A heat treatment method for eliminating tempering bending of 38CrMoAl steel

Abstract

The application provides a heat treatment method for eliminating bending of 38CrMoAl steel, relates to the field of metal material heat treatment, and uses rolled material as raw material and comprises a slow cooling step, an annealing step and a quenching and tempering step, wherein the annealing step comprises: annealing the slow-cooled rolled material, the holding temperature of the annealing treatment is 900 DEG C-930 DEG C, and then the rolled material is discharged after cooling. The 38CrMoAl produced by the new heat treatment process meets the standard requirements required by subsequent deep processing after bending, eliminates the straightening process after quenching and tempering in the factory, reduces the fixed equipment investment in the factory, shortens the product delivery cycle, reduces the production cost, and improves the product competitiveness.

Description

Technical Field

[0001] This invention relates to the field of heat treatment of metallic materials, specifically to the field of steel rolling, and particularly to a heat treatment method for eliminating quenching and tempering bending of 38CrMoAl steel. Background Technology

[0002] One of the uses of 38CrMoAl steel is in the manufacture of injection molding machine screws and barrels, and the market demand is currently huge. However, after quenching and tempering, 38CrMoAl steel is very prone to bow-shaped bending. According to the quenching and tempering manufacturers, the bending rate has reached about 80%, which makes the steel unsuitable for direct machining. A straightening process is required, which prolongs the processing cycle and increases the cost by about 200 yuan / ton. This seriously restricts the mass production and application of this steel. Summary of the Invention

[0003] To address the problems in the prior art, this invention provides a heat treatment method for eliminating the bending of 38CrMoAl steel after quenching and tempering. 38CrMoAl steel treated using this method exhibits significantly refined and homogenized microstructure in the annealed state, achieving a Brinell hardness of 190-229 HBW in the annealed state and a bending degree ≤2 mm / m after quenching and tempering. This eliminates the need for straightening after quenching and tempering, significantly reducing the factory's processing cycle, lowering production costs, and enhancing product competitiveness.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] This invention provides a heat treatment method for eliminating quenching and tempering bending in 38CrMoAl steel. The heat treatment method uses rolled steel as raw material and includes a slow cooling step, an annealing step, and a quenching and tempering step.

[0006] The annealing step includes:

[0007] The rolled material after slow cooling is annealed at a holding temperature of 900℃-930℃, and then cooled before being taken out of the furnace.

[0008] Furthermore, based on the above-described technical solution of this invention, in the annealing step,

[0009] The preferred holding temperature for the annealing treatment is 905℃-915℃;

[0010] And / or, an annealing process is performed using a bogie furnace as the heating equipment;

[0011] And / or, the heating rate for raising the equipment used for heating to the holding temperature is ≤80℃ / h;

[0012] And / or, the holding time for the annealing treatment is calculated based on the diameter of the rolled material, and the holding time is ≥ (1.4-1.6)h / 100mm × diameter;

[0013] And / or, the cooling is performed by slowly cooling at ≤30℃ / h to 700℃-720℃, and then cooling in the furnace to 640℃-660℃ before being removed from the furnace.

[0014] Furthermore, based on the above-described technical solution of this invention, the conditioning step includes:

[0015] The annealed rolled material is subjected to quenching and tempering treatment at a quenching temperature of 920℃-940℃. After quenching, it is tempered to obtain quenched and tempered 38CrMoAl steel.

[0016] Furthermore, based on the above-described technical solution of this invention, in the conditioning step,

[0017] A pit furnace was used as the heating equipment for quenching and tempering treatment;

[0018] And / or, the heat preservation time at 920℃-940℃ is 4.5h-5.5h;

[0019] And / or, use a stepped heating method to raise the temperature of the equipment to 920℃-940℃;

[0020] And / or, the holding temperature for the tempering treatment is 625℃-635℃.

