CN105936978A - Slag system for preparing high-nitrogen austenitic stainless steel through pressurizing electroslag remelting gas phase nitriding - Google Patents

Abstract

The invention belongs to the technical field of smelting high-nitrogen steel, and in particular relates to a slag system for smelting high-nitrogen austenitic stainless steel by pressurized electroslag remelting gas-phase nitriding. The mass percentages of the chemical components of the slag system in the present invention are: CaF ₂ : 57-62%, CaO: 16-20%, Al ₂ O ₃ : 12-16%, MgO: 3-6%, SiO ₂ : 0.5-1.5% , and the rest are impurities, and the impurity content does not exceed 0.8%; among them, CaO/Al ₂ O ₃ is 1.00-1.67. By controlling CaO, adjusting the ratio of CaO/Al ₂ O ₃ , and configuring a reasonable content of CaF ₂ , MgO and SiO ₂ , the nitrogen capacity and nitrogen permeability of the slag system can be improved, and the nitrogen content of austenitic stainless steel can be effectively increased. , and can make nitrogen evenly distributed along the ingot body, which is conducive to obtaining high-quality high-nitrogen austenitic stainless steel.

Description

A slag system for the preparation of high-nitrogen austenitic stainless steel by pressurized electroslag remelting gas-phase nitriding

technical field

The invention relates to a slag system for smelting high-nitrogen steel, in particular to a slag system for smelting high-nitrogen austenitic stainless steel by gas phase nitriding in the process of pressurized electroslag remelting.

Background technique

Nitrogen is a strong austenite stabilizing element, which can promote the formation of austenite structure in stainless steel. In the calculation of nickel equivalent, the nitrogen equivalent is 30 times that of nickel, so the precious metal nickel can be replaced by cheap nitrogen, or even completely replaced by nickel to obtain austenitic stainless steel. In general, austenitic stainless steel with a nitrogen content of more than 0.4% in steel is called high-nitrogen austenitic stainless steel. In high-nitrogen austenitic stainless steel, nitrogen, as an interstitial element in steel, can improve the strength, toughness, and creep of steel through synergistic effects with other alloying elements (Mn, Cr, Mo, V, Nb, and Ti, etc.) Resistance, wear resistance, corrosion resistance and other properties, so high nitrogen austenitic stainless steel is considered to be the most valuable new material for research and development in recent years.

At present, the methods of smelting high-nitrogen austenitic stainless steel are mainly divided into adding nitride alloy method and composite electrode method. Among them, although the method of adding nitrided alloys (such as ferrochromium nitride, manganese nitride, silicon nitride, etc.) is simple and easy to operate, it has high production costs, is easily contaminated by impurities in the nitrided alloy, and uses Si ₃ N ₄ It is easy to make Si exceed the standard, the uniformity of nitrogen content is poor, and the nitrogen content is difficult to control and many other problems; the composite electrode method has the advantages of stable electroslag remelting process and good surface quality, but the preparation of composite electrodes is complicated, and welding composite electrodes is easy to make electroslag ingots Oxygenation. Compared with the above two methods, the gas phase nitriding method has the advantages of low cost and avoiding the contamination of the product by the nitrided alloy, so it is currently a key development technology for high nitrogen austenitic stainless steel.

Pressurized electroslag remelting is currently the main method for the commercial production of high nitrogen austenitic stainless steels. In the process of pressurized electroslag remelting, the slag system plays a pivotal role and directly affects the effect of gas phase nitriding. The nitrogen capacity is a measure of the ability of the slag system to accommodate nitrogen, and a slag system with a high nitrogen capacity is conducive to gas-phase nitriding. However, the traditional electroslag remelting slag system (such as 70% CaF ₂ +30% Al ₂ O ₃ , 60% CaF ₂ +20% CaO + 20% Al ₂ O ₃ and 40% CaF ₂ +30% CaO +30% Al ₂ O ₃ ) have low nitrogen capacity. During the process of pressurized electroslag remelting high-nitrogen austenitic stainless steel, the nitrogen in the gas phase is not easy to enter the steel through the slag, so it is difficult to obtain high-nitrogen Austenitic stainless steel.

