Weldable ultrahigh-strength protective steel plate with tensile strength of 1650 MPa or above and manufacturing method of weldable ultrahigh-strength protective steel plate

Abstract

The invention discloses a weldable ultrahigh-strength protective steel plate with the tensile strength of 1650 MPa or above and a manufacturing method of the weldable ultrahigh-strength protective steel plate. The weldable ultrahigh-strength protective steel plate comprises the following chemical components in percentage by weight: 0.25-0.45% of C, 0.6-1.10% of Si, 1.10-1.70% of Mn, 0-0.20% of Al, less than 0.02% of P, less than 0.01% of S, 0.85-1.35% of Cr, 0.01-0.08% of Nb, 0.10-0.20% of V, 0.01-0.03% of N and the balance of Fe and other inevitable impurities. And the elements need to meet the following relational expression: 840C + 140Mn + 21Si + 168Cr + 60V is less than or equal to 714. The steel plate can obtain ultrahigh strength and good plasticity under the condition that traditional heat treatment processes such as quenching and tempering are not needed, the tensile strength of the steel plate at the room temperature is larger than or equal to 1650 MPa, and the V-type Charpy impact energy at the temperature of-40 DEG C is larger than or equal to 25 J.

Description

Technical Field

[0001] The present invention belongs to the technical field of ultra-high strength steel, and specifically relates to a weldable ultra-high strength protective steel plate with a tensile strength of more than 1650 MPa and a manufacturing method thereof. Background Art

[0002] Ultra-high strength special protective steel plates are mainly used for various special vehicle bodies and structural parts with high requirements for protective performance. With the rapid development of the modern automobile industry, wheeled vehicles have demonstrated the characteristics of strong mobility, easy operation and high consumption ratio in various actions, and wheeled vehicles have entered the "golden age" of their own development. Enhanced protection and mobility are two of the characteristics of the development of wheeled vehicles. The body of wheeled vehicles mostly adopts ultra-high strength steel welding structure. The increase in the thickness and hardness level of the steel plate is beneficial to improving the protection ability of the vehicle; the general body weight accounts for about one-third of the total weight of the vehicle. The thinning of the steel plate reduces the weight of the body, which is beneficial to improving the mobility and equipment carrying capacity of the vehicle. Therefore, corresponding to the development trend of wheeled vehicles, the development characteristics of special protective steel plates are ultra-high strength and thin specifications.

[0003] At present, special protective steel plates with tensile strength of 1650MPa or above have been developed, but there are still some problems with the shape and welding performance of the steel plates. Foreign major steel mills rarely promote their protective steel plate products. The ones publicly introduced are the Armox series produced by Swedish Steel Oxeslound Co., Ltd. (SSAB Oxeslound), the Ramor series produced by Finnish Ruukki (acquired by SSAB), and the secure series produced by Thyssenkrupp.

[0004] Chinese patent CN102925803A discloses "a method for producing ultra-high strength steel plates". The chemical composition of steel is as follows: C: 0.26-0.28%, Si: 1.15-1.25%, Mn: 1.50-1.60%, P≤0.015%, S≤0.010%, Alt≤0.020%, Nb: 0.02-0.03%, V: 0.055-0.065%, Ti: 0.045-0.060%, Ni: 0.25-0.035%, Mo: 0.55-0.65%, B: 0.0017-0.0022%, and the rest is Fe and other unavoidable impurities. The process route is converter steelmaking → refining outside the furnace → continuous casting → heating → rolling → cooling after rolling → heat treatment. After the steel plate is quenched by the roller quenching machine, the plate shape is good; the process is simple and the technology is easy to implement, which solves the problem that traditional quenching and tempering cannot produce ultra-high-strength steel plates, and realizes the mass production of ultra-high-strength steel plates above 1650MPa. The steel plate is produced through rolling, quenching and tempering processes. Due to the existence of heat treatment links such as quenching and tempering, the production efficiency of the steel plate is reduced and the production cost of the steel plate is increased. At the same time, for thin-gauge materials, there will be problems with poor plate shape.

