Ultrahigh-strength steel strip and manufacturing method therefor

Abstract

Disclosed is an ultrahigh-strength steel strip, containing Fe and unavoidable impurity elements. Furthermore, the ultrahigh-strength steel strip further contains the following chemical elements in percentage by mass: C: 0.13-0.20%; Si: 0.15-0.50%; Mn: 1.4-2.0%; B: 0.001-0.004%; Al: 0.01-0.04%; Cr: 0.1-0.4%; Mo: 0.1-0.4%; and Ti+V+Nb≤0.02%. The microstructure thereof comprises granular bainite uniformly dispersed in a coral pattern. Correspondingly, further disclosed is a manufacturing method for the ultrahigh-strength steel strip. The ultrahigh-strength steel strip of the present invention has low anisotropy and high in-roll mechanical uniformity while achieving a tensile strength of 1000 MPa.

Description

Technical field

[0001] The present disclosure relates to an ultra-high strength steel and a manufacturing method therefor, in particular to an ultra-high strength steel with a coating layer and a manufacturing method therefor.Background Art

[0002] With the development of the automobile industry, automotive parts are being designed to be increasingly compact, lightweight, and precise, while requiring mechanical properties to be as uniform as possible in dual, even multiple directions to ensure high-speed production, high-precision assembly, and high service stability of the parts. This not only requires materials to have ultra-high strength and good formability, but also relatively low mechanical anisotropy or high isotropy, and even higher performance uniformity.

[0003] At the current stage, an ultra-high-strength steel is increasingly being used in automotive parts. At the same time, since the ultra-high-strength steel is currently produced through large-scale integrated production, it places very high demands on the mechanical uniformity in the same coil of the steel.

[0004] However, in the prior art, as the strength of the high-strength steel reaches 1000 MPa, it becomes extremely difficult to obtain steel strips with low anisotropy and high mechanical uniformity in the same coil. Taking data from the publicly available "Journal of Plasticity Engineering, 2021, 28(7): 124-130" as an example, for a dual-phase steel with a tensile strength of 800 MPa, the difference in strength between the transverse and longitudinal directions is about 8 MPa. However, when the tensile strength of a dual-phase steel reaches the 1000 MPa level, the difference in strength between the transverse and longitudinal directions can be as high as approximately 40 MPa.

[0005] In addition, the Chinese patent publication CN104018092A, published on March 8, 2017, with a title of "750MPa-level high-strength high-mechanical-uniformity steel plate, and application and manufacturing method thereof" discloses a steel grade with a tensile strength of only around 800MPa, and its mechanical uniformity in the transverse, longitudinal, and 45° directions can only reach less than 20MPa.

[0006] Another example is Chinese patent publication CN102397891A, published on March 23, 2018, with a title of "A method for improving the performance uniformity of steel strip". Although it can achieve relatively high strength uniformity in the width direction of the steel strip, with strength fluctuations of only 5-15 MPa, the tensile strength of this steel type is only about 350-600 MPa.

[0007] Based on the above, it can be seen that it is technically difficult to provide an ultra-high-strength steel with low anisotropy (or high isotropy), high mechanical uniformity in the same coil, and a strength level of 1000 MPa grade.Summary

[0008] One of the objectives of the present disclosure is to provide an ultra-high strength steel strip that has a tensile strength of 1000MPa while exhibiting low anisotropy and high mechanical uniformity in the same coil.

[0009] In order to achieve the above objective, the present disclosure provides an ultra-high strength steel strip, which comprises Fe and unavoidable impurity elements, and further comprises the following chemical elements in mass percentages as follows: C: 0.13-0.20%; Si: 0.15-0.50%; Mn: 1.4-2.0%; B: 0.001-0.004%; Al: 0.01-0.04%; Cr: 0.1-0.4%; Mo: 0.1-0.4%; Ti+V+Nb ≤0.02%; wherein its microstructure includes uniformly dispersed granular bainite in a coral sea-like pattern.

[0010] Correspondingly, the present disclosure further provides an ultra-high strength steel strip, comprises the following chemical elements in mass percentages as follows: C: 0.13-0.20%; Si: 0.15-0.50%; Mn: 1.4-2.0%; B: 0.001-0.004%; Al: 0.01-0.04%; Cr: 0.1-0.4%; Mo: 0.1-0.4%; Ti+V+Nb ≤0.02%; with a balance of Fe and unavoidable impurity elements.

[0011] Further, in the ultra-high strength steel strip of the present disclosure, the mass percentage of each chemical element meets at least one of the following items: C: 0.14-0.18%; Ti+V+Nb≤0.01%.

[0012] For the ultra-high strength steel strip of the present disclosure, the principles for designing various chemical elements will be described in detail as follows: C: In the ultra-high strength steel strip of the present disclosure, the element C not only controls the phase transformation of the material but can also form alloy carbides with other alloying elements, thereby affecting the strength, formability, and performance uniformity of the steel plate. In the present disclosure, when the C content in the steel is lower than 0.13%, on one hand, it will lead to the steel not reaching the target strength, and on the other hand, it will result in insufficient formation of granular bainite; if the C content in steel is higher than 0.20%, martensitic structures, coarse carbides, and other types of bainite (non-granular bainite, such as upper bainite) are prone to form, thereby deteriorating the performance and performance uniformity of the steel strip. Therefore, in the present disclosure, the mass percentage of C element is controlled at 0.13-0.20%. Si: In the ultra-high strength steel strip of the present disclosure, Si is an essential element for deoxidation in steelmaking, and it has a certain solid solution strengthening effect. At the same time, it also affects the formation of polygonal ferrite and bainite. In the present disclosure, when the Si content in the steel is lower than 0.15%, it is difficult to achieve sufficient deoxidation; when the Si content in the steel is higher than 0.5%, on one hand, iron oxide scale or color difference with tiger stripe patterns are prone to form, which is unfavorable for the surface quality of the steel plate, and on the other hand, it affects the formation of granular bainite, leading to the formation of polygonal ferrite and carbon-free bainite in the steel, thereby impacting the performance uniformity. Therefore, in the present disclosure, the mass percentage of Si is controlled at 0.15% -0.5%. Mn: In the ultra-high strength steel strip of the present disclosure, Mn is one of the key controlling elements for phase transformations in the steel. When the Mn content is too low, it can result in the steel not reaching the target strength and insufficient formation of granular bainite. When the Mn content is too high, it can not only deteriorate corrosion resistance and weldability, and promote the formation of non-granular bainite microstructures such as martensite, but also exacerbate the tendency for grain coarsening and the formation of banded structures or center segregation, thereby reducing the formability of the steel and degrading the isotropy and performance uniformity of the steel strip. Therefore, in the present disclosure, the mass percentage of Mn is controlled at 1.4-2.0%. B: In the ultra-high strength steel strip of the present disclosure, the element B is not only beneficial for the formation of bainite in the steel, but it also has a significant impact on the strength and hardness of the steel plate. If the B content in the steel is lower than 0.001%, the strength of the steel may not meet the target requirements; if the B content is higher than 0.004%, brittle borides are prone to form, affecting the formability and performance uniformity of the steel plate. Therefore, in the present disclosure, the mass percentage of B is controlled at 0.001-0.004%. AI: In the ultra-high strength steel strip of the present disclosure, the element Al is added to the steel only as a deoxidizing element. It can remove oxygen from the steel to ensure the performance and quality of the steel. Therefore, in the present disclosure, the mass percentage of Al is controlled at 0.01-0.04%. Although in some prior art, a large amount of Al (≥0.1%) is added to the steel as an element for forming ferrite and suppressing carbide precipitation, aiming to achieve solid solution strengthening, or to alter phase transformation temperatures (such as A1, A3), bainite formation kinetics, and carbide precipitation kinetics to change the phase transformation of steel, form retained austenite or carbon-free bainite, and ultimately improve steel strength. However, it does not help improve the performance uniformity and isotropy of the steel strip. Therefore, there is no need to add a large amount of Al, in order to avoid significantly increasing costs and the difficulty of steelmaking. Cr and Mo: In the ultra-high strength steel strip of the present disclosure, Cr and Mo can increase the hardenability of the steel strip, extend the incubation period of pearlite and ferrite, inhibit the formation of ferrite in pearlite, and make it easier to obtain a bainitic structure during cooling. Therefore, if the Cr and Mo content is too low, it will lead to insufficient formation of granular bainite; if the Cr and Mo content is too high, it can on one hand easily result in the formation of more microstructures such as martensite, tempered martensite, and the like, worsening the formability of the steel strip, and on the other hand, it can lead to the formation of banded structures or center segregation in the steel strip, thereby deteriorating the isotropy and performance uniformity of the steel strip. Therefore, in the present disclosure, Cr is controlled at 0.1-0.5% and Mo is controlled at 0.1-0.5%. Ti, Nb and V: In the ultra-high strength steel strip of the present disclosure, Ti, Nb, and V are not intentionally added as common microalloying elements beneficial to the properties of the steel, but are controlled as residual elements from steelmaking. Since these microalloying elements begin to form carbonitride precipitates from the start of steelmaking and continuous casting into slabs, especially the precipitation of relatively large carbonitrides such as TiN, and persist throughout the entire steel strip production process, it is extremely difficult to control the uniformity, stability, and precipitation on demand of these carbonitrides. This can adversely affect the performance uniformity of the steel strip. Therefore, in the present disclosure, Ti, Nb, and V are controlled at Ti+Nb+V≤0.02%, and further preferably Ti+Nb+V≤0.01%.

