U.S. patent application number 13/314916 was filed with the patent office on 2012-04-05 for link chain superior in low temperature toughness and heat treatment method of the same.
This patent application is currently assigned to Kito Corporation. Invention is credited to Tomoya Ishihara.
Application Number | 20120080125 13/314916 |
Document ID | / |
Family ID | 38188613 |
Filed Date | 2012-04-05 |
United States Patent
Application |
20120080125 |
Kind Code |
A1 |
Ishihara; Tomoya |
April 5, 2012 |
LINK CHAIN SUPERIOR IN LOW TEMPERATURE TOUGHNESS AND HEAT TREATMENT
METHOD OF THE SAME
Abstract
The present invention provides a link chain superior in low
temperature toughness comprised of low carbon steel, said link
chain characterized in that two-stage high frequency
quenching-tempering is performed to give a link chain which has a
martensite single phase structure, has an austenite grain size of 9
to 12, further contains in said structure a precipitate comprised
of 40 nm or smaller TiC in 90% or more and 80 nm or smaller Ti(C,N)
in 10% or less, contains AlN in not more than 0.0003%, and has a
-40.degree. C. Charpy impact value of at least 45 J.
Inventors: |
Ishihara; Tomoya;
(Nakakoma-gun, JP) |
Assignee: |
Kito Corporation
Yamanashi
JP
|
Family ID: |
38188613 |
Appl. No.: |
13/314916 |
Filed: |
December 8, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12086755 |
Jun 19, 2008 |
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PCT/JP2006/325315 |
Dec 13, 2006 |
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13314916 |
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Current U.S.
Class: |
148/622 |
Current CPC
Class: |
C21D 9/00 20130101; Y02P
10/253 20151101; C22C 38/06 20130101; C21D 1/42 20130101; C22C
38/02 20130101; Y02P 10/25 20151101; C22C 38/14 20130101; F16G
15/12 20130101; C21D 2211/008 20130101 |
Class at
Publication: |
148/622 |
International
Class: |
C21D 9/00 20060101
C21D009/00; C21D 6/02 20060101 C21D006/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2005 |
JP |
2005-366305 |
Jul 31, 2006 |
JP |
2006-208640 |
Claims
1. A heat treatment method of a link chain superior in low
temperature toughness characterized by comprising the steps of:
heating a link chain consisting of, by mass%, C:0.18 to 0.28%,
Si:0.10 to 0.30%, Mn: 1.00 to 1.70%, Cu:0.10% or less, Ni:0.40% or
less, Al:0.02 to 0.10%, Ti:0.015 to 0.10%, B:0.0003 to 0.0050%, and
a balance of Fe and unavoidable impurities, to a temperatures of
Ac.sub.3 to Ac.sub.3+150.degree. C., rapidly cooling the heated
link chain for quenching, heating the cooled link chain to a
temperature of Ac.sub.3 to Ac.sub.3+50.degree. C., and tempering
the heated link chain at a temperature of Ac.sub.1 or less, wherein
the link chain having a martensite single phase structure
containing precipitates with a ration of number of precipitates of
40 nm or smaller TiC in 90% or more, and 80 nm or smaller Ti(C,N)
in 10% or less, an austenite grain size of 9 to 12, and further
containing AlN in not more than 0.0003 mass %, wherein said link
chain has a -40.degree. C. Charpy impact value of at least 45 J,
and a tensile strength of more than 1000 N/mm.sup.2.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a U.S. divisional application filed
under 37 USC 1.53(b) claiming priority benefit of U.S. Ser. No.
12/086,755 filed in the United States on Jun. 19, 2008, which
claims the benefit under 35 U.S.C. Section 371, of PCT
International Application Number PCT/JP2006/325315, filed Dec. 13,
2006 and Japanese Application Nos. 2005-366305 filed Dec. 20, 2005
and 2006-208640 filed Jul. 31, 2006 in Japan, the contents of which
are incorporated herein by reference.
BACKGROUND
[0002] 1. Field
[0003] The present invention relates to a link chain superior in
low temperature toughness comprised of one or more links connected
together and a heat treatment method for obtaining this link
chain.
[0004] 2. Background
[0005] In the past, steel links or link chains have been given the
necessary strength and toughness by heating in an electric furnace
etc. to 900.degree. C., holding, then oil quenching them for rapid
cooling, then by tempering them at about 450.degree. C. for about 1
hour to obtain a 100% tempered martensite structure free of any
ferrite structure and having a grain size of a relatively coarse
6.5 or so and to obtain characteristics of a hardness of 54 HRC, a
breakage stress of 930 N/mm.sup.2, and a -40.degree. C. toughness
value of 0.034 J/mm.sup.3. However, link chains are required to be
satisfactory in both strength and toughness. With this conventional
material, it was impossible to secure a proper balance of strength
and toughness.
