A low yield ratio 785MPa grade ultra-high strength steel and its preparation method

Abstract

The present invention relates to a low yield ratio 785MPa grade ultra-high strength steel and a preparation method thereof, belonging to the technical field of ultra-high strength steel, and used to solve the problem that the yield ratio of existing ultra-high strength steel is too high. The chemical composition of the low yield ratio 785MPa grade ultra-high strength steel of the present invention is calculated by mass percentage as follows: C: 0.04% to 0.06%, Si: 0.10% to 0.20%, Mn: 0.3% to 0.6%, Cu: 1.5% to 2.0%, Ni: 3.0% to 3.5%, Cr: 0.8% to 1.2%, Ti: 0.008% to 0.015%, Mo: 0.50% to 0.65%, Als: 0.015% to 0.05%, N≤0.004%, P≤0.005%, S≤0.005%, and the rest are Fe and unavoidable impurity elements. The steel yield ratio of the present invention is lower than 0.92, and has the comprehensive characteristics of high strength, excellent low temperature toughness, etc.

Description

Technical Field

[0001] The present invention relates to the technical field of ultra-high strength steel, and in particular to a low yield ratio 785 MPa grade ultra-high strength steel and a preparation method thereof. Background Art

[0002] With the development of deep-sea oil and gas resources, ships and offshore equipment are continuously developing towards large-scale, which puts forward higher requirements on the mechanical properties of ultra-high strength ship and offshore steel. Among them, the yield strength ratio refers to the ability of the material to deform uniformly, that is, the deformation capacity from plastic deformation to final fracture, which represents the fracture safety of the material. Therefore, the control of the yield strength ratio of ultra-high strength steel is also receiving more and more attention. Although some existing ultra-high strength steels have high strength and high yield strength ratio, they cannot meet the use requirements. Summary of the invention

[0003] In view of the above analysis, the present invention aims to provide a low yield ratio 785MPa grade ultra-high strength steel and a preparation method thereof, so as to solve the problem of excessively high yield ratio of existing ultra-high strength steel.

[0004] The purpose of the present invention is mainly achieved through the following technical solutions:

[0005] The invention provides a low yield ratio 785MPa grade ultra-high strength steel, the chemical composition of which is calculated by mass percentage: C: 0.04%-0.06%, Si: 0.10%-0.20%, Mn: 0.3%-0.6%, Cu: 1.5%-2.0%, Ni: 3.0%-3.5%, Cr: 0.8%-1.2%, Ti: 0.008%-0.015%, Mo: 0.50%-0.65%, Als: 0.015%-0.05%, N≤0.004%, P≤0.005%, S≤0.005%, and the rest are Fe and unavoidable impurity elements.

[0006] Furthermore, in the chemical composition of the low yield ratio 785 MPa grade ultra-high strength steel, the mass ratio of Ni to Cu, Ni / Cu, is controlled to be 1.5 to 2.3.

[0007] Furthermore, the microstructure of the low yield ratio 785 MPa grade ultra-high strength steel includes tempered martensite, ferrite and carbide.

[0008] The present invention also provides a method for preparing a low yield ratio 785 MPa grade ultra-high strength steel, which is used to prepare the above-mentioned low yield ratio 785 MPa grade ultra-high strength steel, and the preparation method comprises:

[0009] Step 1: molten iron pretreatment;

[0010] Step 2: converter smelting;

[0011] Step 3: LF refining;

[0012] Step 4: RH refining;

[0013] Step 5: continuous casting;

[0014] Step 6: controlled rolling and controlled cooling: heating the continuous casting billet, and performing controlled rolling and controlled cooling on the continuous casting billet;

[0015] 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 low yield ratio 785MPa grade ultra-high strength steel.

[0016] Furthermore, in step 6, the heating temperature of the continuous casting billet is ≤1250°C.

[0017] Furthermore, in step 6, controlled rolling and controlled cooling are performed in two stages, the first stage of rolling is rough rolling, the final rolling temperature is ≥1050°C, and the cumulative deformation of the first stage of rolling is ≤50%.

[0018] Furthermore, in step 6, the second stage rolling is finish rolling, the final rolling temperature of the finish rolling is ≥950° C., and the final rolling deformation is ≥15%.

