CN112129631B - Cold deformation mold working curve design method based on full-size strain reinforcement - Google Patents

Abstract

The invention relates to a cold deformation die working curve design method based on full-size strain reinforcement, and belongs to the technical field of stainless steel tube processing. The method comprises the following steps: 1. preparing a nearly cylindrical sample; 2. placing the sample in a pressure testing machine stably; 3. the load is stably applied by a pressure testing machine, the strain rate and the strain quantity are controlled, and strain strengthening samples with different parameters are obtained; 4. the test sample is cut along the longitudinal section, a transverse full-size tensile test sample is processed, and a tensile test is carried out, so that the yield strength of the material after strain strengthening is obtained; 5. fitting the intensity values of different strain strengthening parameters to obtain a full-size strain strengthening characteristic curve of the material; 6. and (5) derivative is obtained on the strain strengthening characteristic curve, and the strain strengthening rate characteristic curve is obtained. According to the method, the strain strengthening characteristic curve of the austenitic and duplex stainless steel thick-wall pipe can be quickly and accurately established; the strength value of materials such as austenite, duplex stainless steel and the like can be effectively predicted.

Description

A method for designing working curve of cold deformation die based on full-size strain hardening

Technical Field

The invention relates to a cold deformation die working curve design method based on full-size strain strengthening, and belongs to the technical field of stainless steel pipe processing.

Background technique

In deep oil and gas production, the oil well pipe string usually reaches 5,000 meters or even deeper. Therefore, the oil well pipe body is usually required to have very high strength, such as 110 steel grade (758MPa), 125 steel grade (862MPa), 140 steel grade (965MPa). In addition, the aerospace industry usually requires the pipeline system to have high strength and light weight.

At present, strain hardening is the main strengthening mechanism of austenitic and duplex stainless steel pipes. Therefore, cold deformation processes such as cold rolling and cold drawing are usually used to design and manufacture high-strength austenitic and duplex stainless steel pipes.

In addition, due to safety considerations and requirements, strict requirements are usually placed on the uniformity of energy industry pipeline systems, such as requiring that the yield strength in the length direction should not fluctuate by more than 75MPa between batches, and sometimes even by less than 50MPa.

Therefore, how to accurately design and control the cold deformation process and cold deformation mold of austenitic and duplex stainless steels to achieve precise control of strain strength is an important direction for the design and manufacturing of high-precision and high-requirement stainless steel pipes.

Relevant data show that uniaxial tension or uniaxial torsion methods can be used to characterize the constitutive model of materials. However, due to the influence of material plasticity, the strain is generally not large. When the strain is greater than a certain value (such as about 30%), the material will produce obvious necking and break, which cannot effectively characterize and predict the strengthening characteristics of the material at large strain (such as strain > 50%). In addition, uniaxial compression is often only used for the study of the constitutive model of materials, and the sample size is very small (usually no more than 15mm in diameter). In addition, the above three methods are usually used to characterize the deformation resistance of the material and to verify the load capacity of the equipment. In summary, the above methods cannot quickly and intuitively reflect the changes in the strength value of the material itself after strain strengthening, and lack guidance for the design of cold deformation molds.

There is still a large gap between the current research ideas of obtaining material constitutive models and stress-strain curves from small-size samples and engineering practice. In particular, when the material size is enlarged, the size effect caused by the unevenness of the material itself is encountered, and the results often cannot be accurately guided and applied to engineering practice.

In addition, in order to accurately obtain the strain hardening value of the material, it is necessary to reasonably design the working curve of the cold deformation mold to match the material hardening rate characteristic curve. When the material strain hardening characteristic curve is inconsistent with the mold working curve required for cold processing, the material deformation process and difficulty are often counterproductive. At the same time, different materials have different hardening rate characteristic curves. Therefore, the design of the cold processing mold working curve often needs to be adjusted according to material changes. However, the current mold cold deformation working curve design is often based on empirical rules, such as using polynomial, exponential, or linear design methods, which leads to problems such as reduced efficiency and increased costs.

Summary of the invention

In view of the shortcomings of the prior art, the purpose of the present invention is to provide a cold deformation die working curve design method based on full-size strain hardening. By mastering the strain hardening law of materials such as stainless steel, the design and manufacture of high-precision strain-hardened stainless steel pipes can be quickly and accurately guided.

The technical solution of the present invention to solve the above problems is as follows:

A cold deformation die working curve design method based on full-size strain hardening includes the following steps:

Step 1: Prepare a nearly cylindrical specimen by mechanical processing;

Step 2: Place the sample stably on the lower die seat of the pressure testing machine so that the bottom of the sample and the lower die seat fit closely; then turn the upper die seat upside down on the top of the sample;

Step 3: Use a pressure testing machine to steadily apply a load to the upper die seat, and control the strain rate and strain amount, so as to obtain strain hardening specimens with different parameters;

Step 4: Cut the strain-hardened specimen along the longitudinal section, process the transverse full-size tensile specimen and conduct a tensile test to obtain the yield strength of the material after strain hardening;

Step 5: Obtain the full-size strain hardening characteristic curve of the material by fitting the strength values of different strain hardening parameters;

Step 6: Take the derivative of the strain hardening characteristic curve to obtain the strain hardening rate characteristic curve.

