Forming method of titanium alloying high-boron stainless steel plate blank

Abstract

The invention discloses a forming method of a titanium alloying high-boron stainless steel slab, which comprises the following steps: (1) melting molten steel through vacuum induction according to set components, and pouring to obtain an induction cast ingot; the alloy comprises the following components in percentage by mass: 0.2-2.5% of B, 0.5-5.0% of Ti, 17.0-21.0% of Cr, 10.0-16.0% of Ni, 0.5-2.5% of Mn, 0.1-1.5% of Si, less than or equal to 0.07% of C, less than or equal to 0.04% of P, less than or equal to 0.03% of S, less than or equal to 0.1% of N and other inevitable impurities, and the balance of Fe. (2) electroslag remelting is conducted on the induction cast ingot, an electroslag ingot is obtained, after diffusion annealing is conducted on the electroslag ingot at the temperature of 1100-1220 DEG C, hot forging cogging is conducted, and a thick plate intermediate billet is obtained; and (3) the thick plate intermediate blank is subjected to hot rolling and solution treatment with the single-pass rolling reduction being 3%-18%, and the titanium alloying high-boron stainless steel plate blank is obtained. In order to solve the problems that the high-boron stainless steel plate blank is small in specification, difficult to machine and limited in industrialization, the alloy component design is optimized, the forming process of titanium alloying, electroslag, hot forging and hot rolling is provided, and an effective technical route is provided for industrial production of the large-size and thick-specification plate blank.

Description

Technical Field

[0001] The invention belongs to the technical field of steel alloy materials, and in particular relates to a forming method for a titanium alloyed high-boron stainless steel slab. Background Art

[0002] High-boron stainless steel slabs are important thermal neutron absorption and gamma-ray shielding materials in the fields of nuclear pressure water reactors and fusion reactors. Compared with other neutron absorption materials, it has excellent mechanical properties and corrosion resistance, and can be stably immersed in the highly corrosive spent fuel storage pool for a long time. Studies have shown that the higher the boron content in stainless steel, the higher its thermal neutron capture ability, that is, the better the thermal neutron absorption performance. At the same time, as the boron content increases, the slab thickness can be thinner and the use environment can be more severe, that is, the application range is wider. However, it cannot be ignored that due to the extremely low solid solubility of boron in steel, excess boron atoms usually combine with metal atoms to form new phases, thereby changing the microstructure characteristics, mechanical properties and processability of stainless steel. When the boron content in stainless steel is greater than 0.1wt.%, a large amount of eutectic boride (Fe,Cr)2B with high hardness, high brittleness and difficulty in deformation will precipitate in the matrix structure, which seriously deteriorates the thermoplasticity of the material and brings great challenges to the hot processing process, especially for large-size and thick-gauge high-boron stainless steel slabs mainly formed by hot deformation.

[0003] Patents ZL201910460658.3, ZL201910773871.X, etc. disclose a method for preparing high-plasticity and high-boron austenitic stainless steel sheets by titanium alloying. A high-plasticity and high-boron austenitic stainless steel sheet is prepared by adding a certain Ti element to the high-boron austenitic stainless steel and combining it with a thin strip continuous casting process. The basic idea is to introduce submicron TiB2 particles and combine them with a sub-rapid solidification process to refine, spheroidize, and reduce (Fe, Cr)2B borides, thereby improving the plasticity and toughness of the material. This has also been proven by related basic research, such as Wang ZJ et al. Materials Science and Engineering A, 2021, 811: 141067. However, since the high-boron stainless steel prepared by this process is small in size and thin in slab, and the thin strip continuous casting process has not been widely promoted on this type of material, its industrial application is very limited. In addition, since borides are mainly formed during the solidification stage of molten steel, controlling the smelting alloy composition, smelting process and the quality of the ingot is also a very important means to improve the hot workability of high-boron stainless steel slabs, such as introducing the electroslag remelting process (patent CN115921577A). However, such reports and applications are still relatively rare, especially in the application of high-boron stainless steel hot-rolled slabs.

[0004] Based on the above research background, the present invention provides a forming method for titanium alloyed high-boron stainless steel slabs. Aiming at the problems of small specifications, difficult processing and limited industrialization of high-boron stainless steel slabs, the alloy composition design and forming process route are optimized, and Ti borides (TiB2) with smaller size and hardness are introduced by titanium alloying, thereby reducing the proportion of Cr borides with larger size and higher hardness; at the same time, vacuum induction melting and electroslag remelting processes are used to reduce element segregation and solidification defects (inclusions, harmful elements, shrinkage, etc.) during solidification, greatly improving the quality of the ingot; finally, the hot forging, hot rolling and solution treatment processes, especially the heating temperature and deformation amount, are finely regulated to provide an effective process route for the forming of high-boron stainless steel slabs. Summary of the invention

[0005] The purpose of the present invention is to provide a forming method for a titanium alloyed high-boron stainless steel slab in view of the above problems.

