BR0214816B1 - austenitic steel possibly hardening or quenching with precipitation. - Google Patents

Abstract

The present invention relates to a stainless steel alloy, more precisely a highstrength stainless, precipitation hardenable, austenitic, stainless alloy, containing a well adjusted amount of aluminium and a high silicon content and which has the following composition (in weight-%): C 0-0.07 Si 0.5-3.0 N 0-0.1 Cr 15.0-20.0 Ni 7.0-12.0 Al 0.25-1.5 Cu 0<Cu<4.0 Mn 0-3.0 Mo 0-2.0 Ti 0-1.0 and the balance Fe together with normally occurring impurities and additives and a product that is reduced by cold working, especially drawing, without intermediate heat treatment, the strength of which increases by final heat treatment at 300° C. to 500° C. by not less than 14%, that shows a M<SUB>d30</SUB>-value of between -55 and -100, a loss of force that is smaller than 3.0% at 1 N during 24 h and which is very suitable for use in spring applications, such as springs of round wire and strip steel and in medical applications, such as surgical and dental instruments.

Description

"possible hardening austenitic steel emerges with precipitation".

Technical area

The present invention relates to an austenitic stainless steel alloy, more precisely to a high strength austenitic stainless steel alloy, possible to harden or to precipitate with precipitation, containing a well balanced aluminum content and a high silica content, and also to a a product which is reduced by cold processing, especially stretching, without intermediate heat treatment, the strength of which increases by the final heat treatment at a temperature of 300 °: -500 ° C by not less than 14%, showing a value of Md30 between - 55 and -100, a strength loss that is less than 3% at 1400 N for 24 hours and is well suited for use in spring applications such as round and steel strip springs and medical applications. such as surgical and dental instruments.

Background of the Invention

In the stainless steel spring market, austenitic stainless steel for AISI 302 cold-processed springs are dominant. This is based on a combination of relatively satisfactory corrosion resistance and a possibility of cold processing of the material with considerable resistance, which is a prerequisite for satisfactory spring material. Based on the state of cold processing, mechanical properties can be further increased by simple heat treatment. The AISI 631 type steel is alloyed with aluminum in order to intensively increase the resistance to heat treatment. During cold processing, a transformation occurs from the main constituent of the annealed austenite structure to deformation of the martensite, which is harder than the phase from which the same is formed. This rapid stage of

Hardening / deformation simultaneously decreases the material's friction and for this reason soft annealing has to be performed in one or several steps in the production chain. This makes the production process more expensive as well as increases the risk of introducing surface effects into the material. For type AI3I6 31 steel, the addition of aluminum makes the material able to form ferrite in the structure during solidification after casting. The resulting deaustenite / ferrite structure and relatively low alloy content thus provide a rapid stage of hardening / deformation, meaning that only moderate reductions are possible in order to avoid crack formation during the production process. Alternatively, types AISI 304 and AISI 316 are used with spring steels. These steels are more highly alloyed and have a lower carbon content than AiSI 302 and EISI 6 31 type steels. This requires that a higher deduction velocity be allowed on this type of steel. The advantage of these steels is that the properties of the resulting producer, which are essential for a good working of the shell, are often worse than those of AISI 302 and AISI 631 grades. An example of such a property is the resistance to rest, which describes the capacity of a spring. in retaining the strength of the meda over time.

The Fater.te US-A-6. 10 6,339 describes a Cr-Ni-Cu steel which can be strongly reduced between annealing. For example, a force of 18 56 MPa is indicated at a reduction of ε = 3.41 (5.5 to 1 mm). This is compared to a specified force according to the 20 50MPa standard. In accordance with US-A-6. 106. 139, heat treatment should be performed to allow the alloy to obtain strength values in accordance with that standard. Aliga according to Fatenoe US-A-6,106,139 contains a cover as a resistance enhancing element under heat treatment.

