U.S. patent application number 16/320651 was filed with the patent office on 2019-06-06 for steel for machine structural use.
The applicant listed for this patent is Nippon Steel & Sumitomo Metal Corporation. Invention is credited to Makoto Egashira, Masayuki Hashimura, Takanori Iwahashi, Shouji Toudou.
Application Number | 20190169723 16/320651 |
Document ID | / |
Family ID | 61016225 |
Filed Date | 2019-06-06 |
United States Patent
Application |
20190169723 |
Kind Code |
A1 |
Hashimura; Masayuki ; et
al. |
June 6, 2019 |
Steel for Machine Structural Use
Abstract
A steel for machine structural use according to the present
embodiment has a chemical composition which consists of, in mass %,
C: 0.15 to less than 0.30%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.00%,
P: 0.030% or less, S: 0.010 to 0.100%, Pb: 0.010 to 0.100%, Al:
0.010 to 0.050%, N: 0.015% or less, O: 0.0005 to 0.0030% and Cr:
0.50 to 2.00%, with the balance being Fe and impurities, the
chemical composition satisfying Formula (1). The total number of
specific inclusions included in the steel which are any of MnS
inclusions, Pb inclusions and composite inclusions containing MnS
and Pb and which have an equivalent circular diameter of 5 .mu.m or
more is 40 per mm.sup.2 or more. Mn/S.gtoreq.8.0 (1) Where, a
content (mass %) of a corresponding element is substituted for each
symbol of an element in Formula (1).
Inventors: |
Hashimura; Masayuki;
(Chiyoda-ku, Tokyo, JP) ; Egashira; Makoto;
(Chiyoda-ku, Tokyo, JP) ; Toudou; Shouji;
(Chiyoda-ku, Tokyo, JP) ; Iwahashi; Takanori;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Steel & Sumitomo Metal Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
61016225 |
Appl. No.: |
16/320651 |
Filed: |
July 27, 2017 |
PCT Filed: |
July 27, 2017 |
PCT NO: |
PCT/JP2017/027154 |
371 Date: |
January 25, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22D 11/124 20130101;
C22C 38/06 20130101; C22C 38/44 20130101; C22C 38/22 20130101; B22D
11/001 20130101; C22C 38/00 20130101; C22C 38/28 20130101; C21D
2211/004 20130101; C22C 38/32 20130101; C21C 7/04 20130101; C21C
7/00 20130101; C21C 7/0006 20130101; C22C 38/04 20130101; C22C
38/20 20130101; C21C 7/10 20130101; C22C 38/26 20130101; C22C 38/60
20130101; C22C 38/24 20130101; C21C 7/06 20130101; C22C 38/001
20130101; C22C 38/02 20130101; C22C 38/002 20130101 |
International
Class: |
C22C 38/60 20060101
C22C038/60; C21C 7/04 20060101 C21C007/04; C22C 38/44 20060101
C22C038/44; C22C 38/32 20060101 C22C038/32; C22C 38/28 20060101
C22C038/28; C22C 38/26 20060101 C22C038/26; C22C 38/22 20060101
C22C038/22; C22C 38/20 20060101 C22C038/20; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/24 20060101 C22C038/24; C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 27, 2016 |
JP |
2016-147194 |
Claims
1. A steel for machine structural use having a chemical composition
consisting of, in mass %: C: 0.15 to less than 0.30%, Si: 0.01 to
0.80%, Mn: 0.20 to 2.00%, P: 0.030% or less, S: 0.010 to 0.100%,
Pb: 0.010 to 0.100%, Al: 0.010 to 0.050%, N: 0.015% or less, O:
0.0005 to 0.0030%, Cr: 0.50 to 2.00%, Ni: 0 to 3.50%, B: 0 to
0.0050%, V: 0 to 0.70%, Mo: 0 to 0.70%, W: 0 to 0.70%, Nb: 0 to
less than 0.050%, Cu: 0 to 0.50%, Ti: 0 to 0.100%, and Ca: 0 to
0.0030%, with the balance being Fe and impurities, the chemical
composition satisfying Formula (1), wherein, in the steel, a total
number of specific inclusions which are any of MnS inclusions, Pb
inclusions and composite inclusions containing MnS and Pb, and
which have an equivalent circular diameter of 5 .mu.m or more is 40
per mm.sup.2 or more; Mn/S.gtoreq.8.0 (1) where, a content (mass %)
of a corresponding element is substituted for each symbol of an
element in Formula (1).
2. The steel for machine structural use according to claim 1,
wherein the chemical composition contains one or more types of
element selected from a group consisting of: Ni: 0.02 to 3.50%, B:
0.0005 to 0.0050%, V: 0.05 to 0.70%, Mo: 0.05 to 0.70%, W: 0.05 to
0.70%, Nb: 0.001 to less than 0.050%, Cu: 0.05 to 0.50%, and Ti:
0.003 to 0.100%.
3. The steel for machine structural use according to claim 1,
wherein the chemical composition contains: Ca: 0.0001 to
0.0030%.
4. The steel for machine structural use according to claim 1,
wherein: a ratio of a number of the composite inclusions with
respect to the specific inclusions is 40% or more.
5. The steel for machine structural use according to claim 2,
wherein the chemical composition contains: Ca: 0.0001 to
0.0030%.
6. The steel for machine structural use according to claim 2,
wherein: a ratio of a number of the composite inclusions with
respect to the specific inclusions is 40% or more.
7. The steel for machine structural use according to claim 3,
wherein: a ratio of a number of the composite inclusions with
respect to the specific inclusions is 40% or more.
8. The steel for machine structural use according to claim 5,
wherein: a ratio of a number of the composite inclusions with
respect to the specific inclusions is 40% or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to steel, and more
particularly relates to a steel for machine structural use.
BACKGROUND ART
[0002] Excellent rolling contact fatigue properties may be required
for machine components to be used for structural use and power
transmission use, such as components of general machinery and
automobiles. An example of a method for producing such kind of
machine components is as follows. A steel for machine structural
use is subjected to hot working (hot forging or the like) to
produce an intermediate product. The intermediate product is
subjected to machining (cutting or grinding) to produce a machine
component. As necessary, a case hardening treatment may also be
performed on the machine component. The case hardening treatment
is, for example, a carburizing. A steel for machine structural use
for producing such kind of machine components is required to be
excellent not only in hot workability, but also in
machinability.
[0003] A steel for machine structural use that is excellent in
machinability is also called a "free-cutting steel", and is defined
in JIS G 4804 (2008) (Non Patent Literature 1). The machinability
of a free-cutting steel is enhanced by containing Pb.
[0004] A steel for machine structural use that contains Pb is
disclosed, for example, in Japanese Patent Application Publication
No. 2000-282172 (Patent Literature 1). The steel material for
machine structural use disclosed in Patent Literature 1 has a
chemical composition which contains, in mass %, C: 0.05 to 0.55%,
Si: 0.50 to 2.5%, Mn: 0.01 to 2.00%, S: 0.005 to 0.080%, Cr: 0 to
2.0%, P: 0.035% or less, V: 0 to 0.50%, N: 0.0150% or less, Al:
0.04% or less, Ni: 0 to 2.0%, Mo: 0 to 1.5%, B: 0 to 0.01%, Bi: 0
to 0.10%, Ca: 0 to 0.05%, Pb: 0 to 0.12%, Ti: 0 to less than 0.04%,
Zr: 0 to less than 0.04% and Ti (%)+Zr (%): 0 to less than 0.04%,
Te: 0 to 0.05%, Nd: 0 to 0.05%, Nb: 0 to 0.1%, Cu: 0 to 1.5%, and
Se: 0 to 0.5%, the chemical composition satisfying a condition that
a value of fn1 represented by a formula hereunder is 100 or less, a
value of fn2 represented by a formula hereunder is 0 or more, and a
value of fn3 represented by a formula hereunder is 3.0 or more,
with the balance being Fe and impurities. In addition, a proportion
that a ferritic phase occupies in the micro-structure is, with
respect to the area ratio, from 10 to 80%, and Hv hardness is in a
range from 160 to 350. Here, fn1=100C+11Si+18Mn+32Cr+45Mo+6V,
fn2=-23C+Si(5-2Si)-4Mn+104S-3Cr-9V+10, and
fn3=3.2C+0.8Mn+5.2S+0.5Cr-120N+2.6Pb+4.1Bi-0.001.alpha..sup.2+0.13.alpha.-
. A symbol of an element in the respective formulas represents the
content in mass % of the corresponding element, and a represents
the area ratio (%) of the ferritic phase in the micro-structure. It
is described in Patent Literature 1 that the steel material for
machine structural use is excellent in machinability and
toughness.
CITATION LIST
Patent Literature
[0005] Patent Literature 1: Japanese Patent Application Publication
No. 2000-282172
Non Patent Literature
[0005] [0006] Non Patent Literature 1: Japanese Industrial
Standards Committee, Standard No.: JIS G 4804 (2008), Standard
Name: Free-cutting Steels
SUMMARY OF INVENTION
Technical Problem
[0007] In this connection, in some cases machining such as cutting
is performed using an automated equipment system. In an automated
equipment system, in the case of producing a large amount of
machine components by cutting intermediate products, such as
producing several hundred or more machine components per day,
excellent chip treatability is required. It is preferable that
chips that are to be discharged accompanying cutting are split into
small pieces and discharged. If the chips remain connected in a
long length, the chips are liable to become entwined around the
intermediate product, and defects are liable to arise on the
surface of the machine component after cutting. If a chip is
entwined around a machine component, it is also necessary to
temporarily stop the production line to remove the chip that is
entwined around the machine component. In this case, it is
difficult to perform unattended production, and it is necessary to
assign personnel to monitor the production process. Thus, chip
treatability affects both the quality of the machine components and
the production cost. In addition, in an automated equipment system,
productivity decreases as tool wear increases. Therefore, a steel
for machine structural use is required to have high machinability,
such as being capable of suppressing tool wear and being excellent
in chip treatability.
[0008] In addition, when cutting is performed using automated
equipment system, in some cases rust occurs in the machine
component. In an automated equipment system, a water-soluble
cutting oil is utilized from the viewpoint of performing unattended
operations. Consequently, in some cases rust occurs in the machine
components. Rust is not only a cause of the occurrence of shape
errors, but is also a cause of quality defects when performing a
plating treatment on the machine component. In addition, after
cutting, the machine components are sometimes stored in a bucket or
the like for a long time period until undergoing the next process
after the cutting process. For example, in a case where machine
components are cut in a certain country and the next process is
performed in a separate factory in a different country, a period of
several days to several months may pass after cutting until the
machine components are subjected to the next process. Therefore, a
steel for machine structural use is required to be not only
excellent in machinability, but also to have characteristics that
suppress the occurrence of rust (hereunder, referred to as "rusting
characteristics").
[0009] An objective of the present invention is to provide a steel
for machine structural use that is excellent in machinability,
rusting characteristics and hot workability, and with which a
machine component that is excellent in rolling contact fatigue
properties is obtained.
Solution to Problem
[0010] A steel for machine structural use according to the present
invention has a chemical composition which consists of, in mass %,
C: 0.15 to less than 0.30%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.00%,
P: 0.030% or less, S: 0.010 to 0.100%, Pb: 0.010 to 0.100%, Al:
0.010 to 0.050%, N: 0.015% or less, O: 0.0005 to 0.0030%, Cr: 0.50
to 2.00%, Ni: 0 to 3.50%, B: 0 to 0.0050%, V: 0 to 0.70%, Mo: 0 to
0.70%, W: 0 to 0.70%, Nb: 0 to less than 0.050%, Cu: 0 to 0.50%,
Ti: 0 to 0.100% and Ca: 0 to 0.0030%, with the balance being Fe and
impurities, the chemical composition satisfying Formula (1). In the
steel, a total number of specific inclusions which are any of MnS
inclusions, Pb inclusions, and composite inclusions containing MnS
and Pb and which have an equivalent circular diameter of 5 .mu.m or
more is 40 per mm.sup.2 or more.
Mn/S.gtoreq.8.0 (1)
[0011] Where, a content (mass %) of a corresponding element is
substituted for each symbol of an element in Formula (1).
Advantageous Effects of Invention
[0012] A steel for machine structural use according to the present
invention is excellent in machinability, rusting characteristics
and hot workability, and is a steel with which a machine component
that is excellent in rolling contact fatigue properties is
obtained.
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIG. 1A is a schematic diagram illustrating an S
distribution in an observation surface, that was obtained by EPMA
analysis.
[0014] FIG. 1B is a schematic diagram illustrating a Pb
distribution in the same observation surface as in FIG. 1A, that
was obtained by EPMA analysis.
[0015] FIG. 1C is a schematic diagram of an image obtained by
combining FIG. 1A and FIG. 1B.
[0016] FIG. 2 is a schematic diagram for describing a criterion for
determining whether or not to regard adjacent inclusions as a
single inclusion.
[0017] FIG. 3 is a transverse sectional view of a starting material
that was cast.
[0018] FIG. 4 is a schematic diagram of a cutting test machine for
describing a cutting test.
