U.S. patent application number 13/583288 was filed with the patent office on 2013-09-19 for tool steel for extrusion.
The applicant listed for this patent is Celso Antonio Barbosa, Rafael Agnelli Mesquita. Invention is credited to Celso Antonio Barbosa, Rafael Agnelli Mesquita.
Application Number | 20130243639 13/583288 |
Document ID | / |
Family ID | 44562754 |
Filed Date | 2013-09-19 |
United States Patent
Application |
20130243639 |
Kind Code |
A1 |
Barbosa; Celso Antonio ; et
al. |
September 19, 2013 |
TOOL STEEL FOR EXTRUSION
Abstract
The present invention relates to a steel for extrusion tools
characterized for lower cost and tempering resistance higher than
that of conventional steel H13, whose chemical composition, in
percentage by mass, comprises the following: Carbon between 0.40
and 0.60, Silicon below 1.0, Phosphorus below 0.030; Chromium
between 2.5 and 4.5; Molybdenum between 0.5 and 0.7, considering
that molybdenum can be replaced by tungsten in a ratio=2W/1Mo;
Vanadium between 0.10 and 1.0; Manganese below 1.0; the remainder
consisting essentially of Fe and inevitable deleterious substances.
As an option to provide high hardness after nitriding, the Al
content of the steel of the present invention can be .ltoreq.1.0;
for high toughness purposes, however, this Al content should be
kept below 0.10.
Inventors: |
Barbosa; Celso Antonio;
(Carolina, BR) ; Mesquita; Rafael Agnelli;
(Carolina, BR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Barbosa; Celso Antonio
Mesquita; Rafael Agnelli |
Carolina
Carolina |
|
BR
BR |
|
|
Family ID: |
44562754 |
Appl. No.: |
13/583288 |
Filed: |
March 4, 2011 |
PCT Filed: |
March 4, 2011 |
PCT NO: |
PCT/BR11/00059 |
371 Date: |
December 13, 2012 |
Current U.S.
Class: |
420/91 |
Current CPC
Class: |
C22C 33/02 20130101;
C22C 38/06 20130101; C22C 38/42 20130101; C22C 38/52 20130101; C22C
38/46 20130101; C21D 9/00 20130101; C22C 38/24 20130101; C22C 21/00
20130101; C22C 38/22 20130101; C22C 38/04 20130101; C22C 38/44
20130101; C22C 38/02 20130101 |
Class at
Publication: |
420/91 |
International
Class: |
C22C 38/52 20060101
C22C038/52; C22C 38/44 20060101 C22C038/44; C22C 38/02 20060101
C22C038/02; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/46 20060101 C22C038/46; C22C 38/42 20060101
C22C038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 8, 2010 |
BR |
PI 1003185-5 |
Claims
1-16. (canceled)
17. STEEL FOR EXTRUSION TOOLS, characterized by a composition of
alloying elements that comprise, in percent by mass: carbon between
0.40 and 0.60, silicon below 1.0, phosphorus below 0.030, chrome
between 2.5 and 4.5, molybdenum between 0.5 and 0.7, vanadium
between 0.10 and 1.0, manganese below 1.0, aluminum between 0.1 and
1.0, being the remaining composed of Fe and inevitable deleterious
substances.
18. STEEL FOR EXTRUSION TOOLS, characterized by a composition of
alloying elements that comprise, in percent by mass, carbon between
0.40 and 0.60, silicon below 0.50, phosphorus below 0.030, chrome
between 3.0 and 4.2, molybdenum between 0.55 and 0.65, vanadium
between 0.30 and 0.8, manganese below 0.8, aluminum between 0.2 and
0.80, being the remaining composed of Fe and inevitable deleterious
substances.
19. STEEL FOR EXTRUSION TOOLS, characterized by a composition of
alloying elements consisting essentially of, in percent by mass,
carbon between 0.45 and 0.55, silicon below 0.5, phosphorus below
0.030, chrome between 3.5 and 4.2, molybdenum between 0.55 and
0.65, vanadium between 0.30 and 0.50, manganese below 0.50,
aluminum between 0.4 and 0.60, being the remaining composed of Fe
and inevitable deleterious substances.
20. STEEL FOR EXTRUSION TOOLS, according to claim 17, characterized
by having molybdenum replaced with tungsten in a ratio of
1Mo=2W.
