Manufacturing process for plate or forging of ferrite-austenite two-phase stainless steel

This invention relates to a manufacturing process for plate or forging (bar, stamp work or the like) of ferrite-austenite two-phase stainless steel, containing C at 0.03% or below, Si at 2.0% or below, Mn at 2.0% or below, Cr at 25 to 35%, Ni at 6 to 15%, N at 0.35% or below, and Fe and inevitable impurity for the remainder with or without adding B at 0.001 to 0.030% with the following nickel balance value specified at -3 to -9 and comprising an average crystal grain size at 0.015 mm or below from heating an ingot of the above mentioned ferrite-austenite two-phase stainless steel at 1,200.degree. C. or below and keeping a forging ratio by hot working at 5 or over. EQU Ni balance value=Ni %+0.5 Mn %+30.times.(C+N) %-1.1(Cr %+1.5 Si %)+8.2

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a manufacturing process for plate or forging 
(bar, stamp work or the like) of ferrite-austenite two-phase stainless 
steel and particularly of ferrite-austenite two-phase stainless steel 
superior in resistance to nitric acid. 
2. Description of the Prior Art 
In a nitric acid environment, a stainless steel having a higher content of 
Cr is strong in resistance thereto accordingly, and an intergranular 
corrosiveness is extremely severe according to the density of nitric acid, 
therefore an extremely-low carbon type and Nb-stabilized high-chrome 
austenite stainless steel, 310 LC (low carbon--25% Cr--20% Ni steel), 310 
LCNb (low carbon--25% Cr--20% Ni--0.2% Nb steel) or the like, for example, 
is employed hitherto. However, in the case of such austenite stainless 
steel having a higher content of Ni, since a solid solubility limit of 
carbon (C) is small, a chrome carbide deposits preferentially onto a 
crystal grain boundary to deteriorate intergranular corrosion resistance 
under the effect of heating at 500.degree. to 900.degree. C. or welding 
heat, and a solidification cracking sensitivity is high at the time of 
welding, thus losing a reliability on the weld zone. On the other hand 
ferrite-austenite two-phase stainless steel having a high Cr content is 
susceptible, due to the heat generated by welding, to selective corrosion 
between the structures. Such corrosion tendency is conspicuous 
particularly in a nitric acid environment, and thus a conventional 
two-phase stainless steel has two properties adversely impacting on its 
ability to work as a nitric acid resistant material having a welded 
structure. 
As the result of having studied on influences of structure and percentages 
of elemental fractions on nitric acid resistance in stainless steel, the 
inventors contrived a high-chrome two-phase stainless steel effective to 
alleviate the above-described defects of austenite stainless steel and 
two-phase stainless steel, and provide superior in nitric acid resistance 
and weldability, and cheap in cost as well. (Japanese Patent Application 
No. 130442/1981 (Japanese Patent Laid-Open No. 3106/1983)). This inventive 
type of steel has a higher Cr and Ni content as compared with a 
conventional ferrite-austenite two-phase stainless steel having Cr of 23 
to 25% and Ni of 4 to 6% generally, and a specific Ni balance value at the 
same time. Moreover, a structure has been found which is superior in 
nitric acid resistance to the above-mentioned materials of 310 LC and 310 
LCNb even though an expensive Ni component is present in lesser amounts. 
Further, the nitric acid resistance is improved by adding B at 0.001 to 
0.03% thereto, which is enhanced more by decreasing P to 0.010% or below 
and S to 0.005% or below which are contained inevitably as impurities, and 
it has the following compositions: 
(1) That for which the composition is: C being 0.03% by weight or below, Si 
being 2.0% or below, Mn being 2.0% or below, P being 0.040% or below, S 
being 0.030% or below, Cr being 25 to 35%, Ni being 6 to 15%, N being 
0.35% or below, Fe and inevitable impurity for the remainder, and also the 
following expression 
EQU -13&lt;Ni eq-1.1.times.Cr eq+8.2&lt;-9 
is satisfied. 
(2) That for which 0.001 to 0.03% B is added to the above mentioned steel. 
(3) That for which P and S are decreased independently or simultaneously to 
0.010% or below and to 0.005% or below respectively in the above mentioned 
steels (1) and (2). 
The inventive steel has superior resistance to nitric acid, this property 
is the result of the elemental composition and the fine grain structure of 
ferrite and austenite peculiar to two-phase stainless steel. That is, the 
superior resistance to nitric acid is due to a superior intergranular 
corrosion resistance, and it is generally known that the intergranular 
corrosion resistance depends on a crystal grain size, and the smaller the 
crystal grain size is, the better the resistance becomes. Thus the 
superior intergranular corrosion resistance of the steel is intrinsically 
related to the fine structure which is a feature of the two-phase 
stainless steel. Originally, the crystal grain size of the two-phase 
stainless steel is influenced largely by its manufacturing history, and 
the larger a forging ratio is, the smaller the grain size becomes. 
