Production method of micro-segregation NAK80 die steel

Abstract

The invention discloses a production method of micro-segregation NAK80 die steel, which comprises the following steps of: smelting high-clean steel, adopting a water-cooling die, and adjusting water-cooling strength and a pouring process to realize micro-defect solidification and obtain a micro-segregation steel ingot, and further carrying out controlled cooling, controlled rolling and heat treatment to finally obtain an NAK80 steel plate with slight segregation and uniform hardness. According to the steel plate obtained through the scheme, the hardness of the whole cross section of the core of the steel plate is 36-40 HRC, the steel plate is uniform in overall hardness and good in welding performance, the requirement that the defect diameter is smaller than or equal to 3 mm in the GB / T 6402-20084 level of forgings is met, the polished surface roughness is smaller than or equal to 0.02 micron, Mn segregation ranges from 0.97 to 1.05, and C segregation ranges from 0.95 to 1.06. And on the premise that the quality of the steel plate is guaranteed, the production efficiency is greatly improved, and good market competitiveness is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of extra-thick plate production, and specifically relates to a production method of micro-segregation NAK80 die steel. Background Art

[0002] Plastic molds are indispensable key equipment in the plastic processing industry. With their increasingly widespread applications, the demand for steel is large, and there are many varieties and specifications. Plastic molds occupy an important share in the mold steel market. For plastic mold steel, it is usually required to have certain strength, hardness, wear resistance and corrosion resistance. In addition, it is also required to have good processing performance, including cutting, polishing, leather grain processing, etc. In order to meet these performance requirements, a variety of special plastic mold steels have been developed, including: non-alloy plastic mold steel, carburized plastic mold steel, pre-hardened plastic mold steel, aging hardened plastic mold steel, corrosion-resistant plastic mold steel, etc. NAK80 mold steel, as a pre-hardened plastic mold steel, is known for its excellent wear resistance, excellent cutting performance, stable mirror processing performance and simple etching processing performance. Its pre-hardening hardness is as high as 40HRC, which ensures high durability in large-scale production. Since NAK80 mold steel needs to meet the high standards of polishing and leather grain processing, extremely strict requirements are put forward for the segregation and inclusion control of the steel plate, which undoubtedly increases the difficulty of production and poses higher challenges to the production technology of various steel mills.

[0003] Chinese patent CN200510027393.6 discloses "NAK80 die steel pre-hardening treatment method", which adopts forging deformation, and through quenching + tempering process, the produced NAK80 can meet the core hardness requirements. Its characteristics are that through hot processing forging technology, repeated forging process, the coarse grains produced by loose bonding and segregation in the steel ingot are broken, so as to produce NAK80 plastic die steel with good internal quality, but the forging process has a long production cycle and poor forging surface quality, which is not conducive to the control of surface quality and yield rate.

[0004] Chinese patent CN202210780440.8 discloses "a mirror plastic mold steel and its preparation method", which adopts continuous casting and electroslag remelting technology, through the electroslag ingot process, combined with solid solution and tempering treatment, to produce plastic mold steel with good mirror properties. Its characteristics are to obtain high cleanliness and high density electroslag ingots through electroslag remelting technology, and then roll them, but the production of electroslag ingots has high production costs and lacks market competitiveness.

[0005] How to ensure the quality of steel plates while improving production efficiency and reducing costs has become the key to current market competition. Summary of the invention

[0006] In order to solve the above technical needs, the purpose of the present invention is to provide a production method of micro-segregation NAK80 die steel, which is high in efficiency and low in cost, and the quality of the obtained steel plate can be guaranteed.

