U.S. patent application number 16/313931 was filed with the patent office on 2019-08-01 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 | 20190233927 16/313931 |
Document ID | / |
Family ID | 60912701 |
Filed Date | 2019-08-01 |
United States Patent
Application |
20190233927 |
Kind Code |
A1 |
Hashimura; Masayuki ; et
al. |
August 1, 2019 |
Steel for Machine Structural Use
Abstract
A steel for machine structural use is provided which is
excellent in machinability, rusting characteristics, and hot
ductility. The steel for machine structural use according to the
present embodiment has a chemical composition which consists of, in
mass %, C: 0.30 to 0.80%, 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 and O: 0.0005 to 0.0030%, 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) ; Iwahashi; Takanori;
(Chiyoda-ku, Tokyo, JP) ; Toudou; Shouji;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Steel & Sumitomo Metal Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
60912701 |
Appl. No.: |
16/313931 |
Filed: |
July 4, 2017 |
PCT Filed: |
July 4, 2017 |
PCT NO: |
PCT/JP2017/024442 |
371 Date: |
December 28, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22D 11/001 20130101;
C22C 38/12 20130101; C21D 8/065 20130101; C22C 38/60 20130101; C22C
38/16 20130101; C22C 38/08 20130101; C22C 38/001 20130101; B22D
11/124 20130101; C22C 38/06 20130101; C22C 38/14 20130101; C21D
2211/004 20130101; C22C 38/38 20130101; C22C 38/02 20130101; C22C
38/04 20130101; C22C 38/002 20130101 |
International
Class: |
C22C 38/60 20060101
C22C038/60; C22C 38/38 20060101 C22C038/38; C22C 38/02 20060101
C22C038/02; C22C 38/04 20060101 C22C038/04; C22C 38/08 20060101
C22C038/08; C22C 38/12 20060101 C22C038/12; C22C 38/16 20060101
C22C038/16; C22C 38/14 20060101 C22C038/14; C22C 38/06 20060101
C22C038/06; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 4, 2016 |
JP |
2016-132902 |
Claims
1. A steel for machine structural use having a chemical composition
consisting of, in mass %: C: 0.30 to 0.80%, 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 to 0.70%, 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: Cr: 0.10 to 0.70%, 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 a steel, and more
particularly relates to a steel for machine structural use.
BACKGROUND ART
[0002] Machine components that are to be used for structural use
and power transmission use such as general machine components and
automobile components are produced using a steel for machine
structural use. 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. There are also cases in which, as necessary, the machine
component is subjected to a heat treatment (normalizing or the
like), a case hardening heat treatment (induction hardening or the
like), or quenching and tempering. 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 .alpha.
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 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.
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: 0.30 to 0.80%, 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 to 0.70%,
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.
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. 3A is a photographic image of an S distribution
obtained by performing EPMA analysis on the steel for machine
structural use of the present embodiment.
[0018] FIG. 3B is a photographic image of a Pb distribution
obtained by performing EPMA with respect to the same visual fields
as in FIG. 3A.
[0019] FIG. 3C is a composite image of the images shown in FIG. 3A
and FIG. 3B.
[0020] FIG. 4 is a transverse sectional view of a starting material
that was cast.
[0021] FIG. 5 is a schematic diagram of a cutting test machine for
describing a cutting test.
[0022] FIG. 6A is a perspective view of a chip.
[0023] FIG. 6B is a planar photographic view of a chip.
DESCRIPTION OF EMBODIMENTS
[0024] 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: 0.30 to
0.80%, 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 to 0.70%, 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,
there is a possibility that excellent machinability and excellent
hot workability are obtained.
[0025] 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.
[0026] On the other hand, most of the Pb in the steel does not
dissolve in the steel, and is present as Pb inclusions (Pb
particles). The MnS inclusions and Pb inclusions each enhance the
machinability of the steel.
[0027] 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 the section "Method of measuring number TN of specific
inclusions and composite ratio RA" that 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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).
[0035] 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.
[0036] Specifically, the Mn content and S content satisfy the
following Formula (1).
Mn/S.gtoreq.8.0 (1)
[0037] Where, the content (mass %) of a corresponding element is
substituted for each symbol of an element in Formula (1).
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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: 0.30 to
0.80%, 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 to 0.70%, 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)
[0042] Where, the content (mass %) of a corresponding element is
substituted for the respective elements in Formula (1).
[0043] 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 Cr: 0.10 to 0.70%, 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%.
[0044] The chemical composition of the steel for machine structural
use that is described above may contain Ca: 0.0001 to 0.0030%.
[0045] 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.
[0046] 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.
[0047] [Chemical Composition]
[0048] The chemical composition of the steel for machine structural
use of the present embodiment contains the following elements.
