Gradient nanostructured stainless steel bar and method of making

Abstract

The present application relates to a kind of gradient nanostructured stainless steel bar and preparation method, belong to metal processing field.The stainless steel bar is gradient nanostructure, core is coarse grain, edge is nanocrystalline, and the main reason for the generation of edge nanocrystalline is that a large number of dispersed precipitates quickly refine grain in the plastic deformation process.The preparation process of the stainless steel bar introduces the precipitation alloying element Ti, Sc in matrix, introduces Al element in a certain range of bar edge, through composition alloying, cyclic cold drawing, cyclic aluminizing, aging local precipitation and the plastic deformation method of free end twist, finally obtains surface aluminum-rich layer, middle aluminum-poor precipitation hardening layer and core original layer, and the chemical composition, organizational features, grain size, precipitate phase quantity present gradient change in thickness direction.While improving the tensile strength of material, the plasticity of material can be kept basically unchanged, greatly widen the application scene of stainless steel bar.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of metal processing, and relates to a gradient nanostructure stainless steel bar and a preparation method. BACKGROUND

[0002] In the production process of stainless steel, a stainless steel black bar is usually produced, which is formed by hot rolling or forging of a stainless steel ingot for several passes. The stainless steel black bar is also a widely used basic material in the market. Common stainless steel materials can be processed on the basis of the black bar. Stainless steel round bar, also known as cold-drawn round bar, is formed by cold rolling and cold drawing of the stainless steel black bar, and then acid pickling and surface polishing. The stainless steel cold-drawn round bar is widely used in hardware kitchenware, petroleum, electronics, chemical industry, construction, nuclear power, aviation and other fields. However, the mechanical properties of the stainless steel bar are usually low. During the process of being processed into a round bar, the strength of the stainless steel bar increases due to plastic deformation, but the toughness is severely reduced.

[0003] With the development of aerospace, shipbuilding, automobile and other industries, there is an urgent need for high-strength and high-toughness materials. From the perspective of microstructure, refining the grain size is the most effective method to simultaneously improve the strength and toughness of metal materials. However, general grain refinement methods usually result in loss of plasticity and toughness. Unlike traditional homogeneous materials, the introduction of gradient nanostructure can optimize the mechanical properties comprehensively. By adjusting the heterogeneity (such as grain size, microstructure, crystal orientation and composition) between components, a variety of heterogeneous structures can be constructed. Through the geometrically necessary dislocation accumulation and back stress strengthening caused by the mutual constraint of heterogeneous components and the incoordination of plastic deformation during the plastic deformation process, high strength and high plasticity and toughness can be achieved, which solves the problem that traditional homogeneous materials cannot achieve. SUMMARY

[0004] Therefore, the purpose of the present application is to provide a gradient nanostructure stainless steel bar and a preparation method. By means of plastic deformation to produce gradient nanostructure, the tensile strength of the material is improved, and the plasticity of the material is maintained.

[0005] To achieve the above purpose, the present application provides the following technical scheme:

[0006] A gradient nanostructure stainless steel bar, which has a gradient structure with gradually decreasing grain size from the inside to the outside along the radial direction of the bar. The outer layer of the bar is nanocrystalline. The nanocrystalline is produced by the refinement of grains during plastic deformation of the dispersed precipitates.

[0007] Optionally, the layer thickness of the gradient structure is 10-1500 microns, and the grain size of the nanocrystalline is 10-150 nanometers.

[0008] Optionally, the components include, by weight components: 0.02%≤C≤0.08%, 0.15%≤Si≤0.45%, 1.0%≤Mn≤2.0%, 0.02≤P≤0.03%, 22%≤Cr≤24%, 0.7%≤Ti≤1.0%, 0.005%≤Sc≤0.3%, and the balance of Fe.

[0009] A method for preparing a gradient nano-structured stainless steel bar, comprising the following steps:

[0010] S1 preparing a bar of a specified size, the bar being prepared according to the following chemical composition and mass fraction: 0.7%≤Ti≤1.0%, 0.005%≤Sc≤0.3%, and the balance of iron, other alloy components, and unavoidable impurities;

[0011] In the preparation process, the components are smelted and sufficiently heat-insulated to ensure uniform diffusion of the alloy elements, and then a bar of a specified size is prepared.

[0012] S2 cold-drawing and embedding treatment, the bar prepared in S1 is subjected to cold-drawing treatment, is polished after the cold-drawing, and is then embedded in an aluminizing powder for aluminizing;

[0013] S2 is repeated at least twice.

