U.S. patent application number 15/286065 was filed with the patent office on 2017-01-26 for high-strength spring steel.
This patent application is currently assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). The applicant listed for this patent is KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). Invention is credited to Tomotada MARUO, Sayaka NAGAMATSU, Nao YOSHIHARA.
Application Number | 20170022580 15/286065 |
Document ID | / |
Family ID | 44195692 |
Filed Date | 2017-01-26 |
United States Patent
Application |
20170022580 |
Kind Code |
A1 |
NAGAMATSU; Sayaka ; et
al. |
January 26, 2017 |
HIGH-STRENGTH SPRING STEEL
Abstract
Provided is a spring steel that contains 0.15-0.40% carbon,
1-3.5% silicon, 0.20-2.0% manganese, 0.05-1.20% chromium, at most
0.030% phosphorus, at most 0.02% sulfur, and at least one of the
following: 0.005-0.10% titanium, 0.005-0.05% niobium, and at most
0.25% vanadium. The remainder of said spring steel comprises iron
and unavoidable impurities. The carbon equivalent (Ceq.sub.1) of
the provided spring steel, as calculated by formula (1), is at most
0.55.
Ceq.sub.1=[C]+0.108.times.[Si]-0.067.times.[Mn]+0.024.times.[Cr]-0.05.ti-
mes.[Ni]+0.074.times.[V] (1) (In the formula (1), each symbol in
brackets represents the content (mass %) of the corresponding
element.)
Inventors: |
NAGAMATSU; Sayaka;
(Kobe-shi, JP) ; MARUO; Tomotada; (Kobe-shi,
JP) ; YOSHIHARA; Nao; (Kobe-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) |
Kobe-shi |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(KOBE STEEL, LTD.)
Kobe-shi
JP
|
Family ID: |
44195692 |
Appl. No.: |
15/286065 |
Filed: |
October 5, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13511541 |
May 23, 2012 |
|
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PCT/JP2010/073003 |
Dec 21, 2010 |
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15286065 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/34 20130101;
C21D 7/06 20130101; C22C 38/02 20130101; C22C 38/50 20130101; C22C
38/42 20130101; C21D 8/065 20130101; C21D 1/02 20130101; C22C 38/04
20130101; C21D 9/02 20130101; C22C 38/46 20130101; C22C 38/002
20130101; C22C 38/54 20130101 |
International
Class: |
C21D 9/02 20060101
C21D009/02; C21D 8/06 20060101 C21D008/06; C22C 38/54 20060101
C22C038/54; C22C 38/00 20060101 C22C038/00; C22C 38/46 20060101
C22C038/46; C22C 38/42 20060101 C22C038/42; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C21D 1/02 20060101
C21D001/02; C22C 38/50 20060101 C22C038/50 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 2009 |
JP |
2009-291143 |
Claims
1. A spring steel wire rod omitting tempering, the spring steel
wire rod comprising: C: 0.15-0.33% (means mass %, hereinafter the
same); Si: 1.9-3.5%; Mn: 0.20-1.5% at least one element selected
from a group consisting of Ti: 0.005-0.10%, Nb: 0.005-0.05% and V:
0.25% or less; Cr: 0.05-1.20%; P: 0.030% or less; S: 0.02% or less;
and iron and unavoidable impurities, wherein: the spring steel wire
rod comprises no Mo; and a carbon equivalent Ceq.sub.1 of the
spring steel wire rod expressed by formula (1) below is 0.55 or
less:
Ceq.sub.1=[C]+0.108.times.[Si]-0.067.times.[Mn]+0.024.times.[Cr]-0.05.tim-
es.[Ni]+0.074.times.[V] (1) with each symbol in brackets
representing the content (mass %) of the corresponding element.
2. The spring steel wire rod according to claim 1, further
comprising: Ni: 0.05-2% and Cu: 0.05-0.50%.
3. The spring steel wire rod according to claim 2, further
comprising: Ni: 0.15-2%.
4. The spring steel wire rod according to claim 1, comprising at
least one element selected from a group consisting of Ti:
0.035-0.10%, Nb: 0.005-0.05%, and V: 0.05-0.25%, wherein a grain
size number of the spring steel wire rod after quenching is 7.5 or
above.
5. The spring steel wire rod according to claim 1, further
comprising: B: 0.005% or less.
6. A method for manufacturing a spring excellent in corrosion
fatigue property, the method comprising: hot-coiling the spring
steel wire rod of claim 1; quenching; and setting thereafter while
omitting tempering.
7. The spring steel wire rod according to claim 1, comprising: Si:
2.01-3.55%.
8. The spring steel wire rod according to claim 1, consisting of:
C: 0.15-0.33% (means mass %, hereinafter the same) Si: 1.90-3.5%;
Mn: 0.20-1.5%; at least one element selected from a group
consisting of Ti: 0.005-0.10%, Nb: 0.005-0.05% and V: 0.25% or
less; Cr: 0.05-1.20%; P: 0.030% or less; S: 0.02% or less; iron and
unavoidable impurities; and optionally at least one of Ni: 2% or
less, Cu: 0.50% or less, and B: 0.005% or less, wherein a carbon
equivalent Ceqx of the spring steel wire rod according to the
following formula is 0.55 or less:
Ceq1=[C]+0.108.times.[Si]-0.067.times.[Mn]+0.024.times.[Cr]-0.05.times.[N-
i]+0.074.times.[V], with each symbol in brackets representing the
content (mass %) of the corresponding element.
9. The spring steel wire rod according to claim 8, consisting of:
C: 0.15-0.33% (means mass %, hereinafter the same); Si: 1.90-3.5%;
Mn: 0.20-1.5%; at least one element selected from a group
consisting of Ti: 0.005-0.10%, Nb: 0.005-0.05% and V: 0.25% or
less; Cr: 0.05-1.20%; P: 0.030% or less; S: 0.02% or less; Ni:
0.15-2%; iron and unavoidable impurities; and optionally at least
one of Cu: 0.50 or less, and B: 0.005% or less,
10. The spring steel wire rod according to claim 8, consisting of
C: 0.15-0.33% (means mass %, hereinafter the same) Si: 1.90-3.5%;
Mn: 0.20-1.5%; at least one element selected from a group
consisting of Ti: 0.035-0.10%, Nb: 0.005-0.05%, and V: 0.05-0.25%;
Cr: 0.05-1.20%; P: 0.030% or less; S: 0.02% or less; iron and
unavoidable impurities; and optionally at least one of Ni: 2% or
less, Cu: 0.50 or less, and B: 0.005% or less, wherein a grain size
number of the spring steel wire rod after quenching is 7.5 or
above.
