U.S. patent application number 15/505505 was filed with the patent office on 2017-09-21 for high strength austenitic stainless steel and production method thereof.
The applicant listed for this patent is Outokumpu Oyj. Invention is credited to Juho Talonen.
Application Number | 20170268076 15/505505 |
Document ID | / |
Family ID | 55350237 |
Filed Date | 2017-09-21 |
United States Patent
Application |
20170268076 |
Kind Code |
A1 |
Talonen; Juho |
September 21, 2017 |
High Strength Austenitic Stainless Steel and Production Method
Thereof
Abstract
An austenitic stainless steel including in weight % 0-0.4% C,
0-3% Si, 3-20% Mn, 10-30% Cr, 0-4.5% Ni, 0-3% Mo, 0-3% Cu,
0.05-0.5% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V, the balance of Fe and
inevitable impurities. The content of at least one of the elements
in the group of niobium (Nb), titanium (Ti) or vanadium (V) is more
than 0.05% so that the total amount of niobium (Nb), titanium (Ti)
and vanadium (V) contents is in the range of 0.05-0.5%. The grain
size of the steel is less than 10 micrometer after annealing the
cold deformed product and the difference between the yield
strengths of the steel measured in transverse and parallel
directions to the rolling direction is lower than 5%. Also, a
method for producing the austenitic stainless steel.
Inventors: |
Talonen; Juho; (Espoo,
FI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Outokumpu Oyj |
Helsinki |
|
FI |
|
|
Family ID: |
55350237 |
Appl. No.: |
15/505505 |
Filed: |
August 21, 2015 |
PCT Filed: |
August 21, 2015 |
PCT NO: |
PCT/FI2015/050539 |
371 Date: |
February 21, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/44 20130101;
C22C 38/46 20130101; C21D 1/26 20130101; C21D 8/0236 20130101; C22C
38/002 20130101; C22C 38/50 20130101; C22C 38/02 20130101; C21D
8/0273 20130101; C21D 9/46 20130101; C22C 38/42 20130101; C22C
38/48 20130101; C22C 38/58 20130101; C22C 38/001 20130101; C21D
2211/001 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/48 20060101 C22C038/48; C21D 8/02 20060101
C21D008/02; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C22C 38/58 20060101 C22C038/58; C22C 38/42 20060101
C22C038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 21, 2014 |
FI |
20145735 |
Claims
1. An austenitic stainless steel, comprising, in weight 0-0.4% C,
0-3% Si, 3-20% Mn, 10-30% Cr, 0-4.5% Ni, 0-3% Mo, 0-3% Cu,
0.05-0.5% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V, the balance Fe and
inevitable impurities, wherein the content of at least one of the
elements in the group consisting of niobium (Nb), titanium (Ti) and
vanadium (V) is more than 0.05% so that the total amount of the
niobium (Nb), titanium (Ti) and vanadium (V) contents is in the
range of 0.05-0.5%, and after annealing of the cold deformed steel,
the grain size is less than 10 micrometers and a difference between
the yield strengths of the steel measured in transverse and
parallel directions to the rolling direction is less than 5%.
2. The austenitic stainless steel according to claim 1, wherein the
grain size of the steel is less than 7 micrometers.
3. The austenitic stainless steel according to claim 1, wherein the
steel comprises 0-1.5% molybdenum.
4. The austenitic stainless steel according to claim 1, wherein the
steel comprises 0.05-0.30% Nb.
5. The austenitic stainless steel according to claim 1, wherein the
steel comprises 0.05-0.30% Ti.
6. The austenitic stainless steel according to claim 1, wherein the
steel comprises 0.05-0.30% V.
7. A method for producing an austenitic stainless steel,
comprising, in weight %, 0-0.4% C, 0-3% Si, 3-20% Mn, 10-30% Cr,
0-4.5% Ni, 0-3% Mo, 0-3% Cu, 0.05-0.5% N, 0-0.5% Nb, 0-0.5% Ti,
0-0.5% V, the balance of Fe and inevitable impurities, wherein the
content of at least one of the elements in the group consisting of
niobium (Nb), titanium (Ti) and vanadium (V) is more than 0.05% so
that the total amount of niobium (Nb), titanium (Ti) and vanadium
(V) is in the range of 0.05-0.5%, the method comprising cold
deforming the steel with a reduction degree of at least 50% and
annealing the steel, wherein after annealing, the steel has a grain
size of less than 10 micrometers, and the difference between the
yield strengths of the steel measured in transverse and parallel
directions to the rolling direction is lower than 5%.
