1300-mpa-grade ultra-high-strength steel having low yield ratio and manufacturing method therefor

Abstract

Disclosed in the present invention is 1300-MPa-grade ultra-high-strength steel having a low yield ratio. The steel comprises Fe and inevitable impurities, and further comprises the following chemical elements in percentage by mass: C: 0.18-0.25%; Si: 0.5-1.0%; Mn: 1.96-2.5%; B: 0.001-0.004%; Al: 0.02-0.25%; and Ti: 0.086-0.15%. The microstructure thereof has retained austenite and dispersed nanoscale carbides and / or borides. Further disclosed in the present invention is a method for manufacturing ultra-high-strength steel. The method comprises the steps of: smelting and casting; hot rolling; acid pickling and cold rolling; and annealing. The steel plate has low yield strength and a low yield ratio, and also has good mechanical properties.

Description

1300mpa grade ultra-high strength steel with low yield ratio and manufacturing method thereof TECHNICAL FIELD

[0001] The steel plate and the manufacturing method thereof, in particular, relate to an ultra-high strength steel and a manufacturing method thereof. BACKGROUND

[0002] With the development of automobile lightweight, more and more automobile structural parts are prepared from ultra-high strength steel, and some cold stamping parts have already used steel with tensile strength of 1300 MPa or above, which puts forward high requirements on the forming performance, blank size precision and flatness, forming springback control and other aspects of the ultra-high strength steel.

[0003] The existing steel with tensile strength of 1300 MPa or above, whether it is introduced into martensite structure by ultra-fast cooling or introduced into bainite structure by adding multiple alloys, has high yield strength and yield ratio. For the steel with tensile strength of 1300 MPa or above, the yield strength is generally more than 1000 MPa, and the yield ratio is generally more than 0.70.

[0004] For example, the Chinese patent document with publication number CN111647732A, publication date September 11, 2020 and name "1300MPa grade complex phase steel, its preparation method and application" discloses a method for obtaining 1300MPa grade ultra-high strength steel by introducing bainite structure, but the yield strength of the product is more than 1000MPa.

[0005] For example, the Chinese patent document with publication number CN117305683A, publication date December 29, 2023 and name "1300MPa or above grade cold-rolled steel plate and manufacturing method thereof" discloses a method for obtaining 1300MPa grade ultra-high strength steel by introducing martensite structure, but the yield strength of the product is also more than 1000MPa.

[0006] High yield strength leads to difficult forming of the steel during cold stamping and forming, resulting in insufficient size precision, and even during blanking, due to the high yield strength of the steel plate and the release of residual stress, it is difficult to flatten, further exacerbating the control of size precision during subsequent forming. SUMMARY

[0007] One of the purposes of the present application is to provide a 1300MPa grade ultra-high strength steel with low yield ratio, which has low yield strength, low yield ratio and excellent mechanical properties. In this paper, 1300MPa grade means that the tensile strength is greater than or equal to 1300MPa.

[0008] In order to achieve the above-mentioned purpose, the present application provides a 1300MPa grade ultra-high strength steel with low yield ratio, which contains Fe and inevitable impurities, and further contains the following chemical elements with mass percentage as follows:

[0009] C: 0.18-0.25%; Si: 0.5-1.0%; Mn: 1.96-2.5%; B: 0.001-0.004%; Al: 0.02-0.25%; Ti: 0.086-0.15%; and the balance of Fe and inevitable impurities.

[0010] The microstructure of the steel has residual austenite and dispersed nanoscale carbides and / or borides.

[0011] In some embodiments, the microstructure of the 1300MPa grade ultra-high strength steel with low yield ratio of the present application comprises residual austenite and dispersed nanoscale carbides, and optionally further comprises dispersed nanoscale borides.

[0012] In the present application, at the initial stage of the bainite or martensite phase transformation of the steel, through relatively high temperature reheating and partitioning process, the solute atoms of C and B in the newly formed bainite and martensite are partitioned from the lattice to the outside of the lattice, and form carbides and borides with the carbide and boride forming elements such as Ti, Nb, V and Mo, thereby reducing the content of C and B elements in the newly formed bainite and martensite, reducing the lattice distortion and the resulting dislocation density, and reducing the hardness of the bainite and martensite; the formed carbides and borides are dispersed because of the lower generation temperature and smaller particle size, so the hindering effect on dislocation slip is limited at the initial stage of deformation. Through the superposition of the above two factors, the yield strength of the steel can be reduced. However, whether it is martensite, bainite or carbide precipitates, especially the latter, will greatly hinder dislocation movement as the degree of deformation of the steel increases, thereby ensuring the high tensile strength of the steel, thereby obtaining an ultra-high strength steel with low yield strength and low yield ratio.

