SE526881C2 - Secretion curable austenitic alloy, use of the alloy and preparation of a product of the alloy - 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

20 25 30; - f_ "¿° ._: s :: - .. 2 deformation hardening which means that only moderate reductions are possible * t avoid cracking during the manufacturing process. As spring steel, alternatively steel of the type AlSl 304 and AlSl 316 are used. steels are higher alloyed and have a lower carbon content than steels of the type AlSl 302 and AlSl 631. This means that a higher degree of reduction can be allowed in this type of steel. worse than for AlSl 302 and AlSl 631 steels.An example of such a property is the relaxation resistance, which describes an fl spring's ability to maintain kraft force over time.

US-A-6,106,639 discloses a Cr-Ni-Cu steel which can be greatly reduced between annealing. In the example, a strength of 1856 MPa is stated at a reduction of s = 3.41 (5.5 to 1 mm). This is set against the specified strength according to the standard of 2050 MPa. A heat treatment must be performed according to US-A-6,106,639 for the alloy to achieve strength values according to this standard. The alloy according to US-A-6,106,639 contains copper as a strength-enhancing element in heat treatment. US-A-6,048,416 discloses a Cr-Ni-Cu steel intended for the reinforcement of vehicle tires in the form of high-strength wire. In order to achieve the desired properties, the alloy according to US-A-6,048,416, in composition must be within a stability range expressed in a so-called JM value (JM = 551462x (C% + N%) - 9.2xSi% -20xMn% -13.7xCr % -29x (Ni% + Cu%) - 18.5xMo%), which should be greater than -55 but less than -30. In the alloy according to the invention, the cumulative logarithmic (e = 2 * ln (S0 / Sf) degree of reduction is limited to a maximum of 4. This corresponds to a maximum area reduction in wire drawing of 98%.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a high strength, precipitation hardenable, austenitic, stainless steel alloy containing a well-balanced amount of aluminum and a high silicon content, a product which is 0 000 10 15 20 25 30 526 sar * vu an. 3 reduced by cold working, in particular drawing, without intermediate xpharm treatment, the strength of which is increased by final heat treatment at 300 ° C to 500 ° C by at least 14%, showing a power loss of less than 3.0% at 1400 N for 24 hours for use in capercaillie applications, such as round wire and strip steel springs and in medical applications, such as surgical and dental instruments.

These objects are achieved according to the present invention with a high strength, precipitation hardenable, austenitic stainless steel alloy containing (in% by weight): C O-0.07 Si 0.5-3.0 N 0-0.1 Cr 15.0-20 .0 Ni 7.0-12.0 AI 0.25-1.5 Cu 0 s Cu s 4.0 Mn 0-3.0 Mo 0-2.0 Ti 0-1.0 residue Fe and normally occurring impurities and additives.

Brief Description of the Figures Figure 1 shows the power loss of the springs after 24 hours for materials according to the invention compared to AlS1302 and charge No. 150725.

Figure 2 shows the yield strength of materials according to the invention compared with AlSl 302 * (* - with intermediate heat treatment) and charge no. 150725.

Figure 3 shows the yield strength as a logarithmic function of the cumulative degree of reduction for materials according to the invention compared to charge no. 150725.. n n n o c a. a n c n nu n n »avan- n 00000 10 15 20 25 30 526 881 få 'i -' Éï * - 4 Figure 4 schematically shows a segment of a possible design of an expander ring in a side view.

Figure 5 shows in Figure 5a the ring seen from above. The ends are pressed against each other with the force F, Figure 5b shows the ring seen from the side, the ends are pressed against each other with the force F and Figure 5c shows a part of the expander ring which forms a spring element and how this is affected by the force F.

Figure 6 shows different embodiments for band springs.

Detailed description of the invention The significance of the alloying elements for the present alloy is as follows: 591 (C) has a high tendency to combine with chromium, which means that chromium carbides precipitate in the crystal grain boundaries, whereby the surrounding matrix is depleted of chromium. At high carbon contents, the corrosion properties of the material thus deteriorate, there are also problems with embrittlement, which mainly causes problems when the wire is formed into springs. The carbon content should therefore be kept as low as possible, max. 0.07% by weight, preferably 0.025% by weight. _K_isgl (Si) has a ferrite stabilizing effect, which means that too high a silicon content gives a two-phase structure. The silicon content should therefore not exceed 3.0% by weight. However, silicon is also favorable in that it contributes to a greater increase in strength during heat treatment of the cold-worked product.

