U.S. patent application number 09/892370 was filed with the patent office on 2002-01-24 for ferritic stainless steel.
This patent application is currently assigned to Kawasaki Steel Corporation. Invention is credited to Hirasawa, Junichiro, Miyazaki, Atsushi, Muraki, Mineo, Satoh, Susumu.
Application Number | 20020007876 09/892370 |
Document ID | / |
Family ID | 18699838 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020007876 |
Kind Code |
A1 |
Hirasawa, Junichiro ; et
al. |
January 24, 2002 |
Ferritic stainless steel
Abstract
Ferritic stainless steel comprising all three of Co, V, and B,
having a Co content of about 0.01 mass % to about 0.3 mass %, a V
content of about 0.01 mass % to about 0.3 mass %, and a B content
of about 0.0002 mass % to about 0.0050 mass %, and having superior
secondary working embrittleness resistance and superior high
temperature fatigue characteristics.
Inventors: |
Hirasawa, Junichiro; (Chiba,
JP) ; Miyazaki, Atsushi; (Chiba, JP) ; Muraki,
Mineo; (Chiba, JP) ; Satoh, Susumu; (Chiba,
JP) |
Correspondence
Address: |
IP Department
Schander Harrison Segal & Lewis
36th Floor
1600 Market Street
Philadelphia
PA
19103
US
|
Assignee: |
Kawasaki Steel Corporation
|
Family ID: |
18699838 |
Appl. No.: |
09/892370 |
Filed: |
June 27, 2001 |
Current U.S.
Class: |
148/325 ;
420/38 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/52 20130101; C22C 38/54 20130101; C22C 38/44 20130101; C22C
38/48 20130101; C22C 38/46 20130101; C22C 38/001 20130101; C22C
38/004 20130101 |
Class at
Publication: |
148/325 ;
420/38 |
International
Class: |
C22C 038/18 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 7, 2000 |
JP |
2000-202296 |
Claims
What is claimed is:
1. A ferritic stainless steel or welded part thereof, which has a
composition, on a weight percentage basis, comprising about: 0.2%
or less of C; 0.2% to 1.0% of Si; 0.1% to 1.5% of Mn; 0.04% or less
of P; 0.01% or less of S; 11.0% to 20.0% of Cr; 0.1% to 1.0% of Ni;
1.0% to 2.0% of Mo; 1.0% or less of Al; 0.2% to 0.8% of Nb; 0.02%
or less of N; 0.01% to 0.3% of Co; 0.01% to 0.3% of V; 0.0002% to
0.0050% of B; and the remainder being Fe and incidental
impurities.
2. A ferritic stainless steel according to claim 1, wherein
contents of Co, V, and B fall substantially within the range
represented by the following formula
0.1.ltoreq.[Co]+0.5.times.[V]+100.times.[B].ltoreq.0.5 where [Co],
[V] and [B] indicate contents by weight percentage.
3. A ferritic stainless steel according to claim 1, which has a
composition, on a weight percentage basis, further comprising at
least one element selected from the group consisting of about 0.05%
to 0.5% of Ti, about 0.05% to 0.5% of Zr, and about 0.05% to 0.5%
of Ta.
4. A ferritic stainless steel according to claim 1, which has a
composition, on a weight percentage basis, further comprising about
0.1% to 2.0% of Cu.
5. A ferritic stainless steel according to claim 1, which has a
composition, on a weight percentage basis, further comprising at
least one element selected from the group consisting of about 0.05%
to 1.0% of W and about 0.001% to 0.1% of Mg.
6. A ferritic stainless steel according to claim 1, which has a
composition, on a weight percentage basis, further comprising about
0.0005% to 0.005% of Ca.
7. The ferritic stainless steel defined in claim 1, wherein the
limits of the formula [Co]+0.5[V]+100[B] are from 0.07-0.57, where
[Co], [V] and [B] indicate contents by weight percentage.
8. The steel or welded part thereof, as defined in claim 1, wherein
the weight percentage Co is about 0.02 to 0.3%, the weight
percentage V is about 0.05 to 0.3%, and the percentage B is about
0.0005 to 0.0050%.
9. The steel or welded part thereof, as defined in claim 1, wherein
the weight percentage Cr is about 14-16
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a novel ferritic stainless
steel. It particularly includes a welded ferritic stainless steel
and welded product having superior secondary working embrittleness
resistance and superior high temperature fatigue characteristics,
and concerns welded parts that are suitable for applications in
which a welded pipe or a welded plate, after having undergone
forming work, is used.
[0003] The expression "secondary working" as used herein refers to
the processing of a specified part after having already having
subjected it to forming work. For example, a welded pipe may be
subjected to bending work (primary working), and thereafter, to
pipe diameter enlargement work (secondary working).
[0004] In known ferritic stainless steels, cracks due to
brittleness are likely to form during secondary working.
[0005] The expression "high temperature fatigue" as used herein
refers to a phenomenon wherein fatigue fracture of a material
occurs due to repetitive bending at high temperatures of
600.degree. C. or more.
[0006] For example, welded parts of components of an exhaust pipe
system in an automobile undergo secondary working and high
temperature fatigue. Among them, an exhaust manifold, as shown in
FIG. 1 of the drawings, is subjected to severe conditions during
operation, and undergoes intense vibration at high temperatures of
600.degree. C. or more due to the action of engine exhaust gas.
