A method for producing a pipeline steel by a five-stage differential continuous cooling method

Abstract

The present application relates to the technical field of metallurgy, and particularly relates to a method for producing pipeline steel by adopting a five-stage differential continuous cooling mode. The method at least comprises the step of five-stage differential continuous cooling after plastic deformation of the steel plate, wherein the first stage adopts air cooling mode; the second stage adopts high-density quick cooling nozzle jet cooling; the third stage adopts air cooling mode; the fourth stage adopts high-density quick cooling nozzle jet cooling; and the fifth stage adopts air cooling to room temperature. Through effective control of each stage process, the steel plate obtains more uniform and refined specific microstructure, and finally realizes the improvement of the microstructure uniformity of the whole mother plate. While meeting the physical property indexes of the steel plate, the flatness level of the steel plate is also greatly improved.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a method for producing pipeline steel using a five-stage differential continuous cooling method. Background Technology

[0002] Pipeline steel refers to a type of steel with special requirements used in the manufacture of pipelines for transporting oil, natural gas, and other commodities. In recent years, as the focus of oil and gas field development has shifted to remote areas, the harsh environment has placed more stringent demands on the performance of pipeline steel. Currently, advanced manufacturing technologies have led to continuous improvements in the strength levels of pipeline steel, and its application fields are becoming increasingly widespread. A typical steel grade, such as X70M pipeline steel, requires high strength, high ductility and toughness, high crack arrest, and excellent weldability. It is usually designed with a low-carbon composition and is a typical TMCP (controlled rolling and controlled cooling) steel grade, with relatively mature production processes in the industry. However, the traditional continuous cooling method used for this type of steel can easily cause problems such as poor uniformity of performance between the base plate and the steel plate, resulting in large strength deviations. Furthermore, it can easily cause the steel plate to warp and have poor straightness.

[0003] To address the issue of poor straightness in pipeline steel plates, Chinese invention patent application CN 112246881 A provides a method for controlling the shape of X70 thin and wide plates suitable for ultra-rapid cooling. This method includes a pre-straightening step, a water-cooling step, a reheating step, and a hot straightening machine step. While this method can achieve the technical effect of stable production of thin, ultra-wide, and high-strength pipeline steel X70, it still has certain limitations in controlling the temperature uniformity of the plate and precisely regulating the microstructure. Summary of the Invention

[0004] To address the issues of poor uniformity and straightness in pipeline steel plates produced by traditional continuous cooling methods, this invention provides a method for producing pipeline steel using a five-stage differential continuous cooling process. Through effective control of the process at each stage, the steel plate achieves a more uniform and refined specific microstructure, ultimately improving the uniformity of the entire base plate. While meeting the physical property indicators of the steel plate, the straightness level of the steel plate is also greatly improved.

[0005] The technical solution of this invention is as follows:

[0006] A method for producing pipeline steel using a five-stage differential continuous cooling method includes at least the step of performing five-stage differential continuous cooling after the plastic deformation of the steel plate, wherein the first stage adopts air cooling, the cooling rate is 0.5~6℃ / s, and the final cooling temperature is Ar3+(10~15)℃.

[0007] The second stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 18~25℃ / s, and a final cooling temperature of Bf+(50~60)℃;

[0008] The third stage uses air cooling, with a cooling time of 2~5s and a final cooling temperature of Bf+(40~50)℃.

[0009] The fourth stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 20~25℃ / s, and a final cooling temperature of Bf-(15~30)℃;

[0010] The fifth stage uses air cooling to bring the temperature to room temperature at a rate of 0.5~4℃ / s.

[0011] The meanings of the important phase transition temperatures Ar3 and Bf involved in the above five-stage differential continuous cooling are as follows:

[0012] Ar3—the phase transformation temperature during the material cooling process, where austenite transforms into ferrite.

[0013] Bf — the temperature at which the ferrite structure transforms into bainite structure during the material cooling process.

[0014] Furthermore, the methods specifically include:

[0015] (1) KR desulfurization: improves the purity of smelting raw materials. After this process, the S content in the raw materials is guaranteed to be ≤0.003%;

[0016] (2) Converter smelting: Bottom blowing adopts full-process argon blowing, and the slag is blocked by the slide plate in a double-barrel method. The corresponding alloys are added according to the chemical composition.