[0021] Furthermore, based on the above-mentioned technical solution of the present invention, the stepped heating refers to first heating the heating equipment to 620℃-640℃ at a rate of 90℃ / h-110℃ / h and holding it at that temperature for 2.5h-3.5h, then heating it to 820℃-840℃ at a rate of 90℃ / h-110℃ / h and holding it at that temperature for 2.5h-3.5h, then heating it to 920℃-940℃ at a rate of 90℃ / h-110℃ / h and holding it at that temperature, and finally oil cooling to room temperature;

[0022] And / or, the oil cooling rate is 30℃ / s-35℃ / s.

[0023] Furthermore, based on the above-mentioned technical solution of the present invention, the slow cooling step includes transferring the rolled material into a slow cooling pit for cooling.

[0024] Furthermore, the temperature of the rolled material entering the slow cooling pit is ≥670℃;

[0025] And / or, keep warm in the slow-cooling pit for more than 40 hours before removing it from the pit;

[0026] And / or, the rolled material is round steel, preferably, the diameter of the rolled material is 110mm-240mm.

[0027] Furthermore, based on the above-mentioned technical solution of the present invention, the rolled material used as raw material is obtained from steel billet through a heating step and a rolling step;

[0028] And / or, the cross-sectional dimensions of the steel billet are 410mm × 530mm;

[0029] And / or, in the heating step, the billet is heated to 1220℃-1280℃, and the total heating time is 320min-500min;

[0030] And / or, during the rolling process, the initial rolling temperature of the billet is 980℃-1150℃;

[0031] And / or, between the heating step and rolling, descaling is also included, specifically including: sending the heated billet into a high-pressure water descaling machine to remove the iron oxide scale on the surface, and the temperature of the billet after descaling is reduced so that the initial rolling temperature of the billet is lower than the temperature after heating.

[0032] Furthermore, based on the above-mentioned technical solution of the present invention, the steel billet comprises, by mass percentage: C 0.35%-0.42%; Si 0.20%-0.45%; Mn 0.30%-0.60%; Cr 1.35%-1.65%; Al 0.70%-1.10%; Mo 0.18%-0.25%; V≤0.10%; Ni≤0.30%; Cu≤0.25%; P≤0.025%; S≤0.025%; with the balance being Fe and impurities unavoidable in the smelting process.

[0033] Furthermore, based on the above-described technical solution of this invention, the following steps are included more specifically:

[0034] S1: Heating, heat the steel billet to 1220℃-1280℃, heat for 320min-500min and then remove it from the furnace;

[0035] S2: Rolling, the heated steel billet is transferred to the rolling mill for rolling. The initial rolling temperature of the steel billet is 980℃-1150℃ to obtain rolled material;

[0036] S3: Slow cooling, the rolled material is transferred into a slow cooling pit to cool down, and then removed from the pit after being kept at the temperature for 40 hours;

[0037] S4: Annealing. After slow cooling, the rolled material is transferred to a bogie hearth for annealing. The temperature is increased to 900℃-930℃ at a rate of ≤80℃ / h. The temperature is held for a time of ≥1.5h / 100mm×diameter. After the holding time is completed, the temperature is slowly cooled to 710℃ at a rate of ≤30℃ / h. The material is then cooled to 650℃ in the furnace before being taken out of the furnace.

[0038] S5: Quenching and tempering. The annealed rolled material is transferred to a pit furnace and heated to 630℃ at a rate of 100℃ / h and held for 3 hours. Then it is heated to 830℃ at a rate of 100℃ / h and held for 3 hours. Then it is heated to 930℃ at a rate of 100℃ / h and held for 5 hours. After oil cooling, it is heated to 630℃ at a rate of ≤100℃ / h and held for 8 hours. Then it is air cooled to obtain quenched and tempered 38CrMoAl steel.

