A method for improving band structure of a rolled and forged elongated round steel

Abstract

The present application belongs to the technical field of metal pressure processing, and particularly relates to a strip structure improvement method for elongation of round steel by rolling and forging, and the specific steps are as follows: S1: combined production of an initial rolling mill and a radial forging machine: the initial rolling mill is mainly used for initial rolling and radial forging compression ratio, except that the radial forging single-process forging ratio is designed as 2-3, all the rest compression ratios are arranged in the initial rolling blooming process; S2: initial rolling and blooming of a steel ingot: the steel ingot is red sent and loaded into a heat accumulating soaking furnace for heating in a steelmaking mode, initial rolling and blooming is adopted to roll a square billet; S3: initial rolling billet radial forging: the cooled initial rolling billet is loaded into a chamber furnace for heating, and radial forging is adopted to forge the material; S4: hot treatment of the forged round: the forged round is laid flat and loaded into a car type annealing furnace for normalizing and tempering treatment in the furnace. The method adopts steelmaking mode, initial rolling and blooming, flat laying air cooling, low temperature heating, radial forging machine forging and normalizing and tempering treatment, so that the strip structure reaches level 1 and below, and meets the requirements.

Description

Technical Field

[0001] This invention belongs to the field of metal pressure processing technology, and particularly relates to a method for improving the banded structure of long, thin-diameter round steel produced by combined rolling and forging. Background Technology

[0002] Conventional alloy structural steel is mainly used for various mechanical parts and engineering components that bear loads, transmit power, and provide support. It is widely used in construction, bridges, vehicles, ships, engineering machinery, petrochemicals, energy equipment and other fields. It meets the requirements of different working conditions such as high strength, high toughness, wear resistance, fatigue resistance and weldability. It is the steel grade with the largest amount of use and the widest application in industrial manufacturing and infrastructure construction.

[0003] Small-sized alloy steel forged round bars are mainly used to manufacture high-speed pump shafts, with a specification range of Ø100-Ø150mm and a length of 4m-5m. This product has a special production purpose, requiring high strength and high toughness. To prevent displacement, torsion, and bending deformation during high-speed operation, users have very strict requirements for the banded structure of the forged round steel. The finished product's banded structure inspection requirement is less than level 1. However, based on existing equipment conditions, the main process used is forging from ingot casting. The specific process flow is: steelmaking ingot casting - initial rolling mill forging - stacking cooling - high-temperature heating (1180±20℃) - radial forging - annealing - inspection and delivery. The entire process uses direct drawing and forming, and the unidirectional large deformation will elongate the segregation bands, which is very detrimental to improving the banded structure. The finished product's banded structure is grade 2.0-3.0, which does not meet the requirements. Summary of the Invention

[0004] The purpose of this invention is to provide a method for improving the banded structure of slender round steel bars produced by combined rolling and forging. This method addresses the problem that the existing die-casting steel ingot forging process uses direct drawing to form the entire product, which results in large unidirectional deformation that elongates the segregation bands, making it very unfavorable for improving the banded structure. The finished product has a banded structure grade of 2.0-3.0, which does not meet the requirements.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for improving the banded microstructure of slender round steel bars obtained through combined rolling and forging, comprising the following specific steps: S1: Joint production of primary rolling mill and radial forging mill: The primary rolling mill rolls the billet into a primary rolled square billet, and the radial forging mill forges the finished forged round. When allocating the compression ratio, the compression ratio of primary rolling and radial forging is planned and designed in advance, with the primary rolling mill as the main component. Except for the radial forging single-process forging ratio designed to be 2-3, all other compression ratios are placed in the primary rolling billet opening process. S2: Initial rolling of steel ingots: The steel ingots cast in the steel plant are sent into a regenerative soaking furnace for heating. After loading into the furnace, the heating temperature is raised to 1260±20℃ and held for 2-4 hours. Then, the initial rolling mill is used for rolling and billet making. A special die is used for rolling to produce finished square billets. The rolling compression ratio accounts for 90-95% of the total compression ratio. The rolled square billets are directly placed on a cooling bed for air cooling. After cooling to a surface temperature of 580-600℃, they are naturally cooled in the air. S3: Initial rolling billet radial forging: After cooling, the initial rolling billet is loaded into a chamber furnace and heated to 1100±50℃. After holding at the temperature for 2-4 hours, it is forged into a product using a radial forging machine. After being forged into round steel, it is first placed on a cooling bed for air cooling with an interval of more than 500mm, and then air cooled to below 300℃. S4: Heat treatment of forged rounds: The forged rounds are laid flat and placed into a bogie-type annealing furnace for normalizing and tempering treatment inside the furnace.

