An economical high-mirror P40 plastic mold steel plate and its manufacturing method

Abstract

The present invention provides an economical high-mirror P40 plastic mold steel plate and its manufacturing method. The composition is C 0.16 - 0.35%, Si 0.5 - 0.6%, Mn 1.3 - 1.45%, P ≤ 0.015%, S ≤ 0.008%, Alt 0.015 - 0.035%, Cr 1.6 - 1.8%, Mo 0.3 - 0.4%, Ni 0.55 - 0.65%, V 0.15 - 0.3%, RE 0.0015 - 0.003%, N 0.003 - 0.005%, and the rest is Fe and residual elements. Rare earth is added to the composition, and the hot charging and stepwise heat preservation heating process, high-temperature normalizing rolling + water cooling after rolling + residual heat tempering in a slow cooling pit process are adopted. The surface hardness of the steel plate reaches 40 - 43 HRC, the surface finish is ≤ 0.05 μm, the whole cross-section is bainite, the structure grains are uniform and fine, and the hardness difference of the whole cross-section is ≤ 3 HRC.

Description

Technical Field

[0001] The present invention belongs to the field of plastic mold steels, and particularly relates to an economical high-mirror P40 plastic mold steel plate and a manufacturing method thereof. Background Art

[0002] With the improvement of the requirements for the appearance quality of plastic products, the surface finish of mold steels needs to be improved to Ra less than 0.050 μm; China is a major manufacturing country with a large demand for plastic products, which promotes the increasing market demand for high-mirror plastic mold steels. High-mirror plastic mold steels are different from ordinary plastic mold steels. To ensure excellent surface brightness performance, the hardness of the steel plate needs to be evenly maintained in the range of 39-44 HRC across the entire section, and at the same time, the grain size of the structure must be fine and uniform to ensure that the surface finish of the processed mold meets the standard.

[0003] At present, the high-mirror plastic mold steels commonly used in China are NAK80 developed by Japanese steel mills and its improved variety 10Ni3MnCuAl. This variety adopts quenching and tempering treatment. By adding a relatively high amount of Ni and Cu, on the basis of improving the hardenability of the steel plate, the precipitation strengthening effect of Al3Ni and Cu-rich phases is used to refine the grain size of the steel plate, obtaining a bainite structure and uniformly increasing the overall hardness of the steel plate.

[0004] At present, high mirror surface plastic mold steel plates are produced from 250mm and 300mm continuous casting billets. The chemical composition is as follows: C: 0.06 - 0.16%, Si: 0.05 - 0.2%, Mn: 1.4 - 1.7%, P ≤ 0.02%, S ≤ 0.01%, Alt: 0.7 - 1.3%, Mo: 0.2 - 0.5%, Ni: 2.5 - 3.5%, Cu: 0.8 - 1.25%, and the rest are Fe and residual elements. During production, in the smelting process, converter smelting - LF furnace refining - RH furnace vacuum degassing are carried out. Among them, in the LF furnace, the slag is electrified for 6 - 10 minutes, the temperature is measured and samples are taken, deoxidation is carried out to make white slag (FeO + MnO in the slag ≤ 1.0%), and the holding time of the white slag is ≥ 10 minutes; in the RH furnace, the vacuum treatment time is ≥ 25min, the ultimate vacuum degree is ≤ 20Pa, the holding time of the ultimate vacuum is ≥ 20min, and the pure degassing time is ≥ 15min to ensure that [H] in the molten steel is ≤ 2ppm when leaving the station. In the heating process, anti - oxidation coatings are applied to the upper and lower surfaces before entering the furnace, the furnace inlet temperature is ≤ 60°C, and then it enters the walking beam heating furnace for heating. The heating temperature in the pre - heating section of the billet is ≤ 800°C, the temperature in the first heating section is 800°C - 1150°C, the temperature in the second heating section is 1200°C - 1255°C, and the soaking section temperature is 1200 - 1250°C. To ensure the full solution of alloying elements, the total heating time for 250 and 300mm cast billets is ≥ 700 minutes. To prevent "copper cracking", the billet stays in the pre - heating section for ≥ 120 minutes and in the soaking section for ≥ 200 minutes. In the rolling process, two - stage controlled rolling is adopted (in the first stage, high - temperature recrystallization rolling is used, the starting rolling temperature is ≥ 1050°C, the finishing rolling temperature is > 950°C, and the cumulative reduction ratio is 50% - 60%; in the second stage, non - recrystallization rolling is used, the starting rolling temperature is ≤ 880°C, the finishing rolling temperature is 800 - 840°C, and the cumulative reduction ratio is 30% - 40%, and the reduction per pass is ≤ 20mm), and it is air - cooled after rolling. The steel plate is stacked and air - cooled for 24 - 48 hours. In the heat treatment process, quenching + tempering is adopted. The quenching holding temperature is: 880 - 900°C, the furnace holding time is 3×H minutes (H represents the thickness of the steel plate, in mm), the temperature of the quenched steel plate is 20 - 40°C, and the tempering process: the tempering temperature is: 550 - 570°C, and the furnace holding time is: 1.5×H + 150 minutes (H represents the thickness of the steel plate, in mm).

[0005] Although the above - mentioned varieties can obtain a uniform and fine structure, good mirror surface performance and a long mold service life, they need to add a high content of precious metal Ni and carry out quenching and tempering treatment for solution strengthening. The production cost is high, the process flow is complex, and the manufacturing cycle is long. They are not suitable for some products with high surface finish requirements but fast product replacement and short mold service life for products.

[0006] Therefore, it is very necessary to develop an economical high - mirror - surface P40 plastic mold steel plate to reduce the manufacturing cost and improve the production efficiency. Summary of the Invention

[0007] The object of the present invention is to provide an economical high-mirror P40 plastic mold steel plate and its manufacturing method. Through composition design and addition of rare earth, the obtained structure has uniform and fine crystal grains. Combined with the production process design of the present application, using the hot charging and hot delivery process with a car-bottom heating furnace + step-by-step heat preservation heating process, and the high-temperature normalizing rolling + post-rolling water cooling + residual heat tempering process in a slow cooling pit, the obtained steel plate not only has a surface hardness of 40 - 43 HRC and a surface finish of ≤ 0.05 μm, but also has bainite throughout the cross-section, with uniform and fine crystal grains and a cross-sectional hardness difference of ≤ 3 HRC.

[0008] The specific technical solution of the present invention is as follows:

[0009] An economical high-mirror P40 plastic mold steel plate, including the following components by mass percentage:

[0010] C: 0.16 - 0.35%, Si: 0.5 - 0.6%, Mn: 1.3 - 1.45%, P ≤ 0.015%, S ≤ 0.008%, Alt: 0.015 - 0.035%, Cr: 1.6 - 1.8%, Mo: 0.3 - 0.4%, Ni: 0.55 - 0.65%, V: 0.15 - 0.3%, RE: 0.0015 - 0.003%, N: 0.003 - 0.005%, and the rest are Fe and residual elements.

[0011] Preferably, the RE includes yttrium Y: 0.0009 - 0.002%, lanthanum La: 0.0003 - 0.0005%, cerium Ce: 0.0003 - 0.0005%; the total amount of RE rare earth elements is controlled at 0.0015 - 0.003%;

[0012] The thickness of the economical high-mirror P40 plastic mold steel plate is 6 - 70 mm.

[0013] The structure of the economical high-mirror P40 plastic mold steel plate is bainite.

[0014] The cross-sectional hardness of the economical high-mirror P40 plastic mold steel plate is 40 - 43 HRC, the cross-sectional hardness difference is ≤ 3 HRC, the grain size is 9 - 10 grades, and the surface finish Ra ≤ 0.050 μm.

[0015] Preferably, the hardness difference between the surface and the core of the economical high-mirror P40 plastic mold steel plate is ≤

[0016] 1.4 HRC.

[0017] A manufacturing method of an economical high-mirror P40 plastic mold steel plate provided by the present invention includes smelting, heating, rolling, post-rolling cooling, and stacking and cooling.