[0013] Further, in the ultra-high strength steel of the present application, the mass percentage of each chemical element is as follows:

[0014] C: 0.18-0.25%; Si: 0.5-1.0%; Mn: 1.96-2.5%; B: 0.001-0.004%; Al: 0.02-0.25%; Ti: 0.086-0.15%; and the balance of Fe and inevitable impurities.

[0015] In the low-alloy ultra-high strength steel of the present application, the design principles of each chemical element are as follows:

[0016] C: In the low-alloy ultra-high-strength steel described in the present application, the C element not only controls the phase transformation, affects the formation of martensite and bainite, and determines the hardness of martensite and bainite, but also directly affects the precipitation, distribution and particle size of carbides. The C element has a decisive influence on the yield strength and yield strength ratio of the steel in the present application. It is necessary to match the addition amount of alloying elements, especially carbide elements, to relatively reduce the solid solution C content in the martensite or bainite while ensuring a certain degree of carbide precipitation, thereby reducing the yield strength and yield strength ratio of the product. Therefore, in the low-alloy ultra-high-strength steel described in the present application, the mass percentage content of C element is controlled between 0.18-0.25%.

[0017] Si: In the low-alloy ultra-high-strength steel described in the present application, Si element is not only a necessary element for steelmaking deoxidization and a solid solution strengthening element, but also directly affects the precipitation of carbides and the formation of residual austenite. When the mass percentage content of Si element is too low, it will lead to the failure to form residual austenite. When the mass percentage content of Si element is too high, it will affect the precipitation of carbides. Therefore, in the low-alloy ultra-high-strength steel described in the present application, the mass percentage content of Si element is controlled between 0.5-1.0%.

[0018] Mn: In the low-alloy ultra-high-strength steel described in the present application, Mn is one of the core elements affecting strength and controlling phase transformation. When the mass percentage content of Mn element is too high, it will lead to easier transformation of martensite and bainite, and the hardness and yield strength will increase. When the mass percentage content of Mn element is too low, the transformation of martensite and bainite is insufficient, resulting in that the strength does not meet the design requirements. Therefore, in the low-alloy ultra-high-strength steel described in the present application, the mass percentage content of Mn element is controlled between 1.96-2.5%.

[0019] B: In the low-alloy ultra-high-strength steel described in the present application, B element not only helps the formation of martensite and bainite in the steel, but also affects the hardness of martensite and bainite, thereby having a greater impact on the strength and yield strength ratio of the steel. When the mass percentage content of B element is too low, the strength of the steel will not meet the design requirements. When the mass percentage content of B element is too high, brittle borides are easily generated, thereby affecting the forming performance and performance uniformity of the steel plate. Therefore, in the low-alloy ultra-high-strength steel described in the present application, the mass percentage content of B element is controlled between 0.001-0.004%.

[0020] Al: In the low-alloy ultra-high-strength steel described in the present application, the Al element can be used as a deoxidizing element and a trace strengthening element to improve the quality and strength of the steel, and can also be used as a phase transformation control element for inducing the formation of residual austenite. When the mass percentage content of the Al element is too low, the molten steel is not sufficiently deoxidized, and the purity of the steel is affected. When the mass percentage content of the Al element is too high, a large amount of carbide-free bainite and residual austenite is easily formed in the steel, which is not conducive to the design of low yield strength and low yield ratio. Therefore, in the low-alloy ultra-high-strength steel described in the present application, the mass percentage content of the Al element is controlled to be between 0.02 and 0.25%.

[0021] Ti: In the low-alloy ultra-high-strength steel described in the present application, Ti is a strong carbonitride-forming element. The carbonitride formed can refine the grains and hinder dislocation slip in the steel, thereby improving the strength and forming performance of the steel. However, when the mass percentage content of the Ti element is too high, coarse TiN particles and coarsened titanium carbonitride are easily formed in the steel, which is not conducive to the forming performance of the steel. Therefore, in the low-alloy ultra-high-strength steel described in the present application, the mass percentage content of the Ti element is controlled to be between 0.086 and 0.15%.

