Martensitic stainless steel and preparation method of medium-thickness plate of martensitic stainless steel

Abstract

The invention discloses a preparation method of a martensitic stainless steel medium plate. The stainless steel comprises the following chemical elements in percentage: 0.21-0.28% of C; 0.15% to 0.30% of Si; 0.50% to 0.80% of Mn; less than or equal to 0.020% of P; s < = 0.005%; cr: 13.00% to 14.00%; 0.20% to 0.30% of Mo; 0.60% to 0.80% of Ni; the Cu accounts for 0.20%-0.25%, the V accounts for 0.10%-0.20%, and the Nb accounts for 0.03%-0.08%; 0.080% to 0.12% of N; and the balance of Fe and inevitable impurities. The preparation method comprises the steps that molten steel is smelted, steel ingot pouring is conducted, the steel ingot cast into an electrode blank is subjected to double-slag-system electroslag remelting under the protective atmosphere, and a slag system for the first time of electroslag remelting comprises a CaF2-Al2O3-CaO-MgO quaternary slag system with the proportion being 40%: 25%: 30%: 5%; a slag system for secondary electroslag remelting comprises a five-membered slag system of CaF2-Al2O3-CaO-MgO-SiO2 with the proportion of 40%: 30%: 23%: 5%: 2%, and the processes of forging cogging, blank hot rolling, cooling, low-temperature normalizing and spheroidizing annealing heat treatment are carried out. The martensitic stainless steel prepared through the method has the characteristics of being good in corrosion resistance, high in heat treatment hardness, uniform in structure and stable in performance.

Description

Technical Field

[0001] The present invention relates to the technical field of hot processing of metal materials, in particular to a martensitic stainless steel and a method for preparing a medium and thick plate thereof. Background Art

[0002] Existing optical products, automotive lights, LED lights, mobile phone displays, light guide plates, etc. are made of acrylic and PC plastic materials. These materials release halogen ions during the processing of products, which are corrosive to the molds used. Therefore, the mold steel material must be made of martensitic stainless steel with excellent corrosion and rust resistance. At the same time, these products must meet a series of optical requirements such as transmittance and refractive index.

[0003] To ensure that the lens meets these standards, the mold steel must be mirror-polished and grain-treated to ensure its surface is free of pitting, sand holes, grain lines, dark spots, dark shadows, bright shadows, and other undesirable phenomena. This requires that martensitic stainless steel must have extremely high steel purity, structural uniformity, and material stability. However, existing martensitic stainless steel and its corresponding product performance cannot meet the requirements for corrosion resistance, hardness, and stability. Summary of the Invention

[0004] In order to solve some or all of the technical problems existing in the above-mentioned prior art, the present invention provides a martensitic stainless steel and a method for preparing medium and thick plates thereof, which have excellent characteristics of good corrosion resistance, high hardness during heat treatment, uniform structure, fine grains and stable anisotropic properties.

[0005] The technical solutions of the present invention are as follows:

[0006] In a first aspect, a method for preparing a medium and thick martensitic stainless steel plate is provided, comprising the following chemical elements in the following percentages:

[0007] C: 0.21%~0.28%; Si: 0.15%~0.30%; Mn: 0.50%~0.80%; P≤0.020%; S≤0.005%;

[0008] Cr: 13.00% to 14.00%; Mo: 0.20% to 0.30%; Ni: 0.60% to 0.80%; Cu: 0.20% to 0.25%; V: 0.10% to 0.20%; Nb: 0.03% to 0.08%; N: 0.080% to 0.12%; the rest is Fe and unavoidable impurities;

[0009] The preparation method comprises the following steps:

[0010] Molten steel smelting: K-OBM-S+VOD vacuum degassing+LF refining is used to smelt raw materials into molten steel, and the percentage of chemical elements in the molten steel reaches: C: 0.21%-0.28%; Si: 0.15%-0.30%; Mn: 0.50%-0.80%; P≤0.020%; S≤0.005%; Cr: 13.00%-14.00%; Mo: 0.20%-0.30%; Ni: 0.60%-0.80%; Cu: 0.20%-0.25%, V: 0.10%-0.20%, Nb: 0.03%-0.08%; N: 0.080%-0.12%; the rest is Fe and unavoidable impurities;

[0011] Ingot casting: casting the smelted molten steel into electrode billet ingots;

