Control method for microscopic structure of large-size blooming blank of high-rare-earth ferritic stainless steel

Abstract

The invention discloses a control method for a microscopic structure of a large-size blooming blank of high-rare-earth ferritic stainless steel. The control method comprises the following steps: designing a die casting process, wherein the superheat degree of molten steel is 45-60 DEG C, the casting speed of an ingot body is 180-220 mm / min, the casting time of a cap opening is more than or equal to 6 minutes, and the complete solidification time of cap opening molten steel is more than or equal to 60 minutes; a cogging process is designed, specifically, a steel ingot is put into a furnace to be heated to 1150-1200 DEG C, cogging is conducted after heat preservation is conducted for 2-3 h, cogging is conducted at the first stage, the single-pass deformation is 5% or below, and rolling is conducted for 6-8 passes; in the second stage, cogging is conducted, the single-pass deformation is 10% or above, rolling is conducted for 4-6 passes, and the temperature is controlled to be 850 DEG C or above; in the third stage, cogging is conducted, the target size is obtained through small deformation rolling, and the temperature of a blooming blank is controlled to be 750-800 DEG C; and the annealing process is designed as follows: the annealing temperature is 600-650 DEG C, heat preservation is performed for 2-4 h, furnace cooling is performed to below 150 DEG C, and discharging and air cooling are performed. The microstructure of the high-rare-earth ferritic stainless steel large-size blooming blank manufactured through the method is isometric crystals, the grain size is larger than or equal to 00 level, the center porosity is smaller than or equal to 0.5 level, the surface and the edge are free of cracks, and large-scale industrial production is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of steel manufacturing, and in particular relates to a method for controlling the microstructure of a large-size initial rolling billet of high-rare-earth ferrite stainless steel. Background Art

[0002] Ferritic stainless steel has been increasingly used in high-temperature fields such as solid oxide battery systems, automobile exhaust systems, exhaust gas recirculation systems, and electric heating alloys due to its high thermal conductivity, low thermal expansion coefficient, and good oxidation resistance. Rare earth elements have the functions of strengthening grain boundaries and changing high-temperature oxidation mechanisms. Adding them to steel can significantly improve the high-temperature service performance of ferritic stainless steel.

[0003] However, due to the very strong deoxidation and desulfurization capabilities of rare earths, during the smelting process, the rare earth oxide particles in the molten steel are difficult to float and remove, and are easily adhered to the inner wall of the nozzle, causing serious nozzle blockage, resulting in poor castability of the steel and making continuous casting impossible.

[0004] If the die casting process is adopted, especially for large-sized ingots, the following process difficulties are faced: ① The columnar crystals in the cast structure of ferritic stainless steel are well developed, which makes it difficult to open the steel ingot, easily produces surface and edge cracks, and has a high level of central looseness; ② The initial rolled billet of ferritic stainless steel has coarse grains, and hot charging and red delivery are required during the production process. The production rhythm is very tight, which affects the overall production efficiency. If it cannot be delivered in time, production accidents such as transverse cracks may also occur.

[0005] Therefore, it is necessary to develop a control method for the microstructure of large-size initial rolled billets of high rare earth ferritic stainless steel to ensure that the microstructure of the initial rolled billets meets the requirements of grain size and central porosity level, and there are no cracks on the surface and edges of the initial rolled billets, thereby breaking the limitations of hot delivery and hot loading of traditional ferritic stainless steel initial rolled billets and realizing large-scale industrial production of large-size high rare earth ferritic stainless steel products. Summary of the invention

[0006] In order to solve the above technical problems existing in the prior art, the present invention provides a method for controlling the microstructure of a large-size bloom of high-rare-earth ferrite stainless steel, comprising:

[0007] (1) Die casting process design: After the target composition is reached through K-OBM-S+VOD+LF smelting, die casting is carried out. During the die casting process, the superheat of the molten steel is controlled at 45-60°C, argon protection is used throughout the pouring process, the average pouring speed of the ingot body is controlled at 180-220 mm / min, the cap mouth shrinkage feeding pouring time is controlled at ≥6 min, and the cap mouth molten steel complete solidification time is controlled at ≥60 min, thereby producing a large-sized steel ingot;

