Process for smelting 200-series stainless steel in VOD (Vacuum Oxygen Decarburization) furnace

Abstract

The invention aims to provide a process for smelting 200-series stainless steel in a VOD (Vacuum Oxygen Decarburization) furnace, which comprises the following steps: adding manganese mineral powder into a steel ladle in advance, then pouring molten stainless steel subjected to primary decarburization in an AOD converter into the steel ladle after slag stopping and tapping, and transporting the steel ladle to a VOD furnace station for vacuum decarburization treatment and reduction treatment after vacuum breaking. The addition amount of the manganese mineral powder is limited by the fact that the thickness of slag in a steel ladle at the initial stage of the vacuum decarburization step is smaller than or equal to 70 mm. The manganese ore is used as one of the manganese alloy sources, the cost is low, the manganese ore powder is firstly added to the bottom of the steel ladle and then poured into the molten steel, the manganese ore powder can be rapidly melted and prevented from escaping, the manganese ore powder is fully utilized in the VOD smelting process, and the yield of the manganese ore powder is increased; in addition, the manganese mineral powder serves as slag and an oxidizing agent in the vacuum decarburization process, the amount of blown oxygen needed by VOD decarburization is reduced, the VOD smelting time is shortened, and the VOD smelting efficiency is improved.

Description

Technical Field

[0001] The invention relates to a steel smelting process, in particular to a process for smelting 200 series stainless steel using a VOD furnace. Background Art

[0002] The manganese content of 200 series stainless steel is relatively high, usually between 5% and 16%. At present, the production process of 200 series austenitic stainless steel is as follows: scrap steel and ferroalloys are melted in an electric arc furnace or alloy melting furnace, and then enter the AOD and VOD furnaces for secondary refining. The molten steel obtained after secondary refining is cast into billets in a continuous casting machine. The manganese content of 200 series stainless steel is relatively high, and manganese ore is usually added to AOD for direct alloying. However, manganese alloys are easily volatilized during low vacuum decarburization in the subsequent VOD (vacuum smelting furnace), which not only greatly reduces the manganese alloy recovery rate, but also the volatilized manganese alloy adheres to the inner wall of the VOD furnace, increasing the equipment maintenance load and reducing the effective operation time.

[0003] Chinese patent CN102168160A discloses a converter steelmaking process for direct reduction and alloying of manganese ore. This process is a production method for producing finished carbon steel products with a manganese content of less than 1.0% under normal pressure. It directly reduces and alloys the manganese ore by adding manganese ore in the middle and late stages of normal pressure blowing in the converter. Compared with vacuum conditions, manganese alloys are more stable and less volatile under normal pressure conditions, but the manganese alloy yield of this process is still low (41-47%), and is not suitable for 200 series stainless steel with a high manganese content.

[0004] In addition, there are no reports on the process of using VOD direct alloying to produce 200 series stainless steel with a high manganese content at home and abroad. Summary of the invention

[0005] The object of the present invention is to provide a process for smelting 200 series stainless steel in a VOD furnace.

