KR20080090424A - 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

Spring steel, method for manufacturing spring using same and spring made therefrom {SPRING STEEL, METHOD FOR PRODUCING A SPRING USING SAID STEEL AND A SPRING MADE FROM SUCH STEEL}

The present invention relates to steel manufacture, and more particularly to the field of spring steel.

Generally, springs require ever-increasing hardness and tensile strength as the fatigue strain applied to the springs increases. As a result, the sensitivity to breakage originating from defects such as inclusions or surface defects that occur during spring manufacture is increased, and fatigue resistance is likely to reach a limit. Next, springs used in highly corrosive environments, such as suspension springs, should use at least equivalent to, or preferably better than, fatigue properties in corrosive conditions because they use steel with higher hardness and tensile strength. Thus, such springs are prone to failure immediately during defects during the fatigue cycle in air and later during corrosion cycles in the corrosion medium. In particular, in the case of fatigue in corrosive conditions, defects may occur in the corrosive grooves. Moreover, the fact that the spring surface defects, which may occur during spring coiling, or the concentration of deformation forces on the corrosion grooves in nonmetallic inclusions, among other steps in the manufacturing process, have a more important effect when the spring hardness increases. From this, it is more difficult to improve or maintain fatigue life under corrosion conditions as the applied strain increases.

According to the prior art, the documents FR-A-2740476 and JP-3474373B disclose spring steel grades with good resistance to hydrogen embrittlement and fatigue resistance, wherein among the elements of titanium, niobium, zirconium, tantalum, or hafnium The diameter of the carbonitrosulfide containing at least one is to have a smaller average size of less than 5 μm, and the number is also very large (more than 10,000 at the cutting plane) to control corrosion.

However, this kind of steel, after quenching and tempering according to the industrial spring manufacturing process, shows only 50 HRC or slightly higher hardness levels corresponding to 1700 MPa or slightly higher tensile strength, but 1900 MPa corresponding to 53.5 HRC hardness. It does not exceed much. Due to this moderate level of hardness, such steels only have adequate deflection resistance, and improved deflection resistance is required for steels with higher tensile strengths. Thus, such steels have an excellent compromise between high resistance over 2100 MPa, hardness greater than 55 HRC, high fatigue resistance in the air, and fatigue resistance at least equivalent corrosion conditions if not higher than required for springs. can not do.

Compared with known steels, the present invention simultaneously increases the spring hardness and tensile strength, the fatigue properties in air, the fatigue resistance at corrosion conditions is at least equal or otherwise higher, the spring deflection resistance is increased, and the spring It is an object to propose means for reducing the sensitivity to surface defects that may occur during coiling.

In view of the object of the present invention,

C = 0.45 to 0.70%

Si = 1.65-2.50%

Mn = 0.20 to 0.75%

Cr = 0.60 to 2%

Ni = 0.15 to 1%

Mo = trace to 1%

V = 0.003 to 0.8%

Cu = 0.10 to 1%

Ti = 0.020 to 0.2%

Nb = trace to 0.2%

Al = 0.002 to 0.050%

P = trace to 0.015%

S = trace to 0.015%

O = trace to 0.0020%

N = 0.0020 to 0.0110%

The balance has iron and a composition which is an impurity derived from the steel manufacturing process,

The carbon equivalent (Ceq) content calculated according to Equation 1 is 0.80 to 1.00%,

Hardness after quenching and tempering is 55HRC or more,

Provides spring steel with high resistance to fatigue and cyclic sag in air and in corrosive conditions.

[Equation 1]

Ceq% = [C%] + 0.12 [Si%] + 0.17 [Mn%]-0.1 [Ni%] + 0.13 [Cr%]-0.24 [V%]

The maximum size of titanium nitride or carbonitride observed at a surface area of 1.5 ± 0.5 mm of bars, wire rods, slugs or springs with a surface area of at least 100 mm ² is preferably 20 μm or less, where the size is regarded as square It is the square root of the surface area of inclusions.

