WO2007080256A1

WO2007080256A1 - Spring steel, method for producing a spring using said steel and a spring made from such steel

Info

Publication number: WO2007080256A1
Application number: PCT/FR2006/002700
Authority: WO
Inventors: Nao Yoshihara; Kazuhisa Kawata; Julie Mougin; Jacques Languillaume
Original assignee: Ascometal; Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.)
Priority date: 2005-12-15
Filing date: 2006-12-11
Publication date: 2007-07-19
Also published as: NO20082766L; CA2633153A1; RS51070B; US20080308195A1; RU2397270C2; JP2007224413A; KR20080090424A; KR101048946B1; FR2894987A1; ES2331539T3; PL1966407T3; ATE445026T1; BRPI0619892B1; CA2633153C; EP1966407A1; EP1966407B1; NO341748B1; CN101400818B; CN101400818A; JP4869051B2

Abstract

The invention relates to spring steel having increased fatigue strength in air and in corrosive environments and high resistance to cyclical slackening. The composition of said steel contains the following components as expressed in percentages by weight, namely: C = 0.45 0.7 %; Si = 1.65 2.5 %; Mn = 0.2 0.75 %; Cr = 0.6 - 2 %; Ni = 0.15 - 1 %; Mo = trace - 1 %; V = 0.003 0.8 %; Cu = 0.1 - 1 %; Ti = 0.02 0.2 %; Nb = trace 0.2 %; Al = 0.002 0.05 %; P = trace 0.015 %; S = trace 0.015 %; O = trace 0.002 %; N = 0.002 0.011 %, the remainder comprising iron and impurities resulting from production. Moreover, the equivalent carbon content Ceq, which is calculated using formula Ceq % = [C %] + 0.12 [Si %]+ 0.17 [Mn %] 0.1 [Ni %] + 0.13[Cr %] 0.24 [V %], is between 0.8 and 1 % and hardness following quenching and tempering is greater than or equal to 55HRC. The invention also relates to a method for producing a spring using said steel and to the spring thus produced.

Description

Spring steel, and method of making a spring using this steel, and spring made of such steel.

The invention relates to iron and steel, and more specifically to the field of spring steels.

In general, with the increase in fatigue loadings applied to the springs, the hardness and tensile strength required for the springs increases continuously. As a result, the fracture sensitivity initiated on defects, such as inclusions or surface defects generated during the manufacture of the springs, increases, and the fatigue strength tends to become limited. On the other hand, springs used in a highly corrosive environment, such as suspension springs, must have at least equivalent and even higher fatigue corrosion properties since they use steels with hardness and tensile strength. higher. Thus, such springs tend to break at the level of defects, immediately during air fatigue cycles, and later during fatigue cycles in a corrosive environment. In particular, for fatigue under corrosion, defects can be initiated on pitting corrosion. In addition, with the increase in stress applied, it is more difficult to improve the fatigue life under corrosion or to maintain it at an equivalent level, given that the effects of the stress concentration on the bites corrosion, surface defects of the springs that may be generated during winding of the spring or other stages of its manufacture, or non-metallic inclusions, become more critical when the hardness of the spring increases.

According to the state of the prior art, FR-A-2,740,476 and JP-A-3,474,373 disclose a spring steel grade having good resistance to embrittlement by hydrogen and good strength. in the case of fatigue, in which the inclusions of carbonitrosulfides comprising at least one of titanium, niobium, zirconium, tantalum or hafnium, are controlled so as to have a reduced average size, less than 5 μm in diameter, and be very numerous (10,000 or more on a section of cut). However, this type of steel leads, after quenching and tempering according to the industrial process of manufacturing springs, to a hardness level of only 50HRC or a little more, corresponding to a tensile strength of 1700 MPa or a little more , but hardly greater than 1900MPa, corresponding to a hardness of 53.5HRC. Because of this moderate hardness level, this steel has only moderate scuff resistance, and steel with higher tensile strength is required to improve the scuff resistance. Thus, such a steel does not provide an excellent compromise between a high strength, which would be greater than 2100MPa, a hardness that would be greater than 55HRC, a high resistance to fatigue in the air and a resistance to fatigue under corrosion. less equivalent, and even superior, to what is necessary for springs.

