High-thermal-strength steel and method for manufacturing the same

Abstract

A kind of high thermal strength steel, the alloy composition and its mass percentage content are as follows: C 0.85~0.95%, Si 0.8~1.2%, Mn 12.5~15.5%, Cr 4.5~5.5%, Mo 1.7~2.3%, W 1.2~1.8%, V 0.8~1.2%, N 0.08~0.12%, P≤0.010%, S≤0.008%, the balance is Fe and inevitable impurities.The application adds 0.08~0.12% nitrogen in the traditional Fe-Mn-Cr-C austenitic steel system, ensures that the matrix of steel maintains single-phase austenite structure in the range of 20~800 DEG C, guarantees the consistency of material performance;Meanwhile, carbide forming elements such as W, Mo, V are added, which improves the matrix strength.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of metal materials, and further belongs to a high thermal strength steel. BACKGROUND

[0002] With the continuous progress of the die industry, the high-temperature forming manufacturing industry has developed rapidly, especially in the fields of light metal (copper and its alloys) die casting, stamping and extrusion. However, under the service condition of high temperature of 650-800℃, the die fatigue life is the main factor limiting its development. Therefore, it is an important task to develop a high thermal strength die steel that can be used in high temperature environment service, has a longer fatigue life, and has higher strength and toughness.

[0003] Most of the high thermal strength die steels commonly used at present are martensitic hot work die steels represented by H13, but when the service temperature is higher than 650℃, this kind of die steel will produce martensite tempering phenomenon, which will cause the hardness and strength of the matrix to decrease sharply, and the service life to decrease significantly. Austenitic hot work die steel can avoid the sharp decrease of the hardness and strength of the matrix because it can maintain stable austenitic matrix structure at room temperature and high temperature without phase change. Therefore, it is an ideal hot work die steel material.

[0004] Manganese is an element that expands austenite, so general austenitic hot work die steel contains manganese element. In addition, the addition of Cr can increase the hardenability and tempering stability, and also form Cr2O3 on the surface of the steel matrix, thereby improving the high temperature oxidation resistance. Therefore, austenitic hot work die steel is generally a steel of Fe-Mn-Cr composition system, and Mo, V and other carbonitride forming elements are added to increase the matrix strength by means of element solid solution, intermetallic compound and carbide precipitation.

[0005] However, due to the addition of a large number of alloys with high content, the composition and structure uniformity of austenitic hot work die steel is difficult to control accurately, and problems such as eutectic carbide, chromium-molybdenum segregation often occur during solidification. Some defects are difficult to eliminate even after the later heat treatment process. In view of these problems, many researchers have conducted in-depth and meticulous research and developed many new composition systems and production processes. However, most of the steel types are conceived for a certain problem, and it is difficult to be widely applied in the field of application.

[0006] Patent with publication number CN101942606A discloses a kind of nitrogen-containing austenitic hot die steel and its preparation method, its composition is C 0.3~0.7%, Si 0.5~1.1%, Mn 10.0~15.0%, Cr 2.0~6.0%, Mo 1.5~3.5%, V 0.5~2.0%, P <0.02%, S <0.005%, N 0.15~0.30%, Fe balance.The invention adopts manganese and nitrogen to stabilize the austenite phase of steel, obtain stable austenitic hot die steel at room temperature, while adding Cr, Mo, V and other carbonitride forming elements to improve the strength and hardness of steel matrix, it is a very novel technical solution, but according to the composition provided by the invention, the saturation solubility of N is 0.11% at 1600 DEG C and 0.1 MPa, to reach the target content of 0.15~0.30%, it needs to use supercharged induction furnace + supercharged electroslag furnace smelting, which is difficult to be popularized in industry.

[0007] Patent with publication number CN107974632A discloses an austenitic hot die steel and its preparation method, the alloy composition is C 0.55~0.75%, Si 0.45~0.85%, Mn 13.5~18.5%, Cr 3.00~4.50%, Mo 1.00~3.00%, V 1.00~2.00%, P≤0.01%, S≤0.003%, T.[O]≤0.002%, Als≤0.020%, the rest is Fe.The invention controls the organization and composition by reasonable preparation process to achieve the purpose of refining carbide size, quantity, morphology and distribution, but the strict composition requirements such as T.[O]≤0.002%, S≤0.003% make the smelting process extremely difficult, which is not convenient for large-scale production. SUMMARY

[0008] The purpose of the present application is to provide a high thermal strength die steel, first of all, by reasonable composition design, 0.08~0.12% of nitrogen is added in the traditional Fe-Mn-Cr-C austenitic steel system, which ensures that the matrix of the steel maintains single-phase austenite organization in the range of 20~800 DEG C, and guarantees the consistency of material performance; at the same time, carbide forming elements such as W, Mo and V are added to improve the matrix strength.

