U.S. patent application number 11/332910 was filed with the patent office on 2006-07-20 for cold working die steel.
This patent application is currently assigned to DAIDO STEEL CO., LTD.. Invention is credited to Toshimitsu Fuji, Yasutaka Ikeuchi, Takayuki Shimizu.
Application Number | 20060157163 11/332910 |
Document ID | / |
Family ID | 36682651 |
Filed Date | 2006-07-20 |
United States Patent
Application |
20060157163 |
Kind Code |
A1 |
Shimizu; Takayuki ; et
al. |
July 20, 2006 |
Cold working die steel
Abstract
This invention is aimed at providing a cold working die steel
successfully reduced in the hardness, and increased in readiness in
the cold workability (forging, pressing and so forth) and
machinability (milling, drilling, endmilling processing, grinding
and lathe turning). A cold working die steel aimed at solving the
above-described subject consists essentially of, in % by mass,
0.6%.ltoreq.C.ltoreq.1.60%, 0.10%.ltoreq.Si.ltoreq.1.20%,
0.10%.ltoreq.Mn.ltoreq.0.60%, 5.5%.ltoreq.Cr.ltoreq.13.0%,
0.80%.ltoreq.Mo+0.5W.ltoreq.2.10%, 0.10%.ltoreq.V.ltoreq.0.40%,
0.0002%.ltoreq.O.ltoreq.0.0080%, 0.001%.ltoreq.A1.ltoreq.0.10%, and
the balance of Fe and inevitable impurities; has transformation
point Ar3 in the range from 750.degree. C. to 850.degree. C., with
both ends inclusive; has a mean circle-equivalent diameter of a
carbide, which belongs to a circle-equivalent diameter range from
0.1 .mu.m to 3 .mu.m observed in a section of a structure obtained
after spherodizing a sample that was heated at a temperature of
(Ar3+50.degree. C.) or above and 1,050.degree. C. or below, of 0.25
.mu.m to 0.8 .mu.m, with both ends inclusive; has a Brinell
hardness attained after the spherodizing of HB179 to HB235, with
both ends inclusive; has a steel cleanliness of (dB+dC)60.times.400
in the group C inclusion and the group B inclusion specified by JIS
G0555 of 0.05% or less; and has a K value defined as
Cr(%)-6.8.times.C(%) of 0.1 to 3.5, both ends inclusive.
Inventors: |
Shimizu; Takayuki;
(Nagoya-shi, JP) ; Ikeuchi; Yasutaka; (Nagoya-shi,
JP) ; Fuji; Toshimitsu; (Nagoya-shi, JP) |
Correspondence
Address: |
TOWNSEND & BANTA;c/o PORTFOLIO IP
PO BOX 52050
MINNEAPOLIS
MN
55402
US
|
Assignee: |
DAIDO STEEL CO., LTD.
|
Family ID: |
36682651 |
Appl. No.: |
11/332910 |
Filed: |
January 17, 2006 |
Current U.S.
Class: |
148/334 |
Current CPC
Class: |
C22C 38/22 20130101;
C22C 38/06 20130101; C22C 38/04 20130101; C22C 38/24 20130101; C22C
38/02 20130101 |
Class at
Publication: |
148/334 |
International
Class: |
C22C 38/22 20060101
C22C038/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 14, 2005 |
JP |
2005-007198 |
Claims
1. A cold working die steel consisting essentially of, in % by
mass, 0.6%.ltoreq.C.ltoreq.1.60%, 0.10%.ltoreq.Si.ltoreq.1.20%,
0.10%.ltoreq.Mn.ltoreq.0.60%, 5.5%.ltoreq.Cr.ltoreq.13.0%,
0.80%.ltoreq.Mo+0.5W.ltoreq.2.10%, 0.10%.ltoreq.V.ltoreq.0.40%,
0.0002%.ltoreq.O.ltoreq.0.0080%, 0.001%.ltoreq.Al.ltoreq.0.10%, and
the balance of Fe and inevitable impurities; having transformation
point Ar3 in the range of from 750.degree. C. to 850.degree. C.,
both ends inclusive; having a mean circle-equivalent diameter of a
carbide, which belongs to a circle-equivalent diameter range from
0.1 .mu.m to 3 .mu.m observed in a section of a structure obtained
after spherodizing a sample that was heated at a temperature of
(Ar3+50.degree. C.) or above and 1,050.degree. C. or below, of 0.25
.mu.m to 0.8 .mu.m, with both ends inclusive; having a Brinell
hardness attained after the spherodizing of HB179 to HB235, with
both ends inclusive; having a steel cleanliness of
(dB+dC)60.times.400 in the group C inclusion and the group B
inclusion specified by JIS G0555 of 0.05% or less; and having a K
value defined as Cr(mass %)-6.8.times.C(mass %) of 0.1 to 3.5, with
both ends inclusive.
2. The cold working die steel as claimed in claim 1, wherein the
steel components are 0.60%.ltoreq.C.ltoreq.0.80%,
0.10%.ltoreq.Si.ltoreq.1.20%, 0.10%.ltoreq.Mn.ltoreq.0.60%,
5.5%.ltoreq.Cr.ltoreq.8.5%, 0.80%.ltoreq.Mo+0.5W.ltoreq.2.10%,
0.10%.ltoreq.V.ltoreq.0.40%, 0.0002%.ltoreq.O.ltoreq.0.0080% and
0.001%.ltoreq.Al.ltoreq.0.10%.
3. The cold working die steel as claimed in claim 1, wherein the
steel components are 0.90%.ltoreq.C.ltoreq.1.10%,
0.8%.ltoreq.Si.ltoreq.1.20%, 0.10%.ltoreq.Mn.ltoreq.0.60%,
7.0%.ltoreq.Cr.ltoreq.9.0%, 1.50%.ltoreq.Mo+0.5W.ltoreq.2.10%,
0.10%.ltoreq.V.ltoreq.0.40%, 0.0002%.ltoreq.O.ltoreq.0.0080% and
0.001%.ltoreq.Al.ltoreq.0.10%.
4. The cold working die steel as claimed in claim 1, wherein the
steel components are 1.40%.ltoreq.C.ltoreq.1.60%,
0.10%.ltoreq.Si.ltoreq.0.40%, 0.10%.ltoreq.Mn.ltoreq.0.60%,
11.0%.ltoreq.Cr.ltoreq.13.0%, 0.80%.ltoreq.Mo+0.5W.ltoreq.1.20%,
0.10%.ltoreq.V.ltoreq.0.40%, 0.0002%.ltoreq.O.ltoreq.0.0080% and
0.001%.ltoreq.Al.ltoreq.0.10%.
