An ultra-long mandrel steel and its manufacturing method

Abstract

This invention discloses an ultra-long mandrel steel containing the following chemical elements in the following mass percentages: C: 0.28–0.32%, Si: 0.80–1.25%, Mn: 0.20–0.30%, Cr: 4.75–5.00%, Mo: 1.10–1.60%, V: 0.30–0.60%, S≤0.010%, P≤0.015%, Cu≤0.08%, Ni≤0.08%, Ti≤0.010%. Accordingly, this invention also discloses a method for manufacturing this ultra-long mandrel steel, comprising the steps of: heating: heating a continuously cast billet with a circular cross-section at 1200–1250℃ and holding for 2–4 hours; rolling: controlling the final rolling temperature at 900–1050℃; and cooling. This invention can effectively solve the problems of poor straightness and low yield of mandrel steel.

Description

Technical Field

[0001] This invention relates to a type of steel and its preparation method, and more particularly to a mandrel steel and its preparation method. Background Technology

[0002] Extra-long mandrels, such as those exceeding 30m in length, are prone to bending during cooling, which affects straightness and consequently impacts subsequent processing and use.

[0003] Poor straightness leads to two main consequences: first, it makes it impossible to process finished mandrels of the required diameter with a small amount of machining, resulting in the need to increase the size of the billet and leave more machining allowance; second, it makes it unsuitable for continuous rolling mandrel insertion, which can easily lead to problems such as mandrel sticking and uneven wall thickness on one side of the rolled steel pipe.

[0004] Based on this, it is desirable to provide an ultra-long mandrel steel and its manufacturing method, which solves the bending problem of the mandrel during the cooling process by controlling the composition of the mandrel material and the manufacturing process. Summary of the Invention

[0005] One of the objectives of this invention is to provide an ultra-long mandrel steel, which, through a reasonable chemical composition design, influences the phase transformation process of the mandrel, thereby reducing bending caused by the phase transformation.

[0006] To achieve the above objectives, this invention proposes an ultra-long mandrel steel containing Fe and unavoidable impurities, as well as the following chemical elements in the following mass percentages:

[0007] C: 0.28~0.32%, Si: 0.80~1.25%, Mn: 0.20~0.30%, Cr: 4.75~5.00%, Mo: 1.10~1 .60%, V: 0.30~0.60%, S≤0.010%, P≤0.015%, Cu≤0.08%, Ni≤0.08%, Ti≤0.010%.

[0008] Furthermore, the present invention also provides an ultra-long mandrel steel, wherein the mass percentage content of each chemical element is as follows:

[0009] C: 0.28–0.32%, Si: 0.80–1.25%, Mn: 0.20–0.30%, Cr: 4.75–5.00%, Mo: 1.10–1.60%, V: 0.30–0.60%, S≤0.010%, P≤0.015%, Cu≤0.08%, Ni≤0.08%, Ti≤0.010%; the balance is Fe and other unavoidable impurities.

[0010] As described above, the ultra-long mandrel steel of this invention, through chemical composition design, influences the phase transformation process of the mandrel, thereby reducing bending caused by phase transformation. Specifically:

[0011] C: In this invention, the carbon content is mainly used to improve strength through solid solution strengthening, precipitation strengthening, and phase transformation strengthening. If the carbon content is too low, the strength and thermal strength of the mandrel will be insufficient, failing to meet the requirements for subsequent tooling / mold use. If the carbon content is too low, the martensitic transformation temperature will be too high, easily leading to simultaneous bainite + martensite transformation during cooling. This results in uneven microstructure transformation at different temperatures, causing larger volume expansion in areas with more martensite transformation and smaller volume expansion in areas with less martensite transformation, thus causing the mandrel to bend. Furthermore, if the carbon content is too high, it can easily lead to delayed cracking during prolonged storage after the mandrel has cooled to room temperature. Therefore, this invention controls the carbon content to 0.28-0.32%.

