Method for improving continuous casting castability of rare earth structural steel

Abstract

The invention provides a method for improving continuous casting castability of rare earth structural steel, which is mainly characterized in that La-Ce rare earth alloy is added into a steel ladle for La-Ce rare earth treatment after deoxidation and desulfurization treatment of molten steel is completed, the La-Ce rare earth content is 20-30ppm, the argon flow in the bottom argon blowing process is 200-300L / min, the soft blowing time is not less than 8min, and the argon pressure is 0.3-0.4 MPa. Through deoxidation process optimization, alloy element content control, refining outbound temperature control, calcium treatment process optimization, continuous casting protection pouring process optimization and the like, the purpose of improving the transverse impact performance of the plate is achieved on the premise that the continuous casting castability is not affected, and the key process for improving the continuous casting castability of the rare earth structural steel is formed. Compared with the prior art, the excellent transverse impact performance of the plate is obtained, and the castability of continuous casting is not affected.

Description

Technical Field

[0001] The invention belongs to the technical field of high-strength steel, and in particular relates to a method for improving the continuous casting castability of rare earth structural steel. Background Art

[0002] Rare earth structural steel is a type of structural steel whose properties are improved by adding rare earth elements (REE). The addition of rare earth elements can significantly improve the strength, toughness, corrosion resistance and other properties of steel, making it perform well in many applications. Although the addition of rare earth elements can significantly improve the performance of steel, it may also bring some adverse effects. Rare earth elements can form stable compounds such as rare earth sulfides and rare earth oxides in molten steel. These compounds may form fine particles in molten steel, increase the viscosity of molten steel and reduce fluidity. Rare earth elements react with sulfur, oxygen and other elements in molten steel to generate fine inclusions; these inclusions may be deposited in the tundish, nozzle and crystallizer during the pouring process, causing blockage and interruption of flow. Therefore, the continuous casting castability of rare earth structural steel is an issue that needs to be solved urgently. Summary of the invention

[0003] The purpose of the present invention is to provide a method for improving the castability of rare earth structural steel continuous casting to solve the above technical problems. The specific scheme is as follows:

[0004] A method for improving the continuous casting castability of rare earth structural steel comprises the following steps: after the molten steel is deoxidized and desulfurized, La-Ce rare earth alloy is added into the ladle for La-Ce rare earth treatment, the La-Ce rare earth content is 20-30 ppm, the argon flow rate in the bottom blowing argon process is 200-300 L / min, the soft blowing time is not less than 8 min, and the argon pressure is 0.3-0.4 MPa.

[0005] In the present invention, the molten steel is first deoxidized, and the low free oxygen in the molten steel is a core parameter for whether the La-Ce rare earth treatment is effective; then rare earth treatment is carried out, and the purpose of modifying the inclusions in the molten steel without affecting the continuous casting castability is achieved by controlling key parameters such as the rare earth content, and then bottom argon blowing is carried out, and the argon gas flow rate and the bottom argon blowing time, that is, the soft blowing time, are strictly controlled to purify the molten steel, ensure that the inclusions in the molten steel are removed as much as possible without the occurrence of slag rolling, and make the molten steel have excellent castability after the rare earth treatment.

[0006] Further preferably, the above-mentioned molten steel deoxidation and desulfurization treatment includes the steps of: adding ferrosilicon and aluminum ingots after the converter is tapped for Al-Si composite deoxidation, and the Al element content in the molten steel in the whole smelting process is controlled in the range of 0.02-0.05wt%; then adding lime for slag making and desulfurization; the final oxygen content of the molten steel after deoxidation and desulfurization treatment is less than 50ppm, and the sulfur content is 0.010-0.012wt%.

[0007] The present invention controls the efficient removal of inclusions in the molten steel under the condition of ensuring extremely low free oxygen in the molten steel through the Al-Si composite deoxidation process, thereby achieving the purpose of smelting high-purity molten steel. The purpose of avoiding slag rolling in the molten steel after rare earth treatment is to obtain the final high-purity molten steel. Too high Al content in the molten steel will not only make the slag interface in the ladle always in an unbalanced state, but also cause nodules in the submerged nozzle during continuous casting, thereby affecting castability. Too low Al content will cause too high dissolved oxygen in the molten steel, which will cause new inclusions to be continuously generated in the molten steel, affecting the cleanliness of the molten steel. Control of Al content is a key parameter to ensure the effect of rare earth treatment, and provides an important guarantee for ensuring the castability of molten steel.