In the above technical solution of the present invention, the strain hardening rate characteristic curve is obtained by taking the derivative of the hardening characteristic curve, which can be used for the deformation curve design of the working section of the cold deformation die. When the strain hardening parameters change, the strength values of materials such as austenite and duplex stainless steel can be effectively predicted by combining the interpolation method and the extrapolation method.

As a preferred embodiment of the above technical solution, the diameter of the sample is not less than 70 mm, and the ratio of height to diameter is between 1.2 and 3.0; the deviation value of the sample height and outer diameter is ±0.50 mm, and the parallelism of the upper and lower planes is ≤0.50 mm; the upper and lower planes of the sample are both machined with R10 arc guide surfaces.

As a preferred embodiment of the above technical solution, the lower die base of the pressure testing machine includes an inner lining layer, a middle lining layer and an outer lining layer, wherein the inner lining layer is a pressure-bearing layer, the middle lining layer is a protective layer, and the outer lining layer is a supporting layer.

In the above technical scheme of the present invention, the inner lining layer is a pressure-bearing layer, which mainly bears the load during the deformation process; the middle lining layer is a protective layer, which mainly prevents abnormal fragmentation of the pressure-bearing layer during loading, thereby improving the safety of the mold base system; the outer lining layer is a supporting layer, which plays the role of supporting the middle lining layer.

As a preferred embodiment of the above technical solution, a small hole is provided on the side of the middle lining layer and the outer lining layer; during the test, the cooling liquid passes through the small hole to quickly cool the sample.

In the above technical solution of the present invention, during the loading process, the sample is subjected to force and produces extrusion deformation, and at the same time generates high heat, causing the sample temperature to rise. When the temperature rises, the strain hardening effect of the material will decrease. Therefore, in order to offset the temperature rise effect, the cooling liquid can flow out from the small hole and quickly cool the sample, so that the effect of precise control of strain hardening can be achieved. The cooling liquid flow rate is not less than 50 ml/s.

As a preferred embodiment of the above technical solution, the upper die base system of the pressure testing machine includes an inner lining layer and an outer lining layer, wherein the inner lining layer is a pressure-bearing layer and the outer lining layer is a supporting layer.

In the above technical solution of the present invention, the inner lining layer is a pressure-bearing layer and the outer lining layer is a supporting layer. During the strain hardening process, the inner lining layer is the main pressure-bearing component and the outer lining layer mainly plays the role of supporting the inner lining layer.

As a preferred embodiment of the above technical solution, the inner lining layer of the upper die base and the lower die base is provided with a guide groove with a depth not exceeding 10 mm; during the specimen loading test, grease is pre-coated in the guide groove.

In the above technical solution of the present invention, the inner lining layer of the upper die seat and the lower die seat is provided with a guide groove with a depth of no more than 10 mm, which is convenient for close cooperation with the nearly cylindrical specimen during installation and the specimen loading test, and plays a role in metal guide. At the same time, the angle of the groove is 5-15°, and a layer of solid grease is coated in the groove to facilitate demoulding after strain hardening.

As a preferred embodiment of the above technical solution, when the pressure testing machine applies load, the displacement sensor and pressure sensor are set to control the moving speed and strain of the upper die seat. The maximum axial strain can reach 85%. The pressure sensor is used to monitor the load change in a timely manner.

As a preferred embodiment of the above technical solution, in step 4, the diameter of the processed standard transverse tensile specimen is at least 15 mm, and a tensile test is carried out to obtain the yield strength of the specimen after strain hardening.

As a preferred embodiment of the above technical solution, in step five, the yield strength of the sample obtained after different strain hardening parameters is fitted to obtain a mathematical expression of the full-size strengthening characteristic curve of the stainless steel material.

In summary, the present invention has the following beneficial effects:

1. According to the method of the present invention, the strain hardening characteristic curve of austenitic and duplex stainless steel thick-walled pipes (wall thickness greater than 10 mm) can be established quickly and accurately.

2. According to the strain hardening characteristic curve obtained by the method of the present invention, the corresponding mathematical expression is obtained; when the strain hardening parameters change, the strength values of materials such as austenite and duplex stainless steel can be effectively predicted by combining interpolation and extrapolation.