5. The cold working die steel as claimed claim 1, further
containing, as the steel component, either one of or both of
0.0030%.ltoreq.N.ltoreq.0.0500% and
0.001%.ltoreq.P.ltoreq.0.040%.
6. The cold working die steel as claimed in claim 1, further
containing any one of, or two or more of steel components selected
from 0.01%.ltoreq.Cu.ltoreq.1.0%, 0.01%.ltoreq.Ni.ltoreq.1.0%,
0.2%.ltoreq.Co.ltoreq.1.0% and 0.0003%.ltoreq.B.ltoreq.0.010%.
7. The cold working die steel as claimed claim 1, further
containing any one of, or two or more of steel components selected
from 0.001%.ltoreq.S.ltoreq.0.20%, 0.005%.ltoreq.Se.ltoreq.0.10%,
0.005%.ltoreq.Te.ltoreq.0.10%, 0.0002%.ltoreq.Ca.ltoreq.0.010%,
0.005%.ltoreq.Pb.ltoreq.0.10% and
0.005%.ltoreq.Bi.ltoreq.0.10%.
8. The cold working die steel as claimed claim 1, further
containing any one of, or two or more of steel components selected
from 0.01%.ltoreq.Nb.ltoreq.0.12%, 0.005%.ltoreq.Ta.ltoreq.0.10%,
0.005%.ltoreq.Ti.ltoreq.0.10%, 0.005%.ltoreq.Zr.ltoreq.0.10%,
0.005%.ltoreq.Mg.ltoreq.0.10% and
0.005%.ltoreq.REM.ltoreq.0.10%.
9. The cold working die steel as claimed in claim 1, wherein the
spherodizing comprises keeping the steel heated, and cooling the
steel to as low as 750.degree. C. or lower at a cooling rate slower
than 60.degree. C./h.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a cold working die steel used for
cold die and structural components processed by forging or pressing
in cold working; mechanical components demanding wear resistance;
punch and die for cold forging; mold die for high tensile steel
sheets; bending dies; cold forging dies; swaging dies; thread
rolling dies; punch components; slitter knives; punch dies for lead
frames; gauges; deep-drawing punches; bender punches; shear blades;
benders for stainless steel; drawing dies; tools for plastic
working such as heading; punches for gears; cam components; press
punch dies; progressive punch dies; seal plates for soil conveyers;
screw components; rotary plates for concrete spraying machines; IC
molding dies; precision press mold demanding high dimensional
accuracy; and dies used for the above-described applications after
being surface-treated by CVD, PVD or TD.
BACKGROUND ART
[0002] SKD11, SKD12 and so forth, which are representative steel
types for high-alloy cold dies specified by JIS G4404 have
conventionally been used as cold die steel. These cold die steels
are subjected to cold working after being subjected to hot working
(forging, rolling), and then to annealing, in which the steel is
heated to a temperature range from point Ar3 to Ar3+50.degree. C.
and then slowly cooled. The hardness obtained after annealing under
such annealing conditions falls in the range approximately from
HB241 to HB255 on the Brinell hardness basis.
[0003] Japanese Laid-Open Patent Publication "Tokkaihei" No.
6-322439 discloses that the hardness can be lowered as compared
with that attained by conventional annealing, by raising the
annealing temperature higher than in the conventional process.
Although the document describes that the lowering of the hardness
can increase the machinability, it is hardly said that the hardness
of HV262 to 278 (approximately 250 to 265 on the HB basis) listed
in Table 2 is small enough. Moreover, no description is made on the
hardness after quench-and-temper, despite a known problem in that
annealing carried out at a temperature higher than hardening
temperature results in lowering in the hardness after quench and
temper than usual.
[0004] In recent years, shorter delivery periods and lower costs in
the fabrication process have been more strongly demanded, and there
have been increased demands on cold working die steel more easily
used in machinability such as post-anneal machinability after
annealing or cold workability, in view of achieving
near-net-shaping and shortening of the time period for fabricating
the structural and mechanical components. With cold tool steel,
however, it is hard to achieve a low level of hardness by annealing
intrinsically because of the composition thereof.
[0005] Steel having large amounts of coarse carbide are used for
applications in particular demanding wear resistance such as dies,
tools and mechanical components, in view of the wear resistance,
but making too much account of wear resistance inevitably results
in degradation of the material characteristics, such as a lowering
of the overall toughness. On the other hand, a decrease in the
coarse carbide or an addition of a large amount of element
enhancing free-cutting properties, such as S, aiming at improving
the machinability, lowers the wear resistance required for tool
steel. It is to be noted herein that the "coarse carbide (primary
carbide)" refers to a carbide having a diameter of approximately 10
.mu.m or larger on a circle-equivalent basis.
[0006] The present invention was conceived taking the
above-described problems into account, and an object thereof
resides in providing a cold working die steel which can be reduced
in its hardness by annealing, and is improved in readiness in the
cold workability (forging, pressing and so forth) and machinability
(milling, drilling, endmilling processing, grinding and
turning).
SUMMARY OF THE INVENTION
[0007] Aiming at solving the aforementioned problems, cold working
die steel according to the first aspect of this invention consists
essentially of, in % by mass and by both ends inclusive,
0.6%.ltoreq.C.ltoreq.1.60%, 0.10%.ltoreq.Si.ltoreq.1.20%,
0.10.ltoreq.Mn.ltoreq.0.60%, 5.5%.ltoreq.Cr.ltoreq.13.0%,
0.80%.ltoreq.Mo+0.5W.ltoreq.2.10%, 0.10%.ltoreq.V.ltoreq.0.40%,
0.0002%.ltoreq.O.ltoreq.0.0080%, 0.001%.ltoreq.Al.ltoreq.0.10%, and
the balance of Fe and inevitable impurities; having transformation
point Ar3 in the range from 750.degree. C. to 850.degree. C., with
both ends inclusive; having a mean circle-equivalent diameter of a
carbide, which belongs to a circle-equivalent diameter range from
0.1 .mu.m to 3 .mu.m observed in a section of a structure obtained
after spherodizing a sample heated at a temperature of
(Ar3+50.degree. C.) or above and 1,050.degree. C. or below, of 0.25
.mu.m to 0.8 .mu.m, with both ends inclusive; having a Brinell
hardness attained after the spherodizing of HB179 to HB235, with
both ends inclusive; having a steel cleanliness level of
(dB+dC)60.times.400 in group C inclusion and group B inclusion
specified by JIS G0555 of 0.05% or less; and having a K value
defined as Cr(%)-6.8.times.C(%) of 0.1 to 3.5, with both ends
inclusive.