[0014] S3 heat diffusion of the bar obtained after the S2 cycle;

[0015] S4 free-end torsion treatment, the bar obtained in S3 is twisted and is allowed to rebound;

[0016] S5 aging treatment.

[0017] Optionally, the other alloy components of the bar in S1 include, by weight components: 0.02%≤C≤0.08%, 0.15%≤Si≤0.45%, 1.0%≤Mn≤2.0%, 0.02≤P≤0.03%, and 22%≤Cr≤24%.

[0018] Optionally, the Ti and Sc raw materials in S1 are in the form of particles or blocks.

[0019] Optionally, in S2, the deformation amount of the cold-drawing treatment in a single pass is 10%-30%.

[0020] Optionally, in S2, the aluminizing temperature is 600°C-850°C, and the aluminizing time is 0.5h-24h each time.

[0021] Optionally, in S2, the aluminizing agent includes, by weight components: aluminum powder, 2-35%; iron powder, 5-10%; ammonium chloride, 1%-4%; and the balance of aluminum oxide.

[0022] Optionally, the number of cycles in step S2 is not more than 10.

[0023] Optionally, after the cycle of step S2, the aluminizing thickness of the rod is 50-600 μm.

[0024] Optionally, in step S3, the heat treatment temperature is 200-350 °C, and the holding time is 5-72 h.

[0025] Optionally, the heat treatment temperature in step S3 is lower than the temperature at which Al and other components in stainless steel form an intermediate phase.

[0026] Optionally, in step S4, the torsion angle is 30-360 °.

[0027] Optionally, in step S4, the rod is clamped and torsion force is applied to one segment of the rod to twist it.

[0028] Optionally, in step S4, it further includes cryogenic treatment. After the rod is twisted by the free end, before it fully rebounds, the rod is immersed in liquid nitrogen for cryogenic treatment, and after sufficient cooling, the rod is taken out and placed at room temperature.

[0029] After the free end is twisted, a large number of point defects, dislocations, and stacking faults are generated in the material. After twisting, the material will rebound, and this process is actually a process in which some defects disappear and dislocations recover. Therefore, liquid nitrogen cryogenic treatment is added in this stage. At the liquid nitrogen temperature, the atomic movement speed is very slow, so more dislocation density can be retained in the matrix, thereby providing more nucleation sites during the aging treatment in step S5, and more Al3Ti and Al3Sc intermediate phases are precipitated to improve the strength of the material. Al3Ti and Al3Sc precipitates are usually nanoscale.

[0030] Optionally, the treatment time of the rod in liquid nitrogen is 10-12 h, and the placement time at room temperature is 1-72 h.

[0031] Optionally, in step S4, the rod is kept for 0.5-24 h after twisting to release the torsion stress.

[0032] Optionally, in step S5, the aging treatment temperature is 350-850 °C, and the holding time is 0.5-12 h.

[0033] Optionally, it further includes step S6, in which the rod is cold-drawn to the designed size of the product after step S5.

[0034] Optionally, the deformation amount in step S6 is 20-80%.

[0035] Optionally, it further includes step S7, in which the rod is subjected to surface oil removal, polishing, and polishing treatment to obtain the final product.

[0036] The beneficial effects of the present application are:

[0037] The present application is based on a non-precipitation strengthened stainless steel composition, introducing precipitation alloying elements Ti, Sc in the matrix, through cyclic cold drawing + cyclic pack aluminizing treatment technology, introducing Al elements in a certain range of the edge of the bar, through thermal diffusion treatment, making the aluminum content of the edge and the core significantly different, and then producing different amounts of nanoscale precipitates, the present application finally obtains a surface aluminum-rich layer, a middle aluminum-poor precipitation hardened layer and a core original layer, the chemical composition, microstructure characteristics, grain size and precipitation phase quantity present gradient changes in the thickness direction, a large number of ultra-fine nanocrystals are found in the range of 10-150 μm of the edge of the bar, which can significantly improve the tensile strength while maintaining almost unchanged plasticity, greatly widening the use range of stainless steel bars.

