Low-cost cold-rolled high-strength steel having low carbon equivalent, and manufacturing method therefor

Abstract

Disclosed in the present invention is low-cost cold-rolled high-strength steel having low carbon equivalent. The steel comprises Fe and inevitable impurities, and further comprises the following chemical elements in percentage by mass: C: 0.04-0.08%; 0<Si≤0.2%; Mn: 0.65-1.15%; Al: 0.02-0.05%; and Ti: 0.10-0.16%. The microstructure thereof comprises ferrite, granular bainite, and precipitated carbonitrides. Further disclosed in the present invention is a method for manufacturing cold-rolled high-strength steel. The method comprises the steps of: smelting and casting; hot rolling; acid pickling and cold rolling, wherein the cold-rolling reduction rate is controlled to be 45-90%; and annealing, involving: heating strip steel to 770-820°C and maintaining the temperature, then cooling same to 450-550°C at a cooling rate of 10-40°C / s and maintaining the temperature, and then cooling same to room temperature at a cooling rate of 1-5°C / s.

Description

Low-cost low-carbon-equivalent cold-rolled high-strength steel and manufacturing method thereof TECHNICAL FIELD

[0001] The present application relates to a steel sheet and a manufacturing method thereof, in particular to a cold-rolled 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 high-strength steel, and the required yield strength gradually reaches or exceeds 700 MPa.

[0003] The existing cold-rolled high-strength steel with a yield strength of 700 MPa is mostly designed as dual-phase steel or multi-phase steel, which obtains higher strength by introducing martensite into the microstructure of the steel. However, the alloy addition amount and carbon equivalent of this method are high, which not only increases the manufacturing cost but also deteriorates the weldability of the steel. In addition, a small amount of designs are to improve the strength of the steel by adding low alloy / micro alloy, such as low alloy high strength steel or precipitation strengthened steel, but the yield strength of such steel is generally not more than 500 MPa.

[0004] In the prior art, if the yield strength is further improved to 700 MPa, there are two ways: one is still to increase the alloying elements in the low alloy steel, and the other is to use semi-annealing or incomplete annealing to improve the yield strength.

[0005] For example, the Chinese patent document with the publication number CN105200332A and the publication date of December 30, 2015, and the name of "700 MPa grade thin gauge high strength steel strip and its production method" discloses a 700 MPa grade thin gauge high strength steel strip and its production method, which adds at least 0.2% of Cr and up to 2.0% of Mn. It can be seen that this method still cannot avoid the problems of high alloy cost and high carbon equivalent.

[0006] For example, the Chinese patent document with the publication number CN109207843A and the publication date of January 15, 2019, and the name of "high-strength cold-rolled steel strip and manufacturing method thereof" discloses a high-strength cold-rolled steel strip, when the tensile strength of the patent fluctuates within a small range of 850±15 MPa, the fluctuation range of the yield strength is as high as 69 MPa. SUMMARY

[0007] One of the purposes of the present application is to provide a low-cost low-carbon-equivalent cold-rolled high-strength steel, which realizes low alloy cost while keeping low carbon equivalent and has good mechanical properties by lower alloy addition.

[0008] In order to achieve the above-mentioned purpose, the present application provides a low-cost low-carbon equivalent cold-rolled high-strength steel containing Fe and inevitable impurities, which further contains the following chemical elements with mass percentage as follows:

[0009] C: 0.04-0.08%;

[0010] 0

[0011] Mn: 0.65-1.15%;

[0012] Al: 0.02-0.05%;

[0013] Ti: 0.10-0.16%;

[0014] The microstructure thereof contains ferrite + granular bainite + precipitated carbonitride.

[0015] Further, in the low-cost low-carbon equivalent cold-rolled high-strength steel according to the present application, the mass percentage of each chemical element is as follows:

[0016] C: 0.04-0.08%;

[0017] 0

[0018] Mn: 0.65-1.15%;

[0019] Al: 0.02-0.05%;

[0020] Ti: 0.10-0.16%;

[0021] the balance being Fe and inevitable impurities.

[0022] The present application obtains the microstructure of ferrite + granular bainite + precipitated carbonitride through the cooperation of components and process design, thereby obtaining good mechanical properties while maintaining low alloy cost and low carbon equivalent.

