A low cost pre-hardened 718 plastic mold steel of 150-280 mm thickness and a production method thereof

Abstract

This invention discloses a low-cost pre-hardened 718 plastic mold steel with a thickness of 150-280 mm, comprising the following chemical composition (in %) by mass fraction: C: 0.40-0.44, Si: 0.20-0.40, Mn: 1.20-1.50, Cr: 1.50-1.90, Mo: 0.25-0.40, Ni: 0.90-1.0, P≤0.015, S≤0.002, Ti: 0.020-0.040, and satisfying 3.44≤C+Mn+Cr≤3.84, with the remainder being Fe and residual elements. The production method includes: smelting → water-cooled mold casting → steel ingot heating → controlled rolling → online cooling → tempering. Water-cooled mold casting is used to accelerate the solidification and cooling of the steel ingot. The online cooling of the steel plate is achieved through the optimization of the element ratio of C, Mn and Cr to meet the pre-hardening requirements, reducing the heat treatment process and enabling low-cost mass production of pre-hardened 718 plastic mold steel plates. The steel plate has a hardness of 32-34 HRC, and the hardness difference within the same plate is ≤2 HRC. After testing, the polishing score (12000#) of the processed surface is ≤90, and the surface roughness Ra is ≤0.20μm, which meets the requirements for the production of pre-hardened 718 plastic molds.

Description

Technical Field

[0001] This invention relates to the field of metal material manufacturing technology, specifically to a low-cost pre-hardened 718 plastic mold steel with a thickness of 150-280mm and its production method. Background Technology

[0002] In recent years, most steel mills in my country have been mass-producing some internationally recognized steel grades with strong versatility and excellent performance. Pre-hardened 718 plastic mold steel plate, as a type of high-end plastic mold steel, possesses excellent polishability and photo-etching properties; excellent machinability; good wear resistance; and the pre-hardened large mold material has uniform hardness, dense structure, no risk of quenching cracks or heat treatment deformation, and requires no heat treatment, thus shortening the mold manufacturing cycle.

[0003] In the market for pre-hardened 718 plastic mold steel, most mold steels with a thickness of more than 150mm are produced by forging ingots or large-thickness electroslag ingots. This involves many processes and complex heat treatment, resulting in high costs and low output. This fails to meet the market's demand for short delivery times and high economic benefits in the mold steel industry.

[0004] Therefore, developing a high-quality 718 plastic mold steel with low-cost pre-hardening treatment, uniform material, high cleanliness, excellent polishing performance, and photolithography properties is key to improving the market competitiveness of steel mills in the high-end plastic mold steel market.

[0005] Chinese patent application 202310791119.4 discloses "a pre-hardened plastic mold steel plate and its preparation method," wherein the steel plate contains the following chemical composition by mass fraction: C: 0.35–0.45%, Si: 0.20–0.40%, Mn: 1.20–1.60%, P ≤ 0.025%, S ≤ 0.015%, Als: 0.020–0.045%, Cr: 1.70–2.00%, Mo: 0.10–0.20%, Ti: 0.015–0.02%. 5%, B: 0.0010~0.0020%, balance being Fe and unavoidable impurities; and the above element contents must simultaneously satisfy the following relationship: 3.0%≤Mn+Cr≤3.2%; its preparation method includes converter smelting → LF refining → vacuum degassing → casting → hot billet heating → rolling → stacking cooling; the steel plate produced by this invention has a thickness of 30~60mm, center segregation C1.0 grade or below, steel plate hardness of 30~36HRC, hardness fluctuation within the same plate ≤3HRC, and can meet the pre-hardening processing requirements without heat treatment. Practice shows that its production method is only suitable for thin plates less than 80mm thick. If this composition is used for steel plates thicker than 150mm, relying solely on post-rolling air cooling + stacking cooling pre-hardening results in insufficient hardenability and hardness, and without tempering or annealing to uniformly refine the grain structure, the hardness value is uneven, which does not meet the hardness uniformity requirements of 718 plastic mold steel.

