A method for preparing stainless steel micro-wire and foil with high strength, toughness, elasticity and stability

Abstract

A method for preparing stainless steel micro-wires and foils with high strength, toughness, elasticity and stability, comprising the following steps: 1) preparing wires or strips; 2) reverse transformation annealing; 3) surface treatment; 4) stabilization treatment; 5) cold working. According to the method of the present invention, the cold-worked wires or strips are subjected to reverse transformation annealing and then cold-worked again to obtain ultrafine-grained martensite + ultrafine-grained reverse transformation austenite + residual austenite, so that the strength of the material is greater than 1900MPa, and the material has high strength and toughness; and the holding temperature of the reverse transformation annealing treatment is not greater than 850°C, and the energy consumption is low.

Description

Technical Field

[0001] The present invention relates to the technical field of metal wire and strip processing, and in particular to a method for preparing a stainless steel micro-wire and foil with high strength, toughness, elasticity and stability, and in particular to a method for preparing a stainless steel micro-wire and foil with high strength, toughness, elasticity and stability based on a reverse transformation annealing process. Background Art

[0002] Deformation-induced martensitic transformation strengthening is an important strengthening mechanism for stainless steel. Metastable austenitic stainless steel and semi-austenitic stainless steel usually obtain high strength through cold working. Cold working causes the austenite in the matrix to undergo martensitic transformation, especially the tensile strength of cold-drawn micro-wires or cold-rolled foils can reach more than 2GPa. However, the plasticity of cold-worked micro-wires or foils will decrease sharply, making it difficult to continue deep processing.

[0003] Regarding the strength-toughness trade-off of steel materials, except for fine grain strengthening, traditional strengthening methods all sacrifice the toughness and plasticity of the material. Fine grain strengthening can improve the strength of steel materials while obtaining good toughness and plasticity. In industrial practice, fine grain strengthening is often used to improve the comprehensive mechanical properties of steel materials. Since austenitic stainless steel has a high recrystallization temperature, the grains are easy to grow, and the possibility of obtaining fine grain structure through static recrystallization is low. Therefore, it is difficult to obtain ultrafine grains by traditional thermomechanical processing methods (such as solution treatment).

[0004] After cold deformation, the unstable austenite of metastable austenite or semi-austenite stainless steel undergoes martensite transformation to form a fine-sized martensite structure. The reversibility of the martensite transformation is utilized to reversely transform the martensite into fine-sized austenite grains through annealing at a certain temperature and time. The key point of reverse transformation annealing is to control the processing technology to obtain sufficient martensite, strictly control the annealing process, and delay the growth of austenite grains, thereby obtaining an ultrafine grain structure. The advantages of this method are: (1) The reverse transformation of martensite to austenite is a solid-state phase transformation with an extremely fast speed, so the holding time is short, the heat treatment temperature is lower than the general solution treatment temperature, the efficiency is high, and energy is saved; (2) The strength of stainless steel is significantly improved without significantly reducing its plasticity.

[0005] However, the reverse transformation annealing method still has the following problems: (1) The size and quantity of reverse transformed austenite are mainly determined by the size and content of deformation-induced martensite obtained in the previous processing. In order to obtain small-sized and sufficient martensite, the methods currently used mainly include low-temperature (below -10°C) rolling, repeated reverse transformation treatment, severe plastic deformation processing (RCSR, ARB, ECAP), etc. Most of these methods are still in the laboratory stage. (2) Current research shows that when reverse transformation annealing is applied to 3 series metastable austenitic stainless steel or PH series semi-austenitic stainless steel, the tensile strength of the material can be increased by 300-400MPa compared with general solution treatment, about 1200-1400MPa, while slightly reducing the elongation. However, it still cannot meet the high strength requirements of special components (such as precision springs), especially the small cross-sectional area of ​​micro-wires or foils, which often require higher strength.

