A forming method for increasing the grain size of Ni2 steel

Abstract

The present invention discloses a forming method for improving the grain size of Ni2 steel forgings. This method addresses the problems of poor internal quality, low grain size, and large grain size variations in Ni2 steel during the forging process. The present invention provides an effective method for improving the grain size of Ni2 steel. The forming method employed by the present invention to solve the technical problems includes the following steps: 1) pre-forging homogenization heat treatment; 2) using a steel plate; 3) upsetting; 4) drawing and rounding; 5) forging an intermediate billet; 6) complete annealing of the intermediate billet; 7) heating the intermediate billet; 8) forging the finished product; and 9) complete annealing of the finished product. The Ni2 steel forging forming method of the present invention can effectively cut off the Ni2 steel microstructure, improve the internal quality and grain size uniformity of the forging, and reduce grain size variations.

Description

Technical Field

[0001] The invention belongs to the technical field of metal plastic processing, and particularly relates to a forming method for improving the grain size of Ni2 steel, which can effectively improve the internal quality and grain uniformity of Ni2 steel forgings and reduce grain size differences. Background Art

[0002] Ni2 steel's high Ni content gives it high hardenability and good comprehensive mechanical properties. It's often used to manufacture large, high-strength, high-toughness forgings, and is widely used in key load-bearing and pressure-bearing components of large-scale equipment such as oil equipment, power station equipment, and transportation equipment. Therefore, Ni2 steel forgings are subject to harsh service environments and complex stress conditions during their service life. Although this steel grade has good inherent advantages, it also has strong structural inheritance. The austenite grains produced during the heating process are difficult to refine, reducing both the strength and toughness of the material. This can significantly shorten the service life of Ni2 steel forgings, causing the entire equipment to be shut down, resulting in significant economic losses.

[0003] At present, the traditional forming method of Ni2 steel is to heat it to the forging temperature and then perform a upset and pull-out process to separate the blank. When the surface temperature of the forging drops below 800℃, it is returned to the furnace for heating for two hours and then forged again. Although this forming method can effectively form the finished product size and shape, the grain size of the forged product is about 5 levels, and there are grains with a grain size of 3 levels, which cannot meet the requirements of high-end products. In order to effectively improve the internal quality and grain size uniformity of Ni2 steel forgings and reduce the grain size difference, it is necessary to make full use of the "forming" and "forming properties" characteristics of forging to refine the internal structure grains, cut off the tissue inheritance, and obtain Ni2 steel forgings with fine and uniform grains. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a forming method for improving the grain size of Ni2 steel, so as to improve the uniformity of the internal grain size of Ni2 steel forgings, reduce the grain size difference, and at the same time improve the forging efficiency, reduce the overall production cost, and improve the overall quality of Ni2 steel forgings.

[0005] The present invention achieves the above-mentioned purpose through the following technical solutions:

[0006] A forming method for increasing the grain size of Ni2 steel, the forming steps are as follows:

[0007] Step 1) Homogenization heat treatment before forging: Place the electroslag ingot into the heating furnace and heat it in three stages to 1230℃~1260℃ for 20~25 hours to ensure uniform heating;

[0008] Step 2) Offsetting the ingot guard plate: Place the electroslag ingot after being taken out of the furnace on the hydraulic press, use the upper and lower flat anvils to offset the electroslag ingot tail guard plate, and level the electroslag ingot tail end surface;

[0009] Step 3) Upsetting: The electroslag ingot obtained in step 2) is upset on a hydraulic press using an upsetting plate and a lower plate. The upsetting deformation is 50% to 60%, and the height-to-diameter ratio of the electroslag ingot after upsetting is 0.6 to 0.75.

[0010] Step 4) Lengthening: The electroslag ingots obtained in step 3) are stretched, squared and rounded on a hydraulic press using upper and lower flat anvils. The deformation of each stretching pass is 20% to 30%.

[0011] Step 5) Forging the intermediate billet: Forge the intermediate billet obtained in step 4) to the shape and size of the intermediate billet, with a deformation of 25% to 45% per pass; a forging ratio of 1.5 to 3.0 is reserved from the size of the intermediate billet to the size of the finished product;

[0012] Step 6) Full annealing of the intermediate billet: normalizing, hydrogen expansion and full annealing of the intermediate billet after forging to room temperature;

[0013] Step 7) Heating the intermediate billet: Rapidly heat the intermediate billet obtained in step 6) to 950°C~1050°C at low temperature and keep it at this temperature for 4~6 hours to ensure uniform heating;

[0014] Step 8) Finished product forging: Forge the intermediate billet obtained in step 7) to the finished product shape and size; the deformation amount of each pass is 25%~45%;

[0015] Step 9) Complete annealing of the finished product: normalize and completely anneal the finished product obtained in step 8) to room temperature.