[0005] Chinese patent CN106756495A announced "A 1760MPa ultra-high strength anti-bullet steel and its manufacturing method". The main steps of the method for the steel plate are: KR molten iron pretreatment; converter smelting; LF refining outside the furnace; RH vacuum degassing converter secondary refining; slab continuous casting; slab reheating; rough rolling by roughing mill; finishing rolling by finishing mill; quenching and tempering inspection. The 6-25mm ultra-high strength alloy steel plate produced by the method of the present invention achieves excellent strength and toughness matching, and has excellent low-temperature impact toughness at -40°C. Through controlled rolling, quenching and tempering heat treatment, the metallographic structure of the steel plate is mainly tempered martensite, with a tensile strength of ≥1760MPa, a yield strength of ≥1270MPa, a -40°C transverse low-temperature impact energy of ≥20J, and a surface Brinell hardness of ≥490HBW.

[0006] In summary, for ultra-high strength steel materials with tensile strength above 1650MPa, carbide indenters are often used for hardness test, and the Brinell hardness is expressed in HBW. At present, the composition of ultra-high hardness 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. At the same time, for thin-gauge materials, there will be problems with poor plate shape.

[0007] However, there is still no high-strength steel with a tensile strength of more than 1650MPa, which is produced only through controlled rolling of steel plates without traditional tempering and heat treatment. Summary of the invention

[0008] The object of the present invention is to provide a weldable ultra-high strength protective steel plate with a tensile strength of more than 1650MPa and a manufacturing method thereof, wherein the steel plate has ultra-high strength and good plasticity and toughness, wherein the tensile strength of the steel plate at room temperature reaches more than 1650MPa, and the -40°C V-type Charpy impact energy is ≥25J.

[0009] In order to achieve the above object, the technical solution provided by the present invention is:

[0010] A weldable ultra-high strength protective steel plate with a tensile strength of more than 1650 MPa, wherein the chemical composition by weight is as follows: C: 0.25-0.45%, Si: 0.6-1.10%, Mn: 1.10-1.70%, Al: 0-0.20%, P < 0.02%, S < 0.01%, Cr: 0.85-1.35%, Nb: 0.01-0.08%, V: 0.10-0.20%, N: 0.01-0.03%, and the balance includes Fe and other unavoidable impurities; and the above elements must also satisfy the following relationship:

[0011] 840C+140Mn+21Si+168Cr+60V≤714.

[0012] Furthermore, the balance is Fe and other inevitable impurities.

[0013] The metallographic structure of the steel plate of the present invention is lath martensite + residual austenite film existing between martensite laths + dispersed precipitated vanadium nitride.

[0014] The tensile strength of the steel plate of the present invention at room temperature is ≥1650MPa, and the -40°C V-type Charpy impact energy is ≥25J.

[0015] In the composition design of the steel plate of the present invention:

[0016] 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 hardness but poorer toughness. At the same time, a higher carbon content will increase the tendency of welding cracking of the steel plate during the subsequent welding process, which is not conducive to the welding performance of the steel plate. Therefore, the carbon content of the present invention is controlled at 0.25-0.45%.

[0017] Mn: Manganese is the main element for stabilizing austenite in steel. A higher manganese content can ensure that the material matrix structure is a stable austenite structure, thereby ensuring that the material can have a higher untransformed austenite content at room temperature. However, the martensitic transformation temperature of steel can be reduced by about 35 to 50°C for every 1% of manganese, and the ability of manganese to stabilize austenite is second only to carbon, which is beneficial to improving the stability of austenite. In addition, an increase in the content of solid-solution manganese in martensite will increase the strength of martensitic laths. However, an excessively high manganese content will cause the martensitic transformation temperature of the steel plate to decrease, increase the content of residual austenite in the steel, and reduce the strength of the steel plate. Therefore, the manganese content of the present invention is controlled at 1.10 to 1.70%.