The silicon content should therefore not be less than 0.5% by weight and should be in the range 0.5 to 3.0% by weight, preferably between 0.5 and 2.5% by weight, preferably 0.5 to 1.5% by weight. %.

Nitrogen (N) is an alloying element which, together with aluminum, forms undesirable brittle slag in the form of aluminum nitrides. Nitrogen also increases the deformation hardening during cold working, which is to the disadvantage of the present unit 526 881 u n I. | p u n n - u ø a ø o ø ø ø ø oc 5 invention. It is therefore of the utmost importance that the nitrogen content is kept at as low a level as possible, 0.1. weight Vd, preferably 0.05 wt%. _K_ro_m (Cr) is a very important alloying element in terms of the corrosion resistance of the material. This is due to the ability of chromium to form a passive layer of Cr2O3 on the steel surface. In order for this passive layer to form, it is required that the chromium content exceeds about 12.0% by weight, in addition, the corrosion resistance is improved with increased chromium content. Another advantage of chromium is that the austenitic structure of the material is stabilized against conversion to maitensite during cold working. However, chromium is 10 ferrite stabilizing, so the content must not be too high. Therefore, in the alloy of the present invention, the chromium content should not be lower than 15.0% by weight and not higher than 20.0% by weight, preferably in the range 16.0 to 19.0% by weight. fl jçkej (Ni) is an alloying element which in sufficient quantity guarantees that the material has an austenitic structure at room temperature. In addition, ductility is improved with increased nickel content. However, nickel is an expensive alloying element and high levels lead to a slow deformation hardening, which in turn causes difficulties in achieving sufficient strength. The nickel content should therefore be in the range 7.0 to 12.0% by weight, preferably between 8.0 to 11.0% by weight, most preferably in the range 9.0 to 10.0% by weight.

Aluminum (Al) is a key alloying element in the present invention.

Aluminum was added as a precipitation hardening element to increase the strength, which in turn affects the relaxation resistance. Upon precipitation hardening at 350 DEG-50 DEG C. of 500 ° C of machined wire, deposits are formed in the form of β-NiAl, which increases the mechanical properties in contrast to hitherto known materials. This effect is of the greatest importance when the wire is to be used as springs whose relaxation resistance must meet very high requirements. A disadvantage of aluminum is that it is ferrite stabilizing, which is why the aluminum content is limited to a maximum of 1.5% by weight. In view of the above, however, the aluminum content should be at least 0.25% by weight and preferably in the range 0.4-1.0% by weight. 526 881 ~ r Q a n c n n o o n »o. 6 ßggg (Cu) is an alloying element that has two important properties. Firstly, copper is an austenite stabilizing element and secondly, copper lowers the deformation hardening of the material and leads to improved ductility.

Since the material must withstand extreme reductions without intermediate annealing, the copper content must be as high as possible. With increasing copper content, however, the risk of unwanted precipitations increases, which impairs the ductility of the material. Therefore, the copper content should be in the range 0 s Cu 2 4.0 wt%, preferably between 2.0 to 3.5 wt%, most preferably between 2.4 to 3.0 wt% />. Manganese (Mn) has a similar effect as nickel both in terms of forming austenite during solidification and in stabilizing it against martensite conversion during cold working. Manganese, on the other hand, increases deformation hardening, which nickel does not. This leads to a faster deformation hardening and reduces the maximum possible degree of reduction between the annealing. The manganese content should therefore be limited to a maximum of 3.0% by weight, preferably to 1.0% by weight.

Molybdenum (Mo) is a ferrite stabilizing element that has a strong beneficial effect on corrosion resistance in chloride environments. Established PRE (Pitting Resistance Equivalent) formulas assign molybdenum a factor ~ 3 compared with chromium effect. However, a high molybdenum content stabilizes the ferrite phase in steel.

In addition, the risk of precipitation of intermetallic phases, such as sigma phase, increases.

The molybdenum content is therefore limited upwards to 2.0% by weight.

