A method for controlling liquidation of NbC in a niobium microalloyed medium carbon non-quenched and tempered steel

Abstract

The application belongs to the technical field of medium-carbon non-quenched and tempered steel production, and discloses a control method of liquid precipitation NbC in niobium micro-alloyed medium-carbon non-quenched and tempered steel. In view of the problem of NbC precipitation in the solidification process of medium-carbon niobium micro-alloyed steel, the Ti and N contents in the steel are controlled, the fine TiN particles are used as the core of liquid precipitation NbC, and the purpose of inhibiting the large-size NbC with dozens of microns in the continuous casting billet is achieved. Through the process, the size of liquid precipitation NbC in the continuous casting billet can meet the requirement of ≤10 μm.

Description

Technical Field

[0001] This invention relates to a method and process for controlling NbC precipitation in niobium microalloyed carbon non-quenched and tempered steel. Background Technology

[0002] Non-quenched and tempered steel refers to medium-carbon alloy structural steel whose phase transformation mechanism is altered by adding microalloying elements (such as V, Nb, Ti, etc.) and whose properties are achieved or approached those of quenched and tempered steel through rolling, forging, and controlled cooling. Because the quenching and tempering process can be omitted when using non-quenched and tempered steel, it can significantly save energy and reduce manufacturing costs by approximately 25%-38% compared to quenched and tempered steel. It is currently widely used in the automotive industry, for example, in components such as crankshafts, connecting rods, and camshafts for automobile engines.

[0003] Niobium (Nb) is a commonly used microalloying element in non-quenched and tempered steels. Due to the significant difference in atomic size between niobium and iron, and the large difference in its diffusion coefficient within austenite compared to iron's self-diffusion coefficient, it can significantly induce solute-drafted recrystallization. Furthermore, due to the strong bonding between niobium and C and N elements, it exists in the steel matrix as second-phase particles, which can act as grain boundary pinning or precipitation strengthening. Therefore, the effective function of Nb requires either solid solution in the steel matrix or precipitation as a nanoscale second phase within the steel matrix. However, in actual production, due to the strong segregation behavior of Nb in molten steel during solidification, large-sized NbC phases, reaching tens or even hundreds of micrometers in size, are frequently found in continuously cast niobium-containing steels (e.g., [example missing]). Figure 1 As shown in the figure, the three-dimensional morphology of the NbC phase in the continuously cast billet of medium-carbon niobium microalloyed steel is shown. The presence of this large-sized phase not only affects the effective utilization of Nb element, but also has an adverse effect on the fatigue and other properties of steel products. The paper "Precipitation Control of NbC in Medium-Carbon Niobium Microalloyed Steel" studied the principle of NbC precipitation. The study found that when the niobium mass fraction in medium-carbon steel is 0.03%, niobium carbide with a diameter of more than 5 μm will precipitate during the continuous casting solidification process. It has high thermal stability and can only be dissolved when the billet rolling heating temperature is about 1250℃ and the high-temperature holding time is more than 4 hours. However, such a high heating temperature and time will aggravate the burning loss and decarburization of steel materials, and this process is difficult to implement. Furthermore, many current technologies have studied the precipitation of titanium nitride particles during the solidification process of continuously cast billets, using them as nucleation sites for heterogeneous precipitation to reduce or eliminate coarse columnar crystal structures in the billets. However, for niobium microalloying, the effects of changes in Ti and N content on the size of NbC during solidification in carbon non-quenched and tempered steel have not been specifically studied. The patent authors' research found that unreasonable Ti and N content can actually lead to insignificant or even worsened NbC refinement. In summary, there are currently no relevant patents or studies involving effective methods for controlling large-sized NbC precipitation. Summary of the Invention

[0004] The application provides a method for controlling heterogeneous nucleation of liquid-precipitated niobium carbide, which optimizes the content of Ti and N, and matches a reasonable continuous casting process, so that fine TiN cores are generated in the steel, the heterogeneous nucleation rate of NbC is improved, and the purpose of refining liquid-precipitated NbC is achieved.

[0005] The application realizes the technical scheme as follows:

[0006] By controlling the content of Ti and N in the steel and matching a suitable continuous casting cooling process, fine TiN particles with a certain number are formed in the solidification stage of the molten steel, so that the heterogeneous nucleation rate of NbC is improved, and the purpose of reducing the size of liquid-precipitated NbC is achieved.

[0007] The application relates to a control method and production process of liquid-precipitated NbC in a niobium micro-alloyed medium-carbon non-quenched and tempered steel.

[0008] The application relates to a control method and production process of liquid-precipitated NbC in a niobium micro-alloyed medium-carbon non-quenched and tempered steel, and comprises the following steps: primary refining in a primary refining furnace, LF refining, vacuum treatment and continuous casting.

