CN111748742A - Super-thick-wall X70 grade marine acid-resistant pipeline steel and preparation method thereof - Google Patents
Super-thick-wall X70 grade marine acid-resistant pipeline steel and preparation method thereof Download PDFInfo
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
Abstract
The invention discloses extra-thick-wall X70 grade marine acid-resistant pipeline steel which comprises the following chemical components in percentage by mass: 0.03-0.05% of C, 0.10-0.30% of Si, 1.0-1.3% of Mn, 0.2-0.3% of Cr, 0.2-0.4% of Ni, 0.10-0.20% of Mo, 0.04-0.07% of Nb, 0.03-0.05% of V, 0.01-0.02% of Ti, 0.01-0.045% of Al and the balance of Fe. The obtained steel matrix structure is granular bainite/acicular ferrite and a small amount of small-size quasi-polygonal ferrite, so that the high strength, high toughness, hydrogen sulfide corrosion resistance and stress corrosion resistance are effectively considered, and the technical index requirement of X70MOS with the thickness of more than 25mm is met; and the related preparation process is simple, low in energy consumption and suitable for popularization and application.
Description
Technical Field
The invention belongs to the technical field of low-carbon microalloy steel materials, and particularly relates to extra-thick-wall X70 grade marine acid-resistant pipeline steel and a preparation method thereof.
Background
Ocean oil and gas resources account for 70% of the total amount of global oil and gas resources, and with the gradual exhaustion of land oil resources, the ocean becomes a main battlefield for acquiring oil and gas resources in the future. As pipeline steel for marine service, the pipeline steel needs to bear the service environments such as marine low temperature, high pressure, ocean current movement, external/internal corrosive media and the like, and strict requirements are put forward on low-temperature toughness, strength, corrosion resistance, large wall thickness and the like of materials. At present, the pipeline steel for marine service engineering in China is mainly made of steel grade below X70, submarine pipeline steel with the thickness of 20-31.8mm and different thicknesses of X65 and X70 is made of the submarine pipeline steel, and the application of batch engineering is realized. For example, the most advanced submarine pipeline steel in China is X70 grade pipeline steel with the thickness of 31.8mm for Nanhai litchi Bay submarine pipeline engineering, the submarine pipeline steel is jointly supplied by Bao steel (Wu Steel Limited, Bao mountain steel), saddle steel and Nanhai steel, the submarine acid-resistant pipeline steel only has X65MOS engineering application at present, and X70 grade H-resistant pipeline steel is not available yet2Record of X70MOS application of extra-thick-wall submarine pipeline steel engineering for S corrosion and deep/ultra-deep sea, high strength, high toughness and high resistanceH2S corrosion submarine pipeline steel completely depends on import and becomes one of the most key factors for restricting the development of marine energy and national energy safety in China.
The patent CN 106566991A discloses X65MOS acid-resistant submarine pipeline steel and a preparation method thereof, the compression ratio of the steel plate is more than or equal to 11, the thickness of the obtained steel plate is 34-48 mm, but the yield strength is not 485 MPa; CN 105132807A discloses pipeline steel with excellent acid corrosion resistance on the seabed, the accumulated reduction rate of two stages of rough rolling and finish rolling is not less than 70%, and continuous casting billets with the thickness of 250mm and below can only be used for producing seabed acid-resistant pipelines below 23mm and can not be used for producing seabed acid-resistant pipelines with the super-thick specification of 25mm and above; CN 108624811A discloses a large thick-wall acid corrosion resistant pipeline steel and a production method thereof, TMCP technology is adopted for rolling, ultra-fast cooling is carried out to 280-300 ℃, polygonal and bainite structures with the core part of 30 mu m are obtained, the acid corrosion resistant performance requirements can be met, but the mechanical property and the steel plate thickness specification are not published, the content of a strengthening element C is considered to be low, the size of polygonal ferrite is large, and the strength grade is not high; CN 105132833A discloses an economical high-strength submarine pipeline steel and a production method thereof, wherein the high-carbon component system is easy to cause H due to the component and structure segregation of carbon2S, testing the incompatibility, and the incompatibility is caused to the components of the thick-specification seabed acid-resistant pipeline; CN 105648327A discloses an HIC and SSC resistant pipeline steel plate with a small compression ratio and a preparation method thereof, the obtained steel plate has the advantages of excellent HIC and SSC resistance, less alloy addition, low production cost, effective compression ratio of less than or equal to 10, good structure uniformity, small limitation condition on the thickness of a casting blank and the like, and is suitable for manufacturing oil and natural gas conveying pipelines containing acidic media, but only steel with the thickness below 25mm can be prepared.
