Low-carbon equivalent ferrite type pipeline steel and construction method thereof

Abstract

The invention discloses low-carbon equivalent ferrite type pipeline steel and a construction method, and belongs to the technical field of metallurgical production processes. The invention provides the low-carbon equivalent ferrite type pipeline steel which has ultralow carbon equivalent and meets the matching requirement of strength and toughness of X52 pipeline steel and the construction method. The low-carbon equivalent ferrite type pipeline steel is a hot-rolled steel strip with the thickness specification of 8.0-12.0 mm, and comprises the following components in percentage by weight: 0.020-0.050% of C, 0.10-0.25% of Si, 0.70-1.00% of Mn, less than or equal to 0.008% of P, less than or equal to 0.003% of S, 0.020-0.040% of Als, 0.015-0.025% of Nb, 0.010-0.020% of Ti, less than or equal to 0.0050% of N and the balance of Fe and inevitable impurities, and the cold crack sensitivity index Pcm of the hot-rolled steel strip is less than or equal to 0.1%. The construction method comprises the steps of molten steel smelting, calcium treatment, plate blank preparation, plate blank reheating, plate blank rolling, cooling and coiling, in the plate blank preparation process, molten steel which is smelted to be qualified is directly and continuously cast on a continuous casting machine to form a plate blank meeting the requirement, and after the continuous casting plate blank is slowly cooled for more than 48 hours, the plate blank is fed into a heating furnace to be heated in the furnace for 150-230 min; and after discharging, carrying out rough rolling, finish rolling and laminar cooling, and coiling to obtain the low-carbon equivalent ferrite type pipeline steel.

Description

Technical Field

[0001] The present invention relates to a ferrite pipeline steel, in particular to a low carbon equivalent ferrite pipeline steel, belonging to the technical field of metallurgical production process. The present invention also relates to a construction method for the low carbon equivalent ferrite pipeline steel. Background Art

[0002] The natural gas mined in my country is associated with a large amount of acidic substances such as H2S gas. In order to ensure the safe operation of the gathering and transportation pipelines and prevent the corrosion and damage of the pipelines by acidic gases, the pipeline materials need to have certain HIC resistance performance. A higher carbon equivalent is not conducive to the HIC resistance of the pipe. By reducing the carbon equivalent in the chemical composition of the pipe and improving the level of organizational regulation, the HIC resistance can be improved to achieve good service performance.

[0003] Patent document CN114645198A "A low-cost easy-to-weld pipeline steel hot-rolled coil and its preparation method" discloses a chemical composition of 0.050-0.080% C, 0.10-0.25% Si, 0.50-0.70% Mn, P≤0.020%, S≤0.0040%, 0.045-0.065% Ti, 0.020-0.050% Als, N≤0.00 50%, using smelting-refining-continuous casting-heating-rolling-cooling-coiling process, preparing a pipeline steel hot-rolled coil with a thickness of 6-14mm, a ferrite structure, a yield strength of 421-480MPa, a tensile strength of 513-556MPa, an elongation of ≥30%, a yield strength ratio of ≤0.85, a transverse full-size impact energy of 240-310J at -20℃, and a drop hammer shear surface ratio of ≥85% at -10℃. The invention adopts a low-carbon, low-manganese titanium microalloying component system. Due to the high Ti content and the complete absence of Nb, it is easy to cause grain coarsening, mixed crystals and large performance fluctuations.

[0004] Patent document CN104694823B "An ultra-low carbon, high-strength and toughness HIC pipeline resistant steel plate and its preparation method" discloses a method for preparing a HIC pipeline resistant steel plate with a chemical composition of 0.03-0.05% C, 0.15-0.30% Si, Mn≤1.20%, S≤0.002%, P≤0.01%, 0.02-0.08% Nb, Ti≤0.05%, independent addition of Ni≤0.50% or mixed addition of Ni≤0.50% and Cr≤0.50%, and two-stage rolling and accelerated cooling using a medium and thick plate rolling mill to obtain a thickness of 12-20mm, a yield strength of 501-533MPa, a tensile strength of 564-585MPa, a yield strength ratio of 0.87-0.92, a -20℃ transverse impact energy of 266-427J, a -20℃ drop hammer shear surface ratio ≥85%, and a crack rate of 0. This invention adopts a component system suitable for rolling medium and thick plates. The organizational type is ferrite + degenerate pearlite, which has good organizational uniformity. However, its carbon equivalent is high and the yield strength ratio is high, so it is not suitable for the production of thin-gauge steel plates.

