CN112553433B - Steel with bionic multi-layer structure and processing technology thereof - Google Patents

Steel with bionic multi-layer structure and processing technology thereof Download PDF

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CN112553433B
CN112553433B CN202011415419.5A CN202011415419A CN112553433B CN 112553433 B CN112553433 B CN 112553433B CN 202011415419 A CN202011415419 A CN 202011415419A CN 112553433 B CN112553433 B CN 112553433B
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杨盛超
张新宇
刘志勇
刘伟
黄超
吴建宁
孟桂花
李文娟
曲欣
崔林
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Xinjiang Tianfu Energy Co ltd
Shihezi University
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Shihezi University
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
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    • C23C8/22—Carburising of ferrous surfaces

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Abstract

The invention relates to a steel with a bionic multi-layer structure and a processing technology thereof. A processing technology of steel with a bionic multi-level structure comprises the following steps: s10: decarburizing and carburizing the steel raw material in the range of an austenite phase region; the carbon content of the austenite phase region is 0.4-0.8 wt%, and the temperature is 700-1500 ℃; s20, cooling: cooling the steel raw material treated in the step S20 under the protection of argon, and reducing the temperature to be not more than 250 ℃ within 20-50 min; s30: and cooling the steel raw material treated in the step S20 to room temperature, and then tempering in an argon atmosphere to obtain the steel with the bionic multi-level structure. The invention adopts the decarburization and carburization technology to change the spatial distribution of carbon elements in steel to control the phase transformation interval, further to hierarchically transform the microstructure of the martensitic steel, and finally to control the spatial mechanical property distribution so as to meet the fatigue design requirement.

Description

Steel with bionic multi-layer structure and processing technology thereof
Technical Field
The invention belongs to the technical field of metal materials, and particularly relates to a steel with a bionic multi-level structure and a processing technology thereof.
Background
At the end of 2018, the national railway mileage reaches 13.1 kilometers, and the composite increase rate of mileage of newly added railways in 2013 and 2018 is 8%. The steel has the advantages of price, mechanical property, recycling and the like, and is used as a main structural material of railway engineering. High performance rails have broad application prospects, and the application of many high strength steels in moving structures is limited by their fatigue properties. This form of loading occurs in almost all engineering applications, with about 90% of metal material failures being due to fatigue failure. Therefore, a new fatigue design mode and idea to improve the fatigue life of the mobile structure has great engineering requirements and social value.
Fatigue failure can be simply divided into three stages of crack nucleation, crack propagation and rapid fracture. Generally, fatigue cracks tend to nucleate and propagate inwards on the surface of a material, higher material strength is beneficial to preventing the crack nucleation process, but the crack propagation is accelerated, and higher material toughness is beneficial to slowing the crack propagation but is not beneficial to preventing the crack nucleation. Excellent fatigue performance requires that the material has both excellent strength and toughness, but the two are difficult to obtain in material design, which is also the most classical problem in material design.
In view of the above, the invention provides a new structural design method and a new processing method for improving the fatigue resistance of steel, which are a brand new fatigue design idea, and are inspired from the structure of teeth, and the bionic material is manufactured so that the macroscopic mechanical properties in the material are distributed in a multi-level manner, and the fatigue resistance of the material can be greatly improved.
Disclosure of Invention
The invention aims to provide a processing technology of a steel with a bionic multi-hierarchy structure, which controls the diffusion of carbon element at an austenite temperature through a decarburization and carburization technology, so that the carbon element has designed distribution in a sample, and further the tissue phase change and the mechanical property distribution in the sample cooling process are influenced, the bionic steel with the hierarchical structure at a macroscopic scale is formed, the mechanical property has diversity in the whole space to meet different requirements of fatigue crack growth stages on the material property, and the classic problem that the material strength and the toughness are difficult to obtain simultaneously in fatigue design is avoided, namely, the important problem that the nucleation resistance and the long crack propagation resistance of a homogeneous high-strength steel fatigue crack cannot be obtained simultaneously in the traditional fatigue design is solved by utilizing the hierarchical structure design.