Preferably, the steel

C = 0.45 to 0.65%

Si = 1.65-2.20%

Mn = 0.20 to 0.65%

Cr = 0.80 to 1.7%

Ni = 0.15 to 0.80%

Mo = trace to 0.80%

V = 0.003 to 0.5%

Cu = 0.10 to 0.90%

Ti = 0.020 to 0.15%

Nb = trace to 0.15%

Al = 0.002 to 0.050%

P = trace to 0.010%

S = trace to 0.010%

O = trace to 0.0020%

N = 0.0020 to 0.0110%

The balance has iron and a composition which is an impurity derived from the steel manufacturing process.

The invention also relates to making liquid steel in converters or electric furnaces, controlling their composition, casting into bloom or continuous flow billets or ingots, allowing them to cool to room temperature, rolling them into bars, wire rods or slugs, and deforming into springs. Therefore, in the method of manufacturing spring steel having high fatigue resistance in air and corrosion conditions, and resistance to cyclic deflection,

The steel is of the same kind as before,

After becoming solid, the bloom, billet or ingot has a minimum average cooling rate of 0.3 ° C./s at 1450 to 1300 ° C.,

The bloom, billet or ingot is rolled at 1200 to 800 ° C. with a reheating and rolling cycle of 1 or 2,

A process in which the wire or slug, or spring, made is austenitized at 850-1000 ° C. and then water quenched, polymer quenched or oil quenched and tempered at 300-550 ° C. to give a steel having a hardness of at least 55 HRC. To provide.

The invention also provides a spring made of such steel, and a spring made of steel produced by the method.

Unexpectedly, the inventors have found that steels with the above-described content of constituents and morphologies are characterized by high levels of durability against airborne fatigue and fatigue under corrosive conditions, high resistance to cyclic deflection, and during spring manufacture. It has been found to have a good compromise between low sensitivity to surface defects, while at the same time securing a hardness of at least 55 HRC after steelmaking, casting, rolling, quenching and tempering under certain conditions.

The invention may be better understood upon reading the following detailed description with reference to the accompanying drawings in which:

1 shows the hardness and cyclic deflection test results for steel and reference steel according to the present invention;

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

3 shows Charpy impact test results as a function of steel hardness for steels and reference steels according to the present invention;

4 shows the results of fatigue test at corrosion conditions as a function of steel hardness for steel and reference steel according to the invention.

The steel composition according to the present invention must satisfy the following conditions.

The carbon content should be 0.45% to 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%, the quenching and tempering treatments in the usual temperature ranges used for spring production do not result in the high strength and hardness of the steels described herein. Next, if the carbon content exceeds 0.7%, preferably 0.65%, during the austenitization introduced before quenching, the crude, very hard carbide, combined with chromium, molybdenum, and vanadium, does not dissolve. May remain and can have a significant impact on fatigue life in air, fatigue resistance and roughness under corrosive conditions. As a result, a carbon content above 0.7% should be avoided, preferably not exceeding 0.65%.

Silicone content is between 1.65% and 2.5%. Silicon is present in solid solution so that not only high levels of strength and hardness but also high carbon equivalents are important elements for securing Ceq and sag resistance. In order to have the tensile strength and hardness values of the steel according to the invention, the silicon content must be at least 1.65%. Furthermore, silicon contributes at least in part to the reduction of the steel. If the content exceeds 2.5%, preferably 2.2%, the oxygen content in the steel may be at least 0.0020%, preferably at least 0.0025% by thermodynamic reaction. This involves the formation of oxides of various compositions, which are harmful to fatigue resistance in air. Moreover, when the silicon content is greater than 2.5%, various bonded elements, such as manganese, chromium or others, can be separated during solidification after casting. Such isolates are very detrimental to fatigue behavior in air and fatigue resistance in corrosive conditions. Finally, if the silicon content is greater than 2.5%, the decarbonization at the spring bar or wire surface becomes too high for the in-service nature of the spring. This is why the silicon content should not exceed 2.5%, preferably 2.2%.