[0023] Specifically, in the low-alloy high-strength steel according to the present application, the design principles of each chemical element are as follows:

[0024] C: In the low-alloy high-strength steel according to the present application, the C element simultaneously affects phase transition and carbonitride precipitation. When the mass percentage content of C element is too low, effective phase transition strengthening cannot be formed, and the amount of carbonitride formation is also very limited; when the mass percentage content of C element is too high, although phase transition occurs more easily, it also causes a substantial increase in carbon equivalent and the size of carbonitride to be coarse, affecting the formability of weldability. Therefore, in the low-alloy high-strength steel according to the present application, the mass percentage content of C element can be controlled between 0.04-0.08%.

[0025] Si: In the low-alloy high-strength steel described in the present application, the Si element is a necessary element for steelmaking deoxidation and a solid solution strengthening element, and is also a ferrite forming and stabilizing element. However, when the mass percentage content of the Si element is too high, it will affect the formation of granular bainite, and at the same time, it will also worsen the weldability performance, and increase the alloy cost of steel manufacturing. Therefore, in the low-alloy high-strength steel described in the present application, the mass percentage content of the Si element can be controlled to be 0 < Si ≤ 0.2%. In some embodiments, in the low-alloy high-strength steel described in the present application, the mass percentage content of the Si element is controlled to be between 0.05 ~ 0.2%.

[0026] Mn: In the low-alloy high-strength steel described in the present application, the Mn element is one of the core elements that affect the strength and control the phase change. When the mass percentage content of the Mn element is too low, it will cause insufficient granular bainite transformation, resulting in insufficient strength; when the mass percentage content of the Mn element is too high, although it is beneficial to improve the strength of the steel, on the one hand, it will cause the carbon equivalent to rise, affecting the weldability, and on the other hand, it will also increase the manufacturing cost. Therefore, in the low-alloy high-strength steel described in the present application, the mass percentage content of the Mn element can be controlled to be between 0.65 ~ 1.15%.

[0027] Al: In the low-alloy high-strength steel described in the present application, the Al element can be used as a deoxidizing element and a carbonitride forming element to improve the quality of the steel and improve the strength of the steel. At the same time, the Al element is also a ferrite forming element and a stabilizing element. When the mass percentage content of the Al element is too low, the molten steel is not fully deoxidized, and the steel purity will be affected; when the mass percentage content of the Al element is too high, it is not conducive to the formation of granular bainite. Therefore, in the low-alloy high-strength steel described in the present application, the mass percentage content of the Al element can be controlled to be between 0.02 ~ 0.05%.

[0028] Ti: In the low-alloy high-strength steel described in the present application, the Ti element is a main carbonitride forming element, which is used to refine the grains and improve the strength. When the mass percentage content of the Ti element is too low, the formed carbonitride precipitates are not enough, and the required strength cannot be achieved; when the mass percentage content of the Ti element is too high, it is easy to form coarse TiN particles and coarse titanium carbonitride in the steel, which is not conducive to the forming performance of the steel. Therefore, in the low-alloy high-strength steel described in the present application, the mass percentage content of the Ti element can be controlled to be between 0.10 ~ 0.16%.

[0029] Further, in the low-cost low-carbon equivalent cold-rolled high-strength steel described in the present application, it also contains at least one of the following elements:

[0030] 0 < Nb ≤ 0.015 wt%;

[0031] 0 < Ce≤ 0.006 wt%;

[0032] 0 < La≤ 0.006 wt%;

[0033] 0 < V≤ 0.1 wt%;

[0034] 0 < Cr≤ 0.2 wt%.

[0035] In the present application, Nb, Ce, La, V and Cr can be selectively added to the steel as optional elements. Among them, La element and Ce element as rare earth elements have the effect of improving steel quality, modifying inclusions, and refining grains; Nb element and V element as strong carbonitride forming elements can form a large amount of carbonitride precipitates, refine grains, and improve strength; Cr element can adjust the phase transformation of steel, and in addition, can improve the hardenability, which is beneficial to the formation of granular austenite.