[0006] Preferably, the ingot temperature in step S2 is 600-700℃.

[0007] Preferably, the heating rate in step S2 is 120-150℃ / h.

[0008] Preferably, the finished billet in step S2 has a size of 150*150-180*180mm.

[0009] Preferably, in step S2, the flat air cooling uses 2-4 fans for cooling.

[0010] Preferably, in step S3, the initial temperature of the chamber furnace is set to 500℃-600℃, and the heating rate is set to 80-100℃ / h.

[0011] Preferably, in step S3, the reduction rate of the intermediate pass of the forging mill is set to 18-22%.

[0012] Preferably, in step S4, the heating temperature for normalizing is 880℃-900℃, and after holding at this temperature for 4-6 hours, it is air-cooled to ensure uniformity of the microstructure. After air-cooling for 4-5 hours, the surface temperature reaches 350-450℃. It is then heated to 600-650℃, held at this temperature for 6-8 hours for tempering, and then removed from the furnace and air-cooled.

[0013] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: (1) By designing the change of deformation distribution in rolling and forging, a division of labor mechanism is established between rolling main deformation and forging to maintain shape, so as to achieve the optimal matching of total compression ratio segmentation, concentrate the large compression ratio for high temperature rolling, and improve the internal band structure by combining small deformation of radial forging. (2) After the initial rolling, flat air cooling is adopted to optimize the rolling cooling process, accelerate the cooling speed of the high-temperature hot steel billet, suppress the formation of coarse grains, create conditions for the internal structure stability of the billet after forging, and reduce the tendency of the finished round steel strip structure to deteriorate. (3) A two-stage microstructure improvement technique of air cooling pre-control and normalizing and tempering is adopted to induce the formation of fine and uniform microstructure and achieve overall homogenization of microstructure and composition; (4) This method uses steel casting ingots, rolling and billet opening on a primary rolling mill, flat air cooling, low temperature heating, forging on a radial forging mill, and normalizing and tempering treatment to make the banded structure reach level 1 or below, which meets the requirements. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the banded structure of the long alloy steel bar obtained by combined rolling and forging in Example 1 of the present invention. Detailed Implementation

[0015] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0016] Taiyuan Iron & Steel's Profiles Division uses a 1000mm primary rolling mill and an 1800-ton radial forging mill to jointly forge small-diameter CrMo alloy steel round bars. The raw material is an 8.4-ton square ingot (806mm*806mm at the large end and 673mm*673mm at the small end), with a forged round diameter of 120mm and a total compression ratio of 52.1. The entire process employs longitudinal drawing and high compression ratio deformation forming. The method for improving the banded microstructure of slender round bars using a combined rolling and forging method provided by this invention is described in the following steps: Example 1

[0017] S1: Joint production of primary rolling mill and radial forging mill: The compression ratio of primary rolling and radial forging is mainly based on the primary rolling mill, with a rolling specification of 160*160mm. The single-process forging ratio of radial forging is 2.2, and the rest are all placed in the primary rolling billet opening process. S2: Initial rolling of steel ingots: 8.4 tons of square ingots are loaded into a soaking furnace for heating. The ingot temperature is 600℃, the heating rate is 120℃ / h, the heating temperature is raised to 1260℃, and the temperature is held for 4 hours. The billet is rolled on a 1000mm primary rolling mill. The finished square billet has a specification of 160*160mm. The rolling compression ratio accounts for 90% of the total compression ratio. After rolling, the billet is placed on a cooling bed and cooled by two fans to 595℃. The fans are then turned off and the billet is allowed to cool naturally. S3: Initial rolling billet radial forging: The initial rolling billet is loaded into a chamber furnace for heating. The initial temperature of the chamber furnace is set to 500℃, the heating rate is set to 80℃ / h, the heating temperature is raised to 1150℃, and it is held for 2 hours. The billet is forged into a product using an 1800-ton radial forging machine. The reduction rate of the intermediate pass of the radial forging machine is set to 22%. After the radial forging machine produces round bars, they are first placed on a cooling bed for air cooling with an interval of more than 500mm, and then air cooled to 210℃. S4: Heat Treatment of Forged Rounds: The forged rounds are laid flat and placed into a bogie-type annealing furnace for normalizing and tempering as a preparatory heat treatment. The normalizing temperature is 880℃, held for 4 hours, and then air-cooled to ensure uniform microstructure. After air-cooling for 4 hours, the surface temperature is 380℃. Then, the temperature is raised to 600℃, held for 6 hours, and tempered before being removed from the furnace and air-cooled. The finished forged rounds are tested for banded microstructure grade 1.0. Figure 1 As shown. Example 2

