MX2008007563A - Spring steel, method for producing a spring using said steel and a spring made from such steel - Google Patents

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

STEEL FOR SPRINGS, AND PROCESS OF MANUFACTURING A SPRING USING THIS STEEL, AND SPRING MADE OF SUCH STEEL Description of the invention The invention relates to the steel industry, and more precisely, the field of steel for springs. In general, with the increase of the continuous efforts in fatigue applied to the springs, the hardness and the resistance to the traction, required for the springs, are continuously increased. Consequently, the sensitivity to rupture initiated in defects, such as inclusions or surface defects generated during the manufacture of springs, increases, and fatigue resistance tends to be limited. On the other hand, springs used in a strongly corrosive environment, such as suspension springs, must have fatigue properties under corrosion at least equivalent and also higher since they use steels that have a hardness and tensile strength superiors Thus, such springs tend to break at the level of defects, immediately during fatigue cycles in the air, and later during fatigue cycles in a corrosive environment. In particular, for fatigue under corrosion, defects can be initiated in corrosion pits. In addition, with the increase in applied stresses, it is more difficult to improve the duration of Ref..193976 life in fatigue under corrosion or to maintain it at an equivalent level, taking into account in fact the effects of the stress concentration in the pitting corrosion, in the surface defects of the springs occasionally generated during the winding of the spring or other stages of its manufacture, or in the non-metallic inclusions, become more critical when the hardness of the spring increases. According to the known prior art, documents FR-A-2 740 476 and JP-A-3 474 373 describe a class of steel for springs which has a good resistance to embrittlement by hydrogen and good resistance to stress. Fatigue, in which carbonitrosulfide inclusions comprising at least one element between titanium, niobium, zirconium, tantalum or hafnium, are controlled so as to avoid a reduced average size, less than 5 μm in diameter, and to be more numerous (10,000 or even more in a section of cut). However, this type of steels lead, after tempering and tempering according to the industrial process of making springs, to a hardness level of only 50HRC or a little more, which corresponds to a tensile strength of 1700MPa or a little more, but barely more than 1900MPa, which corresponds to a hardness of 53.5HRC. Because of this moderate level of hardness, this steel it does not have more than a moderate deterioration resistance, a steel that presents a higher tensile strength is necessary to improve the resistance to deterioration. Thus, a steel does not ensure an excellent compromise between a high resistance, which will be higher than 2100MPa, a hardness which will be higher than 55HRC, a resistance to high fatigue in the air and a resistance to fatigue under at least equivalent corrosion, and even higher, for what is necessary for the springs. The object of the invention is to propose means for simultaneously performing, in relation to the known spring steels, an increase in the hardness and the tensile strength of springs, superior fatigue properties in the air, fatigue properties under corrosion to the less equivalent and even superior, a resistance to the deterioration of the upper spring and a lower sensitivity to the surface faults which can be generated during the torsion of the spring. For this purpose, the invention has as its object a steel for springs with high fatigue behavior with air and under corrosion and high resistance to cyclic deterioration, of composition, in weighted 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 rest is iron and impurities that result from the elaboration, and whose quantity of carbon equivalent Ceq, is calculated under the formula Ceq% = [C%] + 0.12 [Yes%] + 0.17 [Mn%] - 0.1 [Ni%] + 0.13 [Cr%] - 0.24 [V%] is between 0.80 and 1.00%, and whose hardness, after tempering and tempered, is greater than or equal to 55HRC. The maximum size of Ti nitrides or carbonitrides observed at 1.5 _ + 0.5 mm from the surface of a bar, or from a wire rod, a block or a spring above 100 mm2 of the cutting surface is preferably less than or equal to 20 μm, the size is the square root of the surface of inclusions considered as square.

