CN115287428B - Method for regulating and controlling X70 grade pipeline steel dual-phase structure and increasing low-temperature toughness - Google Patents
Method for regulating and controlling X70 grade pipeline steel dual-phase structure and increasing low-temperature toughness Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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- C—CHEMISTRY; METALLURGY
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0231—Warm rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Abstract
The invention relates to a method for regulating and controlling a dual-phase structure of X70 grade pipeline steel and increasing low-temperature toughness. The method comprises steelmaking, continuous casting, controlled rolling and controlled cooling, wherein the thickness of the obtained X70 grade pipeline steel is not more than 35mm, and the steel contains ferrite and granular bainite, wherein the ferrite accounts for 40-60%, and the granular bainite accounts for 40-60%. The invention prepares the pipeline steel containing ferrite and granular bainite dual-phase structure by adopting a three-stage controlled rolling and three-stage controlled cooling method based on low-carbon low-alloying components, has low yield ratio, good low-temperature impact toughness and low-temperature crack arrest performance, good low-temperature impact toughness stability and smaller impact toughness reduction range under the condition of greatly reducing the environmental temperature. The pipeline steel produced by the invention has excellent comprehensive mechanical properties, and can improve the reliability and safety of the oil gas transmission pipeline in the region with extremely cold and large environmental temperature variation range.
Description
Technical Field
The invention relates to the field of pipeline steel production, and particularly provides a method for regulating and controlling a dual-phase structure of an X70 grade pipeline steel plate and increasing low-temperature toughness.
Background
At present, fossil energy sources such as petroleum, natural gas and the like still occupy the main position in the world energy structure, with the development of economy and society, the oil and gas demand of human beings is increasing, but the oil and gas resource distribution between countries and regions is extremely unbalanced, and the oil and gas resource needs to be transported from mining places to demand places through long-distance conveying pipelines. The oil gas resources and the requirements are reversely distributed, so that a large amount of imported oil gas is required for ensuring the energy safety, or the oil gas resources of the geographical and climate-bad remote areas are developed, and the construction strength of the long-distance oil gas transmission pipeline is required to be enhanced. In order to improve the conveying capacity of the oil and gas long-distance conveying pipeline, measures of improving conveying pressure and expanding pipe diameter are generally adopted, so that high requirements are put on the wall thickness, strength and welding performance of the steel pipe. Increasing the C content is the most direct and effective way to increase the strength of the steel, but the increase in carbon equivalent has a very detrimental effect on the welding performance. For long-distance oil and gas conveying pipelines, geological and climatic conditions along the way are complex, and some oil and gas conveying pipelines, such as medium Russian pipeline oil and gas conveying pipelines, must pass through extremely cold climatic regions such as ice and snow lands, permafrost, forest regions of little Khingan and the like, and in order to ensure the safety of pipeline conveying, high requirements are put on the low-temperature toughness, low-temperature crack arrest performance and yield ratio of pipeline steel. With the rapid change of global climate, the sudden drop of the temperature in local areas occurs, and the possibility of further extreme weather in extremely cold areas of pipeline paths is improved, so that the low-temperature impact toughness is required to be reduced to a smaller extent when the temperature of pipeline steel is greatly reduced, namely the low-temperature impact toughness is good in stability, so that the safe operation of the pipeline is ensured.
In the prior art, patent CN106319390A discloses an X70 grade large deformation resistant pipeline steel and a manufacturing method thereof, wherein a two-phase zone quenching and tempering process is added after a controlled rolling and controlled cooling process to obtain a soft and hard two-phase structure with proper proportion, so that good matching of performances such as strength, low-temperature toughness, yield ratio and the like is realized. However, this method has a heat treatment step added to obtain a two-phase structure in a proper ratio, and is disadvantageous in terms of cost and economical efficiency.
Patent CN103658170a discloses a rolling method of X80 pipeline steel, which obtains a mixed structure of granular bainite, M/a components, a small amount of ferrite and a small amount of pearlite by three-stage rolling and laminar cooling. However, this method does not show the performance index range of the produced pipeline steel, does not list specific examples and comparative examples, and increases the production cost by adding a large amount of alloying elements such as Mo, cu, cr, etc. from the viewpoint of composition design.
Patent CN110205553B discloses a production method of thick-specification X70 grade pipeline steel with excellent low-temperature DWTT performance, which adopts a transverse-longitudinal rough rolling and finish rolling by adopting a controlled rolling process, adopts a single cooling speed by adopting a controlled cooling process, and the produced pipeline steel has high strength and good low-temperature crack arrest performance. However, since a proper tissue control process is not adopted, a proper proportion of soft and hard two-phase tissue is not obtained, resulting in a higher yield ratio.
