Super-long core rod steel and manufacturing method thereof

Abstract

The invention discloses ultra-long core rod steel which comprises the following chemical elements in percentage by mass: 0.28 to 0.32 percent of C, 0.80 to 1.25 percent of Si, 0.20 to 0.30 percent of Mn, 4.75 to 5.00 percent of Cr, 1.10 to 1.60 percent of Mo, 0.30 to 0.60 percent of V, less than or equal to 0.010 percent of S, less than or equal to 0.015 percent of P, less than or equal to 0.08 percent of Cu, less than or equal to 0.08 percent of Ni and less than or equal to 0.010 percent of Ti. Correspondingly, the invention also discloses a manufacturing method of the ultra-long core rod steel, which comprises the following steps: heating: heating a continuous casting billet with a circular section at 1200-1250 DEG C and preserving heat for 2-4 hours; rolling, wherein the finish rolling temperature is controlled to be 900-1050 DEG C; and cooling. By adopting the method, the problems of poor straightness and low percent of pass of the core rod steel can be effectively solved.

Description

Technical Field

[0001] The invention relates to a steel material and a preparation method thereof, and in particular to a core rod steel and a preparation method thereof. Background Art

[0002] Extra-long mandrels, such as those exceeding 30 m in length, are prone to bending during the cooling process, thereby affecting the straightness and further processing and use.

[0003] The deterioration of straightness will lead to two major consequences. First, it is impossible to produce a finished mandrel rod of the required diameter specification with a small processing volume, which requires increasing the size of the billet and leaving more processing allowance. Second, it cannot be used for continuous rolling mandrel rod insertion, which can easily lead to problems such as sticking and uneven wall thickness on one side of the rolled steel pipe.

[0004] Based on this, it is desired to provide an ultra-long mandrel steel and a manufacturing method thereof, which can solve the bending problem of the mandrel during the cooling process by controlling the material composition of the mandrel and the manufacturing process. Summary of the invention

[0005] One of the purposes of the present invention is to provide an ultra-long mandrel steel, which has a reasonable chemical composition design to affect the phase change process of the mandrel, thereby reducing the bending caused by the phase change.

[0006] In order to achieve the above object, the present invention provides an ultra-long mandrel steel, which contains Fe and inevitable impurities, and further contains 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, the mass percentage of each chemical element of which is:

[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 inevitable impurities.

[0010] As described above, the ultra-long mandrel steel of the present invention is designed through chemical composition to affect the phase change process of the mandrel, thereby reducing the bending caused by the phase change. Specifically:

[0011] C: In the present invention, the C content mainly improves the strength through solid solution strengthening, precipitation strengthening and phase transformation strengthening. If the C content is too low, the strength and thermal strength of the core rod will be insufficient and cannot meet the use requirements of subsequent tools / dies. If the C content is too low, the martensitic transformation temperature will be too high, and bainite + martensitic transformation will easily occur simultaneously during the cooling process, resulting in uneven microstructure transformation at locations with different temperatures, resulting in large volume expansion in places with more martensitic transformation and small volume expansion in places with less martensitic transformation, thereby causing the core rod to bend. In addition, if the C content is too high, it will easily lead to delayed cracking problems during the long-term storage of the core rod after it is cooled to room temperature. Based on this, the present invention controls the C content to 0.28-0.32%.

[0012] Si: In the present invention, if the Si content is too low, the core rod will have insufficient tempering softening resistance and insufficient thermal strength, and will not meet the use requirements of subsequent tools and molds. If the Si content is too high, the material's impact resistance and accident resistance will be reduced. Therefore, the present invention controls the Si content to 0.80-1.25%.

[0013] Mn: Mn mainly improves strength through solid solution strengthening. In the present invention, if the Mn content is too low, the content of other alloys to be added will increase, the cost will increase, and the martensitic transformation temperature will be too high, which will easily cause bainite + martensitic transformation to occur simultaneously during the cooling process, resulting in uneven structural transformation at locations with different temperatures, which will in turn cause large volume expansion in places with more martensitic transformation and small volume expansion in places with less martensitic transformation, thereby causing the core rod to bend. If the Mn content is too high, segregation will easily form, which will reduce the impact resistance of the material and reduce the material's ability to resist accidents. Segregation caused by too high Mn will affect the isotropic uniformity of the material's properties, thereby causing non-uniformity of plastic deformation, which will cause the core rod to bend during cooling. Based on this, the present invention controls the Mn content to be 0.20-0.30%.