Contents of the invention

Aiming at the problems existing in the traditional slag system, the present invention provides a slag system for smelting high-nitrogen austenitic stainless steel by pressurized electroslag remelting gas phase nitriding. By adjusting the composition of the slag system, the nitrogen capacity and nitrogen permeability of the slag system can be improved , so as to ensure the high efficiency of gas phase nitriding in the process of pressurized electroslag remelting high nitrogen austenitic stainless steel, reduce the amount of nitrided alloy added, and reduce the Cost of production.

The mass percent of the chemical composition of the slag system used in the pressurized electroslag remelting smelting of high-nitrogen austenitic stainless steel of the present invention is: CaF ₂ : 57-62%, CaO: 16-20%, Al ₂ O ₃ : 12-16%, MgO: 3-6%, SiO ₂ : 0.5-1.5%, and the rest are impurities, the impurity content not exceeding 0.8%; among them, CaO/Al ₂ O ₃ is 1.00-1.67.

The melting temperature of the slag system of the above-mentioned pressurized electroslag remelting gas phase nitriding smelting high nitrogen austenitic stainless steel is 1250-1325°C.

The viscosity of the slag system of the above-mentioned pressurized electroslag remelting gas phase nitriding smelting high-nitrogen austenitic stainless steel is 0.028-0.035 Pa·s at 1500°C.

The resistivity of the slag system of the above-mentioned pressurized electroslag remelting gas phase nitriding smelting high-nitrogen austenitic stainless steel at 1500°C is 0.316-0.408Ω·cm.

The nitrogen capacity of the slag system of the above-mentioned pressurized electroslag remelting gas phase nitriding smelting high nitrogen austenitic stainless steel at 1500°C is (0.906～1.309)×10 ^-13 .

A kind of preparation method of the slag system of pressurized electroslag remelting gas-phase nitriding smelting high-nitrogen austenitic stainless steel completed by the present invention is: using high-purity fluorite, lime, industrial alumina, fused magnesia and silica as raw materials, according to After weighing the raw materials, mix them evenly; pre-melt and stir at 1550°C to 1580°C for at least 30 minutes, then pour the slag into a steel tank to cool and solidify to room temperature; finally crush to a particle size of 0-10mm, Thus, the slag system for high-nitrogen austenitic stainless steel was prepared by pressurized electroslag remelting.

A method for using the slag system of the pressurized electroslag remelting gas phase nitriding smelting high nitrogen austenitic stainless steel of the present invention is as follows: an arc starting ring made of the same material as the smelted high nitrogen austenitic stainless steel and 0.45±0.05kg Put the arc strike scraps on the pressurized electroslag furnace bottom water tank under the consumable electrode; bake the prepared pre-slag at 500℃～700℃ for 4h～6h, and then add them all to the pressurized electroslag furnace for crystallization inside the smelting chamber; fill the smelting chamber with nitrogen to evacuate the air in the smelting chamber, and perform solid-state arc slagging; when the slagging is completed, gradually increase the nitrogen pressure in the smelting chamber, simultaneously increase the cooling water pressure of the crystallizer, and increase the current and Voltage, for low melting speed pressurized electroslag remelting gas-phase nitriding smelting; through the step-by-step feeder, aluminum particles or silicon-calcium alloys are continuously added at a ratio of 0.4kg to 0.7kg/ton of steel for deoxidation; pressurized electroslag remelting After the feeding is completed, open the pressurized electroslag furnace release valve to release the pressure, and simultaneously reduce the pressure of the cooling water to normal pressure. After the steel ingot is completely cooled, the steel ingot is released.

The slag system of the pressurized electroslag remelting gas-phase nitriding smelting of high-nitrogen austenitic stainless steel of the present invention is based on _CaF2 , and the content is 57-62%, which not only helps to reduce the melting point of slag, but also helps to improve Nitrogen capacity and nitrogen permeability of the slag system; in order to ensure a high basicity, obtain a good electroslag remelting desulfurization effect, and increase the nitrogen capacity, 16-20% CaO is added to the slag; For dissolved nitrogen, adjust CaO/Al ₂ O ₃ to 1.00-1.67, and make the addition of Al ₂ O ₃ 12-16%. In order to ensure the nitrogen capacity and nitriding effect of the slag system, the content of SiO ₂ in the slag needs to be controlled At 0.5-1.5%; at the same time, add 3-6% MgO to the slag to replace part of CaO.

