CN113702613B - A method for determining critical conditions for dynamic recrystallization of materials - Google Patents

Abstract

The invention relates to a method for determining critical conditions for dynamic recrystallization of a material, comprising: 1) Carrying out a single compression experiment on the experimental material to obtain a stress-strain curve in the deformation process; 2) Taking absolute values of stress sigma and strain epsilon data, and then redrawing a stress-strain curve in a sigma-lg epsilon semi-logarithmic coordinate system or in a lg sigma-lg epsilon double-logarithmic coordinate system; 3) Calibrating the linear section part interval, and selecting different subintervals to perform multiple linear regression; selecting a linear regression equation obtained from subintervals with regression coefficients R more than or equal to 0.99; 4) Drawing xi in rectangular coordinate system ₂ -epsilon curve; 5) The critical strain value at which dynamic recrystallization of the material occurs is determined. The invention can rapidly and accurately determine the critical condition of dynamic recrystallization of the material during compression deformation, and provides a basis for grasping the technological parameters of the steel material during the hot working process and optimizing the hot working process.

Description

A method for determining critical conditions for dynamic recrystallization of materials

Technical field

The present invention relates to the technical field of thermal processing of metal materials, and in particular to a method for determining critical conditions for dynamic recrystallization of materials.

Background technique

When metal materials are plastically deformed at high temperatures, on the one hand, work hardening will occur as the amount of deformation increases, causing a large number of dislocations inside the material; on the other hand, the softening process of dynamic recovery and dynamic recrystallization will occur to offset this work hardening. Dynamic recrystallization has a great influence on the subsequent phase transformation behavior and the mechanical properties of the final product. One of the current mathematical modeling efforts to study the dynamic recrystallization of metals and alloys during hot deformation is to determine the critical conditions for dynamic recrystallization to occur.

Initially, some scholars regarded the strain corresponding to the peak stress in the material's true stress-strain curve as the critical strain for dynamic recrystallization. Later studies found that the metal recrystallized before reaching the peak stress. Therefore, it is inappropriate to use the strain corresponding to the stress peak as the critical strain for dynamic recrystallization. Another direct method is to determine the critical strain of dynamic recrystallization by observing the metallographic microstructure under different strains. This method is difficult to operate, and there is a certain deviation between the determined critical strain and the actual critical strain.

The Chinese invention patent with patent number ZL201811110795.6 discloses a "method for predicting the critical reduction of dynamic recrystallization during hot rolling of microalloy steel". It is based on the observation of metallographic structure and through the deformation of cylindrical samples. Conduct high-temperature compression experiments in the temperature range of 850 to 1250°C, read the peak strains at different temperatures on the flow stress curve obtained from the experiment, and calculate the range of critical strains for dynamic recrystallization at different temperatures. The two are combined to determine the occurrence of dynamic recrystallization. The critical strain of dynamic recrystallization is then obtained through linear fitting to obtain the relationship between the strain corresponding to the peak stress and the critical strain. The critical strain range selected in this method is a wide data interval, and this interval cannot cover the rheological behavior characteristics of all materials. It is difficult to observe the metallographic structure, and there is also a certain deviation in the linear fitting. Therefore, this interval The method is difficult to give accurate critical strain.

Based on the differences between dynamic recovery, dynamic recrystallization, and strain hardening behaviors, Ryan, McQueen, and Kocks defined the stress at which θ and σ on the θ-σ curve begin to deviate from the linear relationship as the critical stress, and thereby determined the critical strain. However, when the true stress is less than the critical stress, the linear relationship between θ and σ is not inevitable, which makes it difficult to determine the location of the inflection point, that is, the critical stress. And obtaining the θ-σ curve requires operations such as fitting, differentiation, and curve transformation of experimental data.

In summary, in order to quickly and accurately determine the critical conditions for dynamic recrystallization, a new determination method needs to be found.