[0049] C: 0.30 to 0.80%
[0050] Carbon (C) enhances the strength of the steel. When
producing a component using a steel for machine structural use, as
necessary, a heat treatment (normalizing or the like), a case
hardening heat treatment (induction hardening or the like), or
quenching and tempering are performed after forging the steel for
machine structural use. In such a case, C increases the strength of
the steel. If the C content is less than 0.30%, sufficient strength
will not be obtained. On the other hand, if the C content is more
than 0.80%, a large amount of retained austenite will form after
tempering. In such a case, not only will an increase in the
strength be saturated, but hard cementite will form and the
machinability of the steel will decrease. Accordingly, the C
content is in a range of 0.30 to 0.80%.
[0051] A preferable lower limit of the C content in the case of
using a component as it is in a normalized state is 0.34%, and more
preferably is 0.40%. In the case of performing quenching such as
induction hardening, a preferable upper limit of the C content is
0.70%. In such a case, a strength that is commensurate with the C
content is obtained. Further, in the steel for machine structural
use of the present embodiment, a temperature region in which an
austenite single-phase structure is formed during quenching is
extremely narrow. Accordingly, in the case of large-scale
production, a preferable upper limit of the C content is 0.60%.
[0052] Si: 0.01 to 0.80%
[0053] Silicon (Si) deoxidizes the steel. By adding Si after adding
Mn at the time of deoxidation, the Si modifies oxides.
Specifically, Si added to molten steel modifies oxides that are
mainly composed of Mn into oxides that are mainly composed of Si.
By adding Al after adding Si, composite oxides containing Si and Al
form in the steel. The composite oxides serve as nuclei for
crystallization of MnS inclusions. Therefore, the composite 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%.
[0054] 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%.
[0055] Mn: 0.20 to 2.00%
[0056] Manganese (Mn) forms MnS inclusions and composite inclusions
containing MnS and Pb, and enhances the machinability of the
steel.
[0057] 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 modified. Hereunder, the modified oxides are
referred to as "composite oxides". The Mn that is discharged into
the molten steel from the oxides combines with S to form MnS
inclusions. Note that, composite oxides formed by modification of
oxides easily become nuclei for crystallization of MnS inclusions.
Therefore, if composite oxides are formed, crystallization of MnS
inclusions is promoted. The MnS inclusions formed by
crystallization easily form composite inclusions also.
[0058] 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. 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%.
[0059] P: 0.030% or less
[0060] 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%.
[0061] S: 0.010 to 0.100%
[0062] 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%.
[0063] Pb: 0.010 to 0.100%
[0064] Lead (Pb) forms Pb inclusions (Pb particles) 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%.
[0065] Al: 0.010 to 0.050%
[0066] 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 modified and
composite oxides containing Si and Al are formed. The composite
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 decreases. 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%.
[0067] On the other hand, if the Al content is more than 0.050%,
coarse composite oxides are liable to form. The composite oxides
are liable to become coarse. If coarse composite oxides are formed
in the steel, surface defects are liable to occur on the steel. If
coarse composite 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).
[0068] N: 0.015% or less
[0069] 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).
[0070] O: 0.0005 to 0.0030%
[0071] Oxygen (O) is contained not only in oxides, but also in MnS
inclusions. O forms composite oxides that serve as nuclei for
crystallization of MnS inclusions. If the O content is less than
0.0005%, the formed amount of composite 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. On the other hand, 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. Furthermore, if the O content is more than
0.0030%, in some cases coarse oxides that become the starting
points for fractures are formed. In this case, the rolling contact
fatigue properties of the machine component 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).
[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 Cr, Ni, B, V, Mo,
W, Nb, Cu and Ti.
[0075] Cr: 0 to 0.70%
[0076] Chromium (Cr) is an optional element and need not be
contained. If contained, Cr dissolves in the steel and increases
the hardenability and temper softening resistance of the steel and
enhances the steel strength. In a case where nitriding is performed
as a case hardening treatment, Cr also deepens the hardened layer
depth. The aforementioned effects are obtained to a certain extent
if even a small amount of Cr is contained. On the other hand, if
the Cr content is more than 0.70%, if quenching and tempering are
performed, cementite in the steel will coarsen. In addition, if the
Cr content is more than 0.70%, if induction hardening is performed,
cementite in the steel will not dissolve. Furthermore, if the Cr
content is more than 0.70%, austenite will stabilize at even a low
temperature. In such a case, the steel will become brittle.
Accordingly, the Cr content is in a range from 0 to 0.70%. A
preferable lower limit of the Cr content for increasing
hardenability is 0.10%, and more preferably is 0.30%. A preferable
upper limit of the Cr content is 0.60%.
