Online heat treatment method for large-specification low-alloy structural steel round rolled piece

Abstract

The invention relates to the field of preparation of structural steel. Specifically, the invention relates to an on-line heat treatment method of a large-specification low-alloy structural steel round rolled piece and a large-specification low-alloy structural steel round rolled piece obtained by the on-line heat treatment method. According to the online heat treatment method, after-rolling waste heat of a large-specification low-alloy high-strength structural steel round rolled product with the diameter being about 200 mm or above is utilized, and a fog cooling fan used for rapidly cooling the rolled product in the red hot rolling state is designed and manufactured on an after-rolling cooling bed; and under the condition that the finish rolling temperature, the upper cooling bed temperature, the air inlet fog cooling area temperature, the air fog cooling finishing temperature and the lower cooling bed temperature of the rolled material are controlled, the large-specification low-alloy structural steel circular steel is obtained. Compared with steel obtained through an off-line normalizing process in the prior art, the obtained large-specification low-alloy structural steel round steel is equivalent in mechanical property, the purpose of process cost reduction is achieved, and the delivery period is shortened by at least about three days.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of structural steel production. Specifically, the present application relates to an on-line heat treatment method of large-diameter low-alloy structural steel round rolling piece, and a large-diameter low-alloy structural steel round steel obtained by the on-line heat treatment method. BACKGROUND

[0002] The general requirements of chemical composition (mass percent) of low-alloy high-strength structural steel are: C≤0.22wt%, Si≤0.55wt%, Mn≤1.60wt%, Cr≤0.40wt%, P≤0.025wt%, S≤0.025wt%, Ni≤0.30wt%, Mo≤0.08wt%, Cu≤0.30wt%, Al: 0.020-0.060wt%, N≤0.0120wt%, CEV≤0.49wt% (wherein CEV = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15). Generally, the large-diameter low-alloy high-strength structural steel produced with a diameter of about 200 mm or more needs to be treated by an “off-line normalizing” process to ensure that its mechanical properties meet the standard requirements, for example, by off-line normalizing at a process temperature of about 860℃, to ensure that the mechanical properties of the steel meet the standard requirements of “yield strength of about 285 MPa or more, tensile strength in the range of about 450 to about 600 MPa, elongation after fracture of about 17% or more, and minimum impact energy at -40℃ of about 27 J or more”.

[0003] However, the off-line normalizing process for obtaining large-diameter low-alloy high-strength structural steel that meets the standard requirements is relatively high in cost and long in delivery cycle. SUMMARY

[0004] OBJECTIVE In view of the problems existing in the prior art described in the background section above, the purpose of the present application is to provide an on-line heat treatment method of low-alloy structural steel round rolling piece with a diameter of 200 mm or more, and to provide a large-diameter low-alloy structural steel round steel obtained by the on-line heat treatment method.

[0005] TECHNICAL SCHEME In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme: Scheme 1: An on-line heat treatment method of low-alloy structural steel round rolling piece with a diameter of about 200 mm or more, wherein the on-line heat treatment method comprises: cooling the low-alloy structural steel round rolling piece with a diameter of about 200 mm or more in a red-hot state after rolling at a temperature in the range of about 850℃ to about 870℃ in the following order of cooling steps: Air-cooling step (1): The low-alloy structural steel round rolled piece with a diameter of about 200 mm or more, which is in a post-rolling red-hot state with a temperature in the range of about 850°C to about 870°C, is cooled to a temperature of about 550°C to about 600°C by air-cooling at a cooling rate in the range of about 0.7°C to about 0.8°C / second; and Air cooling step (2): The low alloy structural steel round rolled piece with a diameter of about 200 mm or more, which has been cooled to a temperature of about 400 to about 450°C by air cooling at a cooling rate of about 0.3 to about 0.4°C / second, having passed through the air cooling step (1) where the temperature is in the range of about 550°C to about 600°C.

