Ultra-high-strength protective steel plate with tensile strength of 1950 MPa or above and manufacturing method of ultra-high-strength protective steel plate

Abstract

The invention discloses an ultrahigh-strength protective steel plate with the tensile strength being 1950 MPa or above and a manufacturing method of the ultrahigh-strength protective steel plate. The ultrahigh-strength protective steel plate comprises the following chemical components in percentage by weight: 0.30-0.60% of C, 1.00-1.50% of Si, 0.80-1.30% of Mn, 0-0.20% of Al, less than 0.02% of P, less than 0.01% of S, 0.45-1.45% of Cr, 1-3% of Ni, 0.01-0.05% of Nb, 0.01-0.08% of Ti, 0.02-0.10% of V, 0.30-0.90% of Mo and the balance of Fe and other inevitable impurities. In addition, the elements need to meet the following relational expression: 340 > = 550-350C-40Mn-35V-20Cr-17Ni + 30Al > = 290 at the same time. The steel plate obtains more excellent obdurability matching performance under the condition that traditional heat treatment processes such as quenching and tempering are not needed, the yield strength of the steel plate is larger than or equal to 1350 MPa, the tensile strength of the steel plate is larger than or equal to 1950 MPa, and the Charpy impact energy of the steel plate at the temperature of-40 DEG C is larger than or equal to 20 J.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgy, and in particular relates to an ultra-high strength protective steel plate with a tensile strength of more than 1950 MPa and a manufacturing method thereof. Background Art

[0002] Ultra-high strength steel is an alloy steel with ultra-high strength and high toughness developed on the basis of ordinary alloy structural steel. It occupies a very important position in modern industry. The main characteristics of ultra-high strength steel are extremely high strength and sufficient plasticity and toughness, so that it can be used to manufacture important structural parts that withstand high stress. At present, it is generally believed that steel plates with a yield strength exceeding 1350MPa belong to ultra-high strength steel. Its application range is very wide, including steel for aircraft landing gear in the aerospace field, solid rocket engine casings, and penetrating projectiles of various tactical missiles.

[0003] Since the Second World War, in order to improve the survivability of special vehicles in modern warfare and reduce casualties, the development of special protective materials has been paid attention to by the world's military powers and has been comprehensively developed. From the initial homogeneous steel to today's aluminum alloys, titanium alloys, ceramics, composite materials, etc., special protective materials have developed rapidly.

[0004] Metal ceramic materials, as one of the excellent anti-ballistic materials, have the characteristics of high hardness, wear resistance, corrosion resistance, high temperature resistance, low density, etc. They have strong defense capabilities against kinetic bullets and ammunition fragments. Therefore, they have always been valued by various military powers. At present, ceramic materials can be used in the protection of equipment such as bulletproof vests, vehicles and aircraft. The composite material composed of ceramics and homogeneous steel (special steel sandwiched with ceramics) has better anti-penetration (anti-armor penetration performance) than traditional homogeneous steel.

[0005] Metal ceramics also have significant disadvantages. They have low toughness and are brittle. At the same time, compared with traditional homogeneous steel materials, the cost of ceramics is very high. Although the composite materials containing ceramic materials have a much better defense capability than special steels of the same thickness, they have a fatal weakness. Once they are hit for the first time, the ceramics will break, and their defense effect against the next shell will drop significantly, and the drop is higher than that of steel. There is still no better solution. These shortcomings have caused many obstacles to the application of ceramic materials in special protection.

[0006] Due to the brittleness of metal ceramic materials, their ability to resist continuous attacks in actual combat is relatively low. Therefore, it is necessary to have a material with ultra-high hardness and a certain toughness to replace ceramic materials to a certain extent, so as to ensure the special protection ability of equipment or personnel in continuous combat.

[0007] When designing ultra-high strength steel, the improvement of alloy elements and processes can ensure that the martensite formed in the steel plate has ultra-high strength, generally twin martensite (carbon mass fraction greater than 0.6%). At the same time, compared with ceramic materials, this type of steel plate also has a certain low-temperature toughness, and will not break like ceramics when hit by artillery shells, which can ensure the ability of equipment or personnel to resist secondary attacks.