11. The spring steel wire rod according to claim 1, comprising: Si:
2.01-3.5%,
12. A method for manufacturing a spring excellent in corrosion
fatigue property, the method comprising: hot-coiling the spring
steel wire rod of claim 8: quenching; and setting thereafter while
omitting tempering.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. patent
application Ser. No. 13/511,541, which was filed on May 23, 2012.
U.S. patent application Ser. No. 13/511,541 was a National Stage of
PCT/JP2010/073003, which was filed on Dec. 21, 2010. This
application is based upon and claims the benefit of priority to
Japanese Application No. 2009-291143, which was filed on Dec. 22,
2009.
TECHNICAL FIELD
[0002] The present invention relates to a spring steel useful as a
raw material for a coil spring, and relates specifically to a
spring steel used in manufacturing a coil spring and having the
tensile strength of 1,900 MPa class under a condition as
quenched.
BACKGROUND ART
[0003] With respect to a spring used for automobiles and the like
(suspension spring and the like), weight reduction is required in
order to reduce the exhaust gas and to improve the fuel
consumption, and, as a part thereof, high-strengthening is directed
to. In a high-strengthened spring, the sensitivity against a defect
rises, breakage from a corrosion pit caused by adhesion of a snow
melting agent is liable to occur for example, and early breakage
due to corrosion fatigue becomes a problem. Therefore, a spring
with high strength and excellent in corrosion fatigue property has
been required. For example, "UHS1900" developed by the present
applicant previously as a spring steel can achieve excellent
corrosion fatigue property although the tensile strength is as high
as 1,900 MPa class by performing quenching and tempering after
coiling into a spring shape. Accordingly, a spring obtained from
the spring steel can satisfy both the high strength and excellent
corrosion fatigue property.
[0004] In general, such spring is manufactured by drawing of a
spring steel (wire rod), cold-finishing, thereafter heating,
hot-coiling, then quenching, tempering, and setting. The quenching
and tempering treatment after hot-coiling is performed in order to
adjust the strength of the spring. In the heat treatment such as
the quenching and tempering treatment, much CO.sub.2 is discharged.
In recent years however, aiming to reduce the load against the
earth environment, reduction of CO.sub.2 has been strongly required
as one of the global warming preventive measures. Therefore, even
in the manufacturing steps for a coil spring, reduction of the
discharge amount of CO.sub.2 has been required.
[0005] Also, in the patent document 1, a steel for a stabilizer is
proposed in which the toughness in the ordinary temperature and the
toughness in the low temperature are improved by performing
water-quenching soon after hot forming and being left as
water-quenched without performing tempering. The steel for a
stabilizer is featured that the componential composition is
adjusted to a low C-high Mn--Cr system or a low C-high Mn--B--Cr
system added with one element or two elements or more of Ti, V, Nb.
The stabilizer which is the object of the patent document 1 is
different in the technical field from that of a coil spring. For
example, the strength level is 800 MPa class where the corrosion
fatigue property does not become a problem, and the stabilizer is
not related with the spring of a high strength range (1,900 MPa
class for example) which is required to satisfy the corrosion
fatigue property simultaneously.
[0006] Also, in general, the strength of iron and steel materials
increases as the hardness increases, and, as the hardness
increases, the toughness drops. That is, when the strength of the
iron and steel materials increases, the toughness drops, however,
as a material for a spring, a fracture property capable of enduring
a severe use environment of a spring is required, and it becomes
necessary to secure the toughness, or the low temperature toughness
in particular which becomes important when used in a cold weather
region, even in springs such as a high-strengthened suspension
spring and the like.
[0007] For example, the patent document 2 discloses that the
ductility and toughness were improved in a high-strength spring
steel by adjusting various compositions, and the patent document 3
discloses that a spring steel having both of the hardness and
toughness was obtained by adjusting various compositions. However,
both of the patent documents 2 and 3 focused only on the toughness
in the ordinary temperature, and did not consider the low
temperature toughness. Usually, the toughness in the low
temperature is inferior to the toughness in the ordinary
temperature, and considering the ordinary temperature toughness
disclosed in the patent documents 2 and 3, the low temperature
toughness in the technology of the patent documents 2 and 3 is
insufficient.
DOCUMENT ON PRIOR ART
Patent Document
[0008] [Patent Document 1] Japanese Patent No. 4406341 [0009]
[Patent Document 2] Japanese Patent No. 3577411 [0010] [Patent
Document 3] Japanese Patent No. 3246733
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0011] The present invention was developed considering such
circumstances as described above, and its object is to provide a
spring steel capable of manufacturing a coil spring achieving both
of the high strength and excellent corrosion fatigue property as
well as excellent in the low temperature toughness even when the
tempering treatment after quenching is omitted in working into the
coil spring. Also, another object of the present invention is to
provide a spring obtained from the spring steel.
Means for Solving the Problem
[0012] A high-strength spring steel in relation with the present
invention that can solve the problems contains C: 0.15-0.40% (means
mass %, hereinafter the same), Si: 1-3.5%, Mn: 0.20-2.0%, and
contains at least one element selected from a group consisting of
Ti: 0.005-0.10%, Nb: 0.005-0.05% and V: 0.25% or less, Cr:
0.05-1.20%, P: 0.030% or less, S: 0.02% or less, with the remainder
including iron and unavoidable impurities, in which a carbon
equivalent Ceq.sub.1 expressed by a formula (1) below is 0.55 or
less.
Ceq.sub.1=[C]+0.108.times.[Si]-0.067.times.[Mn]+0.024.times.[Cr]-0.05.ti-
mes.[Ni]+0.074.times.[V] (1)
(In the formula (1), each symbol in brackets represents the content
(mass %) of the corresponding element.)