Preferably, the steel composition is: C = 0.45 - 0.65% Si = 1.65 - 2.50% 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 is iron and impurities that result from processing. Another subject of the invention is a manufacturing process for a steel for springs with high fatigue behavior with air and low corrosion and high resistance to cyclic deterioration, according to which a liquid steel is produced in a converter or an electric furnace , its composition is adjusted, it is emptied in the form of palancones or billets of continuous casting or of ingots that are left cooling to room temperature, it is laminated in the form of bars, wire rods or blocks and is transformed into springs, characterized in that: the steel is of the preceding type; it is imposed on the palancones, billets or ingots during or after solidification, a minimum average cooling speed of 0.3 ° C / s between 1450 and 1300 ° C; the palancones, billets or ingots are rolled between 1200 and 800 ° C in one or two cycles of reheating and rolling; and they are realized in the rods, the wire rods or the blocks, or in the springs which are resulting, an austenitization between 850 and 1000 ° C, followed by a tempering with water, a tempering with polymer or an oil tempering, and by a tempering at 300-550 ° C, to give the steel a hardness greater than or equal to 55HRC. The invention also relates to springs made of steel, and springs in a steel obtained by the preceding process. Unexpectedly, the inventors have perceived that a steel having the characteristics of composition and inclusion morphology specified allow to ensure, after processing, casting, rolling, tempering and tempering made under the specific conditions, a hardness above 55HRC, making an excellent compromise between a long life span in fatigue in the air and fatigue under corrosion, a resistance to high cyclic deterioration and a reduced sensitivity to surface defects that occur during the manufacture of the spring. The invention will be better understood with the reading of the description which follows, given with reference to the following appended figures: - Figure 1 which shows the results of hardness and cyclic deterioration tests for steels according to the invention and steels of reference; Figure 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; - Figure 3 which shows the results of Charpy resilience tests depending on the hardness of the steel, for steels according to the invention and reference steels; - Figure 4 which shows the results of fatigue tests under corrosion as a function of the hardness of the steel for steel according to the invention and reference steels. The composition of the steel according to the invention must meet the following requirements. The amount of carbon must be between 0. 45 and 0.7%. The carbon allows, after tempering and tempering, to increase the tensile strength and hardness of steel. If the amount of carbon is less than 0.45%, in the range of temperature usually used for the manufacture of springs, no tempering and tempering treatment leads to a high strength and a high hardness of the steel described in the invention. On the other hand, if the amount of carbon exceeds 0.7% even 0.65%, the coarse and very hard carbides, combined with chromium, molybdenum and vanadium, may remain in the undissolved state during the austenitization performed before tempering, and may significantly affect the duration of life in fatigue in the air, the resistance to fatigue under corrosion and equally the tenacity. Consequently, carbon quantities above 0.7% should be excluded. Preferably, they should not exceed 0.65%. The amount of silicon is between 1.65 and 2.5%. Silicon is an important element that allows to ensure, due to its presence in solid solution, high levels of strength and hardness, as well as the values of carbon equivalent Ceq and resistance to deterioration, high. To obtain the values of tensile strength and hardness of the steel according to the invention, the amount of silicon should not be less than 1.65%. In addition, silicon contributes at least partially to deoxidation of steel. If its amount exceeds 2.5%, or even 2.2%, the oxygen amount of the steel can be, by thermodynamic reaction, higher than 0.0020 or even 0.0025%. This translates the formation of oxides of various compositions which are per udicial for the resistance to fatigue in the air. In addition, for silicon amounts greater than 2.5%, segregations of different combined elements such as manganese, chromium or others may occur during solidification, after casting. These segregations are very detrimental to the fatigue behavior in the air and to the resistance to fatigue under corrosion. Finally, for a quantity of silicon higher than 2.5%, the decarburization to the surface of the bars or to the threads intended to form the springs becomes too important for the properties in service of the spring. This is because the amount of silicon should not exceed 2.5%, and preferably 2.2%. The amount in manganese is between 0.20 and 0.75%. Manganese, in combination with the residual sulfur comprised between traces and 0.015%, must be added to an amount at least greater than ten times the amount of sulfur to avoid the formation of iron sulfur extremely harmful to the rolling of the steel. Consequently, a minimum amount of manganese of 0.20% is necessary. In addition, manganese contributes to hardening in solid solution during the tempering of steel, with the same title as nickel, chromium, molybdenum and vanadium, which allows to