Method for improving the homogeneity of small bloom billets of high-carbon steel based on a completely new reduction mode

Abstract

The present invention belongs to the technical field of metallurgy, and particularly relates to a method for improving the homogeneity of a small billet of high carbon steel based on a completely new reduction mode, which includes the steps of starting in-mold electromagnetic stirring and electromagnetic stirring at the final stage of solidification during the casting process, controlling the central solid fraction of the slab to fs=0.1-0.2 and the drawing speed to 1.6-3.0 m / min, corresponding to the electromagnetic stirring at the final stage of solidification, performing soft reduction at the final stage of solidification of the slab, the central solid fraction of the slab to fs=0.4-0.85 and the reduction amount to 8-16 mm corresponding to the soft reduction section, and distributing the reduction amount among multiple rolls to perform a small amount of multi-roll continuous soft reduction operation, and performing single-roll heavy reduction using the first press roll corresponding to the central solid fraction of the slab being 1. The method of the present invention realizes homogenization while ensuring efficient production of small billets of high carbon steel, and not only creates a completely new production process path for the energy-saving, low-consumption, low-cost production of high-quality high-carbon steel cast pieces, but also lays an important foundation for the production of high-quality high-carbon special steel from small billets.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgy, and specifically relates to a method for improving the homogeneity of small bloom billets of high-carbon steel based on a completely new rolling reduction mode.

Background Art

[0002] High-carbon steel is likely to form internal defects such as severe central macrosegregation, central porosity, shrinkage cavities, and internal cracks during the continuous casting process. The formation of these defects significantly reduces the homogeneity of the billet. If subsequent processes are not excluded from having further adverse effects, these defects cannot be eliminated or significantly improved by the heating and rolling processes, then significant differences or abnormalities in the structure and performance will occur in the subsequent rolled products. Furthermore, there may be cup-and-cone fractures during the steel processing process, such as drawing, or other failure problems may occur in other usage processes.

[0003] For improving the homogeneity of billets, generally used technical means mainly include low superheat pouring, high-intensity secondary cooling and withdrawal speed control, electromagnetic stirring, and rolling reduction technology, etc. [1-6] Among them, low superheat pouring has very high requirements for temperature control during the pouring process, and easily causes fluctuations in smooth production and pouring stability. High-intensity secondary cooling and withdrawal speed control can play a positive role in central segregation, etc., but the probability of crack defects in billets increases significantly, and production efficiency is also impaired. Electromagnetic stirring technology, especially electromagnetic stirring technology, after continuous development and improvement, realizes a very high industrial application rate in the continuous casting process. However, single electromagnetic stirring has limitations in improving internal quality.

[0004] The continuous casting solidification end-stage rolling reduction technology has been widely applied in recent years. This technology is generally considered an effective method for improving the central segregation and density of continuous casting billets. However, currently, this method mainly applies to circular [7] with large cross-sections, [8、9] square [10、11]It is intensively applied to the slab, and there are few reports and application examples for small billets (less than 200 mm). The reason is that in the case of a small cross-section, mainly because its liquid core is small and it is difficult to grasp the soft reduction effect, the effect of improving by performing soft reduction is not significant, and it is not obvious compared with the conditions of a large cross-section with a large liquid core. Therefore, the soft reduction process is mainly applied to slabs with a large cross-section. Regarding the form of soft reduction, the current soft reduction technology generally focuses on performing soft reduction before the slab is completely solidified, or only performing heavy soft reduction at the end of solidification or after complete solidification. Also, the setting of the parameters of the soft reduction process is a decisive factor affecting the soft reduction effect. If the soft reduction is not appropriate, not only can the quality inside the slab not be improved, but it may also cause deterioration of the segregation distribution inside the slab and slab defects such as cracks.

Prior Art Documents

Non-Patent Documents

[0005]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

Non-Patent Document 8

Non-Patent Document 9

Non-Patent Document 10

Non-Patent Document 11

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention provides a method for improving the homogeneity of small billet slabs of high-carbon steel based on a completely new reduction mode, innovatively using an effective combination of dynamic light reduction and heavy reduction of small billets, and a combination of continuous reduction and single-point reduction in the solidification zone. Furthermore, the synergistic effect of electromagnetic stirring on the control of slab structure is further used to effectively improve the homogeneity of small billets of high-carbon steel.