8. The method according to claim 7, wherein the steel is annealed
at a temperature of 700-1050.degree. C. for 1-400 seconds.
9. The method according to the claim 7 or 8, wherein the
deformation is cold rolling.
10. The austenitic stainless steel according to claim 1, wherein
the grain size of the steel is less than 5 micrometers.
11. The austenitic stainless steel according to claim 1, wherein
the steel comprises 0-0.5% molybdenum.
12. The austenitic stainless steel according to claim 1, wherein
the steel comprises 0.05-0.20% Nb.
13. The austenitic stainless steel according to claim 1, wherein
the steel comprises 0.05-0.20% Ti.
14. The austenitic stainless steel according to claim 1, wherein
the steel comprises 0.05-0.20% V.
15. The method according to claim 8, wherein the steel is annealed
for 1-200 seconds.
Description
[0001] This invention relates to a high strength austenitic
stainless steel exhibiting good combination of strength and
elongation and high isotropy of the mechanical properties. The
invention relates also to the production method of the steel.
[0002] Yield strength of austenitic stainless steel in annealed
condition is relatively low. A conventional method for increasing
the yield strength of austenitic stainless steels strip is temper
rolling, i.e., strengthening of the steel strip by cold-rolling.
Temper rolling, however, has an important disadvantage: mechanical
properties of temper-rolled steel tend to be highly anisotropic.
For instance, yield strength of temper-rolled austenitic stainless
steel may be up to 20% higher in transverse direction compared to
direction parallel to the rolling direction. The anisotropy is a
drawback that, for instance, makes the forming of the austenitic
stainless steel more difficult.
[0003] Furthermore, temper rolling increases the strength at the
expense of elongation. For some austenitic stainless steel grades,
remaining elongation and formability after the temper rolling
process may be too low.
[0004] The refinement of grain size of steel is a well-known and
efficient method to increase yield strength of austenitic stainless
steels. The method can be utilized instead of temper rolling. Yield
strength of the steel increases with decreasing grain size
according to the well-known Hall-fetch relationship. The refinement
of grain size compared to temper rolling has also the advantage
that the anisotropy of the mechanical properties is substantially
lower. However, the production of fine grained steel is difficult,
because the grain growth is very fast at its initial stages, and
thus, the process window, i.e. the allowable time and temperature
range to reach a certain small grain size and strength level, may
be too small. If the process window is too small, the mechanical
properties may vary too much along the steel strip. In the case the
target mechanical properties cannot be reached, substantial yield
losses may occur.
[0005] It is well known that grain growth can be restricted by
addition of carbide and nitride forming elements to the austenitic
stainless steel. These elements form carbides and nitrides, which
limit the grain growth due to so called Zener pinning effect. For
instance, the JP publication 2010215953 discloses an austenitic
stainless steel containing niobium (Nb), titanium (Ti) or vanadium
(V). However, a drawback of this steel is that it contains at least
4.5 weight % nickel (Ni). The JP publication 2014001422 relates to
an austenitic stainless steel plate, with an average crystal grain
size in the parent phase 10 .mu.m or less, and to its manufacturing
method, which steel contains in weight % C: 0.02 to 0.30%, Cr: 10.0
to 25.0%, Ni: 3.5 to 10.0%, Si: 3.0% or less, Mn: 0.5% to 5.0%, N:
0.10 to 0.40%, C+3.times.N: 0.4% or more and Fe and impurities as
the balance, and further optionally Mo: <3%, Cu: <3%, Nb:
<0.5%, Ti: <0.1% and V: <1 so that the sum of Nb+Ti+V is
0-1.6%. According to this JP publication 2014001422 when using Nb,
Ti and V as the alloying components the nickel content is at the
range of 5.0-6.6 weight %. Due to the high and fluctuating nickel
price, such austenitic stainless steel is not enough cost
efficient. There is market demand for more cost efficient
low-nickel high strength austenitic stainless steels.
[0006] The object of the present invention is to prevent drawbacks
of the prior art and to produce a cost efficient high strength
austenitic stainless steel exhibiting small grain size, high
strength and isotropic mechanical properties. The invention relates
also to the method of processing of the steel, and on the alloying
of the steel with carbide and nitride forming elements in order to
restrict grain growth and thus improve the processability of the
steel. The essential features of the present invention are enlisted
in the appended claims.
