Method for controlling long cracks on surface of peritectic steel round steel

Abstract

The invention particularly discloses a method for controlling long cracks on the surface of peritectic steel round steel, which comprises the steps of LD converter, LF external refining, RH vacuum degassing, continuous casting, stepping type heating furnace heating, high-pressure water descaling, rough rolling, pre-finish rolling, PSM rolling mill reducing sizing, feeding to a cooling bed, cutting sizing and feeding into a slow cooling room. An R30 crystallizer is adopted, casting powder QB3-2 with optimized components is used, the degree of superheat of molten steel in the continuous casting process is 30-40 DEG C, the casting speed of a casting blank is 0.90 m / min, small nozzle aerial fog cooling is adopted, the specific water flow of secondary cooling water is 0.15 L / Kg, and the tail end electromagnetic stirring parameter is 250 A / 7 Hz. And the high-grade and high-quality 12Cr1MoVG steel with the specification of phi 80-100 mm and the long crack proportion not larger than 3% is produced.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and particularly relates to a method for controlling surface long cracks of peritectic steel 12Cr1MoVG round steel with a specification of Φ80-100mm for manufacturing high-pressure boiler tubes. Background Art

[0002] 12Cr1MoVG is a steel grade developed in the Soviet Union. It belongs to a low-alloy heat-resistant peritectic steel with a carbon content of 0.12% and an alloy content of less than 2%. This material has a low manufacturing cost, good process performance and welding performance. It has high heat-resistant performance and comprehensive performance below 580°C and is a mature steel grade in China's GB / T5310 standard. Large and small diameter tubes of this steel are widely used in domestic boilers. Its small diameter tubes are mainly used for heating surfaces such as superheaters and reheaters of boilers with a working temperature not exceeding 580°C; large diameter tubes are used for boiler headers, power station pipelines, etc. with a working temperature not exceeding 565°C.

[0003] Round steel of 12Cr1MoVG with a specification of Φ80-100mm was developed and produced by conventional processes. During the inspection process, it was found that other indicators of this steel met the requirements. However, when the surface magnetic flux leakage inspection of the round steel was carried out in accordance with the GB / T31212-2014 standard (the depth requirement of inspection defects is ≤0.30mm), it was found that the proportion of long cracks with a depth greater than >0.30mm accounted for about 15%, which was relatively high. Long cracks with a surface defect depth >0.30mm not only affect the surface quality of the pipe piercing of customers, but also easily cause waste products for customers. Moreover, if the proportion of long cracks is high, the round steel needs to be peeled, which not only has a high cost, but also affects the delivery period and causes customer complaints.

[0004] In order to reduce the proportion of long cracks in 12Cr1MoVG round steel, it is urgent to develop a new method for controlling long cracks in peritectic steel round steel. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for controlling surface long cracks of peritectic steel round steel to solve the problem of a relatively high proportion of long cracks in 12Cr1MoVG steel with a specification of Φ80-100mm.

[0006] To achieve the above purpose, the present invention provides the following technical solutions:

[0007] A method for controlling long surface cracks of peritectic steel round bars, comprising the steps of LD converter → LF ladle furnace refining → RH vacuum degassing → continuous casting → heating in a walking beam reheating furnace → high-pressure water descaling → rough rolling → pre-finishing rolling → sizing and reducing in a PSM rolling mill → entering a cooling bed → splitting and sizing → entering a slow cooling chamber. In the continuous casting step, the oscillation frequency of the mold is increased from 130 Hz / min to 150 Hz / min, the R30 mold is used instead of the previous R20 mold, the mold powder QB3-2 with optimized composition is used, the superheat of the molten steel during continuous casting is increased from 15 - 30 °C to 30 - 40 °C, the casting speed of the billet is increased from 0.80 m / min to 0.90 m / min, aerosol cooling with small nozzles is adopted, the specific water volume of secondary cooling water is reduced from 0.25 L / Kg to 0.15 L / Kg, and the parameters of the final electromagnetic stirring are increased from the original 150 A / 7 Hz to 250 A / 7 Hz.

[0008] Preferably, the composition of the mold powder QB3-2 contains Fe 2 0 3 1.12%, F 1.15%, A1 2 0 3 10.84%, CaO 32.11%, Si0 2 24.52%, C 14.68%, MgO 1.07%, viscosity 0.98 Pa·s, melting rate 50 mm / s, melting point 1205 °C, basicity 1.31.