[0012] Electroslag remelting: The electroslag remelting comprises remelting the steel ingot cast into the electrode blank using a double slag system electroslag under a protective atmosphere to obtain an electroslag ingot; wherein the protective gas comprises an inert gas, and the slag system of the first electroslag remelting in the double slag system electroslag remelting comprises a quaternary slag system of CaF2-Al2O3-CaO-MgO in a ratio of 40%:25%:30%:5%, and the slag system of the second electroslag remelting comprises a quinary slag system of CaF2-Al2O3-CaO-MgO-SiO2 in a ratio of 40%:30%:23%:5%:2%;

[0013] Forging and blanking: The electroslag ingot obtained by electroslag remelting is heated to 1250℃ at 100℃ / h and kept at this temperature for 5-8 hours, and then forged into a blank of preset specifications through a three-upsetting and three-drawing forging process;

[0014] Hot rolling of billets: The billets are heated to 1250-1270℃ at a heating rate of 80℃ / h, kept at this temperature for 12-15 hours to achieve high temperature diffusion and homogenization, and the billets after being kept at this temperature are widened and longitudinally rolled into medium and thick plates;

[0015] Cooling: The hot-rolled medium and thick plates are quickly water-cooled to 550-600℃ and then straightened. After straightening, they are air-cooled to 20-50℃;

[0016] Microstructure refinement heat treatment: The medium and thick plates after air cooling are subjected to low-temperature normalizing to refine the grains, and are heated to 970-990℃ for austenitization at a heating rate of 80℃ / h. The holding time is calculated as 2min / mm, and then quickly water-cooled to 550-600℃ and straightened. After straightening, the medium and thick plates are air-cooled to 80-150℃, and finally spheroidizing annealing is performed. The medium and thick plates are heated to 820-840℃ at a heating rate of ≤80℃ / h and kept warm for 6-12 hours, and then cooled to 740-760℃ at 15-25℃ / h and kept warm for 18-30 hours. After holding, the furnace is cooled to 400℃ and then taken out of the furnace and air-cooled to room temperature.

[0017] Furthermore, in the above-mentioned method for preparing a medium and thick martensitic stainless steel plate, during the smelting of molten steel, the ratio of Ni element to Cu element must meet the following requirements:

[0018] Ni:Cu≥3.

[0019] Furthermore, in the above-mentioned method for preparing a medium and thick martensitic stainless steel plate, annealing is required after the ingot is cast to obtain the electrode billet ingot, after the first electroslag remelting and after forging to open the billet, and the annealing temperature includes 750°C.

[0020] Furthermore, in the above-mentioned method for preparing a medium and thick martensitic stainless steel plate, the electroslag remelting comprises:

[0021] The electrode blank steel ingot is annealed at 750° C. to prepare an electrode blank. After the oxide scale is stripped, a quaternary slag system of CaF2-Al2O3-CaO-MgO in a ratio of 40%:25%:30%:5% is used for the first electroslag remelting. A nitrogen atmosphere is used for protection during the electroslag remelting process, and the melting rate is 9-11 kg / min. The electroslag ingot obtained by the electroslag remelting is annealed at 750° C. for 15 hours to prepare an electrode blank with a diameter of φ690 mm. Subsequently, a quinary slag system of CaF2-Al2O3-CaO-MgO-SiO2 in a ratio of 40%:30%:23%:5% is used for the second electroslag remelting. A nitrogen atmosphere is used for protection during the electroslag remelting process, and the melting rate is 11-14 kg / min.

[0022] Furthermore, in the above method for preparing a medium and thick martensitic stainless steel plate, during the hot rolling of the billet, it is required that the reduction ratio of at least one pass is greater than or equal to 15%.

[0023] Furthermore, in the above-mentioned method for preparing a medium and thick martensitic stainless steel plate, the flow rate of the cooling water during water cooling is 4000m 3 / h, the cooling water pressure is 0.2MPa.

[0024] Furthermore, in the above-mentioned method for preparing a medium and thick martensitic stainless steel plate, the preset specifications of the length, width and thickness of the blank include: dimensions of 250×1300×(1700-2000) mm.

[0025] Furthermore, in the above-mentioned method for preparing a medium and thick martensitic stainless steel plate, the upsetting ratio in the forging process is required to be greater than or equal to 2.0.