The method integrates the research and development and production difficulties and characteristics of the existing extra-thick high-strength submarine acid-resistant pipeline, and mainly has four problems: 1) the purity of the molten steel is extremely high, and in the process of hydrogen sulfide corrosion, hydrogen atoms are easy to gather on a two-phase interface and form cracks, so that the higher the purity of the molten steel is, the better the purity of the molten steel is; 2) for thick pipeline steel plates, a high-homogeneity continuous casting billet with large thickness is needed to ensure that crystal grains are sufficiently refined, but the larger the thickness of the continuous casting billet is, the more serious the segregation is, so that the poorer the corrosion resistance of wet hydrogen sulfide is, the contradiction between the two is caused, and the difficulty is extremely high; 3) for the pipeline steel with the thickness of more than 25mm, the deformation in the thickness direction in the rolling process has large difference, the deformation is difficult to permeate to the center, the temperature gradient and the cooling speed difference in the cooling process are also large, and the tissue uniformity is difficult to control; 4) the high-strength seabed acid-resistant pipeline also requires a lower yield ratio and higher impact drop hammer toughness on the basis of high strength, so that the requirements of high strength, high toughness and low yield ratio are considered, and the high-strength seabed acid-resistant pipeline provides higher technical challenges for component, tissue design and precise control of a technological process on the basis of matching of mechanical properties of materials.
Disclosure of Invention
The invention mainly aims to provide extra-thick-wall X70 grade marine acid-resistant pipeline steel aiming at the defects of the conventional X65 grade submarine acid-resistant pipeline steel, and the obtained pipeline steel meets the requirements of high strength, large wall thickness, high toughness and H resistance2The corrosion of S and other pipeline steel service technical requirements, and the related production process is simple, convenient to operate and suitable for popularization and application.
In order to achieve the purpose, the invention adopts the technical scheme that:
an extra-thick wall X70 grade ocean acid-resistant pipeline steel comprises the following chemical components in percentage by mass: 0.03-0.05% of C, 0.10-0.30% of Si, 1.0-1.3% of Mn, 0.2-0.3% of Cr, 0.2-0.4% of Ni, 0.10-0.20% of Mo, 0.04-0.07% of Nb, 0.03-0.05% of V, 0.01-0.02% of Ti, 0.01-0.045% of Al and the balance of Fe.
Preferably, the extra-thick wall has a thickness of 25mm or more; can also be used for pipeline steel with the thickness less than or equal to 25mmX70 grade.
More preferably, the extra-thick wall has a thickness of 25 to 40 mm.
In the above scheme, the metallographic structure of the low compression ratio thick-gauge pipeline steel comprises: small size polygonal ferrite, bainite, and acicular ferrite.
The preparation method of the extra-thick wall X70 grade marine acid-resistant pipeline steel comprises the processes of smelting, continuous casting, casting blank heating, controlled rolling, relaxation and controlled cooling.
In the scheme, the clean steel smelting technology is adopted for smelting, the content of harmful elements in the molten steel is controlled, the molten steel is treated by adding Ca, and the Ca/S ratio in the molten steel is controlled to be 1.5-3.0.
In the above scheme, the content requirements of the hazardous elements are as follows: less than or equal to 0.0050 percent of N, less than or equal to 0.0002 percent of H, less than or equal to 0.0025 percent of O, less than or equal to 0.015 percent of P, and less than or equal to 0.0020 percent of S.
In the scheme, the superheat degree of the continuous casting process is controlled to be 10-25 ℃; when the condition is met, electromagnetic stirring and dynamic soft reduction are adopted to improve the segregation of the continuous casting billet, and the low power rating of the casting billet is controlled within C1.0 level
In the scheme, the heating temperature of the casting blank is 1150-1180 ℃, and the uniform heating is carried out for 60-120 min.
In the above scheme, the rolling control adopts a small-compression-ratio rolling process, and the rolling control process adopts three-stage rolling control: in the first stage, 1-3 times of large deformation rolling is carried out in a high-temperature austenite region with the temperature of more than 1080 ℃, the single-pass reduction rate is more than or equal to 10 percent, and the first-stage reduction rate is more than or equal to 30 percent, so that austenite grains are fully crushed; in the second stage, 1-5 times of large deformation rolling is adopted within the temperature range of 1000-1080 ℃ close to the recrystallization stop temperature, the single-pass reduction rate is more than or equal to 15 percent, and the second stage reduction rate is more than or equal to 30 percent, so that the original austenite grains are subjected to dynamic/static recrystallization at a lower temperature, and the grains are inhibited from growing up in the temperature waiting process after recrystallization; the third stage is a finish rolling stage, the initial rolling temperature is less than or equal to 950 ℃, rolling is carried out in an austenite non-recrystallization region, the accumulated reduction rate is more than 55 percent, austenite crystal grains are fully flattened, a large number of deformation zones, twin crystals and other crystal defects are formed in the crystal grains, the effective crystal grain area is increased, so that phase change particles of a new phase are improved in the continuous cooling phase change process after rolling, and the finished product structure is refined.
