High toughness stainless steels and the method of producing the same

A high toughness ferritic stainless steel having the excellent cold workability and the excellent heading workability suitable for the plastic working of screws, which consists essentially of, by weight percentage, C.ltoreq.0.03%, P.ltoreq.0.040%, S.ltoreq.0.010%, Si.ltoreq.1.0%, Mn.ltoreq.1.0%, 11.5%.ltoreq.Cr.ltoreq.22.0%, 0.05%.ltoreq.Nb.ltoreq.0.80%, N.ltoreq.0.025%, if necessary at least one selected from 0.2%.ltoreq.Cu.ltoreq.1.0%, 0.01.ltoreq.Mo.ltoreq.2.00%, 0.020%.ltoreq.Ni.ltoreq.1.50% and the balance being Fe and inevitable impurities, and the number of inclusions larger than 20 .mu.m amoung inclusions composed of carbo-nitrides of Nb, Ti and/or Zr in the steel is not more than 20. In the production of the high toughness ferritic stainless steel, the rolling material having the above-mentioned chemical compositions is heated to 1200.degree. C. or above at the rod rolling.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to ferritic stainless steel having an excellent 
toughtness and the method of producing the same, and more particularly to 
high toughness ferritic stainless steel excellet in the cold workability 
and suitable for use as materials of screws which are manufactured so as 
to form the heads by header prosessing (plastic working for the head of 
the screw) and to form the screw parts by, for example, rolling (plastic 
working for the screw body) and the method of producing the same. 
2. Description of the Prior Art 
Recent years, the consumption of screws manufactured by header processing 
and rolling has been increasing instead of screws made by machining 
process, and ferritic stainless steels have a tendency to be adopted as 
materials for the plastic-worked screws because of the cheapness and the 
excelletn corrosion resistance. 
In the case of manufacturing the screws using such ferritic stainless 
steels as materials by the header processing and the rolling, though the 
conventional ferritic stainless steels have the excellent heading 
wokability, the neck-breakage resistance at the head portion of the cerew 
is not always sufficient even now because the head portion of the screw is 
subjected to heavy plastic deformation by the header processing. 
Accordingly, there is a problem since there is the possibility that the 
screw may be broken down at the head portion along the grain flow produced 
by the header processing. 
SUMMARY OF THE INVENTION 
Therefore, this invention is made in view of the afore-mentioned problem of 
the prior art, it is an object to provide a high toughness ferritic 
stainless steel which is excellent in the heading workability in the case 
of manufacturing the screw by the header processing and the rolling for 
example, and also excellent in the neck-breakage resistance of the screw 
head formed by the header processing. And another object of this invention 
is to provide a method for producing the high toughness ferritic stainless 
steel having excellent properties as described above. 
The construction of the high toughness stainless steel according to this 
invention for attaining the aforementioned object is characterized in that 
it consists essentially of not more than 0.03 wt % of C, not more than 
0.040 wt % of P, not more than 0.010 wt % of S, not more than 1.0 wt % of 
Si, not more than 1.0 wt % of Mn, 11.5 to 22.0 wt % of Cr, 0.05 to 0.80 wt 
% of Nb, not more than 0.025 wt % of N, and if necessary at least one 
selected from 0.2 to 1.0 wt % of Cu, 0.01 to 2.00 wt % of Mo and 0.02 to 
1.50 wt % of Ni, and the balance being Fe and inevitable inpurities, and 
the number of inclusons larger than 20 .mu.m among inclusions composed of 
carbonitrides of Nb and, Ti and Zr contained as inevitable impurities is 
not more than 20 per 300 mm.sup.2, and preferably a percentage of area of 
the carbo-nitrides is not more than 0.05%. And the construction of the 
method of producing the high toughness stainless steel according to this 
invention for attaining the aforementioned object is characterized by 
heating the stainless steel material having the aforementioned composition 
at a temperature of 1200 or above at the time of the rod rolling.

DETAILED DESCRIPTION OF THE INVENTION 
The reason why the chemical composition (by weight percetntage) of the high 
toughness stainless steel according to this invention is limited to the 
above range will be described below. 
C: not more than 0.03% 
C is an element conducive to improve the strength of the steel, but 
sometimes deteriorates the corrosion resistance by the formation of 
carbides combined with carbide forming elements such as Nb added, Ti and 
Zr contained as impurities, and so on because the precipitated carbides 
function as a starting point of the corrosion. And C lowers the effect of 
Nb by combining with Nb added and forming carbide NbC so that the C 
content is limited to not more than 0.03%. 
P: not more than 0.040% 
It is necessary to reduce the content of P as much as possible because p is 
deteriorates the cold workability of ferritic stainless steels and impairs 
the formability of the screw head by header processing, so that the P 
content is limited to not more than 0.040%. 
S: not more than 0.010% 
It is necessary to reduce the content of S as much as possible because S is 
deteriorates the cold workability of ferritic stainless steels and impairs 
the formability of the screw head by header processing, therefore the S 
content is limited to not more than 0.010%. 
