Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
21 62704
DESCRIPTION
HIGH-STRENGTH AUSTENITIC HEAT-RESISTANT STEEL
EXCELLENT IN WELDABILITY AND GOOD IN HIGH-
TEMPERATURE CORROSION RESISTANCE PROPERTY
Technical Field
This invention relates to an austenitic heat-
resistant steel exhibiting outstanding high-temperature
strength, excellent weldability and good high-temperature
corrosion resistance property and displaying excellent
performance when utilized in boilers, which are experiencing
increasingly harsh use environments.
Background Art
From the points of improved economy and the recent
move to suppress carbon dioxide gas emissions`, thermal power
plants are planning extra super critical temperature boilers
with high-temperature, high-pressure steam conditions. As
pointed out in "Iron and Steel" No.70, p.S-1409 and "Thermal
and Nuclear Power Generation" vol.38, p.75, high-strength
steels developed for withstanding use in such harsh
environments include austenitic heat-resistant steels
utilizing precipitation strengthening by carbo-nitrides of
Nb, Ti and the like and solution strengthening by Mo.
Since these heat-resistant steels contain large
amounts of alloying elements, however, they cannot be
2 1 62704
considered easy to weld in comparison with conventional
austenitic heat-resistant steel such as SUS347H and, as
such, have a problem regarding welding workability.
Increasing steel purity, specifically, reducing P
and S content together with reduction of C content, is know
as an effective means of improving weldability. Since as
just mentioned, however, most heat-resistant steels are
strengthened by carbo-nitrides, reduction of C content leads
to reduction of high-temperature strength.
On the other hand, it is known that increasing the
content of Mo frequently added for the purpose of solution-
strengthening a steel degrades high-temperature corrosion
resistance property.
The object of this invention is to provide an
austenitic heat-resistant steel that exhibits good
weldability and is excellent in high-temperature strength
and high-temperature corrosion resistance property.
Disclosure of the Invention
The inventors conducted various experiments
regarding steel added with Mo and W in order to offset by
solution strengthening the loss of high-temperature strength
caused by reduction of C content and, as a result, succeeded
in developing a heat-resistant steel which maintains high-
temperature strength at a low C content while also securing
high-temperature corrosion resistance property.
Specifically, the gist of this invention is as follows:
2 1 62704
(1) A high-strength austenitic heat-resistant steel
excellent in weldability and good in high-temperature
corrosion resistance property characterized in that it
comprises, in mass percent,
C : less than 0.02~,
Si : not more than 1.5%,
Mn : 0.3 - 1.5%,
P : not more than 0.02%,
S : not more than 0.005~,
Cr : 18 - 26~,
Ni : 20 - 40~,
W : 0.5 - 10.0%,
Nb : 0.05 - 0.4%,
Ti : 0.01 - 0.2%,
B : 0.003 - 0.008%, and
N : 0.05 - 0.3%,
the balance being Fe and unavoidable impurities.
(2) A high-strength austenitic heat-resistant steel
excellent in weldability and good in high-temperature
corrosion resistance property according to paragraph (1)
above further containing
Mo : 0.5 - 2.0%.
(33 A high-strength austenitic heat-resistant steel
excellent in weldability and good in high-temperature
corrosion resistance property according to paragraph (1) or
(2) above further containing one or more of
Mg : 0.001 - 0.05%,
2 1 62704
Ca : 0.001 - 0.05%, and
Rare earth elements (REM) : 0.001 - 0.15~.
Brief Description of Drawings
Figure 1 is a graph showing the effect of Mo and W
on the high-temperature corrosion resistance property of
20 Cr - 25 Ni steel.
Figure 2 is a graph comparing the creep rupture
strengths and high-temperature corrosion weight losses of
invention steels and comparison steels.
Figure 3 is a graph showing the results of
Varestraint tests conducted on steels containing the main
alloying elements other than C within the ranges of the
invention and on SUS347H.
