KR100966068B1 - Precipitation hardenable austenitic steel - 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

Precipitation hardening austenitic steel {PRECIPITATION HARDENABLE AUSTENITIC STEEL}

The present invention provides an austenitic stainless steel alloy, more precisely a hardened precipitation hardening austenitic stainless steel alloy having a well controlled aluminum content and a high silicon content, and reduced cross section by cold working, in particular drawing, without intermediate heat treatment. Its strength increases by more than 14% by the final heat treatment at 300 ° C. to 500 ° C., showing an M _d30 − value of −55 to 100 and a loss of force less than 3.0% at 1400 N for 24 hours. It relates to spring applications such as springs of wire and strip steel and products that are well suited for medical applications such as surgical and dental instruments.

In the market of stainless spring steel, AISI 302 type cold worked austenitic stainless spring steel is mainstream. This is because it has relatively good corrosion resistance and can be cold worked to the high strength required as an excellent spring material. In the cold worked state, the mechanical properties can be further increased by simple heat treatment. Aluminum is alloyed with AISI 631 type steel to further increase strength during heat treatment. During cold working, a phase shift occurs with strained martensite in which the main component of the austenite freed tissue is harder than the phase before formation. This rapid strain hardening simultaneously reduces the ductility of the material, which requires softening annealing in one or several steps in the manufacturing process. This makes the manufacturing process more expensive and increases the risk of surface defects in the material. For AISI 631 type steel, the addition of aluminum tends to cause the material to form ferrite in the structure upon solidification after casting. Due to the austenitic-ferrite structure formed and the relatively low alloy content, rapid strain hardening is achieved, so that only moderate cross-sectional reduction is possible to avoid cracking during the manufacturing process. Alternatively, AISI 304 and AISI 316 type steels are used as spring steel. These steels are more alloyed and have less carbon content than AISI 302 and AISI 631 types of steel. Thus, larger cross-sectional reduction rates are possible with this type of steel. The disadvantage of this steel is that the product properties necessary for good spring function are often inferior to those of AISI 302 and AISI 631. An example of such a property is relaxation resistance, which indicates the ability of a spring to maintain spring strength for a long time.

US-A-6 106 639 discloses a Cr-Ni-Cu steel which can be reduced in cross section between annealing. This steel exhibits, for example, an intensity of 1856 MPa when the cross-sectional reduction of? = 3.41 (5.5 to 1 mm) is reduced. This is in contrast to the specific strength 2050 MPa according to the standard. According to US-A-6 106 639, the heat treatment must be carried out such that the alloy has a strength according to this standard. The alloy according to US-A-6 106 639 contains copper as a strength increasing element during heat treatment.

US-A-6 048 416 discloses Cr-Ni-Cu-steel for reinforcing vehicle tires in the form of high strength steel wires. In order to obtain the desired properties, the composition of the alloy according to US-A-6 048 416 must be greater than -55 and less than -30, the so-called JM value (JM = 551-462 × (C% + N%)-9.2 × Si It should be within the stability interval, expressed as%-20 x Mn%-13.7 x Cr%-29 x (Ni% + Cu%)-18.5 x Mo%). In the alloy according to the invention, the logarithmic cumulative cross-sectional reduction rate ε = 2 x ln (S _o / S _f ) is limited to a maximum of four. This corresponds to the maximum cross-sectional reduction in 98% wire drawing. Alloys according to US-A-6 048 416 do not contain precipitation hardening elements.

Accordingly, an object of the present invention is to reduce the cross section by cold working, in particular drawing, with a high strength precipitation hardening austenitic stainless steel alloy having a well controlled aluminum content and a high silicon content, without intermediate heat treatment, the strength of which is 300 ℃ Increased by more than 14% by the final heat treatment at -500 ° C, showing a loss of M _{d30 -value} of -55 to -100 and a force loss of less than 3.0% at 1400 N for 24 hours, such as springs of round wire and strip steel It is to provide a product that is very suitable for medical applications such as spring applications and surgical instruments and dental instruments.

According to the invention, this object is achieved in weight percent.

