Low-alloy ultrahigh-strength steel with excellent drawing performance and manufacturing method of low-alloy ultrahigh-strength steel

Abstract

The invention discloses a low-alloy ultrahigh-strength steel with excellent drawing performance, which contains Fe and inevitable impurities, and also contains the following chemical elements in percentage by mass: 0.18-0.25% of C; 0.4 to 0.9 percent of Si; mn: 1.55 to 1.95%; b: 0.001 to 0.004 percent of the total weight of the raw materials; 0.03 to 0.3 percent of Al; ti: 0.086 to 0.15%; the microstructure of the steel plate comprises retained austenite and nanoscale carbide. The invention further discloses a manufacturing method of the low-alloy ultrahigh-strength steel. The manufacturing method comprises the following steps: smelting and casting; hot rolling; pickling and cold rolling; and annealing.

Description

Technical Field

[0001] This invention relates to a steel plate and its manufacturing method, and more particularly to a low-alloy ultra-high-strength steel and its manufacturing method. Background Technology

[0002] With the development of lightweight vehicles, more and more automotive structural components are made of ultra-high strength steel, which puts forward increasingly higher requirements for the forming performance, especially the drawing performance, of ultra-high strength steel.

[0003] In the existing technology, the solution to improve the drawing performance of ultra-high strength steel is to add a large proportion of Mn, Si and / or Al elements to the steel composition design, thereby introducing a large proportion of retained austenite into the steel matrix.

[0004] For example, Chinese patent document CN116732448A, published on September 12, 2023, entitled "A 1000MPa Grade Hot-Dip Galvanized Reinforced Formed Multiphase Steel and Its Preparation Method," discloses a 1000MPa grade hot-dip galvanized reinforced formed multiphase steel and its preparation method. This steel exhibits excellent drawing properties with a uniform elongation of 8%, but it contains 2.4–2.9% Mn, 1.0–1.5% Al, and 0.4–0.8% Cr. However, the addition of large amounts of Mn, Cr, Si, and / or Al elements significantly increases the alloy cost of the steel and impairs its weldability, severely limiting its application in automotive parts manufacturing. Summary of the Invention

[0005] One of the objectives of this invention is to provide a low-alloy ultra-high-strength steel with excellent drawing performance. This low-alloy ultra-high-strength steel is designed with a lower alloy composition to achieve excellent drawing performance and mechanical strength.

[0006] To achieve the above objectives, the present invention provides a low-alloy ultra-high-strength steel with excellent drawing properties, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

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

[0008] Its microstructure contains retained austenite and nanoscale carbides.

[0009] In this invention, the inventors designed a relatively low alloy composition to generate bainite, retained austenite, and nanoscale carbides in the microstructure. They then utilized the TRIP effect of the transformation of retained austenite to martensite and the hindering effect of nanoscale carbides on the high-density dislocation slip in bainite and newly formed martensite to improve the drawing performance of ultra-high strength steel.

[0010] Furthermore, in the low-alloy ultra-high-strength steel described in this invention, the mass percentage content of each chemical element is as follows:

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

[0012] The design principles of each chemical element in the low-alloy ultra-high-strength steel described in this invention are as follows:

[0013] C: In the low-alloy ultra-high-strength steel described in this invention, carbon (C) not only controls the phase transformation of the microstructure and affects the formation of retained austenite, but also directly affects the precipitation, distribution, and grain size of carbides. Simultaneously, carbon plays a decisive role in the strength and drawing properties of the steel in this invention. Therefore, in the low-alloy ultra-high-strength steel described in this invention, the mass percentage content of carbon is controlled between 0.18% and 0.25%.

[0014] Si: In the low-alloy ultra-high-strength steel described in this invention, Si is not only an essential element for deoxidation and solid solution strengthening in steelmaking, but also directly affects carbide precipitation and the formation of retained austenite. When the mass percentage content of Si is too high, it will affect carbide precipitation and bainite formation. Therefore, in the low-alloy ultra-high-strength steel described in this invention, the mass percentage content of Si is controlled between 0.4% and 0.9%.

