A 280 mm thick low cost high polish p20 plastic mold steel and its production method

Abstract

The application discloses a 280mm-thick low-cost high-polish P20 plastic mold steel and a production method thereof, and contains the following mass fractions of chemical components (unit, %): C=0.35-0.38, Si=0.20-0.40, Mn=1.5-1.6, P=0.013-0.020, S<=0.002, Cr=1.8-1.9, Mo=0.25-0.30, Ti=0.020-0.040, Al=0.030-0.060, B=0.0008-0.0010, the rest is Fe and inevitable impurities, and 300g of rare earth silicon alloy (containing Ce elements) is added in batches per 100kg. The alloy ratio and the rare earth elements are added, and the water cooling intensity in the pouring process is controlled, so that the grain is refined, the dendritic segregation and the regional segregation are reduced, the macroscopic quality of the steel plate is improved, the inclusion particles and the distribution in the steel are best by adding in this way, the structure is uniform after rolling, the grain refinement effect is obvious, and the polishing property of the steel plate is obviously improved.

Description

Technical Field

[0001] This invention relates to the field of metal material manufacturing technology, specifically to a 280mm thick, low-cost, high-polish P20 plastic mold steel and its production method. Background Technology

[0002] The performance of mold steel is not only related to its chemical composition and metallographic structure, but also closely related to the inclusions in the steel. Treating molten steel with mixed rare earth elements can not only homogenize the chemical composition and refine the grains, but also change the type, morphology, size, and distribution of inclusions in the steel. Because rare earth elements are chemically very reactive, they have a strong affinity for nitrogen and oxygen in steel, forming refractory compounds with low specific gravity. Therefore, rare earth elements have a purifying effect, including deoxidation, denitrification, and reduction of non-metallic inclusions. Simultaneously, as surface-active elements, they can adsorb onto the surface of growing solid crystal nuclei, forming a thin film that hinders the supply of atoms required for crystal growth, thereby refining the grains and reducing dendrite segregation and zonal segregation, improving the uniformity of the steel's chemical composition. This patent is mainly based on the above theoretical guidance, and by adjusting the method and amount of rare earth addition, combined with the control of water cooling intensity during the casting process of our water-cooled molds, we produce P20 mold steel with high polishability.

[0003] Chinese patent application CN202011347020.8, "A method for producing P20 steel for extra-thick plastic molds," is characterized in that: the steel plate is 200-300mm thick and contains the following chemical composition by mass percentage: C: 0.45-0.51%, Si: 0.3-0.6%, Mn: 1.50-1.6%, P≤0.006%, S≤0.001%, Cr: 1.70-1.80%, V: 0.15-0.20%, B: 0.0010-0.0018%, with the balance being Fe and residual elements; the steel plate undergoes rough rolling, finish rolling, rapid cooling to 500-520℃ after rolling, stacking cooling, and tempering heat treatment to obtain a steel plate with an internal quality that meets the requirements. The hardness difference of the forging is within 40HB; the production method of the steel plate also includes: clean steel smelting, water-cooled mold casting, steel ingot cleaning and heating, rolling, post-rolling rapid cooling, stacking cooling, and tempering heat treatment; a. Clean steel smelting: molten iron is desulfurized by KR, smelted in a converter, treated with argon blowing, refined by LF and treated with VD vacuum, with a hydrogen concentration of 1.2 ppm. After VD vacuum breaking, calcium wire of 1.6~2.0 m / t is added according to the Al content in the molten steel for calcium treatment to ensure W(Ca) / W(Al) = 0.08~0.12. A covering agent is added before the molten steel leaves the station to ensure that it covers the surface of the molten steel and the temperature at the station is controlled at 1545±15℃; b. Water-cooled mold casting: 42-48 ton fixed thickness and adjustable width water-cooled steel ingot molds are used, with an ingot thickness of 80 0-840mm, casting temperature controlled at 1540-1548℃; c. Ingot cleaning and heating: Ingots are cleaned within 24 hours after demolding, with a cleaning temperature ≥240℃; When loading ingots into the furnace, the furnace temperature is less than 400℃, and the ingots are steamed for 3 hours at a furnace temperature of 450-550℃. The first stage heating rate is 60℃ / h, and when the temperature reaches 860℃, it is held for 8 hours. The second stage heating rate is 70-80℃ / h, and the heating rate is unlimited when the furnace temperature reaches 1000℃. The third stage furnace temperature is 1220-1240℃, and it is held for 16 hours. After turning the ingots over, the temperature is raised again to 1220-1240℃ and held for 4 hours before unloading; d. Rolling: Two-stage rolling is adopted. The first stage is high-temperature, high-pressure, rapid rolling, with an initial rolling temperature of 1050-1100℃. The rolling process begins with a first rolling pass at a reduction rate of 9-12%, followed by cooling the steel at 980℃-1020℃ until it reaches a thickness 1.5-2 times that of the finished product. The second rolling stage begins at 930-950℃, with a reduction rate exceeding 12%. The reduction rate for the first four passes is guaranteed to be no less than 15%, with a total reduction exceeding 50%. The final rolling temperature is 850-870℃. After final rolling, the steel plate is rapidly cooled in a quenching water tank. The third stage involves rapid cooling after rolling, with the quenching water temperature controlled at 50-60℃. The initial steel plate temperature upon immersion is 780-800℃, with a cooling rate of 1-2℃ / s and a reddening temperature of 500-520℃. The fourth stage involves slow cooling after rapid cooling in the quenching water tank, with a stacking temperature of 500±20℃ and a stacking time ≥48 hours.Tempering heat treatment: The steel plate is tempered in an external mechanized furnace to adjust its hardness to a suitable range. The tempering temperature is 550-580℃, and the holding time is 3-6 min / mm. After exiting the furnace, it is air-cooled to room temperature. Although this method uses V instead of Mo to increase the C content and improve strength, it results in a high carbon content and increases the risk of water-cooling cracking.