[0006] Option 2: According to the online heat treatment method described in Option 1 above, the low-alloy structural steel round rolled piece with a diameter of about 200 mm or more in the post-rolled red-hot state at a temperature in the range of about 850°C to about 870°C is obtained by the following air-cooling step: Air cooling step (0): The low alloy structural steel round rolled piece with a diameter of about 200 mm or more and a temperature of about 950°C or more after rolling is cooled to a temperature of about 850 to about 870°C by air cooling on a cooling bed at a cooling rate of about 0.4 to about 0.5°C / second.

[0007] Option 3: According to the online heat treatment method described in Option 1 or 2 above, the low alloy structural steel contains approximately 0.020 to approximately 0.060% by weight of element Al based on the total weight of the low alloy structural steel, and the balance element Fe and other unavoidable elements.

[0008] Option 4: According to the online heat treatment method described in Option 3 above, the total amount of elements other than Fe contained in the low alloy structural steel is less than about 5% by weight, preferably less than about 3% by weight, based on the total weight of the low alloy structural steel.

[0009] Option 5: The online heat treatment method according to Option 3 or 4 above, wherein, based on the total weight of the low-alloy structural steel, the low-alloy structural steel contains other elements that are unavoidable. The content of element C is less than approximately 0.22% by weight. The content of element Si is less than approximately 0.55% by weight. The content of element Mn is less than approximately 1.60% by weight. The content of element Cr is less than approximately 0.40% by weight. The content of element P is less than approximately 0.025% by weight. The content of element S is less than approximately 0.025% by weight. The content of element Ni is less than approximately 0.30% by weight. The content of element Mo is less than approximately 0.08% by weight. The content of element Cu is less than approximately 0.30% by weight, and / or The content of element N is less than approximately 0.0120% by weight.

[0010] Option 6: According to the online heat treatment method described in Option 5 above, the amount of CEV of the low alloy structural steel is controlled to be below about 0.49% by weight, wherein CEV = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15.

[0011] Option 7: The online heat treatment method according to any one of Options 1 to 6 above, wherein the online heat treatment method further includes the following cooling step: Slow cooling pit slow cooling step (3): The low alloy structural steel round rolled piece, which has been cooled to about 400 to about 450°C after the air cooling step (2), is put into the slow cooling pit for slow cooling, and cooled to a temperature of about 200°C or below at a cooling rate of about 10 to about 10.5°C / hour.

[0012] Option 8: A round low-alloy structural steel bar with a diameter of about 200 mm or more, obtained by the online heat treatment method according to any one of Options 1 to 7 above, wherein the round steel bar has one or more of the following properties: The yield strength is approximately ≥285 MPa, preferably approximately ≥295 MPa, and more preferably approximately ≥300 MPa. The tensile strength is in the range of about 450 to about 600 MPa, preferably in the range of about 516 to about 535 MPa. The elongation after fracture is approximately ≥17%, preferably approximately ≥29%. The minimum impact absorption energy at -40℃ is approximately ≥27 J, preferably approximately ≥30 J, more preferably ≥40 J, and The hardness is in the range of about 200 to about 220 HBW, preferably in the range of 205 to 210 HBW.

[0013] Technical effect The online heat treatment method of this invention, namely the "online normalizing" process that replaces "offline normalizing," achieves mechanical properties in large-size low-alloy high-strength structural steel equivalent to those in the "offline normalized state"—that of "hot-rolled steel." Specifically, utilizing the residual heat after rolling of large-size low-alloy high-strength structural steel circular rolled products with a diameter of approximately 200 mm or more, a mist-cooling fan is designed and manufactured on the post-rolling cooling bed to rapidly cool the red-hot rolled product. The red-hot rolled steel is placed on the cooling bed for "mist cooling," thus replacing the "offline normalizing process" with an "online normalizing method utilizing residual heat after rolling." This achieves cost reduction (saving approximately 400 yuan / ton in normalizing costs) while ensuring the mechanical properties of the steel, and shortens the delivery cycle by at least three days, which is of great significance. Specifically, the present invention achieves the above objectives by controlling the final rolling temperature, upper cooling bed temperature, inlet air mist cooling zone temperature (comparable to the temperature of offline normalizing process), air mist cooling completion temperature, and lower cooling bed temperature of large-size low alloy high-strength structural steel rolled products with a diameter of approximately 200 mm or more. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely for the purpose of aiding understanding of this invention and should not be considered as specific limitations on this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. Process parameters in the following embodiments that are not specifically specified are generally performed under conventional conditions.