[0008] (2) Billeting process design: The steel ingot is demoulded at a temperature above 700°C, heated to 1150-1200°C in a furnace, and kept at this temperature for 2-3 hours before billeting. The billeting process is divided into three stages: In the first stage, the deformation of a single pass is controlled below 5%, and 6-8 passes are rolled; In the second stage, the deformation of a single pass is controlled above 10%, and 4-6 passes are rolled. During the billeting process in the second stage, the temperature is controlled above 850°C; In the third stage, the billet is rolled with a small deformation amount to achieve the target size and flatness. After billeting is completed, the temperature of the billet is controlled at 750-800°C;

[0009] (3) Annealing process design: The initial rolled billet is annealed at a temperature of 600-650°C. After being kept at this temperature for 2-4 hours, it is cooled to below 150°C in the furnace and then air-cooled.

[0010] Furthermore, in the above-mentioned method for controlling the microstructure of large-size initial rolled billet of high rare earth ferrite stainless steel, the weight of the die-cast steel ingot is 5.8 to 9.6 tons, and the size specification of the initial rolled billet is (1000 to 1100) mm×(120 to 150) mm×(6200 to 8000) mm.

[0011] Furthermore, in the above-mentioned method for controlling the microstructure of large-size initial rolled slab of high rare earth ferritic stainless steel, the high rare earth ferritic stainless steel includes, by mass percentage: C≤0.030%, N≤0.030%, Si: 0.10-0.80%, Mn: 0.20-0.60%, Cr: 16.0-26.0%, Nb: 0.10-0.60%, W: 1.0-4.0%, and a total amount of 0.05-0.20% of one or more of the rare earth elements La, Ce, Y and Hf, and the rest is Fe and unavoidable impurities.

[0012] Furthermore, in the above-mentioned method for controlling the microstructure of large-size rough rolled slab of high rare earth ferrite stainless steel, the microstructure of the rough rolled slab is equiaxed crystal, the grain size is ≥00 level, the center porosity is ≤0.5 level, and there are no cracks on the surface and edges.

[0013] The control method of the microstructure of the large-size rough rolled billet of high rare earth ferritic stainless steel of the present invention has the following advantages and beneficial effects: the microstructure of the large-size rough rolled billet of high rare earth ferritic stainless steel produced by the present invention is equiaxed crystal, grain size ≥00 grade, center porosity ≤0.5 grade, and no cracks on the surface and edge, breaking the limitation of hot delivery and hot loading of the traditional ferritic stainless steel rough rolled billet, thereby easing the production rhythm, improving production efficiency, and realizing the large-scale industrial production of large-size products of high rare earth ferritic stainless steel. DETAILED DESCRIPTION

[0014] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention. 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 work are within the scope of protection of the present invention.

[0015] The method for controlling the microstructure of a large-size bloom of high-rare-earth ferrite stainless steel of the present invention comprises:

[0016] (1) Mold casting process design: After the target composition is achieved through K-OBM-S+VOD+LF smelting, mold casting is carried out. During the mold casting process, the superheat of the molten steel is controlled at 45-60°C, argon protection is used throughout the pouring process, the average pouring speed of the ingot body is controlled at 180-220 mm / min, the cap mouth shrinkage compensation pouring time is controlled at ≥6 min, and the cap mouth steel liquid complete solidification time is controlled at ≥60 min, thereby producing a large-sized steel ingot.

[0017] Through the above-mentioned die casting process design, the as-cast structure and internal quality of the steel ingot can be effectively optimized. Since rare earth elements are very easy to oxidize, low-temperature fast pouring is adopted for pouring. Reducing the superheat and increasing the pouring speed can effectively control the growth of columnar crystals during the solidification of the steel ingot, which is conducive to the growth of equiaxed crystals and reduces metal oxidation; extending the cap-mouth shrinkage feeding pouring time and the solidification time can reduce the central segregation, looseness, shrinkage cavity and other low-magnification defects of the steel ingot, thereby improving the internal strength of the steel ingot.

