A method for avoiding fatigue failure caused by oxide inclusions in steel

Abstract

The present invention relates to the technical field of steel fatigue fracture failure technology and metallurgy, and in particular to a method for preventing fatigue failure caused by oxide inclusions in steel. The method for preventing fatigue failure caused by oxide inclusions in steel comprises the following steps: S1, obtaining the radius r of the largest oxide inclusion in the detected steel. oxide ; S2, the radius r of the largest oxide inclusion passing through S1 oxide Calculate the safe thickness r of the sulfide shell formed on it sulfide ; S3, r obtained according to S2 sulfide Calculate the cooling rate R corresponding to the solidification process of the casting C The present invention avoids fatigue failure caused by oxide inclusions in steel by regulating the cooling rate parameters of the continuous casting process, providing corresponding experimental evidence and theoretical basis for material selection, design and reasonable control of inclusion size to improve the service life of engineering materials.

Description

Technical Field

[0001] The present invention relates to the technical field of steel fatigue fracture failure technology and metallurgy, in particular to a method for avoiding fatigue failure caused by oxide inclusions in steel. Background Art

[0002] The rapid development of high-end equipment in my country has placed increasingly stringent demands on steel performance, including fatigue resistance and long life. However, large oxide inclusions are unavoidable during the steel smelting process. These brittle oxide inclusions are a major cause of cracking during service and ultimately lead to premature fatigue failure. Consequently, the control of large oxide inclusions in steel has received increasing attention in recent years.

[0003] Prior art has developed various methods to reduce the fatigue fracture hazard of oxide inclusions to steel. While these can be achieved by reducing the size of oxide inclusions, lowering the probability of their occurrence, and converting oxide inclusions into other substances that do not cause fatigue fracture in steel, Chinese patent CN118064679A discloses a method for rare earth-modified MnS inclusions in 20CrMo gear steel. The addition of a lanthanum-iron alloy during the middle stage of RH vacuum treatment causes La to react with S in the steel to form a series of rare earth sulfides. This significantly reduces or even eliminates the number of elongated MnS inclusions, transforming them into small, circular MnS inclusions that cling to the rare earth sulfides and become more dispersed in the molten steel. The resulting rare earth sulfides more closely resemble the properties of the steel matrix, reducing stresses caused by deformation during hot working and improving the mechanical properties of the material. However, the high cost and poor economic applicability of rare earth elements hinder their widespread application in industrial large-scale production.

[0004] Chinese patent CN112485118A discloses a method for estimating the critical inclusion size for fatigue failure in high-strength steel. This method proposes an energy-based mechanism for inclusion-induced fatigue fracture in high-strength steel. High-strength steels with varying strength-toughness profiles are obtained using different heat treatment regimes and subjected to tensile, fracture toughness, and elastic performance tests. The method then uses the energy criterion to estimate the critical inclusion size for fatigue failure in high-strength steel. However, this method does not provide a method for controlling the inclusion size to be smaller than this critical size, nor does it provide a prediction of whether the control mechanism is suitable for industrial production.

[0005] Although Chinese patent CN118127402A discloses a method for controlling inclusions in martensitic stainless steel with high fatigue performance and corrosion resistance, this method controls the main inclusions Al2O3, CaO-Al2O3, CaS, and TiN in the stainless steel to dispersed Mg-Al-RE-O type inclusions. The control method is very complex, the operation is difficult, and a large amount of high-cost alloying elements are added.

[0006] Obviously, the above methods are not very practical for reducing the damage of oxide inclusions to fatigue fracture of steel. The process is complicated, the cost is high and the efficiency is low.

[0007] Another effective approach is to allow sulfides to precipitate and grow around oxides during the cooling and solidification of the continuous casting, ultimately forming composite inclusions in the form of oxides + sulfides. This method reduces stress concentration between the composite inclusions and the matrix by fully enveloping the brittle oxides with a layer of soft sulfides, effectively preventing crack initiation and propagation in the oxides, thereby improving the fatigue performance of the steel.

[0008] However, there has been no quantitative calculation or effective guidance for the safe thickness range and control strategy for the precipitation of sulfide shells, making it difficult to completely overcome the technical difficulties of fatigue failure caused by oxides in production. Considering that steel billets usually need to undergo a quenching heat treatment process before they are finally processed into components to optimize their mechanical properties. However, due to the different thermal expansion coefficients of composite inclusions and the steel matrix, a certain amount of residual stress will be generated at the interface between the composite inclusions and the matrix after quenching heat treatment, and the residual stress is concentrated at the interface, which is a major cause of premature fatigue failure of steel components.

[0009] Therefore, it is necessary to accurately calculate the safe thickness of the outer sulfide shell so that the oxides are fully encapsulated by the sulfides, reducing the risk of fatigue fracture of the steel parts caused by the oxides during service. At the same time, the cooling rate of the continuous casting steel during solidification should be properly controlled to ensure that a sufficiently thick sulfide shell is precipitated on the oxide inclusions, thereby significantly reducing stress concentration at the inclusion-matrix interface, preventing fatigue crack initiation, and extending service life.

[0010] However, there is currently no simple and effective method to calculate the safe thickness of the sulfide shell and its corresponding control strategy to inform the design of production processes to avoid fatigue failure caused by oxide inclusions in steel.

[0011] Therefore, the present invention proposes an accurate calculation method for the safe thickness of the sulfide shell and the cooling rate of the corresponding solidification process, thereby completely avoiding fatigue failure caused by oxide inclusions in steel. Summary of the Invention

[0012] In order to solve many technical problems in the prior art, such as premature fatigue failure of steel caused by hard oxide inclusions, modifying oxide inclusions and controlling the safe thickness of sulfide shell precipitation, and regulating the cooling rate during the solidification stage of continuous casting, the present invention provides a method for preventing fatigue failure caused by oxide inclusions in steel. The technical solution is as follows:

[0013] A method for preventing fatigue failure caused by oxide inclusions in steel, the method for preventing fatigue failure caused by oxide inclusions in steel comprising the following steps:

[0014] S1. Get the radius r of the largest oxide inclusion in the tested steel oxide ;

[0015] S2, the radius r of the largest oxide inclusion passing through S1 oxide Calculate the safe thickness r of the sulfide shell formed on it sulfide ;

[0016] S3, r obtained according to S2 sulfide Calculate the cooling rate R corresponding to the solidification process of the casting C .