[0022] Further, in the ultra-high-strength steel described in the present application, it further contains at least one of the following elements:

[0023] 0 < Mo ≤ 0.25 wt%

[0024] 0 < Cr ≤ 0.25 wt%;

[0025] 0 < Nb ≤ 0.06 wt%;

[0026] 0 < Cu ≤ 0.2 wt%;

[0027] 0 < Ce ≤ 0.006 wt%;

[0028] 0 < V ≤ 0.2 wt%.

[0029] In the present application, Mo, Cr, Nb, Cu, Ce and V can be selectively added to the steel as optional elements. Among them:

[0030] Cr, Mo: In the ultra-high strength steel described in the present application, the Cr element and the Mo element can increase the hardenability of the steel strip, increase the incubation period of pearlite and ferrite, inhibit the formation of pearlite and ferrite, and make it easy to obtain bainite structure during cooling. At the same time, the Mo element is also a weak carbide forming element, and the effect of its content on carbide precipitation and size also needs to be considered. When the mass percentage content of Cr and Mo is too low, it will lead to insufficient formation of bainite and martensite; when the mass percentage content of Cr and Mo is too high, it is easy to form more martensite structure, which deteriorates the forming performance of the steel strip. Based on this, considering the proportion of bainite, martensite and other hard phases and carbide precipitation, in the ultra-high strength steel described in the present application, the mass percentage content of Mo element can be controlled to 0 < Mo ≤ 0.25wt%, and the mass percentage content of Cr element can be controlled to 0 < Cr ≤ 0.25wt%. In some embodiments, the mass percentage content of Mo element in the ultra-high strength steel described in the present application is controlled between 0.05-0.25%. In some embodiments, the mass percentage content of Cr element in the ultra-high strength steel described in the present application is controlled between 0.05-0.25%.

[0031] In addition, as optional elements, Nb, Cu, Ce, V elements can refine the grain, improve the stability of austenite, and Cu is also an austenite stabilizing element. In the present application, although the addition of the above optional elements is beneficial to improve the proportion of residual austenite and improve the forming performance of the steel, but from the perspective of cost control and ensuring weldability, the addition amount of the above elements needs to be controlled. Therefore, in the low-alloy high-strength steel described in the present application, the mass percentage content of Nb element can be controlled to Nb ≤ 0.06wt%, the mass percentage content of Cu element can be controlled to Cu ≤ 0.2wt%, the mass percentage content of Ce element can be controlled to Ce ≤ 0.006wt%, and the mass percentage content of V element can be controlled to V ≤ 0.2wt%. In some embodiments, the mass percentage content of Nb element in the ultra-high strength steel described in the present application is controlled between 0.02-0.06%. In some embodiments, the mass percentage content of Cu element in the ultra-high strength steel described in the present application is controlled between 0.05-0.2%. In some embodiments, the mass percentage content of Ce element in the ultra-high strength steel described in the present application is controlled between 0.002-0.006%. In some embodiments, the mass percentage content of V element in the ultra-high strength steel described in the present application is controlled between 0.05-0.2%.

[0032] Further, in the ultra-high strength steel described in the present application, the mass percentage content of each chemical element satisfies at least one of the following:

[0033] 0.1% ≤ C-Ti / 2-Nb-V / 4-Mo / 10+30xB ≤ 0.3%;

[0034] (Mo+Cr / 4) / (Ti+Nb+V / 2)≤2;

[0035] Wherein, each element symbol is brought into the mass percentage value of the corresponding element.

[0036] In the present application, since Ti, V and Nb are strong carbide forming elements, and Mo and Cr are both weak carbide forming elements and strong phase transformation controlling elements, the synergistic effect of the above two types of elements on carbide precipitation and phase transformation needs to be considered, therefore, 0.1≤C-Ti / 2-Nb-V / 4-Mo / 10+30×B≤0.3; (Mo+Cr / 4) / (Ti+Nb+V / 2)≤2 needs to be controlled.

[0037] Further, in the unavoidable impurities of the ultra-high strength steel described in the present application, P≤0.015wt%, S≤0.003wt%, N≤0.004wt%.

