Ultra-high strength steel with an elongation of not less than 20% and preparation method thereof

Abstract

The present invention relates to an ultra-high strength steel with an elongation of not less than 20% and a preparation method thereof, and belongs to the technical field of ultra-high strength steel. The chemical composition of the ultra-high strength steel with an elongation of not less than 20% is calculated by mass percentage as follows: C: 0.08%-0.11%, Si: 0.12%-0.25%, Mn: 1.0%-2.0%, Ni: 9.5%-10.5%, Cr: 0.8%-1.2%, Ti: 0.011%-0.015%, Mo: 0.85%-1.2%, V: 0.085%-0.15%, Als: 0.4%-0.6%, N≤0.004%, P≤0.005%, S≤0.005%, and the rest are Fe and inevitable impurity elements. The steel of the present invention has the characteristics of an elongation of not less than 20%, ultra-high strength, high plasticity and excellent low temperature toughness.

Description

Technical Field

[0001] The present invention relates to the technical field of ultra-high strength steel, and in particular to an ultra-high strength steel with an elongation of not less than 20% and a preparation method thereof. Background Art

[0002] Ships and offshore engineering equipment are iconic equipment in marine resource development projects, supporting engineering equipment with a total weight of more than hundreds of tons. These usage characteristics determine that the steel used for ships and offshore engineering equipment must have high strength, high toughness, fatigue resistance, lamellar tear resistance, good weldability and cold workability, and seawater corrosion resistance. As ships and offshore engineering equipment develop towards large-scale, in order to improve the safety and mobility of steel used in offshore platforms, the proportion of high-strength / ultra-high-strength steel used has increased year by year.

[0003] The strength and plasticity (elongation) of ultra-high strength steel are a pair of contradictions. Improving both strength and plasticity has long been a major problem in the field of steel materials, and is also a bottleneck from basic research to technological innovation and application practice. Especially when the strength reaches 1100MPa, the plasticity drops drastically, and the elongation is generally less than 15%.

[0004] Therefore, how to improve the plasticity of ultra-high strength steel has become an urgent problem to be solved. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide an ultra-high strength steel with an elongation of not less than 20% and a preparation method thereof, so as to solve the problem of low elongation and strength-ductility product of existing ultra-high strength steel.

[0006] The purpose of the present invention is mainly achieved through the following technical solutions:

[0007] The present invention provides an ultra-high strength steel with an elongation of not less than 20%, wherein the chemical composition of the ultra-high strength steel is as follows by mass percentage: C: 0.08%-0.11%, Si: 0.12%-0.25%, Mn: 1.0%-2.0%, Ni: 9.5%-10.5%, Cr: 0.8%-1.2%, Ti: 0.011%-0.015%, Mo: 0.85%-1.2%, V: 0.085%-0.15%, Als: 0.4%-0.6%, N≤0.004%, P≤0.005%, S≤0.005%, and the rest are Fe and unavoidable impurity elements.

[0008] Furthermore, in the chemical composition of the ultra-high strength steel with an elongation of not less than 20%, Ni=-5Als+0.125, wherein Ni and Als represent the mass percentages of the corresponding elements.

[0009] Furthermore, the chemical composition of the ultra-high strength steel with an elongation of not less than 20% is, by mass percentage, as follows: C: 0.085% to 0.11%, Si: 0.12% to 0.24%, Mn: 1.1% to 1.9%, Ni: 9.5% to 10.4%, Cr: 0.9% to 1.2%, Ti: 0.011% to 0.014%, Mo: 0.9% to 1.2%, V: 0.09% to 0.15%, Als: 0.45% to 0.6%, N: 0.002% to 0.004%, P≤0.005%, S≤0.005%, and the rest are Fe and unavoidable impurity elements.

[0010] Furthermore, the microstructure of the ultra-high strength steel having an elongation of not less than 20% includes tempered martensite, ferrite and reversed austenite, and NiAl precipitated phase.

[0011] Furthermore, in the microstructure of the ultra-high strength steel with an elongation of not less than 20%, the volume percentage of tempered martensite is not less than 65%, the volume percentage of ferrite is not less than 10%, and the volume percentage of reversed austenite is not less than 15%.