This is a typical example. The present invention is preferably
applied to, for example, an exhaust manifold of ferritic stainless
steel, and other welded products.
[0007] 2. Description of the Related Art
[0008] When a welded pipe that has been subjected to complicated
bending work, or pipe diameter enlargement or reduction is used,
for example, as an exhaust manifold of an automobile, problems
arise because cracks occur in welded parts that had already become
brittle due to secondary working. Fatigue cracks occur in welded
parts during use, due to insufficient strength at a high
temperature.
[0009] The primary reason cracks are likely to occur in welded
parts, rather than base materials, is that the toughness and
strength of the welded parts deteriorate because crystal grains of
the welded parts become coarse due to heat input during
welding.
[0010] A ferritic steel containing an intervening material,
Al.sub.2O.sub.3, has been suggested in Japanese Unexamined Patent
Publication No. 11-172369. However, the aforementioned kind of
steel exhibits insufficient secondary working embrittleness which
causes cracks in the welded parts. Whether or not high temperature
fatigue characteristics are achieved, serious cracks frequently
occur as a result of the harmful secondary working
embrittleness.
[0011] In order to reduce an intervening material introduced into
the steel, Al.sub.2O.sub.3, Si or Mn must be used as a deoxidizer
in the steel making process. Accordingly, Al, widely used as a
deoxidizer, cannot be used in production of welded products free of
defects caused by harmful secondary working embrittleness.
[0012] A ferritic stainless steel having improved secondary working
embrittleness resistance by adding phosphide, and controlling its
size and amount, was suggested in Japanese Unexamined Patent
Publication No. 7-126812. When P is added, however, degradation of
toughness of the welded product cannot be avoided. It is believed
that this is a result of segregation of P at the grain boundaries
of the welded part, due to heat input during welding.
[0013] Furthermore, high temperature fatigue characteristics of a
welded part are not improved by controlling the amount of
phosphide. Accordingly, high temperature fatigue cracks cannot be
prevented by the addition of P to the steel.
[0014] As described above, regarding improvements of secondary
working embrittleness resistance and high temperature fatigue
characteristics, various suggestions have been made. However, no
ferritic stainless steel having both of these advantageous
properties has been discovered.
[0015] It is an object of this invention to do so.
SUMMARY OF THE INVENTION
[0016] It is an object of the present invention to meet the
aforementioned demand and to provide the significant advantages
heretofore detailed.
[0017] It is a further object of the present invention to provide a
ferritic stainless steel in which both secondary working
embrittleness resistance and high temperature fatigue
characteristic of welded parts are improved.
[0018] A ferritic stainless steel and a ferritic stainless steel
welded part are provided with both superior secondary working
embrittleness resistance and high temperature fatigue
characteristic in accordance with this invention.
[0019] The ferritic stainless steel of this invention has a
composition, on a weight percentage basis, composed of about: 0.02%
or less of C, 0.2% to 1.0% of Si, 0.1% to 1.5% of Mn, 0.04% or less
of P, 0.01% or less of S, 11.0% to 20.0% of Cr, 0.1% to 1.0% of Ni,
1.0% to 2.0% of Mo, 1.0% or less of Al, 0.2% to 0.8% of Nb, 0.02%
or less of N, 0.01% to 0.3% of Co, 0.01% to 0.3% of V, 0.0002% to
0.0050% of B, and the remainder Fe and incidental impurities.
[0020] The ferritic stainless steel contents of Co, V, and B
preferably fall within the range represented by the following
formula
0.1.ltoreq.[Co]+0.5.times.[V]+100.times.[B].ltoreq.0.5
[0021] where [Co], [V] and [B] designate the contents by weight
percentages of the respective elements.
[0022] The aforementioned ferritic stainless steel preferably has a
composition, on a weight percentage basis, further comprising at
least one element selected from the group consisting of about 0.05%
to 0.5% of Ti, about 0.05% to 0.5% of Zr, and about 0.05% to 0.5%
of Ta.
[0023] The aforementioned ferritic stainless steel preferably has a
composition, on a weight percentage basis, further comprising about
0.1% to 2.0% of Cu.
[0024] The aforementioned ferritic stainless steel preferably has a
composition, on a weight percentage basis, comprising at least one
element selected from the group consisting of about 0.05% to 1.0%
of W and about 0.001% to 0.1% of Mg.
[0025] The aforementioned ferritic stainless steel preferably has a
composition, on a weight percentage basis, further comprising about
0.0005% to 0.005% of Ca.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a schematic diagram of an exhaust manifold
comprising a ferritic stainless steel in accordance with this
invention.
[0027] FIG. 2 is a graph showing the effects of Co, V, and B on
secondary working embrittleness transition temperatures of welded
parts such as the exhaust manifold of FIG. 1.
[0028] FIG. 3 is a graph similar to FIG. 2 showing effects of Co,
V, and B on high temperature fatigue characteristics (10.sup.7
fatigue limit (MPa)) of such welded parts.
[0029] FIG. 4 is a schematic diagram illustrating a test for
evaluation of secondary working embrittleness resistance of such
welded parts.
[0030] FIG. 5 is a schematic diagram illustrating one example of a
shape of a test piece used in a high temperature fatigue test, and
a bending direction thereof.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] In order to achieve the aforementioned objects, we have
closely investigated effects of various additive elements on the
secondary working embrittleness resistance and the high temperature
fatigue characteristic of welded parts of ferritic stainless
steel.
