly decreased strength.
Example 3
[0096] The effects of other components were examined. Steels shown
in Table 3 were produced by vacuum melting. The test methods used
were the same as those used in Example 1. The test results are
shown in Table 3, which shows that excessive contents of chromium
and titanium resulted in decreased strength and excessive contents
of nickel, vanadium, and niobium resulted in a saturated
effect.
Example 4
[0097] The effect of the rate of temperature rise in heating for
quenching was examined. A steel having the same composition as
Steel No. 1-4 of Example 1 was subjected to furnace heating rather
than high-frequency heating and was tempered under the same
conditions as used in Example 1. This steel was examined for
structure and properties. Table 4 shows a comparison of the rates
of temperature rise, structures, and properties of the steel
subjected to furnace heating (Steel No. 4-1) with the steel
subjected to high-frequency heating (Steel No. 1-4 in Table 1).
[0098] According to Table 4, the use of furnace heating with a low
rate of temperature rise for quenching resulted in formation of
large martensite blocks. The steel could not achieve an elongation
of 10% or more at a strength of 2,000 MPa or more and also had
decreased toughness.
Example 5
[0099] The effect of tempering temperature was examined. Steels
having the same composition as Steel No. 1-4 of Example 1 and
steels having the same composition as Steel No. 1-12 of Example 1
were quenched under the same conditions as used in Example 1 and
were tempered at varying temperatures, namely, 260.degree. C.,
380.degree. C., and 450.degree. C. The test results are shown in
Table 5.
[0100] Table 5 shows that the tempering temperature exceeding
400.degree. C. resulted in a decreased concentration of boron at
grain boundaries and significantly decreased toughness.
Example 6
[0101] Practical endless metal belts were evaluated for fatigue
strength. The cold-rolled sheets of Example 1 with a thickness of
0.4 mm were cut to a width of 20 mm, were welded into an annular
shape, and were quenched and tempered to prepare samples. These
samples were suspended on SUJ2 pulleys shown in FIG. 1 and were
rotated at 2,000 rpm under a predetermined tensile load (P=3,500
N). The samples were evaluated for the number of revolutions before
fracturing (the number of reciprocations of a particular point on
the belts between the two pulleys). The materials used in the test
were Steel Nos. 1-1 to 1-16 of Example 1 and Steel Nos. 5-1 to 5-6
of Example 5. The quenching and tempering conditions used for Steel
Nos. 1-1 to 1-16 and Steel Nos. 5-1 to 5-6 were the same as those
used in Examples 1 and 5, respectively. The test was performed
three times for each material. The test results are shown in Table
6, which shows that the numbers of revolutions of the steels of the
invention examples were nearly equivalent to that of the maraging
steel. The steels of the comparative examples had decreased fatigue
strength when used for practical components because of low tensile
strength or ductility. The steels tempered at more than 400.degree.
C. had decreased fatigue strength. Addition of more than 0.5% of
molybdenum, namely, Steel No. 1-14, provided only a limited effect.