obtain high tensile and hardness values and carbon equivalent values Ceq of steel according to the invention. For manganese amounts greater than 0.75% even 0.65%, the segregations, in combination with silicon, can occur during the solidification phase followed by the working and casting of the steel. These segregations are per udicial for the properties in service of the steel and for the homogeneity of the steel. This is because the amount of manganese in the steel should not exceed 0.75%, or better 0.65%. The amount of chromium should be between 0.60 and 2%, and preferably between 0.80 and 1.70%. The chromium is added to obtain, in solid solution after austenitization, tempering and tempering, high values of tensile strength and hardness, and to contribute to obtain the carbon equivalent Ceq, but also to increase the resistance to fatigue under corrosion. To ensure these properties, the amount of chromium must be at least 0.60%, and preferably at least 0.80%. Above 2%, even 1.7% of particular chrome carbides, coarse and very hard, in combination with vanadium and molybdenum, can survive after the treatment of austenitization executed before tempering. Such carbides greatly affect the resistance to fatigue in the air. This is because the amount of chromium should not exceed 2%. The amount of nickel 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 tempering and tempering. As it does not form carbides, nickel contributes to the hardening of the steel, such as chromium, molybdenum and vanadium, without the formation of coarse and hard particular carbides which will not be dissolved during the austenitization carried out before tempering and could be detrimental to the resistance to fatigue in the air. It also allows adjusting the equivalent carbon between 0.8 and 1% in the steel according to the invention as necessary. As a non-oxidizable element, nickel improves resistance to fatigue under corrosion. To ensure that these effects are significant, the amount of nickel should not be less than 0.15%. On the contrary, above 1% even 0.80%, nickel can lead to a too high amount of residual austenite, whose presence is very detrimental to fatigue resistance under corrosion. In addition, high amounts of nickel significantly increase the cost of steel. For all these reasons, the amount of nickel should not exceed 1%, better 0.80%.

The amount of molybdenum must be between traces and 1%. Like chromium, molybdenum increases the hardenability of steel, as does its strength. In addition, it has a reduced oxidation potential. For these two reasons, molybdenum is favorable for fatigue behavior in the air and under corrosion. But for quantities greater than 1%, even 0.80%, of coarse and very hard molybdenum carbides can subsist, optionally combined with vanadium and with chromium, after the austenitization that precedes the annealing. These particular carbides are very detrimental to the fatigue behavior in the air. Finally, an addition of molybdenum that exceeds 1% unnecessarily increasing the cost of steel. This is because the amount of molybdenum should not exceed 1%, better 0.80%. The amount of vanadium must be between 0.003 and 0.8%. Vanadium is an element that allows to increase the hardenability, the tensile strength and the hardness after tempering and tempering. In addition, in combination with nitrogen, vanadium allows the formation of a large number of fine vanadium or vanadium nitrides and submicroscopic titanium, which allows the grain to be refined and the levels of tensile strength and hardness increased due to structural hardening. . To obtain the formation of submicroscopic nitrides of V and Ti for refining of the grain, the vanadium must be present with a minimum amount of 0.003%. But this element is expensive and should be kept close to this lower limit if a compromise is sought between the cost of processing and the fine-tuning of the grain. Vanadium should not exceed 0.8% and, preferably, 0.5%, because beyond this value, a precipitation of carbides containing coarse and very hard vanadiums, combined with chromium and molybdenum, may remain in the undissolved state during the austenitization that takes place before tempering. This can be very unfavorable in the amount of fatigue with air, for the high values of strength and hardness of the steel according to the invention. And an addition of vanadium beyond 0.8% unnecessarily increases the cost of steel. The amount of copper must be between 0. 10 and 1%. Copper is an element which hardens the steel when it is in solid solution after the tempering and tempering treatment. Thus, it can be added with other elements that contribute to increase the strength and hardness of steel. As it is not combined with carbon, it provides a hardening of the steel without the formation of hard and thick carbides per udicial to the resistance to fatigue in the air. From the electrochemical point of view, its passivation potential is higher than that of iron and, consequently, it is favorable to low fatigue resistance corrosion of steel. To ensure that its effects are significant, the amount of copper should not be less than 0.10%. On the contrary, for amounts greater than 1%, even 0.90%, copper has a very detrimental influence on the behavior of hot rolling. This is because the copper amounts should not exceed 1%, better 0.90%. The amount of titanium should be between 0.020 and 0.2%. Titanium is added to form, in combination with nitrogen, even also carbon and / or vanadium, fine nitrides or submicroscopic carbonitrides which