Electroslag remelting method of high-nitrogen non-magnetic steel

Abstract

The invention relates to the technical field of metallurgy, and particularly discloses a high-nitrogen non-magnetic steel electroslag remelting method which comprises the following steps: S110, preparing an electrode bar base material required by electroslag remelting, and loading the electrode bar base material into an electroslag remelting furnace; s120, a reducing agent is added into the CaF2-CaO-Al2O3-MgO-SiO2 five-membered slag system electroslag, and the CaF2-CaO-Al2O3-MgO-SiO2 five-membered slag system electroslag is fully mixed to S130, an electroslag remelting furnace under the protective atmosphere is filled with electroslag mixed with a reducing agent, and after electrification and arcing, the melting speed of an electrode bar base metal is controlled for smelting; and S140, after smelting is finished, furnace cooling is conducted firstly, then air cooling is conducted, and an electroslag ingot is prepared. By means of the method, Al can be prevented from being increased in the high-nitrogen non-magnetic steel electroslag remelting process, a large number of AlN inclusions are prevented from being generated, head-tail segregation of the electroslag ingot is improved, and the solidification quality of the electroslag ingot is improved.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to an electroslag remelting method for high-nitrogen non-magnetic steel. Background Art

[0002] High nitrogen non-magnetic steel is a new type of steel with high nitrogen content and no magnetism. Due to its excellent mechanical properties, corrosion resistance and non-magnetism, it has been widely used in many fields such as medical, aerospace, marine engineering, electronic industry, energy, etc. Among them, the exploration and development of offshore oil and gas in my country has become one of the main sources of crude oil in my country, but the high-strength, high-toughness seawater corrosion-resistant materials required for offshore drilling platforms and marine drilling still need to be imported in large quantities. For example, the non-magnetic drill collar material is mainly high-nitrogen chromium-manganese stainless steel, which is used for oil drilling in a small diameter range. It is characterized by non-magnetism, high endurance and toughness, and high corrosion resistance in drilling environments. At present, Cr-Mn austenitic stainless steel alloyed with high N elements is one of the most widely used materials for non-magnetic drilling tools in the international market. The main alloying elements in this series of austenitic stainless steel include Cr, Mn, Ni, Mo and N.

[0003] The non-magnetic steel for non-magnetic drilling tools has high requirements for cleanliness. The main production process is AOD (argon oxygen decarburization) + LF (ladle refining) + mold casting / continuous casting + electroslag remelting. The most commonly used electroslag composition in the electroslag remelting process is 70% CaF2 + 30% Al2O3. At the same time, in order to remove the oxygen in the slag, Al particles are usually mixed into the slag. Since non-magnetic steel contains a large amount of Mn (about 20%) and less impurity Si, on the one hand, the Al2O3 in the slag is reduced to the steel during the high-temperature smelting process, and on the other hand, the Al particles mixed in the slag are directly dissolved into the steel, resulting in the Al content in the final steel exceeding the standard. Due to the high nitrogen content in non-magnetic steel, when the Al content in the steel exceeds the standard, a large amount of AlN inclusions will be generated. Electroslag remelting is the last link of smelting. Once a large amount of AlN is generated in the steel, it will no longer be removed after the electroslag ingot solidifies. At the same time, when high-nitrogen steel is electroslag remelted, N is easy to escape, resulting in component segregation and even unqualified product composition. These all seriously affect the corrosion resistance, mechanical properties and hot working properties of steel.

[0004] Therefore, how to achieve stable production of high-quality, high-nitrogen, non-magnetic steel electroslag ingots has become a technical problem that needs to be urgently solved in the metallurgical field. Summary of the invention

[0005] In view of the shortcomings of the prior art, the main purpose of the present invention is to provide a method for electroslag remelting of high-quality high-nitrogen non-magnetic steel, to prevent the increase of Al during the electroslag remelting process of high-nitrogen non-magnetic steel, to avoid the generation of a large number of AlN inclusions, to improve the head and tail segregation of the electroslag ingot, and to improve the solidification quality of the electroslag ingot.

