RU2397270C2 - Spring steel, procedure for fabrication of spring out of this steel ans spring out of this steel - Google Patents

Abstract

FIELD: metallurgy. ^ SUBSTANCE: invention refers to ferrous metallurgy, particularly to fabricating spring steel. In a converter or electric furnace there is melted liquid steel containing the following elements, wt %: carbon 0.45 - 0.70, silicon 1.65 - 2.50, manganese 0.20-0.75, chromium 0.60 - 2, nickel 0.15 - 1, molybdenum (traces) - 1.0, vanadium 0.003 - 0.8, copper 0.10 - 1, titanium 0.020 - 0.2, niobium (traces) - 0.2, aluminium 0.002 - 0.050, phosphorus (traces) - 0.015, sulphur (traces) - 0.015, oxygen (traces) - 0.0020, nitrogen 0.0020 - 0.0110, iron and associated impurities at melting - the rest. Steel is cast and there are produced blooms, ingots or flat bars. During their solidification or after it they are cooled at rate 0.3C/s within the range 1450 - 1300C. The said blooms, ingots or flat bars are rolled at temperature 1200 - 800C per one or two cycles of heating and rolling. Rods, rolled bars, work-pieces or springs made out of them are subject to austenisation within the range 850 - 1000C and to successive quenching in water, polymer or oil and tempering at 300 - 550C to strengthen steel to 55 HRC or more and to maximal dimension of nitrides or carbonitrides of titanium at depth 1.5 mm 0.5 mm from surface of rod, rolled bar, work-piece or spring with cross section of 100 mm2 area constituting 20 mcm or less. Also the said dimension is a value of square root of surface area of the said inclusions, shape of which is taken as square. ^ EFFECT: steel possesses upgraded tensile strength and durability, fatigue strength in air and in corrosion medium and high resistance to cyclic fatigue softening. ^ 5 cl, 4 tbl, 4 dwg

Description

The invention relates to the field of ferrous metallurgy, in particular to spring steels.

As a rule, with the growth of fatigue loads in springs, the required hardness and tensile strength constantly increase. As a result, the tendency to fracture at the places of defects, such as inclusions or surface defects formed during the manufacturing of springs, is increased, and fatigue strength is limited. However, springs used in conditions of severe corrosion, for example, such as suspension springs, must have such fatigue properties in a corrosive environment that are at least equivalent or even higher, since they use steels with high hardness and tensile strength . Therefore, such springs tend to break at defect sites directly during fatigue load cycles in air, and also later during fatigue load cycles in a corrosive environment. In particular, during corrosion fatigue, defects can occur in etching figures. In addition, with an increase in the applied loads, it becomes more difficult to increase the fatigue life in a corrosive medium or maintain it at an equivalent level, since the consequences of stress concentration in etching figures, on surface defects of springs formed, possibly during their winding or at other stages of their manufacture , or on non-metallic inclusions, become more critical with increasing spring hardness.

In the prior art, FR-A-2740476 and JP-A-3474373 describe a spring steel grade with high resistance to hydrogen embrittlement and high fatigue strength, in which inclusions of carbonitrosulfides containing at least one element selected from titanium, niobium, zirconium, tantalum or hafnium are controlled to ensure their reduced average size with a diameter of less than 5 microns while providing a very large amount (10,000 or more in cross-sectional area).

However, after quenching and tempering produced industrially in the manufacture of springs, this type of steel acquires a Rockwell hardness of only 50 HRC or slightly greater, which corresponds to a tensile strength of 1700 MPa or slightly greater, but not more than 1900 MPa, which corresponds to a hardness of 53 5 HRC. Due to such moderate hardness, this steel has only moderate resistance to softening, and to increase the resistance against softening, steel with higher tensile strength is required. Therefore, such steel does not provide the necessary compromise between high strength above 2100 MPa, hardness above 55 HRC, high fatigue strength in air and fatigue strength in a corrosive environment, which is at least equivalent to or even exceeds the strength required for springs.

The aim of the invention is to provide means that simultaneously provide, in comparison with known spring steels, an increase in tensile strength and hardness of springs, better fatigue properties in air, at least the same or better fatigue properties in a corrosive environment, better resistance to spring softening and less sensitivity to surface defects formed during winding of the spring.