The object of the invention is to propose means for producing, simultaneously with known spring steels, an increase in the hardness and the tensile strength of the springs, the higher fatigue properties in the air, the properties in corrosion fatigue at least equivalent and even higher, a resistance to slackening of the upper spring and less sensitivity to surface defects that can be generated during the twisting of the spring. To this end, the subject of the invention is a spring steel with high fatigue strength in air and under corrosion and with high resistance to cyclic wear, of composition, in weight percentages:

C = 0.45 - 0.70%

Si = 1, 65 - 2.50% Mn = 0.20 - 0.75%

Cr = 0.60 - 2%

Ni = 0.15 - 1%

Mo = traces - 1%

V = 0.003 - 0.8% Cu = 0.10 - 1%

Ti = 0.020 - 0.2%

Nb = traces - 0.2% Al = 0.002 - 0.050%

P = traces - 0.015%

S = traces - 0.015%

O = traces - 0.0020% N = 0.0020 - 0.0110% the remainder being iron and impurities resulting from the preparation, and whose equivalent carbon content Ceq, calculated according 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.

The maximum size of Ti nitrides or carbonitrides observed at 1, 5 +

0.5mm of the surface of a bar, or a wire rod, a piece or a spring on 100mm ² of the cutting surface is preferably less than or equal to 20μm, said size being the square root of the surface inclusions considered as squares.

Preferably, the composition of the steel is:

C = 0.45 - 0.65%

If = 1.65 - 2.20% Mn = 0.20 - 0.65%

Cr = 0.80 - 1.7%

Ni = 0.15 - 0.80%

Mo = traces - 0.80%

V = 0.003 - 0.5% Cu = 0.10 - 0.90%

Ti = 0.020 - 0.15%

Nb = traces - 0,15%

Al = 0.002 - 0.050%

P = traces - 0.010% S = traces - 0.010%

O = traces - 0.0020%

N = 0.0020 - 0.0110% the rest being iron and impurities resulting from the elaboration.

The invention also relates to a method of manufacturing a spring steel with high fatigue resistance in air and under corrosion and with high resistance to cyclic scumming, according to which a liquid steel is produced in a converter or an electric furnace, its composition is adjusted, it is cast in the form of blooms or continuous casting billets or ingots which are allowed to cool to room temperature, it is rolled in the form of bars, wire rods or slugs and it is transformed into springs, characterized in that:

- the steel is of the previous type; blooms, billets or ingots are imposed during or after their solidification, a minimum average cooling rate of 0.3 ° C / s between 1450 and 1300 ⁰ C;

these blooms, billets or ingots are laminated between 1200 and 800 ^° C. in one or two cycles of reheating and rolling; and on the bars, the machine wires or the plots, or on the springs resulting therefrom, austenitization is carried out between 850 and 1000 ^° C., followed by water quenching, polymer quenching or quenching. oil, and by an income at 300-550 ° C, so as to give the steel a hardness greater than or equal to 55HRC. The invention also relates to springs made of such steel, and springs made of a steel obtained by the above method.

Unexpectedly, the inventors have found that a steel having the above-mentioned characteristics of inclusionary composition and morphology made it possible, after elaboration, casting, rolling, quenching and tempering, to obtain, under specific conditions, a hardness greater than 55HRC, while achieving an excellent compromise between a long fatigue life in the air and fatigue under corrosion, a high resistance to cyclic sagging and a low sensitivity to surface defects occurring during the manufacture of the spring. The invention will be better understood on reading the description which follows, given with reference to the following appended figures: FIG. 1, which shows the results of hardness and cyclic wear tests for steels according to the invention and reference steels;

FIG. 2 which shows the results of fatigue tests in the air as a function of the hardness of the steel, for steels according to the invention and reference steels;

FIG. 3 which shows the results of Charpy resilience tests as a function of the hardness of the steel, for steels according to the invention and reference steels; - Figure 4 which shows the results of corrosion fatigue tests as a function of the hardness of the steel for steels according to the invention and reference steels.

