Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Dual Phase Steel Strip Suitable for Galvanizing
Technical Field
Dual phase galvanized steel strip is made utilizing a thermal profile
involving a
two-tiered isothermal soaking and holding sequence. The strip is at a
temperature
close to that of the molten metal when it enters the coating bath.
Background of the Invention
Prior to the present invention, the galvanizing procedure whereby steel strip
is
both heat treated and metal coated has become well known and highly developed.
Generally a cold rolled steel sheet is heated into the intercritical regime
(between
Acl and Ac3) to form some austenite and then cooled in a manner that some of
the
austenite is transformed into martensite, resulting in a microstructure of
ferrite and
martensite. Alloying elements such as Mn, Si, Cr and Mo are in the steel to
aid
in martensite formation. Various particular procedures have been followed to
accomplish this, one of which is described in Omiya et al US Patent 6,312,536.
In the Omiya et al patent, a cold rolled steel sheet is used as the base for
hot dip
galvanizing, the steel sheet having a particular composition which is said to
be
beneficial for the formation, under the conditions of the process, of a
microstructure composed mainly of ferrite and martensite. The Omiya et al
patent describes a galvanized dual phase product.
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According to the Omiya et al patent, a dual phase galvanized steel sheet is
made
by soaking the cold rolled steel sheet at a temperature of 780 C (1436 F) or
above, typically for 10 to 40 seconds, and then cooling it at a rate of at
least 5 C
per second, more commonly 20-40 C per second, before entering the galvanizing
bath, which is at a temperature of 460 C (860 F). The steel, according to the
Omiya et al patent, should have a composition as follows, in weight percent:
Carbon: 0.02-0.20 Aluminum: 0.010-0.150
Titanium: 0.01 max Silicon: 0.04 max
Phosphorous: 0.060 max Sulfur: 0.030 max
Manganese: 1.5-2.40 Chromium: 0.03-1.50
Molybdenum:0.03-1.50 with the provisos that the amounts of
manganese, chromium and molybdenum should have the relationship:
3Mn+6Cr+Mo: 8.1%max,and
Mn+6Cr+10Mo: atleast3.5%
The Omiya et al patent is very clear that an initial heat-treating (soaking)
step is
conducted at a temperature of at least 780 C (1436 F). See column 5, lines 64-
67; col 6, lines 2-4: "In order to obtain the desired microstructure and
achieve
stable formability, it is necessary to heat the steel sheet at 780 C or above,
which
is higher than the Acl point by about 50 C. ... Heating should be continued
for
more than 10 seconds so as to obtain the desired microstructure of ferrite +
austenite." The process description then goes on to say the steel sheet is
cooled
to the plating bath temperature (usually 440-470 C, or 824-878 F) at an
average
cooling rate greater than 1 C/second, and run through the plating bath. After
plating, cooling at a rate of at least 5 C/second will achieve the desired
microstructure of predominantly ferrite and martensite. Optionally, the plated
sheet may be heated prior to cooling, in an alloying procedure (often called
galvannealing) after metal coating but prior to the final cooling.
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Omiya et al clearly do not appreciate that it is possible to achieve a dual
phase
product without the high temperatures of their soaking step, or that a
particular
holding step following a lower temperature soak can facilitate the desired
microstructure formation.
Brief Description of the Drawings
FIGURE 1 is a graph illustrating the general thermal cycle used in the steel
sheet
processing of Example 1;
FIGURE 2 is a graph showing ultimate tensile strength (UTS) as a function of
soak
temperature and hold time from samples of Example 1;
FIGURE 3 is a graph showing the yield ratio as a function of soak temperature
of the
samples of Figure 1;
FIGURE 4 is a graph showing the effect of soak temperature on yield ratio for
the
steel of Example 2;
FIGURE 5 is a graph showing the yield ratio of the material of Example 3 as a
function of soak temperature for the holding time of 70 seconds; and
FIGURE 6 shows galvannealed and galvanized thermal cycles in accordance with
an
embodiment of the instant invention.