In order to realize the purpose, the adopted technical scheme is as follows:
a processing technology of steel with a bionic multi-level structure comprises the following steps:
s10: decarburizing and carburizing the steel raw material in the range of an austenite phase region; the carbon content of the austenite phase region is 0.4-0.8 wt%, and the temperature is 700-1500 ℃;
s20, cooling: cooling the steel raw material treated in the step S20 under the protection of argon, and reducing the temperature to be not more than 250 ℃ within 20-50 min;
s30: and (5) cooling the steel raw material treated in the step (S20) to room temperature, and then tempering in an argon atmosphere to obtain the steel with the bionic multi-level structure.
Further, in step S10, the phase diagram of the steel raw material is plotted by Thermo-Calc thermodynamic phase diagram calculation software to determine the austenite phase region;
in the step S30, the tempering temperature is 460 ℃, the time is 2-4h, and the flow of argon is 500cm 3 /min。
Still further, in step S30, when the grain size of the material processed in step S20 is 100 μm, the tempering time is 2 h; when the grain size of the material treated in the step S20 is 200 μm, the tempering time is 4 h; the temperature rising rate in the tempering process is controlled to be 5-10 ℃/s.
Further, the decarburizing and carburizing step in step S10 includes:
designing the spatial distribution of carbon elements by a carbon profile curve obtained by simulating the numerical solution of the Fick second law of a diffusion equation;
on the basis of the spatial distribution of carbon elements, the technical parameters for determining the time, temperature, gas flow rate and ratio of decarburization and carburization are calculated, and then decarburization and carburization are performed under the technical parameters.
Still further, the formula of the second law of the simulated diffusion equation fick is as follows:
Figure BDA0002819979960000021
the diffusion coefficient of carbon atoms in the space fraction of the designed carbon element is determined by calculation based on the following formula, wherein the temperature in the formula is Fahrenheit, and the calculation accuracy of the lattice is to divide the width of 1mm into 1000 parts;
Figure BDA0002819979960000031
still further, in step S10, the carbon content of the austenite phase region is 0.5-0.6 wt%, and the temperature is 930 ℃.
Still further, the decarbonizing step in step S10 is: putting the steel raw material into a vertical tube furnace, exhausting air, heating to 930 ℃, and introducing 430cm 3 Wet hydrogen decarburization for 4-60 h;
the carburizing step in step S10 is: after hydrogen removal, in CO/CO 2 Ratio of 25, CO flow 300cm 3 /min、CO 2 The amount is 12cm 3 Carburizing for 1-6h under the condition of/min.
Still further, in step S10: the decarburization time is 4h, or 6h, or 8h, or 10h, and the carburization time is 15-75 min;
in the decarburization in step S10: when the temperature of the steel raw material is raised to 700 ℃, the temperature raising rate is controlled to be 1-2 ℃/s, and after the temperature reaches 930 ℃, the temperature is kept for 10min, and then moist hydrogen is introduced.
Still further, the steel raw material is AISI L6 martensite tool steel, the thickness is 3mm, and the carbon content is 0.56 wt%.
Another object of the present invention is to provide a steel material having a biomimetic multi-level structure, which is produced by the above-mentioned process, and which is a biomimetic multi-level steel material imitating the hierarchical structure of human teeth by utilizing the excellent anti-fatigue performance of the multi-level structure of teeth.
Compared with the prior art, the invention has the beneficial effects that:
the traditional material design usually changes the material performance by designing the microstructure, material composition and macrostructure of the material, and compared with the traditional mode, the multi-hierarchy can introduce a new design freedom degree, thereby adjusting the material performance on the space distribution of the mechanical performance.
Multi-tiered materials are a viable route to increasing the fatigue limit of materials, especially in ferrous materials. Steels are particularly mentioned because the transformation of the structure in steel materials and the mechanical properties of the structure are exceptionally sensitive to the distribution and content of carbon elements. The invention is inspired from the structure of teeth, utilizes the excellent performance of the multi-level structure of the teeth in fatigue resistance to prepare the bionic multi-level steel which simulates the hierarchical structure of human teeth, and prepares the bionic material to ensure that the macroscopic mechanical property in the material is in multi-level distribution, and perhaps the fatigue property of the material can be greatly improved, thereby being a brand new fatigue design idea.