[0017] Optionally, the radius r of the largest oxide inclusion in the detected steel is obtained in S1 oxide The method is to calculate the radius r of the largest oxide inclusion in the tested steel based on the national standard rating results. oxide , or directly obtain the radius r of the largest oxide inclusion from the inclusion data obtained by various microscopic observation and analysis methods oxide , or calculate the radius r of the largest oxide inclusion in steel by the national standard extreme value analysis method oxide .

[0018] Optionally, in S1, the radius r of the largest oxide inclusion in the tested steel is calculated based on the national standard rating results. oxide The national standard GB / T 10561-2023 assesses the grade of Ds-type large oxide inclusions, with N ranging from 0.5 to 5, with a total of 10 grades at intervals of 0.5. The grade N is related to the oxide radius r oxide The relationship is shown in the following formula:

[0019] r oxide =0.5[10 0.302N+0.972 ] (1)

[0020] The calculation results in the square brackets in the formula are rounded off to get the maximum oxide inclusion radius in the tested steel;

[0021] The radius r of the largest oxide inclusion is directly obtained from the inclusion data obtained by various microscopic observation and analysis methods. oxide To directly obtain the maximum oxide inclusion size r using the ASPEX inclusion automatic scanner oxide ;

[0022] Calculate the radius r of the largest oxide inclusion in steel using the national standard extreme value analysis method oxide The maximum oxide inclusion size r in steel is calculated by the extreme value analysis method of GB / T40281-2021. oxide .

[0023] Alternatively, in S2, when the outer layer of oxide envelops the sulfide to obtain a composite inclusion, the radius r of the composite inclusion is complex Satisfy the following formula:

[0024] r complex =r oxide +r sulfide (2).

[0025] Optionally, in S2, when the thermal expansion coefficient of the composite inclusion is greater than that of the steel matrix, residual stress will be generated at the interface between the composite inclusion and the matrix during the quenching heat treatment, which can be calculated as follows:

[0026] (3)

[0027] Among them, σ R It is the residual stress generated at the interface between the composite inclusion and the steel matrix after quenching; C is related to the Young's modulus and Poisson's ratio of the composite inclusion and the matrix, and is always greater than 0; α M is the thermal expansion coefficient of the substrate; △T is the temperature difference of quenching heat treatment; α complex is the thermal expansion coefficient of the composite inclusion.

[0028] Optionally, the thermal expansion coefficient α of the composite inclusion in S2 complex It can be calculated by the following formula:

[0029] (4)

[0030] Among them, α oxide is the thermal expansion coefficient of the oxide, α sulfide is the thermal expansion coefficient of sulfide.

[0031] Alternatively, if the thermal expansion coefficient α of the composite inclusion in S2 is complex If the stress is not greater than that of the matrix, no residual stress will be generated at the interface between the matrix and the steel matrix, that is, α M ≤α complex When σ R =0, so the safe thickness of the sulfide shell should satisfy the following formula:

[0032] (5).

[0033] Alternatively, combining equation (5) with equation (1) yields:

[0034] (6).

[0035] Optionally, the thickness of the sulfide shell in S3 is determined by the cooling rate during the solidification process of the ingot, and the relationship between the thickness of the sulfide precipitation shell and the cooling rate is calculated using the Scheil solidification model;

[0036] The diffusion of element i in molten steel is the limiting link in the formation of sulfides. Therefore, the diffusion of element i controls the growth rate of sulfides. It can be obtained that:

[0037] (7)

[0038] Among them, J i is the diffusion flux of limiting element i; D i is the diffusion coefficient of limiting element i in the liquid phase, ρ Fe is the density of Fe liquid; M i is the molar mass of the limiting element i; w[i] L is the mass fraction of limiting element i at the solidification front of molten steel; w[i] eq is the mass fraction of limiting element i at equilibrium; r sulfide is the radius of the sulfide;

[0039] In the model, the inclusions are assumed to be spherical after precipitation. According to the mass balance, the relationship between the diffusion flux of element i and the shell thickness of the sulfide is:

[0040] (8)

[0041] Combining formula (1), we can get:

[0042] (9)

[0043] By integrating Equation (9), we can obtain that the precipitation growth of sulfide and the shell thickness r sulfide expression: (10)

[0044] In formula (10), M sulfide is the molar mass of sulfide; ρ Fe is the density of Fe liquid, 7070kg / m 3 ;M i is the molar mass of the limiting element i; ρ sulfide is the density of sulfide; Di is the diffusion coefficient of the limiting element i;

[0045] In formula (10), w[i] L is the mass fraction of limiting element i at the solidification front of molten steel, which can be calculated using the Scheil microsegregation model as follows:

[0046] (11)

[0047] Where w[i]0 is the content of solute i in the molten steel before solidification begins; k0 is the equilibrium distribution coefficient of element i; f s is the solid phase ratio of molten steel, which is calculated as follows:

[0048] (12)

[0049] Among them, T0 is the melting point of pure iron, which is 1809K; T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel;

[0050] In formula (10), w[i] eq It can be calculated using the following formulas (13) to (16):

[0051] [M] + [S] = MS, lgK=A / TB (13)

[0052] (14)

[0053] (15)

[0054] (16)

[0055] Wherein, formula (13) is the reaction equilibrium equation of the reaction between elements M and S to form sulfide MS, and in formula (14) α M , α S , α MS are the activities of [M], [S] and MS respectively; f in equations (15) and (16) M and f S are the activity coefficients of [M] and [S], respectively; and are the interaction coefficients of other elements [j] in the molten steel on [M] and [S] respectively;

[0056] In formula (10), is the solidification time, calculated according to the following formula:

[0057] (17)

[0058] Among them, T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel; R C is the cooling rate during solidification;

[0059] To achieve a safe thickness r sulfide , Substituting Equation (17) into Equation (10), the cooling rate R C The following formula should be satisfied:

[0060] (18)

[0061] Among them, M sulfide and M i are the molar masses of sulfide and limiting element i, respectively; ρ Fe and ρ sulfide are the densities of Fe liquid and sulfide respectively; D i is the diffusion coefficient of limiting element i in the liquid phase; w[i] L is the mass fraction of limiting element i at the solidification front of molten steel; w[i] eq is the mass fraction of limiting element i at equilibrium; T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel.