Means for Solving the Problems

[0007] As specific technical means used to achieve the object of the present invention, a method for improving the homogeneity of small billet slabs of high-carbon steel based on a completely new reduction mode is such that the components of the small billets of the relevant high-carbon steel grades are C: 0.67 to 1.0%, Si: 0.12 to 0.50%, Mn: 0.02 to 0.80%, P ≤ 0.025%, S ≤ 0.025%, and the remainder is the alloy and Fe specifically added to each steel grade, and the specification of the small billet is 145 to 200 mm (side length of the cross-section, preferably the cross-section of the slab is 175 mm × 175 mm).

[0008] Under a drawing speed of 1.6 to 3.0 m / min and appropriate secondary cooling conditions, molten steel is poured into a small billet with a cross-section of 175 mm × 175 mm.

[0009] During the ladling process, in-mold electromagnetic stirring (M-EMS) and final solidification electromagnetic stirring (F-EMS) are initiated. The central solid fraction fs of the slab corresponding to the position of the final solidification electromagnetic stirring is 0.1 - 0.2. At this time, the liquid core at the center of the slab still has high fluidity and can perform a horizontal rotational movement under the stirring action. The stirring serves to homogenize the composition and temperature of the molten steel at the end of solidification and reduce the accumulation of solute elements in the solid-liquid two-phase region. Electromagnetic stirring uses alternating stirring, i.e., intermittent stirring with direction changes. When stirring in this way, a regular and strong stirring movement is not continuously formed in the molten steel at the solidification front, avoiding the occurrence of "banded segregation". Also, intermittent stirring is beneficial for the recovery and continuous growth of columnar crystals, and can control the proportion of equiaxed crystals occupying too large a proportion, avoiding the adverse effects of large shrinkage, thick dendrites, dendrite collapse, and severe V-shaped segregation caused by too large a proportion of equiaxed crystals.

[0010] The setting of the stirring current and frequency is based on the generation of electromagnetic torque. Preferably, the setting range of the electromagnetic torque of in-mold electromagnetic stirring is 13 - 20 N·mm, and the setting range of the electromagnetic torque of final solidification electromagnetic stirring is 15 - 30 N·mm.

[0011] Soft reduction is carried out at the end of solidification of the slab. The central solid fraction fs of the slab corresponding to the reduction interval is 0.4 - 0.85, the reduction amount is 8 - 16 mm, and it is distributed to multiple rolls to perform a small amount and multiple roll continuous soft reduction operation. By effectively feeding, it avoids concentrating the molten steel to the center of the slab due to negative pressure suction caused by shrinkage, while reducing the subsequent feeding pressure of heavy reduction. At the same time, the solute element-rich molten steel between the dendrites in the central region of the slab is appropriately refluxed to the liquid phase to re-perform solute distribution (excessive reflux cannot be carried out, otherwise, negative segregation will increase), and it also prevents the occurrence of reduction cracks at the solidification front at a low central solid fraction and the adverse effects caused by the swelling of the slab.

[0012] Preferably, the press rolls for carrying out soft reduction are 5 - 10 pairs of press rolls continuously distributed, and the range of the reduction speed is controlled to be 2 mm / m - 6 mm / m.

[0013] After that, when the central solid fraction fs of the slab becomes 1, that is, after complete solidification, single-roll heavy reduction is carried out using the corresponding first press roll. Preferably, the reduction amount is 10 to 15 mm. At this time, by performing heavy reduction with a large reduction amount, the risk of central cracking caused by reduction can be significantly avoided. Also, by utilizing the temperature difference between the center and the surface, the reduction can be effectively transmitted to the center of the slab without causing significant widening of the slab, the central shrinkage cavity can be sufficiently crimped and removed, which is advantageous for eliminating porosity.

Advantages of the Invention

[0014] Compared with the prior art, the present invention has the following technical advantages.

[0015] The method of the present invention innovatively performs dynamic mechanical reduction on small square billets, combines light reduction and heavy reduction, and utilizes a reduction method that combines continuous reduction and single-point reduction in a reasonable reduction interval. It fully utilizes the positive effect on slab segregation by light reduction, the positive effect on the density of the slab by heavy reduction, and the synergistic effect, and realizes a significant and effective improvement in the internal quality of small square billet slabs of high-carbon steel under high production efficiency conditions. The implementation of dynamic reduction and the reasonable online adjustment of reduction parameters create favorable conditions for the stable production of high-quality slabs under the casting conditions where the steel type changes, the temperature changes, the cooling changes, and the withdrawal speed changes, and greatly improve the applicability of the steel type and casting conditions of the inventive method. Also, due to the synergistic effect on the control of the internal structure of the slab by electromagnetic stirring, the control and improvement of the homogenization of the center of the slab are further strengthened. Finally, the central segregation index of the small square billet slab of high-carbon steel is stably controlled within 1.08. Regarding the central density, there are no obvious shrinkage cavities and large-scale porosity, and there are no obvious cracks visible to the naked eye on the surface and inside of the slab. While ensuring the efficient production of small square billets of high-carbon steel, homogenization is realized, and not only a completely new production process route is created for the energy-saving, low-consumption, and low-cost production of high-quality high-carbon steel slabs, but also an important foundation is built for producing high-quality high-carbon special steel from small square billets.