[0012] The present invention also provides a method for preparing ultra-high strength steel with an elongation of not less than 20%, comprising:

[0013] Step 1: molten iron pretreatment;

[0014] Step 2: converter smelting;

[0015] Step 3: LF refining;

[0016] Step 4: RH refining;

[0017] Step 5: Continuously casting to obtain a continuous casting billet;

[0018] Step 6: heating the continuous casting billet, and performing controlled rolling and controlled cooling on the continuous casting billet;

[0019] Step 7: Offline quenching and tempering: The continuous casting billet after controlled rolling and controlled cooling is subjected to offline quenching and tempering to obtain ultra-high strength steel with an elongation of not less than 20%.

[0020] Furthermore, in step 6, the heating temperature of the continuous casting billet is ≤1200°C.

[0021] Furthermore, in step 6, the controlled rolling adopts a two-stage rolling method: divided into a first stage rolling and a second stage rolling, the first stage rolling is rough rolling, and the first stage rolling performs rapid deformation in the austenite recrystallization temperature range.

[0022] Furthermore, in step 6, the final rolling temperature of the first stage rolling is controlled to be ≥ 950°C.

[0023] Furthermore, in step 7, the offline tempering includes two quenching processes and one tempering process.

[0024] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0025] 1. The ultra-high strength steel with an elongation of not less than 20% of the present invention ensures excellent mechanical properties of the steel by precisely controlling the types and contents of elements, especially the element ratios of Al, Ni, Cr and Mo, and realizes ultra-high strength, high plasticity and excellent low-temperature toughness of the steel.

[0026] 2. The preparation method of the present invention combines rolling, cooling, and off-line quenching and tempering processes to ensure that the microstructure of the steel includes tempered martensite, ferrite, reversed austenite, and NiAl precipitated phase, the volume percentage of tempered martensite is not less than 65%, the volume percentage of ferrite is not less than 10%, the volume percentage of reversed austenite is not less than 15%, and the NiAl phase particle number density is not less than 5×10 23 m -3 The excellent comprehensive mechanical properties of the steel are ensured. The preparation method of the present invention has a simple production process and can be applied on a large scale.

[0027] 3. The yield strength of the ultra-high strength steel with an elongation of not less than 20% of the present invention is ≥1100MPa (e.g. 1120-1150MPa), the tensile strength is ≥1200MPa (e.g. 1220-1250MPa), the elongation is ≥20.0% (e.g. 20.5%-22.0%), the impact energy at -40°C is ≥80J (e.g. 100-120J), and the strength-plasticity product is 25000MPa·% or more (e.g. 25200-26000MPa·%), which solves the technical problem that the elongation of 1000MPa-grade ultra-high strength steel is less than 15%. The ultra-high strength steel prepared by the present invention can be used for large hull structures or marine engineering equipment.

[0028] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0030] Figure 1 It is the EBSD structure of 1# steel of the present invention;

[0031] Figure 2 This is the 3D-ATP morphology of the NiAl precipitation phase of the 1# steel of the present invention. DETAILED DESCRIPTION

[0032] Preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used to explain the principles of the present invention together with the embodiments of the present invention.

[0033] The present invention provides an ultra-high strength steel with an elongation of not less than 20%. The chemical composition of the ultra-high strength steel with an elongation of not less than 20% is as follows by mass percentage: C: 0.08%-0.11%, Si: 0.12%-0.25%, Mn: 1.0%-2.0%, Ni: 9.5%-10.5%, Cr: 0.8%-1.2%, Ti: 0.011%-0.015%, Mo: 0.85%-1.2%, V: 0.085%-0.15%, Als: 0.4%-0.6%, N≤0.004%, P≤0.005%, S≤0.005%, and the rest are Fe and unavoidable impurity elements.

[0034] Specifically, in the chemical composition of the ultra-high strength steel with an elongation of not less than 20%, Ni=-5Als+0.125 is controlled, where Ni and Als represent the mass percentage of the corresponding elements. Ni and Als phases form a nano-scale NiAl precipitation phase, which can significantly improve the yield strength of the steel, but will reduce the toughness and plasticity of the test steel. Therefore, it is necessary to match a rich Ni element to improve the low-temperature toughness of the steel plate. The main function of the rich Ni content is to promote the formation of reverse transformation austenite structure and improve the low-temperature toughness of the steel plate.

