15CrNiMo bar with reduced hot rolling hardness and preparation method of 15CrNiMo bar

Abstract

The invention relates to the field of metallurgy, in particular to the field of preparation of special steel bars. The invention particularly provides a preparation method of a 15CrNiMo bar with reduced hot rolling hardness and the bar prepared by the method. According to the preparation method, control over the hot rolling hardness of the 15CrNiMo bar is achieved through temperature control over the rolling process and the follow-up cooling process, the preparation method is simple and easy to implement and convenient to popularize, and the hot rolling hardness of the prepared 15CrNiMo bar ranges from 180 HBW to 200 HBW, namely the hardness level after off-line annealing is achieved. According to the method, the long-process off-line annealing procedure needed before the 15CrNiMo bar is applied is avoided, carbon emission is reduced, the production and manufacturing cost of the 15CrNiMo bar is greatly reduced, and the comprehensive yield of the 15CrNiMo bar is increased.

Description

Technical Field

[0001] This invention relates to the field of metallurgy, specifically to the field of special steel bar preparation. In particular, this invention provides a method for preparing 15CrNiMo bars with reduced hot-rolled hardness and the bars prepared by this method. Background Technology

[0002] 15CrNiMo is a high-strength alloy steel. The Chinese national standard GB / T 3077-2015, "Alloy Structural Steel," specifies its chemical composition by mass percentage as follows: C: 0.13-0.18%; Si: 0.17-0.37%; Mn: 0.70-0.90%; Cr: 0.45-0.65%; Mo: 0.45-0.60%; Ni: 0.70-1.00%; P≤0.025%; S≤0.020%; the remainder being Fe, totaling 100% by mass. 15CrNiMo contains a high proportion of nickel and molybdenum, which significantly improves the steel's hardness, wear resistance, and corrosion resistance. Due to its high hardness and excellent mechanical properties and wear resistance, 15CrNiMo steel is particularly suitable for manufacturing high-load mechanical parts such as gears, balance wheels, crankshafts, and bearings, as well as components operating in harsh environments, such as oil drill pipes, drill collars, and drill bits. Furthermore, 15CrNiMo steel exhibits high thermal strength and oxidation resistance at high temperatures, and possesses a certain degree of resistance to hydrogen corrosion. Therefore, it is widely used in the petroleum, petrochemical, and high-pressure boiler industries. Because the hot-rolled hardness of typical 15CrNiMo steel is relatively high, it is necessary to pre-process the raw material with softening annealing to reduce its hot-rolled hardness, for example, to the range of approximately 180 to 200 HBW, before use in the manufacture of industrial parts, to facilitate subsequent machining.

[0003] Chinese utility model patent CN219253673U, entitled "Intelligent Waste Heat Online Heat Treatment Device for Special Steel Bars," discloses an online heat treatment device capable of performing online heat treatment on cooled rolled products. However, this utility model patent does not disclose the specific type of special steel bar or the process for reducing hot-rolled hardness.

[0004] There are no existing technology reports on a process for reducing the hot-rolled hardness of 15CrNiMo online. Summary of the Invention

[0005] Purpose of the invention

[0006] In view of the problems existing in the prior art described in the background section above, the object of the present invention is to provide a method for preparing 15CrNiMo bars with reduced hot rolling hardness and the bars prepared by the method.

[0007] Technical solution

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] Option 1: A method for preparing 15CrNiMo bars having a hot-rolled hardness in the range of about 180 to about 200 HBW, wherein the method includes subjecting the 15CrNiMo bars obtained after a rolling process to a cooling process, the temperature of the 15CrNiMo bars obtained after the rolling process being in the range of about 700 to about 800°C, the cooling process including a cooling bed cooling process and a slow cooling pit cooling process performed sequentially, wherein...

[0010] The cooling bed cooling process includes cooling the 15CrNiMo bar obtained after the rolling process on a cooling bed at a cooling rate of about ≤8°C / min to about 550 to about 600°C, and then air cooling on the cooling bed to about 400°C to about 450°C; and

[0011] The slow cooling pit cooling process includes placing the 15CrNiMo rod obtained after the cooling bed cooling process into a slow cooling pit at a temperature of approximately 400°C to approximately 450°C, and then cooling it at approximately...

[0012] Cool slowly to below approximately 100°C at a cooling rate of ≤10°C / hour.