Manganese content is 0.20% to 0.75%. In order to prevent the formation of iron sulfide, which is extremely harmful to the rolling of the steel, in combination with traces of sulfur to levels of 0.015%, the content of manganese must be at least 10 times higher than the sulfur content. As a result, a manganese content of at least 0.20% is required. Moreover, manganese contributes to solid solution hardening during the quenching of not only steel, but also nickel, chromium, molybdenum and vanadium, providing steels with the high tensile strength and hardness values and carbon equivalent Ceq values described herein. A manganese content of greater than 0.75%, preferably 0.65%, may be combined with the silicon and separated during the solidification step after steelmaking and casting. These isolates are detrimental to the properties and homogeneity of the steel in use. This is why the manganese content should not exceed 0.75%, preferably 0.65%.

The chromium content should be 0.60% to 2%, preferably 0.80% to 1.70%. Chromium is added to the solid solution after austenitization, quenching and tempering to obtain high tensile strength and hardness and to obtain a carbon equivalent Ceq, as well as to increase fatigue resistance in corrosion conditions. In order to secure these properties the chromium content should be at least 0.60%, preferably at least 0.80%. If it exceeds 2%, preferably 1.7%, it may be combined with vanadium and molybdenum after austenitization prior to quenching, so that a specific preparation and hard chromium carbide may remain. Such carbides greatly affect fatigue resistance in air. This is why the chromium content should not exceed 2%.

Nickel content is 0.15% to 1%. Nickel is added to increase the tensile strength and hardness as well as the hardenability of the steel after quenching and tempering. Since nickel does not form carbide, it does not dissolve during austenitization prior to quenching, such as chromium, molybdenum, and vanadium, and therefore, does not form specific carbides, which do not dissolve in the air. Contribute. This also means that the carbon equivalent in the steel according to the invention can be adjusted from 0.8% to 1% as needed. Nickel, as a non-oxidizing element, improves fatigue resistance in corrosive conditions. To ensure this important effect, the nickel content should not be lower than 0.15%. On the contrary, if it exceeds 1%, preferably 0.80%, nickel can excessively increase the content of residual austenite, which is very harmful to fatigue resistance even in the presence of corrosion. Moreover, high nickel levels significantly increase the cost of the steel. For all these reasons the nickel content should not exceed 1%, preferably 0.80%.

The content of molybdenum should be trace to 1%. Like chromium, molybdenum increases the strength as well as the hardenability of the steel. Moreover, chromium has a low oxidation potential. For these two reasons, molybdenum is beneficial for fatigue resistance in air and in corrosive conditions. However, if the content is above 1%, preferably 0.80%, a coarse and very hard molybdenum carbide may remain optionally in combination with vanadium and chromium after austenitization prior to quenching. These specific carbides are very harmful to air fatigue resistance. Finally adding molybdenum above 1% unnecessarily increases the cost of the steel. This is why the molybdenum content should not exceed 1%, preferably 0.80%.

The vanadium content should be 0.003% to 0.8%. Vanadium is an element that increases hardenability, tensile strength, and hardness after quenching and tempering. Moreover, vanadium combines with nitrogen to refine the grains and form many microscopic microscopic vanadium or vanadium and titanium nitrides that increase the tensile strength and hardness levels through structural hardening. To obtain the formation of ultra-microscopic vanadium, or vanadium and titanium nitride, which refines the grains, vanadium must be present in a content of at least 0.003%. However, such an element is expensive and should be kept at the lower limit if a trade-off between steelmaking cost and grain refinement is sought. Vanadium should not exceed 0.8%, preferably 0.5%. Beyond this range, the precipitates of crude, very hard vanadium-containing carbides, combined with chromium and molybdenum, may remain undissolved during austenitization prior to quenching. This can be very disadvantageous for air fatigue resistance, high strength and hardness values in the steel according to the invention. In addition, adding 0.8% more vanadium unnecessarily increases the cost of the steel.

