CN110923430A - Preparation method of high-strength and high-plasticity 304 austenitic stainless steel with low martensite content - Google Patents
Preparation method of high-strength and high-plasticity 304 austenitic stainless steel with low martensite content Download PDFInfo
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Abstract
The invention discloses a preparation method of high-strength and high-plasticity 304 austenitic stainless steel with low martensite content, and belongs to the technical field of metal material strengthening. The method comprises the steps of performing small-angle circular reciprocating torsional deformation treatment on a coarse grain 304 stainless steel bar or pipe, so that a dislocation and nanometer twin deformation microstructure in gradient distribution is introduced into an original coarse grain structure of 304 stainless steel, and the martensite structure content on the surface layer of the material is lower than 7%; the small-angle cyclic torsional deformation treatment is as follows: one end of the bar or the pipe is fixed, force is applied to enable the other end of the bar or the pipe to rotate around the central axis in a reciprocating mode, and the cycle of the reciprocating rotation is one cycle of the twisting. Compared with a uniform coarse-grain structure with the same components, the yield strength of the metal material treated by the cyclic reciprocating torsion process is improved by more than 1.5 times, and the metal material has uniform plasticity equivalent to that of coarse grains.
Description
Technical Field
The invention relates to the technical field of metal material strengthening, in particular to a preparation method of high-strength and high-plasticity 304 austenitic stainless steel with low martensite content.
Background
304 austenitic stainless steel is used as the largest and most widely applied austenitic stainless steel, has the advantages of high toughness, plasticity, easy cutting property, good corrosion resistance and the like, and is widely applied to almost all industrial fields of aerospace, petrochemical industry, transportation and the like. In recent years, with the development of society, resources are increasingly in short, energy consumption and environmental pollution are increasingly aggravated, which forces metal components and equipment to be continuously developed towards high performance, light weight, energy conservation and environmental protection, and makes the service environment more and more complex. However, the austenitic 304 stainless steel has low yield strength and hardness, and has insufficient fatigue resistance, frictional wear resistance and local corrosion resistance, thereby severely limiting the application of the austenitic 304 stainless steel in severe working condition environments.
Over the last century, the core problem of material research has been how to improve the strength of materials. To date, a series of techniques for reinforcing a metal material by regulating the composition, microstructure, and internal defects of the material have been developed, including solid solution reinforcement, deformation reinforcement, dispersion reinforcement, and the like. Among these techniques, the use of deformation strengthening (e.g., cold rolling) to obtain the martensite phase is currently the most common way to strengthen 304 stainless steel. Although the introduction of a high-strength martensite structure can improve the strength of the material to some extent, the plasticity and the toughness of the material are deteriorated, and the work hardening capacity is reduced. Furthermore, the martensite/austenite phase interface negatively affects the plasticity and corrosion resistance of the material. More importantly, the martensite structure is ferromagnetic at normal temperature and shows strong magnetization in a magnetic field, so that the martensite structure is not beneficial to being used in the magnetic field environments of superconducting power generation, large nuclear fusion devices, magnetic suspension trains, submarines and the like.
In the last 80 s, the german scientist Gleiter taught the concept of nanostructured materials, i.e., the reduction of structural units of materials (e.g., grain size in polycrystalline materials) to the nanometer scale, which were characterized by a significant structural feature containing a large number of grain boundaries or other interfaces. A large number of research results show that the micron grain size of the 304 austenitic stainless steel can be uniformly refined to the nanometer scale (namely a nano-structure material) by utilizing severe plastic deformation technologies such as equal channel angular pressing, high-pressure torsion and the like. The strength of the obtained uniform nanostructured material is several times that of the coarse crystalline structure, but its uniform plasticity is almost 0. This is mainly due to the fact that the high density of defects in the homogeneous nanostructured material itself severely inhibits the propagation of dislocations and the occurrence of strain localization, resulting in a loss of work hardening capability. In addition, these nanostructures are mainly martensitic due to severe plastic deformation and have significant magnetic properties. Therefore, how to prepare the 304 austenitic stainless steel with high strength and high plasticity and less or no martensite in order to meet the use requirements under the more severe working condition environment is an important technical problem to be solved at present.