0 <C ≤0.07,                 

0.5 ≤ Si ≤ 3.0,

0 <N ≤ 0.1,

15.0 ≤ Cr ≤ 20.0,

7.0 ≤Ni ≤12.0,

0.25 ≤ Al ≤ 1.5,

0 ≤ Cu ≤ 4.0,

0 <Mn ≤ 3.0,

0 <Mo ≤2.0,

0 <Ti ≤1.0

It is achieved by a high-strength precipitation hardening austenitic stainless steel alloy containing a balance and remainder Fe and the impurities and additives commonly generated.

Figure 1 shows the loss of spring force after 24 hours in a material according to the invention compared to AISI 302 and charge number 150725.

Figure 2 shows the ultimate tensile strength of a material according to the invention in comparison with AISI 302 (intermediate heat treated) and charge number 150725.

Figure 3 shows the ultimate tensile strength of a material according to the invention as a function of logarithmic cumulative reduction in cross section compared to charge number 105725.

4 is a side view schematically showing a section of a ring elongated in a possible embodiment.                 

FIG. 5A is a view of the ring from above, where the ends are receiving force F in a direction facing each other, FIG. 5B is a view of the ring from the side, and an end is a force F facing each other. 5c shows how some of the elongated rings that make up the flat spring element are affected by the force F.

6 shows another embodiment of a strip spring.

In the case of this alloy, the importance of alloying elements is as follows.

Carbon (C) has a high tendency to bond with chromium, and chromium carbide precipitates at grain boundaries, resulting in a lack of chromium in the surroundings. Thus, the high carbon content deteriorates the corrosive properties of the material and, above all, embrittlement problems may also arise which can cause problems when forming the wires with springs. Therefore, the carbon content should be limited as small as possible, more than 0.0% by weight but up to 0.07% by weight, preferably 0.05% by weight, most preferably at most 0.035% by weight.

Since silicon (Si) has a ferrite stabilizing effect, when the silicon content is too large, a biphasic structure is formed. Therefore, the silicon content should not exceed 3.0% by weight. However, silicon is preferred in that it further increases the strength upon heat treatment of the cold worked product. Therefore, the silicon content should not be less than 0.5% by weight, but should be 0.5 to 3.0% by weight, preferably 0.5 to 2.5% by weight, most preferably 0.5 to 1.5% by weight.

Nitrogen (N) together with aluminum is an alloying element which forms undesirable brittle slag in the form of aluminum nitride. Moreover, nitrogen increases the strain hardening during cold working, which is disadvantageous to the present invention. Therefore, it is very important to keep the nitrogen content as small as possible, up to 0.1% by weight, preferably 0.05% by weight.

Chromium (Cr) is a very important alloying element with respect to the corrosion resistance of the material. This is because chromium can form a passive layer of Cr ₂ O ₃ on the steel surface. In order for the passivation layer to form, the chromium content must exceed about 12.0% by weight, and the corrosion resistance increases as the chromium content increases. Another advantage of chromium is that the austenite structure of the material is stabilized against phase transition to martensite during cold working. However, since chromium stabilizes ferrite, its content should not be too high. Thus, for the alloy according to the invention the chromium content should not be less than 15.0% by weight and not greater than 20.0% by weight, with 16.0-19.0% by weight being preferred.

Nickel (Ni) is an alloying element that ensures the austenite structure of a material at room temperature when it is in sufficient quantity. In addition, the ductility improves as the nickel content increases. However, nickel is an expensive alloying element, and at high contents, it is difficult to obtain sufficient strength because slow deformation hardening occurs. Therefore, the nickel content should be 7.0 to 12.0% by weight, preferably 8.0 to 11.0% by weight, more preferably 9.0 to 10.0% by weight.

Aluminum (Al) is a major alloying element in the present invention. Aluminum is added as a precipitation hardening element to increase strength, and also affects relaxation resistance. During precipitation hardening at 350 to 500 ° C. of the cold worked wire, precipitation in the form of β-NiAL occurs, which improves mechanical properties unlike materials known to date. This effect is most important when the wire is used as a spring, and the spring's relaxation resistance must meet very high requirements. The disadvantage of aluminum is that it stabilizes ferrite, so the aluminum content is limited to a maximum of 1.5% by weight. In view of the foregoing, however, the aluminum content should be at least 0.25% by weight, preferably 0.4 to 1.0% by weight.