Summsiry of the Invention
I have found, contraty to the above quoted reaitation in the Omiya et al
patent,
that not only is it not necessary to maintain the initial heat treatcnent
temperature
at 780 C (1436 F) or higher, but that the desired dual phase microstructure
can be
achieved by maintaining the temperature during an initial heat treat[nent
(soalring)
in the range from AC1+45 F, but at least 1340 F (727 C), to AC1+135 F, but no
more than 1425 F (775 C). One does not need to maintain the temperature at
780 C or higher, contrary to the Omiya et al patent, provided the rest of my
proaedure is followed. For convenience hereafter, my initial heat treatment
will
be referred to as the "soak" However, my process does not rely only on a lower
tcmperature for the soak as compared to Omiya et al; nather, the soak
temperature
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of (Aci+45 F) to 1425 F, usually 1340-1420 F, must be coupled with a
subsequent substantially isothermal heat treatment, termed the holding step,
in the
range of 850-920 F (454-493 C). In the holding step, the sheet is maintained
at
850-920 F (454-493 C), sometimes herein expressed as 885 Ff35 F, for a period
of 20 to 100 seconds, bafore cooling to room (ambient) temperature. Cooling to
ambient temperature should be conducted at a rate of at least 5 C per second.
It
is important to note, once again,.that the Omiya et al patent says nothing
about a
holding step at any temperature or for any time in their thermal process.
Furthermore, my work has shown that if a steel as defined in the Omiya et al
patent is soaked within Omiya's defined, higher, soalcing range (for example
1475 F) and further processed through a thermal cycle including a holding step
as
described herein (850-920F), the resultant steel will ggi achieve the desired
predominantly ferrite-marbonsite microstrachre but will contain a significant
amount of bainite and/or pearlite.
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I express the lower temperature limit of the soak step as "Ac1+45 F, but at
least
1340 F (727 C)", because virtually all steels of Composition A will have an
Acl
of at least 1295 F.
The steel sheet should have a composition similar to that of the Omiya et al
patent:
Carbon: 0.02-0.20 Aluminum: 0.010-0.150
Titanium: 0.01 max Silicon: 0.04 max
Phosphorous: 0.060 max Sulfur: 0.030 max
Manganese: 1.5-2.40 Chromium: 0.03-1.50
Molybdenum:0.03-1.50 with the provisos that the amounts of
manganese, chromium and molybdenum should have the relationship:
Mn + 6Cr + 10 Mo: at least 3.5%
For my purposes, the silicon content may be as much as 0.5%, and, preferably,
carbon content is 0.03-0.12% although the Omiya et al carbon range may also be
used. This composition, as modified, may be referred to hereafter as
Composition A.
Thus my invention is a method of making a dual phase steel sheet comprising
soaking a steel sheet at a temperature of in the range from AC1+45 F, but at
least
1340 F (727 C), to AC1+135 F, but no more than 1425 F (775 C), for a period of
20 to 90 seconds, cooling the sheet at a rate no lower than 1 C/second to a
temperature of 454-493 C, and holding the sheet at temperatures in the range
of
850-920 F (454-493 C) for a period of 20 to 100 seconds. The holding step may
be prior to the hot dip or may begin with the hot dip, as the galvanizing pot
will be
at a temperature also in the range 454-493 C (850-920 F). Immediately after
the
holding step, whether or not the sheet is galvanized, the sheet can be cooled
to
ambient temperature at a rate of at least 5 C/second. Alternatively, after the
sheet
is coated, the sheet may be galvannealed in the conventional manner - that is,
the
sheet is heated for about 5-20 seconds to a temperature usually no higher than
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about 960 F and then cooled at a rate of at least 5 C/second. My galvannealed
and galvanized thermal cycles are shown for comparison in Figure 6.
The actual hot dip step is conducted more or less conventionally - that is,
the steel
is contacted with the molten galvanizing metal for about 5 seconds; while a
shorter time may suffice in some cases, a considerably longer time may be used
but may not be expected to result in an improved result. The steel strip is
generally about 0.7 mm thick to about 2.5 mm thick, and the coating will
typically
be about 10 m. After the holding and coating step, the coated steel may be
either
cooled to ambient temperature as described elsewhere herein or conventionally
galvannealed, as described above. When the above protocol is followed, a
product having a microstructure comprising mainly ferrite and martensite will
be
obtained.
Commercially, it is common to perform hot dip galvainizing substantially
continuously by using coils of steel strip, typically from 1000 to 6000 feet
long.