According to the invention, by a decarburization and carburization process, the diffusion of carbon element is controlled at an austenite temperature, so that the carbon element has a designed distribution in a sample, the tissue phase change and the mechanical property distribution in the sample cooling process are further influenced, and the bionic steel with a hierarchical structure under a macroscopic scale is formed, so that the mechanical property has diversity in the whole space to meet different requirements of fatigue crack growth stages on material properties, and the hierarchical structure has great operability and diversity and can meet the requirements of different fatigue designs.
Designing the spatial distribution of carbon elements can be realized by guiding parameters in the decarburization and carburization technology through a carbon profile curve obtained by simulating a diffusion equation (Fick's second law). The invention combines the characteristics of a multi-level structure material to solve the traditional fatigue problem, and researches the phase change control of steel, the exploration of the processing technology-level structure-level material performance relation, and the research of the basic diffusion equation simulation and fatigue theory. Meanwhile, the decarburization and carburization process is mature in technology, low in price and free of requirements on the shape of a workpiece, so that the method has large-scale industrial potential. The achievable layering scale of the project is larger, the crack nucleation and expansion are limited, and the design size and the stress service environment of the steel rail component are met.
Drawings
FIG. 1 is a process flow diagram of the present invention;
FIG. 2 is a steel material having a multi-layered structure processed from AISI L6 steel having a thickness of 3mm according to example 1 of the present invention;
FIG. 3 is a graph showing the Hardness (HV) of a steel material processed according to example 1 of the present invention as a function of Depth (Depth); a is the hardness of a surface martensite area (0-200 mu m) prepared under different conditions changes with the depth, b is the hardness of a bainite transition area (200-600 mu m) prepared under different conditions changes with the depth, and the design range of the mechanical property of the multi-level structure is large.
Detailed Description
In order to further illustrate the steel material with a bionic multi-level structure and the processing technology thereof, and achieve the intended purpose of the invention, the following detailed description is provided for the steel material with a bionic multi-level structure and the processing technology thereof according to the present invention with reference to the preferred embodiments. In the following description, different "one embodiment" or "an embodiment" refers to not necessarily the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
The steel material with a bionic multi-level structure and the processing technology thereof of the present invention will be further described in detail with reference to the following specific embodiments:
the inspiration of the invention is derived from the successful application of the multilayer structure of human teeth in the fatigue load environment, and provides a complete thought and a method for manufacturing the multilayer structure in steel, which clarifies the feasibility of the bionic hierarchical structure in fatigue design, introduces new fatigue design freedom and aims to improve the fatigue resistance of the steel by using the bionic multilayer structure. The process flow chart is shown in figure 1, and the technical scheme is as follows:
a processing technology of steel with a bionic multi-level structure comprises the following steps:
s10: decarburizing and carburizing the steel raw material in the range of an austenite phase region; the carbon content of the austenite phase region ranges from 0.4 to 0.8 wt%, and the temperature ranges from 700 ℃ to 1500 ℃.
S20, cooling: cooling the steel raw material treated in the step S20 under the protection of argon, and reducing the temperature to be not more than 250 ℃ within 20-50 min;
s30: and (5) cooling the steel raw material treated in the step (S20) to room temperature, and then tempering in an argon atmosphere to obtain the steel with the bionic multi-level structure.
Preferably, in step S10, the phase diagram of the steel raw material is drawn by Thermo-Calc thermodynamic phase diagram calculation software to determine the austenite phase region.
In the step S30, the tempering temperature is 460 ℃, the time is 2-4h, and the flow of argon is 500cm 3 /min。
Further preferably, in the step S30, when the grain size of the material processed in the step S20 is 100 μm, the tempering time is 2 h; when the grain size of the material treated in the step S20 is 200 μm, the tempering time is 4 h; the temperature rising rate in the tempering process is controlled to be 5-10 ℃/s.
Preferably, the step of decarburizing and carburizing in the step S10 is:
designing the spatial distribution of carbon elements by a carbon profile curve obtained by simulating the numerical solution of the Fick second law of a diffusion equation;
on the basis of the spatial distribution of carbon elements, the technological parameters for determining the time, temperature, gas flow rate and ratio of decarburization and carburization are calculated, and then decarburization and carburization are performed under the technological parameters.