Embodiments for Carrying Out the Invention

[0016] The present invention is not limited to the following specific embodiments. Those skilled in the art can implement the present invention using various other specific embodiments based on the disclosure of the present invention, or any modifications or changes obtained by simply using the design structure and concept of the present invention are all included in the protection scope of the present invention. In addition, the examples in the present invention and the features in the examples can be combined with each other as long as they do not conflict.

[0017] Hereinafter, the present invention will be described in more detail with reference to examples.

[0018] For each of the above representative steel grades, during the ladle pouring process, the withdrawal speed is 2.0 - 2.8 m / min, the specific water ratio is 0.25 - 1.2 L / kg, the degree of superheat is 20 - 35 °C, and the specific basic ladle pouring parameters corresponding to each steel grade are shown in Table 1.

[0019] During the ladle pouring process, in-mold electromagnetic stirring and electromagnetic stirring at the end of solidification are started, and the implementation parameters such as the electromagnetic torque of in-mold electromagnetic stirring and electromagnetic stirring at the end of solidification for each steel grade are shown in Table 2. Electromagnetic stirring at the end of solidification uses direction-changing intermittent stirring, and the specific interval is set to 10 s - 3 s - 10 s, that is, the forward rotation lasts for 10 s, stops for 3 s, and the reverse rotation lasts for 10 s, and it operates as one cycle in this way. The central solid fraction fs of the slab corresponding to the action position of electromagnetic stirring at the end of solidification is 0.12, 0.15, 0.13, and 0.18 respectively.

[0020] Perform continuous multi-roll dynamic soft reduction (continuous multi-roll soft reduction within the range where fs is 0.4 to 0.85) at the end of solidification of the slab, and the parameters regarding the dynamic soft reduction of each representative steel grade are shown in Table 3. The range of the central solid fraction fs of the slab corresponding to the reduction interval mainly concentrates on 0.4 to 0.85, and the corresponding values for specific steel grades slightly vary due to the differences in the distribution of the two-phase region according to the characteristics of the steel grade and the casting conditions. The total dynamic soft reduction amount increases from 10 mm to 16 mm with the increase in the carbon content of the representative steel grades, the number of press rolls increases from 5 pairs to 10 pairs, and the reduction rate in the soft reduction interval varies between 2 to 6 mm / m due to the differences in the reduction amount of each roll and the roll interval.

[0021] In the casting process of each steel grade, after the slab is completely solidified, that is, when fs = 1, single-roll heavy reduction is performed again using the first roll, and the reduction amount of each representative steel grade is 10 to 15 mm.

[0022] Basic implementation parameters of continuous casting

Table 1

[0023] Implementation parameters of electromagnetic stirring

Table 2

[0024] Implementation parameters of a completely new combined reduction mode

Table 3

[0025] Under the above implementation conditions, the verification results of the homogenization indexes of the slabs of each representative steel grade are shown in Table 4. As can be seen from this table, the central segregation indexes corresponding to the 5-point drilling method of the 175mm×175mm small billets after implementation are all controlled within 1.08. Based on the standard YB / T153-2015, the uniformity evaluation is carried out. The central porosity does not exceed 0.5 grade, no central shrinkage cavity is observed, almost no various internal cracks occur, and the maximum grade does not exceed 0.5 grade.

[0026] Indexes of the homogeneity of the slabs in the examples

Table 4

[0027] In Comparative Example 1, the drawing speed is low, no rolling reduction is performed, only electromagnetic stirring is carried out, and other parameters are after optimization adjustment.

[0028] The components of the steel grade are C: 0.72%, Si: 0.23%, Mn: 0.56%, P: 0.011%, S: 0.009%, and the rest is Fe.