The composition of the steel according to the invention must meet the following requirements. The carbon content must be between 0.45 and 0.7%. Carbon, after quenching and tempering, increases the tensile strength and hardness of steel. If the carbon content is less than 0.45%, in the temperature range usually used for the manufacture of the springs, no quenching and tempering treatment leads to a high strength and high hardness of the steel described in the invention. On the other hand, if the carbon content exceeds 0.7% or even 0.65%, coarse and very hard carbides combined with chromium, molybdenum and vanadium may remain undissolved during austenitization performed prior to quenching, and can significantly affect fatigue life in air, fatigue resistance under corrosion and also toughness. As a result carbon contents above 0.7% are to be excluded. Preferably, they should not exceed 0.65%.

The silicon content is between 1.65 and 2.5%. Silicon is an important element for ensuring, thanks to its presence in solid solution, high levels of strength and hardness, as well as high Ceq equivalent carbon and resistance values. To obtain the values of tensile strength and hardness of the steel according to the invention, the Silicon content should not be less than 1.65%. In addition, silicon contributes at least partially to the deoxidation of the steel. If its content exceeds 2.5% or even 2.2%, the oxygen content of the steel may be, by thermodynamic reaction, greater than 0.0020 or even 0.0025%. This reflects the formation of oxides of various compositions that are detrimental to the fatigue resistance in the air. In addition, for silicon contents greater than 2.5%, segregations of various combined elements such as manganese, chromium or the like may occur during solidification after casting. These segregations are very damaging to fatigue behavior in the air and resistance to fatigue under corrosion. Finally, for a silicon content greater than 2.5%, the decarburization on the surface of the bars or wires intended to form the springs becomes too important for the properties in service of the spring. This is why the silicon content should not exceed 2.5%, and preferably 2.2%. The manganese content is between 0.20 and 0.75%. Manganese, in combination with residual sulfur between trace amounts and 0.015%, must be added at a level at least greater than ten times the sulfur content so as to avoid the formation of iron sulphides which are extremely damaging to the laminability of the iron. 'steel. As a result, a minimum manganese content of 0.20% is required. In addition, manganese contributes to solid solution hardening during the quenching of steel, in the same way as nickel, chromium, molybdenum and vanadium, which makes it possible to obtain the values of tensile strength and high hardness and equivalent carbon Ceq values of the steel according to the invention. For manganese contents greater than 0.75% or even 0.65%, segregations, in combination with silicon, may occur during the solidification phase following the preparation and casting of the steel. These segregations are damaging to the service properties of steel and the homogeneity of steel. This is why the manganese content of the steel must not exceed 0.75%, or better still 0.65%. 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, high values of resistance tensile strength and hardness, and to help achieve the equivalent value of carbon Ceq, but also to increase resistance to fatigue under corrosion. To ensure these properties, the chromium content should be at least 0.60%, and preferably at least 0.80%. Above 2%, or even 1.7%, particular, coarse and very hard chromium carbides, in combination with vanadium and molybdenum, may remain after the austenitization treatment performed prior to quenching. Such carbides greatly affect the fatigue resistance in the 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 the hardenability of the steel, as well as the tensile strength and hardness after quenching and tempering. As it does not form carbides, nickel contributes to the hardening of steel, just like chromium, molybdenum and vanadium, without the formation of particular coarse and hard carbides that would not be dissolved during the austenitization performed before quenching. and could be harmful to the fatigue resistance in the air. It also makes it possible to adjust the equivalent carbon between 0.8 and 1% in the steel according to the invention as necessary. As a non-oxidizable element, nickel improves fatigue resistance under corrosion. To ensure that these effects are significant, the nickel content should not be less than 0.15%. On the contrary, above 1% or even 0.80%, nickel can lead to a too high residual austenite content, the presence of which is very damaging to the resistance to fatigue under corrosion. In addition, high levels of nickel significantly increase the cost of steel. For all these reasons, the nickel content should not exceed 1%, better 0.80%