Iitg (Ti), like aluminum, is a precipitation hardening element that is added to increase the strength, which in turn affects the relaxation resistance. In addition, titanium together with silicon gives a strong heat treatment effect already at Ijz; low levels of titanium. However, titanium is a strong ferrite stabilizer, so the content must not be too high. The titanium content is therefore limited to 1.0% by weight, preferably a maximum of 0.75% by weight. j :: í_ so 526 881 2 "u | cuna | ooa - o ø au nu Description of the test procedure The test materials were manufactured by melting in a high frequency oven. Then all test ingots were completely ground before forging. was in the range between 1240 ° C and 1260 ° C. The holding time at full temperature was 1 hour.

The rolling wire in the dimension range ø 5.50 mm - ø 5.60 mm was manufactured by heating the blanks to 1200 ° C - 1240 ° C, after which they were rolled to a finished dimension and then cooled by water quenching. The hot rolled yarns were then cold worked by drawing in a conventional drawing machine.

The chemical composition, weight%, of the alloys in the experimental program and reference materials are found in Table 1.

Table 1. Chemical composition (in% by weight) 1 2 3 4 0 Aisi 302 150725 c 0.023 0.021 0.023 0.027 0.033 g 0.12 0.011 sr 0.90 1.40 1.37 0.59 0.90 g 2.0 0.51 N 0.021 0.020 0.019 0.010 0.020 g 0.1 0.012 cr 10.45 10.35 10.42 10.40 10.73 2 10.0 17.44 g 19.0 No. 9.03 9.01 9, 73 9.02 9.02 2 0.0 9.40 g 9.5 Ai 0.42 0.93 0.01 0.03 0.44 ----------- __ <0.003 * Iïf ou 2.99 2.97 2.90 3.00 2.40 ----------- __ 3.02 Mn 0.00 0.93 0.73 0.70 0.93 g 2, 0 0.00 M0 <0.01 <0.01 <0.01 <0.01 <0.01 g 0.00 0.10 10 15 I binic 20 526 881 8 The strength of the alloys in the cold-worked state and after heat treatment in single-axis tensile test is shown in Table _, where the breaking limit corresponds to the maximum value of the load in the force strain diagram. All alloys have been reduced to a logarithmic cumulative degree of reduction of s = 3.95 (corresponding to 98% area reduction) without intermediate annealing. AlSl 302 could not be cold worked to s = 3.95 without cracking, so an annealing operation must be performed before drawing to the finished dimension. However, all alloys have the same wire diameter.

The heat treatment was carried out for the same purpose as for stainless steel of the AlSl 302 type, when an increase in the mechanical properties is obtained. This affects your important spring properties, such as the resistance to relaxation, but in a stronger way than hitherto known.

Table 2. Fracture limit before and after the heat treatment.

Charge no. Breaking limit after Breaking limit after Heat treatment - cold processing Heat treatment effect [MPa] [MPa] [%] 1 2014 2298 * 14.1 2 2132 2496 * 17.1 3 2136 2442 * 14.3 4 1942 2502 * 28.8 5 2162 2482 ” 14.8 AlSl 302 2140 2370 * 10.7 150725 1760 1953 * 11.0 * Heat treatment time = 1.5 h, Heat treatment temperature = 350 ° C ** Heat treatment time = 1.0 h, Heat treatment temperature = 480 ° C For evaluation of the relaxation resistance was manufactured fi springs of the cylindrical screw type fl springs with non-closed turns. The experimental results are shown in Table 3. 10 15 20 25 30 526 881 et seq .; Q nnu ~ Q ona - ø ooo aa Table 3. Spring spring Wire diameter (Di) 0.762 mm Inner diameter of the springs 6.84 mm Average diameter of the springs (Dm) 7.6 mm Slope 1.52 mm Number of turns (NV) 50.5 Spring force ( F) and the total suspension (fi) was determined at room temperature using a force versus elongation curve. Then, the spring 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 * Df,) / (ft * Df) The relaxation test was performed by loading tempered springs with a constant elongation. The load was read every minute for the first five minutes, then the number of readings decreased. Each sample was stopped after one day. Springs from each charge were initially loaded at four different levels.