[0038] In the present application, the unavoidable impurities mainly include phosphorus (P), sulfur (S), and nitrogen (N), and it is desirable to have as little content as possible under the technical conditions.

[0039] Further, in the ultra-high strength steel described in the present application, the volume fraction of residual austenite in the microstructure is 2-6%.

[0040] Further, in the ultra-high strength steel described in the present application, the volume fraction of carbides and / or borides in the microstructure is 4-9%. Herein, the meaning of "the volume fraction of carbides and / or borides in the microstructure is 4-9%" is: when the microstructure only includes carbides and does not include borides, the volume fraction of carbides is 4-9%; when the microstructure only includes borides and does not include carbides, the volume fraction of borides is 4-9%; when the microstructure includes both carbides and borides, the sum of the volume fraction of carbides and the volume fraction of borides is 4-9%.

[0041] Further, in the ultra-high strength steel described in the present application, the size of the carbides and / or borides is ≤100nm. Herein, when the microstructure only includes carbides and does not include borides, "the size of the carbides and / or borides is ≤100nm" means the size of the carbides is ≤100nm; when the microstructure only includes borides and does not include carbides, "the size of the carbides and / or borides is ≤100nm" means the size of the borides is ≤100nm; when the microstructure includes both carbides and borides, "the size of the carbides and / or borides is ≤100nm" means the size of the carbides is ≤100nm and the size of the borides is ≤100nm.

[0042] Further, in the ultra-high strength steel according to the present application, the microstructure further comprises acicular bainite and tempered martensite, and optionally granular bainite.

[0043] Further, in the ultra-high strength steel according to the present application, the microstructure further comprises acicular bainite, granular bainite and tempered martensite.

[0044] Further, in the ultra-high strength steel according to the present application, the yield strength is ≤ 850 MPa, the tensile strength is ≥ 1300 MPa, the yield ratio is ≤ 0.65, and the elongation at break is ≥ 9%.

[0045] Further, in the ultra-high strength steel according to the present application, the yield strength is ≤ 750 MPa, and the yield ratio is ≤ 0.55.

[0046] In some embodiments, the yield strength of the ultra-high strength steel according to the present application is ≥ 650 MPa.

[0047] In some embodiments, the tensile strength of the ultra-high strength steel according to the present application is ≤ 1450 MPa.

[0048] In some embodiments, the elongation at break of the ultra-high strength steel according to the present application is ≤ 13%.

[0049] Another object of the present application is to provide a manufacturing method of an ultra-high strength steel, which can obtain a 1300 MPa grade ultra-high strength steel with low yield strength and low yield ratio.

[0050] To achieve the above object, the present application provides a manufacturing method of an ultra-high strength steel, comprising the steps of:

[0051] smelting and casting;

[0052] hot rolling;

[0053] pickling and cold rolling;

[0054] annealing: heating the strip steel to 860-920℃ and holding, then cooling to 720-820℃ at a slow cooling rate of 5-20℃ / s and holding, then cooling to 260-370℃ at a fast cooling rate of 40-80℃ / s and holding, then reheating to 350-470℃ at a heating rate of 5-30℃ / s and holding, then cooling to 300-460℃ at a second fast cooling rate of 5-30℃ / s and holding, and then cooling to room temperature at a final average cooling rate of ≤ 10℃ / s.

[0055] In the annealing step of the present application, the strip steel is heated to a temperature of 860-920℃ to ensure the formation of austenite parent phase and the secondary dissolution of carbide, refine the grain size and the size of carbide; the slow cooling rate of 5-20℃ / s to 720-820℃ and holding can avoid the formation of ferrite and the precipitation of carbide. The fast cooling rate of 40-80℃ to 260-370℃ and holding can make the strip steel enter the martensite or bainite phase transition region; the heating rate of 5-30℃ / s to 350-470℃ and holding, and then the secondary fast cooling rate of 5-30℃ / s to 300-460℃ and holding can ensure the C element partitioning in the martensite or bainite to form a small amount of residual austenite and a large amount of fine dispersed carbide; the final cooling average cooling rate of ≤10℃ / s to room temperature can further promote the fine and dispersed precipitation of carbide.