[0018] (2) Billeting process design: The steel ingot is demoulded at a temperature above 700℃, heated to 1150-1200℃ in the furnace, and kept warm for 2-3 hours before billeting. The billeting process is divided into three stages: in the first stage, the deformation of a single pass is controlled below 5%, 6-8 passes are rolled, the steel is turned over and the above process is repeated; in the second stage, the deformation of a single pass is controlled above 10%, 4-6 passes are rolled, the steel is turned over and the above process is repeated. During the second stage of billeting, the temperature must be controlled above 850℃; in the third stage, the billet is rolled with a small deformation to achieve the target size and flatness. After billeting, the temperature of the billet is controlled at 750-800℃.

[0019] Through the above-mentioned blanking process design, the microstructure of the initial rolled billet can be effectively controlled. The columnar crystals are well developed in the cast structure of the steel ingot. In the first stage of blanking, it is necessary to first perform multiple small deformation rolling to break the columnar crystals; in the second stage of blanking, large deformation rolling is adopted and the number of rolling passes is reduced, so that the deformation can fully penetrate into the center, the steel ingot is fully deformed as a whole, the microstructure is regulated, and the temperature needs to be controlled above 850℃, on the one hand, so that the steel ingot can be recrystallized, and on the other hand, the precipitation of harmful second phases is prevented; in the third stage of blanking, small deformation rolling is adopted to make the initial rolled billet reach the target size and flatness, and provide qualified raw materials for the subsequent process.

[0020] (3) Annealing process design: After the steel ingot is formed, the initial rolled billet is annealed at a temperature of 600-650°C. After being kept at this temperature for 2-4 hours, it is cooled to below 150°C and then air-cooled.

[0021] Through the above-mentioned annealing process design, the internal stress of the initial rolling billet can be effectively eliminated. In the existing conventional process route, the initial rolling billet of ferritic stainless steel is easy to crack, and it is sent to the next process for production by hot charging and hot delivery, and the production rhythm is very tight. On the basis of regulating the microstructure of the initial rolling billet, the present invention eliminates the internal stress in the initial rolling billet through the process design of "annealing + furnace cooling", breaking the limitation of traditional hot delivery and hot charging of the initial rolling billet of ferritic stainless steel.

[0022] As a specific implementation method, in the above-mentioned method for controlling the microstructure of large-size initial rolled billet of high rare earth ferrite stainless steel, the weight of the die-cast steel ingot is 5.8 to 9.6 tons, and the size specification of the initial rolled billet is (1000 to 1100) mm×(120 to 150) mm×(6200 to 8000) mm.

[0023] As a specific implementation method, in the above-mentioned high rare earth ferritic stainless steel large-size bloom microstructure control method, the high rare earth ferritic stainless steel includes, by mass percentage: C≤0.030%, N≤0.030%, Si: 0.10-0.80%, Mn: 0.20-0.60%, Cr: 16.0-26.0%, Nb: 0.10-0.60%, W: 1.0-4.0%, and a total amount of 0.05-0.20% of one or more of the rare earth elements La, Ce, Y and Hf, and the rest is Fe and unavoidable impurities.

[0024] As a specific implementation method, in the above-mentioned method for controlling the microstructure of large-size rough rolled billets of high rare earth ferrite stainless steel, the microstructure of the rough rolled billets obtained is equiaxed crystals, with a grain size ≥00, a central porosity ≤0.5, and no cracks on the surface and edges.

[0025] The method for controlling the microstructure of a large-size bloom of high-rare-earth ferrite stainless steel of the present invention is described below in conjunction with specific embodiments.

[0026] Example 1

[0027] In Example 1 of the present invention, the chemical composition of the molten steel to be poured, except for iron and inevitable impurity elements, is as follows in terms of mass percentage:

[0028] C: 0.015%, N: 0.020%, Si: 0.24%, Mn: 0.25%, Cr: 22.11%, Nb: 0.49%, W: 1.8%, La: 0.04%, Ce: 0.03%.

