980MPa grade high toughness steel with a thickness of 80-120mm for deep-sea submersibles and production method

Abstract

The chemical composition and wt% of 980MPa grade high-toughness steel with a thickness of 80-120mm for deep-sea submersibles are: C: 0.051-0.086%, Si: 0.13-0.175%, Mn: 0.49-0.85%, Ni: 2.95-4.2%, Cr: 0.18-0.33%, Mo: 0.30-0.55%, V: 0.072-0.088%, B: 0.0021-0.0033%, Nb :0.025%~0.055%, Ti: 0.0078~0.012%, Al: 0.008~0.015%, H≤0.0002%, O≤0.002%, N: 0.005~0.009%, As, Sb, Bi, Sn, Pb, P, S are controlled at ≤0.005% respectively; production method: pouring into billet after smelting; two-stage rolling after heating the billet; cooling; quenching after heating again; tempering; standby. The yield strength of the invention is ≥800MPa, tensile strength is ≥980MPa, elongation is ≥17.5%, impact energy at -84℃ is ≥155J, thickness direction fluctuation is small, heat treatment process is simple, and one quenching is saved.

Description

Technical Field

[0001] The present invention relates to a steel for mechanical equipment and a production method, in particular to a 980MPa grade high-toughness steel with a thickness of 80-120mm for deep-sea submersibles and a production method. Background Art

[0002] Deep-sea manned submersibles are important tools for human beings to explore the mysteries of the deep sea. They can complete a variety of complex tasks, including surveying seabed resources through video and photography, executing the fixed-point deployment of underwater equipment, and testing submarine cables and pipelines. Deep-sea submersibles, especially deep-sea manned submersibles, are one of the frontiers and commanding heights of marine development. Their level can reflect a country's comprehensive scientific and technological strength in materials, control, oceanography and other fields.

[0003] The diving depth is the most important indicator to measure the level of deep-sea submersibles. However, as the diving depth increases, the submersible shell needs to withstand greater pressure. The pressure of 5,000 meters underwater is equivalent to 500 atmospheres, which is equivalent to 5,000 tons of weight on an area of ​​1 square meter. In addition, the structural design of the submersible needs to comprehensively consider the internal space of the shell, the structural weight, and the performance of the shell material. In general, in order to meet the pressure and safety requirements of deep-sea submersibles, the materials used have high strength and large thickness characteristics, and the low temperature of the deep sea has strict requirements on the toughness of the materials. However, there is a common phenomenon of "inversion" of strength and toughness in structural materials. After increasing the strength, it often leads to a decrease in toughness, causing brittle fracture and reducing the reliability and service life of the structure. The more serious problem is that as the thickness of the material increases, its strength and toughness will show unevenness along the thickness direction, which will cause difficulties in the preparation and application of ultra-thick plates and even cause serious safety accidents. Therefore, there are technical bottlenecks in the strength, impact toughness, and preparation of thick plates for deep-sea submersibles.

[0004] Due to the stringent performance requirements of steel plates used in deep-sea submersibles, the alloy content is usually added at a high level, resulting in low heat transfer efficiency; and due to the large thickness of the steel plates, there are uneven temperature and deformation during the rolling and cooling process of the steel plates, resulting in poor uniformity of the structure and performance in the thickness direction of the Hong Kong version.

[0005] In order to meet the production needs of steel plates with a thickness of more than 80 mm, the production process is not only complicated, resulting in low production efficiency, but also requires at least three quenchings in the steel plate heat treatment process, which not only increases production costs and energy consumption, and prolongs the production cycle, but also increases the risk of cracking due to quenching.

[0006] After searching:

[0007] The manned hulls of the "Sea Cliff" and "Alvin" submersibles developed by the United States are both built with HY100 steel used to make submarine hulls. The maximum diving depth of the "Alvin" with three people on board is 2,200m, and its tensile strength grade is 800MPa. In addition, the spherical shell of the manned cabin of the "Deepsea Challenger" is made of 820MPa Australian standard EN26 steel, which is the steel material for deep-sea submersible hulls with the highest strength grade in current public data and literature.

[0008] In addition to steel materials, titanium alloy materials are also widely used, and their strength level is only 800MPa. For example, the manned cabin of the "Limiting Factor" uses 90mm thick 800MPa Ti64 titanium alloy. It can be said that the strict requirements of large thickness and high toughness of metal materials used in deep-sea submersibles limit their further improvement in high strength.

