Steel nitrogen-increasing strengthening treatment method

Abstract

The invention belongs to the field of metal steel production, and discloses a steel nitrogen-increasing strengthening treatment method. The production process of the steel comprises the steps of steelmaking, steel ladle alloying, continuous casting, casting blank cutting and rolling. After the steelmaking procedure is finished, when molten steel is tapped and enters a steel ladle, coke nuts are added into the steel ladle along with flowing of the molten steel, the molten steel and the coke nuts are stirred under the dynamic conditions in the tapping process, the coke nuts are melted in the molten steel, and nitrogen increasing is achieved; and adding a vanadium-nitrogen alloy, a silicon-iron alloy, a silicon-manganese alloy, a manganese-iron alloy and an optional deoxidizer into the steel ladle along with the inflow of the molten steel, and alloying the steel ladle. According to the method, the nut coke is added in the early stage of the tapping process, then alloying is conducted, nitrogen increasing is achieved through the nut coke, microalloy elements added into the steel have sufficient conditions to be separated out, second-phase particles are formed, and improvement of the mechanical property of the steel is promoted.

Description

Technical Field

[0001] This invention belongs to the field of metal steel production, and more specifically, relates to a method for strengthening steel by adding nitrogen. Background Technology

[0002] Currently, common steel strengthening technologies include microalloying, controlled rolling and cooling, and fine-grained rolling. Among these, microalloying relies on the precipitation of microalloying elements combined with nitrogen in the steel to improve its mechanical properties. Increasing the nitrogen content in steel is key to fully utilizing the role of microalloying elements. Therefore, under current technological conditions, steel mills generally use methods such as wire feeding and alloying to increase nitrogen in molten steel. These technologies all require the use of expensive cored wire and nitriding alloys, with a price exceeding 10,000 yuan per ton, resulting in high nitrogen-increasing costs and a tendency to form oxide inclusions in the steel.

[0003] CN105400927B discloses a multi-element nitride alloy cored wire and its application and application method in the strengthening process of HRB400 steel. It discloses the composition of the multi-element nitride alloy cored wire and the process of tapping steel at the converter end point, refining molten steel, continuous casting and billet rolling. In the process of refining molten steel, the composition and temperature of molten steel are adjusted and degassing and impurity removal purification processes are carried out on the molten steel. Then, the multi-element nitride alloy cored wire is fed into the molten steel and then the refined molten steel is cast into a billet.

[0004] The disadvantages of traditional nitrided cored wire alloying technology are as follows: During production, after the steel is tapped from the converter, the ladle is moved to the argon blowing station to begin feeding the wire. Soft argon blowing is used to add nitrogen to the molten steel during the feeding process. This process is difficult to control due to factors such as feeding speed, argon blowing, and large fluctuations in the weight of the core powder per meter. This results in unstable yield of nitrided cored wire. While a small amount of nitride can refine the grains and strengthen the alloying, an increase in nitride in the molten steel reduces its plasticity and toughness, leading to unstable steel performance. Production is also affected by equipment failures in the wire feeding machine, causing interruptions. Furthermore, the effective utilization rate of improving steel performance through the action of a single element is low. The vanadium ferronitride used for micro-alloying the molten steel, besides combining with nitrogen in the fed nitrided cored wire, leaves approximately 30% of the vanadium dissolved in the matrix without playing a role (failing to form vanadium dispersed particles for precipitation strengthening). Other components in the cored wire oxidize, forming oxide inclusions. Additionally, the actual production cost is high, failing to achieve the goal of efficient and low-cost operation.

[0005] Therefore, there is an urgent need to propose a new method for strengthening steel by adding nitrogen. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a nitrogen-enriched strengthening treatment method for steel. This invention involves adding coking coal in the early stages of the steelmaking process, followed by alloying. The coking coal is used to achieve nitrogen enrichment, allowing the added microalloying elements in the steel to precipitate and form second-phase particles, thereby promoting the improvement of the steel's mechanical properties.

