U.S. patent application number 13/510205 was filed with the patent office on 2012-12-20 for steel with high tempering resistance.
Invention is credited to Celso Antonio Barbosa, Rafael Agnelli Mesquita.
Application Number | 20120321505 13/510205 |
Document ID | / |
Family ID | 44059134 |
Filed Date | 2012-12-20 |
United States Patent
Application |
20120321505 |
Kind Code |
A1 |
Barbosa; Celso Antonio ; et
al. |
December 20, 2012 |
STEEL WITH HIGH TEMPERING RESISTANCE
Abstract
STEEL WITH HIGH TEMPERING RESISTANCE comprising a composition of
alloying elements consisting essentially of, in percent by mass, C
between 0.20 and 0.50, Si lower than 1.0, P lower than 0.030, Cr
between 3.0 and 4.0, Mo between 1.5 and 4.0, V between 0.1 and 2.0,
Co lower than 1.5, being the remaining composed of Fe and
inevitable deleterious substances. The steel is produced by
processes involving ingot casting and hot/cold forming, or used
with the cast structure; or by processes involving atomization or
dispersion of the molten metal, such as powder metallurgy, powder
injection or spray forming.
Inventors: |
Barbosa; Celso Antonio;
(Parque Nove Campinas-Campinas, BR) ; Mesquita; Rafael
Agnelli; (Sao Manoel - American - SP, BR) |
Family ID: |
44059134 |
Appl. No.: |
13/510205 |
Filed: |
November 10, 2010 |
PCT Filed: |
November 10, 2010 |
PCT NO: |
PCT/BR2010/000375 |
371 Date: |
August 30, 2012 |
Current U.S.
Class: |
420/107 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/22 20130101; C22C 38/52 20130101; C22C 38/24 20130101; C22C
38/44 20130101; C22C 38/46 20130101; C22C 38/04 20130101 |
Class at
Publication: |
420/107 |
International
Class: |
C22C 38/52 20060101
C22C038/52; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 17, 2009 |
BR |
PI0904607-0 |
Claims
1-10. (canceled)
11. STEEL WITH HIGH TEMPERING RESISTANCE comprising a composition
of alloying elements consisting essentially of, in percentage by
mass, C between 0.20 and 0.50, Si at maximum 0.30, P lower than
0.030, Cr between 3.0 and 4.0, Mo between 1.5 and 4.0, V between
0.1 and 2.0, Co lower than 1.5, being the remaining composed of Fe
and inevitable deleterious substances.
12. STEEL WITH HIGH TEMPERING RESISTANCE, according to claim 11,
comprising a composition of alloying elements consisting
essentially of, in percentage by mass, C between 0.30 and 0.50, Si
at maximum 0.30, P lower than 0.020, Cr between 3.0 and 4.0, Mo
between 2.0 and 3.0, V between 0.1 and 1.0, Co lower than 1.0,
being the remaining composed of Fe and inevitable deleterious
substances.
13. STEEL WITH HIGH TEMPERING RESISTANCE, according to claim 12,
comprising a composition of alloying elements consisting
essentially of, in percentage by mass, C between 0.30 and 0.45, Si
at maximum 0.30, P lower than 0.015, Cr between 3.2 and 3.9, Mo
between 2.0 and 3.0, V between 0.3 and 1.0, Co lower than 1.0,
being the remaining composed of Fe and inevitable deleterious
substances.
14. STEEL WITH HIGH TEMPERING RESISTANCE, according to claim 13,
comprising a composition of alloying elements consisting
essentially of, in percentage by mass, C between 0.30 and 0.40, Si
at maximum 0.30, P lower than 0.010, Cr between 3.5 and 3.9, Mo
between 2.2 and 2.8, V between 0.3 and 0.8, Co lower than 0.5,
being the remaining composed of Fe and inevitable deleterious
substances.
15. STEEL WITH HIGH TEMPERING RESISTANCE according to claim 11,
wherein a Mo/W replacement ratio is 1 Mo:2 W.
16. STEEL WITH HIGH TEMPERING RESISTANCE according to claim 11,
wherein a Vanadium: Niobium/Titanium replacement ratio is 1 V:2
Nb/1 Ti.
17. USE OF STEEL WITH HIGH TEMPERING RESISTANCE according to claim
11, for molds, dies and multiple-use tools, for formation of solid
or liquid materials, at room temperature or at temperatures up to
1300.degree. C.