[0019] FIG. 5A is a perspective view of a chip.
[0020] FIG. 5B is a planar photographic view of a chip.
[0021] FIG. 6 shows a front view and a side view of a rolling
contact fatigue test specimen used in a rolling contact fatigue
test.
[0022] FIG. 7 is a schematic diagram of a thrust-type rolling
contact fatigue test machine for describing a rolling contact
fatigue test.
DESCRIPTION OF EMBODIMENTS
[0023] The present inventors conducted investigations and studies
regarding the machinability, rusting characteristics and hot
workability of steels for machine structural use. As a result, the
present inventors found that if a steel for machine structural use
has a chemical composition consisting of, in mass %, C: 0.15 to
less than 0.30%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.00%, P: 0.030% or
less, S: 0.010 to 0.100%, Pb: 0.010 to 0.100%, Al: 0.010 to 0.050%,
N: 0.015% or less, O: 0.0005 to 0.0030%, Cr: 0.50 to 2.00%, Ni: 0
to 3.50%, B: 0 to 0.0050%, V: 0 to 0.70%, Mo: 0 to 0.70%, W: 0 to
0.70%, Nb: 0 to less than 0.050%, Cu: 0 to 0.50%, Ti: 0 to 0.100%,
and Ca: 0 to 0.0030%, with the balance being Fe and impurities,
excellent machinability and excellent hot workability are obtained,
and that there is a possibility that excellent rolling contact
fatigue properties will be obtained after a carburizing.
[0024] Mn in the steel combines with S to form MnS. The MnS is
divided into MnS inclusions and MnS precipitates according to the
formation process. MnS inclusions crystallize in the molten steel
before solidification. On the other hand, MnS precipitates
precipitate in the steel after solidification. The MnS inclusions
form in the molten steel. Therefore, the size of the MnS inclusions
tends to be large in comparison to the MnS precipitates that form
after solidification.
[0025] On the other hand, most of the Pb in the steel does not
dissolve in the steel, and is present as Pb inclusions (Pb grains).
The MnS inclusions and Pb inclusions each enhance the machinability
of the steel.
[0026] In addition, in a case where Mn and Pb are present in the
steel, in addition to the aforementioned MnS inclusions and Pb
inclusions, the Mn and Pb also form composite inclusions containing
MnS and Pb (hereinafter, also referred to simply as "composite
inclusions"). The term "composite inclusions" means inclusions that
contain MnS and Pb, with the balance being impurities. More
specifically, there are cases where composite inclusions are
composed by MnS and Pb that are adjacent to each other, and there
are also cases where Pb dissolves into MnS to form a composite
inclusion. In the present description, "MnS inclusions", "Pb
inclusions" and "composite inclusions" are identified by a method
described in a section "Method of measuring number of inclusions TN
and RA" which is described later. In the present description, the
term "MnS inclusions" refers to inclusions that contain Mn and S
and do not contain Pb. The term "Pb inclusions" refers to
inclusions which are composed of Pb and impurities and which do not
contain Mn. The term "composite inclusions" refers to inclusions
that contain Mn, S and Pb.
[0027] MnS inclusions are known as inclusions that enhance
machinability. On the other hand, the fusing point of Pb inclusions
is lower than the fusing point of MnS inclusions. Therefore, Pb
inclusions exert a lubricating action during cutting, and as a
result the machinability of the steel is enhanced.
[0028] In addition, it is considered that composite inclusions
enhance the machinability of steel more than individual MnS
inclusions and Pb inclusions. In a case where a fissure has arisen
at the periphery of a composite inclusion, liquefied Pb enters into
the open crack. By this means, propagation of the crack is promoted
and machinability is enhanced. Accordingly, if composite inclusions
are also formed, and not just MnS inclusions and Pb inclusions, the
machinability is enhanced further.
[0029] The mechanism by which composite inclusions are formed is
considered to be as follows. It is easier for Pb to move in liquid
phase than in solid phase. Therefore, almost no composite
inclusions can be formed from MnS precipitates which form after
solidification of the steel, and the composite inclusions are
instead formed by adherence of Pb to MnS inclusions that are formed
in the molten steel before solidification. Accordingly, in order to
form a large number of composite inclusions, it is desirable to
form a large number of MnS inclusions in the molten steel rather
than forming MnS precipitates after solidification.
[0030] As described above, in order to enhance the machinability of
steel, it suffices to form a large number of MnS inclusions, Pb
inclusions and composite inclusions. As described above, MnS
inclusions are formed in molten steel by crystallization. In
addition, as described above, the greater the number of MnS
inclusions that are present, the greater the number of composite
inclusions that will be formed. Therefore, it is considered that
the machinability of the steel is enhanced by causing a large
number of MnS inclusions to crystallize in the molten steel.
[0031] On the other hand, a steel for machine structural use that
contains MnS inclusions, MnS precipitates, Pb inclusions and
composite inclusions is susceptible to rusting. However, hitherto
the mechanism of rusting with respect to a steel for machine
structural use had not been studied in detail. Therefore, the
present inventors conducted investigations and studies regarding
the rusting mechanism. As a result, the present inventors obtained
the following finding.
[0032] MnS inclusions, MnS precipitates, Pb inclusions and
composite inclusions become starting points for rust. In this case,
the susceptibility to rusting depends more on the total number of
MnS inclusions, MnS precipitates, Pb inclusions and composite
inclusions than the size of the MnS inclusions, MnS precipitates,
Pb inclusions and composite inclusions. Specifically, the
susceptibility to rusting of the steel increases as the total
number of MnS inclusions, MnS precipitates, Pb inclusions and
composite inclusions increases. Based on the above finding, the
present inventors concluded that in order to suppress rusting while
obtaining excellent machinability, it is effective to decrease the
total number of MnS inclusions, MnS precipitates, Pb inclusions and
composite inclusions. Therefore, the present inventors studied
methods for decreasing the total number of MnS inclusions, MnS
precipitates, Pb inclusions and composite inclusions.
[0033] As described above, MnS inclusions that are formed by
crystallization in molten steel are liable to grow (coarsen) in the
molten steel. Therefore, the size of MnS inclusions is larger than
the size of MnS precipitates that are formed by precipitation in
the steel after solidification. That is, the MnS precipitates
precipitate more finely than the MnS inclusions. Therefore, in a
steel having a certain Mn content and S content, if a case in which
MnS inclusions are caused to crystallize and a case in which MnS
precipitates are caused to precipitate are supposed, the number of
MnS precipitates that are formed by precipitation will be
noticeably greater than the number of MnS inclusions that are
formed by crystallization. Accordingly, to improve the rusting
characteristics of a steel, it suffices to suppress precipitation
of MnS precipitates by crystallizing MnS inclusions in the molten
steel and causing the MnS inclusions to grow (coarsen).
[0034] In order to cause MnS inclusions to crystallize and grow in
molten steel and suppress precipitation of MnS precipitates and, as
a result, decrease the total number of MnS inclusions, MnS
precipitates, Pb inclusions and composite inclusions, it suffices
to significantly increase the Mn content in comparison to the S
content. If the Mn content is sufficiently higher than the S
content, coarse MnS inclusions are likely to form in the molten
steel. In this case, because S is consumed by the crystallization
of the coarse MnS inclusions, the amount of dissolved S in the
steel after solidification is lowered. Consequently, precipitation
of MnS precipitates can be suppressed, and the total number of MnS
inclusions, MnS precipitates, Pb inclusions and composite
inclusions can be decreased. As a result, excellent rusting
characteristics are obtained.
[0035] Specifically, the Mn content and S content satisfy the
following Formula (1).
Mn/S.gtoreq.8.0 (1)
[0036] Where, the content (mass %) of a corresponding element is
substituted for each symbol of an element in Formula (1).
[0037] Here, it is defined that F1=Mn/S. If F1 is less than 8.0, it
is difficult for MnS inclusions to adequately crystallize in the
molten steel. Therefore, the amount of dissolved S in the steel
after solidification cannot be adequately decreased, and a large
number of fine MnS precipitates are formed after solidification. In
this case, because the total number of MnS inclusions, MnS
precipitates, Pb inclusions and composite inclusions cannot be
decreased, the rusting characteristics of the steel decline. On the
other hand, if F1 is 8.0 or more, the Mn content is adequately high
in comparison with the S content. In this case, by using an
appropriate production method, MnS inclusions in the molten steel
adequately crystallize and grow. As a result, the amount of
dissolved S in the steel after solidification is adequately
decreased, and precipitation of MnS precipitates in the steel after
solidification can be suppressed. Therefore, the total number of
MnS inclusions, MnS precipitates, Pb inclusions and composite
inclusions can be adequately reduced, and the rusting
characteristics of the steel are enhanced.
[0038] Here, inclusions which are any of MnS inclusions, Pb
inclusions and composite inclusions and which have an equivalent
circular diameter of 5 .mu.m or more are defined as "specific
inclusions". In the present description, the term "equivalent
circular diameter" means the diameter of a circle in a case where
the area of an inclusion or a precipitate that is observed during
micro-structure observation is converted into a circle having the
same area. In this case, in the present embodiment, in addition, in
a steel for machine structural use having the aforementioned
chemical composition and satisfying Formula (1), the total number
of specific inclusions is 40 per mm.sup.2 or more.
[0039] If the total number of specific inclusions in the steel is
40 per mm.sup.2 or more, coarse MnS inclusions adequately
crystallize and formation of MnS precipitates can be suppressed. As
a result, the total number of MnS inclusions, MnS precipitates, Pb
inclusions and composite inclusions which become starting points
for rusting can be adequately reduced. Therefore, excellent
machinability and excellent rusting characteristics can both be
realized in a compatible manner. On the other hand, if the total
number of specific inclusions in the steel is less than 40 per
mm.sup.2, MnS inclusions do not adequately crystallize, and a large
number of MnS precipitates form. As a result, the formation of MnS
precipitates can be suppressed. Consequently, the total number of
MnS inclusions, MnS precipitates, Pb inclusions and composite
inclusions which become starting points for rusting cannot be
adequately reduced. Therefore, although excellent machinability is
obtained, adequate rusting characteristics are not obtained.
[0040] A steel for machine structural use according to the present
embodiment that was completed based on the above findings has a
chemical composition which consists of, in mass %, C: 0.15 to less
than 0.30%, Si: 0.01 to 0.80%, Mn: 0.20 to 2.00%, P: 0.030% or
less, S: 0.010 to 0.100%, Pb: 0.010 to 0.100%, Al: 0.010 to 0.050%,
N: 0.015% or less, O: 0.0005 to 0.0030%, Cr: 0.50 to 2.00%, Ni: 0
to 3.50%, B: 0 to 0.0050%, V: 0 to 0.70%, Mo: 0 to 0.70%, W: 0 to
0.70%, Nb: 0 to less than 0.050%, Cu: 0 to 0.50%, Ti: 0 to 0.100%
and Ca: 0 to 0.0030%, with the balance being Fe and impurities, the
chemical composition satisfying Formula (1). In the steel, the
total number of specific inclusions which are any of MnS
inclusions, Pb inclusions, and composite inclusions containing MnS
and Pb, and which have an equivalent circular diameter of 5 .mu.m
or more is 40 per mm.sup.2 or more.
Mn/S.gtoreq.8.0 (1)
[0041] Where, the content (mass %) of a corresponding element is
substituted for the respective elements in Formula (1).
[0042] The chemical composition of the steel for machine structural
use that is described above may contain one or more types of
element selected from a group consisting of Ni: 0.02 to 3.50%, B:
0.0005 to 0.0050%, V: 0.05 to 0.70%, Mo: 0.05 to 0.70%, W: 0.05 to
0.70%, Nb: 0.001 to less than 0.050%, Cu: 0.05 to 0.50% and Ti:
0.003 to 0.100%.
[0043] The chemical composition of the steel for machine structural
use that is described above may contain Ca: 0.0001 to 0.0030%.
[0044] In the steel for machine structural use that is described
above, a ratio of the number of the composite inclusions to the
specific inclusions may be 40% or more.
[0045] Hereunder, the steel for machine structural use according to
the present embodiment is described in detail. The symbol "%" in
the chemical composition means "mass percent" unless specifically
stated otherwise.
[0046] [Chemical Composition]
[0047] The chemical composition of the steel for machine structural
use of the present embodiment contains the following elements.
[0048] C: 0.15 to less than 0.30%
[0049] Carbon (C) increases the strength of steel. In the case of
producing a component using steel for machine structural use, in
some cases a carburizing is performed after forging of the steel
for machine structural use. In this case, C increases the strength
of the outer layer of the steel. If the C content is less than
0.15%, sufficient strength is not obtained in the steel. If a soft
steel is used, in some cases chips connect during cutting, and the
chips become entwined around the tools or material and cause damage
thereto. In addition, if the C content is less than 0.15%, the
cutting resistance of the steel increases. On the other hand, if
the C content is 0.30% or more, the hardness of a core part of the
component will increase after carburizing, and the balance between
the toughness and the strength of the outer layer will decrease.