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

[0016] B: In the low-alloy ultra-high-strength steel described in this invention, element B not only promotes the formation of bainite in the steel but also significantly affects the strength and hardness of the steel plate. When the mass percentage content of element B is too low, the strength of the steel will not meet the design requirements. When the mass percentage content of element B is too high, brittle borides are easily formed, thereby affecting the formability and uniformity of the steel plate. Therefore, in the low-alloy ultra-high-strength steel described in this invention, the mass percentage content of element B is controlled between 0.001% and 0.004%.

[0017] Al: In the low-alloy ultra-high-strength steel described in this invention, Al can be used as a deoxidizing element and a trace strengthening element to enhance the quality and strength of the steel. It can also be used as a phase transformation control element to induce the formation of retained austenite. Therefore, in the low-alloy ultra-high-strength steel described in this invention, the mass percentage content of Al is controlled between 0.03% and 0.3%.

[0018] Ti: In the low-alloy ultra-high-strength steel described in this invention, Ti is a strong carbonitride forming element. The carbonitrides formed can refine the grains on the one hand, and hinder dislocation slip in the steel on the other hand, thereby improving the strength and formability of the steel. Therefore, in the low-alloy ultra-high-strength steel described in this invention, the mass percentage content of Ti is controlled between 0.086% and 0.15%.

[0019] Furthermore, the low-alloy ultra-high-strength steel described in this invention also contains at least one of the following elements:

[0020] 0 < Mo ≤ 0.2 wt%

[0021] 0 < Cr ≤ 0.2 wt%;

[0022] 0 < Nb ≤ 0.06 wt%;

[0023] 0 < Cu ≤ 0.2 wt%;

[0024] 0 < Ce ≤ 0.006 wt%;

[0025] 0 < V ≤ 0.2 wt%.

[0026] In this invention, Mo, Cr, Nb, Cu, Ce, and V can be selectively added to the steel as optional elements. Wherein:

[0027] Cr and Mo: In the low-alloy ultra-high-strength steel described in this invention, Cr and Mo elements can increase the hardenability of the steel strip, prolong the incubation period of pearlite and ferrite, inhibit the formation of pearlite and ferrite, and facilitate the formation of bainite structure during cooling. When the mass percentage of Cr and Mo is too low, insufficient bainite formation will occur; when the mass percentage of Cr and Mo is too high, more martensite structure will easily form, deteriorating the formability of the steel strip. Based on this, in the low-alloy ultra-high-strength steel described in this invention, the mass percentage of Mo element can be controlled to Mo≤0.2wt%, and the mass percentage of Cr element can be controlled to Cr≤0.2wt%.

[0028] In addition, as optional elements, Nb, Cu, Ce, and V can refine grains and improve austenite stability, with Cu being an austenite stabilizing element. In this invention, although the addition of these optional elements is beneficial for increasing the bainite and retained austenite proportions and improving the formability of the steel, the amount of these elements added needs to be controlled to an upper limit from the perspective of cost control and ensuring weldability. Therefore, in the low-alloy high-strength steel described in this invention, the mass percentage content of Nb can be controlled to Nb≤0.06wt%, the mass percentage content of Cu can be controlled to Cu≤0.2wt%, the mass percentage content of Ce can be controlled to Ce≤0.006wt%, and the mass percentage content of V can be controlled to V≤0.2wt%.

[0029] Furthermore, in the low-alloy ultra-high-strength steel described in this invention, the mass percentage content of each chemical element satisfies at least one of the following conditions:

[0030] Mn+Si+Cr+Al+Mo+Cu≤3.5%;

[0031] 1.6≤Mn / (Si+Al)≤3.2;

[0032] 0.15≤(Ti+V+2Nb+Mo / 4) / (Si+Al)≤0.40.

[0033] In this invention, controlling Mn+Si+Cr+Al+Mo+Cu≤3.5% can ensure low manufacturing cost, low carbon equivalent level and good weldability of steel.

