CN114086051B - High-strength high-toughness easily-welded nano steel with thickness of 60-120 mm and thickness of 850MPa and preparation method thereof - Google Patents
High-strength high-toughness easily-welded nano steel with thickness of 60-120 mm and thickness of 850MPa and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a high-strength high-toughness easy-to-weld steel with the thickness of 60-120 mm and 850MPa, and a preparation method thereof, wherein the composition is as follows: and C:0.05 to 0.08, si:0.25 to 0.5, mn: 0.8-1.5, P is less than or equal to 0.01, S is less than or equal to 0.0015, cu:1.0 to 1.5, ni:3.5 to 5.0, cr:0.7 to 1.5, mo:0.5 to 1.0, nb:0.02 to 0.1, ti:0.01 to 0.05, al: 0.005-0.05, and the balance of Fe and unavoidable impurities, comprising the following steps: smelting and refining-casting-rolling-heat treatment; the nano steel has the yield strength of more than or equal to 850MPa, the Charpy V notch impact energy of more than or equal to 150J at-84 ℃, the elongation rate of more than or equal to 15 percent, and the characteristics of high strength, high toughness, high plasticity and easy welding.
Description
Technical Field
The invention belongs to the field of alloy steel, and particularly relates to high-strength high-toughness easy-to-weld steel with the thickness of 60-120 mm and the thickness of 850MPa, which can be used in the fields of ships, ocean engineering equipment, pipelines, heavy machinery equipment and the like.
Background
In China, a steel plate with the thickness of more than 60mm is generally called as an extra thick plate with the thickness of 60-120 mm, and the extra thick plate can be widely applied to the fields of ocean engineering equipment, high-rise buildings, pressure vessels, chemical reaction synthesis towers, bridges, armors and the like. The strengthening mechanism of the traditional super-thick high-strength steel mainly comprises a martensite or lower bainite structure with higher carbon content. In order to ensure sufficient hardenability, a relatively high content of an alloy element such as C, ni, cr, mo is generally added, resulting in poor toughness of the material, and also in poor weldability due to an increase in carbon equivalent.
The nano-phase strengthening is a strengthening method for effectively improving the strength without losing the toughness, the Cu nano-precipitation phase strengthening is used for replacing the traditional carbon strengthening, a good strengthening effect can be achieved at the center of the super-thick plate, and meanwhile, better welding performance can be achieved due to the ultra-low carbon content. At present, high-strength steel with the grade of more than 800MPa still mostly adopts the component design of low carbon and micro-alloy elements, and the toughness and welding performance are not ideal due to the higher carbon content.
In the patent document with publication number of CN112143958A, a 1000 MPa-grade steel plate with super-thick, super-high toughness and excellent weldability is disclosed, the yield strength is more than 890MPa, a process of rolling in a non-recrystallized zone is adopted to obtain smaller grain size, meanwhile, a TMCP+ off-line modulation process is adopted, the nickel element content is high, vanadium and boron are required to be added to improve the hardenability, and the carbon content is high and is more than 0.08%. In addition, the steel only has the impact performance of minus 60 ℃, and compared with the high-strength high-toughness easy-to-weld steel with the thickness of 60-120 mm and the thickness of 850MPa, the steel has better welding performance and low-temperature toughness and can be used under more severe conditions.
In the patent document with publication number CN106544590A, a 1000 MPa-grade high-toughness high-performance uniformity easily-welded super-thick steel plate and a manufacturing method thereof are disclosed, wherein the thickness of the steel plate reaches 180mm thick while the yield strength is kept to be more than 1000MPa, but the carbon content is higher than 0.1%, and the impact energy is less than 100J at minus 40 ℃.
In the patent document with publication number of CN106636961A, a Cu nano-phase reinforced easy-to-weld steel and a preparation method are disclosed, wherein the content of aluminum is relatively high, meanwhile, the preparation of steel with the thickness of less than 15mm is only carried out, and the steel plate with the thickness of more than 15mm is not explained.
In view of the above, there is no large-thickness steel material capable of satisfying high strength, high toughness and good welding performance at the same time, which is a great challenge for the conventional organization design concept and heat treatment process.