21. STEEL FOR EXTRUSION TOOLS, according to claim 17, characterized
by having vanadium replaced with niobium or titanium in a ratio of
1V=2Nb or 1Ti.
22. STEEL FOR EXTRUSION TOOLS, according to claim 17, characterized
by being applied to molds, dies and general usage tools, for
forming solid and liquid materials, at room temperature or
temperatures at 1300.degree. C.
23. STEEL FOR EXTRUSION TOOLS, according to claim 17, characterized
by being applied to tools for forming metals at temperatures
between 300 and 1300.degree. C., in applications of forging,
extrusion or casting of ferrous or non-ferrous alloys.
24. STEEL FOR EXTRUSION TOOL, according to claim 17, characterized
by being applied to non-ferrous alloy hot extrusion tools,
particularly aluminum alloys, and to solid shape or pipe extrusion
dies.
25. STEEL FOR EXTRUSION TOOL, according to claim 17, characterized
by being produced for processes involving casting of billets and
hot and cold forming, or even used with the gross structure of
melting.
26. STEEL FOR EXTRUSION TOOL, according to claim 17, characterized
by being produced for processes involving fragmentation of liquid
metal, such as powder metallurgy, powder injection or the process
of forming by spray
Description
[0001] The present invention relates to a steel intended for use in
various hot form tools and dies, particularly for extrusion of
aluminum alloys or other non-ferrous metals. Although initially
designed for extrusion processes, the material can also be employed
in other hot forming processes, in which the metal to be formed
withstands temperatures above 600.degree. C., although the said
steel can be employed in processes at lower temperatures or even at
ambient temperature. The composition of the steel in question
allows it to be classified as hot work tool steel, whose primary
characteristic the lower content of high-cost alloying elements,
such as molybdenum and vanadium, but with tempering resistance (or
resistance to loss of hardness) greater than that of conventional
steels of prior art concept. An additional alternative to the steel
of the present invention is provided to increase hardness after
nitriding, and may result in performance levels even greater than
those of conventional steels, at the same time that the cost is
kept low due to a simpler chemical composition. Such an effect is
possible by carefully designing the alloy, and setting the optimum
ranges of the elements: carbon, chromium, molybdenum and
aluminum.
[0002] The term hot work tools is applied to a large number of
hot-forming operations, employed in industries and focused on the
production of parts for mechanical applications, especially
automotive parts. The most popular hot-forming processes are
forging of steel, and the extrusion or casting of non-ferrous
alloys. Other applications performed at high temperature, typically
above 500/600.degree. C., can also be classified as hot work. In
these applications, molds, dies, punches, inserts and other forming
devices are classified by the generic term: hot work tools. These
tools are usually made of steels, which require special properties
to withstand high temperatures and the mechanical efforts of the
processes in which those tools are employed.
[0003] Among their key properties of hot work steels, the following
stand out: resistance after high temperature tempering, the
resistance to the loss of hardness called tempering resistance, the
toughness, the hardenability and physical properties such as
thermal conductivity and specific heat.
[0004] The extrusion dies used for non-ferrous alloys, especially
aluminum alloys are main hot work target for applying the steel of
the present invention. These typical dies comprise an important
segment of the tool steel market both in Brazil and abroad. In this
application, the steels are very standardized, based on steels such
as ABNT H13 (see Table 1), with quality requirements not as strict
as those of other applications, e.g., pressure die casting, but
with emphasis on lower production costs.
[0005] The increased cost of metal alloys, especially Mo and V,
significantly impaired this segment, making it eager for low-cost
alternatives. Low-alloy steels have been employed, such as DIN
1.2714 (chemical composition given in Table 1). However, their low
wear resistance due to reduced hot strength and lower
post-nitriding hardness prevents them from being applied.
[0006] Recent developments, such as US 2009/0191086, were focused
on the reduction of alloying elements, by means of reduced Cr, Mo
and V content. However, negative effects are produced by reducing
the Cr content. First, the alloys' composition is not sufficient to
achieve high hardness after tempering (at least 45 HRC after
tempering at 600.degree. C.). Second, a reduced Cr content can also
generate lower hardness after nitriding, which is not suitable for
extrusion applications, considering the apparent gain produced by
nitridation in these applications (virtually all extrusion dies are
currently nitrified).