My invention permits more convenient control over the process not only because
the soak step takes place at a lower temperature, but also because the strip
may be
more readily kept at the same temperature as the hot dip vessel entering and
leaving it, with little concern about significant heat transfer occurring
between
steel strip and zinc pot that could heat up the molten zinc and limit
production.
As applied specifically to a continuous steel strip galvanizing line, which
includes
a strip feeding facility and a galvanizing bath, my invention comprises
feeding a
cold rolled coil of steel strip of Composition A to a heating zone in the
galvanizing line, passing the strip through a heating zone continuously to
heat the
strip to within the range of AC1+45 F, but at least 1340 F (727 C), to AC1+135
F,
but no more than 1425 F (775 C), passing the strip through a soaking zone to
maintain the strip within the range of AC1+45 F, but at least 1340 F (727 C),
to
AC1+135 F, but no more than 1425 F (775 C), for a period of 20 to 90 seconds,
passing the strip through a cooling zone to cool the strip at a rate greater
than
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1 C/second, discontinuing cooling the strip when the temperature of the strip
has
been reduced to a temperature in the range 885 7:05 F, but also 30 degrees F
of the temperature of the galvanizing bath, (preferably within 20 degrees F:k
the
temperature of the bath, and more preferably within 10 degrees F the
temperature of the bath), holding the strip within 30 degrees F of the
temperature
of the galvanizing bath (again preferably within 20 degrees F f the
temperature of
the bath, and more preferably within 10 degrees F the temperature of the
bath)
for a period of 20 to 100 seconds, passing the strip through the galvanizing
bath,
optionally galvannealing the coated strip, and cooling the strip to ambient
temperature. The galvanizing bath is typically at about 870 F (850-920 F), and
may be located at the beginning of the holding zone, or near the end of the
hold
zone, or anywhere else in the holding zone, or immediately after it. Residence
time in the bath is normally 3-6 seconds, but may vary somewhat, particularly
on
the high side, perhaps up to 10 seconds. As indicated above, after the steel
is
dipped into and removed from the zinc bath, the sheet can be heated in the
conventional way prior to cooling to room temperature to form a galvanneal
coating, if desired.
In one embodiment, there is provided a method of making an incipient dual
phase
steel sheet, wherein said steel sheet has the composition, in weight percent,
carbon: 0.02-0.20; aluminum: 0.010-0.150; titanium: 0.01 max; silicon: 0.5
max;
phosphorous: 0.060 max; sulfur: 0.030 max; manganese: 0.8-2.40; chromium:
0.03-1.50; molybdenum:0.03-1.50; with the provisos that the amounts of
manganese, chromium and molybdenum have the relationship: (Mn + 6Cr + 10
Mo) = at least 3.5%, comprising soaking said steel sheet for 20 to 90 seconds
at a
temperature within the range of Acl+45 F, Acl+135 F, cooling said steel sheet
at
a rate of at least 1 C per second to a temperature in the range 850-940 F, and
holding said steel sheet in the range 850-940 F for 20 to 100 seconds.
In one embodiment, there is provided a method of substantially continuously
galvanizing steel strip in a galvanizing line including a galvanizing bath,
comprising feeding a coil of steel strip having the composition, in weight
percent,
carbon: 0.02-0.20; aluminum: 0.010-0.150; titanium: 0.01 max; silicon: 0.5
max;
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phosphorous: 0.060 max; sulfur: 0.030 max; manganese: 0.8-2.40; chromium:
0.03-1.50; molybdenum:0.03-1.50; with the provisos that the amounts of
manganese, chromium and molybdenum have the relationship (Mn + 6Cr + 10
Mo) at least 3.5%, to a heating zone in said galvanizing line, passing said
strip
through a heating zone continuously to heat said strip to 1340-1425 F, passing
said strip through a soaking zone to maintain said strip within the range of
1340-
1425 F for a period of 20 to 90 seconds, passing said strip through a cooling
zone to cool said strip at a rate greater than 1 C per second, discontinuing
cooling said strip when the temperature of said strip has been reduced to a
temperature 30 degrees F of the temperature of said galvanizing bath,
holding
said strip at a temperature between 850-940 F and within 30 degrees F of the
temperature of said galvanizing bath for a period of 20 to 100 seconds,
passing
said strip through said galvanizing bath, and cooling said strip to ambient
temperature.