In the scheme, after finish rolling, relaxation is carried out for a period of time before accelerated cooling, the start cooling temperature is ensured to be 760-780 ℃, the start cooling temperature cannot be lower than 760 ℃, and the large-size ferrite proportion in the structure is prevented from being greatly increased.
In the scheme, an ultra-fast cooling process is adopted in the cooling control step, the cooling rate is 10-35 ℃/s, the final cooling temperature is designed to be 350-550 ℃, the temperature fluctuation in the whole board width direction is controlled to be less than or equal to 30 ℃, and the anisotropy of longitudinal and transverse mechanical properties is prevented.
In the scheme, the thickness of the casting blank obtained by continuous casting is 150-250mm of the thickness of a conventional continuous casting blank; the pipeline steel for the seabed acid-resistant steel with the thickness of 20-41mm and above X70 grade is produced by adopting a low compression ratio (6-9) process.
The principle of the invention is as follows:
principle of designing components
Low-carbon-medium-low-manganese components are adopted to be matched with micro-alloying of Nb, V, Ti and the like, and the HIC and SSCC resistance of the steel is improved by reducing the contents of easily segregated components C, Mn and S; the strength and toughness of the material are effectively guaranteed by combining the effects of precipitation strengthening of Nb, V and Ti alloy elements and the like; meanwhile, Nb and V are used in a matching way, so that the comprehensive improvement effect of the obtained steel can be further improved; the specific functions and contents of the added chemical elements comprise:
c: carbon is a cheap and effective strengthening element, and carbon is a main element influencing the toughness and weldability of pipeline steel. The carbon content is increased, the weldability is deteriorated, the toughness is reduced, the segregation is intensified, and the HIC resistance is reduced; with the improvement of the strength grade of the steel, the content of C in the pipeline steel is in a descending trend, so that the content of C is controlled to be 0.03-0.05 percent;
si: si is dissolved in steel in a solid solution mode and plays a role in solid solution strengthening, and the solubility of carbon in austenite can be reduced in the steel; si element strongly inhibits carbide from being precipitated along grain boundaries in the bainite transformation process, increases the grain boundary binding force and improves the toughness; when the silicon content is too high, the plasticity and toughness of the material are obviously reduced, and the weldability of the steel is also reduced, so that in order to avoid the obvious deterioration of the plasticity and toughness of the steel caused by adding excessive silicon, the Si content is controlled within the range of 0.10-0.30%;
mn: mn is a basic alloy element of HSLA steel for pipelines; the main role of manganese in steel is four: 1) the gamma → alpha phase transition temperature is reduced, the austenite phase transition is delayed, the pearlite amount is reduced, the ferrite grain size is refined, and the acicular ferrite nucleation can be promoted by high Mn; 2) the solubility product of Nb (C, N) in austenite is improved, the early precipitation tendency of Nb in austenite is reduced, the size of precipitated carbide is reduced, and the precipitation strengthening effect is promoted; 3) an inherent beneficial effect on toughness; 4) when the Mn content in steel is excessively increased, component segregation is easily caused, a pearlite strip is formed in the center of the plate thickness, when a banded structure is larger than or equal to 2%, acid-resistant HIC and SSCC experiments cannot be generally passed, Mn and S in the steel form easy MnS inclusions, and the segregation and MnS inclusions easily cause Hydrogen Induced Cracking (HIC) and stress corrosion cracking (SSCC) in the service environment of the acid-resistant submarine pipeline at the seabed, and the acid-resistant submarine pipeline generally reduces the manganese content in the steel on the basis of the same steel grade pipeline steel, so that the addition amount of Mn is generally controlled to be 1.0-1.3%;
p, S: phosphorus is easy to cause segregation in pipeline steel, and the phosphorus also can deteriorate the welding performance, obviously reduce the low-temperature impact toughness of the steel and increase the brittle transition temperature; sulfur is a main element influencing the HIC and SSC resistance of the pipeline steel, and is easy to combine with manganese to generate MnS inclusions and influence the low-temperature impact toughness of the pipeline steel; the adverse effect of P, S element on the performance of the steel should be reduced as much as possible, the P content is controlled below 0.015 percent, the S content is controlled below 0.002 percent, and the adverse effect is reduced by applying the techniques of inclusion denaturation treatment and the like to spheroidize and uniformly distribute inclusions in the steel;
nb: niobium can obviously improve the recrystallization temperature of steel, and the high Nb content can ensure that the high austenite recrystallization temperature is obtained, so that a fine structure containing a large amount of deformation zones is obtained; meanwhile, after Nb reaches a certain content in microalloy controlled rolling steel, the fine Nb (C, N) particle precipitation strengthening can be separated out in the rolling and cooling process, thereby improving the strength of the steel; the content of Nb is controlled to be 0.04-0.07%;
