Low-alloy ultra-high-strength steel having excellent drawability, and manufacturing method therefor

Abstract

Disclosed in the present invention is a low-alloy ultra-high-strength steel having excellent drawability. The low-alloy ultra-high-strength steel contains Fe and inevitable impurities, and further contains the following chemical elements in mass percentage: 0.18-0.25% of C; 0.4-0.9% of Si; 1.55-1.95% of Mn; 0.001-0.004% of B; 0.03-0.3% of Al; and 0.086-0.15% of Ti. The microstructure of the steel has retained austenite and nanoscale carbides. Also disclosed in the present invention is a manufacturing method for the low-alloy ultra-high-strength steel, the method comprising the steps of: smelting and casting; hot rolling; pickling and cold rolling; and annealing.

Description

Low-alloy ultra-high-strength steel with excellent drawing performance and manufacturing method thereof TECHNICAL FIELD

[0001] The present application relates to a steel sheet and a manufacturing method thereof, in particular to a low-alloy 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 by using ultra-high-strength steel, which puts forward higher and higher requirements on the forming performance of the ultra-high-strength steel, especially the drawing performance.

[0003] In the prior art, the scheme for improving the drawing performance of the ultra-high-strength steel is to add a large proportion of Mn, Si and / or Al elements in the composition design of the steel material, so as to introduce a large proportion of residual austenite in the matrix of the steel material.

[0004] For example, a Chinese patent document with the publication number CN116732448A, the publication date of September 12, 2023, and the name of "1000MPa grade hot-dip galvanized enhanced forming complex phase steel and preparation method thereof" discloses a 1000MPa grade hot-dip galvanized enhanced forming complex phase steel and a preparation method thereof, which can obtain excellent drawing performance with a uniform elongation of 8%, but the complex phase steel adds 2.4-2.9% Mn, 1.0-1.5% Al and 0.4-0.8 Cr. The addition of a large amount of Mn, Cr, Si and / or Al elements on the one hand greatly increases the alloy cost of the steel material, and on the other hand damages the welding performance of the steel material, which greatly limits its application in automobile part manufacturing. SUMMARY

[0005] One of the purposes of the present application is to provide a low-alloy ultra-high-strength steel with excellent drawing performance, which uses a lower alloy composition design to obtain excellent drawing performance and mechanical strength.

[0006] In order to achieve the above-mentioned purpose, the present application provides a low-alloy ultra-high-strength steel with excellent drawing performance, which contains Fe and inevitable impurities, and further contains the following chemical elements with mass percentage as follows:

[0007] C: 0.18-0.25%; Si: 0.4-0.9%; Mn: 1.55-1.95%; B: 0.001-0.004%; Al: 0.03-0.3%; Ti: 0.086-0.15%;

[0008] The microstructure thereof has residual austenite and nanoscale carbides.

[0009] In the present application, the inventors generate bainite, retained austenite and nanoscale carbides in the microstructure by a relatively low alloying design, and utilize the TRIP effect of the retained austenite transforming to martensite and the hindering effect of the nanoscale carbides on high-density dislocation slip in bainite and newly generated martensite to improve the drawing performance of the ultra-high strength steel.

[0010] Further, in the low-alloy ultra-high strength steel described in the present application, the mass percentage of each chemical element is:

[0011] C: 0.18-0.25%; Si: 0.4-0.9%; Mn: 1.55-1.95%; B: 0.001-0.004%; Al: 0.03-0.3%; Ti: 0.086-0.15%; the balance being Fe and unavoidable impurities.

[0012] In the low-alloy ultra-high strength steel described in the present application, the design principles of each chemical element are as follows:

[0013] C: In the low-alloy ultra-high strength steel described in the present application, the C element not only controls the phase transformation and affects the formation of retained austenite, but also directly affects the precipitation, distribution and particle size of carbides. At the same time, the C element plays a decisive role in the strength and drawing performance of the steel in the present application. Therefore, in the low-alloy ultra-high strength steel described in the present application, the mass percentage of C is controlled between 0.18-0.25%.