[0017] (3) LF treatment: Ensure argon blowing time;

[0018] (4) RH treatment: Ensure vacuum level, soft blowing and calming time, and fully feed calcium-aluminum wire;

[0019] (5) Continuous casting: Protect the casting process throughout and ensure constant casting speed;

[0020] (6) Slab heating: Ensure heating temperature and furnace time to allow alloying elements to fully dissolve in austenite;

[0021] (7) First stage rolling: to make the material undergo large-scale plastic deformation under high pressure in the recrystallization temperature region;

[0022] (8) Second stage rolling: Based on the physical properties of the material and the characteristics of the production line configuration, plastic deformation is carried out in the low temperature region as much as possible on the basis of a temperature higher than Ar3.

[0023] (9) Five-stage differential continuous cooling.

[0024] Furthermore, after RH treatment, the low magnification sample of the billet is no less than Class C 1.0 for central segregation and Class 0.5 for central porosity.

[0025] Furthermore, the reduction amount in the first stage of rolling shall account for no less than 60% of the total plastic deformation reduction amount, and the final deformation detection temperature in this stage shall be 1030~1050℃.

[0026] Furthermore, the furnace exit temperature of the slab heating should not exceed 1190℃.

[0027] Furthermore, the elemental composition and mass percentage content of the pipeline steel are as follows: C 0.04%~0.10%, Si 0.10%~0.50%, Mn 1.20%~1.80%, P≤0.20%, S≤0.10%, Cr 0.10%~0.50%, Nb 0.01%~0.06%, Mo≤0.50%, Ti 0.008%~0.30%, N≤0.004%, O≤0.003%, H≤0.0003%, with the remainder being Fe and other unavoidable impurity elements.

[0028] Furthermore, the Ar3 of the aforementioned pipeline steel is 759℃ and the Bf is 532℃. The five-stage differential continuous cooling is as follows:

[0029] The first stage uses air cooling at a rate of 5℃ / s, with the final cooling temperature controlled at 769~774℃.

[0030] The second stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 20℃ / s, and a final cooling temperature controlled at 582~592℃.

[0031] The third stage uses air cooling, with a cooling time of 2-5 seconds and a final cooling temperature controlled at 572-582℃.

[0032] The fourth stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 25℃ / s, and a final cooling temperature controlled at 502~517℃.

[0033] The fifth stage uses air cooling to bring the temperature to room temperature, with the cooling rate controlled within the range of 0.5~4℃ / s.

[0034] The beneficial effects of this invention are as follows:

[0035] The pipeline steel production method provided by this invention employs a five-stage differential continuous cooling process. After the second stage of cooling, the microstructure is dominated by low-carbon bainite, with a small amount of acicular ferrite present. The third stage of cooling aims to release the internal stress generated locally in the steel plate during the second stage of cooling, effectively improving the uniformity of steel plate performance and the flatness level. After the fourth stage of cooling, the steel plate forms a refined and uniform low-carbon bainite structure. The fifth stage can fully release the structural stress formed inside the steel plate, improving the flatness level and overall performance of the steel plate. Compared with traditional processes, the uniformity of strength performance within the same plate is higher, and the flatness level of the steel plate is superior. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 This is a photograph of the metallographic structure of the edge of the steel plate obtained in Example 1.

[0038] Figure 2 This is a metallographic photograph of the steel plate at one-quarter thickness obtained in Example 1.

[0039] Figure 3 This is a metallographic image of the center of the thickness of the steel plate obtained in Example 1.

[0040] Figure 1 , Figure 2 , Figure 3 The magnification is 500x. Detailed Implementation

[0041] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0042] Example 1

[0043] X70M steel with finished product dimensions of 14.3mm (thickness) * 2270mm (width) * 12000mm (length) * 4 times is produced using a billet with dimensions of 150mm * 2380mm * 4830mm. The billet composition by mass fraction is as follows: C 0.049%, Si 0.27%, Mn 1.43%, P 0.01%, S 0.002%, Cr 0.33%, Nb 0.032%, Ti 0.010%, Mo 0.10%, N 0.001%, O 0.001%, H 0.0001%, with the remainder being Fe and other unavoidable impurity elements.

[0044] The process route is as follows: KR desulfurization → converter smelting → LF → RH → continuous casting → billet cutting to length → billet cooling → slab heating → high-pressure water descaling → first stage rolling on 3500mm hot roll mill → second stage rolling on 3500mm hot roll mill → five-stage differential continuous cooling → straightening → flaw detection → cutting → inspection and marking → warehousing.

