Method for controlling niobium segregation in niobium-containing ferritic stainless steel

Abstract

The present application relates to a method for controlling the solidification segregation of niobium in ferritic stainless steel containing niobium, and the smelting of the ferritic stainless steel comprises the following process flow: primary smelting, VOD furnace refining, LF furnace refining, continuous casting and casting, and the method comprises the following steps: after the molten steel is well reduced after primary smelting, the prepared niobium metal is added to the molten steel at one time, and the stirring intensity of bottom blowing is greater than 600 NL / min, and the stirring time is more than 10 min; and the parameters of the continuous casting process are controlled to improve the equiaxed crystal ratio of the casting blank. The present application controls the solidification segregation of niobium in ferritic stainless steel containing niobium from the aspects of molten steel niobium alloying process and continuous casting process, reduces the intergranular segregation of niobium in the primary blank shell of continuous casting, improves the equiaxed crystal ratio of the casting blank, and further improves the banded structure defects of the rolled material caused by the solidification segregation of niobium.

Description

Technical Field

[0001] This invention belongs to the field of steel production technology, and specifically relates to a method for controlling niobium solidification segregation in niobium-containing ferritic stainless steel. Background Technology

[0002] Niobium-containing ferritic stainless steel is a type of ferritic stainless steel that uses niobium (Nb) as a key microalloying element. Niobium-containing ferritic stainless steel improves corrosion resistance, high-temperature strength, and formability through the formation of carbonitrides and grain boundary strengthening by niobium. The niobium content in general steel ranges from 0.20% to 0.60%, with a maximum of 1.2%. However, in actual production and use, it has been found that banded structures exist in the microstructure of cold-rolled stainless steel sheets, and the cause of this microstructure inhomogeneity is compositional segregation.

[0003] The general production process of niobium-containing ultra-pure ferritic stainless steel is smelting → continuous casting → hot rolling → hot plate annealing and pickling → cold rolling → cold plate annealing and pickling. Relevant scholars have conducted systematic research on the segregation behavior of Nb in high-niobium ultra-pure ferritic stainless steel and the inheritance behavior of the microstructure throughout the entire process. They believe that the formation of banded microstructure in cold-rolled plates is due to Nb segregation. Nb segregation is formed during the solidification of the continuously cast billet, and the main influencing factor is the low proportion of equiaxed crystals during the continuous casting process. Nb segregation has microstructure inheritance, and the segregation is inherited from the continuously cast billet to the hot-rolled annealed plates and cold-rolled annealed plates, forming banded microstructure. Summary of the Invention

[0004] In order to solve all or part of the above problems, the present invention aims to provide a method for controlling niobium solidification segregation in niobium-containing ferritic stainless steel.

[0005] According to one aspect of the present invention, a method for controlling niobium solidification segregation in niobium-containing ferritic stainless steel is provided. The smelting of ferritic stainless steel includes the following process flow: primary smelting → VOD furnace refining → LF furnace refining → continuous casting → billet grinding. The method includes:

[0006] After the molten steel has undergone primary refining and achieved good reduction, the prepared niobium metal is added to the molten steel in one go, and the mixture is stirred at a bottom-blowing stirring intensity greater than 600 NL / min for at least 10 minutes. Good reduction is defined as a Cr₂O₃ content in the slag of less than 0.1%, an Al content in the molten steel of greater than or equal to 0.03%, and a S content in the molten steel of less than 0.001%.

[0007] Controlling the parameters of the continuous casting process can increase the proportion of equiaxed crystals in the billet.

[0008] Furthermore, the step of adding the prepared niobium metal to the molten steel all at once after primary refining and good reduction, and stirring at a bottom-blowing stirring intensity of greater than 600 NL / min for more than 10 minutes specifically involves:

[0009] After the molten steel has undergone primary refining and good reduction, the niobium yield is calculated to be 95%. The prepared niobium metal is added to the molten steel all at once and stirred at a bottom blowing stirring intensity of more than 600 NL / min for more than 10 minutes.

[0010] Furthermore, the method also includes:

[0011] The total time from the addition of niobium metal to the end of continuous casting stirring and the start of casting should be greater than 40 minutes.

[0012] Furthermore, the control of parameters in the continuous casting process to increase the equiaxed crystal ratio of the billet includes at least one of the following:

[0013] The superheat of molten steel in the continuous casting tundish should be controlled at 40℃-60℃.

