|Title of Invention||
STEEL FOR SPRINGS, PROCESS OF MANUFACTURE FOR SPRING USING THIS STEEL, AND SPRING MADE FROM SUCH STEEL
|Abstract||A spring steel with higfetigue resistance in air and in contusive conditions and with high resistance to cyclic sag, having the composition in weight percent: the balance being iron, and impurities resulting from the steel making process, where the carbon equivalent Ceq content calculated acceding to the formula: Cecf/o = [C%] + 0.12 [Si%] + 0.17 [Mn%] - 0.1 [Ni%] + 0.13 [0%] 0.24 [V%] is between 0.80 and 1.00%, and whose hardness, after quenching and tempering, is greater than or equal to 55HRC.|
STEEL FDR SPRINGS, PROCESS OF MANUFACTURE FOR SPRING USING THIS STEEL, AND SPRING MADE FROM SUCH STEEL
 The invention relates to steel making, and more specifically, the field of spring steel.
 Generally, as increasing fetigue stresses are applied to springs, springs need continually increasing hardness and tensile strength. Consequently, susceptibility to filatures that bin on defects, such as inclusions or surfece defects generated during spring manufactures, increases, and fetigue resistance tends to become limited. Secondly, springs used in highly corrosive environments, such as suspension springs, must have at least equivalent and preferably better fatigue properties in corrosive conditions since they use steels having higher hardness and tensile strength. Accordingly, such springs tend to facture at the defects, immediately during the fetigue cycles in air, and more late during fatigue cycles in a corrosive medium. In particular, for fetigue in corrosive conditions, defects can begin in corrosion pits. Furthermore, with increasing pied stress, it is more difficult to improve the fetigue life in corrosive conditions or to maintain it at an equivalent level, given the feet that the effects of the concentration of stresses on the corrosion pits, on the surface defects of the brings that may be generated during spring cowling, in other steps in the manufacturing process, or in non-metallic inclusions, become more critical when spring hairiness increases.  According to the prior art, documents FR-A-2740476 and JP-3474373B describe a spring sted grade with good resistance to hydrogen embrittlement and good fatigue resistance, in which inclusions of carbonitrosulfides containing at least one of the elements titanium, niobium, zirconium, tantalum or hafnium are controlled so as to have bowers mean size, less than 5 nm in diameter, and to be very numerous (10,000
or more on a cutting section).
 However, this type of steels leads, after quenching and tempering according to the industrial spring manufecturing process, to a hardness kvd of only 50HRC or a little higher, corresponding to a tensile strength of 1700 MPa or a little higher, but not much over 1900 MPa, corresponding to a hardness of 53.5HRC. Because of this moderate hardness level, this sted only has moderate sag resistance, steel with a higher tensile strength being needed to improve sag resistance. According^, such steel does not ensure an excellent compromise between high resistance, which would be above 2100 MPa, a hardness that would be higher than 55HRC, a higji fetigue resistance in air and fetigue resistance in corrosive conditions that is at least equivalent, if not higjier than that needed for springs.
 The purpose of the invention is to propose means to simultaneously increase, as compared to known steels, spring hardness and tensile strength, fetigue properties in air, making fatigue resistance in corrosive conditions at least equivalent, if not higher, increase spring sag resistance and to reduce susceptibility to surfece defects that can be generated during spring coiling.
 With this in mind the object of the invention is a spring steel with hi^ fatigue resistance in air and in corrosive conditions and with hi^ resfetance to cyclic sag, having the composition in weight percent
C = 0.45-0.70%
Mn = 0.20-0.75%
a = 0.60-2%
Ni = 0.15-1%
Mo = traces-1%
V = 0.003-0.5%
Ti = 0.020-0.15%
Nb = traces-0.15%
Al = 0.002-0.050%
P = traces-0.010%
S = traces-0.010%
0 = traces-0.0020%
N = 0.0020-0.0110%  The balance being iron and impurities resulting from the steel making process. [OOlpI A fiorther object of the invention is a manufecturing process for a spring steel with hi^ fatigue resistance in air and in corrosive conditions and high resistance to cyclic sag, according to which a liquid sted is made in a converter or an electric furnace, its composition is adjusted, it is cast into blooms or continuous flow billets or ir^ots that are left to cod to room temperature; that are rolled into bars, wire rods or slugs and transformed into springs, characterized in that:
- the sted is of the previous type;
- after they become solid the blooms, billets or ingots have a minimum mean cooling rate of 0.3°C/s between 1450- 1300°C;
- the blooms, billets or ingots are rolled between 1200-800°C in one or two reheating and rolling cydes;
- and bars, wire rods or slu^, or springs made from these, are austenitized between 850-1000°C, followed b>' a water quench, a polymer quench or an oil quench, and by tempering at 300-550'C, so as to deliver steel with hairiness greater than or equal to 55HRC.
