CN111057965B - Ocean engineering steel with low yield ratio and preparation method thereof - Google Patents
Ocean engineering steel with low yield ratio and preparation method thereof Download PDFInfo
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Abstract
The invention relates to low-yield-ratio steel for ocean engineering and a preparation method thereof, belongs to the technical field of steel for ocean engineering, and solves the problems of high yield ratio, low-temperature toughness and low service safety of the conventional steel for ocean engineering. The ocean engineering steel comprises the following components in percentage by mass: c: 0.045% -0.05%, Ni: 3.0% -3.2%, Cu: 1.4% -1.5%, Mo: 0.20-0.50%, Cr: 0.62-1.50%, Mn: 0.82% -0.90%, Si: 0.05-0.15%, Nb: 0.010% -0.030%, Ti: 0.008-0.018%, Al: 0.02-0.04%, P is less than or equal to 0.007%, S is less than or equal to 0.002%, and the balance is Fe and inevitable impurities. The preparation method of the steel for ocean engineering adopts a heat treatment process of twice quenching and three times tempering. The invention can reduce the yield ratio of the steel for ocean engineering and improve the low-temperature toughness of the steel.
Description
Technical Field
The invention relates to the technical field of steel for ocean engineering, in particular to steel for ocean engineering with low yield ratio and a preparation method thereof.
Background
The service conditions of ships and ocean engineering equipment are very severe, and the influence of various severe sea conditions such as storms, sea waves, tides and glaciers must be fully considered. In recent years, along with the large-scale development of ocean engineering equipment, higher and higher requirements are provided for the safety of ocean engineering structures and the performance of steel plates used, wherein the high strength is an important trend of large-scale ocean engineering spare steel, however, for the traditional low alloy steel, along with the improvement of the strength, the yield ratio is often higher and even exceeds 0.95 due to the unicity of the structure; in order to improve the safety of equipment, the yield ratio of steel needs to be controlled to the maximum extent, and a lower yield ratio means that the steel plate has higher bearing capacity when being subjected to external force, so that structural deformation or cracking can not occur, and the safety of the equipment is improved. However, for the design of high strength steel, it is usually difficult to obtain a low yield ratio by adding more alloying elements to the steel or by performing a second phase precipitation to obtain a high strength. The increase of soft phase structures (such as retained austenite, reversed transformed austenite and the like) in the steel is beneficial to reducing the yield ratio of the steel and also can obviously improve the low-temperature toughness of the steel, wherein the retained austenite is usually formed in the quenching process, and the content of the retained austenite is directly related to the alloy components of the steel; the reverse transformation austenite is usually formed in the high-temperature tempering or aging process, has a large relation with the heat treatment process, and is promoted to form in the steel by adopting methods of quenching in a critical zone, tempering, raising the tempering temperature and the like in the prior art, but the problem of large strength loss is brought, so that the optimal performance process window is narrow, and therefore, the method for obviously increasing the content of the reverse transformation austenite is designed under the condition of ensuring small strength loss, and has very important significance.
Disclosure of Invention
In view of the above analysis, the present invention aims to provide a steel for ocean engineering with low yield ratio and a preparation method thereof, which can solve at least one of the following technical problems: (1) the yield ratio of the existing steel for ocean engineering is high; (2) low-temperature toughness and low service safety.
The purpose of the invention is mainly realized by the following technical scheme:
on one hand, the invention discloses ocean engineering steel with low yield ratio, which comprises the following components in percentage by mass: c: 0.045% -0.05%, Ni: 3.0% -3.2%, Cu: 1.4% -1.5%, Mo: 0.20-0.50%, Cr: 0.62-1.50%, Mn: 0.82% -0.90%, Si: 0.05-0.15%, Nb: 0.010% -0.030%, Ti: 0.008-0.018%, Al: 0.02-0.04%, P is less than or equal to 0.007%, S is less than or equal to 0.002%, and the balance is Fe and inevitable impurities.
In one possible design, the microstructure of the steel for ocean engineering is mainly tempered martensite and reverse transformed austenite.
On the other hand, the invention discloses a preparation method of the ocean engineering steel with low yield ratio, which adopts a heat treatment process of twice quenching and three times tempering.
