Method for improving anisotropy of low-alloy ultrahigh-strength steel bar

Abstract

The invention discloses a method for improving anisotropy of a low-alloy ultrahigh-strength steel bar, which comprises the following steps of: improving the structure and texture, refining grains, controlling the size and form of plastic non-metallic inclusions and reducing the anisotropy of the structure and the inclusions in a specific high-temperature diffusion and step-by-step cooling forging manner to obtain a matrix with uniform structure and high purity; and the difference between plasticity and impact absorbing energy of the bar in the transverse and longitudinal directions is reduced to be less than or equal to 5%.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metallurgy, in particular to a method for improving anisotropy of low-alloy ultra-high-strength steel bars. BACKGROUND

[0002] Low-alloy ultra-high-strength steel refers to steel with a total content of alloying elements such as Mn, Si, Ni, Cr, Mo, and V being less than or equal to 5% and a tensile strength at room temperature being greater than or equal to 1380 MPa. Typical steel grades include 40CrNi2Si2MoVA, 30Cr3SiNiMoVA, 30CrMnSiNi2A, 40CrMnSiMoVA, and AISI4340. The core structure of these steels is tempered martensite, which is strengthened by a small amount of alloying elements such as Cr, Ni, and Mo. Combined with a simple quenching and tempering process, these steels achieve a balance between high strength and toughness, and have fatigue resistance and wear resistance. Compared with high-alloy steels and alloy materials, low-alloy ultra-high-strength steels have lower costs, higher cost-effectiveness, and mature production processes, making them suitable for lightweight applications.

[0003] Low-alloy ultra-high-strength steels are mainly used in scenarios that require high strength, high reliability, and lightweightness. Typical applications include aircraft landing gear and fuselage load-bearing components in the aerospace industry, tank armor and casings in the weapons industry, high-performance vehicle safety structures and new energy battery casings in the automotive industry, and high-pressure containers and crane arms in the engineering machinery industry. These steels are key materials for reducing weight and increasing efficiency in high-end equipment.

[0004] “GJB5063-2001 Specification for Ultra-High-Strength Steel Bars for Aerospace” is a typical industry material specification for low-alloy ultra-high-strength steel bars. It specifies the requirements for ultra-high-strength steel bars for aerospace applications, including low-alloy steels such as 40CrNi2Si2MoVA (code: 300M), 45CrNiMo1VA (code: D6AC), and 40CrNi2MoA (code: 4340).

[0005] Taking the typical steel grade 40CrNi2Si2MoVA as an example, there is a significant anisotropy in the plasticity (reduction of area) and impact energy in the longitudinal and transverse directions of the steel bar. Other steel grades have similar anisotropy. For example, with a nominal cross-sectional area of 645-920 cm2, the anisotropy in the reduction of area is 30%, and the impact energy is 40%. As research deepens and production capacity improves, this problem is particularly pronounced in large-sized bars, severely limiting the further application and promotion of the products. SUMMARY

[0006] The present application provides a method for improving the anisotropy of steel bars, which solves the problems of low-alloy ultra-high-strength steels in the prior art, reduces the anisotropy of the materials, and improves the product quality and competitive advantage.

[0007] In view of the above, the present application provides the following technical scheme: A method for improving anisotropy of low-alloy ultra-high strength steel bars, comprising the following steps: S1. obtaining a steel ingot with a target composition meeting the material requirements of 40CrNi2Si2MoVA by smelting; S2. homogenizing the steel ingot obtained in step S1 at a temperature of 1180°C or higher for 10-15 hours at a heating rate of 150°C / h or lower after preheating; S3. reducing the temperature of the heating furnace to 700-850°C for 3-5 hours of low-temperature holding, then increasing the temperature to 1100-1150°C at a rate of 100-150°C / h for 3-6 hours of holding for forging preparation; S4. performing multi-pass step-down forging on the steel ingot, with a total forging ratio of >8 and a single-pass forging ratio of ≥3.5, to obtain low-alloy ultra-high strength steel bars with improved anisotropy.

[0008] Further, in step S1, the steel ingot has a composition by mass percentage of C 0.38-0.43, Si 1.45-1.80, Mn 0.60-0.90, P≤0.010, S≤0.010, Cr 0.70-0.95, Ni 1.65-2.00, Mo 0.30-0.50, Cu≤0.35, V 0.05-0.10, and the balance of Fe. More preferably, the composition is C 0.38-0.43, Si 1.45-1.80, Mn 0.60-0.90, P≤0.010, S≤0.001, Cr 0.70-0.95, Ni 1.65-2.00, Mo 0.30-0.50, V 0.05-0.10, and the balance of Fe.