[0008] This invention has major features described below.
(1) Lowering of the Hardness After Annealing
[0009] Hardness after annealing is determined mainly by state of
distribution of carbide having a diameter ranging from 0.1 .mu.m to
3 .mu.m, with both ends inclusive, on the circle-equivalent basis
(referred to as secondary carbide, hereinafter), so that the
hardness can be decreased by adjusting the mean circle-equivalent
diameter of the carbide to the range from 0.2 .mu.m to 0.8 .mu.m,
with both ends inclusive. This range of the mean circle-equivalent
diameter corresponds to the enlargement of the secondary carbide.
In other words, an absolute amount of the secondary carbide
produced by the annealing is the same, so that a larger mean
circle-equivalent diameter results in a smaller hardness due to
scarceness of the secondary carbide having relatively large
sizes.
[0010] On the contrary, a smaller mean circle-equivalent diameter
results in a larger hardness by virtue of the denseness of the
secondary carbide having relatively small sizes. More specifically,
adjustment of the mean circle-equivalent diameter of the secondary
carbide in the above-described range makes it possible to obtain a
Brinell hardness of as low as HB179 to HB235, with both ends
inclusive (conventional steels showed a Brinell hardness of as
large as HB241 to HB255 or around), and to distinctively improve
the efficiency in machining and cold working.
(2) Control of the Size of the Secondary Carbide by Annealing
Conditions
[0011] Size of the secondary carbide is controlled by spherodizing,
or keeping the steel heated at a temperature of (Ar3+50.degree. C.)
or above and 1,050.degree. C. or below. In more detail, the steel
is heated at a temperature (from Ar3+50.degree. C. to a hardening
temperature (1,050.degree. C. or around)) higher than the
conventional annealing temperature (from Ar3 to Ar3+50.degree. C.)
so as to allow more secondary carbides to dissolve into the matrix.
The reason why the temperature at which the steel is heated is
adjusted to a temperature lower than the hardening temperature
(1,050.degree. C. or around) is to avoid lowering of the hardness
after quench-and-temper.
[0012] Thereafter, the steel is gradually cooled at a cooling rate
slower than 60.degree. C./h to as low as 750.degree. C. or below,
in order to allow the dissolved secondary carbide to grow into
larger grains (gradual cooling method). Other known treatment
methods include the "repetitive heating-and-gradual-cooling method"
in which heating and cooling are repeated at least twice within
temperature ranges from 650.degree. C. to transformation point Ar1,
and from transformation point Ar3 to 1,050.degree. C.; "prolonged
annealing method" in which the steel is kept at a temperature lower
than Ar3 for a long period of time; and "isothermal transformation
method" in which the steel is kept at a constant temperature during
the gradual cooling process in the gradual cooling method.
[0013] Any of these methods can successfully control the size of
the secondary carbide. Low temperature annealing at a temperature
of Ar1 or below before the spherodizing annealing makes it possible
to reduce variation in the post-anneal hardness after annealing and
to obtain still an even lower hardness. It is to be noted that the
above-described spherodized annealing will not largely alter the
size and population of the coarse carbide (primary carbide) which
affects the wear resistance required for applications such as dies
and tools.
[0014] Transformation point "Ar3" herein expresses the A3
transformation point (austenization temperature), wherein "r" means
cooling (refroidissement). In the spherodized annealing, the
temperature at which the steel is heated is necessarily set higher
than that in the conventional process in view of thoroughly
dissolving the secondary carbides into the matrix, and the
transformation point of Ar3 is necessarily low enough so as to
adjust the temperature to the quenching temperature or below. In
other words, it is required that there is a large enough difference
between the transformation point of Ar3 and the quenching
temperature.
[0015] The steel is therefore necessarily adjusted in the
composition thereof so as to make transformation point Ar3 fall
within the range of 750.degree. C. to 850.degree. C. If the Ar3 is
too high only a small difference from the normal quenching
temperature (1,050.degree. C. or around) will result, and
consequently fail in raising the annealing temperature thereby
resulting in only an insufficient dissolution of the secondary
carbide (not so different from the conventional annealing). On the
other hand, if the Ar3 is too low, a vastly longer duration time
for the depositing and growing of the secondary carbide. The
annealing temperature herein is determined to a desirable level
(Ar3+50.degree. C. to quenching temperature (at around
1,050.degree. C.)), based on results of the measurements of the
transformation point Ar3 using an apparatus such as DTA
(differential thermal analyzer).
(3) Influence of Oxide-Base Inclusion on Machinability
[0016] Increases in groups B and C (oxide-based ones in particular)
inclusions (conforming to JIS G0555) are known to decrease the
machinability. These inclusions have extremely high hardness of
their own, which exceeds the hardness of the matrix. By coming into
contact with these inclusions, tool edges can be chipped and the
lifetime of tools can be considerably decreased. Both group B and C
inclusions become less affective as the contents of which reduce
closer to 0, so that it is necessary, for the purpose of obtaining
more sufficient machinability than conventional steel, to adjust
the steel cleanliness to (dB+dC)60.times.400.ltoreq.0.05%. This
cleanliness level can be obtained by adjusting mainly the contents
of O and Al within the range described later.
[0017] The following paragraphs will describe reasons for
limitations of the individual numerical ranges.
(4) 0.6%.ltoreq.C.ltoreq.1.60%
[0018] C is an essential element for raising the hardness of
martensite after quenching. The element forms carbide through
binding with carbide-forming elements such as Cr, Mo and V, and
thereby make the crystal grain more fine. The carbide also
contributes to the improvement of the wear resistance. An addition
to an amount of the lower limit or more is necessary in order to
realize a hardness after quench and temper of HRC55 or above. On
the other hand, the element is added to as much as the upper limit
or less, because excessive addition results in excessive content of
the carbide, which thereby decreases the toughness.
(5) 0.10%.ltoreq.Si.ltoreq.1.20%
[0019] Si is added as a deoxidizing element. Because the element
contributes to the increase in hardness in the
high-temperature-temper, it is added to as much as the lower limit
or more, so as to obtain the effect. On the other hand, the element
is added to as much as the upper limit or less, because excessive
addition degrades the hot workability, and the
after-quench-and-temper toughness.
