Alloy steel 40Cr for hot-rolled plate mechanical structure and preparation method of alloy steel 40Cr

Abstract

The invention discloses alloy steel 40Cr for a hot-rolled plate mechanical structure and a preparation method of the alloy steel 40Cr, the alloy steel 40Cr comprises the following components: 0.375-0.39% of C, 0.18-0.25% of Si, 0.55-0.65% of Mn, 0.010-0.040% of Al, 0.85-0.95% of Cr, less than or equal to 0.015% of P, less than or equal to 0.005% of S and the balance of iron and unavoidable impurity elements, the thickness of the alloy steel 40Cr for the hot-rolled plate mechanical structure is 2-16 mm, the width is 900-1500 mm, the yield strength is less than or equal to 500 MPa, the A-D type inclusions are less than or equal to 1.5, the DS type inclusions are less than or equal to 1.0, and the balance is Fe. A fully decarburized layer is not allowed to appear on the surface of the steel plate, the thickness of a single-face semi-decarburized layer is not larger than 1% of the nominal thickness, and the product is excellent in plate shape, roll shape, surface quality, machining performance and the like and is highly accepted by users.

Description

Technical Field

[0001] The present application relates to the technical field of metal materials, and in particular to an alloy steel 40Cr for hot-rolled coil mechanical structure and a preparation method thereof. Background Art

[0002] 40Cr steel is a medium carbon alloy structural steel. After quenching and tempering, it has good strength, plasticity and toughness, and has low cost. It is widely used in industries such as machinery and automobile manufacturing. The raw materials of 40Cr steel plates are coils and wide and thick plates. Due to the low production cost of coils, 2-16mm specifications are generally hot-rolled. The general processing flow of 40Cr hot-rolled coils is: Kaiping-laser cutting (or stamping)-shot blasting-heat treatment. 40Cr has high strength, and it is difficult for general leveling equipment to level it. The leveling process of steel coils has high requirements for coil shape. If the steel coil is flat, the steel coil will jump in the uncoiler during high-speed leveling, resulting in scratches on the surface of the steel plate. Severe flat coils cause the inner diameter of the steel coil to be smaller than the mandrel of the uncoiler, resulting in the inability to level the steel plate on the uncoiler. The leveling process has high requirements for the surface quality of the steel plate. If there is iron oxide scale peeling during the leveling process of the steel plate, the iron oxide scale will accumulate on the leveling roller, which will crush the subsequent leveling strip steel. Laser cutting has high requirements for the flatness of the steel plate and the iron oxide scale of the strip. If the shape of the steel plate is not good, the laser nozzle is easily damaged during the cutting process. If there is iron oxide scale peeling, the laser nozzle will be blocked by the iron oxide scale. The stamping process of the steel plate has high requirements for the cleanliness of the material. The steel plate has problems such as inclusions and segregation, and cracks are prone to occur during the stamping process. The heat treatment process has requirements for the depth of the decarburization layer of the steel plate. If there is a decarburization layer, the surface hardness will be low after quenching. Summary of the invention

[0003] Technical problem to be solved: The main purpose of the present invention is to improve the steel quality, plate shape, surface quality, decarburization layer and other control levels of the existing hot-rolled coil mechanical structure alloy steel 40Cr, and to improve the processing performance and product qualification rate of 40Cr. To achieve the above purpose, the present invention provides a hot-rolled coil mechanical structure alloy steel 40Cr and a preparation method thereof.

[0004] Technical solution:

[0005] The purpose of the present invention and the solution to the technical problem are achieved by adopting the following technical solutions.

[0006] A hot-rolled coil alloy steel 40Cr for mechanical structure, comprising the following components and weight percentages: C 0.375-0.39%, Si 0.18-0.25%, Mn 0.55-0.65%, Al 0.010-0.040%, Cr 0.85-0.95%, P≤0.015%, S≤0.005%, and the rest being iron and unavoidable impurity elements.

[0007] Furthermore, the hot-rolled coil alloy steel 40Cr for mechanical structure has a thickness of 2-16 mm, a width of 900-1500 mm, a yield strength of ≤500 MPa, AD inclusions ≤1.5, DS inclusions ≤1.0, no full decarburization layer is allowed on the steel plate surface, and the thickness of the single-sided semi-decarburization layer is not greater than 1% of the nominal thickness.