The molybdenum content must be between traces and 1%. Like chromium, molybdenum increases the hardenability of steel, as does its strength. In addition it has a low oxidation potential. For these two reasons, molybdenum is conducive to fatigue resistance in the air and under corrosion. But for contents higher than 1%, even 0.80%, coarse and very hard molybdenum carbides may remain, possibly combined with vanadium and chromium, after austenitization prior to quenching. These Particular carbides are very damaging to fatigue resistance in the air. Finally, addition of molybdenum exceeding 1% unnecessarily increases the cost of steel. This is why the molybdenum content should not exceed 1%, better 0.80%. The vanadium content should be between 0.003 and 0.8%. Vanadium is an element that increases quenchability, tensile strength and hardness after quenching and tempering. In addition, in combination with nitrogen, vanadium makes it possible to form a large number of fine vanadium or vanadium nitrites and submicroscopic titanium to refine the grain and increase the levels of tensile strength and hardness. , thanks to a structural hardening. To obtain submicroscopic V and Ti nitride formation for grain refinement, vanadium must be present with a minimum content of 0.003%. But this element is expensive and must be kept close to this lower limit if a compromise between the cost of elaboration and the refining of the grain is sought. The vanadium should not exceed 0.8% and preferably 0.5%, because above this value a precipitation of carbides containing coarse and very hard vanadium combined with chromium and molybdenum can remain in the undissolved state during the austenitization which takes place before quenching. This can be very unfavorable to the resistance to air fatigue, for the high values of strength and hardness of the steel according to the invention. And an addition of vanadium above 0.8% unnecessarily increases the cost of steel.

The copper content must be between 0.10 and 1%. Copper is an element that hardens steel when in solid solution after tempering and tempering treatment. Thus, it can be added with other elements contributing to increase the strength and hardness of the steel. Since it does not combine with carbon, it provides hardening of the steel without formation of hard and coarse carbides damaging the fatigue resistance in the air. From the electrochemical point of view, its passivation potential is higher than that of iron and, consequently, it is favorable to the resistance to corrosion fatigue of steel. To ensure that its effects are significant, the copper content should not be less than 0.10%. On the contrary, for contents above 1% even 0.90%, copper has a very damaging influence on the hot rolling behavior. This is why the copper content should not exceed 1%, better 0.90%.

The titanium content should be between 0.020 and 0.2%. Titanium is added to form, in combination with nitrogen, or even carbon and / or vanadium, fine submicroscopic nitrides or carbonitrides to refine the austenitic grain during the austenitization treatment that takes place before quenching. Thus, it increases the surface of the grain boundaries in the steel, thus leading to a reduction in the amount of unavoidable impurities segregated at grain boundaries, such as phosphorus. Such intergranular segregation would be very detrimental to toughness and fatigue resistance in the air if present at high concentrations per unit area at the grain boundaries. In addition, combined with carbon and nitrogen, or even with vanadium and niobium, titanium leads to the formation of other nitrides or fine carbonitrides producing an irreversible trapping effect of certain elements, such as the hydrogen formed during corrosion reactions, which can be extremely damaging to fatigue resistance under corrosion. For good efficiency, the titanium content should not be less than 0.020%. On the contrary, above 0.2% or even 0.15%, titanium can lead to the formation of coarse and hard nitrides or carbonitrides, very damaging to the resistance to fatigue in the air. This last effect is even more damaging to the high levels of tensile strength and hardness of the steel according to the invention. For these reasons the titanium content should not exceed 0.2%, better 0.15%. The niobium content must be between traces and 0.2%. Niobium is added to form, in combination with carbon and nitrogen, extremely fine submicroscopic precipitates of nitrides and / or carbides and / or carbonitrides which allow, especially when the aluminum content is low (0.002% by example), to complete refinement of the austenitic grain during the austenitization treatment performed before quenching. Thus, niobium increases the surface of grain boundaries in steel, and contributes to the same favorable effect as titanium with respect to embrittlement. grain boundaries by unavoidable impurities such as phosphorus, the effect of which is very damaging to the tenacity and resistance to fatigue under corrosion. In addition, extremely fine precipitates of niobium nitrides or carbonitrides contribute to the hardening of the steel by structural hardening. However, the niobium content must not exceed 0.2% or even 0.15%, so that the nitrides or carbonitrides remain very fine, to ensure refinement of the austenitic grain and avoid the formation of cracks or crevices during rolling. hot. For these reasons, the niobium content should not exceed 0.2%, better still 0.15%. The aluminum content must be between 0.002 and 0.050%.

