Method for reducing Brinell hardness of 40CrNiMo hot-rolled round steel

Abstract

The invention discloses a method for reducing the Brinell hardness of 40CrNiMo hot-rolled round steel. The method specifically comprises the following steps: controlling the working procedures of heating, rolling, cooling bed cooling and slow cooling in the production process of the 40CrNiMo hot-rolled round steel; wherein in the heating process, the temperature of a soaking zone is 1180-1250 DEG C, and the heating time of the soaking zone is 2.5-5.5 hours. By adopting a reasonable controlled rolling and controlled cooling process, the hot-rolled 40CrNiMo round steel obtains a metallographic structure mainly comprising granular bainite, the hardness of the round steel is effectively reduced, the brinell hardness of the round steel in the hot rolling state is 230-246 HBW, that is, the hardness of the round steel in the hot rolling state can meet the standard requirement that the hardness is 269 HBW or below specified by the national standard without annealing or high-temperature tempering, and the production cost is remarkably reduced.

Description

Technical Field

[0001] The invention relates to the technical field of steel production, and in particular to a method for reducing the Brinell hardness of 40CrNiMo hot-rolled round steel. Background Art

[0002] 40CrNiMo belongs to Cr-Ni-Mo alloy structural steel, which is mainly used for high-strength, high-toughness, large-section and important quenched and tempered parts, such as steam turbines, shafts, gears, transmission eccentric shafts of horizontal forging machines, large generator rotors, etc.

[0003] The implementation standard of 40CrNiMo is GB / T 3077, and its main chemical composition is C: 0.37-0.44%, Si: 0.17-0.37%, Mn: 0.50-0.80%, Cr: 0.60-0.90%, Ni: 1.25-1.65%, Mo: 0.15-0.25%. Due to the high content of CrNiMo alloy elements in the chemical composition, the stability of supercooled austenite is increased, the ferrite transformation is inhibited, and the critical cooling rate is reduced. The mixed structure of lath bainite and martensite can be obtained within a wide cooling rate range, resulting in a high hot-rolled Brinell hardness of more than 300HBW, which is difficult to meet the user's direct shearing and cutting processing requirements. The national standard stipulates that the Brinell hardness must be lower than 269HBW before delivery and use; in order to reduce the hardness, steel manufacturers generally meet the standard requirements through annealing or high-temperature tempering heat treatment, and the production cost is high. Summary of the invention

[0004] The object of the present invention is to provide a method for reducing the Brinell hardness of 40CrNiMo hot-rolled round steel. Without annealing or high-temperature tempering, the hot-rolled hardness of the round steel can meet the national standard requirement of less than 269HBW, thereby reducing the production cost of steel enterprises.

[0005] To achieve the above object, the present invention adopts the following technical solutions: A method for reducing the Brinell hardness of 40CrNiMo hot-rolled round steel, the method specifically comprising: controlling the heating, rolling, cooling on a cooling bed and slow cooling processes in the production process of the 40CrNiMo hot-rolled round steel; wherein, in the heating process, the soaking section temperature is 1180-1250°C, and the soaking section heating time is 2.5-5.5 hours.

[0006] Furthermore, in the rolling process of the present invention, the steel billet is continuously rolled into round steel through rough rolling, intermediate rolling and finish rolling in sequence, wherein the starting rolling temperature is 1080-1150°C and the final rolling temperature is 980-1040°C.

[0007] Furthermore, in the cooling process of the cooling bed of the present invention, the rolled round steel is slowly cooled after entering the cooling bed, the conveying roller table entering the cooling bed is covered with a heat preservation cover, a wind shield is installed around the cooling bed, and the round steel is quickly collected when it passes the cooling bed for more than 3 steps to ensure that the collection temperature is ≥650°C.

[0008] Furthermore, in the slow cooling process described in the present invention, the collected round steels are hoisted into a slow cooling pit for slow cooling, wherein the temperature entering the slow cooling pit is ≥580°C, hot steel is first placed on the bottom of the slow cooling pit before entering the pit, and hot steel is covered on top after entering the pit, and finally an insulation cover is put on, the temperature out of the slow cooling pit is ≤100°C, and the slow cooling time is ≥24 hours.

[0009] The microstructure of the 40CrNiMo hot-rolled round steel produced by the method of the invention is mainly granular bainite without martensite hard phase structure; the hot-rolled Brinell hardness is 230-246 HBW.