[0008] The present application also discloses a method for preparing alloy steel 40Cr for hot-rolled coil mechanical structure, the preparation method comprising the following steps:

[0009] Step 1: Desulfurization of molten iron by mechanical stirring with KR. After KR desulfurization treatment, the S content of molten iron is ≤ 0.008%;

[0010] The second step: converter oxygen smelting, LF furnace refining, after the LF refining furnace treatment, the S content of the molten steel is ≤0.005%;

[0011] Step 3: Degassing in RH vacuum degassing furnace, degassing under vacuum degree less than 2mBar;

[0012] Step 4: After RH is completed, calcium silicon wire is fed for calcium treatment, and soft stirring is performed after calcium treatment;

[0013] Step 5: Continuous casting of ingots. The superheat of the tundish is controlled at 25±5℃. High carbon steel protective slag is used for the mold protective slag. The fluctuation amplitude of the steel liquid level during normal casting is ≤±3mm. Electromagnetic stirring is used in the secondary cooling zone. The ingot is lightly pressed at the end of solidification. The continuous casting speed is 1.10±0.05m / min.

[0014] Step 6: hot charging of the ingot into the furnace, the furnace top temperature and furnace side temperature of the preheating recovery section of the heating furnace are ≤600℃, the furnace top temperature of the preheating section is 1050±20℃, the furnace side temperature of the preheating section is 1030±20℃, the furnace top temperature of the first heating section is 1230±20℃, the furnace side temperature of the first heating section is 1210±20℃, the furnace top temperature of the second heating section is 1270±20℃, the furnace side temperature of the second heating section is 1250±20℃, the furnace top temperature of the soaking section is 1250±20℃, the furnace side temperature of the soaking section is 1230±20℃, and the furnace time is 130-200min;

[0015] Step 7: After the ingot is removed from the furnace, it is descaled once and then enters the rough rolling mill for rolling; the finished product thickness is 2.0mm≤h<6.0mm, the rough rolling outlet setting thickness is 36mm, and the rough rolling outlet temperature is 1050-1090℃; the finished product thickness is 6.0mm≤h<10.5mm, the rough rolling outlet setting thickness is 40mm, and the rough rolling outlet temperature is 1040-1080℃; the finished product thickness is 10.5mm≤h≤16mm, the rough rolling outlet setting thickness is 45mm, and the rough rolling outlet temperature is 1030-1070℃;

[0016] Step 8: After rough rolling, the slab is descaled for the second time. After descaling, the slab enters the finishing mill for rolling. The finished product thickness is 2.0mm≤h<6.0mm, and the final rolling temperature is 880-920℃; the finished product thickness is 6.0mm≤h<10.5mm, and the final rolling temperature is 870-910℃; the finished product thickness is 10.5mm≤h≤16mm, and the final rolling temperature is 860-900℃;

[0017] Step 9: After finishing rolling, the strip is cooled to the target coiling temperature by laminar flow cooling. The finished product thickness is 2.0mm≤h<6.0mm, and the coiling temperature is 680-720℃; the finished product thickness is 6.0mm≤h<10.5mm, and the coiling temperature is 670-710℃; the finished product thickness is 10.5mm≤h≤16mm, and the coiling temperature is 660-700℃; the length of the non-cooling section at the head of the strip is controlled within 3m;

[0018] Step 10: The strip is coiled into a coil by the coiler, and after staying in the coiler for 30-60 seconds, it is unloaded and transported out to obtain the hot-rolled coil 40Cr alloy steel for mechanical structure.

[0019] Furthermore, in the eighth step, for finished product thickness ≥ 6 mm, the cooling water between the stands of the finishing mill must be turned on.

[0020] Furthermore, in the ninth step of laminar cooling of the strip steel, the water ratio of the upper and lower surfaces of the cooling steel plate is adjusted to reduce the cooling intensity of the upper surface and improve the bowl shape caused by the high cooling intensity of the upper surface. The finished product thickness is 2.0mm≤h<6.0mm, and the water ratio of the upper and lower surfaces is 0.75-0.80; the finished product thickness is 6.0mm≤h<10.5mm, and the water ratio of the upper and lower surfaces is 0.80-0.85; the finished product thickness is 10.5mm≤h≤16mm, and the water ratio of the upper and lower surfaces is 0.85-0.90.