[0026] In a second aspect, a martensitic stainless steel is also provided, wherein the martensitic stainless steel contains the following chemical elements in the following percentages:

[0027] C: 0.21%~0.28%; Si: 0.15%~0.30%; Mn: 0.50%~0.80%; P≤0.020%; S≤0.005%;

[0028] Cr: 13.00%~14.00%; Mo: 0.20%~0.30%; Ni: 0.60%~0.80%; Cu: 0.20%-0.25%, V: 0.10%~0.20%, Nb: 0.03%-0.08%; N: 0.080%~0.12%; the rest are Fe and unavoidable impurities.

[0029] The main advantages of the technical solution of the present invention are as follows:

[0030] The present invention discloses a martensitic stainless steel and a method for preparing medium and thick plates thereof. By reducing the carbon content and increasing the nitrogen content, synergistically improving corrosion resistance through the elements Cr, Mo, Cu, and N, and adding V and Nb as composite grain-refining elements, a fine-grained martensitic stainless steel with high corrosion resistance and heat-treatment hardness is designed. The "dual-slag electroslag remelting" refining process achieves ultrapure steel quality, reducing the level of various non-metallic inclusions to ≤ 0.5. The thermal deformation process of "electroslag ingot three-upsetting and three-stretching for blanking + widening and final rolling" achieves stable isotropic properties of the material. The uniformity of the annealed microstructure reaches A1-A3 levels, with a grain size finer than level 10, through the uniformly refining heat treatment process of "intermediate billet high-temperature diffusion heating + rapid cooling after rolling + low-temperature normalizing + spheroidizing annealing." The martensitic stainless steel prepared by the present invention exhibits excellent corrosion resistance, high heat-treatment hardness, uniform microstructure, fine grains, and stable isotropic properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The drawings described herein are used to provide a further understanding of the embodiments of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0032] Figure 1 A schematic flow chart of a method for preparing a medium and thick martensitic stainless steel plate according to an embodiment of the present invention;

[0033] Figure 2 The figure is a metallographic diagram of a medium and thick plate of martensitic stainless steel after annealing using a method for preparing the medium and thick plate provided by one embodiment of the present invention. DETAILED DESCRIPTION

[0034] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments of the present invention and corresponding drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0035] The following is combined with Figure 1-2 , describes in detail the technical solution provided by the embodiments of the present invention.

[0036] Example 1

[0037] As attached Figure 1-2 As shown, in order to make the martensitic stainless steel material have the excellent characteristics of good corrosion resistance, high heat treatment hardness, uniform structure, fine grains and stable anisotropic properties. In the embodiment of the present invention, on the basis of the martensitic stainless steel component system, the carbon content is reduced and the nitrogen element is increased, and the corrosion resistance, heat treatment hardness and wear resistance of the material are synergistically improved by the Cr+Mo+Cu+N elements; V+Nb composite refines the grains to ensure that the grains do not grow at higher quenching temperatures. The embodiment of the present invention provides a method for preparing a medium and thick plate of martensitic stainless steel, which contains the following chemical elements in the following percentages:

[0038] C: 0.21%~0.28%; Si: 0.15%~0.30%; Mn: 0.50%~0.80%; P≤0.020%; S≤0.005%;

[0039] Cr: 13.00% to 14.00%; Mo: 0.20% to 0.30%; Ni: 0.60% to 0.80%; Cu: 0.20% to 0.25%; V: 0.10% to 0.20%; Nb: 0.03% to 0.08%; N: 0.080% to 0.12%; the rest is Fe and unavoidable impurities;

[0040] After actual testing, the pitting corrosion resistance equivalent of the martensitic stainless steel with the above-mentioned ratio of elements in the embodiment of the present invention is PREN=Cr+3.3Mo+30N=17.3, which is 3.8 higher than that of conventional martensitic stainless steel 2-4Cr13 (13.5).

[0041] In some optional implementations of this embodiment, the method for preparing the martensitic stainless steel medium and thick plate includes the following steps S1 to S7:

[0042] Step S1: molten steel smelting, using K-OBM-S+VOD vacuum degassing+LF refining to smelt the raw materials into molten steel, and the percentage of chemical elements in the molten steel reaches: C: 0.21% to 0.28%; Si: 0.15% to 0.30%; Mn: 0.50% to 0.80%; P≤0.020%; S≤0.005%; Cr: 13.00% to 14.00%; Mo: 0.20% to 0.30%; Ni: 0.60% to 0.80%; Cu: 0.20%-0.25%, V: 0.10% to 0.20%, Nb: 0.03%-0.08%; N: 0.080% to 0.12%; the rest is Fe and unavoidable impurities;

[0043] In some optional implementations of this embodiment, the ratio of Ni to Cu must meet the following requirements:

[0044] Ni:Cu≥3.