[0006] The technical solution to achieve the purpose of the present invention is: a process for smelting 200 series stainless steel in a VOD furnace, comprising the following steps: (1) adding manganese ore powder into a ladle, wherein the mass percentages of manganese oxide, iron oxide and other impurities in the manganese ore powder correspond to 25% to 65%, 10% to 40% and 10% to 30% respectively; (2) pouring the stainless steel liquid after primary decarburization in the AOD converter into the ladle described in step (1) after slag blocking and tapping, wherein the temperature of the stainless steel liquid in the ladle is in the range of 1550°C to 1630°C, and the mass fraction of the chemical composition of the stainless steel liquid in the ladle is controlled as follows: C: 0.15% to 0.45%, Si: 0.05% to 0.2%, Mn: 0% to 1%, Cr: 10% to 20%, Ni: 0% to 10%, and the rest is iron and other residual elements; (3) The ladle after adding the stainless steel liquid in step (2) is transported to the VOD furnace station for vacuum decarburization treatment; during the vacuum decarburization treatment process, the vacuum degree in the VOD furnace reaches ≤1000Pa, and the top blowing oxygen flow rate is 200-300Nm 3 / h, the bottom blowing argon flow rate is 20~30Nm 3 / h for blowing, and the vacuum decarburization treatment time is 15 to 35 minutes; (4) breaking the vacuum of the ladle after the vacuum decarburization treatment, adding reducing materials into the ladle for reduction, and the mass fractions of the chemical composition of the molten steel out of the VOD furnace after the reduction treatment are as follows: C: 0.03% to 0.25%, Si: 0.1% to 0.5%, Mn: 3% to 10%, Cr: 10% to 20%, Ni: 0% to 10%, and the rest are iron and other residual elements; The amount of manganese ore powder added in step (1) is limited to ensure that the slag thickness in the ladle at the initial stage of the vacuum decarburization step in step (3) is ≤70 mm.

[0007] The present invention adds manganese ore powder to the ladle in advance, and the manganese ore powder acts as slag during the vacuum decarburization process. The stainless steel liquid after the initial decarburization in the AOD converter contains a small amount of slag after the slag blocking and steel tapping. Generally speaking, the inlet slag thickness of the VOD furnace is required to be no more than 70 mm. Therefore, the amount of manganese ore powder added in step (1) of the present invention is limited to make the slag thickness in the ladle at the initial stage of the vacuum decarburization step in step (3) ≤ 70 mm. The manganese element in the manganese ore powder exists in the ladle in the form of manganese oxide. The stability of manganese oxide is stronger than that of metallic manganese. It remains stable and not easy to volatilize during the vacuum decarburization process. During the subsequent reduction, the manganese oxide will be converted into metallic manganese with stability deviation under the action of the reducing agent and enter the molten steel. However, at this time, the ladle breaks the vacuum and is in a normal pressure state. Therefore, the manganese metal in the molten steel will still not volatilize, thereby effectively curbing the volatilization of metallic manganese in the VOD smelting process, which is beneficial to improving the yield of manganese alloy and also avoiding the volatilized manganese alloy from adhering to the inner wall of the VOD furnace to bring a series of negative effects. Moreover, since the present invention uses manganese ore as one of the sources of manganese alloy, the use amount and investment cost of manganese alloy are reduced compared with directly using purchased manganese alloy. Meanwhile, the present invention first adds manganese ore powder at the bottom of the ladle, and then pours molten steel, which can quickly melt the manganese ore powder, avoid the manganese ore powder from escaping, make the manganese ore powder fully utilized in the VOD smelting process, help to improve the yield of manganese ore powder, and finally improve the yield of manganese alloy to more than 90%. In addition, the present invention adds manganese ore powder in the ladle in advance, and the oxidizability of slag can also be improved. The manganese ore powder itself can be used as one of the decarburization oxygen sources in the VOD smelting process, thereby reducing the amount of oxygen blown in required for VOD decarburization, thereby shortening the VOD smelting time, and helping to improve the VOD smelting efficiency.

[0008] Furthermore, the oxygen blowing amount of the VOD in step (3) is determined by formula 1: Formula 1: V = M1*(MC*2665.71+MSi*1142.86)-M2*218.62 Formula 1: V-VOD total oxygen blowing volume, Nm 3 ; M1-VOD incoming molten steel volume, t; MC-VOD incoming carbon content, mass fraction; MSi-VOD incoming silicon content, mass fraction; M2-manganese ore addition amount.

[0009] Furthermore, the bottom blowing of nitrogen during the whole process of vacuum decarburization and the top blowing of oxygen last for 10 to 25 minutes, which reduces the amount of oxygen blown in for VOD decarburization and shortens the oxygen blowing time.