[0014] Cr: Cr can play the role of phase transformation strengthening and precipitation strengthening, thereby improving the resistance to temper softening and increasing thermal strength. In the present invention, if the Cr content is too low and the martensite transformation temperature is too high, it is easy for bainite + martensite transformation to occur simultaneously during the cooling process, resulting in uneven tissue transformation at locations with different temperatures, resulting in large volume expansion in places with more martensite transformation and small volume expansion in places with less martensite transformation, thereby causing the core rod to bend. If the Cr content is too high, it is easy to cause delayed cracking problems during the long-term storage of the core rod after the core rod is cooled to room temperature, and if the content is too high, a dense oxide film is easily formed locally during the cooling process. The uneven oxide film on the surface will lead to uneven cooling, resulting in uneven plastic deformation, which in turn causes the core rod to bend during cooling. Therefore, the present invention controls the Cr content to 4.75-5.00%.

[0015] Mo: Mo mainly plays the role of solid solution strengthening, phase transformation strengthening, and precipitation strengthening, thereby improving the resistance to tempering softening and increasing thermal strength. In the present invention, if the Mo content is too low and the martensite transformation temperature is too high, it is easy for bainite + martensite transformation to occur simultaneously during the cooling process, resulting in uneven microstructure transformation at locations with different temperatures, resulting in large volume expansion in places with more martensite transformation and small volume expansion in places with less martensite transformation, thereby causing the core rod to bend. If the Mo content is too high, it is easy to cause delayed cracking during the long-term storage of the core rod after it is cooled to room temperature. Based on this, the present invention controls the Mo content to 1.10-1.60%.

[0016] V: V can play a role in precipitation strengthening, thereby improving the tempering softening resistance and improving the thermal strength. In the present invention, if the V content is too low, the core rod will have insufficient tempering softening resistance and insufficient thermal strength, and cannot meet the use requirements of subsequent tools / tools. If the V content is too high, it is easy to form hard and brittle liquid precipitation carbides, reduce the impact performance of the core rod, and reduce the material's ability to resist accidents. Based on this, the present invention controls the V content to 0.30-0.60%

[0017] S and P are the main impurity elements of the present invention, which are not good for the impact properties of the material, so their upper limits need to be controlled. S and P are elements that easily form segregation in steel. Excessive content of S and P will lead to segregation, affecting the uniformity of material properties, thereby leading to non-uniformity of plastic deformation, and further causing the mandrel to bend during cooling. Based on this, the present invention controls S≤0.010% and P≤0.015%.

[0018] Cu, Ni and Ti are unavoidable residual elements in the present invention. When the content of Cu and Ni is high, their presence will promote the formation of an oxide film with good surface bonding during the cooling process of the hot core rod, but it is impossible to achieve a completely consistent coverage of the oxide film on the entire surface of the super-long core rod. Therefore, an oxide film with good bonding is formed on the surface, but it is not formed uniformly everywhere, which is not conducive to the uniformity of cooling, resulting in bending of the core rod during cooling. Based on this, the present invention controls Cu≤0.08% and Ni≤0.08%.

[0019] When the Ti content is too high, it will form nitrides and carbides due to its combination with N and C, which will increase the anisotropy of the super-long mandrel during rolling, thereby causing non-uniform plastic deformation and further causing the mandrel to bend during cooling. Therefore, the present invention controls Ti≤0.010%.

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

[0021] Furthermore, the microstructure of the ultra-long mandrel steel of the present invention includes air-cooled martensite+bainite.

[0022] Furthermore, the super-long mandrel steel of the present invention has a full-length straightness of ≤0.1%L after it is finally cooled to room temperature, wherein L represents the length of the super-long mandrel steel.

[0023] Furthermore, the super-long mandrel steel of the present invention has a local curvature of ≤1.5 mm / m after it is finally cooled to room temperature.

[0024] Another object of the present invention is to provide a method for manufacturing an ultra-long mandrel steel, wherein the method is designed with components to control the straightness of the ultra-long mandrel steel from a process perspective, thereby avoiding the problem of reduced straightness.