[0013] Option 2: According to the preparation method described in Option 1 above, the 15CrNiMo rod comprises, based on the total weight of the 15CrNiMo rod,

[0014] Approximately 0.13 to approximately 0.18% by weight of carbon;

[0015] About 0.17 to about 0.37 wt% of Si element;

[0016] Mn element, approximately 0.70 to approximately 0.90 wt%;

[0017] Approximately 0.45 to approximately 0.65% by weight of Cr element;

[0018] Mo element: about 0.45 to about 0.60 wt%;

[0019] Ni element, approximately 0.70 to approximately 1.00 wt%;

[0020] The remainder consists of Fe and unavoidable impurities.

[0021] The amount of phosphorus (P), which is an unavoidable impurity, will be controlled to below approximately 0.025% by weight.

[0022] The sulfur content is controlled to be below approximately 0.020% by weight.

[0023] Option 3: According to the preparation method described in Option 1 or 2 above, the rolling process includes sequentially subjecting a 15CrNiMo billet obtained after a descaling process at a temperature in the range of about 980 to about 1050°C to a roughing process, a continuous rolling process, and a finishing rolling process to obtain 15CrNiMo bars.

[0024] Option 4: According to the preparation method described in Option 3 above, the inlet temperature of the roughing process is in the range of about 980 to about 1050°C, and the number of roughing passes is 5, 6 or 7.

[0025] Option 5: The preparation method according to Option 3 or 4 above, wherein the continuous rolling process includes sequentially subjecting the 15CrNiMo billet obtained after the roughing process to an intermediate rolling process and a pre-finishing rolling process, wherein...

[0026] The entry temperature of the intermediate rolling process is in the range of approximately 900 to approximately 950°C, and the intermediate rolling passes are 5, 6, or 7; and

[0027] The exit temperature of the pre-finishing rolling process is in the range of about 900 to about 950°C, and the number of pre-finishing rolling passes is 3, 4 or 5.

[0028] Option 6: The preparation method according to any one of Options 3 to 5 above, wherein the finishing rolling process includes subjecting the 15CrNiMo round steel obtained after the continuous rolling process to a sizing mill for finishing rolling, wherein the inlet temperature of the sizing mill for finishing rolling is in the range of about 700 to about 800°C.

[0029] Option 7: The preparation method according to any one of Options 3 to 6 above, wherein the finishing rolling process includes subjecting the 15CrNiMo round steel that has undergone the continuous rolling process to finishing rolling on a KOCKS reducing sizing mill.

[0030] Option 8: The preparation method according to any one of Options 3 to 7 above, wherein the descaling process includes descaling the 15CrNiMo continuous casting billet obtained after the heating process with high-pressure water in the range of about 18 to about 20 MPa.

[0031] Option 9: The preparation method according to Option 7 above, wherein the heating process includes sequentially subjecting the 15CrNiMo continuous casting billet to optional preheating, heating, and soaking stages, wherein...

[0032] The optional preheating section heating includes holding the 15CrNiMo continuous casting billet at a temperature of about 550 to about 700°C for a period of about 180 minutes or more.

[0033] The heating section includes heating the 15CrNiMo continuous casting billet to a temperature of about 1180 to about 1200°C, and the total heating time of the heating section is about 240 minutes or more;

[0034] The soaking zone heating includes holding the 15CrNiMo continuous casting billet, which has undergone heating in the heating zone, at a temperature of about 1200 to about 1250°C for about 30 to about 90 minutes, and

[0035] Option 10: A 15CrNiMo bar prepared by any one of the preparation methods of Options 1 to 9 above, wherein the 15CrNiMo bar has a hot-rolled hardness in the range of about 180 to about 200 HBW.

[0036] Option 11: The 15CrNiMo bar according to Option 10 above, wherein the 15CrNiMo bar has one or more of the following properties:

[0037] The equivalent diameter of the cross-section of the 15CrNiMo bar is in the range of approximately 50 to approximately 130 mm; and / or

[0038] The hot-rolled microstructure of the 15CrNiMo bar is ferrite + pearlite; and / or

[0039] The 15CrNiMo bar has a tensile strength of approximately ≥985 MPa, and / or

[0040] The 15CrNiMo bar has a yield strength of approximately ≥790 MPa, and / or

[0041] The 15CrNiMo bar has an elongation at break of approximately ≥15%, and / or

[0042] The 15CrNiMo bar has a reduction of area of ​​approximately ≥56%, and / or

[0043] The 15CrNiMo bar has an impact absorption energy of approximately ≥56J, and / or

[0044] The 15CrNiMo rod has a grain size level of ≥8.