Contents of the invention

The present invention provides a method for determining the critical conditions for dynamic recrystallization of materials, which can quickly and accurately determine the critical conditions for dynamic recrystallization of materials during compression deformation. In order to master the process parameters of steel materials in the thermal processing process and optimize the thermal processing Processing technology provides the basis.

In order to achieve the above objects, the present invention adopts the following technical solutions:

A method for determining critical conditions for dynamic recrystallization of materials, including the following steps:

1) Conduct a single-pass compression experiment on the experimental material through a thermal simulation experiment to obtain the stress-strain curve during the deformation process of the experimental material; smooth the obtained stress-strain curve to remove the influence of noise on the experimental curve;

2) Take the absolute value of the stress σ and strain ε data corresponding to the stress strain curve described in step 1), that is, the stress σ and strain ε are both positive values; then in the σ-lgε semi-logarithmic coordinate system or in lgσ -Redraw the stress-strain curve in the lgε logarithmic coordinate system;

3) According to the shape characteristics of the stress-strain curve redrawn in step 2), calibrate some intervals of the linear segment, and select different sub-intervals to perform multiple linear regressions; select the linear regression equation obtained from the sub-interval with a regression coefficient R≥0.99, The regression equation is expressed by the following formula:

σ ₁ =A+Blgε (1)

lgσ ₂ =A ₁ +B ₁ lgε (2)

In the formula, σ ₁ is the stress value; lgσ ₂ is the logarithm of the stress value; A, B, A ₁ and B ₁ are regression coefficients;

If the subinterval is selected under the σ-lgε semi-logarithmic coordinate system, then formula (1) is used; if the subinterval is selected under the lgσ-lgε double logarithmic coordinate system, then formula (2) is used;

4) Using the strain ε described in step 2) as the independent variable, use the formula (1) or formula (2) described in step 3) to calculate the stress value σ ₁ or the logarithm of the stress value lgσ ₂ , and calculate the stress value or Compare the logarithm of the stress value with the stress σ or lgσ described in step 2) to obtain the following formula:

ξ ₁ =σ ₁ -σ (3)

ξ ₂ =lgσ ₂ -lgσ (4)

In the formula, ξ ₁ is the characteristic stress difference under the σ-lgε semi-logarithmic coordinate system, ξ ₂ is the increment caused by the characteristic stress difference under the lgσ-lgε double logarithmic coordinate system;

If the subinterval is selected under the σ-lgε semi-logarithmic coordinate system, use formula (3) to calculate ξ ₁ and draw the ξ ₁ -ε curve under the rectangular coordinate system; if the subinterval is selected under the lgσ-lgε double logarithmic coordinate system If selected, use formula (4) to calculate ξ ₂ and draw the ξ ₂ -ε curve in the Cartesian coordinate system;

5) Analyze the ξ ₁ -ε curve or ξ ₂ -ε curve obtained in step 4). It can be seen that the value of this curve is zero in the selected strain sub-interval; when the strain exceeds the selected strain sub-interval, as the When the strain increases to a certain value, the calculated value will be greater than the experimental value. This value is the critical strain value for dynamic recrystallization of the material.

Compared with the prior art, the beneficial effects of the present invention are:

Based on the process of work hardening, recovery softening and dynamic recrystallization softening experienced by the material during the deformation process, the reducing effect of dynamic recrystallization on the stress value is highlighted through the logarithmic coordinate system, and the stress-related stress values are found through curve fitting. The difference between the characteristic calculated value and the actual value can quickly and accurately determine the critical conditions for dynamic recrystallization, which lays the foundation for studying the dynamic recrystallization process of materials.

Description of the drawings

Figure 1 is the stress-strain curve of the experimental steel during deformation at 1000°C in the Cartesian coordinate system in Example 1.

Figure 2 is the stress-strain curve of the experimental steel during deformation at 1000°C in the σ-lgε semi-logarithmic coordinate system in Example 1.

Figure 3 is a comparison chart between the calculated values and experimental values of the deformation stress-strain curve of the experimental steel in Example 1 at 1000°C.

