CN115287428A - Method for regulating and controlling dual-phase structure of X70-grade pipeline steel and increasing low-temperature toughness - Google Patents
Method for regulating and controlling dual-phase structure of X70-grade pipeline steel and increasing low-temperature toughness Download PDFInfo
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- 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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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 the steps of 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 obtained X70-grade pipeline steel contains ferrite and granular bainite, wherein the proportion of the ferrite is 40-60%, and the proportion of the granular bainite is 40-60%. The invention is based on low-carbon low-alloying components, and adopts a three-stage controlled rolling and three-stage controlled cooling method to prepare the pipeline steel containing a ferrite and granular bainite dual-phase structure, which has low yield ratio, good low-temperature impact toughness and low-temperature crack arrest performance, good low-temperature impact toughness stability and small reduction amplitude of the impact toughness under the condition of greatly reducing the environmental temperature. The pipeline steel produced by the method has excellent comprehensive mechanical properties, and can improve the reliability and safety of oil and gas conveying pipelines in extremely cold regions and regions with large environmental temperature change amplitude.
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 such as oil, natural gas and the like still occupies the main position in the world energy structure, and with the development of economic society, the human oil and gas demand is increased, but the oil and gas resource distribution between countries and regions is extremely unbalanced, and the oil and gas resource must be transported from a mining place to a demand place through a long-distance transportation pipeline. China belongs to the countries with oil shortage and gas shortage, oil gas resources and demands are distributed in a reverse direction, a large amount of imported oil gas is needed for guaranteeing energy safety, or the construction strength of long-distance oil gas conveying pipelines must be strengthened for developing the oil gas resources in remote areas with severe geographical and climatic conditions. In order to improve the conveying capacity of the oil and gas long-distance pipeline, two measures of improving conveying pressure and enlarging the pipe diameter are generally adopted, so that higher requirements on the wall thickness, the strength and the welding performance of the steel pipe are provided. Increasing the C content is the most direct and effective way to increase the strength of the steel, but increasing the carbon equivalent has a very adverse effect on the welding performance. For long-distance oil and gas transmission pipelines, geological and climatic conditions along the way are complex, some oil and gas transmission pipelines, such as middle and Russian east oil and gas transmission pipelines, must pass through extremely cold climatic regions such as ice and snow places, permafrost, lesser Khingan forest regions and the like, and in order to ensure the safety of pipeline transmission, high requirements are provided for the low-temperature toughness, the low-temperature crack arrest performance and the yield ratio of pipeline steel. Along with the drastic change of global climate, the sudden temperature drop of local areas occurs at times, the possibility of further extreme climate of extremely cold areas of pipeline approaches is improved, and the requirement is that when the temperature of pipeline steel is greatly reduced, the reduction range of low-temperature impact toughness is small, namely the stability of the low-temperature impact toughness is good, so as to ensure the safe operation of pipelines.
In the prior art, patent CN106319390A discloses an X70-grade pipeline steel with large deformation resistance and a manufacturing method thereof, which obtains a proper proportion of soft-hard two-phase structure by adding two-phase zone quenching and tempering processes after a controlled rolling and cooling process, and realizes good matching of properties such as strength, low-temperature toughness, yield ratio and the like. However, this method has a disadvantage in that the number of heat treatment steps is increased to obtain a two-phase structure at an appropriate ratio, which increases production cost and is economically disadvantageous.
Patent CN103658170A discloses a rolling method of X80 pipeline steel, which through three-stage rolling and laminar cooling, obtains a mixed structure of granular bainite, M/a component, a small amount of ferrite and a small amount of pearlite. However, the method does not show the performance index range of the prepared pipeline steel, does not list specific examples and comparative examples, and increases the production cost by adding more alloying element components such as Mo, cu, cr and the like from the component design.
