Work hardened stainless steel for springs

A metastable austenitic, cold deformed "work hardened stainless steel for springs", with 17.0 to 19.0% Cr, 8.0 to 10.0% Ni, up to 0.03% C, 0.006 to 0.16% N, up to 1.0% Si, 1.0 to 2.0% Mn, up to 0.8% Mo, up to 0.045% P, up to 0.030% S, iron (Fe) and residuals, the alloy being used for spring manufacture, exhibiting good resistance to corrosion after cold deformation, exhibiting high mechanical properties and better resistance to corrosion than UNS S30200 steel, even when exposed to a tempered heat treatment. The steel is appropriate for use as wire rod, bars, wires, sheets and strip forms.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The current invention relates to an improved stainless steel obtained by 
cold deformation, such as wire drawing and rolling. As a result, the steel 
provides a structure, made up of martensite and austenite, with high 
resistance to corrosion. Such properties suit its main application in the 
field of spring manufacture. 
2. Background of the Art 
Springs are submitted to a load cycle, and therefore require good fatigue 
resistance. A number of factors affect this resistance, but it is the 
superficial quality, without any doubt, that most regulates the spring's 
performance when submitted to fatigue conditions. The presence of 
superficial irregularities favors the nucleation of fatigue cracks. 
Nevertheless, resistance to fatigue is not guaranteed just by avoiding 
these defects, because superficial defects can be formed during spring 
use. One of the most prejudicial superficial defects created during spring 
use is corrosion. So, when the design conditions demand and the costs 
permit, stainless steel should be used in the manufacture of springs. 
Stainless steel for springs was developed in order to increase the 
mechanical strength of springs, which was very low in the solubilized 
condition. Compositions that allow for hardening mechanisms and strength 
levels that exceed 2000 MPa, in some alloys and gauge, were developed. In 
addition, stainless steel provides the capacity to be cold worked, which 
eases the manufacturing process such as rolling and drawing. 
Stainless steels that form martensite during cold deformation are called 
metastable. They provide high strength after cold deformation, as occurs 
during wires drawing, so they are the main stainless steels Used in spring 
manufacture. Strength is the result of a microstructure consisting of 
hardened martensite and austenite, having carbon as the main hardening 
element. 
However, metastable austenitic stainless steel, or the current technical 
state, most used in spring manufacture, UNS S30200 steel, with up to 0.15% 
of C, 17.0 to 19.0% Cr, 8.0 to 10.0% Ni, up to 0.75% Si, up to 2.0% Mn, up 
to 0.045% P and up to 0.030% S, does not provide enough resistance to 
intergranular and pitting corrosion. Besides, due to the high carbon 
content, normally over 0.08%, these steels most be heat treated in a cycle 
known as solubilization, at higher temperatures and longer periods than 
other stainless steels. So, working with UNS S30200 steel involves more 
care and higher cost. 
Also, the standard stainless steel for springs provides problems in 
durability when used in applications that require high resistance to 
corrosion. In the spring manufacturing process, a tempering heat treatment 
is normally carried out in order to increase the spring strength and 
durability. Depending on the temperature used, chromium carbide 
precipitation can occur, which reduces the resistance to corrosion. 
The current invention solves these problems.

DETAILED DESCRIPTION OF THE INVENTION 
The object of this invention is to produce a cold deformed stainless steel 
composition for spring manufacture, with a microstructure composed of a 
mixture of martensite and austenite, which yields better resistance to 
intergranular and pitting corrosion and does not require special care for 
solution heat treatment. 
Specifically, the current invention provides a metastable stainless steel 
for spring manufacture that, after cold deformation, has a microstructure 
composed of austenite and martensite. This steel has 17.0 to 19.0% Cr, 8.0 
to 10.0% Ni, 0.06 to 0.16% N, up to 0.03% up to 1.0% Si, 1.0 to 2.0% Mn, 
up to 0.80% Mo, up to 0.075% P and up to 0.030% S; the rest is iron and 
inevitable impurity. 
The stainless steel according to the current invention provides high 
strength after cold deformation and high resistance to intergranular and 
pitting corrosion. Besides, the solution heat treatment of this steel does 
not involve special care, and can be eventually eliminated. 
The chemical composition range of the new steel must have hardening 
properties similar to UNS S30200, where the high resistance is a result of 
the martensite formation during the cold deformation when drawing or 
rolling occurs, and the hardening by carbon. 
The martensite level created depends on the alloy stability degree, which 
is a function of chemical composition. One of the equations that rules 
this dependence is the following: 
EQU Md(30/50) (.degree. C.)=497-462.vertline.(% C)+(% N).vertline.-9.2(% 
Si)-8.1(% Mn)-13.7(% Cr)-20(%Ni)-18.8(% Mo) 
where Md (30/50) is temperature, in degrees Celsius (centigrade), that 
occurs in the formation of 30% martensite, after 50% cold deformation. 
