A high-toughness seamless pipe mandrel steel, a produced seamless pipe mandrel and a heat treatment process

Abstract

The application provides a high-toughness seamless pipe mandrel steel, a produced seamless pipe mandrel and a heat treatment process, and the components include C, Si, Mn, Cr, Mo, V, Ni, Al, Ti, P, S, N, O, the rest is Fe and other inevitable impurities, the components satisfy 60xC+10xMn+12xSi+20xCr+25xMo+32xV >=225, 50x(N-2.1xAl-3.4xTi) / 14 >=0.015, (63xNi+25xV+22xMo)+(3.6xV+6.5xMo)x120xN >=155. After the heat treatment of the steel provided by the application, the steel has good toughness and high-temperature fatigue performance, the thermal fatigue life is >=4200 times, and the service life of the pipe mandrel is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of alloy steel, specifically relating to a high-strength and high-toughness seamless mandrel steel for medium and high temperature range, the production of seamless mandrels, and the heat treatment process. Background Technology

[0002] The mandrel is a crucial tool in the forming of seamless steel pipes. During the rolling process, the mandrel and rolls work together to roll the rough tube into a blank that meets specific dimensional requirements. When the mandrel comes into contact with the steel pipe, its surface temperature rises sharply, reaching 700-800℃. After rolling, to ensure lubrication, the mandrel needs to be forcibly cooled and its temperature controlled at around 100℃. Therefore, the mandrel must not only withstand high rolling forces but also frequent thermal shocks.

[0003] Currently, most mandrels on the market fail due to severe thermal fatigue after 2000-3000 cycles of thermal fatigue. Mandrels have a significant impact on the rolling efficiency of seamless steel pipes, the length and specifications of the tube, product quality, and cost. Therefore, improving the strength and lifespan of mandrel materials is an urgent problem to be solved in the production of seamless steel pipes.

[0004] Patent CN102162071A, published on August 24, 2011, discloses a mandrel steel material for pipe rolling and its manufacturing method. Its chemical composition (mass percentage) is: C 0.40-0.48%, Mn 0.30-0.60%, Si 0.40-0.80%, P≤0.010%, S≤0.008%, Cr 2.30-3.00%, Mo 2.0-2.5%, V 1.0-1.5%, Ni 0.50-0.80%, Nb 0.04-0.10%, Al 0.015-0.035%, with the balance being Fe and unavoidable impurities. It is manufactured through vacuum refining in an electric arc furnace and ladle furnace, electroslag remelting, forging, annealing, and tempering treatment. This process improves the high-temperature yield strength and resistance to tempering softening of the mandrel steel material, thereby increasing mandrel life. However, its performance does not meet current production requirements. Summary of the Invention

[0005] The purpose of this invention is to provide a high-strength, high-toughness seamless mandrel steel for medium- and high-temperature ranges. By improving the overall performance of the steel through composition design, it can be used to produce high-strength, high-toughness, and high-thermal-fatigue-life seamless mandrels.

[0006] Another objective of this invention is to provide a seamless mandrel and a heat treatment process. By using the aforementioned high-strength and high-toughness seamless mandrel steel in the medium- and high-temperature range to produce seamless mandrels, and designing a matching heat treatment method, the product yield strength is ≥1350MPa, tensile strength is ≥1580MPa, 0℃ impact energy KV2 is ≥25J, and thermal fatigue life is ≥4200 cycles, thereby meeting the requirements for long-life rolled mandrels, reducing costs and increasing efficiency.

[0007] The specific technical solution of this invention is as follows:

[0008] A high-strength, high-toughness, seamless mandrel steel for medium- and high-temperature applications comprises the following components by weight:

[0009] C 0.36%-0.42%, Mn 0.33%-0.60%, Cr 4.60%-5.20%, Mo 1.65%-2.10%, Ni 0.50%-0.80%, V 1.20%-1.40%, Si 0.90%-1.20%, Al ≤0.0080%, Ti ≤0.0050%, P ≤0.013%, S≤0.005%, N 0.015%-0.030%, O≤0.0015%, with the remainder being Fe and other unavoidable impurities.