(6) 0.10%.ltoreq.Mn.ltoreq.0.60%
[0020] Mn is added as a deoxidizing element. Because the element
contributes to the enhancement of the hardenability, and increasing
in the hardness and strength, it is added to as much as 0.10% or
more. On the other hand, the upper limit of the element is set to
0.60%, because excessive addition degrades the hot workability.
(7) 5.5%.ltoreq.Cr.ltoreq.13.0%
[0021] Cr dissolves into the matrix which raises the hardenability
and increases the hardness, and forms carbides which increase the
wear resistance. Additions to as much as the lower limit or more
are necessary for obtaining these effects. On the other hand, the
element is added to as much as the upper limit or less, because
excessive addition results in formation of an excessive amount of
carbides which decrease the toughness and machinability after
quench-and-temper.
(8) 0.80%.ltoreq.Mo+0.5W.ltoreq.2.10%
[0022] Mo and W dissolve into the solid matrix to thereby raise the
hardenability and to contribute to increase hardness, and forms
carbides to which increase the wear resistance. The elements also
have an effect of raising the anti-softening hardness in
quench-and-temper. Addition to as much as the lower limit or more,
on the Mo-equivalent basis expressed as Mo(%)+0.5W(%), is necessary
for obtaining these effects. On the other hand, the element is
added to as much as the upper limit or less, because excessive
addition decreases the hot workability, toughness and
machinability.
(9) 0.10%.ltoreq.V.ltoreq.0.40%
[0023] V forms a stable carbide, and thereby effectively prevents
the crystal grains from coarsening. The element forms a fine
carbide, and thereby contributes to increases in the wear
resistance and the hardness. Addition to as much as 0.10% or more
is necessary for obtaining these effects. On the other hand, the
upper limit is set to 0.40%, because excessive addition increases
the carbide content, and thereby decreases the machinability and
hot workability.
(10) 0.0002%.ltoreq.O.ltoreq.0.0080%,
0.001%.ltoreq.Al.ltoreq.0.10%
[0024] O and Al are inevitably contained in the steel. The elements
are constituent elements of group B and C inclusions, and large
contents of which can decrease the toughness, so that it is
necessary to suppress the contents to as much as the upper limit or
less. Positive efforts of reducing these elements, although
depending on balance with the production cost, makes it possible to
maintain a high, stable toughness. Excessive lowering of the
contents only results in an increase in the production cost and a
saturation of influences exerted on the toughness, so that the
elements are added to as much as the lower limit or more.
(11) Transformation Point Ar3 Adjusted to 750.degree. C. to
850.degree. C.
[0025] Transformation point Ar3 adjusted in the above-described
range can expand the temperature range ranging from Ar3+50.degree.
C., at which spherodizing is effected. The quenching temperature
(about 1,050.degree. C.), ensures a sufficient range of annealing
temperature allowing the secondary carbides to thoroughly dissolve.
An excessively high Ar3, however, narrows the temperature range
from Ar3+50.degree. C. to the quenching temperature (about
1,050.degree. C.), and fails to ensure a sufficient range of an
annealing temperature. This, as a consequence, extremely reduces
the amount of dissolution of the secondary carbides, and fails in
obtaining low hardness required for raising machinability. An
excessively low Ar3 takes a longer time to allow the secondary
carbides to deposit and grow in the process of gradual cooling at a
temperature as low as Ar3 or below, and this extremely raises the
cost from the industrial viewpoint. It is to be understood that Ar3
can be measured typically by DTA, and defined as being obtained at
a cooling rate of 5.degree. C./h or more to 60.degree. C./h,
because Ar3 varies depending on conditions of the measurement.
[0026] (12) The mean circle-equivalent diameter of the carbide,
which belongs to the circle-equivalent diameter range of 0.1 .mu.m
to 3 .mu.m was observed in the section of the structure obtained
after spherodizing a sample that was heated at a temperature of
(Ar3+50.degree. C.) or above and 1,050.degree. C. or below,
adjusted to 0.25 .mu.m or more and 0.8 .mu.m or less.
[0027] The mean circle-equivalent diameter of the carbide is
calculated by an image analysis of a polished section of the steel
structure. More specifically, a circle-equivalent diameter is
calculated for every carbide grain having a circle-equivalent
diameter ranging from 0.1 .mu.m to 3 .mu.m which can be seen in a
magnified field of view under a scanning electron microscope or an
optical microscope, and a mean value of which is determined as the
mean circle-equivalent diameter. It is necessary that the
observation in the magnified field of view is carried out at least
in an area of 1 mm.sup.2 or larger, at randomly selected positions
excluding the surface and center portions of the material.
Alternatively, it is also allowable to use 20 or more fields of
view, when each field contains 20 to 50 carbide grains having a
circle-equivalent diameter ranging from 0.1 .mu.m to 3 .mu.m. The
carbide having a diameter ranging from 0.1 .mu.m to 3 .mu.m means a
carbide (secondary carbide) contributive to the hardness.
[0028] A mean grain size of the carbide belonging this range of
less than 0.25 .mu.m results in a high hardness, and fails in
obtaining the effect of increasing the machinability (this applies
to the case where the conventional style of annealing was carried
out). On the other hand, an excessively large mean grain size
results in an extremely small number of carbide grains grown during
the process of gradual cooling in the annealing, and this makes the
regenerative perlite more likely to deposit in the cooling process,
and conversely increases the hardness. It is therefore necessary to
set the upper limit of the mean circle-equivalent diameter of
carbide to 0.8 .mu.m.
(13) Brinell Hardness Attained After Spherodizing of HB179 to
HB235
[0029] Adjustment of the Brinell hardness after the spherodizing to
HB179 to HB235 makes it possible to obtain the machinability and
cold workability superior to those in the prior art. Excessive
lowering of the hardness results in a high cost from the industrial
point of view, so that a hardness of HB179 or more is enough for
the purpose.
(14) Steel Cleanliness in Group B and C Inclusions Specified by JIS
G0555 of (dB+dC)60.times.400.ltoreq.0.05%
[0030] The class B inclusion (mainly alumina and so forth) refers
to grains forming an agglomerate discontinuously arrayed in the
working direction, and the group C inclusion (such as granular
oxide) refers to grains irregularly dispersed without causing
viscous deformation. Excessive amounts of these inclusions
decreases the machinability, and a desirable level of machinability
can be obtained by adjusting the cleanliness (dB+dC)60.times.400 of
the steel with respect to group B and C inclusions, determined by
the test method as specified in JIS G0555, of 0.05% or less.