Meanwhile, the research also shows that the related records of how to obtain X70 grade pipeline steel with high low-temperature impact toughness, small fluctuation of the low-temperature impact toughness along with temperature change and excellent welding performance and other mechanical properties are not related in the prior art.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a method for regulating and controlling the dual-phase structure of X70 grade pipeline steel and increasing the low-temperature toughness. The invention provides considerable strength margin while guaranteeing welding performance based on a low-carbon low-alloying component design scheme; the three-stage controlled rolling and the three-stage controlled cooling process are adopted, the ferrite and granular bainite two-phase structure is regulated and controlled, the yield ratio is reduced, grain refinement is promoted, M/A components with proper size are obtained, the pipeline steel has good strength, low-temperature toughness, low-temperature crack arrest performance and low-temperature impact toughness stability, the impact toughness reduction amplitude is smaller under the condition that the environmental temperature is greatly reduced, and the adaptation of an oil gas conveying pipeline to a low-temperature environment is facilitated.
The invention relates to a method for regulating and controlling the dual-phase structure of X70 grade pipeline steel and increasing low-temperature toughness, wherein the X70 grade pipeline steel obtained by the method contains ferrite and granular bainite; the ferrite accounts for 40 to 60 percent, preferably 45 to 55 percent, and more preferably 45 to 46 percent; the proportion of the granular bainite is 40-60%, preferably 45-55%, and more preferably 54-55%; the C content in the obtained X70 grade pipeline steel is 0.04 to 0.08 weight percent;
the method comprises the following steps:
raw material proportioning, primary smelting and refining are carried out according to chemical components of X70 grade pipeline steel, rapid stokehold component analysis and component adjustment are carried out, and then continuous casting and rolling control and cooling control (TMCP) are carried out.
The rolling control and cooling control (TMCP) comprises three-stage controlled rolling and three-stage controlled cooling, and the three-stage controlled cooling is performed after the three-stage controlled rolling is completed.
The three-stage controlled rolling comprises rough rolling, intermediate rolling and finish rolling.
The rough rolling is carried out at the initial rolling temperature of 1100-1200 ℃, the final rolling temperature of 980-1000 ℃, the effective rolling pass of 6-8 passes and the pass reduction rate of 10-18%. The rough rolling of the present invention is high temperature rolling performed in an austenite recrystallization region, which is advantageous in promoting recrystallization of the austenitic structure of the line steel.
The initial rolling temperature is 870-890 ℃, the final rolling temperature is 790-810 ℃, the effective rolling pass is 7-9 passes, the pass reduction rate is 6-15%, the rolling speed is 4-6 m/s, and the pass interval time is 9-13 s. The pipeline steel is slowly cooled in the deformation process, ferrite is gradually precipitated in austenite to form a ferrite/austenite dual-phase structure, and the process is favorable for the precipitation of ferrite, the refinement of crystal grains and the deformation along the rolling direction.
The finish rolling is carried out at the initial rolling temperature of 780-800 ℃, the final rolling temperature of 740-750 ℃, the number of rolling passes of 2-3 passes, the pass reduction rate of 5-10%, the rolling speed of 2-4 m/s and the inter-pass interval time of 20-40 s. Preferably, the number of passes of the finish rolling is 2, the rolling speed is 2-3 m/s, and the pass interval time is 20-30 s. The finish rolling of the invention adopts slow speed, few times, small pass reduction and long time interval rolling, so that the pipeline steel is effectively recovered, more substructures are generated, the formation of large-angle grain boundaries is promoted, the grain refinement is promoted, and the improvement of the toughness of the pipeline steel is facilitated. Meanwhile, the finish rolling process is carried out at a proper temperature, coarsening of the M/A component can be controlled to a certain extent, and then other parameters are matched, so that the proper M/A component size and morphology can be obtained, the product still keeps a larger obstruction to crack propagation in extremely cold environment, and the stability of impact toughness of the pipeline steel at low temperature fluctuation, namely the low temperature impact toughness is not great along with the temperature change, is facilitated.
The three-stage controlled cooling comprises controlled cooling of three stages of a front stage, a middle stage and a rear stage, the cooling temperature is 720-740 ℃, and the final cooling temperature is 450-520 ℃; the average cooling rate of the three stages is 12-16 ℃ per second, preferably 14 ℃ per second.
The three-stage control cooling is water cooling, and the cooling water quantity ratio of the three water cooling areas of the front section, the middle section and the rear section is 1.2:1:1.2-1.3:1:1.3, preferably 1.2:1:1.2; the cooling speed is fast, slow and fast, the one-stage fast cooling is favorable for inhibiting pearlite generation, promoting bainite phase transformation, the two-stage slow cooling is favorable for relieving residual stress of the plate in the cooling process, the three-stage fast cooling is favorable for inhibiting grain growth, fine grains are finally obtained, and homogenization of the structure of the steel plate in the thickness direction is promoted.
And after the three-stage controlled cooling is finished, air cooling to room temperature.
Preferably, the method for regulating and controlling the dual-phase structure of the X70 grade pipeline steel and increasing the low-temperature toughness comprises the steps of obtaining a steel billet through continuous casting, preheating the steel billet, and performing controlled rolling and controlled cooling to obtain the X70 grade pipeline steel with ferrite and bainite; the preheating treatment process is that the furnace chamber temperature of the heating furnace is not more than 1200 ℃, the soaking temperature is around 1160 ℃, and the soaking time is 1-1.5 h. The setting of the hearth temperature and the soaking temperature mainly considers the full solid solution and homogenization of alloy elements, increases the stability of an austenite structure and obtains finer and uniform austenitizing initial grains.