[0013] The spring steel of the present invention may contain (a)
0.05-2% and Cu: 0.05-0.50%, (b) Ni: 0.15-2% and Cu: 0.05-0.50%, (c)
B: 0.005% or less and/or Mo: 0.60% or less according to the
necessity.
[0014] Further, it is also preferable that the spring steel of the
present invention contains at least one element selected from a
group consisting of Ti: 0.035-0.10%, Nb: 0.005-0.05%, and V:
0.05-0.25%, and the grain size number after quenching is 7.5 or
above.
[0015] When the spring steel is used, the spring steel is
hot-coiled, is quenched, and is thereafter subjected to setting
while omitting tempering, a spring achieving both of the properties
can be manufactured.
Effects of the Invention
[0016] In the spring steel of the present invention, the element
amount of a specific alloy element and the balance of mixing are
properly controlled, therefore in manufacturing a coil spring using
the spring steel, the tempering treatment after quenching can be
omitted, and a spring achieving both of the high strength and
excellent corrosion fatigue property and having excellent low
temperature toughness in a state as quenched can be
manufactured.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a graph showing the relation between the carbon
equivalent (Ceq.sub.1) and the time until the hydrogen
embrittlement cracking of a specimen obtained in the example 1.
[0018] FIG. 2 is a graph showing the relation between the tensile
strength and the low temperature toughness (vE.sub.-50) of a
specimen obtained in the example 2.
BEST MODE FOR CARRYING OUT THE INVENTION
[0019] In manufacturing a spring by coiling a spring steel, the
present inventors have repeatedly carried out intensive studies in
order to provide a spring steel capable of manufacturing a spring
achieving both of the high strength and excellent corrosion fatigue
property as well as excellent in the low temperature toughness
while omitting the tempering treatment after quenching performed
after coiling. As a result, it was found out that, when the kind of
the fundamental alloy elements contained in a spring steel was
limited to C, Si, Mn, Cr, and at least an element out of Ti, Nb and
V, or was limited to one in which the group of these elements was
added further with (i) Ni and Cu or added further with (ii) B
and/or Mo, and Si and Mn were positively contained while reducing
the amount of C, Ti, Nb, V, Cr, Ni and Cu out of these elements as
much as possible, the tempering treatment after quenching could be
omitted in manufacturing a spring by coiling the spring steel, and
a spring achieving both of the tensile strength of 1,900 MPa class
and excellent corrosion fatigue property in a state as quenched as
well as excellent in the low temperature toughness also achieved by
more strictly adjusting the content of Ti, Nb and V could be
provided, and the present invention was completed.
[0020] The spring steel of the present invention is characterized
that the C amount in particular is reduced than the C amount used
for an ordinary spring steel. By reducing the C amount, the amount
of carbide precipitated in steel can be reduced, and therefore
tempering after quenching which is performed in a usual spring
manufacturing process can be omitted. That is, as described above,
the spring is manufactured usually by drawing of a spring steel
(wire rod), cold-finishing, thereafter heating, hot-coiling, then
quenching, tempering, and setting. According to the necessity, shot
peening is performed after setting, and painting is performed
thereafter. However, because the C amount of the spring steel of
the present invention is reduced, the strength of the spring can be
secured even when setting is performed while omitting tempering
after quenching.
[0021] On the other hand, in the spring steel of the present
invention, Si and Mn are positively added. Si and Mn are easily
obtainable elements, and stable supply is assured even when the
amounts of Si and Mn are increased. Also, because Si and Mn have an
action of enhancing the strength without dropping the toughness,
both of the high strength and excellent corrosion fatigue property
can be achieved by adding Si and Mn positively.
[0022] In order to surely achieve both of the high strength and
excellent corrosion fatigue property based on the knowledge
described above, it is necessary to strictly stipulate the amount
of each element and to also stipulate the relation thereof. That
is, in the present invention, the componential composition of the
spring steel was designed as described below, and the carbon
equivalent Ceq.sub.1 expressed by the equation (1) below was made
0.55 or less. In the formula (1) below, each symbol in brackets
represents the content (mass %) of the corresponding element.
<Componential Composition of Spring Steel>
[0023] The spring steel contains C: 0.15-0.40%, Si: 1-3.5%, Mn:
0.20-2.0%, and contains at least one element selected from a group
consisting of Ti: 0.005-0.10%, Nb: 0.005-0.05% and V: 0.25% or
less, and Cr: 0.05-1.20%.
Ceq.sub.1=[C]+0.108.times.[Si]-0.067.times.[Mn]+0.024.times.[Cr]-0.05.ti-
mes.[Ni]+0.074.times.[V] (1)
[0024] The reason for setting the adding amount of each element and
the reason for stipulating the carbon equivalent Ceq.sub.1 are as
described below.
[0025] The reason why C is made 0.15% or more is to enhance the
quenchability and to secure the strength. Also, the reason why C is
made 0.40% or less is to prevent deterioration of the toughness and
corrosion fatigue property. The lower limit of the C amount is
preferable to be 0.2% or more, more preferably 0.25% or more, and
the upper limit of the C amount is preferable to be 0.35% or less,
more preferably 0.34% or less, and especially preferably 0.33% or
less.
[0026] The reason why Si is made 1% or more is to make Si act as a
solid solution strengthening element and to secure the strength.
When Si is below 1%, the matrix strength is insufficient. On the
other hand, when the Si amount becomes excessive, dissolution of
carbide becomes insufficient in heating for quenching, heating at a
higher temperature becomes necessary in order to make the steel
austenitic uniformly, decarbonization of the surface progresses,
and the fatigue property of the spring deteriorates. Therefore, by
making Si 3.5% or less, decarbonization described above can be
suppressed, occurrence of oxidation of the grain boundary and the
like can be suppressed, and drop of the strength due to formation
of an abnormal structure can be prevented. Si is preferable to be
1.5% or more and 3.0% or less, more preferably 1.80% or more and
2.5% or less.
[0027] By making Mn 0.20% or more, the quenchability can be
enhanced and the strength can be secured. Also, by formation of
sulfide-based inclusions, embrittlement of the grain boundary due
to S can be prevented, and the toughness and corrosion fatigue
property can be improved. Also, by making Mn 2.0% or less, a
supercooled structure is generated, and deterioration of the
toughness and corrosion fatigue property can be prevented.