[0034] In this invention, Mn is the main austenite forming element, while Si and Al are the main ferrite forming elements. To ensure that there is a suitable ratio of retained austenite to acicular / granular bainite and tempered martensite in the microstructure, the ratio of the two types of elements needs to be considered in a coordinated manner. Therefore, 1.6≤Mn / (Si+Al)≤3.2 is controlled.

[0035] In this invention, since Ti, V, and Nb are strong carbide-forming elements, Mo is a weak carbonitride-forming element, and the addition of Al and Si inhibits carbide formation in bainite, it is necessary to consider the combined effects of these two types of elements on carbide precipitation and size. Therefore, it is necessary to control 0.15≤(Ti+V+2Nb+Mo / 4) / (Si+Al)≤0.4.

[0036] Furthermore, in the unavoidable impurities of the low-alloy ultra-high-strength steel described in this invention, P ≤ 0.015 wt%, S ≤ 0.003 wt%, and N ≤ 0.004 wt%.

[0037] In this invention, unavoidable impurities mainly include phosphorus (P), sulfur (S), and nitrogen (N), and it is desirable to have as little of these as possible when technical conditions permit.

[0038] Furthermore, in the low-alloy ultra-high strength steel described in this invention, the volume ratio of retained austenite in its microstructure is 4-12%.

[0039] Furthermore, in the low-alloy ultra-high-strength steel described in this invention, the volume ratio of nanoscale carbides in its microstructure is 3-8%.

[0040] Furthermore, in the low-alloy ultra-high-strength steel described in this invention, its microstructure also includes acicular bainite, granular bainite, and tempered martensite.

[0041] Furthermore, in the low-alloy ultra-high-strength steel described in this invention, the number of nanoscale carbides satisfies the following condition: per 1 μm 2 Within the detection field of view, there exists an area of ​​10000nm. 2 The region contains 1 to 20 nanoscale carbides.

[0042] Furthermore, in the low-alloy ultra-high strength steel described in this invention, its tensile strength is ≥980MPa, uniform elongation is ≥10%, and elongation at break is ≥14%.

[0043] Another objective of this invention is to provide a method for manufacturing low-alloy ultra-high-strength steel, which can obtain a low-alloy ultra-high-strength steel with excellent drawing properties.

[0044] To achieve the above objectives, the present invention provides a method for manufacturing low-alloy ultra-high-strength steel, comprising the following steps:

[0045] Smelting and casting;

[0046] Hot-rolled;

[0047] Pickling and cold rolling;

[0048] Annealing: Heat the strip steel to 860-950℃ and hold it at that temperature. Then, cool it to 720-820℃ at a slow cooling rate of 5-20℃ / s and hold it at that temperature. Next, cool it to 280-380℃ at a rapid cooling rate of 20-80℃ and hold it at that temperature. Then, reheat it to 390-480℃ at a heating rate of 5-30℃ / s and hold it at that temperature. Then, cool it to 300-380℃ at a secondary rapid cooling rate of 5-50℃ / s and hold it at that temperature. Finally, cool it to room temperature at a final cooling average rate of ≤10℃ / s.

[0049] In the annealing step of this invention, controlling the strip heating temperature at 860–950°C ensures the full formation of the austenite parent phase and the secondary redissolution of carbides, refining the grain size and carbide size. Cooling to 720–820°C at a slow cooling rate of 5–20°C / s and holding at that temperature can prevent the formation of ferrite and regulate the precipitation of carbides. Cooling to 280–380°C at a rapid cooling rate of 20–80°C and holding at that temperature allows the strip to enter the martensitic or bainitic phase transformation region. Reheating to 390–480°C at a heating rate of 5–30°C / s and holding at that temperature, followed by a second rapid cooling to 300–380°C at a rapid cooling rate of 5–50°C / s and holding at that temperature, ensures the fine and dispersed precipitation of carbides while the residual austenite forms. Cooling to room temperature at a final cooling average rate of ≤10°C / s further promotes the fine and dispersed precipitation of carbides.