(15) K Value Expressed by Cr(mass %)-6.8.times.C(mass %) of 0.1 or
More and 3.5 or Less
[0031] K value expresses the amount of Cr solubilized in the matrix
at an appropriate quenching temperature. Adjustment within the
above range results in the hardness obtained after
quench-and-temper almost equivalent to the previous, and therefore
makes it possible to adjust the amount of deposition of carbides
depending on the wear resistance, toughness and machinability
required for the cold working die steel. In contrast, if the K
value is out of the above range, it results in only an insufficient
amount of the secondary carbide which deposits during tempering and
contributes to the hardness, and consequently fails in maintaining
the hardness necessary for the cold working die steel. FIG. 1 shows
a relation between the C content and the Cr content. The straight
lines ascending from the lower left towards the upper right in the
drawing relate to the K value, and the straight lines descending
from the upper left towards the lower right in the drawing relate
to the L value described later.
[0032] The cold working die steel of this invention can further
contain, as steel components, either one of or both of
0.0030%.ltoreq.N.ltoreq.0.0500% and
0.001%.ltoreq.P.ltoreq.0.040%.
[0033] These elements are inevitably contained in the steel. Large
contents of these elements decrease the toughness, so it is
preferable to control them to as much as the upper limit or below.
A positive effort of reducing these elements, in a trade-off manner
with the manufacturing cost, makes it possible to maintain a stable
and high toughness. It is to be understood that excessive lowering
in the contents only results in an increase in cost while allowing
an effect on the toughness to saturate, so that it is preferable to
control them to the lower limit or above.
[0034] The cold working die steel of this invention can further
contain, as steel components, any one of, or two or more of steel
components selected from 0.01%.ltoreq.Cu.ltoreq.1.0%,
0.01%.ltoreq.Ni.ltoreq.1.0%, 0.2%.ltoreq.Co.ltoreq.1.0% and
0.0003%.ltoreq.B.ltoreq.0.010%.
[0035] These elements have an effect of increasing the
hardenability by dissolving themselves into the matrix. They have
also an effect of increasing the toughness by lowering the impact
transition temperature, and by consequently preventing the
weldability from degrading. Co has an effect of increasing the high
temperature strength. Addition to as much as the lower limit or
above is preferable in view of obtaining these effects. The
elements are added to as much as the upper limit or less, because
excessive addition only results in a saturated effect.
[0036] The cold working die steel of this invention can further
contain, as steel components, any one of, or two or more of
0.001%.ltoreq.S.ltoreq.0.20%, 0.005%.ltoreq.Se.ltoreq.0.10%,
0.005%.ltoreq.Te.ltoreq.0.10%, 0.0002%.ltoreq.Ca.ltoreq.0.010%,
0.005%.ltoreq.Pb.ltoreq.0.10% and
0.005%.ltoreq.Bi.ltoreq.0.10%.
[0037] S can be added as an element increasing the free cutting
property. Depending on the contents of the carbide-forming elements
such as Cr, Mo and V, addition to as much as 0.04% or more is
preferable in view of obtaining the effect of increasing the free
cutting property. Excessive addition of the element considerably
decreases the toughness, or mechanical properties including surface
roughness after discharge processing and cutting, so that the upper
limit is preferably adjusted to 0.20%. For applications making a
great account of machinability, the element is added considering
the balance with the mechanical properties. On the other hand, for
applications in which a greater account is placed on the mechanical
properties rather than on the machinability, the amount of addition
of S is set to 0.02% or less, and more preferably to 0.01% or less,
considering the balance with the cost for manufacturing. The
mechanical properties can be satisfied by adjusting the amount of
the addition to 0.01% or more and 0.02% or less for the practical
operation. This makes it possible to obtain the S content described
in the above.
[0038] Also Se, Te, Ca, Pb and Bi can be added for the purpose of
increasing machinability. Se and Te can be used as substitutive
elements of S in Mn sulfide. Ca improves the machinability by
forming a protective film on the surface of a tool during cutting,
by forming an oxide or by dissolving itself into Mn sulfide. Pb and
Bi segregate in the grain boundary, to thereby lower the grain
boundary strength and to improve the machinability. The elements
are necessarily added to as much as the lower limits or more in
view of obtaining these effects. On the other hand, excessive
addition results in degraded mechanical properties, so that the
upper limits should be met.
[0039] The cold working die steel of this invention can further
contain, as the steel components, any one of, or two or more of
0.01%.ltoreq.Nb.ltoreq.0.12%, 0.005%.ltoreq.Ta.ltoreq.0.10%,
0.005%.ltoreq.Ti.ltoreq.0.10%, 0.005%.ltoreq.Zr.ltoreq.0.10%,
0.005%.ltoreq.Mg.ltoreq.0.10% and
0.005%.ltoreq.REM.ltoreq.0.10%.
[0040] These elements can be added for the purpose of obtaining an
effect of increasing the toughness through fineness of the carbide
grains and fineness of the crystal grains. Mg and REM have an
effect of increasing the toughness and machinability, through
formation of oxides, which contribute to the reduction of the O
content and consequently in coarse oxide grains. Addition to as
much as the lower limit or above is preferable in view of obtaining
these effects. On the other hand, excessive addition results in
decreasing in the toughness and weldability, so that the amount of
addition is preferably set to the upper limit or less. A single
species or two or more species of rare earth metals may be used as
REM.
[0041] Next, the cold working die steel according to the second
aspect may contain, as the steel components,
0.60%.ltoreq.C.ltoreq.0.80%, 0.10%.ltoreq.Si.ltoreq.1.20%,
0.10%.ltoreq.Mn.ltoreq.0.60%, 5.5%.ltoreq.Cr.ltoreq.8.5%,
0.80%.ltoreq.Mo+0.5W.ltoreq.2.10%, 0.10%.ltoreq.V.ltoreq.0.40%,
0.0002%.ltoreq.Al.ltoreq.0.0080%. In other words, further
limitations are added to C, Si, Cr and Mo, out of the steel
components according to the first aspect.
[0042] It is essential for cold working die steel die applications,
that the toughness and fine machining are particularly required to
decrease the coarse carbide. More specifically, it is necessary to
avoid as much as possible, the formation of coarse carbides mainly
composed of M.sub.7C.sub.3 (where, M represents Cr, Mo or V), by
adjusting the contents of C, Si, Cr and Mo within the
above-described ranges. The amount of coarse carbide corresponds to
0.01 to 5% in % by mass. Assuming now L value as an index
expressing the amount of coarse carbide as Cr(mass
%)+15.5.times.C(mass %), the amount of coarse carbide corresponds
to 14.9.ltoreq.(L value).ltoreq.21.0 (see FIG. 1).