Furthermore, formation and coarsening of excessive sulfide-based
inclusions can be suppressed, and deterioration of the toughness
and corrosion fatigue property can be prevented. The lower limit of
the Mn amount is preferable to be 0.5% or more, more preferably
0.80% or more, and the upper limit of the Mn amount is preferable
to be 1.8% or less, and especially preferably 1.5% or less.
[0028] The reason why Ti is made 0.005% or more is to miniaturize
the former austenite grains after quenching, to improve the
strength and proof stress ratio, and to improve the toughness and
corrosion fatigue property. By improving the toughness, the sag
resistance can be improved. Also, the reason for making Ti 0.10% or
less is to prevent precipitation of coarse inclusions (Ti-nitride
for example) and to suppress deterioration of the corrosion fatigue
property. The lower limit of the Ti amount is preferable to be
0.01% or more (especially preferable to be 0.05% or more), and the
upper limit of the Ti amount is preferable to be 0.080% or less,
more preferably 0.07% or less.
[0029] V is an element effectively acting for further enhancing the
quenchability and enhancing the strength. Also, V is an element
enhancing the toughness to contribute to improvement of the sag
resistance, and miniaturizing the grains to improve the strength
and proof stress ratio. In order to exert such actions, V is
preferable to be contained by 0.05% or more, more preferably 0.08%
or more, and further more preferably 0.1% or more. However, when V
becomes excessive, coarse carbonitride is formed, and the toughness
and corrosion fatigue property deteriorate. Accordingly, V is to be
0.25% or less, preferably 0.22% or less, and more preferably 0.2%
or less.
[0030] Nb is an element enhancing the toughness to contribute to
improvement of the sag resistance, and is an element miniaturizing
the grains to improve the strength and proof stress ratio. In order
to exert such actions, the Nb amount is to be made 0.005% or more,
preferably 0.008% or more, and more preferably 0.01% or more. On
the other hand, when the Nb amount becomes excessive, the toughness
is influenced adversely. Accordingly, the Nb amount is to be 0.05%
or less, preferably 0.04% or less, and more preferably 0.03% or
less.
[0031] Ti, V and Nb may be added solely, or may be added combining
two elements or more. The content of Ti, V and Nb is to be Ti:
0.035-0.10%, Nb: 0.005-0.05%, V: 0.05-0.25% respectively, and it is
preferable to contain at least one element thereof. Also, by
containing Ti, V and Nb by these ranges, the grain miniaturizing
effect can be effectively exerted, and the grain size number after
quenching can be made 7.5 or above, which results in exertion of
excellent low temperature toughness. The grain size number after
quenching is preferable to be 8.0 or above, more preferably 9.0 or
above. The low temperature toughness of the spring steel of the
present invention is, for example, 50 J/cm.sup.2 or more in terms
of Charpy absorbed energy at -50.degree. C., preferably 70
J/cm.sup.2 or more, and more preferably 80 J/cm.sup.2 or more.
[0032] By making Cr 0.05% or more, the steel matrix is strengthened
by strengthening solid solution, the quenchability is improved, and
the strength can be secured. Also, Cr is an element making the rust
formed on the surface layer part under a corroding condition
amorphous and dense and contributing to improvement of the
corrosion resistance. On the other hand, by making Cr 1.20% or
less, drop of the Ms point and formation of a supercooled structure
are prevented, the toughness and corrosion fatigue property can be
secured, and reduction of the strength and hardness due to
insufficient dissolution of Cr-carbide in quenching can be
prevented. Cr is preferable to be 0.1% or more and 1.10% or less,
more preferably 0.5% or more and 1.05% or less.
[0033] The remainder of the spring steel of the present invention
is substantially iron. However, it is rightly allowed that the
unavoidable impurities brought in due to the situation of the
material such as the iron material (including scraps), auxiliary
raw material, and the like as well as the manufacturing facilities
and the like are included in the steel. Out of the unavoidable
impurities, it was stipulated particularly that P was to be 0.030%
or less and S was to be 0.02% or less. The reason why such ranges
were stipulated is as described below.
[0034] The reason why P is made 0.030% or less is to suppress
segregating on the former austenite grain boundary and making the
grain boundary brittle and to prevent deterioration of the
toughness and corrosion fatigue property. P is preferable to be
0.02% or less, more preferably 0.01% or less. Although P is
preferable to be as little as possible, it is usually included by
approximately 0.001%.
[0035] The reason why S is made 0.02% or less is to prevent that
sulfide-based inclusions are formed in the steel, coarsened, and
deteriorate the corrosion fatigue property. S is preferable to be
0.015% or less, especially preferably 0.01% or less. Similar to P,
S is also preferable to be as little as possible, however it is
usually included by approximately 0.001%.
[0036] The total amount of P and S is preferable to be 0.015% or
less, more preferably 0.010% or less.
[0037] The reason why the carbon equivalent Ceq.sub.1 is made 0.55
or less is to achieve both of the strength and corrosion fatigue
property of the spring even when the tempering treatment after
quenching is omitted in manufacturing a coil spring by coiling the
spring steel. That is, the carbon equivalent Ceq.sub.1 represents
the contribution degree of an alloy element exerting influence on
the hardness after quenching, the hardness of the core part of the
spring can be secured by omitting the tempering treatment after
quenching while reducing the value, and high strengthening can be
achieved. Also, by suppressing the carbon equivalent Ceq.sub.1 to
0.55 or less, the degree of dependability of the alloy element can
be lowered, and stable supply can be ensured more. The carbon
equivalent Ceq.sub.1 is preferable to be 0.53 or less, more
preferably 0.50 or less. Also, the cost can be reduced as the
composition is designed so that the carbon equivalent Ceq.sub.1
becomes as small as possible, however, in order to achieve both of
the strength and corrosion fatigue property, it is necessary to add
alloy elements to some degree. Therefore, the lower limit of the
carbon equivalent Ceq.sub.1 is 0.30. Also, in calculation of the
formula (1) below, when there is an element not contained,
calculation should be executed assuming that the content of the
element is 0 mass %.
[0038] The spring steel of the present invention is to satisfy the
chemical componential composition and the carbon equivalent
Ceq.sub.1, however aiming to further improve the property, Ni and
Cu may be contained, and B and/or Mo may also be contained.