[0010] The added amount of the manganese ore powder is preferably 2 to 5 tons. DETAILED DESCRIPTION

[0011] The specific implementation method of the VOD furnace smelting 200 series stainless steel process with the addition of manganese ore powder of the present invention is described in detail below: Example 1

[0012] This embodiment adopts the process of smelting 200 series stainless steel by VOD furnace with manganese ore powder added, and the steel grade produced is 08Cr18Mn6Ni3, and the smelting steps are as follows: (1) adding 3.01 tons of manganese ore powder into a ladle, wherein the mass percentages of manganese oxide, iron oxide and other impurities in the manganese ore powder are 25% to 65%, 10% to 40% and 10% to 30% respectively; (2) pouring the stainless steel liquid after primary decarburization in the converter (AOD) into the ladle described in step (1) after slag blocking and tapping, wherein the temperature of the stainless steel liquid in the ladle is 1593° C., and the mass fraction of the chemical composition of the stainless steel liquid in the ladle is shown in Table 1 below; Table 1 C To Mn Cr In 0.27 0.068 0.65 17.96 3.1 (3) The ladle after adding the stainless steel liquid in step (2) is transported to the VOD station for vacuum decarburization treatment; during the vacuum decarburization treatment, the vacuum degree in the VOD furnace reaches below 1000Pa, and the top blowing oxygen flow rate is 200-300Nm 3 / h, the bottom blowing argon flow rate is 20~30Nm 3 / h for blowing, top blowing oxygen 90Nm 3 The vacuum decarburization treatment time is 31 minutes, and argon is blown at the bottom during the vacuum decarburization process; (4) After the vacuum decarburization treatment is completed, the vacuum is broken, and reducing materials are added for reduction according to the product composition requirements. Specifically, 8.91 tons of silicon manganese alloy, 1.1 tons of lime and 0.7 tons of fluorite are added. The mass fraction of the chemical composition of the molten steel discharged from the VOD furnace after the reduction treatment is shown in Table 2 below. Table 2 C To Mn Cr In 0.053 0.43 5.91 18.06 3.25 The VOD smelting treatment time of Example 1 is 45 minutes in total; after calculation, the manganese alloy yield is 90.09%. Example 2

[0013] This embodiment adopts the process of smelting 200 series stainless steel by VOD furnace with manganese ore powder added, and produces 08Cr18Mn6Ni3 steel. The smelting steps are different from those in Embodiment 1 in that: In step (1), 2.12 tons of manganese ore powder is added to the ladle; The temperature of the stainless steel liquid in the ladle in step (2) is 1615° C., and the mass fractions of the chemical components of the stainless steel liquid in the ladle are shown in Table 3 below; In step (3), 119 tons of stainless steel liquid is transported to the VOD station for vacuum decarburization treatment, and top blowing oxygen is 75Nm 3 , vacuum decarburization treatment time is 29min; Table 3 C To Mn Cr In 0.21 0.072 0.59 17.87 3.16 In step (4), reducing materials are added for reduction according to the product composition requirements. Specifically, 8.95 tons of silicon manganese alloy, 1.8 tons of lime and 1.1 tons of fluorite are added. The mass fractions of the chemical composition of the molten steel out of the VOD furnace are shown in Table 4 below. Table 4 C To Mn Cr In 0.072 0.47 5.67 18.04 3.29 The VOD smelting treatment time of Example 2 is 49 minutes in total; after calculation, the manganese alloy yield is 90.57%. Example 3

