3cr seamless steel pipe and production method for controlling surface crack of continuous casting billet

Abstract

The present application belongs to the technical field of seamless steel pipe manufacturing, and particularly relates to a 3Cr seamless steel pipe and a production method for controlling surface cracks of continuous casting billets, wherein the chemical composition of the 3Cr seamless steel pipe is as follows: C: 0.16%-0.23%, Si: 0.15%-0.35%, Mn: 0.48%-0.70%, Cr: 2.80%-3.50%, Ni: 0.10%-0.15%, Al: 0.010%-0.030%, N: 0.0040%-0.0080%, P≤0.012%, S≤0.010%, 3.0≤Mn / C≤3.5, and the rest is Fe and trace impurities. The scheme controls the steelmaking and continuous casting parameters, reduces the surface cracks of the billets, controls the heating process of the continuous casting billets, prevents the expansion of a small amount of surface cracks, reduces the surface defects of the round pipe billets after rolling, thereby ensuring the qualified surface quality of the seamless steel pipe, and finally realizing the good surface quality of the continuous casting billets and the surface crack grade of the continuous casting billets ≤1.0.

Description

Technical Field

[0001] This invention belongs to the field of seamless steel pipe manufacturing technology, specifically relating to a 3Cr seamless steel pipe and a production method for controlling surface cracks in continuously cast billets. Background Technology

[0002] According to API Spec 5CT, all steel used in casing and tubing should undergo grain refinement treatment, requiring the steel to contain one or more grain-refining elements, such as a certain amount of aluminum, niobium, vanadium, or titanium, to refine the austenite grains. Al, as an inexpensive grain-refining element, is widely used, with beneficial effects including fixing nitrogen in the steel, controlling the austenite grain size during reheating, improving weldability, and lowering the ductile-brittle transition temperature. However, in medium-carbon 3Cr alloy steel, increasing the Cr content reduces the steel's hot plasticity. Simultaneously, increased Cr leads to a larger phase transformation volume change during the austenite-to-ferrite transformation. This phase transformation volume change, combined with cooling stress, significantly increases the risk of cracking. After adding Al to refine the grains, Al combines with C, N, and O elements to form fine, dispersed carbides and nitrides, which easily precipitate at austenite grain boundaries or in the thin film of ferrite at the grain boundaries, worsening the steel's high-temperature plasticity and increasing the probability of crack formation.

[0003] CN116213668A discloses a method for controlling surface cracks in VN microalloyed steel continuously cast billets. The VN microalloyed steel continuously cast billets have a thickness of 400–700 mm and a width of 2000–2600 mm, and are produced using a vertical continuous casting machine. By controlling process parameters such as casting speed, ladle pouring time, and cooling water volume in the secondary cooling zone, the probability of billet cracks caused by liquid phase precipitation inclusions such as AlN is reduced. This technical solution is only applicable to the production of large square billets and not to small square billet processes.

[0004] CN113699431A discloses a method for reducing surface cracks in low-alloy steel. This method precisely controls the content of Ti, Mn, Al, and N in the low-alloy steel, especially by not adding aluminum or aluminum alloys throughout the refining process, controlling the Al content in the low-alloy steel to below 0.01%, and simultaneously performing nitrogen control treatment during converter smelting, LF refining, and continuous casting to control the N content to below 50 ppm. This reduces AlN and eliminates grain boundary brittleness caused by NbC and NbCN precipitation, which can greatly reduce the risk of surface cracking in low-alloy steel. In addition, by optimizing the configuration of the secondary cooling water during continuous casting and the billet heating process, surface cracks caused by thermal stress in low-alloy steel are further reduced.

[0005] CN102399929A ​​discloses a method for reducing surface cracks in aluminum-deoxidized high-carbon steel billets. This method controls Als and N in the molten steel during each process of converter smelting, tapping deoxidation, slag formation, refining, and continuous casting to inhibit the formation of AlN, thereby reducing surface cracks in high-carbon steel billets. This method has high requirements for N content control after converter smelting and is suitable for steel grades with C content of 0.6% to 1.0%.

[0006] CN105018677A discloses a continuous casting production method for Q345B hot-rolled structural steel. This method can avoid the problems of surface defects such as lines and peeling on Q345B hot-rolled structural steel. This solution reduces the possibility of subsurface microcracks in the billet caused by Al and N combining to form AlN and precipitating at grain boundaries during the solidification process of continuously cast steel by controlling the content of Al and N in the molten steel.

