Large-thickness pre-hardened plastic alloy mold steel plate and production method thereof

Abstract

The invention discloses a large-thickness pre-hardened plastic alloy mold steel plate and a production method thereof, a blank adopted by the steel plate is LF + RH refined steel, and the steel plate comprises the following chemical components: 0.35 to 0.40 percent of C, 0.20 to 0.40 percent of Si, 1.30 to 1.60 percent of Mn, less than or equal to 0.018 percent of P, less than or equal to 0.005 percent of S, 0.012 to 0.040 percent of Als, 1.50 to 1.80 percent of Cr, 0.15 to 0.25 percent of Mo, 0.80 to 0.90 percent of Ni, 0.0015 to 0.0030 percent of Ca, 0.0010 to 0.0020 percent of B and the balance of Fe and inevitable impurities; according to the pre-hardened plastic alloy mold steel plate, the limitation of the rolling compression ratio is broken through, a large-thickness steel plate with the specification of 100-180 mm can be rolled through a continuous casting blank, the Ni-B strengthened alloy design is provided for improving the hardenability and refining crystal grains, steel rolling is conducted through an intermediate water-cooling three-stage controlled rolling mode, the rolling force of a super-thick plate is conducted to the center, the performance of the center is uniform, and the steel plate can be used for manufacturing the steel plate. And after tempering, the steel plate has a tempered martensite and tempered sorbite structure, so that the hardness range of the steel plate is 30-36 HRC, the core structure uniformity is good, the hardness fluctuation of the same plate is less than or equal to 3 HRC, the mold etching property and the heat stability are good, and the steel plate can be qualified for manufacturing large-size molds.

Description

Technical Field

[0001] The invention relates to the technical field of steel material manufacturing, in particular to a thick pre-hardened plastic alloy die steel plate and a production method thereof. Background Art

[0002] Moulds are important basic process equipment in the manufacturing industry. They are widely used due to their high production efficiency, good product quality, low material consumption and low production cost. They are indispensable process equipment in modern industry, especially in the fields of automobiles, aviation, instruments, medical equipment, household appliances, daily necessities, etc. Mould steel includes cold working, hot working and plastic moulds, etc. Plastic moulds are the most widely used, accounting for about 80% of the market. Plastic products facilitate people's daily life and are closely related to people's livelihood. Therefore, the development of plastic mould steel is particularly important.

[0003] Pre-hardened plastic mold steel is favored by the mold market because of its special delivery method. The hardness of the steel plate must meet the use requirements when it leaves the factory. It can be used directly without heat treatment after cutting. With the development of science and technology, mold products are gradually developing in the direction of ultra-thick large size and high polishability, and the demand for ultra-thick mold steel is also increasing. However, with the increase of mold thickness, due to the limitations of steel hardenability and compression ratio of pressed steel, the problems of uneven cross-section structure and large hardness fluctuation in the core of the steel are particularly prominent. Usually, more Ni, Mo, and V precious alloy elements need to be added for fine grain strengthening, which greatly increases the production cost. On the other hand, for ultra-thick mold steels above 100mm, in order to ensure the core performance and internal quality, continuous casting billets are rarely used for direct production. Large-section die casting or composite billet rolling is usually used, which has low production efficiency and is not conducive to cost control.

[0004] The Chinese patent with publication number CN102896466A discloses "a method for producing 150-400mm thick steel plates for plastic molds". A large thickness billet is formed by gas shielded welding, submerged arc welding and vacuum electron beam combined welding process or full vacuum electron beam welding. Then, after heating, rolling, slow cooling and heat treatment processes, the ultrasonic flaw detection qualified rate of the produced plastic mold steel plate reaches more than 98%, the hardness distribution is uniform, and it has excellent processability, wear resistance and polishing properties, which is suitable for the manufacture of various types of plastic molds. The ultra-thick steel plate described therein adds a casting billet composite process, which requires the composite billet to be welded and then rolled, with a long heating time and low production efficiency. In addition, the gas shielded welding process is very likely to produce surface oxidation, which affects the welding effect and the composite strength is difficult to guarantee. The Chinese invention patent with publication number CN117448681A proposes "a pre-hardened plastic mold steel and its manufacturing method". The chemical composition is designed with Cr~Mo~V, and no Ni or B elements are added. There is a problem of insufficient hardenability in the core, which is not conducive to the production of ultra-thick plates. The invention adopts die casting production, which requires a forging ratio of ≥6.0 times. Die casting has lower production efficiency and higher cost than continuous casting. The requirement of a forging ratio of ≥6.0 further limits its application in continuous casting billets.