[0029] During the pouring process, the drawing speed is 1.6m / min, the specific water volume is 0.56L / kg, and the superheat degree is 15~20℃.

[0030] Start electromagnetic stirring in the mold and electromagnetic stirring at the end of solidification. The electromagnetic torque of the electromagnetic stirring in the mold is 15N mm, the electromagnetic torque of the electromagnetic stirring at the end of solidification is set to 30N mm, and the electromagnetic stirring at the end of solidification uses continuous stirring in a single direction. The central solid fraction fs of the slab corresponding to the action position of the electromagnetic stirring at the end of solidification is 0.35.

[0031] In Comparative Example 2, the rolling reduction range covers 0.4≦fs<1, heavy rolling reduction is carried out at 0.9≦fs<1.0, and other parameters are after optimization adjustment.

[0032] The components of the steel grade are C: 0.82%, Si: 0.25%, Mn: 0.68%, P: 0.022%, S: 0.010%, and the rest is Fe.

[0033] During the pouring process, the drawing speed is 2.5 m / min, the specific water ratio is 0.25 L / kg, and the degree of superheat is 20 - 35°C.

[0034] Start the electromagnetic stirring in the mold and the electromagnetic stirring at the end of solidification. The electromagnetic torque of the electromagnetic stirring in the mold is 15 N·mm, and the electromagnetic torque of the electromagnetic stirring at the end of solidification is set to 23 N·mm. The electromagnetic stirring at the end of solidification uses direction-changing intermittent stirring, and the specific interval is set to 10 s - 3 s - 10 s, that is, the forward rotation lasts for 10 s, stops for 3 s, and the reverse rotation lasts for 10 s, and it operates as one cycle in this way. The central solid fraction fs of the slab corresponding to the action position of the electromagnetic stirring at the end of solidification is 0.15.

[0035] During the end stage of solidification of the slab, dynamic soft reduction and heavy reduction are continuously carried out (soft reduction is carried out within the range where fs is 0.2 - 0.9, and heavy reduction is continuously carried out within the range where fs is 0.9 - 1.0). The range of the central solid fraction fs of the slab corresponding to the reduction section is 0.4 - 1. In the action section of soft reduction, fs is 0.4 - 0.9, the amount of soft reduction is 14 mm, and it is carried out by 6 sets of continuous press rolls, and the reduction rate in this process is 1 - 5 mm / m. Then, when 0.9 ≤ fs < 1.0, single-roll heavy reduction is carried out again for the section, and the amount of reduction is 7 mm.

[0036] In Comparative Example 3, the amount of reduction is concentrated at low fs, and other parameters are after optimized adjustment.

[0037] The components of the steel grade are C: 0.86%, Si: 0.22%, Mn: 0.54%, P: 0.014%, S: 0.009%, and the rest is Fe.

[0038] In the process of pouring hot metal, the drawing speed is 2.6 m / min, the specific water ratio is 0.60 L / kg, and the superheat degree is 20 - 35°C.

[0039] Start electromagnetic stirring in the mold and electromagnetic stirring at the end of solidification. The electromagnetic torque of the electromagnetic stirring in the mold is 15 N·mm, and the electromagnetic torque of the electromagnetic stirring at the end of solidification is set to 23 N·mm. The electromagnetic stirring at the end of solidification uses direction-changing intermittent stirring, and the specific interval is set to 10 s - 3 s - 10 s, that is, the forward rotation lasts for 10 s, stops for 3 s, and the reverse rotation lasts for 10 s, and it operates as one cycle in this way. The central solid fraction fs of the slab corresponding to the action position of the electromagnetic stirring at the end of solidification is 0.15.

[0040] Implement dynamic soft reduction and heavy reduction step by step at the end of solidification of the slab. The range of the central solid fraction fs of the slab corresponding to the soft reduction interval is 0.2 - 0.6, the soft reduction amount is 14 mm, the reduction is implemented by 4 pairs of rolls, the reduction rate in this process is 2.5 - 5 mm / m, and the distribution of the reduction amount and the reduction rate are optimized and adjusted based on the low central solid fraction. Then, after complete solidification, that is, when fs = 1, single-roll heavy reduction is implemented again, and the reduction amount is 14 mm.

[0041] In Comparative Example 4, no electromagnetic stirring is performed, the soft reduction amount is too large, and no heavy reduction is performed.

[0042] The components of the steel grade are C: 0.92%, Si: 0.24%, Mn: 0.33%, Cr: 0.27%, P: 0.021%, S: 0.016%, and the rest is Fe.