[0045] KR desulfurization improves the purity of smelting raw materials, ensuring that S ≤ 0.003% in the raw materials after this process. Converter smelting bottom blowing uses full-process argon blowing with a dual-stage sliding plate slag blocking method, and appropriate alloys are added according to the chemical composition. Billet smelting employs an LF+RH dual refining process, ensuring controlled argon blowing time, vacuum level, soft blowing, and calming time, and adequate feeding of calcium-aluminum wire to ensure that the low-magnification sample of the cast billet has a center segregation grade of C 1.0 and a center porosity grade of 0.5, and is free from obvious inclusions and other internal cracks or defects. The billet is produced using a low-temperature heating process with a dual-regenerative walking beam furnace intelligent combustion control system, ensuring uniform heating of the billet. The surface of the billet after exiting the furnace is free of visible black marks, and the exit temperature does not exceed 1190℃. Plastic deformation employs a two-stage rolling process. The first stage involves large-scale plastic deformation with a reduction in the recrystallization temperature range. This reduction should account for no less than 60% of the total plastic deformation reduction, and the final measurement temperature for this stage is 1030–1050℃. The second stage involves low-temperature deformation in the non-recrystallization region. In principle, this stage utilizes low-temperature rolling deformation as much as possible, above the Ar3 temperature (the transformation temperature from austenite to ferrite), until the desired rectangular dimensions are achieved. The key phase transformation temperatures derived from this selected composition mass fraction are as follows: Ar3: 759℃, Bf: 532℃.

[0046] The post-rolling water cooling process adopts a five-stage differential continuous cooling process. The starting cooling temperature of the second, third, fourth, and fifth stages is the final cooling temperature of the previous stage. The specific cooling process is as follows:

[0047] The first stage uses air cooling at a rate of 5℃ / s, with the final cooling temperature controlled at 769~774℃.

[0048] The second stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 20℃ / s, and a final cooling temperature controlled at 582~592℃.

[0049] The third stage uses air cooling, with a cooling time of 4 seconds and a final cooling temperature controlled at 572~582℃.

[0050] The fourth stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 25℃ / s, and a final cooling temperature controlled at 502~517℃.

[0051] The fifth stage uses air cooling to bring the temperature to room temperature, with the cooling rate controlled within the range of 0.5~4℃ / s.

[0052] Meanwhile, a steel plate with the same composition and specifications as Example 1 was treated using conventional water cooling process as Comparative Example 1.

[0053] Example 2

[0054] X70M steel with finished product dimensions of 14.3mm (thickness) * 2270mm (width) * 12000mm (length) * 4 times is produced using a billet with dimensions of 150mm * 2380mm * 4830mm. The billet composition by mass fraction is as follows: C 0.097%, Si 0.44%, Mn 1.69%, P 0.01%, S 0.002%, Cr 0.18%, Nb 0.030%, Mo 0.15%, Ti 0.022%, N 0.001%, O 0.001%, H 0.0001%, with the remainder being Fe and other unavoidable impurity elements.

[0055] The process route is as follows: KR desulfurization → converter smelting → LF → RH → continuous casting → billet cutting to length → billet cooling → slab heating → high-pressure water descaling → first stage rolling on 3500mm hot roll mill → second stage rolling on 3500mm hot roll mill → five-stage differential continuous cooling → straightening → flaw detection → cutting → inspection and marking → warehousing.

[0056] KR desulfurization improves the purity of smelting raw materials, ensuring that S ≤ 0.003% in the raw materials after this process. Converter smelting bottom blowing uses full-process argon blowing with a dual-stage sliding plate slag blocking method, and appropriate alloys are added according to the chemical composition. Billet smelting employs an LF+RH dual refining process, ensuring controlled argon blowing time, vacuum level, soft blowing, and calming time, and adequate feeding of calcium-aluminum wire to ensure that the low-magnification sample of the cast billet has a center segregation grade of C 1.0 and a center porosity grade of 0.5, and is free from obvious inclusions and other internal cracks or defects. The billet is produced using a low-temperature heating process with a dual-regenerative walking beam furnace intelligent combustion control system, ensuring uniform heating of the billet. The surface of the billet after exiting the furnace is free of visible black marks, and the exit temperature does not exceed 1190℃. Plastic deformation employs a two-stage rolling process. The first stage involves large-scale plastic deformation with a reduction in the recrystallization temperature range. This reduction should account for no less than 60% of the total plastic deformation reduction, and the final measurement temperature for this stage is 1030–1050℃. The second stage involves low-temperature deformation in the non-recrystallization region. In principle, this stage utilizes low-temperature rolling deformation as much as possible, above the Ar3 temperature (the transformation temperature from austenite to ferrite), until the desired rectangular dimensions are achieved. The key phase transformation temperatures derived from this selected composition mass fraction are as follows: Ar3: 759℃, Bf: 532℃.

