CN109652733B - 690 MPa-grade super-thick steel plate and manufacturing method thereof - Google Patents
690 MPa-grade super-thick steel plate and manufacturing method thereof Download PDFInfo
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- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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- C21D6/008—Heat treatment of ferrous alloys containing Si
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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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
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- 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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
Abstract
The invention discloses a 690 MPa-grade super-thick steel plate and a manufacturing method thereof, wherein the steel plate comprises the following chemical components in percentage by mass: c: 0.04-0.08%, Mn: 5.2-6.0%, Si: 0.1-0.4%, Mo: 0.1-0.5%, Ni: 0.2-0.6%, Cr: 0.2-0.6%, Ti: 0.01-0.05%, S: less than or equal to 0.005%, P: less than or equal to 0.010 percent and the balance of Fe and impurities. The steel sheet uses manganese as a main alloying element, and the addition amount of expensive elements such as nickel and the like is reduced during the manufacture of the super-thick steel sheet, thereby reducing the alloying cost. By adopting a specific manufacturing process, the prepared super-thick steel plate has excellent core mechanical properties of high strength, high plasticity and high toughness and lamellar tearing resistance, and can meet the requirements of severe service environments such as ocean engineering and the like on the high-performance super-thick steel plate.
Description
Technical Field
The invention relates to a super-thick steel plate and a manufacturing method thereof, in particular to a 690MPa super-thick steel plate and a manufacturing method thereof.
Background
With the implementation of national ocean development strategy and the development of oil and gas resource from land to deep sea and polar region, the requirements on the performance and the structural safety of the ocean platform are higher and higher. Steel materials required by manufacturing of the ocean platform develop towards high strength and high toughness, and the demand of the thick plate for the high-strength and high-toughness ocean platform with the yield strength of 690MPa is increasing. The mechanical property of the core of the traditional 690MPa extra-thick plate for ocean engineering is difficult to improve. In order to improve the uniformity of the properties of the entire steel sheet, a large amount of elements such as Ni, Mo, Cr, and Cu are generally added, and the total addition amount of these elements exceeds even 4%, so that the alloy cost is high. In addition, multiple quenching processes are usually required in the manufacturing method, which makes the manufacturing difficult. In recent years, in order to meet the increase in material demand in marine engineering construction, development of high-strength-grade super-thick steel plates has received much attention.
The chinese invention patent No. 201510125485.1 discloses a low yield ratio high toughness thick steel plate with excellent low temperature impact toughness and a manufacturing method thereof, wherein the chemical components of the low yield ratio high toughness thick steel plate contain 3.6-5.5% of Ni, and the cost is high.
The Chinese patent application with the application number of 201610026446.0 discloses a high-strength steel plate for ocean engineering and a production method thereof, wherein a Nb and V microalloying method is adopted, but because the content of elements such as Mn and the like for improving hardenability is not high, the maximum thickness of the steel plate cannot exceed 100 mm.
From the current situation of material development, the performance of the current high-performance extra-thick steel plate for ocean engineering needs to be improved.
Disclosure of Invention
The purpose of the invention is as follows: in order to overcome the defects of the prior art, the invention provides a 690 MPa-grade super-thick steel plate which has excellent core mechanical property and can meet the requirement of severe service environments such as ocean engineering and the like on a high-performance super-thick steel plate.
The invention also aims to provide a manufacturing method of the 690 MPa-grade super-thick steel plate.
The technical scheme is as follows: the 690 MPa-grade super-thick steel plate disclosed by the invention comprises the following chemical components in percentage by mass: c: 0.04-0.08%, Mn: 5.2-6.0%, Si: 0.1-0.4%, Mo: 0.1-0.5%, Ni: 0.2-0.6%, Cr: 0.2-0.6%, Ti: 0.01-0.05%, S: less than or equal to 0.005%, P: less than or equal to 0.010 percent and the balance of Fe and impurities.
Furthermore, the thickness of the steel plate is 80-150 mm.
Further, the microstructure of the steel sheet has martensite and austenite; wherein the volume fraction of austenite is 4-10%.