[0056] Further, in the annealing step of the manufacturing method, the strip steel is heated to 860-920℃ and held for 90-180s, then cooled to 720-820℃ at a slow cooling rate of 5-20℃ / s and held for less than 10s, then cooled to 260-370℃ at a fast cooling rate of 40-80℃ and held for 1-20s, then reheated to 350-470℃ at a heating rate of 5-30℃ / s and held for 5-30s, then cooled to 300-460℃ at a secondary fast cooling rate of 5-30℃ / s and held for 60-180s, and then cooled to room temperature at a final cooling average cooling rate of ≤10℃ / s.

[0057] Further, in the annealing step of the manufacturing method, the hot dip galvanizing of the strip steel surface is completed in the process of reheating to 350-470℃ at a heating rate of 5-30℃ / s and holding.

[0058] Further, in the hot rolling step of the manufacturing method, the slab discharge temperature is controlled to 1220-1300℃, the rough rolling temperature is 1030-1120℃, and the finish rolling temperature is 880-970℃.

[0059] In the hot rolling step of the present application, the slab discharge temperature is controlled to 1220-1300℃ to ensure the full dissolution of the first formed carbonitride in the slab; the rough rolling temperature is controlled to 1030-1120℃, and the finish rolling temperature is controlled to 880-970℃ to avoid the batch precipitation and coarsening of carbide, and only form a small amount of fine carbide, which is beneficial to the improvement of the elongation and the drawing forming performance of the product.

[0060] Further, in the hot rolling step of the manufacturing method, the strip is first cooled to 680-740℃ at a first cooling rate of 20-50℃ / s after rolling, and then cooled to 460-520℃ at a second cooling rate of 50-100℃ / s before coiling.

[0061] In the hot rolling step of the present application, the above-mentioned post-rolling cooling process can avoid the formation of uneven martensite structure in the hot-rolled structure; and the coiling temperature controlled at 460-520℃ can avoid excessive precipitation and coarsening of carbides in the hot-rolled coil, thereby affecting the subsequent microstructure regulation during annealing.

[0062] The 1300MPa grade ultra-high strength steel with low yield ratio and the manufacturing method thereof have the following advantages and beneficial effects:

[0063] The 1300MPa grade ultra-high strength steel and the manufacturing method thereof can obtain a 1300MPa grade ultra-high strength steel with low yield strength and low yield ratio by adding lower amounts of alloying elements and utilizing the kinetic balance of martensite and bainite phase transformation and carbide precipitation during annealing. Meanwhile, the nano-sized carbides can hinder the high-density dislocation slip in bainite and martensite to improve the tensile strength and elongation of the ultra-high strength steel.

[0064] In some embodiments, the 1300MPa grade ultra-high strength steel has a yield strength of ≤850MPa, a tensile strength of ≥1300MPa, a yield ratio of ≤0.65, and a fracture elongation of ≥9%.

[0065] In some embodiments, the yield strength is ≤750MPa and the yield ratio is ≤0.55. Meanwhile, due to the low amount of alloying elements, the steel can be widely used in the preparation of various automobile structural parts. BRIEF DESCRIPTION OF DRAWINGS

[0066] Figure 1 shows the microstructure morphology of the ultra-high strength steel of Example 1. DETAILED DESCRIPTION

[0067] The 1300MPa grade ultra-high strength steel with low yield ratio and the manufacturing method thereof will be further explained and described in conjunction with specific examples and the accompanying drawings, but the explanation and description do not constitute an improper limitation on the technical solutions of the present application.

[0068] Examples 1-6 and Comparative Examples 1-3

[0069] Table 1-1 and Table 1-2 list the mass percentages of each chemical element in the 1300MPa grade ultra-high strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3.

[0070] Table 1-1. (The balance is Fe and other inevitable impurities other than P, S, N)

[0071] Table 1-2. (The balance is Fe and other inevitable impurities other than P, S, N)

[0072] The 1300 MPa grade ultra-high strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3 were prepared by the following steps:

[0073] (1) Smelting and continuous casting;

[0074] (2) Hot rolling: the slab discharge temperature was controlled at 1220-1300℃, the rough rolling temperature was 1030-1120℃, and the finish rolling temperature was 880-970℃. After rolling, the strip was cooled at a slow speed first and then at a fast speed, i.e. the strip was cooled to 680-740℃ at a cooling rate of 20-50℃ / s, and then cooled to 460-520℃ at a cooling rate of 50-100℃ / s for coiling.