[0015] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. The term "about" as used in this invention indicates that the number it modifies may fluctuate within ±20%, ±15%, ±10%, ±5%, or ±2% of that number. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and the individual point values ​​contained within them, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0016] According to a first aspect of the present invention, the present invention provides an online heat treatment method for large-size low-alloy structural steel circular rolled pieces.

[0017] The large-size low-alloy structural steel round rolled pieces targeted by the online heat treatment method according to the first aspect of the present invention refer to low-alloy structural steel round rolled pieces with a diameter of about 200 mm or more, and particularly refer to low-alloy structural steel round rolled pieces with a diameter of about 200 mm or more in the post-rolling red-hot state at a temperature in the range of about 850°C to about 870°C.

[0018] Here, the large-size low-alloy structural steel round rolled piece is preferably obtained through the following air-cooling process: Air cooling step (0): The large-size low-alloy structural steel round rolled piece with a temperature of about 950°C or above (e.g., about 1000°C or above) obtained after rolling is cooled on a cooling bed by air cooling at a cooling rate of about 0.4 to about 0.5°C / second (e.g., about 0.45°C / second) to a temperature of about 850 to about 870°C, for example, to about 860°C.

[0019] The online heat treatment method according to a first aspect of the present invention comprises cooling the large-size low-alloy structural steel round rolled piece in a post-rolled red-hot state at a temperature ranging from about 850°C to about 870°C in the following sequence of cooling steps: Air-cooling step (1): The large-size low-alloy structural steel round rolled piece, which is in the red-hot state after rolling and has a temperature in the range of about 850°C to about 870°C, is cooled to a temperature of about 550°C to about 600°C by air-cooling at a cooling rate of about 0.7 to about 0.8°C / second; and Air cooling step (2): The large-size low alloy structural steel round rolled piece, which has been cooled to a temperature of about 400 to about 450°C by air cooling at a cooling rate of about 0.3 to about 0.4°C / second, is cooled to a temperature of about 550°C to about 600°C after the air cooling step (1).

[0020] In the above-mentioned air-cooling step (1), the temperature of the large-size low-alloy structural steel round rolled piece in the red-hot state before air-cooling should be in the range of about 850°C to about 870°C, for example, about 855°C, about 860°C, or about 865°C. Here, the temperature of the air-cooling zone of the red-hot round rolled piece should not be too low, for example, not lower than about 850°C, otherwise it may lead to an excessively high tensile strength of the obtained low-alloy structural steel; the temperature should also not be too high, for example, not higher than about 870°C, otherwise it may lead to coarse grains in the obtained low-alloy structural steel, resulting in a decrease in the strength and toughness of the material.

[0021] In the above-mentioned air-cooling step (1), the cooling rate of the air-cooling should be in the range of about 0.7 to about 0.8 °C / second, for example, about 0.75 °C / second. Here, the cooling rate of the air-cooling step should not be too fast, for example, it should not exceed about 0.8 °C / second, otherwise it may lead to the high tensile strength and hardness of the obtained low alloy structural steel; the cooling rate should also not be too slow, for example, it should not be lower than about 0.7 °C / second, otherwise it may lead to the low tensile strength and hardness of the material.

[0022] In the above-mentioned air-cooling step (1), the temperature of the rolled piece after air-cooling should be in the range of about 550 to about 600°C, for example, about 560°C, about 570°C, about 580°C, or about 590°C. Here, the temperature after air-cooling should not be too high, for example, it should not exceed about 600°C, otherwise it may lead to the tensile strength and hardness of the obtained low-alloy structural steel being too high; the temperature should also not be too low, for example, it should not be lower than about 550°C, otherwise it may lead to the tensile strength and hardness of the obtained low-alloy structural steel being too low.