[0045] Technical effect

[0046] The preparation method of this invention is simple, easy to implement, and readily applicable. Through the online controlled cooling process of this invention, 15CrNiMo bars with a hot-rolled hardness in the range of 180 to 200 HBW can be directly obtained, meaning the hot-rolled hardness of the 15CrNiMo bars reaches the hardness level after offline annealing. This method not only avoids the lengthy offline annealing process required before applying 15CrNiMo bars, reducing carbon emissions, but also significantly reduces the production cost of 15CrNiMo bars and improves the overall yield. Attached Figure Description

[0047] To more clearly illustrate the technical solutions in the specific embodiments of the present invention, the accompanying drawings used in the specific embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0048] Figure 1 Metallographic images of the 15CrNiMo steel prepared in Example 1 of this invention are shown.

[0049] Figure 2 A photograph illustrating the grain size of the 15CrNiMo steel prepared in Example 1 of this invention. Detailed Implementation

[0050] 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.

[0051] 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 individual point values, and 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.

[0052] According to a first aspect of the present invention, the present invention provides a method for preparing 15CrNiMo bars, wherein the 15CrNiMo bars have a hot-rolled hardness in the range of about 180 to about 200 HBW (e.g., about 185 HBW, about 190 HBW, or about 195 HBW).

[0053] In the preparation method according to the first aspect of the present invention, the 15CrNiMo rod preferably comprises, based on the total weight of the 15CrNiMo rod,

[0054] Approximately 0.13 to approximately 0.18% by weight of carbon;

[0055] About 0.17 to about 0.37 wt% of Si element;

[0056] Mn element, approximately 0.70 to approximately 0.90 wt%;

[0057] Approximately 0.45 to approximately 0.65% by weight of Cr element;

[0058] Mo element: about 0.45 to about 0.60 wt%;

[0059] Ni element, approximately 0.70 to approximately 1.00 wt%;

[0060] The remainder consists of Fe and unavoidable impurities.

[0061] The amount of phosphorus (P), which is an unavoidable impurity, will be controlled to below 0.025% by weight.

[0062] The sulfur content was controlled to be below 0.020% by weight.

[0063] The preparation method according to the first aspect of the present invention includes subjecting a 15CrNiMo bar obtained after a rolling process to a cooling process, wherein the temperature of the 15CrNiMo bar obtained after the rolling process is in the range of about 700 to about 800°C, for example, about 720°C, about 740°C, about 760°C, or about 780°C, and the cooling process includes a cooling bed cooling process and a slow cooling pit cooling process performed sequentially. Here, since the microstructure transformation of the 15CrNiMo bar requires an initial temperature to begin cooling, and then the cooling rate needs to be controlled to reduce hardness, the cooling process includes two sequential cooling processes: a cooling bed cooling process and a slow cooling pit cooling process.

[0064] The cooling bed cooling process includes slowly cooling the 15CrNiMo bar obtained after the rolling process on a cooling bed at a cooling rate of less than about 8°C / min (e.g., about 7°C / min, about 6°C / min, or about 5°C / min) to about 550 to about 600°C, for example, about 560°C, about 570°C, about 580°C, or about 590°C, and then air cooling on the cooling bed to about 400°C to about 450°C, for example, about 410°C, about 420°C, about 430°C, or about 440°C.

[0065] Here, since the 15CrNiMo bar undergoes the transformation from austenite to ferrite and pearlite at a temperature of approximately 550 to 600°C, thereby increasing the ferrite content and reducing hardness, it is necessary to first slowly cool the 15CrNiMo bar to approximately 550 to 600°C. If the temperature is not cooled below approximately 600°C, the microstructure transformation may not be fully completed. This slow cooling is preferably carried out within a cooling bed insulation hood to ensure that the cooling rate is not too fast, for example, not exceeding 8°C / minute. Otherwise, an excessively rapid cooling rate may lead to the formation of bainite and / or martensite structures, resulting in excessively high bar hardness. Furthermore, since further slow cooling may affect the production schedule, after reaching the target temperature, air cooling on a cooling bed can be used instead of slow cooling on a cooling bed to reduce production schedule losses. Here, the air cooling bed cools the bar to a temperature of about 400°C to about 450°C, thereby ensuring that the core and surface temperatures tend to be uniform and avoiding core cracking caused by thermal stress. Excessively high cooling temperatures (e.g., exceeding about 450°C) may also lead to poor consistency of hot-rolled hardness at various locations of the obtained bar.