[0029] The specific implementation process of the method for controlling the microstructure of the large-size bloom of high rare earth ferrite stainless steel in Example 1 is as follows:

[0030] (1) Die casting process: the superheat of molten steel is 50°C, the average pouring speed of the ingot body is 200 mm / min, the cap-mouth shrinkage feeding pouring time is 7 min, the cap-mouth molten steel complete solidification time is 110 min, and the weight of the obtained die-cast steel ingot is 7.2 tons;

[0031] (2) Billeting process: The steel ingot is demoulded at 720℃, heated to 1200℃ in the furnace, and kept warm for 3 hours before billeting. In the first stage of billeting, the deformation of a single pass is controlled below 5%, 8 passes are rolled, the steel is turned over and the above process is repeated; in the second stage of billeting, the deformation of a single pass is controlled above 10%, 6 passes are rolled, the steel is turned over and the above process is repeated. After the second stage of billeting is completed, the surface temperature is 860℃; in the third stage of billeting, through rolling with a small deformation, the initial billet reaches the target size and flatness, and the size of the initial billet obtained is 1050×125×6800mm. After the billeting is completed, the surface temperature of the initial billet is 780℃;

[0032] (3) Annealing process: The initial rolled billet is kept at 650℃ for 2.5h, then cooled to 120℃ and air-cooled.

[0033] The large-size rough rolled billet of high rare earth ferrite stainless steel produced in Example 1 was subjected to low-power and metallographic examination. The results showed that the microstructure of the rough rolled billet was equiaxed crystals, the grain size was level 0, the center porosity was level 0.5, and there were no cracks on the surface and edges.

[0034] Example 2

[0035] In Example 2 of the present invention, the chemical composition of the molten steel to be poured, excluding iron and inevitable impurity elements, is as follows by mass percentage:

[0036] C: 0.012%, N: 0.015%, Si: 0.37%, Mn: 0.20%, Cr: 18.43%, Nb: 0.55%, W: 2.1%, La: 0.06%, Ce: 0.02%.

[0037] The specific implementation process of the method for controlling the microstructure of large-size bloom of high rare earth ferrite stainless steel in Example 2 is as follows:

[0038] (1) Die casting process: the superheat of molten steel is 58°C, the average pouring speed of the ingot body is 185 mm / min, the cap-mouth shrinkage feeding pouring time is 10 min, the cap-mouth molten steel complete solidification time is 150 min, and the weight of the obtained die-cast steel ingot is 9.6 tons;

[0039] (2) Billeting process: The steel ingot is demoulded at 700℃, heated to 1180℃ in the furnace, and kept warm for 3 hours before billeting. In the first stage of billeting, the deformation of a single pass is controlled below 5%, 8 passes are rolled, the steel is turned over and the above process is repeated; in the second stage of billeting, the deformation of a single pass is controlled above 10%, 6 passes are rolled, the steel is turned over and the above process is repeated. After the second stage of billeting is completed, the surface temperature is 900℃; in the third stage of billeting, through rolling with a small deformation amount, the initial billet reaches the target size and flatness, and the size of the initial billet obtained is 1100×150×7250mm. After the billeting is completed, the surface temperature of the initial billet is 800℃;

[0040] (3) Annealing process: The initial rolled billet is kept at 600℃ for 3h, then cooled to 150℃ and air-cooled.

[0041] The large-size rough-rolled billet of high rare earth ferrite stainless steel produced in Example 2 was subjected to low-power and metallographic examination. The results showed that the microstructure of the rough-rolled billet was equiaxed crystals, the grain size was level 0, the center porosity was level 0.5, and there were no cracks on the surface and edges.

[0042] Example 3

[0043] In Example 3 of the present invention, the chemical composition of the molten steel to be poured, except for iron and inevitable impurity elements, is as follows by mass percentage:

[0044] C: 0.020%, N: 0.022%, Si: 0.25%, Mn: 0.40%, Cr: 25.06%, Nb: 0.25%, W: 1.3%, La: 0.06%.

[0045] The specific implementation process of the method for controlling the microstructure of large-size bloom of high rare earth ferrite stainless steel in Example 3 is as follows:

[0046] (1) Die casting process: the superheat of molten steel is 55°C, the average pouring speed of the ingot body is 210 mm / min, the cap-mouth shrinkage feeding pouring time is 6 min, the cap-mouth molten steel complete solidification time is 90 min, and the weight of the obtained die-cast steel ingot is 5.8 tons;

[0047] (2) Billeting process: The steel ingot is demoulded at 750℃, heated to 1200℃ in the furnace, and kept warm for 2.5h before billeting. In the first stage of billeting, the deformation of a single pass is controlled below 5%, 6 passes are rolled, the steel is turned over and the above process is repeated; in the second stage of billeting, the deformation of a single pass is controlled above 10%, 4 passes are rolled, the steel is turned over and the above process is repeated. After the second stage of billeting is completed, the surface temperature is 920℃; in the third stage of billeting, through rolling with a small deformation, the initial billet reaches the target size and flatness, and the size of the initial billet obtained is 1000×120×6200mm. After the billeting is completed, the surface temperature of the initial billet is 790℃;

[0048] (3) Annealing process: The initial rolled billet is kept at 650℃ for 2h, then cooled to 150℃ and air-cooled.