Accordingly, the C content is from 0.15 to less than 0.30%. A
preferable lower limit of the C content is 0.16%, and more
preferably is 0.18%. A preferable upper limit of the C content is
0.25%, and more preferably is 0.23%.
[0050] Si: 0.01 to 0.80%
[0051] Silicon (Si) deoxidizes the steel. By adding Si after adding
Mn at the time of deoxidation, the Si reforms oxides. Specifically,
Si added to molten steel reforms oxides that are mainly composed of
Mn into oxides that are mainly composed of Si. By adding Al after
adding Si, complex oxides containing Si and Al form in the steel.
The complex oxides serve as nuclei for crystallization of MnS
inclusions. Therefore, the complex oxides enhance the rusting
characteristics of the steel. Si also enhances temper softening
resistance and raises the strength. The aforementioned effects are
not obtained if the Si content is less than 0.01%.
[0052] On the other hand, Si is a ferrite forming element. If the
Si content is more than 0.80%, the outer layer of the steel may be
decarburized. Furthermore, if the Si content is more than 0.80%,
the ferrite fraction may increase and the strength decrease in some
cases. Accordingly, the Si content is from 0.01 to 0.80%. A
preferable lower limit of the Si content for increasing the temper
softening resistance is 0.10%, and more preferably is 0.20%. A
preferable upper limit of the Si content for keeping the ferrite
fraction low is 0.70%, and more preferably is 0.50%.
[0053] Mn: 0.20 to 2.00%
[0054] Manganese (Mn) forms MnS inclusions and composite inclusions
containing MnS and Pb, and enhances the machinability of the
steel.
[0055] Mn also deoxidizes the steel. The deoxidizing power of Mn is
weak compared to Si or Al. Therefore, a large amount of Mn may be
contained. In a case where another strong deoxidizing element is
not present in the molten steel, oxides that are mainly composed of
Mn form in the molten steel. Thereafter, if another strong
deoxidizing element (Si or Al) is added to the molten steel, the Mn
contained in the oxides is discharged into the molten steel, and
the oxides are reformed. Hereunder, the reformed oxides are
referred to as "complex oxides". The Mn that is discharged into the
molten steel from the oxides combines with S to form MnS
inclusions. Note that, complex oxides formed by reformation of
oxides easily become nuclei for crystallization of MnS inclusions.
Therefore, if complex oxides are formed, crystallization of MnS
inclusions is promoted. The MnS inclusions formed by
crystallization easily form composite inclusions also.
[0056] If the Mn content is less than 0.20%, it is difficult for
MnS inclusions to adequately crystallize. Therefore, a large number
of MnS precipitates will form in the steel after solidification. In
this case, the total number of MnS inclusions, MnS precipitates, Pb
inclusions and composite inclusions will increase. Consequently,
the rusting characteristics of the steel will decrease. In
contrast, if the Mn content is more than 2.00%, the hardenability
of the steel will be too high and, as a result, the hardness of the
steel will be too high. In this case, the machinability of the
steel will decrease. Furthermore, in this case, the hot workability
of the steel will also decrease. Accordingly, the Mn content is
from 0.20 to 2.00%. A preferable lower limit of the Mn content is
0.50%. A preferable upper limit of the Mn content is 1.50%, and
more preferably is 1.20%.
[0057] P: 0.030% or less
[0058] Phosphorus (P) is unavoidably contained. P embrittles the
steel and enhances the machinability. On the other hand, if the P
content is more than 0.030%, hot ductility decreases. In such a
case, rolling defects and the like occur, and the productivity
decreases. Accordingly, the P content is 0.030% or less. A
preferable lower limit of the P content for enhancing the
machinability is 0.005%. In this case, the machinability,
particularly the chip treatability, is enhanced. A preferable upper
limit of the P content is 0.015%.
[0059] S: 0.010 to 0.100%
[0060] Sulfur (S) forms MnS in the steel and enhances the
machinability. In particular, MnS suppresses tool wear. If the S
content is less than 0.010%, MnS will not crystallize adequately
and it will be difficult for composite inclusions containing MnS
and Pb to form. As a result, the rusting characteristics will
decrease. On the other hand, if the S content is more than 0.100%,
S will segregate at grain boundaries and the steel will become
brittle, and the hot workability of the steel will decrease.
Accordingly, the S content is from 0.010 to 0.100%. When
prioritizing mechanical properties over machinability, a preferable
lower limit of the S content is 0.015%, and a preferable upper
limit is 0.030%. When prioritizing machinability, a preferable
lower limit of the S content is 0.030%, and a preferable upper
limit is 0.050%.
[0061] Pb: 0.010 to 0.100%
[0062] Lead (Pb) forms Pb inclusions (Pb grains) by itself, and
enhances the machinability of the steel. Pb also combines with MnS
inclusions to form composite inclusions and enhance the
machinability of the steel, and in particular enhance the chip
treatability. The aforementioned effects are not obtained if the Pb
content is less than 0.010%. On the other hand, if the Pb content
is more than 0.100%, although the machinability will be enhanced,
the steel will become brittle. As a result, the hot workability of
the steel will decrease. In addition, if the Pb content is more
than 0.100%, because the Pb inclusions will excessively increase,
the rusting characteristics of the steel will decrease.
Accordingly, the Pb content is from 0.010 to 0.100%. A preferable
lower limit of the Pb content for promoting the formation of
composite inclusions and enhancing the machinability is 0.020%, and
more preferably is 0.025%. A preferable upper limit of the Pb
content for enhancing the rusting characteristics is 0.050%.
[0063] Al: 0.010 to 0.050%
[0064] Aluminum (Al) deoxidizes the steel. In the steel for machine
structural use according to the present invention, in order to
suppress the formation of holes and surface defects during
solidification, deoxidation is performed by aluminum killing. As
described later, if deoxidation is performed by adding Al into the
molten steel after Mn and Si, oxides in the steel are reformed and
complex oxides containing Si and Al are formed. The complex oxides
easily become nuclei for crystallization of MnS inclusions.
Therefore, it is easy for MnS inclusions to disperse and
crystallize, and to grow and coarsen, and it is also easy for
composite inclusions containing MnS and Pb to form. In this case,
the machinability of the steel improves. In addition, in a case
where MnS inclusions disperse and crystallize, precipitation of
fine MnS precipitates is suppressed. In this case, the total number
of MnS inclusions, MnS precipitates, Pb inclusions and composite
inclusions increases. Consequently, the rusting characteristics of
the steel improve. Al also combines with N to form AlN and thereby
suppress coarsening of austenite grains in various kinds of heat
treatment. The aforementioned effects are not obtained if the Al
content is less than 0.010%.
[0065] On the other hand, if the Al content is more than 0.050%,
coarse complex oxides are liable to form. If coarse complex oxides
are formed in the steel, surface defects are liable to occur on the
steel. If coarse complex oxides are formed in the steel, the
fatigue strength of the steel will also decrease. In addition, if
the Al content is more than 0.050%, deoxidation will proceed
excessively, and the amount of oxygen in the molten steel will
decrease. In this case, it will be difficult to form MnS
inclusions, and the machinability (particularly, suppression of
tool wear) of the steel will decrease. In such a case, in addition,
it will be difficult for composite inclusions in which Pb is
combined with MnS inclusions to form, and a large number of
independent Pb inclusions will remain in the steel. As a result,
the total number of MnS inclusions, MnS precipitates, Pb inclusions
and composite inclusions will increase, and the rusting
characteristics will decrease. Accordingly, the Al content is from
0.010 to 0.050%. A preferable lower limit of the Al content for
obtaining a further effect of suppressing the coarsening of grains
by formation of AlN is 0.015%, and more preferably is 0.020%. A
preferable upper limit of the Al content is 0.035%. In the present
description, the term "Al content" means the content of
acid-soluble Al (sol. Al).
[0066] N: 0.015% or less
[0067] Nitrogen (N) is unavoidably contained. N combines with Al to
form AlN to thereby suppress coarsening of austenite grains during
heat treatment and enhance the strength of the steel. On the other
hand, if the N content is more than 0.015%, the cutting resistance
of the steel increases and the machinability decreases. If the N
content is more than 0.015%, the hot workability also decreases.
Accordingly, the N content is 0.015% or less. A preferable lower
limit of the N content is 0.002%, and more preferably is 0.004%. A
preferable upper limit of the N content is 0.012%, and more
preferably is 0.008%. In the present description, the term "N
content" means the total content of N (t-N).
[0068] O: 0.0005 to 0.0030%
[0069] Oxygen (O) is contained not only in oxides, but also in MnS
inclusions. O forms complex oxides that serve as nuclei for
crystallization of MnS inclusions. If the O content is less than
0.0005%, the formed amount of complex oxides will be insufficient,
and it will be difficult for MnS inclusions to crystallize in the
molten steel. In such a case, the machinability of the steel will
decrease. Furthermore, in such a case, a large number of fine MnS
precipitates will form after solidification. As a result, the total
number of MnS inclusions, MnS precipitates, Pb inclusions and
composite inclusions will increase, and rusting characteristics
will decrease. Furthermore, if the O content is more than 0.0030%,
coarse alumina-based oxides will form and promote cutting wear of
the tools, and hence the machinability of the steel will decrease.
Accordingly, the O content is from 0.0005 to 0.0030%. A preferable
lower limit of the O content for further improving the
machinability of the steel as well as the rusting characteristics
of the steel is 0.0007%, and more preferably is 0.0010%. A
preferable upper limit of the O content is 0.0025%, and more
preferably is 0.0020%. In the present description, the term "O
content" means the total content of oxygen (t-O).
[0070] Cr: 0.50 to 2.00%
[0071] Chromium (Cr) dissolves in the steel and enhances the
hardenability and temper softening resistance of the steel to
thereby increase the strength of the steel. As a result, the
rolling contact fatigue properties of the steel are enhanced. If a
carburizing is performed as a case hardening treatment, Cr also
deepens the hardened layer depth. The aforementioned effects are
not obtained if the Cr content is less than 0.50%. On the other
hand, if the Cr content is more than 2.00%, the hardenability will
be too high and a supercooled microstructure (martensite) will form
during cooling and the steel will become too hard. In such a case,
the machinability of the steel will decrease. If the Cr content is
more than 2.00%, in some cases austenite may stabilize at even a
low temperature and the steel will become brittle. Accordingly, the
Cr content is from 0.50 to 2.00%. A preferable lower limit of the
Cr content is 0.70%, and more preferably is 0.90%. A preferable
upper limit of the Cr content is 1.80%, and more preferably is
1.60%.
[0072] The balance of the chemical composition of the steel for
machine structural use according to the present embodiment is Fe
and impurities. Here, the term "impurities" refers to elements
which, during industrial production of the steel for machine
structural use, are mixed in from ore or scrap that is used as a
raw material, or from the production environment or the like, and
which are allowed within a range that does not adversely affect the
steel for machine structural use of the present embodiment.
[0073] [Optional Elements]
[0074] The chemical composition of the steel for machine structural
use of the present embodiment may further contain one or more types
of element selected from the group consisting of Ni, B, V, Mo, W,
Nb, Cu and Ti.
[0075] Ni: 0 to 3.50%
[0076] Nickel (Ni) is an optional element and need not be
contained. If contained, Ni dissolves in the steel and increases
the hardenability of the steel, and enhances the steel strength. Ni
also improves the ductility of the matrix. In addition, Ni
increases the toughness of the steel. Furthermore, Ni increases the
corrosion resistance of the steel. The aforementioned effects are
obtained to a certain extent if even a small amount of Ni is
contained. On the other hand, if the Ni content is more than 3.50%,
a large amount of retained austenite will remain. In such a case, a
part of the retained austenite will transform into martensite by
strain induced transformation, and the ductility of the steel will
decrease. Accordingly, the Ni content is from 0 to 3.50%.
[0077] A preferable lower limit of the Ni content for stably
obtaining the aforementioned effects is 0.02%, and more preferably
is 0.05%. A preferable upper limit of the Ni content for further
suppressing the formation of retained austenite is 2.50%, and more
preferably is 2.00%. When prioritizing toughness, a preferable
lower limit of the Ni content is 0.20%. Note that, Ni detoxifies Cu
and enhances the toughness. If the steel contains Cu, a preferable
lower limit of the Ni content is equal to or more than the Cu
content.
[0078] B: 0 to 0.0050%
[0079] Boron (B) is an optional element and need not be contained.
If contained, B increases the hardenability of the steel and
increases the steel strength. B also suppresses segregation at the
grain boundaries of P and S that decrease toughness, and thus
enhances the fracture characteristics. The aforementioned effects
are obtained to a certain extent if even a small amount of B is
contained. On the other hand, if the B content is more than
0.0050%, a large amount of BN will be formed and the steel will
become brittle. Accordingly, the B content is from 0 to 0.0050%. A
preferable lower limit of the B content in a case where Ti or Nb
that are nitride-forming elements is contained is 0.0005%. A
preferable upper limit of the B content is 0.0020%.
