Note: Descriptions are shown in the official language in which they were submitted.
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WARM ROLLING OF STEELS CONTAINING METASTABLE AUSTENITE
100011
BACKGROUND
[0002] Cold rolling of' steels containing metastable austenite can be
challenging due to
deformation induced transformation of metastable austenite to a higher
strength
martensite phase. Cold rolling of such steel leads to a significant increase
in mill
loads. Steel also needs to undergo anneal ing(s) to partial or full
austenitization
before further cold reduction can be performed.
SUMMARY
[0003] The present invention involves warming the material before or
during cold rolling
to suppress the transformation of austenite to martensite. This can result in
lower
mill loads and higher amounts of reduction at similar loads. The ability to
reduce
material more can also lead to fewer intermediate anneals before material can
get
to final gauge. Surprisingly, as-warm rolled steel has shown enhanced
mechanical
properties when compared to steel reduced the same amount by cold rolling.
Warm rolling followed by subsequent annealing also results in better
mechanical
properties than those achieved in material cold rolled the same amount and
then
annealed. Steel that has been warm rolled, on subsequent room temperature
rolling (cold rolling), shows enhancement in both strength and ductility.
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100041 Previously, warm rolling has been avoided in the production
environment because
of concern that it may cause damage to rolling equipment as well as present
risks
related to warming of the oils used as lubricants. The present application
shows
that the benefits of warm rolling can be achieved at moderate temperatures and
without extensive line modifications.
DESCRIPTION OF FIGURES
[0005] Fig. 1 depicts percent martensite in a metastable steel as a
function of percent
reduction resulting from warm rolling and cold rolling.
[0006] Fig. 2 depicts percent elongation in a metastable steel as a
function of percent
reduction resulting from cold rolling and warm rolling.
[0007] Fig. 3(a) depicts true stress-true strain curves for a metastable
steel that was warm
rolled and then cold rolled.
[0008] Fig. 3(b) depicts true stress -true strain curves for a
metastable steel that was cold
rolled in two passes.
DETAILED DESCRIPTION
100091 This invention pertains to steels containing significant amount
of metastable
austenite (10(l/0-1000.Y0 austenite), referred to as "metastable steel."
Austenite is
deemed metastable if it transforms to martensite upon mechanical deformation.
Such martensite is called deformation-induced martensite. Steels containing
such
metastable austenite can be carbon steel or stainless steel.
100101 There are several ways to characterize the stability of
austenite. One way is to
calculate an Instability Factor (IF) of the austenite based on its chemical
composition. This factor was described in U.S. Patent 3,599,320, which defines
IF
as:
100111 IF=37.193 -51.248(%C) -0.4677(%Cr) -1.0174(%Mn) -34.396 (%N) -
2.5884(%Ni) Equation 1
2
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100121 Steels with calculated IF values from 0-2.9 are categorized as
"slightly
metastable" and steels with IF greater than 2.9 are categorized as "moderately
metastable". The method of the present invention has the most significance for
steel containing metastable austenite with an IF greater than 2.9.
[0013] Another technique to characterize the stability of austenite is
to calculate or
measure what is known as the Md30 temperature. For a given metastable steel
composition, on deformation to 0.3 true strain at the Md30 temperature, 50% of
the austenite transforms to martensite. For a given metastable steel
composition,
the Md temperature is the temperature above which no martensite is formed upon
deformation. Md and Md30 temperatures are well-known in the art. In addition
to
being empirically determined, the Md30 temperature for a particular steel
composition can also be calculated by one of the several equations that can be
found in literature, including the following:
[0014] As taught by Nohara, K., Ono, Y. and Ohashi, N. 1977. Composition
and Grain-
Size Dependencies of Strain-Induced Martensitic Transformation in Metastable
Austenitic Stainless Steels. Journal of' Iron and Steel Institute of Japan,
63(5), pp.
7P-722:
Md30= 551 -462(%C+%N) -68*%Cb -13.7*Cr - 29(%Cu+%Ni) -8.1*%Mn -
18.5*%Mo -9.2*%Si Equation 2
[0015] As taught by Angel, T. 1954. Formation of Martensite in
Austenitic Stainless
Steels. Journal of the Iron and Steel Institute, 177 (5), pp. 165-174:
Md30¨ 413 -462*(%C+%N) -13.7*%Cr -8.1*%Mn -18.5*0/Mo -9.5*%Ni -
9.2*9/0Si Equation 3
100161 The higher is the Md30 temperature of the austenite of a given
metastable steel
composition, the more unstable is the austenite. Md30 temperature in such
metastable austenite is above the Ms temperature (martensite start temperature
of
thermal martensite).