[0049] The large-size rough-rolled billet of high rare earth ferrite stainless steel produced in Example 3 was subjected to low-power and metallographic examination. The results showed that the microstructure of the rough-rolled billet was equiaxed crystals, the grain size was 0.5, the center porosity was 0.5, and there were no cracks on the surface and edges.

[0050] In summary, compared with the prior art, the method for controlling the microstructure of large-size bloom of high rare earth ferrite stainless steel of the present invention has the following advantages and beneficial effects:

[0051] The present invention combines the design and control of the three process steps of "die casting + blanking + annealing" to produce large-size rough-rolled billets of high rare earth ferritic stainless steel with equiaxed crystal microstructure, grain size ≥00, center porosity ≤0.5, and no cracks on the surface and edges, thus breaking the limitation of hot-rolled billets of traditional ferritic stainless steel, thereby easing the production rhythm, improving production efficiency, and realizing large-scale industrial production of large-size products of high rare earth ferritic stainless steel.

[0052] It should be noted that, in this article, the term "comprise" or any other variation thereof is intended to cover non-exclusive inclusion, so that an article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such article or device.

[0053] It should also 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 cause the essence of the corresponding technical solutions to deviate from the scope of the present invention.

Claims

1. A method for controlling the microstructure of a large-size bloom of high-rare-earth ferrite stainless steel, characterized in that: include: (1) Die casting process design: After the target composition is reached through K-OBM-S+VOD+LF smelting, die casting is carried out. During the die casting process, the superheat of the molten steel is controlled at 45-60°C, argon protection is used throughout the pouring process, the average pouring speed of the ingot body is controlled at 180-220 mm / min, the cap mouth shrinkage feeding pouring time is controlled at ≥6 min, and the cap mouth molten steel complete solidification time is controlled at ≥60 min, thereby producing a large-sized steel ingot; (2) Billeting process design: The steel ingot is demoulded at a temperature above 700°C, heated to 1150-1200°C in a furnace, and kept at this temperature for 2-3 hours before billeting. The billeting process is divided into three stages: In the first stage, the deformation of a single pass is controlled below 5%, and 6-8 passes are rolled; In the second stage, the deformation of a single pass is controlled above 10%, and 4-6 passes are rolled. During the billeting process in the second stage, the temperature is controlled above 850°C; In the third stage, the billet is rolled with a small deformation amount to achieve the target size and flatness. After billeting is completed, the temperature of the billet is controlled at 750-800°C; (3) Annealing process design: The initial rolled billet is annealed at a temperature of 600-650°C. After being kept at this temperature for 2-4 hours, it is cooled to below 150°C in the furnace and then air-cooled.

2. The method for controlling the microstructure of a large-size bloom of high rare earth ferrite stainless steel according to claim 1, characterized in that: The weight of the die-cast steel ingot is 5.8 to 9.6 tons, and the size of the initial rolled billet is (1000 to 1100) mm × (120 to 150) mm × (6200 to 8000) mm.

3. The method for controlling the microstructure of a large-size bloom of high rare earth ferrite stainless steel according to claim 1, characterized in that: The high rare earth ferritic stainless steel comprises, by mass percentage, C≤0.030%, N≤0.030%, Si: 0.10-0.80%, Mn: 0.20-0.60%, Cr: 16.0-26.0%, Nb: 0.10-0.60%, W: 1.0-4.0%, and one or more of the rare earth elements La, Ce, Y and Hf with a total amount of 0.05-0.20%, and the rest is Fe and unavoidable impurities.

4. The method for controlling the microstructure of a large-size bloom of high rare earth ferrite stainless steel according to claim 1, characterized in that: The microstructure of the initial rolled billet is equiaxed crystal, the grain size is ≥00, the center is loose ≤0.5, and there is no crack on the surface and edge.