[0007] Existing methods for controlling crack formation all involve controlling the nitrogen content in each steelmaking process to reduce AlN precipitation at grain boundaries and thus reduce surface cracks on the billet. However, these methods do not disclose the production process of 3Cr steel or subsequent rolling processes, and therefore cannot solve the problem of crack propagation in continuously cast billets during subsequent heating and rolling. Summary of the Invention

[0008] To address the technical problem of surface cracks in continuously cast billets during the continuous casting process using existing crack control methods, this invention provides a 3Cr seamless steel pipe and a production method for controlling surface cracks in continuously cast billets. This solution can control the generation of surface cracks, especially corner cracks, in continuously cast billets. By using a heating process for continuously cast billets, the expansion and addition of cracks in the billets are suppressed, thereby improving the surface quality of round billets and seamless steel pipes.

[0009] To solve the above-mentioned technical problems, the present invention provides a 3Cr seamless steel pipe, the chemical composition of which, based on a total mass percentage of 100%, is as follows: C: 0.16%~0.23%, Si: 0.15%~0.35%, Mn: 0.48%~0.70%, Cr: 2.80%~3.50%, Ni: 0.10%~0.15%, Al: 0.010%~0.030%, N: 0.0040%~0.0080%, P≤0.012%, S≤0.010%, 3.0≤Mn / C≤3.5, the remainder being Fe and trace impurities.

[0010] Alloy element design principles: C: As a basic strengthening element in steel, it is used in this invention to ensure the strength of the steel pipe after heat treatment. Excessive C content increases the risk of quenching cracking and reduces the steel's corrosion resistance. Therefore, this invention precisely controls the C content to be 0.16%~0.23%.

[0011] Si plays a role in solid solution strengthening in steel, increasing its hardness and strength. However, excessive Si reduces the toughness and plasticity of steel pipes. Therefore, this invention precisely controls the Si content to be 0.15%~0.35%.

[0012] Mn: Since carbon (C) reduces the corrosion resistance of steel, the Mn content can be increased to ensure the strength of the steel. However, the ratio of Mn to C should be strictly controlled at 3.0 ≤ Mn / C ≤ 3.5. Excessive Mn easily forms banded structures and micro-segregation. Furthermore, excess Mn readily combines with sulfur (S) in the steel to form MnS inclusions, which are detrimental to strength, toughness, and corrosion resistance. This invention precisely controls the Mn content to 0.48%~0.70%.

[0013] Cr: is the core alloying element of quenched and tempered steel, and also an important element for resisting CO2 corrosion. It mainly forms Cr7C3 carbides to resist CO2 corrosion and slow down the corrosion of steel in the CO2-H2S-Cl phase. - The corrosion rate in the environment is important to ensure overall mechanical properties, but Cr ≥ 3.5% will promote M corrosion. 23 The formation of coarse C6 carbides is detrimental to the corrosion resistance of steel. This invention precisely controls the Cr content to be 2.80%~3.50%.

[0014] Ni can increase the electrode potential of the steel matrix. When combined with 3Cr steel, it makes the passivation film denser and more stable, making the steel more difficult to corrode. Ni alloys are relatively expensive, and adding 0.15% can improve corrosion resistance, especially resistance to CO2 pitting corrosion. In this invention, the Ni content is precisely controlled to be 0.10%~0.15%.

[0015] Al, as a strong deoxidizer, can effectively remove oxygen from molten steel, fix nitrogen in the molten steel to form AlN, and inhibit the growth of austenite grains at high temperatures. However, excessive Al or poor process control can generate a large amount of Al2O3 inclusions, clogging the nozzle and causing defects such as surface cracks and inclusions in the billet; increasing the probability of surface cracks in the billet. This invention precisely controls the Al content to be 0.010%~0.030%.

[0016] Nitrogen (N): Appropriate amounts of nitrogen (N) react with Al to form AlN, refining the grain structure of the cast billet and improving the strength, toughness, and weldability of the steel. Excessive N content leads to increased AlN precipitation, increasing the likelihood of surface cracks in the cast billet. This invention precisely controls the N content to be between 0.0040% and 0.0080%.