[0043] Next, the cold working die steel according to the third
aspect may contain, as the steel components,
0.90%.ltoreq.C.ltoreq.1.10%, 0.8%.ltoreq.Si.ltoreq.1.20%,
0.10%.ltoreq.Mn.ltoreq.0.60%, 7.0%.ltoreq.Cr.ltoreq.9.0%,
1.50%.ltoreq.Mo+0.5W.ltoreq.2.10%, 0.10%.ltoreq.V.ltoreq.0.40%,
0.0002%.ltoreq.Al.ltoreq.0.0080%. In other words, further
limitations are added to C, Si, Cr and Mo, out of the steel
components according to the first aspect.
[0044] It is necessary for cold working die steel die applications,
that the wear resistance and the toughness must be well-balanced,
to ensure a certain amount of coarse carbides. More specifically,
the coarse carbides mainly composed of M.sub.7C.sub.3, are formed
by adjusting the contents of C, Si, Cr and Mo within the
above-described ranges. The amount of coarse carbides corresponds
from 5 to 10% in % by mass, and to 21.0.ltoreq.(L
value).ltoreq.27.0 (see FIG. 1).
[0045] Next, the cold working die steel according to the fourth
aspect may contain, as the steel components,
1.40%.ltoreq.C.ltoreq.1.60%, 0.10%.ltoreq.Si.ltoreq.0.40%,
0.10%.ltoreq..ltoreq.Mn.ltoreq.0.60%, 11.0%.ltoreq.Cr.ltoreq.13.0%,
0.80%.ltoreq.Mo+0.5W.ltoreq.1.20%, 0.10%.ltoreq.V.ltoreq.0.40%,
0.0002%.ltoreq.Al.ltoreq.0.0080% In other words, further
limitations are added to C, Si, Cr and Mo, out of the steel
components according to the first aspect.
[0046] It is necessary for cold working die steel die applications,
where the wear resistance is particularly required, to contain a
large amount of coarse carbide. More specifically, large amounts of
coarse carbides mainly composed of M.sub.7C.sub.3 are formed by
adjusting the contents of C, Si, Cr and Mo within the
above-described ranges. The amount of coarse carbide corresponds to
10 to 15% in % by mass, and to 27.0.ltoreq.(L value).ltoreq.37.8
(see FIG. 1).
BRIEF DESCRIPTION OF THE DRAWING
[0047] FIG. 1 is a drawing expressing relations between C content
and Cr content (K value, L value).
BEST MODES FOR CARRYING OUT THE INVENTION
[0048] First, 200 kg of each of the inventive and comparative steel
materials having compositions listed in Table 1 were melted in a
vacuum induction furnace, made into an ingot, and the obtained
steel ingot was hot-forged so as to produce a 70 mm.times.70 mm
square rod. The rod was then annealed at the temperature listed in
Table 2 (cooling rate: 18.degree. C./h).
[0049] Of the compositions of comparative steels listed in Table 1,
those departing from the compositional ranges specified by this
invention are indicated by a downward arrow (.dwnarw.) if they came
short of the lower limits, by an upward arrow (.uparw.) if they
exceeded the upper limits.
[0050] [Table 1] TABLE-US-00001 TABLE 1 Components (wt %) No. C Si
Mn P S Cu Ni Cr Mo W 1 Comparative steel 0.71 0.38 .uparw.0.93
0.028 .uparw.1.32 0.11 7.03 0.84 2 Inventive steel 1.23 0.53 0.34
0.06 0.43 10.32 1.44 3 Comparative steel 0.71 0.38 0.37 0.028 0.002
0.09 .uparw.1.55 7.03 0.84 4 Comparative steel 0.66 0.42 0.32 0.013
0.001 0.23 0.53 10.5 0.32 5 Inventive steel 0.61 0.95 0.47 0.004
0.063 0.05 0.13 5.7 2.01 6 Inventive steel 0.78 0.75 0.21 0.035
0.035 0.85 0.53 5.81 2.09 7 Inventive steel 0.93 0.23 0.15 0.021
0.001 0.34 0.43 6.53 0.82 8 Inventive steel 0.79 0.35 0.32 0.003
0.23 0.03 8.82 0.93 9 Inventive steel 0.62 0.11 0.11 0.028 0.058
0.53 0.84 7.53 1.56 10 Inventive steel 0.72 0.69 0.55 0.013 0.008
0.13 0.08 7.03 1.03 11 Inventive steel 0.78 1.17 0.59 0.018 0.17
0.25 0.31 6.53 0.81 0.02 12 Inventive steel 0.71 1.18 0.58 0.022
0.008 0.023 0.89 8.2 0.88 0.83 13 Inventive steel 0.83 0.11 0.42
0.006 0.004 0.14 0.67 5.96 1.65 14 Comparative steel 0.93 0.89 0.31
0.022 .uparw.0.25 0.08 0.33 .uparw.13.94 1.83 15 Comparative steel
0.99 0.89 0.52 0.012 0.003 0.35 0.46 6.34 1.93 16 Inventive steel
0.94 0.93 0.41 0.011 0.001 0.02 0.04 6.63 1.84 17 Inventive steel
1.19 0.83 0.22 0.001 0.14 0.75 0.23 8.32 1.74 0.11 18 Inventive
steel 1.07 0.85 0.39 0.013 0.003 0.05 0.13 10.71 1.63 19 Inventive
steel 0.81 0.93 0.18 0.032 0.002 0.13 0.45 8.83 1.99 0.03 20
Inventive steel 0.92 1.05 0.59 0.021 0.095 0.34 0.63 8.21 2.03 21
Inventive steel 0.88 1.17 0.11 0.009 0.103 0.23 0.09 7.58 1.54 22
Inventive steel 0.99 1.08 0.58 0.013 0.003 0.02 0.75 9.03 2.08 23
Inventive steel 0.99 0.42 0.11 0.023 0.38 0.08 0.09 8.25 0.97 24
Inventive steel 0.86 0.93 0.11 0.025 0.001 0.011 0.98 9.32 1.55 25
Comparative steel .uparw.1.83 0.32 0.53 0.001 0.09 0.31 11.95
.dwnarw.0.43 26 Comparative steel 1.53 0.13 0.43 0.011 0.001 0.21
0.33 9.31 0.88 0.31 27 Inventive steel 1.23 0.29 0.46 0.018 0.001
0.04 0.08 8.53 0.83 0.22 28 Inventive steel 1.58 0.11 0.23 0.005
0.11 0.32 0.32 11.44 0.95 29 Inventive steel 1.56 0.18 0.12 0.029
0.002 0.03 0.98 12.98 0.93 0.31 30 Inventive steel 1.44 0.21 0.18