TABLE-US-00001 TABLE 1 Structure Dissolved B intensity ratio Volume
Average (boundary/ Steel Chemical composition (mass %) % of M block
interior; No. C Si Mn Al Mo B Others phase size (.mu.m) mass ppm)
1-1 0.0025 0.002 0.006 0.084 4.87 0.0007 Ni: 17.99 Co: 10.44 Cr:
0.19 Ti: 0.48 1-2 0.20 0.74 0.65 -- 0.39 0.0019 -- 93 1.80 19, 2.5
1-3 0.35 0.72 0.66 -- 0.41 0.0020 -- 94 1.48 15, 2.0 1-4 0.43 0.71
0.65 -- 0.40 0.0020 -- 96 0.90 15, 2.0 1-5 0.55 0.72 0.64 -- 0.40
0.0021 -- 96 0.85 12, 1.2 1-6 0.44 0.31 0.64 -- 0.40 0.0019 -- 96
0.90 14, 2.0 1-7 0.43 0.95 0.65 -- 0.39 0.0022 -- 95 0.90 18, 2.0
1-8 0.44 1.12 0.64 -- 0.41 0.0020 -- 96 0.80 18, 2.0 1-9 0.43 0.71
1.10 -- 0.40 0.0020 -- 95 0.90 18, 2.0 1-10 0.43 0.72 1.66 -- 0.41
0.0019 -- 95 0.95 18, 2.0 1-11 0.42 0.71 0.65 -- 0.25 0.0018 -- 94
1.60 14, 2.5 1-12 0.44 0.75 0.66 -- 0.31 0.0019 -- 95 0.90 15, 2.0
1-13 0.43 0.73 0.64 -- 0.49 0.0020 -- 94 0.80 10, 1.8 1-14 0.43
0.72 0.64 -- 0.60 0.0020 -- 95 0.80 8, 1.0 1-15 0.43 0.71 0.66 --
0.41 0.0002 -- 90 1.55 0, -- 1-16 0.44 0.70 0.65 -- 0.42 0.0060 --
96 0.90 45, 3.0 Charpy test (J) Tensile Total Test Test Steel
strength elogation temperature: temperature: No. (MPa) (%)
-40.degree. C. 40.degree. C. Remarks 1-1 2100 12.5 41.3 45.5 Com.
Ex. 1-2 1750 16.5 45.2 48.1 Com. Ex. 1-3 2125 15.0 45.5 48.3 In.
Ex. 1-4 2230 12.5 42.7 45.3 In. Ex. 1-5 2285 8.5 11.0 18.2 Com. Ex.
1-6 2205 15.0 46.3 49.0 In. Ex. 1-7 2190 12.0 43.6 45.5 In. Ex. 1-8
2200 8.0 9.2 14.1 Com. Ex. 1-9 2220 11.5 41.8 44.3 In. Ex. 1-10
2235 6.5 8.5 12.3 Com. Ex. 1-11 1650 8.5 10.0 12.4 Com. Ex. 1-12
2080 12.0 43.5 46.6 In. Ex. 1-13 2265 14.5 47.2 48.6 In. Ex. 1-14
2265 14.0 45.6 46.9 Com. Ex. 1-15 1900 11.5 42.1 44.7 Com. Ex. 1-16
2285 14.5 47.3 49.1 In. Ex. * The underlined items are beyond the
scope of the present invention.
[0102] TABLE-US-00002 TABLE 2 Structure Charpy test (J) Volume
Tensile Total Test Test Steel Chemical composition (mass %) % of M
strength elogation temperature: temperature: No. C Si Mn Al Mo B
Others phase (MPa) (%) -40.degree. C. 40.degree. C. Remarks 2-1
0.43 0.71 0.65 -- 0.40 0.0020 -- 96 2230 12.5 40.1 45.1 In. Ex. 2-2
0.43 0.71 0.65 -- 0.40 0.0020 -- 91 2040 15.0 44.6 47.2 In. Ex. 2-3
0.43 0.71 0.65 -- 0.40 0.0020 -- 85 1840 16.5 49.1 50.6 Com. Ex.
2-4 0.43 0.71 0.65 -- 0.40 0.0020 -- 70 1650 19.0 52.2 53.4 Com.
Ex. * The underlined items are beyond the scope of the present
invention.