[0005] The Chinese invention patent with application number 201910047103.6 discloses "a pre-hardened plastic mold steel and its preparation method", which contains C: 0.32% ~ 0.40%, Si: 0.20% ~ 0.35%, Mn: 1.30% ~ 1.60%, Cr: 1.80% ~ 2.10%, Ni: 0.90% ~ 1.15%, P ≤ 0.015%, S ≤ 0.005%, V: 0.10 ~ 0.35%, and the balance is Fe; its isotropy is ≥ 0.80. The pre-hardened plastic mold steel prepared by the method of the invention has more Ni and V precious alloys added and better hardenability, but too much precious alloy addition increases the alloy cost. The heat treatment of the invention adopts normalizing + tempering process. Although it refines the microstructure grains and improves local segregation, it adds a normalizing process, a long production cycle, is not conducive to cost control, and affects market competitiveness.

[0006] Therefore, the development of a low-cost, good comprehensive performance, large-thickness pre-hardened plastic alloy mold steel is of great significance to the development of the existing plastic alloy mold steel industry. Summary of the invention

[0007] The purpose of the present invention is to provide a thick pre-hardened plastic alloy mold steel plate and its production method in view of the common problems of high production cost, complicated process, long production cycle and difficulty in ensuring comprehensive performance in the current production of pre-hardened thick plastic mold steel plates. The plastic mold steel produced by the present invention is a uniform mixed structure of tempered martensite + tempered troostite, with a steel plate hardness performance of 30-36HRC, a cross-section hardness deviation of ≤3HRC, sufficient stress elimination, and a plastic mold steel plate that can be directly sawed and used.

[0008] The invention discloses a thick pre-hardened plastic alloy mold steel plate, characterized in that the steel plate comprises the following chemical components in mass fraction: C: 0.35-0.40%, Si: 0.20-0.40%, Mn: 1.30-1.60%, P≤0.018%, S≤0.005%, Als: 0.012-0.040%, Cr: 1.50-1.80%, Mo: 0.15-0.25%, Ni: 0.80-0.90%, Ca: 0.0015-0.0030%, B: 0.0010-0.0020%, and the rest is Fe and unavoidable impurities.

[0009] The finished steel plate produced by the present invention has a thickness specification of 100-180 mm, strong heat treatment hardenability, tempered martensite + tempered troostite structure after tempering, a steel plate hardness range of 30-36HRC, good core structure uniformity, hardness fluctuation of the same plate ≤3HRC, and good mold etching performance and thermal stability.