[0062] Optionally, in order to obtain the safe thickness of the sulfide shell in S3, the safe thickness r sulfide , Substituting equation (6) into equation (18), the cooling rate R C The following formula should be satisfied:

[0063] (19)

[0064] Among them, M sulfide and M i are the molar masses of sulfide and limiting element i, respectively; ρ Fe and ρ sulfide are the densities of Fe liquid and sulfide respectively; D i is the diffusion coefficient of limiting element i in the liquid phase; w[i] L is the mass fraction of limiting element i at the solidification front of molten steel; w[i] eq is the mass fraction of limiting element i at equilibrium; T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel.

[0065] Compared with the prior art, the above technical solution has at least the following beneficial effects:

[0066] The above scheme, the present invention proposes a method for preventing fatigue failure caused by oxide inclusions in steel, which can solve many technical problems in the prior art, such as the lack of quantitative calculation and effective guidance on the safe thickness range and control strategy for the precipitation of sulfide shells in steel inclusions, which leads to the fact that oxide inclusions in steel are very likely to cause fatigue failure during service.

[0067] The present invention first obtains the radius r of the largest oxide inclusion in steel oxide Then, based on the principle of residual stress generation during quenching, the safe thickness of the sulfide shell wrapped around it was obtained. Finally, combined with the relationship between sulfide precipitation and growth and the cooling rate of the ingot solidification process, a corresponding control strategy was given, which solved the problem of accurate calculation and control of the safe thickness of sulfides in the production process. Subsequently, automatic inclusion scanning and rotary bending fatigue tests were carried out to verify the effectiveness of the present invention.

[0068] The present invention calculates the radius of the largest oxide inclusion in the detected steel according to the national standard rating results or directly obtains the radius r of the largest oxide inclusion in the steel from the inclusion data obtained by various microscopic observation and analysis methods. oxide The obtained values ​​are accurate and reliable, which is further proved after actual technical verification.

[0069] The solidification cooling rate R of the present invention C Safe thickness of sulfide shell r sulfide There is a relationship defined by the following formula:

[0070] , which has also been verified by actual technology to be consistent with the actual situation.

[0071] In the present invention, if the thermal expansion coefficient α of the composite inclusion is complex If the stress is not greater than that of the matrix, no residual stress will be generated at the interface between the matrix and the steel matrix, that is, α M ≤α complex When σ R =0, so the safe thickness of the sulfide shell should satisfy the following formula:

[0072] , which has also been verified by actual technology to be consistent with the actual situation.

[0073] In summary, compared with other traditional methods, the method of the present invention avoids fatigue failure caused by oxide inclusions in steel by regulating the cooling rate parameters of the continuous casting process, providing corresponding experimental evidence and theoretical basis for material selection, design and reasonable control of inclusion size to improve the service life of engineering materials; and this method can effectively improve the production efficiency and product quality of steel preparation, reduce the defective rate, is suitable for production line regulation of mass production of steel, and is conducive to large-scale industrial production and promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0075] Figure 1 This is a typical composite inclusion morphology of oxide + sulfide shell in a GCr15 bearing steel metallographic sample in a method for preventing fatigue failure caused by oxide inclusions in steel in Example 1 of the present invention;

[0076] Figure 2 The large-sized composite inclusions and energy spectrum results that cause failure fracture in the fatigue fracture source region of GCr15 bearing steel in a method for preventing fatigue failure caused by oxide inclusions in steel in Example 1 of the present invention;

[0077] Figure 3 This is a statistical result graph of the thickness of all sulfide shells of GCr15 bearing steel in a method for preventing fatigue failure caused by oxide inclusions in steel in Example 1 of the present invention;

[0078] Figure 4 is a curve showing the relationship between the solidification cooling rate and the thickness of the sulfide shell of GCr15 bearing steel in a method for preventing fatigue failure caused by oxide inclusions in steel in Example 1 of the present invention;

[0079] Figure 5 This is a stress intensity factor-fatigue life (K-N) diagram of fatigue failure caused by composite inclusions in GCr15 bearing steel in a method for preventing fatigue failure caused by oxide inclusions in steel in Example 1 of the present invention;

[0080] Figure 6 This is a relationship diagram between the oxide radius of GCr15 bearing steel and the thickness of the sulfide containment layer in a method for avoiding fatigue failure caused by oxide inclusions in steel in Example 1 of the present invention. DETAILED DESCRIPTION

[0081] The technical solution of the present invention is described below in conjunction with the accompanying drawings.

[0082] In the embodiments of the present invention, words such as "exemplarily" and "for example" are used to indicate examples, illustrations, or explanations. Any embodiment or design described as an "exemplary" in the present invention should not be interpreted as being preferred or advantageous over other embodiments or designs. Rather, the use of the word "exemplary" is intended to present concepts in a concrete manner. Furthermore, in the embodiments of the present invention, "and / or" can mean both or either of the two.

[0083] In the embodiments of the present invention, "image" and "picture" may sometimes be used interchangeably. It should be noted that, when the distinction between them is not emphasized, the meanings they convey are the same. "of," "corresponding," and "corresponding" may sometimes be used interchangeably. It should be noted that, when the distinction between them is not emphasized, the meanings they convey are the same.

[0084] In the embodiments of the present invention, sometimes a subscript such as W1 may be written as a non-subscript such as W1. When the difference is not emphasized, the meanings to be expressed are the same.