[0039] The present invention also provides a 38CrMoAl steel obtained by the heat treatment method described above for eliminating the bending of 38CrMoAl steel after quenching and tempering. The 38CrMoAl steel after annealing meets the Brinell hardness of 190-229 HBW, and the quenched and tempered 38CrMoAl steel meets the standard of bending degree ≤2mm / m.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] This invention provides a heat treatment method suitable for eliminating bending during tempering of 38CrMoAl steel. This method adjusts the annealing temperature of the steel to 900℃-930℃, causing recrystallization annealing, re-austenitization, and slow cooling to obtain an equilibrium microstructure. This reduces the steel's hardness, eliminates internal stress, and significantly improves the uniformity of the grain size in the cross-section of the hot-rolled steel. It solves the problem of quenching bending caused by uneven microstructure and grain size parallel to the rolling direction. The annealed steel meets a Brinell hardness of 190-229 HBW, and the annealed 38CrMoAl steel, after tempering, meets the bending degree standard of ≤2mm / m, satisfying the requirements for subsequent deep processing. This eliminates the need for straightening after tempering in the factory, reduces investment in fixed equipment, shortens the finished product delivery cycle, lowers production costs (approximately 200 RMB / ton), and improves product competitiveness. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments 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 illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0043] 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. 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.

[0044] A first aspect of the present invention provides a heat treatment method for eliminating heat-induced bending in 38CrMoAl steel, the heat treatment method using rolled steel as raw material, comprising a slow cooling step, an annealing step, and a heat treatment step, wherein...

[0045] The annealing step includes:

[0046] The rolled material after slow cooling is annealed at a holding temperature of 900℃-930℃ (e.g., 905℃, 910℃, 915℃, 920℃, 925℃, etc.), and then cooled before being taken out of the furnace.

[0047] In the existing technology, stress-relief annealing is conventionally carried out at 630℃, 650℃ or 680℃, which can only reduce hardness, eliminate dislocations and internal stress, but the annealing structure does not change significantly and still retains the original banded structure.

[0048] The annealing holding temperature of the present invention is maintained at 900℃-930℃, preferably 905℃-915℃, because at 910℃, the material will undergo recrystallization annealing, and the material's structure will be re-austenitic, thereby reducing hardness, eliminating internal stress, and significantly improving the banded structure in the hot-rolled state, thus solving the problem of quenching bending caused by the banded structure parallel to the rolling direction.

[0049] Annealing temperatures below 900℃ will result in insufficient austenitization of the steel structure, making it impossible to obtain an equilibrium structure and thus reducing the hardness of the steel. Annealing temperatures above 930℃ will affect production capacity and energy consumption. The higher the heating temperature, the longer the temperature drop time, and the longer the overall production time.

[0050] By adjusting the annealing temperature of the steel to 900℃-930℃, preferably 905℃-915℃, recrystallization annealing and re-austenitization occur, followed by slow cooling to obtain an equilibrium microstructure. This reduces the steel's hardness, eliminates internal stress, and significantly improves the uniformity of the grain size in the cross-section of the hot-rolled steel. It also solves the problem of quenching bending caused by uneven microstructure and grain size parallel to the rolling direction. This results in the steel meeting a Brinell hardness of 190-229 HBW after annealing. Simultaneously, the annealed 38CrMoAl, after tempering, meets the bending requirement of ≤2mm / m, satisfying the standards required for subsequent deep processing. This eliminates the need for straightening after tempering in the factory, reduces investment in fixed equipment, shortens the finished product delivery cycle, lowers production costs (approximately 200 RMB / ton), and enhances product competitiveness.

[0051] In the above heat treatment method, as an optional approach, in the annealing step,

[0052] The preferred holding temperature for the annealing treatment is 905℃-915℃;

[0053] And / or, an annealing process is performed using a bogie furnace as the heating equipment;

[0054] And / or, the heating rate for raising the equipment used for heating to the holding temperature is ≤80℃ / h;

[0055] And / or, the holding time for the annealing treatment is calculated based on the diameter of the rolled material, and the holding time is ≥ (1.4-1.6)h / 100mm × diameter;

[0056] And / or, the cooling is performed by slowly cooling at ≤30℃ / h to 700℃-720℃ (e.g., 705℃, 710℃, 715℃, etc.), and then cooling in the furnace to 640℃-660℃ (e.g., 645℃, 650℃, 655℃, etc.) before being removed from the furnace.