In one possible design, the preparation method comprises the following steps:
step 1, smelting in a converter;
step 2, continuously casting into a steel billet;
step 3, rolling the billet in stages; water cooling is carried out after rolling;
step 4, quenching for the first time;
step 5, quenching for the second time;
step 6, tempering for the first time;
step 7, tempering for the second time;
and 8, tempering for the third time.
In a possible design, in the step 3, the billet is heated to 1100-1150 ℃ before being rolled in stages, and is subjected to homogenization treatment after being kept for 2-3 hours.
In one possible design, in step 3, the staged rolling includes rough rolling and finish rolling; wherein the rough rolling initial rolling temperature is higher than the finish rolling initial rolling temperature.
In one possible design, in step 4, the first quenching includes: and heating the rolled steel plate to an austenitizing temperature T1, preserving the heat for 60-120 min, and then carrying out water quenching.
In one possible design, in step 5, the second quench includes: and (3) heating the steel plate subjected to the primary quenching again to the temperature T2 of the two-phase region, preserving the heat for 60-120 min, and then performing water quenching.
In one possible design, step 6, the first tempering includes: loading the steel plate into a heat treatment furnace at room temperature, raising the temperature of the heat treatment furnace to T3 at a speed V1, preserving the temperature for T1 time, discharging the steel plate out of the furnace, and air-cooling the steel plate to the room temperature;
in step 7, the second tempering comprises: loading the steel plate into a heat treatment furnace at room temperature, raising the temperature of the heat treatment furnace to T4 at a speed V2, preserving the temperature for T2 time, discharging the steel plate out of the furnace, and air-cooling the steel plate to the room temperature;
in step 8, the third tempering comprises: and (3) loading the steel plate into a heat treatment furnace at room temperature, raising the temperature of the heat treatment furnace to T5 at a speed V3, preserving the temperature for T3 time, discharging the steel plate out of the furnace, and air-cooling the steel plate to room temperature.
In one possible design, T5> T4> T3.
In one possible design, t1< t2 ═ t 3.
Compared with the prior art, the invention can realize at least one of the following beneficial effects:
(1) the ocean engineering steel with low yield ratio provided by the invention has the advantages that through the optimized design of C, Si, Cr, Ni, Cu and other elements and the combination of the heat treatment process of twice quenching and three times tempering, the yield ratio of the steel is effectively reduced to be below 0.9 on the premise of ensuring that the strength of the steel is not greatly reduced, the low-temperature toughness of the steel can be greatly improved, for example, the low-temperature impact energy at-80 ℃ is more than or equal to 260J, and the safety of large-scale marine engineering equipment under the actual service working condition is further improved.
(2) According to the invention, through accurately controlling various process parameters, for example, controlling the third tempering temperature to be higher than the second tempering temperature to be higher than the first tempering temperature, the main formation of the reverse transformation austenite is ensured in the first tempering stage, the second tempering (the reverse transformation austenite nucleation can be further promoted by prolonging the time and increasing the temperature) is used for promoting the reverse transformation austenite to be re-nucleated in a new element enrichment region, the third tempering is used for further promoting the nucleation, increasing the volume fraction of the reverse transformation austenite, and ensuring the volume fraction of the reverse transformation austenite to be more than 6%, so that the yield ratio of the steel is effectively reduced to be less than 0.9, and the low-temperature toughness of the steel is greatly improved.
In the invention, the technical schemes can be combined with each other to realize more preferable combination schemes. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and drawings.
Drawings
The drawings are only for purposes of illustrating particular embodiments and are not to be construed as limiting the invention, wherein like reference numerals are used to designate like parts throughout.
FIG. 1 is a schematic view of the process of the present invention of two quenching plus three tempering;
FIG. 2 is a microstructure diagram of sample No. 1 in example of the present invention.
Detailed Description
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate preferred embodiments of the invention and together with the description, serve to explain the principles of the invention and not to limit the scope of the invention.