[0014] Si: In the low-alloy ultra-high strength steel described in the present application, the 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 retained austenite. When the mass percentage of Si is too high, it will affect the precipitation of carbides and the formation of bainite. Therefore, in the low-alloy ultra-high strength steel described in the present application, the mass percentage of Si is controlled between 0.4-0.9%.

[0015] Mn: In the low-alloy ultra-high strength steel described in the present application, Mn is one of the core elements that affect strength and control phase transformation. In the present application, Mn, together with Si, determines the formation of retained austenite and bainite. Therefore, in the low-alloy ultra-high strength steel described in the present application, the mass percentage of Mn is controlled between 1.55-1.95%.

[0016] B: In the low-alloy ultra-high-strength steel according to the present application, the element B is not only beneficial to the formation of the bainite in the steel, but also has a great influence on the strength and hardness of the steel plate. When the mass percentage content of the element B is too low, the strength of the steel cannot meet the design requirements. When the mass percentage content of the element B 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 according to the present application, the mass percentage content of the element B is controlled to be between 0.001% and 0.004%.

[0017] Al: In the low-alloy ultra-high-strength steel according to the present application, the element Al can not only serve as a deoxidizing element and a trace strengthening element to improve the quality and strength of the steel, but also serve as a phase transformation control element for inducing the generation of residual austenite. Therefore, in the low-alloy ultra-high-strength steel according to the present application, the mass percentage content of the element Al is controlled to be between 0.03% and 0.3%.

[0018] Ti: In the low-alloy ultra-high-strength steel according to the present application, Ti is a strong carbonitride-forming element. The carbonitride formed by Ti can not only refine the grains, but also hinder the dislocation slip in the steel, thereby improving the strength and forming performance of the steel. Therefore, in the low-alloy ultra-high-strength steel according to the present application, the mass percentage content of the element Ti is controlled to be between 0.086% and 0.15%.

[0019] Further, in the low-alloy ultra-high-strength steel according to the present application, the steel further contains at least one of the following elements:

[0020] 0 < Mo≤ 0.2 wt%

[0021] 0 < Cr≤ 0.2 wt%;

[0022] 0 < Nb≤ 0.06 wt%;

[0023] 0 < Cu≤ 0.2 wt%;

[0024] 0 < Ce≤ 0.006 wt%;

[0025] 0 < V≤ 0.2 wt%.

[0026] In some embodiments, the low-alloy ultra-high-strength steel according to the present application contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages: C: 0.18-0.25%; Si: 0.4-0.9%; Mn: 1.55-1.95%; B: 0.001-0.004%; Al: 0.03-0.3%; Ti: 0.086-0.15%; Mo≤0.2wt%; Cr≤0.2wt%; Nb≤0.06wt%; Cu≤0.2wt%; Ce≤0.006wt%; and V≤0.2wt%; and the microstructure thereof has residual austenite and nanoscale carbides. In some embodiments, the low-alloy ultra-high-strength steel according to the present application contains the following chemical elements in the following mass percentages: C: 0.18-0.25%; Si: 0.4-0.9%; Mn: 1.55-1.95%; B: 0.001-0.004%; Al: 0.03-0.3%; Ti: 0.086-0.15%; Mo≤0.2wt%; Cr≤0.2wt%; Nb≤0.06wt%; Cu≤0.2wt%; Ce≤0.006wt%; V≤0.2wt%; and the balance is Fe and inevitable impurities.

[0027] In the present application, Mo, Cr, Nb, Cu, Ce and V can be selectively added to the steel as optional elements. Among them:

[0028] Cr and Mo: In the low-alloy ultra-high-strength steel according to the present application, Cr and Mo can increase the hardenability of the steel strip, increase the incubation period of pearlite and ferrite, inhibit the formation of pearlite and ferrite, and facilitate the formation of bainite structure during cooling. When the mass percentage of Cr and Mo is too low, it can lead to insufficient bainite formation; when the mass percentage of Cr and Mo is too high, it can easily form more martensite structure, which can deteriorate the formability of the steel strip. Therefore, in the low-alloy ultra-high-strength steel according to the present application, the mass percentage of Mo can be controlled to be Mo≤0.2wt%, and the mass percentage of Cr can be controlled to be Cr≤0.2wt%.