Further preferably, the formula of the simulated diffusion equation fick's second law is as follows:
Figure BDA0002819979960000051
where Δ t in the equation represents the time increment step between time n and n +1, and Δ X is the grid spacing. At any time step n, Ci-1, Ci and Ci +1 are known, the diffusion coefficient of each grid point i can be determined; finally, Ci for the next time step n +1 will be determined by Ci-1, Ci and Ci +1 for time step n.
The diffusion coefficient of carbon atoms in the space fraction of the designed carbon element is determined by calculation based on the following formula, wherein the temperature in the formula is Fahrenheit, and the calculation accuracy of the lattice is to divide the width of 1mm into 1000 parts;
Figure BDA0002819979960000061
wherein yc ═ xc/(1-xc), xc is the mole fraction of carbon; t is the Kelvin temperature.
Further preferably, the carbon content of the austenite phase region in the step S10 is 0.5-0.6 wt%, and the temperature is 930 ℃.
Further preferably, the step of decarbonizing in step S10 is: putting the steel raw material into a vertical tube furnace, removing air, heating to 930 ℃, and introducing 430cm 3 Wet hydrogen decarburization for 4-60 h;
the carburizing step in step S10 is: after hydrogen removal, in CO/CO 2 The ratio was 25, the CO flow was 300cm 3 /min、CO 2 The amount is 12cm 3 Carburizing for 1-6h under the condition of/min.
Further preferably, in step S10: the decarburization time is 4h, or 6h, or 8h, or 10h, and the carburization time is 15-45 min;
in the decarburization in step S10: when the temperature of the steel raw material is raised to 700 ℃, the heating rate is controlled to be 1-2 ℃/s, and after the temperature reaches 930 ℃, the temperature is kept for 10min, and then moist hydrogen is introduced.
Further preferably, the steel raw material is AISI L6 martensite tool steel, the thickness is 3mm, and the carbon content is 0.56 wt%.
The decarbonization and carburization temperature is determined by comprehensively considering the components of the raw materials, the carbon content and the temperature interval of an austenite phase region and the upper temperature limit born by an instrument. In principle, the decarburization carburization is carried out at as high a temperature as possible within the range which can be tolerated by the apparatus to shorten the working time. The upper limit of the temperature of the tubular furnace equipment used by the invention is 1000 ℃, meanwhile, the austenite interval of AISI L6 is 700-1500 ℃ at 0.56 wt% C, and 930 ℃ is selected comprehensively. For the decarburization and carburization time, after the space distribution of the corresponding carbon element is designed according to the size of the required multi-level structure, a carbon profile curve obtained by numerical solution of Fick's second law of simulated diffusion equation guides the time parameter in the decarburization and carburization technology.
The invention plots a fatigue design strategy map based on component dimensions. For small-size samples, the fatigue period is dominated by surface crack nucleation, so compared with homogeneous martensite, the hierarchical surface high-strength martensite layer can effectively delay the fatigue crack nucleation, improve the fatigue life by 5 times under the stress of 900MPa, and improve the fatigue limit by 100MPa (15%). For large size samples, whose fatigue cycle is dominated by long crack growth in the pariglaw region, the lower bainite, which is graded, may reduce the long crack growth rate by 60% compared to homogeneous martensite. Whereas for the intermediate size samples, the most effective strategy is to prepare a "dental steel" of an overall hierarchical structure, following the multi-layer structure of the tooth, forming high strength martensite at the surface to limit crack nucleation, and lower bainite in the middle to retard long crack propagation, while maintaining martensite in the core region to support the material bulk strength, increasing the number of "dental steel" fatigue cycles from 1984 to about 20000 times by a factor of 10 at 700MPa test stress compared to homogeneous martensite. For a certain size of steel construction, the hierarchical structure of the 'dental steel' is an effective fatigue design strategy, and can limit crack nucleation and crack propagation at the same time. Compared with the traditional variables such as change of microstructure, surface strengthening, crack closing effect and the like, the multi-level structure can effectively improve the fatigue resistance.