[0009] Further, in step S1, the smelting process is completed by vacuum induction and vacuum consumable.

[0010] Further, in step S2, the homogenization process is performed at a temperature of 1180-1250°C for 10-12 hours. And / or, the homogenization heating rate is 80-100°C / h.

[0011] Further, in step S3, the low-temperature holding process is performed at 800-850°C for 4 hours. And / or, the temperature before forging preparation is increased to 1130-1150°C for holding.

[0012] Further, in step S4, the ingot heating temperature of the first heating process of the forging process is ≥ 1100℃, ensuring that the opening forging temperature is ≥ 1050℃; the first heating process is 2-up 2-drawing opening, and the forging ratio is ≥ 5; in the first heating process, after the first up-drawing, the second up-drawing is directly performed without temperature recovery.

[0013] Further, in step S4, the ingot heating temperature of the last heating process is ≥ 1050℃, ensuring that the opening forging temperature is ≥ 1000℃, and the final forging temperature is ≥ 850℃.

[0014] Further, in step S4, the up-drawing of the first heating process of the forging process is as follows: the original height is up-drawn by 1 / 2H at a pressing rate of 50-80 mm / s, then transverse drawing is performed, the drawing is pressed at 100-140 mm / s, the main deformation adopts single-anvil pressing amount of 200-350 mm, and the feeding amount is 1 / 2-3 / 4 anvil width.

[0015] Preferably, in step S4, the ingot heating temperature of the second heating process of the forging process is lower than that of the first heating process.

[0016] Preferably, in step S4, the up-drawing mode of the second heating process is the same as that of the first heating process.

[0017] Compared with the prior art, the present application has the following beneficial effects: The method of the present application improves the organization texture, refines the grain, controls the size and morphology of plastic non-metallic inclusions, reduces the anisotropy of the organization and inclusions, obtains a uniform and high-purity matrix, and reduces the difference between the plasticity and impact absorption energy of the rod in the transverse and longitudinal directions to ≤ 5%. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The transverse and longitudinal grain size detection results of the rod prepared in Example 1 of the present application.

[0019] Figure 2 The transverse and longitudinal grain size detection results of the rod prepared in Example 2 of the present application.

[0020] Figure 3 The transverse and longitudinal grain size detection results of the rod prepared in Example 3 of the present application.

[0021] Figure 4 The grain size detection results of the rod prepared in Comparative Example 1 of the present application.

[0022] Figure 5 The grain size detection results of the rod prepared in Comparative Example 2 of the present application.

[0023] Figure 6 The grain size detection results of the rod prepared in Comparative Example 3 of the present application.

[0024] Figure 7 SEM images and elemental analysis results of the inclusions in the rod prepared in Example 1 of the present application.

[0025] Figure 8 SEM images and elemental analysis results of the inclusions in the rod prepared in Example 2 of the present application.

[0026] Figure 9 SEM images and elemental analysis results of the inclusions in the rod prepared in Example 3 of the present application.

[0027] Figure 10 SEM images and elemental analysis results of the inclusions in the rod prepared in Comparative Example 1 of the present application.

[0028] Figure 11 SEM images and elemental analysis results of the inclusions in the rod prepared in Comparative Example 2 of the present application.

[0029] Figure 12 SEM images and elemental analysis results of the inclusions in the rod prepared in Comparative Example 3 of the present application. DETAILED DESCRIPTION

[0030] The technical solutions of the present application will be described in detail below in combination with preferred embodiments. Obviously, the described embodiments are only a 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 those of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0031] Example 1

[0032] The steel ingot has a composition by mass percentage of C 0.41, Si 1.65, Mn 0.80, P 0.007, S 0.001, Cr 0.83, Ni 1.86, Mo 0.45, V 0.08, and the balance of Fe.