allow the austenitic grain to be refined during the austenitizing treatment which takes place before tempering. Thus, the surface of the grain joints in the steel increases, thus leading to a reduction in the amount of unavoidable impurities secreted to the grain joints, such as phosphorus. Such intergranular segregations will be very detrimental to the tenacity and resistance to fatigue in the air if they are present at concentrations per unit area raised to the level of grain joints. In addition, combined with carbon and nitrogen, or even with vanadium and niobium, titanium leads to the formation of other nitrides or fine carbonitrides that produce an irreversible capture effect of certain elements, such as hydrogen formed during the reactions of corrosion, and which can be extremely damaging to fatigue resistance under corrosion. For good efficiency, the amount of titanium should not be less than 0.020%. On the other hand, above 0.2% or even 0.15%, titanium can lead to the formation of coarse and hard nitrides or carbonitrides, which are very harmful to the fatigue resistance in the air. This latter effect is even more detrimental to the high levels of tensile strength and hardness of the steel according to the invention. For these reasons the amount of titanium should not exceed 0.2%, better 0.15%. The amount in niobium 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, particularly when the amount in aluminum is low (0.002% for example ), finish the refining of the austenitic grain during the austenitization treatment carried out before tempering. Thus, niobium increases the surface area of grain seams in steel, and contributes to the same favorable effect as titanium in terms of the embrittlement of grain seals by unavoidable impurities such as phosphorus, the effect of which is very low. harmful to the tenacity and resistance to fatigue under corrosion. In addition, the extremely fine precipitates of nitrides or carbonitrides of niobium contribute to the hardening of the steel by structural hardening. However, the amount of niobium should not exceed 0.2% or even 0.15%, so that the nitrides or carbonitrides remain very thin, to ensure the fine tuning of the austenitic grain and to avoid the formation of cracks or cracks during hot rolling . For these reasons, the amount of niobium should not exceed 0.2%, better 0.15%. The amount of aluminum must be between 0.002 and 0.050%. The aluminum can be added to finish the deoxidation of the steel and obtain oxygen amounts as low as possible, and in any case lower than 0.0020% in the steel according to the invention. In addition, combined with nitrogen, aluminum contributes to the fine tuning of the grain by the formation of submicroscopic nitrides. To ensure these two functions, an amount of aluminum which is not less than 0.002% is required. Conversely, an amount of aluminum exceeding 0.05% may lead to the presence of thick isolated inclusions or to finer, but hard and angulated aluminates, in the form of long rosaries, detrimental to the lifetime in fatigue in the air and the cleaning of steel This is because the amount of aluminum should not exceed 0.05%. The amount in phosphorus must be between traces and 0.015%. Phosphorus is an inevitable impurity in steel. During a tempering and tempering treatment, co-segregated with elements such as chromium or manganese to the old austenitic grain joints. Resulting in a reduction of the cohesion of the grain joints and an intergranular embrittlement very detrimental to the tenacity and to the resistance to fatigue in the air. These effects are even more detrimental 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 spring steel and good resistance to fatigue in air and fatigue under corrosion, the amount in phosphorus should also be as low as possible and should not be exceed 0.015%, preferably 0.010%. The amount of sulfur is between the traces and 0.015%. Sulfur is an inevitable impurity in steel. Its quantity should be as low as possible, between traces and 0.015%, and preferably at a maximum of 0.010%. It is thus desired to avoid the presence of unfavorable sulfur to the fatigue resistance 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 amount of oxygen must be between traces and 0.0020%. Oxygen is also an impurity l inevitable in steels. Combined with deoxidizing elements, oxygen can lead to the appearance of thick, isolated inclusions, very hard and angular, or to finer inclusions but in the form of long rosaries which are very harmful to the resistance to fatigue in the air. These effects are even more detrimental 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 the 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 amount of oxygen should not exceed 0.0020%. The amount of nitrogen must be between 0.0020 and 0.0110%. Nitrogen must be controlled in this range to form, in combination with titanium, niobium, aluminum or vanadium of very fine nitrides, carbides or submucriscopic carbonitrides in sufficient number, which allows fine tuning of the grain. Thus, for this purpose, the minimum amount of nitrogen should be 0.0020%. Its quantity should not exceed 0.0110% to avoid the formation of thick and hard titanium nitrides or carbonitrides larger than 20 μm, observed at 1.5 mm + 0.5 mm from the surface of bars or wire rods used for the manufacture of springs. This site is the place which is the most critical in regards to fatigue solicitation of springs. In fact, such large nitrides or carbonitrides are very unfavorable for the fatigue resistance in