Nitrided round steel and production method thereof

Abstract

The invention relates to nitrided round steel and a production method thereof. The production method comprises the steps of heating, descaling, rolling and cooling bed cooling. In the heating procedure, the second heating temperature ranges from 1040 DEG C to 1070 DEG C, and the soaking temperature ranges from 1040 DEG C to 1070 DEG C; in the descaling process, the descaling pressure is 23 + / -1 MPa; and in the rolling procedure, the rolling speed of each pass is controlled according to 80-90% of the rolling speed designed by the process. The requirement of a user for the hardness of the round steel is met, the softening annealing procedure after rolling can be omitted, the production cost is reduced, and the delivery cycle is shortened.

Description

Technical Field

[0001] The present invention relates to the technical field of alloys, and more particularly to a nitrided round steel and a production method thereof. Background Art

[0002] Nitrided round steel contains 0.35-0.42% carbon (C), 0.2-0.45% silicon (Si), 0.30-0.60% manganese (Mn), 1.35-1.65% chromium (Cr), 0.15-0.25% molybdenum (Mo), and 0.7-1.1% aluminum (Al), with the balance being iron and minor impurities. Nitrided round steel has a metallurgical structure of ferrite, pearlite, and bainite, offering excellent wear resistance, high fatigue resistance, and high strength. It has a wide range of applications, including machinery manufacturing, injection molding machines, construction machinery, petroleum and chemical industries, aerospace, automotive manufacturing, mold making, new energy, electric power, and national defense.

[0003] Nitrided round steel requires machining before use. However, due to its high hardness, turning efficiency is low during turning. Therefore, to facilitate cutting, the hardness of the nitrided round steel is subject to certain requirements. For example, the hardness of nitrided round steel with a diameter of Φ40-Φ80 must be ≤260 HBW. However, the hardness of nitrided round steel produced by conventional hot rolling is ≥290 HBW. To meet these requirements, soft annealing is commonly used after rolling, which increases production costs, increases finishing time, and extends delivery cycles.

[0004] Based on this, the present invention proposes a new method for producing nitrided round steel to solve the above problems. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a nitrided round steel and a production method thereof, which not only meets the user's hardness requirements for round steel, but also eliminates the softening annealing process after rolling, reduces production costs, and shortens delivery cycle.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions: A method for producing nitrided round steel, comprising heating, descaling, rolling and cooling on a cooling bed; In the heating process, the second heating temperature is 1040-1070°C, and the soaking temperature is 1040-1070°C; In the descaling process, the descaling pressure is 23 ± 1 MPa; During the rolling process, the rolling speed of each pass is controlled at 80-90% of the process design rolling speed.

[0007] In one embodiment, during the cooling process, the cooling bed cover is used to keep the round steel warm and cool it slowly, and the cooling rate of the round steel is controlled to be 0.5-0.8°C / s.

[0008] In one embodiment, the step of keeping warm and cooling slowly is 3-4 minutes.

[0009] In one embodiment, the rolling process includes rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling and final rolling. The rolling speed of each pass of rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling and final rolling is controlled at 80-90% of the process design rolling speed. Among them, the rough rolling adopts dry head rolling, and the final rolling temperature is ≤850℃.

[0010] In one embodiment, the rough rolling is performed in 6 passes.

[0011] In one embodiment, the intermediate rolling is performed for 4-6 passes.

[0012] In one embodiment, the pre-finish rolling is performed for 2-4 passes.

[0013] In one embodiment, the finishing rolling is performed for 2-6 passes.

[0014] In one embodiment, the rough rolling is performed in 3-4 passes.

[0015] A nitrided round steel is produced by the above-mentioned production method of nitrided round steel.