18. USE OF STEEL HIGH RESISTANCE TEMPERING according to claim 11,
for metal-forming tools subject to temperatures between 300 and
1300.degree. C., and also to other applications such as forging,
extrusion or casting ferrous or nonferrous alloys.
Description
[0001] This report deals with a steel designed for hot metal
forming tools, typically in cases where the metal to be formed
withstands temperatures above 600.degree. C., even though processes
at lower temperatures or even at room temperature can be used with
the said steel. The steel in question has a composition that allows
ranking it as hot work tool steel, whose main characteristic is
increased resistance to loss of hardness at high
temperature--called tempering resistance, while retaining high
toughness and adequate thermal conductivity and hardenability. Such
an effect is possible by carefully designing the alloy, and setting
the optimum ranges of the elements P, Si, Mo and Cr.
[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, 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 characteristics
of the processes in which they are employed.
[0003] Among their key properties of hot work steels, the following
stand out: hot resistance, more specifically tempering resistance,
toughness, hardenability and physical properties such as thermal
conductivity, specific heat, both correlated to thermal
diffusivity, and thermal expansion coefficient.
[0004] For forging applications, hot forging of steels stands out,
especially steels for mechanical construction applied to auto
parts. In such operations, the forged billet withstands
temperatures above 1100.degree. C. During the forming process, it
heats up the surface of the tool, given that the higher the contact
period, the higher the temperature. Consequently, the heat
generated requires high hot resistance from the steel used. The
steel hardening mechanism for hot die forming is chiefly induced by
the precipitation of fine carbides. Noteworthy are the Mo or W
carbides, M.sub.2C-type, or V carbides, MC-type. For high-Cr
steels, Cr-rich M.sub.7C.sub.3 carbides also stand out, but also
with Mo and V in solid solution. Despite their high stability,
these carbides tend to coalesce after long periods of time at high
temperatures, typically above 550.degree. C.--conditions easily
reached within the tool operating range. As a consequence, the
hardness of the area decreases, causing wear and hot plastic
deformation, resulting in tool failure.
[0005] Improving the resistance of the material against loss of
hardness or tempering resistance, is thus critical to improving the
performance of the tools that operate under high temperature
conditions. Examples of such applications are tools used in hot
forging of steel parts or other metal alloys, extrusion of
non-ferrous alloys and dies for casting non-ferrous alloys (being
the latter two applications more important to Al alloys). The same
goes for other applications, such as extrusion or casting of
non-ferrous alloys. E.g., in forging steel applications, the
temperature of the pre-forms to be forged is about 1200.degree. C.
Even considering the short contact time with the tools (seconds),
their surface heat up significantly, causing loss of hardness due
to tempering of these surfaces. Considering the extrusion of
aluminum alloys or other non-ferrous alloys, the billet temperature
is lower, ranging from 400 to 600.degree. C. However, in these
applications, the contact time is significantly longer (tens of
minutes to hours of operation). Moreover, local friction generated
by the tool/aluminum contact intensifies heating, increasing the
loss of hardness of the tool steel and, consequently, leading to
steel wear. In pressurized casting dies, the molten metal is
injected at high pressure and temperature (around 700.degree. C.),
also heating the surface of the die. In this case, failure is due
mainly to thermal fatigue cracking caused by the successive heating
and cooling of the working surface of the die. But, the high heat
exchange between molten aluminum and the die surface favors the
heating of the surface areas, generating, as in other applications,
loss of hardness and, consequently, triggering the fatigue cracking
process.
[0006] This mechanism of post-heating loss of hardness is,
therefore, critically important to hot work tool steels; thus,
increasing the strength of the material against this phenomenon is
something desirable: Concerning the steel employed, improved hot
resistance is usually obtained through the use of higher grades of
those elements that form secondary carbides such as Mo, W and V, or
by solid solution hardening. Although effective in increasing hot
resistance, the excessive increase of the grade of these elements
implies reduced toughness, poor thermal diffusivity and
conductivity or significant increase of production costs. This
latter economic factor is really important nowadays, given the high
cost of raw materials used as a source of alloy elements Mo, W and
V.