[0006] Chinese patent application CN202210090893.8 discloses "a production process for pre-hardened high-polish plastic mold steel." The post-forging heat treatment consists of two steps: First, after multi-directional forging, the workpiece is air-cooled or wind-cooled to a surface temperature of 250–350°C. Then, it is placed in an annealing furnace at 280–320°C and held for 2–8 hours. Next, the temperature is raised to 640–660°C and held for 5–10 hours. Finally, the temperature is raised to 900–930°C and held for 2 hours. h, D is the maximum thickness of the workpiece in the furnace, and its unit is dm. After the heat treatment is completed, the workpiece is lifted off the trolley and air-cooled or air-cooled to 250-350℃. Second step: The cooled workpiece is put back into the annealing furnace at 280-320℃ and held for 5-10 hours. The temperature is raised to 740-760℃ and held for 4-5 hours. Then the workpiece is cooled with the furnace to 640-660℃ and held for 5D-7D hours. Then it is cooled with the furnace to below 250-300℃ and air-cooled. The heat treatment involves two heating and cooling processes, which are complex and occupy heat treatment furnace resources, resulting in high costs.

[0007] Chinese patent application CN202210607582.4 discloses "a pre-hardened mirror-finish plastic mold steel," wherein the mold steel is composed of the following elements in the indicated mass percentages: C: 0.20-0.25%, Si: 0.50-0.70%, Mn: 1.80-2.00%, S≤0.005%, P≤0.015%, Cr: 3.0-5.0%, Mo: 0.30-0.50%, Ni: 1.00-1.50%, Al≤0.01%, N≤0.005%, B: 0.0008-0.0015%, with the remainder being Fe and unavoidable impurities. Furthermore, the above chemical composition must satisfy the following formula:

[0008] B=0.043%+0.625S-0.128C-0.0031Cr-0.0036Mo,

[0009] Al = -0.034% -0.167C + 5.69S + 0.696Ti + 9.87N. The composition range is set, especially since the yield of Al is unstable during the RH vacuum furnace smelting process, which increases the difficulty of smelting. Secondly, the produced steel plates are less than 120mm thick, and the composition contains a high content of precious alloys such as Cr, Mo, and Ni, which increases the cost per ton of steel and reduces the competitiveness of the product.

[0010] Compared with the aforementioned patents, the pre-hardened 718 plastic mold steel plate has the disadvantages of high alloy cost and complicated heat treatment process leading to slow production pace, which increases the production cycle and cost of the steel plate and reduces the competitiveness of its products. Summary of the Invention

[0011] To address the shortcomings of existing technologies, and through experimental exploration and production application practice, the purpose of this invention is to provide a low-cost pre-hardened 718 plastic mold steel with a thickness of 150-280mm. Compared with existing technologies, the difference in producing pre-hardened 718 plastic mold steel of the same thickness lies in the fact that, during the smelting process, the alloy content of hardenable elements C, Mn, and Cr is adjusted according to the element ratio formula 3.44≤C+Mn+Cr≤3.84 for steel plate thickness, thereby minimizing alloy costs. At the same time, smelting according to this element ratio formula, and online cooling after rolling, can meet the cooling rate required for pre-hardening of the mold steel plate, while reducing the normalizing step and lowering heat treatment costs.

[0012] Another objective of this invention is a method for producing 150-280mm thick, low-cost pre-hardened 718 plastic mold steel.

[0013] To achieve the above objectives, the technical solution adopted by this invention is: a low-cost pre-hardened 718 plastic mold steel with a thickness of 150-280mm, comprising the following chemical composition (unit, %) by mass fraction: C: 0.40-0.44, Si: 0.20-0.40, Mn: 1.20-1.50, Cr: 1.50-1.90, Mo: 0.25-0.40, Ni: 0.90-1.0, P≤0.015, S≤0.002, Ti: 0.020-0.040, and satisfying 3.44≤C+Mn+Cr≤3.84, with the remainder being Fe and residual elements.

