CN108595827A - A kind of determination method of C-Mn-Al high strength steels Hot Deformation Microstructure evolution mechanism and hot-working character - Google Patents

Abstract

The invention belongs to the technical field of high-strength steel processing engineering, and in particular relates to a method for determining the thermal deformation microstructure evolution mechanism and thermal processing performance of C-Mn-Al high-strength steel. The present invention first carries out high-temperature compression experiment to novel C-Mn-Al high-strength steel, obtains the true stress-true strain curve data of steel, then establishes the flow stress prediction model of steel, and model selection is based on creep theory, has considered Young's A type of constitutive model with physical basis for the relationship between modulus and austenite self-diffusion coefficient and temperature. The established constitutive model can accurately predict the flow stress of steel; establish the thermal deformation processing map of steel, combined with the microstructure Identify mechanisms of tissue evolution in different regions of the processing map. Combining the thermal deformation constitutive model with the processing map, the thermal deformation flow stress and thermal deformation power dissipation efficiency under arbitrary deformation conditions are analyzed, so as to obtain the corresponding microstructure evolution mechanism and thermal processing performance information. Process control is of great significance.

Description

Microstructure evolution mechanism and hot working performance of a C-Mn-Al high-strength steel during hot deformation determination method

technical field

The invention belongs to the technical field of high-strength steel processing engineering, and particularly relates to a method for determining the thermal deformation microstructure evolution mechanism and thermal processing performance of C-Mn-Al high-strength steel.

Background technique

In order to save energy and protect the environment, it is urgent to develop high-strength steels with good ductility and toughness, including TRIP steels. Conventional TRIP steels were developed based on the C-Mn-Si alloy system. The purpose of using high silicon content is to suppress the formation of cementite during cooling so as to increase the stability and quantity of retained austenite. However, high silicon content may cause steel defects such as hard oxide layer, poor surface properties and low coating ability. Since the substitution of Al for Si can eliminate these harmful effects of Si, C-Mn-Al-Si or C-Mn-Al-based TRIP steels have attracted more and more attention, and people have studied the microstructure and mechanical behavior of such steels. A lot of research has been done, but the research on its thermal deformation behavior is relatively scarce.

Dynamic recrystallization is a very common and important deformation mechanism. In the process of dynamic recrystallization, obvious tissue reconstruction occurs, which can greatly eliminate various defects in the original tissue. Microscopic structural changes lead to macroscopic processing plasticity. Improvement and reduction of deformation resistance. In the early stage of the dynamic recrystallization process, there will always be dynamic recovery, because the elimination and rearrangement of dislocations during the dynamic recovery process can form a certain size cell structure and a large-angle misorientation interface, which is precisely for dynamic recrystallization. Crystallization provides the conditions for new nuclei. Dynamic recrystallization is a safe deformation mechanism during thermal processing. In order to obtain a good thermal deformation microstructure, the deformation parameters should be controlled as much as possible within the range where the dynamic recrystallization mechanism of the alloy plays a role when formulating the thermal processing process.

The constitutive equation of thermal deformation can express the relationship between physical quantities such as stress, strain, temperature and strain rate, which can be measured on macroscopic objects. The advantage of the constitutive equation is that the flow stress under a certain deformation condition can be obtained intuitively, but the microstructure evolution mechanism and thermal processing performance of the material cannot be determined simply through the information of the flow stress curve. The dynamic material model is established based on the basic principles of irreversible thermodynamics, physical system simulation and continuum mechanics under the condition of large strain plastic deformation. The processing diagram of the dynamic material model can accurately reflect the microstructure evolution law and mechanism of the material under different deformation conditions through a small number of experiments, and can be used to optimize the thermal processing process of the material, but the information of the flow stress curve cannot be obtained through the processing diagram.

After retrieval, the Chinese patent application No. 201610131725.3 discloses a method for evaluating the thermal processing performance of metal materials. The method includes the following steps: (1) Under the designed thermal deformation conditions, conduct a high-temperature compression test of the metal material, Obtain the true stress-true strain data of metal materials; (2) Establish an improved hyperbolic sinusoidal constitutive model describing the high temperature flow stress of metal materials, and implement it through programming; (3) Establish a thermal processing dissipation efficiency evaluation model for metal materials and instability criterion model, and realized by programming; (4) using the high temperature flow stress prediction model, thermal processing dissipation efficiency evaluation model and instability criterion model established in steps (2) and (3), it is possible to predict The flow stress, thermal processing dissipation efficiency and instability coefficient of metal materials under arbitrary deformation conditions, so as to realize the comprehensive evaluation of the thermal processing performance of metal materials under arbitrary deformation conditions. However, this application still adopts the traditional hyperbolic sine constitutive model, and the accuracy of its prediction results still needs to be further improved.