[0006] To solve at least one of the above technical problems, the present invention adopts the following technical solutions: According to the present invention, there is provided an electroslag remelting method for a high-nitrogen non-magnetic steel, comprising the following steps: S110, preparing an electrode bar base material required for electroslag remelting and loading it into an electroslag remelting furnace; S120, adding a reducing agent to the electroslag of a CaF2-CaO-Al2O3-MgO-SiO2 five-component slag system and mixing it evenly; S130, filling the electroslag mixed with the reducing agent in the electroslag remelting furnace under a protective atmosphere, starting an arc after power-on, and controlling the melting rate of the electrode bar base material for smelting; S140, after the smelting is completed, first performing furnace cooling and then air cooling to prepare an electroslag ingot.

[0007] According to an embodiment of the present invention, in step S110, the electrode bar base material is prepared by a die casting or continuous casting process.

[0008] According to an embodiment of the present invention, in step S110, the electrode bar base material is composed of chemical components with the following weight percentages: C: 0.02% to 0.06%, Si ≤ 0.4%, Mn: 18% to 22%, Cr: 16% to 22%, Ni: 1.0% to 4.5%, Mo: 0.5% to 3.0%, Al ≤ 0.02%, O ≤ 0.003%, N: 0.45% to 0.75%, H ≤ 0.002%, and the balance is Fe and inevitable impurities.

[0009] According to an embodiment of the present invention, in step S120, the electroslag is composed of chemical components with the following weight percentages: CaF2: 48% to 55%, Al2O3: 20% to 25%, CaO: 18% to 23%, SiO2: 1.0% to 4.0%, MgO: 1.0% to 5.0%.

[0010] According to an embodiment of the present invention, in step S120, the reducing agent is a Ni-Mg alloy, and the mass ratio of the Ni-Mg alloy to the electroslag is 1:20 to 30.

[0011] According to an embodiment of the present invention, in step S120, the electroslag is baked at 700 to 800 °C for more than 6 hours before mixing with the reducing agent.

[0012] According to an embodiment of the present invention, in step S130, the melting rate of the electrode bar base material is controlled to be 9 to 11 kg / min.

[0013] According to an embodiment of the present invention, in step S130, the protective atmosphere is dry N2.

[0014] According to an embodiment of the present invention, in step S130, before filling the protective atmosphere, the electroslag remelting furnace is evacuated to below 5 Pa.

[0015] According to an embodiment of the present invention, in step S140, the furnace cooling time is controlled to be 50 - 80 min.

[0016] By adopting the above technical solution, the electroslag remelting method of high nitrogen non-magnetic steel according to the present invention can prevent Al addition during melting, effectively avoid the formation of a large amount of AlN inclusions, reduce the content of gas impurities in the electroslag ingot, prevent the escape of N during melting, improve the cleanliness and uniformity of the steel, and achieve stable production of high-quality high nitrogen non-magnetic steel electroslag ingots by designing the electroslag composition, optimizing the smelting parameters in the electroslag remelting process, and controlling the protective atmosphere of the electroslag furnace. Description of the Drawings

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0018] Figure 1 It is a schematic diagram of the steps of the electroslag remelting method of high nitrogen non-magnetic steel according to an embodiment of the present invention; Figure 2 It is a typical inclusion picture of the electroslag ingot obtained in Example 1 of the present invention; Figure 3 It is a typical inclusion picture of the electroslag ingot obtained in the comparative example of the present invention. Detailed Embodiments

[0019] To make the purpose, technical solutions, and advantages of the present invention clearer, the following will further describe the embodiments of the present invention in detail with reference to specific embodiments and the accompanying drawings.

[0020] It should be understood that the embodiments of the present invention shown in the exemplary embodiments are merely illustrative. Although only a few embodiments of the present invention are described in detail, those skilled in the art can easily understand that various modifications are feasible without substantially departing from the teachings of the subject matter of the present invention. Accordingly, all such modifications should be included within the scope of the present invention. Without departing from the gist of the present invention, other substitutions, modifications, changes, and deletions can be made to the design, operating conditions, parameters, etc. of the following exemplary embodiments.