Preferably, the invention relates to a method for regulating and controlling the dual-phase structure and increasing the low-temperature toughness of X70 grade pipeline steel, which comprises the following chemical components in percentage by mass:
C：0.04～0.08％，
Si：0.15～0.25％，
Mn：1.50～1.80％，
Cr：0.15～0.25％，
Mo：0.10～0.20％，
Nb：0.04～0.06％，
Ti：0.01～0.03％，
Al：0.02～0.04％，
Cu：0.10～0.20％，
P：≤0.015％，
S：≤0.004％，
ceq:0.35 to 0.48 percent, and the balance of Fe and other unavoidable impurities.
Under the process conditions defined by the invention, the content of the C element is controlled within the range of 0.04-0.08 wt%, specifically, 0.04wt%,0.05wt%,0.06wt%,0.07wt% and 0.08wt%; after optimization, the ratio can be 0.05 to 0.07 percent; c is the most main element for improving strength at low cost, the strength of steel is in direct proportion to the carbon content, the welding performance and the plasticity and toughness are in inverse proportion to the carbon content, and when the C content exceeds 0.04wt%, the strength of a bainite matrix in the steel is improved, precipitation of a NbC second phase and grain refinement are facilitated on the one hand, and the decarburization capacity of a converter is improved on the other hand, but when the C content is higher than 0.08wt%, the plasticity and toughness of the steel are reduced, and the welding performance of the steel is influenced. As a further preferred aspect, the C content in the X70 grade pipeline steel obtained by the present invention is controlled to 0.6wt%.
Under the process conditions defined by the invention, the content of Si element is controlled within the range of 0.15-0.25 wt%, such as 0.15wt%,0.16wt%,0.17wt%,0.18wt%,0.19wt%,0.20wt%,0.21wt%,0.22wt%,0.23wt%,0.24wt% and 0.25wt%; when the Si content is 0.15wt% or more, it is advantageous to improve the hardenability and strength of the steel, but when the Si content is more than 0.25wt%, it is easy to cause cold embrittlement, and the welding quality of the steel is seriously affected.
Under the process conditions defined by the invention, the content of Mn element is controlled within the range of 1.50-1.80 wt%, specifically, for example, 1.50wt%,1.60wt%,1.70wt% and 1.80wt%; when the Mn content is 1.50wt% or more, it is useful for deoxidization and sulfur removal in the smelting process, producing solid solution strengthening and refining grains, and is useful for improving the strength and toughness of steel sheets, but when the Mn content is more than 1.80wt%, center segregation is easily produced, thereby producing defects such as banding and microcracking, and affecting the impact toughness and the internal quality of steel.
Under the process conditions defined by the invention, the content of Mo element is controlled within the range of 0.10-0.20 wt%, such as 0.10wt%,0.11wt%,0.12wt%,0.13wt%,0.14wt%,0.15wt%,0.16wt%,0.17wt%,0.18wt%,0.19wt%,0.20wt%, and preferably 0.12-0.18 wt%; when the content of Mo is more than or equal to 0.10wt%, the austenite stability and the hardenability of the steel are improved, the structure uniformity of the steel plate in the thickness direction in the cooling process is improved, so that the strength and the toughness are improved, but when the content of Mo is more than 0.20wt%, the contribution to the strength improvement is small, the welding performance of the steel is not facilitated, and the economy is not good; as a further preferred aspect, the Mo content in the X70 grade pipeline steel obtained by the present invention is controlled to 0.15wt%.
Under the process conditions defined by the invention, the content of Cr element is controlled within the range of 0.15-0.25 wt%, specifically, for example, 0.15wt%,0.16wt%,0.17wt%,0.18wt%,0.19wt%,0.20wt%,0.21wt%,0.22wt%,0.23wt%,0.24wt%,0.25wt%, and preferably 0.18-0.22 wt%; when the content of Cr is more than or equal to 0.15wt%, the transformation point of the steel is favorably reduced, fine grain strengthening is generated, the hardenability of the steel is improved, but when the content of Cr is more than 0.25wt%, coarse carbide is easy to be separated out on a grain boundary, and the toughness and the welding performance of the steel are obviously reduced; as a further preferred aspect, the Cr content in the X70 grade pipeline steel of the present invention is controlled to 0.20wt%.
Under the process conditions defined by the invention, the content of Cu element is in the range of 0.10-0.20 wt%, specifically, for example, 0.10wt%,0.11wt%,0.12wt%,0.13wt%,0.14wt%,0.15wt%,0.16wt%,0.17wt%,0.18wt%,0.19wt% and 0.20wt%; when the Cu content is more than 0.10wt%, precipitation strengthening occurs, and further, improvement of the corrosion resistance, weldability, low-temperature toughness and the like of the steel is very beneficial, but when the Cu content is more than 0.20wt%, thermal embrittlement is easily caused.