The patent CN110205553B discloses a production method of thick X70-grade pipeline steel with excellent low-temperature DWTT performance, wherein a controlled rolling process adopts horizontal-longitudinal rough rolling and finish rolling, and a controlled cooling process adopts single cooling speed rapid cooling, so that the prepared pipeline steel is high in strength and good in low-temperature crack arrest performance. However, since a proper tissue control process is not adopted, a soft-hard two-phase tissue with a proper ratio is not obtained, so that the yield ratio is high.
Meanwhile, the search also finds that the prior art does not relate to the relevant records of how to obtain the X70 grade pipeline steel with high low-temperature impact toughness, small fluctuation of the low-temperature impact toughness along with the temperature change and excellent welding performance and other mechanical properties.
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 allowance while ensuring welding performance based on a low-carbon low-alloying component design scheme; the three-stage controlled rolling and three-stage controlled cooling process is adopted to regulate and control ferrite and granular bainite two-phase structures, reduce yield ratio, promote grain refinement and obtain M/A component with proper size, so that the pipeline steel obtains good strength, low-temperature toughness, low-temperature crack arrest performance and low-temperature impact toughness stability, the reduction range of the impact toughness is small under the condition that the environmental temperature is greatly reduced, and the adaptability of an oil-gas conveying pipeline to a low-temperature environment is facilitated.
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 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 to 60%, preferably 45 to 55%, and more preferably 54 to 55%;
the C content in the obtained X70 grade pipeline steel is 0.04 to 0.08wt%;
the method comprises the following steps:
the method comprises the steps of proportioning raw materials, carrying out primary smelting and refining according to chemical components of X70 grade pipeline steel, carrying out rapid stokehole component analysis and component adjustment, and then carrying out continuous casting and controlled rolling and controlled cooling (TMCP).
The controlled rolling and controlled cooling (TMCP) comprises three-stage controlled rolling and three-stage controlled cooling, and the three-stage controlled cooling is carried out after the three-stage controlled rolling is finished.
The three-stage controlled rolling comprises rough rolling, medium rolling and finish rolling.
The rough rolling is carried out at the rolling temperature of 1100-1200 ℃, the final rolling temperature of 980-1000 ℃, the effective pass of rolling is 6-8 passes, and the pass reduction rate is 10-18%. The rough rolling of the present invention is high temperature rolling performed in an austenite recrystallization zone, which is advantageous in promoting austenite recrystallization of the line steel.
In the intermediate rolling, the rolling temperature is 870 to 890 ℃, the final rolling temperature is 790 to 810 ℃, the effective pass of rolling is 7 to 9 passes, the pass reduction rate is 6 to 15 percent, the rolling speed is 4 to 6m/s, and the pass interval time is 9 to 13s. The temperature of the pipeline steel is slowly reduced in the deformation process, ferrite is gradually separated out from austenite, a ferrite/austenite dual-phase structure is formed, and the process is favorable for separation of the ferrite, refinement of grains and deformation along the rolling direction.
The finish rolling is carried out, wherein the rolling temperature is 780-800 ℃, the finishing temperature is 740-750 ℃, the number of rolling passes is 2-3 passes, the pass reduction rate is 5-10%, the rolling speed is 2-4 m/s, and the time interval between passes is 20-40s. Preferably, the number of passes of the finish rolling is 2, the rolling speed is 2 to 3m/s, and the time interval between passes is 20 to 30s. The finish rolling of the invention adopts low speed, few times, small pass secondary pressing amount 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 obdurability 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 degree, and then the proper size and shape of the M/A component can be obtained by matching with other parameters, so that the product still keeps larger obstruction to crack propagation in an extremely cold environment, and the stability of the impact toughness of the pipeline steel during low-temperature fluctuation is facilitated, namely the low-temperature impact toughness does not change greatly along with the temperature.
The three-stage controlled cooling comprises three stages of controlled cooling of a front stage, a middle stage and a rear stage, wherein the open cooling temperature is 720-740 ℃, and the final cooling temperature is 450-520 ℃; the average cooling rate in three stages is 12 to 16 ℃/s, preferably 14 ℃/s.
