U.S. patent application number 12/657473 was filed with the patent office on 2010-05-20 for machine structural steel excellent in machinability and strength properties.
This patent application is currently assigned to Nippon Steel Corporation. Invention is credited to Masayuki Hashimura, Kenichiro Miyamoto, Kei Miyanishi, Atsushi Mizuno.
Application Number | 20100124515 12/657473 |
Document ID | / |
Family ID | 39608640 |
Filed Date | 2010-05-20 |
United States Patent
Application |
20100124515 |
Kind Code |
A1 |
Miyanishi; Kei ; et
al. |
May 20, 2010 |
Machine structural steel excellent in machinability and strength
properties
Abstract
The invention provides a machine structural steel excellent in
machinability and strength properties that has good machinability
over a broad range of machining speeds and also has high impact
properties and high yield ratio, which machine structural steel
comprises, in mass %, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05
to 2.0%, P: 0.005 to 0.2%, S: 0.001 to 0.15%, total Al: greater
than 0.05% and not greater than 0.3%, Sb: less than 0.0150%
(including 0%), and total N: 0.0035 to 0.020%, solute N being
limited to 0.0020% or less, and a balance of Fe and unavoidable
impurities.
Inventors: |
Miyanishi; Kei; (Tokyo,
JP) ; Hashimura; Masayuki; (Tokyo, JP) ;
Mizuno; Atsushi; (Tokyo, JP) ; Miyamoto;
Kenichiro; (Tokyo, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Assignee: |
Nippon Steel Corporation
Tokyo
JP
|
Family ID: |
39608640 |
Appl. No.: |
12/657473 |
Filed: |
January 20, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12225897 |
Sep 30, 2008 |
|
|
|
PCT/JP2007/075350 |
Dec 25, 2007 |
|
|
|
12657473 |
|
|
|
|
Current U.S.
Class: |
420/84 ; 420/104;
420/121; 420/87; 420/89; 420/90 |
Current CPC
Class: |
C21D 1/28 20130101; C21D
2211/004 20130101; C21D 2211/005 20130101; C21D 8/005 20130101;
C21D 7/13 20130101; C21D 6/02 20130101 |
Class at
Publication: |
420/84 ; 420/87;
420/89; 420/90; 420/104; 420/121 |
International
Class: |
C22C 38/60 20060101
C22C038/60; C22C 38/16 20060101 C22C038/16; C22C 38/20 20060101
C22C038/20; C22C 38/18 20060101 C22C038/18; C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 25, 2006 |
JP |
2006-347928(PAT. |
Claims
1-2. (canceled)
3. A machine structural steel excellent in machinability and
strength properties comprising, in mass %: C: 0.1 to 0.85%, Si:
0.01 to 0.26%, Mn: 0.05 to 2.0%, P: 0.005 to 0.2%, S: 0.001 to
0.15%, total Al: 0.080% to 0.3%, total N: 0.0035 to 0.020%, solute
N: 0.0009 to 0.0020%, and a balance of Fe and unavoidable
impurities.
4. A machine structural steel excellent in machinability and
strength properties according to claim 1, further comprising, in
mass %, one or more of Ca: 0.0003 to 0.0015%, Ti: 0.001 to 0.1%,
Nb: 0.005 to 0.2%, W: 0.01 to 1.0%, V: 0.01 to 1.0%, Mg: 0.0001 to
0.0040%, Zr: 0.0003 to 0.01%, REMs: 0.0001 to 0.015%, Sn: 0.005 to
2.0%, Zn: 0.0005 to 0.5%, B: 0.0005 to 0.015%, Te: 0.0003 to 0.2%,
Bi: 0.005 to 0.5%, Pb: 0.005 to 0.5%, Cr: 0.01 to 2.0%, Mo: 0.01 to
1.0%, Ni: 0.05 to 2.0% and Cu: 0.01 to 2.0%.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a machine structural steel that is
to be machined and particularly to a machine structural steel
excellent in machinability and strength properties that is amenable
to machining over a broad spectrum of machining speeds ranging from
relatively low-speed machining with a high-speed steel drill to
relatively high speed machining such as longitudinal turning with a
super-steel coated tool.
DESCRIPTION OF THE RELATED ART
[0002] Although recent years have seen the development of steels of
higher strength, there has concurrently emerged a problem of
declining machinability. An increasing need is therefore felt for
the development of steels that maintain excellent strength without
experiencing a decline in machining performance. Addition of
machinability-enhancing elements such as S, Pb and Bi is known to
be effective for improving steel machinability. However, while Pb
and Bi are known to improve machinability and to have relatively
little effect on forgeability, they are also known to degrade
strength properties.
[0003] Moreover, Pb is being used in smaller quantities these days
owing to the tendency to avoid use because of concern about the
load Pb puts on the natural environment. S improves machinability
by forming inclusions, such as MnS, that soften in a machining
environment, but MnS grains are larger than the those of Pb and the
like, so that it readily becomes a stress concentration raiser. Of
particular note is that at the time of elongation by forging or
rolling, MnS produces anisotropy, which makes the steel extremely
weak in a particular direction. It also becomes necessary to take
such anisotropy into account during steel design. When S is added,
therefore, it becomes necessary to utilize a technique for reducing
the anisotropy.
[0004] As pointed out in the foregoing, it has been difficult to
achieve good strength properties and good machinability
simultaneously because addition of machinability-enhancing elements
degrades the strength properties. Further technological innovation
is therefore needed to enable simultaneous realization of
satisfactory steel machinability and strength properties.
[0005] This situation has led to efforts to provide a machine
structural steel enabling prolongation of machine tool life by, for
example, incorporating a total of 0.005 mass % or greater of at
least one member selected from among solute V, solute Nb and solute
Al, and further incorporating 0.001% or greater of solute N,
thereby enabling nitrides formed by machining heat during machining
to adhere to the tool to function as a tool protective coating (see
Japanese Patent Publication (A) No. 2004-107787). In addition,
there has been proposed a machine structural steel that achieve
improved shavings disposal and mechanical properties by defining C,
Si, Mn, S and Mg contents, defining the ratio of Mg content to S
content, and optimizing the aspect ratio and number of sulfide
inclusions in the steel (see Japanese Patent No. 3706560). The
machine structural steel taught by Patent No. 3706560 defines the
content of Mg as 0.02% or less (not including 0%) and the content
of Al, when included, as 0.1% or less.
SUMMARY OF THE INVENTION
[0006] However, the foregoing existing technologies have the
following drawbacks. The steel taught by Japanese Patent
Publication (A) No. 2004-107787 is liable not to give rise to the
aforesaid phenomenon unless the amount of heat produced by the
machining exceeds a certain level. The machining speed must
therefore be somewhat high to realize the desired effect, so the
invention has a problem in the point that the effect cannot be
anticipated in the low speed range. Japanese Patent No. 3706560 is
totally silent regarding the strength properties of the steel it
teaches. Moreover, the steel of this patent is incapable of
achieving adequate strength properties because it gives no
consideration to machine tool life or yield ratio.
[0007] The present invention was achieved in light of the foregoing
problems and has as its object to provide a machine structural
steel that has good machinability over a broad range of machining
speeds and also has high impact properties and high yield
ratio.
[0008] The machine structural steel excellent in machinability and
strength properties according to the present invention comprises,
in mass %, C: 0.1 to 0.85%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.0%, P:
0.005 to 0.2%, S: 0.001 to 0.15%, total Al: greater than 0.05% and
not greater than 0.3%, Sb: less than 0.0150% (including 0%), and
total N: 0.0035 to 0.020%, solute N being limited to 0.0020% or
less, and a balance of Fe and unavoidable impurities.
[0009] The machine structural steel can further comprise, in mass
%, Ca: 0.0003 to 0.0015%.
[0010] The machine structural steel can further comprise, in mass
%, one or more elements selected from the group consisting of Ti:
0.001 to 0.1%, Nb: 0.005 to 0.2%, W: 0.01 to 1.0%, and V: 0.01 to
1.0%.
[0011] The machine structural steel can further comprise, in mass
%, one or more elements selected from the group consisting of Mg:
0.0001 to 0.0040%, Zr: 0.0003 to 0.01%, and REMs: 0.0001 to
0.015%.
[0012] The machine structural steel can further comprise, in mass
%, one or more elements selected from the group consisting of Sn:
0.005 to 2.0%, Zn: 0.0005 to 0.5%, B: 0.0005 to 0.015%, Te: 0.0003
to 0.2%, Bi: 0.005 to 0.5%, and Pb: 0.005 to 0.5%.