[0045] Step S2: Steel ingot casting: casting the smelted molten steel into electrode billet steel ingots;

[0046] In some optional implementations of this embodiment, K-OBM-S smelting, VOD vacuum degassing, and LF refining are performed, and after the weight percentage of the molten steel reaches the percentage requirements of the above-mentioned chemical elements, the steel is tapped and cast into φ600×3000mm electrode billet ingots.

[0047] Step S3: electroslag remelting: The electroslag remelting comprises remelting the steel ingot cast into the electrode blank using a double slag system electroslag under a protective atmosphere to obtain an electroslag ingot; wherein the protective gas comprises an inert gas, and the slag system of the first electroslag remelting in the double slag system electroslag remelting comprises a quaternary slag system of CaF2-Al2O3-CaO-MgO in a ratio of 40%:25%:30%:5%, and the slag system of the second electroslag remelting comprises a quinary slag system of CaF2-Al2O3-CaO-MgO in a ratio of 40%:30%:23%:5%:2%;

[0048] In order to ensure that the prepared martensitic stainless steel meets the performance requirements, annealing heat treatment is required after the steel ingot is cast to obtain the electrode billet ingot, the first electroslag remelting and the forging billet.

[0049] In some optional implementations of this embodiment, the annealing temperature is 750°C.

[0050] In practical applications, electroslag remelting involves annealing the steel ingot at 750°C to produce an electrode blank. The surface of the electrode blank is then polished and descaled. The first electroslag remelting step is performed using a quaternary slag system composed of CaF2-Al2O3-CaO-MgO with a slag ratio of 40%:25%:30%:5%. The crystallizer is sized at φ710×2600mm. A nitrogen atmosphere is used for the electroslag remelting process, with a melting rate of 9-11kg / min. The steel after the first electroslag remelting step is primarily composed of Class D non-metallic inclusions (≤ Grade 1.0), with S ≤ 0.001%. The resulting electroslag ingot is then annealed at 750°C for 15 hours, polished, and descaled to produce an electrode blank with a diameter of φ690mm. The second electroslag remelting adopts a five-element slag system of CaF2-Al2O3-CaO-MgO-SiO2 with a ratio of 40%:30%:23%:5%:2%. The crystallizer is φ850×2600mm. The electroslag remelting process is protected by nitrogen atmosphere, and the melting rate is 11~14kg / min.

[0051] Therefore, the level of various inclusions can be reduced to level 0.5 through double slag system and double electroslag remelting.

[0052] Step S4: Forging and blanking: The electroslag ingot obtained by electroslag remelting is heated to 1250°C at 100°C / h and kept at this temperature for 5-8 hours, and then forged into a blank of preset specifications through a three-upsetting and three-drawing forging process;

[0053] In some optional implementations of this embodiment, the preset specifications of the length, width and thickness of the blank include: dimensions of 250×1300×(1700-2000) mm.

[0054] The electroslag ingots obtained by double electroslag remelting are heated to 1250°C at 100°C / h and held for 5-8 hours. They are then forged using a high-speed forging machine through a three-upsetting and three-drawing process to specifications of 250×1300×1700-2000mm. To ensure the performance of the forged martensitic stainless steel, an upsetting ratio of at least 2.0 is required. The forged billets are then annealed at 750°C.

[0055] Therefore, this process can improve the isotropy of the billet and the density of the steel.

[0056] Step S5: hot rolling of the billet: heating the billet to 1250-1270°C at a heating rate of 80°C / h, holding the billet at this temperature for 12-15 hours to achieve high-temperature diffusion and homogenization, and then widening and longitudinally rolling the billet after holding the temperature to form medium and thick plates;

[0057] Specifically, based on practical application, the billet is loaded into a trolley-type heating furnace and heated to 1250-1270°C at a heating rate of 80°C / h, where it is held for 12-15 hours for high-temperature diffusion and homogenization. The ingot is then widened and longitudinally rolled in a plate mill into medium and heavy plates measuring 20-100×1500×Lmm. To better control the rolling deformation and improve the material's microstructure and properties, a reduction of at least 15% is required in at least one pass, and the final rolling temperature is controlled at 950-1050°C.