Further, the mechanical properties of the core of the steel plate are as follows: the yield strength is not less than 690MPa, the tensile strength is not less than 770MPa, the elongation after fracture is not less than 14%, and the impact absorption energy of a V-shaped sample in a Charpy pendulum impact test at-60 ℃ is not less than 80J.
Further, the steel sheet has a reduction of area of not less than 50% by drawing in the sheet thickness direction.
The invention relates to a method for manufacturing a 690 MPa-grade super-thick steel plate, which adopts the technical scheme that the method comprises the following steps:
(1) performing converter smelting after molten iron desulfurization treatment, and reducing the S, P content in the molten iron to less than or equal to 0.005 percent of S and less than or equal to 0.010 percent of P;
(2) LF refining is carried out to complete the alloying of the required mass fractions of C, Mn, Si, Mo, Ni and Ti elements, and then RH treatment is carried out, wherein the vacuum degree is less than or equal to 4mbar, and the treatment time is more than or equal to 20 min;
(3) casting to obtain a plate blank, wherein the ratio of the thickness of the plate blank to the thickness of a steel plate is more than or equal to 4;
(4) heating the plate blank at 1060-1140 ℃ for 40-90 min;
(5) rolling the heated plate blank, wherein the initial rolling temperature is less than or equal to 1020 ℃, the final rolling temperature is more than or equal to 930 ℃, and the pass deformation is more than or equal to 10%;
(6) immediately cooling the rolled steel plate by water, wherein the surface re-reddening temperature of the cooled steel plate is less than or equal to 360 ℃, and the average cooling rate is 1-5 ℃/s;
(7) quenching heat treatment, namely reheating the steel plate to 780-830 ℃, soaking for 5-15min, cooling by water until the surface re-reddening temperature of the steel plate is less than or equal to 110 ℃, and the average cooling rate is 2-8 ℃/s;
(8) tempering heat treatment, heating the quenched steel plate to 610-640 ℃, soaking for 40-70min, and air cooling the tempered steel plate to room temperature.
Has the advantages that: the steel plate of the invention takes manganese as a main alloy element, and reduces the addition amount of expensive elements such as nickel and the like during the manufacture of the super-thick steel plate, thereby reducing the alloy cost. By adopting a specific manufacturing process, the prepared super-thick steel plate has excellent core mechanical properties of high strength, high plasticity and high toughness and lamellar tearing resistance, and can meet the requirements of severe service environments such as ocean engineering and the like on the high-performance super-thick steel plate.
Drawings
FIG. 1 is a transmission electron micrograph of the core structure of the steel sheet of example 1.
Detailed Description
The chemical composition, production process, structure, and performance of the present invention will be described with reference to examples.
In the chemical composition design of the 690MPa grade super-thick steel plate of the present invention, C is an important strengthening element capable of remarkably improving the structure strength by interstitial solid solution strengthening and is also an important austenite stabilizing element, but the addition amount thereof needs to be controlled at a low level in order to ensure low-temperature impact toughness and weldability. Mn can improve the structural strength by substitution solid solution strengthening, and also can significantly improve the austenite stability. The proper amount of C and Mn can obviously improve hardenability, reduce the phase transition temperature of the super-cooled austenite and obtain a high-strength martensite structure. On the other hand, during the martensite tempering process, the addition of C and Mn lowers the temperature required to form a certain amount of reverse transformed austenite. C and Mn are enriched in the reverse transformed austenite, so that the structure can still keep stable at low temperature, and the C and Mn become important tissues for improving the plasticity and the toughness of the invention. It is to be noted that when the tempering temperature is lower than 600 ℃ in the present invention, particularly when the tempering is performed at around 550 ℃, grain boundary segregation of elements such as Mn, P, etc. is easily caused and the toughness is lowered. The inventor fully considers the action mechanism of C and Mn elements in the invention and determines the design of the low-carbon medium-manganese component with the C content of 0.04-0.08% and the Mn content of 5.2-6.0%.