[0013] Further, in the ultra-high strength steel strip of the present disclosure, the contents of the various elements also satisfy: 2.7≤(Cr+Mo) / C≤3.3, where each chemical element represents the value before the percent sign of the mass percentage of the chemical element.

[0014] In the present disclosure, since C, Cr, and Mo play a key role in phase transformation of bainite, it is further preferable to control 2.7 ≤ (Cr + Mo) / C ≤ 3.3. If (Cr + Mo) / C is too low, it easily leads to a too small bainite phase region, resulting in insufficient hardenability of the steel strip. During the production process of annealing, excessive microstructures such as ferrite, pearlite, and upper bainite will be produced due to insufficient cooling rate. Conversely, if (Cr + Mo) / C is too high, it will lead to excessively high hardenability of the steel strip and a higher martensite transformation point temperature, resulting in excessive martensite in the steel strip.

[0015] Further, in the ultra-high strength steel strip of the present disclosure, the content of the inevitable impurity elements in mass percentage satisfies at least one of the following items: P≤0.012%, S≤0.004%, N≤0.004%.

[0016] The impurity elements in the present disclosure are only P, S, and N. Under permissible technical conditions, it is preferable that their contents are as low as possible. Furthermore, the content can be controlled as P ≤ 0.012%, S ≤ 0.004%, N ≤ 0.004%. In some embodiments, S is ≤ 0.003%.

[0017] Further, in the ultra-high strength steel strip of the present disclosure, the volume phase fraction of the granular bainite is ≥ 95.0%.

[0018] Further, in the ultra-high strength steel strip of the present disclosure, the granular bainite has an area of ≤5µm 2< , an aspect ratio of ≤2:1.

[0019] Further, in the ultra-high strength steel strip of the present disclosure, granular bainite is distributed in any cross-sectional area of ≤50 2< µm within the entire region that is no less than 30 µm away from both the top and bottom surfaces of the steel strip in the thickness direction.

[0020] Still further, in the ultra-high strength steel strip of the present disclosure, granular bainite is distributed in any cross-sectional area of ≤ 10 2< µm within the entire region that is no less than 20 µm away from both the top and bottom surfaces of the steel strip in the thickness direction.

[0021] This indicates that granular bainite is basically distributed throughout the middle region that is no less than 30 µm away from the surface of the steel strip.

[0022] Further, in the ultra-high strength steel strip of the present disclosure, its microstructure also comprises ferrite, with a volume fraction of ferrite ranging from 0.1% to 4.5%.

[0023] In addition to granular bainite and ferrite, the microstructure of the ultra-high strength steel strip according to the present disclosure may also comprise retained austenite, other forms of bainite (such as acicular bainite), tempered martensite, carbonitride of titanium (or carbonitride of niobium, or carbonitride of vanadium), and martensite. However, the content of these microstructures is relatively small, with the total volume phase fraction not exceeding 0.5%, and the fraction of each phase satisfies the following order: retained austenite > other forms of bainite > tempered martensite > carbonitride of titanium (or carbonitride of niobium, or carbonitride of vanadium) > martensite.

[0024] Further, the ultra-high strength steel strip of the present disclosure has a tensile strength of ≥1000MPa, a yield strength of≥780MPa; a low anisotropy that satisfies: the difference in yield strength is ≤25 MPa, and the difference in tensile strength is ≤20 MPa in the transverse and longitudinal directions at the same position of the steel strip; and a mechanical uniformity in the same coil that satisfies: for the same steel coil, along its length or width direction, the difference in yield strength is ≤25 MPa, and the difference in tensile strength is ≤20 MPa at different positions in the same stretching direction.

[0025] Still further, the ultra-high strength steel strip of the present disclosure has a tensile strength of ≥1000MPa, a yield strength of ≥780MPa; and a low anisotropy that satisfies: the difference in yield strength is ≤20 MPa, and the difference in tensile strength is ≤15 MPa in the transverse and longitudinal directions at the same position of the steel strip; and a mechanical uniformity in the same coil that satisfies: for the same steel coil, along its length or width direction, the difference in yield strength is ≤20 MPa, and the difference in tensile strength is ≤15 MPa at different positions in the same stretching direction .

[0026] In some embodiments, the ultra-high strength steel strip of the present disclosure has a yield strength of ≥800MPa. In some embodiments, the ultra-high strength steel strip of the present disclosure has a yield strength of ≥850MPa. In some embodiments, the ultra-high strength steel strip of the present disclosure has a yield strength of ≥880MPa.

[0027] In some embodiments, the ultra-high strength steel strip of the present disclosure has a tensile strength of 1000-1080MPa, and a yield strength of 780-950MPa.

[0028] Further, the ultra-high strength steel strip of the present disclosure has an elongation at break of ≥10.0%, and / or a hole expansion ratio of ≥50%.