[0013] The machine structural steel can further comprise, in mass
%, one or two elements selected from the group consisting of Cr:
0.01 to 2.0% and Mo: 0.01 to 1.0%.
[0014] The machine structural steel can further comprise, in mass
%, one or two elements selected from the group consisting of Ni:
0.05 to 2.0% and Cu: 0.01 to 2.0%.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a diagram showing a region from which a Charpy
impact test piece was cut.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Preferred embodiments of the present invention are explained
in detail in the following. The machine structural steel excellent
in machinability and strength properties according to the present
invention achieves the foregoing object by providing a machine
structural steel wherein solute N acting to degrade machinability
and impact properties is minimized by adjusting the amounts of
added N and nitride-forming elements such as Al, wherein effective
cutting performance is established with respect to a broad cutting
speed range extending from low to high speed by ensuring presence
of suitable amounts of solute Al serving to improve
high-temperature embrittlement property and machinability, and Sb
serving to produce a matrix embrittlement effect, and forming a
crystal structure exhibiting high-temperature embrittlement effect
and cleavage, thereby ensuring an appropriate amount of AlN serving
to improve machinability, and wherein high impact properties are
also realized by increasing Al addition so that at the slab stage
segregation is made smaller and MnS of highly uniform
dispersibility (type III MnS by SIMS analysis) is made more
abundant than in conventional Al-killed steel. Moreover, the steel
further achieves a high yield ratio owing to fine precipitation of
AlN and presence of solute Al.
[0017] Specifically, the machine structural steel according to the
present invention comprises, in mass %, C: 0.1 to 0.85%, Si: 0.01
to 1.5%, Mn: 0.05 to 2.0%, P: 0.005 to 0.2%, S: 0.001 to 0.15%,
total Al: greater than 0.05% and not greater than 0.3%, Sb: less
than 0.0150% (including 0%), and total N: 0.0035 to 0.020%, solute
N being limited to 0.0020% or less, and a balance of Fe and
unavoidable impurities.
[0018] The individual elements constituting the machine structural
steel of the present invention and the contents thereof of will
first be explained. In the ensuing explanation, percentage
composition by mass of the steel components is denoted simply by
the symbol %.
C: 0.1 to 0.85%
[0019] C has a major effect on the fundamental strength of the
steel. When the C content is less than 0.1%, adequate strength
cannot be achieved, so that large amounts of other alloying
elements must be incorporated. When C content exceeds 0.85%,
machinability declines markedly because carbon concentration
becomes nearly hypereutectoid to produce heavy precipitation of
hard carbides. In order to achieve sufficient strength, the present
invention therefore defines C content as 0.1 to 0.85%. The
preferred lower limit of C content is 0.2%.
Si: 0.01 to 1.5%
[0020] Si is generally added as a deoxidizing element but also
contributes to ferrite strengthening and temper-softening
resistance. When Si content is less than 0.01%, the deoxidizing
effect is insufficient. On the other hand, an Si content in excess
of 1.5% degrades the steel's embrittlement and other properties and
also impairs machinability. Si content is therefore defined as 0.01
to 1.5%. The preferred upper limit of Si content is 1.0%.
Mn: 0.05 to 2.0%
[0021] Mn is required for its ability to fix and disperse sulfur
(S) in the steel in the form of MnS and also, by dissolving into
the matrix, to improve hardenability and ensure good strength after
quenching. When Mn content is less than 0.05%, the steel is
embrittled because S therein combines with Fe to form FeS. When Mn
content is high, specifically when it exceeds 2.0%, base metal
hardness increases to degrade cold workability, while its strength
and hardenability improving effects saturates. Mn content is
therefore defined as 0.05 to 2.0%.
P: 0.005 to 0.2%
[0022] P has a favorable effect on machinability but the effect is
not obtained at a P content of less than 0.005%. When P content is
high, specifically when it exceeds 0.2%, base metal hardness
increases to degrade not only cold workability but also hot
workability and casting properties. P content is therefore defined
as 0.005 to 0.2%,
S: 0.001 to 0.15%
[0023] S combines with Mn to produce MnS that is present in the
steel in the form of inclusions. MnS improves machinability but S
must be added to a content of 0.001% or greater for achieving this
effect to a substantial degree. When S content exceeds 0.15%, the
impact value of the steel declines markedly. In the case of adding
S to improve machinability, therefore, the S content is made 0.001
to 0.15%.
Total Al: greater than 0.05% and not greater than 0.3%
[0024] Al not only forms oxides but also promotes precipitation of
AlN, which contributes to grain size control and machinability, and
further improves machinability by passing into solid solution. Al
must be added to a content of greater than 0.05% in order to form
solute Al in an amount sufficient to enhance machinability. Al also
affects the form of MnS grains/precipitation. Moreover, when Al is
added in an amount exceeding 0.05%, segregation at the slab stage
can be made smaller and MnS of highly uniform dispersibility (type
III MnS by SIMS analysis) be made more abundant than in a
conventional Al-killed steel. This makes it possible to obtain a
machine structural steel also having high impact properties and
further to achieve a high yield ratio owing to fine precipitation
of AlN and the presence of solute Al. However, machinability starts
to decline when total Al content exceeds 0.3%. Total Al content is
therefore defined as greater than 0.05% and not greater than 0.3%.
The lower limit of total Al content is preferably 0.08% and more
preferably 0.1%.
Sb: less than 0.0150% (including 0%)
[0025] Sb improves machinability by suitably embrittling ferrite.
This effect of Sb is pronounced particularly when solute Al content
is high but is not observed when Sb content is less than 0.0005%.
When Sb content is high, specifically when it reaches 0.0150% or
greater, Sb macro-segregation becomes excessive, so that the impact
value of the steel declines markedly. Sb content is therefore
defined as 0.0005% or greater and less than 0.0150%. When high
machinability is not required or total Al is greater than 0.1%,
addition of Sb can be omitted (Sb content of 0%).
Total N: 0.0035 to 0.020%
[0026] N, which is present not only as solute N but also in
nitrides of Ti, Al V and the like, suppresses austenite grain
growth. However, no substantial effect is obtained at a total N
content of less than 0.0035%. When total N content exceeds 0.020%,
it leads to the occurrence of roll marks during rolling. Total N
content is therefore defined as 0.0035 to 0.020%.
Solute N: 0.0020% or less
[0027] Solute N hardens the steel. Of particular concern is that it
shortens cutting tool life by causing steel near the cutting edge
to harden under dynamic strain aging. It also causes occurrence of
roll marks during rolling. High solute N content, specifically a
content in excess of 0.0020%, aggravates tool wear during cutting
because cutting resistance rises due to increased local hardness.
Solute N content is therefore held to 0.0020% or less. This helps
to reduce tool wear. Moreover, high solute N content also degrades
impact properties by causing matrix embrittlement, but such matrix
embrittlement can also be mitigated by holding solute N content to
0.0020% or less. Solute N content as termed here means the value
obtained by subtracting the N content of AlN, NbN, TiN, VN and
other such nitrides from total N content. It can be calculated, for
example, in accordance with Equation (1) shown below, using the
total N content determined by the inert gas fusion thermal
conductivity method and the N content of nitrides determined by
SPEED (Selective Potentiostatic Etching by Electrolytic
Dissolution) analysis and indophenol absorbency analysis of residue
electrolytically extracted using a 0.1 .mu.m filter.
(Solute N content)=(Total N content)-(Nitride N content) (1)
[0028] Solute N content can be lowered by the methods explained
below: [0029] 1) Hold total N content to a low level within the
range defined by the present invention. Although total N is defined
as 0.020% or less, it is preferably held to 0.01% or less and more
preferably to 0.006% or less. [0030] 2) When total N content is
high, it is helpful to increase the amount of N compounds by adding
suitable amounts of Al, a nitride-forming element, as well as other
nitride-forming elements. [0031] 3) Solute N reduction by fine
precipitation of nitrides is preferable in a machine structural
steel from the viewpoint of inhibiting grain coarsening. Taking
into account that reduction of solute N content by fine
precipitation of nitrides requires holding at a high temperature
enabling more complete solution treatment into N and
nitride-forming element content, solution heat treatment is
conducted at a temperature of 1100.degree. C. or greater,
preferably 1200.degree. C. or greater, and more preferably
1250.degree. C. or greater, whereafter precipitation is performed
by conducting a heat treatment such as normalizing or
carburization. Of particular note is that in the case of AlN,
solute N can be reduced by utilizing prolonged retention near
850.degree. C. to increase precipitation. By "prolonged" here is
meant 0.8 hr or greater, preferably 1 hr or greater and more
preferably 1.2 hr or greater.