[0058] Step S6: Cooling: The hot-rolled medium and thick plates are rapidly water-cooled to 550-600°C and then straightened. After straightening, they are air-cooled to 20-50°C.

[0059] In order to avoid the formation of network and chain carbides in the steel plate during the cooling process due to the slow cooling rate, the steel plate is rapidly cooled to 550-600℃ using the ADCOS-PM system with a cooling water flow of 4000m 3 / h, pressure 0.2MPa. After cooling, the steel plate is straightened and air-cooled to 20-50℃ to complete the martensitic transformation.

[0060] Step S7: Microstructure Refining Heat Treatment: The air-cooled medium and thick plates are placed in an online chamber furnace for low-temperature normalizing to refine the grains. The plates are heated to 970-990°C at a heating rate of 80°C / h for austenitization, with a holding time of 2 min / mm. After removal from the furnace, they are rapidly water-cooled to 550-600°C. After straightening, the steel plates (in this embodiment, medium and thick plates) are air-cooled to 80-150°C. Finally, spheroidizing annealing is performed, heating the plates to 820-840°C at a heating rate of ≤80°C / h and holding for 6-12 hours. The plates are then cooled to 740-760°C at a rate of 15-25°C / h and held for 18-30 hours. After this holding time, the plates are cooled to 400°C in the furnace and air-cooled to room temperature.

[0061] In some optional implementations of this embodiment, such as Figure 2 As shown, Figure 2 The figure is a metallographic diagram of a medium and thick plate of martensitic stainless steel after annealing using a method for preparing the medium and thick plate provided by one embodiment of the present invention.

[0062] In order to ensure the water cooling effect and the mechanical properties of martensitic stainless steel, the cooling water flow rate is 4000m 3 / h, the cooling water pressure is 0.2MPa.

[0063] In combination with actual application, the above spheroidizing annealing includes timely loading the steel plate cooled to 20-50℃ into the online chamber furnace, heating it to 970-990℃ at 80℃ / h for austenitization, holding time is 2min / mm, and then the steel plate is quickly cooled to 550-600℃ by ADCOS-PM system after being taken out of the furnace, with cooling water flow rate of 4000m3 / h pressure 0.2MPa. After straightening, the steel plate is air-cooled to 20-50℃. The air-cooled steel plate is promptly loaded into the annealing furnace for spheroidizing annealing. The steel plate is heated to 820-840℃ at a heating rate of ≤80℃ / h and kept at this temperature for 6-12 hours. Then, it is cooled to 740-760℃ at a heating rate of 15-25℃ / h and kept at this temperature for 18-30 hours. After the heat preservation, it is cooled to 400℃ and then removed from the furnace and air-cooled to room temperature.

[0064] Therefore, in the embodiment of the present invention, by adopting the above process, the homogenization of the annealing structure and the refinement of the grains can be achieved, so that the annealing hardness reaches 185-220 HB.

[0065] In summary, the annealing microstructure uniformity of the martensitic stainless steel medium and thick plates prepared by the above steps reaches the A1-A4 level of corrosion-resistant steel in GB-T35840.3-2018-Plastic Mold Steel Part 3, all types of non-metallic inclusions are ≤0.5 level, the grain size is finer than level 10, the anisotropic properties of the steel plate are stable, and the hardness of the processed mold after quenching and tempering heat treatment is 52-54HRC.

[0066] Example 2

[0067] In combination with practical applications, the method of the above embodiment 1 is used to prepare martensitic stainless steel, including:

[0068] K-OBM-S smelting + VOD vacuum degassing + LF refining;

[0069] After K-OBM-S smelting, VOD vacuum degassing and LF refining processes, the weight percentage of the molten steel reaches the requirements of Table 1 below, and the steel is tapped and cast into φ600×3000mm electrode billets.

