Manufacturing method of high-fatigue-resistance long-service-life core rod and core rod

Abstract

The invention discloses a manufacturing method of a core rod with high fatigue resistance and long service life. The manufacturing method comprises the following steps: obtaining a quenched and tempered core rod; and surface pressure stress treatment is conducted on the surface of the core rod through a surface plasticity micro-deformation method, so that annular pressure stress ranging from-100 MPa to-800 MPa and axial pressure stress ranging from-100 MPa to-800 MPa are formed from the surface of the core rod to the depth of 1 mm. The invention further discloses the core rod which is manufactured by adopting the manufacturing method disclosed by the invention. The high-fatigue-resistance long-service-life core rod manufactured through the manufacturing method can be applied to high-temperature rolling of seamless steel pipes, the performance can be improved, the service life can be prolonged, and the use cost can be reduced.

Description

Technical Field

[0001] The present invention relates to a steel material and a preparation method thereof, and particularly to a mandrel steel and a preparation method thereof. Background Art

[0002] The mandrel is mainly used to further roll the pierced tube blank into a rough tube with a certain outer diameter and wall thickness during the continuous rolling process, and can act as an internal mold. Due to the action of deformation resistance, friction, and high temperature during the rolling process, as well as the subsequent water tank cooling, the mandrel will be affected by cyclic loads and temperature changes, that is, it will be affected by thermal mechanical fatigue during use.

[0003] Under such working conditions, damage will gradually appear on the surface of the mandrel, that is, in the form of annular microcracks. With further use, the annular microcracks will develop into annular macro-cracks relatively quickly, resulting in the failure of this tool, the mandrel.

[0004] Therefore, the mandrel with annular cracks needs to be taken offline for scrapping, otherwise it will lead to unqualified internal surface quality of the steel pipe. When annular cracks appear prematurely, that is, when the mandrel life is relatively low, it will greatly increase the requirement for the processing turnover speed of the mandrel, increase the number of spare parts, and at the same time increase the cost of rolling steel per ton. It not only affects the internal surface quality of the product but also weakens the competitiveness of the product.

[0005] Currently, there are already references studying the mandrel:

[0006] For example, the Chinese patent document with the publication number CN1415439, the publication date of May 7, 2003, and the name "Method for Reducing the Rolling Friction Coefficient between the Mandrel and the Hollow Billet" discloses a method for reducing the rolling friction coefficient between the mandrel and the hollow billet. After spraying a lubricant on the mandrel, a drying process is set. This drying process can be to set a drying table for natural drying, or to set a dryer for drying, or to use radiation drying, etc. After adopting the process technology of the present invention, the rolling friction coefficient between the mandrel and the hollow billet is effectively reduced, and the longitudinal wall thickness unevenness phenomenon of the continuous rolling tube of the floating mandrel continuous rolling mill is fundamentally eliminated. By attaching a lubricant to the surface of the mandrel, this patent can achieve the effect of reducing the friction coefficient, but it cannot fundamentally solve the problem of low thermal mechanical fatigue life.

[0007] For another example, a Chinese patent document with the publication number CN101994114A, publication date of March 30, 2011, and title "Process for Laser Cladding Wear-Resistant and Thermal Fatigue-Resistant Alloy Coating on Limiting Mandrel of Hot-Rolled Seamless Steel Tube Mill" discloses a method for laser cladding a wear-resistant and thermal fatigue-resistant alloy coating on the limiting mandrel of a hot-rolled seamless steel tube mill. The characteristics are as follows: First, the oil stain and rust layer on the limiting mandrel are cleaned on the surface; then, cobalt-based alloy powder is selected, and with a laser device and an automatic powder feeding device, laser cladding processing is carried out on the surface of the limiting mandrel to form a coating with a thickness of 0.2 - 2 mm that resists high-temperature oxidation, thermal fatigue, and thermal wear; finally, the laser-cladded limiting mandrel is subjected to high-temperature tempering treatment in a heat treatment furnace to reduce the residual stress after the transformation of the coating and the matrix structure after laser cladding. This patent uses the method of laser cladding high-temperature alloys to improve the surface material properties of the mandrel, with a complex process and high cost, and is not suitable for batch applications.

[0008] However, the prior art does not provide a solution for a mandrel with high fatigue resistance and high life. Summary of the Invention

[0009] One object of the present invention is to provide a manufacturing method for a mandrel with high fatigue resistance and high life. This method forms a certain magnitude of compressive stress on the surface of the mandrel steel through the method of surface plastic micro-deformation and reaches a certain depth of the residual compressive stress penetration layer, which plays a role in reducing the load on the working layer of the mandrel surface, aiming to achieve high fatigue resistance performance and high life of the mandrel.