[0085] In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0086] A method for preventing fatigue failure caused by oxide inclusions in steel, the method for preventing fatigue failure caused by oxide inclusions in steel comprising the following steps:

[0087] S1. Get the radius r of the largest oxide inclusion in the tested steel oxide ;

[0088] S2, the radius r of the largest oxide inclusion passing through S1 oxide Calculate the safe thickness r of the sulfide shell formed on it sulfide ;

[0089] S3, r obtained according to S2 sulfide Calculate the cooling rate R corresponding to the solidification process of the casting C .

[0090] In particular, the radius r of the largest oxide inclusion in the detected steel is obtained in S1 oxide The method is to calculate the radius r of the largest oxide inclusion in the tested steel based on the national standard rating results. oxide , or directly obtain the radius r of the largest oxide inclusion from the inclusion data obtained by various microscopic observation and analysis methods oxide , or calculate the radius r of the largest oxide inclusion in steel by the national standard extreme value analysis method oxide .

[0091] In particular, the radius r of the largest oxide inclusion in the tested steel is calculated based on the national standard rating results in S1. oxide The national standard GB / T 10561-2023 assesses the grade of Ds-type large oxide inclusions, with N ranging from 0.5 to 5, with a total of 10 grades at intervals of 0.5. The grade N is related to the oxide radius r oxide The relationship is shown in the following formula:

[0092] r oxide =0.5[10 0.302N+0.972 ] (1)

[0093] The calculation results in the square brackets in the formula are rounded off to get the maximum oxide inclusion radius in the tested steel;

[0094] The radius r of the largest oxide inclusion is directly obtained from the inclusion data obtained by various microscopic observation and analysis methods. oxide To directly obtain the maximum oxide inclusion size r using the ASPEX inclusion automatic scanner oxide ;

[0095] Calculate the radius r of the largest oxide inclusion in steel using the national standard extreme value analysis method oxide The maximum oxide inclusion size r in steel is calculated by the extreme value analysis method of GB / T40281-2021. oxide .

[0096] In particular, when the outer layer of oxide in S2 is wrapped with sulfide to obtain a composite inclusion, the radius of the composite inclusion r complex Satisfy the following formula:

[0097] r complex =r oxide +r sulfide (2).

[0098] In particular, when the thermal expansion coefficient of the composite inclusions in S2 is greater than that of the steel matrix, residual stress will be generated at the interface between the composite inclusions and the matrix during the quenching heat treatment process, which can be calculated as follows:

[0099] (3)

[0100] Among them, σ R It is the residual stress generated at the interface between the composite inclusion and the steel matrix after quenching; C is related to the Young's modulus and Poisson's ratio of the composite inclusion and the matrix, and is always greater than 0; α M is the thermal expansion coefficient of the substrate; △T is the temperature difference of quenching heat treatment; α complex is the thermal expansion coefficient of the composite inclusion.

[0101] In particular, the thermal expansion coefficient α of the composite inclusions in S2 complex It can be calculated by the following formula:

[0102] (4)

[0103] Among them, α oxide is the thermal expansion coefficient of the oxide, α sulfide is the thermal expansion coefficient of sulfide.

[0104] In particular, if the thermal expansion coefficient α of the composite inclusion in S2 complex If the stress is not greater than that of the matrix, no residual stress will be generated at the interface between the matrix and the steel matrix, that is, α M ≤α complex When σ R =0, so the safe thickness of the sulfide shell should satisfy the following formula:

[0105] (5).

[0106] In particular, combining Equation (5) with Equation (1) yields:

[0107] (6).

[0108] In particular, the thickness of the sulfide shell in S3 is determined by the cooling rate during the solidification process of the ingot, and the Scheil solidification model is used to calculate the relationship between the thickness of the sulfide precipitation shell and the cooling rate;

[0109] The diffusion of element i in molten steel is the limiting link in the formation of sulfides. Therefore, the diffusion of element i controls the growth rate of sulfides. It can be obtained that:

[0110] (7)

[0111] Among them, J i is the diffusion flux of limiting element i; D i is the diffusion coefficient of limiting element i in the liquid phase, ρ Fe is the density of Fe liquid; M i is the molar mass of the limiting element i; w[i] L is the mass fraction of limiting element i at the solidification front of molten steel; w[i] eq is the mass fraction of limiting element i at equilibrium; r sulfide is the radius of the sulfide;

[0112] In the model, the inclusions are assumed to be spherical after precipitation. According to the mass balance, the relationship between the diffusion flux of element i and the shell thickness of the sulfide is:

[0113] (8)

[0114] Combining formula (1), we can get:

[0115] (9)

[0116] By integrating Equation (9), we can obtain that the precipitation growth of sulfide and the shell thickness r sulfide expression:

[0117] (10)

[0118] In formula (10), M sulfide is the molar mass of sulfide; ρ Fe is the density of Fe liquid, 7070kg / m 3 ;M i is the molar mass of the limiting element i; ρ sulfide is the density of sulfide; D i is the diffusion coefficient of the limiting element i;

[0119] In formula (10), w[i] L is the mass fraction of limiting element i at the solidification front of molten steel, which can be calculated using the Scheil microsegregation model as follows:

[0120] (11)

[0121] Where w[i]0 is the content of solute i in the molten steel before solidification begins; k0 is the equilibrium distribution coefficient of element i; f s is the solid phase ratio of molten steel, which is calculated as follows:

[0122] (12)

[0123] Among them, T0 is the melting point of pure iron, which is 1809K; T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel;

[0124] In formula (10), w[i] eq It can be calculated using the following formulas (13) to (16):

[0125] [M] + [S] = MS, lgK=A / TB (13)

[0126] (14)

[0127] (15)

[0128] (16)

[0129] Wherein, formula (13) is the reaction equilibrium equation of the reaction between elements M and S to form sulfide MS, and in formula (14) α M , α S , α MS are the activities of [M], [S] and MS respectively; f in equations (15) and (16) M and f S are the activity coefficients of [M] and [S], respectively; and are the interaction coefficients of other elements [j] in the molten steel on [M] and [S] respectively;

[0130] In formula (10), is the solidification time, calculated according to the following formula:

[0131] (17)

[0132] Among them, T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel; R C is the cooling rate during solidification;

[0133] To achieve a safe thickness r sulfide , Substituting Equation (17) into Equation (10), the cooling rate R C The following formula should be satisfied:

[0134] (18)

[0135] Among them, M sulfide and M i are the molar masses of sulfide and limiting element i, respectively; ρ Fe and ρ sulfide are the densities of Fe liquid and sulfide respectively; D i is the diffusion coefficient of limiting element i in the liquid phase; w[i] L is the mass fraction of limiting element i at the solidification front of molten steel; w[i] eq is the mass fraction of limiting element i at equilibrium; T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel.