[0010] The present invention provides a method for producing a thick pre-hardened plastic alloy mold steel plate, comprising: converter smelting → LF refining → vacuum degassing → pouring → hot delivery and heating of the ingot → rolling → stack cooling → heat treatment, wherein: (1) Converter smelting: nickel and molybdenum alloys are blown into the furnace along with scrap steel, and carbon is pulled out of the steel. The final composition of the smelting is controlled at [C]: 0.08-0.012%, [P] ≤ 0.015%, and the steel tapping temperature is 1620-1660°C; (2) LF refining: Deoxidation of aluminum wire feeding, slag formation and heating. The heating is carried out in two stages. In the early stage, the high-end position of the arc is rapidly heated for 10 minutes until the molten steel temperature reaches 1570±10℃. According to the slag removal and temperature conditions, the secondary heating is carried out for 5-10 minutes until the molten steel temperature reaches 1600±10℃. The slag basicity is controlled at 5-8. White slag appears at temperatures above 1580℃. Calcium carbide is added at an interval of 1-2 minutes at a time of 20-40kg / time, and a total of 0.8-2.0kg / t of steel is added. The white slag is kept for ≥15 minutes. (3) Vacuum degassing: RH treatment ≤ 67Pa high vacuum conditions, pressure holding time ≥ 15min / furnace, feed 300-400m / furnace of calcium silicon wire after breaking the air, calcium silicon wire diameter φ12mm, soft argon blowing treatment after calcium treatment, argon flower diameter of 50-100mm on the steel liquid surface, soft blowing time ≥ 5min; (4) Casting: The casting section is 360×2200mm. The electromagnetic stirring and dynamic light pressure are turned on. The steel is drawn in the weak cooling and slow drawing mode. The electric stirring parameters are set to 420A, 6Hz, the drawing speed is 0.6-0.65m / min, the primary cooling water supply is 500 / 4300L / min, the secondary cooling is controlled by zones, the cooling water ratio is 0.4-0.5L / kg, the straightening temperature is controlled at 920-950℃, and the light pressure is 6.0mm in the fan-shaped section 7-8 to further reduce the degree of central segregation. After the billet is cut, it is promptly sent to the heating furnace for loading; (5) Hot delivery heating of the ingot: the ingot temperature is controlled at 400-650°C; the first heating temperature is 600-1000°C, the first heating time is 60-90 min, the second heating temperature is 1150±50°C, the third heating temperature is 1250±30°C, the soaking section temperature is 1230±20°C, the soaking time is controlled at 40 min, the total heating rate is (0.9-1.1)×Hmin / mm, the furnace discharge temperature is 1180-1220°C, where H is the ingot thickness in millimeters; (6) Rolling: The three-stage steel rolling process with intermediate water cooling is adopted. The single-pass reduction in the first stage is ≤35mm, the billet thickness is set to 260mm, and the billet is put into the intermediate water cooler for accelerated cooling. The cooling rate is 2-5℃ / s. After water is added, wait for 10 seconds for the temperature to return to the temperature for the second stage rolling. The single-pass reduction is controlled at 30-40mm. The intermediate billet thickness is the finished product thickness + 30mm. The second stage rolling temperature is ≤980℃. Multi-pass small reduction rolling is adopted. The single-pass reduction is ≤8mm. The final rolling temperature is 940±20℃, and the total billet compression ratio is ≥2.0. (7) Cooling: After the steel plate is rolled, ACC is used for watering, with a water ratio of 1:1.3, a roller speed of 0.9m / s, a cooling rate of 2-4℃ / s, and a red-return temperature of 650-700℃; after straightening, the steel plate is air-cooled to 200-400℃ on the cooling bed, and the steel plate is piled down for cooling for ≥48h to avoid tuyere. The upper and lower pads are required to be insulated with hot steel; (8) Heat treatment: high temperature tempering at 580±30℃, holding time 3.5×Hmin, H is the thickness of the ingot in millimeters.

[0011] The reasons for limiting the amounts of the main chemical components in the present invention are described in detail below: In the present invention, C is an important element in steel to ensure hardness and hardenability, which can effectively improve hardenability and is the most economical. If the C content is low, the uniformity of cross-section hardness cannot be guaranteed. If the C content is too high, segregation and cracking are likely to occur. Therefore, the present invention controls the C content in the steel within the range of 0.35% to 0.40%; Mo is a strong hardenability element, which can significantly improve the strength of the steel, while reducing the temper brittleness of the steel plate and preventing cracks from occurring; Mn and Cr are both toughening and hardenability improving elements, which can be well dissolved in the Fe matrix to improve the strength and hardness of the steel plate; Ni can strengthen ferrite and refine pearlite in steel, especially for the core structure of thick gauges. While maintaining good plasticity and toughness, uniform core performance, and comprehensively considering cost factors, the present invention controls the Ni content in the steel to be 0.80% to 0.90%; B exists in the steel in the form of interstitial atoms, and its solubility in the steel is very small. B can delay the formation of ferrite and pearlite, and has extremely strong hardenability. When the B content is too high, it will cause the grain boundary energy to be too low to form boron brittleness. The B content control range of the present invention is 0.0010 to 0.0020%; Ca is a strong deoxidizer modifier, which can spheroidize the form and quantity of non-metallic inclusions, improve the fluidity of molten steel, and at the same time refine the grains, reduce the number of hard large particle inclusions, and improve polishing and cutting processability.

[0012] The following is a detailed description of the process and reasons for setting process parameters of the mold steel plate of the present invention: (1) Converter smelting: Molybdenum and nickel alloys are highly stable and can be added to the converter with scrap steel without burning. At the same time, the temperature drop loss caused by the addition of alloys in the subsequent process is reduced, and high-temperature steelmaking is avoided, which is not conducive to P removal. Carbon pulling is used for steelmaking, mainly considering the carbon increase effect when high-carbon ferromanganese and medium-high-carbon ferrochrome are used when economically matching Mn and Cr. However, too low carbon in steelmaking will increase the oxidation of molten steel, increase aluminum consumption, and increase the difficulty of removing inclusions. The end point temperature is preferably 1620-1660℃, and the end point C is preferably 0.08%-0.12%. The end point P is mainly to prevent the phosphorus content from being abnormal due to the return of phosphorus in the slag during the steelmaking process.

