U.S. patent application number 16/072757 was filed with the patent office on 2019-01-31 for steel.
This patent application is currently assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION. The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Atsushi MONDEN, Shinya TERAMOTO, Shingo YAMASAKI.
Application Number | 20190032178 16/072757 |
Document ID | / |
Family ID | 59624929 |
Filed Date | 2019-01-31 |
United States Patent
Application |
20190032178 |
Kind Code |
A1 |
TERAMOTO; Shinya ; et
al. |
January 31, 2019 |
STEEL
Abstract
According to an aspect of the present invention, there is
provided a steel containing, by mass %, C: 0.08% to 0.12%, Si:
0.05% to 0.50%, Mn: 1.00% to 3.00%, P: 0.040% or less, S: 0.020% or
less, Cr: 1.0% to 2.5%, Cu: 0.01% to 0.50%, Ni: 0.75% to 3.20%, Mo:
0.10% to 0.50%, Nb: 0.005% to 0.050%, Al: 0.010% to 0.100%, N:
0.0050% to 0.0150%, V: 0% to 0.300%, Ca: 0% to 0.0100%, Zr: 0% to
0.0100%, Mg: 0% to 0.0100%, and a remainder including Fe and
impurities, in which a number density of Mn sulfides having an
equivalent circle diameter of more than 5 m is 0 pieces/mm.sup.2 to
10 pieces/mm.sup.2, and an average aspect ratio of the Mn sulfides
having an equivalent circle diameter of 1.0 m to 5.0 .mu.m is 1.0
or more and 10.0 or less.
Inventors: |
TERAMOTO; Shinya; (Tokyo,
JP) ; YAMASAKI; Shingo; (Tokyo, JP) ; MONDEN;
Atsushi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL & SUMITOMO METAL
CORPORATION
Tokyo
JP
|
Family ID: |
59624929 |
Appl. No.: |
16/072757 |
Filed: |
February 19, 2016 |
PCT Filed: |
February 19, 2016 |
PCT NO: |
PCT/JP2016/054852 |
371 Date: |
July 25, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/44 20130101; C22C 38/46 20130101; C22C 38/00 20130101; C22C
38/48 20130101; C22C 38/58 20130101; C21D 8/065 20130101; C22C
38/50 20130101; C22C 38/04 20130101; C22C 38/06 20130101; C22C
38/02 20130101; C22C 38/42 20130101; C21D 8/06 20130101; C22C
38/002 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/42 20060101 C22C038/42; C22C 38/00 20060101
C22C038/00; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/06 20060101 C22C038/06 |
Claims
1. A steel comprising, by unit mass %, C: 0.08% to 0.12%; Si: 0.05%
to 0.50%; Mn: 1.00% to 3.00%; P: 0.040% or less; S: 0.020% or less;
Cr: 1.00% to 2.50%; Cu: 0.01% to 0.50%; Ni: 0.75% to 3.20%; Mo:
0.10% to 0.50%; Nb: 0.005% to 0.050%; Al: 0.010% to 0.100%; N:
0.0050% to 0.0150%; V: 0% to 0.300%; Ca: 0% to 0.0100%; Zr: 0% to
0.0100%; Mg: 0% to 0.0100%; and a remainder including Fe and
impurities, wherein a number density of Mn sulfides having an
equivalent circle diameter of more than 5 .mu.m is 0
pieces/mm.sup.2 to 10 pieces/mm.sup.2, and an average aspect ratio
of the Mn sulfides having an equivalent circle diameter of 1.0
.mu.m to 5.0 .mu.m is 1.0 or more and 10.0 or less.
2. The steel according to claim 1 comprising, by unit mass %, V:
0.010% to 0.300%.
3. The steel according to claim 1 comprising, by unit mass %, one
or more selected from the group consisting of Ca: 0.0005% to
0.0100%; Zr: 0.0005% to 0.0100%; and Mg: 0.0005% to 0.0100%.
4. The steel according to claim 2 comprising, by unit mass %, one
or more selected from the group consisting of Ca: 0.0005% to
0.0100%; Zr: 0.0005% to 0.0100%; and Mg: 0.0005% to 0.0100%.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to steel having a high
strength and an excellent low-temperature toughness after quenching
and tempering.
RELATED ART
[0002] Recently, in response to changes in energy situation, active
efforts have been made across the globe in order to develop new
energy sources. In such circumstances, offshore oil fields have
drawn attentions as sources developed onshore have been depleted,
and development using oil-drilling rigs has been conducted across a
broad range of regions, mainly, continental shelves. In particular,
recently, the number of marine structures represented by offshore
oil-drilling rigs that are operated in the depths of the sea has
been rising, and, in order to prevent damage to drilling rigs by
large-scale hurricanes, there has been a demand for increasing the
strength of chains for mooring drilling rigs. Broken chains lead
directly to serious accidents such as the collapse of rigs. In
order to ensure safety which is a vital object, an increase in both
the strength and toughness of chains has been pursued.
Specifically, there has been a demand for chains having a tensile
strength of 1,200 MPa or more and a Charpy impact value at
-20.degree. C. of 75 J/cm.sup.2 or more.
[0003] Such chains are manufactured by cutting a hot rolled steel
bar having a diameter of .phi.50 mm or more to a predetermined
length, forming the steel bar to an annular shape, and welding
butted end surfaces through flash butt welding. After flash butt
welding, there are cases where a stud is press-fitted into the
center of the annular chain. After that, the chain is quenched and
tempered, thereby imparting a high strength and a high toughness to
the chain.
[0004] Patent Documents 1 to 6 and the like can be exemplified as
invention examples of steel for a high strength and high toughness
chain. However, all of the documents aim to provide a chain having
a tensile strength of 800 MPa to 1,000 MPa and do not study a case
where the strength of steel is set to 1,200 MPa or more. In recent
years, although an additional increase in strength has been
demanded for chains, it is known that an increase in the strength
of steel generally degrades the toughness of steel and thus
decreases the impact value of steel. When the strength of the steel
proposed by the above described documents is set to 1,200 MPa or
more, it is not possible to obtain an intended impact value.
PRIOR ART DOCUMENT
Patent Document
[0005] [Patent Document 1] Japanese Unexamined Patent Application.
First Publication No. S58-22361 [0006] [Patent Document 2] Japanese
Unexamined Patent Application, First Publication No. S58-96856
[0007] [Patent Document 3] Japanese Unexamined Patent Application.
First Publication No. S59-159972 [0008] [Patent Document 4]
Japanese Unexamined Patent Application, First Publication No.