[0007] To provide a better understanding of this invention, we
describe below some of the state-of-the-art steels currently used
(chemical composition is summarized in Table 1). The H11 and H13
steels stand out, as these are the tool steels mostly used for hot
works. These materials contain 5% Cr providing adequate
hardenability and to assist with hot resistance, 0.9% V and 1.2% Mo
to improve hot resistance, and generally low grades of P and S to
promote adequate toughness. However, for improving tempering
resistance, higher Mo grades would be needed. The DIN 1.2365 and
DIN 1.2367 are steels used for such purpose. They have a high Mo
grade to improve hot resistance. However, if the content of this
element is increased within the structure of the DIN 1.2367 steel,
toughness and thermal conductivity and diffusivity tend to drop. In
the 1.2365 steel, this thermal conductivity reduction is
counterbalanced by increasing the Mo content and decreasing the Cr
content. However, the lower the Cr content, the lower the
hardenability, limiting the applications in large tools. It is
important emphasize that attention should be paid to the thermal
conductivity and toughness properties. During the work, the
increase in thermal conductivity is important such that the tool
steel is able to homogenize quickly the difference of temperature
between the material formed and the tool core, thus reducing
stresses and thermal cracking. And, in case cracking occurs, the
toughness of the material is also critical because it delays
propagation and, consequently, thermal fatigue damage. Thus, it is
clear that only increasing the Mo content, as exemplified by steel
DIN 1.2367 and DIN 1.2365, is not sufficient for overall
improvement of the properties of hot work steels.
TABLE-US-00001 TABLE 1 Typical chemical composition of
state-of-the-art steels. 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 weight and
Fe balance. The sum Mo + W + V + Co is shown because these elements
impact the alloy cost the most. Mo + W + Designation C Si Cr Mo W V
Co V + Co H11 0.36 1.0 5.0 1.2 -- 0.5 -- 1.7 H13 0.38 1.0 5.0 1.2
-- 1.0 -- 2.2 DIN 1.2365 0.36 0.3 2.8 2.8 -- 0.5 -- 3.3 DIN 1.2367
0.38 0.3 5.0 3.0 -- 0.5 -- 3.5 PI 9909160-7 0.36 0.2 5.0 2.3 -- 0.5
-- 2.8
[0008] A new steel type has been developed more recently and is
described in PI 9909160-7. This material has, similarly to DIN
1.2367, higher Mo content, but lower Si and P content to improve
toughness. In this case, cost increase is avoided by not using a
high Mo content, but the hot resistance gain is not significant in
comparison to steel H13.
[0009] Given this scenario, it is evident the need for a tool steel
with hot resistance higher than that of state-of-the-art steel H13,
but without using excessive alloying elements which might affect
thermal conductivity and material cost. Also, the material used
should feature high hardenability, which allows it to be applied to
large tools.
[0010] Therefore, the steel of the present invention will fulfill
all these needs.
[0011] The initial purpose of the invention was to investigate the
influence of the Cr and Mo content on hot work tool steels that
allowed identifying some synergy between the two elements and hot
resistance. More specifically, when an increase of the Mo content
is followed by a reduction of the Cr content, a more significant
effect on the hot strength can be observed. In addition, a reduced
Cr content improves thermal conductivity, thereby reversing the
negative effect of a higher Mo content. On the other hand, the Cr
content must be carefully balanced, because very low values, as
previously mentioned, impair hardenability and limit the
application to tools with average section dimension above 100 mm.
Therefore, the material of the present invention shows a chemical
composition with ideal arrangement of the Cr and Mo contents,
capable of overcoming the tempering resistance properties of
state-of-the-art steels, with no significant cost increase and
proper thermal conductivity, toughness and hardenability.
[0012] 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] 0.20 to 0.50 C, preferably
0.3 to 0.45 C, typically 0.36 C [0014] 3.0 to 4.0 Cr, preferably
3.5 to 3.9 Cr, typically 3.8 Cr [0015] 1.5 to 4.0 Mo, preferably
2.0 to 3.0 Mo, typically 2.5 Mo. Given its chemical similarity to
W, Mo can be replaced with W, a 2 W: 1 Mo ratio by mass. [0016] 0.1
to 2.0 V, preferably 0.3 to 1.0 V, typically 0.5 V; V can be
partially or fully replaced with Nb, following a 1.0% Nb: 0.5% V
ratio. [0017] up to 1.0 Si, preferably up to 0.5 Si, typically 0.3
Si [0018] Max 0.030 P, preferably max 0.015 P, typically max 0.010
P.