Therefore, the object of the invention is spring steel with increased fatigue strength in air and in a corrosive environment and with high resistance against cyclic softening, having the following composition, wt.%:

C = 0.45-0.70,

Si = 1.65-2.50;

Mn = 0.20-0.75;

Cr = 0.60-2.0;

Ni = 0.15-1.0;

Mo = traces-1.0;

V = 0.003-0.8;

Cu = 0.10-1.0;

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 associated with the smelting, while the content of Ceq equivalent carbon in steel, calculated by the formula:

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

ranges from 0.80 to 1.00 wt.%, the hardness of the steel after quenching and tempering is 55 HRC or more.

The maximum size of titanium nitrides or carbonitrides, noted at a depth of 1.5 ± 0.5 mm from the surface of a bar, wire rod, workpiece or spring with a cross-section of 100 mm ² , is preferably 20 μm or less, the size being referred to as the square root of the area inclusions, the shape of which was taken as a square.

Preferably, the steel has the following composition, wt.%:

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 rest is iron and the impurities associated with the smelting.

Another object of the invention is a method for producing spring steel with increased fatigue strength in air and in a corrosive environment and with high resistance against cyclic softening, in which steel is smelted in a converter or an electric furnace, the composition is adjusted, cast in the form of blooms or sutures by continuous casting or in the form ingots, which are cooled to ambient temperature, rolled to produce rods, wire rods or billets and made from them springs, characterized in that

- steel is the above type of steel,

- blooms, stoops or ingots during their solidification or after it is subjected to cooling at an average rate of 0.3 ° C / s in the range from 1450 to 1300 ° C,

- blooms are rolled, mimics or ingots at a temperature of 1200-800 ° C for one or two heating and rolling cycles,

- rods, wire rod or billets or springs made from them are subjected to austenization at a temperature of from 850 to 1000 ° C, followed by quenching in water, polymer or oil and tempering at 300-550 ° C to give steel a hardness of 55 HRC or more.

Also an object of the invention are springs of such steel and springs of steel obtained in the above manner.

Unexpectedly, the inventors found that steel with properties determined by the above composition and morphology of inclusions, after smelting, casting, rolling, hardening and tempering, carried out in a special mode, can achieve a hardness of more than 55 HRC, providing an excellent compromise between high fatigue life in air and in a corrosive environment, increased resistance against cyclic softening and low sensitivity to surface defects formed during the manufacture of the spring.

Below the invention is explained in more detail description with reference to the accompanying figures, which represent:

figure 1 - test results for hardness and resistance against cyclic softening for steels according to the invention and control steels;

figure 2 - the results of fatigue tests in air depending on the hardness of the steels according to the invention and control steels;

figure 3 - test results for impact strength according to Charpy depending on the hardness of the steels according to the invention and control steels;

figure 4 - the results of fatigue tests in a corrosive environment depending on the hardness of the steels according to the invention and control steels.

The composition of the steel according to the invention must meet the following requirements.

The carbon content should be 0.45-0.7%. After quenching and tempering, carbon can increase tensile strength and hardness of steel. If the carbon content is less than 0.45%, then in the temperature range commonly used in the manufacture of springs, neither quenching nor tempering will impart high strength and hardness to the steel according to the invention. On the other hand, if the carbon content exceeds 0.7% or even 0.65%, the large and very hard carbides associated with chromium, molybdenum and vanadium can remain undissolved during austenization before quenching and can significantly affect the fatigue life air, fatigue strength in a corrosive environment, as well as viscosity. Therefore, carbon content in excess of 0.7% must be avoided. Preferably, this content does not exceed 0.65%.

The silicon content is 1.65-2.5%. Silicon is an important element that allows one to achieve in its presence in a solid solution high strength and hardness values, as well as increased Ceq equivalent carbon and resistance to cyclic softening. To obtain these indicators of tensile strength and hardness of the steel according to the invention, the silicon content in it should not be less than 1.65%. In addition, silicon contributes, at least in part, to the deoxidation of steel. If the silicon content is more than 2.5% or even 2.2%, the oxygen content in the steel due to the thermodynamics of the reaction can exceed 0.0020 and even 0.0025%. This leads to the formation of oxides of different compositions, which reduce the fatigue resistance in air. In addition, with a silicon content of more than 2.5%, segregations of various related elements, such as manganese, chromium, etc. can form during solidification and after casting. These segregations have a very adverse effect on fatigue properties in air and fatigue strength in a corrosive environment. . Finally, with a silicon content of more than 2.5%, too much decarburization occurs on the surface of the rods or wire rod designed for the manufacture of springs, which affects the performance of the spring. Therefore, the silicon content should not exceed 2.5%, preferably 2.2%.