[0080] V: 0 to 0.70%
[0081] Vanadium (V) is an optional element and need not be
contained. If contained, V precipitates as V carbides, V nitrides,
or V carbo-nitrides during tempering and during nitriding, and
enhances the strength of the steel. The V precipitates (V carbides,
V nitrides and V carbo-nitrides) also suppress coarsening of
austenite grains and increase the toughness of the steel. In
addition, V dissolves in the steel and thereby increases the temper
softening resistance of the steel. The aforementioned effects are
obtained to a certain extent if even a small amount of V is
contained.
[0082] On the other hand, if the V content is more than 0.70%, V
precipitates will form even at a temperature equal to or higher
than the A.sub.3 point. It is difficult for V precipitates that are
formed at a temperature equal to or higher than the A.sub.3 point
to dissolve in the steel, and such V precipitates remain in the
steel as insoluble precipitates. When insoluble precipitates
remain, the amount of dissolved V decreases. Consequently, the
temper softening resistance of the steel decreases. In addition,
when insoluble precipitates remain, it is difficult for fine V
precipitates to precipitate by means of a heat treatment conducted
thereafter. In such a case, the strength of the steel decreases.
Accordingly, the V content is from 0 to 0.70%. A preferable lower
limit of the V content for stably obtaining the aforementioned
effects is 0.05%, and more preferably is 0.10%. A preferable upper
limit of the V content is 0.50%, and more preferably is 0.30%.
[0083] Mo: 0 to 0.70%
[0084] Molybdenum (Mo) is an optional element and need not be
contained. If contained, Mo precipitates as Mo carbides during a
heat treatment at a low temperature that is not more than the Ai,
such as a heat treatment for tempering or nitriding. Therefore, the
strength and temper softening resistance of the steel increase. Mo
also dissolves in the steel and increases the hardenability of the
steel. The aforementioned effects are obtained to a certain extent
if even a small amount of Mo is contained. On the other hand, if
the Mo content is more than 0.70%, the hardenability of the steel
will be too high. In such a case, a supercooled microstructure is
liable to form during rolling or a softening heat treatment before
wire drawing or the like. Accordingly, the Mo content is from 0 to
0.70%.
[0085] A preferable lower limit of the Mo content for stably
obtaining the aforementioned effects is 0.05%, more preferably is
0.10%, and further preferably is 0.15%. A preferable upper limit of
the Mo content for stably obtaining ferrite, pearlite and bainite
in the micro-structure of the steel is 0.40%, and more preferably
is 0.30%.
[0086] W: 0 to 0.70%
[0087] Tungsten (W) is an optional element and need not be
contained. If contained, W precipitates as W carbides in the steel
and enhances the strength and temper softening resistance of the
steel. W carbides form at a low temperature that is not more than
the A.sub.3 point. Therefore, unlike V, Nb, Ti and the like, it is
difficult for W to form insoluble precipitates. Consequently, W
carbides increase the strength and temper softening resistance of
the steel by precipitation strengthening. W also dissolves in the
steel and thereby increases the hardenability of the steel and
increases the steel strength. The aforementioned effects are
obtained to a certain extent if even a small amount of W is
contained.
[0088] On the other hand, if the W content is more than 0.70%, a
supercooled microstructure is liable to form, and the hot
workability of the steel will thus decrease. Accordingly, the W
content is from 0 to 0.70%. A preferable lower limit of the W
content for stably increasing the temper softening resistance of
the steel is 0.05%, and more preferably is 0.10%. A preferable
upper limit of the W content for stably obtaining ferrite, pearlite
and bainite in the micro-structure of the steel is 0.40%, and more
preferably is 0.30%.
[0089] It is difficult for W and Mo to form nitrides. Therefore,
these elements can enhance the temper softening resistance of the
steel without being influenced by the N content. A preferable total
content of W and Mo for obtaining a high temper softening
resistance is from 0.10 to 0.30%.
[0090] Nb: 0 to less than 0.050%
[0091] Niobium (Nb) is an optional element and need not be
contained. If contained, Nb forms Nb nitrides, Nb carbides, or Nb
carbo-nitrides and suppresses coarsening of austenite grains during
quenching or during normalizing. Nb also increases the strength of
the steel by precipitation strengthening. The aforementioned
effects are obtained to a certain extent even if a small amount of
Nb is contained. On the other hand, if the Nb content is more than
0.050%, insoluble precipitates form and the toughness of the steel
decreases. In addition, if the Nb content is more than 0.050%, a
supercooled microstructure is liable to form and consequently the
hot workability of the steel will decrease. Accordingly, the Nb
content is from 0 to less than 0.050%. A preferable lower limit of
the Nb content for stably obtaining the aforementioned effects is
0.001%, and more preferably is 0.005%. A preferable upper limit of
the Nb content is 0.030%, and more preferably is 0.015%.
[0092] Cu: 0 to 0.50%
[0093] Copper (Cu) is an optional element and need not be
contained. If contained, Cu prevents decarburization. Cu also
increases corrosion resistance, similarly to Ni. The aforementioned
effects are obtained to a certain extent if even a small amount of
Cu is contained. On the other hand, if the Cu content is more than
0.50%, the steel will become brittle and rolling defects are liable
to arise. Accordingly, the Cu content is from 0 to 0.50%. A
preferable lower limit of the Cu content for stably obtaining the
aforementioned effects is 0.05%, and more preferably is 0.10%. In a
case where 0.30% or more of Cu is contained, the hot ductility can
be maintained if the Ni content is higher than the Cu content.
[0094] Ti: 0 to 0.100%
[0095] Titanium (Ti) is an optional element and need not be
contained. If contained, Ti forms nitrides, carbides or
carbo-nitrides, and suppresses coarsening of austenite grains
during quenching and during normalizing. Ti also increases the
strength of the steel by precipitation strengthening. Ti also
deoxidizes the steel. In addition, in a case where B is contained,
Ti combines with dissolved N and maintains the amount of dissolved
B. In this case, the hardenability increases. The aforementioned
effects are obtained to a certain extent if even a small amount of
Ti is contained.
[0096] On the other hand, because Ti forms the aforementioned
nitrides and sulfides, Ti influences MnS inclusions and composite
inclusions. Specifically, if the Ti content is more than 0.100%,
the crystallized amount of MnS inclusions decreases, and formation
of composite inclusions also decreases. In this case, the rusting
characteristics of the steel decrease. In addition, if the Ti
content is too high, the Ti forms nitrides and sulfides, and the
fatigue strength decreases. Accordingly, the Ti content is from 0
to 0.100%. A preferable lower limit of the Ti content for
effectively obtaining the aforementioned effects is 0.003%. In
particular, in a case where B is contained, a preferable lower
limit of the Ti content for reducing dissolved N is 0.005%. A
preferable upper limit of the Ti content for increasing corrosion
resistance is 0.090%, and more preferably is 0.085%.
[0097] The steel for machine structural use of the present
embodiment may further contain Ca.
[0098] Ca: 0 to 0.0030%
[0099] Calcium (Ca) is an optional element and need not be
contained. If contained, Ca forms CaS or (Mn, Ca)S and spheroidizes
MnS inclusions, and reduces the amount of tool wear. As a result,
the machinability of the steel increases. The aforementioned
effects are obtained to a certain extent if even a small amount of
Ca is contained. On the other hand, if the Ca content is more than
0.0030%, oxide-based inclusions coarsen and the fatigue strength of
the steel decreases. Accordingly, the Ca content is from 0 to
0.0030%. A preferable lower limit of the Ca content for further
enhancing the machinability is 0.0001%. When prioritizing fatigue
strength over machinability, a preferable upper limit of the Ca
content is 0.0015%, and more preferably is 0.0003%.
[0100] [Regarding Formula (1)] The chemical composition of the
steel for machine structural use of the present embodiment also
satisfies Formula (1).
Mn/S.gtoreq.8.0 (1)
[0101] Where, a content (mass %) of a corresponding element is
substituted for each element in Formula (1).
[0102] Here, it is defined that F1=Mn/S. F1 means the Mn content
relative to the S content. If F1 is less than 8.0, it will be
difficult for MnS inclusions to adequately crystallize.
Consequently, the amount of dissolved S in the steel after
solidification will not adequately decrease, and a large number of
fine MnS precipitates will form after solidification. In such a
case, the rusting characteristics of the steel will decrease
because the total number of MnS inclusions, MnS precipitates, Pb
inclusions and composite inclusions cannot decrease. If the amount
of dissolved S in the steel after solidification cannot be
adequately decreased, dissolved S will remain at crystal grain
boundaries after solidification. As a result, in some cases the hot
workability of the steel will decrease.
[0103] On the other hand, if F1 is 8.0 or more, the Mn content will
be adequately high in comparison to the S content. In this case,
MnS inclusions in the molten steel will adequately crystallize and
grow. As a result, the amount of dissolved S in the steel after
solidification will be adequately decreased, and precipitation of
MnS precipitates in the steel after solidification can be
suppressed. Therefore, the total number of MnS inclusions, MnS
precipitates, Pb inclusions and composite inclusions in the steel
can be adequately reduced, and the rusting characteristics of the
steel improve. A preferable lower limit of F1 for improving the
rusting characteristics of the steel is 10.0, and more preferably
is 20.0.
[0104] [Regarding the Steel Micro-Structure]
[0105] The micro-structure of the steel for machine structural use
according to the present invention is mainly composed of ferrite,
pearlite and bainite. Specifically, a total area fraction of
ferrite, pearlite and bainite in the micro-structure of the steel
for machine structural use having the aforementioned chemical
composition is 99% or more.
[0106] The total area fraction of ferrite, pearlite and bainite in
the micro-structure can be measured by the following method. A
sample is taken from the steel for machine structural use. For
example, in a case where the steel for machine structural use is a
steel bar or a wire rod, in a transverse section (a face
perpendicular to the axial direction), a sample is taken from a
middle part of a radius R (hereunder, referred to as "R/2 part")
that links the external surface and the central axis. Of the entire
area of a transverse section (surface) of the sample of the R/2
part, a surface that is perpendicular to the central axis of the
steel for machine structural use is adopted as an observation
surface. After polishing the observation surface, the observation
surface is subjected to etching using 3% nitric acid-alcohol (nital
etching reagent). The etched observation surface is observed with
an optical microscope having a magnification of .times.200, and
photographic images of an arbitrary five visual fields are
generated.
[0107] In each visual field, the contrast differs for each of the
respective phases of ferrite, pearlite, bainite and the like.
Accordingly, the respective phases are identified based on the
contrast. The total area (.mu.m.sup.2) of ferrite, pearlite and
bainite among the identified phases is determined for each visual
field. The total area in the respective visual fields is totaled
for all of the visual fields (five visual fields), and the ratio
relative to the gross area of all the visual fields is determined.
The determined ratio is defined as the total area fraction (%) of
ferrite, pearlite and bainite.
[0108] [Number TN of Specific Inclusions]
[0109] According to the steel for machine structural use of the
present invention, a total number TN of inclusions (that is,
specific inclusions) which are any of MnS inclusions, Pb inclusions
and composite inclusions containing MnS and Pb and which have an
equivalent circular diameter of 5 .mu.m or more in the steel is 40
per mm.sup.2 or more.
[0110] If the number TN of specific inclusions is 40 per mm.sup.2
or more, coarse MnS inclusions having an equivalent circular
diameter of 5 .mu.m or more will adequately crystallize, and as a
result the total number of MnS inclusions, MnS precipitates, Pb
inclusions and composite inclusions can be adequately reduced.
Therefore, excellent machinability and excellent rusting
characteristics can both be realized in a compatible manner. On the
other hand, if the number TN of specific inclusions in the steel is
less than 40 per mm.sup.2, coarse MnS inclusions having an
equivalent circular diameter of 5 .mu.m or more do not adequately
crystallize, and as a result the total number of MnS inclusions,
MnS precipitates, Pb inclusions and composite inclusions cannot be
adequately reduced. Therefore, adequate rusting characteristics are
not obtained. A preferable lower limit of the number TN of specific
inclusions is 80 per mm.sup.2, and more preferably is 150 per
mm.sup.2. A preferable upper limit of the number TN of specific
inclusions is 300 per mm.sup.2. Note that, although an upper limit
of the equivalent circular diameter of the specific inclusions is
not particularly limited, for example, the upper limit is 200
.mu.m.
[0111] [Ratio of Number of Composite Inclusions Among Specific
Inclusions (Composite Ratio) RA]
[0112] Preferably, a ratio (hereunder, also referred to as
"composite ratio") RA of the total number (number per mm.sup.2) of
composite inclusions having an equivalent circular diameter of 5
.mu.m or more with respect to the number (number per mm.sup.2) of
specific inclusions is 40% or more.
[0113] As described above, the susceptibility of the steel to
rusting increases as the total number of MnS inclusions, MnS
precipitates, Pb inclusions and composite inclusions increases. In
this case, the larger the number of composite inclusions that the
MnS inclusions and Pb inclusions form, the more that the total
number of MnS inclusions, MnS precipitates, Pb inclusions and
composite inclusions can be reduced. In particular, the total
number of Pb inclusions in the steel can be reduced. Pb inclusions,
in particular, are liable to decrease the rusting characteristics.