Under the technological conditions defined by the invention, the content of Nb is within the range of 0.04-0.06 wt%, the invention has obvious precipitation strengthening effect and grain refining effect, is beneficial to tissue dispersion and homogenization, and improves the strength and toughness of steel; as a further preferred, the Nb content in the X70 grade pipeline steel obtained by the present invention is controlled to 0.05wt%.
Under the process conditions defined by the invention, the content of Ti element is controlled within the range of 0.01-0.03 wt%, so that the invention can generate the functions of precipitation strengthening and grain refinement, and can improve the distribution form of sulfide, thereby being beneficial to improving the strength and toughness of steel; as a further preferred aspect, the Ti content in the X70 grade pipeline steel of the present invention is controlled to 0.02wt%.
Under the technological conditions defined by the invention, the content of Al element is controlled within the range of 0.02-0.04 wt%, and Al and O, N have extremely strong affinity, can play a positive deoxidizing role, and form AlN with N, so that not only can grains be thinned, but also N can be effectively solidified.
In the invention, the content of S is less than 0.015wt percent, the content of P is less than 0.004wt percent, and high toughness is obtained by strictly controlling S, P and other impurity elements, so that the hot cracking and cold cracking tendencies are reduced.
Preferably, the invention relates to a method for regulating and controlling the dual-phase structure of X70 grade pipeline steel and increasing the low-temperature toughness, wherein the grain size of granular bainite is 2-6 mu m; the grain size of the ferrite is 3-12 mu m; the granular bainite is distributed with M/A components, and the size is 0.2-0.7 mu M.
The fine grain size, the proper M/A island size and the proper M/A island morphology are favorable for blocking dislocation movement and crack propagation under the low-temperature condition, the strength, the low-temperature impact toughness and the low-temperature crack arrest performance of the pipeline steel are improved, meanwhile, as the environmental temperature is further reduced, the grain boundary, the proper M/A island and the proper M/A island have better blocking effect on crack propagation, and the pipeline steel has good low-temperature impact toughness stability when the temperature is further reduced. When the grain size is too large and the M/A island size is too small, dislocation movement and crack propagation are difficult to block, the pipeline steel strength, low-temperature impact toughness and low-temperature crack stopping performance are reduced, and the low-temperature impact toughness stability is also reduced; when the M/A island size is too large, the interface between the M/A island and the matrix structure is likely to become a crack nucleation site, and the low-temperature impact toughness is also reduced.
Preferably, the X70 grade pipeline steel dual-phase structure regulation and control method for increasing low-temperature toughness is adopted, and the obtained pipeline steel is plate-shaped and has the thickness of not more than 35mm. The specific thickness is adjusted according to the actual requirements when the method is applied to industry and engineering.
The invention relates to a method for regulating and controlling a dual-phase structure of X70 grade pipeline steel and increasing low-temperature toughness, wherein the yield strength of the obtained pipeline steel is 480-525 MPa, the tensile strength is 650-670 MPa, the yield ratio is 0.74-0.79, the impact absorption power at minus 20 ℃ is 285-325J, the impact absorption power at minus 45 ℃ is 260-310J, the impact absorption power at minus 60 ℃ is 245-290J, and when the material composition and the preparation process are the same, the fluctuation of the impact absorption power at minus 60 ℃ and the impact absorption power at minus 20 ℃ of a product is not more than 40J; the shearing area of the DWTT low-temperature drop hammer test at the temperature of minus 20 ℃ is not less than 90 percent.
In the invention, after ensuring that the strength of the product is within the set range of the invention, controlling the fluctuation of the impact absorption power of the product at minus 60 ℃ and the impact absorption power of the product at minus 20 ℃ to be less than or equal to 40J, and ensuring that the impact absorption power of the product at minus 60 ℃ is greater than or equal to 245MPa, and can be greater than or equal to 268MPa after optimization; the method is favorable for ensuring that the impact toughness of the pipeline steel in service in extremely cold environments is not greatly reduced under the condition that global climate is possibly changed and extremely changed, and further ensuring the reliability and safety of pipeline operation.
As a preferable scheme, the yield strength of the obtained pipeline steel is more than or equal to 500MPa, the tensile strength is more than or equal to 645MPa, the yield ratio is 0.75-0.79, the impact absorption power at minus 20 ℃ is more than or equal to 300J, the impact absorption power at minus 45 ℃ is more than or equal to 270J, the impact absorption power at minus 60 ℃ is more than or equal to 263J, and the fluctuation of the impact absorption power at minus 60 ℃ and the impact absorption power at minus 20 ℃ of the product is less than or equal to 38J;
the shearing area of the DWTT low-temperature drop hammer test at the temperature of minus 20 ℃ is not less than 91 percent.
As a further preferable scheme, the fluctuation of the impact absorption power of the obtained pipeline steel at the temperature of 60 ℃ below zero and the impact absorption power of the pipeline steel at the temperature of 20 ℃ below zero is less than or equal to 32J; and the impact absorption power is greater than or equal to 268J at the temperature of minus 60 ℃.