[0018] For the smelting process, rare earth elements are added in two processes: RH furnace vacuum degassing smelting and tundish continuous casting.

[0019] The specific smelting process is as follows: converter smelting - LF furnace refining - RH furnace vacuum degassing - continuous casting.

[0020] For the LF furnace refining, the LF furnace is electrified to slag for 6 - 10 minutes for temperature measurement and sampling, deoxidized to form a white slag, with FeO + MnO ≤ 1.0% in the slag, and the white slag holding time ≥ 10 minutes.

[0021] For the RH furnace vacuum degassing, control the RH furnace vacuum treatment time ≥ 15 minutes, the vacuum degree ≤ 133 Pa, the ultimate vacuum holding time ≥ 10 minutes. After breaking the vacuum, feed seamless calcium wire (0.50 - 0.70) kg / ton of steel within 2 minutes, with a wire feeding speed of (1.6 - 1.8) m / s. After feeding the calcium wire, feed yttrium - based heavy rare earth alloy wire (0.5 - 0.6) kg / ton of steel, with a wire feeding speed of (1.3 - 1.5) m / s. After wire feeding, perform soft blowing, with a soft blowing time ≥ 10 minutes, and ensure that [H] ≤ 2 ppm when leaving the station. The yttrium - based heavy rare earth alloy wire includes the following mass percentage components:

[0022] REO(RE) 31.05(25.15)%, Si 39.39%, Ca 1.42%, and Fe 28.14%.

[0023] For the continuous casting, full - protection casting is adopted, with the tundish superheat degree of 10 - 30 °C. During the casting process, add 300 - mesh RE rare earth fine powder into the tundish through the tundish powder injection pipe. The rare earth powder is pushed by argon gas, with an argon gas delivery flow rate of 4 - 8 m 3 / h, and the addition amount of RE rare earth fine powder is (0.2 - 0.3) kg / ton of steel. The RE rare earth fine powder refers to a mixture of La and Ce, with the mass proportion of La and Ce more than 95%, and the balance being trace elements such as Fe and C.

[0024] By adding rare earth elements in two processes of RH vacuum treatment and tundish casting, the present invention improves the rare earth recovery rate. Utilizing the property of rare earth elements to purify molten steel, modify the inclusions of (Mn,Ca)S into RE2O2S and RE x S y inclusions with smaller size and more quantity, modify large - size harmful inclusions into fine and dispersed beneficial inclusions, provide more nucleation sites for subsequent rolling, promote recrystallization nucleation, refine the grain of the steel plate, and at the same time improve the center segregation of the steel plate to make the grain of the whole cross - section tissue uniform and fine.

[0025] The present invention significantly reduces the content of precious metal Ni and eliminates the addition of Cu element. By increasing the C content and adding Cr element, the hardenability and hardenability of the steel plate during water cooling are improved. Combined with the uniformly fine grain structure obtained by adding rare earth elements to the steel plate, it replaces the precipitation strengthening of Al3Ni and Cu-rich phases in the original composition to refine the grain of the steel plate and improve the hardness of the steel plate.

[0026] Due to the elimination of Cu element and the significant reduction of the content of precious metal Ni in the P40 die steel produced by the present invention, the Ni content limit is below 0.65%. The alloy cost is lower. During the heating process, not only the occurrence of "copper cracking" is avoided, but also the surface quality of Ni-containing steel can be improved by shortening the holding time above 1200°C, avoiding the situation where the billet cannot be hot-charged into the furnace due to high Ni content (Ni content ≥ 0.7%) and the billet needs to be cooled and coated with anti-oxidation coating. The heating and temperature-rising time of the billet is shortened, preventing the grain growth due to too long heating and holding time, and refining the original grain of the steel plate.

[0027] The heating adopts hot charging and step-by-step holding process;

[0028] For the hot charging, the maximum hot charging temperature of the steel billet is 700°C, and the furnace inlet temperature is 500 - 600°C;

[0029] The step-by-step holding process: after entering the furnace, the steel billet is heated at 200°C ± 20°C / h to 800°C ± 10°C and held for 0.2×H minutes. After the holding is completed, it is heated at 200 ± 20°C / h to 1000°C ± 10°C and held for 0.2×H. After the holding is completed, it is heated at 200 ± 20°C to 1100°C ± 10°C and held for 0.2×H. After the holding is completed, it is heated at 200 ± 20°C to 1200°C ± 10°C and held for 0.4×H. After the holding is completed, it is heated at 200 ± 20°C to 1230°C ± 10°C and held for 0.4×H. After the holding is completed, it is taken out of the furnace for rolling; where H represents the thickness of the steel billet, in mm.

[0030] The present invention adopts the hot charging + step-by-step holding heating process with a car-bottom heating furnace, shortens the holding time of the slab at temperatures above 1200°C, reduces the melting amount of the eutectic low-melting substance of FeO - SiO2 with a melting point of 1208°C, thereby effectively reducing the proportion of high-viscosity scale generated by the combination of NiS and FeO - SiO2 melt, improving the surface quality of the steel plate. At the same time, the overall temperature of the billet is uniform when it enters the furnace, which can significantly shorten the heating time, not only reducing the heating energy consumption and improving the production efficiency, but also reducing the risk of grain growth, providing a finer grain size for subsequent rolling and heat treatment.

[0031] The rolling process adopts a high-temperature normalizing rolling process, specifically: the starting rolling temperature is ≥1080°C, the reduction ratios in the 1st, 2nd, 3rd, and 4th passes are 15-20%, during the rolling processes of the 2nd and 4th passes, the upper and lower surfaces of the steel plate are cooled by the high-pressure water devices in front of and behind the rolling mill, the high-pressure water pressure is ≥15 MPa, and the high-pressure water flow rate is ≥400 m 3 / h, ensuring that the temperature difference between the surface and the core reaches 100-150°C, enabling a large reduction amount to be better transmitted to the core of the steel plate, promoting the recrystallization of the core tissue of the steel plate and refining the grains. The finishing rolling temperature is 900-930°C, and water cooling is carried out after rolling.

[0032] For the post-rolling cooling, ultra-fast cooling is adopted, specifically: the starting cooling temperature is ≥880°C, the speed of the cooling roller table is 0.3-0.5 m / s, the water volume in the high-pressure section is 5000-6000 m 3 / h, the water ratio is 1.2-1.6, the water volume in the low-pressure section

[0033] is 3000-4000 m 3 / h, the water ratio is 1.2-1.6, the recrystallization temperature is 300-340°C, and it is immediately lifted into the slow-cooling pit after hot straightening, and heat treatment is carried out using the waste heat of the steel plate.

[0034] For the stacking cooling, a slow-cooling pit waste heat tempering process is adopted, specifically: before the steel plate is put into the pit, first lift 4-6 pads with a temperature of 400-450°C and a thickness of 50-80 mm to cover the ground of the slow-cooling pit, then lift the steel plate to be tempered into the slow-cooling pit, cover the pit cover and heat the steel plate, raise the temperature of the steel plate to 550-610°C, and keep it warm for 3×H-4×H minutes. Since the steel plate has a waste heat of 250°C-300°C before being lifted in, compared with the off-line tempering process, the heating time can be reduced by 1.5×H, improving the production efficiency of the steel plate and reducing energy consumption.

[0035] During stacking cooling, the holding temperatures and times for steel plates of different thicknesses are as follows:

[0036] For steel plates with a thickness of 6 mm ≤ H ≤ 16 mm, the holding temperature is 600 ± 10°C, and the holding time is 4×H minutes, where H represents the thickness of the steel plate in mm;

[0037] For steel plates with a thickness of 16 mm < H ≤ 40 mm, the holding temperature is 575 ± 10°C, and the holding time is 3×H minutes, where H represents the thickness of the steel plate in mm;

[0038] For steel plates with a thickness of 40 mm < H ≤ 70 mm, the holding temperature is 560 ± 10°C, and the holding time is 3×H minutes, where H represents the thickness of the steel plate in mm;

[0039] When calculating the above holding time, the value of the thickness H in mm can be directly substituted into the formula for calculation.