[0018] S1: Joint production of primary rolling mill and radial forging mill: The compression ratio of primary rolling and radial forging is mainly based on the primary rolling mill, with a rolling specification of 180*180mm. The single-process forging ratio of radial forging is 2.8, and the rest are all placed in the primary rolling billet opening process. S2: Initial rolling of steel ingots: 8.4 tons of square ingots are loaded into a soaking furnace for heating. The ingot temperature is 650℃, the heating rate is 130℃ / h, and the heating temperature is raised to 1260℃. The temperature is held for 4 hours, and the billet is rolled on a 1000mm primary rolling mill. The finished square billet has a specification of 180*180mm. The rolling compression ratio accounts for 90% of the total compression ratio. After rolling, the billet is placed on a cooling bed and cooled by 3 fans to 595℃. The fans are then turned off for natural cooling. S3: Initial rolling billet radial forging: The initial rolling billet is loaded into a chamber furnace for heating. The initial temperature of the chamber furnace is set to 550℃, the heating rate is set to 90℃ / h, the heating temperature is raised to 1150℃, and it is held for 2 hours. The billet is forged into a product using an 1800-ton radial forging machine. The reduction rate of the intermediate pass of the radial forging machine is set to 22%. After the radial forging machine forges the round steel, it is first placed on a cooling bed for air cooling with an interval of more than 500mm, and air cooled to 210℃. S4: Heat treatment of forged rounds: The forged rounds are laid flat and placed into a bogie-type annealing furnace. They are normalized and tempered in the furnace as a preparatory heat treatment. The heating temperature for normalizing is 890℃. After holding at this temperature for 4 hours, they are air-cooled to ensure uniformity of the microstructure. After air-cooling for 4 hours, the surface temperature is 360℃. Then, the temperature is raised to 610℃ and held at this temperature for 5 hours. After tempering, the forged rounds are removed from the furnace and air-cooled. The finished forged rounds are tested for banded microstructure grade 1.0. Example 3

[0019] S1: Joint production of primary rolling mill and radial forging mill: The compression ratio of primary rolling and radial forging is mainly based on primary rolling mill, with a rolling specification of 150*150mm. The single-process forging ratio of radial forging is 2.2, and the rest are all placed in the primary rolling billet opening process. S2: Initial rolling of steel ingots: 8.4 tons of square ingots are loaded into a soaking furnace for heating. The ingot temperature is 700℃, the heating rate is 130℃ / h, the heating temperature is raised to 1260℃, and the temperature is held for 4 hours. The billet is rolled on a 1000mm primary rolling mill. The finished square billet has a specification of 150*150mm. The rolling compression ratio accounts for 95% of the total compression ratio. After rolling, the billet is placed on a cooling bed and cooled by 4 fans to 595℃. The fans are then turned off and the billet is allowed to cool naturally. S3: Initial rolling billet radial forging: The initial rolling billet is loaded into a chamber furnace for heating. The initial temperature of the chamber furnace is set to 550℃, the heating rate is set to 90℃ / h, the heating temperature is raised to 1150℃, and it is held for 2 hours. The billet is forged into a product using an 1800-ton radial forging machine. The reduction rate of the intermediate pass of the radial forging machine is set to 22%. After the radial forging machine forges the round steel, it is first placed on a cooling bed for air cooling with an interval of more than 500mm to air cool to 210℃. S4: Heat treatment of forged rounds: The forged rounds are laid flat and placed into a bogie-type annealing furnace. They are normalized and tempered in the furnace as a preparatory heat treatment. The heating temperature for normalizing is 880℃. After holding at this temperature for 4 hours, they are air-cooled to ensure uniformity of the microstructure. After air-cooling for 4 hours, the surface temperature drops to 380℃. Then, they are heated to 600℃, held at this temperature for 6 hours, tempered, and then removed from the furnace and air-cooled. The finished forged rounds are inspected and found to have a banded microstructure of grade 1.0 or lower.