[0035] The effects and dosages of the chemical components in the present invention are described in detail below, and only % is used to represent the mass percentage in the composition.

[0036] C: The C content in the steel of the present invention needs to be less than 0.11% to ensure the toughness and plasticity of the steel plate; when the C content is ≤ 0.08%, it will not meet the requirements of ultra-high strength. Therefore, the C content is controlled at 0.08% to 0.11%.

[0037] Mn: Mn solid solution in steel will increase the strength of the steel. The Mn content should be controlled above 1.0% to ensure the strength of the steel. When the Mn content exceeds 2.0%, center segregation will occur, reducing the low-temperature toughness of the parent material. Therefore, the Mn content is controlled at 1.0% to 2.0%.

[0038] Si: Si is usually used as a deoxidizer during steelmaking. In the present invention, when the silicon content is less than 0.12%, the molten steel is easily oxidized; Si is also a solid solution strengthening element, but a large amount of Si is usually unfavorable to welding performance. In order to ensure the toughness of the welding heat affected zone, the Si content should be controlled below 0.25%. Therefore, in the present invention, the Si content is controlled at 0.12% to 0.25%.

[0039] Als: Als is an important strengthening element. When the Als content is less than 0.4%, it is difficult to play the role of NiAl phase precipitation strengthening. Therefore, the Als content should be controlled at 0.4% to 0.6%.

[0040] Ni: Ni can improve the low temperature toughness of the material and combine with Al to form NiAl strengthening phase. In order to improve the low temperature toughness requirements of ultra-high strength steel, it is necessary to add a certain amount of Ni element to improve the low temperature toughness and precipitation strengthening effect of the steel plate. However, too high Ni content will increase Ceq and Pcm, causing the preheating temperature of the steel plate to be too high, so the Ni content cannot be higher than 10.5%. Therefore, the Ni content is controlled at 9.5% to 10.5%.

[0041] V: V element usually acts through solute drag and precipitation particles (V(C,N)). On the one hand, it pins the original austenite grain boundary during the material deformation process, and on the other hand, precipitation strengthening improves the strength of the material. The V content is controlled at 0.085% to 0.15%.

[0042] Cr: Cr can improve the hardenability of the material and can increase the strength of the material in the form of carbides. If the Cr content is lower than 0.8%, it is difficult to play its role. However, if the Cr content exceeds 1.2%, it will reduce the low-temperature toughness of the base material and the welding heat-affected zone, and increase the manufacturing cost of the material. Therefore, Cr is controlled at 0.8% to 1.2%.

[0043] Mo: Mo can improve the hardenability of materials, promote low-temperature microstructure transformation, and improve the strength of materials. If the Mo content is less than 0.85%, the effect of Mo is low. If the Mo content exceeds 1.2%, it not only increases the manufacturing cost, but also damages the weldability of the steel plate. Therefore, Mo is controlled at 0.85% to 1.2%.

[0044] Ti: Ti combines with N to form TiN, which inhibits the growth of austenite grains during the heating process of the continuous casting billet and pins the austenite grain size during the welding thermal cycle, thereby improving the toughness of the steel plate and the welding heat affected zone. When the Ti content is lower than 0.011%, it is not easy to play the above role; excessive Ti reduces the precipitation time of TiN and increases the temperature, reducing the pinning effect on austenite grains. Therefore, the Ti content is controlled at 0.011% to 0.015%.

[0045] P: Phosphorus is an impurity element in steel, which will damage the toughness of the steel plate and the heat-affected zone of welding. Therefore, the P content is controlled below 0.005%.

[0046] S: Sulfur is an impurity element in steel, which will form sulfide inclusions and become a crack source. Therefore, the S content is controlled below 0.005%.

[0047] N: A certain amount of N can form TiN with Ti, improve the toughness of the steel plate and the heat-affected zone of welding, and form carbonitrides with Ti to improve strength; but too high a N content will affect the low-temperature toughness of the material. Therefore, the N content is controlled at ≤0.004%.