[0025] Based on the above invention object, the present invention also provides a method for manufacturing the ultra-long mandrel steel as described above, which comprises the steps of:

[0026] Heating: Heat the circular cross-section continuous casting billet at 1200-1250℃ for 2-4h;

[0027] Rolling: Control the final rolling temperature to 900-1050℃, and no local dense oxide film will be formed on the surface after rolling;

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

[0029] The manufacturing method described in the present invention designs a reasonable high-temperature heating rolling process in order to reduce the material anisotropy caused by easily segregated elements such as C, Mn, P, S, which leads to uneven plastic deformation during the cooling process of the mandrel, thereby causing the mandrel to bend. The first is to reduce the deformation resistance of the mandrel during hot processing, and the second is to 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 the segregation elements is very long, and the segregation cannot be effectively improved, resulting in the segregation being retained in the subsequent cooling process, thus affecting the uniformity of deformation and causing bending in the cooling process. If the heating temperature is too high, the austenite grains will be coarse and uneven. The uneven austenite grains will undergo inconsistent phase transformations in the subsequent cooling process, that is, the bainite and martensite phase transformations at different positions are inconsistent, resulting in uneven volume expansion, which in turn causes bending in the cooling process.

[0031] The reason for controlling the heating and holding time to 2-4h is that: if the heating and holding time is too short, the diffusion of segregation elements is insufficient, and the segregation problem cannot be effectively improved, resulting in the segregation being retained in the subsequent cooling process, thus affecting the uniformity of deformation and causing bending during the cooling process. If the heating and holding time is too long, the austenite grains will be coarse and uneven. The uneven austenite grains will undergo inconsistent phase transformations during the subsequent cooling process, that is, the bainite and martensite phase transformations at different positions are inconsistent, resulting in uneven volume expansion, which in turn causes bending during the cooling process.

[0032] The final rolling temperature is controlled at 900-1050℃ because: if the final rolling temperature is too high, the temperature of the mandrel will be too high when it is placed on the cooling rack after rolling, and the oxide film will not be destroyed during rolling. Local oxide film coverage is easily generated at various positions, resulting in uneven cooling at various positions, which leads to bending. If the final rolling temperature is too low, the residual rolling stress is too large, and the mandrel is prone to bending as the stress is released during the cooling process.

[0033] The above cooling method can avoid the local temperature of the core rod being too low due to the local contact time of the core rod steel with the cooling rack for too long, thereby avoiding the problem of core rod bending and straightness reduction caused by different temperature drop speeds and phase change speeds at different positions of the core rod. It can effectively ensure that the temperature fluctuation of the core rod steel at all locations during the entire cooling process is ≤30°C.

[0034] Among them, the control interval time is ≤20s because: the interval time is too long, local temperature drop has already occurred, the temperature fluctuation at all locations during the cooling process is ≤30℃, and local and overall bending will be inevitable.

[0035] Among them, the total duration of alternating operation is controlled to be ≥6h because: the total operation time is too short, the temperature of the core rod steel has not cooled to room temperature after rolling, and after stopping the operation, local temperature unevenness will still occur, resulting in poor bending and straightness.

[0036] Furthermore, in the manufacturing method of the present invention, in the cooling step, the temperature fluctuation of each location of the core rod steel is controlled to be ≤30°C during the entire cooling process.

[0037] Furthermore, in the manufacturing method described in the present invention, in the cooling step, the core rod steel with a temperature higher than 880° C. is placed alternately back and forth 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 the manufacturing method thereof of the present invention have the following advantages and beneficial effects:

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

[0040] In terms of process parameters, the present invention improves segregation and avoids the formation of uneven coarse austenite structure by controlling the heating temperature and heating time to ensure the uniformity of martensite phase transformation in the subsequent cooling process.

[0041] In some embodiments, the mandrel is placed alternately on different cooling racks after rolling to ensure uniform cooling of the mandrel, thereby achieving uniform deformation of the material during cooling and avoiding the occurrence of bending problems. This process can be effectively applied to mass production of mandrel steel.

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

[0043] The ultra-long mandrel steel and the manufacturing method thereof according to the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.

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

[0045] The super-long mandrel steels of Examples 1-10 are all prepared by the following steps:

[0046] (1) A continuous casting billet with a circular cross section is prepared. Table 1 lists the mass percentages of chemical elements in the continuous casting billets of various embodiments of the present invention and comparative examples;

[0047] (2) Heating: The circular cross-section continuous casting billet is heated at 1200-1250°C for 2-4 hours;

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

[0049] (4) Cooling: The rolled mandrel steel is placed alternately 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°C, no air-cooled martensite is formed by transformation.

