1000MPa-grade ultrahigh-strength steel and production method thereof

Abstract

The invention discloses 1000MPa-grade ultrahigh strength steel and a production method thereof. The 1000MPa-grade ultrahigh strength steel comprises the following chemical components in percentage by weight: 0.15 to 0.21 percent of C, 0.20 to 1.00 percent of Si, 0.80 to 1.50 percent of Mn, 0.010 to 0.045 percent of Ti, 0.061 to 0.120 percent of V, 0.010 to 0.030 percent of Nb, 0.40 to 1.20 percent of Cr, less than or equal to 0.30 percent of Mo, less than or equal to 0.80 percent of W, less than or equal to 0.30 percent of Ni, less than or equal to 0.40 percent of Cu, 0.0005 to 0.0030 percent of B, less than or equal to 0.0020 percent of RE, 0.02 to 0.06 percent of Al, 0.001 to 0.004 percent of Ca, less than or equal to 0.005 percent of N, less than or equal to 0.020 percent of P, less According to the method, fine grain strengthening and precipitation strengthening are fully utilized, the ultra-high strength and toughness of the steel plate are guaranteed, and then the ultra-high strength steel with good comprehensive performance is obtained; and meanwhile, a rapid heat treatment process is combined, so that the total time of quenching and tempering treatment is greatly shortened, the production efficiency is improved, and the production cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of ultra-high strength steel production, and in particular to 1000 MPa ultra-high strength steel and a production method thereof. Background Art

[0002] Hot-rolled ultra-high-strength steel with a yield strength of 960MPa is mainly used to manufacture components such as crane booms and pump truck fabric booms, which have very high requirements on the strength, plasticity, low-temperature toughness and cold bending properties of the steel plate.

[0003] Chinese patent CN108774681A discloses "ultra-fast heat treatment method for high-strength steel", which rapidly heats high-strength steel to above Ac3+50℃ at a heating rate of 400℃ / s, keeps it warm for 5-10s, and instantly cools it to room temperature at an extremely high cooling rate of 3000℃ / s. This method is suitable for extremely thin strips and is difficult to implement for hot-rolled strip steel. On the one hand, the heating rate is too fast, making it difficult to control the temperature of the strip steel. On the other hand, the cooling rate is very high, and thick strip steel generates latent heat of phase change during the cooling process, which makes it impossible to achieve the expected cooling rate.

[0004] Chinese patent CN1039621A discloses "a heat treatment method for producing fine-grained dual-phase steel". This method uses induction heating and contact electric heating to heat low-carbon low-alloy steel to the dual-phase region and then quench to obtain certain strength, plasticity, cold formability and deformation strengthening ability. For hot-rolled ultra-high-strength steel, dual-phase heating will inevitably form ferrite, and the strength cannot reach the expected target.

[0005] Chinese patent CN112553526A discloses "a 960MPa grade ultra-high strength structural steel, steel pipe and its working method and application", the main components of which are C: 0.12-0.17%, Mn: 0.7-1.3%, Cr: 0.5-1.0%, Mo: 0.70-1.0%, W: 0.1-0.3%, etc. The invention adds a higher Mo element, which has a higher cost, and the impact energy obtained at -40℃ is only 45J.

[0006] Chinese patent CN111455269A discloses "Marine steel plate with yield strength of 960 MPa or even higher and its manufacturing method". The main chemical composition is C: 0.07-0.15%, Si: 0.02-0.15%, Mn: 0.80-1.3%, Mo: 0.40-0.70%, Ni: 1.0-2.0%, V: 0.03-0.06%, Cu: 0.20-0.50%, etc., and conventional heat treatment process is adopted. This patent adds higher Mo, Ni and Cu elements, which is more expensive.