[0039] When Ni and Cu are to be contained (that is, using both of
Ni and Cu simultaneously), the Ni amount is to be 0.05-2%, and the
Cu amount is to be 0.05-0.50%. The reason why Ni is made 0.05% or
more is to enhance the toughness, to lower the defect sensitivity,
and to improve the corrosion fatigue property. Also, Ni has an
action of making the rust formed amorphous and dense to improve the
corrosion resistance, and has also an action of improving the
setting resistance which is important as the spring property. On
the other hand, by making Ni 2% or less, dropping of the Ms point
and formation of a supercooled structure can be prevented, and the
toughness and corrosion fatigue property can be secured. Ni is
preferable to be 0.15% or more and 2% or less, more preferably
0.18% or more and 1.5% or less, further more preferably 0.20% or
more and 1% or less, especially 0.5% or less.
[0040] Because Cu is an element electrochemically nobler than iron,
it is an element having an action of making the rust dense and
improving the corrosion resistance. Therefore, when Cu is
contained, the Cu amount is to be 0.05% or more. However, even if
Cu is added excessively, its effect saturates, and embrittlement of
the raw material due to hot rolling may be caused adversely.
Accordingly, the upper limit of the Cu amount was made 0.50% or
less. Cu is preferable to be 0.1% or more and 0.4% or less, more
preferably 0.15% or more (especially 0.18% or more) and 0.3% or
less.
[0041] B is an element enhancing the quenchability further to
enhance the grain boundary strength, enhancing the toughness to
improve the setting resistance, and making the rust formed on the
surface dense to improve the corrosion resistance. In order to
exert such actions, B is preferable to be contained by 0.0005% or
more, more preferably 0.001% or more, and further more preferably
0.0015% or more. However, when B becomes excessive, the effects
saturate and coarse carbonitride is formed, and the toughness and
corrosion fatigue property deteriorate. Therefore, B is to be
0.005% or less, preferably 0.004% or less, and more preferably
0.003% or less.
[0042] Mo is an element enhancing the toughness and contributing to
improvement of the setting resistance, and is an element securing
the quenchability and enhancing the strength and toughness of the
steel. In order to exert such actions effectively, the Mo amount is
preferable to be 0.05% or more, more preferably 0.08% or more, and
further more preferably 0.10% or more. On the other hand, even if
the Mo amount becomes excessive, the effects saturate. Therefore,
the Mo amount is preferable to be 0.60% or less, more preferably
0.50% or less, and further more preferably 0.35% or less. B and Mo
may be contained solely, or both may be used simultaneously.
[0043] As described above, the spring steel of the present
invention is characterized that the amount of each alloy element is
strictly stipulated and the relation thereof is stipulated, and
when the spring steel is used, the tempering treatment after
quenching performed after coiling can be omitted, and the spring
achieving both of the high strength of 1,900 MPa or above tensile
strength even in a state as quenched and excellent corrosion
fatigue property can be manufactured. Also, by more strictly
controlling the content of the elements having a grain
miniaturizing action (Ti, Nb and V), the low temperature toughness
can be improved. Below, a method for manufacturing a spring from
the spring steel will be described.
[0044] In manufacturing a spring from the spring steel of the
present invention, it is necessary to omit tempering after
quenching. That is, although the process is the same with that of
the prior art including the steps of drawing of a spring steel
(wire rod) satisfying the chemical componential composition,
cold-finishing, thereafter heating, hot-coiling to form into a
spring shape, and quenching, it is necessary to perform setting
while omitting tempering after quenching. Because the C amount of
the spring steel of the present invention is reduced than that of
the spring steel of the prior arts, when tempered after quenching,
the spring steel is softened excessively, and the toughness and
corrosion fatigue property deteriorate. Accordingly, it is
necessary to omit tempering after quenching.
[0045] Here, "omission of tempering" means the spring is not heated
to a temperature exceeding 350.degree. C. after quenching.
[0046] The setting may be performed either in a cold state or in a
warm state. The temperature of cold setting can be an ordinary
temperature, and the temperature of warm setting can be
approximately 200-250.degree. C.
[0047] After setting, painting may be performed after shot peening
according to the necessity. The condition of shot peening and
painting is not limited particularly, and the ordinary condition
can be employed.
[0048] The spring thus obtained can achieve both of the high
strength and excellent corrosion fatigue property, and is excellent
in the low temperature toughness as well.
[0049] The manufacturing condition of the spring steel in relation
with the present invention is not particularly limited, however in
order to make the grain size number 7.5 or above which is a
preferable aspect of the present invention, it is recommendable to
make the heating temperature before quenching 925.degree. C. or
below and to make the heating time 15 min or less. The lower limits
of the heating temperature before quenching and the heating time
are not particularly limited, however the lower limit of the
heating temperature is approximately 850.degree. C., and the lower
limit of the heating time is approximately 10 min usually.
EXAMPLES
[0050] Although the present invention will be described below more
specifically referring to examples, the present invention is not to
be limited by the examples described below, it is a matter of
course that the present invention can also be implemented with
modifications added appropriately within the range adaptable to the
purposes described previously and later, and any of them is to be
included within the technical range of the present invention.
Example 1
[0051] After the steel of the chemical componential composition
shown in Table 1 below (the remainder is iron and unavoidable
impurities) was molten by a vacuum melting furnace of 150 kg, the
steel was held at 1,200.degree. C., was thereafter hot forged into
a billet of 155 mm square, the billet was hot-rolled, and the
spring steel with 13.5 mm diameter (wire rod for a spring) was
manufactured. The wire rod for a spring was subjected to
finish-rolling so that the diameter became 12.5 mm, was thereafter
cut to the length of 70 mm, and was then quenched. Quenching was
performed by heating for 10 min at the temperature of 925.degree.
C., and thereafter being dipped into an oil bath with the
temperature of 50.degree. C. After quenching, a specimen of 10 mm
width.times.1.5 mm thickness.times.65 mm length was cut out by
machining.
[0052] No. 29 and No. 30 shown in Table 1 are the data imitating
the wire rod for a spring "UHS1900" made by Kobe Steel, Ltd., and
in No. 30 out of them, the specimen was manufactured by quenching,
thereafter tempering by being held for 1 hour at 400.degree. C.,
and then being machined in a condition the same with the above.