The application provides a low-yield-ratio steel for ocean engineering, which comprises the following components in percentage by mass: c: 0.045% -0.05%, Ni: 3.0% -3.2%, Cu: 1.4% -1.5%, Mo: 0.20-0.50%, Cr: 0.62-1.50%, Mn: 0.82% -0.90%, Si: 0.05-0.15%, Nb: 0.010% -0.030%, Ti: 0.008-0.018%, Al: 0.02-0.04%, P is less than or equal to 0.007%, S is less than or equal to 0.002%, and the balance is Fe and inevitable impurities.
Specifically, the microstructure of the low-yield-ratio steel for ocean engineering mainly comprises tempered martensite, a small amount of reverse transformation austenite, residual austenite and carbide, wherein the volume fraction of the tempered martensite is 80-92%, and the volume fraction of the reverse transformation austenite is 6-20%; the mass percent of Cr element in the reverse transformed austenite is 20-30%, the mass percent of Ni element is 40-50%, the sum of the mass percent of Cr element and Ni element in the reverse transformed austenite is more than 70% (for example 77-80%), the high content of Cr element and Ni element in the reverse transformed austenite can increase the stability of the reverse transformed austenite, ensure the stable existence of the reverse transformed austenite at room temperature, further effectively reduce the yield ratio of steel, and greatly improve the low-temperature toughness of steel.
The action and the proportion of each element in the application are as follows:
c: has obvious solid solution strengthening effect and can improve the hardenability of the steel, but the increase of the content of C in the steel is very unfavorable for the low-temperature toughness and weldability of the steel. Therefore, the invention adopts the ultra-low carbon design, and the content of C is controlled to be 0.045-0.05%.
Cu: after solid solution, the obvious precipitation strengthening effect is generated in a nanometer precipitation form in the tempering and aging process, the strength loss caused by C reduction can be compensated, and meanwhile, the seawater corrosion resistance of the steel can be improved by Cu. In order to ensure that the steel has certain strength, the content of Cu is controlled to be 1.4-1.5 percent.
Ni: the Ni is a key element of the invention, on one hand, the Ni can improve the hardenability and the low-temperature toughness of the steel, is an important element for forming reverse transformation austenite, and has a certain solid solution strengthening function; secondly, the surface hot-embrittlement phenomenon caused by the addition of Cu can be suppressed by the addition of Ni, and thirdly, Ni and Cu can be synergistically precipitated during the temper aging process, and coarsening of a Cu-rich phase is suppressed, thereby enhancing the Cu precipitation strengthening effect. The Ni content is controlled to be 3.0-3.2%.
Mo: the hardenability of the steel can be obviously improved, a certain solid solution strengthening effect is achieved, meanwhile, the tempering stability can be improved by Mo, and the tempering brittleness is obviously reduced. The content of Mo in the invention is controlled to be 0.20-0.50%.
Cr: the hardenability of the steel can be obviously improved, and meanwhile, Cr can form a compact oxidation film, so that the corrosion resistance of the steel can be obviously improved. The Cr content of the invention is controlled between 0.62 percent and 1.50 percent.
Mn: the hardenability of the steel can be obviously improved, and meanwhile, the steel has a certain solid solution strengthening effect, but when the content of Mn is too high, the corrosion resistance of the steel is reduced, and coarse M/A islands are easily formed in a welding heat affected zone, so that the low-temperature toughness of the welding heat affected zone is obviously reduced. In the invention, the Mn content is controlled to be 0.82-0.90%.
Si: one of deoxidizing elements in the steel, and simultaneously, silicon is also a non-carbide forming element, exists in a steel matrix in a solid solution form, has a certain solid solution strengthening effect, but excessive silicon is unfavorable for the low-temperature toughness of the steel, the welding cold crack and hot crack sensitivity of the steel are increased, and the Si content is controlled to be 0.05-0.15%.
Nb: the solid-dissolved Nb can improve the hardenability of the steel during high-temperature soaking, and the deformation-induced precipitation of Nb can inhibit the recrystallization of deformed austenite, so that flat austenite can be obtained during rolling in a non-recrystallization zone; the effect is not obvious when the Nb content is too low, and the effect of damaging the toughness of a welding heat affected zone is caused when the Nb content is too high. The Nb content is controlled to be 0.010-0.030 percent.