[0007] The existing 960MPa grade ultra-high strength steel is generally produced by "direct quenching + tempering" or "offline quenching and tempering" to obtain tempered martensite. For offline quenching and tempering heat-treated ultra-high strength steel, the traditional heat treatment process has a slow heating speed and a long holding time during the quenching and tempering process. The holding time of the quenching and tempering process is generally more than 90 minutes, which makes the production cycle too long, the production efficiency is low, and the cost is high. In addition, long-term heat preservation during the quenching stage is also prone to abnormal grain growth, reducing the strength and plastic toughness of the steel plate, causing the steel plate to crack when the user bends. In order to improve toughness, this type of ultra-high strength steel often adds a large amount of precious metal elements such as Ni and Mo, which makes the alloy cost very high; in addition, the V and Ti content of this type of steel is low, which is to avoid the formation of large particles of VN and TiN, which reduces toughness. At present, ultra-high strength steel is developing in the direction of low cost, high performance or additional performance. Summary of the invention

[0008] The object of the present invention is to provide a 1000MPa grade ultra-high strength steel and a production method thereof, wherein the ultra-high strength steel has ultra-high strength, high plastic toughness and high cold bending performance, wherein the yield strength is ≥960MPa, the tensile strength is ≥1000MPa, the elongation is >20%, the transverse and longitudinal impact energy at -60°C is >100J, and the cold bending performance satisfies D=3a, 90°; at the same time, a rapid heat treatment process is adopted to reduce the total time of quenching and tempering heat treatment from the traditional 90min or more to 3-10min, thereby greatly improving the production efficiency and reducing the production cost.

[0009] To achieve the above object, the technical solution of the present invention is:

[0010] A 1000MPa grade ultra-high strength steel, the chemical composition weight percentage of which is: C: 0.15-0.21%, Si: 0.20-1.00%, Mn: 0.80-1.50%, Ti: 0.010-0.045%, V: 0.061-0.120%, Nb: 0.010-0.030%, Cr: 0.40-1.20%, Mo≤0.30%, W≤0.80%, Ni≤0.30%, Cu≤0.40%, B: 0.0005-0.0030%, RE≤0.0020%, Al: 0.02-0.06%, Ca: 0.001-0.004%, N≤0.005%, P≤0.020%, S≤0.0050%, and the rest includes Fe and other inevitable impurities;

[0011] The ultra-high strength steel microstructure is tempered troostite + nano-scale precipitates, wherein the original austenite size is 5-10 μm, the tempered troostite lath width and lath gap are 30-90 nm, the precipitate size is 2-5 nm, and the precipitate types are TiC, VC and Fe3C.

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

[0013] The ultra-high strength steel of the present invention has a yield strength of ≥960MPa, a tensile strength of ≥1000MPa, an elongation of >20%, a -60°C transverse and longitudinal impact energy of >100J, and a cold bending performance that satisfies D=3a, 90°.

[0014] In the composition design of the 1000MPa grade ultra-high strength steel of the present invention:

[0015] Carbon: C is a basic element of steel, plays an important role in improving the strength of steel, and also determines the yield strength and tensile strength of steel after quenching. After quenching, tempering at an appropriate temperature and time can further adjust the strength and plasticity to obtain the expected performance. For ultra-high-strength steel with high strength, a higher C content is required, but if the C content is too high, welding cracking will occur. Therefore, the C content of the present invention is controlled at 0.15-0.21%.

[0016] Silicon: A certain amount of Si inhibits the diffusion of carbon during tempering, thereby inhibiting the growth of VC, TiC and Fe3C, forming nano-scale carbides to precipitate and improve the strength of steel. Therefore, the silicon content is controlled within the range of 0.20-1.00% in the present invention.

[0017] Manganese: Mn is the most basic element of steel and one of the important elements in the present invention. Mn can improve the hardenability of steel when it is above 0.8%, but when the content exceeds 1.6%, segregation is likely to occur during steelmaking, resulting in inclusions such as MnS, which seriously affects the toughness of ultra-high strength steel. Therefore, the Mn content of the present invention is controlled at 0.80-1.50%.

[0018] Titanium: Ti is a microalloying element and one of the important elements in the present invention. Ti reacts with elements such as C and N in steel to produce TiC and TiN. TiC can produce nano-scale precipitates when coiled at a suitable temperature, producing a greater precipitation strengthening effect. TiN easily forms large-sized cubic compounds, which will deteriorate the toughness and plasticity of steel. Therefore, the Ti content of the present invention is controlled at 0.010-0.045%.