[0009] To solve the above technical problems, the technical scheme of the present application is as follows: a high thermal strength steel, the alloy composition and its mass percentage are as follows: C 0.85~0.95%, Si 0.8~1.2%, Mn 12.5~15.5%, Cr 4.5~5.5%, Mo 1.7~2.3%, W 1.2~1.8%, V 0.8~1.2%, N 0.08~0.12%, P≤0.010%, S≤0.008%, the balance is Fe and unavoidable impurities.

[0010] A method for preparing the high-thermal-strength steel, comprising the following steps: obtaining an electroslag ingot by electroslag remelting, and then obtaining a forged material by forging the electroslag ingot, wherein the forging adopts the following steps:

[0011] 1) homogenization treatment: the electroslag ingot is heated to 1190±10℃ at a furnace temperature, and is kept at the temperature for (2-3) min×thickness / diameter (mm) of the blank;

[0012] 2) forging: the forging is performed at 960-1160℃, and the forged material is air-cooled after the forging;

[0013] 3) solid solution and aging treatment: the forged material is subjected to solid solution treatment at 1180±20℃ for (1-2) min×thickness / diameter (mm) of the blank, and then is water-cooled to room temperature; and then is subjected to aging treatment at 700±50℃ for (2-3) min×thickness / diameter (mm) of the blank, and then is air-cooled to room temperature.

[0014] Further, in the homogenization treatment process: the heating furnace has a heating rate of ≤200℃ / h, and is first heated to 650±10℃, and then is heated to 1190±10℃ after being kept at 650±10℃ for 2-3 h.

[0015] Further, in the step 2: the ingot is forged by adopting a three-die three-drawing and one-way elongation method, the first drawing deformation is 25-30%, the second drawing deformation is 30-35%, and the third drawing deformation is 35-40%; the elongation operation has a single-pass deformation of 30-45%; and the total forging pressure ratio is ≥6.

[0016] Further, in the electroslag remelting process, the electroslag remelting is performed under nitrogen protection, the ingot is removed after the mold is cooled for 30-60 min after the smelting is completed, and the ingot is pit-cooled.

[0017] Further, in the electroslag remelting process, the melting rate v is determined according to the following formula 1, and the metal bath depth H is determined according to the following formula 2,

[0018] v=(0.5-0.6)D 结 (1)

[0019] H=(0.43-0.45)D 结 (2)

[0020] wherein D 结 in the above formulas 1 and 2 is a diameter of a crystallizer, v in the formula 1 is in kg / h, and D 结 is in mm.

[0021] The consumable electrode used in the electroslag remelting process can be obtained by die casting or continuous casting. Before the continuous casting or die casting, the smelting of the consumable electrode liquid steel can use metallurgical equipment such as electric furnace, converter, LF, AOD, RH according to the composition needs. The application embodiment provides a method for obtaining a consumable electrode by induction furnace smelting and die casting.

[0022] The beneficial effects produced by the above technical solution are:

[0023] (1) The composition design is more comprehensive. First, compared with the traditional Fe-Mn-Cr-C austenitic hot die steel, 0.08-0.12% nitrogen is added to stabilize the austenite together with manganese, so that the matrix structure is more stable at high temperature; second, the Cr content is increased to effectively improve the high-temperature oxidation resistance; third, 0.08-0.12% W is added to increase the room temperature and high temperature strength of the matrix, so that the material maintains the consistency of the microstructure and performance at 20-800°C.