[0036] However, the addition of the above alloying elements will increase the manufacturing cost of the steel on the one hand, and will also increase the carbon equivalent, deteriorating the weldability of the steel. Therefore, in the low-alloy high-strength steel described in the present application, the mass percentage content of Nb element can be controlled to be Nb≤0.015wt%, the mass percentage content of Ce element can be controlled to be Ce≤0.006wt%, the mass percentage content of La element can be controlled to be La≤0.006wt%, the mass percentage content of V element can be controlled to be V≤0.1wt%, and the mass percentage content of Cr element can be controlled to be Cr≤0.2wt%. In some embodiments, in the low-alloy high-strength steel described in the present application, the mass percentage content of Nb element is controlled to be between 0.006 and 0.015%. In some embodiments, in the low-alloy high-strength steel described in the present application, the mass percentage content of Ce element is controlled to be between 0.001 and 0.006%. In some embodiments, in the low-alloy high-strength steel described in the present application, the mass percentage content of La element is controlled to be between 0.001 and 0.006%. In some embodiments, in the low-alloy high-strength steel described in the present application, the mass percentage content of V element is controlled to be between 0.03 and 0.1%. In some embodiments, in the low-alloy high-strength steel described in the present application, the mass percentage content of Cr element is controlled to be between 0.05 and 0.2%.

[0037] Further, in the low-cost low-carbon equivalent cold-rolled high-strength steel described in the present application, the mass percentage content of each chemical element satisfies at least one of the following:

[0038] Ti / (Al+Nb)≥3.0;

[0039] 0.5%≤Mn+Cr-Ti-Nb-V≤1.0%;

[0040] wherein each element symbol is substituted with the mass percentage content value of the corresponding element.

[0041] In the present application, a certain balance needs to be found among the carbonitride-forming abilities of the three elements Al, Ti, and Nb. Al, Ti, and Nb are all carbonitride-forming elements, but in terms of carbonitride-forming ability, Nb > Ti > Al. If the content of Al in the steel is too high and the contents of Ti and Nb are too low, then during the manufacturing process, less carbonitride is formed, and the strength of the steel is not enough; if the content of Nb in the steel is too high, although it is easier to form carbonitride during the manufacturing process, on the one hand, it will cause the carbonitride to be coarse, and on the other hand, it will consume more C elements, resulting in insufficient or inability to form granular bainite; if the content of Ti is too low and the contents of Nb or Al are too high, it will cause the carbonitride precipitation and the granular bainite phase transformation to be unbalanced, resulting in deterioration of the strength and forming performance of the product. Therefore, in the low-alloy high-strength steel described in the present application, Ti / (Al+Nb)≥3.0 is also controlled.

[0042] In the chemical composition design of the present application, while controlling the content of a single element, the ratio of alloying elements can also be controlled, such that 0.5%≤Mn+Cr-Ti-Nb-V≤1.0%, in order to regulate the optimal ratio of carbonitride precipitation and granular bainite formation. Among them, Mn and Cr are beneficial to the formation of granular bainite, and Ti, Nb, and V are strong carbide-forming elements, therefore, the higher the value of the above formula, the more beneficial to the formation of granular bainite, and the lower the value of the above formula, the more beneficial to the precipitation of carbonitride.

[0043] Further, in the unavoidable impurities of the low-cost low-carbon equivalent cold-rolled high-strength steel described in the present application, P≤0.02wt%, S≤0.006wt%, N≤0.006wt%.

[0044] 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.

[0045] Further, in the low-cost low-carbon equivalent cold-rolled high-strength steel described in the present application, the volume fraction of ferrite in the microstructure is ≥90%, the volume fraction of granular bainite is ≤4%, and the volume fraction of carbonitride is ≥6%. In some embodiments, in the low-cost low-carbon equivalent cold-rolled high-strength steel described in the present application, the volume fraction of granular bainite in the microstructure is 0.5-4%. In some embodiments, in the low-cost low-carbon equivalent cold-rolled high-strength steel described in the present application, the volume fraction of carbonitride in the microstructure is 6-9%.

[0046] Further, in the low-cost low-carbon equivalent cold-rolled high-strength steel according to the present application, the carbon equivalent Cev is ≤0.28, wherein Cev = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu + Si) / 15, and each chemical element is substituted by the value before the mass percentage.

[0047] In the present application, by controlling the carbon equivalent Cev to be ≤0.28, the steel can be ensured to have excellent weldability.

[0048] Further, in the low-cost low-carbon equivalent cold-rolled high-strength steel according to the present application, the yield strength is ≥700 MPa, for example, >700 MPa, the elongation at break is ≥10%, and the ratio of 90° bending radius to plate thickness r / t is ≤0.3.