[0009] Therefore, in order to further increase the diving depth of deep-sea submersibles and ensure their safe service life, the present invention has developed a high-toughness steel with a thickness of 80-120mm and a tensile strength of 980MPa for the pressure hull of a deep-sea submersible with a large diving depth, and a production method thereof. While ensuring uniform performance in the thickness direction of the 80-120mm thick steel plate, the present invention solves the problems of low strength level and poor low-temperature toughness of existing materials. Summary of the invention

[0010] The present invention aims to overcome the shortcomings of the prior art, such as thin steel plate thickness, tensile strength not exceeding 850MPa, poor uniformity of performance in the thickness direction, and complicated heat treatment process, and provide a 980MPa-grade high-toughness steel with a thickness of 80-120mm for deep-sea submersibles, which has a yield strength of ≥800MPa, a tensile strength of ≥980MPa, an elongation of ≥17.5%, an impact energy of ≥155J at -84°C, small fluctuation in the thickness direction, and a simple heat treatment process, and a production method.

[0011] Measures to achieve the above objectives:

[0012] The components and weight percentage of 980MPa grade high-toughness steel with a thickness of 80-120mm for deep-sea submersibles are as follows: C: 0.051-0.086%, Si: 0.13-0.175%, Mn: 0.49-0.85%, Ni: 2.95-4.2%, Cr: 0.18-0.33%, Mo: 0.30-0.55%, V: 0.072-0.088%, B: 0.0021-0.0033%, Nb: 0.025%-0. 055%, Ti: 0.0078~0.012%, Al: 0.008~0.015%, H≤0.0002%, O≤0.002%, N: 0.005~0.009%, As, Sb, Bi, Sn, Pb, P, S are all controlled at ≤0.005%, the rest are Fe and unavoidable impurities; Mechanical properties: yield strength ≥800MPa, tensile strength ≥980MPa, elongation ≥17.5%, -84℃ impact energy ≥155J.

[0013] Preferably, the weight percentage of Nb is 0.025% to 0.046%.

[0014] Preferably, the weight percentage content of Ti is 0.0078-0.0098%.

[0015] Preferably, the weight percentage of Al is 0.008-0.011%.

[0016] The production method of 980MPa grade high toughness steel with a thickness of 80-120mm for deep-sea submersibles comprises the following steps:

[0017] 1) After smelting, the billet is cast into a billet, and the thickness of the billet is controlled to be no less than 250mm;

[0018] 2) After conventional heating of the ingot, two-stage rolling is performed:

[0019] The rolling temperature in the first stage is 1050-1080°C, and the cumulative reduction rate is 40-60%;

[0020] The second stage rolling temperature is 950-980℃, the pass reduction rate is 10-18%, and the final rolling temperature is 880-920℃;

[0021] 3) cooling to 250-350° C. at a cooling rate of 20-45° C. / s;

[0022] 4) After the steel plate is heated again and quenched, the heating temperature is controlled at 870-890°C and kept at this temperature for 70-130 minutes;

[0023] 5) Tempering, controlling the tempering temperature at 650-680°C and the tempering time at 180-320min;

[0024] 6) Set aside.

[0025] Preferably, the reheating temperature is between 870°C and 883°C, and the temperature is kept at 70 to 115 minutes.

[0026] Preferably, the tempering temperature is between 657 and 673° C., and the tempering time is between 186 and 305 min.

[0027] Functions and mechanisms of various raw materials and main processes in the present invention

[0028] C: The reason why the present invention controls C to 0.051-0.086% is that C is the most effective interstitial solid solution strengthening element and can form alloy carbides with other alloy elements to increase strength. Increased C content will lead to deterioration of low-temperature toughness and cause segregation, resulting in uneven performance.

[0029] Si: It has a certain solid solution strengthening effect, but it will cause brittleness. Therefore, the Si content is reduced as much as possible in the composition design. However, auxiliary materials will bring in a certain amount of Si during the production process. The Si content is controlled within the range of 0.13-0.175%.

[0030] Mn: It can improve the hardenability of steel and the deformation during quenching is relatively small. Mn is easy to induce brittleness and quenching cracking, Mn promotes its temper brittleness and enhances the sensitivity of steel to white spots. These factors easily lead to reduced toughness of steel. Therefore, the Mn content of this steel is designed to be within the range of 0.49-0.85%. If it is lower than 0.49%, it will not achieve sufficient strengthening effect, and if it is higher than 0.85%, it will lead to reduced toughness.

[0031] Ni: It can strengthen steel, improve its low temperature performance, especially toughness, and improve its hardenability. Nickel steel is also very rust-resistant and has high corrosion resistance to acid, alkali and seawater. The lower limit is set at 2.95%. However, nickel is an important strategic material and is expensive. In order to control manufacturing costs, its upper limit is set at 4.2%.