S59-159969 [0009] [Patent Document 5] Japanese Unexamined Patent
Application, First Publication No. 562-202052 [0010] [Patent
Document 6] Japanese Unexamined Patent Application, First
Publication No. S63-203752
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0011] An object of the present invention is to provide a steel
having a high strength and an excellent low-temperature toughness
(particularly, fracture toughness at a low temperature) after
quenching and tempering. Specifically, the object of the invention
is to provide a steel in which the Charpy impact value at
-20.degree. C. reaches 75 J/cm.sup.2 or more, when quenching and
tempering are carried out so that the tensile strength reaches
1,200 MPa or more.
Means for Solving the Problem
[0012] The gist of the present invention is as described below.
[0013] (1) According to an aspect of the present invention, there
is provided a steel containing, by unit mass %, C: 0.08% to 0.12%,
Si: 0.05% to 0.50%, Mn: 1.00% to 3.00%, P: 0.040% or less, S:
0.020% or less, Cr: 1.00% to 2.50%, Cu: 0.01% to 0.50%, Ni: 0.75%
to 3.20%, Mo: 0.10% to 0.50%, Nb: 0.005% to 0.050%, Al: 0.010% to
0.100%, N: 0.0050% to 0.0150%, V: 0% to 0.300%, Ca: 0% to 0.0100%,
Zr: 0% to 0.0100%, Mg: 0% to 0.0100%, and a remainder including Fe
and impurities, in which a number density of Mn sulfides having an
equivalent circle diameter of more than 5 .mu.m is 0 pieces/mm2 to
10 pieces/mm2, and an average aspect ratio of the Mn sulfides
having an equivalent circle diameter of 1.0 .mu.m to 5.0 .mu.m is
1.0 or more and 10.0 or less.
[0014] (2) The steel according to (1) may contain, by unit mass %,
V: 0.010% to 0.300%.
[0015] (3) The steel according to (1) or (2) may contain, by unit
mass %, one or more selected from the group consisting of Ca:
0.0005% to 0.0100%, Zr: 0.0005% to 0.0100%, and Mg: 0.0005% to
0.0100%.
Effects of the Invention
[0016] According to the present invention, it is possible to
provide steel having a tensile strength of 1,200 MPa or more and a
Charpy impact value at -20.degree. C. of 75 J/cm.sup.2 or more
after quenching and tempering.
EMBODIMENTS OF THE INVENTION
[0017] The present inventors have continued a variety of researches
in order to realize steel having a high strength and an excellent
low-temperature toughness, as a result, the present inventors
obtained the following findings.
[0018] (a) In order to impart a tensile strength of 1,200 MPa or
more to quenched and tempered steel, a C content in the steel needs
to be set to 0.08% or more.
[0019] (b) When steel contains all of Ni, Mo, and Nb, the
low-temperature toughness of the steel is improved. The present
inventors found that, when steel contains all of Ni, Mo, and Nb,
the impact value of the steel is improved. This is considered to be
because, when steel contains all of Ni, Mo, and Nb, cementite in
the steel which may generally act as an origin of fracture is
refined to a level at which the cementite does not act as a
fracture origin. In addition, when steel contains all of Ni, Mo,
and Nb, the block size of a martensite becomes small, and thus it
is assumed that the ductile-brittle transition temperature of the
steel is decreased and brittle fracture does not easily occur even
at a low temperature.
[0020] (c) The present inventors found that the low-temperature
toughness of steel can be improved by decreasing the grain size and
aspect ratios of Mn sulfides which may act as an origin of
fracture.
[0021] On the basis of the above described findings, the present
inventors found the chemical composition, inclusion state, and
manufacturing method for steel that can be used to manufacture a
structural component having a high strength and a high
low-temperature toughness, particularly, chains. Hereinafter, a
specific aspect of steel according to the present embodiment will
be described. In addition, although the steel according to the
present embodiment is steel having an effect in which the tensile
strength reaches 1,200 MPa or more and the Charpy impact value at
-20.degree. C. reaches 75 J/cm.sup.2 or more after quenching and
tempering, the strength and the impact value before quenching and
tempering are not particularly limited. Hereinafter, unless
particularly otherwise described, description of mechanical
properties such as strength and toughness relates to the steel
according to the present embodiment after quenching and
tempering.
[0022] Hereinafter, the reasons for limiting the amounts of
individual alloying elements of the steel according to the present
embodiment will be described. The unit "%" of the amounts of the
alloying elements indicates mass %.
[0023] C: 0.08% to 0.12%
[0024] C is an important element that determines the strength of
the steel. In order to obtain a tensile strength of 1,200 MPa or
more after quenching and tempering, the lower limit of the C
content is set to 0.08%. On the other hand, when the C content is
excessive, the strength of the steel is excessively increased, and
thus the toughness of the steel is degraded. In addition, when the
C content is excessive, the amount of cementite which acts as an
origin of fracture is increased, and the toughness of the steel is
significantly degraded. Therefore, the upper limit of the C content
is set to 0.12%. The upper limit of the C content is preferably
0.11%. The lower limit of the C content is preferably 0.09%.
[0025] Si: 0.05% to 0.50%
[0026] Si has an action for ensuring the strength of the steel and
also an action as a deoxidizing agent. When the Si content is less
than 0.05%, the deoxidizing action cannot be sufficiently obtained,
the number of non-metallic inclusions in the steel is increased,
and the toughness of the steel is degraded. On the other hand, when
the Si content is more than 0.50%, Si causes the degradation in the
toughness of the steel. Therefore, the Si content is set to 0.05%
to 0.50%. The upper limit of the Si content is preferably 0.40%,
0.30%, or 0.20%. The lower limit of the Si content is preferably
0.06%, 0.07%, or 0.08%.
[0027] Mn: 1.00% to 3.00%
[0028] Mn is an essential element for ensuring a desired
hardenability. In order to ensure sufficient hardenability for
setting a tensile strength of the steel after quenching and
tempering to 1,200 MPa or more, the lower limit of the Mn content
is set to 1.0%. On the other hand, when the Mn content is
excessive, the toughness of the steel is degraded, and thus the
upper limit of the Mn content is set to 3.00%. The upper limit of
the Mn content is preferably 2.90%, 2.80%, or 2.70%. The lower
limit of the Mn content is preferably 1.10%, 1.20%, or 1.30%.