[0008] Usually, for materials with a tensile strength higher than 1950MPa, a carbide indenter is used to test the hardness value. The Brinell hardness is expressed in HBW. The Brinell hardness of ultra-high strength steel plates above 1950MPa ranges from about 600-650HBW.

[0009] Chinese patent CN102953016A discloses "a 650HB grade wear-resistant steel plate and its manufacturing method", the composition percentage of the steel plate is: C: 0.45-0.55%, Si: 0.10-0.35%, Mn: 0.20-1.00%, P≤0.02%, S≤0.01%, Cr: 0.20-1.00%, Mo: 0.1-0.8%, Ni: 0.50-2.00%, Nb: 0.0 1~0.08%, Ti: 0.001~0.06%, Al: 0.01~0.10%, B: 0.0005~0.0040%, Mg: 0.001~0.010%, Ca: 0.001~0.010%, and also meet: (Si+Mn)≤1.10%, (Cr+Mo)≥0.80%, 0.04%≤(Al+Ti)≤0.11%, the balance is Fe and unavoidable impurities. Its manufacturing method includes: smelting, heating, rolling, quenching near Ac3, and tempering between 100~400℃. The typical Brinell hardness of the wear-resistant steel obtained by this patent is 650HB, which matches high strength, high hardness and good toughness.

[0010] Chinese patent CN108034889A discloses "a high hardness wear-resistant steel plate and its manufacturing method", the composition percentage of which is: C: 0.47-0.49%, Si: 0.41-0.43%, Mn: 0.23-0.25%, P≤0.002%, S≤0.003%, Cr: 1.13-1.15%, Mo: 0.62-0.64%, Ni: 0.71-0.73%, Ti: 0.033-0.035%, Nb: 0.011-0.013%, V: 0.16-0.18%, B: 0.003-0.005%, N≤0.003%, O≤0.0025%, and the rest is Fe and unavoidable impurities. The steel plate obtained by this invention has an ultra-high Brinell hardness of 650HB and a low-temperature impact toughness of -20°C ≥30J. While ensuring extremely high wear resistance, it also has good corrosion resistance.

[0011] At present, the composition of ultra-high strength steel is mainly medium-high carbon steel, supplemented by Cr, Ni, Mo, Nb, V, Ti or a small amount of B element that significantly improves hardenability. Usually, such steel plates are produced through rolling, quenching and tempering processes. Due to the existence of heat treatment links such as quenching and tempering, the production efficiency of steel plates is reduced and the production cost of steel plates is increased.

[0012] From the existing research on ultra-high-strength steel, there is still no high-strength steel product with a tensile strength of more than 1950MPa that is produced only by rolling steel plates without heat treatment. High-strength steel with a tensile strength of more than 1950MPa is mainly obtained by adding alloy elements and superimposing quenching and tempering treatment. In addition, the existing ultra-high-strength steels either have no requirements for Charpy impact energy or require -20℃ impact energy, which cannot meet the -40℃ impact requirements of ultra-high-strength steel for protection. Therefore, it is necessary to develop an ultra-high-strength protective steel with a tensile strength of more than 1950MPa, a simple production process, and low cost to meet the demand for ultra-high-strength materials for protective equipment. Summary of the invention

[0013] The object of the present invention is to provide an ultra-high strength protective steel plate with a tensile strength of more than 1950MPa and a method for manufacturing the same. The steel plate has ultra-high strength and good plasticity and toughness. The yield strength of the steel plate is ≥1350MPa, the tensile strength is ≥1950MPa, and the Charpy impact energy at -40°C is ≥20J.

[0014] In order to achieve the above object, the technical solution provided by the present invention is:

[0015] An ultra-high strength protective steel plate with a tensile strength of more than 1950 MPa, wherein the chemical composition by weight is: C: 0.30-0.60%, Si: 1.00-1.50%, Mn: 0.80-1.30%, Al: 0-0.20%, P < 0.02%, S < 0.01%, Cr: 0.45-1.45%, Ni: 1-3%, Nb: 0.01-0.05%, Ti: 0.01-0.08%, V: 0.02-0.10%, Mo: 0.30-0.90%, and the balance includes Fe and other unavoidable impurities; and the above elements must also satisfy the following relationship:

[0016] 340≥550-350C-40Mn-35V-20Cr-17Ni+30Al≥290.