[0040] The present invention adopts the process of hot normalizing rolling + water cooling after rolling + residual heat tempering in a slow cooling pit. Since the step-by-step preheating shortens the heating time, the grains are refined. At the same time, the fine and dispersed rare earth modified inclusions in the steel provide nucleation sites for recrystallization rolling, promoting the tissue recrystallization during hot normalizing rolling. During rolling, a high-pressure water descaling system before and after the rolling mill is used to spray high-pressure water on the upper and lower surfaces of the steel plate in the 2nd and 4th rolling passes, so that the temperature difference between the surface and the core of the steel plate is 100 - 150 °C. The large reduction ratio of more than 15% in a single pass is used to increase the recrystallization driving force of the steel plate core, making the grains of the whole cross-section tissue tend to be fine and uniform. After rolling, an on-line ultra-fast cooling device is used for on-line quenching and cooling to 300 - 340 °C to obtain a bainite structure throughout the cross-section, with the overall hardness difference ≤ 3 HRC. After on-line straightening, it is hoisted to the slow cooling pit for residual heat tempering process, reducing the reheating process of off-line quenching and tempering, lowering the production cost, shortening the production cycle, and improving the production efficiency.

[0041] Compared with the prior art, through composition design and production process optimization, the economical high mirror surface P40 plastic mold steel plate produced by the present invention not only has a surface hardness of 40 - 43 HRC and a surface finish ≤ 0.05 μm, but also has bainite throughout the cross-section, with uniform and fine tissue grains and an overall cross-section hardness difference ≤ 3 HRC. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 Microstructure at the surface of the steel in Example 1;

[0043] Figure 2 Microstructure at 1 / 2 thickness of the steel in Example 1;

[0044] Figure 3 Microstructure at the surface of the steel in Example 2;

[0045] Figure 4 Microstructure at 1 / 2 thickness of the steel in Example 2.

[0046] Figure 5 Microstructure at the surface of the steel in Example 3;

[0047] Figure 6 Microstructure at 1 / 2 thickness of the steel in Example 3.

[0048] Figure 7 Microstructure at the surface of the steel in Comparative Example 1;

[0049] Figure 8 Microstructure at 1 / 2 thickness of the steel in Comparative Example 1;

[0050] Figure 9 Microstructure at the surface of the steel in Comparative Example 2;

[0051] Figure 10 Microstructure at 1 / 2 thickness of the steel in Comparative Example 2.

[0052] Figure 11 The microstructure of the surface of the steel for Comparative Example 3;

[0053] Figure 12 The microstructure at the 1 / 2 thickness of the steel for Comparative Example 3. Specific Embodiments

[0054] An economical high-mirror P40 plastic mold steel plate provided by the present invention comprises the following components by mass percentage:

[0055] C: 0.16 - 0.35%, Si: 0.5 - 0.6%, Mn: 1.3 - 1.45%, P ≤ 0.015%, S ≤ 0.008%, Alt: 0.015 - 0.035%, Cr: 1.6 - 1.8%, Mo: 0.3 - 0.4%, Ni: 0.55 - 0.65%, V: 0.15 - 0.3%, RE: 0.0015 - 0.003% (where Y: 0.0009 - 0.002%,

[0056] La: 0.0003 - 0.0005%, Ce: 0.0003 - 0.0005%), N: 0.003 - 0.005%, and the balance is Fe and residual elements.

[0057] The thickness of the economical high-mirror P40 plastic mold steel plate is 6 - 70 mm.

[0058] The microstructure of the economical high-mirror P40 plastic mold steel plate is bainite.

[0059] The full-section hardness of the economical high-mirror P40 plastic mold steel plate is 40 - 43 HRC, the full-section hardness difference ≤ 3 HRC, the grain size is 9 - 10 grades, and the surface finish Ra ≤ 0.050 μm.

[0060] A manufacturing method of the economical high-mirror P40 plastic mold steel plate provided by the present invention comprises smelting, heating, rolling, post-rolling cooling and stacking cooling processes; specifically as follows:

[0061] The smelting process is as follows: converter smelting - LF furnace refining - RH furnace vacuum degassing - continuous casting. During LF furnace operation, the electric slag melting lasts for 6 - 10 minutes for temperature measurement and sampling. Then, deoxidation is carried out to produce white slag (FeO + MnO in the slag ≤ 1.0%), and the white slag holding time is ≥ 10 minutes. The vacuum treatment time in the RH furnace is ≥ 15 minutes, the vacuum degree is ≤ 133 Pa, the ultimate vacuum holding time is ≥ 10 minutes. After breaking the vacuum, seamless calcium wire is fed at a rate of (0.50 - 0.70) kg / ton of steel within 2 minutes, with a feeding speed of (1.6 - 1.8) m / s. After feeding the calcium wire, yttrium-based heavy rare earth alloy wire is fed at a rate of (0.5 - 0.6) kg / ton of steel, with a feeding speed of (1.3 - 1.5) m / s. The yttrium-based heavy rare earth alloy wire contains the following mass percentage components: REO(RE) 31.05(25.15)%, Si 39.39%, Ca 1.42%, and Fe 28.14%. After wire feeding, soft blowing is carried out for ≥ 10 minutes to ensure that [H] ≤ 2 ppm when leaving the station. During continuous casting, full protection casting is adopted, the superheat degree of the tundish is 10 - 30 °C. During the casting process, 300-mesh RE rare earth fine powder is added to the tundish through the tundish powder injection pipe, and the rare earth powder is pushed by argon gas, with an argon gas delivery flow rate of 4 - 8 m 3 / h, and the addition amount of RE rare earth fine powder is (0.2 - 0.3) kg / ton of steel. The RE rare earth fine powder refers to a mixture of La and Ce, with the mass ratio of La and Ce accounting for more than 95%, and the balance being trace elements such as Fe and C.

[0062] The heating process: hot charging and step-by-step heat preservation processes are adopted. The maximum hot charging temperature of the billet is 700 °C, and the furnace inlet temperature is 500 - 600 °C. The step-by-step heat preservation heating process: after entering the furnace, the billet is heated at a rate of 200 ± 20 °C / h to 800 °C ± 10 °C and held for 0.2H minutes (H represents the billet thickness, in mm). After the heat preservation is completed, it is heated at a rate of 200 ± 20 °C / h to 1000 °C ± 10 °C and held for 0.2H (H represents the billet thickness, in mm). After the heat preservation is completed, it is heated at a rate of 200 ± 20 °C to 1100 °C ± 10 °C and held for 0.2H (H represents the billet thickness, in mm). After the heat preservation is completed, it is heated at a rate of 200 ± 20 °C to 1200 °C ± 10 °C and held for 0.4H (H represents the billet thickness, in mm). After the heat preservation is completed, it is heated at a rate of 200 ± 20 °C to 1230 °C ± 10 °C and held for 0.4H (H represents the billet thickness, in mm). After the heat preservation is completed, it is taken out of the furnace for rolling.

[0063] The rolling process: high-temperature normalizing rolling process is adopted. The starting rolling temperature is ≥ 1080 °C, the reduction ratios for the 1st, 2nd, 3rd, and 4th passes are 15 - 20%. During the rolling of the 2nd and 4th passes, the upper and lower surfaces of the steel plate are cooled by the high-pressure water devices in front of and behind the rolling mill, with the high-pressure water pressure ≥ 15 MPa and the high-pressure water flow rate ≥ 400 m 3 / h, ensure that the temperature difference between the surface and the core reaches 100 - 150 °C, so that the large reduction can be better transmitted to the core of the steel plate, promoting the recrystallization of the core structure of the steel plate and refining the grains. The finish rolling temperature is 900 - 930 °C, and water cooling is carried out after rolling.