[0033] The above steel ingot with a diameter of 650 mm and a height of 1800 mm is used to prepare a 400 mm round rod, and the specific process is as follows: 1. High temperature diffusion and pre-preparation before forging The steel ingot is preheated at 550℃ for 4h, then heated to 1180℃ at a rate of 100℃ / h for homogenization treatment, and the steel ingot is through-burned for 12h. After homogenization, the temperature of the heating furnace is reduced to 800℃ for 4h, then heated to 1140℃ at a rate of 130℃ / h, and kept for 3h to prepare for furnace-out forging; 2. Forging 2.1 The first fire performs 2 upsetting 2 drawing bloom, the forging ratio is 5.02, the ingot heating temperature before forging is 1140°C, and the guaranteed opening forging temperature is 1085°C; the ingot is first longitudinally upset, and is upset to the ingot height of 900mm at the pressing speed of 80mm / s, the single drum diameter is 1030mm, after turning 90°, it is transversely drawn, the drawing is at the pressing speed of 120mm / s, the single anvil pressing amount of main deformation is 300mm, the feeding amount is 600mm (the anvil width is 800mm), and the drawing is into an octagon with the diameter of 640mm; after the first upsetting and drawing, it is not returned to the furnace for temperature rising, and continues the second upsetting and drawing, and the process requirements are the same as the first upsetting and drawing.

[0034] 2.2 The second fire performs one upsetting and one drawing, the forging ratio is 6.70, the ingot heating temperature is 1080°C, the guaranteed opening forging temperature is 1025°C, the final forging temperature is 878°C, the ingot is first longitudinally upset, and is upset to the ingot height of 900mm at the pressing speed of 75mm / s, the single drum diameter is 1035mm, after turning 90°, it is transversely drawn, the drawing is at the pressing speed of 130mm / s, the single anvil pressing amount of main deformation is 280mm, the feeding amount is 650mm (the anvil width is 800mm), and the drawing is into a round bar with the diameter of 400mm after adopting a round anvil to spin a round into a round bar with the diameter of 405mm. After forging, the round bar is subjected to the conventional uniform structure 900°C normalizing treatment and the stress relieving 680°C annealing treatment.

[0035] Example 2

[0036] The ingot is composed of C 0.39, Si 1.60, Mn 0.78, P 0.006, S 0.001, Cr 0.85, Ni 1.79, Mo 0.48, V 0.06 and the balance of Fe in percentage by mass.

[0037] The diameter of the above-mentioned ingot is 650mm, and the height is 1800mm. The 380mm round bar is prepared, and the specific process is as follows: 1. High temperature diffusion and preparation before forging The ingot is preheated at 600°C for 3h, and then is homogenized at 1250°C at the temperature rising rate of 120°C / h, and the ingot is through-burned for 10h. After the homogenization, the heating furnace temperature is reduced, the ingot is kept at 800°C for 4h, and then is heated to 1150°C at the temperature rising rate of 150°C / h, and is kept for 3h to prepare for the furnace-out forging; 2. Forging 2.1 The first fire performs 2 upsetting 2 drawing bloom, the forging ratio is 5.02, the ingot heating temperature before forging is 1150°C, and the guaranteed opening forging temperature is 1090°C; the ingot is first longitudinally upset, and is upset to the ingot height of 900 mm at the pressing speed of 60 mm / s, the single drum diameter is 1030 mm, after turning 90°, it is transversely drawn, the drawing is at the pressing speed of 120 mm / s, the single anvil pressing amount of main deformation is 300 mm, the feeding amount is 600 mm (the anvil width is 800 mm), and the drawing is into an octagon with the diameter of 640 mm; after the first upsetting and drawing, it is not returned to the furnace for temperature rising, and continues the second upsetting and drawing, and the process requirements are the same as the first upsetting and drawing.

[0038] 2.2 The second fire performs one upsetting and one drawing, the forging ratio is 7.41, the ingot heating temperature is 1070°C, the guaranteed opening forging temperature is 1015°C, the final forging temperature is 865°C, the ingot is first longitudinally upset, and is upset to the ingot height of 900 mm at the pressing speed of 70 mm / s, the single drum diameter is 1035 mm, after turning 90°, it is transversely drawn, the drawing is at the pressing speed of 130 mm / s, the single anvil pressing amount of main deformation is 280 mm, the feeding amount is 650 mm (the anvil width is 800 mm), and the drawing is into a round bar with the diameter of 380 mm after adopting a round anvil to spin a round into a round bar with the diameter of 380 mm. After forging, the round bar is subjected to the conventional uniform structure 900°C normalizing treatment and the stress relieving 680°C annealing treatment.