[0029] In some embodiments, the ultra-high strength steel strip of the present disclosure has an elongation at break of 10.0-15.0%, and / or a hole expansion ratio of 50-75%.

[0030] Another objective of the present disclosure is to provide a method for manufacturing the ultra-high strength steel strip. The steel strip that has a tensile strength of 1000MPa while exhibiting low anisotropy and high mechanical uniformity in the same coil can be prepared by the manufacturing method.

[0031] To achieve the above purpose, the present disclosure provides a manufacturing method for the ultra-high strength steel strip, comprising steps of: smelting and casting; hot-rolling; cooling after rolling and coiling: Cooling after rolling is carried out in multiple stages using alternating fast and slow cooling. Finally, for the areas beyond 100 meters away from the head and tail of the steel strip, the main body of the steel strip is cooled to the main body coiling temperature of 440°C-520°C at a cooling rate of 5-100°C for coiling; for the areas within 100 meters away from the head and tail of the steel strip, the head and tail of the steel strip is cooled to the head and tail coiling temperature of 480°C-560°C at a cooling rate of 5-25°C for coiling; pickling and cold rolling; annealing: the steel is heated at a heating rate of ≤50°C / s to 840-900°C and held; then cooled at a cooling rate of 2-20°C / s to 700-780°C and held; then cooled at a cooling rate of 20-50°C / s to 360-430°C and held; then reheated at a heating rate of 5-30°C / s to 440-480°C; then cooled at a cooling rate of 0.1-0.5°C / s to 330-400°C; and finally cooled at a cooling rate of 15-50°C / s to room temperature and coiled.

[0032] The present disclosure adopts the above-mentioned cooling after rolling and coiling process in order to obtain a uniform cross-sectional shape of the steel strip, good plate profile, and uniform precursor structure, ensuring that during the annealing stage, the steel strip develops uniformly dispersed granular bainite in a coral sea-like pattern along the length, width, and thickness directions, thereby ensuring that the final steel strip has low anisotropy and high performance-uniformity in the same coil.

[0033] The adoption of a multi-stage cooling scheme is mainly to ensure uniform microstructure of the hot-rolled coil after coiling. During the cooling after the rolling stage, the introduction of cooling water can lead to extremely efficient heat exchange and very rapid temperature changes, which can easily cause non-uniform microstructure and properties in local areas of the hot-rolled coil. Moreover, due to fluctuations in the cross-sectional shape and plate profile of the strip after finishing rolling, water may accumulate in local areas of the hot-rolled coil, resulting in abnormal microstructure and properties in local areas. Therefore, the main idea of the multi-stage cooling scheme of the present disclosure is to use a stepwise alternating fast and slow cooling method, reducing the impact of introducing cooling water on the microstructure and properties of the hot coil. The cooling rate gradually decreases, and after fast cooling, slow cooling always follows to eliminate the effects of heat exchange fluctuations and water accumulation caused by the cooling water.

[0034] Regarding the final coiling temperature, the present disclosure adopts a method of low-temperature coiling for the main body of the steel strip, with increased coiling temperatures within 100 meters away from the head and tail of the steel strip. This is mainly to consider the impact of environmental heat exchange after coiling on the microstructure and properties of the hot-rolled coil, and to form a bainite structure within a relatively stable temperature range as much as possible: First, keeping the main body at the lowest possible coiling temperature greatly reduces the effect of environmental heat exchange on the kinetics of hot-rolled coil microstructure formation; second, slightly increasing the coiling temperature at the head and tail (≤100 m) also considers that environmental heat exchange is higher at the head and tail of the hot-rolled coil, which easily leads to a significant temperature drop. Finally, the temperature range of 440°C to 520°C is the main temperature range for bainite formation. If the temperature is too low, martensite structure is easily formed; if the temperature is too high, uneven heat exchange with the environment can occur, leading to a heterogeneous microstructure (possibly forming pearlite, bainite, or martensite in different areas).

[0035] The annealing process used in the present disclosure is mainly to ensure that uniformly dispersed granular bainite in a coral-sea-like pattern in length, width, and thickness directions is obtained in the steel strip.

[0036] Based on the above, the steel strip is first heated at a heating rate of ≤50°C / s to 840-900°C and held to ensure that the steel strip has a primary austenite structure as uniform as possible. The steel strip is then cooled at a cooling rate of 2-20°C / s to 700-780°C and held; on one hand, the purpose is to form a certain amount of ferrite at this stage, thereby significantly reducing the possibility of martensite formation during the subsequent low-temperature bainite transformation, and on the other hand, to lower the rapid cooling start temperature and reduce temperature drop during rapid cooling, thereby minimizing the impact of heat exchange fluctuations on the microstructure and properties during rapid cooling. Then the steel strip is subjected to an annealing process within the bainite phase region of rapid cooling, heating, and slow cooling. Only through such an annealing process design, a coral sea-like, uniformly dispersed granular bainite can be formed. During the rapid cooling stage, if the temperature is too low, martensite is likely to form; if the temperature is too high, upper bainite is likely to form. At the same time, controlling the cooling rate is also crucial. An excessively fast cooling rate is unfavorable for controlling the microstructure uniformity of the steel strip; an excessively slow cooling rate easily leads to the formation of ferrite, pearlite, and upper bainite. The design of the reheating stage and the slow cooling stage aims to allow the steel strip to undergo phase transformation over a longer period within the granular bainite transformation region, thereby forming uniformly dispersed granular bainite with a coral sea-like structure.

[0037] Furthermore, in the hot rolling step of the manufacturing method of the present disclosure, the heating temperature is controlled at 1180-1280°C, and the final rolling temperature of finishing rolling is 870-970°C.

[0038] In this embodiment, the heating temperature is controlled at 1180-1280°C. On one hand, this ensures that the slab is heated to a uniform temperature throughout, and on the other hand, the higher heating temperature also ensures that the final rolling temperature of the subsequent finishing rolling is 870-970°C. If the heating temperature is too low, it will lead to uneven heating of the slab and an overly low final rolling temperature of finishing rolling; if the heating temperature is too high, it will cause severe surface oxidation of the slab, ultimately resulting in abnormal surface microstructure of the slab after annealing.

[0039] The final rolling temperature of finishing rolling is controlled at 870-970°C. On one hand, the purpose is to prevent the transformation of austenite to ferrite in the steel strip during finishing rolling, and on the other hand, higher final rolling temperature of finishing rolling is conductive to the control of the cross-sectional dimensions and shape of the steel strip. If the final rolling temperature is too low, ferrite transformation may occur during or after finishing rolling, which is unfavorable for microstructure control during subsequent annealing; if the final rolling temperature is too high, more cooling water will be required to cool the steel strip after rolling, and introducing excessive cooling water is detrimental to uniformly controlling the hot-rolled microstructure during coiling.

[0040] Further, in the hot rolling step of the manufacturing method of the present disclosure, the thickness difference between the central point and a point within 40 cm away from the edge of the steel strip in the width direction of the cross section of the steel strip after finishing rolling is controlled at ≤50 µm.