Disclosure of Invention
The invention aims to: the invention aims to provide high-strength high-toughness easy-to-weld steel with the thickness of 60-120 mm and the thickness of 850MPa and the preparation method thereof, which can meet the requirements on high strength, high toughness and good welding performance of a steel plate, wherein the yield strength of the steel plate is more than or equal to 850MPa, the elongation is more than or equal to 15 percent, and the Charpy V notch impact energy at minus 84 ℃ is more than or equal to 150J through a solution treatment and aging treatment two-step heat treatment process.
The technical scheme of the invention is as follows:
a preparation method of a high-strength high-toughness easily-welded nano steel with the thickness of 60-120 mm and the thickness of 850MPa comprises the following steps:
(1) Smelting and refining: molten iron smelted by adopting an electric furnace, oxygen blowing dephosphorization decarburization, aluminum deoxidation, refining in a ladle furnace, and adding alloy materials at the same time, wherein the mass percentages of elements in the alloy are as follows: 0.05 to 0.08, si:0.25 to 0.5, mn: 0.8-1.5, P is less than or equal to 0.01, S is less than or equal to 0.0015, cu:1.0 to 1.5, ni:3.5 to 5.0, cr:0.7 to 1.5, mo:0.5 to 1.0, nb:0.02 to 0.1, ti:0.01 to 0.05, al: 0.005-0.05, and balancing Fe and unavoidable impurities, adjusting the components to target components, and then performing dehydrogenation and deoxidation in a VD vacuum furnace;
(2) Casting into steel ingots: casting molten iron after smelting into steel ingots by adopting a die casting method, stacking and slowly cooling the ingot for more than 24 hours;
(3) Cogging: heating the steel ingot at 1150-1200 ℃ for 18-24 h, hot rolling and cogging, and rolling at high temperature and high pressure for ensuring the strength of the super-thick plate, wherein the reduction of at least 3 passes is more than 45mm, and the final rolling temperature is higher than 900 ℃; rolling: heating the billet to 1150-1200 ℃, preserving heat for 2-6 hours, removing iron scales by using high-pressure water before rolling, removing phosphorus by using high-pressure water in the rolling process, wherein the rolling comprises rough rolling and finish rolling, and the rough rolling temperature is controlled to be 1000-1150 ℃; the initial rolling temperature of the finish rolling is 950-1050 ℃, the final rolling temperature is higher than 900 ℃, and the finish rolling is performed to obtain a 60-120 mm thick steel plate;
(4) And (3) heat treatment: after the steel plate is subjected to heat preservation for 60-300 minutes at 800-950 ℃, carrying out ultra-fast quenching to room temperature, wherein the quenching maintenance time is tquenching maintenance=30+ (H-10) multiplied by 1.5, the unit is min, and the H is the thickness of the finished steel plate, and the unit is mm; and tempering at 550-700 ℃ for 120-540 minutes, and air cooling to room temperature, wherein the tempering maintaining time is t tempering maintaining=60+ (H-10) ×2.5, the unit is min, and the unit is mm of the thickness of the finished steel plate.
A60-120 mm thick 850MPa high-strength high-toughness easily-welded nano steel microstructure consists of an ultralow-carbon lath martensitic structure and flaky contravariant austenite.
The yield strength of the nano steel is more than or equal to 850MPa, the Charpy V notch impact energy at-84 ℃ is more than or equal to 150J, and the elongation is more than or equal to 15%.
Compared with the prior art, the invention has the following beneficial effects:
1. the high-strength high-toughness easy-to-weld steel with the thickness of 60-120 mm and the height of 850MPa replaces the traditional carbon reinforcement by using the copper-rich nano precipitation phase reinforcement, and has low carbon content and good welding performance; meanwhile, the content of alloy elements is low, and the cost is low. The high strength and toughness improves the safety and stability of the large and heavy steel structure, the good weldability saves the cost of manufacturing components, particularly for the ultra-high strength steel plate, the sensitivity of welding cold cracks is greatly reduced, the welding preheating, the post-heating temperature is reduced, the heat input and output range is wider, and the cost is greatly reduced.