[0007] To achieve the above object, the technical solution adopted by the present invention is: a method for producing micro-segregation NAK80 mold steel, the mold steel adopts the following chemical composition in mass percentage (unit: wt%): C: 0.08-0.13, Si: 0.15-0.25, Mn: 1.70-2.00, P≤0.010, S≤0.003, Ni: 3.20-3.40, Mo: 0.35-0.55, Al: 0.65-1.0, Cu: 0.75-1.15, and the rest is Fe and unavoidable impurities;

[0008] The production method of the above die steel includes smelting, water-cooled mold pouring, heating, controlled cooling and controlled rolling, and heat treatment, as follows:

[0009] 1) Smelting: The P content of the converter steel is controlled below 0.008%; LF refining uses high-basicity slag, and the basicity is controlled at 5-6. The amount of lime used in the whole process is more than 1200kg, and the aluminum particles are more than 100kg. Calcium carbide is used to assist deoxidation to ensure that the final slag is white foam slag. The white slag is kept for more than 40 minutes during the whole refining process; VD requires low vacuum pressure maintenance, and the vacuum is below 45pa. The first 20-22 minutes use 500-700L / min argon flow rate and large stirring mode to fully remove H, and the last 5 minutes use 50-100L / min soft blowing mode to ensure that inclusions float up, to provide a guarantee for obtaining clean steel, to ensure that the end point H is ≤0.8PPm, and it is forbidden to add other alloys after VD breaks the air;

[0010] 2) Water-cooled mold pouring: The temperature of molten steel is controlled between liquidus + 40 and 50 °C. 5 seconds before pouring, "full flow" pouring is carried out above the slag basin to remove drainage sand and foreign inclusions. After 10 seconds, the slide is opened to 30% of the "full flow" state, and the steel pouring car is moved to the center column pipe to start pouring. When the molten steel rises to a height of about 80 to 100 mm in the ingot mold, the pouring speed is increased to 80% of the "full flow". After pouring for 2 minutes, the pouring speed is gradually increased to "full flow". When it is poured to 100 mm below the cap mouth line, the pouring speed is gradually reduced to 20 to 30% of the "full flow" until the cap mouth is filled. After the pouring is completed, the insulation agent is added above the cap mouth and evenly spread; the pouring time of the ingot body is controlled at 20 to 24 minutes, the pouring time of the cap mouth is controlled at 6 to 9 minutes, and the water pouring volume is 500 to 550 m 3 / h control, within 30 minutes after the pouring, open the wide surface extrusion device, adjust the gap between the wide surface crystallizer and the ingot, and increase the original water volume by 50-100m 3 / h; demoulding within 8 hours after pouring, the steel ingot is transferred to the slow cooling pit for slow cooling, and the steel ingot is cleaned when the surface temperature drops to 150-200℃;

[0011] It should be noted that the use of high cooling rate combined with appropriate casting process can accelerate the solidification rate of the steel ingot and reduce the uneven hardness caused by the segregation of Mn and other elements.

[0012] 3) Heating: The steel ingot is heated in the soaking pit. When the furnace temperature is below 700°C, the heating rate is <50°C / h, the soaking temperature is 1260±20°C, and the soaking time is 1.1-1.5min / mm;

[0013] 4) Controlled cooling and rolling: two-stage rolling is adopted, the starting temperature of rough rolling is greater than 980℃, high-pressure water is applied once for each rolling pass, and the reduction amount per pass is 45-50mm. When the steel is rolled to 300mm, the steel is cooled, and the starting temperature of finishing rolling is 860-920℃. At the same time, vertical roller edge extrusion is carried out, and the vertical roller edge extrusion single pass is required to be controlled within 10mm. After rolling, the steel plate is transferred to the cooling bed for cooling. When the temperature drops to 400℃, it is transferred to the heat treatment furnace for heat treatment;

[0014] 5) Heat treatment: the solution temperature is controlled at 876-884°C for 5-6 hours, and the steel is quenched to 250-300°C after being taken out of the furnace; then the steel is put into the furnace again, and the aging temperature is controlled at 507-513°C for 8-10 hours. After the aging is completed, the steel plates are stacked and slowly cooled to room temperature.