[0029] 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 bainite and residual austenite and improve the forming performance of the steel material, but from the perspective of cost control and ensuring weldability, the addition amount of the above elements needs to be controlled within the upper limit. Therefore, in the low alloy high strength steel described in the present application, the mass percentage content of Nb element can be controlled as Nb≤0.06wt%, the mass percentage content of Cu element can be controlled as Cu≤0.2wt%, the mass percentage content of Ce element can be controlled as Ce≤0.006wt%, and the mass percentage content of V element can be controlled as V≤0.2wt%.

[0030] Further, in the low alloy ultra-high strength steel described in the present application, the mass percentage content of each chemical element satisfies at least one of the following:

[0031] Mn+Si+Cr+Al+Mo+Cu≤3.5%;

[0032] 1.6≤Mn / (Si+Al)≤3.2;

[0033] 0.15≤(Ti+V+2Nb+Mo / 4) / (Si+Al)≤0.40.

[0034] In the present application, controlling Mn+Si+Cr+Al+Mo+Cu≤3.5% can ensure low manufacturing cost, low carbon equivalent level and good welding performance of the steel material. In some embodiments, Mn+Si+Cr+Al+Mo+Cu≤3.0% is controlled.

[0035] In the present application, Mn is the main austenite forming element, and Si and Al are the main ferrite forming elements. In order to ensure the presence of appropriate residual austenite and the proportion of acicular / particle bainite and tempered martensite in the microstructure, the ratio of the above two types of elements needs to be considered, therefore, 1.6≤Mn / (Si+Al)≤3.2 is controlled. In some embodiments, Mn / (Si+Al) is controlled within the range of 1.8-2.6.

[0036] In the present application, since Ti, V and Nb are strong carbide forming elements, Mo is a weak carbonitride forming element, and the addition of Al and Si will inhibit the formation of carbides in bainite, the influence of the above two types of elements on carbide precipitation and size needs to be considered. Therefore, 0.15≤(Ti+V+2Nb+Mo / 4) / (Si+Al)≤0.4 is controlled. In some embodiments, (Ti+V+2Nb+Mo / 4) / (Si+Al) is controlled within the range of 0.20-0.37.

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

[0038] In the present application, the unavoidable impurities mainly include phosphorus (P), sulfur (S), and nitrogen (N), and it is desirable that their contents are as low as possible under the technical conditions.

[0039] Further, in the low-alloy ultra-high-strength steel of the present application, the microstructure thereof includes acicular bainite, granular bainite, residual austenite, and nanoscale carbide, and optionally tempered martensite; preferably, the sum of the volume proportions of acicular bainite and granular bainite is greater than or equal to 60%, such as 60-92% or 80-92%. In some other embodiments, the microstructure of the low-alloy ultra-high-strength steel of the present application is acicular bainite, granular bainite, residual austenite, and nanoscale carbide. In some embodiments, the microstructure of the low-alloy ultra-high-strength steel of the present application is acicular bainite, granular bainite, tempered martensite, residual austenite, and nanoscale carbide; preferably, the sum of the volume proportions of acicular bainite + granular bainite + tempered martensite is greater than or equal to 60%, such as 60-90% or 80-90%.

[0040] Further, in the low-alloy ultra-high-strength steel of the present application, the volume proportion of residual austenite in the microstructure thereof is 4-12%.

[0041] Further, in the low-alloy ultra-high-strength steel of the present application, the volume proportion of nanoscale carbide in the microstructure thereof is 3-8%.

[0042] Further, in the low-alloy ultra-high-strength steel of the present application, the number of nanoscale carbides satisfies: in each 1 μm 2 of the detection field of view, the number of nanoscale carbides is 1-20.