[0019] Furthermore, in step 7, the offline tempering is a quenching and a tempering process, and the heating temperature of the quenching process is within the two-phase region.

[0020] Furthermore, the heating temperature of the quenching process is 720-760°C.

[0021] Furthermore, the heating temperature of the tempering process is 540-560°C.

[0022] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0023] 1) The present invention achieves excellent mechanical properties of the steel, as well as high yield strength, low yield ratio and excellent low-temperature toughness by precisely controlling the element types and contents of the low yield ratio 785MPa grade ultra-high strength steel, especially the element ratios of Mn, Ni and Cr.

[0024] 2) The preparation method of the present invention combines two-stage rolling, air cooling and offline quenching and tempering processes, and controls the quenching of the quenching and tempering process to adopt two-phase zone quenching, which ensures that the microstructure of the steel includes tempered martensite, ferrite and carbide, as well as trace Cu precipitation particles; the volume percentage of tempered martensite is 70.0% to 85.0%, and the volume percentage of ferrite is 15.0% to 30.0%, which ensures the excellent comprehensive mechanical properties of the steel. The production process of the preparation method of the present invention is simple, and the low yield ratio 785MPa grade ultra-high strength steel produced can be used for the construction of large ships.

[0025] 3) The yield strength of the low yield ratio 785MPa grade ultra-high strength steel of the present invention is ≥785MPa (e.g. 820-850MPa), the elongation is ≥21% (e.g. 21%-23%), the yield ratio is lower than 0.92 (e.g. 0.88-0.91), and the ductile-brittle transition temperature is not higher than -100°C (e.g. -100--120°C).

[0026] 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

[0027] 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.

[0028] Figure 1 The microstructure diagram of the steel of Example 1 of the present invention;

[0029] Figure 2 This is a diagram of the structure content of the steel of Example 1 of the present invention. DETAILED DESCRIPTION

[0030] 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.

[0031] The present invention provides a low yield ratio 785MPa grade ultra-high strength steel. The chemical composition of the low yield ratio 785MPa grade ultra-high strength steel is calculated by mass percentage as follows: C: 0.04%-0.06%, Si: 0.10%-0.20%, Mn: 0.3%-0.6%, Cu: 1.5%-2.0%, Ni: 3.0%-3.5%; Cr: 0.8%-1.2%, Ti: 0.008%-0.015%, Mo: 0.50%-0.65%, Als: 0.015%-0.05%, 0<N≤0.004%, P≤0.005%, S≤0.005%, and the rest are Fe and unavoidable impurity elements.

[0032] Specifically, in the chemical composition of the above-mentioned low yield ratio 785 MPa grade ultra-high strength steel, the Ni / Cu value is controlled to be 1.5-2.3, and preferably, the Ni / Cu value is controlled to be 1.9-2.3.

[0033] 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.

[0034] C: Carbon can ensure the strength of the steel plate and significantly affect the low-temperature toughness and weldability of the material. The C content needs to be less than 0.06%. When the C content is less than 0.04%, the strength of the steel plate cannot be improved. Therefore, the C content is controlled at 0.04% to 0.06%.

[0035] Mn: The solid solution of Mn element will increase the yield strength and tensile strength of steel, and affect the microstructure type and proportion of the material after heat treatment by affecting the phase transition point. For example, when the Mn content increases, the Ac1 point of the steel decreases. After the process of the present invention, the tempered martensite content in the material increases, and the ferrite content decreases; when the Mn content decreases, the Ac1 point of the steel increases. After the process of the present invention, the tempered martensite content in the material decreases, the ferrite content increases, and the yield strength ratio of the material decreases. Therefore, a reasonable Mn content plays an important role in reducing the yield strength ratio of the material. Therefore, after in-depth research, the Mn content is controlled at 0.3% to 0.6%.

[0036] Si: Si is usually used as a deoxidizer in steelmaking. When the silicon content is less than 0.1%, 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 to less than 0.2%. Therefore, the Si content is controlled at 0.1% to 0.20%.