[0043] In the process of pouring hot metal, the drawing speed is 2.7 m / min, the specific water ratio is 1.12 L / kg, and the superheat degree is 20 - 35°C.

[0044] Start electromagnetic stirring in the mold. The electromagnetic torque of the electromagnetic stirring in the mold is 15 N·mm, and no electromagnetic stirring is performed at the end of solidification.

[0045] Dynamic soft reduction with a large reduction amount is carried out at the end of solidification of the slab. The range of the central solid fraction fs of the slab corresponding to the soft reduction section is 0.4 - 0.7, the soft reduction amount is 32 mm, and it is carried out by 8 sets of continuous press rolls. The reduction rate in this process is 4.5 - 8.5 mm / m. (In this comparative example, both the central solid fraction and the reduction rate of the slab corresponding to the soft reduction section are optimized and adjusted based on a large reduction amount).

[0046] Heavy reduction is not carried out after complete solidification.

[0047] Indicators of the homogeneity of the slab in the comparative example

Table 5

[0048] As can be seen from Table 5, under the conditions of the comparative example, the central segregation of the slab clearly increases. All positive segregations are 1.13 or more, the highest value reaches 1.21, and when the central solid fraction fs is low, central negative segregation due to an overly large reduction amount appears. On the surface of the central density, different degrees of central porosity still exist, and more obvious central shrinkage cavities appear. Regarding internal cracks, due to inappropriate reduction in different comparative examples, different degrees of internal cracks appear in different parts.

[0049] The above content is only a preferred specific embodiment of the present invention, and the protection scope of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the technical means and inventive concept of the present invention within the technical scope disclosed by the present invention should all be included within the protection scope of the present invention.

Claims

1. A method for improving the homogeneity of a bloom billet of high-carbon steel based on a completely new rolling reduction mode, where the mass components of the bloom are C: 0.67 - 1.0%, Si: 0.12 - 0.50%, Mn: 0.02 - 0.80%, P ≤ 0.025%, S ≤ 0.025%, and the balance is specifically added alloy and Fe, The method includes: In the casting process, starting electromagnetic stirring in the mold and electromagnetic stirring at the end of solidification, and controlling the central solid fraction fs of the billet corresponding to the electromagnetic stirring at the end of solidification to be 0.1 - 0.2 and the withdrawal speed to be 1.6 - 3.0 m / min; Performing soft reduction at the end of solidification of the billet, where the central solid fraction fs of the billet corresponding to the soft reduction section is 0.4 - 0.85, the reduction amount is 8 - 16 mm, and distributing it to multiple rolls to perform a small - amount multi - roll continuous soft reduction operation; Performing single - roll heavy reduction using the first press roll corresponding to when the central solid fraction fs of the billet is 1, The setting range of the electromagnetic torque of the electromagnetic stirring in the mold is 13 - 20 N mm, and the setting range of the electromagnetic torque of the electromagnetic stirring at the end of solidification is 15 - 30 N mm. The electromagnetic torque of the electromagnetic stirring at the end of solidification is set larger than the electromagnetic torque of the electromagnetic stirring in the mold. The electromagnetic stirring at the end of solidification stirs with one cycle being 10 s of continuous forward rotation, 3 s of stop, and 10 s of continuous reverse rotation. The press rolls for performing soft reduction are 5 - 10 pairs of continuously distributed press rolls, controlling the range of the reduction rate to 2 mm / m - 6 mm / m, and specifying the soft reduction amount according to the carbon content of the steel such that the soft reduction amount increases as the carbon content of the steel increases. Method.

2. The single - roll heavy reduction amount is 10 - 15 mm. A method for improving the homogeneity of a bloom billet of high - carbon steel based on the completely new rolling reduction mode according to Claim 1, characterized in that.

3. During the casting process, control the drawing speed to 2.0 - 2.8 m / min, A method for improving the homogeneity of small bloom billets of high-carbon steel based on the completely new reduction mode according to claim 1, characterized in that.

4. During the casting process, the specific water volume is 0.25 - 1.2 L / kg and the superheat degree is 20 - 35 °C, A method for improving the homogeneity of small bloom billets of high-carbon steel based on the completely new reduction mode according to claim 1, characterized in that.

5. The cross-section of the small bloom billet is 175 mm × 175 mm, A method for improving the homogeneity of small bloom billets of high-carbon steel based on the completely new reduction mode according to any one of claims 1 to 4, characterized in that.

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