[0136] In particular, in order to obtain the safe thickness of the sulfide shell in S3, the safe thickness r sulfide , Substituting equation (6) into equation (18), the cooling rate R C The following formula should be satisfied:

[0137] (19)

[0138] Among them, M sulfide and M i are the molar masses of sulfide and limiting element i, respectively; ρ Fe and ρ sulfide are the densities of Fe liquid and sulfide respectively; D i is the diffusion coefficient of limiting element i in the liquid phase; w[i] L is the mass fraction of limiting element i at the solidification front of molten steel; w[i] eq is the mass fraction of limiting element i at equilibrium; T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel.

[0139] In order to verify a method for avoiding fatigue failure caused by oxide inclusions in steel, the solidification cooling rate R C Safe thickness of sulfide shell r sulfide There is a relationship defined by the following formula (1):

[0140] (1)

[0141] This verification scheme takes GCr15 bearing steel as an example and specifically includes the following steps:

[0142] (1) Experimentally determine the types of core oxides and outer sulfides in steel, and calculate the thickness of the sulfide shell:

[0143] For the selected GCr15 bearing steel, the total preparation area is 810mm 2 Metallographic samples were automatically statistically analyzed for inclusions under a scanning electron microscope. Rotary bending fatigue specimens were also prepared and tested to observe and count composite inclusions in the fatigue source zone. The geometry of the fatigue specimens and the testing procedures were processed and conducted in accordance with the national standard GB / T 4337-2015. The tests were conducted on a QBWP-6000J rotary bending fatigue testing machine with a stress ratio of R = -1.

[0144] Figure 1 The SEM images of the metallographic inclusions were automatically analyzed. Combined with the results of the energy spectrum, the core oxide components of the composite inclusions in the steel were mainly Al-Mg-O, and the outer sulfides were mainly MnS. Figure 2 This is a large-scale composite inclusion observed in the fatigue fracture source area that caused the failure fracture. Similarly, the oxide is Al-Mg-O and the outer sulfide is MnS. In addition, the shell thickness of all oxide outer sulfides is counted and plotted on Figure 3 It was found that the shell thickness of sulfides was about 2 μm.

[0145] (2) From the known solidification cooling rate RC Calculate the sulfide shell thickness r sulfide :

[0146] In this example, the composition of the GCr15 bearing steel used is shown in Table 1. The ingot size is 300 × 340 mm. From the surface to the core, it is divided into a chill layer, a columnar crystal zone, a mixed crystal zone, and a central equiaxed crystal zone. The cooling rate decreases from fast to slow, with a clear cooling rate gradient. Because fatigue specimens were cut from the columnar crystal zone, mixed crystal zone, and central equiaxed zone of the ingot, it is important to focus on the precipitation radius of sulfides in these three zones. The corresponding cooling rate can be calculated using the average secondary dendrite spacing in each zone, as shown in Table 2.

[0147] Table 1 GCr15 steel composition (mass%)

[0148]

[0149] Table 2 Secondary dendrite spacing and calculated cooling rate of the ingot

[0150]

[0151] Since the diffusion of element S in molten steel is the limiting link for the formation of MnS, the diffusion of element S controls the growth rate of sulfides, so it can be obtained:

[0152] (2)

[0153] Among them, J S is the diffusion flux of the limiting element S; D S is the diffusion coefficient of the limiting element S in the liquid phase, ρ Fe is the density of Fe liquid; M S is the molar mass of the limiting element S; w[S] L is the mass fraction of the limiting element S at the solidification front of the molten steel; w[S] eq is the mass fraction of the limiting element S at equilibrium; r MnS is the radius of the sulfide.

[0154] In the model, the inclusions are assumed to be spherical after precipitation. According to the mass balance, the relationship between the diffusion flux of element S and the shell thickness of MnS is:

[0155] (3)

[0156] Combining formula (1), we can get:

[0157] (4)

[0158] By integrating Equation (3), we can obtain the shell thickness r of MnS:MnS expression:

[0159] (5)

[0160] In formula (4), M MnS is the molar mass of MnS, 0.087 kg / mol; ρ Fe is the density of Fe liquid, 7070kg / m 3 ;M S is the molar mass of S, 0.032 kg / mol; ρ MnS is the density of MnS, 3990 kg / m 3 ;D S is the diffusion coefficient of S, ;

[0161] In formula (4), w[S] L is the mass fraction of the limiting element S at the solidification front of the molten steel, which can be calculated using the Scheil microsegregation model as follows (5):

[0162] (6)

[0163] Where w[S]0 is the content of solute S in the molten steel before solidification begins; k0 is the equilibrium distribution coefficient of element S, 0.035; f s is the solid phase ratio of the molten steel, which is calculated as follows (6):

[0164] (7)

[0165] Among them, T0 is the melting point of pure iron, which is 1809K; T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel. Calculated by thermodynamic software Thermal-Calc, in this case, T start and T S They are 1625K and 1605K respectively.

[0166] In formula (4), w[S] eq It can be calculated using the following formulas (7) to (10):

[0167] [Mn]+[S]=MnS,lgK=8627 / T-4.745 (8)

[0168] (9)

[0169] (10)

[0170] (11)

[0171] Wherein, Equation (7) is the chemical reaction equilibrium equation of the element Mn and S to generate MnS and the corresponding standard Gibbs free energy change, α Mn , α S and α MnS are the activities of [Mn], [S] and MnS respectively; f Mn and f s are the activity coefficients of [Mn] and [S], respectively; and are the interaction coefficients of other elements [j] in the molten steel on [Mn] and [S], respectively, and their values ​​are shown in Table 3 below:

[0172] Table 3 Interaction coefficients of elements in molten steel

[0173]

[0174] In formula (4), is the local solidification time, calculated according to the following formula (11):

[0175] (12)

[0176] Among them, T start is the temperature at which MnS begins to precipitate, T S is the solidus temperature in steel, which are 1625K and 1605K respectively, R C is the cooling rate during solidification.