[0008] Further preferably, the above rare earth treatment also includes a titanium treatment step: the aluminum supplementation operation is completed 3 minutes before adding ferrotitanium, the soft blowing time after adding ferrotitanium is greater than 8 minutes, the Ti element content in the molten steel is controlled in the range of 0.038-0.050wt%, and the calming time of the molten steel after leaving the station is greater than 10 minutes.

[0009] Further preferably, the above also includes a calcium treatment step: Ca treatment is performed within 2 minutes after adding titanium iron, the argon flow rate during the treatment process is controlled at 100-300NL / min, the soft blowing time after Ca treatment is greater than 8 minutes, the control range of Ca element content in molten steel is 0.008-0.010wt%, and the calming time of molten steel after leaving the station is greater than 10 minutes. The calcium treatment step is performed when any abnormal situation occurs, such as single-stream casting, poor continuous casting castability, or LF refining power transmission time greater than 15 minutes.

[0010] Further preferably, the main chemical components of the molten steel when it leaves the station are C: 0.15-0.17wt%, Si: 0.10-0.20wt%, Mn: 0.40-0.50wt%, Ti: 0.038-0.050wt%, S<0.012wt%, P≤0.030wt%, Al: 0.020-0.050wt%, N≤0.0050wt%, Ca: 0.008-0.010wt%, and the remainder is Fe element and impurity elements that are difficult to remove.

[0011] Preferably, the above also includes continuous casting process adjustment: target working casting speed 0.85-1.20 m / min, tundish superheat 15-35° C. Further, the tundish superheat of the open casting furnace is 20-35° C., and the tundish superheat of the continuous casting furnace is 15-30° C.

[0012] Preferably, before the continuous casting process begins, the tundish is purged with argon to exhaust the air, and the argon purge time is ≥ 3 minutes. The continuous casting process adjustment also includes cleaning the tundish before baking, and checking whether the tundish cover is well sealed; before baking the tundish, each argon sealing gas path is confirmed to ensure that each argon gas pipeline is not leaking or blocked.

[0013] Preferably, a double-layer covering agent is used in the continuous casting process, wherein the lower covering agent is an alkaline covering agent and the upper covering agent is carbonized rice husk. The use of the double-layer covering agent further ensures the oxygen isolation effect of the molten steel, avoids excessive slag production during the continuous casting of the molten steel, and further avoids overoxidation of the molten steel during the continuous casting process.

[0014] Further preferably, the molten steel level of the tundish during pouring is normally controlled at 1000±100mm, and the molten steel level is ≥600mm in abnormal cases, and the remaining molten steel level in the tundish is ≥200mm. 1000±100mm is a normal process requirement, and ≥600mm is an abnormal situation. When an abnormal situation occurs, the product quality needs to be downgraded.

[0015] Further preferably, the argon control of the above continuous casting process is: the pressure range of the argon gas sealing the long nozzle and the argon gas between the plates is 0-1MPa. In addition, it is strictly prohibited to cast steel without the argon gas sealing the long nozzle, the argon gas of the stopper rod, the argon gas of the upper nozzle and the argon gas sealing the nozzle.

[0016] Further preferably, the rare earth structural steel includes Q345D-RE.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The method for improving the continuous casting castability of rare earth structural steel provided by the present invention mainly achieves the purpose of improving the lateral impact performance of the plate without affecting the continuous casting castability by optimizing the deoxidation process, controlling the alloy element content, controlling the refining exit temperature, optimizing the calcium treatment process, and optimizing the continuous casting protection pouring process, thereby forming a key process for improving the continuous casting castability of rare earth structural steel. Compared with the prior art, the present invention obtains excellent lateral impact performance of the plate without affecting the continuous casting castability. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present drawings or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present drawings. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0020] Figure 1This is a curve diagram of continuous casting stopper rod variation of Q345D-RE rare earth structural steel produced by the method of Comparative Example 1;

[0021] Figure 2 This is a curve diagram of the continuous casting stopper rod change of Q345D-RE rare earth structural steel produced by the method of Example 1. The purpose, functional characteristics and advantages of this figure will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application is described and illustrated below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not intended to limit the present application. Based on the embodiments provided in the present application, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present application.