[0019] Balance by 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: [0020] Max 0.10 S,
preferably max 0.020 S, typically max 0.008 S. [0021] Max 1.5 Al,
Mn or Co, preferably up to 1.0 Al, Mn or Co, typically below 0.5
Mn, Al and Co.
[0022] Next, we describe the ratios of the specification of the
composition of the new material. The percentages listed refer to
percent by mass.
[0023] C: Carbon is primarily responsible for martensite hardening
under low temperature conditions. Together with the alloying
elements, carbon acts in the secondary hardening, important for the
hardening at high temperature. For such effects, carbon contents of
at least 0.20% are recommended, preferably above 0.30%. 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), and result in increased hardness and
volume of secondary carbides. Thus, toughness is generally
impaired. the C content should be limited to a maximum value of
0.50%, preferably below 0.40%. This limitation also plays a role in
the reduction of the amount of retained austenite, preventing
problems associated with dimensional instability and
embrittlement.
[0024] Cr: The chromium content should be higher than 3.0%,
preferably greater than 3.5%, 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. This is an important effect, because the
final tempering resistance is higher than that of state-of-the-art
steels. The mechanisms that cause this effect are thought to 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
volume of M.sub.7C.sub.3 carbides and, thus, the greater the amount
of Mo and V available for secondary hardening. The end result is a
significantly higher tempering resistance when the alloy Cr content
is lower than that of state-of-the-art alloys. Even in relation to
the PI 9909160-7 alloy, there is a significant tempering resistance
gain. This is significant because the alloys have Mo equivalent
grades (Mo is a pricey alloying element), showing that the present
invention is able to reach high hot resistance values without
excessively increasing the Mo content. For all such effects, the Cr
content should be below the 5.0% content of conventional steels,
being the preferred Cr content lower than 4.0%. Finally, the ideal
Cr content required to maximize tempering resistance identified in
the present invention should be set between 3.0% and 4.0%. In
addition to the heat resistance property, a lower Cr content
improves thermal conductivity, also preserving this property as the
Mo content rises. Therefore, this shorter Cr range aims at a
careful adjustment for maximum tempering resistance and adequate
thermal conductivity.
[0025] Mo and W: high Mo contents are used in the alloy of the
present invention to improve tempering resistance properties. This
is possible by the formation of chemically-stable, Mo-rich
tempering carbides, especially the M.sub.2C carbide. Thus, the
alloy of the present invention must contain at least 1.5%,
preferably above 2.0%. On the other hand, excessively high Mo
grades can harm toughness due to precipitation of pro-eutectic
carbides at the time of quenching, and may significantly increase
the cost of the alloy, making its application in many tools
unfeasible. Hence, the Mo content should be limited to 4.0%,
preferably below 3.0%. 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.
[0026] 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. Hence, the V content should be lower than 2.0%,
preferably below 1.0%.
[0027] Si: silicon produces a strong effect on secondary hardening
and toughness. At high levels, Si increases the secondary hardness
up to quenching temperatures of 600.degree. C. However, the study
of the present invention showed that a lower Si content was
important to reduce the loss of hardness under high temperature
conditions, thereby increasing tempering resistance. A lower Si
content also results in significant increase of toughness, having
this effect been applied to the present invention. Therefore, the
Si content of the material of the present invention must be lower
than 1.0%, typically below 0.5%.
[0028] P: reduction of the P content also results in significant
increase of toughness, because this element can be segregated on
the grain surface and, thereby, diminishes cohesion in these
surfaces. Therefore, P content should be lower than 0.030%,
typically below 0.015%.
[0029] Residual Elements: Other elements such as Mn and Al should
be considered as deleterious substances associated with the
steelmaking deoxidation processes or inherent to the manufacturing
processes. Hence, the Mn and Al content should be limited to 1.5%,
preferably below 1.0%. The Co content should also be limited to the
same values, due to its beneficial effect on hot resistance and
strong impact on the alloy cost. 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%.
[0030] 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.
[0031] The figures attached herein have been referenced to in the
description of the experiments carried out, and their contents are
listed below:
[0032] FIG. 1 shows the effects of P and Si on alloys 1-8, in terms
of post-tempering toughness and hardness.
[0033] FIG. 2 compares alloys 1 to 8, but showing P effect on
toughness, depending on the quenching temperature.