[0014] The mold steel has a center segregation of C1.0 grade or below, the steel plate is tempered to form a uniform structure and remove structural stress, the steel plate hardness is 32-34HRC, the hardness difference within the same plate is ≤2HRC, which meets the pre-hardening requirements, and the polishing score (12000#) of the processed forming surface is ≤90 and the surface roughness Ra is ≤0.20μm, which meets the requirements for the production of pre-hardened 718 plastic molds.

[0015] The production method of the aforementioned 150-280mm thick low-cost pre-hardened 718 plastic mold steel includes smelting → water-cooled mold casting → ingot heating → controlled rolling → online cooling → tempering, wherein...

[0016] (1) Smelting: Clean steel technology is used to smelt pure steel that meets the above composition requirements, with the gas content in the steel being [H]≤1.2ppm, [O]≤30ppm, and [N]≤50ppm;

[0017] (2) Water-cooled mold casting: Argon gas protection is used for casting. Argon gas is introduced into the ingot mold before casting. Water-cooled ingot mold with a thickness of more than 600mm is used for casting. The casting temperature is controlled at the liquidus line +20~30℃. The water flow rate is controlled at 420~560L / min to accelerate the cooling of the steel ingot. At the same time, the steel output rate is controlled at 5~6t / min during casting to achieve a uniform rise of the steel liquid level. After casting, the ingot is demolded and cooled slowly for 7~8 hours.

[0018] (3) Steel ingot heating: heating rate ≤50℃ / h, holding temperature 1290±20℃, holding time = steel ingot body thickness * (5~10)min / mm;

[0019] (4) Controlled rolling: Steel ingot billets with a compression ratio >3 are used for rolling. The initial rolling temperature is >950℃. Large reduction rolling is used to control the plate shape. Rolling is stopped when the thickness reaches 300-450mm. The plate is cooled by ACC water 1-2 times. When the temperature drops to 850-890℃, rolling is started again. At the same time, vertical roller extrusion is performed to achieve the purpose of delivering the plate with flush edges. Online cooling is performed after rolling is completed.

[0020] (5) Online cooling: During the ACC cooling stage, the cooling rate of the steel plate is controlled at 15-20℃ / min, and the final red temperature of the whole plate is controlled between 450 and 550℃; During the air cooling stage of the cooling bed, the cooling rate of the steel plate is controlled at 2℃ to 10℃ / min, and the highest red temperature of the steel plate surface is ≤550℃. Finally, the steel plate is taken offline for tempering when the temperature is below 250℃.

[0021] (6) Tempering process: The tempering temperature is controlled at 600~610℃, the holding time is equal to the thickness * 4min / mm, the temperature of the steel plate is controlled uniformly, and the steel plates are stacked and air-cooled to room temperature after being taken out of the furnace.

[0022] Furthermore, argon gas protection is used during water-cooled mold casting. Specifically, the argon gas pressure is >0.5 kPa, the argon gas flow rate is 150-220 L / min, argon gas is introduced into the mold for 10-15 minutes before casting, and the air content in the mold is <10%.

[0023] During rolling, the large reduction rolling is specifically 50-60mm per pass. To control the plate shape, descaling is performed once after each of the first three passes, and water is not applied in subsequent passes. The vertical roll edge extrusion is specifically controlled at 10-15mm per pass, requiring multiple edge extrusions.

[0024] The ACC cooling speed control is specifically based on the steel plate thickness, with a roller speed of 0.6 to 1.0 m / s, the upper and lower ACC nozzles fully open, the side spray closed, and the red glow time 1 minute for each ACC water cooling cycle. The number of water cooling cycles is 5 to 8 depending on the steel plate thickness.

[0025] The cooling rate control of the steel plates in the air-cooling stage of the cooling bed is specifically set up as follows: the cooling bed is 20m long and 50m wide, with 5 fans arranged in each direction of the steel plates. Each fan has an outlet diameter of 1.5m, an air pressure of ≥500pa, and an air volume of 100,000~200,000M3 / h. The fans are placed 1m apart. At one end, 5 fans are tilted upward at 45°, with the top of the air outlet flush with the lower surface of the steel plate. At the other end, 5 fans are tilted downward at 45°, with the bottom of the air outlet flush with the upper surface of the steel plate. The front of the 10 fans is 1-2m away from the beginning and end of the steel plate. Depending on the width of the steel plate, 3-5 steel plates are air-cooled at a time.