[0032] The machine structural steel of the present invention can
contain Ca in addition to the foregoing components.
Ca: 0.0003 to 0.0015%
[0033] Ca is a deoxidizing element that forms oxides in the steel.
In the machine structural steel of the present invention, which has
a total Al content of greater than 0.05% and not greater than 0.3%,
Ca forms calcium aluminate (CaOAl.sub.2O.sub.3). As
CaOAl.sub.2O.sub.3 is an oxide having a lower melting point than
Al.sub.2O.sub.3, it improves machinability by constituting a tool
protective film during high-speed cutting However, this
machinability-improving effect is not observed when the Ca content
is less than 0.0003%. When Ca content exceeds 0.0015%, CaS forms in
the steel, so that machinability is instead degraded. Therefore,
when Ca is added, its content is defined as 0.0003 to 0.0015%.
[0034] When the machine structural steel of the present invention
needs to be given high strength by forming carbides, it can include
in addition to the foregoing components one or more elements
selected from the group consisting of Ti: 0.001 to 0.1%, Nb: 0.005
to 0.2%, W: 0.01 to 1.0%, and V: 0.01 to 1.0%.
Ti: 0.001 to 0.1%
[0035] Ti forms carbonitrides that inhibit austenite grain growth
and contribute to strengthening. It is used as a grain size control
element for preventing grain coarsening in steels requiring high
strength and steels requiring low distortion. Ti is also a
deoxidizing element that improves machinability by forming soft
oxides. However, these effects of Ti are not observed at a content
of less than 0.001%, and when the content exceeds 0.1%, Ti has the
contrary effect of degrading mechanical properties by causing
precipitation of insoluble coarse carbonitrides that cause hot
cracking. Therefore, when Ti is added, its content is defined as
0.001 to 0.1%.
Nb: 0.005 to 0.2%
[0036] Nb also forms carbonitrides. As such, it is an element that
contributes to steel strength through secondary precipitation
hardening and to austenite grain growth inhibition and
strengthening. Ti is therefore used as a grain size control element
for preventing grain coarsening in steels requiring high strength
and steels requiring low distortion. However, no high strength
imparting effect is observed at an Nb content of less than 0.005%,
and when Nb is added to a content exceeding 0.2%, it has the
contrary, effect of degrading mechanical properties by causing
precipitation of insoluble coarse carbonitrides that cause hot
cracking. Therefore, when Nb is added, its content is defined as
0.005 to 0.2%.
W: 0.01 to 1.0%
[0037] W is also an element that forms carbonitrides and can
strengthen the steel through secondary precipitation hardening.
However, no high strength imparting effect is observed when W
content is less than 0.01%, Addition of W in excess of 1.0% has the
contrary effect of degrading mechanical properties by causing
precipitation of insoluble coarse carbonitrides that cause hot
cracking. Therefore, when W is added, its content is defined as
0.01 to 1.0%.
V: 0.01 to 1.0%.
[0038] V is also an element that forms carbonitrides and can
strengthen the steel through secondary precipitation hardening. It
is suitably added to steels requiring high strength. However, no
high strength imparting effect is observed when V content is less
than 0.01%, Addition of V in excess of 1.0% has the contrary effect
of degrading mechanical properties by causing precipitation of
insoluble coarse carbonitrides that cause hot cracking. Therefore,
when V is added, its content is defined as 0.01 to 1.0%.
[0039] When the machine structural steel of the present invention
is subjected to deoxidization control for controlling sulfide
morphology, it can comprise in addition to the foregoing components
one or more elements selected from the group consisting of Mg:
0.0001 to 0.0040%, Zr: 0.0003 to 0.01%, and REMs: 0.0001 to
0.015%.
Mg: 0.0001 to 0.0040%
[0040] Mg is a deoxidizing element that forms oxides in the steel.
When Al deoxidization is adopted, Mg reforms Al.sub.2O.sub.3, which
impairs machinability, into relatively soft and finely dispersed
MgO and Al.sub.2O.sub.3--Mg. Moreover, its oxide readily acts as a
precipitation nucleus of MnS and thus works to finely disperse MnS.
However, these effects are not observed at an Mg content of less
than 0.0001%. Moreover, while Mg acts to make MnS spherical by
forming a metal-sulfide complex therewith, excessive Mg addition,
specifically addition to a content of greater than 0.0040%,
degrades machinability by promoting simple MgS formation.
Therefore, when Mg is added, its content is defined as to 0.0001 to
0.0040%.
Zr: 0.0003 to 0.01%.
[0041] Zr is a deoxidizing element that forms an oxide in the
steel. The oxide is thought to be ZrO.sub.2, which acts as a
precipitation nucleus for MnS. Since addition of Zr therefore
increases the number of MnS precipitation sites, it has the effect
of uniformly dispersing MnS. Moreover, Zr dissolves into MnS to
form a metal-sulfide complex therewith, thus decreasing MnS
deformation, and therefore also works to inhibit MnS grain
elongation during rolling and hot forging. In this manner, Zr
effectively reduces anisotropy. But no substantial effect in these
respects is observed at a Zr content of less than 0.0003%. On the
other hand, addition of Zr in excess of 0.01% radically degrades
yield. Moreover, by causing formation of large quantities of
ZrO.sub.2, ZrS and other hard compounds, it has the contrary effect
of degrading mechanical properties such as machinability, impact
value, fatigue properties and the like. Therefore, when Zr is
added, its content is defined as to 0.0003 to 0.01%.
REMs: 0.0001 to 0.015%
[0042] REMs (rare earth metals) are deoxidizing elements that form
low-melting-point oxides that help to prevent nozzle clogging
during casting and also dissolve into or combine with MnS to
decrease MnS deformation, thereby acting to inhibit MnS shape
elongation during rolling and hot forging. REMs thus serve to
reduce anisotropy. However, this effect does not appear at an REM
content of less than 0.0001%. When the content exceeds 0.015%,
machinability is degraded owing to the formation of large amounts
of REM sulfides. Therefore, when REMs are added, their content is
defined as 0.0001 to 0.015%.
[0043] When the machine structural steel of the present invention
is to be improved in machinability, it can include in addition to
the foregoing components one or more elements selected from the
group consisting of Sn: 0.005 to 2.0%, Zn: 0.0005 to 0.5%, B:
0.0005 to 0.015%, Te: 0.0003 to 0.2%, Bi: 0.005 to 0.5%, and Pb:
0.005 to 0 5%.
Sn: 0.005 to 2.0%
[0044] Sn extends tool life by embrittling ferrite and also
improves surface roughness. These effects are not observed when the
Sn content is less than 0.005%, and the effects saturate when Sn is
added in excess of 2.0%. Therefore, when Sn is added, its content
is defined as 0.005 to 2.0%.
Zn: 0.0005 to 0.5%
[0045] Zn extends tool life by embrittling ferrite and also
improves surface roughness. These effects are not observed when the
Zn content is less than 0.0005%, and the effects saturate when Zn
is added in excess of 0.5%. Therefore, when Zn is added, its
content is defined as 0.0005 to 0.5%.
B: 0.0005 to 0.015%
[0046] B, when in solid solution, has a favorable effect on grain
boundary strength and hardenability. When it precipitates, it
precipitates as BN and therefore helps to improve machinability.
These effects are not notable at a B content of less than 0.0005%.
When B is added to a content of greater than 0.015%, the effects
saturate and mechanical properties are to the contrary degraded
owing to excessive precipitation of BN. Therefore, when B is added,
its content is defined as 0.0005 to 0.015%.
Te: 0.0003 to 0.2%
[0047] Te improves machinability. It also forms MnTe and, when
co-present with MnS, decreases MnS deformation, thereby acting to
inhibit MnS shape elongation. Te is thus an element effective for
reducing anisotropy. These effects are-not observed when Te content
is less than 0.0003%, and when the content thereof exceeds 0.2%,
the effects saturate and hot rolling ductility declines, increasing
the likelihood of flaws. Therefore, when Te is added, its content
is defined as: 0.0003 to 0.2%.
Bi: 0.005 to 0.5%
[0048] Bi improves machinability. This effect is not observed when
Bi content is less than 0.005%. When it exceeds 0.5%, machinability
improvement saturates and hot rolling ductility declines,
increasing the likelihood of flaws. Therefore, when Bi is added,
its content is defined as 0.005 to 0.5%.
Pb: 0.005 to 0.5%
[0049] Pb improves machinability. This effect is not observed when
Pb content is less than 0.005%. When it exceeds 0.5%, machinability
improvement saturates and hot rolling ductility declines,
increasing the likelihood of flaws. Therefore, when Pb is added,
its content is defined as 0.005 to 0.5%.