Ti: the strong carbonitride forming element, trace Ti can be combined with N in steel to form TiN, thereby preventing austenite grains from growing in soaking and also preventing austenite grains from growing in a welding heat affected zone, and improving weldability. The Ti content is controlled to be 0.008-0.018 percent.
Al: is a strong deoxidizing element, can be combined with N to form AlN, and has the functions of preventing the aging brittleness of steel and refining crystal grains. The Al content of the invention is controlled between 0.02 percent and 0.04 percent.
P and S: the impurity elements in the steel are very unfavorable for the performance of the steel, particularly the low-temperature toughness, and the content of the impurity elements in the steel is strictly controlled and is not higher than 0.007 percent and 0.002 percent respectively.
The application also provides a manufacturing method of the ocean engineering steel with low yield ratio, and a heat treatment process of twice quenching and three times tempering is adopted. The process schematic of the double quench plus triple temper is shown in figure 1 below.
Specifically, the manufacturing method of the steel with low yield ratio for ocean engineering comprises the following process steps:
step 1, smelting in a converter;
step 2, continuously casting into a steel billet;
specifically, an electromagnetic stirring process is adopted in the continuous casting process, the superheat degree of the molten steel is 10-20 ℃ (the superheat degree is larger at the position, segregation is easily caused, the superheat degree is smaller, and the molten steel is easily solidified, so that the proper superheat degree is controlled, the non-solidification of the molten steel in the casting process can be ensured, the segregation can be controlled, and the uniformity of the structure can be ensured) casting, illustratively, the superheat degree of the molten steel is 12 ℃ casting, and the blank drawing speed is 0.9 m/min; the thickness of the cast steel billet is 230 mm.
Step 3, rolling the billet in stages; water cooling is carried out after rolling;
specifically, in the step 3, the billet needs to be heated to 1100-1150 ℃ before being rolled in stages, the temperature is kept for 2-3 hours for homogenization treatment, then the billet is taken out of the furnace and subjected to high-pressure water descaling, and then the steel plate is rolled in stages (including rough rolling and finish rolling); wherein, the rough rolling is performed for 3-5 times, illustratively, the rough rolling is performed for 3 times, the rough rolling initial rolling temperature is higher than the finish rolling initial rolling temperature, specifically, the rough rolling initial rolling temperature is not lower than 1000 ℃ (for example, 1076-1100 ℃), the pass deformation of each pass of the rough rolling is 15% -25%, and the deformation of the last two passes of the rough rolling is controlled to be more than or equal to 20% (for example, 20% -25%) in order to ensure the deformation and the penetration of the core; rough rolling for 4-6 times, wherein the initial rolling temperature of finish rolling is not higher than 970 ℃ (for example 936-963 ℃), and the final rolling temperature is 700-850 ℃; and (3) after rolling, performing water cooling at the water cooling speed of 5-15 ℃/s and at the final cooling temperature of 500-650 ℃.
Specifically, the thickness of the rolled steel plate is 10-100 mm.
Step 4, quenching for the first time;
specifically, in step 4, the first quenching process includes: the rolled steel sheet is heated to an austenitizing temperature T1 (Ac)3And (3) keeping the temperature for 60-120 min at the temperature of 30-50 ℃, ensuring that the material is fully austenitized, and then performing water quenching (the water temperature of the water quenching is lower than 20 ℃, and the quenching roller speed is lower than 4m/min) to obtain martensite or lath bainite.
Step 5, quenching for the second time;
specifically, in step 5, the second quenching process includes: heating the steel plate subjected to the primary quenching again to a two-phase region temperature T2 (namely above Ac1 and below Ac 3), keeping the temperature for 60-120 min, and then performing water quenching (the water temperature of the water quenching is lower than 20 ℃, and the quenching roller speed is less than 4 m/min); the purpose is to refine grains and improve the toughness of steel, and the structure of the quenched steel plate is secondary martensite and a small amount of ferrite. When the two-phase region is heated, C diffuses into austenite, and because the temperature is relatively high, the enrichment degree of C is low in the heating process and can reach balance in the austenite, and elements such as Ni can only diffuse to the vicinity of an austenite interface. After the two-phase zone quenching, austenite is transformed into secondary martensite, and at this time, although C in the secondary martensite is relatively uniform, the alloying elements such as Ni and Mn are concentrated in the vicinity of the boundary of the secondary martensite, so that the alloying elements in this region are most concentrated, causing fluctuations in the composition of the steel and lowering the actual Ac1 point in this region.