3. The present invention can provide a basis for the manufacturing process parameters and cold deformation die design of thick-walled stainless steel pipes, and accurately predict the strain strength of stainless steel pipes, greatly saving human resources and time costs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the upper and lower mold base system of the present invention;

FIG2 is a hardening rate curve of material 1;

FIG3 is a hardening rate curve of material 2.

Detailed ways

The present invention is further described in detail below with reference to the embodiments.

Embodiment 1

The material 1 of the first embodiment is super duplex stainless steel, the seamless steel pipe specification is ∅251mm×25mm×10000mm, and the chemical composition (mass percentage, %) is: C 0.015%, Cr 25.7%, Ni 6.9%, Mo 3.8%, N 0.27%, Fe balance.

The cold deformation die working curve design method based on full-size strain hardening mainly consists of the following steps:

Step 1: Use mechanical processing to prepare a nearly cylindrical specimen. The maximum diameter of the nearly cylindrical specimen is 80mm and the height is 180mm. The deviation of the specimen height and outer diameter is ±0.15mm, and the parallelism of the upper and lower planes is 0.25mm. The upper and lower surfaces of the nearly cylindrical specimen are machined into R10 arc guide surfaces.

Step 2: Place the nearly cylindrical specimen steadily on the lower die seat of the pressure testing machine, with its bottom closely fitting the lower die seat. The upper die seat is inverted on the top of the nearly cylindrical specimen (as shown in Figure 1).

The upper die base system of the pressure testing machine consists of two layers, of which the inner lining layer is the pressure-bearing layer and the outer lining layer is the supporting layer. During the strain hardening process, the inner lining layer is the main pressure-bearing component, and the outer lining layer mainly plays the role of supporting the inner lining layer (as shown in Figure 1).

The lower die base system of the pressure testing machine is mainly composed of a three-layer structure, in which the inner lining layer is the pressure-bearing layer, which mainly bears the mechanical load during the deformation process. The middle lining layer is a protective layer, which mainly prevents abnormal fragmentation of the pressure-bearing layer during loading. The outer lining layer is a supporting layer, which plays a role in supporting the middle lining layer (as shown in Figure 1).

A small hole is set on the side of the lining and outer lining of the lower die base system of the pressure testing machine (as shown in Figure 1). The cooling liquid flows out of the small hole and quickly cools the sample, so that the effect of precise control of strain hardening can be achieved. The cooling liquid flow rate is 100 ml/s.

The inner lining of the upper and lower die base systems of the pressure testing machine is equipped with a guide groove, which is convenient for close cooperation with the installation of the nearly cylindrical specimen. It plays a role of metal guide when the specimen is loaded for testing. At the same time, the angle of the groove is 10°, and a layer of solid grease is coated in the groove to facilitate demoulding after strain strengthening.

Step 3: Use a pressure testing machine to steadily apply a load to the upper die seat, and control the strain rate to 0.15s ^-1 , and the strain amounts to 10-60%, thereby obtaining strain-hardened specimens with different parameters.

Step 4: Cut the nearly truncated cone-shaped specimen along the longitudinal section after strain hardening, process the transverse tensile specimen (a standard rod-shaped specimen with a parallel section diameter of 10 mm) and conduct a tensile test to obtain the corresponding yield strength of the material after different strain hardening.

Step 5: By fitting the strength values of different strain hardening parameters and taking derivatives, the full-size hardening rate characteristic curve of material 1 is quickly obtained, as shown in Figure 2. Its mathematical expression is y=-25.6+3108.6e ^(-3.39x) , where x is the strain and y is the hardening rate. Therefore, the development curve of the cold deformation die working section can be designed into a curvature shape.

Through interpolation calculation, it can be obtained that in order to obtain a high-strength super duplex stainless steel pipe with a yield strength between 862 and 1000 MPa, when the strain rate is about 0.15 s-1, the strain value should be designed to be 15 ± 3%.

Embodiment 2

The material 2 of the second embodiment is an austenitic iron-nickel based alloy, the seamless steel pipe specification is ∅219mm×42mm×3000mm, and the chemical composition (mass percentage, %) is: C 0.018%, Cr 26.8%, Ni 30.5%, Mo 3.3%, Cu 0.95%, N0.09%, Fe balance.

It mainly consists of the following steps:

Step 1: Use mechanical processing to prepare a nearly cylindrical specimen. The maximum diameter of the nearly cylindrical specimen is 100mm and the height is 220mm. The deviation of the specimen height and outer diameter is ±0.25mm, and the parallelism of the upper and lower planes is 0.30mm. The upper and lower surfaces of the nearly cylindrical specimen are machined into R10 arc guide surfaces.

Step 2: Place the nearly cylindrical specimen steadily on the lower die seat of the pressure testing machine, with its bottom closely fitting the lower die seat. The upper die seat is inverted on the top of the nearly cylindrical specimen (as shown in Figure 1).