[0007] To achieve the above objectives, the present invention provides a method for strengthening steel by adding nitrogen, characterized in that the steel production process includes: steelmaking, ladle alloying, continuous casting, billet cutting and rolling;

[0008] After the steelmaking process is completed, when the molten steel is tapped into the ladle, coking coal is first added into the ladle along with the molten steel. The kinetic conditions during the tapping process are used to stir the molten steel and coking coal, so that the coking coal melts in the molten steel and increases nitrogen content. Subsequently, vanadium-nitrogen alloy, ferrosilicon alloy, ferrosilicon alloy, ferromanganese alloy and optional deoxidizer are added into the ladle along with the molten steel to carry out the ladle alloying process.

[0009] According to the present invention, preferably, the steelmaking process is converter smelting or electric furnace smelting.

[0010] According to the present invention, preferably, the particle size of the coke is 2-7 mm; based on the total weight of the coke, the coke comprises fixed C 80-85%, S≤1.0%, H2O≤1.0%, and N≥1.0%.

[0011] According to the present invention, preferably, the amount of coke added per ton of steel is determined based on the nitrogen content of the finished steel product, the carbon content of the finished steel product, and the carbon control level at the end of the steelmaking process.

[0012] According to the present invention, preferably, when the steel is HRB400 steel, the amount of coking coal added per ton of steel is 0.5-1.5 kg.

[0013] According to the present invention, preferably, when 1 / 4 to 1 / 2 of the steel is tapped, a vanadium-nitrogen alloy, a ferrosilicon alloy, a ferrosilicon alloy, a ferromanganese alloy, and optionally a deoxidizer are added to the ladle.

[0014] According to the present invention, preferably, the vanadium-nitrogen alloy, ferrosilicon alloy, ferrosilicon alloy, ferromanganese alloy, and optionally the deoxidizer are added starting at 1 / 4 of the steel being tapped and completed at 1 / 2 of the steel being tapped.

[0015] According to the present invention, preferably, the vanadium-nitrogen alloy is at least one of ferrovanadium nitride, vanadium-nitrogen alloy, and manganese silicon nitride.

[0016] According to the present invention, preferably, the deoxidizer is at least one selected from aluminum manganese calcium, calcium barium silicon, and calcium aluminum barium silicon.

[0017] According to the present invention, preferably, the ladle alloying and continuous casting process further includes an argon blowing station and an LF furnace refining.

[0018] In this invention, nitrogen is used for cooling in dry quenching coke, resulting in a nitrogen content >1% in the coke. Coke briquettes, a byproduct of dry quenching coke production (dry quenching coke undersize), have the same physicochemical properties as dry quenched coke, except for particle size. Therefore, this invention adds coke briquettes in the early stages of the steelmaking process, followed by alloying, increasing both carbon and nitrogen in the molten steel, fully utilizing the nitrogen in the coke briquettes. After converter blowing, the molten steel contains a large amount of oxygen, with an oxygen activity of 300-400 ppm, indicating a supersaturated state and low gas solubility. Upon addition of coke briquettes, a CO reaction occurs with the molten steel, generating CO or CO2 which escapes. Subsequently added alloys and deoxidizers react with the oxygen in the molten steel, further reducing the oxygen activity and creating conditions suitable for nitrogen dissolution. Simultaneously, the impact of the tapping steel stream on the molten steel surface agitates the coke briquettes, causing them to melt rapidly. Coke briquettes that do not undergo a CO reaction melt within the molten steel, achieving the goal of increasing both carbon and nitrogen in the molten steel.

[0019] The beneficial effects of the technical solution of this invention are as follows: This invention involves adding coking coal in the early stage of the steelmaking process, followed by alloying. Utilizing the kinetic conditions during steelmaking and the non-uniformity during steel deoxidation, the coking coal rapidly melts in the steel. During this process, the carbon in the coking coal reacts with the residual oxygen in the steel, producing CO and CO2, both gaseous elements that do not form oxide inclusions that contaminate the molten steel. The remaining carbon and nitrogen elements in the coking coal dissolve into the steel, achieving nitrogen enrichment. This allows the added microalloying elements to precipitate and form second-phase particles, promoting improved mechanical properties of the steel. Specifically:

[0020] 1. Taking the production of rebar as an example, the vanadium-nitrogen alloy added to the steel is mainly ferrovanadium nitride. In traditional technology, due to limitations in its production process, more than 30% of the ferrovanadium in ferrovanadium nitride has not been nitrided. Therefore, after being added to the steel, this portion of vanadium is dissolved in austenite and cannot exert its maximum effect. Artificial intervention is required to forcibly increase the nitrogen content in the molten steel so that this portion of vanadium has the conditions to form second-phase particles. This invention adds coking coal to increase the nitrogen and carbon content of the molten steel, making it suitable for the formation of V(CN) and fully exerting its strengthening effect.