TABLE-US-00001 TABLE 1 Typical chemical composition of steels of
prior art concept. The sum Mo + V + Co is shown because these
elements have the highest cost, and are closely related to the
final cost of the tool steel. Content in percentage by mass and Fe
balance. For all extrusion applications the W content is
low,usually <0.1%. Designation C Si Mn Ni P Cr Mo V Al H13 0.38
1.0 0.3 0.3 0.025 5.0 1.2 1.0 <0.05 DIN 1.2714 0.56 0.3 0.7 1.7
0.025 1.1 0.5 0.1 <0.05 US 2009/0191086 0.38 0.5 1.3 0.3 0.009
2.4 0.6 0.5 <0.05 or 0.53
[0007] A third problem of invention US 2009/0191086 relates to the
hardness of the die core, which may be lower due to decreased
hardenability as a result of reduced Cr and Mo contents. To avoid
this, the alloys of invention US 2009/0191086 have higher Mn
content, which lead to higher hardenability, potential segregation
problems (banding) and excessive austenite retention. Both effects
may impair the final hardness and toughness and, thus, the tool
life. A final aspect can also be mentioned, with regard to the high
Mn content: Scrap from this steel can hardly be incorporated into
the production of conventional, low-Mn-content hot work steels.
[0008] Given all these drawbacks, invention US 2009/0191086 is
considered by the authors as a cost-reducing solution, but with
inferior properties. In the text of the patent, the authors
quantify the expected efficiency loss, of about 20 to 30% lower
than that of steel H13. Considering the machining and heat
treatment costs associated with dies, this efficiency loss can be
considered quite significant, thus requiring a reduction of the
cost of the material by more than 30% to compensate the
substitution. For example, considering that only 60% of the final
die cost is associated with the tool steel used, a 30% lower life
can only be viable if the cost of the new material is half the cost
of the conventional material. From 2005 to 2008, when the cost of
raw materials peaked, this could be true (though still difficult to
occur, because the cost difference required is too high). However,
for the current scenario, such cost reduction can hardly be
achieved for steel H13, considering only the reduction of the Mo
and Cr contents. Thus, reduction in cost associated with efficiency
loss of the alloy of patent US 2009/0191086 can be currently
considered impractical for such application.
[0009] Given this scenario, it is evident the need for a tool steel
which effectively has a positive effect on tool life by means of an
equivalent performance, but at a cost lower than that of steel H13.
This is only possible if the steel in question has tempering
resistance and hardness after tempering at 600.degree. C. (typical
heat treatment condition) equivalent to those of steel H13, but
with lower content of alloying elements and suitable hardness after
nitriding. In addition, the material used must have high
hardenability, but free of problems associated with high Mn
content, thus allowing it to be applied to tools larger than
extrusion dies.
[0010] Therefore, the steel of the present invention will fulfill
all these needs.
[0011] To achieve the cost reduction/zero quality loss goal, the
effect of the key elements related to hot strength, Cr and Mo, was
studied separately. Apart from significant findings, this study
also showed that the variation of the content of these elements is
not sufficient to promote the hot strength required. Thus, the C
content could be increased up to levels that did not impact
toughness, especially whit the accompaniment of low P and Si
contents. Finally, the Al effect was used to compensate for the
reduction of Cr, hence, a potential lower hardness after nitriding.
This work also focused on this issue because the nitrified layer is
critical to providing wear resistance to various hot forming tools,
especially extrusion and hot forging tools.
[0012] Therefore, in order to satisfy the above conditions, the
steel of the present invention has a composition of alloying
elements, which, in percentage by mass consists of: [0013] 0.40 to
0.60 C, preferably 0.45 to 0.55 C, typically 0.50 C [0014] 2.5 to
4.5 Cr, preferably 3.0 to 4.2 Cr, typically 3.8 Cr [0015] 0.30 to
0.90 Mo, preferably 0.50 to 0.70 Mo, typically 0.60 Mo. Given its
chemical similarity to W, Mo can be replaced with W, 2W:1Mo ratio
by mass. [0016] 0.1 to 1.0 V, preferably 0.3 to 0.8 V, typically
0.4 V; V can be partially or fully replaced with Nb, following a 1
Nb:0.5 V ratio by mass. [0017] up to 1.0 Si, preferably up to 0.50
Si, typically 0.30 Si Max 1.0 Mn, preferably max 0.80 Mn, typically
max 0.50 Mn.