[0066] The slow cooling pit cooling process involves placing the 15CrNiMo rods obtained from the cooling bed cooling process into a slow cooling pit at a temperature of approximately 400°C to approximately 450°C, and then slowly cooling them to below approximately 100°C at a cooling rate of less than approximately 10°C / hour (e.g., approximately 9°C / hour, approximately 8°C / hour, or approximately 7°C / hour). Here, the temperature of the rods placed in the slow cooling pit should not be too low, for example, not below 400°C, otherwise thermal stress cracks may occur in the core of the rods. Furthermore, the cooling rate in the slow cooling pit should not exceed approximately 10°C / hour, the purpose of which is to homogenize the hardness of the entire cross-section through low-temperature tempering. Additionally, the final temperature reached by the 15CrNiMo rods after cooling in the slow cooling pit needs to be below 100°C, for example, it can be cooled to room temperature.

[0067] In some preferred embodiments of the preparation method according to the first aspect of the present invention, the rolling process includes sequentially subjecting a 15CrNiMo billet, obtained after a descaling process at a temperature in the range of about 980 to about 1050°C (e.g., about 1000°C, about 1020°C, or about 1040°C), to a roughing process, a continuous rolling process, and a finishing process to obtain 15CrNiMo bars.

[0068] In a preferred embodiment of the roughing rolling process, the inlet temperature of the roughing rolling process is controlled within the range of approximately 980 to approximately 1050°C. Here, the inlet temperature of the roughing rolling process should not be too high, for example, not higher than approximately 1050°C, otherwise it may affect the temperature control of subsequent processes, resulting in the inlet temperature of the finishing rolling process not meeting the standard; nor should it be too low, for example, not lower than approximately 980°C, otherwise the temperature is too low, which increases the deformation resistance of the 15CrNiMo billet, making it unable to bite into the rolling, thereby causing the temperature of the subsequent rolling process to rise and making it impossible to reach the optimal temperature for controlling hardness.

[0069] In another preferred embodiment of the roughing rolling process, the total reduction in the roughing rolling process can be in the range of about 40% to about 60%, for example, about 50%, and the number of roughing rolling passes can be set to 5 to 7, preferably 6. Here, too many passes (e.g., more than 7 passes) will result in some passes having no reduction, and / or insufficient reduction in a single pass, leading to core defects; conversely, too few passes (e.g., less than 5 passes) will result in uneven dimensional rolling. Therefore, a 6-pass reduction distribution is most reasonable.

[0070] The continuous rolling process includes sequentially subjecting the 15CrNiMo billet obtained after the roughing process to intermediate rolling and pre-finishing rolling processes after cooling.

[0071] In a preferred embodiment of the intermediate rolling process, the inlet temperature of the intermediate rolling process is 900 to 950°C, for example, about 910°C, about 920°C, about 930°C, or about 940°C. Here, the inlet temperature of the continuous rolling process should not be too high, for example, not higher than about 950°C, otherwise it may affect the temperature control of subsequent processes, resulting in the finishing rolling inlet temperature not meeting the standard; nor should it be too low, for example, not lower than about 900°C, otherwise the excessively low temperature will increase the deformation resistance of the 15CrNiMo billet, causing it to be unable to bite into the rolling, thereby causing the temperature of subsequent rolling processes to rise and failing to reach the optimal temperature for hardness control.

[0072] In a preferred embodiment, the total reduction in the intermediate rolling process can be in the range of about 20% to about 30%, for example, about 25%, and the number of intermediate rolling passes can be set to 5 to 7 passes, preferably 6 passes. Here, too many passes (e.g., more than 7 passes) will result in some passes having no reduction, and / or insufficient reduction in a single pass, leading to core defects; conversely, too few passes (e.g., less than 5 passes) will result in uneven dimensional rolling. Therefore, a 6-pass reduction distribution is most reasonable.