[0032] As a consequence, we have discovered that the secondary
working embrittleness resistance and the high temperature fatigue
characteristics of a welded part were both remarkably improved by
the addition of very small amounts of Co, V, and B.
[0033] Results of the investigation regarding the effect of the
addition of Co, V, and B on secondary working embrittleness
transition temperatures of the welded parts are summarized as shown
in FIG. 2.
[0034] As is clear from FIG. 2, in the case in which all three
elements Co, V, and B are added, secondary working embrittleness
transition temperatures are surprisingly lower than those where
only two of the aforementioned three elements are added. This
indicates that cracks due to brittleness do not occur during use at
a lower temperature.
[0035] In particular, when contents of Co, V, and B fall within the
range represented by the following formula
0.1.ltoreq.[Co]+0.5.times.[V]+100.times.[B].ltoreq.0.5
[0036] where [Co], [V], and [B] designate the contents of the
stated elements by weight percentage of the respective elements, a
further decrease in brittleness transition temperature was
discovered.
[0037] Furthermore, when the relationship among the high
temperature fatigue characteristics of welded parts and the Co, V,
and B contents were also investigated, we discovered that the
addition of Co, V, and B surprisingly had a beneficial effect on
the high temperature fatigue characteristics of the product.
[0038] Results of the investigation regarding the effect of Co+V+B
on the high temperature fatigue characteristics are summarized as
shown in FIG. 3.
[0039] The expression "10.sup.7 fatigue limit" as used herein means
the maximum bending stress with which bending was repeated 10.sup.7
times without any occurrence of any fatigue crack of welded
parts.
[0040] As is clear from FIG. 3, in the case in which all three
elements Co, V, and B, were added, the 10.sup.7 fatigue limits were
substantially improved, compared to those where only two of those
elements were added. This indicates that the welded part can
withstand higher stresses created by highly repetitive bending.
[0041] In particular, when the contents of those elements fall
approximately within the range represented by the following
formula,
0.1.ltoreq.[Co]+0.5.times.[V]+100.times.[B].ltoreq.0.5
[0042] significantly higher 10.sup.7 fatigue limits were
exhibited.
[0043] Reasons for limiting the components of the steel of this
invention are as follows. The term "%" means the weight percentage
(mass %) unless otherwise specified.
[0044] C: about 0.02% or less
[0045] C, when added in an appropriate amount, functions to
strengthen the grain boundaries of the steel and improves the
secondary working embrittleness resistance of welded parts.
However, when C is increased and carbide is produced and deposited
at the grain boundaries, the secondary working embrittleness
resistance is adversely affected. In particular, when C exceeds
about 0.02%, the adverse effect becomes remarkable. Therefore, C is
specified to be about 0.02% or less. In particular, from the
viewpoint of improving the secondary working embrittleness
resistance, the content is preferably within the range of about
0.003%<C.ltoreq.0.01%.
[0046] Si: about 0.2% to 1.0%
[0047] Si is useful in this invention in that it contributes
effectively to an increase in strength and to improve the high
temperature fatigue characteristics. In order to achieve this
advantage, the Si content must be about 0.2% or more, although when
the Si content exceeds about 1.0%, the steel becomes brittle, and
the secondary working embrittleness resistance of the welded part
is degraded. Therefore the Si content is specified to be about 0.2%
to 1.0%. However, from the viewpoint of improving the secondary
working embrittleness resistance of the welded part, the Si content
is preferably about 0.6% or less.
[0048] Mn: about 0.1% or more, but about 1.5% or less
[0049] Since Mn is effective in improving oxidation resistance, it
is necessary in materials used at high temperatures. The Mn content
must be about 0.1% or more. However, when there are excessive
amounts of Mn, not only the toughness of steel, but also the
secondary working embrittleness resistance of a welded part is
degraded. Therefore the Mn content is specified to be about 1.5% or
less. However, from the viewpoint of improving the secondary
working embrittleness resistance, the Mn content is preferably
about 0.5% or less.
[0050] P: about 0.04% or less
[0051] P is likely to segregate at grain boundaries of the steel so
as to reduce the strengthening effect at the grain boundaries by B
as described below. Therefore, by minimizing the content of P, the
secondary working embrittleness resistance and the high temperature
fatigue characteristic of the welded part can be improved. However,
when the P content is reduced too much, steel production costs
increase. As a consequence, the upper limit of the P content is
specified to be about 0.04%.
[0052] S: about 0.01% or less
[0053] When S is reduced, corrosion resistance, which is a
characteristic of the stainless steel, is improved. However, the S
content is specified to be about 0.01% or less due to economic
constraints relating to desulfurization treatment in the steel
making.
[0054] Cr: about 11.0% to 20.0%
[0055] Cr is effective in improving high temperature strength,
oxidation resistance, and corrosion resistance. In order to exhibit
sufficient high temperature strength, oxidation resistance, and
corrosion resistance, Cr must be about 11.0% or more. On the other
hand, Cr degrades the toughness of steel. In particular, when the
Cr content exceeds about 20.0%, the toughness is remarkably
degraded, and the secondary working embrittleness resistance of the
welded part is also degraded. Therefore the Cr content is specified
to be within the range of about 11.0% to 20.0%. In particular, from
the viewpoint of improving high temperature fatigue characteristic,
the Cr content is preferably about 14.0% or more. On the other
hand, from the viewpoint of improving secondary working
embrittleness resistance, the Cr content is preferably about 16.0%
or less.