The manganese content is 0.20-0.75%. Manganese must be added taking into account residual sulfur, the content of which ranges from traces to 0.015%, in an amount that is at least ten times higher than the sulfur content, to prevent the formation of iron sulfides, which sharply reduce the rolling ability of steel. Therefore, the minimum content of manganese should be 0.20%. In addition, manganese contributes to the hardening of the solid solution during hardening of steel to the same extent as nickel, chromium, molybdenum and vanadium, which allows to obtain in the steel according to the invention increased tensile strength and hardness and equivalent carbon Ceq. With a manganese content of more than 0.75% or even 0.65%, segregations in combination with silicon can form at the solidification stage after steelmaking and casting. These segregations adversely affect the operational properties of steel and its uniformity. Therefore, the manganese content in the steel should not exceed 0.75%, preferably 0.65%.

The chromium content should be 0.60-2%, preferably 0.80-1.70%. Chromium is added to obtain in solid solution after austenization, quenching and tempering high tensile strength and hardness, Ceq equivalent carbon index, as well as to increase fatigue strength in a corrosive environment. To achieve these properties, the chromium content should be at least 0.60%, preferably at least 0.80%. With a content of more than 2% or even 1.7% after treatment for austenization, special, large and very hard chromium carbides can be formed in combination with vanadium and molybdenum before quenching. Such carbides significantly affect the fatigue strength in air. Therefore, the chromium content should not exceed 2%.

Nickel content is 0.15-1%. Nickel is added to increase the hardenability of steel, as well as to increase tensile strength and hardness after hardening and tempering. Since nickel does not form carbides, it contributes to the hardening of steel in the same way as chromium, molybdenum and vanadium, without the formation of special, large and hard carbides, which do not dissolve during austenization before quenching and can reduce fatigue resistance in air. It also allows you to adjust the required equivalent carbon content from 0.8 to 1% in the steel according to the invention. As a non-oxidizing element, nickel increases fatigue strength in a corrosive environment. To achieve such effects, the nickel content should not be less than 0.15%. However, with a content of more than 1%, even 0.80%, nickel can cause a very high residual austenite content, the presence of which is very harmful for fatigue strength in a corrosive environment. In addition, a high nickel content significantly increases the cost of steel. For these reasons, the nickel content should not exceed 1%, preferably 0.80%.

The molybdenum content should be from traces to 1%. Like chromium, molybdenum increases the hardenability of steel and its strength. In addition, it has a low oxidation potential. For these reasons, molybdenum has a positive effect on the fatigue strength in air and in a corrosive environment. However, with a content of more than 1%, or even 0.80%, large and very hard molybdenum carbides can form, in certain cases associated with vanadium and chromium, after the previous austenization quenching. These special carbides are very detrimental to fatigue resistance in air. Finally, the addition of molybdenum over 1% increases the cost of steel. Therefore, the molybdenum content should not exceed 1%, preferably 0.80%.

The vanadium content should be 0.003-0.8%. Vanadium is an element that increases hardenability, tensile strength and hardness after hardening and tempering. In addition, in combination with nitrogen, vanadium allows the formation of a large number of small vanadium nitrides or submicroscopic vanadium and titanium nitrides, which allow grinding grain and increase tensile strength and hardness due to dispersion hardening. For the formation of submicroscopic vanadium and titanium nitrides in order to grind the grain, vanadium must be contained in a minimum amount of 0.003%. However, this element is expensive and its content must be maintained close to this lower limit in the case when a compromise is required between the cost of smelting and grinding grain. The vanadium content should not exceed 0.8%, preferably 0.5%, since above this value the precipitation of large and very solid vanadium carbides in combination with chromium and molybdenum can remain undissolved during austenization before quenching. This can have a very adverse effect on the fatigue strength in air at high strength and hardness values of the steel according to the invention. In addition, the addition of vanadium in an amount of more than 0.8% increases the cost of steel.