[0017] P and S are impurity elements that segregate at grain boundaries, reducing the toughness and sulfide stress corrosion resistance of steel. To ensure the corrosion resistance of steel, their content should be minimized. For oil well pipes used in acidic environments, P ≤ 0.012% and S ≤ 0.010%.

[0018] Based on the above technical solution, the specifications of the 3Cr seamless steel pipe are Φ60mm~Φ180mm×4~20mm.

[0019] Based on the above technical solution, the yield strength of the 3Cr seamless steel pipe is ≥758MPa, the tensile strength is ≥862MPa, the elongation is ≥18%, the longitudinal impact performance at 0℃ reaches 45J or more, and the microstructure is tempered sorbite.

[0020] To solve the above-mentioned technical problems, the present invention also provides a production method for controlling surface cracks in continuously cast billets, comprising the following steps: steelmaking - continuous casting - billet rolling - seamless steel pipe rolling - seamless steel pipe heat treatment. Steelmaking: Use low-sulfur molten iron or desulfurized molten iron with slag removal. The sulfur content of the molten iron entering the furnace should be ≤0.040%. Remove slag before tapping. The slag layer thickness after tapping should be ≤100mm. The nitrogen content of the steel tapped from the converter should be controlled below 40ppm. LF refining adjusts the composition except for Al. VD vacuum pressure is below 100Pa. The holding time is 15~20min. After holding, adjust the Al content to 0.010%~0.030%. Continuous casting: molten steel superheat 10~20℃, primary cooling water flow rate 3200~3500NL / min, secondary cooling water flow rate 1.0~1.3L / kg; end electromagnetic stirring current 300~500A; continuous casting billets are required to be slowly cooled after leaving the line, and slow cooling for at least 48 hours before being loaded into the furnace for rolling. The cross-sectional dimensions of the continuous casting billets are 300~340mm×390~420mm. Billet rolling: The billet heating time is greater than 350~450min. The furnace temperature in the preheating section is controlled to drop below 800℃ before loading into the furnace. The continuous casting billet includes a preheating section, a heating section, and a soaking section. After exiting the furnace, the continuous casting billet is rolled into a round tube billet with a diameter of Φ185mm~Φ210mm.

[0021] Based on the above technical solution, the preheating section and heating section in the continuous casting billet are specifically: heated from room temperature to 950~1050℃ at a heating rate of 15~18min / cm. The soaking zone in the continuously cast square billet is specifically heated from 950~1050℃ to 1200~1240℃ at a heating rate of 3.5~4.5min / cm.

[0022] Based on the above technical solution, seamless steel pipe rolling: the uniform heating temperature of the round billet is 1220~1250℃, the total heating time is 200~270min, the uniform heating period is 40~55min, and after piercing, rolling and sizing, the rolled steel pipe is obtained.

[0023] Based on the above technical solution, the heat treatment of seamless steel pipes is as follows: the rolled steel pipes are quenched and tempered at high temperature; the quenching is specifically as follows: the rolled steel pipes are heated to a quenching temperature of 900~930℃ and held for 40~75 minutes, and then water quenched. The high-temperature tempering treatment specifically involves heating the water-quenched steel pipe to a tempering temperature of 580~680℃ and holding it for 50~120 minutes to obtain a 3Cr seamless steel pipe.

[0024] Beneficial effects 1. The technical solution disclosed in this invention reduces surface cracks on billets by controlling steelmaking and continuous casting parameters; it also controls the heating process of continuously cast billets to prevent the propagation of a small number of surface cracks, thereby reducing surface defects in rolled round tube billets and ensuring the surface quality of seamless steel pipes. Ultimately, it achieves good surface quality of continuously cast billets, with a surface crack grade of ≤1.0 and a surface crack defect rate of ≤0.5% for steel pipes.

[0025] 2. The 3Cr seamless steel pipe produced by the technical solution disclosed in this invention has a specification of Φ60mm~Φ180mm×4~20mm. The 3Cr seamless steel pipe has a yield strength ≥758MPa, a tensile strength ≥862MPa, an elongation ≥18%, a longitudinal impact performance of 0℃ of ≥45J, and a microstructure of tempered sorbite. Attached Figure Description

[0026] Figure 1 The microstructure of 3Cr prepared in Example 1 is shown. Detailed Implementation

[0027] To make the objectives and technical solutions of this invention clearer, the following embodiments are provided for further explanation. However, the scope of protection of this invention is not limited to these embodiments; the embodiments are merely for illustrative purposes. Those skilled in the art should understand that any changes or equivalent substitutions that do not depart from the concept of this invention are included within the scope of protection of this invention.