However, when it is heated at high temperatures of 1,250.degree. C. and 
over for hot working, the structure comes near to a single phase structure 
of ferrite with the resulting excessive coarsening of the crystal grain. 
SUMMARY OF THE INVENTION 
Now, in consideration of such characteristic of the two-phase stainless 
steel, a principal object of this invention is to manufacture a plate or 
forging of ferrite-austenite two-phase stainless steel superior 
particularly in resistance to nitric acid. 
That is, the invention is to improve nitric acid resistance and 
particularly intergranular corrosion resistance by controlling the crystal 
grain size of a product to 0.015 mm or below through hot working of a 
two-phase stainless steel having the above-mentioned composition. 
The above-mentioned object and features of this invention will be apparent 
from the following detailed description thereof taken with the 
accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
It has been found that a resistance to nitric acid and particularly a 
intergranular corrosion resistance can be improved by controlling the 
crystal grain size of a product to 0.015 mm or below. 
The present invention improves intergranular nitric acid corrosion 
resistance by adjusting the heating temperature of the ingot to 1,200 
degrees Centigrade or below in the hot working process and maintain a 
forging ratio during hot working of no less than about 5. The "forging 
ratio" refers to the overall working rate from the ingot, which is 
expressed by the ratio of the ingot sectional area to the product 
sectional area. It has been found that these conditions keep the average 
crystal grain size at no greater than about 0.015 mm in manufacturing a 
plate and a forging of the ferrite-austenite stainless steel, containing C 
at 0.03% or below, Si at 2.0% or below, Mn at 2.0% or below, Cr at 25 to 
35%, Ni at 6 to 15%, N at 0.35% or below, Fe and inevitable impurity for 
the remainder with or without adding B at 0.001 to 0.030%, and having Ni 
balance value adjusted to -13 to -19. 
This invention has found that a steel component containing higher elemental 
percentages of Cr and Ni, as compared with a conventional 
ferrite-austenite two-phase stainless steel having Cr of 23 to 25% and Ni 
of 4 to 6% generally, and having a specific Ni balance value at the same 
time, improves the nitric acid resistance so as to be superior in 
resistance to nitric acid to 310 LC and 310 LCNb even though an expensive 
Ni component is kept less, and further that enhances the resistance to 
nitric acid by adding B thereto as occasion demands, and furthermore by 
decreasing P to 0.010% or below and S to 0.005% or below which are 
contained inevitably as impurities. In manufacturing a plate and forging 
of the ferrite-austenite two-phase stainless steel having such a 
composition, a steel material superior remarkably in resistance to nitric 
acid is thus obtainable through regulating heating temperature and forging 
ratio in the process of hot working as described above, and the reason why 
the chemical composition is limited as above will be described first. 
C: C is an effective element for formation of austenite, however, since it 
forms a carbide which acts to increase the intergranular corrosion 
sensitivity, it must be contained as small as possible. Still, in 
consideration of its being easy to manufacture, the upper limit will be 
0.03%. 
Si and Mn: Si and Mn are elements used as deoxidizers during the process of 
steel manufacture, and Si and Mn will have to be added normally at 2.0% or 
below to facilitate manufacture industrially, therefore each will be 
limited to 2.0% or below. 
Cr: Cr is a ferrite forming element and is important not only for formation 
of a two-phase structure of austenite and ferrite but also for increasing 
corrosion resistance and particularly resistance to nitric acid, therefore 
it must be added at 25% or over for a satisfactory resistance to nitric 
acid. The resistance to nitric acid increases as a Cr content increases 
under proper structural balance, however, when it exceeds 35%, workability 
deteriorates and manufacture of steel material and fabrication of 
equipment become difficult to lose a practical applicability, therefore 
the upper limit will be specified at 35%. 
Ni: Ni is an austenite forming element and is also important along with Cr 
for formation of a two-phase structure, and further it is a very important 
element for decreasing an active dissolution rate including general 
corrosion, therefore it must be added at 6% to 15% to obtain a preferable 
structural balance of ferrite-austenite correspondingly to the content of 
Cr which is a principal ferrite forming element. 
N: N is a powerful austenite forming element like C and Ni, and is also 
effective for enhancement of a corrosion resistance such as pitting 
resistance, however, when N exceeds 0.35%, a blowhole may arise on ingot 
during the process for manufacturing steel and a hot workability will 
deteriorate, therefore it is limited to 0.35% or below. 
In this invention, it is meaningless to specify these elements 
independently, and an excellent effect will be obtainable only under an 
optimum combination, therefore it is necessary to limit the range of each 
component so that the following expression will be satisfied, which is so 
found as will feature the invention in a sense. 
