Low alloy steels for use in pressure vessel

Low alloy steels for use in pressure vessel comprising on the weight % basis: PA0 C: from 0.05% to 0.30%, PA0 Si: less than 0.10%, PA0 Mn: from 0.3% to 1.5%, PA0 Ni: from inevitably incorporated content to 0.55%, PA0 Cr: from 1.5% to 5.5%, PA0 Mo: from 0.25% to 1.5%, PA0 V: in excess of 0.10% and less than 0.6%, and PA0 the balance of iron and inevitably incorporated impurities. The steels are excellent in hardenability, the hot strength, toughness weldability and hydrogen attack and embrittlement resistance, as well as show excellent toughness after the use in the temper brittle temperature region.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns low alloy steels for use in pressure vessel and, 
more specifically, it relates to Cr-Mo low alloy steels which are 
excellent in hardenability, hot strength, toughness, weldability and 
hydrogen attack and embrittlement resistance, as well as have excellent 
toughness even after the use in the temper brittle temperature region and, 
accordingly, are suitable to pressure vessel such as coal liquefying 
apparatus used in a hydrogen atmosphere under high temperature and high 
pressure. 
2. Description of the Prior Art 
Cr-Mo steels have hitherto been employed generally for pressure vessel such 
as in petroleum refining facilities used in the hydrogen atmosphere under 
high temperature and high pressure. By the way, new energy sources have 
recently been looked for as the substitutes for petroleum and study and 
experiment have been made, for example, on coal liquefication. In the case 
of coal liquefication, however, since the reaction is taken place under 
high temperature and pressure as compared with the conventional petroleum 
refining, reaction vessels used therefor have to satisfy the requirement 
for higher creep strength. Further, as the pressure vessel have become 
larger in the scale and increased in the thickness from the economical 
point of view, they tend to reduce the cooling rate and increase the time 
for post weld heat treatment thus making it difficult to provide steel 
materials with great hot strength. In addition, inevitable increase has 
been imposed to the material cost, and production or transportation cost 
due to the increase in the weight of the steel materials. Further, since 
the operation condition in the coal liquefication, for example, that of 
the temperature which is higher than 450.degree. C. corresponds to 
so-called the temper brittle temperature region, the toughness of the 
steels is degraded during use. 
In order to overcome the foregoing problems, there have been proposed low 
alloy steels for use in pressure vessel, for instance, in Japanese Patent 
Publication No. 57946/1982 (Kokai 57-57946), in which the sulfur content 
is decreased to improve the toughness and the silicon content is decreased 
to suppress the sensitivity to embrittlement in Cr-Mo steels and, further, 
vanadium and niobium contents are added to compensate the reduction in the 
hot strength caused by the decrease in the silicon content. However, even 
these proposed steels have no sufficient hot strength and creep strength. 
OBJECT OF THE INVENTION 
Accordingly, it is an object of this invention to provide low alloy steels 
for use in pressure vessel which are excellent in the hardenability and 
the toughness. 
Another object of this invention is to provide low alloy steels for use in 
pressure vessel which are improved in the hot strength and the creep 
strength. 
SUMMARY OF THE INVENTION 
The present inventors have made an earnest study for overcoming the 
foregoing problems in the prior art and attained this invention based on 
the finding that the toughness of steel materials can be improved by 
decreasing the silicon content while ensuring the hardenability by 
increasing the addition amount of manganese and, optionally, nickel and 
that the hot strength and the creep strength can significantly be improved 
by the addition of at least one element selected from niobium and titanium 
in combination with vanadium. 
As the main feature, the low alloy steels for use in pressure vessel 
according to this invention comprises on the weight % basis: 
C : from 0.05 % to 0.30 % 
Si : less than 0.10 % 
Mn : from 0.3 % to 1.5 % 
Ni : from inevitably incorporated content to 0.55 % 
Cr : from 1.5 % to 5.5 % 
Mo : from 0.25 % to 1.5 % 
V : in excess of 0.10 % and less than 0.6 %, and the balance of iron and 
inevitably incorporated impurities.

DETAILED DESCRIPTION OF THE INVENTION 
Description will at first be made to the reason in definiting the amount of 
alloying elements added to the steel materials according to this 
invention. 
Carbon (C) has to be added at least by 0.05 % for securing the strength of 
the steel materials. However, since excess addition results in the 
degradation for the toughness and the weldability, the upper limit for the 
addition amount is defined as 0.30 %. 