[0029] P: 0.040% or Less
[0030] P is an impurity that is incorporated into the steel during
the manufacturing process of the steel. When the P content exceeds
0.040%, the toughness of the steel is degraded more than a
permissible limit, and thus the P content is limited to 0.040% or
less. The upper limit of the P content is preferably 0.030%,
0.025%, or 0.020%. The steel according to the present embodiment
does not need P, and thus the lower limit of the P content is 0%;
however, when the capability of a refining facility and the like
are taken into account, the lower limit of the P content may be set
to 0.001%, 0.002%, or 0.003%.
[0031] S: 0.020% or Less
[0032] S is, similar to P, an impurity that is incorporated into
the steel during the manufacturing process of the steel. When the S
content exceeds 0.020%, S forms a large amount of Mn sulfides in
the steel, and the toughness of the steel is degraded. Therefore,
the S content is limited to 0.020% or less. When the S content is
0.020% or less, the number density of the Mn sulfides is
sufficiently decreased, and the toughness of the steel is
maintained at a high level. The upper limit of the S content is
preferably 0.015%, 0.012%, or 0.010%. The steel according to the
present embodiment does not need S, and thus the lower limit of the
S content is 0%; however, when the capability of a refining
facility and the like are taken into account, the lower limit of
the S content may be set to 0.001%, 0.002%, or 0.003%.
[0033] Cr: 1.00% to 2.50%
[0034] Cr has an action for enhancing the hardenability of the
steel. In order to ensure sufficient hardenability for setting a
tensile strength of the steel after quenching and tempering to
1,200 MPa or more, the lower limit of the Cr content is set to
1.00%. On the other hand, when the Cr content is excessive, the
toughness of the steel is degraded. Therefore, the upper limit of
the Cr content is set to 2.50%. The upper limit of the Cr content
is preferably 2.40%, 2.30%, or 2.20%. The lower limit of the Cr
content is preferably 1.30%, 1.40%, or 1.50%.
[0035] Cu: 0.01% to 0.50%
[0036] Cu is an effective element for improving the hardenability
and corrosion resistance of the steel. In order to ensure
sufficient hardenability and corrosion resistance for setting a
tensile strength of the steel after quenching and tempering to
1,200 MPa or more, the lower limit of the Cu content is set to
0.01%. On the other hand, when the Cu content is excessive, the
toughness of the steel is degraded. Therefore, the upper limit of
the Cu content is set to 0.50%. The upper limit of the Cu content
is preferably 0.40%, 0.30%, or 0.20%. The lower limit of the Cu
content is preferably 0.02%, 0.03%, or 0.05%.
[0037] Ni: 0.75% to 3.20%
[0038] Ni is an extremely effective element for improving the
toughness of the steel and an essential element for increasing the
toughness of the steel according to the present embodiment in which
a tensile strength after quenching and tempering is 1,200 MPa or
more. When the Ni content is less than 0.75%, it is difficult to
sufficiently exhibit the effects. On the other hand, when the Ni
content exceeds 3.20%, the effect for improving toughness is
saturated. Therefore, the Ni content is set to 0.75% to 3.20%. The
upper limit of the Ni content is preferably 3.15%, 3.10%, or 3.05%.
The lower limit of the Ni content is preferably 0.80%, 0.85%, or
0.90%.
[0039] Mo: 0.10% to 0.50%
[0040] The present inventors found that Mo has an effect for
improving the low-temperature toughness of the steel, when Mo is
contained in the steel together with Ni and Nb. This is considered
to be because, when the steel contains Mo together with Ni and Nb,
cementite in the steel which may generally act as an origin of
fracture is refined to a level at which the cementite does not act
as a fracture origin. In addition, when the steel contains Mo
together with Ni and Nb, the block size of a martensite becomes
small, and thus it is assumed that the ductile brittle transition
temperature of the steel is decreased and brittle fracture does not
easily occur even at a low temperature. When the Mo content is less
than 0.10%, it is difficult to sufficiently exhibit the effects. On
the other hand, when the Mo content exceeds 0.50%, the effect for
improving toughness is saturated. Therefore, the Mo content is set
to 0.10% to 0.50%. The upper limit of the Mo content is preferably
0.47%, 0.45%, or 0.42%. The lower limit of the Mo content is
preferably 0.15%, 0.20%, or 0.25%.
[0041] Nb: 0.005% to 0.050%
[0042] Nb has an effect for improving the low-temperature toughness
of the steel, when Nb is contained in the steel together with Ni
and Mo. This is considered to be because, when the steel contains
Nb together with Ni and Mo, cementite in the steel which may
generally act as an origin of fracture is refined to a level at
which the cementite does not act as a fracture origin. In addition,
when the steel contains Nb together with Ni and Mo, the block size
of a martensite becomes small, and thus it is assumed that the
ductile brittle transition temperature of the steel is decreased
and brittle fracture does not easily occur even at a low
temperature. When the Nb content is less than 0.005%, it is
difficult to sufficiently exhibit the effects. On the other hand,
when the Nb content exceeds 0.050%, the effect for improving
toughness is saturated. Therefore, the Nb content is set to 0.005%
to 0.050%. The upper limit of the Nb content is preferably 0.045%,
0.040%, or 0.035%. The lower limit of the Nb content is preferably
0.007%, 0.010%, or 0.015%.
[0043] Al: 0.010% to 0.100%
[0044] In addition to a deoxidizing action, A1 has an action for
adjusting the crystal grain size of a metallographic structure and
miniaturizing the metallographic structure when A1 is precipitated
as AlN. When the A1 content is less than 0.010%, it is not possible
to obtain a sufficient miniaturizing effect, and thus the toughness
of the steel is degraded. On the other hand, when the A1 content in
the steel exceeds 0.100%, the amount of AlN precipitated is
saturated, the number of alumina based non-metallic inclusions in
the steel is increased, and the toughness of the steel is degraded.
Therefore, the A1 content is set to 0.010% to 0.100%. The upper
limit of the A1 content is preferably 0.090%, 0.070%, or 0.050%.
The lower limit of the A1 content is preferably 0.012%, 0.015%, or
0.018%.
[0045] N: 0.0050% to 0.0150%
[0046] N has an action for precipitating AlN, which is effective
for adjusting the crystal grain size of the metallographic
structure, by bonding to A1. When the N content is less than
0.0050%, this action is not sufficiently exhibited. On the other
hand, when the N content in the steel exceeds 0.0150%, the number
of solute N is increased, and the toughness of the steel is
degraded. Therefore, the N content is set to 0.0050% to 0.0150%.
The upper limit of the N content is preferably 0.0140%, 0.0130%, or
0.0120%. The lower limit of the N content is preferably 0.0055%,
0.0060%, or 0.0065%.