[0021] Furthermore, in the ninth step of laminar cooling of the strip, after the strip exits laminar cooling, a combined blowing mode of side water spray, top water spray and air spray is adopted to remove residual water on the surface of the strip, reduce cooling water retention on the upper surface, and reduce thermal stress of surface iron oxide.

[0022] Furthermore, in the combined purging mode of side water spraying, top water spraying and air spraying in the ninth step, compressed air is used for air spraying.

[0023] Furthermore, if the RH process in the third step needs to add alloy, the net cycle time after adding the alloy is 5-15 minutes, and the degassing time is 10-15 minutes.

[0024] Furthermore, the stirring time of the soft stirring in the fourth step is 10-15 minutes.

[0025] Furthermore, in the sixth step, the ingot is hot-charged into the furnace, ensuring that the furnace temperature is 400-500°C.

[0026] Beneficial effects:

[0027] Compared with the prior art, the present application adopts the above technical solution and can achieve the following technical effects:

[0028] 1. By optimizing the steelmaking process, adopting electromagnetic stirring in the secondary cooling zone for continuous casting, and lightly pressing the ingot at the end of solidification, the AD and DS inclusions of the material are ≤1.0, which is much better than the market requirement (AD and DS inclusions ≤1.5).

[0029] 2. By adopting hot charging, increasing the furnace temperature, reducing the heating temperature and shortening the heating time, the thickness of the 40Cr decarburization layer is effectively reduced. The maximum thickness of the single-sided decarburization layer is 0.44%, and the maximum thickness of the double-sided total decarburization layer is 0.75%, which is much better than the market requirements (the total thickness of the single-sided decarburization layer is not more than 1.5% of the nominal thickness, and the total thickness of the double-sided decarburization layer is not more than 2.5% of the nominal thickness);

[0030] 3. By adopting high temperature coiling, the yield strength of 40Cr is effectively reduced, the yield strength is 402-456MPa, the average is 419MPa, and the yield strength is much higher than the customer's requirements (not more than 500MPa), which is convenient for Kaiping;

[0031] 4. By optimizing the descaling process, opening the cooling water between the racks for thick specifications, and adopting the side spray water + top spray water + air spray combined purge mode for laminar cooling of the strip, the problem of iron oxide peeling and clogging the laser nozzle when users open the flat and laser cut 40Cr is solved;

[0032] 5. By optimizing the upper and lower water ratio of laminar cooling and the retention time of steel coils in the coiler, the quality of strip shape and coil shape can be effectively improved; 6. The product Shagang 40Cr of the present invention is excellent in plate shape, coil shape, surface quality, processing performance, etc., and is highly recognized by users;

[0033] 7. More than 9,000 tons have been delivered to customers in 2023, with a pass rate of 99.75%. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a photo of the descaling of alloy steel 40Cr for hot-rolled coil for mechanical structure in this application;

[0035] Figure 2 The photos of the decarburization of the alloy steel 40Cr for hot-rolled coil mechanical structure in this application are shown, wherein the left photo is a photo of the decarburization layer on the upper surface, and the right photo is a photo of the decarburization layer on the lower surface;

[0036] Figure 3The Kaiping tracking diagram of the hot-rolled coil mechanical structure alloy steel 40Cr and other mainstream steel mills 40Cr on the market shows that the surface quality of Shagang is better than that of a mainstream steel mill on the market;

[0037] Figure 4 This is the metallographic cross section of 40Cr iron oxide alloy steel for hot-rolled coil mechanical structure of this application, where the left picture is the upper metallographic picture and the right picture is the lower metallographic picture;

[0038] Figure 5 This is a schematic diagram of the structure of the alloy steel 40Cr for hot-rolled coil mechanical structure in this application when gas spraying is used;

[0039] Figure 6 This is a diagram of the surface quality of 40Cr alloy steel after optimization for hot-rolled coil mechanical structure in this application. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the present invention is further described in detail in conjunction with the embodiments below. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0041] A hot-rolled coil alloy steel 40Cr for mechanical structure, comprising the following components and weight percentages: C 0.386%, Si 0.213%, Mn 0.57%, Al 0.019%, Cr 0.87%, P ≤ 0.013%, S ≤ 0.005%, and the rest being iron and unavoidable impurity elements.