[0048] Specifically, in order to further improve the comprehensive performance of the ultra-high strength steel with an elongation of not less than 20%, the chemical composition of the ultra-high strength steel with an elongation of not less than 20% is as follows by mass percentage: C: 0.085% to 0.11%, Si: 0.12% to 0.24%, Mn: 1.1% to 1.9%, Ni: 9.5% to 10.4%, Cr: 0.9% to 1.2%, Ti: 0.011% to 0.014%, Mo: 0.9% to 1.2%, V: 0.09% to 0.15%, Als: 0.45% to 0.6%, N: 0.002% to 0.004%, P≤0.005%, S≤0.005%, and the rest are Fe and unavoidable impurity elements.

[0049] The present invention also provides a method for preparing the ultra-high strength steel having an elongation of not less than 20%, comprising the following steps:

[0050] Step 1: molten iron pretreatment;

[0051] Step 2: converter smelting;

[0052] Step 3: LF refining;

[0053] Step 4: RH refining;

[0054] Step 5: Continuously casting to obtain a continuous casting billet;

[0055] Step 6: heating the continuous casting billet, and performing controlled rolling and controlled cooling on the continuous casting billet;

[0056] Step 7: Offline quenching and tempering: The continuous casting billet after controlled rolling and controlled cooling is subjected to offline quenching and tempering to obtain ultra-high strength steel with an elongation of not less than 20%.

[0057] Specifically, in the above step 1, the impurity elements in the steel are reduced by pretreatment of the molten iron to ensure that after KR desulfurization, the S content is ≤ 0.007% and the slag thickness meets the slag removal level 1.

[0058] Specifically, in the above step 2, during converter smelting, the slag layer thickness is required to be ≤100mm when slag blocking and steel tapping, the added alloy material and auxiliary materials such as lime, sintered ore, dolomite, etc. must be dry, and the order of alloying and deoxidation of the ladle is from strong to weak.

[0059] Specifically, in the above step 3, during the LF refining process, the added slag-making materials and deoxidizers must be dried and kept in the silo for ≤24 hours to ensure that the slag has good fluidity; the white slag keeping time is ≥10 minutes, and Ti and Fe must be added after deoxidation is complete.

[0060] Specifically, in the above step 4, during the RH refining process, calcium treatment is performed before steel tapping, and the static argon blowing time after wire feeding is ≥10 min to ensure that the Ca content in the molten steel is 0.001% to 0.0015%.

[0061] Specifically, in the above step 5, the continuous casting process adopts full-process protected casting, electromagnetic stirring and light pressing, the superheat of the tundish is ≤35°C, and after continuous casting, it enters the slow cooling pit for treatment to obtain a continuously cast billet.

[0062] Specifically, in the above step 5, the thickness ratio of the continuous casting billet to the final steel plate is controlled to be greater than 5, so as to achieve the purpose of refining the core structure of the steel plate.

[0063] Specifically, in the above step 6, the heating temperature of the continuous casting billet is ≤ 1200°C to ensure that the austenite grains do not grow. Preferably, the heating temperature of the continuous casting billet is 1150-1200°C, such as 1160°C, 1170°C, 1180°C, 1190°C.

[0064] Specifically, in the above step 6, the controlled rolling adopts a two-stage rolling method: divided into a first stage rolling and a second stage rolling, so as to give full play to the effect of the controlled rolling and controlled cooling process on refining the grain size of the material.

[0065] Specifically, in the above step 6, the first stage rolling is rough rolling, and the first stage rolling is a rapid deformation in the austenite recrystallization temperature range; considering that the final rolling temperature of the first stage rolling is too high, it is not conducive to the rolling process design, and too small austenite cannot be fully refined in the high temperature stage. Therefore, the final rolling temperature of the first stage rolling is controlled to be ≥950°C, such as 950-1040°C, such as 960°C, 970°C, 980°C, 990°C, 1000°C, 1010°C, 1020°C, 1030°C.

[0066] Specifically, in step 6, considering that the single-pass deformation of the first stage rolling is too large, resulting in a significant increase in the torque of the rolling mill, and too small a deformation cannot reach the critical deformation for dynamic recrystallization of austenite, the single-pass deformation is controlled to be 10% to 15%.