A typical composition of UNS S30200 steel used by experts consists of 0.10% 
C, 0.40% Si, 1.70% Mn, 17.5% Cr, 8.3% Ni, 0.03% N and 0.4% Mo. Using the 
above equation will result in Md (30/50) equal to 6.34.degree. C. The 
alloy of this current invention must have the same content of the Cr, Ni, 
Si, Mn and Mo elements present in UNS S30200. Supposing a carbon content 
equal to 0.02% (the required specification is up to 0.03%) and calculating 
the Md (30/50) for the new alloy, obtained is: 
EQU Md(30/50)=57.16-462(% N). 
For the new alloy to have an equivalent martensite value, after cold 
deformation, to UNS S30200, its Md (30/50) must be the same, which 
involves a desirable typical content of 0.11% nitrogen. 
In relation to hardening effect, the nitrogen is at least as efficient as 
carbon, because the nitrogen interactions with the dislocations are much 
stronger than those obtained with carbon. 
The reason for the current stainless steel chemical composition 
specification is described as follows: 
Cr: 17.0% to 19.0%--Chromium is the essential element to promote resistance 
to corrosion through a superficial protector layer formation turning the 
steel stainless. 
Ni: 8.0% to 10.0%--Nickel is the element that provides stability to 
austenite and resistance to corrosion. Its content should be balanced with 
chromium content to guarantee a starting microstructure completely 
austenitic after the solution heat treatment or the rolling. Besides, the 
composition range must be stabilized in order for the martensite formation 
to occur after cold deformation. 
C: up to 0.03%--Carbon is a gamagenic element that is dissolved when its 
concentration is low. However, when the C content increases, the M23C6 
carbide type can precipitate in grain boundaries, consuming chromium that 
is useful to intergranular corrosion resistance. In the current invention 
the limit of this element, at most 0.03%, will be compensated, as will be 
seen below, by the nitrogen content. 
N: 0.06% to 0.16%--Nitrogen is the most critical element of the current 
invention and is particularly important to obtain simultaneously the 
mechanical properties necessary for stainless steel spring manufacture 
with improved resistance to corrosion. The nitrogen works as a stabilizer 
of the austenitic phase and as a hardener. During cold deformation, the 
nitrogen hardens the formed martensite, assuring a high work hardening 
behavior. This element increases the resistance to pitting corrosion and 
delays the kinetics of M23C6 precipitation, increasing, therefore, the 
resistance to intergranular corrosion. After heat treatment of the 
hardened material, by cold drawing or rolling, the nitrogen creates an 
atmosphere in the vicinity of the dislocations, raising still more the 
steel, strength. The effect can not be obtained with a nitrogen content 
below 0.06%; on the other hand, it can not be over 0.16% because the Md 
(30/50) value reaches values that damage the alloy metastability, and as a 
result, the mechanical property levels reached. 
Si: up to 1.0%--Silicon is a deoxidizing element and its presence is 
related with the Steel manufacturing process. 
Mn: 1.0% to 2.0%--manganese is a gamagenic element and helps to assure a 
completely austenitic structure after solution heat treatment. The 
manganese is also used in steel deoxidation. 
P, S and other residual elements inevitably mixed up in the steel 
manufacturing process, should be at the lowest levels possible. 
The alloy, as described, can be manufactured as rolling or forged products 
by a standard or a special process, such as powder metallurgy or 
continuous casting wire rod, bars, wires, sheets and strips. 
In the following Example, the steel properties of the current invention 
will be described and compared with those of the UNS S30200 steel. 
EXAMPLE 
In Table 1, displayed is a comparison of alloys that were casted and rolled 
to 8 millimeter diameter wire rod and solubilized. The materials were cold 
deformed by wire drawing up to a 3.0 millimeter diameter wire, and in 
each, reduction samples were taken. In Table 2, the work hardening 
behavior of the two steels is displayed. The new steel presents sufficient 
metastability to reach high levels of strength necessary for spring 
application. In spite of situations where the strength values of the 
current invention are below the values obtained for UNS S30200, it can be 
seen in the Example that they still meet the minimum levels required by 
the standards that establish spring manufacture from drawn wires. The 
spring, during its manufacturing, is submitted to a tempering heat 
treatment at temperatures around 400.degree. C. Table 3 displays that the 
new steel presents, in its final condition, more hardening than the UNS 
S30200 steel, showing the effective action of nitrogen as a hardening 
element. 
