U.S. patent application number 13/510236 was filed with the patent office on 2012-12-13 for stainless mold steel with lower delta ferrite content.
Invention is credited to Celso Antonio Barbosa, Rafael Agnelli Mesquita.
Application Number | 20120315181 13/510236 |
Document ID | / |
Family ID | 44059135 |
Filed Date | 2012-12-13 |
United States Patent
Application |
20120315181 |
Kind Code |
A1 |
Barbosa; Celso Antonio ; et
al. |
December 13, 2012 |
STAINLESS MOLD STEEL WITH LOWER DELTA FERRITE CONTENT
Abstract
STAINLESS MOLD STEEL WITH LOWER DELTA-FERRITE CONTENT comprising
a composition of alloying elements consisting essentially of, in
percentage by mass, Carbon between 0.01 and 0.20; Nitrogen between
0.01 and 0.07; Manganese between 2.0 and 4.0; Nickel between 0.01
and 1.0; Chromium between 11.0 and 13.0; Molybdenum+Tungsten lower
than 1.0; Copper between 0.01 and 1.5; Vanadium between 0.01 and
1.0; Sulfur between 0.01 and 0.20; Calcium at maximum 0.01;
Aluminum lower than 0.05; Silicon lower than 1.0; the remainder
consisting essentially of Fe and inevitable impurities to the
preparation process.
Inventors: |
Barbosa; Celso Antonio;
(Parque Nova Campinas-Campinas, BR) ; Mesquita; Rafael
Agnelli; (Sao Monoel-Americana-SP, BR) |
Family ID: |
44059135 |
Appl. No.: |
13/510236 |
Filed: |
November 10, 2010 |
PCT Filed: |
November 10, 2010 |
PCT NO: |
PCT/BR2010/000376 |
371 Date: |
August 30, 2012 |
Current U.S.
Class: |
420/42 |
Current CPC
Class: |
C22C 38/38 20130101;
C22C 38/44 20130101; C21D 2211/005 20130101; C22C 38/001 20130101;
C22C 38/42 20130101; C22C 38/06 20130101; C22C 38/02 20130101; C22C
38/58 20130101; C22C 38/04 20130101; C22C 38/46 20130101 |
Class at
Publication: |
420/42 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/60 20060101 C22C038/60; C22C 38/42 20060101
C22C038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 17, 2009 |
BR |
PI 0904608.9 |
Claims
1-9. (canceled)
10. STAINLESS MOLD STEEL WITH LOWER DELTA-FERRITE CONTENT,
comprising a composition of alloying elements consisting
essentially of, in percentage by mass, Carbon between 0.01 and
0.20; Nitrogen between 0.01 and 0.07; Manganese between 2.0 and
4.0; Nickel between 0.01 and 1.0; Chromium between 11.0 and 13.0;
Molybdenum+Tungsten lower than 1.0; Copper between 0.01 and 1.5;
Vanadium between 0.01 and 1.0; Sulfur between 0.01 and 0.2; Calcium
at maximum 0.01; Aluminum lower than 0.50; Silicon lower than 1.0;
the remainder consisting essentially of Fe and inevitable
impurities to the preparation process.
11. STAINLESS MOLD STEEL WITH LOWER DELTA-FERRITE CONTENT,
according to claim 10, comprising a composition of alloying
elements consisting essentially of, in percentage by mass, Carbon
between 0.03 and 0.10; Nitrogen between 0.03 and 0.06; Manganese
between 2.2 and 3.0; Nickel between 0.10 and 0.5; Chromium between
11.0 and 13.0; Molybdenum+Tungsten lower than 0.5; Copper between
0.1 and 0.8; Vanadium between 0.02 and 0.10; Sulfur between 0.05
and 0.14; Calcium between 0.01 and 0.003; Aluminum lower than 0.10;
Silicon lower than 0.50; the remainder consisting essentially of Fe
and inevitable impurities to the preparation process.