[0006] On the other hand, recently, the usage environments of link
chains have become tougher. For example, there is a rising need for
use under low temperature environments of -40.degree. C. or less.
Link chains are being exposed to low temperatures or ultralow
temperatures in increasing instances. Along with such changes in
the usage environments, tough performances of a tensile strength of
1000 N/mm.sup.2 and a -40.degree. C. low temperature toughness of
45 J or more are being demanded.
[0007] Attempts are being made to develop link chains meeting these
requirements. For example, Japanese Patent Publication (A) No.
2003-221621 proposes the art of making the structure of a link
chain after heat treatment a mixed structure of a 60 to 90%
tempered martensite structure and a 10 to 40% tempered ferrite
structure and making the grain size 12.5, that is, a fine
structure. To obtain the mixed structure proposed in this patent,
it is necessary to treat the link chain by high frequency induction
heating or another means in the austenite+ferrite coexisting region
by rapid heating to about 830.degree. C., then rapid cooling for
quenching, then tempering at 400.degree. C. or less. However, in
such a method, while it is possible to obtain a link chain provided
with both strength and toughness, obtaining the mixed structure of
a 60 to 90% tempered martensite structure and a 10 to 40% tempered
ferrite structure stably at all times requires strict temperature
control, so this really cannot be said to be a practical means.
DISCLOSURE OF THE INVENTION
[0008] The present invention provides a link chain having a fine
austenite grain size after heat treatment, superior in strength and
toughness by strict control of the precipitates contained in the
link chain, and giving a high impact value even under a -40.degree.
C. low temperature environment and a heat treatment method of the
same. It has as its gist the following:
[0009] (1) A link chain superior in low temperature toughness
comprised of low carbon steel, said link chain characterized in
that said link chain has a martensite single phase structure, has
an austenite grain size of 9 to 12, and has a -40.degree. C. Charpy
impact value of at least 45 J.
[0010] (2) A link chain superior in low temperature toughness
comprised of low carbon steel, said link chain characterized in
that two-stage high frequency quenching-tempering is performed to
give a link chain which has a martensite single phase structure,
has an austenite crystal grain size of 9 to 12, further contains in
said structure a precipitate comprised of 40 nm or smaller TiC in
90% or more and 80 nm or smaller Ti(C,N) in 10% or less, contains
AlN in not more than 0.0003%, and has a -40.degree. C. Charpy
impact value of at least 45 J.
[0011] (3) A link chain superior in low temperature toughness as
set forth in (1) or (2), characterized-in that said link chain
comprises, by mass %, C: 0.18 to 0.28%, Si: 0.10 to 0.30%, Mn: 1.00
to 1.70%, Cu: 0.10% or less, Ni: 0.40% or less, Al: 0.02 to 0.10%,
Ti: 0.015 to 0.10%, B: 0.0003 to 0.0050%, and a balance of Fe and
unavoidable impurities.
[0012] (4) A heat treatment method of a link chain superior in low
temperature toughness characterized by heating a link chain
comprised of low carbon steel to a temperature of Ac.sub.3 to
Ac.sub.3+150.degree. C., then rapid cooling it to quench it, then
heating it to a temperature of Ac.sub.3 to Ac.sub.3+50.degree. C.,
then rapid cooling it to quench it, and further tempering it at a
temperature of Ac.sub.1 or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1(a) is a view showing the state of distribution of the
precipitated size of TiC, while FIG. 1(b) is a view showing the
state of distribution of the precipitated size of Ti(C,N).
[0014] FIG. 2 is a view showing the ratio of precipitation of TiC
and Ti(C,N).
BEST MODE FOR CARRYING OUT THE INVENTION
[0015] The present invention provides a link chain having a
superior low temperature toughness of a -40.degree. C. Charpy
impact value of 45 J or more by treating a link chain produced from
low carbon steel (MnB steel) wire by a two-stage
quenching-tempering treatment so as to make the link chain
structure a martensite single phase and to make the austenite grain
size a finer 9 to 12 and a heat treatment method of the same.
[0016] First, as the link chain wire used in the present invention,
a low carbon steel (MnB steel) wire is preferable. Wire having a
composition, by mass %, of C: 0.18 to 0.28%, Si: 0.10 to 0.30%, Mn:
1.00 to 1.70%, Cu: 0.10% or less, Ni: 0.40% or less, Al: 0.02 to
0.10%, Ti: 0.015 to 0.10%, B: 0.0003 to 0.0050%, and a balance of
Fe and unavoidable impurities was used as the starting material.
That is, this MnB steel is increased in strength by raising the C,
Si, and Mn and including B for raising the quenchability.