[0005] Patent document CN106636888A "A low carbon equivalent high strength X70 pipeline steel hot rolled coil and its manufacturing method" discloses a method for preparing a coil with a chemical composition of 0.04-0.08% C, 0.10-0.30% Si, 1.40-1.60% Mn, Nb≤0.06%, Ti≤0.02%, 0.1-0.2% Mo, Ceq≤0.38, using smelting, continuous casting, secondary heating, rough rolling, finishing rolling, cooling, and coiling processes to obtain granular bainite and ferrite structures. The invention is suitable for X70 coils with a thickness of 4.0-9.0 mm, and solves a series of problems caused by the high carbon equivalent and low coiling temperature of conventional X70 steel grades, but cannot be applied to the production of low-grade ferrite pipeline steel.

[0006] Conventional X52 pipeline steel is generally made by submerged arc welding. The microstructure type is ferrite + pearlite, and it uses a component system with a higher carbon equivalent. It is not suitable for ERW pipe making and is not suitable for use in scenarios where the transport medium contains a lot of acidic gases such as H2S. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a low-carbon equivalent ferritic pipeline steel having an ultra-low carbon equivalent and meeting the strength and toughness matching requirements of X52 pipeline steel, and a construction method for the low-carbon equivalent ferritic pipeline steel.

[0008] The technical solution adopted to solve the above technical problems is: a low carbon equivalent ferrite pipeline steel, the low carbon equivalent ferrite pipeline steel is a hot rolled steel strip containing the following weight percentage components and a thickness specification of 8.0-12.0 mm,

[0009] The weight percentage components are 0.020-0.050% C, 0.10-0.25% Si, 0.70-1.00% Mn, P≤0.008%, S≤0.003%, 0.020-0.040% Als, 0.015-0.025% Nb, 0.010-0.020% Ti, N≤0.0050%, and the rest are Fe and unavoidable impurities.

[0010] The cold crack sensitivity index P of the hot rolled steel strip cm ≤0.1%, the calculation formula is,

[0011] P cm =C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B, where the symbol of each element in the formula represents its weight percentage in the steel plate.

[0012] Furthermore, the microstructure of the hot rolled steel strip is ferrite or ferrite + a small amount of bainite.

[0013] The preferred embodiment of the above scheme is that the transverse yield strength of the finished hot-rolled steel strip is ≥360MPa, the tensile strength is ≥420MPa; the transverse full-size impact energy is ≥250J at -20°C, the ductile-brittle transition temperature is lower than -60°C; and the drop hammer tearing area ratio is ≥85% at -10°C.

[0014] A construction method for preparing the low carbon equivalent ferritic pipeline steel, the construction method comprising the steps of molten steel smelting, calcium treatment, slab preparation, slab reheating, slab rolling, cooling and coiling.

[0015] When preparing the slab, qualified molten steel is directly cast on the continuous casting machine into slabs that meet the requirements. After the continuous cast slab is slowly cooled for more than 48 hours, it is sent to the heating furnace and heated in the furnace for 150-230 minutes. After it is taken out of the furnace, it is rough rolled, finish rolled, and coiled after laminar cooling to obtain low carbon equivalent ferritic pipeline steel.

[0016] Furthermore, molten steel smelting includes converter smelting, refining outside the furnace and vacuum smelting.

[0017] During converter smelting, argon is blown throughout the process, oxygen is blown until C≤0.040%, then alloy Si, Mn, Nb are added, and the exit temperature is>1615℃;

[0018] During refining outside the furnace, the content of elements C, Si, Mn and Als alloy is fine-tuned, and the outlet temperature is >1600℃;

[0019] The processing time of vacuum smelting is ≥20min, the cycle time under vacuum degree <3mbar is ≥10min, and the cycle time after alloying is ≥6min.

[0020] The preferred embodiment of the above scheme is that 100-300 m of Ca-Al wire is fed during calcium treatment, and the circulation time after wire feeding is ≥8 min.