Aluminum can be added to complete the deoxidation of the steel and obtain oxygen levels as low as possible, and in any case less than 0.0020% in the steel according to the invention. In addition, combined with nitrogen, aluminum contributes to refinement of the grain by the formation of submicroscopic nitrides. To ensure these two functions, an aluminum content of not less than 0.002% is required. On the contrary, an aluminum content exceeding 0.05% can lead to the presence of large isolated inclusions or to finer but hard and angular aluminates, in the form of long strings, damaging to the fatigue life in the air and cleanliness of the steel. Therefore, the aluminum content must not exceed 0.05%.

The phosphorus content must be between traces and 0.015%. Phosphorus is an inevitable impurity in steel. During tempering and tempering treatment, it co-segregates with elements such as chromium or manganese with the former austenitic grain boundaries. This results in a reduction of the cohesion of the grain boundaries and an intergranular embrittlement which is very damaging to the tenacity and to the resistance to fatigue in the air. These effects are even more damaging to the high tensile strengths and hardness required for the steels according to the invention. In order to simultaneously obtain a high tensile strength and a high hardness of the spring steel and a good resistance to fatigue in the air and fatigue under corrosion, the phosphorus content must be as low as possible and should not exceed 0.015%, preferably 0.010%. The sulfur content is between traces and 0.015%. Sulfur is an inevitable impurity in steels. Its content should be as low as possible, between traces and 0.015%, and preferably not more than 0.010%. It is thus desired to avoid the presence of sulfides that are unfavorable to the resistance to fatigue under corrosion and to the resistance to fatigue in the air, for the high values of strength and hardness of the steel according to the invention.

The oxygen content must be between traces and 0.0020%. Oxygen is also an inevitable impurity in steels. Combined with deoxidizing elements, oxygen can lead to the appearance of coarse inclusions, isolated, very hard and angular, or to thinner inclusions but in the form of long chains which are very damaging to the fatigue resistance in the air. These effects are even more damaging to the high values of tensile strength and hardness of the steels according to the invention. For these reasons, in order to ensure a good compromise between high tensile strength and hardness and high resistance to fatigue in the air and fatigue under corrosion for the steel according to the invention, the oxygen 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 very fine nitrides, carbides or carbonitrides submicroscopic sufficient, allowing a grain refinement. Thus, for this purpose, the minimum nitrogen content should be 0.0020%. Its content must not exceed 0,0110% so as to avoid the formation of coarse and hard titanium nitrides or carbonitrides larger than 20 μm, observed at 1.5 mm + 0.5 mm from the surface of the bars or wire rods used to the manufacture of springs. This location is the most critical location for fatigue loading of springs. In fact, such nitrides or carbonitrides of large size are very unfavorable to the resistance to fatigue in the air for the high values of strength and hardness of the steels according to the invention, given that during fatigue tests in the air, spring break occurs at the location of such large inclusions precisely located in the vicinity of the surface of the springs as mentioned, when these inclusions are present.

To estimate the size of titanium nitrides and carbonitrides, inclusions are considered squares and their size is equal to the square root of their surface.

A method of manufacturing springs according to the invention will now be described.

A non-limiting example of a process for producing a steel according to the invention is as follows. The liquid steel is produced either in a converter or in an electric furnace and then undergoes a pocket metallurgy treatment during which alloying element additions and deoxidation are performed, and in general all secondary metallurgy operations. to obtain a steel having the composition according to the invention and avoiding the formation of complex sulfides or "carbonitrosulfides" of elements such as titanium and / or niobium and / or vanadium. In order to avoid the formation of such coarse precipitates during production, the inventors unexpectedly discovered that the contents of the various elements, in particular those of titanium, nitrogen, vanadium and sulfur, must be carefully controlled. within the aforementioned limits. After the development which has just been described, the steel is then cast, either by continuous casting in the form of blooms or billets, or in the form of ingots. But to avoid completely or as much as possible the formation of coarse titanium nitrides or carbonitrides during and after the solidification of these products, it has been found that the average cooling rate of these products (blooms, billets or ingots) must be regulated in such a way that to be equal to 0.3 ° C / s or more between 1450 and 1300 ⁰ C. When operating under these conditions during the solidification and cooling step, it is unexpectedly observed that the size of the largest nitrides or carbonitrides of Ti observed on the springs is always less than 20 microns. We will talk further about the situation and the size of these titanium precipitates.