The copper content should be 0.10-1%. Copper is an element that hardens steel in solid solution after quenching and tempering. Therefore, it can be added along with other elements that improve the strength and hardness of steel. Since it does not combine with carbon, it causes the hardening of steel without the formation of large and hard carbides, which impair the fatigue strength in air. From the point of view of electrochemistry, the passivation potential of copper exceeds the same passivation potential of iron and, therefore, it has a beneficial effect on the fatigue strength of steel in a corrosive environment. To achieve such significant effects, the copper content should not be less than 0.10%. However, with a content of more than 1% or even 0.9%, copper has a very adverse effect on the hot rolling ability of steel. Therefore, the copper content should not exceed 1%, preferably 0.90%.

The titanium content should be 0.020-0.2%. Titanium is added in combination with nitrogen, even carbon and / or vanadium, to form fine nitrides or submicroscopic carbonitrides, which provide grinding of the austenitic grain during austenitization before quenching. Therefore, it increases the surface along the grain boundaries in steel, which leads to a decrease in the amount of inevitable impurities, such as phosphorus, released along the grain boundaries. Such intergranular segregations are very harmful for viscosity and fatigue strength in air when their quantity per surface unit along grain boundaries is large. In addition, when combined with carbon and nitrogen, as well as with vanadium and niobium, titanium forms other small nitrides or carbonitrides, which have an irreversible effect of trapping some elements, such as hydrogen generated during corrosion reactions, which can be extremely unfavorable for fatigue strength. To achieve high efficiency, the titanium content should not be less than 0.020%. Above 0.2%, even 0.15%, titanium can cause the formation of large and solid nitrides or carbonitrides, which are very unfavorable for fatigue resistance in air. They have an even more unfavorable effect on the achievement of high tensile strength and hardness of the steel according to the invention. For these reasons, the titanium content should not exceed 0.2%, preferably 0.15%.

The niobium content should be from traces to 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, in particular, at a low content (for example, 0.002%) of aluminum to complete grinding of austenitic grain during austenization before quenching. Consequently, niobium increases the grain boundary surface in steel and contributes, like titanium, to a beneficial effect on preventing grain boundary embrittlement caused by inevitable impurities such as phosphorus, which is very unfavorable for viscosity and fatigue strength in a corrosive environment. In addition, extremely fine precipitations of niobium nitrides or carbonitrides contribute to the hardening of steel due to dispersion hardening. However, the niobium content should not exceed 0.2% or even 0.15% so that the nitrides or carbonitrides are kept very small and that the austenitic grain is crushed and the formation of cracks or tears during hot rolling is avoided. For these reasons, the niobium content should not exceed 0.2%, preferably 0.15%.

The aluminum content should be 0.002-0.050%. Aluminum can be added to complete the deoxidation of the steel and to obtain the lowest possible oxygen content in the steel according to the invention, in any case less than 0.002%. In addition, in combination with nitrogen, aluminum contributes to grain refinement by the formation of submicroscopic nitrides. To perform both of these functions, the aluminum content must be at least 0.002%. When the aluminum content is above 0.05%, individual large inclusions or smaller, but hard and uneven aluminates can form in the form of long splices, which adversely affect the fatigue life in air and the purity of the steel. Therefore, the aluminum content should not exceed 0.05%.

The phosphorus content should be from traces to 0.015%. Phosphorus is an inevitable admixture of steel. During quenching and tempering, it forms joint segregations with elements such as chromium or manganese, along the former boundaries of austenitic grains. Because of this, there is a decrease in adhesion along the grain boundaries and intergranular embrittlement, which is very harmful for viscosity and fatigue strength in air. These effects have an even more negative effect on obtaining the high tensile strength and hardness required in the steel according to the invention. In order to simultaneously achieve high tensile strength and high hardness of spring steel, as well as good fatigue strength in air and in a corrosive environment, the phosphorus content should be as low as possible and not exceed 0.015%, preferably 0.010%.

The sulfur content is from traces to 0.015%. Sulfur is an inevitable impurity in steels. Its content should be as low as possible and range from traces to 0.015%, preferably not more than 0.010%. Therefore, it is necessary to exclude the presence of sulfur, which adversely affects the fatigue strength in a corrosive environment and in air, in order to obtain high strength and hardness in steel according to the invention.

The oxygen content should be from traces to 0.0020%. Oxygen is also an inevitable impurity in steels. In combination with deoxidants, oxygen can lead to the appearance of separate large, very hard and uneven inclusions or smaller inclusions in the form of long splices, which are very unfavorable for fatigue resistance in air. These effects are even more unfavorable for obtaining high tensile strength and hardness in steels according to the invention. For these reasons, for an optimal compromise between high tensile strength, hardness and high fatigue strength in air and in a corrosive environment, the oxygen content in the steel according to the invention should not exceed 0.0020%.