[0177] A point was selected every 5K from the range of 50-120K / min, and the MnS shell thickness corresponding to 15 points was calculated. The calculation results are as follows Figure 4 As shown, the shell thickness of MnS is in the range of 1.5-2.3 μm.

[0178] (3) Comparison of experimental observations and calculation results of sulfide shell thickness:

[0179] Figure 3 The experimental statistics of the MnS shell thickness is about 2μm, and the calculated value of the MnS shell thickness is as follows: Figure 4 As shown, it is in the range of 1.5-2.3μm. This verifies the solidification cooling rate R of S3 in the method for avoiding fatigue failure caused by oxide inclusions in steel of the present invention. C and the sulfide shell thickness r sulfide satisfies the derived relationship.

[0180] In order to verify the safety thickness of the sulfide shell in S2 calculated according to the following formula, when it is attached to the outer layer of the oxide, inclusion-induced fatigue failure will not occur.

[0181] (13)

[0182] This example takes GCr15 bearing steel as an example and verifies the solidification cooling rate R of S3. C and the sulfide shell thickness r sulfide The materials used to satisfy the derived relationship are the same material. The validity of the S2 relationship (13) of the present invention is verified by a high-cycle rotation bending fatigue test, which specifically includes the following steps:

[0183] (1) Extrapolating the fatigue test results, the critical size of composite inclusions causing fatigue failure is obtained:

[0184] If the fatigue specimen does not break after more than 10^7 cycles under the applied stress, it is considered that the specimen will no longer break and the test is terminated. This also indicates that the size of all composite inclusions in the specimen is smaller than the critical size and that the composite inclusions are insufficient to cause fatigue fracture. The geometry of the fatigue specimen and the test steps are processed and conducted in accordance with the national standard GB / T 4337-2015. The test is completed on a QBWP-6000J rotary bending fatigue testing machine with a stress ratio of R=-1. After the fatigue test, the fatigue failure data points caused by composite inclusions are plotted as Figure 5 SN curve, the horizontal axis is lgN f , represents the number of cycles; the vertical axis is the stress intensity factor K, which can be calculated as follows:

[0185] (14)

[0186] Where σ0 is the value of the applied cyclic stress, Represents the diameter of the composite inclusion observed in the source area of ​​the fatigue fracture.

[0187] After fitting the fatigue failure data points caused by composite inclusions, the linear relationship is satisfied: K=-0.76·lgN f +9.7. When lgN f ≥7, that is, the number of cycles is greater than 10 7 , it is believed that the sample will no longer fatigue fracture, so the stress intensity factor needs to meet K≤4.38. Since fatigue tests have found that when the external cyclic load is greater than 1197MPa, fatigue failure is caused by surface cracking. Therefore, fatigue failure caused by composite inclusions only occurs when the external cyclic load is lower than 1197MPa. Therefore, the maximum value of the external cyclic load σ0 is taken as 1197MPa. Since K≤4.38, the extrapolation method is calculated to be ≤17.0μm, that is, in this example, when the radius of the composite inclusion is less than 8.5μm, fatigue failure will not occur.

[0188] (2) Calculation of the critical size of composite inclusions:

[0189] If α M ≥α complex After quenching, the interface between the composite inclusions and the steel matrix will produce a certain amount of residual stress. Subsequent cyclic loading will cause stress concentration at the interface, which will increase the possibility of failure of steel parts in the early stage of service. Therefore, if α can be satisfied M ≤α complex , then no residual stress is generated at the interface between the composite inclusion and the steel matrix after quenching heat treatment, which makes it difficult for cracks to initiate around the composite inclusion, reducing the probability of it serving as a fatigue crack initiation source. Therefore, it can be deduced that the thickness of the sulfide containment layer should satisfy the following formula:

[0190] (15)

[0191] In this case, since the sample comes from Example 1, and it has been verified that the shell thickness of MnS is about 2μm, it can be obtained that the radius of the oxide should satisfy:

[0192]

[0193] where α oxide and α MnS The thermal expansion coefficients of oxide and MnS are 6×10 -6 ℃ -1 and 1.61×10 -5 ℃ -1 ; α M is the thermal expansion coefficient of the substrate, 1.25×10 -5 ℃ -1 ;

[0194] The calculated oxide radius should be less than 6.7μm. Since the shell thickness of MnS is about 2μm, the radius of the composite inclusion should be less than 8.7μm. At this time, it is believed that the hard Al-Mg-O oxide wrapped by MnS will not produce residual stress at the interface with the matrix after quenching, that is, it will not cause fatigue crack initiation and sample fracture. The relationship between the oxide radius and the sulfide containment layer thickness is as follows: Figure 6 shown.

[0195] (3) Comparison of experimental and calculated results of critical size of composite inclusions:

[0196] Extrapolation from fatigue tests shows that the critical radius of composite inclusions that will not cause fatigue fracture is 8.5μm, and based on the principle of no residual stress, the critical size of composite inclusions calculated is 8.7μm. The results are very consistent, thus proving the effectiveness of this method.

[0197] Example 1

[0198] A method for preventing fatigue failure caused by oxide inclusions in the GCr15 steel composition shown in Table 1, characterized in that the method for preventing fatigue failure caused by oxide inclusions in the steel comprises the following steps:

[0199] S1. Calculate the radius r of the largest oxide inclusion in the GCr15 steel tested in Table 1 according to the rating results of the national standard GB / T 10561-2023. oxide ;

[0200] S2, the radius r of the largest oxide inclusion passing through S1 oxide Calculate the safe thickness r of the sulfide shell formed on it sulfide ;

[0201] S3, r obtained according to S2 sulfide Calculate the cooling rate R corresponding to the solidification process of the casting C The calculated range should be met. The final results are shown in Table 4 below.