The method can be applied to the fatigue performance improvement of the mobile construction with steel as the base material, can also be applied to the anti-fatigue service environment of the iron-based material, and can limit the fatigue crack nucleation and the fatigue crack propagation simultaneously by a multi-level structure.
Example 1.
The specific operation steps are as follows:
s10: AISI L6 martensite tool steel (0.56 wt% C, 1.1 wt% Cr, 1.7 wt% Ni, 0.5 wt% Mo, 0.1 wt% V) with the thickness of 3mm is taken as a research object, the decarburization and carburization technology is adopted to change the spatial distribution of carbon elements in steel materials to control a phase transformation interval, further the microstructure of the multi-level martensite steel is further changed, and the spatial mechanical property distribution of the multi-level martensite steel is finally controlled to meet the requirement of fatigue design. This inspiration stems from the successful application of the multi-layer structure of human teeth in fatigue-loaded environments. The method comprises the following three steps:
(1) the AISI L6 martensite tool steel is subjected to phase diagram drawing by Thermo-Calc thermodynamic phase diagram calculation software to determine the austenite phase region (FCC _ A1) and is subjected to decarburization and carburization in the phase region, the carbon content of the phase region ranges from 0.4 to 0.8 wt%, and the temperature ranges from 700- ℃ 。
(2) The spatial distribution of carbon elements can be designed by numerical solution of Fick's second law (Fick's second law) of simulated diffusion equation
Figure BDA0002819979960000081
The obtained carbon profile curve guides the parameters in the decarburization and carburization technology to realize.
(3) And determining the technical parameters of the decarburizing time, temperature, gas flow and ratio through the calculated spatial distribution of the carbon element. The diffusion coefficient of carbon atoms in the space fraction of the designed carbon element is determined by calculation based on the following formula, wherein Fahrenheit is used for the temperature, and the calculation accuracy of the lattice is to divide the width 1mm into 1000 parts.
Figure BDA0002819979960000082
The obtained technical parameters are as follows: firstly, decarbonization: putting the steel raw material into a vertical tube furnace, exhausting air, heating to 930 ℃, and introducing 430cm 3 The wet hydrogen gas is decarbonized for 4 to 60 hours at a/min time. ② carburizing: after hydrogen removal, in CO/CO 2 Ratio of 25, CO flow 300cm 3 /min、CO 2 The amount is 12cm 3 Carburizing for 1-6h under the condition of/min.
Wherein, when the temperature of 930 ℃ is set as the decarburization and carburization temperature, the heating rate is controlled to be 1-2 ℃/s after the temperature of the steel raw material is raised to 700 ℃, and gas is introduced after the temperature reaches 930 ℃ and is kept for 10 min.
S20: cooling the decarburized and carburized material under the protection of argon, wherein the cooling rate needs to be within 20-50min, and the temperature is reduced to 250 ℃ from 930 ℃;
s30: after the material is cooled to room temperature (25 ℃), the material is tempered under the protection of argon, the temperature is 460 ℃, and the time is 2 h.
FIG. 2 shows a steel product processed in this example. As can be seen, the produced steel has different structural layers formed in different depth ranges and has a multi-level structure. The martensite region at the surface of-120 μm can limit crack nucleation, the bainite transition region between-120 μm and-400 μm can limit long crack propagation, and the remaining martensite region provides the overall strength.
FIG. 3 shows the Hardness (HV) of the steel material processed according to this example as a function of the Depth (Depth); a is the hardness of the surface martensite region (0-200 μm) prepared under different conditions (different carburizing times) as a function of depth, and b is the hardness of the bainite transition region (200 μm-600 μm) prepared under different conditions (different decarburization times) as a function of depth. As can be seen from the figure, the mechanical property design range of the multi-level structure is large, and the hierarchical structure has great operability and diversity.
Example 2.
A bearing factory is tested in a standard fatigue component style, the adopted sample is made of common homogeneous steel, and the width of a fatigue testing part is 3 mm. The tensile strength is 1500MPa, the fatigue strength is 700MPa, and the tensile strength is less than 50 percent.
For small size samples, the fatigue cycle is dominated by surface crack nucleation, so the hierarchical surface high strength martensite layer is effective in retarding fatigue crack nucleation compared to homogeneous martensite.