Si is a deoxidizing element in the steel-making process, and a proper amount of Si can inhibit segregation of Mn and P and improve toughness. Si can also produce solid solution strengthening, but when the content exceeds 0.3%, the toughness is significantly reduced. The invention controls Si to be 0.1-0.4%.
Mo can improve the strength of the martensite after tempering, and can weaken the grain boundary segregation of Mn within a certain content range so as to improve the toughness. The invention controls the Mo content to be 0.1-0.5%, and does not increase the cost obviously while playing the Mo role.
Ni can stabilize austenite phase, improve hardenability, reduce ductile-brittle transition temperature, is an effective element for improving low-temperature toughness, and is also beneficial to improving weldability. But Ni is expensive, the invention controls the Ni content to be 0.2-0.6%, and the beneficial effect of Ni element is fully exerted without increasing the cost obviously.
Cr can generate obvious solid solution strengthening effect, is beneficial to improving the strength and can improve the corrosion resistance. However, in the case where Mn is added in a large amount in the present invention, Cr and Mn carbides are easily formed at grain boundaries during tempering due to an excessively high Cr content, and the inhibition of grain boundary propagation to crack growth is reduced, resulting in a reduction in plasticity and toughness. The invention controls the content range of Cr in a proper range of 0.2-0.6%.
In the invention, a trace amount of Ti is added, and the grain boundary migration at high temperature can be hindered by a fine and dispersed second phase precipitation form, so that the grains are refined and the mechanical property is improved, and the adding amount is controlled within the range of 0.01-0.05%.
The contents of P and S are strictly controlled, and in the case of the invention, the medium content of Mn element is added, S is easy to form MnS with Mn and reduce plasticity. P is easily segregated in the grain boundary, and the crack propagation resistance of the grain boundary is reduced, thereby reducing the toughness. The invention requires that S is less than or equal to 0.005 percent and P is less than or equal to 0.010 percent.
The remainder of the present invention is Fe, however, it is inevitable to introduce impurities from the raw materials or the surrounding environment in the usual manufacturing process. Since these impurities are obvious to those skilled in the art, their names and contents are not specifically described in the present specification.
In the manufacturing method, the content of S and P is reduced to be less than or equal to 0.005 percent and P is less than or equal to 0.010 percent by converter smelting after molten iron desulphurization treatment, the content of gas impurity elements is reduced by RH treatment with enough high vacuum degree (the vacuum degree is less than or equal to 4mbar) and enough long vacuum time (the treatment time is more than or equal to 20min), and the addition of alloys such as C, Mn, Si, Mo, Ni, Ti and the like is finished by LF refining, so that the smelting effect with high purity can be realized.
The slab casting can adopt a continuous casting or die casting plus forging mode to obtain slabs with different dimensions. The condition that the thickness of the plate blank is less than or equal to 320mm can be obtained by a continuous casting mode, and the production efficiency is high; slabs of greater thickness (> 320mm) can be die cast + forged. In order to achieve the core mechanical properties required by the present invention, a sufficient total rolling deformation amount is required as a requirement. The ratio of the thickness of the plate blank to the thickness of the steel plate is required to be more than or equal to 4, and the rolling total row variable can be ensured to be more than or equal to 75%. When the thickness of the steel plate is 80mm, the required thickness of the plate blank is more than or equal to 320 mm; when the thickness of the steel plate is 150mm, the required thickness of the plate blank is more than or equal to 600 mm. The obtained plate blank obtains the required structure and performance through rolling and heat treatment processes.
Within the composition range of the invention, the steel has an Ac3 temperature of not more than 770 ℃. When the slab is heated to 1060-1140 ℃, a high-temperature austenite structure is formed, and meanwhile, alloy elements such as C, Mn are homogenized through a diffusion mode. In the soaking process that the core temperature of the plate blank is close to the surface temperature and heat preservation is continuously carried out, the whole plate blank realizes the homogenization of austenite, and the uniform temperature-equalizing time is 40-90min, so that the uniform diffusion of elements can be ensured. At 1140 deg.C and below, the second phase particles of Ti can act as a barrier to grain growth. However, when the temperature is lower than 1060 ℃, the diffusion of elements is too slow, and the austenite homogenization efficiency is too low.