[0012] Si: In this invention, if the Si content is too low, the mandrel will lack sufficient resistance to tempering softening and thermal strength, failing to meet the requirements for subsequent tooling / dies. If the Si content is too high, it will reduce the material's impact resistance and accident resistance. Therefore, this invention controls the Si content to be between 0.80% and 1.25%.

[0013] Mn: Mn primarily enhances strength through solid solution strengthening. In this invention, excessively low Mn content necessitates the addition of more other alloys, increasing costs. Furthermore, excessively high martensite transformation temperatures can lead to simultaneous bainite and martensite transformations during cooling, resulting in uneven microstructure transformation at different temperatures. This manifests as greater volume expansion in areas with higher martensite transformation and less in areas with lower transformation, causing the mandrel to bend. Conversely, excessively high Mn content can lead to segregation, reducing the material's impact resistance and accident resistance. Furthermore, excessive Mn segregation can affect the anisotropic uniformity of material properties, resulting in uneven plastic deformation and ultimately, mandrel bending during cooling. Therefore, this invention controls the Mn content to be 0.20-0.30%.

[0014] Cr: Cr can play a role in phase transformation strengthening and precipitation strengthening, thereby improving resistance to tempering softening and increasing hot strength. In this invention, if the Cr content is too low, the martensitic transformation temperature will be too high, and bainite + martensite transformation will easily occur simultaneously during cooling. This will lead to uneven transformation of the microstructure at different temperatures, resulting in large volume expansion in areas with more martensite transformation and small volume expansion in areas with less martensite transformation, thus causing the mandrel to bend. If the Cr content is too high, it will easily cause delayed cracking during the long-term storage of the mandrel after cooling to room temperature. Moreover, if the Cr content is too high, a dense oxide film will easily form locally during the cooling process. The uneven oxide film on the surface will lead to uneven cooling, resulting in uneven plastic deformation, and thus causing the mandrel to bend during cooling. Therefore, this invention controls the Cr content to be 4.75-5.00%.

[0015] Mo primarily functions as a solid solution strengthener, phase transformation strengthener, and precipitation strengthener, thereby improving resistance to tempering softening and enhancing hot strength. In this invention, if the Mo content is too low, the martensitic transformation temperature will be too high, easily leading to simultaneous bainite + martensite transformation during cooling. This results in uneven microstructure transformation at different temperatures, causing larger volume expansion in areas with more martensite transformation and smaller volume expansion in areas with less martensite transformation, thus causing the mandrel to bend. If the Mo content is too high, it can easily lead to delayed cracking during prolonged storage after the mandrel has cooled to room temperature. Therefore, this invention controls the Mo content to be 1.10-1.60%.

[0016] V (V): V can play a role in precipitation strengthening, thereby improving resistance to tempering softening and increasing hot strength. In this invention, if the V content is too low, the mandrel's resistance to tempering softening is insufficient, and its hot strength is inadequate, failing to meet the requirements for subsequent tooling / dies. If the V content is too high, it easily forms hard and brittle liquid carbides, reducing the mandrel's impact resistance and the material's resistance to accidents. Based on this, the V content is controlled at 0.30-0.60% in this invention.

[0017] S and P are the main impurity elements in this invention, which are detrimental to the impact performance of the material, thus their upper limits need to be controlled. S and P are elements that easily form segregation in steel. Excessive content of these elements can lead to segregation, affecting the uniformity of material properties, resulting in uneven plastic deformation, and consequently causing the mandrel to bend during cooling. Based on this, this invention controls S ≤ 0.010% and P ≤ 0.015%.

[0018] Cu, Ni, and Ti are unavoidable residual elements in this invention. When Cu and Ni contents are high, their presence promotes the formation of a well-bonded oxide film during the cooling process of the hot mandrel, but it cannot achieve a completely uniform oxide film coverage across the entire surface of the ultra-long mandrel. Therefore, while a well-bonded oxide film forms on the surface, its formation is uneven, which is detrimental to the uniformity of cooling and causes the mandrel to bend during cooling. Based on this, this invention controls Cu ≤ 0.08% and Ni ≤ 0.08%.

