Steel plate for high-toughness flange used under low-cost, high-hardness and ultralow-temperature conditions and manufacturing method of steel plate

Abstract

The invention relates to a high-toughness flange steel plate used under low-cost, high-hardness and ultralow-temperature conditions and a manufacturing method thereof, and belongs to the technical field of iron-based metallurgy. The steel plate comprises the following chemical components in percentage by mass: 0.10 to 0.30 percent of C and 0.10 to 0.50 percent of Si; p < = 0.005%; s < = 0.002%; 1.20% to 1.80% of Mn; 0.02 to 0.04 percent of Al; 0.04% to 0.08% of Nb; v is less than or equal to 0.01%; 0.50 to 0.80 percent of Mo and Ni; the microstructure of the steel plate is composed of fine needle-shaped ferrite, carbide and Nb-containing precipitates. The production thickness of the steel plate is 100-150mm, the impact energy at-80 DEG C is greater than or equal to 90J, and the Rockwell hardness is greater than or equal to 40HRC. The steel plate for the flange is produced by adopting a low-speed large-rolling-reduction rolling + high-temperature DQ + tempering process, so that the impact property of the steel plate for the flange at-80 DEG C is greater than or equal to 90J, the Rockwell hardness is greater than or equal to 40HRC, and the use requirements of the steel for the flange under the service condition of an ultralow-temperature environment are met.

Description

TECHNICAL FIELD

[0001] The present invention relates to the metallurgical technical field, and involves a flange steel plate used under ultra-low temperature conditions and a manufacturing method. BACKGROUND

[0002] In recent years, the petroleum chemical industry, hydrogen energy, wind power, shipbuilding and other industries have developed rapidly, driving a substantial increase in flange production and providing a broad market space for the flange manufacturing industry. Flanges are important components for connecting industrial pipelines, valves, pumps and other equipment, and are mainly used in pipeline systems, pressure vessels, mechanical equipment and wind power fields, and are generally used in pairs with gaskets fastened by bolts. The global flange market size has exceeded 6.5 billion US dollars, with a compound annual growth rate of 6%, mainly driven by the expansion of oil and gas pipelines (Russia, the Middle East, North American shale gas), European liquefied natural gas infrastructure construction, hydrogen energy pipeline flanges, offshore wind power and emerging market industrial markets. However, traditional flange manufacturing enterprises also face market / product transformation and cost pressure. Currently, flanges are mainly made from ingots through free forging, with a material utilization rate of ≤50%. New precision forming technologies such as die forging and ring rolling can improve material utilization rate ≥90% and reduce subsequent machining loss. At the same time, by optimizing steel composition (micro-alloying) and adopting new processes, the performance of steel can be improved, or the flange wall thickness can be reduced under the premise of ensuring steel performance, reducing installation cost and improving transportation efficiency. Currently, many oil and gas pipelines pass through severe cold or harsh environments (e.g. Norway, Finland, Canada, Russia, etc.), i.e. pipeline flanges are used in ultra-low temperature environments. The extreme temperature in these places is around -80°C, so when designing flange connectors using steel plates, ultra-low temperature performance needs to be considered to better meet engineering needs and improve engineering safety. Therefore, the hardness, low temperature and toughness requirements of various flange steels used in pipeline transportation are becoming increasingly high. Therefore, flange forming part customers tend to choose low-cost, high-toughness and high-hardness flange steel plates for processing. The research on high-toughness flange steels used under ultra-low temperature conditions using continuous casting billets for direct rolling is still in the exploratory stage.

[0003] Chinese Patent Publication No. CN115786808A proposes a 420Mpa grade wind power flange steel and its preparation method, which adopts a V+Nb+Cr micro-alloy composite composition design, a forging process, and a microstructure composed of ferrite+pearlite+tempered sorbite, without high hardness and -80°C ultra-low temperature performance. Chinese Patent Publication No. CN118222935A proposes a continuous casting large round billet production wind power tower flange steel and its manufacturing method, which adds V micro-alloy to the continuous casting large round billet and adopts a forging process to forge the material. The product hardness and toughness under ultra-low temperature conditions are insufficient. SUMMARY

[0004] According to the requirements of low cost, high hardness, high toughness under ultra-low temperature conditions for flange steel, a high toughness flange steel plate and a manufacturing method are provided.