The technical scheme of the invention is adopted to improve the fatigue resistance of the multi-level structure, and comprises the following steps:
(1) the steel was subjected to a composition test to determine the carbon content to be 0.5 wt%, and then the member was placed in a tube furnace at 500cm 3 Introducing argon gas at a rate of/min to remove air, heating to 930 deg.C for 10min, and introducing 430cm 3 Wet hydrogen decarburization for 20 h;
(2) before starting carburization, 500cm 3 Min argon gas was introduced for 30min to remove hydrogen, CO/CO in carburization 2 Ratio of 25, CO flow 300cm 3 /min，CO 2 The amount is 12cm 3 The carburization time is 30 min;
(3) cooling under the protection of argon after the decarburization and carburization are finished, wherein the cooling rate needs to be controlled within 20-50min and is reduced to 250 ℃ from 930 ℃;
(4) after cooling the component to 25 ℃, the component is tempered under the protection of argon at 460 ℃ for 2h (grain size of 100 μm).
The component with the multi-level structure is placed in a high-cycle fatigue performance testing instrument, the testing frequency is 20Hz, the testing stress is respectively selected from 500MPa, 600MPa, 700MPa and 800MPa, and the testing is performed according to the parameters, at the moment, the fatigue strength of the common component is 700MPa, the fatigue strength of the component with the multi-level structure is 800MPa, and the fatigue strength is improved by 14.2%. The fatigue life of the component with the multi-level structure is 10 under 900MPa 5 Weekly, the common construction is half of it.
Example 3.
A bearing factory is tested in a standard fatigue component style, the adopted sample is made of common homogeneous steel, and the width of a fatigue testing part is 10 mm. The tensile strength is 1000MPa, the fatigue strength is 400MPa, and the tensile strength is less than 50 percent.
For the intermediate-size sample, the most effective strategy is to imitate the multi-layer structure of the tooth, form high-strength martensite on the surface to limit crack formation, form lower bainite in the middle to delay the growth of the crack, simultaneously keep martensite in the core region to support the material adult strength,
the method for improving the fatigue resistance of the multi-level structure comprises the following steps:
(1) performing component detection on steel, determining that the carbon content is 0.6 wt% C, and placing the component in a tube furnace for 500cm 3 Introducing argon gas at a rate of 430 cm/min, heating to 930 deg.C for 10min, and decarbonizing to obtain carbon steel 3 Introducing humid hydrogen at a flow rate of/min for 60 hours;
(2) before starting carburization, 500cm 3 Introducing argon gas for 30 min/min to remove hydrogen, and then adding CO/CO in carburization 2 Ratio of 25, CO flow 300cm 3 /min，CO 2 The amount is 12cm 3 The carburizing time is 75 min;
(3) cooling under the protection of argon after the decarburization and carburization are finished, wherein the cooling rate needs to be controlled within 20min to 50min and is reduced from 930 ℃ to 250 ℃;
(4) and cooling the component to 25 ℃, and then performing argon protection tempering treatment at 460 ℃ for 2 h.
The component with the multi-level structure is placed in a high-cycle fatigue performance testing instrument, the testing frequency is 20Hz, the testing stress is respectively selected from 400MPa, 500MPa, 600MPa and 700MPa, and the testing instrument operates according to the parameters, at the moment, the fatigue strength of the common component is 400MPa, the fatigue strength of the component with the multi-level structure is 600MPa, and the fatigue strength is improved by 50%. The fatigue life of the component with the multi-level structure is 10 under 700MPa 4 Weekly, common construction was 25% thereof. Meanwhile, the fatigue crack propagation area of the component with the multi-level structure is remarkably increased by 50%, and the crack propagation is effectively slowed down.
Example 4.
A bearing factory is tested in a standard fatigue component style, the adopted sample is made of common homogeneous steel, and the width of a fatigue testing part is 30 mm. For large size samples, the fatigue crack growth rate is typically tested.
For large gauge samples, the fatigue cycle is controlled primarily by long crack propagation in the pariglaw region, and the lower bainite, which is graded, may reduce the rate of long crack propagation compared to homogeneous martensite.