Step 6, tempering for the first time; and (3) loading the steel plate into a heat treatment furnace at room temperature, heating the heat treatment furnace to the temperature T3 at the speed V1, preserving the temperature for T1 time, discharging the steel plate out of the furnace, and air-cooling the steel plate to the room temperature.
Specifically, in step 6, if the temperature increase rate is too high, the surface and the core of the steel sheet are heated unevenly; the excessively low speed can cause the excessively long temperature rise time and waste of energy, so that the V1 is controlled to be 8-12 ℃/min, and the preferable V1 is 10 ℃/min.
Specifically, in the step 6, the steel plate is tempered in the heat treatment furnace in the first tempering, the temperature T3 is controlled to be 10-50 ℃ below Ac1, and the temperature T1 is as follows: t1 ═ d1 × h, wherein d1 is 2.0 to 2.5min/mm, h is the plate thickness, and the unit of h is mm; this is because A in the alloy element-enriched region is presentc1Below the tempering temperature, reverse transformed austenite is formed (the reverse transformed austenite is mainly formed in the first tempering stage because of the more abundant thermodynamic and kinetic conditions in this stage); at the moment, C can still diffuse at a relatively low tempering temperature, so that C in the reversed austenite is further enriched, the stability of the austenite is greatly improved, and the temperature is kept to room temperature.
Step 7, tempering for the second time; and (3) loading the steel plate into a heat treatment furnace at room temperature, heating the heat treatment furnace to the temperature T4 at the speed V2, preserving the temperature for T2 time, discharging the steel plate out of the furnace, and air-cooling the steel plate to the room temperature.
Specifically, in step 7, if the temperature increase rate is too high, the surface and the core of the steel sheet are heated unevenly; the excessively low speed can cause the excessively long temperature rise time and waste of energy, so that the V2 is controlled to be 8-12 ℃/min, and the preferable V2 is 10 ℃/min.
Specifically, in the step 7, T4> T3, the temperature difference between T4 and T3 is 5-10 ℃, and T2 is as follows: t2 ═ d2 × h, wherein d2> d1, and illustratively, d2 is 2.5 to 3.0 min/mm; h is the plate thickness, and the unit of h is mm; the reason is that the reversed transformed austenite formed in the first stage tempering process causes a certain area of Ni-poor and Mn-poor areas around the reversed transformed austenite, which is not beneficial to the growth of the reversed transformed austenite after formation, so that the second tempering (the time is prolonged and the temperature is increased to further promote the nucleation of the reversed transformed austenite) promotes the re-nucleation of the reversed transformed austenite in a new element enrichment area, and further improves the volume fraction of the reversed transformed austenite in the steel.
8, tempering for the third time; and (3) loading the steel plate into a heat treatment furnace at room temperature, heating the heat treatment furnace to the temperature T5 at the speed V3, preserving the temperature for T3 time, discharging the steel plate out of the furnace, and air-cooling the steel plate to the room temperature.
Specifically, in step 8, if the temperature increase rate is too high, the surface and the core of the steel sheet are heated unevenly; the excessively low speed can cause the excessively long temperature rise time and waste of energy, so that the V3 is controlled to be 8-12 ℃/min, and the preferable V3 is 10 ℃/min.
Specifically, in the step 8, T5> T4, the temperature difference between T5 and T4 is 5-10 ℃, T3 is T2, and the third tempering can further promote nucleation and increase the volume fraction of the reverse transformed austenite.
Example 1
The steel plate for ocean engineering in the same furnace is adopted in the embodiment of the invention, and the chemical components of the steel plate are shown in the following table 1.