[0023] In the air-cooling step (2) described above, the cooling rate should be in the range of approximately 0.3 to approximately 0.4 °C / second, for example, approximately 0.35 °C / second. Here, the air-cooling rate should not be too fast, for example, it should not exceed approximately 0.4 °C / second, otherwise it may lead to excessively high hardness of the obtained low-alloy structural steel or even surface cracks; the air-cooling rate should also not be too slow, for example, it should not be lower than approximately 0.3 °C / second, otherwise it may lead to lower strength and hardness of the obtained low-alloy structural steel, and poorer energy absorption in the -40 °C impact test.

[0024] In the above-mentioned air-cooling step (2), the temperature of the lower cooling bed reached by the rolled piece after air-cooling should be in the range of about 400 to about 450°C, for example, about 410°C, about 420°C, about 430°C, or about 440°C. Here, the temperature of the lower cooling bed should not be too high, for example, it should not exceed about 450°C, otherwise it may cause the obtained low-alloy structural steel to bend and the steel to be difficult to cool down to the specified temperature (below about 200°C) within the specified slow cooling time; the temperature of the lower cooling bed should also not be too low, for example, it should not be lower than about 400°C, otherwise it may cause the obtained low-alloy structural steel to crack and fail ultrasonic testing, as well as quality problems such as high steel strength and hardness.

[0025] The large-size low-alloy structural steel targeted by the online heat treatment method according to the first aspect of the present invention preferably contains about 0.020 to about 0.060% by weight of element Al (aluminum) based on the total weight of the low-alloy structural steel, and the balance element Fe and other unavoidable elements.

[0026] Here, Al is an essential element in low-alloy structural steel. Adding an appropriate amount of Al to the low-alloy structural steel of the present invention can refine the grain size of the steel, improve the mechanical properties of the steel and the impact absorption energy value at -40°C. Therefore, the content of Al in the low-alloy structural steel of the present invention is generally set in the range of about 0.020 to about 0.060% by weight, for example about 0.030% by weight, about 0.040% by weight or about 0.050% by weight.

[0027] Furthermore, the total amount of elements other than Fe (i.e., alloying elements) in the low-alloy structural steel of the present invention is generally required to be less than about 5% by weight, preferably less than about 3% by weight. Typically, the alloying elements present in the low-alloy structural steel of the present invention mainly include Mn (manganese), and contain small amounts of Si (silicon), Cr (chromium), Ni (nickel), Mo (molybdenum), Cu (copper), and P (phosphorus). The presence of Mn and Si strengthens ferrite; the presence of small amounts of Cr, Ni, and Mo forms carbides and nitrides, which can provide dispersion strengthening and grain refinement, improving the toughness of the steel; the presence of small amounts of Cu and P improves the corrosion resistance of the steel.

[0028] Therefore, in the low-alloy structural steel of the present invention, it is further preferred that the content of element Si, which is an unavoidable other element, is controlled to be less than about 0.55 wt%, the content of element Mn is controlled to be less than about 1.60 wt%, the content of element Cr is controlled to be less than about 0.40 wt%, the content of element P is controlled to be less than about 0.025 wt%, the content of element Ni is controlled to be less than about 0.30 wt%, the content of element Mo is controlled to be less than about 0.08 wt%, and / or the content of element Cu is controlled to be less than about 0.30 wt%, preferably less than 0.22 wt%, and / or the content of element C (carbon) is controlled to be less than about 0.22 wt%, the content of element S (sulfur) is controlled to be less than about 0.025 wt%, and / or the content of element N (nitrogen) is controlled to be less than about 0.0120 wt%.

[0029] In a further preferred embodiment, the CEV (Carbon Equivalency), which represents the total quantitative strength of carbon and alloying elements in the low-alloy structural steel, is preferably controlled to be below approximately 0.49% by weight, i.e., calculated by chemical composition as: CEV = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 ≤ 0.49% by weight. In the above formula, the element symbol enclosed in square brackets indicates the element's weight percentage content in the low-alloy structural steel. Here, the CEV value should not exceed approximately 0.49% by weight, otherwise it may lead to a decrease in the strength, hardness, and minimum impact absorption energy at -40°C of the obtained low-alloy structural steel round steel.