Table 2 shows whether tempering was performed or not.
[0053] Also, the chemical componential amount in the steel and the
calculation result of the carbon equivalent (Ceq.sub.1) calculated
from the formula (1) are shown in Table 1 below.
[0054] The strength and corrosion fatigue property of the specimen
obtained were examined as described below.
[0055] The strength and corrosion fatigue property of the specimen
were measured imitating that setting was performed in a cold state
or in a warm state. That is, when cold setting was imitated, the
specimen was used for each test as it was, and when warm setting
was imitated, the specimen heated for 60 min at 200.degree. C. was
used for each test. Table 2 below shows which of cold setting and
warm setting was imitated.
<Strength>
[0056] The strength of the specimen was evaluated by measuring the
hardness of the specimen with a Rockwell hardness tester using the
C scale. The measurement result of the C hardness is shown in Table
2 below. In the present invention, those with HRC of 51 or more are
to be evaluated to have passed.
<Corrosion Fatigue Property>
[0057] The corrosion fatigue property was evaluated by performing a
hydrogen embrittlement cracking test. In the hydrogen embrittlement
cracking test, the specimen was immersed in the aqueous solution of
a mixture of sulfuric acid (0.5 mol/L) and potassium thiocyanate
(KSCN: 0.01 mol/L) while applying the stress of 1,400 MPa on the
specimen by 4-point bending, the voltage of -700 mV which is lower
than the SCE electrode was applied using a potentiostat, and the
time until cracking occurred (hereinafter referred to as "the time
until hydrogen embrittlement cracking") was measured. The
measurement result of the hydrogen embrittlement cracking test is
shown in Table 2 below. In the present invention, those in which
the time until cracking occurs is 600 sec or more are evaluated to
have passed.
[0058] Also, the criteria of 51 or above of HRC and 600 sec or more
of the time until cracking occurs mean to have the property equal
or better than that of a suspension spring according to the prior
art (No. 30 of the Table 2 below) obtained performing tempering
after quenching.
[0059] In FIG. 1, the relation between the carbon equivalent
(Ceq.sub.1) and the time until hydrogen embrittlement cracking
(sec) is shown. In FIG. 1, the results of No. 1-15, 31, 33 were
shown by .quadrature., the results of No. 16-29, 32 were shown by ,
and the result of No. 30 (with tempering) was shown by
.largecircle..
[0060] As is clear from FIG. 1, it is known that there is a
tendency that, as the carbon equivalent (Ceq.sub.1) is reduced, the
time until hydrogen embrittlement cracking can be made longer and
the corrosion fatigue property can be improved.
[0061] From Table 2, the following study is possible.
[0062] No. 30 is an example in which tempering was performed after
quenching. In the example, the hardness of the core part was
secured, the strength was high, the time until hydrogen
embrittlement cracking was excellent, and the corrosion fatigue
property was improved. However, because the tempering treatment was
performed after quenching, the discharge amount of CO.sub.2 could
not be reduced.
[0063] Although the componential composition of No. 29 is similar
to that of the No 30, it is an example of omitting tempering after
quenching. In the example, because the tempering treatment is
omitted, the discharge amount of CO.sub.2 can be reduced, however,
because the carbon equivalent exceeds 0.55 and tempering is omitted
in spite that the amount of the alloy element is much, the hardness
of the core part becomes too hard, the toughness drops, the time
until hydrogen embrittlement cracking becomes short, and the
corrosion fatigue property deteriorates.
[0064] No. 16-28, 32 are the examples not satisfying the
requirement stipulated in the present invention, and the high
strength and excellent corrosion fatigue property were not achieved
simultaneously. That is, the carbon equivalent (Ceq.sub.1) of the
spring steel exceeded the range stipulated in the present
invention, tempering after quenching was omitted, and the discharge
amount of CO.sub.2 therefore could be reduced, however the hardness
of the core part became too hard, the toughness dropped, the time
until hydrogen embrittlement cracking became short, and the
corrosion fatigue property deteriorated.
[0065] No. 1-15, 33 are the examples satisfying the requirement
stipulated in the present invention, and both of the high strength
and excellent corrosion fatigue property were achieved. That is,
while the carbon equivalent (Ceq.sub.1) was suppressed to 0.55 or
below, the discharge amount of CO.sub.2 could be reduced because
tempering after quenching was omitted, the hardness of the core
part could be secured appropriately, and the high strength was
achieved. Also, the time until hydrogen embrittlement cracking was
long, and the corrosion fatigue property was also improved.
Furthermore, because the carbon equivalent (Ceq.sub.1) of the
spring steel was suppressed to 0.55 or below, the degree of
dependability of the alloy element can be lowered, and stable
supply can be achieved. Accordingly, it is known that, when the
spring steel of the present invention is used, a spring exerting
the property of the same level with the No. 30 imitating the
"UHS1900" or higher can be provided.