[0018] As described below, Al can be added simultaneously to the
alloys of the present invention to provide gains in terms of
hardness after nitriding, but also negative effects in terms of
toughness and complexity of the steel-making process. Thus, the Al
content must be dosed as follows, in percentage by mass: [0019] Max
1.0 Al, preferably max 0.80 Al, typically max 0.60 Al. For
compositions in which the effects of Al are not targeted, this
element should be treated as residual impurity, limited to 0.10,
typically <0.05.
[0020] The compositions should be characterized by balance by Fe
(iron) and metallic or non-metallic deleterious substances
inevitable to the steelmaking process, in which said non-metallic
deleterious substances include but are not limited to the following
elements, in percentage by mass:
[0021] Max 0.030 P, preferably max 0.015 P, typically max 0.010
P.
[0022] Max 0.10 S, preferably max 0.030 S, typically max 0.008
S.
[0023] Max 1.5 Ni or Co, preferably up to 1.0 Ni or Co, typically
below 0.5 Ni and Co.
[0024] Next, we describe the ratios of the specification of the
composition of the new material. The percentages listed refer to
percent by mass.
[0025] C: Carbon is primarily responsible for martensite hardening
under low temperature conditions. However, together with the
alloying elements, carbon also plays a role in the secondary
hardening, important for the hardening at high temperature. In
these cases, the C content is more important for hardness at
temperatures below 600.degree. C., when hardness still depends on
the martensite hardness or formation of cementite or Cr carbides.
Furthermore, carbon is an important hardenability-promoting
element, and causes no increase in cost. It is also considered
important to increase hardness to 45 HRC and up, carbon contents of
at least 0.40% are recommended, preferably above 0.45%. On the
other hand, very high C contents, cause excessive precipitation of
grain-shaped carbides at the time of quenching (especially when Mo
and V contents are high), as well as lead to increased hardness and
volume of secondary carbides. Thus, toughness is generally
impaired, the C content should be limited to a maximum value of
0.60%, preferably below 0.55%. This limitation also plays a role in
the reduction of the amount of retained austenite, preventing
problems associated with dimensional instability and
embrittlement.
[0026] Cr: The chromium content should be higher than 2.5%,
preferably greater than 3.0%, because this element favors
hardenability, which is important for application in large tools.
However, the Cr content should be limited. The present invention
has incorporated the concept of reducing the Cr content to improve
tempering resistance. The mechanisms of this effect are not fully
understood but they may be related to the formation of secondary Cr
carbides, M.sub.7C.sub.3-type, which dissolve Mo and V are the
first carbides to be formed. Therefore, the lower the Cr content,
the lower the amount of M.sub.7C.sub.3 carbides and, thus, the
greater the amount of Mo and V available for the formation of fine
carbides M.sub.2C and MC, which are also important for secondary
hardening. The end result is a significantly higher tempering
resistance in steels with lower Cr content, thus enabling the
reduction of the Mo content when compared to steels of prior art
concept. [0027] Mo and W: low concentrations of Mo have been
employed in the present invention not only for the purpose of cost
reduction, but also to promote the highest secondary hardness and
tempering resistance equivalent or even greater than that of steel
H13 in association with Cr and C contents. To do so, the alloy of
the present invention must contain at least 0.30%, preferably above
0.50%. On the other hand, an extremely high Mo content might harm
toughness due to deposition of pro-eutectic carbides during the
quenching phase and can increase significantly the alloy cost, in
an opposite direction to the cost-reduction goal of the present
invention. Hence, the Mo content should be limited to 0.90%,
preferably below 0.70%. Tungsten and molybdenum produce similar
effects in the tool steel of the present invention, forming
M.sub.2C or M.sub.6C secondary carbides. Thus, they can be jointly
specified through the tungsten equivalent relationship (W.sub.eq)
given by the sum W+2Mo, which normalizes the differences in atomic
weight between the two elements. [0028] V: Vanadium is primarily
important for the formation of MC secondary carbides. Because they
are very thin, these carbides block the movement of dislocation
lines, increasing mechanical strength. V also improves grain
growth, allowing high austenitizing temperatures (above
1000.degree. C.). For such effects, V must be above 0.1%,
preferably above 0.3%. However, excessively high V grades may
generate primary, difficult-to-solubilize carbides, thus reducing
toughness, and also promote significant increase of costs. Hence,
the V content should be lower than 1.0%, preferably below 0.6%.