[0017] Furthermore, the balance is Fe and other inevitable impurities.

[0018] The metallographic structure of the steel plate of the present invention is a lath martensite structure and a residual austenite film between the martensite laths.

[0019] The steel plate of the present invention has a yield strength of ≥1350MPa, a tensile strength of ≥1950MPa, and a Charpy impact energy of ≥20J at -40°C.

[0020] In the composition design of the steel plate of the present invention:

[0021] C: It can ensure the strength of the material while stabilizing austenite. When the carbon content in the steel is too high, twin martensite will form during the cooling process of the steel plate. Compared with lath martensite, twin martensite has higher strength but poorer toughness. However, a higher carbon content can ensure that more carbon is enriched in the untransformed austenite of the steel plate during the stack cooling process, thereby improving the stability of the residual austenite, preventing martensitic transformation at room temperature, and further improving the plastic toughness of the steel plate. Therefore, the carbon content of the present invention is controlled at 0.30-0.60%.

[0022] Mn: Manganese is the main element for stabilizing austenite in steel. A higher manganese content can ensure that the material matrix is ​​a stable austenite structure, thereby ensuring that the material can have a higher untransformed austenite content at room temperature. However, according to the fact that every 1% of manganese can reduce the martensitic transformation temperature of steel by about 35 to 50°C, and the ability of manganese to stabilize austenite is second only to carbon, so too much manganese content will cause the untransformed austenite in the steel to be too stable, and will not produce the effect of deformation-induced martensitic phase transformation during the subsequent room temperature deformation process, and will not be able to improve the plasticity and toughness of the steel plate, which will lead to a low actual deformation capacity of the steel plate. Therefore, the manganese content of the present invention is controlled at 0.80 to 1.30%.

[0023] Al: In the present invention, aluminum can effectively prevent the formation of carbides in steel, facilitate the diffusion of carbon in martensite into austenite without being consumed by the formation of carbides, improve the stability of austenite during cooling, and greatly improve the toughness of the steel plate. However, if the aluminum content in the steel is too high, it will increase the difficulty of smelting and pouring the steel, increase the manufacturing cost, and form excessive oxides to deteriorate the quality of the steel plate. Therefore, the aluminum content of the present invention is controlled at 0-0.2%.

[0024] V: Vanadium is a strong carbide-forming element, which has the effects of precipitation strengthening and grain refinement strengthening in the material. Because too high Mn content in steel can easily lead to grain coarsening, adding a trace amount of vanadium is conducive to refining the structure and improving the strength of the alloy. At the same time, the precipitation of V carbides has the effect of dispersion strengthening, which can further improve the strength of the steel. Therefore, the vanadium content of the present invention is controlled at 0.02-0.10%.

[0025] Si: Silicon has the effect of solid solution strengthening, which can improve the corrosion resistance and high temperature oxidation resistance of steel, and the addition of Si can effectively improve the elastic modulus of the steel plate; but too high a content will cause serious decarburization on the steel surface and reduce welding performance. Therefore, the silicon content of the present invention is controlled at 1.00-1.50%.

[0026] Ti: Titanium is a strong carbide-forming element. It can form TiN in steel, which serves as a particle for austenite nucleation and plays a role in refining austenite grains. The addition of a trace amount of Ti in the present invention mainly plays a role in refining austenite grains. If the Ti content is too high, it will lead to the precipitation of TiC, consume C in the steel, reduce the solid solubility of C in austenite, and cause the stability of austenite to decrease. Therefore, the Ti content of the present invention is controlled at 0.01-0.08%.