[0067] Specifically, in the above step 6, considering that if the cumulative deformation of the first stage rolling is too large, the cumulative deformation of the finishing stage will be reduced, and if it is too small, the austenite grains cannot undergo dynamic recrystallization. Therefore, the cumulative deformation of the first stage rolling is controlled to be ≤50%, for example, 30% to 40%.

[0068] Specifically, in the above step 6, the second stage of rolling is finishing rolling. Considering that if the start rolling temperature of finishing rolling is too high, the austenite grains will undergo static recrystallization quickly, and if it is too low, the deformation resistance will increase and the rolling process cannot be completed. Therefore, the start rolling temperature of finishing rolling is controlled to be ≤950℃, such as 900~950℃, such as 910℃, 920℃, 930℃, 940℃.

[0069] Specifically, in the above step 6, if the final rolling temperature of the finishing rolling is too high, the deformed austenite grains cannot accumulate enough deformation bands and dislocation density, and if it is too low, the torque of the rolling mill increases and the steel plate is difficult to deform. Therefore, the final rolling temperature of the finishing rolling is controlled to be below 850°C, such as 780-840°C, such as 790°C, 800°C, 810°C, 820°C, and 830°C.

[0070] Specifically, in the above step 6, in order to increase the deformation band and dislocation density, the final rolling deformation of the finishing rolling is controlled to be ≥15%, for example, 15% to 18%.

[0071] Specifically, in the above step 6, considering that the cumulative deformation of the finishing rolling is too large, the deformation in the rough rolling stage is too small to achieve austenite grain refinement in the high temperature stage, and too small is not conducive to the refinement of the final phase transformation structure. Therefore, the cumulative deformation of the finishing rolling is controlled to be ≥50%, for example, 60% to 70%.

[0072] Specifically, in step 7, the offline tempering includes two quenching processes and one tempering process.

[0073] Specifically, in step 7, if the heating temperature of the primary quenching process is too high, the austenite grains will grow, which is not conducive to the low-temperature toughness of the steel plate, and if it is too low, it will not be conducive to the homogenization of the austenite grains. Therefore, the heating temperature of the primary quenching process is controlled to be 760-790°C, and the holding time is 1-2h.

[0074] Specifically, in step 7, the heating temperature of the secondary quenching process is too high, resulting in too high secondary martensite content, and although the strength increases, the toughness decreases. If the heating temperature is too low, the secondary martensite content is insufficient and the strength is low. Therefore, the secondary quenching temperature is controlled to be 620-650°C, and the holding time is 1-2h.

[0075] Specifically, in step 7, the heating temperature of the tempering process is too high, resulting in dislocation recovery and coarsening of the precipitate phase of the steel plate, which reduces the strength. If the heating temperature is too low, the strength of the steel plate is too high and the toughness is insufficient. Therefore, the heating temperature of the tempering process is controlled to be 560-580°C, the holding time is 1-2h, and the steel plate is cooled to room temperature after the holding time.

[0076] It should be noted that the content of Al and Ni is the most important element affecting the strength, plasticity and low temperature performance of the steel in the present invention. In combination with the preparation method of the present invention, a certain proportion of reverse austenite can be formed to achieve plasticization; since the introduction of the soft phase "reverse austenite" and ferrite leads to a decrease in strength, the NiAl precipitation is used in the composition design to compensate for the strength loss.

[0077] The microstructure of the steel of the present invention comprises tempered martensite, ferrite and reversed austenite, and NiAl precipitated phase, wherein the volume percentage of tempered martensite is not less than 65%, the volume percentage of ferrite is not less than 10%, the volume percentage of reversed austenite is not less than 15%, and the particle number density of NiAl phase is not less than 5×10 23 m -3 For example, the volume percentage of tempered martensite is 65% to 70%, the volume percentage of ferrite is 10% to 15%, the volume percentage of reversed austenite is 16% to 23%, and the particle number density of NiAl phase is (7 to 9)×10 23 m -3 .