In recent years, the concept of microstructure gradient is gradually applied to engineering material design, wherein a gradient nanocrystalline structure (i.e. a continuous increase of the grain size from nanometer scale to micrometer scale, GNG) is a typical construction structure for optimizing the comprehensive performance of metal materials. For example, GNGs 304 prepared using surface mechanical milling techniques have not only higher strength, but also good tensile plasticity, primarily due to the constraining effect of the macrocrystalline matrix on strain localization of the GNG surface layer. However, since several surface deformation processes such as surface mechanical milling and surface rolling process techniques developed at present require high-speed milling or rolling of metal surfaces with cemented carbide tips having specific geometrical dimensions, the process techniques are severe and the process efficiency is low. More importantly, the 304 austenitic stainless steel sample treated by the deformation technology still has a high-density martensite structure on the surface, thereby severely limiting the wide application in the industrial environment with the magnetic field.
Disclosure of Invention
The invention aims to provide a preparation method of high-strength and high-plasticity 304 austenitic stainless steel with low martensite content, which is characterized in that coarse grain 304 stainless steel is subjected to cyclic torsion processing, so that no or only a small amount of austenite structure on the surface layer of the material is converted into martensite, and meanwhile, the processing process can obviously improve the strength of the 304 stainless steel and does not obviously reduce the uniform plasticity and the fracture elongation of the 304 stainless steel.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a preparation method of high-strength and high-plasticity 304 austenitic stainless steel with low martensite content comprises the steps of carrying out small-angle cyclic reciprocating torsional deformation treatment on a bar or a pipe of coarse grain 304 stainless steel, thereby introducing dislocation and nanometer twin deformation microstructures in gradient distribution into an original coarse grain structure of the 304 stainless steel, and simultaneously, no or only a small amount of austenite structures on the surface layer of the material are converted into martensite structures; the small-angle cyclic torsional deformation treatment is as follows: one end of the bar or the pipe is fixed, force is applied to enable the other end of the bar or the pipe to rotate around the central axis in a reciprocating mode, and the cycle of the reciprocating rotation is one cycle of the twisting.
The axial length of the coarse-grain 304 stainless steel bar or pipe is more than 10mm, and the diameter of the coarse-grain 304 stainless steel bar or pipe is more than 1 mm.
The one-time reciprocating rotation means that the other end of the bar or the pipe rotates around the central axis, the bar or the pipe rotates clockwise by a rotation angle theta from the initial position and then returns to the initial position by a counterclockwise rotation angle theta, and the bar or the pipe completes one-time reciprocating rotation, namely one cycle of twisting; and then the bar or the pipe is subjected to the next reciprocating rotation process from the initial position, and the process is repeatedly circulated until the required cycle twisting cycle is reached.
The angle theta of each rotation of the bar or the pipe is the amplitude of the torsion angle, the amplitude of the torsion angle is 2-30 degrees, the torsion rate is 100-4000 degrees/min, and the cycle torsion frequency is 2-500. And theta is taken as a fixed value in the process of each cycle of reciprocating torsional deformation.
The invention introduces deformation microstructures such as dislocation, nanometer twin crystal and the like in gradient distribution from the surface to the inside of 304 stainless steel through a cyclic reciprocating torsional deformation process, and keeps a crude crystal structure in an original state; the volume content of the martensite structure of the surface layer of the 304 stainless steel is less than 7 percent.
In the direction vertical to the surface of the 304 stainless steel, the microhardness shows a continuous gradient change from high to low from outside to inside; compared with the uniform coarse-grain structure of the original 304 stainless steel before processing, the yield strength of the 304 stainless steel after the treatment of the cyclic reciprocating torsion process is improved by more than 1.5 times, the uniform plasticity and the elongation at break which are equivalent to the original structure are kept, and the good strength and plasticity matching is realized.