[0027] Nb: In the technical solution described in the present invention, niobium is a strong carbide-forming element, which can refine the rolled austenite grains through deformation-induced precipitation during high-temperature rolling deformation. At the same time, the precipitated carbides can also play a precipitation strengthening effect by pinning dislocations. Too low Nb content will have weak precipitation strengthening and grain refinement effects in steel. As the niobium content in steel increases, its strengthening effect is enhanced, but when the niobium content is too high, its precipitation strengthening and grain refinement effects are no longer obvious. Therefore, the niobium content of the present invention is controlled at 0.01-0.05%.

[0028] Cr: Chromium can increase the hardenability of steel. In steel with a high carbon content, chromium can also form chromium carbide with carbon, thereby improving the strength and wear resistance of carbon steel without making the steel brittle, and can also increase the thermal strength of steel. At the same time, the addition of chromium can significantly delay the transformation of ferrite and pearlite, so that the process window of the steel plate during the cooling process is enlarged, and the structure of the steel after quenching is only martensite and untransformed austenite. Therefore, the chromium content in the present invention is controlled at 0.45-1.45%.

[0029] Ni: Nickel can increase the hardenability of steel in steel. As the nickel content in steel increases, the strength of the steel continues to rise, but the plastic toughness does not decrease significantly. In medium-high carbon steel, nickel can reduce the pearlite transformation temperature, refine the pearlite, and improve the strength of the steel plate without significantly reducing the toughness of the steel plate. At the same time, nickel can also increase the low-temperature toughness of the steel plate. In the present invention, the main role of nickel is to increase the stability of austenite and improve the low-temperature toughness of the steel plate. However, since nickel is a precious metal, its cost is relatively high. Therefore, the nickel content of the present invention is controlled at 1-3%.

[0030] Mo: Molybdenum in steel can improve the hardenability and heat resistance of steel plates, make thick steel plates with larger sections hardened deeply and thoroughly, and increase the thickness of steel plates that can be produced. Similarly, molybdenum is also a precious metal. Because of its high cost, the molybdenum content in the present invention is controlled at 0.30-0.90%.

[0031] At the same time, in order to meet the martensitic transformation temperature of the steel plate between 100 and 200°C, so that the austenite in the steel will not completely undergo martensitic transformation, and a part of the austenite will still remain at room temperature, the above composition should also meet the following requirements during design: 340≥550-350C-40Mn-35V-20Cr-17Ni+30Al≥290.

[0032] The method for manufacturing the ultra-high strength protective steel plate with a tensile strength of 1950 MPa or above of the present invention comprises the following steps:

[0033] 1) Smelting and casting

[0034] According to the above chemical composition, steel is smelted and refined by converter or electric furnace, and then cast into billets;

[0035] 2) Heating

[0036] The heating temperature of the casting is 1140~1160℃;

[0037] 3) Controlled rolling

[0038] The controlled rolling process is divided into two stages. The first stage has a rolling temperature of 1040-1120°C, and the thickness of the steel plate after rolling is 3-5 times the thickness of the finished steel plate. The second stage rolling begins when the temperature reaches 840-880°C, and the final rolling temperature is ≥820°C, and the steel plate is rolled to the thickness of the finished steel plate.

[0039] 4) Online water cooling

[0040] The final cooling temperature is 200±20℃, and the off-line stack is slowly cooled to room temperature to obtain the finished steel plate.

[0041] Preferably, in step 3), the thickness of the finished steel plate is 4.5 to 20 mm.

[0042] In the method for manufacturing the ultra-high strength protective steel plate with a tensile strength of 1950 MPa or above described in the present invention:

[0043] Controlled rolling is divided into two stages. The starting rolling temperature of the first stage is 1040-1120℃, and the thickness of the steel plate after rolling is 3-5 times the thickness of the finished steel plate. The second stage of rolling begins when the temperature reaches 840-880℃. The final rolling temperature is ≥820℃, and the steel plate is rolled to the thickness of the finished steel plate.