The invention performs recrystallization rolling on the heated slab at 930 ℃ or above to refine grains. The initial rolling temperature is less than or equal to 1020 ℃, so that the growth rate of the recrystallized grains can be prevented from being too high. The pass deformation amount is more than or equal to 10 percent, so that enough distortion energy can be accumulated after austenite is deformed, and the recrystallization refining effect is ensured.
After rolling of the steel sheet, in order to avoid excessive growth of recrystallized grains refined after rolling deformation, the rolled steel sheet needs to be immediately water-cooled. Martensite transformation will also occur during water cooling. Because the sufficient Mn element is added, the martensite transformation critical cooling rate is lower than 1 ℃/s, and the martensite structure can be obtained under the condition of lower cooling rate. In the case of a steel sheet with a greater thickness, the core cooling is generally significantly slower than the surface cooling, and the composition design of the invention ensures that the martensitic transformation also occurs in the core of a steel sheet with a thickness of 80-150 mm. However, the excessively high cooling speed easily causes excessively high thermal stress of the steel plate and even cracks of the steel plate, so that the average cooling rate is controlled to be not higher than 5 ℃/s by the method. The surface re-reddening temperature of the steel sheet cooled after rolling is selected to be 360 ℃ or lower, so that the remarkable element segregation in the cooling process can be avoided, the precipitation of coarse carbides is inhibited, and the temperature is lower than the martensite transformation starting temperature under the composition of the invention. The cooling process after rolling selected by the invention can provide a proper precursor structure for the subsequent heat treatment process.
The invention carries out quenching and tempering heat treatment on the steel plate. 780-830 ℃ quenching temperature is higher than Ac3, and the austenite structure is obtained by soaking. Because the steel plate is cooled by water after rolling, the element segregation and the formation of coarse carbides in the cooling process are avoided, and the element homogenization time in the austenite is greatly shortened. The soaking time of quenching and heating is selected to be 5-15min, so that the grain size can be effectively refined while the austenite homogenization is ensured, and the mechanical property of the steel plate is favorably improved. The quenching cooling rate is selected from the same reasons as those of the cooling rate after rolling, and is controlled to be 2-8 ℃/s. However, the surface reddening temperature of the quenched steel sheet is required to be 110 ℃ or lower, which is lower than the martensite finish temperature in the present invention, and a high-strength quenched martensite structure can be ensured in the entire steel sheet.
And tempering and heat treating the quenched steel plate. In the tempering process with the tempering temperature of 610-640 ℃ and the soaking time of 40-70min, the reverse transformed austenite with the volume fraction of 4-10 percent can be obtained besides improving the toughness matching of martensite, and is mainly in a film shape and distributed among martensite laths. In the tempering process, austenite stabilizing elements such as C, Mn and the like are enriched in austenite, so that the stability of the austenite is improved, and in the air cooling process to room temperature after tempering, even under the condition of lower temperature, the austenite can still keep the stability of the crystal structure without martensite transformation. The air cooling of the steel plate after tempering can reduce the thermal stress of the super-thick steel plate and improve the quality of the steel plate.
After the heat treatment of the invention, a martensite + austenite structure is obtained in the whole thickness direction of the steel sheet, especially in the core of the steel sheet. During the tensile deformation, the martensite matrix provides a yield strength of 690MPa or more, and the plasticity of the tempered martensite is also improved. Austenite acts as a soft phase to relieve local stress concentrations in the early and middle stages of deformation, while martensite can occur and produce strengthening effects in the later stages of deformation. Therefore, the existence of austenite delays the initiation and propagation of cracks, and plays an important role in improving the tensile strength and the elongation after fracture. In the impact deformation process, the existence of austenite hinders the crack propagation and improves the crack propagation work, thereby improving the impact toughness. The austenite of the present invention is still able to exert a beneficial effect on impact toughness at-60 ℃ due to its sufficient stability. The beneficial effect of the austenite structure is closely related to the volume fraction and the element enrichment degree of the austenite structure, and the selection of the process parameters of the manufacturing method, particularly the process parameters of the tempering heat treatment, most directly determines the properties of the austenite structure.