[0019] When the Ti content is too high, it combines with N and C to form nitrides and carbides, increasing the anisotropy of the ultra-long mandrel during rolling, thus leading to uneven plastic deformation and consequently, bending of the mandrel during cooling. Therefore, this invention controls the Ti content to be ≤0.010%.

[0020] Furthermore, the ultra-long mandrel steel described in this invention has a length of 25-35m.

[0021] Furthermore, the microstructure of the ultra-long mandrel steel described in this invention comprises air-cooled martensite + bainite.

[0022] Furthermore, the ultra-long mandrel steel described in this invention, after being finally cooled to room temperature, has a total straightness of ≤0.1%L, where L represents the length of the ultra-long mandrel steel.

[0023] Furthermore, the ultra-long mandrel steel described in this invention, after being finally cooled to room temperature, has a local curvature of ≤1.5mm / m.

[0024] Another objective of this invention is to provide a method for manufacturing ultra-long mandrel steel, which, in conjunction with composition design, controls the straightness of the ultra-long mandrel steel from a process perspective, thereby avoiding the problem of reduced straightness.

[0025] Based on the above-mentioned objectives, the present invention also provides a method for manufacturing ultra-long mandrel steel as described above, comprising the following steps:

[0026] Heating: Heat the continuously cast billet with a circular cross-section at 1200-1250℃ and hold for 2-4 hours;

[0027] Rolling: The final rolling temperature is controlled at 900-1050℃, and no local dense oxide film is formed on the surface after rolling;

[0028] Cooling: The rolled mandrel steel is placed alternately on different cooling racks at intervals of ≤20s to air cool to room temperature, and the total alternating operation time is controlled to be ≥6h.

[0029] In order to reduce the material anisotropy caused by easily segregating elements such as C, Mn, P, and S, which leads to uneven plastic deformation during mandrel cooling and thus causes the mandrel to bend, the manufacturing method described in this invention designs a reasonable high-temperature heating and rolling process. This process aims to reduce the deformation resistance of the mandrel during hot working and ensure sufficient temperature and time to improve segregation.

[0030] The heating temperature is controlled at 1200-1250℃ because: if the heating temperature is too low, the uniform diffusion time of segregated elements is too long, which cannot effectively improve segregation. As a result, the segregation is retained during the subsequent cooling process, affecting the uniformity of deformation and causing bending during cooling. If the heating temperature is too high, the austenite grains become coarse and non-uniform. Non-uniform austenite grains will undergo inconsistent phase transformations during the subsequent cooling process, that is, the bainite and martensite phase transformations are inconsistent at different locations, resulting in non-uniform volume expansion, which in turn causes bending during cooling.

[0031] The heating and holding time is controlled at 2-4 hours because: if the heating and holding time is too short, the diffusion of segregated elements is insufficient, failing to effectively improve the segregation problem. This causes the segregation to be retained during subsequent cooling, affecting the uniformity of deformation and leading to bending during cooling. If the heating and holding time is too long, the austenite grains become coarse and uneven. These uneven austenite grains undergo inconsistent phase transformations during subsequent cooling, meaning that the bainite and martensite phase transformations are inconsistent at different locations. This results in uneven volume expansion, leading to bending during cooling.

[0032] The final rolling temperature is controlled at 900-1050℃ because: if the final rolling temperature is too high, the mandrel temperature will be too high when it is placed on the cooling rack after rolling. Without the oxide film being broken down during rolling, localized oxide film formation can easily occur at various locations, leading to uneven cooling and thus bending. If the final rolling temperature is too low, the residual rolling stress will be too high, and the mandrel will easily bend during the cooling process as the stress is released.

[0033] The above cooling method can avoid the local temperature from being too low due to prolonged contact between the mandrel steel and the cooling rack. This avoids the problem of mandrel bending and reduced straightness caused by different temperature drop rates and phase transformation rates at different locations of the mandrel. It can effectively ensure that the temperature fluctuation at all points of the mandrel steel is ≤30℃ throughout the entire cooling process.