[0006] The publication number is CN115710678A, which discloses a method for solving the longitudinal cracks in 17-7PH steel wire and improving the strength and toughness. The strength can reach 1900MPa, but the solution treatment temperature is as high as 1150℃. This method is still fundamentally a traditional solution treatment + cold deformation. The publication number is CN110872641A, which discloses a method of austenite reverse transformation and sub-temperature forming. This method only uses heat treatment for strengthening, which has limited effect and the strength of the material is low, about 1200MPa.

[0007] The publication number is CN117344111A, which discloses a method for producing manganese high-strength and tough medium and thick plates with controllable yield ratio. The yield ratio of the material is regulated by changing the reverse transformation austenite content and stability, but the strength is relatively low at about 1000MPa. The publication number is CN116377184A, which discloses a method for refining low-carbon steel austenite grains. By increasing the reverse transformation austenite nucleation rate, excellent strength-toughness-plasticity matching is obtained. The grain size can be refined to less than 10μm, but the heating temperature needs to be greater than 1100℃, and the energy consumption is relatively high. The publication numbers are CN114480952B, CN116219286A, and CN109023130A, all of which improve the strength and plasticity of the material, but the strength of the material is relatively low, all below 1200MPa. Although the above patents enable the material to obtain a good strength-toughness match, and the elongation of the material can reach 20% to 45%, the strength is still relatively low, all below 1900Mpa, and is only applicable to the preparation of some large-sized components; it is not applicable to some special components (such as precision springs) with limited installation space, miniaturization, and lightweight. In addition, the above patents all involve heat treatment above 1000℃, which consumes a lot of energy. Summary of the invention

[0008] The purpose of the present invention is to provide a method for preparing stainless steel micro-wires and foils with high strength, toughness, high elasticity and stability in view of the deficiencies in the prior art. By adopting the method of the present invention, the cold-processed wire or strip is subjected to reverse transformation annealing and then cold-processed again to obtain ultrafine-grained martensite + ultrafine-grained reverse transformation austenite + residual austenite, so that the strength of the material is greater than 1900MPa and has high strength and toughness; and the holding temperature of the reverse transformation annealing treatment is not greater than 850°C, and the energy consumption is low.

[0009] The object of the present invention is achieved in that:

[0010] The method for preparing stainless steel micro-wires and foils with high strength, toughness, elasticity and stability comprises the following steps:

[0011] 1) Preparation of wire or strip

[0012] The stainless steel wire is cold drawn to obtain the wire, or the stainless steel strip is cold rolled to obtain the strip;

[0013] 2) Reverse transformation annealing

[0014] The wire or strip obtained in step 1) is treated by the following method:

[0015] Treatment method ①: Continuous annealing in a protective atmosphere, with a holding temperature of 650-850°C for 180-360s, followed by cooling;

[0016] Treatment method ②: annealing in air atmosphere, keeping temperature at 650-850℃ for 120-300s, then keeping in water at 60-100℃ for 30-60min, and cooling;

[0017] 3) Surface treatment

[0018] Removing the surface oxide scale of the wire or strip after being treated in step 2);

[0019] 4) Stabilization treatment

[0020] The surface treated wire or strip is placed at room temperature for ≥ 1 day;

[0021] 5) Cold working

[0022] The stabilized wire is cold drawn to a desired size, or the stabilized strip is cold rolled to a desired size.

[0023] The wire or strip is semi-austenitic stainless steel or metastable austenitic stainless steel.

[0024] The stainless steel is 07Cr17Ni7Al or 07Cr15Ni7Mo2Al or 12Cr17Ni7 or 1Cr18Ni9 or 06Cr19Ni10.

[0025] The chemical composition of the stainless steel needs to satisfy the stacking fault energy:

[0026] SEF=39 + 1.59Ni - 1.34Mn + 0.06Mn 2 - 1.75Cr + 0.01Cr 2 + 15.21Mo -5.59Si- 0.69(C+1.2N) 1 / 2 + 26.27(C+1.2N)×(Cr+Mn+Mo) 1 / 2 + 0.61[Ni(Cr+Mn)] 1 / 2 ≤18mJ / m 2 .