[0041] In this embodiment, after finishing rolling, the thickness difference between the center point and a point within 40 cm away from the edge of the steel strip in the width direction of the cross section of the steel strip is ≤50 µm. This is mainly to work in conjunction with subsequent cold rolling steps to control the cross-sectional shape and good plate profile of the steel strip during the final continuous annealing, thereby reducing the non-uniformity in microstructure and properties of the final coil caused by fluctuations in the cross-sectional shape and plate profile of the steel strip during the final continuous annealing.

[0042] Furthermore, in the manufacturing method of the present disclosure, throughout the hot rolling, cooling after rolling and coiling steps, the temperature difference between the non-middle regions and the middle region of the steel strip in the width direction at the same moment is controlled to be ≤30°C, and the temperature fluctuation between the middle region and the non-middle regions in the length direction is controlled to be ≤15°C.

[0043] The process in this embodiment is designed to ensure the uniformity in microstructure and properties of the steel strip in both width and length directions by controlling temperature fluctuations.

[0044] In some embodiments, in the cooling after rolling step described herein, the cooling rate for rapid cooling is 50-200°C / s; the cooling rate for slow cooling is 5-30°C / s, preferably 5-25°C / s.

[0045] In some embodiments, in the cooling after rolling step described herein, rapid cooling is performed, followed by slow cooling, with multi-stage cooling carried out in an alternating manner of rapid and slow cooling.

[0046] In some embodiments, the cooling end temperature for the first rapid cooling is 680-760°C. In some embodiments, the cooling end temperature for the final slow cooling is 490-570°C. In some embodiments, except for the first rapid cooling, the temperature of the steel strip decreases by 20-160°C during each cooling, preferably by 30-80°C.

[0047] Further, in the cooling after rolling step of the manufacturing method of the present disclosure, the multi-stage cooling carried out in an alternating manner of rapid and slow cooling includes: cooling the steel strip to 680-760°C at a cooling rate of 100-200°C / s in a first stage; cooling the steel strip to 600-670°C at a cooling rate of 5-30°C / s, preferably 5-25°C / s in a second stage; cooling the steel strip to 520-590°C at a cooling rate of 50-150°C / s in a third stage; and cooling the steel strip to 490-570°C at a cooling rate of 5-30°C / s, preferably 5-25°C / s in a fourth stage.

[0048] The selection of the temperatures of each stage of the cooling step as mentioned above is designed within the ferrite transformation region of the steel strip, so that a small amount of ferrite and pearlite preferentially form at this stage to ensure the uniformity in microstructure and properties of the hot-rolled coil, thereby significantly reducing the possibility of martensite formation during subsequent low-temperature coiling.

[0049] Further, in the cold rolling step of the manufacturing method described in the present disclosure, the cold rolling reduction ratio is controlled to be ≥30%, such as 30-75%, and the target thickness of the steel strip is controlled, so that the thickness difference between the central point and the point at any position within 40 cm away from the edge in the width direction of the cross-section of the steel strip is ≤30 µm.

[0050] In this embodiment, the setting of process parameters is intended to control the cross-sectional shape and good plate profile of the cold-rolled steel strip, in order to reduce the non-uniformity in microstructure and properties of the final coil caused by fluctuations in the cross-sectional shape and plate profile of the steel strip during the final continuous annealing.

[0051] Further, in the annealing step of the manufacturing method according to the present disclosure, during heating and holding, when the heating temperature is ≥870°C, the holding time is ≤2 min; when the heating temperature is <870°C, the holding time is >2 min.

[0052] Further, in the manufacturing method of the present disclosure, throughout the entire annealing process, the temperature difference between the non-middle regions and the middle region in the width direction of the steel strip at the same moment is controlled to be ≤10°C, and the temperature fluctuation between the middle regions and the non-middle regions in the length direction of the steel strip is controlled to be ≤5°C.

[0053] The process in this embodiment is also designed to ensure uniformity in microstructure and properties of the steel strip in both width and length directions after coiling by controlling temperature fluctuations.

[0054] Further, in the annealing step of the manufacturing method of the present disclosure, the steel is heated at a heating rate of ≤50°C / s, such as 10-50°C / s, to 840-900°C and held for 1-4 minutes; then cooled at a cooling rate of 2-20°C / s to 700-780°C and held for 10-40 seconds; then cooled at a cooling rate of 20-50°C / s to 360-430°C and held for 2.5-10 seconds.

[0055] The ultra-high strength steel strip and its manufacturing method of the present disclosure has the following advantages and beneficial effects: Ensuring that the chemical composition and process remain relatively simple and controllable, the present disclosure provides an ultra-high strength steel with low anisotropy and high uniformity in the same coil through innovation of composition design, microstructure regulation and manufacturing methods.

[0056] In some embodiments, the steel strip has a tensile strength of ≥1000MPa, a yield strength of ≥780MPa; a low anisotropy that satisfies: the difference in yield strength is ≤25 MPa, and the difference in tensile strength is ≤20 MPa in the transverse and longitudinal directions at the same position of the steel strip; and a mechanical uniformity in the same coil that satisfies: for the same steel coil, along its length or width direction, the difference in yield strength is ≤25 MPa, and the difference in tensile strength is ≤20 MPa at different positions in the same stretching direction.

[0057] In some embodiments, the ultra-high-strength steel strip of the present disclosure not only possesses the above-mentioned properties but also has good formability, with an elongation at break of ≥10% and a hole expansion ratio of ≥50%.

[0058] The ultra-high strength steel strip according to the present disclosure can be used for automotive parts that have stringent requirements for part dimensional accuracy and service stability, such as automotive structural parts like the front seat rails in vehicle cabin systems. It represents a new design concept of "high, precise, and advanced", emphasizing fine, stabilized, and differentiated products, and has good prospects for promotion and application value.Description of the Drawings

[0059] Fig. 1 is a microstructure photograph of the ultra-high strength steel strip of Example 1 at a magnification of 3000.Detailed Description

[0060] The ultra-high strength steel strip and the manufacturing method therefor will be further interpreted and explained below in combination with specific embodiments, but the interpretation and explanation do not constitute an undue limitation to the technical solution of the present disclosure.Example 1-7 and Comparative Example 1-2

[0061] The ultra-high strength steel strips of Examples 1-7 and the comparative steel strips of Comparative Examples 1-2 were prepared by the following steps: (1) smelting and casting were performed according to the chemical compositions shown in Table 1. (2) hot-rolling: the heating temperature was controlled at 1180-1280°C, and the final rolling temperature of finishing rolling was controlled at 870-970°C. In some embodiments, it is preferable to control the thickness difference between the central point and the point within 40 cm away from the edge in the width direction of the cross-section of the steel strip after finishing rolling to be ≤50 µm. (3) cooling after rolling and coiling: Cooling after rolling was performed in multiple stages using alternating fast and slow cooling. Finally, for the areas beyond 100 meters away from the head and tail of the steel strip, the main body of the steel strip is cooled to the main body coiling temperature of 440°C-520°C at a cooling rate of 5-100°C for coiling; for the areas within 100 meters away from the head and tail of the steel strip, the head and tail of the steel strip is cooled to the head and tail coiling temperature of 480°C-560°C at a cooling rate of 5-30°C for coiling.