Contents of the invention

1. The technical problem to be solved by the invention

The purpose of the present invention is to overcome the deficiencies of the prior art, and to provide a method for determining the evolution mechanism of hot-deformed microstructure and hot-working performance of C-Mn-Al high-strength steel. Adopting the method of the present invention can more accurately judge the evolution mechanism and thermal processing performance of thermal deformation structure of C-Mn-Al high-strength steel under different conditions, which is of great importance for the rational formulation of C-Mn-Al high-strength steel thermal processing technology guiding significance.

2. Technical solution

In order to achieve the above object, the technical scheme provided by the invention is:

A kind of C-Mn-Al high-strength steel hot deformation microstructure evolution mechanism of the present invention and the determination method of hot workability, this method comprises the following steps:

Step 1: Carry out high-temperature compression experiments on steel at different deformation temperatures and strain rates to obtain the true stress-true strain curve data of the steel;

Step 2: Establish a constitutive model for predicting the high-temperature flow stress of C-Mn-Al high-strength steel. The constitutive model is selected based on creep theory, taking into account the relationship between Young's modulus and austenite self-diffusion coefficient and temperature A class of models with a physical basis, as shown in the following formula:

In the above formula, is the strain rate (s ^-1 ), T is the temperature (K), σ is the flow stress (MPa), D(T) is the self-diffusion coefficient of austenite, D(T)=D ₀ exp(Q _sd / (RT)), D ₀ is the diffusion constant, Q _sd is the self-diffusion activation energy, E(T) describes the relationship between Young's modulus and temperature; B', α' and n' are material constants;

Introduce the influence of strain ε on the flow stress of the experimental steel into the constitutive equation, and calculate the constitutive equations under the corresponding stresses of a series of different strains one by one. The obtained material constants lnB', α' and n' and the strain The relationship of ε is fitted with a polynomial of degree 5;

Substituting the fitting results into the model, the rheological stress prediction model is obtained:

Among them: α′ _ε ＝ α ₀ + α ₁ ε + α ₂ ε ² + α ₃ ε ³ + α ₄ ε ⁴ + α ₅ ε ⁵ , n′ _ε = N ₀ +N ₁ ε+N ₂ ε ² +N ₃ ε ³ +N ₄ ε ⁴ +N ₅ ε ⁵ , (lnB′) _ε ＝B ₀ ′+B ₁ ′ε+B ₂ ′ε ² +B ₃ ′ε ³ +B ₄ ′ε ⁴ +B ₅ ′ ε ⁵ ;

Step 3: The dynamic material model is established based on the basic principles of irreversible thermodynamics, physical system simulation and continuum mechanics under the condition of large strain plastic deformation. According to the dynamic material model, the criteria for defining the power dissipation efficiency factor η and processing instability are as follows:

where m is the strain rate sensitivity factor, Under the same strain, on the temperature-strain rate two-dimensional plane, draw the contour map of η, that is, the power dissipation map, and then draw the area where the parameter ξ is negative, that is, the thermal processing instability map, that is, The processing map of the material is obtained; the microstructure of steel under different deformation conditions is observed, combined with the processing map, the rheological instability zone, dynamic recrystallization zone and dynamic recovery zone are determined in the processing map;

Step 4: Combine the constitutive model and the processing map to study the thermal deformation behavior of the material: use the established constitutive model to predict the stress-strain curves under different deformation conditions, different deformation conditions correspond to different positions in the processing map, determine the different The power dissipation efficiency factor η under deformation conditions, so as to obtain the flow stress curve information and thermal deformation power dissipation efficiency η value under arbitrary deformation conditions. The invention utilizes two methods of processing diagram and constitutive equation to verify each other, thereby more accurately judging the thermal deformation microstructure evolution mechanism and thermal processing performance under different conditions.

Furthermore, the deformation temperature in the step 1 is 900-1100°C, the deformation temperature interval is 50°C, and the strain rate is 0.01-30s ^-1 , which are respectively 0.01, 0.1, 1, 10 and 30s ^-1 .