[0030] The online heat treatment method according to the first aspect of the present invention may preferably further include a slow cooling pit slow cooling step (3) after the air cooling step (2): the low alloy structural steel round rolled piece cooled to about 400 to about 450°C after the air cooling step (2) is placed in the slow cooling pit for slow cooling, and cooled to a temperature of about 200°C or below at a cooling rate of about 10 to 10.5°C / hour.

[0031] In the above-mentioned slow cooling pit step (3), the cooling rate of the slow cooling pit is preferably in the range of about 10 to about 10.5°C / hour, for example, about 10.1°C / hour, about 10.2°C / hour, about 10.3°C / hour, or about 10.4°C / hour. Here, the cooling rate of the slow cooling pit should not be too fast, for example, it should not exceed about 10.5°C / second, otherwise it may cause cracks and ultrasonic flaw detection failures in the obtained low alloy structural steel round steel, as well as quality problems such as excessive strength and hardness of the steel; the cooling rate of the slow cooling pit should also not be too slow, for example, it should not be lower than about 10°C / second, otherwise it may significantly increase production costs and affect the production rhythm and product contract delivery time.

[0032] In the slow cooling step (3) described above, the temperature to which the rolled piece is cooled by slow cooling in the slow cooling pit is preferably below about 200°C. Here, the exit temperature of the slow cooling pit after slow cooling should not be too high, for example, it should not exceed about 200°C, otherwise it may cause cracks and failure of ultrasonic testing of the obtained low alloy structural steel round steel, as well as quality problems such as excessive strength and hardness of the steel.

[0033] According to a second aspect of the present invention, the present invention provides a large-diameter (diameter greater than about 200 mm) low-alloy structural steel round steel bar obtained by the online heat treatment method according to the first aspect of the present invention, wherein the large-diameter low-alloy round steel bar has one or more of the following properties: The yield strength is approximately 285 MPa or higher, preferably approximately 295 MPa or higher, and more preferably approximately ≥300 MPa. The tensile strength is in the range of about 450 to about 600 MPa, preferably in the range of about 516 to about 535 MPa. The elongation after fracture is approximately 17% or higher, preferably approximately 29% or higher. The minimum impact absorption energy at -40℃ is approximately 27 J or more, preferably approximately 30 J or more, and more preferably approximately 40 J or more. The hardness is in the range of about 200 to about 220 HBW, preferably in the range of about 205 to about 210 HBW.

[0034] The above performance data can be verified, for example, according to Chinese national standards GB / T 1591-2018 "Low Alloy High Strength Structural Steel" and GB / T 231.1-2018 "Britton Hardness Test for Metallic Materials".

[0035] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0036] The low-alloy structural steel continuous casting billets used in the following examples and comparative examples, based on the total weight of the low-alloy structural steel continuous casting billets, have approximately 0.040% by weight of element Al, with the balance being element Fe and other elements, wherein the other elements include: Approximately 0.16% by weight of element C, Approximately 0.30% by weight of element Si, Approximately 1.32% by weight of element Mn, Approximately 0.33% by weight of element Cr, Approximately 0.012% by weight of element P, Approximately 0.005% by weight of element S, Approximately 0.02% by weight of element Ni, Approximately 0.02% by weight of the element Mo, Approximately 0.02% by weight of the element Cu, and Approximately 0.010% by weight of element N.

[0037] Where CEV = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15 = 0.45 by weight.

[0038] Example 1 (E1): Obtaining rolled parts: A square continuous casting billet of low alloy structural steel with a cross-sectional size of approximately 410×530 mm is selected. It is rolled into a round low alloy structural steel billet with a diameter of approximately 200 mm at a suitable heating temperature. The final rolling temperature is controlled at approximately 1000℃ or higher. After rolling, the billet is quickly moved through the transverse section and sawn to ensure that the appropriate upper cooling bed temperature is above approximately 950℃.

[0039] Cooling bed cooling: The low-alloy structural round steel roll is placed into a cooling bed and cooled by air cooling at a rate of approximately 0.45°C / second until the temperature of the roll reaches the inlet mist cooling zone temperature of approximately 860°C.