[0047] V: 0% to 0.300%
[0048] The steel according to the present embodiment does not need
V. Therefore, the lower limit of the V content is 0%. However, V
has an action for adjusting the crystal grain size of the
metallographic structure and miniaturizing the metallographic
structure when V is precipitated as VN. Therefore, as an optional
element, the steel may contain 0.010% or more, 0.020% or more, or
0.030% or more of V. On the other hand, when the V content in the
steel exceeds 0.300%, coarse VN remains in the steel after heating
for quenching, and this coarse VN degrades the toughness of the
steel after quenching and tempering. Therefore, the V content is
set to 0.300% or less. The upper limit of the V content is
preferably 0.250% or less, 0.200%, or 0.150%.
[0049] One or more selected from the group consisting of Ca: 0% to
0.0100%, Zr: 0% to 0.0100% or less, and Mg: 0% to 0.0100%
[0050] The steel according to the present embodiment does not need
Ca, Zr, and Mg. Therefore, the lower limit of the V content is 0%.
However, all of Ca. Zr, and Mg have an effect for forming an oxide,
acting as a crystallization nucleus of MnS, and uniformly and
finely dispersing MnS so as to improve the impact value of the
steel. Therefore, as an optional element, the steel may contain
0.0005% or more, 0.0010% or more, or 0.0015% or more of Ca, may
contain 0.0005% or more, 0.0010% or more, or 0.0015% or more of Zr,
and may contain 0.0005% or more, 0.0010% or more, or 0.0015% or
more of Mg. On the other hand, when each of the Ca content, the Zr
content, and the Mg content exceeds 0.0100%, an excess amount of a
hard inclusion such as an oxide and a sulfide is generated, and the
toughness of the steel is degraded. Therefore, the upper limits of
each of the Ca content, the Zr content, and the Mg content is set
to 0.0100% or less. The upper limit of the Ca content is preferably
0.0090%, 0.0070%, or 0.0050%, the upper limit of the Zr content is
preferably 0.0090%, 0.0070%, or 0.0050%, and the upper limit of the
Mg content is preferably 0.0090%, 0.0070%, or 0.0050%.
[0051] Remainder: Fe and Impurities
[0052] The remainder of the chemical composition of the steel
according to the present embodiment consists of Fe and impurities.
The impurities refer to elements which are incorporated by a raw
material such as an ore or a scrap, or a variety of causes in the
manufacturing process during the industrial manufacturing of the
steel, and the impurities are permitted to an extent in which the
steel according to the present embodiment is not adversely
influenced.
[0053] Next, the reason for limiting the inclusion state of the
steel according to the present embodiment will be described.
[0054] Number density of Mn sulfides having an equivalent circle
diameter of more than 5 .mu.m is 0 pieces/mm.sup.2 to 10
pieces/mm.sup.2
[0055] A Mn sulfide having an equivalent circle diameter of more
than 5 .mu.m (hereinafter, referred to as a "coarse Mn sulfide")
significantly degrades the low-temperature toughness of the steel,
and thus the number density of the coarse Mn sulfides is preferably
set to substantially 0 pieces/mm.sup.2. Therefore, the lower limit
of the number density of the coarse Mn sulfides is 0
pieces/mm.sup.2. However, when the number density is 10
pieces/mm.sup.2 or less, the low-temperature toughness is not
seriously impaired. Therefore, the upper limit of the number
density of the coarse Mn sulfides is set to 10 pieces/mm.sup.2. The
upper limit of the number density of the coarse Mn sulfides is
preferably 9 pieces/mm.sup.2, 8 pieces/mm.sup.2, or 7
pieces/mm.sup.2.
[0056] Average aspect ratio of the Mn sulfides having an equivalent
circle diameter of 1.0 .mu.m to 5.0 .mu.m is 1.0 or more and 10.0
or less
[0057] A Mn sulfide having an equivalent circle diameter of 1.0
.mu.m to 5.0 .mu.m (hereinafter, referred to as a "fine Mn
sulfide") has a smaller adverse influence on the toughness of the
steel than the coarse Mn sulfide. However, a fine Mn sulfide, in
which the aspect ratio of the Mn sulfide that can be calculated by
dividing the major axis of the Mn sulfide by the minor axis of the
Mn sulfide is excessively large, may act as an origin of fracture
and degrade the toughness of the steel, similar to the coarse Mn
sulfide. The present inventors found that, when the average aspect
ratio of the fine Mn sulfides is set to 10.0 or less, it is
possible to make the fine Mn sulfides almost harmless. A preferred
upper limit of the average aspect ratio of the fine Mn sulfides is
9.0, 7.5, or 6.0. When the major axis and the minor axis of the
fine Mn sulfide are equal to each other, the aspect ratio of the
fine Mn sulfide reaches 1.0, and thus the lower limit of the
average aspect ratio of the fine Mn sulfides is set to 1.0.
[0058] The fine dispersion of Mn sulfides which may act as origins
of fracture and a decrease in the aspect ratios thereof are
extremely effective for improving the low-temperature toughness of
the steel. In addition, since the state of the Mn sulfides does not
change before and after quenching and tempering that are carried
out under ordinary conditions, the state of the Mn sulfides is
maintained even after quenching and tempering as long as the state
of the Mn sulfides is controlled as described above before
quenching and tempering, and the above described effects can be
obtained.
[0059] In addition, in the steel according to the present
embodiment, there is no need for limiting the number density of the
fine Mn sulfides. Although there is concern that an extremely large
amount of the fine Mn sulfides impairs the toughness of the steel,
the number density of the fine Mn sulfides does not increase to an
extent in which the toughness of the steel is impaired, as long as
the S content is in the above described range. Furthermore, in the
steel according to the present embodiment, a Mn sulfide having an
equivalent circle diameter of less than 1.0 .mu.m (hereinafter,
referred to as an "ultrafine Mn sulfide") does not act as an origin
of fracture, and thus the aspect ratio and number density of the
ultrafine Mn sulfide are not particularly specified. Furthermore,
in the steel according to the present embodiment, the Mn sulfides
(the coarse Mn sulfides and the fine Mn sulfides) are almost
uniformly dispersed, and thus a place where the state of the Mn
sulfide is specified is not particularly limited.