[0062] In some embodiments, the multi-stage cooling carried out in an alternating manner of rapid and slow cooling specifically includes: cooling the steel strip to 680-760°C at a cooling rate of 100-200°C / s in a first stage; cooling the steel strip to 600-670°C at a cooling rate of 5-30°C / s in a second stage; cooling the steel strip to 520-590°C at a cooling rate of 50-150°C / s in a third stage; and cooling the steel strip to 490-570°C at a cooling rate of 5-30°C / s in a fourth stage, ensuring that the steel strip temperature at the end of the fourth stage is lower than that at the end of the third stage. Subsequently, the steel strip is further cooled to the coiling temperature, which constitutes the fifth stage of cooling, ensuring that the coiling temperature at any position of the steel strip is lower than the steel strip temperature at the corresponding position at the end of the fourth stage.

[0063] (4) pickling and cold rolling: the cold rolling reduction was controlled at ≥30%.

[0064] In some embodiments, the target thickness of the steel strip is controlled so that the thickness difference between the center point and any position within 40 cm away from the edge is ≤ 30 µm in the width direction of the cross-section of the steel strip.

[0065] In some preferred embodiments, throughout the hot rolling step, cooling after rolling and coiling steps, the temperature difference between non-middle regions and the middle region in the width direction of the steel strip at the same moment is controlled to be ≤ 30°C, and the temperature fluctuation between the middle region and non-middle regions in the length direction is ≤ 15°C.

[0066] In some more specific embodiments, throughout the hot rolling, cooling after rolling and coiling steps, 7-point equidistant temperature measurement and control can be used in the width direction of the steel strip. All temperatures in the hot rolling step, as well as in the cooling after rolling and coiling steps are the temperature at the middlemost region of the 7-point equidistant temperature measurement and control. The temperature difference between the middle region and the other 6 regions in the width direction can be controlled at ≤30°C. All temperatures in the hot rolling step, as well as in the cooling after rolling and coiling steps, refer to the average temperature of the middlemost region of the steel strip within the required length range, with a temperature fluctuation within the length range of ≤15°C. Unless a specific length range is indicated, it refers to the entire length of the steel strip.

[0067] (5) Annealing: the steel was heated at a rate of ≤50°C / s to 840-900°C and held for 1-4 minutes; then cooled at a cooling rate of 2-20°C / s to 700-780°C and held for 10-40 seconds; then cooled at a cooling rate of 20-50°C / s to 360-430°C and held for 2.5-10 seconds; then reheated at a heating rate of 5-30°C / s to 440-480°C; then cooled at a cooling rate of 0.1-0.5°C / s to 330-400°C; finally, cooled at a cooling rate of 15-50°C / s to room temperature for coiling; wherein, when the heating temperature is ≥870°C, the holding time is ≤2 minutes; when the heating temperature is <870°C, the holding time is >2 minutes.

[0068] In some embodiments, throughout the entire annealing step, the temperature difference between the non-middle regions and the middle region in the width direction of the steel strip at the same moment is controlled to be ≤10°C, and the temperature fluctuation between the middle region and the non-middle regions in the length direction of the steel strip is controlled to be ≤5°C.

[0069] In some more specific embodiments, during the entire annealing step, a 7-point equidistant temperature measurement and control can be used in the width direction of the steel strip. All temperatures in the annealing step are the temperature of the middlemost region of the 7-point equidistant temperature measurement and control, and the temperature difference between the middle region and the other 6 regions in the width direction can be controlled at ≤10°C. All temperatures in the annealing step refer to the average temperature of the middlemost region of the steel strip within the required length range, with temperature fluctuations within this length range of ≤5°C.If no specific length range is indicated, it refers to the entire length of the steel strip.

[0070] It should be noted that the chemical composition design and related process parameters of the ultra-high strength steel strips of Examples 1-7 all met the requirements of the design specification of the present disclosure. Correspondingly, there are process parameters that did not meet the requirements of design specification of the present disclosure in the chemical composition design and related process of the comparative steel strips of Comparative Examples 1-2.

[0071] Table 1 lists the mass percentages of various chemical elements in the ultra-high strength steel strips of Examples 1-7 and the comparative steels of Comparative Examples 1-2. Table 1.( wt%, a balance of Fe and other unavoidable impurities except P, S, N)No.CSiMnBAlCrMoPSNTiNbv(Cr+Mo) / CEx. 10.160.351.70.00250.020.30.20.0080.0020.0040.0010.0020.0043.13Ex. 20.140.52.00.0040.040.20.250.0120.0030.00380.0020.0010.0063.21Ex. 30.150.41.80.0030.0250.350.10.0110.0010.00350.0040.0030.0033.00Ex. 40.190.151.40.0010.010.40.150.0060.0040.00250.0030.0040.0012.89Ex. 50.200.251.60.0020.030.250.350.0090.0010.00350.0010.0020.0053.00Ex. 60.130.451.90.00350.0350.10.30.010.0010.00150.0060.0050.0033.08Ex. 70.180.21.50.00150.0150.150.40.0050.0010.00180.0050.0060.0023.06CEx.10.210.251.60.00150.020.150.150.0080.0020.0040.0010.0020.0011.43CEx.20.150.41.80.0030.0250.350.10.0110.0010.00350.0040.0030.0033.00