[0040] Turn on the mist cooling fan and control the appropriate cooling bed stepping speed to keep the cooling rate of the rolled piece within the range of about 0.75℃ / second, so that the rolled piece reaches a mist cooling completion temperature of about 575℃ after passing through the mist cooling zone.

[0041] The mist cooling fan is turned off, and the temperature of the rolled piece is reduced to the lower cooling bed temperature of about 425°C by air cooling at a cooling rate of about 0.35°C / second.

[0042] Slow cooling pit for slow cooling: After being removed from the cooling bed, the rolled piece is placed in a slow cooling pit to a temperature below approximately 200°C, with the slow cooling rate in the slow cooling pit being approximately 10.2°C / hour.

[0043] Example 2-15 (E2-15): The low-alloy structural steel round rolled pieces with a diameter of 200 mm or more were subjected to a cooling process similar to that in Example 1 above, with the cooling process parameters shown in Table 1 below.

[0044] Comparative Examples 1-10 (CE1-10): The low-alloy structural steel round rolled pieces with a diameter of 200 mm or more were subjected to a cooling process similar to that in Example 1 above, with the cooling process parameters shown in Table 1 (continued) below.

[0045] After the large-size low-alloy structural steel round steels that underwent the online heat treatment processes of Examples 1-15 and Comparative Examples 1-10 were slowly cooled and removed from the pit, their various performance data were tested according to the Chinese national standards GB / T 1591-2018 "Low-alloy high-strength structural steel" and GB / T231.1 "Metallic materials Brinell hardness test". The performance test results are shown in Table 2 and Table 2 (continued) below.

[0046] Table 1

[0047] Table 1 (continued)

[0048] Table 2:

[0049] Table 2 (continued)

[0050] As can be seen from the results in Table 2 above, the yield strength of the large-size low-alloy structural steel round steel obtained by Examples 1 to 15 of the present invention is in the range of about 295 to about 310 MPa, the tensile strength is in the range of about 516 to about 535 MPa, the elongation after fracture is in the range of about 29% to about 32%, and the minimum impact absorption energy at -40℃ is in the range of about 30 to about 50 J. All properties meet the mechanical property standard requirements of steel obtained by the "offline normalizing process".

[0051] As can be seen from the results in Table 2 (continued), the mechanical and technological properties of the large-size low-alloy structural steel round steel obtained from Comparative Examples 1 to 10 cannot simultaneously meet the requirements of the national standards.

[0052] In summary, by utilizing the residual heat after rolling of low-alloy high-strength structural steel round rolled pieces with a diameter of approximately 200 mm, and controlling the following steps—namely, maintaining the steel's temperature on the upper cooling bed at at least approximately 950°C, the temperature in the air-cooling zone at approximately 850 to approximately 870°C, cooling it at a rate of approximately 0.7 to approximately 0.8°C / s to approximately 550 to approximately 600°C, then shutting off the air-cooling fan, cooling it at a rate of approximately 0.3 to approximately 0.4°C / s to the lower cooling bed temperature (approximately 400 to approximately 450°C), and then allowing it to cool slowly in the pit for approximately 24 hours before removing it—the physical quality of the steel and its mechanical and technological properties can be guaranteed to meet the requirements of GB / T1591 standard. Therefore, the "online heat treatment method for rolled pieces" of this invention can replace the existing "offline normalizing process," achieving the expected results of reducing process costs (saving approximately 400 RMB / ton of steel compared to the existing "offline normalizing process") and shortening the production cycle (eliminating the at least three days required to produce one batch of steel using the existing "offline normalizing process") while ensuring comparable mechanical properties.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions claimed by the present invention.