air for the high values of strength and hardness of steels according to the invention, taking into account the fact that during the tests of fatigue in the air, the rupture of the springs occurs at the site of such thick inclusions precisely located near the surface of the springs as mentioned, when these inclusions are present. To estimate the size of titanium nitrides and carbonitrides, inclusions are considered as squares and their size is equal to the square root of their surface. A process for manufacturing springs according to the invention will now be described. A non-limiting example of the process for making a steel according to the invention is as follows. The liquid steel is produced either in a converter, or in an electric furnace, then it undergoes a metallurgical treatment in a bucket during which the additions of alloying elements and the deoxidation are executed, and in general all the secondary metallurgical operations that they make it possible to obtain a steel having the composition according to the invention and which prevent the formation of complex sulfides or of «carbonitrosulfides» of elements such as titanium and / or the niobium and / or the vanadium. In order to avoid the formation of such coarse precipitates during processing, the inventors have discovered, unexpectedly, that the amounts of the different elements, in particular those of titanium, nitrogen, vanadium and sulfur, must be carefully controlled in the above limits. After processing which will be described, the steel is then cast, either by continuous casting in the form of palancones or billets, or in the form of ingots. But to completely avoid or as long 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 speed of these products (crowns, billets or ingots) must be regulated to be equal to 0.3 ° C / s or even more between 1450 and 1300 ° C. When operating under these conditions during the solidification and cooling step, it is unexpectedly observed that the size of the thickest nitrides or Ti carbonitrides observed in the springs is always less than 20 μm. The situation and the size of these titanium precipitates will be discussed further. After their passage to ambient temperature, the products having the precise composition according to the invention (levers, billets or ingots) are then reheated and laminated between 1200 and 800 ° C in the form of wire rod, or bars in a single or double sequence of heating and rolling. To obtain the properties of the steel specific to the invention, the bars, the threads, the pieces, or also the springs produced from these rods or wire rods, are then subjected to a treatment of water quenching, polymer quenching or tempering in oil after austenitization in a temperature range of 850 to 1000 ° C, so that a fine austenitic grain is obtained such that there are no grains thicker than 9 on the ASTM scale of grain size. This tempering treatment is then followed by a tempering treatment executed specifically between 300 and 550 ° C, which allows to obtain the high levels of tensile strength and steel hardness required, and to avoid on the one hand a microstructure which will lead to a fragility to the tempering, and on the other hand a too high presence of residual austenite. It has been found that a quenching embrittlement and a too strong presence of residual austenite are extremely perceptible to the fatigue resistance under corrosion of the steel according to the invention. In the case where the springs are manufactured from bars not heat treated or from wire rods or from pieces left from such rods, the previously mentioned treatments (tempering and tempering) must be executed in the springs by themselves in the conditions which have been mentioned. In the case where the springs are manufactured by cold forming, these heat treatments can be conducted on the rods, or on the brones wing or the pieces left from these rods before the manufacture of the spring. It is well known that the hardness of a steel depends not only on its composition, but also on the tempering temperature to which it has been subjected. It should be understood that for all the compositions of the invention, it is possible to find tempering temperatures in the industrial range of 300-550 ° C which allows to obtain the minimum hardness of 55HRC objective. The nitrides and carbonitrides are very hard, their size as defined above does not evolve practically during the transformation stages of the steel. It is therefore unimportant that it be measured on the semi-finished product (bar, wire or piece) which will be used to manufacture the spring or spring itself. The invention makes it possible to obtain spring steels able to reconcile a high and improved hardness and tensile strength with respect to the prior art, at the same time as improved fatigue properties in the air and resistance to deterioration, properties in fatigue under corrosion at least equivalent to those of steels known for this use, or even better, and a lower sensitivity to concentrations of difficulties caused by surface defects that may occur during the manufacture of the spring, due to an addition of micro-alloy elements, a reduction of residual elements and a control of the analysis and of the production row of the steel. The invention will now be illustrated by means of examples and reference examples. Table 1 shows the steel 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 [Cr] -0.24 [V] where [C], [Si], [Mn], [Ni], [Cr] and [V] represent the quantity of each element in weight percentages.