[0057] Specifically, the heating rate of the equipment used for heating to the holding temperature should not be too fast, otherwise excessive thermal stress will cause core stress cracks.

[0058] Specifically, choosing the aforementioned stepwise slow cooling during annealing can ensure sufficient phase transformation, more uniform microstructure, and has significant advantages over the plasticity and anisotropy of steel.

[0059] In the above heat treatment method, as an optional approach, the tempering step includes:

[0060] The annealed rolled material is subjected to quenching and tempering treatment at a quenching temperature of 920℃-940℃ (e.g., 925℃, 930℃, 935℃, etc.). After quenching, it is tempered to obtain quenched and tempered 38CrMoAl steel.

[0061] In the above heat treatment method, as an optional approach, in the tempering step,

[0062] A pit furnace was used as the heating equipment for quenching and tempering treatment;

[0063] And / or, the heat preservation time at 920℃-940℃ is 4.5h-5.5h (e.g., 4.7h, 5.0h, 5.2h, etc.);

[0064] And / or, use a stepped heating method to raise the temperature of the equipment to 920℃-940℃;

[0065] And / or, the holding temperature for the tempering treatment is 625℃-635℃.

[0066] In the above heat treatment method, as an optional approach, the stepped heating refers to first heating the heating equipment to 620℃-640℃ (e.g., 625℃, 630℃, 635℃, etc.) at a rate of 90℃ / h-110℃ / h (e.g., 95℃ / h, 100℃ / h, 105℃ / h, etc.) and holding it at that temperature for 2.5h-3.5h (e.g., 2.7h, 3.0h, 3.2h, etc.), then heating it to 820℃-840℃ (e.g., 825℃, 830℃, 835℃, etc.) at a rate of 90℃ / h-110℃ / h and holding it at that temperature for 2.5h-3.5h, then heating it to 920℃-940℃ at a rate of 90℃ / h-110℃ / h and holding it at that temperature, and finally oil cooling to room temperature;

[0067] And / or, the oil cooling rate is 30℃ / s-35℃ / s (e.g., 31℃ / s, 32℃ / s, 33℃ / s, 34℃ / s, etc.).

[0068] Specifically, the use of a pit furnace in the quenching and tempering process occupies a small area and is more suitable for small-scale production; furthermore, the use of a stepped heating method helps to ensure that the various components in the steel are heated more fully.

[0069] In the above heat treatment method, as an optional approach, the slow cooling step includes transferring the rolled material into a slow cooling pit for cooling;

[0070] Furthermore, the temperature at which the rolled material enters the slow cooling pit is ≥670℃ (e.g., 700℃, 710℃, 720℃, 730℃, etc.);

[0071] And / or, keep warm in the slow cooling pit for more than 40 hours (e.g., 45 hours, 50 hours, 55 hours, etc.) before removing it from the pit;

[0072] And / or, the rolled material is round steel, preferably, the diameter of the rolled material is 110mm-240mm (e.g., 130mm, 150mm, 170mm, 200mm, 220mm, etc.).

[0073] Specifically, the purpose of slow cooling is to prevent stress cracks in the steel and ensure the quality of the steel core and surface.

[0074] In one optional embodiment of the present invention, the rolled material used as raw material is obtained from a steel billet through a heating step and a rolling step;

[0075] And / or, the cross-sectional dimensions of the steel billet are 410mm × 530mm;

[0076] And / or, in the heating step, the billet is heated to a temperature of 1220℃-1280℃ (e.g., 1220℃, 1230℃, 1240℃, 1250℃, 1260℃, 1270℃ or 1280℃), and the total heating time is 320min-500min (e.g., 320min, 420min, 500min). The purpose of selecting the appropriate heating temperature and heating time is to ensure that the billet is fully heated without overheating.

[0077] Specifically, the total heating time includes the time difference between entering and exiting the furnace, as well as all time spent inside the furnace. It is further required that the preheating section be ≥120 min, and the sum of the heating sections and the soaking section be ≥120 min.