[0032] The composite addition of Cr and Mo can reduce the critical cooling rate, improve the hardenability of steel, and is conducive to obtaining lath martensite structure within a certain cooling rate range, and can refine the grains, improve strength and toughness. Cr can form a variety of carbides in steel to improve strength and hardness, and Mo is solid-dissolved in ferrite and austenite in steel, which has the effects of solid solution strengthening and carbide dispersion strengthening. Cr and Mo carbide-forming elements can hinder the diffusion of carbon, thereby significantly increasing the decomposition temperature of martensite and maintaining the strength of the martensite matrix. The contents of Cr and Mo in the present invention are Cr: 0.18-0.33%, Mo: 0.30-0.55%.

[0033] V, Ti, Nb: are strong carbide forming elements, with fine grain strengthening, precipitation strengthening and solid solution strengthening effects. Dispersed precipitation of C and N compounds can pin the austenite grain boundaries, hinder the migration of austenite grain boundaries, that is, hinder the growth of austenite grains, and inhibit austenite recrystallization, refine grains, and produce a comprehensive effect of fine grain strengthening and precipitation strengthening. At the same time, hard particles have a significant strengthening effect and can reduce the difference in performance in the thickness direction of thick steel plates. Therefore, V: 0.072~0.088%, Nb: 0.025%~0.055%, Ti: 0.0078~0.012%, Nb, V, Ti meet the requirements of adding to increase the number of precipitated phases and reduce the size of precipitated phases, and the strengthening effect is more significant without reducing toughness.

[0034] The addition of Nb plays an important role in the uniformity of the performance of the 80-120mm thick steel plate of the patent. Generally, a large amount of alloying elements are added to the low-alloy high-strength thick steel plate of 980MPa grade polar marine engineering, resulting in low heat conduction efficiency of the steel plate during the preparation process. Its rolling process adopts two-stage controlled rolling, namely recrystallization zone rolling and non-recrystallization zone rolling. According to the general composition calculation, the recrystallization temperature is about 890℃. When rolling 80-120mm thick steel plates, the thickness of the intermediate billet is generally greater than 130mm. Due to the low heat conduction efficiency, the cross-sectional temperature difference exceeds 120℃. When rolling in the non-recrystallization zone, in order to make the entire cross section below the recrystallization temperature (less than 890℃), the surface temperature has dropped to below 800℃. As the rolling progresses, the temperature is further reduced to the two-phase zone, resulting in abnormal organization, and the deformation resistance increases at a lower temperature, which increases the load of the rolling mill, making it difficult to meet the reduction rate requirements, and even difficult to roll. If the temperature is increased, the temperature of the core area is still above the recrystallization temperature, which is easy to cause mixed crystals during rolling. Whether the structure is abnormal or mixed crystal, it will affect the low temperature impact toughness. Adding 0.025% to 0.055% can increase the recrystallization temperature, avoid the above situation, make the structure and grains uniform, and improve the low temperature impact toughness.

[0035] Al is also a strong nitride forming element. It can work together with Ti to increase the effective B content, thereby improving the hardenability of thick steel plates and ensuring the uniformity of performance in the thickness direction. However, it is easy to form large particle inclusions with sharp edges during the pouring process, which will not only cause fluctuations in low-temperature impact toughness, but also cause cracks due to stress concentration. Therefore, the range of Al is 0.008-0.015%.

[0036] B: In order to improve the hardenability, a trace amount of B element is added to the steel. The solid solution B segregates to the austenite grain boundary before quenching, and improves the hardenability by inhibiting the ferrite phase transformation. B can also precipitate with Cr and Ti to form high-temperature refractory particles. However, the increase in B content will segregate to the grain boundary and increase the crack sensitivity. Therefore, the B content range of this steel is designed to be B: 0.0021~0.0033%.

[0037] N, H, and O are gas-forming elements in steel. These gas elements mainly come from raw materials and enter during contact with air. They generally have a small amount of solid solution in solid steel at room temperature and will form bubbles or inclusions. Therefore, a large amount of gas in steel will affect the quality of steel. The type and content of gas elements have different effects on the performance of steel. Some gas elements will even have a beneficial effect in some steels within an appropriate range.

[0038] H: is dissolved in steel in the form of interstitial atoms. The solubility of hydrogen in solid steel is very small and decreases with decreasing temperature. Hydrogen reduces the toughness of steel and easily forms hydrogen-induced cracks, causing fracture. This patent controls the H range to 0-0.0002% to avoid the adverse effects of H.