[0064] The post-rolling cooling: adopts the ultra-rapid cooling process, the starting cooling temperature ≥ 880 °C, the speed of the cooling roller table is 0.3 - 0.5 m / s, the water volume in the high-pressure section is 5000 - 6000 m 3 / h, the water ratio is 1.2 - 1.6, and the water volume in the low-pressure section

[0065] 3000 - 4000 m 3 / h, the water ratio is 1.2 - 1.6, the return red temperature is 300 - 340 °C, and it is immediately lifted into the slow cooling pit after hot straightening, and heat treatment by tempering is carried out using the waste heat of the steel plate.

[0066] The stacking and cooling process: adopts the waste heat tempering process in the slow cooling pit. Before the steel plate enters the pit, first lift 4 - 6 pads with a temperature of 400 - 450 °C and a thickness of 50 - 80 mm to cover the ground of the slow cooling pit, and then lift the steel plate to be tempered into the slow cooling pit. After covering the pit cover, heat the steel plate to raise the temperature of the steel plate to 550 - 610 °C, and keep it warm for 3×H - 4×H minutes. The specific temperature and time parameters for heat preservation of steel plates with different thicknesses are shown in Table 1. Since the steel plate has a waste heat of 250 °C - 300 °C before being lifted in, compared with the off-line tempering process, the heating time can be reduced by 1.5×H, the production efficiency of the steel plate can be improved, and the energy consumption can be reduced.

[0067] Table 1 Parameters of waste heat tempering process in slow cooling pit

[0068]

[0069] In the existing technical solutions, by increasing a large amount of elements Ni, Cu, and Al, the precipitation strengthening effect of forming Al3Ni and Cu-rich phases in the steel is used to refine the grains of the steel plate, improve the hardenability and tissue uniformity of the steel plate, so as to ensure that the entire cross-section structure of the steel plate is fine and uniform, the grain size is 9 - 10 levels, the hardness is 40 - 44 HRC, and the hardness difference of the entire cross-section ≤ 3 HRC. However, the price of element Ni alloy is high. At the same time, NiS network structure will be generated in the Ni-containing steel billet during the heating process, which combines with the eutectic low-melting substance of FeO - SiO2 generated in the steel billet, increasing the viscosity of the scale on the surface of the steel billet and making it difficult to remove in the follow-up process. It is necessary to apply anti-oxidation coatings, increasing the manufacturing cost of the steel plate.

[0070] In the present invention, by adding rare earth elements in the two processes of RH vacuum treatment and tundish casting, the recovery rate of rare earth elements is improved. Utilizing the property of rare earth elements to purify molten steel, the inclusions of (Mn,Ca)S are modified into RE2O2S and RE with smaller size and more quantity x S yInclusions, modify large-sized harmful inclusions into fine and dispersed beneficial inclusions, provide more nucleation sites for subsequent rolling, promote recrystallization nucleation, refine the grains of the steel plate, and at the same time improve the segregation in the core of the steel plate to make the grains of the whole cross-section structure fine and uniform. In combination with the increase in C content and the addition of Cr element, improve the hardenability and hardenability of the steel plate during water cooling, and replace the role of using the precipitation and precipitation strengthening of Al3Ni and Cu-rich phases in the original composition to refine the grains of the steel plate and improve the hardness of the steel plate.

[0071] Due to the high Ni content in the billet (content ≥ 0.7%) in the existing technical solution, an anti-oxidation coating needs to be applied before entering the furnace, and the hot charging and hot delivery process cannot be adopted. The maximum furnace inlet temperature is only 60°C, which not only increases the heating energy consumption, but also to achieve the temperature uniformity of the steel billet, it is necessary to extend its time in the heating section and soaking section, increasing the risk of tissue coarsening and affecting the hardenability of the steel plate.

[0072] However, the present invention greatly reduces the content of Ni element (content ≤ 0.65%). At the same time, the hot charging and hot delivery + step-by-step heat preservation heating process of the car-bottom heating furnace is adopted to shorten the heat preservation time of the slab at temperatures above 1200°C, and reduce the melting amount of the eutectic low-melting point of FeO-SiO2 with a melting point of 1208°C, thereby effectively reducing the proportion of highly viscous scale generated by the combination of NiS and FeO-SiO2 melt, improving the surface quality of the steel plate. At the same time, the overall temperature of the billet is uniform when it enters the furnace, which can greatly shorten the heating time, not only reducing the heating energy consumption and improving the production efficiency, but also reducing the risk of tissue growth, providing a finer tissue grain size for subsequent rolling and heat treatment.

[0073] In the existing technical solution, in order to obtain uniform hardness performance across the cross-section to ensure the high mirror performance and smoothness of the steel mill, an offline quenching + tempering process is adopted. This process requires reheating and water cooling of the steel plate, resulting in a long production cycle and high process costs.

[0074] However, the present invention adopts a high-temperature normalizing rolling + post-rolling water cooling + slow-cooling pit waste heat tempering process. Since the previous step-by-step heat preservation shortens the heating time, the grains are refined. At the same time, the fine and dispersed rare earth-modified inclusions in the steel provide nucleation sites for recrystallization rolling, promoting the recrystallization of the structure during high-temperature normalizing rolling. During rolling, the high-pressure water descaling system before and after the rolling mill is used to spray high-pressure water on the upper and lower surfaces of the steel plate at the 2nd and 4th rolling passes to make the temperature difference between the surface and the core of the steel plate 100-150°C, and use a large reduction of 15% per pass to increase the recrystallization driving force of the core of the steel plate, making the grains of the whole cross-section structure tend to be fine and uniform. After rolling, an online ultra-fast cooling device is used for online quenching and cooling to 300-340°C to obtain a bainite structure across the cross-section, with an overall hardness difference ≤ 3HRC. After online straightening, it is lifted to the slow-cooling pit for waste heat tempering, reducing the reheating process of offline quenching and tempering, reducing the production cost, shortening the production cycle, and improving the production efficiency.

[0075] The present invention is further described below with reference to specific embodiments.

[0076] Example 1

[0077] An economical high-mirror P40 plastic mold steel plate with a thickness of 6mm and the following components by mass percentage:

[0078] C: 0.28%, Si: 0.55%, Mn: 1.38%, P: 0.008%, S: 0.007%, Alt: 0.03%, Cr: 1.66%, Mo: 0.34%, Ni: 0.58%, V: 0.18%, RE: 0.0016% (including Y: 0.001%, La: 0.0003%, Ce: 0.0003%), N: 0.0034%, and the rest are Fe and residual elements.

[0079] The manufacturing method of the economical high-mirror P40 plastic mold steel plate of this embodiment includes smelting, heating, rolling, post-rolling cooling and stack cooling processes. The specific process steps are as follows:

[0080] Smelting process: using a 100-ton molten steel capacity converter ladle, the LF furnace is electrified for 8 minutes to slag, deoxidize and make white slag, the white slag holding time is 14 minutes, and the FeO+Mn0 in the slag is 0.6%; the RH furnace vacuum treatment time is 15 minutes, the vacuum degree is 95Pa, the ultimate vacuum holding time is 10 minutes, and 55Kg of seamless calcium wire is fed 2 minutes after breaking the air, the feeding speed is 1.6m / s, and then 55kg of yttrium-based heavy rare earth alloy wire is fed after feeding, the feeding speed is 1.3m / s, and soft blowing is performed after feeding the wire, the soft blowing time is 12 minutes, and [H]≤2ppm when leaving the station; continuous casting is fully protected casting, the tundish superheat is 18℃, and 300-mesh RE rare earth fine powder is added to the tundish through the tundish powder spraying pipe during casting. The rare earth powder is pushed by argon gas, and the argon gas conveying flow rate is 4.5m 3 / h, the amount of RE rare earth fine powder added is 22kg.

[0081] Heating process: billet hot delivery temperature: 680℃, furnace entry temperature 550℃, adopting step-by-step insulation process: billet thickness H is 230mm, after entering the furnace, the billet is heated to 800℃±10℃ at 200℃ / h and kept warm for 46 minutes, after the insulation is completed, the billet is heated to 1000℃±10℃ at 200℃ / h and kept warm for 46 minutes, after the insulation is completed, the billet is heated to 1100℃±10℃ at 200℃ and kept warm for 46 minutes, after the insulation is completed, the billet is heated to 1200℃±10℃ at 200℃ and kept warm for 92 minutes, after the insulation is completed, the billet is heated to 1230℃±10℃ at 200℃ and kept warm for 92 minutes, after the insulation is completed, the billet is taken out of the furnace and rolled.