[0038] Other advantages, objects, and features of the present application will be apparent to those skilled in the art from the following specification, and it is intended to be covered by the following claims, insofar as is not inconsistent with the prior art. The objects and other advantages of the present application will be realized and attained by the structure particularly pointed out in the specification. BRIEF DESCRIPTION OF DRAWINGS

[0039] In order to make the objects, technical solutions and advantages of the present application clearer, the preferred detailed description of the present application will be combined with the drawings as follows, wherein:

[0040] Figure 1 Process flow chart for some embodiments of the present application. DETAILED DESCRIPTION

[0041] The embodiments of the present application will be described in detail below through specific concrete examples, and those skilled in the art can easily understand other advantages and effects of the present application from the disclosure of the present specification. The present application can also be implemented or applied by different specific embodiments, and various modifications or changes can be made to the details in the specification based on different views and applications without departing from the spirit of the present application. It should be noted that the diagrams provided in the following embodiments only illustrate the basic concept of the present application in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0042] Among them, the drawings are only used for illustrative explanation, and the representation is only a schematic diagram, not a physical diagram, and cannot be understood as a limitation on the present application; in order to better illustrate the embodiments of the present application, some components in the drawings will be omitted, enlarged or reduced, and do not represent the size of the actual product; it is understandable for those skilled in the art that some known structures and their descriptions in the drawings can be omitted.

[0043] In the description of the present application, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "left", "right", "front", "back" and the like is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore the positional relationship described in the drawings is only used for exemplary illustration, and cannot be understood as a limitation on the present application, and for those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0044] The present application aims to provide a gradient nanostructured stainless steel bar, which is gradient nanostructured, with coarse grains in the core and nanocrystalline in the edge. The layer thickness of the gradient structure is controllable, and the nanocrystalline grain size is controllable. The main reason for the nanocrystalline in the edge is that a large amount of dispersed precipitates rapidly refine the grains during plastic deformation.

[0045] Please refer to Figure 1 In some embodiments of the present application, the following process flow is adopted, specifically comprising the following steps:

[0046] 1) Ti, Sc micro-alloying

[0047] A stainless steel, comprising, in percentage by weight: 0.02%≤C≤0.08%, 0.15%≤Si≤0.45%, 1.0%≤Mn≤2.0%, 0.02≤P≤0.03%, 22%≤Cr≤24%, 0.7%≤Ti≤1.0%, 0.005%≤Sc≤0.3%, the balance being Fe and unavoidable impurities.

[0048] According to the above composition, smelting and sufficient heat preservation are carried out to ensure uniform diffusion of alloying elements, and then a bar of specified size is prepared.

[0049] 2) Cyclic cold drawing + cyclic aluminizing treatment

[0050] The single pass cumulative deformation is 10-30%, and after deformation, the surface of the stainless steel bar is polished and placed in the aluminizing powder. The aluminizing temperature is 600-850°C, the aluminizing time is 0.5-24h, and the proportion of aluminum powder in the aluminizing powder is 5-65%. The above is one cycle, and a total of 2-10 cycles are carried out.

[0051] 3) Low temperature thermal diffusion treatment

[0052] After the above step 2 treatment, the aluminizing thickness of the bar reaches 50-600μm, in order to diffuse the aluminizing layer to the core, the heat treatment temperature is 200-350°C, and the heat preservation time is 5-72h.

[0053] 4) Free end torsion treatment

[0054] The two ends of the bar are clamped by a twisting processing device, and one end of the bar is twisted in one direction by 30°-360°. After the twisting is completed, the bar is allowed to fully rebound. In other embodiments, the bar can also be twisted in two directions by applying a twisting force to the two ends of the bar respectively.

[0055] 5) Aging treatment

[0056] The treated bar is subjected to aging treatment at a temperature of 350-850°C for 0.5-12h.

[0057] 6) Cold drawing

[0058] The bar obtained in step 5 is subjected to cold drawing again, and the deformation amount is 20%-80% to reach the final product size.

[0059] 7) Post-treatment

[0060] The cold-drawn bar is subjected to surface oil removal, polishing, polishing and other treatments to obtain the final product.

[0061] The present application is further illustrated by specific examples as follows:

[0062] Example 1

[0063] A method for preparing a gradient structure nanostructured stainless steel bar comprises the following steps:

[0064] 1) Ti, Sc micro-alloying

[0065] A stainless steel is melted according to the following table of ingredients and is fully heat treated to ensure uniform diffusion of alloying elements, and then a bar of a specified size is prepared.

[0066] The chemical composition table (wt%) of the bar produced in this example

[0067]

[0068] 2) Cyclic cold drawing + cyclic aluminizing treatment

[0069] The cold drawing deformation amount is 10%, and after deformation, the surface of the stainless steel bar is polished and placed in the aluminizing powder. The aluminizing temperature is 600°C, the aluminizing time is 5h, and the proportion of aluminum powder in the aluminizing powder is 25%. The above is one cycle, and a total of 2 cycles are performed.