The mechanical properties of the starting material, solubilized wire rod 
with an 8.0 millimeter diameter, are shown in Table 4. The alloy in the 
current invention has a greater yield strength and the same ductility as 
the UNS S30200 steel. There is no difference in the tensile strength. 
Some pitting corrosion tests were conducted in the solubilized material and 
in the wire, with 82% deformation. The tests were conducted according to 
ASTM G48 rule, mass loss in a ferric chloride solution after 72h. The 
results are displayed in Table 5. It is clear that the new steel is 
superior to UNS S30200 in terms of resistance to pitting corrosion, 
maintaining this benefit in the work hardened condition as well. The 
results confirm the strong effect of nitrogen in resistance to pitting 
corrosion. 
The tests of intergranular corrosion were also conducted in the solubilized 
material, in the=wire with 82% deformation, and in the wire after 
treatment at 400.degree. C. during 40 minutes. The test was conducted 
according to ASTM A 262-C rule, mass loss in boiling nitric acid. The 
results are displayed in Table 6. In all conditions, the steel of the 
present invention was superior to UNS S30200 steel. The difference was 
greater after treatment at 400.degree. C. during 40 minutes, due to 
precipitation of carbide in grain boundaries in the UNS S30200 steel. One 
must be aware of the fact that, in the current Example, the UNS S30200 
steel was solubilized (1060.degree. C. during 3h). One fault in the UNS 
S30200 steel solution heat treatment reduces its resistance to 
intergranular corrosion. Even in the as rolled condition, the wire rod of 
the current invention did not present intergranular corrosion. 
To evaluate fatigue life, springs were manufactured from drawn wires of 1.0 
mm diameter. The manufacturing process was conducted under the same 
conditions normally used for UNS S30200 steel. The springs made with the 
two steels were tested in compression, with load varying from 287N to 
988N, according to DIN 2089 standard. The steel of the current invention 
showed a fatigue life, up to breakage, of 120,000 cycles, as compared to 
80,000 cycles of UNS S30200 steel. 
It will be obvious to experts that the principles of the invention, herein 
described in relation to a specific Example, will allow for many other 
changes and applications. It is also desirable that, when analyzing the 
scope of the appended claims, they not be limited to the specific Example 
of the invention herein described. 
The following Tables were referred to in the EXAMPLE: 
TABLE 1 
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CHEMICAL COMPOSITION IN WEIGHT PERCENTAGES 
ALLOY Cr Ni Mn Si N C Mo Cu P S 
__________________________________________________________________________ 
UNS S30200 18.1 
8.72 
1.42 
0.60 
0.041 
0.08 
0.09 
0.1 
0.027 
0.014 
Steel of the Invention 
17.45 
8.21 
1.88 
0.45 
0.10 
0.01 
0.35 
0.18 
0.03 
0.024 
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TABLE 2 
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WORK HARDENING BEHAVIOR 
Tensile Strength (MPa) 
Reduction 
(%) 0 35 52 59 68 75 80 82 
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Steel of the 
595 935 1190 1345 1455 1595 1640 1755 
Invention 
UNS S30200 
600 940 1210 1400 1580 1690 1780 1820 
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TABLE 3 
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WIRE HARDENING AFTER ANNEALING 
Material Condition Hardness (HV1) 
______________________________________ 
Steel of the 
82% deformed 463 
Invention 82% deformed + 
547 
400.degree. C. .times. 40 min. 
UNS S30200 82% deformed 485 
82% deformed + 
517 
400.degree. C. .times. 40 min. 
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TABLE 4 
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MECHANICAL PROPERTIES OF THE 
SOLUBILIZED WIRE ROD 
TEST TEMPERATURE 
25.degree. C. AND .epsilon. = 0.001 s.sup.-1 
UNS 
Steel of the Invention 
S30200 
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Yield Strength 0.2% (MPa) 
332.1 254.6 
Tensile Strength (MPa) 
654.5 653.9 
Elongation 5d (%) 
78.6 83.1 
Reduction in area (%) 
79.7 79.3 
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TABLE 5 
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PITTING CORROSION TESTS RESULTS - ASTM G48 
Material Condition Mass Loss (mg/cm.sup.2) 
______________________________________ 
Steel of the 
solubilized 24.06 
Invention 82% deformed 
44.03 
UNS S30200 solubilized 46.15 
82% deformed 
56.38 
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TABLE 6 
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INTERGRANULAR CORROSION TESTS 
RESULTS - ASTM A262-C 
Material Condition Mass Loss (.mu.g/cm.sup.2) 
______________________________________ 
Steel of the 
solubilized 1160 
Invention 82% deformed 1420 
82% deformed + 
1660 
400.degree. C./40 min. 
UNS S30200 solubilized 1300 
82% deformed 1640 
82% deformed + 
5070 
400.degree. C./40 min. 
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