[0042] The alloy steel 40Cr for hot rolled coil mechanical structure has a thickness of 2-16mm, 900-1500mm, a yield strength of ≤500MPa, AD inclusions ≤1.5, DS inclusions ≤1.0, no full decarburization layer is allowed on the steel plate surface, and the thickness of the single-sided semi-decarburization layer is not greater than 1% of the nominal thickness.

[0043] The present application also discloses a method for preparing alloy steel 40Cr for hot-rolled coil mechanical structure, the preparation method comprising the following steps:

[0044] Step 1: Desulfurization of molten iron by mechanical stirring with KR. After KR desulfurization treatment, the S content of molten iron is ≤ 0.008%;

[0045] The second step: converter oxygen smelting, LF furnace refining, after the LF refining furnace treatment, the S content of the molten steel is ≤0.005%;

[0046] Step 3: Degassing in RH vacuum degassing furnace. When the vacuum degree is less than 2mBar, the degassing time is 10-15min. If the RH process needs to add alloy, the net cycle time after adding alloy is 5-10min.

[0047] Step 4: After RH is finished, feed the silicon calcium wire for calcium treatment, and then perform soft stirring for 10-15 minutes;

[0048] Step 5: Continuous casting of ingots. The superheat of the tundish is controlled at 25±5℃. High carbon steel protective slag is used for the mold protective slag. The fluctuation amplitude of the steel liquid level during normal casting is ≤±3mm. Electromagnetic stirring is used in the secondary cooling zone. The ingot is lightly pressed at the end of solidification. The continuous casting speed is 1.10±0.05m / min.

[0049] Step 6: Hot charge the ingot into the furnace, ensuring that the temperature is ≥400℃. The top temperature and side temperature of the preheating recovery section of the heating furnace are ≤600℃, the top temperature of the preheating section is 1050±20℃, the side temperature of the preheating section is 1030±20℃, the top temperature of the first heating section is 1230±20℃, the side temperature of the first heating section is 1210±20℃, the top temperature of the second heating section is 1270±20℃, the side temperature of the second heating section is 1250±20℃, the top temperature of the soaking section is 1250±20℃, the side temperature of the soaking section is 1230±20℃, and the time in the furnace is 130-200min;

[0050] Step 7: After the ingot is removed from the furnace, it is descaled once and then enters the rough rolling mill for rolling. The finished product thickness is 2.0mm≤h<6.0mm, the rough rolling outlet thickness is set to 36mm, and the rough rolling outlet temperature is 1050-1090℃; the finished product thickness is 6.0mm≤h<10.5mm, the rough rolling outlet thickness is set to 40mm, and the rough rolling outlet temperature is 1040-1080℃; the finished product thickness is 10.5mm≤h≤16mm, the rough rolling outlet thickness is set to 45mm, and the rough rolling outlet temperature is 1030-1070℃;

[0051] Step 8: After rough rolling, the slab is descaled for the second time, and then the slab enters the finishing mill for rolling. For finished product thickness ≥6mm, the cooling water between the finishing mill stands must be turned on; for finished product thickness 2.0mm≤h<6.0mm, the final rolling temperature is 880-920℃; for finished product thickness 6.0mm≤h<10.5mm, the final rolling temperature is 870-910℃; for finished product thickness 10.5mm≤h≤16mm, the final rolling temperature is 860-900℃;

[0052] Step 9: After finishing rolling, the strip is cooled by laminar flow to the target coiling temperature and then coiled. The water ratio of the upper and lower surfaces of the cooling steel plate is adjusted to reduce the cooling intensity of the upper surface and improve the bowl shape caused by the high cooling intensity of the upper surface. The finished product thickness is 2.0mm≤h<6.0mm, and the water ratio of the upper and lower surfaces is 0.75-0.80; the finished product thickness is 6.0mm≤h<10.5mm, and the water ratio of the upper and lower surfaces is 0.80-0.85; the finished product thickness is 10.5mm≤h≤16mm, and the water ratio of the upper and lower surfaces is 0.85-0.90. The finished product thickness is 2.0mm≤h<6.0mm, and the coiling temperature is 680-720℃; the finished product thickness is 6.0mm≤h<10.5mm, and the coiling temperature is 670-710℃; the finished product thickness is 10.5mm≤h≤16mm, and the coiling temperature is 660-700℃; the length of the non-cooling section at the strip head is controlled within 3m; after the strip is laminar cooled, a combined blowing mode of side water spray + top water spray + air spray is adopted to remove residual water on the surface of the strip, reduce the retention of cooling water on the upper surface, and reduce the thermal stress of surface iron oxide. Compressed air is used for air spraying.