The invention has the following advantages:
1. the method of the invention fully uses the characteristics of the traditional fatigue test cyclic deformation, and utilizes the twisting equipment to carry out cyclic twisting deformation treatment on the metal bar and the pipe under a smaller twisting angle (less than 30 degrees), so that the shearing plastic strain with gradient distribution can be generated from the surface to the inside of the metal material. Compared with large-angle (>30 ℃) torsional deformation, the single small-angle cycle torsional strain and stress are small, the martensite phase transformation cannot be caused, or only a small amount of martensite phase is generated, and the macroscopic shape and the surface appearance of the sample are not obviously changed; but by increasing the twisting frequency, the twisting strain can be increased, so that high-density dislocation, nanometer twin crystal and other microstructures are introduced, the gradient distribution deformation microstructure is introduced, and the original state structure is reserved by regulating and controlling the twisting frequency.
2. The high-strength and high-plasticity 304 austenitic stainless steel with low martensite content has no or only a small amount of martensite phase (the volume content is less than 7 percent) on the surface layer, so the invention has wide application prospect in a plurality of industrial fields with magnetic fields, such as electric power, rail transit, buildings, national defense and military industry and the like. For example, the non-magnetic steel for the power industry can reduce the magnetic leakage at the end part of the rotor and improve the energy utilization efficiency; in surface ships and underwater submarines, the use of non-magnetic steel can effectively avoid the tracking of radar and has good effect on stealth.
3. Different from the tiny nanocrystalline or nano twin crystal structure obtained on the surface of a sample by the traditional surface mechanical deformation process, the invention introduces deformation microstructures such as dislocation, nano twin crystal and the like in gradient distribution from the surface to the inside of a metal material through the cyclic reciprocating torsional plastic deformation, but retains the original coarse crystal structure, and part of austenite structure on the outermost surface of the material is converted into a martensite structure. Therefore, the 304 stainless steel material treated by the cyclic reciprocating torsion process has uniform plasticity and elongation at break equivalent to the original structure, and simultaneously has higher yield strength and tensile strength and good strength and plasticity matching.
4. The traditional severe plastic deformation and surface mechanical deformation process uses complicated equipment, for example, the surface mechanical grinding technology needs to use a cemented carbide tool bit with special geometric dimensions to grind or roll the metal surface, the process technology is harsh, the treatment efficiency is low, and the treatment time of one sample is about 2 hours or more. In addition, since the surface plastic deformation technology mainly refines the structure by grinding or rolling the sample surface through the tool bit to generate plastic deformation, the sample diameter is often required to be large (at least larger than 4mm) in order to prevent the sample from bending deformation during the processing. The invention has simple process, low requirement on equipment and high treatment efficiency, and the time for treating one sample is about tens of seconds to several minutes. Due to the process characteristics, the technology has small size limitation on the processed samples, can process bar and pipe samples with different sizes to meet the service requirements of workpieces, has important significance on light weight of mechanical equipment, energy conservation and emission reduction, and has wide application prospect in industry.
Drawings
FIG. 1 is a schematic view of the cyclic torsion process of the present invention.
FIG. 2 is a scanning electron micrograph of a surface microstructure obtained after a 304 stainless steel bar sample is twisted for 200 weeks under a condition of a twist angle amplitude of 5 ° in example 1; wherein: the lower three figures (a), (b) and (c) are enlarged views of the three positions a, b and c in the upper figure, respectively.
FIG. 3 is a scanning electron micrograph of a surface microstructure obtained after a 304 stainless steel bar sample is twisted for 200 weeks under a condition of a twist angle amplitude of 15 ° in example 2; wherein: the lower three figures (a), (b) and (c) are enlarged views of the three positions a, b and c in the upper figure, respectively.
Fig. 4 is a phase distribution diagram of a surface microstructure obtained after twisting a 304 stainless steel rod sample for 200 cycles under a torsion angle amplitude of 15 ° in example 2.
FIG. 5 is the XRD results of the skin, subsurface and core portions after twisting a 304 stainless steel bar sample for 200 weeks under the conditions of twisting angle amplitudes of 5 ° and 15 ° in examples 1 and 2; wherein: (a) comparative example 1; (b) example 1.
FIG. 6 is a transmission electron micrograph and a microstructure size statistical chart of a surface microstructure obtained after a 304 stainless steel bar sample is twisted for 200 weeks under a condition of a torsion angle amplitude of 15 degrees in example 2; wherein: (a) a core microstructure; (b) mean dislocation wall and dislocation cell size; (c) twinning in the crystal grains; (d) average twin slice thickness.