Copper (Cu) is an alloying element having two important properties. First copper is an austenite stabilizing element, and secondly copper improves ductility by reducing strain hardening of the material. The copper content should be as large as possible, since the material must withstand severe cross-sectional reduction without intermediate annealing. However, as the copper content increases, undesirable precipitation increases, thereby reducing the ductility of the material. Therefore, the copper content should be 0 wt% or more and 4.0 wt% or less, preferably 2.0 to 3.5 wt%, most preferably 2.4 to 3.0 wt%.

Manganese (Mn) has a similar effect to nickel in that it forms austenite during setting and stabilizes austenite against martensite transformation during cold working. However, manganese increases strain hardening, while nickel does not. This results in faster strain hardening and reduces the maximum possible rate of cross-sectional reduction between loosenings. Therefore, the manganese content should be greater than 0.0% by weight but limited to at most 3.0% by weight, preferably at most 1.0% by weight.

Molybdenum (Mo) is a ferrite stabilizing element having a very desirable effect on corrosion resistance in chloride environments. In contrast to the effect of chromium in the established PRE (pitting resistance index) formula, molybdenum gives a factor of about 3. However, when the content of molybdenum is large, the ferrite phase in the steel is stabilized. In addition, the risk of precipitation of intermetallic phases, such as sigma phases, is increased. Therefore, the molybdenum content should be greater than 0.0% by weight but limited to a maximum of 2.0% by weight.

Titanium (Ti), like aluminum, is a precipitation hardening element added to increase strength, and also affects relaxation resistance. Moreover, when titanium coexists with silicon, it shows a large heat treatment effect even with a small titanium content. However, titanium is very stable in ferrite, so the content should not be too high. Therefore, the titanium content should be greater than 0.0% by weight but limited to at most 1.0% by weight, preferably at most 0.75% by weight.

Explanation of the Experiment Process

Experimental materials were prepared by melting in a high frequency furnace. Next, all experimental ingots were ground completely before forging. The ingot was forged to a 103 x 103 mm long material. Heating temperature was 1240 degreeC-1260 degreeC. The holding time at the highest temperature was 1 hour. At the next blank treatment, the blank was ground thoroughly and tested by ultrasound.

The blank was heated to 1200 ° C. to 1240 ° C., then rolled to final dimensions and water cooled to prepare a wire rod having a diameter of 5.50 mm to 5.60 mm. Then, the hot rolled wire was drawn in a conventional drawing machine and cold worked.                 

The chemical composition of the alloy and reference material in the experimental program is given in Table 1 in weight percent.

Table 1. Chemical Composition (Unit: wt%)

The strengths of the cold worked and heat treated alloys in the uni-axial tensile test are shown in Table 2, where the ultimate tensile strength corresponds to the maximum value of the load in the stretch-load diagram. All the alloys were sectioned with a logarithmic cumulative section reduction rate of ε = 3.95 (corresponding to a 98% section reduction) without intermediate annealing. Since AISI 302 could not be cold worked to ε = 3.95 without cracking, it had to be unannealed before drawing to final dimensions. However, all alloys have the same wire diameter.

The heat treatment was carried out for the same purpose as for the AISI 302 type spring steel, increasing the mechanical properties. As such, some important spring characteristics, for example relaxation resistance, are affected in a stronger direction than is known so far.                 

Table 2. Ultimate Tensile Strength Before and After Heat Treatment

^* Heat treatment time: 1.5 hours, heat treatment temperature: 350 ℃

^** Heat treatment time: 1.0 hours, heat treatment temperature: 480 ℃

To evaluate the relaxation resistance, springs of cylindrical spiral spring type with no lined-up turn were made. The experimental results are shown in Table 3.

Table 3. Spring Dimensions

The force versus load curve was used to determine the spring force (F) and total spring suspension (f _t ) at room temperature. And, the spring constant (C) and shear modulus (G) were calculated using Equations 1 and 2.

Equation 1: C = (F × N _v ) / f _t

Equation 2: G = (8 × F × N _v × D ³ _M ) / (f _t × D ⁴ _t )

Relaxation experiments were performed by applying a constant load to the blued springs. The load was measured every minute for the first five minutes, and then the number of measurements was reduced. After 24 hours each experiment was stopped. Initially four different loads were applied to the springs from each charge. Relaxation was calculated using Equation 3, and the results are summarized in FIG.