The three-stage controlled cooling is water cooling, and the water cooling volume ratio of the front-stage water cooling area to the middle-stage water cooling area to the rear-stage water cooling area is (1.2); the cooling speed is high, low and high, the rapid cooling in the first stage is favorable for inhibiting the generation of pearlite and promoting the transformation of bainite, the slow cooling in the second stage is favorable for relieving the residual stress of the plate in the cooling process, and the rapid cooling in the third stage is favorable for inhibiting the growth of crystal grains, finally obtaining fine crystal grains and promoting the homogenization of the structure of the steel plate in the thickness direction.
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 and increasing the low-temperature toughness of the X70-grade pipeline steel comprises the steps of continuously casting to obtain a steel billet, preheating the steel billet, and then performing controlled rolling and controlled cooling to obtain the X70-grade pipeline steel with ferrite and bainite; the preheating treatment process is that the temperature of a hearth of the heating furnace is not more than 1200 ℃, the soaking temperature is around 1160 ℃, and the soaking time is 1 to 1.5 hours. 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 relatively fine and uniform austenitizing initial grains.
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 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%，
0.35 to 0.48 percent of Ceq, and the balance of Fe and other inevitable impurities.
Under the process conditions defined by the invention, the content of the element C is controlled within the range of 0.04 to 0.08wt%, specifically, 0.04wt%,0.05wt%,0.06wt%,0.07wt%,0.08wt%; after optimization, the content can be 0.05 to 0.07 percent; c is the most important element for improving the strength at low cost, the strength of the steel is in direct proportion to the carbon content, the welding performance and the ductility and toughness are in inverse proportion to the carbon content, when the C content exceeds 0.04wt%, the improvement of the bainite matrix strength in the steel, the precipitation and grain refinement of a second phase NbC are facilitated on one hand, and the decarburization capability of a converter is facilitated on the other hand, but when the C content is higher than 0.08wt%, the ductility and toughness of the steel are reduced, and the welding performance of the steel is influenced. As a further preference, the C content in the X70 grade pipeline steel obtained by the invention is controlled to be 0.6wt%.
Under the process conditions defined by the invention, the content of Si element is controlled within the range of 0.15 to 0.25wt%, specifically, 0.15wt%,0.16wt%,0.17wt%,0.18wt%,0.19wt%,0.20wt%,0.21wt%,0.22wt%,0.23wt%,0.24wt%,0.25wt%; when the Si content is 0.15wt% or more, it is advantageous to improve hardenability and strength of steel, but when the Si content is more than 0.25wt%, cold embrittlement is easily caused, and the welding quality of 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 to 1.80wt%, specifically, 1.50wt%,1.60wt%,1.70wt%,1.80wt%; when the content of Mn is more than or equal to 1.50wt%, the steel is beneficial to deoxidation and desulphurization in the smelting process, solid solution strengthening is generated, crystal grains are refined, and the steel plate strength and toughness are beneficial to be improved, but when the content of Mn is more than 1.80wt%, central segregation is easily generated, so that defects such as banded structures and microcracks are generated, and the impact toughness and the internal quality of the steel are influenced.
Under the process conditions defined by the invention, the content of the Mo element is controlled within the range of 0.10 to 0.20wt%, specifically, 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 to 0.18wt%; when the content of Mo is more than or equal to 0.10wt%, the improvement of austenite stability and steel hardenability is facilitated, and the improvement of the structural uniformity in the thickness direction of the steel plate in the cooling process is facilitated, so that the strength and toughness are improved, but when the content of Mo is more than 0.20wt%, the contribution to the improvement of the strength is small, the welding performance of the steel is not facilitated, and the economical efficiency is poor; as a further preference, the Mo content in the X70 grade pipeline steel obtained by the invention is controlled to be 0.15wt%.