ti: 0.02 percent of titanium is added into the controlled rolling low-carbon pipeline steel to refine grains, so that the yield strength and the toughness of the steel are improved; this improvement is mainly related to the fact that titanium increases the recrystallization temperature and austenite grain coarsening temperature of the steel, thereby controlling the grain size during continuous casting and heating; meanwhile, the precipitation incubation period of NbC can be prolonged by adding Ti into Nb steel, so that the precipitation starting time of carbide of the Nb-Ti composite steel is later than that of the Nb steel, and precipitates are finer and more dispersed; because Ti can be combined with N at high temperature to form TiN particles, the addition of Ti is beneficial to the grain control of a heat affected zone during welding and is beneficial to improving the toughness of the welding heat affected zone; the invention controls the Ti content to be 0.01-0.02%;
al: al is a main deoxidizing element in steel, so that the oxygen content in the steel can be remarkably reduced, and meanwhile, AlN is formed by combining aluminum and nitrogen, so that grains can be effectively refined; however, when the aluminum content in the steel exceeds a certain amount, the oxide inclusions of aluminum are easily increased obviously, the cleanliness of the steel is reduced, and the toughness is not favorable, so that the content of Als is controlled to be 0.01-0.045%;
mo: the Mo element is a main element for expanding a gamma phase region, delaying ferrite formation during gamma → alpha phase transition and promoting acicular ferrite formation, can improve the hardenability of a thick steel plate, is beneficial to austenite grain refinement and fine bainite formation during rolling, plays an important role in controlling a phase transition structure, and can obtain an obvious acicular ferrite structure by adding 0.10-0.20% into low-carbon pipeline steel, so that the Mo content is generally controlled to be 0.1-0.2%;
v: v can supplement the defect of Nb precipitation strengthening in steel, can also improve the toughness of steel after welding, can exert stronger precipitation strengthening and weaker fine grain effect, and improve the toughness, especially the tensile strength, of the steel; in addition, the composite addition of niobium, vanadium and titanium has more obvious effect than the respective independent addition, not only improves the strength and toughness of the steel, but also enhances the hydrogen sulfide corrosion resistance of the pipeline steel; the invention controls the content of V at 0.03-0.05%.
Cr: the addition of Cr is very important for improving the corrosion resistance of the material, and the addition of Cr can improve the strength of steel, and the Cr is controlled to be 0.20-0.30 percent.
Ni can improve the strength of the steel through the solid solution strengthening effect, and simultaneously can improve the structural transformation uniformity of thick pipeline steel during ultra-fast cooling as a hardenability element, improve the low-temperature toughness of the steel, and also can compensate the strength reduction caused by the increase of the thickness of the pipeline steel; the invention controls the Ni content to be 0.2-0.4%. The low-temperature toughness of the steel is improved, and the strength reduction caused by the increase of the thickness of the pipeline steel can be compensated.
Second, the principle of process improvement
Smelting and continuous casting process
The content of harmful elements in steel is controlled by adopting a clean steel smelting technology, the harmful elements N are less than or equal to 0.0050 percent, H is less than or equal to 0.0002 percent, O is less than or equal to 0.0025 percent, P is less than or equal to 0.015 percent, and S is less than or equal to 0.0020 percent, meanwhile, the precise control and homogenization of alloy elements in molten steel are enhanced, and the heat preservation, oxidation resistance and N increase control of the molten steel are enhanced; after elements and harmful elements in steel are accurately controlled, the type and the size of inclusions in molten steel are also required to be controlled, and large-size inclusions with the size of more than 10 mu m are ensured not to exist in the molten steel solidification process; the calcium treatment can convert low-melting-point easily-deformed MnS inclusions generated in the solidification process into high-melting-point difficultly-deformed CaS spherical inclusions, so that cluster Al is formed2O3The inclusion turns into low melting point (CaO)12(Al2O3)7Promoting the steel-making process to float upwards to remove purified molten steel and reduce Al2O3The water gap nodulation caused by inclusions is solved, the Ca treatment process of the inclusions can improve the anisotropy of steel, and particularly solves the problems of Hydrogen Induced Cracking (HIC) and stress corrosion cracking (SSCC) of the seabed acid-resistant pipeline, so that the inclusion treatment denaturation technology must be adopted in the production of the seabed acid-resistant pipeline, the soft blowing time after calcium treatment is not less than 4min, and the Ca/S in the molten steel is controlled according to 1.5-3.0;
in the process of molten steel solidification, the casting temperature and the superheat degree have direct influence on solidification segregation, the higher the superheat degree is, the more serious the component and structure segregation is, and once macro-segregation in the casting process is generated in hot rolling and hot processing, the problem that a large amount of strip-shaped segregation structures exist in the hot rolled plate, particularly in the center part, the acid-resistant performance of the seabed acid-resistant pipeline is not good is caused, so that in order to reduce the macro-segregation in the casting process, the casting temperature needs to be reduced on the premise of ensuring the castability, the superheat degree of a component system of the seabed acid-resistant pipeline is designed to be 10-25 ℃, and Hydrogen Induced Cracking (HIC) and stress corrosion cracking (SSCC) caused by component structure segregation caused by high-temperature steel casting; meanwhile, the segregation of the casting blank is reduced by the electromagnetic stirring and soft reduction technology in the solidification process, and the low-power rating of the casting blank is controlled within C1.0 level.