Specifically, the mechanical properties of the core of the steel plate are as follows: the yield strength is not less than 690MPa, the tensile strength is not less than 770MPa, the elongation after fracture is not less than 14%, and the impact absorption energy of a V-shaped sample in a Charpy pendulum impact test at-60 ℃ is not less than 80J.
The definition of the mechanical property index in the invention is in accordance with the standard GB/T228.1, GB/T229 and GB/T5313, and the definition of the technical index is clear to the skilled person, so the description is not excessive in the specification.
It should be noted that the invention realizes excellent mechanical property of the core of the steel plate, and the mechanical property of other positions of the thickness of the steel plate can also reach the mechanical property of the core. The invention effectively controls the structure and the performance of each position of the super-thick steel plate, so that the steel plate has very high reduction of area in the thickness direction, the reduction of area based on the stretching in the plate thickness direction is not less than 50%, and the lamellar tearing resistance is very excellent.
The steel sheet and the method for producing the same will be further described with reference to specific examples.
Example 1: a690 MPa-grade super-thick steel plate with excellent core mechanical properties is 80mm thick, and comprises the following chemical components (the content is expressed by mass percent): 0.06% C, 5.7% Mn, 0.22% Si, 0.35% Mo, 0.2% Ni, 0.31% Cr, 0.02% Ti, S.ltoreq.0.005%, P.ltoreq.0.010%, Fe as a remainder, and other unavoidable impurity elements.
The method for manufacturing the steel plate comprises the following steps:
performing converter smelting after molten iron desulfurization treatment, and reducing the S, P content in the molten iron to less than or equal to 0.005 percent of S and less than or equal to 0.010 percent of P; LF refining is carried out to complete alloying of required mass fractions of elements such as C, Mn, Si, Mo, Ni, Ti and the like, and then RH treatment is carried out, wherein the vacuum degree is 3mbar, the treatment time is 23min, and the content of gas impurity elements in molten steel is reduced; a slab with the thickness of 320mm is obtained by adopting a continuous casting mode. Heating the plate blank to 1140 ℃, and soaking for 60 min; rolling the heated plate blank, wherein the initial rolling temperature is 1005 ℃, the final rolling temperature is 952 ℃, and the rolling schedule of a rolling mill is 320-280-240-200-165-135-110-90-80 mm; and (3) immediately cooling the rolled steel plate by water, wherein the surface of the cooled steel plate returns to the red temperature of 350 ℃, and the average cooling rate is 3.1 ℃/s. And carrying out quenching and tempering heat treatment on the steel plate. The quenching temperature is 810 ℃, the soaking time is 10min, the steel plate is cooled by water to the surface re-reddening temperature of 77 ℃, and the average cooling rate is 2.9 ℃/s; tempering temperature 626 ℃, soaking time 55min, and air cooling the steel plate to room temperature after tempering.
The steel sheet structure obtained contained martensite and austenite, and the volume fraction of austenite was 6.5%. FIG. 1 is a transmission electron micrograph of the core structure of a steel plate, in which martensite and austenite are observed at intervals, wherein the light-color contrast lath-shaped structure is martensite, and the dark-color contrast thin-film structure is austenite. The yield strength of the center of the steel plate is 758MPa, the tensile strength is 842MPa, the elongation after fracture is 16 percent, and the impact absorption energy of a V-shaped test sample in a Charpy pendulum impact test at the temperature of-60 ℃ is 135J. The reduction of area in the thickness direction of the steel plate was 63%.
Example 2: a690 MPa-grade super-thick steel plate with excellent core mechanical properties is 80mm thick, and comprises the following chemical components (the content is expressed by mass percent): 0.04% of C, 5.2% of Mn, 0.4% of Si, 0.1% of Mo, 0.6% of Ni, 0.6% of Cr, 0.01% of Ti, S less than or equal to 0.005%, P less than or equal to 0.010%, Fe as a residue, and other inevitable impurity elements.