[0050] When the machine structural steel of the present invention
is to be imparted with strength by improving its hardenability
and/or temper-softening resistance, it can include in addition to
the foregoing components one or two elements selected from the
group consisting of Cr: 0.01 to 2.0% and Mo: 0.01 to 1.0%.
Cr: 0.01 to 2.0%
[0051] Cr improves hardenability and also imparts temper-softening
resistance. It is therefore added to a steel requiring high
strength. These effects are not obtained at a Cr content of less
than 0.01%. When Cr content is high, specifically when it exceeds
2.0%, the steel is embrittled owing to formation of Cr carbides.
Therefore, when Cr is added, its content is defined as 0.01 to
2.0%.
Mo: 0.01 to 1.0%
[0052] Mo imparts temper-softening resistance and also improves
hardenability. It is therefore added to a steel requiring high
strength. These effects are not obtained at an Mo content of less
than 0.01%. When Mo is added in excess of 1.0%, its effects
saturate. Therefore, when Mo is added, its content is defined as
0.01 to 1.0%.
[0053] When the machine structural steel of the present invention
is to be subjected to ferrite strengthening, it can include in
addition to the foregoing components one or two elements selected
from the group consisting of Ni: 0.05 to 2.0% and Cu: 0.01 to
2.0%.
Ni: 0.05 to 2.0%
[0054] Ni strengthens ferrite, thereby improving ductility, and is
also effective for hardenability improvement and anticorrosion
improvement. These effects are not observed an Ni content of less
than 0.05%. When Ni is added in excess of 2.0%, mechanical property
improving effect saturates and machinability is degraded.
Therefore, when Ni is added, its content is defined as 0.05 to
2.0%.
Cu: 0.01 to 2.0%
[0055] Cu strengthens ferrite and is also effective for
hardenability improvement and anticorrosion improvement. These
effects are not observed a Cui content of less than 0.01%. When Cu
is added in excess of 2.0%, mechanical property improving effect
saturates. Therefore, when Cu is added, its content is defined as
0.01 to 2.0%. A particular concern regarding Cu is that its effect
of lowering hot rollability may lead to occurrence of flaws during
rolling. Cu is therefore preferably added simultaneously with
Ni.
[0056] As explained in the foregoing, the machine structural steel
of the present invention is minimized in solute N content and
therefore achieves better machinability and impact properties than
conventional machine structural steels. Moreover, total Al content
and Sb content are controlled to suitable levels to ensure presence
of proper amounts of solute Al, Sb and AlN serving to improve
machinability, thereby establishing effective cutting performance
with respect to a broad cutting speed range extending from low to
high speed. The steel also achieves a high yield ratio owing to
fine precipitation of the AlN and presence of solute Al. In
addition, excellent impact properties are realized by appropriately
regulating the contents of elements affecting MnS precipitation so
as to obtain an abundance of MnS of highly uniform
dispersibility.
[0057] The machine structural steel excellent in machinability and
strength properties according to the present invention can be
produced by hot-forging a billet having the aforesaid steel
composition into a bar at a temperature of 1200.degree. C. or
greater, subjecting the bar to solution heat treatment at a
temperature of 1100.degree. C. or greater, and then to a heat
treatment such as normalizing or carburization. Of particular note
is that in the case of a steel containing the carbide AlN, a
machine structural steel markedly reduced in solute N can be
obtained by prolonged retention following the solution heat
treatment at 1100.degree. C. or greater for 0.8 hr or greater,
preferably 1 hr or greater, and more preferably 1.2 hr or
greater.
EXAMPLES
First Set of Examples
[0058] The effects of the present invention will now be
specifically explained giving Examples and Comparative Examples. In
this set of Examples, steels of the compositions shown in Table 1
and Table 2, 150 kg each, were produced in a vacuum furnace,
hot-forged under a temperature condition of 1250.degree. C., and
elongation-forged into 65-mm diameter bars. The properties of the
Example and Comparative Example steels were evaluated by subjecting
them to machinability testing, Charpy impact testing and tensile
testing by the methods set out below. In Table 2, underlining
indicates a value outside the invention range.
TABLE-US-00001 TABLE 1 Composition (mass %) No. C Si Mn P S Cr V Sb
Ca Ttl Al Ttl N Sol N Other Invention 1 0.42 0.19 0.80 0.014 0.022
-- -- 0.0100 0.0012 0.110 0.0052 0.0012 Examples 2 0.40 0.25 0.76
0.012 0.034 -- -- 0.0089 0.0008 0.051 0.0060 0.0013 3 0.41 0.24
0.76 0.013 0.038 -- 0.1 0.0086 -- 0.051 0.0060 0.0013 4 0.40 0.23
0.78 0.015 0.038 -- -- 0.0067 -- 0.052 0.0045 0.0014 5 0.43 0.23
0.75 0.011 0.022 -- -- 0.0087 -- 0.060 0.0049 0.0012 Mg: 0.0020 6
0.43 0.20 0.77 0.013 0.039 -- -- 0.0074 -- 0.051 0.0065 0.0013 Ti:
0.04 7 0.44 0.20 0.78 0.012 0.040 -- -- 0.0068 -- 0.052 0.0075
0.0016 Nb: 0.02 8 0.41 0.21 0.77 0.011 0.047 -- -- 0.0083 -- 0.090
0.0058 0.0014 W: 0.2 9 0.45 0.22 0.79 0.012 0.045 -- -- 0.0058 --
0.080 0.0055 0.0013 Ni: 0.2 10 0.43 0.23 0.71 0.011 0.051 -- --
0.0071 -- 0.110 0.0045 0.0017 Cu: 0.5 11 0.44 0.22 0.72 0.014 0.041
-- -- 0.0087 -- 0.053 0.0052 0.0010 Sn: 0.05 12 0.45 0.20 0.74
0.010 0.033 -- -- 0.0069 -- 0.070 0.0051 0.0014 Zn: 0.007 13 0.43
0.24 0.76 0.015 0.041 -- -- 0.0077 -- 0.090 0.0053 0.0019 B: 0.002
14 0.45 0.22 0.71 0.011 0.043 -- -- 0.0073 -- 0.080 0.0046 0.0015
Te: 0.002 15 0.43 0.19 0.74 0.011 0.051 1.0 -- 0.0051 -- 0.090
0.0047 0.0016 16 0.44 0.21 0.72 0.013 0.023 0.1 -- 0.0085 -- 0.070
0.0048 0.0013 17 0.42 0.21 0.73 0.012 0.048 -- -- 0.0088 -- 0.110
0.0071 0.0010 Ti: 0.03, Mg: 0.0025 18 0.41 0.20 0.72 0.012 0.035 --
-- 0.0059 -- 0.090 0.0075 0.0011 Ti: 0.04, Zn: 0.004 19 0.42 0.24
0.74 0.013 0.040 1.0 -- 0.0083 -- 0.060 0.0071 0.0012 Ti: 0.03 20