A billet heating temperature schedule; when the billet is heated in the heating furnace, the heating system is strictly controlled, so that the micro-alloy elements are fully dissolved in the solution, and the austenite grains are prevented from being obviously coarsened. As shown in Table 1, the Nb (C, N) second phase solution Temperature (TDISS) in this patent is 1124 ℃ and the heating temperature should be 30 to 50 ℃ higher than the Nb (C, N) second phase solution temperature to ensure sufficient solution heat of the alloying elements. The austenite grain size is easy to grow when the soaking time exceeds 2 hours, so the soaking time is controlled within 60-120 min. Therefore, the heating temperature system of the steel is determined to be 1150-1180 ℃, and soaking is carried out for 60-120 min.
Controlling the binding process; aiming at the special performance requirements and technical difficulties of the high-strength thick-specification submarine acid-resistant pipeline, a unique three-stage controlled rolling technology is developed: in the first stage, 1-3 times of large deformation rolling is carried out in a high-temperature austenite region with the temperature of more than 1080 ℃, the single-pass reduction rate is more than or equal to 10 percent, and the first-stage reduction rate is more than or equal to 30 percent, so that austenite grains are fully crushed; in the second stage, 1-5 times of large deformation rolling is adopted within the temperature range of 1000-1080 ℃ close to the recrystallization stop temperature, the single-pass reduction rate is more than or equal to 15 percent, and the second stage reduction rate is more than or equal to 30 percent, so that the original austenite grains are subjected to dynamic/static recrystallization at a lower temperature, and the grains are inhibited from growing up in the temperature waiting process after recrystallization; the finishing temperature of rough rolling is in a complete recrystallization zone and cannot enter a partial recrystallization zone so as to avoid generating mixed crystal tissues, and the accumulated reduction rate of rough rolling is more than 50 percent; the third stage is a finish rolling stage, in order to avoid mixed crystals caused by partial recrystallization of thick pipeline steel and ensure that accumulated deformation in the finish rolling stage can be transferred from the surface of the steel plate to the center of the steel plate, the finish rolling initial temperature is set to be less than or equal to 950 ℃, austenite non-recrystallization region rolling is carried out, the accumulated reduction rate is more than 55%, austenite crystal grains are fully flattened, a large number of deformation zones, twin crystals and other crystal lattice defects are formed in the crystal grains, the effective crystal grain area is increased, so that phase change particles of a new phase are improved in the continuous cooling phase change process after rolling, and the finished product structure is refined.
A relaxation process; in order to obtain an ideal ferrite and bainite dual-phase structure (or ferrite, bainite and MA component dual-phase structure), good matching of high strength, high toughness and low yield ratio is realized, relaxation is carried out for a period of time before accelerated cooling after finish rolling, the open cooling temperature is ensured to be 760-789 ℃, a part of fine pro-eutectoid ferrite is precipitated in the air cooling relaxation process before accelerated cooling, and diffusion carburization is carried out to residual austenite which does not undergo phase transformation, so that the carbon is enriched.
An ultra-fast cooling process; accelerated cooling to refine and determine the bainite complex phase structure after final cooling phase transformation as much as possible, so that the residual austenite without phase transformation is converted into fine and uniform bainite/acicular ferrite structure; the cooling speed of the adopted accelerated cooling is 10-25 ℃/s, and the final cooling temperature is 350-550 DEG C
A post-rolling controlled cooling process, in order to obtain an ideal ferrite and bainite dual-phase structure (a small amount of small-size ferrite or no ferrite), realize good matching of high strength, high toughness and low yield ratio, and ensure that the open cooling temperature is 760-789 ℃ after finishing rolling, a part of fine pro-eutectoid ferrite is precipitated in the air cooling process before accelerated cooling (dual-phase structure is required), and is diffused and carburized into residual austenite which does not undergo phase transformation to enable the residual austenite to be rich in carbon, and then is accelerated cooling to enable the residual austenite which does not undergo phase transformation to be converted into a fine and uniform bainite/acicular ferrite structure; the starting cooling temperature of the single-phase acicular ferrite structure is 789-830 ℃, the cooling rate of the accelerated cooling is 10-25 ℃/s, and the final cooling temperature is 350-550 DEG C
Compared with the prior art, the invention has the beneficial effects that:
1) by adopting reasonable component design and optimized rolling TMCP production process, the toughness of the submarine acid-resistant pipeline steel can be obviously improved, the submarine acid-resistant pipeline steel has excellent hydrogen sulfide corrosion resistance, the problems that the strength, the hydrogen sulfide corrosion resistance and the toughness of the existing pipeline steel product are difficult to match and the like are solved, and the requirement of the marine acid-resistant pipeline steel on the service environment is met.