In Figure 3, 1 is the calculated value of the stress-strain curve, and 2 is the experimental value of the stress-strain curve.

Figure 4 is a schematic diagram for determining the critical strain of dynamic recrystallization of the experimental steel deformed at 1000°C in Example 1.

In Figure 4, 1 is the critical point, 2 is the stress increment, and 3 is the straight line where the stress increment is zero.

Figure 5 is the stress-strain curve of the experimental steel during deformation at 950°C in the Cartesian coordinate system in Example 2.

Figure 6 is the stress-strain curve of the experimental steel during deformation at 950°C in the lgσ-lgε double logarithmic coordinate system in Example 2.

Figure 7 is a comparison chart between the calculated values and experimental values of the deformation stress strain curve of the experimental steel at 950°C in Example 2.

In Figure 7, 1 is the calculated value of the stress-strain curve, and 2 is the experimental value of the stress-strain curve.

Figure 8 is a schematic diagram for determining the critical strain of dynamic recrystallization of the experimental steel deformed at 950°C in Example 2.

In Figure 8, 1 is the critical point, 2 is the stress increment, and 3 is the straight line where the stress increment is zero.

Detailed ways

The method of determining critical conditions for dynamic recrystallization of materials according to the present invention includes the following steps:

1) Conduct a single-pass compression experiment on the experimental material through a thermal simulation experiment to obtain the stress-strain curve during the deformation process of the experimental material; smooth the obtained stress-strain curve to remove the influence of noise on the experimental curve;

2) Take the absolute value of the stress σ and strain ε data corresponding to the stress strain curve described in step 1), that is, the stress σ and strain ε are both positive values; then in the σ-lgε semi-logarithmic coordinate system or in lgσ -Redraw the stress-strain curve in the lgε logarithmic coordinate system;

3) According to the shape characteristics of the stress-strain curve redrawn in step 2), calibrate some intervals of the linear segment, and select different sub-intervals to perform multiple linear regressions; select the linear regression equation obtained from the sub-interval with a regression coefficient R≥0.99, The regression equation is expressed by the following formula:

σ ₁ =A+Blgε (1)

loσ ₂ =A ₁ +B ₁ lgε (2)

In the formula, σ ₁ is the stress value; loσ ₂ is the logarithm of the stress value; A, B, A ₁ and B ₁ are regression coefficients;

If the subinterval is selected under the σ-lgε semi-logarithmic coordinate system, then formula (1) is used; if the subinterval is selected under the lgσ-lgε double logarithmic coordinate system, then formula (2) is used;

4) Using the strain ε described in step 2) as the independent variable, use the formula (1) or formula (2) described in step 3) to calculate the stress value σ ₁ or the logarithm of the stress value lgσ ₂ , and calculate the stress value or Compare the logarithm of the stress value with the stress σ or lgσ described in step 2) to obtain the following formula:

ξ ₁ =σ ₁ -σ (3)

ξ ₂ =lgσ ₂ -lgσ (4)

In the formula, ξ ₁ is the characteristic stress difference under the σ-lgε semi-logarithmic coordinate system, ξ ₂ is the increment caused by the characteristic stress difference under the lgσ-lgε double logarithmic coordinate system;

If the subinterval is selected under the σ-lgε semi-logarithmic coordinate system, use formula (3) to calculate ξ ₁ and draw the ξ ₁ -ε curve under the rectangular coordinate system; if the subinterval is selected under the lgσ-lgε double logarithmic coordinate system If selected, use formula (4) to calculate ξ ₂ and draw the ξ ₂ -ε curve in the Cartesian coordinate system;

5) Analyze the ξ ₁ -ε curve or ξ ₂ -ε curve obtained in step 4). It can be seen that the value of this curve is zero in the selected strain sub-interval; when the strain exceeds the selected strain sub-interval, as the When the strain increases to a certain value, the calculated value will be greater than the experimental value. This value is the critical strain value for dynamic recrystallization of the material.