Equation 3: R = ((F ₁ -F ₂ ) / F ₁ ) × 100

Where R is the relaxation, F ₁ is the initial load, and F ₂ is the load at a given time.

In Fig. 1, it can be seen that an alloy having a very small aluminum content, that is, charge number 150725, relaxes considerably more than the alloy of the experimental program having all of aluminum as an effective alloying element. In addition, all alloys of the experimental program have equivalent or better relaxation resistance than AISI 302.

Let M _d30 / No (M _d30 / Nohara) is in a cold reduction of area of 30% and 50% of the austenite phase of the steel is transformed - represents the temperature at which transformation to martensite (martensite-transformation). The higher this temperature, the more stable the structure (easier martensite formation), the greater the cold strain in the steel.

The value of M _d30 according to No Hara is calculated by the following equation.

M _d30 / Nohara = 551-462 × (C + N)-9.2 × Si-8.1 × Mn-13.7 × Cr-20 × (Ni + Cu)-18.5 × Mo-68 × Nb-1.42 × (ASTM particle size - 8 )

Table 4 shows the results of Experiment Charges 1-7. The steel of the composition according to the present invention showed the best heat treatment effect at the M _d30 -value of -55 to -100, and the ultimate tensile strength increased the most after only cold working without intermediate heat treatment.

Table _{4.M d30} / Nohara

Description of the Preferred Embodiments

In the following, some embodiments of the present invention are described. It is intended to illustrate the invention and not to limit it.

The steel according to the invention is very susceptible to cold deformation. For example, it can be molded into different cross-sectional structures, such as round wires, egg-shaped wires, profiles of different cross sections (eg squares, triangles or more complex shapes and structures). Round wire can even be rolled flat.                 

Example 1: Spring of Round Wire

As described above, the wire spring made of the alloy according to the invention is wound. These springs have good spring characteristics in terms of relaxation and retain their spring forces for a long time, for example locking springs, ie mechanical parts of the locking device, springs of aerosol containers, pens (especially ballpoint pens), pump springs, industrial looms. It can be advantageously used for certain spring applications, such as springs in industrial looms, springs in the automotive industry, electronics, computers and precision machinery.

Example 2 Spring of Strip Steel

In the case of plane torsion springs, torque is an important quantity. The torque can be expressed as follows.

Where M = torque of the spring,

I = moment of bending inertia (b × t ^3/12)

B = spring strip width

T = spring strip thickness

L = extended spring length

n ₀ = number of turns on free spring (not mounted)

n = number of working turns

In order to increase the torque in a given spring structure, so-called back-winding can be done. In so-called "resilient" windings, the spring is wound in the opposite direction of the operating direction. The spring is then heat treated and then wound in the opposite direction into the spring housing. In the case of so-called "cross curve" windings, a strip is formed on the tack and then heat treated. Then, the spring is wound in the opposite direction in the spring housing. By this process, a smaller n ₀ value, often even a negative n ₀ value, can be obtained in contrast to a simply wound spring (see FIG. 6). Since the strength during the heat treatment increases very much, the alloy according to the present invention is very suitable for use as a torsion spring where a large torque and good relaxation resistance are required.

Example 3 expander wire

The expander is a kind of wire, shaped as a wavy, flat spring connected in series. This spring is used for example to regulate the pressure of the oil scraper ring against the cylinder wall of the internal combustion engine. A common expander for vehicle motors is a wavy wire between two piston rings. A possible embodiment of such a wavy ring is schematically shown in FIG. 4.

The disadvantage of today's motorized vehicles is the large energy consumption required to achieve the desired performance. The simplest way to reduce energy consumption is, among other things, to reduce the internal friction of the drive and reduce the total weight of the vehicle. The piston core accounts for more than half of the motor friction. As a result, we are continuing to improve the materials and precision of the rings, pistons and cylinder walls to reduce body weight and withstand pressure. The expander is a spring that regulates the oil scrap and some of the internal friction of the motor by regulating the pressure of the oil scraper ring against the cylinder wall. The load of the expander wire is composed of a force F as shown in Figs. 5A to 5C.

For flat springs, the following relationship applies when a load is applied at 90 ° to the rear face under maximum load.