[0202] Table 4

[0203]

[0204] According to the rating results of the national standard GB / T 10561-2023, since the oxygen content in this steel is only 4.4ppm, it is a high-cleanliness bearing steel with the highest rating of N=1.0, so the solidification cooling rate of the continuous casting billet should be controlled to be less than 17K / min.

[0205] Example 2

[0206] A method for preventing fatigue failure caused by oxide inclusions in Cr-Mo gear steel, characterized in that the method for preventing fatigue failure caused by oxide inclusions in steel comprises the following steps:

[0207] S1. The radius r of the largest oxide inclusion in the Cr-Mo gear steel tested was obtained using the ASPEX inclusion automatic scanner as shown in Table 5 below. oxide ;

[0208] Table 5 Composition of CrMo gear steel (mass%)

[0209]

[0210] S2, the radius r of the largest oxide inclusion passing through S1 oxide Calculate the safe thickness r of the sulfide shell formed on it sulfide ;

[0211] S3, r obtained according to S2 sulfideCalculate the cooling rate R corresponding to the solidification process of the casting C The final results are shown in Table 6 below.

[0212] Table 6

[0213]

[0214] Example 3

[0215] A method for preventing fatigue failure caused by oxide inclusions in 65Mn spring steel, characterized in that the method for preventing fatigue failure caused by oxide inclusions in steel comprises the following steps:

[0216] S1. According to the extreme value analysis method of GB / T 40281-2021, the radius r of the largest oxide inclusion in the 65Mn spring steel with five different oxygen contents in Table 7 was calculated. oxide ;

[0217] Table 7 Composition of 65Mn spring steel (mass%)

[0218]

[0219] S2, the radius r of the largest oxide inclusion passing through S1 oxide Calculate the safe thickness r of the sulfide shell formed on it sulfide ;

[0220] S3, r obtained according to S2 sulfide Calculate the cooling rate R corresponding to the solidification process of the casting C The final results are shown in Table 8 below.

[0221] Table 8

[0222]

[0223] The above scheme, the present invention proposes a method for preventing fatigue failure caused by oxide inclusions in steel, which can solve many technical problems in the prior art, such as the lack of quantitative calculation and effective guidance on the safe thickness range and control strategy for the precipitation of sulfide shells in steel inclusions, which leads to the fact that oxide inclusions in steel are very likely to cause fatigue failure during service.

[0224] The present invention first obtains the radius r of the largest oxide inclusion in steel oxideThen, based on the principle of residual stress generation during quenching, the safe thickness of the sulfide shell wrapped around it was obtained. Finally, combined with the relationship between sulfide precipitation and growth and the cooling rate of the ingot solidification process, a corresponding control strategy was given, which solved the problem of accurate calculation and control of the safe thickness of sulfides in the production process. Subsequently, automatic inclusion scanning and rotary bending fatigue tests were carried out to verify the effectiveness of the present invention.

[0225] The present invention calculates the radius of the largest oxide inclusion in the detected steel according to the national standard rating results or directly obtains the radius r of the largest oxide inclusion in the steel from the inclusion data obtained by various microscopic observation and analysis methods. oxide The obtained values ​​are accurate and reliable, which is further proved after actual technical verification.

[0226] The solidification cooling rate R of the present invention C Safe thickness of sulfide shell r sulfide There is a relationship defined by the following formula:

[0227] , which has also been verified by actual technology to be consistent with the actual situation.

[0228] In the present invention, if the thermal expansion coefficient α of the composite inclusion is complex If the stress is not greater than that of the matrix, no residual stress will be generated at the interface between the matrix and the steel matrix, that is, α M ≤α complex When σ R =0, so the safe thickness of the sulfide shell should satisfy the following formula:

[0229] , which has also been verified by actual technology to be consistent with the actual situation.

[0230] In summary, compared with other traditional methods, the method of the present invention avoids fatigue failure caused by oxide inclusions in steel by regulating the cooling rate parameters of the continuous casting process, providing corresponding experimental evidence and theoretical basis for material selection, design and reasonable control of inclusion size to improve the service life of engineering materials; and this method can effectively improve the production efficiency and product quality of steel preparation, reduce the defective rate, is suitable for production line regulation of mass production of steel, and is conducive to large-scale industrial production and promotion.

[0231] It should be understood that the term "and / or" as used herein simply describes a relationship between associated objects, indicating that three possible relationships exist. For example, "A and / or B" can represent: A alone, A and B together, or B alone. A and B can be singular or plural. Furthermore, the character " / " as used herein generally indicates an "or" relationship between the associated objects, but it may also indicate an "and / or" relationship. For specific understanding, please refer to the context.

[0232] In this disclosure, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" can mean: a, b, c, ab, ac, bc, or abc, where a, b, and c can be single or plural.