[0006] The object of the present invention is achieved in this way: a forming method of a titanium alloyed high-boron stainless steel slab, comprising the following steps: step 1: smelting titanium alloyed high-boron stainless steel molten steel in a vacuum induction furnace according to a set composition, and pouring to obtain an induction ingot; the set composition is as follows by mass percentage: B: 0.2-2.5%, Ti: 0.5-5.0%, Cr: 17.0-21.0%, Ni: 10.0-16.0%, Mn: 0.5-2.5%, Si: 0.1-1.5%, C≤0.07%, P≤0.04%, S≤0.03%, N and other unavoidable impurities≤0.1%, and the rest is Fe; step 2: removing the head and tail of the induction ingot with casting defects, performing electroslag remelting in an argon protective atmosphere, and demolding after natural cooling in the air to obtain an electroslag ingot; step 3: heating the electroslag ingot at 1100 ~1220℃ diffusion annealing t1=(1.0~6.0)h1; where h1 is the thickness of the electroslag ingot, unit mm, t1 is min; Step 4: The electroslag ingot after heat treatment is taken out of the furnace for hot forging and blanking, wherein the single-pass reduction does not exceed 20%, and after forging, it is returned to the furnace and kept at 1100~1220℃ for 120~240min to obtain a thick plate intermediate billet; Step 5: The thick plate intermediate billet is hot rolled, wherein the single-pass reduction is between 3%~18%, and the total reduction is between 50%~95%, and air-cooled to room temperature after rolling to obtain a hot-rolled plate; Step 6: After the hot-rolled plate is solution treated at 1030~1150℃ t2=(0.5~4.0)h2, it is taken out of the furnace and water-cooled to room temperature to obtain a titanium alloyed high-boron stainless steel slab; where h2 is the thickness of the thick plate intermediate billet, unit mm, t2 is min; In the above step 1, the raw materials used for smelting should be free of oxidized impurities before use, electromagnetic stirring can be applied during the smelting process, and argon gas protection can be added after alloying Mn and Ti consumable elements.

[0007] In the above step 2, the ingot starter plate and arc starter chips with the same composition as the above high-boron stainless steel should be used for electroslag remelting, and no oil or water stains are allowed on the surface.

[0008] In the above step 4, the surface temperature of the electroslag ingot is not less than 900°C during hot forging after being taken out of the furnace, otherwise it is returned to the furnace and reheated for 30 to 150 minutes before forging.

[0009] In the above step 5, the surface temperature of the intermediate billet is not less than 920°C during hot rolling, otherwise it is returned to the furnace and reheated for 30 to 150 minutes before hot rolling, and the yield rate of the hot-rolled plate is not less than 70%.

[0010] In the above step six, the elongation of the obtained titanium alloyed high-boron stainless steel slab is increased by 2% to 14% compared with that without adding Ti, and the impact energy AKv is increased by 3 to 20 J (25°C).

[0011] The beneficial effects of the present invention are as follows: (1) by optimizing the alloy composition design, a forming process of "titanium alloying + electroslag + hot forging + hot rolling" is proposed, which can produce large-size, thick-gauge, high-boron stainless steel slabs with good plasticity and toughness, breaking through the bottleneck of small specifications and small batches of such materials.

[0012] (2) The present invention organically combines the beneficial effects of titanium alloying of high-boron stainless steel and the advantages of various hot forming methods. It has strong process operability and universality, can be applied industrially, and provides an effective technical route for the large-scale production of high-boron stainless steel slabs. DETAILED DESCRIPTION

[0013] By optimizing the alloy composition design, a forming process of "titanium alloying + electroslag + hot forging + hot rolling" was proposed, which can produce large-size, thick, high-boron stainless steel slabs with good plasticity and toughness, breaking through the bottleneck of small specifications and small batches of this type of material.

[0014] The present invention combines the beneficial effects of titanium alloying of high-boron stainless steel with the advantages of various hot forming methods, has strong process operability and universality, can be industrially applied, and provides an effective technical route for the large-scale production of high-boron stainless steel slabs.

[0015] A forming method of a titanium alloyed high-boron stainless steel slab comprises the following steps: 1. Smelting titanium alloyed high-boron stainless steel molten steel in a vacuum induction furnace according to a set composition, and casting to obtain an induction ingot. The composition is as follows by mass percentage: B: 0.2-2.5%, Ti: 0.5-5.0%, Cr: 17.0-21.0%, Ni: 10.0-16.0%, Mn: 0.5-2.5%, Si: 0.1-1.5%, C≤0.07%, P≤0.04%, S≤0.03%, N and other unavoidable impurities≤0.1%, and the rest is Fe.