[0034] FIG. 3 shows the distribution of carbides in the high and
low Si content alloys, demonstrating a better distribution in low
Si content alloys, which explains their superior toughness.
[0035] FIG. 4 compares the reduction of hardness versus time at
600.degree. C., showing tempering resistance. The greater the
displacement to the right, the higher the alloy's tempering
resistance.
[0036] FIG. 5 shows a comparison of thermal conductivity values for
some of the alloys investigated.
[0037] FIG. 6 shows a comparison of toughness of alloys 9 to 12 and
alloys IP 1 and IP 2; the data produced by the un-notched impact
test (7.times.10 mm test specimens) and Charpy V
[0038] FIG. 7 shows the hot forging punch to which the
industrially-produced IP 2 steel was applied and compared with the
state-of-the-art H13 steel. Note: a) wear failures and cracking; b)
hardness profile, showing drop in working areas (distance from
surface=zero).
EXAMPLE 1
Effect of Silicon and Phosphorus
[0039] Eight experimental ingots were initially produced to
evaluate the effect of Si and P on the state-of-the-art H11 steel.
The compositions are shown in Table 2. The hardness and impact
results are shown in FIG. 1. Note the strong Si influence on
hardness for quenching temperatures below 500.degree. C., even
though the same effect is not observed for quenching temperatures
above 600.degree. C.; the hardness of both the alloys with high and
low Si content is about the same. The P effect is compared in FIG.
2 for different tempering temperatures. In this case, it was
possible to observe that a decreasing P content improves
significantly the toughness of Si-rich alloys, but this effect is
less significant on alloys with low Si content.
[0040] Therefore, the results show that the best combination, in
terms of toughness, would be alloys with low P and Si content.
Alloys of high Si content are only viable for situations when
hardness values higher than 52 HRC are employed and, for such,
tempering temperatures below 600.degree. C. are also adopted. In
these cases, the decrease of the P content is even more
critical.
[0041] The reasons for these significant Si and P effects have not
been fully defined, but early scientific results conducted by the
inventors of this patent show a relationship with the formation of
secondary carbides. In alloys with high Si content, the secondary
carbides tend to concentrate in areas of high diffusion (lath or
grain surfaces), because of the difficulties imposed by Si onto the
cementite formation process. On the other hand, in alloys with low
Si content, cementite is rapidly formed, leading to a better
distribution of secondary carbides formed at higher temperatures.
FIG. 3 shows the images of transmission electron microscopy that
illustrate these observations.
TABLE-US-00002 TABLE 2 Chemical composition of the various Si and P
contents analyzed for the state-of-the-art H11 alloy. Alloy: 1 2 3
4 5 6 7 8 C 0.36 0.34 0.36 0.36 0.36 0.35 0.36 0.35 Si 0.05 0.32
0.98 1.92 0.05 0.33 1.01 1.90 Mn 0.35 0.35 0.35 0.35 0.34 0.35 0.35
0.35 P 0.023 0.028 0.024 0.012 0.012 0.012 0.011 0.008 S 0.004
0.004 0.004 0.003 0.004 0.004 0.005 0.003 Co 0.06 0.05 0.05 0.05
0.05 0.05 0.05 0.06 Cr 5.09 5.13 5.06 5.08 5.08 5.03 5.10 5.05 Mo
1.28 1.31 1.33 1.24 1.32 1.32 1.33 1.23 Ni 0.20 0.19 0.19 0.20 0.19
0.20 0.19 0.20 V 0.44 0.44 0.42 0.41 0.44 0.44 0.45 0.43 W 0.10
0.11 0.11 0.10 0.11 0.10 0.11 0.10 Nb <0.01 <0.01 <0.01
<0.01 <0.01 <0.01 <0.01 <0.01 Al 0.029 0.020 0.023
0.036 0.024 0.022 0.036 0.043 W.sub.eq 1.48 1.53 1.55 1.44 1.54
1.52 1.55 1.43 (=W + 2Mo)
EXAMPLE 2
Effect of Cr and Mo
[0042] To assist in the definition of the effect of Cr and Mo,
seven additional experimental ingots have been produced,
comprehending four state-of-the-art steels: H11, H13 and the steel
described in PI 9909160-7 and two alloys proposed for the present
invention (see Table 3). These two compositions lead to a reduction
of the Si and P contents described in example 1 but also to
distinct balances of Cr and Mo.