Manganese (Mn) has to be added by more than 0.5 % for securing the 
hardenability of the steel material, and it also contributes to the 
improvement in the resistance to stress relief cracks (SR crack 
resistance). However, its upper limit is defined as 1.5 % since excess Mn 
addition over 1.5 % reduces the hot strength, increase the sensitivity to 
the temperature embrittlement and further degrades the weldability. 
Nickel (Ni) is usually contained by a trace amount in the steels as 
inevitable impurities. In this invention, nickel may positively be added 
for improving the toughness and the hardenability of the steels. The upper 
limit for the Ni addition is defined as 0.55 % since the addition in 
excess of the above-defined limit reduces the creep strength. 
Chromium (Cr) is added at least by 1.5 % for providing the steel materials 
with the resistance to oxidation and hydrogen attack. If the Cr content is 
less than the above level, neither the intended effect nor sufficient hot 
strength can be obtained. On the other hand, since excess Cr addition 
leads to the degradation in the weldability and the workability, the upper 
limit is defined as 5.5 %. 
Molybdenum (Mo) is an element effective to the significant improvement in 
the hot strength of the steel materials and also the improvement in the 
resistance to the hydrogen attack and embrittlement. In this invention, Mo 
is added by more than 0.25 % in order to obtain such effects 
substantially. However, since excess Mo addition reduces the weldability 
and increases the material cost, the upper limit is defined as 1.5 %. 
Vanadium (V) is an essential alloying element in the steels according to 
this invention for improving the cold and hot strength of the steels due 
to its function of forming carbides and nitrides. V is added in excess of 
1.0 % and less than 0.6 % in this invention, but more preferably in excess 
of 0.25% and less than 0.5%. Vanadium in excess of 0.3% and less than 0.6% 
is also preferred. 
FIG. 1 shows the tensile strength (at 25.degree. C.) and the rupture 
strength of the steels according to this invention when heated at 
500.degree. C. for 1000 hours while varying the addition amount of V. It 
will be apparent from the figure that the cold strength and the hot 
strength can remarkably be improved, particularly upon adding V by more 
than 0.2 %. If the addition amount of vanadium is lower than 0.10 % only 
an insufficient improvement can be attained in the creep strength and the 
hot strength of the steels. On the other hand, addition of vanadium in 
excess of 0.6 % is neither desired since this degrades the toughness and 
the weldability of the steels. More preferably, vanadium is added in 
excess of 0.25% and less than 0.5% when considering creep strength and 
hydrogen attack and embrittlement. 
In the steel materials according to this invention, it is possible, in 
addition to the elements as described above, to incorporate at least one 
ingredients selected from: 
(i) from 0.01 % to 0.6 % of at least one element selected from Nb and Ti in 
total, 
(ii) from 0.0005 % to 0.02 % of at least one element selected from Ca and 
Zr in total and/or from 0.01 % to 0.20 % of at least one of rare earth 
elements, and 
(iii) from 0.0005 % to 0.002 % B. 
Niobium (Nb) and titanium (Ti), like vanadium, form carbides and nitrides 
to significantly increase the cold strength and the hot strength of the 
steel materials. As described above, addition of at least one of them in 
combination with vanadium can significantly improve the cold strength and 
the hot strength of the steel materials. In the steels according to this 
invention, at least one element selected from Nb and Ti can be added in a 
range between 0.01 % - 0.6 %. However, excess addition thereof degrades 
the toughness and the weldability of the steels. 
FIG. 2 shows the creep strength of the steels according to this invention 
having the chemical compositions shown in steel Nos. 21-23 and that of 
SA336F2 which is a typical example of conventional Cr-Mo steels shown in 
Table 1 below. 
TABLE 1 
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Steel 
Chemical composition (wt %) 
No. 