Si: not more than 1.0% 
Although Si has a deozidation action in melting process of the steel and 
has an action for improving the oxidation resistance, the toughness is 
degraded if Si is contained too much so that the Si content is limited to 
not more than 1.0%. 
Mn: not more than 1.0% 
Mn has a deoxidation and desulfurization action in melting process of the 
steel and has an action for improving the mechanical properties. However 
if Mn is contained too much, the heading workability is harmed, so that 
the content of Mn is defined as not more than 1.0%. 
Cr: 11.5 to 22.0% 
Cr is an foundamental element of ferritic stainless steels, and is defined 
as not less than 11.5% in order to obtain the good corrosion resistance. 
However the Cr content is limited to not more than 22.0% because the 
workability is degraded and it becomes impossible to perform the forming 
of the screw head satisfactorily by the header processing when Cr is 
contained in excess. 
Nb: 0.05 to 0.80% 
Nb is an ellement effective for improving the toughness of ferritic 
stainless steels and improving the heading workability, and is defined as 
not less than 0.05%. However, if Nb is contained too much, the brittleness 
transition temperature becomes higher and the toughness is rather 
degraded, so that it is limited to not more than 0.80%. 
N: not more than 0.025% 
N changes into nitrides by combining with nitride former such as Nb added, 
Ti and Zr contained as impurities and the like, and the corrosion 
resistance is sometimes degraded because the precipitated nitrides 
function as a starting point of the corrosion. And the Nb added in the 
steel becomes ineffective since the nitride NbN is formed by combining Nb 
with N, so that the content of N is limited to not more than 0.025%. 
Cu: 0.2 to 1.0%, Mo: 0.01 to 0.50%, Ni: 0.02 to 1.50% 
Cu, Mo and Ni are elements conductive to improve the corrosion resistance 
of ferritic stainless steels, it is preferably to contain at least one 
selected from not less than 0.2% of Cu, not less than 0.01% of Mo and not 
less than 0.02% of Ni at need. However, if these elements are contained 
too much, the workability, the toughness and the ductility are degraded, 
especially the strength is improved in excess and the formability of the 
screw head by the header processing is deteriorated when Mo is contained 
too much. Therefore, it is necessary to limit the Cu content to not more 
than 1.0%, the Mo content to not more than 2.00%, and the Ni content to 
not more than 1.50% in case of containing these elements. 
The high toughness stainless steel according to this invention has the 
abovementioned chemical compositions, and the number of inclusions larger 
than 20 .mu.m among inclusions composed of carbo-nitrides of Nb and, Ti 
and Zr contained as inevitable impurities is not more than 20 per 300 
mm.sup.2 in the stainless steel for the reason that the screw becomes easy 
to be broken down by occurence of cracks starting from coarse 
carbon-nitrides at the head portion formed by header processing and the 
neck-breakage resistance is degraded when the number of coarse-granular 
inclusions larger than 20 .mu.m are not more than 20 per 300 mm.sup.2, 
which are observed in accordance with "Microscopic Testing method for the 
Non-metallic inclusions in Steel" prescribed in Japanese Industrial 
Standard G-0555. 
And, it is possible to further improve the heading workability by 
decreasing an area percentage of the carbo-nitrides (total of B.sub.2 type 
inclusions and C.sub.2 type inclusions prescribed by JIS G 0555) into not 
more than 0.05% preferably. 
Furthermore, in the method of producing the high toughness stainless steel 
according to this invention, a temperature at the time of rolling the high 
toughness stainless steel for header processing having above-mentioned 
compositions (extracting temperature of the rolling material) is made 
higher into 1200.degree. C. or above, and is kept for 5 to 20 minutes or 
so preferably so as not to precipitate the carbo-nitrides such as Nb 
(C,N), Ti (C,N) and Zr (C,N) detected as B.sub.2 type inclusions and 
C.sub.2 type inclusions by dissolving the carbo-nitrides in the rolling 
material perfectly. 
The high toughness stainless steel according to this invention has the 
aforementioned construction, therefore it is excellent in the 
neck-breakage resistance at the screw head formed by the header processing 
as wll as the heading wokability by controlling the amount of the 
carbo-nitrides in the steel. 
EXAMPLE 
Each of ferritic stainless steels having chemical compositions shown in 
Table 1 was melted and then cast into ingots. Each ingots was heated at 
respective extracting temperatures as shown in table 2 and kept at the 
temperatures for 20 minutes, and then was rolled into wire rods with 
diameters of 4.0 mm. And the wire rods were coiled up at coiling 
temperatures shown also in Table 2. Further, some of them were anealed 
under conditions shown in Table 2 after the rolling. 