[0024] (2) A feasible high-thermal-strength die steel production process is provided. The process flow is simple, and harmful components such as P, S, and inclusions can be effectively controlled. The hit rate of elements such as C, Si, Mn, Cr, W, Mo, and V is also relatively high. After electroslag remelting, the composition and structure uniformity is effectively improved. In view of the possible eutectic carbides in the solidification process, a relatively low electroslag melting speed (v=(0.5-0.6)D 结 ) and a relatively shallow metal bath (H=(0.43-0.45)D 结 ) are used to ensure a large temperature gradient and a small local solidification time during dendrite production, effectively inhibiting the formation of eutectic carbides.

[0025] (3) Reasonable homogenization, forging, solid solution and aging treatment processes are given. High temperature and long time homogenization treatment can make the carbon and nitrides of elements such as Cr, W, Mo, and V fully dissolve, obtaining a high-temperature austenite structure with uniform composition; narrow temperature range and large forging reduction ensure that the material effectively breaks the large eutectic carbides and coarse grains during forging, obtaining a fine and uniform structure; air cooling after forging also avoids the generation of large particle blocky and network carbides; during high-temperature aging, high-hardness carbide phases are dispersedly precipitated in the structure, and their number increases continuously with the increase of aging temperature. When the temperature reaches a certain value, the number of carbide particles reaches a peak, and then the temperature increases, causing the carbide particles to begin to aggregate and grow, resulting in a decrease in the number of carbide particles and a decrease in hardness. Therefore, appropriate time and temperature of solid solution and aging ensure the dispersion of fine carbides, achieving the best precipitation strengthening effect. DETAILED DESCRIPTION

[0026] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in details below with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not used to limit the present application.

[0027] Examples 1-3

[0028] Example 1

[0029] The high thermal strength die steel is produced by using 1t non-vacuum induction furnace, 1t protective atmosphere electroslag furnace, 2000t hydraulic quick forging machine and other equipments. The processes are as follows:

[0030] 1) Non-vacuum induction furnace smelting

[0031] ① Furnace charging: 30kg pre-melted slag (W CaO = 90%, W CaF2 = 10%) is laid at the bottom of the crucible, 300kg pure iron, 28kg molybdenum iron (W Mo = 60%), 17kg tungsten iron (W W = 70%) are sequentially placed, and 410kg pure iron is placed at the upper part of the crucible;

[0032] ② Melting and alloying: power is sent to melt, and it is cleaned after 3h; slag is removed, 30kg pre-melted slag (W CaO = 90%, W CaF2 = 10%) is added to form slag, and then 3kg silicon iron powder (W Si = 70%), 2kg low-carbon ferromanganese (W Mn = 70%) are added for deoxidation; 45kg metal chromium is added in two batches, and after cleaning, 11.5kg silicon iron (W Si = 70%), 155kg metal manganese, 9kg carbon powder, 16kg vanadium iron (W V = 50%), 10kg chromium nitride (W N = 7.6%) are sequentially added; and it is cleaned;

[0033] ③ Sampling: sampling and fine-tuning of components; after the components are qualified, the temperature is increased to 1540℃;

[0034] ④ Tapping: casting into Ф300*1800mm electrode.

[0035] 2) Electroslag remelting

[0036] ① Preparation: after the electrode is stress relieved and annealed, the iron oxide skin is removed and welded; 50kg pre-melted slag is weighed and placed in a 600℃ heating furnace for 8h baking, and the slag composition is 60% CaF2, 20% CaO, 14% Al2O3, 3% SiO2, 3% MgO;

[0037] ② Melting: using Ф400*1500mm crystallizer to melt; solid slag starting, slagging period voltage 25-39V, current 2500-4500A, slagging time 45min; remelting period voltage 49-51V, current 7500-7800A, maintaining the melting rate 200kg / h, the depth of the molten bath 176mm; starting continuous top-up when the electrode remains 100kg, top-up period 50min, voltage uniformly adjusting from 49V to 35V, current uniformly adjusting from 7500A to 2800A;

[0038] ③ Demoulding: demoulding 30min after the melting ends, and the ingot pit is cooled.

[0039] 3) Homogenization

[0040] Placing the electroslag ingot in a heating furnace, heating to 660℃ for 3.5h, keeping for 2h, then heating to 1180℃ for 3.5h, keeping for 800min.