[0010] The inventive principle and beneficial effects of the present invention are: The present invention adopts a higher heating temperature and a longer heating time in the soaking section, promotes the diffusion of NiCrMo alloy elements, improves the uniformity of chemical composition, and reduces the hard phase martensite generated by the segregation of alloy components. After the round steel is rolled, a combination of slow cooling on a cooling bed and slow cooling pit cooling is adopted to slowly cool it in the granular bainite phase transformation zone to perform a sufficient organizational transformation, thereby obtaining a metallographic structure dominated by granular bainite and no martensitic hard phase structure, effectively reducing the hardness of the round steel, and its hot-rolled Brinell hardness is 230-246 HBW, that is, without annealing or high-temperature tempering, the hot-rolled hardness can meet the national standard requirement of less than 269 HBW, omitting the annealing heat treatment process, and the user can directly cut and process the material for use, reducing the production cost of steel enterprises. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 This is the metallographic structure of the 40CrNiMo hot-rolled round steel in Example 1.

[0012] Figure 2 This is the metallographic structure of 40CrNiMo hot-rolled round steel in Example 2.

[0013] Figure 3 This is the metallographic structure of 40CrNiMo hot-rolled round steel in Example 3. DETAILED DESCRIPTION

[0014] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described in conjunction with the embodiments below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. Example 1

[0015] The chemical composition of the 40CrNiMo hot-rolled round steel in this embodiment is: C: 0.41%, Si: 0.26%, Mn: 0.75%, P: 0.018%, S: 0.013%, Cr: 0.80%, Ni: 1.50%, Mo: 0.15%, Al: 0.020%, and the rest are iron and unavoidable impurity elements.

[0016] The production method of reducing the Brinell hardness of 40CrNiMo hot-rolled round steel in this embodiment includes heating, rolling, cooling on a cooling bed and slow cooling steps, which are specifically as follows: (1) The 40CrNiMo ingot with the above composition is fed into a walking beam heating furnace for heating, and the soaking section temperature is controlled at 1200°C and the soaking section heating time is 3 hours; (2) After the heated billet is taken out of the furnace, it is continuously rolled into round steel through rough rolling, intermediate rolling and finish rolling, with the starting rolling temperature being 1080°C and the final rolling temperature being 980°C; (3) The rolled round steel is slowly cooled after entering the cooling bed. The conveying roller of the cooling bed is covered with a heat preservation cover. A wind shield is installed around the cooling bed. The steel is quickly collected at a distance of 5 steps over the cooling bed. The collection temperature is 650℃. (4) The collected round steel is hoisted into the slow cooling pit for slow cooling. The temperature entering the slow cooling pit is 590℃. Before entering the pit, hot steel is placed on the bottom of the slow cooling pit. After entering the pit, hot steel is placed on top to cover it. Finally, an insulation cover is put on. The temperature out of the slow cooling pit is 80℃ and the slow cooling time is 25 hours.

[0017] After testing, the Brinell hardness of the 40CrNiMo hot-rolled round steel in this embodiment is 240 HBW.

[0018] The metallographic structure of the 40CrNiMo hot-rolled round steel in this embodiment is as follows: Figure 1 shown by Figure 1 It can be seen that its microstructure is mainly granular bainite without martensite. Example 2

[0019] The chemical composition of the 40CrNiMo hot-rolled round steel in this embodiment is: C: 0.40%, Si: 0.24%, Mn: 0.70%, P: 0.020%, S: 0.015%, Cr: 0.85%, Ni: 1.45%, Mo: 0.20%, Al: 0.022%, and the rest are iron and unavoidable impurity elements.

[0020] The production method of reducing the Brinell hardness of 40CrNiMo hot-rolled round steel in this embodiment includes heating, rolling, cooling on a cooling bed and slow cooling steps, which are specifically as follows: (1) The 40CrNiMo ingot with the above composition is fed into a walking beam heating furnace for heating, and the soaking section temperature is controlled at 1180°C and the soaking section heating time is 5.5 hours; (2) After the heated billet is taken out of the furnace, it is continuously rolled into round steel through rough rolling, intermediate rolling and finish rolling, with the starting rolling temperature being 1100°C and the final rolling temperature being 1010°C; (3) The rolled round steel is slowly cooled after entering the cooling bed. The conveying roller of the cooling bed is covered with a heat preservation cover. A wind shield is installed around the cooling bed. The steel is quickly collected at a distance of 5 steps over the cooling bed. The collection temperature is 670℃. (4) The collected round steel is hoisted into the slow cooling pit for slow cooling. The temperature of the slow cooling pit is 600℃. Hot steel is placed on the bottom of the slow cooling pit before entering the pit. After entering the pit, hot steel is placed on the top and covered. Finally, an insulation cover is put on. The temperature out of the slow cooling pit is 90℃ and the slow cooling time is 26 hours.

[0021] After testing, the Brinell hardness of the 40CrNiMo hot-rolled round steel in this embodiment is 230 HBW.