EQU -13&lt;Ni balance value&lt;-9 
where Ni balance value=Ni eq-1.1.times.Cr eq+8.2, Ni 
eq=Ni%+0.5.times.Mn%+30.times.(C+N)%, Cr eq=Cr%+1.5.times.Si%. 
Where Ni balance value falls below -13, there is an increased tendency for 
selective corrosion between structures, and under such condition not only 
the resistance to nitric acid cannot be improved even if the Cr content 
may be increased but also the Ni balance value is shifted in the direction 
more disadvantageous for corrosion resistance, thereby actually 
accelerating corrosion. On the other hand, if the Ni balance value is 
taken greater than -9, then not only is there incurred a disadvantage 
economically from increasing the addition rate of expensive Ni, but also a 
hot workability is prevented thereby and the corrosion resistance 
deteriorates consequently, therefore the Ni balance value is limited to 
-13 to -9. 
B: The effect of improving resistance to nitric acid will be increased when 
B is added at 0.001% or over, however, workability and weldability will 
deteriorate when it exceeds 0.03%, therefore it is limited to 0.001 to 
0.03%. 
P and S: P and S which are impurity elements will be desirable, as they are 
kept less, however, as will be apparent from Japanese Industrial Standards 
and the like, P being 0.040% or below and S being 0.030% or below are 
normally permissible. However, when P is limited to 0.010% or below and S 
to 0.005% or below, the effect of improving resistance to nitric acid will 
be enhanced. 
An effect equivalent to decreasing P and S was ensured from adding rare 
earth elements (REM) such as La, Ce and the like a small quantity or, for 
example, at about 0.02%. 
Next, the reason why heating temperature and forging ratio are regulated as 
described hereinabove in a manufacturing process of this invention will be 
described. 
In the case of two-phase stainless steel, the austenite phase decreases to 
nearly a single phase structure of ferrite as the heating temperature 
rises to 1,100.degree. C. or over, which is a feature on the structure, 
and the above-mentioned steel is turned to a ferrite structure at about 
1,350.degree. C. In the ferrite-austenite two-phase structure, the growth 
of the ferrite crystal grain is suppressed by austenite crystal grain, 
however, when austenite decreases in volume, the effect of the suppression 
fades to turn the crystal grain coarse, and thus the austenite crystal 
grain becomes coarse at the same time. Further, as will be apparent from 
FIG. 2 representing a relation between heating temperature and .gamma. 
(austenite phase) content, the .gamma. content decreases abruptly at 
1,200.degree. C. or over, and coarsening increases sharply, therefore the 
upper limit is specified at 1,200.degree. C. in the invention, however, in 
the case of two-phase stainless steel, cracking occurs during the hot work 
at 900.degree. C. or below and thus the product yield deteriorates, 
therefore it is preferable to maintain a high heating temperature. 
Then, in the hot working process, it is difficult to obtain a fine crystal 
size where there is a small amount of working, even if the heating 
temperature is maintained at less than about 1,200 degrees Centigrade, and 
where the hot working is less than about 10% it provides a driving force 
for the growth of crystal grains and thus to promote coarsening. Therefore 
the degree of working more than that will be necessary therefor, further 
where the degree of working is small, heating-working process must be 
repeated to obtain the required forging ratio, which may result in 
coarsening of the crystal grain, and on the other hand, it is difficult to 
obtain the forging ratio at 5 or over through single working, therefore 
the heating-working process must be repeated more than once, and in such 
case it is preferable that the degree of working per heating be kept at 
50% or over. As will be apparent from the example described later, it is 
ensured by a manufacturing scale test that there may be a case where a 
desired average crystal grain size is not obtainable through the degree of 
working at 50% or below, 40% for example. 
Generally, the ingot structure is coarse as compared with forging material, 
and a crystal is made fine by repetition of working-recrystallization. 
However, it is found in this invention that the average crystal grain size 
at 0.015 mm or below as described above may minimize a intergranular 
corrosion depth to 0.010 mm or below, thus indicating a superior 
resistance to nitric acid (FIG. 1), and as will be apparent further from 
FIG. 3 representing a relation between forging ratio and crystal grain 
size, it is necessary to keep the forging ratio from ingot at 5 or over 
for obtaining the average crystal grain size at 0.015 mm or below. 
As described, this invention relates to a manufacturing technique for plate 
and forging of ferrite-austenite two-phase stainless steel superior in 
resistance to nitric acid ensured by a specific component and 
manufacturing process. 
An example of this invention will be described as follows: 
EXAMPLE 
Table 1 shows an example according to this invention, describing steels in 
this invention and comparative steels, SUS 329 J1 steel and extremely-low 
carbon 310 steel (310 ELC). 