12. STAINLESS MOLD STEEL WITH LOWER DELTA-FERRITE CONTENT,
according to claim 11, comprising a composition of alloying
elements consisting essentially of, in percentage by mass, Carbon
between 0.03 and 0.08; Nitrogen between 0.03 and 0.06; Manganese
between 2.2 and 2.8; Nickel between 0.10 and 0.50; Chromium between
11.5 and 12.5; Molybdenum+Tungsten lower than 0.1; Copper between
0.3 and 0.7; Vanadium between 0.03 and 0.08; Sulfur between 0.08
and 0.12; Calcium between 0.0015 and 0.0025; Aluminum lower than
0.05; Silicon lower than 0.50; the remainder consisting essentially
of Fe and inevitable impurities to the preparation process.
13. STAINLESS MOLD STEEL WITH LOWER DELTA-FERRITE CONTENT according
to claim 10, wherein Vanadium is replaced with Niobium or Titanium
in a ratio corresponding to 1V:2Nb and 1V:1 Ti.
14. STAINLESS MOLD STEEL WITH LOWER DELTA-FERRITE CONTENT according
to claim 10, wherein delta-ferrite content in the microstructure is
lower than 10%.
15. STAINLESS MOLD STEEL WITH LOWER DELTA-FERRITE CONTENT according
to claim 10, wherein the stainless mold steed is homogenized,
forged or hot rolled at temperatures higher than 1160.degree. C.,
but with delta-ferrite content in the microstructure lower than
10%.
16. STAINLESS MOLD STEEL WITH LOWER DELTA-FERRITE CONTENT according
to claim 10, wherein the stainless mold steel is applicable to
molds, dies and multiple-use tools, for formation of solid or
liquid materials, at room temperature or at temperatures up to
1300.degree. C.
17. STAINLESS MOLD STEEL WITH LOWER DELTA-FERRITE CONTENT according
to claim 16, wherein the stainless mold steel is applicable to
plastic molds and plastic mold components.
18. STAINLESS MOLD STEEL WITH LOWER DELTA-FERRITE CONTENT according
to claim 17, wherein the stainless mold steel is applicable to hot
chambers or other devices of plastic molds, in which high corrosion
resistance and high machinability are required.
Description
[0001] This invention is a stainless steel for general applications
in plastic-forming molds, particularly, but not limited, to hot
chambers molds. Its main feature is the combination of properties
related to the mold fabrication, such as machinability, weldability
and low cost (associated with low nickel (Ni) content) and for
being easy to process, in terms of control of an undesirable
microstructural phase called delta-ferrite. Due to these mold- and
steel-making advantages, this invention allows a considerable
reduction of the mold cost.
[0002] The tools and molds are usually operated to form other
materials, either thermoplastic polymer materials (commonly known
as plastic materials) or metallic materials. Depending on the
properties of the material used to make the tools, these are used
in processes at room or high temperatures, around 700.degree. C.
The steel of this invention is especially applied to molds or mold
devices, which are exposed to room temperature or temperatures
below 500.degree. C. and must be corrosion-resistant. A typical
example of such applications is the hot chambers used in
plastic-forming molds, which do not exceed 300.degree. C. For such
cases, the combined temperature/water-cooling effect may lead to
corrosion, which explains the need for stainless steels. And, due
to the high content of machined material, the machinability
property should be optimized.
[0003] In addition to these two features, corrosion resistance and
machinability, welding is many times applied on mold steels, minor
repairs and mold modifications. However, conventional martensitic
stainless steels with high content of chromium (12 to 17%) and
medium content of carbon (approx. 0.4%) have an extremely high
hardenability causing significant hardness and potential cracking
in welded areas (see Table 1). Thus, the development of a
low-carbon alloy is something desirable.