[0072] Table 2-1, Table 2-2, Table 2-3 and Table 2-4 list the specific process parameters in the above process steps of the ultra-high strength steel strips of Examples 1-7 and the comparative steel strips of Comparative Examples 1-4. Table 2-1.No.Heating temperature (°C)Final rolling temperature (°C)Maximum thickness difference between the center point and any point within 40 cm away from the edge of the finishing rolled plate (µm)Cooling rate in the first stage after rolling (°C / s)Temperature in the first stage after rolling (°C)Cooling rate in the second stage after rolling (°C / s)Temperature in the second stage after rolling (°C)Cooling rate in the third stage after rolling (°C / s)Temperature in the third stage after rolling (°C)Cooling rate in the fourth stage after rolling (°C / s)Temperature in the fourth stage after rolling (°C)Ex. 112209304020071015620605808570Ex. 2128096030180760206705053020500Ex.3120089045190700116507059030540Ex. 4126095025140740306001405205510Ex. 5124091035160680864010054010520Ex. 6118087050120690566015055025490Ex. 71270970201007502561012057015550CEx. 111958904512071076707057010555CEx. 2118087050220690506601805305490 Table 2-2. No.Cooling rate in the fifth stage after rolling: within 100 meters away from the head (°C / s)Cooling rate in the fifth stage after rolling: within 100 meters away from the tail (°C / s)Cooling rate in the fifth stage after rolling: other regions (°C / s)Coiling temperature: within 100 meters from the head (°C / s)Coiling temperature: within 100 meters away from the tail (°C / s)Coiling temperature: other regions (°C / s)Maximum temperature difference of the regions in the width direction of the steel strip in each stage (°C)Maximum temperature difference in the length direction of the steel strip (°C)Ex. 15525560560520176Ex. 22020804904904602712Ex. 31510505255304902914Ex. 425251005005004703015Ex. 58864510510440208Ex. 65554804804802510Ex. 7101040540540510155CEx. 11212365455455252013CEx. 235357404804804505024 Table 2-3 No.Cold rolling reduction (%)Maximum thickness difference between the center point and any point within 40 cm away from the edge of the cold rolled plate (µm)Heating rate of annealing (°C / s)Holding temperature (°C) and holding time (min) of annealingSlow cooling rate of annealing (°C / s)Slow cooling temperature (°C) and holding time (s) of annealingRapid cooling rate of annealing (°C / s)Rapid cooling temperature (°C) and holding time (s) of annealingEx. 1302441840×42.5740×4024380×10Ex. 2751826900×1.513.3700×1548430×3.75Ex. 3602728890×28.5720×2044390×5Ex. 4701520870×28710×2040410×5Ex. 5352123860×3.52.9760×3525.9420×8.75Ex. 6403011865×33.2770×3036.4360×7.5Ex. 7501215850×2.52.8780×2543.7370×6.25CEx. 1402741840×43.5700×4018.0430×10CEx. 2703026900×113770×10709.3360×2.5 Table 2-4 No.Re-heating rate of annealing (°C / s)Re-heating temperature of annealing (°C)Slow cooling rate after reheating of annealing (°C / s)Slow cooling temperature after reheating of annealing (°C)Final cooling rate of annealing (°C / s)Maximum temperature difference of the regions in the width direction of the steel strip in each stage during annealing (°C)Maximum temperature difference in the length direction of the steel strip during annealing (°C)Ex. 1134400.14001952Ex. 25.34400.438048105Ex. 328.04600.353903794Ex. 424.04700.53703584Ex. 513.74800.434018.2953Ex. 625.34550.3535022.0073Ex. 725.64500.4833024.8062CEx. 164600.2536017.0085CEx. 2764551.05350662012

[0073] The ultra-high strength steel strips obtained in Examples 1-7 were sampled, observed and tested for the microstructure, and the results of the microstructure observations are listed in Table 3 below.

[0074] The microstructure testing methods were as follows: Preparation of metallographic samples: 4% nitric acid in alcohol was used to pre-etch the polished metallographic samples for about 10 seconds; further, based on the above, the color metallographic sample preparation method was used: a solution of 1g sodium metabisulfite in 100ml water and a solution of 4g picric acid in 100ml ethanol were mixed in equal proportions and kept still, then used for etching for 30-40 seconds.

[0075] Observation: An optical microscope or scanning electron microscope was used to observe the microscopic structure and take photos to analyze the microstructure. Table 3.No.Uniformly dispersed granular bainite with in a coral sea-like pattern (%)Volume fraction of ferrite (%)Types of other microstructuresEx.197.52Retained austenite, acicular bainite, tempered martensiteEx.295.14.5Retained austenite, upper bainiteEx.396.04.0NoEx.496.73.0Upper bainite, acicular bainiteEx.598.81.0Upper bainite, acicular bainiteEx.696.53.4Tempered martensite, martensiteEx.799.10.5Acicular bainite, tempered martensite, titanium carbonitride, niobium carbonitride

[0076] In addition, based on observations of each Example, the area of granular bainite in the ultra-high-strength steel strips of all Examples is ≤5 µm 2< , with an aspect ratio of ≤2:1.

[0077] For all ultra-high-strength steel strips of Examples 1-7, the granular bainite is distributed in any cross-sectional area of ≤50 µm 2< within all regions that are no less than 30 µm away from both the top and bottom surfaces of the steel strip in the thickness direction.

[0078] For all ultra-high-strength steel strips of Examples 1-7, the granular bainite is distributed in any cross-sectional area of ≤10 µm 2< within all regions that are no less than 20 µm away from both the top and bottom surfaces of the steel strip in the thickness direction.

[0079] In addition, Fig. 1 also shows the microstructure of the ultra-high strength steel strip of Example 1 at a magnification of 3000. As it can be seen from Fig. 1, the ultra-high strength steel strip has uniformly dispersed granular bainite with a coral sea-like pattern.

[0080] In addition, the ultra-high strength steel strips of Examples 1-7 and the comparative steel strips of Comparative Examples 1-2 were sampled respectively, and tested for their mechanical properties, in order to evaluate their anisotropy and uniformity in the same coil. The anisotropy and uniformity in the same coil of each example and comparative example are listed in Tables 4-1 to 4-9. The relevant methods for testing mechanical properties are as follows: Tensile performance test was conducted in accordance with GB / T 228.1-2021 "Metallic materials-Tensile testing-Part 1: Method of test at room temperature".