[0028] Unless otherwise specified, all reagents and raw materials used in this invention are obtained through purchase.

[0029] Example A 3Cr seamless steel pipe, based on a total mass percentage of 100%, has the following chemical composition: C: 0.16%~0.23%, Si: 0.15%~0.35%, Mn: 0.48%~0.70%, Cr: 2.80%~3.50%, Ni: 0.10%~0.15%, Al: 0.010%~0.030%, N: 0.0040%~0.0080%, P≤0.012%, S≤0.010%, 3.0≤Mn / C≤3.5, with the remainder being Fe and trace impurities. The specific chemical composition of the 3Cr seamless steel pipes in Examples 1~7 is shown in Table 1. In addition, the production method for controlling surface cracks in continuously cast square billets includes the following steps: steelmaking (hot metal pretreatment → converter smelting → ladle refining (LF+VD)) - continuous casting (square billet continuous casting → continuous casting billet slow cooling) - billet rolling (continuous casting billet heating → rolling round tube billet) - seamless steel pipe rolling (round tube billet heating → piercing → continuous rolling → sizing) - seamless steel pipe heat treatment. Steelmaking: Use low-sulfur molten iron or desulfurized molten iron with slag removal. The sulfur content of the molten iron entering the furnace should be ≤0.040%. Remove slag before tapping. The slag layer thickness after tapping should be ≤100mm. The N content of the steel tapped from the converter should be controlled below 40ppm. LF refining adjusts the composition except for Al. VD vacuum pressure is below 100Pa. The holding time is 15~20min. After the holding time, adjust the Al content to 0.010%~0.030%. Specific steelmaking parameters are shown in Table 2.

[0030] Continuous casting: The superheat of molten steel is 10-20℃, the primary cooling water flow rate is 3200-3500 NL / min, and the secondary cooling water flow rate is 1.0-1.3 L / kg to reduce the cooling intensity and reduce the precipitation of AlN between grain boundaries; the final electromagnetic stirring current is 300-500A; the continuously cast billet is required to be slowly cooled after leaving the line, and the slow cooling should last for at least 48 hours before being loaded into the furnace for rolling. In order to avoid stress cracking on the surface of the billet, it is required to be cooled slowly in a sheltered place and other hot billets should be placed to reduce the cooling rate. The cross-sectional dimensions of the continuously cast billet are 300-340mm × 390-420mm. Specific continuous casting parameters are shown in Table 3.

[0031] Billet rolling: The billet heating time is greater than 350~450min. The furnace temperature in the preheating section is controlled to drop below 800℃ before loading into the furnace. The continuously cast square billet includes a preheating section, a heating section, and a soaking section. The preheating and heating sections are heated slowly to reduce the temperature difference between the core and the surface, thereby reducing the heating stress of the billet and avoiding surface cracks. The preheating section heats from room temperature to 950~1050℃ at a heating rate of 15~18min / cm. The soaking section heats from 950~1050℃ to 1200~1240℃ at a heating rate of 3.5~4.5min / cm. As the thermal conductivity increases, the heating rate is increased and the temperature is held for a short time to avoid grain growth. After exiting the furnace, the continuously cast billet is rolled into a round tube billet with a diameter of Φ185mm~Φ210mm. Specific billet rolling parameters are shown in Table 4. Seamless steel pipe rolling: The homogenization temperature of the round billet is 1220~1250℃, the total heating time is 200~270min, and the homogenization period is 40~55min. The homogenization temperature of the round billet affects the grain size and properties of the final product. Lowering the homogenization temperature is beneficial to controlling the final rolling temperature and preventing excessive grain growth. Below 1220℃, it is easy to cause excessive consumption of equipment and dies. After piercing, rolling and sizing, the round billet is rolled into a steel pipe. Seamless steel pipe heat treatment: The rolled steel pipe is subjected to quenching and high-temperature tempering. The quenching specifically involves heating the rolled steel pipe to a quenching temperature of 900-930℃ and holding it for 40-75 minutes, followed by water quenching. The high-temperature tempering specifically involves heating the water-quenched steel pipe to a tempering temperature of 580-680℃ and holding it for 50-120 minutes. Specific heat treatment parameters for the seamless steel pipe are shown in Table 5, resulting in a 3Cr seamless steel pipe. The mechanical properties of the 3Cr seamless steel pipe are shown in Table 6. The microstructure of the 3Cr seamless steel pipe prepared in Example 1 is shown in Table 6. Figure 1 ,Depend on Figure 1 It can be seen that the microstructure of 3Cr seamless steel pipe is tempered sorbite.