[0057] The post-rolling water cooling process adopts a five-stage differential continuous cooling process. The starting cooling temperature of the second, third, fourth, and fifth stages is the final cooling temperature of the previous stage. The specific cooling process is as follows:

[0058] The first stage uses air cooling at a rate of 5℃ / s, with the final cooling temperature controlled at 769~774℃.

[0059] The second stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 20℃ / s, and a final cooling temperature controlled at 582~592℃.

[0060] The third stage uses air cooling, with a cooling time of 3 seconds and a final cooling temperature controlled at 572~582℃.

[0061] The fourth stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 25℃ / s, and a final cooling temperature controlled at 502~517℃.

[0062] The fifth stage uses air cooling to bring the temperature to room temperature, with the cooling rate controlled within the range of 0.5~4℃ / s.

[0063] Meanwhile, a steel plate with the same composition and specifications as Example 2 was treated using conventional water cooling process as Comparative Example 2.

[0064] The properties and flatness of the steel plates obtained in Examples 1 and 2 and Comparative Examples 1 and 2 were tested, and the results are shown in Table 1 below.

[0065] Table 1 Steel Plate Properties and Flatness

[0066] Serial Number Water cooling process Difference in yield strength between the mother plate and the slab Difference in tensile strength between the mother slab and the slab Steel plate straightness level Comparative Example 1 conventional 70MPa 60MPa ≤10mm / 2m Comparative Example 2 conventional 75MPa 70MPa ≤10mm / 2m Example 1 5-stage cooling 55MPa 50MPa ≤9mm / 2m Example 2 5-stage cooling 60MPa 55MPa ≤8mm / 2m

[0067] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for producing pipeline steel using a five-stage differential continuous cooling process, characterized in that, The methods specifically include: (1) KR desulfurization; (2) converter smelting; (3) LF treatment; (4) RH treatment; (5) continuous casting; (6) slab heating; (7) first stage rolling, the reduction of the first stage rolling is not less than 60% of the total plastic deformation reduction, and the final detection temperature of this stage is 1030~1050℃; (8) second stage rolling; (9) five-stage differential continuous cooling, of which the first stage adopts air cooling, the cooling rate is 0.5~6℃ / s, and the final cooling temperature is Ar3+10~15℃; The second stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 18~25℃ / s, and a final cooling temperature of Bf+50~60℃. The third stage uses air cooling, with a cooling time of 2-5 seconds and a final cooling temperature of Bf+40-50℃. The fourth stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 20~25℃ / s, and a final cooling temperature of Bf-15~30℃; The fifth stage uses air cooling to bring the temperature to room temperature at a rate of 0.5~4℃ / s; The elemental composition and mass percentage content of pipeline steel are as follows: C 0.04%~0.10%, Si 0.10%~0.50%, Mn 1.20%~1.80%, P≤0.20%, S≤0.10%, Nb 0.01%~0.06%, Cr 0.10%~0.50%, Mo≤0.50%, Ti 0.008%~0.30%, N≤0.004%, O≤0.003%, H≤0.0003%, with the remainder being Fe and other unavoidable impurity elements; The straightness of the pipeline steel plate is ≤9mm / 2m.

2. The method as described in claim 1, characterized in that, After RH treatment, the low magnification sample of the billet should not be lower than the center segregation grade C 1.0 and the center porosity grade 0.

5.

3. The method as described in claim 1, characterized in that, The furnace exit temperature of the slab heating should not exceed 1190℃.

4. The method as described in claim 1, characterized in that, The Ar3 of pipeline steel is 759℃ and the Bf is 532℃.

5. The method as described in claim 4, characterized in that, The five-stage differential continuous cooling is as follows: The first stage uses air cooling at a rate of 5℃ / s, with the final cooling temperature controlled at 769~774℃. The second stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 20℃ / s, and a final cooling temperature controlled at 582~592℃. The third stage uses air cooling, with a cooling time of 2-5 seconds and a final cooling temperature controlled at 572-582℃. The fourth stage uses high-density rapid cooling nozzles for spray cooling, with an inlet water pressure of 0.5MPa, a cooling rate of 25℃ / s, and a final cooling temperature controlled at 502~517℃. The fifth stage uses air cooling to bring the temperature to room temperature, with the cooling rate controlled within the range of 0.5~4℃ / s.

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