The relaxation was calculated using Equation 3 and the results were compiled in Figure 1.

Equation 3. R = ((F1-F2) / F1) * 100 where R = Relaxation F1 = Initial load F; = The load at a given time Figure 1 shows that the alloy with a very low aluminum content, ie charge no. 150725, relaxes much more strongly than the alloys in the experimental program, all of which have aluminum as an active alloying element. In addition, all alloys in the 526 sei 10 test program have equivalent or better relaxation resistance than Al-S! 302.

Description of Preferred Embodiments In the following, some embodiments of the invention are described. These are intended to illustrate the invention, but not to limit it.

The steel according to the present invention is subjected to a strong cold deformation. It can be formed into different cross-sectional geometries, such as round, oval wire, profiles of different cross-sections such as rectangular, triangular or more complicated designs and geometries. Round wire can also be flat rolled.

Example 1: Springs of round wire As described above, springs of wire made of the alloy according to the invention are wound. These springs exhibit good spring properties in the form of relaxation, ie the maintenance of spring force for a long period of time and are advantageously used in typical spring applications, such as e.g. springs in locking applications, ie. Mechanical parts within the locking device, springs in aerosol packs, pens, in particular ballpoint pens, pump springs, springs for industrial looms, springs for the automotive industry, electronics, data and precision mechanics.

Example 2: Strip steel springs 25 - For flat torsion springs, the torque is a decisive quantity. The torque can be expressed as **** _ å. MZEIZÉM no): ...: 39 000 I 0 c 00 10 15 20 25 30 526 881 oocnaaunano Q .n 11 Where: lvl = capercaillie xfridmcment I = bending moment of inertia (b * t ^ 3/12) B = spring band width T = capercaillie band thickness L = spread fi spring length no = number of revolutions at free spring (unassembled) n = number of working revolutions To increase the torque at a given spring geometry, so-called reverse winding can be performed. In so-called "resilient" winding, the spring is preformed by being wound in the opposite direction to the working direction. Thereafter, a heat treatment of the spring takes place, after which it is wound in the opposite direction in the spring housing. In so-called "cross curve" winding, the strip is formed over a pin, after which heat treatment takes place.

Then the spring is wound in the reverse direction into the capercaillie housing. By this method a lower and sometimes even a negative value of no can be obtained in comparison with a single wound spring, see Figure 6. Due to the very good increase in strength during heat treatment, the alloy according to the present invention is very well suited for use with torsion springs. , where high torque and good relaxation durability are required.

Example 3: Expander wire An expander is a piece of wire that is folded and shaped into a series-shaped spring. This spring is used, for example, to regulate the pressure of the oil scrapers against the cylinder path in an internal combustion engine. A typical expander for passenger car engines is seen as the pleated wire between two piston rings. A possible embodiment of such a pleated ring is shown schematically in Figure 4.

A disadvantage of motor vehicles today is the large energy consumption required to give the vehicle its desired performance. The simplest ways to achieve a reduced energy consumption are, among other things, to reduce the internal friction of the drive and to reduce the total mass of the vehicle. The piston package accounts for more 526 881 n> a u o n ~ ~ o a. o o o o now 12 than half of an engine's friction. It is therefore an ongoing effort to improve the material and precision of the rings, pistons and cylinder walls in order to reduce dead weights and abutment pressures. The expander is the spring that regulates the pressure of the oil scrapers against the inner wall and thus also oil consumption and part of an engine's internal friction. The load of the expander wire consists of the force F, as shown in Figures 5a to 5c.

For a shape där spring where the load is applied at a 90 ° angle to the maximum loaded back, the following relationship applies: if, Permissible maximum load in the back of the spring F the loading force which is determined by the length of the expander wire in relation to the piston diameter Thread width , how much the expander is deformed FUWfTIIIJ-I radius of curvature in each capercaillie element 6FR o '= - m ”BTZ 42R3F s: Eßrt (3) the combination of (1) and (2) gives: (1) (2) _ 4218F _ 6FR 3 T _ vzazam EsT3 o 'Tz Es m fl X B Expressions (3) show that the wire thickness required for a given property depends on the design of the expander. If the permissible tension of the material is increased, a smaller radius of curvature can be allowed, which is of great interest as rings of smaller models can be manufactured. The possibility of being able to manufacture smaller rings is becoming increasingly important, as the demand for small motors increases as environmental requirements are tightened.