If the composite ratio is 40% or more, the total number of MnS
inclusions, MnS precipitates, Pb inclusions and composite
inclusions can be reduced, and the number of Pb inclusions that are
independently present can also be reduced. As a result, the rusting
characteristics of the steel can be further enhanced. Accordingly,
the composite ratio RA is preferably 40% or more. In this case, the
rusting characteristics of the steel can be further enhanced. A
more preferable lower limit of the composite ratio RA is 60%, and
further preferably is 75%.
[0114] [Method of Measuring Number TN of Specific Inclusions and
Composite Ratio RA]
[0115] The number TN of specific inclusions and the composite ratio
RA can be measured by the following methods. A sample is taken from
the steel for machine structural use by the method described above.
Using a scanning electron microscope (SEM), 20 visual fields at a
transverse section (surface) of the sample of the R/2 part are
randomly observed at a magnification of .times.1000. Specific
inclusions (any of MnS inclusions, Pb inclusions and composite
inclusions for which an equivalent circular diameter is 5 .mu.m or
more) are identified in the respective visual fields (referred to
as "observation surfaces"). It is possible to distinguish specific
inclusions and other inclusions based on contrast. In addition,
among the specific inclusions, MnS inclusions, Pb inclusions and
composite inclusions are respectively identified by the following
method.
[0116] For each observation surface, an image of the S distribution
and Pb distribution in the observation surface is obtained by means
of a wavelength dispersive x-ray spectroscopy device (EPMA). FIG.
1A is a schematic diagram illustrating the S distribution in an
observation surface, which was obtained by EPMA analysis. FIG. 1B
is a schematic diagram illustrating the Pb distribution in the same
observation surface as in FIG. 1A, which was obtained by EPMA
analysis.
[0117] Reference numeral 10 in FIG. 1A denotes a region in which S
is present. Because S is almost entirely present as MnS, MnS can be
regarded as being present at the locations indicated by each
reference numeral 10 in FIG. 1A. Reference numeral 20 in FIG. 1B
denotes a region in which Pb is present.
[0118] As illustrated in FIG. 1B, in some cases, as shown by
reference numeral 20A, Pb is divided by rolling or the like and is
arranged in the rolling direction. The same applies with respect to
S. As illustrated in FIG. 2, in an image obtained by EPMA analysis,
in a case where adjacent inclusions IN each have an equivalent
circular diameter of 5 .mu.m or more, if a distance D between the
adjacent inclusions IN is not more than 10 .mu.m, these inclusions
IN are regarded as a single inclusion. Note that, as described
above, the term "equivalent circular diameter" means the diameter
of a circle in a case where the area of the respective inclusions
or respective precipitates is converted into a circle that has the
same area. Even when an inclusion group is defined as a single
inclusion, the equivalent circular diameter is the diameter of a
circle having the same total area as the inclusion group.
[0119] FIG. 1C is an image obtained by combining FIG. 1B with FIG.
1A. Referring to FIG. 1C, in the case where the Pb inclusions 20
overlap with the MnS inclusions 10, the relevant inclusions are
recognized as being composite inclusions 30. On the other hand,
referring to FIG. 1C, in the case where the MnS inclusion 10 and
the Pb inclusion 20 do not overlap (region A1 and region A2 in FIG.
1C), the relevant inclusions are identified as an MnS inclusion 10
and a Pb inclusion 20.
[0120] By the above method, MnS inclusions, Pb inclusions and
composite inclusions are identified using a scanning microscope and
EPMA. The area of each inclusion that is identified is determined,
and the diameter of a circle with the same area is determined as
the equivalent circular diameter (m) for each of the
inclusions.
[0121] Among the respective inclusions, specific inclusions for
which the equivalent circular diameter is 5 .mu.m or more are
identified. The total number (number in 20 visual fields) of the
specific inclusions that are identified is determined, and is
converted to a number TN per mm.sup.2 (inclusions/mm.sup.2). The
number TN of specific inclusions is determined by the above method.
In addition, among the identified specific inclusions, a number MN
of composite inclusions (inclusions/mm.sup.2) for which the
equivalent circular diameter is 5 .mu.m or more is determined, and
the composite ratio RA (%) is determined based on the following
Formula (2).
RA=MN/TN.times.100 (2)
[0122] [Production Method]
[0123] An example of a method for producing the steel for machine
structural use according to the present invention will now be
described. According to the present embodiment, a method for
producing a steel bar or a wire rod as an example of the steel for
machine structural use will be described. However, a steel for
machine structural use according to the present invention is not
limited to a steel bar or a wire rod.
[0124] One example of the production method includes a steel making
process of refining and casting molten steel to produce a starting
material (a cast piece or an ingot), and a hot working process of
subjecting the starting material to hot working to produce a steel
for machine structural use. Hereunder, each of these processes is
described.
[0125] [Steel Making Process]
[0126] The steel making process includes a refining process and a
casting process.
[0127] [Refining Process]
[0128] In the refining process, firstly, hot metal that was
produced by a well-known method is subjected to refining (primary
refining) using a converter. Molten steel that was tapped from the
converter is subjected to secondary refining. In the secondary
refining, an alloy whose components have been adjusted is added to
the molten steel to thereby produce a molten steel having the
aforementioned chemical composition.
[0129] Specifically, Mn is added to the molten steel that was
tapped from the converter. As a result, oxides that are mainly
composed of Mn form in the molten steel. After addition of the Mn
is completed, Si which has a stronger deoxidizing power than Mn is
added. As a result, the oxides that are mainly composed of Mn are
reformed to oxides that are mainly composed of Si. After addition
of the Si is completed, Al which has an even stronger deoxidizing
power than Si is added. As a result, the oxides that are mainly
composed of Si are reformed to complex oxides containing Si and Al
(hereinafter, also referred to simply as "complex oxides").
[0130] The complex oxides that were formed by the above described
refining process serve as nuclei for crystallization of MnS
inclusions. Therefore, by forming the complex oxides, MnS
inclusions adequately crystallize and grow coarse. That is, if
complex oxides form, it is easy for specific inclusions that are
inclusions having an equivalent circular diameter of 5 .mu.m or
more to form, and the number TN of specific inclusions becomes 40
per mm.sup.2 or more. As a result, the amount of dissolved S in the
steel after solidification is adequately reduced, and precipitation
of MnS precipitates in the steel after solidification can be
suppressed. Therefore, the total number of MnS inclusions, MnS
precipitates, Pb inclusions and composite inclusions can be
adequately reduced, and the rusting characteristics of the steel
are enhanced.
[0131] After deoxidation, slag is removed as well known. After
removing slag, secondary refining is performed. For example,
composite refining is performed as the secondary refining. For
example, first a primary treatment that uses an LF (ladle furnace)
or VAD (vacuum arc degassing) is performed. In addition an RH
(Ruhrstahl-Hausen) vacuum degassing may be performed. In the
secondary refining, Mn, Si, and other elements are added as
necessary to adjust the components of the molten steel. After
adjusting the components of the molten steel, a casting process is
performed.
[0132] [Casting Process]
[0133] A starting material (a cast piece or an ingot) is produced
using the molten steel produced by the above described refining
process. Specifically, a cast piece is produced by a continuous
casting process using the molten steel. Alternatively, an ingot may
be produced by an ingot-making process using the molten steel.
Hereinafter, a cast piece and an ingot are referred to generically
as "starting material". A cross-sectional area of the starting
material in this case is, for example, 200 to 350 mm.times.200 to
600 mm.
[0134] A solidification cooling rate RC during casting is
100.degree. C./min or less. If the solidification cooling rate RC
is 100.degree. C./min or less, MnS inclusions adequately
crystallize and grow in the molten steel. Therefore, it is easy for
specific inclusions to form, and the number TN thereof becomes 40
per mm.sup.2 or more. As a result, the amount of dissolved S in the
steel after solidification is adequately reduced, and precipitation
of MnS precipitates in the steel after solidification can be
suppressed. Therefore, the total number of MnS inclusions, MnS
precipitates, Pb inclusions and composite inclusions can be
adequately reduced, and the rusting characteristics of the steel
are enhanced.
[0135] On the other hand, if the solidification cooling rate RC is
more than 100.degree. C./min, MnS inclusions do not adequately
crystallize, and MnS inclusions also do not adequately grow.
Therefore, it will be difficult for specific inclusions to be
formed, and the number TN of specific inclusions will be less than
40 per mm.sup.2. In this case, the amount of dissolved S in the
steel after solidification cannot be adequately reduced, and a
large number of fine MnS precipitates will form after
solidification. As a result, since the total number of MnS
inclusions, MnS precipitates, Pb inclusions and composite
inclusions cannot be reduced, the rusting characteristics of the
steel will decline. Accordingly, the solidification cooling rate RC
is 100.degree. C./min or less.
[0136] A preferable solidification cooling rate RC is from 8 to
less than 50.degree. C./min. In this case, it is even easier for
MnS inclusions to crystallize and grow. Furthermore, if the
solidification cooling rate RC is from 8 to less than 50.degree.
C./min, because the time period until solidifying is long, a
sufficient time period for Pb to move through the molten steel and
adhere to MnS inclusions can be secured. Therefore, it is easy for
composite inclusions containing MnS and Pb to form, and the
composite ratio RA becomes 40% or more. A more preferable upper
limit of the solidification cooling rate RC is 30.degree. C./min. A
more preferable lower limit of the solidification cooling rate RC
is 10.degree. C./min, and further preferably is 15.degree.
C./min.
[0137] The solidification cooling rate RC can be determined based
on the starting material that was cast. FIG. 3 is a transverse
sectional view of a starting material that was cast. In the
starting material having a thickness W (mm), at a point P1 located
at a position at a depth of W/4 towards the center of the starting
material from the surface, the cooling rate from the liquidus
temperature to the solidus temperature is defined as the
solidification cooling rate RC (.degree. C./min) in the casting
process. The solidification cooling rate RC can be determined by
the following method. After solidification, the starting material
is cut in the transverse direction. In the transverse section of
the starting material, a secondary dendrite arm spacing 22 (.mu.m)
in the thickness direction of the solidification structure at the
point P1 is measured. Using the measurement value 22, the
solidification cooling rate RC (.degree. C./min) is determined
based on the following Formula (3).
RC=(.lamda.2/770).sup.-(1/0.41) (3)
[0138] The secondary dendrite arm spacing 22 depends on the
solidification cooling rate RC. Accordingly, the solidification
cooling rate RC can be determined by measuring the secondary
dendrite arm spacing 22.
[0139] [Hot Working Process]
[0140] In the hot working process, hot working is usually performed
one or a plurality of times. The starting material is heated before
each hot working operation is performed. Thereafter, the starting
material is subjected to the hot working. The hot working is, for
example, hot forging or hot rolling. In the case of performing hot
working a plurality of times, the initial hot working is, for
example, blooming or hot forging, and the next hot working is
finish rolling using a continuous mill. In the hot rolling mill, a
horizontal stand having a pair of horizontal rolls, and a vertical
stand having a pair of vertical rolls are alternately arranged in a
row. The starting material after hot working is cooled by a
well-known cooling method such as air cooling.
[0141] The steel for machine structural use according to the
present embodiment is produced by the above described processes.
The steel for machine structural use is, for example, a steel bar
or a wire rod.
[0142] The steel for machine structural use produced by the above
described method is excellent in machinability and rusting
characteristics. Production of the steel for machine structural use
into a machine component is performed, for example, by the
following method.
[0143] The steel for machine structural use is subjected to hot
forging to produce an intermediate product having a rough shape. As
necessary, the intermediate product is subjected to a normalizing.
The intermediate product is also subjected to machining. The
machining is, for example, cutting. The intermediate product that
underwent machining may be subjected to a thermal refining
treatment (quenching and tempering). In a case where a thermal
refining treatment is performed, the machining such as cutting may
be performed on the intermediate product after the thermal refining
treatment. A machine component is produced by the above process. A
machine component may also be produced by performing cold forging
instead of hot forging.
EXAMPLES
[0144] Molten steels having the chemical compositions shown in
Table 1 were produced.