[0010] The composition of the steel used for the high-strength, high-toughness, medium- and high-temperature range seamless mandrel also meets the following requirements:

[0011] M≥225, M=60×%C+10×%Mn+12×%Si+20×%Cr+25×%Mo+32×%V.

[0012] The composition of the steel used for the high-strength, high-toughness, medium- and high-temperature range seamless mandrel also meets the following requirements:

[0013] Q≥0.015,Q=50×(N%-2.1×Al%-3.4×Ti%) / 14.

[0014] The composition of the steel used for the high-strength, high-toughness, medium- and high-temperature range seamless mandrel also meets the following requirements:

[0015] X≥155, X=(63×%Ni+25×%V+22×%Mo)+(3.6×%V+6.5%×Mo)×120×%N.

[0016] The symbols for each element above represent the mass percentage of each element, expressed as a percentage (%). When calculating, simply substitute the value before the percentage into the formula.

[0017] This invention provides a seamless mandrel, produced using high-strength, high-toughness, medium- and high-temperature seamless mandrel steel. The seamless mandrel has a yield strength ≥1350 MPa, tensile strength ≥1580 MPa, elongation A ≥15%, reduction of area Z ≥54%, impact energy at 0℃ KV2 ≥25 J, and thermal fatigue life ≥4200 cycles. It is suitable for medium- and high-temperature conditions ≤700℃, and at 700℃, its yield strength ≥490 MPa and tensile strength ≥600 MPa.

[0018] The present invention provides a heat treatment process for a seamless mandrel, including quenching and tempering.

[0019] The quenching holding time t1 is related to the mandrel diameter D and the quenching heating temperature T1; the tempering holding time t2 is related to the mandrel diameter D and the tempering heating temperature T2.

[0020] The quenching process involves heating the mandrel to T1 980-1030℃, holding it at that temperature, and then air cooling. The holding time t1 is determined by the mandrel diameter D and the heating temperature T1, where 250+D×2-T1 / 4≤t1≤270+D×2-T1 / 4. The unit of t1 is min, the unit of D is mm, and the unit of T1 is ℃. During heating, the mandrel is heated to 980-1030℃ at a rate of 10-30℃ / min.

[0021] The tempering process involves heating the mandrel to T2 (570-620℃), holding it at that temperature for t2, and then air cooling. The holding time t2 is determined by the mandrel diameter D and the heating temperature T2. The heat treatment process parameters should satisfy: 300 + D × 3 - T2 / 2 ≤ t2 ≤ 320 + D × 3 - T2 / 2. The unit of t2 is min, the unit of D is mm, and the unit of T2 is ℃. During heating, the mandrel is heated to 570-620℃ at a rate of 10-30℃ / min.

[0022] The depth of the compressive stress layer on the surface of the seamless mandrel after tempering is ≥2 mm, and the hardness of the compressive stress layer is ≥50HRC.

[0023] When calculating the above formulas, simply substitute the numerical value before the unit into the formula.

[0024] Preferably, the present invention provides a method for producing seamless mandrels, which is obtained by using the above-mentioned high-strength and high-toughness seamless mandrel steel in the medium-high temperature range, including the above-mentioned heat treatment process. The specific production process includes: electric arc furnace or converter smelting → LF furnace refining → RH or VD vacuum degassing → continuous casting billet / ingot → heating → rolling → machining → heat treatment → finishing → packaging and warehousing.

[0025] The design concept of this invention is as follows:

[0026] C: Carbon is the most effective element for increasing the strength of steel, and it also affects the segregation of steel components and the uniformity of its microstructure. Furthermore, it strengthens the matrix by precipitating various carbides during tempering, in conjunction with elements such as chromium, molybdenum, and vanadium. However, excessive carbon can lead to segregation during solidification, thus affecting impact toughness. Therefore, this invention controls the C content to be 0.36%-0.42%.

[0027] Mn: An appropriate amount of Mn can effectively improve the strength of steel, eliminate the hot brittleness of steel caused by sulfur and oxygen, improve the hot working performance of steel, and improve the cold brittleness tendency of steel. However, excessive Mn content makes steel brittle. Therefore, in order to ensure that the microstructure does not coarsen and segregation is light, the present invention controls Mn to be 0.33%-0.60%.