[0043] Further, in the low-alloy ultra-high-strength steel of the present application, the tensile strength thereof is greater than or equal to 980 MPa, the uniform elongation thereof is greater than or equal to 10%, and the elongation at break thereof is greater than or equal to 14%. In some embodiments, the tensile strength of the low-alloy ultra-high-strength steel of the present application is greater than or equal to 1000 MPa. In some embodiments, the tensile strength of the low-alloy ultra-high-strength steel of the present application is 980-1160 MPa, such as 1000-1160 MPa. In some embodiments, the yield strength of the low-alloy ultra-high-strength steel of the present application is greater than or equal to 650 MPa. In some embodiments, the yield strength of the low-alloy ultra-high-strength steel of the present application is 650-900 MPa, such as 720-900 MPa.

[0044] Another object of the present application is to provide a manufacturing method of low-alloy ultra-high-strength steel, which can obtain a low-alloy ultra-high-strength steel with excellent drawing performance.

[0045] In order to achieve the above-mentioned object, the present application provides a manufacturing method of low-alloy ultra-high-strength steel, which comprises the steps of:

[0046] smelting and casting;

[0047] hot rolling;

[0048] pickling and cold rolling;

[0049] annealing: heating the strip steel to 860-950℃ and holding, then cooling to 720-820℃ at a slow cooling rate of 5-20℃ / s and holding, then cooling to 280-380℃ at a fast cooling rate of 20-80℃ and holding, then reheating to 390-480℃ at a heating rate of 5-30℃ / s and holding, then cooling to 300-380℃ at a second fast cooling rate of 5-50℃ / s and holding, and then cooling to room temperature at a final average cooling rate of ≤10℃ / s.

[0050] In the annealing step of the present application, controlling the heating temperature of the strip steel to 860-950℃ can ensure the sufficient formation of austenite parent phase and the secondary resolubilization of carbides in the strip steel, refining the grain size and the size of carbides; cooling to 720-820℃ at a slow cooling rate of 5-20℃ / s and holding can avoid the formation of ferrite on the one hand and regulate the precipitation of carbides on the other hand; cooling to 280-380℃ at a fast cooling rate of 20-80℃ and holding can make the strip steel enter the martensite or bainite transformation region; reheating to 390-480℃ at a heating rate of 5-30℃ / s and holding, and then cooling to 300-380℃ at a second fast cooling rate of 5-50℃ / s and holding can ensure the fine and dispersed precipitation of carbides while the residual austenite is formed; cooling to room temperature at a final average cooling rate of ≤10℃ / s can further promote the fine and dispersed precipitation of carbides.

[0051] Further, in the annealing step of the manufacturing method described in the present application, the strip steel is heated to 860-950℃ and held for 90-180s, then cooled to 720-820℃ at a slow cooling rate of 5-20℃ / s and held for 3-20s, then cooled to 280-380℃ at a fast cooling rate of 20-80℃ and held for 1-10s, then reheated to 390-480℃ at a heating rate of 5-30℃ / s and held for 3-20s, then cooled to 300-380℃ at a second fast cooling rate of 5-50℃ / s and held for 100-200s, and then cooled to room temperature at a final average cooling rate of ≤10℃ / s.

[0052] Further, in the annealing step of the manufacturing method, the hot-dip galvanizing of the strip surface is completed in the process of reheating to 390-480°C at a heating rate of 5-30°C / s.

[0053] Further, in the hot-rolling step of the manufacturing method, the slab discharge temperature is controlled to 1260-1300°C, the rough-rolling temperature is controlled to 1070-1120°C, and the finish-rolling temperature is controlled to 890-970°C.

[0054] In the present application, controlling the slab discharge temperature to 1260-1300°C can ensure that the carbonitride formed first in the slab can be fully redissolved; controlling the rough-rolling temperature to 1070-1120°C and the finish-rolling temperature to 890-970°C can avoid batch precipitation and coarsening of the carbide, and only form a small amount of fine carbide, which is beneficial to the improvement of the elongation at break.