The copper content should be 0.10% to 1%. Copper is an element that hardens steel when in solid solution after quenching and tempering. Thus, it can be added together with other elements that contribute to increasing the strength and hardness of the steel. Since it does not bind with carbon, the steel is cured without the formation of a hard carbide which is harmful to fatigue resistance in the air. From an electrochemical point of view, its passivation potential is higher than that of iron, which consequently benefits the fatigue resistance of the steel under corrosive conditions. To ensure these important effects, the copper content should not be lower than 0.10%. In contrast, if the copper content is higher than 1%, preferably 0.90%, copper has a very detrimental effect on the behavior during hot rolling. This is why the copper content should not exceed 1%, preferably 0.90%.

Titanium content should be 0.020% to 0.2%. Titanium is added to combine with nitrogen, preferably also carbon and / or vanadium, to form a microscopic microscopic nitride or carbonitride that refines austenite grains during austenitization prior to quenching. Thus, it increases the surface area at grain boundaries in the steel, thereby reducing the amount of unavoidable impurities that separate at the grain boundaries, such as phosphorus. If such intergranular separation is present at high concentrations per unit surface area at grain boundaries, it will be very detrimental to roughness and fatigue resistance in air. Moreover, titanium combines with carbon and nitrogen, preferably with vanadium and niobium, resulting in the formation of other fine nitrides or carbonitrides which produce an irreversible trapping effect for some elements such as hydrogen formed during the corrosion reaction and However, it can be extremely harmful to fatigue resistance under corrosive conditions. For good efficiency, the titanium content should not be less than 0.020%. Conversely, if it exceeds 0.2%, preferably 0.15%, titanium is a preparation, which is very detrimental to fatigue resistance in air, and may lead to the formation of hard carbonitrides. The latter effect is also much more detrimental to the high levels of tensile strength and hardness of the steels according to the invention. For this reason the content of titanium should not exceed 0.2%, preferably 0.15%.

Niobium content should be trace to 0.2%. When niobium is added, extremely fine microscopic nitrides and / or carbides, which combine with carbon and nitrogen during austenitization prior to small grains, especially when the aluminum content is low (eg 0.002%), refine the austenite grains and And / or forms a precipitate of carbonitride. Thus, niobium increases the surface area of grain boundaries in steel and contributes to a similarly beneficial effect as titanium on embrittlement of grain boundaries by unavoidable impurities such as phosphorus, which have a very detrimental effect on roughness and fatigue resistance under corrosion conditions. . Moreover, extremely fine precipitates of niobium nitride or carbonitride contribute to hardening of the steel through structural hardening. However, the content of niobium should not exceed 0.2%, preferably 0.15%, in order for the nitrides or carbonitrides to remain very fine so as to refine the austenite grains and to prevent cracking or cracking formation during hot rolling. For this reason the niobium content should not exceed 0.2%, preferably 0.15%.

The content of aluminum should be 0.002% to 0.050%. Aluminum is added to reduce the steel and provide the lowest possible oxygen content, exactly less than 0.0020% in the steel according to the invention. Moreover, aluminum combines with nitrogen to form microscopic nitrides, contributing to the refinement of grains. In order to secure these two functions, the aluminum content should not be lower than 0.002%. In contrast, the content of aluminum in excess of 0.05% in the form of long stringers can lead to the presence of large, isolated inclusions or finer but harder, angled aluminates, which are detrimental to the fatigue life and purity of the steel in the air. This is why the aluminum content should not exceed 0.05%.

The content of phosphorus should be trace to 0.015%. Phosphorus is an inevitable impurity in steel. During the quenching and tempering treatment, they are simultaneously separated with elements such as chromium or manganese at the previous austenite grain boundary. The result is reduced intergranular embrittlement, which is very detrimental to cohesion at the grain boundaries and fatigue resistance in air. These effects are even more detrimental to the high tensile strength and hardness required in the steel according to the invention. In order to simultaneously obtain high spring steel tensile strength and hardness, and good fatigue resistance under air and corrosion conditions, the phosphorus content should be as low as possible and not exceed 0.015%, preferably 0.010%.