[0013] (2) LF refining treatment: The aluminum wire is fed into the station to create a reducing atmosphere for deoxidation, which is beneficial to desulfurization; the temperature is raised in two stages, with high-grade and large current in the early stage and medium-grade temperature adjustment in the later stage. The main purpose is to increase the slag-forming speed in the early stage, slag-forming and slag-forming as early as possible, and improve the refining effect. The temperature is accurately compensated in the later stage. When the basicity of the refined slag is 5-8, a 7 (Al2O3) · 12 (Ca0) slag system can be formed, which has a better effect on adsorbing inclusions; after the white slag is removed, calcium carbide powder is added at intervals multiple times to maintain the slag system, enhance the removal of inclusions, and improve the purity of the steel.

[0014] (3) Vacuum degassing: Under high vacuum conditions of ≤67Pa, the pressure holding time is ≥15min / furnace, which can effectively ensure the RH degassing effect. Calcium treatment after breaking the air can reduce calcium oxidation and increase the yield. The soft blowing controls the size and time of argon flowers on the liquid surface mainly to avoid secondary oxidation of molten steel while ensuring the circulation effect of argon-calcium blowing treatment and avoiding local aggregation that is detrimental to steel quality.

[0015] (4) Pouring: During pouring, electromagnetic stirring and dynamic light pressure are enabled, and the steel is poured in a weak cooling and slow pulling mode. The pouring section is preferably 360×2200mm, the electric stirring parameters are set to 420A 6Hz, the pulling speed is 0.6-0.65m / min, the primary cooling water supply is 500 / 4300L / min, the secondary cooling is controlled by zones, the cooling water ratio is 0.4-0.5L / kg, the straightening temperature is controlled at 920-950℃, and the light pressure is completed at 6.0mm in the fan-shaped section 7-8, which can further reduce the degree of central segregation. The primary and secondary cooling water supply both adopt the weak cooling mode, and the straightening temperature should not be too low. The brittle pocket interval should be avoided to effectively control the occurrence of cracks.

[0016] (5) Hot delivery heating of ingot: In order to avoid thermal stress cracks caused by extreme cold and heat and reduce temperature drop losses, it is advisable to load the ingot into the furnace hot. The charging temperature should not be too low, otherwise the hot charging effect can be exerted. The heating gradient should be controlled during heating, and it should not be too fast. The first, second and third additions should be heated slowly according to a certain gradient. Because the temperature of the hot delivery ingot fluctuates greatly, the heating gradient in the first stage is large, and it crosses the second brittle zone of the steel, the first acceleration rate should not be too fast, and the heating time should be ensured to be 60 to 90 minutes. In order to fully dissolve alloy elements such as Cr, Mn, Mo, and Ni, the soaking section temperature is 1230±20℃, the soaking time is 40min, and the total heating rate is 9 to 11min / cm. Too long soaking time and slow heating rate have a greater impact on the production rhythm. The furnace discharge temperature of 1180 to 1220℃ is mainly for descaling temperature drop and hit rolling process considerations.

[0017] (6) Rolling: The three-stage steel rolling process with intermediate water cooling is adopted. It is different from the traditional two-stage controlled rolling. After rough rolling, an intermediate water cooling process is added to reduce the surface temperature of the billet. The high temperature and low strength of the core are conducive to the transmission of rolling force to break the core grains, which is beneficial for large thickness rolling. After cooling, the temperature is returned to normal for 10 seconds to avoid serious edge cracking caused by two-dimensional overcooling black edges at the corners of the billet. In addition, the alloy content of the mold steel is high and the crack sensitivity is strong. The single pass reduction should not be too large to avoid rolling cracks caused by excessive mechanical stress during rolling. The three-stage rolling temperature is low, the plasticity is reduced, and the total reduction should not be too large. Multi-pass small reduction mode rolling should be adopted, with the intermediate billet thickness equal to the finished product thickness + 30mm, and the single pass reduction of finishing rolling should be controlled within 8mm. In order to simulate normalizing rolling to control the grain size of the structure and ensure the hot straightening effect of the steel plate after rolling, the final rolling temperature should not be too low, and the final rolling temperature should be 940﹢20℃; in order to ensure the uniform structure of the core of the ingot and the appropriate refinement of the grain size, the total compression ratio of the ingot should be ≥2.0.

[0018] (7) Cooling and stacking: To further refine the grain size, ACC cooling is used immediately after the steel plate is rolled. Due to the large stress of alloy steel, the cooling rate should not be too fast and should be controlled at 2-4℃ / s and 650-700℃. Air cooling to 200-400℃ on the cooling bed and then stacking the steel can promote the transformation of bainite and martensite and avoid the insufficient transformation of high-temperature stacking structure affecting the subsequent heat treatment process. On the other hand, the magnetic attraction of the steel is poor, and the high temperature is not conducive to the operation of electromagnetic crane lifting steel.

