阅读说明：本技术 一种测定材料静态再结晶体积分数的方法 (Method for measuring static recrystallization volume fraction of material ) 是由 赵宝纯 黄磊 李广龙 王晓峰 周敬 马惠霞 于 2019-09-30 设计创作，主要内容包括：本发明公开了一种测定材料静态再结晶体积分数的方法,应用热力模拟试验机进行一组单道次压缩试验,得到应力应变曲线,进行数据拟合、微分操作,得到加工硬化率与相应的应力之间的关系曲线,从曲线中的拐点判断发生动态再结晶的临界应变ε<Sub>c</Sub>；应用热力模拟试验机进行一组双道次压缩试验,双道次的总变形量小于单道次的变形量,且双道次压缩试验中的每一道次变形量ε<Sub>0</Sub>均小于单道次压缩试验确定出的动态再结晶临界应变量ε<Sub>c</Sub>；采用多项式分别对单道次、双道次中应力应变曲线进行拟合,分别进行积分操作,得到相应的应变能；最后计算静态再结晶体积分数。本发明考虑了回复、动态再结晶对静态再结晶体积分数的影响,能够更准确测定钢铁材料奥氏体静态再结晶体积分数。(The invention discloses a method for measuring the volume fraction of static recrystallization of a material, which comprises the steps of carrying out a group of single-pass compression tests by using a thermal simulation testing machine to obtain a stress-strain curve, carrying out data fitting and differential operation to obtain a relation curve between the work hardening rate and corresponding stress, and judging the critical strain epsilon of dynamic recrystallization from the inflection point in the curve c (ii) a A group of double-pass secondary compression tests are carried out by using a thermal simulation testing machine, the total deformation of double passes is less than that of single pass, and the deformation epsilon of each pass in the double-pass secondary compression tests 0 All are less than the critical strain epsilon of dynamic recrystallization determined by single-pass compression test c (ii) a Fitting stress-strain curves in single pass and double passes respectively by adopting a polynomial, and performing integral operation respectively to obtain corresponding strain energy; and finally calculating the static recrystallization volume fraction. The invention considers the volume of recovery, dynamic recrystallization and static recrystallizationThe influence of the fraction can more accurately determine the austenite static recrystallization volume fraction of the steel material.)

1. A method for determining the static recrystallization volume fraction of a material, comprising the steps of:

1) performing a single-pass compression test by using a thermal simulation testing machine, heating and preserving heat of a group of samples, cooling to different deformation temperatures at a set cooling speed, preserving heat, compressing the samples at a set strain rate and a set deformation, and collecting stress and strain data in the test process to obtain a stress-strain curve; performing data fitting and differential operation on the obtained single-pass stress-strain curve to obtain a relation curve between the work hardening rate and corresponding stress, and judging the critical strain epsilon of dynamic recrystallization from the inflection point in the curve_c；

2) Performing a double-pass compression test by using a thermal simulation testing machine, wherein the states of a group of test samples for the test are consistent with those of the test samples in the step 1, and the conditions of heating, heat preservation, cooling, deformation temperature and deformation speed are the same as those of the single-pass compression test in the step 1; the total deformation of the double passes is less than that of the single pass, and the deformation epsilon of each pass in the double-pass compression test₀All are less than the critical strain epsilon of dynamic recrystallization determined by single-pass compression test_c；

3) Fitting the single-pass stress-strain curve obtained at a certain deformation temperature and strain rate in the step 1 by adopting a polynomial, and selecting the strain in the single-pass stress-strain curve as epsilon according to the fitted curve₀-2ε₀The following equation is obtained:

σ_m＝A_m+B_mε+C_mε²+D_mε³+… (1)

wherein ε is the dependent variable σ_mIs stress, A_m，B_m，C_m，D_m… is a constant;

and (3) fitting the stress-strain curve of each pass obtained by the two-pass test in the step (2) by adopting a polynomial to obtain the following formula:

σ₁＝A+Bε+Cε²+Dε³+… (2)