Under the process conditions defined by the invention, the content of Cr element is controlled within the range of 0.15 to 0.25wt%, specifically, 0.15wt%,0.16wt%,0.17wt%,0.18wt%,0.19wt%,0.20wt%,0.21wt%,0.22wt%,0.23wt%,0.24wt%,0.25wt%, preferably 0.18 to 0.22wt%; when the content of Cr is more than or equal to 0.15wt%, the phase transformation point of steel is favorably reduced, fine grain strengthening is generated, and the hardenability of the steel is improved, but when the content of Cr is more than 0.25wt%, coarse carbides are easily precipitated on grain boundaries, and the toughness and the welding performance of the steel are obviously reduced; further preferably, the Cr content of the X70 grade pipeline steel obtained by the present invention is controlled to 0.20wt%.
Under the process conditions defined by the invention, the content of the Cu element is within the range of 0.10 to 0.20wt%, specifically such as 0.10wt%,0.11wt%,0.12wt%,0.13wt%,0.14wt%,0.15wt%,0.16wt%,0.17wt%,0.18wt%,0.19wt%,0.20 wt%; when the Cu content is more than 0.10wt%, precipitation strengthening occurs and, in addition, improvement of corrosion resistance, weldability, low temperature toughness and the like of the steel is very advantageous, but when the Cu content is more than 0.20wt%, hot embrittlement is easily caused.
Under the process conditions limited by the invention, the content of Nb in the steel is within the range of 0.04-0.06wt%, so that the steel has obvious precipitation strengthening effect and grain refining effect, is favorable for tissue dispersion and homogenization, and improves the strength and toughness of the steel; as a further preference, the Nb content in the X70 grade pipe 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 to 0.03wt%, precipitation strengthening and grain refining effects can be generated, and the distribution form of sulfide can be improved, which is beneficial to improving the strength and toughness of steel; further preferably, the Ti content in the X70 grade pipeline steel obtained by the invention is controlled to be 0.02wt%.
Under the process conditions defined by the invention, the content of Al element is controlled within the range of 0.02 to 0.04wt%, al, O and N have strong affinity, a positive deoxidation effect can be achieved, alN is formed with N, crystal grains can be refined, and N can be effectively fixed.
In the invention, the content of S is less than 0.015wt%, the content of P is less than 0.004wt%, and high toughness is obtained by strictly controlling impurity elements such as S, P and the like, thereby reducing the hot cracking tendency and the cold cracking tendency.
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 following steps of (1) controlling the grain size of granular bainite grains to be 2-6 mu m; the grain size of the ferrite is 3 to 12 mu m; M/A components are distributed on the granular bainite, and the size of the M/A components is 0.2 to 0.7 mu M.
The fine grain size, the proper M/A island size and the proper M/A island shape in the invention are beneficial to hindering dislocation motion 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, along with further reduction of the ambient temperature, the grain boundary, the M/A island with the proper size and the M/A island with the proper shape still keep a good hindering effect on the crack propagation, and the low-temperature impact toughness stability of the pipeline steel is good when the temperature is further reduced. When the grain size is too large and the size of the M/A island is too small, dislocation movement and crack propagation are difficult to block, the strength, the low-temperature impact toughness and the low-temperature crack arrest performance of the pipeline steel are reduced, and the stability of the low-temperature impact toughness is also reduced; when the size of the M/A island is too large, crack nucleation sites are likely to be formed at the interface between the M/A island and the matrix structure, and the low-temperature impact toughness is also lowered.
Preferably, the invention provides a method for regulating and increasing low-temperature toughness of a dual-phase structure of X70 grade pipeline steel, and the obtained pipeline steel is plate-shaped and has the thickness of not more than 35mm. The specific thickness is adjusted according to actual requirements when the material 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, the yield strength of the obtained pipeline steel is 480 to 525MPa, the tensile strength is 650 to 670MPa, the yield ratio is 0.74 to 0.79,
the work of-20 ℃ impact absorption is 285 to 325J, the work of-45 ℃ impact absorption is 260 to 310J, the work of-60 ℃ impact absorption is 245 to 290J, and when the material composition and the preparation process are the same, the fluctuation of the work of-60 ℃ impact absorption and-20 ℃ impact absorption of the product is not more than 40J;
the shear 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 the strength of the product is ensured to be in the set range of the invention, the fluctuation of the-60 ℃ impact absorption power and-20 ℃ impact absorption power of the product is controlled to be less than or equal to 40J, the impact absorption power of the product at-60 ℃ is controlled to be more than or equal to 245MPa, and the optimized impact absorption power can be more than or equal to 268MPa; the method is favorable for ensuring that the impact toughness of the pipeline steel in service in an extremely cold environment cannot be greatly reduced under the condition that the global climate is possibly changed drastically and extremely, and further ensuring the reliability and safety of the pipeline operation.