2) An ideal organization structure of granular bainite and quasi-polygonal ferrite can be obtained, the high strength, the high toughness, the hydrogen sulfide corrosion resistance and the stress corrosion resistance are effectively considered, and the X70MOS technical index requirements are met; yield strength Rt of the resulting Steel0.5485 to 635MPa, tensile strength Rm570-760 MPa, low yield ratio, impact-30 ℃ KV2 not less than 250J, impact-20 ℃ DWTTSA not less than 85%, hardness HV10 is less than or equal to 250, and has excellent HIC and SSCC acid corrosion resistance.
3) The invention adopts a small compression ratio (6-9 times) production technology, directly uses a main flow continuous casting machine of 150 plus 250mm to produce the thick-specification pipeline steel, and reduces the cost for newly introducing a large-section continuous casting machine or modifying equipment for producing the thick-specification pipeline; meanwhile, the problem of core macrosegregation easily caused by a large-section continuous casting billet is effectively solved; and can effectively promote hot rolling process production rhythm and efficiency, reduce equipment load, improve production line productivity, have higher practical economic value.
Drawings
FIG. 1 is a metallographic structure diagram of a rolled core portion 1/2 of a 25mm X70 MOS.
FIG. 2 is a metallographic structure diagram of a 40mm X70MOS rolled core 1/2.
Detailed Description
In order to better understand the present invention, the following examples are further provided to illustrate the present invention, but the present invention is not limited to the following examples.
Examples 1 to 5
Rolling 25mm thick X70 grade seabed acid-resistant pipeline steel by adopting a 150mm casting blank, and specifically implementing the steps as follows:
1) smelting according to the component requirements in the table 1, and controlling the content of molten steel specifically comprises the following steps: n is less than or equal to 0.0050 percent, H is less than or equal to 0.0002 percent, O is less than or equal to 0.0025 percent, P is less than or equal to 0.015 percent, and S is less than or equal to 0.0020 percent; after the components of the molten steel are qualified, Ca is added into the molten steel, the Ca/S ratio in the molten steel is controlled to be 1.5-3.0, the casting temperature is controlled to be 1533-1548 ℃, the segregation of a continuous casting blank is improved by adopting electromagnetic stirring and dynamic soft reduction, and the low-power rating of the casting blank is controlled to be within C1.0 level;
2) casting into a casting blank with the thickness of 150mm, and then heating and preserving heat in a heating furnace; the heating temperature is 1150-1180 ℃; according to different charging temperatures, the in-furnace time is 130-230 min, and the heat preservation time at the high temperature of 1150-1180 ℃ is 60-90 min;
3) hot rolling for three stages; in the first stage, 2-pass large deformation rolling is carried out in an austenite region at a high temperature of above 1050 ℃, the first-pass reduction rate is 13%, the second-pass reduction rate is 23%, and the first-stage reduction rate is 33% so that austenite grains are fully crushed; in the second stage, 2-pass large deformation rolling is adopted within the temperature range of 1000-1050 ℃ close to the recrystallization stop temperature, the third-pass reduction is 20%, the fourth-pass reduction is 25%, and the accumulated reduction rate of rough rolling is 60%; the third stage is a finish rolling stage, rolling and cooling control are carried out, the finish rolling initial rolling temperature is set to be less than or equal to 950 ℃, rolling of an austenite non-recrystallization region is carried out, the cumulative four-pass reduction rate of finish rolling is 58%, and the final rolling temperature is 810-830 ℃;
4) after finishing finish rolling, relaxing for a period of time before accelerated cooling, ensuring the open cooling temperature to be 760-780 ℃, separating out a part of fine pro-eutectoid ferrite in the air cooling relaxation process before accelerated cooling, diffusing and carburizing into the residual austenite which does not undergo phase transformation to enrich carbon, then accelerating cooling, converting the residual austenite which does not undergo phase transformation into fine and uniform bainite/acicular ferrite tissues, wherein the open cooling temperature cannot be lower than 760 ℃; the large-size ferrite in the structure is prevented from being greatly increased, the cooling rate of accelerated cooling is 10-25 ℃/s, the final cooling temperature is designed to be 350-550 ℃, the temperature fluctuation in the whole board width direction is controlled to be less than or equal to 30 ℃, and the anisotropy of longitudinal and transverse mechanical properties is prevented.