[0016] 2. After removing the heads and tails of multiple induction casting ingots and welding them together, electroslag remelting is carried out under argon protective atmosphere. After natural cooling in the air, demoulding is carried out to obtain electroslag ingots.

[0017] 3. Diffusion anneal the electroslag ingot at 1100~1220℃ with t1=(1.0~6.0)h1 to homogenize the as-cast structure; where h1 is the thickness of the electroslag ingot, in mm, and t1 is in min.

[0018] 4. The electroslag ingot after heat treatment is taken out of the furnace for hot forging, wherein the reduction in a single pass does not exceed 20%. After forging, it is returned to the furnace and kept at 1100-1220°C for 120-240 minutes to obtain a thick plate intermediate billet.

[0019] 5. Hot-roll the intermediate billet of the thick plate, wherein the single-pass reduction is between 3% and 18%, and the total reduction is between 50% and 95%. After rolling, air-cool it to room temperature to obtain a hot-rolled plate.

[0020] 6. After the hot-rolled plate is solution treated at 1030~1150℃ for t2=(0.5~4.0)h2, it is cooled to room temperature by water to obtain titanium alloyed high-boron stainless steel slab. In the formula, h2 is the thickness of the intermediate slab of the thick plate, in mm, and t2 is in min.

[0021] In the above step 1, the raw materials used for smelting should be free of oxidized impurities before use, electromagnetic stirring can be applied during the smelting process, and argon gas protection can be added after alloying Mn, Ti and other consumable elements.

[0022] In the above step 2, the ingot starter plate and arc starter chips with the same composition as the above high-boron stainless steel should be used for electroslag remelting, and no oil or water stains are allowed on the surface.

[0023] In the above step 4, the surface temperature of the electroslag ingot should not be lower than 900°C during hot forging after being taken out of the furnace, otherwise it should be returned to the furnace and reheated for 30 to 150 minutes before forging.

[0024] In the above step 5, the surface temperature of the intermediate billet during hot rolling should not be lower than 920°C, otherwise it should be returned to the furnace and reheated for 30 to 150 minutes before hot rolling. The yield rate of the hot-rolled plate should not be lower than 70% (the ratio of the effective yield area to the total slab volume).

[0025] In the above step 6, the elongation of the obtained titanium alloyed high-boron stainless steel slab is increased by 2% to 14% compared with that without adding Ti, and the impact energy AKv is increased by 3 to 20 J (25°C).

[0026] The smelting raw materials used in the embodiments of the present invention have a clean surface and are free of oxidized impurities, and the allowable deviation of the chemical composition of the finished steel product adopts the standard GB / T222.

[0027] In the embodiment of the present invention, a box-type resistance furnace is used for heat treatment, and timing starts after the furnace temperature rises to a set temperature.

[0028] In the embodiment of the present invention, the elongation test adopts the standard GB / T228.1, and the impact energy test adopts the standard GB / T229. Example

[0029] The raw materials are smelted according to the set composition ratio (mass fraction): B: 2.1%, Ti: 3.0%, Cr: 18.0%, Ni: 13.5%, Mn: 2.0%, Si: 0.7%, C: 0.03%, P≤0.03%, S≤0.02%, N and other unavoidable impurities≤0.08%, and the rest is Fe; pure iron, electrolytic chromium, nickel and other metal raw materials are added into the vacuum induction melting furnace respectively, electromagnetic stirring is applied during the smelting process, and argon gas protection is filled after alloying elements such as Mn and Ti are added; the molten steel is melted and poured to obtain an induction ingot.

[0030] The two induction ingots are welded together after the heads and tails are sawn off and removed, and used as electrode blanks. They are electroslag remelted in an argon protective atmosphere, and demoulded after natural cooling in the air to obtain electroslag ingots (cylindrical) with a diameter of 350mm. During the electroslag process, there is no oil or water stain on the surface of the raw materials, and the same steel grade ingot starter plate and arc starter chips are used.

[0031] After the electroslag ingot is kept at 1160℃ for t1=720min (2.1h1), it is taken out of the furnace for hot forging and blanking, where the single pass reduction is between 5% and 20%, and a thick plate intermediate blank (150mm thick) is obtained. During the forging process, when the surface temperature of the billet drops to 930℃, it is returned to the furnace for reheating for 90min before forging.

[0032] The thick plate intermediate billet was kept at 1170℃ for 180min, and then hot-rolled to 50mm thickness (total reduction 66.7%), with a single-pass reduction of 4% to 15%. After rolling, it was air-cooled to room temperature to obtain a hot-rolled plate.

[0033] The hot-rolled plate was solution treated at 1100°C for 90 min (1.8 h2), and then water-cooled to room temperature to obtain a titanium alloyed high-boron stainless steel slab.