0.44 0.23 0.75 0.010 0.034 -- -- 0.0089 -- 0.110 0.0077 0.0015 Ti:
0.03, Cu: 0.3 21 0.40 0.20 0.71 0.010 0.037 -- -- 0.0074 -- 0.110
0.0054 0.0010 Ti: 0.02, Mg: 0.0025, Sn: 0.04 22 0.42 0.21 0.73
0.012 0.053 1.1 -- 0.0098 -- 0.110 0.0074 0.0019 Ti: 0.03, Mg:
0.0025 23 0.43 0.21 0.77 0.014 0.052 -- -- 0.0071 -- 0.070 0.0062
0.0019 Ti: 0.03, Mg: 0.0025, Cu: 0.4 24 0.41 0.19 0.74 0.013 0.054
1.0 -- 0.0076 -- 0.100 0.0061 0.0015 Ti: 0.02, Mg: 0.0025, Sn: 0.04
25 0.43 0.23 0.71 0.014 0.021 -- -- 0.0058 -- 0.060 0.0060 0.0012
Ti: 0.03, Mg: 0.0025, Sn: 0.04, Cu: 0.3 26 0.43 0.25 0.76 0.013
0.024 1.0 -- 0.0085 -- 0.070 0.0074 0.0010 Ti: 0.03, Mg: 0.0025,
Cu: 0.4 27 0.45 0.23 0.72 0.015 0.034 1.0 -- 0.0086 -- 0.100 0.0055
0.0012 Ti: 0.03, Sn: 0.04 28 0.41 0.19 0.78 0.011 0.025 -- --
0.0087 -- 0.080 0.0061 0.0016 Ti: 0.03, Sn: 0.04. Cu: 0.3 29 0.41
0.21 0.70 0.015 0.025 0.9 -- 0.0057 -- 0.051 0.0062 0.0017 Ti:
0.04, Sn: 0.04. Cu: 0.3 30 0.44 0.23 0.71 0.012 0.036 1.0 -- 0.0052
-- 0.060 6.0056 0.0012 Ti: 0.03, Cu: 0.3
TABLE-US-00002 TABLE 2 Composition (mass %) No. C Si Mn P S Cr V Sb
Ca Ttl Al Ttl N Sol N Other Examples 31 0.44 0.20 0.77 0.014 0.055
-- -- 0.0081 -- 0.110 0.0047 0.0010 Mg: 0.0025, Zn: 0.003 32 0.45
0.21 0.79 0.011 0.029 1.0 -- 0.0099 -- 0.060 0.0049 0.0013 Mg:
0.0019, Zn: 0.003 33 0.43 0.21 0.74 0.010 0.038 -- -- 0.0078 --
0.110 0.0048 0.0014 Mg: 0.0022, Ca: 0.3 34 0.42 0.25 0.77 0.011
0.036 1.0 -- 0.0087 -- 0.090 0.0046 0.0014 Mg: 0.0020, Sn: 0.04 35
0.44 0.25 0.78 0.015 0.055 -- -- 0.0079 -- 0.100 0.0050 0.0018 Mg:
0.0025, Sn: 0.04, Cu: 0.1 36 0.42 0.19 0.74 0.013 0.022 1.0 --
0.0062 -- 0.052 0.0047 0.0019 Mg: 0.0021, Sn: 0.02, Cu: 0.1 37 0.41
0.19 0.77 0.010 0.025 1.1 -- 0.0050 -- 0.110 0.0049 0.0019 Mg:
0.0029, Cu: 0.1 38 0.43 0.20 0.79 0.011 0.020 1.0 -- 0.0060 --
0.060 0.0049 0.0010 Sn: 0.04 39 0.42 0.23 0.80 0.015 0.048 -- --
0.0086 -- 0.070 0.0046 0.0016 Sn: 0.03, Cu: 0.1 40 0.41 0.19 0.78
0.010 0.042 1.0 -- 0.0069 -- 0.100 0.0046 0.0009 Sn: 0.04, Cu: 0.1
41 0.43 0.21 0.79 0.010 0.035 0.9 -- 0.0080 -- 0.080 0.0046 0.0014
Cu: 0.2 42 0.44 0.19 0.77 0.013 0.042 -- -- 0.0087 -- 0.060 0.0055
0.0014 Nb: 0.01, Mg: 0.0026, Sn: 0.04, Ca: 0.3 Comparative 43 0.45
0.24 0.78 0.010 0.025 -- -- 0.0069 -- 0.025 0.0052 0.0018 Examples
44 0.43 0.25 0.76 0.010 0.041 -- -- 0.0092 -- 0.035 0.0051 0.0019
45 0.41 0.24 0.73 0.011 0.035 -- -- 0.0098 -- 0.040 0.0053 0.0017
46 0.44 0.25 0.78 0.014 0.022 -- -- 0.0059 -- 0.030 0.0034 0.0019
47 0.41 0.24 0.72 0.011 0.051 -- -- 0.0087 -- 0.003 0.0049 0.0034
48 0.44 0.25 0.77 0.015 0.052 -- -- 0.0062 -- 0.358 0.0062 0.0011
49 0.41 0.21 0.72 0.013 0.021 -- -- 0.0055 -- 0.103 0.0058 0.0025
50 0.42 0.20 0.73 0.013 0.037 -- -- 0.0077 -- 0.153 0.0057 0.0026
51 0.44 0.24 0.79 0.013 0.038 -- -- 0.0157 -- 0.067 0.0054 0.0016
52 0.45 0.23 0.76 0.010 0.036 -- -- 0.0175 -- 0.103 0.0049 0.0010
53 0.44 0.19 0.73 0.014 0.044 -- -- 0.0211 -- 0.243 0.0046 0.0016
54 0.45 0.19 0.71 0.010 0.025 -- -- 0.0223 -- 0.060 0.0046
0.0009
Machinability Test
[0059] Machinability testing was conducted with respect to Example
and Comparative Example steels that had been elongation-forged
under heating at 1250.degree. C. by first subjecting them to heat
treatment consisting of normalization under temperature condition
of 850.degree. C. for 1 hr, 0.5 hr in the case of Comparative
Examples No. 49 and No. 50, followed by air-cooling. A
machinability evaluation test piece was then cut from each
heat-treated steel and the machinabilities of the Example and
Comparative Example steels were evaluated by conducting drill
boring testing under the cutting conditions shown in Table 3 and to
longitudinal turning testing under the conditions shown in Table 4.
The maximum cutting speed VL1000 enabling cutting up to a
cumulative hole depth of 1000 mm was used as the evaluation index
in the drill boring test, and the maximum width VB_max of wear of
the relief flank after 10 min was used as the evaluation index in
the longitudinal turning test.
TABLE-US-00003 TABLE 3 Cutting Speed: 10-120 m/min conditions Feed:
0.25 mm/rev Cutting fluid: Water-soluble cutting oil Drill Drill
diameter: 3 mm NACHI ordinary drill Overhang: 45 mm Other Hole
depth: 9 mm Tool life: Until breakage
TABLE-US-00004 TABLE 4 Cutting Cutting speed: 250 m/min conditions
Feed: 0.3 mm/rev Depth of cut: 1.5 mm Dry cutting Tool Holder:
PTGNR2525M16 Tool shape: TNMG160408N-UZ Material: AC2000
Charpy Impact Test
[0060] FIG. 1 is a diagram showing a region from which a Charpy
impact test piece was cut. In the Charpy impact test, first, as
shown in FIG. 1, a cylinder 2 measuring 25 mm in diameter was cut
from a steel 1 heat-treated by the same method and under the same
conditions as the aforesaid machinability test piece so that its
axis was perpendicular to the elongation-forging direction of the
steel 1. Next, the cylinder 2 was held under temperature condition
of 850.degree. C. for 1 hr, 0.5 hr in the case of Comparative
Examples No. 49 and No. 50, then oil-quenched by cooling to
60.degree. C., and further subjected to tempering with water
cooling in which it was held under temperature condition of
550.degree. C. for 30 min. Next, the cylinder 2 was machined to
fabricate a Charpy test piece 3 in conformance with JIS Z 2202,
which was subjected to a Charpy impact test at room temperature in
accordance with the method prescribed by JIS Z 2242. Absorbed
energy per unit area (J/cm.sup.2) was adopted as the evaluation
index.
Tensile Test
[0061] A cylinder 2 sampled parallel to the elongation-forging
direction was oil-quenched and tempered by the same methods and
under the same conditions as in the aforesaid Charpy impact test,
whereafter it was processed into a tensile test piece measuring 8
mm in parallel section diameter and 30 mm in parallel section
length, and then tensile tested at room temperature in accordance
with the method prescribed by JIS Z 2241. Yield ratio (=(0.2% proof
stress YP)/(tensile strength TS) was adopted as the evaluation
index.
[0062] The results of the foregoing tests are shown in Tables 5 and
6.