[0010] To achieve the above object, the present invention provides a manufacturing method for a mandrel with high fatigue resistance and high life, including the steps of:

[0011] 4]Obtain a mandrel that has been quenched and tempered.

[0012] Use the surface plastic micro-deformation method to perform surface compressive stress treatment on the mandrel surface, so as to form a circumferential compressive stress of -100 to -800 MPa and an axial compressive stress of -100 to -800 MPa within the depth range from the mandrel surface to 1 mm.

[0013] In the present invention, after the mandrel steel is completed with quenching and tempering heat treatment, a circumferential compressive stress of -100 to -800 MPa and an axial compressive stress of -100 to -800 MPa are formed at a depth of 1 mm on the surface of the mandrel steel through ultrasonic impact or ultrasonic rolling or other surface plastic micro-deformation methods, which plays a role in reducing the actual operating load on the working layer of the mandrel surface, reducing the magnitude of the tensile load and even transforming it into a compressive load, thereby improving the tensile-tensile fatigue performance or tensile-compressive fatigue performance of the mandrel.

[0014] It should be noted that in the present invention, the negative sign before the numerical values of the circumferential compressive stress and the axial compressive stress indicates that they are "compressive" stresses.

[0015] In the present invention, when the value of the residual compressive stress on the surface layer of 1 mm (including axial and circumferential) is less than 100 MPa, it cannot effectively counteract the action of the external tensile load, and the improvement of the anti-fatigue life is not obvious; when the value of the residual compressive stress is greater than 800 MPa, it will greatly increase the dislocation density in the surface layer metal, resulting in easy occurrence of recovery softening at high temperatures, thus accelerating the occurrence of tissue evolution and performance degradation, and will also cause embrittlement of the mandrel, which is not conducive to the anti-fatigue performance. Based on this, in the present invention, it is necessary to control the residual compressive stress on the surface layer of 1 mm to be between -100 MPa and -800 MPa.

[0016] Furthermore, in the manufacturing method of the high anti-fatigue and high-life mandrel of the present invention, after the surface compressive stress treatment of the mandrel, the penetration depth of the surface residual compressive stress is ≥ 2 mm.

[0017] In the present invention, when the penetration depth of the surface residual compressive stress is ≥ 2 mm, that is, to ensure that the stress state at the 2 mm position is a compressive stress state, a crack initiation and propagation resistance layer area will be formed, thus prolonging the crack propagation life. When the penetration depth of the residual compressive stress is less than 2 mm, it is not sufficient to provide enough crack propagation resistance, which is not conducive to the anti-fatigue life. Therefore, in the present invention, it is necessary to control the penetration depth of the surface residual compressive stress to be ≥ 2 mm.

[0018] Furthermore, in the manufacturing method of the high anti-fatigue and high-life mandrel of the present invention, for a mandrel with an initial hardness of 42 - 48 HRC, after the surface compressive stress treatment, its hardness is 49 - 60 HRC.

[0019] In the present invention, the hardness of the mandrel is highly correlated with its wear resistance. Increasing the hardness of the mandrel can improve its wear resistance. During the use of the mandrel, fatigue and wear will couple and promote each other. Therefore, improving the wear resistance of the mandrel can also improve its anti-fatigue performance. In addition, the increase in hardness can also improve the fatigue strength limit of the mandrel against crack initiation. When the hardness of the mandrel is lower than 49 HRC, this improvement effect is not obvious. When the hardness of the mandrel is higher than 60 HRC, it will greatly increase the dislocation density in the surface layer metal, resulting in easy occurrence of recovery softening at high temperatures, thus accelerating the occurrence of tissue evolution and performance degradation, and will also cause embrittlement of the mandrel, which is not conducive to the anti-fatigue performance. Therefore, in the present invention, in order to improve the wear resistance of the mandrel, avoid the coupling effect of wear and fatigue, and thus further improve the fatigue life, it is necessary to control the surface hardness level of the mandrel after surface treatment to be between 49 - 60 HRC.

[0020] Furthermore, in the manufacturing method of the high anti-fatigue and high-life mandrel of the present invention, after the surface compressive stress treatment of the mandrel, its surface finish is ≤ Ra0.8.

[0021] In the present invention, the surface finish of the mandrel steel is improved by ultrasonic peening, ultrasonic rolling or other surface plastic micro-deformation methods, and the notch sensitivity of the mandrel surface is reduced, that is, the problem of easy crack initiation caused by stress concentration due to machining tool marks is reduced, thereby prolonging the crack initiation life. If the surface finish > Ra0.8, as the finish deteriorates, the notch sensitivity increases, resulting in a reduction in fatigue performance. Therefore, in the present invention, it is necessary to control the surface finish level at ≤ Ra 0.8.