σ₂＝A₂+B₂ε+C₂ε²+D₂ε³+… (3)

wherein ε is the dependent variable σ₁Is the first pass stress, σ₂Stress of the second pass, A, B, C, D, A₂，B₂，C₂，D₂… is a constant;

4) integrating the formulas (1), (2) and (3) in the step 3 respectively to obtain the following formulas:

wherein S is₁、S₂And S_mThe strain energy of the first pass in the double pass deformation, the strain energy of the second pass in the double pass deformation and the strain of the single pass are respectively from epsilon₀-2ε₀Strain energy in time;

5) if the relation curve between the work hardening rate and the corresponding stress obtained by single compression has no inflection point or the epsilon determined at the inflection point_c﹥2ε₀And calculating the volume fraction of the generated static recrystallization according to the strain energy obtained in the step 5 and the following formula:

wherein, X_sIs the static recrystallization volume fraction;

if 2 ε₀>ε_cThen, the deformation of the single compression does not reach 2 epsilon₀Then dynamic recrystallization has occurred and the calculated softening value already contains the softened portion of the dynamic recrystallization, the formula for calculating the volume fraction of static recrystallization can be changed to the following formula:

wherein, X_dIs the dynamic recrystallization volume fraction.

2. The method of claim 1, wherein the step of determining the static recrystallization volume fraction of the material comprises: the dynamic recrystallization volume fraction X_dThe calculation formula of (2) is as follows:

wherein k and n are material constants epsilon_c、ε_pThe critical strain and peak strain for dynamic recrystallization are shown.

3. The method of claim 1, wherein the step of determining the static recrystallization volume fraction of the material comprises: samples for single-pass and double-pass two-group compression tests are obtained from a blank.

Technical Field

The invention relates to a method for measuring the tissue evolution of a material in a rolling process, in particular to a method for measuring the static recrystallization volume fraction of the material.

Background

During the hot working and forming process of the material, two processes of work hardening, recovery, recrystallization and softening simultaneously occur inside the material. The recrystallization modes include dynamic recrystallization, sub-dynamic recrystallization and static recrystallization, and the static recrystallization is also an important softening mode in the processing process. The research on the static recrystallization rule of the material in the forming process has important significance for controlling the structure and the performance during hot processing.

The precondition of the mathematical model study of the structure change during the rolling pass interval is to obtain the static recrystallization volume fraction for new steel grades or new processes. The austenite static recrystallization softening rate can be measured by metallographic structure observation and a double-pass deformation method, and the former method is not generally adopted because of too large workload. Therefore, the latter is commonly used. However, based on the two-pass deformation method, many methods for calculating the static recrystallization softening rate have appeared. Mainly comprises a 0.2% compensation method, a 2% compensation method, a 5% total strain method, a rear insertion method, an average stress method and an area method.

The principle of the double pass deformation to determine the static recrystallization volume fraction is based on the difference between the two passes of stress or strain energy caused by the softening of austenite between the two passes. For example, the 0.2% compensation method, the 2% compensation method and the 5% total strain method are used for calculation based on a certain stress value, so that accidental errors and system errors are easily generated, the conventional area method or average stress method does not clearly specify the deformation conditions of double passes and the pass deformation amount used for calculation, and an extrapolation model is required, so that different researchers can obtain different results, particularly, the dynamic recrystallization of the material during deformation is still calculated according to the conventional method, and the result is unscientific and unreasonable. Therefore, the results obtained by the different methods are inconsistent and sometimes even far from each other, and in order to more accurately predict the static recrystallization fraction at the rolling pass interval to establish a mathematical model of the structure change at the rolling pass interval, a more accurate measurement method is required.

Disclosure of Invention

The invention aims to provide a method for measuring the static recrystallization volume fraction of a material, which obtains real strain energy according to actual single-pass test data to eliminate the influence of dynamic recrystallization and can more accurately measure the austenite static recrystallization volume fraction of the material.