[0037] N: A certain amount of N can form Ti(C,N) with Ti, improve the toughness of the steel plate and the heat affected zone of welding, and form carbonitride with Ti to improve the strength; but if the N content is too high, N will exist in the form of solid solution nitrogen, thus affecting the low temperature toughness of the material. Therefore, the N content is controlled at 0<N≤0.004%.

[0038] Ti: Ti combines with N to form TiN. When the Ti content is less than 0.008%, it is not easy to play the above role; excessive Ti causes the TiN precipitation time to decrease, the temperature to increase, and the pinning effect on the austenite grains to decrease. Therefore, the Ti content is controlled at 0.008% to 0.015%.

[0039] Als: Als is an important deoxidizing element in the steelmaking process. When the Als content is less than 0.015%, it is difficult to control the oxygen content below 0.004%. When the Als content is higher, coarse Al oxide inclusions will form and aggregate into clusters, causing the steelmaking nozzle to be blocked or serving as a crack source, resulting in a reduction in toughness. Therefore, the Als content should be controlled between 0.015% and 0.05%.

[0040] Cr: The Cr element affects the microstructure type and proportion of the material after heat treatment by affecting the phase transition point. For example, when the Cr content decreases, the Ac1 point of the test steel decreases, and the tempered martensite content in the material increases and the ferrite content decreases after the process of the present invention; when the Cr content increases, the Ac1 point of the test steel increases, and the tempered martensite content in the material decreases and the ferrite content increases after the process of the present invention, and the yield strength ratio of the material decreases. Therefore, a reasonable Cr content plays an important role in reducing the yield strength ratio of the material. Therefore, after in-depth research, the Cr content is controlled at 0.8% to 1.2%.

[0041] Cu: Cu and Ni elements are usually added in combination. The strength of the steel plate can be improved by using Cu aging precipitation particles, but a certain Ni content needs to be matched to eliminate hot cracks during rolling. At this time, the minimum Ni content is 1.5Cu, and the maximum Ni content does not exceed 2.3Cu; in addition, too much Cu element will increase the carbon equivalent (Ceq) and cold crack sensitivity index (Pcm), causing the preheating temperature of the material to increase during welding. Therefore, the Cu content of the present invention is controlled at 1.5% to 2.0%, and the Ni / Cu value is controlled to be 1.5 to 2.3.

[0042] Ni: In order to meet the requirements of high-strength steel for low-temperature toughness, a certain amount of Ni element needs to be added to improve the low-temperature toughness of the heat-affected zone of the steel plate and the welded joint. However, too high a 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 3.5%. Therefore, the Ni content is controlled at 3.0% to 3.5%.

[0043] Mo: Mo can improve the hardenability of the material, promote low-temperature microstructure transformation, and improve the strength of the material. At the same time, Mo promotes the formation of acicular ferrite under non-recrystallization rolling conditions. If the Mo content is less than 0.50%, the effect of Mo is low. If the Mo content exceeds 0.65%, it not only increases the manufacturing cost, but also damages the weldability of the steel plate, especially the low-temperature toughness under high-line energy welding. Therefore, Mo is controlled at 0.50% to 0.65%.

[0044] 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%.

[0045] 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%.

[0046] Specifically, in order to further improve the comprehensive performance of the above-mentioned low yield ratio 785MPa grade ultra-high strength steel, the chemical composition of the above-mentioned low yield ratio 785MPa grade ultra-high strength steel is calculated by mass percentage as follows: C: 0.04%~0.055%, Si: 0.10%~0.20%, Mn: 0.35%~0.55%, Cu: 1.5%~1.8%, Ni: 3.0%~3.5%, Cr: 0.8%~1.1%, Ti: 0.009%~0.015%, Mo: 0.50%~0.65%, Als: 0.015%~0.04%, N: 0.002%~0.0035%, P≤0.005%, S≤0.005%, and the rest are Fe and unavoidable impurity elements.

[0047] Specifically, the microstructure of the low yield ratio 785 MPa grade ultra-high strength steel includes tempered martensite, ferrite and carbide.

[0048] Specifically, the microstructure of the above-mentioned low yield ratio 785 MPa grade ultra-high strength steel also includes trace amounts of Cu precipitated particles.