[0019] Vanadium: V is a microalloying element and one of the important elements in the present invention. V is a strong carbide-forming element and can react with C to form VC precipitation particles. Through the rapid heat treatment process, nano-scale VC precipitates are formed to give full play to the precipitation strengthening effect of VC. Therefore, the vanadium content of the present invention is controlled at 0.061-0.120%.

[0020] Niobium: Nb is a microalloying element. After Nb is added to steel, it can refine austenite during the heating and rolling stages, and the grain refinement effect is obvious. Therefore, the niobium content of the present invention is controlled at 0.010-0.030%.

[0021] Chromium: Cr is an element that stabilizes austenite, improves the hardenability of steel, and is more likely to form martensite during quenching. During tempering, Cr carbides are formed, which have the effect of resisting tempering softening. If the Cr content exceeds 1.2%, large sparks will appear during welding, affecting the welding quality. Therefore, the Cr content of the present invention is controlled at 0.40-1.2%.

[0022] Molybdenum: Mo is similar to Cr, and can improve the hardenability of steel and has the effect of resisting high temperature tempering softening. Too high Mo content will lead to an increase in carbon equivalent and deteriorate welding performance. At the same time, Mo is a precious metal and will increase the cost. Therefore, the present invention controls the Mo content to ≤ 0.30%.

[0023] Tungsten: The W element can improve the hardenability of steel, form carbide particles during tempering, and has a significant effect of resisting temper softening, and also has a role in resisting temper brittleness. Therefore, the present invention controls the W content to ≤0.80%.

[0024] Nickel: 0.10% or more of Ni element has the effect of refining martensite structure and improving toughness. Too high Ni content will increase carbon equivalent and deteriorate welding performance. At the same time, Ni is a precious metal and will increase cost. Therefore, the present invention controls Ni content ≤ 0.30%.

[0025] Copper: The Cu element can produce a certain precipitation strengthening effect during tempering. In addition, adding a certain amount of Cu element can improve the corrosion resistance of ultra-high strength steel for engineering machinery. Therefore, the present invention controls the Cu content to ≤ 0.40%.

[0026] Boron: A small amount of B can improve the hardenability and strength of steel. B exceeding 0.0030% is prone to segregation and forms carbon-boron compounds, which seriously deteriorates the toughness of steel. Therefore, the present invention controls the boron content to 0.0005-0.0030%.

[0027] Rare earth elements: Rare earth elements can improve the morphology of inclusions in steel, making the inclusions smaller and more dispersed. Especially for the steel containing Ti component of the present invention, the size and morphology of TiN inclusions can be significantly improved, and the toughness and fatigue performance of steel can be improved. Too much rare earth elements will increase the cost. Therefore, the present invention controls the RE content to ≤0.0020%.

[0028] Aluminum: Al is used as a deoxidizer. Adding more than 0.02% Al to steel can refine grains and improve impact toughness. When the Al content exceeds 0.06%, Al oxide inclusion defects are easily generated. Therefore, the present invention controls the Al content to 0.02-0.06%.

[0029] Calcium: Ca elements exceeding 0.001% can act as a purifier in the steel smelting process and improve the toughness of the steel; too high a Ca content is likely to form larger Ca compounds, which will deteriorate the toughness. Therefore, the present invention controls the Ca content to 0.001-0.004%.

[0030] Nitrogen: N is an impurity element in the present invention, and the lower its content, the better. N must be fixed in the form of nitride during steelmaking, otherwise free N atoms are very detrimental to the impact toughness of steel. Therefore, the present invention controls the N content to ≤0.005%.

[0031] Phosphorus and sulfur: P and S, as impurity elements, affect the plasticity and toughness of steel. The present invention controls P≤0.020% and S≤0.0050% respectively.

[0032] The composition design of the present invention adopts a high V design, adds 0.061-0.120% of V elements, controls VC to be fully precipitated after rolling and coiling, and at the same time, by adding 0.20-1.00% of Si elements, the diffusion of C elements and the growth of VC are inhibited, and VC precipitate particles of 2-5nm are generated, so that V produces a precipitation strengthening effect; Si elements can also inhibit the growth of Fe3C and TiC, and VC, Fe3C and TiC interact with each other to produce a better precipitation strengthening effect. The present invention makes full use of the precipitation strengthening effect of precipitates, ensures the ultra-high strength and toughness of the steel plate, and then obtains ultra-high strength steel with good comprehensive performance, and at the same time avoids the addition of a large amount of precious metal elements such as Ni and Mo in the composition design.