[0021] Furthermore, the pulling speed during continuous casting is 0.9-1.5 m / min, the pulling speed change within 10 s is ≤ 0.05 m / min, 5 mm light pressure is adopted, the continuous casting overheat is 15-20° C., and the thickness of the continuous casting slab is 200-230 mm.

[0022] The preferred embodiment of the above scheme is that the furnace temperature of the slab after reheating is 1180-1200°C, high pressure descaling is performed before rough rolling, the number of rough rolling passes is 5, the deformation of a single pass is ≥20%, and the thickness of the intermediate slab is 35-48mm.

[0023] Furthermore, the intermediate billet after rough rolling is finished by 7-stand continuous rolling, the F1-F3 stands open inter-stand cooling water to cool the strip, the cumulative deformation of the F1-F3 passes is ≥60%, and the final rolling temperature ranges from 800-840°C.

[0024] The preferred embodiment of the above scheme is that the front section of laminar cooling adopts sparse cooling, and the strip is cooled to 560-620°C at a cooling rate of 5-15°C / s for coiling.

[0025] The beneficial effects of the present invention are as follows: the weight percentage components of the low carbon equivalent ferrite pipeline steel provided by the present application are 0.020-0.050% C, 0.10-0.25% Si, 0.70-1.00% Mn, P≤0.008%, S≤0.003%, 0.020-0.040% Als, 0.015-0.025% Nb, 0.010-0.020% Ti, N≤0.0050%, and the rest are Fe and unavoidable impurities. The thickness specification of the rolled hot-rolled steel strip is 8.0-12.0 mm, and the steel strip is prepared according to the steps of molten steel smelting, calcium treatment, slab preparation, slab reheating, slab rolling, cooling and coiling. Since the present application strictly controls the chemical composition, especially controls the cold crack sensitivity index P cm ≤0.1%, and in addition to smelting the molten steel with controlled component content, calcium treatment is also carried out when preparing the slab, and then the slab is heated again, rough rolled, finish rolled and laminar cooled, and finally air-cooled and coiled, so as to control the carbon equivalent of the pipeline steel to a lower level, and then combined with the corresponding controlled rolling and controlled cooling parameters, the hot-rolled coil that meets the requirements of X52 steel grade pipeline steel, is suitable for ERW pipe making and has good HIC resistance and welding performance is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The microstructure diagram of Example 1 of the low carbon equivalent ferrite pipeline steel and the construction method thereof of the present invention;

[0027] Figure 2 This is a microstructure diagram of Example 2 involving the low carbon equivalent ferritic pipeline steel and its construction method of the present invention. DETAILED DESCRIPTION

[0028] like Figure 1 , Figure 2 The present invention provides a low carbon equivalent ferritic pipeline steel having an ultra-low carbon equivalent and meeting the strength and toughness matching requirements of X52 pipeline steel, and a construction method for the low carbon equivalent ferritic pipeline steel. The low carbon equivalent ferritic pipeline steel is a hot-rolled steel strip containing the following weight percentage components, with a thickness specification of 8.0-12.0 mm, and the weight percentage components are 0.020-0.050% C, 0.10-0.25% Si, 0.70-1.00% Mn, P≤0.008%, S≤0.003%, 0.020-0.040% Als, 0.015-0.025% Nb, 0.010-0.020% Ti, N≤0.0050%, and the rest are Fe and unavoidable impurities. The cold crack sensitivity index P of the hot-rolled steel strip is 0.010-0.020% C, 0.010-0.020% Nb, 0.010-0.020% Ti, N≤0.0050%, and the rest are Fe and unavoidable impurities. cm ≤0.1%, calculated as follows: P cm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B, where the symbol of each element in the formula represents its weight percentage in the steel plate. The construction method includes several steps, namely, molten steel smelting, calcium treatment, slab preparation, slab reheating, slab rolling, cooling and coiling. During the slab preparation, the molten steel that has been smelted and qualified is directly continuously cast on a continuous casting machine into a slab that meets the requirements. After the continuous casting slab is slowly cooled for more than 48 hours, it is sent to a heating furnace, heated in the furnace for 150-230 minutes, and after being taken out of the furnace, it is subjected to rough rolling, finish rolling, and coiled after laminar cooling to obtain a low carbon equivalent ferrite pipeline steel. The weight percentage components of the low carbon equivalent ferritic pipeline steel provided in the present application are 0.020-0.050% C, 0.10-0.25% Si, 0.70-1.00% Mn, P≤0.008%, S≤0.003%, 0.020-0.040% Als, 0.015-0.025% Nb, 0.010-0.020% Ti, N≤0.0050%, and the rest are Fe and unavoidable impurities. The thickness specification of the rolled hot-rolled steel strip is 8.0-12.0 mm, and it is prepared according to the steps of molten steel smelting, calcium treatment, slab preparation, slab reheating, slab rolling, cooling and coiling. Since the present application strictly controls the chemical composition, especially controls the cold crack sensitivity index Pcm≤0.1%, and in addition to smelting the molten steel with controlled component content, calcium treatment is also performed when preparing the slab, and then the slab is reheated, rough rolled, finish rolled and laminar cooled, and finally air-cooled and coiled, the carbon equivalent of the pipeline steel is controlled to a lower level, and combined with the corresponding controlled rolling and controlled cooling parameters, a hot-rolled coil that meets the requirements of X52 steel grade pipeline steel, is suitable for ERW pipe making, and has good HIC resistance and welding performance is obtained.