In US-A-6,048,416, a Cr-Ni-Cu steel is designed to make vehicle tires stronger in the form of a high strength steel wire. In order to achieve the desired properties, the alloy according to US-A-6. 04 8. 416, must have a composition within a stability intervenale »expressed by a value called JM: JM = 551-462 χ f% N) - 9.2 χ% Si - 20 χ% Mn - 13.7 χ

Cr - 29 χ (% Ni +? Cu) - 18.5 χ% Mo),

which must be greater than -55 but less than -30. In accordance with the invention, the cumulative reduction rate as a logarithmic function (ε - 2 + ln (S0 / Sf)) is limited to a maximum of 4. Issc- corresponds to a max_ma reduction area in the wire stretch of 98%. of copper, the alloy according to US-A-6 Ci 4 S 4 1 (5, does not contain any hardening elements with precipitating occurrence.

Summary of the Invention

Therefore, it is an object of the present invention to provide a high strength, hardenable or precipitation hardening austenitic stainless steel alloy containing a well-balanced aluminum content and a high silica content, and also to a product which is reduced by cold processing. , especially drawing, without intermediate heat treatment, the strength of which is increased by the final heat treatment at a temperature of 300 ° C - 500 ° C not less than 14 'ofc [ug mostiTa]. a vsIor cIg GntirG —55 θ100, a strength loss that is less than 3% at 1400 N 24 hours and is well suited for use in spring applications such as round wire springs and strip steel and medical applications such as as surgical and dental instruments.

According to the present invention such objects are achieved by means of a high strength austenitic stainless steel alloy which can be hardened or tempered with precipitation which contains (in% by weight):

- C: more than 0 to 0.07;

- Si: 0.5-3.0;

- N:> 0-0.1;

Cr: 15.0-20.0;

- Ni: 7.0-12.0;

- Al: U, 25-1.5, - Cu: O <Cu <4.0;

- Mn:> 0-3.0;

- Mo:> 0-2.0;

- Ti:> 0-1.0;

Fe balance and commonly occurring impurities and additives.

Brief Description of the Drawings

Figure 1 shows the loss of strength of the material springs according to the invention after 24 hours compared to AISI 302 type steel and Mo load. 150725.

Figure 2 shows the ultimate tensile strength of the materials according to the invention compared to AISI 302 * (* - with intermediate heat treatment) and Mo load. 150725.

Figure 3 shows the ultimate tensile strength as a logarithmic function of the cumulative reduction velocity of the materials according to the invention compared to load No. 150725.

Figure 4 schematically shows a segment of a possible embodiment of an expansion ring in a lateral view.

Figure 5 shows in Figure 5a the ring seen from above. The ends are pressed contraryly between sipela force F; In Figure 5b the ring is shown, in a lateral view, with the ends being pressed against each other by force F, and in Figure 5c, there is shown an expansion ring part which constitutes a flat spring element and how it is influenced by force F.

Figure 6 shows different embodiments for strip-shaped asmolas.

Detailed Description of the Invention

The importance of alloying elements for the present alloy is described below:

Carbon (C):

Carbon has a high propensity to combine with chromium, which means the precipitation of chromium carbide, which looks close to crystal granules, skin, the volume of which is chromium free. Thus, in the presence of high carbon contents, the properties of iecorrosion of the material deteriorate, also arising from the occurrence of brittle properties, which mainly causes problems when the wire is modeled for springs. Therefore, the carbon content should be kept as low as possible, being greater than 0.0% by weight, but at most 0.07% by weight, preferably 0.05% by weight, more preferably, by weight. 0.035% by weight.

Silicon (Si):

Silicon has a deferred stabilizing effect, which implies that an excessively high silicon content produces a two-phase structure.Therefore, the Si content does not rise above 3.0% by weight ·. However, silicon is also favorable in that it contributes to a further increase in the heat treatment resistance of the cold processed product. Therefore, the silicon content should not be less than 0.5 wt%, and should vary in the ole range 0.5 to 5.0 wt%, preferably between 0.02 and 2.5%. by weight, more preferably from 0.5 to 1.5% by weight.