[0005] The technical scheme adopted by the present application is: a high toughness flange steel plate and a manufacturing method for use under low cost, high hardness and ultra-low temperature conditions, the chemical composition of the steel plate is C 0.10-0.30%, Si 0.10-0.50%; P≤0.005%; S≤0.002%; Mn 1.20-1.80%; Al 0.02-0.04%; Nb 0.04-0.08%; V≤0.01%; Mo+Ni 0.50-0.80%; the balance is Fe and inevitable impurities.

[0006] The production steps are: the manufacturing process is KR molten iron pretreatment-BOF smelting-LF and RH furnace refining-continuous casting-slab heating-low speed large reduction rolling-high temperature DQ cooling-straightening-pile slow cooling-tempering-steel plate shearing-inspection into the warehouse. The process steps include: (1) The steelmaking process adopts KR molten iron pretreatment, BOF converter smelting, LF refining, RH vacuum degassing treatment to produce high-purity molten steel, and then uses a 450mm thickness continuous casting machine to produce continuous casting slabs. After continuous casting, the continuous casting blank is covered and slow-cooled for ≥120 hours.

[0007] (2) The continuous casting blank is heated to 1200-1220℃, and high-pressure water is used to remove the scale after the continuous casting blank is discharged; then two-stage rolling is carried out: (3) The first stage is the rough rolling stage: the opening rolling temperature is 1060-1150℃, wherein the down-line temperature is 1060℃=the finishing rolling temperature 1020℃+5 passes of rough rolling*8℃ temperature drop per pass, and the up-line temperature is 1150℃, which is determined according to the 1220℃ discharge temperature after the initial scale removal by vertical roll and high-pressure water, with a temperature drop of ≥70℃; the rough rolling is ≥2 passes with a single pass reduction of ≥50mm, and the rough rolling speed is 0.35-0.8m / s. The intermediate slab thickness in this stage is 1.3h-2.0h (h is the thickness of the finished steel plate), which can ensure the heating effect of the heating furnace and provide 3-5 rolling passes for the finishing rolling; at the same time, the cumulative comprehensive reduction rate in this stage is ≥90%, which can ensure that the austenite grains are completely broken and provide ultra-fine grains for the finishing rolling.

[0008] (4) The second stage is a finishing rolling stage: high temperature rolling must ensure that the finishing rolling temperature is greater than or equal to 1020 DEG C, according to the pass temperature drop on site, the finishing rolling open rolling is set to 1020-1080 DEG C, the cumulative pass reduction rate of the finishing rolling is greater than or equal to 30%, so as to ensure that the deformation can be deep into the center of the steel plate and more refined grains are obtained; after the finishing rolling (the water inlet temperature is greater than or equal to 980 DEG C), DQ cooling is carried out, the cooling rate is 25-35 DEG C / S, and the water outlet temperature is 200-350 DEG C; the tempering process adopts 600-680 DEG C, the holding time is 4.5-6.8 Min / mm, and air cooling is adopted.

[0009] The reasons for selecting the contents of all the components contained in the application are specifically described as follows: C: C in the steel can increase the strength of the steel, but the ductility, forgeability, machinability and welding performance of the steel will be reduced when the content of C is increased, and the content of C in the application is selected in the range of 0.10-0.30%.

[0010] Si: Si improves the oxidation resistance, can promote the ferrite microstructure and improve the strength, but too high content will form intermetallic compounds at high temperature, thereby causing embrittlement, and the content of Si in the application is determined in the range of 0.10-0.50%.

[0011] Mn: Mn is a deoxidizing agent and a degassing agent in the steel, which can improve the toughness of the steel and reduce the ductile-brittle transition temperature; but too high content of Mn will cause embrittlement and adversely affect the toughness of the steel, and the content of Mn in the application is designed in the range of 1.20-1.80%.

[0012] P, S: P is easy to cause segregation during solidification in the steel, and is not conducive to the ductility and toughness of the steel, and the content of P in the application is determined in the range of ≤0.005%; S is easy to produce thermal embrittlement and reduce the ductility and toughness of the steel, and the content of S in the application is determined in the range of ≤0.002%.

[0013] Al: Al is a deoxidizing agent and a degassing agent, which can delay grain growth and be used for controlling the austenite grain size, but too much Al will form brittle inclusions during the molten steel smelting process and reduce the purity of the molten steel, and the content of Al in the application is selected in the range of 0.02-0.04%.