[0028] Si: Si is an effective solid solution strengthening element in steel, which improves the strength and hardness of steel. However, Si tends to segregate at austenite grain boundaries, which reduces the grain boundary bonding force and causes brittleness. Therefore, the composition of this invention is designed to be low in Si, with a control range of 0.90%-1.20%.

[0029] Cr: Cr is a carbide-forming element that can improve the hardenability and strength of steel. However, because its carbides are prone to coarsening, they have a certain detrimental effect on toughness. Therefore, the content of chromium is appropriately controlled to 4.60%-5.20% in this invention.

[0030] Mo: Mo dissolved in the matrix enables the steel microstructure to maintain high stability during tempering and effectively reduces the segregation of impurity elements such as P, S, and As at grain boundaries, thereby improving the toughness of the steel and reducing temper brittleness. Mo reduces the stability of M7C3; when the Mo content is high, acicular Mo2C will form, leading to a reduction in the Mo content in the matrix. Mo can improve the strength of steel through the combined effects of solid solution strengthening and precipitation strengthening, and can also change the toughness of steel by altering the precipitation of carbides. Therefore, the Mo content in this invention is controlled at 1.65%-2.10%.

[0031] Vanadium (V) primarily refines grain size and reduces overheating sensitivity, while also increasing tempering stability and toughness, thereby improving service life. This invention increases V to 1.20-1.40% and adds 0.015-0.030% N to form a high-hardness VN precipitate (hardness ≥2400 HV), whose thermal stability is significantly superior to traditional VC or V4C3 phases. VN can effectively pin dislocations at high temperatures, enhancing high-temperature creep resistance.

[0032] Ni: Ni can refine ferrite grains, improve the ductility and toughness of steel, and enhance the hardening properties of steel. In this invention, the range of Ni is controlled to be 0.50%-0.80%.

[0033] Nitrogen (N) can form compounds with V, Ti, and Al, refining grain size. Simultaneously, trace amounts of nitrogen can promote V(C,N) precipitation, increasing the quantity and stability of undissolved carbides in quenched steel. As a nucleation core, it reduces the size of eutectic carbides such as Cr23C6, thereby enhancing the overall performance of the steel through increased grain refinement and precipitation strengthening, and improving the material's thermal strength and toughness. This invention controls [N] to be 150-300 ppm.

[0034] This invention utilizes a deoxidation process with O ≤ 0.0015% and Al ≤ 0.0080% and Ti ≤ 0.0050%, resulting in a D-class inclusion level ≤ 0.5 (ISO 4967), which increases fatigue crack initiation resistance by 40-50%. Simultaneously, S ≤ 0.005% optimizes sulfide morphology and improves transverse impact toughness.

[0035] To ensure the high-temperature strength of the mandrel material during use, this invention incorporates beneficial alloying elements in specific proportions. In this compositional system, C is the most effective element for improving the hardness and strength of steel, with a contribution coefficient of 60. Mn, Si, Cr, Mo, and V can all effectively improve the hardness of steel, and their coefficients are defined as 10, 12, 20, 25, and 32 respectively, based on their contribution. The strength factor of the steel is defined as M, and M should be ≥ 225.

[0036] M=60×%C+10×%Mn+12×%Si+20×%Cr+25×%Mo+32×%V.

[0037] This invention, by adding nitrogen element, can promote the precipitation of V(C,N), increase the quantity and stability of undissolved quenched carbides in steel, and reduce the size of eutectic carbides such as Cr23C6 as nucleation cores, thereby further improving the strength and toughness of steel. To ensure sufficient precipitation effect, the precipitation coefficient Q≥0.015.

[0038] Q=50×(N%-2.1×Al%-3.4×Ti%) / 14.

[0039] In this invention, Ni effectively improves the impact toughness of the material, V effectively refines the grains and improves toughness, and Mo greatly improves tempering stability, thus ensuring toughness. Based on their respective contributions to the system's toughness, their influence coefficients are defined as 63, 25, and 22, respectively. Furthermore, the elements in the system also interact, affecting performance. V, Mo, and other elements combine with C to form extremely fine (V, Mo) N-type carbides, greatly improving the bulk toughness, with an influence coefficient of 120. The contributions of the remaining individual elements are also cumulative. To ensure the material's usability and toughness, the toughness factor X ≥ 155.