After their passage at ambient temperature, the products having the precise composition according to the invention (blooms, billets or ingots) are then heated and rolled between 1200 and 800 ⁰ C in the form of son son, or bars in a single or double sequence of heating and rolling. In order to obtain the properties of the specific steel of the invention, the bars, the wires, the plots, or even the springs produced from these bars or machine wires, are then subjected to a quenching treatment at the same time. water, polymeric quenching or oil quenching after austenitization in a temperature range of 850 to 1000 ⁰ C ₁ so as to obtain a fine austenitic grain such that there are no grains coarser than 9 on the ASTM grain size scale. This quenching treatment is then followed by a treatment of income executed specifically between 300 and 55O ⁰ C, to obtain the high levels of the required tensile strength and hardness of the steel, and to avoid on the one hand a microstructure that would lead to income fragility, and on the other hand an excessively high presence of residual austenite. It has been found that an embrittlement to the income and an excessive presence of residual austenite are extremely damaging to the resistance to corrosion fatigue of the steel according to the invention. Where the springs are made from untreated heat bars or from machine wires or slugs from such bars, the above mentioned treatments (quenching and tempering) shall be carried out on the springs themselves under the conditions which have been said. In the case where the springs are manufactured by cold forming, these heat treatments can be conducted on the bars, or on the machine wires or slugs from these bars before the spring is made.

It is well known that the hardness of a steel depends not only on its composition, but also on the temperature of the income to which it has been subjected. It should be understood that for all compositions of the invention, it is possible to find tempering temperatures in the industrial range of 300-550 ^° C to obtain the minimum hardness of 55HRC referred.

Nitrides and carbonitrides being very hard, their size as defined above hardly evolves during the stages of steel processing. It is therefore irrelevant whether it is measured on the half-product (bar, wire rod or billet) which will be used to manufacture the spring or the spring itself. The invention makes it possible to obtain spring steels capable of reconciling a high and improved hardness and tensile strength with respect to the prior art, together with fatigue properties in the air and resistance to fatigue. improved wear properties, corrosion-resistant properties at least equivalent to those of the steels known for this use, or even better, and less sensitivity to the stress concentrations produced by the surface defects that may occur during the manufacture of the spring, thanks to addition of microalloy elements, a reduction of residual elements and a control of the analysis and production chain of the steel.

The invention will now be illustrated by way of examples and reference examples. Table 1 shows the steel compositions according to the invention and reference steels. The equivalent carbon Ceq is given by the following formula:

Ceq = [C] + 0.12 [Si] + 0.17 [Mn] - 0.1 [Ni] + 0.13 [Cr] - 0.24 [V] where [C], [Si], [ Mn], [Ni], [Cr] and [V] represent the content of each element in percentages by weight.

Table 1: Chemical compositions of the steels tested (in%) Table 2 shows the hardness values obtained for steels according to the invention and reference steels, as a function of the tempering temperature applied to them.

Table 2: Hardness and tensile strength as a function of the tempering temperature.

Table 3 shows the maximum size of titanium nitride or carbonitride inclusions observed at 1.5mm of the surface of steels according to the invention and of reference steels, as defined above. The titanium contents of the various steels have also been reported.

The maximum size of inclusions of nitrides or carbonitrides of titanium is determined as follows. On a section of rod or wire rod from a given steel casting, a 100mm ² surface is examined at a location 1.5mm + 0.5mm below the surface of the bar or wire rod. After these observations, the size of the inclusion of titanium nitride or carbonitride with the largest area is determined by considering that the inclusions are squares and that the size of each of these inclusions, including inclusion having the largest surface, is equal to the square root of this area. All inclusions are observed on a bar or wire rod cut for springs, observations being made on 100mm ² of this section. The steel casting is in accordance with the invention when the maximum size of the abovementioned inclusions observed over 100mm ² at 1.5mm + 0.5mm below the surface is less than 20 μm. The corresponding results obtained on steels according to the invention and reference steels are given in Table 3. With regard to the reference tests 1 and 3, their titanium content is practically zero and the size of the nitrides and carbonitrides observed is not applicable.