[0053] Step 10: The strip is coiled by the coiler and stays in the coiler for 30-60 seconds before being unloaded and transported out. The hot-rolled coil with a thickness of 9.5-14mm is produced.

[0054] like Figure 2 As shown, Figure 2 This is a photo of the decarburization of 40Cr alloy steel for hot-rolled coil mechanical structure for this application, with a thickness specification of 12 mm. The photo shows that no complete decarburization layer was found on the upper and lower surfaces. The left photo shows the thickness of the decarburization layer on the upper surface: 38 μm, 0.32% of the nominal thickness, and the right photo shows the thickness of the decarburization layer on the lower surface: 41 μm, 0.34% of the nominal thickness.

[0055] like Figure 3 As shown, Figure 3 From the Kaiping tracking chart of hot-rolled coil alloy steel 40Cr for mechanical structure in this application and 40Cr from other mainstream steel mills on the market, it can be seen that the surface quality of Shagang is better than that of a mainstream steel mill on the market.

[0056] like Figure 4 As shown, Figure 4 This is the cross-sectional metallographic image of 40Cr oxidized iron alloy steel for hot-rolled coil mechanical structure for this application. The left picture is the upper table, and the right picture is the lower table. The figures show that the thickness of the oxidized iron scale on the upper and lower surfaces is within 30μ, and the cut is tightly bonded to the matrix without peeling.

[0057] The following table shows the actual performance of 40Cr produced this time. The yield strength of 40Cr produced in Volume 1 is 402-456MPa, with an average of 419MPa, which meets the requirements.

[0058] Table 1 Mechanical properties

[0059]

[0060]

[0061] The inclusions and decarburization layer depth meet the standard requirements.

[0062] Table 2 Inclusions and decarburized layer

[0063]

[0064] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A hot rolled coil mechanical structure alloy steel 40Cr, characterized in that: The hot-rolled coil mechanical structure alloy steel 40Cr contains the following components and weight percentages: C 0.375-0.39%, Si 0.18-0.25%, Mn 0.55-0.65%, Al0.010-0.040%, Cr 0.85-0.95%, P≤0.015%, S≤0.005%, and the rest are iron and unavoidable impurity elements.

2. The hot rolled coil mechanical structure alloy steel 40Cr according to claim 1, characterized in that The hot rolled coil mechanical structure alloy steel 40Cr has a thickness of 2-16 mm, a width of 900-1500 mm, a yield strength of ≤500 MPa, AD inclusions ≤1.5, DS inclusions ≤1.0, a full decarburized layer is not allowed to appear on the steel plate surface, and the thickness of the single-sided semi-decarburized layer is not more than 1% of the nominal thickness.