[0075] (3) Pickling and cold rolling;

[0076] (4) Annealing: the strip was heated to 860-920℃ and held for 90-180s, then cooled to 720-820℃ at a slow cooling rate of 5-20℃ / s and held for less than 10s, then cooled to 260-370℃ at a fast cooling rate of 40-80℃ / s and held for 1-20s, then reheated to 350-470℃ at a heating rate of 5-30℃ / s and held for 5-30s, then cooled to 300-460℃ at a second fast cooling rate of 5-30℃ / s and held for 60-180s, and then cooled to room temperature at a final cooling average cooling rate of ≤10℃ / s.

[0077] In addition, in some embodiments, the hot-dip galvanizing of the surface of the strip can be completed in the process segment of reheating to 350-470℃ at a heating rate of 5-30℃ / s and holding in step (4), so as to obtain a hot-dip galvanized product.

[0078] It should be noted that the composition and process of the embodiments 1-6 of the present application all meet the requirements of the present application, the composition of the comparative example 1 is the same as that of the embodiment 2, and the process parameters thereof do not meet the present application, and although the process parameters of the comparative examples 2-3 meet the design of the present application, the composition design thereof does not meet the design of the present application.

[0079] Tables 2-1, 2-2 and 2-3 list the specific process parameters of the 1300 MPa grade ultra-high strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3.

[0080] Table 2-1.

[0081] Table 2-2 Process parameters of step (4)

[0082] Table 2-3 Process parameters of step (4)

[0083] In order to verify the implementation effect of the present application, the 1300MPa grade ultra-high strength steel of the present application examples 1-6 and the comparative steel of the comparative examples 1-3 were sampled, and the metallographic sample after polishing was eroded by 4% nitric acid alcohol solution for about 10s, and after clear blowing and drying, the microstructure was observed by scanning electron microscope, and the observation results are listed in Table 3.

[0084] Table 3 lists the microstructure observation results of the 1300MPa grade ultra-high strength steel of the present application examples 1-6 and the comparative steel of the comparative examples 1-3.

[0085] Table 3.

[0086] Note: When the microstructure only includes carbides and does not include borides, the "volume phase ratio of carbides and / or borides in the microstructure" refers to the volume phase ratio of carbides; when the microstructure includes carbides and borides, the "volume phase ratio of carbides and / or borides in the microstructure" refers to the sum of the volume phase ratio of carbides and the volume phase ratio of borides.

[0087] As can be seen from the above Table 3, the microstructure of the 1300MPa grade ultra-high strength steel of the present application examples 1-6 contains residual austenite and dispersedly distributed carbides, wherein the volume phase ratio of residual austenite is between 2-6%, the volume phase ratio of carbides and / or borides is between 4-9%, and the size of carbides and / or borides is ≤100nm.

[0088] In addition, Figure 1 shows the microstructure morphology of Example 1.

[0089] As shown in Figure 1, the microstructure of the 1300MPa grade ultra-high strength steel contains residual austenite and dispersedly distributed carbides and borides, wherein the volume phase ratio of residual austenite is 5%, the sum of the volume phase ratio of carbides and the volume phase ratio of borides is 5%, and the size of carbides and / or borides is ≤100nm.

[0090] The 1300MPa grade ultra-high strength steel of the present application examples 1-6 and the comparative steel of the comparative examples 1-3 were resampled, and their mechanical properties were detected, and the test results are listed in Table 4. Among them, the tensile property detection was carried out according to GB / T228.1-2021 "Metallic materials-tensile testing-Part 1:room temperature testing methods".

[0091] Table 4.

[0092] As can be seen from Table 4 above, the yield strength of the 1300 MPa grade ultra-high strength steel of the embodiments 1-6 of the present application is less than 850 MPa, the tensile strength is greater than 1300 MPa, the yield strength ratio is less than 0.65, and the fracture elongation is greater than or equal to 9%.

[0093] It should be noted that the combination of the technical features in the present case is not limited to the combination manner described in the claims of the present case or the combination manner described in the specific embodiments, and all the technical features described in the present case can be freely combined or combined in any manner, unless contradictory to each other.