The method for manufacturing the steel plate comprises the following steps:
performing converter smelting after molten iron desulfurization treatment, and reducing the S, P content in the molten iron to less than or equal to 0.005 percent of S and less than or equal to 0.010 percent of P; LF refining is carried out to complete the alloying of the required mass fractions of C, Mn, Si, Mo, Ni, Ti and other elements, then RH treatment is carried out, the vacuum degree is 3mbar, the treatment time is 20min, and the content of gas impurity elements in the molten steel is reduced; a slab with the thickness of 320mm is obtained by adopting a continuous casting mode. Heating the plate blank to 1105 ℃ and soaking for 40 min; rolling the heated plate blank, wherein the initial rolling temperature is 1001 ℃, the final rolling temperature is 930 ℃, and the rolling mill reduction schedule is 320-280 mm-240mm-200mm-165mm-135mm-110mm-90mm-80 mm; and immediately cooling the rolled steel plate by water, wherein the surface of the cooled steel plate returns to the red temperature of 271 ℃ and the average cooling rate is 4.7 ℃/s. And carrying out quenching and tempering heat treatment on the steel plate. Quenching temperature is 830 ℃, soaking time is 5min, water cooling is carried out until the surface of the steel plate returns to the red temperature of 51 ℃, and the average cooling rate is 4.2 ℃/s; tempering temperature is 640 ℃, soaking time is 40min, and the steel plate is air-cooled to room temperature after tempering.
The steel sheet structure obtained contained martensite and austenite, and the volume fraction of austenite was 10%. The yield strength of the center of the steel plate is 741MPa, the tensile strength is 821MPa, the elongation after fracture is 17.5 percent, and the impact absorption energy of a V-shaped sample in a Charpy pendulum impact test at the temperature of-60 ℃ is 165J. The reduction of area in the thickness direction of the steel plate was 71%.
Example 3: a690 MPa-grade super-thick steel plate with excellent core mechanical properties is 150mm thick, and comprises the following chemical components (the content is expressed by mass percent): 0.08% of C, 6.0% of Mn, 0.1% of Si, 0.5% of Mo, 0.5% of Ni, 0.2% of Cr, 0.05% of Ti, S < 0.005%, P < 0.010%, Fe as a remainder and other inevitable impurity elements.
The method for manufacturing the steel plate comprises the following steps:
performing converter smelting after molten iron desulfurization treatment, and reducing the S, P content in the molten iron to less than or equal to 0.005 percent of S and less than or equal to 0.010 percent of P; LF refining is carried out to complete alloying of required mass fractions of elements such as C, Mn, Si, Mo, Ni, Ti and the like, and then RH treatment is carried out, wherein the vacuum degree is 3mbar, the treatment time is 26min, and the content of gas impurity elements in molten steel is reduced; and obtaining a plate blank with the thickness of 610mm by adopting a die casting and post-forging mode. Heating the plate blank to 1060 ℃, and soaking for 90 min; rolling the heated plate blank, wherein the initial rolling temperature is 1015 ℃, the final rolling temperature is 942 ℃, and the rolling mill reduction schedule is 610mm-540mm-470mm-400mm-340mm-290mm-245mm-215mm-190mm-170mm-150 mm; and immediately cooling the rolled steel plate by water, wherein the surface of the cooled steel plate returns to the red temperature of 327 ℃ and the average cooling rate is 1.5 ℃/s. And carrying out quenching and tempering heat treatment on the steel plate. The quenching temperature is 780 ℃, the soaking time is 15min, the steel plate is cooled by water to the surface re-reddening temperature of 102 ℃, and the average cooling rate is 1.2 ℃/s; tempering temperature is 610 ℃, soaking time is 70min, and the steel plate is air-cooled to room temperature after tempering.
The steel sheet structure obtained contained martensite and austenite, and the volume fraction of austenite was 4%. The yield strength of the center of the steel plate is 745MPa, the tensile strength is 819MPa, the elongation after fracture is 15 percent, and the impact absorption energy of a V-shaped sample in a Charpy pendulum impact test at the temperature of-60 ℃ is 106J. The reduction of area in the thickness direction of the steel plate was 57%.
