A continuous casting billet for high carbon steel wire and a preparation method thereof

Abstract

The present invention discloses a method for preparing a continuous casting billet for high-carbon steel wire rod, comprising subjecting refined molten steel to a continuous casting process to obtain the continuous casting billet; in the continuous casting process, the straightening position of the continuous casting machine includes six straightening machines, which are, in order along the billet-casting direction, a first straightening machine, a second straightening machine, a third straightening machine, a fourth straightening machine, a fifth straightening machine, and a sixth straightening machine; each straightening machine has a limited pressure and a reduction ratio. The present invention also provides a continuous casting billet obtained by the above-mentioned preparation method. In the preparation method of the present invention, by limiting the pressure and reduction ratio of the straightening machine during the straightening process in the continuous casting process, the force on the solidification front of the billet at the straightening position is controlled to be less than the stress value of the allowable deformation of the billet, and the resulting continuous casting billet has a middle crack level of ≤0.5.

Description

Technical Field

[0001] The present invention relates to the field of steel smelting, and in particular to a continuous casting billet for high-carbon steel wire and a preparation method thereof. Background Art

[0002] High-carbon steel wire rod boasts high strength, a high work-hardening rate, and excellent wire drawing properties. It is widely used in applications such as stranded wire, cables, wire ropes, steel cords, bearings, and tools. However, quality defects in continuous casting billets often carry over to high-carbon steel wire rod, resulting in defects such as scarring, pitting, folding, cracking, and carbon smearing. Therefore, stringent quality requirements are imposed on the continuous casting billets used in the production of high-carbon steel wire rod.

[0003] The quality of continuously cast ingots is primarily categorized into surface quality and internal quality. Internal quality includes characteristics such as center cracks, central porosity, and central segregation. The location of center cracks in continuously cast ingots is often associated with compositional segregation. If the cracking level is high and rolling fails to close, a network of cementite tends to form at the corresponding crack location in the wire rod, leading to brittle fracture during subsequent drawing and substandard performance. Therefore, the level of center cracks in continuously cast ingots for high-carbon steel wire rod should be controlled. Research on the mechanisms of center crack formation currently encompasses three main perspectives: mechanics, metallurgy, and solidification theory. The mechanistic perspective includes the critical stress theory and the critical strain theory; the metallurgical perspective includes the grain boundary embrittlement theory, the notch effect theory of columnar crystal regions, the sulfide brittleness theory, the particle precipitation theory, and the solidification shrinkage theory; and the solidification perspective includes the liquid film theory and the strength theory. These theories offer different insights into the causes of center crack formation, but fundamentally, the ultimate cause of crack formation is that the stress and strain experienced at the solidification front exceed the critical stress and strain that it can withstand. Generally speaking, during the casting process of a large square billet, the following four strains exist: (1) bulging strain; (2) straightening strain; (3) thermal stress strain; and (4) phase transformation stress strain. That is, when the sum of the above four stresses applied to the solidification front of the molten steel exceeds the maximum stress that the solid-liquid interface can withstand, crack defects will occur at the solid-liquid interface inside the billet, biased towards the solid phase. On the other hand, from the metallurgical point of view, the grain boundary embrittlement theory and the particle precipitation theory are both related to precipitates: the grain boundary embrittlement theory believes that at the solidification front of the continuous casting billet, a liquid film rich in solutes (such as sulfides) with a liquid fraction of about 10% surrounds the dendrites, reducing the ductility and strength of the steel near the solidus temperature. When subjected to external forces, cracks will occur along the grain boundaries, causing cracks to form at the solidification front; the particle precipitation theory believes that during the cooling process of the continuous casting billet, precipitates such as AlN and (Ti,V)N precipitate at the grain interface, increasing grain boundary brittleness and crack sensitivity.

[0004] Chinese patent document CN107537989A provides a manufacturing method for controlling the middle cracks of boron-containing steel slabs. By controlling the water volume in the secondary cooling zone during the continuous casting of the boron-containing steel slabs according to the width, thickness and casting speed of the slabs and reasonably distributing the water volume in each zone, the middle cracks of the slabs are improved. This method mainly improves the middle cracks caused by excessive cooling and reheating of the slab surface, and does not explain how to improve the middle cracks caused by precipitates and straightening stress.