In one embodiment, there is provided a method of making a galvanized steel
strip having a predominantly martensite and ferrite microstructure,
wherein said steel has the ingredients, in weight percent, carbon: 0.02-
0.20; aluminum: 0.010-0.150; titanium: 0.01 max; silicon: 0.5 max;
phosphorous: 0.060 max; sulfur: 0.030 max; manganese: 0.8-2.40;
chromium: 0.03-1.50; molybdenum:0.03-1.50, comprising soaking said
steel strip at ACt+45 F, to AC1+135 F, for at least 20 seconds, cooling said
strip at a rate of at least 1 C per second, passing said strip through a
galvanizing vessel for a residence time therein of 2-9 seconds to coat said
strip at any time while holding said strip at 895 F 45 F for 20 to 100
seconds, and cooling the strip so coated to ambient temperature.
In one embodiment, there is provided a method of making an incipient dual
phase
steel sheet, wherein said steel sheet has the composition, in weight percent,
carbon: 0.02-0.20; aluminum: 0.010-0.150; titanium: 0.01 max; silicon: 0.5
max;
phosphorous: 0.060 max; sulfur: 0.030 max; manganese: 1.5-2.40; chromium:
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0.03-1.50; molybdenum:0.03-1.50; with the provisos that the amounts of
manganese, chromium and molybdenum have the relationship: (Mn + 6Cr + 10
Mo) = at least 3.5%, comprising soaking said steel sheet for 20 to 90 seconds
at a
temperature within the range of AC1+45 F, but at least 1340 F (727 C), to
AC1+135 F, but no more than 1425 F (775 C), cooling said steel sheet at a rate
of
at least 1 C per second to a temperature in the range 850-920 F, and holding
said
steel sheet in the range 850-920 F for 20 to 100 seconds.
In one embodiment, there is provided a method of substantially continuously
galvanizing steel strip in a galvanizing line including a galvanizing bath,
comprising feeding a coil of steel strip having the composition carbon: 0.02-
0.20;
aluminum: 0.010-0.150; titanium: 0.01 max; silicon: 0.5 max; phosphorous:
0.060 max; sulfur: 0.030 max; manganese: 1.5-2.40; chromium: 0.03-1.50;
molybdenum:0.03-1.50; with the provisos that the amounts of manganese,
chromium and molybdenum have the relationship (Mn + 6Cr + 10 Mo) at least
3.5%, to a heating zone in said galvanizing line, passing said strip through a
heating zone continuously to heat said strip to 1340-1425 F, passing said
strip
through a soaking zone to maintain said strip within the range of 1340-1420 F
for a period of 20 to 90 seconds, passing said strip through a cooling zone to
cool
said strip at a rate greater than 1 C per second, discontinuing cooling said
strip
when the temperature of said strip has been reduced to a temperature 30
degrees F of the temperature of said galvanizing bath, holding said strip at a
temperature between 850-920 F and within 30 degrees F of the temperature of
said galvanizing bath for a period of 20 to 100 seconds, passing said strip
through
said galvanizing bath, and cooling said strip to ambient temperature.
In one embodiment, there is provided a method of making a galvanized steel
strip
having a predominantly martensite and ferrite microstructure, wherein said
steel
has the ingredients, in weight percent, carbon: 0.02-0.20; aluminum: 0.010-
0.150; titanium: 0.01 max; silicon: 0.5 max; phosphorous: 0.060 max; sulfur:
0.030 max; manganese: 1.5-2.40; chromium: 0.03-1.50; molybdenum:0.03-1.50,
comprising soaking said steel strip at AcI+45 F, but at least 1340 F, to
Aci+135 F, but no more than 1425 F, for at least 20 seconds, cooling said
strip
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at a rate of at least 1 C per second, passing said strip through a galvanizing
vessel
for a residence time therein of 2-9 seconds to coat said strip at any time
while
holding said strip at 885 F t35 F for 20 to 100 seconds, and cooling the strip
so
coated to ambient temperature.