[0027] The ferrite content in the blank of the wire or strip is less than 5%, and the sum of the ratings of type B + type D inclusions is less than level 1.5.

[0028] Step 1) the cold drawing has a surface reduction rate greater than 55%;

[0029] Preferably, the area reduction rate is 60% to 90%;

[0030] The thickness reduction rate of the cold rolling is greater than 50%;

[0031] Preferably, the thickness reduction rate is 55% to 85%.

[0032] Step 2) The protective atmosphere is ammonia decomposition gas or pure H2.

[0033] Step 2) the cooling is air cooling to room temperature;

[0034] Preferably, the holding temperature of the treatment method ① in step 2) is 700-850°C;

[0035] Preferably, the insulation temperature of step 2) the treatment method ② is 700-800°C.

[0036] Step 3) The surface treatment uses a stainless steel pickling solution.

[0037] Step 5) the cold drawing has a surface reduction rate greater than 65%;

[0038] The thickness reduction rate of the cold rolling is greater than 60%.

[0039] Beneficial effects of the present invention:

[0040] (1) The method of the present invention is used to treat a cold-worked metastable austenite or semi-austenite stainless steel wire or strip, and then cold-work it again after reverse transformation annealing, so as to adjust the microstructure of the material and obtain ultrafine-grained martensite + ultrafine-grained reverse transformation austenite + residual austenite, so that the material has high strength while reducing the degree of sacrifice of its plasticity, and the prepared micro-wire or foil has stable elastic properties;

[0041] Among them, the content of ultrafine-grained martensite is greater than 85%, and has a high dislocation density, which ensures the strength of the material; the content of ultrafine-grained reverse-transformed austenite + residual austenite is greater than 5%, which is used to coordinate plastic deformation and ensure the toughness and plasticity of the material.

[0042] (2) The wire or strip prepared in step 1) of the present invention can obtain a martensite structure with a volume fraction of more than 80%, which lays a structural foundation for reverse transformation annealing; and during the cold drawing or cold rolling process, the martensite participates in plastic deformation to form a smaller high dislocation density martensite, which is conducive to reverse transformation annealing to obtain ultrafine grained reverse transformation austenite.

[0043] (3) The reverse transformation annealing treatment of the present invention transforms the martensite in the wire or strip into a reverse austenite structure with an ultrafine grain size, thereby improving the strength and ensuring the plasticity required for subsequent cold working.

[0044] (4) In the reverse transformation annealing treatment of the present invention, the holding temperature is set to 650-850°C. If the holding temperature is higher than 850°C, the material may recrystallize to obtain a recrystallized austenite structure. The recrystallized austenite grains are relatively coarse, and it is difficult to obtain an ultrafine grain structure below 10 μm, which is not conducive to improving the strength. If the holding temperature is lower than 650°C, the thermodynamic conditions required for reverse transformation cannot be met, and the reverse transformation austenite structure cannot be produced. The present invention adopts a low holding temperature and a short holding time, which has high efficiency and low energy consumption.

[0045] (5) The stabilization treatment of the present invention stabilizes the reverse transformed austenite structure, further ensures the plasticity of the material, and is beneficial to subsequent cold processing.

[0046] By adopting the method of the present invention, wires with a minimum diameter of up to 0.2 mm and foils with a minimum thickness of up to 0.025 mm can be prepared, with high dimensional accuracy and good surface quality; the strength can reach more than 1900 MPa, with high toughness and high elastic stability, the elastic aftereffect is less than 0.5‰, and the elastic stability can reach more than 98%. The method is mainly used for the preparation of precision springs with complex structures, such as energy storage springs for sealing, spiral springs, foils for bearings, diaphragms, hairsprings, wave springs and scroll springs, etc., and can also be used for the preparation of extremely thin seamed pipes, metal bellows, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1is a flow chart of the present invention;

[0048] Figure 2 The following is a process flow chart for preparing the material of the present invention;

[0049] Figure 3 Metallographic photographs of materials prepared in the present invention;

[0050] Figure 4 The room temperature tensile curves of the examples of the present invention and the comparative examples are shown.