[0015] It should be noted that when the solution time is insufficient, the temperature of the core of the steel plate cannot reach the solution temperature requirement, the core structure cannot complete austenitization, and undissolved ferrite in the rolled state is still retained. After quenching and cooling, the ferrite in the core cannot be transformed into bainite, and the central hardness is only about 30HRC, which cannot meet the use requirements; at the same time, within the aging temperature range of this scheme, as the aging time increases, the amount of precipitation of the strengthening phase continues to increase, thereby increasing the hardness of the steel plate accordingly. During the aging time, the hardness reaches a peak value and can reach more than 40HRC.

[0016] Preferably, in step ②, when the C content is 0.08-0.10 and the Mn content is 1.90-2.00, the pouring temperature is controlled at liquidus + 40-43°C, and the pouring time of the steel ingot body is controlled at 20-22 minutes; when the C content is 0.12-0.13 and the Mn content is 1.70-1.80, the pouring temperature is controlled at liquidus + 48-50°C, and the pouring time of the steel ingot body is controlled at 22-24 minutes.

[0017] Compared with the prior art, the present invention uses a water-cooled mold to adjust the water-cooling strength to achieve micro-defect solidification through high-clean steel smelting, and adjusts the pouring process to obtain a micro-segregation steel ingot, and combines the rolling and heat treatment processes to finally obtain a NAK80 steel plate with slight segregation and uniform hardness. The steel plate obtained by this scheme is tested and meets the following performance indicators: the hardness of the entire cross-section of the core is 36-40HRC and the overall hardness of the steel plate is uniform, the welding performance is good, and it meets the requirements of defect diameter ≤3mm in the forging GB / T 6402-20084 grade, the polished surface roughness is ≤0.02μm, the Mn segregation is between 0.97 and 1.05, and the C segregation is between 0.95 and 1.06. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0019] Figure 1 It indicates that the present invention takes samples at the positions of 30mm, 630mm and 1260mm away from the edge of the steel plate to analyze the segregation of C and Mn.

[0020] Figure 2 The flaw detection waveform diagram of the 152 mm thick NAK80 steel plate of the present invention is shown.

[0021] Figure 3 A low-magnification image of a 152 mm thick NAK80 cap sample of the present invention is shown. DETAILED DESCRIPTION

[0022] In order to make the objectives, technical solutions and advantages of the present invention more clear, a detailed description will be given below in conjunction with specific embodiments.

[0023] The ingot type is 30t+30t+29t steel ingot, and the smelting composition is controlled according to Table 1:

[0024] Table 1 NAK80 steel plate finished product composition (wt%)

[0025]

[0026] The production process is as follows:

[0027] 1) Converter smelting: The output of steel is controlled according to the casting ingot + 8t casting residue, and the P of steel is controlled at ≤0.007%.

[0028] 2) LF refining: Use high basicity slag and fully deoxidize. The amount of lime used in the whole process is 1250kg and aluminum particles are 110kg. Calcium carbide is used to assist deoxidation to ensure that the final slag is white foam slag. The white slag is kept for 45 minutes during the whole refining process.

[0029] 3) VD refining: low vacuum pressure is required, the vacuum is below 45pa, during the pressure holding period, the argon flow rate is controlled at 600-700L / min for the first 21 minutes, and the argon flow rate is controlled at 50-80L / min for soft blowing for the last 5 minutes. No other alloys are added after VD is broken.

[0030] 4) Pouring: Before casting, the molten steel pouring temperature is controlled between liquidus + 43 and 45°C. The specific operation is as follows:

[0031] Follow the principle: "When the carbon (C) content is low and the manganese (Mn) content is high, the pouring temperature should be at the lower limit and the pouring speed should be at the upper limit". The liquidus is 1504℃ and the pouring temperature is between 1544 and 1547℃.

[0032] 5 seconds before pouring, start pouring at a "full flow" above the slag basin to remove drainage sand and foreign inclusions.