[0019] (8) Heat treatment: In order to obtain the final hardness performance of 30-36HRC, uniform structure and sufficient stress release, high temperature tempering at 580±30℃ is adopted, and the holding time is preferably 3.5×Hmin (H is the plate thickness / mm).

[0020] Compared with the existing 2311 and other similar pre-hardened plastic mold steel plates and their production processes, the present invention has the following advantages: (1) The addition of Ni and B alloys further improves the hardenability. While ensuring high strength, it improves the high-temperature plasticity and toughness of the steel, refines the internal structure, and makes the cross-sectional hardness uniform, making it more suitable for making large-size and thick molds.

[0021] (2) The three-stage steel rolling process with intermediate water cooling can increase the temperature difference between the billet surface and the core, promote the penetration of rolling force into the core, refine the core grains, break the limit of rolling reduction ≥ 3.0 times, and achieve rolling of continuous casting billets with a reduction of 2.0 times, thereby expanding the thickness range of steel plates produced by continuous casting billets.

[0022] (3) The mold steel plate of the present invention has the advantages of wide thickness range of continuous casting billet direct rolling, uniform hardness, fine grains, good machinability, etc. It is suitable for manufacturing high-quality, large-size, and ultra-thick plastic molds and has good application prospects.

[0023] The present invention proposes a method for directly rolling 100-180mm ultra-thick pre-hardened plastic alloy mold steel plates using continuous casting billets and a manufacturing method, introduces how to control the steelmaking, continuous casting, rolling and heat treatment processes to obtain economical pre-hardened plastic mold steel with moderate hardness, uniform core cross-sectional performance and good polishing properties, breaking through the 3.0 times rolling compression ratio limit and providing a working idea for steel companies to solve the problem of uniformity of core performance of ultra-thick plates.

[0024] Pre-hardened alloy plastic mold steel first requires hardness to meet standard requirements. Therefore, the present invention can make the matrix structure hardness meet the standard through reasonable design of strong chemical elements such as C, Mn, Cr, Mo, and then add appropriate amounts of Ni and B alloys to further improve hardenability, promote uniformity of steel plate cross-section structure, and reduce core hardness fluctuations. The purpose of the present invention is to improve the purity of steel by reasonably designing the chemical composition of steel, controlling the cooling process of refining and continuous casting process, and promoting the crushing and refinement of the core structure of the steel plate through three-stage controlled rolling and intermediate water cooling process to improve the uniformity of cross-section performance. After stack cooling, high-temperature tempering is performed to fine-tune the performance to obtain a uniform mixed structure of tempered martensite + tempered troostite, with a steel plate hardness performance of 30 to 36HRC, a cross-section hardness deviation of ≤3HRC, sufficient stress elimination, and a plastic mold steel plate that can be directly sawed and used. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the metallographic structure of the plastic mold steel plate produced in Example 1 of the present invention. DETAILED DESCRIPTION

[0026] In order to better explain the technical solution of the present invention, the technical solution of the present invention is further described below in conjunction with specific embodiments. The following embodiments are merely exemplary of the technical solution of the present invention and do not limit the present invention in any form. The serial numbers of the following embodiments are only for description and do not represent the advantages and disadvantages of the embodiments.

[0027] Table 1 below is a list of chemical composition values ​​(wt, %) of the steel plates of various embodiments of the present invention and the comparative examples; Table 2 below is a list of main process parameter values ​​of steel plates of various embodiments of the present invention and comparative examples; Table 3 below is a list of main performance test and surface quality analysis results of the steel plates of various embodiments of the present invention and the comparative example steel plates.