Claims

1. An online heat treatment method for low-alloy structural steel circular rolled pieces with a diameter of 200 mm or more, characterized in that, The online heat treatment method includes cooling a round low-alloy structural steel workpiece with a diameter of 200 mm or more in its post-rolled red-hot state at a temperature ranging from 850°C to 870°C in the following cooling sequence: Air-cooling step (1): The low-alloy structural steel round rolled pieces with a diameter of 200 mm or more, which are in a red-hot state after rolling and have a temperature in the range of 850℃ to 870℃, are cooled to a temperature of 550 to 600℃ by air-cooling at a cooling rate in the range of 0.7 to 0.8℃ / second; and Air cooling step (2): The low alloy structural steel round rolled piece with a diameter of more than 200 mm, which has been cooled to a temperature of 400 to 450°C by air cooling at a cooling rate of 0.3 to 0.4°C / second, is cooled to a temperature of 550°C to 600°C after the air cooling step (1).

2. The online heat treatment method according to claim 1, characterized in that, The low-alloy structural steel round rolled pieces with a diameter of 200 mm or more in the post-rolling red-hot state at a temperature range of 850°C to 870°C are obtained through the following air-cooling process: Air cooling step (0): The low alloy structural steel round rolled piece with a diameter of more than 200 mm and a temperature of more than 950℃ obtained after rolling is cooled to a temperature of 850 to 870℃ by air cooling on a cooling bed at a cooling rate of 0.4 to 0.5℃ / second.

3. The online heat treatment method according to claim 1 or 2, characterized in that, The low-alloy structural steel comprises, based on the total weight of the low-alloy structural steel: 0.020 to 0.060 wt% of element Al, and The remaining element is Fe, as well as other unavoidable elements.

4. The online heat treatment method according to claim 3, characterized in that, Based on the total weight of the low-alloy structural steel, the total amount of elements other than Fe contained in the low-alloy structural steel is less than 5% by weight, preferably less than 3% by weight.

5. The online heat treatment method according to claim 3, characterized in that, Based on the total weight of the low-alloy structural steel, the low-alloy structural steel contains other elements that are unavoidable. The content of element C is ≤ 0.22% by weight, preferably ≤ 0.16% by weight. The content of element Si is ≤ 0.55% by weight, preferably ≤ 0.30% by weight. The content of element Mn is ≤ 1.60% by weight, preferably ≤ 1.32% by weight. The content of element Cr is ≤ 0.40% by weight, preferably ≤ 0.33% by weight. The content of element P is ≤ 0.025% by weight, preferably ≤ 0.012% by weight. The content of element S is ≤ 0.025% by weight, preferably ≤ 0.005% by weight. The content of element Ni is ≤ 0.30% by weight, preferably ≤ 0.02% by weight. The content of element Mo is ≤ 0.08% by weight, preferably ≤ 0.02% by weight. The content of element Cu is ≤ 0.30% by weight, preferably ≤ 0.02% by weight, and / or The content of element N is ≤ 0.012% by weight, preferably ≤ 0.010% by weight.

6. The online heat treatment method according to claim 5, characterized in that, The CEV of the low-alloy structural steel is controlled to be ≤ 0.49% by weight, wherein... CEV = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15, In the above formula, the element symbol enclosed in square brackets indicates the element's weight percentage content in the low-alloy structural steel.

7. The online heat treatment method according to claim 1 or 2, characterized in that, The online heat treatment method further includes the following cooling steps: Slow cooling pit slow cooling step (3): The low alloy structural steel round rolled piece, which has been cooled to 400 to 450°C by the air cooling step (2), is put into the slow cooling pit for slow cooling, and cooled to a temperature below 200°C at a cooling rate of 10 to 10.5°C / hour.

8. A round low-alloy structural steel bar with a diameter of 200 mm or more, obtained by the online heat treatment method according to any one of claims 1 to 7, characterized in that... The round steel material has one or more of the following properties: The yield strength is ≥ 285 MPa, preferably ≥ 295 MPa, and more preferably ≥ 300 MPa. The tensile strength is in the range of 450 to 600 MPa, preferably in the range of 516 to 535 MPa. The elongation after fracture is ≥ 17%, preferably ≥ 29%. The minimum impact absorption energy at -40℃ is ≥ 27 J, preferably ≥ 30 J, more preferably ≥ 40 J, and The hardness is in the range of 200 to 220 HBW, preferably in the range of 205 to 210 HBW.

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