[0005] Chinese patent document CN113088613A provides an operating method for controlling the middle cracks of 400 series stainless steel slabs. By controlling the process parameters of molten iron dephosphorization, TSR furnace smelting, LF refining, continuous casting and other processes, the middle cracks of 400 series stainless steel slabs are improved. However, it does not explain how to improve the middle cracks caused by precipitates. Summary of the Invention

[0006] Therefore, the technical problem to be solved by the present invention is that it is difficult to improve the problem of intermediate cracks caused by precipitates and straightening stress in the existing technology, and thus a continuous casting billet for high carbon steel wire and a preparation method thereof are provided.

[0007] To this end, the present invention adopts the following technical solutions:

[0008] A method for preparing a continuous casting billet for high carbon steel wire rod,

[0009] The method comprises subjecting the refined molten steel to a continuous casting process to obtain the continuous casting billet;

[0010] In the continuous casting process, the straightening position of the continuous casting machine includes 6 straightening machines, which are the first straightening machine, the second straightening machine, the third straightening machine, the fourth straightening machine, the fifth straightening machine and the sixth straightening machine in the order of the straightening machine along the casting direction;

[0011] Among them, the pressure of the first tension leveler is 20-35 bar, and the reduction rate is 0-0.2 mm / m;

[0012] The pressure of the third tension leveler is 25-40 bar, and the reduction rate is 0.1-0.3 mm / m;

[0013] The pressure of the fifth tension leveler is 35-50 bar, and the reduction rate is 0.3-0.5 mm / m;

[0014] The pressure of the sixth tension leveler is 50-65 bar, and the reduction rate is 0.6-1.0 mm / m.

[0015] Furthermore, in the molten steel, Ti≤0.0035%, and N≤0.0040%, calculated in mass percentage.

[0016] Among the tension and leveling machines, the first tension and leveling machine, the third tension and leveling machine, the fifth tension and leveling machine and the sixth tension and leveling machine have upper and lower rollers, and the second tension and leveling machine and the fourth tension and leveling machine have only lower rollers.

[0017] In the continuous casting process, the argon pressure of the ladle long nozzle is 4 to 6 bar, the flow rate is 200 to 400 L / min, and the tundish adopts an integral submerged four-hole nozzle.

[0018] In the continuous casting process, the casting speed of the continuous casting machine is 0.75-0.90 m / min, the electromagnetic stirring current of the crystallizer is 600-800 A, the frequency is 1-3 Hz, and the water content in the secondary cooling zone is 0.2-0.4 L / kg.

[0019] The continuous casting machine is a straight arc rectangular billet continuous casting machine with a cross section of 300mm×390mm and an arc radius of 8m.

[0020] Furthermore, the continuous casting process also includes the steps of KR desulfurization, primary refining and refining.

[0021] The present invention also provides a continuous casting billet for high carbon steel wire rod, which is prepared by the above preparation method, and the middle crack of the continuous casting billet is ≤ level 0.5.

[0022] Furthermore, in the continuous casting billet, the following percentages by mass are: 0.80%≤C≤1.00%, 0.20%≤Si≤1.20%, 0.30%≤Mn≤0.80%, 0.10%≤Cr≤0.50%, P≤0.015%, S≤0.01%, 0.01%≤Al≤0.05%, 0.02%≤V≤0.08%, O≤0.0025%, and the balance is Fe and other unavoidable impurities.

[0023] The technical solution of the present invention has the following advantages:

[0024] Aiming at the problem of middle cracks in continuous casting billets for high carbon steel wire rod,

[0025] (1) The present invention limits the pressure and reduction rate of the straightening machine during the straightening process in the continuous casting process, and controls the force on the solidification front of the billet at the straightening position to be less than the stress value of the allowable deformation of the billet.

[0026] (2) The present invention limits the contents of Ti and N. Through the above measures, the formation of (Ti, V) N inclusions during the solidification process of the continuous casting billet is reduced, and the embrittlement of the grain boundaries at the solidification front of the casting billet due to the (Ti, V) N inclusions is prevented, resulting in poor strength and plasticity.

[0027] (3) The present invention further reduces the intermediate cracks caused by straightening stress by adjusting process parameters such as the casting speed and water content of the continuous casting machine.