[0028] A method for producing a thick pre-hardened plastic alloy mold steel plate according to each embodiment of the present invention comprises: converter smelting → LF refining → vacuum degassing → pouring → hot delivery and heating of the ingot → rolling → stack cooling → heat treatment, wherein: (1) Converter smelting: nickel and molybdenum alloys are blown into the furnace along with scrap steel, and carbon is pulled out of the steel. The final composition of the smelting is controlled at [C]: 0.08-0.012%, [P] ≤ 0.015%, and the steel tapping temperature is 1620-1660°C; (2) LF refining: Deoxidation of aluminum wire feeding, slag formation and heating. The heating is carried out in two stages. In the early stage, the high-end position of the arc is rapidly heated for 10 minutes until the molten steel temperature reaches 1570±10℃. According to the slag removal and temperature conditions, the secondary heating is carried out for 5-10 minutes until the molten steel temperature reaches 1600±10℃. The slag basicity is controlled at 5-8. White slag appears at temperatures above 1580℃. Calcium carbide is added at an interval of 1-2 minutes at a time of 20-40kg / time, and a total of 0.8-2.0kg / t of steel is added. The white slag is kept for ≥15 minutes. (3) Vacuum degassing: RH treatment ≤ 67Pa high vacuum conditions, pressure holding time ≥ 15min / furnace, feed 300-400m / furnace of calcium silicon wire after breaking the air, calcium silicon wire diameter φ12mm, soft argon blowing treatment after calcium treatment, argon flower diameter of 50-100mm on the steel liquid surface, soft blowing time ≥ 5min; (4) Casting: The casting section is 360×2200mm. The electromagnetic stirring and dynamic light pressure are turned on. The steel is drawn in the weak cooling and slow drawing mode. The electric stirring parameters are set to 420A, 6Hz, the drawing speed is 0.6-0.65m / min, the primary cooling water supply is 500 / 4300L / min, the secondary cooling is controlled by zones, the cooling water ratio is 0.4-0.5L / kg, the straightening temperature is controlled at 920-950℃, and the light pressure is 6.0mm in the fan-shaped section 7-8 to further reduce the degree of central segregation. After the billet is cut, it is promptly sent to the heating furnace for loading; (5) Hot delivery heating of the ingot: the ingot temperature is controlled at 400-650°C; the first heating temperature is 600-1000°C, the first heating time is 60-90 min, the second heating temperature is 1150±50°C, the third heating temperature is 1250±30°C, the soaking section temperature is 1230±20°C, the soaking time is controlled at 40 min, the total heating rate is (0.9-1.1)×Hmin / mm, the furnace discharge temperature is 1180-1220°C, where H is the ingot thickness in millimeters; (6) Rolling: The three-stage steel rolling process with intermediate water cooling is adopted. The single-pass reduction in the first stage is ≤35mm, the billet thickness is set to 260mm, and the billet is put into the intermediate water cooler for accelerated cooling. The cooling rate is 2-5℃ / s. After water is added, wait for 10 seconds for the temperature to return to the temperature for the second stage rolling. The single-pass reduction is controlled at 30-40mm. The intermediate billet thickness is the finished product thickness + 30mm. The second stage rolling temperature is ≤980℃. Multi-pass small reduction rolling is adopted. The single-pass reduction is ≤8mm. The final rolling temperature is 940±20℃, and the total billet compression ratio is ≥2.0. (7) Cooling: After the steel plate is rolled, ACC is used for watering, with a water ratio of 1:1.3, a roller speed of 0.9m / s, a cooling rate of 2-4℃ / s, and a red-return temperature of 650-700℃; after straightening, the steel plate is air-cooled to 200-400℃ on the cooling bed, and the steel plate is piled down for cooling for ≥48h to avoid tuyere. The upper and lower pads are required to be insulated with hot steel; (8) Heat treatment: high temperature tempering at 580±30℃, holding time 3.5×Hmin, H is the thickness of the ingot in millimeters.

[0029] Table 1 List of chemical composition values ​​of steel plates of various embodiments of the present invention and comparative examples (wt, %) Table 2 List of main process parameter values ​​of steel plates of various embodiments of the present invention and comparative examples

[0030] Table 3 List of performance test and surface quality analysis results of various embodiments of the present invention and comparative examples