Furthermore, the value of α' in step 2 is obtained by using the formula α'=β'/n ₁ ', and n ₁ ' and β' are obtained by and The slope is obtained, and the values of n ₁ ' and β' are obtained according to linear regression; according to the constitutive model, linear fitting The resulting slope and intercept were used to calculate the values of n' and lnB', respectively.

Furthermore, the value of the dependent variable ε in step 2 is from 0.05 to 0.80, and the interval interval is 0.05.

Furthermore, E(T) in step 2 is calculated according to the following formula:

Among them, E ₀ and G ₀ represent the Young's modulus and shear modulus of the material at 300K, respectively, G is the shear modulus of the material at temperature T, and T _M is the melting point of the material.

3. Beneficial effect

Compared with the prior art, the technical solution provided by the invention has the following remarkable effects:

(1) A method for determining the thermal deformation microstructure evolution mechanism and thermal processing performance of a C-Mn-Al high-strength steel of the present invention, combined with the physical-based thermal deformation constitutive equation and thermal deformation processing diagram, using the processing diagram and constitutive The two methods of the equation confirm each other, so that the evolution mechanism of hot deformation microstructure and hot processing performance under different conditions can be judged more accurately.

(2) A kind of C-Mn-Al high-strength steel thermal deformation structure evolution mechanism of the present invention and the determination method of thermal processing performance utilize the constitutive model of establishment to predict the stress-strain curve under different deformation conditions, and different deformation conditions correspond to According to different positions in the processing diagram, determine the power dissipation efficiency factor η under different deformation conditions, so as to obtain the flow stress curve information and thermal deformation power dissipation efficiency η value under arbitrary deformation conditions, and understand the material performance under certain deformation conditions. Thermal deformation and dynamic recrystallization information provide important references for the thermal processing of materials.

Description of drawings

Figures 1 to 4 show the comparison between the predicted values and experimental values of the constitutive model under different deformation conditions.

Fig. 5 is the thermal deformation processing diagram and the thermal deformation microstructure corresponding to different regions in the processing diagram.

Fig. 6 shows the calibration of positions in the corresponding processing maps under two different deformation conditions (1080°C, 0.03s ^-1 and 930°C, 6s ^-1 ).

Figure 7 is the flow stress curve predicted by the constitutive model under the conditions of 1080°C and 0.03s ^-1 .

Fig. 8 is the rheological stress curve predicted by the constitutive model under the conditions of 930°C and 6s ^-1 .

Detailed ways

In order to further understand the content of the present invention, the present invention will be described in detail in conjunction with specific embodiments.

Example 1

A kind of C-Mn-Al high-strength steel hot-deformed microstructure evolution mechanism and the determination method of hot-working performance of the present embodiment, the method comprises the following steps:

Step 1: Thermal deformation at a deformation temperature of 900-1100°C (the interval is 50°C) and a strain rate of 0.01-30s ^-1 (0.01, 0.1, 1, 10, 30s ^-1 ) with an engineering strain of 0.6 Under the same conditions, high-temperature compression experiments were carried out on C-Mn-Al high-strength steel to obtain the true stress-true strain curve data of C-Mn-Al high-strength steel;

Step 2: Establish a constitutive model for predicting the thermal deformation flow stress of C-Mn-Al high-strength steel. The selected constitutive equation is:

in, is the strain rate (s ^-1 ), T is the temperature (K), σ is the flow stress (MPa), B′, α′ and n′ are material constants, D(T) is the self-diffusion coefficient of austenite, D(T)=D ₀ exp(Q _sd /(RT)), D ₀ is the diffusion constant, Q _sd is the self-diffusion activation energy, and E(T) describes the relationship between Young's modulus and temperature. γ-Fe is the closest material to the experimental steel. Substituting the data of γ-Fe into it, we get:

Among them, E ₀ and G ₀ represent the Young's modulus and shear modulus of the material at 300K, respectively, and T _M is the melting point of the material.

There are three unknowns B', α' and n' in the equation to be determined. The value of α' can be obtained by using the formula α'=β'/n ₁ ', and n ₁ ' and β' can be calculated by and The slope is obtained, and the values of n ₁ ' and β' are obtained by linear regression. According to the constitutive equation, a linear fit The resulting slope and intercept can be used to calculate the values of n' and lnB', respectively.

Introduce the influence of the strain on the flow stress of the experimental steel into the constitutive equation, the value of the strain ranges from 0.05 to 0.80, and the interval interval is 0.05, and calculate the constitutive equations under the stresses corresponding to the series of different strains one by one. The relationship between the obtained material constants α', n' and lnB' and the strain ε is fitted by a polynomial of degree 5.