[0073] During the intermediate rolling pass and pre-finishing rolling process, the billet temperature hardly drops. Therefore, the exit temperature of the pre-finishing rolling process is still in the range of 900 to about 950°C, for example, about 910°C, about 920°C, about 930°C or about 940°C. Here, the exit temperature of the pre-finishing rolling process should not be too high, for example, it should not be higher than about 950°C or lower than about 900°C. Too high or too low a temperature will result in the finishing rolling temperature being too high or too low, causing the initial cooling temperature of the subsequent cooling bed to be substandard.

[0074] In a preferred embodiment, the total reduction in the pre-finishing rolling process can be in the range of about 30% to about 40%, for example, about 35%, and the number of pre-finishing rolling passes can be set to 3 to 5 passes, preferably 4 passes. Here, since the main function of pre-finishing rolling is to ensure the dimensional ovality of the 15CrNiMo round steel, 3 to 5 passes can evenly distribute the reduction, ensuring that the ovality meets the requirements.

[0075] After roughing and continuous rolling, the continuously cast billet is cooled (e.g., water-cooled) to form 15CrNiMo round steel suitable for subsequent finishing rolling.

[0076] In a preferred embodiment of the finishing rolling process, the inlet temperature of the finishing rolling process is controlled within the range of about 700 to about 800°C, for example, about 720°C, about 740°C, about 760°C, or about 780°C. Here, the inlet temperature of the finishing rolling process should not be too high, for example, it should not be higher than about 800°C, otherwise the initial cooling temperature of the cooling bed will be too high, affecting production efficiency; nor should it be too low, for example, it should not be lower than about 700°C, otherwise the initial cooling temperature of the subsequent cooling bed will not meet the standard.

[0077] In another preferred embodiment of the finishing rolling process, the finishing rolling process can be finishing rolling on a reducing sizing mill, preferably a KOCKS reducing sizing mill. Reducing sizing mill finishing rolling applies triaxial compressive stress, ensuring better dimensional tolerances, and also facilitates control of the finishing rolling temperature to allow for the precipitation of more ferrite.

[0078] In some preferred embodiments of the preparation method according to the first aspect of the present invention, the descaling process includes descaling the 15CrNiMo continuous casting billet obtained after the heating process with high-pressure water at a pressure in the range of about 18 to about 20 MPa (e.g., about 19 MPa).

[0079] Here, the descaling process is to remove the iron oxide scale from the surface of the continuously cast billet obtained after the heating process. Specifically, after the continuously cast billet undergoes the heating process, when it passes the descaling point, high-pressure water generated by the high-pressure pump station is sprayed onto the surface of the continuously cast billet through nozzles in the form of a high-speed jet. Under the action of water flow erosion, peeling, and thermal explosion effect, the iron oxide scale on the surface of the continuously cast billet quickly falls off its surface, and the descaling work is completed at this time. In the descaling process of the present invention, the pressure of the high-pressure water is in the range of about 18 to about 20 MPa. Here, the pressure of the high-pressure water used should not be too high, for example, it should not be greater than about 20 MPa. If the pressure is too high, it may cause the surface temperature of the continuously cast billet to be too low, the deformation resistance to be strong, and it will not be able to bite into the rolling mill; it should also not be too low, for example, it should not be less than about 18 MPa. If the pressure is too low, it may cause the iron oxide scale on the surface of the continuously cast billet to be incompletely removed, which will be pressed into the surface of the bar during subsequent rolling, resulting in surface defects.

[0080] In some preferred embodiments of the preparation method according to the first aspect of the present invention, the heating process includes subjecting the 15CrNiMo continuous casting billet to optional preheating, heating, and soaking in sequence.

[0081] In the heating process, the optional preheating stage includes holding the 15CrNiMo continuous casting billet at a temperature of about 550 to about 700°C (e.g., about 600°C or about 650°C) for more than 180 minutes to make the core and surface temperatures of the 15CrNiMo continuous casting billet tend to be uniform and to avoid core defects caused by thermal stress.

[0082] In the heating process, the heating section includes heating the 15CrNiMo continuous casting billet, or optionally the 15CrNiMo continuous casting billet that has undergone the preheating section, to a temperature ≤1200°C, and the total heating time of the heating section is approximately 240 minutes or more. Here, the temperature reached in the heating section should not be too high, for example, not exceeding approximately 1200°C, otherwise overheating defects may occur in the continuous casting billet; however, it should also not be too low, for example, not below approximately 1180°C, otherwise it may affect the grain size of the continuous casting billet, leading to coarse grains. The heating section time should not be too short, for example, not less than approximately 240 minutes, otherwise it will affect the rolling dimensions and the uniformity of the microstructure.