[0081] The hole expansion ratio of the steel is determined by a hole expansion test. By using a punch to press a specimen with a central hole into a die, the central hole of the specimen is enlarged until necking or through cracks appear at the edge of the hole in the plate. Because the preparation method of the original central hole of the specimen and the corresponding edge quality of the original hole significantly affect the results of the hole expansion test, the test and measurement methods are carried out in accordance with the hole expansion test method specified in ISO / DIS 16630. The original central hole is made by stamping (corresponding to the processing method which results in the worst quality at the edge of the original hole). Table 4-1. Example 1Ex. 1Width positionYield strengthTensile strengthElongation (%)Strength difference in transverse and longitudinal directionHole expansion ratio (%)Length positionlongitudinal-transverselongitudinaltransverselongitudinaltransverseyieldtensileheadedge8808871041104811117765center8698781034104011.511.59665edge8818871044104911116564within 100 meters away from the headedge8848921042104711118562center8758831046105011.5118465edge8858931048105211118460middle of the lengthedge8768761040104111.511.50161center8738761041104311.511.53262edge8788801043104511.5112261within 100 meters away from the tailedge88289410461055111112958center8788931041105011.51115963edge8738851042104811.51112666tailedge8888951049105511117657center8738811040104511.5118566edge8768841041104911.5118864strength difference19191515----- Table 4-2. Example 2 Ex. 2Width positionYield strengthTensile strengthElongation (%)Strength difference in transverse and longitudinal directionHole expansion ratio (%)Length positionlongitudinaltransverselongitudinaltransverselongitudinaltransverseyieldtensileheadedge830836102310341211.561155center8208281015102612.51281152edge833837101810301211.541257within 100 meters away from the headedge836842102210331211.561159center8238331028103712.51210955edge835843103010411211.581160middle of the lengthedge826825102210301211.5-1856center821825101910261211.54754edge82883010171032121221555within 100 meters away from the tailedge832843102410381211.5111459center828843102010391211.5151953edge8238351017103412.512121752tailedge838841102510391211.531461center8268311017103112.51251454edge82783410231036121271354strength difference18181515----- Table 4-3. Example 3 Ex. 3Width positionYield strengthTensile strengthElongation (%)Strength difference in transverse and longitudinal directionHole expansion ratio (%)Length positionlongitudinaltransverselongitudinallongitudinallongitudinaltransverseyieldtensileheadedge84685710051007121211268center8428531000100212.51211268edge852860100110031211.58267within 100 meters away from the headedge853865100510081211.512360center8448561006101012.51212461edge852865100910121211.513369middle of the lengthedge8458531000100412.5128473center8408511001100312.51211272edge8448531003100512129271within 100 meters away from the tailedge851862101010121211.511270center845859100510101211.514572edge8398531002100512.51214369tailedge853864100910121211.511372center8438551004100512.51212170edge846860100310121211.514969strength difference14141010----- Table 4-4. Example 4 Ex.4Width positionYield strengthTensile strengthElongation (%)Strength difference in transverse and longitudinal directionHole expansion ratio (%)Length positionlongitudinaltransverselongitudinaltransverselongitudinaltransverseyieldtensileheadedge875883102710321110.58565center8688771020102511119566edge8788841021102811116765100 meters away from the headedge8808891034103811119468center8698801027103011.51111368edge88089010331039111110667middle of the lengthedge8718741025102911113470center8688721022102611114469edge8738771024103011114669100 meters away from the tailedge877889102510361110.5121166center871886102310371111151469edge868882102010321111141267tailedge87388810291037111115868center8648781022103011.51114868edge872885102810391110.5131166strength difference16181414----- Table 4-5. Example 5 Ex. 5Width positionYield strengthTensile strengthElongation (%)Strength difference in transverse and longitudinal directionHole expansion ratio (%)Length positionlongitudinaltransverselongitudinaltransverselongitudinaltransverseyieldtensileheadedge895906104810571110.511968center8879001042105011.51113870edge899911105110591110.512867within 100 meters away from the headedge901909105010541110.58462center892900105110551110.58466edge893901105710631110.58663middle of the lengthedge891895104610501110.54470center8908951044104711.5115369edge892898104510501110.56568within 100 meters away from the tailedge903911104710541110.58765center900910104610551110.510969edge8959051042105011.51110870tailedge900911105410621110.511864center888900104610551110.512971edge889901104610541110.512870strength difference16161516----- Table 4-6. Example 6 Ex. 6Width positionYield strengthTensile strengthElongation (%)Strength difference in transverse and longitudinal directionHole expansion ratio (%)Length positionlongitudinaltransverselongitudinaltransverselongitudinaltransverseyieldtensileheadedge8608711043105311.511111058center8518641036104711.511131160edge8638731038104811.511101057within 100 meters away from the headedge865878104610551110.513955center8558691048105711.51114960edge865878105110611110.5131056middle of the lengthedge8538631042105211.511101059center8508631039105011.511131162edge8558661040105111.511111157within 100 meters away from the tailedge862876104610581110.5141259center8568701040105311.511141358edge8528661038105011.511141258tailedge865878104510571110.5131254center8508651036105011.511151461edge8578721043105511.511151256strength difference15151514----- Table 4-7. Example 7 Ex.7Width positionYield strengthTensile strengthElongation (%)Strength difference in transverse and longitudinal directionHole expansion ratio (%)Length positionlongitudinaltransverselongitudinaltransverselongitudinaltransverseyieldtensileheadedge9259341069107610.5109762center916925107110781110.59766edge9269351076108310.5109763within 100 meters away from the headedge926935106710741110.59765center9259341065107210.5109769edge919928106110681110.59770middle of the lengthedge916922106510731110.56870center915921106210681110.56669edge918926106410701110.58668within 100 meters away from the tailedge9209281071107610.5108568center914923106210691110.59770edge9239321064107110.5109767tailedge9259351069108010.510101164center911923106210731110.5121171edge914926106610771110.5121170strength difference15141515----- Table 4-8. Comparative Example 1 CEx.1Width positionYield strengthTensile strengthElongation (%)Strength difference in transverse and longitudinal directionHole expansion ratio (%)Length positionlongitudinaltransverselongitudinaltransverselongitudinaltransverseyieldtensileheadedge79382099810201312272225center775801978100113.512262329edge799827100110241311.5282321within 100 meters away from the headedge79982699910201311.5272123center778803983100513.512252230edge801827100410261311262224middle of the lengthedge78180899710221312272524center769792978100113.512.5232332edge78881499210171312262523within 100 meters away from the tailedge80082699510161311.5262122center783810100110251312272428edge778805983100613.512272327tailedge800827100110241311.5272320center778804979100313.512262427edge79381599810211312222326strength difference32352625 Table 4-9. Comparative Example 2 CEx.2Width positionYield strengthTensile strengthElongation (%)Strength difference in transverse and longitudinal directionHole expansion ratio (%)Length positionlongitudinaltransverselongitudinaltransverselongitudinaltransverseyieldtensileheadedge874888103310421110.514957center8508631010102211.511131270edge881895103610481110141250within 100 meters away from the headedge879892102910411110131253center848861100510181211.5131373edge8748881022103711.510.5141554middle of the lengthedge883897103310431110.5141054center8548661012102011.51112872edge879891103310421110.512954within 100 meters away from the tailedge878892102910391110.5141057center8528651014102411.511131069edge884895103610471110111152tailedge876889103610471110131150center852866101010221211141268edge883898103510451110151051strength difference36373130

[0082] Note: The 'strength difference' in Tables 4-1 to 4-9 refers to the maximum-minimum difference of all testing points under longitudinal or transverse stretching along the entire length direction and entire width direction of the steel strip.

[0083] From the above Tables 4-1 to 4-9, it can be seen that all examples of the present disclosure satisfy that: the tensile strength is ≥1000 MPa, the yield strength is ≥780 MPa, the elongation at break is ≥10%, and the hole expansion rate is ≥50%; at the same time, the anisotropy meets: at the same position on the steel strip, the difference in yield strength in the transverse and longitudinal directions is ≤25 MPa, and the difference in tensile strength in the transverse and longitudinal directions is ≤20 MPa; and the mechanical uniformity in the same coil meets: for the same steel coil, along its length or width direction, the difference in yield strength at different positions in the same stretching direction is ≤25 MPa, and the difference in tensile strength at different positions in the same stretching direction is ≤20 MPa.

[0084] For Comparative Example 1, not only does the carbon content exceed the upper limit as designed, but it also fails to meet the requirement of 2.7≤(Cr Mo) / C≤3.3, as (Cr Mo) / C is only 1.43. In this case, the composition imposes very high demands on the cooling rate. In actual manufacturing processes, the cooling rate in the fast-cooling section of annealing does not meet the requirements, ultimately leading to abnormal product performance. Not only are yield strength and tensile strength too low, and the hole expansion ratio insufficient, but anisotropy is also relatively high, and uniformity in the same coil is low.

[0085] Although the composition design of Comparative Example 2 meets the requirements of the present disclosure, the cooling rate during the manufacturing process is too high and does not meet the requirements of the present disclosure. Although the mechanical properties of the product meet the requirements, the uniformity in the same coil is poor, and the strength difference between different positions is too significant.

[0086] It should be noted that the combination of the technical features in the present disclosure is not limited to the combination described in the claims or the specific embodiments, and all the technical features recorded herein may be freely combined or combined in any way, unless there is a contradiction between them.

[0087] It should also be noted that the examples listed above are only specific embodiments of the present disclosure. Obviously, the present disclosure is not limited to the above embodiments, and similar changes can be made thereby. Any similar changes or modifications directly derived or easily envisaged by those skilled in the art from the contents disclosed in the present disclosure shall fall within the protection scope of the present disclosure.