Table 3: Maximum sizes of the largest inclusions of titanium nitrides or carbonitrides found at 1.5mm from the surface of the samples.

The size of the inclusions of the reference steels 1 and 3 was not measured, as their Ti content was low and not in accordance with the invention: the result would have been meaningless.

Samples for fatigue tests were taken from bars, the final diameter of specimen samples being 11mm. Fatigue test sample preparation includes coarse machining, austenitization, oil quenching, tempering, grinding, and shot blasting. These samples were tested for fatigue-torsion in the air. The shear stress applied was 856 + 494MPA and the number of cycles to failure was counted. The tests were stopped after 2.10 ⁶ cycles if the samples were not broken.

The samples for fatigue testing under corrosion were taken from bars, the final diameter of the test pieces being 11 mm. Sample preparation for fatigue testing includes coarse machining, austenitization, oil quenching, tempering, grinding, and shot blasting. These samples were tested for corrosion fatigue, that is, corrosion was applied at the same time as a fatigue load. Load in fatigue is a shear stress equal to 856 + 300MPa. The applied corrosion was cyclic corrosion in two stages alternating:

a step being a wet step with spraying a saline solution containing 5% NaCl for 5 minutes at 35 ° C; a step being a dry step without spraying, of duration 30 minutes at a temperature maintained at 35 ° C.

The number of cycles to failure was considered the fatigue life under corrosion.

Moisture resistance was determined using a cyclic compression test on cylindrical samples. The diameter of the samples was 7mm and their height 12mm. They were taken from the steel bars.

The manufacture of the specimens of chill tests included rough machining, austenitization, oil quenching, tempering and final fine grinding. The height of the sample was measured precisely before the start of the test using a comparator with a precision of 1 μm. A preload was applied to simulate the pretensioning of the springs, this pretensioning being a compressive stress of 2200MPa.

Then the fatigue load cycle was applied. This constraint was 1270 + 730 MPa. The loss of sample height was measured during the execution of a number of cycles, up to 1 million. At the end of the test, the total loss was determined by an accurate measurement of the remaining height compared to the initial height, the wear resistance being all the better that the decrease in height, as a percentage of the initial height, was lower.

The results of fatigue, corrosion fatigue and wear tests on the steels of the invention and the reference steels are given in Table 4.

Table 4: Results of fatigue tests, fatigue under corrosion and slump.

From these tables, it appears that the various reference steels are unsatisfactory, especially for the following reasons.

The reference steel 1 has, in particular, a sulfur content too high to achieve a good compromise between the fatigue resistance in the air and the fatigue content under corrosion. In addition, its manganese content is too high, resulting segregations damaging to the homogeneity of the steel and fatigue resistance in the air.

Reference steel 2 has a carbon content and a carbon equivalent that is too low to ensure high hardness. Its tensile strength is too low for good resistance to fatigue in the air.