[0082] Rolling process: The starting rolling temperature is 1125°C, the reduction rate in the first pass is 18%, the reduction rate in the second pass is 17%, the reduction rate in the third pass is 16%, the reduction rate in the fourth pass is 16%. During the rolling process of the second and fourth passes, the upper and lower surfaces of the steel plate are cooled by the high-pressure water devices in front of and behind the rolling mill. The high-pressure water pressure is 20 MPa, and the high-pressure water flow rate is 500 m 3 / h. The temperature difference between the measured surface and the core reaches 140°C, the finishing rolling temperature is 930°C, and after rolling, it enters the ultra-fast cooling device for water cooling.

[0083] Post-rolling cooling process: The starting cooling temperature is 890°C, the speed of the cooling roller table is 0.5 m / s, the water volume in the high-pressure section is 5000 m 3 / h, the water ratio is 1.4, the water volume in the low-pressure section is 3000 m 3 / h, the water ratio is 1.3, the recrystallization temperature is 330°C. Immediately after hot straightening, it is lifted into the slow cooling pit and heat treatment is carried out using the waste heat of the steel plate.

[0084] Heat treatment process: Before the steel plate enters the pit, first lift 4 pads with a thickness of 60 mm and a temperature of 400 - 450°C to cover the ground of the slow cooling pit, then lift the steel plate into the slow cooling pit, cover the heat preservation pit cover, heat the steel plate to 600°C ± 10°C. The thickness of the steel plate is 6 mm, and the heat preservation time is 24 minutes.

[0085] The microscopic structure of the steel plate in this example is shown in Figure 1 、 Figure 2 . The properties of the steel plate are shown in Table 2.

[0086] Table 2 Properties of 6mm XFP40 Steel Plate in Example 1

[0087]

[0088] It can be seen from the performance test results in Table 2 that the performance indexes of the surface and the 1 / 2 thickness of the 6mm P40 steel plate are good, fully meeting the delivery requirements of high mirror surface die steel. At the same time, the hardness difference between the surface and the core is ≤ 1.5 HRC, the whole cross-section is bainite, and the structure and hardness of the whole cross-section are uniform.

[0089] Example 2

[0090] An economical high mirror surface P40 plastic mold steel plate with a thickness of 30 mm, including the following mass percentage components: C: 0.25%, Si: 0.54%, Mn: 1.38%, P: 0.009%, S: 0.007%, Alt: 0.025%, Cr: 1.68%, Mo: 0.38%, Ni: 0.58%, V: 0.23%, RE: 0.0025% (where Y: 0.0015%, La: 0.0005%, Ce: 0.0005%), N: 0.0038%, and the rest are Fe and residual elements.

[0091] The manufacturing method of the economy type high mirror surface P40 plastic mold steel plate in this embodiment includes smelting, heating, rolling, post-rolling cooling and stacking cooling processes. The specific process steps are as follows:

[0092] Smelting process: A 100-ton converter ladle is used. The LF furnace is electrified to slag for 9 minutes, then the temperature is measured and samples are taken. Deoxidation is carried out to make white slag, with FeO+MnO in the slag being 0.7%. The white slag holding time is 13 minutes. The vacuum treatment time of the RH furnace is 16 minutes, the vacuum degree is 85 Pa, the ultimate vacuum holding time is 13 minutes. After breaking the vacuum, seamless calcium wire of 62 kg is fed in 2 minutes, the wire feeding speed is 1.7 m / s. After feeding, yttrium-based heavy rare earth alloy wire of 54 kg is fed, the wire feeding speed is 1.4 m / s. After wire feeding, soft blowing is carried out, and the soft blowing time is 15 minutes. [H] at the time of leaving the station is 1.4 ppm. The whole continuous casting process is carried out under protective casting. The superheat degree of the tundish is 15 °C. During the casting process, 300-mesh RE rare earth fine powder is added into the tundish through the tundish powder injection pipe, and the rare earth powder is pushed by argon. The argon delivery flow rate is 6 m 3 / h, and the addition amount of RE rare earth fine powder is 28 kg.

[0093] Heating process: The thickness of the billet is 250 mm, the hot delivery temperature is 700 °C, and the furnace inlet temperature is 545 °C. The step-by-step heat preservation heating process: After entering the furnace, the steel billet is heated at a rate of 200 °C / h to 800 °C ± 10 °C and kept warm for 50 minutes. After the heat preservation is completed, it is heated at a rate of 200 °C / h to 1000 °C ± 10 °C and kept warm for 50 minutes. After the heat preservation is completed, it is heated at a rate of 200 °C to 1100 °C ± 10 °C and kept warm for 50 minutes. After the heat preservation is completed, it is heated at a rate of 200 °C to 1200 °C ± 10 °C and kept warm for 100 minutes. After the heat preservation is completed, it is heated at a rate of 200 °C to 1230 °C ± 10 °C and kept warm for 100 minutes. After the heat preservation is completed, it is taken out of the furnace for rolling.

[0094] Rolling process: The rolling start temperature is 1136 °C, the reduction rate of the first pass is 17%, the reduction rate of the second pass is 16%, the reduction rate of the third pass is 16%, the reduction rate of the fourth pass is 17%. During the rolling of the second and fourth passes, the upper and lower surfaces of the steel plate are cooled by the high-pressure water devices in front of and behind the rolling mill. The high-pressure water pressure is 20 MPa, and the high-pressure water flow rate is 500 m 3 / h. The temperature difference between the surface and the core is measured to reach 130 °C, and the final rolling temperature is 926 °C. After rolling, it is sent into the ultra-fast cooling device for water cooling.

[0095] Post-rolling cooling process: The starting cooling temperature is 900 °C, the speed of the cooling roller table is 0.4 m / s, the water volume in the high-pressure section is 5300 m 3 / h, the water ratio is 1.5, the water volume in the low-pressure section is 3500 m 3 / h, the water ratio is 1.3, the red-return temperature is 310 °C. Immediately after hot straightening, it is lifted into the slow cooling pit and heated and tempered using the waste heat of the steel plate.

[0096] Pile cooling process: Before the steel plate enters the pit, first lift 4 cushion plates with a thickness of 60 mm and a temperature of 400 - 450 °C to cover the ground of the slow cooling pit, then lift the steel plate into the slow cooling pit, cover the heat preservation pit cover, heat the steel plate to 575 °C ± 10 °C, the thickness of the steel plate is 30 mm, and the heat preservation time is 90 minutes.

[0097] The microscopic structure of the steel plate in this example is shown in Figure 3 , Figure 4 , and the performance of the steel plate is shown in Table 3.

[0098] Table 3 Performance of 30 mm P40 steel plate in Example 2

[0099]

[0100] It can be seen from the performance test results in Table 3 that the performance indexes of the surface and the 1 / 2 thickness of the 30 mm P40 steel plate are good, fully meeting the delivery requirements of high mirror surface die steel. At the same time, the hardness difference between the surface and the core is ≤ 0.8 HRC, and the organization and hardness of the whole section are uniform.

[0101] Example 3

[0102] An economical high mirror surface P40 plastic mold steel plate with a thickness of 70 mm, including the following mass percentage components: C: 0.31%, Si: 0.52%, Mn: 1.4%, P: 0.008%, S: 0.006%, Alt: 0.028%, Cr: 1.72%, Mo: 0.37%, Ni: 0.6%, V: 0.24%, RE: 0.0026% (where Y: 0.0018%, La: 0.0005%, Ce: 0.0003%), N: 0.0037%, and the rest are Fe and residual elements.