[0009] Preferably, the converter does not perform supplementary blowing. Ferro-aluminum is added 1 minute after tapping, and then ferro-silicon, ferro-silico-manganese, ferro-chrome, ferro-vanadium, and ferro-molybdenum alloys are added in sequence.

[0010] Preferably, the LF refining produces a white slag and maintains it all the time, while ensuring the fluidity of the top slag. In order to prevent the oxygen in FeO and MnO in the slag from increasing the oxygen content in the molten steel, FeO + MnO ≤ 0.50%. According to the content of Al, an aluminum wire can be supplemented once in the early stage of refining, and the addition of aluminum is strictly prohibited in the later stage.

[0011] Preferably, alloy addition and returning to the LF furnace for temperature raising are strictly prohibited in the RH. The vacuum treatment time ≥ 15 min, and the ultimate vacuum degree ≤ 133 Pa.

[0012] Preferably, an integral tundish is used in the continuous casting. The tundish is cleaned thoroughly, purged with argon before casting, the argon blowing time ≥ 2 minutes, and casting is protected throughout the process, with casting starting outside the ladle.

[0013] Preferably, the remaining molten steel in the continuous casting ladle is not less than 4 t.

[0014] Compared with the prior art, the beneficial effects of the present invention are:

[0015] Increase the vibration frequency of the mold, use the mold powder with optimized composition and the R30 mold, increase the superheat degree of molten steel and the drawing speed during continuous casting, cool the continuous casting billet by aerosol with small nozzles, reduce the specific water volume of secondary cooling water, and increase the electromagnetic stirring parameters at the end of continuous casting, so as to produce 12Cr1MoVG round steel with a specification of Φ80 - 100mm and a long crack ratio of not more than 3%, providing high-quality and high-performance steel products for the high-end equipment field. Specific embodiments

[0016] To make the above objects, features and advantages of the present invention more obvious and understandable, the following detailed description of the specific embodiments of the present invention is given through examples.

[0017] In the following description, many specific details are set forth in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Those skilled in the art can make similar generalizations without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0018] Secondly, the so-called "one embodiment" or "embodiment" herein refers to a specific feature, structure or characteristic that can be included in at least one implementation manner of the present invention. The "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it a separate or selectively exclusive embodiment from other embodiments.

[0019] The following further elaborates on the present invention in combination with embodiments. The range of chemical composition requirements for 12Cr1MoVG round steel with a specification of Φ80 - 100mm in the embodiments of the present invention is shown in Table 1.

[0020] Table 1 Chemical composition (wt%)

[0021]

[0022] Example 1:

[0023] Consecutively cast 5 heats of 12Cr1MoVG steel. After 1 minute of tapping from the converter, ferrosilicon aluminum is added, and then ferro-silicon, ferro-silico-manganese, ferro-chrome, ferro-vanadium, ferro-molybdenum and other alloys are added in sequence. The LF furnace makes white slag and maintains it all the time, while ensuring the fluidity of the top slag. The content of FeO+MnO in the slag of 5 heats of steel is between 0.31% and 0.37%. The vacuum treatment time is 16 - 18 minutes, and the ultimate vacuum degree is 125 - 131 Pa. The continuous casting uses an integral tundish. The tundish is cleaned thoroughly, filled with argon and emptied before casting, and the argon blowing time is 3 - 5 minutes. The whole process is protected by casting, and casting starts outside the ladle. The mold uses an R30 mold, the mold vibration frequency is 150 Hz / min, the mold powder uses the optimized QB3-2 mold powder, the casting superheat is 31 - 36 °C, the casting speed of the billet is 0.90 m / min, the billet is cooled by using small nozzles for aerosol cooling, the specific water volume of secondary cooling is 0.15 L / Kg, the parameters of electromagnetic stirring at the end of continuous casting are 250 A / 7 Hz, and the remaining steel in the ladle is 4.5 - 6 t. The composition of the mold powder QB3-2 contains Fe 2 0 3 1.12%, F 1.15%, A1 2 0 3 10.84%, CaO 32.11%, Si0 2 24.52%, C1 4.68%, MgO 1.07%, viscosity 0.98 Pa·s, melting rate 50 mm / s, melting point 1205 °C, basicity 1.31.