 A further object of the invention is springs made fcom such steel, and springs
made of steel obtained ly the previous process.
 In an unexpected way, the inventors realized that a sted with the
characteristics of the previously dted indusion composition and morpholep ensured,
after stedmaking, casting, roDir^, quenching and tempering done in specific conditions,
a hardness greater than 55HRC, while assuring escdlent compromise between high
endurance level to fetigue in air and to fetigue in corrosive conditions, high resistance to
cydic sag and low sensitivity to surfece defects arising during manufecture of the
 The invention will be better understood upon reading the description that
follows, given in reference to the following appended figures:
- Figure 1 which shows the results of hardness and cyclic sag tests for steels according to the invention and reference steels;
- Figure 2 which shows the results of fatigue tests in air as a fianction of sted hardness for steds according to the invention and reference steds;
- Figure 3 which shows the results of Charpy impact tests as a function of the steel hardness for steds according to the invention and reference steds; and
- Figure 4 which shows the results of fetigue tests in corrosive conditions as a function of steel hardness for steels according to the invention and reference steds.  The sted composition according to the invention must meet the following conditions.
 The carbon content must be between 0.45% and 0.7%. After quenching and tempering, carbon increases the tensile strength and hardness of the steel. If the carbon content is less than 0.45%, in the temperature range usually used to manufacture springs, no quendiing and tempering treatment leads to the high
strength and hardness of the sted described in the invention. Secondly, if the carbon content exceeds 0.7% preferably 0.65%, coarse and very hard cariDides, combined with chromium, molybdenum and vanadium, can remain undissolved during the austenitization conducted before the quench, and can agnificantly affect fetigue lifetime in air, fetigue resistance in corrosive conditions and also toughness. Consequentfy^ carbon contents above 0.7% must be avoided. Preferably, it should not exceed 0.65%.  The siliccn content is between 1.65% and 2.5%. Silicon is an important element that ensures, throu^ its presence in solid solution, high levels of strength and hardness, as well as hi^ carbon equivalent values Ceq and sag resistance. To have the tensile strength and hardness values of the steel according to the invention, the silicon content must not be less than 1.65%. Furthermore, silicon contributes at least partially to steel deoxidation. If its content exceeds 2.5%, preferably 2.2%, the oxygen content of the steel can be, by thermodynamic reaction, greater than 0.0020%, preferably 0.0025%. This involves formation of oxides of various compositions which are harmful to fatigue resistance in air. Furthermore, for silicon contents greater than 2.5%, the various combined elements such as manganese, chromium or others can segre^te during solidification, after casting. This segr^^tion is very harmful to fet^e behavior in air and to fetigue resistance in corrosive conditions. Finally, for silicon content greater than 2.5%, decarburization at the surfece of bars or mre iDds for springs becomes too higji for the in-service properties of the springs. This is why the silicon content must not exceed 2.5%, and preferably 2.2%.
[OOlTj The manganese content is between 0.20% and_0.75%. In combination with residual sulfur at level of traces to 0.015%, the man^nese content must be at least ten times higher than the sulfur content so as to avoid formation of iron sulfides that are extremely harmful to steel rolling. Consequentfy", a minimum manganese content of
0.20% is required. Furthermore, mangianese contributes to solid solution hardening during the quenching of the steel as well as nickel, chromium, molybdenum and vanadium, which delivers high tensile strength and hardness values and the carbon equivalent Ceq value of the steel described in the invention. Mangpnese contents greater than 0.75%, preferably 0.65%, in combination with silicon, can s^regale during the solidification stage, after steel making and casting. These s^r^ptions are harmful to the in-service properties and to the homogeneity of the steel. This is why the manganese content must not exceed 0.75%, and preferably 0.65%. [OOIS] The chromium content must be between 0.60% and 2%, and preferably between 0.80% and 1.70%. Chromium is added to obtain, in solid solution after austenitization, quenching and tempering, hi^ values for tensile strength and hardness, and to contribute to obtaining the carbon equivalent Ceq value, but also to increase fetgue resistance in corrosive conditions. To ensure these properties the chromium content must be at least 0.60%, and preferabfy at least 0.80%. Above 2%, preferably 1.7%, specific coarse, very hard chromium carbides, in combination with vanadium and molybdenum^ can remain after the austenitization treatment that precedes the quench. Such carbides greatly affect the fetgue resistance in air. This is why the chromium content must not exceed 2%.