[0077] Ni: 0 to 3.50%
[0078] 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%.
[0079] 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.
[0080] B: 0 to 0.0050%
[0081] 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%.
[0082] V: 0 to 0.70%
[0083] Vanadium (V) is an optional element and need not be
contained. If contained, V precipitates as carbides, nitrides, or
carbo-nitrides during tempering and during nitriding, and enhances
the strength of the steel. The V precipitates (nitrides, carbides
and 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.
[0084] 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%.
[0085] Mo: 0 to 0.70%
[0086] 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
A.sub.1, 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 structure 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%.
[0087] 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 structure is
0.40%, and more preferably is 0.30%.
[0088] W: 0 to 0.70%
[0089] 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.
[0090] On the other hand, if the W content is more than 0.70%, a
supercooled structure 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 structure is 0.40%,
and more preferably is 0.30%.
[0091] 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%.
[0092] Nb: 0 to less than 0.050%
[0093] Niobium (Nb) is an optional element and need not be
contained. If contained, Nb forms nitrides, carbides, or
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 structure 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%.
[0094] Cu: 0 to 0.50%
[0095] 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.
[0096] Ti: 0 to 0.100%
[0097] 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.
[0098] 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%.
[0099] The steel for machine structural use of the present
embodiment may further contain Ca.
[0100] Ca: 0 to 0.0030%
[0101] 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%.
[0102] [Regarding Formula (1)]
[0103] The chemical composition of the steel for machine structural
use of the present embodiment also satisfies Formula (1).
Mn/S.gtoreq.8.0 (1)
[0104] Where, a content (mass %) of a corresponding element is
substituted for each element in Formula (1).
[0105] 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.
[0106] 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.
[0107] [Regarding the Steel Micro-Structure]
[0108] The micro-structure of the steel for machine structural use
according to the present invention is mainly composed of ferrite
and pearlite. Specifically, a total area fraction of ferrite and
pearlite in the micro-structure of the steel for machine structural
use having the aforementioned chemical composition is 99% or
more.
[0109] The total area fraction of ferrite and pearlite 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.
[0110] 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 and pearlite
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 and pearlite.
[0111] [Number TN of Specific Inclusions]
[0112] 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.
[0113] 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.
[0114] [Ratio of Number of Composite Inclusions Among Specific
Inclusions (Composite Ratio) RA]
[0115] 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.
[0116] 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%.
[0117] [Method of Measuring Number TN of Specific Inclusions and
Composite Ratio RA]
[0118] 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.
[0119] For each observation surface, an image of the S distribution
and Pb distribution in the observation surface is obtained by means
of an X-ray spectroscopy wavelength dispersion 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.
[0120] 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.
[0121] 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.
[0122] 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 A.sub.1 and region A2 in
FIG. 1C), the relevant inclusions are identified as an MnS
inclusion and a Pb inclusion.
[0123] FIG. 3A is a photographic image of an S distribution
obtained by performing EPMA analysis on the steel for machine
structural use of the present embodiment, and FIG. 3B is a
photographic image of the Pb distribution. FIG. 3C is a
photographic image obtained by superposing the images in FIG. 3A
and FIG. 3B. Referring to FIG. 3A to FIG. 3C. The MnS inclusions 10
are observed in an area A10 in FIG. 3A, and the Pb inclusions 20
are observed in the area A10 in FIG. 3B. Accordingly, it can be
recognized that the composite inclusions 30 are present in the area
A10 in FIG. 3C. Further, the MnS inclusions 10 are not observed in
an area A20 in FIG. 3A, and the Pb inclusions 20 are observed in
the area A20 in FIG. 3B. Therefore, it can be recognized that the
inclusions present in the area A20 in FIG. 3C are the Pb inclusions
20.
[0124] 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 (.mu.m) for each of the
inclusions.
[0125] 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)
[0126] [Production Method]
[0127] 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.
[0128] 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.
[0129] [Steel Making Process]
[0130] The steel making process includes a refining process and a
casting process.
[0131] [Refining Process]
[0132] 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.
[0133] 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
modified 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 modified to composite oxides containing Si and
Al (hereinafter, also referred to simply as "composite
oxides").
[0134] The composite oxides that were formed by the above described
refining process serve as nuclei for crystallization of MnS
inclusions. Therefore, by forming the composite oxides, MnS
inclusions adequately crystallize and grow coarse. That is, if
composite 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.
[0135] A well-known removing slag is performed after performing the
deoxidation. After the 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
treatment 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.
[0136] [Casting Process]
[0137] 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.
[0138] 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.
[0139] 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.
[0140] 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.