[0004] Chinese patent application CN202211248487.6, "A method for preparing rare earth microalloyed low-cost high-hardness plastic mold steel P20", includes the following steps: smelting and continuous casting: molten iron undergoes desulfurization pretreatment, decarburization and dephosphorization are carried out in a top-and-bottom blowing converter, followed by LF ladle refining and RH furnace vacuum degassing, slab continuous casting, slab cleaning, slow cooling, and slab quality inspection; heating and controlled rolling and cooling: heating to 1180℃~1255℃ After being removed from the furnace, the material undergoes high-pressure water descaling followed by rolling. The roughing rolling temperature is 1090℃~1175℃, and the finishing rolling temperature is 975℃~1040℃, ensuring a single-pass reduction rate ≥11% and a cumulative reduction rate ≥61%. The thickness after finishing is 2.3~3.8 times the finished product thickness. The finishing rolling temperature is ≤955℃, ensuring a single-pass reduction rate ≥12% and a cumulative reduction rate ≥63%. The finishing rolling temperature range is 830℃~880℃, and the final rolling temperature is 10~19℃. The steel plate is cooled to 590–640℃ at a cooling rate of / s, and then sent to a straightening machine for straightening. Heat treatment: After surface quality inspection, the steel plate undergoes shot blasting and tempering at 500–570℃ for 30 minutes. Hardness testing shows that the pre-hardened hardness of the steel plate is HRC 30–36. The sample quenching temperature is 850–880℃, the coolant is oil, and the sample hardness is HRC ≥ 60. The mass fraction of the hot-rolled raw material composition is: C: 0.2%. The composition is as follows: 8-0.40%, Si: 0.20-0.80%, Mn: 0.60-1.00%, P≤0.015%, S≤0.010%, B: 0.0010-0.0060%, Cr: 1.40-2.00%, Al: 0.020-0.050%, RE: 0.0015-0.030%, N≤0.003%, O≤0.002%, with the remainder being iron and other unavoidable impurities. This patent document does not discuss how rare earth elements are added or controlled; it only notes that the finished product contains rare earth elements. Furthermore, this patent is more suitable for continuous casting of thin plates than for thick plates.