TABLE-US-00001 TABLE 1 Typical chemical composition of traditional
steels approached in the state of art. The approximate hardness of
martensite is shown in order to highlight the difficult weldability
caused by the high content of carbon. Content in mass percentage
and Fe balance. Denom- Martensite ination C S Mn Ni Cr Mo V
Hardness AISI 1.0 0.003* 0.30 -- 17.5 0.5 -- 65 HRC 440 C AISI 420
0.40 0.003* 0.30 -- 13.5 -- 0.25 55 HRC mod. (DIN 1.2083) DIN 0.38
0.005* 0.60 0.80 16.0 1.0 -- 55 HRC 1.2316 DIN 0.35 0.15 1.0 --
15.0 -- -- 55 HRC 1.2085 *Typical values; not specified by
standard
[0004] In addition to these metallurgical properties, the cost
issues have become even more critical. Strong competitiveness,
especially considering low-cost molds available worldwide, makes
the mold manufacturers look for low-cost options. Under these
conditions, a negative metallurgical factor is the microstructural
stability in terms of absence of delta-ferrite. Carbon and nickel
are the most important elements to promote the austenitic phase and
the elimination of delta-ferrite in martensitic steels. However,
there is a limitation for carbon, as mentioned above, with regard
to weldability problems. And, in case of nickel, cost limitation is
significant. The higher the carbon content, the lower the need for
nickel and, thus, the higher the alloy cost.
[0005] New developments are under way to solve such problem. For
instance, U.S. Pat. No. 6,358,334 and U.S. Pat. No. 6,893,608 B2
address the production of low-nickel and carbon stainless steels
employing high levels of copper and nitrogen (see Table 2).
However, the occurrence of delta-ferrite is significant for both of
them, with levels of up to 10% being common. On the other hand, the
control of delta-ferrite in these alloys influences the alloy
forging and laminations temperatures. Table 2 shows the equilibrium
temperature calculated by the "Thermocalc" thermodynamic
calculation software for these alloys. When combined with high
sulfur content, low temperatures may easily create cracking or
excessive power in the forming equipment (usually a forging press
or lamination mill). So, considering all those items, there are
some state-of-the-art low-carbon and nickel steels, but processing
them is not an easy task, which results in costlier processes and
consequent increase of the alloy cost.
[0006] Therefore, the need for a stainless steel with high
machinability, low-nickel and carbon content and increased
processing capacity is evident. In order to allow the reduction of
the steel-making process cost, the forming temperatures of the
material should be significantly higher than those of
state-of-the-art steels.
[0007] The steel of this invention will fulfill all those
needs.
TABLE-US-00002 TABLE 2 State-of-the-art steels developed more
recently than the steels shown in Table 1. Content in mass
percentage and Fe balance. The hardness of martensite in these
alloys, due to the low content of carbon, is about 35 HRC. Maximum
Forming Pat. C N Mn S Ni Cr Mo Cu V Temperature* U.S. Pat. No. 0.05
0.04 1.3 0.12 0.10 12.6 0.05 0.95 0.08 1150.degree. C. 6,358,334
U.S. Pat. No. 0.05 0.04 0.30 0.15 0.70 13.5 0.40 0.25 0.06
1100.degree. C. 6,893,608 *For AISI 420 steel, the forming
temperature may reach up to 1260.degree. C.
[0008] The stainless steel for molds, proposed by this invention,
can be produced with a lower content of delta-ferrite and at
temperatures about 30.degree. C. higher during forging or
lamination processes. Its chemical composition also lacks high-cost
elements such as nickel and molybdenum, but the chromium content is
sufficient to ensure inoxidability. And, as previously mentioned,
weldability requirements can be achieved due to lower carbon
content.