[0039] O: Its solubility in solid steel is much lower than that in molten steel. When the molten steel solidifies, it will precipitate in the form of oxide inclusions, reducing the toughness of the steel. Oxygen will also react with carbon to generate CO bubbles, which will affect the density of the steel and also reduce the toughness of the steel. Therefore, this patent controls the range of O to 0-0.002%

[0040] N: A large amount of V is added to the steel. The appropriate N content can control the type, shape and size of the second phase particles, improve the precipitation strengthening effect and do not affect the low temperature impact toughness. This patent limits N to 0.005-0.009% based on the amount of V added in the steel (0.05%-0.07%).

[0041] The reason why the present invention controls As, Sb, Bi, Sn, Pb, P, and S to be ≤0.005% is that there are inevitably elements in the steel material that are not in the composition design. Some of these elements have little effect on the performance of the steel material, while others have a significant effect on the steel material, resulting in uncontrollable performance. They are called impurity elements or harmful elements. The harmful elements of different steel grades vary depending on the performance requirements of the steel grade. To produce 80-120mm thick 980MPa deep-sea submersible steel plates, thicker billets are generally required. Thicker billets are more likely to produce center segregation during the casting process, and center segregation will deteriorate material properties, especially significantly affecting the performance of the core of the 80-120mm thick steel plate. Therefore, elements that are easy to segregate and have a greater impact on the performance of the steel after segregation should be strictly controlled. Through experimental research, As, Sb, Bi, Sn, Pb, P, and S are easy to segregate and easily form low-melting point inclusions, which have a great influence on impact toughness. They are determined to be impurity elements of 980MPa deep-sea submersible steel plates. According to the degree of influence of these impurity elements on the performance, this patent determines that the content of these elements must be controlled at 0-0.005%. An upper limit below 0.005% will greatly increase the production difficulty and manufacturing cost.

[0042] The conventional process of this steel is ordinary rolling + two quenching + tempering. This patent adopts two-stage controlled rolling + controlled cooling after rolling + quenching and tempering heat treatment process to prepare 80-120mm thick 980MPa steel plates, aiming to improve the uniformity of organization and performance, low-temperature toughness, shorten the process flow, improve production efficiency, and reduce the probability of quenching cracks. Specifically:

[0043] The reason why the present invention controls the first stage rolling temperature at 1050-1080°C and the cumulative reduction rate at 40-60% is that the cumulative reduction in the austenite recrystallization region reaches a certain amount to fully refine the austenite grains, and fully deform the thickness center to improve the uniformity of deformation, thereby increasing the fine grain strengthening effect of the entire thickness section, while also improving toughness, and inducing Ti, Nb, V second phase particles to be fully refined and precipitated in the austenite region.

[0044] The rolling temperature in the second stage is between 950 and 980°C, the pass reduction rate is between 10 and 18%, and the final rolling temperature is between 880 and 920°C. This is because this temperature range belongs to the non-recrystallization zone of austenite. Increasing the pass reduction in this area can flatten the austenite and increase the deformation energy storage, providing a favorable nucleation position and kinetic energy for the subsequent phase transformation process, making the structure fully refined. A pass reduction rate of 10 to 18% can allow the deformation to penetrate into the interior, thereby improving the uniformity of performance.

[0045] The reason why the present invention controls cooling to 250-350°C at a cooling rate of 20-45°C / s is to obtain a deformed martensite structure and simultaneously inhibit the precipitation of Ti, Nb, and V second phase particles in the ferrite region, thereby improving the refinement degree and precipitation strengthening effect of the subsequent heat treatment structure.

[0046] The reason why the present invention controls the heating temperature after reheating to 870-890°C and keeps it at this temperature for 70-130 minutes is that this temperature range and holding time can fully austenitize and limit the excessive growth of austenite grains, and can make Ti, Nb, and V that cannot be precipitated during cooling after rolling fully precipitate without growing.

[0047] The reason why the present invention controls the tempering temperature at 650-680°C and the tempering time at 180-320min is that the quenched martensite structure is improved, a uniform tempered troostite structure with high toughness is obtained, and carbides can continue to precipitate, thereby improving the strength and toughness of the material.

[0048] Compared with the prior art, the present invention has comprehensive control from aspects such as component design, determination of impurity element types and content control, gas element content control, inclusion types, shapes, and sizes, and has a yield strength of ≥800MPa, a tensile strength of ≥980MPa, an elongation of ≥17.5%, an impact energy of -84°C of ≥155J, small fluctuation in thickness direction, a simple heat treatment process, saving one quenching, improving production efficiency, and reducing the probability of quenching cracks. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is the metallographic structure diagram of the present invention. DETAILED DESCRIPTION

[0050] The present invention is described in detail below:

[0051] Table 1 is a list of chemical components of various embodiments and comparative examples of the present invention;

[0052] Table 2 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0053] Table 3 is a table of performance test results of various embodiments of the present invention and comparative examples.