[0040] X=(63×%Ni+25×%V+22×%Mo)+(3.6×%V+6.5%×Mo)×120×%N.

[0041] The steel provided by this invention possesses excellent strength, toughness, and high-temperature fatigue performance, significantly improving the service life of rolled mandrels. This high-strength, high-toughness seamless mandrel steel for medium- and high-temperature applications is suitable for the preparation of rolled mandrels with limited movement. This innovative approach integrates high Cr, high Mo, and high V, and introduces VN synergy. In particular, through N element regulation, it solves the problem of insufficient V carbide precipitation and coarse eutectic carbides in high-Cr steel, ultimately achieving a yield strength ≥1350 MPa, tensile strength ≥1580 MPa, yield strength ≥490 MPa and tensile strength ≥600 MPa at 700℃, impact energy KV2 ≥25 J at 0℃, and a thermal fatigue life ≥4200 cycles, thus meeting the requirements for long-life rolled mandrels and improving production efficiency. Attached Figure Description

[0042] Figure 1 This is a microstructure diagram of the mandrel in Example 1. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] Examples 1-3

[0045] A high-strength and high-toughness seamless mandrel steel for medium and high temperature range comprises the following composition by weight: as shown in Table 1, the remainder not shown in Table 1 is Fe and other unavoidable impurities.

[0046] Comparative Examples 1-4

[0047] A seamless mandrel steel comprises the following components by weight: as shown in Table 1, with the remainder not shown in Table 1 being Fe and other unavoidable impurities.

[0048] Table 1. Composition (wt%) of steels in each embodiment and comparative example

[0049]

[0050] The production of high-strength, high-toughness, medium- and high-temperature seamless mandrel steel in each embodiment and comparative example includes the following steps:

[0051] Electric arc furnace smelting: oxygen is determined before tapping, and slag discharge is strictly controlled during the tapping process;

[0052] LF furnace: C, Si, Mn, Cr, Mo, Ni, V and other elements are adjusted to the target values;

[0053] Vacuum degassing: Pure degassing time ≥ 20 minutes, ensuring that the [H] content after vacuum treatment is ≤ 1.5 ppm;

[0054] Casting: The target temperature of molten steel in the ladle is controlled at 20-40℃ above the liquidus temperature, for round billets / square billets.

[0055] The production of seamless mandrels using steel produced in the above embodiments and comparative examples specifically includes: heating → rolling → machining → heat treatment → finishing → packaging and warehousing. The processing steps include quenching and tempering.

[0056] The quenching process involves heating the mandrel to T1 (980-1030℃), holding it at that temperature for t1, and then air-cooling it. The holding time t1 is determined by the mandrel diameter D and the heating temperature T1, where 250 + D × 2 - T1 / 4 ≤ t1 ≤ 270 + D × 2 - T1 / 4. The unit for t1 is min, the unit for D is mm, and the unit for T1 is ℃. During heating, the mandrel is heated to 980-1030℃ at a rate of 10-30℃ / min.

[0057] The tempering process involves heating the mandrel to T2 (570-620℃), holding it at that temperature for t2, and then air cooling. The holding time t2 is determined by the mandrel diameter D and the heating temperature T2. The heat treatment process parameters should satisfy: 300 + D × 3 - T2 / 2 ≤ t2 ≤ 320 + D × 3 - T2 / 2; t2 is in min, D is in mm, and T2 is in ℃. During heating, the mandrel is heated to 570-620℃ at a rate of 10-30℃ / min. The heat treatment process parameters for each embodiment and comparative example are shown in Table 2.

[0058] Table 2. Heat treatment processes of various embodiments and comparative examples of the present invention.

[0059]

[0060] The performance testing methods are as follows:

[0061] Performance: After heat treatment of the mandrel, samples were taken from the finished product and mechanical property tests were conducted in accordance with GB / T 230.1 and GB / T 229. The mechanical properties are shown in Table 3.

[0062] Table 3. List of mechanical property test results for each embodiment and comparative example of the present invention.