[0009] In order to satisfy the abovementioned conditions, the
alloys of this invention have a composition of alloying elements,
which, in percentage by mass, consist of: [0010] Carbon: between
0.01 and 0.2, preferably, and between 0.03 and 0.10, typically
0.05. [0011] Nitrogen: between 0.01 and 0.07, preferably between
0.03 and 0.06, typically 0.055. [0012] Manganese: between 2.0 and
4.0, preferably between 2.2 and 3.0, typically 2.5 [0013] Nickel:
between 0.01 and 1.0, preferably between 0.1 and 0.5, typically 0.3
[0014] Chromium: between 11.0 and 13.0, preferably between 11.5 and
12.5, typically 12.0 [0015] Molybdenum and Tungsten: the sum should
be below 1.0, preferably below 0.5, typically below 0.2. [0016]
Copper: between 0.01 and 1.5, preferably between 0.1 and 0.8,
typically 0.55. [0017] Vanadium: between 0.01 and 1.0, preferably
between 0.02 and 0.10, typically 0.05. [0018] Sulfur: between 0.01
and 0.20, preferably between 0.05 and 0.14, typically 0.09. [0019]
Calcium: below 0.010, preferably between 0.001 and 0.003, typically
0.002. [0020] Aluminum: below 0.50, typically below 0.10,
preferably below 0.050. [0021] Silicon: below 0.1, preferably below
0.05, typically between 0.1 and 0.6.
[0022] Balance by Fe and metallic or non-metallic impurities are
inevitable to the steel-making process.
[0023] Next, we present the ratios of the specification of the
composition of the new material and a description of the effect of
each of the alloying elements. The percentages listed refer to
percentage by mass.
[0024] C: carbon is the main responsible for the response to the
heat treatment, and also for the hardness of martensite produced by
quenching. Due to the intense heating and quick cooling, the
welding process can be considered similar to quenching. Thus, the
carbon content controls the final hardness created in the welded
zone of the steel of this invention. Therefore, to achieve the
required hardness, the carbon content should be at least 0.01%,
preferably above 0.03%. However, the carbon content should be below
0.2%, preferably below 0.1%, such that hardness in the welded zones
is below 40 HRC to prevent cracking and facilitate the machining
process.
[0025] N: nitrogen is necessary in the alloy of this invention
because it is a powerful austenitizer and reduces the amount of
delta-ferrite. Moreover, nitrogen increases pitting corrosion
resistance. On the other hand, a nitrogen surplus may generate
gases, given that delta-ferrite is the first solid phase in the
steel of this invention, considering limited nitrogen solubility.
Thus, the nitrogen content should lie between 0.01% and 0.08%,
preferably between 0.02% and 0.06%, typically around 0.05%.
[0026] Mn: as Mn is not a costlier element, but is a powerful
austenitizer, it should be employed at high levels in the steel of
this invention. Therefore, its content should be above 2.0%,
preferably above 22%, typically 2.5%. However, when employed in
excess, manganese increases the content of retained austenite, as
well as the coefficient of material hardening, decreasing the
machinability, besides increasing hydrogen solubility and promoting
flake formation; thus, the manganese content should not exceed
4.0%, preferably below 3.0%.
[0027] Ni: nickel is a powerful austenitizer, but makes the alloy
to become costlier. In order to get both aspects under control, the
nickel content should remain between 0.01 and 1.0%, preferably
between 0.10 and 0.50%, and typically, 0.30%.
[0028] Cr: chromium confers inoxidability to the steel of this
invention, being the most important element as far as this property
is concerned (due to the low content of Mo and Ni in this alloy).
Thus, the chromium content should be above 11.0%, typically above
12.0%. However, chromium is also a major ferritizer, contributing
to increase the delta-ferrite content and to reduce the austenitic
field. In order to counterbalance such effects, the Cr content
should be lower than 13.0%, preferably below 12.5%.
[0029] Molybdenum and Tungsten: when combined, the total content
should be below 1.0% because they increase the cost of the alloy
and the ferrite content. Preferably, the sum should be below 0.5%,
typically below 0.2%.
[0030] Copper: it is an austenitizer and also promotes
precipitation hardening required for the response to heat
treatment. However, if employed in excess, copper may have a
negative effect on the cost and is a major scrap contaminant. Thus,
the copper content should lie between 0.01% and 1.5%, preferably
between 0.1% and 0.8%, and typically, 0.55%.