[0014] Mo, Ni: Mo can form a more stable passivation film, at the same time, refine the grains of the steel and improve the hardenability, but too much will cause the toughness to decrease; Ni can improve the strength while ensuring good toughness, and Ni can improve the electrode potential of the matrix and thus improve the corrosion resistance, but Ni is scarce and expensive; and the content of Mo+Ni in the application is designed in the range of 0.50-0.80%.

[0015] Nb: Nb is a grain refining element, which can improve toughness and improve the resistance to sensitization, has good welding strength and creep resistance, but the cost of Nb is higher; considering comprehensively, the content of Nb in the application is determined to be 0.04-0.08%.

[0016] The flange blank under the condition of ultra-low temperature is produced by adopting continuous casting billet rolling, and the chemical composition, production process, high toughness, high hardness and microstructure and performance of the flange steel are systematically researched; the steel plate with the thickness of 100-150mm is produced by adopting the process of medium-low C, composite addition of Mn, Nb, Ni and Mo and other alloys, low-speed large reduction rolling (single pass reduction≥50mm), high temperature DQ and tempering, unique microstructure structure such as fine acicular ferrite, fine carbide and Nb-containing precipitates is obtained, the impact property of the flange steel plate at-80℃ is greater than or equal to 90J, the Rockwell hardness is greater than or equal to 40HRC (the surface hardness of the steel plate), and the use requirements of the flange steel under the service condition of ultra-low temperature environment are met.

[0017] Compared with the prior art, the advantages of the application are as follows: the thick steel plate is produced by adopting the process of medium-low C content design, composite addition of Mn, Nb, Ni and Mo and other alloys, low-speed large reduction rolling (single pass reduction≥50mm, rolling speed 0.35-0.8m / s), high temperature DQ and tempering, and the technical mechanism is as follows: The metallographic structure of the traditional flange steel plate is ferrite+pearlite+small amount of cementite, compared with the metallographic structure of the traditional flange steel plate, the low-speed large reduction rolling breaks the austenite grains+high temperature DQ rapid cooling can form fine acicular ferrite, the microstructure has more ideal low-temperature toughness, the fine grain brings fine-grain strengthening effect; the fine molybdenum carbide dispersedly distributed in the grain boundary is precipitated during the tempering at 600-680℃, which improves the strength of the steel plate; at the same time, the residual fine vanadium carbide dispersedly distributed in the grain boundary is also precipitated during the tempering at 600-680℃, which has very high hardness and helps to improve the strength of the steel plate; the Nb-containing precipitate (mainly NbC) dispersedly distributed in the grain boundary and locally clustered in a small amount is precipitated during the tempering, which refines the grains, promotes the formation of fine acicular ferrite, increases the surface hardness of the steel plate and improves the strength, and the content of the precipitate is 0.03-0.05%. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 It is the microstructure photo of the embodiment of the application at 1 / 4 of the plate thickness direction of 105mm specification; Figure 2 It is the microstructure photo of the embodiment of the application at 1 / 4 of the plate thickness direction of 148mm specification; Figure 3 It is the electron scanning Nb compound photo of the embodiment of the application at 1 / 4 of the plate thickness direction of 148mm specification. DETAILED DESCRIPTION

[0019] The application will be further described in conjunction with the following examples which are illustrative and are intended to explain the application and not to be understood to limit the application.

[0020] Example 1-2: According to the chemical composition range and manufacturing method of the present application, a new type of high hardness, high toughness under ultra-low temperature condition flange steel plate with unique microstructure is manufactured by KR hot metal pretreatment-BOF smelting-LF and RH furnace refining-continuous casting-slab heating-low speed large reduction rolling-high temperature DQ cooling-straightening-pile slow cooling-tempering-steel plate shearing-inspection into warehouse.