As one of the best schemes, the yield strength of the obtained pipeline steel is 510MPa, the tensile strength is 663MPa, the yield ratio is 0.77, the impact absorption power of the obtained pipeline steel at-20 ℃ is 301J, the impact absorption power at-45 ℃ is 284J, the impact absorption power at-60 ℃ is 270J,
the shearing area of the DWTT low-temperature drop hammer test at the temperature of minus 20 ℃ is not less than 91 percent.
The technical scheme of the invention has the following beneficial effects:
1. according to the invention, by designing low-carbon low-alloying components and matching with a proper controlled rolling and cooling process, ferrite and granular bainite two-phase structures with proper proportion are prepared, the best matching of high strength and toughness and low yield ratio is realized, the yield strength is 480-525 MPa, the tensile strength is 640-670 MPa, the yield ratio is 0.75-0.79, the impact absorption power at-20 ℃ is 285-325J, the impact absorption power at-45 ℃ is 260-310J, the impact absorption power at-60 ℃ is 245-290J, and the shearing area of a DWTT low-temperature drop hammer experiment at-20 ℃ is not less than 90%.
2. The pipeline steel prepared by the invention has fine grain structure, moderate M/A component size and reasonable morphology, has smaller low-temperature impact toughness reduction range under the condition of greatly reducing the environmental temperature, has better low-temperature impact toughness stability, and further ensures the reliability and the safety of the pipeline in extreme environments when the fluctuation of the impact absorption power of-60 ℃ and the impact absorption power of-20 ℃ is not more than 40J when the material composition and the preparation process are the same.
Drawings
FIG. 1 is a schematic diagram of a rolling and cooling control (TMCP) process flow and related parameters according to the invention;
FIG. 2 is an OM photograph of a pipeline steel obtained in example 7 of the present invention;
FIG. 3 is an OM photograph of pipeline steel obtained in example 8 of the present invention;
FIG. 4 is an OM photograph of pipeline steel obtained in example 9 of the present invention;
FIG. 5 is an OM photograph of the pipeline steel obtained in comparative example 8 of the present invention;
FIG. 6 is an OM photograph of the pipeline steel obtained in comparative example 9 of the present invention.
Detailed Description
In order to further clarify the technical problems, technical solutions and technical effects to be solved by the present invention, a detailed description will be given below with reference to the drawings and examples, which are only for aiding in understanding the present invention and should not be construed as limiting the present invention in any way.
Aiming at the existing problems, the invention provides a method for regulating and controlling the dual-phase structure of X70 grade pipeline steel and increasing the low-temperature toughness, which adopts a low-carbon low-alloy composition design, and obtains the preparation method of X70 grade pipeline steel with large wall thickness through an optimized rolling and cooling control (TMCP) process as shown in figure 1. Wherein the furnace hearth temperature of the heating furnace in the preheating treatment process is not more than 1200 ℃, the soaking temperature is close to 1160 ℃, and the soaking time is 1.5h; the initial rolling temperature of rough rolling is 1160 ℃, the final rolling temperature is 990 ℃, the rolling passes are 7 passes, the average pass reduction rate is 15.5%, and the average pass interval time is 20s; the initial rolling temperature of the intermediate rolling is 880 ℃, the final rolling temperature is 800 ℃, the rolling passes are 8, the average pass reduction is 12.4%, the rolling speed is about 5m/s, and the average pass interval time is 10s; the initial rolling temperature of the finish rolling is 790 ℃, the final rolling temperature is 750 ℃, the rolling passes are 2-4 passes (see table 2 in detail), the rolling speed is about 3m/s, and the inter-pass interval time is 20-50 s (see table 2 in detail); the cooling temperature of the sectional cooling is 710-750 ℃ (see in particular table 2), and the final cooling temperature is 450-520 ℃; the sectional cooling adopts water cooling; wherein, when the front section, the middle section and the rear section are water-cooled, the water quantity ratio of the water-cooling area is 1:1:1-1.4:1:1.4, and the average cooling speed of the water-cooling area is 12-18 ℃/s (see in particular table 2); and then air-cooled to room temperature.
Table 1 lists the mass percentages of the main alloying elements of the examples and comparative examples of this technical scheme. The elements not listed in table 1 and the corresponding contents are as follows: wherein Si is 0.20wt%, mn is 1.70wt%, al is 0.03wt%, cu is 0.15wt%, and the balance is Fe and unavoidable impurities, wherein P is less than 0.01wt%, S is less than 0.003wt%; these elements and amounts are consistent in the comparative examples and examples of the present invention.