[0033] After 10 seconds, open the slide to 30% of the "full flow" state, move the steel pouring car, and start pouring on the center column pipe.

[0034] When the molten steel overflows the spherical area at the bottom of the mold, maintain the flow rate to allow the molten steel to rise steadily. When the molten steel rises to a height of 80-100mm in each ingot, increase the injection speed to 80% of the "full flow" for pouring.

[0035] After pouring for 2 minutes, gradually increase the flow rate to "full flow".

[0036] When pouring to 100mm below the cap opening line, reduce the flow rate to 20-30% of "full flow" until the cap opening is filled. After pouring, add the insulation above the cap opening and spread it evenly.

[0037] The pouring time of the main body is controlled within 21 minutes, and the pouring time of the cap is controlled within 8 minutes. The watering volume is 530m 3 / h control, within 30 minutes after the pouring, open the wide surface extrusion device, adjust the gap between the wide surface crystallizer and the ingot, and adjust the water flow rate, adding 80m3 on the basis of the original water volume. 3 / h.

[0038] The mold is removed 8 hours after pouring, and the ingot is transferred to a slow cooling pit for insulation and slow cooling. When the surface temperature drops to about 195°C, the iron oxide scale and defects on the surface of the ingot are cleaned.

[0039] 5) Heating: The soaking pit temperature is 320℃, the soaking pit temperature is below 700℃, the heating rate is less than 50℃ / h, the holding temperature is 1260-1270℃, and the holding time is 1.3min / mm.

[0040] 6) Controlled cooling and rolling: When rolling starts, the surface temperature of the ingot is 1030℃. Rough rolling adopts differential temperature rolling. High-pressure water is applied once for each rolling pass. The reduction in a single pass is 50mm. When the thickness reaches 300mm, rolling is stopped and the steel is cooled. When the temperature drops to 890℃, rolling is started again. At the same time, vertical roller edge extrusion is carried out. The vertical roller edge extrusion is required to be controlled at 10mm per single pass. Multiple edge extrusions are performed to achieve the purpose of delivering the steel with the same edges. After rolling, the steel plate is transferred to the cooling bed for cooling. When the steel plate is cooled to 400℃, it is transferred to heat treatment.

[0041] 7) Heat treatment: The solution temperature of the steel plate is 881°C, the solution time is 5.5 hours, and then it is quenched to 260°C; the steel plate is put into the furnace again, the aging temperature is controlled at 510°C, and the aging time is 9 hours. After the aging is completed, the steel plate is stacked and slowly cooled until the temperature drops to room temperature.

[0042] The steel plates were then destackered, sawed, and hardness tested. The mechanical properties of the steel plates obtained according to the example are shown in Table 2.

[0043]

[0044] Figure 1 For the steel plate obtained in Example 2, samples were taken at 30mm, 630mm, and 1260mm away from the edge of the steel plate, and the C and Mn segregation were analyzed. It can be seen that the C segregation is between 0.95 and 1.06, and the Mn segregation is between 0.97 and 1.05.

[0045] pass Figure 1 Resulting C element segregation distribution (%)

[0046]

[0047]

[0048] Mn element segregation distribution (%)

[0049]

[0050]

[0051] Attached Figure 2 The flaw detection waveform of the 152mm NAK80 steel plate of the present invention shows no defects; Figure 3 The low-magnification image of the 152mm NAK80 cap mouth sample of the present invention shows no obvious signs of segregation.