Next, the number of inclusions larger than 20 .mu.m which are contained in 
the rolled wire rod and composed of carbo-nitrides Nb (C,N), Ti (C,N), Zr 
(C,N) was measured per 300 mm.sup.2 in accordance with "Microscopic 
Testing Method for Non-Metallic inlusions in steel" prescribed in JIS G 
0555. The results are shown also in table 2. And percentage of the total 
area of B.sub.2 type inclusions (inclusions composed of carbo-nitrides of 
Nb, Ti and Zr among B type inclusions) and C.sub.2 type inclusions 
(inclusions composed of carbo-nitrides of Nb, Ti and Zr among C type 
inclusions) prescribed in JIS G 0555 was investigated. The results are 
shown in Table 2. 
Subsequently, fifty screw materials having head portions were prepared as 
specimeans from the respective rolled wire rod by header processing. Then 
the screw material 1 was set into a hole 2b of a jig 2 having an inclined 
slope 2a by 30 degrees as shown in FIG. 1, and neck-breakage resistance 
test was carried out by striking a head portion 1a of the screw material 1 
with a hummer 3 and bending the screw material at a shank 1b just under 
the head portion 1a. After the bending, an appearance of the breakage at 
the neck portion of respective screw material 1 was investigated by 
macroscopic observation. The observed results are also shown in Table 2. 
As the results obtained by such investigations, the relationship between 
the number of inclusions larger than 20 .mu.m and the number of fractured 
specimens among tested fifty specimens is shown in FIG. 2, and the 
relationship between the temperature of the rolling material (extracting 
temperature) and the percentage of area of carbo-nitrides (B.sub.2 type 
inclusions and C.sub.2 type inclusions) is shown in FIG. 3. 
TABLE 1 
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Chemical composition (wt %) 
Kind of steel 
C P S Si Mn Cr Nb Cu Ni N O Fe 
__________________________________________________________________________ 
Example 
A 0.010 
0.033 
0.005 
0.35 
0.41 
19.50 
0.40 
-- -- 0.015 
0.015 
bal. 
B 0.009 
0.037 
0.003 
0.23 
0.33 
20.05 
0.38 
0.38 
0.31 
0.020 
0.017 
bal. 
Comparative 
C 0.008 
0.035 
0.004 
0.31 
0.35 
18.80 
0.02 
-- -- 0.021 
0.016 
bal. 
example 
D 0.006 
0.036 
0.003 
0.28 
0.36 
19.70 
1.21 
-- -- 0.016 
0.018 
bal. 
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TABLE 2 
__________________________________________________________________________ 
Manufacturing conditions 
Rolling conditions The number of 
Extracting Anealing 
inclusions Area percentage 
The number of 
temperature 
Coiling 
temper- 
larger than 
of B.sub.2 and C.sub.2 
Heading 
fractured 
Kind of 
(kept for 20 min.) 
temperature 
ature 
20 .mu.m type inclusions 
work- 
spesimens 
No. 
steel 
(.degree.C.) 
(.degree.C.) 
(.degree.C.) 
(the number/300 mm.sup.2) 
(%) ability 
(the 
__________________________________________________________________________ 
number/50) 
1 A 1000 805.about.830 
-- 27 0.17 good 8 
2 A 1000 805.about.830 
780 29 0.17 good 8 
3 A 1150 805.about.830 
-- 21 0.14 good 4 
4 A 1200 803.about.860 
-- 0 0.05 good 0 
5 A 1250 870.about.900 
-- 0 0 good 0 
6 A 1300 915.about.985 
-- 0 0 good 0 
7 B 1000 805.about.830 
-- 36 0.17 good 10 
8 B 1000 805.about.830 
780 60 0.17 good 18 
9 B 1150 805.about.830 
-- 25 0.13 good 4 
10 B 1200 830.about.850 
-- 9 0.04 good 0 
11 B 1250 870.about.900 
-- 0 0.01 good 0 
12 B 1300 915.about.985 
-- 0 0 good 0 
13 C 1250 880.about.905 
-- 0 0 almost 
-- 
good 
14 D 1250 870.about.900 
-- 0 0 no good 
-- 
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As shown in Table 2 and FIG. 2, the number of fractured specimens increases 
as the number of coarse carbo-nitrides larger than 20 .mu.m increases, and 
it is confirmed as shown in Table 2 and FIG. 3 that the number of the 
coarse carbo-nitrides larger than 20 .mu.m decreases into not more than 20 
and the neck-breakage of the screw material is solved by making the 
temperature at the rolling higher to 1200.degree. C. or above. 
As described above, the high toughness stainless stell according to this 
invention is a ferritic stainless steel having specified chemical 
composition including Cr and Nb, and is so controlled that the number of 
inclusions larger than 20 .mu.m among inclusions composed carbo-nitrides 
of Nb and, Ti and Zr contained as inevitable impurities may be not more 
than 20 per 300 mm.sup.2. Therefore, it has high toughness and is 
excellent in the cold workability, especially in the heading workability 
and the neck-breakage resistance at the screw head in the case in which 
the screw is manufactured by header processing. And an excellent effect 
can be obtained since it is suitable to be used as a material for making 
screws with high reliability by plastic working. And another excellent 
effect can be obtained since it is possible to produce the high toughness 
stainless steel having aforementioned good characteristics by the method 
of producing the high toughness stainless steel according to this 
invention.