[0055] Further, in the hot-rolling step of the manufacturing method, the strip is cooled to 450-510°C at a cooling rate of 30-80°C / s after rolling and then coiled.

[0056] In the present application, the above-mentioned cooling process after rolling can avoid the formation of uneven martensite structure in the hot-rolled structure; and controlling the coiling temperature to 450-510°C can avoid excessive precipitation and coarsening of the carbide in the hot-rolled coil, thereby affecting the organization regulation during subsequent annealing.

[0057] The low-alloy ultra-high-strength steel with excellent drawing performance and the manufacturing method thereof have the following advantages and beneficial effects:

[0058] In the low-alloy ultra-high-strength steel with excellent drawing performance, through relatively low alloy composition design, bainite, residual austenite and nanoscale carbide are generated in the microstructure, and the TRIP effect of the transformation of residual austenite to martensite and the hindering effect of nanoscale carbide on high-density dislocation slip in bainite and newly formed martensite are used to improve the drawing performance of the ultra-high-strength steel.

[0059] In some embodiments, the low-alloy ultra-high-strength steel with excellent drawing performance has a tensile strength ≥980 MPa, a uniform elongation ≥10%, and an elongation at break ≥14%. At the same time, due to its low alloy content, it can be widely used in the preparation of various automobile structural parts. BRIEF DESCRIPTION OF DRAWINGS

[0060] Figure 1 shows the microstructure morphology of nanoscale carbide observed under a transmission electron microscope in Example 1 of the present application.

[0061] Figure 2 schematically shows the engineering stress-strain curves of Example 1 and Example 2. DETAILED DESCRIPTION

[0062] The low-alloy ultra-high-strength steel with excellent drawing performance and the manufacturing method thereof will be further explained and described in connection with specific examples and the accompanying drawings of the specification, however, the explanation and description does not constitute undue limitation on the technical solutions of the present application.

[0063] Examples 1-6 and Comparative Examples 1-3

[0064] Table 1-1 and Table 1-2 list the mass percentages (wt%) of each chemical element in the low-alloy ultra-high-strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3.

[0065] Table 1-1. (The balance is Fe and other unavoidable impurities other than P, S, and N)

[0066] Table 1-2. (The balance is Fe and other unavoidable impurities other than P, S, and N)

[0067] The low-alloy ultra-high-strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3 are both prepared by the following steps:

[0068] (1) Smelting and continuous casting;

[0069] (2) Hot rolling: the slab discharge temperature is controlled at 1260-1300℃, the rough rolling temperature is 1070-1120℃, the final rolling temperature of the finishing mill is 890-970℃, and after rolling, the strip steel is cooled to 450-510℃ at a cooling rate of 30-80℃ / s for coiling;

[0070] (3) Pickling and cold rolling;

[0071] (4) Annealing: the strip steel is heated to 860-950℃ and held for 90-180s, then cooled to 720-820℃ at a slow cooling rate of 5-20℃ / s and held for 3-20s, then cooled to 280-380℃ at a fast cooling rate of 20-80℃ / s and held for 1-10s, then reheated to 390-480℃ at a heating rate of 5-30℃ / s and held for 3-20s, then cooled to 300-380℃ at a second fast cooling rate of 5-50℃ / s and held for 100-200s, and then cooled to room temperature at a final cooling average cooling rate of ≤10℃ / s.

[0072] In addition, in some embodiments, the hot-dip galvanizing of the surface of the strip steel can also be completed in the process segment of reheating to 390-480℃ at a heating rate of 5-30℃ / s and holding in step (4), so as to obtain a hot-dip galvanized product.

[0073] It should be noted that the components and processes of the embodiments 1-6 of the present application all meet the requirements of the present application, the components of the comparative example 1 are the same as those of the embodiment 1, the process parameters thereof do not meet the present application, the process parameters and component design of the comparative example 2 do not meet the design of the present application, and the components of the comparative example 3 do not meet the present application, but the process parameters thereof meet the present application.

[0074] Table 2-1, Table 2-2 and Table 2-3 list the specific process parameters of the low-alloy ultra-high-strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3.