In the present invention, by analyzing the ξ ₁ -ε curve or ξ ₂ -ε curve obtained in step 4), it can be seen that the value of the curve is zero in the selected strain sub-interval, because in this interval, according to the formula (3 ) or formula (4) is highly consistent with the experimental value, so the difference between the two is zero. When the strain exceeds the selected strain sub-interval, the calculated value will be greater than the experimental value as the strain increases to a certain value. This is because the material undergoes processes such as work hardening, recovery softening, and dynamic recrystallization softening during the deformation process. What occurs in the initial stage is the process of work hardening and recovery softening. The stress increases rapidly with the increase in strain, but The increase gradually decreases. When dynamic recrystallization occurs, the change pattern of stress with strain is bound to change, which will destroy the original change pattern and cause a sudden change. This sudden change corresponds to when the above calculated values begin to deviate from the experimental values. strain value, from which the critical strain at which dynamic recrystallization occurs can be determined.

The specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings:

The following examples are implemented on the premise of the technical solution of the present invention and provide detailed implementation modes and specific operating processes. However, the protection scope of the present invention is not limited to the following examples.

[Example 1]

In this embodiment, the process of determining the critical conditions for dynamic recrystallization of materials is as follows:

1. The sample material is an alloy steel containing nickel-chromium-molybdenum alloy elements. The sample size is A single-pass compression test was performed on the sample through a thermal simulation testing machine. The sample was heated to 1200°C and kept at this temperature for 3 minutes, and then cooled to the deformation temperature of 1000°C. At this temperature, the strain rate was 0.1s ^{- 1.} Perform compression deformation to obtain the stress-strain curve during the deformation process of the sample, and smooth the curve to remove the influence of noise on the curve during the experiment, as shown in Figure 1;

2. Take the absolute values of the stress σ and strain ε data corresponding to the stress strain curve described in step 1, that is, change the corresponding values to positive values, and then redraw the stress strain curve in the σ-lgε semi-logarithmic coordinate system, such as As shown in Figure 2;

3. Calibrate some intervals of the linear segment in Figure 2, select the sub-interval (0.056, 0.180) for linear regression, and obtain the regression coefficient R = 0.9996. The regression equation is:

σ ₁ =147.228+52.544lgε (5)

4. Use the strain described in step 2 as the independent variable, use formula (5) to calculate the stress value σ ₁ , compare the calculated stress value with the experimental value, and draw the curve corresponding to the experimental value and the calculated value in the same coordinate system , as shown in Figure 3;

5. Difference the ordinate stress values of the curve in Figure 3 to obtain the ξ ₁ -ε curve. It can be seen that the value of this curve is zero in the selected strain sub-interval. Draw a ξ ₁ = 0 straight line in the coordinate system, then in Within the selected strain sub-interval, the straight line ξ ₁ =0 coincides with the curve ξ ₁ -ε. As the strain further increases, the two curves begin to deviate. The starting point of deviation is determined as the critical point for dynamic recrystallization of the sample. At this time The strain is determined as the critical strain for dynamic recrystallization of the sample, as shown in Figure 4.

[Example 2]

In this embodiment, the process of determining the critical conditions for dynamic recrystallization of materials is as follows:

1. The sample material is a low carbon microalloy steel, and the sample size is A single-pass compression test was performed on the sample through a thermal simulation testing machine. The sample was heated to 1200°C and kept at this temperature for 3 minutes, and then cooled to the deformation temperature of 950°C. At this temperature, the strain rate was 0.1s ^{- 1.} Perform compression deformation to obtain the stress-strain curve during the deformation process of the sample, and smooth the curve to remove the influence of noise on the curve during the experiment, as shown in Figure 5;

2. Take the absolute values of the stress σ and strain ε data corresponding to the stress strain curve described in step 1, that is, change the corresponding values to positive values, and then redraw the stress strain curve in the lgσ-lgε double logarithmic coordinate system, such as As shown in Figure 6;