Table 1: Chemical compositions of steels tested (in%) Table 2 shows the hardness values obtained for steels according to the invention and reference steels, depending on the tempering temperature that has been applied to them.

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

Table 3 shows the maximum size of titanium nitride or carbonitride inclusions observed at 1.5 mm from the surface of steels according to the invention and from reference steels, as defined above. The amounts of titanium of the various steels have also been reported. The maximum size of titanium nitride or carbonitride inclusions is determined as follows. In a bar or wire rod section that comes from a given steel casting, a surface of 100 mm2 is examined for a location located at 1.5 inm + 0.5 mm below the surface of the bar or wire rod. After these observations, the inclusion size of titanium nitride or carbonitride having the largest surface, is determined considering that the inclusions are square and that the size of each of these inclusions, including the inclusion that has the largest surface , is equal to the square root of this surface. All inclusions are observed on a rod cut or wire rod for springs, the observations are executed in 100 mm2 of this section. The steel casting is in accordance with the invention when the maximum size of previously mentioned inclusions observed above 100 mm2 at 1.5 mm + 0.5 mm below the surface is less than 20 μm. The corresponding results obtained in the steels according to the invention and reference steels are given in table 3. As regards the reference tests 1 and 3, their amount of titanium is practically nil and the size of nitrides and carbonitrides observed It is without an object.

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

The size of inclusions of reference steels 1 and 3 has not been measured, as the amount of Ti that is reduced and not according to the invention: the result would have been without significance. The samples for fatigue tests have been taken in bars, the final diameter of specimen samples is 11 mm. The preparation of fatigue test samples comprises coarse fabrication, austenitization, oil quenching, tempering, roughing and blasting. These samples have been tested in fatigue-torsion in the air. The applied shear stress is 856 + _ 494MPA and the number of cycles until the break has been counted. The tests were stopped after 2,106 cycles if the samples had not been broken. The samples for corrosion fatigue test have been taken in the bars, the final diameter of the specimens is 11 mm. The preparation of the samples for fatigue tests comprises coarse fabrication, austenitization, tempering in oil, tempering, roughing and blasting. These samples have been tested in fatigue under corrosion, meaning that a corrosion has been applied at the same time as a load in fatigue. The load in fatigue is a shear stress equal to 856 + _ 300MPa. Applied corrosion is a cyclic corrosion of two alternating stages: a stage is a wet stage with the spraying of a saline solution containing 5% NaCl for 5 minutes at 35 ° C; - a stage is a dry stage without spraying, lasting 30 minutes at a temperature maintained at 35 ° C. The number of cycles until rupture has been considered as the duration of life in fatigue under corrosion. The resistance to deterioration has been determined using a cyclic compression test on samples cylindrical The diameter of the samples is 7 mm and its height is 12 mm. They have been taken on steel bars. The manufacture of samples of deterioration tests comprising a coarse fabrication, an austenitization, an oil tempering, an annealing and a final fine grinding. The height of the sample was measured just before the start of the test using a comparator with a precision of 1 μm. A preload has been applied to simulate the pretension of the springs, this pre-tension is a compression tension of 220? MPa. Then the fatigue load cycle was applied. This tension was 1270 + 730 MPa. The height loss of the sample has been measured during the execution of a certain number of cycles, up to 1 million. At the end of the test, the total deterioration was determined by a precise measurement of the subsistent height compared to the initial height, the resistance to deterioration that was so much better than the decrease in height, as a percentage of the initial height, was more reduced. The results of the fatigue, corrosion fatigue and deterioration tests on the steels of the invention and the reference steels are given in table 4.