TABLE-US-00005 TABLE 5 Impact VL1000 VB_max value No. (m/min)
(.mu.m) (J/cm.sup.2) YP/TS Examples 1 87 118 27 0.85 2 89 119 22
0.83 3 85 124 20 0.89 4 88 126 25 0.82 5 85 125 20 0.84 6 89 123 21
0.86 7 89 128 22 0.83 8 92 129 22 0.86 9 89 126 20 0.86 10 92 122
21 0.82 11 91 121 21 0.83 12 87 130 22 0.84 13 90 127 22 0.82 14 90
125 21 0.84 15 90 125 24 0.86 16 85 121 26 0.87 17 93 128 24 0.86
18 86 124 22 0.85 19 90 128 25 0.86 20 89 126 20 0.82 21 89 121 24
0.84 22 96 125 21 0.82 23 93 129 22 0.83 24 94 126 23 0.82 25 82
126 26 0.82 26 86 120 25 0.84 27 89 130 26 0.85 28 86 127 20 0.86
29 83 129 20 0.84 30 86 122 24 0.86 31 95 129 26 0.86 32 89 130 22
0.83 33 89 123 21 0.87 34 90 126 24 0.85 35 94 121 22 0.82 36 83
127 20 0.85 37 83 121 20 0.85 38 82 127 30 0.83 39 93 127 21 0.83
40 90 124 27 0.86 41 89 127 23 0.84 42 91 126 21 0.87
TABLE-US-00006 TABLE 6 Impact VL1000 VB_max value No. (m/min)
(.mu.m) (J/cm.sup.2) YP/TS Comparative 43 54 149 21 0.68 Examples
44 62 141 24 0.66 45 60 142 25 0.67 46 53 158 22 0.68 47 64 178 9
0.65 48 48 178 22 0.82 49 62 149 16 0.83 50 69 150 15 0.85 51 99
122 14 0.85 52 98 124 11 0.87 53 104 128 12 0.84 54 100 128 13
0.85
[0063] The Steels No.1 to No. 42 shown in Tables 1, 2 and 5 are
Examples of the present invention, and the steels No. 43 to No. 51
shown in Tables 2 and 6 are Comparative Example steels. As shown in
Tables 5 and 6, the steels of Examples No 1 to No. 42 exhibited
good values for all of the evaluation indexes, namely VL1000,
VB_max, impact value (absorbed energy), and YP/TS (yield ratio),
but the steels of the Comparative Examples were each inferior to
the Example steels in at least one of the properties. Specifically,
the steels of Comparative Examples No. 43 to No. 46 had total Al
contents below the range of the present invention and were
therefore inferior to the Example steels in machinability
evaluation index VL1000 and yield ratio (YP/TS). Moreover, the
steel of Comparative Example No.47 had a total Al content far below
the range of the present invention, so that its solute N content
was above the range of the present invention, and the steel was
therefore inferior to the steels of the Examples in machinability
(VL1000, VB_max), impact value, and yield ratio (YP/TS).
[0064] The steel of Comparative Example No. 48 had a total Al
content above the range of the present invention, so that its
hardness increased, and the steel was therefore inferior in
machinability (VL1000, VB_max). The steels of Comparative Examples
No. 49 and No. 50 were maintained at 850.degree. C., the
temperature at which AlN readily precipitates, for a shorter
holding time than the steels of the Examples, so that their solute
N contents were above the range of the present invention, and the
steels were therefore inferior to the steels of the Examples in
machinability (VL1000, VB_max) and impact value. The steels of
Comparative Examples No. 51 to No. 54 had Sb contents above the
range of the present invention and were therefore inferior to the
steels of the Examples in impact value.
Second Set of Examples
[0065] In this set of Examples, steels of the compositions shown in
Table 7 and Table 8, 150 kg each, were produced in a vacuum
furnace, hot-forged under a temperature condition of 1250.degree.
C., and elongation-forged into 65 mm diameter bars. The properties
of the Example and Comparative Example steels were evaluated by
subjecting them to machinability testing, Charpy impact testing and
tensile testing by the methods set out below. In Tables 7 and 8,
underlining indicates a value outside the invention range.
TABLE-US-00007 TABLE 7 Composition (mass %) No C Si Mn P S Cr Ca
Ttl Al Ttl N Sol N Other Examples 1 0.44 0.25 0.76 0.015 0.017 --
0.0000 0.121 0.0052 0.0012 2 0.44 0.26 0.76 0.015 0.012 -- 0.0006
0.101 0.0052 0.0012 3 0.44 0.25 0.75 0.016 0.010 -- 0.0008 0.250
0.0060 0.0013 4 0.44 0.25 0.76 0.015 0.008 -- 0.0010 0.075 0.0045
0.0011 5 0.46 0.26 0.76 0.015 0.013 -- 0.0006 0.099 0.0049 0.0012
Mg: 0.0020 6 0.44 0.24 0.74 0.015 0.011 -- 0.0008 0.193 0.0065
0.0013 Ti: 0.04 7 0.45 0.25 0.74 0.015 0.013 -- 0.0008 0.178 0.0075
0.0016 Nb: 0.02 8 0.44 0.24 0.74 0.015 0.011 -- 0.0006 0.169 0.0058
0.0014 W: 0.2 9 0.45 0.24 0.74 0.016 0.010 -- 0.0012 0.175 0.0055
0.0013 Ni: 0.2 10 0.46 0.26 0.76 0.015 0.014 -- 0.0005 0.142 0.0045
0.0017 Cu: 0.5 11 0.44 0.26 0.75 0.015 0.015 -- 0.0007 0.127 0.0052
0.0010 Sn: 0.05 12 0.44 0.25 0.76 0.015 0.011 -- 0.0004 0.147
0.0051 0.0014 Zn: 0.007 13 0.45 0.24 0.76 0.014 0.011 -- 0.0012
0.144 0.0053 0.0019 B: 0.002 14 0.45 0.26 0.75 0.015 0.011 --
0.0012 0.187 0.0046 0.0015 Te: 0.002 15 0.41 0.24 0.78 0.015 0.014
1.0 0.0010 0.108 0.0047 0.0016 16 0.44 0.25 0.76 0.015 0.013 0.1
0.0012 0.112 0.0048 0.0013 17 0.44 0.24 0.74 0.015 0.010 -- 0.0006
0.131 0.0071 0.0010 Ti: 0.03, Mg: 0.0025 18 0.45 0.26 0.75 0.016
0.010 -- 0.0009 0.109 0.0075 0.0010 Ti: 0.04, Zn: 0.004 19 0.41
0.24 0.75 0.016 0.010 1.0 0.0008 0.168 0.0071 0.0012 Ti: 0.03 20
0.44 0.26 0.74 0.016 0.010 -- 0.0011 0.113 0.0077 0.0015 Ti: 0.03,
Cu: 0.3 21 0.44 0.24 0.75 0.016 0.014 -- 0.0008 0.104 0.0054 0.0010
Ti: 0.02, Mg: 0.0025, Sn: 0.04 22 0.41 0.25 0.75 0.015 0.010 1.1
0.0005 0.192 0.0074 0.0019 Ti: 0.03, Mg: 0.0025 23 0.45 0.24 0.75
0.015 0.013 -- 0.0009 0.119 0.0062 0.0019 Ti: 0.03, Mg: 0.0025, Cu:
0.4 24 0.40 0..26 0.75 0.015 0.012 1.0 0.0005 0.198 0.0061 0.0015
Ti: 0.02, Mg: 0.0025, Sn: 0.04 25 0.44 0.26 0.75 0.014 0.015 --
0.0008 0.169 0.0060 0.0012 Ti: 0.03, Mg: 0.0025, Sn: 0.04, Cu: 0.3
26 0.42 0.24 0.75 0.014 0.013 1.0 0.0011 0.116 0.0074 0.0010 Ti:
0.03, Mg: 0.0025, Cu: 0.4 27 0.41 0.24 0.74 0.015 0.014 1.0 0.0004
0.198 0.0055 0.0012 Ti: 0.03, Sn: 0.04 28 0.46 0.25 0.75 0.015
0.010 -- 0.0010 0.179 0.0061 0.0016 Ti: 0.03, Sn: 0.04, Cu: 0.3 29
0.41 0.25 0.75 0.016 0.011 0.9 0.0008 0.156 0.0062 0.0017 Ti: 0.04,
Sn: 0.04, Cu: 0.3 30 0.41 0.26 0.75 0.014 0.012 1.0 0.0009 0.137
0.0056 0.0012 Ti: 0.03, Cu: 0.3 31 0.45 0.24 0.75 0.015 0.013 --
0.0013 0.109 0.0047 0.0010 Mg: 0.0025, Zn: 0.003 32 0.41 0.25 0.76
0.016 0.015 1.0 0.0011 0.104 0.0049 0.0013 Mg: 0.0019, Zn: 0.003 33