[0007] According to invention an austenitic stainless steel is
alloyed with carbide and nitride forming elements, such as niobium
(Nb), titanium (Ti) and vanadium (V). These elements for carbide
and nitride precipitates effectively restrict grain growth. Thus,
during the annealing process carried out to produce a fine grain
size for a cold deformed product made of the austenitic stainless
steel of the invention, the presence of these carbide precipitates
and nitride precipitates enables a larger process window and
processability. In order to provide a sufficiently strong effect,
more than 0.05 weight % of at least one of the elements in the
group of niobium (Nb), titanium (Ti) or vanadium (V) shall be
added. In order to keep the austenitic stainless steel cost
efficient, the total amount of niobium (Nb), titanium (Ti) and
vanadium (V) is lower than 0.5 weight %.
[0008] The austenitic stainless steel according to the invention is
made cost efficient by the reduction of the nickel content compared
to conventional nickel-containing austenitic stainless steels.
Therefore, the steel according to the invention does not contain
more than 4.5 weight % nickel.
[0009] The stainless steel of the invention is an austenitic
stainless steel containing in weight % 0-0.4% C, 0-3% Si, 3-20% Mn,
10-30% Cr, 0-4.5% Ni, 0-0.5% Mo, 0-3% Cu, 0.05-0.5% N, 0-0.5% Nb,
0-0.5% Ti, 0-0.5% V. the total amount of the niobium (Nb), titanium
(Ti) and vanadium (V) contents being at the range of 0.05-0.5% so
that the content of at least one of the elements in the group of
niobium (Nb), titanium (Ti) or vanadium (V) is more than 0.05%, the
balance of Fe and inevitable impurities, such as phosphorus,
sulphur and oxygen. In order to ensure desirable mechanical
properties, the grain size after annealing for a cold deformed
product is lower than 10 micrometers, preferably lower than 7
micrometers, and more preferably lower than 5 micrometers. The
difference between the yield strengths of the stainless steel
measured in transverse and parallel directions to the rolling
direction is less than 5%.
[0010] The high strength austenitic stainless steel according to
the invention is produced via the conventional stainless steel
process route including among others melting, AOD (Argon Oxygen
Decarburization) converter and ladle treatments, continuous
casting, hot rolling, cold rolling, annealing and pickling.
However, the austenitic stainless steel according to the invention
is annealed below the temperature of 1050.degree. C., which
temperature is lower than in a conventional production process.
Lowering of the annealing temperature slows the grain growth, and
thus smaller grain size and higher yield strength can be achieved.
However, in order to avoid harmful sensitization phenomenon, the
annealing temperature shall be higher than 700.degree. C. The
desired annealing temperature range is thus 700-1050.degree. C.,
and the annealing time is 1-400 seconds, preferably 1-200 seconds.
The cold deformation reduction, such as the cold rolling reduction,
before the annealing process shall be high enough to enable
formation of fine grain size. The deformation reduction degree,
such as cold rolling reduction degree shall be at least 50%.
[0011] The present invention is described in more details referring
to the following drawings, in which
[0012] FIG. 1 shows influence of annealing time and temperature on
grain size of a reference alloy containing no niobium,
[0013] FIG. 2 shows influence of annealing time and temperature on
grain size of a test alloy according to the invention containing
0.05% niobium,
[0014] FIG. 3 shows influence of annealing time and temperature on
grain size of a test alloy according to the invention containing
0.11% niobium,
[0015] FIG. 4 shows influence of annealing time and temperature on
grain size of a test alloy according to the invention containing
0.28% niobium,
[0016] FIG. 5 shows influence of annealing time and temperature on
grain size of a test alloy according to the invention containing
0.45% niobium and
[0017] FIG. 6 shows the annealing window, i.e., combinations of
annealing time and temperature, corresponding to reaching 2-3
micrometer (.mu.m) grain size in test alloys containing no niobium
and 0.11% niobium.
[0018] Five austenitic test alloys 1-5 with varying amounts of
niobium were studied. The chemical compositions of the test alloys
are shown in Table 1.
TABLE-US-00001 TABLE 1 Chemical compositions of the test alloys 1-5
C Si Mn P S Cr Ni Nb Cu N (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) 1
0.079 0.40 9.0 0.032 0.004 15.2 1.1 0 1.7 0.115 2 0.070 0.30 9.1
0.006 0.005 15.2 1.1 0.05 1.7 0.165 3 0.072 0.28 9.2 0.006 0.005
15.2 1.1 0.11 1.7 0.130 4 0.083 0.28 9.2 0.006 0.004 15.1 1.1 0.28
1.7 0.160 5 0.100 0.30 8.9 0.008 0.006 15.2 1.1 0.45 1.7 0.160
[0019] The alloy 1 was produced in full-scale production and the
alloys 2-5 in a pilot scale production unit. After melting, casting
and hot rolling, the materials were subjected to a 60% cold rolling
reduction. Annealing tests were performed on the cold rolled
materials at different temperatures and for varying annealing times
with a Gleeble 1500 thermomechanical simulator. The heating rate
was 200.degree. C./s and the cooling rate 200.degree. C./s down to
400.degree. C. before natural air cooling.