[0049] In some embodiments, the tensile strength of the low-cost low-carbon equivalent cold-rolled high-strength steel according to the present application is ≥750 MPa, for example, 750-900 Pa, 750-860 MPa.

[0050] In some embodiments, the yield strength of the low-cost low-carbon equivalent cold-rolled high-strength steel according to the present application is ≥700 MPa, for example, 700-850 MPa, 700-810 MPa.

[0051] In some embodiments, the elongation at break of the low-cost low-carbon equivalent cold-rolled high-strength steel according to the present application is 10-16%.

[0052] In some embodiments, the ratio of 90° bending radius to plate thickness r / t of the low-cost low-carbon equivalent cold-rolled high-strength steel according to the present application is ≤0.3, for example, 0.1-0.3.

[0053] Another object of the present application is to provide a manufacturing method of cold-rolled high-strength steel, which is green and efficient, and the low-alloy high-strength steel produced by the method has the advantages of green low-carbon and low-alloy addition amount, and has good mechanical strength.

[0054] To achieve the above object, the present application provides a manufacturing method of cold-rolled high-strength steel, which comprises the steps of:

[0055] smelting and casting;

[0056] hot rolling;

[0057] pickling and cold rolling: controlling the cold rolling reduction rate to be 45-90%;

[0058] annealing: heating the strip steel to 770-820℃ and holding, then cooling to 450-550℃ at a first cooling rate of 10-40℃ / s and holding, and then cooling to room temperature at a second cooling rate of 1-5℃.

[0059] The manufacturing method can find the optimal balance point of phase transition strengthening and precipitation strengthening by introducing a small amount of granular bainite, so that higher strength and excellent forming performance than traditional ferrite-based low alloy high strength steel can be obtained without introducing additional alloy and using incomplete annealing.

[0060] In the pickling and cold rolling step of the present application, by controlling the cold rolling reduction to 45-90%, it can be ensured that the subsequent steel strip can be fully recrystallized during annealing.

[0061] In the annealing step of the present application, higher heating and holding temperature can ensure the secondary dissolution of carbonitride in the steel strip, and also ensure the formation of a small amount of austenite parent phase; the first cooling and short holding stage can ensure the formation of a small amount of granular bainite and carbonitride precipitates; the second slow cooling and final cooling can further promote the fine and dispersed precipitation of carbonitride. Through the annealing process described in the present application, the steel can obtain a large amount of ferrite + a small amount of granular bainite + submicron-sized carbonitride structure, and ensure that the content of ferrite is ≥ 90%, the proportion of granular bainite is ≤ 4%, and the proportion of carbonitride is ≥ 6%.

[0062] Further, in the annealing step of the manufacturing method of the cold rolled high strength steel described in the present application, the steel strip is heated to 770-820℃ and held for 90-180s, then cooled to 450-550℃ at a first cooling rate of 10-40℃ / s and held for 5-15s, then cooled to room temperature at a second cooling rate of 1-5℃.

[0063] Further, in the annealing step of the manufacturing method of the cold rolled high strength steel described in the present application, hot dip galvanizing of the steel strip surface is completed in the process of cooling to 450-550℃ at a first cooling rate of 10-40℃ / s and holding.

[0064] Further, in the hot rolling step of the manufacturing method of the cold rolled high strength steel described in the present application, the slab discharge temperature is controlled to 1250-1300℃, and the finish rolling temperature is 900-970℃.

[0065] Further, in the hot rolling step of the manufacturing method of the cold rolled high strength steel described in the present application, the steel strip is cooled to 440-530℃ (e.g. 460-530℃) at a cooling rate of ≥ 80℃ / s after rolling for coiling. In some embodiments, the cooling rate after rolling is 80-150℃ / s.

[0066] In the hot rolling step of the present application, the higher slab out-furnace temperature can ensure that the carbonitride formed first in the slab can be fully redissolved; the higher finishing temperature and ultra-fast cooling speed can avoid batch precipitation and coarsening of the carbonitride; and through low-temperature coiling at 440-530℃ (for example, 440-520℃, 460-530℃), a trace amount of fine carbonitride can be formed in the strip.

[0067] The low-cost low-carbon equivalent cold-rolled high-strength steel and the manufacturing method thereof have the following advantages and beneficial effects:

[0068] The cold-rolled high-strength steel has low alloy addition and low carbon equivalent, and also has high strength.