2. The high-strength high-toughness easy-to-weld steel with the thickness of 60-120 mm and the thickness of 850MPa fully plays the potential of alloy element hardenability through regulating and controlling rolling and heat treatment processes, effectively refines the sizes of prior austenite and martensite lath bundles, ensures high-density large-angle grain boundaries, and obtains excellent performances that the yield strength is more than or equal to 850MPa, the elongation is more than or equal to 15%, and the Charpy V notch impact energy at minus 84 ℃ is more than or equal to 150J.
Drawings
FIG. 1 is an optical micrograph of example 1;
FIG. 2 is an engineering stress strain curve for example 1.
Detailed Description
The invention will now be further described with reference to the accompanying drawings and examples:
the invention relates to a high-strength high-toughness easy-to-weld steel with the thickness of 60-120 mm and the thickness of 1000MPa, and a preparation method thereof, wherein the high-strength steel comprises the following components: and C:0.05 to 0.08, si:0.25 to 0.5, mn: 0.8-1.5, P is less than or equal to 0.01, S is less than or equal to 0.0015, cu:1.0 to 1.5, ni:3.5 to 5.0, cr:0.7 to 1.5, mo:0.5 to 1.0, nb:0.02 to 0.1, ti:0.01 to 0.05, al: 0.005-0.05, and the balance of Fe and unavoidable impurities.
The invention principle and component design basis of the 850 MPa-level high-strength high-toughness easy-to-weld steel are as follows:
the principle of the invention: the microstructure of the high-strength high-toughness easy-to-weld steel with the thickness of 60-120 mm and the height of 850MPa is lath martensite, copper-rich nickel-aluminum composite nano precipitated phase and inverted austenite. The high strength of the steel of the invention comes mainly from four aspects: the method comprises the steps of precipitation strengthening of a copper-rich nano phase, solid solution strengthening of alloy elements, fine grain strengthening of lath martensite and dislocation strengthening. The precipitation strengthening mainly comes from the fact that copper, nickel and manganese elements added into the alloy are separated out in the aging process and are uniformly distributed in a matrix phase, dislocation movement is prevented to play a role in strengthening, and in the invention, the strengthening effect of more than 200MPa can be achieved at the center and the surface simultaneously. The uniform distribution of alloy components can be ensured through LF ladle refining, thereby ensuring the effects of precipitation strengthening and solid solution strengthening and ensuring that the two strengthening modes can play the same role on the core part and the surface of the medium plate. The fine grain strengthening firstly is to pin grain boundaries in a recrystallization rough rolling stage and then further refine austenite grain sizes in a non-crystallization region finish rolling stage due to the fact that the effective grain sizes of lath martensite are lath bundles, and the lath bundles are only a fraction of the original austenite sizes, so that the fine grain strengthening brings great strength contribution. Dislocation enhancement results mainly from high density dislocations in the lath martensite.
In order to ensure that enough lath martensite or lower bainite can be obtained after quenching, the ideal critical diameter DI is adopted to simulate and calculate the hardenability of different alloy components, the alloy components are adjusted to enable DI to be larger than 1.5 times of the thickness of the steel plate, so that the core part of the medium plate is ensured to have enough proportion of martensite to provide fine grain strengthening and dislocation strengthening, and the strength of the material is ensured.
The parameters of the ideal critical diameter DI formula adopted by the invention are obtained by the experimental summary of the series of high-strength steels, which are more in line with the types of the steels, and the ideal critical diameter DI is calculated according to the factors of carbon and alloy elements.
D I ＝25.4·f C ·f Mn ·f Si ·f Ni ·f Cr ·f Mo ψf Cu (mm)
The toughness of the steel mainly comes from the blocking effect of lath martensite or lower bainite on crack propagation and the passivation of crack tips by reverse transformed austenite to inhibit crack initiation and propagation, and simultaneously, the content of impurity elements such as phosphorus, sulfur and the like and the control of impurities are controlled. The quantity density of lath martensite at the steel core of the medium-thickness plate is controlled by regulating and controlling the ideal critical diameter DI of the alloy, and the content of impurity elements and inclusions is controlled by VD vacuum degassing, so that the toughness of the steel core of the medium-thickness plate can reach a higher level.