[0022] The metallographic structure of the 40CrNiMo hot-rolled round steel in this embodiment is as follows: Figure 2 shown by Figure 2 It can be seen that its microstructure is mainly granular bainite without martensite. Example 3

[0023] The chemical composition of the 40CrNiMo hot-rolled round steel in this embodiment is: C: 0.39%, Si: 0.26%, Mn: 0.68%, P: 0.019%, S: 0.020%, Cr: 0.74%, Ni: 1.52%, Mo: 0.18%, Al: 0.028%, and the rest are iron and unavoidable impurity elements.

[0024] The production method of reducing the Brinell hardness of 40CrNiMo hot-rolled round steel in this embodiment includes heating, rolling, cooling on a cooling bed and slow cooling steps, which are specifically as follows: (1) The 40CrNiMo ingot with the above composition is sent into a walking beam heating furnace for heating, and the soaking section temperature is controlled at 1250°C and the soaking section heating time is 2.5 hours; (2) After the heated billet is taken out of the furnace, it is continuously rolled into round steel through rough rolling, intermediate rolling and finish rolling, with the starting rolling temperature being 1150°C and the final rolling temperature being 1040°C; (3) The rolled round steel is slowly cooled after entering the cooling bed. The conveying roller of the cooling bed is covered with a heat preservation cover, and a wind shield is installed around the cooling bed. The 4-step distance is fast over the cooling bed for rapid collection, and the collection temperature is 680°C; (4) The collected round steel is hoisted into the slow cooling pit for slow cooling. The temperature entering the slow cooling pit is 610℃. Before entering the pit, hot steel is placed on the bottom of the slow cooling pit. After entering the pit, hot steel is placed on top to cover it. Finally, an insulation cover is put on. The temperature out of the slow cooling pit is 100℃ and the slow cooling time is 24 hours.

[0025] After testing, the Brinell hardness of the 40CrNiMo hot-rolled round steel in this embodiment is 246 HBW.

[0026] The metallographic structure of the 40CrNiMo hot-rolled round steel in this embodiment is as follows: Figure 3 shown by Figure 3 It can be seen that its microstructure is mainly granular bainite without martensite.

[0027] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for reducing the Brinell hardness of 40CrNiMo hot-rolled round steel, characterized in that: The method specifically comprises: controlling the heating, rolling, cooling on a cooling bed and slow cooling processes in the production process of 40CrNiMo hot-rolled round steel; wherein, in the heating process, the soaking section temperature is 1180 to 1250° C., and the soaking section heating time is 2.5 to 5.5 hours.

2. The method for reducing the Brinell hardness of 40CrNiMo hot-rolled round steel according to claim 1, characterized in that: In the rolling process, the steel billet is continuously rolled into round steel through rough rolling, intermediate rolling and finish rolling in sequence, wherein the starting rolling temperature is 1080-1150°C and the final rolling temperature is 980-1040°C.

3. The method for reducing the Brinell hardness of 40CrNiMo hot-rolled round steel according to claim 1, characterized in that: In the cooling process of the cooling bed, the rolled round steel is slowly cooled after entering the cooling bed, the conveying roller table entering the cooling bed is covered with a heat preservation cover, a wind shield is installed around the cooling bed, and the steel is quickly collected when it passes the cooling bed at a distance of more than 3 steps to ensure that the collection temperature is ≥650°C.

4. The method for reducing the Brinell hardness of 40CrNiMo hot-rolled round steel according to claim 1, characterized in that: In the slow cooling process, the collected round steels are hoisted into a slow cooling pit for slow cooling, wherein the temperature entering the slow cooling pit is ≥580°C, hot steel is first placed on the bottom of the slow cooling pit before entering the pit, hot steel is laid on top after entering the pit, and finally an insulation cover is put on, the temperature out of the slow cooling pit is ≤100°C, and the slow cooling time is ≥24 hours.

5. The method for reducing the Brinell hardness of 40CrNiMo hot-rolled round steel according to claim 1, characterized in that: The chemical composition and mass percentage of the 40CrNiMo hot-rolled round steel are: C: 0.37-0.44%, Si: 0.17-0.37%, Mn: 0.50-0.80%, S≤0.025%, P≤0.025%, Cr: 0.60-0.90%, Ni: 1.25-1.65%, Mo: 0.15-0.25%, Al: 0.020-0.035%, and the rest are iron and unavoidable impurity elements.

6. The method for reducing the Brinell hardness of 40CrNiMo hot-rolled round steel according to claim 1, characterized in that: The microstructure of the 40CrNiMo hot-rolled round steel produced by the method is mainly granular bainite without martensite hard phase structure.

7. The method for reducing the Brinell hardness of 40CrNiMo hot-rolled round steel according to claim 1, characterized in that: The hot-rolled Brinell hardness of the 40CrNiMo hot-rolled round steel produced by the method is 230-246 HBW.

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