[0005] Chinese patent application CN202011254778.7 discloses a smelting process for lanthanum-cerium rare earth alloy mold steel. The process is characterized by being applicable to alloy mold steels of one of the following: H11, H13, 1.2343, 1.2344, 1.2083, 1.2316, 718, 136, P15, and P20. It is not applicable to high-aluminum steels with an aluminum content greater than or equal to 0.5% or steels requiring calcium treatment. The specific process is as follows: Step 1) Prepare sealed packaging of lanthanum-cerium composite rare earth, with a weight ratio of lanthanum:cerium = 35:65; Step 2) Calculate the total addition amount based on adding 0.12-0.15 kg of composite rare earth per ton of mold steel; Step 3) The VD vacuum degree is small... When the temperature is maintained at 0.67 mbr for 12 minutes, the rare earth packaging is opened, and the calculated amount is weighed and added to the vacuum-sealed intermediate silo. Step 4) When the VD vacuum degree is maintained at less than or equal to 0.67 mbr for 18 minutes, the vacuum-sealed intermediate silo is activated to add the rare earth to the molten steel. The conditions for adding composite rare earth are: the carbon weight percentage of the initial steel is not less than 0.05%; the refining slag composition by weight percentage is CaO: 0.47-58%, Al2O3 = 25-32%, SiO2 = 8-10%; dissolved oxygen before VD is not greater than 5 ppm; aluminum is fed before VD, but not after VD, and the aluminum content is controlled at 0.015-0.025% by weight. This method involves adding rare earth elements during the VD pressure holding process, but this method is easily limited by equipment, and there is an urgent need to study another addition method that can also meet the requirements. Summary of the Invention

[0006] To address the aforementioned technical deficiencies, the present invention aims to provide a 280mm thick, low-cost, high-polish P20 plastic mold steel. Through a variety of unique methods, including the addition of rare earth elements in two stages, the use of water-cooled casting technology, multiple water cooling and multiple reheating processes, and two-stage tempering, the strength and toughness of the material can be effectively enhanced, the crystallization rate and yield of the ingot can be controlled, and the risk of quenching cracking can be reduced.

[0007] Another object of the present invention is to provide a method for producing 280mm thick, low-cost, highly polishable P20 plastic mold steel.

[0008] To achieve the above objectives, the technical solution adopted by this invention is: a 280mm thick, low-cost, high-polish P20 plastic mold steel, comprising the following chemical composition by mass fraction (unit, %): C = 0.35-0.38, Si = 0.20-0.40, Mn = 1.5-1.6, P = 0.013-0.020, S ≤ 0.002, Cr = 1.8-1.9, Mo = 0.25-0.30, Ti = 0.020-0.040, Al = 0.030-0.060, B = 0.0008-0.0010, with the balance being Fe and unavoidable impurities. In batches, 300g of rare earth silicon alloy (containing Ce) is added per 100kg.

[0009] The above-mentioned production method for 280mm thick, low-cost, high-polish P20 plastic mold steel is achieved through the following steps:

[0010] 1) Ingot selection: Water-cooled mold casting technology is adopted and the ingot is rolled for production. The ingot size is made of steel ingot with a thickness of 800 / 840mm and an ingot width between 1600-2000mm. The compression ratio is 2.7-3. The width is adjusted according to the tonnage of the steel plate, which is suitable for the production of 30-35 ton steel plates.

[0011] 2) Rare earth addition: Rare earth alloys are added in the following ways, following common smelting processes and according to the above composition requirements:

[0012] After VD breaks the air, add rare earth silicon alloy. The amount added is 200g of rare earth silicon alloy (containing Ce element) per 100kg. After adding, gently blow for 8 minutes.

[0013] Alternatively, after the soft blow molding process is completed, add rare earth silicon alloy at a rate of 100g of rare earth silicon alloy (containing Ce) per 100kg of molded product, let stand for 5 minutes, and then start casting.

[0014] 3) Steel ingot casting: The casting temperature is controlled at 1535-1540℃, and the casting process is completed in the following three stages:

[0015] Phase 1: Initial watering volume: 300-320 ml 3 / h control, start pouring at full flow when starting pouring;

[0016] Second stage: When the ingot height is 1 / 5, adjust the water volume to 350-360 ml. 3 / h control, at which point the pouring flow rate is reduced to 2 / 3 of the total flow;

[0017] Third stage: When the ingot height is halfway up the casting, adjust the water volume to 400-410 ml. 3 The injection rate is controlled at / h and remains constant until the pouring is complete.