[0033] The production method of 1000MPa grade ultra-high strength steel of the present invention comprises the following steps:

[0034] 1) Smelting and casting

[0035] According to the above chemical composition, steel is smelted and refined by converter or electric furnace, and then cast to form ingot;

[0036] 2) Heating

[0037] The heating temperature of the ingot is 1230-1300℃. When the core of the ingot reaches the heating temperature, the insulation begins. The insulation time is >2.0h.

[0038] 3) Rolling and coiling

[0039] The controlled rolling and controlled cooling process is adopted, the coiling temperature is 510-580℃, and air cooling to room temperature after coiling;

[0040] 4) Heat treatment

[0041] Quenching treatment: the steel plate is rapidly heated to Ac3~(Tvc+150)℃ at a speed of more than 20℃ / s, kept warm for a short time for 10~30s, and rapidly cooled to room temperature at a cooling rate of ≥150℃ / s;

[0042] After tempering, the temperature is heated to 530-630°C at a rate of more than 15°C / s, kept at this temperature for 20-60s, and cooled to room temperature in air;

[0043] Ac3 is the end temperature of austenite transformation, Ac3 = 955-350C-25Mn+51Si+106Nb+100Ti+68Al-11Cr-33Ni-16Cu+67Mo, unit: °C;

[0044] Tvc is the melting temperature of VC, Tvc=7041 / (4.7-lg(V×C)), unit: °C; wherein V and C are the weight percentages of the corresponding elements in the chemical composition of the ultra-high strength steel, respectively.

[0045] In the production method of 1000MPa grade ultra-high strength steel of the present invention:

[0046] In the billet heating process, the slab is heated to 1230-1300°C and the holding time is greater than 2 hours to allow the alloy elements to diffuse evenly.

[0047] The controlled rolling and controlled cooling process is adopted during rolling, and the steel is coiled at 510-580℃ and air-cooled to room temperature. After rolling, a hot-rolled substrate is obtained. Ferrite and a small amount of pearlite structure are obtained when coiled at 510-580℃, so that VC and TiC are fully precipitated in the ferrite structure during coiling.

[0048] The heat treatment of the present invention adopts a rapid heat treatment process, mainly considering the following points:

[0049] 1) Rapid heat treatment process is adopted, and quenching begins immediately after the steel plate completes austenitization. In the quenching process, the steel plate is rapidly heated to Ac3~(Tvc+150)℃ at a heating rate of more than 20℃ / s, and kept warm for 10~30s. The extremely short time of 10~30s keeps the VC and TiC precipitated during rolling from dissolving and coarsening in time, and keeps the nano-size state. At the same time, because the holding time is very short, the austenite is very fine, only 5~10μm, and then a finer martensite structure is obtained after quenching, which has the effect of fine grain strengthening.

[0050] The grain size of austenite in traditional heat treatment is usually 25-50μm, resulting in a coarse martensite structure, and the strength and plasticity of the steel plate are relatively low; and the heating time before quenching is very long, usually more than 20 minutes. The long heating dissolves the VC precipitated during rolling and coiling. The higher the heating temperature, the shorter the time required for VC to dissolve. When heated at 900℃ for more than 3 minutes, VC will dissolve; the dissolution temperature of TiC is relatively high, generally above 1000℃, but long-term heating and insulation will coarsen the TiC precipitated during rolling and coiling, reducing the precipitation strengthening effect.

[0051] 2) During the tempering treatment, the steel plate is rapidly heated to 530-630°C at a heating rate of more than 15°C / s, kept at this temperature for 20-60s, and then air-cooled to room temperature. During the tempering period, the martensite structure is transformed into the tempered troostite structure. Rapid tempering can also obtain fine nano-scale Fe3C precipitates, while ensuring that the nano-scale VC and TiC precipitated by rolling will not coarsen.