[0026] The rationale for specifying the main chemical components in this invention is as follows:

[0027] (1) C, Mn and Cr elements can improve the hardenability of steel and significantly improve the strength and hardness of steel. Through the screening and proportioning of alloying elements, and the adjustment of process parameters in steel ingot rolling, cooling and heat treatment, a large number of systematic experimental studies have been carried out. When the chemical composition of the mold steel meets 3.44≤C+Mn+Cr≤3.84, the hardness of the steel plate after the above-mentioned ACC water cooling and cooling bed air cooling is between 34-38HRC. After tempering, the structural stress is removed and the internal structure of the steel plate is uniform. The final hardness of the steel plate is 32-34HRC, and the hardness difference of the same plate is ≤2HRC. At the same time, the alloy content of hardenability elements C, Mn and Cr is adjusted according to the thickness of the steel plate to meet the requirement of 3.44≤C+Mn+Cr≤3.84, so as to maximize the reduction of alloy cost.

[0028] (2) Ni improves the low-temperature toughness of steel and can improve the uniformity of hardness of steel plate. Mo can significantly improve the hot strength and wear resistance of steel and improve the fatigue life of mold steel. Adding necessary Ni and Mo elements can make 718 plastic mold steel uniform in hardness, with excellent polishing and photo-etching properties, and improve the service life of mold steel materials.

[0029] The rationale for setting the production process parameters in this invention is as follows:

[0030] (1) Argon gas is used for protection during water-cooled casting. Argon gas is introduced into the ingot mold before casting to remove air impurities. During casting, the casting speed is uniform and the cooling water volume is controlled to improve the cooling intensity during solidification, resulting in a high-quality steel ingot with a dense interior. After testing, the low-magnification center segregation at the 1 / 2 position of the 800mm thick steel ingot is below grade C2.0, and there are no defects such as porosity or inclusions.

[0031] (2) During rolling, the large reduction rolling is specifically a single pass reduction of 50-60mm to ensure that the rolling force of the thick steel plate fully penetrates into the core, refines the grains, and increases the uniformity of the internal structure. The plate shape is controlled to require descaling once after each of the first three passes, and no water is sprayed in the subsequent passes to prevent black heads or warping during rolling. The surface unevenness of the steel plate is ensured to be ≤10mm. The single pass of vertical roll edge extrusion is controlled at 10-15mm. Multiple edge extrusions are required to prevent edge folding caused by a large single extrusion amount. At the same time, the edge-trimmed delivery improves the yield of steel plates and meets the needs of mold plate customers.

[0032] (3) During the ACC cooling stage, the cooling rate of the steel plate is controlled at 15-20℃ / min, and the final red temperature of the whole plate is controlled between 450 and 550℃; during the cooling bed air cooling stage, the cooling rate of the steel plate is controlled at 2℃ to 10℃ / min, and the highest red temperature of the steel plate surface is ≤550℃. Finally, when the temperature of the steel plate is below 250℃, it is taken off the line for tempering. By controlling the cooling rate of the steel plate in stages, it meets the requirements of variable speed cooling of high alloy steels such as mold steel, and prevents stress cracking of the steel plate while pre-hardening the steel plate.

[0033] This invention has the following advantages: by designing appropriate chemical composition and strictly controlling water-cooled casting, high-quality steel ingots with dense structure are produced. Through precise control, especially online cooling and subsequent tempering processes, pre-hardened 718 plastic mold steel plates with uniform cross-sectional hardness can be mass-produced at low cost. The steel plate hardness is 32-34 HRC, the hardness difference within the same plate is ≤2 HRC, the polishing fraction (12000#) of the machined surface is ≤90, and the surface roughness Ra is ≤0.20 μm, which meets the requirements for manufacturing pre-hardened high-quality 718 plastic molds. Detailed Implementation

[0034] 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.