C Si Mn Ni Cr Mo V etc. Remarks 
__________________________________________________________________________ 
1 0.14 
0.23 
0.45 
0.10 
2.20 
1.02 
-- -- Conventional 
2 0.14 
0.07 
0.47 
0.08 
2.88 
0.97 
-- -- steels 
3 0.15 
0.08 
0.46 
0.07 
2.89 
0.99 
0.25 
-- Comparative 
4 0.14 
0.06 
0.49 
0.73 
2.98 
0.95 
0.23 
-- steels 
5 0.14 
0.07 
0.55 
0.40 
2.98 
1.00 
0.24 
-- Invented 
6 0.14 
0.07 
0.74 
0.07 
3.02 
1.00 
0.25 
-- steels 
7 0.14 
0.08 
1.26 
0.07 
3.05 
0.93 
0.27 
-- 
8 0.13 
0.09 
0.92 
0.20 
2.98 
0.97 
0.39 
-- 
9 0.14 
0.08 
0.98 
0.18 
3.01 
1.04 
0.26 
Nb: 0.08 
10 0.14 
0.07 
1.00 
0.09 
3.04 
0.98 
0.25 
Nb 0.01 
Ti 0.03 
11 0.14 
0.05 
1.03 
0.07 
3.00 
0.98 
0.22 
Ti: 0.04 
12 0.15 
0.07 
1.01 
0.20 
2.99 
1.03 
0.34 
Ca: 0.0037 
13 0.14 
0.07 
1.04 
0.10 
3.00 
1.00 
0.35 
Ca 0.0040 
Ce 0.030 
14 0.15 
0.08 
0.98 
0.09 
3.02 
0.97 
0.34 
Ca 0.0040 
Zr 0.018 
15 0.14 
0.07 
0.93 
0.18 
3.02 
0.95 
0.48 
Zr: 0.058 
16 0.14 
0.08 
1.02 
0.15 
2.95 
0.98 
0.25 
B: 002 
17 0.14 
0.07 
1.02 
0.10 
3.01 
0.98 
0.25 
Ca 0.0040 
B 0.0018 
18 0.13 
0.07 
0.98 
0.07 
3.02 
0.97 
0.24 
Zr 0.018 
B 0.0020 
19 0.14 
0.07 
0.99 
0.07 
2.98 
0.99 
0.23 
Ce 0.030 
B 0.0015 
21 0.14 
0.07 
1.04 
0.07 
2.91 
1.01 
0.26 
Nb: 0.07 
Ca: 0.0044 
20 0.14 
0.08 
0.82 
0.10 
2.56 
0.93 
0.26 
Ca: 0.0035 
22 0.14 
0.07 
0.70 
0.30 
2.99 
0.99 
0.25 
Nb: 0.05 
Ca: 0.0045 
23 0.13 
0.054 
0.82 
-- 3.01 
0.99 
0.29 
Nb: 0.057 
Ca: 0.0050 
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The steels according to this invention have extremely high creep strength, 
as well as much higher hot strength as compared with that of the 
conventional steels and comparative steels at the same level of the cold 
strength and, accordingly, the invented steels are practically superior. 
Calcium (Ca), Zirconium (Zr) and rare earth elements, being sulfide-forming 
elements, can significantly reduce the sensitivity of steels to the 
welding cracks by decreasing the solid-soluted sulfur content in the 
steels. In order to effectively attain this effect, at least one of Ca and 
Zr has to be added within a range of 0.005 %-0.02 % in total. While on the 
other hand,the rare earth element is added within a desired range of 0.01 
%-0.2 %. However, if these elements are added in excess of the above 
defined ranges, the purify of the steels becomes poor and the toughness is 
reduced. 
Boron (B) is added for improving the hardenability of the steels. According 
to this invention, this improvement can be attained effectively by boron 
alone without using titanium together. A preferred range for the addition 
of boron is between 0.0005 %-0.02 %. 
The steels according to this invention can be manufactured by conventional 
procedures of melting, ingot preparation and hot rolling, and by applying 
conventional heat treatment subsequently or continuously thereto. 
In the steels according to this invention, the toughness can be improved by 
decreasing the Si content while securing the hardenability by increasing 
the addition amount of manganese and, optionally, nickel, as well as the 
hot strength and the creep strength can significantly be improved by 
adding vanadium together with at least one element preferably selected 
from niobium and titanium. Further, since the steels according to this 
invention have excellent resistance to the hydrogen attack and 
embrittlement and the weldability, as well as have excellent toughness 
after the use in the temper brittle temperature regin, they are suitable 
as the steel materials for use in pressure vessel used in hydrogen 
atmosphere under the high temperature and high presssure. 
This invention will now be described referring to Examples. 
EXAMPLE 
After melting steels having chemical compositions respectively as shown in 
Table 1 in an induction vacuum furnace into steel ingots, they were forged 
and rolled to steel sheets. Then, they were subjected to austenizing at 
950.degree.-1050.degree. C., cooling at an average cooling rate of 
10.degree. C./sec., tempering at 675.degree. C., and then applied with an 
after heat treatment through by heating at 690.degree. C. for 25 hours. 