[0023] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0024] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0025] Comparative Example 1

[0026] A method for improving the continuous casting castability of rare earth structural steel Q345D-RE, comprising the following process steps:

[0027] 1) Deoxidation process: After the converter is tapped, ferrosilicon and aluminum ingots are added for Al-Si composite deoxidation, and the Al element content in the molten steel during the entire smelting process is 0.06wt%; then lime is added for slag making and desulfurization; the final oxygen content of the molten steel after deoxidation and desulfurization treatment is 35ppm, and the sulfur content is 0.015wt%.

[0028] 2) After the deoxidation and desulfurization treatment of the molten steel is completed, La-Ce rare earth alloy (purity of 99.5%, La:Ce=1:2 (atomic ratio)) is added to the ladle for La-Ce rare earth treatment. The La-Ce rare earth content is 40ppm. At this time, the bottom blowing argon process parameters are: argon flow rate 200L / min, soft blowing time 10min, and argon pressure 0.3MPa.

[0029] 3) Alloying process: The ladle that has completed rare earth treatment is supplemented with aluminum and ferrotitanium. The aluminum supplementation operation is completed 3 minutes before adding ferrotitanium. The soft blowing time after adding ferrotitanium is 10 minutes. The Al element content in the molten steel is controlled in the range of 0.04wt%, and the Ti element content is in the range of 0.06wt%. The molten steel is discharged after the temperature reaches 1620℃. The molten steel is cooled for 12 minutes after leaving the station.

[0030] 4) Calcium treatment process: Ca treatment is carried out, and Ca treatment is carried out within 2 minutes after adding titanium iron. The argon flow rate during the treatment process is controlled at 100NL / min. The soft blowing time after Ca treatment is 10 minutes. The control range of Ca element content in the molten steel is 0.015wt%. The molten steel is discharged from the station after the temperature reaches 1610°C. The molten steel is cooled for 10 minutes after it leaves the station. The chemical composition of the molten steel when it leaves the station is: C: 0.15wt%, Si: 0.20wt%, Mn: 0.50wt%, Ti: 0.06wt%, S: 0.010wt%, P: 0.020wt%, Al: 0.04wt%, N: 0.004wt%, Ca: 0.015wt%, Ca: 0.015wt%, and the remainder is Fe element and impurity elements that are difficult to remove.

[0031] 5) Continuous casting process: Continuous casting is performed at a target working speed of 0.85 m / min and a tundish superheat of 15°C to obtain ingots.

[0032] The tundish of the continuous casting process should be cleaned before baking, and the tundish cover should be checked to see if it is well sealed; before baking the tundish, each argon sealing gas path should be confirmed to ensure that each argon gas pipeline is not leaking or blocked.

[0033] In the continuous casting process, before pouring begins, argon is blown into the tundish in advance to exhaust the air, and the argon blowing time is 3 minutes.

[0034] The covering agent of the continuous casting process uses an alkaline covering agent (lower layer, (CaO-SiO 2 -Al 2 O 3 Slag system, w (CaO) / w (SiO2) 为4.2 ) and carbonized rice husk (upper layer) double covering.

[0035] The liquid level of the tundish in the continuous casting process is normally controlled at 1000±100 mm, and the final height of the remaining molten steel in the tundish is 200 mm.

[0036] 6) The ingot produced by the continuous casting process is rolled according to the controlled cooling and controlled rolling process (the starting rolling temperature is 1150°C, the final rolling temperature is 966°C, the reduction procedure is: 60mm→40mm→23mm→12mm, and ultra-fast cooling to room temperature (25°C) is adopted after rolling) to obtain Q345D-RE rare earth structural steel plate.

[0037] During the implementation of the scheme of comparative example 1, the change curve of the stopper rod during the continuous casting process is as follows: Figure 1As shown, the stopper rod swelled in the sixth heat of casting. The transverse impact performance of the Q345D-RE rare earth structural steel plate obtained in comparative example 1 was tested, and the test results are shown in Table 1.

