Medium-carbon high-chromium steel continuous casting round tube blank and preparation method thereof

Abstract

The invention belongs to the field of steel for high-chromium medium-carbon seamless steel tubes, and discloses a medium-carbon high-chromium steel continuous casting round tube blank and a preparation method thereof.The high-chromium medium-carbon steel continuous casting round tube blank comprises, by weight, 0.24%-0.29% of C, 0.20%-0.35% of Si, 0.40%-0.60% of Mn, 2.90%-3.10% of Cr, 0.05%-0.15% of Mo, 0.010%-0.040% of Al, smaller than or equal to 0.015% of P, smaller than or equal to 0.005% of S and the balance Fe and inevitable impurities; the continuous casting low superheat degree of 15-25 DEG C and the low pulling speed of 0.65-0.75 m / min are combined to protect pouring in the whole process, electromagnetic pulse magnetic oscillation is matched to promote solid-liquid interface front edge nucleation, the solidification structure is remarkably refined, the center segregation, shrinkage cavity and looseness defects are reduced, the internal density and component uniformity of a casting blank are improved, a high-quality pipe blank foundation is provided for subsequent perforation and heat treatment, and the production cost is reduced. The comprehensive mechanical property and the service life of the seamless steel pipe are improved.

Description

Technical Field

[0001] This invention belongs to the field of high-chromium medium-carbon seamless steel pipes, specifically relating to a medium-carbon high-chromium steel continuously cast round billet and its preparation method. Background Technology

[0002] Medium-carbon high-chromium steel is a type of alloy steel with a medium carbon content and a relatively high proportion of chromium. It possesses high strength, wear resistance, and corrosion resistance, and is widely used in oil well pipes, casings, and related pressure-bearing pipes. This type of steel is typically produced using converter or electric furnace smelting, combined with ladle refining and continuous casting processes. Its microstructure and properties are highly sensitive to the metallurgical process. High-quality medium-carbon high-chromium billet steel not only requires stable chemical composition control but also needs to maintain a uniform and dense microstructure at both the macroscopic and microscopic levels, avoiding significant central porosity and compositional segregation. As oil and gas wells develop towards high temperature, high pressure, and deep wells, the requirements for the uniformity, reliability, and service stability of steel pipes are constantly increasing. This places higher demands on the solidification control capabilities of medium-carbon high-chromium steel continuously cast billets, especially on the microstructure density and compositional uniformity of the central region of the billet, which require stricter control targets.

[0003] Currently, medium-carbon high-chromium steel billets are mostly produced using round billet continuous casting. Electromagnetic stirring in the crystallizer, secondary cooling, and necessary end-solidation control measures improve the internal quality of the billet. This process can mitigate center defects in general carbon steel or low-alloy steel to some extent. However, for medium-carbon high-chromium steel, its higher alloy content and wider solidification range lead to a significant tendency for columnar crystal growth, an expansion of the pasty zone at the end of solidification, and enrichment of solute elements in the central region, resulting in more pronounced center segregation and porosity defects. These defects are easily amplified during subsequent piercing and rolling processes, leading to quality problems such as annular folds on the inner surface of the steel pipe. Existing common methods mostly rely on single or combined electromagnetic stirring and empirical parameter adjustments, but their adaptability to different composition systems and cross-sectional specifications is limited, making it difficult to effectively reduce the degree of segregation in medium-carbon high-chromium steel billets while ensuring production stability.

[0004] In summary, it is evident that solidification conditions and electromagnetic interaction methods during continuous casting significantly affect the microstructure and compositional distribution of medium-carbon high-chromium steel billets. However, existing technologies are still not ideal in their comprehensive control of segregation and porosity defects when dealing with the specific steel grade and cross-sectional shape of medium-carbon high-chromium tube billets. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing medium-carbon high-chromium steel continuously cast round billets. By rationally controlling the key processes of smelting, refining and continuous casting, the solidification behavior of the billet is improved, the degree of central segregation and porosity is reduced, thereby obtaining medium-carbon high-chromium steel round billets with stable internal quality and uniform structure.

