U.S. patent application number 10/898963 was filed with the patent office on 2005-02-03 for low-carbon free cutting steel.
This patent application is currently assigned to Sumitomo Metal Industries, Ltd.. Invention is credited to Hasegawa, Tatsuya, Kato, Toru, Matsui, Naoki, Nishi, Takayuki, Watari, Koji.
Application Number | 20050025658 10/898963 |
Document ID | / |
Family ID | 33562752 |
Filed Date | 2005-02-03 |
United States Patent
Application |
20050025658 |
Kind Code |
A1 |
Matsui, Naoki ; et
al. |
February 3, 2005 |
Low-carbon free cutting steel
Abstract
The invention provides a low-carbon free cutting steel
containing no lead and is at least comparable in machinability to
the conventional leaded free cutting steels and composite free
cutting steels and furthermore has excellent finished surface
characteristics. The steel is a low-carbon free cutting steel which
comprises, on the percent by mass basis, C: 0.05 to under 0.20%,
Mn: 0.4-2.0%, S: 0.21-1.0%, Ti: 0.002-0.10%, P: 0.001-0.30%, Al:
not higher than 0.2%, 0: 0.001-0:03% and N: 0.0005-0.02%, with the
balance being Fe and impurities, and which satisfies the relations
(a) and (b) given below concerning the inclusions contained in the
steel: (A+B)/C.gtoreq.0.8 (a), N.sub.A.gtoreq.5 (b), wherein, A:
the total area occupied by substantial MnS with Ti carbide and/or
Ti carbonitride included therein among the inclusions not smaller
than 1 .mu.m in circle-equivalent diameter per mm.sup.2 of a cross
section parallel to the direction of rolling; B: the total area
occupied by substantial MnS with neither Ti carbide nor Ti
carbonitride included therein among the inclusions not smaller than
1 .mu.m in circle-equivalent diameter per mm.sup.2 of a cross
section parallel to the direction of rolling; C: the total area
occupied by all the inclusions not smaller than 1 .mu.m in
circle-equivalent diameter per mm.sup.2 of a cross section parallel
to the direction of rolling; N.sub.A: the number of substantial MnS
inclusions with Ti carbide and/or Ti carbonitride included therein
among the inclusions not smaller than 1 .mu.m in circle-equivalent
diameter per mm.sup.2 of a cross section parallel to the direction
of rolling.
Inventors: |
Matsui, Naoki;
(Amagasaki-shi, JP) ; Nishi, Takayuki;
(Kashima-shi, JP) ; Kato, Toru; (Kashima-shi,
JP) ; Watari, Koji; (Kobe-shi, JP) ; Hasegawa,
Tatsuya; (Kitakyushu-shi, JP) |
Correspondence
Address: |
BURNS DOANE SWECKER & MATHIS L L P
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
Sumitomo Metal Industries,
Ltd.
Osaka-shi
JP
|
Family ID: |
33562752 |
Appl. No.: |
10/898963 |
Filed: |
July 27, 2004 |
Current U.S.
Class: |
420/124 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/04 20130101 |
Class at
Publication: |
420/124 |
International
Class: |
C22C 038/14 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 1, 2003 |
JP |
2003-285463 |
Claims
What is claimed is:
1. A low-carbon free cutting steel which comprises, on the percent
by mass basis, C: 0.05 to under 0.20%, Mn: 0.4-2.0%, S: 0.21-1.0%,
Ti: 0.002-0.10%, P: 0.001-0.30%, Al: not higher than 0.2%, O
(oxygen): 0.001-0.03% and N: 0.0005-0.02%, with the balance being
Fe and impurities, and which satisfies the relations (a) and (b)
given below concerning the inclusions contained in the steel:
(A+B)/C.gtoreq.0.8 (a), N.sub.A.gtoreq.5 (b), wherein, in the
relations (a) and (b), A, B, C and NA denote as follows: A: the
total area occupied by substantial MnS with Ti carbide and/or Ti
carbonitride included therein among the inclusions not smaller than
1 .mu.m in circle-equivalent diameter per mm.sup.2 of a cross
section parallel to the direction of rolling; B: the total area
occupied by substantial MnS with neither Ti carbide nor Ti
carbonitride included therein among the inclusions not smaller than
1 .mu.m in circle-equivalent diameter per mm.sup.2 of a cross
section parallel to the direction of rolling; C: the total area
occupied by all the inclusions not smaller than 1 .mu.m in
circle-equivalent diameter per mm.sup.2 of a cross section parallel
to the direction of rolling; N.sub.A: the number of substantial MnS
inclusions with Ti carbide and/or Ti carbonitride included therein
among the inclusions not smaller than 1 .mu.m in circle-equivalent
diameter per mm.sup.2 of a cross section parallel to the direction
of rolling.
2. A low-carbon free cutting steel according to claim 1, which
further contains at least one element selected from among Se:
0.0005-0.10%, Te: 0.0005-0.10%, Bi: 0.01-0.3%, Sn: 0.01-0.3%, Ca:
0.0001-0.01%, Mg: 0.0001-0.005%, B: 0.0002-0.02% and rare earth
elements: 0.0005-0.02% in lieu of part of Fe.
3. A low-carbon free cutting steel according to claim 1, which
further contains at least one element selected from among Cu:
0.01-1.0%, Ni: 0.01-2.0%, Mo: 0.01-0.5%, V: 0.005-0.5% and Nb:
0.005-0.5% in lieu of part of Fe.
4. A low-carbon free cutting steel according to claim 1, which
further contains at least one element selected from among Se:
0.0005-0.10%, Te: 0.0005-0.10%, Bi: 0.01-0.3%, Sn: 0.01-0.3%, Ca:
0.0001-0.01%, Mg: 0.0001-0.005%, B: 0.0002-0.02% and rare earth
elements: 0.0005-0.02% and at least one element selected from among
Cu: 0.01-1.0%, Ni: 0.01-2.0%, Mo: 0.01-0.5%, V: 0.005-0.5%, and Nb:
0.005-0.5% in lieu of part of Fe.
5. A low-carbon free cutting steel according to claim 1, which
further contains either one or both of Si: 0.1-2.0% and Cr:
0.03-1.0% in lieu of part of Fe.
6. A low-carbon free cutting steel according to claim 2, which
further contains either one or both of Si: 0.1-2.0% and Cr:
0.03-1.0% in lieu of part of Fe.
7. A low-carbon free cutting steel according to claim 3, which
further contains either one or both of Si: 0.1-2.0% and Cr:
0.03-1.0% in lieu of part of Fe.
8. A low-carbon free cutting steel according to claim 4, which
further contains either one or both of Si: 0.1-2.0% and Cr:
0.03-1.0% in lieu of part of Fe.
Description
[0001] The disclosure of Japanese Patent Application No.
2003-285463 filed in Japan on Aug. 1, 2003 including specification,
drawings and claims is incorporated herein by reference in its
entirety.
FIELD OF THE INVENTION
[0002] This invention relates to a low-carbon free cutting steel
which is free of lead (Pb). In particular, it relates to a
low-carbon free cutting steel which, when machined with a carbide
tool, has superior machinability compared to conventional leaded
free cutting steels and composite free cutting steels in which lead
and one or more machinability improving elements are used together,
in spite of its being free of lead, and also which has excellent
hot workability and finished surface characteristics after
machining, and can be produced at a low cost. The present invention
also relates to a low-carbon free cutting steel excellent, not only
in the above-mentioned characteristics, but also in carburizing
characteristics.
BACKGROUND OF THE INVENTION
[0003] In manufacturing soft small articles, not required to have
very high strength, steel materials excellent in machinability,
namely the so-called free cutting steels, have so far been used for
the improvement of productivity. The best known free cutting steels
include resulfurized free cutting steels which are improved in
machinability by MnS due to the addition of a large amount of S;
leaded free cutting steels obtained by an addition of Pb; and
composite free cutting steels containing both of S and Pb. In
particular, leaded free cutting steels are characterized in that
they contribute toward prolonging the tool life and are excellent
in chip disposability and also in that the steel products after
machining are excellent in surface roughness, etc. Further, there
are free cutting steels containing Te (tellurium) and/or Bi
(bismuth) for the purpose of machinability improvement. These
steels are used in large amounts in small parts such as automotive
breaks, personal computers and their accompanying parts, and other
various machine parts such as electric appliance parts and
molds.
[0004] In recent years, the ability of cutting machines has been
improved and, as a result, it has become possible to increase the
speed of machining. Accordingly, steel materials, used as raw
materials in the above-mentioned parts, are also intensely
requested to show further improved machinability in high-speed
machining.
[0005] Furthermore, in some instances, such parts as mentioned
above, after being completed to a desired shape and form by
machining, are subjected to carburizing in order to secure a
desired level of surface strength. Therefore, the steel material to
be used in manufacturing such parts is sometimes required, not only
to have high machinability, but also to be excellent in carburizing
characteristics.
[0006] Steels to be used as materials of such parts as mentioned
above are required to have good machinability. As for such
machinability, a special emphasis is placed not only on prolonging
of the tool life but also on the properties, such that chips are
separated in small pieces or, in other words, "chip disposability".
The chip disposability is indispensable for working line automation
and also essential for increasing productivity. Further, in
addition to the tool life and chip disposability requirements, it
is desired, from the working precision viewpoint, that the steel
surface after machining should be good in finished surface
characteristics, namely the finished surface roughness should be as
low as possible. Among the free cutting steels mentioned above,
leaded cutting steels and composite free cutting steels, containing
Pb and one or more machinability improving elements, are superior
due to such characteristics and are deemed the best in
machinability among the currently available steels.
[0007] In view of the recent increasing concern about the
environmental problems, a non-leaded free cutting steel is
earnestly desired because those steels which contain Pb, which is
hazardous to human beings and the global environment, require
installation of large-scaled exhausters for the process of
production and, further, the trend toward suppression of the use of
Pb for the preservation of the environment is increasing.
[0008] To meet the above demand, various proposals have been made
concerning non-leaded low-carbon resulfurized free cutting steels,
which are to serve as alternatives to leaded free cutting steels.
However, any free cutting steel capable of contributing to
prolonging the life of the tool and having, to a satisfying extent,
all the characteristic of the Pb-containing free cutting steels,
such as good chip disposability and low levels of surface
roughness, has not yet been developed.
[0009] In Japanese Patent Laid-open Application (JP Kokai)
2003-49240 (Patent Document 1), there is disclosed a free cutting
steel improved in machinability, by causing Ti and/or Zr
carbosulfide type inclusions to exist therein. Since the Ti
carbosulfide or the Zr carbosulfide is dispersed, together with
MnS, in this free cutting steel, the pseudo lubricating effect of
the MnS can hardly be obtained and the frictional force between the
tool and the work material increases. As a result, the cutting
force increases and a built-up edge formation on the edge of
cutting tool is facilitated. Once a built-up edge has been formed,
the finished surface roughness after finish machining increases and
the working precision of the parts is impaired.