[0038] Table 1 Q345D-RE lateral impact performance test results

[0039]

[0040] Embodiment 1:

[0041] A method for improving the continuous casting castability of rare earth structural steel Q345D-RE, comprising the following process steps:

[0042] 1) Deoxidation process: After the converter is tapped, ferrosilicon and aluminum ingots are added for Al-Si composite deoxidation. The Al element content in the molten steel during the entire smelting process is controlled within a range of 0.03wt%. Subsequently, lime is added for slag making and desulfurization. After deoxidation and desulfurization, the final oxygen content of the molten steel is 35ppm and the sulfur content is 0.015wt%.

[0043] 2) After the deoxidation and desulfurization treatment of the molten steel is completed, La-Ce rare earth alloy (purity of 99.5%, La:Ce=1:2 (atomic ratio)) is added to the ladle for La-Ce rare earth treatment. The La-Ce rare earth content is 20ppm. At this time, the bottom blowing argon process parameters are: argon flow rate 300L / min, soft blowing time 12min, and argon pressure 0.4MPa.

[0044] 3) Alloying process: The ladle that has completed rare earth treatment is supplemented with aluminum and ferrotitanium. The aluminum supplementation operation is completed 3 minutes before adding ferrotitanium. The soft blowing time after adding ferrotitanium is 11 minutes. The Al element content in the molten steel is controlled in the range of 0.025wt%, and the Ti element content is in the range of 0.045wt%. The molten steel is discharged after the temperature reaches 1620℃. The molten steel is cooled for 12 minutes after leaving the station.

[0045] 4) Calcium treatment process: Ca treatment is carried out within 2 minutes after adding titanium iron, and the argon flow rate during the treatment process is controlled at 300NL / min. The soft blowing time after Ca treatment is 10 minutes, and the control range of Ca element content in the molten steel is 0.008wt%. The molten steel leaves the station after the temperature reaches 1610°C. The molten steel calming time after leaving the station is 12 minutes; the composition of the molten steel at this time is: C: 0.16wt%, Si: 0.15wt%, Mn: 0.47wt%, Ti: 0.045wt%, S: 0.08wt%, P: 0.016wt%, Al: 0.025wt%, N: 0.0048wt%, Ca: 0.008wt%; the remainder is Fe element and impurity elements that are difficult to remove.

[0046] 5) Continuous casting process: Continuous casting is performed at a target working speed of 1.20 m / min and a tundish superheat of 15°C to obtain ingots.

[0047] The tundish of the continuous casting process must be cleaned before baking, and the tundish cover must be checked for good sealing; before baking the tundish, each argon sealing gas path must be confirmed to ensure that each argon gas pipeline is not leaking or blocked.

[0048] Before pouring begins, the tundish is purged with argon to exhaust the air. The argon purge time is 3 minutes.

[0049] The covering agent of the continuous casting process uses an alkaline covering agent (lower layer, (CaO-SiO 2 -Al 2 O 3 Slag system, w (CaO) / w (SiO2) It is double-layer covered with 4.2) and carbonized rice husk (upper layer), and a middle bag covering agent is added appropriately to prevent the molten steel from being exposed.

[0050] The liquid level of the tundish in the continuous casting process is normally controlled at 1000±100 mm, and the final height of the remaining molten steel in the tundish is 300 mm.

[0051] Argon control in the continuous casting process: casting steel without argon sealing at the shroud, argon at the stopper rod, argon at the upper shroud and argon sealing at the shroud is strictly prohibited; the pressure range of argon sealing at the shroud and argon between the plates is 0.5 MPa.

[0052] 6) The ingot produced by the continuous casting process is rolled according to the controlled cooling and controlled rolling process (the starting rolling temperature is 1150°C, the final rolling temperature is 966°C, the reduction procedure is: 60mm→40mm→23mm→12mm, and ultra-fast cooling to room temperature (25°C) is adopted after rolling) to obtain Q345D-RE rare earth structural steel plate.

[0053] During the implementation of the scheme of Example 1, the change curve of the stopper rod during the continuous casting process is as follows: Figure 2 As shown in Table 2, no expansion of the rod occurs during the pouring process, and the continuous casting castability is guaranteed. The transverse impact performance of the Q345D-RE rare earth structural steel plate obtained in Example 1 is tested, and the test results are shown in Table 2.