0.45 0.24 0.75 0.015 0.011 -- 0.0013 0.109 0.0048 0.0014 Mg:
0.0022, Cu:: 0.3 34 0.40 0.25 0.75 0.016 0.015 1.0 0.0008 0.105
0.0046 0.0014 Mg: 0.0020, Sn: 0.04 35 0.45 0.24 0.74 0.015 0.014 --
0.0009 0.110 0.0050 0.0018 Mg: 0.0025, Sn: 0.04, Cu: 0.1 36 0.42
0.25 0.75 0.014 0.012 1.0 0.0012 0.107 0.0047 0.0019 Mg: 0.0021,
Sn: 0.02, Cu: 0.1 37 0.41 0.24 0.75 0.015 0.014 1.1 0.0005 0.104
0.0049 0.0019 Mg: 0.0029, Cu:: 0.1 38 0.42 0.25 0.76 0.015 0.011
1.0 0.0009 0.102 0.0049 0.0010 Sn: 0.04 39 0.44 0.24 0.76 0.015
0.012 -- 0.0010 0.110 0.0046 0.0016 Sn: 0.03, Cu: 0.1 40 0.41 0.25
0.75 0.015 0.011 1.0 0.0009 0.108 0.0046 0.0009 Sn: 0.04, Cu: 0.1
41 0.41 0.25 0.75 0.015 0.011 0.9 0.0003 0.102 0.0046 0.0014 Cu:
0.2 42 0.46 0.25 0.76 0.015 0.011 -- 0.0003 0.102 0.0055 0.0014 Nb:
0.01, Mg: 0.0026, Sn: 0.04, Cu: 0.3 Comparative 43 0.44 0.24 0.76
0.014 0.011 -- 0.0006 0.025 0.0052 0.0018 Examples 44 0.45 0.25
0.76 0.015 0.014 -- 0.0006 0.035 0.0051 0.0019 45 0.45 0.24 0.75
0.015 0.014 -- 0.0008 0.040 0.0053 0.0017 46 0.45 0.25 0.76 0.014
0.011 -- 0.0010 0.030 0.0034 0.0019 47 0.46 0.25 0.74 0.016 0.011
-- 0.0008 0.003 0.0043 0.0034 48 0.44 0.24 0.75 0.014 0.009 --
0.0007 0.103 0.0058 0.0025
TABLE-US-00008 TABLE 8 Composition (mass %) No C Si Mn P S Cr Ca
Ttl Al Ttl N Sol N Other Examples 52 0.45 0.26 0.75 0.016 0.025 --
0.0002 0.101 0.0052 0.0012 53 0.44 0.25 0.76 0.015 0.030 -- 0.0000
0.250 0.0060 0.0013 54 0.45 0.25 0.74 0.015 0.042 -- 0.0001 0.123
0.0048 0.0012 55 0.45 0.24 0.75 0.015 0.090 -- 0.0002 0.106 0.0049
0.0013 56 0.45 0.24 0.75 0.014 0.042 -- 0.0001 0.102 0.0052 0.0011
Mg: 0.0020 57 0.45 0.24 0.74 0.015 0.042 -- 0.0001 0.190 0.0065
0.0016 Ti: 0.04 58 0.46 0.25 0.76 0.016 0.047 -- 0.0001 0.154
0.0075 0.0012 Nb: 0.02 59 0.45 0.25 0.74 0.015 0.044 -- 0.0001
0.129 0.0058 0.0017 W: 0.2 60 0.44 0.25 0.76 0.015 0.044 -- 0.0001
0.109 0.0055 0.0015 Ni: 0.2 61 0.45 0.26 0.74 0.016 0.041 -- 0.0001
0.148 0.0045 0.0015 Cu: 0.5 62 0.46 0.25 0.75 0.016 0.047 -- 0.0000
0.111 0.0052 0.0013 Sn: 0.03 63 0.46 0.25 0.75 0.015 0.051 --
0.0001 0.188 0.0051 0.0012 Zn: 0.007 64 0.45 0.24 0.76 0.015 0.073
-- 0.0002 0.197 0.0053 0.0011 B: 0.002 65 0.44 0.25 0.75 0.015
0.092 -- 0.0002 0.109 0.0046 0.0010 Te: 0.002 66 0.45 0.25 0.74
0.015 0.062 -- 0.0000 0.200 0.0046 0.0011 Cr: 0.1 67 0.45 0.26 0.76
0.014 0.049 -- 0.0001 0.109 0.0070 0.0012 68 0.45 0.26 0.76 0.016
0.040 -- 0.0000 0.172 0.0072 0.0010 Ti: 0.03, Mg: 0.0025 69 0.45
0.25 0.75 0.014 0.040 -- 0.0001 0.110 0.0068 0.0010 Ti: 0.04, Zn:
0.004 70 0.41 0.25 0.75 0.015 0.043 0.9 0.0000 0.125 0.0075 0.0009
Ti: 0.03 71 0.45 0.25 0.76 0.015 0.043 -- 0.0002 0.110 0.0069
0.0009 Ti: 0.03, Cu: 0.3 72 0.45 0.24 0.76 0.015 0.047 -- 0.0000
0.125 0.0062 0.0018 Ti: 0.03, Mg: 0.0015, Sn: 0.04 73 0.40 0.26
0.75 0.014 0.049 1.0 0.0001 0.142 0.0065 0.0017 Ti: 0.03, Mg:
0.0025 74 0.45 0.24 0.75 0.015 0.044 -- 0.0001 0.149 0.0062 0.0017
Ti: 0.03, Mg: 0.0025, Cu: 0.4 75 0.41 0.26 0.76 0.016 0.041 1.0
0.0001 0.129 0.0059 0.0019 Ti: 0.05, Mg: 0.0025, Sn: 0.04 76 0.44
0.24 0.76 0.015 0.043 -- 0.0001 0.188 0.0061 0.0014 Ti: 0.03, Mg:
0.0025, Sn: 0.04, Cu: 0.3 77 0.40 0.26 0.75 0.016 0.046 0.9 0.0001
0.172 0.0064 0.0013 Ti: 0.03, Mg: 0.0025, Cu: 0.4 78 0.41 0.25 0.75
0.016 0.042 1.0 0.0000 0.111 0.0063 0.0013 Ti: 0.03, Sn: 0.04 79
0.46 0.25 0.76 0.015 0.047 -- 0.0001 0.151 0.0067 0.0012 Ti: 0.03,
Sn: 0.04, Cu: 0.3 80 0.40 0.26 0.76 0.016 0.043 0.9 0.0001 0.120
0.0072 0.0017 Ti: 0.02, Sn: 0.04, Cu: 0.3 81 0.41 0.26 0.74 0.015
0.046 1.1 0.0001 0.144 0.0069 0.0010 Ti: 0.03, Cu: 0.3 82 0.46 0.24
0.76 0.014 0.040 -- 0.0001 0.105 0.0051 0.0010 Mg: 0.0028, Zn:
0.003 83 0.41 0.24 0.76 0.014 0.047 0.9 0.0000 0.102 0.0052 0.0013
Mg: 0.0019, Zn: 0.003 84 0.45 0.24 0.76 0.015 0.041 -- 0.0001 0.102
0.0069 0.0011 Mg: 0.0022, Cu:: 0.3 85 0.41 0.26 0.75 0.016 0.041
1.0 0.0000 0.109 0.0055 0.0012 Mg: 0.0020, Sn: 0.04 86 0.44 0.25
0.76 0.016 0.047 -- 0.0001 0.103 0.0062 0.0010 Mg: 0.0025, Sn:
0.04, Cu: 0.1 87 0.42 0.26 0.75 0.015 0.042 1.0 0.0001 0.101 0.0057
0.0011 Mg: 0.0017, Sn: 0.04, Cu: 0.1 88 0.41 0.25 0.75 0.015 0.046
1.1 0.0001 0.106 0.0067 0.0013 Mg: 0.0025, Cu:: 0.1 89 0.41 0.25
0.74 0.014 0.046 1.0 0.0000 0.109 0.0059 0.0016 Sn: 0.02 90 0.45
0.26 0.75 0.015 0.042 -- 0.0001 0.100 0.0066 0.0013 Sn: 0.04, Cu:
0.1 91 0.41 0.24 0.74 0.015 0.046 1.1 0.0001 0.105 0.0065 0.0012
Sn: 0.04, Cu: 0.1 92 0.41 0.26 0.75 0.015 0.040 1.1 0.0000 0.109
0.0058 0.0014 Cu: 0.1 93 0.44 0.24 0.75 0.015 0.057 -- 0.0001 0.101
0.0051 0.0017 Nb: 0.01, Mg: 0.0025, Sn: 0.04, Cu: 0.3 Comparative
94 0.45 0.25 0.74 0.014 0.026 -- 0.0001 0.025 0.0051 0.0017
Examples 95 0.45 0.24 0.75 0.014 0.043 -- 0.0001 0.024 0.0052
0.0018 96 0.46 0.24 0.75 0.016 0.046 -- 0.0002 0.032 0.0051 0.0019
97 0.46 0.24 0.76 0.015 0.046 -- 0.0002 0.104 0.0078 0.0034 98 0.45
0.25 0.74 0.016 0.043 -- 0.0001 0.103 0.0058 0.0025 99 0.44 0.26
0.76 0.016 0.051 -- 0.0000 0.243 0.0057 0.0026 100 0.45 0.24 0.75
0.014 0.073 -- 0.0001 0.111 0.0067 0.0031 101 0.46 0.25 0.75 0.014
0.099 -- 0.0001 0.142 0.0077 0.0035
Machinability Test
[0066] Machinability testing was conducted with respect to Example
and Comparative Example steels that had been elongation-forged
under heating at 1250.degree. C. by first subjecting them to heat
treatment consisting of normalization under temperature condition
of 850.degree. C. for 1 hr, 0.5 hr in the case of Comparative
Examples No. 48, No. 49 and No. 97 to No. 101, followed by
air-cooling. A machinability evaluation test piece was then cut
from each heat-treated steel and the machinabilities of the Example
and Comparative Example steels were evaluated by conducting drill
boring testing under the cutting conditions shown in Table 9 and to
longitudinal turning testing under the conditions shown in Table
10. The maximum cutting speed VL1000 enabling cutting up to a
cumulative hole depth of 1000 mm was used as the evaluation index
in the drill boring test, and the maximum width VB_max of wear of
the relief flank after 10 min was used as the evaluation index in
the longitudinal turning test.