[0018] Al: Aluminum can effectively prevent the formation of carbides in steel, is conducive to the solid solution of carbon in austenite, improves the stability of austenite during cooling, and is of great benefit to improving the toughness of steel plates. However, if the aluminum content in steel is too high, it will increase the difficulty of smelting and pouring steel, increase the manufacturing cost, and form excessive oxides to deteriorate the quality of steel plates. Therefore, the aluminum content of the present invention is controlled at 0-0.2%.

[0019] V: Vanadium is a strong carbide-forming element, and has the effects of precipitation strengthening and fine grain strengthening in the material. Because too high a Mn content in steel easily leads to grain coarsening, adding a trace amount of vanadium is beneficial to refining the structure and improving the strength of the alloy. At the same time, in the present invention, by adding a certain amount of N element, the vanadium nitrogen dispersed precipitates play a role of precipitation strengthening, which can further improve the strength of the steel. Therefore, the vanadium content of the present invention is controlled at 0.10-0.20%.

[0020] 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 0.60-1.10%.

[0021] Nb: In the technical solution described in the present invention, niobium is a strong carbide-forming element. In the process of high-temperature rolling and deformation in steel, it can play a role in refining the rolled austenite grains through deformation-induced precipitation. 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 0.01-0.08%.

[0022] Cr: Chromium can increase the hardenability of steel. In steels with a high carbon content, chromium can also form chromium carbides with carbon, thereby improving the hardness and wear resistance of carbon steel without making the steel brittle, and can also increase the thermal strength of the steel. At the same time, the addition of chromium can significantly delay the ferrite-pearlite transformation, and the steel plate will not undergo ferrite phase transformation during the air cooling process after rolling, so that the steel plate can maintain a low final rolling temperature during the rolling process, which is conducive to increasing the defect density in the steel, improving the nucleation position of martensite during the quenching process, reducing the size of martensite laths, and ultimately improving the strength of the steel plate. Therefore, the chromium content of the present invention is controlled at 0.85-1.35%.

[0023] N: Nitrogen in steel can be dissolved in the body-centered cubic structure like carbon, improving the solid solution strengthening effect. Nitrogen expands the austenite phase and has a strong effect of stabilizing austenite. Nitrogen in steel can also form alloy compounds with elements such as chromium, vanadium, and titanium, which have the effect of precipitation strengthening. The present invention improves the strength of the steel plate by using the strengthening effect of vanadium nitrogen precipitation through the composite addition of vanadium and nitrogen. However, if the residual nitrogen content in the steel is too high, it will lead to loose macroscopic structure or the formation of pores. Therefore, the nitrogen content of the present invention is controlled at 0.01-0.03%.

[0024] At the same time, in order to ensure that the steel plate has good weldability, the above composition should also meet the requirements of 840C+140Mn+21Si+168Cr+60V≤714 when designed. On the one hand, the addition of alloying elements such as C and Mn improves the stability of austenite, while the addition of Cr can effectively delay the transformation of ferrite and pearlite, maintain the full austenite structure of the steel plate, increase the defect density in the deformed austenite, and is conducive to the formation of refined lath martensite, thereby improving the strength of the steel plate.

[0025] On the other hand, the above alloying elements are basically dissolved in the martensite laths, which plays a positive role in improving the strength of martensite. In addition, vanadium and nitrogen elements are added to the steel, and through deformation-induced precipitation, dispersed vanadium nitrides are produced in the steel, which has the effect of precipitation strengthening. The Si element added to the steel can effectively prevent the precipitation and growth of cementite in the steel plate during air cooling, avoid the consumption of carbon in the steel, and provide a prerequisite for the stable existence of austenite at room temperature.