[0063]

[0064] Figure 1 The figure shows the microstructure of the mandrel in Example 1. As can be seen from the figure, the microstructure of the mandrel in this example is fine and uniform with high isotropy, indicating that the mandrel steel designed by the composition and heat treatment of the present invention has good comprehensive performance.

[0065] In Examples 1-3, the chemical composition and heat treatment methods of the steel were appropriately controlled, resulting in good strength, toughness, and service life. Comparative Example 1 had a reasonable composition design, but the quenching temperature and time were too low, leading to insufficient strength and toughness, inadequate surface layer properties, and a short service life. Comparative Examples 2 and 3 had chemical compositions that did not meet the requirements of this invention, resulting in insufficient strength and toughness. Comparative Example 4 had a low toughness factor in its chemical composition, and the short holding time during the quenching and tempering processes of the mandrel resulted in unsatisfactory strength, impact resistance, and other properties. The thickness and hardness of the surface compressive stress layer did not meet the requirements, leading to a low mandrel service life.

[0066] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A seamless mandrel, characterized by: The seamless mandrel is made of high-toughness middle-high-temperature-range seamless mandrel steel; The high-toughness middle-high-temperature-range seamless mandrel steel comprises the following components in percentage by mass: C 0.36%-0.42%, Mn 0.33%-0.60%, Cr 4.60%-5.20%, Mo 1.65%-2.10%, Ni 0.50%-0.80%, V 1.20%-1.40%, Si 0.90%-1.20%, Al ≤0.0080%, Ti ≤0.0050%, P ≤0.013%, S ≤0.005%, N 0.015%-0.030%, O ≤0.0015%, and the rest Fe and other inevitable impurities; and satisfies: M ≥ 225, wherein M=60×%C+10×%Mn+12×%Si+20×%Cr+25×%Mo+32×%V; Q ≥ 0.015, wherein Q=50×(N%-2.1×Al%-3.4×Ti%) / 14; X ≥ 155, wherein X=(63×%Ni+25×%V+22×%Mo)+(3.6×%V+6.5%×Mo)×120×%N. The high-toughness middle-high-temperature-range seamless mandrel steel is suitable for production of seamless mandrels under ≤700℃ middle-high-temperature working conditions, and the yield strength of the seamless mandrel under 700℃ high-temperature conditions is ≥490MPa, and the tensile strength is ≥600MPa; and the impact energy KV2 at 0℃ is ≥25J. The room-temperature yield strength of the seamless mandrel is ≥1350 MPa, the tensile strength is ≥1580 MPa, the elongation A is ≥15%, the reduction of area Z is ≥54%, and the thermal fatigue life is ≥4200 times.

2. A heat treatment process for a seamless tube mandrel as claimed in claim 1, characterized in that, The heat treatment process comprises quenching and tempering, the quenching holding time t1 is related to the mandrel diameter D and the quenching heating temperature T1, and the tempering holding time t2 is related to the mandrel diameter D and the tempering heating temperature T2.

3. The heat treatment process of claim 2, wherein, In the quenching, the mandrel is heated to T1 980-1030℃, held for t1, and then air-cooled; the holding time t1 is determined by the mandrel diameter D and the heating temperature T1, and 250+D×2-T1 / 4≤t1≤270+D×2-T1 / 4; t1 is in units of min, D is in units of mm, and T1 is in units of ℃.

4. The heat treatment process of claim 3, wherein, In the quenching, the heating is at a speed of 10-30℃ / min to 980-1030℃.

5. The heat treatment process of claim 2, wherein, In the tempering, the mandrel is heated to T2 570-620℃, held for t2, and then air-cooled; the holding time t2 is determined by the mandrel diameter D and the heating temperature T2, and 300+D×3-T2 / 2≤t2≤320+D×3-T2 / 2; t2 is in units of min, D is in units of mm, and T2 is in units of ℃.

6. The heat treatment process of claim 4, wherein, In the tempering, the heating is at a speed of 10-30℃ / min to 570-620℃.

7. Heat treatment process according to claim 5 or 6, characterized in that: After the tempering, the surface compressive stress layer depth of the seamless mandrel is ≥2 mm, and the hardness of the compressive stress layer is ≥50HRC.

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