[0078] And / or, during the rolling step, the billet initial rolling temperature is 980℃-1150℃ (e.g., 980℃, 1000℃, 1020℃, 1040℃, 1060℃, 1080℃, 1100℃, 1120℃ or 1150℃).

[0079] And / or, between the heating step and rolling, descaling is also included, specifically including: sending the heated billet into a high-pressure water descaling machine to remove the iron oxide scale on the surface, and the temperature of the billet after descaling is reduced so that the initial rolling temperature of the billet is lower than the temperature after heating.

[0080] As an optional embodiment of the present invention, the steel billet comprises, by mass percentage: C 0.35%-0.42%; Si 0.20%-0.45%; Mn 0.30%-0.60%; Cr 1.35%-1.65%; Al 0.70%-1.10%; Mo 0.18%-0.25%; V≤0.10%; Ni≤0.30%; Cu≤0.25%; P≤0.025%; S≤0.025%; with the balance being Fe and impurities unavoidable during the smelting process.

[0081] As an optional embodiment of the present invention, the C content in the steel billet can be 0.35%, 0.37%, 0.39%, 0.40%, 0.41%, or 0.42%, etc.

[0082] As an optional embodiment of the present invention, the Si content in the steel billet can be 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, or 0.45%, etc.

[0083] As an optional embodiment of the present invention, the Mn content in the steel billet can be 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, or 0.60%, etc.

[0084] As an optional embodiment of the present invention, the Cr content in the steel billet can be 1.35%, 1.40%, 1.45%, 1.50%, 1.55%, 1.60%, or 1.65%, etc.

[0085] As an optional embodiment of the present invention, the Al content in the steel billet can be 0.70%, 0.80%, 0.90%, 1.00%, or 1.10%, etc.

[0086] As an optional embodiment of the present invention, the Mo content in the steel billet can be 0.18%, 0.19%, 0.20%, 0.21%, 0.22%, 0.23%, 0.24%, or 0.25%, etc.

[0087] More specifically, as an optional implementation, the heat treatment method of the present invention includes:

[0088] S1: Heating, heat the steel billet to 1220℃-1280℃, heat for 320min-500min and then remove it from the furnace;

[0089] S2: Rolling, the heated steel billet is transferred to the rolling mill for rolling. The initial rolling temperature of the steel billet is 980℃-1150℃ to obtain rolled material;

[0090] S3: Slow cooling, the rolled material is transferred into a slow cooling pit to cool down, and then removed from the pit after being kept at the temperature for 40 hours;

[0091] S4: Annealing. After slow cooling, the rolled material is transferred to a bogie hearth for annealing. The temperature is increased to 900℃-930℃ at a rate of ≤80℃ / h. The temperature is held for a time of ≥1.5h / 100mm×diameter. After the holding time is completed, the temperature is slowly cooled to 710℃ at a rate of ≤30℃ / h. The material is then cooled to 650℃ in the furnace before being taken out of the furnace.

[0092] S5: Quenching and tempering. The annealed rolled material is transferred to a pit furnace and heated to 630℃ at a rate of 100℃ / h and held for 3 hours. Then it is heated to 830℃ at a rate of 100℃ / h and held for 3 hours. Then it is heated to 930℃ at a rate of 100℃ / h and held for 5 hours. After oil cooling, it is heated to 630℃ at a rate of ≤100℃ / h and held for 8 hours. Then it is air cooled to obtain quenched and tempered 38CrMoAl steel.

[0093] Specifically, the holding time required in step S4 is greater than or equal to 1.5h / 100mm×diameter, in order to fully heat the steel and allow it to undergo austenitization from the inside to the surface.

[0094] The present invention also provides a 38CrMoAl steel obtained by the above heat treatment method. The 38CrMoAl steel after annealing meets the Brinell hardness of 190-229 HBW, and the 38CrMoAl steel after quenching and tempering meets the standard of bending degree ≤2mm / m.