Claims

1. A method for producing micro-segregation NAK80 die steel, characterized in that: The die steel adopts the following chemical composition in mass percentage (unit: wt%): C: 0.08-0.13, Si: 0.15-0.25, Mn: 1.70-2.00, P≤0.010, S≤0.003, Ni: 3.20-3.40, Mo: 0.35-0.55, Al: 0.65-1.0, Cu: 0.75-1.15, and the rest are Fe and unavoidable impurities; The production method of the above die steel includes smelting, water-cooled mold pouring, heating, controlled cooling and controlled rolling, and heat treatment, as follows: ① Smelting: The P content of the converter steel is controlled below 0.008%; LF refining uses high-basicity slag, and the basicity is controlled at 5-6. The amount of lime used in the whole process is more than 1200kg, and the aluminum particles are more than 100kg. Calcium carbide is used for auxiliary deoxidation to ensure that the final slag is white foam slag. The white slag is kept for more than 40 minutes during the whole refining process; VD requires low vacuum pressure maintenance, and the vacuum is below 45pa. The first 20-22 minutes use 500-700L / min argon flow rate and large stirring mode to fully remove H, and the last 5 minutes use 50-100L / min soft blowing mode to ensure that inclusions float up, and the end point H≤0.8PPm. It is forbidden to add other alloys after VD breaks the air; ② Water-cooled mold pouring: The temperature of molten steel is controlled between liquidus + 40 and 50°C. 5 seconds before pouring, "full flow" pouring is carried out above the slag basin to remove drainage sand and foreign inclusions. After 10 seconds, the slide is opened to 30% of the "full flow" state, and the steel pouring car is moved to the center column pipe to start pouring. When the molten steel rises to a height of about 80 to 100 mm in the ingot mold, the pouring speed is increased to 80% of the "full flow". After pouring for 2 minutes, the pouring speed is gradually increased to "full flow". When it is poured to 100 mm below the cap mouth line, the pouring speed is gradually reduced to 20 to 30% of the "full flow" until the cap mouth is filled. After the pouring is completed, the insulation agent is added above the cap mouth and evenly spread; the pouring time of the ingot body is controlled at 20 to 24 minutes, the pouring time of the cap mouth is controlled at 6 to 9 minutes, and the water pouring volume is 500 to 550 m 3 / h control, within 30 minutes after the pouring, open the wide surface extrusion device, adjust the gap between the wide surface crystallizer and the ingot, and increase the original water volume by 50-100m 3 / h; demoulding within 8 hours after pouring, the steel ingot is transferred to the slow cooling pit for slow cooling, and the steel ingot is cleaned when the surface temperature drops to 150-200℃; ③ Heating: The steel ingot is heated in the soaking pit. When the furnace temperature is below 700℃, the heating rate is <50℃ / h, the soaking section insulation temperature is 1260±20℃, and the insulation time is 1.1~1.5min / mm; ④Controlled cooling and controlled rolling: Two-stage rolling is adopted, the starting temperature of rough rolling is greater than 980℃, high-pressure water is applied once for each rolling pass, and the reduction amount per pass is 45-50mm. When the steel is rolled to 300mm, the steel is cooled, and the starting temperature of finishing rolling is 860-920℃. At the same time, vertical roller extrusion is carried out, and the vertical roller extrusion single pass is required to be controlled within 10mm. After rolling, the steel plate is transferred to the cooling bed for cooling. When the temperature drops to 400℃, it is transferred to the heat treatment furnace for heat treatment; ⑤ Heat treatment: The solution temperature is controlled at 876-884°C for 5-6 hours. After being taken out of the furnace, it is quenched to 250-300°C. Then it is put into the furnace again, and the aging temperature is controlled at 507-513°C for 8-10 hours. After the aging is completed, the steel plates are stacked and slowly cooled to room temperature.

2. The method for producing micro-segregation NAK80 die steel according to claim 1, characterized in that: In step ②, when the C content is between 0.08 and 0.10 and the Mn content is between 1.90 and 2.00, the pouring temperature is controlled at liquidus + 40 to 43°C, and the pouring time of the steel ingot body is controlled at 20 to 22 minutes; when the C content is between 0.12 and 0.13 and the Mn content is between 1.70 and 1.80, the pouring temperature is controlled at liquidus + 48 to 50°C, and the pouring time of the steel ingot body is controlled at 22 to 24 minutes.

Images