[0021] 2. This invention solves the problem of increasing carbon and nitrogen in molten steel at the same time, makes full use of coking coal resources, enables molten steel to obtain nitrogen, saves vanadium-nitrogen alloy, reduces the production cost of rebar, and improves production efficiency.

[0022] 3. This invention stabilizes the nitrogen content fluctuation problem caused by residual oxygen content fluctuations after steel tapping. Specifically: The steel tapping process involves complex kinetic and thermodynamic conditions, resulting in significant fluctuations in the basic nitrogen content. By adding coking coal, it combines with residual oxygen in the steel, making the molten steel more susceptible to absorbing nitrogen from the air, thus stabilizing the nitrogen content in the molten steel. After the nitrogen in the coking coal enters the steel, the basic nitrogen content increases and becomes more stable, providing more sufficient bonding conditions for Nb, V, and Ti elements. These microalloying elements are dissolved in austenite and continuously precipitate as the temperature decreases, combining with nitrogen and carbon elements in the steel to form second-phase ions, namely V(CN), Nb(CN), and TiN, giving the steel a stable precipitation strengthening effect.

[0023] 4. This invention saves on the cost of adding nitrogen to molten steel.

[0024] Other features and advantages of the present invention will be described in detail in the following detailed description section. Detailed Implementation

[0025] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0026] Example 1

[0027] This embodiment provides a method for nitrogen-enhancing steel to prepare HRB400 steel. The steel production process includes: steelmaking, ladle alloying, argon blowing station, LF furnace refining, continuous casting, billet cutting and rolling.

[0028] After the steelmaking process is completed, when the molten steel is tapped into the ladle, coke is first added to the ladle along with the molten steel. The kinetic conditions during the tapping process are used to stir the molten steel and coke, causing the coke to melt in the molten steel and achieving nitrogen addition. Subsequently, vanadium-nitrogen alloy, ferrosilicon alloy, silicon-manganese alloy, ferromanganese alloy, and deoxidizer are added to the ladle along with the molten steel to carry out the ladle alloying process. The vanadium-nitrogen nitride (V53N10), ferrosilicon alloy (Si75), silicon-manganese alloy (Mn65Si17), ferromanganese alloy (Mn65), and silicon-calcium-barium (Si48) are added starting at 1 / 4 of the tapping time and completed at 1 / 2 of the tapping time.

[0029] The steelmaking process is described as converter smelting.

[0030] The coke particles have a particle size of 2-7 mm; based on the total weight of the coke particles, the coke particles comprise 80-85% C, ≤1.0% S, ≤1.0% H2O, and ≥1.0% N. The amount of coke particles added per ton of steel is 0.5-1.5 kg.

[0031] Steelmaking: The temperature of molten steel is controlled at 1615-1680℃; the composition of molten steel is: C 0.08%~0.15%, P 0%~0.035%, S 0%~0.035%, O 0~100ppm;

[0032] LF furnace refining: The basic composition of the molten steel is adjusted to C 0.23%~0.25%, Si 0.20%~0.30%, Mn 1.40%~1.52%, P 0%~0.043%, and S 0%~0.043%, and the temperature of the molten steel is adjusted to control it at 1560℃~1580℃;

[0033] Continuous casting: The temperature of the intermediate tank is controlled at 1521℃~1536℃, and the billet size is 160mm*160mm*L.