[0103] The manufacturing method of the economical high mirror surface P40 plastic mold steel plate in this example includes smelting, heating, rolling, post-rolling cooling and pile cooling processes, and the specific process steps are as follows:

[0104] Smelting process: Use a 100-ton converter ladle, the LF furnace is powered on to melt the slag for 10 minutes for temperature measurement and sampling, deoxidize to make white slag, the FeO + Mn0 in the slag is 0.8%, and the white slag holding time is 12 minutes; the RH furnace vacuum treatment time is 16 minutes, the vacuum degree is 75 Pa, the ultimate vacuum holding time is 12 minutes, 66 kg of seamless calcium wire is fed 2 minutes after breaking the vacuum, the wire feeding speed is 1.75 m / s, after feeding, 56 kg of yttrium-based heavy rare earth alloy wire is fed again, the wire feeding speed is 1.5 m / s, soft blowing is carried out after wire feeding, the soft blowing time is 15 minutes, and [H] at the time of tapping is 1.4 ppm; the whole continuous casting process is carried out under protective casting, the superheat of the tundish is 20 °C, and during the casting process, 300-mesh RE rare earth fine powder is added to the tundish through the tundish powder injection pipe, and the rare earth powder is pushed by argon, and the argon delivery flow rate is 7 m3 / h, The addition amount of RE rare earth fine powder is 27 kg.

[0105] Heating process: The billet thickness is 300 mm, the hot delivery temperature is 690 °C, the furnace inlet temperature is 540 °C, step-by-step heat preservation heating process: After entering the furnace, the steel billet is heated to 800 °C ± 10 °C at a rate of 200 °C / h and kept warm for 60 minutes. After the heat preservation is completed, it is heated to 1000 °C ± 10 °C at a rate of 200 °C / h and kept warm for 60 minutes. After the heat preservation is completed, it is heated to 1100 °C ± 10 °C at a rate of 200 °C and kept warm for 60 minutes. After the heat preservation is completed, it is heated to 1200 °C ± 10 °C at a rate of 200 °C and kept warm for 120 minutes. After the heat preservation is completed, it is heated to 1230 °C ± 10 °C at a rate of 200 °C and kept warm for 120 minutes. After the heat preservation is completed, it is taken out of the furnace for rolling.

[0106] Rolling process: The rolling start temperature is 1132 °C, the reduction ratio of the first pass is 18%, the reduction ratio of the second pass is 17%, the reduction ratio of the third pass is 19%, the reduction ratio of the fourth pass is 15%. During the rolling process of the second pass and the fourth pass, the upper and lower surfaces of the steel plate are cooled by using the high-pressure water devices in front of and behind the rolling mill. The high-pressure water pressure is 20 MPa, and the high-pressure water flow rate is 500 m 3 / h, The temperature difference between the surface and the core is measured to reach 126 °C, the final rolling temperature is 924 °C, and after rolling, it is sent into the ultra-fast cooling device for water cooling.

[0107] Post-rolling cooling process: The cooling start temperature: 896 °C, the speed of the cooling roller table is 0.3 m / s, the water volume in the high-pressure section is 6000 m 3 / h, the water ratio is 1.3, the water volume in the low-pressure section is 4000 m 3 / h, the water ratio is 1.2, the red-return temperature is 326 °C, and immediately after hot straightening, it is lifted into the slow-cooling pit and heated and tempered by using the waste heat of the steel plate.

[0108] Stack cooling process: Before the steel plate enters the pit, first lift 4 pads with a thickness of 60 mm and a temperature of 400 - 450 °C to cover the ground of the slow-cooling pit, and then lift the steel plate into the slow-cooling pit, cover the heat preservation pit cover, heat the steel plate to 560 °C ± 10 °C, the steel plate thickness is 70 mm, and the heat preservation time is 210 minutes.

[0109] The microscopic structure of the steel plate in this embodiment is shown in Figure 5 , Figure 6 , The performance of the steel plate is shown in Table 4.

[0110] Table 4 Performance of 70 mm P40 steel plate in Example 3

[0111]

[0112] It can be seen from the performance test results in Table 4 that the performance indexes of the surface and the 1 / 2 thickness of the 70 mm P40 steel plate are good, fully meeting the delivery requirements of high mirror surface die steel. At the same time, the hardness difference between the surface and the core is ≤ 2 HRC, and the organization and hardness of the whole cross-section are uniform.

[0113] It can be seen from the microstructural pictures of the steel plates in Example 1, Example 2 and Example 3 that the surface and core tissues of the steel plates are bainite, and the tissue grains are finer and more uniform, ensuring the uniformity of the properties of the whole steel plate tissue.

[0114] Comparative Example 1

[0115] An economical high-mirror P40 plastic mold steel plate with a steel plate thickness of 70 mm, including the following mass percentage components: C: 0.30%, Si: 0.53%, Mn: 1.41%, P: 0.007%, S: 0.006%, Alt: 0.027%, Cr: 1.78%, Mo: 0.35%, Ni: 0.61%, V: 0.22%, N: 0.0033%, and the rest are Fe and residual elements. (RE rare earth elements not added)

[0116] The manufacturing method of the economical high-mirror P40 plastic mold steel plate in this example includes smelting, heating, rolling, post-rolling cooling and stacking cooling processes. The specific process steps are as follows:

[0117] Smelting process: Use a 100-ton converter ladle. The LF furnace is electrified to slag for 10 minutes for temperature measurement and sampling, deoxidized to make white slag, with FeO + Mn0 = 0.7% in the slag, and the white slag holding time is 11 minutes; the RH furnace vacuum treatment time is 17 minutes, the vacuum degree is 55 Pa, the ultimate vacuum holding time is 12 minutes, 63 kg of seamless calcium wire is fed 2 minutes after breaking the vacuum, the wire feeding speed is 1.70 m / s, soft blowing is carried out after wire feeding, the soft blowing time is 14 minutes, and [H] at the time of tapping is 1.6 ppm; the continuous casting is protected by casting throughout the process, and the superheat of the tundish is 21°C.

[0118] Heating process: The billet thickness is 300 mm, the hot charging temperature is 685°C, the furnace inlet temperature is 552°C, and the step-by-step heat preservation heating process: After entering the furnace, the steel billet is heated to 800°C ± 10°C at a rate of 200°C / h and held for 60 minutes. After the heat preservation is completed, it is heated to 1000°C ± 10°C at a rate of 200°C / h and held for 60 minutes. After the heat preservation is completed, it is heated to 1100°C ± 10°C at a rate of 200°C and held for 60 minutes. After the heat preservation is completed, it is heated to 1200°C ± 10°C at a rate of 200°C and held for 120 minutes. After the heat preservation is completed, it is heated to 1230°C ± 10°C at a rate of 200°C and held for 120 minutes. After the heat preservation is completed, it is taken out of the furnace for rolling.

[0119] Rolling process: The rolling start temperature is 1123°C, the reduction rate of the first pass is 18%, the reduction rate of the second pass is 18%, the reduction rate of the third pass is 16%, the reduction rate of the fourth pass is 15%. During the rolling of the second pass and the fourth pass, the upper and lower surfaces of the steel plate are cooled by using the high-pressure water devices in front of and behind the rolling mill. The high-pressure water pressure is 20 MPa, and the high-pressure water flow rate is 500 m 3 / h, the temperature difference between the surface and the core is measured to reach 124 °C, the finish rolling temperature is 928 °C, and it is water-cooled in the ultra-fast cooling device after rolling.

[0120] Cooling process after rolling: Starting cooling temperature: 893 °C, cooling roller table speed 0.3 m / s, water volume in the high-pressure section 6000 m 3 / h, water ratio 1.3, water volume in the low-pressure section 4000 m 3 / h, water ratio 1.2, red-return temperature 322 °C, immediately lifted into the slow cooling pit after hot straightening, and heat treatment by tempering is carried out using the residual heat of the steel plate.

[0121] Piling and cooling process: Before the steel plate enters the pit, first lift 4 pads with a thickness of 60 mm and a temperature of 400 - 450 °C to cover the ground of the slow cooling pit, then lift the steel plate into the slow cooling pit, cover the heat preservation pit cover, heat the steel plate to 560 °C ± 10 °C, the thickness of the steel plate is 70 mm, and the heat preservation time is 210 minutes.