[0094] It should also be noted that the above-mentioned embodiments are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned embodiments, and similar changes or modifications made in accordance with the disclosure of the present application are directly derived or easily thought of by those skilled in the art, and should all fall within the protection scope of the present application.

Claims

1. A 1300 MPa grade ultra-high strength steel with low yield ratio, containing Fe and inevitable impurities, characterized in that, It also contains the following chemical elements in the following mass percentages: C: 0.18-0.25%; Si: 0.5-1.0%; Mn: 1.96-2.5%; B: 0.001-0.004%; Al: 0.02-0.25%; Ti: 0.086-0.15%. Its microstructure has residual austenite and nanometer-sized carbides and / or borides dispersedly distributed.

2. The ultra-high strength steel according to claim 1, characterized in that, It contains the following chemical elements in the following mass percentages: C: 0.18-0.25%; Si: 0.5-1.0%; Mn: 1.96-2.5%; B: 0.001-0.004%; Al: 0.02-0.25%; Ti: 0.086-0.15%; the balance being Fe and inevitable impurities.

3. The ultra-high strength steel according to claim 1, characterized in that, It also contains at least one of the following elements: 0 0 0 0 4. The ultra-high strength steel according to claim 1, wherein 0 5. The ultra-high strength steel according to claim 1, wherein The mass percentages of its chemical elements satisfy at least one of the following: 0.1%≤C-Ti / 2-Nb-V / 4-Mo / 10+30×B≤0.3%; (Mo+Cr / 4) / (Ti+Nb+V / 2)≤2.

6. The ultra-high strength steel according to claim 1, wherein Among its inevitable impurities, P≤0.015wt%, S≤0.003wt%, N≤0.004wt%.

7. The ultra-high strength steel according to claim 1, wherein The volume fraction of residual austenite in its microstructure is 2-6%.

8. The ultra-high strength steel according to claim 1, wherein The volume fraction of carbides and / or borides in its microstructure is 4-9%.

9. The ultra-high strength steel according to claim 1, wherein The size of the carbides and / or borides is ≤100nm.

10. The ultra-high strength steel according to claim 1, wherein Its microstructure also has acicular bainite and tempered martensite, and optionally granular bainite.

11. The ultra-high strength steel according to claim 10, wherein Its yield strength is ≤850MPa, tensile strength is ≥1300MPa, yield strength ratio is ≤0.65, and elongation at break is ≥9%.

12. The method of producing ultra-high-strength steel according to any one of claims 1 to 11, characterized in that, Its yield strength is ≤750MPa, and yield strength ratio is ≤0.

55. It comprises the steps of: smelting and casting; hot rolling; pickling and cold rolling; annealing: heating the strip to 860-920°C and holding, then cooling to 720-820°C at a slow cooling rate of 5-20°C / s and holding, then cooling to 260-370°C at a fast cooling rate of 40-80°C / s and holding, then reheating to 350-470°C at a heating rate of 5-30°C / s and holding, then cooling to 300-460°C at a second fast cooling rate of 5-30°C / s and holding, and then cooling to room temperature at a final average cooling rate of ≤10°C / s.

13. The production method according to claim 12, wherein In the annealing step, the strip steel is heated to 860-920℃ and kept for 90-180s, then cooled to 720-820℃ at a slow cooling rate of 5-20℃ / s and kept for less than 10s, then cooled to 260-370℃ at a fast cooling rate of 40-80℃ / s and kept for 1-20s, then reheated to 350-470℃ at a heating rate of 5-30℃ / s and kept for 5-30s, then cooled to 300-460℃ at a second fast cooling rate of 5-30℃ / s and kept for 60-180s, and then cooled to room temperature at a final cooling average cooling rate of ≤10℃ / s.

14. The production method according to claim 12, wherein In the annealing step, the hot-dip galvanizing of the surface of the strip steel is completed in the process segment of reheating to 350-470℃ at a heating rate of 5-30℃ / s and keeping.

15. The production method according to claim 12, wherein In the hot rolling step, the slab tapping temperature is controlled to be 1220-1300℃, the rough rolling temperature is 1030-1120℃, and the finish rolling temperature is 880-970℃; and / or In the hot rolling step, the strip steel is first cooled to 680-740℃ at a first cooling rate of 20-50℃ / s after rolling, and then cooled to 460-520℃ at a second cooling rate of 50-100℃ / s for coiling.

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