TABLE 1 tabulated chemical compositions and weight percents of the steels of examples 1-5
| Element(s)Components | C | Si | Mn | P | S | Cr | Ni | Mo | Nb | V | Alt | Ti |
| Example 1 | 0.053 | 0.27 | 1.14 | 0.0091 | 0.0016 | 0.2 | 0.33 | 0.17 | 0.053 | 0.042 | 0.023 | 0.017 |
| Example 2 | 0.036 | 0.24 | 1.10 | 0.0090 | 0.0013 | 0.19 | 0.33 | 0.17 | 0.053 | 0.041 | 0.013 | 0.014 |
| Example 3 | 0.049 | 0.24 | 1.11 | 0.0085 | 0.0012 | 0.22 | 0.34 | 0.17 | 0.056 | 0.041 | 0.020 | 0.014 |
| Example 4 | 0.048 | 0.10 | 1.30 | 0.0086 | 0.0010 | 0.11 | 0.21 | 0.10 | 0.059 | 0.044 | 0.035 | 0.010 |
| Example 5 | 0.044 | 0.19 | 1.15 | 0.0085 | 0.0020 | 0.16 | 0.26 | 0.16 | 0.067 | 0.050 | 0.010 | 0.020 |
Table 2 shows the rolling and cooling temperatures of the preparation processes of examples 1 to 5 of the present invention
The mechanical properties, low-temperature impact properties, corrosion resistance and the like of the steel materials obtained in examples 1 to 5 were respectively tested, and the results are shown in Table 3.
Table 3 shows the results of the performance tests of the steels obtained in examples 1 to 5 of the present invention
The results show that the comprehensive performance of the steel obtained in the embodiments 1 to 5 can completely meet the technical index requirements of X70-grade submarine acid-resistant pipelines (X70MOS), and the steel has a low yield ratio (less than or equal to 0.80), good toughness and H resistance2And S has good corrosion performance. The representative microscopic metallographic structure is shown in figure 1, and the structure type is small-size quasi-polygonal ferrite + bainite/acicular ferrite.
Examples 6 to 10
40mmX 70-grade pipeline steel is rolled by adopting a 230mm casting blank, and the specific implementation steps are as follows:
1) smelting according to the component requirements in the table 4, and controlling the content of molten steel specifically comprises the following steps: n is less than or equal to 0.0050 percent, H is less than or equal to 0.0002 percent, O is less than or equal to 0.0025 percent, P is less than or equal to 0.015 percent, and S is less than or equal to 0.0020 percent; after the components of the molten steel are qualified, Ca is added into the molten steel, the Ca/S in the molten steel is controlled to be 1.5-3.0, the casting temperature is controlled to be 1533-1548 ℃, and when the conditions are met, the segregation of a continuous casting billet is improved by adopting electromagnetic stirring and dynamic soft reduction, and the low power rating of the casting billet is controlled to be within the C1.0 level;
2) casting into a casting blank with the thickness of 230mm, and then heating and preserving heat in a heating furnace; the heating temperature is 1150-1180 ℃; according to different charging temperatures, the in-furnace time is 160-260 min, and the heat preservation time at the high temperature of 1150-1180 ℃ is 60-100 min;
3) hot rolling for three stages; in the first stage, 1-pass large deformation rolling is carried out in an austenite region at a high temperature of above 1050 ℃, and the first-pass reduction rate is 13%; in the second stage, 5-pass large deformation rolling is adopted within a temperature range of 950-1050 ℃ close to the recrystallization stop temperature, the reduction rate of each pass is greater than 10%, the accumulated reduction rate of rough rolling is 57%, and the thickness of the intermediate blank after rough rolling is 100 mm; the third stage is a finish rolling stage, rolling and cooling control are carried out, the finish rolling initial rolling temperature is set to be less than or equal to 950 ℃, rolling is carried out on an austenite non-recrystallization region, the accumulated reduction rate of finish rolling is 60%, and the finish rolling temperature is 810-830 ℃;
4) after finishing finish rolling, before accelerated cooling, relaxation is carried out for a period of time, the open cooling temperature is ensured to be 760-780 ℃, a part of fine pro-eutectoid ferrite is separated out in the air cooling relaxation process before accelerated cooling, and diffusion carburization is carried out to the residual austenite which does not generate phase transformation, so that the residual austenite is rich in carbon, then accelerated cooling is carried out, the residual austenite which does not generate phase transformation is converted into a fine and uniform bainite/acicular ferrite structure, the open cooling temperature cannot be lower than 760 ℃, the large-size ferrite proportion in the structure is prevented from being greatly increased, the cooling rate of accelerated cooling is 10-25 ℃/s, the final cooling temperature is designed to be 350-550 ℃, the temperature fluctuation of the whole plate in the width direction is controlled to be less than or equal to 30 ℃, and the anisotropy.