The component design is based on:
c: carbon is a solid solution strengthening element and plays an important role in improving strength. The traditional steel materials are mainly enhanced in strength by solid solution strengthening of carbon, but excessive carbon can form massive brittle cementite during tempering to seriously affect toughness, and meanwhile, the increase of high carbon content can affect weldability. The invention replaces the traditional carbon reinforcement by using nano phase reinforcement, so the carbon content is controlled between 0.05 and 0.08 percent.
Cu: copper is the most important forming element of the precipitated phase, the strength can be improved under the condition of not losing the toughness by forming the nano-scale precipitated phase, and meanwhile, copper has the effect of refining grains. Too low copper content can affect the strengthening effect, and too high copper content can easily produce thermal embrittlement, affecting welding and thermal processing. Therefore, the copper content of the invention is controlled to be 1.0-1.5%.
Ni: nickel is one of main elements formed by nano precipitated phases, and forms a B2 ordered structure to wrap the surface of a precipitated phase formed by copper elements, so that the thermal stability of the precipitated phase can be improved; meanwhile, nickel is used as an austenite stabilizing element, so that the size of the martensite lath bundles can be thinned, and the low-temperature toughness is remarkably improved; nickel in the copper-containing steel can also eliminate copper embrittlement and reduce intergranular cracking in the hot rolling process; for extra thick plates, sufficient nickel content is necessary to improve hardenability. Therefore, the nickel content of the invention is controlled to be 3.5-5.0%.
Mn: manganese is one of main constituent elements of nano precipitated phase, and can refine crystal grains, improve strength and low-temperature toughness of steel, but the content is too high, so that casting blank segregation, large structural stress, welding performance reduction and the like are easily caused, and the manganese content is controlled to be 0.8-1.5%.
Al: aluminum is a strong deoxidizing element in the steelmaking process, and can play a role in refining grains, but when the content is too high, the graphitization tendency of carbon in steel is promoted, the effect of refining grains is reduced, and the aluminum content is controlled to be 0.005-0.05%.
Cr: chromium can increase corrosion resistance of steel while improving hardenability and tempering stability of steel. The thickness of the steel plate is thicker, so the chromium content is controlled to be 0.7-1.5%.
Mo: molybdenum can increase the hardenability of steel, refine grains, form carbide to improve strength, and promote nucleation of nano-precipitated phase. The thickness of the steel plate is thicker, and the molybdenum content is controlled to be 0.5-1.0.
Nb: niobium can form a carbonitride pinning austenite grain boundary to prevent the growth of grains, and can play a role in precipitation strengthening to improve the strength. The niobium content of the invention is controlled between 0.02 and 0.1 percent.
Ti: titanium may form carbonitride pinning grain boundaries, refining the grains. The niobium content of the invention is controlled to be 0.01 to 0.05 percent
The invention relates to 850 MPa-level high-strength high-toughness easy-to-weld steel and a preparation method thereof, comprising the following steps:
smelting molten iron in an electric furnace, oxygen blowing dephosphorization decarburization, LF ladle refining, VD vacuum furnace treatment, casting, cogging, rolling, quenching, tempering, flaw detection and performance inspection;
the specific operation of the main procedures is as follows:
1) Smelting and refining: molten iron smelted by adopting an electric furnace is subjected to oxygen blowing dephosphorization decarburization, aluminum deoxidation, then transferred into a ladle furnace for refining, alloy materials are added simultaneously, the components are adjusted to target components, and then dehydrogenation deoxidation is carried out in a VD vacuum furnace;
2) Casting into steel ingots: casting molten iron after smelting into steel ingots by adopting a die casting method, stacking and slowly cooling the ingot for more than 24 hours;
3) Cogging: the steel ingot is heated for 18 to 24 hours at 1150 to 1200 ℃, hot rolling cogging is adopted after heating, high-temperature large-pressure rolling is adopted for ensuring the strength of the super-thick plate, the reduction of at least 3 passes is more than 45mm, and the final rolling temperature is more than 900 ℃.
3) Rolling: heating the billet to 1150-1200 ℃, preserving heat for 2-6 hours, removing iron oxide scale by high-pressure water before rolling, and removing phosphorus by high-pressure water in the rolling process.