[0010] In Patent Document 1, no example is given of the case where
the Ti content is 0.1% or less. This indicates that the invention
disclosed in Patent Document 1 aims at forming Ti carbosulfide
inclusions by causing a large amount of Ti to be contained in the
steel. Actually, it is described therein that the Ti carbosulfide
type inclusions, together with MnS, are dispersed in a granular
form in the matrix. In this case, those performance
characteristics, such as tool life, chip disposability and
low-level finished surface roughness, which are required in the
steel to be used in manufacturing the parts mentioned above, cannot
be acquired.
[0011] JP Kokai 2003-49241 (Patent Document 2) discloses a free
cutting steel containing Ti and/or Zr within the content range of
(Ti+0.52 Zr)/S<2, containing Ti or Zr carbosulfide as an
inclusion component and contributing to a prolonged tool life in
turning and drilling. The invention described in Patent Document 2
aims at improving the tool life in turning by causing a formation
of Ti carbosulfide in the steel. This technology can indeed improve
the tool life to a certain extent but the presence of Ti or Zr
carbosulfide makes it difficult to obtain the lubricating effect of
the MnS, hence the frictional force between the tool and the work
material increases. As a result, the cutting force increases and a
built-up edge formation on the edge of cutting tool is facilitated.
Once a built-up edge has been formed, the finished surface
roughness after cutting increases, with the result that the working
precision of the parts is impaired.
[0012] In Patent Document 2, there is no example found of the free
cutting steel containing S in the range of not lower than 0.21% and
Ti at a level not higher than 0.1%, as specified later herein in
relation to the present invention. Due to this, it is evident that
the invention in Patent Document 2 is not an invention aiming at
improving the finished surface roughness and/or chip disposability.
According to the invention in Patent Document 2, Ti or Zr
carbosulfide, together with MnS, is dispersed in the matrix and,
therefore, the desired level of finished surface roughness and of
chip disposability cannot be attained.
[0013] In JP Kokai 2000-319753, there is disclosed a low-carbon
resulfurized free cutting steel in which the amount of MnS is
increased through an S content exceeding 0.4% to which Pb is not
added. However, this steel is low in improving the tool life of
carbide tools. Further, there is no improvement in chip
disposability, which is important, nor in performance
characteristics, as compared with the conventional resulfurized
free cutting steels.
[0014] JP Kokai H09-53147 discloses a free cutting steel excellent
in carbide tool machinability and, in particular, in tool life,
which contains C: 0.01-0.2%, Si: 0.10-0.60%, Mn: 0.5-1.75%, P:
0.005-0.15%, S: 0.15-0.40%, O (oxygen): 0.001-0.010%, Ti:
0.0005-0.020% and N: 0.003-0.03%. This invention aims at improving
the carbide tool life only due to the content of 0.1-0.6% of Si,
together with Ti, as the essential components. Accordingly, this
invention does not aim at improving the tool life or the chip
disposability or the finished surface roughness level because of
the "substantial MnS with Ti carbide and/or Ti carbonitride
included therein" exists in the steel without adding Si, as
included in the present invention.
[0015] Japanese Patent No. 3390988 discloses an invention relating
to a low-carbon resulfurized free cutting steel improved in
mechanical anisotropy which contains C: 0.02-0.15%, Mn: 0.3-1.8%,
S: 0.225-0.5%, Ti: 0.1-0.6% and Zr: 0.1-0.6%, with the proviso that
Ti+Zr: 0.3-0.6% and (Ti+Zr)/S ratio: 1.1-1.5. The steel of this
invention is improved in mechanical anisotropy and machinability by
employing the above composition to cause a formation of Ti and/or
Zr sulfide, which are high in hot deformation resistance. However
such sulfides, high in deformation resistance, can hardly afford a
sulfide-due lubricating effect during machining, hence the cutting
force increases, the deterioration of the tool life takes place and
the finished surface roughness after finish machining
increases.
SUMMARY OF THE INVENTION
[0016] It is an objective of the present invention to provide a
low-carbon free cutting steel which is free of Pb, which is
hazardous to the environment, shows superior machinability, in
particular when carbide tools are used, as compared with the
conventional leaded free cutting steels and composite free cutting
steels containing Pb and one or more machinability improving
elements, is excellent in hot workability and further in surface
characteristics after machining, and can be produced at a low cost.
Another objective of the present invention is to provide a
low-carbon free cutting steel, having good carburizing
characteristics, in addition to the characteristics mentioned
above.
[0017] As is well known, the states of inclusions, such as
sulfides, exert great influences on the machinability of steels.
Various inclusions are observed in steels containing C, Ti, S, N
and O. For example, this includes Ti sulfide, Ti carbosulfide, Ti
carbide, Ti carbonitride, Ti nitride and Ti oxide. When Mn is
further contained, Mn sulfide represented by the chemical formula
"MnS" is also present. When Al and/or Si are contained in addition
to those elements mentioned above, the oxides thereof are also
present. The states of these inclusions are diverse, and the
compositions and the states of these inclusions greatly influence
the machinability and other mechanical characteristics of
steels.
[0018] Previously, the present inventors filed a patent application
in Japan under Application No. 2002-26368 concerning a non-leaded,
low-carbon resulfurized free cutting steel. This free cutting steel
is characterized in that it contains C, Mn, S, Ti, Si, P, Al, O and
N in respective specified proportions and satisfies the expressions
(A) below concerning the contents of Ti and S, and the expression
(B) given below concerning the atomic ratio between Mn and S, and
contains MnS with Ti sulfide and/or Ti carbonsulfide included
therein.
Ti (% by mass)/S (% by mass)<1 (A)
Mn/S.gtoreq.1 (B)
[0019] The steel according to this senior invention is much better
in tool life than leaded free cutting steels and shows good chip
disposability. This steel, however, still has some drawbacks with
respect to its surface characteristics after machining. Namely, it
has been revealed that when the finish machining was carried out,
the finished surface roughness became large in some instances.
[0020] It is considered that when substantial Ti sulfide and/or Ti
carbosulfide exist in the matrix, it becomes difficult to obtain
the pseudo lubricating effect of MnS, hence the cutting force
increases and a built-up edge is formed on the edge of cutting
tool, which results in the steel surface after machining, loses
brightness and that the finished surface smoothness is
deteriorated. The term "substantial Ti sulfide and/or Ti
carbosulfide" as used herein means those inclusions in each of
which the total area in percent occupied by Ti sulfide and Ti
carbosulfide amounts to not less than 50%. Some of them are shown
in FIG. 1A attached hereto.
[0021] The present inventors made investigations to solve this
problem and, as a result, obtained new findings as mentioned
below.
[0022] (1) When a steel in which "substantial Ti sulfide and/or Ti
carbosulfide" existing in the matrix is machined, a built-up edge
is formed on the edge of cutting tool, resulting in a deteriorated
finished surface smoothness.
[0023] (2) When the formation of "substantial Ti sulfide and/or Ti
carbosulfide" are prevented as far as possible and a large amount
of MnS is allowed to exist in the steel, the built-up edge
formation can be prevented and the finished surface can be improved
from the roughness viewpoint.
[0024] (3) However, with steels not containing Ti but containing
only MnS, the carbide tool life is deteriorated. For improving the
carbide tool life, it is necessary to add Ti and, at the same time,
allow the existence of "substantial MnS with Ti carbide and/or Ti
carbonitride included therein".
[0025] (4) The "substantial MnS with Ti carbide and/or Ti
carbonitride included therein" improves the tool life, while it
does not impair the pseudo lubricating effect of MnS.
[0026] Based on the above findings, the relation between the
chemical composition and the states of existence of inclusions was
investigated in detail. As a result, a low-carbon free cutting
steel, specified below, was invented. This low-carbon free cutting
steel is comparable or superior in machinability to the leaded free
cutting steels and composite free cutting steels. The "%"
indicating the content of each individual component means "% by
mass".
[0027] A low-carbon resulfurized free cutting steel, that is
characterized by comprising C: 0.05 to under 0.20%, Mn: 0.4-2.0%,
S: 0.21-1.0%, Ti: 0.002-0.10%, P: 0.001-0.30%, Al: not higher than
0.2%, O (oxygen): 0.001-0.03% and N: 0.0005-0.02%, with the balance
being Fe and impurities, and which satisfies the relations (a) and
(b) given below concerning the inclusions contained in the
steel.
(A+B)/C.gtoreq.0.8 (a),
N.sub.A.gtoreq.5 (b),
[0028] where A, B, C and NA denote as follows:
[0029] A: The total area occupied by substantial MnS with Ti
carbide and/or Ti carbonitride included therein, among the
inclusions not smaller than 1 .mu.m in circle-equivalent diameter
per mm.sup.2 of a cross section parallel to the direction of
rolling.
[0030] B: The total area occupied by substantial MnS with neither
Ti carbide nor Ti carbonitride included therein, among the
inclusions not smaller than 1 .mu.m in circle-equivalent diameter
per mm.sup.2 of a cross section parallel to the direction of
rolling.
[0031] C: The total area occupied by all the inclusions not smaller
than 1 .mu.m in circle-equivalent diameter per mm.sup.2 of a cross
section parallel to the direction of rolling.
[0032] N.sub.A: The number of substantial MnS inclusions with Ti
carbide and/or Ti carbonitride included therein, among the
inclusions not smaller than 1 .mu.m in circle-equivalent diameter
per mm.sup.2 of a cross section parallel to the direction of
rolling.
[0033] The above low-carbon free cutting steel may contain one or
more components selected from at least one group out of the
following first to third groups:
[0034] First group:
[0035] Se: 0.0005-0.10%, Te: 0.0005-0.10%, Bi: 0.01-0.3%, Sn:
0.01-0.3%, Ca: 0.0001-0.01%, Mg: 0.0001-0.005%, B: 0.0002-0.02% and
rare earth elements: 0.0005-0.02%.
[0036] Second group:
[0037] Cu: 0.01-1.0%, Ni: 0.01-2.0%, Mo: 0.01-0.5%, V: 0.005-0.5%
and Nb: 0.005-0.5%.
[0038] Third group:
[0039] Si: 0.1-2.0% and Cr: 0.03-1.0%.
[0040] The term "substantial MnS with Ti carbide and/or Ti
carbonitride included therein" as used herein means those
inclusions in each of which the area percentage of MnS amounts to
not less than 50% and Ti carbide and/or Ti carbonitride are
included (i.e. coexist). On the other hand, the term "substantial
MnS with neither Ti carbide nor Ti carbonitride included therein"
means those inclusions in each of which the area percentage of MnS
amounts to not less than 50% and neither Ti carbide nor Ti
carbonitride are included (i.e. coexist). In each of these
"substantial MnS with Ti carbide and/or Ti carbonitride included
therein" and "substantial MnS with neither Ti carbide nor Ti
carbonitride included therein", there may be included any of
sulfides, carbosulfides, carbides, nitrides and other compounds
other than Ti carbide and Ti carbonitride.