[0122] The microstructure of the steel plate in this example is shown in Figure 7 、 Figure 8 , and the properties of the steel plate are shown in Table 5.

[0123] Table 5 Properties of 70-mm P40 steel plate in Comparative Example 1

[0124]

[0125] It can be seen from the performance test results in Table 5 that for the 70-mm P40 steel plate without adding RE rare earth elements, the surface performance indicators are good and fully meet the delivery requirements of high mirror surface die steel, but the performance indicators and surface finish at 1 / 2 of the thickness are on the low side and cannot meet the delivery requirements of high mirror surface die steel. At the same time, the hardness difference between the surface and the core reaches 7.0HRC , and the hardness of the entire cross-section is uneven.

[0126] It can be seen from the microstructure picture of the steel plate in Comparative Example 1 that the surface structure of the steel plate is bainite, but in addition to bainite, there is also a small amount of pearlite structure in the core structure, and the grains of some structures are coarse and uneven, which not only affects the hardness performance of the core but also has a poor surface finish.

[0127] Comparative Example 2

[0128] An economical high mirror surface P40 plastic mold steel plate with a thickness of 30 mm, including the following mass percentage components: C: 0.29%, Si: 0.57%, Mn: 1.4%, P: 0.009%, S: 0.005%, Alt: 0.029%, Cr: 1.7%, Mo: 0.38%, Ni: 0.61%, V: 0.23%, RE: 0.0019% (where Y: 0.0019%), N: 0.0036%, and the rest are Fe and residual elements.

[0129] Manufacturing method of the economy type high mirror surface P40 plastic mold steel plate in this embodiment, including smelting, heating, rolling, post-rolling cooling and stacking cooling processes, and the specific technological steps are as follows:

[0130] Smelting process: Use a 100-ton converter ladle. The LF furnace is electrified to slag for 9 minutes for temperature measurement and sampling, deoxidize to make white slag, with FeO + MnO = 0.9% in the slag, and the white slag holding time is 15 minutes; the RH furnace vacuum treatment time is 16 minutes, the vacuum degree is 85 Pa, the ultimate vacuum holding time is 11 minutes, and 55 kg of seamless calcium wire is fed 2 minutes after breaking the vacuum, with the wire feeding speed of 1.6 m / s. After feeding, yttrium-based heavy rare earth alloy wire is fed 80 kg , with the wire feeding speed of 1.5 m / s. After wire feeding, soft blowing is carried out, and the soft blowing time is 14 minutes. [H] at the time of tapping: 1.6 ppm; the whole continuous casting process is protected by casting, and the superheat degree of the tundish is 18 °C. (RE rare earth elements not added for the second time, only added during vacuum treatment)

[0131] Heating process: The billet thickness is 250 mm, the hot charging temperature is 690 °C, the furnace inlet temperature is 565 °C, and the step-by-step heat preservation heating process: After entering the furnace, the steel billet is heated to 800 °C ± 10 °C at a rate of 200 °C / h and held for 50 minutes. After the heat preservation is completed, it is heated to 1000 °C ± 10 °C at a rate of 200 °C / h and held for 50 minutes. After the heat preservation is completed, it is heated to 1100 °C ± 10 °C at a rate of 200 °C and held for 50 minutes. After the heat preservation is completed, it is heated to 1200 °C ± 10 °C at a rate of 200 °C and held for 100 minutes. After the heat preservation is completed, it is heated to 1230 °C ± 10 °C at a rate of 200 °C and held for 100 minutes. After the heat preservation is completed, it is taken out of the furnace for rolling.

[0132] Rolling process: The rolling start temperature is 1132 °C, the reduction rate of the first pass is 19%, the reduction rate of the second pass is 18%, the reduction rate of the third pass is 16%, the reduction rate of the fourth pass is 16%. During the rolling of the second pass and the fourth pass, the upper and lower surfaces of the steel plate are cooled by the high-pressure water devices in front of and behind the rolling mill. The high-pressure water pressure is 20 MPa, and the high-pressure water flow rate is 500 m 3 / h, and the temperature difference between the surface and the core is measured to reach 131 °C. The final rolling temperature is 929 °C, and after rolling, it enters the ultra-fast cooling device for water cooling.

[0133] Post-rolling cooling process: The starting cooling temperature: 887 °C, the cooling roller table speed is 0.4 m / s, the water volume in the high-pressure section is 5300 m 3 / h, the water ratio is 1.5, the water volume in the low-pressure section is 3500 m 3 / h, the water ratio is 1.3, the recalescence temperature is 310 °C, and it is immediately lifted into the slow cooling pit after hot straightening, and heat treatment for tempering is carried out using the waste heat of the steel plate.

[0134] Pile cooling process: Before the steel plate is put into the pit, first lift 4 pads with a thickness of 60 mm and a temperature of 400 - 450 °C to cover the ground of the slow cooling pit, then lift the steel plate into the slow cooling pit, cover the heat preservation pit cover, heat the steel plate to 575 °C ± 10 °C, the thickness of the steel plate is 30 mm, and the heat preservation time is 90 minutes.

[0135] The microstructure of the steel plate in this embodiment is shown in Figure 9 , Figure 10 , and the performance of the steel plate is shown in Table 6.

[0136] Table 6 Performance of 30mm P40 steel plate in Comparative Example 2

[0137]

[0138] It can be seen from the performance test results in Table 6 that for the 30mm P40 steel plate without secondary addition of RE rare earth elements, the surface performance indicators are good and fully meet the delivery requirements of high mirror surface die steel, but the performance indicators and surface finish at 1 / 2 of the thickness are on the low side and cannot meet the delivery requirements of high mirror surface die steel. At the same time, the hardness difference between the surface and the core reaches 5.5HRC , and the hardness of the whole section is uneven.

[0139] Comparative Example 3

[0140] An economical high mirror surface P40 plastic mold steel plate with a thickness of 70 mm, including the following mass percentage components: C: 0.30%, Si: 0.55%, Mn: 1.35%, P: 0.007%, S: 0.005%, Alt: 0.022%, Cr: 1.75%, Mo: 0.38%, Ni: 0.63%, V: 0.2%, RE: 0.0024% (where Y: 0.0017%, La: 0.0004%, Ce: 0.0003%), N: 0.0033%, and the rest are Fe and residual elements.

[0141] The manufacturing method of the economical high mirror surface P40 plastic mold steel plate in this embodiment includes smelting, heating, rolling, post-rolling cooling and pile cooling processes. The specific process steps are as follows:

[0142] Smelting process: A 100-ton molten steel capacity converter ladle is used. In the LF furnace, the slag is melted by electrifying for 8 minutes, and then the temperature is measured and samples are taken. Deoxidation is carried out to produce white slag, with FeO + MnO in the slag being 0.9%. The white slag retention time is 11 minutes. The vacuum treatment time in the RH furnace is 15 minutes, the vacuum degree is 70 Pa, and the ultimate vacuum retention time is 11 minutes. After breaking the vacuum, seamless calcium wire of 64 kg is fed in 2 minutes at a wire feeding speed of 1.7 m / s. After feeding the calcium wire, yttrium-based heavy rare earth alloy wire of 55 kg is fed at a wire feeding speed of 1.5 m / s. After wire feeding, soft blowing is carried out for 13 minutes. At the time of tapping, [H] is 1.1 ppm. The continuous casting is carried out under full protection. The superheat degree of the tundish is 19 °C. During the casting process, 300-mesh RE rare earth fine powder is added into the tundish through the tundish powder injection pipe, and the rare earth powder is pushed by argon gas. The argon gas conveying flow rate is 7 m 3 / h, and the addition amount of RE rare earth fine powder is 26 kg.