The rolling comprises rough rolling and finish rolling, and the rough rolling temperature is controlled to be 1000-1150 ℃; the initial rolling temperature of the finish rolling is 950-1050 ℃, the final rolling temperature is higher than 900 ℃, and the steel plate with 60-120 mm thickness is rolled.
And (3) heat treatment: after the steel plate is subjected to heat preservation for 60-300 minutes at 800-950 ℃, carrying out ultra-fast quenching to room temperature, wherein the quenching maintenance time is tquenching maintenance=30+ (H-10) multiplied by 1.5, the unit is min, and the H is the thickness of the finished steel plate, and the unit is mm; and tempering at 550-700 ℃ for 120-540 minutes, and air cooling to room temperature, wherein the tempering maintaining time is t tempering maintaining=60+ (H-10) ×2.5, the unit is min, and the unit is mm of the thickness of the finished steel plate.
The chemical compositions of the embodiment of the invention are shown in table 1 (mass percent), and the balance is Fe and unavoidable impurities.
TABLE 1
| C | Si | Mn | P | S | Cu | Ni | Cr | Mo | Nb | Ti | Al | |
| Example 1 | 0.055 | 0.32 | 1.0 | 0.006 | 0.0008 | 1.5 | 4.0 | 1.05 | 0.98 | 0.045 | 0.017 | 0.037 |
The embodiment adopts electric furnace smelting, oxygen blowing dephosphorization decarburization, aluminum deoxidation, ladle furnace refining, deep desulfurization, heating, refining treatment and component adjustment to target components, argon is blown into molten steel from an air brick at the bottom of a ladle to stir so as to ensure uniform components, refining treatments such as degassing, inclusion removal and the like are carried out in a VD vacuum furnace, gas and inclusion are fully removed, the purity of the molten steel is ensured, steel ingots are finally cast, and stacking and slow cooling are carried out for 48 hours;
the steel ingot is heated for 18 to 24 hours at 1150 to 1200 ℃, hot rolling cogging is adopted after heating, high-temperature large-pressure rolling is adopted for ensuring the strength of the super-thick plate, the reduction of at least 3 passes is more than 45mm, and the final rolling temperature is more than 900 ℃.
And then reheating the billet to 1160-1200 ℃, preserving heat for 2-6 hours, and rolling. The scale is removed by high-pressure water before rolling, and the high-pressure water is used for removing phosphorus in the rolling process. The rolling comprises two steps of rough rolling and finish rolling, wherein the rough rolling temperature is controlled to be 1000-1150 ℃; the initial rolling temperature of the finish rolling is 950-1050 ℃, and the final rolling temperature is higher than 900 ℃ to roll the steel plate.
After the steel plate is subjected to heat preservation for 60-300 minutes at 800-950 ℃, ultra-fast water cooling quenching is carried out to room temperature; and tempering at 550-700 deg.c for 120-540 min and air cooling to room temperature. Table 2 shows the main rolling process parameters of the examples.
TABLE 2
Table 3 shows the heat treatment process parameters.
TABLE 3 Table 3
The heat-treated steel sheet was subjected to transverse sampling processing into tensile and impact test pieces, and mechanical properties were measured, and the results are shown in Table 4.
TABLE 4 Table 4
FIG. 1 is an optical micrograph of the steel sheet of example 1, which is lath martensite. The structure not only ensures that the steel has better toughness, but also ensures better elongation.
Fig. 2 shows the tensile curve of the steel sheet of example 1.
The invention has wide application, and can be applied to key structures such as ships, ocean engineering, aerospace engineering and the like.