[0029] Correspondingly, the microstructure of the hot-rolled steel strip prepared according to the above parameters and steps with controlled chemical composition is ferrite or ferrite + a small amount of bainite. The transverse yield strength of the finished hot-rolled steel strip is ≥360MPa, the tensile strength is ≥420MPa; the transverse full-size impact energy at -20°C is ≥250J, the ductile-brittle transition temperature is lower than -60°C; the drop hammer tearing area ratio at -10°C is ≥85%.

[0030] Furthermore, in order to control the physical and chemical properties of the finished X52 pipe-grade pipeline steel to the maximum extent, the molten steel smelting of this application includes converter smelting, furnace refining and vacuum smelting. During converter smelting, argon and oxygen are blown throughout the process until C≤0.040%, and then alloys Si, Mn and Nb are added, and the exit temperature is>1615℃; during furnace refining, the content of elements C, Si, Mn and Als alloy is fine-tuned, and the exit temperature is>1600℃; the processing time of vacuum smelting is ≥20min, the cycle time under vacuum degree <3mbar is ≥10min, and the cycle time after alloying is ≥6min. During calcium treatment, 100-300m of Ca-Al wire is fed, and the cycle time after wire feeding is ≥8min. The pulling speed during continuous casting is 0.9-1.5m / min, and the pulling speed change within 10s is ≤0.05m / min. 5mm light pressure is used, the continuous casting overheat is 15-20℃, and the thickness of the continuous casting slab is 200-230mm. The reheated slab temperature is 1180-1200℃. It is first descaled under high pressure before rough rolling. The number of rough rolling passes is 5 passes, and the deformation of a single pass is ≥20%. The thickness of the intermediate slab is 35-48mm. After rough rolling, the intermediate slab is finished by continuous rolling of 7 stands. The inter-stand cooling water of F1-F3 stands is turned on to cool the strip steel. The cumulative deformation of F1-F3 passes is ≥60%, and the final rolling temperature range is 800-840℃. The front section of laminar cooling adopts sparse cooling method, and the strip steel is cooled to 560-620℃ at a cooling rate of 5-15℃ / s for coiling.

[0031] The technical solution of the present application is further described below through specific embodiments:

[0032] In order to solve the above problems, the present application provides a hot-rolled pipeline steel coil having an ultra-low carbon equivalent and meeting the strength-toughness matching of X52 pipeline steel, and a ferrite microstructure type.

[0033] The present application provides a low carbon equivalent ferritic pipeline steel with a thickness specification of 8.0-12.0 mm, which includes, by mass percentage, 0.020-0.050% C, 0.10-0.25% Si, 0.70-1.00% Mn, P≤0.008%, S≤0.003%, 0.020-0.040% Als, 0.015-0.025% Nb, 0.010-0.020% Ti, N≤0.0050%, and the rest is Fe and unavoidable impurities.