In order to achieve the purpose of the invention, the technical scheme of the invention comprises the following steps:

1) performing a single-pass compression test by using a thermal simulation testing machine, heating and preserving heat of a group of samples, cooling to different deformation temperatures at a set cooling speed, preserving heat, compressing the samples at a set strain rate and a set deformation, and collecting stress and strain data in the test process to obtain a stress-strain curve; performing data fitting and differential operation on the obtained single-pass stress-strain curve to obtain a relation curve between the work hardening rate and corresponding stress, and judging the critical strain epsilon of dynamic recrystallization from the inflection point in the curve_cIf the curve has no inflection point, no dynamic recrystallization occurs, and if the curve has the inflection point, the dynamic recrystallization occurs; critical strain epsilon at which dynamic recrystallization occurs_cThe method is used for setting the pass deformation of double-pass secondary compression and determining the selection of a calculation method of the integral number of the static recrystallization, and a stress-strain curve of single-pass secondary compression is used for calculating strain energy consumed in the deformation process.

2) Performing a double-pass compression test by using a thermal simulation testing machine, wherein the states of a group of test samples for the test are consistent with those of the test samples in the step 1, and the conditions of heating, heat preservation, cooling, deformation temperature and deformation speed are the same as those of the single-pass compression test in the step 1; the total deformation of the double passes is less than that of the single pass, and the deformation epsilon of each pass in the double-pass compression test₀All are less than the critical strain epsilon of dynamic recrystallization determined by single-pass compression test_c。

3) Fitting the single-pass stress-strain curve obtained at a certain deformation temperature and strain rate in the step 1 by adopting a polynomial, and selecting the strain in the single-pass stress-strain curve as epsilon according to the fitted curve₀-2ε₀(ε₀For each pass of the two pass compression test), the following equation is obtained:

σ_m＝A_m+B_mε+C_mε²+D_mε³+… (1)

wherein ε is the dependent variable σ_mIs stress, A_m，B_m，C_m，D_m… is a constant;

the method aims to calculate the static recrystallization volume fraction by using a real test curve so as to eliminate the influence of reversion and dynamic recrystallization on the calculation of the static recrystallization volume fraction;

and (3) fitting the stress-strain curve of each pass obtained by the two-pass test in the step (2) by adopting a polynomial to obtain the following formula:

σ₁＝A+Bε+Cε²+Dε³+… (2)

σ₂＝A₂+B₂ε+C₂ε²+D₂ε³+… (3)

wherein ε is the dependent variable σ₁Is the first pass stress, σ₂Stress of the second pass, A, B, C, D, A₂，B₂，C₂，D₂And … is a constant.

4) Integrating the formulas (1), (2) and (3) in the step 3 respectively to obtain the following formulas:

wherein S is₁、S₂And S_mThe strain energy of the first pass in the double pass deformation, the strain energy of the second pass in the double pass deformation and the strain of the single pass are respectively from epsilon₀-2ε₀Strain energy in time;

S_mto test forThe true value of the strain energy is consumed, rather than being obtained by extrapolation or calculation, and the difference between the calculated value and the true value is 2 epsilon₀>ε_cThe condition shows larger difference, which causes larger deviation of the calculation result.

5) If the relation curve between the work hardening rate and the corresponding stress obtained by single compression has no inflection point or the epsilon determined at the inflection point_c﹥2ε₀And calculating the volume fraction of the generated static recrystallization according to the strain energy obtained in the step 4 and the following formula:

wherein, X_sIs the static recrystallization volume fraction;

if 2 ε₀>ε_cThen, the deformation of the single compression does not reach 2 epsilon₀Then dynamic recrystallization has occurred and the calculated softening value already contains the softened portion of the dynamic recrystallization, the formula for calculating the volume fraction of static recrystallization can be changed to the following formula:

wherein, X_dIs the dynamic recrystallization volume fraction.

X_dThe calculation formula of (2) is as follows:

wherein k and n are material constants epsilon_c、ε_pCritical strain and peak strain for dynamic recrystallization

The invention has the beneficial effects that: the method considers the influence of reversion and dynamic recrystallization on the static recrystallization volume fraction, can more accurately measure the austenite static recrystallization volume fraction of the steel material so as to accurately establish a mathematical model of the structure change of the steel material at the rolling pass interval time, and particularly can formulate a control rolling process of reasonable microalloy steel.