[0056] Ni: about 0.1% or more, but about 1.0% or less
[0057] Ni improves corrosion resistance, which is a characteristic
of the stainless steel, and in order to improve the corrosion
resistance, the Ni content must be about 0.1% or more. However,
when the Ni content exceeds about 1.0%, the steel became hard, and
the secondary working embrittleness resistance and the high
temperature fatigue characteristic of the welded part are adversely
affected.
[0058] Mo: about 1.0% to 2.0%
[0059] Mo is effective in improving high temperature strength and
corrosion resistance. In order for the invented steel to exhibit
sufficient high temperature strength and corrosion resistance, a Mo
content must be about 1.0% or more. On the other hand, when the Mo
content exceeds about 2.0%, the toughness is degraded, and the
secondary working embrittleness resistance of the welded part is
also degraded. Therefore the Mo content is specified to be within
the range of about 1.0% to 2.0%. From the viewpoint of improving
high temperature fatigue characteristic, the Mo content is
preferably about 1.5% or more.
[0060] Al: about 1.0% or less
[0061] Al is essential as a deoxidizer in the steelmaking process,
although excessive addition thereof causes production of an
intervening material resulting in degradation of the secondary
working embrittleness resistance. Therefore the Al content is
specified to be about 1.0% or less. From the viewpoint of improving
the secondary working embrittleness resistance, the Al content is
preferably about 0.1% or less.
[0062] Nb: about 0.2% to 0.8%
[0063] Nb is effective in improving high temperature strength of
the steel. In order for the invented steel to exhibit sufficient
high temperature strength, a Nb content must be about 0.2% or more.
On the other hand, when the Nb content exceeds about 0.8%, the
toughness is degraded, and the secondary working embrittleness
resistance of the welded part is also degraded. Therefore the Nb
content is specified to be within the range of about 0.2% to 0.8%.
From the viewpoint of improving the high temperature fatigue
characteristic of the welded part, the Nb content preferably
exceeds about 0.4%. On the other hand, from the viewpoint of
improving the secondary working embrittleness resistance, the Nb
content is preferably about 0.6% or less.
[0064] N: about 0.02% or less
[0065] When added in appropriate amounts, N functions to strengthen
the grain boundaries and improves the secondary working
embrittleness resistance of the steel. However, when nitride is
produced and deposited at the grain boundaries, the secondary
working embrittleness resistance is adversely affected particularly
when the N content exceeds about 0.02%. Therefore, the N content is
specified to be about 0.02% or less. From the viewpoint of
improving the secondary working embrittleness resistance of the
welded part, the N content is preferably about 0.01% or less.
[0066] Co: about 0.01% to 0.3%, V: about 0.01% to 0.3%, and B:
about 0.0002% to 0.0050%
[0067] Both the secondary working embrittleness resistance and the
high temperature fatigue characteristic of the welded part are
remarkably improved by this compound addition of Co, V, and B. The
aforementioned effect is exhibited when both the Co content and the
V content are about 0.01% or more and the B content is about
0.0002% or more. In order for the steel of this invention to
exhibit especially superior advantages, it is preferable that the
Co content is about 0.02% or more, the V content is about 0.05% or
more, and the B content is about 0.0005% or more. On the other
hand, when the Co content exceeds about 0.3%, the V content exceeds
about 0.3%, and the B content exceeds about 0.0050%, the effect
reaches saturation even though the cost is increased. Therefore the
contents of Co, V, and B are specified to be within the
aforementioned range.
[0068] The mechanism by which the compound addition of Co, V, and B
effectively contributes to improvement of the secondary working
embrittleness resistance and the high temperature fatigue
characteristic has not yet been exactly clarified, although it is
believed to be as follows.
[0069] It is believed that Co improves the internal strength of
grains which become coarse due to heat input during welding, and
prevents cracks from occurring therein. It is believed that B
coacts by segregating at the grain boundaries of the steel due to
heat input, so as to strengthen the grain boundaries and to prevent
formation of intergranular fractures. It is further believed that V
also coacts by producing carbide due to the heat input so as to
inhibit movement of the grain boundaries and to prevent crystal
grains from becoming coarse, and that at the same time, V coacts by
fixing C to prevent reduction of strengthening of the grain
boundaries by B by deposition of carbide produced from B.
[0070] In the present invention, Co, V, and B interact with each
other so as to exhibit a remarkable effect. If there is an
insufficiency of the amount present of at least one of them, the
aforementioned advantages cannot be enjoyed.
[0071] As described above, the addition of all of Co, V, and B
results in a remarkable improvement in the secondary working
embrittleness resistance of the welded part. Furthermore, it is
believed that the aforementioned strengthening of the inside of the
grain and the grain boundaries also contributes to the effects on
the high temperature fatigue exhibited when Co, V, and B are added
in approximately the following relationship:
0.1.ltoreq.[Co]+0.5.times.[V]+100.times.[B].ltoreq.0.5
[0072] In addition, since the secondary working embrittleness
resistance and the high temperature fatigue characteristic can be
further improved by the addition of Co, V, and B with contents
falling within the range represented substantially by the
aforementioned formula, as shown in the aforementioned FIGS. 2 and
3, it is preferable that contents of these elements are made to
fall within the approximate range represented by the aforementioned
formula.