[0022] Furthermore, in the manufacturing method of the high-fatigue-resistant and high-life mandrel described in the present invention, the surface plastic micro-deformation includes ultrasonic peening or ultrasonic rolling.

[0023] Furthermore, in the manufacturing method of the high-fatigue-resistant and high-life mandrel described in the present invention, the rotational speed of the tool for surface compressive stress treatment is 100 - 300 r / min.

[0024] In the present invention, the mandrel does not rotate, and the surface compressive stress treatment of the mandrel is carried out by rotating the tool, and the rotational speed of the tool is controlled at 100 - 300 r / min. If the rotational speed of the tool is too low, the processing efficiency is low. If the rotational speed is too high, the requirement for the equipment power is high, and it is not beneficial to the surface finish, and it will reduce the depth of the residual compressive stress penetration layer, unable to meet the requirement of a depth ≥ 2 mm. Therefore, in the present invention, it is necessary to control the rotational speed of the tool at 100 - 300 r / min.

[0025] Furthermore, in the manufacturing method of the high-fatigue-resistant and high-life mandrel described in the present invention, the power of the tool for surface compressive stress treatment is 400 - 1200 W.

[0026] In the present invention, when the power of the tool is too low, insufficient force can be generated to form the required residual compressive stress on the mandrel surface. When the power of the tool is too high, it exceeds the requirement to reach the required compressive stress level or hardness level, increasing the cost. Therefore, in the present invention, it is necessary to control the power of the tool for surface compressive stress treatment at 400 - 1200 W.

[0027] Furthermore, in the manufacturing method of the high-fatigue-resistant and high-life mandrel described in the present invention, the speed at which the tool for surface compressive stress treatment travels along the axial direction of the mandrel is 0.1 - 0.8 mm / r.

[0028] In the present invention, it is also necessary to control the speed at which the tool for surface compressive stress treatment travels along the axial direction of the mandrel. When the speed at which the tool travels along the axial direction of the mandrel is too low, the processing efficiency is low. When the speed at which the tool travels along the axial direction of the mandrel is too high, it is not conducive to the surface finish, and the depth of the residual compressive stress penetration layer will be reduced, unable to meet the requirement of a depth ≥ 2 mm. Therefore, in the present invention, it is necessary to control the speed at which the tool for surface compressive stress treatment travels along the axial direction of the mandrel to be between 0.1 - 0.8 mm / r.

[0029] Another object of the present invention is to provide a mandrel with high fatigue resistance and high lifespan. The mandrel manufactured by this method can be applied to the hot rolling of seamless steel pipes, which can improve performance, extend lifespan, and reduce usage costs.

[0030] To achieve the above object, the present invention also provides a mandrel with high fatigue resistance and high lifespan, which is obtained by the above manufacturing method.

[0031] The mandrel with high fatigue resistance and high lifespan and its manufacturing method according to the present invention have the following advantages and beneficial effects:

[0032] The mandrel with high fatigue resistance and high lifespan according to the present invention, manufactured by this method, can be applied to the hot rolling of seamless steel pipes, which can improve performance, extend lifespan, and reduce usage costs.

[0033] The manufacturing method of the mandrel with high fatigue resistance and high lifespan according to the present invention forms a certain amount of compressive stress on the surface of the mandrel steel through ultrasonic impact or ultrasonic rolling or other surface plastic micro-deformation methods and reaches a certain depth of the residual compressive stress penetration layer, which plays a role in reducing the load on the working layer of the mandrel surface. At the same time, it can improve the surface finish level of the mandrel, as well as enhance the hardness of the surface layer material to improve wear resistance, and ultimately achieve high fatigue resistance and high lifespan of the mandrel. Detailed Embodiments

[0034] The following will further explain and illustrate the manufacturing method of the mandrel with high fatigue resistance and high lifespan according to the present invention with specific embodiments. However, this explanation and illustration do not unduly limit the technical solution of the present invention.

[0035] Examples 1 - 9 and Comparative Examples 1 - 4

[0036] The mandrels with high fatigue resistance and high lifespan in Examples 1 - 9 are all obtained by the following steps:

[0037] (1) Obtain a mandrel with a circular cross-section by continuous rolling. In some more specific embodiments, a mandrel made of ASTM A681 H11 material is obtained by rolling;

[0038] (2) Perform quenching and tempering treatment on the rolled mandrel, and the initial hardness after quenching and tempering treatment is between 42 - 48 HRC.

[0039] (3) Adopt the surface plastic micro - deformation method to conduct surface compressive stress treatment on the mandrel surface, where the surface plastic micro - deformation can adopt ultrasonic impact or ultrasonic rolling.