[0014] This embodiment adopts the process of smelting 200 series stainless steel by VOD furnace with manganese ore powder added, and produces 08Cr18Mn6Ni3 steel. The smelting steps are different from those in Embodiment 1 in that: In step (1), 4.21 tons of manganese ore powder is added to the ladle; The temperature of the molten steel in the ladle in step (2) is 1569° C., and the mass fractions of the chemical components of the molten steel in the ladle are shown in Table 5 below; In step (3), 125 tons of stainless steel liquid is transported to the VOD station for vacuum decarburization treatment, and top-blown oxygen 102Nm 3 , vacuum decarburization treatment time is 33min; Table 5 C To Mn Cr In 0.34 0.093 0.53 17.83 3.15 In step (4), reducing materials are added for reduction according to the product composition requirements. Specifically, 8.75 tons of silicon manganese alloy, 1.68 tons of lime and 1.16 tons of fluorite are added. The mass fractions of the chemical composition of the molten steel out of the VOD furnace are shown in Table 6 below. Table 6 C To Mn Cr In 0.073 0.49 6.07 18.11 3.25 The VOD smelting treatment time of Example 2 is 51 minutes in total; after calculation, the manganese alloy yield is 91.97%.

[0015] Comparative Example 1 The VOD furnace smelting process of 200 series stainless steel in Comparative Example 1 produces 08Cr18Mn6Ni3 steel, and the smelting steps are as follows: The primary decarburized stainless steel liquid smelted by AOD was tapped with slag blocking and poured into a ladle (without adding manganese ore powder). The temperature of the liquid steel in the ladle was 1586°C. The mass fractions of the chemical components of the liquid steel in the ladle are shown in Table 7 below. Table 7 C To Mn Cr In 0.25 0.079 1.98 17.89 3.09 (2) 126 tons of stainless steel liquid was transported to the VOD station for vacuum decarburization treatment, and 708 Nm of oxygen was blown into the top. 3 , vacuum treatment time 76min, bottom blowing nitrogen throughout the process; (3) After the air decarburization treatment is completed, the vacuum is broken, and reducing materials are added for reduction according to the product composition requirements. Specifically, 9.7 tons of silicon manganese alloy, 1.6 tons of electrolytic manganese, 1.7 tons of lime and 1.3 tons of fluorite are added. The mass fraction of the chemical composition of the molten steel discharged from the VOD furnace after the reduction treatment is shown in Table 8 below. Table 8 C To Mn Cr In 0.069 0.48 6.03 18.01 3.21 The VOD smelting treatment time of Comparative Example 1 is 93 minutes in total; after calculation, the manganese alloy yield is 75.4%.

[0016] Comparative Example 2 The VOD furnace smelting process of 200 series stainless steel in Comparative Example 2 produces 08Cr18Mn6Ni3 steel, and the smelting steps are as follows: (1) The primary decarburized stainless steel liquid produced by AOD was tapped with slag blocking and poured into a ladle (without adding manganese ore powder). The temperature of the liquid steel in the ladle was 1623°C. The mass fractions of the chemical components of the liquid steel in the ladle are shown in Table 9 below. Table 9 C To Mn Cr In 0.23 0.068 2.13 17.91 3.06 (2) 121 tons of stainless steel liquid was transported to the VOD station for vacuum decarburization treatment, and 593 Nm of oxygen was blown into the top gun. 3 , vacuum treatment time 67min, bottom blowing nitrogen throughout the process; (3) After the air decarburization treatment is completed, the vacuum is broken, and reducing materials are added for reduction according to the product composition requirements. Specifically, 9.4 tons of silicon manganese alloy, 1.3 tons of electrolytic manganese, 1.8 tons of lime and 1.4 tons of fluorite are added. The mass fraction of the chemical composition of the molten steel leaving the VOD furnace after the reduction treatment is shown in Table 10 below. Table 10 C To Mn Cr In 0.075 0.46 6.13 18.13 3.16 The VOD smelting treatment time of Comparative Example 2 is 83 minutes in total; after calculation, the manganese alloy yield is 74.21%.

[0017] The oxygen blowing amount of VOD in step (3) of the present invention is determined by formula 1: Formula 1: V = M1*(MC*2665.71+MSi*1142.86)-M2*218.62 Formula 1: V-VOD total oxygen blowing volume, Nm 3 ; M1-VOD incoming molten steel volume, t; MC-VOD incoming carbon content, mass fraction; MSi-VOD incoming silicon content, mass fraction; M2-manganese ore addition amount.