[0031] It can be seen from Tables 1 to 3 that, in terms of the hardness performance and internal and external quality of the five embodiments, the six HRC values ​​measured according to the test standard of the hardness test, the surface and cross-section core hardness range 30 to 36HRC, the hardness difference of the same plate ≤3HRC, the fluctuation is small, the hardness target is hit, the GB / T2970 flaw detection standard II level flaw detection is qualified, and the internal and external quality control is good. On the other hand, in Comparative Example 1, the Mo content of 0.15% and the Ni content of 0.60% are low, the hardenability is insufficient, and the hardness strength is insufficient; no intermediate water cooling is performed during steel rolling, and two-stage rolling is adopted, which is not conducive to the transmission of rolling force to the center, affecting the cross-section grain and performance uniformity. After tempering, the surface hardness and core hardness are 27.8-32.4HRC, which cannot meet the 30-36HRC hardness requirement, and the cross-section hardness deviation is 4.6HRC, and the performance uniformity is poor. In Comparative Example 2, the dephosphorization in the converter is not ideal, the P content at the end of smelting is 0.021%, and the P content in the finished product is 0.026%. P is a grain boundary segregation element, which is prone to local stress concentration and internal cracks; during LF treatment, the lime slag is poor, the basicity of the refined slag is 3.5, which is too low, the slag liquid is too thin, the desulfurization effect is not good, and the S content of the finished product is 0.013%, which is too high; the furnace is not treated with calcium, the inclusions cannot be modified and spheroidized, and are easy to aggregate and grow; the continuous casting secondary cooling water ratio of 0.65 is too strong, and the straightening temperature of 880℃ is too low. For crack-sensitive steel with high alloy content, excessive cooling rate and low-temperature straightening are prone to internal cracks, which is basically consistent with the non-conformity of the flaw detection. The rolling specification of the comparative sample is 150mm, the tempering temperature is 580℃, and the furnace time is 435min. The tempering time is relatively short, which does not reach 3.5 times the plate thickness min / cm. The carbide precipitation is insufficient during tempering, which affects the hardness uniformity, which is basically consistent with the surface and core hardness difference of 4.3HRC>3HRC. In comparative example 3, the steel rolling is not water-cooled in the middle, which is not conducive to the transmission of the rolling force in the core to crush and refine the grains, and reduces the hardness uniformity of the core; the cooling bed is directly air-cooled after rolling, and ACC water cooling is not used. The temperature of the upper and lower cooling beds is high, and the grains are easy to grow during the organizational transformation, which is not conducive to organizational refinement and reduces the rolled hardness value; 610℃ tempering is used during tempering, the temperature is high, and the carbides are first precipitated and then gradually grow, which reduces the strength of the steel plate, and the hardness is 25-28HRC, which cannot meet the requirements of 30-36HRC.

[0032] Figure 1 This is the metallographic structure of the plastic mold steel plate produced in Example 1 of the present invention. It can be seen from the figure that the matrix structure of the steel is tempered martensite + tempered troostite, the grain size is uniform, and the uniformity is good, which provides conditions for hardness uniformity.

[0033] Pre-hardened plastic mold steel is mainly used for the production of mold cavities for air conditioners, refrigerators, and automotive plastic parts. The steel purity is required to be high. For ultra-thick, large-size, and excellent polishing mold steel, conventional CrMo alloy molds are difficult to meet due to the large fluctuation of the core section performance. It is necessary to add a large amount of high hardenability Ni elements or V elements for grain refinement to improve it, which increases the production cost. On the other hand, due to the limitations of the continuous casting billet size and the rolling compression ratio, in order to ensure uniform performance, large-thickness alloy mold steels are usually rolled using composite billets, which has low production efficiency and is not conducive to production cost control. The present invention introduces a pre-hardened plastic mold steel and a production method thereof, which adopts a Ni-B alloy combination to improve hardenability, adopts an intermediate water-cooled three-stage controlled rolling mode to refine the core grain size, and directly tempers to adjust the hardness, so as to achieve a surface and cross-section core hardness range of 30 to 36 HRC, and a hardness deviation of ≤3HRC on the same plate. The steel of the invention has the characteristics of good comprehensive mechanical properties, strong hardenability, uniform cross-sectional hardness distribution after quenching and tempering, and excellent polishability, and is suitable for the production of large-size and ultra-thick molds with polishability requirements.

[0034] The above embodiments are only specific examples for explaining the present invention and do not limit the present invention in any form. Any non-substantial changes made by anyone based on the above contents and forms without departing from the protection scope of the claims of the present invention shall be deemed to fall within the protection scope of the claims of the present invention. The present invention is not limited to the above specific implementation examples.

Claims

1. A thick pre-hardened plastic alloy mold steel plate, characterized in that The steel plate contains the following chemical components in mass fraction: C: 0.35-0.40%, Si: 0.20-0.40%, Mn: 1.30-1.60%, P≤0.018%, S≤0.005%, Als: 0.012-0.040%, Cr: 1.50-1.80%, Mo: 0.15-0.25%, Ni: 0.80-0.90%, Ca: 0.0015-0.0030%, B: 0.0010-0.0020%, and the rest is Fe and unavoidable impurities.

2. The thick pre-hardened plastic alloy mold steel plate according to claim 1, characterized in that: The finished steel plate produced has a thickness specification of 100-180mm, strong heat treatment hardenability, and after tempering, it has tempered martensite + tempered troostite structure. The hardness range of the steel plate is 30-36HRC, the core structure is uniform, the hardness fluctuation of the same plate is ≤3HRC, and the mold etching and thermal stability are good.