[0009] LF refining: the content of Nb is adjusted to the product requirement range by using a niobium-containing alloy in the LF refining process; the content of aluminum is adjusted to 0.025-0.045% at the end of the LF refining process;

[0010] Vacuum treatment: the molten steel after the LF refining process is subjected to RH vacuum treatment, the gas flow is 60-100 Nm 3 / h, the vacuum degree is less than or equal to 1 KPa, the pressure maintaining time is greater than or equal to 10 min; after the vacuum is broken, the content of Ti is adjusted to 0.010-0.018% by using a titanium alloy wire, the molten steel is poured after continuous soft blowing, and the content of nitrogen in the ladle is 50-80 ppm.

[0011] Continuous casting: the cross-sectional size of the continuous casting blank is between 200 mm and 300 mm, the pulling speed is 0.70-0.90 m / min, the secondary cooling water quantity is 0.30 L / kg, the current and frequency of the electromagnetic stirring in the crystallizer are 300 A / 2.5 Hz, and the current and frequency of the electromagnetic stirring at the end are 200 A / 6 Hz.

[0012] The technical scheme principle of the application is as follows:

[0013] (1) Through a large number of theoretical calculations and experimental analyses of the author (such as the papers "The characteristics and generating mechanism of large precipitates in Nb-Ti-microalloyed H13 tool steel", "Generating Mechanism of Large Heterogeneous Carbonitrides with Multiple Layers in H13+Nb Ba", etc.), the micron-sized large-size NbC is precipitated in the liquid steel due to the enrichment of Nb, C and other elements in the liquid steel during solidification. Its formation period is at the end of solidification, usually after 0.90 of the solidification rate, in order to control its size, the effective core can be selected from the aspect of heterogeneous nucleation, the number of nucleation of NbC is increased, so as to achieve the purpose of reducing its size.

[0014] (2) The carbonitride crystal lattice structures of Nb, V and Ti are similar, and can be mutually dissolved, so they can be used as nucleation cores. However, the precipitation of V element is usually after Nb, and cannot be used as a core, while the precipitation of Ti element can be before Nb, so the precipitation of Ti can be controlled to achieve the purpose of controlling the precipitation of Nb. Ti usually precipitates in the form of TiN during solidification, and the precipitation timing is before NbC, so it can be used as an effective core of NbC. However, if the amount of Ti in the steel is too small, it cannot achieve the purpose of refining NbC; if the content of Ti or N is too high, large-size TiN will be formed in the steel, which is even more harmful to the performance of the product than NbC. Therefore, it is necessary to reasonably design the content of Ti and control the content of N, so as to effectively control NbC while avoiding the formation of large-size TiN. Through theoretical calculation and practical experience, the content of Ti is designed to be 0.010-0.018%, and the content of N is designed to be 50-80ppm.

[0015] (4) For the precipitated phase during solidification, appropriate high cooling speed helps to increase the nucleation driving force, improve the nucleation number and refine the size; in addition, the use of electromagnetic stirring helps to alleviate the aggregation and coarsening of the liquid precipitated phase caused by the positive segregation in the center. However, for Nb-containing steel, due to the high temperature brittleness itself, too high cooling is easy to cause cracks in the cast blank, so it is necessary to reasonably design the continuous casting process parameters. For high-end non-quenched and tempered steel products, more than bar is mainly used, and the cross section of the continuous casting blank is usually not too small, so the continuous casting process involved in the present patent is that the cross section of the continuous casting blank is between 200-300mm, the withdrawal speed is 0.70-0.90m / min, the secondary cooling water quantity is 0.30L / kg, the current and frequency of the crystallizer electromagnetic stirring are 300A / 2.5Hz, and the current and frequency of the terminal electromagnetic stirring are 200A / 6Hz. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 Typical morphology of NbC in Example 1.

[0017] Figure 2 Typical morphology of NbC in Example 1.

[0018] Figure 3 Typical morphology of NbC in Comparative Example 1.

[0019] Figure 4 Typical morphology of TiN in Comparative Example 2.

[0020] Figure 5 Two-dimensional size distribution of liquid precipitated NbC in Example and Comparative Example. DETAILED DESCRIPTION

[0021] The application will be further described in connection with the detailed method, but the scope of the application is not limited to the described content.

[0022] Example 1

[0023] This example illustrates the actual control effect of the application on liquid precipitated niobium carbide in the production process of niobium micro-alloyed F40MnVS. In addition to the standard required components, the Nb content in the steel is 0.029%.

[0024] (1) Converter smelting: the converter tapping amount is 145 t, and the tapping C content is 0.06%; the deoxidizer aluminum cake, ferrosilicon, manganese silicon, carbon additive and slag are added in sequence; the slag is lime and slagging agent.