[0016] In summary, the present invention has the following beneficial effects: The present invention adopts a series of measures such as lowering the secondary heating temperature, lowering the soaking temperature, lowering the rolling speed, increasing the high-pressure water dephosphorization pressure, and dry rolling, thereby ultimately reducing the final rolling temperature of the round steel to ≤850°C and controlling the generation of bainite structure. After covering the cooling bed with a heat-insulating cover for 3 to 4 minutes of slow cooling, the cooling speed of the round steel is controlled to 0.5-0.8°C / S, further reducing the generation of bainite structure. Ultimately, the hardness of the round steel is ≤260 HBW. The present invention can meet the user's hardness requirements for the round steel and can eliminate the softening annealing process after rolling, thereby reducing production costs and shortening delivery cycles. The present invention can be promoted and applied in similar production lines of high-quality special steel. DETAILED DESCRIPTION

[0017] The present invention provides a production method of nitrided round steel, comprising the following steps: S1. Heating: The continuous casting billet is heated in a walking beam heating furnace, wherein the second heating temperature is 1040-1070°C and the soaking temperature is 1040-1070°C; In the prior art, the second heating temperature of the heating process is generally 1140-1220°C, and the soaking temperature is generally 1160-1200°C. Step S1 of the present invention reduces the temperature of the second heating section and the soaking section in the heating process, and cooperates with subsequent steps to control the rolling temperature of the finished round steel to ≤850°C.

[0018] S2. High-pressure water descaling: Descaling is performed on the continuous casting billet after the heating treatment, and the descaling pressure is 23±1 MPa.

[0019] In the conventional descaling process, the pressure of high-pressure water is 18±1 MPa. The present invention increases the descaling pressure to solve the problem of poor phosphorus removal effect caused by the low temperature of the heating process, and increases the descaling pressure to improve the descaling effect.

[0020] S3. Rough rolling: The continuous casting billet is subjected to rough rolling to obtain rolled products. During the rough rolling process, the rolling speed of each pass is controlled at 80-90% of the process design rolling speed.

[0021] Preferably, in step S3, dry rolling is employed during the rough rolling process. Dry rolling involves turning off the roller cooling water before the workpiece enters the Nxth rolling mill to prevent warping and steel accumulation. When the workpiece enters the N(x+1)th rolling mill, turning on the roller cooling water of the Nxth rolling mill is performed. The low-temperature continuous casting slab of the present invention is susceptible to warping of the workpiece head during the rolling process. Dry rolling can effectively prevent this.

[0022] Preferably, in step S3, the rough rolling is performed for 6 passes.

[0023] S4, Flying shear head: removes defects on the head of the workpiece after rough rolling.

[0024] Preferably, in step S4, the cut length is ≥100 mm.

[0025] S5, intermediate rolling: intermediate rolling is performed on the workpiece. During the intermediate rolling process, the rolling speed of each pass is controlled at 80-90% of the process design rolling speed.

[0026] Preferably, in step S5, the intermediate rolling is performed for 4-6 passes.

[0027] S6, Flying shear head: removes defects at the head and tail of the rolled piece after intermediate rolling.

[0028] Preferably, in step S6, the length of the cut head is ≥200 mm, and the length of the cut tail is ≥200 mm.

[0029] S7, pre-finishing rolling: If pre-finishing rolling is required for the rolled piece, the rolling speed of each pass is controlled at 80-90% of the process design rolling speed during the pre-finishing rolling process, otherwise enter S10 or S11.

[0030] Preferably, in step S7, the pre-finish rolling is performed for 2-4 passes.

[0031] S8, Flying shear head: removes defects at the head and tail of the rolled piece after pre-finish rolling.

[0032] Preferably, in step S8, the length of the cut head is ≥300 mm, and the length of the cut tail is ≥300 mm.

[0033] S9, finishing rolling: If the workpiece needs to be finish rolled, the rolling speed of each pass is controlled at 80-90% of the process design rolling speed during the finishing rolling process, otherwise enter S10 or S11.

[0034] Preferably, in step S9, the finishing rolling is performed for 2-6 passes.

[0035] S10, Flying shear head: removes defects at the head and tail of the rolled piece after finishing rolling.

[0036] Preferably, in step S10, the length of the cut head is ≥300 mm, and the length of the cut tail is ≥300 mm.

[0037] S11. Finish rolling: The rolled piece is subjected to finish rolling to obtain round steel. During the rolling process, the rolling speed of each pass is controlled at 80-90% of the process design rolling speed, and the rolling temperature is ≤850℃.

[0038] Preferably, in step S11, the finishing rolling is performed for 3-4 passes.

[0039] Through the above-mentioned heating treatment, descaling treatment and measures of reducing the rolling speed, the final rolling temperature of the round steel is made ≤850°C, which inhibits the formation of bainite structure.