[0060] A method for specifying the state of the Mn sulfide is as
described below. First, a cross section of the steel is
mirror-polished, and then optical microscopic photographs are
captured at 10 or more random places on the cross section at a
magnification of 1,000 times. The ten photographs obtained in the
above described manner are processed using image analysis software,
for example, Luzex (registered trademark) or the like, whereby the
state of Mn sulfides in the steel, that is, the number density of
the coarse Mn sulfides and the average aspect ratio of the fine Mn
sulfides can be obtained. In the steel according to the present
embodiment, Mn sulfides are elongated in a processing direction.
For example, when the steel is hot rolled, the Mn sulfides are
elongated in a hot rolling direction. Therefore, the cross section
where the optical microscopic photographs are captured needs to be
formed parallel to the processing direction (for example, the hot
rolling direction). On the other hand, since the Mn sulfides are
almost uniformly dispersed in the steel according to the present
embodiment, and a place where the optical microscopic photographs
are captured is not particularly specified.
[0061] Next, a method for manufacturing the steel according to the
present embodiment will be described.
[0062] The method for manufacturing the steel according to the
present embodiment includes a process of continuously casting
molten steel having the chemical composition of the steel according
to the present embodiment so as to obtain a slab and a process of
soaking the slab twice or more. The conditions for continuously
casting the molten steel are not particularly limited. In the
process of soaking the slab, firstly, the slab is heated up to a
temperature range of 1,300.degree. C. to 1,350.degree. C., and
then, the temperature of the slab is held in this temperature range
for 300 seconds to 18,000 seconds, and furthermore, the slab is
cooled to 900.degree. C. or lower. In addition, the soaking is
carried out twice or more.
[0063] (Soaking)
[0064] The soaking is carried out in order to finely disperse Mn
sulfides included in the slab. During the continuous casting,
coarse Mn sulfides are crystallized in the slab. When the slab is
heated up to a temperature range of 1,300.degree. C. to
1,350.degree. C. and then held in this temperature range for 300
seconds to 18,000 seconds, the coarse Mn sulfides are solutionized,
and the Mn sulfides are precipitated when the slab is cooled to
900.degree. C. or lower. The Mn sulfides are refined by
solutionizing and precipitation.
[0065] When the holding temperature of the slab is lower than
1,300.degree. C. and when the holding time of the slab at the
temperature is shorter than 300 seconds, the Mn sulfides are not
sufficiently solutionized. In addition, when the soaking is carried
out only once, the Mn sulfides are not sufficiently refined. In
order to set the dispersion state of the Mn sulfides in the steel
in the above described range, the soaking under the above described
conditions needs to be carried out twice or more. When the cooling
stop temperature of the slab is set to higher than 900.degree. C.
and the subsequent soaking is initiated, the Mn sulfides are not
precipitated during cooling, and thus the refinement of the Mn
sulfides becomes insufficient.
[0066] Meanwhile, when the heating temperature of the slab is
higher than 1,350.degree. C., the ductility of the slab is
degraded, and a problem of cracking is caused. In addition, when
the heating time of the slab is longer than 18,000 seconds, it is
not preferable in consideration of the economic efficiency.
[0067] On the slab in which the Mn sulfides are sufficiently
refined by the above described treatment, it is possible to carry
out an optional processing and an optional heat treatment
afterwards. For example, blooming and hot rolling are performed on
this slab so as to produce a steel bar, and a chain processing is
performed on this steel bar, whereby a chain can be obtained. In
addition, during or after the chain processing, it is possible to
quench and temper the chain. Since the Mn sulfides included in the
slab obtained using the above described method are sufficiently
refined, and it is assumed that the fine Mn sulfides included in
the slab are not outside the specification range described above
due to blooming, hot rolling, the chain processing, and quenching
and tempering that are carried out under ordinary conditions.
[0068] Even when the steel according to the present embodiment is
quenched and tempered so that the tensile strength reaches 1,200
MPa or more, the Charpy impact value at -20.degree. C. can be
maintained at 75 J/cm.sup.2 or more. Therefore, the steel according
to the present embodiment is particularly preferably used as steel
for quenching and tempering.
[0069] For example, when a quenching treatment in which steel is
heated to 900.degree. C., held for 30 minutes, and then cooled with
water is performed on the steel according to the present
embodiment, and furthermore, a tempering treatment in which the
steel is heated to 135.degree. C. and held for 30 minutes is
performed on the steel, steel having a tensile strength of 1,200
MPa or more and a Charpy impact value at -20.degree. C. of 75
J/cm.sup.2 or more is obtained. In the steel according to the
present embodiment on which the heat treatment under the above
described quenching and tempering conditions is performed, the
number density of Mn sulfides having an equivalent circle diameter
of more than 5 .mu.m is 0 pieces/mm.sup.2 to 10 pieces/mm.sup.2,
the average aspect ratio of the Mn sulfides having an equivalent
circle diameter of 1.0 .mu.m to 5.0 .mu.m is 1.0 or more and 10.0
or less, the average grain size of cementite is 0.05 .mu.m or less,
and the average size of martensite blocks is 5.5 .mu.m or less. The
steel according to the present embodiment contains 0.08% or more of
C and thus has a tensile strength of 1,200 MPa or more, when the
heat treatment under the above described quenching and tempering
conditions is performed. Generally, the low-temperature toughness
(particularly, low-temperature toughness) is impaired when the
tensile strength of the steel is 1,200 MPa or more. However, the
steel according to the present embodiment contains 0.75% to 3.20%
of Ni, 0.10% to 0.50% of Mo, and 0.005% to 0.050% of Nb, and thus,
when the heat treatment under the above described quenching and
tempering conditions is performed, martensite blocks and cementite
are sufficiently refined, and the steel has a high low-temperature
toughness. In addition, in the steel according to the present
embodiment on which the heat treatment under the above described
quenching and tempering conditions is performed, similar to the
steel according to the present embodiment before quenching and
tempering, the number density of Mn sulfides having an equivalent
circle diameter of more than 5 .mu.m is 0 pieces/mm.sup.2 to 10
pieces/mm.sup.2, and the average aspect ratio of the Mn sulfides
having an equivalent circle diameter of 1.0 .mu.m to 5.0 .mu.m is
1.0 or more and 10.0 or less, and thus the steel has a high
low-temperature toughness.
[0070] Meanwhile, quenching and tempering under the above described
conditions are simply an example of the use of the steel according
to the present embodiment. According to the purposes, a heat
treatment under optional conditions can be performed on the steel
according to the present embodiment. In addition, the properties of
the steel according to the present embodiment on which the heat
treatment is performed on the basis of an example of the above
described quenching and tempering conditions do not limit the
technical scope of the steel according to the present embodiment.