The steel plate provided by the embodiment of the invention is produced according to the following steps:
1) smelting in a converter, continuously casting into a steel billet, adopting an electromagnetic stirring process in the continuous casting process, casting the molten steel at the superheat degree of 12 ℃, and drawing at the speed of 0.9 m/min;
2) heating the steel billet to 1100-1150 ℃, and carrying out homogenization treatment for 2-3 h;
3) descaling the steel billet after discharging by high-pressure water, and then rolling the steel plate by stages; the rolled steel plates are divided into 4 types with the thicknesses of 20mm, 32mm, 35mm and 50mm, and are respectively marked as 1-4# samples; after rolling, water cooling is carried out, the water cooling speed is 5-15 ℃/s, the final cooling temperature is 500-650 ℃, and the rolling process of the 1-4# sample is shown in the following table 2;
4) the 1-4# samples were subjected to the process treatment of two times of quenching and three times of tempering, and the specific process treatment parameters are shown in the following table 3.
The microstructure of sample # 1 of this example is shown in FIG. 2, the microstructure results of sample # 1-4 are shown in Table 4 below, and the mechanical properties of sample # 1-4 are shown in Table 5 below.
Comparative example 1
The steel sheet composition of this comparative example was the same as that of example 1, as shown in table 1 below, the first 3 steps of the comparative example were the same as those of example 1, and the rolled steel sheets were divided into 4 kinds of samples having thicknesses of 20mm, 32mm, 35mm and 50mm, which were respectively designated as # 5-8; the quenching and tempering process comprises the following steps:
4) heating the steel plate to 880 ℃, preserving heat for 100-120 min to fully austenitize, and then quenching;
5) the quenched steel plate was tempered at a temperature of 620 ℃ and the specific process parameters of the 5-8# samples are shown in table 3 below.
The microstructure of the sample No. 5-8 of the comparative example is shown in the following table 4, and the mechanical properties are shown in the following table 5.
Table 1 chemical composition (mass percentage,%) of steel sheets of example 1 and comparative example 1
Table 2 steel sheet rolling process parameters of example 1 and comparative example 1
TABLE 3 Process parameters of Steel sheets of example 1(1-4#) and comparative example 1(5-8#)
TABLE 4 microstructure results of the steel sheets of example 1(1-4#) and comparative example 1(5-8#)
TABLE 5 mechanical property results of the steel sheets of example 1(1-4#) and comparative example 1(5-8#)
From the above table 4, it can be seen that the microstructure of the matrix of the samples # 1 to # 4 in example 1 of the present invention is mainly tempered martensite (for example, the volume fraction of the tempered martensite is 88.5% to 90.5%) and reversed austenite (for example, the volume fraction of the reversed austenite is 6.39% to 7.25%), the sum of the mass percentages of the Cr element and the Ni element in the reversed austenite is greater than 70% (for example, 77% to 80%), and the higher content of the Cr element and the Ni element in the reversed austenite can increase the stability of the reversed austenite, ensure the stable existence of the reversed austenite at room temperature, further effectively reduce the yield ratio of the steel, and greatly improve the low-temperature toughness of the steel.
From Table 5 above, the tensile strength Rm of the steel sheet of samples # 1 to # 4 of example 1 is not less than 820MPa (e.g., 821 to 833MPa) at room temperature; yield strength Rp of steel sheet0.2Not less than 720MPa (for example, 728 to 747 MPa); the low-temperature impact energy at minus 80 ℃ is more than or equal to 260J (for example, 268 to 299J). Therefore, by adopting the ocean engineering steel with the low yield ratio, the strength of the ocean engineering steel with the low yield ratio is ensured, the yield ratio of the steel plate is obviously reduced to be below 0.9 (such as 0.881-0.898) at the same time, and the-80 ℃ low-temperature impact toughness of the steel plate is obviously improved by optimally designing elements such as C, Si, Cr, Ni and Cu and combining the heat treatment process of twice quenching and three times of tempering and accurately controlling each process parameter. The content of the reverse transformed austenite in the steel is tested by adopting an X-ray diffraction method, and the results in Table 5 show that the content of the reverse transformed austenite in the embodiment of the invention is more than 6 percent, and the content of the reverse transformed austenite in the conventional heat treatment process is less than 2 percent. The reversed austenite phase is shown in FIG. 2 and is formed primarily at lath boundaries.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention.