[0041] The main characteristic features of the low-carbon free
cutting steel of the present invention are as follows:
[0042] (1) It contains C at a level of 0.05 to under 0.20%, S at a
level within the range of 0.21-1.0%, and Ti at a content level of
0.002-0.1%.
[0043] (2) Ti binds to C, S, N and O to form the sulfide,
carbosulfide, carbide, carbonitride and oxide. Ti, which shows a
stronger tendency toward sulfide formation than Mn, readily forms
Ti sulfide and Ti carbosulfide. However, when the content balance
among Mn, Ti, S and N is carefully considered and adjusted, it is
possible to allow the abundant existence of "substantial MnS with
Ti carbide and/or Ti carbonitride included therein" and
"substantial MnS with neither Ti carbide nor Ti carbonitride
included therein" while not allowing the abundant formation of
"substantial Ti carbosulfide and/or Ti sulfide".
[0044] (3) When the chemical composition given above (1) is
employed and the state of existence of inclusions as specified
above (2) are obtained, "substantial MnS" capable of softening and
producing a lubricating effect during machining accounts for most
of all the inclusions existing in the matrix, and the content of
other sulfides other than this "substantial MnS", namely
"substantial Ti sulfide and/or Ti carbosulfide", becomes nearly
zero. For attaining a favorable finished surface roughness level on
that occasion, the amount of "substantial MnS" formed must account
for most of all the inclusions formed. More specifically, it is
necessary, for the total area occupied by "substantial MnS"
inclusions not smaller than 1 .mu.m in circle-equivalent diameter
per mm.sup.2 of a sectional surface for observation in the
direction of rolling, to amount to not less than 80% of the total
area of all the inclusions not smaller than 1 .mu.m in
circle-equivalent diameter. Only in that case, the built-up edge
formation on the edge of cutting tool, which is caused by the
existence of "substantial Ti sulfide and/or Ti carbosulfide", can
be prevented and the good finished surface roughness features can
be obtained.
[0045] The above-mentioned "substantial MnS" comprises those
inclusions in each of which the area percentage of MnS is not less
than 50%, including "substantial MnS with Ti carbide and/or Ti
carbonitride included therein" and "substantial MnS with neither Ti
carbide nor Ti carbonitride included therein".
[0046] As shown in the expression (a) given above, it is "A+B" in
expression (a) that "accounts for at not less than 80%". And, A and
B are respectively defined as the area occupied by "substantial MnS
with Ti carbide and/or Ti carbonitride included therein" and the
area occupied by "substantial MnS with neither Ti carbide nor Ti
carbonitride included therein" among the sulfides not smaller than
1 .mu.m in circle-equivalent diameter per mm.sup.2 of a cross
section parallel to the direction of rolling.
[0047] And, C in the expression (a) is the total area of
"substantial MnS with Ti carbide and/or Ti carbonitride included
therein", "substantial MnS with neither Ti carbide nor Ti
carbonitride included therein", "substantial Ti sulfide and/or Ti
carbosulfide", other sulfides, carbosulfides, carbides, nitrides,
oxides, Al.sub.2O.sub.3, SiO.sub.2, and so forth.
[0048] (4) When the steel containing such inclusions as specified
above term
[0049] (3), namely a steel in which almost no "substantial Ti
sulfide and/or Ti carbosulfide" is present and the inclusions
contained therein mostly comprise "substantial MnS" but there
exists "substantial MnS with Ti carbide and/or Ti carbonitride
included therein", is machined in the high-speed range where the
cutting temperature increases, a hard TiN film is formed on the
tool surface and the prolonged tool life can be obtained
accordingly.
[0050] (5) In the steel in which "substantial MnS with Ti carbide
and/or Ti carbonitride included therein" exists, that "substantial
MnS" is finer in size, hence larger in number of inclusions, than
the MnS contained in the conventional JIS SUM 22L to 24L composite
free cutting steels. In this case, such fine "substantial MnS"
inclusions serve as stress concentration points in chips during
machining and promote crack propagation, so that such a level of
chip disposability that is at least comparable to that of the
composite free cutting steels can be obtained.
[0051] (6) The steel in which "substantial MnS with Ti carbide
and/or Ti carbonitride included therein" exists is no problem about
its hot workability and, therefore, the content of S, which is
effective in machinability improvement, can be increased and, in
that case, there is no trouble in producing the same in a
continuous casting plant, for instance. As for the level of an
addition amount of Ti, a small amount thereof can produce
satisfactory effects, hence the production cost can remain at a low
level and the steel can be applied as an inexpensive one.
[0052] As mentioned hereinabove, good machinability can be obtained
by restricting the ranges of the respective alloying elements and
adjusting the morphology of inclusions. However, the steels to be
used in manufacturing automotive parts are desired to be excellent
not only in machinability but also in carburizing characteristics
in some cases. Therefore, the influences of Si and Cr on such steel
characteristics were investigated and, as a result, it was revealed
that the carburizing characteristics can be improved by adjusting
the Si content and Cr content without impairing the above-mentioned
morphology of inclusions, hence without deteriorating the
machinability of the steel.
[0053] Si and Cr are dissolved in austenite and this increases the
hardenability of steels and thereby increases the depth of
carburizing and the hardness of the carburized layer in the
carburizing treatment. In addition to Si and Cr, there are other
hardenability increasing elements, for example Mn, Mo and P.
However, from the machinability or hot workability viewpoint, Mn is
required to be added at a sufficiently high level relative to the
content of S. In that case, the addition of a further amount of Mn
for hardenability improvement only means an additional cost. Mo is
effective in increasing the hardenability of the steels but is more
expensive than Si or Cr, hence the addition of Mo in an equally
effective amount results in an increase in production cost. P also
has the same effect but the addition thereof results in a rapid
increase in hardness of the steels themselves, hence in the
deterioration in machinability. However, in cases where there is no
restriction in the material cost these elements may be added at
levels not leading to deterioration in machinability or in
mechanical properties. On the other hand, Si and Cr are desirous as
carburization characteristics improving elements when the desired
steels are to be produced at a low cost without deterioration in
machinability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] FIG. 1A is a schematic representation of the morphology of
inclusions in a steel for comparison, and FIG. 1B in a steel
according to the present invention.
[0055] FIG. 2 is a graphic representation of the relationship
between ratio (A+B)/C and mean finished surface roughness for
steels according to the present invention and for steels for
comparison.
[0056] FIG. 3 is a graphic representation of the relationship
between mean finished surface roughness and tool life for steels
according to the present invention and for steels for
comparison.
[0057] FIG. 4 is a graphic representation of the relationship
between chip disposability and tool life for steels according to
the present invention and for steels for comparison.
DETAILED DESCRIPTION OF THE INVENTION
[0058] 1. "Substantial MnS with Ti Carbide and/or Ti Carbonitride
Included Therein"
[0059] Ti binds to S, C, N and O to form Ti sulfide and Ti
carbosulfide represented by the chemical formulas such as TiS and
Ti.sub.4C.sub.2S.sub.2, as well as Ti-based inclusions such as Ti
carbide, Ti carbonitride, Ti nitride and Ti oxide represented by
the chemical formulas TiC, Ti(CN), TiN and TiO, respectively. In
some cases, Ti is dissolved in MnS and exists as (Mn, Ti)S; since,
however, the solubility of Ti in MnS is low, this sulfide is
substantially MnS.
[0060] On the other hand, at times Ti is not dissolved in MnS but
exists as a phase distinct from MnS. Then Ti exists in the form of
TiC and/or Ti(C, N), namely in a form distinctly different in
composition from MnS, and the mode of existence thereof is
diversified, for example in the vicinity around one sulfide, or
surrounded by MnS.
[0061] FIG. 1A is a schematic representation of inclusions existing
in a Ti-containing free cutting steel for comparison, and FIG. 1B
in a free cutting steel according to the present invention. In the
steel shown in FIG. 1A, the independently existing Ti sulfide
and/or Ti carbosulfide inclusions and those inclusions in each of
which the area percentage of Ti sulfide and/or Ti carbosulfide is
not less than 50%, even when they coexist with MnS, and which can
be regarded substantially as Ti sulfide and/or Ti carbosulfide,
namely the above-mentioned "substantial Ti sulfide and/or Ti
carbosulfide", exist in large numbers.
[0062] On the other hand, in the steel of the present invention as
shown in FIG. 1B, those inclusions with Ti carbide and/or Ti
carbonitride taken up on the periphery or in the inside of MnS,
namely "substantial MnS with Ti carbide and/or Ti carbonitride
included therein", and "substantial MnS with neither Ti carbide nor
Ti carbonitride included therein" exist in large numbers.
[0063] In case Ti sulfide, Ti carbosulfide, Ti carbide, Ti
carbonitride, Ti nitride, Ti oxide and other inclusions are
included in the steel shown in FIG. 1B each in a form resulting
from distinct phase separation from MnS, those in which the area
percentage of MnS is not less than 50%, are each judged as
substantially one MnS inclusion, namely "substantial MnS"
inclusion. Conversely, those inclusions in each of which the area
percentage of such a Ti-based inclusion compound or some other
element-based oxide, nitride, carbide, etc. amounts to not less
than 50% are each regarded not as "substantial MnS" but as
substantially one Ti-based inclusion or other element-based oxide,
nitride, carbide or the like.
[0064] Among the MnS inclusions mentioned above, those in which Ti
carbide and/or Ti carbonitride, in particular, are included in a
form resulting from the distinct phase separation from MnS, and the
area percentage of MnS is not less than 50%, are defined as
"substantial MnS with Ti carbide and/or Ti carbonitride included
therein". On the other hand, "substantial MnS with neither Ti
carbide nor Ti carbonitride included therein" includes MnS in those
inclusions in each of which such a Ti-based inclusion other than Ti
carbide and Ti carbonitride as mentioned above, or some other
element-based oxide, nitride, carbide or like compound and MnS
exist in distinctly separate phases, and the area percentage of MnS
is not less than 50%, and which substantially play the role as MnS,
and MnS entirely free of the above-mentioned Ti-based inclusions or
other element-based oxide, nitride, carbide, etc. Thus, the sum of
"substantial MnS with Ti carbide and/or Ti carbonitride included
therein" and "substantial MnS with neither Ti carbide nor Ti
carbonitride included therein" represents the sum of inclusions
regarded substantially as MnS (the above-mentioned "substantial
MnS" species) while the other inclusions include Ti-based
inclusions such as Ti sulfide, Ti carbosulfide, Ti carbide, Ti
carbonitride, Ti nitride, and Ti oxide, and other element-based
oxides, carbides and nitrides, etc.