[0021] The present invention provides an electroslag remelting method for high nitrogen non-magnetic steel. As Figure 1 shown, the electroslag remelting method of high nitrogen non-magnetic steel according to an embodiment of the present invention generally includes the following steps: S110, prepare the electrode bar base material required for electroslag remelting and load it into the electroslag remelting furnace; S120, add a reducing agent to the electroslag of the CaF2-CaO-Al2O3-MgO-SiO2 quinary slag system and mix it thoroughly; S130, fill the electroslag mixed with the reducing agent into an electroslag remelting furnace under a protective atmosphere. After powering on and arcing, control the melting rate of the electrode bar base material for smelting; S140, after the smelting is completed, first perform furnace cooling and then air cooling to prepare an electroslag ingot.

[0022] Step S110 can use die casting or continuous casting processes to prepare the electrode bar base material. In the embodiments of the present invention, the electrode bar base material is composed of chemical components with the following weight percentages: C: 0.02% - 0.06%, Si ≤ 0.4%, Mn: 18% - 22%, Cr: 16% - 22%, Ni: 1.0% - 4.5%, Mo: 0.5% - 3.0%, Al ≤ 0.02%, O ≤ 0.003%, N: 0.45% - 0.75%, H ≤ 0.002%, and the balance is Fe and unavoidable impurities. The specific component design of the electrode bar base material required for electroslag remelting needs to be determined by comprehensively considering material properties, process characteristics, and final application requirements. In other embodiments, those skilled in the art can use the teaching content disclosed herein to seek the required characteristics and appropriately change the values of each element. It should be understood that the use of a numerical range represented by endpoints includes all numbers within that range and any range within that range. Taking C as an example, 0.02% - 0.06% can include 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, and so on.

[0023] In step S120, the electroslag can be composed of chemical components with the following weight percentages: CaF2: 48% - 55%, Al2O3: 20% - 25%, CaO: 18% - 23%, SiO2: 1.0% - 4.0%, MgO: 1.0% - 5.0%. Among them, the addition of SiO2 can not only reduce the melting point of the electroslag system, improve the fluidity of the slag system, facilitate the reaction at the slag-steel interface and the floating and removal of inclusions, but also react with CaO to form calcium silicate (CaSiO3), enhance the desulfurization ability of the slag system, reduce the sulfur content in the steel, and improve the purity of the steel. During the electroslag remelting process, chemical reactions will occur between the molten steel and the slag system. For example, Si in the molten steel will reduce Al2O3 in the slag, generating Al to enter the molten steel, and at the same time generating SiO2 to enter the slag system. By adding SiO2, the activity of SiO2 in the slag increases, inhibiting the reduction of Al2O3 in the slag by the steel, reducing the fluctuation of the Al content in the molten steel, and being beneficial to preventing the increase of Al in the electroslag ingot during the smelting process. The addition of MgO can increase the basicity of the slag system, enhance the desulfurization and dephosphorization abilities of the slag system, further reduce the sulfur and phosphorus contents in the steel, that is, better remove inclusions and impurity element S in the non-magnetic steel; at the same time, an appropriate amount of MgO can improve the fluidity of the slag system, promote the reaction at the slag-steel interface and the floating and removal of inclusions, which is beneficial to improving the cleanliness and composition uniformity of the electroslag ingot. In addition, MgO can also react with nitrogen in the steel to form stable nitrides, helping to control the nitrogen content in the steel and prevent the loss or excess of nitrogen.