[0078] The yield strength of the ultra-high strength steel prepared by the present invention is ≥1100MPa (e.g. 1120-1150MPa), the tensile strength is ≥1200MPa (e.g. 1220-1250MPa), the elongation is ≥20.0% (e.g. 20.5%-22.0%), the impact energy at -40°C is ≥80J (e.g. 100-120J), and the strength-plasticity product is 25000MPa·% or more (e.g. 25200-26000MPa·%), which solves the technical problem that the elongation of 1000MPa-grade ultra-high strength steel is less than 15%. The ultra-high strength steel prepared by the present invention can be used for large hull structures or marine engineering equipment.

[0079] The advantages of precise control of the chemical composition, content and preparation method parameters of the present invention will be demonstrated below with specific examples and comparative examples.

[0080] Example

[0081] This embodiment discloses three types (1#-3# steels) of ultra-high strength steels with elongation not less than 20% and preparation methods thereof.

[0082] The preparation method of 1#-3# steel includes the following steps:

[0083] (1) Hot metal pretreatment: Hot metal S≤0.008%;

[0084] (2) Converter smelting: slag layer thickness ≤ 100 mm;

[0085] (3) LF refining: The standby time in the silo is ≤ 24 hours to ensure that the slag has good fluidity; the white slag holding time is ≥ 10 minutes, and Ti and Fe must be added after deoxidation is complete;

[0086] (4) RH refining: Calcium treatment is carried out before steel is tapped, and the static argon blowing time after wire feeding is ≥10 min to ensure that the Ca content in the molten steel is 0.001% to 0.0015%;

[0087] (5) Continuous casting: The target superheat of the molten steel in the tundish is ≤35°C, and after continuous casting, it enters the slow cooling pit for treatment; the thickness ratio of the continuous casting billet to the final steel is controlled to be greater than 5;

[0088] (6) Controlled rolling and controlled cooling: the reheating temperature of the continuous casting billet is ≤1200℃; two-stage rolling, the first stage (rough rolling) is rolling for rapid deformation in the austenite recrystallization temperature range, the final rolling temperature is ≥950℃, the single-pass deformation is 10%-15%, and the cumulative deformation of the first stage rolling is ≤50%; the second stage rolling is finishing rolling, the rolling temperature is ≤950℃, the final rolling temperature is not higher than 850℃, the final rolling deformation is ≥15%, and the cumulative deformation of the second stage rolling is ≥50%;

[0089] (7) Offline quenching and tempering: primary quenching process: heating temperature 760-790℃, holding time 2h; secondary quenching process: 620-650℃, holding time 1-2h; tempering process: 560-580℃, holding time 1-2h.

[0090] The inventors conducted a large number of experimental studies during the research process, using some solutions with poor performance (4#, 5#) as comparison steels.

[0091] The chemical composition of 1# to 5# steels is shown in Table 1, and the specific process parameters of the preparation method are shown in Table 2;

[0092] The microstructure is shown in Table 3, and the mechanical properties test results are shown in Table 4.

[0093] The microstructure of 1# steel is shown in Figure 1 As shown in Figure 5, the 3D-ATP morphology of NiAl precipitation phase in 1# steel is as follows: Figure 2 shown.

[0094] Table 1 Chemical composition of 1#-5# steel (wt, %)

[0095] Steel Grade C Si Mn S P V Ti Ni Cr Mo Als N 1# 0.10 0.15 1.2 0.0030 0.0050 0.10 0.011 9.5 1.0 1.0 0.6 0.0035 2# 0.11 0.12 1.5 0.0035 0.0040 0.11 0.012 10.0 1.0 1.0 0.5 0.0025 3# 0.11 0.20 1.5 0.0044 0.0035 0.12 0.012 9.6 1.0 1.1 0.58 0.0030 4# 0.10 0.15 1.5 0.0050 0.0050 0.10 0.010 10.2 1.0 1.0 0.03 0.0028 5# 0.05 0.14 1.5 0.0033 0.0045 0.11 0.010 10.0 1.0 1.0 0.80 0.0033

[0096] Table 2 Specific process parameters of 1#-5# steel

[0097]

[0098] Table 3 Microstructure of 1#-5# steel

[0099]

[0100] Table 4 Mechanical properties of 1#-5# steel

[0101]

[0102] In Table 3, since the amount of NiAl precipitated phase is small, the NiAl precipitated phase is not included in the calculation of the microstructure content, that is, the sum of the contents of tempered martensite, ferrite and reversed austenite is 100%.