Theoretical analysis and research results show that:

The main forms of nitrogen in the slag are free nitrogen (N ^3- ) and non-bridging nitrogen (N ^- ), and these two forms of nitrogen will form the following network structure with aluminum and silicon in the slag:

Adding CaO can depolymerize the above-mentioned network structure, which helps nitrogen dissolve ⁱⁿ the form of N ^3- and N-, and diffuse into molten steel. At the same time, when CaO/Al ₂ O ₃ is low, adding CaF ₂ is helpful for slag system to dissolve nitrogen.

The nitrogen capacity definition formula of the slag system of a kind of pressurized electroslag remelting gas phase nitriding smelting high nitrogen austenitic stainless steel of the present invention is:

${\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \%</span>}\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{{\mathrm{<span\; class="notranslate">\; o</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}$

Among them, C _N is the nitrogen capacity; (%N) is the nitrogen content in the slag when the slag and nitrogen reach equilibrium; is the partial pressure of oxygen; is the nitrogen partial pressure.

The invention provides a slag system for smelting high-nitrogen austenitic stainless steel by pressurized electroslag remelting gas-phase nitriding, which is characterized in that the slag system is a high-nitrogen slag-tolerant system. At 1500°C, the formula for calculating the nitrogen capacity of slag system is as follows:

logC _N =7.442Λ-19.05

In the formula: Λ=(Σx _A n _A Δ _A +x _B n _B Δ _B +....)/(Σx _A n _A +x _{B n B} +....); mole fraction of components; n _i is the mole number of each component in the slag system; _Δi is the optical basicity of each component in the slag system; C _N is the nitrogen capacity of the slag system.

Table 1 shows the nitrogen capacity calculation results of the slag system of the pressurized electroslag remelting gas phase nitriding smelting high nitrogen austenitic stainless steel of the present invention and the traditional electroslag remelting slag system. It can be seen from the table that, compared with the traditional slag system, the nitrogen capacity of the slag system of a pressurized electroslag remelting gas phase nitriding smelting high nitrogen austenitic stainless steel of the present invention is higher, which is beneficial to pressurized electroslag The gas phase nitriding is carried out smoothly during the remelting process.

Table 1 Comparison of nitrogen capacity between the slag system of the present invention and the traditional electroslag remelting slag system, wt.%

A small amount of SiO ₂ is added to the slag system of the pressurized electroslag remelting gas phase nitriding smelting high nitrogen austenitic stainless steel of the present invention. Since low melting speed and pressure electroslag remelting can easily cause surface defects of electroslag ingots, adding a small amount of SiO ₂ can prevent the viscosity of the slag system from changing suddenly with temperature, and make the slag system have good strength and high temperature plasticity, thereby improving the Surface quality of high nitrogen austenitic stainless steel electroslag ingots. However, a higher content of SiO ₂ can easily lead to burning of easily oxidized elements such as Al and Ti in the steel, and greatly reduce the nitrogen capacity and nitriding effect of the slag system. Therefore, this slag system limits SiO ₂ to 0.5-1.5%. .

In the slag system of the pressurized electroslag remelting gas phase nitriding smelting high nitrogen austenitic stainless steel of the present invention, 3-6% MgO is added to replace part of CaO, so as to prevent the slag system from resurging due to excessive CaO, increasing the The water content of the slag causes the hydrogen and oxygen content in the high-nitrogen austenitic stainless steel smelted by the pressurized electroslag furnace to exceed the standard.

The present invention proposes a slag system for smelting high-nitrogen austenitic stainless steel by pressurized electroslag remelting gas phase nitriding. On the basis of the traditional slag system, the ratio of CaO/Al ₂ O ₃ is adjusted by controlling CaO, and With reasonable content of CaF ₂ , MgO and SiO ₂ , a slag system for smelting high-nitrogen austenitic stainless steel by pressurized electroslag remelting gas-phase nitriding is obtained. The slag system has reasonable physical properties and higher nitrogen content. It can effectively increase the nitrogen content of austenitic stainless steel and make the nitrogen evenly distributed along the ingot body, which is beneficial to obtain high-quality high-nitrogen austenitic stainless steel.

detailed description

The specific implementation of the present invention will be described in detail below in conjunction with the examples, but the specific implementation of the present invention is not limited to the following examples.