[0014] After the molten steel in the tundish enters the crystallizer, the primary cooling process uses a water flow rate of 2500-3000 L / min·m. 2 The strong cooling process;

[0015] The secondary cooling process employs a forced cooling technique with a water flow rate of 1.2-1.4 L / t.

[0016] A dual electromagnetic stirring process is adopted, which includes electromagnetic stirring in the curved section and electromagnetic stirring at the solidification end.

[0017] The continuous casting speed is controlled within the range of 0.90-1.10 m / min.

[0018] Furthermore, the parameters for controlling the continuous casting process to increase the equiaxed crystal ratio of the billet also include: controlling the basicity of the mold flux to be 0.8-0.9 and the viscosity of the mold flux to be 0.2-0.4 Pa·s at 1300℃.

[0019] Furthermore, the secondary cooling process employs a forced cooling technique with a water flow rate of 1.2-1.4 L / t, specifically as follows:

[0020] The secondary cooling adopts a forced cooling process with a water flow rate of 1.2-1.4 L / t, and the inlet temperature of the cooling water is controlled at 20-30℃.

[0021] Furthermore, the dual electromagnetic stirring process, which includes electromagnetic stirring at the curved section and electromagnetic stirring at the solidification end, specifically refers to:

[0022] The stirring current is controlled at 1000-1600A and the stirring frequency is 3-5Hz. Electromagnetic stirring is performed on the curved section, and the stirring direction is changed every 15 seconds.

[0023] Control the stirring current to 300-500A for electromagnetic stirring at the end of solidification.

[0024] As can be seen from the above technical solution, the method for controlling niobium solidification segregation in niobium-containing ferritic stainless steel provided by the present invention has the following beneficial effects:

[0025] This invention addresses the problem of niobium solidification segregation in niobium-containing ferritic stainless steel by controlling it from two aspects: the niobium alloying process in molten steel and the continuous casting process. This reduces intergranular segregation of niobium in the initial shell of the continuously cast billet, increases the proportion of equiaxed crystals in the billet, and thus improves the banded structure defects in the rolled material caused by niobium solidification segregation. Attached Figure Description

[0026] Figure 1 This is a flowchart of a method for controlling niobium solidification segregation in niobium-containing ferritic stainless steel according to an embodiment of the present invention;

[0027] Figure 2 This represents the stirring curve corresponding to any bottom blowing line;

[0028] Figure 3 The equiaxed crystal control of the cast billet is obtained in Example 1. Detailed Implementation

[0029] To fully understand the purpose, features, and effects of this invention, the following detailed embodiments are provided. Except as described below, the process methods of this invention employ conventional methods or apparatus in the art. Unless otherwise specified, the terms and expressions used below have the meanings commonly understood by those skilled in the art.

[0030] When a range of values ​​is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0031] like Figure 1 As shown, it illustrates a method for controlling niobium solidification segregation in niobium-containing ferritic stainless steel according to an embodiment of the present invention, specifically targeting ultra-pure ferritic stainless steel. The smelting of ferritic stainless steel includes the following process flow: primary smelting → VOD furnace refining → LF furnace refining → continuous casting → billet grinding.

[0032] The ultrapure ferritic stainless steel mentioned here refers to stainless steel with a total carbon and nitrogen content of less than 300 ppm. The raw materials for smelting can be molten iron or scrap iron as needed. When molten iron is selected, it needs to be dephosphorized before smelting in the AOD furnace. When scrap iron is selected, it needs to be smelted in the EAF furnace before smelting in the AOD furnace.

[0033] For the primary refining process, one can choose from AOD furnace, KOBM-S furnace, GOR furnace, induction furnace, or EAF furnace as needed. AOD furnace refers to argon-oxygen decarburization furnace, LF furnace refers to ladle refining furnace, VOD furnace refers to vacuum oxygen blowing decarburization furnace, and EAF furnace refers to electric arc furnace.

[0034] The method for controlling niobium solidification segregation in niobium-containing ferritic stainless steel according to embodiments of the present invention includes the following steps:

[0035] Step S001: After the molten steel has undergone primary refining and achieved good reduction, the prepared niobium metal is added to the molten steel all at once, and stirred at a bottom-blowing stirring intensity greater than 600 NL / min for at least 10 minutes; and

[0036] Step S002: Control the parameters of the continuous casting process to increase the equiaxed crystal ratio of the billet.