Nitrogen (H):

Nitrogen is an alloying element which together with aluminum forms undesirable brittle slag in the form of aluminum nitrides. In addition, nitrogen increases deformation / hardening in cold processing, which is disadvantageous for the present invention. Therefore, it is of utmost importance that the denitrogen content be kept as low as possible, with a maximum of 0.1 wt.%, Preferably of) 0 wt.%.

Chrome (Cr)

Chromium is a very important alloying element as regards the corrosion resistance of the material. This is due to the ability of chromium to form a passive layer of Cr2Oa on the surface of steel. In order for this passive layer to be formed, the chromium ionizer must exceed approximately 12% by weight, and corrosion resistance increases with the chromoaddiction content. Another disadvantage of chromium is that the austenitic structure of the material is stabilized against the transition to martensite in cold processing. However, it is a ferrite stabilizer and therefore its content should not be too high. Thus, in the alloy according to the present invention, the chromium content should not be less than 15 wt% and more than 20.0 wt% and should preferably be in the range of 16.0 to 19.0 wt%. Nickel (Ni):

Nickel is an Iiqa forming element which when sufficient in quantity ensures that the material has an austenitic structure at room temperature.

In addition, ductility is enhanced with an increased nickel dowel. However, nickel is a high cost alloy forming element, and yet high nickel contents provide slow deformation / hardening which in turn provides difficulties in obtaining sufficient strength. Therefore, the nickel content should be within the range of 7.0 to 12.0% by weight, preferably between 8.0 and 11.0% by weight, more preferably within the range of 9.0 to 10, 0% by weight.

Aluminum (Al):

Aluminum is the main alloying element of the present invention. Aluminum is added as a precipitating hardening element to increase strength, which in turn influences the resistance to rest. During cold-tempering / hardening at 1-50-500 ° C cold-formed wire precipitations are formed in the β-NiAl form, which improves the mechanical properties of materials probably unknown to date. This effect is of great importance when wire is to be used in springs, whose resistance to rest has to meet very severe requirements. A disadvantage of aluminum is that it is a ferrite stabilizer, whereby the aluminum impeller must be limited to a maximum of 1.5% by weight. However, in view of the above, the aluminum content should be at least 0.25 wt%, preferably in the range of 0.4-1.0 wt%. Copper (Cu):

Copper is an alloying element that has two important properties. The first is that copper is an austenite stabilizing element and the second is that crust decreases material deformation / hardening and provides improved ductility. Since the material has to withstand extreme reductions without the occurrence of intermediate annealing, the coating content must be as high as possible. However, with a high copper content, the risk of unwanted precipitation increases, which decreases the ductility of the material. Therefore, the copper content should be in the range of 0 <Cu <4.0% by weight, preferably between 2.0 and 3.5% by weight, more preferably between 2.4 and 3.0? by weight

Manganese (Mn):

Manganese has a similar effect to nickel, both in relation to the formation of austenite, as to its stabilization against the demarcensite transformation in cold processing. However, manganese increases deformation / hardening, unlike nickel. This results in a faster deformation / hardening action and decreases the highest possible reduction speed between annealing. Therefore, the manganese content should be greater than 0.0% by weight but limited to a maximum of 3.0% by weight, preferably to a maximum of; 1.0% by weight.

Molybdenum (Mc ·): Molybdenum is a ferrite stabilizing element that has a highly favorable effect on corrosion resistance in environments where chlorides are present. Established Formulas for Equivalent Corrosion Resistance (PRE) - Pitting Resistance Equivalent - give molybdenum a factor of approximately 3 compared to the effect of chromium. However, urr. High molybdenum content stabilizes the ferrite phase in steel. In addition, there is a risk of precipitation of the internet phase, such as the sigma phase. Therefore, the molybdenum content should be greater than 0.0% by weight but limited to more than 2.0% by weight.

Titanium (Ti):

Titanium, like aluminum, is a precipitation-hardening element that is added in order to increase strength, which in turn influences resting resistance. In addition, titanium together with silicon provides a marked heat treatment effect, even with low levels of detitanium. However, the titanium content should be greater than 0.0% by weight, but limited to 1.0% by weight, preferably to a maximum of 0.75% by weight.