Another way to see the benefit of a higher strength in the expander ring is by making an energy observation according to the reasoning below.

A Elastic energy K Material utilization constant E E module V The effective volume of the spring (how much of the spring material works) cr Applied voltage 0.2 (4) A = VK -É- Expression (4) shows that a certain elastic energy for a given E module is a function of the power volume, material utilization and the maximum allowable voltage. An increased maximum permissible voltage usually increases the material utilization constant, which in combination gives a large effect on the necessary effective volume. It is thus possible to reduce the volume of material by increasing the permissible voltage for maintaining the level of elastic energy.

Shaping an expander ring into its complex shape is only possible with soft materials. Formability is the primary reason why stainless steel is used at all. For the function of the expander, however, the yield strength and yield strength are at least as important as in all fi spring applications. This has previously been an unmanageable opposition. By using the steel according to the invention, the material can be formed in a relatively soft state and then heat-treated in a finished form, whereupon the desired capercaillie properties are obtained by precipitation hardening. o n ~ u n o. Q u o a. o a ø o oo Dacia 10 15 20 25 30 526 881 o o o c o c o u n o o n Q o ø o c oo 14 Example 4: Flat wire This embodiment according to the present invention is used especially in applications that place high demands on the steel's relaxation properties, as it must withstand a force without being deformed. This makes the steel particularly suitable for use as e.g. wire for windscreen wipers, where good punchability of the starting material must be combined with a good relaxation resistance of the finished product.

Example 5: Round and flat wire and strip steel for medical applications Wire made from the alloy according to the invention can also be used in medical applications, such as in the form of dental instruments such as files, such as root canals, nerve extractors and the like, and surgical needles. Flat-rolled wire of the steel according to the invention can be advantageously used for the manufacture of dental and surgical instruments.

All these applications have in common that they have complicated geometries, which are manufactured by grinding, bending, and / or twisting advantageously before the last heat treatment and which then have a sharp increase in the mechanical properties, ie a high breaking strength in combination with good ductility.

Claims (1)

526 ss1 f: Patent claims High-strength austenitic stainless steel alloy, characterized in that it is precipitation-curable and has the following composition (in% by weight): C 0-0.07 Si 0.5-3.0 N 0-0 , 1 Cr 15.0-20.0 Ni 8.0-11.0 Al 0.25-1.5 Cu 2.4-3.0 Mn 0-1.0 Mo 0-2.0 Ti 0-1 , 0 and the rest Fe together with normally occurring impurities and additives. High-strength austenitic stainless steel precipitation-hardenable alloy according to Claim 1, characterized in that it contains nickel in a content of between 9.0 and 10.0% by weight. High-strength austenitic stainless steel precipitation-curable alloy according to Claim 1, characterized in that it contains chromium in a content of between 16.0 and 19.0% by weight. High-strength austenitic stainless steel precipitation-hardenable alloy according to Claim 1, characterized in that it contains aluminum in a content of 0.4 to 1.0% by weight. 10. 11. 12. 526 881 / e High-strength austenitic stainless steel precipitation curable alloy according to claim 1, characterized in; that it contains silicon in a content of 0.5 to 2.5% by weight. High-strength austenitic stainless steel precipitation-curable alloy according to claim 1, characterized in that it contains silicon in a content of 0.5 to 1.5% by weight. Use of an alloy with a composition according to any one of the preceding claims as a product in the form of wire, threads and / or ribbons. Use of an Alloy with a composition requirements 1-6 in fi spring applications, such as fi round wire and strip steel springs. Use of an Alloy with a Composition Claims 1-6 in medical applications, such as surgical and dental instruments. Production of a product from the alloy according to any one of claims 1-6 is characterized in that the product is reduced by cold working, especially by drawing, without intermediate heat treatment. The production of a product of the alloy according to claim 10 is characterized in that it can be reduced by cold working by more than 99.0 percentage points without intermediate heat treatment. Production of a product of the alloy according to claim 10 or 11, characterized in that it is subjected to a final heat treatment at 300 ° C to 500 ° C, the strength increasing by at least 14%.