[0018] 4) Steel ingot heating: A slow heating and high-temperature short-time holding process is adopted, specifically as follows:

[0019] Steel ingot charging: Steel ingot temperature > 300-500℃, furnace temperature > 450-600℃;

[0020] Steel ingot heating: The heating rate should be controlled at <30~50℃ / h;

[0021] Ingot heat preservation: After a long heating process, the internal temperature gradient of the ingot is small. After proper heat preservation, it can be tapped and rolled. The heat preservation temperature is controlled at ≤1260~1280℃, and the heat preservation time is controlled according to the ingot thickness (mm)*(0.3~0.6)min / mm.

[0022] 5) Steel ingot rolling: The rolling process employs a combination of longitudinal rolling, hot rolling, and reheating. The specific production process is as follows: initial rolling temperature ≥ 1020℃, rolling deformation ratio per pass ≥ 15%, and final rolling temperature of the first stage ≥ 950℃. The first-stage rolling thickness = steel plate thickness + 50mm; after the first stage, the steel is water-cooled once, reheated for 3 minutes, and then the second-stage rolling begins. The second-stage rolling primarily involves small reductions and leveling passes.

[0023] 6) Online water cooling: A multi-stage online water cooling + reheating + air cooling process is employed. Specific production requirements: The initial water immersion temperature is controlled at 800-850℃. Multiple online water cooling cycles are performed, with reheating control required after every two water cooling cycles to minimize the temperature difference between the steel ingot surface and core. Reheating time > 2 minutes. When the side center temperature ≤ 500℃, switch to the air cooling area for air cooling. Cooling is complete when the temperature drops below 300℃.

[0024] 7) Steel plate heat treatment: A two-stage tempering process is adopted, specifically as follows:

[0025] The first stage of tempering process: 590-610℃, the holding time is based on the steel plate thickness (mm) * (4-4.5) min / mm;

[0026] The second stage tempering process uses 540-550℃ and the holding time is calculated as steel plate thickness (mm) * (1.5-2) min / mm.

[0027] Compared with the prior art, the present invention has the following differences or advantages:

[0028] The composition design of this invention adopts a low-carbon design to prevent cracking caused by high carbon content, while reducing C segregation and improving the overall plasticity and toughness of the material; at the same time, the Mo content is reduced to reduce costs, the appropriate addition of B element improves the hardenability of the steel plate, the addition of Ti element plays a role in nitrogen fixation, and the addition of rare earth alloys inhibits grain growth and plays a role in refining the grains.

[0029] This invention employs a two-stage rare earth addition method, which can effectively increase the purification effect of rare earth and the nucleation of rare earth oxides, refine the grain size, and enhance the strength and toughness of the material.

[0030] The casting method of this invention adopts water-cooled mold casting technology. By controlling the water volume and casting speed, the crystallization rate and yield of the ingot can be controlled, and the component segregation can be effectively reduced, creating favorable conditions for the subsequent manufacturing of high-quality steel plates.

[0031] The online cooling method of this invention employs multiple water cooling and multiple reheating processes to reduce the temperature difference between the core and the surface, ensuring uniform microstructure throughout the thickness direction. This method results in a small temperature gradient between the steel plate surface and core, which is beneficial for controlling microstructure uniformity. Simultaneously, an online air-cooling process replaces the original strong quenching process, improving production speed and reducing the risk of quenching cracking.

[0032] The unique feature of the heat treatment in this invention is the use of two-stage tempering. Compared with the existing process, a two-stage low-temperature tempering process is added. The purpose is to: adjust the hardness in the first tempering to meet customer needs, and ensure the full release of stress during the second tempering to reduce the risk of processing cracks.

[0033] The inventors have refined the grains and reduced dendritic and regional segregation by adjusting the alloy ratio and adding rare earth elements, combined with controlling the water cooling intensity during the casting process. This improves the low-magnification quality of the steel plate. Furthermore, when added in this way, the inclusion particles and distribution in the steel are optimal, resulting in a uniform microstructure after rolling and a significant grain refinement effect. In particular, the polishability of the steel plate is significantly improved. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the microstructure of a 280mm P20 steel plate in an embodiment of the present invention.