[0065] The above-mentioned area percentage of MnS and Ti-based
inclusions in one inclusion can be understood by area analysis and
quantitative analysis, using an EPMA (electron probe
microanalyzer), an EDX (energy dispersive X-ray microanalyzer) or
the like, of a microstructure test specimen cut out from a round
bar to be subjected to a machining test. The existence of
"substantial MnS with Ti carbide and/or Ti carbonitride included
therein", "substantial MnS with neither Ti carbide nor Ti
carbonitride included therein" and other inclusions can be
confirmed by the same methods, and the total area and numbers of
inclusions can be determined by such a technique as image analysis
and, on that occasion, measurements are made in a plurality of
fields of view so that the sum of all the observation areas may
exceed 1 mm.sup.2, and the total areas and numbers of respective
inclusions per mm.sup.2 are converted into the mean total areas and
mean numbers, respectively.
[0066] 2. Reasons Why the Expression (A+B)/C.gtoreq.0.8 Should be
Satisfied
[0067] In the above-mentioned expression (a), A is the total area
occupied by "substantial MnS with Ti carbide and/or Ti carbonitride
included therein" among the inclusions not smaller than 1 .mu.m in
circle-equivalent diameter per mm.sup.2 of a cross section parallel
to the direction of rolling. B is the total area occupied by
"substantial MnS with neither Ti carbide nor Ti carbonitride
included therein" among the inclusions not smaller than 1 .mu.m in
circle-equivalent diameter per mm.sup.2 of a cross section parallel
to the direction of rolling. The term "circle-equivalent diameter"
indicates the diameter resulting from the conversion of the area of
one inclusion as determined by the technique mentioned above, for
example by image analysis, to a circle having the same area. The
restriction "not smaller than 1 .mu.m in circle-equivalent
diameter" is applied since those inclusions which are smaller than
1 .mu.m exert little influence on the machinability.
[0068] The expression (a) given above indicates that it is
necessary for the sum of A and B to be not less than 80% of the
total area occupied by all the inclusions not smaller than 1 .mu.m
in circle-equivalent diameter. Although good machinability can be
obtained within this range, a more preferred lower limit is 90%.
Further, as mentioned above, the inclusions other than the
inclusions represented by A and B include independently existing
nitrides, carbides, oxides, "substantial Ti sulfide and/or Ti
carbosulfide", etc. Thus, the expression (a) indicates that the
total area of such inclusions other than "substantial MnS" should
be lower than 20% of the total area (i.e. C in expression (a))
occupied by all inclusions. More preferably, the total area in
question is less than 10%.
[0069] When Ti is added to a steel containing a large amount of S
for the purpose of machinability improvement; Ti, which has a
stronger tendency toward sulfide formation than Mn, readily forms
Ti sulfide and/or Ti carbosulfide. However, the expression (a),
specified in accordance with the present invention, aims at
preventing the formation of Ti sulfide and Ti carbosulfide on the
assumption that Ti is added. This is because Ti sulfide and Ti
carbosulfide interfere with the pseudo lubricating effect of MnS
during machining. If the pseudo lubricating effect of MnS is
weakened, it is presumable that the frictional force between the
tool and work material increase and a built-up edge is formed on
the edge of cutting tool, it results in the deterioration of the
finished surface roughness characteristics. Therefore, the
formation of Ti sulfide and Ti carbosulfide must be prevented.
Thus, as shown in expression (a), when independently existing
"substantial Ti sulfide and/or Ti carbosulfide" are almost absent
in the steel and "substantial MnS" accounts for not less than 80%
of the inclusions contained in the steel, the pseudo lubricating
effect can be obtained during machining.
[0070] Thus, when the steel composition range specified in
accordance with the present invention is employed and the
expression (a) is satisfied, the steel can be at least comparable
in the finished surface roughness level in finish machining to the
conventional leaded free cutting steels and composite free cutting
steels. On the other hand, when the expression (a) is not satisfied
even if the chemical composition is within the range specified in
accordance with the present invention, good machinability cannot be
obtained.
[0071] 3. Reasons Why the Expression N.sub.A.gtoreq.5 Should be
Satisfied
[0072] In the above-mentioned expression (b), NA is "the number of
substantial MnS inclusions with Ti carbide and/or Ti carbonitride
included therein among the inclusions not smaller than 1 .mu.m in
circle-equivalent diameter per mm.sup.2 of a cross section parallel
to the direction of rolling". The term "substantial MnS inclusions
with Ti carbide and/or Ti carbonitride included therein" indicates
those inclusions in each of the area percentage of MnS is not less
than 50%, as mentioned hereinabove. The "substantial MnS with Ti
carbide and/or Ti carbonitride included therein" does not
substantially impair the pseudo lubricating effect, hence the
built-up edge hardly forms and the finished surface roughness
feature of the machined material will not deteriorate.
[0073] When a steel containing "substantial MnS with Ti carbide
and/or Ti carbonitride included therein" was machined in a
high-speed range exceeding 100 m/min using a carbide tool, and the
tool surface was observed in detail, it was found that TiN had
formed on the tool surface. Presumably, the hard TiN in a laminar
form, having a thickness of several micro-meters to scores of
micro-meters, is formed on the tool surface which come into contact
with the work material as a result of the reaction and the change
in quality occurring with the increase of temperature due to
friction. The existence of the hard TiN in a laminar form can be
confirmed by an area analysis and a point quantitative analysis, by
AES (Auger electron spectroscopy) or using an EPMA (electron probe
microanalyzer), of the tool surface deprived of carbon-containing
contaminants (e.g. oils and fats etc.) by Ar sputtering or the like
after completion of machining. As a result, it was found that the
surface area of TiN, adhering to the tool which covers 10 to 80% of
the contact area between the work material and the tool. The
remainder of the contact area between the work material and the
tool being covered by MnS or Fe resulting from adhesion thereof on
the occasion of machining or being the tool surface as such without
any adhering matter. Presumably, the hard TiN formed on the tool
surface prevents tool wear, such as a thermal diffusion wear and a
mechanical friction wear due to the hard inclusions, so that a
markedly superior tool life, compared with the conventional
resulfurized free cutting steels and Pb-containing composite free
cutting steels, can be obtained.
[0074] In order to obtain such effects as mentioned above, it is
only necessary that not less than 5 inclusions, preferably not less
than 10 inclusions, consisting of "substantial MnS with Ti carbide
and/or Ti carbonitride included therein" be present in each
mm.sup.2 of a cross-sectional observation face in the direction of
rolling.
[0075] On the other hand, when the expression (b) is not satisfied,
even if the chemical composition is within the range specified in
the invention in the expressed application, good machinability
cannot be obtained.
[0076] In a steel in which the forms of the inclusions satisfy the
expressions (a) and (b), as a result of the addition of Ti, MnS
exists in a very fine form. Thus, the number of MnS inclusions is
very large. These fine MnS inclusions serve as stress concentration
points in chips formed during machining and promote crack
propagation in the chips, whereby the chip disposability is
improved.
[0077] In summary, when not less than five "substantial MnS with Ti
carbide and/or Ti carbonitride included therein" exist stably in
each mm.sup.2 of a sectional observation face in the direction of
rolling in steel, and the sum of the total areas of "substantial
MnS with Ti carbide and/or Ti carbonitride included therein" and
"substantial MnS with neither Ti carbide nor Ti carbonitride
included therein" in each mm.sup.2 of a sectional observation face
in the direction of rolling is not less than 80% of the total area
of all inclusions, it is possible to obtain such tool life,
finished surface roughness characteristics, and chip disposability,
that are comparable or superior to those of the leaded free cutting
steels and composite free cutting steels. For realizing such
morphology of inclusions more stably and producing steels having
good machinability at a low cost by continuous casting or a like
method, it is necessary to take the balance among the contents of
Mn, Ti, S and N into consideration. More specifically, the
following conditions should be met.
Ti (%)/S (%).gtoreq.0.25 (a)
[0078] If Ti is added in a large amount relative to the amount of
S, namely if the Ti/S ratio exceeds 0.25, Ti sulfide and Ti
carbosulfide will exist in large amounts. As a result, the
expression (a) is not satisfied, and the pseudo lubricating effect
of MnS will be reduced. On such occasion, the cutting force tends
to increase and the built-up edge formation on the edge of cutting
tool easily tends to occur resulting in deterioration in surface
roughness characteristics in the finish machining and in working
precision.
[0079] Conversely, when a slight amount of Ti is added relative to
the amount of S, that is, when the Ti/S ratio is not higher than
0.25, Ti forms Ti carbide or Ti carbonitride while "substantial Ti
sulfide and/or Ti carbosulfide" in the form of independent
inclusions are almost absent.
[0080] While Ti carbide and Ti carbonitride precipitate in various
forms, they exist in a form included in MnS in some instances. And,
a steel containing "substantial MnS with Ti carbide and/or Ti
carbonitride included therein" is machined in high-speed range
using a carbide tool, a satisfactory tool life can be obtained.
Thus, the Ti (%)/S (%) ratio should be adjusted to 0.25 or below so
that the formation of singly existing inclusions consisting of
"substantial Ti sulfide and/or Ti carbosulfide" may be
prevented.
Atomic ratio between Mn and S: [Mn]/[S].gtoreq.1 (b)
[0081] S is an element which causes cracking in a step of hot
working. However, when an appropriate composition is maintained to
meet the condition that the atomic ratio between the amount of Mn
and S, namely the ratio in the number of atoms (number of moles)
between Mn and S, should be not lower than 1, i.e.
[Mn]/[S].gtoreq.1, Mn crystallizes as MnS and there arises no hot
workability problem even if Ti (%)/S (%).gtoreq.0.25. Supposing
that the steel is produced by continuous casting, for instance, no
hot workability problem arises within the above range and,
therefore, it is possible to add S in large amounts to thereby
increase the amount of MnS, which is effective in the machinability
improvement; even at high S content levels, the morphology of
inclusions as specified by the expressions (a) and (b) are not
impaired.
[0082] When [Mn]/[S]<1, sulfides resulting from abundant
dissolution of FeS in the MnS and the TiS are mainly formed and the
hot workability cannot be further improved unless more Ti is added
in an amount exceeding the amount of S. Even when [Mn]/[S]<1,
the addition of Ti in a range exceeding the amount of S can improve
the hot workability. In that case, however, the main sulfide
product is not MnS but mainly Ti sulfide and/or Ti carbosulfide,
which are harder than MnS, because Ti has a stronger tendency
toward sulfide formation than Mn. In that case, the soft
sulfide-due pseudo lubricating effect between the tool and work
material during machining cannot be obtained, but the cutting force
increases, resulting in deterioration in surface roughness level,
as mentioned above. Thus, it is a desirable condition for producing
the machinability improving effect of MnS and simultaneously
obtaining good hot ductility so that the atomic ratio between the
amounts of Mn and S should be not less than 1, namely
[Mn]/[S].gtoreq.1.