[0024] In the embodiments of the present application, an Ni-Mg alloy can be used as a reducing agent. During the electroslag remelting process, Mg will react preferentially with oxygen (O) and sulfur (S) to form MgO and MgS, and will not significantly reduce Al2O3; while Ni, as an inert metal, can dilute the activity of Mg, slow down the reaction rate of Mg with Al2O3 in the slag, and further inhibit the reduction of Al2O3. Thus, using an Ni-Mg alloy as a reducing agent can prevent the increase of the Al content in the molten steel due to the addition of the reducing agent. Moreover, Mg reacts with oxygen (O) in the molten steel to form MgO, which can quickly reduce the oxygen content in the molten steel, prevent the oxygen potential in the slag from being too high, and oxidize the molten steel during the refining process; Ni, as an inert metal, can dilute the activity of Mg, making the deoxidation reaction of Mg more gentle and avoiding process instability caused by violent reactions. In the embodiments of the present invention, the mass ratio of the Ni-Mg alloy to the electroslag is preferably 1:20 - 30. To prevent the moisture content in the slag from being too high, resulting in too high an H content in the steel and increasing the risk of subsequent forging cracking, the electroslag can be baked at 700 - 800 °C for more than 6 hours before mixing with the reducing agent.

[0025] In step S130, the melting rate of the electrode bar base material can be controlled at 9-11 kg / min. By adopting a high melting rate, the morphology of the molten pool is improved, the fluidity of the molten steel is enhanced, the removal ability of inclusions at the solidification front is improved, and the cleanliness of the molten steel is ensured. The protective atmosphere is dry N2. Before filling the protective atmosphere, the electroslag remelting furnace is evacuated to below 5 Pa. The water and air in the furnace are removed, and then dry N2 gas is filled to atmospheric pressure to prevent the escape of N in the steel during the melting process, improve the uniformity of the electroslag ingot, and at the same time prevent air from entering and contaminating the molten steel, resulting in an increase in the content of gas impurities in the steel and improving the cleanliness of the molten steel.

[0026] In step S140, the furnace cooling time is controlled at 50-80 min to facilitate the formation of a uniform structure, reduce the residual stress, and avoid cracking or deformation of the electroslag ingot.

[0027] The following are specific examples and specific process parameters of the electroslag remelting method of high-nitrogen non-magnetic steel according to the present invention.

[0028] The specific chemical compositions of the electrode bar base materials prepared in Examples 1-5 are shown in Table 1: Table 1 Chemical compositions of the electrode bar base materials in each example, the balance is iron (wt.%)

[0029] Example 1: An electroslag remelting method of high-nitrogen non-magnetic steel disclosed in this example includes the following steps: Step S110, after the electrode bar base material obtained by die casting is polished and turned, it is loaded into the electroslag remelting furnace; Step S120, add 15 kg of Ni-Mg alloy to the electroslag after baking at 700 °C for 7 h and mix evenly. The electroslag composition is 50% CaF2, 23% Al2O3, 19% CaO, 3.0% SiO2, 5.0% MgO; Step S130, evacuate the protective atmosphere electroslag remelting furnace to 1 Pa, then fill it with dry N2 gas to atmospheric pressure, and then fill the slag material mixed with the reducing agent. After starting the arc by electrifying, start melting, and control the melting rate of the electrode bar base material at 9 kg / min; Step S140, after the melting is completed, cool in the furnace for 60 min, and then air-cool to obtain an electroslag ingot.

[0030] Figure 2 This is the finished product metallographic diagram of the electroslag ingot obtained in this example after forging, and no AlN inclusions are generated in the finished product.

[0031] Example 2: An electroslag remelting method of high-nitrogen non-magnetic steel disclosed in this example includes the following steps: Step S110: Grind and turn the electrode bar base material obtained by die casting, and then load it into the electroslag remelting furnace; Step S120: Add 18 kg of Ni-Mg alloy to the electroslag after baking at 750 °C for 6.5 h and mix evenly. The electroslag composition is 52% CaF2, 21% Al2O3, 20% CaO, 3.0% SiO2, and 4.0% MgO; Step S130: Evacuate the protective atmosphere electroslag remelting furnace to 1 Pa, then fill it with dry N2 gas to atmospheric pressure, and then fill the slag material mixed with the reducing agent. After energizing and starting the arc, start melting. The melting rate of the electrode bar base material is controlled at 10 kg / min; Step S140: After the melting is completed, cool the furnace for 70 min, and then perform air cooling to prepare the electroslag ingot.