 The nickel content is between 0.15% and 1%. Nickel is added to increase steel hardenability, as well as tensile strength and hardness after quenching and tempering. Since it does not form carbides, nickel contnl)Utes to steel hardening, just like chromium, molybdenum and vanadium, without forming specific coarse, 'hard carbides which would not be dissolved during the austenitization that precedes the quench, and could be harmful to fetigue resistance in air. It also means that the carbon equivalent can be adjusted between 0.8% and 1% in the steel according to the invention
as needed. As a non-oxidizable element, nickel improves fetigue resistance in corrosive conditions. To ensure that these effects are significant, the nickel content must not be lower than 0.15%. In contrast, above 1%, preferabfy^ 0.80%, nickel can lead to oveiiy higji residual austenite content, whose presence is veiy harmful to fetigue resistance in corrosive conditions. Furthermore, hi^ nickel levels significantly increase the cost of the steel. For all these reasons the nickel content must not exceed 1%, preferably 0.80%.
[0020^ The molytxienum content must be between traces and 1%. As for chromium, mofybdenum increases steel hardenability, as well as strength. Furthermore, it has bw oxidation potential For these two reasons, mofybdenum is fevorable to fetigue resistance in air and in corrosive conditions. But for contents above 1%, preferabfy 0.80%, coarse, veiy hard molybdenum carbides can remain, optionally combined with vanadium and chromium, after the austenitization that precedes the quench. These particular carbides are very harmful for fetigue resistance in air. Finally, adding more than 1% molybdenum increases the cost of the steel unnecessarily. This is why the mofybdenum content must not eweed 1%, and preferably 0.80%.  The vanadium content must be between 0.003% and 0.8%. Vanadium is an element that increases hardenability, tensile strength and handnesis aftier quenching and tempering. Furthermore, in comtanation with nitrogen, vanadium forms a large number of fine submicroscopic vanadium or vanadium and titanium nitrides that refine the grain and increase tensile strength and hardness levels, through stiiictural hardening. To obtain formation of submicroscopic vanadium or vanadium and titanium nitrides that refine the grain, vanadium must be present vAUi a minimum content of 0.003%. But this element is expensive and it has to be kept at this lower limit if a compromise is sought between the cost of steel making and the grain
refinement. Vanadium must not exceed 0.8% and, preferably, 0.5%, because beyond this value a precipitate of coarse, veiy hard vanadium-containing carbides, in combination with chromium and mofybdenum, can remain undissolved during the austenitization that precedes the quench. This can be very unfevorable for fetigue resistance in air, for high values of strength and hardness in the sted according to the invention. Further, adding more than 0.8% vanadium increases the cost of the steel unnecessarify.
 The copper content must be between 0.10% and 1%. Copper is an element that hardens steel when it is in solid solution after the quenching and tempering treatment Accordingly, it can be added along with other elements that contribute in increasing the strength and hardness of tiie steel As it does not combine with carbon, it hardens the steel without forming coarse, hard carbides that harm fetigue resistance in air. From the electrochemical point of view, its passivation potential is higjier than that of iron and, consequently, it fevors sted fetigue resistance in corrosive conditions. To ensure that these effects are sgnificant, the copper content must not be tower than 0.10%. In contrast, at contents of more than 1%, preferably 0.90%, copper has a very harmful influence on the behavior during hot rolling This is why the copper content must not exceed 1%, and preferabfy 0.90%.