[0034] The cold crack sensitivity index Pcm of the pipeline steel is ≤ 0.1%, and the calculation formula is:

[0035] Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B, wherein: the symbol of each element in the formula represents its weight percentage in the steel plate.

[0036] The microstructure of the pipeline steel is ferrite or ferrite + a small amount of bainite.

[0037] The thickness specification of the rolled plate is 8.0-12.0 mm.

[0038] The transverse yield strength of the rolled plate is ≥360MPa and the tensile strength is ≥420MPa.

[0039] The transverse full-size impact energy of the rolled plate at -20°C is ≥250J, and the ductile-brittle transition temperature is lower than -60°C.

[0040] The drop hammer tearing area ratio of the coil at -10°C is ≥85%.

[0041] In addition, the present application also provides a smelting method for the above-mentioned low carbon equivalent ferrite pipeline steel, the process flow includes: converter smelting → refining outside the furnace → vacuum smelting → calcium treatment → continuous casting, the specific steps are as follows:

[0042] (1) Converter smelting: argon blowing throughout the process, oxygen blowing until C ≤ 0.040%, then adding alloy Si, Mn, Nb, the exit temperature is > 1615℃;

[0043] (2) Refining outside the furnace: fine-tuning of the alloy contents of elements C, Si, Mn, and Als, with the outlet temperature > 1600°C;

[0044] (3) Vacuum smelting: treatment time ≥ 20 min, vacuum degree < 3 mbar, cycle time ≥ 10 min, cycle time after alloying ≥ 6 min;

[0045] (4) Calcium treatment: feed Ca-Al line 100-300m, and the circulation time after feeding is ≥8min;

[0046] (5) Continuous casting: pulling speed 0.9-1.5 m / min, pulling speed change within 10 s ≤ 0.05 m / min, 5 mm light pressure, continuous casting overheat 15-20 ° C, to obtain a slab with a thickness of 230 mm.

[0047] The present application also provides a hot rolling method for the above-mentioned low carbon equivalent ferrite pipeline steel, the process flow includes: slab reheating → rough rolling → finishing rolling → cooling → coiling, the specific steps are as follows:

[0048] (6) After being slowly cooled for more than 48 hours, the continuous casting slab is sent to the heating furnace, with a furnace temperature of 1180-1200°C and a furnace time of 150-230 minutes;

[0049] (7) After high-pressure descaling, the rough rolling is carried out, and five passes are used, with a deformation of ≥20% in a single pass to obtain an intermediate billet with a thickness of 35-48 mm;

[0050] (8) The intermediate billet enters the finishing rolling process, using 7 stands for continuous rolling. The inter-stand cooling water of the F1-F3 stands is turned on to cool the strip steel. The cumulative deformation of the F1-F3 passes is ≥60%, and the final rolling temperature range is 800-840°C.

[0051] (9) After finishing rolling, the strip enters the laminar cooling section and adopts the front-stage sparse cooling method to cool the strip to 560-620℃ at a cooling rate of 5-15℃ / s for coiling.

[0052] Compared with the prior art, the beneficial effect of the present invention is that a lower carbon equivalent is adopted, and through corresponding controlled rolling and controlled cooling parameters, a hot-rolled coil is obtained which meets the requirements of X52 steel grade pipeline steel, is suitable for ERW pipe making and has good HIC resistance and welding performance.

[0053] Embodiment 1

[0054] The chemical composition restrictions of the hot-rolled coil of the present invention are described below:

[0055] Carbon (C): The main function of carbon is to produce solid solution strengthening and form fine carbide points with microalloying elements such as niobium and titanium, which plays a role in refining grains. The carbon content will directly affect the carbon equivalent. Under the premise of not affecting the performance, the present invention controls the C content to 0.020-0.050%.

[0056] Silicon (Si): Silicon acts as a reducing agent and deoxidizer during the smelting process of pipeline steel, and also provides a certain solid solution strengthening effect. However, too high a silicon content will reduce the plasticity, toughness and welding performance of the steel. Therefore, the designed addition amount of Si is 0.10-0.25%.