Table 1 mass percent of the major alloying elements of the examples and comparative examples
| Alloy element table | C | Cr | Mo | Nb | Ti |
| Example 1 | 0.05 | 0.20 | 0.15 | 0.05 | 0.02 |
| Example 2 | 0.07 | 0.20 | 0.15 | 0.05 | 0.02 |
| Example 3 | 0.06 | 0.18 | 0.12 | 0.05 | 0.02 |
| Example 4 | 0.06 | 0.22 | 0.18 | 0.05 | 0.02 |
| Example 5 | 0.06 | 0.20 | 0.15 | 0.04 | 0.01 |
| Example 6 | 0.06 | 0.20 | 0.15 | 0.06 | 0.03 |
| Example 7 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Example 8 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Example 9 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Example 10 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Example 11 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Example 12 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Example 13 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Example 14 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Comparative example 1 | 0.02 | 0.20 | 0.15 | 0.05 | 0.02 |
| Comparative example 2 | 0.10 | 0.20 | 0.15 | 0.05 | 0.02 |
| Comparative example 3 | 0.06 | 0.10 | 0.05 | 0.05 | 0.02 |
| Comparative example 4 | 0.06 | 0.30 | 0.25 | 0.05 | 0.02 |
| Comparative example 5 | 0.06 | 0.20 | 0.15 | 0.02 | - |
| Comparative example 6 | 0.06 | 0.20 | 0.15 | 0.08 | 0.04 |
| Comparative example 7 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Comparative example 8 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Comparative example 9 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Comparative example 10 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Comparative example 11 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Comparative example 12 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Comparative example 13 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
| Comparative example 14 | 0.06 | 0.20 | 0.15 | 0.05 | 0.02 |
。
Proportioning according to the determined chemical composition ratio, melting, dephosphorizing, refining outside the furnace, stirring by argon, casting into billets, preheating, and performing three-stage controlled rolling and three-stage controlled cooling. Table 2 shows the finish rolling and controlled cooling schemes of the examples and comparative examples.
TABLE 2 finish rolling and controlled cooling schemes for examples and comparative examples
| Finish rolling and cooling control scheme | Number of tracks | Time/s between passes | Cooling temperature/°c | Three stage water ratio | Average cooling rate/. Degree.C/s |
| Example 1 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Example 2 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Example 3 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Example 4 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Example 5 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Example 6 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Example 7 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Example 8 | 3 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Example 9 | 2 | 30 | 730 | 1.2∶1∶1.2 | 14 |
| Example 10 | 2 | 20 | 720 | 1.2∶1∶1.2 | 14 |
| Example 11 | 2 | 20 | 740 | 1.2∶1∶1.2 | 14 |
| Example 12 | 2 | 20 | 730 | 1.3∶1∶1.3 | 14 |
| Example 13 | 2 | 20 | 730 | 1.2∶1∶1.2 | 13 |
| Example 14 | 2 | 20 | 730 | 1.2∶1∶1.2 | 15 |
| Comparative example 1 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Comparative example 2 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Comparative example 3 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Comparative example 4 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Comparative example 5 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Comparative example 6 | 2 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Comparative example 7 | 4 | 20 | 730 | 1.2∶1∶1.2 | 14 |
| Comparative example 8 | 2 | 50 | 730 | 1.2∶1∶1.2 | 14 |
| Comparative example 9 | 2 | 20 | 710 | 1.2∶1∶1.2 | 14 |
| Comparative example 10 | 2 | 20 | 750 | 1.2∶1∶1.2 | 14 |
| Comparative example 11 | 2 | 20 | 730 | Single cooling speed | 14 |
| Comparative example 12 | 2 | 20 | 730 | 1.4∶1∶1.4 | 14 |
| Comparative example 13 | 2 | 20 | 730 | 1.2∶1∶1.2 | 10 |
| Comparative example 14 | 2 | 20 | 730 | 1.2∶1∶1.2 | 18 |
The steel plate was tested for yield strength, tensile strength according to standard GB/T228.1-2010, core impact absorption work at-20℃according to GB/T229-2020, full thickness DWTT drop hammer performance at-20℃according to GB/T8363-2018, and ferrite duty ratio was calibrated by Image-Pro Plus software, and the results are shown in Table 3.
TABLE 3 mechanical Properties index and ferrite ratio of examples and comparative examples
As can be seen from Table 3, the yield strength of the pipeline steel of the embodiment of the invention is 480-525 MPa, the tensile strength is 640-670 MPa, the yield ratio is 0.75-0.79, the impact absorption power at minus 20 ℃ is 285-325J, the impact absorption power at minus 45 ℃ is 260-310J, the impact absorption power at minus 60 ℃ is 245-290J, and when the material composition and the preparation process are the same, the fluctuation of the impact absorption power at minus 60 ℃ and the impact absorption power at minus 20 ℃ of the product is not more than 40J; the shearing area of the DWTT low-temperature drop hammer test at the temperature of minus 20 ℃ is not less than 90 percent. The ferrite accounts for 40-60%, the granular bainite accounts for 40-60%, the grain size of the ferrite is 3-15 μm, the grain size of the granular bainite is 2-6 μm, and the M/A component size is 0.2-0.7 μm.
As is clear from examples 1, 2, 1 and 2, the carbon content is increased, the strength of the pipeline steel is improved, the toughness is reduced, and examples 3, 4, 3 and 4 show that the Cr and Mo content is increased, the strength of the pipeline steel is improved, the toughness is reduced, and the comprehensive mechanical properties of the pipeline steel are excellent when the Cr and Mo content is within the protection range of the present application, but when the Cr and Mo content is beyond the protection range of the present application, the strength and toughness of the pipeline steel are difficult to be compatible. The comparative example 5 has too low Nb and Ti contents, the pipeline steel has poor strength, the comparative example 6 has too high Nb and Ti contents, and the pipeline steel has reduced toughness.