Preferably, 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 to 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 shear area of the DWTT low-temperature drop hammer test at the temperature of-20 ℃ is not less than 91 percent.
As a further preferable scheme, the fluctuation of the impact absorption work of the obtained pipeline steel at-60 ℃ and the fluctuation of the impact absorption work at-20 ℃ are less than or equal to 32J; and the impact absorption work at-60 ℃ is more than or equal to 268J.
As one of the best proposal, the yield strength of the obtained pipeline steel is 510MPa, the tensile strength is 663MPa, the yield ratio is 0.77, the impact absorption work of the obtained pipeline steel at minus 20 ℃ is 301J, the impact absorption work at minus 45 ℃ is 284J, the impact absorption work at minus 60 ℃ is 270J,
the shear area of the DWTT low-temperature drop hammer test at the temperature of-20 ℃ is not less than 91 percent.
The technical scheme of the invention has the following beneficial effects:
1. the invention prepares a ferrite and granular bainite two-phase structure with a proper proportion by designing low-carbon and low-alloying components and matching with a proper rolling and cooling control process, realizes the optimal matching of high toughness and low yield ratio, and has the yield strength of 480 to 525MPa, the tensile strength of 640 to 670MPa, the yield ratio of 0.75 to 0.79, the impact absorption power of 285 to 325J at-20 ℃, the impact absorption power of 260 to 310J at-45 ℃, the impact absorption power of 245 to 290J at-60 ℃ and the shear area of DWTT low-temperature drop hammer test at-20 ℃ of not less than 90 percent.
2. The pipeline steel prepared by the invention has the advantages of fine grain structure, moderate M/A component size, reasonable appearance, small reduction range of low-temperature impact toughness and good low-temperature impact toughness stability under the condition of greatly reducing the environmental temperature, and when the material composition and the preparation process are the same, the fluctuation of the impact absorption power of the product at minus 60 ℃ and the fluctuation of the impact absorption power at minus 20 ℃ are not more than 40J, thereby further ensuring the reliability and the safety of the pipeline in the extreme environment.
Drawings
FIG. 1 is a process flow and related parameter diagram of controlled rolling and controlled cooling (TMCP) design according to the present invention;
FIG. 2 is a photograph of OM of the pipeline steel obtained in example 7 of the present invention;
FIG. 3 is a photograph of OM of the pipeline steel obtained in example 8 of the present invention;
FIG. 4 is a photograph of OM of the pipeline steel obtained in example 9 of the present invention;
FIG. 5 is a photograph OM of the pipeline steel obtained in comparative example 8 of the present invention;
fig. 6 is a photograph of OM of the pipeline steel obtained in comparative example 9 of the present invention.
Detailed Description
To further clarify the technical problems, technical solutions and technical effects to be solved by the present invention, the following detailed description is given with reference to the accompanying drawings and examples, which are only for the purpose of understanding the present invention and should not be construed as specifically limiting the present invention.