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[0017] Steels with a significant amount of metastable austenite work
hardens rapidly as
the austenite transforms to higher strength martensite. This work hardening,
and
resulting martensite, can present a challenge when further cold rolling such
steels
because they can require loads that may exceed a mill's capability. Such
metastable steels then need to be annealed to form some or all austenite
before
they can be cold rolled further. If during rolling, transformation of
austenite to
martensite can be suppressed, the steel can be rolled to thinner gauges, with
lower
mill loads. One way to suppress such transformation is to warm the material
prior
to or during cold rolling. Waim rolling has shown to have additional benefit
resulting in better mechanical properties.
[0018] The methods of the present application involve rolling such
metastable steels
while the steel is warm. It is considered warm when the metastable steel
temperature is above room temperature (typically about 80 F). For certain
embodiments, the steel is warmed to a temperature near or above the Ma
temperature for the particular metastable steel composition. In other
embodiments, the steel is warmed to a temperature above the Ma30 temperature
for the particular metastable steel composition. In other embodiments, the
metastable steel is warmed to a temperature less than or equal to 250 F.
[0019] The coils of such material can be warmed in ways that will be
apparent to one of
skill in the art, including one of or a combination of the following methods.
[0020] I. Warm the coil in a furnace/oven prior to putting it on the
rolling line.
[0021] II. Warm the coil on the line by using heaters, before it is cold
rolled.
[0022] III. Warm the coolant on the mill before rolling the steel
material. This can be
performed in several ways. One way is to turn off the cooling tower on rolling
mill and run some other material to warm up the coolant prior to rolling the
metastable steel. Other methods of warming the coolant prior to rolling will
be
apparent to those of skill in the art.
[0023] The metastable steel is melt, cast, hot rolled, and annealed prior
to cold rolling (if
applicable) in accordance with typical metal-making processing for the
particular
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composition. During the cold rolling processing of the metastable steel, at
least
one "cold rolling" pass is a "warm rolling" pass that is performed while the
steel
is wami, i.e., while the steel is at a temperature above 80 F. In some
embodiments, the steel is warmed to a temperature no greater than 250 F. In
other
embodiments, the metastable steel is warmed to a temperature near or above the
Ma temperature for the particular metastable steel composition. And in other
embodiments, the metastable steel is warmed to a temperature near or above the
WO temperature for the particular metastable steel composition. Such warm
rolling passes can be one or more of the first, second, or any subsequent
"cold
rolling" steps.
[0024] In some embodiments of the present invention, the metastable steel
may be
annealed after one or more warm rolling step. For example, during the "cold
rolling" processing, the metastable steel may be warm rolled in a first pass,
annealed, and then cold rolled (at room temperature) in a second pass.
[0025] Example 1
[0026] A metastable steel was prepared by melting a heat with a chemistry
that had an
Instability Factor of 8.5 and WO (Nohara) = 447.6 F. The heat was
continuously cast into slabs. The slabs were re-heated to 2300 F and hot
rolled to
a thickness of 0.175", with a coiling temperature of 1000 F. The hot band was
the then pickled to remove the scale. Sections of the pickled hot bands were
cold
rolled and warm rolled. For purpose of waini rolling, the hot band sections
were
warmed to desired temperatures in a furnace and rolled to desired gauges.
[0027] Figure 1 shows the amount of martensite transformation from cold and
warm
rolling of such metastable steel. At the same amount of reduction, the amount
of
martensite in each warm rolled steel is significantly less than in cold rolled
steel,
which was rolled at room temperature. The benefits of warm rolling can be seen
at low temperatures (150 F in this example) but the higher the temperature
during warm rolling (250 F in this example), the lower is the amount of
martensite formed.
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[0028] Figure 2 shows the % elongation of the metastable steel, after warm
rolling and
cold rolling to different reduction amounts. Surprisingly, warm rolling led to
an
increase in % elongation till certain amount of reduction before starting to
drop.
The benefits of warm rolling can be tailored by either varying the amount of
reduction performed at a temperature or by varying the temperature. On the
other
hand, cold rolling at room temperature always leads to a decrease in ?/0
elongation
for metastable steels.
[0029] Example 2
[0030] Another metastable steel was prepared by selecting a chemistry with
an Instability
Factor of 13.11 and Md30 (Nohara) = 227.6 F. The heat was cast into ingots.
After trimming the ingots, four bars of 5.75" (W) x 2.75" (T) x 2.75" (L) were
obtained. These trimmed ingots were soaked at 2200 F and hot rolled to 0.2"
with a finishing temperature of 1100 F. The hot band was then pickled to
remove
the scale. Sections of the pickled hot bands were cold rolled and walm rolled
at
different temperatures. For purposes of warm rolling, the hot band sections
were
warmed to the desired temperatures in a furnace and rolled to desired gauges.
[0031] In such metastable steel, waini rolling followed by cold rolling
showed an
increase in both strength and % elongation. Without prior warm rolling, the
same
steel showed an increase in strength but a decrease in % elongation, as
expected.