[0073] The indispensable components of the invented steel have been
explained above, although in the present invention, other elements
as described below can be added:
[0074] Ti: about 0.05% or more, but about 0.5% or less, Zr: about
0.05% or more, but about 0.5% or less, and Ta: about 0.05% or more,
but about 0.5% or less
[0075] The elements Ti, Zr, and Ta are useful in that they deposit
as carbide due to heat input during welding, and so contribute to
improvement of high temperature fatigue characteristics by
strengthening due to the deposition thereof. When these elements
are added, the content of each must be about 0.05% or more.
However, when content of each exceeds about 0.5%, the effect
reaches saturation, and surface properties of the steel plate are
remarkably degraded. Therefore, each of the contents is specified
to be about 0.5% or less.
[0076] Cu: about 0.1% or more, but about 2.0% or less
[0077] Cu is effective in improving corrosion resistance and
toughness of steel. When Cu is added, the Cu content must be about
0.1% or more. When the Cu content exceeds about 2.0%, however,
workability of steel is degraded. Therefore, the upper limit of the
Cu content is specified to be about 2.0%.
[0078] W: about 0.05% or more, but about 1.0% or less, Mg: about
0.001% or more, but about 0.1% or less
[0079] Each of W and Mg is effective in improving high temperature
fatigue characteristics. When W and Mg are added, the W content and
the Mg content must be about 0.05% or more and about 0.001% or
more, respectively. When the W content and the Mg content exceed
about 1.0% and about 0.1%, respectively, however, toughness is
degraded, and the secondary working embrittleness resistance of the
welded part is also degraded. Therefore, the W content and the Mg
content are specified to be within the aforementioned range,
respectively.
[0080] Ca: about 0.0005% or more, but about 0.005% or less
[0081] Ca has an effect of preventing nozzle plugging due to a
Ti-based intervening material during slab casting, and Ca is added
if necessary. When Ca is added, the Ca content must be about
0.0005% or more. However, when the Ca content exceeds about 0.005%,
the effect reaches saturation, and corrosion resistance is
degraded, since an intervening material containing Ca becomes a
starting point of development of pitting corrosion. Therefore, the
Ca content is specified to be about 0.005% or less.
[0082] The remainder is essentially composed of Fe and incidental
impurities. This means that very small amounts of, for example,
alkali metals, alkaline-earth metals, rare earth elements, and
transition metals, other than Fe, will inevitably be present as
admixed components. When very small amounts of these elements are
present, the effects of the present invention are not affected.
[0083] Next, a method for manufacturing the steel of this invention
will be explained.
[0084] The method for manufacturing the invented steel is not
specifically limited, and a generally adopted method for
manufacturing ferritic stainless steel can be applied as it is
conventionally used. For example, regarding steel making, a method
in which a molten steel having a composition in the aforementioned
range is preferably refined with a converter or an electric
furnace, etc., and is then subjected to a secondary refining by VOD
(Vacuum Oxygen Decarburization). The refined molten steel can be
made into a steel raw material by known methods for casting,
although continuous casting is preferably applied, from the
viewpoint of productivity and quality.
[0085] The resulting steel raw material produced by the continuous
casting is heated to 1,000.degree. C. to 1,250.degree. C., and made
into a hot rolled plate having a predetermined thickness. The
resulting hot rolled plate is, if necessary, preferably subjected
to continuous annealing at a temperature of 900.degree. C. to
1,100.degree. C., and thereafter subjected to pickling and cold
rolling so as to produce a cold rolled plate. The resulting cold
rolled plate is preferably continuously annealed at 900.degree. C.
to 1,100.degree. C., and thereafter, is pickled so as to produce a
cold rolled annealed plate which becomes a product.
[0086] The product, which is produced by way of hot rolling,
annealing, and thereafter pickling, etc., for removing scales, can
also be used depending on the purpose intended.
[0087] Any conventional method for welding, for example, arc
welding, e.g. TIG, MIG, and MAG, high frequency resistance welding
and high frequency induction welding used for producing electric
resistance weld pipes, and laser welding, can be applied.
EXAMPLES
[0088] Each of 50 kg steel ingots, which become test specimens
having compositions as shown in Tables 1 to 3, was refined by a
vacuum melting furnace, and was made into a hot rolled plate of 4
mm in thickness by the usual hot rolling. The resulting plate was
subjected to annealing at 1,000.degree. C. for 60 seconds. Scale
was removed from the surface by pickling, and thereafter, a cold
rolled plate 1.5 mm in thickness was produced by cold rolling.
Subsequently, annealing finishing at 1,000.degree. C. for 60
seconds and pickling for removing scales were performed so as to
produce a cold rolled, annealed, and pickled plate 1.5 mm in
thickness as a test specimen.
[0089] Butt TIG welding was applied to each of the resulting test
specimens, and thereafter, each welded test specimen was subjected
to secondary working embrittleness testing and high temperature
fatigue testing. The TIG welding was performed under the following
conditions; current 240 A, voltage 12 V, welding speed 10 mm/s, and
shield gas 100% Ar.