[0039] Example 3

[0040] The ingot is composed of the following components in percentage by mass: C 0.42, Si 1.70, Mn 0.82, P 0.008, S 0.001, Cr 0.83, Ni 1.82, Mo 0.41, V 0.07 and the balance of Fe.

[0041] The above ingot with the diameter of 650 mm and the height of 1800 mm is used to prepare a 300 mm round bar, and the specific process is as follows: 1. High temperature diffusion and preparation before forging The ingot is preheated at 530°C for 4 h, and then is homogenized at 1200°C at the temperature rising rate of 120°C / h, and the ingot is through-burned for 11 h. After the homogenization, the furnace temperature is lowered, the ingot is kept at 850°C for 4 h, and then is heated to 1130°C at the temperature rising rate of 120°C / h, and is kept at 1130°C for 3 h for preparing the ingot for forging; 2. Forging 2.1 The first fire performs 2 upsetting and 2 drawing of the bloom, the forging ratio is 5.02, the ingot heating temperature before forging is 1130°C, and the guaranteed opening forging temperature is 1075°C; the ingot is first longitudinally upset, and is upset to an ingot height of 900 mm at a pressing rate of 80 mm / s, the single drum diameter is 1030 mm, after turning 90°, it is transversely drawn, the drawing is at a pressing rate of 120 mm / s, the single anvil pressing amount of main deformation is 300 mm, the feeding amount is 600 mm (anvil width 800 mm), and the drawing is into an eight-corner with a diameter of 640 mm; after the first upsetting and drawing, it is not returned to the furnace for temperature rising, and continues to be secondly upset and drawn, and the process requirements are the same as the first upsetting and drawing.

[0042] 2.2 The second fire performs one upsetting and one drawing, the forging ratio is 11.9, the ingot heating temperature is 1080°C, the guaranteed opening forging temperature is 1025°C, and the final forging temperature is 878°C; the ingot is first longitudinally upset, and is upset to an ingot height of 900 mm at a pressing rate of 75 mm / s, the single drum diameter is 1035 mm, after turning 90°, it is transversely drawn, the drawing is at a pressing rate of 130 mm / s, the single anvil pressing amount of main deformation is 280 mm, the feeding amount is 650 mm (anvil width 800 mm), and the drawing is into a round bar with a diameter of 300 mm after adopting a round anvil to spin a round bar with a diameter of 305 mm. After forging, the round bar is subjected to a conventional uniform structure normalizing treatment at 900°C and a stress relief annealing treatment at 680°C.

[0043] Comparative Example 1

[0044] The difference from Example 1 is that high-temperature diffusion is not performed, the ingot is directly heated to 1140°C, the ingot is taken out of the furnace after heating for 8 h, and the other processes are performed according to Example 1 to prepare a 400 mm round bar; it is found that there are slender strip MnS inclusions, and the forging transverse and longitudinal impact difference is 10%.

[0045] Comparative Example 2

[0046] The difference from Example 2 is that the ingot is taken out of the furnace to perform one upsetting and one drawing in the first fire, the forging ratio is 4, and the other processes are performed according to Example 2 to prepare a 380 mm round bar, the forging transverse impact difference is 8%, and the grain size is coarsened longitudinally.

[0047] Comparative Example 3

[0048] The difference from Example 3 is that the ingot heating temperature in the last fire is 1130°C without temperature reduction forging, and the other processes are performed according to Example 3 to prepare a 400 mm round bar, the grain size is coarsened, and the performance is reduced.

[0049] Detection Example

[0050] The annealed rod of the above examples and comparative examples was sampled for detection, the grain size sample was quenched and then detected by etching, the non-metallic inclusion SEM sample was directly sampled on the rod for detection, the mechanical property sample was quenched and then detected after 2 times of tempering treatment (part heat treatment process), and the mechanical property detection was performed according to the standard of GJB 5063-2001 Specification for Ultra-high Strength Steel Rod for Aerospace.

[0051] 1) Grain size detection results

[0052] The grain size detection results of examples 1-3 are shown in Figures 1-3 respectively (the left picture in each figure is the transverse grain size, and the right picture is the longitudinal grain size). It can be obviously seen that the transverse and longitudinal grain sizes are uniform and fine.

[0053] The grain size detection results of comparative examples 1-3 are shown in Figures 4-6 respectively, which present different degrees of grain size coarsening.