[0057] Manganese (Mn): The main function of manganese is solid solution strengthening, which compensates for the decrease in strength caused by the decrease in carbon content. At the same time, manganese can reduce the austenite phase transition temperature, stabilize the austenite phase, promote low-temperature phase transition, refine the phase transition structure, and reduce the tough-brittle transition temperature. When the manganese content is lower than 0.70%, it cannot effectively play its role in stabilizing austenite and improving strength. If the manganese content is too high, it will cause segregation in the center of the molten pool and reduce the welding performance of the steel. Therefore, the Mn content is controlled at 0.70%-1.00%.

[0058] Niobium (Nb) and titanium (Ti): Niobium and titanium are the main microalloying elements. The carbonitrides precipitate at different stages of slab heating and hot rolling, which play a role in grain refinement and precipitation strengthening. Niobium can refine austenite grains and increase the austenite recrystallization temperature. Titanium can precipitate carbonitrides at higher temperatures to pin grain boundaries and refine weld grains. To ensure the effect of grain refinement, the present invention controls the Nb content to 0.015-0.025% and the Ti content to 0.010-0.020%; in order to increase the effective titanium content, the nitrogen content should be limited to less than 0.005%.

[0059] Aluminum (Als) is a deoxidizer in the smelting process. Als within a certain range can ensure sufficient deoxidation and affect the slag-gold reaction, while too high a content will lead to an increase in non-metallic inclusions. Therefore, the Als content is controlled within the range of 0.020-0.040%.

[0060] The reasons for the manufacturing process limitations of the hot-rolled coil described in the present invention are explained below:

[0061] Slab reheating: The main function is to dissolve the alloy elements and make them homogenized. In order to meet the conditions of alloy element homogenization and carbide precipitation at the same time, the heating temperature is controlled at 1180-1200℃ and the heating time is controlled between 150-230min.

[0062] Rough rolling: The main function is to refine the austenite grains by controlling the dynamic recrystallization of austenite. The deformation of a single rough rolling pass must be greater than the critical deformation, otherwise it will easily lead to insufficient deformation at the thickness center. Therefore, the deformation of a single rough rolling pass is controlled to be ≥20%, in order to fully recrystallize the austenite grains.

[0063] Finishing rolling: The main function is to promote the deformation of non-recrystallized austenite and provide nucleation energy and nucleation particles for ferrite phase transformation. During the finishing rolling process, the cooling water between the F1-F3 stands is turned on to control the pass deformation temperature, inhibit the growth of recrystallized austenite, and promote austenite recrystallization by increasing the cumulative deformation amount of F1-F3. When entering the F4 stand, the temperature is reduced to the austenite non-recrystallization zone. F4-F7 can use a smaller pass deformation amount to optimize the plate shape and reduce anisotropy.

[0064] The key points of the process control provided by the present invention are to ensure that the deformation amount of a single pass is ≥20% and the deformation temperature is not lower than 970°C in the rough rolling stage; in the finishing rolling stage, the F1-F3 frames open the cooling water between the frames to cool the strip, inhibit the growth of recrystallized austenite, and at the same time increase the cumulative deformation amount to promote austenite recrystallization.

[0065] In the cooling stage, the cooling rate can reach the critical cooling rate of ferrite phase transformation. Therefore, the front-stage sparse cooling method is adopted to cool the strip to 560-620℃ at a cooling rate of 5-15℃ / s for coiling.

[0066] The specific implementation of the present invention is further described below in conjunction with the examples, but the present invention is not limited to the scope of the examples. Table 1 shows the composition of the examples of the present invention, Table 2 shows the hot rolling process parameters of the examples of the present invention, and Table 3 shows the performance of the examples of the present invention.

[0067] Table 1 Example composition (mass percentage, wt-%)

[0068] C Si Mn P S Nb Ti Als Example 1 0.022 0.19 0.73 0.005 0.003 0.025 0.018 0.030 Example 2 0.045 0.19 1.03 0.005 0.003 0.022 0.018 0.035 Example 3 0.022 0.19 0.73 0.005 0.003 0.025 0.018 0.030 Example 4 0.045 0.19 1.03 0.005 0.003 0.022 0.018 0.035 ;

[0069] Table 2 Key hot rolling parameters of the embodiment

[0070]

[0071] Table 3 Performance of the embodiment

[0072] <![CDATA[R t0.5 / MPa]]> <![CDATA[R m / MPa]]> Elongation / % -20℃ impact energy / J -10℃DWTT / % Yield-to-strength ratio Example 1 394 467 30 244 95 0.84 Example 2 405 485 32 240 98 0.84 Example 3 382 463 29 237 92 0.83 Example 4 413 497 31 261 99 0.83 .