The invention discloses a high-strength high-toughness easy-to-weld steel with the thickness of 60-120 mm and 850MPa, and a preparation method thereof, wherein the high-strength high-toughness easy-to-weld steel comprises the following components: and C:0.05 to 0.08, si:0.25 to 0.5, mn: 0.8-1.5, P is less than or equal to 0.01, S is less than or equal to 0.0015, cu:1.0 to 1.5, ni:3.5 to 5.0, cr:0.7 to 1.5, mo:0.5 to 1.0, nb:0.02 to 0.1, ti:0.01 to 0.05, al: 0.005-0.05, and the balance of Fe and unavoidable impurities. The preparation method of the high-strength high-toughness easy-to-weld steel comprises the following steps: smelting and refining-casting-rolling-heat treatment. According to the ultra-thick plate steel, under the condition of ultra-low carbon content, the content of nano precipitated phase forming elements and a thermo-mechanical treatment process are adjusted, a large amount of nano precipitated phases are separated out to improve strength, and meanwhile, the plasticity and low-temperature toughness are optimized by controlling inverted austenite form, distribution and volume fraction, so that the Charpy V notch impact energy at-84 ℃ is more than or equal to 150J, the elongation is more than or equal to 15%, and the ultra-thick plate steel has the characteristics of high strength, high toughness, high plasticity and easiness in welding. The high-strength high-toughness easy-to-weld steel can be widely applied to key structures such as ships, ocean engineering, engineering machinery, bridges, oil pipelines, aerospace engineering and the like.
It should be noted that the above is merely an example of the present invention, and it is obvious that the present invention is not limited to the above example, but many similar variations are possible as required. All modifications and variations therein may occur to those skilled in the art upon the direct derivation or communication from this disclosure.
Claims (3)
1. The preparation method of the high-strength high-toughness easily-welded nano steel with the thickness of 60-120 mm and the thickness of 850MPa is characterized by comprising the following steps:
(1) Smelting and refining: molten iron smelted by adopting an electric furnace, oxygen blowing dephosphorization decarburization, aluminum deoxidation, refining in a ladle furnace, and adding alloy materials at the same time, wherein the mass percentages of elements in the alloy are as follows: 0.055 to 0.08, si:0.25 to 0.5, mn: 0.8-1.5, P is less than or equal to 0.01, S is less than or equal to 0.0015, cu:1.0 to 1.5, ni:3.5 to 5.0, cr:1.05 to 1.5, mo:0.5 to 1.0, nb:0.02 to 0.1, ti:0.01 to 0.05, al: 0.005-0.037, the balance being Fe and unavoidable impurities, adjusting the components to target components, and then performing dehydrogenation and deoxidation in a VD vacuum furnace;
(2) Casting into steel ingots: casting molten iron after smelting into steel ingots by adopting a die casting method, stacking and slowly cooling the ingot for more than 24 hours;
(3) Cogging: heating the steel ingot at 1150-1200 ℃ for 18-24 h, hot rolling and cogging, and rolling at high temperature and high pressure for ensuring the strength of the super-thick plate, wherein the reduction of at least 3 passes is more than 45mm, and the final rolling temperature is higher than 900 ℃; rolling: heating the billet to 1150-1200 ℃, preserving heat for 2-6 hours, removing iron scales by using high-pressure water before rolling, removing phosphorus by using high-pressure water in the rolling process, wherein the rolling comprises rough rolling and finish rolling, and the rough rolling temperature is controlled to be 1000-1150 ℃; the initial rolling temperature of the finish rolling is 950-1050 ℃, the final rolling temperature is higher than 900 ℃, and the finish rolling is performed to obtain a 60-120 mm thick steel plate;
(4) And (3) heat treatment: after the steel plate is subjected to heat preservation for 60-300 minutes at 800-950 ℃, carrying out ultra-fast quenching to room temperature, wherein the quenching maintenance time is tquenching maintenance=30+ (H-10) multiplied by 1.5, the unit is min, and the H is the thickness of the finished steel plate, and the unit is mm; and tempering at 550-700 ℃ for 120-540 minutes, and air cooling to room temperature, wherein the tempering maintaining time is t tempering maintaining=60+ (H-10) ×2.5, the unit is min, and the unit is mm of the thickness of the finished steel plate.
2. A nanosteel prepared by the method of claim 1 wherein the microstructure consists of ultra-low carbon lath martensitic structure and lamellar inverse austenite.
3. The nanosteel of claim 2, wherein the yield strength is not less than 850MPa, the charpy V-notch impact energy at-84 ℃ is not less than 150J, and the elongation is not less than 15%.
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