[0054] 2) MnS inclusion detection results

[0055] The MnS inclusions in examples 1-3 and comparative examples 1-3 were observed by SEM, and the element composition was analyzed, and the results are shown in Figures 7-9 , Figures 10-12 The sizes of the MnS inclusions are as follows:

[0056] 3) Mechanical properties:

[0057] From the above detection results, it can be seen that the rods obtained in examples 1-3 have obvious advantages in grain size, MnS inclusion, transverse and longitudinal difference and mechanical properties, which meet the requirements of ultra-high strength steel rod for aerospace. Comparative example 1 does not produce elongated MnS inclusions by high temperature diffusion, and the transverse and longitudinal impact difference is 10%; comparative example 2 executes one upsetting and one drawing after the first furnace discharge of the ingot, which causes the transverse impact difference of 8%, and the grain size in the longitudinal direction is coarse; comparative example 3 does not cool the last ingot for forging, which leads to coarse grain size and reduced performance. The above conclusions fully demonstrate the effects of the specific high temperature diffusion and forging method of the present application.

[0058] Although the embodiments of the present application have been shown and described, it can be understood by those skilled in the art that various changes, modifications, replacements and variations can be made to the embodiments without departing from the principles and spirits of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A method of improving the anisotropy of low alloy ultra-high strength steel bar material, characterized by the steps of The application relates to a method for preparing a low-alloy super-high-strength steel rod. S1. obtaining a steel ingot with a target composition meeting the material requirements of 40CrNi2Si2MoVA through smelting; S2. after the steel ingot obtained in step S1 is preheated, the steel ingot is homogenized at a temperature above 1180 DEG C at a rate below 150 DEG C / h for 10-15 h; S3. after the homogenization treatment, the temperature of the heating furnace is lowered to 700-850 DEG C for low-temperature holding for 3-5 h, then the temperature is raised to 1100-1150 DEG C at a rate of 100-150 DEG C / h for holding for 3-6 h for forging preparation; S4. the steel ingot is forged through multiple fire times and step-by-step temperature reduction, the total forging ratio is greater than 8, and the single fire time forging ratio is greater than or equal to 3.5, so that the low-alloy super-high-strength steel rod with improved anisotropy is obtained.

2. The method of claim 1, wherein, In step S1, the steel ingot is composed of the following components in percentage by mass: C 0.38-0.43, Si 1.45-1.80, Mn 0.60-0.90, P <=0.010, S <=0.010, Cr 0.70-0.95, Ni 1.65-2.00, Mo 0.30-0.50, Cu <=0.35, V 0.05-0.10 and the balance of Fe.

3. The method of claim 1, wherein, In step S1, the smelting process is completed through vacuum induction and vacuum consumable.

4. The method of claim 1, wherein, In step S2, the homogenization temperature is 1180-1250 DEG C, and the homogenization time is 10-12 h; And / or, the homogenization temperature rising rate is 80-100 DEG C / h.

5. The method of claim 1, wherein, In step S3, the low-temperature holding process is 4 h at 800-850 DEG C; And / or, the temperature rising to 1130-1150 DEG C for holding before forging.

6. The method of claim 1, wherein, In step S4, the ingot heating temperature of the first fire time is greater than or equal to 1100 DEG C, 2-up 2-down breakdown is carried out in the first fire time, the forging ratio is greater than or equal to 5, and the second-up 2-down elongation is directly carried out after the first-up 2-down elongation without temperature recovery.

7. The method of claim 1, wherein, In step S4, the ingot heating temperature of the last fire time is greater than or equal to 1050 DEG C, and the final forging temperature is greater than or equal to 850 DEG C.

8. The method of claim 1, wherein, In step S4, the first-up 2-down elongation of the first fire time is carried out in the following manner: the original height is halved H at a pressing rate of 50-80 mm / s, then transverse elongation is carried out, the elongation is pressed at a rate of 100-140 mm / s, the main deformation adopts single-anvil pressing of 200-350 mm, and the feeding amount is 1 / 2-3 / 4 of the anvil width.

9. The method of claim 8, wherein, In step S4, the ingot heating temperature of the second fire time is lower than that of the first fire time.

10. The method of claim 8, wherein, In step S4, the second-up 2-down elongation is carried out in the same manner as the first-up 2-down elongation.

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