[0052] 3) Rapid heat treatment process is used to obtain fine original austenite structure with a size of 5 to 10 μm, and then tempered troostite and nano-scale precipitates with small lath width and lath spacing are obtained. The tempered troostite lath width and lath spacing are 30 to 90 nm. At the same time, the precipitate size is refined to form nano-scale TiC, VC and Fe3C precipitates with a size of 2 to 5 nm. The precipitation strengthening effect of the precipitates is fully utilized to ensure the ultra-high strength and toughness of the steel plate, thereby obtaining ultra-high strength steel with good comprehensive performance.

[0053] Compared with the prior art, the present invention has the following beneficial effects:

[0054] In the composition design of the present invention, relatively high V and Ti elements are added, and at the same time, Si element is added to inhibit the diffusion of carbon elements and the growth of VC, thereby producing VC precipitate particles of 2 to 5 nm, so that VC produces a precipitation strengthening effect of more than 100 MPa. Si element can also inhibit the growth of Fe3C and TiC. VC, Fe3C and TiC work together to produce a precipitation strengthening effect of more than 150 MPa compared to steel types without precipitation strengthening effect. The precipitation strengthening effect of precipitates is fully utilized to ensure the ultra-high strength and toughness of the steel plate, thereby obtaining ultra-high strength steel with good comprehensive performance. However, in conventional composition design, a large amount of precious metal elements such as Ni and Mo need to be added to improve the toughness of steel.

[0055] The present invention, based on the component design, utilizes controlled rolling and controlled cooling to promote the full precipitation of VC and TiC, and then combines with a rapid heat treatment process to obtain fine tempered troostite and nanometer-scale carbides, avoids the dissolution and coarsening of the fully precipitated VC and TiC, and simultaneously precipitates nanometer-scale Fe3C precipitates, fully utilizing the precipitation strengthening effect of nanometer-scale precipitates including TiC, VC and Fe3C, and ensures the ultra-high strength and toughness of the steel plate from the perspectives of fine grain strengthening and precipitation strengthening, thereby obtaining ultra-high strength steel with good comprehensive performance.

[0056] The present invention greatly reduces or avoids the addition of precious metal elements such as Ni and Mo in component design. In process design, a rapid heat treatment process is adopted, and the total time of quenching and tempering heat treatment is reduced from the traditional 90 minutes or more to 3 to 10 minutes, which greatly improves production efficiency, significantly shortens the production cycle, and greatly improves production efficiency. From the perspective of composition and process, the production cost of producing ultra-high strength, high plasticity and high cold-bent steel plates is reduced.

[0057] The ultra-high strength steel of the present invention has a yield strength of ≥960MPa, a tensile strength of ≥1000MPa, an elongation of >20%, a transverse and longitudinal impact energy of >100J at -60°C, and a cold bending performance satisfying D=3a, 90°. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 This is a typical optical microscope metallographic structure diagram of the ultra-high strength steel plate of Example 3 of the present invention.

[0059] Figure 2 This is a typical scanning electron microscope metallographic structure diagram of the ultra-high strength steel plate of Example 3 of the present invention.

[0060] Figure 3 This is a typical transmission electron microscope metallographic structure diagram of the ultra-high strength steel plate of Example 3 of the present invention.

[0061] Figure 4 This is the cold bending performance of the ultra-high strength steel plate of Example 3 of the present invention. DETAILED DESCRIPTION

[0062] The present invention will be further described below in conjunction with the embodiments and drawings.

[0063] The composition and specific production process parameters of the embodiments of the present invention are shown in Table 1 and Table 2, and the properties of the steel plates of each embodiment are shown in Table 3.

[0064] The production process of the ultra-high strength steel plate of the present invention is: smelting, casting → heating → rolling → cooling → rapid heat treatment.

[0065] Figure 1 to Figure 3 The microstructure photos of the steel plate of Example 3 of the present invention under an optical microscope, a scanning electron microscope and a transmission electron microscope are shown respectively.

[0066] from Figure 1 From the metallographic photos, it can be seen that the metallographic microstructure of the finished steel plate is fine tempered troostite, and the original austenite size is 5 to 10 μm. Figure 2 , Figure 3 A large number of dispersed nanoscale precipitates can be seen, and the size of the precipitates is 2 to 5 nm.