[0041] 4) Forging

[0042] Taking the ingot out of the furnace, and forging at 960-1060℃. The forging process is as follows: first upsetting: Ф400mm→Ф500mm→300*300mm 2 billet, reheating and keeping for 3h; second upsetting: 300*300mm 2 →400*400mm 2 →260*260mm 2 billet, reheating and keeping for 3h; third upsetting: 260*260mm 2 →300*300mm 2 →220*220mm 2 billet, reheating and keeping for 3h; then elongating to the specification of Ф50mm round steel through several passes, and air cooling to room temperature; during the forging process, cutting off the billet in time according to the operating stroke of the forging machine.

[0043] 5) Solid solution and aging

[0044] Placing the Ф50mm round steel in a heating furnace, keeping at 1160℃ for 50min, and water cooling to room temperature; then keeping at 650℃ for 100min, and then air cooling to room temperature.

[0045] After the above process, the composition and performance of the steel are detected, and the results are as follows. Table 1 is the composition of the round steel and its mass percentage, and Table 2 is the mechanical properties of the round steel.

[0046] Table 1

[0047] C / ％ Si / % Mn / % S / ％ P / ％ Cr / ％ V / ％ Mo / % W / ％ N / ％ 0.85 0.80 15.5 0.003 0.008 4.50 0.8 1.7 1.2 0.080

[0048] Table 2

[0049]

[0050] Example 1 employs 1t induction furnace and 1t electroslag furnace and other smaller equipment for smelting, and the alloy is basically added according to the lower limit, but the performance of the forged material is also very outstanding. As can be seen from Table 1-2, the hardness at room temperature is 54HRC, and the hardness after 48h holding at 750℃ reaches 50HRC, and the yield strength at 800℃ reaches 847.2MPa, indicating excellent performance at high temperature.

[0051] Example 2:

[0052] A 2t non-vacuum induction furnace, a 2t protective atmosphere electroslag furnace, a 2000t hydraulic quick forging machine and other equipment are used to produce high-thermal-strength die steel. The processes are as follows:

[0053] 1) Non-vacuum induction furnace smelting

[0054] ① Furnace charging: 40kg of pre-melted slag (W CaO =90%, W CaF2 =10%) is laid at the bottom of the crucible, followed by 600kg of pure iron, 76.7kg of molybdenum iron (W Mo =60%), 51.4kg of tungsten iron (W W =70%), and 775kg of pure iron at the top of the crucible;

[0055] ② Melting and alloying: power-on melting, clear after 4h; slagging, adding 40kg of pre-melted slag (W CaO =90%, W CaF2 =10%) for slagging, followed by 5kg of silicon iron powder (W Si =70%), 4kg of low-carbon manganese iron (W Mn =70%) for deoxidation; adding 110kg of metallic chromium in two batches, and after clear, adding 36kg of silicon iron (W Si =70%), 250kg of metallic manganese, 20kg of carbon powder, 48kg of vanadium iron (W V =50%), and 30kg of chromium nitride (W N =7.6%); clear;

[0056] ③ Sampling: sampling, fine-tuning of composition; after the composition is qualified, heating to 1550℃;

[0057] ④ Tapping: casting into a Ф400*2000mm electrode.

[0058] 2) Electroslag remelting

[0059] ① Preparation: electrode stress relief annealing, removing iron oxide skin, welding; weighing 100 kg of pre-melted slag, placing in a 600°C heating furnace for 8h, the slag composition is 58% CaF2, 18% CaO, 12% Al2O3, 7% SiO2, 5% MgO;

[0060] ② Melting: using Ф550*1500mm crystallizer for melting; solid slag starting, slagging period voltage 35-42V, current 3000-7500A, slagging time 60min; remelting period voltage 51-55V, current 9500-10800A, maintaining the melting rate 330kg / h, the molten bath depth 237mm; starting continuous feeding with the remaining 180kg of electrode, feeding period 80min, voltage gradually adjusting from 51V to 42V at a uniform speed, current gradually adjusting from 9500A to 3800A at a uniform speed;

[0061] ③ Demoulding: demoulding 60min after the end of melting, ingot pit cooling.

[0062] 3) Homogenization

[0063] Placing the electroslag casting blank in a heating furnace, first heating to 650°C for 5h, then heating to 1190°C for 5h after 2.5h of heat preservation, and heat preservation for 1400min.