[0020] FIGS. 1-5 show the influence of the annealing time and the
annealing temperature on the resulting grain size for alloys 1, 2,
3, 4 and 5 with different niobium (Nb) contents, respectively. From
the figures it can be observed, that grain growth was substantially
restricted by niobium alloying, because the area of for instance
under 5 micrometer (.mu.m) in the time-temperature coordinate
system of the FIGS. 1-5 will increase in accordance with the
increase of the niobium content. Correspondingly, the contour lines
corresponding to different grain sizes were shifted to the top
right direction, indicating that the allowable range of annealing
temperatures and times became larger when niobium (Nb) was added to
the austenitic stainless steel according to the invention.
Furthermore, it can be observed that relatively large effect was
achieved already with 0.11 weight % niobium (Nb) alloying. Further
increase in the niobium (Nb) content did not have a strong further
effect on the grain growth.
[0021] FIG. 6 further demonstrates the beneficial effect of the
niobium (Nb) content. FIG. 6 presents the annealing window, i.e.,
the allowable combinations of the annealing temperature and the
annealing time for reaching the grain size of 2-3 micrometers
defined based on the experimental results. It is obvious that the
annealing window is much larger for the alloy 3 with 0.11 weight %
niobium (Nb). For instance, at the temperature range around
900.degree. C. the allowable annealing time range for the alloy 1
without niobium (Nb) was only about 1-10 s, whereas for the alloy 3
with 0.11 weight % niobium (Nb) the allowable annealing time range
was 1-100 s. Such difference makes processing of the alloy 3 more
feasible, resulting in more uniform product quality and better
yield and efficiency.
[0022] In order to study the effect of the production method
according to the present invention on mechanical properties of
stainless steels, the two more alloys were tested. The chemical
compositions of these alloys are shown in Table 2.
TABLE-US-00002 TABLE 2 Chemical compositions of the test alloys 6
and 7 C Si Mn P S Cr Ni Mo Nb Cu N (%) (%) (%) (%) (%) (%) (%) (%)
(%) (%) (%) 6 0.77 0.28 9.11 0.007 0.006 16.0 2.0 1.1 0.17 2.0
0.212 7 0.10 0.27 9.14 0.007 0.004 17.0 1.0 0 0.16 2.1 0.241
[0023] The alloys 6 and 7 were produced in a pilot scale production
unit. As the alloys 1-5 after melting, casting and hot rolling, the
alloys 6 and 7 were subjected to a 60% cold rolling reduction.
Tensile test samples were cut from the cold rolled sheets in the
angles 0.degree., 45.degree. and 90.degree. to the rolling
direction. The tensile test samples were subsequently annealed in a
laboratory furnace at temperatures of 900.degree. C. and
950.degree. C. for 300 seconds and water quenched.
[0024] Table 3 presents test results of these samples measured in
the tensile test directions having the angles of 0.degree.,
45.degree. and 90.degree. to the rolling direction. Also the grain
sizes of the materials are shown. It can be observed that the
measured yield strength values measured in different directions are
close to each other, i.e., the properties do not exhibit high
anisotropy. The difference between the yield strengths of the
alloys 6 and 7 measured in transverse and parallel directions to
the rolling direction is less than 5%. Furthermore, the grain size
of the alloys 6 and 7 has remained at low levels despite the rather
long annealing time due to the beneficial effect of the Nb
alloying, which has resulted in attractive mechanical
properties.
TABLE-US-00003 TABLE 3 Results for mechanical properties for the
alloys 6 and 7 Yield Tensile Elongation Grain Tensile strength
strength to fracture Annealing size test Rp0.2 Rm A80 mm Alloy
temperature .mu.m direction MPa MPa % 6 900.degree. C. 4.1 0 522
842 38 6 900.degree. C. 4.1 45 536 816 31 6 900.degree. C. 4.1 90
518 816 39 6 950.degree. C. 3.7 0 478 833 39 6 950.degree. C. 3.7
45 477 802 34 6 950.degree. C. 3.7 90 481 802 40 7 900.degree. C.
2.9 0 566 886 38 7 900.degree. C. 2.9 45 539 859 39 7 900.degree.
C. 2.9 90 544 864 33 7 950.degree. C. 3.2 0 523 862 38 7
950.degree. C. 3.2 45 522 837 39 7 950.degree. C. 3.2 90 504 827
25
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