[0028] (3) The middle crack of the continuous casting billet for high carbon steel wire obtained by the present invention is ≤ level 0.5. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0030] Figure 1 This is a low-magnification longitudinal cross-section of the continuous casting slab obtained in heat number (a) in an embodiment of the present invention;

[0031] Figure 2 This is a low-magnification longitudinal cross-section of the continuous casting slab obtained in heat number (c) in an embodiment of the present invention;

[0032] Figure 3 This is a low-magnification longitudinal cross-section of the continuous casting slab obtained in furnace number (d) in the comparative example of the present invention;

[0033] Figure 4 This is a low-magnification longitudinal cross-section of the continuous casting slab obtained in furnace number (e) in the comparative example of the present invention;

[0034] Figure 5 This is a low-magnification longitudinal cross-section of the continuous casting slab obtained in furnace number (f) in the comparative example of the present invention;

[0035] Figure 6 This is a low-magnification longitudinal cross-section of the continuous casting slab obtained in furnace number (g) of the comparative example of the present invention;

[0036] Figure 7 This is a low-magnification longitudinal cross-section of the continuous casting slab obtained in furnace number (h) in the comparative example of the present invention;

[0037] Figure 8 This is a low-magnification view of the longitudinal section of the continuous casting slab obtained in furnace number (i) in the comparative example of the present invention. DETAILED DESCRIPTION

[0038] The following examples are provided for a better understanding of the present invention and are not intended to limit the best mode of implementation. They do not limit the content and scope of protection of the present invention. Any product identical or similar to the present invention obtained by anyone under the guidance of the present invention or by combining the features of the present invention with other prior arts shall fall within the scope of protection of the present invention.

[0039] If no specific experimental steps or conditions are specified in the examples, the experiments can be carried out according to the conventional experimental steps or conditions described in the literature in the art. The reagents are all commercially available standard reagents.

[0040] The following specific examples are further explanations of the present invention. The examples given here do not represent all embodiments of the present invention. Only some of the embodiments are used as examples for illustration. The specific examples are as follows:

[0041] Example 1

[0042] This embodiment provides a continuous casting billet for high carbon steel wire rod. The molten iron used comprises, by mass percentage, the following: C 4.50%, Si 0.50%, Mn 0.35%, P 0.12%, S 0.022%, V 0.0050%, with the balance being Fe and other unavoidable impurities. The specific preparation method is as follows:

[0043] KR desulfurization process: The molten iron is mechanically stirred and lime powder and fluorite are sprayed into the molten iron for desulfurization;

[0044] BOF primary refining process: The molten iron obtained by KR desulfurization is poured into the converter, and oxygen is blown into the molten iron to increase the temperature, decarburize and dephosphorize it to obtain molten steel;

[0045] LF refining process: The molten steel is hoisted to the refining process for degassing, alloying, and temperature control to obtain molten steel with the required temperature and composition;

[0046] CC continuous casting process: The molten steel with the required temperature and composition is hoisted to the continuous casting platform for protected pouring. The cross-sectional size of the continuous casting billet is 300mm×390mm, and the arc radius is 8m to obtain the continuous casting billet.

[0047] in:

[0048] (1) During the tapping process of the BOF primary refining process, ferrosilicon and metallic manganese are added to the ladle for deoxidation and alloying. The ladle is hoisted to the slag removal position and the top slag is removed. Argon is blown from the bottom during the hoisting process. The argon flow rate is shown in Table 1.

[0049] Table 1 Argon gas flow rate at the bottom of the ladle during lifting

[0050] Furnace number Bottom blowing argon flow rate, L / min (a) 150 (b) 100 (c) 50

[0051] (2) After slagging, the ladle is hoisted to the LF refining process for power-on heating, deoxidation and alloying, and carbon addition. The bottom blowing argon flow rate for each operation is shown in Table 2.

[0052] Table 2 Flow rate of argon gas from bottom blowing of ladle in refining process

[0053] Furnace number Power on temperature rise, L / min Carburization, alloying, L / min Soft stirring, L / min (a) 100 300 30 (b) 200 450 50 (c) 300 600 80

[0054] (3) After LF refining, the ladle is hoisted to the continuous casting platform. The argon pressure and flow rate of the long nozzle are shown in Table 3. The tundish adopts an integral submerged four-hole nozzle;

[0055] Table 3 Argon pressure and flow rate at long shroud

[0056] Furnace number Argon pressure at shroud, bar Long shroud flow rate, L / min (a) 6 400 (b) 5 300 (c) 4 200

[0057] (4) The molten steel in the ladle enters the continuous casting machine mold through the long nozzle, tundish, and submerged nozzle. The molten steel is sampled and analyzed for Ti and N contents, as shown in Table 4.