Table 4: Results of tests on fatigue, fatigue under corrosion and deterioration From these tables, it turns out that the different reference steels are unsatisfactory, mainly for the following reasons. The steel of reference 1 has, mainly, a very high amount of sulfur to make a good compromise between the fatigue behavior in the air and the amount of fatigue under corrosion. In addition, its amount of manganese is very high, which implies harmful segregations for the homogeneity of the steel and the fatigue behavior in the air. The reference steel 2 has a very low amount of carbon and a carbon equivalent to ensure a high hardness. Its tensile strength is very low for good fatigue behavior in the air. The reference steel 3 has, mainly, a very low amount of silicon to ensure a good resistance to deterioration, and also a good fatigue behavior in the air. The resistance to deterioration is higher for the steels of the invention than for the steels of reference, as shown in Figure 1, where it is clear that, according to the aforementioned deterioration measures, the deterioration values are at least 32% lower for the worst case of steels of the invention (steel of the invention 1) with respect to the best case of reference steels (reference steel 1). The duration of life in fatigue in the air is clearly higher for the steels of the invention with respect 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. Actually, so Generally, high hardness steels that are so much more sensitive to defects, such as inclusions and surface defects, that the hardness is higher. Thus, the steels according to the invention are less sensitive to defects, in particular to thick inclusions such as titanium nitrides or carbonitrides, bearing in mind that the invention prevents the appearance of such very large inclusions. As shown in Table 3, the thickest inclusions found in the steels according to the invention do not exceed the size of 14.1 μm, whereas inclusions thicker than 20 μm are found in the reference steel 2. In addition, the lower 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 the reference steels that have undergone a treatment thermal and having hardness of 55HRC or even more, see figure 3. The values measured during Charpy resilience tests on the steels of the invention (where the notch of the specimen simulates a stress concentration as other concentrations of stresses that can be found in surface defects produced during the manufacture of the spring 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 stress concentrations 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 hardness above 55HRC as shown in the figure 4. Thus, the invention allows obtaining a higher hardness with a good compromise between the life duration in fatigue in the air and a resistance to deterioration that are strongly increased, and a life duration in fatigue under corrosion that is better than that of the reference steels according to the prior art. In addition, a lower sensitivity to possible surface defects is also obtained, in particular those generated during the manufacture of the spring or other operations. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (6)

CLAIMS Having described the invention as above, the contents of the following claims are claimed as property:

1. Steel for springs with high fatigue behavior in the air and low corrosion and high resistance to cyclic deterioration, characterized in that it has the 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 rest is iron and impurities that result from the processing, and whose quantity of carbon equivalent Ceq, is calculated under 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 tempering and tempering, is greater than or equal to 55HRC. Spring steel according to claim 1, characterized in that the maximum size of Ti nitrides or carbonitrides observed at 1.5 + 0.5 mm from the surface of a bar, or from a wire rod, a block or a spring above 100 mm2 of the cut surface is less than or equal to 20 μm, the size is the square root of the surface of the inclusions considered as squares. 3. Steel for springs according to claim 1 or 2, characterized in that its composition is: C = 0.45 - 0.65% Si = 1.65 - 2.50% 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 is iron and impurities that result from processing. 4. Manufacturing process of a steel for springs with high fatigue behavior in the air and under corrosion and with high resistance to cyclic deterioration, according to which a liquid steel is made in a converter or an electric furnace, its composition is adjusted , it is emptied in the form of palancones or billets of continuous casting or ingots that are allowed to cool to room temperature, rolled in the form of bars, wire rods or blocks and transformed into springs, characterized in that: steel is of the type according to one of claims 1 to 3; it is imposed on the palancones, billets or ingots during or after solidification, a minimum average cooling speed of 0.3 ° C / s between 1450 and 1300 ° C; - the palancones, billets or ingots between 1200 and 800 ° C in one or two cycles of reheating and rolling; and they are realized in the rods, the wire rods or the blocks, or in the springs which are resulting, an austenitization between 850 and 1000 ° C, followed by a tempering in water, a tempering in polymer or an oil tempering, and by a tempering at 300-550 ° C, to give the steel a hardness greater than or equal to 55HRC. 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 made of a steel obtained by the process according to claim 4.