[0035] A low-cost pre-hardened 718 plastic mold steel with a thickness of 280mm is produced, containing the following chemical composition by mass fraction (unit, %): C: 0.43, Si: 0.35, Mn: 1.45, Cr: 1.75, Mo: 0.35, P: 0.14, S: 0.02, Ni: 0.95, Ti: 0.035, wherein C+Mn+Cr=3.63.

[0036] (1) Smelting: Clean steel technology is used to smelt pure steel that meets the above composition requirements. The gas content in the molten steel is [H] = 0.84 ppm, [O] = 25 ppm, and [N] = 35 ppm.

[0037] (2) Water-cooled mold casting: Argon gas protection is used for casting. Argon gas is introduced into the ingot mold before casting. A water-cooled ingot mold with a thickness of 910mm is used for casting. The casting temperature is 1535℃. The water flow rate is controlled at 520L / min. The steel output rate is controlled at 6t / min during casting. The steel liquid level rises at a uniform speed. The mold is demolded and cooled slowly 8 hours after casting is completed.

[0038] (3) Steel ingot heating: heating rate ≤ 50℃ / h, holding temperature 1290℃, holding time = 910*10min / mm.

[0039] (4) Controlled rolling: Steel ingot billets with a compression ratio of 3.25 are used for rolling. The initial rolling temperature is 1050℃. The reduction in the first three passes is 55 / 55 / 50. Rolling is stopped when the thickness reaches 450mm. The billets are cooled twice by ACC water. When the temperature drops to 890℃, rolling is started again. At the same time, vertical roller extrusion is performed. The extrusion amount per pass is 10mm. The side of the steel plate is flat. Online cooling is performed after rolling.

[0040] (5) Online cooling: During the ACC cooling stage, the cooling rate of the steel plate is controlled at 15℃ / min, and the final red temperature of the whole plate is 550℃. During the cooling bed air cooling stage, a total of 3 sets of fans are turned on at the head and tail of the steel plate, the cooling rate of the steel plate is 5℃ / min, the highest red temperature of the steel plate surface is 530℃, and the steel plate is taken offline for tempering when the temperature is below 250℃.

[0041] (6) Tempering process: The tempering temperature is controlled at 610℃, the holding time is 280*4min / mm, the steel plate temperature is uniform, and the steel plates are stacked and air-cooled to room temperature after being taken out of the furnace.

[0042] (7) After the steel plate temperature drops to room temperature, external inspection, flaw detection and hardness test are carried out.

[0043] This implementation example uses 280mm thick 718 plastic mold steel. The actual material quality is as follows, meeting the parameter requirements of 718 plastic mold steel:

[0044]