[0031] Vanadium: vanadium plays an important role in secondary
hardening that, despite not being intense in the steel of this
invention, is essential for reaching the post-tempering hardness
required at high temperature. However, as vanadium is also a
ferritizer and has a negative impact on the cost of the alloy, its
content should be controlled. Thus, the vanadium content should lie
between 0.01% and 1.0%, preferably between 0.05% and 0.50%,
typically around 0.1%.
[0032] S: in the steel of this invention, sulfur forms manganese
sulfide (MnS) inclusions that become elongated through the hot
forming process. As the inclusions become malleable at temperatures
developed in the machining process, they facilitate the
chip-breaking process and lubricate the cutting tool, thus
improving machinability. In order to produce this effect, the
sulfur content must be higher than 0.01%, preferably above 0.05%,
typically above 0.09%. Despite being beneficial to the machining
process, the MnS inclusions have a negative effect on the
mechanical properties, especially toughness and corrosion
resistance. Hence, the sulfur content should be limited to 0.20%,
preferably below 0.15%.
[0033] Ca: calcium also has an effect on inclusions by modifying
hard alumina inclusions that hinder machinability and by reducing
the size (spheroidal) of inclusions in general. This effect is
mostly important for the control of MnS inclusions, making them
more distributed and less elongated, thus favoring the machining
process and the mechanical properties. However, controlling the
calcium content is quite complex due to its high reactivity. Thus,
the use of calcium can be considered optional for those cases in
which high machinability and polishability are required. If
employed, the calcium content should not exceed 100 ppm (0.01%)
because its solubility in the molten metal and high reactivity
(when in contact with refractories) limit higher values.
Preferably, the Ca content should lie between 10 and 30 ppm (0.001
and 0.003%), typically 20 ppm (0.002%).
[0034] Al: due to the formation of hard alumina inclusions, the Al
content should not be excessively high to hinder machinability. It
should be below 0.5%, typically below 0.1%, preferably below
0.05%.
[0035] Si: silicon is used as a deoxidizer, an important agent in
situations of low Al content, which is the case of the steel of
this invention. However, this element is a ferritizer and if used
in excess, favors the formation of delta-ferrite. Thus, the silicon
content should remain between 0.1% and 1.0%, preferably between
0.2% and 0.7%, typically 0.40%.
[0036] The figures attached herein have been referenced to in the
description of the experiments carried out, and their contents are
listed below:
[0037] FIG. 1 shows the increase of the amount of delta-ferrite for
state-of-the-art alloy 1 and alloys PI 1 and PI 2 of this
invention. Representative microstructures have also been added.
[0038] FIG. 2 shows the tempering curves obtained for the three
alloys, alloy 1, PI 1 and PI 2--the alloys' hardness is low after
quenching, changing from 30 to 34 HCR after tempering.
[0039] FIG. 3 shows a comparison of the microstructure of alloys PI
1 and PI 2 for two contents of sulfur--note that the increase of
the number of inclusions is directly proportional to the increase
of the sulfur content.
EXAMPLE 1
[0040] The "Thermo-calc" software was used to simulate the effect
of N and Mn on the increase of the delta-ferrite formation
temperature to allow defining the composition of the steel of this
invention. Simulations 1 to 4 show the strong effect of nitrogen,
at a composition equivalent to that of U.S. Pat. No. 6,358,334.
However, extremely high N content, above 0.06%, already anticipate
the formation of gas during the solidification stage, which
generates voids in the billets, making their use unfeasible. On the
other hand, for simulation 5, the Mn effect associated with a
higher and safe N content, can be analyzed. In this alloy steel, we
estimate that there is a gain of 30 to 90.degree. C. in the maximum
formation temperature in relation to state-of-the-art alloys. This
indicates the possibility of better hot formation and elimination
of delta-ferrite, (as mentioned above, by reducing the mechanical
and corrosion resistance).