[0064] 4) Forging

[0065] Blank discharge, forging at 1010-1080°C, three-die three-drawing, single-drawing and other multiple passes to the specification of Ф60mm round steel, air cooling to room temperature.

[0066] 5) Solution, aging

[0067] Heating the Ф60mm round steel in a 1180°C furnace for 90min, water cooling to room temperature; then heat preservation for 150min at 700°C, air cooling after discharge.

[0068] After the above process, the composition and performance of the steel are detected, and the results are as follows. Table 3 is the composition of round steel and its mass percentage, and Table 4 is the mechanical properties of round steel.

[0069] Table 3

[0070] C / ％ Si / % Mn / % S / ％ P / ％ Cr / ％ V / ％ Mo / % W / ％ N / ％ 0.94 1.20 12.5 0.006 0.008 5.5 1.2 2.3 1.8 0.12

[0071] Table 4

[0072]

[0073] The experiment is to smelt by using 2t induction furnace, 2t electroslag furnace and other medium scale equipment, to add alloy according to upper limit, and the performance basically reaches the best. From table 3-4, the hardness of the forging material at room temperature is 56HRC, and the hardness reaches 52HRC after 48h at 750℃, and the yield strength at 800℃ reaches 859.7MPa.

[0074] Example 3

[0075] By using 5t non-vacuum induction furnace, 5t protective atmosphere electroslag furnace, 2000t hydraulic fast forging machine and other equipment, 6 high thermal strength die steels with different compositions are produced, and all are forged into Φ80mm round steel.

[0076] The processes are as follows:

[0077] 1) Non-vacuum induction furnace smelting

[0078] ① Furnace charging: 60kg pre-melted slag (W CaO =90%, W CaF2 =10%) is laid at the bottom of the crucible, pure iron with half weight according to respective composition, molybdenum iron and tungsten iron with full weight are placed in turn, and the remaining half weight of pure iron is placed at the upper part of the crucible;

[0079] ② Melting and alloying: power is sent to melt, and it is cleaned after 5h; slag is removed, 60kg pre-melted slag (W CaO =90%, W CaF2 =10%) is added to build slag, and then 10kg silicon iron powder (W Si =70%) and 10kg low-carbon manganese iron (W Mn =70%) are added for deoxidation; metal chromium is added in two batches, and silicon iron, metal manganese, carbon powder, vanadium iron and chromium nitride are added in turn after cleaning;

[0080] ③ Sampling: sampling after cleaning, and adjusting composition; after the composition is qualified, the temperature is raised to 1540℃;

[0081] ④ Tapping: casting into Φ500*3300mm electrode.

[0082] 2) Electroslag remelting

[0083] ① Preparation: electrode stress relief, removal of iron oxide skin, welding; 250kg pre-melted slag is weighed and placed in a 600℃ heating furnace for 8h, and the slag composition is 60% CaF2, 20% CaO, 14% Al2O3, 3% SiO2 and 3% MgO;

[0084] ② Melting: using Ф700*1800mm crystallizer to melt; solid slag starting, voltage 55-70V, current 4500-13500A, slagging time 105min; remelting period voltage 70-85V, current 16000-18000A, maintaining the melting rate 385kg / h, the depth of the molten bath 315mm; starting continuous top-up when the electrode remains 500kg, top-up period 200min, voltage gradually adjusting from 70V to 44V at a uniform speed, current gradually adjusting from 16000A to 5800A at a uniform speed;

[0085] ③ Demoulding: demoulding 55min after the end of melting, and the electric slag ingot pit is cold.

[0086] 3) Homogenization

[0087] The heating furnace for placing the electric slag billet is first heated to 657℃ for 5h, then heated to 1200℃ for 5h after keeping warm for 3h, and kept warm for 1800min.

[0088] 4) Forging

[0089] The billet is discharged from the furnace, and forged at 1090-1160℃, after three-die three-drawing, lengthening and other multiple passes, forged to the specification of Φ80mm round steel, and air-cooled to room temperature.

[0090] 5) Solid solution and aging

[0091] The Φ80mm round steel is kept warm in a 1200℃ heating furnace for 160min, and water-cooled to room temperature; then kept warm in a 750℃ furnace for 240min, and discharged for air-cooling.

[0092] The chemical composition of the 6 furnace steels and the mass percentage are shown in Table 5, and the mechanical properties are shown in Table 6.