3. Calibrate some intervals of the linear segment in Figure 6, select the sub-interval (0.030, 0.110) for linear regression, and obtain the regression coefficient R = 0.9993. The regression equation is:

lgσ ₂ =2.12244+0.1252lgε (6)

4. Use the strain described in step 2 as the independent variable, use formula (6) to calculate the stress value σ ₂ , compare the calculated stress value with the experimental value, and draw the curve corresponding to the experimental value and the calculated value in the same coordinate system , as shown in Figure 7;

5. Difference the ordinate stress values of the curve in Figure 7 to obtain the ξ ₂ -ε curve. It can be seen that the value of this curve is zero in the selected strain sub-interval. Draw a ξ ₂ =0 straight line in the coordinate system, then in Within the selected strain sub-interval, the straight line ξ ₂ =0 coincides with the curve ξ ₂ -ε. As the strain further increases, the two curves begin to deviate. The starting point of deviation is determined as the critical point for dynamic recrystallization. The strain at this time The critical strain at which dynamic recrystallization occurs is determined, as shown in Figure 8.

The above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person familiar with the technical field can, within the technical scope disclosed in the present invention, implement the technical solutions of the present invention. Equivalent substitutions or changes of the inventive concept thereof shall be included in the protection scope of the present invention.

Claims (1)

1. A method for determining critical conditions for dynamic recrystallization of materials, characterized by comprising the following steps: 1) Conduct a single-pass compression experiment on the experimental material through a thermal simulation experiment to obtain the stress-strain curve during the deformation process of the experimental material; smooth the obtained stress-strain curve to remove the influence of noise on the experimental curve; 2) Take the absolute value of the stress σ and strain ε data corresponding to the stress strain curve described in step 1), that is, the stress σ and strain ε are both positive values; then in the σ-lgε semi-logarithmic coordinate system or in lgσ -Redraw the stress-strain curve in the lgε logarithmic coordinate system; 3) According to the shape characteristics of the stress-strain curve redrawn in step 2), calibrate some intervals of the linear segment, and select different sub-intervals to perform multiple linear regressions; select the linear regression equation obtained from the sub-interval with a regression coefficient R≥0.99, The regression equation is expressed by the following formula: σ ₁ =A+Blgε (1) lgσ ₂ =A ₁ +B ₁ lgε (2) In the formula, σ ₁ is the stress value; lgσ ₂ is the logarithm of the stress value; A, B, A ₁ and B ₁ are regression coefficients; If the subinterval is selected under the σ-lgε semi-logarithmic coordinate system, then formula (1) is used; if the subinterval is selected under the lgσ-lgε double logarithmic coordinate system, then formula (2) is used; 4) Using the strain ε described in step 2) as the independent variable, use the formula (1) or formula (2) described in step 3) to calculate the stress value σ ₁ or the logarithm of the stress value lgσ ₂ , and calculate the stress value or Compare the logarithm of the stress value with the stress σ or lgσ described in step 2) to obtain the following formula: ξ ₁ =σ ₁ -σ (3) ξ ₂ =lgσ ₂ -lgσ (4) In the formula, ξ ₁ is the characteristic stress difference under the σ-lgε semi-logarithmic coordinate system, ξ ₂ is the increment caused by the characteristic stress difference under the lgσ-lgε double logarithmic coordinate system; If the subinterval is selected under the σ-lgε semi-logarithmic coordinate system, use formula (3) to calculate ξ ₁ and draw the ξ ₁ -ε curve under the rectangular coordinate system; if the subinterval is selected under the lgσ-lgε double logarithmic coordinate system If selected, use formula (4) to calculate ξ ₂ and draw the ξ ₂ -ε curve in the Cartesian coordinate system; 5) Analyze the ξ ₁ -ε curve or ξ ₂ -ε curve obtained in step 4). It can be seen that the value of this curve is zero in the selected strain sub-interval; when the strain exceeds the selected strain sub-interval, as the When the strain increases to a certain value, the calculated value will be greater than the experimental value. This value is the critical strain value for dynamic recrystallization of the material.