[0054] Each embodiment of the present invention is produced according to the following steps

[0055] 1) After smelting, the billet is cast into a billet, and the thickness of the billet is controlled to be no less than 250mm;

[0056] 2) After conventional heating of the ingot, two-stage rolling is performed:

[0057] The rolling temperature in the first stage is 1050-1080°C, and the cumulative reduction rate is 40-60%;

[0058] The second stage rolling temperature is 950-980℃, the pass reduction rate is 10-18%, and the final rolling temperature is 880-920℃;

[0059] 3) cooling to 250-350° C. at a cooling rate of 20-45° C. / s;

[0060] 4) After the steel plate is heated again and quenched, the heating temperature is controlled at 870-890°C and kept at this temperature for 70-130 minutes;

[0061] 5) Tempering, controlling the tempering temperature at 650-680°C and the tempering time at 180-320min;

[0062] 6) Set aside.

[0063] Table 1 Chemical composition list of various embodiments and comparative examples of the present invention (wt%)

[0064]

[0065] Table 1

[0066]

[0067]

[0068] Table 2 List of main process parameters of various embodiments of the present invention and comparative examples

[0069]

[0070] Table 3 List of mechanical properties test results of various embodiments of the present invention and comparative examples

[0071]

[0072]

[0073] From the results in Table 3, the thickness specification range of the steel plate of the present invention is 80-120 mm, and the standard sampling results of the mechanical properties are: yield strength ≥800 MPa, tensile strength ≥980 MPa, elongation ≥17.5%, -84°C impact energy ≥155 J, and the fluctuation in the thickness direction is small.

[0074] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. The components and weight percentage of 980MPa grade high-toughness steel with a thickness of 80-120mm for deep-sea submersibles are as follows: C: 0.051-0.086%, Si: 0.13-0.175%, Mn: 0.49-0.85%, Ni: 2.95-4.2%, Cr: 0.18-0.33%, Mo: 0.30-0.55%, V: 0.072-0.088%, B: 0.0021-0.0033%, Nb: 0.025%~0.055%, Ti: 0.0078-0.012%, Al: 0.008-0.015%, H≤0.0002%, O≤0.002%, N: 0.005-0.009%, As, Sb, Bi, Sn, Pb, P, S are controlled at ≤0.005% respectively, and the rest are Fe and unavoidable impurities; Mechanical properties: yield strength ≥800MPa, tensile strength ≥980MPa, elongation ≥17.5%, -84℃ impact energy ≥155J, the metallographic structure is tempered troostite structure.

2. The 980MPa grade high toughness steel with a thickness of 80-120mm for deep-sea submersibles as claimed in claim 1, characterized in that: The weight percentage of Nb is 0.025% to 0.046%.

3. The 980MPa grade high toughness steel with a thickness of 80-120mm for deep-sea submersibles as claimed in claim 1, characterized in that: The weight percentage content of Ti is 0.0078-0.0098%.

4. The 980MPa grade high toughness steel with a thickness of 80-120mm for deep-sea submersibles as claimed in claim 1, characterized in that: The weight percentage content of Al is 0.008-0.011%.

5. The method for producing the 980MPa grade high toughness steel with a thickness of 80-120mm for deep-sea submersibles as claimed in claim 1, comprising the following steps: 1) After smelting, the billet is cast into a billet, and the thickness of the billet is controlled to be no less than 250mm; 2) After conventional heating of the ingot, two-stage rolling is carried out: The rolling temperature in the first stage is 1050~1080℃, and the cumulative reduction rate is 40~60%; The second stage rolling temperature is 950~980℃, the pass reduction rate is 10~18%, and the final rolling temperature is 880~920℃; 3) Cooling to 250-350°C at a cooling rate of 20-45°C / s; 4) After reheating the steel plate and quenching, control the heating temperature at 870~890℃ and keep it at this temperature for 70~130min; 5) Tempering, controlling the tempering temperature at 650~680℃ and the tempering time at 180~320min; 6) Set aside.

6. The method for producing the 980MPa grade high toughness steel with a thickness of 80-120mm for deep-sea submersibles as claimed in claim 5, characterized in that: The reheating temperature is between 870 and 883°C and the temperature is kept at 70 to 115 minutes.

7. The method for producing the 980MPa grade high toughness steel with a thickness of 80-120mm for deep-sea submersibles as claimed in claim 5, characterized in that: The tempering temperature is between 657 and 673°C, and the tempering time is between 186 and 305 minutes.

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