Sulfur content is trace to 0.015%. Sulfur is an inevitable impurity in steel. Its content should be as low as possible and should be traces to 0.015%, preferably up to 0.010%. The present inventors therefore desire to prevent the presence of sulfides which are detrimental to fatigue resistance and fatigue resistance in air, in corrosion conditions, such that the steel according to the invention has high strength and hardness values.

Oxygen content should be trace to 0.0020%. Oxygen is also an inevitable impurity in steel. Oxygen, in combination with reducing elements, can lead to the emergence of isolated, formulated, very hard angular inclusions, or have a fine but long stringer form, which can lead to inclusions that are very harmful to fatigue fatigue in air. These effects are even more detrimental in steels with high tensile strength and hardness according to the present invention. For these reasons, the oxygen content should not exceed 0.0020% in order to ensure a good compromise between high tensile strength and hardness and high fatigue resistance in air and corrosion conditions in the steel according to the invention.

The nitrogen content should be 0.0020% to 0.0110%. In combination with titanium, niobium, aluminum or vanadium, nitrogen must be controlled within this range to sufficiently form very fine microscopic nitrides, carbides or carbonitrides that refine the grains. Thus, the minimum nitrogen content should thus be 0.0020%. The content should not exceed 0.0110% in order to prevent the formation of prepared hard titanium nitride or carbonitride larger than 20 μm when observed at 1.5 mm ± 0.5 mm from the surface of the bar or wire used for spring manufacture. This position is very important for the fatigue load of the spring. Indeed such large nitrides or carbonitrides, in the presence of such inclusions, are broken at the location of such large inclusions located precisely at the quoted area of the spring surface, with respect to the steel according to the invention with high strength and hardness. Very disadvantageous to air fatigue

In order to assess the size of titanium nitride and carbonitride, we consider inclusions to be square and say that their size is equal to the square root of their surface area.

The manufacturing process of the spring according to the invention will be described below.

Non-limiting steelmaking process according to the present invention is as follows. After the production of liquid steel in a converter or electric furnace, alloying elements are added to provide a ladle metallurgical treatment, and a steel having the composition according to the invention during the reduction, and to form sulfides or titanium and / or niobium and / or vanadium All common secondary metallurgical operations are performed to prevent the formation of "carbonitrosulfide" composites of elements such as The inventors have unexpectedly found that in order to prevent the formation of such preparation precipitates during steelmaking, the content of various elements, in particular the contents of titanium, nitrogen, vanadium and sulfur, must be carefully controlled in the above-mentioned limits. After the described process, the steel is cast in the form of bloom or billet or ingot. However, during and after the solidification of these products, the average cooling rate for these products (bloom, billet or ingot) has been reduced to completely prevent the formation of crude titanium nitride or carbonitride or to form as much as possible. It was found that it should be adjusted to more than 0.3 ℃ / s at 1450-1300 ℃. The inventors unexpectedly observed that when working at these conditions during the solidification and cooling steps, the size of the prepared titanium nitride or carbonitride observed in the spring was always less than 20 μm. The location and size of such titanium precipitates will be described below.

When returned to room temperature, products with the correct composition according to the invention (bloom, billet, ingot) are reheated in a single or dual heating and rolling process and rolled in the form of wire or bar at 1200-800 ° C. In order to obtain the characteristics of the steel specified in the present invention, after austenitizing at a temperature of 850-1000 ° C., water quenching, polymer quenching or oil quenching may be performed on bars, rods, slugs, or even springs produced from these bars or wires , Fine austenide grains can be obtained which are no coarser (prepared) grains than 9 on the ASTM grain size scale. This quenching treatment is followed by a tempering treatment at 300-550 ° C. specifically. The tempering treatment provides the high levels of tensile strength and hardness required of the steel, firstly preventing the microstructure causing brittleness during tempering, and secondly the excessively high retained austenite. The inventors have found that embrittlement during tempering and excessively high levels of retained austenite are extremely harmful to fatigue resistance under the corrosion conditions of the steel according to the invention. In the case of a bar which has not been heat treated, or is made from wire rods or slag made from such bars, the above-mentioned treatments (quenching and tempering) must be carried out on the springs themselves under the conditions mentioned above. In the case of cold forming the springs, such heat treatment processes may be carried out on the bar, or such straight wire or slug, prior to spring manufacture.