Example 1
Samples of steel sheet were processed, with various "soak" temperatures
according to the general thermal cycle depicted in Figure 1 - one set of
samples
followed the illustr ated curve with a 35 second "hold" at 880 F and the other
set
of samples were held at 880 F for 70 seconds. The samples were cold rolled
steel of composition A as described above - in particular, the carbon was
0.67,
Mn was 1.81, Cr was 0.18 and Mo was 0.19, all in weight percent. The other
elemental ingredients were typical of low carbon, Al Icilled steel. Soak
temperatures were varied in increments of 20 F within the range of 1330 to
1510 F. After cooling, the mechanical properties and microstructures of the
modified samples were determined. Ultimate tensile strength ("UTS") of the
resulting products as a fiinction of soak temperature and hold time is shown
in
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Figure 2. For this particular material, a minimum UTS of 600 MPa was the
target
and was achieved over a range of soak temperatures from about 1350 F to 1450 F
for both hold times.
A goal of Sxamiple 1 was to achieve a predominantly ferrite-marteasite
microstracture. The yield ratio, i.e. the ratio of yield strength to ultimate
tensile
strength, is an indication whether or not a dual phase ferrite-martensite
microstructure is present. When processed as in Example 1, a fenite-martensite
microstructure is indicated when the yield ratio is 0.5 or less. If the yield
ratio is
greater than about 0.5, a significant volume fraction of other deleterious
constituents such as bainite, pearlite, and/or Fe3C may be expected in the
microstracture. Figure 3 shows the yield ratio as a function of soak
temperature
for both the 35 and 70 second holding zones for the samples. Note that a very
low
yield ratio of about 0.45 is achieved over a range of temperatures for both
curves
from about 1350-1430 F, indicating optimum dual phase properties over this
soak
temperature range. Metallographic analyses of the samples performed on steels
soaked within this 1350-1430 F soak range confirmed a ferrite-martensite
micsostructure. Quantitative metallography using point counting techniques
revealed martensite contents of 14.5 and 13.5% respectively, for the steel
soaked
at 1390 F and held at 880 F for 70 and 35 seconds, respectively, with no other
constituents observed in the microstructure. (The images were constructed
using
the Lepera etching technique for which ferrite appears light gray, martensite
white, and such as pearlite and bainite appearing black). For soak
temperatures
below about 1350 F, as expected, iron carbide (FesC) remains in the
nucrostructure due to insufficient carbide dissolution which results in
limited
martensite formation during cooling.
Unexpected, however, is the appearance of bainite in the microstiucture when
soak tcmperatmras get above about 1430 F. For example, metallographic analyses
reveal a bainite content of 8.5% for the steel soaked at 1510 F and held at
880 F
for 70 seconds. These results contrast strongly with Omiya. According to
Omiya,
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it is in this soak temperature range, i.e. necessarily above 1436 F, that a
ferrite-
martensite microstructure should be expected. My work indicates that a
significant amount of bainite is present in the microstructure when the
annealing
soak temperature is in the Omiya recommended range and a hold zone in the
vicinity of 880 F is present in the thermal process. For the particular steel
used
in this example, the necessary annealing range for ferrite-martensite
microstructures is from about 1350 to 1430 F. Table 1 summarizes the
relationships between the thermal process, yield ratio and microstructural
constituents for this example at the different soak temperature regimes.
Table 1
Soak Temp Hold Temp Hold Time Yield Ratio Percent Percent
F F (sec) Martensite Bainite
1330 880 35 0.50 <3 <1
1330 880 70 0.52 <3 <1
1390 880 35 0.45 14.5 <1
1390 880 70 0.44 13.5 <1.
1510 880 35 0.52 4.5 11
1510 880 70 0.56 4.5 8.5
Example 2
A different cold rolled sheet steel of Composition A was subjected to the same
set
of thermal cycles a described in Example 1 and shown in Figure 1. This steel
also lay within the stated composition range, in this case specifically
containing
the following, in weight percent: 0.12%C, 1.96%Mn, 0.24%Cr, and 0.18%Mo,
and the balance of the composition typical for a low carbon Al-killed steel.
Once
again, the mechanical properties of the material were measured. The effect of
soak temperature on yield ratio for this steel for the 70 second holding
sequence at
880 F is shown in Figure 4. This curve exhibits a shape similar to the curves
in
Figure 3, with metallographic analyses revealing identical metallogical
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phenomena occurring at the different soak temperature regimes as in the
previous
example. Also as demonstrated in the previous example, the annealing soak
temperature range necessary for a predominantly ferrite-martensite
microstructure
to be obtained is from about 1350 to 1425 F when a hold step is conducted at
about880 F.