[0095] The specific rolling method of the steel in this invention is the conventional round steel rolling method in the field. For example, it can be a 1350 reversible rolling mill for billet opening + 6 consecutive rolling. The specific rolling methods of the steel in the following embodiments and comparative examples are basically the same, with only slight differences in the opening rolling temperature. Please refer to the various embodiments and comparative examples for details.

[0096] Example 1

[0097] A heat treatment method for eliminating quenching and tempering bending in 38CrMoAl steel is disclosed. The billet cross-section is selected as 410mm × 530mm. The chemical composition of the 38CrMoAl steel billet, by mass percentage, is: C 0.40%; Si 0.31%; Mn 0.39%; Cr 1.52%; Al 0.892%; Mo 0.22%; V 0.013%; Ni 0.04%; Cu 0.02%; P 0.011%; S 0.002%, with the balance being iron and unavoidable impurities.

[0098] Heating steps: The steel billet is heated to 1241-1258℃ in the heating furnace, and the total heating time is 368min;

[0099] Rolling steps: The initial rolling temperature of the billet after descaling is 1101℃;

[0100] Slow cooling step: For finished products with a diameter of 135mm, the rolled parts are quickly fed into the slow cooling pit. The temperature of the rolled parts is 670-695℃ when they enter the slow cooling pit, and the slow cooling time is 41 hours and 23 minutes.

[0101] Annealing steps: After entering the annealing furnace, heat to 910℃ at a rate of ≤80℃ / h, hold at 905-915℃ for 3 hours, and then slowly cool to 710℃ at a rate of ≤30℃ / h. Finally, cool to 650℃ in the furnace before removing from the furnace.

[0102] The quenching and tempering process is as follows: heat to 630℃ at a rate of 100℃ / h and hold for 3 hours, then heat to 830℃ at a rate of 100℃ / h and hold for 3 hours, then heat to 930℃ at a rate of 100℃ / h and hold for 5 hours, followed by oil cooling, then heat to 630℃ at a rate of 100℃ / h and hold for 8 hours, followed by air cooling to obtain quenched and tempered 38CrMoAl steel.

[0103] Example 2-3

[0104] In Examples 2-3, the chemical composition of the 38CrMoAl steel billet is the same as that in Example 1;

[0105] The parameters for the remaining heating, rolling, slow cooling, annealing, and tempering steps, as well as the finished product specifications, differ from those in Example 1. Specific parameters and specifications are shown in Table 1. The hardness values ​​at half the radius of the annealed section and the flatness measurements in the tempered state are also shown in Table 1.

[0106] Comparative Examples 1-3

[0107] In Comparative Examples 1-3, the chemical composition of the 38CrMoAl steel billet was the same as that in Example 1.

[0108] In Comparative Examples 1-3, the parameters for heating, rolling, slow cooling, annealing, and tempering steps, as well as the finished product specifications, differ from those in Example 1. The main difference is that the annealing temperatures are 630±10℃ (Comparative Example 1), 650±10℃ (Comparative Example 2), and 680±10℃ (Comparative Example 3), respectively. Other specific parameters and specifications are shown in Table 1. The hardness values ​​at half the radius of the cross-section in the annealed state and the flatness measurements in the tempered state are also shown in Table 1.

[0109] Comparative Example 4

[0110] In this comparative example, the chemical composition of the 38CrMoAl steel billet is the same as that in Example 1.

[0111] The main difference between this comparative example and Example 1 is that in the annealing step, after entering the annealing furnace, the temperature is increased to 910°C at a rate of ≤80°C / h, the holding temperature is 910±5°C, the holding time is 3h, and then the furnace is removed and air-cooled.

[0112] The remaining steps and parameters are the same as in Example 1. The specific parameters and steel properties are shown in Table 1.

[0113] Table 1 shows the parameters for each step in Examples 1-3 and Comparative Examples 1-4, as well as the properties of the resulting steel.