[0034] The reason for controlling the interval time to be ≤20s is that if the interval time is too long, local temperature drop will have already occurred, and the temperature fluctuation throughout the cooling process will be ≤30℃, making local and overall bending inevitable.

[0035] The reason for controlling the total alternating operation time to be ≥6h is that if the total operation time is too short, the mandrel steel will not have cooled to room temperature after rolling, and uneven local temperature will still occur after the operation stops, resulting in problems such as bending and poor straightness.

[0036] Furthermore, in the manufacturing method described in this invention, during the cooling step, the temperature fluctuation of the mandrel steel is controlled to be ≤30°C throughout the entire cooling process.

[0037] Furthermore, in the manufacturing method described in this invention, during the cooling step, mandrel steel with a temperature higher than 880°C is alternately placed on different cooling racks at intervals of ≤20 seconds, and the total duration of the alternating operation is controlled to be ≥6 hours.

[0038] The ultra-long mandrel steel and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0039] In terms of composition design, this invention improves the anisotropy problem caused by segregation by controlling C, Mn, P and S, avoids the problem of uneven phase transformation caused by excessively high martensite transformation temperature by controlling C, Mn, Cr and Mo, and avoids the problem of uneven cooling caused by unevenly distributed and locally dense oxide film on the surface by controlling Cu, Ni and Cr.

[0040] In terms of process parameters, this invention improves segregation and avoids the formation of uneven coarse austenite structures by controlling heating temperature and heating time, thus ensuring the uniformity of martensitic phase transformation during subsequent cooling.

[0041] In some implementations, the uniformity of cooling of the mandrel is ensured by alternating the placement of the rolled mandrel on different cooling racks, which achieves uniform deformation during the material cooling process, avoids bending problems, and can be effectively applied to the mass production of mandrel steel.

[0042] Based on this, the present invention can obtain ultra-long mandrel steel with a total straightness of ≤0.1%L and a local curvature of ≤1.5mm / m after cooling to room temperature. It can effectively solve the problems of poor straightness and low pass rate of mandrel steel, which is conducive to cost saving and has a very good prospect for promotion and application value. Detailed Implementation

[0043] The following will provide further explanation and description of the ultra-long mandrel steel and its manufacturing method according to the present invention with reference to specific embodiments. However, such explanation and description do not constitute an undue limitation on the technical solution of the present invention.

[0044] Examples 1-10 and Comparative Examples 1-8

[0045] The extra-long mandrel steels in Examples 1-10 were all prepared using the following steps:

[0046] (1) A continuous casting billet with a circular cross section is obtained. Table 1 lists the mass percentage of each chemical element in the continuous casting billets of each embodiment and comparative example of the present invention.

[0047] (2) Heating: Heat the round cross-section continuous casting billet at 1200-1250℃ and hold for 2-4 hours;

[0048] (3) Rolling: The final rolling temperature is controlled at 900-1050℃, and no local dense oxide film is generated on the surface after rolling;

[0049] (4) Cooling: The rolled mandrel steel is placed back and forth on different cooling racks at intervals of ≤20s to air cool to room temperature. The total duration of the alternating operation is controlled to be ≥6h. When cooled to 350℃, no air-cooled martensite is formed.

[0050] In some implementations, in step (4), mandrel steel with a temperature higher than 880°C (i.e., the incoming material temperature in Table 2) is alternately placed on different cooling racks at intervals of ≤20s, and the total duration of the alternating operation is controlled to be ≥6h, so that the temperature fluctuation of the mandrel steel throughout the cooling process is ≤30°C.

[0051] The comparative mandrel steels of Comparative Examples 1-8 were prepared using the same steps and processes described above, but the chemical composition of Comparative Examples 1-3 did not meet the requirements of this invention, and the specific process parameters in each step of Comparative Examples 4-8 failed to meet the design requirements of this invention.

[0052] Table 1 lists the mass percentage of each chemical element in the ultra-long mandrel steels of Examples 1-10 and the comparative mandrel steels of Comparative Examples 1-8.