The nitrogen content should be between 0.0020 and 0.0110%. Nitrogen must be controlled in this range so that when combined with titanium, niobium, aluminum or vanadium, very small nitrides, carbides or submicroscopic carbonitrides are obtained in sufficient quantities to grind the grain. Therefore, the minimum nitrogen content should be 0.0020%. Its content should not exceed 0.0110% so that it is possible to exclude the formation of large and very solid titanium nitrides or carbonitrides larger than 20 μm, marked at a depth of 1.5 ± 0.5 mm from the surface of rods or wire rods intended for manufacture springs. This arrangement depth is most critical with respect to the fatigue stress of the springs. Indeed, such large-sized nitrides or carbonitrides have a very adverse effect on the fatigue strength in air at high strength and hardness values of the steels according to the invention, it must be taken into account that during the fatigue strength tests in air, the springs broke at the location such large inclusions located just near the surface of the springs, as mentioned, in those cases where these inclusions were present.

To estimate the size of titanium nitrides and carbonitrides, it was believed that the inclusions had the shape of squares and assumed that the size of their sides was equal to the square root of their surface.

Below will be described a method of manufacturing springs according to the invention.

The following is a non-limiting example of a method for steelmaking according to the invention. Liquid steel is produced either in a converter or in an electric furnace, then it is subjected to metallurgical treatment in a ladle, in which alloying elements and deoxidizers are introduced, and, in principle, any secondary metallurgy operations are necessary to obtain steel with the composition according to the invention and to prevent the formation of complex sulfides or carbonitrosulfides of elements such as titanium and / or niobium and / or vanadium. The inventors unexpectedly found that to prevent the formation of large precipitates during smelting, the content of various elements, in particular titanium, nitrogen, vanadium and sulfur, should be carefully monitored to comply with the above limit values. After the described smelting, the steel is poured either in a continuous manner to produce blooms or slides or in the form of ingots. However, in order to completely or as completely as possible eliminate the formation of large titanium nitrides or carbonitrides during or after the solidification of these products, it was found that the average cooling rate of these products (blooms, storages or ingots) should be 0.3 ° C / s or more at a temperature from 1450 to 1300 ° C. When they work in such conditions at the stage of solidification and cooling, it turned out that the size of the largest titanium nitrides or carbonitrides in the springs is always less than 20 microns. Below we will talk about the location and size of these titanium emissions.

After cooling to ambient temperature, the products with the exact composition according to the invention (blooms, pimps, ingots) are again heated and rolled in the temperature range of 1200-800 ° C to obtain wire rod or rods in one or two passes of heating and rolling. In order to impart specific properties of the rod according to the invention, wire rods, billets and even springs made from rods and wire rods are quenched in water, polymer or oil after austenization in the temperature range of 850-1000 ° С in order to obtain fine austenitic grain whose size does not exceed 9 according to ASTM (American Society for the Testing of Materials) table. After hardening, a specific tempering is carried out at a temperature of 300-550 ° C to obtain the required high tensile strength and hardness of steel, as well as to eliminate, on the one hand, the microstructure leading to brittleness during tempering, and, on the other hand, the presence of too much the amount of residual austenite. It was found that embrittlement during tempering and the presence of too much residual austenite are extremely detrimental to the fatigue strength of the steel according to the invention in a corrosive environment. In the event that the springs were made from rods that did not undergo heat treatment, or from wire rod or billets obtained from such rods, the springs themselves should be subjected to the above types of processing (quenching and tempering) under the conditions described above. If the springs are made by cold stamping, then before the manufacture of the springs, the specified types of heat treatment can be subjected to rods, wire rod or billets obtained from these rods.

It is well known that the hardness of steel is determined not only by its composition, but also by the temperature of its tempering. It should be noted that for any alloy composition according to the invention, it is possible to choose the tempering temperature in the range of industrially applicable temperatures from 300 to 550 ° C, at which it is possible to obtain the required minimum hardness of 55 HRC.

Since nitrides and carbonitrides are very solid, their pre-determined grain size remains virtually unchanged during subsequent steel treatments. Therefore, it does not matter whether the grain size is measured in the semi-finished product (rods, wire rod or workpiece) intended for the manufacture of springs, or in the spring itself.