[0114]

[0115]

[0116] As shown in Table 1, in Comparative Examples 1-3, the hardness values ​​at half the radius of the cross section of ordinary softening annealing at 630, 650 and 680℃ are 201-220 HBW, which meets the requirements. However, the curvature is 1.7-5.0 mm / m, and the on-site measurement shows that about 80% of the curvature exceeds the standard.

[0117] In Examples 1-3, the annealing process at 910℃ resulted in uniform grain size of the cross section, all at grade 5-4. The hardness at half the radius of the cross section was 191-202 HBW, which met the requirements. The curvature was 1.0-1.6 mm / m, which also met the requirements. On-site measurements showed that there was basically no curvature exceeding the standard.

[0118] As shown in Table 1, in Comparative Example 4, after heating and holding in the annealing process, the steel was directly taken out of the furnace and air-cooled without slow cooling treatment. During the air cooling process, the cooling rate of the steel stacked at different positions in the trolley varied greatly, resulting in uneven cross-sectional grain size and hardness. The hardness dispersion was large, ranging from 239 to 285 HBW, and the cross-sectional grain size was uneven, with a curvature of 1.7 to 4.8 mm / m.

[0119] Therefore, this invention provides a heat treatment method suitable for eliminating the bending of 38CrMoAl steel after quenching and tempering. This heat treatment method adjusts the annealing temperature of the steel to be maintained at 900℃-930℃, causing the steel to recrystallize and anneal, re-austenitize, and slowly cool to obtain an equilibrium microstructure. This reduces the hardness of the steel, eliminates internal stress, and significantly improves the uniformity of the cross-sectional grain size of the hot-rolled steel. It solves the problem of quenching bending caused by microstructure and grain size inhomogeneity parallel to the rolling direction. This ensures that the annealed steel meets a Brinell hardness of 190-229 HBW, and the annealed 38CrMoAl steel, after quenching and tempering, meets the bending degree standard of ≤2mm / m.

[0120] 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. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A heat treatment method for eliminating quenching and tempering bending in 38CrMoAl steel, characterized in that, The heat treatment method uses rolled steel as raw material and includes a slow cooling step, an annealing step, and a quenching and tempering step. The slow cooling step includes transferring the rolled material into a slow cooling pit to reduce its temperature. The temperature of the rolled material entering the slow cooling pit is ≥670℃; After being kept in the slow-cooling pit for more than 40 hours, it is removed from the pit. The annealing step includes: The rolled material after slow cooling is annealed at a holding temperature of 900℃-930℃, and then cooled before being taken out of the furnace. The cooling process involves slowly cooling the furnace at a rate of ≤30℃ / h to 700℃-720℃, then further cooling it in the furnace to 640℃-660℃ before removing it from the furnace. The conditioning step includes: The annealed rolled material is subjected to quenching and tempering treatment, which includes quenching and tempering treatment. The holding temperature of the quenching treatment is 920℃-940℃. After quenching treatment, tempering treatment is performed to obtain quenched and tempered 38CrMoAl steel. A stepped heating method is used to raise the temperature of the equipment to 920℃-940℃; The stepped heating method refers to first heating the heating equipment to 620℃-640℃ at a rate of 90℃ / h-110℃ / h and holding it at that temperature for 2.5h-3.5h, then heating it to 820℃-840℃ at a rate of 90℃ / h-110℃ / h and holding it at that temperature for 2.5h-3.5h, then heating it to 920℃-940℃ at a rate of 90℃ / h-110℃ / h and holding it at that temperature, and finally cooling it to room temperature with oil. The holding temperature for the tempering treatment is 625℃-635℃.

2. The heat treatment method for eliminating quenching and tempering bending of 38CrMoAl steel according to claim 1, characterized in that, In the annealing step, the holding temperature for the annealing treatment is 905℃-915℃; And / or, an annealing process is performed using a bogie furnace as the heating equipment; And / or, the heating rate for raising the equipment used for heating to the holding temperature is ≤80℃ / h; And / or, the holding time for the annealing treatment is calculated based on the diameter of the rolled material, and the holding time is ≥1.4h / 100mm×diameter.