[0067] Figure 4The cold bending results of the steel plate under different elbow diameters of Example 6 of the present invention are given. When the elbow diameter D=3a, after cold bending by 90°, there is no crack on the surface of the steel plate.

[0068] The present invention adopts controlled rolling and controlled cooling + rapid heat treatment process, starting from the perspective of fine grain strengthening and precipitation strengthening, by designing the chemical composition design, regulating the substrate organization, obtaining fine tempered troostite and nano-scale carbides, these nano-scale precipitates include TiC, VC and Fe3C, and finally producing ultra-high strength steel with ultra-high strength, high plastic toughness and high cold bending performance. The yield strength of the steel plate of the present invention is ≥960MPa, the tensile strength is ≥1000MPa, the elongation is >20%, the transverse and longitudinal impact energy at -60℃ is >100J, and the cold bending performance meets D=3a, 90°.

[0069]

[0070]

[0071]

Claims

1. A 1000MPa grade ultra-high strength steel, the chemical composition of which is as follows by weight: C: 0.15-0.21%, Si: 0.20-1.00%, Mn: 0.80-1.50%, Ti: 0.010-0.045%, V: 0.061-0.120%, Nb: 0.010-0.030%, Cr: 0.40-1.20%, Mo≤0.30%, W≤0.80%, Ni≤0.30%, Cu≤0.40%, B: 0.0005-0.0030%, RE≤0.0020%, Al: 0.02-0.06%, Ca: 0.001-0.004%, N≤0.005%, P≤0.020%, S≤0.0050%, and the rest includes Fe and other unavoidable impurities; The microstructure of the ultra-high strength steel is tempered troostite + nano-scale precipitates; wherein, The original austenite size is 5-10 μm, the tempered troostite lath width and lath gap are 30-90 nm, the precipitate size is 2-5 nm, and the precipitate types are TiC, VC and Fe3C.

2. The 1000MPa grade ultra-high strength steel according to claim 1, characterized in that: The rest is Fe and other inevitable impurities.

3. The 1000MPa grade ultra-high strength steel according to claim 1 or 2, characterized in that: The ultra-high strength steel has a yield strength of ≥960MPa, a tensile strength of ≥1000MPa, and an elongation of >20%. -60℃ transverse and longitudinal impact energy>100J, cold bending performance meets D=3a, 90°.

4. The method for producing 1000 MPa grade ultra-high strength steel according to any one of claims 1 to 3, 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 then cast to form a casting ingot; 2) Heating The heating temperature of the ingot is 1230-1300℃. When the core of the ingot reaches the heating temperature, the insulation begins. The insulation time is >2.0h. 3) Rolling and coiling The controlled rolling and controlled cooling process is adopted, the coiling temperature is 510-580℃, and air cooling is performed to room temperature after coiling; 4) Rapid heat treatment Quenching treatment: the steel plate is rapidly heated to Ac3~(Tvc+150)℃ at a speed of more than 20℃ / s, kept warm for 10~30s, and then rapidly cooled to room temperature at a cooling rate of ≥150℃ / s; After tempering, the steel is heated to 530-630°C at a speed of more than 15°C / s, kept warm for 20-60s, and air-cooled to room temperature. Ac3 is the end temperature of austenite transformation, Ac3 = 955-350C-25Mn+51Si+106Nb+100Ti+68Al-11Cr-33Ni-16Cu+67Mo, unit: °C; Tvc is the melting temperature of VC, Tvc=7041 / (4.7-lg(V×C)), unit: °C; wherein V and C are the weight percentages of the corresponding elements in the chemical composition of the ultra-high strength steel, respectively.

5. The production method according to claim 4, characterized in that The microstructure of the obtained ultra-high strength steel is tempered troostite + nanoscale precipitates, wherein the original austenite size is 5-10 μm, the tempered troostite lath width and lath gap are 30-90 nm, the precipitate size is 2-5 nm, and the precipitate types are TiC, VC and Fe3C.

6. The production method according to claim 4 or 5, characterized in that The yield strength of the obtained ultra-high strength steel is ≥960MPa, the tensile strength is ≥1000MPa, the elongation is >20%, the transverse and longitudinal impact energy at -60°C is >100J, and the cold bending performance meets D=3a, 90°.

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