[0049] Specifically, in the microstructure of the above-mentioned low yield ratio 785 MPa grade ultra-high strength steel, the volume percentage of tempered martensite is 70.0% to 85.0%, and the volume percentage of ferrite is 15.0% to 30.0%.

[0050] The present invention also provides a method for preparing the above-mentioned low yield ratio 785MPa grade ultra-high strength steel, comprising the following steps:

[0051] Step 1: molten iron pretreatment;

[0052] Step 2: converter smelting;

[0053] Step 3: LF refining;

[0054] Step 4: RH refining;

[0055] Step 5: continuous casting;

[0056] Step 6: controlled rolling and controlled cooling: heating the continuous casting billet, and performing controlled rolling and controlled cooling on the continuous casting billet;

[0057] 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 low yield ratio 785MPa grade ultra-high strength steel.

[0058] 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.

[0059] Specifically, in the above step 2, during converter smelting, the slag layer thickness is required to be ≤100 mm when slag blocking and steel tapping, and the added alloy material and auxiliary materials such as lime, sintered ore, dolomite, etc. must be dry.

[0060] 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 holding time is ≥10 minutes, and the alloy material must be added after deoxidation is complete.

[0061] 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 minutes to ensure that the Ca content in the molten steel is 0.001% to 0.0015%.

[0062] Specifically, in the above step 5, the continuous casting process adopts full-process protected casting, electromagnetic stirring and light pressing are performed, the superheat of the tundish is ≤30°C, and after continuous casting, it enters the slow cooling pit for treatment to obtain a continuously cast billet.

[0063] Specifically, in the above step 5, the thickness ratio of the continuous casting billet to the finally obtained low yield ratio 785 MPa grade ultra-high strength steel is controlled to be greater than 8, so as to refine the core structure of the steel plate by rolling with a large compression ratio.

[0064] Specifically, in the above step 6, the heating temperature of the continuous casting billet is ≤ 1250°C to ensure that the austenite grains do not grow. Preferably, the heating temperature of the continuous casting billet is 1210-1250°C, for example, 1220°C, 1230°C, 1240°C.

[0065] Specifically, in the above step 6, the controlled rolling and controlled cooling 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.

[0066] Specifically, in the above step 6, the first stage of rolling is rough rolling, and rapid deformation is performed in the austenite recrystallization temperature range, and the final rolling temperature is ≥1050°C. Considering that a single-pass deformation is too large, the rolling torque is too large and easy to damage the rollers, and austenite grains cannot be refined if the pass deformation is too small. Therefore, the single-pass deformation is controlled at 10% to 15%. Considering that the cumulative deformation of the first stage of rolling is too large, the finishing deformation is too small, affecting the grain refinement effect, and the cumulative deformation is too small, resulting in the inability to fully refine the austenite in the high temperature stage. Therefore, the cumulative deformation of the first stage of rolling is controlled to be ≤50%. For example, 30% to 50%.

[0067] Preferably, in the above step 6, considering that if the final rolling temperature of the first stage rolling is too low, the austenite grains cannot be fully refined, the final rolling temperature of the first stage rolling is controlled to be 1050-1070°C.

[0068] Specifically, in the above step 6, the second stage rolling is finishing rolling. Considering that the starting temperature of finishing rolling is too high and the deformation is high, the austenite grains undergo rapid static recrystallization and grow, resulting in the inability to refine the grains, it is necessary to control the starting temperature of finishing rolling to below 1000°C, for example, 970-1000°C.

[0069] Specifically, in the above step 6, if the final rolling temperature of the finishing rolling is too high, the austenite grains will grow in the subsequent cooling process; if the final rolling temperature is too low, the deformation resistance of the steel plate will increase and deformation will be difficult. Therefore, the final rolling temperature of the finishing rolling is controlled to be ≥ 950°C, for example, 955-985°C.