[0032] Table 1. Specific chemical composition (wt%) of 3Cr seamless steel pipe

[0033] Table 2 Steelmaking related parameters

[0034] Table 3 Relevant parameters for continuous casting

[0035] Table 4 Relevant parameters for billet rolling

[0036] Table 5. Relevant parameters for heat treatment of seamless steel pipes

[0037] Table 6 Mechanical properties of 3Cr seamless steel pipes

[0038] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A 3Cr seamless steel pipe, characterized in that, The chemical composition of the 3Cr seamless steel pipe, calculated as 100% by total mass, is as follows: C: 0.16%~0.23%, Si: 0.15%~0.35%, Mn: 0.48%~0.70%, Cr: 2.80%~3.50%, Ni: 0.10%~0.15%, Al: 0.010%~0.030%, N: 0.0040%~0.0080%, P≤0.012%, S≤0.010%, 3.0≤Mn / C≤3.5, the remainder being Fe and trace impurities.

2. The 3Cr seamless steel pipe according to claim 1, characterized in that, The specifications of the 3Cr seamless steel pipe are Φ60mm~Φ180mm×4~20mm.

3. The 3Cr seamless steel pipe according to claim 1, characterized in that, The 3Cr seamless steel pipe has a yield strength ≥758MPa, tensile strength ≥862MPa, elongation ≥18%, longitudinal impact performance at 0℃ reaches 45J or more, and its microstructure is tempered sorbite.

4. A production method for controlling surface cracks in continuously cast square billets, characterized in that, The process includes the following steps: steelmaking, continuous casting, billet rolling, seamless steel pipe rolling, and seamless steel pipe heat treatment. Steelmaking: Use low-sulfur molten iron or desulfurized molten iron with slag removal. The sulfur content of the molten iron entering the furnace should be ≤0.040%. Remove slag before tapping. The slag layer thickness after tapping should be ≤100mm. The nitrogen content of the steel tapped from the converter should be controlled below 40ppm. LF refining adjusts the composition except for Al. VD vacuum pressure is below 100Pa. The holding time is 15~20min. After holding, adjust the Al content to 0.010%~0.030%. Continuous casting: molten steel superheat 10~20℃, primary cooling water flow rate 3200~3500NL / min, secondary cooling water flow rate 1.0~1.3L / kg; end electromagnetic stirring current 300~500A; continuous casting billets are required to be slowly cooled after leaving the line, and slow cooling for at least 48 hours before being loaded into the furnace for rolling; the cross-sectional dimensions of continuous casting billets are 300~340mm×390~420mm. Billet rolling: The billet heating time is greater than 350~450min. The furnace temperature in the preheating section is controlled to drop below 800℃ before loading into the furnace. The continuous casting billet includes a preheating section, a heating section, and a soaking section. After exiting the furnace, the continuous casting billet is rolled into a round tube billet with a diameter of Φ185mm~Φ210mm.

5. The production method according to claim 4, characterized in that, The preheating and heating sections in the continuously cast square billet are specifically: heated from room temperature to 950-1050℃ at a heating rate of 15-18 min / cm. The soaking zone in the continuously cast square billet is specifically heated from 950~1050℃ to 1200~1240℃ at a heating rate of 3.5~4.5min / cm.

6. The production method according to claim 4, characterized in that, Seamless steel pipe rolling: The round billet is heated at a temperature of 1220~1250℃, the total heating time is 200~270min, the heating period is 40~55min, and after piercing, rolling and sizing, the rolled steel pipe is obtained.

7. The production method according to claim 4, characterized in that, Seamless steel pipe heat treatment: Quenching and high-temperature tempering treatment of rolled steel pipes; Quenching: The rolled steel pipe is heated to a quenching temperature of 900~930℃ and held for 40~75 minutes, and then water quenched. High-temperature tempering treatment: Heat the water-quenched steel pipe to the tempering temperature of 580~680℃ and hold for 50~120 minutes.

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