[0044] In the process of controlled rolling, since the carbon, manganese, nickel, chromium and molybdenum added to the steel have the effect of delaying the transformation of pearlite and improving the stability of austenite, a lower final rolling temperature can be set, which is conducive to the accumulation of enough defects such as dislocations inside the deformed austenite. On the one hand, in the subsequent online water cooling process, the deformed austenite with a higher dislocation density provides more nucleation sites for the phase transformation of martensite, refines the martensite lath structure, and improves the strength of the steel plate; on the other hand, the defect density of austenite increases and its stability also improves. In the subsequent online water cooling process, the content of untransformed austenite in the steel plate increases, which is conducive to increasing the proportion of plastic and tough phases in the steel.

[0045] In addition, carbon, manganese, nickel, chromium and molybdenum elements cool down quickly during the online water cooling process. Since there is no time for the diffusion of large atoms to occur, these elements are basically dissolved in the martensite laths, which plays a positive role in improving the strength of martensite.

[0046] The stop cooling temperature of the online water cooling is 200±20°C. The present invention controls the composition to be 340≥550-350C-40Mn-35V-20Cr-17Ni+30Al≥290, so that the martensite start transformation temperature of the steel plate is between 100 and 200°C, so as to ensure that there is a certain temperature difference between the martensite start transformation temperature and the stop cooling temperature of the steel plate during the online water cooling process, and the microstructure of the steel is lath martensite and residual austenite structure, so that the steel plate has ultra-high strength and good plastic toughness.

[0047] After the steel plate is water-cooled online, it is stacked offline and slowly cooled to room temperature to obtain the finished steel plate. On the one hand, the stop temperature of online water cooling is 200±20℃, and other alloy elements in the steel cannot diffuse. However, the supersaturated carbon in the martensite can diffuse in a short range during the subsequent stack cooling process due to its small atoms. According to Gibbs free energy, the carbon atoms at this time will diffuse from the residual austenite that has not been transformed by the martensite, increase the carbon richness of the residual austenite, and then improve the stability of the residual austenite, forming carbon-rich austenite at the martensite-austenite interface, so that it will not undergo martensite transformation after finally cooling to room temperature, and remain between the martensite laths to form a residual austenite film. These residual austenites can relieve stress concentration during deformation, delay the generation and expansion of cracks, and thus improve the plastic toughness of the steel plate. On the other hand, the thermal stress and structural stress concentration of the steel plate after online water cooling can also be relieved to a certain extent by slow cooling, reducing the residual stress of the steel plate, so that the steel plate has good plastic toughness while maintaining high strength.

[0048] The Si element added to the steel can effectively prevent the precipitation and growth of cementite during the stacking and slow cooling process of the steel plate, avoid the consumption of carbon in the steel, and provide a prerequisite for the stable existence of austenite at room temperature.

[0049] Beneficial effects of the present invention:

[0050] 1. In the composition design of the present invention, more alloy elements that can effectively delay the phase transformation of pearlite and ferrite are added to the steel, and 340≥550-350C-40Mn-35V-20Cr-17Ni+30Al≥290 is controlled, and the starting transformation temperature of martensite is controlled between 100 and 200°C, which is conducive to the formation of a complex phase structure of the steel plate with lath martensite as the matrix structure, and at the same time, a film-like residual austenite is enriched between the martensite laths. While the supersaturated martensite ensures the high strength of the steel plate, the residual austenite can effectively improve the plastic toughness of the steel plate, achieving a good combination of strength and plasticity.

[0051] 2. In terms of composition design, the present invention adopts a two-stage controlled rolling process for rolling, which can effectively increase the defect density in the steel through deformation in the finishing stage, increase the nucleation position of martensite in the subsequent online water cooling process, further refine the size of the martensite lath, and improve the strength of the steel plate. Combined with subsequent online water cooling, the solid solution of elements in the martensite is increased, and the strength of the steel plate is further improved. A certain temperature difference is controlled between the stop cooling temperature and the martensite start transformation temperature to ensure the presence of residual austenite structure in the steel, and the residual austenite is carbon-enriched in the subsequent slow cooling process to improve the stability of the residual austenite, so that it will not undergo martensite transformation at low temperatures, and finally a residual austenite film can be formed between the martensite laths to improve the low-temperature impact toughness of the steel plate. The steel plate finally obtained does not require traditional heat treatment processes such as quenching and tempering, and the performance of the obtained steel plate in terms of strength and toughness matching is better, and its yield strength is ≥1350MPa, tensile strength is ≥1950MPa, and -40℃ Charpy impact energy is ≥20J.