[0070] 3) Low-temperature thermal diffusion treatment

[0071] After the above step 2 treatment, the aluminizing thickness of the bar reaches 50μm. In order to diffuse the aluminizing layer to the core, the heat treatment temperature is 200°C, and the heat preservation time is 12h.

[0072] 4) Free end twisting treatment

[0073] Using a torsion processing device, the rod is clamped at both ends and twisted 180° in one direction. After the torsion is completed, the rod is allowed to fully rebound.

[0074] 5) Aging treatment

[0075] The treated rod is subjected to aging treatment at a temperature of 450°C for 12h.

[0076] 6) Cold drawing

[0077] The rod obtained in step 5 is subjected to cold drawing again, with a deformation of 60%, to achieve the final product size.

[0078] 7) Post-treatment

[0079] The cold-drawn rod is subjected to surface oil removal, polishing, and polishing, etc., to obtain the final product.

[0080] Example 2

[0081] A method for preparing a gradient structure nanostructured stainless steel rod comprises the following steps:

[0082] 1) Ti, Sc micro-alloying

[0083] A stainless steel is melted according to the following table of ingredients and is fully heat treated to ensure uniform diffusion of alloying elements, and then a rod of a specified size is prepared.

[0084] The chemical composition table of the rod produced in this example (wt%)

[0085]

[0086] 2) Cyclic cold drawing + cyclic aluminizing treatment

[0087] The cold drawing deformation is 20%, and after deformation, the stainless steel rod is polished and placed in an aluminizing powder, with an aluminizing temperature of 850°C, an aluminizing time of 4h, and an aluminum powder proportion of 25% in the aluminizing powder. The above is one cycle, and a total of 4 cycles are performed.

[0088] 3) Low-temperature thermal diffusion treatment

[0089] After the above step 2 treatment, the aluminizing thickness of the rod reaches 150μm. In order to diffuse the aluminizing layer to the core, the heat treatment temperature is 200°C, and the heat preservation time is 12h.

[0090] 4) Free end torsion treatment

[0091] Using a torsion processing device, the rod is clamped at both ends and twisted 90° in one direction. After the torsion is completed, the rod is allowed to fully rebound.

[0092] 5) Aging treatment

[0093] The treated rod is subjected to aging treatment at a temperature of 450°C for 12h.

[0094] 6) Cold drawing

[0095] The rod obtained in step 5 is subjected to cold drawing again, with a deformation of 40%, to reach the final product size.

[0096] 7) Post-treatment

[0097] The cold-drawn rod is subjected to surface oil removal, polishing, polishing and other treatments to obtain the final product.

[0098] Example 3

[0099] A method for preparing a gradient structure nanostructured stainless steel rod comprises the following steps:

[0100] 1) Ti, Sc micro-alloying

[0101] A stainless steel is melted according to the following table of ingredients and is fully heat treated to ensure uniform diffusion of alloying elements, and then a rod of a specified size is prepared.

[0102] The chemical composition table of the rod produced in this example (wt%)

[0103]

[0104] 2) Cyclic cold drawing + cyclic aluminizing treatment

[0105] The cold drawing deformation is 30%, and after deformation, the stainless steel rod is polished and placed in the aluminizing powder, the aluminizing temperature is 650°C, the aluminizing time is 6h, the proportion of aluminum powder in the aluminizing powder is 25%, and the above is one cycle, a total of 5 cycles.

[0106] 3) Low temperature thermal diffusion treatment

[0107] After the above step 2 treatment, the aluminizing thickness of the rod reaches 300μm, in order to make the aluminizing layer diffuse to the core, the heat treatment temperature is 200°C, and the heat preservation time is 12h.

[0108] 4) Free end torsion treatment

[0109] Using a torsion processing device, the rod is clamped at both ends and twisted in one direction by 240°, and after the torsion is completed, the rod is allowed to fully rebound.

[0110] 5) Aging treatment

[0111] The treated rod is subjected to aging treatment at a temperature of 450°C for 12h.

[0112] 6) Cold drawing

[0113] The rod obtained in the step 5 is subjected to cold drawing again, and the deformation amount is 80%, so as to reach the final product size.

[0114] 7) Post-treatment

[0115] The cold-drawn rod is subjected to surface oil removal, polishing, polishing and other treatments, so as to obtain the final product.