Description »of Test Procedure

The test materials were produced by melting in a high frequency furnace. Then all the test plots were fully ground before being forged. The ornament was made in a stock ingot, 103 χ 103 mm long. The heating temperature was in the range between 1240 and 1260 ° C. Maintenance time at maximum temperature was 1 hour. Subsequent treatment of the comparison material (white), such materials were fully ground and ultrasonically tested.

A wire bar of 5.50 mm - 5.60 mm diameter was produced by heating the comparison material to a temperature of 1200 ° C-1240 ° C, after which these materials were laminated to a final dimension and then cooled rapidly. with water. The hot rolled wires were then cold processed by stretching on a conventional drawing machine.

Chemical compositions by weight% of the alloys in the test program and reference materials are given in Table 1.

Table 1 - Chemical Composition (in% by weight)

<table> table see original document page 12 </column> </row> <table>

The strength of the alloys in the processed state and after heat treatment in the axial tensile test can be observed in Table 2, where the ultimate tensile strength corresponds to a maximum load value in the load elongation diagram. All the alloys were reduced by a degree of cumulative reduction in bjgarhythmic function of ε = 3.95 (corresponding to an area reduction of 981;), without intermediate annealing. AISI 302 steel could not be cold processed to ε = 3.95 without cracking because an annealing operation would have to be performed before drawing to the finished dimension. However, all alloys have the same wire diameter.

The heat treatment was obtained with the same purpose as that of spring steel type AISI 302, when an increase in mechanical properties was obtained. Thus, several important spring properties, such as resistance to rest, are influenced to a considerable extent in relation to the hitherto known.

Table 2 - Final Tensile Strength before and after

Heat treatment

<table> table see original document page 13 </column> </row> <table>

* Heat treatment time = 1.5 hours; Thermal breakdown temperature = 350 ° C;

Heat treatment time = 1.0 hour; Heat Treatment Temperature = 480 ° C

For the evaluation of the resistance to rest, cylindrical helical springs were produced that do not present aligned spirals. Test results are observed in Table 3.

Table 3 - Spring Dimensions

<table> table see original document page 14 </column> </row> <table>

Spring force (F) and total spring suspension (ft) are diti determined at room temperature by a force versus load curve. Then the damola constant (c) and the shear modulus (G) were calculated using equations 1 and 2.

Equation 1: C = (F * Nv) / ft;

Equation 2: G = (8 + F * NV * D3m) / (ft * D4t).

The rest test was obtained by recharging the annealed springs with a constant load. The charge was read every minute for the first five minutes and then the number of readings was eliminated. Each test was interrupted after 24 hours. Springs from the corresponding loads were initially charged at four different levels. Rest level was calculated using equation 3 and the results are summarized in Figure 1. Equation 3: R = ((F1-F2) / F1) * 10 0 where:

F; = rest level;

F1 = initial load;

F2 = load at a given time.

In Figure 1 it is observed that the alloy which has a very low aluminum content, that is, the load of no.150725, has a considerably higher resting level than the alloys of the test program, which all have aluminum as an alloy. alloy forming element. In addition, all alloys in the detest program have an equivalent or better resting strength level than AISI 302 alloy steel.

The Md30 index according to Nohara shows the temperature at which at a cold reduction rate of 30%, a 50% percentage of austenite in steel is transformed to martensite transformation. A higher temperature value indicates that the structure is more stable (more willing to form martensite) and provides a higher speed of cold deformation in the steel.

The value of Md30 according to Nohara is calculated by the following formula:

Md30 / Nohara = 551 - 462 χ (C + N) - 9.2 χ Si - 8.1 χ Mn -13.7 x Cr - 2 0 χ (Ni + Cu) - 18.5 χ Mo - 68 χ Nb - 1,4 2 χ (ASTM - 8 grain size).

Table 4 shows the results for the test loads 1 to 7. It has been surprisingly shown that steel as a composition according to the present invention has the best heat treatment effect at Md30 between -55 and -100 ° C and the largest increase in strength. ultimate tensile strength only after cold processing, without intermediate heat treatment.