The invention allows to obtain spring steels that combine high hardness and tensile strength, improved compared to the prototype, with improved fatigue properties in air and resistance to softening, fatigue properties in a corrosive environment, at least equivalent to such properties of steel of known grades for the same purpose, or even better, and with less sensitivity to stress concentrations caused by surface defects formed during the manufacture of springs, giving alloying additive trace elements, reducing the content of residual elements and control of a composition and a process of production of steel chain.

Below the invention is illustrated using examples. Table 1 shows the compositions of the steel according to the invention and control steels. The equivalent carbon content Ceq is calculated by the formula:

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

where: [C], [Si], [Mn], [Ni], [Cr], [V] is the content of each element in wt.%.

Table 1 The chemical composition of the tested steels (%) FROM Si Mn Ni Cr V Ti Cu Mo No P S Al N O Ceq Steel 1 according to the invention 0.48 1.82 0.21 0.15 1.48 0.204 0,072 0.20 0.02 0 0.006 0.006 0,034 0.0051 0,0007 0.86 Steel 2 according to the invention 0.58 1.79 0.22 0.15 0.98 0.216 0,073 0.20 0,03 0 0.006 0.008 0,032 0.0051 0,0007 0.89 Steel 3 according to the invention 0.59 1.80 0.22 0.15 0.99 0.212 0,025 0.20 0,03 0,022 0.007 0.008 0,032 0.0066 0,0008 0.91 Steel 4 according to the invention 0.48 2.10 0.21 0.70 1.50 0.152 0,069 0.51 0,03 0 0.005 0.005 0,032 0.0042 0,0008 0.86 Steel 5 according to the invention 0.54 1.81 0.23 0.34 1.25 0,098 0,077 0.42 0.02 0 0.006 0.008 0,031 0.0041 0,0007 0.90 Control steel 1 0.60 1.73 0.88 0.08 0.20 0.154 0.002 0.19 0,03 0,020 0.010 0.019 0.002 0.0084 0.0010 0.94 Control steel 2 0.40 1.79 0.17 0.53 1,04 0.166 0,064 0.20 0.01 0 0.013 0.004 0,020 0.0034 0.0011 0.69 Reference steel 3 0.48 1.45 0.89 0.11 0.47 0.136 0.002 0.19 0.02 0 0.011 0.013 0.003 0.0062 0.0010 0.82

Table 2 shows the hardness values of the steels according to the invention and control steels depending on the tempering temperature.

table 2 Hardness and tensile strength depending on tempering temperature Tempering temperature, ° С Hardness HRC Tempering temperature, ° С Hardness HRC Steel 1 according to the invention 350 56.9 400 55.3 Steel 2 according to the invention 350 58.5 400 57.1 Steel 3 according to the invention 350 59.0 400 57.2 Steel 4 according to the invention 350 56.7 400 55.6 Steel 5 according to the invention 350 57.6 400 55.8 Control steel 1 350 57.9 400 55.1 Control steel 2 350 54,2 400 52,5 Reference steel 3 350 54.8 400 51.3

Table 3 shows the maximum value of inclusions of titanium nitrides or carbonitrides at a depth of 1.5 mm from the surface of the steels according to the invention and the control steels described above. The titanium content in different steels is also indicated.

The maximum size of inclusions of titanium nitrides or carbonitrides was determined as follows. At a depth of 1.5 mm ± 0.5 mm from the surface of the bar or wire rod, a section of 100 mm ^{2 was} examined on a section of a bar or wire rod made of steel of this casting. As a result of this study, the size of the inclusion of titanium nitride or carbonitride with a maximum surface was determined, provided that the inclusions are squares and that the size of each inclusion, incl. inclusions with the largest surface is the square root of the magnitude of this surface. All inclusions were observed on a slice of a bar or wire rod designed for springs per 100 mm ^{2 of} this slice. Steel casting took place according to the invention, the maximum size of the inclusions observed on an area of 100 mm ² at a depth of 1.5 mm ± 5 mm was less than 20 microns. The corresponding results obtained on the steels according to the invention and on the control steels are shown in table 3.

Regarding the tests of control steels 1 and 3, it should be noted that the titanium content in them was practically 0 and the size of nitrides and carbonitrides was not determined.