The object of the steel according to the present embodiment is to
obtain a Charpy impact value at -20.degree. C. of 75 J/cm.sup.2 or
more after a heat treatment is carried out so that the tensile
strength reaches 1,200 MPa. As described above, in order to achieve
this object, it is necessary to control the chemical composition
and the Mn sulfide state before the heat treatment. However, other
constitutions, for example, the states of martensite and cementite
before the heat treatment and the like do not need to be controlled
in order to achieve the object of the steel according to the
present embodiment.
[0071] In addition, quenching and tempering under ordinary
conditions do not have any influences on the Mn sulfide state.
Therefore, when the Mn sulfide state in quenched and tempered steel
is in the specification range described above, it is assumed that
the Mn sulfide state before quenching and tempering in the steel is
also in the specification range described above.
[0072] The steel according to the present embodiment is capable of
exhibiting particularly excellent effects, when the steel according
to the present embodiment is used as a material for chains for
mooring offshore oil-drilling rigs which needs to have a high
tensile strength and a high low-temperature toughness.
EXAMPLES
[0073] Hereinafter, the present invention will be described in
detail using examples. Meanwhile, these examples are intended to
describe the technical meaning and effects of the present invention
and do not limit the scope of the present invention.
Example 1
[0074] Steel A having a chemical composition shown in Table 1 was
continuously cast so as to obtain a slab, then, a soaking was
performed on the slab once or more, and furthermore, a blooming was
performed on the slab, thereby obtaining a 162 mm.times.162 mm
rolled material. The conditions for the soaking and the number of
times of the soaking are shown in Table 2. After that, hot rolling
was performed on the rolled material, thereby producing a round
steel bar having a diameter of 86 mm. Next, a quenching treatment
in which the round steel bar was cut, heated to 900.degree. C.,
held for 30 minutes, and furthermore, cooled with water was carried
out, and then a tempering treatment in which the round steel bar
was heated to 135.degree. C. and held for 30 minutes was carried
out, thereby obtaining round steel bars Nos. A1 to A5. These
quenching conditions and tempering conditions are the same as heat
treatment conditions that are recommended for the production of
chains using the present invention steel.
[0075] Three JIS No. 14A tensile test pieces and four JIS No. 4
V-notch Charpy impact test pieces were produced from a 1/4D portion
(a region at a depth of approximately 1/4 of a diameter D of the
round steel bar from the surface of the round steel bar) of a C
cross section of each of the quenched and tempered round steel bars
Nos. A1 to A5. A tensile test was carried out at normal temperature
and a rate of 20 mm/min according to JIS Z 2241. A Charpy impact
test was carried out at -20.degree. C. according to JIS Z 2242.
[0076] Furthermore, a 10 mm.times.10 mm sample was cut out from the
1/4D portion of the C cross section of each of the quenched and
tempered round steel bars Nos. A1 to A5, and the metallographic
structure of the steel and the state of inclusions were observed on
a cross section parallel to a rolling direction. In order to
observe Mn sulfides present in the steel, the cross section was
mirror-polished, then, 10 metallographic photographs were captured
using an optical microscope at a magnification of 1,000 times, and
the equivalent circle diameters and aspect ratios of Mn sulfides
included in the photographs were obtained by means of an image
analysis (Luzex (registered trademark)). In addition, in order to
observe cementite present in the steel, the cross section was
corroded with a nital etching solution, five metallographic
photographs were captured using a scanning electron microscope at a
magnification of 5,000 times, and the average grain size of the
cementite included in the photographs was obtained by means of an
image analysis (Luzex (registered trademark)). Furthermore, a
crystal orientation analysis was carried out on the sample using an
electron backscatter diffraction pattern, and the area-weighted
average equivalent circle diameter of crystal grains surrounded by
high angle grain boundaries which had an orientation difference
angle of 15 degrees, which was obtained from the above described
analysis, was considered as the average grain size of martensite
blocks.
[0077] The results of the above described experiments are shown in
Table 1 and Table 2. Table 1 shows the chemical compositions of the
steel A (that is, the chemical compositions of the steels No. A1 to
No. A5). Table 2 shows soaking conditions and the number of times
of soaking during the manufacturing of the steels No. A1 to No. A5,
and the average aspect ratios of Mn sulfides having an equivalent
circle diameter of 1.0 .mu.m to 5.0 .mu.m, the number densities of
Mn sulfides having an equivalent circle diameter of more than 5.0
.mu.m, the tensile strengths, the impact values, the average grain
sizes of the cementite, and the average sizes of the martensite
blocks in the steels No. A1 to No. A5 which were quenched and
tempered under the above described conditions. In Table 2, values
outside the specification ranges of the present invention are
underlined. Meanwhile, quenching and tempering under the above
described conditions do not have any influences on the state of the
Mn sulfides, and thus the states of the Mn sulfides in the quenched
and tempered steels No. A1 to No. A5 disclosed in Table 2 are the
same as those of the steels No. A1 to No. A5 before quenching and
tempering.
TABLE-US-00001 TABLE 1 Chemical composition (mass %) Steel type C
Si Mn P S Cr Cu Ni Mo Nb Al N A 0.10 0.21 1.81 0.006 0.007 1.72
0.09 1.52 0.33 0.015 0.023 0.0085
TABLE-US-00002 TABLE 2 Number density Average aspect of Mn sulfides
Average Soaking conditions ratio of Mn sulfides having equivalent
grain size Average Heating Number of having equivalent circle
diameter of Tensile Impact of size of temperature Holding times of
circle diameter of more than 5.0 .mu.m strength value cementite
martensite No. (.degree. C.) time (sec.) soaking 1.0 .mu.m to 5.0
.mu.m (pieces/mm.sup.2) (MPa) (J/cm.sup.2) (.mu.m) blocks (.mu.m)
Classification A1 1300 7200 3 1.3 0.0 1308 171 0.03 3.7 Invention
A2 1350 7200 2 4.5 1.1 1321 154 0.04 3.8 Example A3 1290 7200 2
10.8 10.2 1324 68 0.03 3.8 Comparative A4 1350 290 2 10.5 10.8 1311
54 0.03 3.8 Example A5 1300 7200 1 10.3 11.1 1284 65 0.03 4.1
[0078] As shown in Table 1 and Table 2, in the steels No. A1 and
No. A2 which were the present invention example, the chemical
compositions and the manufacturing conditions were appropriate, and
thus the form of the Mn sulfides was in the specification range of
the present invention. Therefore, the steels No. A1 and A2 had a
tensile strength of 1,200 MPa or more and a Charpy impact value at
-20.degree. C. of 75 J/cm.sup.2 or more after quenching and
tempering. In contrast, in the steels No. A3 to A5 which were
comparative examples, the manufacturing conditions were not
appropriate, and thus the Mn sulfides coarsened or the aspect
ratios of the Mn sulfides increased, and the low-temperature
toughness after quenching and tempering was insufficient.