[0026] The method for manufacturing a weldable ultra-high strength protective steel plate with a tensile strength of 1650 MPa or more according to the present invention comprises the following steps:

[0027] 1) Smelting and casting

[0028] According to the above chemical composition, steel is smelted and refined by converter or electric furnace, and then cast into billets;

[0029] 2) Heating

[0030] The heating temperature of the casting is 1100~1150℃;

[0031] 3) Controlled rolling

[0032] Controlled rolling is divided into two stages. The first stage starts at a rolling temperature of 1050-1100°C, and the thickness of the steel plate after rolling is 3-4 times the thickness of the finished steel plate. When the temperature reaches 860-900°C, the second stage of rolling begins, and the thickness is rolled to the finished steel plate thickness.

[0033] 4) Online water cooling

[0034] The starting cooling temperature is ≥830℃, the final cooling temperature is 250~280℃, and the finished steel plates are obtained by slowly cooling the stacked steel plates off the line to room temperature.

[0035] Preferably, the finished steel plate has a thickness of 5 to 20 mm.

[0036] In the method for manufacturing a weldable ultra-high strength protective steel plate having a tensile strength of 1650 MPa or more described in the present invention:

[0037] Controlled rolling is divided into two stages. The rolling temperature of the first stage is 1050-1100°C. The thickness of the steel plate after rolling is 3-4 times the thickness of the finished steel plate. When the temperature reaches 860-900°C, the second stage rolling begins and the steel plate is rolled to the thickness of the finished steel plate. Since alloy elements such as C and Mn are added in the present invention, the stability of austenite is improved. At the same time, the added Cr element can effectively delay the transformation of ferrite and pearlite, and the rolling temperature can be reduced while maintaining the full austenite structure of the steel plate. The defect density in the deformed austenite is increased, which is conducive to the formation of refined lath martensite and the strength of the steel plate. In addition, vanadium and nitrogen elements are added to the steel in combination, which can induce precipitation through deformation during the first stage of high-temperature rolling, and produce dispersed vanadium nitrides in the steel, which has the effect of precipitation strengthening. The Si element added to the steel can effectively avoid the precipitation and growth of cementite in the steel plate during the slow cooling process, avoid the consumption of carbon elements in the steel, and provide a prerequisite for the stable existence of austenite at room temperature.

[0038] After rolling, online water cooling is carried out, and the start cooling temperature is controlled to be ≥830℃, and the final cooling temperature is 250~280℃, so as to form a refined lath martensite structure and a residual austenite film between the martensite laths. In the first water cooling process, the alloy elements such as C, Mn, Cr, which have a faster cooling rate, are basically dissolved in the lath martensite because they do not have time to diffuse large atoms, which plays a positive role in improving the strength of martensite. The final cooling temperature is controlled at 250~280℃, 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 because of its small atoms. According to Gibbs free energy, carbon atoms will diffuse from martensite to the adjacent residual austenite, forming carbon-rich austenite at the martensite-austenite interface, further improving the chemical stability of the residual austenite, so that it will not undergo martensitic transformation after finally cooling to room temperature, and will remain between the martensite laths to form a residual austenite film. These residual austenites can relieve stress concentration and delay the generation and expansion of cracks during deformation, thereby improving the plasticity and toughness of the steel plate.

[0039] On the other hand, the thermal stress and structural stress concentration of the steel plate after online water cooling can also be alleviated to a certain extent through slow cooling, reducing the residual stress of the steel plate, so that the steel plate has good plasticity while maintaining high strength.

[0040] Beneficial effects of the present invention:

[0041] 1. In terms of composition design, the present invention adds elements such as C and Mn to stabilize austenite in steel, and controls 840C+140Mn+21Si+168Cr+60V≤714. By utilizing stable austenite, the pearlite-ferrite transformation temperature is postponed, the full austenite structure of the steel plate is maintained, the defect density in the deformed austenite is increased, which is conducive to the formation of refined lath martensite, and a part of the austenite is retained without transformation. While the steel plate obtains higher strength, it can also maintain good plastic toughness. The tensile strength at room temperature is ≥1650MPa, and the -40℃ V-type Charpy impact energy is ≥25J.