[0083] In the heating process, the soaking zone heating includes holding the 15CrNiMo continuously cast billet, which has undergone soaking in the heating zone, at a temperature of approximately 1200 to approximately 1250°C (e.g., approximately 1210°C, approximately 1220°C, approximately 1230°C, or approximately 1240°C) for approximately 30 to approximately 90 minutes (e.g., approximately 40 minutes, approximately 50 minutes, approximately 60 minutes, approximately 70 minutes, or approximately 80 minutes). Here, the temperature of the soaking zone should not be too high, for example, not exceeding approximately 1250°C, otherwise the rolling process temperature will be too high; nor should it be too low, for example, not below approximately 1200°C, otherwise it will affect the tapping temperature, and thus affect the rolling process temperature. The soaking zone time should not be too long, for example, not exceeding 90 minutes, otherwise it will waste energy and affect the production rhythm; nor should it be too short, for example, not less than approximately 30 minutes, otherwise it will affect the rolling dimensions and microstructure uniformity.

[0084] In a preferred embodiment of the preparation method of the first aspect of the present invention, the heating process can be carried out in a four-stage walking beam furnace. The segmented heating of the four-stage walking beam furnace allows for individual control of the heating rate of a specific segment, precisely controlling the tapping temperature and facilitating temperature control in the subsequent rolling process.

[0085] According to a second aspect of the present invention, the present invention provides a 15CrNiMo bar prepared by the preparation method of the first aspect of the present invention as described above, the 15CrNiMo bar having a hot-rolled hardness in the range of about 180 to about 200 HBW, for example about 185 HBW, about 190 HBW or about 195 HBW.

[0086] In a preferred embodiment of the 15CrNiMo bar material of the second aspect of the present invention described above, the 15CrNiMo bar material has one or more of the following properties:

[0087] The microstructure in the hot-rolled state is ferrite + pearlite; and / or

[0088] Tensile strength of approximately ≥985 MPa, and / or

[0089] Yield strength of approximately ≥790 MPa, and / or

[0090] Elongation at break of approximately ≥15%, and / or

[0091] Reduction of area of ​​approximately ≥56%, and / or

[0092] Impact absorption energy of approximately ≥56J, and / or

[0093] Grain size level ≥ 8.

[0094] Here, the above-mentioned performance can be measured according to conventional methods in the field, for example, with reference to Chinese National Standard GB / T3077-2015.

[0095] In a preferred embodiment of the 15CrNiMo bar material of the second aspect of the present invention, the equivalent diameter of the cross-section of the bar material may be in the range of about 50 to about 130 mm, for example, about 70 mm, about 90 mm or about 110 mm.

[0096] Furthermore, the preferred technical features described in the first aspect of the present invention for the various technical features of the preparation method are also applicable to the corresponding technical features in the 15CrNiMo bar material technical solution described in the second aspect of the present invention.

[0097] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0098] Example 1: Preparation of 15CrNiMo rods

[0099] A 15CrNiMo continuous casting billet with a cross-sectional dimension of approximately 300 × 400 mm is provided, the total weight of which is based on the 15CrNiMo continuous casting billet.

[0100] Approximately 0.172% by weight of C;

[0101] Approximately 0.251% by weight of Si;

[0102] Approximately 0.786% by weight of Mn;

[0103] Approximately 0.552% by weight of Cr;

[0104] Approximately 0.556% by weight of Mo;

[0105] Approximately 0.851% by weight of Ni;

[0106] Approximately 0.015% by weight of P;

[0107] Approximately 0.012% by weight of S;

[0108] The remainder consists of Fe and unavoidable impurities.

[0109] After the 15CrNiMo continuously cast billet is cut into ingots approximately 5 to 9 meters long, it is placed into a heating furnace for heating using either cold or hot feeding methods. It then undergoes a series of processes including descaling, rolling, and cooling.

[0110] The heating process includes using a four-stage walking beam furnace to sequentially subject the continuously cast billet to four stages: a preheating stage of approximately 180 minutes at approximately 700°C, a first-stage heating stage of approximately 1030°C, a second-stage heating stage of approximately 1200°C, and a soaking stage of approximately 60 minutes at approximately 1220°C; the total duration of the heating process is approximately 8.5 hours.