[0141] The solidification cooling rate RC can be determined based
on the starting material that was cast. FIG. 4 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 .lamda.2
(.mu.m) in the thickness direction of the solidification structure
at the point P1 is measured. Using the measurement value .lamda.2,
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)
[0142] The secondary dendrite arm spacing .lamda.2 depends on the
solidification cooling rate RC. Accordingly, the solidification
cooling rate RC can be determined by measuring the secondary
dendrite arm spacing .lamda.2.
[0143] [Hot Working Process]
[0144] 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.
[0145] 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.
[0146] 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.
[0147] 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
treatment. 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
[0148] 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 0.49 0.80 0.94 0.014 0.046 0.048 0.026
0.004 0.0015 -- -- -- -- -- -- -- -- -- -- 2 0.38 0.38 0.20 0.014
0.013 0.034 0.033 0.004 0.0007 -- -- -- -- -- -- -- -- -- -- 3 0.69
0.34 1.33 0.007 0.045 0.045 0.050 0.005 0.0012 -- -- -- -- -- -- --
-- -- -- 4 0.48 0.25 1.17 0.005 0.033 0.025 0.022 0.005 0.0030 --
-- -- -- -- -- -- -- -- -- 5 0.63 0.39 0.65 0.015 0.010 0.041 0.029
0.008 0.0015 -- -- -- -- -- -- -- -- -- -- 6 0.54 0.50 1.66 0.010
0.022 0.051 0.029 0.006 0.0005 -- -- -- -- -- -- -- -- -- -- 7 0.80
0.42 1.36 0.012 0.021 0.035 0.029 0.005 0.0018 -- -- -- -- -- -- --
-- -- -- 8 0.30 0.39 1.01 0.010 0.021 0.025 0.020 0.007 0.0019 --
-- -- -- -- -- -- -- -- -- 9 0.53 0.01 1.48 0.009 0.025 0.035 0.032
0.007 0.0019 -- -- -- -- -- -- -- -- -- -- 10 0.56 0.24 1.90 0.014
0.033 0.045 0.023 0.007 0.0017 -- -- -- -- -- -- -- -- -- -- 11
0.55 0.41 1.55 0.030 0.024 0.043 0.029 0.006 0.0009 -- -- -- -- --
-- -- -- -- -- 12 0.56 0.46 0.92 0.009 0.100 0.040 0.026 0.007
0.0008 -- -- -- -- -- -- -- -- -- -- 13 0.47 0.29 1.59 0.017 0.017
0.100 0.023 0.006 0.0011 -- -- -- -- -- -- -- -- -- -- 14 0.66 0.32
1.59 0.014 0.026 0.010 0.023 0.005 0.0011 -- -- -- -- -- -- -- --
-- -- 15 0.51 0.43 0.90 0.005 0.021 0.035 0.010 0.005 0.0016 -- --
-- -- -- -- -- -- -- -- 16 0.43 0.49 1.22 0.007 0.047 0.049 0.026
0.015 0.0009 -- -- -- -- -- -- -- -- -- -- 17 0.57 0.34 1.68 0.014
0.040 0.030 0.025 0.005 0.0015 0.70 -- -- -- -- -- -- -- -- -- 18
0.53 0.44 0.85 0.012 0.031 0.045 0.031 0.007 0.0018 -- 3.50 -- --
-- -- -- -- -- -- 19 0.69 0.29 1.08 0.014 0.028 0.047 0.023 0.005
0.0013 -- -- 0.0050 -- -- -- -- -- -- -- 20 0.42 0.34 1.01 0.015
0.035 0.029 0.034 0.006 0.0013 -- -- -- 0.70 -- -- -- -- -- -- 21
0.46 0.32 0.66 0.015 0.020 0.025 0.029 0.008 0.0011 -- -- -- --
0.70 -- -- -- -- -- 22 0.41 0.50 1.44 0.014 0.027 0.039 0.035 0.005
0.0013 -- -- -- -- -- 0.70 -- -- -- -- 23 0.44 0.42 0.49 0.011
0.052 0.031 0.036 0.008 0.0013 -- -- -- -- -- -- 0.049 -- -- -- 24
0.50 0.28 1.32 0.010 0.016 0.040 0.026 0.006 0.0011 -- -- -- -- --