Best Mode for Carrying out the Invention
The reasons for setting the ranges of the alloying
elements in the invention in the foregoing manner will be
explained.
C:
It is necessary to reduce C content as far as
possible for preventing high-temperature cracking during
welding and ductility degradation. Based on tests, the
upper limit of C content was set as follows for securing
good weldability. Figure 3 shows the results of an
evaluation of weldability by Varestraint tests conducted on
steels containing the main alloying elements other than C
- 21 62704
within the ranges of the invention (Cr : 20~, Ni : 25%, W :
3~) and having varied C content (- in the drawing) and on
SUS347H (corresponding to comparison steel K in the examples
set out later; ~ in the drawing). The conditions of the
test were, test piece thickness : 5 mm, welding method :
GTAW, welding voltage : 10 V, welding current : 80 A,
welding velocity : 80 mm/min, and applied strain : 2%.
Based on the tests results, and aiming at a content on a par
with SUS347H, the upper limit of C content for securing good
weldability is set at less than 0.02%.
si:
Si not only is effective as a deoxidizing agent but
is also an element which improves oxidation resistance and
high-temperature corrosion resistance property, but an
excessive Si content reduces creep rupture strength,
toughness and weldability. The upper limit is therefore
set at 1.5~.
Mn :
Mn is an element which has deoxidizing activity and
improves weldability and hot workability. For obtaining
sufficient deoxidation and a sound ingot, the lower limit of
Mn is set at 0.3%. Since an excessive Mn content degrades
oxidation resistance, however, the upper limit is set to
1.5%.
Cr :
Cr is an indispensable element for oxidation
resistance, water vapor oxidation resistance and high-
21 62704
temperature corrosion resistance property. For securingproperties at least as good as prior art austenitic
stainless steels, the lower limit of Cr content is set at
18%, which is the same as the Cr content of austenitic
stainless steels. However, since increasing Cr content
lowers the stability of the austenite and weakens the high-
temperature strength and further promotes formation of an
intermetallic compound ~ phase and reduces toughnèss, the
upper limit is set at 26%.
Ni :
Ni is an element required for increasing the
stability of the austenite and suppressing formation of an
intermetallic compound ~ phase. An Ni content of not
less than 20% is necessary for ensuring stability of the
austenite against the content of Cr and other ferrite
forming elements. On the other hand, since an Ni content
exceeding 40% is disadvantageous from the aspect of price,
the Ni content is set at 20 - 40%.
Mo, W :
Mo and W are both elements which markedly increase
high-temperature strength as by entering solid solution.
Neither has much effect when added at less than 0.5%, while
addition of W at more than 10% leads to precipitation of
intermetallic compounds such as Laves phase and reduces
creep rupture ductility. When Mo is added alone, the high-
temperature corrosion resistance property worsens as the
Mo content increases. On the other hand, tests show that
2 1 62704
adding W alone does not degrade the high-temperature
corrosion resistance property and that adding it in
combination with Mo improves the high-temperature corrosion
resistance property over that of a steel added with Mo
alone. Therefore, W is always added, and the range thereof
is set at 0.5 - 10~. As Mo in particular degrades the high-
temperature corrosion resistance property when added in
excess of 2.0%, even when added in combination with W, it
is added, when required, at 0.5 - 2.0%.
Nb, Ti :
Nb and Ti markedly improve long-term creep rupture
strength by forming minute carbo-nitrides. Since this
effect is not obtained when the Nb content is less than
0.05% or the Ti content is less than 0.01%, the lower limits
of Nb and Ti content are set at 0.05% and 0.01%. Although
the aforesaid effect becomes more pronounced as the content
of Nb and Ti soluble at the solid solution treatment
temperature increases, adding Nb and Ti in excess of the
solution limit degrades the creep rupture strength owing to
the undissolved carbo-nitrides that remain. Therefore, the
upper limits of Nb and Ti content are set at 0.4% and 0.2%,
and for increasing the solid solution (Nb + Ti) content
within these ranges, Nb and Ti are added in combination.