[0042] 2. In terms of composition design, the present invention postpones the pearlite-ferrite transformation temperature, reduces the final rolling and cooling temperature of the steel plate, increases the defect density in the deformed austenite, increases the nucleation position of the martensitic phase transformation, is conducive to the formation of refined martensitic laths, and improves the strength of the steel plate; at the same time, the above-mentioned alloy elements improve the chemical stability of the austenite. Since the austenite transformation temperature decreases, online cooling is directly adopted after rolling. During the cooling process, it can be ensured that part of the austenite does not undergo transformation and is retained to room temperature, which has a significant promoting effect on the toughness improvement of the steel plate, so that the final steel plate can obtain ultra-high strength and good plastic toughness without the need for traditional heat treatment processes such as quenching and tempering.

[0043] 3. The steel plate of the present invention adopts conventional rolling + direct online water cooling process, which improves the rolling efficiency of the steel plate, reduces the energy consumption of the rolling process, greatly reduces the production cost, and has a simple production process and strong operability. However, the traditional ultra-high strength steel production also requires an offline tempering heat treatment process, which has high energy consumption, low production efficiency, and high comprehensive cost of the steel plate. From the implementation effect, the ultra-high strength steel plate of the present invention is suitable for stable mass production in medium and thick plate production lines. DETAILED DESCRIPTION

[0044] The present invention will be further described below in conjunction with the embodiments.

[0045] The specific composition of the steel plate according to the embodiment of the present invention is shown in Table 1, and the balance includes Fe and other inevitable impurities.

[0046] The process parameters of the steel plates of the embodiments of the present invention are shown in Table 2, and the properties of the steel plates of the embodiments are shown in Table 3.

[0047] The steel plate produced by the present invention has a tensile strength of ≥1650MPa, and a V-type Charpy impact energy of ≥25J at -40°C. Compared with the steel plates of the same strength level currently produced by the quenching and tempering process, the mechanical properties are similar, but the alloy elements of the steel plate itself are significantly reduced, and the production process of the steel plate is simplified, eliminating the traditional offline quenching + tempering treatment, reducing the production energy consumption of the steel plate, improving the production efficiency of the steel plate, and significantly reducing the comprehensive production cost.

[0048]

[0049]

[0050]

Claims

1. A weldable ultra-high strength protective steel plate with a tensile strength of more than 1650 MPa, wherein the chemical composition by weight is as follows: C: 0.25-0.45%, Si: 0.6-1.10%, Mn: 1.10-1.70%, Al: 0-0.20%, P < 0.02%, S < 0.01%, Cr: 0.85-1.35%, Nb: 0.01-0.08%, V: 0.10-0.20%, N: 0.01-0.03%, and the balance includes Fe and other unavoidable impurities; Moreover, the above elements must satisfy the following relationship at the same time: 840C+140Mn+21Si+168Cr+60V≤714.

2. The weldable ultra-high strength protective steel plate with a tensile strength of 1650 MPa or more according to claim 1, characterized in that: The balance is Fe and other inevitable impurities.

3. The weldable ultra-high strength protective steel plate with a tensile strength of 1650 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 + dispersed precipitated vanadium nitride.

4. The weldable ultra-high strength protective steel plate with a tensile strength of 1650 MPa or more as claimed in claim 1, 2 or 3, characterized in that: The steel plate has a room temperature tensile strength of ≥1650 MPa, and a -40°C V-type Charpy impact energy of ≥25 J.

5. The method for manufacturing a weldable ultra-high strength protective steel plate having a tensile strength of 1650 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 1100~1150℃; 3) Controlled rolling Controlled rolling is divided into two stages. The first stage starts at a rolling temperature of 1050-1100°C, and the thickness of the steel plate after rolling is 3-4 times the thickness of the finished steel plate. When the temperature reaches 860-900°C, the second stage of rolling begins, and the thickness is rolled to the finished steel plate thickness. 4) Online water cooling The starting cooling temperature is ≥830℃, the final cooling temperature is 250~280℃, and the finished steel plates are obtained by slowly cooling the stacked steel plates off the line to room temperature.

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

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