[0075] Table 2-1.

[0076] Table 2-2 Process parameters of step (4)

[0077] Table 2-3 Process parameters of step (4)

[0078] In order to verify the implementation effect of the present application, the low-alloy ultra-high-strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3 are sampled, and the metallographic sample after polishing is etched by 4% nitric acid alcohol solution for about 10s, and then observed under an optical microscope or a scanning electron microscope after being clearly blown dry, and the observation results are listed in Table 3. Among them:

[0079] The number of nanoscale carbides in each 1μm 2 of the detection field of view is detected by the following method: a transmission electron microscope sample is prepared parallel to the surface of the steel plate or perpendicular to the surface of the steel plate, the sample edge thickness is ≤30μm, then the sample center thin area is prepared by FIB, ion thinning, double-spray thinning and the like, and then the distribution of nanoscale carbides is detected under a transmission electron microscope by bright field image.

[0080] Table 3 lists the microstructure observation results of the low-alloy ultra-high-strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3.

[0081] Table 3.

[0082] As can be seen from the above Table 3, the microstructure of the low-alloy ultra-high-strength steel of the embodiments 1-6 of the present application has residual austenite and nanoscale carbides, and also has acicular bainite and granular bainite, and optionally tempered martensite. Among them, the volume phase ratio of residual austenite is between 4-12%, the volume phase ratio of carbides is between 3-8%, and the number of nanoscale carbides contained in each 1μm 2 of the detection field of view is between 1-20.

[0083] Figure 1 shows the microstructure morphology of nanoscale carbides observed under a transmission electron microscope in Example 1 of the present application.

[0084] As shown in Figure 1, the nanoscale carbides in Example 1 hindered high-density dislocation slip.

[0085] The low-alloy ultra-high-strength steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3 were resampled and tested, and the test results are listed in Table 4. Among them, the tensile property test was carried out according to GB / T228.1-2021 "Metallic Materials Tensile Test Part 1: Room Temperature Test Method". The yield strength and tensile strength were tested according to GB / T228.1-2021 "Metallic Materials Tensile Test Part 1: Room Temperature Test Method".

[0086] Table 4.

[0087] As can be seen from the above Table 4, the tensile strength of the low-alloy ultra-high-strength steels of Examples 1-6 of the present application is greater than 980 MPa, the uniform elongation is greater than 10%, and the fracture elongation is greater than or equal to 14%.

[0088] In addition, Figure 2 schematically shows the engineering stress-strain curves of Example 1 and Example 2.

[0089] As shown in Figure 2, the tensile strength of Examples 1 and 2 is more than 980 MPa, the uniform elongation is greater than 10%, and the fracture elongation is greater than 14%.

[0090] It should be noted that the combination of the technical features in the present case is not limited to the combination mode described in the claims of the present case or the combination mode described in the specific embodiments. All the technical features described in the present case can be freely combined or combined in any way, unless contradictory to each other.

[0091] It should also be noted that the above-mentioned examples are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned examples, and similar changes or modifications made on the basis of the disclosure of the present application are directly derived or easily conceived by those skilled in the art, and should all fall within the scope of protection of the present application.

Claims

1. A low-alloy ultra-high-strength steel with excellent drawing properties, containing Fe and unavoidable impurities, characterized in that, It also contains the following chemical elements in the following mass percentages: C: 0.18~0.25%; Si: 0.4~0.9%; Mn: 1.55~1.95%; B: 0.001~0.004%; Al: 0.03~0.3%; Ti: 0.086–0.15%; Its microstructure contains retained austenite and nanoscale carbides.

2. The low-alloy ultra-high-strength steel as described in claim 1, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.18~0.25%; Si: 0.4~0.9%; Mn: 1.55~1.95%; B: 0.001~0.004%; Al: 0.03~0.3%; Ti: 0.086–0.15%; balance Fe and unavoidable impurities.