The method for improving the fatigue resistance of the multi-level structure comprises the following steps:
(1) the steel was subjected to a composition test to determine the carbon content to be 0.56 wt% C, and then the member was placed in a tube furnace at 500cm 3 Introducing argon gas at a rate of 430 cm/min, heating to 930 deg.C, maintaining for 10min, and decarbonizing 3 The flow rate of/min is followed by the introduction of humidified hydrogen for 8 h.
(2) Before starting carburization, at 500cm 3 Introducing argon gas for 30min to remove hydrogen gas and CO/CO in carburization 2 Ratio of 25, CO flow 300cm 3 /min，CO 2 The amount is 12cm 3 The carburization time is 30 min;
(3) cooling under the protection of argon after the decarburization and carburization are finished, and reducing the temperature from 930 ℃ to 250 ℃ within 20-50 min;
(4) and cooling the component to 25 ℃, and then performing argon protection tempering treatment at 460 ℃ for 2 h.
For large-size samples, the fatigue crack growth resistance can only be tested by a fatigue crack growth tester, and under the conditions that the load ratio R is 0.1, the frequency is 5Hz, and the maximum load is 5kN, the fatigue crack growth speed of the component with the multi-layer structure is 6 multiplied by 10 -8 m/cycle, whereas the ordinary homogenization construction is 9.5X 10 -8 m/cycle, the crack growth rate decreased significantly by 36.8%.
Example 5.
The test is carried out in a standard fatigue component style in a certain factory, the adopted sample is made of common homogeneous steel, and the width of a fatigue test part is 80 mm. For large size samples, the fatigue crack growth rate was typically tested.
The method for improving the fatigue resistance of the multi-level structure comprises the following steps:
(1) the steel was subjected to a composition test to determine the carbon content to be 0.6 wt% C, and then the member was placed in a tube furnace at 500cm 3 Introducing argon gas at a rate of 430 cm/min, heating to 930 deg.C, maintaining for 10min, and decarbonizing 3 The flow rate of/min is followed by the introduction of humidified hydrogen for 20 h.
(2) Before starting carburization, 500cm 3 Min argon gas was introduced for 30min to remove hydrogen, CO/CO in carburization 2 Ratio of 25, CO flow 300cm 3 /min，CO 2 The amount is 12cm 3 The carburization time is 45 min;
(3) cooling under the protection of argon after the decarburization and carburization are finished, wherein the cooling rate needs to be controlled within 20min to 50min and is reduced from 930 ℃ to 250 ℃;
(4) and cooling the component to 25 ℃, and then carrying out argon protection tempering treatment at 460 ℃ for 2 h.
The fatigue crack growth resistance is tested by a fatigue crack growth tester, the fatigue crack growth speed of the component with the multi-level structure is 4 multiplied by 10 under the conditions that the load ratio R is 0.1, the frequency is 5Hz and the maximum load is 5kN -8 m/cycle, whereas the ordinary homogenization construction is 9.5X 10 -8 m/cycle, the crack growth rate decreased significantly by 57.8%.
Materials in nature can reasonably combine units with different performances through a multi-level structure, and the combined performance is far better than the addition of the performances of the units. For example, teeth have excellent fatigue performance due to a special multi-level biological structure, the outer layer tooth enamel is extremely hard, cracks are difficult to nucleate, and after the cracks are nucleated on the tooth enamel, the cracks can rapidly expand into the second stage of crack growth due to poor toughness. However, the underglaze layer is a soft and tough dentine structure, so that it is difficult for the crack tips to continue to propagate, and the layered tooth structure as a whole has excellent fatigue properties. The invention uses the technology with large-scale industrialization potential, namely a decarburization and carburization process, as a carrier, controls the spatial distribution of carbon elements in steel by changing the chemical potential of carbon at a material interface, and further controls the stability, phase change and mechanical properties of various phases, thereby constructing the multi-level structural steel imitating the tooth structure. The designed multi-level structure can simultaneously limit fatigue crack nucleation and fatigue crack propagation, and the stress service environment of the steel structure fatigue component is met. The decarburization and carburization process further expands the multi-level structure scale of the bionic design, and is expected to be applied to larger steel structural members.