[0090] A method for evaluating secondary working embrittleness
resistance is shown in FIG. 4. That is, a disk 49.5 mm in diameter,
in which the bead of welding passed through the center of the disk,
was stamped out. Then, the disk was subjected to deep drawing with
a draw ratio of 1.5 using a cylindrical punch 33.0 mm in diameter.
The resulting cylindrical cup was placed, so that the welded part
on the side thereof facing upward, then a weight of 3 kg was
dropped from a height of 800 mm directly above the cylindrical cup.
Thereafter, the welded part was observed to determine whether or
not cracks were present. The aforementioned drop weight tests were
performed, while temperatures of the cylindrical cup were varied in
the range of -60.degree. C. to +50.degree. C. at intervals of
10.degree. C., in order to determine the temperatures (secondary
working embrittleness transition temperature) at which cracking did
not occur.
[0091] Regarding the high temperature fatigue test, the 10.sup.7
fatigue limit (the maximum bending stress with which bending was
repeated 10.sup.7 times without the occurrence of a fatigue crack)
was measured by a flex (reversed stress) test at 800.degree. C. in
conformity with JIS Z 2275 using a test piece in which a TIG welded
bead is located at the center as shown in FIG. 5. Herein, the
bending stress .sigma. was determined as described below. Bending
deformation was applied to each test piece, and a bending moment M
(Nm) was measured regarding the section at which the maximum stress
was generated (a section of the TIG welded bead part as shown in
FIG. 5). Subsequently, the value of the bending moment was divided
by the modulus of the section in order to calculate the value of
the bending stress.
[0092] The results of the aforementioned tests are shown in Tables
4 and 5.
[0093] As is clear from Tables 4 and 5, each of the steels of this
invention Nos. 1 to 36, was proved to be superior in both secondary
working embrittleness resistance and high temperature fatigue
characteristics of the welded part.
[0094] On the other hand, regarding each of Comparative Steels Nos.
37 to 56, the secondary working embrittleness resistance and the
high temperature fatigue characteristic were sharply inferior to
the steels Nos. 1-36.
[0095] As described above, according to the present invention, a
ferritic stainless steel, including a welded part having superior
secondary working embrittleness resistance and superior high
temperature fatigue characteristic, was stably produced. As a
consequence, in the case in which a welded pipe or a welded plate
after forming work is used, cracks during use were effectively
prevented from occurring.
[0096] The steel of this invention is suitable for many purposes,
for example, components relating to automobile exhaust gas, in
particular, exhaust manifolds, etc., in which a welded pipe is
subjected to complicated bending work and used at a high
temperature. The welded part of the steel of this invention
exhibits excellent toughness and high temperature fatigue
characteristics when it is used without further working or after
primary working, so that it can also be applied to such a use with
advantage.
1TABLE 1 Chemical Component (mass %) No. C Si Mn P S Cr Ni Mo Al Nb
N Co V B Formula 1 Others 1 0.004 0.35 0.22 0.03 0.003 11.3 0.3 1.1
0.03 0.45 0.004 0.07 0.05 0.0009 0.19 2 0.005 0.25 0.28 0.02 0.003
11.8 0.2 1.5 0.03 0.58 0.004 0.08 0.06 0.0008 0.19 Ti: 0.13 3 0.008
0.23 0.18 0.03 0.001 11.2 0.3 1.0 0.02 0.22 0.006 0.05 0.07 0.0005
0.14 Ca: 0.0012 4 0.016 0.36 0.21 0.01 0.003 11.3 0.4 1.2 0.02 0.48
0.009 0.12 0.06 0.0012 0.27 Mg: 0.0010 5 0.004 0.30 0.21 0.03 0.003
14.8 0.3 1.6 0.03 0.45 0.006 0.02 0.05 0.0006 0.11 6 0.003 0.95
0.43 0.03 0.005 14.5 0.4 1.5 0.08 0.55 0.002 0.14 0.09 0.0006 0.25
7 0.004 0.47 0.18 0.01 0.002 14.2 0.5 2.0 0.03 0.46 0.004 0.22 0.21
0.0008 0.41 W: 0.14 8 0.006 0.38 0.32 0.03 0.005 13.5 0.2 1.6 0.06
0.52 0.009 0.01 0.16 0.0009 0.18 Ti: 0.12 9 0.004 0.23 0.43 0.04
0.002 14.8 0.1 1.7 0.03 0.43 0.006 0.11 0.07 0.0002 0.17 Zr: 0.06
10 0.006 0.31 0.46 0.02 0.003 14.2 0.2 1.7 0.05 0.55 0.005 0.02
0.09 0.0009 0.16 Ti: 0.13 11 0.008 0.43 0.12 0.03 0.005 15.8 0.3
1.8 0.01 0.48 0.009 0.05 0.14 0.0007 0.19 Cu: 0.15 12 0.005 0.38
0.32 0.03 0.003 14.6 0.5 1.5 0.03 0.42 0.006 0.06 0.12 0.0005 0.17
W: 0.08 13 0.002 0.28 0.22 0.02 0.002 14.8 0.3 1.6 0.02 0.48 0.006
0.08 0.07 0.0005 0.17 Ta: 0.05 14 0.005 0.25 0.26 0.03 0.003 14.1
0.3 1.7 0.02 0.47 0.008 0.02 0.04 0.0003 0.07 15 0.004 0.35 0.23
0.01 0.004 15.3 0.5 1.5 0.04 0.53 0.004 0.18 0.01 0.0025 0.44 Cu:
0.25 16 0.006 0.36 1.46 0.02 0.008 14.8 0.2 1.7 0.02 0.43 0.003
0.03 0.07 0.0005 0.12 Ca: 0.0007 17 0.016 0.31 0.20 0.02 0.003 15.8
0.3 1.3 0.02 0.45 0.006 0.05 0.07 0.0006 0.15 18 0.009 0.68 0.23
0.01 0.003 15.3 0.9 1.5 0.01 0.43 0.006 0.05 0.06 0.0008 0.16 Ti:
0.11, Cu: 0.53 Formula 1 = [Co] + 0.5 .times. [V] + 100 .times.