[0070] Table 1 Parameters of composition weight percentage of molten steel after K-OBM-S smelting, VOD vacuum degassing and LF refining process

[0071]

[0072]

[0073] Protective atmosphere double slag system electroslag remelting

[0074] The steel ingot is heated to 750℃ at 120℃ / h and kept at this temperature for 15 hours. After cooling to 500℃, it is taken out of the furnace and air-cooled to room temperature. The surface of the annealed steel ingot is polished, and the oxide scale is peeled off to prepare a φ590mm electrode blank for the first electroslag remelting. The slag system adopts a quaternary slag system of CaF2-Al2O3-CaO-MgO with a ratio of 40%:25%:30%:5%. The crystallizer size is φ710×2600mm. Nitrogen gas (30N) is filled before electroslag remelting. 3 / h) for 90 minutes, exhaust the air in the crystallizer, and use nitrogen atmosphere protection during the electroslag re-dissolution process with a nitrogen flow rate of 8N 3 / h, melting rate 10kg / min. Then two electroslag remeltings were carried out, wherein the electroslag ingot obtained from the first electroslag remelting was heated to 750℃ at 120℃ / h and kept warm for 15 hours, then cooled to 500℃ in the furnace and air-cooled to room temperature. The surface of the ingot was polished, and the oxide scale was peeled off to prepare an electrode blank with a diameter of φ690mm. The second electroslag remelting used a five-element slag system of CaF2-Al2O3-CaO-MgO-SiO2 with a ratio of 40%:30%:23%:5%:2%, and the crystallizer specification was φ850×2600mm. Nitrogen was filled before electroslag remelting (30N 3 / h) for 120 minutes, exhaust the air in the crystallizer, and use nitrogen protection during the electroslag re-dissolution process with a nitrogen flow rate of 10N 3 / h, melting rate 12kg / min.

[0075] Electroslag ingot upsetting, forging, blanking + rolling

[0076] Electroslag ingot upsetting and drawing forging: The electroslag ingots obtained by double electroslag remelting are transferred to the forging workshop, heated to 1250°C at 100°C / h and held for 6 hours. They are then forged using a high-speed forging machine through three upsetting and drawing cycles to form billets with a size of 250×1300×1700-2000mm, with an upsetting ratio of 2.0. The forged billets are heated to 750°C at 120°C / h, held for 8 hours, furnace-cooled to 500°C, and then air-cooled to room temperature.

[0077] Billet Heating and Rolling: The billet is placed in a trolley-type heating furnace and heated to 1265°C at a rate of 80°C / h, where it is held for 15 hours for high-temperature diffusion and homogenization. The ingots are then widened and longitudinally rolled in a plate mill to produce 37, 72, and 100×1500×Lmm plates. The rolling process parameters for the 37, 72, and 100×1500×Lmm plates are shown in Table 2.

[0078] Table 2 Rolling process parameters

[0079] Specifications (mm) Finish rolling temperature ℃ Maximum reduction rate per single pass (%) 37×1500×L 1006 18.37 72×1500×L 1013 22.03 97×1500×L 1027 15.43

[0080] The steel plate structure is refined into heat treatment

[0081] The rolled steel plate is rapidly cooled to 500-600℃ using the ADCOS-PM system, with a cooling water flow of 4000m 3 / h, pressure 0.2MPa. After straightening, the cooled steel plate is air-cooled to 20-50℃ to complete the martensitic transformation. Among them, the cooling parameters for cooling martensitic stainless steel medium and thick plates of different specifications are given.

[0082] Table 3 Cooling parameters for cooling martensitic stainless steel medium and thick plates of different specifications

[0083]

[0084]

[0085] The air-cooled steel plate is placed in an online chamber furnace and heated to 990℃ for austenitization at 80℃ / h. The holding time is 2min / mm. Then, it is taken out of the furnace and rapidly cooled to 550-600℃ using the ADCOS-PM system. The cooling water flow rate is set to 4000 3 / h pressure 0.2MPa. After straightening, the steel plate is air-cooled to 80-150℃ and then placed in a furnace for spheroidizing annealing. The medium and thick plates are heated to 835℃ at 30℃ / h and held at that temperature for 8 hours, then cooled to 750℃ at 18℃ / h and held at that temperature for 25 hours. Finally, the plates are furnace-cooled to 400℃ and air-cooled to room temperature. The different normalizing heat treatment parameters used for different specifications of martensitic stainless steel are shown in Table 4 below.