The upper die base system of the pressure testing machine consists of two layers, of which the inner lining layer is the pressure-bearing layer and the outer lining layer is the supporting layer. During the strain hardening process, the inner lining layer is the main pressure-bearing component, and the outer lining layer mainly plays the role of supporting the inner lining layer (as shown in Figure 1).

The lower die base system of the pressure testing machine is mainly composed of a three-layer structure, in which the inner lining layer is the pressure-bearing layer, which mainly bears the mechanical load during the deformation process. The middle lining layer is a protective layer, which mainly prevents abnormal fragmentation of the pressure-bearing layer during loading. The outer lining layer is a supporting layer, which plays a role in supporting the middle lining layer (as shown in Figure 1).

A small hole is set on the side of the lining and outer lining of the lower die base system of the pressure testing machine (as shown in Figure 1). The cooling liquid flows out of the small hole and quickly cools the sample, so that the effect of precise control of strain hardening can be achieved. The cooling liquid flow rate is 150 ml/s.

The inner lining of the upper and lower die base systems of the pressure testing machine is equipped with a guide groove, which is convenient for close cooperation with the installation of the nearly cylindrical specimen. It plays a role of metal guide when the specimen is loaded for testing. At the same time, the angle of the groove is 10°, and a layer of solid grease is coated in the groove to facilitate demoulding after strain strengthening.

Step 3: Use a pressure testing machine to steadily apply a load to the upper die seat, and control the strain rate to 0.15s-1, and the strain amounts to 10-75%, thereby obtaining strain-hardened specimens with different parameters.

Step 4: Cut the nearly truncated cone-shaped specimen along the longitudinal section after strain hardening, process the transverse tensile specimen (a standard rod-shaped specimen with a parallel section diameter of 10 mm) and conduct a tensile test to obtain the corresponding yield strength of the material after different strain hardening.

Step 5: By fitting the strength values of different strain hardening parameters, the full-size hardening rate characteristic curve of material 2 is quickly obtained, as shown in Figure 3. Its mathematical expression is y=1509.1-114.5x, where x is the strain and y is the hardening rate. Therefore, the development curve of the cold deformation die working section can be designed to be conical.

Through interpolation calculation, it can be obtained that in order to obtain an austenitic iron-nickel-based stainless steel pipe with a yield strength between 758-965MPa, when the strain rate is about 0.15s-1, the strain value should be designed to be 42±6%.

Claims (2)

1. A cold deformation die working curve design method based on full-size strain hardening includes the following steps: Step 1: Prepare a nearly cylindrical specimen by mechanical processing; Step 2: Place the sample stably on the lower die seat of the pressure testing machine so that the bottom of the sample and the lower die seat fit closely; then turn the upper die seat upside down on the top of the sample; Step 3: Use a pressure testing machine to steadily apply a load to the upper die seat, and control the strain rate and strain amount, so as to obtain strain hardening specimens with different parameters; Step 4: Cut the strain-hardened specimen along the longitudinal section, process the transverse full-size tensile specimen and conduct a tensile test to obtain the yield strength of the material after strain hardening; Step 5: Obtain the full-size strain hardening characteristic curve of the material by fitting the strength values of different strain hardening parameters; Step 6, taking the derivative of the strain hardening characteristic curve to obtain the strain hardening rate characteristic curve; The diameter of the specimen is not less than 70mm, and the ratio of height to diameter is between 1.2 and 3.0; the deviation of the specimen height and outer diameter is ±0.50mm, and the parallelism of the upper and lower planes is ≤0.50mm; the upper and lower planes of the specimen are both machined with R10 arc guide surfaces; When the pressure testing machine applies load, the displacement sensor and pressure sensor are used to control the moving speed and strain of the upper die seat. In step 4, the diameter of the processed standard transverse tensile specimen is at least 15 mm, and a tensile test is performed to obtain the yield strength of the specimen after strain hardening; In step 5, the yield strength of the samples obtained after different strain hardening parameters is fitted to obtain the mathematical expression of the full-size hardening characteristic curve of the stainless steel material; The lower die base of the pressure testing machine includes an inner lining layer, a middle lining layer and an outer lining layer, wherein the inner lining layer is a pressure-bearing layer, the middle lining layer is a protective layer, and the outer lining layer is a supporting layer; a small hole is arranged on the side of the middle lining layer and the outer lining layer, and during the test, the cooling liquid passes through the small hole to quickly cool the sample; The upper die base system of the pressure testing machine includes an inner lining layer and an outer lining layer, wherein the inner lining layer is a pressure-bearing layer and the outer lining layer is a supporting layer. 2. According to the method for designing a working curve of a cold deformation die based on full-size strain hardening according to claim 1, it is characterized in that: the inner lining layer of the upper die seat and the lower die seat is provided with a guide groove with a depth not exceeding 10 mm; during the specimen loading test, grease is pre-coated in the guide groove.