Furthermore, it is made to include Al for preventing coarsening of
the grains and is made to include Ti for securing the efficacy of
the B. Further, to secure the toughness, it is preferable to use
MnB reducing the P, S, and Cu and further, considering the economic
merits, greatly reducing the Ni to be substantially less than 0.4
by mass % or almost not present in the MnB used. Below, the reasons
for limitation of the amounts of the above composition will be
explained.
[0017] C is an element essential for raising the strength of the
wire material of the chain link. If less than 0.18%, sufficient
strength cannot be obtained, while if over 0.28%, cementite
precipitates at the grain boundaries and, further, the toughness is
remarkably lowered.
[0018] Si is an element which solid-solves in steel as a
substitution type element and is effective for raising the strength
and heat resistance of the wire material. If less than 0.10%, the
effect is not sufficient, while if over 0.30%, it will not be able
to completely dissolve in the steel and will cause the workability
to remarkably deteriorate.
[0019] Mn improves the quenchability, immobilizes the S, and
prevents problems unique to S. If less than 1.00%, the effect is
almost completely absent, while if over 1.70%, the toughness
falls.
[0020] Cu degrades the hot workability, so should be as small as
possible and is made not more than 0.10%. Ni is preferably
substantially not contained at all, but is included as an impurity.
The upper limit is made not more than 0.40%, preferably not more
than 0.1 by mass %.
[0021] Al forms AlN and makes the grains finer. If less than 0.02%,
that effect does not appear, while if over 0.10%, it connects with
N to form nitrides, produces a large amount of AlN, and further
obstructs the effect of increasing the amount of dissolved N, so
the content is made not more than 0.10%.
[0022] Ti is an element forming carbides (TiC) and carbonitrides
(Ti(C,N)) and is effective for making the grains finer. Further, it
raises the efficacy of B. If less than 0.015%, this effect is
small, while if over 0.10%, hard high melting point inclusions of
TiO are produced and the workability is degraded, so 0.10% is made
the upper limit.
[0023] B is an element essential for raising the quenchability. If
0.0003% or more, that effect is exhibited, but if added over
0.0050%, it forms a large amount of BN and degrades the
workability.
[0024] Note that P and S lower the ductility, so are preferably as
small as possible. The upper limits are 0.02%.
[0025] Steel links or a plurality of these are connected to a link
chain are produced by welding this wire. In the present invention,
the thus formed steel links or link chain connecting a plurality of
these is quenched-tempered, preferably two-stage quenched-tempered,
that is, the link chain is heated to a temperature of Ac.sub.3 to
Ac.sub.3+150.degree. C., then rapidly cooled to quench it, then is
heated to a temperature of Ac.sub.3 to Ac.sub.3+50.degree. C., then
rapidly cooled for quenching and further is tempered at a
temperature of Ac.sub.1 or less.
[0026] The first stage heat treatment uses a high frequency
induction heating, electric heating, laser heating, or another
device to rapidly heat the chain to an austenite single phase
region of Ac.sub.3 to Ac.sub.3+150.degree. C., specifically
930.degree. C. or less, then rapidly cools it to quench it. The
reason for setting the above temperature region in this rapid
heating is to make an austenite single phase in a solid-solved
state with C uniformly diffused in this temperature region. Above
this temperature, the austenite grains become coarser and it
becomes difficult to secure an austenite grain size of 9 to 12
after the subsequent step of quenching. Further, with heating at
below the above temperature, the result becomes not an austenite
single phase region, but a coexisting region of austenite+ferrite.
Note that the heating time is not particularly limited, but if
considering the time for passing through the induction heating
coil, it is about 10 to 40 sec. The thus rapidly heated link chain
is quenched from the above austenite single phase region to
ordinary temperature. The coolant used in this quenching is oil,
water, mist, etc. Note that in the present invention, to ease the
internal stress at the time of this first stage quenching or to
prevent season cracking, the chain may also be tempered.
[0027] Next, after the above first stage heat treatment, as second
stage heat treatment, the chain is rapidly heated to a temperature
of Ac.sub.3 to Ac.sub.3+50.degree. C., specifically 830.degree. C.
or less, by similar means as the heating device used in the first
stage. The reason for making the temperature of the second stage
rapid heating Ac.sub.3 to Ac.sub.3+50.degree. C. is obtain a
uniform austenite single phase structure without causing coarsening
of the grains and, further, to secure an austenite grain size of 9
to 12.
[0028] The above-mentioned two-stage quenched link chain is then
treated to stabilize the steel structure and to restore the
toughness and obtain the desired strength and hardness by tempering
it at a temperature of the Ac.sub.1 or less, preferably
200.degree., for about 1 hour to make the link chain a martensite
structure.