-- -- 0.50 -- -- 25 0.51 0.44 1.42 0.012 0.026 0.030 0.029 0.006
0.0010 -- -- -- -- -- -- -- -- 0.100 -- 26 0.60 0.27 0.56 0.015
0.047 0.048 0.030 0.004 0.0012 -- -- -- -- -- -- -- -- -- 0.0030 27
0.80 0.43 1.35 0.011 0.023 0.032 0.033 0.006 0.0017 -- -- -- -- --
-- -- -- -- -- 28 0.54 0.01 1.47 0.008 0.027 0.036 0.033 0.007
0.0018 -- -- -- -- -- -- -- -- -- -- 29 0.57 0.39 1.59 0.030 0.026
0.044 0.031 0.004 0.0008 -- -- -- -- -- -- -- -- -- -- 30 0.48 0.30
1.60 0.015 0.018 0.100 0.024 0.007 0.0013 -- -- -- -- -- -- -- --
-- -- 31 0.50 0.42 0.91 0.006 0.019 0.033 0.010 0.004 0.0017 -- --
-- -- -- -- -- -- -- -- 32 0.55 0.49 1.68 0.009 0.024 0.049 0.031
0.005 0.0005 -- -- -- -- -- -- -- -- -- -- 33 0.54 0.45 0.82 0.013
0.030 0.042 0.033 0.008 0.0019 -- 3.50 -- -- -- -- -- -- -- -- 34
0.43 0.33 1.04 0.014 0.036 0.030 0.034 0.005 0.0012 -- -- -- 0.70
-- -- -- -- -- -- 35 0.43 0.44 0.51 0.009 0.049 0.034 0.035 0.007
0.0014 -- -- -- -- -- -- 0.049 -- -- -- 36 0.49 0.22 0.57 0.029
0.076 0.044 0.045 0.006 0.0017 -- -- -- -- -- -- -- -- -- -- 37
0.51 0.22 0.34 0.021 0.056 0.055 0.031 0.009 0.0012 -- -- -- -- --
-- -- -- -- -- 38 0.55 0.27 0.27 0.011 0.038 0.041 0.026 0.006
0.0013 -- -- -- -- -- -- -- -- -- -- 39 0.58 0.55 2.14 0.012 0.044
0.043 0.024 0.006 0.0013 -- -- -- -- -- -- -- -- -- -- 40 0.57 0.28
0.16 0.006 0.012 0.047 0.026 0.004 0.0015 -- -- -- -- -- -- -- --
-- -- 41 0.42 0.25 1.18 0.008 0.113 0.048 0.021 0.008 0.0020 -- --
-- -- -- -- -- -- -- -- 42 0.44 0.22 0.58 0.013 0.008 0.030 0.029
0.006 0.0010 -- -- -- -- -- -- -- -- -- -- 43 0.46 0.23 0.86 0.007
0.014 0.113 0.025 0.005 0.0015 -- -- -- -- -- -- -- -- -- -- 44
0.49 0.37 1.03 0.014 0.012 0.008 0.020 0.005 0.0007 -- -- -- -- --
-- -- -- -- -- 45 0.49 0.25 0.89 0.008 0.022 0.034 0.004 0.008
0.0014 -- -- -- -- -- -- -- -- -- -- 46 0.63 0.31 0.58 0.008 0.043
0.041 0.030 0.019 0.0014 -- -- -- -- -- -- -- -- -- -- 47 0.45 0.37
1.41 0.007 0.021 0.029 0.027 0.005 0.0034 -- -- -- -- -- -- -- --
-- -- 48 0.45 0.26 0.28 0.007 0.014 0.021 0.034 0.007 0.0003 -- --
-- -- -- -- -- -- -- -- 49 0.45 0.31 1.61 0.015 0.016 0.035 0.033
0.007 0.0016 -- -- -- -- -- -- -- -- -- -- 50 0.48 0.25 1.42 0.007
0.013 0.039 0.035 0.005 0.0018 -- -- -- -- -- -- -- -- -- --
[0149] 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.
[0150] For the molten steels of test numbers other than Test
Numbers 49 and 50, 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 49, 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 50, after tapping the molten steel from the converter, Mn,
Al and Si were added in that order to perform deoxidation.
[0151] After deoxidation, slag is removed. After removing slag, a
primary treatment was performed using VAD, and thereafter an RH
vacuum degassing treatment was performed. After the RH vacuum
degassing treatment, final adjustment of alloying elements was
performed. Molten steels having the chemical compositions shown in
Table 1 were produced by the above described process.