[0043] As mentioned, the purpose of the IP 1 and IP 2 alloys is to
obtain greater resistance to loss of hardness, i.e., tempering
resistance. Therefore, hardness reduction after different exposure
periods at 600.degree. C. was evaluated and the results are shown
in Table 4. The time increments followed a logarithmic scale, as
depicted in the chart of FIG. 4b. These results show that, when
comparing alloys IP 1 and H11, hot resistance rises simply by
modifying the Si and P contents (but the effect is likely to be
related only to Si, since P does not play a role in the carbide
formation process)
[0044] However, this gain in hot resistance by reducing the Si
content is not sufficient to produce results significantly higher
than those of H13. Therefore, a higher Mo content together with a
lower Cr content in the IP 2 alloy was used. For that case, a
significant variation of the hot resistance could be verified,
which provided increased hardness after the same exposure period.
And, as shown in FIG. 4a, the same drop in hardness obtained for
the H13 steel occurs after far longer periods in the case of alloy
IP 2. For example, hardness reduces from 45 HRc to 35 HRc at a
temperature of 600.degree. C. after an exposure period of 25 hours,
whereas the same phenomenon takes place in alloy IP 2 after only 60
hours.
[0045] This significant tempering resistance improvement is related
not only to the increase of the Mo content but also to the
reduction of the Cr content. This effect is clear after comparing
the differences between alloy PI2 and alloy 12 (patent PI
9909160-7). It also explains the high tempering resistance results
obtained for alloy 11.
TABLE-US-00003 TABLE 3 Chemical composition of state-of-the-art
steels and those proposed for the present invention. Alloy: 9 10 11
12 IP 1 IP 2 Note: H11 H13 DIN Patent Current 1.2365 9909160-7
invention C 0.36 0.4 0.31 0.35 0.35 0.35 Si 1.02 0.96 0.3 0.13 0.3
0.31 Mn 0.48 0.34 0.3 0.49 0.27 0.3 P 0.025 0.023 0.023 0.009 0.007
0.01 S 0.005 0.006 0.005 0.005 0.005 0.006 Co 0.02 0.02 0.01 0.02
0.01 0.02 Cr 5.03 5.23 2.85 4.99 4.96 3.78 Mo 1.4 1.31 2.8 2.28
1.39 2.49 Ni 0.19 0.20 0.20 0.19 0.20 0.19 V 0.34 0.85 0.5 0.57
0.42 0.52 W 0.03 0.02 0.02 0.02 0.02 0.02 Nb <0.010 0.02 0.02
0.01 <0.010 0.005 Al <0.005 0.014 0.019 0.009 <0.005 0.005
W.sub.eq 2.8 2.6 5.6 4.6 2.8 5.0 (=W + 2Mo) W + Mo + 1.79 2.2 3.33
2.89 1.84 3.05 Co + V
TABLE-US-00004 TABLE 4 Loss of hardness after exposure at
600.degree. C. for various exposure periods Initial hardness around
45 HRC Alloy 9 Alloy 10 Alloy 11 Alloy 12 IP 1 IP 2 Cr and 5Cr
5.2Cr 2.8Cr 5.0Cr 5.0Cr 3.8Cr Mo %: 1.4Mo 1.3Mo 2.8Mo 2.3Mo 1.4Mo
2.5Mo Initial 45.5 44.6 45.3 45.7 44.9 45.4 3 h 43.3 43.5 44.7 44.1
43.3 44.8 10 h 39.6 41.3 43.8 41.8 40.8 43.7 30 h 34.4 36.1 41.0
37.6 36.9 40.5 100 h 31.1 30.7 36.0 32.0 32.1 34.5
[0046] Despite the interesting effect on tempering resistance, Cr
contents should not drop to excessively low levels to prevent
damaging the hardenability and thus, limiting its application in
large tools. This can be considered the major setback of the
state-of-the-art DIN 1.2365 steel (alloy 11), i.e., excellent
tempering resistance but low hardenability. Table 5 illustrates
these Cr.times.hardenability issues based on dilatometer test
results. The IP 2 composition can be considered ideal under this
aspect, with Cr content lower than that of steel H13 (alloy 10), to
provide increased tempering resistance, but not as low as that of
steel DIN 1.2365 (alloy 11). The higher Mo content of alloy IP 2
also helps achieving proper hardenability levels, which compensates
the effect resulting from the Cr content reduction and ensures its
application in large tools.