[0021] The specific process of the above heating, low speed large reduction rolling, high temperature DQ cooling and tempering is as follows: the 450m thickness continuous casting slab is heated to 1210℃, the soaking section is kept for 45min (Example 1) or the 450m thickness continuous casting slab is heated to 1220℃, the soaking section is kept for 45min (Example 2), and high pressure water is used to remove scale after the continuous casting slab is discharged; then two-stage rolling is carried out, the first stage rolling temperature is 1080-1120℃, the cumulative comprehensive reduction is 92%, the rough rolling is 2 single pass reduction amount≥55mm, the rolling speed is 0.45m / s, and the intermediate slab thickness is 240mm (Example 1) or the first stage rolling temperature is 1090-1140℃, the cumulative comprehensive reduction is 96%, the rough rolling is 3 single pass reduction amount≥50mm, the rolling speed is 0.45m / s, and the intermediate slab thickness is 210mm (Example 2); the second stage rolling temperature is 1050℃, the cumulative pass reduction is 40% (Example 1) or the second stage rolling temperature is 1080℃, the cumulative pass reduction is 36% (Example 2), and the final steel plate thickness is 105mm (Example 1) and 148mm (Example 2); high temperature DQ rapid cooling is carried out after rolling, the DQ temperature is 990℃, the cooling speed is 31℃ / s, and the water outlet temperature is 280℃ (Example 1) or the DQ temperature is 985℃, the cooling speed is 29℃ / s, and the water outlet temperature is 250℃ (Example 2); then hot straightening is carried out; the steel plate is subjected to pile slow cooling after hot straightening; tempering treatment: the tempering temperature is 650℃, the holding time is 480min, and air cooling (Example 1) or the tempering temperature is 650℃, the holding time is 668min, and air cooling (Example 2).

[0022] The chemical composition of the test steel plate is shown in Table 1, the hardness and impact performance is shown in Table 2, and the microstructure of the steel plate is shown in Figure 1 and Figure 2 .

[0023] Table 1 Chemical composition of steel plate in Examples 1 and 2 (wt.%)

[0024] Table 2 Hardness and impact properties of the steel sheets in Examples 1 and 2

[0025] In addition to the above embodiments, the present application also includes other embodiments, and any technical solutions formed by equivalent transformation or equivalent replacement shall fall within the protection scope of the claims of the present application.

Claims

1. A steel sheet for a flange, characterized by: The chemical composition is C 0.10-0.30%, Si 0.10-0.50%; P≤0.005%; S≤0.002%; Mn 1.20-1.80%; Al 0.02-0.04%; Nb 0.04-0.08%; V≤0.01%; Mo+Ni 0.50-0.80%; the balance is Fe and inevitable impurities, and the microstructure is fine acicular ferrite + carbide + Nb-containing precipitates.

2. Steel sheet for flanges according to claim 1, characterized in that: The production thickness of the steel plate is 100-150 mm.

3. Steel sheet for flanges according to claim 1, characterized in that: The carbide includes vanadium carbide, molybdenum carbide and niobium carbide, the vanadium carbide and molybdenum carbide are dispersedly distributed at the grain boundaries, the niobium carbide is dispersedly distributed at the grain boundaries and locally clustered in a small amount, and the content of the niobium carbide precipitate is 0.03-0.05%.

4. Steel sheet for flanges according to claim 1, characterized in that: The steel plate has an impact energy of ≥90 J at-80 ℃, and the surface hardness of the steel plate is ≥40 HRC.

5. A method of manufacturing a steel sheet for a flange according to claim 1, characterized by: The method comprises the following steps: Step one, smelting of molten steel: smelting molten steel according to the element composition design; Step two, continuous casting: using a continuous casting process to cast the molten steel into a continuous casting billet with a thickness of more than 450 mm, and performing cover stacking and slow cooling on the continuous casting billet, and the slow cooling time is ≥120 hours; Step three, heating of the continuous casting billet: heating the continuous casting billet to 1200-1220 ℃, and removing scales by using high-pressure water after uniform heating; Step four, rolling: using a two-stage rolling process of rough rolling and finish rolling, setting the rough rolling starting temperature at 1060-1150 ℃, the rough rolling speed at 0.35-0.8 m / s, the single pass reduction amount at ≥50 mm, the cumulative reduction rate at ≥90%, the thickness of the intermediate billet after rough rolling at 1.3h-2.0h (h is the thickness of the finished steel plate), and using 3-5 rolling passes for finish rolling, high-temperature rolling, a starting temperature of 1020-1080 ℃, a finish rolling temperature of ≥1020 ℃, and a cumulative pass reduction rate of ≥30% for finish rolling; Step five, DQ cooling: performing DQ cooling on the steel plate after finish rolling, the water inlet temperature of the steel plate is ≥980 ℃, the cooling rate is 25-35 ℃ / s, and the water outlet temperature is 200-350 ℃; Step six, tempering: 600-680 ℃, holding time 4.5-6.8 Min / mm, air cooling.

6. The method of manufacturing a steel sheet for a flange according to claim 5, characterized in that: Step one, using KR molten iron pretreatment, BOF converter smelting, LF refining, and RH vacuum degassing to smelt the molten steel.

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