[0050] Furthermore, in the annealing step of the manufacturing method described in this invention, the strip steel is heated to 860-950°C and held for 90-180 seconds, then cooled to 720-820°C at a slow cooling rate of 5-20°C / s and held for 3-20 seconds, then cooled to 280-380°C at a rapid cooling rate of 20-80°C and held for 1-10 seconds, then reheated to 390-480°C at a heating rate of 5-30°C / s and held for 3-20 seconds, then cooled to 300-380°C at a secondary rapid cooling rate of 5-50°C / s and held for 100-200 seconds, and finally cooled to room temperature at a final cooling average rate of ≤10°C / s.

[0051] Furthermore, in the annealing step of the manufacturing method described in this invention, hot-dip galvanizing of the strip surface is completed in a process segment where the strip is reheated to 390-480°C at a heating rate of 5-30°C / s and held at that temperature.

[0052] Furthermore, in the hot rolling step of the manufacturing method described in this invention, the slab exit temperature is controlled at 1260–1300°C, the rough rolling temperature at 1070–1120°C, and the finishing rolling temperature at 890–970°C.

[0053] In this invention, controlling the slab exit temperature at 1260–1300°C ensures that the carbonitrides formed in the slab can be fully re-dissolved; controlling the roughing temperature at 1070–1120°C and the finishing rolling temperature at 890–970°C avoids the bulk precipitation and coarsening of carbides, resulting in only a small amount of fine carbides, which is beneficial to improving the elongation at break.

[0054] Furthermore, in the hot rolling step of the manufacturing method described in this invention, the strip is cooled to 450-510°C at a cooling rate of 30-80°C / s after rolling and then coiled.

[0055] In this invention, the above-mentioned post-rolling cooling process can avoid the formation of uneven martensite structure in the hot-rolled structure; controlling the coiling temperature at 450-510°C can avoid excessive precipitation and coarsening of carbides in the hot-rolled coil, thereby affecting the microstructure control during subsequent annealing.

[0056] The low-alloy ultra-high-strength steel with excellent drawing properties and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0057] In the low-alloy ultra-high strength steel with excellent drawing performance described in this invention, bainite, retained austenite and nanoscale carbides are generated in the microstructure through a relatively low alloy composition design. The drawing performance of the ultra-high strength steel is improved by utilizing the TRIP effect of the transformation of retained austenite to martensite and the hindering effect of nanoscale carbides on the high-density dislocation slip in bainite and newly formed martensite.

[0058] In some embodiments, the low-alloy ultra-high-strength steel with excellent drawing properties described in this invention has a tensile strength ≥980MPa, a uniform elongation ≥10%, and a fracture elongation ≥14%. Furthermore, due to its low alloy content, it can be widely used in the preparation of various automotive structural components. Attached Figure Description

[0059] Figure 1 The microstructure of nanoscale carbides observed under a transmission electron microscope in Example 1 of the present invention is shown.

[0060] Figure 2 The engineering stress-strain curves of Examples 1 and 2 are shown schematically. Detailed Implementation

[0061] The following will further explain and illustrate the low-alloy ultra-high-strength steel with excellent drawing properties and its manufacturing method with reference to specific embodiments and accompanying drawings. However, this explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0062] Examples 1-6 and Comparative Examples 1-3

[0063] Tables 1-1 and 1-2 list the mass percentages of each chemical element in the low-alloy ultra-high-strength steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3.

[0064] Table 1-1. (The balance is Fe and other unavoidable impurities besides P, S, and N)

[0065]

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

[0067]

[0068] The low-alloy ultra-high-strength steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3 of this invention were all prepared using the following steps:

[0069] (1) Smelting and continuous casting;

[0070] (2) Hot rolling: The slab exit temperature is controlled at 1260~1300℃, the rough rolling temperature is 1070~1120℃, the finishing rolling temperature is 890~970℃, and the strip is cooled to 450~510℃ after rolling at a cooling rate of 30~80℃ / s before coiling.

[0071] (3) Pickling and cold rolling;

[0072] (4) Annealing: Heat the strip steel to 860-950℃ and hold for 90-180s, then cool it to 720-820℃ at a slow cooling rate of 5-20℃ / s and hold for 3-20s, then cool it to 280-380℃ at a fast cooling rate of 20-80℃ and hold for 1-10s, then reheat it to 390-480℃ at a heating rate of 5-30℃ / s and hold for 3-20s, then cool it to 300-380℃ at a secondary fast cooling rate of 5-50℃ / s and hold for 100-200s, and then cool it to room temperature at a final cooling average rate of ≤10℃ / s.