 The titanium content must be between 0.020% and 0.2%. Titaniimi is added to form, in combination with nitrogen, preferably also carbon and/or vanadium, fine, submicroscopic nitrides or carbonitrides that refine the austenitic grain during the austenitization that precedes the quench. Accordingfc^, it increases the surface area of the grain boundaries in the steel, tiiereb^^ reducing the quantity of unavoidable impurities that segr^ate in the grain boundaries, such as phosphorus. Such intergranular segregations would be very harmful to toughness and fetigue resistance
in air if they are present at high concentrations per unit of surface area at the grain boundaries. Furthermore, combined with carbon and nitnc^en, preferabfy with vanadium and nbbium, titanium leads to the formation of other fine nitrides or carbonitrides producing an irreversible tr^ping effect for some elements, such as hydrogen formed during corrosion reacticxis, and which can be extreme^ harmful to fetigue resistance in corrosive conditions. For good efiOdency the titanium content must not be lower than 0.020%. In contrast, above 0.2%, preferably 0.15%, titanium can lead to the formation of coarse, hard carbonitrides that are veiy harmful to fatigue resistance in air. The latter effect is yet more harmful for high levels of tensile strength and hardness in the sted according to the invention. For these reasons the titanium content must not exceed 0.2%, preferably 0.15%.
 The niobium content must be between traces and 0.2%. Niobium is added to form, in combination with carbon and nitrogjsn, extreme^ fine, submicroscopic precipitates of nitrides and/or carbides and/or carbonitrides that refine the austenitic grain during the austenitization that precedes the quench, especially when the aluminum content is low (0.002% for example). According, nbbium increases the suiiaoe area of the grain boundaries in the steel, and contributes to the same favorable effect as titanium as re^rds embrittlement of grain boundaries by unavoidable impurities such as phosphorus, whose effect is very harmful to toughness and fetigue resistance in corrosive conditions. Furthermore, extremely fine precipitates of niobium nitrides or carbonitrides contribute to steel hardening through structural hardening. However, the niobium content must not exceed 0.2%, preferably 0.15%, so that the nitrides or carbonitrides remain very fine, to ensure austenitic grain refining and to avoid cracks or splits forming during hot rolling. For these reasons the niobium content must not exceed 0.2%, preferably 0.15%.
 The aluminum content must be between 0.002% and 0.050%. Aluminum can be added to complete steel deoxidation and to obtain the lowest possible axygen. contents, certainly less than 0.0020% in the steel according to the invention. Furthermore, in combination with nitrogen, aluminum contributes to refining the grain by forming submicroscopic nitrides. To ensure these two functions, the aluminum content must not be bwer than 0,002%. In contrast, an aluminum content exceeding 0.05% can lead to the presence of laiigs, isolated inclusions or to aluminates that are finer but hard and angular, in the foim of long stimgprs tiiat are harmfiil to the fetigue lifetime in air and to the cleanliness of the steel. This is why the aluminum content must not exceed 0.05%.
 The phosphorus content must be between traces and 0.015%. Phosphorus is an unavoidable impurity in steel During a quenching and tempering treatment, it co-s^rie^tes witii elements such as chromium or manganese in the former austenitic grain boundaries. The result is reduced cohesbn in the grain boundaries and intei^nanular embrittlement that is very harmful to fatigue resistance in air. These effects are even more harmful for the high tensile strengths and hardnesses required in steels according to the invention. With the aim of simultaneously obtaining high spring steel tensile strength and hardness and good fatigue resistance in air and in corrosive conditions, the fdiosphorus content must be as low as possible and must not exceed 0.015%, preferably 0.010%.
 The sulfur content is between traces and 0.015%. Sulfur is an unavoidable impurity in steel Its content must be as low as possible, between traces and 0.015%, and preferably 0.0 lO^/o at most. Accordingly, we wish to avoid the presence of sulfides that are unfavorable to fetigue resistance in corrosi\^e conditions and fetigue resistance in air, for high values of strength and hardness in the steel according to the invention.
[002S| The ojg^en content must be between traces and 0.0020%. Qj^en is also an unavoidable impurity in sted. In combination with deoxidizing elements, oxygen can lead to isolated, coarse, very hard, angular inclusions appearing, or to inclusions that are finer but in the form of long stringers which are very hannflil to fetigue resistance in air. These efifects are even more haraiful at the high tensile strengths and hardnesses of the steels according to the invention. For these reasons, to ensure a good compromise between high tensile strength and hardness and hi^ fatigue resistance in air and in corrosive conditions in the steel according to the invention, the ce^en content must not exceed 0.0020%.