0.003 0.08 0.09 0.75 12.84 1.18 31 Inventive steel 1.08 0.31 0.22
0.002 0.002 0.21 0.63 10.73 1.09 32 Inventive steel 1.43 0.37 0.34
0.009 0.083 0.85 0.15 12.59 1.05 0.09 33 Inventive steel 1.23 0.23
0.57 0.028 0.052 0.43 0.44 10.39 1.02 34 Inventive steel 1.45 0.97
0.36 0.038 0.034 0.88 0.1 12.78 0.76 1.33 35 Inventive steel 1.3
0.29 0.43 0.024 0.27 0.91 0.06 12.01 1.85 No. V Al O N K value L
value Others 1 0.35 0.028 0.0009 2.202 18.035 2 0.15 0.093 0.0053
1.956 29.385 3 .uparw.1.34 0.028 0.0009 0.019 2.202 18.035 4 0.11
0.033 0.0021 0.011 .uparw.6.012 20.73 5 0.21 0.008 0.0023 0.015
1.552 15.155 6 0.39 0.03 0.0053 0.004 0.506 17.9 7 0.11 0.09 0.0003
0.0033 0.206 20.945 8 0.12 0.053 0.0009 0.0085 3.448 21.065 9 0.11
0.08 0.0004 0.009 3.314 17.14 10 0.14 0.032 0.0063 0.013 2.134
18.19 11 0.33 0.021 0.0078 0.024 1.226 18.62 12 0.13 0.012 0.0013
0.012 3.372 19.205 B = 0.0032 13 0.22 0.096 0.0043 0.048 0.316
18.825 Zr = 0.022, Ta = 0.07, Mg = 0.09 14 0.22 0.053 0.0031 0.011
.uparw.7.616 28.355 15 0.32 0.023 0.0039 0.016 .dwnarw.-0.392
21.685 16 0.31 0.011 0.0019 0.009 0.238 21.2 17 0.21 0.022 0.0043
0.0395 0.228 26.765 18 0.25 0.034 0.0041 0.0312 3.434 27.295 19
0.33 0.048 0.0077 0.0213 3.322 21.385 20 0.29 0.028 0.0063 0.0183
1.954 22.47 21 0.19 0.035 0.0031 0.018 1.596 21.22 22 0.37 0.059
0.0019 0.031 2.298 24.375 23 0.18 0.001 0.0002 0.0035 1.518 23.595
Mo = 0.02, Ti = 0.043, REM = 0.05 24 0.39 0.032 0.0079 0.01 3.472
22.65 Ca = 0.0053, Te = 0.026, Bi = 0.024 25 0.34 0.099 0.0009
.dwnarw.-0.494 40.315 26 0.25 0.008 0.0019 0.011 .dwnarw.-1.094
33.025 27 0.27 0.005 0.0023 0.014 0.166 27.595 28 0.16 0.008 0.0003
0.0185 0.696 35.93 29 0.33 0.005 0.0053 0.0045 2.372 37.16 30 0.23
0.093 0.0005 0.0093 3.048 35.16 31 0.38 0.053 0.0078 0.023 3.386
27.47 32 0.11 0.083 0.0015 0.0075 2.866 34.755 33 0.32 0.053 0.0034
0.0053 2.026 29.455 34 0.25 0.012 0.0035 0.011 2.92 35.255 Co =
0.36 35 0.33 0.004 0.0031 0.013 3.17 32.16 Pb = 0.09, Se = 0.09
[0051] The inventive steels and comparative steels were subjected
to the tests and evaluations below. Results are shown in Table
2.
(a) Surface Analysis and Evaluation of Carbide
[0052] The polished surface of each steel was subjected to image
analysis and the mean grain size of the carbide was measured. The
image analysis was made on an image observed under a SEM, wherein a
total of 1 mm.sup.2 area was observed at an appropriate
magnification ranging from 500.times. to 5,000.times.. The
circle-equivalent diameters were calculated for all carbide grains
having a diameter ranging from 0.1 .mu.m to 3.0 .mu.m seen in the
field of view, and thereby obtained an average mean value. The
polished surface herein was etched with a picric acid-ethanol
solution to a depth allowing observation of the carbide grains
having a diameter of about 0.1 .mu.m, without causing dropping of
the carbide grains.
(b) Amounts of Group B and C Inclusions
[0053] According to the test method specified by JIS G0555,
(dB+dC)60.times.400 was measured.
(c) Machinability Test
[0054] A test piece was cut out from each of the manufactured
inventive steels and comparative steels, and subjected to the
machinability test.
Conditions for Endmill Processing Test
[0055] Tool: carbide solid endmill (.phi.=10 mm), six-flute
[0056] Cutting speed: 120 m/min
[0057] Feed: 0.06 mm/rev
[0058] Cutting width: 0.5 mm, 10 mm in height
[0059] Lubricant: air blow
[0060] Cutting length: up to 60,000 mm
[0061] Judgment: marked with .smallcircle. if the tool caused no
breakage, and marked with x if the tool caused breakage or sparking
during the cutting.
(c) Cold Workability
[0062] Test pieces measuring 12.times.18 mm were cut out from each
of the inventive steels and the comparative steels, and pressed by
a single stroke to 60% height of the test piece using a 600-t
hydraulic press machine. Ten pressed test pieces of the individual
steels were observed, and the number of test pieces causing
breakage was found.
(e) Maximum Hardness
[0063] Hardness was measured using a Rockwell C scale, under varied
annealing conditions in the quench-and-temper.
(f) Specific Wear
[0064] Specific wear was measured using a pin-on-disk friction and
wear tester. Two 8-mm-diameter pins were cut out from the
individual manufactured inventive steels and the comparative
steels. A disk was cut out from S45C. The inventive steels and the
comparative steels were subjected to quench-and-temper under which
the maximum hardness can be attained. Test conditions included a
slipping rate of 1.6 m/s, and a press load of 10.5 kgf, with no
lubricant. The weight of the pins was measured before and after the
test, and loss of weight by wear was measured. The specific wear of
the inventive steels and the comparative steels was evaluated,
assuming that the weight loss by the wear of comparative steel was
1 as one.