Ti (%)/N (%).gtoreq.1.35 (c)
[0083] It is an outstanding feature of the free cutting steel of
the present invention that it contains "substantial MnS with Ti
carbide and/or Ti carbonitride included therein". In cases where Ti
(%)/N (%)<1.35, this "substantial MnS with Ti carbide and/or Ti
carbonitride included therein" cannot be obtained in a sufficient
amount. In such cases, it is presumed that most of Ti added
crystallizes as TiN in the early stage of solidification, hence a
sufficient amount of Ti to form "substantial MnS with Ti carbide
and/or Ti carbonitride included therein" cannot be secured.
Therefore, the ratio Ti (%)/N (%) is desirably not less than 1.35
and, in order to stably obtain "substantial MnS with Ti carbide
and/or Ti carbonitride included therein", the ratio Ti (%)/N (%) is
more preferably not less than 1.5.
[0084] 4. Grounds for Restriction of the Chemical Composition
[0085] In the following, the grounds for restriction of the
chemical composition in accordance with the present invention will
be explained, together with the effects of the respective
components.
[0086] C: 0.05 to under 0.20%
[0087] C is an important element exerting a great influence on the
machinability. C content of 0.20% or above increases the strength
of the steel but deteriorates its machinability, thus rendering the
steel inappropriate to use where the machinability is regarded as
important. At a C content level under 0.05%, however, the steel
becomes excessively soft, allowing the occurrence of plucking
during machining, and the wear on the tool is promoted and the
finished surface roughness increases. Therefore, an adequate
content of C is 0.05 to under 0.20%. A more adequate C content for
obtaining better machinability is within the range of
0.07-0.18%.
[0088] Mn: 0.4-2.0%
[0089] Mn is an important element which forms sulfide inclusions
with S and thus exerts a great influence on the machinability. At a
Mn content level less than 0.4%, the absolute quantity of the
sulfide is insufficient, therefore a satisfactory level of
machinability cannot be obtained. Since Mn is an element increasing
the hardenability of steels, the content of Mn may be increased
when it is desired to obtain good carburizing characteristics.
However, it is necessary for the steel of the present invention,
which contains a large amount of S, to contain a large amount of Mn
so that the Mn may form MnS with S. The addition of Mn for
improving the carburizing characteristics means a further addition
to the Mn content and therefore is undesirable from a production
cost viewpoint. Therefore, the upper limit of the Mn content is set
to 2.0%. At a Mn content level exceeding 2.0%, the strength of the
steel increases and, accordingly, the cutting force increases, so
that the tool life is shortened. In order to further reduce the
cutting force and improve the tool life, chip disposability,
finished surface roughness characteristics and hot workability, the
relationship with the content of S is important. Therefore in order
to ensure those performance characteristics, the Mn content is
preferably within the range of 0.6-1.8%.
[0090] S: 0.21-1.0%
[0091] S is an element capable of forming sulfide inclusions with
Mn and thus effective in machinability improvement. The
machinability improving effect of MnS increases with the increase
in the amount of S and, therefore, the selection of the S content
level is important. At a S content level below 0.21%, it is
impossible to obtain a sufficient amount of sulfide inclusions,
hence no satisfactory machinability can be expected. On the other
hand, generally, S contents exceeding 0.35% deteriorate the hot
workability and promote the segregation of S in the center of the
steel ingot and induce cracking in a step of forging. By
maintaining an appropriate composition, however, it is possible to
increase the upper limit to 1.0%. For machinability improvement by
MnS, it is preferable to add a further amount of S, more preferably
to a level of not lower than 0.35%. An addition level exceeding
0.40% is still more preferred for further machinability
improvement. However, an excessive addition level results in an
increase in production cost due to a decrease in yield. Therefore,
a preferred upper limit to the S content is 0.70%.
[0092] Ti: 0.002-0.10%
[0093] Ti is an indispensable and important element for forming Ti
carbide and/or Ti carbonitride with N and/or C, causing the MnS,
which includes them, to exist in the steel. When "substantial MnS
with Ti carbide and/or Ti carbonitride included therein" exists in
the steel, the tool life of the high-speed machining using a
carbide tool is markedly improved, as described above. For causing
such MnS to exist in the steel, a Ti content not lower than 0.002%
is required. In order to stably disperse it in the steel and
obtaining a satisfactory tool life without deterioration in the
finished surface roughness level, it is necessary to take the
balance between the Ti content and the contents of S and N into
consideration. At a Ti content level exceeding 0.10%, "substantial
Ti sulfide and/or Ti carbosulfide" exists in the steel and
deteriorates the finished surface roughness level in finish
machining. Therefore, the upper limit of the Ti content is set to
0.10%. For obtaining good finished surface roughness levels more
stably, the Ti content is preferably not higher than 0.08%, more
preferably less than 0.03%.
[0094] On the other hand, when the Ti content is less than 0.002%,
it is impossible to form "substantial MnS with Ti carbide and/or Ti
carbonitride included therein" in a sufficient amount for improving
the tool life. For forming such MnS more reliably and thereby
improving the carbide tool life, a Ti content exceeding 0.01% is
desired.
[0095] P: 0.001-0.30%
[0096] P increases the hardenability of the steel and at the same
time increases the strength. For producing such effects, the
content of P should be not lower than 0.001%. At a P content level
not higher than 0.30%, the hardenability and strength can be
secured without deteriorating the machinability. At a P content
level exceeding 0.30%, the strength becomes excessively high and
the machinability is somewhat deteriorated and, in addition, the
hot workability is deteriorated through promoted segregation in the
steel ingot. Therefore, the content of P should be 0.001-0.30%. A
more preferred content of P for stably maintaining good
machinability and strength levels is 0.005-0.13%.
[0097] Al: not more than 0.2% (may not be added)
[0098] Al is used as a potent deoxidizing element and may be
contained up to a level of 0.2%. However, the oxide formed upon
deoxidation is hard and, when the content of Al exceeds 0.2%, the
hard oxide is formed in large amounts, deteriorating the
machinability. Therefore, content of not more than 0.1% is more
preferred. In cases where a sufficient deoxidation is possible by
the addition of C and Mn, Al may not be added and the content of Al
may have an impurity level not higher than 0.002%.
[0099] O (oxygen): 0.001-0.03%
[0100] When an appropriate amount of oxygen is contained in the
steel, it is dissolved in the MnS and this prevents the elongation
of MnS upon rolling and thus reduces the anisotropy in mechanical
properties, although the effects of the oxygen in the steel of the
present invention are not affected by the state of the deoxidation.
Further, oxygen is also effective in improving the machinability
and hot workability and also prevents the segregation of S.
However, at an O content level exceeding 0.03%, it causes problems;
for example, it causes deterioration of and damage to the
refractory material in the melting stage. Therefore, the oxygen
content should be within the range of 0.001-0.03%. A preferred
range for properly obtaining the above effects is 0.0015-0.01%.
[0101] N: 0.0005-0.02%
[0102] N easily forms hard nitrides with Al and/or Ti. These
nitrides are effective in making grains finer. However, when
present in large amounts, these nitrides tend to promote the wear
of the tool and deteriorate the machinability. Since Ti is added,
as an essential component, to the steel of the present invention,
the content of N should preferably be as low as possible. However,
for obtaining the above effect, N is allowable to be contained at a
level of not lower than 0.0005%. When, on the other hand, the
content of N becomes excessive, coarse TiN may be formed, possibly
impairing the machinability, hence the upper limit of the N content
is set to 0.02%. For securing better machinability, the upper limit
of the N content is preferably set to 0.015%. Since the present
invention aims at improving the machinability by the occurrence of
"substantial MnS with Ti carbide and/or Ti carbonitride included
therein", it is desirable that the contents of Ti and N satisfy the
condition of Ti (%)/N (%).gtoreq.1.35 so that such MnS may exist
stably in the steel. This is because when Ti (%)/N (%)<1.35,
most of Ti added forms TiN in the early stage of solidification
and, as a result, "substantial MnS with Ti carbide and/or Ti
carbonitride included therein" cannot be stably obtained, as
described hereinabove.
[0103] Owing to the chemical composition comprising the elements
whose contents are respectively adjusted as mentioned above and the
morphology of inclusions as specified by the expressions (a) and
(b), a low-carbon free cutting steel excellent in machinability,
hot workability and finished surface characteristics can be
obtained.
[0104] The low-carbon free cutting steel of the present invention
may further contain one or more elements selected from at least one
group among the first to third groups mentioned hereinabove (refer
to page 9).
[0105] (1) Elements of the First Group
[0106] The elements of the first group are elements capable of
further improving the machinability of the steel without impairing
the effects obtained by the above-mentioned main composition
according to the present invention. Therefore, for obtaining
further improved machinability, one or more of them may be
contained in the steel.
[0107] Se: 0.0005-0.10% and Te: 0.0005-0.10%
[0108] Se and Te form Mn(S, Se) and Mn(S, Te) with Mn. These play
the same role as that of MnS in producing the pseudo lubricating
effect during machining and, therefore, Se and Te are elements
which are effective in machinability improvement and, for further
machinability improvement, they may be contained in the steel
within the above respective ranges. When their content levels are
below 0.0005%, their effects are insignificant. On the other hand,
at levels exceeding 0.10%, not only the effect of each of the Se
and Te reaches a point of saturation but also due to the addition
it becomes uneconomical, and also the hot workability deteriorates.
For simultaneously obtaining good hot workability and more stable
machinability, the additional level of each element is preferably
0.0010-0.05%.
[0109] Bi: 0.01-0.3% and Sn: 0.01-0.3%
[0110] Bi and Sn are effective in improving the machinability of
the steel. This is presumably because they produce a lubricating
effect during the machining as low-melting metal inclusions,
similar to Pb. In order to attain that effect, the content of each
is preferably set to not lower than 0.01%. When the addition level
of each of them exceeds 0.30%, however, not only the effect of each
of them reaches a point of saturation but also the hot workability
deteriorates. For simultaneously obtaining good hot workability and
more stable machinability, the additional level of each element is
preferably 0.03-0.1%.
[0111] Ca: 0.0001-0.01%
[0112] Ca has a high affinity for S and O (oxygen) and forms the
corresponding sulfide and oxide in the steel. Further, Ca is
dissolved in MnS to form (Mn, Ca) S but the amount of Ca soluble
therein is slight, hence the effects of MnS are not impaired. The
oxide formed by Ca is a low-melting oxide and, thus, Ca is an
additive element effective in further improving the machinability
of the steel of the present invention. In order to improve the
machinability produced by the addition of Ca, the lower limit of
the Ca content is preferably set to 0.0001%. Since, however, the
yield of the additional Ca is low, the addition of large amounts of
Ca is required and, this is unfavorable from the production cost
viewpoint. Therefore, the upper limit to the Ca content is set to
0.01%. A more preferred upper limit is 0.005%.