[0040] Among them, the power of the tool for surface compressive stress treatment is 400 - 1200 W, the rotational speed of the tool is 100 - 300 r / min, and the speed at which the tool walks along the axial direction of the mandrel is 0.1 - 0.8 mm / r.

[0041] It should be noted that the comparison mandrel steels of Comparative Examples 1 - 3 are also prepared by the above - mentioned step process, but their specific process parameters do not meet the design requirements of the present invention.

[0042] Table 1 lists the specific process parameters of the high - fatigue - resistance and high - life mandrels of Examples 1 - 9 and the comparison mandrels of Comparative Examples 1 - 4 in the above steps.

[0043] Table 1.

[0044]

[0045]

[0046] In order to verify the effect of the present invention, samples are taken from the high - fatigue - resistance and high - life mandrels of Examples 1 - 9 and the comparison mandrels of Comparative Examples 1 - 4 respectively, and the various performance aspects of the hot - rolled strip steel samples of each example prepared by the following method are tested, and the measurement results are listed in Table 2 below. Among them:

[0047] Hardness test: Conduct hardness test and hardness conversion using a surface Richter hardness tester or measure using a Rockwell hardness tester.

[0048] Residual compressive stress test: Detect the circumferential residual compressive stress and longitudinal residual compressive stress using an X - ray analyzer.

[0049] Depth of residual compressive stress penetration layer: Detect the circumferential residual compressive stress and longitudinal residual compressive stress using an X - ray analyzer, and the penetration layer depth is equal to the distance from the surface to where the residual compressive stress is 0.

[0050] Surface finish test: Measure using a surface finish tester (or surface roughness tester).

[0051] Table 2 lists the number of rolled steel pipes (i.e., service life or fatigue - resistance life) of the high - fatigue - resistance and high - life mandrels of Examples 1 - 9 and the comparison mandrels of Comparative Examples 1 - 4

[0052] Table 2.

[0053]

[0054]

[0055] Note: The “-” in Table 2 above indicates that the value is “compressive” stress rather than tensile stress. Therefore, “-101” of Example 1 falls within the range of -100 to -800 MPa of claim 1 of the present invention, while “-56” of Comparative Example 1 is outside the range of -100 to -800 MPa of claim 1 of the present invention.

[0056] As can be seen from Table 2, for core rods with an initial hardness of 42-48 HRC, after surface compressive stress treatment, their hardness is between 49-60 HRC, the axial compressive stress formed in the range from the core rod surface to a depth of 1 mm is between -100 and -800 MPa, the hoop compressive stress is between -100 and -800 MPa, the residual compressive stress penetration depth of the core rod surface is greater than 2 mm, and the surface finish is less than or equal to Ra0.8.

[0057] At the same time, the fatigue life of the mandrels was determined by rolling more than 5,000 steel pipes at a continuous rolling temperature of 900-1100° C. using the high fatigue resistance and high life mandrels of Examples 1-9 of the present invention, i.e., no ring cracks with a depth greater than 0.5 mm appeared.

[0058] In addition, the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0059] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A manufacturing method of a mandrel with high fatigue resistance and high service life, characterized in that, Including the steps: Obtain a quenched and tempered mandrel; Adopt the surface plastic micro-deformation method to perform surface compressive stress treatment on the surface of the mandrel, so as to form a circumferential compressive stress of -100 to -800 MPa and an axial compressive stress of -100 to -800 MPa in the range from the surface of the mandrel to a depth of 1 mm.

2. The manufacturing method according to claim 1, characterized in that, After the surface compressive stress treatment of the mandrel, the penetration depth of the surface residual compressive stress is ≥ 2 mm.

3. The manufacturing method according to claim 1, characterized in that, For a mandrel with an initial hardness of 42 - 48 HRC, after the surface compressive stress treatment, its hardness is 49 - 60 HRC.

4. The manufacturing method according to claim 1, characterized in that, After the surface compressive stress treatment of the mandrel, its surface finish is ≤ Ra0.

8.

5. The manufacturing method according to claim 1, characterized in that, The surface plastic micro-deformation includes ultrasonic impact or ultrasonic rolling.

6. The manufacturing method according to claim 1, characterized in that, The rotational speed of the tool for surface compressive stress treatment is 100 - 300 r / min.

7. The manufacturing method according to claim 1, characterized in that, The power of the tool for surface compressive stress treatment is 400 - 1200 W.

8. The manufacturing method according to claim 1, characterized in that, The speed at which the tool for surface compressive stress treatment travels along the axial direction of the mandrel is 0.1 - 0.8 mm / r.

9. A mandrel, which is manufactured by the manufacturing method described in any one of claims 1 - 8.

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