[0018] The vacuum decarburization treatment is performed with bottom blowing of nitrogen throughout the process, and the top blowing of oxygen for a time of 10 to 25 minutes.

[0019] Compared with Comparative Examples 1 and 2, Examples 1 to 3 of the present invention add manganese ore powder to the ladle to perform VOD smelting of 200 series stainless steel, which effectively shortens the VOD oxygen blowing time, improves the VOD smelting efficiency, and shortens the VOD smelting time of 200 series stainless steel to 55 minutes. At the same time, the manganese alloy yield is increased from 75% to more than 90%, which reduces the volatilization of manganese alloy in the vacuum treatment process and greatly reduces the cost of manganese alloying.

[0020] Since the inlet slag thickness of VOD during stainless steel smelting is required to be no more than 70 mm, the amount of manganese ore powder added in step (1) of the present invention is limited to a slag thickness of ≤70 mm in the ladle at the initial stage of the vacuum decarburization step in step (3). When the inlet slag thickness of VOD is 70 mm, the corresponding total amount of slag in the ladle is 5 tons. The amount of manganese ore powder added is preferably 2 to 5 tons.

[0021] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent process changes made using the contents of the present invention specification, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A process for smelting 200 series stainless steel in a VOD furnace, characterized in that: The following steps are involved: (1) adding manganese ore powder into a ladle, wherein the mass percentages of manganese oxide, iron oxide and other impurities in the manganese ore powder correspond to 25% to 65%, 10% to 40% and 10% to 30% respectively; (2) pouring the stainless steel liquid after primary decarburization in the AOD converter into the ladle described in step (1) after slag blocking and tapping, wherein the temperature of the stainless steel liquid in the ladle is in the range of 1550°C to 1630°C, and the mass fraction of the chemical composition of the stainless steel liquid in the ladle is controlled as follows: C: 0.15% to 0.45%, Si: 0.05% to 0.2%, Mn: 0% to 1%, Cr: 10% to 20%, Ni: 0% to 10%, and the rest is iron and other residual elements; (3) The ladle after adding the stainless steel liquid in step (2) is transported to the VOD furnace station for vacuum decarburization treatment; during the vacuum decarburization treatment process, the vacuum degree in the VOD furnace reaches ≤1000Pa, and the top blowing oxygen flow rate is 200-300Nm 3 / h, the bottom blowing argon flow rate is 20~30Nm 3 / h for blowing, and the vacuum decarburization treatment time is 15 to 35 minutes; (4) breaking the vacuum of the ladle after the vacuum decarburization treatment, adding reducing materials into the ladle for reduction, and the mass fractions of the chemical composition of the molten steel out of the VOD furnace after the reduction treatment are as follows: C: 0.03% to 0.25%, Si: 0.1% to 0.5%, Mn: 3% to 10%, Cr: 10% to 20%, Ni: 0% to 10%, and the rest are iron and other residual elements; The amount of manganese ore powder added in step (1) is limited to ensure that the slag thickness in the ladle at the initial stage of the vacuum decarburization step in step (3) is ≤70 mm.

2. The process for smelting 200 series stainless steel by VOD furnace according to claim 1, characterized in that: The oxygen blowing amount of VOD in step (3) is determined by formula 1: Formula 1: V = M1*(MC*2665.71+MSi*1142.86)-M2*218.62 Formula 1: V-VOD total oxygen blowing volume, Nm 3 ; M1-VOD incoming molten steel volume, t; MC-VOD incoming carbon content, mass fraction; MSi-VOD incoming silicon content, mass fraction; M2-manganese ore addition amount.

3. The process for smelting 200 series stainless steel by VOD furnace according to claim 1, characterized in that: The vacuum decarburization treatment is performed with bottom blowing of nitrogen and top blowing of oxygen for 10 to 25 minutes.

4. The process for smelting 200 series stainless steel by VOD furnace according to claim 1, characterized in that: The added amount of the manganese ore powder is 2 to 5 tons.