[0006] The present invention also aims to provide a medium-carbon high-chromium steel continuously cast round billet, which has high internal density and compositional uniformity, and can meet the requirements of subsequent processing steps such as piercing, rolling and heat treatment for the quality of the raw material billet.

[0007] This invention provides a method for preparing a continuous casting round tube billet of medium carbon high chromium steel, the method comprising the following steps:

[0008] Converter → LF refining furnace → RH vacuum treatment → continuous casting → slow cooling → warehousing;

[0009] In the continuous casting process, the molten steel in the tundish is superheated to 15-25℃, and the casting speed is 0.65-0.75m / min. The pulsed magnetostrictive oscillation (PMO) voltage is 250-270V / 17Hz, and the electromagnetic effect acts on the solid-liquid interface. Through electromagnetic induction, it promotes nucleation at the solid-liquid interface front near the slab solid-liquid interface, thereby refining the solidification structure and effectively improving segregation and shrinkage defects. The crystallizer electric stirring (M-EMS) parameter is 150-250A / 3Hz, which acts on the initial solidification stage of the slab. It mainly homogenizes the composition and temperature of the molten steel, promotes the flotation of inclusions and gases, and enhances the initial solidification uniformity. The end electromagnetic stirring (F-EMS) parameter is 350-450A / 8Hz, which acts on the end of solidification. At this time, the solidification shrinkage is significant, and the paste-like area is enriched with solute. Its main function is to disperse the enriched solute and expand the central equiaxed zone, thereby reducing internal defects such as central segregation, shrinkage, and porosity.

[0010] In the converter smelting process, 100-120 kg of aluminum blocks are added to the tapped steel for deoxidation, and 5.5 kg / t of silicon manganese, 14.5 kg / t of high-carbon ferrochrome, 35.5 kg / t of low-carbon ferrochrome and 1.5 kg / t of ferromolybdenum are alloyed. The tapped slag consists of 500 kg of quicklime and 200 kg of refining slag. The bottom blowing argon flow rate of the ladle is 120-160 NL / min for stirring to improve the uniformity of the steel composition. The converter uses a dual-baffle tapping system to ensure slag blocking effect and avoid phosphorus return after the furnace.

[0011] In the LF refining furnace process, the refining deoxidation and slag-forming cycle is 60-80 minutes. 400-600 kg of quicklime and 200 kg of refining slag are added to the refining deoxidation and slag-forming process to create foamy slag. Diffusion deoxidation is carried out throughout the refining process. In the early and middle stages, 80-100 kg of aluminum granules and 100-150 kg of silicon carbide are added for mixed deoxidation, and in the middle and later stages, 50-100 kg of silicon carbide is used for deoxidation to ensure that the final refining slag sample is white slag.

[0012] In the RH vacuum process, the vacuum level is ≤67Pa, the process time is ≥15min, and the lifting gas flow rate is ≥1000Nm. 3 / min, after RH refining, the soft blowing time is ≥15min.

[0013] In the continuous casting process, the continuous casting cross-section is a φ300mm round continuous casting billet, and the entire pouring process is protected by argon gas.

[0014] The present invention also provides a medium-carbon high-chromium steel continuously cast round tube billet prepared by the above preparation method, wherein the medium-carbon high-chromium steel continuously cast round tube billet comprises the following chemical composition and weight percentage:

[0015] C 0.24-0.29%; Si 0.20-0.35%; Mn 0.40-0.60%; Cr 2.90-3.10%; Mo 0.05-0.15%; Al 0.010-0.040%; P≤0.015%; S≤0.005%; and the balance Fe and unavoidable impurities.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] 1) By adopting process parameters of low superheat and low casting speed, the solidification and shrinkage process of molten steel was effectively controlled, the segregation of the billet was significantly reduced, and the internal quality and uniformity of the billet were improved.