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 low-carbon low-alloying component design and obtains a preparation method of X70-grade pipeline steel with large wall thickness by an optimized controlled rolling and controlled cooling (TMCP) process as shown in figure 1. Wherein the furnace temperature of the heating furnace in the preheating process is not more than 1200 ℃, the soaking temperature is around 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 pass is 7 passes, the average pass reduction rate is 15.5 percent, and the average pass interval time is 20s; the initial rolling temperature of the middle rolling is 880 ℃, the final rolling temperature is 800 ℃, the rolling passes are 8, the average pass reduction rate is 12.4%, the rolling speed is about 5m/s, and the average pass interval time is 10s; the initial rolling temperature of finish rolling is 790 ℃, the final rolling temperature is 750 ℃, the rolling pass is 2 to 4 passes (see table 2 specifically), the rolling speed is about 3m/s, and the inter-pass interval time is 20 to 50s (see table 2 specifically); the open cooling temperature of the segmented cooling is 710 to 750 ℃ (see table 2), and the final cooling temperature is 450 to 520 ℃; water cooling is adopted for sectional cooling; when the front section, the middle section and the rear section are water-cooled, the water-cooling area water volume ratio is 1 to 1.4; then air-cooled to room temperature.
Table 1 shows the mass percentages of the main alloying elements in the examples and comparative examples of the present invention. The elements and corresponding contents not listed in table 1 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 contents are the same in the comparative examples and examples of the present invention.
The method comprises the steps of proportioning according to a determined chemical component proportion, melting, dephosphorizing, performing external refining, stirring by argon gas, casting into a billet, preheating, and performing three-stage controlled rolling and three-stage controlled cooling. Table 2 shows the finish rolling and controlled cooling schedule for the examples and comparative examples.
Testing the yield strength and the tensile strength according to the standard GB/T228.1-2010, and testing the steel plate-20 according to the GB/T229-2020 ° C core impact absorption power, steel plate-20 is tested according to GB/T8363-2018 ° The drop weight performance of the full-thickness DWTT under C was calibrated by Image-Pro Plus software, and the results are shown in Table 3.
As can be seen from Table 3, the yield strengths of the steel for pipeline of the examples of the present invention are 480 to 525MPa, and the tensile strengths thereof are 640 to 670MPa, the yield ratio is 0.75 to 0.79, the impact absorption work at 20 ℃ is 285 to 325J, the impact absorption work at 45 ℃ is 260 to 310J, the impact absorption work at 60 ℃ is 245 to 290J, and when the material composition and the preparation process are the same, the fluctuation of the impact absorption work at 60 ℃ below zero and the impact absorption work at 20 ℃ below zero of the product is not more than 40J; -20 ° The shearing area of the C DWTT low-temperature drop hammer test is not less than 90 percent. The proportion of ferrite is 40-60%, the proportion of granular bainite is 40-60%, the grain size of the ferrite is 3-15 μ M, the grain size of the granular bainite is 2-6 μ M, and the component size of M/A is 0.2-0.7 μ M.
It is understood from examples 1, 2, 1 and 2 that the steel for line pipes has increased carbon content, increased strength, and decreased toughness, and from examples 3, 4, 3 and 4, that the steel for line pipes has increased Cr and Mo content, increased strength, and decreased toughness, and that the steel for line pipes has excellent comprehensive mechanical properties when the Cr and Mo content is within the range of the present application, but the steel for line pipes has poor balance between strength and toughness when the Cr and Mo content is outside the range of the present application. In comparative example 5, the strength of the pipeline steel is poor due to too low Nb and Ti contents, and in comparative example 6, the toughness of the pipeline steel is reduced due to too high Nb and Ti contents.
When the finish rolling parameters are within the protection range of the application, as shown in example 7 and shown in FIG. 2, the pipeline steel has fine grains, clear grain boundaries and ferrite which is mainly in a quasi-polygonal characteristic; the M/A component is slightly coarsened and is distributed more uniformly. The fine grain size and the properly coarsened M/A component are beneficial to hindering dislocation motion and crack propagation, so that the pipeline steel keeps better toughness while the strength is improved, the two-phase structure with a proper proportion is beneficial to reducing the yield ratio, and simultaneously, in an extreme low-temperature environment, a large-angle grain boundary and the properly coarsened M/A component have an inhibiting effect on crack propagation, so that the pipeline steel prepared by the method has excellent low-temperature impact toughness stability.