[0069] In some embodiments, the yield strength is ≥700 MPa, for example, >700 MPa, the elongation at break is ≥10%, and the ratio of the 90° bending radius to the plate thickness r / t is ≤0.3, and it can be widely used in the preparation of various automobile structural parts. BRIEF DESCRIPTION OF DRAWINGS

[0070] Figure 1 shows the microstructure morphology of the cold-rolled high-strength steel of Example 1. DETAILED DESCRIPTION

[0071] The low-cost low-carbon equivalent cold-rolled high-strength steel and the manufacturing method thereof will be further explained and described in conjunction with specific examples, but the explanation and description do not constitute undue limitations on the technical solutions of the present application.

[0072] Examples 1-6 and Comparative Examples 1-3

[0073] Table 1-1 and Table 1-2 list the mass percentages of each chemical element in the cold-rolled high-strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3.

[0074] Table 1-1. (the balance is Fe and other unavoidable impurities other than P, S and N)

[0075] Table 1-2. (the balance is Fe and other unavoidable impurities other than P, S and N)

[0076] The cold-rolled high-strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3 are both prepared by the following steps:

[0077] (1) smelting and continuous casting;

[0078] (2) Hot rolling: the slab discharge temperature is controlled to be 1250-1300 °C, and the finish rolling temperature is controlled to be 900-970 °C; after rolling, the ultra-fast cooling and low-temperature coiling method is adopted to cool the steel strip to 460-530 °C at a cooling rate of ≥80 °C / s and then coiling;

[0079] (3) Pickling and cold rolling: the cold rolling reduction is controlled to be 45-90 %;

[0080] (4) Annealing: the strip is heated to 770-820 °C and held for 90-180 s, then cooled to 450-550 °C at a first cooling rate of 10-40 °C / s and held for 5-15 s, and then cooled to room temperature at a second cooling rate of 1-5 °C / s.

[0081] In addition, in some embodiments, the hot-dip galvanizing of the surface of the strip can be completed in the process segment of cooling to 450-550 °C at a first cooling rate of 10-40 °C / s and holding in step (4), so as to obtain a hot-dip galvanized product.

[0082] 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, while the components of the comparative example 1 are the same as those of the embodiment 6, and the process parameters thereof do not meet the present application, and although the process parameters of the comparative examples 2-3 meet the design of the present application, the component design thereof does not meet the design of the present application.

[0083] Table 2-1 and Table 2-2 list the specific process parameters of the cold-rolled high-strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3.

[0084] Table 2-1.

[0085] Table 2-2. Process parameters of the annealing step

[0086] In order to verify the implementation effect of the present application, the cold-rolled high-strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3 are sampled, and after polishing, the metallographic samples are etched by 4 % nitric acid alcohol solution for about 10 s, washed and dried, and then microstructure observation is carried out by using an optical microscope and a scanning electron microscope, and the observation results are listed in Table 3. Among them, the volume phase ratio of ferrite and granular bainite is observed by an optical microscope, and the volume phase ratio of carbonitride is confirmed by a scanning electron microscope.

[0087] Table 3 lists the microstructure observation results of the cold-rolled high-strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3.

[0088] Table 3.

[0089] As can be seen from the above Table 3, the microstructure of the cold-rolled high-strength steel of the embodiments 1-6 of the present application contains ferrite + granular bainite + precipitated carbonitride, the volume phase ratio of ferrite is greater than or equal to 90%, the volume phase ratio of granular bainite is less than or equal to 4, and the volume phase ratio of carbonitride is greater than or equal to 6%.

[0090] In addition, Figure 1 shows the microstructure morphology of Example 1.

[0091] As described in Figure 1, the microstructure of Example 1 is ferrite + granular bainite + carbonitride, wherein the volume phase ratio of carbonitride is 6%, the volume phase ratio of granular bainite is 0.5%, and the rest is ferrite.

[0092] The cold-rolled high-strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3 were resampled and tested for various properties, and the test results are listed in Table 4. Among them, the various property tests include:

[0093] Tensile property test: GB / T 228.1-2021 "Metallic materials - Tensile test - Part 1: Method of test at room temperature" was used to test the low-cost cold-rolled low-alloy high-strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3.

[0094] Bending property test: GB / T 232-2010 "Metallic materials - Bending test methods" was used to test the low-cost cold-rolled low-alloy high-strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3.