Under the working conditions given in Table 1, each 1-ton ingot of the 
above steels (2 kinds of steels of this invention and SUS 329 J1, 310 ELC) 
are heated twice by each forging ratio and hot rolled (sample No. 8 being 
heated three times), each is heated at 1,050.degree. C. and water-cooled 
for solid solution annealing, and corrosion samples 3.times.20.times.30 mm 
(general-grinding #03) are then sampled to a 48-hour boiling test in 65% 
HNO.sub.3 +Cr.sup.+6 100 ppm solution 5 times, and then a intergranular 
corrosiveness in the nitric acid environment is evaluated from the 
intergranular corrosion depth. 
Then, FIG. 1 illustrates a test result of sample Nos. 1 to 4, and as will 
be apparent from FIG. 1, the intergranular corrosion depth and the crystal 
grain size are correlated with each other, and the average grain sizing 
coming below 0.015 mm will minimize the intergranular corrosion depth to a 
superior resistance to nitric acid. Further, as shown in Table 1, 
corrosion resistance cannot be improved satisfactorily even if the forging 
ratio is 7 or over when the heating temperature works at 1,250.degree. C. 
or over, therefore the working must be carried out at 1,200.degree. C. or 
below, and an enhancement of the intergranular corrosion resistance is 
difficult even working at the heating temperature of 1,200.degree. C. or 
below where the forging ratio is 3. Furthermore, a formation of the fine 
crystal grain is too insufficient to obtain a satisfactory corrosion 
resistance even the heating at 1,200.degree. C. and the forging ratio at 5 
where the degree of working at every times of heating comes below 40%. 
Then, the intergranular corrosion resistance cannot be improved any more 
from employing the working process according to this invention on SUS 329 
J1 and 310 ELC. 
Although the invention has been described in its preferred embodiment, it 
will be obvious to those skilled in the art that modification and 
variation is possible in light of the above description. 
TABLE 1 
__________________________________________________________________________ 
Sample Chemical component % 
No. Classification 
C Si Mn P S Cr Ni N Others 
Ni-bal 
__________________________________________________________________________ 
1 Process of the 
0.011 
0.52 
0.58 
0.028 
0.008 
26.75 
8.02 
0.10 
-- -10.44 
invention 
2 Process of the 
" " " " " " " " " 
invention 
3 Process, 
" " " " " " " " " 
comparative 
4 Process, 
" " " " " " " " " 
comparative 
5 Process of the 
0.009 
0.55 
0.51 
0.025 
0.006 
27.32 
7.90 
0.10 
B -11.33 
invention 0.0011 
6 Process of the 
" " " " " " " " B " 
invention 0.0011 
7 Process, 
" " " " " " " " B " 
comparative 0.0011 
8 Process, 
" " " " " " " " B " 
comparative 0.0011 
9 Process, 
0.017 
0.65 
0.60 
0.022 
0.007 
25.07 
5.10 
0.11 
Mo -8.84 
comparative 1.80 
10 Process, 
0.011 
0.51 
1.06 
0.026 
0.005 
25.14 
20.56 
-- -13.22 
comparative 
__________________________________________________________________________ 
Working conditions 
Degree 
Average 
Intergranular 
of crystal 
corrosion 
Sample Heating 
Forging** 
working 
grain size 
depth 
No. Classification 
temp. 
ratio % mm mm Remarks 
__________________________________________________________________________ 
1 Process of the 
1200.degree. C. 
6 .gtoreq.60 
0.012 
0.009 Steel of the invention 
invention 
2 Process of the 
" 12 .gtoreq.70 
0.007 
0.009 " 
invention 
3 Process, 
1250.degree. C. 
7 .gtoreq.60 
0.030 
0.018 " 
comparative 
4 Process, 
1200.degree. C. 
3 .gtoreq.70 
0.022 
0.016 " 
comparative 
5 Process of the 
1200.degree. C. 
6 .gtoreq.60 
0.013 
0.008 " 
invention 
6 Process of the 
" 11 " 0.008 
0.010 " 
invention 
7 Process, 
1250.degree. C. 
8 " 0.027 
0.019 " 
comparative 
8 Process, 
1200.degree. C. 
5 .ltoreq.40 
0.020 
0.015 " 
comparative (heated 
three 
times) 
9 Process, 
1200.degree. C. 
7 .gtoreq.60 
0.012 
0.018 Comparative steel 
comparative (SUS 329J1) 
10 Process, 
1200.degree. C. 
7 .gtoreq.70 
0.080 
0.100 Comparative steel 
comparative (SUS 310ELC) 
__________________________________________________________________________ 
*Workability at every times of heating 
**Ingot sectional area/finished product sectional area