[0113] Mg: 0.0001-0.005%
[0114] Mg also has a high affinity for S and O (oxygen) in the
steel and forms the corresponding sulfide and oxide. The
Mg-containing sulfide and oxide function as nucleating agents in
the crystallization of MnS and are effective in preventing the
elongation of MnS. When such effects are desired, Mg may be added.
In order to sufficiently produce those effects, the lower limit of
the Mg content is preferably set to not lower than 0.0001%. Since,
however, the oxide formed by Mg is hard, and excessively high Mg
content deteriorates machinability. Therefore, the upper limit of
the Mg content is set to 0.005%. A preferred upper limit for
preventing the elongation of MnS and simultaneously obtaining good
machinability is 0.002%.
[0115] B: 0.0002-0.02%
[0116] B binds to O (oxygen) or N to form the oxide or nitride and
is effective in the improvement of machinability, hence may be
added according to need. In order to obtain that effect, the B
content of not lower than 0.0002% is required and in order to be
more effective, a level of not lower than 0.0010% is desirable. At
a B content level exceeding 0.02%, that effect reaches a point of
saturation and, in addition, the hot workability is somewhat
deteriorated.
[0117] Rare earth elements: 0.0005-0.02%
[0118] Rare earth elements constitute a group of elements
classified as lanthanoids. When they are added, a misch metal or
the like containing them as main components is generally used. The
content of rare earth elements, so referred to herein, is expressed
in terms of the total content of one or more elements among the
rare earth elements. The rare earth elements form oxides with
oxygen and also bind to S to form sulfides, and thereby improve the
machinability. In order to be effective, their content should be
not less than 0.0005%. However, at content levels exceeding 0.02%,
the effect reaches a point of saturation. Further, the yield of the
addition of rare earth elements is low, hence the addition of rare
earth elements in large amounts is uneconomical.
[0119] (2) Elements of the Second Group
[0120] The elements of the second group all increase the strength
of steel. The steel may contain one or more of these elements
according to need.
[0121] Cu: 0.01-1.0%
[0122] Cu is effective in improving the strength of the steel
through precipitation hardening. In order to obtain this effect, a
Cu content of not less than 0.01% is required and an addition level
of not lower than 0.1% is desirable. However, at a Cu content level
exceeding 1.0%, the hot workability deteriorates. And moreover the
above effect reaches a point of saturation due to the coarsening of
the Cu precipitates. In addition, this brings about a decrease in
machinability.
[0123] Ni: 0.01-2.0%
[0124] Ni is effective in improving the strength of the steel
through solid-solution strengthening. In order to be effective, its
content is preferably not lower than 0.01%. At a Ni content level
exceeding 2.0%, however, the machinability deteriorates and, at the
same time, the hot workability also deteriorates.
[0125] Mo: 0.01-0.5%
[0126] Mo is an element capable of improving hardenability, but
when Mo is added in an amount sufficient to produce the carburizing
characteristics equivalent to that obtainable by the addition of Si
and/or Cr, the production cost disadvantageously increases since Mo
is more expensive than Si or Cr. However, Mo is also effective in
rendering the microstructure finer and improving the toughness,
therefore when it is desired that these effects be produced, Mo may
be added. In order to obtain the effects, a content of Mo of not
less than 0.01% is desirable, however, at levels exceeding 0.5%,
the effects reaches a point of saturation and, in addition, the
steel production cost increases.
[0127] V: 0.005-0.5%
[0128] V precipitates as fine nitrides or carbonitrides and
improves the strength of the steel. This effect can be obtained if
the V content is not less than 0.005% but a V content of not less
than 0.01% is preferred. At a V content level exceeding 0.5%,
however, the above effect reaches a point of saturation and, in
addition, the excessively formed nitride and/or carbide bring out a
decrease in machinability.
[0129] Nb: 0.005-0.5%
[0130] Nb precipitates as fine nitrides or carbonitrides and
improves the strength of the steel. This effect can be obtained if
the Nb content is not less than 0.005% but a Nb content of not less
than 0.01% is preferred. At a Nb content level exceeding 0.5%,
however, the above effect reaches a point of saturation and, in
addition, the excessively formed nitride and/or carbide bring out a
decrease in machinability and the Nb addition at such level is also
uneconomical.
[0131] (3) Elements of the Third Group
[0132] The elements of the third group are elements either one or
both of which may be contained in the steel when the carburizing
characteristics thereof need improvement.
[0133] Si: 0.1-2.0%
[0134] In the free cutting steel of the present invention, as set
forth in any of Claims 1 to 4, no positive addition of Si is made.
This is due to Si being one of the impurities and the content of Si
is less than 0.1%. In the case of the free cutting steel of the
present invention, as set forth in any of Claims 1 to 4, Si is
added in some instances as a deoxidizing element for adjusting the
oxygen content in the steel to an appropriate level. Even in those
cases, it is not necessary to positively allow Si to remain; the Si
remains in the steel is an impurity and its content is less than
0.1%.
[0135] Through its dissolution in ferrite, Si is effective in
improving the strength of the steel and it is also effective in
improving the hardenability of the steel. By increasing the
hardenability of the steel, it becomes possible to also improve the
carburizing characteristics which are desired in manufacturing
automotive parts. In this only case, Si can be added at a level of
not lower than 0.1% and in order to further improve the carburizing
characteristics more reliably, a content level exceeding 0.6% is
desirable. However, at a Si content level exceeding 2.0%, the
machinability is adversely affected, for example the hot
workability deteriorates and the cutting force increases, because
of solid solution hardening of the ferrite phase. Even when the Si
content is at an impurity level of lower than 0.1%, the oxygen
content in the steel can be adjusted to an appropriate level by
properly adding C, Mn and/or Al.
[0136] Cr: 0.03-1.0%
[0137] Cr is an element capable of improving the carburizing
characteristics thereby increasing the hardenability of the steel
through additional small amounts. When it contains Cr, the steel
shows improved carburizing characteristics; the carburized layer
hardness, after carburizing treatment, is high, and the effective
hardening depth can be increased. In order to obtain such effects,
the Cr content should be set to not less than 0.03% and when more
reliable improvements in carburizing characteristics are desired,
content exceeding 0.05% is desirable. However, at a Cr content
level exceeding 1.0%, the machinability deteriorates and the
production cost also increases.
[0138] When the above Si and/or Cr are contained, it becomes
possible to obtain a steel having good machinability and hot
workability and, further, good carburizing characteristics.
EXAMPLES
[0139] 1. Preparation of Test Specimens
[0140] Using a high frequency induction furnace, 150-kg steel
ingots (diameter: about 220 mm), having various respective
compositions shown in Tables 1 and 2 were prepared. The steels
according to the present invention are shown in Table 1, and the
conventional steels and comparative steels are shown in Table
2.
[0141] For the stable formation of "substantial MnS with Ti carbide
and/or Ti carbonitride included therein", these steel ingots were
heated to a temperature as high as 1,250.degree. C. and kept for 2
hours or a longer period at that temperature. After that, in order
to simulate the rolling process, forging was performed at a
finishing temperature of not lower than 1,000.degree. C. and, then,
the forgings were air-cooled to produce round bars having a
diameter of 65 mm. These forged bars were normalized by heating to
950.degree. C. and maintaining that temperature for 1 hour,
followed by air cooling.
[0142] The comparative steels Nos. 51 to 53 were poor in hot
workability and cracking occurred during the forging, making it
impossible to produce any forged bars; hence subsequent
investigations were not performed with them.
1TABLE 1 [Mn]/ Ti/S Ti/N [S] (mass (mass Steel Chemical Composition
(mass %; bal.: Fe and impurities) (Atomic % % (A + B)/ No. C Si Mn
P S Al Ti Cr N O Others Ratio) Ratio) Ratio) C N.sub.A .gtoreq. 5 1
0.05 <0.01 0.89 0.015 0.42 <0.001 0.092 -- 0.0012 0.0096 --
1.11 0.22 17.00 0.95 .largecircle. 2 0.06 <0.01 1.13 0.019 0.41
<0.001 0.060 -- 0.0085 0.0089 -- 1.61 0.15 7.02 0.96
.largecircle. 3 0.09 0.06 0.97 0.032 0.44 0.002 0.009 -- 0.0028
0.0018 -- 1.28 0.02 3.21 0.99 .largecircle. 4 0.10 0.04 1.49 0.029
0.55 0.001 0.061 -- 0.0047 0.0033 -- 1.58 0.11 12.98 0.98
.largecircle. 5 0.10 0.05 1.58 0.032 0.49 <0.001 0.018 -- 0.0047
0.0027 -- 1.88 0.04 3.83 0.97 .largecircle. 6 0.10 <0.01 0.96
0.016 0.36 <0.001 0.087 -- 0.0063 0.0046 -- 1.56 0.24 13.79 0.88
.largecircle. 7 0.10 <0.01 0.85 0.018 0.35 <0.001 0.080 --
0.0085 0.0079 -- 1.42 0.23 9.41 0.94 .largecircle. 8 0.12 0.04 1.10
0.030 0.38 <0.001 0.026 -- 0.0125 0.0018 -- 1.69 0.07 2.08 0.96
.largecircle. 9 0.13 <0.01 1.59 0.032 0.48 <0.001 0.024 --
0.0050 0.0043 -- 1.95 0.05 4.80 0.98 .largecircle. 10 0.15 0.07
1.49 0.030 0.49 0.025 0.014 -- 0.0047 0.0020 -- 1.77 0.03 2.98 0.99
.largecircle. 11 0.18 0.03 1.52 0.058 0.47 0.002 0.021 -- 0.0130
0.0036 -- 1.89 0.04 1.62 0.96 .largecircle. 12 0.09 0.02 1.17 0.022
0.40 0.005 0.053 0.20 0.0054 0.0035 -- 1.71 0.13 9.81 0.95
.largecircle. 13 0.11 0.46 1.60 0.020 0.60 <0.001 0.014 --
0.0101 0.0026 -- 1.56 0.02 1.39 0.98 .largecircle. 14 0.12 1.28
1.46 0.015 0.44 0.002 0.024 -- 0.0058 0.0027 -- 1.93 0.05 4.14 0.98
.largecircle. 15 0.15 0.04 1.12 0.026 0.40 <0.001 0.027 0.50
0.0125 0.0038 -- 1.63 0.07 2.16 0.96 .largecircle. 16 0.18 0.85
1.36 0.040 0.39 <0.001 0.021 -- 0.0134 0.0031 -- 2.04 0.05 1.57
0.97 .largecircle. 17 0.18 0.01 1.46 0.028 0.46 0.002 0.025 0.15
0.0048 0.0064 -- 1.85 0.05 5.21 0.98 .largecircle. 18 0.14 0.06
1.65 0.028 0.45 0.002 0.018 -- 0.0049 0.0024 Se: 0.010 2.14 0.04
3.67 0.99 .largecircle. 19 0.08 0.13 1.60 0.030 0.42 <0.001
0.014 -- 0.0095 0.0028 Te: 0.015 2.22 0.03 1.47 0.98 .largecircle.