Substituting the coefficient value obtained by fitting into the formula, the prediction model of rheological stress based on physics is obtained:

Among them, α′ _ε = α ₀ +α ₁ ε+α ₂ ε ² +α ₃ ε ³ +α ₄ ε ⁴ +α ₅ ε ⁵ , n′ _ε =N ₀ +N ₁ ε+N ₂ ε ² +N ₃ ε ³ +N ₄ ε ⁴ +N ₅ ε ⁵ , (lnB′) _ε ＝B ₀ ′+B ₁ ′ε+B ₂ ′ε ² +B ₃ ′ε ³ +B ₄ ′ε ⁴ +B ₅ ′ ε ⁵ , whose coefficients are shown in Table 1.

Using the prediction model to calculate the thermal deformation rheological stress under different deformation conditions, the comparison diagrams of the obtained prediction values and experimental values are shown in Figure 1-Figure 4. It can be seen from the figure that the prediction results are good, so the established constitutive model can be accurate Prediction of thermal deformation flow stress curve of steel.

Each coefficient value in table 1 quintic polynomial

Step 3: The dynamic material model is established based on the basic principles of irreversible thermodynamics, physical system simulation and continuum mechanics under the condition of large strain plastic deformation. According to the dynamic material model, the power dissipation efficiency factor η and the criterion of processing instability are defined.

where m is the strain rate sensitivity factor, Under the same strain, on the temperature-strain rate two-dimensional plane, draw the contour map of η (that is, the power dissipation map), and then draw the area where the parameter ξ is about negative (that is, the thermal processing instability map) , the processing map of the material is obtained.

In order to verify the accuracy of the thermal deformation processing map, the microstructure of steel under different deformation conditions was observed, combined with the processing map, the rheological instability zone, dynamic recrystallization zone and dynamic recovery zone in the processing map were determined. Figure 5 shows the obtained processing map and the corresponding microstructure map of different regions in the processing map. It can be determined that the shaded area in the figure is the processing instability area (hot processing should be avoided in this area), and the rectangular frame line in the figure The included region (1000-1100°C, 0.01-1s ^-1 ) is the dynamic recrystallization region, and the other regions are the dynamic recovery region.

Step 4: Combining the constitutive equation and the processing diagram to study the thermal deformation behavior of the material: use the established constitutive equation to predict the stress-strain curve under different deformation conditions. Different deformation conditions correspond to different positions in the processing map and have different power dissipation efficiencies. The two methods of processing diagram and constitutive equation can confirm each other, and can more accurately judge the microstructure evolution mechanism and thermal processing performance under different deformation conditions. Examples will be given below for specific description.

Two different deformation conditions (1080°C, 0.03s ^-1 and 930°C, 6s ^-1 ) were arbitrarily selected as examples for analysis. The positions marked (1080°C, 0.03s ^-1 and 930°C, 6s ^-1 ) from the processing diagram are shown in Fig. 6 . It can be seen from Figure 3 that the deformation condition of 1080°C and 0.03s- ¹ corresponds to a power dissipation efficiency of 0.25, which is located in the dynamic recrystallization zone, while the deformation condition of 930°C and 6s ^-1 corresponds to a power dissipation efficiency of 0.19, in the dynamic reply area. The stress-strain curves under the two deformation conditions predicted by the constitutive equation are shown in Fig. 7 and Fig. 8. It can be seen that the flow stress curve under the deformation conditions of 1080 ℃ and 0.03s ^-1 is a dynamic recrystallization type, so both the processing diagram and the constitutive equation determine that the material will undergo dynamic recrystallization under this deformation condition. However, there is no obvious dynamic recrystallization peak in the flow stress curve under the deformation condition of 930°C and 6s ^-1 , so it is preliminarily judged that the material undergoes dynamic recovery under this deformation condition, which can be confirmed with the processing diagram, and it can be more accurately judged that the deformation condition corresponds to The dynamic reply area of the material. Similarly, this method can be used to obtain the flow stress curve information and thermal deformation power dissipation efficiency η value under arbitrary deformation conditions, so as to determine the thermal deformation microstructure evolution mechanism and thermal processing performance of the material.