Claims

1. A method for producing 150-280mm thick, low-cost pre-hardened 718 plastic mold steel, characterized in that: The mold steel contains the following chemical composition by mass percentage: C: 0.40~0.44, Si: 0.20~0.40, Mn: 1.20~1.50, Cr: 1.50~1.90, Mo: 0.25~0.40, Ni: 0.90-1.0, P≤0.015, S≤0.002, Ti: 0.020~0.040, and satisfies 3.44≤C+Mn+Cr≤3.84, with the remainder being Fe and residual elements; The production method of the aforementioned 150-280mm thick low-cost pre-hardened 718 plastic mold steel includes smelting → water-cooled mold casting → ingot heating → controlled rolling → online cooling → tempering, specifically as follows: 1) Smelting: Clean steel technology is used to smelt pure steel that meets the above composition requirements, with gas content in the molten steel of [H]≤1.2ppm, [O]≤30ppm, and [N]≤50ppm; 2) Water-cooled mold casting: Argon gas is used for protective casting. Argon gas is introduced into the ingot mold before casting. A water-cooled ingot mold with a thickness of more than 600mm is used for casting. The casting temperature is controlled at the liquidus line +20~30℃. The initial water flow rate is controlled at 420~560L / min to accelerate the cooling of the steel ingot. At the same time, the steel output rate is controlled at 5~6t / min during casting to achieve a uniform rise of the molten steel level. After casting, the ingot is demolded and cooled slowly for 7~8 hours. 3) Steel ingot heating: heating rate ≤ 50℃ / h, holding temperature 1290±20℃, holding time = steel ingot body thickness * (5~10) min / mm; 4) Controlled rolling: Use steel ingot billets with a compression ratio >3 for rolling, with an initial rolling temperature >950℃. Use high reduction rolling to control the plate shape. Stop rolling when the thickness reaches 300-450mm. Cool the billets with ACC water 1-2 times. When the temperature drops to 850-890℃, start rolling again. At the same time, perform edge extrusion with vertical rollers to achieve the purpose of delivering the product with flush edges. After rolling, perform online cooling. 5) Online cooling: During the ACC cooling stage, the cooling rate of the steel plate is controlled at 15-20℃ / min, and the final red temperature of the whole plate is controlled between 450 and 550℃; during the air cooling stage of the cooling bed, the cooling rate of the steel plate is controlled at 2℃ to 10℃ / min, and the highest red temperature of the steel plate surface is ≤550℃. Finally, the steel plate is taken offline for tempering when the temperature is below 250℃. (6) Tempering process: The tempering temperature is controlled at 600-610℃, the holding time is equal to the thickness * 4min / mm, the temperature of the steel plate is controlled uniformly, and the steel plates are stacked and air-cooled to room temperature after being taken out of the furnace.

2. The method for producing 150-280mm thick low-cost pre-hardened 718 plastic mold steel according to claim 1, characterized in that: The mold steel has a center segregation grade of C1.0 or lower, a plate hardness of 32-34 HRC, and a hardness difference of ≤2 HRC within the same plate.

3. The method for producing 150-280mm thick low-cost pre-hardened 718 plastic mold steel according to claim 1, characterized in that: Argon gas is used for protection during water-cooled mold casting. Specifically, the argon gas pressure is >0.5kPa, the argon gas flow rate is 150-220L / min, argon gas is introduced into the mold for 10-15min before casting, and the air content in the mold is <10%.

4. The method for producing 150-280mm thick low-cost pre-hardened 718 plastic mold steel according to claim 1, characterized in that: During rolling, the large reduction rolling is specifically a single-pass reduction of 50-60mm. To control the plate shape, the first three passes require descaling after each rolling pass, and subsequent passes do not require water spraying. The single-pass edge extrusion of the vertical rolls is controlled at 10-15mm, and multiple edge extrusions are required.

5. The method for producing 150-280mm thick low-cost pre-hardened 718 plastic mold steel according to claim 1, characterized in that: The ACC cooling speed control is specifically set at a roller speed of 0.6 to 1.0 m / s, with the upper and lower ACC nozzles fully open and the side spray closed. The red glow time is 1 minute after each ACC water cooling cycle, and the number of water cooling cycles is 5 to 8 depending on the thickness of the steel plate.

6. The method for producing 150-280mm thick low-cost pre-hardened 718 plastic mold steel according to claim 1, characterized in that: The cooling rate control of the steel plates during the air-cooling stage of the cooling bed is specifically implemented by setting up a cooling bed with a length of 20m and a width of 50m, with 5 fans arranged in each direction along the head and tail of the steel plates. Each fan has an outlet diameter of 1.5m, an air pressure ≥500Pa, and an air volume of 100,000~200,000 M³ / h. 3 / h, each fan is placed 1m apart, with 5 fans at one end tilted upwards at 45°, the top of the air outlet being flush with the bottom surface of the steel plate, and 5 fans at the other end tilted downwards at 45°, the bottom of the air outlet being flush with the top surface of the steel plate. The front of the 10 fans is 1-2m away from the beginning and end of the steel plate. Depending on the width of the steel plate, 3-5 steel plates are cooled at a time.

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