[0029] The thus treated link chain had a structure in which 40 nm
or smaller TiC precipitates were contained in 80% or more and 80 nm
or smaller Ti(C,N) precipitates were contained in 20% or less in a
good balance. If the size of the TiC is over 40 nm and the size of
the Ti(C,N) is over 80 nm, coarse TiC and Ti(C,N) precipitate and
the austenite grain size of 9 to 12 prescribed in the present
invention can no longer be stably obtained.
EXAMPLES
[0030] Link chains produced from a low carbon steel (MnB steel)
wire of a wire diameter of 16.0 mm having the chemical compositions
shown in Table 1 were, as a comparative example, treated by the
conventional single-stage quenching-tempering and, according to the
present invention, treated by two-stage quenching-tempering. The
characteristics obtained were compared.
[0031] First, as the single-stage quenching-tempering of the
comparative example, a high frequency induction heating device was
used to rapidly heat the link chain to the 930.degree. C. austenite
region, then the chain was rapidly cooled from the austenite single
phase for quenching, then was tempered at 200.degree. C. for 1
hour. On the other hand, as an example of the present invention, 1)
high frequency induction heating was used to rapidly heat the link
chain to the 930.degree. C. austenite region, then the chain was
rapidly cooled from the austenite single phase for quenching, 2)
next, high frequency induction heating was used to further rapidly
heat the link chain to 830.degree. C., then the chain was rapidly
cooled from the austenite single phase for second quenching, 3)
furthermore, the chain was tempered at 200.degree. C. for 1 hour.
This two-stage quenching-tempering was performed to make the
structure of the link chain a martensite structure. A sample was
extracted from the straight shank part of the heat treated link
chain obtained in this way. A Charpy test piece (JIS V-notch test
piece) was extracted from the straight shank part of the link
chain, then the metallic structure was investigated and an impact
test was conducted. The results of examination of the metal
structure are shown in FIGS. 1(a), (b) and FIG. 2.
[0032] FIG. 1(a) shows the results of analysis of the state of
distribution of the precipitated size of TIC by a transmission type
electron microscope (TEM), while FIG. 1(b) shows the results of
analysis of the state of distribution of the precipitated size of
Ti(C,N) by a TEM. As will be understood from FIGS. 1(a) and (b), in
the present invention, almost no TiC particles of over 40 nm size
precipitated, while similarly almost no Ti(C,N) particles of over
80 nm size precipitated. Further, as shown in FIG. 2, the ratio of
distribution of precipitation of TiC and Ti(C,N) is TiC: 92% and
Ti(C,N): 8% or overwhelmingly TiC. The austenite grain size is
believed to be unambiguously dependent on the state of distribution
of precipitation of TiC and its ratio.
[0033] Next, the above Charpy test pieces were subjected to an
impact test. The results are shown in Table 2 and Table 3. As will
be understood from Table 2 and Table 3, the Charpy impact value (J)
at -40.degree. C. was a high value of 72.7 J, while the value due
to the conventional single-stage heat treatment was an extremely
low value of 19.0 J. It will be understood that the two-stage heat
treatment according to the present invention gives a superior low
temperature toughness of 2 to 3 times that of the past.
TABLE-US-00001 TABLE 1 Chemical Compositions of Test Materials (by
mass %) Material C Si Mn P S Cu Ni Cr Mo Al Ti N B Range of 0.18 to
0.10 to 1.00 to 0.02 or 0.02 or 0.10 or 0.10 or Not Not 0.02 to
0.015 to Not 0.0003 to ingredient 0.28 0.30 1.70 less less less
less limited limited 0.10 0.10 limited 0.0050 Measured 0.26 0.12
1.38 0.011 0.005 0.01 0.02 0.03 0.029 0.029 0.0031 0.0022 value
TABLE-US-00002 TABLE 2 Chain Performance Breakage Total stress
elongation Hardness Austenite (N/mm.sup.2) at break (%) (Hv) grain
size Comp. ex. 1039 25.5 481 8 to 9 Inv. ex. 1025 25.3 478 10 to
11
TABLE-US-00003 TABLE 3 Results of Charpy Impact Test (J)
-80.degree. C. -60.degree. C. -40.degree. C. -20.degree. C.
0.degree. C. +20.degree. C. +40.degree. C. +60.degree. C. Comp. ex.
12.3 17.7 19.0 32.3 45.3 60.0 75.3 79.3 Inv. ex. 44.7 62.7 72.7
82.3 107.7 117.0 130.3 129.0
INDUSTRIAL APPLICABILITY
[0034] According to the present invention, it becomes possible to
provide, at low cost, a link chain having high impact value even in
a usage environment under a low temperature condition of
-40.degree. C. by making the structure a martensite structure with
the finer grains of the link chain.
* * * * *