[0032] Example 3: An electroslag remelting method for high nitrogen non-magnetic steel disclosed in this example includes the following steps: Step S110: Grind and turn the electrode bar base material obtained by die casting, and then load it into the electroslag remelting furnace; Step S120: Add 15 kg of Ni-Mg alloy to the electroslag after baking at 800 °C for 6 h and mix evenly. The electroslag composition is 49% CaF2, 20% Al2O3, 22% CaO, 4.0% SiO2, and 5.0% MgO; Step S130: Evacuate the protective atmosphere electroslag remelting furnace to 1 Pa, then fill it with dry N2 gas to atmospheric pressure, and then fill the slag material mixed with the reducing agent. After energizing and starting the arc, start melting. The melting rate of the electrode bar base material is controlled at 11 kg / min; Step S140: After the melting is completed, cool the furnace for 80 min, and then perform air cooling to prepare the electroslag ingot.

[0033] Example 4: An electroslag remelting method for high nitrogen non-magnetic steel disclosed in this example includes the following steps: Step S110: Grind and turn the electrode bar base material obtained by die casting, and then load it into the electroslag remelting furnace; Step S120: Add 12 kg of Ni-Mg alloy to the electroslag after baking at 750 °C for 6.5 h and mix evenly. The electroslag composition is 48% CaF2, 25% Al2O3, 23% CaO, 3% SiO2, and 1% MgO; Step S130: Evacuate the protective atmosphere electroslag remelting furnace to 1 Pa, then fill it with dry N2 gas to atmospheric pressure, and then fill the slag material mixed with the reducing agent. After energizing and starting the arc, start melting. The melting rate of the electrode bar base material is controlled at 10 kg / min; Step S140: After the melting is completed, cool the furnace for 50 min, and then perform air cooling to obtain an electroslag ingot.

[0034] Example 5: An electroslag remelting method for high-nitrogen non-magnetic steel disclosed in this example includes the following steps: Step S110: Grind and turn the electrode bar base material obtained by die casting, and then load it into the electroslag remelting furnace. Step S120: Add 18 kg of Ni-Mg alloy to the electroslag that has been baked at 780 °C for 7 h, and mix evenly. The electroslag composition is 55% CaF2, 20% Al2O3, 18% CaO, 2% SiO2, and 5% MgO. Step S130: Vacuum the protective atmosphere electroslag remelting furnace to 1 Pa, then fill it with dry N2 gas to atmospheric pressure, and then fill the slag material mixed with the reducing agent. After starting the arc by electrification, start melting, and control the melting rate of the electrode bar base material at 10 kg / min. Step S140: After the melting is completed, cool the furnace for 65 min, and then perform air cooling to obtain an electroslag ingot.

[0035] Comparative example: Grind and turn the electrode bar base material obtained by die casting, and then load it into the electroslag remelting furnace. After baking the slag material at 650 °C for 7 h, mix 5 kg of Al powder into it. The electroslag composition is 70% CaF2 and 30% Al2O3. Vacuum the protective atmosphere electroslag remelting furnace to 1 Pa, then fill it with dry Ar gas and then add the slag material mixed with the reducing agent. After starting the arc by electrification, start melting, and control the melting rate of the electrode bar base material at 13 kg / min. After the melting is completed, cool the furnace for 50 min, and then perform air cooling to obtain an electroslag ingot.

[0036] Figure 3 This is the metallographic diagram of the finished product after forging the electroslag ingot obtained in this comparative example. The AlN inclusions in the finished product seriously exceed the standard.