[0018] 2) By combining electromagnetic stirring in the crystallizer and electromagnetic stirring at the end with electromagnetic pulse magnetostrictive oscillation technology, the component segregation defects caused by selective crystallization are effectively reduced, the internal density of the product is improved, and a good foundation is provided for subsequent tube threading and heat treatment processes. Attached Figure Description

[0019] Figure 1 The image shows a low-magnification microstructure of the medium-carbon high-chromium steel continuously cast round tube billet in Example 1. The central porosity of the medium-carbon high-chromium steel continuously cast round tube billet is 0.5 mm, and the shrinkage cavity is 0 mm.

[0020] Figure 2 The image shows a low-magnification microstructure of a medium-carbon high-chromium steel continuously cast round tube billet in Example 2. The central porosity of the medium-carbon high-chromium steel continuously cast round tube billet is 0.5 mm, and the shrinkage cavity is 0 mm.

[0021] Figure 3 The image shows a low-magnification microstructure of a medium-carbon high-chromium steel continuously cast round tube billet in Example 3. The central porosity of the medium-carbon high-chromium steel continuously cast round tube billet is 1.0, and the shrinkage cavity is 0.

[0022] Figure 4 The image shows a low-magnification microstructure of a medium-carbon high-chromium steel continuously cast round tube billet in Comparative Example 1. The central porosity of the medium-carbon high-chromium steel continuously cast round tube billet is 1.5 mm, and the residual shrinkage cavity is 0.5 mm.

[0023] Figure 5 The image shows a low-magnification microstructure of a medium-carbon high-chromium steel continuously cast round tube billet in Comparative Example 2. The billet has a central porosity of 2.0 mm and a shrinkage cavity residue of 0.5 mm. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] A medium-carbon high-chromium steel continuously cast round tube billet, wherein the medium-carbon high-chromium steel continuously cast round tube billet comprises the following chemical composition and weight percentage:

[0026] C 0.24-0.29%; Si 0.20-0.35%; Mn 0.40-0.60%; Cr 2.90-3.10%; Mo 0.05-0.15%; Al 0.010-0.040%; P≤0.015%; S≤0.005%; and the balance Fe and unavoidable impurities.

[0027] The method for preparing the medium-carbon high-chromium steel continuous casting round tube billet includes the following steps:

[0028] In the converter smelting process, 120 kg of aluminum blocks are added to the tapped steel for deoxidation, and 5.5 kg / t of silicon manganese, 14.5 kg / t of high-carbon ferrochrome, 35.5 kg / t of low-carbon ferrochrome and 1.5 kg / t of ferromolybdenum are alloyed. The tapped slag consists of 500 kg of quicklime and 200 kg of refining slag. Argon is blown into the ladle at a flow rate of 160 NL / min for stirring. The converter taps steel in two stages.

[0029] In the LF refining furnace process, the refining deoxidation and slag-making cycle is 60 minutes, during which 400-600 kg of quicklime and 200 kg of refining slag are added to create foamy slag. Diffusion deoxidation is carried out throughout the refining process. In the early and middle stages, 80-100 kg of aluminum particles and 100-150 kg of silicon carbide are added for mixed deoxidation, and in the middle and later stages, 50-100 kg of silicon carbide is used for deoxidation to ensure that the final slag sample of the refining is white slag.

[0030] In the RH vacuum treatment step, the vacuum level is maintained at ≤67Pa for 15 minutes, and the gas flow rate is increased to 1000 Nm³. 3 / min, the soft blowing time after RH refining is 15min.

[0031] In the continuous casting step, the continuous casting cross-section is a φ300mm round continuous casting billet. The entire pouring process is under argon protection. The molten steel in the tundish is superheated to 15-25℃, the casting speed is 0.65-0.75m / min, the PMO voltage is 250-270V / 17Hz, the M-EMS parameters are 150-250A / 3Hz, and the F-EMS parameters are 350-450A / 8Hz. The invention will now be described in detail with reference to specific embodiments.

[0032] Table 1. Chemical composition (%) of medium-carbon high-chromium steel continuously cast round tube billets in the examples and comparative examples.