Example 1

The mass percentages of chemical components of the slag system of the pressurized electroslag remelting gas phase nitriding smelting of high nitrogen austenitic stainless steel in this embodiment are: CaF ₂ : 59.3%, A1 ₂ O ₃ : 15%, CaO: 18%, MgO : 6%, SiO ₂ : 1%, and the rest are impurities. The melting temperature of the slag system is 1270°C; at a temperature of 1500°C, the resistivity of the slag system is 0.381Ω·cm, the viscosity is 0.033Pa·s, and the nitrogen capacity is 1.046×10 ^-13 .

The preparation method of the slag system is as follows: use high-purity fluorite, lime, industrial alumina, fused magnesia and silica as raw materials, weigh the raw materials according to the above-mentioned composition ratio, and mix them evenly; Pre-melt for 35 minutes, melting current is 1500A, voltage is 35V, then pour the molten slag into the steel tank to cool and solidify to room temperature; finally crush it to 0-10mm, so as to prepare pressurized electroslag remelted high-nitrogen austenitic stainless steel Use the slag system.

200kg of 18Cr2Mo0.5N steel (the target nitrogen content is 0.4-0.6%) was remelted by using the above-mentioned pre-melted slag to pressurize the electroslag. The consumable electrode base material is obtained by smelting in a nitrogen-protected vacuum melting furnace, and forged into a consumable electrode with a diameter of Φ130mm. Its composition is as follows:

The method of using this slag system is as follows: put the arc striking ring and 0.44kg arc striking scraps of the same material as 18Cr2Mo0.5N steel on the pressurized electroslag furnace bottom water tank under the consumable electrode; put the prepared pre-melted slag After baking at 600°C for 5 hours, all of them were added to the crystallizer with a diameter D of 220mm, and the amount of slag was 9kg; the smelting chamber of the pressurized electroslag furnace was sealed, filled with nitrogen and evacuated; The slagging current is 38V, the slagging current is 2300A, and the slagging time is 24 minutes to complete the slagging; when the slagging is completed, gradually increase the nitrogen pressure in the smelting chamber to 1.6MPa, and simultaneously increase the cooling water pressure of the pressurized electroslag furnace crystallizer to 1.6MPa, pressurized electroslag remelting vapor phase nitriding smelting, smelting current and voltage are 3800A and 42V respectively, melting speed is 89kg/h; at the same time, use a step-by-step feeder to add aluminum particles with a total weight of 110g for deoxidation. After the pressurized electroslag remelting and feeding are completed, the pressurized electroslag furnace release valve is opened to release the pressure, and the cooling water pressure is simultaneously reduced to normal pressure. After the steel ingot is completely cooled, the steel ingot is released.

The composition of the pressurized electroslag remelting 18Cr2Mo0.5N steel ingot is as follows:

After smelting, the nitrogen content in different parts was analyzed, and the results are shown in the table below:

It can be seen from the above table that the electroslag ingot obtained in this example meets the nitrogen content requirements of the target steel grade 18Cr2Mo0.5N, the nitrogen is evenly distributed along the ingot body, the surface quality of the electroslag ingot is good, and there are no slag grooves, nodules, and heavy skin , wrinkles, slag inclusions and other defects.

Example 2

The mass percentages of the chemical components of the slag system of the pressurized electroslag remelting gas phase nitriding smelting of high nitrogen austenitic stainless steel in this embodiment are: CaF ₂ : 61.7%, A1 ₂ O ₃ : 12%, CaO: 20%, MgO : 5%, SiO ₂ : 0.5%, and the rest are impurities. The melting temperature of the slag system is 1260°C; at a temperature of 1500°C, the resistivity of the slag system is 0.318Ω·cm, the viscosity is 0.030Pa·s, and the nitrogen capacity is 1.288×10 ^-13 .