[B]
[0097]
2TABLE 2 Chemical Component (mass %) No. C Si Mn P S Cr Ni Mo Al Nb
N Co V B Formula 1 Others 19 0.004 0.38 0.33 0.03 0.007 15.0 0.2
1.2 0.05 0.23 0.008 0.03 0.08 0.0008 0.15 Ti: 0.06, Zr: 0.08 20
0.009 0.35 0.27 0.02 0.003 14.3 0.2 1.6 0.03 0.43 0.005 0.08 0.03
0.0007 0.17 Ca: 0.0009 21 0.007 0.53 0.25 0.03 0.006 14.9 0.3 1.7
0.02 0.52 0.009 0.05 0.07 0.0008 0.17 Ti: 0.15, Cu: 0.35 22 0.004
0.33 0.28 0.004 0.003 15.3 0.3 1.9 0.002 0.51 0.007 0.07 0.09
0.0007 0.19 Ti: 0.11, W: 0.13 23 0.006 0.46 0.19 0.01 0.005 15.2
0.4 1.7 0.04 0.49 0.008 0.13 0.05 0.0003 0.19 Ti: 0.12, Zr: 0.07 24
0.008 0.39 0.21 0.03 0.003 14.8 0.1 1.8 0.002 0.48 0.006 0.16 0.15
0.0009 0.33 Ti: 0.05, Ca: 0.0008 25 0.006 0.28 0.22 0.02 0.002 15.4
0.2 1.5 0.02 0.45 0.009 0.20 0.23 0.0025 0.57 26 0.007 0.53 0.25
0.03 0.006 14.9 0.3 1.7 0.02 0.52 0.009 0.05 0.07 0.0008 0.17 Ti:
0.11, Cu: 0.22, Ca: 0.0010 27 0.009 0.35 0.27 0.02 0.003 14.3 0.2
1.6 0.03 0.43 0.005 0.08 0.08 0.0012 0.24 Ti: 0.13, Zr: 0.09, Ca:
0.0012 28 0.005 0.38 0.32 0.03 0.003 14.6 0.5 1.5 0.03 0.42 0.006
0.06 0.12 0.0005 0.17 Cu: 0.18, W: 0.12 29 0.005 0.29 0.35 0.03
0.008 14.1 0.4 1.6 0.03 0.53 0.017 0.24 0.06 0.0007 0.34 Zr: 0.12,
Cu: 0.24 30 0.010 0.33 0.23 0.01 0.006 14.8 0.5 1.7 0.31 0.49 0.009
0.10 0.26 0.0005 0.28 31 0.004 0.38 0.42 0.02 0.003 15.3 0.1 1.5
0.03 0.57 0.010 0.28 0.08 0.0009 0.41 Cu: 0.12, Ca: 0.0014 32 0.008
0.43 0.14 0.03 0.007 15.4 0.2 1.8 0.05 0.48 0.006 0.02 0.05 0.0046
0.51 Cu: 0.61 33 0.003 0.28 0.26 0.02 0.008 14.3 0.6 1.7 0.03 0.78
0.005 0.08 0.06 0.0008 0.19 Ti: 0.10 34 0.009 0.31 0.71 0.01 0.003
15.2 0.3 1.6 0.95 0.42 0.007 0.05 0.08 0.0006 0.15 35 0.005 0.21
0.31 0.03 0.005 18.2 0.2 1.7 0.05 0.48 0.008 0.12 0.11 0.0008 0.26
Ti: 0.15 36 0.006 0.39 0.37 0.03 0.005 19.8 0.1 1.5 0.02 0.41 0.004
0.07 0.05 0.0010 0.20 Formula 1 = [Co] + 0.5 .times. [V] + 100
.times. [B]
[0098] It is noted that, in the foregoing Examples 1-36, the values
of the formula [Co]+0.5[V]+100[B], in accordance with this
invention, can range between 0.07 and 0.57, with excellent results.
As stated, in the formula the expressions [Co], [V] and [B]
represent the contents by weight percentage.