[0016] The entire forging process is completed in two passes: steps 2), 3), 4), and 5) are the first pass, and step 8) is the second pass. In steps 4), 5), and 8, the electroslag ingot is formed individually on a hydraulic press, or after being forged to the desired size on a hydraulic press, it is forged on a precision forging machine.

[0017] The beneficial effects of the present invention are as follows:

[0018] Before forging, the electroslag ingot undergoes a homogenization heat treatment to improve internal segregation and coarse mixed crystals caused by component segregation through high-temperature diffusion. A hydraulic press is used for high-deformation upsetting and stretching to fully deform the core of the electroslag ingot, breaking up the core's as-cast structure and welding together loose shrinkage cavities. High temperature and high deformation allow dynamic recrystallization, sub-dynamic recrystallization, and static recrystallization after deformation to fully refine the core grains. Squaring and rounding fully deform the surface of the electroslag ingot, refining the surface grains, thereby producing an intermediate billet with uniform overall deformation and fine grains. After the intermediate billet is fully annealed to room temperature to cut off microstructure inheritance, it is rapidly heated to the final product deformation temperature to prevent grain coarsening within the intermediate billet due to heating. A larger deformation margin is reserved for the intermediate billet relative to the final product, and low-temperature deformation further refines the internal grain structure. After forging to the final shape and size, it is fully annealed again to cut off microstructure inheritance and provide a good internal grain structure for subsequent tempering heat treatment. For example, in steps 3 and 4), a hydraulic press uses an upsetting plate and a lower flat plate to upset the ESR ingot with a large deformation of 50% to 60%. After upsetting, the ESR ingot has a height-to-diameter ratio of 0.6 to 0.75. Upper and lower flat anvils are used to stretch the ESR ingot with a large deformation, with each pass achieving a deformation of 20% to 30%. By slab-forming the ESR ingot with a large deformation, the internal as-cast structure is fully broken down, core defects are compacted and welded, and the internal deformation of the ESR ingot is uniform, allowing for sufficient dynamic recrystallization and grain refinement.

[0019] The invention only requires one firing step from the electroslag ingot to the intermediate blank and from the intermediate blank to the finished product, which effectively prevents the grain coarsening of the internal grains of the Ni2 steel caused by heating and effectively improves the production efficiency.

[0020] The Ni2 steel deformation method of the present invention can effectively improve the internal quality and grain size uniformity of forgings, reduce grain size differences, and simultaneously improve forging production efficiency and reduce comprehensive production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic flow chart of a forming method for improving the grain size of Ni2 steel according to the present invention.

[0022] Figure 2 This is a dimension diagram of an intermediate billet forging of a hydraulic press according to an embodiment of the forming method for improving the grain size of Ni2 steel of the present invention.

[0023] Figure 3 This is a dimension diagram of an intermediate billet forging of a precision forging machine according to an embodiment of the forming method for improving the grain size of Ni2 steel of the present invention.

[0024] Figure 4 This is a dimension diagram of a semi-finished forging of a hydraulic press according to an embodiment of the forming method for improving the grain size of Ni2 steel of the present invention.

[0025] Figure 5 This is a dimension diagram of a semi-finished forging of a precision forging machine according to an embodiment of the forming method for improving the grain size of Ni2 steel of the present invention.

[0026] Figure 6 This is a dimension diagram of a finished forging of a hydraulic press according to an embodiment of a forming method for increasing the grain size of Ni2 steel described in the present invention.

[0027] Figure 7 This is a dimension diagram of finished forgings of a precision forging machine according to an embodiment of a forming method for increasing the grain size of Ni2 steel described in the present invention.

[0028] Figure 8 This is a grain size diagram at 1 / 2 radius of three finished electroslag ingots produced in Example 1 of a forming method for improving the grain size of Ni2 steel described in the present invention.

[0029] Figure 9 This is a grain size diagram at 1 / 2 radius of three finished electroslag ingots produced in Example 2 of a forming method for improving the grain size of Ni2 steel described in the present invention. DETAILED DESCRIPTION

[0030] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be derived from these drawings without inventive effort. The embodiments of the present invention and the features in the embodiments can be combined with each other unless there is a conflict.