Table 4 - Md3o / Nohara

<table> table see original document page 16 </column> </row> <table>

Description of Preferred Modalities

In the following, some embodiments of the invention will be described for the purpose of illustrating the invention without limitation thereof.

Steel according to the present invention is subjected to strong cold deformation. The steel can be shaped into different cross-sectional geometries, eg round wire, oval, profiles of different cross sections, eg rectangular, triangular or more complicated shapes and geometries. The round wire can be rolled flat.

Example 1: Round Wire Springs

As described above, the wire springs made from the alloy according to the present invention are wound. These springs have satisfactory resting spring properties, ie long-term spring force retention, and are advantageously used in typical spring applications, such as, for example, springs in locking applications, ie mechanical parts in the device. locking springs, aerosol container springs, pens, in particular, ballpoint pens, pump springs, industrial loom springs, vehicle industry springs, electronic device industry springs, thin mechanical computers and devices.

Example 2: Steel Strip Springs

For flat torsion springs, torque is a decisive amount. 0 torque can be expressed as:

<formula> formula see original document page 17 </formula> where:

M = spring torque;

I = bending moment of inertia (b * tJ / 12);

B = width of spring strip;

T = spring strip thickness;

L = extended spring length;

n0 = number of coils in free spring (not assembled) η = number of processing coils.

In order to increase the torque in a given spring measurement, an effect called reverse winding can be obtained. In an effect called "elastic" winding, the spring is previously shaped by being wound in a direction opposite to the processing direction. Thereafter, a heat treatment of the spring occurs, after which it is wound in the opposite direction to the spring housing. In the so-called "transverse curve" winding, the strip is modeled on a cable, after which heat treatment occurs. The spring is then wound in the opposite direction of the spring housing. By this procedure a lower and sometimes even negative value for nD can be obtained compared to a single mode coiled spring (see Figure 6). Due to the quite satisfactory increase of the resistance under heat treatment, the lightness according to the present invention is very suitable for use as a material in the production of torsion springs, where a high torque and a satisfactory rest resistance is required.

Example 3: Expander Wire

An expander wire is a piece of wire that is corrugated and shaped like a flat spring connected in series. The spring is used, for example, to regulate the pressure of oil segment rings against the cylinder wall in an internal combustion engine. A typical expansion of car engines is seen as corrugated wire between two piston rings. One possible embodiment of corrugated lug is shown schematically in Figure 4.

A drawback of today's warm-powered vehicles is the energy-saving grate that; necessary in order to provide the vehicle with the desired performance. The easiest way to achieve low power consumption, among other ways, is to reduce the internal friction of the transmission and reduce the total mass of the vehicle. The piston core contributes more than half the friction of an engine. Therefore, it is a continuous objective to improve the material and appreciation of the rings, pistons and cylinder walls, in order to reduce the tare weights and pressure of the bearings. The expander is the spring that regulates the pressure of the oil segment rings against the cylinder wall, thus also the oil consumption and part of the internal friction of an engine. The load of the expanding wire consists of the force F as shown in figures 5a to 5c.

90 ° to the rear with a maximum load, the following ratios apply:

For a flat spring when the load is applied on

<formula> formula see original document page 19 </formula>

(3) the combination of (1) and (2) provides:

<formula> formula see original document page 19 </formula>

on what:

<formula> formula see original document page 19 </formula> = maximum permissible load at the rear of the spring F = loading force which is determined by the length of the expander wire relative to the piston diameter;

T = wire thickness;

B = wire width;

E = modulus of elasticity of wire material;

s = suspension path, how much of the expander is deformed;

R = radius of curvature in each spring element.

Expression 3 shows that the wire thickness required for a given property depends on the expander model. If the allowable stress of the material is increased, a smaller bend radius may be allowed, which is of great interest as smaller type rings may be manufactured. The possibility of manufacturing smaller rings becomes more and more important as the demand for smaller motors increases in the face of increasing environmental demands.