[0233] It should be understood that in various embodiments of the present invention, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0234] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for preventing fatigue failure caused by oxide inclusions in steel, characterized in that: The method for preventing fatigue failure caused by oxide inclusions in steel comprises the following steps: S1. Get the radius r of the largest oxide inclusion in the tested steel oxide ; S2, the radius r of the largest oxide inclusion passing through S1 oxide Calculate the safe thickness r of the sulfide shell formed on it sulfide ; When the outer layer of oxide wraps the sulfide to obtain a composite inclusion, the radius of the composite inclusion r complex Satisfy the following formula: r complex =r oxide +r sulfide （2）； When the thermal expansion coefficient of the composite inclusions is greater than that of the steel matrix, residual stress will be generated at the interface between the composite inclusions and the matrix during the quenching heat treatment process, which can be calculated as follows: （3） Among them, σ R It is the residual stress generated at the interface between the composite inclusion and the steel matrix after quenching; C is related to the Young's modulus and Poisson's ratio of the composite inclusion and the matrix, and is always greater than 0; α M is the thermal expansion coefficient of the substrate; △T is the temperature difference of quenching heat treatment; α complex is the thermal expansion coefficient of the composite inclusion; Thermal expansion coefficient α of composite inclusions complex It can be calculated by the following formula: （4） Among them, α oxide is the thermal expansion coefficient of the oxide, α sulfide is the thermal expansion coefficient of sulfide; If the thermal expansion coefficient of the composite inclusion is α complex If the stress is not greater than that of the matrix, no residual stress will be generated at the interface between the matrix and the steel matrix, that is, α M ≤α complex When σ R =0, so the safe thickness of the sulfide shell should satisfy the following formula: （5） S3, r obtained according to S2 sulfide Calculate the cooling rate R corresponding to the solidification process of the casting C ; The thickness of the sulfide shell is determined by the cooling rate during the solidification process of the ingot. The Scheil solidification model is used to calculate the relationship between the thickness of the sulfide precipitation shell and the cooling rate. The diffusion of element i in molten steel is the limiting link in the formation of sulfides. Therefore, the diffusion of element i controls the growth rate of sulfides. It can be obtained that: （7） Among them, J i is the diffusion flux of limiting element i; D i is the diffusion coefficient of limiting element i in the liquid phase, ρ Fe is the density of Fe liquid; M i is the molar mass of the limiting element i; w[i] L is the mass fraction of limiting element i at the solidification front of molten steel; w[i] eq is the mass fraction of limiting element i at equilibrium; r sulfide is the safe thickness of the sulfide shell; In the model, the inclusions are assumed to be spherical after precipitation. According to the mass balance, the relationship between the diffusion flux of element i and the shell thickness of the sulfide is: （8） Combining formula (1), we can get: （9） By integrating Equation (9), we can obtain that the precipitation growth of sulfide and the shell thickness r sulfide expression: （10） In formula (10), M sulfide is the molar mass of sulfide; ρ Fe is the density of Fe liquid, 7070kg / m 3 ;M i is the molar mass of the limiting element i; ρ sulfide is the density of sulfide; D i is the diffusion coefficient of the limiting element i; In formula (10), w[i] L is the mass fraction of limiting element i at the solidification front of molten steel, which can be calculated using the Scheil microsegregation model as follows: （11） Where w[i]0 is the content of solute i in the molten steel before solidification begins; k0 is the equilibrium distribution coefficient of element i; f s is the solid phase ratio of molten steel, which is calculated as follows: （12） Among them, T0 is the melting point of pure iron, which is 1809K; T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel; In formula (10), w[i] eq It can be calculated using the following formulas (13) to (16): [M] + [S] = MS, lgK=A / TB (13) （14） （15） （16） Among them, formula (13) is the reaction equilibrium equation for the reaction of elements M and S to form sulfide MS, and in formula (14) α M , α S , α MS are the activities of [M], [S] and MS respectively; f in equations (15) and (16) M and f S are the activity coefficients of [M] and [S], respectively; and are the interaction coefficients of other elements [j] in the molten steel on [M] and [S] respectively; In formula (10), is the solidification time, calculated according to the following formula: （17） Among them, T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel; R C is the cooling rate during solidification; To achieve a safe thickness r sulfide , Substituting Equation (17) into Equation (10), the cooling rate R C The following formula should be satisfied: （18） Among them, M sulfide and M i are the molar masses of sulfide and limiting element i, respectively; ρ Fe and ρ sulfide are the densities of Fe liquid and sulfide respectively; D i is the diffusion coefficient of limiting element i in the liquid phase; w[i] L is the mass fraction of limiting element i at the solidification front of molten steel; w[i] eq is the mass fraction of limiting element i at equilibrium; T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel.

2. The method for preventing fatigue failure caused by oxide inclusions in steel according to claim 1, characterized in that: The radius r of the largest oxide inclusion in the detected steel is obtained in S1 oxide The method is to calculate the radius r of the largest oxide inclusion in the tested steel based on the national standard rating results. oxide , or directly obtain the radius r of the largest oxide inclusion from the inclusion data obtained by various microscopic observation and analysis methods oxide , or calculate the radius r of the largest oxide inclusion in steel by the national standard extreme value analysis method oxide .

3. The method for preventing fatigue failure caused by oxide inclusions in steel according to claim 2, characterized in that: In S1, the radius r of the largest oxide inclusion in the tested steel is calculated based on the national standard rating results. oxide The national standard GB / T10561-2023 for the grading of Ds-type large-size oxide inclusions is N, which ranges from 0.5 to 5, with a total of 10 levels at intervals of 0.

5. The level N is related to the oxide radius r oxide The relationship is shown in the following formula: r oxide =0.5[10 0.302N+0.972 ] （1） The calculation results in the square brackets in the formula are rounded off to get the maximum oxide inclusion radius in the tested steel; The radius r of the largest oxide inclusion is directly obtained from the inclusion data obtained by various microscopic observation and analysis methods. oxide To directly obtain the maximum oxide inclusion size r using the ASPEX inclusion automatic scanner oxide ; Calculate the radius r of the largest oxide inclusion in steel using the national standard extreme value analysis method oxide To calculate the maximum oxide inclusion size r in steel using the extreme value analysis method of GB / T 40281-2021 oxide .

4. The method for preventing fatigue failure caused by oxide inclusions in steel according to claim 3, characterized in that: Combining formula (5) with formula (1) we can get: （6）。 5. The method for preventing fatigue failure caused by oxide inclusions in steel according to claim 4, characterized in that: In order to obtain the safe thickness of the sulfide shell in S3, the safe thickness r sulfide , Substituting equation (6) into equation (18), the cooling rate R C The following formula should be satisfied: （19） Among them, M sulfide and M i are the molar masses of sulfide and limiting element i, respectively; ρ Fe and ρ sulfide are the densities of Fe liquid and sulfide respectively; D i is the diffusion coefficient of limiting element i in the liquid phase; w[i] L is the mass fraction of limiting element i at the solidification front of molten steel; w[i] eq is the mass fraction of limiting element i at equilibrium; T start is the temperature at which sulfide begins to precipitate; T S is the solidus temperature in steel.

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