3. The heat treatment method for eliminating quenching and tempering bending of 38CrMoAl steel according to claim 1, characterized in that, In the conditioning step, a pit furnace is used as the heating equipment for conditioning treatment; And / or, the heat preservation time at 920℃-940℃ is 4.5h-5.5h.

4. The heat treatment method for eliminating quenching and tempering bending of 38CrMoAl steel according to claim 1, characterized in that, The oil cooling rate is 30℃ / s-35℃ / s.

5. The heat treatment method for eliminating quenching and tempering bending of 38CrMoAl steel according to claim 1, characterized in that, The rolled material is round steel.

6. The heat treatment method for eliminating quenching and tempering bending of 38CrMoAl steel according to claim 5, characterized in that, The diameter of the rolled material is 110mm-240mm.

7. The heat treatment method for eliminating quenching and tempering bending of 38CrMoAl steel according to claim 1, characterized in that, The rolled material used as raw material is obtained from steel billets through heating and rolling processes; And / or, the cross-sectional dimensions of the steel billet are 410mm × 530mm; And / or, in the heating step, the billet is heated to 1220℃-1280℃, and the total heating time is 320min-500min; And / or, during the rolling process, the initial rolling temperature of the billet is 980℃-1150℃; And / or, between the heating step and rolling, descaling is also included, specifically including: sending the heated billet into a high-pressure water descaling machine to remove the iron oxide scale on the surface, and the temperature of the billet after descaling is reduced so that the initial rolling temperature of the billet is lower than the temperature after heating.

8. The heat treatment method for eliminating quenching and tempering bending of 38CrMoAl steel according to claim 1, characterized in that, The steel billet comprises, by weight percentage: C 0.35%-0.42%; Si 0.20%-0.45%; Mn 0.30%-0.60%; Cr 1.35%-1.65%; Al 0.70%-1.10%; Mo 0.18%-0.25%; V≤0.10%; Ni≤0.30%; Cu≤0.25%; P≤0.025%; S≤0.025%; with the balance being Fe and unavoidable impurities from the smelting process.

9. The heat treatment method for eliminating quenching and tempering bending of 38CrMoAl steel according to claim 1, characterized in that, Includes the following steps: S1: Heating, heat the steel billet to 1220℃-1280℃, heat for 320min-500min and then remove it from the furnace; S2: Rolling, the heated steel billet is transferred to the rolling mill for rolling. The initial rolling temperature of the steel billet is 980℃-1150℃ to obtain rolled material; S3: Slow cooling, the rolled material is transferred into a slow cooling pit to cool down, and then removed from the pit after being kept at the temperature for more than 40 hours; S4: Annealing. After slow cooling, the rolled material is transferred to a bogie hearth for annealing. The temperature is increased to 900℃-930℃ at a rate of ≤80℃ / h. The temperature is held for a time of ≥1.5h / 100mm×diameter. After the holding time is completed, the temperature is slowly cooled to 710℃ at a rate of ≤30℃ / h. The material is then cooled to 650℃ in the furnace before being taken out of the furnace. S5: Quenching and tempering. The annealed rolled material is transferred to a pit furnace and heated to 630℃ at a rate of 100℃ / h and held for 3 hours. Then it is heated to 830℃ at a rate of 100℃ / h and held for 3 hours. Then it is heated to 930℃ at a rate of 100℃ / h and held for 5 hours. After oil cooling, it is heated to 630℃ at a rate of ≤100℃ / h and held for 8 hours. Then it is air cooled to obtain quenched and tempered 38CrMoAl steel.

10. A 38CrMoAl steel obtained by the heat treatment method for eliminating quenching and tempering bending of 38CrMoAl steel as described in any one of claims 1-9, characterized in that, After annealing, 38CrMoAl steel meets the Brinell hardness standard of 190-229 HBW, and after quenching and tempering, 38CrMoAl steel meets the standard of bending ≤2mm / m.

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