3TABLE 3 Chemical Component (mass %) No. C Si Mn P S Cr Ni Mo Al Nb
N Co V B Formula 1 Others 37 0.006 0.14 0.43 0.03 0.004 14.5 0.4
1.4 0.04 0.45 0.004 0.13 0.07 0.0003 0.20 38 0.004 0.34 0.24 0.03
0.003 14.9 1.1 1.6 0.03 0.42 0.004 0.06 0.06 0.0008 0.17 39 0.008
0.35 1.62 0.02 0.005 15.2 0.3 1.7 0.05 0.58 0.005 0.08 0.15 0.0002
0.18 40 0.025 0.45 0.26 0.01 0.008 15.9 0.4 1.3 0.03 0.51 0.006
0.12 0.06 0.0005 0.20 41 0.008 0.43 0.12 0.05 0.005 15.8 0.3 1.8
0.01 0.48 0.009 0.05 0.14 0.0007 0.19 Cu: 0.10 42 0.008 0.72 0.21
0.03 0.003 14.2 0.4 0.8 0.04 0.42 0.009 0.11 0.21 0.0005 0.27 43
0.010 0.35 0.28 0.02 0.007 14.8 0.3 1.4 1.19 0.48 0.005 0.09 0.06
0.0007 0.19 44 0.006 0.37 0.31 0.01 0.007 14.7 0.2 1.2 0.05 0.14
0.007 0.04 0.09 0.0010 0.19 45 0.004 0.35 0.19 0.03 0.010 15.7 0.2
1.4 0.02 0.48 0.026 0.08 0.14 0.0005 0.20 Ti: 0.13 46 0.006 0.42
0.29 0.04 0.008 15.3 0.3 1.2 0.03 0.52 0.004 <0.01 0.08 0.0012
0.16 47 0.004 0.32 0.21 0.03 0.004 15.2 0.1 1.3 0.03 0.54 0.005
0.07 <0.01 0.0006 0.13 Cu: 0.21 48 0.009 0.45 0.26 0.01 0.005
14.0 0.4 1.4 0.06 0.48 0.007 0.05 0.09 <0.0002 0.10 Ti: 0.15,
Cu: 0.31 49 0.007 0.42 0.27 0.02 0.003 14.9 0.2 1.4 0.04 0.45 0.010
<0.01 <0.01 0.0007 0.07 Zr: 0.13 50 0.005 0.27 0.18 0.01
0.007 15.5 0.1 1.3 0.03 0.42 0.006 <0.01 <0.01 <0.0002
0.00 Ti: 0.12, Ca: 0.0011 51 0.004 0.27 0.16 0.03 0.007 15.3 0.3
1.3 0.02 0.51 0.004 0.13 <0.01 <0.0002 0.13 W: 0.09 52 0.007
0.39 0.19 0.01 0.005 14.3 0.3 1.1 0.04 0.50 0.008 <0.01 0.07
<0.0002 0.04 Ca: 0.0017 53 0.006 1.12 0.23 0.03 0.008 15.8 0.2
1.7 0.002 0.52 0.004 0.06 0.06 0.0009 0.18 54 0.003 0.37 0.26 0.02
0.005 15.2 0.3 1.8 0.15 0.90 0.006 0.08 0.07 0.0005 0.17 Ti: 0.15
55 0.004 0.35 0.36 0.03 0.010 14.7 0.2 2.2 0.02 0.43 0.003 0.12
0.14 0.0010 0.29 56 0.005 0.35 0.21 0.03 0.007 21.1 0.3 1.6 0.01
0.58 0.006 0.08 0.08 0.0008 0.20
[0099]
4TABLE 4 10.sup.7 fatigue Secondary working limit of embrittleness
transition welded temperature of welded No. part (MPa) part
(.degree. C.) Remarks 1 31 -30 Present Invention 2 33 -30 Present
Invention 3 30 -30 Present Invention 4 30 -20 Present Invention 5
38 -30 Present Invention 6 35 -20 Present Invention 7 41 -30
Present Invention 8 33 -20 Present Invention 9 32 -20 Present
Invention 10 43 -30 Present Invention 11 38 -40 Present Invention
12 41 -30 Present Invention 13 37 -20 Present Invention 14 31 -20
Present Invention 15 32 -20 Present Invention 16 35 -20 Present
Invention 17 30 -20 Present Invention 18 31 -20 Present Invention
19 33 -30 Present Invention 20 33 -20 Present Invention 21 43 -40
Present Invention 22 45 -30 Present Invention 23 34 -20 Present
Invention 24 41 -30 Present Invention 25 32 -20 Present Invention
26 43 -40 Present Invention 27 44 -30 Present Invention 28 42 -40
Present Invention
[0100]
5TABLE 5 10.sup.7 fatigue Secondary working limit of embrittleness
transition welded temperature of welded No. part (MPa) part
(.degree. C.) Remarks 29 40 -20 Present Invention 30 37 -20 Present
Invention 31 36 -40 Present Invention 32 31 -20 Present Invention
33 39 -20 Present Invention 34 36 -20 Present Invention 35 39 -20
Present Invention 36 35 -20 Present Invention 37 18 -30 Comparative
Example 38 15 +10 Comparative Example 39 33 +10 Comparative Example
40 26 +10 Comparative Example 41 18 +10 Comparative Example 42 15
-20 Comparative Example 43 35 +10 Comparative Example 44 15 -30
Comparative Example 45 29 +10 Comparative Example 46 16 +10
Comparative Example 47 15 0 Comparative Example 48 16 0 Comparative
Example 49 13 +10 Comparative Example 50 14 +10 Comparative Example
51 18 +10 Comparative Example 52 16 +10 Comparative Example 53 36
+10 Comparative Example 54 38 +10 Comparative Example 55 39 +10
Comparative Example 56 35 +10 Comparative Example
* * * * *