It is well known that the hardness of steel depends not only on its composition but also on the quenching temperature to be treated. It can be seen that the quenching temperature to have a minimum desired hardness of 55 HRC for all compositions of the present invention is in the industrial range of 300-550 ° C.

Since nitrides and carbonitrides are very hard, as defined above, their size does not change at all during the strong transformation step. Thus, it does not matter whether it is observed in intermediate products (bars, wire rods, or slugs) that will be used to manufacture the spring or to be used by the spring itself.

The present invention not only has improved high hardness and tensile strength compared to the prior art, but also has improved fatigue properties and deflection resistance in air, fatigue properties under corrosion conditions that are at least equivalent to or better than those of known steels for such applications. The addition of microalloyed elements, reduction in residual elements and analysis of the steel and control of the fabrication path provide a spring steel that is less sensitive to the concentration of strain generated by surface defects that may form during spring manufacture. .

Hereinafter, the present invention will be described using 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 equation:

[Equation 1]

Ceq = [C] + 0.12 [Si] + 0.17 [Mn]-0.1 [Ni] + 0.13 [Cr]-0.24 [V]

In the above formula, the contents (% by weight) of each of [C], [Si], [Mn], [Ni], [Cr], and [V] are shown.

Table 2 shows the hardness values of the inventive and reference steels as a function of the quenching temperature used.

Quenching temperature (℃) HRC hardness Quenching temperature (℃) HRC hardness Inventive River 1 350 56.9 400 55.3 Inventive River 2 350 58.5 400 57.1 Present invention river 3 350 59.0 400 57.2 Present invention river 4 350 56.7 400 55.6 Inventive River 5 350 57.6 400 55.8 Reference river 1 350 57.9 400 55.1 Reference river 2 350 54.2 400 52.5 Reference river 3 350 54.8 400 51.3

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 of the surface of the invention and reference steels as defined above, and we also reported titanium content in various steels.

The maximum size of such titanium nitride or carbonitride inclusions is determined as follows. The cross section of the bar or wire from a given steel casting was tested for a surface area of 100 mm ² at a point located 1.5 mm ± 0.5 mm below the bar or wire surface. After observation the inclusions were square and the size of each of these inclusions, including inclusions with the largest surface area, was considered to be equal to the square root of the surface area and the size of the titanium nitride or carbonitride inclusions with the maximum surface area was determined. All inclusions were observed in the cross section of the spring bars or wire rods and observations were made for each cross section 100 mm ² . When the maximum size of the aforementioned inclusions observed for 100 mm ² at 1.5 mm ± 0.5 mm below the surface is less than 20 μm, the steel casting corresponds to the invention example. The corresponding results obtained for the present invention and reference steels are shown in Table 3.

For Reference Experiments 1 to 3, their titanium content was actually zero, and no nitrides and carbonitrides were observed.

Ti (%) Maximum size of nitride or carbonitride observed at 100 mm ² (μm) Inventive River 1 0.072 11.8 Inventive River 2 0.073 12.4 Present invention river 3 0.025 13 Present invention river 4 0.069 11.9 Inventive River 5 0.077 14.1 Reference River 1 0.002 - Reference Steel 2 (Experiment 1) 0.064 20.8 Reference Steel 2 (Experiment 2) 0.064 29 Reference River 3 0.002 -

Table 3: Maximum size of the largest titanium nitride or carbonitride at 1.5 mm from the sample surface

The inventors did not measure the size of inclusions for the reference steels 1 and 3 because their titanium content was low and because the results were not important because they do not correspond to the present invention.