(g) Charpy Impact Value
[0065] A 10R-notched Charpy test piece was cut out from each of the
manufactured inventive steels and the comparative steels. The
direction of the test pieces was aligned to the longitudinal
direction of the material. Under annealing conditions yielding the
maximum hardness, the test was carried out according to the method
described in JIS Z2242. The test was carried out under room
temperature.
[0066] [Table 2] TABLE-US-00002 TABLE 2 Mean circle- Charpy Ar3
equivalent diameter Annealing After-SA Maximum Cold impact
temperature of the carbide Inclusion temperature hardness hardness
work- Machin- Specific value No. (.degree. C.) (.mu.m) dB + dC
(.degree. C.) (HB) (HRC) ability ability wear (J/cm2) 1 Comparative
steel .dwnarw. 740 .sup. .dwnarw. 0.21 .sup. 0.005 .dwnarw. 750
.sup. .uparw. 269 .sup. 60.3 x8 x 1 53 2 Inventive steel 793 0.71
0.002 980 204 63.2 .smallcircle. .smallcircle. 0.32 21 3
Comparative steel .dwnarw. 733 .sup. .uparw. 1.53 .sup. .uparw.
0.41 .sup. .uparw. 1140 .sup. .dwnarw. 170 .sup. 60.4 .smallcircle.
x 1.03 46 4 Comparative steel 831 0.53 0.003 950 204 54.3
.smallcircle. .smallcircle. 5.93 53 5 Inventive steel 805 0.45
0.001 970 212 61.3 .smallcircle. .smallcircle. 0.92 63 6 Inventive
steel 822 0.58 0.034 1020 208 63.3 .smallcircle. .smallcircle. 0.93
68 7 Inventive steel 803 0.38 0.027 960 198 63.5 .smallcircle.
.smallcircle. 0.89 73 8 Inventive steel 755 0.35 0.046 960 199 62.9
.smallcircle. .smallcircle. 0.93 58 9 Inventive steel 790 0.26
0.003 970 203 64.2 .smallcircle. .smallcircle. 0.89 66 10 Inventive
steel 809 0.44 0.002 970 203 63.8 .smallcircle. .smallcircle. 0.88
67 11 Inventive steel 833 0.58 0.011 990 185 62.87 .smallcircle.
.smallcircle. 0.84 93 12 Inventive steel 813 0.65 0.001 1030 204
62.7 .smallcircle. .smallcircle. 0.93 81 13 Inventive steel 836
0.28 0.006 1030 233 62.9 .smallcircle. .smallcircle. 0.99 88 14
Comparative steel .uparw. 876 .sup. 0.39 .uparw. 0.39 .sup. 970 226
51.1 .smallcircle. x 6.33 67 15 Comparative steel 843 0.67 0.008
950 193 53.2 .smallcircle. .smallcircle. 6.29 93 16 Inventive steel
848 0.52 0.003 990 198 62.5 .smallcircle. .smallcircle. 0.63 43 17
Inventive steel 845 0.78 0.048 990 183 64.3 .smallcircle.
.smallcircle. 0.55 44 18 Inventive steel 833 0.47 0.036 990 195
64.1 .smallcircle. .smallcircle. 0.59 48 19 Inventive steel 828
0.36 0.021 930 199 64.8 .smallcircle. .smallcircle. 0.63 53 20
Inventive steel 773 0.31 0.012 840 225 63.8 .smallcircle.
.smallcircle. 0.73 41 21 Inventive steel 819 0.63 0.001 930 203
63.7 .smallcircle. .smallcircle. 0.72 39 22 Inventive steel 835
0.65 0.003 980 211 63.2 .smallcircle. .smallcircle. 0.72 46 23
Inventive steel 832 0.71 0.007 890 191 62.8 .smallcircle.
.smallcircle. 0.73 52 24 Inventive steel 825 0.68 0.001 930 200
62.7 .smallcircle. .smallcircle. 0.72 37 25 Comparative steel
.uparw. 883 .sup. .dwnarw. 0.11 .sup. 0.002 .dwnarw. 900 .sup.
.uparw. 266 .sup. 48.7 x10 x 3.14 11 26 Comparative steel 817 0.73
0.004 980 213 51.3 .smallcircle. .smallcircle. 3.82 13 27 Inventive
steel 795 0.71 0.002 890 200 62.7 .smallcircle. .smallcircle. 0.32
19 28 Inventive steel 808 0.68 0.043 1000 201 63.3 .smallcircle.
.smallcircle. 0.31 23 29 Inventive steel 813 0.57 0.007 1000 205
63.2 .smallcircle. .smallcircle. 0.46 22 30 Inventive steel 803
0.55 0.011 940 209 64.7 .smallcircle. .smallcircle. 0.49 28 31
Inventive steel 755 0.49 0.028 920 205 62.1 .smallcircle.
.smallcircle. 0.28 27 32 Inventive steel 761 0.68 0.032 990 217
61.8 .smallcircle. .smallcircle. 0.38 17 33 Inventive steel 783
0.73 0.003 910 218 63.3 .smallcircle. .smallcircle. 0.37 19 34
Inventive steel 796 0.41 0.006 870 231 63.6 .smallcircle.
.smallcircle. 0.31 20 35 Inventive steel 807 0.29 0.007 860 223
64.4 .smallcircle. .smallcircle. 0.45 29
[0067] As is known from Table 2, comparative steel 1, having a
composition departing from the compositional ranges specified by
this invention, showed an extremely lowered Ar3 temperature. The
steel annealed under conventional annealing conditions failed to
fully solubilize the carbide into solid, and failed in allowing the
carbide to grow larger under gradual cooling, so that the carbide
became smaller in size, and the steel became harder. The steel was
consequently poor in the cold workability, showing cracks in 8 out
of ten test pieces.
[0068] Comparative steel 3, having a composition departing from the
compositional ranges specified by this invention, showed an
extremely lowered Ar3 temperature. Too high a temperature in the
quench-and-temper resulted in an extremely lowered hardness,
increased ductility of the material, and conversely degraded
machinability, although the cold workability was judged as
desirable by virtue of a large carbide size and a considerably
lowered hardness.
[0069] Comparative steel 4 showed a K value largely departing from
the inventive range, despite having a composition within the
compositional ranges specified by this invention. The steel
therefore failed in achieving a maximum hardness of as large as
HRC60 or above after annealing by the quench-and-temper, and failed
in obtaining a level of hardness required for cold working die
steel. The low hardness also resulted in a large specific wear.
Comparative steels 14, 15, 25 and 26, again having K values
departing from the inventive range, also resulted in small maximum
hardness and larger specific wear.
* * * * *