[0033]

[0034] Table 2. Process and Parameters for Continuously Cast Round Tube Billets of High-Carbon High-Chromium Steel

[0035]

[0036] Comparative Example 1, due to its increased casting speed, and Comparative Example 2, due to the shutdown of pulsed magnetostrictive oscillation, both exhibited billet performance inferior to the Example. Specifically, the segregation index of the Example improved from 0.92-1.12 in the Comparative Example to 0.95-1.07, with a significantly reduced fluctuation range; simultaneously, the low-magnification center porosity level of the billet decreased from 1.5-2.0 in the Comparative Example to 0.5-1.0.

[0037] The above detailed description of a medium-carbon high-chromium steel continuous casting round tube billet and its preparation method, with reference to the embodiments, is illustrative rather than limiting. Several embodiments can be listed according to the defined scope. Therefore, changes and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A method for preparing a continuous casting round tube billet of medium carbon high chromium steel, characterized in that, The preparation method includes the following steps: Converter → LF refining furnace → RH vacuum treatment → continuous casting → slow cooling → warehousing; In the continuous casting process, the molten steel in the tundish is superheated to 15-25°C, and the casting speed is 0.65-0.75 m / min; the PMO voltage is 250-270V / 17Hz, the M-EMS parameters are 150-250A / 3Hz, and the F-EMS parameters are 350-450A / 8Hz.

2. The method for preparing medium-carbon high-chromium steel continuously cast round tube billets according to claim 1, characterized in that, In the converter smelting process, 100-120 kg of aluminum blocks are added to the tapped steel for deoxidation, 5.5 kg / t of silicon manganese, 14.5 kg / t of high-carbon ferrochrome, 35.5 kg / t of low-carbon ferrochrome and 1.5 kg / t of ferromolybdenum for alloying; the tapped slag consists of 500 kg of quicklime and 200 kg of refining slag.

3. The method for preparing medium-carbon high-chromium steel continuously cast round tube billets according to claim 1, characterized in that, In the converter smelting process, the ladle bottom blowing argon flow rate is 120-160 NL / min for stirring, and the converter taps steel in two stages.

4. The method for preparing medium-carbon high-chromium steel continuously cast round tube billets according to claim 1, characterized in that, In the LF refining furnace process, the refining, deoxidation, and slag-forming cycle is 60-80 minutes.

5. The method for preparing a medium-carbon high-chromium steel continuously cast round tube billet according to claim 1, characterized in that, In the LF refining furnace process, 400-600 kg of quicklime and 200 kg of refining slag are added to the refining deoxidation slag-making process to create foamed slag.

6. The method for preparing a medium-carbon high-chromium steel continuously cast round tube billet according to claim 1, characterized in that, In the LF refining furnace process, diffusion deoxidation is carried out throughout the refining process. In the early and middle stages, 80-100 kg of aluminum granules and 100-150 kg of silicon carbide are added for mixed deoxidation, and in the middle and later stages, 50-100 kg of silicon carbide is used for deoxidation to ensure that the final slag sample of the refining is white slag.

7. The method for preparing medium-carbon high-chromium steel continuously cast round tube billets according to claim 1, characterized in that, In the RH vacuum process, the vacuum level is ≤67Pa, the process time is ≥15min, and the lifting gas flow rate is ≥1000Nm. 3 / min, after RH refining, the soft blowing time is ≥15min.

8. The method for preparing medium-carbon high-chromium steel continuously cast round tube billets according to claim 1, characterized in that, In the continuous casting process, the continuous casting cross-section is a φ300mm round continuous casting billet, and the entire pouring process is protected by argon gas.

9. A medium-carbon high-chromium steel continuously cast round tube billet prepared by the preparation method described in claims 1-8, characterized in that, The medium-carbon high-chromium steel continuously cast round tube billet mentioned above has the following chemical composition and weight percentage: C 0.24-0.29%; Si 0.20-0.35%; Mn 0.40-0.60%; Cr 2.90-3.10%; Mo 0.05-0.15%; Al 0.010-0.040%; P≤0.015%; S≤0.005%; and the balance Fe and unavoidable impurities.

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