TABLE-US-00001 TABLE 1 Test Num- Chemical Composition (unit is mass
%; balance is Fe and impurities) ber C Si Mn P S Pb Al t-N t-O Cr
Ni B V Mo W Nb Cu Ti Ca 1 0.21 0.76 0.93 0.010 0.042 0.070 0.038
0.010 0.0011 0.90 -- -- -- 0.07 -- -- -- -- -- 2 0.18 0.35 0.25
0.012 0.021 0.024 0.018 0.011 0.0013 0.93 -- -- -- 0.06 -- -- -- --
-- 3 0.22 0.29 1.12 0.009 0.090 0.052 0.047 0.005 0.0016 1.12 -- --
-- 0.19 -- -- -- -- -- 4 0.18 0.21 0.58 0.013 0.056 0.012 0.015
0.006 0.0009 1.09 -- -- -- 0.08 -- -- -- -- -- 5 0.21 0.49 0.99
0.007 0.018 0.058 0.047 0.015 0.0017 1.29 -- -- -- 0.27 -- -- -- --
-- 6 0.19 0.30 0.62 0.012 0.020 0.058 0.030 0.005 0.0005 1.10 -- --
-- 0.11 -- -- -- -- -- 7 0.16 0.28 0.44 0.010 0.041 0.033 0.034
0.012 0.0010 0.55 -- -- -- 0.57 -- -- -- -- -- 8 0.29 0.48 1.14
0.011 0.057 0.032 0.015 0.010 0.0012 0.94 -- -- -- 0.12 -- -- -- --
-- 9 0.15 0.33 0.61 0.010 0.016 0.054 0.034 0.010 0.0010 1.25 -- --
-- 0.30 -- -- -- -- -- 10 0.21 0.01 0.73 0.009 0.048 0.041 0.024
0.009 0.0012 1.16 -- -- -- 0.26 -- -- -- -- -- 11 0.22 0.42 1.96
0.010 0.056 0.064 0.043 0.006 0.0007 0.96 -- -- -- 0.16 -- -- -- --
-- 12 0.19 0.37 1.15 0.029 0.044 0.055 0.016 0.009 0.0012 0.97 --
-- -- 0.20 -- -- -- -- -- 13 0.19 0.49 0.85 0.004 0.026 0.028 0.049
0.009 0.0008 0.97 -- -- -- 0.07 -- -- -- -- -- 14 0.21 0.41 0.59
0.011 0.012 0.039 0.028 0.011 0.0017 1.19 -- -- -- 0.15 -- -- -- --
-- 15 0.19 0.42 0.58 0.012 0.044 0.095 0.026 0.011 0.0012 1.02 --
-- -- 0.23 -- -- -- -- -- 16 0.19 0.26 0.67 0.006 0.049 0.067 0.046
0.006 0.0011 1.31 -- -- -- 0.18 -- -- -- -- -- 17 0.21 0.39 0.63
0.014 0.024 0.025 0.013 0.008 0.0015 0.92 -- -- -- 0.09 -- -- -- --
-- 18 0.22 0.40 1.15 0.008 0.038 0.019 0.022 0.006 0.0026 1.02 --
-- -- 0.06 -- -- -- -- -- 19 0.21 0.21 0.55 0.014 0.050 0.024 0.036
0.005 0.0007 1.24 -- -- -- 0.25 -- -- -- -- -- 20 0.19 0.33 1.16
0.009 0.028 0.021 0.044 0.008 0.0011 1.85 -- -- -- -- -- -- -- --
-- 21 0.19 0.31 1.01 0.012 0.036 0.059 0.049 0.005 0.0011 0.55 --
-- -- 0.24 -- -- -- -- -- 22 0.21 0.25 0.97 0.012 0.044 0.059 0.015
0.009 0.0021 1.25 -- -- -- 0.05 -- -- -- -- -- 23 0.24 0.24 0.40
0.012 0.048 0.026 0.026 0.008 0.0014 0.98 2.26 -- -- 0.09 -- -- --
-- -- 24 0.22 0.45 1.01 0.008 0.044 0.031 0.015 0.008 0.0017 0.95
-- 0.0040 -- 0.13 -- -- -- -- -- 25 0.19 0.31 0.45 0.005 0.033
0.067 0.040 0.010 0.0017 1.25 -- -- 0.65 0.14 -- -- -- -- -- 26
0.17 0.32 0.42 0.012 0.030 0.039 0.040 0.009 0.0007 0.95 -- -- --
0.08 0.65 -- -- -- -- 27 0.18 0.30 0.70 0.015 0.026 0.031 0.019
0.011 0.0014 1.11 -- -- -- 0.09 -- 0.046 -- -- -- 28 0.21 0.41 0.63
0.005 0.040 0.052 0.044 0.004 0.0011 0.94 -- -- -- 0.08 -- -- 0.27
-- -- 29 0.20 0.40 0.73 0.005 0.045 0.028 0.031 0.009 0.0018 1.09
-- -- -- 0.29 -- -- -- 0.080 -- 30 0.20 0.21 0.83 0.007 0.048 0.056
0.021 0.006 0.0015 1.24 -- -- -- 0.07 -- -- -- -- 0.0004 31 0.21
0.30 0.93 0.008 0.056 0.043 0.025 0.009 0.0020 1.07 0.48 -- -- 0.27
-- -- 0.45 -- -- 32 0.29 0.49 1.15 0.010 0.058 0.034 0.016 0.011
0.0013 0.92 -- -- -- 0.11 -- -- -- -- -- 33 0.19 0.01 0.71 0.007
0.042 0.042 0.025 0.008 0.0015 1.15 -- -- -- 0.27 -- -- -- -- -- 34
0.18 0.38 1.10 0.029 0.043 0.054 0.017 0.011 0.0010 0.95 -- -- --
0.21 -- -- -- -- -- 35 0.20 0.39 0.57 0.015 0.012 0.041 0.032 0.010
0.0019 1.23 -- -- -- 0.16 -- -- -- -- -- 36 0.20 0.24 0.66 0.006
0.051 0.061 0.046 0.007 0.0012 1.26 -- -- -- 0.17 -- -- -- -- -- 37
0.20 0.42 1.17 0.006 0.035 0.020 0.022 0.006 0.0026 0.98 -- -- --
0.05 -- -- -- -- -- 38 0.20 0.31 1.11 0.011 0.029 0.025 0.046 0.006
0.0009 1.85 -- -- -- -- -- -- -- -- -- 39 0.18 0.28 0.91 0.011
0.041 0.061 0.016 0.008 0.0017 1.18 -- -- -- 0.05 -- -- -- -- -- 40
0.20 0.25 0.68 0.014 0.050 0.028 0.024 0.007 0.0014 0.96 2.26 -- --
0.08 -- -- -- -- -- 41 0.21 0.41 0.98 0.006 0.048 0.035 0.016 0.006
0.0019 0.91 -- 0.0040 -- 0.12 -- -- -- -- -- 42 0.18 0.38 0.71
0.006 0.045 0.026 0.028 0.008 0.0020 1.10 -- -- -- 0.30 -- -- --
0.080 -- 43 0.22 0.21 0.50 0.028 0.077 0.041 0.030 0.005 0.0015
1.21 -- -- -- -- -- -- -- -- -- 44 0.22 0.23 0.30 0.024 0.051 0.057
0.025 0.013 0.0014 1.14 -- -- -- -- -- -- -- -- -- 45 0.18 0.49
0.31 0.007 0.042 0.041 0.026 0.006 0.0013 1.09 -- -- -- 0.12 -- --
-- -- -- 46 0.22 0.32 2.17 0.008 0.044 0.043 0.024 0.006 0.0013
1.27 -- -- -- 0.07 -- -- -- -- -- 47 0.22 0.35 2.07 0.015 0.038
0.023 0.021 0.006 0.0014 1.07 -- -- -- 0.15 -- -- -- -- -- 48 0.20
0.47 0.18 0.008 0.012 0.047 0.026 0.004 0.0015 1.13 -- -- -- 0.29
-- -- -- -- -- 49 0.20 0.40 0.18 0.011 0.012 0.056 0.029 0.008
0.0009 1.07 -- -- -- 0.24 -- -- -- -- -- 50 0.18 0.47 0.96 0.014
0.113 0.048 0.021 0.008 0.0020 1.06 -- -- -- 0.13 -- -- -- -- -- 51
0.20 0.48 0.86 0.012 0.152 0.032 0.047 0.011 0.0018 1.10 -- -- --
0.15 -- -- -- -- -- 52 0.18 0.39 0.84 0.009 0.007 0.030 0.029 0.006
0.0010 1.07 -- -- -- 0.12 -- -- -- -- -- 53 0.22 0.28 0.86 0.011
0.007 0.036 0.015 0.012 0.0018 1.24 -- -- -- 0.18 -- -- -- -- -- 54
0.21 0.24 0.95 0.014 0.028 0.113 0.025 0.005 0.0015 0.93 -- -- --
0.19 -- -- -- -- -- 55 0.18 0.44 0.62 0.015 0.028 0.201 0.025 0.008
0.0018 1.14 -- -- -- 0.25 -- -- -- -- -- 56 0.18 0.46 1.17 0.013
0.036 0.008 0.020 0.005 0.0007 1.24 -- -- -- 0.22 -- -- -- -- -- 57
0.20 0.43 0.72 0.014 0.043 0.004 0.024 0.008 0.0010 0.92 -- -- --
0.19 -- -- -- -- -- 58 0.18 0.24 0.55 0.011 0.025 0.024 0.008 0.005
0.0029 0.95 -- -- -- 0.07 -- -- -- -- -- 59 0.18 0.39 0.90 0.008
0.033 0.041 0.030 0.019 0.0014 1.30 -- -- -- 0.19 -- -- -- -- -- 60
0.18 0.36 0.96 0.011 0.027 0.029 0.027 0.005 0.0034 1.27 -- -- --
0.08 -- -- -- -- -- 61 0.20 0.25 0.99 0.008 0.017 0.043 0.047 0.008
0.0032 0.91 -- -- -- 0.18 -- -- -- -- -- 62 0.18 0.28 0.99 0.012
0.032 0.020 0.035 0.007 0.0003 0.91 -- -- -- 0.19 -- -- -- -- -- 63
0.18 0.30 1.07 0.008 0.027 0.047 0.025 0.010 0.0015 2.11 -- -- --
0.08 -- -- -- -- -- 64 0.19 0.24 1.03 0.005 0.032 0.066 0.042 0.008
0.0018 0.42 -- -- -- 0.09 -- -- -- -- -- 65 0.22 0.40 0.66 0.007
0.041 0.065 0.041 0.005 0.0012 0.92 -- -- -- 0.09 -- -- -- -- -- 66
0.24 0.44 0.68 0.009 0.048 0.059 0.035 0.009 0.0011 0.98 -- -- --
0.08 -- -- -- -- --
[0145] The molten steel of each test number was produced by the
following method. Hot metals produced by a well-known method were
subjected to primary refining under the same conditions using a
converter to thereby produce the molten steels of the respective
test numbers.
[0146] For the molten steels of test numbers other than Test
Numbers 65 and 66, after tapping the molten steel from the
converter, Mn, Si and Al were added in that order to perform
deoxidation. For the molten steel of Test Number 65, after tapping
the molten steel from the converter, Si, Al and Mn were added in
that order to perform deoxidation. For the molten steel of Test
Number 66, after tapping the molten steel from the converter, Mn,
Al and Si were added in that order to perform deoxidation.
[0147] After deoxidation, slag was removed. After removing slag, a
primary treatment was performed using VAD, and thereafter an RH
vacuum degassing was performed. After the RH vacuum degassing,
final adjustment of alloying elements was performed. Molten steels
having the chemical compositions shown in Table 1 were produced by
the above described process.
[0148] Each of the molten steels was cast to produce an ingot for
test use that had a rectangular parallelepiped shape. The cross
sectional shape of the ingot was a rectangular shape with
dimensions of 190 mm.times.190 mm. The solidification cooling rates
RC (.degree. C./min) for the respective test numbers were as listed
in Table 2. The solidification cooling rate RC was determined by
measuring a secondary dendrite arm spacing of the ingot and
applying the determined value to the aforementioned Formula
(3).
TABLE-US-00002 TABLE 2 Rolling Contact Test TN Machinability
Fatigue Hot Num- Deoxidation RC F1 = Micro- (inclusions/ RA Rusting
CL1000 Chip Life Ductility Overall ber Order (.degree. C./min) Mn/S
structure mm.sup.2) (%) Characteristics (m/min) Treatability
(hours) (%) Evaluation 1 Mn.fwdarw.Si.fwdarw.Al 18.3 21.8 F + P + B
116.7 92.2 .circle-w/dot. 52 .largecircle. 6.2 70 .largecircle. 2
Mn.fwdarw.Si.fwdarw.Al 41.9 11.9 F + P + B 81.2 90.0 .circle-w/dot.
63 .largecircle. 5.2 70 .largecircle. 3 Mn.fwdarw.Si.fwdarw.Al 28.0
12.4 F + P + B 102.9 85.3 .circle-w/dot. 91 .largecircle. 7.1 77
.largecircle. 4 Mn.fwdarw.Si.fwdarw.Al 19.3 10.2 F + P + B 144.8
86.5 .circle-w/dot. 57 .largecircle. 7.9 80 .largecircle. 5
Mn.fwdarw.Si.fwdarw.Al 47.8 55.6 F + P + B 104.8 64.7
.circle-w/dot. 67 .largecircle. 5.1 79 .largecircle. 6
Mn.fwdarw.Si.fwdarw.Al 23.1 31.5 F + P + B 223.5 74.8
.circle-w/dot. 71 .largecircle. 5.8 85 .largecircle. 7
Mn.fwdarw.Si.fwdarw.Al 39.0 10.7 F + P + B 147.4 74.8
.circle-w/dot. 69 .largecircle. 5.0 81 .largecircle. 8
Mn.fwdarw.Si.fwdarw.Al 72.4 20.0 F + P + B 47.6 26.1 .largecircle.