[0047] A further advantage of using a lower Cr content than that of
alloy 12 and other state-of-the art alloys is the ability to
maintain adequate thermal conductivity. As shown in FIG. 5, this
property tends to fall as the Mo content increases (compare alloys
12 and 10), and to rise as the Cr content goes down (alloys 11 and
IP 2). Therefore, in addition to being considered ideal with regard
to hot resistance, the combination of the Cr and Mo contents of
alloy IP 2 allows maintaining thermal conductivity at levels even
higher than those of the traditional H13 steel (alloy 10).
TABLE-US-00005 TABLE 5 Results of the TRC curve developed for the
investigated steels, used in hardenability evaluation. The lower
the critical rate and the higher the hardness after quenching at
0.1.degree. C., the higher will be the hardenability. Alloy 11
Alloy 12 Alloy 10 DIN PI H13 1.2365 9909160-7 IP1 IP 2 Critical
rate to start 0.2 8.0 0.3 0.5 0.5 the bainite-forming process
(.degree. C./s) Hardness after 538 389 534 512 486 quenching at
0.1.degree. C./s (HV)
[0048] Superior tenacity is another gain of alloys IP 1 and IP 2 in
relation to the state-of-the-art H11, H13 and DIN 1.2365 alloys
(alloys 9 to 11). These results can be compared in FIG. 6. Note the
gain of alloy IP 2 in relation alloy 11, which, similarly, has high
tempering resistance. That is, in addition to superior
hardenability, the balance of the IP 2 chemical composition makes
this alloy significantly tougher than alloy 11. The effect, in this
case, is primarily associated with lower Si and P contents, as
discussed in example 1.
EXAMPLE 3
Field Test
[0049] A field study is detailed next, with allow IP 2 being
compared to steel H13 in forging tools. The results were analyzed
based on the failure modes and on the properties of materials.
[0050] The process in question deals with high-speed warm forging
(see FIG. 7a). Despite the fact that the forged billets are exposed
to a temperature lower than the usual hot forging temperature, high
processing speed makes the contact between the heated billet and
the matrix to extend, thus heating its surface. The process is also
developed under high cooling conditions, thereby promoting thermal
shock on the surface region.
[0051] Process data
[0052] Product: shaft end.
[0053] Tool: Warm forging precision punch
[0054] Forged Material: Modified SAE 1045 and 1050 steels
[0055] Billet temperature: approximately 900.degree. C.
[0056] Cooling: Intense, water-cooled.
[0057] Blow application speed: high.
[0058] Tool steel previously used: AISI H13 (hardness: 53 HRC).
[0059] Steel tested: IP 2, at same hardness.
[0060] FIG. 7a shows the punch analyzed after the end of its life.
Since this type of forging produces parts of high dimensional
accuracy, deviations of tenths of mm compromise the quality of the
part produced. The end of its life is caused by wear on the
protruding and rounded surfaces and occurrence of thermal cracking
(see FIG. 7a). After the end of its life, the matrix was destroyed
and analyzed. FIG. 7b shows the data concerning hardness vs.
distance from the contact surface; note hardness decrease close to
subsurface areas. Wear is actually related to this loss of hardness
during the work, regardless of core hardness. The occurrence of
thermal cracking is also associated with loss of hardness, as
surfaces of lower hardness become more sensitive to the occurrence
of thermal cracking. Therefore, increased tempering resistance is
essential to further increasing the tool's life time.
[0061] The steel of the present invention, IP 2, was then tested
and approved for the application, increasing the tools life time by
56%. In numerical values, 5000 parts made of H13 steel could be
forged up to the end of the tool's life and this figure increased
to 7500 parts made of steel IP 2; the comparative analysis of the
tempering curves and hardness vs. time developed for steels H13
(alloy 10) and IP 2, FIGS. 4 and 6, provides a better understanding
of the phenomenon. For both cases the steel hardness decreases when
subjected to high temperatures, the greater the drop, the longer
the time and the higher the temperature employed. However, there is
greater stability of alloy IP 2 at high temperature. Thus, during
the forging process, the failure will occur after a higher number
of strokes, resulting in the yield gain that was observed.
* * * * *