When the number of passes is out of the range protected by the present application, as in comparative example 7, it can be seen from FIG. 3 that the grain size and M/A component in the line steel are both coarsened greatly and the ferrite content is increased as compared with example 7. The excessively coarsened M/A component is easy to become a crack source, and the formation of microcracks in the pipeline steel structure is promoted; the coarsened crystal grains cause the reduction of the number of crystal boundaries, reduce the inhibition effect on crack propagation, and especially reduce the inhibition effect on crack propagation in a very low temperature environment, thereby damaging the low temperature impact toughness, the low temperature crack arrest performance and the low temperature impact toughness stability of the pipeline steel.
When the finishing pass interval time is out of the range of the present application, as in comparative example 8, it can be seen from FIG. 4 that the content of ferrite in the line steel is increased, the crystal grains are coarsened, the M/A component is excessively coarsened and the distribution is not uniform, as compared with example 7. The increase of ferrite content and coarsening of crystal grains lead to the reduction of the strength of the pipeline steel, the excessive coarsening and segregation of the M/A component cause the increase and aggregation of micro-crack nucleation points in the pipeline steel, which leads to the reduction of the low-temperature impact toughness of the pipeline steel, the excessive coarsening and coarsening of the crystal grains of the M/A component cause the easier nucleation and expansion of cracks under the extremely low-temperature environment, and the low-temperature crack arrest performance and the low-temperature impact toughness stability are reduced.
When the open cooling temperature is lower than the protection range of the application, as shown in a comparative example 9 and shown in a figure 5, the ferrite content is greatly increased, the M/A component is coarse, and a certain aggregation phenomenon is generated, so that the pipeline steel has lower strength and poor toughness; when the open cooling temperature exceeds the protection range of the application, as shown in fig. 6 of comparative example 10, the ferrite content is greatly reduced to about 30%, which results in high yield ratio and poor toughness of the pipeline steel, and the requirements are difficult to meet.
From comparative examples 11, 7, 12 and 12, it is understood that the strength of the line steel is increased, the low-temperature impact toughness and the low-temperature crack arrest performance are decreased, and the yield ratio is increased as the water content ratio in the three stages is increased. When the water content ratio in the three stages is higher than the range protected by the application, as in comparative example 12, the low-temperature impact toughness and the low-temperature crack arrest performance are poor, and the yield ratio is higher; when the pipeline steel is cooled at a single cooling speed, as in comparative example 11, the strength is lower, the toughness is not improved, and the comprehensive mechanical property is reduced.
From comparative examples 13, 7, 14 and 14, it is understood that as the average cooling rate is increased, the low-temperature impact toughness and the low-temperature crack arrest property of the line pipe steel are decreased and the yield ratio is increased. When the average cooling rate is lower than the range protected by the present application, as in comparative example 13, the strength of the line steel is too low; when the average cooling rate is higher than the range protected by the present application, the toughness of the steel for line pipe is too poor as in comparative example 14, and it is difficult to satisfy the performance requirements.
In conclusion, the mechanical property indexes of the pipeline steel are low due to the components which are not in the scope of the invention or the controlled rolling process and the controlled cooling process which are provided by the invention are not adopted, 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 embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.