Table 3 The maximum size of the largest inclusions of titanium nitrides or carbonitrides at a depth of 1.5 mm from the surface of the samples Ti,% The size of the largest nitride / carbonitride in an area of 100 mm ² , microns Steel 1 according to the invention 0,072 11.8 Steel 2 according to the invention 0,073 12,4 Steel 3 according to the invention 0,025 13 Steel 4 according to the invention 0,069 11.9 Steel 5 according to the invention 0,077 14.1 Control steel 1 0.002 - Reference Steel 2 (Test 1) 0,064 20.8 Reference Steel 2 (Test 2) 0,064 29th Reference steel 3 0.002 -

For control steels 1 and 3, the size of the inclusions was not measured, since the titanium content in them was low and did not correspond to the invention.

Fatigue test specimens were made from bars with a final specimen diameter of 11 mm. The preparation of samples for fatigue testing included roughing, austenization, oil quenching, tempering, grinding and shot blasting. Samples were tested for fatigue by twisting in air. The applied shear stress was 856 ± 494 MPa, the number of cycles before failure was calculated. The tests were stopped after 2 · 10 ⁶ cycles if the samples were not destroyed.

Samples for fatigue testing in a corrosive environment were made of rods, with a final diameter of 11 mm. The preparation of samples for fatigue testing included roughing, austenization, oil quenching, tempering, grinding and shot blasting. These samples were tested for fatigue under corrosive conditions, i.e. corrosion acted simultaneously with fatigue. Fatigue load was a shear stress of 856 ± 300 MPa. The applied corrosion was cyclic corrosion with two alternating stages:

- one stage was wet, in which a salt solution with a content of 5% NaCl was sprayed for 5 min at 35 ° C,

- the other stage was dry without the specified spraying, lasting 30 minutes at a temperature of 35 ° C.

The number of cycles before failure was considered fatigue life in a corrosive environment.

Resistance against softening was determined by testing in the form of cyclic compression of cylindrical samples. The diameter of the samples was 7 mm, the height was 12 mm, and the samples were made of steel bars.

The preparation of samples for softening testing included roughing, austenization, oil quenching, tempering and final fine grinding. Prior to the test, the sample height was accurately measured using a comparator with an accuracy of 1 μm. A preload was used to modulate the prestress of the springs, while the pre-stress was a compression force of 2200 MPa.

Then a fatigue load cycle was carried out. This loading was 1270 ± 730 MPa. The decrease in the height of the sample was measured when conducting the number of cycles reaching 1 million. At the end of the test, the total softening was determined by comparing the accurately measured obtained height with the initial height, while the resistance to softening was the higher, the smaller the decrease in the initial height in percent.

The results of fatigue tests, as well as fatigue in a corrosive environment and softening of steels according to the invention and control steels are shown in table 4.

Table 4 Test results for fatigue, as well as fatigue in a corrosive environment and softening Hardness HRC Tensile strength, MPa Fatigue life, number of cycles Fatigue life in a corrosive environment, number of cycles Softening,% Steel 1 according to the invention 56.7 2129 1742967 192034 0,025 Steel 2 according to the invention 56.4 2106 > 2,000,000 138112 0.01 Steel 3 according to the invention 56.5 2118 > 2,000,000 135562 0.015 Steel 4 according to the invention 56.9 2148 > 2,000,000 202327 0,025 Steel 5 according to the invention 57.0 2156 > 2,000,000 139809 0,025 Control steel 1 56.7 2131 514200 96672 0,03 Control steel 2 53.8 1898 217815 241011 0.10 Reference steel 3 55.6 2062 301524 150875 0,075

From the above tables it follows that the control steels were unsatisfactory, in particular, for the following reasons.

In control steel 1, in particular, the sulfur content was too high to compromise between fatigue strength in air and fatigue strength in a corrosive environment. In addition, the manganese content in it was too large, which led to segregations, harmful to the uniformity of steel and fatigue strength in air.

The carbon and equivalent carbon content of control steel 2 was too low to achieve high hardness. Its tensile strength was too small to provide good fatigue strength in air.

In control steel 3, the content, in particular, of silicon was too low to achieve good resistance against softening and, therefore, good fatigue resistance in air.

The resistance to softening turned out to be higher in steels according to the invention than in control steels, as shown in Fig. 1, which shows that according to the above measurements of softening, its performance is less than at least 32% in the worst steel according to the invention (steel 1 according to the invention) than in the best control steel (control steel 1).