 The nitrogen content must be between 0.0020% and 0.0110%. The nitrogen must be controlled in this range so as to form, in combination with titanium, niobium, aluminum or vanadium, a sufficient number of very fine submicroscopic nitrides, carbides or carbonitrides that refine the grain. Accordingly, to do so the minimum nitrogen content must be 0.0020%. Its content must not exceed 0.0110% so as to avoid forming coarse, hard titanium nitrides or carbonitrides larger than 20 |iim, observed at 1.5 mm ± 0.5 mm from, the surfece of the bars or wire rods used to manufecture the springs. This position is the place that is most critical as r^ards the fetigue loading of the springs. Indeed, such large nitrides or carbonitrides are very unfevorable to fetigue resistance in air for higji strength and hardness values for steels according to the invention, given the fact that during the tests on fatigue in air, these springs fiBctured at the location of such large inclusions that were located precisely in the dted area of the surfece of the spring, when these inclusions were present.  To estimate the size of the titanium nitrides and carbonitrides, we consider the inclusions as squares and we suggest that their size is equal to the square root of their surface area.
 A manufacturing process for spring according to the invention will now be described.
 A non-limiting steel making process tiiat conforms to the invention is as follows. liquid sted is produced either in a converter, or in an electric fLunace, then undergoes a ladle metallurgy treatment during which alloy elements are added and deoxidation is performed, and in general all secondary metallurgy operations delivering a sbed having the composition according to the invention and avoiding fcrmation of sulfide or "carbonitrosulfide" compleuses of elements such as titanium and/or niobium and/or vanadium. To avoid formation of such coarse precipitates during steel making, the inventors have discovered, in an unexpected way, that the contents of the various elements, in particular those of titanium, nitrogen, vanadium and sulfiir, must be carefiilly controlled in the previously dted limits. After the process that has just been described the sted is cast in the form of blooms or billets, or into ingots. But to completely avoid forming, or to avoid fomning as much as posstole, coarse titanium nitrides or carbonitrides during and after the solidification of these products, we have found that the mean cooling rate of these products (blooms, billets or ingots) must be controlled so as to be 0.3°C/s or higher between 1450-1300°C. When we operate in these conditions during the solidification and cooling stages, we observe in an unexpected way that the size of the coarsest titanium nitrides or carbonitrides observed on the springs is always less than 20 pm. The location and size of these titanium predpitates wiH be discussed hereinafter.
 When they have returned to room temperature, products having tiie precise composition according to the invention (blooms, billets or ingots) are next reheated and rolled between 1200-800°C into the form of wire rods or bars in a single or double heating and rolling process. So as to obtain the properties of the steel that is specific to
the invention, the bars, rods, slugs, or even springs produced fiiom these bars or wire rods, are next subjected to a water quench treatment, a polymer quench or an oil quench after austenitization in a temperature range firjm 850-1000 C, so as to obtain a fine ausbenitic grain where there are no grains coarser than 9 on the ASTM grain size scale. This quenching treatment is then followed by a tempering treatment specifically performed between 300-550°C, that delivers the high levels of tensile strength and hardnisss required for the steel, and avoids firstly a microstructure that would lead to embrittiement during tempering, and secondly, overly high residual austenite. We found that embrittiement during tempering and an oveiiy h^ level of residual austentite are extremely harmful to fetigue resistance in corrosive conditions of the steel according to the invention. In the case where the springs are manufactured Sum bars that have not been heat treated or from wire rods or slugs made fijom such bars, the abovementioned treatments (quenching and tempering) must be performed on the springs themselves under the abovementioned conditions. In the case where the springs are manufactured from using cold forming, these heat treatments can be done on the bars, wire rods or slugs made fiTom these bars before manufacturing the spring.  It is well known that the hardness of steel depends not onb^^ on its composition, but also on the quenching temperature that it was subjected to. It must be understood that for all the compositions of the invention, it is possible to find quenching temperatures in the industrial range of 300-550°C that deliver the minimum targeted hardness of 55HRC.
 Since nitrides and carbonitrides are very hard, their size as previously defined does not change at all during the steel transformation steps. Therefore it is not important whether it is measured on the intermediate product (bar, wire rod or slu^ which will be used to manuJacture the spring or on the spring itsdf.