When the finish rolling parameters are within the protection scope of the present application, as can be seen from example 7, as shown in fig. 2, the grains in the pipeline steel are finer, the grain boundaries are clear, and the ferrite mainly has quasi-polygonal characteristics; the M/A components are slightly coarsened and distributed uniformly. The fine grain size and the properly coarsened M/A component are beneficial to blocking dislocation movement and crack propagation, so that the pipeline steel keeps better toughness while the strength is improved, the two-phase structure with proper proportion is beneficial to the reduction of the yield ratio, and meanwhile, in an extremely low-temperature environment, the large-angle grain boundary and the properly coarsened M/A component have an inhibition effect on crack propagation, so that the pipeline steel prepared by the invention has excellent low-temperature impact toughness stability.
When the finish rolling pass number is out of the range of the present application, as can be seen from comparative example 7, as compared with example 7, both the crystal grains and the M/a components in the pipeline steel are coarsened greatly, and the ferrite content is increased, as can be seen from fig. 3. The M/A component which is excessively coarsened is easy to become a crack source, and promotes the formation of microcracks in the pipeline steel structure; the coarsened crystal grains lead to a reduction in the number of grain boundaries, reduce the impeding effect on crack growth, and especially reduce the impeding effect on crack growth in an extremely low temperature environment, thereby damaging the low temperature impact toughness, low temperature crack arrest performance and low temperature impact toughness stability of the pipeline steel.
When the finish pass interval time exceeds the protection range of the present application, as can be seen from comparative example 8, as compared with example 7, the iron element content in the pipeline steel is increased, the crystal grains are coarsened, and the M/A components are excessively coarsened and unevenly distributed. The increase in ferrite content and coarsening of grains lead to the decrease in strength of the pipeline steel, the excessive coarsening and segregation of the M/A components increase and aggregate micro-crack nucleation sites in the pipeline steel, the low-temperature impact toughness of the pipeline steel is reduced, the excessive coarsening and coarsening of the M/A components lead to easier nucleation and expansion of cracks in an extremely low-temperature environment, and the low-temperature crack-stopping performance and the low-temperature impact toughness stability are reduced.
When the cooling temperature is lower than the protection range of the application, as can be seen from comparative example 9, as shown in fig. 5, the ferrite content is greatly increased, the M/A components are coarse, a certain aggregation phenomenon is generated, the strength of the pipeline steel is lower, and meanwhile, the toughness is poor; when the cooling temperature exceeds the protection range of the present application, as can be seen from comparative example 10, the ferrite content is greatly reduced to about 30%, resulting in high yield ratio of the pipeline steel and poor toughness, which are all difficult to meet the requirements.
From comparative examples 11, 7, 12 and 12, it is understood that as the water ratio in the three stages is increased, the strength of the pipeline steel is increased, the low-temperature impact toughness and the low-temperature crack-stopping performance are lowered, and the yield ratio is improved. When the three-stage water ratio is higher than the protection range of the application, such as comparative example 12, the low-temperature impact toughness and the low-temperature crack arrest performance are poorer, and the yield ratio is higher; when the pipeline steel is cooled at a single cooling rate, as in comparative example 11, the strength is lower without improvement in toughness, and the comprehensive mechanical properties are lowered.
From comparative examples 13, 7, 14 and 14, it is understood that as the average cooling rate increases, the strength of the pipeline steel increases, the low-temperature impact toughness and the low-temperature crack-stopping performance decreases, and the yield ratio increases. When the average cooling rate is below the range protected herein, as in comparative example 13, the pipeline steel strength is too low; when the average cooling rate is higher than the range protected in this application, as in comparative example 14, the toughness of the pipeline steel is too poor, and it is difficult to satisfy the performance requirements.
In conclusion, the components which are not in the range of the invention, or the controlled rolling process and the controlled cooling process which are not provided by the invention, have lower mechanical property indexes, the defects generated in the preparation process are difficult to eliminate, and the comprehensive mechanical property is poor.
While the foregoing is directed to the preferred embodiments of the present invention, it will be appreciated by those skilled in the art that various modifications and adaptations can be made without departing from the principles of the present invention, and such modifications and adaptations are intended to be comprehended within the scope of the present invention.