[0031] Example 1: This example provides a method for improving the grain size of Ni2 steel. The electroslag ingot specification is Φ725mm / Φ790mm, length is 1461mm, and the finished product size is as follows: Figure 6 、 7 As shown, the forming steps are as follows:

[0032] Step 1) Place the electroslag ingot into the heating furnace and heat it to 1230±10℃ in three stages. Keep it warm for 25 hours to allow the high temperature to diffuse and ensure uniform heating.

[0033] Step 2) Place the electroslag ingot after being taken out of the furnace on the hydraulic press, use the upper and lower flat anvils to remove the electroslag ingot tail guard plate, and level the electroslag ingot tail end surface;

[0034] Step 3) The electroslag ingot obtained in step 2) is upset on a hydraulic press using an upsetting plate and a lower plate, upsetting to H = 700 mm (~Φ1120 mm);

[0035] Step 4) The electroslag ingot obtained in step 3) is stretched and squared on a hydraulic press using upper and lower flat anvils; the deformation amount of each stretching pass is 20%, the ingot is squared to 400mm, squared to 420mm, and rounded to Φ420mm;

[0036] Step 5) Forge the electroslag ingot obtained in step 4) on a hydraulic press. Figure 2 Forging size drawing of intermediate billet of hydraulic press, notch at the minor diameter position and stretch from Φ420mm to Φ300mm;

[0037] Step 6) Air-cool the intermediate billet after step 5) to 250°C~280°C, load it into the furnace for normalizing + hydrogen expansion + complete annealing to room temperature;

[0038] Step 7) Rapidly heat the intermediate billet obtained in step 6) to 1050±10°C at low temperature and keep it at this temperature for 4 hours to ensure uniform heating;

[0039] Step 8) Forge the intermediate billet obtained in step 7) on a hydraulic press in two passes to the finished product shape and size; the first pass presses the forging into Figure 4 The semi-finished product of the hydraulic press is stretched according to the forging size drawing. The large diameter is forged from Φ420mm to Φ355mm, and the small diameter is forged from Φ300mm to Φ240mm. The second pass presses the forging into Figure 4 The finished product forging dimensions of the hydraulic press are forged to be lengthened, with the major diameter forged from Φ355mm to Φ305mm, and the minor diameter forged from Φ240mm to Φ195mm;

[0040] Step 9) The finished product of step 8) is air-cooled to 250°C~280°C, placed in a furnace for normalizing and complete annealing to room temperature.

[0041] The electroslag ingot is forged to the intermediate billet size in one pass without returning to the furnace for heating. The intermediate billet is forged to the finished product size in one pass without returning to the furnace for heating.

[0042] In this embodiment 1, three electroslag ingots are produced. Figure 8 This is the grain size diagram of the finished forging at 1 / 2 radius. The grains are uniform and fine, and the grain size is about 8~9.

[0043] Example 2: This example provides a method for improving the grain size of Ni2 steel. The electroslag ingot specification is Φ725mm / Φ790mm, length is 1461mm, and the finished product size is as follows: Figure 6 、 7 shown.

[0044] Step 1) Place the electroslag ingot into the heating furnace and heat it in three stages to 1230±10℃ for 25 hours to ensure high temperature diffusion and uniform heating.

[0045] Step 2) Place the electroslag ingot after being taken out of the furnace on the hydraulic press, use the upper and lower flat anvils to remove the electroslag ingot tail guard plate, and level the electroslag ingot tail end surface;

[0046] Step 3) The electroslag ingot obtained in step 2) is upset on a hydraulic press using an upsetting plate and a lower plate, upsetting to H = 700 mm (~Φ1120 mm);

[0047] Step 4) The electroslag ingot from step 3) is stretched and squared on a hydraulic press using upper and lower flat anvils. The deformation per stretching pass is 20%, and the ingot is squared to 480mm, squared to 500mm, and rounded to Φ500mm. The ingot is then transferred to a fine forging machine for radial forging, where it is forged to Φ420mm in one pass at a speed of ≥3m / min and a forging frequency of 180 strokes / minute.