The method can be applied to the fatigue performance improvement of the mobile construction with steel as the base material, can also be applied to the anti-fatigue service environment of the iron-based material, and can limit the fatigue crack nucleation and the fatigue crack propagation simultaneously by a multi-level structure.
The technical scheme of the invention has the following beneficial effects:
(1) the multi-level structure processing method based on the decarburization and carburization technology utilizes gas to perform decarburization and carburization, can meet the requirements of processing workpieces with different sizes, particularly processing the hierarchical structure of large-size workpieces, and can also process workpieces with complex shapes. The technology has no requirements on the size and the shape of the workpiece, and has great industrialization potential. (2) The decarburization and carburization main body technology is mature, meanwhile, the price of the used gas is low, and compared with other processing modes, such as multi-step heat treatment and mechanical processing, the decarburization and carburization main body technology has a large cost advantage. (3) The decarburization and carburization equipment is basic equipment in the steel industry, the multi-level structure processing method based on the technology can effectively enhance and improve the fatigue resistance of steel without carrying out large-scale engineering transformation on the original equipment, and the method is simple, low in engineering investment cost and easy to realize in engineering. (4) The present technique may limit both crack nucleation and crack propagation. Compared with the traditional variables such as change of microstructure, surface strengthening, crack closing effect and the like, the multi-level structure can effectively improve the fatigue resistance.
The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention in any way, and any simple modification, equivalent change and modification made to the above embodiments according to the technical spirit of the present invention are within the scope of the technical solution of the present invention.

Claims (7)

1. A processing technology of steel with a bionic multi-level structure is characterized by comprising the following steps:
s10: decarburizing and carburizing the steel raw material in the range of an austenite phase region; the carbon content of the austenite phase region is 0.4-0.8 wt%, and the temperature is 700-1500 ℃;
the decarburization step comprises: placing the steel raw material into a vertical tube furnace, after air is exhausted, when the temperature of the steel raw material is raised to 700 ℃, controlling the heating rate at 1-2 ℃/s, after the temperature reaches 930 ℃, keeping for 10min, and introducing 430cm 3 Wet hydrogen decarburization for 4h or 6h or 8h or 10 h;
the carburizing step comprises the following steps: after hydrogen elimination, in CO/CO 2 The ratio was 25, the CO flow was 300cm 3 /min、CO 2 The amount is 12cm 3 Carburizing for 15-75min under the condition of min;
s20, cooling: cooling the steel raw material treated in the step S20 under the protection of argon, and reducing the temperature to be not more than 250 ℃ within 20-50 min;
s30: and (5) cooling the steel raw material treated in the step (S20) to room temperature, and then tempering in an argon atmosphere to obtain the steel with the bionic multi-level structure.
2. The process of claim 1,
in the step S10, drawing a phase diagram of the steel raw material through Thermo-Calc thermodynamic phase diagram calculation software to determine an austenite phase region of the steel raw material;
in the step S30, the tempering temperature is 460 ℃, the time is 2-4h, and the flow of argon is 500cm 3 /min。
3. The process of claim 2,
in the step S30, when the grain size of the material treated in the step S20 is 100 μm, the tempering time is 2 h; when the grain size of the material treated in the step S20 is 200 μm, the tempering time is 4 h; the temperature rise rate in the tempering process is controlled to be 5-10 ℃/s.
4. The process of claim 1,
the decarburization and carburization step in step S10 is:
designing the spatial distribution of carbon elements by a carbon profile curve obtained by simulating the numerical solution of the Fick second law of a diffusion equation;
on the basis of the spatial distribution of carbon elements, the technical parameters for determining the time, temperature, gas flow rate and ratio of decarburization and carburization are calculated, and then decarburization and carburization are performed under the technical parameters.
5. The process of claim 4,
in step S10, the carbon content of the austenite phase region is 0.5-0.6 wt%, and the temperature is 930 ℃.
6. The process of claim 1,
the steel raw material is AISI L6 martensite tool steel, the thickness is 3mm, and the carbon content is 0.56 wt%.
7. A steel product with a biomimetic multi-level structure, characterized in that it is obtained by the process according to any of claims 1-6.
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