[0051] Reference numeral: M s is the martensitic transformation starting temperature, M d is the martensitic transformation end temperature, A s is the austenitizing start temperature, T 再 is the recrystallization temperature, A f is the austenitizing completion temperature, A is austenite, M is martensite, and F is ferrite. DETAILED DESCRIPTION Embodiment 1:

[0052] See also Figure 1 , preparing a microfilament with a diameter of 0.3 mm, comprising the following steps:

[0053] 1) Preparation of wire

[0054] The soft 07Cr17Ni7Al wire with a diameter of 1.0 mm was cold drawn to a diameter of 0.6 mm through 7 passes, with a surface reduction rate of 64%;

[0055] 2) Reverse transformation annealing

[0056] The Φ0.6mm wire was placed in a box-type resistance furnace for annealing treatment at a holding temperature of 700°C for 300 seconds. After being taken out of the furnace, it was immediately kept in boiling water at 100°C for 60 minutes and then air-cooled to room temperature.

[0057] 3) Surface treatment

[0058] Step 2) using conventional stainless steel pickling liquid to remove the oxide scale on the surface of the obtained wire;

[0059] 4) Stabilization treatment

[0060] The surface treated Φ0.6 mm wire was left to stand naturally for 1 day to stabilize the reverse transformed austenite structure.

[0061] 5) Cold working

[0062] The Φ0.6mm wire after stabilization treatment is cold drawn through 5 passes to a diameter of 0.3mm, and the finished product reduction rate is 75.0%.

[0063] Comparative Example 1:

[0064] The preparation of microfilaments with a diameter of 0.3 mm includes the following steps:

[0065] 1) Solution treatment

[0066] The 07Cr17Ni7Al wire with a diameter of 0.7 mm was solution treated in a box-type resistance furnace with a holding temperature of 1050°C and a holding time of 300s. After being taken out of the furnace, it was cooled in normal temperature water.

[0067] 2) Surface treatment

[0068] Step 1) The obtained wire is treated with a conventional stainless steel pickling solution to remove the oxide scale on the wire surface.

[0069] 3) Continue cold drawing

[0070] The surface treated Φ0.7mm wire was cold drawn through 6 passes to a diameter of 0.3mm, and the surface reduction rate of the finished product was 81.6%. Embodiment 2:

[0071] See also Figure 1 , preparing a foil with a thickness of 0.15 mm, comprising the following steps:

[0072] 1) Strip preparation

[0073] The soft 07Cr17Ni7Al strip with a thickness of 3.0 mm was cold rolled to a thickness of 1.0 mm through 9 passes, with a thickness reduction rate of 67%;

[0074] 2) Reverse transformation annealing

[0075] The δ1.0 mm strip was placed in a continuous furnace for annealing treatment. The protective atmosphere was ammonia decomposition gas (N2+H2), the holding temperature was 850°C, the furnace time was 180 s, and it was air-cooled to room temperature in a water-cooling jacket;

[0076] 3) Surface treatment

[0077] The surface of the strip under protective atmosphere is bright and no surface treatment is required;

[0078] 4) Stabilization treatment

[0079] The annealed δ1.0 mm strip was left to stand naturally for 1 day to stabilize the reverse transformed austenite structure;

[0080] 5) Cold working

[0081] The δ1.0 mm strip after stabilization treatment was cold rolled to a thickness of 0.15 mm in 8 passes, and the thickness reduction rate of the finished product was 85%.

[0082] Comparative Example 2:

[0083] The preparation of a foil having a thickness of 0.15 mm comprises the following steps:

[0084] 1) Solution treatment

[0085] The 07Cr17Ni7Al strip with a thickness of 1.0 mm was solution treated in a continuous furnace with a protective atmosphere of ammonia decomposition gas (N2+H2), a holding temperature of 1050°C, a furnace time of 300 s, and air cooling in a water cooling jacket.

[0086] 2) Continue cold rolling

[0087] The δ1.0 mm strip after solution treatment was cold rolled to a thickness of 0.15 mm in 8 passes, and the thickness reduction rate of the finished product was 85%.