[0053] Table 1. (wt%, balance Fe and other unavoidable impurities besides P and S)

[0054]

[0055]

[0056] Tables 2-1 and 2-2 list the specific process parameters for the ultra-long mandrel steels of Examples 1-10 and the comparative mandrel steels of Comparative Examples 1-8 in the above steps.

[0057] Table 2-1.

[0058]

[0059]

[0060] Note: The surface martensite content at 350℃ is a range value rather than a point value because the surface martensite content varies along the length of the mandrel steel.

[0061] Table 2-2.

[0062]

[0063] Samples of the mandrel steels of Examples 1-10 and Comparative Examples 1-8 were taken respectively. The straightness was measured by the method of straightness = maximum deviation value / total length * 100%, and the local curvature was calculated by the method of local curvature = maximum deviation value within 3m / 3m * 100%. The measurement results are listed in Table 3.

[0064] Table 3.

[0065]

[0066] It should be noted that after cooling is completed, the microstructure of the mandrel steel in each embodiment of the present invention is air-cooled martensite + bainite. However, during the cooling process, when the temperature is cooled to 350°C, air-cooled martensite does not form on the surface of the mandrel steel.

[0067] As can be seen from Table 3, the overall straightness of the mandrel steel obtained by using the technical solution described in this invention is ≤0.1%L, and the local curvature is ≤1.5mm / m, indicating that this invention can effectively solve the problems of poor straightness and low pass rate of mandrel steel.

[0068] In contrast, Comparative Example 1, due to its low C and Mn content, cannot meet the requirements of this invention in terms of overall straightness and local curvature.

[0069] Comparative Example 2 has low Cr and Mo content, therefore its overall straightness and local curvature cannot meet the requirements of this invention.

[0070] Comparative Example 3 has high Cu, Ni, and Cr content, therefore its overall straightness and local curvature cannot meet the requirements of this invention.

[0071] The heating temperatures of Comparative Examples 4 and 5 are higher or lower than the requirements of this invention, therefore their overall straightness and local curvature cannot meet the requirements of this invention.

[0072] The alternation interval and total duration of the alternation operation during cooling in Comparative Examples 6-8 do not meet the requirements of this invention, therefore their overall straightness and local curvature cannot meet the requirements of this invention.

[0073] Furthermore, the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0074] 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 thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. An ultra-long mandrel steel, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.28–0.32%, Si: 0.80–1.25%, Mn: 0.20–0.30%, Cr: 4.75–5.00%, Mo: 1.10–1.60%, V: 0.30–0.60%, S≤0.010%, P≤0.015%, Cu≤0.08%, Ni≤0.08%, Ti≤0.010%; the balance is Fe and other unavoidable impurities. The length of the ultra-long mandrel steel is 25-35m; After the ultra-long mandrel steel is finally cooled to room temperature, the overall straightness is ≤0.1%L and the local curvature is ≤1.5mm / m, where L represents the length of the ultra-long mandrel steel; and the overall straightness = maximum deviation value / total length * 100%, and the local curvature = maximum deviation value within 3m / 3m * 100%.

2. The ultra-long mandrel steel as described in claim 1, characterized in that, Its microstructure consists of air-cooled martensite and bainite.

3. The method for manufacturing ultra-long mandrel steel as described in any one of claims 1-2, characterized in that: Heating: Heat the continuously cast billet with a circular cross-section at 1200-1250℃ and hold for 2-4 hours; Rolling: The final rolling temperature is controlled at 900-1050℃, and no local dense oxide film is formed on the surface after rolling; Cooling: The rolled mandrel steel is placed alternately on different cooling racks at intervals of ≤20s to air cool to room temperature, and the total alternating operation time is controlled to be ≥6h.

4. The manufacturing method as described in claim 3, characterized in that, During the cooling process, the temperature fluctuation of the mandrel steel is controlled to be ≤30℃ throughout the entire cooling process.

5. The manufacturing method as described in claim 3, characterized in that, During the cooling process, mandrel steel with a temperature higher than 880℃ is alternately placed on different cooling racks at intervals of ≤20s, and the total duration of the alternating operation is controlled to be ≥6h.

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