[0070] Specifically, in the above step 6, in order to increase the accumulation of dislocations and deformation bands during the finishing rolling process, the final rolling deformation of the finishing rolling is controlled to be ≥15% (e.g., 15% to 17%). Considering that the cumulative deformation of the second stage rolling is too large, the rolling passes are increased and the rolling of the steel plate cannot be completed; the cumulative deformation of the second stage rolling is too small, resulting in the steel plate being unable to accumulate enough dislocations and deformation bands to provide positions for phase transformation. Therefore, the cumulative deformation of the second stage rolling is controlled to be ≥50% (e.g., 60% to 70%).

[0071] Specifically, in the above step 6, the steel is air-cooled to room temperature after finish rolling.

[0072] Specifically, in the above step 7, the offline quenching and tempering is a quenching and a tempering process, the heating temperature of the quenching process is in the two-phase region, specifically, the heating temperature of the quenching process is 720-760°C, and the holding time is 1-2 hours. Subsequently, the heating temperature of the tempering process is 540-560°C, the holding time is 1-2 hours, and air cooling to room temperature is performed after holding.

[0073] It should be noted that the content of Mn and Cr are the most important elements that affect the yield ratio of the steel of the present invention. The present invention significantly reduces the yield ratio of the material by reducing the Mn content to less than 0.7% and increasing the Cr content to no more than 1.2%. However, after reducing the Mn content, the solid solution strengthening ability of the steel plate is reduced, and the strength cannot be guaranteed. It is necessary to make up for the strength loss of the material by supplementing alloy elements and strengthening the structure. Therefore, in terms of component composition, the present invention adopts NiCrMoCu component design, improves the strength by aging and precipitating dispersed Cu particles of Cu element during tempering, and improves the strength of the material by using Cr carbide; in the production process, a two-phase zone quenching and tempering process is adopted to give play to the effect of tempered lath martensite on improving strength.

[0074] The microstructure of the steel of the present invention includes tempered martensite, ferrite and carbide, as well as trace Cu precipitated particles; the volume percentage of the tempered martensite is 70.0% to 85.0%, and the volume percentage of the ferrite is 15.0% to 30.0%. The steel of the present invention has excellent low-temperature toughness while ensuring material strength and yield strength ratio.

[0075] The low yield ratio 785MPa grade ultra-high strength steel of the present invention has a yield strength of ≥785MPa (e.g. 820-850MPa), an elongation of ≥21% (e.g. 21%-23%), a yield ratio lower than 0.92 (e.g. 0.88-0.91), and a ductile-brittle transition temperature not higher than -100°C (e.g. -100°C--120°C).

[0076] The advantages of precise control of chemical composition, content and preparation process parameters of the steel of the present invention will be demonstrated below with specific examples and comparative examples.

[0077] Example

[0078] This embodiment provides three types (1#-3# steels) of low yield ratio 785MPa grade ultra-high strength steels and preparation methods thereof.

[0079] The preparation method of 1#-3# steel includes the following steps:

[0080] (1) Hot metal pretreatment: Hot metal S≤0.008%;

[0081] (2) Converter smelting: slag layer thickness ≤ 100 mm;

[0082] (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;

[0083] (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 0.001-0.0015%;

[0084] (5) Continuous casting: The target superheat of molten steel in the tundish is ≤30°C, and after continuous casting, it enters the slow cooling pit for treatment;

[0085] (6) Controlled rolling and controlled cooling: The reheating temperature of the continuous casting billet is 1210-1250°C; two-stage rolling, the first stage (rough rolling) is rolling in the austenite recrystallization temperature range for rapid deformation, the single pass deformation is 10%-15%, the final rolling temperature is 1050-1070°C, and the cumulative deformation is ≤50%; the second stage (finishing rolling) rolling start temperature is 970-1000°C, the final rolling temperature is 955-985°C, the final rolling deformation is 15%-17%, and the cumulative deformation is 60%-70%;

[0086] (7) Offline quenching and tempering: Quenching process: heating temperature 720-740℃, holding time 2h; tempering process: heating temperature 540-560℃, holding time 2h; plate thickness after rolling ≤40mm.

[0087] The inventors conducted a large number of experimental studies during the research process, using some solutions with poor performance (4#, 5#) as comparison steels.

[0088] The chemical composition of 1#~5# steel is shown in Table 1, the specific process parameters of the preparation method are shown in Table 2; the microstructure and mechanical properties test results are shown in Table 3.