Table 4 shows the chemical compositions and weight percentages of the steels of examples 6 to 10 of the present invention
| Elemental composition | C | Si | Mn | P | S | Cr | Ni | Mo | Nb | V | Alt | Ti |
| Example 6 | 0.040 | 0.20 | 1.15 | 0.011 | 0.0010 | 0.15 | 0.25 | 0.17 | 0.060 | 0.045 | 0.025 | 0.018 |
| Example 7 | 0.044 | 0.25 | 1.25 | 0.012 | 0.0008 | 0.10 | 0.29 | 0.25 | 0.048 | 0.035 | 0.035 | 0.017 |
| Example 8 | 0.050 | 0.23 | 1.10 | 0.011 | 0.0009 | 0.20 | 0.34 | 0.17 | 0.042 | 0.033 | 0.033 | 0.015 |
| Example 9 | 0.046 | 0.15 | 1.18 | 0.013 | 0.0010 | 0.16 | 0.20 | 0.20 | 0.04 | 0.05 | 0.034 | 0.011 |
| Example 10 | 0.045 | 0.30 | 1.25 | 0.012 | 0.0009 | 0.2 | 0.30 | 0.15 | 0.05 | 0.03 | 0.019 | 0.018 |
Table 5 is a table of rolling and cooling temperatures for the preparation processes of examples 6 to 10 of the present invention
The mechanical properties, low-temperature impact properties, corrosion resistance and the like of the steel materials obtained in examples 6 to 10 were respectively tested, and the results are shown in Table 6.
Table 6 shows the results of the performance tests of the steels obtained in examples 6 to 10 of the present invention
The results show that the comprehensive performance of the steel obtained in the embodiments 6-10 of the invention can completely meet the requirement of X70MOS technical index, the yield ratio is low (less than or equal to 0.82), the toughness is good, and the steel has H resistance2And S has good corrosion performance. The representative microstructure is shown in figure 2, and the microstructure type is small-size quasi-polygonal ferrite + bainite/acicular ferrite.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the present invention. The invention can be realized by all the raw materials listed in the invention, and the invention can be realized by the upper and lower limit values and interval values of all the raw materials, and the examples are not listed. Further, the applicant intends to point out that modifications and variations may be made in accordance with the above teachings while remaining within the spirit and principles of the present invention, and all such modifications and variations are intended to fall within the scope of the appended claims.
Claims (8)
1. The X70 grade ocean acid-resistant pipeline steel with the extra-thick wall is characterized by comprising the following chemical components in percentage by mass: 0.03-0.05% of C, 0.10-0.30% of Si, 1.0-1.3% of Mn, 0.2-0.3% of Cr, 0.2-0.4% of Ni, 0.10-0.20% of Mo0.04-0.07% of Nb0.03-0.05% of V, 0.01-0.02% of Ti, 0.01-0.045% of Al and the balance of Fe.
2. The extra thick wall X70 grade marine acid-resistant pipeline steel according to claim 1, wherein the extra thick wall is 25mm or more in thickness.
3. A low compression ratio thick gauge pipeline steel as claimed in claim 1, wherein the metallographic structure thereof includes quasi-polygonal ferrite, bainite and acicular ferrite.
4. The preparation method of the extra-thick-wall X70 grade marine acid-resistant pipeline steel as claimed in any one of claims 1-3, which comprises smelting, continuous casting, billet heating and controlled rolling and controlled cooling processes, wherein the controlled rolling and controlled cooling processes comprise steps of controlled rolling, relaxation and ultra-fast cooling, and the controlled rolling process adopts three-stage controlled rolling: in the first stage, 1-3 times of large deformation rolling is carried out in a high-temperature austenite region with the temperature of more than 1080 ℃, the single-pass reduction rate is more than or equal to 10 percent, and the first-stage reduction rate is more than or equal to 30 percent; in the second stage, 1-5 times of large deformation rolling is adopted within the temperature range of 1000-1080 ℃, the single-pass reduction rate is more than or equal to 15%, and the second stage reduction rate is more than or equal to 30%; the third stage is a finish rolling stage, the initial rolling temperature is less than or equal to 950 ℃, and the accumulated reduction rate is more than 55 percent;
the relaxation step ensures that the cooling starting temperature is 760-780 ℃ and the cooling starting temperature cannot be lower than 760 ℃.
5. The production process according to claim 4, wherein the ultrafast cooling rate is 10 to 35 ℃/s, the final cooling temperature is 350 to 550 ℃, and the temperature fluctuation in the full width direction of the plate is controlled to be less than or equal to 30 ℃.
6. The production process of claim 4, wherein the clean steel smelting technology is adopted for smelting, the content of harmful elements in the molten steel is controlled, Ca is added into the molten steel, and the Ca/S ratio in the molten steel is controlled to be 1.5-3.0; the content requirements of the harmful elements are as follows: less than or equal to 0.0050 percent of N, less than or equal to 0.0002 percent of H, less than or equal to 0.0025 percent of O, less than or equal to 0.015 percent of P, and less than or equal to 0.0020 percent of S.
7. The production process according to claim 4, wherein the degree of superheat of the continuous casting process is controlled to be 10-25 ℃; and electromagnetic stirring and dynamic soft reduction are adopted to improve the segregation of the continuous casting billet, and the low power rating of the casting billet is controlled within C1.0 level.
8. The production process according to claim 4, wherein the heating temperature of the casting blank is 1150-1180 ℃, and the soaking is carried out for 60-120 min.
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