Detailed Description

The specific implementation mode of the invention is as follows:

1. selecting a low-carbon microalloyed steel as a material to be tested, and performing a group of single-pass compression tests by using a thermal simulation testing machine, wherein the test process of the sample comprises the steps of heating to 1150 ℃ at a speed of 20 ℃/S, preserving heat for 5 minutes, then cooling to 850 ℃ at a speed of 5 ℃/S, preserving heat for 1 minute at the temperature, performing compression deformation, wherein the deformation amount is 70%, and the strain rate is 5S^-1(ii) a Collecting stress and strain data in the test process to obtain a stress-strain curve;

performing data fitting and differential operation on a single-pass stress-strain curve obtained by deformation at the temperature of 850 ℃ and 950 ℃ to obtain a relation curve between the work hardening rate and corresponding stress, wherein the curve shows that an inflection point appears when the curve is deformed at the temperature of 950 ℃, which indicates that dynamic recrystallization occurs at the inflection point, and the critical strain for the dynamic recrystallization is determined as epsilon_c0.29, and the value is used for setting the deformation of each pass of the subsequent two passes; when the alloy is deformed at 850 ℃, the relation curve between the work hardening rate and the corresponding stress is obtained without inflection points, which indicates that no dynamic recrystallization occurs under the condition;

2. and performing another set of double-pass compression tests on the thermal simulation testing machine, wherein the set of samples and the samples of the single-pass compression tests are obtained from a blank. The test process comprises heating to 1150 deg.C at 20 deg.C/s, maintaining for 5 min, cooling to 850 deg.C at 5 deg.C/s, maintaining for 1 min, performing compression deformation at each temperature for two times, and generating dynamic recrystallization critical strain epsilon_c0.29, the deformation of each pass in the two-pass test is epsilon₀20%, the time interval between passes is 100 and 3 seconds respectively, and the strain rate is 5S^-1。

3. And (3) fitting the single-pass stress-strain curve obtained in the step (1) by adopting a polynomial under the conditions that the deformation temperature is 850 ℃ and 950 ℃ to obtain the following formula:

the corresponding curve fitting result of the formula (10) under the condition that the corresponding deformation temperature is 850 ℃; the curve fitting result corresponding to the deformation temperature of 950 ℃ in the formula (11)

And fitting the stress-strain curve of the two-pass test deformed at the temperature of 850 ℃ in the step 2 by adopting a polynomial to obtain the following formula:

the formulas (12) and (13) respectively correspond to the first pass curve and the second pass curve

And fitting the stress-strain curve of the double-pass test deformed at the temperature of 950 ℃ in the step 2 by adopting a polynomial to obtain the following formula:

the formulas (14) and (15) respectively correspond to the curves of the first pass and the second pass

4. The equations (10) to (15) in step 3 were integrated respectively to obtain strain energies as shown in table 1:

TABLE 1 Strain energy at different temperatures

5. The integrated static recrystallization volume calculated by step 4 is shown in table 2 in comparison with the results calculated by other methods.

TABLE 2 static recrystallization fraction comparison

The relation curve between the work hardening rate and the corresponding stress obtained by single-pass compression of the material at 850 ℃ has no inflection point, epsilon_c﹥2ε₀And (3) calculating the volume fraction of the statically recrystallized grains to be 66.7% according to the formula (7) according to the strain energy obtained in the step 4:

the curve of the relation between the work hardening rate and the corresponding stress obtained by single-pass compression of the material at 950 ℃ has an inflection point, epsilon_c＝0.29﹤2ε₀The volume fraction of the crystals in which static recrystallization occurred was calculated to be 66.9% according to the equation (8) from the strain energy obtained in step 4, 0.4.

As can be seen from the comparison in the table, the static recrystallization volume fraction measured by the method is closest to the result measured by the metallographic method, the error is within 5 percent, and the error generated by other measuring methods is close to 10 percent, so that the static recrystallization volume fraction can be more accurately measured by the method.