[0048] Step 5) Radial forging of the electroslag ingot obtained in step 4) on a precision forging machine. Figure 3 The forging size of the intermediate billet of the precision forging machine is drawn at the small diameter position, from Φ420mm to Φ320mm, with a drawing speed of ≥3m / min and a forging frequency of 180 times / min;

[0049] Step 6) Air-cool the intermediate billet after step 5) to 250°C~280°C, load it into the furnace for normalizing + hydrogen expansion + complete annealing to room temperature;

[0050] Step 7) Rapidly heat the intermediate billet obtained in step 6) to 1050±10°C at low temperature and keep it at this temperature for 2.5 hours to ensure uniform heating;

[0051] Step 8) The intermediate billet obtained in step 7) is radially forged on a finishing forging machine in two passes to the finished product shape and size; the first pass presses the forging into Figure 5 The semi-finished product is stretched according to the forging size drawing of the precision forging machine. The large diameter is forged from Φ420mm to Φ355mm, and the small diameter is forged from Φ320mm to Φ240mm. The pulling speed is ≥3m / min, and the forging frequency is 180 times / min. The forging is pressed into Figure 5 The finished product forging dimensions of the precision forging machine are drawn out, with the major diameter forged from Φ355mm to Φ305mm, and the minor diameter forged from Φ240mm to Φ195mm. The drawing speed is ≥6m / min, and the forging frequency is 180 times / min.

[0052] Step 9) The finished product of step 8) is air-cooled to 250°C~280°C, placed in a furnace for normalizing and complete annealing to room temperature.

[0053] The electroslag ingot is forged to the intermediate billet size in one pass without returning to the furnace for heating. The intermediate billet is forged to the finished product size in one pass without returning to the furnace for heating.

[0054] In this embodiment 2, three electroslag ingots are produced. Figure 9 This is the grain size diagram of the finished forging at 1 / 2 radius. The grains are uniform and fine, and the grain size is about 8~9.

Claims

1. A forming method for increasing the grain size of Ni2 steel, characterized in that: It includes the following steps: Step 1) Homogenization heat treatment before forging: Place the electroslag ingot into the heating furnace and heat it in three stages to 1230℃~1260℃, keep it warm for 20~25 hours to ensure uniform heating; Step 2) Offsetting the ingot guard plate: Place the electroslag ingot after being taken out of the furnace on the hydraulic press, use the upper and lower flat anvils to offset the electroslag ingot tail guard plate, and level the electroslag ingot tail end surface; Step 3) Upsetting: The electroslag ingot obtained in step 2) is upset on a hydraulic press using an upsetting plate and a lower plate. The upsetting deformation is 50% to 60%, and the height-to-diameter ratio of the electroslag ingot after upsetting is 0.6 to 0.

75. Step 4) Lengthening: The electroslag ingots obtained in step 3) are stretched, squared and rounded on a hydraulic press using upper and lower flat anvils. The deformation of each stretching pass is 20% to 30%. Step 5) Forging the intermediate billet: Forge the intermediate billet obtained in step 4) to the shape and size of the intermediate billet, with a deformation of 25% to 45% per pass; a forging ratio of 1.5 to 3.0 is reserved from the size of the intermediate billet to the size of the finished product; Step 6) Full annealing of the intermediate billet: normalizing, hydrogen expansion and full annealing of the intermediate billet after forging to room temperature; Step 7) Heating the intermediate billet: Rapidly heat the intermediate billet obtained in step 6) to 950°C~1050°C at low temperature and keep it at this temperature for 4~6 hours to ensure uniform heating; Step 8) Finished product forging: Forge the intermediate billet obtained in step 7) to the finished product shape and size; the deformation amount of each pass is 25%~45%; Step 9) Complete annealing of the finished product: normalize and completely anneal the finished product obtained in step 8) to room temperature.

2. A forming method for improving the grain size of Ni2 steel according to claim 1, characterized in that: The entire forging process is completed by two firings, of which step 2), step 3), step 4), and step 5) are the first firing, and step 8) is the second firing to prevent the internal grain coarsening of the forging caused by reheating.

3. A forming method for increasing the grain size of Ni2 steel according to claim 1, characterized in that: Step 1) Before forging, the electroslag ingot is subjected to homogenization heat treatment, and the internal segregation of the electroslag ingot is improved by high-temperature diffusion through heating the electroslag ingot to 1230°C~1260°C in three stages and keeping the temperature for 20~25 hours.

4. The forming method for increasing the grain size of Ni2 steel according to claim 1, characterized in that: In step 4), step 5), and step 8, the electroslag ingot is formed separately in a hydraulic press, or after being forged to a suitable size in a hydraulic press, it is forged in a precision forging machine.

5. The forming method for increasing the grain size of Ni2 steel according to claim 1, characterized in that: In step 6), the electroslag ingot is forged to the intermediate billet size. After a forging ratio of 1.5 to 3.0 is reserved from the intermediate billet size to the finished product size, the intermediate billet is air-cooled to 250°C to 280°C and loaded into a furnace for normalizing + hydrogen expansion + complete annealing to room temperature to cut off the internal tissue inheritance of the intermediate billet.

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