3. The low-alloy ultra-high-strength steel as described in claim 1 or 2, characterized in that, It also contains at least one of the following elements: 0 < Mo ≤ 0.2 wt% 0 < Cr ≤ 0.2 wt%; 0 < Nb ≤ 0.06 wt%; 0 < Cu ≤ 0.2 wt%; 0 < Ce ≤ 0.006 wt%; 0 < V ≤ 0.2 wt%.

4. The low-alloy ultra-high-strength steel as described in claim 1 or 2, characterized in that, Its mass percentage content of each chemical element satisfies at least one of the following conditions: Mn+Si+Cr+Al+Mo+Cu≤3.5%; 1.6≤Mn / (Si+Al)≤3.2; 0.15≤(Ti+V+2Nb+Mo / 4) / (Si+Al)≤0.

40.

5. The low-alloy ultra-high-strength steel as described in claim 1 or 2, characterized in that, Of its unavoidable impurities, P ≤ 0.015 wt%, S ≤ 0.003 wt%, and N ≤ 0.004 wt%.

6. The low-alloy ultra-high-strength steel as described in claim 1 or 2, characterized in that, The volume ratio of retained austenite in its microstructure is 4-12%.

7. The low-alloy ultra-high-strength steel as described in claim 1 or 2, characterized in that, The volume ratio of the nanoscale carbides in its microstructure is 3-8%.

8. The low-alloy ultra-high-strength steel as described in claim 1 or 2, characterized in that, Its microstructure also includes acicular bainite, granular bainite, and optional tempered martensite.

9. The low-alloy ultra-high-strength steel as described in claim 1 or 2, characterized in that, The number of nanoscale carbides satisfies the following condition: per 1 μm 2 The number of nanoscale carbides contained in the detection field of view is 1 to 20.

10. The low-alloy ultra-high-strength steel as described in claim 1 or 2, characterized in that, Its tensile strength is ≥980MPa, uniform elongation is ≥10%, and elongation at break is ≥14%; preferably, its yield strength is ≥650MPa.

11. The method for manufacturing low-alloy ultra-high-strength steel according to any one of claims 1-10, characterized in that, It includes the following steps: Smelting and casting; Hot-rolled; Pickling and cold rolling; Annealing: Heat the strip steel to 860-950℃ and hold it at that temperature. Then, cool it to 720-820℃ at a slow cooling rate of 5-20℃ / s and hold it at that temperature. Next, cool it to 280-380℃ at a rapid cooling rate of 20-80℃ and hold it at that temperature. Then, reheat it to 390-480℃ at a heating rate of 5-30℃ / s and hold it at that temperature. Then, cool it to 300-380℃ at a secondary rapid cooling rate of 5-50℃ / s and hold it at that temperature. Finally, cool it to room temperature at a final cooling average rate of ≤10℃ / s.

12. The manufacturing method as described in claim 11, characterized in that, In the annealing step, the strip steel is heated to 860-950℃ and held for 90-180s, then cooled to 720-820℃ at a slow cooling rate of 5-20℃ / s and held for 3-20s, then cooled to 280-380℃ at a rapid cooling rate of 20-80℃ and held for 1-10s, then reheated to 390-480℃ at a heating rate of 5-30℃ / s and held for 3-20s, then cooled to 300-380℃ at a secondary rapid cooling rate of 5-50℃ / s and held for 100-200s, and finally cooled to room temperature at a final cooling average rate of ≤10℃ / s.

13. The manufacturing method as described in claim 11, characterized in that, In the annealing step, hot-dip galvanizing of the strip surface is completed in a process where the strip is reheated to 390-480°C at a heating rate of 5-30°C / s and held at that temperature.

14. The manufacturing method as described in claim 11, characterized in that, In the hot rolling process, the slab exit temperature is controlled at 1260–1300℃, the rough rolling temperature at 1070–1120℃, and the finishing rolling temperature at 890–970℃.

15. The manufacturing method as described in claim 14, characterized in that, In the hot rolling process, the strip is cooled to 450-510°C at a cooling rate of 30-80°C / s after rolling and then coiled.

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