[0073] In addition, in some embodiments, the hot-dip galvanizing of the strip surface can be completed in step (4) by reheating to 390-480°C at a heating rate of 5-30°C / s and holding the temperature thereafter, thereby obtaining a hot-dip galvanized product.

[0074] It should be noted that the components and processes of Examples 1-6 of the present invention all meet the requirements of the present invention. The components of Comparative Example 1 are the same as those of Example 1, but its process parameters do not conform to the present invention. The process parameters and component designs of Comparative Examples 2-3 do not conform to the design of the present invention.

[0075] Tables 2-1, 2-2, and 2-3 list the specific process parameters for the low-alloy ultra-high-strength steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3.

[0076] Table 2-1.

[0077]

[0078] Table 2-2 Process parameters for step (4)

[0079]

[0080]

[0081] Table 2-3 Process parameters for step (4)

[0082]

[0083] To verify the effectiveness of this invention, samples were taken from the low-alloy ultra-high-strength steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3. The polished metallographic samples were etched with a 4% nitric acid alcohol solution for approximately 10 seconds. After clear drying, the microstructure was observed using an optical microscope or a scanning electron microscope, and the observation results are listed in Table 3. Wherein:

[0084] per 1μm 2 The method for detecting the number of nanoscale carbides in the detection field of view is as follows: A transmission electron microscope (TEM) sample is prepared by taking a polished steel plate and placing it parallel or perpendicular to the surface of the steel plate. The thickness of the sample edge is ≤30μm. Then, a thin region in the center of the sample is prepared by using methods such as FIB, ion thinning, and double-jet thinning. The distribution of nanoscale carbides is then detected by bright-field imaging under a TEM.

[0085] Table 3 lists the microstructure observation results of the low-alloy ultra-high strength steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3 of the present invention.

[0086] Table 3.

[0087]

[0088]

[0089] As can be seen from Table 3 above, the microstructure of the low-alloy ultra-high-strength steels of Examples 1-6 of the present invention contains retained austenite and nano-sized carbides, as well as acicular bainite, granular bainite, and tempered martensite. The volume fraction of retained austenite is between 4-12%, and the volume fraction of carbides is between 3-8%, with a density of approximately 1 μm. 2 The number of nanoscale carbides contained in the detection field ranges from 1 to 20.

[0090] Figure 1The microstructure of nanoscale carbides observed under a transmission electron microscope in Example 1 of the present invention is shown.

[0091] like Figure 1 As shown, the nanoscale carbides in Example 1 hindered the slip of high-density dislocations.

[0092] The low-alloy ultra-high-strength steels of Examples 1-6 and the control steels of Comparative Examples 1-3 were sampled again and subjected to various tests. The test results are listed in Table 4. Among them, the tensile properties were tested according to GB / T228.1-2021 "Metallic materials, tensile testing - Part 1: Test at room temperature".

[0093] Table 4.

[0094]

[0095]

[0096] As can be seen from Table 4 above, the tensile strength of the low alloy ultra-high strength steels in Examples 1-6 of the present invention is greater than 980 MPa, the uniform elongation is greater than 10%, and the fracture elongation is greater than or equal to 14%.

[0097] also, Figure 2 The engineering stress-strain curves of Examples 1 and 2 are shown schematically.

[0098] like Figure 2 As shown, the tensile strength of Examples 1 and 2 both exceed 980 MPa, the uniform elongation is greater than 10%, and the elongation at break is greater than 14%.

[0099] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0100] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

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

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

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

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

40.

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

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

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

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

9. The low-alloy ultra-high-strength steel as described in claim 1 or 2, characterized in that, The number of nanoscale carbides satisfies the following condition: per 1 μm 2 Within the detection field of view, there exists an area of ​​10000nm. 2 The region contains 1 to 20 nanoscale carbides.

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

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

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

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

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

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

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