[0037] Good steel reduction means that the Cr2O3 content in the slag is less than 0.1%, the Al content in the molten steel is greater than or equal to 0.03%, and the S content in the molten steel is less than 0.001%. Specifically, for example, niobium metal can be added all at once 3 minutes before the end of VOD furnace smelting, or 5 minutes before the end of LF furnace refining. The purpose of adding niobium metal all at once after good steel reduction and controlling the stirring time and intensity is to make the niobium metal more evenly distributed in the molten steel.

[0038] This invention addresses the problem of niobium solidification segregation in niobium-containing ferritic stainless steel by controlling it from two aspects: the niobium alloying process in molten steel and the continuous casting process. This reduces intergranular segregation of niobium in the initial shell of the continuously cast billet, increases the proportion of equiaxed crystals in the billet, and thus improves the banded structure defects in the rolled material caused by niobium solidification segregation.

[0039] For step S001, after the molten steel has undergone initial refining and achieved good reduction, the prepared niobium metal is added to the molten steel all at once, and stirred at a bottom-blowing stirring intensity of greater than 600 NL / min for more than 10 minutes. Specifically:

[0040] After the molten steel has undergone primary refining and good reduction, the niobium yield is calculated to be 95%. The prepared niobium metal is added to the molten steel all at once and stirred at a bottom blowing stirring intensity of more than 600 NL / min for more than 10 minutes.

[0041] The method in this embodiment of the invention further includes controlling the total time from the addition of niobium metal to the end of continuous casting stirring and the start of casting to be greater than 40 minutes.

[0042] In specific implementation, for example, the bottom blowing stirring intensity is 600NL / min, 700NL / min, 800NL / min, 900NL / min, 1000NL / min, 1100NL / min, 1200NL / min, 1300NL / min, 1400NL / min or 1500NL / min. The higher the bottom blowing stirring intensity, the more uniform the mixing of niobium and molten steel. The stirring time is, for example, 15min, 11min, 12min, 13min, 14min or 15min. The total time from the addition of niobium metal to the end of continuous casting stirring and the start of casting is, for example, 45min, 50min, 55min, 60min, 65min, 70min, 80min, 90min, 100min, 110min or 120min.

[0043] Since ultrapure ferritic stainless steel is a high-alloy steel, its viscosity in molten steel is higher than that of ordinary low-alloy steel. Therefore, after niobium is added to molten steel, a certain stirring intensity and time must be ensured so that the niobium metal can be more evenly distributed in the molten steel, and the total stirring time should be controlled to be greater than 40 minutes to achieve sufficient stirring.

[0044] Controlling the parameters of the continuous casting process in step S002 to increase the equiaxed grain ratio of the cast billet includes at least one of the following:

[0045] S0021: Control the superheat of molten steel in the continuous casting tundish to 40℃-60℃;

[0046] S0022: After the molten steel in the tundish enters the crystallizer, the primary cooling process uses a water flow rate of 2500-3000 L / min·m. 2 The strong cooling process;

[0047] S0023: Secondary cooling adopts a forced cooling process with a water flow rate of 1.2-1.4L / t;

[0048] S0024: A dual electromagnetic stirring process is adopted, including electromagnetic stirring in the bending section and electromagnetic stirring at the solidification end.

[0049] S0025: Control the continuous casting speed range to 0.90-1.10 m / min.

[0050] Specifically, for example, the superheat of molten steel in the continuous casting tundish is 40℃, 45℃, 50℃, 55℃ or 60℃.

[0051] For example, the water flow rate used for primary cooling is 2500 L / min·m 2 2600L / min·m 2 ,

[0052] 2700L / min·m 22800L / min·m 2 2900L / min·m 2 Or 3000 L / min·m 2 2500 L / min·m 2 This indicates that the cooling water volume per square meter of steel plate is 2500L per minute.

[0053] For example, the water flow rate used for secondary cooling is 1.2L / t, 1.25L / t, 1.3L / t, 1.35L / t, and 1.4L / t. Here, 1.2L / t means that the cooling water volume per ton of molten steel is 1.2L.

[0054] For example, the continuous casting speed is 0.90 m / min, 0.95 m / min, 1.00 m / min, 1.05 m / min or 1.10 m / min.

[0055] Through research and follow-up experiments, it has been found that strong cooling in continuous casting combined with a suitable electromagnetic stirring process can effectively solve the problem of intergranular Nb segregation caused by coarse columnar crystals in continuous casting billets. Therefore, in the embodiments of this invention, both primary and secondary cooling adopt strong cooling processes, thereby reducing intergranular segregation of niobium in continuous casting billets.