[0152] 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 Machinability Amount of Chip Rusting Hot
Test Deoxidation RC F1 = Micro- TN RA Hardness Tool Wear Treat-
Charac- Ductility Overall Number Order (.degree. C./min) Mn/S
structure (inclusions/mm.sup.2) (%) HV (.mu.m) ability teristics
(%) Evaluation 1 Mn.fwdarw.Si.fwdarw.Al 8.3 20.4 F + P 254.4 89.0
198 115 .largecircle. .circle-w/dot. 75 .largecircle. 2
Mn.fwdarw.Si.fwdarw.Al 21.1 15.4 F + P 66.3 89.6 168 192
.largecircle. .circle-w/dot. 76 .largecircle. 3
Mn.fwdarw.Si.fwdarw.Al 40.5 29.6 F + P 138.1 79.2 275 167
.largecircle. .circle-w/dot. 79 .largecircle. 4
Mn.fwdarw.Si.fwdarw.Al 30.8 35.5 F + P 58.0 71.3 200 181
.largecircle. .circle-w/dot. 81 .largecircle. 5
Mn.fwdarw.Si.fwdarw.Al 33.2 65.0 F + P 91.7 79.8 255 181
.largecircle. .circle-w/dot. 79 .largecircle. 6
Mn.fwdarw.Si.fwdarw.Al 11.8 75.5 F + P 106.4 77.8 219 168
.largecircle. .circle-w/dot. 83 .largecircle. 7
Mn.fwdarw.Si.fwdarw.Al 54.2 64.8 F + P 59.8 38.3 272 188
.largecircle. .largecircle. 74 .largecircle. 8
Mn.fwdarw.Si.fwdarw.Al 68.2 48.1 F + P 55.9 32.6 188 181
.largecircle. .largecircle. 71 .largecircle. 9
Mn.fwdarw.Si.fwdarw.Al 65.3 59.2 F + P 61.2 36.5 211 163
.largecircle. .largecircle. 82 .largecircle. 10
Mn.fwdarw.Si.fwdarw.Al 72.7 45.5 F + P 57.4 35.8 270 179
.largecircle. .largecircle. 84 .largecircle. 11
Mn.fwdarw.Si.fwdarw.Al 88.2 64.6 F + P 49.1 35.6 215 162
.largecircle. .largecircle. 75 .largecircle. 12
Mn.fwdarw.Si.fwdarw.Al 59.9 9.2 F + P 110.2 38.0 228 110
.largecircle. .largecircle. 72 .largecircle. 13
Mn.fwdarw.Si.fwdarw.Al 72.5 93.5 F + P 48.3 37.2 244 106
.largecircle. .largecircle. 78 .largecircle. 14
Mn.fwdarw.Si.fwdarw.Al 65.1 61.2 F + P 50.5 35.8 269 197
.largecircle. .largecircle. 81 .largecircle. 15
Mn.fwdarw.Si.fwdarw.Al 64.3 42.9 F + P 59.8 31.4 191 179
.largecircle. .largecircle. 76 .largecircle. 16
Mn.fwdarw.Si.fwdarw.Al 88.0 26.0 F + P 68.7 21.6 177 187
.largecircle. .largecircle. 73 .largecircle. 17
Mn.fwdarw.Si.fwdarw.Al 39.4 42.0 F + P 125.4 74.8 265 187
.largecircle. .circle-w/dot. 79 .largecircle. 18
Mn.fwdarw.Si.fwdarw.Al 77.5 27.4 F + P 52.3 36.1 243 161
.largecircle. .largecircle. 79 .largecircle. 19
Mn.fwdarw.Si.fwdarw.Al 42.4 38.6 F + P 121.4 65.0 285 187
.largecircle. .circle-w/dot. 75 .largecircle. 20
Mn.fwdarw.Si.fwdarw.Al 59.6 28.9 F + P 48.7 33.1 268 156
.largecircle. .largecircle. 79 .largecircle. 21
Mn.fwdarw.Si.fwdarw.Al 61.9 33.0 F + P 47.3 31.2 231 188
.largecircle. .largecircle. 82 .largecircle. 22
Mn.fwdarw.Si.fwdarw.Al 9.3 53.3 F + P 88.2 60.4 266 198
.largecircle. .circle-w/dot. 81 .largecircle. 23
Mn.fwdarw.Si.fwdarw.Al 69.7 9.4 F + P 66.9 36.2 218 129
.largecircle. .largecircle. 79 .largecircle. 24
Mn.fwdarw.Si.fwdarw.Al 72.1 82.5 F + P 41.6 32.0 232 182
.largecircle. .largecircle. 82 .largecircle. 25
Mn.fwdarw.Si.fwdarw.Al 44.1 54.6 F + P 101.6 59.2 231 162
.largecircle. .circle-w/dot. 79 .largecircle. 26
Mn.fwdarw.Si.fwdarw.Al 54.6 11.9 F + P 51.1 39.4 244 149
.largecircle. .largecircle. 76 .largecircle. 27
Mn.fwdarw.Si.fwdarw.Al 148.5 58.7 F + P 33.2 28.4 285 190
.largecircle. X 75 X 28 Mn.fwdarw.Si.fwdarw.Al 195.6 54.4 F + P
37.2 32.8 219 174 .largecircle. X 85 X 29 Mn.fwdarw.Si.fwdarw.Al
102.5 61.2 F + P 31.5 30.4 229 165 .largecircle. X 79 X 30
Mn.fwdarw.Si.fwdarw.Al 111.3 88.9 F + P 28.9 38.1 246 107
.largecircle. X 80 X 31 Mn.fwdarw.Si.fwdarw.Al 188.7 47.9 F + P