[0143] Heating process: The thickness of the billet is 300 mm, the hot charging temperature is 682 °C, and the furnace inlet temperature is 545 °C. Adopting the conventional walking beam furnace type Heating process: The heating temperature in the billet preheating section is ≤800°C, the temperature in the first heating section is 800°C - 1150°C, the temperature in the second heating section is 1200°C - 1255°C, the soaking section temperature is 1200 - 1250°C, total heating time: for 300 mm cast billet ≥500 minutes After the heat preservation is completed, it is taken out of the furnace for rolling.

[0144] Rolling process: In the first stage, high-temperature recrystallization rolling is adopted. The rolling start temperature is 1066°C, the finishing rolling temperature is 959°C, cumulative Reduction ratio 57%, the second stage adopts non-recrystallization rolling, the rolling start temperature is 878°C, the finishing rolling temperature is 834°C, cumulative reduction Ratio is 36%. (The step-by-step heat preservation heating process and high-temperature normalizing rolling process are not adopted)

[0145] Post-rolling cooling process: Cooling start temperature: 821°C, The speed of the cooling roller table is 0.3 m / s, the water volume in the high-pressure section is 5700 m 3 / h, the water ratio is 1.3, the water volume in the low-pressure section is 3800 m 3 / h, the water ratio is 1.2, the return red temperature is 316 °C, and it is immediately lifted into the slow cooling pit after hot straightening, and heat treatment is carried out by using the waste heat of the steel plate.

[0146] Stack cooling process: Before the steel plate enters the pit, first lift 4 pads with a thickness of 60 mm and a temperature of 400 - 450 °C to cover the ground of the slow cooling pit, then lift the steel plate into the slow cooling pit, cover the heat preservation pit cover, heat the steel plate to 560 °C ± 10 °C, the thickness of the steel plate is 70 mm, and the heat preservation time is 210 minutes.

[0147] The microscopic structure of the steel plate in this embodiment is shown in Figure 11 and Figure 12 , and the performance of the steel plate is shown in Table 7.

[0148] Table 7 Performance of 70 mm P40 steel plate in Comparative Example 3

[0149]

[0150] From the performance test results in Table 7, it can be seen that for the P40 steel plate without using the stepwise heat preservation heating process and the high-temperature normalizing rolling process, the surface performance indicators are good and fully meet the delivery requirements of high mirror finish die steel. However, the performance indicators and surface finish at the 1 / 2 thickness are on the low side and cannot meet the delivery requirements of high mirror finish die steel. At the same time, the hardness difference between the surface and the core reaches 6.5HRC , and the hardness of the entire cross-section is uneven.

[0151] The underlined data above do not meet the requirements of the present invention.

[0152] From the microstructural photographs of the steel plates in Comparative Example 2 and Comparative Example 3, it can be seen that the surface structure of the steel plate is bainite, but there is a certain amount of pearlite and composition segregation in the core structure. The uneven structure in the thickness direction of the steel plate causes performance differences.

Claims

1. An economical high-mirror P40 plastic mold steel plate, characterized in that, The described economical high mirror surface P40 plastic mold steel plate comprises the following components by mass percentage: C: 0.16 - 0.35%, Si: 0.5 - 0.6%, Mn: 1.3 - 1.45%, P ≤ 0.015%, S ≤ 0.008%, Alt: 0.015 - 0.035%, Cr: 1.6 - 1.8%, Mo: 0.3 - 0.4%, Ni: 0.55 - 0.65%, V: 0.15 - 0.3%, RE: 0.0015 - 0.003%, N: 0.003 - 0.005%, and the balance is Fe and residual elements; The production method of the described economical high mirror surface P40 plastic mold steel plate includes smelting, heating, rolling, post-rolling cooling, and stacking cooling; For the described smelting, rare earth is added in two processes of RH furnace vacuum degassing smelting and tundish continuous casting; For the described heating, hot charging and stepwise heat preservation process are adopted; The described stepwise heat preservation process: After the billet is put into the furnace, it is heated at 200°C ± 20°C / h to 800°C ± 10°C and kept warm for 0.2×H minutes. After the heat preservation is completed, it is heated at 200 ± 20°C / h to 1000°C ± 10°C and kept warm for 0.2×H. After the heat preservation is completed, it is heated at 200 ± 20°C to 1100°C ± 10°C and kept warm for 0.2×H. After the heat preservation is completed, it is heated at 200 ± 20°C to 1200°C ± 10°C and kept warm for 0.4×H. After the heat preservation is completed, it is heated at 200 ± 20°C to 1230°C ± 10°C and kept warm for 0.4×H. After the heat preservation is completed, it is taken out of the furnace for rolling; where H represents the thickness of the billet, in mm; For the rolling process, the starting rolling temperature is ≥1080°C, the reduction ratios for the 1st, 2nd, 3rd, and 4th passes are 15 - 20%. During the rolling processes of the 2nd and 4th passes, high-pressure water devices in front of and behind the rolling mill are used to cool the upper and lower surfaces of the steel plate. The high-pressure water pressure is ≥15 MPa, and the high-pressure water flow rate is ≥400 m 3 / h, ensuring that the temperature difference between the surface and the core reaches 100 - 150°C, the finishing rolling temperature is 900 - 930°C, and water cooling is carried out after rolling; The post-rolling cooling adopts ultra-fast cooling, with the starting cooling temperature ≥ 880°C, the speed of the cooling roller table being 0.3 - 0.5 m / s, the water volume in the high-pressure section being 5000 - 6000 m 3 / h, the water ratio being 1.2 - 1.6, the water volume in the low-pressure section being 3000 - 4000 m 3 / h, the water ratio being 1.2 - 1.6, and the recalescence temperature being 300 - 340°C; For the described stacking cooling, a slow cooling pit waste heat tempering process is adopted, specifically: Before the steel plate is put into the pit, first lift 4 - 6 pads with a temperature of 400 - 450°C and a thickness of 50 - 80mm to cover the ground of the slow cooling pit, then lift the steel plate to be tempered into the slow cooling pit, cover the pit cover and heat the steel plate to raise the steel plate temperature to 550 - 610°C and keep warm for 3×H - 4×H minutes.

2. The economical high mirror surface P40 plastic mold steel plate according to claim 1, wherein The full-section hardness of the described economical high mirror surface P40 plastic mold steel plate is 40 - 43HRC, the full-section hardness difference ≤ 3HRC; the surface finish Ra ≤ 0.050μm; the surface and core hardness difference ≤ 1.4HRC.

3. A manufacturing method of the economical high mirror surface P40 plastic mold steel plate as claimed in claim 1, including smelting, heating, rolling, post-rolling cooling, and stacking cooling.

4. The manufacturing method according to claim 3, characterized in that, The specific process of the described smelting is: converter smelting - LF furnace refining - RH furnace vacuum degassing - continuous casting; For the described RH furnace vacuum degassing, control the RH furnace vacuum treatment time ≥ 15min, the vacuum degree ≤ 133Pa, the ultimate vacuum holding time ≥ 10min. After breaking the vacuum, feed seamless calcium wire (0.50 - 0.70) kg / ton of steel within 2min, the wire feeding speed is (1.6 - 1.8) m / s. After feeding, feed yttrium-based heavy rare earth alloy wire (0.5 - 0.6) kg / ton of steel, the wire feeding speed is (1.3 - 1.5) m / s; after wire feeding, perform soft blowing, the soft blowing time ≥ 10min, and ensure that [H] ≤ 2ppm when leaving the station.

5. The manufacturing method according to claim 3, characterized in that, For the continuous casting, full-process protected casting is adopted. The superheat of the tundish is 10~30°C. During the casting process, 300-mesh RE rare earth fine powder is added into the tundish through the tundish powder injection pipe. The rare earth powder is pushed by argon gas, and the argon gas delivery flow rate is 4~8m 3 / h, and the addition amount of RE rare earth fine powder is (0.2~0.3) kg / ton of molten steel.

6. The manufacturing method according to claim 3, characterized in that, For the heating, hot charging and step-by-step heat preservation processes are adopted; for the hot charging, the maximum hot charging temperature of the billet is 700 °C, and the furnace charging temperature is 500 - 600 °C.

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