Example 2
[0079] Each of steel B to AH having a chemical composition shown in
Table 3 was continuously cast so as to obtain a slab, then, a
soaking in which the holding temperature was 1,300.degree. C. and
the holding time was 7,200 seconds was performed on the slab twice,
and furthermore, blooming was performed on the slab, thereby
obtaining a 162 mm.times.162 mm rolled material. After that, hot
rolling was performed on the rolled material, thereby producing a
round steel bar having a diameter of 86 mm. Next, a quenching
treatment in which the round steel bar was cut, heated to
900.degree. C., held for 30 minutes, and furthermore, cooled with
water was carried out, and then a tempering treatment in which the
round steel bar was heated to 135.degree. C. and held for 30
minutes was carried out, thereby obtaining round steel bars Nos. B
to AH. These quenching conditions and tempering conditions are the
same as heat treatment conditions that are recommended for the
production of chains using the present invention steel.
[0080] Three JIS No. 14A tensile test pieces and four JIS No. 4
V-notch Charpy impact test pieces were produced from a 1/4D portion
of a C cross section of each of the quenched and tempered round
steel bars Nos. B to AH. A tensile test was carried out at normal
temperature and a rate of 20 mm/min according to JIS Z 2241. A
Charpy impact test was carried out at -20.degree. C. according to
JIS Z 2242.
[0081] Furthermore, a 10 mm.times.10 mm sample was cut out from the
1/4D portion of the C cross section of each of the quenched and
tempered round steel bars Nos. B to AH, and the metallographic
structure of the steel and the state of inclusions were observed on
a cross section parallel to a rolling direction. In order to
observe Mn sulfides present in the steel, the cross section was
mirror-polished, then, 10 metallographic photographs were captured
using an optical microscope at a magnification of 1,000 times, and
the equivalent circle diameters and aspect ratios of Mn sulfides
included in the photographs were obtained by means of an image
analysis (Luzex (registered trademark)). In addition, in order to
observe cementite present in the steel, the cross section was
corroded with a nital etching solution, five metallographic
photographs were captured using a scanning electron microscope at a
magnification of 5,000 times, and the average grain size of the
cementite included in the photographs was obtained by means of an
image analysis (Luzex (registered trademark)). Furthermore, a
crystal orientation analysis was carried out on the sample using a
backscattered electron beam diffraction pattern, and the
area-weighted average equivalent circle diameter of crystal grains
surrounded by high angle grain boundaries which had an orientation
difference angle of 15 degrees, which was obtained from the above
described analysis, was considered as the average grain size of
martensite blocks.
[0082] The results of the above described experiments are shown in
Table 3 and Table 4. Table 3 shows the chemical compositions of the
steels Nos. B to AH. Table 4 shows the aspect ratios of Mn sulfides
having an equivalent circle diameter of 1.0 .mu.m to 5.0 .mu.m, the
number densities of Mn sulfides having an equivalent circle
diameter of more than 5.0 .mu.m, the tensile strengths, the impact
values, the average grain sizes of the cementite, and the average
sizes of the martensite blocks in the steels Nos. B to AH which
were quenched and tempered under the above described conditions. In
Table 3 and Table 4, values outside the specification ranges of the
present invention are underlined. Meanwhile, quenching and
tempering under the above described conditions do not have any
influences on the state of the Mn sulfides, and thus the states of
the Mn sulfides in the quenched and tempered steels Nos. B to AH
disclosed in Table 4 are the same as those of the steels Nos. B to
AH before quenching and tempering.
TABLE-US-00003 TABLE 3 Steel Chemical composition (mass %) type C
Si Mn P S Cr Cu Ni Mo Nb Al N V Others Classification B 0.10 0.07
2.07 0.009 0.007 1.82 0.07 0.75 0.36 0.019 0.023 0.0059 -- --
Invention C 0.08 0.09 1.78 0.008 0.003 1.81 0.10 2.52 0.35 0.013
0.024 0.0093 0.038 -- Example D 0.12 0.10 1.33 0.007 0.007 1.78
0.08 3.18 0.35 0.015 0.017 0.0113 -- -- E 0.10 0.11 1.66 0.008
0.005 1.56 0.09 2.45 0.33 0.022 0.018 0.0075 0.031 -- F 0.10 0.05
1.37 0.005 0.004 1.57 0.10 2.90 0.37 0.020 0.016 0.0119 -- Ca:
0.003% G 0.10 0.09 1.79 0.008 0.004 1.66 0.08 2.50 0.40 0.021 0.020
0.0109 0.042 Mg: 0.005% H 0.10 0.50 1.92 0.008 0.006 1.56 0.10 2.62
0.33 0.021 0.024 0.0080 -- Zr: 0.009% I 0.10 0.10 1.01 0.007 0.005
1.57 0.10 2.42 0.34 0.019 0.017 0.0054 -- -- J 0.10 0.09 2.98 0.006
0.004 1.03 0.09 2.74 0.35 0.015 0.021 0.0113 -- -- K 0.10 0.09 1.51
0.005 0.005 2.47 0.11 2.77 0.35 0.016 0.021 0.0087 -- -- L 0.09
0.07 1.35 0.007 0.005 1.58 0.07 2.46 0.11 0.015 0.017 0.0118 0.053
-- M 0.09 0.10 1.40 0.007 0.005 1.54 0.10 2.62 0.50 0.022 0.018
0.0077 -- -- N 0.10 0.08 1.34 0.008 0.006 1.55 0.08 2.64 0.31 0.005