32.8 28.4 203 187 .largecircle. X 73 X 32 Mn.fwdarw.Si.fwdarw.Al
175.0 70.0 F + P 39.4 31.4 222 162 .largecircle. X 84 X 33
Mn.fwdarw.Si.fwdarw.Al 173.2 27.3 F + P 29.2 29.9 244 167
.largecircle. X 81 X 34 Mn.fwdarw.Si.fwdarw.Al 109.9 28.9 F + P
37.0 39.6 255 176 .largecircle. X 73 X 35 Mn.fwdarw.Si.fwdarw.Al
102.1 10.4 F + P 25.9 38.4 221 133 .largecircle. X 81 X 36
Mn.fwdarw.Si.fwdarw.Al 31.2 7.5 F + P 31.5 58.1 201 62
.largecircle. X 49 X 37 Mn.fwdarw.Si.fwdarw.Al 71.5 6.1 F + P 29.6
33.8 198 122 .largecircle. X 53 X 38 Mn.fwdarw.Si.fwdarw.Al 200.0
7.1 F + P 27.5 34.6 225 148 .largecircle. X 57 X 39
Mn.fwdarw.Si.fwdarw.Al 79.6 35.0 F + P 67.7 79.1 304 288
.largecircle. .largecircle. 82 X 40 Mn.fwdarw.Si.fwdarw.Al 187.4
13.3 F + P 30.9 34.2 237 144 .largecircle. X 58 X 41
Mn.fwdarw.Si.fwdarw.Al 63.1 10.4 F + P 113.0 79.8 171 102
.largecircle. .largecircle. 51 X 42 Mn.fwdarw.Si.fwdarw.Al 54.4
72.5 F + P 37.8 32.8 181 144 .largecircle. X 83 X 43
Mn.fwdarw.Si.fwdarw.Al 21.7 59.7 F + P 118.3 35.8 188 111
.largecircle. X 61 X 44 Mn.fwdarw.Si.fwdarw.Al 198.5 85.8 F + P 7.1
92.8 205 270 X .largecircle. 79 X 45 Mn.fwdarw.Si.fwdarw.Al 56.2
115.6 F + P 35.6 35.8 226 134 .largecircle. X 75 X 46
Mn.fwdarw.Si.fwdarw.Al 94.4 13.5 F + P 43.0 84.7 260 256
.largecircle. .largecircle. 63 X 47 Mn.fwdarw.Si.fwdarw.Al 187.8
67.1 F + P 38.7 81.1 193 210 .largecircle. .largecircle. 80 X 48
Mn.fwdarw.Si.fwdarw.Al 177.4 67.1 F + P 15.7 61.1 181 215 X
.largecircle. 81 X 49 Si.fwdarw.Al.fwdarw.Mn 42.8 100.6 F + P 29.8
37.2 244 106 .largecircle. X 78 X 50 Mn.fwdarw.Al.fwdarw.Si 44.6
109.2 F + P 31.1 33.7 204 158 .largecircle. X 77 X
[0153] 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 treatment in a range of 800 to 950.degree. C. was
performed on the produced steel bar. The cooling method adopted in
the normalizing treatment 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.
[0154] [Evaluation Tests]
[0155] [Micro-Structure Observation]
[0156] 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 and
pearlite 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" in Table 2.
[0157] [Number TN of Specific Inclusions and Composite Ratio
RA]
[0158] 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.
[0159] [Vickers Hardness Test]
[0160] A Vickers hardness test was performed in conformity with JIS
Z 2244 (1981) at an arbitrary five points of the R/2 part of the
steel bar of each test number. The test force was set to 100 N. The
average of the obtained five values was defined as the Vickers
hardness (HV) of the steel bar of the relevant test number. If the
Vickers hardness was HV 160 or more, it was determined that the
steel bar had sufficient strength. On the other hand, if the
Vickers hardness was less than HV 160, it was determined that the
steel bar had insufficient strength. The results are shown in Table
2. The results show that the Vickers hardness was HV 160 or more
for each test number, indicating sufficient strength.
[0161] [Machinability]
[0162] The machinability was evaluated by evaluating the amount of
tool wear (.mu.m) and the chip treatability. Specifically, a steel
bar having a diameter of 50 mm was cut to a predetermined length
and adopted as a cutting test specimen. The cutting test specimen
was subjected to outer circumferential lathe turning as illustrated
in FIG. 5. The conditions of the outer circumferential lathe
turning are shown in Table 3.