[0024] Temperature measurement shows that during the continuous casting process, the temperature of the billet entering the straightener plane is 1000 - 1050 °C, and the temperature of the billet corner is 960 - 1000 °C. The continuous casting produces billets of 240 mm×300 mm. The 5 heats of steel are simultaneously rolled into 12Cr1MoVG round steel with a diameter of Φ80 mm. Ultrasonic flaw detection and magnetic flux leakage flaw detection are carried out on these 5 heats of Φ80 mm 12Cr1MoVG round steel. The ultrasonic flaw detection is carried out according to the B-level accuracy of GB / T4162 standard, and no central shrinkage defect is found in the flaw detection. The magnetic flux leakage flaw detection is carried out according to GB / T 31212 standard, the flaw detection requirement is ≤0.30 mm, and the average proportion of long crack flaw detection is 2.3%. The specific detection results are shown in Table 2.

[0025] Example 2:

[0026] Continuously cast 7 heats of 12Cr1MoVG steel. After 1 minute of tapping from the converter, ferrosilicon aluminum is added, and then ferro-silicon, ferro-silico-manganese, ferro-chrome, ferro-vanadium, ferro-molybdenum and other alloys are added in sequence. The LF furnace makes white slag and maintains it all the time, while ensuring the fluidity of the top slag. The content of FeO + MnO in the slag of 7 heats of steel is between 0.34% and 0.41%. The vacuum treatment time is 17 - 20 minutes, and the ultimate vacuum degree is 121 - 129 Pa. The continuous casting uses an integral tundish. The tundish is cleaned thoroughly, purged with argon before casting, and the argon blowing time is 3 - 4 minutes. The whole process is protected by casting, and casting starts outside the ladle. The mold uses an R30 mold, the mold vibration frequency is 150 Hz / min, the mold powder uses the optimized QB3-2 mold powder, the casting superheat is 32 - 36 °C, the casting billet drawing speed is 0.90 m / min, the casting billet is cooled by using small nozzle aerosol cooling, the specific water volume of secondary cooling water is 0.15 L / Kg, the parameters of electromagnetic stirring at the end of continuous casting are 250 A / 7 Hz, and the remaining steel in the ladle is 4 - 5 t. The composition of the mold powder QB3-2 contains Fe 2 0 3 1.12%, F 1.15%, A1 2 0 3 10.84%, CaO 32.11%, Si0 2 24.52%, C1 4.68%, MgO 1.07%, viscosity 0.98 Pa·s, melting rate 50 mm / s, melting point 1205 °C, basicity 1.31.

[0027] Temperature measurement shows that during the continuous casting process, the temperature of the casting billet entering the straightening machine plane is 990 - 1060 °C, and the temperature of the casting billet corner is 950 - 990 °C. The continuous casting is made into a 240 mm × 300 mm casting billet, and 7 heats of steel are simultaneously rolled into 12Cr1MoVG round steel with a Φ90 mm specification. Ultrasonic flaw detection and magnetic flux leakage flaw detection are carried out on these 7 heats of 12Cr1MoVG round steel with a Φ80 mm specification. Ultrasonic flaw detection is carried out according to the B-level accuracy of the GB / T4162 standard, and no central shrinkage defect is found in the flaw detection. Magnetic flux leakage flaw detection is carried out according to the GB / T 31212 standard, the flaw detection requirement is ≤0.30 mm, and the average proportion of long crack flaw detection is 2.7%. The specific detection results are shown in Table 2.

[0028] Example 3:

[0029] Continuously cast 6 heats of 12Cr1MoVG steel. After 1 minute of tapping from the converter, ferrosilicon aluminum is added, and then ferro-silicon, ferro-silico-manganese, ferro-chrome, ferro-vanadium, ferro-molybdenum and other alloys are added in sequence. The LF furnace produces white slag and maintains it continuously, while ensuring the fluidity of the top slag. The content of FeO + MnO in the slag of 7 heats of steel is between 0.36% and 0.40%. The vacuum treatment time is 15 - 22 minutes, and the ultimate vacuum degree is 126 - 131 Pa. The continuous casting uses an integral tundish. The tundish is cleaned thoroughly, purged with argon before casting, and the argon blowing time is 2 - 4 minutes. The whole process is protected by casting, and casting starts outside the ladle. The mold uses an R30 mold, the mold vibration frequency is 150 Hz / min, the mold powder uses the optimized QB3-2 mold powder, the casting superheat is 33 - 39 °C, the casting speed of the billet is 0.90 m / min, the billet is cooled by using small nozzle aerosol cooling, the specific water volume of secondary cooling is 0.15 L / Kg, the parameters of electromagnetic stirring at the end of continuous casting are 250 A / 7 Hz, and the remaining steel in the ladle is 4 - 6 t. The composition of the mold powder QB3-2 contains Fe 2 0 3 1.12%, F 1.15%, A1 2 0 3 10.84%, CaO 32.11%, Si0 2 24.52%, C1 4.68%, MgO 1.07%, viscosity 0.98 Pa·s, melting rate 50 mm / s, melting point 1205 °C, basicity 1.31.