[0058] Table 4 Ti and N contents in molten steel in the crystallizer

[0059] Furnace number Ti, wt% N, wt% (a) 0.0035 0.0020 (b) 0.0025 0.0030 (c) 0.0015 0.0040

[0060] (5) The molten steel forms a shell of a certain thickness in the crystallizer. The continuous casting billet is pulled by the straightening machine and then enters the secondary cooling zone for water spray cooling. The continuous casting machine pulling speed, crystallizer electromagnetic stirring parameters and the water content in the secondary cooling zone are shown in Table 5.

[0061] Table 5 Casting speed of continuous casting machine, electromagnetic stirring parameters of crystallizer and water content in secondary cooling zone

[0062] Furnace number Casting speed, m / min Current, A Frequency, Hz Specific water volume in the secondary cooling zone, L / kg (a) 0.75 600 3 0.2 (b) 0.80 700 2 0.3 (c) 0.90 800 1 0.4

[0063] (6) After the secondary cooling zone, the continuous casting billet enters the air cooling section, straightening zone, and horizontal section to obtain the continuous casting billet. The center temperature of the wide surface of the continuous casting billet in the straightening zone and the pressure and reduction rate of the 1#, 3#, 5#, and 6# straightening machines are shown in Table 6;

[0064] Table 6 The center temperature of the wide surface in the straightening zone of the continuous casting machine and the pressure and reduction rate of the straightening machine

[0065]

[0066] like Figure 1 and Figure 2 As shown, they are low-magnification longitudinal cross-section images of the continuous casting billets obtained from heat number (a) and heat number (c), respectively.

[0067] Comparative Example 1

[0068] Compared with furnace number (a) in this comparative example and the embodiment, some parameters are changed. The specific changes are shown in Table 7, and the others remain unchanged:

[0069] Table 7 Change parameters of each furnace in comparative example 1

[0070]

[0071] like Figure 3 、 Figure 4 and Figure 5 As shown, they are low-magnification longitudinal cross-section images of the continuous casting billets obtained from heat number (d), heat number (e) and heat number (f).

[0072] Comparative Example 2

[0073] Compared with furnace number (a) in this comparative example and the embodiment, the pressure and reduction rate of the straightening machine in the straightening zone of the continuous casting machine were changed. The specific changes are shown in Table 8, and the other changes remain unchanged:

[0074] Table 8 Pressure and reduction rate of the straightening machine in the straightening zone of the continuous casting machine in comparative example 2

[0075] Furnace number 1#, bar 3#, bar 5#, bar 6#, bar 1#, mm / m 3#, mm / m 5#, mm / m 6#, mm / m (g) 35 40 60 75 0.2 0.3 0.7 1.2 (h) 35 50 60 65 0.2 0.6 0.7 1.0 (i) 45 50 50 65 0.5 0.6 0.5 1.0

[0076] like Figure 6 、 Figure 7 and Figure 8 As shown, they are low-magnification longitudinal cross-section images of the continuous casting billets obtained from heat number (g), heat number (h) and heat number (i).

[0077] Test example

[0078] The performance tests were performed on the continuous casting slabs obtained in the examples and comparative examples:

[0079] (1) Take a low-magnification sample and analyze the density of (Ti, V)N inclusions ≥ 20 μm at a depth of 50 to 90 mm from the inner arc surface in the thickness direction, 20 mm from the center of the wide surface, as shown in Table 9:

[0080] Table 9 (Ti, V)N inclusion density in the center thickness direction of the wide surface of the continuous casting machine

[0081]

[0082]

[0083] (2) The low-magnification intermediate cracks of continuous casting billets were rated according to YB-T 4002-2013 “Rating Chart of Low-magnification Microstructure Defects of Continuously Cast Steel Billets”. The rating results are shown in Table 10.

[0084] Table 10 Rating results of intermediate cracks in continuous casting slabs

[0085] Furnace number Rating results of cracks in the middle of continuous casting slabs (a) Level 0 (b) Level 0 (c) Level 0.5 (d) Level 2.0 (e) Level 2.5 (f) Level 3.0 (g) Level 1.0 (h) Level 2.0 (i) Level 2.5

[0086] Compared with the furnace number (a) in the embodiment, in the furnace numbers (d) and (e) in comparative example 1, the Ti and N contents in the molten steel are increased, so that the formation temperature of (Ti, V) N inclusions is higher than the solidus temperature of the continuous casting billet, and larger and more numerous (Ti, V) N inclusions are formed at the solidification front of the continuous casting billet, resulting in embrittlement of the grain boundaries at the solidification front and poor plasticity. Under the action of straightening stress, the grade of the middle cracks in the continuous casting billet is higher; the pulling speed of furnace number (f) is high, and at the same straightening position, the solidified billet shell is thin and the strength is low. Under the action of straightening stress, the grade of the middle cracks is high.