Claims (10)
1. A method for regulating and controlling a dual-phase structure of X70 grade pipeline steel and increasing low-temperature toughness is characterized by comprising the following steps: 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 0.08wt%;
the method comprises the following steps:
carrying out raw material proportioning, primary smelting and refining according to chemical components of X70 grade pipeline steel, carrying out rapid stokehold component analysis and component adjustment, and then carrying out continuous casting and controlled rolling and controlled cooling;
the controlled rolling and controlled cooling is three-stage controlled rolling and three-stage controlled cooling;
the three-stage controlled rolling comprises rough rolling, medium rolling and finish rolling;
the rough rolling is carried out at the rolling temperature of 1100-1200 ℃, the final rolling temperature of 980-1000 ℃, the rolling pass is 6-8, and the pass reduction rate is 10-18%;
in the middle rolling, the rolling temperature is 870 to 890 ℃, the final rolling temperature is 790 to 810 ℃, the rolling pass is 7 to 9 passes, the pass reduction rate is 6 to 15 percent, the rolling speed is 4 to 6m/s, and the pass interval time is 9 to 13s;
the finish rolling is carried out, wherein the rolling temperature is 780-800 ℃, the final rolling temperature is 740-750 ℃, the rolling pass is 2-3, the pass reduction rate is 5-10%, the rolling speed is 2-4 m/s, and the pass interval time is 20-40s;
the three-stage controlled cooling comprises three stages of controlled cooling of a front stage, a middle stage and a rear stage;
the controlled cooling is carried out, wherein the open cooling temperature is 720 to 740 ℃, and the final cooling temperature is 450 to 520 ℃; controlling cooling by water cooling; wherein the water volume ratio of the front section to the middle section to the rear section is 1.2 to 1.3.
2. The method for regulating and increasing the low-temperature toughness of the dual-phase structure of the X70-grade pipeline steel according to claim 1, wherein the method comprises the following steps: the rolling pass of the finish rolling is 2 passes, the rolling speed is 2-3 m/s, and the time interval between passes is 20-30s.
3. The method for regulating and increasing the low-temperature toughness of the dual-phase structure of the X70-grade pipeline steel according to claim 1, wherein the method comprises the following steps: the three-stage controlled cooling has the water quantity ratio of the front-stage water cooling area to the middle-stage water cooling area to the rear-stage water cooling area of 1.2.
4. The method for regulating and increasing the low-temperature toughness of the dual-phase structure of the X70-grade pipeline steel according to claim 3, wherein the method comprises the following steps: the average cooling rate of the water cooling zone is 14 ℃/s.
5. The method for regulating and increasing the low-temperature toughness of the dual-phase structure of the X70-grade pipeline steel according to claim 1, wherein the method comprises the following steps: 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%，
0.35 to 0.48 percent of Ceq, and the balance of Fe and other inevitable impurities.
6. The method for regulating and increasing the low-temperature toughness of the dual-phase structure of the X70-grade pipeline steel according to claim 1, wherein the method comprises the following steps: the size of the granular bainite crystal grain is 2 to 6 mu m; the grain size of the ferrite is 3 to 12 mu m; M/A components are distributed on the granular bainite, and the size of the M/A components is 0.2 to 0.7 mu M.
7. The method for regulating and increasing the low-temperature toughness of the dual-phase structure of the X70-grade pipeline steel according to claim 1, wherein the method comprises the following steps: the thickness of the pipeline steel is not more than 35mm.
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 any one of claims 5 to 7, wherein the method comprises the following steps: the yield strength of the pipeline steel is 480 to 525MPa, the tensile strength is 640 to 670MPa, the yield ratio is 0.75 to 0.79,
the work of-20 ℃ impact absorption is 285 to 325J, the work of-45 ℃ impact absorption is 260 to 310J, the work of-60 ℃ impact absorption is 245 to 290J, and when the material composition and the preparation process are the same, the fluctuation range of the-60 ℃ impact absorption and the-20 ℃ impact absorption of the product is not more than 40J;
the shear area of the DWTT low-temperature drop hammer test at the temperature of-20 ℃ is not less than 90 percent.
9. The method for regulating and increasing the low-temperature toughness of the dual-phase structure of the X70-grade pipeline steel according to claim 8, wherein the method comprises the following steps: 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 to 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 shear area of the DWTT low-temperature drop hammer test at the temperature of minus 20 ℃ is not less than 91 percent.
10. The method for regulating and increasing the low-temperature toughness of the dual-phase structure of the X70-grade pipeline steel according to claim 9, wherein the method comprises the following steps: the fluctuation of the impact absorption work of the pipeline steel at minus 60 ℃ and the fluctuation of the impact absorption work at minus 20 ℃ are less than or equal to 32J; and the impact absorption work at-60 ℃ is more than or equal to 268J.
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