[0086] Table 4 Normalizing heat treatment parameters of martensitic stainless steel of different specifications

[0087] Specifications (mm) Normalizing temperature ℃ Holding time Water cooling temperature ℃ Air cooling temperature ℃ 37×1500×L 993 74 517 83 72×1500×L 993 144 524 97 97×1500×L 993 194 589 126

[0088] The corrosion-resistant die steel plate prepared by the above steps, non-metallic inclusions, annealing structure and hardness, grain size (the sample was kept at 1030°C for 30 minutes and then air-cooled to room temperature), and heat treatment hardness (the sample was kept at 1030°C for 30 minutes, oil quenched, and tempered twice at 180°C, each time for 2 hours) are shown in the following Tables 5 and 6, where Table 5 is a table of non-metallic inclusion parameters in the martensitic stainless steel medium and thick plate prepared by the method of Example 1 of the present invention, and Table 6 is a table of performance parameters of the martensitic stainless steel medium and thick plate prepared by the method of Example 1 of the present invention.

[0089] Table 5 Parameters of non-metallic inclusions in martensitic stainless steel medium and thick plates prepared by the method of Example 1 of the present invention

[0090]

[0091] Table 6 Performance parameters of martensitic stainless steel medium and thick plates prepared by the method of Example 1 of the present invention

[0092]

[0093] In summary, the martensitic stainless steel medium and thick plates in the embodiments of the present invention undergo a process of K-OBM-S smelting + VOD vacuum degassing + LF refining → casting steel ingots → protective atmosphere double slag system double electroslag remelting → forging and blanking → rolling → steel plate refinement heat treatment, and have excellent characteristics of good corrosion resistance, high heat treatment hardness, uniform structure, fine grains and stable anisotropic properties.

[0094] In a second aspect, an embodiment of the present invention further provides a martensitic stainless steel comprising the following chemical elements in the following percentages:

[0095] C: 0.21%~0.28%; Si: 0.15%~0.30%; Mn: 0.50%~0.80%; P≤0.020%; S≤0.005%;

[0096] Cr: 13.00%~14.00%; Mo: 0.20%~0.30%; Ni: 0.60%~0.80%; Cu: 0.20%-0.25%, V: 0.10%~0.20%, Nb: 0.03%-0.08%; N: 0.080%~0.12%; the rest are Fe and unavoidable impurities.

[0097] Through the above-mentioned percentage ratio of chemical elements, martensitic stainless steel has excellent characteristics of good corrosion resistance, high hardness during heat treatment, uniform structure, fine grains and stable anisotropic properties.

[0098] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In addition, "front", "back", "left", "right", "upper" and "lower" in this document are all referenced to the placement states shown in the accompanying drawings.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for preparing a medium and thick martensitic stainless steel plate, characterized in that: Contains the following chemical elements in the following percentages: C: 0.21% to 0.28%; Si: 0.15% to 0.30%; Mn: 0.50% to 0.80%; P ≤ 0.020%; S ≤ 0.005%; Cr: 13.00% to 14.00%; Mo: 0.20% to 0.30%; Ni: 0.60% to 0.80%; Cu: 0.20% to 0.25%, V: 0.10% to 0.20%, Nb: 0.03% to 0.08%; N: 0.080% to 0.12%; the rest is Fe and unavoidable impurities; The preparation method comprises the following steps: Molten steel smelting: K-OBM-S+VOD vacuum degassing+LF refining is used to smelt raw materials into molten steel, and the percentage of chemical elements in the molten steel reaches: C: 0.21%-0.28%; Si: 0.15%-0.30%; Mn: 0.50%-0.80%; P≤0.020%; S≤0.005%; Cr: 13.00%-14.00%; Mo: 0.20%-0.30%; Ni: 0.60%-0.80%; Cu: 0.20%-0.25%, V: 0.10%-0.20%, Nb: 0.03%-0.08%; N: 0.080%-0.12%; the rest is Fe and unavoidable impurities; Ingot casting: casting the smelted molten steel into electrode billet ingots; Electroslag remelting: The electroslag remelting comprises remelting the steel ingot cast into the electrode blank using a double slag system electroslag under a protective atmosphere to obtain an electroslag ingot; wherein the protective gas comprises an inert gas, and the slag system of the first electroslag remelting in the double slag system electroslag remelting comprises a quaternary slag system of CaF2-Al2O3-CaO-MgO in a ratio of 40%:25%:30%:5%, and the slag system of the second electroslag remelting comprises a quinary slag system of CaF2-Al2O3-CaO-MgO-SiO2 in a ratio of 40%:30%:23%:5%:2%; Forging and blanking: The electroslag ingot obtained by electroslag remelting is heated to 1250℃ at 100℃ / h and kept at this temperature for 5-8 hours, and then forged into a blank of preset specifications through a three-upsetting and three-drawing forging process; Hot rolling of billets: The billets are heated to 1250-1270℃ at a heating rate of 80℃ / h, kept at this temperature for 12-15 hours to achieve high temperature diffusion and homogenization, and the billets after being kept at this temperature are widened and longitudinally rolled into medium and thick plates; Cooling: The hot-rolled medium and thick plates are quickly water-cooled to 550-600℃ and then straightened. After straightening, they are air-cooled to 20-50℃; Microstructure refinement heat treatment: The medium and thick plates after air cooling are subjected to low-temperature normalizing to refine the grains, and are heated to 970-990℃ for austenitization at a heating rate of 80℃ / h. The holding time is calculated as 2min / mm, and then quickly water-cooled to 550-600℃ and straightened. After straightening, the medium and thick plates are air-cooled to 80-150℃, and finally spheroidizing annealing is performed. The medium and thick plates are heated to 820-840℃ at a heating rate of ≤80℃ / h and kept warm for 6-12 hours, and then cooled to 740-760℃ at 15-25℃ / h and kept warm for 18-30 hours. After holding, the furnace is cooled to 400℃ and then taken out of the furnace and air-cooled to room temperature.