[0030] Temperature measurement shows that during continuous casting, the temperature of the billet entering the straightener plane is 1000 - 1060 °C, and the corner temperature of the billet is 970 - 1000 °C. The continuous casting produces billets of 240 mm × 300 mm. The 6 heats of steel are simultaneously rolled into 12Cr1MoVG round steel with a diameter of Φ100 mm. Ultrasonic flaw detection and magnetic flux leakage flaw detection are carried out on these 5 heats of Φ100 mm 12Cr1MoVG round steel. Ultrasonic flaw detection is carried out according to the B-level accuracy of GB / T4162 standard. No center shrinkage defect is found in the flaw detection. Magnetic flux leakage flaw detection is carried out according to GB / T 31212 standard. The flaw detection requirement is ≤0.30 mm, and the average proportion of long crack flaw detection is 2.9%. The specific detection results are shown in Table 2.

[0031] Table 2 Flaw Detection Results of Examples 1 - 3

[0032]

[0033] As can be seen from the above embodiments, by adopting the present invention, during the smelting of 12Cr1MoVG steel, the oscillation frequency of the mold is increased, the mold powder with optimized composition and the R30 mold are used, the superheat degree of molten steel and the drawing speed during continuous casting are increased, the small nozzle aerosol cooling of the billet is carried out, the specific water volume of secondary cooling water is reduced, and the electromagnetic stirring parameters at the end of continuous casting are increased, thus solving the problem of a relatively high proportion of long cracks in 12Cr1MoVG steel produced by the conventional process, and producing high-quality 12Cr1MoVG steel with a specification of Φ80 - 100mm and a proportion of long cracks not greater than 3%.

[0034] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A method for controlling long cracks on the surface of peritectic steel round steel, comprising the steps of LD converter → LF refining outside the furnace → RH vacuum degassing → continuous casting → heating in a walking beam furnace → high-pressure water descaling → rough rolling → pre-finishing rolling → PSM rolling mill sizing → cooling bed → cutting to size → entering a slow cooling room, characterized in that: The continuous casting step has a crystallizer vibration frequency of 150Hz / min, an R30 crystallizer, and a protective slag QB3-2 with optimized composition. The molten steel superheat during the continuous casting process is 30-40°C, the billet pulling speed is 0.90m / min, a small nozzle mist cooling is adopted, the secondary cooling water volume is 0.15L / Kg, and the terminal electromagnetic stirring parameter is 250A / 7Hz.

2. The method for controlling long cracks on the surface of peritectic steel round steel according to claim 1, characterized in that: The components of the protective slag QB3-2 include Fe2031.12%, F 1.15%, A120310.84%, CaO 32.11%, Si0224.52%, C14.68%, MgO 1.07%, viscosity 0.98 Pa·s, melting speed 50 mm / s, melting point 1205°C, and basicity 1.

31.

3. The method for controlling long cracks on the surface of peritectic steel round steel according to claim 1, characterized in that: The converter is not reblown, and ferroaluminum is added 1 minute after steel is tapped, followed by ferrosilicon, silicon manganese, ferrochrome, ferrovanadium and ferromolybdenum alloy in sequence.

4. The method for controlling long cracks on the surface of peritectic steel round steel according to claim 1, characterized in that: The LF refining produces white slag and keeps it all the time, while ensuring the fluidity of the top slag. In order to prevent FeO and MnO in the slag from adding oxygen to the molten steel, FeO+MnO≤0.50%. According to the Al content, aluminum wire can be added once in the early stage of refining, and aluminum addition operations are strictly prohibited in the later stage.

5. The method for controlling long cracks on the surface of peritectic steel round steel according to claim 1, characterized in that: The RH is strictly prohibited from adding alloys and returning to the LF furnace for temperature increase. The vacuum treatment time is ≥15min and the ultimate vacuum degree is ≤133Pa.

6. The method for controlling long cracks on the surface of peritectic round steel according to claim 1, characterized in that: The continuous casting adopts an integral tundish, the tundish is cleaned, filled with argon and emptied before pouring, the argon blowing time is ≥2 minutes, the casting is protected throughout the whole process, and the pouring is started outside the tank.

7. The method for controlling long cracks on the surface of peritectic round steel according to claim 6, characterized in that: The residual steel in the continuous casting ladle is not less than 4t.

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