[0056] The implementation of the dual electromagnetic stirring process in this embodiment of the invention enables the equiaxed crystal ratio of the cast billet to reach more than 60%.

[0057] Furthermore, by reducing intergranular segregation of niobium in continuous casting and increasing the proportion of equiaxed crystals in the billet, the banded structure defect caused by niobium solidification segregation in the rolled material was improved.

[0058] Controlling the parameters of the continuous casting process to increase the proportion of equiaxed crystals in the billet also includes controlling the basicity of the mold flux to 0.8-0.9 and the viscosity of the mold flux to 0.2-0.4 Pa·s at 1300℃. For example, the protective slag with the composition shown in Table 1 below can be used. According to Table 1, the protective slag used in the embodiments of the present invention includes the following components in parts by mass: SiO2 30-40 parts, CaO 27-35 parts, Al2O3 5.9-7.9 parts, MgO 2.0-4.0 parts, F 4.9-7.9 parts, Na2O 8.4-11.4 parts. The protective slag components in Table 1 correspond to the indicators of the protective slag shown in Table 2 below. As can be seen from Table 2, the basicity of the protective slag is 0.8-0.9, where basicity refers to the weight percentage of basic oxide (CaO) and acidic oxide (SiO2). The viscosity of the protective slag at 1300℃ is 0.2-0.4 Pa·s, and the melting point of the protective slag is 1070-1170℃.

[0059] The acidic high-viscosity protective slag used in this embodiment of the invention is intended to ensure the cooling intensity of the primary cooling and the surface quality of the cast billet.

[0060] Table 1: Composition of niobium-ferritic stainless steel protective slag

[0061] scope 34-40 27-35 5.9-7.9 2.0-4.0 4.9-7.9 8.4-11.4

[0062] Table 2: Indicators of the protective slag corresponding to Table 1

[0063] scope 0.8-0.9 0.2-0.4 1070-1170

[0064] The secondary cooling process employs a forced refrigeration technique with a water flow rate of 1.2-1.4 L / t, specifically as follows:

[0065] The secondary cooling process of S0023 adopts a forced cooling process with a water flow rate of 1.2-1.4L / t. Specifically, the secondary cooling adopts a forced cooling process with a water flow rate of 1.2-1.4L / t, and the inlet temperature of the cooling water is controlled at 20-30℃.

[0066] S0024 employs a dual electromagnetic stirring process, including electromagnetic stirring in the curved section and electromagnetic stirring at the solidification end, specifically as follows:

[0067] The stirring current is controlled at 1000-1600A and the stirring frequency is 3-5Hz. Electromagnetic stirring is performed on the curved section, and the stirring direction is changed every 15 seconds.

[0068] Control the stirring current to 300-500A for electromagnetic stirring at the end of solidification.

[0069] This invention successfully solves the problem of niobium solidification segregation by employing a process of high superheat in molten steel, strong cooling during continuous casting, and dual electromagnetic stirring. This invention also develops a method for niobium alloying before LF furnace smelting to address the high viscosity of high-alloy steel, thus solving the problem of uneven steel composition. Furthermore, this invention reduces intergranular segregation of niobium in the initial shell of the continuously cast billet, achieving an equiaxed crystal ratio of over 60% in the billet, thereby improving the banded structure defects in the rolled material caused by niobium solidification segregation.

[0070] The specific embodiments of the present invention are described in detail below with reference to examples, but the specific embodiments of the present invention are not limited to the following examples.

[0071] Example 1

[0072] After roughing and deep decarburization in a VOD furnace, the molten steel is adjusted to achieve the content of all elements except iron as shown in Table 3. Once the molten steel meets the composition control requirements for the corresponding steel grade, it undergoes LF smelting. Upon arrival at the LF furnace, the molten steel is either bottom-blown for slag breaking or electrolytically slag-forming. Niobium iron is added in a single batch at a Nb recovery rate of 95%, and stirring is performed using two bottom-blowing pipelines. The stirring curves of each bottom-blowing pipeline are as follows: Figure 2 As shown, by Figure 2It is known that the stirring operation of each bottom blowing pipeline includes stirring at a stirring intensity of 50 NL / min for 10 minutes, stirring at a stirring intensity of 300 NL / min for 15 minutes, stirring at a stirring intensity of 200 NL / min for 5 minutes, stirring at a stirring intensity of 150 NL / min for 10 minutes, stirring at a stirring intensity of 200 NL / min for 5 minutes, and stirring at a stirring intensity of 50 NL / min for 15 minutes. After the molten steel composition and temperature reach the target requirements, continuous casting is carried out.