Claims (9)
1. A method for regulating and controlling the dual-phase structure of X70 grade pipeline steel and increasing the low-temperature toughness is characterized in that: the obtained X70 grade pipeline steel contains ferrite and granular bainite, wherein the ferrite accounts for 40-60%, and the granular bainite accounts for 40-60%; the C content in the obtained X70 grade pipeline steel is 0.04 to 0.08 weight percent;
the method comprises the following steps:
raw material proportioning, primary refining and refining are carried out according to chemical components of X70 grade pipeline steel, rapid stokehold component analysis and component adjustment are carried out, and then continuous casting, controlled rolling and controlled cooling are carried out;
the rolling control and cooling control comprises three-stage rolling control and three-stage cooling control;
the three-stage controlled rolling comprises rough rolling, intermediate rolling and finish rolling;
the rough rolling is carried out at the initial rolling temperature of 1100-1200 ℃, the final rolling temperature of 980-1000 ℃, the rolling pass of 6-8 passes and the pass reduction rate of 10-18%;
the initial rolling temperature is 870-890 ℃, the final rolling temperature is 790-810 ℃, the rolling passes are 7-9, the pass reduction rate is 6-15%, the rolling speed is 4-6 m/s, and the pass interval time is 9-13 s;
the finish rolling is carried out at the initial rolling temperature of 780-800 ℃, the final rolling temperature of 740-750 ℃, the rolling pass of 2-3 passes, the pass reduction rate of 5-10%, the rolling speed of 2-4 m/s and the pass interval time of 20-40 s;
the three-stage control cooling comprises three-stage control cooling of a front stage, a middle stage and a rear stage;
the controlled cooling is carried out, the opening cooling temperature is 720-740 ℃, and the final cooling temperature is 450-520 ℃; the control cooling adopts water cooling; wherein the method comprises the steps of
The water quantity ratio of the front section, the middle section and the rear section of the three water cooling areas is 1.2:1:1.2-1.3:1:1.3, and the average cooling speed is 12-16 ℃/s;
the pipeline steel comprises the following chemical components in percentage by mass:
C:0.04～0.08％，
Si:0.15～0.25％，
Mn:1.50～1.80％，
Cr:0.15～0.25％，
Mo:0.10～0.20％，
Nb:0.04～0.06％，
Ti:0.01～0.03％，
Al:0.02～0.04％，
Cu:0.10～0.20％，
P:≤0.015％，
S:≤0.004％，
ceq is 0.35-0.48%, and the balance is Fe and other unavoidable impurities.
2. The method for regulating and controlling the dual-phase structure of X70 grade pipeline steel and increasing the low-temperature toughness according to claim 1, wherein the method comprises the following steps of: the rolling pass of the finish rolling is 2, the rolling speed is 2-3 m/s, and the pass interval time is 20-30 s.
3. The method for regulating and controlling the dual-phase structure of X70 grade pipeline steel and increasing the low-temperature toughness according to claim 1, wherein the method comprises the following steps of: the three-stage control cooling is carried out, and the water quantity ratio of the front section, the middle section and the rear section of the three water cooling areas is 1.2:1:1.2.
4. A method for dual phase structure conditioning and low temperature toughness enhancement of an X70 grade pipeline steel according to claim 3, wherein: the average cooling rate of the water cooling zone is 14 ℃/s.
5. The method for regulating and controlling the dual-phase structure of X70 grade pipeline steel and increasing the low-temperature toughness according to claim 1, wherein the method comprises the following steps of: the grain size of the granular bainite is 2-6 mu m; the grain size of the ferrite is 3-12 mu m; the granular bainite is distributed with M/A components, and the size is 0.2-0.7 mu M.
6. The method for regulating and controlling the dual-phase structure of X70 grade pipeline steel and increasing the low-temperature toughness according to claim 1, wherein the method comprises the following steps of: the thickness of the pipeline steel is not more than 35mm.
7. The method for regulating and controlling the dual-phase structure of the X70 grade pipeline steel and increasing the low-temperature toughness according to any one of claims 5 to 6, which is characterized in that: the yield strength of the pipeline steel is 480-525 MPa, the tensile strength is 640-670 MPa, the yield ratio is 0.75-0.79,
the impact absorption power at the temperature of minus 20 ℃ is 285-325J, the impact absorption power at the temperature of minus 45 ℃ is 260-310J, the impact absorption power at the temperature of minus 60 ℃ is 245-290J, and when the material composition and the preparation process are the same, the fluctuation range of the impact absorption power at the temperature of minus 60 ℃ and the impact absorption power at the temperature of minus 20 ℃ is not more than 40J;
the shearing area of the DWTT low-temperature drop hammer test at the temperature of minus 20 ℃ is not less than 90 percent.
8. The method for regulating and controlling the dual-phase structure of the X70 grade pipeline steel and increasing the low-temperature toughness according to claim 7, wherein the method comprises the following steps of: the yield strength of the pipeline steel is more than or equal to 500MPa, the tensile strength is more than or equal to 645MPa, the yield ratio is 0.75-0.79, the impact absorption power at-20 ℃ is more than or equal to 300J, the impact absorption power at-45 ℃ is more than or equal to 270J, the impact absorption power at-60 ℃ is more than or equal to 263J, and the fluctuation of the impact absorption power at-60 ℃ and the impact absorption power at-20 ℃ of the product is less than or equal to 38J;
the shearing area of the DWTT low-temperature drop hammer test at the temperature of minus 20 ℃ is not less than 91 percent.
9. The method for regulating and controlling the dual-phase structure of the X70 grade pipeline steel and increasing the low-temperature toughness according to claim 8, wherein the method comprises the following steps of: the fluctuation of the impact absorption power of the pipeline steel at the temperature of 60 ℃ below zero and the impact absorption power at the temperature of 20 ℃ below zero is less than or equal to 32J; and the impact absorption power is greater than or equal to 268J at the temperature of minus 60 ℃.
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