Example 4: design 4 sets of parallel tests, component content and preparation method are basically the same as example 1, except that the rolling temperature is as shown in table 3 below.
TABLE 1 mechanical Properties of the steel sheet of example 4
As is apparent from Table 1, the elongation after fracture, low-temperature impact energy and reduction of area in the sheet thickness direction of the produced steel sheet are inferior in the groups 1 to 2, which are the initial rolling temperatures within the range of the present invention, and the groups 3 to 4, which are the initial rolling temperatures outside the range of the present invention.
Example 5: 3 sets of parallel tests were designed, and the component contents and the preparation method were substantially the same as those in example 2, except that the cooling rate of water cooling after quenching was as shown in table 2 below.
Table 2 mechanical properties of the steel sheets of example 5
As can be seen from Table 2, group 1 is the average cooling rate of water cooling after quenching within the range of the present invention, and groups 2 to 3 are the average cooling rates outside the range of the present invention. The steel plate of group No. 2 has poor yield strength and low-temperature impact energy performance; the steel sheet of group No. 3 had poor low-temperature impact power properties, and cracks were generated in the steel sheet due to thermal stress.
Claims (5)
1. The 690 MPa-grade super-thick steel plate is characterized by comprising the following chemical components in percentage by mass: c: 0.04-0.08%, Mn: 5.2-6.0%, Si: 0.1-0.4%, Mo: 0.1-0.5%, Ni: 0.2-0.6%, Cr: 0.2-0.6%, Ti: 0.01-0.05%, S: less than or equal to 0.005%, P: less than or equal to 0.010 percent and the balance of Fe and impurities;
the thickness of the steel plate is 150mm, and the preparation method comprises the following steps:
(1) performing converter smelting after molten iron desulfurization treatment, and reducing the S, P content in the molten iron to less than or equal to 0.005 percent of S and less than or equal to 0.010 percent of P;
(2) LF refining is carried out to complete the alloying of the required mass fractions of C, Mn, Si, Mo, Ni and Ti elements, and then RH treatment is carried out, wherein the vacuum degree is less than or equal to 4mbar, and the treatment time is more than or equal to 20 min;
(3) forging a plate blank after die casting, wherein the ratio of the thickness of the plate blank to the thickness of a steel plate is more than or equal to 4;
(4) heating the plate blank at 1060-1140 ℃ for 40-90 min;
(5) rolling the heated plate blank, wherein the initial rolling temperature is less than or equal to 1020 ℃, the final rolling temperature is more than or equal to 930 ℃, and the pass deformation is more than or equal to 10%;
(6) immediately cooling the rolled steel plate by water, wherein the surface re-reddening temperature of the cooled steel plate is less than or equal to 360 ℃, and the average cooling rate is 1-5 ℃/s;
(7) quenching heat treatment, namely reheating the steel plate to 780-830 ℃, soaking for 5-15min, cooling by water until the surface re-reddening temperature of the steel plate is less than or equal to 110 ℃, and the average cooling rate is 2-8 ℃/s;
(8) tempering heat treatment, heating the quenched steel plate to 610-640 ℃, soaking for 40-70min, and air cooling the tempered steel plate to room temperature.
2. The 690MPa grade super thick steel plate according to claim 1, wherein the microstructure of the steel plate has martensite and austenite; wherein the volume fraction of austenite is 4-10%.
3. The 690MPa class super thick steel plate of claim 2, wherein the austenite is in a film shape, and the austenite is distributed among martensite laths.
4. The 690MPa grade super thick steel plate according to claim 1, wherein the steel plate has a core mechanical property: the yield strength is not less than 690MPa, the tensile strength is not less than 770MPa, the elongation after fracture is not less than 14%, and the impact absorption energy of a V-shaped sample in a Charpy pendulum impact test at-60 ℃ is not less than 80J.
5. The 690MPa grade super thick steel plate according to claim 1, wherein the reduction of area of the steel plate based on plate thickness direction stretching is not less than 50%.
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