[0073] Table 3: Content of elements other than iron in steel grades

[0074] 441 ≤0.03 ≤1.00 ≤1.00 ≤0.040 ≤0.030 17.50-18.50 ＜0.60 0.10-0.60 ≥0.30+3C ＜0.015 442D ≤0.015 ≤0.80 ≤0.50 ≤0.040 ≤0.010 19.00-20.00 ＜0.60 0.45-0.70 ＜0.015

[0075] Before continuous casting, the water nozzles for the two cooling processes should be inspected to ensure that the water nozzle unobstructed rate is >98% to avoid billet cracks caused by uneven cooling. The parameters during the continuous casting process should be set according to Table 4.

[0076] Table 4: Process parameters during continuous casting

[0077] 40-55 0.9-1.1 1.4 1600A 350A

[0078] In this embodiment, the equiaxed crystal ratio of the cast billet reached over 60%, such as Figure 3 As shown, this illustrates that the embodiments of the present invention improve the banded structure defect in rolled materials caused by niobium solidification segregation.

[0079] The present invention has been disclosed above with reference to preferred embodiments. However, those skilled in the art should understand that these embodiments are merely illustrative of the invention and should not be construed as limiting its scope. It should be noted that any variations and substitutions equivalent to these embodiments should be considered to be covered within the scope of the claims. Therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for controlling niobium solidification segregation in niobium-containing ferritic stainless steel, characterized in that, The smelting of ferritic stainless steel includes the following process flow: primary smelting → VOD furnace smelting → LF furnace smelting → continuous casting → billet grinding, and the method includes: After the molten steel has undergone primary refining and achieved good reduction, the prepared niobium metal is added to the molten steel in one go, and the mixture is stirred at a bottom-blowing stirring intensity greater than 600 NL / min for at least 10 minutes. Good reduction is defined as a Cr₂O₃ content in the slag being less than 0.1%, an Al content in the molten steel being greater than or equal to 0.03%, and a S content in the molten steel being less than 0.001%. Controlling the parameters of the continuous casting process to increase the proportion of equiaxed crystals in the billet; The parameters for controlling the continuous casting process to increase the equiaxed crystal ratio of the billet include the following: The superheat of molten steel in the continuous casting tundish should be controlled at 40℃-60℃. After the molten steel in the tundish enters the crystallizer, the primary cooling process uses a water flow rate of 2500-3000 L / min·m. 2 The strong cooling process; A dual electromagnetic stirring process is adopted, which includes electromagnetic stirring in the curved section and electromagnetic stirring at the solidification end. The continuous casting speed is controlled within the range of 0.90-1.10 m / min.

2. The method according to claim 1, characterized in that, The process of adding the prepared niobium metal to the molten steel in one go after primary refining and good reduction, and stirring at a bottom-blowing stirring intensity of greater than 600 NL / min for more than 10 minutes specifically involves: After the molten steel has undergone primary refining and good reduction, the niobium yield is calculated to be 95%. The prepared niobium metal is added to the molten steel all at once and stirred at a bottom blowing stirring intensity of more than 600 NL / min for more than 10 minutes.

3. The method according to claim 1, characterized in that, The method further includes: The total time from the addition of niobium metal to the end of continuous casting stirring and the start of casting should be greater than 40 minutes.

4. The method according to claim 1, characterized in that, The parameters for controlling the continuous casting process to increase the equiaxed crystal ratio of the billet also include: controlling the basicity of the mold flux to be 0.8-0.9 and the viscosity of the mold flux to be 0.2-0.4 Pa·s at 1300℃.

5. The method according to claim 1, characterized in that, The specific implementation of the dual electromagnetic stirring process, which includes electromagnetic stirring at the curved section and electromagnetic stirring at the solidification end, is as follows: The stirring current is controlled at 1000-1600A and the stirring frequency is 3-5Hz. Electromagnetic stirring is performed on the curved section, and the stirring direction is changed every 15 seconds. Control the stirring current to 300-500A for electromagnetic stirring at the end of solidification.

Images