20 0.12 0.03 1.50 0.031 0.45 0.002 0.022 -- 0.0120 0.0032 Bi: 0.05
1.95 0.05 1.83 0.98 .largecircle. 21 0.16 0.01 1.35 0.025 0.42
0.002 0.028 -- 0.0059 0.0055 Sn: 0.04 1.88 0.07 4.75 0.98
.largecircle. 22 0.06 0.01 0.88 0.017 0.49 0.001 0.082 -- 0.0062
0.0073 Ca: 0.0029 1.05 0.17 13.27 0.99 .largecircle. 23 0.10 0.05
1.02 0.020 0.46 0.004 0.025 -- 0.0037 0.0026 Mg: 0.0015 1.29 0.05
6.76 0.97 .largecircle. 24 0.10 0.01 1.48 0.027 0.47 0.002 0.021 --
0.0095 0.0048 B: 0.0025 1.84 0.04 2.21 0.97 .largecircle. 25 0.11
0.01 1.22 0.029 0.37 0.002 0.022 -- 0.0124 0.0025 Cu: 0.10 1.92
0.06 1.77 0.98 .largecircle. 26 0.15 0.02 1.38 0.035 0.43 0.002
0.025 -- 0.0045 0.0056 V: 0.05 1.87 0.06 5.56 0.98 .largecircle. 27
0.12 <0.01 1.55 0.030 0.46 <0.001 0.026 -- 0.0047 0.0050 Nb:
0.12 1.97 0.06 5.53 0.98 .largecircle. 28 0.16 0.07 1.00 0.025 0.25
0.002 0.060 -- 0.0101 0.0092 Ni: 0.10 2.33 0.24 5.93 0.89
.largecircle. 29 0.10 0.03 1.40 0.038 0.41 0.002 0.029 -- 0.0052
0.0064 Mo: 0.10 1.99 0.07 5.58 0.97 .largecircle. 30 0.11 0.01 1.25
0.035 0.46 <0.001 0.025 -- 0.0130 0.0048 Ca: 0.0015, 1.59 0.05
1.92 0.97 .largecircle. Mg: 0.0018 31 0.09 0.05 1.44 0.027 0.43
0.003 0.022 -- 0.0112 0.0030 Bi: 0.07, 1.95 0.05 1.96 0.98
.largecircle. Nb: 0.10 32 0.10 0.02 1.46 0.026 0.42 0.003 0.022 --
0.0132 0.0024 Sn: 0.07, 2.03 0.05 1.67 0.98 .largecircle. V: 0.10
33 0.12 0.05 0.99 0.031 0.40 0.001 0.029 -- 0.0085 0.0030 Ca:
0.001, 1.44 0.07 3.41 0.97 .largecircle. Mo: 0.12 34 0.10 0.03 1.52
0.032 0.45 <0.001 0.019 -- 0.0090 0.0035 Te: 0.012, 1.97 0.04
2.11 0.98 .largecircle.
[0143]
2 TABLE 2 [Mn]/ Ti/S Ti/N Chemical Composition (mass %; bal.: Fe
and impurities) [S] (mass (mass (A + Steel Oth- (Atomic % % B)/
N.sub.A .gtoreq. No. C Si Mn P S Al Ti Cr N O ers Ratio) Ratio)
Ratio) C 5 35 0.07 <0.01 1.02 0.070 0.32 0.002 -- -- 0.0052
0.0160 *Pb: 1.86 -- -- -- -- 0.31 36 0.08 0.01 1.12 0.060 0.31
0.002 -- -- 0.0084 0.0145 *Pb: 2.11 -- -- -- -- 0.18 37 0.08 0.01
1.02 0.067 0.33 0.002 -- -- 0.0066 0.0150 -- 1.80 -- -- -- -- 38
0.06 0.06 0.86 0.020 0.42 0.001 *0.280 -- 0.0050 0.0068 -- 1.20
*0.67 56.0 *0.75 .largecircle. 39 0.08 0.02 1.15 0.033 0.35 0.001
*0.250 -- 0.0079 0.0052 -- 1.92 *0.71 31.6 *0.70 .largecircle. 40
0.10 0.01 1.12 0.028 0.36 <0.001 *0.330 -- 0.0085 0.0053 -- 1.82
*0.92 38.8 *0.65 .largecircle. 41 0.18 0.03 0.47 0.016 0.29 0.003
*0.420 -- 0.0059 0.0019 -- *0.95 *1.45 71.2 *0.20 .largecircle. 42
0.10 0.01 1.05 0.013 0.39 0.001 0.006 -- 0.0099 0.0035 -- 1.57 0.02
*0.6 0.99 x* 43 0.09 0.35 1.21 0.025 0.30 0.001 0.009 -- 0.0138
0.0084 -- 2.35 0.03 *0.7 0.98 x* 44 *0.52 0.17 0.52 0.016 0.21
<0.001 0.050 -- 0.0079 0.0180 -- 1.45 0.24 6.4 0.96
.largecircle. 45 *0.45 <0.01 0.85 0.019 0.32 0.002 0.078 --
0.0046 0.0058 -- 1.55 0.24 17.0 0.95 .largecircle. 46 *0.01
<0.01 0.98 0.016 0.33 0.002 0.067 -- 0.0048 0.0048 -- 1.73 0.20
14.0 0.96 .largecircle. 47 0.10 0.02 1.45 0.016 0.49 *0.35 0.028 --
0.0057 0.0017 -- 1.73 0.06 4.9 0.97 .largecircle. 48 0.07 0.01 0.92
0.015 0.47 <0.001 0.056 *2.50 0.0075 0.0036 -- 1.14 0.12 7.5
0.99 .largecircle. 49 0.09 *2.55 1.56 0.020 0.45 0.001 0.065 --
0.0058 0.0024 -- 2.02 0.14 11.2 0.98 .largecircle. 50 0.05 <0.01
0.48 0.015 *0.15 <0.001 0.022 -- 0.0095 0.0193 -- 1.87 0.15 2.3
0.92 .largecircle. 51 0.09 0.01 1.85 0.018 *1.14 0.001 0.102 --
0.0078 0.0128 -- *0.95 0.09 13.1 -- -- 52 0.06 <0.01 *0.21 0.016
0.33 0.001 0.074 -- 0.0073 0.0078 -- *0.37 0.22 10.1 -- -- 53 0.08
<0.01 1.25 0.025 0.44 0.002 0.072 -- 0.0050 0.0049 *Te: 1.66
0.16 14.4 -- -- 0.12 54 0.06 <0.01 1.03 0.016 0.46 <0.001
0.045 -- 0.0059 0.0085 *V: 1.31 0.10 7.6 0.99 .largecircle. 2.0 55
0.15 <0.01 1.11 0.015 0.46 0.002 0.080 -- 0.0078 0.0063 *Mo:
1.41 0.17 10.3 0.98 .largecircle. 1.50 The mark "*" indicates that
the value is outside the relevant range specified by the present
invention.
[0144] 2. Investigation of the Morphology of Inclusions
[0145] Many of the inclusions observed in a cross section parallel
to the direction of rolling show a form which is elongated in the
direction of the rolling or an unspecific form. In investigating
the number of inclusions and the area occupied thereby, test
specimens for microscopic observation were taken from each of the
forged bars at a site corresponding to Df/4 (Df: diameter of the
forged bar) in the longitudinal sectional direction, and embedded
in a resin and, after mirror-like polishing, photographed under an
optical microscope magnified 400 times.
[0146] Each photograph was observed by image processing and the
number of inclusions and the area occupied by them were determined.
Those inclusions, which were not smaller than 1.mu.m in
circle-equivalent diameter upon conversion of the area of each
inclusion into a circle, having the same area, were employed as the
targets. The reason the targets were restricted to those having a
circle-equivalent diameter of not smaller than 1 .mu.m, is that
those inclusions smaller than 1 .mu.m have no substantial effect on
the machinability, as described hereinabove.
[0147] The composition of each of these inclusions was confirmed in
the following manner. Namely, as mentioned above, each test
specimen was prepared for microscopic observation by cutting out
from the forged bar at a site corresponding to Df/4 (Df is the
diameter of each forged bar) in a longitudinal sectional direction,
embedded in a resin, mirror-like polished and then was subjected to
area analysis and quantitative analysis using an EPMA (electron
probe microanalyzer), an EDX (energy dispersive X-ray
spectroscope), etc. The magnification for observation had a range
not exceeding 10,000 times and, at the selected observation
magnification, an inclusion in which MnS and Ti carbide and/or Ti
carbonitride were observed in distinctly separated phases, and in
which the area percentage of MnS was not lower than 50%,
corresponds to "substantial MnS with Ti carbide and/or Ti
carbonitride included therein".
[0148] Based on the results of such observation, the areas of
"substantial MnS with Ti carbide and/or Ti carbonitride included
therein" and "substantial MnS with neither Ti carbide nor Ti
carbonitride included therein" were determined for individual
inclusions having a circle-equivalent diameter of not smaller than
1 .mu.m. Then, the sums of the areas of these inclusions per
mm.sup.2 in the section in the direction of rolling were calculated
and, the sum of the areas occupied by these inclusions per mm.sup.2
in the section in the direction of rolling was calculated, and then
(A+B)/C was calculated.
[0149] From the results obtained in the above manner, the number of
inclusions consisting of "substantial MnS with Ti carbide and/or Ti
carbonitride included therein" was determined. When the average
number per mm.sup.2 of the section in the direction of rolling was
5 or more, the relevant steel was evaluated as ".largecircle.".
Conversely, when the number of inclusions consisting of
"substantial MnS with Ti carbide and/or Ti carbonitride included
therein" was less than 5, the relevant steel was evaluated as
".times.". The comparative steels Nos. 35 to 37 given in Table 2
are Ti-free leaded or resulfurized free cutting steels. They are
substantially free of "substantial MnS with Ti carbide and/or Ti
carbonitride included therein", hence such calculations were not
carried out.
[0150] 3. Machinability Testing
[0151] In the machinability testing, each forged bar was externally
machined into a round bar with a diameter of 60 mm and was
subjected to tests for tool life and finished surface roughness.
The tool life test was carried out using an uncoated JIS P20
carbide tool under the following dry turning conditions;
[0152] Cutting speed: 150 m/min,
[0153] Feed: 0.10 mm/rev, and
[0154] Depth of cut: 2.0 mm.