3. A method for producing a thick pre-hardened plastic alloy mold steel plate as claimed in claim 1 or 2, comprising: Converter smelting → LF refining → vacuum degassing → pouring → hot heating of ingot → rolling → stack cooling → heat treatment, characterized by: (1) Pouring: The pouring section is 360×2200mm. The electromagnetic stirring and dynamic soft pressure are turned on. The steel is drawn in the weak cooling and slow drawing mode. The electric stirring parameters are set to 420A, 6Hz, the drawing speed is 0.6-0.65m / min, the primary cooling water supply is 500 / 4300L / min, the secondary cooling is controlled by zones, the cooling water ratio is 0.4-0.5L / kg, the straightening temperature is controlled at 920-950℃, and the soft pressure is 6.0mm in the fan-shaped section 7-8 to further reduce the degree of central segregation. After the billet is cut, it is promptly sent to the heating furnace for hot loading; (2) Hot delivery heating of the ingot: the ingot temperature is controlled at 400-650°C; the first heating temperature is 600-1000°C, the first heating time is 60-90 min, the second heating temperature is 1150±50°C, the third heating temperature is 1250±30°C, the soaking section temperature is 1230±20°C, the soaking time is controlled at 40 min, the total heating rate is (0.9-1.1)×Hmin / mm, the furnace discharge temperature is 1180-1220°C, where H is the ingot thickness in millimeters; (3) Rolling: The three-stage steel rolling process with intermediate water cooling is adopted. The single-pass reduction in the first stage is ≤35mm, the billet thickness is set to 260mm, and the billet is put into the intermediate water cooler for accelerated cooling. The cooling rate is 2-5℃ / s. After water is added, wait for 10 seconds for the temperature to return to the second stage for rolling. The single-pass reduction is controlled at 30-40mm. The intermediate billet thickness is the finished product thickness + 30mm. The second stage rolling temperature is ≤980℃. Multi-pass small reduction rolling is adopted. The single-pass reduction is ≤8mm. The final rolling temperature is 940±20℃, and the total billet compression ratio is ≥2.

0. (4) Cooling: After the steel plate is rolled, ACC is used for watering, with a water ratio of 1:1.3, a roller speed of 0.9m / s, a cooling rate of 2-4℃ / s, and a red-return temperature of 650-700℃; after straightening, the steel plate is air-cooled to 200-400℃ on the cooling bed, and the steel plate is piled down for cooling for ≥48h to avoid tuyere. The upper and lower pads are required for heat insulation; (5) Heat treatment: high temperature tempering at 580±30℃, holding time 3.5×Hmin, H is the thickness of the ingot in millimeters.

4. The method for producing a thick pre-hardened plastic alloy mold steel plate according to claim 3, characterized in that: During the converter smelting process, nickel and molybdenum alloys are blown into the furnace along with scrap steel, carbon is pulled out of the steel, and the composition at the smelting end point is controlled to be [C]: 0.08-0.012%, [P] ≤ 0.015%, and the steel tapping temperature is 1620-1660°C.

5. The method for producing a thick pre-hardened plastic alloy mold steel plate according to claim 3, characterized in that: During the LF refining process: aluminum wire is fed into the station for deoxidation, slag is formed and heated, and the heating is carried out in two stages. In the early stage, the high-end submerged arc position is rapidly heated for 10 minutes until the molten steel temperature reaches 1570±10°C, and the mid-range compensation secondary heating is performed for 5 to 10 minutes according to the slag removal and temperature conditions until the molten steel temperature reaches 1600±10°C. The slag basicity is controlled at 5 to 8, and white slag appears at temperatures above 1580°C. 20 to 40 kg of calcium carbide is added every 1 to 2 minutes during the process, and a total of 0.8 to 2.0 kg of calcium carbide is added per t of steel, and the white slag is kept for ≥15 minutes.

6. The method for producing a thick pre-hardened plastic alloy mold steel plate according to claim 3, characterized in that: During the vacuum degassing process: under high vacuum conditions of RH treatment ≤ 67Pa, the pressure holding time is ≥ 15min / furnace, 300-400m / furnace of calcium silicon wire is fed after breaking the air, the diameter of the calcium silicon wire is φ12mm, soft argon blowing treatment is performed after calcium treatment, the diameter of the argon flower on the steel liquid surface is 50-100mm, and the soft blowing time is ≥ 5min.

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