[0116] The mechanical property test results of the above examples are shown in the following table:

[0117] Table 1 Mechanical property table

[0118]

[0119] From the above data, it can be seen that the preparation method of the gradient nano-structured stainless steel rod provided in the application can improve the tensile strength of the material while keeping the plasticity of the material basically unchanged through the methods of component alloying, cyclic cold drawing, cyclic aluminizing, aging local precipitation and plastic deformation of free end twisting.

[0120] Finally, it should be pointed out that the above examples are only used to illustrate the technical solutions of the application and are not limiting. Although the application has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the application can be modified or replaced equivalently without departing from the purpose and scope of the technical solutions, and they should be covered in the scope of the claims of the application.

Claims

1. A method for producing a gradient nanostructured stainless steel bar, characterized by, The method comprises the following steps: S1: preparing a sized bar, and ingredients of the bar are formulated according to the following chemical components and mass fractions: 0.7%≤Ti≤1.0%, 0.005%≤Sc≤0.3%, 0.02%≤C≤0.08%, 0.15%≤Si≤0.45%, 1.0%≤Mn≤2.0%, 0.02%≤P≤0.03%, 22%≤Cr≤24%, and the balance is iron and inevitable impurities; S2: cold-drawing and embedding treatment, the bar prepared in S1 is subjected to cold-drawing treatment, and then is polished after the cold-drawing, and then is placed in an aluminizing powder for aluminizing embedding; S2 is cyclically performed for at least 2 times; S3: heat diffusion of the bar after the cycle of S2; S4: free-end torsion treatment, the bar obtained in S3 is subjected to torsion, and the bar is allowed to rebound; S5: aging treatment.

2. The method of claim 1, wherein the method further comprises: The Ti and Sc raw materials in step S1 are in the form of particles or blocks. ​ 3. The method of claim 1, wherein the method further comprises: In step S2, the deformation amount of the single pass of the cold-drawing treatment is 10%-30%. ​ 4. The method of claim 1, wherein the method further comprises: In step S2, the aluminizing temperature is 600℃-850℃, and the aluminizing time of each time is 0.5h-24h. ​ 5. The method of claim 1, wherein the gradient nanostructured stainless steel bar is prepared by the steps of: In step S2, the aluminizing agent comprises, in terms of weight components: aluminum powder, 2%-35%; iron powder, 5-10%; ammonium chloride, 1%-4%; and the balance is aluminum oxide. ​ 6. The method of claim 1, wherein the gradient nanostructured stainless steel bar is prepared by the steps of: In step S2, the cycle number of step S2 is not more than 10 times. ​ 7. The method of claim 1, wherein the gradient nanostructured stainless steel bar is prepared by the steps of: After the cycle of step S2, the aluminizing thickness of the bar is 50μm-600μm. ​ 8. The method of claim 1, wherein the gradient nanostructured stainless steel bar is produced by the steps of: In step S3, the heat treatment temperature is 200℃-350℃, and the holding time is 5h-72h. ​ 9. The method of claim 1, wherein the gradient nanostructured stainless steel bar is produced by the steps of: The heat treatment temperature in step S3 is lower than the temperature at which an intermediate phase is formed between Al and other components in the stainless steel. ​ 10. The method of claim 1, wherein the gradient nanostructured stainless steel bar is produced by the steps of: In step S4, the torsion angle is 30°-360°. ​ 11. The method of claim 1, wherein the gradient nanostructured stainless steel bar is produced by the steps of: In step S4, the deep cooling treatment is further included, after the bar is subjected to the free-end torsion treatment, before the bar is fully rebounded, the deep cooling treatment is performed, the bar is immersed in liquid nitrogen, after the bar is fully cooled, the bar is taken out, and the bar is fully placed at room temperature. ​ 12. The method of claim 4, wherein the gradient nanostructured stainless steel bar is prepared by the steps of: In step S4, the bar is kept for 0.5h-24h after the torsion, and the torsion stress is released. ​ 13. The method of claim 1, wherein the gradient nanostructured stainless steel bar is produced by the steps of: In step S5, the aging treatment temperature is 350℃-850℃, and the holding time is 0.5h-12h. ​ 14. The method of claim 1, wherein the gradient nanostructured stainless steel bar is produced by the steps of: Step S6 is further included, after step S5, the bar is subjected to cold-drawing to reach the product design size. ​ 15. The method of claim 14, wherein the gradient nanostructured stainless steel bar is produced by the following steps of: The deformation amount of step S6 is 20%-80%. ​ 16. The method of claim 1, wherein the gradient nanostructured stainless steel bar is produced by the steps of: Step S7 is further included, the bar is subjected to surface oil removal, polishing and polishing treatment to obtain a final product. ​

Images