 The invention delivers spring steels that can combine hi^ hardness and tensile strength that are an improvement over the prior art, as well as improved fatigue properties in air and s^ resistance, fatigue properties in corrosive conditions at least equivalent to those of known steels for this use, or even better, and lesser susceptibility to concentrations of stresses produced by surfece defects that can form during spring manufecture, throu^ addition of microaHqyed elements, a reduction in residual elements and control of the analysis and production route of the steel.  The invention is now illustrated using examples and reference examples. Table 1 shows sted compositions according to the invention and reference steels. The carbon equivalent Ceq is given by the following formula;
Ceq= [C] + 0.12 [Si] + 0.17 [Mn]-0.1 [Ni] + 0.13 [Crj-0.24 \Vwhere [C], [Si], [Mn], [Ni], fCr] and [V] represent the content of each element in wd^t percent.
C Si Mn Ni Cr V Ti Cu Mo Nb p s Al ■ N O Cec
ed of the i^ention 1 0.4S 1.82 0.21 015 1.48 0.204 0.072 O^D 0.02 0 0.006 0.006 .0.034 0.0051 0.0007 0.86
eel of the veatkin.2 0,58 1.79 022 0.15 0.98 0.216 0.073 020 0.03 0 0.006 0.008 0.032 0.0051 0.0007 0.89
ed of the i?entian3 0.39 1.80 0.22 0.15 039 0212 0.025 020 0.03 0.022 0.007 0.008 0.032 0.0066 O.OOCB 091
ed of the i7ention4 0.48 2.10 021 0.70 1.50 0152 0.069 051 0.03 0 0.005 0.005 0.032 0.0042 0.0008 0.86
ed of the t^entianS 0.54 1.81 0.23 0.34 1.25 0.098 0.077 0.42 0.02 0 0.006 0.008 0.031 0.0041 0.0007 0.90
fenence 3dl 0.60 1.73 0.88 O.CB 020 0.154 0.002 0.19 0.03 0.020 0.010 0.019 0.002 0.0084 0.0010 om
rference 2d2 0.40 1.79 017 0.53 1.04 a 166 0.064 0.20 0.01 0 0.013 0.004 0.020 0.0034 0.0011 0.69
fenenoe sd3 048 1.45 0.89 0.11 0.47 0.136 0.002 019 0.02 0 0.011 0.013 0.003 0.0062 0.0010 0.82
Table 1: Chemical compositions of the tested steels (in %)
[003S| Table 2 shows the hardness values obtained for steels according to the invention and reference steels as a functioQ of the quenching temperature that was used.
rc) HRC hardness Quenching temperature
rc) HRC hardness
steel of the invention 1 350 56.9 400 55.3
Steel of the invention 2 350 58.5 400 57.1
Steel of the invention 3 350 59.0 400 57.2
Steel of the invention 4 350 56.7 400 55.6
Steel of the invention 5 350 57.6 400 55.8
Reference sted 1 350 57.9 400 55.1
Reference sted 2 350 54.2 400 52.5
Reference sted 3 350 54.8 400 51.3
Table 2: Hardness and tensile strength as a function of the tempering temperature
 Table 3 shows the maximum size of the indusions of titanium nitride or carbonitrides observed at 1.5 mm fiom the surfece of steels according to the inventicxi and reference steels, as previoust^ defined. We have also reported the titanium contents of the various steels.
 The maximum si2e of such titanium nitride or carbonitride inclusions is determined as folbws. On a section of bar or wire nod coming fiDm a given steel cast, a surface area of 100 mm^ is examined at a point boated 1.5 mm ± 0.5 mm below the surface of the bar or wire rod. After the observations, the size of the titanium nitride or carbonitride indusion having the largest surface area is determined by considering tiiat the indusions are squares and that the size of each of these inclusions, including the indusion having the largest surface area, is equal to the square root of the surfeioe area All the indusions are observed on a section of bar or wire rod for springs, and the observations are performed on 100 mm^ of eadi section. The steel cast conforms to the invention when the maximum size of the abovementioned indusions observed on
100 mm2 at 1.5 mm ± 0.5 mm under the surface is less than 20 |im. The corresponding results obtained on steels according to the invention and reference steels are given in table 3.
 As n^prds the reference tests 1 and 3, their titanium content is practically nil and no nitrides and carbonitrides are observed.