A sample for fatigue testing was taken from the bar and the final diameter of the sample was 11 mm. Rough testing, austenitization, oil quenching, tempering, grinding and shot peening were made to prepare samples for fatigue testing. These samples were tested by torsion-fatigue in air. The applied shear stress was 856 ± 494 MPa and the number of cycles until failure was counted. The test was stopped after 2 X 10 ⁶ cycles if the sample was not broken.

A sample was taken from the bar for fatigue testing in corrosion conditions and the final diameter of the sample was 11 mm. Rough processing, austenitization, oil quenching, tempering, grinding and shot peening were prepared samples for fatigue testing. These samples were subjected to a fatigue test under corrosion conditions, i.e., corrosion applied simultaneously with the fatigue load. Fatigue loads were applied with a shear stress of 856 ± 300 MPa, and corrosion was subjected to cyclic corrosion in two alternating steps:

A wet step of spraying a 5% NaCl solution at 35 ° C. for 5 minutes in one step;

Drying in 35 minutes at 35 ° C. without spraying in one step.

The number of cycles until failure was referred to as fatigue life under corrosive conditions.

Sag resistance was determined using periodic compression tests on cylindrical samples. The sample diameter was 7 mm and its height was 12 mm. These are taken from the river bar.

Sagging test samples were prepared by roughing, austenitizing, oil quenching, tempering and final fine grinding. The height of the sample was measured precisely before testing using a comparator with a precision of 1 μm. Preload was applied to simulate a pre-set spring with a compressive strain of 2200 MPa.

Then, a fatigue load cycle was applied. At this time, the deformation force was 1270 ± 730 MPa. The height loss in the samples for the number of cycles up to one million was measured, and the height remaining at the end of the test was precisely measured and compared to the initial height to determine the total deflection. Lower height decreases as a percentage of the initial height indicate better sag resistance.

Table 4 shows the results of fatigue tests, fatigue tests under corrosion conditions and deflection for the present invention and reference steels.

HRC hardness Tensile Strength (MPa) Fatigue Life (cycles) Fatigue Life Under Corrosion Conditions (cycles) sag(%) Inventive River 1 56.7 2129 1742967 192034 0.025 Inventive River 2 56.4 2106 > 2000000 138112 0.01 Present invention river 3 56.5 2118 > 2000000 135562 0.015 Present invention river 4 56.9 2148 > 2000000 202327 0.025 Inventive River 5 57.0 2156 > 2000000 139809 0.025 Reference river 1 56.7 2131 514200 96672 0.03 Reference river 2 53.8 1898 217815 241011 0.10 Reference river 3 55.6 2062 301524 150875 0.075

Table 4. Fatigue and deflection test results under fatigue and corrosion conditions

From this table, the various reference rivers are particularly unsatisfactory for the following reasons:

Reference steel 1 in particular has a sulfur content that is too high to show a good compromise between fatigue resistance in air and fatigue content under corrosive conditions. Moreover, the manganese content is too high, resulting in a detrimental to the homogeneity of the steel and the fatigue resistance in the air.

Reference steel 2 has a very low carbon content and a carbon equivalent to ensure high hardness, and its tensile strength is too low to show good air fatigue resistance.

Reference steel 3 has a silicon content that is too low to exhibit particularly good deflection resistance and good air resistance.

As shown in FIG. 1, the deflection resistance in the present invention steel is higher than the reference steel, which, when compared with the best case of the reference steel (Reference Steel 1), shows that the deflection value according to the deflection measurement described above is the worst of the present invention steel. It is evident that it is lower than at least 32% of the case (present invention steel 1).

The fatigue life in air is clearly higher in the present invention than in the reference steel. This is due to the increased hardness, as shown in FIG. 2, but the increased hardness is not sufficient. In fact, steels with generally high hardness are more sensitive to defects such as inclusions and surface defects as hardness increases. The invention steels are therefore less sensitive to defects, in particular formulation inclusions such as titanium nitride or carbonitride, in that the present invention prevents the appearance of such large inclusions. As shown in Table 3, the largest inclusions found in the present invention steel do not exceed 14.1 μm, and inclusions larger than 20 μm are found in reference steel 2. Moreover, the lower sensitivity to surface defects occurring during spring fabrication or other operations when the invention steel is used, can be explained by strength tests conducted on the invention steel and reference steels that have been heat treated and have a hardness of at least 5 HRC. (See Figure 3). The values measured during the Charpy impact test on the invention steel (sample notches simulate strain concentrations, such as those found in surface defects generated during the manufacture of a spring or other operation), are compared to reference steels. Higher than measured. This shows that the present invention steel is less sensitive to strain concentration on defects than the reference steel according to the prior art.