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

[0051] The comparative mandrel steels of Comparative Examples 1-8 are also prepared by the above-mentioned steps, but the chemical composition of Comparative Examples 1-3 does not meet the requirements of the present invention, and the specific process parameters in each step of Comparative Examples 4-8 fail to meet the design requirements of the present invention.

[0052] Table 1 lists the mass percentages of the chemical elements of the super-long mandrel steels of Examples 1-10 and the comparative mandrel steels of Comparative Examples 1-8.

[0053] Table 1. (wt%, the balance is Fe and other inevitable impurities except P and S)

[0054]

[0055]

[0056] Table 2-1 and Table 2-2 list the specific process parameters of the super-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 proportion of surface martensite when cooled to 350℃ is a range value rather than a point value because the content of surface martensite at different locations along the length direction of the mandrel steel is different.

[0061] Table 2-2.

[0062]

[0063] The core rod steels of Examples 1-10 and Comparative Examples 1-8 were sampled respectively, and their straightness was measured by the measurement method of straightness = maximum deviation value / total length*100%, and their local curvature was calculated by the measurement 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, the microstructure of the mandrel steel in each embodiment of the present invention is air-cooled martensite+bainite. However, during the cooling process, when cooled to 350°C, air-cooled martensite is not formed on the surface of the mandrel steel.

[0067] It can also be seen from Table 3 that by adopting the technical solution described in the present invention, the full-length straightness of the mandrel steel obtained is ≤0.1%L, and the local curvature is ≤1.5mm / m, which shows that the present invention can effectively solve the problems of poor straightness and low qualified rate of mandrel steel.

[0068] In contrast, in Comparative Example 1, due to its low C and Mn contents, its full-length straightness and local curvature cannot meet the requirements of the present invention.

[0069] The Cr and Mo contents of Comparative Example 2 are low, so its full-length straightness and local curvature cannot meet the requirements of the present invention.

[0070] The content of Cu, Ni and Cr in Comparative Example 3 is high, so its full-length straightness and local curvature cannot meet the requirements of the present invention.

[0071] The heating temperatures of Comparative Examples 4 and 5 are higher or lower than the requirements of the present invention, so their full-length straightness and local curvature cannot meet the requirements of the present invention.

[0072] The alternating interval time and the total duration of alternating operation during cooling of Comparative Examples 6-8 do not meet the requirements of the present invention, so their full-length straightness and local curvature cannot meet the requirements of the present invention.

[0073] 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.

[0074] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with the contents disclosed by those skilled in the art from the present invention, and all should belong to the protection scope of the present invention.

Claims

1. An extra-long mandrel steel containing Fe and unavoidable impurities, characterized in that: It also contains 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%.

2. The super-long mandrel steel according to claim 1, characterized in that: The mass percentage of each chemical element is: 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 inevitable impurities.

3. The super-long mandrel steel according to claim 1 or 2, characterized in that: Its length is 25-35m.

4. The super-long mandrel steel according to claim 1 or 2, characterized in that: Its microstructure includes air-cooled martensite + bainite.

5. The super-long mandrel steel according to claim 1 or 2, characterized in that: After it is finally cooled to room temperature, the full length straightness is ≤0.1%L, where L represents the length of the super-length mandrel steel.

6. The super-long mandrel steel according to claim 1 or 2, characterized in that: After it is finally cooled to room temperature, the local curvature is ≤1.5mm / m.

7. The method for manufacturing an ultra-long mandrel steel according to any one of claims 1 to 6, characterized in that: Heating: Heat the circular cross-section continuous casting billet at 1200-1250℃ for 2-4h; Rolling: Control the final rolling temperature to 900-1050℃, and no local dense oxide film will be formed on the surface after rolling; Cooling: Place the rolled mandrel steel alternately on different cooling racks at intervals of ≤20s to air cool to room temperature, and control the total alternating operation time to be ≥6h.

8. The manufacturing method according to claim 7, characterized in that: In the cooling step, the temperature fluctuation of the mandrel steel at each location is controlled to be ≤30°C during the entire cooling process.

9. The manufacturing method according to claim 7, characterized in that: In the cooling step, the core rod steel with a temperature higher than 880°C is placed alternately 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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