Ti (%) Size of the largest nitride or
carbonitride observed on
100 mm2 (jjm)
Steel of the invention 1 0.072 11.8
Steel of the invention 2 0.073 12.4
Steel of the invention 3 0.025 13
Steel of the invention 4 0.069 11.9
Steel of the invention 5 0.077 14.1
Reference sted 1 0.002 -
Reference steel 2 (first exam) 0.064 20.8
Reference steel 2 (second exam) 0.064 29
Reference sted 3 0.002 -
Table 3: Maximum size of the largest titanium nitride or cariDonitride indusions at 1.5
mm from the surface of the samples
 We did not measure the size of the inclusions with reference steels 1 and 3, since thdr titanium content was low and did not conform to the invention: the result would not have been significant.
 Samples for fatigue testing were taken fiDm bars, and the final diameter of the samples was 11 mm. Preparation of the samples for fatigue testing induded rough machining, austenitization, oil quenching, tempering, grinding and shot-peening. These samples were torsion-fetigue tested in air. The shear stress applied was 856 ± 494 MF^ and the number of cydes to fracture was counted. The tests were stopped after 2.10^ cydes if the samples had not broken.
 Samples for fetigue testing in corrosive conditions were taken fix)m bars, and the final diameter of the samples was 11 mm. Preparation of the samples for fatigue testing included rougji machining, austenitization, oil quenching, tempering, grinding and shot-peening. These samples were tested for fetigue in corrosive conditions, i.e. corrosion was applied at the same time as a fetigue load. The fetigue bad was a shear stress of 856 ± 300 MPa. The corrosion applied was cyclic oorrosbn in two alternating stages:
- one stage being a wet stage, with spraying of a 5% NaCl solution for 5 minutes at 35°C;
- one stage being a dry stage without spra^dng, for 30 minutes at a temperature of 35°C.
 The number of cycles to fi:a.cture was considered to be the fetigue life in
 Sag resistance was determined using a cydic compression test on cylindrical
samples. The sample diameter was 7 mm and their height was 12 mm. They were
taken fi^m steel bars.
 Preparation of the samples for sag testing included rough machining,
austenitizing, oil quenching tempering and final fine grinding. The height of the sample
was measured precisely before starting the test by using a comparator basing 1 |jm
precision. A preload was applied so as to simulate spring presetting, this presetting
being a oompressbn stress of 2200 MPa.
 Then tiie fatigue load cyde was applied. This stress was 1270 + 730 MPa. The
height loss in the sample was measured for a number of cycles, up to 1 million. At the
end of the test the total sag was determined b}' a predse measurement of the remaining
height compared to the initial height, sag resistance being better when the reduction in
height, as a percentage of the initial hei^t, was lower.
 The results of the fetigue tests, fetigue tests in corrosive conditions and sag on
steels according to the invention and reference steels are given in table 4.
HRC hardness Tensile strength (MPa) Fatigue life
ofcydes) Fatigue life
Steel of the invention 1 56.7 2129 1742967 192034 0.025
Steel of the invention 2 56.4 2106 > 2000000 138112 0.01
Steel of the invention 3 56.5 2118 >2000000 135562 0.015
Steel of the invention 4 56.9 2148 >2000000 202327 0.025
Steel of the invention 5 57.0. 2156 > 2000000 139809 0.025
1 56.7 2131 514200 96672 0.03
Reference steel 2 53.8 1898 217815 241011 0.10
Reference steel 3 55.6 2062 301524 150875 0.075
Table 4: Results of fetigue, fetigue in corrosive conditions and sag tests
 From these tables, we see that the various reference steels are unsatisfactory, in particular for the foilowir^ reasons.
 Reference sted 1, in particular, has sulfur content that is too high for good compromise between Migue resistance in air and the content for fatigue in corrosive conditions. Furthermore, its manganese content is too high, which leads to s^ne^tions that are hannful for the homogeneity of the steel and fetigue resistance in
 Reference steel 2 has too bw carbon content and carbon equivalent to ensure hi^ hardness. Its tensile strength is too low for good fetigue resistance in air.  Reference steel 3, in particular, has silicon content that is too low for good sag resistance and also good fatigue resistance in air.
 Sag resistance is higher for the steels of the invention than for reference steels, as Figure 1 shows, where it is dear that according to the abovementioned sag measurements, the values for sag are at least 32% lower for the worst case of the steels of the invention (sted of the invention 1) as compared to the best case of the reference steels (reference sted 1).