The invention predicts the flow stress curve of steel by selecting a new type of constitutive model based on physics. Compared with the traditional hyperbolic sine constitutive model, it is not only simple and effective, but also has a certain physical basis. In the present invention, the prediction results of the constitutive model and the analysis results of the processing diagram are compared and studied, and the research results further confirm that the combined use of the two methods of the constitutive model and the processing diagram can more effectively predict the thermal processing behavior of steel. In summary, combined with the processing diagram and constitutive equation, the thermal deformation and dynamic recrystallization information under certain deformation conditions can be fully understood, thus providing an important reference for the thermal processing of materials.

Claims (5)

1. A method for determining a hot deformation structure evolution mechanism and hot working performance of C-Mn-Al high-strength steel is characterized by comprising the following steps:

step 1: carrying out high-temperature compression experiments on the steel at different deformation temperatures and strain rates to obtain true stress-true strain curve data of the steel;

step 2: establishing a constitutive model for predicting the high-temperature rheological stress of the C-Mn-Al high-strength steel, which is shown as the following formula:

in the above formula, the first and second carbon atoms are,is the strain rate(s)^-1) T is temperature (K), σ is rheological stress (MPa), D (T) is the self-diffusion coefficient of austenite, and D (T) ═ D₀exp(Q_sd/(RT))，D₀Is the diffusion constant, Q_sdis self-diffusion activation energy, E (T) describes the relationship between Young's modulus and temperature, B', α 'and n' are material constants;

introducing the influence of the strain epsilon on the rheological stress of the experimental steel into a constitutive equation, calculating the constitutive equations under the stress corresponding to different strains of a series one by one, and fitting the obtained relationship between the material constants lnB ', α ' and n ' and the strain epsilon by using a 5 th-order polynomial;

and substituting the fitting result into the model to obtain a rheological stress prediction model:

wherein α'_ε＝α₀+α₁ε+α₂ε²+α₃ε³+α₄ε⁴+α₅ε⁵，n′_ε＝N₀+N₁ε+N₂ε²+N₃ε³+N₄ε⁴+N₅ε⁵，(lnB′)_ε＝B′₀+B′₁ε+B′₂ε²+B′₃ε³+B′₄ε⁴+B′₅ε⁵；

step 3, defining the criterion of the power dissipation efficiency factor η and the processing instability according to the dynamic material model as follows:

wherein m is a strain rate sensitive factor, and an eta contour map, namely a power dissipation map, is drawn on a two-dimensional plane of temperature-strain rate under the same strain, and a region with a negative parameter ξ, namely a hot processing instability map, is drawn to obtain a processing map of the material;

and 4, combining the constitutive model and the processing diagram to study the thermal deformation behavior of the material, namely predicting stress-strain curves under different deformation conditions by using the established constitutive model, determining power dissipation efficiency factors η under different deformation conditions corresponding to different positions in the processing diagram, so as to obtain rheological stress curve information and thermal deformation power dissipation efficiency η values under any deformation conditions, and further determining a thermal deformation structure evolution mechanism and thermal processing performance of the material.

2. The method for determining the heat deformation structure evolution mechanism and the hot working performance of the C-Mn-Al high-strength steel according to claim 1, wherein the method comprises the following steps: the deformation temperature in the step 1 is 900-1100 ℃, the interval of the deformation temperature is 50 ℃, and the strain rate is 0.01-30s^-1Respectively taking 0.01, 0.1, 1, 10 and 30s^-1。

3. the method for determining the heat deformation structure evolution mechanism and the hot working performance of the C-Mn-Al high-strength steel according to claim 1, wherein the value of α 'in the step 2 is represented by the formula α' ═ β '/n'₁To obtain n'₁and β' are respectively composed ofAndslope of (2)To obtain n 'from linear regression'₁and beta' values, linear fitting according to a constitutive modelThe resulting slope and intercept are used to calculate the values of n 'and ln B', respectively.

4. The method for determining the heat deformation structure evolution mechanism and the hot working performance of the C-Mn-Al high-strength steel according to any one of claims 1 to 3, wherein the method comprises the following steps: in the step 2, the value of the strain epsilon ranges from 0.05 to 0.80, and the interval ranges from 0.05.

5. The method for determining the heat deformation structure evolution mechanism and the hot working performance of the C-Mn-Al high-strength steel according to any one of claims 1 to 3, wherein the method comprises the following steps: e (T) in step 2 is calculated according to the following formula:

wherein E is₀And G₀Respectively represents the Young modulus and the shear modulus of the material at 300K, G is the shear modulus of the material at the temperature T, and T is_MIs the melting point of the material.