Example 3
As in the previous two examples, a third cold-rolled steel of Composition A
was
processed according to the set of thermal cycles shown in Figure 1. This steel
contained, in weight percent, 0.076C, 1.89 Mn, 0.10Cr, 0.094 Mo, and 0.34 Si,
the
balance of which is typical for a low carbon steel. After annealing as in the
other
examples, the mechanical properties and resultant microstructures were again
determined. Figure 5 shows the yield ratio of this material as a function of
soak
temperature for the holding time of 70 seconds. Once again, a curve having a
shape similar to the previous examples is observed, with a precise annealing
range
over which the dual phase ferrite-martensite microstructure is achieved.
However, note that the curve appears to be shifted to the right about 30 F as
compared to the previous examples. This is due to the fact that the Acl
temperature is higher for this steel as compared to the steels in the previous
two
examples due to the higher silicon. Table 2 shows the necessary soak
temperature
range for ferrite-martensite formation for each of the steels along with their
respective Acl temperature according to Andrews. The preferred annealing range
appears to be a function of the Acl temperature as shown. Generically, based
on
this information, the soak temperature range necessary for dual phase
production
depends on the specific steel composition - that is, it should lie within the
range
from AC1+45 F, but at least 1340 F (727 C), to AC1+135 F, but no more than
1425 F (775 C) when a holding step in the vicinity of 880 (885 F 35 F) is
present in the thermal cycle.
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Table 2
C Mn Cr Mo Si A.l AR for Necessary AR for DP
(wt%) (wt%) (wt%) (wt%) (wt%) ( F) FM( F)* Steel re A,,l**
.067 1.81 .18 .19 .006 1304 1350- Ac1+46 to A,l +126
1430
.12 1.96 .24 .18 .006 1303 1350- A.1+47 to Acl +117
1420
.076 1.89 .1 .094 .34 1318 1380- Ac1+62 to Ac1 +132
1450
*Annealing Range for Ferrite-Martensite (degrees Fahrenheit)
** Necessary Annealing Range for Dual Phase Steel with respect to Acl.
Example 4
Table 3 shows the resultant mechanical properties of two additional steels
having
carbon contents lower than shown previously. They were processed as described
in Figure 1 utilizing the individual soak temperatures of 1365, 1400, and 1475
F,
respectively and a hold time of 70 seconds at 880 F. Also shown within the
table
are the expected necessary soak temperature ranges for dual phase steel
production for each steel as calculated from A,,l as described in Example 3.
Note
that for the 1365 and 1400 F soak temperatures, which reside within the
desired
soak temperature range for both respective steels, low yield ratios
characteristic of
ferrite-martensite microstructures are observed. Furthermore, for the steels
soaked at 1475 F, which is outside the range present invention, the yield
ratio is
significantly higher due to the presence of bainite in the microstructure.
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Table 3
C Mn Mo Cr A.l Ac1+45 to Soak Yield UTS Yield
(wt%) (wt%) (wt%) (wt%) Ac1+135 ( F) Temp Strgth (MPa) Ratio
(MPa)
.032 1.81 .2 .2 1305 1350 to 1435 1365 223 473 0.47
.032 1.81 .2 .2 1305 1350 to 1435 1400 226 474 0.48
.032 1.81 .2 .2 1305 1350 to 1435 1475 261 462 0.56
.044 1.86 .2 .2 1304 1349 to 1434 1365 244 559 0.44
.044 1.86 .2 .2 1304 1349 to 1434 1400 239 548 0.44
.044 1.86 .2 .2 1304 1349 to 1434 1475 265 519 0.51
Additional data has been obtained which shows that manganese contents of less
than 1.5% may be used within my invention. Table 3a displays data collected in
a manner similar to that of Table 3:
Table 3a
C Mn Mo Cr Acl Ac1+45 - Soak YS UTS YS/L
Ac1+135 F TS
.058 1.23 0.4 0.2 1316 1361-1451 1400 251 524 0.48
.058 1.23 0.4 0.2 1316 1361-1451 1500 304 520 0.58
.121 1.22 0.4 0.2 1316 1361-1451 1400 291 619 0.47
.121 1.22 0.4 0.2 1316 1361 - 1451 1500 328 614 0.53
It will be seen from Table 3a that yield ratios no greater than 0.5 are
obtainable
with steel of this composition using a soak temperature of 1400 but not with a
soak temperature of 1500 F. Accordingly, contrary to my previous findings, it
is
not necessary to place absolute limits on the soak temperature range as
expressed
in the phrase "AC1+45 F, but at least 1340 F (727 C), to AC1+135 F, but no
more
than 1425 F (775 C)." Instead, the soak range may be defined as "AC1+45 F to
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AC1+135 F." My invention therefore includes the use of a steel composition as
recited above but wherein the manganese content may range from 0.8 - 2.4
weight percent as well as the previously stated range of 1.5 - 2.4 weight
percent.