[0103] TABLE-US-00003 TABLE 3 Structure Charpy test (J) Volume
Tensile Total Test Test Steel Chemical composition (mass %) % of M
strength elogation temperature: temperature: No. C Si Mn Al Mo Cr
Ni Cu V Ti Nb B phase (MPa) (%) -40.degree. C. 40.degree. C. 3-1
0.44 0.70 0.62 0.021 0.40 -- -- -- -- -- -- 0.0021 94 2210 14.0
42.5 45.8 3-2 0.43 0.72 0.62 0.031 0.40 -- -- -- -- -- -- 0.0018 94
2215 13.5 42.0 45.1 3-3 0.44 0.71 0.62 -- 0.40 1.5 -- -- -- -- --
0.0020 94 2235 13.5 42.6 46.1 3-4 0.42 0.70 0.65 -- 0.40 3.0 -- --
-- -- -- 0.0018 95 1765 11.5 38.5 40.5 3-5 0.43 0.72 0.66 -- 0.39
-- 1.6 -- -- -- -- 0.0021 95 2245 14.0 43.5 45.7 3-6 0.44 0.71 0.64
-- 0.41 -- 2.5 -- -- -- -- 0.0018 95 2250 14.0 42.2 46.3 3-7 0.43
0.69 0.63 -- 0.40 -- -- 0.5 -- -- -- 0.0020 94 2245 13.5 41.8 44.7
3-8 0.45 0.71 0.64 -- 0.41 -- -- -- 0.3 -- -- 0.0018 94 2235 13.0
43.6 45.7 3-9 0.43 0.72 0.65 -- 0.40 -- -- -- 0.6 -- -- 0.0021 95
2235 12.5 41.7 44.9 3-10 0.44 0.72 0.64 -- 0.41 -- -- -- -- 0.04 --
0.0020 95 2190 14.5 45.3 47.5 3-11 0.43 0.70 0.65 -- 0.39 -- -- --
-- 0.11 -- 0.0019 94 1650 9.0 11.3 15.5 3-12 0.44 0.69 0.66 -- 0.40
-- -- -- -- -- 0.03 0.0020 94 2200 13.0 43.3 45.9 3-13 0.45 0.70
0.62 -- 0.41 -- -- -- -- -- 0.11 0.0022 96 2210 13.0 43.4 45.5 *
The underlined items are beyond the scope of the present
invention.
[0104] TABLE-US-00004 TABLE 4 Rate of Structure temperature
Dissolved B rise in Average intensity ratio heating for Volume
block (boundary/ Steel Chemical composition (mass %) quenching % of
M size interior; No. C Si Mn Al Mo B Others (.degree. C./a) phase
(.mu.m) mass ppm) 1-4 0.43 0.71 0.65 -- 0.40 0.0020 -- 250 96 0.90
15, 2.0 4-1 0.43 0.71 0.65 -- 0.40 0.0020 -- 40 95 2.50 15, 2.0
Results Charpy test (J) Tensile Total Test Test Steel strength
elogation temperature: temperature: No. (MPa) (%) -40.degree. C.
40.degree. C. Remarks 1 Remarks 2 1-4 2230 12.5 42.7 45.3 In. Ex.
High- frequency quenching 4-1 1710 13.5 32.5 36.4 Com. Ex. Furnace
quenching * The underlined items are beyond the scope of the
present invention.
[0105] TABLE-US-00005 TABLE 5 Structure Dissolved B intensity ratio
Volume Average (boundary/ Steel Chemical composition (mass %)
Tempering % of M block size interior; No. C Si Mn Al Mo B Others
temperature phase (.mu.m) mass ppm) 1-4 0.43 0.71 0.65 -- 0.40
0.0020 -- 170.degree. C. 96 0.90 15, 2.0 5-1 0.43 0.71 0.65 -- 0.40
0.0020 -- 260.degree. C. 94 0.91 15, 1.9 5-2 0.43 0.71 0.65 -- 0.40
0.0020 -- 380.degree. C. 96 0.92 14, 1.6 5-3 0.43 0.71 0.65 -- 0.40
0.0020 -- 450.degree. C. 96 0.94 14, 1.1 1-12 0.44 0.75 0.66 --
0.31 0.0019 -- 170.degree. C. 95 0.90 15, 2.0 5-4 0.44 0.75 0.66 --
0.31 0.0019 -- 260.degree. C. 94 0.91 15, 1.8 5-5 0.44 0.75 0.66 --
0.31 0.0019 -- 380.degree. C. 95 0.93 15, 1.5 5-6 0.44 0.75 0.66 --
0.31 0.0019 -- 450.degree. C. 94 0.93 14, 1.2 Results Charpy test
(J) Tensile Total Test Test Steel strength elogation temperature:
temperature: No. (MPa) (%) -40.degree. C. 40.degree. C. Remarks 1-4
2230 12.5 42.7 45.3 In. Ex. 5-1 2150 15.0 45.6 50.5 In. Ex. 5-2
2085 18.0 50.5 55.0 In. Ex. 5-3 2050 20.0 14.0 18.5 Com. Ex. 1-12
2080 12.0 43.5 46.6 In. Ex. 5-4 2050 13.5 44.2 47.3 In. Ex. 5-5
2030 14.5 44.8 48.2 In. Ex. 5-6 2005 16.0 12.2 16.4 Com. Ex. * The
underlined items are beyond the scope of the present invention.