3. A method for preparing a hot-rolled coil mechanical structure alloy steel 40Cr according to claim 1 or 2, characterized in that: The preparation method comprises the following steps: Step 1: Desulfurization of molten iron by mechanical stirring with KR. After KR desulfurization treatment, the S content of molten iron is ≤ 0.008%; The second step: converter oxygen smelting, LF furnace refining, after the LF refining furnace treatment, the S content of the molten steel is ≤0.005%; Step 3: Degassing in RH vacuum degassing furnace, degassing under vacuum degree less than 2mBar; Step 4: After RH is completed, calcium silicon wire is fed for calcium treatment, and soft stirring is performed after calcium treatment; Step 5: Continuous casting of ingots. The superheat of the tundish is controlled at 25±5℃. High carbon steel protective slag is used for the mold protective slag. The fluctuation amplitude of the steel liquid level during normal casting is ≤±3mm. Electromagnetic stirring is used in the secondary cooling zone. The ingot is lightly pressed at the end of solidification. The continuous casting speed is 1.10±0.05m / min. Step 6: hot charging of the ingot into the furnace, the furnace top temperature and furnace side temperature of the preheating recovery section of the heating furnace are ≤600℃, the furnace top temperature of the preheating section is 1050±20℃, the furnace side temperature of the preheating section is 1030±20℃, the furnace top temperature of the first heating section is 1230±20℃, the furnace side temperature of the first heating section is 1210±20℃, the furnace top temperature of the second heating section is 1270±20℃, the furnace side temperature of the second heating section is 1250±20℃, the furnace top temperature of the soaking section is 1250±20℃, the furnace side temperature of the soaking section is 1230±20℃, and the furnace time is 130-200min; Step 7: After the ingot is removed from the furnace, it is descaled once and then enters the rough rolling mill for rolling; the finished product thickness is 2.0mm≤h<6.0mm, the rough rolling outlet setting thickness is 36mm, and the rough rolling outlet temperature is 1050-1090℃; the finished product thickness is 6.0mm≤h<10.5mm, the rough rolling outlet setting thickness is 40mm, and the rough rolling outlet temperature is 1040-1080℃; the finished product thickness is 10.5mm≤h≤16mm, the rough rolling outlet setting thickness is 45mm, and the rough rolling outlet temperature is 1030-1070℃; Step 8: After rough rolling, the slab is descaled for the second time. After descaling, the slab enters the finishing mill for rolling. The finished product thickness is 2.0mm≤h<6.0mm, and the final rolling temperature is 880-920℃; the finished product thickness is 6.0mm≤h<10.5mm, and the final rolling temperature is 870-910℃; the finished product thickness is 10.5mm≤h≤16mm, and the final rolling temperature is 860-900℃; Step 9: After finishing rolling, the strip is cooled to the target coiling temperature by laminar flow cooling. The finished product thickness is 2.0mm≤h<6.0mm, and the coiling temperature is 680-720℃; the finished product thickness is 6.0mm≤h<10.5mm, and the coiling temperature is 670-710℃; the finished product thickness is 10.5mm≤h≤16mm, and the coiling temperature is 660-700℃; the length of the non-cooling section at the head of the strip is controlled within 3m; Step 10: The strip is coiled into a coil by the coiler, and after staying in the coiler for 30-60 seconds, it is unloaded and transported out to obtain the hot-rolled coil 40Cr alloy steel for mechanical structure.

4. The method for preparing the hot rolled coil alloy steel 40Cr for mechanical structure according to claim 3, characterized in that: In the eighth step, if the finished product thickness is ≥6mm, the cooling water between the finishing mill stands must be turned on.

5. The method for preparing the hot rolled coil mechanical structure alloy steel 40Cr according to claim 3, characterized in that: In the ninth step of laminar cooling of the strip steel, the water ratio of the upper and lower surfaces of the cooling steel plate is adjusted to reduce the cooling intensity of the upper surface and improve the bowl shape caused by the high cooling intensity of the upper surface. The finished product thickness is 2.0mm≤h<6.0mm, and the water ratio of the upper and lower surfaces is 0.75-0.80; the finished product thickness is 6.0mm≤h<10.5mm, and the water ratio of the upper and lower surfaces is 0.80-0.85; the finished product thickness is 10.5mm≤h≤16mm, and the water ratio of the upper and lower surfaces is 0.85-0.

90.

6. The method for preparing the hot rolled coil alloy steel 40Cr for mechanical structure according to claim 3, characterized in that: In the ninth step of laminar cooling of the strip, after the strip has exited laminar cooling, a combined blowing mode of side spraying, top spraying and air spraying is adopted to remove residual water on the surface of the strip, reduce cooling water retention on the upper surface, and reduce thermal stress of surface iron oxide.

7. The method for preparing the hot rolled coil mechanical structure alloy steel 40Cr according to claim 6, characterized in that: In the combined purging mode of side water spraying, top water spraying and air spraying in the ninth step, compressed air is used for air spraying.

8. The method for preparing the hot rolled coil alloy steel 40Cr for mechanical structure according to claim 3, characterized in that: If the RH process in the third step needs to add alloy, the net cycle time after adding the alloy is 5-15 minutes, and the degassing time is 10-15 minutes.

9. The method for preparing hot rolled coil 40Cr alloy steel for mechanical structure according to claim 3, characterized in that: The stirring time of soft stirring in the fourth step is 10-15 minutes.

10. The method for preparing hot rolled coil 40Cr alloy steel for mechanical structure according to claim 3, characterized in that: In the sixth step, the ingot is hot-charged into the furnace, ensuring that the furnace temperature is 400-500°C.

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