Fig. 7 is a graph of the microhardness as a function of depth from the surface of 304 stainless steel samples treated by the cyclic torsional deformation process in examples 1 and 2.
Fig. 8 is a uniaxial tensile engineering stress-strain curve of a 304 stainless steel sample treated by a cyclic torsional deformation process in example 1 and example 2.
Detailed Description
The present invention is described in detail below by way of examples.
The invention provides a preparation method of high-strength and high-plasticity 304 austenitic stainless steel with low martensite content, which is characterized in that small-angle cyclic reciprocating torsional deformation treatment is applied to a coarse grain sample of original 304 stainless steel, so that deformation microstructures such as dislocation, nanometer twin and the like in gradient distribution are introduced into the material, the coarse grain structure of the original state is retained, and meanwhile, the austenitic structure on the outermost surface of the material is not converted into a martensite structure or only a small amount of the austenitic structure is converted into a martensite phase. Wherein the torsion angle amplitude is between 2 degrees and 30 degrees, and the cycle torsion frequency is between 2 and 200.
In the following embodiments, the device for applying a torsional force to a macrocrystalline 304 stainless steel sample (bar or tube) is only required to be capable of fixing one end of the sample and applying a force to the other end of the sample to rotate around its own central axis, and is not limited to a specific structure (e.g., a fatigue tester, a tube torsion tester, etc.) as long as the above-described function is achieved.
The small-angle circulating torsional deformation treatment in the invention is as follows: one end of the bar or the pipe is fixed, force is applied to enable the other end of the bar or the pipe to rotate around the central axis in a reciprocating mode, and the cycle of the reciprocating rotation is one cycle of the twisting.
The one-time reciprocating rotation means that the other end of the bar or the pipe rotates around a central axis, the bar or the pipe rotates clockwise by a rotation angle theta (torsion angle amplitude) from an initial position and then returns to the initial position by the counterclockwise rotation angle theta, and the bar or the pipe completes one-time reciprocating rotation, namely one-cycle torsion cycle; and then the bar or the pipe is subjected to the next reciprocating rotation process from the initial position, and the process is repeatedly circulated until the required cycle twisting cycle is reached. And theta is taken as a fixed value in the process of each cycle of reciprocating torsional deformation.
Example 1:
in this embodiment, the coarse grain 304 austenitic stainless steel bar is subjected to small-angle cyclic reciprocating torsional deformation treatment.
The coarse grain 304 austenitic stainless steel sample was fixed at one end and subjected to cyclic torsional deformation treatment (as shown in fig. 1) at the other end to obtain a gradient structure 304#1 sample.
The process parameters for the gradient structure 304#1 sample were selected as: the diameter of the 304 austenitic stainless steel is 6mm, the amplitude of the torsion angle is 5 degrees, the torsion rate is 9000 degrees/min, the torsion frequency is 200 weeks, the torsion time is about 14 seconds, and the torsion temperature is 25 degrees at room temperature.
The degree of deformation in the material shows a monotonically decreasing trend with increasing depth from the surface, resulting in a graded distribution of deformed microstructures (fig. 2). But the microstructure is still austenitic with no martensite phase formed (fig. 5 (a)). The microhardness of the 304#1 sample of the gradient structure is gradually reduced from 3.2GPa to 2.0GPa along with the increase of the depth from the surface of the material, and the sample has the characteristic of gradient change and is obviously higher than that of a coarse-grained 304 austenitic stainless steel sample (1.8GPa), as shown in FIG. 7.
In this example, a surface gradient structure 304#1 sample was subjected to a uniaxial tensile test at room temperature, and the engineering stress-strain curve is shown in fig. 8, where the yield strength in uniaxial tension is 306MPa, the tensile strength is 606MPa, and the yield strength is about 1.5 and 1.05 times of the original coarse-grained structure; the uniform elongation is 66.8%, and the fracture elongation is 87.6%, which is equivalent to the coarse-grain structure.
Example 2:
in this example, a coarse grain 304 austenitic stainless steel # 2 sample was subjected to a cyclic torsional deformation treatment to obtain a gradient structure 304#2 sample.