56 .largecircle. 5.8 74 .largecircle. 9 Mn.fwdarw.Si.fwdarw.Al 61.1
38.9 F + P + B 56.9 25.2 .largecircle. 51 .largecircle. 7.9 79
.largecircle. 10 Mn.fwdarw.Si.fwdarw.Al 58.1 15.2 F + P + B 141.2
38.6 .largecircle. 68 .largecircle. 7.1 74 .largecircle. 11
Mn.fwdarw.Si.fwdarw.Al 92.5 35.0 F + P + B 46.0 24.0 .largecircle.
72 .largecircle. 5.9 84 .largecircle. 12 Mn.fwdarw.Si.fwdarw.Al
81.9 26.1 F + P + B 46.2 31.2 .largecircle. 53 .largecircle. 4.9 71
.largecircle. 13 Mn.fwdarw.Si.fwdarw.Al 55.8 33.2 F + P + B 246.7
21.6 .largecircle. 56 .largecircle. 6.6 74 .largecircle. 14
Mn.fwdarw.Si.fwdarw.Al 67.5 49.2 F + P + B 58.4 32.8 .largecircle.
57 .largecircle. 6.8 78 .largecircle. 15 Mn.fwdarw.Si.fwdarw.Al
86.2 13.2 F + P + B 52.6 38.2 .largecircle. 63 .largecircle. 5.6 77
.largecircle. 16 Mn.fwdarw.Si.fwdarw.Al 85.2 13.7 F + P + B 61.2
29.8 .largecircle. 76 .largecircle. 6.4 79 .largecircle. 17
Mn.fwdarw.Si.fwdarw.Al 61.7 26.4 F + P + B 114.7 39.9 .largecircle.
51 .largecircle. 5.5 76 .largecircle. 18 Mn.fwdarw.Si.fwdarw.Al
72.2 30.3 F + P + B 69.7 34.1 .largecircle. 52 .largecircle. 6.4 80
.largecircle. 19 Mn.fwdarw.Si.fwdarw.Al 61.5 10.9 F + P + B 92.8
37.8 .largecircle. 61 .largecircle. 7.7 71 .largecircle. 20
Mn.fwdarw.Si.fwdarw.Al 62.2 41.4 F + P + B 82.5 34.8 .largecircle.
54 .largecircle. 5.4 71 .largecircle. 21 Mn.fwdarw.Si.fwdarw.Al
53.9 28.0 F + P + B 136.1 34.2 .largecircle. 55 .largecircle. 7.5
76 .largecircle. 22 Mn.fwdarw.Si.fwdarw.Al 58.7 22.0 F + P + B
102.8 35.8 .largecircle. 58 .largecircle. 6.5 72 .largecircle. 23
Mn.fwdarw.Si.fwdarw.Al 69.1 8.3 F + P + B 97.7 32.8 .largecircle.
68 .largecircle. 5.9 74 .largecircle. 24 Mn.fwdarw.Si.fwdarw.Al
60.2 23.0 F + P + B 111.3 38.6 .largecircle. 66 .largecircle. 7.4
80 .largecircle. 25 Mn.fwdarw.Si.fwdarw.Al 51.9 13.7 F + P + B 48.4
37.7 .largecircle. 65 .largecircle. 7.5 74 .largecircle. 26
Mn.fwdarw.Si.fwdarw.Al 47.4 14.0 F + P + B 66.2 60.9 .circle-w/dot.
64 .largecircle. 6.7 71 .largecircle. 27 Mn.fwdarw.Si.fwdarw.Al
76.4 26.7 F + P + B 73.1 31.0 .largecircle. 67 .largecircle. 5.4 75
.largecircle. 28 Mn.fwdarw.Si.fwdarw.Al 95.5 15.7 F + P + B 50.9
37.6 .largecircle. 69 .largecircle. 7.3 70 .largecircle. 29
Mn.fwdarw.Si.fwdarw.Al 76.4 16.2 F + P + B 87.9 36.4 .largecircle.
68 .largecircle. 6.5 71 .largecircle. 30 Mn.fwdarw.Si.fwdarw.Al
23.7 17.2 F + P + B 106.9 67.2 .circle-w/dot. 50 .largecircle. 6.2
76 .largecircle. 31 Mn.fwdarw.Si.fwdarw.Al 24.8 16.5 F + P + B
124.2 56.6 .circle-w/dot. 78 .largecircle. 7.0 75 .largecircle. 32
Mn.fwdarw.Si.fwdarw.Al 104.1 19.9 F + P + B 38.8 25.2 X 57
.largecircle. 6.0 77 X 33 Mn.fwdarw.Si.fwdarw.Al 192.9 17.1 F + P +
B 37.5 29.3 X 71 .largecircle. 7.9 78 X 34 Mn.fwdarw.Si.fwdarw.Al
109.5 25.6 F + P + B 30.7 20.3 X 54 .largecircle. 5.1 77 X 35
Mn.fwdarw.Si.fwdarw.Al 198.4 47.7 F + P + B 31.6 34.0 X 59
.largecircle. 7.7 83 X 36 Mn.fwdarw.Si.fwdarw.Al 183.3 12.8 F + P +
B 37.1 22.0 X 77 .largecircle. 6.5 80 X 37 Mn.fwdarw.Si.fwdarw.Al
199.8 33.7 F + P + B 23.0 37.3 X 50 .largecircle. 6.3 84 X 38
Mn.fwdarw.Si.fwdarw.Al 131.0 37.9 F + P + B 23.6 20.9 X 60
.largecircle. 5.9 74 X 39 Mn.fwdarw.Si.fwdarw.Al 170.2 22.2 F + P +
B 37.0 39.6 X 60 .largecircle. 7.0 78 X 40 Mn.fwdarw.Si.fwdarw.Al
165.7 13.6 F + P + B 31.3 37.3 X 70 .largecircle. 5.9 79 X 41
Mn.fwdarw.Si.fwdarw.Al 192.1 20.2 F + P + B 22.6 30.2 X 65
.largecircle. 7.7 84 X 42 Mn.fwdarw.Si.fwdarw.Al 137.2 15.9 F + P +
B 31.2 25.0 X 67 .largecircle. 6.9 76 X 43 Mn.fwdarw.Si.fwdarw.Al
23.4 6.5 F + P + B 24.7 36.5 X 53 .largecircle. 4.1 48 X 44
Mn.fwdarw.Si.fwdarw.Al 56.2 5.9 F + P + B 36.1 32.7 X 68
.largecircle. 4.9 56 X 45 Mn.fwdarw.Si.fwdarw.Al 188.3 7.4 F + P +
B 32.7 33.6 X 62 .largecircle. 4.5 44 X 46 Mn.fwdarw.Si.fwdarw.Al
11.2 49.3 F + P + B 76.5 70.1 .circle-w/dot. 21 .largecircle. 6.5
82 X 47 Mn.fwdarw.Si.fwdarw.Al 11.4 54.5 F + P + B 50.0 85.2
.circle-w/dot. 43 .largecircle. 6.1 84 X 48 Mn.fwdarw.Si.fwdarw.Al
8.3 15.0 F + P + B 32.5 24.2 X 63 .largecircle. 4.1 59 X 49
Mn.fwdarw.Si.fwdarw.Al 13.6 15.0 F + P + B 25.6 30.2 X 57
.largecircle. 4.8 55 X 50 Mn.fwdarw.Si.fwdarw.Al 8.6 8.5 F + P + B
109.6 75.2 .circle-w/dot. 51 .largecircle. 7.6 49 X 51
Mn.fwdarw.Si.fwdarw.Al 9.4 5.7 F + P + B 37.9 29.0 X 88
.largecircle. 6.8 49 X 52 Mn.fwdarw.Si.fwdarw.Al 8.9 120.2 F + P +
B 15.7 32.8 X 51 .largecircle. 6.2 84 X 53 Mn.fwdarw.Si.fwdarw.Al
24.1 123.1 F + P + B 27.8 35.5 X 50 .largecircle. 6.8 70 X 54
Mn.fwdarw.Si.fwdarw.Al 10.9 33.9 F + P + B 77.4 35.8 X 84
.largecircle. 5.4 59 X 55 Mn.fwdarw.Si.fwdarw.Al 15.4 22.3 F + P +
B 80.8 41.0 X 86 .largecircle. 7.1 59 X 56 Mn.fwdarw.Si.fwdarw.Al
14.9 32.5 F + P + B 52.9 71.1 .circle-w/dot. 20 X 6.1 80 X 57
Mn.fwdarw.Si.fwdarw.Al 16.2 16.7 F + P + B 85.7 64.5 .circle-w/dot.
32 X 6.3 71 X 58 Mn.fwdarw.Si.fwdarw.Al 51.2 22.0 F + P + B 38.1
34.8 X 55 .largecircle. 5.6 75 X 59 Mn.fwdarw.Si.fwdarw.Al 22.3
27.3 F + P + B 52.9 68.1 .circle-w/dot. 28 .largecircle. 5.4 62 X
60 Mn.fwdarw.Si.fwdarw.Al 29.6 35.6 F + P + B 47.1 87.8
.circle-w/dot. 25 .largecircle. 3.4 81 X 61 Mn.fwdarw.Si.fwdarw.Al
11.9 58.3 F + P + B 54.0 63.8 .circle-w/dot. 40 .largecircle. 1.2
81 X 62 Mn.fwdarw.Si.fwdarw.Al 22.1 30.9 F + P + B 37.2 86.2 X 26 X
5.7 83 X 63 Mn.fwdarw.Si.fwdarw.Al 21.9 39.9 F + P + B 53.8 76.6
.circle-w/dot. 38 .largecircle. 7.8 81 X 64 Mn.fwdarw.Si.fwdarw.Al
11.0 32.1 F + P + B 79.9 87.2 .circle-w/dot. 75 .largecircle. 1.1
83 X 65 Si.fwdarw.Al.fwdarw.Mn 44.5 16.1 F + P + B 29.9 37.6 X 68
.largecircle. 7.2 72 X 66 Mn.fwdarw.Al.fwdarw.Si 41.5 14.2 F + P +
B 36.4 36.5 X 66 .largecircle. 7.3 70 X
[0149] The produced ingots for test use were subjected to hot
working twice to produce a steel bar. In the hot working, blooming
was performed, and thereafter finish rolling (steel bar rolling)
was performed. The produced test ingot was subjected to hot forging
to produce a steel bar having a diameter of 50 mm. Alternatively,
the test ingot was subjected to blooming, and then subjected to
finish rolling to produce a steel bar having a diameter of 50 mm. A
normalizing in a range of 800 to 950.degree. C. was performed on
the produced steel bar. The cooling method adopted in the
normalizing was to allow cooling of the steel bar. A steel bar
(steel for machine structural use) having a diameter of 50 mm was
produced by the above-described production process.
[0150] [Evaluation Tests]
[0151] [Micro-Structure Observation]
[0152] A test specimen for micro-structure observation use was
taken from the R/2 part of the steel bar of each test number. Of
the entire surface of the test specimen, a cross-section parallel
to the longitudinal direction (that is, the rolling direction or
elongation direction) of the steel bar was defined as the
observation surface. The total area fraction (%) of ferrite,
pearlite and bainite were determined based on the method described
above. The total area fraction was 99% or more in the
micro-structure of the steel bar of each test number. A
micro-structure in which the total area fraction was 99% or more is
shown as "F+P+B" in Table 2.
[0153] [Number TN of Specific Inclusions and Composite Ratio
RA]
[0154] A test specimen for observing the micro-structure was taken
from the R/2 part of the steel bar of each test number. Of the
entire surface of the test specimen, a cross-section that was
parallel to the longitudinal direction (that is, the rolling
direction or elongation direction) of the steel bar was defined as
the observation surface. For the observation surface of the test
specimen for observing the micro-structure of each test number, the
specific inclusions number TN (inclusions/mm.sup.2) and the
composite ratio RA (%) were determined based on the above described
method. The results are shown in Table 2.
[0155] [Machinability]
[0156] A service life characteristic in the case of normal
drilling, and the chip treatability were evaluated with respect to
the machinability.
[0157] [Service Life Characteristic CL1000]
[0158] The steel bar having a diameter of 50 mm was cut at a length
of 20 mm to make a piercing test specimen. The piercing test
specimen was subjected to piercing. The piercing conditions are
shown in Table 3.
TABLE-US-00003 TABLE 3 Cutting Speed 10-70 m/min Feed Rate 0.25
mm/rev Wet Type Water-soluble Cutting Oil Drill Diameter 3 mm Point
Angle 118.degree. Material High-speed Steel Straight Drill Other
Hole Depth 9 mm Service Life Until Breakage
[0159] Specifically, the cumulative hole depth was measured up to
drill breakage at different cutting speed of the drill. A
high-speed steel straight drill was used as the drill. The nose R
of the drill was 3 mm, and the point angle was 118.degree..