[0040] S12, segmentation: segment the round steel after final rolling.

[0041] S13. Cooling on cooling bed: The round steel is cooled on the cooling bed. The insulation cover of the cooling bed keeps the round steel warm and cools it slowly. The cooling rate of the round steel is controlled at 0.5-0.8℃ / s.

[0042] Preferably, in step S13, the mixture is kept warm and then slowly cooled for 3-4 minutes.

[0043] After holding and slow cooling, the formation of bainite structure is further reduced, and the hardness of the round steel is finally controlled to ≤260 HBW, which meets the user's hardness requirements for round steel. It also eliminates the softening annealing treatment after conventional rolling, reduces production costs, and shortens delivery cycle.

[0044] S14, post-processing: post-processing the round steel.

[0045] This invention aims to improve the existing nitrided round steel production process. The process design rolling speed mentioned in this invention refers to the rolling speed set for rolling nitrided round steel in the existing process. The rolling process generally includes rough rolling, intermediate rolling, and final rolling. When rolling certain specifications of nitrided round steel, pre-finishing rolling and finishing rolling are required after intermediate rolling and before final rolling.

[0046] The following is a list of existing process design rolling speeds for nitrided round steel of some specifications, as shown in Table 1. The actual rolling speed of the present invention is controlled to be 80-90% accordingly.

[0047] Table 1 Existing rolling procedures and process design rolling speeds for some specifications of nitrided round steel

[0048] The present invention is described in detail below with reference to the following embodiments and comparative examples.

[0049] Taking the production of Φ45mm round bars from the nitrided steel continuous casting billet as an example, the steps include: Select 10 nitrided steel continuous casting billets, with billet numbers from 1 to 10 and a size of 280 mm * 280 mm * 9000 mm; Ten continuous casting billets with good surface quality were divided into two groups, with five continuous casting billets in each group. The first group adopted the process of the present invention as an embodiment, and the second group adopted the conventional process as a comparative example.

[0050] Since the rolling process is to roll Φ45mm round bars, the rolling process in the embodiment and the comparative example includes rough rolling, intermediate rolling and final rolling. The specific rolling speeds are shown in Table 2 below.

[0051]

[0052] Example The first batch of continuous casting billets are continuously loaded into a walking beam heating furnace for heating. The second heating temperature is 1050-1070℃ and the soaking temperature is 1060℃. High-pressure water dephosphorization, descaling pressure 23MPa, increasing high-pressure water descaling pressure, improving the descaling effect of steel billets; Rough rolling (φ850*3+φ750*3) is carried out by alternating horizontal and vertical rolling mills, both of which are fifth-generation short-stress rolling mills. The rolling process is performed in 6 passes, and the material shape after rolling is Φ144. The rolling speed of each pass is controlled at 80% of the process design rolling speed. Dry rolling is adopted in each pass to prevent warping and steel piling caused by low-temperature rolling. 1# flying shear head, cutting head length ≥ 100mm, to remove defects at the end of the rolled billet; Medium rolling (φ750*1+φ550*5) rolling, the rolling mills are arranged alternately horizontally and vertically, all of which are fifth-generation short stress rolling mills, with 6 rolling passes, the material shape after rolling is Φ59.5, and the rolling speed of each pass is controlled at 80% of the process design rolling speed; 2# flying shear head, cutting head and tail length ≥ 200mm, removing defects at the head and tail ends of the rolled billet; 4# flying shear head, cutting length ≥ 300mm, to remove defects at the head and tail of the rolling billet; Three-roll 370++KOCKS sizing reduction rolling is final rolling, with 4 passes. The finished product size is Φ45. The rolling speed of each pass is controlled at 80% of the process design rolling speed, and the rolling temperature is 830-840℃. 5# flying shear segmented and multiple lengths; Cooling bed, straighten and cool the cooling bed, cover with heat preservation cover, keep warm and cool slowly for 3 to 4 minutes, and control the cooling rate of round steel at 0.6℃ / s; Cold shearing, cutting to length, sampling, testing, and hardness test data are shown in Table 3.