[0103] By comparison, it can be seen that the preparation methods and process parameters of 1#~5# steels are the same or similar, but the mass percentages of the elemental components of 1#~3# steels all meet the requirements of the present invention, and the mass percentages of the elemental components of 4# and 5# steels do not meet the requirements of the present invention. The mechanical properties, tempered martensite content, ferrite content, reversed austenite, and NiAl phase of 1#~3# steel meet the requirements of the present invention, the yield strength, tensile strength and NiAl phase number density of 4# steel do not meet the requirements of the present invention, and the ferrite content and elongation of 5# steel do not meet the requirements of the present invention.

[0104] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. An ultra-high strength steel with an elongation of not less than 20%, characterized in that: The chemical composition of the ultra-high strength steel with an elongation of not less than 20% is as follows by mass percentage: C: 0.08% to 0.11%, Si: 0.12% to 0.25%, Mn: 1.0% to 2.0%, Ni: 9.5% to 10.5%, Cr: 0.8% to 1.2%, Ti: 0.011% to 0.015%, Mo: 0.85% to 1.2%, V: 0.085% to 0.15%, Als: 0.4% to 0.6%, N≤0.004%, P≤0.005%, S≤0.005%, and the rest are Fe and unavoidable impurity elements.

2. The ultra-high strength steel with an elongation of not less than 20% according to claim 1, characterized in that: In the chemical composition of the ultra-high strength steel with an elongation of not less than 20%, Ni=-5Als+0.125, wherein Ni and Als represent the mass percentages of the corresponding elements.

3. The ultra-high strength steel with an elongation of not less than 20% according to claim 1, characterized in that: The chemical composition of the ultra-high strength steel with an elongation of not less than 20% is as follows by mass percentage: C: 0.085% to 0.11%, Si: 0.12% to 0.24%, Mn: 1.1% to 1.9%, Ni: 9.5% to 10.4%, Cr: 0.9% to 1.2%, Ti: 0.011% to 0.014%, Mo: 0.9% to 1.2%, V: 0.09% to 0.15%, Als: 0.45% to 0.6%, N: 0.002% to 0.004%, P≤0.005%, S≤0.005%, and the rest are Fe and unavoidable impurity elements.

4. The ultra-high strength steel with an elongation of not less than 20% according to any one of claims 1 to 3, characterized in that: The microstructure of the ultra-high strength steel with an elongation of not less than 20% includes tempered martensite, ferrite, reversed austenite, and NiAl precipitation phase.

5. The ultra-high strength steel with an elongation of not less than 20% according to claim 4, characterized in that: In the microstructure of the ultra-high strength steel with an elongation of not less than 20%, the volume percentage of tempered martensite is not less than 65%, the volume percentage of ferrite is not less than 10%, and the volume percentage of reversed austenite is not less than 15%.

6. A method for preparing ultra-high strength steel with an elongation of not less than 20%, for preparing the ultra-high strength steel with an elongation of not less than 20% as claimed in any one of claims 1 to 5, characterized in that: The preparation method comprises: Step 1: molten iron pretreatment; Step 2: converter smelting; Step 3: LF refining; Step 4: RH refining; Step 5: Continuously casting to obtain a continuous casting billet; Step 6: heating the continuous casting billet, and performing controlled rolling and controlled cooling on the continuous casting billet; Step 7: Offline quenching and tempering: The continuous casting billet after controlled rolling and controlled cooling is subjected to offline quenching and tempering to obtain ultra-high strength steel with an elongation of not less than 20%.

7. The preparation method according to claim 6, characterized in that: In the step 6, the heating temperature of the continuous casting billet is ≤ 1200°C.

8. The preparation method according to claim 6, characterized in that: In step 6, the controlled rolling adopts a two-stage rolling method: it is divided into a first stage rolling and a second stage rolling, the first stage rolling is rough rolling, and the first stage rolling performs rapid deformation in the austenite recrystallization temperature range.

9. The preparation method according to claim 8, characterized in that: In step 6, the final rolling temperature of the first stage rolling is controlled to be ≥ 950°C.

10. The preparation method according to any one of claims 6 to 9, characterized in that: In the step 7, the offline tempering includes two quenching processes and one tempering process.

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