 The latigue lifetime in air is deaiiy hi^er for the steels of the invention as compared to the reference steels. This is due to the increased hardness, as Figure 2 shows, but increased hardness is not enough- In fact, general^, steels with high hardness are more susceptible to defects, such as indusions and surfece defects as the haidness increases. According, steels according to the invention are less susceptible to defects, in particular to coarse indusions such as titanium nitrides or carbonitrides, given that the invention prevents such lai^ indusions spearing. As table 3 shows, the largest indusbns found in steels aoconiing to the invention do not exceed 14.1 ^im, where indusions lai^er than 20 |am are found in reference sted 2. Furthermore, lower susceptibility to surfece defects such as those that arise during spring manufacture or other operations when steels of the invention are used can be illustrated by strength tests performed on steels of the invention and reference steels having undergone a heat treatment and having hardness of 55HRC or higher, see figure 3. The values measured during Charpy impact tests on the steels of the invention (where the sample notch simulates a concentration of stresses like other concentrations of stresses that we can
find on surfece defects produced during the manufacture of the spring or other operations) are higher than those measured on the reference steels. This jowls that the steels according to the invention are less susceptible to concentrations of stresses on defects than reference steels according to the prior art.
 We know that increasing hardness reduces fetigue resistance in corrosive conditions. According^, it seems that steels according to the invention have the advantage that their futile resistance in corrosive conditions is higher than that of reference steels according to the prior art, and in particular hardness greater than 55HRC as Figure 4 shows.
 Accordingly, the invention delivers hiker hardness with a good compromise between fetigue lifetime in air and sag resistance, which are gritty increased, and fetigue lifetime in corrosive conditions which is better than those of reference steels according to the prior art Furthermore, lesser susceptibility to possible surfece defects, in particular those generated during spring manufacture or other operations, is also obtained.
1. A spring steel with fatigue resistance in air and in corrosive conditions and
with high resistance to cyclic sag, having the composition in weight percent
the balance being iron, and impurities resulting films the steel making process, where the carbon equivalent Ceq content calculated accord to the formula: Ceq% = [C%] + 0.12 [Si%} + 0.17 [Mn%] -0.1 [Ni%] + 0.13 [Cr%] - 0.24 [V%] is between 0.80 and 1.00%, and whose hardness, after quenching and tempering, is greater than or equal to 55HRC.
2. The steel for springs according to dame 1, characterized in that the maximum size
of titanium nitrides or carbonitrides observed at 1.5 ± 0.5 mm of the surface area of a bar, a wire rod, a slug or a spring over 100 mm’ of the surfece area of the section is less than or equal to 20 pm, said size being the square root of the surfece area of the inclusions considered as squares.
3. The spring steel according to claim 1 or 2, characterized in that its composition is:
the balance being iron and impurities resulting fix)m the steel making process.
4. A manufacturing process for a spring steel with high fatigue resistance in air and
in corrosive conditions and high resistance to cyclic sag, according to which a liquid
steel is made in a converter or an electric furnace, its composition is adjusted, it is cast into blooms or continuous flow billets or ingots that are left to cool to room temperature; that are rolled into bars, wire rods or slugs and transformed into springs, characterized in that:
- the steed is the type according to one of claims 1 to 3:
- after they become solid the blooms, billets or ingots have a minimum mean cooling rate of 0.3°C/s between 1450-1300°C;
- said blooms, billets cr ingots are rolled between 1200-800°C in one or two reheating
and rolling cedes;
- and bars, wire rods or slugs, or springs made fixed these, are austerities between
850-1000°C, fooled by a water quench, a polymer quench or an oil quench, and by
tempering at 300-550°C, so as to deliver steel with hardness greater than or equal to
5. A spring, characterized in that ft is made of a steel according to one of claims 1 to
6. A spring according to dam 5, characterized in that it is made of a steel obtained by
the process according to claim 4.
|Indian Patent Application Number||2985/CHENP/2008|
|PG Journal Number||51/2013|
|Date of Filing||13-Jun-2008|
|Name of Patentee||KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.)|
|Applicant Address||10-26, WAKINOHAMA-CHO 2-CHOME, CHUO-KU, KOBE-SHI, HYOGO 651-8585,|
|PCT International Classification Number||C22C38/18|
|PCT International Application Number||PCT/FR06/2700|
|PCT International Filing date||2006-12-11|