[0053] Table 3 Hardness test data of the first group of round steel

[0054] Comparative Example The second batch of continuous casting billets are continuously loaded into the walking beam heating furnace for heating. The second heating temperature is 1140-1220℃ and the soaking temperature is 1160-1200℃. High-pressure water dephosphorization, descaling pressure 18MPa, using high-pressure water to remove iron oxide scale on the surface of the billet; Rough rolling (φ850*3+φ750*3) is carried out by alternating horizontal and vertical rolling mills, both of which are fifth-generation short-stress rolling mills. The rolling process is carried out in 6 passes, and the material shape after rolling is Φ144. The rolling speed of each pass is controlled according to the process design rolling speed. 1# flying shear head, cutting head length ≥ 100mm, to remove defects at the end of rolled billet; Medium rolling (φ750*1+φ550*5) rolling, the rolling mills are arranged alternately horizontally and vertically, all of which are fifth-generation short stress rolling mills, with 6 rolling passes, the material shape after rolling is Φ59.5, and the rolling speed of each pass is controlled according to the process design rolling speed; 2# flying shear head, cutting head and tail length ≥ 200mm, removing defects at the head and tail ends of the rolled billet; 4# flying shear head, cutting length ≥ 300mm, to remove defects at the head and tail of the rolling billet; Three-roll 370++KOCKS sizing reduction rolling is final rolling, with 4 passes. The finished product size is Φ45. The rolling speed of each pass is controlled according to the process design rolling speed; the rolling temperature is 880-890℃; 5# flying shear segmented and multiple lengths; Cooling bed, straightening and cooling, without insulation cover, round steel cooling speed control range: 1.25℃ / S; Cold shearing, cutting to length, sampling and testing; hardness test data are shown in Table 4.

[0055] Table 4 Hardness test data of the second group of round steel

[0056] Analysis of the two groups of data shows that the hardness value of the Φ45mm nitrided round steel in the comparative example fluctuates within the range of 291-312 HBW, which exceeds the user requirement of 260 HBW. The hardness value of the Φ45mm nitrided round steel in the embodiment fluctuates within the range of 228-242 HBW, and the hardness values ​​all meet the user requirement of ≤260 HBW. That is, the hardness value of the present invention meets the requirements of the user's technical agreement, and no softening annealing is required, and it can be directly delivered to the user, reducing production costs and shortening delivery cycles. The present invention can not only meet the user's hardness requirements for round steel, but also cancel the softening annealing process after rolling, thereby reducing production costs and shortening delivery cycles. The invention can be promoted and applied in similar production lines of high-quality steel.

[0057] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiment. All technical solutions based on the concept of the present invention are within the scope of protection of the present invention. It should be noted that for those skilled in the art, various improvements and modifications that do not depart from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing nitrided round steel, characterized in that: Including heating, descaling, rolling and cooling on cooling bed; In the heating process, the second heating temperature is 1040-1070°C, and the soaking temperature is 1040-1070°C; In the descaling process, the descaling pressure is 23 ± 1 MPa; During the rolling process, the rolling speed of each pass is controlled at 80-90% of the process design rolling speed.

2. The method for producing nitrided round steel according to claim 1, wherein: During the cooling process, the cooling bed cover keeps the round steel warm and cools it slowly, controlling the cooling rate of the round steel to 0.5-0.8℃ / s.

3. The method for producing nitrided round steel according to claim 2, wherein: Keep warm and cool for 3-4 minutes.

4. The method for producing nitrided round steel according to any one of claims 1 to 3, wherein: The rolling process includes rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling and final rolling. The rolling speed of each pass of rough rolling, intermediate rolling, pre-finishing rolling, finishing rolling and final rolling is controlled at 80-90% of the process design rolling speed. Among them, rough rolling adopts dry rolling and the final rolling temperature is ≤850℃.

5. The method for producing nitrided round steel according to claim 4, wherein: Rough rolling is performed in 6 passes.

6. The method for producing nitrided round steel according to claim 4, wherein: The intermediate rolling process is 4-6 passes.

7. The method for producing nitrided round steel according to claim 4, wherein: Pre-finish rolling is performed in 2-4 passes.

8. The method for producing nitrided round steel according to claim 4, wherein: Finish rolling is performed in 2-6 passes.

9. The method for producing nitrided round steel according to claim 4, wherein: Rough rolling is performed in 3-4 passes.

10. A nitrided round steel, characterized in that: The nitrided round steel is produced by the production method of any one of claims 1 to 9.

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