[0089] Table 1 Chemical composition of 1#-5# steel (wt, %)

[0090]

[0091]

[0092] Table 2 Process parameters of 1#-5# steel

[0093]

[0094] Table 3 Mechanical properties and microstructure of 1#-5# steel

[0095]

[0096] The microstructure of 1# steel is as follows Figure 1 As shown, tissue content Figure 2 shown.

[0097] By comparison, it can be seen that the smelting methods and process parameters of 1# to 5# steels are the same or similar, but the mass percentages of the chemical components of 1# to 3# steels all meet the requirements of the present invention, the mass percentages of the chemical components of 4# steel do not meet the requirements of the present invention, the tempered martensite content and ferrite content of 4# steel do not meet the requirements of the present invention, and the yield strength of 4# steel is low; the chemical composition of 5# steel does not meet the requirements, some process parameters in the preparation method do not meet the requirements, the tempered martensite content and ferrite content of 5# steel do not meet the requirements of the present invention, and the yield strength ratio does not meet the requirements of the present invention.

[0098] By comparison, it can be seen that the present invention, based on precise chemical composition and preparation method, can ensure that the steel of the present invention obtains excellent mechanical properties, with a yield strength ≥785MPa (e.g. 820-850MPa), an elongation ≥21% (e.g. 21%-23%), a yield strength ratio lower than 0.92 (e.g. 0.88-0.91), and a ductile-brittle transition temperature not higher than -100°C (e.g. -100°C to -120°C).

[0099] 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. A low yield ratio 785MPa grade ultra-high strength steel, characterized in that: The chemical composition of the low yield ratio 785MPa grade ultra-high strength steel is calculated by mass percentage as follows: C: 0.04% to 0.06%, Si: 0.10% to 0.20%, Mn: 0.3% to 0.6%, Cu: 1.5% to 2.0%, Ni: 3.0% to 3.5%, Cr: 0.8% to 1.2%, Ti: 0.008% to 0.015%, Mo: 0.50% to 0.65%, Als: 0.015% to 0.05%, N≤0.004%, P≤0.005%, S≤0.005%, and the rest are Fe and unavoidable impurity elements.

2. The low yield ratio 785MPa grade ultra-high strength steel according to claim 1, characterized in that: In the chemical composition of the low yield ratio 785 MPa grade ultra-high strength steel, the mass ratio of Ni to Cu is controlled to be 1.5 to 2.

3.

3. The low yield ratio 785MPa grade ultra-high strength steel according to claim 1, characterized in that: The microstructure of the low yield ratio 785 MPa grade ultra-high strength steel includes tempered martensite, ferrite and carbide.

4. A method for preparing 785MPa grade ultra-high strength steel with low yield ratio, characterized in that: For preparing the low yield ratio 785MPa grade ultra-high strength steel according to any one of claims 1 to 3, the preparation method comprises: Step 1: molten iron pretreatment; Step 2: converter smelting; Step 3: LF refining; Step 4: RH refining; Step 5: continuous casting; Step 6: controlled rolling and controlled cooling: 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 low yield ratio 785MPa grade ultra-high strength steel.

5. The preparation method according to claim 4, characterized in that: In step 6, the heating temperature of the continuous casting billet is ≤ 1250°C.

6. The preparation method according to claim 4, characterized in that: In step 6, controlled rolling and controlled cooling adopt two-stage rolling, the first stage rolling is rough rolling, the final rolling temperature is ≥1050°C, and the cumulative deformation of the first stage rolling is ≤50%.

7. The preparation method according to claim 6, characterized in that: In step 6, the second stage rolling is finish rolling, the final rolling temperature of the finish rolling is ≥950° C., and the final rolling deformation is ≥15%.

8. The preparation method according to any one of claims 4 to 7, characterized in that: In step 7, the offline tempering is a one-time quenching and one-time tempering process, and the heating temperature of the quenching process is within the two-phase region.

9. The preparation method according to claim 8, characterized in that: The heating temperature of the quenching process is 720-760°C.

10. The preparation method according to claim 8, characterized in that: The heating temperature of the tempering process is 540-560°C.

Images