[0034] According to calculation, the hot-rolled plate yield is 80%; the tensile property test and pendulum impact test show that its elongation is 14% and the impact energy is 19 J. Compared with the performance of comparative example 1, the elongation is increased by 6.5% and the impact energy is increased by 10 J, which meets the requirements of the present invention. Comparative Example 1

[0035] Compared with the embodiment, the differences are: (1) no Ti element is added to the high-boron stainless steel composition system.

[0036] (2) The induction cast ingot is not subjected to electroslag remelting, but directly undergoes the same subsequent hot forging, hot rolling and solution treatment processes.

[0037] During the hot rolling process, it was found that after 5 deformation passes, the steel plate had obviously cracked and the surface was flaky, making it impossible to continue hot rolling, and the yield rate was less than 50%. In addition, samples were taken from a relatively intact part of the core of the slab for mechanical property testing, and it was found that its elongation was 7.5% and the impact energy was 9J, both of which did not meet the requirements of the present invention. Comparative Example 2

[0038] Compared with the embodiment, the differences are as follows: (1) the titanium alloyed induction cast ingot is directly hot forged and hot rolled without electroslag remelting process; (2) The holding temperature during hot rolling is 1100°C, and the reduction per pass is between 18% and 25%.

[0039] During the hot rolling process, it was found that after 4 deformation passes, the steel plate had obviously cracked and the surface was in a slag-like state, making it impossible to continue hot rolling. The yield rate was less than 30%, and thus did not meet the requirements of the present invention.

[0040] The above embodiments and comparative examples are used to illustrate the features and effects of the present invention in detail, but are not limited to these embodiments. Without departing from the concept of the present invention, there may be more other equivalent embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for forming a titanium alloyed high-boron stainless steel slab, characterized in that: The following steps are involved: Step 1: melting titanium alloyed high-boron stainless steel liquid according to the set composition in a vacuum induction furnace, and pouring to obtain an induction ingot; the set composition is as follows by mass percentage: B: 0.2-2.5%, Ti: 0.5-5.0%, Cr: 17.0-21.0%, Ni: 10.0-16.0%, Mn: 0.5-2.5%, Si: 0.1-1.5%, C≤0.07%, P≤0.04%, S≤0.03%, N and other unavoidable impurities≤0.1%, and the rest is Fe; Step 2: After removing the head and tail of the induction casting ingot with casting defects, electroslag remelting is performed under an argon protective atmosphere, and demolding is performed after natural cooling in the air to obtain an electroslag ingot; Step 3: Diffusion annealing the electroslag ingot at 1100~1220℃ t1=(1.0~6.0)h1; where h1 is the thickness of the electroslag ingot, in mm, and t1 is in min; Step 4: The electroslag ingot after heat treatment is taken out of the furnace for hot forging, wherein the single pass reduction does not exceed 20%, and after forging, it is returned to the furnace and kept at 1100-1220°C for 120-240min to obtain a thick plate intermediate billet; Step 5: hot rolling the intermediate billet of the thick plate, wherein the single-pass reduction is between 3% and 18%, the total reduction is between 50% and 95%, and air-cooling to room temperature after rolling to obtain a hot-rolled plate; Step 6: After the hot-rolled plate is solution treated at 1030~1150℃ for t2=(0.5~4.0)h2, it is taken out of the furnace and water-cooled to room temperature to obtain a titanium alloyed high-boron stainless steel slab; where h2 is the thickness of the intermediate slab of the thick plate, in mm, and t2 is in min.

2. The method for forming a titanium alloyed high-boron stainless steel slab according to claim 1, characterized in that: In the above step 1, the raw materials used for smelting should be free of oxidized impurities before use, electromagnetic stirring can be applied during the smelting process, and argon gas protection can be added after alloying Mn and Ti consumable elements.

3. The method for forming a titanium alloyed high-boron stainless steel slab according to claim 1, characterized in that: In the above step 2, the ingot starter plate and arc starter chips with the same composition as the above high-boron stainless steel should be used for electroslag remelting, and no oil or water stains are allowed on the surface.

4. The method for forming a titanium alloyed high-boron stainless steel slab according to claim 1, characterized in that: In the above step 4, the surface temperature of the electroslag ingot is not less than 900°C during hot forging after being taken out of the furnace, otherwise it is returned to the furnace and reheated for 30 to 150 minutes before forging.

5. The method for forming a titanium alloyed high-boron stainless steel slab according to claim 1, characterized in that: In the above step 5, the surface temperature of the intermediate billet is not less than 920°C during hot rolling, otherwise it is returned to the furnace and reheated for 30 to 150 minutes before hot rolling, and the yield rate of the hot-rolled plate is not less than 70%.

6. The method for forming a titanium alloyed high-boron stainless steel slab according to claim 1, characterized in that: In the above step six, the elongation of the obtained titanium alloyed high-boron stainless steel slab is increased by 2% to 14% compared with that without adding Ti, and the impact energy AKv is increased by 3 to 20 J (25°C).