The fatigue life of the steels according to the invention in air is significantly higher than the fatigue life of control steels. This is explained by an increase in hardness, as shown in FIG. 2. However, an increase in hardness is not sufficient. Indeed, as a rule, steel with high strength is more susceptible to defects, such as inclusions and surface defects, the higher their hardness. Consequently, the steels according to the invention are less susceptible to defects, in particular to large inclusions, such as titanium nitrides or carbonitrides, especially since the formation of inclusions of too large a size is eliminated in the steels according to the invention. As can be seen from table 3, the largest inclusions found in steels according to the invention did not exceed 14.1 μm, while in control steel 2 contained inclusions larger than 20 μm. In addition, the minimum sensitivity to surface defects that can form during the manufacturing process of the spring or during other operations in which the steels according to the invention are used can be demonstrated by impact tests performed on steels according to the invention and on heat-treated control steels characterized by hardness 55 HRC or more, see figure 3. The values measured during Charpy impact testing of the steels according to the invention (in this case, the notch on the sample simulated the stress concentration similar to other stress concentrations arising in the places of surface defects formed during the manufacture of the spring and during other operations) exceeded the same values measured on control steels. This shows that the steels according to the invention are less sensitive to stress concentrations at the places of defects than control steels known from the prior art.

It is known that fatigue strength in a corrosive environment decreases with increasing hardness. Therefore, the steels according to the invention have the advantage that their fatigue strength in a corrosive environment exceeds the fatigue strength of reference steels from the prior art, in particular at hardness above 55 HRC, as can be seen in Fig.4.

Thus, the invention allows to obtain higher hardness with a positive compromise between the fatigue life in air and the resistance to softening, which have increased dramatically, and the fatigue life in a corrosive environment, which exceeds the durability of control steels known from the prior art. In addition, a lower sensitivity to probable surface defects, in particular to defects formed during the manufacture of the spring or during other operations, has also been achieved.

Claims (5)

1. Spring steel with increased fatigue resistance in air and in
corrosive and highly cyclic resistant
softening of the following composition, wt.%:
FROM 0.45-0.70 Si 1.65-2.50 Mn 0.20-0.75 Cr 0.60-2 Ni 0.15-1 Mo tracks - 1.0 V 0.003-0.8 Cu 0.10-1 Ti 0.020 - 0.2 Nb traces - 0.2 Al 0.002 - 0.050 R footprints - 0.015 S footprints - 0.015 ABOUT traces - 0.0020 N 0.0020-0.0110,
The rest is iron and impurities associated with the smelting, while the content of equivalent Ceq carbon in steel, calculated by the formula:
Ceq% = [C%] + 0.12 [Si%] + 0.17 [Mn%] - 0.1 [Ni%] + 0.13 [Cr%] - 0.24 [V%],
ranges from 0.80 to 1.00%, the hardness of the steel after quenching and tempering exceeds or equal to 55 HRC and the maximum size of titanium nitrides or carbonitrides at a depth of 1.5 ± 0.5 mm from the surface of a bar, wire rod, or billet, or spring a cross-section with an area of 100 mm ² is 20 μm or less, said size being the square root of the surface area of said inclusions whose shape is taken as square. 2. Spring steel according to claim 1, characterized in that it has the following composition, wt.%:
FROM 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 R tracks - 0.010 S tracks - 0.010 ABOUT traces - 0.0020 N 0.0020-0.0110,
The rest is iron and impurities associated with the smelting. 3. A method of manufacturing a spring from spring steel with increased fatigue strength in air and in a corrosive environment and with high resistance against cyclic softening, including smelting liquid steel in a converter or electric furnace, adjusting its composition, casting to produce blooms, or continuous casting to produce ingots or casing, cooling to ambient temperature, rolling to produce rods, wire rods or billets and the manufacture of springs, characterized in that the steel is steel according to any one of claims 1 and 2, blooms, pimps or ingots during their hardening or after it are subjected to cooling at an average rate of 0.3 ° C / s in the range of 1450-1300 ° C, these blooms, pimps or ingots are rolled at a temperature of 1200-800 ° C for one or two heating and rolling cycles, the bars, wire rod or billets or austenitic springs made from them are subjected to temperature ranges of 850-1000 ° C followed by quenching in water, polymer or oil and tempering at 300-550 ° C to impart steel hardness 55 HRC or more. 4. A spring, characterized in that it is made of steel according to any one of claims 1 and 2. 5. The spring according to claim 4, characterized in that it is obtained by the method according to claim 3.

Images