Claims

1. Low carbon equivalent ferrite pipeline steel, characterized by: The low carbon equivalent ferrite pipeline steel is a hot rolled steel strip containing the following components in weight percentage and having a thickness specification of 8.0-12.0 mm. The components by weight are 0.020-0.050% C, 0.10-0.25% Si, 0.70-1.00% Mn, P≤0.008%, S≤0.003%, 0.020-0.040% Als, 0.015-0.025% Nb, 0.010-0.020% Ti, N≤0.0050%, and the rest are Fe and unavoidable impurities. The cold crack sensitivity index P of the hot rolled steel strip cm ≤0.1%, the calculation formula is, P cm =C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B, where the symbol of each element in the formula represents its weight percentage in the steel plate.

2. The low carbon equivalent ferrite pipeline steel according to claim 1, characterized in that: The microstructure of the hot rolled steel strip is ferrite or ferrite + a small amount of bainite.

3. The low carbon equivalent ferrite pipeline steel according to claim 2, characterized in that: The transverse yield strength of the finished hot-rolled steel strip is ≥360MPa, and the tensile strength is ≥420MPa; the transverse full-size impact energy at -20℃ is ≥250J, and the ductile-brittle transition temperature is lower than -60℃; the drop hammer tearing area ratio at -10℃ is ≥85%.

4. A construction method for preparing the low carbon equivalent ferrite pipeline steel according to claim 1, 2 or 3, characterized in that: The construction method includes the steps of molten steel smelting, calcium treatment, slab preparation, slab reheating, slab rolling, cooling and coiling. When preparing the slab, qualified molten steel is directly cast on the continuous casting machine into slabs that meet the requirements. After the continuous cast slab is slowly cooled for more than 48 hours, it is sent to the heating furnace and heated in the furnace for 150-230 minutes. After it is taken out of the furnace, it is rough rolled, finish rolled, and coiled after laminar cooling to obtain low carbon equivalent ferritic pipeline steel.

5. The construction method according to claim 4, characterized in that: Molten steel smelting includes converter smelting, refining outside the furnace and vacuum smelting. During converter smelting, argon and oxygen are blown throughout the process until C≤0.040%, then alloy Si, Mn and Nb are added, and the exit temperature is>1615℃; During refining outside the furnace, the content of elements C, Si, Mn and Als alloy is fine-tuned, and the outlet temperature is >1600℃; The processing time of vacuum smelting is ≥20min, the cycle time under vacuum degree <3mbar is ≥10min, and the cycle time after alloying is ≥6min.

6. The construction method according to claim 4, characterized in that: During calcium treatment, 100-300m of Ca-Al wire is fed, and the circulation time after feeding is ≥8min.

7. The construction method according to claim 6, characterized in that: The pulling speed during continuous casting is 0.9-1.5m / min, the pulling speed change within 10s is ≤0.05m / min, 5mm light pressure is adopted, the continuous casting overheat is 15-20℃, and the thickness of the continuous casting slab is 200-230mm.

8. The construction method according to claim 7, characterized in that: The furnace temperature of the reheated slab is 1180-1200℃. It is descaled under high pressure before rough rolling. The number of rough rolling passes is 5, and the deformation of a single pass is ≥20%. The thickness of the intermediate slab is 35-48mm.

9. The construction method according to claim 8, characterized in that: The intermediate billet after rough rolling is finished by 7-stand continuous rolling. The inter-stand cooling water of F1-F3 stands is turned on to cool the strip. The cumulative deformation of F1-F3 passes is ≥60%, and the final rolling temperature range is 800-840℃.

10. The construction method according to claim 9, characterized in that: The front section of laminar cooling adopts sparse cooling method, and the strip is cooled to 560-620℃ at a cooling rate of 5-15℃ / s and then coiled.

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