[0041] After this evidence of the strong effects of N and Mn, two
compositions have been produced for pilot-scale billets and
compared to the alloy of U.S. Pat. No. 6,358,334, hereinafter
called alloy 1. The alloys of the present invention will be called
PI 1 and PI 2. The chemical compositions of the billets are shown
in table 4. The principal variables in terms of matrix stability
concerning ferrite formation are the Mn and N contents; however the
S content of the alloys also varied, and the respective effects
will be discussed further on.
TABLE-US-00003 TABLE 3 Equilibrium temperature required to produce
10% by volume of delta-ferrite, in several state-of-the-art alloys
and those proposed by this invention, calculated via "Thermo-calc".
Maximum Formation Temperature Designation Approximate composition
** U.S. Pat. No. 0.05C0.04N1.3Mn0.1Ni12.5Cr1.0Cu 1150.degree. C.
6,358,334 U.S. Pat. No. 0.05C0.04N0.3Mn0.7Ni13.5Cr0.25Cu
1100.degree. C. 6,893,608 Simulation 1
0.05C0.05N1.3Mn0.1Ni12.5Cr1.0Cu 1160.degree. C. Simulation 2*
0.05C0.06N1.3Mn0.1Ni12.5Cr1.0Cu 1180.degree. C. Simulation 3*
0.05C0.07N1.3Mn0.1Ni12.5Cr1.0Cu 1190.degree. C. Simulation 4*
0.05C0.08N1.3Mn0.1Ni12.5Cr1.0Cu 1200.degree. C. Simulation 5*
0.05C0.05N2.5Mn0.1Ni12.5Cr1.0Cu 1190.degree. C. *formation of
N.sub.2 gas during solidification
[0042] The results of the delta-ferrite content measured on
rough-cast samples for the three alloys of Table 4 are shown in
Table 5 ND FIG. 6. The increase of the N content proposed results
in significant gain (compare alloy 1 vs. alloy PI 1) in terms of
increase of temperature required to form 10% delta-ferrite.
[0043] However, the strongest effect takes place after combining
the N and Mn effect, with a gain even higher than that calculated
by the thermodynamic software. Apart from the values of Table 4, it
is also worthy observing the evolution of the delta-ferrite content
as a function of temperature. This is shown in FIG. 1, with a clear
reduction of the delta-ferrite content of alloy 1 if compared to
alloy PI 1 and, especially, if compared to alloy PI 2.
TABLE-US-00004 TABLE 4 Chemical composition of pilot-scale billets
that contain the state-of-the-art alloy defined in patent U.S. Pat.
No. 6,358,334, called alloy 1, and two alloys investigated in the
present invention (PI 1 and PI2). Values in percentage by mass and
balance by Fe. Alloy: Alloy 1 PI 1 PI 2 C 0.058 0.055 0.059 N 0.044
0.055 0.056 Si 0.39 0.39 0.40 Mn 1.05 1.05 2.46 P 0.025 0.026 0.025
S 0.085 0.097 0.140 Cr 12.2 12.3 12.3 Mo 0.06 0.06 0.06 Ni 0.3 0.3
0.3 Cu 0.55 0.56 0.55 V 0.04 0.04 0.04 W 0.03 0.04 0.03 Al 0.009
0.009 0.005
TABLE-US-00005 TABLE 5 Volume fraction of delta-ferrite in alloy 1
and alloys PI 1 and PI 2 calculated through quantitative
metallography. The measurements have been performed after 24 hours
at temperature specified. Alloy: 1150.degree. C. 1180.degree. C.
1200.degree. C. 1230.degree. C. 1260.degree. C. Alloy 1 0% 0.6%
8.4% 21.3% 29.1% PI 1 0% 0% 7.3% 15.7% 21.9% PI 2 0% 0% 0.2% 3.2%
21.0%
[0044] In terms of the response to heat treatment as shown in FIG.