[0095] Table 4.

[0096] As can be seen from the above Table 4, the yield strength of the cold-rolled high-strength steel of the embodiments 1-6 of the present application is greater than 700 MPa, the elongation at break is greater than 10%, and the ratio of 90° bending radius to plate thickness r / t is less than or equal to 0.3.

[0097] 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 technical features described in the present case can be freely combined or combined in any way, unless contradictory to each other.

[0098] 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 all should belong to the protection scope of the present application.

Claims

1. A low-cost, low-carbon equivalent cold-rolled high-strength steel, containing Fe and unavoidable impurities, characterized in that, It also contains the following chemical elements in the following mass percentages: C: 0.04–0.08%; 0 < Si ≤ 0.2%; Mn: 0.65–1.15%; Al: 0.02–0.05%; Ti: 0.10–0.16%; Its microstructure contains ferrite, granular bainite, and precipitated carbonitrides.

2. The cold-rolled high-strength steel as described in claim 1, characterized in that, The mass percentage of each chemical element is as follows: C: 0.04–0.08%; 0 < Si ≤ 0.2%; Mn: 0.65–1.15%; Al: 0.02–0.05%; Ti: 0.10–0.16%; The balance is Fe and unavoidable impurities.

3. The cold-rolled high-strength steel as described in claim 1, characterized in that, It also contains at least one of the following elements: 0 < Nb ≤ 0.015 wt%; 0 < Ce ≤ 0.006 wt%; 0 < La ≤ 0.006 wt%; 0 < V ≤ 0.1 wt%; 0 < Cr ≤ 0.2 wt%.

4. The cold-rolled high-strength steel as described in claim 1, characterized in that, The mass percentage content of each chemical element satisfies at least one of the following conditions: Ti / (Al+Nb)≥3.0; 0.5%≤Mn+Cr-Ti-Nb-V≤1.0%.

5. The cold-rolled high-strength steel as described in claim 1, characterized in that, Of its unavoidable impurities, P ≤ 0.02 wt%, S ≤ 0.006 wt%, and N ≤ 0.006 wt%.

6. The cold-rolled high-strength steel as described in claim 1, characterized in that, Its microstructure has a volume fraction of ferrite ≥90%, a volume fraction of granular bainite ≤4%, and a volume fraction of carbonitrides ≥6%.

7. The cold-rolled high-strength steel as described in claim 1, characterized in that, Its carbon equivalent Cev≤0.28, where Cev=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu+Si) / 15, where each chemical element is replaced with the value before the percentage sign of its mass content.

8. The cold-rolled high-strength steel as described in claim 1, characterized in that, Its yield strength is ≥700MPa, its elongation at break is ≥10%, and the ratio of the 90° bending radius to the plate thickness r / t is ≤0.

3.

9. The cold-rolled high-strength steel as described in claim 1, characterized in that, Its tensile strength is ≥750MPa.

10. The method for manufacturing cold-rolled high-strength steel according to any one of claims 1-9, characterized in that, It includes the following steps: Smelting and casting; Hot-rolled; Pickling and cold rolling: control the cold rolling reduction rate to 45-90%; Annealing: Heat the strip to 770-820℃ and hold it at that temperature, then cool it to 450-550℃ at a first cooling rate of 10-40℃ / s and hold it at that temperature, then cool it to room temperature at a second cooling rate of 1-5℃.

11. The manufacturing method as described in claim 10, characterized in that, In the annealing step, the strip is heated to 770-820℃ and held for 90-180s, then cooled to 450-550℃ at a first cooling rate of 10-40℃ / s and held for 5-15s, and then cooled to room temperature at a second cooling rate of 1-5℃.

12. The manufacturing method as described in claim 10, characterized in that, In the annealing step, hot-dip galvanizing of the strip surface is completed in a process section where the strip is cooled to 450-550°C at a first cooling rate of 10-40°C / s and held at that temperature.

13. The manufacturing method as described in claim 10, characterized in that, During the hot rolling process, the slab exit temperature is controlled at 1250–1300℃.

14. The manufacturing method as described in claim 10, characterized in that, In the hot rolling process, the finishing rolling temperature is controlled to be 900–970℃.

15. The manufacturing method as described in claim 10, characterized in that, In the hot rolling process, the strip is cooled to 460-530°C at a cooling rate of ≥80°C / s after rolling and then coiled.

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