[0111] The descaling process includes subjecting the continuously cast billet, which has undergone the heating process described above, to high-pressure water descaling at approximately 20 MPa to remove iron oxide scale.

[0112] The rolling process includes subjecting the continuously cast billet, after descaling, to a series of sequential processes: roughing, intermediate rolling, pre-finishing, water cooling, and finishing. The roughing process has an inlet temperature of approximately 1000°C and consists of 6 passes. The intermediate rolling process has an inlet temperature of 920°C and consists of 6 passes. The pre-finishing process has an outlet temperature of approximately 900°C and consists of 4 passes. The finishing process is performed using a KOCKS reduction sizing mill, with an inlet temperature of approximately 750°C. After the rolling process, the resulting bar is cut into cylindrical bars with a length of approximately 5 to 10 meters and a diameter of approximately 90 mm.

[0113] The cooling process includes sequentially subjecting the obtained bars to a cooling bed cooling process and a slow cooling pit cooling process. In the cooling bed cooling process, the bars are slowly cooled inside the cooling bed insulation cover at a rate of approximately 5°C / minute until the bar temperature reaches below approximately 600°C. Then, the bars are transferred outside the insulation cover for air cooling on the cooling bed until the bar temperature reaches approximately 400 to approximately 450°C. The slow cooling pit cooling process includes placing the bars at a temperature of approximately 400 to approximately 450°C into a slow cooling pit for slow cooling at a rate of approximately 8°C / hour until the bar temperature reaches below approximately 100°C.

[0114] Comparative Example 1: Preparation of 15CrNiMo Rods

[0115] 15CrNiMo rods were prepared using a process similar to that in Example 1, except that the cooling rate in the cooling bed cooling process was set to be greater than or equal to about 10°C / min.

[0116] Comparative Example 2: Preparation of 15CrNiMo Rods

[0117] 15CrNiMo rods were prepared using a process similar to that in Example 1, except that the final cooling temperature in the cooling bed cooling process was approximately 500°C.

[0118] The 15CrNiMo bars with a cross-sectional diameter of approximately 90 mm prepared from Examples 1 and Comparative Examples 1 and 2 were subjected to performance tests in accordance with the relevant provisions of the Chinese National Standard GB / T 3077-2015.

[0119] Depend on Figure 1 It can be seen that the hot-rolled microstructure of the 15CrNiMo bar obtained by Example 1 of the present invention is ferrite + pearlite.

[0120] The hardness and mechanical properties of the 15CrNiMo bars obtained in Example 1 and Comparative Example 1 of this invention are shown in Table 1. In addition to showing the hardness and mechanical properties of the 15CrNiMo bars obtained in Example 1 and Comparative Examples 1 and 2 of this invention, Table 1 also shows the relevant provisions for 15CrNiMo bars in the Chinese National Standard GB / T3077-2015 for comparison purposes.

[0121] Table 1

[0122]

[0123] As can be seen from the data in Table 1, the 15CrNiMo bar obtained in Example 1 of this invention not only has all mechanical properties superior to the relevant provisions in the Chinese national standard, but its hot-rolled hardness is only in the range of approximately 190 to approximately 195 HBW, reaching the hardness level of 15CrNiMo bars after offline annealing. In contrast, the mechanical properties of Comparative Example 1 are not only inferior to those of Example 1, but its hot-rolled hardness also fails to meet the national standard requirements. The 15CrNiMo bar obtained in Comparative Example 2 suffers from core cracking due to thermal stress, and exhibits significant hardness variation from the core to the surface, resulting in uneven hardness at different locations, and its overall hardness also fails to meet the national standard.

[0124] In addition, from Figure 2 As can be seen from the grain size photographs, the 15CrNiMo rods obtained in Example 1 of this invention have a grain size level of 8 or higher, which is relatively high, resulting in higher mechanical properties. In contrast, the 15CrNiMo rods obtained in Comparative Example 1 only have a grain size level of 6 or higher, but less than 8.