The reference steel 3 has, in particular, a silicon content that is too low to ensure good resistance to scumming, and also good resistance to fatigue in the air. The wear resistance is higher for the steels of the invention than for the reference steels, as shown in FIG. 1, where it is clear that, according to the abovementioned abatement measurements, the wear-out values are at least 32% lower for the worst case of the steels of the invention (steel of the invention 1) compared to the best case of the reference steels (reference steel 1). The fatigue life in the air is significantly higher for the steels of the invention compared to the reference steels. This is due to the increase in hardness, as shown in Figure 2. But an increase in hardness is not enough. In fact, in general, steels of high hardness are all the more sensitive to defects, such as inclusions and surface defects, the hardness is higher. Thus, the steels according to the invention are less sensitive to defects, in particular to large inclusions such as titanium nitrides or carbonitrides, in view of the fact that the invention avoids the appearance of such oversized inclusions. As shown in Table 3, the largest inclusions found in the steels according to the invention do not exceed the size of 14.1 μm, whereas inclusions larger than 20 μm are in the reference steel 2. In addition , the slightest sensitivity to surface defects such as those that may occur during the manufacture of the spring or other operations when using steels of the invention can be illustrated by resilience tests performed on the steels of the invention and reference steels heat-treated and having hardnesses of 55HRC or more, see Figure 3. The values measured in Charpy resiliency tests on the steels of the invention (where the notch of the specimen simulates a concentration of stresses as other stress concentrations that may be encountered on surface defects produced during the spring manufacturing or other operations) are higher than those measured on the reference steels. This shows that the steels according to the invention are less sensitive to the concentrations of stress on the defects than the reference steels according to the prior art. It is known that an increase in hardness reduces the resistance to fatigue under corrosion. Thus, it appears that the steels according to the invention have the advantage that their resistance to fatigue under corrosion is higher than that of the reference steels according to the prior art, and in particular for hardnesses greater than 55HRC as shown Thus, the invention makes it possible to obtain a higher hardness with a good compromise between the fatigue life in the air and a resistance to weariness which are greatly increased, and a service life of fatigue under corrosion which is better than that of the reference steel according to the prior art. In addition, less sensitivity to possible surface defects, particularly those generated during spring making or other operations, is also achieved.

Claims

1. Spring steel with high fatigue resistance in air and under corrosion and with high resistance to cyclic wear, of composition, in percentages by weight: C = 0.45 - 0.70%

Si = 1.65 - 2.50%

Mn = 0.20 - 0.75%

Cr = 0.60 - 2%

Ni = 0.15 - 1% Mo = traces - 1%

V = 0.003 - 0.8%

Cu = 0.10 - 1%

Ti = 0.020 - 0.2%

Nb = traces - 0,2% Al = 0,002 - 0,050%

P = traces - 0.015%

S = traces - 0.015%

O = traces - 0.0020%

N = 0.0020 - 0.0110%, the rest being iron and impurities resulting from the preparation, and whose equivalent carbon content Ceq, calculated according to the formula

2. Spring steel according to claim 1, characterized in that the maximum size of Ti nitrides or carbonitrides observed at 1.5 + 0.5mm from the surface of a bar, or a wire rod, of a lopin or spring on 100mm ² of the cutting surface is less than or equal to 20μm, said size being the square root of the surface of the inclusions considered as squares.

3. Spring steel according to claim 1 or 2, characterized in that its composition is: C = 0.45 - 0.65%

Si = 1, 65 - 2.20%

Mn = 0.20 - 0.65%

Cr = 0.80 - 1, 7% Ni = 0.15 - 0.80%

Mo = traces - 0.80%

V = 0.003 - 0.5%

Cu = 0.10 - 0.90%

Ti = 0.020 - 0.15% Nb = traces - 0.15%

Al = 0.002 - 0.050%

P = traces - 0.010%

S = traces - 0.010%

O = traces - 0.0020% N = 0.0020 - 0.0110% the remainder being iron and impurities resulting from the preparation.

4. A method of manufacturing a spring steel with high fatigue resistance in air and under corrosion and with high resistance to cyclic scumming, according to which a liquid steel is produced in a converter or electric furnace; its composition is cast in the form of blooms or continuous casting billets or ingots which are allowed to cool to room temperature, rolled in the form of bars, wire rods or plots and converted into springs characterized in that

the steel is of the type according to one of claims 1 to 3; blooms, billets or ingots are imposed during or after their solidification, a minimum average cooling rate of 0.3 ° C / s between 1450 and 1300 ⁰ C;

these blooms, billets or ingots are laminated between 1200 and 800 ^° C. in one or two cycles of reheating and rolling; and on the bars, the machine wires or the plots, or on the springs resulting therefrom, austenitization is carried out between 850 and 1000 ^° C., followed by water quenching, polymer quenching or quenching. oil, and by a returned at 300-550 ⁰ C, so as to give the steel a hardness greater than or equal to 55HRC.

5. Spring, characterized in that it is a steel according to one of claims 1 to 3.

6. Spring according to claim 5, characterized in that it is a steel obtained by the process according to claim 4.