[0093] Table 5

[0094] Si / % C / ％ Mn / % Mo / % S / ％ P / ％ Cr / ％ V / ％ Si / % Mn / % Mo / % Heat Si / % Mn / % Mo / % W / ％ N / ％ 1# 0.90 1.00 14.0 0.006 0.010 5.0 1.0 2.0 1.5 0.10 2# 0.86 1.04 13.5 0.005 0.008 5.2 1.1 2.2 1.6 0.09 3# 0.92 0.96 14.5 0.008 0.007 4.8 0.9 1.8 1.3 0.11 4# 0.88 0.85 15.0 0.008 0.010 5.4 1.0 1.9 1.4 0.12 5# 0.87 0.82 12.8 0.004 0.006 4.6 0.8 1.85 1.2 0.08 6# 0.94 1.16 15.4 0.006 0.009 5.5 1.2 2.3 1.7 0.105

[0095] Table 6

[0096]

[0097] The mold steel of the present example is smelted by using a large-scale 5t induction furnace and a 5t electric slag furnace, the alloy of 6 components is matched to basically cover the component range of the present application, and the mechanical properties are also very outstanding. As can be seen from Tables 5-6, the hardness of the material at room temperature is 54-56HRC, the hardness after keeping warm at 750℃ for 48h is also reached to 50-51HRC, and the yield strength at 800℃ is also reached to above 840MPa.

[0098] The above examples are only used to illustrate but not to limit the technical solutions of the present application. Although the present application is described in detail with reference to the above examples, those skilled in the art should understand that the present application can be modified or equivalently replaced without departing from the spirit and scope of the present application, and any modification or partial replacement should be covered in the scope of claims of the present application.

Claims

1. A high-heat-strength steel, characterized in that, The alloy composition and its mass percentage are as follows: C 0.85-0.95%, Si 0.8-1.2%, Mn 12.5-15.5%, Cr 4.5-5.5%, Mo 1.7-2.3%, W 1.2-1.8%, V 0.8-1.2%, N 0.08-0.12%, P≤0.010%, S≤0.008%, with the balance being Fe and unavoidable impurities.

2. The method for preparing a high-heat-strength steel according to claim 1, wherein an electroslag ingot is obtained by electroslag remelting, and then the electroslag ingot is forged to obtain a forging, characterized in that, The forging process involves the following steps: 1) Homogenization treatment: The electroslag casting ingot is heated to 1190±10℃ in the furnace and held for (2~3) min × billet thickness / diameter mm; 2) Forging: Forging is carried out at 960-1160℃, followed by air cooling; 3) Solution treatment and aging treatment: The forging is solution treated at 1180±20℃ for (1~2) min × billet thickness / diameter mm, and then water-cooled to room temperature; then it is aged at 700±50℃ for (2~3) min × billet thickness / diameter mm, and then air-cooled to room temperature.

3. The method for preparing a high heat-strength steel according to claim 2, characterized in that, In the homogenization process: the heating furnace has a heating rate of ≤200℃ / h, and is first heated to 650±10℃, held for 2-3 hours, and then heated to 1190±10℃.

4. The method for preparing a high heat-strength steel according to claim 2, characterized in that, In step 2: the steel ingot is forged using a three-stage forging and three-stage drawing method with unidirectional drawing. The deformation amount of the first forging and drawing is 25-30%, the deformation amount of the second forging and drawing is 30-35%, and the deformation amount of the third forging and drawing is 35-40%. The deformation amount of a single drawing operation is 30-45%. The total forging ratio is ≥6.

5. The method for preparing a high heat-strength steel according to claim 2, characterized in that, The electroslag remelting process is carried out under nitrogen protection. After the melting is completed, the ingot is cooled in the mold for 30-60 minutes and then the ingot is cooled in the pit.

6. The method for preparing a high heat-strength steel according to claim 2, characterized in that, During electroslag remelting, the melting rate is maintained at v = (0.5~0.6)D. 结 In the formula, v is in kg / h, and D is in the form of... 结 Where D is the diameter of the crystallizer, in mm; the depth of the molten metal pool H = (0.43~0.45)D 结 In the formula, H is in mm, and D 结 The diameter of the crystallizer is in mm.

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