[0088] Comparison of room temperature tensile engineering stress-strain curves of Example 1-2 and Comparative Example 1-2, see Figure 4 Comparative Example 1-2 is a stainless steel micro-wire or foil prepared by the existing solution treatment + cold working method. When the tensile strength is above 1900MPa, there is no necking zone in the room temperature tensile curve, showing a brittle fracture characteristic; Example 1-2 is a stainless steel micro-wire or foil prepared by the method of the present invention, and the tensile strength can reach above 1900MPa, and the elongation is improved. There is a necking zone in the room temperature tensile curve, showing a plastic fracture characteristic; Compared with Comparative Example 1-2, Example 1-2 increases the elongation of the material by 1 to 2 times while achieving high strength, and the plasticity is improved, which can meet the subsequent deep processing of winding, bending, stamping and other precision springs, and is conducive to the preparation of precision components with complex structures.

[0089] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification made to the present invention by those skilled in the art without departing from the spirit of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for preparing stainless steel micro-wires and foils with high strength, toughness, elasticity and stability, characterized in that: The following steps are involved: 1) Preparation of wire or strip The stainless steel wire is cold drawn to obtain the wire, or the stainless steel strip is cold rolled to obtain the strip; 2) Reverse transformation annealing The wire or strip obtained in step 1) is treated by the following method: Treatment method ①: Continuous annealing in a protective atmosphere, with a holding temperature of 650-850°C for 180-360s, followed by cooling; Treatment method ②: annealing in air atmosphere, keeping temperature at 650-850℃ for 120-300s, then keeping in water at 60-100℃ for 30-60min, and cooling; The cooling is air cooling to room temperature; 3) Surface treatment Removing the surface oxide scale of the wire or strip after being treated in step 2); 4) Stabilization treatment The surface treated wire or strip is placed at room temperature for ≥ 1 day; 5) Cold working Cold drawing the stabilized wire to a desired size, or cold rolling the stabilized strip to a desired size; The area reduction rate of the cold drawing is greater than 65%, and the thickness reduction rate of the cold rolling is greater than 60%.

2. The preparation method according to claim 1, characterized in that: The wire or strip is semi-austenitic stainless steel or metastable austenitic stainless steel.

3. The preparation method according to claim 2, characterized in that: The stainless steel is 07Cr17Ni7Al or 07Cr15Ni7Mo2Al or 12Cr17Ni7 or 1Cr18Ni9 or 06Cr19Ni10.

4. The preparation method according to claim 2, characterized in that: The chemical composition of the stainless steel needs to satisfy the stacking fault energy: <h2 style=";text-align:left;direction:ltr">SEF=39 + 1.59Ni - 1.34Mn + 0.06Mn<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> - 1.75Cr + 0.01Cr<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> + 15.21Mo -5.59Si -0.69(C+1.2N)<h2 style=";text-align:left;direction:ltr"> 1 / 2 <h2 style=";text-align:left;direction:ltr"> + 26.27(C+1.2N)×(Cr+Mn+Mo)<h2 style=";text-align:left;direction:ltr"> 1 / 2 <h2 style=";text-align:left;direction:ltr"> + 0.61[Ni(Cr+Mn)]<h2 style=";text-align:left;direction:ltr"> 1 / 2 <h2 style=";text-align:left;direction:ltr"> ≤18mJ / m<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> 。 5. The preparation method according to claim 1, characterized in that: The ferrite content in the blank of the wire or strip is less than 5%, and the sum of the ratings of type B + type D inclusions is less than level 1.

5.

6. The preparation method according to claim 1, characterized in that: Step 1) The cold drawing area reduction ratio is greater than 55%, and the cold rolling thickness reduction ratio is greater than 50%.

7. The preparation method according to claim 1, characterized in that: Step 2) The protective atmosphere is ammonia decomposition gas or pure H2.

8. The preparation method according to claim 1, characterized in that: Step 3) The surface treatment uses a stainless steel pickling solution.

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