Claims

1. An ultra-high strength steel strip, which comprises Fe and unavoidable impurity elements, and further comprises the following chemical elements in mass percentages as follows: C: 0.13-0.20%; Si: 0.15-0.50%; Mn: 1.4-2.0%; B: 0.001-0.004%; Al: 0.01-0.04%; Cr: 0.1-0.4%; Mo: 0.1-0.4%; Ti+V+Nb ≤0.02%; wherein its microstructure includes uniformly dispersed granular bainite in a coral sea-like pattern.

2. The ultra-high strength steel strip according to claim 1, wherein the chemical elements in mass percentages are as follows: C: 0.13-0.20%; Si: 0.15-0.50%; Mn: 1.4-2.0%; B: 0.001-0.004%; Al: 0.01-0.04%; Cr: 0.1-0.4%; Mo: 0.1-0.4%; Ti+V+Nb ≤0.02%; with a balance of Fe and unavoidable impurity elements.

3. The ultra-high strength steel strip according to claim 1 or 2, wherein the chemical elements in mass percentages satisfy at least one of the following items: C: 0.14-0.18%; Ti+V+Nb ≤0.01%.

4. The ultra-high strength steel strip according to claim 1 or 2, wherein the mass percentages of Cr, Mo and C also satisfy: 2.7≤(Cr+Mo) / C≤3.3.

5. The ultra-high strength steel strip according to claim 1 or 2, wherein the content of the inevitable impurity elements in mass percentage satisfies at least one of the following items: P≤0.012%, S≤0.004%, N≤0.004%.

6. The ultra-high strength steel strip according to claim 1 or 2, wherein the volume phase fraction of the granular bainite is ≥95.0%; perferably, the granular bainite has an area of ≤5µm2, an aspect ratio of ≤2:1; preferably, the microstructure further comprises ferrite with a volume phase fraction of 0.1-4.5%.

7. The ultra-high strength steel strip according to claim 1, wherein the granular bainite is distributed in any cross-sectional area of ≤502 µm within the entire region that is no less than 30 µm away from both the top and bottom surfaces of the steel strip in the thickness direction; preferably, the granular bainite is distributed in any cross-sectional area of ≤ 102 µm within the entire region that is no less than 20 µm away from both the top and bottom surfaces of the steel strip in the thickness direction.

8. The ultra-high strength steel strip according to claim 1 or 2, wherein the steel strip has a tensile strength of ≥1000MPa, a yield strength of ≥780MPa, preferably ≥800MPa, more preferably ≥850MPa, further preferably ≥880MPa; a low anisotropy that satisfies: the difference in yield strength is ≤25 MPa, and the difference in tensile strength is ≤20 MPa in the transverse and longitudinal directions at the same position of the steel strip; and a mechanical uniformity in the same coil that satisfies: for the same steel coil, along its length or width direction, the difference in yield strength is ≤25 MPa, and the difference in tensile strength is ≤20 MPa at different positions in the same stretching direction; preferably, it has an elongation at break of ≥10%, and / or a hole expansion ratio of ≥50%.

9. A manufacturing method for the ultra-high strength steel strip according to any one of claims 1-8, comprising steps of: smelting and casting; hot-rolling; cooling after rolling and coiling: cooling after rolling is carried out in multiple stages using alternating fast and slow cooling; finally, for the areas beyond 100 meters away from the head and tail of the steel strip, the main body of the steel strip is cooled to the main body coiling temperature of 440°C-520°C at a cooling rate of 5-100°C for coiling; for the areas within 100 meters away from the head and tail of the steel strip, the head and tail of the steel strip is cooled to the head and tail coiling temperature of 480°C-560°C at a cooling rate of 5-25°C for coiling; pickling and cold rolling; annealing: the steel is heated at a heating rate of ≤50°C / s to 840-900°C and held; then cooled at a cooling rate of 2-20°C / s to 700-780°C and held; then cooled at a cooling rate of 20-50°C / s to 360-430°C and held; then reheated at a heating rate of 5-30°C / s to 440-480°C; then cooled at a cooling rate of 0.1-0.5°C / s to 330-400°C; and finally cooled at a cooling rate of 15-50°C / s to room temperature and coiled.

10. The manufacturing method according to claim 9, wherein, in the hot rolling step, the heating temperature is controlled at 1180-1280°C, and the final rolling temperature of finishing rolling is controlled at 870-970°C; and / or the thickness difference between the central point and a point within 40 cm away from the edge of the steel strip in the width direction of the cross section of the steel strip after finishing rolling is controlled at ≤50 µm.

11. The manufacturing method according to claim 9, wherein, throughout the hot rolling, cooling after rolling and coiling steps, the temperature difference between the non-middle regions and the middle region of the steel strip in the width direction at the same moment is controlled to be ≤30°C, and the temperature fluctuation between the middle region and the non-middle regions in the length direction is controlled to be ≤15°C; and / or, throughout the entire annealing process, the temperature difference between the non-middle regions and the middle region in the width direction of the steel strip at the same moment is controlled to be ≤10°C, and the temperature fluctuation between the middle region and the non-middle regions in the length direction of the steel strip is controlled to be ≤5°C.

12. The manufacturing method according to claim 9, wherein, in the cooling after rolling step, the cooling rate for rapid cooling is 50-200°C / s; the cooling rate for slow cooling is 5-30°C / s, preferably 5-25°C / s; preferably, the cooling end temperature for the first rapid cooling is 680-760°C, the cooling end temperature for the final slow cooling is 490-570°C; preferably, except for the first rapid cooling, the temperature of the steel strip decreases by 20-160°C during each cooling, preferably by 30-80°C; more preferably, the multi-stage cooling carried out in an alternating manner of rapid and slow cooling specifically includes: cooling the steel strip to 680-760°C at a cooling rate of 100-200°C / s in a first stage; cooling the steel strip to 600-670°C at a cooling rate of 5-30°C / s, preferably 5-25°C / s in a second stage; cooling the steel strip to 520-590°C at a cooling rate of 50-150°C / s in a third stage; and cooling the steel strip to 490-570°C at a cooling rate of 5-30°C / s, preferably 5-25°C / s in a fourth stage.

13. The manufacturing method according to claim 9, wherein, in the cold rolling step, the cold rolling reduction ratio is controlled to be ≥30%, and the target thickness of the steel strip is controlled, so that the thickness difference between the central point and the point at any position within 40 cm away from the edge in the width direction of the cross-section of the steel strip is ≤30 µm.

14. The manufacturing method according to claim 9, wherein, in the annealing step, when the heating temperature is ≥870°C, the holding time is ≤2 min; when the heating temperature is <870°C, the holding time is >2 min.

15. The manufacturing method according to claim 9, wherein, in the annealing step, the steel is heated at a heating rate of ≤50°C / s to 840-900°C and held for 1-4 minutes; then cooled at a cooling rate of 2-20°C / s to 700-780°C and held for 10-40 seconds; then cooled at a cooling rate of 20-50°C / s to 360-430°C and held for 2.5-10 seconds.

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