[0035] Figure 2 This is a schematic diagram of the steel plate structure with a type A (sulfide) inclusion level of 0.5 in an embodiment of the present invention.

[0036] Figure 3 This is a schematic diagram of the steel plate structure with a B-type (alumina-type) inclusion grade of 0.5 in an embodiment of the present invention.

[0037] Figure 4 This is a schematic diagram of the steel plate structure with a D-type (spherical oxide) inclusion level of 0.5 in an embodiment of the present invention. Detailed Implementation

[0038] The following examples are used to illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0039] This invention produces 280mm thick, low-cost, high-polish P20 plastic mold steel, using 800 / 840mm thick steel ingots rolled from steel ingots with a width between 1600-2000mm and a compression ratio of 2.7.

[0040] Rare earth elements are added to the alloy using common smelting processes, according to the above-mentioned composition requirements.

[0041] ① After VD breaks the void, add rare earth silicon alloy. The amount added is 200g of rare earth silicon alloy (containing Ce element) per 100kg. After adding, gently blow for 8 minutes.

[0042] ② After the soft blow molding is completed, add rare earth silicon alloy at a rate of 100g of rare earth silicon alloy (containing Ce element) per 100kg, let stand for 5 minutes and then start casting;

[0043] The actual controlled components in this embodiment of the invention are as follows:

[0044] Table 1. Chemical composition (mass fraction) of steel in the embodiments of the present invention.

[0045] 0.36 0.35 1.54 0.016 0.002 0.040 1.85 0.28 0.00092 0.037

[0046] Casting process: The casting temperature is controlled at 1535-1540℃, and the casting process is completed in the following three stages:

[0047] Phase 1: The water flow rate is controlled at 310 m³ / h, and full-flow irrigation is carried out at the start of irrigation.

[0048] Second stage: When the ingot height is 1 / 5, adjust the water volume to 355m. 3 / h control, at which point the pouring flow rate is reduced to 2 / 3 of the total flow;

[0049] Third stage: When the ingot height is halfway up the casting, adjust the water volume to 408m. 3 The injection rate is controlled at / h and remains constant until the pouring is complete.

[0050] Steel ingot heating: A slow heating and high-temperature short-time holding process is adopted, specifically as follows:

[0051] When the steel ingots are loaded into the furnace, the ingot temperature is 310℃ and the furnace temperature is 460℃.

[0052] The heating rate of the steel ingot is controlled at 35℃ / h;

[0053] Ingot heat preservation: After a long heating process, the internal temperature gradient of the ingot is small. After proper heat preservation, it can be tapped and rolled. The heat preservation temperature is controlled at ≤1260~1280℃, and the heat preservation time is controlled according to the ingot thickness (mm)*0.4min / mm.

[0054] Steel ingot rolling: The rolling process employs a combination of longitudinal rolling, hot rolling, and reheating. The specific production process is as follows: initial rolling temperature 1030℃, rolling deformation ratio per pass ≥15%, and final rolling temperature of 980℃ for the first stage. The first-stage rolling thickness is 330mm; after the first stage, the ingot is water-cooled once, reheated for 3 minutes, and then the second-stage rolling begins. The second-stage rolling primarily involves small reductions and leveling passes.

[0055] Online water cooling: A multi-stage online water cooling + reheating + air cooling process is employed. Specific production requirements: The initial water immersion temperature is controlled at 800-850℃. Multiple online water cooling cycles are performed, with reheating control required after every two water cooling cycles to minimize the temperature difference between the steel ingot surface and core. The reheating time is 4 minutes. When the center temperature reaches 490℃, the process switches to air cooling. Cooling is completed when the temperature drops below 300℃.

[0056] Steel plate heat treatment: A two-stage tempering process is adopted, specifically as follows:

[0057] First-stage tempering process: 590-610℃, holding time according to steel plate thickness (mm) * 4min / mm;

[0058] The second stage tempering process: 540-550℃, with a holding time of 2 min / mm based on the steel plate thickness (mm);

[0059] See appendix Figure 1-4 The resulting steel plate has good low-magnification quality. By adjusting the alloy ratio and adding rare earth elements, combined with the control of water cooling intensity during the casting process, the grains are refined and dendritic segregation and regional segregation are reduced, effectively improving the low-magnification quality of the steel plate. At the same time, the inclusion particles and distribution in the steel are the best, the microstructure after rolling is uniform, the grain refinement effect is obvious, and the polishability of the steel plate is significantly improved.