[0029] Si: silicon produces a strong effect on secondary hardening
and toughness. When a low Si concentration is used, toughness
improves due to a better distribution of secondary carbides.
Therefore, the Si content of the material of the present invention
must be lower than 1.0%, typically below 0.5%.
[0030] Mn: high Mn contents may be considered undesirable for
promoting intense micro-segregation generating banding at different
degrees of hardness, and for increasing the retained austenite
content; therefore Mn is considered a deleterious element in the
present invention. Thus, the Mn content should be limited to 1.0%,
preferably below 0.8%, typically below 0.50%.
[0031] Al: to promote greater hardness of the nitrified layer, the
alloys' Al content can be high. However, the Al content, under
these conditions, should be limited to 1.0% because they lead to
decreased toughness. Thus, Al contents between 0.40% and 0.60% may
be of interest for this purpose. However, for applications in which
the hardness of the nitrified layer is slightly lower than that of
steel H13, but high toughness is required, the Al content of the
alloy of the present invention can be <0.1%, typically below
0.05%.
[0032] Residual Elements: Other elements such as Ni and Co should
be considered as deleterious substances associated with the
steelmaking deoxidation processes or inherent to the manufacturing
processes. Hence, the Ni and Co content should be limited to 1.5%,
preferably below 1.0%. In terms of formation of inclusions, the
sulfur content should be controlled, because such inclusions may
lead to cracking during operation; therefore the S content should
remain below 0.050%, preferably below 0.020%. Also, for high
toughness purposes, embrittling elements such as P should be
avoided, being desirable P<0.030%, preferably P<0.015%,
typically P<0.010%. Indeed, a low Cr content also helps to
reduce the P content in electric arc furnace steelmaking processes,
thus leading to conclusions that are not contradictory to the cost
reduction philosophy desired.
[0033] The alloy, as described above, can be produced as rolled or
forged products through conventional or special processes such as
powder metallurgy, spray forming or continuous casting, such as
wire rods, bars, wires, sheets and strips.
[0034] The experiments carried out are described below, and
reference is made to the following attached figures:
[0035] FIG. 1A shows the effect of the Mo content on hardness after
tempering at 600.degree. C., while FIGS. 1B and 1C show the effect
of the Cr content at 0.60% Mo on usual C contents (FIG. 1B) and
higher C contents (FIG. 1C); the horizontal dashed line of FIGS.
1A, 1B and 1C indicates the Minimum Hardness desirable for the
application.
[0036] Likewise FIG. 1, FIGS. 2A, 2B and 2C show the effect of
molybdenum (FIG. 2A) and chromium (FIG. 2B and FIG. 2C) on
tempering resistance. The higher the hardness at high temperatures
the greater the alloy's tempering resistance. In all cases, the
alloys were first annealed at 600.degree. C.
[0037] FIGS. 3A and 3B show the OCT curve of the compositions of
the present invention, considering two Cr contents. Quantitative
hardenability results can be obtained from the number of formed
phases (pearlite and bainite) and, most importantly, from the final
hardness obtained per rate. The compositions are summarized in
Table 1, base 3, considering Cr contents of 3% and 4% for
comparison purposes. FIG. 3A illustrates the CCT curve for a 0.50%
C, 3.00% Cr composition, and FIG. 3B shows the CCT curve for a
0.50% C, 4.00% Cr composition.
[0038] FIG. 4 shows the CCT curve of H13 steel of the prior art
concept, whose data can be compared to the results of the steel of
the present invention. The same data concerning number of phases
and hardness shown in FIG. 3 can be assessed for different cooling
rates.
[0039] In FIGS. 5A and 5B, the alloys with the final composition of
the present invention, PI 1 to PI 3, are compared in terms of
hardness after tempering (FIG. 5A) and loss in hardness vs. time
(FIG. 5B) at 600.degree. C. (referred to in the tempering
resistance text).