[0020] After implementing the method of this invention, the banded structure of the long and thin alloy steel bars produced by combined rolling and forging reaches grade 1.0 or below, solving the serious problem of banded structure in pure long and thin round steel bars and achieving stable supply of the product.

Claims

1. A method for improving the banded microstructure of slender diameter round steel produced by combined rolling and forging, characterized in that, The specific steps are as follows: S1: Joint production of primary rolling mill and radial forging mill: The primary rolling mill rolls the billet into a primary rolled square billet, and the radial forging mill forges the finished forged round. When allocating the compression ratio, the compression ratio of primary rolling and radial forging is planned and designed in advance, with the primary rolling mill as the main component. Except for the radial forging single-process forging ratio designed to be 2-3, all other compression ratios are placed in the primary rolling billet opening process. S2: Initial rolling of steel ingots: The steel ingots cast in the steel plant are sent into a regenerative soaking furnace for heating. After loading into the furnace, the heating temperature is raised to 1260±20℃ and held for 2-4 hours. Then, the initial rolling mill is used for rolling and billet making. A special die is used for rolling to produce finished square billets. The rolling compression ratio accounts for 90-95% of the total compression ratio. The rolled square billets are directly placed on a cooling bed for air cooling. After cooling to a surface temperature of 580-600℃, they are naturally cooled in the air. S3: Initial rolling billet radial forging: After cooling, the initial rolling billet is loaded into a chamber furnace and heated to 1100±50℃. After holding at the temperature for 2-4 hours, it is forged into a product using a radial forging machine. After being forged into round steel, it is first placed on a cooling bed for air cooling with an interval of more than 500mm, and then air cooled to below 300℃. S4: Heat treatment of forged rounds: The forged rounds are laid flat and placed into a bogie-type annealing furnace for normalizing and tempering treatment inside the furnace.

2. The method for improving the banded microstructure of a thin-diameter round bar produced by combined rolling and forging as described in claim 1, characterized in that, In step S2, the ingot temperature for loading into the furnace is 600-700℃.

3. The method for improving the banded microstructure of a thin-diameter round bar produced by combined rolling and forging as described in claim 2, characterized in that, In step S2, the heating rate is 120-150℃ / h.

4. The method for improving the banded microstructure of a thin-diameter round bar produced by combined rolling and forging as described in claim 1, characterized in that, In step S2, the finished billet has a size of 150*150-180*180mm.

5. The method for improving the banded microstructure of a thin-diameter round bar produced by combined rolling and forging as described in claim 1, characterized in that, In step S2, the flat air cooling system uses 2-4 fans for cooling.

6. The method for improving the banded microstructure of a slender round bar produced by combined rolling and forging as described in claim 1, characterized in that, In step S3, the initial temperature of the chamber furnace is set to 500℃-600℃, and the heating rate is set to 80-100℃ / h.

7. The method for improving the banded microstructure of a slender round bar produced by combined rolling and forging as described in claim 6, characterized in that, In step S3, the intermediate pass reduction rate of the forging mill is set to 18-22%.

8. The method for improving the banded microstructure of a thin-diameter round bar produced by combined rolling and forging as described in claim 1, characterized in that, In step S4, the heating temperature for normalizing is 880℃-900℃. After holding at this temperature for 4-6 hours, the surface is air-cooled to ensure uniformity of the microstructure. After air-cooling for 4-5 hours, the surface temperature reaches 350-450℃. The surface is then heated to 600-650℃, held at this temperature for 6-8 hours for tempering, and then removed from the furnace and air-cooled.

Images