The difference from example 1 is that the process parameters of the 304#2 sample of the gradient structure were selected to be 6mm diameter of the 304 austenitic stainless steel, 15 degrees of torsional angle amplitude, 9000 °/min of torsional rate, 200 cycles of torsion, about 6min of torsional time, and 25 degrees of torsional temperature at room temperature.
The degree of deformation in the material shows a monotonically decreasing trend with increasing depth from the surface, resulting in a graded distribution of deformed microstructures (fig. 2). The outermost surface is still a coarse crystal structure (figure 4), high-density dislocation, a large number of dislocation cells and dislocation walls are distributed in the coarse crystal (figure 6(a)), and the average dislocation wall and cell size is 250nm (figure 6 (b)); in addition, a high density of nano-scale twin bundles was found inside some of the grains (fig. 3 and 6(c)), and the average twin lamella thickness was about 16nm (fig. 6 (d)). At the core of the sample, a large number of dislocation cells and dislocation wall structures were also seen, with the average dislocation wall and cell size being 850 nm. The EBSD results show that the microstructure of the sample is still predominantly austenitic with very little martensite phase formed only at the very surface (fig. 5(b)), and calculations show that the volume fraction of martensite is about 7%.
The microhardness of the sample 304#2 with the gradient structure is still characterized by the gradient change along with the increase of the depth from the surface, and the hardness of the outermost layer is about 4.2GPa, which is obviously higher than that of the coarse-grained 304 austenitic stainless steel sample (1.8GPa), as shown in FIG. 7.
In this example, a surface gradient structure 304#2 sample was subjected to a uniaxial tensile test at room temperature, and the engineering stress-strain curve is shown in fig. 8, where the yield strength in uniaxial tension is 442MPa, the tensile strength is 661MPa, and is about 2.1 and 1.1 times of the coarse-grained structure; the uniform elongation was 53.7% and the elongation at break was 78.5%.
Comparative example 1:
the ordinary annealed coarse grain 304 austenitic stainless steel (grain size about 100 μm) was stretched at room temperature, and had a yield strength of 210MPa, a tensile strength of 581MPa, a uniform elongation of 70.3%, and an elongation at break of 97.1%, as indicated by the dotted line in FIG. 8.
Therefore, the macrocrystalline structure has low yield strength and tensile strength, although it has good tensile plasticity.
Comparative example 2:
mallick et al, an indian scientist, produced a sample of a severely deformed structure 304 using a low temperature (-196 degrees) rolling technique and having a volume fraction of martensite of 44%. Tensile testing indicated that the 304 stainless steel sample had a yield strength as high as 1400MPa, but a uniform elongation of only 9%.
Comparative example 3:
the CG 304 austenitic stainless steel sample is subjected to dynamic plastic deformation treatment by the aid of liquid nitrogen temperature by Haohyu et al, a metal research institute of Chinese academy of sciences, and the 304 stainless steel with the mixed structure consisting of martensite and residual austenite is obtained. Wherein the volume fraction of martensite is 87%, and the retained austenite is mainly composed of nanocrystals with grain size less than 100 nm. The yield strength of this sample was as high as 1250MPa but there was almost no tensile plasticity (< 1%). In order to inhibit the generation of martensite in the deformation process, a nano-twin mixed structure 304 austenitic stainless steel with nano-twin mixed structure samples containing nano-twin crystal austenite grains with different volume fractions is prepared by a temperature rise (150 ℃) dynamic plastic deformation technology. Wherein the average twin lamella thickness of the nano twin crystal is 10nm, the volume fraction is about 58%, and the dislocation structure size is about several hundred nanometers. The sample strength was as high as 1135MPa but again there was no uniform tensile plasticity (< 1%). Therefore, poor plasticity of homogeneous nanostructured metals severely limits their practical applications.
Comparative example 4:
the Weiyujie (see application No. 201310206344.3) of the mechanics research institute of Chinese academy of sciences applies single large-angle (270 ℃) torsional deformation to a cold-rolled 304 austenitic stainless steel sample to obtain a structure with high-density martensite and deformation twin crystals distributed in a gradient manner from the surface to the core of the sample. The tensile test shows that the yield strength of the sample after large-angle torsional deformation is 660MPa, which is 2 times of that of the untreated cold-rolled 304 austenitic stainless steel sample (310 MPa), the uniform strain is about 30 percent, and the plasticity of the sample after the treatment by the process is obviously lower.