[0040] FIG. 6 compares the results of impact toughness tests
conducted for two types of transverse test specimens: unnotched (7
mm.times.10 mm section, as per NADCA) or Charpy V, with 10
mm.times.10 mm section and V notch. All materials treated to
hardness 45 HRC according to the parameters of FIG. 5a.
[0041] FIG. 7 shows the hardness profile of the nitrified layer of
alloys PI 1, PI 2 and PI 3 vs. steel H13. A plasma nitriding
process was conducted for steel H13. Prior to nitriding, all sample
alloys were quenched and tempered such to reach 45 HRC.
EXAMPLE 1
Effect of Molybdenum, Chromium and Carbon
[0042] For this work, samples of approximately 200 g were collected
in an experimental VIM furnace with varied composition for the same
heat. Therefore, three heats were produced by varying the Cr, Mo
and C contents, as shown in Table 1 below (details: Annex 1). Steel
H11 served as a base for these alloys since it already has half of
V content. The materials were always characterized after special
annealing (austenitizing at 1010.degree. C., oil solubilization and
over-annealing at 810.degree. C.). In this process we used
annealing at 1020.degree. C. and tempering between 400 and
650.degree. C. Steel H13, of typical industrial composition, was
used as a base.
[0043] Hardness after tempering at 600.degree. C. is shown in FIG.
1, highlighting the effects of reduced Mo and Cr contents, and also
the effect of higher C content. With regard to the Mo content, a
lower Mo concentration results in lower hardness after tempering.
However, if the Cr content drops, post-tempering hardness rises. A
possibility is that a lower Cr content reduces the amount of
M.sub.7C.sub.3 which, in turn, dissolves Mo. Thus, a higher content
of free Mo should be present in alloys of lower Cr content, which
explains a more intense response to tempering.
[0044] Despite this important Cr effect, just reducing its content
is not sufficient to promote the required hardness (about 45 HRC).
Possibly, the required hardness can be obtained by tempering at
lower temperatures. However, this practice is sometimes not
feasible for hot work because the ideal tempering temperature
should be 50 to 80.degree. C. above the working temperature to
provide proper tempering resistance. Thus, for hot work involving
extruded and cast aluminum, the typical tempering temperature
should be 600.degree. C.
Table 1: Chemical compositions adopted for samples from the same
heat with variation of a single element. The asterisks used in the
Cr and Mo fields of the table below indicate that several
compositions using this base were produced for the same heat,
increasing the content of this element, but keeping the base
composition of the heat.
TABLE-US-00002 Base 1 Base 2 Base 3 H13 Variation of . . . Mo Cr C
-- C 0.36 0.36 0.48 0.37 Si 0.32 0.32 0.32 0.92 Mn 0.26 0.28 0.27
0.31 P 0.007 0.006 0.006 0.022 S 0.001 0.002 0.001 0.001 Co 0.02
0.02 0.02 0.02 Cr 5.00 ** *** 4.82 Mo * 0.65 0.6 1.17 Ni 0.15 0.06
0.06 0.11 V 0.4 0.41 0.41 0.79 W 0.01 0.01 0.01 0.09 Cu 0.02 0.03
0.03 0.03 Al 0.013 <0.005 <0.005 0.02 * Mo variation: 0.05;
0.30; 0.60; 0.90; 1.22; 1.51 ** Cr variation, considering 0.36% C:
2.0; 3.0; 4.0; 5.1; 6.2; 7.1, *** Cr variation, considering 0.48%
C: 2.0; 3.0; 4.0; 5.1; 6.1; 7.0;
[0045] Therefore, to increase hardness after tempering at
600.degree. C., we increased the C content. As shown in FIG. 1, the
result was effective and hardness even higher than those from H13
were obtained. In this case, the C effect is related to increased
formation of secondary carbides and, when associated with a lower
Cr content, it provides the hardness required to start the work,
even in alloys of lower Mo content (half of steel H13). In alloys
of higher C content, a similar Cr effect can be observed.
[0046] Besides hardness after tempering, loss of hardness is also a
key factor to promote adequate response by the alloys in question
to the high temperatures they are subjected to. The results shown
in FIG. 2 demonstrate the important Mo effect in this regard (FIG.
2a), and also that the reduction of the Cr content is also an
interesting option to reduce the loss of hardness, which means
re-plotting the curves to higher hardness levels (see FIG. 2b). In
alloys with higher C content (FIG. 2c), this effect is even
stronger. Thus, the low Cr/high C combination seems
interesting.