[0054] Table 2 Q345D-RE lateral impact performance test results

[0055]

[0056] By comparing the test results of the embodiment and the comparative example and the curve of the change of the stopper rod during the continuous casting process, it can be found that in the comparative example, due to the fact that the process steps such as deoxidation process optimization, alloy element content control, refining exit temperature control, calcium treatment process and continuous casting protection pouring did not reach the optimal value, the stopper rod swelled in the sixth furnace of pouring, and the continuous casting castability had problems. In addition, the lateral impact performance test data of the plate finally obtained in the comparative example was not as good as that in Example 1. It shows that the present invention achieves the purpose of improving the lateral impact performance of the plate without affecting the continuous casting castability by adjusting and optimizing the deoxidation process, alloy element content control, refining exit temperature control, calcium treatment process optimization and continuous casting protection pouring.

[0057] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A method for improving the castability of rare earth structural steel continuous casting, characterized in that: The following steps are involved: After the deoxidation and desulfurization of the molten steel is completed, La-Ce rare earth alloy is added to the ladle for La-Ce rare earth treatment. The La-Ce rare earth content is 20-30ppm. At this time, the argon flow rate in the bottom blowing argon process is 200-300L / min, the soft blowing time is not less than 8min, and the argon pressure is 0.3-0.4MPa.

2. The method according to claim 1, characterized in that The deoxidation and desulfurization treatment of molten steel comprises the following steps: adding ferrosilicon and aluminum ingots to carry out Al-Si composite deoxidation after steel is tapped from the converter, and the Al element content in the molten steel in the whole smelting process is controlled in the range of 0.02-0.05wt%; then adding lime to carry out slag making and desulfurization; the final oxygen content of the molten steel after deoxidation and desulfurization treatment is less than 50ppm, and the sulfur content is 0.010-0.012wt%.

3. The method according to claim 2, characterized in that The rare earth treatment also includes a titanium treatment step: the aluminum supplementation operation is completed 3 minutes before adding titanium iron, the soft blowing time after adding titanium iron is greater than 8 minutes, the Ti element content in the molten steel is controlled in the range of 0.038-0.050wt%, and the calming time after the molten steel leaves the station is greater than 10 minutes.

4. The method according to claim 3, characterized in that The method also includes a calcium treatment step: performing Ca treatment within 2 minutes after adding ferrotitanium, controlling the argon flow rate during the treatment process at 100-300NL / min, performing soft blowing for more than 8 minutes after Ca treatment, controlling the Ca element content in the molten steel at 0.008-0.010wt%, and performing calming for more than 10 minutes after the molten steel leaves the station.

5. The method according to claim 4, characterized in that The main chemical components of the molten steel when leaving the station are C: 0.15-0.17wt%, Si: 0.10-0.20wt%, Mn: 0.40-0.50wt%, Ti: 0.038-0.050wt%, S<0.012wt%, P≤0.030wt%, Al: 0.020-0.050wt%, N≤0.0050wt%, Ca: 0.008-0.010wt%, and the remainder is Fe element and impurity elements that are difficult to remove.

6. The method according to any one of claims 1 to 5, characterized in that: It also includes continuous casting process adjustment: target working casting speed 0.85~1.20m / min, tundish superheat 15~35℃.

7. The method according to claim 6, characterized in that Before the continuous casting process begins, argon is blown into the tundish in advance to exhaust the air, and the argon blowing time is ≥3 minutes.

8. The method according to claim 7, characterized in that A double-layer covering agent is also used in the continuous casting process, wherein the lower layer covering agent is an alkaline covering agent and the upper layer covering agent is carbonized rice husk.

9. The method according to claim 8, characterized in that The molten steel level in the tundish during pouring is normally controlled at 1000±100mm, and in abnormal circumstances the molten steel level is ≥600mm, and finally the remaining molten steel level in the tundish is ≥200mm.

10. The method according to claim 9, characterized in that The argon gas control of the continuous casting process is as follows: the pressure range of the argon gas sealing the long water nozzle and the argon gas between the plates is 0-1MPa.

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