The result shows that the coarse-grained 304 stainless steel sample is subjected to small-angle cyclic reciprocating torsional deformation, so that the deformation microstructures such as dislocation, nanometer twin and the like in gradient distribution are introduced, the original coarse-grained structure is retained, and no or only a small amount of austenite is converted into a martensite phase on the surface of the sample. The yield strength of the metal material treated by the cyclic reciprocating torsion process is improved by more than 1.5 times, the uniform plasticity and the fracture elongation rate which are equivalent to those of the original structure are kept, and the metal material has good strength and plasticity matching.
Claims (7)
1. A method of making a high strength and high plasticity 304 austenitic stainless steel having a low martensite content, characterized by: the method comprises the steps of carrying out small-angle cyclic reciprocating torsional deformation treatment on a coarse grain 304 stainless steel bar or pipe, introducing dislocation and nanometer twin deformation microstructures in gradient distribution into an original coarse grain structure of 304 stainless steel, and simultaneously converting no or only a small amount of austenite structures on the surface layer of the material into martensite structures; the small-angle cyclic torsional deformation treatment is as follows: one end of the bar or the pipe is fixed, force is applied to enable the other end of the bar or the pipe to rotate around the central axis in a reciprocating mode, and the cycle of the reciprocating rotation is one cycle of the twisting.
2. The method of making a high strength and high plasticity 304 austenitic stainless steel with low martensite content according to claim 1, wherein: the axial length of the coarse-grain 304 stainless steel bar or pipe is more than 10mm, and the diameter of the coarse-grain 304 stainless steel bar or pipe is more than 1 mm.
3. The method of making a high strength and high plasticity 304 austenitic stainless steel with low martensite content of claim 2, wherein: the one-time reciprocating rotation means that the other end of the bar or the pipe rotates around the central axis, the bar or the pipe rotates clockwise by a rotation angle theta from the initial position and then returns to the initial position by a counterclockwise rotation angle theta, and the bar or the pipe completes one-time reciprocating rotation, namely one cycle of twisting; and then the bar or the pipe is subjected to the next reciprocating rotation process from the initial position, and the process is repeatedly circulated until the required cycle twisting cycle is reached.
4. A method of making a high strength and high plasticity 304 austenitic stainless steel with low martensite content as claimed in claim 3, characterized in that: the angle theta of each rotation of the bar or the pipe is the amplitude of the torsion angle, the amplitude of the torsion angle is 2-30 degrees, the torsion rate is 100-10000 degrees/min, and the cycle torsion frequency is 2-500.
5. A method of making a high strength and high plasticity 304 austenitic stainless steel with low martensite content as claimed in claim 3, characterized in that: and theta is taken as a fixed value in the process of each cycle of reciprocating torsional deformation.
6. A method of making a high strength and high plasticity 304 austenitic stainless steel with low martensite content as claimed in claim 3, characterized in that: introducing deformation microstructures such as dislocation, nanometer twin crystal and the like in gradient distribution from the surface to the inside of the 304 stainless steel through a cyclic reciprocating torsional deformation process, and keeping a coarse crystal structure in an original state; the volume content of the martensite structure of the surface layer of the 304 stainless steel is less than 7 percent.
7. The method of making a high strength and high plasticity 304 austenitic stainless steel with low martensite content of claim 6, wherein: in the direction vertical to the surface of the 304 stainless steel, the microhardness shows a continuous gradient change from high to low from outside to inside; compared with the uniform coarse-grain structure of the original 304 stainless steel before processing, the yield strength of the 304 stainless steel after the treatment of the cyclic reciprocating torsion process is improved by more than 1.5 times, the uniform plasticity and the elongation at break which are equivalent to the original structure are kept, and the good strength and plasticity matching is realized.
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| CN114749506A (en) * | 2022-03-23 | 2022-07-15 | 南京理工大学 | Gradient structure bar and preparation device and method thereof |
| CN117385149A (en) * | 2023-10-16 | 2024-01-12 | 河北工程大学 | Austenitic stainless steel wire rod with gradient nano dislocation structure and preparation method thereof |
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