[0047] On the other hand, the Cr content cannot be too low, such
that hardenability is not reduced. This effect was studied in the
curves of FIG. 3 and compared to steel H13 in FIG. 4.
Quantitatively, hardness reached after 0.3 and 0.1.degree. C./s
corresponds to steel H13 with 635 HV and 521 HV (FIG. 4), whereas
the 3% Cr alloy corresponds to 595 HV and 464 HV under the same
conditions (FIG. 3a). The scenario changes for the 4% Cr alloy,
which reaches hardness H13, i.e., 696 HV and 523 HV for rates of
0.3 and 0.1.degree. C./s (FIG. 3b). Therefore, Cr contents close to
4% Cr seem to be more interesting. Extremely below this value,
i.e., 3% Cr or less, the volume of bainite and the hardness after
tempering may prevent the application. Thus, a 3.8% Cr content was
selected for all other tests, production of pilot-scale billets and
evaluation of mechanical properties.
EXAMPLE 2
Effect of Al content
[0048] After defining an alloy target, four heats (50 kg cast
billets, 140 mm average section) were produced and forged as plates
(Table 2) with dimensions of 65 mm.times.165 mm. The materials were
then annealed following the same process described in Example 1 and
their properties were evaluated as discussed below.
[0049] The results confirmed the initial results shown in FIGS. 1
and 2, as shown in FIG. 5. Thus, the new alloys can reach similar
results in terms of hardness at 600.degree. C. (FIG. 5a), or even
better, in terms of tempering resistance, if compared to steel H13
(FIG. 5b).
TABLE-US-00003 TABLE 2 experimental 50 kg billets produced for the
alloys of the present invention (PI) and steel H13. PI1 PI2 PI3 H13
C 0.50 0.49 0.51 0.38 Si 0.32 0.31 0.32 0.99 Mn 0.35 0.35 0.35 0.35
P 0.011 0.011 0.011 0.023 S 0.003 0.003 0.004 0.004 Co 0.01 0.01
0.01 0.02 Cr 3.76 3.78 3.81 5.25 Mo 0.62 0.64 0.61 1.32 Ni 0.14
0.13 0.13 0.13 V 0.40 0.39 0.40 0.85 W 0.01 0.01 0.01 0.02 Cu 0.05
0.05 0.05 0.05 Al 0.037 0.51 1.02 0.031
[0050] Another important point can be compared in FIG. 6, in terms
of toughness. The toughness of the alloy of the present invention,
when bearing low Al contents, is equivalent to that of steel H13.
This demonstrates that the low Si and P contents of alloy PI1
compensate for the loss of toughness likely to occur as the C
content increases in relation to steel H13. FIG. 6 also shows that
toughness is inversely proportional to the Al content.
[0051] Al contents are responsible for a significant increase of
hardness after nitriding, as shown in FIG. 7. Thus, for
applications in which high hardness of the nitrified layer is
considered more relevant than toughness (e.g., extrusion of solid
shapes), alloy PI2 becomes interesting for having toughness>200
J and extremely high hardness of the nitrified layer (almost 1400
HV). Alloy PI 3 does not show gains in terms of the nitrified
layer, but toughness is far lower.
[0052] On the other hand, in applications highly susceptible to
cracking, such as pipe extrusion dies, toughness can be considered
a key property. For these cases, alloy PI1 seems more appropriate,
also showing hardness after nitriding similar to that of steel H13,
reaching more than 1000 HV on surface, which is the typical
specification for extrusion tools. Furthermore, as previously shown
in FIG. 5, alloy PI 1 also presents improved hot strength
properties. Therefore, considering the properties required for hot
work applications, the alloys of the present invention show results
equivalent to or better than those of steel H13. Such results are
quite relevant for non-ferrous alloy extrusion dies, e.g., Al
alloys, or hot forging dies. Alloy PI 1 has improved tempering
resistance, but hardness after nitriding and toughness equivalent
to steel H13, while alloy PI 2 has lower toughness, but tempering
resistance and hardness after nitriding significantly higher than
steel H13. The alloy should be selected on the basis of the most
critical properties required for the application. However, in all
cases, significant cost reductions can be obtained due to the low
Mo and V content of the alloys of the present invention.
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