[0034] Comparative Example 1

[0035] This comparative example provides a method for nitrogen-enriching steel to prepare HRB400 steel. The only difference between this comparative example and Example 1 is that:

[0036] The vanadium ferronitride V53N10, ferrosilicon Si75, ferrosilicon Mn65Si17, ferromanganese Mn65, ferromanganese Si48, and coke were added together starting at 1 / 3 of the steel was tapped and finishing at 4 / 5 of the steel was tapped.

[0037] Comparative Example 2

[0038] This comparative example provides a method for nitrogen-enhancing steel to prepare HRB400 steel. The only difference between this comparative example and Example 1 is that no coking coal is added.

[0039] The vanadium ferronitride V53N10, ferrosilicon Si75, ferrosilicon Mn65Si17, ferromanganese Mn65, and barium calcium silicon Si48 are added together starting at 1 / 3 of the steel is tapped and finishing at 4 / 5 of the steel is tapped.

[0040] Test case

[0041] In this test example, the performance of the HRB400 steel obtained in the above embodiments and comparative examples was tested using GB / T 28900 and GB / T 20124. The results are shown in Table 1.

[0042] Table 1

[0043] Example 1 Comparative Example 1 Comparative Example 2 Yield strength (Rel) 454 449 450 Tensile strength (Rm) 623 612 615 Elongation (A) 15.1 15.5 14.7 Nitrogen content in steel (ppm) 101.3 93.1 90.1

[0044] This invention involves sampling and testing 100 heats of HRB400E produced using the coking agent, with a sample size of 10. The average nitrogen content was 101.3 ppm, which was 11.2 ppm higher than that of heats without the coking agent.

[0045] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for nitrogen strengthening of steel, characterized in that: The production process of the steel includes: steelmaking, ladle alloying, continuous casting, billet cutting and rolling; After the steelmaking process is completed, when the molten steel is tapped into a ladle, diced coke is first added into the ladle as the molten steel flows in, and the molten steel and diced coke are stirred using the kinetic conditions during the steel tapping process to melt the diced coke in the molten steel to achieve nitrogen addition; then, vanadium-nitrogen alloy, ferrosilicon alloy, silicon-manganese alloy, ferromanganese alloy and optionally a deoxidizer are added into the ladle as the molten steel flows in to carry out the ladle alloying process.

2. The steel nitrogen strengthening treatment method according to claim 1, wherein: The steelmaking is converter smelting or electric furnace smelting.

3. The steel nitrogen strengthening treatment method according to claim 1, wherein: The particle size of the coke cubes is 2-7 mm; based on the total weight of the coke cubes, the coke cubes contain 80-85% C, ≤1.0% S, ≤1.0% H2O, and ≥1.0% N.

4. The steel nitrogen strengthening treatment method according to claim 1, wherein: The amount of diced coke added per ton of steel is determined according to the nitrogen content of the finished steel product, the carbon content of the finished steel product and the carbon control level at the end of the steelmaking process.

5. The steel nitrogen strengthening treatment method according to claim 4, wherein: When the steel is HRB400 steel, the amount of coke added per ton of steel is 0.5-1.5 kg.

6. The steel nitrogen strengthening treatment method according to claim 1, wherein: When 1 / 4 to 1 / 2 of the steel is tapped, vanadium-nitrogen alloy, ferrosilicon alloy, silicon-manganese alloy, ferromanganese alloy and optionally a deoxidizer are added into the ladle.

7. The steel nitrogen strengthening treatment method according to claim 6, wherein: The vanadium-nitrogen alloy, ferrosilicon alloy, silicon-manganese alloy, ferromanganese alloy and optional deoxidizer are added when 1 / 4 of the steel is tapped and are completely added when 1 / 2 of the steel is tapped.

8. The steel nitrogen strengthening treatment method according to claim 1, wherein: The vanadium-nitrogen alloy is at least one of vanadium iron nitride, vanadium-nitrogen alloy and silicon manganese nitride.

9. The steel nitrogen strengthening treatment method according to claim 1, wherein: The deoxidizer is at least one of aluminum manganese calcium, silicon calcium barium and silicon aluminum barium calcium.

10. The steel nitrogen strengthening treatment method according to claim 1, wherein: An argon blowing station and LF furnace refining are also included between the ladle alloying and continuous casting.