In addition, my invention includes the use of a soak temperature in the range
of
AC1+45 F to AC1+135 F for the defined compositions, without caps. It should
be understood that I use the term Acl in the conventional manner, according to
Andrews: Acl (celsius) = 723-10.7(Mn)-16.9(Ni) +29.1(Si)+16.9(Cr) + 290(As)
+ 6.38(W), where each of the elements is expressed in terms of weight
percentages in the steel. For my purposes, the result is converted to
Fahrenheit.
Also, the elements not listed in the steel I use may possibly be present in
negligible amounts but may be ignored for purposes of Acl calculation.
Example 5
The previous examples were based on laboratory work, but mill trials have also
taken place that have verified the aforementioned thermal processing scheme
for
the production of both hot-dipped galvanized and galvannealed dual phase steel
product. Table 4 shows the results of mill trials for galvannealed steel. Note
that
the steels shown in the table have virtually the same composition and thus
similar
A,,t temperatures. From the Acl temperature, the expected soak temperature
range
for dual phase formation is calculated to be about 1350 to 1440 F.
Furthermore,
in terms of processing, hold temperatures and times are fairly consistent
among
the steels and the annealing (soak) temperature is the main processing
variable
difference between the materials. The mechanical properties are also shown in
the table along with corresponding yield ratios. Note that steels 1 through 4
were
soaked within the soaking range of the invention and exhibited the expected
yield
ratio of less than 0.5. Metallographic examination revealed the presence of
ferrite
martensite microstructures for steels 1 through 4 with martensite contents of
about
15%. Steel 5 was processed outside of the preferred soaking range and
exhibited
a relatively high yield ratio of about 0.61. Metallographic analysis showed a
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bainite content of 11 % in this material. Similar results have been shown for
galvanize as well as galvanneal processing.
Table 4
Steel 1 2 3 4 5
Carbon .067 .067 .067 .067 0.77
Mn 1.81 1.81 1.81 1.81 1.71
Cr .18 .18 .18 .18 .19
Mo .19 .19 .19 .19 .17
Aci 1304 1304 1304 1304 1306
AcI+45 to 1349- 1349- 1349- 1349- 1351-
Ac1+135 1439 1439 1439 1439 1441
( F)
Soak 1370 1383 1401 1421 1475
Temp
Hold Temp 878 881 885 888 890
Hold Time = 70 70 70 70 64
Yield 292 299 294 296 327
Strength
UTS 606 610 614 618 538
Yield Ratio .48 .49 .48 .48 .61
Supplemental laboratory work has shown that I need not be limited to a hold
temperature of 920 F; rather, a hold temperature as high as 940 F may be used
so
long as the soak temperature is within the prescribed range of AC1+45 F to
AC1+135 F. In table 5, where the Acl is 1304, the range is 1349 to 1439 F.
Here, where a 910 F hold temperature is used instead of the 880 F hold
temperature used in the majority of the previous examples, a soak temperature
of
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1500 F results in the undesirable yield ratio of 0.51 while a soak within the
prescribed range, 1400 F, resulted in an acceptable ratio.
Table 5
C Mn Mo
:1: - Cr Acl Soak Hold YS UTS Ratio
F F
0.67 1.81 0.18 0.19 1304 1400 910 278 635 0.44
0.67 1.81 0.4 0.2 1304 1500 910 310 606 0.51
Therefore, the hold temperature may be within the range of 850-940 F (that is,
895 F +45 F), and need not be limited to 850-920 F as previously stated.