[0025] (2) LF refining: conventional deoxidation and slagging operation is carried out, the refining slag binary basicity CaO / SiO2=3.0; Nb-Fe is used in the refining process to adjust the Nb content to the target value; at the end of the refining process, aluminum wire is used to adjust the Al content to 0.035%, and sulfur wire is used to adjust the S content to the target value.

[0026] (3) RH vacuum degassing: argon lifting, lifting gas flow 90 Nm 3 / h; holding time (minimum vacuum degree 43 Pa) 10 min, after breaking the vacuum, 70% Ti-Fe wire is used to adjust the Ti content to 0.010%; after continuing soft blowing for 15 min, the steel is poured. The nitrogen content in the hanging bag is 50 ppm.

[0027] (4) The cross-section size of the continuous casting billet is 220 mm x 260 mm, the pulling speed is 0.80 m / min, the secondary cooling water ratio is 0.30 L / kg, the crystallizer electromagnetic stirring current and frequency are 300 A / 2.5 Hz, and the end electromagnetic stirring current and frequency are 200 A / 6 Hz.

[0028] The finished product composition is Ti 0.010%, N 52 ppm. The typical morphology of the liquid precipitated NbC in the continuous casting billet is shown in Fig. 1, and it can be seen that the core TiN exists, and the core size is mostly below 3 μm. The two-dimensional size statistical results of the liquid precipitated NbC are shown in Fig. 2, and the size distribution is below 10 μm, the proportion of the statistical size of 1-5 μm is 71.3%, and the proportion of the size of 5-10 μm is 28.7%, which is obviously improved compared with the comparative example 1 and the comparative example 2. Figure 2 Figure 5 Figure 1

[0029] Example 2

[0030] This example illustrates the actual control effect of the present application on the liquid precipitated niobium carbide in the production process of the niobium micro-alloyed F40MnVS. In addition to the standard required composition, the Nb content in the steel is 0.029%.

[0031] (1) Converter smelting: the converter tapping amount is 145 t, and the tapping C content is 0.07%; the deoxidizer aluminum cake, ferrosilicon, silicon manganese, carbon additive and slag are added in sequence, and the slag is lime and slagging agent.

[0032] (2) LF refining: the conventional deoxidation and slagging operation is carried out, the refining slag binary basicity CaO / SiO2 is 3.2; the niobium iron is used in the refining process to adjust the Nb content to the target value; the aluminum wire is used at the end of the refining process to adjust the Al content to 0.034%, and the sulfur line is used to adjust the S content to the target value.

[0033] (3) RH vacuum degassing: nitrogen lifting, lifting gas flow is 90 Nm 3 / h; pressure maintaining time (minimum vacuum degree 51 Pa) is 10 min, after breaking the vacuum, the 70% Ti-Fe wire is used to adjust the Ti content to 0.018%; after continuing to soft blow for 15 min, the steel is poured and cast. The nitrogen content in the hanging bag is 77 ppm.

[0034] (4) The cross-sectional size of the continuous casting billet is 220 mm x 260 mm, the target uses the pulling speed of 0.80 m / min, the secondary cooling specific water consumption is 0.30 L / kg, the crystallizer electromagnetic stirring current and frequency are 300 A / 2.5 Hz, and the end electromagnetic stirring current and frequency are 200 A / 6 Hz.

[0035] The finished product composition is Ti 0.018%, N 78 ppm. The size statistical results of the liquid precipitated NbC in the continuous casting billet are shown in Fig. 1, and the size distribution is below 10 μm, the proportion of the statistical size of 1-5 μm is 89.3%, and the proportion of the size of 5-10 μm is 10.7%, which is further reduced compared with the liquid precipitated NbC size of the example 1, and it can be seen that the different matching of Ti and N content has an influence on the size of the liquid precipitated NbC. Figure 5

[0036] Comparative case 1 (low Ti content) ​​​​

[0037] Using steel of the same composition as a comparative example, its production process is as follows:

[0038] (1) Converter smelting: The converter output is 145t, and the C content of the output steel is 0.06%. Deoxidizer aluminum cake, ferrosilicon, ferromanganese, carbon raiser and slag are added successively after the steel is tapped. The slag is lime and slag-forming agent.

[0039] (2) LF refining: Conventional deoxidation and slag-making operations are carried out, and the binary basicity of the refining slag CaO / SiO2 is 3.1; niobium iron is used to adjust the Nb content to the target value during the refining process; aluminum wire is used to adjust the Al content to 0.037% at the end of the refining process, and sulfur wire is used to adjust the S content to the target value.