TABLE-US-00003 TABLE 3 Cutting Speed 200 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 SNMG 120408 Circumference Groove
With Breaker Holder DSBN-R2525
[0163] Specifically, a P20 cemented carbide tool was used as a tool
50. The nose radius 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: 200
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, cutting lathe 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.
[0164] [Service Life Evaluation]
[0165] The amount of tool wear (mm) of the minor flank was measured
with respect to the tool 50 after lathe turning of the 1,000.sup.th
test specimen was completed. The measurement results are shown in
the "amount of tool wear" column in Table 2. The service life was
determined as excellent if the amount of tool wear was 200 .mu.m or
less. On the other hand, if the amount of tool wear was more than
200 .mu.m, the service life was determined as being not
excellent.
[0166] [Chip Treatability Evaluation]
[0167] In the lathe turning for the 1000.sup.th test specimen, a
chip as illustrated in FIG. 6A and FIG. 6B 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 chip was a coil shape of not more than 30 mm in diameter, or
if the chip length 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 chip was
not a coil shape of not more than 30 mm in diameter, and the chip
length was also 50 mm or more, the chip treatability was determined
as being poor ("x" in Table 2).
[0168] [Rusting characteristics (corrosion resistance) evaluation
test] 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.
[0169] [Hot Ductility Evaluation Test]
[0170] A hot tension test was performed by electrical heating, and
the hot ductility 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 screw machining 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 was evaluated as excellent.
On the other hand, if the reduction of area was less than 70%, the
hot ductility was evaluated as not excellent.
[0171] [Test Results]
[0172] In Test Numbers 1 to 26, 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, the amount of tool wear was
200 .mu.m or less, 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. In addition, in the hot
ductility evaluation test, the reduction of area was 70% or more,
showing that excellent hot ductility was obtained.
[0173] In Test Numbers 1 to 6, 17, 19, 22 and 25, 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 7 to 16, 18, 20, 21, 23, 24 and 26 were
obtained.
[0174] On the other hand, in Test Numbers 27 to 35, 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.
[0175] In Test Numbers 36 and 37, 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.
[0176] In Test Number 38, 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.
[0177] In Test Number 39, the Mn content was too high. As a result,
the amount of tool wear was more than 200 .mu.m, and thus excellent
machinability was not obtained.
[0178] In Test Number 40, the Mn content was too low. In addition,
the solidification cooling rate RC was more than 100.degree.
C./min. Therefore, 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 point value was
less than 70%, and thus excellent hot ductility was not
obtained.
[0179] In Test Number 41, the S content was too high. As a result,
the point value was less than 70%, and thus excellent hot ductility
was not obtained.
[0180] In Test Number 42, 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.
[0181] In Test Number 43, the Pb content was too high. As a result,
excellent rusting characteristics were not obtained.
[0182] In Test Number 44, the Pb content was too low. In addition,
the solidification cooling rate RC was more than 100.degree.
C./min. Therefore, the number TN of specific inclusions was less
than 40 per mm.sup.2. As a result, the amount of tool wear was more
than 200 .mu.m and, furthermore, excellent chip treatability was
also not obtained. That is, excellent machinability was not
obtained.
[0183] In Test Number 45, the Al content was too low. Therefore,
the number TN of specific inclusions was less than 40 per mm.sup.2.
As a result, excellent rusting characteristics were not
obtained.
[0184] In Test Number 46, the N content was too high. As a result,
the amount of tool wear was more than 200 .mu.m, and thus excellent
machinability was not obtained. In addition, the point value was
less than 70%, and thus excellent hot ductility was not
obtained.
[0185] In Test Number 47, the O content was too high. In addition,
the solidification cooling rate RC was more than 100.degree.
C./min. Therefore, the number TN of specific inclusions was less
than 40 per mm.sup.2. As a result, the amount of tool wear was more
than 200 .mu.m, and thus excellent machinability was not
obtained.
[0186] In Test Number 48, the O content was too low. In addition,
the solidification cooling rate RC was more than 100.degree.
C./min. Therefore, the number TN of specific inclusions was less
than 40 per mm.sup.2. As a result, the amount of tool wear was more
than 200 .mu.m and, furthermore, excellent chip treatability was
also not obtained. That is, excellent machinability was not
obtained.
[0187] In Test Number 49, although the chemical composition was
appropriate and 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.
[0188] In Test Number 50, although the chemical composition was
appropriate and 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.
[0189] 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
[0190] 10 MnS Inclusions [0191] 20 Pb Inclusions [0192] 30
Composite Inclusions
* * * * *