Increasing hardness reduces fatigue resistance in corrosive conditions. Thus, the present invention steels seem to have the advantages of higher fatigue resistance and especially hardness of 55HRC or higher as shown in FIG. 4 in corrosive conditions compared to reference steels according to the prior art.

Thus, the present invention provides a better compromise between fatigue life and deflection resistance in the air, which is higher, and provides better fatigue life under corrosion conditions compared to the reference steel according to the prior art. Furthermore, it also provides low susceptibility to possible surface defects, especially those that occur during spring manufacture or other operations.

Claims (6)

By weight C = 0.45 to 0.70%, Si = 1.65-2.50%, Mn = 0.20 to 0.75%, Cr = 0.60 to 2%, Ni = 0.15 to 1%, Mo = trace to 1%, V = 0.003 to 0.8%, Cu = 0.10 to 1%, Ti = 0.020 to 0.2%, Nb = trace to 0.2%, Al = 0.002 to 0.050%, P = trace to 0.015%, S = trace to 0.015%, O = trace to 0.0020%, N = 0.0020 to 0.0110%, The balance has iron and a composition which is an impurity derived from the steel manufacturing process, The carbon equivalent (Ceq) content calculated according to Equation 1 is 0.80 to 1.00%, Hardness after quenching and tempering is 55HRC or more, Spring steel with high resistance to fatigue and cyclic deflection under air and corrosive conditions. [Equation 1] Ceq% = [C%] + 0.12 [Si%] + 0.17 [Mn%]-0.1 [Ni%] + 0.13 [Cr%]-0.24 [V%] The method of claim 1, The maximum surface size of titanium nitride or carbonitride observed at 1.5 ± 0.5 mm of bar, wire, slug or spring with a cross-sectional surface area of 100 mm ² or more is 20 µm or less, and the size of the inclusions when considered as square Spring steel, the square root of. The method according to claim 1 or 2, C = 0.45 to 0.65% Si = 1.65-2.20% Mn = 0.20 to 0.65% Cr = 0.80 to 1.7% Ni = 0.15 to 0.80% Mo = trace to 0.80% V = 0.003 to 0.5% Cu = 0.10 to 0.90% Ti = 0.020 to 0.15% Nb = trace to 0.15% Al = 0.002 to 0.050% P = trace to 0.010% S = trace to 0.010% O = trace to 0.0020% N = 0.0020 to 0.0110% A spring steel having a composition in which the balance is iron and an impurity derived from the steel manufacturing process. As liquid steel is made in a converter or electric furnace, its composition is controlled, cast into a bloom or continuous flow billet or ingot, allowed to cool to room temperature, rolled into bars, wire or slug, and transformed into a spring, In the method of manufacturing spring steel having high resistance to fatigue and cyclic deflection in air and corrosion conditions, Said steel according to any one of claims 1 to 3, After being solid, the bloom, billet or ingot has a minimum average cooling rate of 0.3 ° C./s from 1450 to 1300 ° C., The bloom, billet or ingot is rolled at 1200 to 800 ° C. with a reheating and rolling cycle of 1 or 2, A bar, wire or slug, or spring made is austenitized at 850-1000 ° C. and then water quenched, polymer quenched or oil quenched and tempered at 300-550 ° C. to provide a steel having a hardness of at least 55 HRC Method for producing a spring steel, characterized in that.  Spring, characterized in that made of the steel according to any one of claims 1 to 3. The method of claim 5, wherein Spring, characterized in that it is made of steel obtained by the method according to claim 4.

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