[0106] TABLE-US-00006 TABLE 6 Steel Chemical composition (mass %)
Tempering Number of revolutions before No. C Si Mn Al Mo B Others
temperature fracturing (.times.10,000 revolutions) Remarks 1-1
0.0025 0.002 0.006 0.084 4.87 0.0007 Ni: 17.99 -- 780, 750, 725
Con. Ex. Co: 10.44 Cr: 0.19 Ti: 0.48 1-2 0.20 0.74 0.65 -- 0.39
0.0019 -- 170.degree. C. 110, 105, 120 Com. Ex. 1-3 0.35 0.72 0.66
-- 0.41 0.0020 -- 170.degree. C. 720, 715, 720 In. Ex. 1-4 0.43
0.71 0.65 -- 0.40 0.0020 -- 170.degree. C. 750, 715, 760 In. Ex.
1-5 0.55 0.72 0.64 -- 0.40 0.0021 -- 170.degree. C. 450, 400, 430
Com. Ex. 1-6 0.44 0.31 0.64 -- 0.40 0.0019 -- 170.degree. C. 730,
725, 710 In. Ex. 1-7 0.43 0.95 0.65 -- 0.39 0.0022 -- 170.degree.
C. 710, 720, 720 In. Ex. 1-8 0.44 1.12 0.64 -- 0.41 0.0020 --
170.degree. C. 500, 510, 490 Com. Ex. 1-9 0.43 0.71 1.10 -- 0.40
0.0020 -- 170.degree. C. 745, 720, 715 In. Ex. 1-10 0.43 0.72 1.66
-- 0.41 0.0018 -- 170.degree. C. 480, 410, 405, Com. Ex. 1-11 0.42
0.71 0.65 -- 0.25 0.0018 -- 170.degree. C. 125, 110, 130 Com. Ex.
1-12 0.44 0.75 0.66 -- 0.31 0.0019 -- 170.degree. C. 700, 715, 725
In. Ex. 1-13 0.43 0.73 0.64 -- 0.49 0.0020 -- 170.degree. C. 750,
740, 720 In. Ex. 1-14 0.43 0.72 0.64 -- 0.60 0.0020 -- 170.degree.
C. 740, 725, 730 Com. Ex. 1-15 0.43 0.71 0.66 -- 0.41 0.0002 --
170.degree. C. 120, 165, 110 Com. Ex. 1-16 0.44 0.70 0.65 -- 0.42
0.0060 -- 170.degree. C. 750, 735, 720 In. Ex. 5-1 0.43 0.71 0.65
-- 0.40 0.0020 -- 260.degree. C. 720, 735, 715 In. Ex. 5-2 0.43
0.71 0.65 -- 0.40 0.0020 -- 380.degree. C. 700, 705, 720 In. Ex.
5-3 0.43 0.71 0.65 -- 0.40 0.0020 -- 450.degree. C. 425, 440, 450
Com. Ex. 5-4 0.44 0.75 0.66 -- 0.31 0.0019 -- 260.degree. C. 690,
710, 720 In. Ex. 5-5 0.44 0.75 0.66 -- 0.31 0.0019 -- 380.degree.
C. 685, 690, 715 In. Ex. 5-6 0.44 0.75 0.66 -- 0.31 0.0019 --
450.degree. C. 440, 425, 430 Com. Ex. * The underlined items are
beyond the scope of the present invention.
INDUSTRIAL APPLICABILITY
[0107] A steel according to the present invention has a better
balance between tensile strength and elongation and higher
toughness than maraging steel and can therefore be used for
components that to date have been conventionally produced with
maraging steel.
* * * * *