0.021 0.0112 -- -- O 0.09 0.09 1.41 0.008 0.008 1.61 0.09 2.74 0.32
0.048 0.019 0.0091 -- -- P 0.09 0.10 1.62 0.009 0.007 1.63 0.08
2.81 0.35 0.015 0.011 0.0073 -- -- Q 0.10 0.10 1.58 0.006 0.007
1.54 0.10 2.68 0.33 0.016 0.098 0.0077 -- -- R 0.10 0.09 1.63 0.039
0.006 1.55 0.11 2.52 0.32 0.021 0.022 0.0064 0.028 Ca: 0.005% S
0.09 0.09 1.58 0.007 0.019 1.62 0.09 2.37 0.32 0.019 0.020 0.0084
-- -- T 0.10 0.10 1.52 0.008 0.007 1.54 0.02 2.41 0.37 0.018 0.019
0.0093 -- -- U 0.10 0.08 1.42 0.008 0.008 1.42 0.48 2.57 0.35 0.022
0.022 0.0078 -- -- V 0.10 0.10 1.41 0.006 0.005 1.71 0.11 2.82 --
-- 0.022 0.0062 -- -- Comparative W 0.09 0.10 1.61 0.005 0.004 1.82
0.08 2.98 0.32 -- 0.024 0.0096 -- -- Example X 0.09 0.07 1.67 0.007
0.004 1.47 0.08 2.73 -- 0.022 0.021 0.0117 -- -- Y 0.09 0.08 1.51
0.006 0.006 1.64 0.09 0.73 0.45 0.019 0.021 0.0080 -- -- Z 0.10
0.08 1.37 0.006 0.005 1.72 0.08 2.47 0.08 0.014 0.019 0.0081 0.032
Ca: 0.005% AA 0.10 0.09 0.43 0.008 0.006 1.64 0.09 2.72 0.34 0.003
0.022 0.0077 -- Ma: 0.007% AB 0.07 0.10 1.91 0.006 0.005 1.73 0.10
2.65 0.30 0.023 0.025 0.0065 0.045 -- AC 0.13 0.09 1.46 0.007 0.004
1.81 0.07 2.52 0.30 0.017 0.024 0.0055 -- -- AD 0.09 0.51 1.74
0.006 0.007 1.70 0.09 2.55 0.43 0.019 0.022 0.0083 -- -- AE 0.09
0.09 3.20 0.008 0.004 1.68 0.08 2.81 0.40 0.015 0.023 0.0057 -- --
AF 0.09 0.09 1.32 0.009 0.006 0.97 0.09 3.08 0.33 0.022 0.018
0.0063 -- -- AG 0.10 0.10 1.37 0.008 0.021 1.63 0.09 2.74 0.33
0.019 0.020 0.0094 0.024 Zr: 0.009% AH 0.09 0.09 2.08 0.007 0.004
1.84 0.08 2.90 0.38 0.023 0.023 0.0154 -- --
TABLE-US-00004 TABLE 4 Average aspect ratio of Number density of Mn
Mn sulfides having sulfides having equivalent circle equivalent
circle diameter Tensile Impact Average grain Average size of Steel
diameter of 1.0 .mu.m to of more than 5 .mu.m strength value size
of cementite martensite blocks type 5.0 .mu.m (pieces/mm.sup.2)
(MPa) (J/cm.sup.2) (.mu.m) (.mu.m) Classification B 4.1 2.1 1278
156 0.03 3.2 Invention C 3.9 0.0 1222 150 0.04 3.9 Example D 4.5
1.3 1370 170 0.04 3.8 E 4.2 0.7 1315 181 0.04 4.4 F 3.7 0.0 1301
167 0.04 4.8 G 4.3 0.4 1307 153 0.04 3.6 H 4.1 1.1 1290 127 0.04
4.4 I 4.2 1.6 1304 131 0.04 3.4 J 3.8 0.6 1318 187 0.03 3.7 K 4.1
0.8 1285 170 0.04 4.9 L 4.3 1.3 1285 164 0.04 4.1 M 4.5 1.7 1296
128 0.03 3.1 N 3.9 1.1 1311 127 0.03 3.8 O 4.3 0.3 1308 134 0.04
3.7 P 4.4 0.7 1297 130 0.03 3.6 Q 4.1 1.2 1287 136 0.03 3.8 R 3.8
1.8 1303 129 0.04 3.9 S 4.2 1.2 1298 132 0.03 3.6 T 4.0 2.0 1311
138 0.04 3.7 U 4.3 1.4 1318 128 0.04 3.7 V 4.2 2.2 1279 49 0.07
10.2 Comparative W 4.0 0.3 1296 70 0.06 8.7 Example X 3.7 0.0 1294
57 0.06 9.1 Y 4.4 2.1 1267 41 0.06 9.5 Z 4.1 0.8 1302 63 0.07 9.8
AA 4.3 1.2 1297 54 0.06 9.9 AB 4.2 0.7 1193 162 0.04 3.6 AC 4.1 0.6
1415 68 0.04 3.3 AD 4.5 1.4 1282 58 0.04 5.1 AE 4.3 2.1 1294 46
0.04 4.7 AF 4.3 0.2 1266 71 0.04 5.4 AG 4.1 5.6 1309 52 0.03 4.3 AH
4.0 0.0 1282 67 0.03 3.8
[0083] As shown in Table 3 and Table 4, in all of the steels Nos. B
to U which were the present invention example, the chemical
compositions and the states of the Mn sulfides were in the
specification range of the present invention. Therefore, the steels
Nos. B to U had a tensile strength of 1,200 MPa or more and a
Charpy impact value at -20.degree. C. of 75 J/cm.sup.2 or more
after quenching and tempering.
[0084] In contrast, in the steels Nos. V, W, X, Y, Z, and AA which
were comparative examples, the amount of one or more of Mo, Nb, and
Ni was insufficient or one or more of Mo, Nb, and Ni was not
included, and thus, after quenching and tempering, cementite which
acted as an origin of fracture became coarse, furthermore, the
average size of martensite blocks became coarse, and the
low-temperature toughness was insufficient.
[0085] In the steel No. AB which was a comparative example, the C
content was insufficient, and thus a necessary tensile strength
could not be obtained after quenching and tempering. Meanwhile, in
the steel No. AC which was a comparative example, the C content was
excessive, and thus the strength became excessively high, and the
low-temperature toughness after quenching and tempering was
insufficient.
[0086] In the steel No. AD which was a comparative example, the Si
content was excessive, and in the steel No. AE, the Mn content was
excessive. The excess Si or Mn degraded the toughness of the steel,
and thus the low-temperature toughness of the steels No. AD and No.
AE after quenching and tempering was insufficient.
[0087] In the steel No. AF which was a comparative example, the Cr
content was insufficient, and thus sufficient hardenability could
not be obtained, and the low-temperature toughness after quenching
and tempering was insufficient.
[0088] In the steel No. AG which was a comparative example, the S
content was excessive, and thus an excess amount of Mn sulfides
were formed, and the low-temperature toughness after quenching and
tempering was insufficient. In the steel No. AH which was a
comparative example, the N content was excessive, and thus the
amount of solute N became excessive, and the low-temperature
toughness after quenching and tempering was insufficient.
* * * * *