B :
B is an element which has the effect of enhancing
intergranular strength and increasing creep rupture
strength. However, since this effect is small at less than
2 1 62704
0.003% and a content exceeding 0.008% degrades weldability
and hot workability, the B content range is set at 0.003 -
0.008%.
P:
Since P markedly degrades weldability when added in
a large amount, its upper limit is set at 0.02%.
S:
Since S segregates at the grain boundaries and
degrades hot workability and also promotes intergranular
brittleness during creep, its upper limit is set at 0.005%.
N :
N is an element which markedly improves creep
rupture strength by solution strengthening and formation of
nitrides. At a content of less than 0.05%, N cannot offset
the loss of strength resulting from the reduction of C
content for improving weldability, while addition at more
than 0.3% produces little increase in long-term creep
rupture strength but degrades toughness. Therefore, the N
content range is set at 0.05 - 0.3%.
Mg, Ca, rare earth elements (REM)
While these elements purify the steel by deoxidation
and desulfurization, thereby enhancing hot workability, for
obtaining this effect it is necessary to add at least one of
them at not less than 0.001%. However, since addition in
excess of Mg : 0.05%, Ca : 0.05%, REM : 0.15% has the
opposite effect of impairing hot workability, the respective
addition ranges are set at Mg : 0.001 - 0.05%, Ca : 0.001 -
21 6~704
0.05~, REM : 0.001 - 0.15~o
Examples
The invention will now be explained with reference
to specific examples.
Table 1 and Table 2 (continued from Table 1) show
the chemical compositions and material properties of tested
steel specimens. After solution treatment at 1250C , these
steels were subjected to creep rupture test at 700 and 7S0C
and to high-temperature corrosion test at 700C . The creep
rupture strength data was organized using the Larson-Miller
method for estimating the 700C x 100,000 h rupture
strength. The high-temperature corrosion test was conducted
by immersing the steel specimen in simulated coal-fired
boiler ash of K2SOq : Naz S4 : Fe2 ( S04 ) 3 = O . 28 : 0.2 : 0.5
(mass ratio) for 200 h and then measuring the corrosion
weight loss. The test results are shown in Table 2.
Among the steels shown in Tables 1 and 2, A - J are
invention steels and K - U are comparison steels. Among the
comparison steels, K corresponds to the widely used SUS347H.
The invention steels have high-temperature strengths and
high-temperature corrosion resistance properties that are
very superior in comparison with the SUS347H steel. Among
the comparison steels, L - O are examples having low high-
temperature strength because they contain neither Mo or W
and their Nb or B content is outside the range of the
invention. P - U are examples with relatively high high-
2 1 62704
temperature strength but having poor high-temperature
corrosion resistance property notwithstanding addition of Mo
alone or in combination with W, owing to large Mo content.
Figure 1 shows the effect of Mo and W on the high-
temperature corrosion resistance property of 20 Cr - 25 Ni
steel. While corrosion weight loss is large when Mo is
added alone (- in the drawing), it will be noted that the
high-temperature corrosion resistance property is improved
when W is added in combination at 1.5% (- in the figure).
It can further be seen that the corrosion weight loss does
not change when W is added alone (~ in the figure).
Figure 2 compares the creep rupture strengths and
high-temperature corrosion weight losses of invention steels
and comparison steels. It can be seen that the comparison
steels are inferior in one or both of the high-temperature
strength and the high-temperature corrosion resistance
property, while the invention steels excel in both high-
temperature strength and high-temperature corrosion
resistance property.