[0037] Conclusion Detect the main chemical components at the head and tail of the high-nitrogen non-magnetic steel electroslag ingots obtained in Examples 1-5, and at the same time detect the main chemical components at both ends of the high-nitrogen non-magnetic steel ingot obtained in the comparative example. The results are shown in Table 2: Table 2 Chemical components at the head and tail of the electroslag ingots of each example and at both ends of the comparative example, with the balance being iron (wt.%)

[0038] Rate the inclusions in the high-nitrogen non-magnetic steel ingots obtained in Examples 1-5 and the comparative example. The results are shown in Table 3: Table 3 Inclusion rating results

[0039] The results show that the composition uniformity and cleanliness level of the electroslag ingot of high-nitrogen non-magnetic steel are good.

[0040] In summary, the present invention designs a new electroslag remelting slag system and optimizes the slag mixing process to inhibit the reaction between Al2O3 in the slag and the molten steel during electroslag remelting, avoiding Al increase caused by the slag-steel reaction and Al particles in the slag mixing; at the same time, the smelting process is optimized to control the molten pool and composition segregation during smelting, improve the inclusion removal rate, and finally achieve high-quality and stable production of high-nitrogen non-magnetic steel.

[0041] The above is only a preferred embodiment of the present invention and is not used to limit the scope of implementation of the present invention; any modification or equivalent replacement of the present invention without departing from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

[0042] Those of ordinary skill in the art should understand that the discussion of any of the above embodiments is exemplary only and is not intended to imply that the scope disclosed by the embodiments of the present invention is limited to these examples; under the idea of the embodiments of the present invention, the technical features between the above embodiments or different embodiments can also be combined, and there are many other variations in different aspects of the embodiments of the present invention as described above, which are not provided in detail for the sake of brevity. Therefore, any omission, modification, equivalent replacement, improvement, etc. made within the spirit and principle of the embodiments of the present invention shall be included in the protection scope of the embodiments of the present invention.

Claims

1. An electroslag remelting method for high-nitrogen non-magnetic steel, characterized in that, It includes the following steps: S110, prepare the electrode bar base material required for electroslag remelting and load it into the electroslag remelting furnace; S120, add a reducing agent to the electroslag of the CaF2-CaO-Al2O3-MgO-SiO2 five-component slag system and mix it thoroughly; S130, fill the electroslag mixed with the reducing agent in the electroslag remelting furnace under a protective atmosphere, start the arc after energization, and control the melting rate of the electrode bar base material for smelting; S140, after the smelting is completed, first perform furnace cooling and then air cooling to prepare an electroslag ingot.

2. The method according to claim 1, characterized in that In step S110, the electrode bar base material is prepared by an ingot casting or continuous casting process.

3. The method according to claim 1, characterized in that In step S110, the electrode bar base material is composed of chemical components with the following weight percentages: C: 0.02% - 0.06%, Si ≤ 0.4%, Mn: 18% - 22%, Cr: 16% - 22%, Ni: 1.0% - 4.5%, Mo: 0.5% - 3.0%, Al ≤ 0.02%, O ≤ 0.003%, N: 0.45% - 0.75%, H ≤ 0.002%, and the balance is Fe and unavoidable impurities.

4. The method according to claim 1, wherein In step S120, the electroslag is composed of chemical components with the following weight percentages: CaF2: 48% - 55%, Al2O3: 20% - 25%, CaO: 18% - 23%, SiO2: 1.0% - 4.0%, MgO: 1.0% - 5.0%.

5. The method according to claim 4, characterized in that In step S120, the reducing agent is a Ni-Mg alloy, and the mass ratio of the Ni-Mg alloy to the electroslag is 1:20 - 30.

6. The method according to claim 5, wherein In step S120, the electroslag is baked at 700 - 800 °C for more than 6 hours before mixing with the reducing agent.

7. The method according to claim 1, wherein In step S130, control the melting rate of the electrode bar base material to be 9 - 11 kg / min.

8. The method according to claim 1, wherein In step S130, the protective atmosphere is dry N2.

9. The method according to claim 1, characterized in that, In step S130, before filling the protective atmosphere, evacuate the electroslag remelting furnace to below 5 Pa.

10. The method according to claim 1, wherein In step S140, control the furnace cooling time to be 50 - 80 min.

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