Another way to look at the benefits of increased resistance in the expansion ring is to make an energy consideration as follows:

<formula> formula see original document page 20 </formula>

on what:

A = elastic energy;

K = constant of use material;

E = modulus of elasticity;

V = effective spring volume (how much spring material is working);

σ = applied stress. Expression (4) shows that a certain elastic energy for a given modulus of elasticity is a function of the specific volume, material use and maximum allowable stress. An increase in the maximum allowable tension generally increases the material usage constant, which in combination provides a greater impact on the specific volume required. Therefore, it is possible to decrease the material volume from the permissible increased tension to the retained elastic energy level.

To produce an expansion ring in its complex form, it is possible only by using m = cios materials. Manageability is, above all, the main reason for stainless steel to be used. However, for expander operation, the yield strength limit and ultimate tensile strength are at least as important as in all measurement applications. This has been a difficult state of contradiction to administer. By utilizing the steel according to the invention, the material can be shaped into a relatively meltable state so that it is further heat treated in the finished form, after which the desired demolishing properties are obtained by hardening with the occurrence of precipitation.

Sxemplip 4: Flat Wire

This embodiment, according to the present invention, is especially used in applications that make high demands on the relief or rest properties of steel, as it must withstand a certain force without being previously modeled. This makes steel especially suitable for use, such as windshield wiper blade, where a satisfactory punching capacity of the split material should be combined with a satisfactory resting resistance of the finished product.

Example 5: Round and flat wire as well as strip steel for medical applications

The wire made of the alloy according to the present invention may further be used in medical applications, for example, in the form of dental instruments such as files, for example dental root canal and treatment files, esimilar nerve pullers, as well as surgical needles. . The laminated planar wire produced from the steel according to the invention can advantageously be used for the production of dental and surgical instruments.

All of these applications have in common the fact that they have complicated geometries, which are produced by grinding, bending and / or twisting, advantageously at a stage prior to the last heat treatment, and then obtaining a marked increase in mechanical properties, ie a high breaking strength, combined with satisfactory ductility.

Claims (11)

1. High strength austenitic stainless steel alloy, characterized in that it has the same hardening or hardening with precipitation, having the following composition (in% by weight): - C: from 0 to 0,07; 0.5-3.0 - N max 0.1 - Cr: 15.0-20.0 - Ni: 9.0-10.0 - Al: 0.2 5-1.5 - Cu: 2.4-3.0 - Mn max 1.0 - Mo max 2.0 - Ti max 1.0 - Fe balance and normally occurring impurities, the ditaliga being reducible by processing at cold. Altar-resistant austenitic stainless steel alloy according to claim 1, characterized in that it contains a chromium content of between 16.0 and 19.0% by weight. Altar-resistant austenitic stainless steel alloy according to claim 1 or 2, characterized in that it contains an aluminum content between -0.4 and 1.0% by weight. Altar-resistant austenitic stainless steel alloy according to any one of claims 1 to 3, characterized in that it contains a silicon content between 0.5 and 2.5% by weight. Altar-resistant austenitic stainless steel alloy according to any one of claims 1 to 4, characterized in that it contains a silicon content between 0.5 and 1.5% by weight. Altar-resistant austenitic stainless steel alloy according to any one of claims 1 to 5, characterized in that it has an Md30 value of between -55 and -100. Altar-resistant austenitic stainless steel alloy according to any one of claims 1 to 6, characterized in that it has a loss of strength that is less than 3.0% at 1400 N for 25 hours. 8 Altar-resistant austenitic stainless steel alloy according to any one of claims 1 to 7, characterized in that it is in the form of wires, profiles and / or strips. Method of producing a product of a alloy as described in any one of Claims 1 to 8, characterized in that it reduces the cold processing alloy at a reduction rate of more than 99% without intermediate heat treatment. Method according to claim 9, characterized in that the cold processing is stretching. Method according to claim 8 or 9, characterized in that the alloy is subjected to final heat treatment at 300 ° C to 500 ° C, whereby a resistance increase of at least 14% is effected.