2. The method for preparing a medium and thick martensitic stainless steel plate according to claim 1, characterized in that: In molten steel smelting, the ratio of Ni and Cu elements must meet the following requirements: Ni:Cu≥3.

3. The method for preparing a medium and thick martensitic stainless steel plate according to claim 1, characterized in that: Annealing is required after the steel ingot is cast to obtain the electrode billet ingot, the first electroslag remelting and the forging blank, and the annealing temperature includes 750°C.

4. The method for preparing a medium and thick martensitic stainless steel plate according to claim 3, characterized in that: Electroslag remelting includes: The electrode blank steel ingot is annealed at 750° C. to prepare an electrode blank. After the oxide scale is stripped, a quaternary slag system of CaF2-Al2O3-CaO-MgO in a ratio of 40%:25%:30%:5% is used for the first electroslag remelting. A nitrogen atmosphere is used for protection during the electroslag remelting process, and the melting rate is 9-11 kg / min. The electroslag ingot obtained by the electroslag remelting is annealed at 750° C. for 15 hours to prepare an electrode blank with a diameter of φ690 mm. Subsequently, a quinary slag system of CaF2-Al2O3-CaO-MgO-SiO2 in a ratio of 40%:30%:23%:5% is used for the second electroslag remelting. A nitrogen atmosphere is used for protection during the electroslag remelting process, and the melting rate is 11-14 kg / min.

5. The method for preparing a medium and thick martensitic stainless steel plate according to claim 1, characterized in that: During hot rolling of the billet, it is required that the reduction ratio of at least one pass is greater than or equal to 15%.

6. The method for preparing a medium and thick martensitic stainless steel plate according to claim 1, characterized in that: The cooling water flow rate is 4000m 3 / h, the cooling water pressure is 0.2MPa.

7. The method for preparing a medium and thick martensitic stainless steel plate according to claim 1, characterized in that: The preset specifications of the blank in terms of length, width and thickness include: 250×1300×(1700~2000)mm.

8. The method for preparing a medium and thick martensitic stainless steel plate according to claim 7, characterized in that: The upset ratio in forging blanking is required to be greater than or equal to 2.

0.

9. A martensitic stainless steel, characterized in that: The martensitic stainless steel contains the following chemical elements in the following percentages: C: 0.21%~0.28%; Si: 0.15%~0.30%; Mn: 0.50%~0.80%; P≤0.020%; S≤0.005%; Cr: 13.00% ~ 14.00%; Mo: 0.20% ~ 0.30%; Ni: 0.60% ~ 0.80%; Cu: 0.20% - 0.25%, V: 0.10% ~ 0.20%, Nb: 0.03% - 0.08%; N: 0.080% ~ 0.12%; The rest is Fe and inevitable impurities.

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