Fig. 3(a) shows true stress strain data from the metastable steel that had
been
warm rolled 30% and subsequently cold rolled at room temperature to various
reductions. In Fig. 3(a) and 3(b), "WR" refers to warm rolling and "RT" refers
to
cold rolling at room temperature. 30% warm rolling followed by additional 10%
cold rolling showed an increase in both elongation and strength. The same
material when cold rolled by 30% followed by an additional cold rolling at
room
temperature of 0-30%, as shown in Fig. 3(b), showed an increase in ultimate
tensile strength ("UTS") but a decrease in elongation, as one would expect.
Again, the benefits of warm rolling can be tailored by either varying the
amount
of reduction performed at a temperature or by varying the temperature.
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[00321 Example 3
[0033] The metastable steel of Example 1 above shows the effect of warm
rolling on
steel containing metastable austenite as further shown by the test data set
forth in
the Tables 1 and 2 below, which compares properties of the steel containing
metastable austenite that has been fully annealed (Coil 1) with steel
containing
metastable austenite that was 25% warm rolled in the plant (Coil 2).
Table 1
Longitudinal
Yield Strength UTS (MPa) Elong. Hardness
@ 0.2% Offset (Manual in 2-)
(MPa) (%)
Coil 1 Coil 2 Coil 1 Coil 2 Coil 1 Coil 2 Coil 1
Coil 2
(HRB) (HRC)
Avg. 386.2 1197.6 1142.4 1551.8 52.6 21.8 98 46.6
Min. 376.1 1173.7 1126.7 1561.6 48.8 18.9
Max 394 1221.5 1164.8 1540.2 57.9 24
Transverse
0.2% OYS UTS (MPa) Elong. Hardness
(MPa) (Nlanual in 2")
(%)
Coil 1 Coil 2 Coil 1 Coil 2 Coil 1 Coil 2 Coil 1
Coil 2
.................................................... (MB) (HRC)
Avg. 423.6 1111.9 1128.8 1520.8 54.5 19.6 98 47.3
Min. 394.6 1092.7 1104.5 1504.5 50.7 17
Max 435.7 1133.1 1149.3 1531.2 57.7 21.8
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Table 2
Test Average (Coil 1) Average (Coil 2) __
Ultimate Tensile Strength 1188 MPa 1551.8 MPa
Yield Strength @ 0.2% 378 MPa 1197.6 MPa
Offset
Elongation 54.6% 21.8%
Uniform Elongation 51.5% 20.4%
Plastic Strain Ratio -------------- 0.80 ________________ 0.86 ------
LDH (Limiting Dome 2.26" 1.32"
Height)
LDR (Limiting Draw Ratio) 1.9 1.5
HER (Hole Expansion 5%, 10%, 21%, 39%, 45%
3.6%, 11.2%, 20.6%, 20.1%,
Ratio) (0.25mm/s, 8mm/s, 23.2%
28mm/s, 114mm/s, 228mm/s)
Hardness 98 HRB 46.6 I-IRC
[0034] Example 4
[0035] The effect of warm rolling on anistropy was also studied on the
metastable steel
of Example 1. Anisotropy can have a significant effect on subsequent forming.
Warm rolling helpd manage anisotropy in mechanical properties of metastable
steels.
[0036] The effect of warm rolling compared to cold rolling is further
demonstrated by the
data set forth in Table 3 below. The initial hot band was the same for both
sets of
rolling. One set was warm rolled (@ ¨250 F) to various reductions (10, 15 and
20%) , the other was cold rolled to similar reductions. In the case of the
cold
rolled samples, elongations in longitudinal (L) and transverse (T)
orientations
differ quite a bit. The higher the amount of reduction, the bigger is the said
difference. However, in the case of warm rolling, the difference is much
smaller.
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Table 3
% _____________________________________________________________
Reduction % Elongation
Cold Rolling Warm Rolling
________ L T (3/0 Difference L T % Difference
10.0 21.1 15.0 40.7 22.1 16.4 34.8 1
15.0 18.8 10.1 86.1 19.7 12.6 56.3 1
20.0 138.2 19.8 13 6 45.LI
18.1 7.6-L. ' -&-
%
Reduction UTS
Cold Rolling Warm Rolling
:
L T % Difference : L T %
Difference i
1
10.0 1114.0 1073.7 3.8 1065.4 1059.7 0.5 r
4k
15.0 1226.6 1161.7 5.6 1199.3 1147.0 4.6
20.0 _1321.2_1204.3 ...... 97:,1 1226.2 , 1190.5 1 3.pdl
%
Reduction 0.2% OYS
-Ti- ,
Cold Rolling 1 Warm Rolling
L T % Difference 1 L T . % Difference
.
10.0 577.8 600.2 -3.7 1 534.5 596.7 . -10.4
15.0 724.9 688.8 5.2 664.3 674.1 -1.5
20.0 789.3 9.7
719'2-L -..t.
9