[0155] Thirty minutes after the start of turning under the above
conditions, the mean flank wear (VB) was measured. For those test
specimens showing a mean flank wear of not less than 200 .mu.m
within 30 minutes, the time required for arriving at such wear and
the mean flank wear (VB) at that time were measured for each of the
specimens.
[0156] The tool life evaluation was carried out using, as a
measure, the time. required for the mean flank wear (VB) to arrive
at 100 .mu.m. When the test specimen became short during testing
due to its superiority in suppressing the tool wear and extremely
slow wear rate of the tool, the time required for the mean flank
wear to arrive at 100 .mu.m was calculated from the turning
time-tool wear curve by the regression method. The chip
disposability was evaluated by collecting 200 representative chips
among the chips discharged from test specimen, measuring their
mass, and calculating the number of chips per unit mass.
[0157] Since the finished surface roughness is evaluated in terms
of the surface roughness after machining, the surface of each
machined material after machining, under the following conditions,
was evaluated using a versatile instrument for the evaluation of
surface texture. The test for the evaluation of the finished
surface roughness was carried out using a TiAlN multilayer-coated
JIS K type carbide tool. The cutting was carried out in the manner
of wet turning using a lubricating oil of the aqueous emulsion type
under the following condition;
[0158] Cutting speed: 100 m/min,
[0159] Feed: 0.05 mm/rev, and
[0160] Depth of cut: 0.5 mm.
[0161] Each tested steel was machined under these conditions for 1
minute, and the test specimen result was evaluated for finished
surface roughness by measuring the mean finished surface roughness
(Ra), while moving the stylus of the versatile instrument for the
evaluation of surface texture in an axial direction.
[0162] 4. Hot Workability Testing
[0163] The hot workability was evaluated in the following manner.
In order to simulate the production conditions in a continuous
casting plant, a test specimen, 10 mm in diameter and 130 mm in
height for high-temperature tensile test was taken from each 150-kg
steel ingot. The steel ingot was produced in the same manner as
mentioned above. The test specimen was taken in the direction of
the steel ingot height so that the specimen center might be close
to the surface of the steel ingot, namely at a site of Di/8 (Di:
the diameter of the steel ingot). The specimen was heated to
1,250.degree. C. for 5 minutes by a direct charge of an electric
current at a fixation distance of 110 mm, and cooled to
1,100.degree. C. at a cooling rate of 10.degree. C./sec. After 10
seconds of maintaining the temperature at 1,100.degree. C., the
tensile test was carried out at a strain rate of 10.sup.-3/sec. In
the tensile test, the reduction of area at the site of breakage was
determined and the hot workability was evaluated based thereon.
[0164] 5. Carburizing Test
[0165] The carburizing test was carried out in the following
manner. A cylindrical steel material, 24 mm in diameter and 50 mm
in length, was used as the test specimen. This was taken from each
of the above-mentioned normalized materials, 65 mm in diameter, at
a site of R/2 (R: the radius of the normalized material). This test
specimen was heated to 900.degree. C. for the carburizing treatment
and then to 850.degree. C. for the diffusion treatment. In the
above-mentioned carburizing step, the carbon potential (C.P.) value
was 0.8% and the treatment time was 75 minutes. The C.P. value
during diffusion treatment was 0.7% and the treatment time was 20
minutes. The test specimen after carburizing treatment was cooled
in an oil bath at 80.degree. C. for quenching treatment. Finally,
the test specimen was heated to 190.degree. C. and maintained at
that temperature for 60 minutes for the tempering treatment. The
method of evaluation for carburizing characteristics was as
follows.
[0166] The test specimen, after carburizing quenching and
tempering, was measured for Vickers hardness distribution from the
surface to the inside in the cross section at a site of 25 mm
distant from the end of the test specimen (namely the center in the
longitudinal direction). The effective case depth after carburizing
corresponding to Hv 400 was determined, and a judgment was made as
to whether the value was greater or smaller than the value obtained
with a conventional leaded composite free cutting steel. The
conventional leaded composite free cutting steel was the steel No.
25 shown in Table 2, and the effective case depth after carburizing
thereof was 0.25 mm.
[0167] In the evaluation for carburizing characteristics, the steel
showing an effective case depth after carburizing.+-.0.05 mm
relative to the steel No. 35, namely 0.20-0.30 mm, was evaluated as
equivalent, the steel showing a value smaller than 0.20 mm as
inferior, and the steel showing a value greater than 0.30 mm as
superior. The results obtained are shown in Table 3 and Table 4 in
terms of .largecircle., .times. or {circle over (.smallcircle.)};
the case of equivalency is represented by ".largecircle.", the case
of inferiority by ".times.", and the case of superiority by
"{circle over (.smallcircle.)}".
[0168] The results obtained in the above tests are summarized in
Table 3 and Table 4. Further, the relationship between (A+B)/C in
relation (a) and finished surface roughness is shown in FIG. 2, the
relationship between finished surface roughness and tool life in
FIG. 3, and the relationship between chip disposability and tool
life in FIG. 4.
3TABLE 3 Mean Time for Finished Reduction Tool Wear Arriving VB
Chip Surface Evaluation for Steel of Area after 30 min. of 100
.mu.m Disposability Roughness Carburizing No. (%) (.mu.m) (min.)
(number/g) (.mu.m) Characteristics 1 61.8 45 90 18 0.7
.largecircle. 2 64.3 39 98 14 0.6 .largecircle. 3 64.5 48 85 12 0.2
.largecircle. 4 80.2 32 125 18 0.5 .largecircle. 5 76.8 43 96 21
0.3 .largecircle. 6 63.6 44 91 14 0.7 .largecircle. 7 67.2 38 96 16
0.7 .largecircle. 8 65.2 42 98 17 0.3 .largecircle. 9 81.4 36 121
20 0.4 .largecircle. 10 76.8 45 93 19 0.4 .largecircle. 11 73.4 40
103 16 0.4 .largecircle. 12 71.8 42 100 14 0.5 .circleincircle. 13
64.7 45 93 18 0.2 .circleincircle. 14 82.9 46 90 18 0.2
.circleincircle. 15 65.2 42 98 17 0.4 .circleincircle. 16 72.8 44
95 18 0.4 .circleincircle. 17 80.1 42 95 12 0.4 .circleincircle. 18
71.0 38 110 18 0.3 .largecircle. 19 69.5 40 98 19 0.3 .largecircle.
20 62.9 45 92 24 0.3 .largecircle. 21 64.8 46 93 13 0.2
.largecircle. 22 60.8 29 120 19 0.5 .largecircle. 23 62.8 43 95 22
0.6 .largecircle. 24 81.2 38 105 16 0.5 .largecircle. 25 75.9 41
101 20 0.4 .largecircle. 26 78.7 43 97 16 0.3 .largecircle. 27 75.4
40 105 22 0.3 .largecircle. 28 80.5 42 93 14 0.8 .largecircle. 29
78.8 36 115 18 0.3 .largecircle. 30 64.2 37 105 24 0.4
.largecircle. 31 60.9 46 91 20 0.3 .largecircle. 32 61.3 44 90 18
0.3 .largecircle. 33 64.3 24 126 19 0.5 .largecircle. 34 67.2 41 99
17 0.3 .largecircle.
[0169]
4TABLE 4 Mean Time for Finished Reduction Tool Wear Arriving VB
Chip Surface Evaluation for Steel of Area after 30 min. of 100
.mu.m Disposability Roughness Carburizing No. (%) (.mu.m) (min.)
(number/g) (.mu.m) Characteristics 35 49.8 98 36 9 0.4
.largecircle. 36 49.6 99 30 8 0.7 .largecircle. 37 55.4 165 17 6
0.7 .largecircle. 38 74.3 35 113 15 1.3 X 39 73.0 40 98 17 1.4 X 40
68.2 45 92 18 1.5 X 41 64.2 71 68 16 1.7 X 42 67.5 89 38 15 0.5
.largecircle. 43 79.0 85 42 13 0.6 .circleincircle. 44 52.8 93 36
12 1.3 .circleincircle. 45 66.9 88 39 15 1.0 .circleincircle. 46
57.5 103 29 8 1.6 X 47 78.7 101 30 16 0.3 .largecircle. 48 56.0 232
12 12 1.8 .circleincircle. 49 59.8 206 16 14 1.9 .circleincircle.
50 65.3 129 20 9 1.3 .largecircle. 51 6.4 -- -- -- -- -- 52 4.3 --
-- -- -- -- 53 3.4 -- -- -- -- -- 54 59.5 198 17 15 1.8
.largecircle. 55 58.5 264 10 10 1.9 .circleincircle.
[0170] In Table 2, the steels Nos. 35 and 36 are composite free
cutting steels, and the steel No. 37 is a resulfurized free cutting
steel. At this time, these steels are regarded as highest in
machinability. As is evident from Table 3, Table 4, FIG. 2 and FIG.
3, the steels of the present invention are superior in tool life
and finished surface roughness level. Furthermore, the steels Nos.
1-34 according to the present invention have good hot workability,
and are at least comparable to the composite free cutting steels
and resulfurized steels, as shown in Table 3 in terms of the
reduction of area in the high-temperature tensile test simulating
the practical production in a continuous casting plant or the like,
and thus are free of problems from the practical viewpoint.
[0171] The steels Nos. 12-17 according to the present invention
shown in Table 1, contains at least one including Si and Cr within
the specified content range in order to improve the carburizing
characteristics. It is evident that these steels show good
carburizing characteristics, in particular, among the steels of the
present invention.
[0172] On the other hand, as the steels 35-55 in Table 2, it is
evident that those steels which fail to satisfy one or more
requirements concerning the morphology of inclusions and chemical
composition, as specified herein in accordance with the present
invention, are inferior at least one of the following: tool life,
finished surface roughness level, chip disposability and hot
workability to the steels of the present invention.
[0173] Although only some exemplary embodiments of the present
invention have been described in detail above, those skilled in the
art will readily appreciated that many modifications are possible
in the exemplary embodiments without materially departing from the
novel teachings and advantages of the present invention.
Accordingly, all such modifications are intended to be included
within the scope of the present invention.
Industrial Applicability
[0174] In spite of its Non-leadea composition, the free cutting
steel of the present invention is comparable or superior in
machinability to the conventional leaded free cutting steels and
composite free cutting steels and, further, are excellent from the
finished surface characteristics viewpoint. When the free cutting
steel of the present invention contains Si and/or Cr, it shows good
carburizing characteristics. Furthermore, this steel is excellent
in hot workability and can be produced at a low cost by continuous
casting. It produces no environmental problems since it does not
contain Pb. Therefore, it is very well suited for use as a raw
material of various machine parts.
* * * * *