2, alloys PI 1 and PI 2 are both capable of reaching the 30 to 34
HRC levels required for the applications. It is also worth being
emphasized that alloys PI 1 and PI 2 have post-quenching hardness
of about 35 to 40 HRC (value extracted from the chart, for
quenching temperature=0.degree. C.), far below the 55/65 HRC of
state-of-the art conventional steels shown in Table 1.
[0045] The S content of alloys PI 1 and PI 2 is not the same, and
this can be positive or negative for the application, and thus, the
S content should be specified depending on the application. This
issue was investigated for the billets shown in Table 4, but after
hot formation for 70.times.70 mm square section size (4.times.
reduction by area). The low values are due to the low degree of
reduction applied to the trial billets.
[0046] The higher S content of alloy PI 2 results in improved
machinability but lower toughness and corrosion resistance. The
results of such changes can be seen in Table 5 and, in
microstructural terms, the different distribution of the S content
of alloys PI 1 and PI 2 can be observed in FIG. 3. The higher
amount of sulfides (dark gray in FIG. 3) and their persistence
explain the lower values obtained for corrosion resistance and
toughness, respectively. And, in terms of machinability, the
preponderant factor is the higher sulfide content of alloy PI
2.
[0047] Therefore, for applications demanding high machinability and
low toughness and corrosion requirements, high Si alloys (around
0.15%) are recommendable. For cases of stricter toughness and
corrosion requirements, alloys with S content around 0.10% are more
adequate.
TABLE-US-00006 TABLE 5 Values relative to machinability, corrosion
resistance and toughness of alloys PI 1 and PI 2. The differences
observed are associated with the different S content of the alloys.
PI 1 PI 2 Alloy: (97 ppm S) (140 ppm S) Volume machined up to tool
wear (cm.sup.3), 121 199 for cutting speed of 250 m/min and advance
per tooth = 0.10 mm % corroded after 2-hour exposure to NaCl 17 33
5% at 35.degree. C. (fog test as per ASTM B117) and NBR 8094 Izod
Impact Test (Charpy V, cross test 4.8 .+-. 1.8 2.7 .+-. 0.3
specimens, treated to 32 HCR)
EXAMPLE 2
[0048] Due to increased stability in terms of delta-ferrite, the
basic composition of alloy PI 2 has been privileged and made on an
industrial scale. However due to the poorer mechanical and
corrosion properties, the PI 1 sulfur content was applied to that
industrialized product. Table 6 shows the chemical composition of
the alloy, called PI 3, and also the chemical composition of a
conventional 420 steel whose machinability can be compared to the
PI 3's. The machining volume up to the end of the tool's lifespan
is shown on the last row of Table 6; note the higher machined
volume of alloy PI 3, pointing out to a significant gain in
relation to the state-of-the-art 420 steel.
[0049] A key observation can be made with respect to alloy PI 3.
Forging took place at temperatures of 1200.degree. C. and, even so,
the delta-ferrite content remained below 10%.
[0050] Therefore, the two aforementioned examples show that the
steel of the present invention, especially PI 3, is capable of
meeting the weldability, machinability, corrosion resistance and
toughness requirements without creating processing problems, for
allowing higher hot forming temperatures.
TABLE-US-00007 TABLE 6 chemical composition of the steel of the
presentinvention, produced on an industrial scale, and of steel
420, subjected to the machinability test (both with 32 HRC) Alloy:
Steel 420 PI 3 C 0.37 0.046 N 0.008 0.040 Si 0.85 0.32 Mn 0.44 2.49
P 0.030 0.028 S 0.001 0.075 Cr 13.10 12.1 Mo 0.11 0.05 Ni 0.29 0.31
Cu 0.07 0.55 V 0.19 0.05 W 0.02 0.03 Al 0.025 0.005 Volume machined
up to tool wear (cm.sup.3), for 148 261 cutting speed of 250 m/min
and advance per tooth = 0.10 mm
* * * * *