The mechanical properties and the weldability of the steels according to 
this invention, conventional steels and comparative steels are shown in 
Table 2. 
TABLE 2 
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Tensile 
Tensile 
strength at 
strength at 
Creep.sup.(2) SR cracking.sup.(5) 
TRC lower.sup.(6) 
Steel 
K f.sup.(1) 
room temp. 
550.degree. C. 
strength 
vTrs.sup.(3) 
.DELTA.vTrs.sup.(4) 
rate limit stress 
No. 
(.degree.C./ ) 
(kg/mm.sup.2) 
(kg/mm.sup.2) 
(kg/mm.sup.2) 
(.degree.C.) 
(.degree.C.) 
(%) (kg/mm.sup.2) 
Remarks 
__________________________________________________________________________ 
1 8.5 60.4 40.5 15.8 -35 15 20 15 Conventional 
steels 
2 4.5 60.2 40.3 15.5 -35 13 15 16 
3 30.0 61.2 42.1 17.0 -35 15 15 14 Comparative 
steels 
4 0.8 65.2 43.3 16.0 -48 10 12 16 
5 1.8 65.0 43.8 17.0 -55 10 0 18 Invented 
steels 
6 7.0 63.9 43.8 17.0 -66 8 0 20 
7 0.23 64.1 44.4 17.5 -60 13 0 23 
8 1.95 68.6 44.0 22.0 -73 10 22 18 
9 5.0 65.3 47.9 24.55 -45 12 15 18 
10 7.0 67.2 47.0 24.7 -33 10 15 7 
11 6.0 70.2 48.2 25.0 -35 15 10 18 
12 1.6 68.1 46.6 20.1 -44 10 0 20 
13 1.8 68.4 46.2 20.3 -50 8 0 22 
14 1.7 68.2 46.0 20.8 -52 7 0 21 
15 2.0 71.6 44.3 24.3 -70 12 0 20 
16 2.0 63.5 45.1 19.0 -48 5 15 18 
17 2.2 64.0 45.0 19.5 -55 7 0 20 
18 2.4 63.0 44.8 20.1 -58 6 0 22 
19 2.3 63.3 44.8 19.4 -57 5 0 21 
21 5.0 65.1 48.2 27.4 -49 9 0 20 
20 0.30 65.9 44.4 17.4 -65 10 0 20 
22 4.0 68.4 46.0 23.0 -73 9 0 19 
23 6.0 68.0 45.5 24.0 -48 5 0 20 
__________________________________________________________________________ 
.sup.(1) Critical cooling rate forming initial ferrite deposition 
.sup.(2) 550.degree. C. .times. 10.sup.3 hr 
.sup.(3) Transition temperature at Charpy 50% brittle broken face 
.sup.(4) vTrs rising amount by step cooling treatment 
.sup.(5) Orthogonal Ytype weld crack test 
.sup.(6) TRC test 
Steels Nos. 1 and 2 as the typical examples of conventional Cr-Mo steels 
are inferior in the cold strength, the hot strength and the toughness. 
Steels No. 3 as the comparative steels with the Mn content lower than the 
range as specified in this invention is poor in the hardenability. Steels 
No. 4 with an excess Ni content has no improved creep strength. 
Steels No. 5 through No. 23 represents those according to this invention. 
It is recognized that the steels according to this invention are generally 
excellent in the cold strength, the hot strength and the creep strength. 
Steels No. 8 having a somewhat higher V content are inferior to other 
steels according to this invention but still comparable with the 
conventional steels, with regard to the weldability. While on the other 
hand, the hot strength and the creep strength are significantly improved 
in the steels No. 8. Steels Nos. 9, 10, 11 and 20 containing at least one 
element selected from Nb and Ti added in combination with V show 
remarkably improved hot strength and creep strength. 
Steels No. 16 containing B show improved hot strength and creep strength. 
Further, the steels according to this invention, in which Ca, Zr and/or Ce 
are added show a remarkable improvement in the weldability in addition to 
the hot strength and the creep strength. 
Although not shown in the examples, sulfur(S) should preferably be 
suppressed not more than 0.01% so as not to cause hydrogen embrittlement 
or hydrogen induced cracking.