1 0
2 i 62704
Table 1
Chemical composition (mass %)
C Si Mn P S Cr Ni Mo W Nb Ti
A 0.014 0.49 1.05 <0.002 <0.001 Z0.0 24.0 - 1.53 0.20 0.09
N B 0.015 0.49 1.06 <0.002 <0.001 20.7 24.8 - 3.24 0.21 0.11
E C0.016 0.50 1.01 <0.002 0.002 19.8 23.9 1.37 1.500.20 0.08
N
T D 0.018 0.50 1.08 <0.002 <0.001 20.9 24.81.54 3.34 0.21 0.12
0 E 0.016 0.47 1.01 <0.002 <0.001 20.2 24.0 - 4.91 0.23 0.10
N
F 0.013 0.48 0.90<0.002<0.001 20.1 24.51.56 4.64 0.21 0.09
S G 0.015 0.48 1.06<0.002 0.002 20.2 25.0 - 8.12 0.18 0.08
E H 0.017 0.49 1.03<0.002 0.002 24.3 34.6 - 1.50 0.23 0.07
L I 0.011 0.48 0.99<0.002<0.001 24.4 34.61.46 1.47 0.23 0.07
J 0.016 0.47 1.00<0.002<0.001 25.0 34.41.50 3.18 0.23 0.08
K 0.050 * 0.49 1.360.014 0.005 18.3 11.3 * - * - * 0.98 * - *
0 L 0.019 0.98 0.870.025 *0.005 20.6 24.7- * - * 0.42 * 0.07
M
P M 0.019 0.46 1.06<0.002 <0.001 20.0 24.7 - * - * - * 0.17
R N 0.016 0.52 1.010.005 0.003 19.6 24.3 - * - * 0.17 0.06
S O 0.015 0.47 1.000.004 <0.001 19.9 25.0 - * - * 0.21 0.10
N P 0. 019 0.53 1.01<0.002 <0.001 20.3 25.1 1.44 - * 0.21 0.09
Q 0.016 0.49 0.99<0.002 0.002 20.4 25.2 2.79 * - * 0.20 0.10
T R 0.015 0.48 1.00<0.002 0.002 20.0 24.0 2.81 * 1.49 0.20 0.08
E S 0.017 0.46 1.070.002 0.002 20.0 24.4 4.38 * - * 0.20 0.12
S T 0.021 * 0.521.04 <0.002 0.002 20.2 24.8 4.03 * 4.56 0.23 0.10
U 0.019 0.46 0.93<0.002 0.002 24.2 34.2 4.02 * - * 0.21 0.06
* This mark indicates that the content is outside the composition range of this invention
2 1 62704
Table 2 (continued from Table 1)
Chemical composition (mass %) 700C xloo~oooh 700C x 200h
creep rupture corrosion weight
B N Mg Ca REM strength (MPa) loss (mg/cm2)
A0.0052 0.133 - - - 83 386
N B0.0043 0.147 - - - 93 346
E C0.0058 0.125 - 0.0074 - 92 429
N
T D 0.0041 0.147 - 0.0056 - 99 439
0 E 0.0045 0.137 - - - 98 402
N
F 0.0053 0.098 - 0.0045 - 103 444
S G 0.0035 0.140 - - - 99 366
T
E H0.0051 0.0980.00350.0048 - 80 307
L I0.0049 0.139 - - 0.006Ce 80 324
J0.0045 0.141 - - 0.012Ce 91 313
K- * 0.008 * - - - 48 750
0 L0.0051 0.155 - - - 61 394
M
P M0.0051 0.042 - - - 65 402
R N- * 0.170 - - - 63 335
S O0.0042 0.095 - 0.0065 - 64 350
N P0.0041 0.099 - 0.0051 - 78 551
Q0.0044 0.100 - 0.0032 - 80 569
S
T R 0.0055 0.122 - 0.0040 - 96 502
E S 0.0047 0.097 - - - 98 685
S T 0.0051 0.124 - 0.0020 - 105 648
U 0.0052 0.094 - - - 81 528
* This mark indicates that the content is outside the composition range of this invention
1 2
2 1 62704
Industrial Applicability
This invention enables realization of an austenitic
heat-resistant steel that is excellent in weldability and
secures high-temperature strength and high-temperature
corrosion resistance property. It facilitates application
of high-strength steel to high-temperature, high-pressure
boilers and enables a reduction of implementation cost.