Claims (6)
1. The preparation method of the steel with low yield ratio for the ocean engineering is characterized by adopting a heat treatment process of twice quenching and three times tempering; the method comprises the following steps:
step 1, smelting in a converter;
step 2, continuously casting into a steel billet;
step 3, rolling the billet in stages; water cooling is carried out after rolling;
step 4, quenching for the first time;
step 5, quenching for the second time;
step 6, tempering for the first time; the method comprises the following steps: loading the steel plate into a heat treatment furnace at room temperature, raising the temperature of the heat treatment furnace to T3 at a speed V1, preserving the temperature for T1 time, discharging the steel plate out of the furnace, and air-cooling the steel plate to the room temperature; v1 is 8-12 ℃/min;
step 7, tempering for the second time; the method comprises the following steps: loading the steel plate into a heat treatment furnace at room temperature, raising the temperature of the heat treatment furnace to T4 at a speed V2, preserving the temperature for T2 time, discharging the steel plate out of the furnace, and air-cooling the steel plate to the room temperature; v2 is 8-12 ℃/min;
8, tempering for the third time; the method comprises the following steps: loading the steel plate into a heat treatment furnace at room temperature, raising the temperature of the heat treatment furnace to T5 at a speed V3, preserving the temperature for T3 time, discharging the steel plate out of the furnace, and air-cooling the steel plate to the room temperature; v3 is 8-12 ℃/min;
the T5> T4> T3; the temperature of T3 is controlled to be 10-50 ℃ below Ac1, T1 is d1 h, wherein d1 is 2.0-2.5 min/mm, and h is the plate thickness; the temperature difference between T4 and T3 is 5-10 ℃; t2 ═ d2 × h, d2 is 2.5-3.0 min/mm; the temperature difference between T5 and T4 is 5-10 ℃, and T3 is T2;
the ocean engineering steel comprises the following components in percentage by mass: c: 0.045% -0.05%, Ni: 3.0% -3.2%, Cu: 1.4% -1.5%, Mo: 0.20-0.50%, Cr: 0.62-1.50%, Mn: 0.82% -0.90%, Si: 0.05-0.15%, Nb: 0.010% -0.030%, Ti: 0.008-0.018%, Al: 0.02-0.04%, P is less than or equal to 0.007%, S is less than or equal to 0.002%, and the balance of Fe and inevitable impurities; the microstructure of the steel for ocean engineering is mainly tempered martensite and reversed austenite.
2. The method for preparing steel for ocean engineering according to claim 1, wherein in the step 3, the billet is heated to 1100-1150 ℃ before being rolled in stages, and is subjected to homogenization treatment after being kept for 2-3 hours.
3. The method for preparing steel for ocean engineering according to claim 1, wherein in the step 3, the staged rolling includes rough rolling and finish rolling; wherein the rough rolling initial rolling temperature is higher than the finish rolling initial rolling temperature.
4. According to the claimsThe method for preparing steel for ocean engineering according to claim 1, wherein the first quenching in the step 4 comprises: heating the rolled steel plate to austenitizing temperature T1And preserving the heat for 60-120 min, and then performing water quenching.
5. The method for preparing steel for ocean engineering according to claim 1, wherein the second quenching in the step 5 comprises: the steel plate after the first quenching is heated to the temperature T of the two-phase region again2And preserving the heat for 60-120 min, and then performing water quenching.
6. The steel for ocean engineering with low yield ratio is characterized in that the steel for ocean engineering with low yield ratio is prepared by the preparation method of any one of claims 1 to 5, and the steel for ocean engineering comprises the following components in percentage by mass: c: 0.045% -0.05%, Ni: 3.0% -3.2%, Cu: 1.4% -1.5%, Mo: 0.20-0.50%, Cr: 0.62-1.50%, Mn: 0.82% -0.90%, Si: 0.05-0.15%, Nb: 0.010% -0.030%, Ti: 0.008-0.018%, Al: 0.02-0.04%, P is less than or equal to 0.007%, S is less than or equal to 0.002%, and the balance is Fe and inevitable impurities.
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