Piercing was performed at a cutting speed of 10 to 70 m/min, a feed
rate of 0.25 mm/rev, and a hole depth of 9 mm. Piercing with one
drill was ended when the cumulative hole depth (hole
depth.times.number of piercings) reached 1000 mm. In such case, the
drill was replaced, and the cutting speed of the drill was raised
to a higher speed and the test was repeated until drill breakage.
The maximum cutting speed of the drill at which it was possible to
pierce a cumulative hole depth of 1000 mm was defined as CL1000
(m/min), and the CL1000 value was adopted as an index of the
machinability. The results are shown in the "CL1000" column in
Table 2. In a case where CL1000 was 50 m/min or more, the service
life characteristic was determined as being excellent. On the other
hand, when CL1000 was less than 50 m/min, the service life
characteristic was determined as being not excellent.
[0160] [Chip Treatability Evaluation]
[0161] A steel bar having a diameter of 50 mm was cut to a
predetermined length and adopted as a cutting test specimen. Outer
circumferential lathe turning shown in FIG. 4 was performed on the
cutting test specimen. The conditions for the outer circumferential
lathe turning are shown in Table 4.
TABLE-US-00004 TABLE 4 Cutting Speed 250 m/min Depth-of-cut Amount
2 mm Feed Rate 0.2 mm/rev Wet Type Water-soluble Cutting Oil Tool
Chip P20 Cemented Carbide Tool SNMG120408 with breaker Holder
DSBN-R2525
[0162] Specifically, a P20 cemented carbide tool was used as a tool
50. The nose R of the tool 50 was 0.4, and the rake angle thereof
was 5.degree.. Outer circumferential lathe turning was performed
under the following conditions: cutting speed V1: 250 m/min; feed
speed V2: 0.2 mm/rev; depth-of-cut amount D1: 2 mm; and
longitudinal direction cutting length L1: 200 mm. After cutting the
outer circumference, turning was repeated again so as to obtain a
small diameter of D1: 2 mm, and with respect to test specimen 5, a
lathe turning test was performed under the aforementioned
conditions for four minutes.
[0163] In the lathe turning for the 1000.sup.th test specimen, a
chip as illustrated in FIG. 5A and FIG. 5B was obtained. A length
L20 and a diameter D20 of the chip were measured. Based on the
measurement result, the chip treatability was evaluated as follows.
If the diameter D20 of the chip was a coil shape of not more than
30 mm, or if the chip length L20 was less than 50 mm even if the
chip was not a coil shape, the chip treatability was determined as
being excellent (".largecircle." in Table 2). On the other hand, if
the diameter D20 of the chip was not a coil shape of not more than
30 mm, and the chip length L20 was also 50 mm or more, the chip
treatability was determined as being not excellent ("x" in Table
2).
[0164] [Rusting Characteristics (Corrosion Resistance) Evaluation
Test]
[0165] A rust test specimen was prepared by cutting the steel bar
having a diameter of 50 mm to a predetermined length. The rust test
specimen was subjected to lathe turning under similar conditions as
in the aforementioned cutting test. Thereafter, the test specimen
was stored for one hour in an atmosphere with a humidity of 70% and
a temperature of 20.degree. C. while spraying tap water onto the
cut surface. After storage, the cut surface of the test specimen
was observed and the number of rust points was measured. The
measurement results are shown in the "rusting characteristics"
column in Table 2. If the number of rust points was less than 10
(indicated by ".circle-w/dot." in Table 2), and if the number of
rust points was 10 or more and less than 20 (indicated by
".largecircle." in Table 2), the rusting characteristics were
determined as being excellent. On the other hand, if the number of
rust points was 20 or more ("x" in Table 2), the rusting
characteristics were determined as being not excellent.
[0166] [Rolling Contact Fatigue Test]
[0167] Evaluation of the rolling contact fatigue life was performed
by means of a Thrust type rolling contact fatigue test. Ten
disk-shaped rolling contact fatigue test specimens 100 having a
diameter of 60 mm and a thickness of 5 mm as illustrated in FIG. 6
were taken from the R/2 part of the steel bar of each test number.
Each rolling contact fatigue test specimen 100 was subjected to
carburizing to harden the surface thereof. The effective hardened
layer depth was made 0.8 mm or more.
[0168] The carburizing conditions were as follows: Cp=0.7 to 0.8,
930.degree. C..times.2 hours, 870.degree. C..times.1 hour, oil
quenching, washing and low-temperature tempering at 170.degree.
C..times.2 hours. The surface of each rolling contact fatigue test
specimen 100 on which the carburizing was performed was subjected
to grinding to remove a incomplete hardened surface layer, and the
effective hardened layer depth was made 0.7 mm. The depth from the
surface to a position at which the hardness became HV550 was
adopted as the effective hardened layer depth. The amount removed
by grinding was set to 0.2 mm or less. The hardness distribution
and effective hardened layer depth in each rolling contact fatigue
test specimen were adjusted by adjusting the Cp and the cutting
depth.
[0169] As illustrated in FIG. 7, each rolling contact fatigue test
specimen 100 was immersed in lubricating oil 102 composed of 70%
oil and 30% water, and because the moisture in the lubricating oil
evaporated due to heat generation, 30 ml of water was added once
per day. The test interfacial pressure was set to a constant
pressure of 4 kN. The hard balls that were used were
Si.sub.3N.sub.4 ceramic hard balls. Three hard balls were placed in
contact with the top of the rolling contact fatigue test specimen
100, and the number of revolutions was set to 1200 rpm. As a
measure of the rolling contact fatigue life, "time (hours) until
producing pitting at a cumulative failure rate of 10% obtained when
the test result was plotted on Weibull probability paper" was used
as the durable life. The results are shown in the "rolling contact
fatigue life" column in Table 2. In a case where the rolling
contact fatigue life was 4.0 hours or more, the rolling contact
fatigue properties were determined as being excellent. On the other
hand, if the rolling contact fatigue life was less than 4.0 hours,
the rolling contact fatigue properties were determined as being not
excellent.
[0170] [Hot Ductility (Hot Workability) Evaluation Test]
[0171] A hot tension test was performed by electrical heating, and
the hot ductility (hot workability) was evaluated. Specifically,
from the cast piece of each test number, a round bar specimen that
had a diameter of 10 mm and a length of 100 mm and in which both
ends had been subjected to threading was prepared. The round bar
specimen was heated to 1100.degree. C. by electrical heating, and
held at that temperature for three minutes. Thereafter, the round
bar specimen was cooled to 900.degree. C. by being allowed to cool.
The tension test was executed in a state in which the temperature
of the round bar specimen was 900.degree. C., and the reduction of
area (%) at the time of breaking off was determined. The tension
test was performed on three round bar specimens for each test
number, and the average of the three values was defined as the
reduction of area (%) of the relevant test number. The reduction of
area is shown in the "hot ductility" column in Table 2. If the
reduction of area was 70% or more, the hot ductility (hot
workability) was evaluated as excellent. On the other hand, if the
reduction of area was less than 70%, the hot ductility (hot
workability) was evaluated as not excellent.
[0172] [Test Results]
[0173] In Test Numbers 1 to 31, the chemical composition was
appropriate, F1 was 8.0 or more, the deoxidation order was
appropriate, and the solidification cooling rate RC was 100.degree.
C./min or less. Therefore, the number TN of specific inclusions was
40 per mm.sup.2 or more. As a result, CL1000 was 50 m/min or more,
and excellent chip treatability was obtained. That is, excellent
machinability was obtained. In addition, in the rusting
characteristics evaluation test, the number of rust points was less
than 20 for each of these test numbers, and thus excellent rusting
characteristics were obtained. Furthermore, in the rolling contact
fatigue test, for each of these test numbers, the rolling contact
fatigue life was 4.0 hours or more, and thus excellent rolling
contact fatigue properties were obtained. In addition, in the hot
ductility evaluation test, the reduction of area was 70% or more,
showing that excellent hot ductility was obtained.
[0174] In Test Numbers 1 to 7, 26, 30 and 31, the solidification
cooling rate RC was within the range of 8 to 50.degree. C./min.
Therefore, not only was the number TN of specific inclusions 40 per
mm.sup.2 or more, but furthermore the composite ratio RA was 40% or
more. As a result, for each of these test numbers, the number of
rust points was less than 10, and thus rusting characteristics that
were even more excellent in comparison to Test Numbers 8 to 25 and
27 to 29 were obtained.
[0175] On the other hand, in Test Numbers 32 to 42, although the
chemical composition was appropriate and F1 was 8.0 or more and the
deoxidation order was appropriate, the solidification cooling rate
RC was more than 100.degree. C./min. Consequently, the number TN of
specific inclusions was less than 40 per mm.sup.2. As a result,
excellent rusting characteristics were not obtained.
[0176] In Test Numbers 43 and 44, although the chemical composition
was appropriate and the deoxidation order was appropriate and the
solidification cooling rate RC was not more than 100.degree.
C./min, F1 was less than 8.0. Consequently, the number TN of
specific inclusions was less than 40 per mm.sup.2. As a result,
excellent rusting characteristics were not obtained. In addition,
the reduction of area was less than 70%, and thus excellent hot
ductility was not obtained.
[0177] In Test Number 45, although the chemical composition was
appropriate and the deoxidation order was appropriate, the
solidification cooling rate RC was more than 100.degree. C./min and
F1 was less than 8.0. Consequently, the number TN of specific
inclusions was less than 40 per mm.sup.2. As a result, excellent
rusting characteristics were not obtained. In addition, the
reduction of area was less than 70%, and thus excellent hot
ductility was not obtained.
[0178] In Test Numbers 46 and 47, the Mn content was too high. As a
result, CL1000 was less than 50 m/min, and thus excellent
machinability was not obtained.
[0179] In Test Numbers 48 and 49, the Mn content was too low.
Consequently, the number TN of specific inclusions was less than 40
per mm.sup.2. As a result, excellent rusting characteristics were
not obtained. In addition, the reduction of area was less than 70%,
and thus excellent hot ductility was not obtained.
[0180] In Test Number 50, the S content was too high. As a result,
the reduction of area was less than 70%, and thus excellent hot
ductility was not obtained.
[0181] In Test Number 51, the S content was too high. In addition,
F1 was less than 8.0. Consequently, the number TN of specific
inclusions was less than 40 per mm.sup.2. As a result, excellent
rusting characteristics were not obtained. In addition, the
reduction of area was less than 70%, and thus excellent hot
ductility was not obtained.
[0182] In Test Numbers 52 and 53, the S content was too low.
Consequently, the number TN of specific inclusions was less than 40
per mm.sup.2. As a result, excellent rusting characteristics were
not obtained.
[0183] In Test Numbers 54 and 55, the Pb content was too high. As a
result, excellent rusting characteristics were not obtained. In
addition, the reduction of area was less than 70%, and thus
excellent hot ductility was not obtained.
[0184] In Test Numbers 56 and 57, the Pb content was too low. As a
result, CL1000 was less than 50 m/min, and furthermore excellent
chip treatability was not obtained. That is, excellent
machinability was not obtained.
[0185] In Test Number 58, the Al content was too low. Consequently,
the number TN of specific inclusions was less than 40 per mm.sup.2.
As a result, excellent rusting characteristics were not
obtained.
[0186] In Test Number 59, the N content was too high. As a result,
CL1000 was less than 50 m/min, and excellent machinability was not
obtained. In addition, the reduction of area was less than 70%, and
thus excellent hot ductility was not obtained.
[0187] In Test Numbers 60 and 61, the O content was too high. As a
result, CL1000 was less than 50 m/min, and excellent machinability
was not obtained. In addition, the rolling contact fatigue life was
less than 4.0 hours, and thus excellent rolling contact fatigue
properties were not obtained.
[0188] In Test Number 62, the O content was too low. Consequently,
the number TN of specific inclusions was less than 40 per mm.sup.2.
As a result, excellent rusting characteristics were not obtained.
In addition, CL1000 was less than 50 m/min, and thus excellent chip
treatability was also not obtained. That is, excellent
machinability was not obtained.
[0189] In Test Number 63, the Cr content was too high. As a result,
CL1000 was less than 50 m/min, and excellent machinability was not
obtained.
[0190] In Test Number 64, the Cr content was too low. As a result,
the rolling contact fatigue life was less than 4.0 hours, and thus
excellent rolling contact fatigue properties were not obtained.
[0191] In Test Numbers 65 and 66, although the chemical composition
was appropriate, F1 was 8.0 or more and the solidification cooling
rate RC was not more than 100.degree. C./min, the deoxidation order
was inappropriate. Consequently, the number TN of specific
inclusions was less than 40 per mm.sup.2. As a result, excellent
rusting characteristics were not obtained.
[0192] An embodiment of the present invention has been described
above. However, the foregoing embodiment is merely an example for
implementing the present invention. Accordingly, the present
invention is not limited to the above embodiment, and the above
embodiment can be appropriately modified within a range which does
not deviate from the gist of the present invention.
REFERENCE SIGNS LIST
[0193] 10 MnS Inclusions [0194] 20 Pb Inclusions [0195] 30
Composite Inclusions
* * * * *