[0040] (3) RH vacuum degassing: Nitrogen gas is added at a flow rate of 100 Nm3 / h; high vacuum (minimum vacuum degree 21 Pa) is maintained for 15 min; after vacuum breaking, 70% Ti-Fe wire is used to adjust the Ti content to 0.0070%; soft blowing is continued for 15 min before steel casting. The nitrogen content in the ladle is measured to be 71 ppm.

[0041] (4) The cross-sectional dimensions of the continuous casting billet are 220mm×260mm. The target casting speed is 0.8m / min, the secondary cooling water volume is 0.30L / kg, the electromagnetic stirring current and frequency of the crystallizer are 300A / 2.5Hz, and the electromagnetic stirring current and frequency of the end are 200A / 6Hz.

[0042] The finished product composition is Ti 0.0068% and N 73ppm. The typical morphology of NbC precipitated in the continuously cast billet is as follows: Figure 3 As shown, heterogeneous nucleation NbC is rarely observed. The statistical results of the two-dimensional size of NbC obtained from liquid chromatography are as follows: Figure 5 As shown, many of the cores are larger than 15μm. The composition design is too low, the number of cores is insufficient, and the improvement effect is poor.

[0043] Compare with Case 2 (high nitrogen content)

[0044] Using steel of the same composition as a comparative example, its production process is as follows:

[0045] (1) Converter smelting: The converter output is 145t, and the C content of the output steel is 0.06%. Deoxidizer aluminum cake, ferrosilicon, ferromanganese, carbon raiser and slag are added successively after the steel is tapped. The slag is lime and slag-forming agent.

[0046] (2) LF refining: Perform conventional deoxidation and slag-making operations, and the binary basicity of the refining slag is CaO / SiO2 3.2; adjust the Nb content to the target value with ferroniobium during the refining process; use aluminum wire to adjust the Al content to 0.040% and use sulfur wire to adjust the S content to the target value at the end of the refining process.

[0047] (3) RH vacuum degassing: Nitrogen boosting, boosting gas flow rate by 100 Nm3 / h; high vacuum (minimum vacuum degree 31 Pa) time 15 min, after breaking vacuum, using 70% Ti-Fe line to adjust Ti content to 0.015%, using 500 m nitrogen manganese line to adjust N content; after continuing soft blowing for 15 min, casting. Test nitrogen content in tundish 150 ppm.

[0048] (4) The cross-sectional size of the continuous casting billet was 220 mm x 260 mm, the target was to use a casting speed of 0.8 m / min, the secondary cooling water ratio was 0.30 L / kg, the electromagnetic stirring current and frequency of the crystallizer were 300 A / 2.5 Hz, and the electromagnetic stirring current and frequency of the end were 200 A / 6 Hz.

[0049] The finished product composition was Ti 0.015%, N 0.015%. Large-size TiN appeared in the continuous casting billet, the typical morphology was as shown in Figure 4 The size reached 14.6 μm, and it could be seen that the composition design was high, which would cause more large-size TiN in the continuous casting billet, and its harm to the performance of the product, especially the automobile parts, was more than that of the NbC phase.

Claims

1. A method for controlling NbC precipitation in niobium microalloyed medium-carbon non-quenched and tempered steel, characterized in that, By mass percentage, its product composition includes: C 0.30-0.70%, Si 0.15-1.00%, Mn 0.50-1.80%, Nb≤0.035%, V≤0.10%, Al 0.010-0.040%, Ti 0.010-0.018%, N 50-80ppm. By controlling the Ti and N content in the steel, and utilizing the fine TiN particles formed during the solidification stage of the molten steel, the heterogeneous nucleation rate of NbC is increased, so that the size of liquid-precipitated NbC in the continuously cast billet is ≤10μm. The production process of niobium microalloyed medium carbon non-quenched and tempered steel includes the following steps: primary refining in a primary furnace - LF refining - RH vacuum treatment - continuous casting; The LF refining process uses niobium-containing alloys to adjust the Nb content to the required product range; the LF endpoint adjusts the aluminum content to 0.025-0.045%. After LF refining, the molten steel undergoes RH vacuum treatment, increasing the gas flow rate by 60-100 Nm. 3 / h, vacuum degree ≤1KPa, holding time ≥10min; after breaking the vacuum, use titanium alloy wire to adjust the Ti content to 0.010-0.018%, continue soft blowing and then cast steel, with nitrogen content in the ladle at 50-80ppm.

2. The method for controlling NbC precipitation in niobium microalloyed carbon non-quenched and tempered steel as described in claim 1, characterized in that: The cross-sectional dimensions of the continuously cast billet are between 200-300mm. The casting speed is 0.70-0.90m / min, the secondary cooling water volume is 0.30L / kg, the electromagnetic stirring current and frequency of the crystallizer are 300A / 2.5Hz, and the electromagnetic stirring current and frequency of the end are 200A / 6Hz.

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