Claims

1. A method for producing 280mm thick, low-cost, high-polish P20 plastic mold steel, characterized in that: The steel for plastic molds contains the following chemical composition by mass percentage: C=0.35~0.38, Si=0.20~0.40, Mn=1.5~1.6, P=0.013~0.020, S≤0.002, Cr=1.8~1.9, Mo=0.25~0.30, Ti=0.020~0.040, Al=0.030~0.060, B=0.0008~0.0010, with the balance being Fe and unavoidable impurities. In batches, 300g of rare earth silicon alloy containing Ce is added per 100kg. The above-mentioned production method for 280mm thick, low-cost, high-polish P20 plastic mold steel is achieved through the following steps: 1) Ingot selection: Water-cooled mold casting technology is adopted and the ingot is rolled for production. The ingot size is made of steel ingots with a thickness of 800 / 840mm and an ingot width between 1600-2000mm. The compression ratio is 2.7-3. The width is adjusted according to the tonnage of the steel plate, which is suitable for the production of 30-35 ton steel plates. 2) Rare Earth Addition: Rare earth alloys are added in the following ways, following common smelting processes and according to the above composition requirements: After VD breaks the void, add rare earth silicon alloy. The amount added is 200g of rare earth silicon alloy containing Ce element per 100kg. After adding, gently blow for 8 minutes. Alternatively, after the soft blow molding process is completed, add rare earth silicon alloy at a rate of 100g of Ce-containing rare earth silicon alloy per 100kg, let stand for 5 minutes, and then start casting. 3) Steel ingot casting: The casting temperature is controlled at 1535-1540℃, and the casting process is completed in the following three stages: Phase 1: The water volume for initial irrigation should be controlled at 300-320 m³ / h, and irrigation should be carried out at full flow. Second stage: When the ingot height is 1 / 5, the water volume is adjusted to 350-360m³ / h and the pouring flow rate is reduced to 2 / 3 of the total flow. Third stage: When the ingot height is half of the ingot body height, the water volume is adjusted to 400-410m³ / h and the pouring speed remains unchanged until the pouring is completed. 4) Steel ingot heating: A slow heating and high-temperature short-time holding process is adopted, specifically as follows: Steel ingot loading: steel ingot temperature 300~500℃, furnace temperature 450~600℃; Steel ingot heating: The heating rate should be controlled at 30-50℃ / h; Ingot heat preservation: After a long heating process, the internal temperature gradient of the ingot is small. After proper heat preservation, it can be tapped and rolled. The heat preservation temperature is controlled at 1260~1280℃, and the heat preservation time is controlled according to the ingot thickness * (0.3~0.6) min / mm. 5) Steel ingot rolling: The rolling process adopts full longitudinal rolling + hot rolling + reheating. The specific production process is as follows: the initial rolling temperature is ≥1020℃, the rolling deformation ratio per pass is ≥15%, the final rolling temperature of the first stage is ≥950℃, and the thickness of the first stage rolling is equal to the thickness of the steel plate + 50mm. After the first stage, the steel plate is water-cooled once, and after reheating for 3 minutes, the second stage rolling begins. The second stage rolling mainly focuses on small reduction and leveling passes. 6) Online water cooling: The cooling process adopts online multiple water cooling + reheating + air cooling. Specific production requirements: The initial water temperature is controlled at 800-850℃. Online multiple water cooling is carried out. After every two water cooling cycles, reheating control is required to reduce the temperature difference between the surface and core of the steel ingot. The reheating time is >2 minutes. When the side center temperature is ≤500℃, the air cooling area is switched to air cooling. Cooling is completed when the temperature drops below 300℃. 7) Steel plate heat treatment: A two-stage tempering process is adopted, specifically as follows: The first stage of tempering process: 590-610℃, with a holding time of (4-4.5) min / mm based on the steel plate thickness; The second stage tempering process: 540-550℃, and the holding time is calculated as steel plate thickness * (1.5-2) min / mm.

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