TABLE-US-00001 TABLE 1 Carbon Chemical composition (mass %)
equivalent No. C Si Mn Ni Cr Ti Cu P S V B P + S Others (mass %) 1
0.34 1.90 0.25 0.58 1.00 0.081 0.23 0.003 0.008 -- -- 0.011 0.52 2
0.33 1.76 0.21 0.40 1.07 0.061 0.28 0.002 0.004 -- -- 0.006 0.51 3
0.33 1.76 0.21 0.37 1.08 0.060 0.28 0.001 0.004 0.160 -- 0.005 0.53
4 0.34 1.77 0.21 0.33 1.06 0.060 0.28 0.001 0.005 0.158 -- 0.006
0.54 5 0.33 2.01 1.21 0.20 0.05 0.031 0.20 0.003 0.005 -- 0.002
0.008 0.46 6 0.15 2.20 1.00 0.25 0.20 0.090 0.25 0.015 0.005 -- --
0.020 0.31 7 0.29 3.00 1.70 0.20 0.05 0.032 0.20 0.003 0.005 -- --
0.008 0.49 8 0.29 2.17 0.78 0.20 0.20 0.073 0.16 0.007 0.004 -- --
0.011 0.47 9 0.29 1.76 0.21 0.30 1.06 0.061 0.28 0.001 0.003 0.161
-- 0.004 0.49 10 0.15 2.20 1.00 0.25 0.20 0.090 0.25 0.015 0.005 --
0.002 0.020 0.31 11 0.28 2.01 0.43 0.26 1.05 0.060 0.21 0.002 0.006
0.158 0.002 0.008 0.49 12 0.16 2.01 0.80 0.25 1.06 0.061 0.21 0.009
0.007 0.157 0.002 0.016 0.35 13 0.40 2.01 1.20 0.19 0.05 0.031 0.20
0.006 0.005 -- 0.002 0.011 0.53 14 0.29 2.01 1.21 0.20 0.05 0.031
0.20 0.007 0.006 -- 0.002 0.013 0.42 15 0.40 2.01 1.20 0.19 0.05
0.031 0.20 0.006 0.005 -- 0.002 0.011 0.53 16 0.60 1.73 0.88 0.08
0.20 -- 0.19 0.010 0.019 0.154 -- 0.029 Nb: 0.020, Mo: 0.03 0.74 17
0.48 1.82 0.21 0.15 1.48 0.072 0.20 0.006 0.006 0.204 -- 0.012 0.71
18 0.54 1.81 0.20 0.34 1.25 0.077 0.42 0.006 0.008 0.098 -- 0.014
0.74 19 0.45 1.86 0.15 0.50 0.96 0.050 0.19 0.003 0.003 0.104 --
0.006 0.65 20 0.48 2.08 -- 0.78 1.50 0.091 0.69 0.007 0.002 -- --
0.009 Nb: 0.002 0.70 21 0.49 2.10 0.18 0.67 1.22 0.093 0.52 0.004
0.004 -- -- 0.008 0.70 22 0.48 2.10 0.32 0.73 1.12 0.072 0.20 0.007
0.005 -- -- 0.012 0.68 23 0.48 2.08 0.36 0.70 1.06 0.075 0.19 0.003
0.004 0.100 -- 0.007 0.68 24 0.42 1.92 0.15 0.58 1.02 0.080 0.23
0.025 0.002 0.184 -- 0.027 0.63 25 0.40 2.27 1.01 0.25 0.21 0.089
0.26 0.032 0.005 -- -- 0.037 0.57 26 0.42 2.24 1.01 0.25 0.21 0.088
0.26 0.040 0.004 -- 0.002 0.044 0.59 27 0.41 2.17 0.79 0.25 0.21
0.010 0.27 0.014 0.004 -- 0.002 0.018 0.58 28 0.42 2.27 1.21 0.25
0.20 0.015 0.26 0.012 0.005 -- -- 0.017 0.58 29 0.41 1.79 0.16 0.44
1.06 0.063 0.26 0.007 0.003 0.154 -- 0.010 0.61 30 0.41 1.75 0.18
0.51 1.05 0.070 0.21 0.009 0.004 0.160 -- 0.013 0.61 31 0.38 1.71
0.85 0.06 1.02 0.052 0.21 0.003 0.006 0.104 0.009 0.54 32 0.39 2.98
1.70 0.21 0.05 0.035 0.21 0.007 0.005 0.002 0.012 0.59 33 0.32 2.06
1.22 0.05 0.038 0.008 0.006 0.014 0.46
TABLE-US-00002 TABLE 2 Time until hydrogen With/without Hardness
embrittlement cracking No. tempering Setting (HRC) (sec) 1 Without
Cold 55.9 717 2 Without Cold 55.8 613 3 Without Cold 55.6 844 4
Without Cold 56.0 797 5 Without Cold 55.3 876 6 Without Cold 51.4
1800 7 Without Cold 54.6 1797 8 Without Cold 55.5 1280 9 Without
Cold 55.3 1188 10 Without Cold 51.1 1800 11 Without Cold 55.2 1235
12 Without Cold 51.3 1800 13 Without Cold 56.2 1562 14 Without Warm
54.2 1640 15 Without Warm 56.1 1510 16 Without Cold 61.3 40 17
Without Cold 59.5 38 18 Without Cold 61.3 48 19 Without Cold 58.5
466 20 Without Cold 58.7 114 21 Without Cold 59.0 164 22 Without
Cold 58.7 144 23 Without Cold 59.7 246 24 Without Cold 56.0 226 25
Without Cold 55.7 355 26 Without Cold 55.5 128 27 Without Cold 55.9
284 28 Without Cold 56.0 411 29 Without Cold 55.8 575 30 With Cold
52.2 603 31 Without Cold 55.5 954 32 Without Cold 56.8 579 33
Without Cold 55.2 925
Example 2
[0066] After the steel of the chemical componential composition
shown in Table 3 (the remainder is iron and unavoidable impurities)
was molten by a vacuum melting furnace of 150 kg, the steel was
casted by an ingot-making method or a continuous casting method, a
billet of 155 mm square was manufactured thereafter by a blooming
mill, was hot-rolled into a wire rod with the diameter of 13.5 mm
which was made a test material. These test materials were heated
for 10 min at the temperature of 925.degree. C., and were
thereafter put into an oil bath of 50.degree. C. for quenching.
Only No. 2-24 was subjected to tempering treatment for 1 hour at
400.degree. C. after quenching.