σ _max  Permissible stress at the back of the spring F  Load determined by the length of the expander wire relative to the piston diameter T  Thickness of wire B  Width of wire E  Modulus of elasticity of wire material s  Suspension length, deformation of the expander R  Bending radius of each spring element

(3) Combining Formula (1) and Formula (2) is as follows.

⇒

⇒

Equation (3) indicates that the wire thickness required for a given property depends on the configuration of the expander. If the allowable tensile stress of the material is increased, a smaller bending radius can be tolerated, and thus a smaller type of ring can be produced, which is very advantageous. As environmental demands increase the demand for smaller motors, the possibility of making smaller rings becomes even more important.                 

Considering the energy for the following reasons it is also possible to obtain the advantage of greater strength in the expanding ring.

A  Elastic energy K  Material-use constant E  Modulus of elasticity V  Effective volume of spring (amount of spring material in operation)  σ  Applied tensile stress

According to equation (4), the specific elastic energy for a given elastic modulus is a function of specific volume, material use and maximum allowable tensile stress. Increased maximum allowable tensile stress generally increases the material use constant and also has a great impact on the specific volume required. Thus, it is possible to reduce the material volume by increasing the allowable tensile stress with respect to the amount of elastic energy maintained.

The complicated shape of the elongated ring is only possible with soft materials. The main reason for the use of stainless steel is its workability. However, for the function of the expander, the tensile yield limit and ultimate tensile strength are equally important for at least all spring applications. This was a state of contradiction that was difficult to handle in the prior art. By using the steel according to the invention, the material can be formed in a relatively soft state so as to be later heat treated in its final form, and the desired spring properties can be obtained by precipitation hardening.

Example 4: Flat Wire

Since the force must be opposed without preforming, this embodiment according to the invention is used in particular for applications where the demand for the relaxation properties of steel is very high. Thus, the steel of the present invention is particularly suitable for wires for windshield wipers, for example, where both good punchability of the starting material and good relaxation resistance of the final product are required.

Example 5 Medical Round and Flat Wire and Strip Steel

The wire made of the alloy according to the invention can be used for medical purposes, for example in the form of dental instruments, nerve extractors and the like, such as piles such as root canal files, as well as surgical needles. The wire flattened with steel according to the present invention can be usefully used for the production of dental instruments and surgical instruments.

For all these applications, the structure is usually complex, manufactured by grinding, bending and / or torsion prior to the final heat treatment, the mechanical properties being greatly increased, which has a large breaking strength and good ductility.

Claims (11)

As a high strength austenitic stainless alloy, The alloy is precipitation hardening type, the composition of the alloy in weight percent 0 <C ≤0.07, 0.5 ≤ Si ≤ 3.0, 0 <N ≤ 0.1, 15.0 ≤ Cr ≤ 20.0, 9.0 ≤Ni ≤10.0, 0.25 ≤ Al ≤ 1.5, 2.4 ≤Cu ≤3.0, 0 <Mn ≤ 1.0, 0 <Mo ≤2.0, 0 <Ti ≤1.0 And the remainder containing Fe and commonly occurring impurities, the alloy having a cross sectional reduction rate of 99% or more, the cross section being reduced by cold working and drawing without intermediate heat treatment, and the tensile strength of the alloy at 300 ° C. to 500 ° C. High strength austenitic stainless alloy, characterized in that increased by more than 14% by the final heat treatment of. delete delete The high strength steel austenitic stainless precipitation hardening alloy according to claim 1, wherein the alloy contains 16.0 to 19.0% by weight of chromium. The high strength steel austenitic stainless precipitation hardening alloy according to claim 1, wherein the alloy contains 0.4 to 1.0% by weight of aluminum. The high strength steel austenitic stainless precipitation hardening alloy according to claim 1, wherein the alloy contains 0.5 to 2.5% by weight of silicon. The high strength steel austenitic stainless precipitation hardening alloy according to claim 1, wherein the alloy contains 0.5 to 1.5% by weight of silicon. delete The high strength steel austenitic stainless precipitation hardening alloy according to claim 1, which is made in the form of a wire and / or a strip. 2. The high strength steel austenitic stainless precipitation hardening alloy according to claim 1, which is suitable for use in spring applications such as springs of round wire and strip steel. The high strength steel austenitic stainless precipitation hardening alloy according to claim 1, which is for medical use such as surgical instruments and dental instruments.

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