The preparation method of the slag system is as follows: use high-purity fluorite, lime, industrial alumina, fused magnesia and silica as raw materials, weigh the raw materials according to the above-mentioned composition ratio, and mix them evenly; Pre-melt for 40 minutes, melting current is 1500A, voltage is 35V, then pour the molten slag into the steel tank to cool and solidify to room temperature; finally crush it to 0-10mm, so as to prepare pressurized electroslag remelted high nitrogen austenitic stainless steel Use the slag system.

200kg of 23Cr4Ni2Mo0.75N steel (the target nitrogen content is 0.6-0.8%) was remelted by using the above-mentioned pre-melted slag to pressurize the electroslag. The consumable electrode base material is obtained by smelting in a nitrogen-protected vacuum melting furnace, and forged into a consumable electrode with a diameter of Φ130mm. Its composition is as follows:

After smelting, the nitrogen content in different parts was analyzed, and the results are shown in the table below:

It can be seen from the above table that the electroslag ingot obtained in this example meets the nitrogen content requirements of the target steel grade 23Cr4Ni2Mo0.75N, the nitrogen is evenly distributed along the ingot body, the surface quality of the electroslag ingot is good, and there are no slag grooves, nodules, and heavy skin , folds, slag inclusions.

Claims (6)

1. A slag system for smelting high-nitrogen austenitic stainless steel by pressurized electroslag remelting gas phase nitriding, characterized in that the composition of the slag system is CaF ₂ : 57-62%, CaO: 16-20% by weight percentage %, Al ₂ O ₃ : 12-16%, MgO: 3-6%, SiO ₂ : 0.5-1.5%, the rest are impurities, and the impurity content does not exceed 0.8%; among them, CaO/Al ₂ O ₃ is 1.00-1.67 ; The melting temperature of the slag system is 1250 ~ 1325 ℃. 2. A slag system for smelting high-nitrogen austenitic stainless steel by pressurized electroslag remelting gas phase nitriding according to claim 1, characterized in that the viscosity of the slag system at 1500°C is 0.028-0.035Pa·s . 3. A slag system for smelting high-nitrogen austenitic stainless steel by pressurized electroslag remelting gas phase nitriding according to claim 1, characterized in that the resistivity of the slag system at 1500°C is 0.316-0.408Ω· cm. 4. A slag system for smelting high-nitrogen austenitic stainless steel by pressurized electroslag remelting gas phase nitriding according to claim 1, characterized in that the nitrogen capacity of the slag system at 1500°C is (0.906-1.309) ×10 ^-13 . 5. A preparation method for smelting high-nitrogen austenitic stainless steel slag system by pressurized electroslag remelting gas phase nitriding, characterized in that the method comprises: using high-purity fluorite, lime, industrial alumina, fused magnesia and Silica is the main raw material, CaF ₂ : 57-62%, CaO: 16-20%, Al ₂ O ₃ : 12-16%, MgO: 3-6%, SiO ₂ : 0.5-1.5%, and the rest After weighing the raw materials for the proportion of impurities, mix them evenly; pre-melt and stir at 1550 ° C ~ 1580 ° C for at least 30 minutes, then pour the molten slag into the steel tank to cool and solidify to room temperature; finally crush to a particle size of 0 ~ 10mm prepared pressure electroslag remelting slag system for high nitrogen austenitic stainless steel. 6. A method for using the slag system of high-nitrogen austenitic stainless steel smelted by pressurized electroslag remelting gas-phase nitriding, characterized in that, the method comprises: introducing the same material as the smelted high-nitrogen austenitic stainless steel Put the arc ring and 0.45±0.05kg arc strike scraps on the pressurized electroslag furnace bottom water tank located under the consumable electrode; bake the prepared pre-slag at 500℃～700℃ for 4h～6h and then add them all into the pressurized electroslag furnace crystallizer; close the smelting chamber, fill it with nitrogen to evacuate the air in the smelting chamber, and perform solid-state arc slagging; when the slagging is completed, gradually increase the nitrogen pressure in the smelting chamber, and simultaneously increase the crystallizer cooling Water pressure, and increase the current and voltage, low melting speed pressurized electroslag remelting vapor phase nitriding smelting.