TABLE-US-00003 TABLE 3 Carbon Chemical composition (mass %)
equivalent No. C Si Mn Cr V Ti P S Ni Cu P + S Others (mass %) 2-1
0.30 3.00 1.70 0.05 0.037 0.003 0.005 0.20 0.20 0.008 0.50 2-2 0.30
2.01 1.21 0.05 0.036 0.003 0.005 0.20 0.20 0.008 B: 0.002 0.43 2-3
0.15 2.20 1.00 0.20 0.180 0.090 0.010 0.005 0.25 0.25 0.015 0.33
2-4 0.15 2.20 1.00 0.20 0.090 0.010 0.005 0.25 0.25 0.015 B: 0.002
0.31 2-5 0.34 2.15 1.00 0.38 0.098 0.009 0.006 0.24 0.22 0.015 Nb:
0.01 0.50 2-6 0.34 2.21 1.15 0.65 0.150 0.072 0.005 0.005 0.20 0.31
0.010 Mo: 0.23 0.52 2-7 0.38 1.76 0.86 1.08 0.160 0.060 0.001 0.004
0.37 0.28 0.005 0.53 2-8 0.34 1.76 0.86 1.06 0.161 0.061 0.001
0.003 0.30 0.28 0.004 B: 0.001 0.49 2-9 0.37 1.79 0.82 1.06 0.170
0.068 0.002 0.006 0.53 0.22 0.008 0.52 2-10 0.37 1.87 0.80 0.98
0.107 0.052 0.002 0.004 0.51 0.06 0.006 0.52 2-11 0.38 1.75 0.84
1.05 0.160 0.074 0.006 0.003 0.51 0.48 0.009 0.52 2-12 0.34 1.79
0.81 1.06 0.154 0.063 0.005 0.003 0.44 0.06 0.008 0.49 2-13 0.35
1.75 0.82 1.09 0.041 0.004 0.004 0.77 0.21 0.008 0.47 2-14 0.38
1.71 0.85 1.02 0.104 0.052 0.003 0.006 0.06 0.21 0.009 0.54 2-15
0.60 1.73 0.88 0.20 0.154 0.010 0.019 0.08 0.19 0.029 Mo: 0.13, Nb:
0.02 0.74 2-16 0.52 1.89 0.15 1.01 0.179 0.078 0.003 0.023 0.29
0.23 0.026 0.74 2-17 0.46 2.18 0.79 0.21 0.071 0.005 0.004 0.009
0.65 2-18 0.46 2.20 0.41 0.21 0.072 0.007 0.004 0.30 0.011 0.66
2-19 0.45 2.18 0.77 0.069 0.005 0.005 0.20 0.31 0.010 0.62 2-20
0.40 2.21 0.82 0.15 0.005 0.005 0.20 0.31 0.010 0.58 2-21 0.30 3.00
1.70 0.05 0.010 0.005 0.20 0.20 0.015 0.50 2-22 0.30 2.01 1.21 0.05
0.012 0.005 0.20 0.20 0.017 B: 0.002 0.43 2-23 0.54 1.50 0.70 0.70
0.012 0.006 0.018 0.67 2-24 0.54 1.50 0.70 0.70 0.012 0.006 0.018
0.67 2-25 0.32 2.06 1.22 0.05 0.038 0.008 0.006 0.014 0.46 2-26
0.39 2.98 1.70 0.05 0.035 0.007 0.005 0.21 0.21 0.012 B: 0.002
0.59
<Low Temperature Toughness>
[0067] An impact test specimen with 2 mm U-notch was taken from the
test material after the quenching, and the Charpy absorbed energy
at -50.degree. C. (vE.sub.-50) was obtained according to JIS Z
2242. The tests were carried out on two pieces for each steel kind,
and the average value thereof was made the Charpy absorbed energy
of each steel kind.
<Grain Size Number>
[0068] At the D/4 position (D is the diameter of the wire rod) of
the test material after the quenching, an optional area of 15
mm.sup.2 was observed with an optical microscope (magnification:
400 times), and the grain size number was measured according to JIS
G 0551. The measurement was performed on two fields of view, and
the average value thereof was made the austenite grain size.
[0069] The result is shown in Table 4.
TABLE-US-00004 TABLE 4 Charpy absorbed Tensile strength Grain size
energy TS No. number vE.sub.-50 (J/cm.sup.2) (MPa) 2-1 7.5 67.7
1726 2-2 8.5 65.7 1534 2-3 8.7 91.8 1109 2-4 7.8 68.0 1137 2-5 9.0
60.9 1833 2-6 8.2 68.4 1710 2-7 9.5 81.3 1804 2-8 10.0 85.7 1638
2-9 10.8 85.8 1897 2-10 9.5 86.5 1982 2-11 9.0 81.6 1713 2-12 9.0
92.3 1857 2-13 8.8 94.3 1811 2-14 9.5 74.7 1747 2-15 10.5 12.2 2418
2-16 9.5 36.7 2409 2-17 8.2 38.5 2336 2-18 8.4 32.2 2422 2-19 8.2
38.5 2106 2-20 7.0 37.5 1823 2-21 6.0 25.5 1648 2-22 5.4 47.5 1457
2-23 6.3 7.0 2356 2-24 6.3 27.5 1565 2-25 8.2 72.5 1776 2-26 7.0
48.5 2349
[0070] In No. 2-1 to No. 2-14 of Table 4, because the requirement
of the present invention was satisfied and the amounts of Ti, Nb
and V were properly adjusted in particular, the steel with high
strength and excellent in the low temperature toughness could be
obtained.
[0071] On the other hand, in No. 2-15 to No. 2-24, because at least
any one of the requirement of the present invention was not
satisfied, the toughness was insufficient.
[0072] No. 2-15 to No. 2-19 are the examples in which the C amount
was excessive, and the low temperature toughness dropped because
the strength increased too high.
[0073] In No. 2-20 to No. 2-22, because none of Ti, Nb and V was
contained, the grain miniaturizing effect was not exerted, and the
low temperature toughness dropped.
[0074] Either of No. 2-23 and No. 2-24 is the steel kind equivalent
to the standardized steel 9254, and No. 2-24 was subjected to the
tempering treatment after quenching. In No. 2-23, the C amount was
much and the strength increased excessively, none of Ti, Nb and V
was contained, and therefore the low temperature toughness dropped.
Also, in No. 2-24, the strength dropped compared with No. 2-23
because the tempering treatment was performed, but the low
temperature toughness dropped, because none of Ti, Nb and V was
contained.
[0075] FIG. 2 is a graph showing the relation between the strength
and the low temperature toughness (Charpy absorbed energy at
-50.degree. C.) with respect to No. 2-1 to 2-24. In FIG. 2, the
results of No. 2-1 to 2-14, 2-25 were shown by .quadrature., the
results of No. 2-15 to No. 2-23, 2-26 were shown by , and the
result of No. 2-24 was shown by .largecircle.. According to FIG. 2,
it is known that, in all of the steel satisfying the requirement of
the present invention (shown by .quadrature. in FIG. 2), the Charpy
absorbed energy was 50 J/cm.sup.2 or more and higher toughness was
achieved than that in the steel not satisfying any of the
requirement of the present invention (shown by and .largecircle. in
FIG. 2) when compared with same strength.
* * * * *