[0052] 3. The steel plate of the present invention adopts conventional rolling + direct online water cooling process, which improves the production efficiency of the steel plate, reduces energy consumption, greatly reduces the production cost, and has a simple production process and strong operability. The traditional ultra-high strength steel production also requires an offline quenching and tempering heat treatment process, which has high energy consumption, low production efficiency, and high comprehensive cost of the steel plate. DETAILED DESCRIPTION

[0053] The present invention will be further described below in conjunction with the embodiments.

[0054] The production process of the ultra-high strength steel plate of the present invention is: smelting, casting→heating→controlled rolling→online cooling.

[0055] The specific composition of the ultra-high strength steel plate of the embodiment of the present invention is shown in Table 1, and the balance includes Fe and other inevitable impurities.

[0056] The process parameters of the ultra-high strength steel plates of the embodiments of the present invention are shown in Table 2, and the properties of the ultra-high strength steel plates of the embodiments are shown in Table 3.

[0057] The steel plates produced by the present invention have a yield strength greater than 1350MPa, a tensile strength ≥1950MPa, and a -40°C Charpy impact energy value greater than 20J. Compared with the steel plates of the same strength level currently produced by the quenching and tempering process, the steel plates of the present invention have better performance in terms of matching strength and toughness, and the production process of the steel plates is simplified, eliminating the traditional offline quenching + tempering treatment, the production process is shorter, the production efficiency is higher, the process energy consumption is lower, and it is suitable for wide promotion and use.

[0058]

[0059]

[0060]

Claims

1. An ultra-high strength protective steel plate with a tensile strength of more than 1950 MPa, wherein the chemical composition by weight is: C: 0.30-0.60%, Si: 1.00-1.50%, Mn: 0.80-1.30%, Al: 0-0.20%, P < 0.02%, S < 0.01%, Cr: 0.45-1.45%, Ni: 1-3%, Nb: 0.01-0.05%, Ti: 0.01-0.08%, V: 0.02-0.10%, Mo: 0.30-0.90%, and the balance includes Fe and other unavoidable impurities; and the above elements must also satisfy the following relationship: 340≥550-350C-40Mn-35V-20Cr-17Ni+30Al≥290.

2. The ultra-high strength protective steel plate with a tensile strength of 1950 MPa or more according to claim 1, characterized in that: The balance is Fe and other inevitable impurities.

3. The ultra-high strength protective steel plate with a tensile strength of 1950 MPa or more according to claim 1 or 2, characterized in that: The metallographic structure of the steel plate is lath martensite + residual austenite film existing between martensite laths.

4. The ultra-high strength protective steel plate with a tensile strength of 1950 MPa or more according to claim 1, 2 or 3, characterized in that: The steel plate has a yield strength of ≥1350 MPa, a tensile strength of ≥1950 MPa, and a Charpy impact energy of ≥20 J at -40°C.

5. The method for producing an ultra-high strength protective steel plate having a tensile strength of 1950 MPa or more according to any one of claims 1 to 4, characterized in that: The steps include: 1) Smelting and casting According to the chemical composition of claim 1 or 2, steel is smelted and refined in a converter or electric furnace and cast into a billet; 2) Heating The heating temperature of the casting is 1140~1160℃; 3) Controlled rolling Controlled rolling is divided into two stages. The first stage starts with a rolling temperature of 1040-1120°C, and the thickness of the steel plate after rolling is 3-5 times the thickness of the finished steel plate. The second stage of rolling begins when the temperature reaches 840-880°C, and the final rolling temperature is ≥820°C, and the steel plate is rolled to the thickness of the finished steel plate. 4) Online water cooling The final cooling temperature is 200±20℃, and the off-line stack is slowly cooled to room temperature to obtain the finished steel plate.

6. The manufacturing method according to claim 5, characterized in that: The thickness of the finished steel plate is 4.5 to 20 mm.

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