[0087] Compared with furnace number (a) in the embodiment, in furnace numbers (g), (h) and (i) in comparative example 2, under the action of the same continuous casting speed, superheat, secondary cooling water volume and other parameters, the thickness of the solidified shell of the continuous casting blank at each straightening machine position is basically the same. When the straightening machine pressure at the straightening position of the continuous casting blank is increased, the reduction rate increases accordingly, and the stress on the solidification front of the continuous casting blank becomes larger. Under the premise of consistent number density of (Ti, V) N inclusion precipitation, the ability of the solidified shell of the continuous casting blank to resist deformation gradually increases from the first to the sixth straightening machines, and the straightening radius of the continuous casting machine gradually increases. Therefore, with the increase of the pressure of the first and third, the third and fifth, and the fifth and sixth straightening machines, its influence on the formation of internal cracks in the continuous casting blank gradually decreases, and the intermediate crack level decreases, but all are higher than the crack level of furnace number (a) in the embodiment.

[0088] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will readily appreciate that other variations or modifications based on the above descriptions are possible. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for preparing a continuous casting billet for high carbon steel wire, characterized in that: The method comprises subjecting the refined molten steel to a continuous casting process to obtain the continuous casting billet; In the continuous casting process, the straightening position of the continuous casting machine includes 6 straightening machines, which are the first straightening machine, the second straightening machine, the third straightening machine, the fourth straightening machine, the fifth straightening machine and the sixth straightening machine in the order of the straightening machine along the casting direction; Among them, the pressure of the first tension leveler is 20-35 bar, and the reduction rate is 0-0.2 mm / m; The pressure of the third tension leveler is 25-40 bar, and the reduction rate is 0.1-0.3 mm / m; The pressure of the fifth tension leveler is 35-50 bar, and the reduction rate is 0.3-0.5 mm / m; The pressure of the sixth tension leveler is 50-65 bar, and the reduction rate is 0.6-1.0 mm / m.

2. The preparation method according to claim 1, characterized in that In terms of mass percentage, the molten steel contains Ti≤0.0035% and N≤0.0040%.

3. The preparation method according to any one of claims 1 or 2, characterized in that Among the tension and leveling machines, the first tension and leveling machine, the third tension and leveling machine, the fifth tension and leveling machine and the sixth tension and leveling machine have upper and lower rollers, and the second tension and leveling machine and the fourth tension and leveling machine have only lower rollers.

4. The preparation method according to claim 1, characterized in that In the continuous casting process, the argon pressure of the ladle long nozzle is 4 to 6 bar, the flow rate is 200 to 400 L / min, and the tundish adopts an integral submerged four-hole nozzle.

5. The preparation method according to claim 1, characterized in that In the continuous casting process, the casting speed of the continuous casting machine is 0.75-0.90 m / min, the electromagnetic stirring current of the crystallizer is 600-800 A, the frequency is 1-3 Hz, and the water content in the secondary cooling zone is 0.2-0.4 L / kg.

6. The preparation method according to claim 1, characterized in that The continuous casting machine is a straight arc rectangular billet continuous casting machine with a cross section of 300mm×390mm and an arc radius of 8m.

7. The preparation method according to claim 1, characterized in that The continuous casting process also includes KR desulfurization, primary refining and refining steps.

8. A continuous casting billet for high carbon steel wire, produced by the preparation method according to any one of claims 1 to 7, characterized in that: The middle crack of the continuous casting billet is ≤ level 0.

5.

9. The continuous casting billet according to claim 8, characterized in that: In the continuous casting billet, in percentage by mass, 0.80%≤C≤1.00%, 0.20%≤Si≤1.20%, 0.30%≤Mn≤0.80%, 0.10%≤Cr≤0.50%, P≤0.015%, S≤0.01%, 0.01%≤Al≤0.05%, 0.02%≤V≤0.08%, O≤0.0025%, and the balance is Fe and other inevitable impurities.

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