TABLE-US-00009 TABLE 9 Cutting Speed 10-120 m/min conditions Feed
0.25 mm/rev Cutting Water-soluble fluid cutting oil Drill Drill 3
mm diameter (.phi.) NACHI Ordinary drill Overhang 45 mm Other Hole
depth 9 mm Tool life Until breakage
TABLE-US-00010 TABLE 10 Cutting Cutting 250 m/min conditions speed
Feed 0.3 mm/rev Mode Dry cutting Tool Holder PTGNR2525M16 Shape
TNMG160408N-UZ Material AC2000
Charpy Impact Test
[0067] FIG. 1 is a diagram showing a region from which a Charpy
impact test piece was cut. In the Charpy impact test, first, as
shown in FIG. 1, a cylinder 2 measuring 25 mm in diameter was cut
from a steel 1 heat-treated by the same method and under the same
conditions as the aforesaid machinability test piece so that its
axis was normal to the elongation-forging direction of the steel 1.
Next, the cylinder 2 was held under temperature condition of
850.degree. C. for 1 hr, 0.5 hr in the case of Comparative Examples
No. 48, No. 49 and No. 97 to No. 101, then oil-quenched by cooling
to 60.degree. C., and further subjected to tempering with water
cooling in which it was held under temperature condition of
550.degree. C. for 30 min. Next, the cylinder 2 was machined to
fabricate a Charpy test piece 3 in conformance with JIS Z 2202,
which was subjected to a Charpy impact test at room temperature in
accordance with the method prescribed by JIS Z 2242. Absorbed
energy per unit area (J/cm.sup.2) was adopted as the evaluation
index.
Tensile Test
[0068] A cylinder 2 oil-quenched and tempered by the same methods
and under the same conditions as in the aforesaid Charpy impact
test was processed into a tensile test piece measuring 8 mm in
parallel section diameter and 30 mm in parallel section length, and
then tensile tested at room temperature in accordance with the
method prescribed by JIS Z 2241. Yield ratio (=(0.2% proof stress
YP)/(tensile strength TS) was adopted as the evaluation index.
[0069] The results of the foregoing tests are shown in Tables 11
and 12.
TABLE-US-00011 TABLE 11 Impact VL1000 VB_max value No. (m/min)
(.mu.m) (J/cm.sup.2) YP/TS Examples 1 70 121 39 0.87 2 65 121 40
0.82 3 65 123 41 0.84 4 65 125 42 0.80 5 70 115 39 0.83 6 65 121 42
0.83 7 65 123 41 0.86 8 65 120 40 0.85 9 65 116 42 0.85 10 65 122
43 0.86 11 70 123 44 0.85 12 70 120 38 0.83 13 70 119 39 0.84 14 70
120 40 0.85 15 55 132 37 0.89 16 65 124 40 0.86 17 65 125 40 0.82
18 70 124 39 0.85 19 55 131 39 0.84 20 65 126 38 0.82 21 70 118 39
0.83 22 55 133 39 0.84 23 70 128 38 0.83 24 60 130 39 0.85 25 70
119 39 0.83 26 55 131 40 0.83 27 65 132 40 0.83 28 70 121 40 0.84
29 60 131 39 0.86 30 55 131 38 0.85 31 70 120 41 0.83 32 55 133 37
0.86 33 70 125 40 0.87 34 60 134 39 0.86 35 70 120 39 0.87 36 60
133 41 0.87 37 55 131 41 0.84 38 60 130 38 0.86 39 70 119 39 0.86
40 60 131 38 0.84 41 55 132 37 0.84 42 65 124 41 0.83 Comparative
43 45 122 41 0.66 Examples 44 45 116 40 0.67 45 45 117 41 0.67 46
50 110 42 0.67 47 35 156 22 0.68 48 50 149 30 0.87 49 50 140 29
0.85
TABLE-US-00012 TABLE 12 Impact VL1000 VB_max value No. (m/min)
(.mu.m) (J/cm.sup.2) YP/TS Examples 52 85 121 25 0.85 53 85 123 24
0.85 54 95 121 23 0.82 55 105 112 21 0.85 56 95 120 22 0.85 57 95
123 22 0.85 58 90 123 22 0.85 59 95 121 22 0.83 60 90 124 22 0.83
61 95 120 22 0.87 62 95 115 22 0.83 63 95 125 22 0.85 64 100 117 21
0.84 65 105 113 21 0.84 66 95 121 20 0.86 67 80 131 20 0.83 68 95
122 25 0.87 69 100 120 24 0.84 70 80 131 20 0.84 71 95 122 23 0.85
72 100 118 26 0.83 73 80 130 21 0.84 74 95 122 25 0.85 75 85 128 20
0.85 76 100 119 25 0.86 77 80 132 22 0.83 78 85 128 20 0.82 79 100
120 24 0.82 80 85 126 21 0.84 81 80 133 21 0.83 82 105 120 23 0.84
83 80 130 21 0.83 84 95 124 24 0.86 85 85 129 22 0.87 86 95 117 23
0.87 87 85 128 21 0.84 88 80 129 21 0.86 89 85 126 20 0.83 90 95
119 21 0.86 91 85 125 20 0.87 92 80 133 20 0.83 93 100 112 22 0.86
Comparative 94 60 180 22 0.68 Examples 95 65 179 20 0.69 96 65 174
19 0.68 97 70 157 18 0.84 98 75 149 18 0.82 99 70 143 18 0.86 100
75 152 15 0.79 101 75 163 12 0.86
[0070] The steels No.1 in Tables 7 and 11 are embodiments of claim
1 and the steels No.2 to No. 42 in the same tables are embodiments
of claim 2. The steels No. 52 to No. 93 in Table 8 and Table 12 are
embodiments of claim 1. The comparative steels No. 43 to No. 49
satisfy the S content and Ca content requirements of claim 2, and
the comparative steels No. 94 to No. 101 satisfy the S content and
Ca content requirements of claim 1.
[0071] As shown in Tables 11 and 12, the steels of Examples No 1 to
No. 42 and No. 52 to No. 93 exhibited good values for all of the
evaluation indexes, namely VL1000, VB_max, impact value (absorbed
energy), and YP/TS (yield ratio), but the steels of the Comparative
Examples were each inferior to the Example steels in at least one
of the properties. Specifically, the steels of Comparative Examples
No. 43 to No. 46 had total Al contents below the range of the
present invention and were therefore inferior to the Example steels
in machinability (VL1000) and yield ratio (YP/TS). Moreover, the
steel of Comparative Example No. 47 had a total Al content below
the range of the present invention, so that its solute N content
was above the range of the present invention, and the steel was
therefore inferior to the steels of the Examples in machinability
(VL1000, VB_max), impact value, and yield ratio (YP/TS).
[0072] The steels of Comparative Examples No. 48 and No. 49 were
maintained at 850.degree. C., the temperature at which AlN readily
precipitates, for a shorter holding time than the steels of the
Examples, so that their solute N contents were above the range of
the present invention, and the steels were therefore inferior to
the steels of the Examples in machinability (VL1000, VB_max) and
impact value. Moreover, the steels of Comparative Examples No. 94
to No. 96 had a total Al content below the range of the present
invention and were therefore inferior to the steels of the Examples
in machinability (VL1000, VB_max) and yield ratio (YP/TS). Further,
the steels of Comparative Examples No. 97 to No. 101 were
maintained at 850.degree. C., the temperature at which AlN readily
precipitates, for a shorter holding time than the steels of the
Examples, so that their solute N contents were above the range of
the present invention, and the steels were therefore inferior to
the steels of the Examples in machinability (VL1000, VB_max) and
impact value.
INDUSTRIAL APPLICABILITY
[0073] The present invention provides a machine structural steel
that has good machinability over a broad range of machining speeds
and also has high impact properties and high yield ratio.
* * * * *