[0125] 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. A method for preparing 15CrNiMo bars with a hot-rolled hardness in the range of 180 to 200 HBW, characterized in that, The method includes subjecting 15CrNiMo bars, which are heated to a temperature of 700 to 800°C after rolling, to a cooling process. This cooling process comprises a sequential cooling bed cooling process and a slow cooling pit cooling process. The cooling bed cooling process includes cooling the 15CrNiMo bar obtained after the rolling process to 550 to 600°C on a cooling bed at a cooling rate of ≤8°C / min, and then air cooling on the cooling bed to 400 to 450°C; and The slow cooling pit cooling process includes placing the 15CrNiMo bar obtained after the cooling bed cooling process into the slow cooling pit at a temperature of 400°C to 450°C, and then slowly cooling it to below 100°C at a cooling rate of ≤10°C / hour.

2. The preparation method according to claim 1, characterized in that, The 15CrNiMo bar includes the total weight based on the 15CrNiMo bar. 0.13 to 0.18 wt% of C element; 0.17 to 0.37 wt% Si element; 0.70 to 0.90 wt% Mn element; 0.45 to 0.65 wt% Cr element; 0.45 to 0.60 wt% of Mo; 0.70 to 1.00 wt% Ni element; The remainder consists of Fe and unavoidable impurities. The amount of phosphorus (P), which is an unavoidable impurity, will be controlled to below 0.025% by weight. The sulfur content was controlled to be below 0.020% by weight.

3. The preparation method according to claim 1 or 2, characterized in that, The rolling process includes sequentially subjecting a 15CrNiMo billet, which has been descaled and is at a temperature in the range of 980 to 1050°C, to a roughing rolling process, a continuous rolling process, and a finishing rolling process to obtain 15CrNiMo bars.

4. The preparation method according to claim 3, characterized in that, The inlet temperature of the roughing process is in the range of 980 to 1050°C, and the number of roughing passes is 5, 6 or 7.

5. The preparation method according to claim 3, characterized in that, The continuous rolling process includes sequentially subjecting the 15CrNiMo billet obtained after the roughing process to intermediate rolling and pre-finishing rolling processes, wherein... The inlet temperature of the intermediate rolling process is in the range of 900 to 950°C, and the intermediate rolling passes are 5, 6, or 7; and The exit temperature of the pre-finishing rolling process is in the range of 900 to 950°C, and the number of pre-finishing rolling passes is 3, 4 or 5.

6. The preparation method according to claim 3, characterized in that, The finishing rolling process includes subjecting the 15CrNiMo round steel obtained after the continuous rolling process to a sizing mill for finishing rolling, wherein the inlet temperature of the sizing mill is in the range of 700 to 800°C; preferably, the finishing rolling process includes subjecting the 15CrNiMo round steel obtained after the continuous rolling process to a KOCKS sizing mill for finishing rolling.

7. The preparation method according to claim 3, characterized in that, The descaling process includes using high-pressure water with a pressure in the range of 18 to 20 MPa to descal the 15CrNiMo continuous casting billet obtained after the heating process.

8. The preparation method according to claim 7, characterized in that, The heating process includes sequentially subjecting the 15CrNiMo continuous casting billet to an optional preheating section, a heating section, and a soaking section, wherein... The optional preheating section includes holding the 15CrNiMo continuous casting billet at a temperature of 550 to 700°C for more than 180 minutes. The heating section includes heating the 15CrNiMo continuous casting billet to a temperature of 1180 to 1200°C, and the total heating time of the heating section is more than 240 minutes; The soaking zone heating includes maintaining the 15CrNiMo continuous casting billet, which has undergone heating in the heating zone, at a temperature of 1200 to 1250°C for 30 to 90 minutes.

9. A 15CrNiMo bar prepared by the preparation method according to any one of claims 1 to 8, wherein the 15CrNiMo bar has a hot-rolled hardness in the range of 180 to 200 HBW.

10. The 15CrNiMo bar according to claim 9, characterized in that, The 15CrNiMo bar has one or more of the following properties: The equivalent diameter of the cross-section of the 15CrNiMo bar is in the range of 50 to 130 mm; and / or The hot-rolled microstructure of the 15CrNiMo bar is ferrite + pearlite; and / or The 15CrNiMo bar has a tensile strength of ≥985MPa, and / or The 15CrNiMo bar has a yield strength of ≥790MPa, and / or The 15CrNiMo bar has an elongation at break of ≥15%, and / or The 15CrNiMo bar has a reduction of area of ​​≥56%, and / or The 15CrNiMo bar has an impact absorption energy of ≥56J, and / or The 15CrNiMo rod has a grain size level of ≥8.

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